CONTINUOUS HARVEST OF CELLULOSIC FILAMENT DURING CULTIVATION OF ACETOBACTER XYLINUM Seiichi Tokura I, Hiroshi Tamura I, Mitsuo Takai *,Taiichi Higuchi and Hisashi Asano 'Faculty of Engineering, Kansai University and HRC, Yamate-cho, Suita, Osaka, 564-8680 Japan. 'Graduate School of Engineering, llokkaido University, Sapporo 065-0012. Japan. 'Graduate School of Environmental Earth Sci., Hokkaido University, Sapporo 065-0012 Japan.
ABSTRACT A shallow pan cultivation of Acetobactor xylinum has been studied using a continuous wind up roller to give swollen bacterial ccllulosc (BC) filament. The swollen filament was stretched with a twisting mode to rcmove water following the purifying procedures such as boiling in aqucous sodium dodecyl sulfate and in aqueous sodium hydroxidc, respectively. Modified BC filament containing residual N-acctylglucosamine (GlcNAc) was also obtaincd, when A. xylinum subcultured in GlcNAc medium was applied in a medium containing glucose and ammonium chloride. The BC and modified filaments showcd higher tensile strcngth than those of cotton and rayon, but poorer elongation and relatively lowcr Young' s modulus than those of cotton and rayon. Highcr orientation of molecule, as shown by X-ray diffraction pattern analysis, was achievcd by applying a heavier weight during drying procedure. The molecular weights of BC products were estimated viscometrically using coppcr-ammonium solvents.
INTRODUCTION It has been establishcd that A. xylinum produces bacterial cellulosc (BC) under static incubation in Hestrin-Schramm (HS) medium containing glucose as a carbon source 4. Because BC is a pure cellulose, it has attracted much attention in various manufacturing fields. In our previous studies on the preparation of BC, we had developed novel proccdures for producing molecular variants of BC. Incorporation of amino-sugar residues has been successfully attained by incubation of the bacteria that had been subcultured rcpcatedly in a medium containing N-acetylglucosamine (GlcNAc) and glucose (Glc) or only GlcNAc as carbon source 5. The subculture of bactcria was required in GlcNAc mcdium to form thc pcllicle containing GlcNAc. Sincc a similar dcgrcc of GlcNAc incorporation was found by employing media containing Glc and either galactosamine or glucosamine but not by mannosaminc, the activation of transaminase was assumed to be rout of metabolic cycle of thc bacteria 6. Undcr rotatory but not static conditions, a similar degree of GlcNAc incorporation was also observed when cultivation was carricd out with air bubbling in a medium containing Glc and ammonium chloride 6, Although much effort have bccn devoted to the preparation and utilization of BC and its analogs, these methods and agents have not yet bccn incorporatcd as an industrial resource,
4
New sources, structure and properties of cellulose
largely because of the high production costs involved. Given that complicated procedures are required for the preparation of fibers and films from these biosynthesized polysaccharides 7, a simple method, one that can serve to reduce these high production costs, has bccn required. We describe the results of our latest investigations to simplify the production processes, which were prompted by recent successes in regard to surface polymerization of nylon capable of giving supcrpolymer films or filaments directly from the interface of organic and aqucous phases *. We have designed shallow pans to increase the BC yield, to regulate the gel thickness during the incubation and to conserve the total amount of the medium used. The tensile strcngth of thc filamcnt was significantly greater than that of the ordinary cellulose fibers, and good orientation of molecules was revealed in the X-ray diffraction pattern.
EXPERIMENTAL Materials Chemicals wcre purchased from Wako Pure Chemicals Co. Ltd., (Osaka, Japan) and uscd without further purification. A wild type of A. xylinum, ATCC 10245 strain, was subcultured at 28 OC in Hestrin and Schramm (HS) medium containing Glc as a carbon source, and repeatedly transfcrrcd to the new culture medium every 3 days. Incubators A. Aerobic Rotatory Incubator A round vessel of 3.0L volume incubator with air supply was designed as shown in Fig. 1. The optimum rate of rotation was around 10 rpm when 2.5L of medium was applied
Heater
(Volume of medium is 2500 d)
Figure 1. Outline of aerobic rotatory incubator.
Continuous harvest of cellulosic filament 5 under our condition.
B. Shallow Pun Incubator with Wind up Roller Two culture pans (100 mm width x 400 mm lcngth x 7 mm depth and 200 mm width x 400 mm lcngth x 7 mm depth) were worked out by cutting stainless steel board. The inside shape of the pan was spccially designed to facilitate harvesting the thin gcl smoothly. The pan was cquippcd with a winding roller and a bath of 2% aqueous sodium dodccyl sulfate (SDS), as shown in Fig. 2. The whole apparatus was set in a sealed chamber in which the tempcrature was maintained at 28 "C and filtered air was passed through the incubator.
Figure 2. Outline of the culture pan for the direct filamcntation of BC a, sinker; b, roller; c, washing pan; d, wind up roller. Composition of mediums The compositions of medium are listed in Table 1 for BC production and Table 2 for the incorporation of GlcNAc residue into BC. Table 1. Composition of medium components Compounds of mcdium
Concentration (w/v%)
~~
Sugar Bacto peptone Yeast extract Disodium hydrogen phosphate Citric acid
2.0 0.5 0.5 0.27 0.115
Tablc 2. Composition of medium components for the incorporation of GlcNAc residue Compounds of medium Sugar Bacto peptone Yeast extract Disodium hydrogenphosphate Citric acid Ammonium salts or GlcNAc
Concentration (w/v%) 2.0
0.5 0.5 0.27 0.115
0.2
Culture mcdium and cultivation HS medium (150 ml or 300 ml) was added to the smallcr or larger culture pans,
6 New sources, structure and properties of cellulose
Figure 3. Pratical winding-up process of bacterial cellulose during incubation of A. xylinum in a shallow pan.
Figure 4. Pravtical incubation process in aerobic rotatory incubator by the time. respectively. The depth of the culture media in each pan was 3-4 mm. The media were inoculated with the subcultured A. xylinum under static conditions at 28 "C. After 2 days of incubation, the edge of the pellicle produced on the surface was picked up, passed through the SDS bath to denature the bactcrial cell wall, and set on the winding roller as shown in Fig. 3(a) and (b). The winding process was continued for a couple of weeks at the rate of 35-40m d h and 28 "C.During the incubation, the depth of the culture medium was maintained by the stepwise addition of HS medium every 8-12 h. 2.5L of HS medium was applied for aerobic rotatory incubator and incubated for 7-14days under conditions similar to those for shallow pan incubation, as shown in Fig. 4.
Purification of filament
Figure 5. Removal of water from fibrous gel by twisting by hand.
Continuous harvest of cellulosic filament 7 The wound filament was boilcd for 3 h in 2% SDS aqueous solution, washcd with distilled water, boiled again in the 4 % aqueous sodium hydroxide solution for 1.5 h. The wet filament was cxtcnsively rinsed with distillcd watcr and thcn air-dried at less than 60 "C under tension following a hand twist of the fibrous gel to exclud water as shown in Fig. 5.
Tensile strength The stress-strain diagrams of thc filaments were obtained using a Shimadzu Auto Graph AGS-5OOD apparatus at a guide distance of 25 mm,a chart speed of 100 mdrnin, and a load cell spced of 2 mm/rnin. The force at the brcaking point was taken as tensile stress, which was transferred to tensile strength and Young's modulus.
Wide-angle X-ray diffraction (WAXD) analysis WAXD patterns were rccorded by using a MAC M18XHF X-ray diffractometer. The X-rays werc gcncrated at 40 kV and 100 mA using nickel-filtered CuKa radiation. A vacuum camcra equipped with a 0.5 mm pin-holc collimator was used.
Scanning electron microscopy (SEM) SEM wits accomplished using an Akashi S-DS 130 microscope with gold-coated sample. Microsphercs wcrc sprinkled onto double-sidcd tape, sputter-coated with gold, and examined in the microscope at 10 kV. Estimation of molecular weight by viscometric measurement The molecular wcight of the BC produced was achieved according to the mcthod of Gralen and Ebcll in which BC was dissolved in coppcr-ammonium solution ( C U ;1.0 ~ k O.lg/L, ammonium; 210 k 5g/L and 1Og/L of sucrose as rcductant) under nitrogen atmosphere. Viscosity was measurcd in an Ubbelohde viscometer at 25°C g.
RESULTS AND DISCUSSION Design of the cultivation system An aerobic rotatory incubator was designed to prepare fibrous BC due to rotatory force to the alignmcnt of cellulose molecules and to increase the yield of BC (Fig. 2 of outline and Fig. 5 of practical proccss). A shallow pan was also dcviscd to make thinner BC gel suitable for dircct and continuous filamentation during the incubation of A. xylinum together with increase of yield. In a preliminary incubation under static conditions using a pan with 10 mm depth, thin BC gcl was obtained on the surface of thc culture medium, and gels werc strong and elastic enough to pick up and manipulate. On this basis, a direct filamentation system was dcsigncd as shown in Fig. 3, consisting of a shallow pan of 7 mm of dcpth (400 mm or length and 100 or 200 mm of width) with a winding up roller. One side of the pan was curved gently to permit harvesting of thc pellicle through a narrow mouth. The thin BC gel was directly passed through a bath containing aqueous SDS solution to rcducc the bacterial activity and thcn the filament was wound slowly on an attached roller. The shallow pan was also effective in conserving the total amount of culture mcdium used for the incubation.
8
New sources, structure and properties of cellulose
Direct filamentation Using the system described above, A. xylinum was incubated under static conditions at 28 "C in the shallow pan containing HS mcdium containing Glc and GlcNAc mixture or Glc mixcd with ammonium chloride as carbon source. Following static incubation for 2 or 3 days, thin BC gel was formed and harvesting was started on the roller system. Taking the growth of the BC gel into account, the optimum rate of wind up was found to be around 4Omn/hr for continuous filamentation. To maintain the depth of the culture medium, NS medium was supplied in increments during the incubation, without further addition of bacteria. From thc 100 mm pan, a filament of more than 5 m length was obtained by winding-up at a rate of 16 mm/h for 14 days. Fig. 3 shows the process of the direct filamentation. For purification, the BC filament thus obtained was succcssively treatcd with boiling 2% SDS solution and boiling 4 % sodium hydroxide solution. All filaments were subjected to air-drying under tension at less than 60 OC following the hand twist to remove water as shown in Fig. 5. Dependence of BC yields on the cultivation methods Yields of BC and of GlcNAc residues incorporated into BC are listed in Table 3. As seen in the Table, the shallow pan method improved the yield of BC remarkably in addition to the production of filament. The yield of BC was fairly constant on the aerobic rotatory incubator, whereas unstable yields were observed on static incubation. As volumes of medium are the final stage of cultivation, the increase of yield on shallow pan cultivation seems to be due to the freshness of medium by stepwise addition of medium. Sufficient air supply is also one of major factors in regulating yield, because a fresh surface is served every time on the shallow pan cultivation due to wind up thc product. X-ray diffraction patterns of filamcnts from three incubation methods are shown in Fig. 6. As seen, higher orientation is suggested for direct filamentation compared with those for the other two methods. The peak of (020) of a directly filarnented one, howcver, is slightly shifted to a lower value than those of static and aerobic rotatory methods probably due to an almost homogeneous crystalline structure. Table 3. Dependence of BC yield on the cultivation
Static culture Static culture Static culture Static culture Rotatory culture Filamentation Pilamentation Filamentation Filamentation
Mcdium (ml)
Days
Weight (g)
Yield (%)
15 30 50 300 2500 200 350 450 375
7 7 3 7 7 7 7 10 4
0.0026 0.0093 0.00 12 0.2200 1.3658 0.1493 0.3505 0.5001 0.2269
0.86 1.35 0.12 3.60 2.73 3.73 5.01 5.56 5.03
Physical properties of the filament The BC filament obtained by the direct filamentation was first examined by scanning elcctron microscopy (SEM) to confirm the success of the washing process. A SEM image
Continuous harvest of cellulosic filament 9
5
10
15
20
25
20 /degree
Figure 6. X-ray diffraction patterns of BCs produced by the three different cultivation methods. (Fig. 7) showed that the BC filament has good alignment of cellulose moieties and a slightly twisted fiber mode. The cut surface of filament, also shown in Fig. 7, shows the melting of the inner part of the filament. X-ray diffraction patterns of the filament were also examined as shown in Fig. 8. The filament from the shallow pan incubator shows a slightly higher orientation of molecules than that of filament prepared by aerobic rotatory incubation, whereas poor orientation was suggested for the filament from static cultivation.
Figure 7. Scanning electron microscopic pictures of the bacterial cellulose filament. surface view (left), cut surface view (right).
Figure 8. X-ray diffraction patterns of BCs incubated in different medium. Glc (left), Glc:GlcNAc = 7: 3(v/v) (center), Glc and NH4CI (right).
10 New sources, structure and properties of cellulose Thc tensile and the stress-strain properties for the filaments obtained by shallow pan incubation are listed in Table 4 as factors of medium composition.The stress-straindiagram shows that there is a little decrease of strength by the change of medium, though denier of filament becomes smaller probably due to thinner gel formation by poor yield especially in ammonium chloride HS medium, But stress of filament was inforced by the introduction of GlcNAc residue though there is almost similar level of ammonium chloride medium. Average values of the tensile strength and Young's module of GlcNAc incorporated filaments are 4.4 g/dcnier and 9.0 Gpa which are comparable to those of BC filamcnt, cotton and othcr fibers Thcsc tcnsilc properties of BC filaments are possibly improved by the finishing proccdurcs, bccausc ethylcnc glycol significantly changed these tensile properties by the addition at water rinsing process even if the content was small. Table 4. Tensile properties of BC filaments Sample
N1 N2 N3 N4 N5 NAc- 1 NAc-2 NAc-3 NAc-4 NH.Cl-1 NIIICl-2 NH.Cl-3 NH];Cl-4
Size (denier)
Elong (%)
Stress (GPa) (Average)
108.0 169.2 108.0 108.0 140.4 26.3 26.3 60.5 60.5 39.6 39.6 39.6 39.6
4.2 6.0 4.5 3.8 5.8 1.5 1.2 0.9 2.1 4.4 4.1 4.0 5.0
0.27 0.22 0.43 (0.33) 0.24 0.39 0.57 OS2 (0.48) 0.37 0.59 0.18 o-28 (0.33) 0.40 0.47
Strength Young's modulus (g/ denier) (GPd (Average) (Averagc) 3.9 5.6 3.6 (4.6) 3.0 6.1 7.4 5.9 (4.4) 1.3 4.6 2.4 2*8(4.0) 3.7 5.6
6.4 3.7 9.6 (6.7) 6.7 6.3 9.6 (9.0) 10.9 7.0 4.0 6*7 (7.0) 9.9 9.5
Pretreatment :treated with boiling 4% sodium dodecylsulfateaqueous solution for 2 hr and thcn 2% sodium hydroxide aqueous solution for 1.5 hr. Abbreviation : Elong ;Elongation ,Stress ;Tensile stress , Strength ;Tensile strength N: BC filaments cultured in HS medium containing Glc as carbon source. NAc: BC filaments containing N-acetylglucosamine residuces cultured in HS medium containing GlcNAc instead of Glc. NH,Cl: BC filaments containing N-acetylglucosamineresiduces cultured in HS medium containing Glc and 0.2% ammonium chloride.
Molecular weight of BC under static cultivation The molccular weight of BC produced under static conditions was estimated viscometrically by plotting reduced viscosity vs polymcr concentration and logarithm reduced viscosity vs polymer conccntration. Although molecular weight of 1 . 0 5lo6 ~ were given for both BCs of 1 day and 4 days incubations as shown in Fig. 9, this is only preliminary measurement.
11
Continuous harvest of cellulosic filament "4days" M w = 1.05 x lo6
"lday" M w = 1.07 x lo6
y = 5.9029 + 70.758 X R2 = 0.999
I 0
.
6
K =U.YVL
I
I
1 4
0.000
0.004 0.008 Concentration
0.012
y
P
= 6.9527 += 12.775 X
R2 = 0.955
1
0.00 0.02 0.04 0.06 0.08 0.10 C 12 Cvnccntration
Figure 9. Profiles of viscosity.
CONCLUSION Our design of a simple and direct filamentation system for the production of bacterial cellulose is, we believe, the first such procedure to be reported in the literature. The tensile strcngth of the filament was found to be significantly stronger than the ordinary cellulose fibers and a good orientation of molecules was shown both by the X-ray diffraction pattern and SEM obscrvation. Thcse simplificd methods for producing and harvesting BC, are cxpecting to lower the production cost of BC togcthcr with the basic research for the orientation of cellulose molecule.
ACKNOWLEDGEMENTS A part of this research was financially supported by the Kansai Univcrsity Special Research Fund. 1999.
REFERENCES 1. J. Brown, On an Acetic Fermant which forms Cellulose, J. Chern. Soc., 1886.49, 432-439. 2. M. Takai, Y. Tsuta, J. Hayashi & S. Watanabe, Biosynthesis of Cellulose by Acetobactcr Xylinum. 111. X-Ray Studies of Preferential Orientation of the Crystallites in a Bacterial Cellulose Membrane, Polym. J., 1975.7, 157-164. 3. M. Fujiwara, K. Fukushi. M. Fukaya, H. Okumura, Y. Kawamura, M. Takai & J. Hayashi, Construction of Shuttle Vectors Derived from Acetobacter xylinum for Cellulose Producing Bacterium Acetobacter xylinum, Biotech. Let., 1992, 14,539542. 4. S. Hestrin & M. Schramm, Synsethis of Cellulose by Acetobacter xylinum.2. Preparation of Freeze-dried Cells Capable of Polymerizing Glucose, Biochem. 1..
12 New sources, structure and properties of cellulose 1954,58,345-352. 5. R. Ogawa & S. Tokura, Preparation of bacterial cellulose containing NAcetylglucosamine residue, Carbohydy.Polymers, Carbohydr. Polymers, 1992, 19, 171-178. 6. A. Shirai, M. Takahashi, H. Kaneko, S.-I. Nishimura, M. Ogawa, N. Nishi & S. Tokura, Biosynsethis of a Novel Polysaccharide by Acetobaetcr xylinum, Int. J. Bioll. Macromol., 1994, 16,297-300. 7. H. Hibbert, Action of Bacteria and Enzymes of Carbohydrates and their Bearing on Plant Synsethis, Science, 1930,71,419-420. 8. E. E. Magat & R. D. Strachan, U.S. Patent, 1995,2,708,617. 9. N. Gralen & L. Ebell, ,J. Biol. Chem., 1924, GO, 257-266. 10. R. Meredith, A Comparison of the Tensile Elasticity of some Textile Fibers, J. Textile Inst., 1945,36, T 107-130. 11. L. Rebenfeld & W. P. Virgin, Relation bctwccn the X-Ray Angle of Cottons and Their Fibcr Mcchanical Properties, Textile Res. J., 1957,27,286-289.
Oil Palm (Elaeis gcrineensis) Wastes as a Potential Source of Cellulose M o l d Azemi Mohd. Noor' and Harun Sari$ 'Biopolymer Research Group, School of Industrial Technology Universili Sains Malaysia, I 1800 Minden, Penang, Malaysia. 'Fibrotech Sdn. Bhd., No. I Lorong Terasek Kanan, Bangsar Baru, 59100 Kuala Lumpur, Mulaysia.
ABSTRACT Oil palms are an important cash crop in Malaysia and currently the total area under cultivation is approximately 2.5 million hectares. Oil palm cultivation generates a significant amount of lignocellulosic biomass dcrivcd from fronds, empty fruit bunches and trunks. About 36 million tons of these lignocellulosic wastes are generated annually and currently most of these wastes are either left in the plantations or burned illegally. Small amounts are being utilised for fibre production and energy generation. The biomass consists of about 40% cellulose, 40% hemicellulosc, 18% lignin and 2% extractives (sugars and phenolic substances etc.). Attempts are being made to fractionate, isolate and purify the cellulose fraction into microcrystalline cellulose. High pressure steam treatments followed by aqucous extraction and chlorite bleaching were employcd to fractionate, isolate and purify cellulosic materials derived from oil palm biomass. Highly crystalline cellulose derivatives resembling microcrystalline (MCC) with over 95% purity were successfully isolated. The properties are comparable to commercially available MCC and cfforts are being made to commercialise these products. INTRODUCTION The cultivation of oil palm (Elueis guineensis Jacq) is forecast to cover a total area of about 2.8 million hcctarcs in Malaysia by the year 20003. Besides palm oil, the by products of oil palm industry such as oil palm trunk (OPT), fronds, cmpty fruit bunches (EFB) and palm press fibre (PPF) contribute significant amounts of lignocellulosic biomass. Currcntly this biomass has yet to be fully utilised as a source of value added products especially chcmicals and its dcrivatives. Cellulose materials rcpresent about 70-80% (cellulose and hemicellulosc) of oil palm biomass and would certainly serve as raw materials for the production of chemical cellulose and its dcrivativcs3. This report describes the production of microcrystalline cellulose (MCC) through fractionation, isolation and purification of this oil palm biomass using high pressure treatments (steam explosion).
14 New sources, structure and properties of cellulose MATERIALS AND METHODS. Oil palm biomass, supplied by Sabutek (M) Sdn. Bhd. Malaysia, was subjected to high pressure steam treatmcnt and followed by aqueous extraction (Pig. 1). The steam exploded fibers were initially subjected to a watcr extraction process at room tempcrature then followed by alkaline treatment. The alkaline extracted fiber (AEF) fractions wcrc then bleached with 10% sodium chlorite and subsequently treated with alkali followcd by acid hydrolysis to produce MCC'. The composition of the water soluble constituents in the water extract liquor (WEL) and alkaline extract liquor (AEL) were analyscd for carbohydrate content and compared with the untreated lignocellulosic material (UT). The carbohydrate content was analysed using borate-anion cxchange chromatography (BAEC) tcchnique developcd by Simatupang4. The MCC was analysed for particle size distribution, degree of polymerisation (DP), viscosity and stability. The particle size distribution was analyscd using a coulter counter (Gulter Elcctronic, Herb, England), while viscosity measurcrnent and stability studies were carried out using Ubblohde capillary tube viscometer and cellulast enzyme (NOVO) digestion respectively.
RESULTS AND DISCUSSION As shown in Table 1, relative percentage of lignin contents after high pressure steam treatmcnt (ST%) varies according to the source of oil palm wastes and corrcspond with the amount of carbohydrate (exprcsscd in term of glucose)'. The relative percentage of lignin and cellulose (in term of glucose) dctected in high prcssure steam treated fibers somehow increascd; indicating only small amount being converted into water soluble fragments. Rhamnose, mannose, arabinose, galactosc and xylose wcre detected in all sources of oil palm wastes.
Table 1. Chemical analysis of various oil palm lignocellulosics before and after high pressure stcam trcatment and of water soluble parts Compound Palm Press Fiber Empty Bunches Oil Palm Trunk (PPF) (EFB) (OPT) U.T. S.T. W.S. U.T. S.T. W.S. U.T. S.T. W.S.
YO Klason lignin Rhamnose Mannose Arabinose Galactose Xylose Glucose 4 -m-MG
35.7
% 41.3
%
YO
1.0
23.3
% 33.3
% 3.3
1.43 0.38 0.17 1.09 0.40 0.18 1.50 2.05 3.78 0.89 1.03 0.79 1.5 1 3.68 1.40 5.40 0.78 1.97 0.74 0.23 2.30 23.02 13.75 44.68 21.45 15.05 25.45 24.22 37.29 2.48 40.22 43.47 1.93 0.51 0.40 0.74 0.35 U.T.= Untreated lignocellulosic material S.T.= High prcssure steam treated, defiberized and refined fibcrs W.S.= Watcr soluble parts Values are bascd on ovcn dried material
-
-
YO
YO
19.8
24.3
0.34 1.20 0.71 0.48 20.00 44.91 1.02
0.76
-
11.40 57.62 0.63
% 0.9
1.65 3.06 2.83 1.73 48.93 4.97
Oil palm wastes
15
Figure 1. Extraction and Purification of Oil Palm Wastes Lignocellulosic Matcrials
Oil Palm Lignocellulosic Materials
J Steam Explosion
J J
Steam Exploded Fibre (SEF) Hot Watcr Extraction
>
Watcr Extract Liqour (WEL)
Water Extracted Fiber (WEF)
J
Alkaline Extraction
Alkaline Extract Liquor (AEL)
J Acidification
4
Lignin
+
Alkaline Extracted Fiber (AEF) Bleaching
+ + 1
Alpha Ccllulose Mineral Acid Hydrolysis Washing & Pufification Microcrystalline Cellulosc
J
Product Characterization
16 New sources, structure and properties of cellulose
Micrograph 1. + Oil Palm Wastes MCC.
Micrograph 2. + Commcrcial MCC
Oil palm wastes
17
I Iowevcr upon high pressure steam treatment, arabinosc was completely solubiliscd as opposed to 4-rn-methyl-gluconocosylxylosc(4-m-MG). The presence of xylose in all water soluble fractions in increasing order is partly due to dcpolymerisation into water soluble low molecular wcight fragmcnts. The MCC samples obtaincd were evaluated for purity, molecular weight, DP, particle size distribution, degree of crystallinity and instrinsic viscosity (Table 2). The particle size distribution of oil palm waste MCC scemcd to be in thc larger range (3.5 30 pm) than the commercial MCC. However, the degree of polymerisation (DP) is lower than that of the commercial MCC. This is in line with the low instrinsic viscosity mcasurcd as compared to the commercial MCC which may contribute toward stability of colloidal suspension. The degree of crystallinity and purity are similar. As shown in micrograph 1 and 2, the properties of MCC derived from oil palm wastcs closely resemble commercially produced MCC. The DP is dcrivativc valuc of the intrinsic viscosity measurement. Table 2. Propertics of Oil Palm Wastcs MCC and Commercial MCC Properties Oil Palm Wastes MCC Commercial MCC I’article size distribution 3.64 pm-28.90 pm 3.12 pm-9.91pm DP 640 repeat unit 940 rcpcat unit Instrinsic Viscosity 3.36 4.96 Crystallinity Index 87.4 88.8 Purity > 95% > 95%
CONCLUSION Microcrystalline cellulose from oil palm wastes exhibited a closc rcscmblance to those produced from various sourccs of biomass and can be isolated through high pressure stcam treatmcnt.
REFERENCES 1. Mohd. Azcmi B.M.N., Sallch M., Wright R.S,and Glasscr W.G. (1998). Stcam Assisted Fractionation of Oil Palm Trunks Solids. Biomass and Bioenergy (In prcss - Elsevicr, Oxford UK). 2. Baltista 0. A., and Smith P A . (1962). Microcrystalline Cellulose in Industrial and Chemical Engineering. 54(9), pp 20-29. 3. H u s h M., IIassan A.H and Mohamad A.T. (1997). Availability and Potential Utilization of Oil Palm Trunks and Fronds upto Ycar 2000. PORIM Occasional I’apcr No. 20. pp 1 - 20 4. Simatupang, M.N. (1977). Ion exchange chromatography of some neutral monosaccharidcs and uronic acids. J. Chromatogr: 178: pp 588-591
ISOLATION AND CHARACTERISATION OF SAGO ( Metroxylon Sagu ) CELLULOSE Mohd. Zahid A., Mohd. Zulkali M.D. and Azemi B.M.N. Bio-polymer Research Group School of Industrial Technology, Universiti Sains Malaysia I I800 Minden, Penang, Maloysia
ABSTRACT Apart from starch, cellulosic materials constitute the major by products of sago starch extraction plants, notably from starch free fibrous materials and the barks. These potentially commcrcialisable cellulosic materials remain unexploited. This paper describes an attempt to isolate and purify the ccllulose fraction which forms about 40% of the total dry weight of the cellulosic materials from sago barks. Alkaline treatment and acidic precipitation of non-cellulosic materials namely lignin were employed to fractionate the cellulose component. Bleaching was accomplished by chlorite oxidation. Upon acid hydrolysis, a-cellulose was obtained and characterised for particle size distribution, cystallinity index, mean molecular weight, intrinsic viscosity, microscopic appearance and susceptibility to cellulase attack. The properties of thc sago dcrived cellulose were compared with that of commercial products.
INTRODUCTION East Malaysia (Sarawak) in particular is known for sago plantations. There are 32 sago mills supplied by 19,720 hectares of sago plantations4. These quality of the plantations are found in the area of Mukah, Igau and Oya-Dalat districts of Sibu Division and others in the Pusa-Saratok districts of Sri Aman Division6. Sibu contributes about 95% of thc starch exported from the State. A total of over 35,000 tons of sago starch and 4,180 tons of sago flour were exported in 1994 alone, procuring incomes of over RM22 million (USD 8 million) and RM2.4 million (USD 1 million) for the State, respectively’. The total annual export of premium quality sago flour to the world market was approximately 43,000 tons. It is predicted that a sufficient supply of harvestable palms would be availablc in four years’ time to support an annual production of 67,000 tons of sago starch. Besidcs starch, the sago industry generates a large variety of lignocellulosic biomass. These untapped potentially rich sourccs of cellulose and hemicellulose remain unexploited. In this paper attempts to isolatc, fractionate and characterise this cellulosic biomass in the form of microcrystallinecellulose (MCC) are reported.
20 New sources, structure and properties of cellulose MATERIALS AND METHODS
Sago palm barks were appropriately sized and subjected to high pressure steam treatments followcd by aqueous extraction. Thc methods used were similar to thosc employed by Azemi and Harun (199Q3. The MCC obtained was analysed for particle size distribution, crystallinity index, molecular weight, degree of polymerization (DP), viscosity, microscopic appearance and digestibility'. The particle size distribution was analyscd using a coulter counter (Coulter Electronic ,He&, England ). The crystallinity index and instrinsic viscosity were analysed using X-ray crystallography and Ubblohde capillary tube viscometer rcspectively2. Scanning electron micrograph (SEM, Leica Cambridge) was employed for microscopic appearance studies, while product digcstibility was carried out with cellulast enzyme (NOVO). RESULT AND DISCUSSION
The particle size distribution of the MCC obtained was similar to that of the commercial MCC, however, intrinsic viscosity and cellulase susceptibility varied (Table 1). The intrinsic viscosity of sago palm bark MCC was approximately one third that of commercial MCC. The crystallinity index was similar to that of commercial MCC, suggcsting the existcnce of amorphous regions in the sago bark MCC which was reflectcd in the cellulase susceptibility value of 1.5%. Howevcr, cellulase susceptibility value for commercial MCC was higher than that of sago bark MCC, suggesting more amorphous region. Micrographs 1 and 2 show the appearance of commercial MCC and sago bark MCC. They seemed to be similar in appearance*. This short study has elaborated the potential of the barks as a source of MCC, however ,further studies are required to fully exploit the wastes. Table 1: Properties of sago palm bark MCC Properties MCC of Sago Bark Commercial MCC Particle size distribution 2.18-8.72 pm 2.18-6.92 pm *DP 70 rcpeat unit 225 repeat unit Intrinsic viscosity 0.375 dL/g 1.179 dL/g Purity >95% >95% 36 000 kD 11 500 kD *Mean molecular weight 8 1.8% 83.4% Crystallinity Index Cellulase (cclluclast) susceptibility 1SO% 2.65% * = These value were derived from the intrinsic viscosity measurement CONCLUSION
,
The work has highlighted the potential of sago palm bark as a good and economically viable source of MCC.
Isolation and characterisation of sago cellulose 2 1 MICROSCOPIC APPEARANCE
- COMMERCIAL MCC
SAGO BARK MCC -
ACKNOWLEDGEMENT
This work was funded by C R A W Sarawak and Universiti Sains Malaysia
22 New sources, structure and properties of cellulose REFERENCES 1. Mohd. Azemi B.M.N., Salleh M., Wright R.S, and Glasser W.G. (1998). Steam assistcd Fractionation of oil palm trunks solids. Biomass and Bioenergy (in press), Elsevier, Oxford, UK. 2. Battista 0. A., and Smith P.A. (1962). Microcrystalline Cellulose in Industrial and Chemical Engineering. 54(9), 20-29. 3. Azemi B.M.N. and Harun S. (1998). Production and Characterization of Microsrytalline Cellulose from Stcam Exploded Oil Palm Fronds. M.Sc. Thesis USM. 4. Ark P.K. (1996). Inventory and Evaluation of Sago Palm (Metroxylon spp.) Distribution. In: 6Ih Sago Symposium: Sago, The Future Source of Food and Feed. Pckanbaru, Indonesia. 5 . Bujang K., K. Apun and M.A. Salleh (1996). A Study in the Production and Bioconversion of Sago Waste. In: 6Ih Sago Symposium: Sago, The Future Source of Food and Feed. Pekanbaru, Indonesia. 6. K.H. Ong (1976). Sago in Sarawak. In: 1'' Int. Sago Symposium: The Equatorial Swamp as a Natural Resource. Kuching, Sarawak. ,
A highly cellulosic exopolysaccharide produced from sugarcane molasses by a Zougfoea sp. M. Paterson-Beedle', L. L. Lloyd', J. F. Kennedy', F. A. D. Melo: & V. Medeiros2 'Birmingham Carbohydrate and Protein Technology Group, ChembiotechLaboratories, University of Birmingham Research Park, Birmingham, BIS ZSQ, UK 'EstaCSo Experimental de Cana-de-A qucar de Carpina- Universidade Federal Rural de Pernambuco, Carpina, Pernanibuco, CEP 55 810 000, Brazil
Brazilian Zoogloea sp. bacterium produced from sugarcane molasses a polymer substance containing glucose (87.6%), xylose (8.6%), mannose (0.8%), ribose (1.7%), galactose (0. l%), arabinose (0.4%) and glucuronic acid (0.8%). Methylation analysis of the polysaccharide showed mainly 2,3,6-tri-O-methylhexitoI(74.7%) and 2,3-di-Omethylhexitol (17.7%). Enzyme hydrolysis of the polysaccharide with a cellulase confirmed the presence of (1+4)-8-D-glucopyranosyl units.
INTRODUCTION Cellulose is produced by Acetobacter xylinum and other, mainly Gram-negative, bacterial species (1,2), the cellulose being excreted into the medium, where it rapidly aggregates as microfibrils. Bacterial cellulose has long been used in foods in Asian countries (3,4) and its different properties from wood-derived cellulose opens new industrial applications. Bacterial cellulose possesses high crystallinity, high degree of polymerization, high tensile strength and tear resistance, and high hydrophilicity that distinguish it from other forms of cellulose (5). Various potential industrial applications for bacterial cellulose include acoustic diaphragms, artificial skin or wound healing, filter membranes, ultra strength paper and paper additives (5,6,7). In food processing, suspensions of disintegrated bacterial cellulose have been found useful as thickening and binding agents and as a dietary fibre supplement (5). Zoogloea ramigera produces zooglan, a polysaccharide composed of D-glucose, Dgalactose and pyruvic acid in an approximate ratio of 11:3:1.5 (8). It is a long chain polysaccharide consisting of mainly 1,Clinked glucose residues and 1,4- and 1,3- linked galactose residues with branches of glucose residues at the C-3 or C-6 positions of the galactose residues. .The pyruvic acid residues, the acidic component, are linked to the nonreducing end and/or 1,3-linked glucose residues through 4,6-ketal linkages (8). Here, we present recent studies on the composition and structure of an exopolysaccharide produced from sugarcane molasses by a Zoogloea sp. EXPERIMENTAL Materials
The following materials were obtained from sources indicated: sugarcane molasses ( M a c 2 0 Experimental de Cana-de-AGucar, Brazil); yeast extract, peptone and agar (Merck, USA); Celluclast 1.5 L (cellulase) (NOVONordisk, Denmark); cellulose powder (Sigma, UK); monosaccharide standards (myo-inositol, glucose, mannose, arabinose, xylose, ficose, galactose, ribose and glucuronolactone) (Sigma, UK); trifluoroacetic acid
24 New sources, structure and properties of cellulose (Janssen Chimica, UK); formic acid, diethylamine, l-methyl imidazole, dimethyl sulphoxide and acetic anhydride (BDH, UK); barium carbonate (Sigma, UK); sulphur dioxide (Fluka, UK), methyl iodide (Aldrich, UK). Other chemicals of analytical grade were obtained commercially. Microorganism An extracellular polysaccharide producing bacterium Zoogloea sp. was isolated fiom an agro-industrial environment in the north-eastern region of Brazil. The identification of the microorganism was carried out at the Instituto de Antibibticos, Universidade Federal de Pernambuco, Brazil. The Zoogloea sp. was maintained as slant cultures at 4 "C. The yeast extract (5.0 peptone (3.0 culture medium consisted of glucose (20.0 agar (15.0 g/L) in deionised watcr and the pH was adjusted to 6.8 before sterilisation (120 "C, 20 rnin).
a),
a),
a),
Production of the exopolysaccharide sample
For the exopolysaccharideproduction, cells were transferred from agar slants to 100 mL of sterilised medium (120"C, 40 min), consisting of sugarcane molasses (15 "Brix, pH 5.0), in 250 mL Erlenmeyer flasks. Cultures were incubated at 30 "C for 7 days. The polysaccharide pellicles, a gel-like material, were formed at the air-liquid interface. The pellicles were washed in deionised water, sterilised (120"C, 40 min) and dried at 60°C on glass plates to form solid polysaccharide sheets. Determination of the water soluble material of the polysaccharide sheet
The polysaccharide sheet (21 1.04 mg) was cut into small particles (ca. 4 mm') and transferred to a centrifbge tube. Deionised water (6 mL) was added, the material was mixed for 1 min using a vortex mixer, left to stand for 15 min, mixed again for a further 1 min and centrihged at 8,OOOg for I5 rnin. The residue was washed with deionised water (6 mL) and centrihged at 8,000 g for 15 min. This procedure was repeated twice. The supernatants were combined, lyophilised and dried using an Abderhalden dryer at 39 "C. The residue was dried using an Abderhalden dryer (39 "C) and the dried weight of the residue determined. Mild acid hydrolysis of the polysaccharide smmple
The water insoluble residue (47.51 mg) was suspended in trifluoroacetic acid (2 M, 4 mL) in a round bottom flask, allowed to stand for 45 min and refluxed at 97 "C for 3 h. The hydrolysate was left at ambient temperature for 20 h. Trifluoroacetic acid was removed by drying the sample using a rotor evaporator. The hydrolysate was washed with deionised water (2 mL) which was also removed by evaporation. This procedure was repeated six times. The residue was transferred to a centrifbge tube and deionised water (1.3 mL) was added. The sample was centrihged at 8,000 g for 30 An. This procedure was repeated three times and thc supernatants were combined and stored at -20°C prior to anion exchange high performance liquid chromatography analysis. The acid insoluble residue was dried usirig an Abdcrhalden dryer (39 "C) and the dry weight determined.
Highly cellulosic exopolysaccharide 25 Strong acid hydrolysis of the polysaccharide sample
Trifluoroacetic acid (99%, 3 mL) was added to the dried acid insoluble residue (33.5 mg) and allowed to stand at ambient temperature (16 h) to swell. The sample was refluxed for 2 h, at 97 "C, diluted to 80% trifluoroacetic acid with deionised water and refluxed for 30 min. The sample was diluted again to 30% trifluoroacetic acid with deionised water and refluxed for a fbrther 4 h. The trifluoroacetic acid was removed and the sample worked up as for the mild acid hydrolysates. Monosaccharide analysis using anion exchange high performance liquid chromatograplry
The trifluoroacetic acid hydrolysates were neutralized with sodium hydroxide solution prior to the analysis. The analysis of monosaccharides of the hydrolysates was performed on a Dionex DX-500system consisting of a GP40 gradient pump and a Dionex ED40 electrochemical detector. The standard sequence of potentials for carbohydrate detection (50 mV for 200 ms; 750 mV for 200 ms and -150mV for 400 ms) was applied to the Au ED40 working electrode for pulsed amperometric detection. The columns used were a CarboPac PA1 analytical column (250 x 4.0 mm ID) and a CarboPac PA1 guard columns (50 x 4.0 mm ID). All eluents and reagents were prepared using water purified to 18.2 Mi2 using a UHQPS system (Elga). Eluents A, B and C were deionised water, sodium acetate (1 M) and sodium hydroxide (1 M), respectively. Sodium hydroxide (300 mM) at 0.5 a m i n was added as a post column reagent. After equilibrating the system with deionised water for 30 min a sample (50 pL) was injected. The sodium acetate concentration was then increased, linearly, to 200 mM over the following 10 min and held at this level for 10 min. Oligosaccharide analysis using anion exchange high performance liquid chromatography - method 1
The analysis of the oligosaccharide component of the water soluble extract and the mild trifluoroacetic acid hydrolysate was performed on a Waters 625-LC system fitted with a non-metallic flow path, a 464 pulsed amperometric detector (PAD) fitted with a gold working electrode and a base stable reference electrode and a Whisp 712 injector. Sodium hydroxide (300 mM), at a flow rate of 0.7 mWmin, was added to the eluent stream between the columns and detector. The PAD was operated in the cathodic mode with the following sequence of potentials: 50 mV for 200 ms; 800 mV for 200 ms; and 600 mV for 500 ms. The columns used were a CarboPac PA1 analytical column (250 x 4.0 mm ID) and a CarboPac PA1 guard column (50 x 4.0 mm ID). N1 eluents and reagents were prepared using water purified to 18.2 Mi2 quality using a UHQPS system (Elga). Eluents A, B, C, and D were sodium hydroxide (100 mM), sodium hydroxide (100 mM) containing sodium acetate (800 mM), sodium hydroxide (300 mM), and sodium hydroxide (500 mM), respectively. Eluent C was used as the post column reagent. After equilibrating the system with sodium hydroxide (100 mM) a sample (200 pL) was injected: The sodium acetate concentration was then increased, linearly, to 800 mM over the following 60 min and held at this level for 5 min.
26 New sources, structure and properties of cellulose Oligosaccharide analysis using anion exchange high performance liquid chromatography - method 2
The method used for the analysis of the oligosaccharide component of the strong trifluoroacetic acid hydrolysate and the enzyme hydrolysates was similar to method 1. The only difference was that the concentration of sodium acetate was increased, linearly, to 400 mh4 over 60 min. Methylation analysis
The water insoluble polysaccharide was methylated by three cycles according to method described by Isogai et al. (9).
Preparation of polysaccharide siispension The water insoluble polysaccharide (32.4 mg) was dispersed in dimethyl sulphoxide (2.84 mL). Concentrated sulphur dioxide-methyl sulphoxide solution (ca. 0.3 dmL) was prepared as follows: sulphur dioxide (15 g) was bubbled into methyl sulphoxide (50 mL). Sulphur dioxide-methyl sulphoxide solution (165 pL) was added to the sample. Then, diethylamine (61 pL) was added. The suspension was stirred at ambient temperature for 20.3 h. Complete dissolution was not obtained.
Methylation of polysuccharide Freshly powdered sodium hydroxide (389 mg) was added to the sample, at ambient temperature, and the mixture stirred for 1 h under helium. Methyl iodide (390 pL) was added dropwise, at ambient temperature, and the mixture was stirred for 1 h, and then kept at 40 "C for 0.5 h, at 50 "C for 0.5 h and at 60 "C for 1 h. The methylation procedure was repeated three times, starting from the preparation of polysaccharide suspension (from the addition of sulphur dioxide-methyl sulphoxide solution). When solidification occurred during stirring, methyl sulphoxide was added to the sample to regenerate the slurry (1 mL prior to the second methylation and 2 mL prior to third methylation). The methylated products were isolated by dialysis and subsequent lyophilised.
Formic acid and sulljhuric acid hydrorySis The dry methylated product (11.8 mg) was allowed to swell in formic acid solution (90%, 1.18 mL) overnight, at ambient temperature. The sample was then refluxed during 2 h, the formic acid evaporated, the residue washed once with deionised water (1 mL) and then evaporated. Sulphuric acid solution (0.125 M, 2.95 mL) was added to the sample and refluxed during 10 h, neutralised with barium carbonate (ca. 236 mg) and the insoluble barium sulphate removed by centrifhgation (7,500 rpm, 15 min). The partially methylated sugars present in the supernatant were then used for the preparation of alditol acetates.
Highly cellulosic exopolysaccharide 27 Preparation of alditol acetates
The resulting partially methylated aldoses were converted to partially methylated alditol acetates as described by Blakeney et al. (10). Ammonia (80 pL) was added to the hydrolysate (containing ca. 2 mg methylated monosaccharides in 0.5 mL). myo-Inositol solution (4 mg/mL, 100 pL) was added as an internal standard. Sodium borohydride (ca. 70 mg) was added and the sample was allowed to stand at 40 "C for 1.5 h. Excess borohydride was destroyed by addition of glacial acetic acid (0.2 mL). After cooling to ambient temperature, I-methyl imidazole (1.5 mL) was added and the sample shaken vigorously to ensure it was in solution. Acetic anhydride (5 mL) was added cautiously (in a bath containing a mixture of ice and cold water), causing an immediate rise in temperature. The sample was allowed to react for 20 min, with frequent gentle agitation. The excess anhydride was hydrolised by addition of deionised water (12 mL) and shaken to thoroughly mix the sample. It was cooled for 5 min in a bath containing a mixture of ice and water, then extracted twice with dichloromethane (1 mL). The dichloromethane (lower layer) was removed by dropping a pipette; the extracts from the sample were combined and submitted to gas liquid Chromatographyanalysis. Gas liquid chromatography nnalysis
Quantification of the partially methylated alditol acetates was carried out on a Carlo Erba GC 8000 series gas chromatograph fitted with a BPX 70 (70% biscyanopropylpolysilphenylene-siloxane, 25 m x 0.33 mm, 0.25 pm film thickness) capillary column (supplied by SGE, UK) and a flame ionisation detector. The column pressure was 150 kPa and the flow of helium was 2 mL/min. The initial temperature of the oven was 150 "C and was increased at a rate of 10 "Chin to 190 "C and maintained for 2 min. The temperature was then increased at a rate of 5 "Chin to a final temperature of 250 "C and maintained for 10 min. The injection temperature was 260 "C and the detector temperature was 300 "C. Gas cliromatography nnd mass spectrometry
Gas chromatography and mass spectrometry (GC/MS) analyses were carried out on a Prospec (VG Company) equipped with a BPX 70 (70% biscyanopropylpolysilphenylene-siloxane, 50 m x 0.33 mm, 0.25 pm film thickness) capillary column (supplied by SGE, UK). The initial temperature of the oven was 150 "C and was increased at a rate of 10 "Chin to 190 "C and maintained for 2 min. The temperature was then increased at a rate of 5 "Chin to a final temperature of 250 "C and maintained for 5 min. Enzymatic liydrolysis of the polysaccharide sample
Particles of the water insoluble fraction of the polysaccharide sheet (4.3 mg) were suspended in sodium acetate buffer (0.4 M, pH 5.0, containing Celluclast 1.5 L, 5 pL/mL, 860 to give a concentration of 5 mg sample/mL. Cellulose powder (3.5 mg) was also suspended in sodium acetate buffer (0.4 M, pH 5.0, containing Celluclast 1.5 L, 5 pWmL, 700 pL). The samples and control (sodium acetate buffer containing Celluclast 1.5 L) were mixed and incubated in a water bath at 50 "C for 68 h and then at 100 "C for 10 min to deactivate the enzyme. After centrihgation at 10,000 g for 30 min, the
a),
28
New sources, structure and properties of cellulose
supernatants were decanted and deionised water (same volume used for the enzymatic hydrolysis) was added to the residues. The samples were centrifbged at 10,000 g for 30 min. The supernatants were combined and stored at -20 "C prior to anion exchange high performance liquid chromatography analysis. RESULTS AND DISCUSSION
Composition analysis
The contents of the water soluble and insoluble components of the polysaccharide sheet were 12 and 88 % (w/w). respectively. The profile of the oligosaccharides present in the water soluble component, compared with an oligosaccharide fingerprint of a starch hydrolysate reference material, indicated the presence of more than one neutral monosaccharide and a number of disaccharides with different monosaccharide composition or linkage position or type. Polysaccharides are hydrolysed under acid conditions to their component monosaccharides by cleavage of the glycosidic linkage at the bond between the anomeric carbon atom and the glycosidic oxygen. Conditions chosen for acid hydrolysis of polysaccharides are always a compromise between release and destruction of the component monosaccharides. Depending on the type of structural information required, conditions can be selected to give optimum release for monosaccharides or to favour release of oligosaccharide fragments. In this study two acid hydrolysis conditions (mild and strong) were chosen to release the monosaccharides. The percentages of soluble and insoluble components obtained from the trifluoroacetic acid hydrolysis of the polysaccharide sheet, under mild conditions, were 15.8 and 84.2% (w/w), respectively. The percentages of soluble and insoluble components obtained from the trifluoroacetic acid hydrolysis (using strong conditions) of the acid insoluble fraction (obtained using mild conditions) of the polysaccharide sheet were 72.2 and 12.0% (w/w), respectively. The total percentage of soluble material obtained from the trifluoroacetic acid hydrolysis, using mild and strong conditions, was 88% (w/w) of the water insoluble fraction. The monosaccharide compositions of the soluble components obtained from the trifluoroacetic acid hydrolysis of the polysaccharide sheet, under mild and strong conditions, determined by anion exchange high performance liquid chromatography analysis, are shown in Table 1. Glucose accounted for 87.6% of the total monosaccharides present in the acid hydrolysates (mild and strong conditions). Galactose was present in a very small amount (0.1%). These results differ from those obtained in the literature for zooglan, which is composed of D-glucose, D-galactose and pyruvic acid in an approximate ratio of 11:3:1.5 (8). The content of xylose (8.6%) was higher than the other monosaccharides present in the hydrolysates. The percentage of monosaccharides present in the acid hydrolysates (strong and mild conditions), determined by anion exchange high performance liquid chromatography, is shown in Table 2. The total percentage of monosaccharides released by the acid hydrolysis was only 38.4% of the total acid soluble component. The oligosaccharide profile of the trifluoroacetic acid hydrolysate, under mild conditions, showed a main peak in the monosaccharide region and other small peaks in the region of di- and tetra-saccharides, It should be noted that the ratio of the two peaks in the monosaccharide component is significantly different for the trifluoroacetic acid hydrolysate produced using the mild conditions and the water soluble component. This ,
Highly cellulosic exopolysaccharide 29 would suggest that there are differences in the solubility of sample according to the monosaccharide composition. Table 1 Monosaccharide compositions of the soluble fractions produced by trifluoroacetic acid hydrolysis, under mild and strong conditions, determined by anion exchange high performance liquid chromatography
".Monosaccharide ............... Fucose
Mild conditions Strong conditions Total ............................e??.?..Y!!!~.......... ...Y!!?.. !.! ...... !............ !? S%?. I.Y!!.!) .....
Arabinose Rhamnose
Galactose Glucosc
Xylose Mannose Ribose
Glucuronic acid Total
.
0.0 I 1.91 0.22 1.58 55.41 29.7 1 1.96 2.37 6.83 100.00
0.01 0.26
0.03 89.88 7.06 0.74 1.63 0.39 100.00
0.01 0.37 0.01 0.13 87.57 8.58
0.82 1.68 0.83 100.00
Table 2 Percentnges of monosacchnridcs, present in the trifluoroacetic acid hydrolysates of the polysaccharide sheet, using mild and strong conditions, determined by anion exchange high performance liquid chroniatography
Treatment Mild
Monosaccharides (%, w/w)'
14.4 43.7 38.4 * Calculated in relation to the amount of material solubiliscd by thc acid treatmcnt Strnne Total
The trifluoroacetic acid hydrolysate, obtained using strong conditions, was analysed for oligosaccharide content at two different concentrations. The main peak was in the monosaccharide region and its elution position correlates with glucose, in the maltodextrin reference material. It should be noted that even with the higher column load the monosaccharide is a single peak unlike the doublet which was observed in the oligosaccharide profiles of the mild trifluoroacetic acid hydrolysate and of the water soluble component .of the polysaccharide sheet. However, the disaccharide peak in the hydrolysate does not correlate with the elution position of maltose, the disaccharide present in the maltodextrin reference material, which would indicate that although the monosaccharide residue is glucose the polysaccharide linkage is different. As expected, these results confirm that a higher amount of material was released from the polysaccharide sheet by the strong acid conditions compared with the mild conditions. Structure analysis
Methylation analysis was carried out in order to hrther study the polysaccharide sheet. Methylation of the polysaccharide sheet demonstrated the presence of the sugar residues shown in Table 3. The main components of the polysaccharide are (1-4) linked glucose (74.7%) and (1-4-6) linked glucose (17.7%). Other linkages detected are (1-3-4), (1-2-
30 New sources, structure and properties of cellulose 4). (1-3-4-6) and (1-2-4-6) linked branch points. A small amount (2.2%) of (1-4) linked pentose is also present. These results are in agreement with the compositional analysis of the polysaccharide sheet which showed mainly the presence of glucose and a small amount of xylose. Table 3 Partially methylated alditol acetates from the polysaccharidesheet obtained from Zoogloea sp. Peaks were assigned by comparison of the fragmentation patterns (GC-MS) with published data (11) % .............. Compound .................................................................... Typc . .....of .....l..i n k q ............................................................................ e Molar ratio
.2,3,4,6-tetra-O-methyIhcxitol 2,3,5-tri-O-methyl pentitol 2,3,6-tri-O-mcthyI hcxitol 2,6-di-O-methyl hexitol 3,6-di-O-methyl hexitol 2,3-di-O-mcthyl hcxitol 2-0-mcthyl hexitol 3-0-methyl hexitol .
Tcrminal hcxosc (1 - 4) pentose (1 - 4) hcxosc (1 3 - 4) hexose (1 - 2 4) hexose (1 - 4 - 6) hexosc (1 - 3 - 4 6) hexosc (1 2 4 - 6) hcxose
-
-
- -
-
2.0 6.2 208.7 1.1
1.o 49.6 3.6 7.2
0.73 2.23 74.69 0.4 1 0.36 17.74 1.28 2.56
Enzymatic hydrolysis
In order to hrther confirm the monosaccharide linkage position and type in the polysaccharide sheet, hydrolysis using an enzyme with a known specificity was used. The commercial enzyme preparation Celluclast 1.5 L hydrolyses cellulose, (1 +)-I3-Dglucopyranose residues in a polysaccharide. A sample of cellulose powder and of the polysaccharide sheet were incubated with the enzyme preparation Cellulcast 1.5 L and the hydrolysates were subjected to oligosaccharide analysis. The two oligosaccharide profiles were very similar. The monosaccharide peak elutes in the position identified as glucose from the analysis of the maltodextrin reference material. The smaller components, and possibly the dimer cellobiose, are common to both hydrolysates. From these analyses it can therefore be concluded that the polysaccharide sheet contains some (1+4)-fi-D-glucose residues which are hydrolysed to glucose by the enzyme. CONCLUSIONS
The contents of the water soluble and insoluble components of the polysaccharide sheet were 12 and 88%; respectively.The total percentage of soluble material obtained from trifluoroacetic acid hydrolysis of the water insoluble fraction of the polysaccharide sheet, using mild and strong conditions, was 88% (w/w). The main monosaccharides present in the soluble fraction were glucose (87.6%), xylose (8.6%), mannose (0.8%), ribose (1.7%), galactose (0.I%), arabinose (0.4%) and glucuronic acid (0.8%). Methylation analysis of the polysaccharide showed mainly 2,3,6-tri-O-mcthylhexitol (74.7%) and 2,3die-methylhexitol (1 7.7%). Enzyme hydrolysis of the polysaccharide with a cellulase (Celluclast 1.5 L) confirmed the presence of (1+4)-l3-D-glucopyranosyl units. ACKNOWLEDGMENTS
We thank Prof. J. Otamar, from the Instituto de Antibibticos, Universidade Federal de Pemambuco, Recife, Brazil, for his assistance in the identification of the Zoogloea sp.
Highly cellulosic exopolysaccharide 3 1 We also thank Mr. G. Burns, from the School of Chemistry, University of Birmingham, UK, for his assistance in the monosaccharide analysis.
REFERENCES 1 I W Sutherland, Biotechnology of microbial exopolysaccharides, Cambridge, Cambridge University Press, 1990. 2 S Masaoka, T Ohe and N Sakota, ‘Production of cellulose from glucose by Acetobacter xylinirm’, Joiimal of Fermentation and Bioengineering, 1993 75( 1) 18-22. 3 A Okiyama, H Shirae, H Kano and S Yamanaka, ‘Bacterial cellulose I. Two-stage fermentation process for cellulose production by Acetobacter uceti’, Food Hydrocolloids, 1992 6(5) 471-477. 4 A Okiyama, M Motoki, and S Yamanaka, ‘Bacterial cellulose 11. Processing of the gelatinous cellulose for food materials’, Food Hydrocolloids, 1992 6(5) 479-487. 5 S Yamanaka and K Watanabe, ‘Applications of bacterial cellulose’. In R D Gilbert (Ed.), Cellitlosic Polymers, Blends and Composites (pp. 207-2 15). Munich, Hanser Publishers, 1994. 6 S Yamanaka, K Watanabe, N Kitamura, M Iguchi, S Mitsuhashi, Y Nishi, and M Uryu ‘The structure and mechanical properties of sheets prepared from bacterial cellulose’, Journal Material Science, 1989 24 3 141-3 145. 7 M Takai, ‘Bacterial cellulose’. In R D Gilbert (Ed.), Cellulosic Polymers, Blends and Composites (pp. 233-240). Munich, Hanser Publishers, 1994. 8 F Ikeda, H Shuto, T Saito, T Fukui, and K Tomita, ‘An extracellular polysaccharide produced by Zoogloea ramigera 115’, European Journal of Biochemistry, 1982 123 437-445. 9 A Isogai, A Ishizu, and J Nakano, ‘A new facile methylation method for cell-wall polysaccharides’. Carbohydrate Research, 1985 138 99-108. 10 A B Blakeney, P J Harris, R J Henry, and B A Stone, ‘A simple and rapid preparation of alditol acetates for monosaccharide analysis’, carbohydrate Research, 1983 113 291299. 1 1 P-E Jansson, L Kenne, H Liedgren, B Lindberg and J Lonngren, ‘A practical guide to the methylation analysis of carbohydrates’,Chemical Cornmimicatioris, 1976 8 1-74.
A highly cellulosic exopolysaccharide produced from sugarcane molasses by a Zougfoea sp. M. Paterson-Beedle', L. L. Lloyd', J. F. Kennedy', F. A. D. Melo: & V. Medeiros2 'Birmingham Carbohydrate and Protein Technology Group, ChembiotechLaboratories, University of Birmingham Research Park, Birmingham, BIS ZSQ, UK 'EstaCSo Experimental de Cana-de-A qucar de Carpina- Universidade Federal Rural de Pernambuco, Carpina, Pernanibuco, CEP 55 810 000, Brazil
Brazilian Zoogloea sp. bacterium produced from sugarcane molasses a polymer substance containing glucose (87.6%), xylose (8.6%), mannose (0.8%), ribose (1.7%), galactose (0. l%), arabinose (0.4%) and glucuronic acid (0.8%). Methylation analysis of the polysaccharide showed mainly 2,3,6-tri-O-methylhexitoI(74.7%) and 2,3-di-Omethylhexitol (17.7%). Enzyme hydrolysis of the polysaccharide with a cellulase confirmed the presence of (1+4)-8-D-glucopyranosyl units.
INTRODUCTION Cellulose is produced by Acetobacter xylinum and other, mainly Gram-negative, bacterial species (1,2), the cellulose being excreted into the medium, where it rapidly aggregates as microfibrils. Bacterial cellulose has long been used in foods in Asian countries (3,4) and its different properties from wood-derived cellulose opens new industrial applications. Bacterial cellulose possesses high crystallinity, high degree of polymerization, high tensile strength and tear resistance, and high hydrophilicity that distinguish it from other forms of cellulose (5). Various potential industrial applications for bacterial cellulose include acoustic diaphragms, artificial skin or wound healing, filter membranes, ultra strength paper and paper additives (5,6,7). In food processing, suspensions of disintegrated bacterial cellulose have been found useful as thickening and binding agents and as a dietary fibre supplement (5). Zoogloea ramigera produces zooglan, a polysaccharide composed of D-glucose, Dgalactose and pyruvic acid in an approximate ratio of 11:3:1.5 (8). It is a long chain polysaccharide consisting of mainly 1,Clinked glucose residues and 1,4- and 1,3- linked galactose residues with branches of glucose residues at the C-3 or C-6 positions of the galactose residues. .The pyruvic acid residues, the acidic component, are linked to the nonreducing end and/or 1,3-linked glucose residues through 4,6-ketal linkages (8). Here, we present recent studies on the composition and structure of an exopolysaccharide produced from sugarcane molasses by a Zoogloea sp. EXPERIMENTAL Materials
The following materials were obtained from sources indicated: sugarcane molasses ( M a c 2 0 Experimental de Cana-de-AGucar, Brazil); yeast extract, peptone and agar (Merck, USA); Celluclast 1.5 L (cellulase) (NOVONordisk, Denmark); cellulose powder (Sigma, UK); monosaccharide standards (myo-inositol, glucose, mannose, arabinose, xylose, ficose, galactose, ribose and glucuronolactone) (Sigma, UK); trifluoroacetic acid
24 New sources, structure and properties of cellulose (Janssen Chimica, UK); formic acid, diethylamine, l-methyl imidazole, dimethyl sulphoxide and acetic anhydride (BDH, UK); barium carbonate (Sigma, UK); sulphur dioxide (Fluka, UK), methyl iodide (Aldrich, UK). Other chemicals of analytical grade were obtained commercially. Microorganism An extracellular polysaccharide producing bacterium Zoogloea sp. was isolated fiom an agro-industrial environment in the north-eastern region of Brazil. The identification of the microorganism was carried out at the Instituto de Antibibticos, Universidade Federal de Pernambuco, Brazil. The Zoogloea sp. was maintained as slant cultures at 4 "C. The yeast extract (5.0 peptone (3.0 culture medium consisted of glucose (20.0 agar (15.0 g/L) in deionised watcr and the pH was adjusted to 6.8 before sterilisation (120 "C, 20 rnin).
a),
a),
a),
Production of the exopolysaccharide sample
For the exopolysaccharideproduction, cells were transferred from agar slants to 100 mL of sterilised medium (120"C, 40 min), consisting of sugarcane molasses (15 "Brix, pH 5.0), in 250 mL Erlenmeyer flasks. Cultures were incubated at 30 "C for 7 days. The polysaccharide pellicles, a gel-like material, were formed at the air-liquid interface. The pellicles were washed in deionised water, sterilised (120"C, 40 min) and dried at 60°C on glass plates to form solid polysaccharide sheets. Determination of the water soluble material of the polysaccharide sheet
The polysaccharide sheet (21 1.04 mg) was cut into small particles (ca. 4 mm') and transferred to a centrifbge tube. Deionised water (6 mL) was added, the material was mixed for 1 min using a vortex mixer, left to stand for 15 min, mixed again for a further 1 min and centrihged at 8,OOOg for I5 rnin. The residue was washed with deionised water (6 mL) and centrihged at 8,000 g for 15 min. This procedure was repeated twice. The supernatants were combined, lyophilised and dried using an Abderhalden dryer at 39 "C. The residue was dried using an Abderhalden dryer (39 "C) and the dried weight of the residue determined. Mild acid hydrolysis of the polysaccharide smmple
The water insoluble residue (47.51 mg) was suspended in trifluoroacetic acid (2 M, 4 mL) in a round bottom flask, allowed to stand for 45 min and refluxed at 97 "C for 3 h. The hydrolysate was left at ambient temperature for 20 h. Trifluoroacetic acid was removed by drying the sample using a rotor evaporator. The hydrolysate was washed with deionised water (2 mL) which was also removed by evaporation. This procedure was repeated six times. The residue was transferred to a centrifbge tube and deionised water (1.3 mL) was added. The sample was centrihged at 8,000 g for 30 An. This procedure was repeated three times and thc supernatants were combined and stored at -20°C prior to anion exchange high performance liquid chromatography analysis. The acid insoluble residue was dried usirig an Abdcrhalden dryer (39 "C) and the dry weight determined.
Highly cellulosic exopolysaccharide 25 Strong acid hydrolysis of the polysaccharide sample
Trifluoroacetic acid (99%, 3 mL) was added to the dried acid insoluble residue (33.5 mg) and allowed to stand at ambient temperature (16 h) to swell. The sample was refluxed for 2 h, at 97 "C, diluted to 80% trifluoroacetic acid with deionised water and refluxed for 30 min. The sample was diluted again to 30% trifluoroacetic acid with deionised water and refluxed for a fbrther 4 h. The trifluoroacetic acid was removed and the sample worked up as for the mild acid hydrolysates. Monosaccharide analysis using anion exchange high performance liquid chromatograplry
The trifluoroacetic acid hydrolysates were neutralized with sodium hydroxide solution prior to the analysis. The analysis of monosaccharides of the hydrolysates was performed on a Dionex DX-500system consisting of a GP40 gradient pump and a Dionex ED40 electrochemical detector. The standard sequence of potentials for carbohydrate detection (50 mV for 200 ms; 750 mV for 200 ms and -150mV for 400 ms) was applied to the Au ED40 working electrode for pulsed amperometric detection. The columns used were a CarboPac PA1 analytical column (250 x 4.0 mm ID) and a CarboPac PA1 guard columns (50 x 4.0 mm ID). All eluents and reagents were prepared using water purified to 18.2 Mi2 using a UHQPS system (Elga). Eluents A, B and C were deionised water, sodium acetate (1 M) and sodium hydroxide (1 M), respectively. Sodium hydroxide (300 mM) at 0.5 a m i n was added as a post column reagent. After equilibrating the system with deionised water for 30 min a sample (50 pL) was injected. The sodium acetate concentration was then increased, linearly, to 200 mM over the following 10 min and held at this level for 10 min. Oligosaccharide analysis using anion exchange high performance liquid chromatography - method 1
The analysis of the oligosaccharide component of the water soluble extract and the mild trifluoroacetic acid hydrolysate was performed on a Waters 625-LC system fitted with a non-metallic flow path, a 464 pulsed amperometric detector (PAD) fitted with a gold working electrode and a base stable reference electrode and a Whisp 712 injector. Sodium hydroxide (300 mM), at a flow rate of 0.7 mWmin, was added to the eluent stream between the columns and detector. The PAD was operated in the cathodic mode with the following sequence of potentials: 50 mV for 200 ms; 800 mV for 200 ms; and 600 mV for 500 ms. The columns used were a CarboPac PA1 analytical column (250 x 4.0 mm ID) and a CarboPac PA1 guard column (50 x 4.0 mm ID). N1 eluents and reagents were prepared using water purified to 18.2 Mi2 quality using a UHQPS system (Elga). Eluents A, B, C, and D were sodium hydroxide (100 mM), sodium hydroxide (100 mM) containing sodium acetate (800 mM), sodium hydroxide (300 mM), and sodium hydroxide (500 mM), respectively. Eluent C was used as the post column reagent. After equilibrating the system with sodium hydroxide (100 mM) a sample (200 pL) was injected: The sodium acetate concentration was then increased, linearly, to 800 mM over the following 60 min and held at this level for 5 min.
26 New sources, structure and properties of cellulose Oligosaccharide analysis using anion exchange high performance liquid chromatography - method 2
The method used for the analysis of the oligosaccharide component of the strong trifluoroacetic acid hydrolysate and the enzyme hydrolysates was similar to method 1. The only difference was that the concentration of sodium acetate was increased, linearly, to 400 mh4 over 60 min. Methylation analysis
The water insoluble polysaccharide was methylated by three cycles according to method described by Isogai et al. (9).
Preparation of polysaccharide siispension The water insoluble polysaccharide (32.4 mg) was dispersed in dimethyl sulphoxide (2.84 mL). Concentrated sulphur dioxide-methyl sulphoxide solution (ca. 0.3 dmL) was prepared as follows: sulphur dioxide (15 g) was bubbled into methyl sulphoxide (50 mL). Sulphur dioxide-methyl sulphoxide solution (165 pL) was added to the sample. Then, diethylamine (61 pL) was added. The suspension was stirred at ambient temperature for 20.3 h. Complete dissolution was not obtained.
Methylation of polysuccharide Freshly powdered sodium hydroxide (389 mg) was added to the sample, at ambient temperature, and the mixture stirred for 1 h under helium. Methyl iodide (390 pL) was added dropwise, at ambient temperature, and the mixture was stirred for 1 h, and then kept at 40 "C for 0.5 h, at 50 "C for 0.5 h and at 60 "C for 1 h. The methylation procedure was repeated three times, starting from the preparation of polysaccharide suspension (from the addition of sulphur dioxide-methyl sulphoxide solution). When solidification occurred during stirring, methyl sulphoxide was added to the sample to regenerate the slurry (1 mL prior to the second methylation and 2 mL prior to third methylation). The methylated products were isolated by dialysis and subsequent lyophilised.
Formic acid and sulljhuric acid hydrorySis The dry methylated product (11.8 mg) was allowed to swell in formic acid solution (90%, 1.18 mL) overnight, at ambient temperature. The sample was then refluxed during 2 h, the formic acid evaporated, the residue washed once with deionised water (1 mL) and then evaporated. Sulphuric acid solution (0.125 M, 2.95 mL) was added to the sample and refluxed during 10 h, neutralised with barium carbonate (ca. 236 mg) and the insoluble barium sulphate removed by centrifhgation (7,500 rpm, 15 min). The partially methylated sugars present in the supernatant were then used for the preparation of alditol acetates.
Highly cellulosic exopolysaccharide 27 Preparation of alditol acetates
The resulting partially methylated aldoses were converted to partially methylated alditol acetates as described by Blakeney et al. (10). Ammonia (80 pL) was added to the hydrolysate (containing ca. 2 mg methylated monosaccharides in 0.5 mL). myo-Inositol solution (4 mg/mL, 100 pL) was added as an internal standard. Sodium borohydride (ca. 70 mg) was added and the sample was allowed to stand at 40 "C for 1.5 h. Excess borohydride was destroyed by addition of glacial acetic acid (0.2 mL). After cooling to ambient temperature, I-methyl imidazole (1.5 mL) was added and the sample shaken vigorously to ensure it was in solution. Acetic anhydride (5 mL) was added cautiously (in a bath containing a mixture of ice and cold water), causing an immediate rise in temperature. The sample was allowed to react for 20 min, with frequent gentle agitation. The excess anhydride was hydrolised by addition of deionised water (12 mL) and shaken to thoroughly mix the sample. It was cooled for 5 min in a bath containing a mixture of ice and water, then extracted twice with dichloromethane (1 mL). The dichloromethane (lower layer) was removed by dropping a pipette; the extracts from the sample were combined and submitted to gas liquid Chromatographyanalysis. Gas liquid chromatography nnalysis
Quantification of the partially methylated alditol acetates was carried out on a Carlo Erba GC 8000 series gas chromatograph fitted with a BPX 70 (70% biscyanopropylpolysilphenylene-siloxane, 25 m x 0.33 mm, 0.25 pm film thickness) capillary column (supplied by SGE, UK) and a flame ionisation detector. The column pressure was 150 kPa and the flow of helium was 2 mL/min. The initial temperature of the oven was 150 "C and was increased at a rate of 10 "Chin to 190 "C and maintained for 2 min. The temperature was then increased at a rate of 5 "Chin to a final temperature of 250 "C and maintained for 10 min. The injection temperature was 260 "C and the detector temperature was 300 "C. Gas cliromatography nnd mass spectrometry
Gas chromatography and mass spectrometry (GC/MS) analyses were carried out on a Prospec (VG Company) equipped with a BPX 70 (70% biscyanopropylpolysilphenylene-siloxane, 50 m x 0.33 mm, 0.25 pm film thickness) capillary column (supplied by SGE, UK). The initial temperature of the oven was 150 "C and was increased at a rate of 10 "Chin to 190 "C and maintained for 2 min. The temperature was then increased at a rate of 5 "Chin to a final temperature of 250 "C and maintained for 5 min. Enzymatic liydrolysis of the polysaccharide sample
Particles of the water insoluble fraction of the polysaccharide sheet (4.3 mg) were suspended in sodium acetate buffer (0.4 M, pH 5.0, containing Celluclast 1.5 L, 5 pL/mL, 860 to give a concentration of 5 mg sample/mL. Cellulose powder (3.5 mg) was also suspended in sodium acetate buffer (0.4 M, pH 5.0, containing Celluclast 1.5 L, 5 pWmL, 700 pL). The samples and control (sodium acetate buffer containing Celluclast 1.5 L) were mixed and incubated in a water bath at 50 "C for 68 h and then at 100 "C for 10 min to deactivate the enzyme. After centrihgation at 10,000 g for 30 min, the
a),
28
New sources, structure and properties of cellulose
supernatants were decanted and deionised water (same volume used for the enzymatic hydrolysis) was added to the residues. The samples were centrifbged at 10,000 g for 30 min. The supernatants were combined and stored at -20 "C prior to anion exchange high performance liquid chromatography analysis. RESULTS AND DISCUSSION
Composition analysis
The contents of the water soluble and insoluble components of the polysaccharide sheet were 12 and 88 % (w/w). respectively. The profile of the oligosaccharides present in the water soluble component, compared with an oligosaccharide fingerprint of a starch hydrolysate reference material, indicated the presence of more than one neutral monosaccharide and a number of disaccharides with different monosaccharide composition or linkage position or type. Polysaccharides are hydrolysed under acid conditions to their component monosaccharides by cleavage of the glycosidic linkage at the bond between the anomeric carbon atom and the glycosidic oxygen. Conditions chosen for acid hydrolysis of polysaccharides are always a compromise between release and destruction of the component monosaccharides. Depending on the type of structural information required, conditions can be selected to give optimum release for monosaccharides or to favour release of oligosaccharide fragments. In this study two acid hydrolysis conditions (mild and strong) were chosen to release the monosaccharides. The percentages of soluble and insoluble components obtained from the trifluoroacetic acid hydrolysis of the polysaccharide sheet, under mild conditions, were 15.8 and 84.2% (w/w), respectively. The percentages of soluble and insoluble components obtained from the trifluoroacetic acid hydrolysis (using strong conditions) of the acid insoluble fraction (obtained using mild conditions) of the polysaccharide sheet were 72.2 and 12.0% (w/w), respectively. The total percentage of soluble material obtained from the trifluoroacetic acid hydrolysis, using mild and strong conditions, was 88% (w/w) of the water insoluble fraction. The monosaccharide compositions of the soluble components obtained from the trifluoroacetic acid hydrolysis of the polysaccharide sheet, under mild and strong conditions, determined by anion exchange high performance liquid chromatography analysis, are shown in Table 1. Glucose accounted for 87.6% of the total monosaccharides present in the acid hydrolysates (mild and strong conditions). Galactose was present in a very small amount (0.1%). These results differ from those obtained in the literature for zooglan, which is composed of D-glucose, D-galactose and pyruvic acid in an approximate ratio of 11:3:1.5 (8). The content of xylose (8.6%) was higher than the other monosaccharides present in the hydrolysates. The percentage of monosaccharides present in the acid hydrolysates (strong and mild conditions), determined by anion exchange high performance liquid chromatography, is shown in Table 2. The total percentage of monosaccharides released by the acid hydrolysis was only 38.4% of the total acid soluble component. The oligosaccharide profile of the trifluoroacetic acid hydrolysate, under mild conditions, showed a main peak in the monosaccharide region and other small peaks in the region of di- and tetra-saccharides, It should be noted that the ratio of the two peaks in the monosaccharide component is significantly different for the trifluoroacetic acid hydrolysate produced using the mild conditions and the water soluble component. This ,
Highly cellulosic exopolysaccharide 29 would suggest that there are differences in the solubility of sample according to the monosaccharide composition. Table 1 Monosaccharide compositions of the soluble fractions produced by trifluoroacetic acid hydrolysis, under mild and strong conditions, determined by anion exchange high performance liquid chromatography
".Monosaccharide ............... Fucose
Mild conditions Strong conditions Total ............................e??.?..Y!!!~.......... ...Y!!?.. !.! ...... !............ !? S%?. I.Y!!.!) .....
Arabinose Rhamnose
Galactose Glucosc
Xylose Mannose Ribose
Glucuronic acid Total
.
0.0 I 1.91 0.22 1.58 55.41 29.7 1 1.96 2.37 6.83 100.00
0.01 0.26
0.03 89.88 7.06 0.74 1.63 0.39 100.00
0.01 0.37 0.01 0.13 87.57 8.58
0.82 1.68 0.83 100.00
Table 2 Percentnges of monosacchnridcs, present in the trifluoroacetic acid hydrolysates of the polysaccharide sheet, using mild and strong conditions, determined by anion exchange high performance liquid chroniatography
Treatment Mild
Monosaccharides (%, w/w)'
14.4 43.7 38.4 * Calculated in relation to the amount of material solubiliscd by thc acid treatmcnt Strnne Total
The trifluoroacetic acid hydrolysate, obtained using strong conditions, was analysed for oligosaccharide content at two different concentrations. The main peak was in the monosaccharide region and its elution position correlates with glucose, in the maltodextrin reference material. It should be noted that even with the higher column load the monosaccharide is a single peak unlike the doublet which was observed in the oligosaccharide profiles of the mild trifluoroacetic acid hydrolysate and of the water soluble component .of the polysaccharide sheet. However, the disaccharide peak in the hydrolysate does not correlate with the elution position of maltose, the disaccharide present in the maltodextrin reference material, which would indicate that although the monosaccharide residue is glucose the polysaccharide linkage is different. As expected, these results confirm that a higher amount of material was released from the polysaccharide sheet by the strong acid conditions compared with the mild conditions. Structure analysis
Methylation analysis was carried out in order to hrther study the polysaccharide sheet. Methylation of the polysaccharide sheet demonstrated the presence of the sugar residues shown in Table 3. The main components of the polysaccharide are (1-4) linked glucose (74.7%) and (1-4-6) linked glucose (17.7%). Other linkages detected are (1-3-4), (1-2-
30 New sources, structure and properties of cellulose 4). (1-3-4-6) and (1-2-4-6) linked branch points. A small amount (2.2%) of (1-4) linked pentose is also present. These results are in agreement with the compositional analysis of the polysaccharide sheet which showed mainly the presence of glucose and a small amount of xylose. Table 3 Partially methylated alditol acetates from the polysaccharidesheet obtained from Zoogloea sp. Peaks were assigned by comparison of the fragmentation patterns (GC-MS) with published data (11) % .............. Compound .................................................................... Typc . .....of .....l..i n k q ............................................................................ e Molar ratio
.2,3,4,6-tetra-O-methyIhcxitol 2,3,5-tri-O-methyl pentitol 2,3,6-tri-O-mcthyI hcxitol 2,6-di-O-methyl hexitol 3,6-di-O-methyl hexitol 2,3-di-O-mcthyl hcxitol 2-0-mcthyl hexitol 3-0-methyl hexitol .
Tcrminal hcxosc (1 - 4) pentose (1 - 4) hcxosc (1 3 - 4) hexose (1 - 2 4) hexose (1 - 4 - 6) hexosc (1 - 3 - 4 6) hexosc (1 2 4 - 6) hcxose
-
-
- -
-
2.0 6.2 208.7 1.1
1.o 49.6 3.6 7.2
0.73 2.23 74.69 0.4 1 0.36 17.74 1.28 2.56
Enzymatic hydrolysis
In order to hrther confirm the monosaccharide linkage position and type in the polysaccharide sheet, hydrolysis using an enzyme with a known specificity was used. The commercial enzyme preparation Celluclast 1.5 L hydrolyses cellulose, (1 +)-I3-Dglucopyranose residues in a polysaccharide. A sample of cellulose powder and of the polysaccharide sheet were incubated with the enzyme preparation Cellulcast 1.5 L and the hydrolysates were subjected to oligosaccharide analysis. The two oligosaccharide profiles were very similar. The monosaccharide peak elutes in the position identified as glucose from the analysis of the maltodextrin reference material. The smaller components, and possibly the dimer cellobiose, are common to both hydrolysates. From these analyses it can therefore be concluded that the polysaccharide sheet contains some (1+4)-fi-D-glucose residues which are hydrolysed to glucose by the enzyme. CONCLUSIONS
The contents of the water soluble and insoluble components of the polysaccharide sheet were 12 and 88%; respectively.The total percentage of soluble material obtained from trifluoroacetic acid hydrolysis of the water insoluble fraction of the polysaccharide sheet, using mild and strong conditions, was 88% (w/w). The main monosaccharides present in the soluble fraction were glucose (87.6%), xylose (8.6%), mannose (0.8%), ribose (1.7%), galactose (0.I%), arabinose (0.4%) and glucuronic acid (0.8%). Methylation analysis of the polysaccharide showed mainly 2,3,6-tri-O-mcthylhexitol (74.7%) and 2,3die-methylhexitol (1 7.7%). Enzyme hydrolysis of the polysaccharide with a cellulase (Celluclast 1.5 L) confirmed the presence of (1+4)-l3-D-glucopyranosyl units. ACKNOWLEDGMENTS
We thank Prof. J. Otamar, from the Instituto de Antibibticos, Universidade Federal de Pemambuco, Recife, Brazil, for his assistance in the identification of the Zoogloea sp.
Highly cellulosic exopolysaccharide 3 1 We also thank Mr. G. Burns, from the School of Chemistry, University of Birmingham, UK, for his assistance in the monosaccharide analysis.
REFERENCES 1 I W Sutherland, Biotechnology of microbial exopolysaccharides, Cambridge, Cambridge University Press, 1990. 2 S Masaoka, T Ohe and N Sakota, ‘Production of cellulose from glucose by Acetobacter xylinirm’, Joiimal of Fermentation and Bioengineering, 1993 75( 1) 18-22. 3 A Okiyama, H Shirae, H Kano and S Yamanaka, ‘Bacterial cellulose I. Two-stage fermentation process for cellulose production by Acetobacter uceti’, Food Hydrocolloids, 1992 6(5) 471-477. 4 A Okiyama, M Motoki, and S Yamanaka, ‘Bacterial cellulose 11. Processing of the gelatinous cellulose for food materials’, Food Hydrocolloids, 1992 6(5) 479-487. 5 S Yamanaka and K Watanabe, ‘Applications of bacterial cellulose’. In R D Gilbert (Ed.), Cellitlosic Polymers, Blends and Composites (pp. 207-2 15). Munich, Hanser Publishers, 1994. 6 S Yamanaka, K Watanabe, N Kitamura, M Iguchi, S Mitsuhashi, Y Nishi, and M Uryu ‘The structure and mechanical properties of sheets prepared from bacterial cellulose’, Journal Material Science, 1989 24 3 141-3 145. 7 M Takai, ‘Bacterial cellulose’. In R D Gilbert (Ed.), Cellulosic Polymers, Blends and Composites (pp. 233-240). Munich, Hanser Publishers, 1994. 8 F Ikeda, H Shuto, T Saito, T Fukui, and K Tomita, ‘An extracellular polysaccharide produced by Zoogloea ramigera 115’, European Journal of Biochemistry, 1982 123 437-445. 9 A Isogai, A Ishizu, and J Nakano, ‘A new facile methylation method for cell-wall polysaccharides’. Carbohydrate Research, 1985 138 99-108. 10 A B Blakeney, P J Harris, R J Henry, and B A Stone, ‘A simple and rapid preparation of alditol acetates for monosaccharide analysis’, carbohydrate Research, 1983 113 291299. 1 1 P-E Jansson, L Kenne, H Liedgren, B Lindberg and J Lonngren, ‘A practical guide to the methylation analysis of carbohydrates’,Chemical Cornmimicatioris, 1976 8 1-74.
THE SUPRAMOLECULAR STRUCTURE OF CELLULOSE 11. STUDIES WITH %-CP/MAS-NMR AND CHEMOMETRICS Helcna Lennholm Dep. of Pulp and Paper Chemistry and Technology KTH, SE-100 44 Stockholm, Sweden
INTRODUCTION Native cellulose I can be turned into cellulose I1 by mercerisation or regeneration. When a sample is mercerised it is swelled in strong alkali solution, and then washcd with water. Regeneration is achieved when the cellulose sample is dissolvcd, and then precipitated in water. Merccrisation is thus a solid state reaction, but regeneration is a reaction from solution state to solid state. Differences in origin of a ccllulose sample can thus be expccted to remain more after a mercerisation than after regencration. 13~-cross-po~arisation magic angle spinning nuclear magnetic resonance (CP/MASNMR)-spcctra of cellulose contain information regarding the amounts and structures of the different cellulose polymorphs and of unordered cellulose [l]. NMR-spectra of lignocellulosic samples, however, contain broad and overlapping peaks. This problem can be 0,vcrcomeby using chemometrics or spectral fitting. NMR-spectra of cellulose I have recently been thoroughly assigncd, using spectral fitting [2]. In earlier work [3] we used chemometrics to evaluate NMR-spectra of cellulose I and 11. In this work we wanted to study more thoroughly which factors determine the production of cellulose I1 from cellulose 1. We chose the starting cellulose materials to represent variations in the proportion of cellulose Ia and Ip, microfibril size and state of order. We used algal ccllulose, bacterial cellulosc, cotton linters and birch kraft pulp. Further, we varied the state of order of the starting materials by ball milling. Thc starting materials and ball-milled starting materials were all mercerised and regenerated. We recordcd 13C-CPAVIAS-NMR-spectraon all samples and evaluated the spectra with chemometrics. EXPERIMENTAL Starting materials
The cotton linters and bleached birch kraft pulp wcre commercially available samples. Bacterial cellulose was generated from commercial Nata de COCO,by boiling in 1 % NaOH for 2 days, then washing with deionised water. Algal cellulose was generated from Cfadophoru from the Baltic: Thc Cladophora was treated with 0.25 M HC1 for 3 days, then washed with deionised watcr. It was then treated with 0.05 g NaC102 and 0.015 ml acetic aced per gram Cfudophorufor 18 h. The chlorite treatment was repeatcd 3 times. After washing with deionised water the algal cellulose was treatcd with 0.1 M NaOH for 2 h, and washed. Mercerisation The samples were immersed in 16 % NaOH and kept at -18 "C for at least 18 h. The mcrcerised samples were then washed with watcr.
34 New sources, structure and properties of cellulose Regeneration A total of 5 g of each sample was dissolved in 50 ml tetrabutylammonium hydroxide (40% in water) and 50 ml dimethylsulphoxide and stirred for 2 h. The solution was then poured into water with some drops of acetic acid during agitation. The regenerate was then washed with water. Milling The samples were milled in a vibratory ball-mill, for 10 min.
NMR-spectroscopy Thc 13C-CJ?/MASNMR spectra were recorded on wet samples on a Bruker AMX-300 instrumcnt at ambient temperature. The spectromctcr operated at 75.47 MHz using a double air-bearing probe and ZrO2 rotors. Spinning rate was 5 kHz, contact time was 0.8 ms, acquisition time was 37 ms, sweep width was 368 ppm and delay betwcen pulses was 2.5 s. For each spectrum, 2000-15000 transients were accumulated with 2048 data points and zero fillcd to 4096 data points. The spectra were referenced to carbonyl in external glycine (6= 176.03 ppm). Chernometrics Principal component analysis (PCA) was carricd out using the SIMCA 7.0 s o b a r c .
RESULTS AND DISCUSSION Fig. 1 shows reference NMR-spcctra of cellulose Ia, cellulose IP, cellulose I1 and unordered cellulosc [3].
110
105
c z , 3,5
c4
GI
100
95
90
85
80
75
70
C6
65
60
55
Figure I I3C-CP/MAS-NMR-spectra representing different cellulose polyinorphs [3].
The NMR-spectra of the starting materials are shown in Fig. 2. Assignments are according to [4]. The spectra of algal and bacterial cellulose show typical appearance of cellulose Ia-pcaks (cf. Fig. 1). The spectrum of cotton show a more typical appcarance of the cellulose IP-peaks (cf. Fig. 1). In the spectra of birch pulp we see thc fcatures of
Supramolecular structure of cellulose I1 35
Cotton linters
110
105
100
95
85
90
70
75
80
65
60
55
50
Figure 2 13C-CP/MAS-NMR-spectra of the starting materials.
Merc linters
A
Merc bacterial
A Alg
A
A -_____
Merc alg Merc birch
A
Bacterial
'TGGrs
A
A
Birch
Figure 3 PCA scores-plot of the NMR-spectra of the starting materials and the mercerised samples.
a more disordered sample (cf. Fig. 1). The spectra of algal and bacterial cellulose reprcscnt samples with high proportion of cellulose Ia,whereas the spectra of cotton and birch represcnt samples with low proportion of cellulose Ia.If we study the peaks originating from thc surfaces o f the microfibrils (80-85ppm, cf. [2]), we furthcr sce that
36
New sources, structure and properties of cellulose
the algal cellulose and cotton have large microfibrils (small surface peaks), whereas bacterial cellulose and birch have small microfibrils (large surface peaks). Fig. 3 shows a principal component analysis (PCA) scores-plot where we can sce the NMR-spectra of the starting materials and the merccrised samples. There is a grouping of the starting materials to the right and mercerised samples to the left. We thus have the merccrisation along principal component 1 (PC1). The birch sample is, however, very close to the merceriscd algal and birch sample. The question is then if the mcrceriscd algal and birch sample still contain some cellulosc I?
