A Practical Guide to Fibre Science

N. S. KAPLAN

A Practical Guide to Fibre Science

"This page is Intentionally Left Blank"

A Practical Guide to Fibre Science

N.S. Kaplan

ABHISHEK PUBLICATIONS QI.AN)IGARH ~ 17 (India)

All rights reserved. No part of this book may be reproduced in any form, electronically or otherwise, in print, photoprint, micro film or by any other means without written permission from the publisher.

ISBN 81-85733-42-2

ISBN : 978-81-85733-42-5

© Publishers First Edition : 2002 Published by: Abhishek Publications, S.C.O. 57-59, Sector 17-C, CHANDIGARH - 160017(lndia) Ph. 707562, Fax 0172-704668

Printed at Mehra Offset, Daryaganj, Delhi - 110002

Contents

1

l.

Textile Fibres

2.

Processing of Textile Fibres

23

3.

Cotton Fibres

93

4.

Cotton Mixing and Blowroom Operations

112

5.

Carding

122

6.

Effects of Fibre Preparation on Instrument Readings

132

7.

Length of Cotton Fibres

150

8.

Cotton Stickiness

166

9.

White Specks

183

10. Fibre Dynamics

194

II.

218

Roving Frame and Draw Frame

12. Metallic Card Clothing

234

13. Winding

248

14. Spinning Geometry

271

"This page is Intentionally Left Blank"

1 Textile Fibres Fibers are natural or chemical structures that can be spun into yarns. Yarns then can be woven, knitted, or bonded into fabrics. Fiber properties and behavior are directly related to fabric performance and care. Learning about fibers and their characteristics will help you to understand fabrics better. Four major natural fibers and 23 man-made fibers are available. Natural fibers come from plants and animals. The plant fibers-cotton and linen-are made of cellulose. Animal fibers, silk and wool, are made of protein. Two classes of man-made fibers are those adapted from cellulose (cellulosic) and those made entirely of chemicals (noncellulosic). Noncellulosic man-made fibers often are called synthetics. Each fiber is identified by a generic name. The Textile Fiber Products Identification Act that officially established the generic fiber classifications became effective in 1960. All fibers (natural or man-made), yarns, fabrics, and household textile articles (includes articles of wearing apparel, draperies,

Textile Technology

2

floorcoverings, furnishings, beddings, and other textiles customarily used in a household), are covered by this Act. Fibrous materials should possess certain properties for them to be useful as textile raw materials. Those properties which are essential for acceptance as a suitable raw material may be classified as "primary properties", while those which add specific desirable character or aesthetics to the end product and its use may be classified as "secondary properties". PRIMARY PROPERTIES

Length; length-width ratio Tenacity (strength) Flexibility (pliability) Acceptable extensibility for processing Cohesion Uniformity of properties Secondary Properties Physical shape (cross-section, surface contour, etc.) Specific gravity (influence weight, cover, etc.) Moisture regain and moisture absorption (comfort, static electricity, etc.) Elastic character - tensile and compression Thermoplasticity (softening point and heat-set character) Dyeability

Textile Fibres

3

Resistance to solvents, corrosive chemicals, microorganisms, and environmental conditions Flammability Luster PROPERTIES DESIRED FOR TEXTILE FIBERS

Apparel and Domestic application: Tenacity: 3 - 7 grams/denier Elongation at break: 10 - 35% Recovery from elongation: 100% at strains up to 5% Modulus of elasticity: 30 - 60 grams/denier Moisture absorbency: 2 - 5% Zero strength temperature (excessive creep and softening point): above 215 0 C High abrasion resistance (varies with type fabric structure) Dyeable Low flammability Insoluble with low swelling in water, in moderately strong acids and bases and conventional organic solvents from room temperature to 1000 C Ease of care INDUSTRIAL REQUIREMENTS

Tenacity: 7 - 8 grams/denier Elongation at break: 8 - 15%

4

Textile Techll%gtj

Modulus of elasticity: 80 grams/denier or more conditioned, 50 grams/denier wet Zero strength tempera ture: 250 0 C or above KEY FIBER PROPERTIES DETERMINED POLYMER COMPOSITION AND STRUCTURE:

• •

Meltwg Point

• • •

Elasticity and recovery from strain Tensile strength

•

Moisture absorption

• •

Dyeability

BY

Modulus

Density

Comfort

The ability of a fiber to withstand the rubbing or abrasion it gets in everyday use POLYESTER

Characteristics Strong Crisp, soft hand Resistant to stretching and shrinkage Washable or dry-cleanable Quick drying Resilient, wrinkle resistant excellent pleat retention (if heat set)

Textile Fibres

5

Abrasion resistant Resistant to most chemicals Because of its low absorbency, stain removal can be a problem Static and pilling problems

Major End Uses Apparel - essentially every form of clothing, dresses, blouses, jackets, separates, sportswear, suits, shirts, pants, rainwear, lingerie, childrenswear Home Fashions - curtains, draperies, floor coverings, fiber fill, upholstery, bedding. Comments - Of all the manufactured fibers, polyester is the most used. Polyester is the best washand-wear fiber. Unfortunately, because of the oversaturation of polyester 20 to 30 years ago, some consumers maintain a negative perception about the fiber. But, that is changing. As with the other manufactured fibers, new developments in polyester have created a new attitude towards manufactured fibers. This is true, not only for manufacturers and designers, but also for consumers. In addition, when polyester is blended with other dry-clean only fibers, like wool, acetate, or rayon, the durability of the blended fabric improves and, in some cases, the fabrics can even be made washable, if the percentage of polyester is high enough. RAYON

Characteristics -

Soft and comfortable

Textile Technology

6

Drapes well Highly absorbent Dyes and prints well No static, no pilling problems Fabric can shrink appreciably if washing dry-cleanonly rayon Washable or dry cleanable. Major End Uses Apparel - Blouses, dresses, jackets, lingerie, linings, millinery, slacks, sportshirts, sportswear, suits, ties, work clothes Home Fashions - bedspreads, blankets, curtains, draperies, sheets, slip covers, table cloths, upholstery. Comments - One of the major characteristics of rayon, also called viscose, centers around the care of the fabric. For those of you who have been around rayon a lot, you know that there are both washable and "dry clean only" rayons in the garments that you sell. Why are there both washable and non-washable rayons? Originally rayon was a "dry clean only" fiber. However, the fiber producers discovered that they could create washability in rayon by putting certain finishes on the surface of the fabric after it was knitted or woven. But this also added to the price. So, today many rayons in the marketplace remain untreated, and are therefore "dry clean only." It's very important to read the labels, and make the customer aware that just because he or she may have purchased a washable rayon last week, that doesn't mean

Textile Fibres

7

that all rayons are washable. Anytime a rayon garment, labeled "dry clean only," is washed, a risk is taken, and one of three things may happen. First the garment can shrink tremendously, sometimes as much as two or three sizes. Second, the garment may fade, or a printed pattern may bleed. And third, the fabric may lose its soft hand. The rayon that was once soft and drapeable may become stiff and harsh. There's also something important to remember when caring for the washable rayons. Most of the washable rayon garments today are labeled "hand wash, cool water, drip dry or dry flat." And, it's important that these directions are followed, because when rayon is wet, it actually loses 30% to 50% of its strength. So, hand washing, like the label says, is the best. The constant agitation of the washer, and tumbling of the dryer will beat the garment against the side of the washer and dryer. And, eventually this causes the fiber to break down, and shorten the life of the rayon garment. So, in order to get the maximum life out of your washable rayei garment, it's best to hand wash and drip/hang dry. ACETATE

Manufactured Fiber - cellulosic-based, made from wood pulp or cotton linters

Characteristics Luxurious appearance Crisp or soft hand

Textile Technology

8

Wide range of colors; dyes and prints well Excellent drapeability and softness Shrink, moth, and mildew resistant Low moisture absorbency, relatively fast' drying No pilling problem, little static problem Most acetate garments require dry-cleaning

Major End Uses Apparel- Blouses, dresses, linings, special occasion apparel, Home Fashions - Draperies, upholstery, curtains, bedspreads. Acetate is a "dry clean only" fiber. However, again, read the label, because many of the new acetate circular knits, that have been out on the market for a few years, are hand washable. So far, there is no washable woven acetate available in the marketplace. Comments - Acetate takes color extremely well. It is also very soft and drapable. Acetate has been blended with a wide variety of other fibers. Experimenting has also been done with different knitting and weaving techniques, which has resulted in the development of a variety of fabrics suitable for many markets. Prior to these developments, the major uses fpr acetate in apparel were as a liner in coats, jackets, blazers, etc., and as a major fabric in special occasion dresses - acetate velvets, crepes, taffetas, and satins were ideal for this market. Because of these new fabric developments, acetate has been elevated to the designer level with many major designers using acetate in their lines.

Textile Fibres

9

ACRYLIC

Characteristics Light-weight, soft, warm with a wool-like hand Dyes to bright colors with excellent fastness Outstanding wickability Machine washable, quick drying Resilient; retains shape; resists shrinkage, & wrinkles Flexible aesthetics for wool-like, cotton-like or blended appearance Excellent pleat retention Resistant to moths, oil and chemicals Superior resistance to sunlight degradation Static and pilling can be a problem Major End Uses Apparel : sweaters, socks, fleece, circular knit apparel, sportswear, childrenswear Home Fashions : Blankets, throws, upholstery, awnings, outdoor furniture, rugs/floor coverings Comments - Acrylic is a soft drapeable fabric which provides warmth without being heavy. It takes color beautifully. Although acrylic has traditionally been a fall/winter fabric, with a wonderful resemblance to wool, acrylic has recently been used in developing some light weight circular knits and blends which lend themselves to trans-seasonal dressing as well. Acrylic is comfortable to wear. It feels like wool. Yet, it is easy care and is machine washable or dry cleanable.

Textile Technology

10

LYOCELL

Characteristics Excellent strength Washable Shrink- and wrinkle-resistant Soft hand Excellent drape Absorbent Dyes and prints well

Major End Uses Apparel - dresses, suits, sportswear, pants, jackets, blouses, skirts. Home Fashions - curtains, draperies, upholstery, bedspreads, table linens, sheets, dish towels, bath towels. Comments - This is the newest cellulosic fiber, and a new brand name that you may have seen recently is Tencel®. This fiber is cellulosic, and it is very similar to rayon in appearance. The major difference between lyocell and rayon is that lyocell is much more durable and has a much stronger wet strength. It can also be machine washable and dryable. MICROFIBERS

Characteristics Ultra fine (less than 1.0 dpf), finer than the most delicate silk

Textile Fibres

11

Extremely drapeable Very soft, luxurious hand with a silken or suede touch Washable, dry cleanable Shrink-resistant High strength (except Rayon) Excellent pleat retention Insulates well against wind, rain and cold

Major End Uses Apparel - hosiery, blouses, dresses, separates, sportswear, ties, scarves, menswear, intimate apparel, activewear, swimwear, outerwear, rainwear. Home Fashions - curtains, draperies, upholstery, sheets, towels, blankets. Comments - Micro-fibers is not really a fiber unto itself. Rather, it refers to an ultra-fine fiber, which can be woven or knit into a very high quality fabric construction. Originally, when DuPont introduced the first microfiber in 1989, it was a polyester microfiber. However, today in addition to polyester microfibers, there are also nylon rhicrofibers that have become important in the pantyhose market, rayon microfibers, and acrylic microfibers. One of the important characteristics of micro fiber fabrics is that they can be woven so tightly that the fabric can't be penetrated by wind, rain, or cold. For this reason, raincoat manufacturers have become big users of polyester microfibers. Microfibers also have a wicking ability, which allows perspiration to pass through the fabriC. Microfibers are comfortable to wear.

Textile Technology

12

NYLON

Characteristics Lightweight Exceptional strength Good drapeability Abrasion resistant Easy to wash Resists shrinkage and wrinkling retentive

resilient, pleat

Fast drying, low moisture absorbency Can be precolored or dyed in a wide range of colors Resistant to damage from oil and many chemicals Static and pilling can be a problem Poor resistance to continuous sunlight Major End Uses Apparel - swimwear, activewear, intimate apparel, foundation garments, hosiery, blouses, dresses, sportswear, pants, jackets, skirts, raincoats, ski and snow apparel, windbreakers, childrenswear. Home Fashions - carpets, rugs, curtains, upholstery, draperies, bedspreads Other - Luggage, back packets, life vests, umbrellas, sleeping bags, tents. Comments - Nylon is one of the strongest fiber. For this reason it's used in garments that take a great deal of hard wear, like panty hose and swimwear.

Textile Fibres

13

The most popular fiber blend used in swimwear today is nylon and spandex. Although nylon is a very strong fiber, it has poor resistance to prolonged exposure to the sun. In addition, spandex breaks down from exposure to chlorine in pool water. Yet, there probably aren't a lot of old, worn out swimsuits being returned because the fabric has wore out. That's because the customer has learned through experience that most swimsuits, if worn a lot, won't last for much more than a season or two. So, in many cases, when a customer shops for swimwear, she may buy as many as two or three at a time in order to get herself through just one season. This is because she has come to know what to expect from these fibers. ELAST ANE FIBERS:(LYCRA) Elastane fibres, better known under their trade names, Lycra and Dorlastan, represent a further high point in the development of man-made fibres. Invented in 1937 in Germany, elastane has properties not found in nature, the most important being an extraordinary elasticity. Compared to rubber, elastane has both greater tear resistance and durability and a tension capacity two or three times greater, at a third of the weight. Elastane is used in all areas where a high degree of permanent elasticity is required, as, for example, in tights, sportswear, swimwear, corsetry, and in woven and knitted fabrics. When stretched, it always reverts to its original form. Elastane thus is a prerequisite for fashionable or functional apparel which is intended to cling to the body, while at the same time remaining comfortable.

Textile Technology

14

Elastane combines its good properties with both natural and man-made fibres. There are two principal methods used in processing elastane. One is to wrap the elastane fibre in a non-elastic thread - either natural or man-made. The resulting yarn has the appearance and feel of the outer fibre used. The second method involves using pure elastane threads, which are worked or woven into fabrics made from other fibres. The added elasticity makes such fabrics more comfortable to wear. Blends with elastane depend on the type of fabric and the end use.