Bacterial cellulose Cotton linters
110
105
I00
95
90
85
80
75
70
65
Figure 4 NMR-spectra of the mercerised samples.
Reg bacterial
A
A
Birch .Linters
Keg birch
A Reg linters
A
A
A Bacterial
60
55
50
Supramolecular structure of cellulose I1 37 The NMR-spectra of the mercerised samples are shown in Fig. 4. Indccd the mcrccrised algal and birch samplcs have NMR-signals corresponding to ccllulose 11, but also some features of ccllulose I (cf. Fig. 1). We further see that the merccrised bacterial and cotton samples show very broad patterns, corresponding to totally unordcrcd cellulose. These samples obviously did not turn into ccllulose I1 upon merccrisation. In Fig. 5 the PCA scores-plot of the NMR-spectra of the starting materials and the rcgeneratcd samples is shown. Here we see a clear grouping of starting materials (to the right) and regeneratcd samples (to the left) along PC1. The corresponding NMR-spectra of the rcgenerated samples are shown in Fig. 6 . These spectra are not as similar as the spectra of the merceriscd samples (Fig. 4). Upon a dissolution and regeneration we expected the samples to lose information about the cellulose structure of the starting materials, but from Fig. 6 we see that this is not the case. The samples with small microfibrils (bacterial and birch) thus seem to yicld regenerated samplcs with low amounts of cellulose 11, compared to the cotton sample (with large microfibrils).
Bacterial cellulose
Cotton linters
110
105
100
95
90
85
80
75
70
65
60
55
50
Figure 6 NMR-spectra of the regenerated samples.
Fig. 7 shows NMR-spcctra of some of the merceriscd and regencratcd milled samples. Here thc pattern is obvious; all the samples are now cellulose I1 (cf. Fig. 1). The disordering or lowering of molecular weight the milling introduced rcsulted in a thorough formation of ccllulose I1 upon mcrccrisation and regeneration, cf. Figs 4 and 6.
38 New sources, structure and properties of cellulose
110
105
I00
95
90
85
80
75
70
65
60
55
50
Figure 7 Some NMR-spectra of the regenerated and mercerised milled samples. CONCLUSIONS
In the study which factors determine the gcneration of cellulosc I1 from cellulose I we have indications that thc origin of the cellulose I-matcrials affected the result of both merccrisation and regeneration. We have so far not obscrved any systematic effect of the proportion cellulose Ia and Ip, neither upon merccrisation nor regeneration. Wc have indications that the microfibril size affect the regeneration of ccllulose I to cellulose 11. Ball milling of the starting materials resulted in increascd formation of cellulose 11, both during mercerisation and regeneration. ACKNOWLEDGEMENTS
Johan Fransson for the skilful expcrimental work. Dr. Tomas Larsson and Professor Tommy Ivcrscn for fruitful discussions. Carl Tryggers Stiftelse for financial support. RFFERENCES 1 A Isogai, M Usuda, T Kato, T Uryu and R H Atalla, Macromolecules, 1989 22 3168. 2 H Lcnnholm and T Iversen, Holzforschung, 1995 49(2) 119-126. 3 P T Larsson, K Wickholm and T Iversen, Carbohydrate Research, 1997 302 19-25. 4 D L Vandcrhart and R €1 Atalla, Macromolecules, 1984 17 1465-1472.
EFFECTS OF PULPING ON CRYSTALLINITY OF CELLULOSE STUDIED BY SOLID STATE NMR T. Liitiii', S. L. Maunu' and B. Hortling'
I Laboratory of Polymer Chemistty, P.O. Box 55, FIN-00014 Universi& of Helsinki, Finland 2 The Finnish Pulp and Paper Research Institute, Paper Science Centre,P.O. Box 70, FIN02151 E S ~ Q O FinIand ,
Solid state NMR spectroscopy has been used in this work to investigate crystallinity of cellulose in spruce wood before and after kraft pulping. Effects of bleaching and refining in water and in weak alkali have also been studied. It has been noticed that the I, crystalline form of cellulose predominates over the Ip form in native spruce and vice versa in all the pulps studied. In pulping part of the cellulose 1, is converted to the more stable Ip form mainly by heat. Cellulose I1 is also formed during pulping and the content of form I1 seems to be higher on the fiber surface. Any measurable changes in the degree of crystallinity or in the relative proportions of different crystalline forms of cellulose could not be seen in pulps after refining or TCF-bleaching. However, it was noticed that the degree of crystallinity was considerably lower in the fines than in the bulk fiber.
INTRODUCTION Cellulose is a scmicrystalline biopolymcr with ordered crystalline and disordered amorphous regions. The crystalline cellulose may crystallize in several different polymorphs. Cellulose I is synthesized natively and exists in two crystalline forms, cellulose I, and Ip.' Cellulose I, is metastable and can be converted to the more stable form Ip by heat?. It is assumed that I, and Ip cellulose have different conformations and that they differ also in hydrogen bonding.'. The difference between cellulose I and I1 is that cellulose chains in cellulose I are thought to be parallel, whereas in cellulose I1 they are probably antiparalle1.d Cellulose I1 is formed when the lattice of cellulose I is destroyed. Cellulose I can be converted to cellulose I1 by alkali treatment called mercerisation or by regeneration of dissolved cellulose. NMR spectroscopy in solid state enables us to investigate wood and pulp fibres in their original state. Thus the physical structure of the samples rcmains unchanged and ordinary "C CPMAS measurements can bc uscd to determine the degree of crystallinity of cellulose. The relative proportions of I,, Ip and I1 crystalline forms of cellulose can be obtained by deconvolution after resolution enhancement of NMR data. Solid statc NMR spectroscopy has been used in this work to investigate the crystallinity of cellulose in spruce wood bcfore and after kraft pulping. Effccts of TCFbleaching and refining in water and in weak alkali have been studicd. We have also separated thc fincs after refining and compared the crystallinity of cellulose between the
'
'
40 New sources, structure and properties of cellulose fines and the bulk fiber to find out whether the crystallinity of cellulose on the fiber surface differs from the bulk fiber.
EXPERIMENTAL Materials
We have studied Wiley-milled (40 mesh) spruce wood (Picea abies) and conventional haft pulp made of the same spruce. The pulp has been refined with a Voith Sulzcr laboratory refiner in water in two different ways (Kraftl, Kraft2) and also in 0.01 M NaOH solution (Kraft3). The first fraction of the pulp has been refined in water so that the fibers are ground more on the surface. The second refining in water has more cutting effect on the fibers. Refining in weak alkali has been made like the refining of Kraftl in water. The refining processes have been explained in more detail elsewhere5and Kraftl, Kraft2 and Kraft3 were referred to as Sal, Sa2 and Sa3, respectively. After refining the fines (Finel, Fine2, Fine3) have been separated from the bulk fibers (Kraftl-F, Kraft2F, Kraft3-F). The effects of TCF-bleaching have been studied after oxygen (0)stage and after oxygen, peroxide, ozone and peroxide treatments (OPZP). Methods
All the ordinary I3C CPMAS NMR measurements have been done with a Varian m"YINOVA 300 NMR spectrometer operating at 75.5 MHz for carbons. The spinning speed was 5000 Hz,contact time 1 ms, acquisition time 20 ms and delay between pulses 2 s for all samples. The time of accumulation has been 20-26 hours for pulps and 2.5 days for spruce wood. The measurements have been long in order to get good signal-tonoise ratio, because the amount of noice increases during resolution enhancement. All samples have been moistened with de-ionised water (-50 w-% H,O), because it is known that water improves resolution and signal-to-noise-ratio in the spectra of cellulose.6 All the spectra have been referenced using the C1 signal as an internal reference (6 105 ppm). The degrce of crystallinity of cellulose (CrI) has been defined from the areas of the crystalline (86-92 ppm) and amorphous (79-86 ppm) C4 signals by deconvolution using Lorenzian line shape.' To be able to see the different crystalline forms of cellulose, the spectra must be resolution enhanced. The resolution enhancement has been done using negative line broadening (lb) together with Gaussian hnction (go. Values of those parameters have been selected experimentally. After the resolution enhancement the relative proportions of different polymorphs have been defined from the deconvolution of the crystalline C4 signal as represented in the figurc 1.b. The signals of the polymorphs arc ovcrlapping doublets and thus the most intense peak at 88.9 ppm is believed to arise from the contribution of all three polymorphs. The fractions of Lorcnzian or Gaussian line shape in deconvolutions have been chosen so that the sum of the areas of the peaks at 89.7 ppm, 88.1 ppm and 87.5 ppm is closest to the area of the peak at 88.9 ppm.
Crystallinity of cellulose 41
110
loo
90
80
70
60
50
PPm
Figure I . a) The deconvolution of KrafiOPZP spectrum for determination of the degree of crystallinity of the cellulose. b) The deconvolution of crystulline C4 signal afier resolution enhancement for the determination of relative proportions of different crystalline forms of cellulose.
RESULTS AND DISCUSSION The crystallinity index of cellulose in spruce wood could not be determined exactly, becausc there are many signals of lignin and hemicelluloses at the same range as the amorphous C4 transition used for determination of the CrI. The CrI values determined for the pulps are not any absolute values of crystallinity either due to solubilisation of unordered materials during kraft pulping* and because pulp contains also some lignin and hemicelluloses. Thcir cffcct on crystallinity index of pulps is anyway thought to be very small and in this work we have used CrI as an approximate value for comparison between pulps. Any measurable changes in the degree of crystallinity could not be seen in pulps after refining or TCF-bleaching. It was noticcd, however, that the degree of crystallinity was considerably lower in the fines than in the bulk fibcr. In all pulps studied the crystallinity iiidcx is approximately 50%, whereas in the fines it is 37-40%. The content of lignin was noticcd to be higher in fines than in the pulps. That may affect little on CrI, but does not explain totally the lower amount of the crystalline cellulose in fines. Mechanical forces may break crystallites during refining and thus lower the CrI in fines. It is also possible that degree of crystallinity is lower on the fiber surface than inside the fiber. Thc "C CPMAS measurements proved that the I, crystalline form of cellulose predominates over the Ipin native spruce. The amount of the cellulosc I, is almost three times the amount of cellulose Ipin spruce wood. In the kraft pulps studicd the amount of cellulose Ip is twice the amount of cellulosc I,. Ccllulose I, is known to be
42 New sources, structure and properties of cellulose metastable and it can be converted to the more stable Ip form by heat. Conversion of the 1, form to the I p form has been found to require 260-280 "C temperatures in highly crystalline algal and bacterial celluloses? Although the temperature is only 170 OC in haft pulping the NaOH media probably accelerates the transformation of cellulose I, into cellulose Ip?. Samples of lower crystallinity, like wood in this case, have also been suggested to convert more easily? According to the NMR spectra small amount of cellulose I1 was also formed during haft pulping. 100% 80% 60%
mCellulose Ib
40%
20% 0% Kraft Pulp
Spruce Wood
Figure 2. Relative proportions of polymofls of the crystalline cellulose in spruce wood and in b a j l pulp.
The relative proportions of the different cellulose polymorphs and proportions of amorphous cellulose in refined and TCF-blcached pulps are represented in figures 3 and 4. Only marginal changes in the amounts of the crystalline forms of cellulose are observed during refining or bleaching. The proportions of cellulose I,, Ip and I1 of all the cellulose in pulps are 11-16%, 28-32% and 5-7%, respectively. In fines the proportions of cellulose I, and Ip are 9-1 1% and 20-23%, respectively, because of the lower CrI. The ratio I& is anyway 0.4 0.5 like in all the other pulps. The relative proportion of cellulose I1 is slightly higher in Finel and Fine3 than in the other samples studied. The ratio 1/11 is 4 in Finel and Fine3, whercas in all the other pulps it is 6-9. In Kraft3-F 1/11 ratio is as large as 11. Finel and Fine3 probably represent the surface of the fiber more than Fine2, because the first and third refining should be more surface active? In the second refining the fibers are more cut. Besides the lower CrI and the higher cellulose I1 content, no significant differences between the bulk fiber and fiber surfaces could be seen.
-
x)O%
90% 00% 70%
60% 50% 40%
30% 20%
UCellulose II .Cellulose Ib pCellulose la
lo% 0%
Figure 3. Relative proportions of different celhlose polymorphs and amorphous cellulose (Am4 in the refinedpulps andfines.
Crystallinity of cellulose 43 .DO% 90%
80% 70%
60% 50% 40%
cellulose II mCellulose Ib
30% 20%
10% 0%
Figure 4. ReIative proporlions of different cellulose polymorphs and amorphous cellulose (Aml) in TCF-bleachedpubs.
CONCLUSIONS
Part of the ccllulose I, is converted to cellulose Ip during haft pulping. This is mainly caused by high tcmperature (170 "C), but the cooking liqour and the lower crystallinity of cellulose in wood probably accelerate the transformation, too. Cellulose I1 is formed also during pulping and the content of cellulose I1 seems to be higher on the fiber surface thhn inside the fiber. The crystallinity index of the surface material of the fiber was detected to be lower than in the bulk fiber. The refining or TCF-bleaching were not observed to cause any signinficant changes in the degree of crystallinity or in the relative proportions of different crystalline forms of cellulose. ACKNOWLEDGEMENT
We are indcbtcd to the Technology Development Ccntrc of Finland (TEKES) for financial support. REFERENCES
1 2 3 4 5
6 7 8
VandcrIIart, D. L. and Atalla, R. H. Macromolecules 17(1984) 1465. Debzi, E. M., Chanzy, H., Sugiyama, J., Tekely, P. and Excoffier, G. Macromolecules 24 (1991) 6816. Sugiyama, J., Pcrsson, J. and Chanzy, H. Macromolecules 24 (1991) 2461. Sjostrom, B. Wood Chemistry, Fundamentals and Applications, Academic Press, Inc. 1981, New York, p.53-55. Hortling, U., Jousimaa, T. ,Hyviirinen, 13.-K. and Holopainen K., Cellucon '98, In this volume. Willis, J. M. and Herring, F. G. Macromolecules 20 (1987) 1554. Tecaiir, I<,, Serimaa, R. and Paakkari, T. Polymer Bull. 17 (1 987) 23 1. I-Iattula,T. Pcperi j a Puu 68 (1986) 926.
44 New sources, structure and properties of cellulose 9
Newman, R. H and Hemmingson, J. A, 8th International Symposium on Wood and Pulping Chemistry, 1995, vol. 1, p. 519.
EFFECTS OF PULPING ON CRYSTALLINITY OF CELLULOSE STUDIED BY SOLID STATE NMR T. Liitiii', S. L. Maunu' and B. Hortling'
I Laboratory of Polymer Chemistty, P.O. Box 55, FIN-00014 Universi& of Helsinki, Finland 2 The Finnish Pulp and Paper Research Institute, Paper Science Centre,P.O. Box 70, FIN02151 E S ~ Q O FinIand ,
Solid state NMR spectroscopy has been used in this work to investigate crystallinity of cellulose in spruce wood before and after kraft pulping. Effects of bleaching and refining in water and in weak alkali have also been studied. It has been noticed that the I, crystalline form of cellulose predominates over the Ip form in native spruce and vice versa in all the pulps studied. In pulping part of the cellulose 1, is converted to the more stable Ip form mainly by heat. Cellulose I1 is also formed during pulping and the content of form I1 seems to be higher on the fiber surface. Any measurable changes in the degree of crystallinity or in the relative proportions of different crystalline forms of cellulose could not be seen in pulps after refining or TCF-bleaching. However, it was noticed that the degree of crystallinity was considerably lower in the fines than in the bulk fiber.
INTRODUCTION Cellulose is a scmicrystalline biopolymcr with ordered crystalline and disordered amorphous regions. The crystalline cellulose may crystallize in several different polymorphs. Cellulose I is synthesized natively and exists in two crystalline forms, cellulose I, and Ip.' Cellulose I, is metastable and can be converted to the more stable form Ip by heat?. It is assumed that I, and Ip cellulose have different conformations and that they differ also in hydrogen bonding.'. The difference between cellulose I and I1 is that cellulose chains in cellulose I are thought to be parallel, whereas in cellulose I1 they are probably antiparalle1.d Cellulose I1 is formed when the lattice of cellulose I is destroyed. Cellulose I can be converted to cellulose I1 by alkali treatment called mercerisation or by regeneration of dissolved cellulose. NMR spectroscopy in solid state enables us to investigate wood and pulp fibres in their original state. Thus the physical structure of the samples rcmains unchanged and ordinary "C CPMAS measurements can bc uscd to determine the degree of crystallinity of cellulose. The relative proportions of I,, Ip and I1 crystalline forms of cellulose can be obtained by deconvolution after resolution enhancement of NMR data. Solid statc NMR spectroscopy has been used in this work to investigate the crystallinity of cellulose in spruce wood bcfore and after kraft pulping. Effccts of TCFbleaching and refining in water and in weak alkali have been studicd. We have also separated thc fincs after refining and compared the crystallinity of cellulose between the
'
'
40 New sources, structure and properties of cellulose fines and the bulk fiber to find out whether the crystallinity of cellulose on the fiber surface differs from the bulk fiber.
EXPERIMENTAL Materials
We have studied Wiley-milled (40 mesh) spruce wood (Picea abies) and conventional haft pulp made of the same spruce. The pulp has been refined with a Voith Sulzcr laboratory refiner in water in two different ways (Kraftl, Kraft2) and also in 0.01 M NaOH solution (Kraft3). The first fraction of the pulp has been refined in water so that the fibers are ground more on the surface. The second refining in water has more cutting effect on the fibers. Refining in weak alkali has been made like the refining of Kraftl in water. The refining processes have been explained in more detail elsewhere5and Kraftl, Kraft2 and Kraft3 were referred to as Sal, Sa2 and Sa3, respectively. After refining the fines (Finel, Fine2, Fine3) have been separated from the bulk fibers (Kraftl-F, Kraft2F, Kraft3-F). The effects of TCF-bleaching have been studied after oxygen (0)stage and after oxygen, peroxide, ozone and peroxide treatments (OPZP). Methods
All the ordinary I3C CPMAS NMR measurements have been done with a Varian m"YINOVA 300 NMR spectrometer operating at 75.5 MHz for carbons. The spinning speed was 5000 Hz,contact time 1 ms, acquisition time 20 ms and delay between pulses 2 s for all samples. The time of accumulation has been 20-26 hours for pulps and 2.5 days for spruce wood. The measurements have been long in order to get good signal-tonoise ratio, because the amount of noice increases during resolution enhancement. All samples have been moistened with de-ionised water (-50 w-% H,O), because it is known that water improves resolution and signal-to-noise-ratio in the spectra of cellulose.6 All the spectra have been referenced using the C1 signal as an internal reference (6 105 ppm). The degrce of crystallinity of cellulose (CrI) has been defined from the areas of the crystalline (86-92 ppm) and amorphous (79-86 ppm) C4 signals by deconvolution using Lorenzian line shape.' To be able to see the different crystalline forms of cellulose, the spectra must be resolution enhanced. The resolution enhancement has been done using negative line broadening (lb) together with Gaussian hnction (go. Values of those parameters have been selected experimentally. After the resolution enhancement the relative proportions of different polymorphs have been defined from the deconvolution of the crystalline C4 signal as represented in the figurc 1.b. The signals of the polymorphs arc ovcrlapping doublets and thus the most intense peak at 88.9 ppm is believed to arise from the contribution of all three polymorphs. The fractions of Lorcnzian or Gaussian line shape in deconvolutions have been chosen so that the sum of the areas of the peaks at 89.7 ppm, 88.1 ppm and 87.5 ppm is closest to the area of the peak at 88.9 ppm.
Crystallinity of cellulose 41
110
loo
90
80
70
60
50
PPm
Figure I . a) The deconvolution of KrafiOPZP spectrum for determination of the degree of crystallinity of the cellulose. b) The deconvolution of crystulline C4 signal afier resolution enhancement for the determination of relative proportions of different crystalline forms of cellulose.
RESULTS AND DISCUSSION The crystallinity index of cellulose in spruce wood could not be determined exactly, becausc there are many signals of lignin and hemicelluloses at the same range as the amorphous C4 transition used for determination of the CrI. The CrI values determined for the pulps are not any absolute values of crystallinity either due to solubilisation of unordered materials during kraft pulping* and because pulp contains also some lignin and hemicelluloses. Thcir cffcct on crystallinity index of pulps is anyway thought to be very small and in this work we have used CrI as an approximate value for comparison between pulps. Any measurable changes in the degree of crystallinity could not be seen in pulps after refining or TCF-bleaching. It was noticcd, however, that the degree of crystallinity was considerably lower in the fines than in the bulk fibcr. In all pulps studied the crystallinity iiidcx is approximately 50%, whereas in the fines it is 37-40%. The content of lignin was noticcd to be higher in fines than in the pulps. That may affect little on CrI, but does not explain totally the lower amount of the crystalline cellulose in fines. Mechanical forces may break crystallites during refining and thus lower the CrI in fines. It is also possible that degree of crystallinity is lower on the fiber surface than inside the fiber. Thc "C CPMAS measurements proved that the I, crystalline form of cellulose predominates over the Ipin native spruce. The amount of the cellulosc I, is almost three times the amount of cellulose Ipin spruce wood. In the kraft pulps studicd the amount of cellulose Ip is twice the amount of cellulosc I,. Ccllulose I, is known to be
42 New sources, structure and properties of cellulose metastable and it can be converted to the more stable Ip form by heat. Conversion of the 1, form to the I p form has been found to require 260-280 "C temperatures in highly crystalline algal and bacterial celluloses? Although the temperature is only 170 OC in haft pulping the NaOH media probably accelerates the transformation of cellulose I, into cellulose Ip?. Samples of lower crystallinity, like wood in this case, have also been suggested to convert more easily? According to the NMR spectra small amount of cellulose I1 was also formed during haft pulping. 100% 80% 60%
mCellulose Ib
40%
20% 0% Kraft Pulp
Spruce Wood
Figure 2. Relative proportions of polymofls of the crystalline cellulose in spruce wood and in b a j l pulp.
The relative proportions of the different cellulose polymorphs and proportions of amorphous cellulose in refined and TCF-blcached pulps are represented in figures 3 and 4. Only marginal changes in the amounts of the crystalline forms of cellulose are observed during refining or bleaching. The proportions of cellulose I,, Ip and I1 of all the cellulose in pulps are 11-16%, 28-32% and 5-7%, respectively. In fines the proportions of cellulose I, and Ip are 9-1 1% and 20-23%, respectively, because of the lower CrI. The ratio I& is anyway 0.4 0.5 like in all the other pulps. The relative proportion of cellulose I1 is slightly higher in Finel and Fine3 than in the other samples studied. The ratio 1/11 is 4 in Finel and Fine3, whercas in all the other pulps it is 6-9. In Kraft3-F 1/11 ratio is as large as 11. Finel and Fine3 probably represent the surface of the fiber more than Fine2, because the first and third refining should be more surface active? In the second refining the fibers are more cut. Besides the lower CrI and the higher cellulose I1 content, no significant differences between the bulk fiber and fiber surfaces could be seen.
-
x)O%
90% 00% 70%
60% 50% 40%
30% 20%
UCellulose II .Cellulose Ib pCellulose la
lo% 0%
Figure 3. Relative proportions of different celhlose polymorphs and amorphous cellulose (Am4 in the refinedpulps andfines.
Crystallinity of cellulose 43 .DO% 90%
80% 70%
60% 50% 40%
cellulose II mCellulose Ib
30% 20%
10% 0%
Figure 4. ReIative proporlions of different cellulose polymorphs and amorphous cellulose (Aml) in TCF-bleachedpubs.
CONCLUSIONS
Part of the ccllulose I, is converted to cellulose Ip during haft pulping. This is mainly caused by high tcmperature (170 "C), but the cooking liqour and the lower crystallinity of cellulose in wood probably accelerate the transformation, too. Cellulose I1 is formed also during pulping and the content of cellulose I1 seems to be higher on the fiber surface thhn inside the fiber. The crystallinity index of the surface material of the fiber was detected to be lower than in the bulk fiber. The refining or TCF-bleaching were not observed to cause any signinficant changes in the degree of crystallinity or in the relative proportions of different crystalline forms of cellulose. ACKNOWLEDGEMENT
We are indcbtcd to the Technology Development Ccntrc of Finland (TEKES) for financial support. REFERENCES
1 2 3 4 5
6 7 8
VandcrIIart, D. L. and Atalla, R. H. Macromolecules 17(1984) 1465. Debzi, E. M., Chanzy, H., Sugiyama, J., Tekely, P. and Excoffier, G. Macromolecules 24 (1991) 6816. Sugiyama, J., Pcrsson, J. and Chanzy, H. Macromolecules 24 (1991) 2461. Sjostrom, B. Wood Chemistry, Fundamentals and Applications, Academic Press, Inc. 1981, New York, p.53-55. Hortling, U., Jousimaa, T. ,Hyviirinen, 13.-K. and Holopainen K., Cellucon '98, In this volume. Willis, J. M. and Herring, F. G. Macromolecules 20 (1987) 1554. Tecaiir, I<,, Serimaa, R. and Paakkari, T. Polymer Bull. 17 (1 987) 23 1. I-Iattula,T. Pcperi j a Puu 68 (1986) 926.
44 New sources, structure and properties of cellulose 9
Newman, R. H and Hemmingson, J. A, 8th International Symposium on Wood and Pulping Chemistry, 1995, vol. 1, p. 519.
ON THE SPECIFIC MASS OF CELLULOSE AND THE CELLULOSE-WATERSYSTEM J.Chirkova, B.Andcrsons, 1.Andersone Latvian Stale Institute of Wood Chemistry, 27 Dzerbenes Str., LV-I 006 Riga, Latvia
ABSTRACT
The specific mass of cellulose depends on the medium properties in which it is located (in which specific mass determination is carried out). A correlation between the cellulose specific mass p and the solubility parameter component Sh (corresponding to hydrogen bonds) of the liquid in which p is located has been established. The dependence of p on 81,is minimum at ~5, 16 (J/m3>’.’, close to the Svalue of cellulose (1 5.6 (J/m3)“’). The highest p values have been obtained for saturated hydrocarbons (1.69 - 1.70 &m3) and the lowest for saturated alcohols (1.57 - 1.59 g/cm3). For cotton and wood cellulose, a common dependence of the p of moist cellulose on the relative content of water p within the p range of 2% to 70% has been obtained for 27 p values: p = 0.916 (k 0.015) - 0.198 (& 0.008) Inp. The extrapolated values of components p are equal to 0.92 dcm3 and 1.70 g/cm3 for water and cellulose, respectively.
-
INTRODUCTION
Specific mass (density) p is an important characteristic of a substance, conditioned by chemical composition and supramolecular structure. In contrast to rigid materials, whose true specific mass (without regard for pores) is constant, the specific mass of non-rigid materials to which cellulose is assigned depends on the nature of the medium in which the cellulose is located. The lowest experimental values of cellulose p are obtained in helium and saturated hydrocarbons (1.55 1.57 g/cm’) and the highest in water (1.60 1.62 g/cm3) [l]. It is commonly agreed that the maximum specific mass of cellulose can be calculated from X-ray measurements (1.63 - 1.64 g/cm3) [2]. In this case, however, it is not taken into account that, as a rule, X-ray patterns are obtained on “air dry” specimens (moisture content 6-8%), and water can deform the crystalline cell dramatically. This fact is confirmed experimentally, in particular, for galactomannan [3]: as the moisture content of fibres increases from 0% to 58%, the cell size a increases more than twice, the size b increases insufficy and the size c remains practically constant (a, b, c - the parameters of the crystalline lattice), although water does not penetrate into the crystallites. However, the obtaining of “oven dry” cellulose is problematic as, owing to the non-rigidity of the structurc, the removal of water from cellulose is accompanied by the collapse of the structure, which includes in it water. Cellulose dewatering by the organic solvent exchange method provides maximum removal of water [4]. However, in this case, the structure and specific mass of the cellulose, in comparison with the initial state, are obviously changed. The processes of ccllulose obtaining and treatment proceed mostly in aqueous media. Moreover, water accompanies cellulose biosynthesis during the plants growth. Therefore it is important to know the regularities of the change of the cellulose-water
-
46 New sources, structure and properties of cellulose system properties depending on the system composition. Such data are also of scientific interest for comprehending the peculiarities and interaction in the cellulose-water system, In connection with the aforementioned, the aim of the work was: 1. To determine the equilibrium values of the cellulose specific mass in liquid media as well as the liquid parameters which determine the cellulose specific mass; 2. To establish the regularities of the change of moist cellulose specific mass as the moisture content changes and, based on the regularity obtained, to estimate the specific mass of the components in the system. EXPERIMENTAL
Materials Cellulose: Cotton cellulose supplied by “Baccay”; sulphate coniferous cellulose supplied by “Tireccll” ( a-cellulose content 98%). Characteristics of solvents are presented in Table 1.
Methods
Determination of the equilibrium specific mass of cellulose A cellulose specimen dried at 103fl”C was suspended on an elastic thread in a hermetically closed retort and vacuum treated at a residual pressure of 100 Pa and, under vacuum, filled in the form of a degassed liquid as a stop-watch was switched on. The vessel with the liquid and the specimen was carried to a balance (accuracy 0.0001 g), and the specimen mass prior to reaching the equilibrium mass was determined over time. The cellulose specific mass peer was calculated based on Archimedes’ law:
-V
mL = mnir
PL= mnir ( 1
- pJ pcci),
where m.i and mL = the mass of the specimen in air and liquid, respectively;p~ and peer = the specific mass of liquid and cellulose, respectively; V = specimen volume.
Determination of the specific mass of moist cellulose Cellulose specimens with different moisture content Q, were obtained by placing the specimens in dessicators with a definite relative humidity of air (above saturated solutions of salts, according to reference data) as well as in liquid water with subsequent removal of interfibrillarwater by pressing with filter paper. The moist cellulose specimen (approx. 0.1 g ) was immersed into methanol (approx. 100 ml) and placed on the balance as a stop-watch was switched on. Changes of the specimen mass in liquid were periodically measured. The peculiarity of the choice of methanol is that it moistens cellulose practically instantly, not penetrating, at the first stage, into the pores filled with water, which had been confirmed in [4]. The initial linear area of the kinetic curve in the coordinates mL = f (7“’5) was analytically extrapolated to p5=0, and the initial mLfo)value of the moist cellulose was obtained. Then the specific mass of moist cellulose was obtained from:
-
pm,i = (mair.pL)/(nt,i mLo)).
Specific mass of cellulose 47 The initial moisture content of cellulose p = (mair- mo.d.)/mo.d.,
-
where nzo.d. mass of ”oven dry” specimen, that was obtained after the equilibrium reaching and drying of the specimen at the temperature 103°C. RESULTS AND DISCUSSION Determination of equilibrium specific mass
These measurements were done only on the wood cellulose. The rate of achieving the equilibrium mass of cellulose in liquid was characterised by the value I/ z 0.5 (z 0.5 time of achieving half the total mass change) (see Table 1). Examples of kinetic curves are shown in Fig. 1. The rate of achieving equilibrium in the liquids under study differ by a factor of lo3.Thus, in methanol, equilibrium is achieved in 2-3 min while in saturated hydrocarbons in 2-3 days.
-
1.7
1.75
n
“E0 4
m
a
/
1.‘65
-
I.7 n -
E
.$ 1.65
1.6
. **
a.
Q
A
.
m
v
Y
.
1.6
1.55
**
.*
.
1.55
1.5
z
10
0
100
0
20
6 h(J/m
(h)
30
3 0.5
Figure 1 (left). Kinetics of establishing cellulose equilibrium specific mass in benzene (a), n-hexane (A)and dioxane (+) Figure 2 (right). Cellulose equilibrium specific mass on liquid solubility parameter component 6,( 0 - formic acid; A acetic acid; - DMSO; - others liquids, see the Table)
-
+
Due to the variety of liquids under study, it is possible to elucidate the factors determining their interaction with cellulose. An experimental dependence of some physico-chemical properties of polymers on the liquid solubility parameter 6 had been
48 New sources, structure and properties of cellulose observed earlier for numerous polymer-solvent systems [5]. In this case, the location of the maximum (minimum) coincided with the polymer solubility parameter. In our experiments, such dependence is absent. However, it is known that the solubility parameter is a geometrical sum of the three components - dispersion (Sd), polarisation (8,)and that corresponding to the contribution of hydrogen bonds (&) and S= (& + S , + 4J0.’[6]. The values of Sand components are also seen in Table 1. Table 1. Characteristics of liquid media and cellulose-liquid systems
Solubility parameter
p of cellulose
(i/~~.~ 1 o4 )~~’.
Liquid Water Methanol Ethanol 1 -propano1 1-butanol 1 -pentanol n-pentane n-hexane Benzene Acetone Dioxan ethylene glycol Dimethylsiloxane formic acid acetic acid
6
6h
46.5 29.1 26.4 24.4 23.0 21.6 14.3 14.8 18.7 19.9 20.4 33.2 26.4 24.7 21.4
34.1 22.2 19.4 17.3 15.7 13.9 0 0 2.1 6.9 7.3 25.9 10.2 16.5 13.5
initial 1.560 1.544 1.541 1.558 1.527 1.543 1 S74 1.540 1.552 1.541 1.566 1.505 1.540 1.684 1.543
equilibrium
1.629 1.593 1.570 1.573 1.577 1.597 1.675 1.656 1.623 1.626 1.626 1.597 1.679 1.715 1.647
0.08
10.25 2.61 0.96 0.28
0.06 0.04 0.91 2.40 1.18
It has been shown that there is a correlation between the equilibrium density of cellulose and the component SA of the solubility parameter (see Fig. 2). Only liquids chemically interacting with cellulose (DMSO, formic and acetic acids) do not submit to this regularity. It is remarkable that the location of the curve minimum shown in Fig. 2 is close to the cellulose solubility parameter (15.6 (J/m3)”.’) [7]. The observed dependence confirms that hydrogen bonds are mainly responsible for the interaction of cellulose with other partncrs. From this point of view, it is possible to analyse the specific mass measurement results. Saturated hydrocarbons (GI= 0), inert with respect to cellulose, do not deform the cellulose crystalline lattice; therefore, the cellulose specific mass in these media is the highest. As the liquid SlI increases, the intermolecular hydrogen bonds in cellulose tend to weaken and partly break, and the specific mass tends to decrease, reaching minimum values in saturated alcohols. With further increasing of Sll, the cellulose structure loosens to such an extent that it becomes accessible to liquid molecules, and the increase in p is observed. Hence, the cellulose specific mass is a function of medium properties, as it had been established earlier in respect of the sorption properties of cellulose [ 8 ] . In this case, this
Specific mass of cellulose 49 dependence is connected not with the original ultraporosity of cellulose, but with the formation of a new loose structure, whose specific mass is conditioned by the thermodynamic properties of the liquid.
Determination of the specijk mass of the cellulose-water syslem Dependence of the specific mass of moist cellulose on relative water content cp for both the celluloses under study, i.e. cotton and wood, is common and is described by the expression (sce Fig. 3,4): p = 0.916 (k0.015)- 0.198 (k0.008) Incp
... ,.. .... (equation 1)
(correlation rate 0.9806).
1.8 I
1.6
--
n
.
1.4
m
E
E
0
'AA
0
\
+
\
rn
m W
Y
0
Q
Q.
1.2
A
A
.A
..
:
.
1
A
A
+ 0.8
0.8
0
0.5
content of water Figure 3 (left).
1
-4
-2
0
In (content of water)
Specific mass of cellulose - water system(+ - cotton cellulose, A wood cellulose) and cellulose (0)on relative water content
-
Figure 4 (right). Specific mass of cellulose-water system on relative water content in semilogarithmic coordinates (+ - cotton cellulose, A -wood cellulose) It is remarkable that, in the observed dependence in the cp value over the range from 2 % to 70%, characteristic points testifjling the change of the water state in the system as the cp value increases are absent. This conclusion does not agree with the widespread
50
New sources, structure and propertics of cellulose
concept of two states of water in moist cellulose (“bound” and “non-bound”, “freezing” and “non-freezing” water) [9]. At the same time, analysing the results of the interaction of hydrophilic polymer (elastin, carotene, methylcellulose) - water systems by NMR, differential scanning calorimetry and heat capacity, Hoeve [lo] concluded that water , in a thermodynamic sense, is always present in the form of one phase in these systems. However, water mobility is limited as a result of the impossibility of realising its volume properties in the fine pores of the swollen polymer, and not the strong bonding with the polymer. From equation 1 it follows that the water specific mass in the cellulose-water system is 0.916 (f0.015) &m3, i.e. disordering of water structure occurs owing to the involving of a part of water molecules in the formation of hydrogen bonds with cellulose. For the other component, “oven dry”(at p = 0) cellulose, it is not possible to obtain the p value analytically. To evaluate the dry cellulose specific mass, the cellulose-water system as a composite was considered from these measurements. The specific mass of the system is plllir,then the components sum volume is:
-
+ pmx i (1 p )lpc,l = 1 where plllix, peel and p,” specific mass of cellulose-water system, cellulose and water, respectively. Then peel = (1 - p) / (l/puiix - p/p,J. Taking into account the continuous character of the dependence shown in Fig. 3, let us assume that the water specific mass in the system does not depend on its content. Then it is possible to calculate peel at different p values (see the symbols (0)in Fig. 3). At low p values, the peel value exceeds 1.7 g/cm3, decreasing exponentially as p increases and passing practically on the plateau at p 0.7 (peel G 1.200 g/cm3)).The value p = 0.7 is the moisture capacity of cellulose, and hrther increase in p results in the appearance of interfibrillar free water, not entering the cellulose-water system. plllirqipw
-
-
CONCLUSIONS
In the present work, to determine cellulose specific mass, a kinetic version of the picnometric method was used, which made it possible to obtain equilibrium and, consequently, comparable values of specific mass in different liquids. A correlation between the value of cellulose specific mass and the liquid solubility parameter component 4,, corresponding to hydrogen bonds, was established for 15 liquids. This result was another confirmation of the principal role of hydrogen bonds in the interaction of cellulose with other partners. Picnometric measurements of moist cellulose in methanol enabled determination of the specific mass of the cellulose-water system. As a result, specific mass values of two celluloses, cotton and wood at 27 values of relative moisture content were obtained, and a continuous dependence of specific mass on water content in the system was found. A lowered specific mass of water in contact with cellulose and a strong dependence of the specific mass of the cellulose itself on moisture contents was observed, namely, specific mass was equal 1.7 gkm3 and 1.2 g/cm3 at the moisture content 2% and 70%, respectively. Hence, cellulose specific mass as well as its other physico-chemical properties, in particular, sorption properties are a hnction of the properties of the media in which cellulose is located.
Specific mass of cellulose 5 1 Since the extrapolation was approximate, the problem of the maximum possible specific mass of cellulose remains vague. As has been mentioned in the Introduction, X-ray experiments on “air dry” cellulose deal with a deformed (stretched) crystalline lattice. As regards the X-ray determination of the specific mass of “oven dry” cellulose, the fact of decreasing the crystallinity degree as moisture content decreased was established for other hydrophilic polymers (proteins) [ 1 I]. REFERENCES 1 . N. I. Klenkova, Structure and Reactivity of Cellulose, Khimija, Leningrad,l967 (in Russian). 2. C. Woodcock & A Sarko, Packing analysis of carbohydrates and polysacharides. 1 1 .Molecular and crystal structure of native ramie cellulose, Macromolecules, 1980, 13, 1183-1 187. 3. R. H. Marchessault, A. Bullcon, J. Deslandes & T. Coto, Comparison of X-ray diffraction data of galactomannans, J Colloid and Interface Sci, 1979, 71, 375-382. 4. J. A. Chirkova, R. E. Reizinsh & G. E. Cakare, Dewatering of cellulose by organic solvents, Khimija drevesiny, 1986, 3, 12-18 (in Russian). 5. A. A. Tager & L. K. Kolmakova; Solubility parameter, its estimation methods and connection with a solubility of polymers, Vysokomolecular. Soed., 1980, ,422, 483496 (in Russian). 6. C. M. Hansen, The universality of the solubility parameter, Industrial and Engineering Chemistry, Product Research and Development, 1969, 8, 2-1 1 . 7. Encyclopedia of Polymers, Moscow, 1972, 1, 1024. 8. J. A. Chirkova, On mechanism of vapour sorption by cellulose,Vysokomolecular. soed.,1989, A31, 1528-1533 (in Russian). 9. S. Deodhar & Ph. Luner, Measurement of bound (non-freezing) water by differential scanning calorimetry, In: Water in Polymers (Stanley P.Rowland, eds.), Washington, 1980,273-287. 10. C. A. J. Hoeve, Structure of water in polymers, In: Water in Polymers (Stanley P.Rowland, eds.), Washington, 1980, 137-149. 1 1 . T. Bluhm, Y. Deslandes, R. H. Marchessault & P. R. Sundararajan, “New understanding of the hydratation of the polysaccharide crystalline structure”, In: Water in Polymers (Stanley P.Rowland, eds.), Washington, 1980, 255-273.
APPLICATION OF SIZE-EXCLUSION CHROMATOGRAPHY TO ENZYMATIC BLEACHING OF WOOD PULP T. Eremeevn", M. Leite, T. Bykovr, A. Treimrnis and U. Viesturs Latvian State Institute of Wood Chemistry, Cellulose Laboratov, 27 Dzerbenes Str., Riga LV-1006, Latvia, Fax: +371 7310135, E-mail: koks(ijetli.lv
ABSTRACT
Size-exclusion chromatography (SEC) of wood pulp constituents (cellulose, hemicellulose and lignin) has become widely used for analysis of these polymers, including transformations occurring under enzymatic processing. Possibilities of the SEC method in the analyses of underivatized wood pulp during enzyme aided bleaching processes with xylanase are presented in this paper. The methodology includes the prefractionation of wood pulp in NaOH (2-18%) and cadoxen, followed by SEC analysis, using aqueous Na0I-I as the eluent. Simultaneous determination of molecular weight distribution (MWD)and the chemical heterogeneity of each fraction is achieved using a double detection system, i.e. a refractometer and a UV detector in the range 254-405 nm. The suitability of the present methodology approach is illustrated by analysis of hardwood and softwood kraft pulps during enzymatic bleaching by xylanase.
INTRODUCTION An increasing interest in biological bleaching suggests that it is vital to gain a better insight into kndamental transformations resulting in delignification during bioblcaching. The limited availability of an adequate analytical technique for the invcstigation of wood pulp presents a significant drawback for the study of the biological bleaching mechanism. In the last few years size-exclusion chromatography (SEC) proved to be an extremely versatile and suitable technique for use in biobleaching process In our earlier the SEC method had been applied to the enzymatic bleaching of wood pulps using both aqueous and organic solvents. The approach used in the present and earlier studies carried out in our laboratory involved prefractionation of wood pulps, followed by direct SEC analysis of the extracted fractions. Variations in the procedure applied depend on the problem to be resolved. In the present paper, two examples of studies on simultaneous quantitative, MWD and chromophores transformations in pulps under biochemical bleaching sequence are described. Thefirst procedure was the monitoring of changes in alkaline extracts (2% followed by 18% NaOH) of pulp, using a fast analytical CGC (cartridge glass column) Separon HEMA Bio 1000 column and 2.5 pM NaOH as the eluent (analysis time 3 min). 0 m e second procedirrre involved the fractional dissolution of pulp in alkali (10% NaOH), then in cadoxen, using for SEC analysis a standard Separon HEMA 1000 column and 1.5 M NaOH as eluent (analysis time 20 min).
56
Application of enzymes to pulp, fibres and cellulose
EXPERIMENTAL Sample preparation. The pulp alkaline extraction procedure has been described previouslys. The pulp residue after extraction was washed twice with alkali solution and decanted. Then cadoxen was added to make a pulp concentration in solution of approx. 1%. The pulp dissolution took about 20 h at 0°C. For SEC analysis the pulp cadoxen solution was diluted with 6% NaOH to obtain an approx. 0.2-0.4% solution. Prior to the analysis, the solution was centrikged to remove the gel fraction. SEC analysis. The analysis was carried out on a liquid chromatograph GPC (Laboratory Instruments, Prague, Czech Republic) with a refractive index RI detector in line with an UV detector equipped with a Rheodyne 7125 fixed-loop (100 or 20 jd) injector. A prepacked Separon HEMA 1000 (10 pm) stainless steel column (250 x 8 mm 1.D) and a compact glass cartridge CGC analytical Separon HEMA BIO 1000 (1Opm) column (150 x 3 mm I.D.) (TESSEK Ltd., Prague, Czech Republic) were used. Alkaline extracts and pulps dissolved in cadoxen were analysed on the stainless steel column using 1.5 M NaOH as the eluent. The alkaline extracts were analysed with the CGC column at a flow rate of 0.3 mumin and injcction volume 5-10 p1 using 2.5 mM NaOH as eluent. The analyses were carried out at room temperature. For molecular mass (MM) determination, the characterised carboxymethyl cellulose and hemicelluloses fractions were used as standards. Quantitative analysis. To quantify the lignin and hemicelluloses solubility, the corresponding calibration graphs of the chromatographic peak area vs. concentration in solution were utilised. The amounts of the lignin in the extracts were determined from UV chromatograms at a wavelength of 290 nm using birch haft and Bjorkman lignin as well as kraft pine lignin as calibrants. Hemicellulose concentrations in the extracts were determined by RI-chromatograms using birch xylan and pine mannan as calibrants. Bleaching. The grey and black alder pulps obtained by extended krafi cooking were bleachcd using two bleaching sequences: XEQPIPZ (X - xylanase treatment, E - alkali extraction, Q - chelating agent stage, PI - first peroxide stage, PZ- second peroxide stage) and DXEQPIPZ (D - chlorine dioxide stage, X - xylanase treatment, E - alkali extraction, Q - chelating agent stage, PI - first peroxide stage, PZ- second peroxide stage). The pine pulps obtained by conventional cooking were bleached according to the XIEX2PJ?zD scheme (XI - first xylanase treatment, E - alkali extraction, XZ- second xylanase treatment, PI, PZ-two subsequent peroxide stages, D - chlorine dioxide stage). A commercial xylanase, Pulpzyme HC (Novo Nordisk NS, Denmark), was used in the enzyme stage. The biobleaching conditions were described in detail
-
RESULTS & DISCUSSION Monitoring of softwood kraft pulp bioblcaching
Two species of unbleached pine krafl pulp with different lignin contents were chosen in expectation of marked distinctions in bleaching results. The first sample, pulp with the yield 40.7%, Klason lignin contcnt 2.6%, and the second sample, pulp with the yield 45.5%, Klason lignin content 3.1% were used in this study. The two samples were bleached according to the XlEX2PlPzD scheme giving quite different final brightness (71.5 and 69.2% IS0 for samples 11, and I respectively). It should be pointed out that
57
Application of size-exclusion chromatography
-
both samples had a similar residual lignin content (measured as Sjostrom lignin) 0.59 and 0.60, respectively. In this part of our study, to gain a better understanding of the mechanism of enzyme attack of hemicelluloses and lignin and/or the lignin-carbohydrate complex (LCC), the accessible part of pulp at each stage of bleaching sequences was characterised more filly by SEC. Pulp fractions were obtained by 2% NaOH (SZ), followed by 18% NaOH (S18) fractional dissolution. These fractions are known to be enriched in hemicelluloses and lignin, SZ being more enriched in lignin destruction products (phenolics) and S ~ in S hemicelluloses. In this part of the work special attention was paid to the chromophore groups behaviour during enzymatic processing. The absorbances of 2% and 18% - NaOHsoluble fractions were measured at four wavelengths: 290, 3 13 (aromatic carbonyls), 365 (carbonyls conjugated with aromatic rings) and 405 (Quinones) nm. Measurements were made after each stage in the sequence to determine where changes occurred. The results revealed essential differences between two samples under investigation. Changes in absorbances at all the wavelengths and differences between enzymatic and control runs were more pronounced in the second sample with a higher lignin content. The absorbances of the Sls fraction were merely affected by the bleaching process, whereas those of the SZ fraction decreased to a much greater extent. An increase in absorbances at the XI and XlEXz stages under the action of xylanase was observed. This increase was more pronounced for the second sample; it was especially typical for 3 13 nm of S ~ fractions. S As the behaviour of the two samples was compared, it was evident that the response to xylanase action was rather different. Changes in the quantity of soluble hemicelluloses were also different for these two samples (Table 1). A more extensive removal of soluble hemicelluloses at the initial stages of the process and a higher increase at the XIEX2 stage were observed for the first sample, whereas the removal of soluble hemicelluloses during the peroxide stages PIPZwas more extensive
-
Table 1. Effect of TCF bleaching upon pine kraft pulp's hemicelluloses and aromatic solubility in 2% then 18% NaOH and chromophores changes
Stage
Hemicelluloses S18,
Initial
x1 XIE XiEXzPi XiEXzPi
%
Aromatics IS0 sz Sl8 content brightS2, S18, ness, 3131 3651 4051 3131 3651 Yo Yo % 290 290 290 290 290 Sample I, y=40.7%, L=Z.~'YO
7.60 8.90 8.30 12.27 12.43
0.58 0.38 0.30 0.37 0.51
1.22 0.97 0.77
9.86 7.28 9.03 10.28 11.29
1.23 1.00 0.93 0.48 0.45
1.55 1.26 1.21 1.04 0.98
1.05 0.78
PzD
33.6
-
36.8 48.4 71.5
1.34 1.21 1.09 0.98 0.81
0.64 0.44 0.50 0.46 0.43
0.28 0.27 0.23 0.20 0.18
4051 290
0.93 1.02 0.90 1.01 0.80
0.53 0.44 0.42 0.43 0.27
0.24 0.24 0.23 0.22 0.14
0.86 0.99 0.84 0.96 0.81
0.43 0.52 0.44 0.45 0.40
0.20 0.21 0.19 0.20 0.18
Sample 11, y=45.5%, k 3 . 1 % Initial XI X1E XiEXd'i XiEXzPi
P,D
33.8 38.3 48.5 69.2
1.06 1.02 0.83 0.90 0.79
0.61 0.54 0.48 0.47 0.69
0.29 0.30 0.29 0.26 0.37
58
Application of enzymes to pulp, fibres and cellulose
for the second sample. In addition to the direct information on the reduction of lignin content and the change of soluble hemicelluloses content during the biobleaching process, the results enable making some suggestions regarding the mechanisms involved. It seems that xylanase, attacking inaccessible hemicelluloses and/or LCC, split them in the two parts: the SISsoluble fraction with a higher MM and the S2 soluble one with a lower MM. In this case, the contents of the chromophoric groups attached to the mass unit in the two fractions are also quite different. The ratio of the two fractions and the residual resistant LCC is supposed to have a distinct influence on the effectiveness of hrther bleaching sequences. SEC study of xylanase bleached hardwood kraft pulps
In the second part of the present paper, two enzymatically bleached hardwood krafl pulps, obtained by extended cooking of black and grey alder wood, are compared. The pulps were bleached using the TCF and ECF bleaching sequences, i.e. by the XEQPIPZ scheme and the DXEQPlP2 one (identification of the sequences is made in the experimental section). The brightness as well as residual lignin and pentosan contents of pulps after different bleaching sequences are shown in Table 2. Marked distinctions in final brightness for grey and black alder pulps were observed. The response on xylanase treatment was higher for grey alder pulp, whereas that for black alder pulp was rather low. Despite a similar residual lignin content and a higher pentosan content, grcy alder pulps, as compared to black alder pulps, had a higher final brightness, in the average by 4-5% I S 0 under TCF sequence. To find the reason for this discrepancy and elucidate the xylanase bleaching effect, alder pulps were investigated by SEC. Pulps were dissolved in 10% NaOH, followed by residues dissolution in cadoxen, hence two fractions were obtained. Analysis was carried out operating the UV detector at two wavelengths: 290 nm (lignin absorbance) and 254 nm (aromatic andor hexenuronic acids). The results presented in Table 2 show that grey alder pulps had a higher solubility in 10% NaOH and a higher difference between enzyme and control results, as compared to black alder pulps. The SEC analysis of residues, dissolved in cadoxen, revealed that black alder pulps Table 2. Changes in properties of fraction soluble in 10% NaOH from kraft grey and black alder pulps under biobleaching sequences I - DXEQPD and I1 - XEQPP
Samples Grey alder I control I Grey alder I1 Control I1 Black alder I Control I Black alder 11 Control 11
ISO, Pentobright- sans, % new, YO 92.3 85.5 91.2 88.8 87.2 86.4 85.9 86.8
19.7 20.0 17.3 18.3 16.8 17.2 15.0 15.5
,310,
%
7.44 9.22 8.40 10.71 5.80 7.22 6.62 6.65
UV29o/RI
Mww
Mn
MwIMn
0.80 1.03 0.85 0.98 0.70 1.02 1.22 1.25
14300 12330 15530 13 400 12090 13020 14830 13 160
9 190 9040 9350 8930 8700 8000 9990 8490
1.55 1.36 1.74 1.43 1.51 1.49 1.48 1.55
Application of size-exclusion chromatography
4
Figure 1.
4.8
5,6
6,4
7,2
8
8,B
59
Ve ml
Effect of TCF bleaching sequence on black alder pulp fractional composition.
had a higher absorbance at 254 nm as compared to the grey alder pulps. As can be seen from Fig. 1, four pcaks were presented on the chromatogram obtained at 254 nm. The relative contribution of every peak, as shown in Table 3, was different for black and grey alder pulps as well as for enzyme and control samples. The content of this first peak in the black alder pulps bleached by the DXEQPlP2 scheme was almost twice as high as that for grcy aldcr pulps. Howevcr, only a small reduction was observed in the enzyme mn as compared to the control. It should be noted that no absorbance at 290 nm was observed in the fractions dissolved in cadoxen, except for a negligible increase of the baseline in the low molecular region. Hence, the absorption at 254 nm was not connected with residual lignin, therefore, the reason for this absorption can be attributed to residual hemicelluloses. It is possible to assume that this first peak contains the most resistant portion of residual hemicelluloses, which were less accessible and had a higher resistance towards the Table 3. Effect on biobleaching upon the composition and molccular weight of cadoxen-
soluble fractions Samples Grey alder I Control I Grey alder II Control 11 Black alder I Control I Black alder 11 Control I1
1,
U,
YO
%
111, Yn
M,"
M.ll
M,JM."
8.4 8.7 8.0 11.3 17.0 17.3 10.7 11.0
51.4 61.5 58.5 61.9 48.2 54.5 53.1 54.5
39.9 30.4 29.8 30.1 34.2 28.2 36.2 34.5
19 480 18 630 18 720 18 680 16 580 19 710 21 080 18 850
1 1 180 10 340 10 210 9 790 8 540 10 000 10 980 10 300
1.74 1.80 1.83 1.91 1.94 1.97 1.92 1.83
Lignin, Y O
0.3 1 0.27 0.35
0.50
60 Application of enzymes to pulp, fibres and cellulose bleaching enzymes, as well as chemical reagents. The MWD of the carbohydrate fraction, which can be extracted by 10% NaOH, is presented in Table 2, and that of the cadoxen-solublehemicelluloses in Table 3. In general, the hemicelluloses in both fractions had a slightly higher & and M,, after enzymatic treatment as compared with control samples. The M, and Mn of alkaliaccessible hemicelluloses ( S ~ Ofraction) were found to be lower than those af alkaliresistant ones (cadoxen-soluble fraction), having DP, 85h5 and 12525 and DP. 55*5 and 75*5, respectively. At the same time, a significant difference in the content of these fractions was determined between enzymatic and control samples. Therefore, it is reasonable to suggest that the ratios of these fractions play an important role and afTect the results of the enzymatic treatment of wood pulps.
CONCLUSIONS The results obtained show that the SEC with UV detection at different wavelengths provides important information concerning the biobleaching process of wood pulps. In addition to direct information on lignin and hemicellulose contents in wood pulps, the results make it possible to draw some conclusions regarding the mechanisms involved.
ACKNOWLEDGEMENTS The authors express their gratitude to Novo Nordisk A / S for providing samples of the enzyme preparation Pulpzyme HC. This research was financed from the Latvian budget, grant No. 578 (Biotechnology)and grant No. 595 (Wood chemistry).
REFERENCES 1. B. Saake, T. Clark and I. Puls, Investigations on the reaction mechanisms of xylanases and mannanases spruce wood chemical pulps, Holzforschiwg, 1995, 49, 60-68. 2. K. K. I. Wong, S. Yokota, J. Saddler and De Jong, Enzyme hydrolysis of lignin carbohydrate complexes isolated from kraft pulp, J. Wood Chern. Technol., 1996, 16 (2), 121-138. 3. M. Leite, T. Eremeeva, A. Treimanis, V. Egle, T. Bykova, L. Purina, U. Viesturs, The effects of xylanase type enzymes on the different layers of birch wood ORGANOSOLV pulp fibre walls, Actu Biotechnol., 1995, 15 (2), 199-210. 4. M. Leite, V. Klevinska, M. Eisimonte, T. Eremeeva, U. Viesturs, G. Telysheva, Extended kraft cooking and biological bleaching of black and grey alder chips, In: Advances in Lignocellulosics Chemistry for Ecologically Friendly Pulping and Bleaching Technologies, Aveiro, Portugal, 1998, pp. 3 13-316. 5 . T. E. Eremeeva & ,T. 0. Bykova, HPSEC of wood hemicelluloses on a poly (2hydroxyethyl methacrylate-co-ethylene dimethacrylate) column with sodium hydroxide solution as eluent, J. Chromatogr., 1993,639, 159-164. 6. M. Leite, T. Eremeeva, M. Eisimonte, A. Treimanis, V. Klevinska, U. Viesturs, Biobleaching - effect of pulp history on the action of the enzymes, In: Advances in Characterisation and Processing of Wood, Non-Wood and Secondary Fibres, Stresa, Italy, 1996, pp. 465-469.
THERMOSTABLE XYLANASES AND THEIR POTENTIAL APPLICATION IN PAPER AND PULP INDUSTRIES M K I%hat'*,S Knlogiannis', N A Bennett', P Biely2, D E Beever3& E Owen3 'Instititle of Food Research. Nonvich Research Park, Colney, Nonvich, NR4 7UA. UK. 2/nsliluteof Chetnistry, Slovak Academy of Sciences, Dubravska cesta. SK-84238, BrAliSlAVA, Slovakia. 'Department of Agriculture, University of Readin& Earley Gate, PO Box 236, Reading, RG6 6AT, UK.
INTRODUCTION Xylan, one of the major hemicelluloses, is prescnt in closc association with cellulose and lignin in plant cell wall (1). Unlike cellulose, xylan is a heteropolysaccharide with a main-chain containing 1,4 linked P-D-xylose rcsidues to which side-groups such as a-Larabinofuranose, 4-0-mcthylglucuronic acid, phenyl and acetyl groups are attached (2). Xylan accounts for up to 30% of the cell wall material of annual plants, 15 30% of hardwoods and 7 - 10% of soft woods (3). Thus, it is the second most abundant and rcnewable energy resource in nature. Neverthcless, the completc hydrolysis of xylan to monosaccharides is a complex process and requires both main- and side-chain cleaving enzymes namely endo-xylanase, P-D-xylosidase, a-L-arabinofuranosidase, a-Dglucuronidase, acetyl and phenyl estcrases (2,4). It has bcen reported that the xylan, which is closcly associated with cellulose and lignin, appcars to physically restrict the removal of high-molccular-mass lignin from the paper pulp (5). Thus, high concentrations of chlorine and other harmful chemicals, which cause serious environmental problems are necessary for bleaching of paper pulp. Use of xylanasc during pre-bleaching of kraft and chemical pulps not only reduces the amount of chlorine required, but also facilitates the easy extraction of lignin and incrcascs the brightness of the final product (6-8). It has been proposed that the action of xylanasc facilitates either the partial hydrolysis of re-precipitated xylan or the removal of xylan from the lignin-carbohydrate complexes (1,9). Both these processes are expected to enhance the removal of entrapped lignin from the fibre cell wall. Although, the conditions uscd during pulping and bleaching are too harsh for simultaneous applications of xylanase, it has been realised that xylanasc free of cellulase active and stable at alkaline pH, and temperatures above 6OoC, could be ideal for pre-bleaching of paper pulp. Recently, we have purified and characterised two thermostable xylanases from thermophilic fungi, Tliermoascus aurantiacus and Tliermomyces lanuginosus (10-13). Intcrestingly, T. lanuginosus produces a thermostable ccllulase-free xylanase, while T. aurantiacus produces both cellulases and hemicellulases. However, the purified xylanases from T.auruntiacus and T.lunuginosus belong to glycosylhydrolase families 10 and 11 respectivcly, and show considerable differences with respcct to their biochcmical and catalytic propcrties. Furthcrmore, the stability, broad pH and tempcrature optima, together with the differcnce in substrate specificity lead to speculation that these xylanases may have potential application in the paper and pulp industry.
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62 Application of enzymes to pulp, fibres and cellulose MATERIALS & METHODS Materials T. aurantiacus IMI 216529 was obtained from the International Mycological Institute, Surrey, UK, whereas T. lanuginosus ATCC 46882 was purchascd from American Type Culture Collection, Maryland, USA. The crude hemicellulase and xylanase rich fractions of T. aurantiacus and T. lanuginosus were kindly supplied by Prof.Tiraby, Cayla, France and Prof.Macris, NTUA, Athens, Greece. All chcmicals and reagents used were analytical grade and purchased cither from Sigma or BDH. Methods Enzyme and protein assays
The xylanase activity was measurcd using birchwood xylan as described (14). Generally the protein was determined by the bicinchoninic acid method (15). while the absorbance at 280 nm was used to monitor the protein in column elucnts. Production and purifcation of extracellular xylanase from T. aurantiacus
For the production of cellulasc and hemiccllulase, T. aurantiacus was grown in Mandels medium (16) containing paper paste (20g I-') and wheat bran (20 g I-') as carbon sources at 50°C for 6 days. The culture was harvested by filtration, concentrated by acetone precipitation and fractionated as cellulasc and hemicellulase rich fractions by ultrafiltration. The hemicellulase rich fraction was subsequently desalted on a Bio-Gel P6DG column and fractionated using a Q-Sepharose fast flow column (20~3.4cm) at pH 8.7. The major xylanase was eluted in the buffer wash. Production and purifcation of extracellular xylanase from T. lanuginosus
For the production of xylanase, T. lanuginosus was grown in a mineral medium containing corn cob (20 g I-') as a carbon source at 50°C for 6 days (13). The culture filtrate containing xylanase activity was spray dried and desalted on a Bio-Gel P6DG column. The xylanasc was purified by ion-exchange chromatography on DEAESepharose fast flow (2.8x2.6 cm) column, followed by gel filtration on Bio-Gel P30 (100~1.6cm) column. Biochemical characterisation: I . Determination of molecular mass and p l
These were determined as described (13.17.18). Determination of stability, p l i and temperature optima
The pH and temperature optima of xylanasc from T. aurantiacus and T. lanuginosus werc determined by measuring the activity in the pH region 2.2 - 10.2 and in the tcmpcrature range 50 - 90°C. The stability of xylanasc from T. aurantiacus was dctermined at pH 3.0 - 9.8 and tcmperatures 50 8OoCfor six days, whereas the stability of xylanase from T. lanuginosus was studied between pH 4.0 - 10.0 and temperatures between 40 - 80°C.
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Thermostable xylanases 63 Determination of substrate specificity This was studied by measuring the activity of xylanase from T. aurantiacus and T. lanuginosus towards various natural polysaccharides as well as 4-nitrophcnyl glycosides. The reducing sugars releascd were either quantified by Nelson/Somogyi method (19) or detected using aniline hydrogen phthalate rcagcnt aftcr separation by TLC. 4-Nitrophenol released from 4-nitrophenyl glycosides was detectcd either by UV or measuring the absorbance at 410 nm using a microtitre plate reader.
Determinution of mode of action This was determined using either [ l-3H] labcllcd or unlabelled linear p-1,4-xylooligosaccharides as Substrates. The hydrolysis products released as a function of time were analysed either by TLC, and subscqucnt mcasurcment of radioactivity in the products and substrates or by HPLC using an NH2-Spherisorb column.
RESULTS & DISCUSSION Production and purification of xyladases from T. aumntiacus and T. lanuginosus Both fungi produced high levels of xylanase when grown on lignocellulosic carbon source. Interestingly, the xylanase produced by T. lanuginoms was free of cellulase activity, whereas T. aurantiacus produced both cellulose and hemicellulose degrading enzymes. Also, the xylanase activity (2840 U/ml) produced by T. Zanuginosus ATCC 46882 on corn cob was the highest recorded so far (13). Xylanase from T. aurantiacus was purified to homogcncity by a single ion-exchange chromatography, while that from T. lanuginosus required ion-exchange and gelfiltration chromatographies. The purification procedures used were relatively simple and yielded mg quantities of pure protein. Also, thcse procedures can be used to produce pure xylanase from T.aumntiacus and T. lanuginosus in bulk quantities.
General properties of xylanases from T. auraritiucus and T. lartirginosus Some of the general properties of both xylanases are presented in Table 1. In addition, the determination of stability of xylanascs at diffcrcnt pH and temperatures as a function of time provided some intcresling rcsults. The xylanase from T. aurantiacus was completely stable up to 80°C and between pH 4.4 - 6.2 for six days. The enzyme also retained 90% of its original activity at 6OoC, between pH 4.4 - 8.0 for 6 days, and 50% of its original activity at 60°C at pH 3.0 and 9.8 for 2 days and 2 h, respectively. In contrast, xylanase from T.lanuginosus was fully stable only between pH 5.0 10.0 and up to 55°C. Also, this xylanase retained only 50 - 60% of its original activity at 65OC for up to 5 h. Nevertheless, the high tcmpcrature optima together with their thermostability indicate that the xylanases from the above two fungi could be useful for many industrial applications.
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64 Application of enzymes to pulp, fibres and cellulose Table 1. Comparison of general properties of xylanases from T. aurantiucus and T. lanuginosus Properties
M,(kDa) PI pH optimum Temp. optimum Stability
Xylanase from T. aurantiacus 32 7.0 4.4 - 5.2 80°C Stable up to 6OoC betwccn pH 4.4 - 8.0 for six days
Xylanase from T. lanuginosus 26.3 3.7 6.0 - 6.5 75OC Stable up to 6OoC between pH 5.0 - 9.0 for 5h
Substrate specificity of xylanases from T.aurantiucus and T. lanuginosits Xylanase from T. aurantiacus showed the same level of activity towards soluble and insoluble xylans from different origins. However, the affinity of this enzyme towards highly substituted xylans such as arabinoxylan and glucuronoxylan was higher compared to less substituted birchwood xylan. Analysis of hydrolysis products by TLC revealed that the xylanase from T. aurantiacus released mainly xylo-oligosaccharides with degree of polymerisation (DP) higher than 4 (xylotetraose) during the initial stages of hydrolysis, but during the latter stages of hydrolysis, the main products were xylobiose and xylotriose. TLC analysis of hydrolysis products also showed the presence of a sugar with Rfhighcr than xylobiose and lower than xylose, and bclieved to be a disaccharide of xylose and arabinose. The concentration of this unknown sugar was higher in reaction mixtures which contained oat spelt xylan than birchwood xyaln. However, the xylanase did not release any xylose or other monosaccharidcs from different xylans, which indicated that it is an endo-xylanase and specific for internal glycosidic linkages. Further studies showed that the enzyme cleaved mainly the internal glycosidic bonds of xylo-oligosaccharidesand appeared to be an endo-xylanase. Xylanasc from T. aurantiacus was active towards 4-nitrophenyl P-D-cellobioside, 4mcthylumbelliferyl 0-D-xylobioside and 4-methylumbelliferyl p-D-xyloglucoside. Nevertheless, its activity towards 4-nitrophenyl a-L-arabinopyranoside and 4nitrophenyl a-L-arabinofuranoside was higher than that towards 4-nitrophenyl p-Dxyloside. These results suggested that the major xylanase from T. auranfiacus belongs to glycosyl hydrolases of family 10. The xylanase from T. funuginosus hydrolysed birchwood and oat spelt xylans. cereal arabinoxylan, 4-0-methyl glucuronoxylan and 0-acctyl-4-0-methylglucuronoxylan from beechwood and rhodymenan (0-D-xylan with ( b 3 ) and (I-A)-linkages). The HPLC analysis using an NHz-Spherisorb column revealed that the cnzyme released mainly xylobiose, xylotriose, xylotetraose and xylopentaose from birchwood, oat spelt and cereal xylans. Nevertheless, TLC analysis showed that the xylanase from T. lanuginosus released mainly acidic xylo-oligosaccharide (methylglucuronic acid xylotctraose) and an isomeric xylotetraose and isomeric xylopentaose, in addition to xylose and xylobiose from beechwood 4-0-methyl glucuronoxylan and rhodymcnan. Also, the enzyme released only small amounts of non-acetylated xylose and xylobiosc from 0-acetyl-4-0-methylglucuronoxylan. Furthermore, the xylanase from T. lanuginosus showed negligible activity towards 4-nitrophcnyl P-D-xylopyranoside and
Thermostable xylanases 65 4-nitrophcnyl P-D-cellobioside. These results strongly indicated that this enzyme belongs to glycosyl hydrolases of family 11. Mode of action of xylunases from T. aurantiacus and T. lanuginosus
The mode of action of xylanasc from T. aurantiacus was studied using xylooligosaccharides with DP 3 to 6. The cnzyme released equimolar amounts of xylose and xylobiose from xylotriose with concentrations up to 2 mM. However, with high concentrations of xylotriose, the ratio of xylobiose to xylose was higher than one, which indicated that the enzyme catalysed both thc hydrolysis and transglycosylation reactions. Xylotctraose and highcr xylo-oligosaccharidcs were found to be better substrates for this xylanase than xylotriosc. The enzyme readily hydrolysed xylotctraose and released xylotriose, xylobiose and xylose, with xylobiose as the main product. Nevertheless, the concentration of xylotriose was considcrably higher than xylosc when the xylotetraose concentration was higher than 2 mM. This again suggested that the xylanase from T. auranliacus catalysed both hydrolysis and transglycosylation reactions. From xylopentaose, the cnzyme released mainly xylobiose and xylotriose, while the main products released from xylohexaosc were xylotriose, and equimolar amounts of xylobiose and xylotetraose. These results revealed that the xylanase from T. auruntiacus is an endo-xylanase. The mode of action of xylanase from T. lanuginosus was studied using [1-3H] labelled and unlabelled xylo-oligosaccharides. The enzyme showed very low activity towards xylotriose, but readily hydrolysed xylotetraose and xylopentaose. Analysis of products as a function of time revealed that the xylotetraose reaction mixture contained mainly xylobiose and xylotriosc, while that of xylopentaose contained mainly xylotriosc, xylotetraose and xylobiose. Thc formation of xylotriosc from xylotetraose and xylotetraose from xylopentaose without thc release of equivalcnt amounts of xylosc suggested that the xylanase from T.lanuginosus catalysed both hydrolysis and transglycosylation rextions. Also, the formation of xylobiose from xylotriose without thc formation of equivalcnt amounts of xylosc strongly supported the above notion. Use of [ 1-3H] labelled xylo-oligosaccharides showcd that the xylanasc from T.lanuginosus cleaved preferentially the first and second glycosidic bonds of xylotriose and xylotetraose from the non-reducing end. However, the enzyme cleavcd the second and third glycosidic bonds of xylopcntaose from the non-reducing end with 60 and 40% efficiencies. These results clearly suggested that thc xylanase from T. lanuginosus is a typical endo-xylanase.
Classification of xylanases from T. aurantiacus and T.lanuginosus based on their biochemical properties Based on their molecular mass and PI, xylanases have been classified into two main groups (7). The group one represents xylanascs with high molccular mass and low PI, while group two contains xylanases with low molecular mass and high PI. Also, the comparison of amino acid sequence and hydrophobic cluster analysis revealed that most xylanases studied so far belong to glycosyl hydrolasc families 10 and 11 (20,21), which is vcry well corrclated with the grouping of xylanascs based on their molccular mass and PI. Furthermorc, the xylanases from families 10 and 11 can be distinguished biochemically based on their action on 4-nitrophcnyl P-D-xylopyranoside and 4nitroplienyl P-D-cellobioside (22). Both these substrates wcre readily hydrolysed at the
66 Application of enzymes to pulp, fibres and cellulose aglyconic linkage by xylanases of family 10, but not by xylanases from family 11. Interestingly, the xylanase from T. uurunriucus rapidly hydrolysed 4-nitrophenyl p-Dxylopyranoside and 4-nitrophenyl P-D-cellobioside, while the xylanase from T. lunuginosus showed negligible activity towards these substrates. These results indicated that xylanases charactcrised from T. uuruntiucus and T. lunuginosus belong to familics 10 and 11, respectively. Recent 3-D structural studics on xylanases from these two thermophilic fungi demonstrated that they belong to families 10 and 11(23,24), and in agreement with our biochemical results.
Potential applications of xylanases from T. uuruntiacus and T. lunuginosus in paper and pulp industry It is generally believed that xylanases active and stable at alkaline pH and at high tempcratures (>55'%) could be ideal for pre-bleaching of paper pulps. Also, it has been predicted that the xylanase preparations free of cellulase and containing mannanase and xylan-dcbranching enzymes could be better than enzyme preparation containing both cellulase and xylanase for pre-bleaching of paper pulps. Interestingly, the two xylanases purified and characterised from T. uuruntiucus and T. Zunuginosus were active over alkaline pH range (up to 9.5) and at temperatures up to 70°C. Also, both xylanases were completely stable between pH 5.0 8.0 and at temperatures up to 6OoC and hydrolysed mainly the internal glycosidic bonds of different xylans. Moreover, T. auruntiucus produced high levels of xylanase and xylan-debranching enzymes, whereas T. lunuginosus produced cellulasc-frce xylanase. These properties strongly suggest that the xylanase preparations from T. uuruntiucus and T. Zanuginosus could be ideal for prebleaching of papcr pulps.
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CONCLUSIONS 1. Both T. uuruntiucus and T. lunuginosus produced high levels of xylanase when grown on lignocellulosic carbon sources. 2. The major xylanase from the culture filtrate of T. auruntiucus and T.Zunuginosus was purified and characteriscd. 3. Xylanases from both fungi were active and stable over a wide range of pH and temperatures. 4. Both xylanases were specific for differcnt xylans and cleaved internal glycosidic linkages of xylans and xylo-oligosaccharides. 5. The purified xylanases from T. uuruntiucus and T. lanuginosus belong to families 10 and 11 of glycosyl hydrolases, rcspectively. 6. Biochemical properties strongly indicate that both xylanases from T. aurantiucus and T. lunuginosus could be ideal for pre-bleaching of paper pulp.
ACKNOWLEDGEMENTS
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The financial support from INCO-Copernicus (projcct numbers CT94-0232 & CT96
IOOO), AIR II (project number CT93-1272) programmes and BBSRC is gratefully
acknowledged.
Thermostable xylanases 67 REFERENCES 1. L Viikari, M Tenkancn, J Buchert, M Ratto, M Bailey, M Siika-Aho, and M Linko, ‘Hemicellulases for industrial applications’, In: Bioconversion offorest and agricultural plant residues, cd. J N Saddler. 1993 131-182,C A B . International, Wallingford, UK. 2. P Biely, ‘Microbial xylanolytic enzymes’, Trends Biotechnol, 1985 3 286-290. 3. E Sjostrom, Wood Chemistry, Fundamentals and applications, Academic press, New York, USA, 1981 pp223. 4.M P Coughlan, ‘Towards an understanding of the mechanism of action of main chainhydrolysing xylanases’, In: Xylans and Xylanases, eds, J Visser, G Beldman, M A Kusters-van Someren, and A G J Voragen, Progress in Biotcchnol, vol. 7, 1992 111139,Elsevier, Amstcrdam, The Netherlands. 5. A M Scallan, ‘The accommodation of water within pulp fibres’, Proceedingsfibrewater interactions in paper making, Oxford, 1977 9-27. 6. K K Y Wong and J N Saddlcr, ‘Trichoderma xylanases, their properties and applications. In: Xylans and Xylanuses, eds, J Visscr, G Beldman, M A Kusters-van Someren, and A G J Voragen, Progress in Biotechnol, vo1.7, 1992 171-186Elsevier, Amsterdam, The Netherlands. 7. K K Y Wong, L U L Tan and J N Saddler, ‘Multiplicity of P-l,l-xylanase in microorganisms: functions and applications’, Microbiol Rev,198852 305-317. 8. A Kantelinen, B Hortling, J Sundquist, M Linko and LViikari, ‘Proposed mechanism of the enzymatic bleaching of kraft pulp with xylanases’, Holi$orschung, 1993 47 318324. 9. J Buchert, T Oksanen, J Pere, M Siika-Aho, A Suurnakki and L Viikari. ‘Applications of Trichoderma reesei enzymes in the pulp and paper industry’, In: Trichoderma and Gliocladium, Enzymes, biological control and commercial applications, eds, G E Harman and C P Kubicek, 1998 vol. 2 pp. 343-363,Taylor & Francis Ltd, London, UK. 10. S Kalogiannis, E Owen, D E Beever and M K Bhat, ‘Screening of ten strains of Thermoascus aurantiacus and characterising a major xylanase’, Med Fac Landbouww Uni Gent, 1995 6014a 1995-1998. 11. S Kalogiannis, E Owcn, D E Beever, M Claeyssens, W Nerinckx and M K Bhat, ‘Characterisation of the xylanolytic system of the thermophilic fungus T.aurantiacus’, 1996 SCI Lecturepaper series no.78 1-1 1. 12. M K Bhat, N J Parry, S Kalogiannis, D E Beever, E Owen, W Ncrinckx and M Claeyssens, ‘Biochemical characterisation of cellulases and xylanascs from Thermoascus aurantiacus’, In: Carbohydrases from Trichoderma reesei and other microorganisms, cds. M Claeyssens, W Nerinckx and K Picns, 1998 102-112, Proceedings of Trice1 ‘97meeting, Royal Society of Chemistry, Cambridge, UK. 13. N A Bennctt, J Ryan, P Biely, M Vrsanska, L Kremnicky, B J Macris, D Kekos. P Christakopoulos, P Katapodis, M Clacysscns, W Nerinckx, P Ntauma and M K Bhat, ‘Biochemical and catalytic propcrties of an endoxylanase purified from the culture filtrate of Thermomyces lanuginosus ATCC 46882’,Carbohydr Res, 1998 306 445-455. 14.M J Bailey, P Biely and K Poutancn, ‘Intcrlaboratory tcsting of methods for assay of xylanase activity’, J Biotechnol, 1992 23 257-270. 15. K S Smith, R L Krohm, G T Hermanson, A K Mallia. F H Gartner, M D Provenzano, E K Fujimoto, N M Goeke, B J Olson and D C Klcnk, ‘Mcasurmcnt of protein using bicinchoninic acid’, Anal Biochem, 1985 150 76-85. 16. M Mandcls and D Sternbcrg, ‘Recent advance in cellulase technology’, J Ferment Techtd, 1976 54 267-286.
68 Application of enzymes to pulp, fibres and cellulose 17. B Lugtenberg, J Meijers, R Peters, P H Vander and L V Alphen, ‘Electrophoretic resolution of the major outer membrane protein of Escherichia coli K12 into four bands’, FEBS Lett, 1975 8 254-258. 18. K M Bhat and T M Wood, ‘Multiple forms of endo-1,4-P-glucanase in the extracellular cellulase of Penicillium pinophilum’, Biotechnol Bioeng, 1989 33 12421248. 19.M Somogyi, ‘Notes on sugar determination’, J Biol Chem, 1952 195 19-23. 20. N R Gilkes, B Henrissat, D G Kilburn, R C Miller Jr and R A J Warren, ‘Domain in microbial P-I,4-glycanases: sequence conservation, function, and families’, Microbiol Rev, 1991 55 303-315. 21. G J Davies and B Henrissat, ‘Structure and mechanisms of glycosyl hydrolases’, Structure, 1995 3 853-859. 22. P Biely, M Vrsanska, M Tenkanen and D Kluepfel, ‘Endo-0 1.4-xylanase families: differences in catalytic properties’, J Biotechnol, 1997 57 151-166. 23. L Lo Leggio, S Kalogiannis, N J Parry, M K Bhat and R W Pickersgill, ‘Structure of Thennoascus aurantiacus xylanase and cellulase, Poster presented in Understanding structure determination conference, 1996, Sweden. 24. K Gruber, G Klintschar, M Hayn, A Schlacher, W Steiner and C Kratky, ‘Thermophilic xylanase from Thermomyces lanuginosus: High-resolution X-ray structure and modelling studies’, Biochemistry, 1998 37 13475-13485.
CELLULASES IN PULP AND PAPER PROCESSING Liisa Viikari, Tarja Oksanen, Anna Suurnakki, Johanna Buchert and Jaakko Pcre VTT Biotechnology and Food Research P.O. BOX 1501 FIN-02044 VTT, Finland
Summary The enzymatic modification of cellulose, the major carbohydrate in different pulp fibres, offers unique opportunities for new applications, due to thc diffcrent modes of action of individual cellulascs. Although thcy all act on the same glycosidic P-1,4-linkage, different structural and functional modifications can be obtaincd depending on the type of fiber, enzyme and process phase used. Thus, cellulase based applications have been developed aiming at energy saving in rcfining, improvement of beatability, drainage and runnability of the paper machine, or enhancing dc-inking processes. In these applications, the enzymatic treatmcnt has bccn carried out on coarse or refined mechanical pulps, chemical pulps or recycled fibers at different phases of the processing. When applying cellulases, it is important to minimize the detrimental effects of these enzymes on the yield and strcngth properties of fibers. In this paper, the mode of action and effccts of different types of cellulases; cellobiohydrolases and endoglucanases, are reviewed in the light of present applications.
Introduction The use of enzymes in the pulp and papcr industry has grown steadily since the mid 1980’s. Increased understanding of the enzymatic reaction mechanisms on the fiber substrates has bcen the basis for development of new commercial enzyme preparations. The establishmcnt of cost cffective production technologics of relevant enzymcs has lead to decreased enzyme prices. The characteristics of enzymatic mcthods mcct wcll the necd of the industry to adopt environmentally benign technologies. Enzymatic methods have, however, to be able to compete with existing and other ncw technologies. Thcrefore, enzymatic mcthods are most applicable while performing reactions with highcr spccificity or with lower costs or environmental impacts than the competing technologies. During rccent years, new applications of cellulases have becn devclopcd especially in the textile industries. In the pulp and papcr industry, the strength propertics of the fibers, described as tensile, burst and tear indcxes, are of primary importance and sensible for impairment by cellulascs. The prcsencc of ccllulases in enzyme preparations used for processing of pulps has traditionally been considered detrimental due to their negative effects on thesc propertics. Prcsenlly, the underlying mechanisms of only few applications have been studicd in delail by using purificd enzymcs or optimizcd
70 Application of enzymes to pulp, fibres and cellulose mixtures. Even mixtures of enzymes can be used to remove certain fiber components, such as hcmicelluloses, whereas in the case of cellulases, non-tailored enzyme mixtures usually lead to random attack and undesired side-effects. Thus, the underlying mechanisms are complex, due to the molecular and structural organization of the amorphous and crystalline regions in different types of fibcrs. Furthermore, the type of fibcrs. the chemical composition, coarseness as well as the outer fibrillation seem to affect the site of the enzymatic attack. The accessibility and interlinkage of carbohydrates with lignin in different types of pulps may also affect the action of cellulases. Purified enzymes in the research phase, howevcr, help to understand and optimize the enzymatic treatments and to develop useful commcrcial enzyme preparations.
Cellulases and their modes of action The endo-ex0 model Cellulascs are classificd according to their mode of attack. Thus, the major groups include the cellobiohydrolases (CBH), acting at the ends of the cellulose chains, endoglucanases (EG), acting randomly within a glucan chain and P-glucosidases, hydrolyzing small cello-oligosaccharides. Trichoderma reesei, one of the most efficient producers of cellulolytic enzymes, produces two cellobiohydrolascs, CBH I and 11, which are known to be especially efficient in hydrolyzing highly crystalline substrates and five endoglucanases (EG I, EG 11, EG 111, EG IV, EG V) acting on amorphous regions of cellulose. The action of different cellulases has been studied on model substrates, such as highly crystalline Valonia ccllulose, using elcctron microscopy (Chanzy et a1. 1983, Chanzy and Henrissat 1985, Hoshino et al. 1994). According to the authors (Chanzy and Henrissat 1985, Iloshino et al. 1994) CBH I dccreased the lateral width of Valonia microcrystals while maintaining the crystal length, whereas CBH I1 hydrolyzed microcrystals unidirectionally from the non-reducing end of cellulose chains. Morphological modification of cotton fibers (Woodward et al. 1992, Hoshino e l al. 1993) and Sinupsis ccllulosc (Sprcy and Bochem 1991, 1992) due to the action of cndoand exo-type cellulascs has been described previously. Treatment of cotton linters with exo-type ccllulascs caused fiber dispersion (Woodward et al. 1992) and deep longitudinal cracks during prolonged incubations, with a low decrease of DP (Hoshino et al. 1993). By contrast, the endo-type ccllulase studied, eroded cotton fibers locally with a simultaneous dccrease in DP (Hoshino et al. 1993). These observations are in accordance with the general endo-cxo model indicating an axial type of action for CBH’s and a local attack for EG’s also in thc case of multilayercd cotton fiber. However, contradictory results have been obtained with Sinupsis cellulose. Treatment of this substrate with EG I resulted in the formation of submicrofibrils (Sprey and Bochem 1991) and a subsequcnt treatment with CBH I created heterogeneous cellulose clusters, which was also observed as an increase in the accessible cellulose surface area (Sprey and Bochem 1992). The action of the four major T. reesei cellulases i.e. EG I, EG 11, CBH I and CBH I1 on different fibers has becn compared by e.g. Pere et ul (1995) and Rahkamo et ul(1996).
Cellulases in pulp and paper processing 7 1 Thc two cellobiohydrolases and endoglucanases studied exhibited significant diffcrences in their mode of action on the pulps. The CBH I treatment had no effect on the handsheet propcrties of chemical pulps evcn after a refining stage, indicating that CBH I caused no structural damage to the fibers. The ccllobiohydrolases had also only a very modcst cffcct on pulp viscosity. This is in agreement with the results of KlcmanLeycr et al. (1996). On the other hand, the endoglucanases (EG) dramatically decreased viscosity, even at low enzyme dosages. Of the endoglucanases, EG I1 was shown to decrease the viscosity most drastically, suggesting that EG I1 attacks cellulose at sites where even low levels of hydrolysis result in large dccreases in viscosity and, consequently, a dramatic dcterioration of the tcnsilc index (Pere et al. 1995). The negative effect of cellulases on pulps is even enhanced when cellulases act synergistically. The degree of synergy obtained, however, depends on the nature of the substrate and the concentration and ratios of the enzymes used. The highest degree of syncrgy is observed between pairs of cellulases; e.g. EG I has been reported to pretreat ccllulose more efficiently for CBH I than for CBH I1 (Nidetzky et al. 1994). Physical determinuntsfor cellulase activity
Although the mode of action of individual cellulases on pure cellulose substrates is well undcrstood today, their mechanistical differcnccs on multilaycrcd and chemically complcx natural fibers have not bccn fully solved. The action of enzymes in pulp is generally affected by the accessibility of substratcs in the fibcr matrix. The main factors limiting the access of enzymes in woody materials are the specific surface area and the porosity, i.e. the median pore size of fibers (Stone et al. 1969, Grethlein 1985). Also the molecular organization of the fiber components and the surface chemistry; i.e. relative amounts of lignin and carbohydrates have a significant impact on the action of cellulases on thc fibcr bound substrates. The hydrolysis experiments of coarse spruce fibers revealed that using the same amount of individual cellulases (CBH I and 11, EG I and 11), the endoglucanase released by far the highest amount of carbohydratcs, whercas the two cellobiohydrolases hydrolyzed the coarse pulps at the same efficiency. Using a cellulasc mixture, a clcar synergistic action was demonstrated. When unbleached softwood kraft pulp was treated with thc same enzymes, generally similar degrees of hydrolysis were obtained, however, within shorter hydrolysis times. Also in this case, thc endoglucanascs hydrolyzcd thc pulp more efficiently (Pcre et al. 1995). Surprisingly, the particle size and hence the available surfacc arca, i.e. fibcr coarseness (measured as freeness, CSF) of the pulps, did not significantly affect thc hydrolytic efficicncy of CBH I. In the case of EG I the liberation of reducing sugars slightly increased (1520%) as the CSF decreased from 580 ml to 130 ml. The higher efficiency of EG I as compared with CBH I was more pronounced when different pulp fractions were used as substrates. This indicatcs thc differcnt modcs of action of thcse two cnzymcs: for CBH I there seems to be a fixed amount of productive binding sites controlling hydrolysis, irrespective of the coarseness of the pulp, whereas the available surface area is a determinant factor for the hydrolytic action of EG I. The superior hydrolytic activity of EG I over CBH I might also be due to thc higher affinity and bettcr binding of this enzyme on the cellulosic substrate (Lindcr et al. 1995). The hydrolytic activity, mcasurcd as soluble oligosaccharides, describe the extent of thc attack, but do not reveal the mode of action of the cellulolytic enzymes. Thc rclationships bctwccn thc rathcr small chemical changes and thc altered technical
72 Application of enzymes to pulp, fibres and cellulose characteristics of pulps are in many cases still poorly understood and can not always be predicted. Thus, the effects of cellulases on pulp substrates can be best evaluated after implementation in the process schcme and after suitable combination with chemical or mechanical treatments. Dcpending on the application, cellulase treatments have been combincd with mechanical or chemical stcps.