Properties High performance and easy care. Elastane fibres can be stretched from four to seven times their length, reverting to their original length when the tension is relaxed. Elastane has the highest stretch tension of all textile raw materials. Two per cent elastane is enough to make trousers, for instance, retain their shape. For body-shaped silhouette and high stretch capacity, i.e. in swimwear, corsetry or sportswear, 15 - 40% elastane is used. Elastane fibres guarantee a high degree of comfort combined with great freedom of movement. In woven and knitted fabrics elastane increases shape retention and accelerates crease recovery. • Elastane is not sensitive to transpiration, make-up, cosmetics, sun cream or sea water. For example swimwear with elastane should be rinsed out after bathing. Elastane is easy to care for.

Textile Fibres

15

Major End Uses Apparel - articles where stretch is desired: athletic apparel, bathing suits, foundation garments, ski pants, slacks, hosiery, socks, belts. Comments - Lycra® is the most familiar spandex fiber, and is DuPont's bran~ name for its spandex fiber. An interesting fact about spandex is that it was developed as a substitute for rubber. And, when it was first introduced in 1959, it totally revolutionized the swimwear and foundations industry. Although it has poor strength, it stretches over 500%. So, the excellent stretch of spandex compensates for the fact that it's a weak fiber. SILK

Characteristics Soft or crisp hand Luxurious Drapes and tailors well Thinnest of all natural fibers Dyes and prints well Hand-washable or dry-cleanable Little problem with static, no pilling problem Only fair abrasion resistance Poor resistance to prolonged exposure to sunlight Major End Uses Apparel - dresses, blouses, skirts, jackets, pants, pants, scarves, ties.

Textile Technology

16

Home Fashion - curtains, draperies, upholstery.

COTTON Characteristics Comfortable Soft hand Absorbent Good color retention, prints well Machine-washable, dry-cleanable Good strength Drapes well Easy to handle and sew Major End Uses Apparel - Wide range of wearing apparel: blouses, shirts, dresses, childrenswear, activewear, separates, swimwear, suits, jackets, skirts, pants, sweaters, hosiery, neckwear. Home Fashions - curtains, draperies, bedspreads, comforters, throws, sheets, towels, table cloths, table mats, napkins FIBER BLENDS

Some reasons of blending are: To facilitate processing To improve properties

Textile Fibres

17

Abrasion resistance strength Absorbency Add bulk and warmth Hand Dimensional stability Resistance to wrinkling To produce multi-color fabrics To reduce cost MICRO FIBER

The first generation of man-made fibresneeded substantial improvements technically and commercially to establisha long lasting favourable relationship between producers and consumers.Ongoing research have produced specialty fibres such as high performencefibres, bio-degradeble fibres, antibiotic fibres, flame retardant fibres,ultrafine fibres etc. which has changed our way of life. The genuine breakthrough occured withthe introduction of micro fibre technology, originated in Japan in the early1970s. It was only subsequently that their advantages were recognised withregard to handle, napping potential and functional characteristics suchas air and moisture vapour permeability and insulation. The developmentof microfibres, the fourth generation of manmade fibres, has even surpassed the dream of Robert Hooke and made possible what for a long time was regarded as unthinkable.

18

Textile Technology

What a microfibre is: Until recently there was no exact definitionof micro fibres. Generally, microfibres refer to staple fibres, or individualfilaments within yarns, which have a finess less than 1 decitex (0.9' denier).Despite the term 'fibre' being used, it is generally applied to filamentyarns. To be classified as microfibre yarn the weight per 10,000 metres of yarn (dtex) is divided by the number of filaments (f), the result mustbe below 1 e.g. dtex 156 f 256 (=0.6). Althoughacrylic, viscose, polypropylene are available for the production of microfibres,polyester and polyamide are mainstream. The fabrics made from them car,be 100% micro fibre or in blends with wool, cotton or viscose. Microfibresare finer than the finest natural fibre i.e. silk (1 dpf =denierper filament) and are ~lso called 'microdenier' or 'ultrafine' fibres.They are so fine that a single filament of Belima X micro fibre weighingjust one pound can circle the earth, while another microfibre Tactel Micro is 60 times stronger and 15 times finer than silk. Production of microfibres: Microfibre spinning is now possible bymany of major fibre producers on their better existing equipments. However,economical production of high quality micro fibres will require significantchanges in future machine design and operation. There are two routes toproduce micro fibres i.e. Direct spinning (conventionalPOY spinning) Bi-component process (segment& Island-in-sea type) In the first method, single componenthiaments are extruded through very fine spinnerets (dia 0.2 mm or

Textile fibreI'

19

lower)and drawn at high draw ratio. However, more finer fibre cannot be producedbecause of filament breakage after extrusion. The second method involvesspinning of conventional sized bicomponent filaments and break them downinto smaller components later. Very fine super micro fibres (upto O.OOldpf) can be prod~ced using bicomponent technique.

Properties and applications The tightly woven microfibre fabrics impede water dropletsfrom penetrating but allow water vapours to permeate. Microfibres offer a great varietyof applications in fashion clothing owing to their extra softness, fullhandle, drape, comfort and easy-care properties. Tightly woven micro fibre fabrics exhibit an exceptional property of obstructing water droplets frompenetrating while allowing water vapour to escape resulting in increased comfort. Their improved water impermeability and lower air permeabilitymake micro fibre fabrics highly suitable for waterproof and windbreakerleisurewear and sportswear market. Microfibres are increasingly used forladies' fashion, outdoor wear and upholstery fields. The fineness of microfibresexcels in producing light weight, flowing drape and silk-like fabrics andtheir handle can be substantially improved by appropriate finishing e.g.emery treatment for peach-skin effect.

Characteristics problems Due to their fineness, the total surfacearea of microfibre yarn or fabric is far greater than ordinary fibres.Threfore

20

Textile Technology

larger quantity of size need to be applied to micro filament warpyarns. Since micro fibres have very small interstitches, with consequentdifficulties of size accessibility and duffusibility, desizing becomesquite difficult and costly. Microfibres have greater absorption area resultingin a dyeing rate four timt;!S higher than that of normal, which can causeunlevelness in dyeing. They also require more dyestuff than standard fibres to obtain the same depth of shade. Larger external surface means an incrreasein number of threads exposed to light which, on destruction of dye, isexpressed as lower lightfastness rating. Staple microfibres offer difficultyin carding. Technological developments: The difficulty in processing microfibrescan be overcome by proper selection of dyestuffs, using appropriate dyeingmachinery (air jet type) and choosing suitable processing parameters. Knowledge of the type of size used isvery important to optimise the desizing process. Pretreatment must be doneeither on tensionless open-width washers or in Overflow or Jet dyeing machine. Control of pH is important for optimum size removal (e.g. pH 10.5-11 forpolyacrylate size). In batch pretreatment process, material is circulatedat 90' C for 30 mins followed by hot, cold rinsing and neutralisation. The fabric is dried at 140150' C on stenter. Materials having 100% rrUcrofibrewarps or have a peach-skin effect should be pre-set at 180' C for 30 secbefore dyeing to ensure dimensional stability and finished appearance.Emerising (sueding) effect, which imparts a slightly napped, peach-likesurface and a pleasant soft handle, has grown in importance for

Textile Fibres

21

microfibrefabrics. Emerising treatment must always be carried out before pre-settingto prevent an uneven surface. Optimesed dyeing cycles can be workedout by controlling the temperature to account for the high rate of dyeingand to eliminate the risk of uneven dyeing. A slower dyebath cooling afterdyeing prevents crease formation. The most useful machine for microfibrefabric processing is a Jet dyeing machine(especially airflow type) as theyallow the fabric to develop a desirable bulk. Examples of such machineare Longclose Ventura Rapid Micro-Tech, Henriksen/Fong's AirJet Thies Luft-roto& soft stream SV etc. Proper dye selection (Le. compatible dyes) eliminatesproblems regarding build-up and fastness properties. Manufacturers likeYorkshire, leI and BASF offer new generation of dyes suitable for processingmicrofibres. An emerging technology for dyeing polyester by using supercriticalfluid (C02) was tested on polyestermicrofibre and initial trials have shown excellent levelness and fastnessof dyeing. Finishing very often consists of both mechanical and chemical treatments. Microfibres are being increasingly usedthroughout the world for various end-uses due to their fineness, high performancecharacteristics and their unique ability to be engineered for a specificrequirement. Extreme care must be taken during processing and handling(lf micro fibre fabrics. This requires specific equipment to be used whendyeing and finishing such delicate type of fabrics. Nylon is claimed tohave advantages over polyester in having a better cover, plus lower density,higher strength and abrasion resistance, where this is very important.Polyester is easier to spin and is available in

22

Textile Technology

finer filaments than nylon.Raised fabrics from polyester are somewhat easier to produce. This hasgiven polyester and economic advantage in apparel and sportswear markets.

2 Processing of Textile Fibres Mixing is the first and important process in the spinning process. Some information regarding "cotton mixing", is given below:-

Cotton mixing: Blow room is the process which takes care of opening, cleaning and blending of different fibres used in the mixing. The technological improvements are remarkable in this process. Important points are highlighted to refere please click the link

Blowroom process: High production carding has now become an established practice for short-staple fibres. Production capacity in recent carding machines can be increased substatially without deterioration in quality.

Carding process: Reading without thinking is like eating without digesting. Metallic card clothing played an important role to improve the production rates on revolving flat cards. High production card would not have been possible without the Metallic card clothing. Moreover the demand for high production and high quality from carding machine put wire manufacturers

Textile Technology

24

under pressure to improve their design continuously to meet the requirements. Modem metallic wire is designed for optimum carding performance during the processing of many tonnes of fibre at the highest proudction rates possible, and with the least amount of maintenance.

Card clothing 90 percent of your work can be done by your subordinates and you genuinely get paid only for 10 percent. Drawframe contributes less than 5% to production cost of yam. But its influence on quality is very big, because drawing is the final process of quality improvement in the spinning mill and quality of drawframe sliver determines the final yam quality.For more information please click

Drawing process Autoievelling Most problems are really the absence of ideas: Combing is the process which serves to improve the raw material.Combed yam is stronger, more uniform, has greater shine and is smoother and purer.The quality improvements are obtained at the cost of additional expenditure on machines, floor and personnel, together with a loss of rawmaterial.

Cambing process The greatest loss is the loss of self-confidence:

Processing of Textile Fibres

25

Roving bobbin is the ideal package form for supply of material to the ring spinning frame. It is very convenient for transport and storing.Eventhough drawframe produces a sliver that already exhibits all the characteristics required for making the yam, the roving frame is forced upon a spinner as a necessary evil for two reasons. They are: 1. higher draft

2. drawframe can feeding creates more problem

Speed frame Do important jobs now before they become urgent The ring spinning will continue to be the most widely used form of spinning machine in the near future, because ring spinning still offers the greatest flexibility in application and supplies yams of a quality that cannot be equalled by the new technologies owing to technological reasons.

Ring frame A pessimist finds difficulty in every opportunity, the optimist finds opportunity in every difficulty. -

Winding is the process which results in producing a good package of long length and fault free yam.

Most of the spinning mills use automatic winding machines. Quality of yam and package and winding machine production are improving day by day. The requirements of package quality and yam quality is also very high for the present knitters and weavers.

Textile Technology

26

Winding The more time spent on self improvement, the less there is to criticise others. Variables in a spinning mill is extremely high e.g., rawmaterial, atmospheric conditions in a plant type of machines, technology adopted, skill level, investment concepts, market requirements, etc. Since the variations are innumerable, it is a must to optimise the process parameters to produce a good quality yarn at a lower manufacturing cost. Some information and guidelines for fixing parameters is given in the following pages. Theory without Practice is sterile, and Practice without Theory is futile. Yarn occupies the intermediate position in the manufacture of fabric from raw material. Yarn tests are therefore essential, both for estimating the quality of the raw material and for controlling the fabric produced.

Characteristics The textile Industry uses compressed air power in some machinery or other, for instance blow room, comber, spinning frame, loom, splicers to name a few. With the use of more sophisticated, high production machinery the need for more and more complicated Pneumatic controls are necessitated. To have minimum breakdowns and reliable performance with minimum failure, in other words, to have good quality products, the compressed air supplied to the machine should be of the highest order, both in QUALITY and QUANTITY.

Processing of Textile Fibres

27

Atmospheric conditions have a decided effect on textile materials, especially during processing.For this reason it is essential that the temperature and moisture content of the air within the mill be controlled. The exact degree of temperature and relative humidity will vary depending upon the material and department.

Humidification It is better to make a first class soup than a second class

painting.