The role of cellulose binding domains (CBD’s) The endoglucanases EG I, EG 11, EG IV and EG V and the cellobiohydrolases CBH I and CBH I1 from T. reesei are composed of two main domains, i.e. active core domain and cellulose binding domain (CBD), which are bound together by a flexiblc linker (Tomme, et al. 1995). Today, using genctic engineering, both domains, i.e. CBD’s and cores, can be produced separately. The rcmoval of CBD from CBH 11 is rcported to have no intluence on the activity of the enzyme when soluble substrates are used but using crystalline substratcs both its binding and activity werc clcarly impaired (Srisodsuk er al., 1993). The disrupture or swelling of cellulose structure by the binding domain has been claimed to be a reason for the highcr activity of the intact enzyme towards crystalline cellulosc in the case of CBH’s. The CBD has been claimed to be less important in the action of endoglucanascs as they are considered to act mainly on noncrystalline cellulose. In order to undcrstand the importance of CBD in the action of T. reesei cellulases in chemical pulp fibers, the effects of monocomponent cellulases (EG I, EG 11, CBH I, CBH 11) and their core protcins on pulp properties has been studied (Suurnakki et al., 1997). It could be anticipatcd that the removal of the CBD would affect the intensity and hydrolytic action of cellulases on fibers and thus minimize their detrimcntal effects in pulp. The role of CBD in cellulasc action in the pulp was monitored mainly by the cellulose hydrolysis levels. The intact enzyme solubilized more ccllulose than thc core protein lacking the CBD. In addition, the CBD of EG I active against both cellulose and xylan, directcd the enzyme action towards hydrolysis of cellulose. Irrcspective of the differences in the hydrolysis efficiency, at the same hydrolysis level the detrimental effect of intact enzymc and the corresponding core protein on pulp properties, such as viscosity of the unbcatcn pulp and strength of the bcaten pulp was, however, approximately the same. Thus, the presence of CBD in T. reesei cellulases appears not to significantly alter the action of cellulases on pulp substrate.
Applications of cellulases Cellulases have been combined with refining in thc production of mechanical or chemical pulps to decrease energy consumption or to improve beatability and with chemical treatments to cnhance dc-inking processes. Enzymatic modification of fibcr properties has becn exploitcd after the refining stage to improve the runnability of paper machine (Table 1). In these applications, the enzymatic treatment has been carried out on coarse or refined mechanical pulps, chemical pulps or recycled fibcrs at differcnt phases of the processing.
Cellulases in pulp and paper processing
73
Table 1. Potential benefits of cellulases in modification of different types of pulps Targeted modification
Type of pulp ~~
Chemical
Beatability Air resistance, density Drainage
Mechanical
Energy saving Increased fines fraction Flexibility
Recycled
De-inking Drainage
Celluluses in mechanical pulping In mechanical pulping, wood logs or chips are fiberized by mechanical means. Refining is comprised of a series of comprcssions and decoinprcssions which lead to the separation and fibrillation of individual fibers with simultaneous generation of fines. The yield in mechanical pulping processes is close to 95%. Mechanical pulps are particularly attractive for printing-grade papers due to their excellent optical properties. The main drawback and hence a limitation to increasing use of mechanical pulps is their high energy requirement. One way to reduce the energy consumption in thermomechanical pulping is to modify the raw material by biotechnical means prior to refining. The aim of the enzymatic treatment has been to reduce the specific energy consumption in the secondary or reject refining stages, after an intermittent cnzymatic step, as visualized in Figure 1. Thus, the enzyme treatment has been carried out on fairly coarse fibers. In the cnzymatic treatment, the target hydrolysis level has generally been adjusted to correspond a hydrolysis level lower than 1% of the pulp, by choosing both the cnzyme dosage and treatment time.
Papcr
Enzymc
Figure 1. Scheme of the enzyme-aided refining process
Reject refining
74 Application of enzymes to pulp, fibres and cellulose Preliminary experiments with different enzymes demonstrated that a slight modification of cellulose by CBH I resulted in to an energy saving of 20% in laboratory-scale disk refiner (Pere et al. in press). Treatment with EG I decreased the energy consumption slightly but only at the expense of pulp quality. Interestingly, no positive effect on energy consumption was detected with CBH 11, neither with a cellulase mixture. When the refining was performed with a low-intensity refiner (wingdefibrator), the positive effect of CBH I in reducing the energy consumption was further enhanced. At a CSF level of 100 ml the energy consumption in the secondary rcfining was reduced by 40% with CBH I, as compared with the untreated reference (Pere et al. 1996). Laboratory refining with different types of refiners indicated that in order to obtain a maximal benefit of the enzymatic treatment, the secondary refining should be performed with a rather low intensity. The results obtained with the laboratory refiners have becn further verified in a pilot-scale experiments. In a two-stage secondary refining an energy saving of 10-15% with CBH I was obtained (Pcre et al. 1996). These results clearly confirmed the previous results in smallcr scale, although no optimization of the process parameters in the pilot experiments has yet been carried out. Neither the cellulase mixture nor CBH I induced evident morphological modifications in the coarse and rigid TMP fibers during a short incubation. Therefore, the possible loosening and unraveling of fiber structure induced by the action of the enzymes was analyzed as fibrillation index in the samples of differcnt CSF levels after secondary refining. Significantly morc intensive fibrillation was observed in the CBH I treated pulp than in that treated with the ccllulase mixture and in the untreated control. This could be an indirect implication of decreased interfibrillar cohesion inside the fiber wall due to the action of CBH I. Thus, it is possible that the CBH I treatmcnt might have activated the mechanism suggested by Karnis (1994), according to which fiber development during refining proceeds through delamination and peeling-off reactions in P- and S-layers of the cell wall as a function of energy input. The results obtained with CBH I and EG I also reflected the different modes of action on mechanical pulp: EG I and the cellulase mixture had a tendency to sensitize fibers to breaking off during refining, whereas CBH I seemed to act more along the fiber axis while maintaining the fiber length. However, it was suprising that the highly lignified and stiff TMP fibers were clearly weakened by the modification of cellulose with EG I and the cellulase mixture. All the cellulase treatments decreased the bulkiness of thc handsheets, but possibly via different mechanisms. Use of CBH I resulted in a pulp with good bonding ability, which gave rise to a higher tensile index as compared with the untreated reference and the pulps treated with EG I and the cellulase mixture. On the other hand, the content of light-scattering material was more favorable in the case of EG I and the cellulase mixture, as compared with CBH I, which gave optical properties comparable with the untreated control. Evidently, EG I and the cellulase mixture created mainly nonfibrillar fines, which acted like a filler and which yielded, instead of an increased bonded area, a high tendency to light scaltering. In fact, the size and shape of fines, i.e. fibrillar vs. agglomcratc, have been shown to affcct significantly cithcr handshect strength properties in the case of fibrillar fines or optical properties in lhe case of non-fibrillar or agglomerate fines (Luukko et al. 1966, Westermarck and Caprctti 1988).
Cellulases in pulp and paper processing 75 Cellulase treatments of chemical pulps
In paper manufacture, bcating. i.e. mechanical refining, is used to obtain the desired properties of chemical pulp fibcrs. Beating leads to improved comformability, flexibility and inter-fiber bonding of fibcrs through a process where both internal delamination and external fibrillation of the fiber cell wall take place. Howevcr, generation of fines during bcating is accompanied by increased water sorption and subsequent decreased dewatering and drainage properties of fibers in the paper machine. When cellulases are used for the modificalion of thc fiber properties, the enzyme stage can be carried out either prior to or after refining of the pulps. By carrying out the enzymatic stage prior to the rcfining process the aim has been to improve the beating response or other fiber properties. The principal challenge in using enzymes to enhance fiber bonding is to increase fibrillation without reducing pulp viscosity (Kirk et al. 1996). However, in the enzymatic treatment of refined or recycled pulps, the main focus is in the improvement of the dewatering, i.e. drainage properties of the pulps, affecting the speed of paper machine operation. The different approaches are schematically represented in Figure 2. Production of special papers with highcr dcnsity and air rcsistancc
Improvement of drainage and runnability
Enzymatic
n Papcr
i Paper
I
Figure 2. Process schemes for modification of chemical pulps with cellulases. According to Noe et al. (1986) and Pommier et al. (1991), improved beatability of kraft pulps has been obtained by using hemicellulases and cellulases. In those investigations, however, enzyme mixtures were used and hence the effects of individual enzymes were not elucidated. The effect of the purified Trichoderma cellulases and hemicellulascs on the beatability and paper technical properties of bleached kraft pulps has been furthcr investigated by Oksanen et al. (1997) and Kamaya (1996). Pretreatment of the pulp with CBH I or CBH I1 had practically no effect on the development of pulp properties, whereas endoglucanases, especially EG 11, were found to improve the beatability of the pulp as measured by SR value, sheet density and Gurley air resistance. A commercial cellulase/hemicellulase enzyme preparation (Pergalase A-40) based on Trichoderma enzymes has been developed and is currently used in several paper mills in the production of rclcase papers and wood-containing printing papers (Pornmier et al. 1990, Jokinen 1991, Freicrmuth et al. 1994). Drainage rate is one of the key parameters controlling the runnability of the paper machine. The potcntial of improving the drainage rates of rccycled fibers by cellulase mixtures was discovered in the late eighties (Fucntes and Robert, 1986, Pommicr et al.,
76 Application of enzymes to pulp, fibres and cellulose 1989, Pommier et al. 1990). According to Stork et al. (1995) the endoglucanase activity is a prercquisite for drainage improvement of recycled pulps. This has been confirmed by Oksanen et al. (1996) and Kamaya (1996) using purified or partially purified Trichoderma cellulases. EG I and EG I1 were equally efficient in decreasing the Schopper-Riegler value (SR) of recyclcd softwood kraft pulp although the amount of solubilized carbohydrates were lower with EG 11. Obviously, the limited hydrolysis of hydrophilic amorphous cellulose by the EGs was essential, because the purified T. reesei xylanases and mannanases had only a slight positive impact on drainage of recycled fibers (Oksanen et al. 1996). The development of cnzymatic fiber modification processes requires profound understanding on the action of different enzymes on different types of pulps. Rccently, many reports on this area have been published. Mansfield et al. (1996) have investigated the impact of a commercial cellulase mixture (Novozyme SP 342 from Humicola insolens) on the different fiber fractions obtained from doughs fir kraft pulp. The cellulase treatment was found to dccrcase the freeness of the fibers indicating defibrillation which in turn lead to reduced fiber coarseness. The strength propcrties (tensile indcx, burst index and tear resistance) were found to decrease with incrcasing cellulase dosage. Other upplications
The application of enzymes in de-inking has been intensely studied in the laboratory and pilot scale in recent years, and numerous patents exist, as reviewed by Welt and Dinus (1995). One of the advantages offered by enzymatic de-inking is the avoidance of alkaline de-inking chemicals. De-inking in a low pH prevents alkaline yellowing of pulp and simplifies the de-inking chemistry. In an industrial operation, the use of enzymes as deinking aids could thus lower the chemical costs and decrease environmental impacts. Carbohydrate hydrolyzing enzymes, such as cellulases, xylanascs or pcctinases have been used to release ink from fiber surfaces (Kim 1991, Prasad et al. 1992). Hydrolyzing the ink carrier releases the individual ink (carbon black) particles which are too small in size to be effectively flotated. Most applications proposed use cellulascs and hemicellulases where the detachment of ink results from a partial enzymatic hydrolysis of carbohydrate molecules on the fiber surface (Heitmann 1992, Prasad et al. 1992, Prasad et al. 1993, Rushing et al. 1993, Jcffries etal. 1994). In general, these studies revealed that cellulases and hemicellulases increase brightness and pulp cleanliness compared to conventional de-inking. The enzymatic de-inking also changes the ink particle size distribution, apparently reducing the particle size. Bcsidcs ink removal, enzymatic de-inking may contribute to improved strength properties and frccness and reduced fines content. Strength improvement has been observed especially with xylanase treatment while cellulases are more efficient in increasing brightness and freeness (Prasad et al. 1993, Rushing et al. 1993, Heitmann et al. 1992). The laboratoryscale results have also been confirmed in pilot trials (Heise et a1 1996). So far, in most reports on enzyme-aidcd de-inking, enzyme mixtures have been used. However, deinking expcrimcnts carried out with monocomponent cellulases of Glocophyllum sepiarium have suggcsted that endoglucanases are the most important enzymes contributing to ink removal from office waste papers (Franks and Munck 1995, Giibitz et al. 1998). Dc-inking results using individual Trichoderma reesei enzymes have
Cellulases in pulp and paper processing 77 indicated that the efficiency of enzymatic de-inking is highly dependent on the waste paper and enzyme type (Suurnakki et al. 1998).
Conclusions Ccllulascs are potential tools for modification of fibers for various purposes. In these applications, thc treatment usually has an optimum betwcen the positive effects and the ncgative cffccts as a function of the treatment efficiency. Therefore, the ncgative effects should be minimized by carefully controlling the composition of the enzyme preparation, the enzyme dosage and the trcatment time. The new concept of using T. reesei CBH I before thc secondary refining of coarsc mechanical pulps has been shown to be successful in laboratory and pilot scale. Most of the cncrgy in refining is consumed in the loosening and partial unraveling of the layered and highly ordered cell wall structure of wood fibers. Thercforc, it can be anticipated that the beneficial cffcct of CBH I treatment on the cnergy consumption is due to thc limited hydrolysis of the surface cellulose chains in the microfibrils leading to enhanced delamination or loosening of the fiber structure. The applicability of this method will depend on further success in optimizing thc enzymatic trcatmcnt and integrating the enzymatic step to present refining systems. Endoglucanases have been shown to be efficicnt in fiber modification for improvcd beatability or drainage. One commcrcial enzyme prcparation is already available for this purpose. The usc of endoglucanases, in spite of their positive effccts, may however be hampered by their negative effcct on pulp viscosity already with low dosages. In future, more targeted commercial cellulase prcparations can be expected to be availablc.
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A Suurnakki, H-J Putz, K Renner, G Galland and J Buchert, “Enzyme-aided deinking of rccovered papers”, Improvement of recyclability and the recycling paper industry of the future, COST Action El , Las Palmas de Gran Canaria, 1998,329-338.
P Tomme, R A J Warren and N R Gilkes, “Cellulose hydrolysis by bacteria and fungi”, Adv. Microb. Physiol. 1995 37, 1-81. T Welt and R J Dinus, “Enzymatic deinking”, Prog. in Paper Recycling 1995 4(2), 3647. U Wcstcrmarck and G Capretti, “Influence of ray cclls on the bleachability and properties of CTMP and kraft pulps”, Nordic Pulp and Pap. Res. J . 1988 3,95-99.
T M Wood and V , Garcia-Campayo, “Enzymology of cellulose degradation”, Biodeegrudation 1990 1, 147-161. J Woodward, “Synergism in cellulase systems”, Bioresource Technology 1991 1,67-75 J Woodward, K A Affholter, K K Noles, N T Troy and S F Gaslightwala, “Does ccllobiohydrolase 11 core protein from Trichoderma reesei disperse ccllulose macrofibrils?’, Enzyme Microb. Technol. 1992 14,625-630.
MODE OF ACTION OF THERMOPHILIC BACTERIAL AND FUNGAL ENDOGLUCANASES ON CARBOXYMETHYL CELLULOSES M K Bhat'. S Bhat'. N J Parry', J F Kennedy', C J Knill', D E Beeve? and E.0wen3 'Food Macromolecular Science Department, Institute of Food Research. Reading Laboratory, Earley Gate, Reading, RGG 6BZ, UK. *Birmingham Carbohydrate and Protein Tcchnology Group, Research Laboratory for thc Chemistry of Bioactive Carbohydrates and Proteins, School of Chcmistry, The University of Birmingham, Birmingham, B15 2TT, UK. 3Department of Agriculture, The University of Reading, Earley Gate, PO Box 236, Rcading, RGG GAT, UK.
ABSTRACT The mode of action of two thermostable endoglucanases from Thermoascus aurantiacus and Clostridium thermocellum on CM-cellulose with varying viscosities and degrees of substitution was studied. Both endoglucanases randomly attacked all thc CM-celluloses tested, but the action of thcsc two endoglucanases decreased with increasing degree of substitution (DS) of CM-cellulose. Thus, the DS rather than viscosity of CM-cellulose appearcd to influcncc the action of both endoglucanases. Further, HPLC analysis showcd that the hydrolysis products released from all CMcclluoses were mainly unsubstituted cellobiose and cellotriose together with small amounts of higher cello-oligosaccharidcs. These results revealed that the endoglucanases not only attacked the glycosidic bonds between unsubstituted glucose residues, but also preferred less substitutcd CM-ccllulose.
INTRODUCTION Carboxymclhyl (CM-) cellulose is an anionic and linear polysaccharide which exists cither as a free acid, sodium salt or as a mixture of both (1). The sodium salt of CMcellulose is generally used in foods, since the free acid form is insoluble in watcr (2). Interesting properties such as viscosity, thixotropy and pseudoplasticity of CMcellulose which can be controlled by varying the chain oxidation and substitution, facilitate its use in food and chemical industrial applications (1,2). In spite of these advantages, the release of large quantities of cellulose derivatives to sewage by chemical industries could cause severe environmental problems. Likewise, the use of incrcasing amounts of CM-cellulose in foods may lead to health disorders, due to their incompatibility with trace elements, interaction with proteins, pseudoplasticity and gelling properties. Therefore, a clear understanding of the enzymatic degradation of CM-cellulose is essential in order to prcpare CM-cellulosc with dcsired biological propertics. In the prescnt paper, wc compare the modc of action of cndoglucanases from a thermophilic bacterium, Clostridium thermocellum and a thermophilic fungus, Thermonscus auruntiacus on various CM-celluloses.
82 Application of enzymes to pulp, fibres and cellulose MATERIALS AND METHODS Materials C. thermocellum strain YS was a kind gift from Professor Raphel Lamed, The University of Tel Aviv, Israel, whereas T. aurantiacus IMI 216529 was purchased from the International Mycological Institute, Surrey, UK. High, medium and low viscosity CM-cellulose (Blanose) samples of different degrees of substitution (DS), 0.78, 0.93, 1.3, 1.7 and 2.1, respectively, were obtained from Hercules Limited, Salford, UK. CM-celluloses labelled as low, medium and high viscosity, rcspectively, were purchased from Sigma. The DS of the CM-cellulose from Sigma was determined, as described below.
Methods Production and purification of a major endoglucanase (Sll subunit) of C. thermocellum cellulosome
For the production of cellulosome, C. thermocellum was cultured in a Wheaton bottle (1 litre) under NZ using GM-medium (3) containing 1% Avicel PH 101, for 96 h at 60°C. The culture was harvested by centrifugation (20,000 g for 20 min) and the cellulosome present in the supernatant was purified by affinity digestion using H3P04swollen cellulose, followed by gel filtration on a Sepharose-2B chromatography , Cellulosome was dissociated at pH 5.0 and in the presence of 0.2% SDS, 10 mM EDTA and 10 mM DTT by incubating at 3OoC for 25 min. Dissociated cellulosome was fractionated by preparative SDS-PAGE using a Model 491 preparative cell (Bio-Rad) according to the instructions given in the Bio-Rad reference manual. Activc fractions corrcsponding to a major endoglucanase (Sl1 subunit) were identificd based on the activity towards medium viscosity CM-cellulose and these fractions were pooled and concentrated. Production and purGcation of an extracellular endoglucanase from
T.aurantiacus
For the production of cellulase, T. auratiacus was cultured in Mandels medium (300 litrcs; 4) containing paper paste (20 g L')and wheat bran (20 g L-')as carbon sources at 50°C for 6 days. Thc culture was harvested by filtration, the filtrate containing cellulase was conccntrated and desalted using a Biogel P6DG column. The major endoglucanase from the cellulase preparation of T. aurantiacus was isolatcd and purified to homogeneity by ion-exchange (DEAE-Sepharose). gel filtration (Ultrogel AcA 44), affinity (p-aminobenzyl cellobioside coupled to Scpharose4B) and Mono Q column chromatographics. Preparation of CM-cellulose
This was prepared by reacting alkali-treated cellulose with sodium monochloroacetate. The reaction was controIled with respect to temperature, time, and substrate to derivatising agent ratio in ordcr to obtain CM-cellulose with desired DS by sodium carboxymethyl groups.
Thermophilic bacterial and fungal endoglucanases
83
Determination of the DS of CM-celluloses This was determined by an alcalimctric method provided by Hercules Inc., Aqualon BV, Rueil Malmaison, France.
Alcohol washing to remove residual sodium chloride CM-ccllulosc (-7 g) was placed in a 500 ml narrow ncck flask and added with 350 ml aqueous methanol (85%, v/v). The flask was capped, and shaken manually, and a few drops of universal indicator added to the contents, which gave a yellow-green colour, indicating the pH to be between 6.5 - 7.5. The flask was placed on a shaking stirrer for 10 tnin and allowcd to stand for 5 min. The supernatant was tcstcd for chloridc by adding a few drops of nitric acid (65%; v/v), and a few drops of silver nitrate, before discarding. This was repeated until lhe complete elimination of residual chlorides. A final washing was pcrformed using anhydrous methanol and the CM-ccllulosc was transferred to a sintered crucible and dried under suction and in an oven at 105+1°C for at least 12 h to obtain dry and purified CM-cellulose.
Calcination and titrution A platinum cruciblc was washed with HCl (0.3 M) and deionised water, heated on a Bunsen burner, and placed in a dcsiccator to cool to room temperature. The crucible was accurately wcighcd and an accurately weighed portion of dry and purified CMccllulose (- 2 g) was placed in the crucible and charred slowly and earcfully on a pilot burncr to avoid inflammation. The crucible was then covered with a lid and the charring continued using Bunsen burner flame. The crucible and the lid were then placed in a muffle furnace at 8505 2OoC for 1 h or until all carbon had disappeared. The crucible was transferred to a desiccator and coolcd to room temperature. After cooling, the crucible and lid were weighed, transferred to a bcakcr containing 300 ml boiled deionised water and boiled for 15 min. At the end of boiling, the solution in thc crucible was cooled to room temperature and titrated with sulphuric acid (0.5 N) using methyl orange as an indicator.
Calculation of DS Thc DS was calculated using the following equation: DS = 162/[(lOOOWl/VN)-0.80],whcre W I = weight of purified CM-cellulose (g), V= sulphuric acid titrc (ml) and N = sulphuric acid conccntration (N). All samplcs wcrc analyscd in triplicatc and thc avcragc rcsulls prcscntcd.
Action of endoglucanases on CM-celluloses This was measured both by reducing sugar and viscosity methods. The experiment for cach CM-cellulose with varying DS and increasing viscosity was done separately. For mcasuring the rcducing sugars released, a 2 ml mixture containing 0.5 ml of cndoglucanase (0.25 pg protein from T. aurantiucus or 1.5 pg protein from C. thermocellum), 1.O ml of 1% CM-cellulose and 0.5 ml of 200 mM 2-(N-morpholino) ethane-sulphonic acid (MES) buffer, pH 6.0 or 50 mM sodium acetate buffer, pH 5.0
84 Application of enzymes to pulp, fibres and cellulose was incubated at 70°C for 15 - 30 min. The reducing sugar released was determincd according to Nelson-Somogyi method (5). The efrect of endoglucanase on the viscosity of CM-cellulose with varying DS and chain length was studied using Ostwald’s viscometer as follows: Endoglucanase (2.5 pg protcin from T. aurantiacus or 11.25 pg protein from C. thennocellurn) was incubated with 15 ml of 0.5% CM-ccllulosc dissolved in 50 mM MES buffer pH 6.0 or 50 mM sodium acetate buffer, pH 5.0, separately at 40°C over a period of 90 - 120 min. The time of outflow of the reaction mixture was measured at different time intcrvals and the spccific viscosity was calculated using the following equation: Specific viscosity (q) = (t-t,,)/t,,.where ‘t’ is the time taken by the reaction mixture to pass between the two markings of thc bulb of thc viscometer while ’,t‘ is the time taken by the distilled water to pass betwecn the same markings of the viscometer (6). Analysis of hydrolysis products by HPLC
This was performed using only the endoglucanase from T. aurantiacus. Initially, the hydrolysis of CM-cellulose by endoglucanase was performed at pH 3.5 and 60°C for 1 h using 0.7 ml reaction volume. The reaction was terminated at different time intervals by placing the samples at 100°C for Srnin, and the unhydrolysed CM-cellulose was precipitated using equal volumcs of propan-2-01. The hydrolysis products present in the supcrnatant wcre analysed by HPLC using an NH2-Spherisorb column (15 x 0.4 cm) with acetonitri1e:watcr(75:25) as the mobile phase.
RESULTS The DS of CM-celluloses purchased from Sigma The low, medium and high viscosity CM-celluloses purchased from Sigma had an average DS of 0.77,0.79 and 0.85, respectivcly.
Action of endoglucanases on CM-cellulose with varying degrees of substitution and viscosity Endoglucanases from C. thermocellum and T. aurantiacus rapidly decreased the specific viscosity of high viscosity CM-cellulose, followed by medium viscosity CMcellulose and CM-cellulose with degrees of substitution 0.78 and 0.93 (Figs.lA and 2A). Nevertheless, both endoglucanases showed a small effect on the viscosity of CMcelluloses with degrees of substitution 1.3 and above (Figs. 1B and 2B). These results indicated that degree of substitution rather than viscosity of CM-cellulosc influenccs the action of both endoglucanases.
Thermophilic bacterial and fungal endoglucanases 85 150-
A
C j 120 5 0 K
z
-
.3
:: 900
.> .0 k
!
6060
8
$
301 30 A
0
0
30
A 60
90
0 1
i
0
30
60
90
Time (mln)
Figure 1. Change in spccific viscosity (q) of CM-celluloses as a function of time by cndoglucanase (S 1 1 subunit) of C.thermocellum cellulosome. A, CM-cclluloses with high (0)and medium (A) viscosities; B, CM-celluloscs with DS 0.78 (O), 0.93 (A), 1.3 (H), 1.7 (0) and 2.1 (+). Each point is the mean of two assays. 600
-
400
-
200
-
B
Kelalionship between change in viscosity and the release of reducing sugars from CM-celluloses by endoglucanases The plot of change in spccific viscosity versus rcducing sugar released is used to distinguish whcther an endoglucanase is a more or lcss randomly acting enzyme (7,8). The endoglucanase (Sl I subunit) of C. thermocellum reduced the specific viscosity of high and mcdium viscosity CM-celluloscs from 138 to 40 cp and 28 to 5 cp, respectively, while releasing almost the same amount of reducing sugar (-160 pg) from both substrates (Fig. 3 A, lines I and 2). However, the determination of change in specific viscosity as a percentage of the initial specific viscosity revealcd that the enzyme reduced the specific viscosity of both CM-celluloses by 80% of their original. This suggested that the major endoglucanase of C. thermocellum cellulosomc attacked both CM-celluloses with equal efficicncy. Nevcrthelcss, the rapid decreasc in the specific viscosity of high viscosity CM-cellulose suggcstcd that the enzyme is a more randomly acting endoglucanase. Similar rcsults wcrc obtained with the cndoglucanase from T,aurantiacus.
86 Application of enzymes to pulp, fibres and cellulose Thc change in specific viscosity and rcducing sugars released by endoglucanases from C. therrnocellurn cellulosome and T. uuruntiacus were inversely proportional to the degree of substitution of CM-celluloses. Thus, with the increase in degree of substitution from C.78 to 2.1, the release of reducing sugars and change in specific viscosity of CM-celluloses by the major endoglucanase from C. therrnocellum cellulosome dccrcased rapidly and reached almost to zero (Fig. 3 A and B). These results demonstrated that the degree of substitution rather than viscosity influences the action of both endoglucanases on CM-cellulose. 150
1
A
3 120 .-2 In g .-
90
u)
Reducing sugar (pg glucose equivalent)
Figure 3. Relationship between the change in specific viscosity (71) and the release of reducing sugars from CM-celluloscs of varying viscosity and DS by the action of endoglucanase (SIIsubunit) of Cthermocellum cellulosome. A, B, symbols 0 and A correspond to CM-celluloses with high and medium viscosities, while symbols 0, A, V and correspond to CM-celluloses with DS 0.78, 0.93, 1.3, 1.7 and 2.1, respectively. Each point is the mean of two assays.
.,
+
IIPLC analysis of hydrolysis products released from CM-celluloses by endoglucanase from T. aurantiacits The enzyme released mainly cellobiose and cellotriose from all CM-cclluloses testcd. Also, the endoglucanase released noticeable amounts of cellotetraose and ccllopentaose from all CM-celluloses during the initial stages of the reaction, and thcir conccntration dccreascd with the increase in incubation time. Nevertheless, the conccntration of all cello-oligosaccharides released from CM-celluloses decreased with their increase in viscosity and degree of substitution. In fact, the level of cellobiose and cellotriose rcleascd from low viscosity CM-cellulose by T. auruntaicus endoglucanase was 2 and 8 folds higher respectively, than that rcleascd from CMcellulosc with high viscosity and dcgrcc of substitution 1.7. Also, thc lcvcl of glucose rcleascd from all CM-celluloses by this endoglucanase was negligible.
CONCLUSIONS 1. Degrce of substitution rathcr than viscosity of CM-ccllulosc influcnccs thc action of major endoglucanases from C. thermocellum and T. uurantiacus. 2. Both endoglucanascs showed similar modcs of action on CM-celluloscs and appcarcd to bc randomly acting cndoglucanascs.
Thermophilic bacterial and fungal endoglucanases
87
3. Endoglucanase from T. aurantiacus released mainly ccllobiose and cellotriose from all CM-celluloses tested, but the level of cello-oligosaccharides released, decrcascd with increase in viscosity and degree of substitution. References 1. D E Coffey, D A Bell and A Henderson, ‘Cellulose and cellulose derivatives’. In: Food polysaccharides and their applications, cd. A M Stephen, 1995 123-153, Marcel Dckker Inc. 2. C H N Sieger, A G M Kroon, J G Batelaan, and G G van Ginkel, ‘Biodegradation of carboxymethyl celluloses by Agrohucterium CM-1’. Carhohydr Poly, 1995 27 137143. 3. D V Garcia-Martinez, A Shinmyo, A Madia, and A L Dcmain, ‘Studies on cellulase production by Clostridium thermocellum’, Eur J Appl Microhiol Biotechriol, 1980 9 189 - 197. 4. M Mandcls and D Sternberg, ‘Recent advance in cellulase technology’, (1976) J Ferment Technol, 1976 54 267-286. 5. M Somogyi. ‘Notes on sugar determination’,J Biol Chem, 1952 195 19-23. 6. T M Wood and K M Bhat, ‘Measurement of ccllulasc activities’, In: Methods Enzymol, eds. W A Wood & S T Kcllogg, 1988 vol. 160 87-112, Academic Press, London. 7. T M’ Wood and S I McCrae, ‘The cellulase of T. koningii: purification and properties of some endoglucanase components with special reference to thcir action on cellulose when acting alone and in syncrgism with the cellobiohydrolasc’, Biochem J, 1978 171 61-72. ’ 8. K M Bhat, S I McCrae and T M Wood, ‘The endo-( 1->4)-P-D-glucanase system of P. pinoplzilum cellulase: isolation, purification and characterisation of five major endoglucanase components’, Carhohydr Res, 1989 190 279-297.
T l l E EFFECT OF ANTIIKAQUINONE ON WOOD CAKSO HYD RATES I)U R 1NG A L KA L1NE PULPI NG IN AQUEOUS ORGANIC SOLVENTS M.F.Kiryushina, M.I.Ermakova, A.S.Olefirenko, E.-M.Bennacer,T.G.Fcdulina, A.B.Nikandrov, M.Ya.Zarubin St.-Pelersburg Academy of Forestry, St.-Petersburg, Russia ABSTRACT The protcctive effect of anthraquinone (AQ) on wood carbohydrates during alkaline pulping of spruce sawdust (25% NaOH, 17OoC, ratio solid-liquid 1:100, 1% AQ) and the samples of kraft pulp in water and aqueous organic solvents: water-EtOH (1: I), water-MeOH (1 :I), water-dioxan (7:3), water-DMSO (7:3), (NaOH 1.25 mol/l, 17OoC,lh, ratio solid-liquid 1:20, AQ 0.016 mol/l) has been investigated. The stabilizing effect of AQ on carbohydrate redox end groups in the above mentioned media is shown on cellobiose an example. The lower content of aliphatic acids in the black liquor and the highcr yield of the wood residue (at the same degree of delignification) after cooking of spruce sawdust and kraft pulp with the addition of AQ was established. It allows confirmation that the prevention of "pccling" reactions by oxidation of aldehyde groups up to carboxyl ones occurs in the presence of AQ more extensively than in its absence. At the same time the decrease of the degree of polymerisation (DP) in nondissolved samples of krafi pulp after treatment indicates that AQ promotes the alkaline solvolysis at arbitrary sites of carbohydrate chains in all investigatcd media and that this effect is grcatcr in the solvents with the highest basicity. INTRODUCTION The use of AQ in alkaline wood delignification has attracted the attention of rcscarchcrs after the publications of Holton, who has offered AQ as a catalyst accelerating the delignification and promoting the increase of the pulp yield ( I ,2). The last fact is provided by the stabilizing of the wood carbohydrates in the presence of AQ (3-5). A large interest to organosolve pulping including thc alkaline ones is connected with the increase of the delignification rate. However, at the same time the selectivity of the process decreases because of a high degrcc of carbohydrate destruction (6). Thc aim of this research was to study the protectivc cffcct of AQ with regard to the wood carbohydrates at the heating in aqueous organic solvents with alkali. RESULTS AND DISCIISSION The samples of spruce sawdust (lignin content 27.6 Ya) were treated by aqueous sodium hydroxide (25%) at 17OoCwith and without AQ (1%) for 0-6 h at a ratio solidliquid of 1 :100. The yield, the amount of dissolved carbohydrates, the lignin content in the wood residue and thc amount of nonvolatile acids in the black liquor were dctcrmined. It was found that the wood residuc yields at the same lignin contcnt (16.5%) are 65.2% (with AQ) against 56.7% (without AQ). Simultaneously, in black
92 Pulp production and processing liquor AQ provides the smaller contcnt of nonvolatilc aliphatic acids (Fig.1). This is confirmcd by the published data on stabilization of carbohydrates in aqueous alkali. It was interesting to ascertain whether this is the case for alkaline pulping with organic solvents. Samples of kraft pulp from pinc wood (lignin content 6.3%, DP 1700) were treated in alkaline solutions (NaOII 1.25 mol/l) of aqueous organic solvents containing AQ (0.016 mol/l) and without AQ for Ih at 170OC for a solid-liquid ratio of 1:20. The yield of nondissolved wood residue and DP were determined (see Table 1). Table 1. The pulp yield and degree of polymerization after alkaline treatment of pine kraft pulp samples in water-organic solvent systems with and without anthraquinone. I
Water-organic solvent systcm
Degree of polymerization, DP with AQ without AQ 840 960 410 460 460 500 400 460 220 220
water watcr-methanol(1:1) watcr-ethanol(1: 1) water-dioxan (7:3) water-DMSO (7:3)
.
Pulp yield, % with AQ 89.6 78.9 82.4 85.0
67.8
without AQ 79.9 66.6 69.8 71.6 59.1
It is shown that in all tested systems: water-EtOH (l:l), water-McOH (I:]), waterdioxan (7:3), water-DMSO (7:3) as well as during pulping in aqueous alkali without solvent, AQ prevents the dissolution of carbohydrates. The yield of the fibrous product after heating with AQ is approximately 10-14% higher than without it (scc Fig. 1). AQ promotes the delay of "peeling" rcactions. This is confirmed by the data on cellobiose cleavage by aqueous NaOH (OSmol/l) in all the above mentioned media during diffcrent times (SCC Fig. 2) in the prcscncc of AQ the proccss occurs slowly. In the
-
0
100
200
300 400 Time, min
-
Figure 1. Dissolution of carbohydrates (% to dry wood weight) 1,2 and change of nonvolatile acids content 3,4. 12- without AQ; 2,4 with AQ.
-
-
Effect of anthraquinone on wood carbohydrates 93 %
100
90 80 70 60 50 40
30 20 10
0 20
0 '
40
60 Time, min
-
-
Figure 2. Degree of cellobiose cleavage (YO):1,2 in water; 3,4 in water:MeOH; 5,6 in water:dioxan; 7,8 in water:DMSO. 1,3,5,7 without AQ; 2,4,6,8 with AQ.
-
-
-
-
absence of organic solvents and heating for 40 min , we observed a stabilization of the glucosidic bond to cleavage. The "peeling" reaction stops and it results in an increase of thc fibrous product yield. Concerning DP, we observed an opposite picture. On the presence of AQ, the DP of the fibrous residue is lower than without it (see Table 1). In water without solvent, AQ decreases the DP more than in water-solvent systems. However, in systems with high basicity as water-DMSO (7:3),the solvent effect covers that of AQ (scc Table 1). The alkaline solvolysis rate of glucosidic bonds in these mcdia is very high (7). The bond cleavage in arbitrary sites of carbohydrates results in DP reduction. Apparently, AQ promotes this process as it can oxidize primary and secondary alcoholic groups (8). AQ oxidizes them to carbonyl groups neighboring with the glucosidic groups and creates the additional possibility for the cleavage of carbohydrates chains through the carbonylic mechanism. This promotes the decrease of the product degree of polymerisation. CONCLUSION The addition of AQ at alkaline pulping in water and in aqueous organic solvents results in a reduction of the "peeling" reactions of the carbohydrate end groups, but docs not prevcnt and even promotes the cleavage of glucosidic bonds in arbitrary sites of the carbohydrate chains. In solutions with high basicity the effect of AQ is masked by the effect of the medium.
94
Pulp production and processing
REFERENCES
H H Holton and F L Chapman, ‘Kraft-pulping with anthraquinone’, TAPPI Journal, 1977 60(11) 121-125.
H IIolton, ‘Better cooking with anthraquinone’, Pulp and Paper Internat, 1978 20(9) 49-52.
L Lowcndahl and 0 Samuelson, ‘Carbohydrate stabilization during krall cooking with addition of anthraquinone’, Svenskpapperstidn, 1977 SO( 17) 549-551. L Lowendahl and 0 Samuelson, ‘Carbohydrate stabilization with anthraquinone during pulping’, Polymer Bull., 1978 l(3) 205-210. L Lowendahl and 0 Samuelson, ‘Carbohydrate stabilization during soda pulping with anthraquinone’, TAPPI Journal, 1978 61(2) 19-21. M F Kiryushina, M I Ermakova, E M Bennacer, A S Olefirenko and M Ya Zarubin, ‘Ccllulose destruction in solutions with high basicity’, Wood Chemisrry (in Russian), 1991 (1) 38-42. E M Bennacer, M F Kiryushina and M Ya Zarubin, ‘Effect of organic solvents on cleavage of beta-alkyl-O-arylic bond kinetics’, 5”’ ISWPC, Ralleigh, 1989. A F Wallis and R H Wearne, ‘Oxidation of monohydric alcochols with anthraquinone and its derivatives under soda pulping conditions’, Journal Wood Cliem. and Teclzol., 1987 7(4) 5 13-525.
TWO PHASE EQUILIBRIA OF METAL IONS IN PULPING UNIT OPERATIONS: FROM IMPREGNATION TO OXYGEN BLEACHING J. Karhu, P. Snickars, L. IIarju and A. Ivaska Laboratory of Analytical Chemistry, Process Chemisty Group, Abo Akademi University, Biskopsgatan 8, FIN-20500 Turku-Abo, Finland
ABSTRACT Two phase equilibria of metal ions at diffcrent steps of the pulping process were studied. Pulp samples were taken from different process steps in a modern Finnish kraft pulp mill, that produces batch cooked kraft pulp from a mixture of coniferous trecs. The concentrations of Na, K, Ca, Mg, Zn, Fe, Mn, Al, Ba and Si both in the liquid and the fiber phases were dctermincd mainly using ICP-MS and DCP-AES. Also wood chips were analyzed for metal ions. Conditional distribution coefficients wcre determined for sevcral mctal ions in different pulp samples. The metal ions can be arranged in affinity orders at the different steps of the pulping process. INTRODUCTION The interest in the ion exchange propcrtics of kraft pulps has strongly grown during the 1990's as totally chlorine free (TCF) bleaching processes have become more common in pulping industry. Some cations, especially transition metal ions such as manganese, iron, cobalt, copper, nickel and chromium have disturbing effect on the TCF process Other metal ions, such as magncsium and calcium have protecting effect on the fibers in alkaline bleaching with oxygen-based chemicals High concentrations of Na and the non-process elements (NPE) K, C1, Al, Si, Ca, P and Mg can be found in spent process liquors. NPEs can cause problems in the recovery processes. Effluents containing nitrogen, phosphorus and heavy metal ions are detrimental to the environmcnt 3*4. Both desorption of metal ions from pulp and wood chips and sorption of metal ions on pulp fibers have reccived increased interest when the demand to close pulping processes has increased. Furthermore, in certain process steps where thc pH is low thc metal ions may desorb from the fibers and precipitate in the following process steps whcre pH is increased. Thc dcposits are one of the biggest obstacles when closing the processes. Most of the metal ions present in the pulping processcs originate from the wood used as raw material. Some of the metal ions e.g. sodium and calcium come mainly from the chemicals used in the mill. On the other hand also the raw water used and corrosion of equipment increase the metal ion content in the process solutions, The main mechanism for sorption of metal ions to wood and pulp is assumed to be cation exchange 5-8. The main binding sites are the carboxylic groups. Their number in pulps is usually rather low, ca 40 mmol/kg in unbleached and oxygen blcached softwood kraft pulp '. Other potential sites for ion exchange are phenol groups and lignosulfonic groups in sulphite pulps lo. Grubhofcr ' I gives a thorough description of ion exchange resins based on native cellulose.
'.
96 Pulp production and processing Metal ions bound to the functional groups in wood and pulp can be desorbed by sequestration with strong chelating agents like EDTA and DTPA or by stripping with acidic solutions. Both desorption of metal ions from wood chips or pulp and the sorption of metal ions on pulp fibers need to be understood when the water circulation in pulping processes becomes more closed.
EXPERIMENTAL Sampling Samples were taken from a Finnish kraft pulp mill working in .the batch mode. A mixture of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) was used as raw material. The actual mixture at the time of sampling consisted of approx. 25 % of pine chips, 35 % of spruce chips and 40 % of softwood chips from saw mills. The sampling sites at the mill are shown in Figure 1. The process delays were considered in sampling in order to have the same original raw material. The washers mentioned in the figure are wash presses. Pulp samples from two batch cooks were studied. However, it was possible to get only one sample from the cooked unwashed pulp. This sample was also diluted to some degree with tap water in the sampling device. The metal concentrations of this sample are therefore not presented in this work.
Wood chips
I
Impregnation
I
Batch cook Pulp suspension out
Wash liquor Pulp suspension in Pulp out
*I
2nd postox washer Pulp suspension in Pulp out
Figure 1. Sampling sites at the pulp mill.
Filtrate out I
I
Wash liquor to the 2nd shower Filtrate out
Two phase equilibria of metal ions 97 Metal concentrations were determined both in the dried fibres (or chips) and in the liquors. The pulp samples were taken as such or filtered if the sample consistency was lower than ca. 20 %. The liquor samples were taken in plastic bottles and filled to maximum volume. The fiber and liquor samples were stored in the freezer (-18 "C) or in the refrigerator (5 "C).
Sample preparation The wood chip samples and the fiber samples were dried at 105 OC in order to determine the metal ion concentrations on dry wood (d.w.) or dry pulp (d.p.) basis. The samples were ground to a suitable particle size using a Cyclo-Tec (Tecator Inc.) cutter mill. The solid samples (500 mg) were digested in a mixture of HNO3 (5 ml) and H202 (2 ml) by the microwave oven technique before the analytical determinations 12. Impregnation experiments were made in the laboratory with the same chips and liquors that were used in the mill.
Instrumental techniques Concentrations of Na, K, Ca, Mg, Zn, Fe, Mn, Al, Ba and Si in the sample solutions were mainly determined using inductively coupled plasma mass spectrometry (ICP-MS) and direct current plasma atomic emission spectroscopy (DCP-AES). For ICP-MS a Perkin-Elmer Elan 6000 (PE Sciex, Toronto, Canada) instrument and for DCP-AES a Spectraspan IIIB instrument (Spectrametrics Inc., Andover, MA) were used for the elemental analyses. The concentrations of some elements were determined both by ICPMS and DCP-AES for the control of the reliability of the used method. A more detailed description of the methods that can be used for determination of metal ions in wood related materials and pulping solutions has been given by Ivaska and Harju If.
RESULTS AND DISCUSSION Metal ion content in wood chips, pulps and process solutions The metal ion concentrations in the dried wood chip and pulp samples are presented in Table 1. The numbers 1 and 2 after the different process steps in the tables represent samples taken from two different batches in the same point in the process. The fiber mat after filtration including the solution in it is regarded as the solid phase l4,I5. Main cations in wood chips are Ca, K, Mg and Na, and their concentrations exceed 100 mgkg dry wood (d.w.). Other metal ions included in the table are in the concentration range 10 - 50 mgkg d.w. The Si concentration in the impregnated chips varied strongly and therefore two values are shown in the table. More information about the metal ion content in wood can be found in the literature 16-18. As could be expected, the concentration of the main component sodium increases clearly in the impregnation stage - from slightly over 0.1 to over 6 g k g dry pulp (d.p.). The measured sodium concentration is at its highest, ca 77 g/kg d.p., in pulp that is pumped to the second brown stock pulp washer. In this washer the sodium concentration decreases down to ca 35 g/kg d.p. Pulp that goes to the second washer after the two oxygen bleaching stages has a lower sodium concentration (ca 3.2 g k g d.p.). This value remains constant in the washing operations. A similar concentration pattern as for sodium can be found for potassium. The increase in potassium Concentration in unbleached pulp is about 60-fold, if compared with the content in wood chips. The magnesium concentration in pulp- is about twice the
98 Pulp production and processing concentration in the wood raw material. The calcium concentration increases from ca 600 mgkg in wood chips to 1200 - 1500 mgkg in unbleached pulp. For the other metal ions the variations are relatively small and generally a decrease in the metal concentration in pulp can be observed during the pulping process studied. The metal ion concentrations determined in process solutions are given in Table 2. The pH values of the different steps are also included. The highest sodium concentration, ca 40 g/l, was found in the impregnation liquors and it is the highest of all the ions in the process solution studied in this work. The sodium concentration, however, decreases radically down to the level of 1 g/1 in the solution after the first washing of the oxygen-bleached pulp. Similar trends can also be seen for potassium. A distinct decrease in concentrations of the other metal ions can be observed for process solutions at the same process step. This is due to addition of cleaner water to this process stage.
Table 1. Metal contents ( m a g ) in dry wood chips and pulps from different steps in the pulping process. b.d.1. = below detection limit. "a1 138 102 6440 6130 78300 75500 36800 32600 3480 2950 3530 3790
[KI 123 125 1030 990 8770 6430 4220 3710 114 112 61.1 71.9
597 572 661 579 1340 1490 1220 1330 1100 1150 1020 1120
[Mgl 108 131 69.3 75.0 256 224 163 168 255 25 1 215 223
[Znl 15.3 3.71 6.58 5.96 21.6 17.0 29.4 16.7 17.2 15.7 17.6 23.7 -
Wood chip mixture, 1 Wood chip mixture, 2 Chips from lab. impregnation, 1
[Fel [Mnl [All 10.7 53.9 9.62 3.74 58.3 b.d.1. 1.10 80.3 28.1
Pal 11.1 8.56 6.64
Chips from lab. impregnation, 2
1.15
61.9
24.3
4.27
Pulp to 2ndpreox washer, 1 Pulp to 2ndpreox washer, 2 Pulp from 2ndpreox washer, 1 Pulp from 2ndpreox washer, 2 Pulp to 2ndpostox washer, 1 Pulp to 2"dpostox washer, 2 Pulp from 2ndpostox washer, 1 Pulp from 2nd postox washer, 2
24.1 20.5 43.7 19.4 39.7 28.2 27.2 20.6
46.1 41.9 36.1 36.3 23.2 21.6 20.7 20.8
35.6 29.5 29.6 22.2 26.6 21.7 23.3 20.9
7.15 8.10 6.43 7.67 9.49 9.84 9.25 9.46
Pi1 23.3 2.04 8391 66.1 245 I 144 243 206 116 119 177 179 126 137
Wood chip mixture, 1 Wood chip mixture, 2 Chips from lab. impregnation, 1 Chips from lab. impregnation, 2 Pulp to 2ndpreox washer, 1 Pulp to 2ndpreox washer, 2 Pulp from 2ndpreox washer, 1 Pulp from 2ndpreox washer, 2 Pulp to 2ndpostox washer, 1 Pulp to 2ndpostox washer, 2 Pulp from 2ndpostox washer, 1 Pulp from 2ndpostox washer, 2
[Gal
Table 1 continues.
Liquor to impregnation, 1 Liquor to impregnation, 2 Liquor from lab. impregnation, 1 Liquor from lab. impregnation, 2 Liquor from lab. impregnation without wood chips, 1 Liquor from lab. impregnation without wood chips, 2 Solution in pulp suspension to 2ndpreox washer, 1 Solution in pulp suspension to 2ndpreox washer, 2 Washing filtrate to Znd preox washer, 1 Washing filtrate to 2ndpreox washer, 2 Filtrate from 2"dpreox washer, 1 Filtrate from 2ndpreox washer, 2 Solution in pulp suspension to 2"dpostox washer, 1 Solution in pulp suspension to 2"dpostox washer, 2 Last washing filtrate to 2ndpostox washer, 1 Last washing fiItrate to Znd postox washer, 2 Filtrate from Znd postox washer, 1 Filtrate from 2ndpostox washer, 2
13.3 13.2 12.9 12.9 13.3 13.4 12.9 12.9 10.3 10.4 12.8 12.9 9.7 9.7 10.6 10.8 9.7 9.7
PH "a1 41800 42500 30100 32900 40700 38100 18200 18100 0600 0600 6000 8200 1110 1140 1040 1050 1290 1290
Table 2. Metal ion concentrations (mgn) in different process solutions. [KI 6080 6330 4330 4880 5750 5400 1720 2210 1160 1120 2110 2270 47.3 50.5 22.0 20.1 28.6 29.2 [Ca] 46.2 54.3 115 122 85.5 106 46.6 42.4 24.4 41.4 32.0 46.9 4.55 4.54 4.67 2.93 4.95 5.42
[Mg] 42.7 40.6 40.6 38.1 43.5 37.1 37.7 48.0 46.7 47.9 27.2 55.3 13.6 13.1 3.87 3.53 14.1 13.4
0.03 0.27 0.28
[Zn] 1.17 2.91 5.94 4.40 31.4 32.9 2.06 1.91 4.13 4.00 3.16 4.13 0.33 0.33 0.03
[Fel 4.79 19.2 4.75 10.9 23.9 33.9 2.95 2.48 3.78 1.84 2.27 3.74 0.19 0.19 0.08 0.07 0.21 0.36
[Mnl 13.7 15.1 8.67 17.8 12.5 12.8 6.74 6.40 4.21 7.03 4.12 2.83 0.81 0.81 0.06 0.07 0.73 0.72
[All 23.1 27.8 13.4 23.3 43.3 32.8 4.46 5.13 6.38 4.35 4.72 6.59 0.49 0.48 0.10 0.08 0.47 0.42
[Bal 1.64 1.24 2.93 1.99 3.76 3.09 1.24 1.27 1.48 1.36 1.37 1.42 0.24 0.26 0.41 0.40 0.30 0.30
13.7 13.7
10.1
[Sil 125 149 102 152 126 127 36.4 47.4 24.1 25.0 33.5 46.8 13.7 13.4 10.0
5
5-
EL
9
B
a
g-
5
f.
(D
g
"3
4
3
100 Pulp production and processing Determination of conditional distribution coefficients, DM Distribution of a metal ion, M”’, between the solid and the aqueous phase can be described by the conditional distribution coefficient, DM’. It is used in this work as defined by Ringbom l9 according to the following equation:
DM =[M’],/[M’] where [M’], is the total analytical concentration of the metal ion in the solid phase and is expressed as molkg d.p. The total concentration of the species containing the metal ion in the aqueous phase, [M’]. is given as mol/l. The distribution coefficient defined in equation [l]is a kind of conditional constant, which strongly depends on pH and other experimental parameters in the two-phase system. Table 3 gives the results of the dctermination of the conditional distribution coefficients, DM’,for several metal ions in different steps of the pulping process in the particular mill studied. The pH values of the solution phase are also included. DM’ values are also given for impregnated wood chips, but they cannot directly be compared to the DM’values reported for pulp samples. The values of DM’are low indicating a relatively weak binding of metal ions to pulps. Large variations of DM’can be observed for most metal ions at different stages of the process. Generally the value of DM’ is increasing towards the end of the pulping process. This is mainly explained by the decrease in pH during the process from pH ca 13 to 10. At pH around 13 there is a strong interference of hydroxide ions on the reactions of transition metal ions, resulting in decrease in the value of the conditional distribution coefficient. Also precipitation of metal hydroxides, silicates and other salts can take place. The alkali metal ions are an exception. These metal ions form very weak hydroxo complexes at very alkaline conditions. In spite of the great variations of the concentrations of e.g. Na and K in the two-phase system the conditional distribution constants are surprisingly constant. The conditional distribution coefficients can also be used for comparison of the binding strength of different metal ions to pulp in a batch with constant pH and other chemical conditions, i.e. in different steps of the pulping process. Due to the small number of functional groups in the pulp, all metal ions probably have the same stoichiometry for the ion exchange reaction. This holds at least for cations of thc samc valency. The following affinity order for different metal ions can be found for the two oxygen bleached pulp samples that were taken from pulp pumped to the 2”dpostox washer: C a > Fe > Al, Z n > B a > Mn > Mg > Si > N a> K Conditional distribution coefficients can be used for theoretical studies of metal ion desorption. Metal ions can be dcsorbed from pulps by addition of chelating agents. The effect of these agents on the sorption processes can easily be considered by so-called side-reaction coefficients (or-coefficients) introduced by Ringbom 19. A complete desorption (99.9 %) of a metal ion is achieved if DM’< 10” V/m
131
where V is the volume of the aqucous phase and m the mass of the solid phase.
Two phase equilibria of metal ions
101
Table 3. Log D' for the diffcrent samplcs. pH Chips from lab. impregnation, 1 Chips from lab. impregnation, 2 Pulp to preox washer, 1 Pulp to 2"d prcox washer, 2 pulp from 2"d preox washcr, 1 Pulp from Pd preox washer, 2 Pulp to 2ndpostox washer, 1 Pulp to 2ndpostox washer, 2 Pulp from postox washer, 1 Pulp from 2"d postox washer, 2
Na
K
Ca
Mg
Zn
Fe
Mn
A1
Ba
Si
12.9 -0.67 -0.63 0.76 0.23 0.04 -0.64 0.97 0.32 0.36 0.921 -0.19 12.9 -0.73 -0.64 0.68 0.29 0.13 -0.98 0.54 0.02 0.33 0.211 -0.02 12.9 0.63 0.71 1.46 0.83 1.02 0.91 0.84 0.90 0.76 0.82 12.9 0.62 0.46 1.55 0.67 0.95 0.92 0.82 0.76 0.80 0.64 12.8 0.36 0.30 1.58 0.78 0.97 1.28 0.94 0.80 0.67 0.54 12.9 0.25 0.21 1.45 0.48 0.61 0.71 1.11 0.53 0.73 0.41 9.7
0.50 0.38 2.38 1.27 1.72 2.32 1.46 1.73 1.60 1.11
9.7
0.41 0.35 2.40
9.7
0.44 0.33 2.31 1.18 1.81 2.11 1.45 1.70 1.49 0.96
9.7
0.47 0.39 2.32
1.28 1.68 2.17
1.43 1.66 1.58 1.13
1.22 1.93 1.76 1.46 1.70. 1.50 1.00
CONCLUSIONS The results of the present work givc detailed information on the metal ion profilcs in a kraft pulp mill and show how the metal ion concentrations change in different process steps. Impregnated chips, batch-cooked pulps, washed unbleached pulps and oxygenbleached pulps were studied. Also the incoming wood materials were analyzed. Conditional distribution coefficients wcre determined for several metal ions for pulp samples from different steps in the pulping process. One main factor affecting the values of DM and thus the binding strcngth of metal ions to pulps is pH. The main pH governed reactions arc formation of metal hydroxo complexes in the aqueous phase and protonation of the functional groups in the fiber phase. The metal ions can be arrangcd in the order of affinity for the different pulp batches studied. Conditional distribution coefficients can be useful for theoretical optimization of the desorption of metal ions from pulp with complexing agents. Howevcr, the ion exchange reactions in pulps are very complicated if compared for instance with synthetic cation exchangers. Further research is still nceded for a better undcrstanding of these processes.
ACKNOWLEDGEMENTS The financial support of The Finnish Technology Development Centre (TEKES), Ahlstrom Machinery Corporation, Enso Group and Metsii-Rauma Oy is gratefully acknowledged. We also thank Tech. Lic. Paul Ek for performing the ICP-MS analyses.
102 Pulp production and processing REFERENCES 1. 2. Yuan, M. D’Entremont, Y. Ni and A. R. P. van Heinigen, ‘The role of transition metal ions during peracetic bleaching of chemical pulps’, Pulp & Paper Canada, 1997,98(1 l), T408-T413. 2. 0. Dahl, J. Niinimai, T. Tirri, A.-S. Jabkelainen and H. Kuopanportti, ‘Bleaching softwood kraft pulp: The role of certain common chemical elements in the peracetic acid stage’, Tappi Pulping Conference, San Francisco, California, Tappi Press, Atlanta, 1997, pp. 1061-1067. 3. P. Ulmgren, “on-process elements in a bleached kraft pulp mill with a high degree of system closure - state of the art’, Nordic Pulp Paper Research Journal, 1997, 12(1), 32-41. 4. T. Krantz, ‘Kretsloppsanpassning i massabruket - var stir man?’, Svensk Papperstidning/Nordisk Cellulosa 1998, 101( l), 32-33 (in Swedish) 5. E. Sjostrom, J. Jansson, P. Haglund and B. Enstrom, ‘The acidic groups in wood and pulp as measured by ion exchange’, J. Polym. Sci, 1965, C1 1,221-241. 6. P. S. Bryant and L. L. Edwards, ‘Cation exchange of metals on kraft pulp’, J. Pulp Pap Sci, 1996,22(l), J37-542. 7. G. Eriksson and U. G r h , ‘Pulp washing: sorption equilibria of metal ions on kraft pulps’, Nord Pulp Pap Res J., 1996, 11(3), 164-176. 8. S. M. Abubakr, B. F. Hrutfiord, T. W. Reichert and W. T. McKean, ‘Retention mechanism of metal ions in recycled and never-dried pulps’, Tappi J., 1997, 80(2), 143-148. 9. J. Karhu, P. Forslund, L. Harju and A. Ivaska, ‘Characterization of carboxyl and phenol groups in kraft pulps at different temperatures’, paper in this Proceedings, 1999. 10. T. Lindstrom, ‘Chemical factors affecting the behaviour of fibres during papermaking’, Nordic Pulp Pap. Res. J., 1992,7(4), 181-192. 1 1. N. Grubhofer, Cellulose ion exchangers, in Zon Exchangers, Ed. K . Dorfner, Walter de Gruyter, New York, 1991, pp. 443-460. 12. Anonymous, Milestone application notes for microwave digestions, Milestone Application Lab, Pergamon, Sorisole, 1995. 13. A. Ivaska and L. Harju, Analysis of inorganic constituents, Analytical Methods in Wood Chemistry, Pulping and Papemaking, Eds E. Sjostrom and R. Altn, SpringerVerlag, Berlin, 1998, pp. 287-304. 14. P. S. Bryant, Transition metal measurement and control in closed kraft mills with hyrogen peroxide bleach lines, Doctoral dissertation, Ann Arbor, Michigan, University of Michigan, 1993. 15. R. Jkvinen and 0. Valttilii, ‘A practical method for studying NPEs in a kraft mill’, 1998 International Chemical Recovery Conference, Tampa, Florida, Tappi Press, Atlanta, 1998, Vol. I , 107-1 16. 16. L. Harju, K.-E. Saarela, S.-J. Heselius, F. J. Hernberg and A. Lindroos, ‘Analysis of trace elements in trunk wood by thick-target PIXE using dry ashing for preconcentration’, Fresenius J. Anal. Chem., 1997,358(4), 523-528. 17. P. Koch, ‘Utilization of hardwoods growing on southern pine sites’, US Dep. Agric. For. Serv., 1985, 1,438-445. 18. P. Koch, ‘Lodgepole pine in North America’, Forest Product Society, Madison, Wisconsin, 1996, Vol. 2,621-638. 19. A. Ringbom, Complexation in Analytical Chemistry, Wiley, New York, 1963.
CATALYSIS OF OXYGEN-ACETONE DELIGNIFICATION Ivan Deineko and Inna Deineko StPetersburg Forestry Academy, Institutsky per.5, 194021,St.Petersburg, Russia, E-ma*
[email protected]
ABSTRACT A catalyst for oxygen-acetonedeligniflcationhas been suggested. The research has been carried out on spruce sawdust. Use of the catalyst in wood oxidation by oxygen in 60 % acetone allows transfer to the solution of about 90 % lignin during the reaction time of 24 hours at the reaction temperature of 120°C. Kinetics of delignification and influence of the catalyst concentration (0.2-2.0 g/l, liquid-to-wood ratio 25, P0(Oz) 0.7 MPa) on dissolution of wood components have been investigated. The rate of delignification in the catalytic process (120°C) is an order of magnitude higher than the rate of lignin dissolution in the process carried out in the absence of the catalyst.
INTRODUCTION The process of oxygen-organosolvdelignificationof wood started to be developed in StPetersburg Forestry Academy about ten years ago. The oxygen-organosolv process (oxysolvolysis) consists of processing wood raw material by oxygen in water-organic solutions under high temperature (140-160°C). The detailed researches have shown that this process has essential advantages compared to traditional and newly developed pulping processes /I/. The advantage of oxysolvolysis in comparison with other organosolv processes is the possibility to obtain pulp fiom both hardwood and softwood. In comparison with oxygen-alkaline pulping the offered process does not require application of alkalis and allows use of wood chips as raw material. However, oxygenorganosolv pulping is relatively low quality of cellulose resulted. The rather low mechanical properties of the fibrous material are apparently associated with a strong destroying action of oxygen on cellulose at high temperatures. A possible way to decrease the role of undesirable reactions resulting in cellulose destruction is to use selective catalysts of delignikation allowing one to carry out the process under softer conditions. An effective catalyst for oxygen-organosolvpulping has been recently offered /2/, and results here indicate there is an opportunity to develop a catalytic pulping process. In the present work, the results of preliminary researches on the use of new catalystsof oxysolvolysis are given.
EXPERIMENTAL Spruce sawdust (0.25-0.50 mm) was used as substrate. The sawdust contained extractives 1.1% (diethyloxide) and Klasson lignin 28.1%. Oxidation of wood by oxygen was conducted in a 1 L rocking autoclave (50 min-'). Sawdust (10 g o.d.), a catalyst and 250 ml of 60% (vol.) acetone were placed in the autoclave and then oxygen (0.7 m a ) was introduced into the autoclave. The time to temperature (1 18'C) was 60 min. After
104 Pulp production and processing the oxidation, the wood residue was separated from the solution on a filter, washed, oven-dried (1 03'C) and weighed. Klasson lignin was again determined.
RESULTS AND DISCUSSION To study the action of catalysts, the reaction conditions were chosen so that the degree of deligniiication in the absence of a catalyst was insignificant. The mixture of two inorganic compounds A and B were used as catalysts. Results given in Table 1 indicate that the compounds A and 13 showed a certain catalytic activity. Table 1. The influence of catalysts on the degree of dissolution of wood componcnts during oxysolvolysis (1 18"C, 2 h) Catalyst
Concentration of catalyst, mM/L No 0 A 8.0 B 8.0 8.0 A( 1)B(2) A(5)B(2)* 17.0 * Temperature was 121'C
Residue yield,
Lignin content,
% 94.0 89.1 81.2 73.3 53.4
% 27.4 27.0 18.0 17.6 5.6
Dissolved substances, % from initial Lignin Carbohydrates 8.3 3.7 14.4 8.1 48.0 6.1 54.1 14.7 28.8 89.4
Probably, that delignification process with catalysts is followed according to the next scheme:
+ +
Lo + Ox, L'o +Redl L'o + O2 products Red1 + 0 x 2 0 x 1+ Red2 Red2 + 0 2 90x2 where LOand L'o are, respectively, native and oxidised lignins Oxl and Red, are, respectively, oxidised and reduced forms of compound A 0x2 and Red2 are, respectively, oxidised and reduced forms of compound B. The results testify that the efficiency of the chosen catalysts differ appreciably. In the presence of compound A, the rates of transformation of both lignin and carbohydrates grow. Nevertheless, the rates of their dissolution increase insigniticantly. The compound B strongly increases the rate of lignin oxidation not rendering essential influence on the dissolution of carbohydrates. The greatest effect is reached with joint introduction of these two compounds (AF3). As the data obtained show, the catalytic activity of the AB system is observed at various mass proportions of the compounds used. An increase in the concentration of the catalytic system up to 17 mM/L allows removal of almost 90 % of lignin. The rate of dissolution of wood components largely depends on the conccntration of the catalyst in a solution. The influence of the concentration of the catalytic system on the dissolution of lignin and carbohydrates was investigated with an equal molar ratio of both substances in a mixture. Data given in Table 2 show that
Catalysis of oxygen-acetone delignification 105 acceleration of the oxidising processes is observed even with addition of small amounts of the catalyst to the solution. Table 2. The influence of the AB (1:l) concentration on sawdust dissolution (1 18'C, 2 h) Concentr ation of
Residue yield, %
Lignin content, %
94.0 88.6 85.1 83.0 82.0 78.0 76.2 72.6
27.4 25.2 24.2 23.0 24.2 22.7 21.3 18.1
AB,
mM/L 0 3.2 4.0 5.0 6.0 8.0 10.0 12.0
Figure 1 shows that the dependence of the amount of the dissolved wood components on the catalyst concentration in solution is linear and can be expressed by the following equation:
Y=P+QX where Y is the mass portion of the substrate (lignin, carbohydrates) dissolved in the presence of the catalyst, P is the mass portion of the substrate dissolved in a noncatalytic process, X is the concentration of the catalyst, Q is a constant.
50 40
30 20
10
0
I
I
I
I
I
I
2
4
6
8
1 10
12 mM/L
Figure 1. The dependence of the degree of dissolution on the concentration of AB (1:l) in solution (1 1 8"C,2 h). 1 - Lignin, 2 Carbohydrate
-
106 Pulp production and processing Factor Q in the given equation reflects the catalytic activity of the catalytic system used and can serve as a criterion of the efficiency of the catalysts. In the system investigated, it is equal to 0.06 LImM for lignin dissolution and 0.005 L/mM for dissolution of carbohydrates. These data testifL that the catalyst demonstrates considerably large catalytic activity in lignin substrate reactions as compared with carbohydrate substrate ones. The kinetics of the process have also been investigated and experimental data reflecting dependence of the delignification degree fiom the reaction time are given in Table 3. Table 3. The influence of oxysolvolysistime on sawdust dissolution (1 1S0C,2 h) in the presence of Al3 (SmM, 1 :1) Oxysolvolysis time
Residue yield
Lignin content
YO
%
91.4 81.9 78.0 66.5 55.6
26.1 24.2 22.7 15.8 6.9
m i n '
0 30 60 120 240
For kinetic curve construction, the results of cxperiments conducted under isothermal conditions were used. Therefore, the contents of wood components determined in the wood residues after achievement of the final temperature (oxysolvolysis time 0) have been taken as their initial contents in the substrate. Figure 2 shows that the delignification process is satisfactorily dcscribcd by a first order mathematical equation. The reaction rate constant of the catalytic process (1.3-104s-') conducted at the catalyst concentration of 8 mM/L is more than five times higher than the reaction rate constant of non-catalysed process (2.4.10-5s-'). The process rate can be increased in magnitude by an order and more (see Table 1) with an increase in the catalyst concentration.
I
0
100
200 Time. min
300
-In YNo 2.5 1
100 200 Time. rnin
300
Figure 2. Kinetics of dissolution of lignin (1) and carbohydrates (2)during oxysolvolysis (1 1S'C, AB 8mM/L. 1:1): a - kinetic curve, b - semilogarithmic anamomhoses
Catalysis of oxygen-acetone delignification 107 The kinetic data also show that the rate of the carbohydrate dissolution expressed through the rate constant of a first order reaction (3.10”s-’) is much lower than the rate of lignin dissolution, and an increase in the rate of carbohydrate destruction with introduction of the catalyst into solution is rather insignificant. The results indicate the possibility of obtaining pulp by the oxysolvolysis method with use of the catalytic system suggested. The purpose of research in the near future will be to check this conclusion.
REFERENCES 1. Deineko I. Oxysolvolysis of lignocellulosic materials. Fourth Brazilian Symposium on the Chemistry ofLignins and other Wood Components. 1995, V.5,5-10. 2. Evtuguin D.V., Net0 C.P., Marques V.M. Delignification by oxygen in the presence of polyoxometalates: mcchanism proposal and possible application. 9Ih International Symposium on Wood and Pulp Chemistry. Poster Presentation. 1997,25-1 - 25-4.
CHARGED GROUPS IN WOOD AND MECHANICAL PULPS Bjame Holmbom, Andrey V. Pranovich, Anna Sundberg and Johanna Buchert a b o Akademi University, Laboratory of Forest Products Chemistry, FIN-20500 Turkulabo, Finland
ABSTRACT Analysis by acid methanolysis and gas chromatography revealed that galacturonic acid, which is the main sugar unit in pectin, was the most abundant uronic acid in Nordic spruce, pine, birch and aspen wood, as well as in mechanical pulps. However, since most of the galacturonic acids are methylesterified in native wood, the 4-0-methyl glucuronic acid in xylan is the main charged group in wood and in mechanical pulps. Mechanical pulp fines were found to contain much more galacturonic acid, rhamnose, aiabinose and galactose, and slightly more xylose and methylglucuronic acid, than the long fibre fractions. Alkaline treatment of mechanical pulp more than doubled the fibre charge, and alkaline peroxide bleaching produced still more new acid groups. Charged groups were formed in the fibres with the same kinetics as the release of methanol and acetic acid. It was concluded that the new acid groups originate mainly from demethylation of pectin, and in case of peroxide bleaching also from oxidation of lignin. INTRODUCTION Charged groups are key functional groups in papermaking fibres. They are important both for the various chemical interactions with fibres in papermaking, and for the properties of the paper. Many paper chemicals are cationic and are sorbed to the fibres through interactions with the anionic groups, and the performance of e.g. retention aids and wet-end sizes is strongly affected by the amount of charged groups. Anionic groups interact with metal cations by ion exchange both in pulping, bleaching and papermaking processes, leading to fibre swelling and softening, and consequently strongly affecting many fibre properties (1). Table 1 lists the various charged groups that can occur in wood and mechanical pulps. Table 1. Charged groups in wood and mechanical pulps. Chemical group Carboxyl, uronic acid oxidised lignin fatty and resin acids Phenolic Hydroxyl Sulphonic acid Protein: both carboxyl and amino groups
Structure R-CO2H Ar-OH R-OH R-SO3H R-CO2H R-NH2
Acid constant, PKA 3.5 - 4 ca. 5 5.5-6.4 ca. 10 >12 ca. 1
110 Pulp production and processing Most charged groups in wood and mechanical pulps are carboxyl groups of uronic acids which are units of certain hemicelluloses and pectins (2, 3). Extractives such as fatty and resin acids also contain carboxylic acids. Carboxyl groups in uronic acids have acid constants of 3.5-4,whereas the carboxyl groups in oxidised lignin and in fatty and resin acids have constants in the range 5-6.5 (4,5). Alkaline treatment of mechanical pulps, as in peroxide bleaching, leads to the formation of new carboxyl groups (2,6). We have recently presented evidence that the new carboxyl groups are formed mainly by demethylation of galacturonic acid methyl ester groups in pectins (7, 8). Lignin phenolic groups are weak acids, and aliphatic hydroxyl groups still weaker, and they are not dissociated at common papermaking conditions. Sulphonic acid groups are found only in sulphite pulps, such as CTMP. Wood contains small amounts of proteins which have also positively charged groups. However, their amount is negligible relative to the anionic uronic acid groups. Chemical pulps contain lower amounts of acid groups than mechanical pulps because a large part of the uronic polysaccharides are dissolved and removcd, or degraded in chemical pulping and bleaching. However, chemi-mechanical pulps, such as CTMP, contain many charged groups (9). The total number of acid groups can be determined by acid-base or polyelectrolyte titrations (10). However, such titrations do not provide distinct information about the origin and chemical character of the charged groups, although groups with different acid strengths can be distinguished. Total uronic acid content is traditionally determined by treatment with strong acids and determination of released carbon dioxide (11). Mild cleavage followed by chromatographic analysis is a means to determine the identity of the uronic acid units in wood and pulp samples. Enzymatic hydrolysis combined to e.g. HPLC analysis has been succesfully used for analysis of uronic acids in haft pulps (12). Enzymes are, however, not able to completely degrade hemicelluloses and pectins in wood and mechanical pulp samples (13). Acid hydrolysis is effective, but uronic acid units are degraded to a large extent. Acid methanolysis provides an essentially better protection of the uronic acid units. Acid methanolysis followed by gas chromatographic analysis of the formed methyl glycosides was found to give good yields of hemicellulose sugars, including uronic acids, for both wood and mechanical pulp samples (14). In this study we have applied acid methanolysis and gas chromatography to analyse the hemicelluloses and pectins in Nordic wood and mechanical pulp samples, with the special objective to determine the charged uronic acid groups. The formation of new charged groups in alkaline treatment and peroxide bleaching has also been examined.
WOOD AND PULP SAMPLES Mature, healthy spruce (Picea abies), pine (Pinus silvestris), birch (Betula pubescens) and aspen (Populus tremula) trees growing in the Turku region were felled, discs were cut from the trees and were on the same day placed in a freezer where they were stored at -24°C until analysis. Knot-free parts of the discs, without any reaction wood, were taken for analysis. Small splinters were cut out, fReze-dricd and ground to wood meal in a Cyclo-Tec mill producing particles smaller than about 30 mesh (0.5 mm). Thermomechanical pulp (TMP) was taken after the second refiner in a Finnish paper mill using spruce as wood raw material. The pulp was stored in a freezer until analysis.
Charged groups in wood and mcchanical pulps
111
The TMP was freeze-dried and extracted during 24 h in Soxhlct apparatus with acetone-water (9:1 by vol.). The pulp was furthermore washed extensively with water at 60°Cto obtain clean TMP fibres. Care was taken to avoid loss of fines during washing. The extracted and washed TMP was treated at 2% consistcncy at a constant pH of 11.0 and 60°C for various times. Treatment were made also with addition of 3% hydrogen peroxide. After the treatments, pH was adjusted to 5.5 by addition of sulphur dioxide water. The pulps were then dewatered and washed with distilled water on a Buchner funnel. The drained water was sampled for analysis. Treatment with alkali, and peroxide bleaching was also made at 10% consistency and 60°C with a starting pH of 12. After 60 minutes reaction time, the suspensions were neutralised to pH 5.5 with sulphur dioxide water and the fibres were thoroughly washcd. Fractionation was made of a TMP watcr suspcnsion as follows. TMP, that had been extracted with hexane but not washed, was diluted to 0.5% consistency with distilled water and the suspension was agitated at 60°C with a mixer blade at 150 min-'. The conductivity was adjusted to ca. 1 mS/cm with 1 M NaCI. After stirring for 3 h, the suspension was disintegrated with a household mixer for 2 minutes. The consistency was again adjusted to 0.5%. The fibres were separated from the rest of the suspension by filtration in a Dynamic Drainage Jar.@DJ) equipped with a 100 mesh (0.15 mm) wire. 1000 mL of the suspension was added to the DDJ and the stirring speed was adjusted to 900 mid'. After 10 s the bottom valve was opcncd and about 300 mL of the filtrate was collected. The DDJ was washed and the filtration was repeated. A part of the suspension, now consisting of large fines, small fines, colloidal substances and dissolved substances, was stored in a cold room until the next day. The large fines were removed by filtration in the DDJ, now equipped with a 400 mesh wire (0.045 mm). About 750-1000 mL was filtered and 300-500 mL was collccted. The DDJ was washed and the filtration repeated, if necessary. The small fines wcre separated from the dissolved and colloidal substances by centrifugation at 500 g for 30 minutes. The supernatant was pipetted of. The colloidal substances were removed from the dissolved substances by filtration with a 0.1 pm filter. In an other fractionation procedure, pressurised groundwood (PGW) was fractionated using a Bauer-McNett classifier (SCAN M6-69).The PGW was produced in a Finnish mill from spruce wood and was sampled dircctly after grinding to CSF 45. The fractionation was performed using screens of 30,50, 100 and 200 mesh (0.54.0.29, 0.15 and 0.074 mm, respectively). Thc fines fraction was recovered from the fraction passing the 200 mesh scrcen by filtering on a Buchner funnel with a 400 mesh wire. The filtrate was recycled to recover all fibre material, including the colloidal fines.
ACID GROUP DETERMINATION AND ANALYSIS Hemicellulose and pectin sugar units, including the uronic acid units, wcre determined by acid methanolysis and gas chromatography (GC)(14). The charge of TMP fibres was determined with polyelectrolyte titration. 0.5 g frccze-dried TMP was ripened ovcmight in 29.5 g of distilled water and further suspended with a magnetic stirrcr for 3 hours. 20 g of 0.005 M 1,5-dimethyl-1,5diazaundecamethylene polymethobromide (polybrene) solution was added to the suspcnsion and stirring was continucd for 2 hours. The suspension was centrifuged and an aliquot of the supernatant was titratcd with a MUtek particle charge detector 03 using potassium polyvinyl sulphate (KPVS) as anionic polymer. TMP water samples,
112 Pulp production and processing containing dissolved and colloidal substances, were mixed with polybrene directly in the measuring cell and were then titrated with KPVS.
ACID GROUPS IN WOOD SAMPLES Analysis of spruce sapwood by methanolysis and GC gave the sugar composition shown in Fig. 1. The predominant hemicellulose type in spruce is galactoglucomannan, here seen as large amounts of mannose, glucose and galactose. However, a minor part of the galactose is present in arabinogalactan, and part of the glucose obtained in thc analysis is probably dcrived from starch. Some of the glucosc may also originate from cellulose, although the cellulose is quite stable against acid methanolysis at the conditions uscd, due to inaccessibility and hydrolytic stability of the cellulose glucosidic bonds. Another major hemicellulose in softwoods is 4-O-methyl-glucuronoarabinoxylan, here seen as xylose, arabinose and methylglucuronic acid in the ratio 4.3: 1:0.9. A minor part of the arabinose and galactose is probably present in form of arabinogalactan. Small amounts of an arabinogalactan that also contains glucuronic acid units can be extracted from spruce wood by cold water (15).
Sugar units, mg/g wood 120 100
80
60 40
20 0 Ara
XYl
Gal
Glc
Man
Rha
GlcA
Me GlcA
GalA
Fig. 1. Hemicelluloses and pcctin sugar units in spruce sapwood.
GalA -
-
[4 -0M e G Ic A2
I I
0-Rha 0
- Xyl - Xyl - X y l -
I I A ra Gal I I I Pig. 2. Uronic acid units in softwood hemicelluloses and pectin. I A ra
Charged groups in wood and mechanical pulps
113
Table 2. Amounts of uronic acids in wood samples, in mg/g of oven-dried wood.
Wood Spruce
GlcA
Sapwood (' Heartwood Sapwood Heartwood
MeGlcA
2-3 8-12 2-3 9-13 Pine 1 9 2 10 2 17 Birch 2 15 Aspen (1 range for analysis of three wood samples
GaL4 13-17 15-18 18 16 21 22
Total uronic acids 26-30 28-32 28 28 40 39
The most abundant uronic acid in spruce sapwood, as well as in the other wood samplcs, was galacturonic acid (Table 2). Pectin is made up mainly of galacturonic acid units, but contains also some rhamnose units (Fig. 2). Most of the galacturonic acids are probably methyl esterified in native wood (16), although the degree of methylation is not exactly known. This means that the 4-0-methyl glucuronic (MeGlcA) acid remJns the predominant free acid unit in these woods. Thc glucuronic (GlcA) acid, that is a unit of arabinogalactan,constitutes only a minor part of the uronic acids. Spruce sapwood and heartwood gave a very similar composition of hemicelluloses and pectins, including the amounts of uronic acid units (Table 2). Pine sapwood and heartwood contained about the same amounts of uronic acids as spruce. The hardwoods, birch and aspen, differed considerably from the softwoods, in containing more xylan, and consequently more methyl-glucuronic acid. The pectin contents, seen as galacturonic acid units, were also highcr.
ACID GROUPS IN MECHANICAL PULPS Mechanical pulps are in the Nordic countries produced mainly from Norway spruce. Thermomechanical pulping (TMP) is today the predominant technique. Mechanical pulps are increasingly bleached by alkaline peroxide. Thcre are no extensive chemical reactions in mechanical pulping processes. However, 3-5% of the wood matcrial is dissolved or dispersed into process waters during pulping (7). Acetylatcd galactoglucomannanis the predominant group among the dissolved substances. Acidic hemicelluloses and pectin are, however, dissolved only to a small extent (7). This was observed also now in the fractionation and analysis of a TMP suspension by DDJ filtration, centrifugation and microfiltration (Table 3). The dissolved fraction contained very little glucuronic and methylglucuronic acids. Howevcr, the colloidal fraction containing the so-called microfines, contained remarkably high amounts of both pectin (Rha and GalA) and acidic arabinogalactan (Ara, Gal and GalA). However, this fraction comprised only 0.4% of the total fibre material. These analyses verified that the acidic groups in mechanical pulps are practically the same as in the wood raw material, with the methylglucuronic acid units in arabinoxylan as predominant acid group.
114 Pulp production and processing Table 3. Hemicellulose and pectin sugar units in different fibre fractions of a spruce TMP suspension. Fractionation by DDJ filtration, centrifugation and microfiltration. Data given in mg/g of each fraction. Fibre fraction (mesh)
Ara
Xyl
Gal
Glc
Man
Rha
GlcA
10
54
22
44
108
1.6
1.6
10
10
261
19
69
41
39
86
5
3
13
35
310
21
60
42
34
76
5
1.9
8.5
40
288
52
29
171
104
95
14
36
2.4
33
536
11
4.8
49
109
272'
1
1.7
1.7
16
466
12 58 28 (1 Weight % of total pulp
48
115
2.3
2.0
10
15
290
>loo
81% (' 100-400 7% ~400 8% Coll. 0.4% Diss. 3% Whole pulp
Me GalA GlcA
Tot.
The fines fractions (100-400,and c400 mesh) contained much more arabinose, galactose and galacturonic acid than the coarse fibre fraction (>lo0mesh). The content of glucuronic acid was higher in the larger fines fraction, but not in the smaller fines fraction. The larger fines fraction contained more xylan (Xyl and MeGlcA) than the coarse fibre fraction. Analysis of fibre fractions of PGW separated by Bauer-McNett fractionation showed the same trends in composition (Table 4). Table 4. Hemicellullose and pectin sugar units in different fibre fractions of spruce PGW separated by Bauer-McNett fractionation. Data given in mg/g of each fraction. Fibre fraction (mesh) >30 25% ( I 30-50
Ara
Xyl
Gal
Glc
Man
Rha
GlcA
Me Glc A
GalA
Tot.
10
52
15
43
120
1.2
0.1
5.1
8
255
12
55
23
44
113
2.7
0.6
6.7
9.1
266
11
54
24
44
108
2.2
0.5
5.9
10
259
12
57
31
45
107
2.6
0.5
7.1
11.9
273
18
66
40
47
96
4.1
0.7
7.6
24
303
13 56 25 (1 Weight % of total pulp
45
110
2.2
0.6
6.1
15
274
13% 50-100 13% 100-200 8% c200 Whole pulp
115
Chargcd groups in wood and mechanical pulps
The fines fraction (<200 mesh) had a much higher content of galacturonic acid, than the larger fibre fractions. This was accompanied by correspondingly more rhamnose, indicating a high content of pectin in the fines fraction. The fines also contained much more arabinose and galactose. However, the amount of glucuronic acid was not notably higher in the fines fraction. The fines fraction contained also slightly more xylose and methyl-glucuronic acid, indicating more glucuronoarabinoxylan. The content of mannose decreased when going from coarse fibre fractions to finer fractions. Previous studies of TMP fines (17, 18) have reported a higher content of arabinose and galactose. However, the uronic acids have not been analysed in mechanical pulp fractions in earlier investigations. Ray cells, that constitute a distinct part of the fines, were by polyelectrolyte titration found to be rich in carboxyl groups (19). This was suggested to be due to the higher content of methylglucuronoxylan in the ray cells than in the whole wood. Since pectin occurs mainly in the primary wall and the pores (16), it is not surprising that fines fractions are rich in pcctins.
ACID GROUPS IN ALKALI-TREATED AND PEROXIDE-BLEACHEDTMP Dithionite bleaching is performed in neutral conditions and does not change the fibre charge (20). However, peroxide bleaching made at alkaline conditions, results in dramatically increased fibre charge, as noticed in several previous studies (20-22). The origin and character of these new acidic groups have not been determined. It has been suggested that they emerge from alkaline hydrolysis of ester and lactone groups originally present in wood (2,6),and from oxidation of lignin (9). New acidic groups were formed fast when Th4P was trcated at pH 11 and 60°C, (Fig. 3). The polyelectrolyte titration value for the washed fibres was doubled in 60 minutes treatment at pH 11, but did not incrcasc further by longer treatment. In the presence of peroxide, the acid group formation was still more extensive and continued even beyond 60 minutes. A considerable amount of acidic polymers was dissolved into the water during the alkali treatment, and still much more in the presence of peroxide. CD in TMP fibres
CD in water phase Peq/g
PWg
100 3% P
80 60
150
-
40
20 0 1
10
100
Time, min
1000
' 1
' '
"""'
' '
10
"""' 100
' ' "'"
1000
Time, min
Fig. 3. Formation of new acidic groups in TMP fibres, and release of acidic polymers into the water phase, by treatment at pH 11 and 6OoC without and with peroxide, determined as cationic demand (CD) by polyelectrolyte titration.
116 Pulp production and processing Amount, mg/g
41
15
3
10
2
5 :
Lignin
1
0
0 1
100
Time, min
1000
!