Spinning machines Blowroom Lakshmi Rieter

-

-

-

Trutzschler

-

Crosrol

Carding -

Lakshmi

-

Rieter

-

Trutzschler

-

Crosrol

-

MarzoH

Comber Lakshmi Rieter

28

Textile Technology

-

Toyoda

-

Marzoli

Draw frame Rieter Vouk Lakshmi Speed frame

-

-

Zinser

-

Toyoda

-

Marzoli

-

Lakshmi

Ring spinning Lakshmi Rieter Zinser

-

-

Suessen

-

Toyoda

Open end Schlafhorst Rieter Lakshmi

-

Winding Schlafhorst Savio

Processing of Textile Fibres

29

Textile resources • AKTRIN Textile Information Center

Importance of Rawmaterial in Yam Manufacturing Raw material represents about 50 to 70% of the production cost of a short-staple yarn. This fact is sufficient to indicate the significance of the rawmaterial for the yarn producer. It is not possible to use a problemfree raw material always, because cotton is a natural fibre and there are many properties which will affect the performance. If all the properties have to be good for the cotton, the rawmaterial would be too expensive. To produce a good yarn with this difficulties, an intimate knowledge of the raw material and its behaviour in processing is a must. Fibre characteristics must be classified according to a certain sequence of importance with respect to the end product and the spinning process. Moreover, such quantified characteristics must also be assessed with reference to the following what is the ideal value? what amount of variation is acceptable in the bale material? what amount of variation is acceptable in the final blend Such valuable experience, which allows one to determine the most suitable use for the raw material, can only be obtained by means of a long, intensified and direct association with the raw material, the spinning process and the end product. Low cost yarn manufacture, fulfilling of all quality requirements and a controlled

30

Textile Technology

fibre feed with known fibre properties are necessary in order to compete on the world's textile markets. Yarn prodcution begins with the rawmaterial in bales, whereby success or failure is determined by the fibre quality, its price and availability. Successful yarn producers optimise profits by a process oriented selection and mixing of the rawmaterial, followed by optimisation of the machine settings, production rates, operating elements, etc. Simultaneously, quality is ensured by means of a closed loop control system, which requires the application of supervisory system at spinning and spinning preparation, as well as a means of selecting the most sutable bale mix. Basic fibre characteristccs A textile fibre is a peculiar object. It has not truly fixed length, width, thickness, shape and cross-section. Growth of natural fibres or prodction factors of manmade fibres are responsible for this situation. An individual fibre, if examined carefully, will be seen to vary in cross-sectional area along it length. This may be the result of variations in growth rate, caused by dietary, metabolic, nutrientsupply, seasonal, weather, or other factors influencing the rate of cell development in natural fibres. Surface characteristics also play some part in increasing the variablity of fibre shape. The scales of wool, the twisted arrangement of cotton, the nodes appearing at intervals, along the cellulosic natural fibres etc. Following are the basic chareteristics of cotton fibre: fibre length fineness strength

Processing of Textile Fibres

31

maturity Rigidity fibre friction structural features The atmosphere in which physical tests on textile materials are performed. It has a relative humidity of 65 + 2 per cent and a temperature of 20 + 2° C. In tropical and sub-tropical countries, an alternative standard atmosphere for testing with a relative humidity of 65 + 2 per cent and a temperature of 27 + 2° C, may be used.

Fibre length The "length" of cotton fibres is a property of commercial value as the price is generally based on this character. To some extent it is true, as other factors being equal, longer cottons give better spinning performance than shorter ones. But the length of a cotton is an indefinite quantity, as the fibres, even in a small random bunch of a cotton, vary enormously in length. Following are the various measures of length in use in different countries: mean length upper quartile effective length Modal length 2.5% span length 50% span length

32

Textile Technology

Mean length It is the estimated quantity which theoretically signifies the arithmetic mean of the length of all the fibres present in a small but representative sample of the cotton. This quantity can be an average according to either number or weight.

Upper quartile length It is that value of length for which 75% of all the

observed values are lower, and 25% higher.

Effective length It is difficult to give a clear scientific definition. It may be defined as the upper quartile of a numerical length distribution eliminated by an arbitrary construction. The fibres eliminated are shorter than half the effective length.

Modal length It is the most frequently occurring length of the fibres in the sample and it is related to mean and median for skew distributions, as exhibited by fibre length, in the follwing way.

(Mode-Mean)

= 3(Median-Mean)

where, Median is the particular value of length above and below which exactly 50% of the fibres lie.

33

Processing of Textile Fibres

2.5% Span length It is defined as the distance spanned by 2.5% of fibres in the specimen being tested when the fibres are parallelized and randomly distributed and where the initial starting point of the scanning in the test is considered 100%. This length is measured using "DIGITAL FIBROGRAPH".

50% Span length It is defined as the distance spanned by 50% of fibres in the specimen being tested when the fibres are parallelized and randomly distributed and where the initial starting point of the scanning in the test is considered 100%. This length is measured using "DIGITAL FIBROGRAPH".

The South India Textile Research Association (SITRA) gives the following empirical relationships to estimate the Effective Length and Mean Length from the Span Lengths. Effective length Mean length

= 1.013 x 2.5% Span length + 4.39

= 1.242

x 50% Span length + 9.78

Fibre length variation Eventhough, the long and short fibres both contribute towards the length irregularity of cotton, the short fibres are particularly responsible for increasing the waste losses, and cause unevenness and reduction in strength in the yarn spun. The relative proportions of short fibres are usually different in cottons having different mean

Textile Technology

34

lengths; they may even differ in two cottons having nearly the same mean fibre length, rendering one cotton more irregular than the other.It is therefore important that in addition to the fibre length of a cotton, the degree of irregularity of its length should also be known. Variability is denoted by anyone of the following attributes. 1.

Co-efficient of variation of length (by weight or number)

2.

Irregularity percentage

3.

Dispersion percentage and percentage of short fibres

4.

Uniformity ratio

Uniformity ratio is defined as the ratio of 50% span length to 2.5% span length expressed as a percentage. Several instruments and methods are available for determination of length. Following are some shirley comb sorter Baer sorter A.N. Stapling apparatus Fibrograph uniformity ration == (50% span length / 2.5% span length) x 100 uniformity index == (mean length / upper half mean length) x 100

Short Fibers The negative effects of the presence of a high proportion

Processing of Textile Fibres

35

of short fibres is well known. A high percentage of short fibres is usually associated with: Increased yam irregularity and ends ddown which reduce quality and increase processing costs Increased number of neps and slubs whiich is detrimental to the yam appearance Higher fly liberation and machine conttamination in spinning, weaving and knitting operations. Higher wastage in combing and other opperations. While the detrimental effects of short fibres have been well established, there is still considerable debate on what constitutes a 'short fibre'. In the simplest way, short fibres are defined as those fibres which are less than 12 mm long. Initially, an estimate of the short fibres was made from the staple diagram obtained in the Baer Sorter method: Short fibre content

= (UB/OB)

x 100

While such a simple definition of short fibres is perhaps adequate for characterising raw cotton samples, it is too simple a definition to use with regard to the spinning process. The setting of all spinning machines is based on either the staple length of fibres or its equivalent which does not take into account the effect of short fibres. In this regard, the concept of 'Floating Fibre Index' defined by Hertel (1962) can be considered to ae a better parameter to consider the effect of short fibres on spinning performance. Floating fibres are defined as those fibres which are not clamped by either pair of rollers in a drafting zone.

36

Textile Technology Floating Fibre Index (FFI) was defined as FFI = «2.5% span length/mean length)-l)x(lOO)

The proportion of short fibres has an extremely great impact on yarn quality and production. The proportion of short fibres has increased substantially in recent years due to mechanical picking and hard ginning. In most of the cases the absolute short fibre proportion is specified today as the percentage of fibres shorter than 12mm. Fibrograph is the most widely used instrument in the textile industry, some information regarding fibrograph is given below.

Fibrograph Fibrograph measurements provide a relatively fast method for determining the length uniformity of the fibres in a sample of cotton in a reproducible manner. Results of fibrograph length test do not necessarily agree with those obtained by other methods for measuring lengths of cotton fibres because of the effect of fibre crimp and other factors. Fibrograph tests are more objective than commercial staple length classifications and also 'provide additional information on fibre length uniformity of cotoon fibres. The cotton quality information provided by these results is used in research studies and quality surveys, in checking commercial staple length classifications, in assembling bales of cotton into uniform lots, and for other purposes. Fibrograph measurements are based on the assumptions that a fibre is caught on the comb in proportion to its length as compared to toal length of all

Processing of Textile Fibres

37

fibres in the sample and that the point of catch for a fibre is at random along its length.

Fibre Fineness Fibre fineness is another important quality characteristic which plays a prominent part in determining the spinning value of cottons. If the same count of yarn is spun from two varieties of cotton, the yarn spun from the variety having finer fibres will have a larger number of fibres in its cross-section and hence it will be more even and strong than that spun from the sample with coarser fibres. Fineness denotes the size of the cross-section dimensions of the fibre. AS the cross-sectional features of cotton fibres are irregular, direct determination of the area of croo-section is difficult and laborious. The Index of fineness which is more commonly used is the linear density or weight per unit length of the fibre. The unit in which this quantity is expressed varies in different parts of the world. The common unit used by many countries for cotton is microgrammes per inch and the various airflow instruments developed for measuring fibre fineness are calibrated in this unit. Following are some methods of determining fibre fineness: gravimetric or dimensional measurements air-flow method vibrating string method Some of the above methods are applicable to single fibres while the majority of them deal with a mass of fibres. As

38

Textile Technology

there is considerable variation in the linear density from fibre to fibre, even amongst fibres of the same seed, single fibre methods are time-consuming and laborious as a large number of fibres have to be tested to get a fairly reliable average value. It should be pointed out here that most of the fineness determinations are likely to be affected by fibre maturity, which is an another important characteristic of cotton fibres.

Air-flow method The resistance offered to the flow of air through a plug of fibres is dpendent upon the specific surface area of the fibres. Fineness tester have been evolved on this principle for determininG fineness of cotton. The specific surface area which determines the flow of air through a cotton plug, is dependent not only upon the linear density of the fibres in the sample but also upon their maturity. Hence the micronaire readings have to be treated with caution particularly when testing samples varying widely in maturity. In the micronaire instrument, a weighed quantity of 3.24 gms of well opened cotton sample is compressed into a cylindrical container of fixed dimensions. Compressed air is forced through the sample, at a definite pressure and the volume-rate of flow of air is measured by a rotometer type flowmeter. The sample for Micronaire test should be well opened cleaned and thoroughly mixed( by hand fluffing and opening method). Out of the various air-flow instruments, the Micronaire is robust in construction, easy to operate and presents little difficulty as regards its maintenance.

Processing of Textile Fibres

39

Fibre maturity Fibre maturity is another important characteristic of cotton and is an index of the extent of development of the fibres. As is the case with other fibre properties, the maturity of cotton fibres varies not only between fibres of different samples but also between fibres of the same seed. The causes for the differences observed in maturity, is due to variations in the degree of the secondary thickening or deposition of cellulose in a fibre. A cotton fibre consists of a cuticle, a primary layer and secondary layers of cellulose surrounding the lumen or central canal. In the case of mature fibres, the secondary thickening is very high, and in some cases, the lumen is not visible. In the case of immature fibres, due t? some physiological causes, the secondary deposition of cellulose has not taken sufficiently and in extreme cases the secondary thickening is practically absent, leaving a wide lumen throughout the fibre. Hence to a cotton breeder, the presence of excessive immature fibres in a sample would indicate some defect in the plant growth. To a technologist, the presence of excessive percentage of immature fibres in a sample is undesirable as this causes excessive waste losses in processing lowering of the yarn appearance grade due to formation of neps, uneven dyeing, etc. An immature fibre will show a lower weight per unit length than a mature fibre of the same cotton, as the former will have less deposition of cellulose inside the fibre. This analogy can be extended in some cases to fibres belonging to different samples of cotton also. Hence it is essential to measure the maturity of a cotton sample in addition to determining its fineness, to check

40

Textile Technology

whether the observed fineness is an characteristic or is a result of the maturity.

inherent

Different methods of testing maturity Maturity ratio The fibres after being swollen with 18% caustic soda are examined under the microscope with suitable magnification. The fibres are classified into different maturity groups depending upon the relative dimensions of wall-thickness and lumen. However the procedures followed in different countries for sampling and classification differ in certain respects. The swollen fibres are classed into three groups as follows 1.

Normal: rod like fibres with no convolution and no continuous lumen are classed as "normal"

2.

Dead: convoluted fibres with wall thickness one-fifth or less of the maximum ribbon width are classed as "Dead"

3.

Thin-walled: The intermediate ones are classed as "thin-walled".

A combined index known as maturity ratio is used to express the results. Maturity ratio

= ((Normal

- Dead)/200) + 0.70

where, N - %ge of Normal fibres D - '%ge of Dead fibres

Processing of Textile Fibres

41

Maturity co-efficient Around 100 fibres from Baer sorter combs are spread across the glass slide(maturity slide) and the overlapping fibres are again separated with the help of a teasing needle. The free ends of the fibres are then held in the clamp on the second strip of the maturity slide which is adjustable to keep the fibres stretched to the desired extent. The fibres are then irrigated with 18% caustic soda solution and covered with a suitable slip. The slide is then placed on the microscope and examined. Fibres are classed into the following three categories: 1.

Mature: (Lumen width "L")/(wall thickness"W") less than 1

2.

Half mature: (Lumen width "L")/(wall thickness "W") is less than 2 and more than 1

3.

Immature: (Lumen width "L")/(wall thickness "W") is more than 2

IS

About four to eight slides are prepared from each sample and examined. The results are presented as percentage of Mature, half-mature and immature fibres in a sample. The results are also expressed in terms of "Maturity Coefficient" Maturity Coefficient = (M + 0.6H + 0.4 1)/100 Where, M is percentage of Mature fibres H is percentage of Half mature fibres I is percentage of Immature fibres If maturity coefficient is:

-

Less than 0.7, it is caJled as immature cotton

Textile Technology

42

between 0.7 to 0.9, it is called as medium mature cotton above 0.9, it is called as mature cotton There are other techniques for measuring maturity using Micronaire instrument. As the fineness value determined by the Micronaire is dependent both on the intrinsic fineness(perimeter of the fibre) and the maturity, it may be assumed that if the intrinsic fineness is constant then the Micronaire value is a measure of the maturity

Dyeing methods Mature and immature fibers differ in their behaviour towards various dyes. Certain dyes are preferentially taken up by the mature fibres while some dyes are preferentially absorbed by the immature fibres. Based on this observation, a differential dyeing technique was developed in the United States of America for estimating the maturity of cotton. In this technique, the sample is dyed in a bath containing a mixture of two dyes, namely Diphenyl Fast Red 5 BL and Chlorantine Fast Green BLL. The mature fibres take up the red dye preferentially, while the thin walled immature fibres take up the green dye. An estimate of the average of the sample can be visually assessed by the amount of red and green fibres.

Fibre Strength The different measures available for reporting fibre strength are: 1.

breaking strength

2.

tensile strength and

Processing of Textile Fibres

3.

43

tenacity or intrinsic strength

Coarse cottons generally give higher values for fibre strength than finer ones. In order, to compare strength of two cottons differing in fineness, it is necessary to eliminate the effect of the difference in cross-sectional area by dividing the observed fibre strength by the fibre weight per unit length. The value so obtained is known as "INTRINSIC STRENGTH or TENACITY". Tenacity is found to be better related to spinning than the breaking strength. The strength characteristics can be determined either on individual fibres or on bundle of fibres.

Single fibre strength The tenacity of fibre is dependent upon the following factors chain length of molecules in the, fibre orientation of molecules size of the crystallites distribution of the crystallites gauge length used the rate of loading type of instrument used and atmospheric conditions. The mean single fibre strength determined is expressed in units of "grams/tex". As it is seen the the unit for tenacity has the dimension of length only, and hence this property is also expressed as the "BREAKING LENGTH", which can be considered as the length of the specimen equivalent in weight to the breaking load. Since tex is the mass in grams of one kilometer of the specimen, the tenacity values expressed in grams/tex will correspond to the breaking length in kilometers.

44

Textile Technology

Dundle Fibre Strength In practice, fibres are not used individually but in groups, such as in yarns or fabrics. Thus, bundles or groups of fibres come into play during the tensile break of yarns or fabrics. Further,the correlation between spinning performance and bundle strength is atleast as high as that between spinning performance and intrinsic strength determined by testing individual fibres. The testing of bundles of fibres takes less time and involves less strain than testing individual fibres. In view of these considerations, determination of breaking strength of fibre bundles has assumed greater importance than single fibre strength tests.

Fibre Elongation There are three types of elongation:

Permanent elongation: the length which extended during loading did not recover during relaxation

Elastic elongation:The extensions through which the fibres does return

Breaking elongation:the maximum extension at which the yarn breaks i.e.permanent and elastic elongation together Elongation is specified as a percentage of the starting length. The elastic elongation is of deceisive importance, since textile products without elasticity would hardly be usable. They must be able to deforme, In order to withstand high loading, but they must also return to shatpe. The greater resistance to crease for wool compared to cotton arises, from the difference in their elongation. For cotton it is 6 -10% and for wool it is aroun 25 - 45%. For normal textile

Processing of Textile Fibres

45

goods, higher elongation are neither necessary nor desirable. They make processing in the spinning mill more difficult, especially in drawing operations.

Fibre Rigidity The Torsional rigidity of a fibre may be defined as the torque or twisting force required to twist 1 cm length of the fibre through 360 degrees and is proportional to the product of the modulus of rigidity and square of the area of cross-section, the constant of proportionality being dependent upon the shape of the cross-section of the fibre. The torsional rigidity of cotton has therefore been found to be very much dependent upon the gravimetric fineness of the fibres. As the rigidity of fibres is sensitive to the relative humidity of the surrounding atmosphere, it is essential that the tests are carried out in a conditional room where the relative humidity is kept constant. Fibre stiffness plays a significant role mainly when rolling, revolving, twisting movements are involved. A fibre which is too stiff has difficulty adapting to the movements. It is difficult to get bound into the yarn, which results in higher hairiness. Fibres which are not stiff enough have too little springiness. They do not return to shape after deformation. They have no longitudinal resistance. In most cases this leads to formation of neps. Fibre stiffness is dependent upon fibre substance and also upon the relationship between fibre length and fibre fineness. Fibres having the same structure will be stiffer, the shorter they are. The slendernesss ratio can serve as a measure of stiffness, slender ratio = fibre length / fibre diameter

46

Textile Technology

Since the fibres must wind as they are bound-in during yam formation in the ring spinning machine, the slenderness ratio also determines to some extent where the fibres will finish up.fine and/or long fibres in the middle coarse and/or short fibres at the yam periphery.

Trash content In additon to usuable fibres, cotton stock contains foreign matter of various kinds. This foreign material can lead to extreme disturbances during processing. Trash affects yam and fabric quality. Cottons with two different trash contents should not be mixed together, as it will lead to processing difficulties. Optimising process paramters will be of great difficulty under this situation, therefore it is a must to know the amount of trash and the type of trash before deciding the mixing. A popular trash measuring device is the Shirley Analyser, which separates trash and foreign matter from lint by mechanical methods. The result is an expression of trash as a percentage of the combined weight of trash and lint of a sample. This instrument is used: to give the exact value of waste figures and also the proportion of clean cotton and trash in the material to select the proper processing sequence based upon the trash content to assess the cleaning efficiency of each machine to determine the loss of good fibre in the sequence of opening and cleaning. Stricter sliver quality requirements led to the gradual evolution of opening and cleaning machinery leading to

Processing of Textile Fibres

47

a situation where blow room and carding machinery were designed to remove exclusively certain specific types of trash particles. This necessitated the segregation of the trash in the cotton sample to different grades determined by their size. This was achieved in the instruments like the Trash Separator and the Micro Dust Trash Analyser which could be considered as modified versions of the Shirley Analyser. The high volume instruments introduced the concept of optical methods of trash measurement which utilised video scanning trash-meters to identify areas darker than normal on a cotton sample surface. Here, the trash content was expressed as the percentage area covered by the trash particles. However in such methods, comparability with the conventional method could not be established in view of the non-uniform distribution of trash in a given cotton sample and the relatively smaller sample size to determine such a parameter. Consequently, it is yet to establish any significant name in the industry.

Raw Material as a factor affecting spinning Fineness determines how many fibres are present in the cross-section of a yam of particular linear density. 30 to 50 fibres are needed minimum to produce a yam fibre fineness influences: 1.

spinning limit

2.

yam strength

3.

yarn evenness

4.

yam fullness

48

Textile Technology

5.

drape of the fabric

6.

lustre

7.

handle

8.

productivity

Productivity is influenced by the end breakage rate and twist per inch required in the yarn. Immature fibres(unripe fibres) have neither adequate strength nor adequate longitudinal siffness. They therefore lead to the following: 1.

loss of yarn strength

2.

neppiness

3.

high proportion of short fibres

4.

varying dyeability

5.

processing difficulties at the card and blowroom

Fibre length is one among the most important characteristics. It influnces: 1.

spinning limit

2.

yarn strength

3.

handle of the product

4.

lustre of the product

5.

yarn hairiness

6.

prod uctivi ty

It can be assumed that fibres of under 4 - 5 mm will be

lost in processing(as waste and fly). fibres upto about 12 15 mm do not contribute to strength but only to fullness of the yarn. But fibres above these lengths produce the other positive characteristics in the yarn.

Processing of Textile Fibres

49

The proportion of short fibres has extremely great influence on the following parameters: 1.

spinning limit

2.

yam strength

3.

handle of the product

4.

lustre of the product

5.

yam hairiness 6.productivity

A large proportion of short fibre leads to strong fly contamination, strain on personnel, on the machines, on the work room and on the air-conditioning, and also to extreme drafting difficul ties. A uniform yam would have the same no of fibres in the cross-section, at all points along it. If the fibres themeselves have variations within themselves, then the yam will be more irregular. If 2.5% span length of the fibre increases, the yarn strength also icreases due to the fact that there is a greater contribution by the fibre strength for the yam strength in the case of longer fibres.

Neps are small entanglements or knots of fibres. There are two types of neps. They are: 1.

fibre neps and

2.

seed-coat neps.ln general fibre neps predominate, the core of the nep consists of unripe and dead fibres. Thus it is clear that there is a relationship between neppiness and maturity index. Neppiness is also dependent on the fibre fineness, because fine fibres have less longitudinal stiffness than coarser fibres.

50

Textile Technology

Nature produces countless fibres, most of which are not usable for textiles because of inadequate strength. The minimum strength for a textile fibre is approximately 6gms/tex ( about 6 kn breaking length). Since blending of the fibres into the yarn is achieved mainly by twisting, and can exploit 30 to 70% of the strength of the material, a lower limit of about 3 gms/tex is finally obtained for the yarn strength, which varies linearly with the fibre strength. Low micronaire value of cotton results in higher yarn tenacity. In coarser counts the influence of micronaire to increase yarn tenacity is not as significant as fine count. Fibre strength is moisture dependent. i.e. It depends strongly upon the climatic conditions and upon the time of exposure. Strength of cotton,linen etc. increases with increasing moisture content. The most important property inflencing yarn elongation is fibre elongation. Fibre strength ranks seconds in importance as a contributor to yarn elongation. Fibre fineness influences yarn elongation only after fibre elongation and strength. Other characters such as span length, uniformity ratio, maturity etc, do not contribute significantly to the yarn elongation.Yarn elongation increases with increasing twist. Coarser yarn has higher elongation than finer yarn. Yarn elongation decreases with increasing spinning tension. Yarn elongation is also influenced by traveller weight and high variation in twist insertion. For ring yarns the number of thin places increases, as the trash content and uniformity ratio increased For rotor yarns 50%span length and bundle strength has an ·influence on thin places.

Processing of Textile Fibres

51

Thick places in ringyarn is mainly affected by 50%span length, trash content and shor fibre content. The following expression helps to obtain the yarn CSP achievable at optimum twist multiplier with the available fibre properties. Lea CSP for Karded count = 280 x SQRT(FQI) + 700 13C Lea CSP for combed count = (280 x SQRT(FQI) + 700 13C)x(1+W)/100 where, FQI L

= LSM/F

= 50%

span length(mm)

S = bundle strength (g/tex) M = Maturity ratio measured by shirly FMT F = Fibre fineness (micrograms/inch)

= yam <;:ount W = comber waste'Yo C

Higher FQI values are associated with higher yarn strength in the case of carded counts but in combed count such a relationship is not noticed due to the effect of combing Higher 2.5 % span length, uniformity ratio, maturity ratio and lower trash content results in lower imperfection. FQI does not show any significant influence on the imperfection. The unevenness of carded hosiery yarn does not show any significant relationships with any of the fibre properties except the micronaire value. As the micronaire

52

Textile Technology

value increases, U% also increases. Increase in FQI however shows a reduction in U%. Honey-dew is the best known sticky substance on cotton fibres. This is a secretion of the cotton louse. There are other types of sticky substances also. They are given below: honey dew - secretions fungus and bacteria - decomposition products vegetable substances - sugars from plant juices, leaf nectar, overprodcution of wax, fats, oils - seed oil from ginning pathogens synthetic substances - defoliants, insecticides, fertilizers, oil from harvesting machines In the great majority of cases, the substance is one of a group of sugars of the most variable composition, primarily but not exclusively, fructose, glucose, saccharose, melezitose, as found, for example on sudan cotton. These saccharides are mostly, but not always, prodced by insects or the plants themselves, depending upon the influence on the plants prior to plucking. Whether or not a fibre will stick depends, not only on the quantity of the sticky coating and it composition, but also on the degree of saturation as a solution. Sugars are broken down by fermentation and by microorganisms during storage of the cotton. This occurs more quickly the higher the moisture content. During spinning of sticky cotton, the R.H.% of the air in the production are should be held as low as possible.

Processing of Textile Fibres

53

Cotton fibre growth !improvements in cotton fiber properties for textiles depend on changes in the growth and development of the fiber. Manipulation of fiber perimeter has a potential to impact the length, micronaire, and strength of cotton fibers. The perimeter of the fiber is regulated by biological mechanisms that control the expansion characteristic of the cell wall and establish cell diameter. Improvements in fiber quality can take many different forms. Changes in length, strength, uniformity, and fineness In one recent analysis, fiber perimeter was shown to be the single quantitative trait of the fiber that affects all other traits. Fiber perimeter is the variable that has the greatest affect on fiber elongation and strength properties. While mature dead fibers have an elliptical morphology, living fibers have a cylindrical morphology during growth and development. Geometrically, perimeter is directly determined by diameter (perimeter = diameter x p). Thus, fiber diameter is the only variable that directly affects perimeter. For this reason, understanding the biological mechanisms that regulate fiber diameter is important for the long-term improvement of cotton. A review of the literature indicates that many researchers believe diameter is est:!bllshed at fiber initiation and is maintained throughout the duration of fiber development . A few studies have examined, either directly or indirectly, changes in fiber diameter during development. Some studies indicate that diameter remains constant; while others indicate that fiber diameter increases as the fiber develops.

54

Textile Technology

The first three stages occur while the fiber is alive and actively growing. Fiber initiation involves the initial isodiametric expansion of the epidermal cell above the surface of the ovule. This stage may last only a day or so for each fiber. Because there are several waves of fiber initiation across the surface of the ovule , one may find fiber initials at any time during the first 5 or 6 d post anthesis. The elongation phase encompasses the major expansion growth phase of the fiber. Depending on genotype, this stage may last for several weeks post anthesis. During this stage of development the fiber deposits a thin, expandable primary cell wall composed of a variety of carbohydrate polymers. As the fiber approaches the end of elongation, the major phase of secondary wall synthesis starts. In cotton fiber, the secondary cell wall is composed almost exclusively of cellulose. During this stage, which lasts until the boll opens (50 to 60 d post anthesis), the cell wall becomes progressively thicker and the living protoplast decreases in volume. There is a significant overlap in the timing of the elongation and secondary wall synthesis stages. Thus, fibers are simultaneously elongating and depositing secondary cell wall. The establishment of fiber diameter is a complex process that is governed, to a certain extent, by the overall mechanism by which fibers expand. The expansion of fiber cells is governed by the same related mechanisms occurring in other walled plant cells. Most cells exhibit diffuse cell growth, in which new wall and membrane materials are added throughout the surface area of the cell. Specialized, highly elongated cells, such as root hairs and pollen tubes, expand via tip synthesis where new wall and membrane materials are added only at a specific location that becomes the growing tip of the cell.

Processing of Textile Fibres

55

While the growth mechanisms for cotton fiber have not been fully documented, recent evidence indicates that throughout the initiation and early elongation phases of development, cotton fiber expands primarily via diffuse growth. Later in fiber development, late in cell elongation, and well into secondary cell wall synthesis (35 d post anthesis), the organization of cellular organelles is consistent with continued diffuse growth. Many cells that expand via diffuse growth exhibit increases in both cell length and diameter; but cells that exhibit tip synthesis do not exhibit increases in cell diameter. If cotton fiber expands by diffuse growth, then it is reasonable to suggest that cell diameter might increase during the cell elongation phase of development. Cell expansion is also regulated by the extensibility of the cell wall. For this reason, cell expansion most commonly occurs in cells that have only a primary cell wall . Primary cell walls contain low levels of cellulose. Production of the more rigid secondary cell wall usually signals the cessation of cell expansion. Secondary cell wall formation is often indicated by the development of wall birefringence. Analyses of fiber diameter and cell wall birefringence show that fiber diameter significantly increased as fibers grew and developed secondary cell walls. Both cotton species and all the genotypes tested exhibited similar increases in diameter; however, the specific rates of change differed. Fibers continued to increase in diameter during the secondary wall synthesis stage of development, indicating that the synthesis of secondary cell wall does not coincide with the cessation of cell expansion.