1
, , , , ,,,@ ,
10 Time, min 100
Other carb. 1000
Fig. 4. Release and dissolution of fibre substance from TMP by treatmcnt at pH 11 and 60°C. At these mild alkalinc conditions mechanical pulp fibres will be changed mainly through hydrolysis of ester groups and partial dissolution of certain hcmicelluloses and low-molar-mass lignin material. Analysis of dissolved components gave insight into these alkaline-induced processes. Acid groups were formed in the fibres with a similar kinetics as the release of acetic acid, formed by hydrolysis of acetyl groups in glucomannan, both reaching a maximum level after about 60 minutes (Fig. 4). Also methanol was released into the watcr at a similar ratc. The methanol is most probably released by hydrolysis of the mcthyl groups of the galacturonic acid units in the pectins, resulting in new carboxyl groups. A part of the pectins dissolved into thc water phase during thc alkaline treatment. Dissolved pectic acids, i.e. demethylatcd pectin, contributcd to a large part of the measured cationic dcmand of the watcr phase. For instance, after 60 minutcs at pH 11 and 60"C,the calculated contribution of pectic acids was 18 peq/g, the measured cationic demand being 38 peq/g. In earlier experiments with another TMP (7),an even highcr calculated contribution from the pectic acids was obtained. The contribution of demcthylated pectic acids in fibres to the fibre charge was also substantial. The determined levcl of galacturonic acids in fibres, 10 mglg TMP, after 60 minutes alkaline treatment theorctically corresponds to a cationic demand of about 57 peq/g. That is about half of the measurcd increase in fibre cationic demand. In case of treatment with peroxide the contribution of pectic acids is considerably lower, approximately only 20% of the increase in cationic demand. In another scrics of experiments, TMP was treatcd at 10% consistency with a starting pH of 12. These conditions correspond more closely to those of industrial peroxide bleaching. The pH value decreased during 60 minutes treatment to about 11. The dissolution of galacturonic acid was rathcr small in these treatments (Table 5). Total charge values were determined by polyelectrolyte titration (Tablc 6). The charge increascd from 85 to 196 pmol/g by the alkali treatment and to 220 pmoVg by the peroxide treatment. Thcsc values were detcrmined at 1% consistency and with only one excess dose of polybrenc, and are probably a bit too high. Whcn the same washed
Charged groups in wood and mechanical pulps
117
Tablc 5. Glucuronic acids in fibres after treatmcnt of TMP at G O T for 60 minutes with a starting pH of 12. Data given in mg/g of oven-dried fibres.
TMP, washed Alkali-treated,60 min pH 12.0 - 11.0 P-bleached, 3% P, 60 min
GlcA
MeGlcA
GalA
Total
2 2
12 12
17 15
31 29
2
11
14
27
TMP was litrated at 0.5% consistency and at a salt conccntration of 0.01 M NaCI, with addition of using several doses of polybrenc and extrapolation to equilibrium polybrene dose, the charge value was 56 pmol/g. The contribution of the uronic acids to the total charge was calculated from the analysed amounts. It was assumed that the dcgree of methylation of galacturonic acids was 80% in the untreated TMP, and 0% in the treated pulps. The calculated contribution of uronic acids excceded the titration value for the untreated TMP. This suggests that the degrce of methylation of galaturonic acids may be higher than 80%. The pcctins were completely demethylated in the alkaline treatment and the peroxide bleaching. A part of the formed pectic acids arc dissolved, but most remains in the fibres. Demethylation of galacturonic acids during alkaline treatment corresponds to a calculated incrcase in chargc of 66 pmol/g, supporting our hypothcsis that most of the new acidic groups are due to demcthylation of pectins. In case of peroxide bleaching, additional formation of carboxyl groups is obvious. Analysis of bleached mechanical pulps has shown that carboxyl groups are formcd in lignin during peroxide bleaching (23, 24). New carboxyl groups may also be formed by hydrolysis of lactonised uronic acid units. Table 6. Amounts of uronic acids and and their calculated contribution to the total fibre charge, comparcd to the charge obtained by polylelectrolyte titration.
TMP, washed mg/g pmol/g Alkali-trcated, 60 min pmol/g P-bleached, 3% P, 60 min pmol/g
GlcA
MeGlcA
GalA
2
12 63 12 63 11
17 19 15 85 14 80
11 2 11 2 11
58
Total charge, pmol/g Titration Calculated values uronic acid contribution 85 93 196 159 220 149
118 Pulp production and processing CONCLUSIONS The charged groups in wood, and in mechanical pulps at papermaking conditions, are primarily uronic acid carboxyl groups. These uronic acid units can be determined by acid methanolysis followed by gas chromatographyof the formed methyl glycosides. The main uronic acid units both in softwoods and hardwoods are galacturonic acids in pectins, 4-0-methyl glucuronic acids in xylans, and glucuronic acids present mainly in arabinogalactans. Since most of the galacturonic acids are mcthylesterified in wood, the methylglucuronic acids are the dominating chargcd groups in wood, and also in unbleached or dithionite-bleached mechanical pulps. Mechanical pulp fines contain much more pectin and arabinogalactan, and also slightly more xylan than the larger fibre fractions. Consequently, fincs contain much more galacturonic acid groups and slightly more methylglucuronic and glucuronic acid groups. In alkaline peroxide bleaching the fibre charge is more than doubled. The new chargcd groups emerge mainly from demethylation of the methyl esterified galacturonic acid groups in pectin. A smaller amount of charged groups arise from lignin oxidation reactions. ACKNOWLEDGEMENTS Support from The Technical Development Centre of Finland (TEKES) and several industrial companics is acknowledged. Wc thank M.Sc. Ecro Klemi for the fractionation and analysis of the PGW pulp. REFERENCES
1 A Scallan, ‘The effect of acidic groups on the swclling of pulps: a rcview’, Tappi J, 1983 66( 11) 73-75. 2 E Sjostrtjm, J Jansson, P Haglund and B Enstrom, ‘The acidic groups in wood and pulp as measured by ion exchange’, JF‘OZJ~ Sci, 1965 5(11), 221-241. 3 E Sjostrom, ‘The origin of charge on cellulosic fibers’, Nord Pulp Pap Res J, 1989 4(2), 90-93. 4 J Laine, L Wvgreen, P Stenius and S Sjobcrg, ‘Potentiomctric titration of unblcached kraft cellulose fibre surfaces’, Colloids SurJ 1994 88 277-287. 5 V Nyr6n and E Back, ‘The ionization constant, solubility product and solubility of abietic and dehydroabietic acid’, Acfa Chem Scand, 1958 12 1516. 6 S Katz, N Liebergott and AM Scallan, ‘A mechanism for the alkali strengthening of mechanical pulps’, Tappi, 1981 64(7) 97-100. 7 B Holmbom, ‘Molecular interactions in fibre suspcnsions’, 9th InfernSymp Wood and Pulping Chem ,CPPA, Montreal 1997, pp. PL3-1- PL3-6. 8 B Holmbom and A V Pranovich, ‘Fibre chemistry of alkaline trcatment and peroxidc bleaching of mechanical pulp’, Proc Fijh European Workshop on Lignocellulosics and Pulp, Univ. of Aveiro, Aveiro, 1998, pp. 559-562. 9 Y Zhang, B Sjogren, P Engstrand and M Htun, ‘Determination of charged groups in mechanical pulp fibres and thcir influence on pulp properties’, J Wood Chem Technol, 1994 14(1) 83-102.
Charged groups in wood and mechanical pulps
119
10 J Laine, J Buchert, L Viikari and P Stenius, ‘Characterization of unbleached kraft pulps by enzymatic peeling, potentiometric titration and polylectrolyte adsorption’, Holzforschung 1996 50 208-214. 11 B L Browning, Methods of Wood Chemistry, Vol II,New York, Wiley, 1967, p. 632. 12 M Tenkanen, G Gellerstcdt, T Vuorinen. A Teleman, M Perttula, J Li and J Buchert, ‘Determination of hexenuronic acid in softwood h a f t pulps by three different methods’, J Pulp Pap Sci. 1999 (In press). 13 H Militz, ‘The enzymatic decomposition of neutral and acid polysaccharidcs from spruce wood‘, WoodSci Technol 1993 28(1) 9-22. 14 A Sundberg, K Sundberg, C Lillandt, and B Holmbom, ‘Dctermination of hemicelluloses and pectins in wood and pulp fibres by acid methanolysis and gas chromatography’, NordPulp Pap ResJ, 1996 ll(4) 216-219,226. 15 J Thornton, R Ekman, B Holmbom and F h a , ‘Polysaccharides dissolved from Norway spruce in thermomcchanical pulping and peroxide bleaching’, J Wood Chem Technol 1994 14(2) 159-175. 16 U Westermark and F Vennigerholz, ‘Morphological distribution of acidic and mcthylesterified pectin in the wood ccll wall’, 8th Intern Symp Wood and Pulping Chem. Vol. I, KCL, Espoo 1995, pp. 101-106. 17 H-M Chang, JD Sinkey and JF Yan, ‘Chemical analysis of refiner pulps’, Tappi 1979 62(9) 103-106. 18 J Sorvari, V Pietarila, A Nygren-Konttinen, A Klemola, JE Laine and E Sjiistriim, ‘Attcmpts at isolating anc charactcrizing secondary wall and middlc lamella from spruce wood (Picea abies)’, Pap Puu 1983 65(3) 117-121. 19 U Westermark and G Capretti, ‘Influence of ray cells on the blcachability and propertics of CTMP and kraft pulps’, Nord Pulp P a p Res J, 1988 3(2), 95-99. 20 J Thornton, R Ekman, B Holmbom and C Eckerman, ‘Rclcase of potential “anionic trash” in peroxide bleaching of mechanical pulp’, P a p Puu 1993 75(6) 426-431. 21 P Engstrand, B SjiSgren, K Olander and M Htun, ‘The significance of carboxyl groups for the physical properties of mechnical pulp fibres’, 6th Intern Symp Wood and Pulping Chem, Vol. I, Appita, Melbourne 1991, pp. 75-79. 22 J Lloyd and CW Home, ‘The determination of fibre chargc and acidic groups of radiata pine pulps’, Nord Pulp Pap Res J, 1993 8(1), 48-52. 23 B Holmbom, R Ekman, R Sjiiholm, C Eckerman, and J Thornton, ‘Chemical changcs in peroxide bleaching of mechanical pulps’, Papier, 1991 45(10A) V16-V22. 24 K Kuys, ‘The effect of bleaching on the surface chemistry of high yield pulps’, 46 th Appita ConJ Appita, Melbourne 1992, pp. 53-62.
THE INVESTIGATION OF CELLULOSE POLYMORPHS IN DIFFERENT PULPS USING I3C CPMAS NMR Sirkka Maunu’, Tiina Liitia’, Seppo Kauliomaki’, Bo Hortling’ and Jorma Sundquist’
1) University of Helsinki, Laboratory of Polymer Chemistry, P.O.Box 55, FIN-00014 University of Helsinki, Finland 2) The Finnish Pulp and Paper Research Institute, Paper Science Centre, P.O.Box 70, FIN-02151 Espoo, Finland ABSTRACT In order to obtain information about the crystallinity and polymorphs of cellulose and the occurrence of hemicelluloses in pulp fibers, wood cellulose, bacterial cellulose, cotton linter, viscose and celluloses in different pulps were investigated by solid state 13C CPh4AS NMR spectroscopy. A mixed softwood kraft pulp and a dissolving pulp were treated under strongly alkaline and acidic conditions and the effect on the cellulose crystallinity was studied. The presence of different crystalline polymorphs of cellulose and the amount of hemicelluloses are considered. INTRODUCTION
Solid state NMR can be used to study crystallinity as well in pure native celluloses as in pulp celluloses. It has been determined by IR, X-ray and NMR that crystallinity increases during haft pulping because of the removal of less ordered structures Is’. There exists a closer correlation between IR and X-ray crystallinity than between IR and NMR crystallinity of celluloses because of the sensitivity difference of the methods. Only material within the crystallites will appear as crystalline in Nh4R spectra. A consequence of this is that the N M R crystallinity depends on the size of crystallites. The surface of crystallites has been found to give its own resonances to CPMAS spectra 3*4. Different polymorphs ( Ia,ID, I1 ) of cellulose have been analysed by NMR for pure and native celluloses Some results have been published for cellulose polymorphs in pulps analysed by solid state N M R 6*7. The conversion of Ia to ID during pulping was shown and a low amount of form I1 was also detected
’.
’.
The effect of hydrolysis on the bleached birch kraft pulp was studied by CPMAS N M R ’. The amount of hemicelluloses was found to decrease and the relative crystallinity was found to increase during the hydrolysing process. Strong alkali treatment of the haft pulp was found to convert the crystalline form I to cellulose I1 on the base of CPMAS studies ‘. Strong acid treatment of pulp is used under parchmentising conditions. This kind of heavy treatment should affect the morphology of cellulose but until now it is not known if the cellulose I1 polymorph is formed as in the alkali treatment lo,ll. In this paper the I3C CPMAS Nh4R spectroscopy has been used to investigate cellulose polymorphs in regenerated, parchrnentised and different pulp celluloses. The amount of
122 Pulp production and processing different crystalline cellulose polymorphs (Ia,ID, 11) has been analysed. The effect of pulping conditions and pre-treatment on the cellulose polymorphs and on the occurrence of hemicellulosesin pulp fibers has been investigated. MATERIALS
As examples of native celluloses the samples of spruce, birch, cotton linter and bacterial cellulose from Acefobucfer@inurn were used (Samples 1-4). Fully bleached 2-stage sulphite pulp (Sample no lo), fully bleached kraft pulp (Sample 5 ) and filly bleached prehydrolysed krafi pulp (Sample 6) from spruce were studied also. The bleaching of these pulps was performed after the oxygen delignifigation stage with the DECED-sequence @=chlorine dioxide, E=alkaline extraction, C=chlorine). The brightness of the sulphite pulp was 89.0% and the viscosity 1240 dm3/kg.The brightness of the krafl pulp was 89.4% and the viscosity 870 dm3/kg and 89.3 % and 750 dm3/kg respectively for the prehydrolysed kraft pulp. A commercial TCF-bleached (totally chlorine free) mixed softwood kraft pulp (Sample no 7) was bleached with the sequence of 0-Z-Q-OP-Z-P(0) (O=oxygen, Z=ozone, Q=EDTA, P=hydrogen peroxide). Dissolving pulp (Sample no 11) was manufactured by sulphite pulping from spruce and bleached with the sequence of CEHDH (C=chlorine, E=alkaline extraction, H=hypochlorite, D=chlorine dioxide). Viscose fibres (Sample 14) were prepared from dissolving pulp. The mixed softwood kraft pulp and the dissolving pulp were steeped in 19 'YO NaOH solution at 48-50 "C for 20 min and then regenerated in water. The strong acidic treatment for these pulps was performed under parchmentising conditions in 70 % (weight-%) sulphuric acid on paper sheets (10cm x 10 cm of 80 eJm2) for 10 s and then regenerated in water. (Samples no 8,9, 12 and 13). METHODS
The conventional I3C CPMAS NMR spectra were measured on a VARIAN UNITY INOVA spectrometer operating at 75,47 MHz for carbons. Wetted (-50 %) samples were accumulated about 20 h using 1 ms contact time and 2 or 3 s delay between pulses. Wood samples were measured over weekend. The decoupler power was 50 kHz and the 20 ms data acquisition time was used. The signal intensities are considered as semiquantitative in measurement conditions and hence reasonable for relative comparison. The crystallinity (CrI) of cellulose was calculated on the basis of deconvoluted data using Lorenzian line shape. Different crystalline polymorphs were assigned from the resolution enhanced data using deconvolution and both Lorenzian and Gaussian line shapes were used to find a satisfied fit. RESULTS AND DISCUSSIONS
The I3C CPMAS NMR spectra of samples which represent native celluloses are presented in Figure 1.The degree of crystallinitydiffers a lot as seen from the size of the signals centred at 89 ppm and 84 ppm presenting crystalline and amorphous phase respectively. The high crystallinity of cotton linter affects the sharpness of the signals. The presence of hemicelluloses in the spectrum of spruce can be detected as a clearly
Cellulose polymorphs 123 resolving shoulder at 102 ppm. The pure birch and spruce samples include 27-30 % of lignin which gives its own signals to I3C CPMAS N M R spectra. Thcrefore the crystallinity calculation directly on the basis of 89 pprn and 84 ppm signals area is misleading. If the spectrum of milled wood lignin (MWL) is subtracted from the spectrum of spruce and the crystallinity calculated from the resultant spectrum about 10 % increase of crystallinity can be obtained. The presence of hemicelluloses in these samples still stresses the area of the amorphous cellulose signal. The degree of crystallinity and the amount of different polymorphs found in the studied samples are presented in Table 1. The CrI was analysed from raw NMR data and the polymorphs were calculated from the C4 crystalline part in resolution enhanced spectra after deconvolution. The high I a content for softwood and high Ip content for hardwood were found as documented earlier 12. Cotton linter and bacterial cellulose represent high crystalline celluloses, the first with high Ip and the second with high I a content as published also earlier *.' Low amounts of cellulose I1 were detected in the spectrum of bacterial cellulose.
FIGURE 1. 13CCPMAS NMR spectra of native cellulose samples A) spruce wood, B) birch wood, C) cotton linter and D) bacterial ccllulose.
124 Pulp production and processing TABLE 1. The analysed samples and the crystallinity (CrI) with different polymorphs. NO Description 1 2 3 4
sprucewood Birch wood Cotton lintcr Bacterial ccllulosc
(38)48 (27)36 67 70
5 6 7 8 9
Fully bleachedknfi pulp Fully blcachcdprchydr. hall pulp mxed sonwood kraft pulp parchmenlised mcrccriscd
47
-"---
55 51 25
-
10 Fully blcachcd sulphite pulp 45 I 1 Dissolvingpulp (sulphitc) 53 parchmentised 18 12 - " 13 ~s mcrccrised 14 Viscose
-
- -
--
*
*
74 41 33 62
26 59 67 30
30 23 23 26
61 65 63 61
9 12 14 13 100
39 32 30
49 57 53
12 11 17 100 100
n
*
the value in parentheses is calculated directly and the othcr is calculatedwhen the spcctrum of MWL is subtracted from the spectrum of wood. a vcry low amount is detected. aproximatcd to about 70 %
-
The crystallinity of sulphite pulp cellulose is lower than in kraft pulps. The occurrence of hemicelluloses in the sulphite pulp is higher than in the haft pulp and apparently affects the large intensity of the amorphous phase because the chemical shifts of hemicelluloses appear at the same resonance region. An increase in CrI can be noticed from sample 10 to samples 5 and 6. These pulps are prepared from the same spruce but pulped differently. The acid treatment decreases the crystallinity of the cellulose noticeably for both mixed softwood kr& pulp and dissolving pulp. It is dificult to calculate the crystallinity for mercerised pulp and viscose samples from C4 resonance because of the shape of the signal. The CrI is approximated to be high. The hydrolysed samples, sample 6 and 11, have higher crystallinity than the unhydrolysed ones. Polymorphs of pulp cellulose The different polymorphs, I, Ip and 11, were found in the pulps with varying amounts. After the acid treatment the different polymorphs remain unchanged although the degree of crystallinity decreases, probably due to the short treatment and because strong acid is used. The alkali treatment converts the crystalline form I to cellulose I1 as expected '. Usually the different cellulose polymorphs are analysed on the basis of the form of the C4 signal at 89 ppm presenting the crystalline part of cellulose C4 resonance. Separate polymorphs give their own resonances as follows, cellulose Ia a doublct at 89.9 ppm and 89.2 ppm, cellulose Ip a doublet at 89.2 ppm and 88.3 ppm and cellulose II a doublet at 89.2 ppm and 87.8 ppm. These resonances are detectable directly in the spectra of the native cellulose samples, but for pulps the NMR data requires mathematical enhancement of resolution to resolve the different signals. If all forms are present, this resonance region looks like a quartet where the other parts of every doublet overlap each other at 89.2 ppm presenting the sum of the signals. The other parts resolve separately making it
Cellulose polymorphs 125 possible to calculate the percentage values of different polymorphs. Cellulose Ip polymorph is the dominating form for every pulp sample. During the pulping process the cellulose I a converts to the more stable Ip form. The signal of cellulose I1 at 87.8 pprn is possible to find in the spectrum of every pulp sample. The amount of cellulose I1 form is low and the calculated values are only estimates. The situation is different for viscose and mercerised samples where crystalline cellulose clearly appears as polymorph 11. It could be expected that the acid treatment increases the amount of the more stable Ip form. In parchrnentised samples the amount of this form seems however not to increase in comparison with the untreated pulp samples 7 and 1 1 . Figure 2. presents the changes in polymorph contents after acidic and alkali treatment.
MIXED SOFMlooD KRAFT PULP 100
80 60 40
20 0 Pulp .... ~
.................................
Acid
Alkali ......
I -.__ beta ocetlutosed ........................... ............ ....
FIGURE 2. Differerent polymorphs of mixed softwood haft pulp and after acidic and alkali treatments. IIemicelluloses
Figure 3. presents the spectra for different pulps. The signals at about 85 and 84 ppm assigned to the surfaces of crystalline domains were found for samples 10, 7 and 11 without resolution enhancement as reported recently also by Newman ’. The presence of hemicelluloses is seen very well in the mathematically untreated spectrum of spruce wood (Figwe 1A.) as a shoulder at 102 ppm. The resolution enhancement went far towards the separation of the signal as seen in Figure 3. The amount of hemicelluloses is highest in the sulphite pulp. In these spectra the chemical shift at 83 ppm for hemicelluloses is seen as a shoulder in the amorphous cellulose resonance. The resonances of hemicelluloces at 102 and 83 ppm appear smaller in the spectra of prehydrolysed kraft pulp and dissolving pulp which have low hemicellulose content in comparison with the hlly bleached sulphite, krafl and mixed softwood haft pulp. Table 2. presents the carbohydrate composition analysed for some samples. An extra small signal at about 100 pprn is found in some CPMAS spectra. It could be
126 Pulp production and processing
Ipo
110
im
m
10
FIGURE 3. Resolution enhanced spectra of A) filly bleached sulphite pulp, B) filly bleached haft pulp, C) mixed softwood haft pulp, D) dissolving pulp and E) prehydrolysed haft pulp. supposed that this resonance originates more probably from glucose end groups than hemicelluloses. These signals are present in the spectra of dissolving pulp and samples prepared fiom it as well as in the spectrum of prehydrolysed haft pulp which should not include residues of hemicelluloses.These small signals are present also in the spectra of alkali treated samples. The treatment obviously cuts the polysaccharide chains and hence increases the amount of end groups. No difference was found in the comparison between the resolution enhanced spectra of alkali treated haft pulp cellulose and alkali treated dissolving pulp cellulose. TABLE 2. The carbohydrate composition of samples. Sample
Ara
Glc
XYl
Man
Fully bleached knft pulp Mixed sonwood haft pulp " parchmcntised '' mcrccrised Fully bleached sulphitc pulp Dissolving pulp (sulphite) " parchmentiscd '' mcrccriscd
0.6 0.6
85.2 87.2 87.9 98.2 84.0 96.3 96.6 100.0
7.6 6.2 0.5 5.7 4.2 1.6
6.6 6.0 1.3 6.0 11.8 2.1
1.4
2.0
- - - - -
0.4
Cellulose polymorphs
127
CONCLUSION
The different pulping conditions affect the crystallinity as well as the polymorphological structure of cellulose analysed on the basis of 13C CPMAS NMR measurements. The acid treatment of mixed softwood pulp and dissolving pulp decreased noticeably the crystallinity of cellulose in the pulps. The polymorph structure did not change during parchmentising as it converts to form I1 during alkali treatment. It is possible to detect the presence of hemicelluloses in the CPMAS spectra of pulps. ACKNOWLEDGEMENT
We are indebted to the Technology Development Centre of Finland ( T E E S ) for financial support. REFERENCES 1. Evans, R.,Newman, R.H., Roick, U.C., Suckling, I. D. and Wallis, F. A,, Changes in Cellulose Crystallinity During Krafl Pulping. Comparison of Infrared, X-ray Diffraction and Solid State NMR Result, Holzforschung, 1995,49,498-504. 2. Hattula, T., Effect of kraft cooking on the ultrastructure of wood cellulose, Paperi j a PUU,1986,926-93 1. 3. Newman, R H., Evidcnce for Assignment of I3C NMR Signals to Cellulose Crystallite Surfaces in Wood, Pulp and Isolated Celluloses, Holzforschung, 1998, 52, 157-159. 4. Hemmingson, J. A. and Newman,
R. H., Changes in molecular ordering associated with alkali treatment and vacuum drying of cellulose, Cellulose, 1995,2,71-82. 5 . Isogai, A., Usuda, M., Kato, T., Uryu, T. and Atalla, R. H., Solid-state CP/MAS N M R Study of CellulosePolymorphs,Macromolecules, 1989,22,3 168-3 172. 6. Lennholm, H., Larsson, T. and Iversen, T., Determination of cellulose I, and Ip in lignocellulosiematerials, CarbohydrateResearch, 1994,261, 119- 13 1. 7. Larsson, P.T., Wickholm, K. and Iversen, T., A CP/MAS I3C NMR investigation of molecular ordering in celluloses, Carbohydate Research, 1997,302, 19-25. 8 . Lennholm, H., Wallbacks, L. and Iversen, T., A 13C-CP/MAS-NMR-spectroscopic study of the effect of laboratory krafl cooking on cellulose structure, Nord P u b Pap. Res. J., 1995, 10,46-50. 9. Wickholm, K., Lennholm, H., Larsson, T. and Iversen, T., Hydrolysis of Cellulose Samples Studied by 13C-CP/MAS-NMR-Spectroscopy and Chemometrics, Extended abstracts of 4th EWLP, Stresa, 1996,21-25. 10. Bucher, H. , Zu den Vorgangen bei der Pergamentierung von Zellstoff mit Schwefelsaure, Ilas Papier, 1957, 1I, 125-133. 11. Ryti, N. and Skogman, R., Beitrag zur Kenntnis des Pergamentierprozesses mit Schwefelsaure, Papper och Tra, 1963,4, 18 1-1 90. 12. Newman, R. H., Crystalline Forms of Cellulose in Softwoods and Hardwoods, J. Wood Chem. Technol., 1994,14,451-466.
CHARACTERIZATION OF CARBOXYL AND PHENOL GROUPS IN KRAFT PULPS AT DIFFERENT TEMPERATURES J. Karhu, P. Forslund, L. IIarju and A. Ivaska* Labomtory of Anulytical Chemistry, Process Chemistry Group, Abo Akudemi University, Biskopsgatan 8, FIN-20500 Turku-Abq Finhnd
ABSTRACT Carboxyl and phenol groups in pulps have been studied at differcnt tcmperatures (25, 45, 65 and 85 "C) by potentiomctric titration. The method dcvclopcd allows dctcrmination of both the protonation constants and the concentrations (capacities) of the acid groups. Samples of unbleachcd and oxygcn bleached softwood h a f t pulps were analyzcd. Models with either onc or two carboxyl groups and one phenol group were applied to evaluate the titration data. The temperature was found to have a remarkable influence on the concentration of available phenol groups.
INTRODUCTION The ion cxchangc properties of pulp are rclated to thc acid groups in wood fibers. &aft pulp is known to contain weak acids, i.e. carboxyl and phenol groups. The carboxylic acids in pulp are mainly uronic acids in hcmicelluloses, but the lignin in pulp also contains carboxylic acids as a rcsult of cooking Carboxylic acids in pulp are normally produced in oxygen, pcroxide and ozone bleaching. Chlorine dioxide and ozone bleaching reducc thc amount of carboxylic acids in hemicelluloses. Hydrogcn pcroxide bleached TCF pulps contain normally more carboxyl groups than ECF pulps '. Hexene uronic acids have been found to consumc the bleaching chemicals chlorine, chlorine dioxide, ozone and peracids '. Transition metal ions like manganese, iron and coppcr disturb totally chlorine frce (TCF) bleaching processcs by dccreasing bleaching selcctivity and by breaking down the bleaching chemicals Phenol groups are present in lignin molecules. Extractivcs contain both phenolic and carboxylic acids According to Gierer, oxygen rcacts mainly with phcnolic lignin structures and the phenol groups have an important effect on initiating thc bleaching process '. In ECF blcaching, chlorine dioxidc mainly reacts with phenolic structures in lignin. The reaction bctwecn chlorine dioxide and non-phenolic structures is a minor rcaction The phenolic hydroxyl and carboxyl groups in lignin makc it soluble in alkaline solutions '. The carboxyl groups in pulps have been studied earlier at 25 OC by means of potentiometric acid-base titrations 1*8*9. No valucs for the protonation constants of the phcnol groups in the lignin have so far been reportcd. The concentration (i.c. capacity) of phenol groups in lignin has been studied earlier by modifying lignin to other compounds. These methods have been regarded as vcry slow and the chromatographic mcasurernents involvcd arc difficult and tcdious '. The purpose of this work is to study how temperature affects the acid groups in pulps. This information is of great importance because the pulping proccsscs take place at elcvatcd temperatures. Such studies havc not been reportcd in international pulping literature.
'.
'.
'.
130 Pulp production and processing THEORY A mixture of acids can be studicd by potcntiomctric titration with a strong base. The strength of an acid group (HA) is characterized by the protonation constant K;iA which for reaction [l]is defined by equation [2]:
The brackets dcnote the Concentration of the involved spccies. The constant K,!iA in equation [Z] is expressed as a concentration constant. The measured hydrogen ion activity, denoted as {H'}, can be transformed to hydrogen ion concentration by equation [3].
{ H ' } = 8,.[H']
[31
The activity coefficient, f,,, depends on the ionic strength of the solution and is different for different ions. It depends on temperature and pressure as well. The activity coefficients can be calculated by using the Debye-Huckel exprcssion lo. The traditional way to treat the titration data is to plot pH as function of the addcd volume titrant. In the ideal case this gives a curve with clear potential jumps. The requirements for these ideal titrations are that the protonation constants of the acids, K i i A , are less than lo', and that the diffcrence between the two constants is larger than lo4. Such an experiment can be considered to consist of two or more separate titrations. However, such requircmcnts are not fullilled in titration of acid groups in pulps. Protonation constants of differcnt carboxylic acids in pulp are rather similar and therefore they cannot bc detcrmincd scparately by a conventional acid-base titration because they react simultaneously with the added base. Phcnolic acids are very weak acids. Their protonation constant is larger than lo9 and therefore thcy will not give any distinct pH jump in the titration curve. Due to thcsc problems the data from a titration of a pulp sample must be interpreted differently. In the Gran mcthod the titration curve is transformed into a linear form ' I . The Gran method has been further developed by Ingman and Still for titration of very weak acids I*. Ivaska dcveloped a linear method for titration of binary mixtures of wcak acids with protonation constants close to each other 13. By using those methods it is possible to determine both the concentration and the protonation constants of different acid groups in the pulp. The evaluation of our titration data is based on equation [4], which describes titration of n acids with a strong base in a mixture 13:
%+{If+,,,
v+v
= "-([c,,
H + J-[OH-])+ v
[41
I=1
All formulae that have bcen uscd in our calculations are derived from this equation. In the cquation [4] Vcq,i stands for the equivalence volume of the acid HAi. VOH is the
Carboxyl and phenol groups
131
volume added base and C o is~ the ~ concentration of it. Vo is the initial volume of the solution. The equivalence volumes in equation [4] are calculated from the known addition of strong acid, inflexion point of the titration curve and by the Gran method in the approximate pH range log K f 0.1 for the stronger carboxylic acid and at pH 10-11 for the phenols ". Inflexion point of the titration curve gives the sum of the equivalence volumes of the strong acid and the weak acids with log K 5 7. The protonation constants arc then calculated by an iterative method. EXPERIMENTAL Unbleached and oxygen blcachcd batch cooked softwood hail pulp samples fiom a Finnish pulp mill have been studied. Those samples were taken from different batches. The pulp raw material was a mixture of Scots pine (Pinus sylvcsfris) and Norwegian spruce (Piceaabies). The process solution and the dissolved lignin were washed away Gom the samples with water with the same temperature and p€I as the process solution (94 "C and pII 10.5 for the unbleached pulp and 80 "C and pH 9.5 for the oxygenbleached pulp). The kappa values of the pulp samples were 17.9 and 7.0. The samples were then washed with 0.01 M nitric acid @.a. grade) for 60 minutes. The excess of acid and the desorbed metal cations were washed away fiom the sample with ultrapure water. The pulp was cquilibrated in 0.1 M KCI (pa. grade) solution for 30 minutes. The solution was then filtcred away. A certain amount of the pulp was then titrated in 0.1 M KCI. Every sample to be studicd was first acidified to pH 2 with nitric acid in order to protonate the acid groups and the titration was carried out up to pII 1 1.5 under nitrogen atmosphere. In titrations at 85 "C a known amount of hydroxide (KOH, p.a. grade) was added in the beginning of the titration in order to increase the pH near 3 and the titration procedure was then started from that point. This precaution was done to prevent the possible breakdown of hexenc uronic acids which takes place at pH below 3.5 at elevated temperatures The titrations were performed at 25 f 0.2, 45 f 2, 65 f 2 and 85 f 3 "C. The first titrations were done manually and the remaining titrations with an automatic MettlerToledo DL 50 titrator. Mcttler-Toledo Inlab 412 combined glass electrodes were uscd for the pH mcasuremcnt. Similar results were obtained by both methods. The potential stability criterion in the automatic titration was 0.1 mV/min and the waiting time between additions between 2 and 10 minutes. At high temperature the waiting time was limited to max. 10 minutes as the total titration time for each analysis was limited to 50 hours. In the experiments the masses of pulp (expressed as oven dried pulp) varied between 2 and 25 g and the initial volumes varied between 100 and 800 ml. RESULTS AND DISCUSSION Two equilibrium models were used in the evaluation of the titration data. In model 1 the presence of one carboxyl and one phenol group was assumed. The second model is based on two carboxyl groups and one phenol group. The results given in Tables 1 and 2 are the mean values of two or three determinations, except for the titration of unbleached pulp at 65 "C. Those values are from a single titration of a larger amount of sample.
132 Pulp production and processing Table 1. Protonation constants and capacities (mmol/kg d.p.) for the samples by using the one carboxyl and one phenol group modcl.
"C
Unbleached softwood pulp Oxygen bleached softwood pulp Carboxyl group Phcnol groups Carboxyl group Phenol groups log K Capacity log K Capacity log K Capacity log K Capacity
25 45 65 85
4.45 4.79 4.85 4.47
Temperature
43.9 31.9 33.4 32.6
9.10 9.43 9.53 9.07
54.4 70.2 51.0 263
4.29 4.32 4.35 4.13
42.1 39.9 33.2 39.7
9.40 9.66 9.53 8.78
19.2 39.6 40.6 192
Table 2. Protonation constants and capacities (mmol/kg d.p.) for the samples by using the two carboxyl and one phenol group model. The values for the phenol groups are the same as in Table 1. Unbleached softwood pulp Oxygen bleached softwood pulp Temperature Carboxyl group 1 Carboxyl group 2 Carboxyl group 1 Carboxyl group 2 "C log K Capacity log K Capacity log K Capacity log K Capacity 25 45 65 85
3.79 3.98 4.01 3.83
17.6 10.1 13.3 10.2
5.19 5.31 5.46 4.85
26.3 21.8 20.1 22.2
3.09 3.75 3.58 4.26
11.1 12.9 11.1 21.0
4.91 4.80 4.93 4.83
31.0 26.9 22.2 18.7
Model 1: One carboxyl and one phenol group Results from the titrations are given in Table 1. As can be seen in the table the protonation constant of the carboxyl group is rathcr independent of temperature. It can also be scen in Table 1 that the carboxyl groups in the unbleachcd pulp are slightly weaker than in the oxygen-bleachcd pulp. The capacities of carboxyl groups in the pulp samples were also determined. The capacity (mmol carboxyl groups / kg dry pulp) was found to be approximately constant. In the unbleached and oxygen bleached softwood pulps the average capacities werc 35 and 39 mmoVkg d.p., rcspectively. These values are very low due to the production of low-kappa pulp. This indicates that somc new carboxyl groups wcrc formed in oxygen bleaching. A typical variation in the capacity of carboxylic acid is 5 - 10 % for pulp from the same batch. This equilibrium model includes also the presencc of one phenolic hydroxyl group. The protonation constant of the group in the unbleached pulp was found to be rather independcnt of tempcrature. However, the value of log K in the oxygen bleached pulp decreased from 9.5 to 8.8 when the tcmperature increased from 25 "C to 85 "C. It mcans that the acidity of thc phenol group in this pulp sample incrcascd with temperature. Some variations in the log K values of phenol groups may be due to experimental difficulties in determination of protonation constants for such very wcak acids as phenols. The capacity (i.e. concentration) of the phenol groups varied between 50 and 70 mmol/kg d.p. in the temperature range 25 - 65 "C in unbleached pulp. In the oxygen bleached pulp the capacity was between 20 and 40 mmol/kg d.p. in the same tempcrature range, i.e. definitely less than in the unblcached pulp as could bc expccted. It has bccn reported in the litcrature that the concentration of phenolic acid
Carboxyl and phenol groups
133
groups after pulping is 50-100 mmol/kg d.p. depending on the kappa number '. Pulp is a rather heterogeneous material and the concentrations of different chemical groups may vary significantly between diffcrent samples. Ncvcrthclcss, our values for the unbleached pulp are in good agreement with the values given in the literature. When the temperature was increased to 85 "C a dramatic change in the concentration of phenol groups was observed. In the unbleached pulp the capacity is 260 mmol/kg d.p. and in the oxygen bleachcd pulp it is 190 mmol/kg d.p. We have obtained similar results both with unbleached and oxygen bleached hardwood pulp. One possible explanation to the large change in the phenol capacity is that somc changes in the charactcr of lignin might take place when the temperature is increased from 65 "C to 85 "C. Lignin can be softened and dissolved to the liquor phase at such alkaline conditions. Alkaline hydrolysis may also take place and affect the composition of the sample. Forssk5hl et al. found, that when glucose and xylosc were trcatcd with 0.63 M NaOH at 96 "C in a nitrogen atmosphere, cyclic enols and phenolic compounds were formed 16. Many of these compounds have morc than one hydroxyl group. Model 2: Two carboxyl and one phenol group The protonation constants and the capacities of the carboxylic acids in the pulp were also calculated by using a model with two separate groups. The rcsults arc shown in Table 2. The log K values for the phenol group are the same as in Table 1 and are therefore not included. Both the protonation constants and capacities are relatively constant for the tempcraturc studied, especially for thc unbleached softwood pulp system. Thc differencc in the logarithm of the protonation constants, Alog K, for thc two carboxylic groups is mostly in the range of 1.O to 1.5. For oxygen bleached pulp the temperature effects are larger, e.g. the capacity of the weaker carboxylic group decreases from 31.0 to 18.7 mmol/kg d.p. when the temperature is increased from 25 to 85 "C. The ratios of the capacities for the two carboxyl groups are in the range 0.36 to 0.67. Oxygen bleached pulp at 85 "C makes an exception with a capacity ratio of 1.12. Potentiomctric acid-basc titration was also used by Laine in determination of the carboxyl groups in pulp '*I5. Hc prcfcrred the two carboxylic acid group model. Our values of log K are in good agreement with his values (log K = 3.3 and log K = 5.6 for unbleached pulp and log K = 3.3 and log K = 5.5 for oxygen bleached pulp). The total capacities detcrmined in our work are lower than the capacities he reported (a total of approx. 90 mmol/kg d.p.). In his samples the capacity of the stronger carboxylic acid, obviously 4-o-mcthylglucuronic acid, was higher than the wcakcr carboxylic acid 15. In our samples the ratio between the two carboxylic acids determined by the model 2 was reversed. The determination of protonation constants and capacities for diffcrcnt types of pulps at different temperatures is not an easy task. Pulp is very heterogcncous material with complicated structure and chemical composition. The low capacity of the acid groups rcquircs high accuracy in the measurements of the hydrogen ion concentrations and the added volumes of titrant especially at elevated temperatures. The experiments are also very time consuming.
'
134 Pulp production and processing CONCLUSIONS As shown in this work acid-base properties of pulp can be dctermined by potentiometric pH titration and by using the linearization method in evaluation of the titration data. With the method it was possible to determine concentrations of carboxyl and phenol groups in the analyzed pulp samples. The method also allowed determination of the protonation constants of the groups and distinction between two different carboxyl groups. The value of the constant obtained with model 1 is bctwcen the two values given by model 2. Both equilibrium models can be used to describe the average acid-base properties of the carboxyl groups in the samples. Some differences in the acid-base propertics bctween the unbleached and oxygen bleached pulp samples were found. Oxygen bleaching made the carboxyl groups stronger and also slightly increased their concentration in the pulp. The main effect was naturally found in the concentration of phenol groups which clearly decrcascd from unbleached to oxygen bleached pulp. The concentration of carboxyl groups was rather indepcndent of temperature. However, a large increase in the concentration of phenol groups was observed when the temperature increased from 65 to 85 OC. Also the log K value of the phenolic acid groups was found to decrease with temperature. ’
ACKNOWLEDGEMENTS The financial support of The Finnish Technology Dcvclopmcnt Ccntrc, Ahlstrom Machinery Corporation, Stora Enso Oyj and Oy MetsP-Rauma Ab is gratefully acknowledged.
REFERENCES 1 . J. Laine, J. Buchert, L. Viikari, P. Stenius, ‘Characterization of Unblcached Kraft Pulps by Enzymatic Treatment, Potentiomctric Titration and Polyclcctrolyte Adsorption’, Hol$orschung, 1996, 50(3), 208-214. 2. W. C. Dence and D. W. Reeve, Pulp bleaching, Principles and practice, Tappi Prcss. Atlanta, 1996, pp. 100,684-686. 3. T. Vuorinen, J. Buchert, A. Teleman, M. Tenkanen, P. Fagerstrom, ‘Selective hydrolysis of hexcncuronic acid groups and its application in ECF and TCF bleaching of kraft pulps’, International Bleaching Conference, Book 1, Washington, 1996,43-5 1. 4. J. Colodette and C. Dence, ‘Factors affecting hydrogen peroxide stability in the brightening of mechanical and chemimechanical pulps - part IV: The effect of transition metals in Norway spruce TMP on hydrogen peroxide stability’, J. Pulp Pap. Sci., 1989, 15(3), J79-J83. 5. D. Fengel and G. Wegener, Wood. Chemistry, Ultrastructure, Reactions, Walter de Gruytcr, Berlin, 1984, pp. 26-28, 106, 145. 6. J. Gierer, ‘Basic principles of bleaching, Part 2: Anionic processes’, HolZforschung, 1990,44(6), 395-400. 7. J. Giercr, ‘Chemistry of delignification, Part 2: Rcactions of lignins during bleaching’, Wood Sci. Technol., 1986,20(l), 1-22. 8. T. M. Herrington and B. R. Midmore, ‘Adsorption of ions at the ccllulosc/aqueous electrolyte intcrface, Part 1 .-Chargc/p€Iisotherms’, J. Chern. Soc., Faraduy Truns., 1984,80(1), 1525-1537.
Carboxyl and phenol groups 135 9. T. Q. Y. Zhu, K. Kuys, I. Parker and N. Vandcrhock, ‘Studics on thc surface charge of eucalypt pulps by potentiometric titration’, 8“’ International Symposium on Wood and Pulping Chemistry, Helsinki, 1995,243-248. 10. I. M. Kolthoff and P. J. Elving: Treutise on unulytical chemistry, Part I, Theory and Practice, Vol 1, Intersciencc, New York,1959, 240. 1 1. G. Gran, ‘Determination of the Equivalence Point in Potcntiomctric Titrations. Part 11’, Analyst, 1952,77(1 l), 661-671. 12. F. Ingman and E. Still, ‘Graphic method for the determination of titration endpoints’, Talanta, 1966, 13, 1431-1442. 13. A. Ivaska, ‘Graphic determination of equivalence volumes in potentiometric titrations of mixtures of weak acids - 1’, Tulunta, 1974,21, 1167-1 173. 14. M. Siltala, K. Winberg, K. Henricson and B. Lonnberg, ‘Mill scale application for selective hydrolysis of hexeneuronic acid groups in TCF, bleaching of haft pulp’, International Pulp Bleaching Conference, Helsinki, 1998, Book 1,279-287. 15. J. Laine, ‘Effect of ECF and TCF bleaching on the charge propcrtics of kraft pulp’,, Paper and timber, 1997,79(8), 55 1-559. 16. I. ForsskBhl, T. Popoff and 0. Theander, ‘Rcactions of D-xylose and D-glucose in ’ alkaline, aqueous solutions’, Carbohydrate Research, 1976,48( 1-2), 13-2 1.
Effect of ozone bleaching on the fibre properties of pine and birch kraft pulp Anu Scisto'), Kristiina Poppius-Levlin and Agneta Fuhrmann KCL, P.O. Box 70,0215 1 ESPOO,FINLAND ')Present address: STFI, P.O. Box 5604,11486 Stockholm, SWEDEN
ABSTRACT Pine and birch kraft pulps were bleached with a TCF scqucnce including an ozone stage. Different ozone charges were used to obtain pulps with different final brightnesses (bctwcen 85 and 92%). Pulps bleached with an ECF sequence or a TCF sequence in which ozone was replaced by peracetic acid wcrc used as references. Main focus was on fibre deformation (curl and kink indexes) and how this was affected by ozone charge. The effect of fibre deformation on zero-span index and paper strength was also considered. No major differences in the degree of fibre dcformation were observed directly after the ozone stage. After final bleaching with alkaline hydrogen peroxide, there was a clear trend towards greater curl and kink indexes with increasing ozonc charge. A similar effect was seen in the case of zero-span index. Some straigthening of the fibres occurred in the final alkaline bleaching stage, depending on the carboxyl group content of the pulps. A connection was found betwccn kink index and both zcro-span index and tensile index of pine and birch pulps. There were also indications of a connection between fibre deformation and tear index of pine pulps. INTRODUCTION
Since the first mill scale ozone bleaching plants were introduced in 1992, several mills have invested in ozone bleaching. Ozone is used not only in TCF bleaching, but also in ECF sequences. As a bleaching chemical, ozone is a strong oxidizing agent and it reacts with both lignin and carbohydrates. More carbohydrates have been found in the bleaching effluents when ozone has been used in the bleaching scqucnce instead of Caro's acid or peracetic acid Ill. However, the dissolved carbohydrates in solution can react further with ozone 121and with peracetic acid 131. During bleaching, mechanical forces arc applied to the pulp. When bleaching is carried out in the laboratory scale, these forces mainly refer to pulp mixing. As ozone is in the gas phase, thorough mixing is rcquircd in the bleaching stage in order to achieve a good bleaching result. Typical conditions for ozone bleaching have been reported to be about 6 kg ozone per tonne of pulp (ptp) for 2 min at pH 3, 50°C and 12 or 40% concistency 141. A high pH or temperature in the ozone stage may result in radical reactions and hence a risk for depolymerization and loss of fibre strength increases. As ozone reacts not just with lignin but also with carbohydrates, and as the pulp is subjected to vigorous mixing during ozone bleaching, it is of interest to consider the effects on fibre properties. The combination of chemical and mechanical action on fibres in the ozone stage may also rcsult in fibre deformation (curl and kink), which affects both fibre strength and the paper properties of the pulps 151. It has been found that delignification with ozone to kappa number below 3-4 results in lower fibre strength, as indicated by zero-span strength measured from rewetted paper strips 161.
138 Pulp production and processing In this study, special attention was given to the fibre properties (shape and length) of ozone bleached pine and birch pulps. The aim was to determine the effcct of ozone bleaching on fibre deformation (curl and kink indexes) and whether this affects zcrospan strength and paper strength. Pulps were analysed after each bleaching stage. In the ozone stage, three different ozone charges were used. Two pulps were uscd as references, one bleached with an ECF scquence and the other with a TCF sequence containing peracetic acid instead of ozone. EXPERIMENTAL Purps Unbleached pine (Pinus sylvestris) and birch (Betula sp.) kraft pulps were prepared for thc bleaching experiments. In the pulping of pine, effective alkali was 5 molkg, sulphidity 35% and maximum temperature 170°C. The corresponding values in the pulping of birch were 4.5 mol/kg, 35% and 165°C. Pulps were bleached with TCF and ECF scquenccs according to Fig. 1. Both cooking and bleaching were carried out in the laboratory. Dctails of the bleaching sequenccs and the conditions used are given in Table 1 for both pine and birch pulps. Pulp properties are presented in Table 2.
Fibre properties Zero-span tensile index was measured from rewetted paper strips (60 g/m2) on a Pulmac tester. Fibre length and shape were detcrmined from unbeaten fibres using FiberMaster at STFI (Swedish Pulp and Paper Research Institute). About 10 000 fibres were measured in each samplc and duplicate measurements were performcd in each case. Fibre shape factor is calculated as the ratio between the length of the diagonal in the circumscribed rectangle and the length of the fibre /7,8/. Curl index is calculated from thc shape factor (llshape factor -1). Fibre kinks are defined as distinct angular bends along the length of pulp fibres /9/. Kink index is calculated according to Kibblewhite and Brookes /lo/.
and birch pulp
delignification
D(EP)D, QPPaaP
I QPl
I
Zlow
z medium
Figure 1. Bleaching sequenccs uscd for pine and birch haft pulps.
Ozone bleaching
139
Table 1. Bleaching conditions for pine and birch haft pulps. PINE BIRCH Bleaching Chem. Chem. Temp. Bleaching Chem. Chem. Temp. stage charge consumpt. OC stage charge consumpt. "C kg/BDt k@Dt k@Dt kdBDt 0 ') 95 0 " 100 00 ') 95 00 ') 100 OOP 30 22 90 OOP2) 20 5 85 3) OOPZlOW 2 2 3, OOPZlOW 1 1 OOPZlowP 10 5 90 OOPZlowP 5 2 90 OOPZmed 4 3 3, OOPZmed 2 2 3) OOPZmedP OOPZhigh OOPZhighP OOPPaa OOPPaaP 001) OOD(EP) OOD(EP)D ') O2pressure 8 bar
20 6
7 4
90
20 20 10 24 3 20
8 17 2 24 2 20
10 5
4 4
90
3,
OOPZrnedP OOPZhigh
90 80 90 60 70 75
OOPZhighP OOPPaa OOPPaaP OOD OOD(EP) OOD(EP)D
10 20 20 20 3 3
6 19 3 20 2 2
90 80 90 60 65 70
3)
were removed prior to the P-stage by chelating the pulp with DTPA (0.3%) at 60°C, 8% consistcncy and pH 5 for 60 min. ') Room temperature 2, Metals
Table 2. Propertics of TCF and ECF bleached pine and birch kraft pulps. PINE BIRCH Bleaching Kappa Visc. Brightness Bleaching Kappa Visc. Brightness stage number mYg % stage number mug % Unbl. 1050 25.5 26 Unbl. 20.5 1280 26 0 12.3 870 34 0 11.8 1040 45 9.5 40 00 830 00 9.8 960 51 OOP 5.4 790 80 OOP 6.8 910 80 OOPZlow 3.3 740 81 OOPZlow 5.2 880 82 OOPZlowP 2.3 670 87 OOPZlowP 4.3 870 85 OOPZmed 2.4 700 84 OOPZmed 4.6 880 81 OOPZmedP 1.1 640 90 OOPZrnedP 3.4 740 87 OOPZhigh 1.2 680 86 OOPZhigh 1.9 850 86 OOPZhighP 0.5 600 92 OOPZhighP 1.3 790 90 OOPPaa 1.9 OOPPaa 85 1.8 910 85 780 OOPPaaP 2.3 88 OOPPaaP 760 1.2 890 89 OOD 4.6 830 68 OOD 4.4 900 78 OOD(EP) 3.2 800 80 OOD(EP) 3.5 890 86 OOD(EP)D 0.9 89 OOD(EP)D 760 2.8 870 89
140 Pulp production and processing Otherproperties
Viscosity (SCAN-CM 15:88) and kappa numbcr (SCAN-C 1:77) were determined according to standard methods. Carboxyl group contcnt was determined by conductometrictitration /11/. Laboratory handsheets were preparcd before and after PFI beating. The physical properties of the pulps (tensile and tear indexes, IS0 5270 and Scott bond, TAPPI T833) and pulp brightness (IS0 3688 without the addition of EDTA) were determined according to standard procedures.
RESULTS AND DISCUSSION Fibre deformation Pine and birch haft pulps werc bleached to 8592% brightness with TCF sequences containing different ozone charges. In addition, a lower hydrogen peroxide chargc was used for the low ozone charge (Zlow) pulps than for the other ozone bleached pulps in order to reach final brightness of about 85%. Pulps bleached with an ECF or TCFPaa sequences were uscd as references. The kappa numbers, viscosities and brightnesses of the pulps are given in Table 2. Slightly higher viscosities after final bleaching of the ozone pulps may have been possible if a chelation stage was included bctween the Z and P stages 1121. Regardless of the bleaching sequence, curl and kink indexes increased as bleaching proceeded. The ozone charge had no direct effect on the degree of fibre deformation, but after the following alkaline bleaching stage differences were seen between the pulps. Curl indcx after the ozone stage varied between 0.30-0.32 in the case of pine pulp and between 0.15-0.17 in the case of birch. After final bleaching, the curl indexes were still quite similar (Pig. 2a). Some straightening of the fibres occurred during the alkaline peroxide stage, as the curl indexes of pine pulps varied between 0.26-0.3 1 and those of birch pulps between 0.12-0.15 after final bleaching. Howcvcr, there were indications in both pine and birch pulps that higher ozone charge resulted in greater fibre curl. The reference pulps had curl indexes very close to those of the ozone blcached pulps. After the ozone stages, the kink indexes of pine pulps were very similar (kink index about 5.8). The same was also observed for birch pulps (kink index about 4.5). Difference were seen in the kink indexes of the pulps after final bleaching (Fig. 2b). In the case of pine the differences were quite small, although there was a slight straightening of the fibres in the final alkaline peroxide stage. Increasing the ozone charge from 2 to 4 k@Dt had no effect on the amount of kinks produced, but thc highest ozone charge resultcd in slightly higher kink index. All the TCF blcached pine pulps had a higher kink index than the ECF bleached pulp, but a difference between ozone and peracetic acid bleached pulps was seen only with the highest ozone charge (Fig. 2b). The Z~OW birch pulp had the lowest kink index of all the birch pulps presented in Fig 2b. The ECF pulp had a slightly higher kink index than the Zlow pulp, while that of the peracetic acid pulp was quite similar to the higher charge ozone pulps (Zrnedium and Zhigh).
Ozone bleaching 141
I
Curl lndex
0.2
--------
0.15 0.1
O
Zlow
ECF
a)
Paa
Zlow
Zmediurn Zhigh
ECF
Paa
ECF
Paa
BIRCH
PINE Klnk Index 6
5 4
3 2 Zlow
Zmediurn
Zhigh
ECF
Paa
Zlow
Zmediurn Zhigh
b)
Figure 2. Curl index (a) and kink index (b) after final bleaching with a sequence containing ozone or peracetic acid (Paa) and with an ECF sequence. Pine pulp, white columns and birch pulp, grey columns. After final bleaching, there was a clear trend towards increasing kink index with increasing ozone charge for the birch pulps. This may be partly due to the ozone stage itself, although its effect does not appear before the following alkaline stage. The difference between the results obtained before and aftcr the final peroxide stage may also bc due to the swelling properties of the fibres in alkaline environment in the peroxide stage. As seen from both curl and kink indexes of the pulps, some straightening of the fibres occurred in several pine and birch pulps in the final bleaching stage. Although swelling properties of the pulps were not studied in this work, the carboxyl group contents of the pulps wcrc dctcrmined to give an indication of swelling ability (Table 3). Carboxyl group content docs not affcct the fibre deformation directly, i.e. there is no correlation between carboxyl group content and curl or kink index. However, carboxyl group content affects the swelling properties of fibres, and hence also the ability of fibres to straighten out. The Zlow birch pulp had the lowest kink index, and the highest carboxyl group content. Hence, in the final pcroxidc stage of thc Zlow birch pulp, the fibres might have been able to swell and straighten out to some extent. In fact, the kink index of Zlow pulp decreased from 4.5 after the ozone stage to 4.0 aller the peroxide stage. The kink index of Zmedium pulp was unaffected by the peroxide stage, while that of Zhigh pulp increased to 4.8. The carboxyl group content of pine pulps was lowcr than that of birch pulps, and the difference between the pulps was smaller that in the case of birch pulps. Hence, the alkaline peroxide stage had less effect on the kink indexes of pine pulps. Howcver, some straightening was observed in the pine fibres as well.
142 Pulp production and processing Table 3. Carboxyl group content of pine and birch pulps after final bleaching. PINE PULP Carboxyl groups BIRCH PULP Carboxyl groups, mmollkg mmolikg Zlow 45 Zlow 105 Zmedium 40 Zmedium 95 Zhigh 35 Zhigh 75 ECF 30 ECF 90 Paa 40 Pa 80
Zero-span index As with the curl and kink indexes, the negative effect of incrcasing ozone charge on fibre strength, as indicated by rewetted zero-span index, was seen after final bleaching. No major differences were observed in the zero-span indexes of pine and birch pulps with increasing ozone charge directly after the ozone stage. For pine pulps, zero-span index was 100-102N d g and for birch pulps 100-105 N d g . Zero-span indexes after final bleaching and before and after PFI-beating are presented in Fig. 3a for pine and in Fig 3b. for birch pulps. The trends are vcry similar to those presented in Fig. 2 for curl and kink indexes. For pine pulps, the zero-span index of Zlow and Zmedium were the same before and after beating to tensile index 70 N d g while that of Zhigh was slightly lowcr after beating. In thc case of unbeaten birch pulps, zero-span index decreased with increasing ozone charge. Even though the difference between the ozone bleached birch pulps decreased after beating to 60 N d g , the same trend was observed after beating. Similar trends for ozonc-bleached pine and birch pulps have also been shown by Fuhrmann et al. 14,6/. Fairly similar zero-span indexes were obtained with the different TCF bleaching sequences (Figs. 3a and b). Only the Zhigh pine and birch pulps differed from the other TCF pulps by having a lower zero-span index. Before beating, the ECF bleached pine pulp had a clearly higher zero-span index than the TCF pulps. Also the ECF bleached birch pulp had a higher zero-span index than the TCF bleached pulps at the unbeaten stage. After bcating, the differences betwccn the pulps were smaller for birch pulps than for pine. As the fibres were straightened out during PFI-beating, zero-span indexes were highcr after beating. A correlation was seen between the kink index and zero-span index of the pulps (Figs. 4a and 4b). All bleaching sequences are included in Fig. 4. The higher the kink index as bleaching proceeds, the lower the zero-span index. Even though the trend is very clear, the variation in the results was large, especially in the case of birch. R2value was 59% for pine pulps and 51% for birch pulp. Similar connection between fibre deformation (sum of kinks, twists and angular folds on fibres) and zero-span index has bcen observed for other pulps as wclll81.
Fibre length Fibre length was not affected by ozone charge. The maximum difference between the TCF and ECF bleachcd pulps was only 0.1 mm for both pine and birch pulps. However, aftcr beating, more fines have bcen dctected in ozone bleached pulp than in pulps bleached with an ECF sequence or with peracetic acid 11,131. This would indicate that ozone bleached fibres are more prone to breaking during beating than other fibres. According to Hartlcr 1141, deformed parts on fibrcs can be severe enough to cause fibre breakage in subsequent mechanical treatment. Hence, fibre deformation, as observed
Ozone bleaching
143
especially with the Zhigh pulps, may lead to a decrease in fibre length and an increase in after beatinghefiningio the desired tcnsile strcngth. 140 120
100
80 60 40
20 0
Zlow
Zmediurn
Zhigh
ECF
Paa
a) 7
Zlow
-
Zmediurn
I
Zhigh
Paa
ECF
b)
Figure 3. Zero-span tensile index (rewctted) of a) bleached pine pulp and b) bleached birch pulp. White columns before beating and grey columns after PFI-beating to tensile index 70 Nm/g in the case of pine pulps and 60 Nm/g in the case of birch pulps.
d l
Zero-span index, Nmlg
Zero-span index, Nmlg
150
140 130
120 110
100 90
I
50
3
4
5
6
Kink index 0 before beating .alter
7
70 8o 2
3
4
5
6
Kink index beating
I 0 before beating
affer beating1
Figure 4. Connection between kink index and zero-span strength of a) pine pulps and b) birch pulps.
144 Pulp production and processing Paper properties
In most of the cases, the more kinks produced in the fibres, the lower thc tensile index obtained, both before and after PFI bcating. This is illustrated in Figs. 5a and 5b for pine and birch pulps, respectively. Again, results from all bleaching sequences are included. The correlation betwecn kink index and tensile index of PFI-beaten pine pulps is good, R2= 71%. In birch pulps, the variation betwccn the samples is quite large, but a similar trend can be observed there also (R2= 58%). A similar conncction bctwecn tensile index and fibre deformation has also bccn found with other pulps 18,151. It has been found earlier 161 that, for pine krafi pulp, a clear drop in tear index results if ozone is uscd to delignify the pulp to low kappa number (below kappa number 3-4). Low kappa numbers were obtained after all the ozone stages carricd out for the pulps in this study (kappa number between 3.3 and 1.2 for pine pulps and 5.2-1.9 for birch pulps). Tear index at tensile index 70 N d g for pine pulps and at GO N d g for birch pulps is presented in Figs. 6a and b. The results for pine pulps (Fig. 6a) agree well with those obtained earlier with a TCF sequencc containing ozone /GI. Tcar index drops by 10% whcn kappa number after the ozone stage is reduced to below 3. In the case of birch pulps the effect is not as clear, as no significant reduction in tear index was observed (tear index from 8.9 to 8.5, Fig. 6b). A slightly higher tear index was obtained with the ECF sequence than with any of the TCF sequcnccs uscd for pine pulp, but the tear index of birch pulps was very similar regardlcss of thc blcaching sequence.
Tensile strength, Nmlg
Tensile strength, Nmlg
120
I
ao 70
60
3
4
5
r
6
2
3
[ 0 before beating
after beating
4
5
8
Kink index
Kink index
I
10
before beating
after beating
I
Figure 5. Conncction betwcen kink index and tcnsilc strength of a) pine pulps and b) birch pulps.
Ozone bleaching 145 m
161
1
'.1 Paa
I
Zlow
,-
Zmedium
Z high
ECF
I -
Zlow
Zmedium
Zhigh
ECF
Paa
b)
Figure 6.Tear index of a) bleached pine pulps at tensile index 70 N d g and b) bleachcd birch pulps at tensile index 60 Nm/g. A connection was noted between fibre deformation and tear index of pine pulps, propably duc to the connection between the kink index and zero-span index. This was expected as sheets containing stronger fibres have been found to have higher tear strength because more work is needcd to pull the fibres out from a sheet than to break them 116,17/. In the case of birch pulps the connection was not as clear. Also, the variation in the tear indexcs of the birch pulps was fairly small. Hence, fibre deformation did not greatly affect the tear index obtainable with birch pulps. Scott bond has been found to increase when high ozone chargc is used to bleach pine pulp 141. No clear connection was found here between fibre deformation and Scott bond. On the other had, it has been found that fibrillation may occur in the ozone stage, which can to some extent cxplain the increase in Scott bond 151. Hencc, the fibres may undergo a certain amount of beating in the ozone stage, especially when high ozone charges are used.
C0N CLU S I 0N S 0
0
0
No significant effect of increasing ozonc charge on fibre deformation (curl and kink indexes) was seen directly after the ozone stage, but after the final alkaline peroxide stage, clear diffcrences werc observed between the pulps After final bleaching, fibre deformation increased with increasing ozone charge. Somc straightening of the fibres occurred in the final peroxide stage, depending on the carboxyl group content of the pulp As kink index of the pine and birch pulp increased, zero-span strength and tensile strength decreased
146 Pulp production and processing
0
An increase in kink index resulted in a decrease in tear strength in the case of pine pulp, possibly due to the decrease in zero-span strength. The tear index of birch pulp was not affected by fibre deformation. The shorter length of birch fibres compared with pine may partly explain why the tear index of birch pulps was less affected by fibre deformation Increasing fibre deformation had no direct impact on either fibre length or Scott bond.
ACKNOWLEDGEMENTS Financial support from the Nordic Industrial Foundation (NordPap programme), TEKES and the member companies of KCL is gratefully acknowledged.
REFERENCES
1
2 3
4 5
6 7 8
9 10 11
12 13 14
A Fuhrmann, X-L Li, R. Rautonen, L Toikkanen, T Hausalo and P-E Sigfors, ‘Influence of TCF and ECF bleaching chemicals on softwood haft pulp components’, Pup. Puu, 1996 78(4) 172-179. T Kishimoto and F Nakatsubo, “on-chlorine bleaching of h a f t pulp. 11. Ozonation of methyl-4-O-Ethyl-~-D-Glucopyranoside (2) Quantitative analysis of reaction products’, Holzforschung, 1996 SO(4) 371-378. . A-S Juskelaincn and K Poppius-Levlin, ‘Carbohydrate reactions in peroxyacetic acid bleaching’, Submitted for publication, 1998. B Backlund, G Gellerstedt, A Fuhrmann, R Rautonen, N Soteland and C-A Lindholm, ‘Bleaching of chemical pulp. Results from a nordic research program and “state of the art” 1997’,SCANForsk rapport 705,1997.(In Swedish) A Seisto, K Poppius-Levlin and A Fuhrmann, ‘Correlation between chemical, fibre and paper properties of TCF and ECF blcached kraft pulps’, Int. Pulp Bleaching Conf, Helsinki, 1998. A Fuhrmann, R Malinen, R Rautonen, T Hausalo and P-E SAgfors, ‘Influence of ozonation parameters on delignification and cellulose degradation’, Int. Symposium on Wood and Puking Chemistry, Hclsinki, 1995. H Karlsson and P-I Fransson, ‘STFI FiberMaster ger pappersmakarna nya muskler’, Svensk Papperstidn., 1994 97(10) 26-28. U-B Mohlin, J Dahlblom and J Homatowska, ‘Fiber deformation and sheet strength’, TuppiJ, 1996 79(6) 105-111. R P Kibblewhite, ‘Effects of fiber kinking and pulp bleaching on wet web strength’, TuppiJ, 1974 57(8) 120-121. R P Kibblewhite and D Brookes, ‘Factors which influence the wet web strength of commercial pulps’, Appita, 1975 28(4) 227-143. S Katz, R P Bcatson and A M Scallan, ‘The determination of strong and weak acidic groups in sulphite pulps’, Svensk Papperstidn., 1984 87(6) 48-53. P AxegArd, E Bcrgnor, M Ek and U Ekholm, ‘Bleaching of softwood krait pulps with H202,O3 and ClOz’, Tappi J., 1996 79(1) 113-119. R Malinen, T Rantanen, R Rautonen and L Toikkancn, ‘TCF blcaching to high brightness bleaching sequences and pulp properties’, Int. Pulp Bleaching Conf, Vancouver, 1994. N Hartler, ‘Aspects on curled and microcompressed fibres’, Nord. Pulp Pup. Res.
-
J. 1995 lO(1) 4-7.
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15 A Seisto and K Poppius-Levlin, ‘Fibre characteristics and paper properties of formic acid I peroxyformic acid birch pulps’, Nord. Pulp Pap. Rex J., 1997 12(4) 237-243. 16 J A Van den Akker, A L Lathorp, M H Voelker and L R Dearth, ‘Importance of fiber strength on sheet strength’, Tuppi J., 1958 41(8) 416-425. 17 D H Page and J M MacLeod, ‘Fiber strength and its impact on tear strength’, Tappi J., 1992 75(1) 172-174.
Studies on the use of black liquor evaporation condensates at different bleaching stages Klaus Niemela, Reijo SaunamPki and Raimo Rasimus The Finnish Pulp and Paper Research Institute (KCL) PO BOX 70, FIN-02151 Espoo. Finland
ABSTRACT Black liquor cvaporation condensatcs were used at laboratory scale as wash watcr during blcaching of kraft pulps, and their effects on off-odour and other pulp propcrties were monitorcd. The applied bleaching chcmicals or sequences included hydrogen peroxide, chlorine dioxide, pcroxyacetic acid and ozone. It was found that the applied secondary condensates did not affect typical pulp properties. such as kappa number, viscosity and brightncss. It was also found that the use of thc secondary condensates did not result in increased off-odour levcls of the pulps.
INTRODUCTION Black liquor evaporation condensates are typically used for washing of unbleached pulp and lime sludge and in preparation of cooking liquors. The closing of water cycles in kraft pulp mills would mean replacing these condensates with recycled and purified bleach plant filtrates. In turn, this would result in increascd use of black liquor condensatcs in other applications, prcfcrentially as washing water at differcnt bleaching stages [l, 21. The black liquor condensates always contain varying amounts of volatile organic impurities [see ref. 3 for a review], somc of which are known as odour-causing contaminants. Thus, there is a risk that the use of the condensates, even the cleanest fractions, would result in transfer of odour to pulps, or would have some other undcsired effects [4]. If so, external purification steps might be required to make the condensates reusable at bleaching stagcs. In ordcr to study this question, various black liquor condcnsates or condensate combinations of industrial origin were now used at laboratory scale at several bleaching stages. During these studies, the effect of the condensatcs on odour and othcr pulp properties (such as brightness, kappa numbcr and viscosity) was investigated, and the main rcsults are now reported. The work dcscribed in this paper forms part of a three-year (1996-1999) EU-project [2] ‘Separation methods for closed-loop technology in bleached kraft pulp manufacture’.
EXPERIMENTAL General Several sets of bleaching expcrirnents, with and without condensates, were carried out in ordcr to study the effect of different condensates on diffcrent pulp propertics at different blcaching stages. These includcd ozone, hydrogen pcroxide, chlorine dioxide, and pcroxyacetic acid stages. Different bleaching conditions and two different pulps wcrc used to simulate the position of each stage either in the beginning or at the end of the bleaching sequence.
150 Pulp production and processing Materials
Two softwood kraft pulps (I and 11, Table 1) sampled at a mill after diffcrcnt delignification stages were uscd. Of these, pulp I was used in bleaching experiments where the main attention was paid to transfer of odour from condensates to the pulp, and the first bleaching stage was simulated. In turn, pulp I1 was used in bleaching experiments where the main attention was paid to other pulp propcrties, and the end-bleaching stages were simulatcd. Table 1. Properties of softwood krafi pulps used for the bleaching experiments. Pulp I was sampled after oxygen stage, and pulp I1 after peroxide stage. After 010-Do-Eop stages (11) Pulp property After 0/0 stage (I) Kappa number 10.9 3.4 Viscosity 870 mdl 850 mdl Brightness 39% IS0 72.7% I s 0
Different condensates (Table 2, all collected at the mills) or condensate mixtures were uscd during the bleaching expcrimcnts. Most of the work was carried out with a “typical” secondary condcnsate, but in order to study more pronounced effects of unrealistically high amounts of impurities, the secondary condensate was occasionally spiked with varying amounts of thc foul condensate, In addition, selected experiments were carried out after spiking the secondary condensate with a small amount of black liquor (collected at the same mill as the condensates). Table 2. Typical concentrations (mg/l) of selected compounds in secondary and foul condcnsates used in the bleaching experiments. Constitucnt Secondary condcnsate, mdl Foul condensate, m d l 7,500 Methanol 300 Ethanol 10 300 Acetone 1 130 Acetaldehydc 4 140 Dimethyl sulphide traces 200 2 200 Dimethyl disulphide TOC 200 12,500 Bleaching experiments
General outline of blcaching cxperiments to monitor the transfer of odour from thc condensates to bleached pulp is given in Fig. 1, and the bleaching conditions for the main scts of experiments are compiled in Tables 3 and 4. After bleaching at a given consistency, the pulps were diluted to 5% consistency, filtered and washed. The original condensates and filtrates were subjected to GC or GUMS analyses, and the washed pulps were taken to sensoric evaluation (off-odour tests), both as such and after drying overnight at 60 ‘C. In the first expcrimcntal set (Table 3), the applied condensate was the only water used at the bleaching stage. In thc sccond experimental set (Table 4), however, the amounts of pure watcr and condcnsate varicd (see Results and Discussion).
Black liquor evaporation condensates
[.
11
ondensatc
Chemicals
1
Filtrate for
GUMS
Wet pulp for odour test
151
Dry pulp for odour test
Fig. 1. General outline of the bleaching expcnments. Two types of reference tests were also performed: one set without bleaching chemicals, and another set with pure water instead of the condensatcs.
Table 3. Bleaching conditions for experimcntal set I (oxygcn delignified pulp), simulating the first bleaching stage* (P, hydrogen peroxide; Paa, peroxyacetic acid; D, chlorine dioxide; and 2, ozone). Z P Paa D Condition (charge) 10 10 10 10 Consistcncy, % 70 70 70 90 Temperature, "C Time, min 180 180 180 180 pH (final) I1 6 4 3 NaOH. % of pulp 2.0 2.0 H2S04, % of pulp 1 .o DTPA, % of pulp 0.2 0.2 MgS04, % of pulp 0.2 0.25 Bleaching chemical, % of pulp 2.0 1.5 3.0 0.4 *The condensates included a typical secondary condensate (loo%), the samc condensate with 10%of foul condensate, and the same condensate with 2%of black liquor. '
Tablc 4. Bleaching conditions for cxperimcntal set II (O/O-Do-Eop dclignified pulp), simulating the final bleaching stagc* (P, hydrogen peroxide; Paa, peroxyacetic acid; D, chlorine dioxide; and Z, ozone). P Paa D Z Condition (charge) Consistcncy, % 10 10 10 12.5 Temperaturc, OC 90 80 70 50 Time, min 120 120 120 10 pH (final) 10 5 3 3 NaOH, % of pulp 0.8 0.9 HzS04, % Of pulp 0.5-0.8 DTPA, % of pulp 0.2 MgS04, % of pulp 0.2 Blcaching chcmical, % of pulp 0.8 1.2 0.8-1.2 0.35 *The final scquences included D/Q-P, Paa/Q-D, ZQ-P, and UQ-D stages, and the condensates were uscd as wash water at the sccond last stages. The condensates included a typical secondary condensate, and a mixture of the secondary (67%) and foul condensates (33%).
152 Pulp production and processing Monitoring of odour transfer
Odours of wet and dry pulps (cf; Fig. 1) wcre characteriscd for their intensities and profiles by 10-person panels, as described in dctail in ref. 5. According to this proccdure, each panellist estimates the odour intensity using a scale from 0 to 4, with a precision of 0.5 units. In this project, however, average intensities wcrc then calculated with a precision of 0.1 units. Determination of odour profiles was based on thc use of 86 possible descriptors (instead of 140 mentioned in ref. 5). During data processing, these wcre divided into 25 different hedonic groups ranging from pleasant to unpleasant (Fig. 2). Although the profiles cannot be rcproduced in this paper, they wcre constantly checked to monitor possible changes, particularly in the area of unplcasant odours.
I Appllcablllty,
'1
%
2 ; i
11
;
- 13
-
-
a
21
14
30
7.0
10 0
I
I
~-plsasant
Hedonlc groups
unpleasant
Appllcablllty, %
--+
10
;
19
30 20
10
0 <-pleasant
Hadonlc groups
unpleasant
Fig. 2. Example of odour profiles in wct (upper panel) and dry (lower panel) pulps from the same bleaching experiment. Other analytical determinations
Typical pulp properties, such as kappa numbcr, viscosity, brightness, and PC number, were measured according to standard procedures. In addition, selected pulp samples wcre analyscd for their toxicity using a light bacteria (VibrioJsheri) test developcd at our Institute [6].In this test (based on ISO/CD 11348), the change of luminescence by
Black liquor evaporation condensates 153 culture of V.fisheri is determined with luminomctry by combining specified amounts of test sample in sodium chloride solution with the bacteria suspension, and the inhibitory or induction effect of the sample is determined by means of dilution series. Finally, the cffcct of the sample is expressed as the concentrations which rcsult in 20% and 50% light reductions (ECzo and ECso) after 15 and 30 min compared to the blank. The condensates and filtrates (cJ: Fig. 1) from selected experiments were analysed for TOC and for various organic impurities. The main impuritics were quantified with the help of hcad-space gas chromatography. In addition, selected samples were analysed by combined gas chromatography and mass spectrometry. The sample preparation for these investigations was based either on head-space, or on extraction with ethyl acetate or other suitable solvents, followed by careful concentrations. Dctailed description of the analytical methods will be givcn, however, in a separate forthcoming paper.
RESULTS AND DISCUSSION In the following discussion, the main attention will be paid to the transfer of odour from different condensates to pulp during bleaching (experimental set I, discussed in Section I), and to the effect of the condensate use on other pulp properties (Experimental set 11, discussed in Section II). In addition, mass spcctrornetric identifications are briefly discussed (in Section 111), as this resulted in detection of numerous novel condensate compounds. Effect of condensate use on odour intensity of pulps
The results (Tablcs 5-7) show that the use of the condensates during bleaching had only little effcct on the odour intensity of the bleached pulp. Table 5. Odour intcnsities of wet and dry pulps after bleaching in the presence of the secondary condensate. Without condensate Applied bleaching With condcnsate chemical Wet pulp Dry pulp Wct pulp Dry pulp
Water (start-refcrcnce) Hydrogen peroxide Peroxyacctic acid Chlorine dioxidc Ozone Water (end-reference)
3.3 1.4 1.8 1.9 3.3 3.4
1.3 1.o
1.4 0.8 2.5 1.4
1.4 0.9 1.5 1.7 3.2 1.1
0.9 0.6 0.7 0.7 1.9 1.o
Table 6. Odour intensities of wet and dry pulps after bleaching in the prcsence of thc secondary condensate, spiked with 10% of foul condensatc. Applied bleaching With condensate Without condcnsate chemical Wet pulp Dry pulp Wet pulp Dry pulp
Water (start-reference) Hydrogen peroxidc I’eroxyacetic acid Chlorine dioxide Ozone
3.7 1.2 1.1 2.2 2.7
2.7 0.6 0.1 1.2 2.3
1.1 1.2 0.9 1.4 2.8
0.6 0.6
0.2 0.6
2.2
154 Pulp production and processing Table 7. Odour intensities of wet and dry pulps after bleaching in the prescnce of the secondary condensate, spiked with 2% of black liquor. Applied bleaching With condensate Without condensate chemical Wet pulp Dry pulp Wct pulp Dry pulp Water (start-reference) 3.6 1.3 1.8 1.o I Iydrogcn peroxide 1.7 1.3 1.9 0.8 0.9 1.7 1.4 Peroxyacetic acid 2.2 Chlorine dioxide 1.7 1.2 2.1 1.2 2.6 Ozone 3.4 3.4 2.6
IF only the secondary condcnsate (Table 5) was applied without any bleaching chemical, most of the transferred odour rcadily disappeared during pulp drying. In these cases, the odour intensities (1.3 and 1.4) rcmained below 2 which is regarded as a threshold level for slight odour. The wet pulps from the similar trcatments showcd, however, distinct unpleasant odour (as might easily bc cxpected). Hydrogcn pcroxide and chlorine dioxide seem to slightly decrease odour intensities of the pulp, regardless the use of the condensate. Aftcr the ozone treatments, the pulp always showed strong odour intensities. It appears, however, that the odour is more characteristic of the ozone itself, and its presence is not directly due to thc use of the condensate. The ozone-derived odour can be expected to disappear during the later bleaching stages, as ozone is never applied as the final chcmical in modern bleaching sequences. Spiking the secondary condensatc with 10% of foul condcnsate (Table 2) increased the odour intensities of the reference pulps (Table 6), i.e. those “blcached” without any bleaching chemicals. However, the odour intensities were significantly reduced in the presence of the applied bleaching chemicals. Obviously, this rcfcrs to capabilitics of these chemicals to destroy the main odour-causing compounds, such as mercaptans. Similar results wcre also obtained when the secondary condensate contained a small amount of black liquor (Table 7). In each case, the odour profiles (cf: Fig. 2) were also checked. This indicated that the use of hydrogen pcroxide, chlorine dioxidc and peroxyacetic acid always produccd profiles that diffcrcd from those recordcd for the references. Typically, the bleaching chemicals changed the profiles from unplcasant to neutral (or pleasant) area, although some variation occurred. The profile changes with ozonc were not so clear, as a lot of odour is generatcd from ozone itself. Altogether, the above data show preliminary, but encouraging results indicating that evcn relatively dirty black liquor evaporation condensates may have a potential for use as wash water during pulp bleaching. The conclusion is derivcd from clear indication that the applied bleaching chemicals arc capable of destroying typical odour from the condensates during bleaching. It must be remembered, however, that the USC of condensates may have various other impacts on the performance of bleaching processes. Some rcsults related to these risks are discussed below. Effcct of condensate use on other pulp properties
In another set of expcriments (Table 4), the condensates were applicd at late bleaching stages, and the main attention was paid to the possible efrect of the condensates on various typical pulp properties (in addition to odour). If the rcsults on the use of the
Black liquor evaporation condensates
155
condensates at later stagcs are promising, this should readily make it reasonable to assume that the same condcnsates can be applied at any previous stage. Kappa number, viscosity, and brightness. According to the first results (Table 8), the use of the condensates did not have any undesired effect on the kappa numbcr or viscosity of the bleached pulps. Even the use of highly contaminated mixture of secondary and foul condensates did not, in two investigated cases, decrease kappa numbers or viscosity. Thesc conditions wcrc only applicd to test the cffcct of certain extreme conditions on the bleaching result, although there is no sense to consider such combinations as a realistic alternative.
Table 8. The effect of the condensate (applied as wash water at the second last stage) on selected pulp properties. Sequence* Applied condensate Pulp property Ref. (pure water) Secondary Secondary + foul** O/O-D-E--Z/Q-D Kappa number 0.7 0.7 1.2 Viscosity, m d l 680 660 680 Brightness, % IS0 88.7 87.8 84.6 -_____________----------------------------------------O/O-D-E--UQ-P Kappa number 0.9 1.1 Viscosity, mg/l 630 660 660 Brightness, % IS0 86.8 87.7 86.9 -_---_________-----------------------------------------O/O-D-E--Paa/Q-P 1.2 1.2 Kappa number 640 660 Viscosity, m d l
Kappa number 0.7 0.9 Viscosity, m d l 720 720 Brightness, % IS0 89.6 89.0 * The pulp was sampled at the mill aftcr the O/O-D-E stages, aftcr which the blcaching was continued in the laboratory (this is indicated by the double hyphen in the sequence). **Thc mixture of secondary (67%) and foul (33%) condensates was used at sclccted sequences only. Pulp brightness was occasionally slightly reduced by the presence of the secondary condensate, although the most pronounced effect was caused by the foul condensate during the final UQ-D stage. Further studics are now in progress to investigate in more detail the effect of diffcrcnt condensates on the above pulp properties. In these studies, mainly the sccondary condensate will be applied, but some dirtier and cleaner fractions will also bc used. Among the main questions will be the effect of the condensates on brightncss (to solve some conflicting data, Table 8), chcmical consumption, and off-flavour. The results obtained so far are, however, in good agreement with those reported by Annola et al. [4].They found that the use of sccondary condcnsates had only marginal effect hydrogen pcroxide and ozone bleaching. Orlour intensities. The odour intensities and profiles were also determined for the wet and dry pulps in Table 8. The data were in a good agreement with those already
156 Pulp production and processing discussed (Tables 5 and 6), and are not reproduced. Gencrally, the intensities remained bclow that of slight odour (i.e., level 2). The only exccption was ozone bleached pulp, since there is always some distinct odour derived form ozone itself, but not so much from the applied condensatcs. Toxicity. As already described, selectcd pulp samples were subjected to toxicity tests using light bacteria Vibriofisheri. Although the set of the experiments was quite limited, it could not be shown that the use of the secondary condensate during bleaching would have incrcased toxicity of the pulps. 111. Identification of organic compounds in condensates
During this work, scveral secondary and foul condensate samples wcre analyscd in detail by gas chromatography and mass spectrometry. Using solvent extraction (for example with ethyl acetate) and careful concentration for sample preparation, it was possible to dctcct up to 100 peaks in the chromatograms of certain condensate samples (Fig. 3). In this way, a large number of poorly known or completely new condensate impurities could be detected and identified. In many cases, however, their conccntrations wcre quite low. The new compounds include several aliphatic and cyclic sulphur compounds, cyclopentenones, some aromatic compounds, and various nitrogen-containing hcterocyclic compounds. More detailed results will Later be reported elsewhere. It can be expected, howcver, that many sulphur-bcaring compounds (that are mainly responsible for the odour) are easily oxidiscd by the applied bleaching chemicals.
Fig. 3. Example of a partial total ion chromatogram of a concentrated black liquor condensate sample. Thc main pcaks refer to various mono- and scsquitcrpenes, cyclopentenones, thiophcnes and other sulphur compounds, and phenolics.
CONCLUDING REMARKS The results show that at least thc cleancst condensate fractions from black liquor evaporation, such as “typical” secondary condcnsatcs, should bc potcntially useful at several bleaching stages without interfering with the pulp propcrties or without causing increased odour levels in pulps. The results also seem to demonstrate that these condensates could be equally used either in the beginning or at the end of the bleaching
Black liquor evaporation condensates
157
sequences. Although it thercfore looks promising to use such condensatcs during bleaching without external purification, dctailcd case by casc studies will apparently be required, instead of making too generalised conclusions [cf:ref. 41. Furthermore, extreme effects of the condensate use were searched for by applying unrealistically contaminated condcnsate mixtures. Evcn in most cascs it could be shown that the odour intensities remained lower than might be expccted, the risks of other problems (such as decreased brightness, incrcased chemical consumption) quickly exclude practical use of so heavily contaminated water. Further studies are now in progrcss. The main subjects include thc effect of differcnt condensatcs on brightncss and off-flavour of the pulps, and on consumption of the bleaching chemicals. ACKNOWLEDGEMENTS
The work was funded by European Union FAIR Programme, as part of project FAIR3CT96-1360, which is gratcfully acknowledged. Marja Pitkanen and Riitta-Maija Osmonen organised odour tcsts, and Kirsti Jokinen was responsible for the toxicity tcsts. REFERENCES
1 D W Reevc, ‘The effluent-free bleached kraft pulp mill - Part XIII: The sccond fifteen ycars of development’, Pulp Pap Can, 1984 85(2) T24-T27. 2 J I4Wk and P Tomani, ‘Towards the closed cycle mill for bleachcd pulp - milcstones and stumbling blocks’, Papier, 1997 51(6A) V54-V59. 3 B R Blackwell, W B MacKay, F E Murray and W K Oldham, ‘Review of haft foul condcnsates. Sources, quantities, chemical composition and environmental effects’, Tappi 1979 62( 10) 33-37. 4 L K Annola, P Hynninen and K Henricson, ‘Effect of condensate use on bleaching’, Pap PUU,1995 77(3) 111-1 15. 5 L Soderhjelm and M PBrssinen, ‘The use of dcscriptors for the characterisation of odour in packaging materials’, Pap Puu 1985 67(8) 412416. 6 K Jokinen and M Savolainen, ‘Light bacteria test for the detcrmination of toxicity of fiber products’, unpublished report from the Finnish Pulp and Paper Research Institute [in Swedish], 1998.24 pp.
EVALUATION OF PULP FIBRE BEATING Bruno Lihnberg', Tom Lundin', Kimmo Harju' and Petteri Soini*
'Abo Akademi University,Laboratory of Puking Technology,FI-20500 TurkdAbo, Finland
'Sun& Defibrator Valkeakoski
QY,
FI-37400, Valkeakoski, Finland
ABSTRACT
The pulp beating process has been developed over a long period of time and new evaluation methods are required to improve the understanding of the fibre beating mechanisms. This work is representing an initial attempt to provide a model for evaluation of the low-consistency pulp beating process. The simplified modelling produced a diagram for the relationship between motor load and pulp production, which reminds of a corresponding diagram used in wood grinding. The load-production relationship is expressed by P/n versus m2/c, where P = net motor load, n = rotational speed, m = dry pulp production and c = pulp consistency. Beating at constant specific energy resulted in load-production combinations forming a specific linear correlation irrespective of specific edge load SEL, but provided that the rotational speed was constant. On the other hand, when the rotational speed increased under conditions of constant SEL of 2.5 J/m, the load level remained unchanged, as the pulp production increased. INTRODUCTION
Fibre pulps must normally be beaten to fulfill the specific properties required by certain types of printing and writing paper. The beating process has developed over a long period of time and more specific evaluation of the beating effects is needed. The classical concept of specific edge load (SEL) has been completed by the specific surface load (SSL) proposed by Lumiainen (1). SSL is computed as the ratio between SEL and the impact width of the bars (denoted "impact length" in literature), which is dependent on rotor and stator bar widths and the intersecting angle between them. Current beating theories have been thoroughly reviewed by Ebeling (2). This work is part of a larger project aimed at development of the lowconsistency (LC) pulp beating, including mechanical treatment of chemical pulp fibres and pulp slurry rheology as affected by the beater fillings. The project will establish some correlations between common beating and rheological conditions as they appear in between the rotor and stator fillings. A simplified diagram was devcloped by dimensional analysis applied on the beating parameters. The hypothesis tested is suggesting that any beating impact corresponds to a certain load taken by the fibre, and that it is dependent on the fibre grade (softwoodhardwood, kraWsoda efc.)and beating rheology. Pulp fibre beating
The impacts result in a spectrum of treatments from slipping to true cut-off of the fibres as indicated in Table 1. In high-consistency beating the pulp fibres are strongly flocculated and hence they fiotate each other rather than get in direct contact with the beater bars. This
160 Pulp production and processing type of beating is therefore suggested to result in fibre curl, fibrillation and kink dependent on the beating intensity (3).
Table 1. Assumedjibre treatments Intensitv
Impact
Fibre response
Low Low Medium High High
Slipping Tension Compression Shearing cut-off
Curl Straightening Fibrillation
Kink Shortening
In low-consistency beating the fibres are also flocculated, but may separate due to turbulence and thus probably get in contact also with the beater bars. Hence, beating is expected to cause fibre straightening, fibrillation and kinklshortening. Advanced fibrillation will appear as so-called fines, which by definition contain colloidal and solid particles small enough to pass a 200 mesh wire (< 74 pm). Unbleached and bleached pulp fibres have different surfaces, because unbleached fibres contain significant amounts of lignin and extractives (fatty and resin acids), also on the surface, as bleached fibres are cleaner. Hence beating would create colloidal and solid particles dependent on the fibre grade. Beating of unbleached pulp would improve the hydrophilicity by just removing the surface lignin and extractives, as beating of bleached pulp probably would require some fibrillation to expose for example carboxylic groups for improved hydrophilicity. EXPERIMENTAL
Theory of beating The beating of pulp fibres could be generally modelled by considering the number of impacts obtained by a certain fibre running through the beating zone and by considering also the intensity (impulse) of the impacts. The number of impacts on the fibre should be related to factors such as pulp flow which together with the pulp consistency determines the residence time of the fibre. The impact intensity again is related to forces and speeds involved, and of course to the viscoelastic properties of the fibre. Thus the complete set of parameters includes: fibre properties (pulp grade, conformability) and slurry properties (consistency, temperature, pressure) design of fillings (bar length, number of bars, bar angles, depth of grooves) beating process (bar gap, rotational speed or peripheral speed)
0
0 0
From the viewpoint of production capacity it would be useful to form a model where the flow of pulp slurry (V) is the dependent factor, as the other beating parameters independently affect the flow. The initial modelling is simplified by considering a fixed pulp grade providing a pulp slurry of certain temperature and pressure and a certain type of beater fillings. It is evident that the pulp flow, and hence the number of impacts on the fibre, is affected by the fibre consistency (c) of the pulp slurry, the peripheral speed (w) and the
-
-
Pulp fibre beating 161 bar length (2) of beater fillings. The force (F)involved in the impact (intensity, F/A, force over contact area with the fibre and impulse, Fx t, force by contact time with the fibre) are supposed to be dependent on the gap between rotor and stator bars and hence affect the flow. The fibre conformability is decisive for the impulse time (t) which determines the time of the specific force acting on the fibre; a stiff and elastic fibre absorbs energy differently compared with a conformable and viscoelastic fibre, which rather is collected on the bar edges. Finally, the dynamic viscosity of the pulp slurry (q) affects the fibre flow dependent on the temperature. Thus a general expression:
would by dimensional analysis transform to give dimensionlessquantities as follows:
V/(wl2) =f(F/(cw212), ( w O / l , 7J(cwI))
/2/
The pulp flow V = m/c, where m stands for dry pulp mass flow, and F = 2M/d, where M stands for torque and d for rotor diameter, are introduced into model 121. Further, considering that M = Pl(n d n), where n stands for rotational frequcncy, F in model 121 can be replaced for 2P4x d 2 n), which provides the following relationship: m/(c w 1 ) '
=f
'(P/(d' n c w
'19, ...)
/3d
and after modification
m 2 / ( c 2 w 2 1 1 )= f ' ( P / ( d Z n c w 2 1 ~...),
/3 b/
These two dimensionless quantities are suggested to describe the rheological conditions of beating, and thus the following combined factors might be used in a diagram for beating:
m '/(c 1 2) eP/(d n)
/4d
or if ignoring the constant bar length and rotor diameter
P/n
m2/c
/4b/
Beater
The general expression is proposed for testing of beating data obtaincd by a certain beater, and it can be used, if the net motor effect P (W), rotational frequency of beater n (rls), dry pulp mass flow m (kgls) and pulp consistency c (%) are measured. For this purpose beating experiments were performed by an industrial-scale conical beater shown in Fig. 1 and specified as follows:
162 Pulp production and processing
Figure 1. ConFIo@JC-01 beater
- power 250 kW (1000 rlmin) - rotational speed 750- 1500 rlmin - capacity 5-10 t/h - pulp consistency 2...6 % - fillings: LF, LM, SF, SM, SC, MX (for details see Table 2) The specificationsfor the different fillings used are given in Table 2.
Table 2. Specification of beater fillings Intersection Type Bar Groove Angle Impact width width o length mm Mm mm LF 4.0 10.0 36 4.2 LM 4.5 10.0 36 4.7 SF 2.0 3.0 23 2.0 SM 2.5 7.0 23 2.5 SC 3.0 7.0 23 3.0 Mx 3.5 4.5 23 3.0
RESULTS AND DISCUSSION Several beating series were conducted and the results were plotted in the diagram developed for evaluation of the beating procedure. Both softwood and hardwood pulps were produced by different pulping methods for comparison. Bleached pine and birch kraft pulps were beaten in several steps and the conditions represented by pulp grade, pulp consistency, rotational frequency and specific energy were kept constant. Fig. 2 indicates that the Pln versus m2/cor the loadproduction relationship remained the same in each beating step for a certain type of pulp, as all test points of the beating series hit the same area in the diagram. However, if the specific edge load (SEL) increased also the load-production relationship increased evidently following a linear correlation. Also, when the long-fibre LM-type filling was
Pulp fibre beating 163 changed to LF-type with narrow bars and shorter impact length of the intersection, the load-productionrelationship increased under constant SEL-conditions of 3.O J/m. For Kraft Pulps 12
*
10
-
LF. 31) Vm
LM, 4.0 Urn
W, 3.0 Jim
CSF 690.28S nL
SE 72 k W I
SE 71 k W l SE 72 L W I TI 37.77 N d i
8 A
2
2
4-
W, 1.1 Jh
15 1.3 V m cy5~1WmL
Pine
e 6-
SM, 0.7
Birch
Vm
2-
SE SO k W 1 400.1s Nnli
SE 49 k w M TI 3 6 4 9 NnJl
SC 1.0 Vm CSF 590.192 n L
CSFboo140mL I E 49 k W I TI 36.71 Nnli
cIf6001241d
SE49kWi
n 36-71 N~JI
04
I
0
2
4
6
12
10
8
14
16
m2k (kg?s2)
Figure 2. P/n versus m2/cfor ECF- 'bleached pine and birch krajl pulps; variable SEL andjilling
birch pulp however there was only a minor increase in the load-production relationship, when the short-fibre SC-type filling accordingly was changed to the SM-type under SEL-conditionsof 1.3 J/m. The long-fibered pine pulp and the short-fibered birch pulp were beaten at the specific energy levels of 70 k W t and 50 k W t respectively, &d thus behaved differently in the load-production diagram of Fig. 2.
Pine Kraft Pulps 14
LM.4.5 Jlm C SF 850495 m L SE 77 kWhlt
-
10 -
______I
12 h
3 8-
LM ,2.5 Jlm C S F 725440mL SE 076 kWhlt
C
iz
6-
204 0
LM ,2.5 Jlm C S F 725-620ml SE 7 7 kWhlt
LM.2.5 Jlm C S F 717.3851111 SE 7 8 k W h l l
4-
I
I
I
I
2
4
6
8
I
10
I
12
I
i
14
16
m2/c (kg2/s2) Figure 3. P/n versus m2/c for ECF- bleached pine kraft pulp; variable rotational speed and SEL
164 Pulp production and processing
4I & 3
-
10
-
Pine Kraft Pulps Pine A & B. LF& LM Pine D, LM
8-
2
+plnbu A3. LM. 2 J/m -+p l n b u A4. LM. 4 Jlm -+pine ECF 83. LM. 2.6Jlm +pins ECF 84. LM. 2.6Jlm - c p l n b s s 86. LM. 2.BJ/m A pinbsa C5. LM. 2.0 Jlm -x- pinam C6. LM. 2.7 J/m -a-pinesa C7. LM. 3.9 Jhn -+pinen DB. LM. 1.9 Jlm + pinera D9. LM. 2.7 Jlm
E 6-
zi:
4-
Pine C. LM
20
I 2 10 0
4
6
8
12
14
16
mZ/c(kg2/s2)
Figure 4. P/n versus d / c for different bleachedpine kraflpulps; variable firring and SEL ECF- bleached pine kr& pulp was beaten by application of rotational speeds of 600, 1000 and 1500 rlmin, Fig. 3. Increased rotational speed under constant conditions (LMtype filling, SEL 2.5 J/m, SE 77 kWh/t) results in about the same load, but in significantly increased production. Subsequently, this also leads to remarkably higher pulp freeness due to the shorter time of residence, particularly at 1500 r/min rotational speed. Different pine krafi pulps were beaten and the results plotted in Fig. 4. It confirms the prcvious findings: increased SEL increased the level of the loadproduction relationship. Typical Scandinavian pulps and other pulps followed the same load-production mode provided that the specific energy was the same. Also, bleached hardwood haft pulps were beaten and the results plotted in the load-production diagram, Fig. 5. It is evident that increased SEL increased the level of the load-production relationship. Pulps behaved differently due to the variable specific energy level.
CONCLUSIONS This initial study performed to confirm the energy absorption theory was made by a ConFlo@ JC-01 beater by application of different bleached softwood and hardwood kraft pulps. A diagram for P/n versus m2/c (net motor load versus pulp production) was developed for evaluation of the beating data obtained. It provides the relationship between the load taken by the pulp slurry and the production, and indirectly also the specific energy absorbed by the pulp slurry. It appears that a pulp fibre grade when beaten under certain beating conditions was continuously giving the same combination of load and production independently of pulp freeness. This implies that created fines and developed fibrillation did not affect
Pulp fibre beating
165
Hardwood kraft pulps
-
Mixed hardwood (MHW). SF
11 10 g87h .
SM Birch B. SM
4
2 6 -
-cMrchA?,SC.l.OJm +blrchA8.SC.1.3Jm -+eucB ~ ~ , ' s M ; o . B J ~ -euw 18, SM.1.0 J m +MHW13.SF.O5Jm 4 M H W 14. SF. 0.8 Jim o MHW 15, SM.0.8 J m MHW 16, SM. 1.0 J m
E a 5-
4-
3-
Mixed hardwood, SM Birch A. SM 80SC
2-
0 l
A 0
2
4
6
8
10
12
14
16
18
20
22
24
26
m2/c(kg2/s2)
Figure 5. P/n versus m2/cfor different bleached hardwood krafr pulps; variousfilling and SEL significantly the beating impacts introduced. It was observed that increased specific edge load SEL increased both motor load and pulp production for a certain pulp grade. Fillings with narrower bars and thus shorter impact length of the intersection (Table 1) did also significantly increase both load and production for the pine kraft pulp, as the effect with birch h a f t pulp was minor. It was evident that the plots for a certain pulp grade followed a specific elevation line provided that the specific energy applied was the same - irrespective of specific edgc load and beater filling. The short-fibered birch h a f t pulps behaved differently, as they did not very significantly respond to finer filling. The diagram presented will be developed further.
REFERENCES 1. J Lumiainen, 'Specific surface load theory', 3'* International Refining Conference, Atlanta, Pira International, 1995.
2. K Ebeling, 'A critical review of current theories for the refining of chemical pulps', International Symposium of Fundamental Concepts of Refining, Appleton, Institute of Ppaer Chemistry, 1980. 3. G A Smook, Handbookfor pulp andpaper technologists, Vancouver, Angus Wilde Publications,1992.
Study of flax fibre structure by WAXS, IR and I3C NMR spectroscopy, and SEM N.E. Kotelnikova', E.F. Panarin', R. SerimaaZ,T. Paakkari2, T.E. Sukhanova', A.V. Gribanov' 'Institute of Macromolecular Compounds, Russian Academy of Sciences, St. Petersburg, 199004 Russia zDepartment of Physics, University of Helsinki, SF-00014 Helsinki, Finland
ABSTRACT Two types of standard flax fibres were analysed by chemical methods. Specific features of their supramolecular structure were studied by WAXS, IR and "C NMR spectroscopy in solid state. Morphological structure of flax fibres was examined by SEM. One of the flax fibres has great natural porcs and cavities that can lead to its high reactivity.
INTRODUCTION In spite of tremendous development of the production of chemical synthetic fibres, the constant renewal of fibre materials in nature is still their attractive feature. Thcy are mainly cotton and flax fibres. In recent ycars the interest in the preparation of textile and other materials from natural fibres exhibiting satisfactory consumer propertics has remained high. These fibres include flax fibres the cultivation of which has markedly decreased, although they display a combination of important properties. The study of the technology of textile materials manufacture, which is related both to the physico-mechanical changes in fibres and in the chemical processing, requires the knowlcdgc of chemical and morphological structure of fibres as well as their supramolecular structurc. However, the number of fundamental studies of the above propcrties by up-to-date methods has also considerably decrcased in recent years. The main purpose of this study was to renew the consideration of fundamental aspect in the study of native flax fibres in order to attract attention to these promising fibres. The combination of chemical methods with the advanced methods of WAXS, IR and I3CNMR spectroscopy, and SEM was applied for this purpose.
EXPERIMENTAL PART Materials Two types of standard flax fibres grown in different climatic conditions and manufactured at Russian flax treating plants were used as initial samples*. Flax fibres were subjected to delignification [ 1 , p. 751; lignins were isolated by the method described elsewhcre [ 1, p. 851. Lignin and cellulose content as well as that of substances soluble in cold and boiling water and in organic solvents (ethanol-toluene mixture, 1:2 vol %) was estimated according to [ 1,pp. 63,65,75]. Two types of pure flax cellulose were obtained after delignification. Lignin and cellulose properties were also studied by the above mcthods. *These initial flax fibres will be named below flax I and 11.
170 Structure and properties of fibres Fibre length was determined as described elsewhere [2]. Degree of polymerisation was determined by viscometry in cadoxen [ 1 , p. 2781, and elemental content by elemental analysis using a Hewlett-Packard C, H, N-analyser.
Methods Wide-angleX-ray scattering
The experiments were performed using both reflection and transmission geometry with CuKa radiation monochromatised with a quartz monochromator in the incident beam. Variation in the intensities of the reflcctions indicates that the samples have a strong fibre texture. The mcasurcments were carried out in the scattering angle range 8"<2thetac50" with an angle step of 0.1". The average sizes of the crystalliteswere determined using the well known Scherrer equation taking the instrumental broadening effects into account. The crystallinity indices were determined by a fitting mcthod using both reflection and transmission data. The precision remains low because of the strong preferred orientation. IR spectroscopy
A Bruker JFS 88 IR Fourier spectrometer was used for spectroscopic study in the IR range (400-3600 cm-') of all samples in thc solid state which were preparcd in the form of KI3r pellets. I3CNMR spectroscopy
13CNMR measurements were performed at ambient temperature on a Brukcr CXP-100 spectrometer operating at 2.34 T (frequency of 25.18 MHz) with a Bruker doublebearing CP/MAS probe. The magic angle spinning (MAS) frequency was either 3.5 or 4.0. kHz. All chemical shifts arc given in ppm from tetramethylsilane.
SEM Electron microscopic study was performed with a scanning elcctron microscope MiniSEM-V (Akashi, Japan).
RESULTS AND DISCUSSION Both flax initial fibres have similar chcmical compositions and molecular weights (Table 1). Contents of elements in flax I and I1 and in those subjected to extraction with water and with organic solvents as well as in isolated lignins and celluloses are listcd in Tablc 2. It can be seen that samples extraction with boiling water leads to a decrease in ash content. This means that mineral substances are removed. Nitrogen and impurities in ash are mostly contained in lignin. Thus, lignin content in sample I1 is 1.2 times higher than that in sample I, and nitrogen and ash contents in isolated lignin of sample 11are also 1.2-1.3 times higher. Lignins isolated from the initial samples are enriched with nitrogen and mineral substances. Thus, nitrogcn contents in these lignins are 1.5 and 4.2 times higher than those in initial samples I and 11, respectively. Ash contcnts are 2.2 and 3.4 times highcr than
Flax fibre structure 171 Table I. Chemical composition of initial flax samples Characteristics
Flax fibres
I
I1
Average fibres length, mm
54.4
58.5
Ccllulose content, mass %
74.5
78.5
Lignin content, mass %
6.4
7.8
Substances soluble in boiling water, mass %
5.5
5.0
Substances soluble in cold watcr, mass %
5.5
3.2
Substances soluble in ethanol-toluene
2.6
3.1
6400
5600
mixture, mass % Degree of polymerisation
Table 2. Elementary content of flax samples, of cellulose and lignin isolated from them Samples
Initial flax Extracted with boiling water Extracted with organic solvents Lignin isolated Cellulose isolated
Nitrogen, mass %
Carbon, mass %
Hydrogen, mass %
Ash, mass %
I
I1
I
I1
I
I1
I
I1
0.5
1.1
43.3
42.8
6.2
6.0
1.8
1.4
0.6
0.4
43.6
43.5
5.9
6.4
1.0
0.7
0.5
0.G
41.9
42.9
6.3
6.1
1.0
1.4
1.6
2.1
51.9
62.0
6.9
8.1
3.9
4.8
0
0
42.7
42.8
6.5
6.5
0
0
those in initial samples I and 11,respectively. In contrast, samples of isolated celluloseshave equal carbon and hydrogen contcnts and contain neither nitrogen nor ash.
172 Structure and properties of fibres Results obtained by WAXS
The WAXS intensity curves are presented in Fig. 1 (1-3). Only minor differences in the crystallinity of the samples were obtained (Table 3). Thus, the crystallinity indices of the initial flax samples are about the same within the precision of the determination. The crystallinity of flax I slightly increased after extraction with organic solvents as well as after delignification. The crystallinity of the flax sample I1 slightly increased after extraction but practically no changes could be found after delignification.
8000
$0
Ib
A
;s
o;
2 theta
&
o;
s;
D
8000
7000 -
3 COUUMSB. &lainad from:
6000 -
5000 -
4000
-
3000 -
Flsx I (dots): FIM 11 (solid)
Fig. 1 (1-3). WAXS intensity curvcs of flax fibres I and I1 (l), those of the samc samplcs after extraction (2), and thosc of cclluloses isolated from flax I and I1 (3).
Flax fibre structure 173 Table 3. Crystallinity and average sizes of the crystallites from the reflections [002] and [040] Sample
Crystallinity index
Flax fibre I
Average size of crystallites (A) from the reflection 002 040
0.4 1
41
160
Flax fibre I extracted
Slightly increased
Slightly increased
No changes
Cellulose from fibre I
Slightly increased
No changes
No changes
0.42
43
160
Slightly increascd
Slightly increased
No changes
No changes
No changes
No changes
~~
Flax fibre I1 ~~
~~
Flax fibre I1 extracted ~~
~~
Cellulose from fibre I1
The average size of the crystallites from the reflection [002] is 41 and 43 8, flax I and 11, respectivcly.The size is slightly increased aftcr extraction but delignification does not affect the size of crystallites. The avcrage size of the crystallites from thc reflection [040] is about 160 8, for all samples. The size does not change after both treatments. Therefore, one can conclude that neither extraction nor delignification affect the length and the thickness of elementary fibrils of flax fibres. At small scattering angles thc intcnsity curves measured by transmission geometry diffcr from each other indicating differcnccs in the supramolecular range order after extraction and delignification.
Results obtained by FTIIl spectroscopy Fig. 2 (I and 11) shows the FTIR spectra of initial (1) and treated flax fibre I samples (2,3) (Fig. 2, I); those of lignin isolated from initial fibre (1) and of complex of extractives (2) (Fig. 2,II). The spectra 1-3 (Fig. 2, I) are typical of the spectrum of cellulose combined with lignin. The difference as compared to the spectrum of pure cellulosc obtained from flax fibre can be seen in the following regions: 1600-1750 cm-I, 2850-2920 cm-I, and 3000-3600 cm-I. Intensive absorption in the region 1600-1720 cm-' is caused by stretching vibrations of carbonyl groups. It is known that the exact position of these groups depends on their conjugation with benzene rings (in this case the position is lower than 1700 cm-I). In the case without conjugation the position is higher than 1700 cm-I. One can conclude that there are both types of these groups in flax samples. In addition, thcre is the intensive band with a maximum at 1738 cm-I, which can be attributed to vibrations of carbonyl groups in carboxyl groups. Some acetyl groups and groups of ethers of uronic acids (in polysaccharides)can also have absorbance in this region. The other reason for the appearance of these bands can be the presence of some impurities in the initial fibres, such as fats, waxes, and resins. They all contain different
174 Structure and properties of fibres 1059
1 2
3
L + 3500
1500
SO00
I11
I
I
3500
1000
n
I
3000
500
1500
1000
I
500
WAVENUMBER, CM-I Fig. 2 (1-111). FTIR spcctra of inilia1 (1) and extractcd flax I samples (2-extracted with boiling water, 3-extractcd with organic solvents) (Fig. 2, I); those of lignin isolated from flax I(1) and of complex of extractives (2) (Fin. 2.11); and those of celluloses isolated from flax I (1) and I1 (2)
Flax fibre structure 175 types of acids and many other substances that can exhibit absorbance in this region. Thus, the spectra of extractives (with boiling water as well as with organic solvents) contain the same bands, which confirms that lignin is partly dissolved under these treatments, and these bands shifted to 1657 cm-I and 1726 cm-' in spectrum of lignin isolated from flax fibres (Fig. 2,II). Moreover, the shape of spectrum of the flax sample subjected to extraction with organic solvents is much more similar to that of cellulose obtained from the initial sample (Fig. 2,111) than that to the spectrum of the initial sample. This is especially seen at the regions about 1430 cm-I and 2900 cm-I. The other essential difference is that the conjugated C=O bonds are removed from the sample after extraction. Two intensive bands with maxima at 2850 and 2918 cm-' are attributcd to deformation vibrations of C-H groups in methyl and methylene groups [CH,, CH,, CH,-OH C(6)] belonging to cellulose as well as to lignin. The shape of this band is not typical of cellulose, which usually exhibits three-shouldcr band with a maximum at 2900 cm-' in this region. Extraction of the initial flax samples with water and/or organic solvents leads to partial dissolution of lignin containing these groups because lignin and cxtractiveshave the bands at the same position (Fig. 2.11). Moreover, the band with a maximum at 2900 cm-' exhibits typical cellulose shape. Delignification of both flax samples leads to preparation of pure celluloses (Fig. 2, 111). It should be emphasised that they do not contain any carboxyl groups in contrast to the initial fibres samplcs. The specific feature of these spectra is the division of the broad band of OH groups to two broad bands with maxima at 3285 and 3500 cm-l. This division is not characteristic of ordinary cellulose spectra. The intensity of the second band is lower than that of the first one. This, probably, indicates that these celldoses exhibit different systems of H-bonds as compared to other celldoses preparcd from cotton, wood, etc. It is known [3] that these bands are attributcd to inter- and intra-molecular hydrogen bonds. Most of these bonds arc included in intermolecular H-bonds, such as O(6)H.. .0(3), the smallest part is included in intra-molecular H-bonds, such as O(3)H ...O(6)and O(2)H ...O(6). Results obtained by I3CN M R spectroscopy Fig. 3 (I and 11) shows I3CNMR spectra of the initial flax sample I1 (l), of that subjected to extraction with water (2) and with organic solvents (3) (Fig. 3, I), and spectraof thecelluloses isolated from the initial samples (Fig. 3.11). It can be seen that the spectra of flax I1 and those of extracted samples do not exhibit any essential diffcrcnces either in resolution of bands or in the number of signals. However, it is known that the chemical shifts of signals of C4 atoms depending on the angle of units rotation about the C,-0, linkage and the chemical shift of C, atoms depending on the rotameric composition of oxymethyl groups are the most sensitive to structural rearrangements [4]. The flax sample after extraction contains less amorphous material (perhaps,lignin) as compared to the initial one. Moreover, the resolution of signals is bcttcr in the case of extractcd fibres. This, probably, means that the mobility of C4atoms as well as that of C4-0, bonds bccomcs higher, which is possible after lignin isolation. The flax samples contain a signal at 30-32 ppm corresponding to C atoms in CH, and CH, groups. This signal, probably, belongs to the above groups in lignin and/or in othcr impurities present in the initial fibres. I3CNMR spectraof celluloses isolated from flax I (1) and I1 (2) are similar to those of the initial samplcs (Fig. 3,II). These spectra show that the celluloses have the structure of crystallinc cellulose I.
176 Structure and properties of fibres I
I1 1
1
l2
i
I
2
UU 180
160
iio'%o' loo'
io
'
do
o
20
40
2
1 8 0 160 1 4 0 120 100 8 0
180
160
140
120'
loo' (Vpn) io
6 6 7 0
'
io
'
60
40
20
0
(VV)
o
Fig. 3 (I and 11). "C N M R spcctra of flax I1 samplc (1). extracted with water (2) and with organic solvcnts (3) (Fig. 3, I), and those of celluloses isolated from flax I (1) and I1 (2) (Fig. 3.11).
Results obtained by SEM The structure of flax fibres, even of those already manufactured from native raw material, is complex. This mainly depends on the degree of preliminary purification of flax fibres from residual bark and the parenchymatous layer, which contain lignin and pectin substances. SEM micrographs presented in Fig. 4 (1-8) show that initial flax samples differ markedly in their degree of purification. Flax fibre I contains many fragments of parechymatous layers, which have not been rcmoved (Fig. 4 , l ) . They can be seen as loose regions partially separated from fibres. The surface structure of flax I is not well ordered. It exhibits both entangled loose fibrils and smooth regions (without any visible fibrillation) and inclusions of small particles consisting of inorganics (probably, salts). Slight peeling of the surface lcads to the appearance of the intcrnal structure. The internal longitudinal structure of fibres is also not quite ordercd. Fibrils form relatively ordered laycr regions which alternate with large voids resembling blind pores andor cavities (Fig. 4.2). These cavities and pores can be seen in flax I1 too. However, their sizes are larger in flax I. The initial sample flax I1 has different surface structure from that of the sample flax I and consists of agglomerates of smaller fibres parallcl to each other. Thcsc fibrils are of similar dimensions. The fibre surface is relatively smooth and has few mineral inclusions (Fig. 4, 3). The residual parenchymatous layer can be also secn on the surface of some fibres, although to a smaller extent than in sample I (Fig. 4,4). At high magnification these fibres exhibit regular fibril structure in some parts [ 5 ] . Thus, it was shown that the morphological structure of flax fibres is quite different and dcpcnds on the degree of primary purification. Moreover, initial fibres have layer structure and contain large natural pores, which can favour their high reactivity. Because
m
178 Structure and properties of fibres of difference in their surface and inner structure they, probably, can exhibit different reactive ability in chemical reactions. Extraction of these fibres with boiling water and organic solvents leads to additional disorder of the fibres surface but does not impart any inner disorder. It is in agreement with the litcrature data. For instance, it has been shown in ref. [6] that treatment of flax fibres with NaOH solutions of low concentration leads to disorder on their surface but has no effect on the inner structure. Delignification greatly affects the morphological structure of initial fibres samples. The morphological structure of isolatcd celluloses is quite different from the initial fibres structure. All cellulosefibres have the shape of tubes with round cavities inside. The regular fibril structure on the surface can be clearly seen. The fibrils on the surface of cellulose obtained from flax I forms an angle with fibres axis (Fig. 4,5). The fibrils on the surface of cellulosc obtained from flax I1 are parallel to the fibre axis (Fig. 4,7).There are no other “impurities” on the surface. Hence, the data obtained by chemical methods and by WAXS, IR. I3C NMR spectroscopy, and SEM enabled us to present extensive charactcristicsof two flax fibres.
CONCLUSIONS Comparative analysis of supramolccular structure and chemical composition of two flax fibrcs samples grown in different climatic conditions was carried out. No essential differences were found in supramolecular structure by the WAXS method. Flax samples as wcll as celluloses obtained from them exhibit the structure of cellulose I modification and have similar crystallinity and crystallites size. Chemical compositions revealed by IR and ”C NMR spectroscopy are similar. Delignification leads to the total removing of lignin and preparation of purc cellulose. IR spectra of the flax celluloses have a broad band of OH groups differcnt from that of ordinary celluloses. OH bands are mostly included in the system of intermolecular hydrogen bonds. Flax samples have specific morphological structure as rcvealed by SEM. The flax I has great pores and cavities that can lead to its high reactivity.
ACKNOWLEDGEMENTS This study was supported by the Russian Foundation for Basic Research. Grant number 96-03- 10009. The authors acknowledge the careful experimental work carried out by Mrs. A. Bobasheva performing FTIR measurements and Mrs. M. Vylegzhanina performing SEM study. We also thank students Yu. Puchek performing chcmical experiments and S. Siiria and M. Saraen carried out WAXS measurements.
REFERENCES 1 A V Obolcnskaya, V P Schegolev, G L Akim et al., Practical work on wood and cellulose chemistry, Moscow, Legprom, 1962. 2 A I Kobliakov, G N Kukin, A N Soloviev et al., Luboratorypractice on textile material science, Moscow, Legprombitisdat, 1986.
Flax fibre structure 179 3 D Fengel, Structural changes of cellulose and their effects on the OWCH, valency vibration range in FTIR spectra, In: Cellulose and Cellulose Derivatives: Physico-chemical Aspects and Industrial Applications, int conf “Cellucon’93”,Lund, Sweden, 1993 75-84. 4 N E Kotelnikova, A Yu Elkin, A I Koltzov et al., Application of 13CNMR high rcsolution method in solid state for study of polymorphous modification, of hydrolysis and thermolysis products of cellulose, In: Methods of cellulose research, Riga, Zinatne, 1982, pp. 61-65. 5 V P Alikin, Physico-mechanicalproperties of natural cellulosefibres, Moscow, Wood Promysl, 1969. 6 H S S Sharma, T W Fraser, D McCall et al., Fine structure of chemically modifedflax fibres, Text. Inst, 1995 86 (4) 539-548.
Evaluating The Surface Energy of Hardwood Fibres Using The Wilhelmy and Inverse Gas Chromatography Methods W. Shen', Y.J.Shcng and I.H. Parker Australian P u b and Paper Institute, Department of Chemical Engineering, Monash Universiv, Willington Rd. Clayton, Vic. 3800, Australia
ABSTRACT Surface energy properties of kraft, neutral sulphite semi-chemical and cold soda eucalypt pulps and two polymers, poly-(methyl methacrylate) and polyvinyl alcohol, were obtained from contact angle determination (using the Wilhelmy method for the fibres and the sessile drop technique for the polymers) and the inverse gas chromatography (IGC) method. Three liquids were used in the contact angle methods and the data were analysed using the Good-van Oss approach to obtain the dispersive, acid and base components of surface energy. The same data were also analysed using the work of adhesion approach as were the IGC data. The dispersive component of the surface energy was obtained using diiodomethane for the Wilhelmy method and alkanes for IGC. The level of agreement between the two techniques was reasonable. Water, formamide and diiodomethane were used to obtain the acid-base components with the Wilhelmy technique and monopolar probes were used for IGC. Agreement was poor for the acid-base components. The Good-van Oss approach suggested all fibres and both polymers are monopolar basic whereas the IGC approach suggested that they are bipolar. Using the work of adhesion approach the Wilhelmy and IGC techniques both gave similar results. The chemistry of the surface was determined using XPS and was compared with the acid-base values. It was concluded that there are potentially serious problems with determining the acid-base components of surface energy using the Good-van Oss approach. INTRODUCTION Unbleached chemical and semi-chemical pulp fibres are important raw materials used in the production of corrugated boxes. Often, printing and glueing are required on the outer layer of the box, the linerboard ply. Printing on uncoated linerboards is largely carried out by water-based flexography. Since the surface energy of the substrate is important when using water-based inks, an understanding of the surface energy of the pulp fibres is necessary if its effect on printability and glueability is to be understood in depth. Surface energy and acid-base characteristics of a solid are traceable back to the surface molecular make-up of the solid [1,2]. Recent work on pulp surface chemical composition in this laboratory using X-ray photoelectron spectroscopy (XPS) has shown that the surface concentrations of extractives and lignin on eucalypt pulps are much higher than the average concentrations of extractives and lignin in these pulps. The coverage of pulp surfaces by extractives and lignin affects the surface energetics of the pulp fibres [3,4].
182 Structure and properties of fibres The surface energy of a solid material can be described as the sum of a Lifshitz-van der Waals component and a Lewis acid-base componcnt. Evaluation of the surface energy of a solid, and of the components of this surface energy, relies almost exclusively on the indirect measurement of the interfacial interaction of the solid with liquids or gaseous probes by means of measurement of contact angle, inverse gas chromatography (IGC) or scanning calorimetry [1,4-81. In this laboratory, whilst the main research effort has been directed to the determination of the surface energetics of eucalypt pulp fibres and the effects of the surface chemical compositions of these fibres on their surface energetics [3,4,10], serious attention has also been given to the comparison of surface energetics data obtained using different techniques and treated with different theories. In some cases, surprising discrepancieshave been found. The contact angle method based on the “three-probe” approach proposed by Goodvan Oss is widely used for the determination of the surface energy of solids [l l-141. This approach links the Lifshitz-van der Waals and Lewis acid-base interactions at a solid-liquid interface with the solid/liquid contact angle in the following way:
Some studies have found that the application of equation (1) for evaluating the surface acid-base characteristics of many solids is unsatisfactory [2,10,15]. This approach yields an acidic parameter (f) which is much smaller than the basic parameter (f << y-) for most solid surfaces, i.e., it leads to the conclusion that these solid surfaces are strongly basic [10,11,13,14]. Dclla Volpe and Siboni suggested that a flaw in the Goodvan Oss approach is the arbitrary assumption that the acid and base parameters of water are equal and proposed that these parameters should be y’=65.0 and y-=10.0 mJ/m2, respectively, since water has much stronger acidic nature [151. These new acid and base parameters were tested for calculating the surface energetics of eucalypt fibres in this work. In order to evaluate accurately the surface energetics of eucalypt fibres, IGC was also employed and the results were comparcd with those obtained by the contact angle method and the Good-van Oss approach. To widen the comparison, two polymcr samples (polyvinyl alcohol (PVA) and poly(methy1 methacrylate) (PMMA)) were also studied using both contact angle (scssile drop) and IGC methods. A correlation between the surface chemical composition and the surface energetics of the eucalypt fibres obtained using IGC is presented in this paper. The surface energetics data of eucalypt pulp fibres and polymers obtained using contact angle and IGC methods are compared and the discrepancies discussed.
EXPERIMENTAL Reagents and materials All chemical reagents were eithcr GC or AR grade. Formamide (FA), bromoform (BF), dimethyl sulphoxide (DMSO) and diethyl ether (DEE) were obtained fiom BDH; alkanes (C, C,,),diiodomethane(DIM), chloroform (CF) and polyvinyl alcohol (PVA, 99% hydrolysed) from Aldrich; toluene from Ajax and PMMA from Scientific Polymers.
-
Surface energy of hardwood fibres
183
The pulp samples were obtained from E. regnans, E. globulus and E. nitens chips which were pulped using laboratory equipment. Pulping conditions are detailed elsewhere [4]. Pulp analysis was carried out after displacement washing with distilled water (Table 1). Samples for X P S measurements were cut from 60 g/mz handsheets, which were formed using purified water and a British Handsheet Machine according to Appita standard AS1301.P203s [4]. In order to estimate the surface contents of extractives and lignin and their effects on pulp surface energetics, pulp fibres and handsheets were extracted using a Soxhlet apparatus and methanol, following the standard method (AS/NZS 1301.012~:1994),for 8 hours [4]. Table 1 Results of pulp analysis* Y l d d (%o)
K.R. K.G. NSSC R. NSSC G. C.S. Nit.
K
44.9 55.2 67.6 72.7 81.3
.
11.8 13.4 120 98 148
Extractwes (YO) Lignin (“/o) 0.32 1.8 0.18 2.0 0.87 18.3 1.15 15.0 0.70 22.6
*K= kappa number, K= kraft, C.S.= cold soda. R.= E. regnans, G.=E. globulus, Nit.= E. nitens. Lignin content is estimated using lignin (%) = (~-number)/6.546 [ 161.
Measurement of the Surface Coverages of Extractives and Lignin Using XI’S
XP Spectra were obtained with an AXIS HSI spectrometer (Kratos) using monochromatizcd AlKa radiation. The X-ray beam has a diameter of 2 mm. The photoclectron emission angle was 90’ with respect to the sample surface. This corresponds to a maximum sampling depth of cu. 10 nm. The fraction of the surface covered by extractives and lignin can be estimated using the following equations, assuming the patches of surface extractives and lignin are thicker than the photoelectron cscape depth [ 17,181:
where O/COignin), O/C~cxtmc~ivcs), O/C(purc pulp) and O/C(pu~psninplc) are the oxygen-to-carbon ratios of the isolated lignin, extractives, the extracted bleached pulp and the extracted pulp samples, respectively. The value was dctermined by XPS under the same cxperirnental geometry to be 0.35 [3]. The O/C(pl,m pulp) was determined from extracted and fully bleached eucalypt fibres to be 0.79. The O/C(cx,mcIiv~s) was estimated using the methanol extractives from E. globulus to be 0.08 [4].
184 Structure and properties of fibres Contact angle and IGC measurements All contact angle measurements were conducted in a conditioned test room at 23°C and 50%RH. Single pulp fibrelliquid contact angles were measured using the Wilhelmy method with a Cahn DCA 322 System which has a sensitivity of lpg and the speed of the liquid probe platform was set at 5 pnds. Since wood fibres are fully wetted when receding from a liquid [19], the advancing contact angle between a fibre and a liquid can be calculated from the advancing and the receding forces using the expression:
-
coso = FA
(4)
FR
A detailed description of the procedure for force measurement and the determination of contact angle values is given elsewhere [9]. Twelve single fibrcs of each of the four pulp samples (i.e,, haft E. regnans and E. globulus, NSSC E. regnans and E. globulus) were measured. Since the purpose of determining the contact angles of single fibres was to test the applicability of the Good-van Oss theory, cold soda pulp was not included for contact angle measurement. Polymer filndliquid contact angles were measured using the sessile drop method with a Fibro 1100 dynamic adsorption tester. A 4 p1 liquid drop (2 pl for DIM and BF) was delivered to the surface of the polymer film. Analysis of the data showed that the contact angle reached a stable value in less than 20 seconds of contact and for most probes there was no change in drop volume in the first 60 seconds. The data point at 60 seconds was taken as the advancing contact angle of the liquid on the polymer surface. In the case of BF, which evaporates faster than other probes, the data point at 30 seconds was taken as the advancing contact angle. Ten contact angle readings were collected for each liquid probe on each polymer surface following the above described procedure. The contact angle data are presented in Table 2. Table 2 Contact Angles (in degree) of Liquids with Pulp Fibres and Polymers Solids K.K. K.G. NSSCR. NSSCG. PVA PMMA
WA 52.ga(10.8)b 49.4 (18.1) 47.7(6.4) 53.8(8.6) 52.3 (2.9) 68.2 (1.6)
FA 41.7 (5.5) 42.1 (9.7) 16.6(10.3) 29.0(12.2) 56.2 (1.1) 57.7 (0.9)
DIM 30.9 (9.1) 29.4 (4.5) 27.0(10.2) 19.1 (5.1) 47.4 (1.9) 36.8 (2.7)
BF
DMSO
-
-
-
25.0 (2.5) 12.8 (1.6)
13.1 (1.2) 27.3 (1.9)
'Arithmetic average. standard deviation.
Calculation of y,"". y', and y,' of fibres and polymers were carried out by solving parallel equations derived from equation (1) [lo]. Liquid probes used for contact angle measurement were water, formamide and diiodomethane. Values of ytW*yI+ and y; of liquid probes used in the calculation were taken from reference [12]. An HP 5890 Series I1 gas chromatograph equipped with a flame ionization detector was used for IGC measurements. Temperatures of the injection and detector ports were set at 150 and 180°C rcspectivcly,and the flow rate of the carrying gas at 20 ml/min. A
Surface energy of hardwood fibrcs
185
1 pI SGE Microvolume syringe was used to inject a minute quantity of probe vapour into the column [3,4]. The dead volume of the column was measured by injecting methane. From the retention time measured for a given probe, the corresponding net retention volume, V,, was calculated [7,20]. Since IGC was operated in the infinite dilution regime, the standard molar free energy change upon the adsorption of the probe gas can be calculated from: = -RTln(V,,) AG,l(Lv
+C
where R and T are gas constant and temperature, C is a constant. The work of adhesion of the probes on the solid surface may then be calculated according to the equation proposed by Dorris and Gray [21]:
Where alno, = uN, a is the contact area of the probe molecule w ith the solid and N is Avogadro's constant. For non-specific adsorption, the work of adhesion is given by [20]: RTln(V,) = 2a,,,
-d
(7)
For alkanes, the plot of RTln(V,) against 2 ~ , , , , , ( y ~is~linear ) and is referred to as the reference line. y,"", the Lifshitz-van der Waals componcnt of the surface energy of the solid, can then be calculated from the slope of the reference line. The acid-base contribution to the work of adhesion of a gas probe capable of acidbase interactions with the solid can be obtained by comparing its retention volume to [4,6,22]: that which is obtained with an inert probe with the same value of alnal(~~W)'n
The values of alno,and the calculation.
ytw for the probe gases published in reference [20] were used in
PULP SURFACE CONCENTRATIONS OF EXTRACTIVES AND LIGNIN The high concentrations of extractives and lignin on pulp surfaces are clearly demonstrated by comparing the results of methanol extractives and lignin analysis of whole pulp (Table 1) and the surface of pulp samples (Table 3) of E. regnans, E. globulus and E. nitens obtained using haft, NSSC and cold soda processes. Work reported by other laboratories has also shown that the extractives and lignin tend to be more concentrated on the surface of the pulp than in the bulk [16,17,23]. It has becn postulated that this is the result of precipitation and/or resorption of extractives and lignin from the pulping liquor back onto the pulp surface during pulping.
186 Structure and properties of fibres Table 3 Results of pulp surface analysis by XPS* C1
c2
F1t.P. (extr.)
7.4
75.4
c3 17.2
K.R. K.G. K.R. (extr.) K.G. (extr.1 NSSC R. NSSC G. NSSC R. (extr.) NSSC G. (extr.)
21.6 17.2 14.5 15.4 22.9 24.2 20.4 22.2
63.0 64.6 67.2 64.4 67.9 67.3 69.4 63.7
15.4 18.2 18.3 20.2 9.1 8.5 10.1 14.0
C.S. Nit. C.S.Nit. (extr.)
23.5 22.3
64.2 64.7
12.3 13.0
Lignin
50.0
44.6
2.9
~~
c4
OK
exlr.
0.81
-
2.5
0.66 0.70 0.71 0.74 0.61 0.59 0.66 0.66
0.08 0.06
0.60 0.63
0.07
0.35
4+ie.
-
0.17 0.1 1 0.09 0.13
0.3 1 0.29
-
0.38 1 .o
*Flt.P.= filter paper, K= kraft, C.S.= cold soda. R.= E. rcgnans, G.= E. globulus, Nit.- E. nitens. (extr.) extracted with methanol. CI, C2, C3 and C4 represent respectively carbon atoms in the following chemical environments: Hydrocarbon (eg. C-H, C-C and C=C), carbon-oxygen single bond (eg. hydroxyl and ether groups), carbon-oxygen double bond (eg. 0-C-0 and carbonyl groups) and carboxyl and 4(,i8)are the surface coverages of extractives and lignin respectively. (O=C-0) groups. =
The difference in the total extractives contents of haft and NSSC pulps is likely to be due to the different alkaline levels used in the two pulping processes. Fatty acids, which are one of the constituents of the eucalypt extractives, is removed more thoroughly in kraft pulping than in other processes, since they are highly saponifiable under highly alkaline conditions. The fatty acid soaps also disperse some neutral materials, leading to their removal from the pulp into the liquor. The near neutral condition in NSSC pulping apparently does not favour the removal of the extractives. In cold soda pulping, although around 2.5% NaOH is used to soften the chips, the alkaline level is much lower than that used in kraft pulping. Thus the cold soda E. nitens pulp has a higher content of extractives than the kraft pulps. Since the degree of delignification of these pulping processes are different, the total lignin contents of the pulps increase in the sequence of kraft, NSSC and cold soda. Whilst the total extractives contents of the kraft pulps were much lower than those of the NSSC and cold soda pulps (Table l), the coverages of the surfaces of the kraft pulps by extractives were similar to those of the NSSC and cold soda pulps (Table 3). This suggests that extractives tend to be redeposited to a higher degree on the surface of haft pulps than on NSSC and cold soda pulps. Figure 1 shows the ratios of the surface coverage (expressed in perccntage) to the total content (expressed as perccntage of OD pulp) of extractives as a function of pulp species. This figure demonstrates the highcr degree of rcdeposition of extractives on the surface of h a f t pulps compared to NSSC and cold soda pulps. A possible explanation for this phenomenon is that the more highly alkaline condition in kraft pulping causcs a highcr degree of release of extractives into the liquor and some extractives precipitate back onto the pulp fibre surface when the alkaline level is reduced during the course of delignification and washing. Laine and Stenius reported that, in their study of unbleached kraft pulps, the surface coverage by
Surface energy of hardwood fibres
187
extractives increased as the alkali dosage used in the digestion was increased [17]. In NSSC and cold soda pulping, since the alkaline level is much lower than that used in kraft pulping, a smaller quantity of extractives is released into the liquor and therefore there is a lower degree of resorption of extractives onto the pulp surface. 40.00
,
Figure 1. The ratios of the pulp surface coverage of extractives vs. the percentage of extractives of the pulps (grey bar) and the surface lignin coverage vs. the percentage of lignin contcnt (black bar) of the pulp samples. The surface concentration of lignin relative to the lignin content of the whole pulp was again higher for the kraft pulps than for NSSC and cold soda pulps (Figure 1). This result indicates that the haft pulps have a higher degree of surface enrichment of lignin than the NSSC and cold soda pulps. The surface lignin concentrations of NSSC and cold soda pulps were only about twice those of the total lignin contents. The high coverage of lignin on kraft pulp surfaces has also been reported by Laine and Stenius[16,241. Our results show that the lower degrees of delignification of NSSC and cold soda pulps correlate with lower degrees of surface enrichment of lignin of thcse pulps. This agrees with the proposed reprecipitation mechanism for surface enrichment of lignin, as the lignin reprecipitation would be less intense for these pulps because of the low lignin concentrations in their pulping liquors. Another intcresting observation fiom Figure 1 is that the ratios of surface coverage by extractives to the bulk extractives contents of the pulp samples are in all cases higher than those of the surface coverage by lignin to the bulk lignin contcnt. This suggcsts that the extractivcs may have higher surface activity than lignin.
THE DISPERSIVE COMPONENTS OF EUCALYPT FIBRES AND POLYMER SAMPLES The dispersive component (yLw) of eucalypt fibre and polymer samples determined by a contact angle method and equation (1) using DIM, and by IGC and equation (7) using alkanes, are shown in Figures 2 and 4, rcspcctively. The yLw values produced by the contact angle methods using DIM (except for PMMA) are 2 to 5.6 mJ/m2 greater than those determined by IGC using alkanes. NSSC globulus fibres showed the
188
Structure and properties of fibres
greatest difference, where the yLwdetermined by the Wilhelmy method is 9.1 mJ/m2 higher than that determined by IGC. It is known that the infinite IGC method prcdominantly detects high energy sites of the surface [19], whereas the contact angle methods detect surface sites of all energy levels and thus determine an average energy level of the surface [7,25]. Thus the yLw measured by IGC will often be higher than those measured by contact angle methods. However, in this study the yLwvalues measured by IGC were lower. The most probable reason for this is that the IGC results presented were measured at 30°C, whereas the contact angle measurements were carried out at 23OC. Since the temperature coefficients of yLwof lignocellulosic fibres and polymers are negative [3,26,27], it is expected that the yLwdetermined at the lower temperature will be greater, although this does not explain the large discrepancy found for the NSSC globulus fibres. Both the IGC and the contact angle methods indicate that PVA has the lowest yLw value of all the samples. The present results suggest that yLwvalues of most fibre and polymer samples determined by these two methods are in an acceptable agreement, if the effect of the testing temperature is taken into account.
ACID-BASE CHAMCTEIUSTICS OF FIBRES AND MEASURED BY IGC AND CONTACT ANGLE METHODS
POLYMERS
Prcdiction of the acid-base characteristics of eucalypt fibres and polymers by the Good-van Oss Approach
The Good-van Oss "three-liquid" analysis of contact angle data of the eucalypt pulp fibres suggests that for all fibres the base parameters are far greater than the acid parameters, suggesting that all pulp fibres are almost monopolar basic.
d
1 s
I
Figure 2
Surface energetics data of Eucalypt fibres and polymers calculated from contact angle results using the equation (1) proposed by Good-van Oss.
Most published data, however, indicate that the surface of cellulosic fibres is bipolar with a predominant acidic character [3,4,7,8,25,28]. Bipolarity of cellulosic fibres is essential for paper making, since hydrogen bonding is responsible for the intcrfibre bonding in a papcr sheet. Strong hydrogen bonding requires the surface of
Surface energy of hardwood fibres
189
the fibre to have both acidic and basic functionality. The dominant acidity of cellulose is thought to be related to the proton of OH groups in cellulose [7]. The predominant acidity of cellulose and cellulosic fibres has been observed repeatedly by many authors using both IGC [3,7,25,28]and contact angle method (by comparing the excess energy for wetting of cellulose using various liquids) [8]. Literature data also indicate that lignocellulosic fibres are less acidic than cellulose, but not strongly basic [8,25,29]. Further discussion on this topic is given in a later section. Analysis of the surface energy Characteristics of PVA and PMMA using the contact angle method and the three-liquid procedure of the Good-van Oss approach leads to the conclusion that both polymers are strongly basic, as well. Moreover, PVA is shown to be even more basic than PMMA. This result must be treated,with suspicion. Since the common functional group in PVA and cellulose is the hydroxyl group, PVA should have similar acid-base characteristics to that of cellulose, i.e. be bipolar with perhaps a stronger acidic component. The failure for the Good-van Oss “three-liquid” procedure to show this suggests that this procedure does not correctly determine the surface characteristics of eucalypt fibres and polymer samples. Della Volpe and Siboni suggested that one deficiency in the Good-van Oss approach is that it wrongly assumes that the acid and base parameters of water are equal. These authors have shown that the consequence of this assumption is that the base parameters of most othcr liquid probcs determined relative to water are much greater than their acid parameters [15]. It also leads to the values of the base parameters of solids determined using three liquid probes being systematically greater than the value of their acid parameters. ’
3E
50 40
30
;20 f- 10 3-
L
Figure 3
O
Surface energetics data of Eucalypt fibres and polymers calculated from contact angle results using the equation (1). The acid and base parameters of the liquid probcs used in the calculation are those proposed by Della Volpe and Siboni [ 151. 0 : yLw, a:y+ and
In order for the acid and base parameters of water to reflect the true acidity and basicity of water, Della Volpe and Siboni proposed new values for these parameters, i.e. y* = 65 mJ/m2and y- = 10 mJ/m2.The acid and base parameters of other probes thus have to be recalculated using the new values for water [151.
190 Structure and properties of fibres The surface energy and acid and base parameters of the eucalypt fibres and polymers have been recalculated using the new values. The results are shown in Figure 3. They still suggest that the eucalypt fibres and polymers are strongly basic, although the valucs of their base parameters are smaller than those obtained using the Good vanOss values (Figure 2). This calculation also predicts that PVA has a higher basicity than PMMA. Thus the acid and base values proposed for water by Della Volpe and Siboni do not seem to eliminate all the problems associated with the Good-van Oss approach. Prediction of the acid-base characteristics of eucalypt fibres and polymers using IGC
In the evaluation of the acid-base characteristics of the fibres and polymers using IGC, the WAAB values of diethyl ether and chloroform with the sample surfaces were used as indexes reflecting the basicity and acidity of the substrate, respectively (Figure 4).
Figure 4
Surface energetics data of Eucalypt fibres and polymers determined by IGC. 0 : p";W: WAmwith diethyl ether, reflecting acidity of the samples; WAAB with chloroform, reflecting the basicity of the samples.
The WAAB data obtained using IGC show that all eucalypt fibre samples are bipolar with thc haft pulp fibres being less basic than the NSSC fibres. An explanation of the difference in surface acid-base characteristics of kraft and NSSC fibres will be given in section 6 in the light of hrther experimental results. The surfaces of the polymer samples are also shown to be bipolar by IGC. PVA is more acidic than basic and PMMA is the reversd. This agrees well with the chemical properties of these polymers, the acidity of PVA being due to its hydroxyl functionality and the basicity of PMMA to its electron donating ester groups. We therefore think that WAABdata obtained using IGC reflect the surface acid-base characteristics of the eucalypt fibres and polymers more accurately than the Good-van Oss approach.
Surface energy of hardwood fibres 191 Comparison of the surface energetics results obtained by contact anglc and IGC methods A serious issue is raised as to whether the surface acid-base charactcristics of a solid detcrmined by IGC are at all comparable with those determined by contact angle methods. The uncertainties in the Good-van Oss three-liquid procedure are three fold. First, as has been addrcssed by Della Volpe and Siboni, the arbitrary assumption that the acid and base parameters of water are numerically equal does not reflect the actual acid and base strengths of water [15]. Second, there is some doubt that the acid-base interactions should be in the form of the sum of the square root of thc acid parameter for one phase and the base parameter for the other, as is described in equation (1). Third, it does not take account of the dissociation of acidic groups on the surfaces of solids in water. The form of equation (1) proposed by Good and van Oss has been questioned as solution of the simultaneous equations sometimes yields (y')'" 0 for the acid component. Attempts to explain the physical meaning of this result have so far not been convincing [121. Many solids havc surface acidic groups which dissociate in water. The lignocellulosic fibres studied in this work have weak Brcpnsted acid sites, e.g. carboxylic groups, which dissociate almost completely in water [30]. It has been reported that dissociation of the surface Brcpnsted acid groups affects the waterbibre contact angle [6,9]. The dissociated carboxylic groups no longer act as electron acceptor sites but as donor sites. In addition, dissociation of surface carboxylic groups in liquids like water leads to electric charge separation at the interface. The degree of dissociation of acidic functional groups of lignocellulosic fibres differs in different probe liquids. The validity of solving simultaneous equations to derive the surface energy of a solid is therefore qucstionable. One way of comparing the surface acid-base characteristics of a solid measured by IGC with those measured by contact angle methods is to compare the work of adhesion due to acid-base interactions betwcen monopolar probes and the solid. The equation proposed by Fowkes [3 11 was used to calculate the work of adhesion from the contact angle data:
Where W, and W",' are the total and the Lifshitz-van der Waals component of the arc the total and the Lifshitz-van der Waals work of adhesion, and y and" :y component of the surface energy of the liquid probe. PVA and PMMA were chosen to demonstratc the comparison. WALW was detcrmined using diiodomethane and the acidic and basic liquid probes chosen for contact angle measurements were bromoform and dimethyl sulphoxide. The values of the work of adhesion of bromoform and dimethyl sulphoxide with the polymer samples obtained using this approach are shown in Table 4. The Lifshitzvan der Waals interaction is the major component of the work of adhesion. WAAB(BF), the work of adhesion due to acid-base interactions between bromoform and the polymers, the work of is a reflection of the basicity of the polymer samples, whilst W,"B~oMso~, adhesion with dimethyl sulphoxide, of the acidity. Whereas both polymer samples are bipolar, the WAABvalues suggest that PVA is slightly more acidic and PMMA more
192 Structure and properties of fibres basic. These predictions are in qualitative agreement with those of IGC, although the values are not identical due to the use of different probes.
Table 4 The Work of Adhesion between Liquid Probes and Polymer Samples (mJ/m2) W,T"'(u, WkW(UQ WA""tBn WATo'(DMSO) WALwpMso) WA""(DMS0) 62.5 16.4 64.4 20.3 84.7 78.9 PVA 11.9 67.0 14.7 81.0 69.1 PMMA 81.8 A similarity between this and the IGC method is that both use the values of work of adhesion of one (or more) monopolar acid and one (or more) monopolar base with the solid to predict the acid-base characteristics of the solid. A shortcoming of this approach is that it is difficult to find pure monopolar probes. Most monopolar probes used have a certain (small) degree of bipolarity [1,15]. It is therefore expected that the use of monopolar probes to predict acid-base characteristics of solids still suffers from the problem of probe dependency. However, predictions made using equation (9) and the IGC mcthod reflect the acid-base characteristics more accurately than does the Good-van Oss approach.
THE EFFECTS OF SURFACE EXTRACTIVES AND LICNIN ON THE SURFACE ENERGETICS OF EUCALYPT FIBRES Methanol extraction of eucalypt krafi, NSSC and cold soda pulps was performed using the soxhlet apparatus in order to separately examine the effects of extractives and lignin on the surface energetics of the fibres. Due to uncertainties related to the Goodvan Oss theory, IGC was used to carry out measurements at 310 K. The dispersive component of the surface energy ($") of krafl, NSSC and cold soda pulps, before and after extraction, calculated using equation (7) and the work of adhesion (WAAB)of the pulps with chloroform and diethyl ether calculated using equation (8) are presented in Figure 5. The yLwand W,"" valucs of the extracted cellulose (#1 Whatman filter paper, extracted with benzene-ethanol mixture [3]) and lignin are also presented in Figure 5 for reference. The $", values of cellulose and lignin were 56.6 and 48.2 mJ/m2,respcctively. The yLwvalues of the unextracted pulps were all lower than that of lignin, falling within a relatively small range of 36.2 to 41.8 mJ/m2. Extraction of the pulps resulted in a large increase in their yLwvalues to between 50.7 and 62.2 mJ/m' (Figure 5). The cold soda pulp showed the smallest increase. The increase in yLw value resulting from extraction has been attributed to the removal of extractives from the pulp surface in a number of studies [3,5,18]. This conclusion is supported by XPS studies, which indicate that the major chemical change in the pulp surface caused by extraction is the removal of the extractives [ 16-181. Interesting changes in the acid-base characteristics of pulp surfaces due to extraction were also observed. Extraction caused preferential increases in the W," values of all pulps with diethyl ether, the basic probe, although W," values with chloroform, the acidic probe, also increased but to a lesser degree (Figure 5). This means that extraction caused a preferential increase in the electron acceptor functionality over that of the elcctron donor for all pulps. Since cellulose, the major constituent of the pulp, has a predominantly electron acceptor (acidic) characteristic [3,8,25], the electron acceptor sites on the cellulose surface must be active in the
Surface energy of hardwood fibres
193
molecular interaction. The adsorption of extractives on the pulp surface physically blocks the electron acceptor and donor sites and this is expected to preferentially suppress the electron acceptor characteristics compared with the electron donor characteristics,because of the highcr activity of the former.
F Figure 5
The dispersive component of surface energy (white bars) and the work of adhesion with chloroform (acidic probe, black bars) and dicthyl ether (basic probe, grey bars) of ccllulose, lignin and pulp samples, All measured at 3 10 K.
The surface acid-base characteristics of cellulose and lignin are different [3,8]. Whereas cellulose is strongly acidic, lignin is more evenly bipolar, with a much weaker acidity than and a similar basicity to cellulose (Figure 5). It is likely that the presence of a large number of ether-oxygen functional groups and a small number of acidic groups (e.g. phenolic and carboxylic hydrogen) in lignin are responsible for the weak acidity of lignin. It is thercfore expected that increasing the lignin concentration on the surface of pulp will affect the acid-base characteristicsof the pulp surface. Figure 5 shows that the difference between the acidic and basic characteristics of the samples dccreased in the scqucnce cellulose, the extracted kraft pulps, the extracted NSSC pulps and the extracted cold soda pulp. This sequence correlates very wcll with the increase in lignin concentration on the surface of these samples. The negative correlation between acidity and lignin concentration of the pulp surface is apparently caused by the change in the number of basic sites on the surface of the pulp, which are most likely the ether groups in lignin. It should be emphasiscd that the use of the extracted pulps for this comparison eliminatcs the influence of extractives. To support this interpretation, Shen and Parker have analyzed the work of adhesion of glass beads coated with differcnt amounts of lignin with chloroform and diethyl ether [4]. Their study has shown that whereas the uncoated glass bcads were strongly acidic (the WAAB value with diethyl ether was greater than 50 mJ/mol), coating them with lignin reduced their acidity and at the same time increased their basicity. A heavy coating of lignin (presumably the glass beads were almost completely covered by
194 Structure and properties of fibres lignin) changed the surface acid-base characteristics of the glass beads to that of lignin. This supports the view that the decrease in acidity in the sequence kraft, NSSC and cold soda pulps is attributable to the increase in the population of basic sites on the pulp surface because of the increased surface coverage of lignin.
CONCLUSION From the results of this paper, the following conclusions may be drawn: The surface extractives and lignin concentrations of the kraft, NSSC and cold soda eucalypt pulp fibres werc not proportional to the total amounts of extractives and lignin in these pulp. Kraft pulps showed high degrees of relative surface concentrations of cxtractives and lignin to those of the bulk pulps. The high relative surface concentrations of extractives and lignin of haft pulps is likely to be related to the high alkali levels used in pulping and the resorption of these chemical components onto the fibre surface during digestion and washing. yLwvalues of all fibre and polymer samples determined by IGC using alkanes are in acceptable agreement with those determined by contact angle methods using diiodomethane. The IGC method suggests that all eucalypt fibre and polymer samples are bipolar, with PVA being more acidic and PMMA more basic. The fibre and polymer acid-base characteristics calculated from the contact angle data for thrce liquid probes using the Good-van Oss approach do not agree with those obtained from IGC measurements. The values obtained using the Good-van Oss approach suggest that all fibre and polymer samples are almost monopolar basic. Such a result cannot be regarded as a true reflection of the surface characteristics of these samples. Re-calculation of :y and y; of the fibre and polymer samples using the Good-van Oss approach with new y,+ and y; valucs for the liquid probcs based on the values for water proposed by Della Volpe and Siboni still yields results that suggest that all fibre and polymer samples are strongly basic. The recalculated yI+ and ys-values therefore also do not agree with the IGC data. Determination of the acid-base characteristics of the fibres and polymers by calculating the work of adhcsion (WAAB)from the contact angle data using the equation proposed by Fowkes yields results that show similar trends to those obtained by IGC. Although quantitatively both the work of adhesion method proposed by Fowkes and the IGC method do not overcome the problem of probe dcpendency they do seem to provide a more realistic estimate of the surface properties of solid samples. The increase in pulp acidity detected by IGC is negatively correlated to the surface lignin concentration of the pulps. We propose that the low rclative acidity of the cold soda pulp was associatcd with the high surface lignin coverage, since the acidity of lignin was reported to be lower than that of the cellulose. A potential difficulty with using the Good-van Oss approach to dctermine the surface acid-base characteristics of lignocellulosic fibres is the likely dissociation of weak Brcpnsted acidic sites, such as carboxylic groups, on fibre surfaces by the liquid probes. The weak acid groups which have dissociated will change their characteristics from electron accepting to electron donating. If the degree of dissociation of these groups by differcnt liquid probes is different, the validity of using the three-
Surface energy of hardwood fibres 195 liquid/simultaneous equation procedure to determine the acid-base parameters of lignocellulosic fibres will be questionable. ACKNOWLEDGMENTS
This study was carried out within the research program of the CRC for Hardwood Fibre and Paper Science. Funding received from the Federal Government under the Cooperative Research Centre Program is gratefilly acknowledged. REFERENCES 1.
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11.
12. 13. 14.
J.C. Berg, Role of acid-base interactions in wetting and related phenomena, in Wettability, J.C. Berg Ed., Marcel Dekker Inc., New York, 1993,75-148 M. Morra, Some reflection on the evaluation of the Lewis acid-base properties of polymer surfaces by wetting measurements, J. Colloid Interface Sci. 1996, 182, 3 12-314 W. Shen, I.H. Parker, and Y.J. Sheng, The effects of surface extractives and lignin on the surface energy of eucalypt kraft pulp fibres, J. Adhesion Sci. Technol. 1998, 12, 161-174 W. Shen and I.H. Parker, Surface composition and surface energetics of various eucalypt pulps, Cellulose, 1999,6,41-55 M.N. Belgacem, A. Blayo, and A. Gandini, Surface characterization of polysaccharides, lignins, printing ink pigments, and ink fillers by inverse gas chromatography, J. Colloid Interface Sci. 1996, 182,431-436 J.C. Berg, The importance of acid-base interactions in wetting, coating, adhesion and rclated phenomena, Nord. Pulp Paper Res. J. 1993,8,75-85 J.M. Felix and P. Gatenholm, Characterization of cellulose fibres using inverse gas chromatography, Nord. Pulp Paper Res. J. 1993,8,200-203 S.B. Lee and P. Luncr, The wetting and interfacial properties of lignin, Tappi, 1972,55, 116-121 Y.J. Sheng, W. Shen and I.H. Parker, The influences of pulping method and pH on the surface energy of eucalypt fibres, Proceedings of S2nd Appita Confrence, Brisbane, Australia, 1998, vol. 1,91-98 W. Shen, Y.J. Sheng and I.H. Parker, Comparison of the surface energetics data of eucalypt fibres and some polymers measured by contact angle and inverse gas chromatography methods, J. Adhesion Sci. Technol. submitted R.J. Good, M.K. Chaudhury and C.J. van Oss, Theory of adhesive forces across interfaces, 2. Interfacial hydrogen bonds as acid-base phenomena and as factors enhancing adhesion, in Fundamentals of Adhesion, L-H Lee Ed., Plenum, New York, 1991,153-172 R.J. Good, Contact angle, wetting, and adhesion: a critical review, in Conlact Angle, Wettability and Adhesion, K.L. Mittal Ed., VSP, The Netherlands, 1993,336 F. Dourado, F.M. Gama, E. Chibowski and M. Mota, Characterization of ccllulose surface free energy, J. Adhesion Sci. Tcchnol., 1998, 12, 1081-1090 D.J. Gardner, Application of the Lifshitz-van der Wads acid-base approach to dcterrnine wood surface tension componcnts, Wood Fiber Sci. 1996,28,422-428
196 Structure and properties of fibres 15. 16. 17. 18. 19. 20.
21. 22. 23. 24. 25. 26. 27. 28. 29. 30.
31.
C . Della Volpe and S . Siboni, Some reflections on acid-base solid surface free energy theories, J. Colloid Interface Sci. 1997, 195, 121-136 J. Laine and P. Stenuis, G. Carlsson and G. Stram, The effect of ECF and TCF bleaching on the surface chemical composition of h a f t pulp as determined by ESCA, Nord. Pulp Pap. Res. J. 1996, 11,201-210 J. Laine and P. Stenuis, Surface characterization of unbleached kraft pulps by means of ESCA, Cellulose, 1994, 1,145-160 G. Strom and G. Carlsson, Wettability of kraft pulps effect of surface composition and oxygen plasma trcatment, 1992,6,745-761 K.T. Hodgson and J.C. Berg, Dynamic wcttability properties of single wood pulp fibres and their relationship to absorbency, Wood Fibre Sci. 1988,20,3-17 J. Schultz and J. Lavielle, Interfacial properties of carbon fibre-epoxy matrix composites, in Inverse Gas Chromatography, D.R. Lloyd, T.C. Ward, H.P. Schreiber and C.C. Pizana Ed., ACS Symposium Series 391, ACS, Washington DC, 1989,185-202 G.M. Dorris and D.G. Gray, Adsorption of n-alkanes at zero surface coverage on cellulose paper and wood fibres. J. Colloid Interface Sci. 1980,77,353-362 A. Lundqvist and L. Qdberg, Surface energy characterization of pigment coatings by inverse gas chromatography, J Pulp Paper Sci. 1997,23,5298-5302 A. Treimanis, Wood pulp fibre structure and chemical composition, their influcnce on technological processes, Nord. Pulp Paper Res. J. 1996, 1 1, 146-1 5 1 P. Stenuis and J. Lainc, Studies of cellulose surfaces by titration and ESCA, Appl. Surf Sci. 1994,75,213-219 P.N. Jacob, and J.C. Berg, Acid-base surface energy characterization of microcrystalline cellulose and two wood pulp fiber types using inverse gas chromatography,Langmuir, 1994, 10,3086-3093 B. Riedl, and P.D.Karndem, Estimation of the dispersive component of surface energy of polymer-grafted lignocellulosic fibers with inverse gas chromatography,J. Adhesion Sci. Technol. 1992,6, 1053-1068 E.G. Shafrin in Polymer Handbook, J. Brandrup and E.H. Immergut Ed., Interscience Publishers, New York, 1966,111-113 M.A. Tshabalala, Determination of the acid-base characteristics of lignocellulosic surfaces by inversc gas chromatography, J. Appl. Polym. Sci. 1997, 65, 10131020 M.A. Tshabalala, Determination of the acid-base charactcristics of lignoccllulosic surfaces by inverse gas chromatography, J. Appl. Polym. Sci. 1997, 65, 10131020 E. Sjostrom, The origin of charge on cellulosic fibrcs, Nord. Pulp Pap. Res. J. 1989,4,90-93 F.M. Fowkes in Physicochemical Aspects of Polymer Surfaces, K.L. Mittal Ed., Plenum Press, New York, 1983
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HEAT-INDUCED CHANGES IN FIBRE SURFACES Ingegerd Forsskihll, Taina Korhonenr and Nenrik Tylli2 'The Finnish Pulp and Paper Research Institute (KCL), Paper Science Centre, P.O. Box 70, FIN-02ISI Espoo, Finland 2Deparirnent of Chemistry, University of Helsinki, P. O.Box 55, FIN-00014 Ilelsinki, Finland
ABSTRACT The optical propcrties of shcets of two mechanical pulps - a spruce groundwood (GW) and a thermomechanical pulp (TMP) - were studied after various thermal treatments. The heat trcatments wcre performed at modcrate and high temperatures in either a dry oven or a humidity chamber. The optical changes were followed using UV-VIS reflectance and fluorcscence spectroscopy. The kinetics of thc yellowing was investigated using both techniques, which reveal changes in the fibre surfaces. The effcct of a prolonged humid heat treatment at 80°C was first compared with that of an equally long dry heat trcatment at 105OC using UV-VIS reflectance spectroscopy. It was found that the dry treatment caused more yellowing. The kinetic behaviour of GW pulp differed from that of TMP, apparently because the TMP had already been exposed to some heat during the rcfining process. The thermal reaction at 105OC was also followed using fluorescence spectroscopy. The heat trcatment caused a broadening of the emission bands, suggesting thc formation of new chromophores emitting in the short and long wavelength regions. The kinetics during heat treatments at different temperatures ranging from 105 to 230°C in an oven for much shorter times (1 min to 1 h) was also studied. Reflcctance values at wavclcngths 457 and 557 nm were measured and post-colour numbers (PC) wcre calculated. The sensitivity to heat was thus assessed at different temperatures. At tcmperatures higher than 130°C the rates of colour-forming reactions accelerate considerably. UV-VIS difference reflectance spectra for the heat-treated pulps were calculated and the chromophore changes and thc kinetics wcre evaluated.
Keywords: Aging; brightness; fluorescence; heat; high-yield pulps; humidity; kinctics; mechanical pulp; morphology; optical properties; Picea; reflectance; spectroscopy; ultraviolct; visible; spectrum INTRODUCTION The optical properties of different pulps and of the end products, the papers, are largely dependent on the raw matcrial but arc also devcloped during both the production process and storage. New energy-saving refining processes are being developed that utilize high dcfibration temperatures. However, one limitation here is the simultaneous decrease in brightness accompanied by papcr yellowing. New paper drying processes also utilize much higher tempcratures than before. The use of strong heat during manufacture and drying, as well as long-term humidity in combination with lower temperatures during storage, are factors that can alter the optical properties of fibres. Accelerated tests can be performed to simulate optical and mechanical changes, but correct temperature regions must be applied for drying and aging tests. Heat and humidity cause chemical, optical and morphological changes in fibres. Previous studies on TMP [ 11 heated in an oven at selected temperatures from 60°C to 23OOC for one hour showed visible changes at 100°C. However, significant morphological changes, which were studied using environmental scanning electron microscopy (ESEM). appeared at much higher temperatures. At 180"C, a shrinking of the surface of the fibrcs was observed as well as loosening of the interfibre structure. Coiling of the S1 cell wall matcrial to form spiral-forming lamellas, and some twisting of
198
Structure and properties of fibres
single fibres were observed in the micrographs. At 200°C.cross cracks appeared on the fibres and there was an increase in the number of totally ruptured fibres. At 230°C.a distinct transverse peeling of the surface layers was clearly visible. Strong discolouration and a decrease in the pH value of the TMP were observed at these high temperatures [ 11. In the present work, the heat-induced chromophore changes in GW and TMP were investigated using UV-VIS reflectance and fluorescence spectroscopy, which revealed the optical changes due to heat-scnsitive chromophores. Using spectroscopic methods, the surfaces of the fibres and papers can be studied and the changes during various heat treatment assessed.
EXPERIMENTAL Thick sheets (ca. 400 g/m2) of Norwegian spruce GW and TMP made at KCL’s pilot plant wcre studied. Heat treatments were performed in either an oven at selected temperatures from 60°C to 240°C for one hour (cf. ref [ 11) or kinetically for various times. Humid treatments were performed in an aging chamber at 80°C and 65% relative humidity (RH). UV-VIS reflectance spectra of heat-treated pulps were recorded on a Perkin- Elmer Lambda 15 spectrometcr equipped with an integrating sphere. Diffeience reflectance spectra were calculated by subtracting the spectrum of the treated pulp from the spectrum of the untreated pulp. Brightness values (R-,%) were taken from the spectra at a wavelength of 457 nm and are not identical to IS0 or Tappi brightness values, for which a specific geometry is required. Fluorescence spectra were obtained with a Shimadzu RF-5001PC spectrofluorometer using 350 nm excitation and a band width of 3 nm according to ref. [2]. Appropriate filters were used on both the excitation and emission sides.
RESULTS AND DISCUSSION A comparison between the two heating treatments - humid treatment at 80°C 65% RH, and treatment at 105°C (virtually dry conditions) - showed that dry treatment at 105°C caused more yellowing, Fig. 1.
Roo, 457 nm
68 66 64 62 60 58
!
0
I
I 20
I
I
I
I
40 60 80 HEATING TIME, h
I 100
I 120
Fig. 1. Kinetics of the yellowing of TMP during a dry treatment ( 105°C)and a humid treatment (80°C.65% RH): brightness (R,at 457 nm) versus heating time.
Heat-induced changes in fibre surfaces 199 The kinetic behaviour of GW differed from that of TMP (Fig. 1). The rate of discolouration of GW was faster than that of TMP, particularly in the initial phase of thc reaction, and the rate curve for TMP was more uniform. Thc slightly lowcr degree of yellowing, measured in terms of brightncss for TMP, can be explained by thc fact that TMP had already been exposed to some heat during the refiner stage. Thc thermal reactions of GW and TMP at 105°C were also followed using fluorescence spectroscopy, Figs. 2 and 3, for 96 h. Below 100°C changes in the fluorescence spectra were almost negligible. The heat treatment at 105°C caused a broadening of the emission band, suggesting the formation of new chromophores emitting in the short and long wavelength regions. The changes were more pronounced for GW, Fig. 2, than for TMP, Fig. 3, in the long wavelength rcgion (see inserted figurcs). The kinetics of the yellowing of TMP in an oven for short times (up to one hour) at high temperatures from 105°C to 230°C wcre also studied. The changes werc moderate up to around 100-150°C. Fig. 4 shows that the rate of yellowing increases markedly with increasing temperature. Thc brightncss losscs at 180°C and 200°C are sevcral units after only a fcw minutes. At 165°C a slight bleaching of TMP was observed after a short treatment (a few minutes), Fig. 4. On heating, it is not unusual for certain pulp propcrties such as brightness, scattering coefficient or some mcchanical property to improve momentarily before they start to deteriorate. Changes in the fibrc surface (“hornification”) occur during short-term heating, which can be attributed to crosslinking or I-I-bond redistribution (inter- or intra-bonding). At around 165°C thc total of the rates of the “bleaching” and the yellowing reactions is such that an overall blcaching at 457 nm is obscrvcd. At higher temperatures the colour-forming reactions are much stronger and yellowing is predominant.
30
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- 20
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5 0 400 450 500 5 5 0 600 650
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1 t
l
350
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400
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I
450
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550
I
I
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I
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I
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I1
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W A V E L E N G T H (nm) Fig. 2. Emission spectra of GW treated at 105°C for various times: curve 1,0 h; curve 2, 4 h; curve 3,21 h; curve 4,46 h; curve 5,96 h. Inset: differcnce between the spcctra of GW pulp treated for 96 h and untreated GW pulp.
200 Structure and properties of fibres 50 40
> 30 Iv)
=
W
20
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l
3 50
1
I
400
I
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l
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450 500 550 WAVELENGTH ( n m )
1
I
600
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I
650
Fig. 3. Emission spcctra of TMP treated at 105°C for various times: curve 1,0 h; curve 2 , 4 h; curve 3,21 h; curve 4,46 h; curve 5,96 h. Inset: difference between the spectra of TMP treated for 96 h and untreated TMP pulp.
DRY OVEN
R-. 457 nm V"
TMP
75 70
65 60
55 50 45 40
35 30 0
20
40
60
80
HEATING TIME, min
Fig. 4. Kinetics of the yellowing of TMP at various temperatures (105", 150°C, 165"C, 180°C. 200°C and 230°C): brightness (R,at 457 nm) versus heating timc.
Heat-induced changes in fibre surfaces 20 1 AR 10
-
8 --
6 --
4
_-
2 --
-2 -
250
,
300
350
400
450 500 550 Wavelength, nm
600
650
700
750
Fig. 5 . Kinctics of the yellowing of TMP at 180OC: UV-VIS difference reflectance spectra (unheatcd minus heated) after diffcrent times (up to one hour).
HEATING TEMPERATURE, "C
Fig. 6. Stability of GW and TMP during one hour of heat treatment at different temperatures (ambient to 240°C).
202 Structure and properties of fibres The corresponding UV-VIS difference reflectance spectra of TMP heated at 180°C are shown in Fig. 5. The yellowing maximum is locatcd at 433 nm. The spectra at the beginning of the heat treatment differ considerably from those obtained after prolonged thermal treatment. In the initial stages, there is a significant decreasc in the chromophore content in the UV region at 370-380 nm, as observed previously [3,4]. During heating new chromophores are formed in the long wavelength region and the initial decrease in the chromophore content in the UV region gradually disappears. The brightness stability of GW and TMP is greatly reduced at high temperatures (>130"C). The relative stabilities of the two pulps and their sensitivity to heat can be assessed from Fig. 6. TMP is slightly more stable than GW in accordance with the results of the fluorescence study. At temperatures higher than 130°C. a considerable acceleration is obscrved in the rates of the different colour-forming reactions in GW and TMP.
CONCLUSIONS In an evaluation of the heat stability of mechanical pulps, as for pulps in general, the following practical concepts may be useful. The temperature range is dividcd into three differcnt thermal regions, all of which cause yellowing. The behaviour of the chromophores differs from one region to another, resulting in different types of yellowing with different kinetics. These thermal regions can be roughly defined as follows: The first rcgion comprises moist heat at ambient to modcrate temperaturcs (
ACKNOWLEDGEMENTS The study was partly funded by the Technology Dcvelopment Centre, Finland (TEKES). We thank Ms Eija Ylinen for her valuable assistance.
Heat-induced changes in fibre surfaces 203 REFERENCES [l ] Eklund, 11. and Forsskihl, I., Heat-induced changes in TMP studied by ESEM microscopy. Proc. of the Presymp. Workshop of thc 9th ISWPC: Advances in microscopy and NMR spectroscopy of lignocellulosic materials, Qutbec, Canada, June 5-6, 1997, Posters No 5, p. 1. [2] Tylli, I-I., Forsskihl, I. and Olkkonen, C., The cffect of photoirradiation on highyield pulps: spectroscopy and kinetics, J. Photochem. Photobiol. A: Chem., 1995, 87, 181-191. [3] Forsskihl, I. and Janson, J., Sequential treatmcnt of mechanical and chemimechanical pulps with light and heat. Part 1. UV-VIS rcflcctance spcctroscopy, Nord. Pulp Paper Res. J. ,1991, 6, 118-126. [4] Forsskhhl, I. and Janson, J., Sequential treatment of mechanical and chemimcchanical pulps with light and heat. Part 2. FTIR and UV-VIS absorptionscattering spectra, Nord. Pulp Paper RES.J., 1992, 7 , 48-54.
INVESTIGATIONS OF SPRUCE PULP FIBRES BY SWELLING EXPERIMENTS AND LIGIIT-MICROSCOPY Hortling, B., Jousimaa, T. and Hyvarinen, 11-K. KCI, Finnish Pulp and Paper Research Institute PI3 70, 02151 E.spoo, Finland
ABSTRACT The properties of fibres refined under different conditions are investigated using light microscopy and swelling in iron-sodium-tartrate ( E W " ) and by mesuring their strength properties. Slight differences in refined and unrefined pulps were observed by light microscopy. Increscd rcfining efficiency correlated with more swelling points on the fibre, however earlywood and latewood fibres should be comparcd separately.The limitcd amount of data makes it difficult to draw any further conclusions.
INTRODUCTION Although a lot is known about delignification rcactions, selective delignification is hindered by thc fact that certain phcnomcna are still not understood. This is partly because of the chemical structures involvcd and also becausc of the heterogeneous stucture and composition of the ccll wall, including thc presence of ruptures (nodes)'), rcsulting not just from tree growth conditions but also from the pulping and bleaching proccsses. To obtain more information about fibrewall componcnts, the properties of the fibres are investigated using light microscopy and swelling in iron-sodium-tartrate (EWNN)2*3s4s7) and by measuring their strength properties. Swelling experiments on pulp fibers have earlier bccn performed in a copper(I1)-ethylenediamine solution (CED)".
EXPERIMENTAL Cooking conditions for the spruce kraft pulp (Sa): Liquor/wood = 3.50, Effective alkali = 4.5 molkg, Sulfidity = 35 %. Refining conditions: Following pulping mixing is performed for 10 min of 1.3 kg of 0.d. pulp at 4 % consistency, refining is performed after that in a Voith Sulzer laboratory refiner, as follows: Sal in water: specific edge load of 1.0 Ws/m, specific encrgy = 187 kWNt ; Sa2 in water: specific edge load 4.0 Ws/m, spccific energy = 197 kWNt; Sa3 in 0.01 M NaOH: specific edge load of 1.0 Ws/m, specific energy = 187 kWh/t TCF-bleaching of the pulps: The oxygcn-delignified Sa pulp was bleachcd using the QPZP- sequence. The physical properties of the liand sheets: Determined according to I S 0 5270.
206 Structure and properties of fibres Fibre properties determined by microscopy: All measurements were made in transmitted light. Fibre wall thickness was taken as the avcrage of 200 wet fibres. Page's index for curl was the average of 300 fibres. Fibres were classified into five groups according to the number of dislocations. A fibre was scored 0 if it had practically no dislocations and 4 if it was full of dislocations. The result given is the sum of the dislocation scores of 100 fibres'). Fibre fibrillation was determined from 300 wet fibres, which were placed into three classes: 1. intact or only slightly fibrillated, 2. highly fibrillated, 3. broken fibers. Fibre swelling in iron-sodiumtartrate (EWNN) determined by light microscopy: All fibres were first subjectcd to similar washing in an ultrasonic bath. They were then mounted on a microscopy slide and the free water was allowed to evaporate off before treatment with EWNN. Thc slide was placed under the microscope and swelling monitored by means of photographs taken at certain intervals.
RESULTS AND DISCUSSION Properties of the pulps, fibres and handsheets The properties of the pulps, individual fibres and handsheets are presented in Tables 1 and 2. The results show that the TCF bleached fibres have thinner cell walls, more curl
~~~
_ _ _ _ _ _ ~
1) Determined by the dynamic drainage jar (modified at KCL) (6). 2) Problems with mixing during ozonization.
fi brcs.
Spruce pulp fibres 207 and more dislocations than the unbleached fibres. There was slightly more fibrillation after refining in alkali than in water. The number of dislocations increases during the bleaching. The increase in specific edge load (Salt4 vs. Sa2t4) resulted in a slightly lower tear strength, while the tensile strengths were about the same. There was a large increase in air permeability, a moderate increase in density and a moderate decrease in light scattering. The effect of refining on the TCF-bleaching was as expected, and the differences seen before bleaching were still present after bleaching. The effect of alkali during constant refining conditions (Sal t4 and Sa3t4) was fairly small, although greater differences were seen after bleaching.
Fibre swelling The swelling properties of the different pulp fibercs are presented in Figure 1. Each photograph represcnts the average of several measurements, among which both greatcr and smaller effects are seen. Swelling behaviour depends on the cell wall thickness.
Salt4 Refined in water, low edge load
Sa2t4 Sa3t4 S a l t4OPZP Refined in 0.1 M NaOH, Refined in water, Refined in water, high edge load low edge load low edge load Swelling time
Figure 1.Swelling of pulp fibres Salt4, Sa2t4, Sa3t4 and Sa3t40PZEP (see Table 1) as a function of time monitopred under a light-microscope (magnification 150*).
208
Structure and properties of fibres
The amount of unswollen parts on the fibres (between swelling sites) increases as refining efficiencey is increased. The refining in 0.01 M NaOH seemed to cause slightly slower swelling and produce more short unswollen areas than the same refining in water. The blcachcd pulps swelled almost completely within 10 minutes.
CONCLUSIONS The results obtained with light microscopy show slight differences between the refined and unrefined pulps. The bleached pulps exhibited large numbers of fibre dislocations. Increased refining efficiency correlated with more swelling point on the fibre, probably due to damages on the fibres. Tensile strength was lower for the bleached pulps, which corrclates with the rapid swelling. The limited amount of data makes it difficult to draw any further conclusions. Earlywood and latewood fibres should be compared separately, and overlapping of fibres under the microscope should be minimized.
ACKNOWLEDGEMENTS The authors are greatly indebted to Pirkko Murronmaa and Eino Gronlund for their skill in preparing the specimens and conducting the microscope measurements, and to the Technology Development Centre of Finland (TEKES) for financial support.
REFERENCES 1 0 L Forgacs, Structural Weaknesses in Softwood Pulp Tracheids, Tappi, 1961,2, 112-119.
2 M. Pihlava, M., Fibre deformation and strength loss in kraft pulping of softwood, 1998, PSC Communication 113, 112 pp.
3 L. Valtasaari, Studies on the Improvement of Cellulose Solvents based on Iron Tartaric Acid Complex, Paperi j a Puu ,1957,4,243. 4 Jayme, G. and El-Kodsi, G., Uber Ein neues Verfahren Zur Herstcllung praktisch sauerstoffunempfindlicher Cellulose-EWNN,,d-Lbsungen fur Viskositatsmessungen und damit erhaltene Ergebnisse, Das Papier, 1968,22, 120 - 124.
5 Kyrklund, I3. and Sihtola, H., On Structural Changes in Pulp Fibres Due to Cold Alkali Treatment, Papcri j a Puu, 1963,3,91-98. 6 Tamminen, T., Kleen, M. and Hortling, B., Analysis of Fibre Surface Material Mechanically Separated from Spruce Kraft Pulp, 5th European Workshop on Lignocellulosics and Pulp, Aveiro, Portugal, 1998, p. 241- 244. 7 Pionteck, H., Berger, W., Morgenstern, B. and Fengel, D., Changes in cellulose structure during dissolution in LiCl:N,N-dimethylacetamide and in thc alkaline iron tartrate system EWNN, Cellulose, 1996,3, 127 - 139.