Textile Technology

56

Ginning The generally recommended machinery sequence at gins for spindle-picked cotton is rock and green-boll trap, feed control, tower drier, cylinder cleaner, stick machine, tower drier, cylinder cleaner, extractor feeder, gin stand, lint cleaner, lint cleaner, and press. Cylinder cleaners use rotating spiked drums that open and clean the seedcotton by scrubbing it across a grid-rod or wire mesh screen that allows the trash to sift through. The stick machine utilizes the slingoff action of channel-type saw cylinders to extract foreign matter from the seedcotton by centrifugal force. In addition to feeding seedcotton to the gin stand, the extractor feeder cleans the cotton using the stick machine's sling-off principle. In some cases the extractor-feeder is a combination of a cylinder cleaner and an extractor. Sometimes an impact or revolving screen cleaner is used in addition to the second cylinder cleaner. In the impact cleaner, seedcotton is conveyed across a series of revolving, serrated disks instead of the grid-rod or wire mesh screen. Lint cleaners at gins are mostly of the controlled-batt, saw type. In this cleaner a saw cylinder combs the fibers and extracts trash from the lint cotton by a combination of centrifugal force, scrubbing action between saw cylinder and grid bars, and gravity assisted by an air current Seedcotton-type cleaners extract the large trash components from cotton. However, they have only a small influence on the cotton's grade index, visible liint foreign-matter content, and fiber length distribution when compared with the lint cleaning

Processing of Textile Fibres

57

effects. Also, the number of neps created by the entire seedcotton cleaning process is about the same as the increase caused by one saw-cylinder lint cleaner. Most cotton gins today use one or two stages of sawtype lint cleaners. The use of too many stages of lint cleaning can reduce the market value of the bale, because the weight loss may offset any gain fromgrade improvement. Increasing the number of saw lint cleaners at gins, in addition to increasing the nep count and short-fiber content of the raw lint, causes problems at the spinning mill. These show up as more neps in the card web and reduced yarn strength and appearance. Pima cotton, extra-long-staple cotton, is roller ginned to preserve its length and to minimize neps. To maintain the highest possible quality bale of pima cotton, mill-type lint cleaners were for a long time the predominant cleaner used by the roller-ginning industry. Today, various combinations of impacts, incline, and pneumatic cleaners are used in most roller-ginning plants to increase lint-cleaning capacity.

Cotton Fiber quality Two simple words, fiber quality, mean quite different things to cotton growers and to cotton processors. No after-harvest mechanisms are available to either growers or processors that can improve intrinsic fiber quality. Most cotton production research by physiologists and agronomists has been directed toward improving yields, so the few cultural-input strategies suggested for

58

Textile Technology

improving fiber quality during the production season are of limited validity. Thus, producers have limited alternatives in production practices that might result in fibers of acceptable quality and yield without increased production costs. Fiber processors seek to acquire the highest quality cotton at the lowest price, and attempt to meet processing requirements by blending bales with different average fiber properties. Of course, bale averages for fiber properties do not describe the fiber-quality ranges that can occur within the bales or the resulting blends. Further, the natural variability among cotton fibers unpredictably reduces the processing success for blends made up of low-priced, lower-quality fibers and highpriced, higher-quality fibers. Blends that fail to meet processing specifications show marked increases in processing disruptions and product defects that cut into the profits of the yarn and textile manufacturers. Mill owners do not have sufficient knowledge of. the role classing-office fiber properties play in determining the outcome of cotton spinning and dyeing processes. Even when a processor is able to make the connection between yarn and fabric defects and increased proportions of low-quality fibers, producers have no way of explaining why the rejected bales failed to meet processing specifications when the bale averages for important fiber properties fell within the acceptable ranges. If, on the other hand, the causes of a processing defect are unknown, neither the producer nor the processor will be able to prevent or avoid that defect in the future. Any future research that is designed to

Processing of Textile Fibres

59

predict, prevent, or avoid low-quality cotton fibers that cause processing defects in yarn and fabric must address the interface between cotton production and cotton processing. Every bale of cotton produced in the USA crosses that interface via the USDA-AMS classing offices, which report bale averages of quantified fiber properties. Indeed, fiber-quality data reports from classing offices are designed as a common quantitative language that can be interpreted and understood by both producers and processors. But the meaning and utility of classing-office reports can vary, depending on the instrument used to evaluate. Fiber maturity is a composite of factors, including inherent genetic fineness compared with the perimeter or cross section achieved under prevailing growing conditions and the relative fiber cell-wall thickness and the primary -to- secondary fiber cellwall ratio, and the time elapsed between flowering and boll opening or harvest. While all the above traits are important to varying degrees in determining processing success, none of them appear in classing-office reports. Micronaire, which is often treated as the fiber maturity measurement in classing-office data, provides an empirical composite of fiber cross section and relative wall thickening. But laydown blends that are based solely on bale-average micronaire will vary greatly in processing properties and outcomes. Cotton physiologists who follow fiber development can discuss fiber chronological maturity in terms of days after floral anthesis. But, they must quantify the corresponding fiber physical maturity as micronaire readings for

60

Textile Technology

samples pooled across several plants, because valid micronaire determinations require at least 10 g of individualized fiber. Some fiber properties, like length and single fiber strength, appear to be simple and easily understood terms. But the bale average length reported by the classing office does not describe the range or variability of fiber lengths that must be handled by the spinning equipment processing each individual fiber from the highly variable fiber population found in that bale. Even when a processing problem can be linked directly to a substandard fib~r property, surprisingly little is known about the causes of variability in fiber shape and maturity. For example: Spinners can see the results of excessive variability in fiber length or strength when manifested as yarn breaks and production halts.Knitters and weavers can see the knots and slubs or holes that reduce the value of fabrics made from defective yarns that were spun from poor-quality fibre Inspectors of dyed fabrics can see the unacceptable color streaks and specks associated with variations in fiber maturity and the relative dye-uptake success. The grower, ginner, and buyer can see variations in color or trash content of ginned and baled cotton. But there are no inspectors or instruments that can see or predict any of the above quality traits of fibers while they are developing in the boll. There is no definitive reference source, model, or database to which a producer can turn for information on how cultural inputs could be adapted to the prevailing growth conditions of soil

Processing of Textile Fibres

61

fertility, water availability, and weather (temperature, for example) to produce higher quality fiber. The scattered research publications that address fiber quality, usually in conjunction with yield improvement, are confusing because their measurement protocols are not standardized and results are not reported in terms that are meaningful to either producers or processors. Thus, physiological and agronomic studies of fiber quality frequently widen, rather than bridge, the communication gap between cotton producers and processors. This overview assembles and assesses current literature citations regarding the quantitation of fiber quality and the manner in which irrigation, soil fertility, weather, and cotton genetic potential interact to modulate fiber quality. The ultimate goal is to provide access to the best answers currently available to the question of what causes the annual and regional fiber quality variations From the physiologist's perspective, the fiber quality of a specific cotton genotype is a composite of fiber shape and maturity properties that depend on complex interactions among the genetics and physiology of the plants producing the fibers and the growth environment prevailing during the cotton production season. Fiber shape properties, particularly length and diameter, are largely dependent on genetics. Fiber maturity properties, which are dependent on deposition of photosynthate in the fiber cell wall, are more sensitive to changes in the growth environment. The effects of the growth environment on the genetic potential of a genotype modulate both shape and maturity properties to varying degrees.

62

Textile Technology

Anatomically, a cotton fiber is a seed hair, a single hyperelongated cell arising from the protodermal cells of the outer integument layer of the seed coat. Like all living plant cells, developing cotton fibers respond individually to fluctuations in the macro- and microenvironments. Thus, the fibers on a single seed constitute continua of fiber length, shape, cell-wall thickness, and physical maturity . Environmental variations within the plant canopy, among the individual plants, and within and among fields ensure that the fiber population in each boll, indeed on each seed, encompasses a broad range of fiber properties and that every bale of cotton contains a highly variable population of fibers. Successful processing of cotton lint depends on appropriate management during and after harvest of those highly variable fiber properties that have been shown to affect finished-product quality and manufacturing efficiency . If fiber-blending strategies and subsequent spinning and dyeing processes are to be optimized for specific end-uses and profitability, production managers in textile mills need accurate and effective descriptive and predictive quant.itative measures of both the means and the ranges of these highly variable fiber properties. In the USA, the components of cotton fiber quality are usually defined as those properties reported for every bale by the classing offices of the USDA-AMS, which currently include length, length uniformity index, strength, micronaire, color as reflectance (Rd) and yellowness (+b), and trash content, all quantified by the High Volume Instrument (HVI) line. The classing offices also provide each bale with the more qualitative classers'

Processing of Textile Fibres

63

color and leaf grades and with estimates of preparation (degree of roughness of ginned lint) and content of extraneous matter. The naturally wide variations in fiber quality, in combination with differences in end-use requirements, result in significant variability in the value of the cotton lint to the processor. Therefore, a system of premiums and discounts has been established to denote a specified base quality. In general, cotton fiber value increases as the bulk-averaged fibers increase in whiteness (+Rd), length, strength, and micronaire; and discounts are made for both low mike (micronaire less than 3.5) and high mike (micronaire more than 4.9). Ideal fiber-quality specifications favored by processors traditionally have been summarized thusly: "as white as snow, as long as wool, as strong as steel, as fine as silk, and as cheap as hell." These specifications are extremely difficult to incorporate into a breeding program or to set as goals for cotton producers. Fiberclassing technologies in use and being tested allow quantitation of fiber properties, improvement of standards for end-product quality, and, perhaps most importantly, creation of a fiber-quality language and system of fiber-quality measurements that can be meaningful and useful to producers and processors alike.

Gene and environmental variability Improvements in textile processing, particularly advances in spinning technology, have led to increased emphasis on breeding cotton for both improved yield and improved fiber properties Studies of gene action suggest that, within upland cotton genotypes there is little non-

64

Textile Technology

additive gene action in fiber length, strength, and fineness ; that is, genes determine those fiber properties. However, large interactions between combined annual environmental factors (primarily weather) and fiber strength suggest that environmental variability can prevent full realization of the fiber-quality potential of a cotton genotype. More recently, statistical comparisons of the relative genetic and environmental influences upon fiber strength suggest that fiber strength is determined by a few major genes, rather than by variations in the growth environment . Indeed, spatial variations of single fertility factors in the edaphic environment were found to be unrelated to fiber strength and only weakly correlated with fiber length. Genetic potential of a specific genotype is defined as the level of fiber yield or quality that could be attained under optimal growing conditions. The degree to which genetic potential is realized changes in response to environmental fluctuations such as application of water or fertilizer and the inevitable seasonal shifts such as temperature, day length, and insolation. In addition to environment-related modulations of fiber quality at the crop and whole-plant levels, significant differences in fiber properties also can be traced to variations among the shapes and maturities of fibers on a single seed and, consequently, within a given boll.

Effect of fiber length Comparisons of the fiber-length arrays from different regions on a single seed have revealed that markedly

Processing of Textile Fibres

65

different patterns in fiber length can be found in the micropylar, middle, and chalaza I regions of a cotton seed - at either end and around the middle. Mean fiber lengths were shortest at the micropylar (upper, pointed end of the seed) . The most mature fibers and the fibers having the largest perimeters also were found in the micropylar region of the seed. After hand ginning, the percentage of short fibers less than 0.5 inch or l2.7 mm long on a cotton seed was extremely low. It has been reported that, in ginned and baled

cotton, the short fibers with small perimeters did not originate in the micropylar region of the seed . MEasurements of fibers from micropylar and chalazal regions of seeds revealed that the location of a seed within the boll was related to the magnitude of the differences in the properties of fibers from the micropylar and chalazal regions. Significant variations in fiber maturity also can be related to the seed position (apical, medial, or due to the variability inherent in cotton fiber, there is no absolute value for fiber length within a genotype or within a test sample . Even on a single seed, fiber lengths vary significantly because the longer fibers occur at the chalazal (cup-shaped, lower) end of the seed and the shorter fibers are found at the micropylar (pointed) end. Coefficients of fiber-length variation, which also vary significantly from sample to sample, are on the order of 40% for upland cotton. Variations in fiber length attributable to genotype and fiber location on the seed are modulated by factors in the micro- and macroenvironment. Environmental changes occurring around the time of floral anthesis may limit fiber initiation or retard the onset of fiber

66

Textile Technology

elongation. Suboptimal environmental conditions during the fiber elongation phase may decrease the rate of elongation or shorten the elongation period so that the genotypic potential for fiber length is not fully realized . Further, the results of environmental stresses and the corresponding physiological responses to the growth environment may become evident at a stage in fiber development that is offset in time from the occurrence of the stressful conditions. Fiber lengths on individual seeds can be determined while the fibers are still attached to the seed , by hand stapling or by photoelectric measurement after ginning. Traditionally, staple lengths have been measured and reported to the nearest 32nd of an inch or to the nearest millimeter. The four upland staple classes are: short «21 . mm), medium (22-25 mm), medium-long (26-28 mm) and long (29-34 mm). Pima staple length is classed as long (29-34 mm) and extra-long (>34 mm). Additionally, short fiber content is defined as the percentage of fiber less than 12.7 mm. Cotton buyers and processors used the term staple length long before development of quantitative methods for measuring fiber properties. Consequently, staple length has never been formally defined in terms of a statistically valid length distribution. In Fibrograph testing, fibers are randomly caught on combs, and the beard formed by the captured fibers is scanned photoelectrically from base to tip . The amount of light passing through the beard is a measure of the number of fibers that extend various distances from the combs. Data are recorded as span length (the distance spanned by a specific percentage of fibers in the test beard). Span lengths are usually reported as 2.5 and 50%.