ROLE OF SOFTWOOD FIBRE FORM AND CONDITION ON ITS REINFORCEMENT CAPABILITY Kari Ebcling' 'UPM-Kymmene Oyj Eteldesplanadi 2, P. 0,Box 380 FI-00101 Ilelsinki Finland
ABSTRACT Softwood chemical pulp fibres provide the reinforcement backbone for today's woodcontaining printing papers. The ongoing development of the thermomechanical pulping process has decreased the need of this reinforcement pulp in comparison to the groundwood based sheet structures. However, it is to be expected, that the drive towards lower basis weight and the ever increasing quality demands for the mechanical printing papers will retain the requirement for good softwood chemical pulps. It is also expected, that as the use of recycled fibres increases in the wood-containing printing paper grades, some long fibred reinforcement pulp will be needed to guarantee the trouble free operations of the fast modem paper machines. What kind of a softwood pulp fibres then provide a good reinforcement pulp? The paper focuses on the role of the fibre dimensions and on the topographic structure as the key components of the quality of the reinforcement pulp. The fundamental role of the interfibre bond between the mechanical pulp fibre fractions and the reinforcement pulp is reviewed extensively. This interfibre bond structure and reinforcement behaviour will depend on three classes of factors, which are: ( I ) inherent fibre propcriics, like dimensions, (2) chemical factors like the effect of pulping and bleaching and the role of bonding additives, and (3) mechanical factors like refining, wet pressing, drying strcsscs and flocculation. The interfibre bond strength alone does not seem to be the key factor for good reinforcement pulp. Instead, the load bcaring capacity of the bonded network of mechanical pulp fibre fractions and of the reinforcement pulp fibres, i.e. the local stress concentrations and the local deformation capabilities in the network seem to be as important as the average bond strength. The ultimate reinforcement capability of the softwood pulp is controlled by the cleavage strength of the interfibre bonds and by the actual fibre strengths. If the interfibre bonds are quite strong, and they have been loaded fairly evenly, then the actual rupture line will exhibit a considerable amount of broken fibres from the long fibre fraction of the mechanical pulp and of the reinforcement pulp. If the interfibre bonds have been relatively weak, or loaded very unevenly, then the rupture line will exhibit 'This prcscntation resembles the Friedrich-Gottlob-KcllerMedal lccturc by the author in Dresden, April 9, 1997 at the 4th PTS-Symposium "Papierfaserstofflechnik".The lecturc has been published in Wochcnsblatt rUr Papierfabrikation 16/97, pp. 757 764.
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Structure and properties of fibres
plenty of unbroken fibres that have been pulled out from the fibre network. The important role of fines to the strength and behaviour of the interfibre bond is also analysed. This paper coinpares theoretical derivations for the strong reinforcement pulp to the actual experimental results to verify the relative importance of the various factors for the reinforcement capability of the softwood pulp.
INTRODUCTION We all know that neither the uncoated nor the coated wood-containing printing papers can be manufactured solely from mechanical pulp. Instead, a certain fraction of long fibre chemical pulp is needed to secure the runnability of the web through the paper machine and through the related subprocesses which are needed to provide the paper with proper printing surface. This long fibre cheinical pulp is called reinforcement pulp. The reinforcement pulp allows the ~nechanical pulp to be tailor made for the requireinents of the printing surface whilst the reinforcement pulp guarantees the strength needed for the trouble free runnability through the papcrinaking processes and the subsequent printing process. The reinforceinent pulp thus allows a division of functional responsibilities to take place between the inechanical pulp and the cheinical PulpThe reinforcement pulp is also critical for the economics of the manufacture of the wood-containing printing papers. Thus, it is obvious that the paperinaker would like to know what type of a bleached softwood pulp would be the best pulp for reinforcement purposes.
This paper presents a critical analysis of the factors that - according to today’s understanding - are important for a good quality reinforcement pulp. Both properties of individual reinforcement pulp fibres as well as properties of the fibrous network will be discussed. llackbone network structure
The amount of reinforcement pulp in a wood-containing paper depends on (a) the type of inechanical pulp used’, (b) the type and quality of wood used for mechanical pulp, (c) the sheet structure of uncoated (SC) or coated (LWC/MWC) magazine paper, (d) the basis weight, (e) the amount of fders and (f) the quality of the reinforcement pulp. Normally, the relative amount of reinforcement pulp varies from around 10 % to around 35 % as one moves from a TMP-based SC paper to a GW-based LWC. Discounting for the contribution of fillers and coat weight one can say that the basis weight of the reinforcement pulp sheet structure varies between 5 and 13 s/in’. In other words, the reinforcement fibre network is quite thin in most magazine papers. The statistical geometry of a 2-D random fibre network /1/ makes it possible to derive that the average (theoretical) amount of fibre-to-fibre crossings of a reinforcement pulp fibre with the other crossing reinforcement fibres is given by 20ncc:m stale that tlie relative amount of reinforceinerit pulp dccre:~scsin the following order: GW, RMP/PGW, TMP,CTMP/DIP.2
Softwood fibre form and condition
where
211
W = basis weight of the sheet lf = average fibre length in1 = fibre coarseness
Equation (1) describes that the average number of fibre crossings per fibre, fi, is directly related to the basis weight and to the fibre length and inversely proportional to the coarseness of the fibre. The number of fibre crossings per unit length, i.e. fi/q depends only on the basis weight and fibre coarseness. The formula of Equation (1) is derived for very thin (2-dimensional) sheet structures, i.e. basis weight around 5 din'. For thicker sheets, the derivation does not hold too well. For a 10 &In2reinforcement sheet one obtains an esthnate for the average number of fibre crossings per fibre (fi) as shown hi Table I. Here the effect of the fibre length is also shown.
Table I.
Effect of fibre length on the number of fibre crossings per fibre length (R) in a randoin 10 g/m2 2-D sheet structure
2S
2
3
The above average numbers of fibre crossings actually mean that the intercrossing distance between the centres of the crossing fibres is about 30 pin. This equals to the average fibre width of inany softwood pulp fibres. In other words, there is - on an average - no free fibre length in between the crossings. As one fibre crossing ends the other starts right away. However, since the fibre crossings - on an average - alternatc between the upper and lower side of the fibre, one side of the fibre (under analysis) has an average uncovered interbond distance equal to one fibre width (Fig. 1).
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...... I
A
i ............................
~_.. ."]
iL
........ .............
..........
Intercrosalng distance from double sided analysis)
B = Intercrosslng distaiico from singlo sided analysis) C =Free fibre length
Simplified view along B fibre showing the arrangement of interfibra crossings
Fig. 1.
Shnplified view along a fibre showing the arrangement of interfibre crossing in typical paper.
212 Structure and properties of fibres Experimental results of Page, Tydeman and Hunt 121 some 35 years ago have shown that the intercrossing distance (for opposing side crossings) in a moderately refmed randomly oriented handsheet of cheinical pulp fibres is about 30 pin, i.e. just what the simple theoretical treatment above predicted. This result also means that there are no large holes in between the reinforcement fibres in a well formed and randomly oriented sheet of about 10 &/in*. In the case of an oriented handsheet the number of fibre crossings is greatly reduced 111. In the case of an orientation typical of machine made papers, the amount of fibre crossings can drop close to 113 of what it was in the case of randomly oriented sheet structure (table I). Is the thin reinforcement fibre network theoretically capable of providing a sufficiently dense network so that the mechanical pulp fibres can be bonded to a fairly strong paper structure'! The mechanical pulp fibres act as elements that separate the reinforcement fibres from each other, and thus lower the number of fibre crossings3 between the reinforcement fibres. One may therefore question if the remaining number of interfibre bonds between them is sufficient from the runnability viewpoint.
Thus, a prerequisite for the good reinforcement action of the softwood pulp network structure is that the softwood fibres themselves are sufficiently strong and that they bond well to the various fractions of the mechanical pulp and to theinselves. One should keep in inind two things: (a) G W type mechanical pulp has about 20 % of its weight long fibres while the corresponding figure for TMP is around 40 %, and (b) the average weight of the long mechanical pulp fibres is twice that of the softwood cheinical pulp fibres4. With this information one may conclude that if the reinforcement pulp content is about 15 % (by weight) in a TMP containing inagazine paper, then the number of long mechanical pulp fibres and that of reinforcement pulp fibres is about the same. In the case of a GW containimg magazine paper having a reinforcement pulp content of about 30 %, the number of reinforcement pulp fibres is four times the number of long inechanical pulp fibres.
Practical experiences of the reinforcerncnt capability of softwood fibres Theoretically one may argue, that the reinforcement action of the softwood pulp in the wood-containing printing papers can be evaluated by the cross-machine directional 3Tlieeffect of ~nechanic;~l fibres on the nuinher of interfibre bonds betwecn [he reinfor- cement librcs c:tn be considered to be soinewhat siinil;~to the effect of lowering the basis weight of a thin rcinforccinent fibre shcet structure [see Equation (l)]. Thcoretically one might estimate that if the :unount of reinforcement pulp is around 33 %I,and the actual m o u n t of reinCorcernent pulp equals to 12 g/in2, ilieri the effect of the tncchanical pulp on the nuinbcr of interfibre crossings between the rcinlorceinent pulp fibres would bc equal to the effect of lowering the hasis weight to 4 g/in2. The nutnkr of fibre crossings would thus he 66 I lower th'm that given in T;ible I, and if one also considers the effect of orientation, the combined effect on the nuinkr of interlibre crossings between the reinforceincnt librcs would be lowering the Tahle I numbers by :ibOut XS %. In other words the nuinber of reinforccrnent fibre crossings wilh e:ch other could bc as low as 10 for 2 inin long fibres. 41n cheinicnl pulping and blericliing the cell wall loscs slightly inore than SO 941 of its inass. The stiiteinciit about SO o/n weight reduction is true only for softwood chemical pulp fibres inadc from the same type of wood (sane growth conditions) ;ISthe 1nec1i:uiical pulp.
Softwood fibre form and condition 213 tear strength (TearcD) and by the machine directional tensile strength (TensMu) of the paper. l'he better the reinforcing capability of the softwood fibre is the smaller amount of such reinforcement pulp is needed in order to achieve the necessary levels of TearcD and TensMD. Fig. 2 describes this in greater detail /3,4/.
t TECR'INDEX. CD. mN . m'lg
23SR 20.5SR
A
*
Y 030%Canadlen
50
I 65
60
TENSILE INDEX, MD. rnNlg
Fig. 2.
Reinforcement potential of a Canadian and Fuinish BSKP in LWC base paper (pilot paper inachine trial).
It can be seen froin Fig. 2 that an addition of only about 25 % of the experimental softwood pulp was required into the base paper of LWC- type coated magazine paper, whilst the reference paper required an addition of about 30 % to reach the required Tearcu and TensMD values. For a fast modern LWC paper machine this can mean a inanufacturing cost advantage of 20 - 30 million DEM.
Studies in Finland have shown that the reinforcement potential of a softwood pulp in wood-containing magazine papers can be related to the following factors' /3,4,5/:
'Soine research scientists claiin that Equation (2) 61s actually lxst for reinforceinent capability of LWCpiipcrs. For SC papers according to thcin - tear index at 70 Ninlg tcnsilc indcx is not needed, the rcinforccincnt potential for SC-payers is sullicicntly well described by (hequotient of average fibre Iengih and fibre cowscncss, i.e. by /,fin,.
-
214
Structure and properties of fibres
Reinforcement potential
- -Tear,o 1, m1
where
If = average fibre length in[ = fibre coarseness Tear70 = tear strength of reinforcement pulp handsheet at a tensile strength of 70 Nmn/gb
Equation (2) describes that the reinforcement potential is proportional to the average fibre length and inversely proportional to the fibre coarseness. The more slender the fibre is the better its reinforcement potential. This actually is evident also from Equation (l), which predicts that the slender fibres have more contact points with the adjacent slender reinforcement fibres. This is so because the number of slender reinforcement fibres is higher than the number of coarse fibres in the sheet structure of a given basis weight. We know that the tear strength of a randomly oriented handsheet depends on the fibre strength /6,7,8/ and on the average fibre length /9,10/. The effect of fibre strength apparently has been underestimated earlier, and it could very well be that for inediuin to well bonded sheet structures the tear strength is related to the fibre strength with a power of about 1.5. The role of fibre length apparently also is more than hiearly proportional in weakly bonded sheet structures. The published results for weakly bonded sheet structures seem to support a power dependence of 1.5 to 2 in case of fairly uniform fibre lengths in the sheet structure7. Also the earlier quoted work of Seth and Page /6/ reports a dependence of the tear strength on power 1.7 for weakly bonded softwood h a f t pulp. Thus one may rewrite Equation ( 2 ) by replacing the tear strength with an average fibre length dependent term
where
k = proportionality constant for the dependence of tear strength on the power function of fibre length
Equation (3) indicates that the reinforceinent potential is related to the second to third power of the average fibre length when other fibre properties like fibre coarseness and fibre strength are kept relatively constant. Thus, the average fibre length of the reinforcement pulp is a critical factor for the reinforcement potential of the softwood pulp. Of course the fibre length cannot be too long, because then it starts to lowcr the 6The author is aware of the criticism about the use of Eltncndorf te:u strength 21s a tncasure of tear strength, but several practical applicationshave SIIOWII that the CD Ehnendorf tern strength of the paper web does hiwe a predictive value for the runability of the web. 7 Pnavilaincn claims in her study, that ilie tear strength is line;uly related to fibre strcngtli, but she I i x varied thc avcr:ige fibre length by mixing short and long libres with each other. In her reported results, the tcnr strength triples aid quadruples as the film length doubles.
Softwood fibre form and condition 215 formation and cause stress concentrations into the sheet structure resulting in lower reinforcement potential for the softwood pulp. The effect of fibre coarseness on the tear strength of handsheets /I]/ is shown in Fig. 3. All the studied fibres had the same average fibre length of about 2.45 inin.
fear index, mNmZ/g 20 18 16
14 12
10
8
6 "/,_
5.15
Fig. 3.
0.2
1
I
0.25 Fibre coarseness, mglm
0.3
0.35
Effect of fibre coarseness on tear strength of softwood h a f t pulps.
One can see froin Fig. 3 that for the utu-efied fibres the tear strength maximum is reached with a fibre coarseness of around 0.2 tngfin. Moderate refining (4000 rev. at PFI) increases the tear strength of all other pulps except in the case of the very slender springwood h a f t fibres included in the study. The tear strength of these springwood fibres decrease in moderate refining by 25%. In the case of the high coarseness fibres ( pine h a f t pulp suintnerwood fibres) the maxinuin tear strength perhaps has not yet been reached with the amount refining done in Fig. 3. However, one may conclude froin Fig. 3 that the tear strength of the moderately refined pulps seeins to be fairly independent of fibre coarseness except in the case of the very slender springwood fibres included in the study. Since the coarser pulps had not yet reached the tensile strength level of 70 Ntdg one m y tentatively present a hypothesis, that the very coarse softwood pulp fibres do not in accordance to Equation (2) - provide the best possible reinforcing strength to the tnagazine papers. One should also keep in inind that the very coarse softwood reinforcement fibres decrease the sinoothness of the sheet, i.e. increase the need for strong calendering and may cause stronger fibre rise during coating or offset printing. Retulainen /12/ has shown that the strengthening capability of springwood and suintnerwood h a f t pulp fibres in a TMP furnish do differ somewhat froin each other already when the addition level of h a f t pulp is between 0 and 40 %. The largest deviations were observed in tensile strength, tensile stiffness (elastic modulus) and internal bond strength (Z-tensile). These observations support the hypothesis that the flcxible springwood fibres - due to their inore numerous presence - are capable of
216 Structure and properties of fibres distributing the load more evenly into the sheet structure in comparison to stiffer and less numerous summerwood fibres. Also the rupture line information indicates that the springwood fibres tend to break more often than the summerwood fibres during the fiial breakage of the tensile strip. Surprisingly, at low additon levels of the reinforcement pulp both the TMP long fibres and the reinforcement fibres tend to break at roughly equal 20 % probability. In other words, about 80 % of these fibres stay unbroken and slide past each other at the fiial rupture.
Effect of wood species and wood structure on the reinforcement potential Table I1 describes the reinforcement potential of softwood h a f t pulps made froin different coirunercial wood species at various locations of the globe /5/, It is important to observe that the age and part of trunk also has a very large effect on the reinforcement potential of the softwood chips (Table 111). The reinforcement potential index has been calculated according to Equation (2).
Table 11. The reinforcement potential index of different softwood h a f t pulps (made froin pulpwood) at various global regions. The index is based on the formula of Equation /n\
British Coluinhia, Coast:il inlxulds
Pinus IWiiita
When saw mill chips are usctl as pulping raw material the rcinfiircement intlex grtiws
Even in the case of the same wood species, the part of the trunk and the age of the tree affect greatly the reinforcement potential index as seen froin Table 111 /4/.Table
Table 111. Effect of various types of softwood chip material on the reinforcement
Chip source
Reinforcement factor'
Specially screened chips froin 205 pine sawinill log (periphery) Chips from spruce sawinill log 195 (periphery) Spruce ~ u l ~ w o o d 190
159 133 128 Latewood section of annual 121 rings of pine pulpwood
' According to Equation (2)
Softwood fibre form and condition
217
The role of fibre bonds in long fibre reinforcement of wood-containing papers It is safe to state that the full reinforcement potential of a softwood pulp requires that the reinforcement fibres forin strong bonds both with the longer fractions of the mechanical pulp and with the other reinforcement pulp fibres. The bonded network needs to have such interfibre bonds that - besides being sufficiently strong - they also are ductile. The network structure of the bonds needs to be such that the loading of the bonds is fairly uniform. Otherwise, there is a danger that the interfibre bonds are loaded up successively, i.e. one after the other. Uneven load distribution to the bonds of the fibrous network causes stress concentrations and leads to successive rupture of the individual bonds, because these bonds do not receive collective support from the other neighbouring bonds of the network. Therefore, even the strong interfibre bonds do not guarantee strong reinforcement if the bonds are loaded very unevenly and if the bonds are very fragile. In other words, it is not the interfibre bond strength that counts in the reinforcement pulp, but the combined strength of the fibre network, i.e. of the system. Let us analyse the formation of the interfibre bonds. It is well known that the formation of the interfibre bonds involves shrinkage of the swollen cell wall of the chemical pulp fibre during the consolidation process /13/. This cross-sectional shrinkage of the crossing fibre introduces an axial coinpression to the other fibre. The amount of this axial compression depends on the easiness of the axial shrinkage in the cell wall (swelling degree, dclainination of the cell wall, amount of contact with the crossing fibre and the radial shrinkage potential of the cell wall). Well refined thin cell wall chemical pulp fibres are easy to compress axially while mechanical pulp fibres are difficult to compress axially because of the stiffening action of the cell wall lignin. Thus onc would expect that the interfibrc bonds between the reinforcement pulp fibres are stronger, because they involve a contact between similar cell wall structures and stronger external fibrils than in the case of the surfaces of mechanical pulp fibres. I n addition, the chemical pulp fibres are more flexible and locally more conformable than the mechanical pulp fibres. Besides, the hydrogen bonding capacity of externally fibrillated chemical pulp fibrcs is better than that of mechanical pulp surfaces. One could perhaps spcculate, that if the mechanical pulp fibres would be practically without external fibrillation and fiies fragments, it could very well be that the swollen chemical pulp fibre would partially detach from thc crossing mechanical pulp fibre during the consolidation. This could be so, because the shrinkage forces of well refined cheinical pulp fibres have been noticed to be so strong that they can detach for instance a wet pressed sheet from the glass surface during the drying of the sheet. The above observation could perhaps be interpreted that such softwood chetnical pulp fibres which have a fairly thin cell wall and a large fibre diameter may not be as good reinforcctnent pulp fibres as fibres with smaller diameter, bccause the latter ones d o not collapse so completely on the surface of the crossing mechanical fibre and thus might not create so high shrinkage stresses as the fully collapsible reinforcement fibres. In other words, the interfibre bond strength between such a reinforcement fibre and the mechanical pulp fibre might remain stronger than in the case where the high shrinkage forces could partially rupture part of the bond established with a stiff mechanical pulp fibre.
218
Structure and properties of fibres
Next, let us look how the surface topography of reinforcement pulp inight affect the tear strength. It is known that soda anthraquinone and sulphite pulps have lower tear strength than the kraft pulps made from corresponding wood species. Nelson and Irviie /14/ clahn - based on their experimental results - that the lower tear strength of soda-AQ pulp fibres in comparison to kraft pulp is due to the extraction of heinicelluloses from the outer layers of the cell wall by the higher concentrations needed in the soda-AQ pulping. N o significant differences were observed between the fibre strengths of the soda AQ and h a f t pulp fibres. But the lower amount of heinicelluloses at the interfibre bond region probably lowered the interfibre bond strength and made the bond more fragile. Similarly, it is known that the weak tearing resistance of sulphite pulps in comparison to kraft pulps of comparable species is related to the lower D.P. of carbohydrates at the outer periphery of the cell wall. In other words the weaker external fibrils and molecular fibrils do not provide sufficient anchoring for the interfibre bond involving certain amount of intermixing of external and molecular fibrils from the two crossing fibres. Because of the weaker anchoring of the various sizes of interinixed fibrils the eventual load carrying capacity of the interfibre bond between sulphite fibres is weaker than in the case of h a f t pulp fibres. This last subconclusion has a strong bearing on the reinforcement capability of ECF and TCF bleached reinforcement pulps. Since the oxycheinicals used in the TCF bleaching are not as specific deligni- fying chemicals as chloride dioxide, they will also tend to degrade the cell wall carbohydrates, especially the carbohydrates of the outer cell wall layers siinultaneously as they remove lignin from the cell wall.
The above subconclusion also implies that in the case of TCF-pulps one expects the reinforcement capability of the TCF fibre increases as the brightness of the pulp decreases, i.e. less bleaching ineans less damage to the outer layers of the cell wall. As a matter of fact, comparisons of the reinforcement potential of ECF and TCF pulps have shown /4/ that the actual tear strength of TCF pulp is somewhat lower than the tear strength of the corresponding ECF pulp of the same pulp inill (Fig. 4)'. This figure also shows how the full TCF bleaching has ruined the tear strength of the pulp in comparison to the corresponding ECF pulp, and that leaving the brightness of the T C F pulp somewhat lower will provide a stronger reinforcement potential (tear strength) to the softwood pulp.
8
in this study ECF and TCF piilps lroin various pulp inills were cvalunted cilher in paper inacliine trials or 1ahor:itory haidsheet Icsts.
Softwood fibre form and condition 219 INDEX = 70 Nm/g). mN m'lg
LEGEND:
@ ECF TCF
Efrocl of ECF TCF Vansfonallon
J I = UZON€
70
80
90
BRIGHTNESS (ISO). X
Fig. 4.
Relationship between brightness and tear strength of certain dried ECF and TCF softwood pulps.
Table IV describes also the effect of bleaching degradation on the strength properties of a softwood fibre sheet structure 1151. The fibres in this study were bleached southern pine h a f t fibres beaten to 8000 rev. at the PFI lab refiner and then fractionated to a +35 mesh BMcN fraction. A portion of the refrned fibres was f i s t overbleached with sodiuin hypochlorite and then fractionated. Different inirtures of these two types of fibres were made to handsheets. Normal paperinaking properties and the load-elongation curve of these handsheets were measured.
Table IV. Strength and other properties of refined southern pine fraction +35 mesh fibres inked with overbleached similar fibres.
1
Property
Composition of yuper; umount overbleached pulp,%
I
It can be seen froin Table IV that the tensile index of the handsheet decreased by 17 % as one moved koin the original bleached pulp to the overbleached pulp of the same fibre dimensions. The tear index fcll by 53 % at the same time. The sheet structure was
fairly constant judged by the density and the scattering coefficient. The rupture elongation decreased by 25 % as did the post-yield slope of the load elongation curve. The original slope of the pre-yield slope was unaffected by the change from the original
220 Structure and properties of fibres bleached fibres to the overbleached fibres, i.e. the same amount of fibre inaterial with the same type of load elongation properties was involved in the so called elastic extension of the sheet structures.
The results of Table IV show that the bleaching reactions can have a strong effect on the load carrying capacity of the interfibre bonds. The role of fines in reinforcement of interfibre bonds It is well known that the excellent printing properties of magazine papers, i.e. especially the smoothness, and low print through are to a large degree due to the amount of fines fraction in mechanical pulp. If the fines arc not autoinatically present in the mechanical pulp - like in certain types of TMP - more of them are usually generated through extra expenditure of energy. It has long been known also that the quality of the fines is critical both to the strength and optical properties of the mechanical pulp containing paper grades /16 181. In other words, the fibrillar ("Schlehnstoff') or lamellar fines contribute better to the strength of the sheet structure than the powdery type (flour stuff or "Mehlstoff") fmes. However, the latter ones contribute more to the opacity than the former ones. '-
Recently Retulainen and coworkers /19, 201 have published new inforination about the role of cheinical pulp and mechanical pulp fines to the strength of interfibre bonds. Table V shows results of the strengthening action of TMP and h a f t pulp (BSKP) fines on the sheet structure of TMP long fibres 1201.
Table V.
Effect of fines on long fibre TMP handsheet properties Internal bond str. by Z- Density, tensile, kg/ln3 max.load, N 21 1 22,3
Tensile index, Nidg
Light scatt, coefficient, in2/kg
1X,5
~
TMP long fibres + 30 o/n kraft fines3 I
'
190
333
25,8
352,2
449
16,4
tidncd on 30 incsli wire in BMcN appiuiitus Passed through 200 mesh wire in BMcN :ippamtus Passed through 200 mesh wire in BMcN after 60 inin Valley heating
Results of Table V indicate that sheet structures containing kraft fuies were significantly stronger (7-fold in tensile strength and 1&fold in internal bond strength) than sheet structures containing only long TMP fibres. They were also about two tines stronger than the sheet structures reinforced with TMP fines.
Softwood fibre form and condition 221 The sheet density increased by 100 5% with the inclusion of kraft fines and only by 50 o/o with TMP fines. The increase in sheet density obtained with the addition of TMP
fines was only slightly higher than what one would estimate from filling the voids between the coarse TMP long fibres with fines inaterial without disturbing the interfibre positions of the long TMP fibres. However, when kraft fines were added to the long TMP fibre furnish, the density increase obtained was twice as high as that obtained by addition of TMP fines. In other words, the h a f t fines have really been able to pull together the network structure of long TMP fibres. One should observe that the light scattering coefficient of the TMP long fibre sheet structure was only slightly higher than that of the sheet structure containing both long T M P fibres and h a f t fines. In other words, the kraft fines seemed to reinforce the interfibre bonds of the long TMP fibres without increasing very much the area of interfibre bonds. Thus, the kraft fines increase considerably also the specific bond strength TMP long fibre network. In addition, the s h y and very colloidal fines inaterial of the refuied kraft pulp seeins to form a continuous and membrane-like reinforcement network at the peripheries of the TMP long fibre crossings. This network is capable of relieving the interfibre bonds between the long TMP fibres from stress concentrations and uneven load distribution. Thus, together with the strengthening of the individual bonds this kraft fines network is also responsible for the very large increase in strength values of the long TMP fibre sheet structure. In the case of the TMP fines the light scattering coefficient increased by 40 YO indicating that a lot of light scattering inaterial had been added into the voids of the long T M P fibres. The bulkier TMP files form more discrete connecting bridges between the stiff long TMP fibres and help to distribute the ovcrall load more evenly into the network by avoiding stress concentrations. Fig. 5 shows siinilarly the different effects of refined h a f t pulp - - fines and TMP fines on the tensile strength vs.-light scattcring coefficient interdependence in various types of sheet structures /19/. Tensite index, Nm@ ............ 15 ?A
j
-....... 'i "'*.. .. -._..__ -4..
.
1
i
Light scattering caefficient, m?kg
Fig. 5.
Role of files on tensile strength and light scattering coefficient of TMP and BSKP handsheets.
222 Structure and properties of fibres It can be seen from this figure that the TMP fines contribute mostly hicreased Light scattering into the sheet structure, but also an increase in the tensile strength can be observed. This increase in tensile strength is relatively significant in the case of TMP long fibres (average fibre length 2.5 tnm, fibre coarseness 0.31 tns/mn) and in the case of a 55/45 blend of TMP long fibres and refined BSKP. The effect obtained through the addition of refined haft pulp fines to the sheet structure seems to be mainly to increase the tensile strength and to a lesser degree to lower the light scattering coefficient9. In other words, the kraft fiies are capable of increasing both the bonded area - especially in the case of TMP/BSKP blend - and the specific bond strength of the interfibre crossings as discussed earlier in connection with Table V.
One should also observe from Fig. 5 that by a suitable combination of TMP fines and rcfiied kraft pulp fines and by selecting a proper addition level one is able to cover a large variety of the tensile strength vs. bonded area/specifc bond strength properties. As a matter of fact, the proper combination of fines seeins to allow the paperinaker to select a wider combination of properties than he would obtain by just varying the amount of reinforcement fibre in the blend of mechanical pulp and softwood h a f t pulp. However, the drainage rate will be impaired very much with high addition levels of fines making the practical utilisation of this method difficult. The type of chemical pulp fiies seems to have a large effect on the reinforcement capability of tensile index as seen from Table VI based on results of Retulainen and coworkers 1201. It can be seen from Table VI that the fines generated by moderate to high amount of refining have the strongest reinforcement action.
Table VI. Effect of various types of fines (15 % addition) on the strengthening of interfibre bonds between moderately refined BSKP fibres ( 30 inin. Valley
Type of fines added Kraft fines extracted with 10 % NaOH' TMP Kraft fines from unrefined BSKP Kraft fines extracted with 5 % NaOH' Kraft fines from 30 inin Valley beating Kraft fines from 60 inin Valley beating Kraft fines from 120 min Valley beating Fines froin medium refiied NSAQ pulp
Increase in tensile index due to addition of 15 % of various fines into lightly beaten BSKP 33 42 49 56 91
93
I 96 I 104
' From 30 inin Vallcy bcatcn BSKP
'In Fig. 5 the kraft fines addition caused a significant drop in the light scaltering coel'licientol' Ihe TMP/BSKP blend in coinparison to only TMP or BSKP long fibre shect structures or lo lhe eCkct rccortlcd in Table V.
Softwood fibre form and condition 223 The amount and nature of the heinicelluloses in the fines inaterial seemed to have also a large effect on the strengthening action. The extraction of fines with 5 % NaOH solution lowered the tensile strength of the handsheets by 30 %, whereas the extraction with 10 % NaOH solution lowered the strength by almost 65 %. The best strengthening action was obtained with neutral sulphite antraquinone pulp fines. The NSAQ pulp is known to have a higher heiiucellulose content than the normal kraft pulp.
CONCIAJSIONS Based on the hypothctical reasoning of the role of bleached softwood pulp fibres ~ I I the reinforcement network of magazine papers, and on the verification of these hypotheses by a selective review of the reinforcement fibre literature one may conclude that a strong reinforcement pulp has the following features:
* long fibres * slender fibres * fibres with well anchored and colloidally covered fibrils * sufficient amount fibrillar fines obtained during beating The long fibres increase the probability of fortning a continuous net- work from the softwood reinforcement pulp fibres even at fairly low addition levels; i.e. at a low basis weight of the reinforcement pulp. However, bccause of the formation demand, the reinforcement pulp fibres cannot be too long in order to avoid unnecessary flocculation of them. The sknderticss of the reinforcement fibres has two advantages; (a) it makes the fibres more flexible, i.e. they can inore easily establish contacts with the long mechanical pulp fibres and the other reinforcement fibres, and (b) because of the lower weight of the slender fibres more of thein can be included into the sheet structure. The increased number of slender reinforceinent fibres in comparison to coarser softwood fibres also means a better capability for load distribution in the sheet structure. The slenderness of the reinforcement fibres should not be achieved by very barge diameter and very thin cell wall cross section of the fibre, because such fibres probably tend to get easily damaged in the process and they may have too high shrinkage tendency which might weaken the bond strength to a crossing mechanical pulp fibre. Instead, smaller fibre diameter with inoderate cell thickness is a more preferable cross section for a superior reinforcement fibre. Also, the inore slender the reinforcement fibres are the inore carefully the pulping and bleaching reactions need to be carried out in order not to damage the cell wall. The thinner cell wall has less possibilities to accotnmotlate a cell wall damage than a thicker cell wall. It is self-evident that the external fibrillation of a refined reinforcement fibre needs to be well anchored into the cell wall structure and incorporate enough colloidal carbohydrates (molecular fibrillation) so that strong interfibre bonds will be established between the long mechanicnl pulp and rcinforcement pulp fibres. In practice this ineans that the D.P. of the fibriUnr cellulose i ~ tthe external layers of the cell wall needs to he
224 Structure and properties of fibres sufficiently high. Sunilarly there needs to be suitable amount of swellable carbohydrates of sufficient D.P. around these surface fibrils. Apparently the fines of refined reinforceinent fibres do contribute to the strength of interfibre crossings. Theoretically one could perhaps argue that if the reinforceincnt fibres have a sufficient external and well anchored fibrillation one should not nced any loose fibrillx fines to enhance the interfibre bonding. However, on the other hand one can also state that the colloidal fines of the refined reinforcement fibres have a tendency to deposit themselves on the right place, i.e. into the intersections of the crossing fibres. Thus they will build an extra membrane-like network on the periphery of the interfibre bonds and provide extra fortification and ductility into the bond. By so doing they also relieve stress concentrations of the interfibre bonds and allow the reinforced sheet structure to reach its ultimate strength. Finally, one may conclude that the first two aspects of the good reinforcement pulp, i.e. high enough average fibre length and sufficiently slender cell wall cross section, can best be controlled by selection of wood species, by age of trees and by which parts of the trunk are chipped. The other two aspects of the good reinforcement pulp, i.e. the proper D.P. and heniicellulose content of the surface layers of the reinforcement pulp fibres can best be controlled by the pulping and bleaching reactions and by the method of refining. A good reinforcement pulp thus is a result of good team work between the wood procurement and the pulp inill.
HEFEKENCES
I. Corte, H. and Kalhnes, O.J.: Statistical geometry of a fibrous network, In Bolain’.. The Forination and Structure of Paper, Transactions of the Symposium held at Oxford, September 1961, Vol. 1, Tech. Section of the British Paper and Board Makers’ Association, London, 1962, pp. 13 - 46. Corte, H.: Structure of Paper, In Ranceb IIandbook of Paper Science, Vol. 2, Elsevier Scientific Publ. Coinp., Oxford, 19x2, pp. 175 - 1x2.
2. Page, D.H., Tydeman, P.A., and Hunt, M.: A Study of Fibre-to-Fibre Bonding by Direct Observation, I n Bolain’s The Formation and Structure of Paper, Transactions of the Symposium held at Oxford, September 1961, Vol. 1, Tech. Section of the British Paper and Board Makers’Association, London, 1962, pp. 171 - 193. 3. Unpublished Research Results, The Finnish Pulp and Paper Research Institute, 1989. 4. Unpublished Research Results, Kaukas Oy, A Subsidiary of (then) Kymnene Corp., 1990 - 1993.
5. Which is the Best Fibre for Paperinaking?, A Jaakko Pijyry Oy Report, Stockholm, 1994, pp. 105 - 121. 6 . Seth, R.S., and Page, D.H.: Fibre properties and tearing resistance, Tappi J., 71 (19XX): 2, 103 - 107.
7. Page, D.H. and MacLeod, J.M.: Fibre strenght and its impact on tear strength, Tappi J., 75 (1992): I , 172 - 174. 8. Page, D.H.: A note on the mechanism of tearing strcngth, Tappi J., 77 (1994): 3, 201
- 203. 9. Paavilainen, L.: Importance of particle size - fibre length and fines - for the characterization of softwood h a f t pulp fibres, Paperi ja Puu, 72 (1990): 5, 516 - 526.
Softwood fibre form and condition 225 10. Yan, N. and Kortschot, M.T.: Modelling of out-of-plane tear energy absorption of paper, Appita 49 (1996): 3, 176 - 1x0. I I . Retulainen, E., Vaulot, F. and Ebeling; K.: The effect of fibre properties and some treatments on sheet structure, Paper presented at the 19th International Syinposiuni "Paper, a physiclly engineered product" held at Miami University, Oxford, Ohio May 16 - 19, I9XX,23 p. 12. Retulainen, E.: Strength properties of mechanical and chemical pulp blends, Paperi ja PUU,74 (1992): 5,419 - 426. 13. Page, D.M.and Tydeinan, P.A.: A New Theory of the Shrinkage, Structure and Properties of Paper, In Bolamb The Formation and Structure of Paper, Transactions of the Syinposium held at Oxford, September 1961, Vol. 1, Tech. Section of the British Paper and Board Makers' Associa tion, London, 1962, pp. 307 - 413. 14. Nelson, P.J. and Irvine, G.M.: Tearing resistance in soda-AQ and haft pulps, Tappi J:, 75 (1992): 1, 163 - 166. 15. Houen, P.J.: Prepared contribution, I n BolamS Consolidation of the Paper Web, Transactions of the Syposium held at Cambridge, September 1965, Vol. 2, Tech. Section of the British Paper and Board Makers' Assoc., London, 1966, pp. 801 - X03. 16. Brecht, W. and Holl, M.: Schaffung eines Normalverfahrens ziir Gutebewertung von Holzschliffen, Papierfabr. 37 (1939): 10, 74 - 86. 17. Brecht, W. and Suttinger, R.: Uber die technologische Bedeutung des Forincharakters von Holzschliffen, Wochenbl. Papierfabr. 74 (1943): 1, 3 - 9; 74 (1943): 2,21 - 27. Brecht, W. and Klermn. K: The rnkture of structures in a mechanical pulp as a key to knowledgee of its technological properties, Pulp Paper Mag. Can. 54 (1953): 4, 72 - 80. 18. Forgacs, 0.: The characterization of mechanical pulps, Pulp Paper Mag. Can. 64 (1963): NO. C, TX9 - T118. 19. Retulainen, Moss, P. and Niemninen, K.: Effect of fines on the properties of fibre networks, In Baker's Products of Papermaking, Transactions of the Symposium held at Oxford, September 1993, Vol. 2, Fundamental Research Coirunittee/Pira International, Leatherhead, U.K. 1993, pp. 727 - 769. 20. Moss, P.A., and Retulainen, E.: The effect of fines on fibre bonding cross-sectional dimensions of TMP fibres at potential bonding sites. Proceedings of the 1995 International Paper Physics Conference held at Niagara-on-the-Lake, Ontario, September 1995, TAPPIRech. Section CPPA, pp. 97 - 101.
e.,
COMPOSITIONAL ANALYSIS OF OIL PALM TRUNK FIBRES Putri Faridatul Akmar I , Mohd Nor Mohd Yusoff I , John F. Kennedy & Charles J. Knill
’ChemistryDivision,Forest Research InstituteMalaysia PRIM), Kepong 52109. Kuala Lunipur, Malaysia
Birmingham Carbohydrate & Protein TechnologyGroup (BCPTG). ChembiotechLaboratories. Universiv of Birmingham Research Park, Vincent Drive, Birniingham R15 2SQ, UK
ABSTRACT The pulping of oil palm trunk containing parenchymatous tissues has been reported to give low pulp yield and poor pulp quality. In view of these observations, a machine known as the ‘j?actioriator’ was developed to produce oil palm trunk fibres free from parenchyma. This investigation examines the chemical composition of the fibres separated from the parenchyma of the oil palm trunk using such technology. Fibres analysed in this study wcre found to have lower lignin (16.9% w/w) and holocellulose (62.3% w/w) contents compared to other hardwoods, using standard test methods. Only 5.4% w/w of the fibres were found to be water-soluble. Hydrolysis of the water insoluble material using concentrated TFA yielded 3 1.9% w/w total monosaccharides, with glucose (16.2% wlw) and xylose (12.7% w/w) being the major components. Loss of carbohydratesis expected during chemical pulping, reducing pulp yield and strength. INTRODUCTION
The oil palm (Elaeis guineensiis)originated in the tropical rain forests of West Africa and is mainly grown for its oil producing fruits. Malaysia is the leading producer of palm oil accounting for approximately half of the total world production. The economic life of the palm is 25 years and many large plantations are due for replanting, hence the huge quantity of waste oil palm trunks currently available. In 1994 alone, 4.6 million tonnes of waste oil palm trunks were generated. This figure was expected to increase to over 7.0 million tonnes by the year 2000. The cost of disposing of unwanted trunks, which are mainly pushed over, shredded and later burnt, is lRM9 (- 11-50) per trunk. Unlike other hardwoods, the oil palm trunk is a monocotyledon. Its physical nature is therefore different from common wood. The trunk is largely composed of parenchymatous tissues with numerous fibres and vascular bundles. Vascular bundles are scattered in parenchyma tissue (Fig. 1). Parenchyma is relatively soft and contains mainly short chain polysaccharides and starch. a-Cellulose, a component of the insoluble carbohydrate, increases in abundance from pith to periphery whilst watersoluble components, i.e. the low molecular weight polysaccharides, decrease in the same direction. This can be explained by the higher quantity of starch and short chain polysaccharides in the parenchyma rich inner portion of the stem Monocot species do not have cambium, growth rings, ring cells, sapwood, heartwood, branches or knots (hardwood and soitwood are dicots). Monocot growth is a result of cell division and enlargement in the parenchyma, and enlargement of vascular bundle fibres.
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228 Structure and properties of fibres
bark
Figure 1. Distribution of the parenchyma and vascular bundles in a crosssection of a mature monocotyledon tree trunk (e.g. an oil palm).
The economic application of oil palm trunk material as a timber substitute for conventional use is highly unlikely. Therefore, the only promising way of utilising oil palm trunk is by using the fibre for production of panel products and pulp. Earlier studies into the feasibility of producing pulps from oil palm trunk waste gave low pulp yields, poor pulp quality and low pulp strength, due to contamination by the carbohydrate rich parenchyma '. Likewise, oil palm trunk chips produced by the conventional cutting method for the production of fibres still contain intact parenchyma, leading to drying problems. If not thoroughly dried, degradation and poor quality of the final product can result due to susceptibility of the parenchyma to attack by hngus and insects. Conventional flaking, hammermilling and screening methods are unsuccessful in separating the fibres and parenchyma. The Forest Research Institute of Malaysia (FRIM)successfully developed a machine, known as the 'ji-actionator', to efficiently and economically shred oil palm trunk at a raw material capacity of 17.0 tonnedhour. The fractionator does not use any blades, but a combination of roiling, crushing and tearing mechanisms '. The shredded parenchyma and fibre produced by the ji-actionator can be segregated efficiently by a hammermill and the produced fibres are free of parenchyma. The oil palm trunk fibres have a mean fibre length of 0.96 mm and a mean diameter of 29.6 pm Apart from physical properties, the quality and pulp yield also depends upon the chemical composition of the fibres. This investigation focused upon compositional analysis of the oil palm trunk fibres produced from thefractionator.
'.
MATERIALS & METHODS Oil Palm Trunk Fibres
Oil palm trunk was obtained from The United Plantation Oil Palm Estates, in Teluk Intan, Malaysia, which was undergoing a replanting programme. The trunk was shredded using an excavator machine before fibre samples were obtained via separation from parenchyma using the fractionator. All subsequent chemical analyses were carried out in quadruplicate and only the averaged results are reported. Proximate analysis was performed according to TAPPI standard methods '.
Oil palm trunk fibres 229 Proximate analysis Alcohol-benzene (A R) extractives anahsis, TAPPI stundard method T6
Fibre samples (- 2 g) were placed in cellulose thimbles and subjected to soxhlet extraction using alcohol-benzene (AB; 2: 1 vlv) for 8 hours. The collected extractives wcre rotary evaporated to dryness under reduced pressure at 3OoC. The extracts were dried at 105°C to constant weight. Average AB extractives content is expressed as a weight percentage of the oven-dried weight of the starting fibre. Substances extracted by this method include fatty acids, resin acids, waxes and tannins. Holocellulose content
Holocellulose is a term used for the product obtained after the removal of lignin from wood. An ideal delignification should result in the total removal of lignin without chemical attack on the polysaccharides, however no delignification process is ideal. Combined residues remaining after AB extraction (- 2 g) were quantitativelytransferred to a round bottom flask (250 mL). Water (100 mL), sodium chlorite (1.5 g) and aqueous acetic acid (10% vlv, 5 I&) were added to the flask. The solution was heated in a boiling water bath for 3 0 minutes. Subsequently, acetic acid (10% v/v, 5 mL) was added and the solution heated for another 30 minutes. After that, hrther sodium chlorite (1.5 g) was added and the solution heated for a fbrther 30 minutes. The alternate addition of acetic acid and sodium chlorite, at intervals of 30 minutes, was continued until a total of 6 g of sodium chlorite had been added and the solution was heated for a final 30 minutes. The suspension was then cooled in an ice bath and filtered using a weighed sintered crucible (porosity 1). The resultant residue was washed with deionised water and then acetone, and was dried in a desiccator until constant weight was obtained. The air-dried holocellulose content is expressed as a weight percentage of the oven-dried weight of the starting fibre. a-Cellulose content, TAPPI stundard method T203
Cellulose cannot be obtained in the pure state via any isolation method, but only as a crude preparation that is generally called a-cellulose. Air-dried holocellulose (isolated above) was placed in a beaker and sodium hydroxide solution (17.5% wlv, 15 mL) was added. The residue was macerated for 1 minute using a flattened glass rod. Further sodium hydroxide solution (17.5% wlv, 10 mL) was added and the beaker contents mixed for 45 seconds. This was repeated using additional sodium hydroxide solution (17.5% w/v, 10 mL), mixed for a hrthcr 15 seconds and the resultant mixture left standing for 3 minutes. Afler this additional sodium hydroxide solution (17.5% wlv, 10 mL) was added and the beaker contents mixed for 2.5 minutes. This final stage was repeated 3 times after which the mixture was left to stand for 30 minutes. Water (100 mL) was added, the contents mixed and left to stand for 30 minutes. The mixture was then suction filtered using a sintercd crucible (porosity 3), washed with sodium hydroxide solution (8.3% wlv, 25 mL) and repeatedly washed with deionised water (650 mL, 20°C). Subsequently, the crucible was filled with acetic acid (2M) and after agitation the acetic acid was removed under suction. Deionised water was then used to remove any residual acid fiom the residue. The residue was dried to constant weight at 105°C. The a-cellulose content is expressed as the weight percentage of the oven-dried weight of the starting fibre.
230 Structure and properties of fibres Klason (acid-insoluble) lignin content, TAPPI standarcimethod T222 Lignin typically occurs in the vascular tissues which are for liquid transport and mechanical strength e.g. xylem, therefore monocotyledons are less lignified compared to dicotyledons. Combined residues remaining after AB extraction (- 1 g) were placed in a glass stoppered conical flask. Aqueous sulphuric acid (72% vlv, 15 mL) was slowly added with constant stirring using a glass rod and cooling to 12-15OC. The thoroughly mixed sample was left standing at 20°C for 2 hours during which time the contents were frequently stirred. The solution was then diluted with deionised water (563 mL). The flask was then fitted with a condenser and heated under reflux for 2 hours to hydrolyse and solubilise all polysaccharide material present. The lignin was filtered on a pre-weighed alundum crucible (RA 360) and washed with hot deionised water (700 mL). The crucible was immersed in a small beaker filled with hot water and heated in a boiling water bath for 30 minutes. It was then washed in a sintered crucible (porosity 3) with hot water (300-500 mL), dried in an oven (105OC) for 2.5 hours, cooled and dried in a desiccator to constant weight. The lignin content is expressed as a weight percentage based on the original unextracted material, calculated as follows:
X w=-(100-2) Y
W X Y
= = =
Z
=
lignin content (% wlw of material) dry weight of lignin residue dry weight of extracted sample AE3 extractives content (YOwlw)
Inorganic material (ash) content, TAPPI stanhrd meihod TI5 A number of inorganic constituents are necessary for plant growth. The composition
of the inorganic matter depends on the environmental conditions under which the tree grew and the location within the tree, generally ranging from 0.1-0.5% wlw in temperate woods and up to 5% wlw in tropical woods The main components of wood ash are K, Ca and Mg as well as Si in the case of some tropical woods. Unextracted fibre (- 2 g) was incinerated at 850°C for 6 hours in a pre-weighed porcelain crucible. The sample was immediately cooled in a desiccator and weighed. The inorganic material content (remaining residue) is reported as the weight percentage of the original dry weight of unextracted fibre starting material.
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Analysis of carbohydrates in water soluble and insoluble fractions
Sample extraction Fibre samples were subjected to triple successive soxhlet extraction (8 hours per solvent) with petroleum ether (BP 4O-6O0C), acetone and mcthanol (70% vlv in water). Fibre residues (- 2 g ) from the triple solvent extraction were further extracted with cold deionised water overnight and filtered using a sintered crucible (porosity 1). The total free monosaccharide content and total neutral carbohydrate content of the filtrate were determined by GC analysis of the reduced acetylated monosaccharides and L-cysteinesulphuric acid assay, respectively, as detailed below. The residue remaining after extraction with cold water was then treated with concentrated trifluoroacetic acid (TFA) to hydrolyse any cold water insoluble oligomcric and polymeric carbohydrate material to its component monosaccharides. Resultant hydrolysates were then reduced, acetylated and their monosaccharideprofiles determined (by GC), as detailed below.
Oil palm trunk fibres 231 Total neutral CarbohyaFate content of water soluble material Neutral carbohydrates in the fibre cold water soluble component were quantified by the L-cysteine-sulphuric acid assay '. Freshly prepared L-cysteine hydrochloride solution (700 mg/L in 86% vlv sulphuric acid, 1 mL) was added to samples, standards and controls (200 pL) containing up to 20 pg of neutral carbohydrate with thorough mixing whilst cooling (ice bath). Solutions were then heated (lOO°C, 3 minutes) in stoppered test tubes, cooled to room temperature and their absorbance (415 nm) determined using a Pye Unicam SP6-550 uvtvis spectrophotometer. The carbohydrate contents of samples were quantified from the calibration curve constructed from the absorbance of the standard solutions (from the line of 'best fit' determined by linear regression
7.
TFA hy&o&sis of water insoluble material Water insoluble material was subjected to concentrated TFA hydrolysis *. Samples (- 50 mg) were left to swell overnight at room temperature in concentrated TFA (4 mL). Each sample was refluxed for 1 hour and then water was added to give an acid concentration of 80% vlv and refluxed for another 15 minutes. Finally the acid was diluted to 30% with water and left to reflux for another 3 hours. TFA was then removed by repeated dilution with water and rotary evaporation (complete "FA removal was confirmed using pH indicator paper).
Preparation of alditol acetate derivativesfor GC analysis Ammonia was added to aliquots of the cold water extraction filtrates and TFA hydrolysates to give an ammonia concentration of 1 M and a final volume of 0.5 mL. Meso-inositol (0.25 mL, 4 mg/mL) was added to samples as an internal standard. Aliquots (0.1 mL) were reduced by addition of sodium borohydride'and acetylated as detailed below '. ( i ) Reduction of monosaccharides in W A hydro&saies (alditolformation)
Monosaccharides were reduced with a solution of borohydride in dimethyl sulphoxide prepared by dissolving sodium borohydride (2 g) in anhydrous dimethyl sulphoxide (100 mL) at 100°C. Monosaccharides were routinely reduced for 90 minutes at 40°C by addition of sodium borohydride solution (1 mL) to the TFA hydrolysate solutions in ammonia (0.1 mL). Excess borohydride was decomposed by the addition of acetic acid (1 8 M) after reduction '. (ii) Acetylation (productionof alditol acetates)
I-Methylimidazole (0.2 mL) and acetic anhydride (2 mL) were added to the reduced hydrolysate solutions and mixed. After 10 minutes at room temperature, water (5 mL) was added to decompose excess acetic anhydride '. Cold dichloromethane (1 mL) was added and the mixture agitated using a vortex mixer. After settling, the lower phase was removed and stored at -20°C until analysed by GC.
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232 Structure and properties of fibres GC anaIyss (of aldirol acetates) Alditol acetates were separated using a BPX-70 (70% bis-cyanopropylpolysilphenylene-siloxane) capillary column (0.32 m m i.d., 0.25 pm film thickness, 25 m) fitted to a Carlo Erba GC series 8000 e uipped with a flame-ionisation detector @ID) and an injection system using split mode 'O. Helium (2.0 mL/minute, 150 @a) was used as the canier gas. Samples (1 pL) were analysed. The oven temperature was kept at 150°C during sample injection and then raised at lO"C/minute to 19O"C, kept at 190°C for 2 minutes, and then raised at 5"C/min to 250°C where it was kept for 10 minutes. The injection port and detector were maintained at 260°C and 3OO0C, respectively.
RESULTS AND DISCUSSION The pulping process endeavours to separate the fibres and subsequently remove the lignin from wood. The amount of lignin determines to a certain extent the pulp yield and quality 11r12. Fibrisation can be accomplished either by mechanical means or by chemical treatment (or a combination of both). In mechanical pulping, lignin modification and removal is not possible. Besides lignin content, cellulose, xylans and mannans are the three main polysaccharides that influence the pulp yield and quality ' I . In the production of chemical pulps, delignification is allowed to proceed until most of the lignin is removed. Higher lignin content in the raw material requires higher chemical consumption. The rate of delignification is also influenced by the type and degree of association of lignin and carbohydrate 12. Initial roximate analyses of the fibres of the oil palm trunk using TAPPI standard methods are displayed in Table 1. These results show that the weight percentage holocellulose and lignin contents of the fibres appear to be significantly lower than for many hardwoods, which generally have holocellulose and lignin contents in the range of 71-86'70 and 24-32% w/w, respectively. The combined results of the proximate analyses only account for 88% wlw of the total material, and only 5.4% wlw of the fibres were cold water soluble. Significant carbohydrate degradation is suspected during the sodium chlorite treatment for the determination of holocellulose content, and would account for the low total amount observed. Therefore, although the 'proximate analysis standard method' has been widely used to assess the composition of woody materials, it is not suitable for holocellulose determination in monocots as it underestimates the carbohydrate composition, since it would appear that a significant proportion of the carbohydrate is being removed with the lignin, hence the low recovery. Hydrolytic degradation of carbohydrates, particularly cellulose and hemicelluloses, lowers the pulp yield and also the pulp strength.
r
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Table 1. Results of the proximate analysis of oil palm trunk fibres. Component AB solubles holocellulose a-cellulose lignin inorganic material
Yo ( W I W ) 6.0 62.3 38.3 16.9 3.0
Oil palm trunk fibres 233 Table 2. Monosaccharide composition of TFA hydrolysates.
Component Ara Gal Glc Man Rha XYl Total
Yo ( W I W )
1.1
0.9 16.2
0.9 0.1 12.7 31.9
The results of the L-cysteine-sulphuric acid assay showed that only 12.8% wlw of the cold water soluble material was in the form of neutral hexose carbohydrate. GC analysis showed that only 7.2% wlw of the cold water soluble material was in the form of free monosaccharides. Such quantities of water soluble material would not have a significant influence upon the pulping of the fibres. TFA hydrolysis of the water insoluble material of oil palm fibres yielded glucose and xylose as the main monosaccharide components (Table 2). Other sugars usually found in plant materials were also detected but in much lower quantities. The TFA hydrolysis data indicated that 31.9% wlw of the carbohydrates were totally hydrolysed with half of it derived from hemicellulose. Assuming that all the glucose in the hydrolysate was derived from cellulose, more than 40% of the cellulose was totally hydrolysable. More detailed investigations on the structural characterisation of the carbohydrate constituents (in fibre and parenchyma) of oil palm trunk have been performed. This has included the effects of variation of TFA hydrolysis conditions on monosaccharide release (monitored by GC and HPLC), and methylation analysis to further characterise the carbohydrate materials present in terms of glycosidic linkage positons 13. CONCLUSIONS Oil palm trunks are waste materials with significant potential as sources of energy, feed, food and chemicals, which have yet to be filly exploited. Like other biomass material, they are composed of cellulose and hemicellulose that are chemically associated with lignin. But unlike other woody materials, these palm wastes are from the monocot species and composed mainly of soft primary cell walls, which constitute valuable carbohydrate resources. Several efforts to utilise these wastes for production of particle-board, charcoal, pulp and paper or extenderlfiller in plywood have been attempted, however, the successful commercial scale production of these are yet to be established I4-l6. The major difficulty in the production of panel products from oil palm trunk fibres has always been getting a complete fragmentation of the fibrous and nonfibrous elements. Although a certain amount of parenchyma can be tolerated in particle-board manufacture, the less, the better. The development of the fiucfionufor has helped to overcome such difficulties. A lot of work has been done on the physical characteristics as well as the down stream utilisation of oil palm trunk, however there are currently few technically feasible processes that could be an outlet for commercialisation. In general, the characteristics of the carbohydrates of oil palm trunk materials have not been fully investigated. Further more detailed investigation of the components is an important prerequisite for optimal utilisation of these materials.
234 Structure and properties of fibres REFERENCES
C. Khoo & T. W.Lee, Preliminary characterisation of oil palm trunk as raw material for pulp and paper, The Malaysian Forester, 1984, 47 (l), 28-42. 2. K. C. Khoo, M. Y. Mohd Nor & T. W. Lee, Pulp and paper, FIUM Research Pamphlet, 1991, 107,51-65. 3. S . Mahmudin & E. Puad, Fibre processing technology fractionation process to produce fibrous strands from oil palm residues, 4Ih National Seminar on Utilisation of Oil Palm Tree: Oil Palm Residues Progress Tmards Commercialisation, 1997, Kuala Lumpur, Malaysia. 4. TAPPI OfficiaI Test Methods, T6, T15, T203 & T222, TAPPI Press, USA, 1979. 5. S. Saka, Chemical composition and distribution, In: Wood and Cellulosic Chemistty, D. N . 4 . Hon & N. Shiraishi (eds.), Marcel Dekker, Inc., New York, 1991, pp. 59-88. 6. C. A. White & J. F. Kennedy, Manual and automated spectrophotometric techniques for the detection and assay of carbohydrates and related molecules (B312), In: Techniques in Lye Sciences, VoI. B3 - Techniques in Carbohydrate Metabolism, Elsevier, The Netherlands, 1981, pp. 1-64. 7. J. C. Miller & J. N. Miller, Statistics for Analytical Chemistry, 2"d Edition, Ellis Horwood, Chichester, 1989, pp. 104-105. 8. D. Fengel & G. Wegener, Hydrolysis of polysaccharides with trifluoroacctic acid and its application to rapid wood and pulp analysis, In: Hydrolysis of Cellulose: Mechanisms of Enzymatic and Acid Cata&sis, Advances in Chemisfty Series 181, R. D. Brown & L. Jurasek (eds.), American Chemical Society (ACS), Washington, 1979, pp. 146-158. 9. A. B. Blakeney, P. J. Harris, R. J. Henry & B. A. Stone, A simple and rapid preparation of alditol acetates for monosaccharide analysis, Carbohydrate Research. 1983. 113.291-299. 10. Analysis of monosaccharide alditol acetates, SGE Publication No. AP-0001-C, SGE Europe Ltd., Milton Keynes, UK. 11. N. Sanyer & G. H. Chidester, Manufacture of wood pulp, In: 7he Chemistry of Wood,B. I. Browning (ed.), Interscience, London, 1963, pp. 441-534. 12. E. Sjostrom, Pulping chemistry, In: Wood Chemistry: Fundamentals and Applications, E. Sjostrom (ed.), Academic Press, Florida, 1981, pp. 104-145. 13. F. A. Putri, Preliminary characterisation of the chemical constituents of oil palm trunk and sago wasfe, PhD thesis, 1998, The University of Birmingham, Birmingham, UK. 14. L. T. Chew & C.L. Ong, Particleboard from oil palm trunk, POXIMBulletin, 1985, 11,99-108. 15. S. Rahim, M. A. Razak & M. A. Zakaria, Chemical components in oil palm trunk influencing wood-cementboard manufacture, Asian Science and Technology Congress, 1987, Ministry Of Science, Kuala Lumpur, Malaysia. 16. K. C. Khoo & T. K. Lee, Sulphate pulping of the oil palm trunk, POxlM Bulletin, 1985, 11, 57-65. 1. M. Y.Mohd Nor, K.
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I
FIBRE BLACKENING IN SUPERCALENDERED PAPERS Tim0 Kosltinen
UPM-Kymmene, P.O. Box 51, FIN-3 7601 Vulkeukoski, FINLAND ABSTRACT Quality targets of supercalcndercd uncoated magazine papers, supercalendercd papers have bccn increased year by year. Very high gloss and smoothness lcvels have been rcached, but simultancously darkening or blackening of the paper surface has become a severe obstacle for many paper producers. However, on given gloss or smoothness level some papers tend to blacken more easily than other apparcntly similar papers. This suggests that in raw materials and/or paper structure there must bc significant diffcrences. The results oblained in pilot scale studies indicate that the impact of papcr structure parameters, mainly formation, on calender blackening is important. In calendering the thickest areas in paper sheet are heavily pressed. In these areas, the light scattering coefficient decreases to such an extent that the thickest places become almost transparcnt. This gives papcr an uneven and blackened appearance. In connection with calender blackcning of paper the light absorption coefficient of fibres and paper changcs only marginally. Formation of paper does clcarly depend on fibre composition. Conscquently chemical fibres, because these increasc more fibre flocculation and additionally are morc comprcssible than rncchanical fibres, tend to increase more blackening of the paper surface in calcndering. Late wood fibres tend to blacken more easily than early wood fibres. This applies as well to chemical pulps as to mechanical pulps.
KEY WORDS Calender blackening, Supercalendcring, SC-paper, Optical propcrties, Late wood fibres, Early wood fibres, Soft wood fibres, Image analyscs, Formation.
INTRODUCTION SC- papers are glossy, supcrcalendcred, uncoated, wood-containing printing papcrs, whcrc filler content is most often around 30 pcr cent. For thcse papcrs high print gloss and even print result on the micro scale can not be achievcd without high paper gloss and low paper roughncss. In order to achieve these, heavy supercalcndering conditions are applied. However, if gloss and smoothness are improved only by using heavier supcrcalendcring conditions calender blackcning easily becomes a limiting factor preventing additional improvements to desired propcrties. However, on given gloss or smoothness level, some papers clcarly show a grcater tendency to blacken than other, apparently almost similar papers. This can be interpreted so that in raw materials orland in paper structure thcrc are significant differences. In certain calendcring conditions these differences result in severe blackening.
236 Structure and properties of fibres Calender blackening of fine paper has been studied by Krenkel (1). The same phenomenon in newsprint paper has been studied by Popil (2). Praast reported on blackening measurement method (3), Ebert et al. on calender blackening studies of SCpaper (4) and Koskinen (5). This paper first discusses calender blackcning as an optical phenomenon and describes methods of visualizing and quantifying it. Then, the main reasons behind the differcnt blackcning tendencies of SC-papers are analyzed, in the light of the results from two types of pilot trial. EXPERIMENTAL Theoretical background
Calender blackening is an optical phenomenon. Thus the terminology of Kubelka Munk theory gives the natural basis for describing it. The strength of this theory is, that it reliably shows how two optical parameters, independent of each other, together with basis weight, fully define the optical behaviour of paper on the grey scale. These two parameters are light absorption coefficient and light scattering coefficient. However, there are some limitations in K-M theory. For example, it assumes that paper is a homogeneous material, which is far from the case. This means that if one wants to study the effect of formation on paper optics, one runs into difficulties. We can, howevcr, overcome these difficulties by assuming that on a small scale, where formation is almost constant, K-M theory is still valid. In trying to explain the darkcning or blackening of paper during calendering, one could easily believe that an incrcase in the light absorption coefficient is taking place. Howcver, experiences of this study showed that the light absorption coefficient increased only marginally, but that the light scattering coefficient decreased quite dramatically when a certain type of paper was calendered within various calendering conditions so that calender blackening increased. Thus a new question arose: is the situation the samc or different when several different types of papcr, rather than one, are calendered to the blackening stage?
Figure 1.
A dark field illuminated picture of heavily calendered SC-paper.
. Fibre blackening in supercalendered papers
237
We know that the darker uncalendered paper has a lower light absorption coefficient. The light scattering coefficient shows how well light is scattered because of fibrelair and fillerlair interfaces. Obviously, during calcndering, the total area of such surfaces is decreased. But why does paper blacken, and why there are such clear differences in the blackening tendency between papers? Some papers lose their ability to scatter light more easily than others: there must be some physical differences between these papers. In order to study these differences, one must produce different kinds of paper which blacken differently and be able to measure the degree of calender blackening that occurs. In order to illustrate calender blackening, light microscopy pictures and SEM pictures were taken. These were unsuccessful until light microscopy pictures with the dark field illuminated surface images were taken. These picturcs, together with pictures takcn with a light source behind the paper samplc, gave a good basis for the understanding of calcnder blackcning (Fig. 1). In the blackened areas of blackened paper, some fibres have lost their ability to scatter light. The light beam goes through the paper in the blackened areas almost like it goes through a piece of glass. In addition to this, some of the fibres on the surfaces of the paper are non-scattering, but light does not go through the paper entirely. So, there is two-sidedness in the blackening of paper, too. The magnification used in light microscopy pictures was most often x50. In order to illustrate how light behaves in a thin, glass like (non-mctallic) parallel plate, Figure 2 shows reflectance and transmittance of a pencil of light 161. In a thin glass plate, only a minor absorption of light takes place.
, Ay ~
.,/
Reflected ray
”.
Refracted ray
Figure 2.
The refraction of light through a glass block 161.
For a fibre to appear blackened, there must be no or low reflectance from it. So, the light beam must penetrate the fibre’s surface, be absorbed by fibres while scattering inside the fibre’s structure or penetrate through the entire sheet. - If the fibre next to blackened fibre reflects light well, then the contrast occurs that gives the paper its undesirable blackened appearance. How fibres look like in the cross section of supercalendered woodcontaining paper is shown in a light microscopy picture in Fig. 3. We can see that many fibres are pressed so that lumen can hardly be seen or it can not be seen at all.
238
Structure and properties of fibres
Figure 3.
Cross section of a 56 dm’ woodcontaining supercalendered (SC) paper.
This was also the starting point both for development of a measuring method and for the experimcntal study of thc reasons behind the diffcrences in blackening. Onc can readily accept that different sets of calcndering parametcrs cause somc differcnces in the glosshlackening ratio. However, real competitive differences must be based on differcnt raw materials and / or paper structures. The method of measurement must record, how the light scattering coefficient of paper has developed as a result of calendering and even so that the unevenness of the locally varying decrease of light scattering can be followed. Looking at the dark ficld illuminated microscopic photos, one can sce that the unevenness of the optical appearance of fibres is characteristic of a blackened papcr sample. If one concludes that the decrease of light scattcring by somc fibres is the explanation for blackening, one might also think that thc locally thickest parts of the paper should be most affected. Thus, high variation of the small scale basis weight, i s . bad formation, should be one reason for an increascd tendcncy to blacken. In uncalendered paper, a locally highcr basis weight also means higher light absorption, while the light scattering coefficient is the same all over the entire sheet. However, when paper is heavily pressed, the higher is the relative change of light scattcring in hcavier placcs, thc highcr will be calender blackening. This was studied by comparing the valucs of the p- radiograms with the results of light microscopy pictures whcn light scattering and light absorption coefficients were measurcd locally. In uncalendcred paper, there was no correlation between the local basis weight and the local light scattering coefficient. In supcrcalendered papcr, whcn high linear loads were applicd there was a clear negative correlation bctween the local basis weight and the local light scattering coefficient. An cxamplc of the results is shown in Fig. 4.
Fibre blackening in supercalendered papers 239 Correlation betwccn local light scattering coefficient and local basis wciglit vs. linear pressure in (last nip of ) supcrcalcndcr (paper A). 0.1
\.,
-0.1
‘ 1
-0.3 -0.5
1
’
-0.7
+ -.-a -‘
.
-0.9
0
50
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160
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250
300
350
400
450
Linear prcssurc (in last) supercalcndcr nip, kN/rn
Correlation between local light scattering coefficient and local basis weight vs. liricar pressure in (last nip of 1 supcrcalender (paper D).
-0.9
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50
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Linear pressure tin last) supercalcndcr nip, kN/m
Change of correlation betwcen local basis weight and local light scattcring with an increasing linear load of supercalender. The developrncnt of two papers is shown. Pilot trials Two pilot trials were done. The first was done by using the pilot paper machine of KCL (FPPRI) in Espoo, Finland. The purpose of this trial was to test differences in raw materials and in the moisture content of paper before calendering. The effect of filler content was tested. Even the effect of load level in wet pressing was checkcd. The trial program is shown in Table 1. The refcrence situation was as follows: in furnish, 67 % GW, 33 % bleached pine haft, 30 % filler clay, retention aid 200 g/t cationic PAM,
240 Structure and properties of fibres moisture after PM 2 %, moisture after remoisterizing 8 %. Paper was moisturized to a moisture content of 8 % after passing through the paper machine, except in two cascs, wcre moisture targets before calcndering were 7 and 9 %. Supercalcndering of these papers was made by using KCL's pilot supercalender. Four linear loads were applied to each trial paper: 120, 180, 240 and 3 10 kN/m. Thc speed was 200 ds.Roll diameters were 375 mm and 400 / 250 mm, in filler rolls and steel rolls respectively. Table 1.
Pilot paper machine trials
Trial no Changed variable 1 Filler content 2 Filler content 3 Reference Dual retention system 4 GW Ibl. kraft 5 PGW 6 PGW I pine kraft- ratio 7 TMP 8 TMP I pine kraft- ratio 9 10 Pine birch ratio Moisture before SC 11 12 Moisture after PM Moisture before SC 13 Formation 14 15 Wet press loads
Value in trial 0% 20 %
Taraet Low filler content Low filler content
(GWIkraft 67/33, filler 30 %) Organopol&-sorb 79/21 % 67 % 79/21 % 67 % 79/21 % 16.5116.5 % 7% 5% 9% Higher than normal High value
Fines flocculation Low chem. pulp cont. PGW vs. GW PGW vs. GW TMP vs. GW TMP vs. GW Birch vs. pine Low moisture Avoiding of the drying High moisture Bad formation Higher w.p. load
Another pilot trial was made in Jiirvenpaa pilot plant of Valmct Inc. The purpose of this trial was especially to test the effect of paper structure on calender blackening tendency of different SC-papers. The effects of supercalendering parameters were also of interest. The aim was not only to produce gloss levels normal in commercial SC-papers, but also above that level, where calender blackening becomes more severe. There were test reels from nine different SC-paper machines. One of the machines was a Foudrinier machine, othcr eight were hybrid former machines. Pilot supercalender was 100 cm wide. Controsteam boxes, two on both sides of paper, were used. Web speed was a constant 600 mlmin. Linear pressures used were 280, 330, 380 and 430 kN/m. The amounts of steam applied wcre 89, 126, 134 and 188 kd(m*h*sidc). The temperature levels of the steel rolls were not intentionally changed. As the surface tempcraturc of no. 10 roll decreased slowly of its own accord, by some 5 to 10 'C, each time a new reel was started, this was included in regression analysis of results. Tcrnperature on no. 10 roll's surface was typically around 90 to 95 'C. For calender blackening measurement a method developed in KCL (FPPRI) was used. By using arrangement that imitates dark field illumination, one can obtain a picture of the paper surface, which to some cxtent exaggerates its blackened appcarance. The information was detected by CCD-cameras applying 45' - 0' geometry and transformed by the image analyzer. The dctected area was G x 6 mm2. As a measure of the amount of calender blackening, was used a value obtained by counting the proportion of pixels from the dark end of the whole grcy scale area 151. The method used was different to that developed by Praast et al. although both arc based on image analyses 131.
Fibre blackening in supercalendered papers
24 1
RESULTS AND DISCUSSION Pilot paper machine trial
A visual evaluation was made to samples supercalendered under the same conditions. The following results were obtained: Changes made in moisture content before calendering had a higher impact on blackening than any changes made in furnish. A lower chemical pulp content gave lower blackening. The moisture content before the final moisturizing did not have any independent effect. Birch haft gave less blackening than pine kraft. TMP- based papers did not blacken as easily as CW or PGW based papers, when a given lincar pressure was applied in supercalendering. When using the measured blackening values, and by interpolating the results to the given gloss (see Table 2) or the given roughness (PPS 10) levels, the picture changes somewhat. Table 2.
Effect of changes on measured calender blackening on given gloss level
Tested chanae Higher filler content Flocculation of fines Worse formation Higher chemical pulp (pine) content Substitution of bl. pine kraft by bl. birch kraft Higher moisture content Moisture before remoisturizing Higher linear loads at wet pressing GW') PGW or TMP') Greater number of coarse fibres
Effect on blackenina Somewhat less blackening No effect Higher blackening level Higher blackening level Lower blackening level Higher blackening level No effect No effect Less blackening than with PGW or TMP More blackening than with GW More blackening
*) Note: the quality of mechanical pulp depends on the production method, but also greatly on raw material quality and on screening, cleaning and post-refining systems not analyzed here.
When the effects of tested variables whcre compared on a given roughness (PPS) lcvcl, the results were in many cases similar with those made on a given gloss level. IIowcver, the effect of filler content was clearer and especially the effcct of substitution of pine kraft with birch h a f t was clearer. Additionally, the impact of higher moisture was even favourable so that at lcast in this test, higher moisturc gave good smoothness in relation to blackening. Pilot supercalendering trials There was a lincar correlation bctwcen the measured blackening and linear pressure of supercalenders. Fig. 5 shows the average dcvelopment of all papers calendered. Gloss and PI'S 10 roughness did not dcvelop in the same linear fashion.
242 Structure and properties of fibres Blackening vs llnear pressure 26
260
300 360 400 Llnsar pressure (In l a d nlp). kNlm
Gloss (rnd) vs llnear pressure
460
Roughness PPS SIO vs h e a r pressure 1.26 .I
62 1
1
:: 1.1 0
I
-_ 300
260
560
400
Llnear proasurn (In last nlp), hNlm
460
260
I
I
400 Llnsar pressure (In laat nlp), kNlm 300
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I 460
Thc dcvelopmcnt of calcndcr blackening, gloss and PPS 10 roughness as a function of the maximum lincar pressure (last, lowest nip) of a pilot supcrcalcndcr. All mill samples and all calendering condition on average.
Figure 5.
Several statistically significant models for calender blackening were obtained by doing rcgression analyses in different ways. For example, by using only those structural parameters which were frcquently significant, and by using the averagc values of both sides, the following model (1) was obtained. B
wherc 13
L V M T U
R F A
=
-70.08 + 0.0496 * L 1.12 * R i4.51 * F
+ 0.0279 * V + 2.47 * M + 0.0746 * T + 2.71 * U + + 0.666 * A , (1)
is calendcr blackening (area) is max. linear prcssure in supercalendering (kN/m) is stcam flow in supcrcalendering (kg/(m*h*side) is moisture of uncalcndcred paper (“A) is surface temperature of no. 10 roll is formation (g/m2), is PPS S 10 roughness (pm) is fibrc length, length weightened (mm) is ash content (%)
Fibre blackening in supercalendered papers 243 In many models obtained, the surface tcmpcrature of the first steel rolls and the stcam flow were not significant. If those are excludcd from modelling, the model changed in regrcssion analyses as follows (2):
B = -122.688+0.0461 * L + 2 . 4 5 * M + 6 . 1 1 * U + 3 . 3 0 * R + 1 . 7 9 * A + 39.5 * J (2)7 where J
is fracture work index (md*cd)” (J/g).
The lattcr model ( 2 ) is even morc significant than thc former, since F-value is 130 instead of 84. There are also some other remarkable fcatures. In the 2nd model, the coefficicnts of linear pressure and moisture content are almost the same as in the first. This may suggcst that these results arc accurately obtained. However, the impact of formation is highcr in 2nd model. In modcl 2, fracture work also becomes significant, which perhaps shows the impact of chemical pulp content and fibre lcngth rather than thc rclcvance of paper strength as such. Since model 2 explains 93.2 % of the variation but model 1, with higher numbcr of parameters only 86.3 % one might be tcmpted to regard modcl 2 as more relevant for the calender blackening of SC-paper. The importance of formation can also be seen in Fig. 6 .
-
’
Regression analyscs were done in scveral different ways. The list of paramctcrs determining calcndcr blackening shown in Table 3 was obtained by using diffcrcnt weight factors for results of regression analysis when a) average structural parameters were used, b) separately top and wire side parameters were used and c) calculations were done by leaving one paper of nine in turn out of analysis.
Calender blackening M e a n of t s & ws with different supercalendering linear pressures. 32
s?
-;
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200 kNlrn
., . . . .. 330 kN/m
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.c
- - - 3 0 0 kN/m PO -430
0)
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Y 0
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Formation, g l t d
Figure 6 .
Calender blackcning of supercalendered magazinc-papcrs as function of formation. The average results of nine papers.
244 Structure and properties of fibres
Table 3.
Parameters influencing calender blackening, in order of importance
Parameter Linear pressure in supercalender Formation Moisture before supercalendering Fibre length PPS S10 roughness (before supercal.) Ash content Chemical pulp content Fracture work Compressibility (uncal. paper, PPS 10/20) Orientation ratio Surface steam flow in supercalender Density (of uncal. paper) Surface temperature of steel roll
Direction
+
Relative irnDortance
15 13 12 11 7 4 4 3 3 3 3 1 1
When fibre analyses of pilot supercalendered production scale papers were made so that the proportion of clearly blackened fibres was compared with that of other fibres, interesting diffcrences were found. Whcn in paper on average 27 % of fibres (of mass of fibres) were chemical pulp fibres, among blackened fibres thc share of chemical fibres was as high as 38 YO.So chemical pulp fibres blackened clearly more easily than mechanical pulp fibres.
Of those chemical pulp fibres which were classified as blackened, of the numbcr of fibres only 15 % were early wood fibres, so that as much as 85 YOwerc late wood fibres. Respectively of blackcned mechanical pulp fibres 68 % were late wood fibres.
Figure 7. Cross sections of early wood fibres (a) and late wood fibres (b) of Norwegian spruce. Fibres were produced by using thermo mechanical pulping method.
Fibre blackening in supcrcalendered papers 245 CONCLUSIONS This paper has discussed the reasons bchind variations in blackening caused by fibrc and othcr raw matcrial parametcrs, paper structure and calcndcring variablcs. Statistically significant models for the blackening of SC magazine paper has been presentcd. Thc impacts of structural paramctcrs, like formation are important on the blackening tcndency. In the calendering thc thicker areas of paper will be heavily pressed. In these areas light scattering coefficient decrcases locally to such an extent that the thickest places bccomc transparent or almost transparent. When lumens of fibres collapse and fibres come very close to each other so called optical contacts occur locally. This givcs the paper an uneven, blackencd appearancc. Calender blackening docs not alter the light absorption Coefficient of paper. Differences between different fibre types are esscntial when blackening is concerned. Because thc thickest arcas undergo the biggest changes, conscquently late wood fibres tend to blackcn more than early wood fibres. Chemical fibres again blackened more easily because they collapsc easily and thcrefore lose heavily their light scattering ability, which is relatively low evcn without calendering. Compared with mechanical fibres chemical fibres also impair formation of paper because of higher avcrage fibre length i.e. the higher flocculation indcx.
ACKNOWLEDGEMENTS IIans Eklund, KCL and Tuija Hell, UPM-Kymmenc, Valkeakoski Research arc acknowledged for skilful preparation of the microscopic pictures. Dr Isko Kajanto, KCL, is acknowledged for preparing the p- radiograms and interpreting the results.
References 1 B Krenkcl, "Calendering study- Relulionship between Calendering parameters and paper properlies." Doctoral Thesis, Technische Hochschule Graz. 1975.
2 R Popil, "The culendering creep equation- u physical model". Fundamentals of paper making. Ninth fundamental research symposium. Cambridge, September 1989. 3 H l'raast, L Gottsching, "Ein bildanalytisches Messgerdt zur Bewertung von Druckqualitdt und Schwatzsafinage". Das Papier 1995 49 (6), 348-354.
4 U Elbert, G Martens, H Praast, J Wcidcnmuller, "Bildunalytische Bewerlung der Schwartzsatinuge", Das Papier 1996 50 (2), 53-59. 5 T Koskincn, "Calender Bluckening", Proccedings of 2nd Ecopapertech Conference, Helsinki, 1998. 6 B Thompson, "Prinling Malerials: Science and Technology". Pira International, Leatherhead, 1998.
Kraft fibers in paper - Effect of beating Kaarlo Niskancn KCL Paper Science Centre, PO Box 70,FIN-02151 Espoo, Finland
ABSTRACT Beating improves mechanical properties in papers made of kraft pulp. The effect is often assumed to arise from increased “bonding” in the dry papcr, perhaps also from the removal of curl and othcr similar defccts in the fibers. In contrast, it is argued here that bonding in dry paper does not explain the effect of beating when considering the elastic modulus of paper. Instead beating primarily influences the elastic modulus of fibers through the drying stresses and bonding in the wet web. The elastic modulus of papcr made of a beaten kraft pulp is essentially cqual to the avcrage modulus of the fibers in the sheet. Because of the strong coupling with drying stresses, paper is dissimilar to ordinary fibcr composite materials. The hypothesis put forward in this paper explains the results of a variety of experiments where beating, grammage, wet pressing, filler content or drying shrinkage were varied.
INTRODUCTION The tensile strength of paper can be improved, among other things, by beating or wet pressing. Both actions are generally believed to primarily improve inter-fiber bonding in paper and thereby the strength properties. Figure 1 shows how the density and tensile strength of paper incrcasc in beating and in wet pressing. The increasc in density reflects the increasing bonding degree of the fiber network even though the precise relationship of relative bonded area to density is unknown. The same qualitative difference between beating and wet pressing would also be seen if one replaced density with the inverse light scattering coefficientanother measure of bonding degree. In this paper density is used as the indicator of changes in the bonding degree of the dry paper because pulps differ more in the light scattering coefficient of paper than in the density. If one is to explain the mechanism through which beating affects the mechanical properties of papcr, then one must also explain why beating diffcrs from wet pressing. For some reason, the slopc of tensile strength vs. density is clearly higher in beating than in wet pressing. This already suggests that part of the beating effect comes from other sources than increasing bonding degree. The strength development in bcating has been discusscd in numerous publications. A systematic analysis was presentcd by Page [2]. Interesting discussions can also be found e.g. in the proceedings of the first Fundamental Research Symposium [3]. Three basic explanations have been put forward: improved bonding, increased elastic modulus of fibers and increased cffcctive length of fibcrs. The bonding explanation refers to the fact that beating increases fiber flexibility and conformability, external fibrillation and fines gencration. As a result, stronger inter-fiber bonds form in the paper sheet. It is even conceivable that beating might improve a “specific bond strength” at the microscopic level while wet pressing might not.
250 Paper fibre production and properties The elastic modulus offibers is important because the tensile strength of paper is linearly proportional to fiber modulus when inter-fiber bond properties remain constant. Fiber modulus increases with drying stress as Jentzen has demonstrated [4]. Since fiber swelling increases in beating, also the drying stress of paper increascs when paper is dried under restraint. In fact, thc elastic modulus of paper is proportional to the drying stress [ 5 ] . An additional factor may also be the “activation” or “tightening” of free fiber segments in the paper sheet [ 6 ] .Also in that case, the net result is an increase in the average elastic modulus of fibers. Finally, it is believed that long fibers give strong paper. Beating and the induced swelling can straighten fiber curl, kinks and other deformations. This corresponds to an increase in the effective length, end-to-end distance of the fibers. In his review of strength development in the beating of dried pulps, Page [2] concluded that the fiber straightening already in the pulp suspcnsion, before sheet making, is the most important factor explaining strength improvements in the beating of dried chemical pulp.
100
0 0
100 200 300 400 500 600 700 800 Density (kg/m3)
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Density (ks/m3)
600 700 800
Figure 1: Tensile index against density for chemical pulps of different lignin content [ 11. For each pulp, line connects points of increasing beating but constant wet pressing (a) or increasing wet pressing but constant beating (b).
Kraft fibers in paper 251 It is difficult to dccidc what is the relative importance of the three microscopic mechanisms of strength development in beating. No reliable theory exists for that purpose so that the analysis of experiments is uncertain. It is believed that new insights are gained from thc clastic modulus of paper as there is a physically sound model for the elastic modulus. Experiments on elastic modulus reveal the same qualitative difference between bcating and wet pressing that is seen in tensile strength (Fig. 2). A significant fraction, though not all, of the changes in the tcnsile strength strength of paper arise from changes in the elastic modulus. When considering elastic modulus one must remember that it is inversely proportional to paper thickness, has the units GPa. Decreasing the thickncss of paper while keeping other factors constant (e.g. calendering) thcrefore induces an increase in the elastic modulus and a trivial coupling between the modulus and dcnsity of paper. The use of the specific modulus of elasticity (units kNm/g), defined as modulus divided by papcr dcnsity, avoids this trivial effect. Specific elastic modulus is analogous to tensile index in terms of the units. Unless otherwise stated, in the following 'elastic modulus' rcfcrs to the specific elastic modulus.
0
100
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300 400 500 600 700 800 Density (ks/m3)
-m
6 -
(b)
Wet pressing
2
40
5 -
E0 4
.g 2 &3 o .?=
-
-
o
2 -
c%
1 -
a,
01 0
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I
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I
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Figure 2: Specific elastic modulus against density in beating (a) and wet pressing (b) for the same samples as in Fig. 1 [l].
252
Paper fibre production and properties
Using experiments on elastic modulus, it is shown next that the bonding degree of dry paper does not is not the cause of the observed density dependence. It is then argued that changes in the elastic modulus of paper arise primarily from changes in the elastic modulus of fibers. Finally it is demonstrated that this hypothesis is consistent with a variety of experimental data. PAPER AS A RANDOM FIBER COMPOSITE According to Page and Seth [7], the effect bonding on the (specific) elastic modulus of paper obeys the Cox thcory [8-91,
where Efis the (specific) elastic modulus of fibers, w is thc width, I the length and Gfthe (specific) shear modulus. M A stands for thc relative bonded area, the fraction of total fiber surface area that is bonded to other fiber surfaces. Equation (1) is widely used in fiber composite materials. Page and Scth compared it with the elastic modulus of paper varied by beating. They uscd the inverse light scattcring coefficient as a mcasure of the relative bonded area RBA. According to Eq. (1) the elastic modulus of paper is reduced at low bonding degrees because fiber ends carry no load. The factor in parenthesis gives the fraction of fiber material that participates in carrying load. This factor is called the “loaded length fraction”, Lloaded. If the elastic modulus of fibers is constant, as in composites, then Lloaded is the only factor through which network structure affects the elastic modulus of paper. Carefil analysis of computer simulations for thin fiber networks has proven [ 101 that Eq. (1) gives a quantitatively incorrect value for Lloaded even though the qualitative effect of fiber length and relative bonded area is correct. Hence leaving out the explicit microscopic factors and replacing RBA with dcnsity (p) gives
where h is some unknown constant that may depend on fiber and bond properties. This expression agrees with computer simulations [lo]. The inverse density depcndence of modulus implied by Eq. (2) applies to any material containing a low concentration of voids in an otherwise homogeneous background. The reduction in elastic modulus is simply proportional to the total void volume. From the mechanical point of view, fiber ends create voids in an otherwise homogeneous paper sheet. The voids are large when RRA is low and vice versa. It is reasonable to assume that density can be used in place of RBA because the mean distance between interfiber bonds decreases with increasing density. A short mean distance bctween bonds is synonymous to a high RBA. Finally, dimensional analysis shows that fiber length must enter as a product with sheet density, as in Eq. (2). This equation is a general expression that holds as long as the equivalent voids at fiber ends are not close to each other. The most important feature of the above discussion is the general and physically obvious coupling between the nctwork density and fiber length. In order to motivate this suppose building a random nctwork by joining together homogeneous fibers. The elastic
Kraft fibers in papcr 253 modulus of such a network is constant as long as the product offibcr length and network density is constant. Lowcr dcnsity suffices when using long fibers than when using short fibers. This prediction compares readily with experiments.
0
1
2
3
4
5
6
Bulk (cd/g) Figure 3: Specific elastic modulus against bulk (= l/p) in thin handsheets where dcnsity increased as gammage ranged from 4 to 20 g/m2 [ 1 13. Measured data are given for 2.2 mm and 1.7 mm long fibers (diamonds and squares, respectively) and the lines show a fit to Eq. (2), as explained in the text.
12 I
\ I
0
1
I
I
I
2 3 4 Bulk (cm3/g)
I
5
6
Figure 4: Spccific elastic modulus against bulk (= l/p) in handsheets of varying wet pressure [ 131. Data are given for 2.7 mm and I .47 mm long fibers (diamonds and squares) and the lines are explained in the text.