Processing of Textile Fibres

67

The 2.5% span length is the basis for machine settings at various stages during fiber processing. The uniformity ratio is the ratio between the two span lengths expressed as a percentage of the longer length. The Fibrograph provides a relatively fast method for reproducibility in measuring the length and length uniformity of fiber samples. Fibrograph test data are used in research studies, in qualitative surveys such as those checking commercial staple-length classifications, and in assembling cotton bales into uniform lots. Since 1980, USDA-AMS classing offices have relied almost entirely on high-volume instrumentation (HVI) for measuring fiber length and other fiber properties (Moore, 1996). The HVI length analyzer determines length parameters by photoelectrically scanning a test beard that is selected by a specimen loader and prepared by a comber /brusher attachment The fibers in the test beard are assumed to be uniform in cross-section, but this is a false assumption because the cross section of each individual fiber in the beard varies significantly from tip to tip. The HVI fiberlength data are converted into the percentage of the total number of fibers present at each length value and into other length parameters, such as mean length, upper-half mean length, and length uniformity . This test method for determining cotton fiber length is considered acceptable for testing commercial shipments when the testing services use the same reference standard cotton samples. All fiber-length methods discussed above require a minimum of 5 g of ginned fibers and were developed for rapid classing of relatively large, bulk fiber samples. For analyses of small fiber samples, fiber property

68

Textile Technology

measurements with an electron-optical particle-sizer, the Zellweger Uster AFIS-A2 have been found to be acceptably sensitive, rapid, and reproducible. The AFISA2 Length and Diameter module generates values for mean fiber length by weight and mean fiber length by number, fiber length histograms, and values for upper quartile length, and for short-fiber contents by weight and by number (the percentages of fibers shorter than 12.7 mm). The AFIS-A2 Length and Diameter module also quantifies mean fiber diameter by number . Although short-fiber content is not currently included in official USDA-AMS classing office reports, short-fiber content is increasingly recognized as a fiber property comparable in importance to fiber fineness, strength, and length . The importance of short-fiber content in determining fiber-processing success, yarn properties, and fabric performance has led the postharvest sector of the U.S. cotton industry to assign top priority to minimizing short-fiber content, whatever the causes. The perceived importance of short-fiber content to processors has led to increased demands for development and approval of a standard short-fiber content measurement that would be added to classing office HVI systems . This accepted classing office-measurement would allow inclusion of short-fiber content in the cotton valuation system. Documentation of post-ginning shortfiber content at the bale level is expected to reduce the cost of textile processing and to increase the value of the raw fiber . However, modulation of short-fiber content before harvest cannot be accomplished until the causes of increased short-fiber content are better understood.

Processing of Textile Fibres

69

Fiber length is primarily a genetic trait, but shortfiber content is dependent upon genotype, growing conditions, and harvesting, ginning, and processing methods. Further, little is known about the levels or sources of pre-harvest short-fiber content . It is essential that geneticists and physiologists understand the underlying concepts and the practical limitations of the methods for measuring fiber length and short-fiber content so that the strong genetic component in fiber length can be separated from environmental components introduced by excessive temperatures and water or nutrient deficiencies. Genetic improvement of fiber length is fruitless if the responses of the new genotypes to the growth environment prevent full realization of the enhanced genetic potential or if the fibers produced by the new genotypes break more easily during harvesting or processing. The reported effects of several environmental factors on fiber length and shortfiber content, which are assumed to be primarily genotype-dependent, are discussed in the subsections that follow.

Fiber Length and Temperature Maximum cotton fiber lengths were reached when night temperatures were around 19 to 20 oc, depending on the genotype. Early-stage fiber elongation was highly temperature dependent; late fiber f'lo~\gation was temperature independent. Fiber length (upper-half mean length) was negatively correlated with the difference between maximum and minimum temperature. Modifications of fiber length by growth temperatures also have been observed in planting-date

70

Textile Technology

studies in which the later planting dates were associated with small increases in 2.5 and 50% span lengths. If the growing season is long enough and other inhibitory factors do not interfere with fiber development, earlyseason delays in fiber initiation and elongation may be counteracted by an extension of the elongation period . Variations in fiber length and the elongation period also were associated with relative heat-unit accumulations. Regression analyses showed that genotypes that produced longer fibers were more responsive to heat-unit accumulation levels than were genotypes that produced shorter fibers. However, the earliness of the genotype was also a factor in the relationship between fiber length (and short-fiber content by weight) and accumulated heat units . As temperature increased, the number of small motes per boll also increased. Fertilization efficiency, which was negatively correlated with small-mote frequency, also decreased. Although fiber length did not change significantly with increasing temperature, the percentage of short-fibers was lower when temperatures were higher. The apparent improvement in fiber length uniformity may be related to increased assimilate availability to the fibers because there were fewer seeds per boll.

Fiber Length and Water Cotton water relationships and irrigation traditionally have been studied with respect to yield . Fiber length was not affected unless the water deficit was great enough to lower the yield to 700 kg ha-l. Fiber elongation was inhibited when the midday water

Processillg of Textile Fibres

71

potential w,as -2.5 to -2.8 mPa. Occurrence of moisture deficits during the early flowering period did not alter fiber length. However, when drought occurred later in the flowering period, fiber length was decreased . Severe water deficits during the fiber elongation stage reduce fiber length, apparently due simply to the direct mechanical and physiological processes of cell expansion. However, water availability and the duration and timing of flowering and boll set can result in complex physiological interactions between water deficits and fiber properties including length.

Fibre Length and Light Changes in the growth environment also alter canopy structure and the photon flux environment within the canopy. For example, loss of leaves and bolls from unfavorable weather (wind, hail), disease, or herbivory and compensatory regrowth can greatly affect both fiber yield and quality. The amount of light within the crop canopy is an important determinant of photosynthetic activity and, therefore, of the source-to-sink relationships that allocate photoassimilate within the canopy . Eaton and Ergle (1954) observed that reduced-light treatments increased fiber length. Shading during the first 7 dafter floral anthesis resulted in a 2% increase in the 2.5'Yo span length. Shading (or prolonged periods of cloudy weather) and seasonal shifts in day length also modulate temperature, which modifies fiber properties, including length. Commercial cotton genotypes are considered to be day-length neutral with respect to both flowering and

Textile Technology

72

fruiting. However, incorporation of day-length data in upland and pima fiber-quality models, based on accumulated heat units, increased the coefficients of determination for the length predictors from 30 to 54% for the upland model and from 44 to 57% for the pima model. It was found that the light wavelengths reflected

from red and green mulches increased fiber length, even though plants grown under those mulches received less reflected photosynthetic flux than did plants grown with white mulches. The longest fiber was harvested from plants that received the highest far red/red ratios.

Fiber Length and Mineral Nutrition Studies of the mineral nutrition of cotton and the related soil chemistry usually have emphasized increased yield and fruiting efficiency. More recently, the effects of nutrient stress on boll shedding have been examined. Also, several mineral-nutrition studies have been extended to include fiber quality . Reports of fiber property trends following nutrient additions are often contradictory due to the interactive effects of genotype, climate, and soil conditions. Potassium added at the rate of 112 kg K ha-lyr-l did not affect the 2.5% span length, when genotype was a significant factor in determining both 2.5 and 50% span lengths . Genotype was not a significant factor in Acala fiber length, but an additional 480 kg K ha-lyr-l increased the mean fiber length. K ha-l yr-l increased the length uniformity ratio and increased 50%, but not 2.5% span length. Genotype and the interaction, genotype-byenvironment, determined the 2.5% span length.

Processing of Textile Fibres

73

As mentioned above, fiber length is assumed to be genotype-dependent, but growth-environment fluctuations - both those resulting from seasonal and annual variability in weather conditions and thos~ induced by cultural practices and inputs - modulate the range and mean of the fiber length population at the test sample, bale, and crop levels. Quantitation of fiber length is relatively straight forward and reproducible, and fiber length (along with micronaire) is one of the most likely fiber properties to be included when cotton production research is extended beyond yield determinations. Other fiber properties are less readily quantified, and the resulting data are not so easily understood or analyzed statistically. This is particularly true of fiber-breaking strength, which has become a crucial fiber property due to changes in spinning techniques.

Fiber Strength The inherent breaking strength of individual cotton fibers is considered to be the most important factor in determining the strength of the yarn spun from those fibers . Recent d~velopments in high-speed yarn spinning technology, specifically open-end rotor spinning systems, have shifted the fiber-quality requirements of the textile industry toward higher-strength fibers that can compensate for the decrease in yarn strength associated with open-end rotor spinning techniques. Compared with conventional ring spinning, openend rotor-spun yarn production capacity is five times greater and, consequently, more economical. Rotor-spun yarn is more even than the ring-spun, but is 15 to 20%

74

Textile Technology

weaker than ring-spun yarn of the same thickness. Thus, mills using open-end rotor and friction spinning have given improved fiber strength highest priority. Length and length uniformity, followed by fiber strength and fineness, remain the most important fiber properties in determining ring-spun yarn strength. Historically, two instruments have been used to measure fiber tensile strength, the Pressley apparatus and the Stelometer. In both of these flat-bundle methods, a bundle of fibers is combed parallel and secured between two clamps. A force to try to separate the clamps is applied and gradually increased until the fiber bundle breaks. Fiber tensile strength is calculated from the ratio of the breaking load to bundle mass. Due to the natural lack of homogeneity within a population of cotton fibers, bundle fiber selection, bundle construction and, therefore, bundle mass measurements, are subject to considerable experimental error. Fiber strength, that is, the force required to break a fiber, varies along the length of the fiber, as does fiber fineness measured as perimeter, diameter, or cross section Further, the inherent variability within and among cotton fibers ensures that two fiber bundles of the same weight will not contain the same number of fibers. Also, the within-sample variability guarantees that the clamps of the strength testing apparatus will not grasp the various fibers in the bundle at precisely equivalent positions along the lengths. Thus, a normalizing lengthweight factor is included in bundle strength calculations. In the textile literature, fiber strength is reported as breaking tenacity or grams of breaking load per tex, where tex is the fiber linear density in grams per kilometer. Both Pressley and stelometer breaking

Processing of Textile Fibres

75

tenacities are reported as 1/8 in. gauge tests, the 1/8 in. (or 3.2 mm) referring to the distance between the two Pressley clamps. Flat-bundle measurements of fiber strength are considered satisfactory for acceptance testing and for research studies of the influence of genotype, environment, and processing on fiber (bundle) strength and elongation. The relationships between fiber strength and elongation and processing success also have been examined using flat-bundle strength testing methods. However cotton fiber testing today requires that procedures be rapid, reproducible, automated, and without significant operator bias. Consequently, the HVI systems used for length measurements in USDA-AMS classing offices are also used to measure the breaking strength of the same fiber bundles (beards) formed during length measurement. Originally, HVI strength tests were calibrated against the lI8-in. gauge Pressley measurement, but the bundle-strengths of reference cotton~ are now established by Stelometer tests that also provide bundle elongationpercent data. Fiber bundle elongation is measured directly from the displacement of the jaws during the bundle-breaking process, and the fiber bundle strength and elongation data usually are reported together (ASTM, 1994, D 4604-86). The HVI bundle-strength measurements are reported in grams-force tex-1 and can range from 30 and above (very strong) to 20 or below (very weak). In agronomic papers, fiber strengths are normally reported as kN m kg-I, where one Newton equals 9.81 kg-force. The BVI bundle-strength and elongation-percent testing methods are satisfactory for acceptance testing

76

Textile Technology

. and research studies when 3.0 to 3.3 g of blended fibers are available and the relative humidity of the testing room is adequately controlled. A 1% increase in relative humidity and the accompanying increase in fiber moisture content will increase the strength value by 0.2 to 0.3 g tex-I, depending on the fiber genotype and maturity. Further, classing-office HVI measurements of fiber strength do not adequately describe the variations of fiber strength along the length of the individual fibers or within the test bundle. Thus, predictions of yarn strength based on HVI bundle-strength data can be inadequate and misleading. The problem of fiber-strength variability is being addressed by improved HVI calibration methods and by computer simulations of bundle-break tests in which the simulations are based on large single-fiber strength databases of more than 20 000 single fiber longelongation curves obtained with MANTIS.

Fiber Strength, Environment, and Genotype: Reports of stelometer measurements of fiber bundle strength are relatively rare in the refereed agronomic literature. Consequently, the interactions of environment and genotype in determining fiber strength are not as well documented as the corresponding interactions that modulate fiber length. Growth environment, and genotype response to that environment, playa part in determining fiber strength and strength variability. Early studies showed fiber strength to be significantly and positively correlated with maximum or mean growth temperature, maximum minus minimum growth temperature, and potential insolation . Increased

Processing of Textile Fibres

77

strength was correlated with a decrease in precipitation. Minimum temperature did not affect fiber strength. All environmental variables were interrelated, and a close general association between fiber strength and environment was interpreted as indicating that fiber strength is more responsive to the growth environment than are fiber length and fineness. Other investigators reported that fiber strength was correlated with genotype only. Square removal did not affect either fiber elongation or fiber strength. Shading, leaf-pruning, and partial fruit removal decreased fiber strength . Selective square removal had no effect on fiber strength in bolls at the first, second, or third position on a fruiting branch. Fiber strength was slightly greater in bolls from the first 4 to 6 wk of flowering, compared with fibers from bolls produced by flowers opening during the last 2 wk of the flowering period . In that study, fiber strength was positively correlated with heat unit accumulation during boll development, but genotype, competition among bolls, assimilatory capacity, and variations in light environment also helped determine fiber strength. Early defoliation, at 20% open bolls, increased fiber strength and length, but the yield loss due to earlier defoliation offset any potential improvement in fiber quality. Of the fiber properties reported by USDA-AMS classing offices for use by the textile industry, fiber maturity is probably the least well-defined and most misunderstood. The term, fiber maturity, used in cotton marketing and processing is not an estimate of the time elapsed between floral anthesis and fiber harvest , However, such chronological maturity can be a useful

78

Textile Technology

concept in studies that follow fiber development and maturation with time . On the physiological and the physical bases, fiber maturity is generally accepted to be the degree (amount) of fiber cell-wall thickening relative to the diameter or fineness of the fiber . Classically, a mature fiber is a fiber in which two times the cell wall thickness equals or exceeds the diameter of the fiber cell lumen, the space enclosed by the fiber cell walls. However, this simple definition of fiber maturity is complicated by the fact that the cross section of a cotton fiber is never a perfect circle; the fiber diameter is primarily a genetic characteristic. Further, both the fiber diameter and the cell-wall thickness vary significantly along the length of the fiber. Thus, attempting to differentiate, on the basis of wall thickness, between naturally thin-walled or genetically fine fibers and truly immature fibers with thin walls greatly complicates maturity comparisons among and within genotypes. Within a single fiber sample examined by image analysis, cell-wall thickness ranged from 3.4 to 4.9 ]lm when lumen diameters ranged from 2.4 to 5.2 ]lm . Based on the cited definition of a mature fiber having a cellwall thickness two times the lumen diameter, 90% of the 40 fibers in that sample were mature, assuming that here had been no fiber-selection bias in the measurements. Unfortunately, none of the available methods for quantifying cell-wall thickness is sufficiently rapid and reproducible to be used by agronomists, the 'classing offices, or fiber processors. Fiber diameter can be quantified, but diameter data are of limited use in determining fiber maturity without estimates of the relationship between lumen width and wall thickness.

Processing of Textile Fibres

79

Instead, processors have attempted to relate fiber fineness to processing outcome.

Estimating Fiber Fineness Fiber fineness has long been recognized as an important factor in yarn strength and uniformity, properties that depend largely on the average number of fibers in the yarn cross section. Spinning larger numbers of finer fibers together results in stronger, more uniform yarns than if they had been made up of fewer, thicker fibers . However, direct determinations of biological fineness in terms of fiber or lumen diameter and cell-wall thickness are precluded by the high costs in both time and labor, the noncircular cross sections of dry cotton fibers, and the high degree of variation in fiber fineness. Advances in image analysis have improved determinations of fiber biological fineness and maturity , but fiber image analyses remain too slow and limited with respect to sample size for inclusion in the HVIbased cotton-classing process. Originally, the textile industry adopted gravimetric fiber fineness or linear density as an indicator of the fiber-spinning properties that depend on fiber fineness and maturity combined. This gravimetric fineness testing method was discontinued in 1989, but the textile linear density unit of tex persists. Tex is measured as grams per kilometer of fiber or yarn, and fiber fineness is usually expressed as millitex or micrograms per meter . Earlier, direct measurements of fiber fineness (either biological or gravimetric) subsequently were replaced by indirect fineness measurements based on the resistance of a bundle of fibers to airflow.

80

Textile Technology

The first indirect test method approved by ASTM for measurement of fiber maturity, lineardensity, and maturity irdex was the causticaire method. In that test, the resistance of a plug of cotton to airflow was measured before and after a cell-wall swelling treatment with an 18% (4.5 M) solution of NaOH (ASTM, 1991, 0 2480-82). The ratio between the rate of airflow through an untreated and then treated fiber plug was taken as indication of the degree of fiber wall development. The airflow reading for the treated sample was squared and corrected for maturity to serve as an indirect estimate of linear density. Causticaire method results were found to be highly variable among laboratories, and the method never was recommended for acceptance testing before it was discontinued in 1992The arealometer was the first dual-compression airflow instrument for estimating both fiber fineness and fiber maturity from airflow rates through untreated raw cotton (ASTM, 1976, 0 1449-58; Lord and Heap, 1988). The arealometer provides an indirect measurement of the specific surface area of loose cotton fibers, that is, the external area of fibers per unit volume (approximately 200-mg samples in four to five replicates). Empirical formulae were developed for calculating the approximate maturity ratio and the average perimeter, wall thickness, and weight per inch from the specific surface area data. The precision and accuracy of arealometer determinations were sensitive to variations in sample preparation, to repeated sample handling, and to previous mechanical treatment of the fibers, e.g., conditions during harvesting, blending, and opening. The arealometer was never approved for acceptance testing, and the ASTM method was withdrawn in 1977 without replacement.

Processing of Textile Fibres

81

The variations in biological fineness and relative maturity of cotton fibers that were described earlier cause the porous plugs used in air-compression measurements to respond differently to compression and, consequently, to airflow. The lIC-Shirley Fineness/Maturity Tester (Shirley FMT), a dual-compression instrument, was developed to compensate for this plug-variation effect (ASTM, 1994, D 3818-92). The Shirley FMT is considered suitable for research, but is not used for acceptance testing due to low precision and accuracy. Instead, micronaire has become the standard estimate of both fineness and maturity in the USDA-AMS classing offices. Whatever the direct or indirect method used for estimating fiber maturity, the fiber property being as sayed remains the thickness of the cell wall. The primary cell wall and cuticle (together »0.1 pm thick) make up about 2.4% of the total wall thickness ( »4.1 pm of the cotton fiber thickness at harvest) . The rest of the fiber cell wall (»98%) is the cellulosic secondary wall, which thickens significantly as polymerized photosynthate is d~posited during fiber maturation. Therefore, any environmental factor that affects photosynthetic C fixation and cellulose synthesis will also modulate cotton fiber wall thickening and, consequently, fiber physiological maturation The dilution, on a weight basis, of the chemically complex primary cell wall by secondary-wall cellulose has been followed with X-ray fluorescence spectroscopy. This technique determines the decrease, with time, in the relative weight ratio of the Ca associated with the pectinrich primary wall . Growth-environment differences between the two years of the studies cited significantly altered maturation rates, which were quantified as rate of

82

Textile Technology

Ca weight-dilution, of both upland and pima genotypes. The rates of secondary wall deposition in both upland and pima genotypes were closely correlated with growth temperature; that is, heat-unit accumulation . Micronaire (micronAFIS) also was found to increase linearly with time for upland and pima genotypes . The rates of micronaire increase were correlated with heatunit accumulations . Rates of increase in fiber crosssectional area were less linear than the corresponding micronaire-increase rates, and rates of upland and pima fiber cell-wall thickening were linear and without significant genotypic effect . Environmental modulation of fiber maturity (micronaire) by temperature was most often identified in planting- and flowering-date studies . The effects of planting date on micronaire, Shirley FMT fiber maturity ratio, and fiber fineness (in millitex) were highly significant in a South African study (Greef and Human, 1983). Although genotypic differences were detected among the three years of that study, delayed planting generally resulted in lower micronaire. The effect on fiber maturity of late planting was repeated in the Shirley FMT maturity ratio and fiber fineness data. Planting date significantly modified degree of thickening, immature fiber fraction, cross-sectional area, and micronaire (micronAFIS) of four upland genotypes that also were grown in South Carolina. In general, micronaire decreased with later planting, but early planting also reduced micronaire of Deltapine 5490, a long-season genotype, in a year when temperatures were suboptimal during the early part of the season. Harvest dates in this study also were staggered so that the length of the growing season was held constant

Processing of Textile Fibres

83

within each year. Therefore, season-length should not have been an important factor in the relationships found between planting date and fiber maturity.

Fiber Maturity and Source-Sink Manipulation Variations in fiber maturity were linked with source-sink modulations related to flowering date, and seed position within the bolls . However, manipulation of source-sink relationships by early-season square (floral bud) removal had no consistently significant effect on upland cotton micronaire in one study. However, selective square removal at the first, second, and third fruiting sites along the branches increased micronaire, compared with controls from which no squares had been removed beyond natural square shedding . The increases in micronaire after selective square removals were associated with increased fiber wall thickness, but not with increased strength of elongation percent. Earlyseason square removal did not affect fiber perimeter or wall thickness (measured by arealometer) . Partial defruiting increased micronaire and had no consistent effect on upland fiber perimeter in bolls from August flowers. Generous water availability can delay fiber maturation (cellulose deposition) by stimulating competition for assimilates between early-season bolls and vegetative growth . Adequate water also can increase the maturity of fibers from mid-season flowers by supporting photosynthetic C fixation. In a year with insufficient rainfall, initiating irrigation when the first-set bolls were 20-d old increased micronaire, but irrigation initiation at first bloom had no effect on fiber maturity.

84

Textile Technology

Irrigation and water-conservation effects on fiber fineness (millitex) were inconsistent between years, but both added water and mulching tended to increase fiber fineness. Aberrations in cell-wall synthesis that were correlated with drought stress have been detected and characterized by glycoconjugate analysis. An adequate water supply during the growing season allowed maturation of more bolls at upper and outer fruiting positions, but the mote counts tended to be higher in those extra bolls and the fibers within those bolls tended to be less mature . Rainfall and the associated reduction in insolation levels during the blooming period resulted in reduced fiber maturity. Irrigation method also modified micronaire levels and distributions among fruiting sites. Early-season drought resulted in fibers of greater maturity and higher micronaire in bolls at branch positions 1 and 2 on the lower branches of rainfed plants. However, reduced insolation and heavy rain reduced micronaire and increased immature fiber fractions in bolls from flowers that opened during the prolonged rain incident. Soil water deficit as well as excess may reduce micronaire if the water stress is severe or prolonged . Micronaire or maturity data now appear in most cotton improvement reports . In a five-parent half-diallel mating design, environment had no effect on HVI micronaire . However, a significant genotypic effect was found to be associated with differences between parents and the PI generation and with differences among the PI generation. The micronaire means for the parents were not significantly different, although HVI micronaire means were significantly different for the PI generation as a group. The HVI was judged to be insufficiently

85

Processing of Textile Fibres

sensitive for detection of the small difference maturity resulting from the crosses.

In

fiber

In another study, F2 hybrids had finer fibers (lower micronaire) than did the parents, but the improvements were deemed too small to be of commercial value. Unlike the effects of environment on the genetic components of other fiber properties, variance in micronaire due to the genotype-by-environment interaction can reach levels expected for genetic variance in length and strength. Significant interactions were found between genetic additive variance and environmental variability for micronaire, strength, and span length in a study of 64 F2 hybrids. The strong environmental components in micronaire and fiber maturity limit the usefulness of these fiber properties in studies of genotypic differences in response to growth environment. Based on micronaire, fiber maturity, cell-wall thickness, fiber perimeter, or fiber fineness data, row spacing had either no or minimal effect on okra-leaf or normal-leaf genotypes . Early planting reduced micronaire, wall-thickness, and fiber fineness of the okra-leaf genotype in one year of that study. In another study of leaf pubescence, nectaried vs. no nectaries, and leaf shape, interactions with environment were significant but of much smaller magnitude than the interactions among traits. Micronaire means for Bt transgenic lines were higher than the micronaire means of Coker 312 and MD51ne when those genotypes were grown in Arizona. In two years out of three, micronaire means of all genotypes in this study, including the controls, exceeded 4.9; in other words, were penalty grade. This apparent undesirable environmental effect on micronaire may have been

86

Textile Technology

caused by a change in fiber testing methods in the one year of the three for which micronaire readings were below the upper penalty limit. Genotypic differences in bulk micronaire may either be emphasized or minimized, depending on the measurement method used.

Grade In U.S. cotton classing, nonmandatory grade standards were first established in 1909, but compulsory upland grade standards were not set until 1915 . Official pima standards were first set in 1918. Grade is a composite assessment of three factors - color. leaf, and preparation. Color and trash (leaf and stem residues) can be quantified instrumentally, but traditional, manual cotton grade classification is still provided by USDA-AMS in addition to the instrumental HVI trash and color values. Thus, cotton grade reports are still made in terms of traditional color and leaf grades; for example, light spotted, tinged, strict low middling.

Preparation There is no approved instrumental measure of preparation-the degree of roughness/smoothness of the ginned lint. Methods of harvesting, handling, and ginning the cotton fibers produce differences in roughness that are apparent during manual inspection; but no clear correlations have been found between degree of preparation and spinning success. The frequency of tangled knots or mats of fiber (neps) may be higher in high-prep lint, and both the growth and processing environments can modulate nep frequency . However,

Processing of Textile Fibres

87

abnormal preparation occurs in less than 0.5% of the U.S. crop during harvesting and ginning.

Trash or Leaf Grade Even under ideal field conditions, cotton lint becomes contaminated with leaf residues and other trash. Although most foreign matter is removed by cleaning processes during ginning, total trash extraction is impractical and can lower the quality of ginned fiber. In HVI cotton classing, a video scanner measures trash in raw cotton, and the trash data are reported in terms of the total trash area and trash particle counts (ASTM, 0 4604-86, 0 4605-86). Trash content data may be used for acceptance testing. In 1993, classer's grade was split into color grade and leaf grade . Other factors being equal, cotton fibers mixed with the smallest amount of foreign matter have the highest value. Therefore, recent research efforts have been directed toward the development of a computer vision system that measures detailed trash and color attributes of raw cotton. The term leaf includes dried, broken plant foliage, bark, and stem particles and can be divided into two general categories: large-leaf and pin or pepper trash . Pepper trash significantly lowers the value of the cotton to the manufacturer, and is more difficult and expensive to remove than the larger pieces of trash. Other trash found in ginned cotton can include stems, burs, bark, whole seeds, seed fragments, motes (underdeveloped seeds), grass, sand, oil, and dust. The growth environment obviously affects the amount of wind-borne contaminants trapped among the fibers. Environmental factors that affect pollination and seed development determine the frequency of undersized seeds and motes.

88

Textile Technology

Reductions in the frequencies of motes and smallleaf trash also have been correlated with semi-smooth and super-okra leaf traits. Environment (crop year), harvest system, genotype, and second order interactions between those factors all had significant effects on leaf grade . Delayed harvest resulted in lower-grade fiber. The presence of trash particles also may affect negatively the color grade.