254 Paper fibre production and properties Figure 3 shows measurements of Hollmark et a1 [ l 11 on thin (4 - 20 g/m2) paper samples. Fiber length was reduced by cutting. The elastic modulus is plotted against bulk (inverse density) in order to match Eq. (2). The linear regression line through the 2.2-mm data yields the parameters Er= 8.26 kNm/g and h = 457 g/m2. The other line for the 1.7-mm fibers uses the same parameters. The good agreement with the measured data implies that Eq. (2) is consistent with these measurements. The situation is quite different at higher, more ordinary, grammages as Fig. 4 demonstrates for a wet pressing series with cut fibers. Fitting the measured moduli with the 2.7-mm fibers to Eq. (2) gives Er = 12.89 kNm/g and h = 406 gm2. The resulting modulus prediction with the 1.47-mm fibers is clearly smaller than the measurcd values. Kimura [ 121 observed the same discrepancy in beating experiments with different fiber lengfhs. Thus, albcit the elastic modulus of (ordinary) paper may slightly dccrease with decreasing fiber length, the effect is much smaller than Eqs. (1) and (2) predict. There is a simple reason why Eq. (2) does not explain the density dependence of clastic modulus at ordinary grammages. The explanation lies in the fact that the number of inter-fiber bonds per fiber is very high (well above ten) in an ordinary paper sheet. Only a small fraction of the total fiber length is not carrying load and Lloaded = 1. As a further demonstration, Llooded is estimated using KCL-PAKKA simulation model [ 141. The simulation builds “virtual” paper. The results in Fig. 5 apply to fiber parameters typical of the long and short fiber fraction of a mildly beaten Scandinavian softwood kraft pulp. Except for grammages below 20 g/m2,more than 90% of the length of an average fiber is between the first and last bond on the fiber, or Lloaded = 1. The effect of fiber length is similar to that seen in Fig. 4. The simulation gives practically the same answer to all relevant values of fiber lcngth, fibcr flexibility or wet pressing pressure. Fiber length becomes important only at very low densities.
0
20
40
60
80
100
120
Grammage (S/m2) Figure 5: Loaded length fraction Lloaded against grammage according to a KCL-PAKKA simulation [ 141 for a lightly beaten softwood kraft pulp. The result is calculated separately for the long and short fiber fractions in the pulp.
Kraft fibers in paper 255 Being confident that Eq. (2) is principally correct, it is inferred that the effective elastic modulus of fibers, Ef, must somehow correlate with the density changes induced in beating and wet prcssing. This means that paper cannot be modelled as a fiber composite. As will be discussed next, drying strcsses provide the coupling betwcen fiber propertics and paper structure.
EFFECT OF DRYING STRESS Htun [ 5 ] has shown that, when using restrained drying, the elastic modulus of paper is approximately linearly proportional to the final drying stress needed to prcvent the shrinkage (Fig. 6). For example, if bcating increases, elastic modulus and drying stress both increase; if beating decreases, elastic modulus and drying stress both approach zero. Furthermore, Fig. 7 demonstrates how beating causes practically no modulus improvement in papers dried under zero restraint (free shrinkagc). One might claim [2] that the relative increase by beating is roughly the same in restrained and free drying but such an interpretation is probably incorrect. The strong coupling between the elastic modulus and drying stress of paper is easy to understand because drying stress increases the elastic modulus of single fibers [4]. It follows, trivially, that the relative bonded area and elastic modulus of paper must correlate. Drying stress can bc non zero only if the wet web has a non zero bonding degree, RBAwe,,a non zero number of inter-fiber contacts. Without mechanical rigidity no drying stress can develop in the wet web. The drying shrinkage and therefore drying stress of paper arises primarily from the transverse shrinkage of fibers (&shrink). The shrinkage force in the axial direction of fibers is equal to the sum of compressive forces crcated at the inter-fiber contact sites. It is therefore proposc&as thc simplest approximation-that the elastic modulus of a restraint-dryed paper shcct is proportional to '
10 I
1
wet preksing Beating
0
2
4
6 8 10 12 Drying stress (Nm/g)
14
16
Figure 6: Effect of beating (black diamonds) and wet prcssing (squares) on the relationship between drying stress and spccific elastic modulus for a bleachcd krafl pulp. Fines rcmoval (open diamonds) after beating is also illustrated [ 5 ] .
256
Paper fibre production and properties
In this model, the elastic modulus of paper increases more with beating than with wet pressing because wet pressing raises only RBA,, and #!&aded but beating raises also &shrink. The factor &shrink includes the transfer of shrinkage forces across inter-fiber contact areas. If these contact areas are small, the shrinkage force is also small. The relative bonded area RBA(d,) of dry paper is close to the RBA,,, of wet paper. Ignoring the distinction between the two RBA values one might rewrite Eq. (3) as Epsper cc RBA x &shrink x Lloadcd. Howevcr, it is important that the relative bonded area in Eq. (3) is that of the wet web because it means that the elastic modulus of paper is practithus arise cally independent of the structure of dry paper. Ordinarily, changes in Epapcr primarily from changes in the elastic modulus of fibers, Ef. The structure of the dry paper is important only at relatively low densities or RBA values where Lloaded differs significantly from unity.
COMPARISON WITH EXPERIMENTS Let me now compare Eq. (3) with experiments. The first test case is the effect of grammage. In the case shown in Fig. 8, the elastic modulus of papcr is constant down to grammages of ca. 40 g/m2 and decreases below that. Since the furnish is constant, the variation of modulus should follow the product RBA,,, x Lloadcd. The prediction from KCL-PAKKA simulations is consistent with the measured elastic modulus. The rapid reduction in RBA,, x Lloaded and hence in elastic modulus at low grammages comes from the sheet surfaces where fibers have bonds only on one side. Thc weak effect of fiber length in the simulations in Fig. 8, above the lowest grammages, is consistent with the experiments in Figs. 3 and 4. However, unlike the experiments, in the simulation the long fibers have been given a slightly higher coarseness and therefore lower flexibility than the short fibers have. This difference agrees with the propcrties of natural fiber fractions and gives the short fiber fraction a slightly higher elastic modulus than the long fiber fraction.
0
100200300400500600700800 Density (kgh-?)
Figure 7: Specific elastic modulus against density in free shrinkage and zero shrinkage. Lines connect points of increasing beating [ 151.
Kraft fibers in paper 257
0' 0
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loo
'0 120
-w(9/d Figure 8: Specific elastic modulus against grammage [I61 in handsheets made of a lightly beaten softwood kraft (squares). The lines give the product RI3A x Lloaded (scale on the right) from KCL-PAKKA simulations [ 141 for the same furnish, evaluated separately for the long fiber fraction (solid line) and short fiber fraction (dashed line).
0'
I
I
I
I
I
I
I
Figure 9: Specific elastic modulus decreases when increasing filler content. The network density was calculated assuming that the filler particles occupy zero volume.
258 Paper fibre production and properties With reference to the data of elastic modulus against density in Fig. 2, it is observed that if beating decreases, elastic modulus decreases rapidly to zero at a finite density, whereas if wet pressing decreases, elastic modulus extrapolates to a positive value at zero density. The most important factor in the early stages of beating seems to be an increase in fiber swelling. At later stages of beating, the rate of increase of elastic modulus, dEldp, approaches the same value as in wet pressing. This suggests that at its later stages, beating increases mainly thc flexibility of fibers but not so much the swelling. In that case it is reasonable that dEldp should approach the same values as in wet pressing. Filler addition reduces the bonding degree of dry paper. Traditionally the decrease in the mechanical properties of paper has bccn explained through this reduction. Figure 9 shows experiments where talc was added to pine haft furnish. The density of the pure fibcr nctwork is estimated by assuming that the volume occupied by filler particles is zero; this corresponds to assuming that their density is much higher than that of the fibers. If the traditional explanation were correct, the effect of adding filler should be similar to that of reducing wet pressing. However, the data in Fig. 9 follows a trend similar to that of reducing beating, Fig. 2(a). It seems that the filler addition not only reduces RBA,,, but also imbedes the transfer of shrinkage forces to the axial direction of fibers. This makes smaller the term Eshrink in Eq. (3). DISCUSSION It has been been shown that the elastic modulus model defined in Eq. (3) is consistent with experimental data where beating, wet pressing, grammage and filler content change. Except for low densities or bonding degrees of dry paper, the model explains any changes in the elastic modulus through changes in the elastic modulus of all the fibers, The network structure of dry paper, the bonding degree and fiber length in particular, influence the elastic modulus of paper only if the bonding degree is low. Other correlations between the network structure and elastic modulus of paper are illusory. This conclusion is corroborated by Seth and Page [ 171 who have already argued that the shape of the stress-strain curve of paper is governed by fiber properties and not network structure. Since fiber length is mostly irrelevant for the elastic modulus, also other aspects of fiber length have little significance. For example, the eventual straightening of fiber curl or removal of fiber kinks in beating should cause little improvement in the elastic modulus of paper. These factors can be relevant only at low dcgrees of bonding. However, only elastic modulus has been considered. The situation may be different with the strength properties of paper. Equation (3) is the simplest possible model dcscribing the coupling between fiber swelling and elastic modulus of paper. The model could well be nonlinear (cf. Fig. 6), but such an complication is unneccssary. The central role of fiber swelling is amply demonstrated by the well-known relationship between the water retention value of a beaten pulp and the mechanical properties of paper made from such a pulp. In addition to increasing flexibility, swelling contributes-directly through drying stress as Eq. (3) assumes. Fiber swelling should increase particularly at the early stages of bcating and cause the rapid increase in elastic modulus (Fig. 2). The influence of drying stress traces back to the Jentzen effect in the elastic modulus of single fibers. Beating contributes to the Jentzcn effcct in paper at least in three ways: increasing fiber flexibility increases RBA,,,, and increasing fiber swelling has the same
&aft fibers in paper 259 effcct on &shrink. The third factor ariscs from thc conformability of the fiber wall matcrial during drying. In a well-beaten fiber, the fibril structure can align more effectively along the drying stress, fiber wall pores can close more easily and defects heal during drying. Finally, it may scem odd that higher compressive shrinkage forces at inter-fiber contacts should increase the clastic modulus of fibers. One might expect the opposite. In order to understand thc experimental effect one has to include the action of transverse fiber segments at the bond sites. Indeed, restraincd drying and a well-beaten pulp give rise to very effective inter-fiber bonding [ 181. As a result, any load in the axial direction of a fiber is partially taken up by the crossing fiber segmcnts. This increases the effective axial modulus of fibcrs. Schulgasser and Page [ 191 found that the all the in-plane elastic properties of a very well-bonded paper can be consistently explained only if one considers paper as a laminate structure formcd of fibcr wall lamella rather than as the traditional network formed of individual fibers. The elastic modulus of such a sheet arises from the fiber wall elastic properties parallel and perpendicular to the fibrils. The latter contribution to the elastic modulus is typically 30% [19]. As Schulgasser and Page pointed out, the laminate model does not apply when the bonding degree of paper is less than pcrfect. Howcvcr, even in such cases the bonded segments of well-beaten fibers certainly behave as a laminate. For unbeaten fibers this is not the case. In summary, it is bclieved that the elastic modulus of paper made of beaten krafl fibers is cqual to the mean elastic modulus of the fibcrs and indepcndent of the structure of paper. Thus papcr is dissimilar to ordinary fibcr composites. The word fiber refers to the effective mechanical unit of the sheet. Of course, the model has not been rigorously proven correct. ACKNOWLEDGMENTS This rescarch was supported in part by the Technology Development Centre. The author is gratehl to Dr. Minoru Kimura who draw my attention to the unexpccted effect of fiber lcngth in 1991. Heikki Kettunen and Antti Korpela are thanked for uscfil criticism and Bjorn Krogerus for the filled sheets. REFERENCES 1. Luner, P., Kama, A.E.U. & Donofrio, C.P., Tappi J. 44 (6):409 (1961). 2. Page, D.H., Sv. Papperstidn. 88(3):R30 (1985). 3. Fundamentals ofPapermaking Fibres (F. Bolam, ed.), British Paper and Board Makers ASSOC., London (1958). 4. Jentzen, C.A., Tappi J. 47(7):412 (1964). 5. Htun, M. and de RUVO, A, Relation between drying stresscs and internal stresses and thc mechanical properties of paper, In Fihre- Water Interactions in Paper-making, (Fundamental Research Committee, ed.), British Paper and Board Ind. Fed., London (1978), pp 477-494. 6 . Giertz, H.W., EUCEPA Conf., Venice (1964); Gicrtz, H.W. and Lobben, T.H., Paper Technology 8(8):211 (1967). 7. Pagc, D. H. and Seth, R. S.,Tappi J. 62(9):99 (1979). 8. Cox, H.L., Br. J. Appl. Phys. 3(3):72 (1952). 9. Perkins, R.W., Micromechanics models for predicting the elastic an strength behavior of paper materials, In :Material Interactions Relevant to the Pulp, Paper and
260 Paper fibre production and properties Wood Industries (Caulfield, D.F., Passaretti, J. D. & Sobczynski, S. F., eds.), Materials Research SOC.,Pittsburgh, 1990, pp. 99-1 18. IO.Raisanen, V. I., Alava, M. J., Niskanen, K. J. and Niemincn, R.M., J. Materials Res. 12(10):2725 (1997). 1I.Hollmark, H., Anderson, H., and Perkins, R. W., Tappi J. 61(9):69 (1978) 12.Kimura, M. and Uchimura, H., Sen-I Gakkaishi 51( 11):550 (1995). 13.Seth, R. S., Fibre quality factors in papennaking I The importance of fibre length and strcngth; I1 The importance of fibre coarseness, In :Material Interactions Relevant to the Pulp, Paper and Wood Zndustries (Caulfield, D.F., Passaretti, J. D. & Sobczynski, S. F., eds.), Materials Research SOC.,Pittsburgh, 1990, pp. 125-162, 14,Niskanen,K., Nilsen, N., Hellh, E., and Alava, M., KCL-PAKKA: Simulation of the 3D structure of papcr, In The Fundamentals of Papermaking Materials (Baker, C.F., Ed.), Pira International, Lcatherhead, 1997, pp. 1273-129 1. 15.Sctterholm, V. C., and Chilson, W.A., Tappi 48(11):634 (1965) 16.Aalt0, M., Pro gradu thesis, University of Jyviiskyla (1996). 17.Seth, R.S., and Page, D.H., The stress-strain curve of papcr, In The Role of Fundamental Research in Paper Making, (Brander, J., ed.), Mechanical Engineering Publ., London, 1983, pp. 42 1-454. 18.Nank0, H. and Ohsawa, J., Structure of fibre bond formation. In Fundamentals of Papermaking (C. F. Baker and V. W. Punton, Eds.) Mech. Eng. Publ, London, 1989, vol. 1, pp. 786-830; Nanko, H., Ohsawa, J., and Okagawa, A., J. Pulp Pap. Sci. 15(1):J17 (1989) 19.Schulgasser, K., and Page, D.H., Composites Sci. Technol. 32(4):279 (1988); Page, D.H., and Schulgasser, K., Evidence for a laminate model for paper. In Mechanics of Cellulosic and Polymeric Materials (Perkins, R.W., Ed.), AMD-Vol. 99; MD-Vol. 13
-
(1989).
EFFECT OF DRY FRACTIONATION ON PULPING CONDITIONS AND FIBRE PROPERTIES OF REED CANARY GRASS Michael Finell, Bjorn Hedman & Carl-Axel Nilsson Swedish Universily of Agricultural Sciences. Department OfAgricultural Research for Northern Sweden Laboratory for Chemistry & Biomass P.O.BOX4097, SE-904 03 Umed@SWEDEN
ABSTRACT
A new harvesting system and the fibre properties of reed canary grass (Phuluris arundinaceu L.) makes this grass an interesting new raw material source for the pulp and paper industry in the Nordic countries. Pilot scale tests in Finland shows that high quality fine paper can successhlly be produced from delayed harvested reed canary grass. Birch pulp can be replaced with reed canary grass pulp in fine paper firnish without any significant differences in the hnctional properties of paper. To achieve good pulp and paper properties the raw material has to be pre-treated (removal of leaves and sheaths by fractionation) before pulping. Fractionating produces a ”chip” fraction of mainly internodes for pulp production and a meal fraction of leaves and sheaths that can be used as biofbel. Fractionation improves the homogeneity of the raw material and the pulp properties. Silica and other minerals, which are considered as a problem in alkaline pulping processes, are concentrated in the meal fraction. Fine material that causes poor drainage of the pulp is also removed in the fractionation process. In this work, different degrees of fractionation have been studied. Pulping conditions and fibre properties for totally unfractionated raw material up to best available raw material (manually fractionated) are compared. INTRODUCTION
A situation in Europe with overproduction of food and large subsidies to the agriculture sector has led to an intensive study of alternative “non-food” production for various industrial use and energy production. Studies of different grasses for bioenergy production in Sweden started in 1981 and revealed that reed canary grass (Phuluris arundinaceu L.) is the most promising species because of its high yield, good quality and sustainability 1v Reed canary grass (RCG) is a tall rhizomatous grass which is native in Sweden and many other northern countries. The fibre properties of RCG and a new harvesting system makes this grass also interesting as a new source of raw material for the pulp and paper industry in Sweden. Delayed harvest means that the crop is left in the field during the winter and harvested as wilted material the following spring. The average annual harvest yield of RCG is about 8 tons DWha with this method. RCG is recommended to grow on humus rich soils to get a low ash and silicon content ’. Pilot scale tests show that high quality fine paper can successfilly be produced from delayed harvested RCG. Birch pulp can be replaced with reed canary grass pulp in
’.
’*
262 Paper fibre production and properties fine paper furnish without any significant differences in the functional properties of paper. To achieve good pulp and paper properties this raw material has to be pre-treated p m o v a l of leaves and sheaths) before pulping. Dry fractionating is a promising method ” to separate parts of the plant unsuitable for pulp production. The two fractions from the dry fractionation process, internode chips and leaf meal, are very bulky. If the fractionation unit is not located close to the pulp mill, these fractions have to be compressed before transport. Briquetting the internode chips fraction * and pelletising the leaf meal fraction can be possible solutions. In this work, a dry fractionation process has been studied for reed canary grass. Fibre properties for different degrees of fractionation are compared. MATERIALS & METHODS Reed canary grass
Reed canary grass of the variety Palaton was used in this trial. The grass was collected in “Hesston” type bales and the moisture content of the grass at harvest time was about 14%. Fractionation
A single line full scale (2.5 tons/h) fractionating process (United Milling Systems AJS) was used for this trial. The process steps are shredding, chopping, milling and screening, Fig. 1. In the disc mill the fragile parts of the straw leaves, sheaths and nodes are ground to a fine meal. The tougher parts of the straw mainly internodes are only cleaved and leave the disc mill as “internode chips”.
-
-
BALE CONVAYOR
--b
BALE SHREDDER
_+I
PRE SEPARATED INTERNODE CHIPS
HAMMER MILL
-
MAGNET SEPARATOR
SEPARATOR
2 STAGE SEPARATED INTERNODE CHIPS
Figure 1.
PLAN SIFTER
Simplified scheme of the fractionation process.
LEAF MEAL
Dry fractionation 263 Table 1. Fractionation parameter settings. Fractionation parameter Hammer mill sieve: Disc mill grinding elements: Disc mill disc clearance: 1'' separator sieve: 2"dseparator:
Setting 0 20 m m HMIlHM3 0.5 m m Mesh sieve, 2.28 m m NO 2"6 separator
In this trial the process parameters were chosen as shown in Tab. 1. These settings gave 56% internode chips and 44% leaf meal. For pulp analysis 30 kg of the internode chips and 30 kg of unseparated (only hammer milled) RCG was collected. The chips fraction from the lst separator was further separated on a laboratory wood chips screen equipped with a perforated screen with 3 mm round holes. Particles retained on the 3 mm screen (10 minutes) was used for pulping experiments. The chips yield after the 2ndstage separation was 39%. As a comparison, a sample of reed canary grass was separated by hand. Sulphate cooking In order to evaluate the effect of fractionation the grass was cooked with the sulphate method in laboratory scale in a 4 x 0.5 L-autoclave digester. The samples were compared under the same cooking conditions, Tab. 2, to evaluate the effect of fractionation. The alkali charge and cooking time was changed to see if it was possible to produce pulp with a low kappa number of all RCG samples. One birch (Betufa verrucosa) sample was also cooked under the same conditions with the exception of time at max. temperature. For birch this time was chosen to be 90 min. Analysis Ash and silicon content in the RCG was determined before and after fractionation (SS 187171 and ASTM D 3682 respectively). After cooking the pulps were disintegrated in a L&W laboratory defibrator using 20 000 revolutions. The pulps were analysed for yield, screening reject (L&W Noram ## 0.15 mm), kappa no. (SCAN-C 1:77 ) and drainage resistance (SCAN-M 3:65). The pulps were also analysed for fibre length distribution, coarseness and fines content in a Kajaani FS-200 fibre analyser (TAPPI T 27 1 pm-9 1). Table 2. Sulphate cooking conditions for RCG and birch. Cooking parameter Temperature rise, "C Heating rate, W m i n Time at max. temp, min Effective alkali, % on raw mtrl Sulphidity, % of chem Liquor : raw mtrl, kglkg
RCG 65-165 1.67 15 18
Birch 65-165 1.67 90
38
18 38
5:l
5:1
264 Paper fibre production and properties RESULTS & DISCUSSION
Ash and silica can be reduced by 40% with 2-stage separation, Tab. 3. The chips yield in the 2-stage separation is however low, 39%. Theoretically the chips yield could be higher since the stem content of hand separated spring harvested RCG is about 80% in average. In the first stage the ash and silicon is reduced by about 30%. The chips yield after the first stage is 56%. By a good separation of the disc milled RCG it is possible to get pulp properties close to those of RCG separated by hand, Tab. 4. The 2-stage separation gives a pulp that is easy to cook to a low kappa number with a high pulp yield and a remarkable improvement of the drainage resistance. The fibre length in the pulp from fractionated material is also higher and the fines content decreases with the degree of fractionation. The length weighted fibre length for RCG pulp is close to birch pulp but the fines content of RCG pulp is much higher, Fig. 2. Calculating only the arithmetic and length weighted fibre lengths does not give enough information about the fibre length distribution. The 2-stage separation gives a pulp with fibre length distribution close to that of hand separated RCG.
Table 3. Ash and Si in RCG at different degrees of fractionation. RCG properties Accept chips, % of whole plant Ash, % SI, Yo
Separated by hand
Separated on 1' separator + 3 mm screen
NIA 3.50 NIA
39 3.43 1.38
Separated on la Whole plant, separator only not separated 56 3.70 1.60
100 5.40 2.30
Table 4. Pulp properties for RCG at different degrees of fractionation. Pulp properties
Kappa no. Screened pulp yield of chips, % Reject # 0.15 mm, % Drainage, "SR Coarseness, mglm Arithrn. fibre length, mm Weigth. fibre length, mm Fines c 0.20 mm,% Pulp yield of RCG in to the fractionation process, %
Separated by hand 9.7
Separated on 1' separator + 3 mm screen 11.0
Syarated on 1 separator only 12.0
47.5
53.2
c 0.5
< 0.5
33.5 0.078 0.33 0.72 43.3
14.0 0.077 0.85 I.oo 6.8
47.5
NIA
54.5
54.5
< 0.5
< 0.5
23.0 0.074 0.40 0.85 33.6
24.0 0.062 0.30 0.79 38.5
50.8 c 0.5 28.0 0.076 0.36 0.77 38.7
NIA
21.3
28.4
Whole Birch, plant, not reference separated (90 min) 15.9 18.0
Dry fractionation 265
25
20
s 15 0 .w a
e
xn 10 5
0 0
42
44
0,s
48
1
1,2
1,4
1,6
1,8
2
Fibrelell@h)lml
Figure 2.
The population distribution for RCG fibres at different degrees of fractionation compared to birch.
If comparing the pulp yield to all RCG entering the fractionation process, the increase in pulp yield of the chip fraction does not compensate the loss of material in the fractionation process. The best fractionated material gives only 21 % pulp yield of the total input. If using the whole plant, we get a pulp yield of 48 %. Optimisation of the fractionation process is in progress. To see if it was possible to cook the 1-stage separated and unseparated RCG to a low kappa number (target kappa lo), the alkali charge and cooking time were changed, Tab. 5 . The unseparated material was not possible to cook to a lower kappa than 12. Fractionated material was relatively easy to cook to kappa about 10. Table 5. Pulping conditions for producing a pulp with low kappa number Pulp properties Kappa no.
Scr. pulp yield, % EA (NaOH),% Time at 165'C, min
Separated on 1" separator + 3 mm screen 10.1 53.8 17.5 30
Separated on 1" separator only 10.3 50.4 19 35
Whole plant, not separated 11.9 47.7 19 65
266
Paper fibre production and properties
CONCLUSIONS
By dry fractionating it is possible to produce a pulp raw material of RCG that is easy to pulp to low kappa numbers. The drainage ability of the fractionated grass is much improved. Fractionation lowers the ash and silicon content of the material with 40 %. Fractionation is important in order to get a raw material with small quality changes. If the material is not fractionated, varying leaf content of RCG entering the pulp mill will cause problems such as kappa number variation, varying drainage ability and variation in the fines content of the pulp. The best results were achieved when the fractionated material was separated in two stages. The chips yield of the process was low in these trials. Optimisation of the process is in progress. Reed canary grass is a different short fibre raw material than birch. RCG pulp however gives comparable paper properties when used instead of birch pulp in fine paper ’. ACKNOWLEDGEMENTS
The authors acknowledge financial support from the European Union (contract AIR3-CT94-2465),the Kempe foundations and Stiftelsen lantbruksforskning. REFERENCES
1. R. Olsson, A new concept for reed canary grass production and its combined processing to energy and pulp. Non-woodflbres for industry, 23-24 March 1994 Silsoe, UK. Conference proceedings, Leatherhead, W. Pira International 1994 2. S.Landstrom, L.Lomakka, and S. Andersson, Harvest in spring improves yield and quality of reed canary grass as a bioenergy crop. Biomass and Bioenergy 1996, vol 11, no 4,pp 333-341. 3. J. Burvall, Influence of harvest time and soil type on fuel quality in reed canary grass. BiomassandBioenergy 1997, vol. 12,No. 3, pp 149-154. 4. L. Paavilainen and R. Torgilsson, Reed canary grass - A new Nordic papermaking fibre. TAPPZ Puking Conference, 6-10 Nov 1994, San Diego, CA, USA. Conference proceedings, Atlanta, GA, USA: TAPPI Press 1994 5. L. Paavilainen, and J. Tulppala, Top-Quality Agro-Based Fine Paper Produced on Pilot Scale. TAPPZ Pulping Conference, 19-23 Oct 1996, Nashville, TN, USA. Conference proceedings, Atlanta, GA, USA: TAPPI Press 1996 6. H. Fuglsang, and B. Lofquist, Progress report on the ECLAIR biorefinery project development of new processes for fractionation and partial defibration. Straw - A valuable raw material, 20-22April 1993,Cirencester, UK. Conference proceedings, Leatherhead, UK: Pira International 1993 7. M. Hemming, M. Jarvenpll, and T. Maunu, On-farm handling techniques for reed canary grass to be used as a raw material in the pulp industry. Non-woodfibresfor industry, 23-24March 1994 Silsoe, UK. Conference proceedings, Leatherhead, UK: Pira International 1994 8. M. Finell, J. Burvall and R. Olsson, Perennial rhizomatous grass Evaluation of techniques for improving transport economy for industrial use of RCG, reed canary grass. Sustainable Agriculfure for Food, Energy and Industry. James & James (Science Publishers) Ltd. 1998, pp 919-921.
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Electrical Properties of Viscose-Kraft Fibre Mixtures Sami Simula and Kaarlo Niskanen* KCL Paper Science Centre, PO Box 70. FIN-02151Espoo. Finland
ABSTRACT We used mixture handsheets of pine kraft and viscose fibres to study how density and moisture content affect the electrical properties of paper. When the viscose content increased from 0% to 70%, papcr density decreased from 650 kg/m3 to 280 kglm3. The effect of density on the dielectric constant of paper was consistent with the ClausiusMossotti relation provided that the relative humidity was low or the measurement frequency high. Under these conditions, the water absorbed by paper had little effect on the dielectric constant or dielectric loss factor. When the moisture content of paper exceeded a certain limit, the dielectric constant began to increase rapidly. Presumably this happened because some of the adsorbed water resembles bulk water in its electrical properties and therefore has a much higher dielectric constant than dry cellulose. The threshold moisture content increased with increasing measurement frequency. The viscose fibres absorbed 2-3%-units more water than the kraft fibres at any given relative humidity. In spite of this, increasing the viscose content decreased the dielectric constant of paper. This indicates that the water molecules are more closely bonded and have less frecdom to rotate in viscose fibres than in kraft fibres. The dielectric loss term was practically zero at low moisture contents, approximately below the same threshold that governs the dielectric constant. The dc resistivity decreased as an exponential function of increasing moisture content, but viscose fibres gave a lower conductivity than kraft fibres at a fixed moisture content. Hence, the water absorbed by viscose fibres influenced the electrical propcrties of paper in all respects less than the water absorbed by kraft fibres.
INTRODUCTION Electrical properties of paper are important in electrophotography, i.e. copying and laser printing. They determine the effectiveness of toner transfer and the severity of static charging problems. Higher speeds and colour printing have increased the importance of electrical properties. Most of the work on the electrical properties of cellulose has been conducted over three decades ago. Good reviews on that work are available [ 1,2], so only a brief summary is given here. The dielectric properties of interest in cellulosic materials are the dielectric constant, dielectric loss and dielectric breakdown strength. Only the first two are considered here. The main mechanisms of polarisation in polymers are atomic, electronic and molecular polarisation. At low frequency, all of these mechanisms are important. Molecular polarisation in cellulosic fibres can be attributcd to the hydroxyl and carboxyl groups as well as water molecules and free ions. The structurc of the sheet is also important because sheet density and the orientation of fibrcs affects the dielectric constant.
268 Paper fibre production and properties Dielectric loss is related to the work required to change the polarisation. It is usually caused by the polar groups in paper and particularly by metallic ions in the presence of moisture. It is generally accepted that the mechanism of electrical conduction in paper and cellulose is ionic in nature and that cations are more important charge carriers than anions. Monovalent cations are most significant. The conductive medium is water associated with the fibre surfaces and the fibre network. In fact, the conductivity of cellulose increases by 14 orders of magnitude when the water content increases from 0 to 20% [3,4].
EXPERIMENTAL Laboratory handsheets were prepared from bleached pine kraft pulp and viscose fibrcs. The viscose fibre content of the sheets varied from 0 to 70%. Sheets made of higher viscose content decompose because viscose fibrcs do not adhere togcther without special treatment. The density of the sheets decreased linearly from 650 to 280 kdm3 as the viscose content increased from 0 to 70%. At high viscose contents density is well below the density of a real paper. Some properties of the sheets are given in Table 1. At fixed relative humidity (RH) the moisture content of the handsheets increased linearly with viscose content. Extrapolation to 100% viscose contcnt at 60%RH yields a moisture content value of 12.2%, which is in agreement with the literature value [ 5 ] . No deviation from linearity was found at relative humidities ranging from 10 to 60%RH. Resistivity was measured according to the standard ASTM D257-93 with 100 V applied potential and 3 0 s charging time. The device used in this study was a commercial HP4339A High Resistance Meter with a HP 16008B Resistivity Testing Cell. Since paper is a dielectric material the mcasurcd resistivity is time-dependent. Thus, the concept of dc rcsistivity is somewhat ill-defined. However, the dc resistivity gives us some idea of the “true” electrical conductivity. Dielectric constant was measured with dielectric analyser DIANA developed by VTT Automation. It allows the measurement of dielectric properties in xyz-directions in the frequency range of 20 Hz to 1 MHz [6]. In all the electrical measurements, relative humidity was controlled within f0.2%units and temperature within f0.2”C. Error bars in the illustrations indicate the statistical uncertainty of the measurements.
Table 1. Some properties of the viscose-kraft fibre handsheets. Viscose content
(%I
Thickness Basis (Pm) weight (g/m2)
Density (kg/m3)
0 25 50 60 70
101 137 183 213 24 1
649 495 368 317 284
65.5 67.8 67.4 67.5 68.5
Air permeance, Gurley (dl00 ml) 6.0 0.5 0.1 0.1 0.0
Electrical properties of viscose-haft fibre mixtures 269 RESULTS AND DISCUSSION Dielectric Properties Figure 1 shows the dielectric constant in the z-direction (perpendicular to the sheet plane) as a function of moisture content at 100 Hz. Figure 2 shows the same curve at I MHz. The effect of moisture content disappears at high frequency. Likewise the asymptotic dielectric constant at low moisture content is independent of frequency. Similar observationshave been reported for cotton [7]. 3.5 . . . . , . . . . , . . . . , . . . . , . . . . , . . . . , . . . . , . . . . , . . . . I
....
Viscose Content 4 0%
-+-
60%
-+-
7 0%
Figure 1. Dielectric constant of viscose-haft handshcets in the z-direction at 100 Hz as a function of moisture content.
Figure 2. Dielectric constant of viscose-haft fibre handsheets in z-direction at 1 MHz against moisture content. For comparison, the axes have the same scale as in Fig. 1.
270 Paper fibre production and properties At low moisture content or high frequency, the rotation of the adsorbed water molecules is restricted by the potential barriers imposed by the cellulosic surfaces. Apparently the uniform viscose fibres are more effective in this than the fibrillated, irregular kraft fibres. Lowering the frequency gives “more time” per cycle for the water molecules to realign themselves and hence increases the dielectric constant. The frcquency dependence found here is analogous to that of ice [7]. Notice that in both figures the dielectric constant decreases as a function of viscose content even though the literature values for dry viscose and dry wood pulp fibres are’ almost identical ( ~ ’ ~ 2 . 6 - 2at . 7 1 kHz)[8]. The decrease is caused by the sheet density which is lower than that of fibres. Since the dielectric constant is relatcd to number of polarised molecules per unit volume, it also depends on the density Pd of the material. This density dependence is given by the Clausius-Mossotti relation [9]
where P d is the density of the sample and Noa/3&0is the polarisability per mole (molar polarisation, [m’]) and M the molecular wcight in [kg]. Fig. 3 shows a plot of Eq. (1) for the data at two values of relative humidity. The straight lines are linear least squares fits of the data with the error bars used as weight of the data points. Except at 100 Hz and 6.0% moisture content, the fits are reasonable. The deviation at 100 Hz and 6.0% moisture contcnt is causcd by the contribution of water. At low moisture content and high frequency, water has little cffect on the dielectric constant.
..................................
0.40
mc=6.0%
o
.:
N
0.20
mc=6.0%
.I
o mc=3.0%
v h
I
0.1 0
0.05 C
0
100
200
300
400
500
600
700
Density (kg/m3)
Figure 3. Dielectric constant ploltcd according to Eq. (1) as a function of density at two frequencies and two moisture contents. Thc quality of the linear fit indicates the validity of the Clausius-Mossotti relation (Eqs. (1) and (2)).
Electrical properties of viscose-haft fibre mixtures 27 1 The Clausius-Mossotti relation assumes that the molecules are electrically neutral or that their dipole moments are in complete disorder [9]. The Clausius-Mossotti relation has been previously applied successfully for dry chemical pulps at 60 Hz and 82°C [lo]. It has also been reported that the dielectric loss should depend linearly on density [lo]. We did not find correlation with density. The dielectric loss seems to behave much in the same way as the dielectric constant (Figs. 4 and 5). Dielectric losses are strongly 1.50 1.25
N
Y
Ea,
t-
1 .oo
-0-
0%
4-
25%
-0- 50%
+60%
0.75
-+-
u)
70%
v)
0 I
0.50 0.25
0.00 0
1
2
3
4
5
6
7
8
9
10
Moisture Content (%)
Figure 4. Dielectric loss term in z-direction at 100 Hz as a hnction of moisture content for viscose-krafl handsheets.
0
1
2
3
4
5
6
7
8
9
10
Moisture Content (%)
Figure 5. Dielectric loss term in z-direction at 1 MHz as a hnction of moisture content for viscose-krafl handshects. For comparison, the x- and y-axis have the same scale as in Fig. 4.
272 Paper fibre production and properties related to the presence of water in the fibres. At high frequency, dielectric loss is practically zero and indcpendent of the moisture content. The dependence of the dielectric loss on the moisture content at different frequencies can either be attributed to dielectric losses of bound water, as in cotton [7], or conduction losses. Electrical Resistance Figure 6 shows the dc volume resistivity of the viscose-krafi handsheets as a function of moisture content. Resistivity decreases exponentially as moisture content increases. The depcndence is usually assumed to be of the form [7, 1 I ] p= Am-"
(3)
where m is the moisture content and po and n are constants. However, the model by Murphy has no physical justification. Murphy [ 111 obtained a value of 9.3 for n and concluded that n is the average number of hydroxyl groups between carboxyl groups or other sites that supply the charge carrying metal ions. According to Murphy, conduction is only allowcd if all the hydroxyl groups between ion sites are occupied by water molecules and this supposcdly lcads to the power-law depcndence in Eq. (3). Unfortunately, the value n=lO corrcsponds to an unrealistically high concentration of ion sitcs. For example, kraft and viscose fibres typically have one carboxylic acid group per hundred glucose units. In viscose fibres it is practically zcro. Furthermore the mathematical model assumes zero conductivity at zero moisture content, which is against experimental evidence. For instance, in wool resistivity is independent of moisture content at very low moisture content levels.
Viscose content -0- 0% 25% -0- 50% &60%
1014
-*-
lot3 : L
Of2
1
-+-
70%
T
0"
F 2
3
4
5
6
7
8
9 1 0 1 1 1 2
M o istu re Content ( O h )
Figure 6. Dc volumc resistivity as a function of moisture content for viscose-kraft handsheets.
Electrical properties of viscose-haft fibre mixtures 273 Table 2. Experimentally determined parameters of Eq. (4) for viscose-kraft handsheets.
1 Viscose content A Mo (W 0 25 50 60 70
(nan) 7.0.1OzJ 3.7.1o~~ 1.7.1OZ6 1.7.102’ 2.4.102’
(W 0.581t0.02 0.63k0.02 0.68k0.02 0.66k0.01 0.691t0.03
..
(eV) 0.43f0.01 0.51f0.01 0.54f0.01 0.52k0.01 0.53f0.01
Although the mathematical form assumed by Murphy is probably incorrect, a sufficiently high concentration of water molecules on the cellulosic surfaces is necessary to provide a “medium” through which the ions move. Hydroxylic groups devoid of water molecules hence represent sites through which the ions cannot move. If we assume that the number of water molecules per hydroxylic group is Poisson distributed and that resistivity is linearly proportional to the fraction of “empty” hydroxylic groups, it follows that resistivity is proportional to exp(-mlMo). Here m/Mo is the average number of water molecules per hydroxylic groups that are available for ionic conduction. The exponential form is evidently consistent with experiments, Fig. 6. The corresponding values of Mo are given in Table 2. Other factors that contribute to resistivity are the ionic content and degree of dissociation of the carboxylic groups, as well as the mobility of the ions. Dissociation and mobility both depcnd on an energy barrier and hence give an exponential dependence on inverse temperature. We therefore conclude that resistivity in paper is given by
where A is a constant, Mo a parameter related to the moisture content, Ea the characteristic activation energy for electrical conduction, k~=1.38.10-23 JK Boltzmann constant, and T temperature in K. The characteristic activation energy for electrical conduction was determined by mcasuring the resistivity as a function of temperature and plotting the natural logarithm of resistivity against inverse temperature. Such a plot yieldcd a straight line from which the activation energy Ea was obtained (see Fig. 7). The values are given in Table 2. They are comparable to the results by Murphy [ 121 who measured a value of 0.46 eV for dry condenser cellulose at room tempcrature.
SUMMARY The dielcctric constant followed accurately the Clausius-Mossotti relation (Eq. (1)) at low moisture content or high frequency. Hence, the differences in the dielectric constants of the viscose-haft sheets at high frequency were caused by density changes only. At low frequency the dielectric constant decreased as a function of viscose content (density), even though the moisture content of viscose fibres was higher than that of kraft fibres. This means that water cannot contribute to the dielectric constant of viscose fibres as much as it does in the case of kraft fibres.
274 Paper fibre production and properties
27.5
27.0
26.5
0
26.0
0
-n, 0
25.5
0.0032
0.0033
.
0.0034
0.0035
Inverse of Temperature (1/K)
Figure 7. The characteristic activation energy for electrical conduction Ea is obtained by plotting the natural logarithm of resistivity as function of the inverse temperature. At high frequency, dielectric loss was practically zero for all viscose contents moisture contents. At low frequency, dielectric losses increase rapidly with moisture content. The overall difference betwccn viscose and kraft was rather small but the loss term still decreased slightly with increasing viscose content. This is consistent with the results on dielectric constant. The apparently higher dc resistivity of viscose was probably caused by the lower density and smaller contact area under the electrode with decreasing density. A new mathematical form was suggested for conduction. The temperature dependence of resistivity was the same for all viscose containing sheets but different for the haft pulp sheet. The corresponding activation energies wcre approximately 0.5 and 0.4 eV for viscose and kraft fibre sheets, respectively.
ACKNOWLEDGEMENTS The authors would like to thank Jorma Sundquist and Tarja Tamminen for valuable comments. This work has been partially financed by the Technology Development Centre of Finland which is gratefully acknowledged.
REFERENCES 1.
2.
Baum, G., Electrical properties: I. Theory, in Handbook of Physical and Mechanical Testing of Paper and Paperboard (R. E. Mark, Ed.), Marcel Dekker, New York, 1984, Vol. 2, pp. 171-139. Matsuda, S., Electrical properties: 11. Practical considerations and methods of mcasurement of electrical properties, in Handbook of Physical and Mechanical
Electrical properties of viscose-haft fibre mixtures 275
3.
4. 5.
6. 7. 8. 9. 10.
11. 12.
Testing of Paper and Paperboard (R. E. Mark, Ed.), Marcel Dekker, New York, 1984, VOI. 2, pp. 201-240. Josefowicz, J. Y. and Deslandes, Y., Electrical Conductivity of Paper: Measurement Methods and Charge Transport Mechanisms, in Colloids and Surfaces in Reprographic Technology (M. Hair and M. Croucher Eds.), American Chemical Society, New York, USA (198 I), pp. 493-530. Murphy, E. J., The Dependence of thc Conductivity of Cellulose, Silk, and Wool on their Water Content. J. Phys. Chem. Solids 16:115( 1960). Fuzek, J. F., Absorption and Desorption of Water by Some Common Fibers, Ind. Eng. Chem. Prod. Res. Dev. 24: 140 (1985). Simula, S., Varpula, T., Ikalainen, S., Seppa. H., Paukku, A,, and Niskanen, K., Measurement of the Dielcctric Properties of Paper, Journal of Imaging Science and Technology, (1999). Morton, W. E. and Hearle, J. W. S., Physical Properties of Textile Fibres, 2nd Ed., The Textile Institute, London, 1975, pp.48 1-501. Verseput, H. W., Studies of Dielectric Properties of Chemical Pulps IV. The Relationship Between the Dielectric Constant and Crystallinity of Cellulose, TAPPI 34(12):572 (1951). von Hippel, A. R., Dielectrics and Waves, M.I.T. Press, Cambridge, Massachusetts, 1954, pp. 97-98. Delevanti, C., Jr. and Hansen, P. B., Studies of Dielectric Properties of Chemical Pulps I. Methods and Effects of Pulp Purity, Paper Trade J. 121(26):241(1 945). Seanor, D. A., Electrical Properties of Polymers in Polymer Science (A. D. Jcnkins, Ed.), North-Holland, Amstcrdam, 1972, pp. 1 187-1280. Murphy, E. J., The Temperature Dependence of the Conductivity of Dry Cellulose. J. Phys. Chem. Solids 15:66(1960).
Effects of retained wood resin and polysaccharides on paper properties Anna Sundbcrg, Bjarne Holmbom, Stefan Willfor and Andrey Pranovich Abo Akademi Univcrsity, Laboratory of Forest Products Chcrnistry, Porthansgatan 3, FIN-20500 TurkulAbo, Finland
ABSTRACT The cffects of retained wood resin and polysaccharides on papcr strength, optical propcrties and fiction tverc studied. Wood rcsin, glucomannans and pectic acids were extractcd from thermomechanical pulp (TMP). The wood resin was addcd in colloidal form to the fibre suspcnsion and aggrcgatcd by the addition of cationic polymers or clcctrolytes. In part of thc tests, polysaccharidcs were also addcd. Hand sheets wcre made and tested. Thc amounts of rcsin and polysaccharides in the shects were dctcrmined by gas chromatography. Thc strength properties of the hand shccts dcleriorated when the amount of wood resin was increased up to 2 mg/g. When the amount of wood resin was increased from 2 mg/g to 10 mdg, only a slight decrease in strcngth properties was obtained. Thc static friction was lowered when the amount of wood resin was increased in the shects. Mowevcr, the optical propcrtics were not significantly affectcd, even at high wood resin Icvels. Whcn polysaccharides were addcd to fibre suspensions, together with wood resin, the strength properties were highcr compared, at thc same resin contcnt, to sheets without addcd polysaccharides. Dcacetylated glucomannans and pcctic acids especially diminishcd the negativc cffect on paper strength caused by high wood resin content. Some of thc polysaccharides were probably adsorbcd onto the surface of the TMP fibres or wood resin droplcls, which will incrcasc the hydrophilicity of the surfaces, thercby promoting hydrogen bonds in the sheet. Aggrcgates of polymcr and polysaccharidcs could also be rctaincd in the fibre network.
IN'I'RODUCTION Wood substances are relcascd from wood and fibrcs during the production of mechanical pulp. Thcsc Dissolved and Colloidal (Disco) substanccs consist mainly of polysaccharides, wood resin, lignin material and lignans [ 11. In intcgrated paper mills, many of these substances arc carried over to the papcr machine, where they may cause runnability problems, and when retained in thc papcr, influence the paper properties. The concentration of Disco substances is increascd in the paper machine white waters by thc closure of water systcms. Alkaline pcroxide bleaching can also cause problcms in papermaking duc to the added bleaching chcmicals and changcs in the chemical composition of the Disco substanccs. In principle, there are three "routcs" by which Disco substanccs can be removed from thc proccss: with the paper, with the effluent or by an internal cleaning stage. Cationic polymers with high charge densities and relatively low molar masscs are commonly uscd for thc fixation andor aggregation of Disco substanccs. Furthermore, some of thc Disco substanccs released from mcchanical pulp can be adsorbcd or depositcd onto the surface of the other pulps and fillers in the stock prcparition, even
278 Paper fibre production and properties without chemical aid. It has been shown that glucomannans released from TMP are easily adsorbed onto cellulosic surfaces, especially onto haft pulp fibrcs [2,3]. The adsorption of glucomannans is accompanied by a decrease in the amount of colloidal wood resin [2]. Pectic acids have been shown to form aggregates with cationic polymers [4,5], and calcium [6]. These aggregates may be retained in the sheet. It has also been shown that certain Disco substances are adsorbed onto fillers [7]. The paper properties may be affected when fibres are contaminated with Disco substances, or when aggregates of process chemicals and Disco substances are retained in the paper. It has long been known that the strength of a paper is impaired by thc prescncc of wood rcsin [8]. It has also been found that Disco substances [9], salts and kraft lignin [9,10] have a deteriorating effect on paper strength, but that the strength is improvcd by hemicclluloses and fines [10,11]. Formation of paper in water contaminated with Disco substances from TMP may cause impaired wet and dry strength, but only slightly affect thc optical propcrtics [12]. Extraction of a papcr with chloroform and acetone results in a highcr friction [13]. In this study, we attempted to simulate the aggregation, sorption and rctcntion of Disco substances in the production of wood-containing papcr. Wood resin and different polysaccharides were isolated from TMP. These substances were thcn addcd systematically to fibre suspensions under controlled conditions and aggrcgated with cationic polymcrs or electrolytes. Hand sheets were prepared and tested, and the retained amounts of wood rcsin and polysaccharides were determined with chromatographic methods. The aim of this study was thus to learn more about how paper propcrties arc affected when thc fibres are contaminated with wood resin and polysaccharides, and whcn aggrcgates of Disco substances and fixation aids are retained in the paper sheet.
EXPERIMENTAL Materials TMP was obtained from a mill in Finland using two-stage refining of Norway spruce. The pulp consistency was about 40%, and the Canadian standard freeness (SCAN-M4) about 140 mL. The pulp was stored at -24°C. Bleached sofnvood krafi pulp was obtained in dry lap form. Thc pulp was beaten i n a Vallcy bcatcr (SCAN-C 25:76) to a Schopper-Riegler value of about 40 and storcd at -24°C. Kaolin, Intrafill C, was delivered by ECC International Europe. Wood resin. TMP was freczc-dried and extracted in a 1.5 L Soxhlet apparatus with n-hexane for 20 h. The hexane was removcd by evaporation, after which the wood resin was re-dissolved in acetonc. Glucomannans. Extracted TMP was suspended in distilled water at 2% consistency. The suspension was stirred with a blade propeller at ca. 200 mid' and 60°C for 3 h. The suspension was then filtered on a paper machine wire. Thc TMP was again suspended in distilled watcr, stirred at ca. 200 min" and 60°C for 3 h and filtcred. The filtrates from the first and second stirrings were mixed and centrifuged at 500 g for 30 min. The supernatant was collected and concentrated by vacuum evaporation using a water bath sct at 40°C. The concentrate was filtcred on a Whatman PolycapTM 75AS filtcr with a 0.2 ym pore sizc to rcmove colloidal
Retained wood resin and polysaccharides
279
substances. Ethanol was addcd to the filtrate, the volume perccntage of cthanol bcing at least 80. The polysaccharides wcre allowed to precipitate. Thc precipitated polysaccharides wcre collectcd and washed twice with ethanol, twicc with mcthanol and once with mcthyl tert-butyl elher. The precipitate was finally dricd in a vacuum drier. The amount of the monomers mannosc, glucose and galactose was about 92% of the total amount of polysaccharide monomers analysed by GC. Pectic acids wcrc preparcd according to Sundberg et al. [14]. TMP was freezedried, extracted with n-hexane and washed with distilled water as above. The TMP was then suspended in distilled water at 2% consistcncy. The suspcnsion was stirred with a blade propeller at ca. 200 min-l and 60°C for 3 h. During thc stirring, pH was adjusted to ca. 11 with 1 M NaOH sevcral times. The fibres were removed by centrifugation at 500 g for 10 min. A frcsh batch of extracted and washed TMP was added to the supcrnatant to a consistency of 2%, and the stirring was repeatcd for 3 h at pH ca. 11. Thc suspcnsion was centrifuged and the supernatant collcctcd. The supernatant was then filtcred on a Whatman PolycapTM75AS filter with a 0.2 ym pore size to remove colloidal substances, and pH was adjusted to ca. 7. CaClz was addcd to the filtered sample to a concentration of 30 mM, and the pectic acids wcre allowcd to precipitate overnight. The precipitate was collectcd aftcr centrifugation. The precipitate was then washed with 30 mM CaC12 in distilled watcr, the sample was left to aggrcgate for at least 3 h and then ccntrifugcd at 500 g for 30 min. The precipitate was thcn washcd one more timc with 30 mM CaC12, twice with 0.05 M HCI in ethanol and three timcs with cthanol. The pI4 was adjusted to 11.0 and thc solution was filtercd on a papcr filter, MN640D. Aftcr neutralisation to pH 7, the pcctic acids were dialyscd on a Spectra/Por Mcmbranc (MWCO: 1 000 cut off) and freeze-dried. The average molar mass of the pectic acids was determined by SEC, calibrated with standard dextrans, to be about 12 000 Dalton. The amount of galacturonic acid monomers was about 94% of the total amount of monomers analysed by GC. The cationic polymers used was a poly-diallyldimethylammonium chloridc (Fennofix Kcmira Chemicals Oy) and a cationic starch bascd polymer (Raifix 25035, Raisio Chemicals Oy). Fennofix had an avcrage molar mass of 47 000 Dalton bascd on intrinsic viscosity mcasuremcnts and a charge density of approximately +7 meq/g. Raifix had a molar mass in thc range of millions and a chargc density of ca. + 2.4 meq/g. The polymers were uscd without further purifications. The electrolytes uscd were of analytical grade.
Methods Wood resin dispersion was prepared according to Sundberg ct al. [15]. A dcsired volumc of wood resin in acetone was injectcd into distillcd watcr under agitation at about 200 min-I, the concentration of wood rcsin bcing about 500 mg/L. Aftcr 1 h of agitation, the dispersion was dialysed overnight by placing the dialysis tube in distilled watcr, agitated with a magnetic stirrer, and thc watcr was exchanged ca. 8 times. The volume of the dispersion was measurcd and the Concentration was adjusted to 500 or 200 mg/L. Polysaccliuride solution. Solutions of acctylated glucomannans or pectic acids werc preparcd by dissolving the substances in distilled water and mixing for 1 h. The pH was adjusted to 7. Deacetylated glucomannans werc preparcd by alkaline treatmcnt of the glucomannan solution. The pII was adjusted to 11 and the solution
280 Paper fibre production and properties stirred at 60°C for 1 h. Thc pH of the solution was then neutraliscd and the salts wcrc removed by dialysis. Solution of cationic polymers. The polymers were diluted in distilled watcr to desired concentration and used without furthcr purification. Determination of polymer doses. The fibres (5 g) were suspended in water as dcscribed bclow. Wood resin and polysaccharides were addcd, and the volume was adjusted to 450 mL with distilled water. The fibre suspension was subsequently dividcd into 95-mL portions, transferred to beakers and heated to 60°C.Different polymer doses were added during agitation and the total volume was adjusted to 100 mL. After 20 min at 60"C, the samplcs were centrifuged for 30 min at 500 g, after which the turbidity was determined. The smallest polymer dose, which gave a turbidity value close to zero, was added to the fibre suspension prior to sheetmaking. Fibre suspensions. The pulp, 10 g of h a f t pulp or a mixture of 7.5 g TMP and 2.5 g kraft pulp, was mixed with 1 L distilled watcr. The suspension was stirred at 150 mid' with a bladc stirrer at 60°C for 90 min. Thc suspension was then disintcgratcd with a household mixer for 2 min and 1 L watcr or a mixture of 0.3 g kaolin-in-water was added. The wood resin dispersion and the polysaccharides were added, and the volume was adjusted to 4 L. Thc suspension was agitated with a blade stirrcr for GO min at 6OOC. The predetermined amount of polymer was addcd during agitation and the volume adjusted to 5 L with distilled water. The agitation continued for 20 min. Then 5 L of distilled water, heated to 60"C, was added and hand sheets were made. Hand sheets (60 dm2) were made on a small sheet former, with an area of 10 cm x 10 cm, equippcd with a ca. 200 mcsh wire. A volumc of 600 m L of the 1 g/L suspension were added to the sheet formcr, gently mixed and de-watered for ca. 10 s. The sheets wcrc not wet prcsscd, but dircctly dried against a convex stecl surface at room temperature. In experiments were the sheets were prcssed, the amount of wood I-esin in the sheets was lower compared to unpressed sheets. Although cationic polymers were used to aggregate the wood resin, the resin was not fixcd to the fibrcs but could be rcmoved upon wet pressing. Analyses
Wood resin. The hand shccts were Soxhlet-extractcd with acctone for about 2 h. Thc amount of lipophilic extractives was determined after silylation by GC as prcviously reported [ 161. Polysaccharides in shects and waters were determined using acid methanolysis to obtain monomcric methyl glycosides of neutral sugars and methyl ester mcthyl glycosides of uronic acids. After silylation, the amounts of thc monomers were dctcrmined by GC [ 171. Tensile index was determined according to SCAN-P 67:93 with the exception that the length of thc paper was 70 mm. Tear index was determincd according to SCAN-P 11:73 with 3 shcet samplcs. Scott bond value was determined according to TAPPI UM 403 on a Internal Bond Tcster Model-B instrument (Petroleum Instruments, Bellwood, L, USA.). Light adsorption and light scattering coefficients were detcrmined according to SCAN-P8 :93. Static friction was determined with an AP PAAR Static and Dynamic Friction tester (Lorcntzen&Wcttre, USA), where psTKr=tan(anglc).
Retained wood resin and polysaccharides 28 1 RESULTS AND DISCUSSION Effects of wood resin on paper properties The wood resin contcnt for sheets madc of bleached haft pulp fibres was ca. 0.1 mg/g (Table 1). The wood resin content of a kraft pulp sheet could only slightly be increased by the addition of mere wood resin. To increase thc wood resin contcnt above 0.2 mg/g, thc wood resin had to be aggregated with cationic polymcrs or elcctrolytes. Evcn whcn the wood rcsin was aggregalcd, only about 10-25% of the addcd wood resin was retained in shccts of kraft pulp. For shccts made of a mixture of TMP, kraft pulp and kaolin, the wood rcsin content was ca. 0.7 mdg. TMP fibres contain wood resin locatcd on the surface of the fibres or insidc parenchyma cells, while some wood resin is present as colloidal droplets in thc watcr phase [18]. The retcntion of the added wood resin was somcwhat higher for shcets of TMP, kraft pulp and kaolin compared to shects of krart pulp, which can be cxplained by a more dcnse fibre mat causcd by the larger amount of fines in TMP. Table 1; Only 10-25% of the added resin was retained in sheets evcn when the resin was aggregated with cationic polymcrs or elcctrolytes.
Wood resin added in sheets mg/g fibre mdg fibre Craft pulp
'MI', kraft pulp nd kaolin
0 46 3.5 10 20 40 0 8 25 10 0 0 1 10 25 0 25 0
25
0.1 0.2 0.5
Fixation aid added mdg fibre 0
4.2 10.3 0.1 0.8 6.7 0.G
0 Fennofix 0.1 Fcnnofix 0.1 Fennofix 0.35 Fennofix 0.4 Raifix 0.8 Raifix 0.2 Raifix 0.8 CaCI2 15 mM
0.7 1.5 1.2 3.9 8.3 1.8 6 1.6 6.6
0 Fennofix 1.7 Fennofix 1.7 Fcnnofix 1.7 Fennofix 1.7 NaCl200 mM NaCl200 mM CaCI, 15 mM CaCI, 15 mM
0.9
282 Paper fibre production and properties
Fennofix 10 0
II(raft1 I
I
I
I
I
2
4
6
8
10
I
Amount of wood resin in sheets, mg/g Figure 1. The tensile index was lower for sheets of haft pulp with high wood resin content compared to sheets with low wood rcsin content. When the amount of resin was incrcased from 2 to 10 mg/g, only a slight dccrease in the tensile index was obtained. No significant differencc in the tensile indcx could be noted when the wood resin was aggrcgated with diffcrcnt cationic polymers or calcium chloridc. The tensile index for sheets of h a f t pulp that contained ca. 0.1 mg wood resin/g fibre was ca. 39 N d g , while a sheet with a resin content of 4 mdg, gave a tensile index of ca. 20 N d g (Fig. I), i.e. a drop to almost half. The strength properties of shccts are lowered by the addition of wood resin components [8], since these substances can be sorbcd on the surface of the fibres and block the formation of hydrogen bonds. When the wood rcsin content of thc sheet was incrcased to ca. 10 mg/g, the tensile indcx remaincd at ca. 20 N d g . This implies that above a certain Icvel, the wood rcsin no longer has a deteriorating cffect on the tcnsile index. This may be due to thc fact that no more resin-polymcr complexes are retained on thc surfacc of the fibres but rather betwccn the fibres i n the fibre-fibre nctwork. Springcr et al. [I)] showed that increasing the concentrations of wood substanccs, that are dissolved or dispersed into the water phase above a certain amount, did not rcsult in further tensilc strength loss. There were no significant differcnccs in tensilc indcx when the wood resin was aggrcgated with differcnt cationic polymers (Fennofix or Raifix) or calcium chloride. Addition of the highest dose of cationic polymer, 0.4 mg/g Fennofix and 0.8 mg/g Raifix, did not lower the tcnsile index significantly. Howcvcr, the retention of the cationic polymers in thc sheets was not determined, thcrefore it is difficult to completely separate the effccts of the cationic polymers and the wood resin. These effects are combined, though, since thc wood resin was not retaincd in the sheet if not aggregated (Table 1).
283
Retained wood resin and polysaccharides
I'ensile index, N d g
1 20
-
10
-
O l 0
ITMI?, kraft pulp and kaolin
I
Fennofm 110 wood
resin added I
I
I
I
I
2
4
6
8
10
Amount of wood resin in sheets, mg/g Figure 2. The tensile indcx was lowcr for sheets of TMP, kraft pulp and kaolin comparcd to sheets made of kraft pulp. Whcn the amount of wood rcsin increased in thc shccts from 0.7 mg/g to ca. 2 mg/g. thc tcnsilc index decreased with ca. 40%. When the wood rcsin was aggregated with electrolytcs, the tensile index was slightly highcr compared to sheets where the rcsin was aggregated with Fennofix. The tensile index was lowcr for sheets made of a rnixturc of TMP, haft pulp and kaolin compared to shccts of only h a f t pulp fibres (Fig. 1 and 2). Thc strength of paper, according to van den Akkcr [20], lies primarily in the bonds formcd betwcen the fibres; the more bonding that can bc achicved, the greater the strength of thc paper. Thc lignin- or resin-covered surfaces of TMP fibrcs docs not form hydrogen bonds with the adjaccnt fibrcs, which results in a lower tensile indcx cornpared to kraft pulp fibrcs. It has also becn shown that fines and fibres of mcchanical pulp are stiffer due to thcir highcr lignin content, which will prohibit thcm from coming into close contact with other fibres and thereby forming internal bonds [11,19]. Fillers also have a disruptive influence on the fibre network, which will lowcr the paper strength [21]. For sheets of TMP, kraft pulp and kaolin, thc tcnsilc index decreased with 30-40% when the wood resin content incrcascd from ca. 0.7 mg/g to 2 mg/g (Fig. 2). Whcn thc wood resin content was incrcascd abovc 2 mg/g, no further decrease of the tensile index was seen. When electrolytes, NaCl and CaC12, wcre used to aggrcgate thc wood rcsin, the tensile index was somewhat highcr compared to the tensile index of shccts where the wood resin was aggregated with Fcnnofix. If Fcnnofix, but no wood rcsin, was added to the suspensions, the tensilc indcx was lowcr compared to a shcct of mcrc fibrcs and kaolin. However, the amount of resin in the shcct was also higher because the resin released from the TMP fibrcs was aggrcgatcd and retained in the sheet.
284 Paper fibre production and properties Table 2. The strength properties for sheets made of kraft pulp and a mixture of TMP, haft pulp and kaolin were lower when the amounts of wood rcsin were increased. The static friction was also lower at high resin content but the optical properties were not iuch affectcd. Fixation aid Wood resin Tcnde Scott
Tear
Static
indcx
friction
coelT.
coeff.
Jh' Nm'k
COCIT.
m2ks
Slm'
kdm'
39 35 31 31 21 20 38 29 21 32
170 170 165 175 147 132 178 161 139
0.72 0.79 0.78 0.75 0.7 1 0.67 0.82 0.68 0.66
0.24 0.19 0.22 0.22 0.21 0.25
47.6 47.3 46.9 49.2 51.9 50.8
*
3.5 4.0 3.2 3.4 2.2 2.2 3.5 3.4 2.3 3.1
*
*
* *
383 385 355 374 346 369 386 365 370 368
Fennofix Fennofix Fennofix NaCl NaCl
0.7 1.5 1.2 3.9 8.3 I .8 5.8
23 16 17 14 13 18 14
93 84 91 80 83 102 86
1.6 1.3 1.6 1.3 1.0 1.5 1.4
0.75 0.72 0.73 0.68 0.68 0.77 0.66
1.26 1.33 1.37 1.35 1.41
62.2 61.5 62.8 64.6 66.4
CaCI, CaCL
1.6 6.6
19 16
94 81
1.5 1.5
0.70 0.69
* * *
*
added
inshccts
indcx
bond
nig/g
Nndg
0.1 0.2 0.5 0.9 4.2 10.3 0.1 0.8 6.7 0.6
Light ads. Light seatt. Densit]
Craft pulp
Fennofix Fennofix Fennofix Fennofix Raifix Raifix Raifix CaC1,
* * *
* *
:MP,kraft pulp and kaolin Fennofix Fennofix
::
*
* * *
269 264 269 255 259 267 258 269 263
Values not measured
The Scott bond value, a measure of the internal strength of a papcr shect, was lower for sheets with a high wood resin content (Table 2). For sheets of kraft pulp fibres the decrease was almost 20% and for sheets of TMP, h a f t pulp and kaolin, the decrease was ca. 10%. Tear index was also 35-45% lower for sheets with a high contcnt of wood rcsin compared to sheets with no wood resin added for the fibre suspcnsions tcsted. Increased amounts of wood resin in thc shects will probably dccrease thc amount of hydrogen bonds, resulting in a lower Scott bond value and tear index. The static friction was decreased with ca. 10% when the wood rcsin content was increased in the shccts. Somc of the oily wood resin could be locatcd at the surface of thc papcr, rcsulting in a lower friction. The added wood rcsin did slightly increasc the light adsorption and light scattering coefficicnts of the sheets from the differcnt fibre suspcnsions. However,.the effect was small. The added wood resin had no visual colour in the conccntrations studicd.
Retained wood resin and polysaccharides
285
Table 3. Sheet properties after addition of polysaccharidcs and wood rcsin to a furnish of TMP, kraft pulp and kaolin at low and high resin content. The polysaccharides diminished the negative effects of wood rcsin especially at high rcsin levels. GM: glucomannans. Substances added to a fibre suspension
Fennofix Tensile Scott Tear Static Llgllt ads. Light scull, Wood resin in sheets udded added Index bond Index friction eocff. coeff. coeff. m’k m’kg mg/g mp/p mg/gfibre Nm/g Jlm’ Nm’k
No additives 100 mug GM 40 mug dcac. G M 60 mug pectic acid
0.1
No additives
8.3 4.8 5.8 4.8
100 mdg G M
40 mp/g dcac. G M GO mdg pectic acid *: Values not measured
0.9 1.0 1.0
0 0
0 0
0
0 0
0
25 25 25
0.7 1.8
25
15.5
I .2
23 24 22 21
93 95 99 94
13 17 18 19
83 81 97 91
1.6 1.8
1.7
*
1
1.3 1.3
*
0.75 0.66 0.82
1.26 1.29 1.41
62.24 57.53 56.88
0.68 0.69 0.70
1.41 1.48 1.43
66.38 63.12 62.83
*
*
*
*
*
*
Effects of wood resin and polysaccharides from TMP on paper properties To simulate and cxamine the effects of the closurc of paper mill white water systems, isolated polysaccharides and wood resin, were added to a fibre suspension and thc resulting sheets were tested. The wood resin contcnt of sheets with added acctylated glucomannans was approximatcly the same as for sheets of mcre fibres and kaolin. The solution of glucomannans did not contain any wood resin and the added glucomannans did not retain more of the wood resin rclcased from thc TMP fibrcs. The tensile and tear indexes were higher for shccts with acctylated glucomannans comparcd to those without wood substances (Fig. 3, Table 3). This was the case at both low and high wood resin contents. The Scott bond value was higher for sheets with acetylated glucomannans at lower wood resin contcnts, but somewhat lower at highcr wood resin contents (Table 3). The amounts of mannans in the shcet were slightly higher when acctylated glucomannans were added (Table 4). The glucomannans that arc rctained or adsorbed will promote new bonds between the fibres and will enhance the mechanical strength. According to McKcnzie [22] there may be some increase in effective contact area as surface microfibrils, or polysaccharidcs, are drawn in towards thc fibre crossings by the retreating watcr menisci. The polysaccharides could thus be concentrated to the fibre crossings, where they would form intermolecular bonds upon drying. When TMP is alkali-trcatcd, as during peroxide bleaching, the glucomannans arc deacetylated and deposited on fibres [ 1,3,4]. Furthermore, acidic pectins are rclcascd from the fibres. Deacetylated glucomannans and pectic acids were addcd to pulp furnishcs and the resulting shccts were tested. The tensile and tear indexes as well as the Scott bond value were higher when deacctylated glucomannans were added, if sheets of the same wood resin content werc compared (Fig. 3, Table 3). Thc amount of mannosc in the sheets increased (Table 4). This indicatcs that deacetylated glucomannans werc adsorbed onto the fibres, which can lcad to the formation of morc hydrogen bonds. It has been shown lhat deacetylatcd glucomannans are adsorbed onto fibres and that this causes a higher tensile index [2].
286 Paper fibre production and properties
ITMP, kraft pulp and kaolin 1 2
4
6
8
Amount of wood resin in sheets, ms/g
10
Figure 3. The ncgative effect of wood resin contamination on the tensile indcx was diminished by the addition of wood substances from TMP to the fibre suspension. Deacetylated glucomannans were probably adsorbed on thc surface of the fibrcs where they promoted new hydrogen bonds. Pectic acids probably formed aggregatcs with the polymer and these aggregates were retained in thc fibre network, increasing the amount of hydrogen bonds between thc fibres. Deac. GM: deacetylated glucomannans. Tablc 4.Retained amounts of polysaccharides in sheets of TMP, kraft pulp and kaolin, dctermined as carbohydratc monomers.
Substances added to
Mannose
Galacturonic acid
81 76
6.7 7.1
No wood substances GM GM and Fennofix Deactylated GM Deaclylatcd GM and Fennofix Pectic acids Pcctic acids and Fennofix
7.0 13.0
Retained wood resin and polysaccharides 287 Upon the addition of pectic acids, the amount of galacturonic acids increased in the sheets only when polymer also was added (Table 4). This indicates that pectic acids had to be aggregated in order to be retained in the sheet. A very high amount of polymer was needcd to aggregate thc pectic acids, ca. 15 mg/g fibre. The tensile index was notably highcr for the resulting sheets when wood resin, pectic acids and Fcnnofix were added, compared to sheets with addcd wood rcsin and Fennofix (Fig. 3, Table 3). The aggregates could act as bridges and bring the fibres into closer contact, thereby facilitating more hydrogen bonds. The Scott bond value was also increased (Table 3).
CONCLUDING REMARKS Incrcased amounts of wood resin in hand sheets decrease the tensile index, as well as the Scott bond value and tcar index. When the wood resin content in the sheets was increased above 2 mg/g only a slight decresc in the tensile index or the Scott bond value was observcd. The static friction decreased slightly with increased amount of wood resin in the sheets. The optical properties of the sheets were not significantly affccted by increasing amounts of wood resin or polysaccharides, even at high resin levels. When polysaccharides were added to fibre suspensions togethcr with resin, the strength properties of the resulting shcets wcre highcr compared, at the same resin content,’ to sheets without added polysaccharides, Deacetylated glucomannans and pectic acids especially diminish the negative effect of high amounts of resin on paper strength. This indicates that polysaccharides were adsorbed onto the surface of the TMP fibres and thereby increased the hydrophilicity of the fibre surface which promoted new hydrogen bonds. Polysaccharides could also form aggregates with the polymer, which wcre retained in the fibre network.
ACKNOWLEDGEMENTS The authors want to thank Danicl Peltonen for preparation and testing of the hand sheets. Financial support was received from the Technical Development Centre of Finland (TEKES), Kemira Chemicals Oy, Raisio Chemicals Oy and UPM-Kymmene.
REFERENCES 1 B Holmbom, R Ekman, R Sjoholm, C Eckerman and J Thornton, ‘Chemical changes in peroxide bleaching of mechanical pulps’, Pupier, 1991 45(10A) V16v22. 2 B Holmbom, A Aman and R Ekman, ‘Sorption of glucomannans and extractives in TMP watcrs onto pulp fibers’, 81h Int Symp Wood Pulping Chern, Helsinki, Appita, Vol 1, 1995, 597404. 3 J Thornton, R Ekman, B Holmbom and I: Ors5, ‘Polysaccharides dissolved from Norway spruce in thermomechanical pulping and peroxide bleaching’, J Wood Chem Tecltnol, 1994,14(2) 159-175. 4 A Sundberg, R Ekman, B Holmbom, K Sundbcrg and J Thornton, ‘Interactions betwcen dissolved and colloidal substances and a cationic fixing agent in mechanical pulp suspensions’, Nord I’ulp Pup Res J, 1993 S(1) 226-231.
288 Paper fibre production and properties 5 6 7
8 9 10 11
12 13
14
15 16 17 18 19 20 21 22
A Sundberg, R Ekman, B Holmbom and H Grijnfors, ‘Interactions of cationic polymers with componcnls in thermomechanical pulp suspensions’, Pup Puu, 1994 76(9) 593-598. K Sundberg, J Thornton, R Ekman and B I-Iolmbom, ‘Interactions between simple electrolytes and dissolved and colloidal substanccs in mechanical pulp’, Nord Pulp Pap Res J , 1994 9(2) 125-128. S Willfor, A Sundberg, B Holmbom and A-L Sihvonen, ‘Interactions between fillers and dissolved and colloidal substances from TMP’, to be submitted. J Brandal and A Lindheim, ‘The influence of extractives in groundwood pulp on fibre bonding’, Pulp Pup Cun, 1966 67(10) T431-T435. A M Springer, J P Dullforce and T H Wegner, ‘The effects of closed white water system contaminants on strength properties of papcr produced from secondary fibers’, Tuppi J, 1985 68(4) 78-82. T Lindstrijm, C SBremark and L Westman, ‘The influence on papcr strength of dissolved and colloidal substances in the white water’, Svensk Pupperstidfi, 1977 SO(11) 341-345. P A Moss and E Retulainen, ‘The effect of fines on fibre bonding: Cross-sectional dimensions of TMP fibrcs at potentional bonding sites’, J Pulp Pup Sci, 1997 23(8) 5382-388. J T Wearing, M C Barbe and M D Ouchi, ‘The cffect of white-watcr contamination on ncwsprint properties’, J Pulp Pap Sci, 1985 ll(4 ) 5113-121. E L Back, ‘Paper-to-paper and paper-to-metal friction’, Intl Pap Physics Conf, Kona, Hawaii 1991, pp. 49-65. K Sundberg, A Sundberg, J Thornton and B Holmbom, ‘Pcctic acids in the production of wood-containing paper’, Tuppi J, 1998 81(7) 131-136. K Sundbcrg, C Pcttersson, C Eckerman and B Holmbom, ‘Preparation and properties of a model dispersion of colloidal wood resin from Norway sprucc’, J Pulp Pup Sci, 1996 22(7) 226-230. F Ors5 and B Holmbom, ‘A convcnient method for dctcrmination of wood extractives in papermaking process waters and cffluents’, J Pulp Pup Sci, 1994 20( 12) 5361-5365. A Sundberg, K Sundberg, C Lillandt and B Holmbom, ‘Determination of Iicmicelluloscs and pcctins in wood and pulp fibres by acid methanolysis and gas chromatography’, Nord Pulp Pap Res J , 1996 ll(4 ) 216-219. L II Allen, ‘Pitch in wood pulps’, Pulp Pup Can, 1975 76(5) T139-TI46. E Retulaincn, ‘Strength properties of mechanical and chemical pulp blends’, Pup PUU,1992 74(5) 419-426. J A Van den Akker, ‘A note on the theory of fiber-fiber bonding in paper; the influence on papcr strength of drying by sublimation’, Tuppi,1952 35(1) 13-15. J C Roberts, ‘Paper Chemistry’, 2”d ed., UK., Blackie Academic and Professionals, Chapman & Hall, 1996, pp. 210-227. A W McKcnzie, ‘The structurc and properties of paper. Part X X I Thc diffusion theory of adhesion applied to interfibre bondning’, Appitu, 1984 37(7) 580-583.
Appearance of rupture surfaces in wood Krister NygArd, Robert Gyllenberg, Bruno Lonnberg and Goran Gros Abo Akademi University, FI-20500 TurkulAbo, Finland
ABSTRACT
-
The Laboratory of Pulping Technology designed a unique apparath “Wood Destructor 96” for shearing of wood blocks under conditions simulating true speeds and temperatures applied in wood grinding and chip refining processes. The aim was to explore the fundamentals of the wood rheology as it appears in the processes, and to determine by SEM the rupture effects on the wood surface. A number of special arrangements were necessary to achieve the high speeds and temperatures needed in the experimental tests. The peripheral speed of a flywheel was transferred by a clutch to a lever producing the true shearing speed. The whole device was enclosed in an autoclave enabling high temperatures, which along with the compression force and speed were recorded during the very short times of shearing This presentation is focusing on SEM images of the rupture surfaces developed in wood under realistic conditions. As expected, the temperature had a significant effect on the appearance of the rupture surfaccs. Low temperaturestendcd to ”cleave” the fibres due to incomplete softening of the wood components, but as soon as the temperature was raised to 120’ C or higher, the wood rupture occurred mainly in between the fibres. These temperatures are known to soften the lignin in sifu, which implies that particularly the primary wall (P) and middle lamella (ML) are activated to rupture. Extremely high wood temperatures of 170’ C again tended to move the rupture from primary wall to middle lamella.
INTRODUCTION Cyclic compression and plain compression of wood has been studied by application of various techniques. Shearing in different wood directions has also been investigated for clarification of the wood deformation and fracture in initial defibration processes (1 , 2), which means that the shearing speeds have been very low. The rheological behaviour of wood under realistic mechanical pulping conditions implying high shearing velocities, from about 25 to 75 d s , has not been explored in the literature. The main goal of the experiments presented in this work was to measure the shear forces obtained in wood at velocities and temperatures used in technical grinding and refining. The aim was to justify the hypothesis that the wood split requires much less energy than the wood cut, which if minimised would savc grinding and refining energy and improve the mechanical pulp properties (3). The Laboratory of Pulping Technology decided therefore to design a device for this purpose implying that shearing speeds up to 75 m/s and temperatures to 170’ C should be applicable and measurable. The basic question was how the grinding and refining processes should be performed as to produce fracture surfaces in the wood with optimum split, i.e. in between the fibres and if possible without rupturing the fibres.
292 Wood, fibre and cellulosic materials EXPERIMENTAL Testing equipment
The testing apparatus uniquely designed for shearing wood samples was named "Wood Destructor -96" (4). The apparatus was aimed to fulfil the following conditions for the testing procedure:
- temperature - overpressure - shearing velocity - specimen size
20 - 170"C O-lba
25 - 75 m / ~ 38 mmm x 38 mm
The Wood Destructor was designed by Gyllenberg. The clamping system, see Fig. 1, for the wood sample was enclosed in an autoclave. The peripheral speed of a flywheel was instantly transferred by a special clutch to a lever, which then introduced shearing in the fixed wood sample at a certain speed. The force generated on the wood sample was measured with 30 kIIz frequency by means of a load cell attached to the static clamp. Simultaneously,the shearing speed, shearing distance and the temperatue were recorded.
--
\
--
-1
hl&wl---..,
-_
Static vice
Fig. 1. The Wood Destructor: Clamping system and load cell. Testing programme
Systematic shearing tests were performed to explore the relative energy demand in longitudinal shearing of spruce wood (along the fibres). Shearing speeds (25,50 and 75 d s ) and temperatures (loo', lZO", 145' and 170' C) were combined to find the best combination for an ideal wood split with a minimum of cleaved fibres. Shearings were also made across the fibres in the wood sample to justify the hypothesis that cutting across the fibres demands much more energy than splitting along
Rupture surfaces in wood
293
the fibres. The splitting mode would of course be preferred as a means of saving the fibre length for better pulp quality. Shearing impulse and energy
The shearing impulse Z was obtained as F dt and indicated indirectly the shearing energy provided that the shearing speed is constant. The initial studies showed however that the shearing speed varied to some extent, i.e. it did not instantly achieve the full speed. The energy W consumed during the shearing test was computed as W = F &, where F is the force recorded by the load cell and s is the shearing distance. The energy was related to the shearing area and reported as a ”relative energy”, because the obtained shearing distance appeared to be longer than the true shearing distance (partly due to the varying speed).
RESULTS AND DISCUSSION Shearing impulse
The shearing forces obtained across and along the fibres in the wood were measured and compared in Fig. 2 as to demonstratethe energy input required for cutting and splitting the wood. It is justificd that shearing across the fibres requires a definitely higher impulse and thus higher energy than shearing along the fibres. A splitting mode applied in mechanical pulping rather than a cutting mode would hence save significantquantities of energy.
-Along IS -
..,.., .. *:.-;. **
* .. 0
10 .-
--,;
*.
fibres
.......Across fibres
I.
;,
Fig. 2. Force versus time obtaincd with spruce wood sheared across and along the fibres (speed 50 d s , temperature 145’ C). Shearing energy
The relative energy per split area was dctemined for spruce wood at different speeds and temperatures, and it is presented in Fig. 3 as a function of the temperature, which indicated a decreasing trend for the energy determined at 75 m f s . The trend was about the same also
294 Wood, fibre and cellulosic materials for 25 and 50 m/s. The relative energy reached a minimum level at the highest temperatures studied, e.g. 145' and 170' C, which was interpreted as a result of a complcte softening of the wood (lignin). The relative energy obtained at 100' C was slightly lower than that at 120" C and displayed a considerable deviation, which evidcntly was due to a premature softening implying that the wood in fact was rather stiff.
Fig. 3. Relative energy per split area versus temperature obtained with spruce wood sheared along the fibres (speed 75 m/s).
Shearing surfaces The rupture surfaces were studied with a scanning electron microscope (SEM) and this investigation showed the significance of the temperature, Figs. 4 and 5. The surface created at 100" C and 25 m/s as in wood grinding, Fig. 4, indicated very clearly that the fracture partly took place in the fibres themselves that were ruptured, not split.
Fig. 4. SEM picture of surface obtained with spruce wood sheared along the fibres (speed 25 m/s, temperature 100' C); a number of ruptured fibres visible.
Rupture surfaces in wood
295
The rise of temperature to a certain level - evidently corresponding fo that providing sufficient softening led to ideal splitting in between the fibres rather than in the fibre walls, see Fig. 5 . It was observed however that ray cells running across the shearing plane were pulled out of the wood matrix due to the softening.
-
Fig. 5. SEM picture of surface obtained with spruce wood sheared along the fibres (spced 25 m/s, temperature 170" C); split occurred in between the fibres without ruptures (ray cells pulled out). Further magnification of the split wood surface showed that the fibre surface created under the high temperature conditions was more or less free of middle lamellae material, see Fig. 6.
Fig. 6. SEM picture of fibre surface obtained with spruce wood sheared along the fibres (speed 75 m / s , temperature 170" C); SZlayer exposed and ML visible only along the fibre edge.
296 Wood, fibre and cellulosic materials CONCLUSIONS Thc perfomied wood shearing tests indicated that the relative shearing energy reported pcr split surface area decreased, dependent on shearing speed, by 40 55 % when the temperature exceeded 120" C. This enerey reduction was suggested to correlate with the wood lignin softening, which in fact was supported by the SEM pictures obtained. As a function of temperature they illustrated a gradual change of thc shearing surface from one with ruptured fibres (100' C ) to one with intact fibres (170' C). It seems that fibres werc ruptured as long as the wood components, mainly the lignin, were stiff. At the highest temperature tcsted the fibre surface displayed the outmost secondary wall (Sz), but only some middle lamella (ML), which implied that the shearing conditions used in this particular case (spced 75 d s , temperature 170' C ) provided prerequisites for both softening of the lignin and significant removal of the middle lamella.
-
REFERENCES Stefan Holmberg, Deformation andjacture of wood in initiul dejibrution prucesses, Lund histihite of Technology, Report TVSM-3019,Lund, 1996. 2 Stefan Holmberg, A numerical and experimental srudy of initial deflhration ofwood, Lund Institute of Technology, Report TVSM-1010, Lund,1998. 3 GOran Gros, Fundamental Mechanisms in Mechanical Pulping, AbolTurku, Abo Akademi University Press (Acta Academiae Abocnsis, Ser. B, Vol. 57,no. 4), 1997. 4 Krister Nygird, Robert Gyllenbcrg, Bruno LUnnberg and GGran Gros, Shear Forces in Wood and Fibre Flocs, Sustainable Paper- research project, The Finnish Pulp and Paper Research Institutc, Espoo, 1998.
1
COMPOSITE MATERIALS FROM PULP AND PAPERMAKING WASTES Vidvuds Lapsa', Talrits Betkers', Galia Shulga"" 'Laboratory of Concrete Mechanics, Technical Urn-versity, I Kdku S k , Rig3 L V-1059, Latvia. 'Laboratory of Lignin Clicmi.~by,Latvian State Institute of Wood Chemisb;y, 27 Dzerbenes Str., Riga LV-1004 Latvia.
ABSTRACT In this work, the results of a study of the effect of a fine dispersed filler as well as the drying conditions for obtaining composite materials on binder properties of a novel adhesive, UF-oligomers partially substituted by the ligninbased interpolymer complex, are presented. It is shown that the application of the filler, irrespective of its naturc (organic or inorganic), implies an increase in the cohesion strength of the novel binder. The lowest value of the crucial concentration of wood flour, which corresponds to the maximum value of the cohesion strength of the binder, is connected with both its high specific surface and the presence of a significant number of reactive hydroxyl groups at the flour particle surface. The temperature and duration of a drying process affect dramatically the adhesive affinity of the binder polymer matrix to filler particles. The remarkable deterioration of the properties of the composite materials dried at elevated temperatures (>60"C) for a long time is explained by the formation of nonequilibrium and defective structures in the binder polymeric matrix as a result of both significant increasing of speed of the structure forming processes and partial dissociation of the interpolymer H-bonds between binder's components.
INTRODUCTION The growing interest in technical lignins, by-products of pulp mills, as a raw polymeric matcrial is conditioned by their availability in large scale and their biodegradability. Undoubtedly, this interest is one of the main reasons for considerable scientific progress being made in the development of new ligninbased polymeric products and resins with the purpose of their successful practical application. At present, lignin-based graft co-polymers [1,2], plastics and elastomer blends [3-61, interpolymer complexes [7,8] and the other lignincontaining polymer products [9-121 are known. The lignin-based products can perform various functions such as an adhesive promoters and regulators of the interfacial tension in polymer systems, fillers and reinforcing agents for engineering plastics etc. The application of lignins for partial substitution of synthetic thermosetting resins (UF, PF) in the manufacture of building composite materials such as
298 Wood, fibre and cellulosic materials fiberboard, particleboard, plywood is well known [13-15]. However, the application of lignins in resins is limited due to a significant worsening of the properties of composite materials and the necessity of additional power consumption for their manufacture at a high lignin content. It has been found 1161 that the coupling of lignosulfonate (LS) with a synthetic water-soluble polymer of linear structure with a high molecular mass in concentrated aqueous solutions results in the formation of a modified LS (MLS) which, from the chemical point of view, represents an interpolymer complex. The LS-based interpolymer complex is able to substitute 45% and more of the UF-oligomers in a composite material composition. Advantages of the manufacture of the composite materials obtained with the novel UF-based binder in comparison with the known technology based on traditional UF-resin application are the following: using of the significantly lower temperatures for pressing (20°C - 30"C), using of a lower pressure (0.1 MPa 0.2 MPa), no necessity in any curing agent. If a papermaking waste containing short cellulosic fibers in the form of granules is used as a filler, the composite materials obtained are characterizcd by a density of 450-500 kg/m3 and a heat conductivity coefficient of 0.080-0.11 W/Km [17,18]. In the present work, the results of the study of the effect of fine dispersed filler properties as well as the drying conditions of the composite materials on the UF-MLS binder properties are discussed.
MATERIALS & METHODS Urea-formaldehyde oligomers were used at pH 7.5 in aqueous solution containing 65.0% of dry matter and up to 0.3% of free formaldehyde. An ammonium lignosulfonate was applied for a substitution of a part of the UFoligomers. This LS is characterized by the following: 50.0% of dry matter, 4.8% of ash, pH 4.4, 5.0% of technical sugar. For the purpose of an LS modification, a synthetic water-soluble polymer with an average molecular mass 2 .lo5 was used. The polymer to the LS mass did not exceed 0.1. The modification was carried out by mixing concentrated water solutions of the LS and the polymer for 30 min at a room temperature. The UF-MLS binder was obtained by blending the oligomers with the modified lignosulfonatc in a mass ratio of the UF-oligomers to MLS of 0.67. A papermaking waste containing more than 50 mass % of short cellulose fibers with the minor addition of sawdust was granulated to aggregates with a size of 5-20 mm and applied as a coarse filler. Fractionated milled sand, coal ash, cement dust and wood flour with particles size of less than 1 mm were used as fine dispersed filler. In the composite material UF-MLS binder and filler were blended in a laboratory mixer. The content of the binder in the composite material depended on the kind of filler used. The specimens with a width of 35 mm, length of 155 mm and height of 35 mm were made by pressing of raw blends at 20°C and a pressure of 0.110.12 MPa for 30 min. The drying process of the specimens obtained was carried out at constant temperature. The specimens made were tested in terms of specific gravity, cohesion strength, bending strength both in air-dried state (R,) and wet state (R,) after soaking for 24 h in cold water as well as for water resistance. The water resistance of the composite materials was characterized by
Composite materials from pulp and papcrmaking wastes 299 the y value reprcscnted as a ratio of R, to R,. The strength values reported are averages from three specimens.
RESULTS & DISCUSSION The physico-mechanical properties of a composite material as well as its durability depend significantly on the properties of the binder. It is possible to improve the properties of UF-based binder by creating optimal conditions for its structure formation. One of the promising methods for development of such conditions may be the application of a fine disperscd filler in a binder composition. On the other hand, it is known that the presence of the filler can promote the reduction of the formaldehyde emission and decrease the shrinkage of the composite material obtained. Fig.1 shows the dependence of cohesion strength of an UF-MLS binder containing different fine dispersed fillers. Irrespective of their nature (organic or inorganic), the inclusions of fillers imply an increase in binder cohesion strength. According to the maximum values of cohesion strength, milled sand is the best reinforcing agent. It doubles the cohesion strength of the UF-MLS binder in comparison with a bindcr containing no filler. Cement dust, in fact, is the worst reinforcing agent. Evidently, in this case the lowest values of the cohesion strength of the filled binder are caused by the dust alkali value @H>7) that significantly inhibits the structure forming processes in the UF-MLS binder. Wood flour and coal ash as reinforcing agents occupy an intermediate position between sand and cement dust and are capable of imparting the same value of maximum cohesion strength to the binder. It is shown (see Fig.1) that the maximum value of cohesion strength is reached by the addition of wood flour to the UF-MLS binder in amounts 4, 5 and 6 times lower than in the case of cement dust, coal ash and milled sand, respectively. It is obvious that a high specific surface of wood flour as well as a considerable number of reactive hydroxyl and carbonyl groups on its particle surface causes the sufficient intensification of structure formation process in the binder. The surface functional groups are capable of forming a multitude of hydrogen links with binder macromolecules at the interface, as shown in Fig. 2. As a result of such an interaction, the cohesion strength of the binder is remarkably increased. However, after reaching the crucial concentration of wood tlour in the binder (which is up to 12 mass %), it falls noticeably owing to the thinning of the bindcr interlayer between the filler particles. It is supposed that the increase of speed of the structure forming processes in the UF-MLS binder can be promoted also by the high hygroscopicity of wood flour filler. As shown in Fig.1, for the unreactive fillers such as milled sand and coal ash, the values of the crucial concentration is much more and vary from 60 to 70 mass %. Table 1 shows the water resistance of the composite materials obtained under different drying conditions. The coarse filler, ganulated cellulose waste, was used for making the composite material. The results reported show a significant effect of thc temperature and duration of drying on the adhesive ability of the UF-MLS binder to the fillcr. According to the data, an incrcasc in thc drying time at 20°C causes an increase in the y value which, after 6 h, makes up 50% of the maximum water resistance attained in 24 h. Specimens dried at 60°C
300 Wood, fibre and ccllulosic materials
6
+wood 2,4
-
flour
cement dust +coal
ash
+sand
u
l,o
1
4.0
18
1
1
1
35
1
l
1
l
69
48
Filler content, mass YO
Figure 1.
Dependence of cohesion strength of the binder on a filler content.
0 0 D
’Y
F
L 0
U R
................
OH
w
1
o
........ ................ .............
\
OH
Figure 2.
.............
The scheme of adhesive “bridges” at the interface boundary
P 0
L Y M E
R M A T
R I
X
Composite materials from pulp and papermaking wastes 301 reach Y,,,~ even after 3 h of drying. At the. same time, an increase in the drying time above 3 h at this temperature results in thc deterioration of the water resistance of the specimens. Table 1. Effect of drying conditions on water resistance (y) of composite materials
Water resistance, y = RJR,
l h
3 h
6 h
24 h
-
0.40
0.77
0.15
0.79
0.65
0.57
0.70
0.39
0.12
20°C
60°C
105%
However, the most considerable deterioration of mechanical strength and the water stability is observed for Composite materials dried at 105°C for over 1 h, indicating a worsening of the adhesive properties of the UF-MLS binder. Obviously, an elevated temperature, in combination with prolonged drying leads to speedier dewatering of the composite material resulting in a significant intensification of structure forming processes in the binder polymer matrix. As a rule, the formation of non- equilibrium polymeric structures and the inhibition of the relaxation processes in binder polymer matrix accompany one anothcr. The formation of defective structures at a high temperature (>60°C) in a combination with prolonged drying is also promoted by partial dissociation of the interpolymer H-bonds between UF oligomers and MLS formed the binder complex macromolecules. It has been found that the effect of high temperature and prolongcd drying upon the properties of composite materials with the fine dispersed organic fillers such as wood flour, sawdust, bark and hydrolyzed lignin (characterized by a high hygroscopicity) is even more dramatic than in the case of the coarse organic filler. Hence, the formation of non-equilibrium and defective structures in the W-MLS binder polymer matrix is one of the main reasons of the unsatisfactory physicomechanical properties of the cornpositc materials obtained at elevated temperatures and prolongcd drying.
302 Wood, fibre and cellulosic materials CONCLUSION Thus, to develop the optimum conditions for structure formation of the UF-MLS binder polymer matrix in order to obtain the composite materials with needed physico-mecanical properties, the nature of a filler, its dispersity, surface as well as the water removal rate and duration of dewatering during the drying process should taken into account the composite material is being manufactured.
ACKNOWLEDGEMENTS The authors acknowlcdgc financial support from the Latvian Council of Science (the grants for the rescarch projects).
REFERENCES 1. J. J. Meister, ‘Review of the synthesis, characterization and testing of graft copolymers of lignin’, In: Polymer scicnce and technology: renewable-resource matebals, C.E. Carraher & L.H. Sperling (eds.), Plenum Press, NewYork, 1986, pp. 305-322. 2. W. de Olivera, W. G. Glasser, ‘Starlike macromers from lignin’, In: Lignin: Prope~iesand Materials, ASC Symp. No. 397, W.G. Glasser & S. Sarkanen (eds.), Am.Chem.Soc., Washington, D.C., 1989, pp. 414-435. 3. D. Feldman, M. Lacasse & L. M. Beznaczuk, Lignin-polymer systems and some applications, Prog. Polym. Sci., 1986, 12, 271-299. 4. D. Feldman, D. Banu, C. Luchian & J. Wang, Epoxy-lignin polyblends: correlation between polymer interaction and curing temperature, .l Appl. Polym. Sci., 1991, 42(5), 1307-1318. 5. J. J. Lindberg, T. A. Kuusela & K. Levon, ‘Specialty polymers from lignin’, In: Lignin Properties and Materials, ASC Symp. No. 397, W.G. Glasser & S. Sarkanen (eds.), Am.Chem.Soc., Washington, D.C., 1989, pp. 190-204. 6. Sh. Hirose, H. Hatakeyma, J. Izuta, T. Yoshida, T. Hatakeyma, ‘Synthesis and thermal properties of epoxy resins derived from lignin and lignin-related phenols’, In: Advances in Lignocellulosics Chemistry for Ecologkally Friendly Pulping and Bleaching TechnoIogies, Sa Eur. Workshop, University of Aveiro, Aveiro, Portugal, 1998, pp. 295-297. 7. G. Strom, P. Barla, P. Stenius, The formation of lignin sulphonate polyethyleneimine complex and its influence on pulp drainage, Svensk pappentidn, 1979, 82 (14), 408-414. 8. G. Shulga. Lignin-based interpolymer complexes: obtaining, reactions, properties and prospects for application, Dr.habil.chem.thesis, Latvian State Institute of Wood Chemistry, Riga, Latvia, 1998. 9. B. KoSikova, A. Revajova, V. Demianova, The effect of adding lignin on modification of surface properties of polypropylene, Em-. Polym. 1,1995, 3 1(10), 995-999. 10. B. KoSikova, V. Demianova, ‘Physico-chemical properties of lignin modified polyolefins’, In: Markets for Sulfur-Free Lignin, 3rd Int. Forum, Working Documents, Fribourg, Switzerland, 1996.
Composite materials from pulp and papermaking wastes
303
11. N. G. Lewis, T. R. Lantzy, ‘Lignin in Adhesives’, In: Adhesives h m renewable nsouzces, ACS Symp. No. 395, P.W. Hemingway & A.II. Conner (eds.), Am.Chem.Soc., Washington, D.C., 1989, pp. 13-26. 12. J. J. Meister, ‘Chemical modification of lignin’, In: Chernicd modification of lignocellulosic rnaten-ds, D.N.-S.Hon (eds.), Marcel Dekker, New York, 1996 pp. 129-157. 13. K. G. Forss, A. Fuhrmann, Finnish plywood, particleboard and fibreboard made with a lignin-based adhesive, Forest P d . 1,1979, 29 (7), 39-43. 14. H. H. Nimz, G. Hitze, The application of spent liquor as an adhesive for particleboard, Cellulose Chem. Technology, 1980, 14(3), 371-382. 15. A. L. Wotten, T. Sellers, Md. Tahir Paridah, Reaction of formaldehyde with lignin, Forest Prod. 1,1988, 6, 45-46. 16. G. M. Shulga , G. M. Telisheva, A. V. Soms, V. H. Lapsa, T. E. Betkers, A. B. Zezin, V. A. Kabanov, ‘Lignosulfonate-based compositions as chemical admixtures in a manufacture of building materials’, In: Application of chemical admixtures in manufkture of cellulose, pap4 timber and building boards h m wood fibrcs, Int. conf., Federation of Scientific and Technical Cooperation, Bulgaria, Sofia, 1990, pp. 30-31. 17. V. H. Lapsa, T. E. Betkers, Thermo- and sound insulating materials, Papermaking industry (Russian), 1990, 4, 21-22. 18. G. Telysheva, G. Shulga, V. Lapsa, T. Betker, A. Zezin, Modified lignosulfonate-based complex binder for building boards, Cellulose. Paper. C d board (Russian), 1994, 4-5, 23-24.
CELLULOSE COMPOSITE MATERIALS AS SORBENTS SORPTION AND RHEOLOGICAL PROPERTIES
-
Sorin Ciovica', Bruno Liinnberg', and Kurt Liinnqvist' 'Abo Akademi University, Laboratory of Pulping Technology, FI-20500 Turku/Abo, Finland
'Cellomeda Oy, vkistdkatu 6 A, FI-20520 Turku, Finland ABSTRACT
In the last few years sorbent cellulose sponges with a tridimensional porous structure obtained by the viscose process have become more important. Their physical properties depend on the characteristics of the initial dissolving pulp, the viscose procedure and coagulation parameters, as well as on the regeneration conditions. Some propcrties can be significantly changed by reinforcing with cotton or man-made cellulose fibres. The properties of viscose sponges made of low-grade dissolving pulps obtained by the IDE and the soda pulping processes were compared with those of a commercial rayon-grade sulphite dissolving pulp. The influence of the amount and orientation of the cotton reinforcing fibres is also presented. A method for evaluation of the water absorption in the sponges and their retention ability was developed and used to test the sponges. All sponges tested absorbed water and reached the water saturation, i.e. the maximum water absorption (at equilibrium) in about 20 s, and this behaviour seemed to follow the sponge structure. The maximum water absorption of the sponges made of birch soda and IDE pulps was 10 - 15 g water/g dry sponge, as the corresponding sponge density was 0.12 - 0.08 g/cm3. A commercial pulp sponge with a density of 0.05 g/cm3 (reinforced by 20 % cotton fibres) absorbed 25 g water/g dry sponge. The birch IDE and soda pulp sponges had a specific compressibility much lower than the corresponding wheat straw pulp sponges, while reinforcement of the commercial pulp sponge with 20 % cotton fibres resulted in a superior compressibility. Microscopy confirmed a preferential orientation of the sponge cavities perpendicularly to the main sponge surface. INTRODUCTION
The state of engineering of biodegradable materials compatible with living tissues is in remarkable progress. Absorbent materials with a porous structure formed by cavities have been studied on a large scale, especially for medical purposes, e.g. as carriers of drugs, bases for cell growth and as implants for tissue regeneration /1-13/. The materials have also been developed for ion-exchange and mass transfer purposes /3, 14-16/. Absorbent materials are characterized by a tridimensional structure of cavities, which is a prerequisite for liquid absorption in the absorbent /17/. The studies in this area have been focusing on synthetic and natural polymers considered biocompatible and biodegradable, e.g. cellulose and its derivatives, modified proteins, alginates /13, 181, copolymers of polylactic and polyglycolic acids 111, polyvinylpyrrolidone, polyvinylalcohol, polyurethanes efc. The structure of cavities is generally created by the
306 Wood, fibre and cellulosic materials introduction of an inert pressurised gas (usually C02) into the polymer solution or melt 11, 19-201, the gas then expanding or separating. In this study, cellulose was converted to viscose and the cavities were created by solid salt crystals mixed with the viscose /4/. The porosity of the cavity-separating cellulose walls is due to the fibrillar structure of the polymers used. Irregular packing of microfibrils creates micropores with 2-7 nm diameters and mesopores with 50-150 nm diameters, as macrofibrils correspondingly create macropores with 1-10 pn diameters. The macropores can be dimensionally modified and controlled by epichlorohydrin-crosslinkingin the case of cellulose /3/, while the mesopore and micropore domains mainly remain unchanged. Chemical reactions as well as drying and freezing affect the hydrogen bond density and thus introduce structural changes to the pores making their specific propcrties particularly suitable for separation, ion-exchange and protein-chromatography procedures. Cellulose properties in general as well as biocompatibility and biodegradability might further be modified and improved by introduction of suitable substituents to the alcoholic hydroxyl groups. The simultaneous hemostatic and tissue-regencration properties of oxycellulose have been studied in many projects 111, 13, 21-22/. Regenerated cellulose with a homogeneous molecular weight distribution and a relatively low DP is supposed to be more suitable for living organisms from the viewpoints of resorption ability and being inert. The behaviour of absorbents obtained with carboxymethyl or hydroxyethyl cellulose crosslinked with cpichlorohydrin /15/ has been studied for this purpose and for structural and rheological properties /16/. The importance of cellulose viscose sponges obtained by coagulation of the viscose in presence of solid electrolytical salts has been emphasized /3,4, 24/. The low mcchanical strength of these sponges when not reinforced may be a drawback for their utilisation in general /l/. Therefore cellulose sponges still require a lot of optimisation of the pore and cavity size distribution for absorption properties, and further development work for better biodegradability.
EXPERIMENTAL Cellulose properties
The dcgree of substitution and viscose filterability are necessarily not required at highquality levels for the production of regenerated cellulose sponges. The hemicellulose content does not in this case seem to be as important as in fibre spinning, because the sponges can be easily reinforced by external cotton fibres. Subsequently, low-grade dissolving pulps might be introduced for this purpose. A new IDE pulping concept developed for both wood and non-wood raw materials was expected to provide a reactive cellulose, unbleached or partly bleached, because of the smooth delignification. Birch and winter wheat straw pulps wcre produced by the IDE pulping process developed in Finland /25-28/ and bleached by a DED-sequence. The pulps appeared degradable as a result of electron-beam treatment, especially at reduced doses, and showed subsequently a high reactivity in the xanthation /24/. The IDE and soda pulp properties are given in Table 1.
Cellulose composite materials as sorbents 307
15:88, 4, TAPPI T203 om-88 Sponge preparation General
The blcached pulps were converted into viscose solutions and finally transformed into sponges .on a laboratory scale. The viscose composition and the ripening conditions corresponded to those applied in common mcmbrane technology. Viscose coagulation and cellulose regcneration were initiated by intensive mixing of the viscose solution into Glauber salt crystals. When a gel structure appeared, the cellulose-salt conglomerate was immersed into hot water (90-95’C), where the cellulose regeneration was completed and the salt rcmoval finished by repeatcd washing with distilled watcr of the same temperature. Cellulose-salt ratio
The cellulose-salt ratio was varied from 22 up to 44 g cellulosekg Glauber salt crystals for optimisation of the cavity structure of the sponge. Provided that the cellulose can be properly distributed in all experiments, irrespective of the cellulose rate, decreasing cellulose would imply thinncr and evidently more “fibrillated” intercavity walls of the sponge with subsequently quicker absorption of water. In this context the cellulose-salt conglomerate was compressed (I 00-250 bar) to reduce the air introduced during thc mixing procedure, which would imply bctter control of the sponge density. Reinforcement
Cotton fibres were introduced in variable proportions (20-60 YO)into the viscose to form a reinforcing fibre network in the final sponge. The aim was to determine the changes in strength and density of the final sponges. The cotton fibres were oriented by certain treatments of the viscose-fibre-salt conglomerate.
308 Wood, fibre and cellulosic materials Sponge characterization Water absorption Viscose sponges of certain fixed dimensions were immersed repeatedly into distilled water of room temperature for short periods of time with intermediate weighing to determine the water absorption rate. The water absorption rate and the maximum water absorption obtained were then related to the sponge density and other characteristics. The data obtained provided some information on the functional properties of the sponges. Routine measurements of the water absorption were also performed, but then the sponges were kept immersed in water until the absorption achieved the maximum absorption corresponding to the state of equilibrium. The sponges were removed from the bath and initially blotted with filter papers on both sides to remove loose surface water and weighed. The water retained and related to the dry sponge weight was reported as the “initial water retention” (IWR, g watertg dry sponge). The sponges were further pressed for 10 s with a weight corresponding to 150 g/crn’ of sponge volume and the “pressed water retention” (PWR, g watertg dry sponge) was reported. Compressibility The specific compressibility oSp was measurcd on the sponges and reported together with the sponge stiffness. Also the yield point was determined. For clarification of the cavity orientation and distribution further compressibility tests were conducted on differently cut sponges as to represent cross-cut, parallel and length section surfaces relative the main sponge surface; cross-cut and length section surfaces are perpendicular to the main surface.
RESULTS AND DISCUSSION Ccllulosc-salt ratio and compression A number of sponges were prepared from the explored birch and straw IDE and soda pulps bleached by the DED-sequence. The cellulose-salt ratio was kept constant and no pressure was applied as to reduce included air. But sponges were produced from a commercial sulphite dissolving pulp with variable cellulose-salt ratio and by doing the compression. The data are compiled in Table 2.
In general, the birch pulps provided better sponges with lower density and higher initial water retention than the straw pulps and the commercial pulp. Fig. 1 shows that the cellulose-salt ratio to a high extent determined the sponge density, which implies that the cellulose viscose was distributed between the salt crystals as expected. The less cellulose, the lower was the sponge density, and the higher was the water retention, as indicated in Fig. 2. It seems that the sponge density was the most important factor for the water retention ability; the commercial pulp sponges followed the same curve in Fig. 2 as well. Table 2 shows however that increased compression slightly increased the sponge density.
Cellulose composite materials as sorbents 309 Table 2. Sponge prcparation parameters (cellulose-salt ratio and compression) and results on sponge density and watcr absorption and retention on compression. ~~
Sample no.
Cellulose
ell./Glaube pressure, salt mtio, bar
& 1 2 3 4 5 6 7 8 ' 9 10
I'
0.8
Birch IDE Birch Soda Straw IDE strawsoda Borregaard begaard Bomgaard Borregaard Bonregaard
I)orregaard
+Sponge
25 25 25 25 25 22 22 22 40 42
-
100 200 250 200 200
0.085 0.122 0.227 0.297 0.144 0.207 0.214 0.217 0.327 0.357
Nater retention, g waterlg sponge Initial 'reSsed(PWR) at W) 150 an3 sponge 'WR PWIWR% 16.5 8.6 52 11.6 7.5 65 6.1 4.7 78 4.2 3.6 85 12.8 8.0 63 4.0 7.3 55 7.9 4.6 59 8.2 4.8 62 6.8 5.4 80 5.7 5.1 89
-
density
-A-PwR/IwR
0.6
0.4 0.2
20 30 40 g celluloselkg Glauber salt
Figure 1. Effect of cellulosc-salt ratio on sponge density.
I
0,05
0,l
0,16
0.2
0,25
0.3
Sponge density, glcrn'
Figure 2. Effect of sponge density on watcr retention.
Water absorption
Fig. 3 is showing the water absorption rate of the sponges made of the commercial sulphite pulp included in the study. It implies that the water is absorbed quickly in the beginning and achieves the equilibrium phase; the initial absorption rate indicates the absorptivity and the equilibrium level of absorption the ability to retain watcr. Both are
3 10 Wood, fibre and cellulosic matcrials important characteristics for porous materials. In comparison with the initial water retention values (IWR) given in Table 2 (for corresponding but not the same samples) the sponges revealed a water absorption not fully explicable by the parameters accepted, i.e. the cellulose-salt ratio and the compression.
]+Sample
0 0
60
120
180
10 240
I 301
The, I
Figure 3. Water absorption rate and maximum absorption in commercial pulp sponges. Sponge reinforcement
The commercial pulp sponges were reinforced with variable proportions of cotton fibres introduced in the viscose so as to strengthen the final sponge. The results are compiled in Table 3. It is evident that the sponge density was dependent on the proportion of cotton fibres charged from 20 to 60 %. The high charge resulted in about 0.07 gkm3 density, as the low charge gave only 0.05 g/cm3, which also provided the best water retention (IWR) whatsoever, about 25 g water/g dry sponge. - The orientation however did not change the sponge density with the same magnitude as did the cotton fibre charge, but slightly increasing though. Sponge compressibility
The samples 1-4 (Table 2) representing unreinforced sponges were testcd for compressibility and stiffness, see Table 4. There was a significant difference in specific between the sponges made of birch and straw pulps. The birch IDE compressibility (ap) pulp sponge (sample 1) showed a low stiffness, as the birch soda pulp sponge (sample 2) was much stiffer and simultaneously stronger. The corresponding straw pulp sponges (samples 3 and 4) had a higher rigidity and a yield point significantly dependent on the pulping process. The cotton reinforced sponges reported in Table 5 had a much higher spccific compression resistance, and in comparison with the sponges made of regenerated cellulose the elastic domain only was significantly larger.
Cellulose composite materials as sorbents 3 11 ose.
The rheological characteristics depended on the sample orientation towards the main surface of the sponge. The strength, stiffness and elastic behaviour of the sponge in the cross-cut direction suggestcd a prevalent orientation of the cavities perpendicularly to the main surface.
Sponge
a,,,
sample
m/m2
Stiffness, m/m3
kNlm2
no.
1 2 3 4
'
Yield Point aEL, oEL,
300 428 1097 1754
7.35 41.5 165 179
300 375 665 887
% asp 100 00 61 51
Post-Yield Point EEL,
aVL
%E,~
kNh2
100 48 23 36
418 89 1 1547
OVB
%asp
98 81 88
%E,~
90 72 85
3 12 Wood, fibre and cellulosic materials
Position
Stiffness,
a,,
GN/m3
in spongc MN/m2
cross-cut parallel-
length-
7.46 2.53 5.21
5.88 3.91 5.16
average , 4.79
,
Yield Point oEL,
GEL,
m/m2 2.97 1.7 1.19
4.61 , 2.10
%or,, 50 44 34 ,
43
,
Post-Yield Point EEL,
OVD
%ESP
mum2
15 31 17
4.44 2.97 3.12
76 76 59
54 77 45
23
3.41
71
62
OVE,
% osp
%ESP
Microscopic structure
Microscopic investigation of birch IDE pulp sponges revealed thin intercavity walls with fibrillar and porous structure which would maintain a good liquid penetration, Fig. 4. The birch soda pulp sponge again had bulkier and thus less fibrillar intercavity walls, Fig. 5 , which also was supported by the highcr specific compressibility and stiffncss. The straw pulp sponges made of IDE pulp, Fig. 6, and soda pulp, Fig. 7, compared in common structure with the respective birch pulps. The microscopic structure of the reinforced sponges made of a commercial softwood sulphite pulp with 20% cotton fibres charged for reinforcement, Figs. 8-10, clearly indicated a homogeneous, fibrillar structure of the intercavity walls, and with most cavities visible in the parallel section (Fig. 9, meaning that they were mainly oriented perpendicularlyto the main sponge surface).
CONCLUSIONS Cellulose sponges used in various medical and clinical products must be pure and strong enough and able to absorb sufficient amounts of water. However, to absorb water and to retain it even under certain pressure, the sponges must have cavities and porosities formed by the salt crystals having the right size distribution and being present in suitable proportions. Accordingly, the intercavity ccllulose walls are formed thin and fibrillar enough to let liquids penetrate sufficiently. All these properties require studies to clarify the sponge forming mechanisms. In this work, it was observed that the sponge density being the most important factor for the absorption properties evidently followed the cellulose-salt ratio, as the applied pressure with the aim to displace the air from the coagulating cellulose-salt conglomerate did not much affect the density. Another very important factor was the cotton fibre charge for reinforcement of the sponge, and it seemed that a charge of 20 % might be optimum. The new IDE pulping concept was tested, and birch and straw were cooked and semibleached (DED) and finally converted to viscose and sponges. The birch IDE pulp sponge had a low density and subsequently a high water absorption ability.
Cellulose composite materials as sorbents 3 13
Fig. 4. Birch IDE pulp sponge.
Fig. 5. Birch soda pulp sponge.
3 14 Wood, fibre and cellulosic materials
.
Fig. 6. Straw IDE pulp sponge.
Fig. 7. Straw soda pulp sponge.
Cellulose composite materials as sorbents 3 15
Fig. 8. Reinforced sponge; cross-cut section.
Fig. 9. Reinforced sponge; parallel scction.
3 16 Wood, fibre and cellulosic materials
Fig. 10. Reinforced sponge; length section.
REFERENCES 1. D.J. Mooney. D.F.Baldwin, N.P.Suh, J.P.Vacanti, R.Langer: “Novel approach to fabricate porous sponges of poly (D,L-lactic-co-glycolicacid) without the use of organic solvents”, Biomaterials, 1996 17 (14) 1417-1422. 2. E.P.Goldberg, Y.Yaacobi, J.W.Burns, M.Staples et al.: “Water soluble polymers for tissue protection during surgery”, Polymer Preprints, 1989 30 ( 2 ) 359. 3. S.Tasker, J.P.S.Badya1: “Influence of Cross-linking upon the macroscopic pore structure of cellulose”, J.Phys.Chem., 1994 98: 7599-7601. 4. O.Pajulo, B.Llinnberg, K.Lonnqvist, J.Viljanto: “Development of a high grade viscose cellulose sponge”, The XYVII Congress of the European Society for Surgical Research (ESSR), Turku-Finland, May 23-26, 1993. Abstract Book, P-156. 5. R.Langer, J.P.Vacanti: “Tissue engineering”, Science, 1993 260 920-926. 6. A.G.Mikos, A.J.Thorsen, L.A.Czerwonka et al.: “Preparation and characterization of poly(L-lactic acid) foams”, Polymer, 1994 35 1068-1077. 7. D.J.Mooney, S.Park, P.M.Kaufmann et al.: “Biodegradable sponges for hepatocyte transplantation”, J BiomedMater.Res., 1995 29 959-966. 8. A.G.Mikos, G.Sakarinos, M.D.Lyman et al.: “Prevascularization of porous biodegradable sponges”, Biotech.Bioeng,, 1993 42 716-723. 9. D.J.Mooney, P.M.Kaufmann, K.Sano et al.: “Transplantation of hepatocytes using porous, biodegradable sponges”, Transplant Proc., 1994 26 3425-3426. 10. L.Freed, G.Vunjak-Novokovic, R.Biron et al.: “Biodegradable polymer scaffolds for tissuc engineering”, Biotechnology , 1994 12 689-693.
Cellulose composite materials as sorbents 3 17 1 1. T.M.Tierney: “Control of bleeding after prostalectomy with special reference to use of oxidized regenerated cellulose”, J. of Urologv, 1964 91 (4) 400-401. 12. R.W.Post1ethwait:“Studies of new absorbable hemostatic material”, Bull Sot.
Internationale de Chirurgie, 1962 (3) 243-250. 13. J.R.Matthew, R.M.Browne, J.W.Frame, B.G.Millar: “Subperiosteal bchaviour of alginate and cellulose wound dressing materials”, Biomaterials, 1995 16 (4) 275-278. 14. F.A.L.Dulien: “Porous media: Fluid Transport and Pore Structure ”,Academic Press, New York, 1979. 15. L.Westman, T.Lindstr6m: “Swelling and mechanical properties of cellulose hydrogels. I. Preparation, characterization, and swelling behavior”, JAppI.PolymSci. 1981 26 (8) 25 19-2532. 16. T.Lindstrom, J.Tulonen, P.Kolseth: “Swelling and mechanical properties of cellulosc hydrogels. Part VI. Dynamic mechanical properties”, Holzforschung 1987 41 (4) 225230. 17. W.0ppemann: “Superabsorbierende Materialen auf Cellulosebasis”, Pupier 1995 49 (12) 765-769. 18. M.Rosdy, L.-C.Clauss: “Cytotoxicity testing of wound dressing using human keratinocytes in culture”, J.Biomed.Mater.Res.1990 24 363-377. 19. C.B.Park, D.F.Baldvin, N.P.Suh: “Ccll nucleation by rapid prcssure drop in continous processing of microccllularplastics”, Polymer Eng.Sci. 1995 35 432-440. 20. D.F.Baldvin, M.Shimbo, N.P.Suh: “The role of gas dissolution and induced crystallization during microcellular polymer processing: a study of poly(ethy1ene terephtalate) and carbon dioxide systems”, J.Eng.Mater.Tech. 1995 117 62. 21. A.Lebendiger, G.F.Gitlitz, E.S.Hunvitt, G.H.Lord, J.Henderson: “Laboratory and clinical evaluation of a new absorbablc hemostatic material prepared from oxidized regenerated cellulose”, Surg. Forum, 1959 10 440-443. 22. E.S.Hurwitt, J.Henderson, G.H.Lord, G.F.Gitlitz, A.Lebendiger: “A new surgical absorbable hemostatic agent”, American J. of Surgery, 1960 100 439-446. 23. A.Holst: “Quellflibige Cellulosederivate, deren Eigenshaften und Anwendungen”, Papier, 1978 32 (10 A) V7-Vl3 24. S.Ciovica, B.Ltinnberg, K.Ltinnqvist: “Dissolving pulp by the IDE concept”, Cellulose Chem. Technol., 1998 32 (3-4) 279-290. 25, K.Ebeling, K.Henricson, T.LaxCn, B.Lonnberg: Method of producing pulp, Patent Application, 1992. 26. M.Backman, B.Lonnberg, K.Ebeling, K.Henricson, T.LaxCn: “Impregnation Depolymerization- Extraction pulping”, Puperi j u Puu, 1994 76 (10) 644-648. 27. L.Robcrtsen, B.Lonnberg, K.Ebeling, K.Henricson, T.LaxCn: “IDE pulping. The impregnation stage”, Paperi j a Puu , 1996 78 (3) 96- 101 . 28. T.E.M.Hultholm, K.B.LBnnberg, K.Nylund, M.Finel1: “The IDE process: a new pulping concept for nonwood annual plants”, in: Proceedings of Pulping Corference, Chicago, Oct. 1-5, 1995, Book 1, TAPPI Press (1995): 85.
NEW CARBOHYDRATE POLYMER DERIVATIVES FROM RENEWABLE BIORESOURCES TARGETED FOR INDUSTRIAL APPLICATION Charles J. Knill, Sabinr F. Rahmrn & John F. Kennedy Birmingham Carbohydrate & Protein Technology Group (BCPl'G), Chernbiotech Laboratories, University ofBirmingham Research Park, Vincent Drive, Birmingham B15 2SQ, UK
ABSTRACT
The results of preliminary investigations into the derivatisation and characterisation of selected renewable carbohydrate polymers, via esterification using cyclic anhydride reagents under basic conditions, are presented. Carbohydrate substrates utilised for such investigations included glucose, maltose, cellobiose, maltodextrin, cotton cellulose and a range of starches of different botanical origin (namely wheat, potato, maize, waxy maize, sago and tapioca). Cyclic anhydride reagents used for esterification purposes included succinic, octenylsuccinic, dodecenylsuccinic, octadecenylsuccinic, maleic, citraconic (methylmaleic), 2,3-dimethylmaleic, and glutaric anhydrides. Triethylamine was utilised as the base and the resultant carbohydrate ester derivatives were isolated as the triethylamine salt of the corresponding carboxylate group liberated via ring-opening of the cyclic anhydride upon ester formation. Successful esterification was confirmed by FT-IR spectroscopic analysis via observation of an ester carbonyl absorbance in the 1750-1720 cm-' region, and carboxylate ion anti-symmetrical and symmetrical stretching signals in the 1610-1550 cm" and 1420-1300 cm-' regions of the spectra, respectively, such peaks being absent in the spectra of the corresponding underivatised substrates. 'H-Nh4R spectroscopic analysis was employed for the determination of average DS via elucidation of the ratio of the integration values for carbohydrate backbone (pyranose ring) protons and ester side chain protons, respectively. Preliminary results demonstrate that this simple room temperature reaction system can be utilised to produce multifunctional ester derivatives of varying DS from a wide variety of carbohydrate substrates. INTRODUCTION
Carbohydrate polymers (polysaccharides) represent a diverse group of renewable bioresources that provide a complex and essentially inexhaustible substrate library for chemical derivatisation. Modification of their physicochemical properties by chemical derivatisation is an important factor in their industrial production and application. Chemical modification of polysaccharides is based upon hydroxyl group chemistry and a modified polysaccharide can be defined as one whose hydroxyl groups have been altered by chemical reaction, e.g. by oxidation, alkylation, esterification, etherification or by cross-linking '. The most important chemical modifications are those of a 'nondegradative' type, where substitution of the free hydroxyl groups takes place. The physicochemical properties of such a modified polysaccharide are largely dependent upon the degree of substitution (DS), which is defined as the average number of substituted hydroxyl groups per anhydroglucoseunit.
'
320 Wood, fibre and cellulosic materials Such chemical modifications affect hydrogen bonding, charge interactions and hydrophobic character thereby altering the nature of the interactions between the polysaccharide chains. For the majority of industrial applications only a low DS value (< 0.2) is required to significantly change the properties of polysaccharides. In the case of amylose and cellulose (linear polysaccharides), there are three hydroxyl groups available for substitution, namely those attached to the C2, C3 and C6 carbon atoms, the C1 and C4 hydroxyl groups being involved in the glycosidic linkages that form the polysaccharide backbone. Thus, the maximum theoretical average DS value, and maximum DS value for a single anhydroglucose monomer unit, is three ’. Obviously, this value is reduced in the case of a branched polysaccharide (e.g. amylopectin), since a branched anhydroglucose unit has a maximum theoretical DS value of two, thus the higher the. degree of branching the lower the maximum theoretical average DS value. Polysaccharide esters are generally prepared in two ways. Aqueous reactions under controlled pH generally produce low DS value esters (< 2), whereas non-aqueous processes can produce higher DS value esters (up to -3). Over the years, considerable attention has focused upon the synthesis of polysaccharide derivatives with particular emphasis given to the investigation of starch and cellulose derivatives. Many diverse starch ester derivatives have found industrial application 3=1. The polysaccharide esters of greatest commercial value are those that provide enhanced hnctional properties compared with the native polysaccharides, such as improved water resistance, stability and improved film forming ability ’. Reactions of cellulose nearly always take place under heterogeneous conditions, i.e. solid cellulose is usually suspended in a liquid reaction medium. The cellulose itself is heterogeneous in nature as different parts of the fibrils have different accessibility to the same reagent. This can often lead to the formation of non-uniform products. There are a number of In this study the lithium solvent systems in which cellulose can be dissolved chloridddimethylacetamide (LiCVDMAC) solvent system was utilised.
’.
MATERIALS & METHODS The methodologies utilised to synthesise the carbohydrate ester derivatives were based upon those described by McCormick and Dawsey ’. Carbohydrates undergo a nucleophilic acyl substitution reaction with cyclic anhydrides in the presence of a base (triethylamine, TEA, in the case of the work presented) to produce ester derivatives that also contain carboxylate hnctionality (figure 1). Carbohydrate substrates
Glucose, maltose and cellobiose were purchased from Sigma-Aldrich Company Ltd., Poole, UK. Maltodextrin (from maize starch, DE 11-14) was supplied by Roquette, Greenwich, UK. Cotton cellulose was supplied by Bundesforschungsanstalt fir Forst und Holzwirtschaft (BFH), Hamburg, Germany. Wheat, potato, maize and waxy maize starches were supplied by Cargill PIC, Tilbury, UK. Sago and tapioca starches were supplied by CRAUN Research Sdn. Bhd. (CRSB), Kuching, Sarawak, Malaysia.
New carbohydrate polymer derivatives
carbohydrate
321
succinic anhydrlde
u+oh&CH2CH3)3
J ()-o/,AX0
carbohvdratesuccinate (trieihyiamine salt)
-0J
LO
Figure 1. Nucleophilic acyl substitution of carbohydrates using cyclic anhydrides. Preparation of solvent exchanged swollen cellulose
Cotton cellulose (20 g) was stirred overnight in deionised water (500 ml). The mixture was suction filtered to remove excess water. The cellulose was added to methanol (500 ml), stirred for 1 hour, then filtered gently under suction. This procedure was repeated four times using methanol (4 x 500 ml). The cellulose was then added to DMAC (500 ml) and stirred for 1 hour. The mixture was filtered gently under suction and the procedure repeated five times with DMAC (5 x 500 ml). The cellulose was purged with nitrogen overnight to produce solvent exchanged cellulose. Preparation of cellulose solution (in LiCVDMAC)
DMAC (1 L) was heated to 80°C using an oil bath. Anhydrous lithium chloride (LiCl, 84 g) was gradually added to the DMAC with continuous gentle stirring until complete dissolution was achieved to afford a 9% wlw solution. The solution was allowed to cool to room temperature prior to use. LiCVDMAC (9% wlw, 1 L) was added to solvent exchanged swollen cellulose with gentle swirling, the mixture purged with nitrogen, and left overnight in a nitrogen atmosphere. DMAC (500 ml) was added to the mixture and it was purged with nitrogen and regularly swirled over several days to produce a highly viscous opaque solution (with a cellulose concentration of- 13.3
a).
Synthesis of carbohydrate ester derivatives
Carbohydrate substrates (5 g) were dissolveddispersed in LiCVDMAC (100 ml) at 80°C (or 200 ml of celluloseLiCV DMAC solution was used). The specific cyclic anhydride (figure 2) was stirred into the mixture until it dissolved (1:l molar equivalent with available hydroxyl groups). Heating was stopped. At 50°C triethylamine (TEA) was gradually added with stirring. Almost immediately the product precipitated out. DMAC was then added to the reaction mixture, which was swirled and left so that the precipitate settled to the bottom of the reaction vessel. The solvent was decanted off and the product washed firther with tetrahydrofiran (THF) until all the solvent soluble impurities were removed by suction filtration. Derivatives wcre then oven dried (6OOC) to remove residual THF.
322 Wood, fibre and cellulosic materials
no o
0
succinic anhydride
a
oBo oEo
o
maleic anhydride
.
methyimaleic anhydride [CitraCOnlC anhydride1
2,3dimethylmaleic anhydride
\
Peten-1-yl succinic anhydride
0 A O J % Pdodecen-I -yl succinic anhydride
glutaric anhydride
(CH315( CH=CH)CHj
.
octadecenyl succinic anhydrlde (mixture of isomers)
Figure 2. Cyclic anhydride reagents utilised for carbohydrate derivatisation (all purchased from Sigma-Aldrich Company Ltd., Poole, UK). Fourier transform infrared (FT-IR) spectroscopic analysis
FT-IR spectroscopic analysis of the synthesised carbohydrate ester derivatives was performed using a Nicolet Avatar 360 FT-IR Spectrometer fitted with a Graseby Specac Golden Gate attenuated total reflectance (ATR) sampling accessory. The Golden Gate sampling accessory is composed of a small diamond embedded in a carborundum plate. A small portion of sample is layered onto the diamond and contact achieved using a sapphire anvil tightened to a predetermined torque value, which depends on the nature of the sample under investigation. All resulting FT-IR spectra were manipulated using Omnic software. Proton nuclear magnetic resonance ('11-NMR)spectroscopic analysis Carbohydrate ester derivativcs (30 mg) were dissolved in deuterated dimethylsulphoxide (d6-DMS0,l ml), and a few drops of trifluroacetic acid (TFA) added to each sample in order to move the signals from any remaining unsubstituted hydroxyl protons upfield to a higher ppm value (i.e. out of the range of interest), so that subsequent integration of the desired signals could be performed using WIN'NMR software to facilitate calculation of average DS values. 'H-NMR spectra of the synthesised carbohydrate ester derivatives were collected using a Brtiker AMX 400 M H Z NMR spectrometer. Samples soluble in d6-DMSO were run using an automated sampling system at room temperature. Other samples were run manually at 80°C in order to ensure complete solubilisation. Some samples were insoluble in available deuterated solvents and could not be analysed All of the chemical shifts are reported in parts per million (ppm) using tetramethylsilane(TMS) as an internal standard..
New carbohydrate polymer derivatives
323
RESULTS
FT-IRspectroscopic analysis Confirmation that esterification had been successfbl was obtained by comparison of the FT-IR spectra of the carbohydrate ester derivatives with those of the unmodified substrates. In all cases, the FT-JR spectra of the synthesised carbohydrate derivatives contained peaks that corresponded to a carboxylate carbonyl C-0 antisymmetrical stretch (- 1610-1550cm-I), a carboxylate carbonyl C-0 symmetrical stretch (- 14201300 cm-I), and an ester carbonyl C-0 stretch (- 1750-1720cm-'). The presence of these three peaks, along with a relative reduction in hydroxyl group peak (- 3500-3200 cm-') and increase in C-H stretching peaks (- 2800-3000 cm-') compared with the unmodified substrate spectra, that confirmed esterification had been successfid. Several bands corresponding to N-H stretches for the TEA salt could also be observed (- 2400-2600 cm-') and the carbohydrate ring ether C-0 stretching (- 11501070 cm-I). An example FT-IR spectrum (for the TEA salt of maltodextrin succinate) is displayed in figure 3. All peaks were identified from absorbence values quoted by Williams and Fleming lo.
---
maltodextrin
;
q
4000
,.,".....,-.
. ..,. ....-, .... .
3 W
succinate
~
, ..,,
3030
.
. ......,...... .. . ,..,..... 2500 MM) wawnunbso(m1)
triethylamine salt
-.--..-
.-..I--
1 W
I..,.. ~
.. *---, -.- ".. I
1WXl
I
Figure 3. FT-IR spectrum of maltodextrin succinate (triethylamine salt).
*
I
500
324 Wood, fibre and cellulosic materials 'H-NMR spectroscopic analysis The resultant 'H-NMR spectra of the carbohydrate ester derivatives were integrated using WINNMR software in order to calculate the average DS values from the ratio of the starch backbone proton signals to the ester substituent protons. The starch backbone protons generally occur as a complex multiplet in the region 3-6 ppm. The signal from any unsubstituted hydroxyl groups has been shifted upfield (> 9 ppm) out of the range of interest by the addition of TFA and is usually observed as a very broad low peak. Two example NMR spectra, for yellow maize starch succinate (TEA salt) and sago starch glutarate (TEA salt) are displayed in figures 4 and 5 , respectively. In the case of the succinate derivative (figure 4) the CH2 groups (a) are equivalent, both being adjacent to a carbonyl group, and hence appear as a single triplet (- 2.4 pprn). In the case of the glutarate derivative, the CH2 groups (b) are equivalent, both being adjacent to a carbonyl group, and hence appear as a single triplet (- 2.2-2.3), however the CH2 group (a) is not adjacent to a carbonyl group and therefore appears as a pentet at 1.7 ppm. Groups next to a carbonyl group are normally located at a higher ppm value because of the electron withdrawing influence of a carbonyl group. Average DS values (calculated from the NMR integration ratios of carbohydrate backbone protons to ester side chain protons) for selected carbohydrate ester derivatives are presented in table 1.
-
0
0
--II
II
().--O-C-CH2-CH2-C-O-
a
starch
a
succinate
+
HN(CH2CH3)3
b c
triethylamine salt
starch backbone protons
Figure 4.
1
H-NMR spectrum of yellow maize starch succinate (triethylamine salt).
New carbohydrate polymer derivatives
starch
glutarate
triethylamine salt
starch anomeric (Cl) protons
Figure 5.
1
Table 1.
Average DS values for selected carbohydrate ester derivatives.
H-NMR spectrum of sago starch glutarate (triethylamine salt).
Carbohydrate ester derivative
Maltose succinate Maltodextrin succinate Yellow maize starch succinate Waxy maize starch succinate Sago starch succinate Wheat starch succinate Tapioca starch succinate Sago starch maleate Yellow maize starch maleate Sago starch glutarate Yellow maize starch glutarate [all TEA salts]
Average DS 1.09 0.51
2.36 0.20 0.18 1.23 0.74 0.91 0.78 3 .OO 2.97
325
326 Wood, fibre and cellulosic materials CONCLUSIONS
Preliminary results demonstrate that this simple organic phase reaction system can be utilised on a homogeneous or heterogeneous basis to synthesise a variety of multifunctional carbohydrate ester derivatives with a wide range of DS values. Many of the carbohydrate esters produced could be analysed using 'H-NMR, however there were a few derivatives that could not be characterised using this technique as they were insoluble in available deuterated solvents. Further investigations are required to carry out NMR analyses using alternative solvents in which the carbohydrate esters are soluble. The starch or cellulose backbone of the derivatives could be partially hydrolysed using DCI (in deuterated DMSO) in order to make the sample more soluble. Ester derivatives prepared using cyclic anhydride derivatives have the added bonus of carboxylate functionality, and therefore their physicochemical properties can be modified by simply changing the counter ion. The derivatives presented in this work have all been isolated as the TEA salt, however, they could easily be converted to the free acid or sodium salt which would have a considerable effect on their solubility profiles. Selected derivatives will be assessed to determine suitability for specific industrial applications, e.g. as surfactants and their suitability as potential surface coating components. A more detailed paper covering other derivatives, oustanding characterisations and assessment of physicochemical properties (e.g. solubility profiles, film formation, etc) will be presentcd at a hture Cellucon conference. REFERENCES 1. G. Fleche, Chemical modification and degradation of starch, In: Starch Conversion
2.
3.
4. 5.
6. 7.
Technology, G. M. A. Van Beynum and J. A. Roels, J. A. (eds.), Marcel Dekker, New York, 1985, pp. 73-99. M. W. Rutenberg & D. Solarek Starch derivatives: production and uses, In: Starch: Chemistry and Technology, 2"dEd, R. L. Whistler, J. N. BeMiller & E. F. Paschal1 (eds.), Academic Press, Orlando, 1984, pp. 3 11-388. J. W. Mullen & E. Pascu, Starch studies: preparation and properties of starch triesters, Ind. Eng. Chem., 1942,2f?, 1209-1207. J. W. Mullen & E. Pascu, Starch studies: possible industrial utilisation of starch esters, Ind. Eng. Chem., 1943,s, 381-384. M. M. Tessler & R. L. Billmers, Preparation of starch esters, J. Environ. Polym. Deg., 1996,4, 85-89. I. A. Wolff, D. W. Olds & G. E. Hilbert, Triesters of corn starch, amylose and amylopectin: properties, Ind. Eng. Chem., 1951,a, 91 1-914. I. A. Wolff, D. W. Olds & G. E. Hilbert, Mixed esters of amylose, Ind. Eng. Chem., 1957,48,1247-1248.
T. P. Nevell & S . H. Zeronian, Cellulose Chemistiy and its Applications, Ellis Horwood, Chichester, 1985. 9. C. L. McCormick & T. R. Dawsey, Preparation of cellulose derivatives via ringopening reactions with cyclic reagents in lithium chloride/ N,N-dimethylacetamide, Macromolecules, 1990,2,3606-3610. 10. D. H. Williams & I. Fleming, SpectroscopicMethods in Organic Chemistry, 5IhEd., McGraw-Hill, London, 1995. 8.
THERMAL AND VISCOELASTIC PROPERTIES OF CELLULOSE- AND LIGNIN-BASED POLYCAPROLACTONES H Hahkeyama', T Yoshida', S Hirose' and T Ihtakeyama' I Fukui
Universiv of Technology,3-6-1 Cakuen, Fukui, Fukui 910-8505,Japan. 'National Insrirure ofMaterials and Chenrical Research, 1-1 IIigashi Tsukuba, Ibaraki 305-8565 Japan.
'Otsuma Women's Universiv, 12 Sanbancho, Chiyoda-ku, Tokyo 102- 8357, Japan ABSTRACT Polycaprolactone (PCL) derivatives were newly synthesized from cellulose acetate (CA) and lignins such as alcoholysis lignin (AL) and &aft lignin (KL). Thermal and viscoelastic properties of the obtained polycaprolactoncs (CA-, AL and KLPCL's) were studied. PCL's were synthesized by the polymerization of E-caprolactone (E-CL) which was initiated by each of the OH groups of cellulose and lignin molecules. The amount of E-CL was varied from 1 to 25 mols / each OH group. A marked change in bascline due to glass transition was obscrvcd in each DSC curve. Tg decreases with increasing CUOH ratio in PCL's, since caprolactonc chains act as soft segments in PCL molecular chains. CA-PCL showed a-,fl-and y-dispersions in dynamic mcchanical analysis. Thermal degradation temperatures (Td's) of PCL's increascd with increasing CL / OH ratio. From this rcsult, it is considered that the thermal degradation of CA-, A L and KLPCLYs with increased PCL chain length occurs with more difficulty than in plant cornponcnts such cellulose and lignin.
INTRODUCTION Since natural polymers are biodegradable and can be circulated in the ecological system, various research groups '-I3) have tried extensively to synthesize polymers which can be derived from plant components such as saccharides and lignin. We have synthesized various types of polyurethancs (PU's) which have plant components in the PU molecular chains. We have also found that their mechanical and thermal properties can be controlled by appropriate molecular design. The PU's were biodegraded by microorganisms when they were placed in soil .?'In our rcccnt study, ") polycaprolactone (PCL) derivatives were synthesized from saccharides such as glucose, fructose and sucrose. Polyurethane sheets were also prepared from the above PCL derivativcs by the reaction with diphenylmcthanc diisocyanate (MDI). In the present study, polycaprolactone (PCL) derivatives wcrc synthcsized from cellulose acctate (CA) and lignins such as alcoholysis lignin (AL) and Kraft lignin (KL). Molecular properties of the obtained polycaprolactoncs (PCL's) were studied by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), thcrrnogravimetry (TG) and TGFourier transform infrared spectromcty (FTIR).
328
Wood, fibre and cellulosic materials
EXPERIMENTAL Materials and sample preparation AL and KL were kindly provided by Repap Co. and Westvaco Co. The provided lignins were used without further purification. Cellulose acetate ( C A acetyl content, 39.87 %; M, = 6.32~10~; M A , ,= 2.27), E-caprolactone (E-CL) and dibutyltin dilaurate (DBTDL) were commercially obtained. PCL's were synthesized by the polymerization of E-CL which was initiated by each of the OH groups of cellulose and lignin molecules. The amount of E-CL was varied from 2 to 20 mol / each OH group in the case of CA-PCL and from 1 to 25 mol / each OH group in the case of AL-PCL and JLPCL. The polymerization was carried out for 12 hr at 150 "C with the presence of a small amount of dibutyltin dilaurate (DBTDL).
Measurements Differential scanning calorimetry (DSC) was performed using a Seiko DSC 220 at a heating rate of 10 "C / min under a nitrogen flow (flow rate = 30 ml/min). Sample mass was ca. 5mg. Aluminum open pans were used. The samples were heated to 120 "C and quenched to -150 "C. DSC heating curves were usually measured at 10 "C/min. However, when the coordination with the heating rate of DMA is needed, DSC heating curves were measured at 2 "C/min. The melting temperature (Tm),melting enthalpy (M,,,), cold crystallization temperature (T=),glass transition temperature (Ta and heat capacity gap (AC,,) were determine by the method reported previously "). Dynamic mechanical properties of PCL's derived from C A (CA-PCL's) were measured using a Seiko dynamic mechanical analyzer DMS 210 equipped with a stretching module. Sample size as follows: width 8 mm, length 20 m m and thickness 0.2 0.7 mm. Temperature was varied from -150 to 100 - 150 "C. The maximum temperature was chosen dcpending on rigidity of each sample. The heating rate was 2 "C/min. Frequency was varied at 0.1, 1.0, 2.0,3.0, and 5.0 Hz, respectively. Dynamic modulus (E'), dynamic loss (E") and tan S were calculated using the equipped software. Activation energy of each dispersion was calculated using the equipped software and/or manual calculation. Thermogravimetry (TG) was carried out in nitrogen flow (flow rate = 200 ml/min) using a Seiko TG 220 at a heating rate of 20 "C / min in the temperature range Gom 20 to 800 "C. Sample mass was ca. 5mg. TG curves and derivatograms were recorded. Mass loss was calculated according to the equation: [(m,/mm)/m20]x 100 (%) where m, is mass at temperature T and mm is mass at 20 "C. Gasses evolved by thermal degradation were analyzed by TG-FTIR using simultaneously a Seiko TG 220 and a JASCO FTnR-420. The heating rate was 20 "C/min in the temperature range from 40 to 800 "C. The flow rate of carricr nitrogen gas was 100 ml/min and the sample weight was 7 to 10 mg. The gas transfer system was maintained at 270 "C. The resolution power of FI'IR was 4 cm-', The number of integration was ten and the data incorporation time was 30 s.
-
Thermal and viscoelastic properties of polycaprolactones 329 RESULTS AND DISCUSSION Figs. 1 shows the phase diagram of CA-PCL's with various CUOH ratios from 2 to 20. As shown in Fig. 1, the glass transition temperature (Tg ) of CA part (Tg1 )in CA-PCL is observed in the initial stage. The Tg of CA observed in this study is 147 "C. It bccomes difficult to detect when CUOH ratio exceeds 15 mol/mol. Tg of PCL part (Tg2) in CA-PCL decreases with increasing CUOH ratio when that in CA-PCL's is below 10, since PCL chains act as soft segments in CAPCL. This softening effect of caprolactone chains was enhanced progressively with increasing chain length of CL chains when CUOH ratio is below 10. However, as also shown in Fig. 1, in the case of CA-PCL's with CWOH ratio over 8, melting of crystalline region is observed. This suggests that CAPCL's with long PCL chains (CUOH ratio is over 8) becomes rigid, since the long PCL chains take regular arrangement, form crystalline structure and restrict the main chain motion. This causes the increase of Tg of CA-PCL(Tg2), when CUOH ratio is over 10. The melting temperature (Tm) of CA-PCL increases with increasing CUOH ration when it is over 8. Fig. 2 shows the phase diagram of A L and KLPCL's with various CUOH ratios from 1 to 25. As shown in Fig. 1, Tg of A L and KLPCL's decreases with increasing CUOH ratio when CUOH ratio is below 10. This softening effect of caprolactone chains was enhanced progressively with increasing chain length of CL chains when CWOH ratio is below 10. However, Tg increases with increasing CUOH ratio when CUOH ratio becomes over 10. Melting of crystalline region of A L and KLPCL's is observed when CWOH ratio is over 2. The above results suggest that the incorporation of PCL chains into cellulose and lignin structures leads to the enhancement of molecular motion of PCL derivatives when the chain length of PCL is below a certain critical value which is specific to cellulose or lignin core structure. However, when the PCL chain becomes over a certain length, a prominent peak of melting of crystals is observed at a specific temperature which is characteristic to each core plant component such cellulose and lignin, respectively in the DSC curve, and the Tm increases with increasing PCL molecular chain. The above facts suggest that the long PCL chain easily takes regular molccular arrangements and form crystalline region. Fig. 3 shows DMA heating curves of CA-PCL with CUOH ratio of 5 measured various frequencies. The dynamic modules E' and tan 6 are clearly observed. From the high to low temperature side, each E' decrease is designated as a-dispcrsion and fl-dispersion, respectivcly. Tan 6 shows a large and broad peak at around -20 "C and a small and broad peak bclow -100 "C. Fig. 4 shows the relationship between frequency and reciprocal temperature corresponding to a-, fl- and y-dispersions. All CA-PCL samples show a-dispersions at almost the same temperature region, but the temperature region of fl-dispersion depends on each sample. The y-dispersion is observed as a small peak at around -120 when CWOH ratio is 5. Fig. 5 shows the relationship between CUOH ratio, thermal degradation tempcrature (Td) and mass residue (WR) of CA-PCL. The Td of CAPCL increased from ca. 350 "C to 390 "C
330 Wood, fibre and cellulosic materials
0
80
-
40
-
0
I-'
0 -
5
0
.15
10
20
25
C V O H Ratio/(mol/mol)
Figure 1. Phase diagram for CA-PCL A; T g 1 , A ; T g 2 , 0 ; Tm
-10
' 0
5
10 '
15
20
25
CL / OH Ratio/(mol/mol)
Figure 2. Phase diagram for A L and KLPCL's 0 ; KLPCL Tgl B; KL-PCL Tm 0 ;ALPCL Tgl 0;AL-PCL T m
30
Thermal and viscoelastic properties of polycaprolactones 33 1 1. ox 10'
0. 40
I . ox 10'
0. 30
. I
,"
'0
1.0XlO'
4
0.20
1.OXlO'
9
0. t o
1. ox 10' -110.0
0. 00
50. 0
-10.0
110.0
Figure 3. E" and tan 6 for CA-PCL (CUOH ratio = 5 moYmol) Numerals in the figure show frequency
1.5
-2
1
0.5
v
'Mz o 0
-0.5
-1 -1.5 2
3
4
5
6.
7
1000/T / K-'
Figure 4. Arrhenius plots for CA-PCL,(CUOH ratio = 5 moYmol)
332 Wood, fibre and cellulosic materials 400
20
380 15
360
;F
10
340 320
0
300
I
I
0
5
I
-
5
I
10 15 20 C Y O H Ratio/ (mol/m ol)
' 2"
0
25
Figure 5. Change of T,and WR with CUOH ratio for CA-PCL ;T d
0;W R
with increasing CUOH ratio. From this result, it is considered that the thermal degradation of CAPCL with increased chain length seems to occur with more difficulty, since the ratio of thermally unstable cellulose structure in CAPCL decreased comparatively with increasing CUOH ratio. The WR at 450 "C decreases with increasing CUOH ratio, suggesting that the CA part in CAPCL constitutes a significant part of the residual products. Fig.6 shows the relationship between CUOH ratio, thermal degradation temperature (Td ) and mass residue (WR) of A L and KLPCL. The Td of CAPCL increased with increasing CUOH ratio. This suggests that the thermal degradation of A L and KL-PCL's seems to occur with more difficulty with increased chain length, since the ratio of thermally unstable lignin structure in lignin PCL derivatives decreased comparatively with increasing CUOH ratio. The WR at 450 "C decreases with increasing CUOH ratio, suggesting that the lignin part constitutes a significant part of the residual products of A L and K L PC L Fig. 7 shows the stacked FTIR spectra of gases evolved at various temperatures during the thermal degradation of AL-PCL (CIJOH ratio = 20). The main peaks observed for the samples are as follows: wavenumber assignment; 1126 cm-' (vC-0-), 1260 cm" (v-C(=o)C), 1517 and 1617 cm" (vC=C), 1718 cm.' (vC=O), 2345 cm-' (vCO,), 2892 cm" (vC-H) and 3700 cm'l (vH,O).
Thermal and viscoelastic properties of polycaprolactones 333 400
1
390
-
380
-
,o \
370
-
360
-
I2
350
-
I 40 40
A
--
p
-
340
30 a? \
20 a 5 10
0 0 0
5
1155 20 CL / OH Ratio/(mol/mol) 10
25
30
Figure 6. Change of Tdand WR with CUOH ratio for AL and KLPCL's A; KLPCL TJT, A; ALPCL TfC, ;KLPCL WW%, 0;ALPCL WR/%
Figure 7. TG-FTIR stacked curves for ALPCL
334 Wood, fibre and cellulosic materials
0.1 u)
m Q
0.05
0 0
20
10
30
CL / OH Ratio/(mol/mol)
Figure 8. Relationship between IR absorption intensity for AL-PCL and CUOH ratio OiC-O-C, O;C=O, O;CO,, A;CH
0.15
0.1 u)
A-
m Q
0.05
-
-+%+
A
v
0
+
A-
30
20 C V O H Ratio/(mol/mol)
0
10
Figure 9. Relationship between IR absorption intensity for KLPCL and CUOH ratio
0 ;C-0-C,
M; C=O,
+; CO,,
A;CH
Thermal and viscoelastic properties of polycaprolactones 335 Figs.8 and 9 show the changes of intensities of characteristic IR absorption peaks of evolved gases from AL- and KL-PCL‘s. The changes of IR absorption intensities for A L and KLPCL’s are almost similar. The evolution of CO, gas is prominent, as shown in Fig. 8. However, the IR absorption intensity of CO, gas from lignin based PCL’s do not show the PCL chain length dependency. This suggests that the cvolution of CO, gas occurs randomly and is not specific to the chemical structure. On the other hand, the IR absorption intensities of C-OX, C=O and CH increase prominenlly with increasing CL/OH ratio. This suggests that gascs having C-0-C, C=O and CH groups are evolved from PCL chains of lignin-based PCL’s. The above facts well accord with the decrease of the mass residue, WR, with increasing PCL chain length.
CONCLUSIONS The decrease of the Tg of cellulose- and lignin-based PCL’s with increasing CWOH ratio is caused by the fact that the PCL chains act as soft segments. When thc PCL chain becomes over a certain length, PCL chains take regular arrangements and crystallization occurs. Melting of crystalline structure occurs at a specific temperature which is characteristic to each core plant component such as cellulose and lignin. Since thermal degradation temperatures (Td’s) of PCL derivatives increased with increasing C L / OH ratio, it i s considered that the thermal dcgradation of CA-, AL- and KLPCL’s with increased chain length seems to occur with more difficulty than in plant components such as cellulose and lignin. The results obtained by TG-FTIR analysis of CA-PCL showed that gases with OH, CH, G O ,C-0-C groups were mainly evolved by thermal degradation.
REFERENCES
1. V.P. Saraf and W.G. Classer, J. Appl. Polym. Sci., 1984,29,1831. 2. V.P. Saraf and W.G. Glasscr, J. Appl. Polym. Sci., 1984,30,2207. 3. K. Nakamura, R. Morck, A. Reimann, K.P. Kringstad and H. Hatakeyama, Polym. Adv. Technol., 1991,2,41. 4. K. Nakamura, T. Hatakeyama and H. Hatakeyama, Polym. Adv. Technol., 1992,3,151. 5. H.Yoshida, R. Morck, K.P. Kringstad and H. Hatakeyama, J. Appl. Polym. Sci., 1990, 40, 1819. 6. K. Nakamura, Y. Nishimura, T. Hatakeyama and H. Hatakeyama., Preparation of Biodegradable Polyurethanes Derived from Coffee Grounds, in Proceedings for International Workshop on Environmentally Compatible Matcrials and Recycling Technology in Tsukuba, Japan, Novembcr 15-16,1993, pp. 239 7. H. Hatakeyama, S. Hirose, K. Nakamura and T. Hatakeyama, New types of Polyurethanes Derived from Lignocellulose and Saccharides, in Cellulosics: Chemical, Biochemical and Material Aspects, J . F. Kennedy, G. 0. Phillips and P. A. Williams, Eds., Ellis Horwood, 1993, pp. 381. 8. H. Yoshida, K. Kobashigawa, S. Hirose and H. Hatakeyama, Molecular Motion of Biodegradable Polyurethanes Derived from Molasses, in Proceedings for International Workshop on Environmentally Compatible Materials and Recycling Tcchnology in Tsukuba, Japan, Novembcr, 1993, pp. 15.
336 Wood, fibre and cellulosic materials 9. N. Morohoshi, S . Hirose, H. Hatakeyama, T. Tokashiki and K. Teruya, Sen-i Gakkaishi, 1995,51,143. 10. T. Nakamura, Y. Nishimura, P. Zctterlund, T. Hatakeyama and H. Hatakeyama, Thermochimica A m , 1996, 2821283,433. 11. P. Zetterlund, S. Hirose, T. Hatakeyama, H. Hatakeyama and A-C. Albertsson, Polymer International, 1997,42, 1. 12. M. J. Donnely, Polymer Internalional, 1992,37,297. 13. H. Hatakeyama, K. Kobashigawa, S . Hirose and T. Hatakeyama, Macromol. Symp., 1998,130,127. 14.T. Hatakeyama and F. X. Quinn, Thermal Analysis, John Wiley & Sons, Chichester 1994, pp.65.
INDEX absorption, of water 305-17 Acetobacter xylinurn 3-12,23, 122 acid group determination 111-19 acid-base characteristics, of fibres 188 adhesive bridges to wood 291-303 aerobic incubator 4-10 aging, of fibre surfaces 197-203 alditol acetates 227 algal cellulose 34-8 alkaline pulping 91-4 anion exchange 26 anthraquinone 91-102 application, of polymer derivatives 31926 bacterial cellulose 3-16 bacterial endoglucanases 81-86 beating, of fibres 249-60 beating, pulp fibre 159-65 biobleaching 56 bioresources 319-26 black liquor 149-57 blackening, of fibres 235-45 bleaching 149-57 bleaching, enzymic 55-60 bleaching, oxygen 95-102 bleaching, ozone 137-47 caprolactones 327-36 carbohydrate, oil palm trunk 227-34 carbohydrates, wood 91-102, 126 carboxyl groups 129-35 catalysis, of delignification 103-7 cellobiose 93 cellulase activity 71 cellulases, in pulp processing 69-80 cellulose acetate 327-36 cellulose binding domains 72 cellulose II, supramolecular structure 33-8 cellulose polymorphs 121-7 cellulose, algal 34-8 cellulose, bacterial 3-12, 36, 37
cellulose, composite material 305-17 cellulose, crystallinity of 39-44 cellulose, from oil palm waste 13-17 cellulose, sago 19-22 cellulose, specific mass 39-44 cellulose-based polycaprolactones 327-36 cellulosic filament 3-12 carboxymethyl cellulose 81-6 charged groups 109-19 chemical derivatives, of polymers 31926 chemical pulp 75 chemistry, of adhesive bridges 291-303 chemometrics 33-8 chlorine dioxide 149-57 Cladophora 33 composite, sorbants 305-17 13CP-CP/MAS-NMR33-8, 39-44, 121-7 crystallinity, of cellulose 39-44 cultivation, of Acetobacterxylinum 3-12 curl index 141 degree of polymerisation 92 de-inking 76 delignification 91, 103-7 derivatives, of polymers 319-26 dielectric constant, of paper 267-75 distribution coefficients 100 dry fractionation, of fibres 261-66 drying stress, of paper 255 elastic modulus, paper 249-60 elastic properties, of fibre 267-75 electrical resistance 272 endoglucanases 81-6 enzyme assay 62 enzymes, in pulp processing 69-80 enzymic bleaching 55-60 enzymic hydrolysis 27, 30
338
Index
eucalypt fibres 181-96 fibre blackening 235-45 fibre length 142 fibre structure 209-25 fibre surfaces 197-203 fibre, flax structure 169-79 fibre, hardwood 181-96 fibre, in paper 235-45, 249-60 fibre, softwood 209-25 fibres properties 137-47 fibres, beating 159-65 fibres, dry fractionation of 261-6 fibres, from palm trunk 227-34 fibres, viscose-Kraft mixtures 267-75 filament, cellulosic 3-12 flax, fibre structure 169-79 fluorescence, of fibre surfaces 197-203 fractionation, dry 261-6 FTIR, of flax fibre 169-79 fungal endoglucanases 81-6 GC analysis 27 glucomannans, in paper 277-88 grass, fibre 261-6
handsheets, of viscose-Kraft mixtures 267-75 hardwood fibres 181-96 heat, effect on fibres 197-203 HPLC 25 industrial biopolymers 319-26 kink index 141 Kraft fibre, in paper 249-60 Kraft lignin, derivatives, 327-36 Kraft pulp 277-88 Kraft, surface energy 169-79 Kraft, viscose mixture fibres 267-75 light microscopy, of pulp fibres 205-8 lignin, of hardwood 185 lignin-based polycaprolactones327-36 mechanical pulping 73
mechanical pulps 109-19 mercerisation 33 metabolism, in pulping 95-102 methylate 26 microcrystalline cellulose 13-17, 19-22 molasses 23-31 monosaccharide analysis 25 morphology, of fibre surfaces 197-203 morphology, of flax fibre 169-79
NMR 39-44 NMR, of flax fibre 169-79 odour transfer 152 oil palm wastes 13-17 oil palm, trunk fibres 227-34 oligosaccharide analysis 25 optical properties, of fibre surfaces 197-203 organic solvents 91-4 organosolve pulping 91, 103-7 oxygen bleaching 95-102 oxygen-acetone delignification 103-7 ozone 149 ozone bleaching 137-47 palm trunk, fibres 227-34 paper density 267-75 paper industry 61-68 paper properties 144 paper, fibres 249-60 paper, from grass, 261-6 paper, Kraft fibres in 249-60 paper, processing 69-80 paper, structure 235-45 paper, wood resin in 277-88 paper, strength 277-88 papermakingfibres 109-19 papers, supercalendered 235-45 pectic acids, in paper 277-88 peroxyacetic acid 149-57 phenol groups 129-35 polycaprolactones327-36 polymorphism, cellulose 121-7 polysaccharides, in paper 277-88 pores, in flax fibre 169-79
Index 339 porous cellulose composites 305-17 protein assay 62 pulp fibre 159-65 pulp industry 61-68 pulp, cellulases in 69-80 pulp, for reinforcement 209-25 pulp, from grass, 261-66 pulp processing 69-80 pulp, sponges from 305-17 pulp, surface energy 169-79 pulp, wood 55-60 pulping conditions 261-66 pulping, alkali 91-102 pulping, effect on crystallinity 39-44 pulps, Kraft 129-35, 137-47, 150-7, 163-5 pulps, polymorphs in 121-7 pulps, woods and mechanical 109-19 reed canary grass 261-6 reflectance, of fibre surfaces, 197-203 reinforcement pulp 209-25 renewable bioresources 319-26 resin, wood 277-88 rheology of wood 291-303 rheology, of cellulose composites 30517 rupture surfaces, in wood 291-303 sago cellulose 19-22 scanning electron microscopy 7, 9 SEM, of flax fibre 169-79 SEM, of wood rupture 291-303 shrinkage, of paper 255 size-exclusion chromatography 55-60 softwood fibre, strength 209-25 softwood Kraft pulp 249-60 solubility parameter 48 specific mass, of cellulose 45-51 specific viscosity 85, 86 sponges, cellulosic 305-17 strength, of fibres 205-8 structure, in paper 235-45 structure, of cellulose II 33-8 structure, of fibre 209-25 structure, of pulp 305-17
sugar cane molasses 23-31 supercalendered papers 235-45 supramolecular structure 33-38 supramolecular structure, of flax 16979 surface chemistry, of hardwood fibres 181-96 surface energy, of hardwood fibres 169-79 surface extractives, pulp 185 surfaces, in wood 291-303 swelling, of fibres 205-8 swelling , of pulp fibres 205-8 tear index 145 tensile index, of paper 277-88 tensile strength 144 thermal properties 197-203, 327-36 thermomechanical pulp 115,277-88 thermophilic endoglucanases 81-6 thermostable xylanases 61-8 two-phase equilibria 95-102 UV-VIS reflectance, of fibre surfaces 197-203 viscoelectric properties 327-36 viscometric measurement 3, 6 viscose-Kraft fibre, mixtures 267-75 viscosity, specific 85,86 water absorption, of fibre 267-75 WAXS, of flax fibre 169-79 wet pressing of fibres 249-60 wide angle x-ray diffraction 3, 5 wood carbohydrates 91-102 wood pulps 55-60, 109-19 wood resin 279-88 wood, rupture surfaces 291-303
XPS, of hardwood fibres 181-96 xylan 61-8 xylanase 58, 61-8 zero-span index 142, 143 Zoogloea sp. 23-31
CELLULOSIC PULPS, FIBRES AND MATERIALS
CELLULOSIC PULPS, FIBRES AND MATERIALS Editors: JOHN F KENNEDY Director of the Birmingham Carbohydrate and Protein Technology Group, School of Chemistry, The University of Birmingham, and Chembiotech Laboratories, The University of Birmingham Research Park, Birmingham, UK and Professor of Applied Chemistry, The North East Wales Institute of Higher Education, Wrexham, Clwyd, Wales, UK
GLYN 0 PHILLIPS Chairman of Research Transfer Ltd Professorial Fellow and formerly Executive Principal of the North East Wales Institute of Higher Education, Wrexham, Clwyd, Wales, UK Formerly Professor of Chemistry, The University of Salford, UK PETER A WILLIAMS Head of the Multidisciplinary Research and Innovation Centre, and The Centre of Expertise in Water Soluble Polymers, and Professor of Polymer and Colloid Chemistry, The North East Wales Institute of Higher Education, Wrexham, Clwyd, Wales, UK Guest Editor: BRUNO LONNBERG Professor of Pulping Technology, and Head of Laboratory of Pulping Technology, Faculty of Chemical Engineering, Abo Akademi University, AbolTurku, Finland
W O O D H E A D P U B L I S H I N G LIMITED
Published by Woodhead Publishing Ltd Abington Hall, Abington, Cambridge CB 1 6AH, England www.woodhead-publishing.com First published 2000 0 2000, Woodhead Publishing Ltd The authors have asserted their moral rights
Conditions of sale All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. While a great deal of carc has been taken to provide accurate and current information, neither the authors, nor the publisher, nor anyone else associated with this publication shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. British Library Cataloguing in Publication Data A cataloguc record for this book is available from the British Library.
ISBN 1 85573 421 4
Printed in Great Britain by Antony Rowe Ltd, Chippcnham, Wiltshire
Contents xi
Preface PART 1: NEW SOURCES, STRUCTUREAND PROPERTIES OF CELLULOSE . 1. Continuous harvest of cellulosic filament during cultivation of Acetobacter Xylinum . S Tokura, H Tamura, M Takai, T Higuchi and H Asano Oil palm (EZueis guineensis) wastes as a potential source of cellulose 2. M A M Noor and H Sarip Isolation and characterisation of sago (Metroxylon Sugu) cellulose . 3. A Mohd Zahid, M D Modh Zulkali and B M N Azemi A highly cellulosic exopolysaccharideproduced from sugarcane 4. molasses by B Zoogloea sp . M Paterson-Beedle. L L Lloyd, J F Kennedy, F A D Melo and V Medeiros The supramolecular structure of cellulose 11. Studies with 5. 13C-CP/MAS-NMRand chemometrics H Lennholm Effects of pulping on crystallinity of cellulose studied by solid state 6. NMR T Liitia, S L Maunu and B Hortling 7. On the specific mass of cellulose and the cellulose-water system . J Chirkova, B Andersons and I Andersone
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PART 2: APPLICATION OF ENZYMES TO PULP, FIBRJ3S AND CELLULOSE . 8. Application of size-exclusion chromatography to enzymatic bleaching of wood pulp . T Eremeeva, M Leite, T Bykova, A Treimanis and U Viesturs Thermostable xylanases and their potential application in paper 9. and pulp industries . M K Bhat, S Kalogiannis, N A Bennctt. P Biely, D E Beevcr and E Owen 10. Cellulases in pulp and paper processing L Viikari, T Oksanen. A Suurnakki, J Buchert and J Pere 11. Mode of action of thermophilic bacterial and fungal endoglucanases on carboxymethyl celluloses . M K Bhat, S Bhat. N J Parry, J F Kennedy, C J Knill, D E Beever and E Owen
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vi
Contents
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PART 3: PULP PRODUCTION AND PROCESSING. 12. The effect of anthraquinone on wood carbohydrates during . alkaline pulping in aqueous organic solvents M F Kiryushina, M I Ermakova, A S Olefirenko, E-M Bennacer, T G Fedulina, A B Nikandrov and M Ya Zarubin 13. Two phase equilibria of metal ions in pulping unit operations: . from impregnation to oxygen bleaching J Karhu, P Snickars, L Harju and A Ivaska 14. Catalysis of oxygen-acetone delignification I Deineko and I Deineko 15. Charged groups in wood and mechanical pulps . B Holmbom, A V Pranovich, A Sundberg and J Buchert 16. The investigation of cellulose polymorphs in different pulps using I3C CPMAS NMR S Maunu, T Liitia, S Kauliorniiki, B Hortling and J Sundquist 17. Characterization of carboxyl and phenol groups in kraft pulps at different temperatures . J Karhu, P Forslund, L Harju and A Ivaska 18. Effect of ozone bleaching on the fibre properties of pine and birch kraft pulp A Seisto, K Poppius-Levlin and A Fuhrmann 19. Studies on the use of black liquor evaporation condensates a t different bleaching stages . K Niemela, R Saunamaki and R Rasimus 20. Evaluation of pulp fibre beating B Lonnberg, T Lundin, K Harju and P Soini
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PART 4: STRUCTURE AND PROPERTIES OF FIBRES . 21. Study of flax fibre structure by WAXS, IR and "C NMR spectroscopy, and SEM N E Kotelnikova, E F Panarin, R Serimaa, T Paakkari, T E Sukhanova and A V Gribanov 22. Evaluating the surface energy of hardwood fibres using the Wilhelmy and inverse gas chromatography methods . W Shen, Y J Sheng and I H Parker 23. Heat-induced changes in fibre surfaces I Forsskihl, T Korhonen and H Tylli 24. Investigation of spruce pulp fibres by swelling experiments and light-microscopy . B Hortling, T Jousimaa and H-K Hyvarinen 25. Role of softwood fibre form and condition on its reinforcement capability . K Ebeling 26. Compositional analysis of oil palm trunk fibres . P F Akmar, M N M Yusoff, J F Kennedy and C J Knill . 27. Fibre blackening in supercalendered papers T Koskinen
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. 205 . 209
. 227 .
235
Contents vii PART 5: PAPER FIBRE PRODUCTION AND PROPERTIES 28. Kraft fibers in paper effect of beating . K Niskanen 29. Effect of dry fractionation on pulping conditions and fibre propertics of reed canary grass M Finell, B Hedman and C-A Nilsson 30. Electrical propertics of viscose-kraft fibre mixtures . S Simula and K Niskanen 31. Effects of retained wood resin and polysaccharidcs on . paper properties A Sundberg. B Holmbom, S Willfor and A Pranovich
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PART 6: WOOD, FIBRE AND CELLULOSIC MATERIALS 32. Appearance of rupture surfaces in wood . K NygHrd, R Gyllenberg, B Lonnberg and G Gros 33. Composite materials from pulp and papermaking wastes V Lapsa, T Betkers and G Shulga 34. Cellulose composite materials as sorbents sorption and . rheological properties S Ciovica, B LZjnnberg and K Lonnqvist 35. New carbohydrate polymer derivatives from renewable bioresources targeted for industrial application . C J h i l l , S F Rahman and J F Kenncdy 36. Thermal and viscoelastic properties of cellulose- and lignin-based polycaprolactones H Hatakeyama, T Yoshida, S Hirose and T Hatakeyama
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Index.
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. 267 . 277 . 289 ,
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MJ
THE CELLUCON TBUST incorporating
CELLUCON CONFERENCES International Educational Scientific Meetings on Cellulose, Cellulosics and Wood
Cellucon Conferences as an organisation was initiated in 1982, and Cellucon '84, which was the original conference, set out to establish the strength of British expertise in the field of cellulose and its derivatives. This laid the foundation for subsequent conferences in Wales (1 986), Japan (1 988), Wales (1 989), Czechoslovakia (1 990)' USA (1991), Wales (1992), Sweden (1993)' Wales (1994), Finland (1998), and Japan (1999). They have had truly international audiences drawn from the major industries involved in the production and use of cellulose pulp and fibre derivatives of cellulose, plus representatives of academic institutions and government research centres. This diverse audience has allowed the cross-fertilisation of many ideas, which has done much to give the field of cellulose in its diverse forms the higher profile that it rightly deserves. Cellucon Conferences are organised by The Cellucon Trust, an official UK charitable trust with world-wide objectives in education in wood and cellulosics. The Cellucon trust is continuing to extend the knowledge of all aspects of ccllulose world-wide. At least one book has been published from each Cellucon Conference as the proceedings thereof. This volume arises from the 1998 conference held in TurkdKbo, Finland and the conferenccs planned to be hcld in Japan, Walcs, etc, will generate further useful books in this area. THE CELLUCON TRUST TRUSTEES AND DIRECTORS
Prof. G.O. Phillips (Chairman) Prof. J.F. Kennedy (Deputy Chairman and Treasurer) Prof. P.A. Williams (Secrctary General)
Research Transfer Ltd, UK The North East Wales Institute, UK, and The University of Birmingham, UK The North East Wales Institute, UK
THE CELLUCON TRUST is a registered charity, UK Registration No: 328582 and a company limited by guarantee, UK Registration No: 2483804 with is registered offices at Chembiotech Laboratories, The University of Birmingham Research Park, Vincent Drive, Birmingham, B15 2SQ, UK.
ACKNOWLEDGEMENTS w i 4 W y - w ’
The 10 Intcrnational Cellucon Conference
CELLUCON ‘98
This book arises from the International conference - CELLUCON ’98 - which was held at the Mauno Koivisto Centre, Biocity, TurkdAbo, Finland. This meeting owed its success to the invaluable work of the Organising Committee and its generous sponsors. SPONSORS OF CELLUCON 98 City of TurkdAbo, Finland DataCity Centcr (since August 1999: T u r k Technology Centre), TurkdAbo, Finland
PULP FOR PAPERMAKING Fibre & Siirface Properties & Other Aspects of Cellulose Technology
Abo Akademi University, TurkdAbo, Finland
MEMBERS OF TIlE ORGANISING COMMITTEE - CELLUCON ’98 Prof. Bruno Lonnberg, Laboratory of Pulping Technology, Abo Akademi University, TwkdAbo, Finland (Chairman)
h4s Outi Rapila, Laboratory of Pulping Technology, Abo Akademi University, TurkdAbo, Finland (Conference Secretary) Ms Agneta Hermansson, Abo Akademi University, TurkdAbo, Finland (Treasurer) Prof. Raimo A h , Laboratory of Applied Chemistry, University of Jyviskyla, Jyviskylii, Finland Prof. John F. Kennedy, Birmingham Carbohydrate and Protein Technology Group, School of Chemistry, The University of Birmingham, Birmingham, UK Dr Charles J. Knill, Birmingham Carbohydrate and Protein Technology Group, School of Chemistry, The University of Birmingham, Birmingham, UK
Prof. Jar1 Rosenhoh, Department of Physical Chemistry, Abo Akademi University, TurkdAbo, Finland Prof. Per Stcnius, Laboratory of Forest Products Chemistry, Helsinki University of Technology, ESPOO,Finland Prof. Jorma Sundqvist, Finnish Pulp and Paper Research Institute (KCL), ESPOO,Finland Prof. Liisa Viikari, VTT Biotechnology, ESPOO,Finland
THE CELLUCON CONFERENCES
1984 Cellucon '84 UK
CELLULOSE AND ITS DERIVATIVES Chemistry, Biochemistry and Applications
1986 Cellucon '86 UK
WOOD AND CELLULOSICS Industrial Technology, Biotechnology, Structure and Properties
1988 Cellucon '88 Japan
CELLULOSICS AND WOOD Fundamentals and Applications
1989 Cellucon '89 UK
CELLULOSE: SOURCES AND EXPLOITATION Industrial Utilisation, Biotechnology and PhysicoChemical Properties
1990 Cellucon '90 Czechoslovakia
CELLULOSE New Trends in the Complex Utilisation of Lignocellulosics (Phytomass)
1991 Cellucon '91 USA
CELLULOSE A Joint Meeting of: ACS Cellulose, Paper and Textile Division, The Cellucon Trust, and 1I* Syracuse Cellulose Conference
1992 Cellucon '93 UK
SELECTIVE PURIFICATION AND SEPERATION PROCESSES
1993 Cellucon '93 Sweden
CELLULOSE AND CELLULOSE DERIVATIVES Physico-Chemical Aspects and Industrial Applications
1994 Cellucon '94 UK
CHEMISTRY AND PROCESSING OF WOOD AND PLANT FIBROUS MATERIALS The Chemistry and Processing of Wood and Plant Fibrous Materials
1998 Cellucon '98 Finland
PULP AND PAPER MAKING Fibre and Surface Properties and other Aspects of Cellulose Technology
1999 Cellucon '99 Japan
RECENT ADVANCES IN ENVIRONMENTALLY COMPATABLE POLYERS
2000 Hyaluronan 2000 Wales
ASPECTS OF HYALURONAN
The procecdings of each conference were formerly published by Ellis Honvood, Simon and Schuster International Group, Prcntice Hall, Campus 400, Maylands Avenue, Hemel IIempstead, Herts, HP2 7EZ and from 1993 are published by Woodhead Publishing Limited, Abington Hall, Abington, Cambridge CB 1 6AH.
PREFACE
The 10’ Cellucon Confercnce was held in thc city of TurkuIAbo, Finland, 1417‘ December 1998 at the Mauno Koivisto Centre, Biocity, in the immediate neighbourhood of the universities in Turku. The conference on “Pulp for Papermaking - Fibre and Surface Properties and other aspects of ccllulose tcchnology” was organised by Abo Akademi University, Faculty of Chemical Engineering, Laboratory of Pulping Technoloa. The background to the organisation of the conference is worth noting. Prof. Glyn 0. Phillips, Chairman of The Cellucon Trust, asked me during a previous Cellucon Conference in Bangor, Wales, 1994, whether Finland would be interested in hosting the Conference in 1997. My answer was not definite, and I wanted to explore the possibilities and the interest in my country, although I realised that thc Confercnce would fit very wcll into the Turku environment. AAcr a number of exchanged E-mails bctwecn Prof. John F. Kennedy (Dcputy Chairman and Treasurer of The Cellucon trust) and myself, we finally ended up with a confcrence datc in Dccember 1998, which maybe was risky. However, I considered that time as suitable, because the City of Turku recently had dcclared itsclf the Christmas Town of Finland, and it would perhaps be an experience for many of the delegates to see the winter and the “kamos” (darkness) almost without any daylight.
The first day, 14Ih Dccember, was a true winter day with beautihl snow fall and decorative illumination of the Aura River and the Porthan Square, when delegates invited by the City of Turku went to the reception in the House of Brinkkala, welcomed by Ms Cay Sev6n, head of the networking activities of the City. Rut winter wcather is very changeable, and a tmible rain storm struck the conference delegates when they were rushing to the technical sessions next morning to be in time for thc opening ceremony and the inaugural speech by Rector Gustav Bjorkstrand of Abo Akadcmi University. Twenty-six oral presentations wcre given in five sessions on “Engineering of fibrcs for papcrmaking”, “Fibre surface properties”, “Biochemical processes in pulping”, “Pulping and bleaching” and “Cellulose”, and in addition thcre wcre as many prcsentations in a special “Poster Session”. All this within two days (13’’ and 16Ih Decembcr)! The Banquet Dinner at Turku Castle entertained by Count Per Brahe, Governor General of Finland in the l?‘ century, and the Countess Kristina Katarina Stenbock, was very enjoyable and the atmosphere was most rclaxing. The last day, 17’ December, conference delegates made a tour of the Laboratories of Forest Products Chemistry, Paper Chemistry, Physical Chemistry and Pulping Tcchnology in the Gadolinia Building of Abo Akademi University. This book marks the end of the 10“ Cellucon Conference, but hopefully it will recall pleasant memories fiom Turku and Finland, and also provide scientific benefits and ncw creative idcas for the more than one hundred delegates who attendcd the meeting and othcr readers of thc proceedings. Personally, I hope to makc new friends and renew old acquaintanccs with delegates at futurc Cellucon Confcrences.
xii Preface This book contains an excerpt of the presentations given at the 10"' Cellucon Conference on "Pulp for Papermaking - Fibre and Surface Properties", and the contents of six parts actually represent a wide range of important research areas of cellulose properties (Part l), application of enzymes (Part 2), pulp production and processing (Part 3), fibre properties (Part 4), paper properties (Part 5 ) and cellulosic materials (Part 6). Since the contents of these proceedings cover the most important subprocesses of the pulp and papermaking process, fkom pulping of differcnt raw materials by delignification and mechanical refining to biochemical and mechanical treatments of the pulp fibre materials by enzymes and beating, and to paper and its properties, this volume will evidently provide a base for innovative research, and is certainly worth reading. Prof Bruno Lonnberg Abo Akadcmi University, TurkdAbo, Finland Chairman of the Cellucon '98 Organising Committee