Fiber Color Raw fiber stock color measurements are used in controlling the color of manufactured gray, bleached, or dyed yarns and fabrics. Of the three components of cotton grade, fiber color is most directly linked to growth environment. Color measurements also are correlated with overall fiber quality so that bright (reflective, high Rd), creamy-white fibers are more mature and of higher quality than the dull, gray or yellowish fibers associated with field weathering and generally lower fiber quality. Although upland cotton fibers are naturally white to creamy-white, pre-harvest exposure to weathering and microbial action can cause fibers to darken and to lose brightness. Premature termination of fiber maturation by applications of growth regulators, frost, or drought characteristically increases the saturation of the yellow (+b) fiber-color component. Other conditions, including insect damage and foreign matter contamination, also modify fiber color. The ultimate acceptance test for fiber color, as well as for finished yarns and fabrics, is the human eye. Therefore, instrumental color measurements must be

Processmg of Textile Fibres

89

correlated closely with visual judgment. In the HVI classing system, color is quantified as the degrees of reflectance (Rd) and yellowness (+b), two of the three tristimulus color scales of the Nickerson-Hunter colorimeter. Fiber maturity has variability in finished grades of raw fibers environmental factors production.

been associated with dye-uptake yarn and fabric, but the color seldom have been linked to or agronomic practices during

Other environmental effects on cotton fiber quality Although not yet included in the USDA-AMS cotton fiber classing system, cotton stickiness is becoming an increasingly important problem . Two major causes of cotton stickiness are insect honeydew from whiteflies and aphids and abnormally high levels of natural plant sugars, which are often related to premature crop termination by frost or drought. Insect honeydew contamination is randomly deposited on the lint in heavy droplets and l->as a devastating production-halting effect on fiber processing. The cost of clearing and cleaning processing equipment halted by sticky cotton is so high that buyers have included honeydew free clauses in purchase contracts and have refused cotton from regions known to have insect-control problems. Rapid methods for instrumental detection of honeydew are under development for use in classing offices and mills .

90

Textile Technology

Fiber quality or fiber yield? Like all agricultural commodities, the value of cotton lint responds to fluctuations in the supply-and-demand forces of the marketplace. In addition, pressure toward specific improvements in cotton fiber quality - for example, ~he higher fiber strength needed for today's high-speed spinning - has been intensified as a result of technological advances in textile production and imposition of increasingly stringent quality standards for finished cotton products. Changes in fiber-quality requirements and increases in economic competition on the domestic and international levels have resulted in fiber quality becoming a value determinant equal to fiber yield . Indeed, it is the quality, not the quantity, of fibers ginned from the cotton seeds that decides the end use and economic value of a cotton crop and, consequently, determines the profit returned to both the producers and processors. Wide differences in cotton fiber quality and shifts in demand for particular fiber properties, based on end-use processing requirements, have resulted in the creation of a price schedule, specific to each crop year, that includes premiums and discounts for grade, staple length, micronaire, and strength. This price schedule is made possible by the development of rapid, quantitative methods for measuring those fiber properties considered most important for successful textile production . With the wide availability of fiber-quality data from HVI, predictive models for ginning, bale-mix selection, and fiber-processing success could be developed for textile mills.

Processing of Textile Fibres

91

Price-analysis systems based on HVI fiber-quality data also became feasible. Quantitation, predictive modeling, and statistical analyses of what had been subjective and qualitative fiber properties are now both practical and common in textile processing and marketing. Field-production and breeding researchers, for various reasons, have failed to take full advantage of the fiber-quality quantitation methods developed for the textile industry. Most field and genetic improvement studies still focus on yield improvement while devoting little attention to fiber quality beyond obtaining bulk fiber length, strength, and micronaire averages for each treatment. Indeed, cotton crop simulation and mapping models of the effects of growth environment on cotton have been limited almost entirely to yield prediction and cultural-input management. Plant physiological studies and textile-processing models suggest that bulk fiber-property averages at the bale, module, or crop level do not describe fiber quality with sufficient precision for use in a vertical integration of cotton production and processing. More importantly, bulk fiber-property means do not adequately and quantitatively describe the variation in the fiber populations or plant metabolic responses to environmental factors during the growing season. Such pooled or averaged descriptors cannot accurately predict how the highly variable fiber populations might perform during processing. Meaningful descriptors of the effects of environment on cotton fiber quality await high-resolution examinations of the variabilities, induced and natural, in fiber-quality averages. Only then can the genetic and

92

Textile Technology

environmental sources of fiber-quality variability be quantified, predicted, and modulated to produce the high-quality cotton lint demanded by today's textile industry and, ultimately, the consumer. Increased understanding of the physiological responses to the environment that interactively determine cotton fiber quality is essential. Only with such knowledge can real progress be made toward producing high yields of cotton fibers that are white as snow, as strong as steel, as fine as silk, and as uniform as genotypic responses to the environment will allow.

3 Cotton Fibres Cotton fibres is defined as white fibrous substance covering seeds harvested from Cotton Plant.

Seed cotton: (called Kapas In India - Paruthi Tamil)harvested from Cotton Plant.

In

Lint cotton: (RUIA in Hind~. PANJU in Tamil) is obtained by removing the seeds in a ginning machine. Lint cotton is spun into Yarn, which is woven or knitted into a Fabric. Researchers have found that cotton was grown more than 9000 years ago. However large scale cultivation commenced during middle of 17th Century AD. Many varieties of Cotton are cultivated mainly from 3 important genetic species of Gossipium. G. Hirsutim- 87% Grown in America, Africa, Asia, Australia Plant grows to a height of 2 Meters. G. Barbadebse- 8% Grown in America, Africa & Asia. Plant grows to a height of 2.5 Meters with yellow flowers, long fibers with good quality, fibers with long staple and fineness

94

Textile Technology

G. Arboreum - 5% Perennial plant grows up to 2 meters with red flowers, poor quality fibers in East Africa and South East Asia. There are four other species grown in very negligible quantities. Cotton harvested from the Plant by hand - picking or machine picking is ginned to remove seeds and the lint is pressed into Bales for delivery to Spinning Mills. Cotton is Roller Ginned (RG) or Saw Ginned (SG) depending varieties and ginning practices. Cotton is cultivated in 75 Countries with an area of 32 Million Hectares. Cultivation period varies from 175 days to 225 days depending on variety. Cotton is harvested in two seasons, summer and winter seasons. Saw ginned cotton is more uniform and cleaner than Roller Ginned Cotton. But fibers quality is retained better quality in Roller Ginning than Saw Ginning which has high productivity. Cotton Fiber is having a tubular structure in twisted form. Now. researchers have developed coloured cotton also. As on date, percentage of Cotton fiber use is more than synthetic fibers. But, its share is gradually reducing. Cotton is preferred for under garments due its comfort to body skin. Synthetics have more versatile uses and advantage for Industrial purposes.

Properties of cotton No other material is quite like cotton. It is the most important of all natural fibres, accounting for half of all the fibres used by the world's textile industry. Cotton has many qualities that make it the best choice for countless. Cotton fibres have a natural twist

Cotton Fibres

95

that makes them so suitable for spinning into a very strong yarn. The ability of water to penetrate right to the core of the fibre makes it easy to remove dirt from the cotton garments, and creases are easily removed by ironing. Cotton fabric is soft and comfortable to wear close to skin because of its good moisture absorption qualities. Charges of static electricity do not build up readily on the clothes.

History of Cotton Nobody seems to know exactly when people first began to use cotton, but there is evidence that it was cultivated in India and Pakistan and in Mexico and Peru 5000 years ago. In these two widely separated parts of the world, cotton must have grown wild. Then people learned to cultivate cotton plants in their fields. In Europe, wool was the only fiber used to make clothing. Then from the Far East came tales of plants that grew "wool". Traders claimed that cotton was the wool of tiny animals called Scythian lambs, that grew on the stalks of a plant. The stalks, each with a lamb as its flower, were said to bend over so the small sheep could graze on the grass around the plant. These fantastic stories were shown to be untrue when Arabs brought the cotton plant to Spain in Middle Ages. In the fourteenth century cotton was grown in Mediterranean countries and shipped from there to mills in the Netherlands in western Europe for spinning and weaving. Until the mid eighteenth century, cotton was not manufactured in England, because the wool

96

Textile Technology

manufacturers there did not want it to compete with their own product. They had managed to pass a law in 1720 making the manufacture or sale of cotton cloth illegal. When the law was finally repealed in 1736, cotton mills grew in number. In the United States though, cotton mills could not be established, as the English would not allow any of the machinery to leave the country because they feared the colonies would compete with them. But a man named Samuel Slater, who had worked in a mill in England, was able to build an American cotton mill from memory in 1790.

Plantation of cotton Cotton plant's leaves resemble maple leaves and flowers look very much like pink mallow flowers that grow in swampy areas. They are relatives and belong in the same plant family. Cotton is grown in about 80 countries, in a band that stretches around the world between latitudes 45 North to 30 South. For a good crop of cotton a long, sunny growing season with at least 160 frost-free days and ample water are required. Well drained, crumbly soils that can keep moisture well are the best. In most regions extra water must be supplied by irrigation. Because of it's long growing season it is best to plant early but not before the sun has warmed the soil enough. Seedlings appear about 5 days after planting the seeds. Weeds have to be removed because they compete with seedlings for water, light and minerals and also encourage pests and diseases. The first flower buds appear after 5-6 weeks, and in another 3-5 weeks these buds become flowers.

COttOIl

Fibres

97

Each flower falls after only 3 days leaving behind a small seed pot, known as the boll. Each boll contains about 30 seeds, and up to 500 000 fibres of cotton. Each fibre grows its full length in 3 weeks and for the following 4-7 weeks each fiber gets thicker as layers of cellulose build up the cell walls. While this is happening the boll matures and in about 10 weeks after flowering it splits open. The raw cotton fibres burst out to dry in the sun. As they lose water and die, each fibre collapses into what looks like a twisted ribbon. Now is time for harvesting. Most cotton is hand-picked. This is the best method of obtaining fully grown cotton because unwanted material, called "trash", like leaves and the remains of the boll are left behind. Also the cotton that is too young to harvest is left for a second and third picking. A crop can be picked over a period of two months as the bolls ripen. Countries that are wealthy and where the land is flat enough usually pick cotton with machines - cotton harvesters. COTTON AND YARN QUALITY CO-RELATION: Instead of buying any cotton available at lowest price, spinning it to produce yarn of highest count possible and selling Yam at any market in random, it is advisable to locate a good market where Yarn can be sold at highest price and select a Cotton which has characteristics to spin Yarn of desired specifications for that market. Essential characteristics of cotton quality and characteristics of Yarn quality of Yarn are given from detailed experimental investigations. Some of the important conclusions which help to find co-relation

98

Textile Technology

between Yarn quality and Cotton quality are given below STAPLE LENGTH: If the length of fiber is longer, it

can be spun into finer counts of Yarn which can fetch higher prices. It also gives stronger Yarn. STRENGTH : Stronger fibers give stronger Yarns.

Further, processing speeds can be higher so that higher productivity can be achieved with less endbreakages. FIBER FINENESS: Finer Fibers produce finer count of

Yarn and it also helps to produce stronger Yarns. FIBER MATURITY : Mature fibers give better evenness of Yarn. There will be less end - breakages . Better dyes' absorbency is additional benefit. UNIFORMITY RATIO: If the ratio is higher. Yam is more even and there is reduced end-breakages. ELONGATION :A better value of elongation will help

to reduce end-breakages in spinning and hence higher productivity with low wastage of raw material. NON-LINT CONTENT: Low percentage of Trash will

reduce the process waste in Blow Room and cards. There will be less chances of Yarn defects. SUGAR CONTENT: Higher Sugar Content will .create

stickiness of fiber and create processing problem of licking in the machines. MOISTURE CONTENT : If Moisture Content is more

than standard value of 8.5%, there will be more invisable loss. If moisture is less than 8.5%, then there will be tendency for brittleness of fiber resulting in frequent Yarn breakages. FEEL: If the feel of the Cotton is smooth, it will be

Cotton Fibres

99

produce more smooth yarn which has potential for weaving better fabric. CLASS : Cotton having better grade in classing will produce less process waste and Yarn will have better appearance. GREY VALUE: Rd. of calorimeter is higher it means it can reflect light better and Yam will give better appearance. YELLOWNESS : When value of yellowness is more, the grade becomes lower and lower grades produce weaker & inferior yarns. NEPPINESS: Neppiness may be due to entanglement of fibers in ginning process or immature fibers. Entangled fibers can be sorted out by careful processing But, Neps due to immature fiber wHl stay on in the end product and cause the level of. Yarndefects to go higher. An analysis can be made of Yarn properties which can be directly attributed to cotton quality. 1.

YARN COUNT: Higher Count of Yarn .can be produced by longer, finer and stronger fibers.

2.

C.V. of COUNT: Higher Fiber Uniformity and lower level of short fiber percentage will be beneficial to keep c.V.(Co-efficient of Variation) at lowest.

3.

TENSILE STRENGTH : This is directly related to fiber strength. Longer Length of fiber will 1180 help to produce stronger yarns.

4.

C.V. OF STRENGTH: is directly related CV of fiber strength.

5.

ELONGATION: Yam elongation will be beneficial for weaving efficiently. Fiber with better elongation have positive co-relation with Yarn elongation.

Textile Technology

100

6.

C.V. OF ELONGATION: C.V. of Yarn Elongation can be low when C.V. of fiber elongation is also low.

7.

MARS VARIA nON: This property directly related to fiber maturity and fiber uniformity.

8.

HAIRINESS: is due to faster processing speeds and

high level of very short fibers, 9.

DYEING QUALITY: will defend on Evenness of Yarn and marketing of cotton fibers.

10. BRIGHTNESS: Yarn will give brighter appearance if cotton grade is higher.

Cotton quality specifications The most important fiber quality is Fiber Length Length Staple

Classification Length mm Length inchesSpinning CountShortLess than 2415/16 -1 Coarse Below 20Medium24- 281.1/132-1.3/ 32Medium Count 20s-34sLong28 -341.3/32 -1.3/8Fine Count 34s - 60sExtra Long34- 401.3/8 -1.9/16Superfine Count 80s - 140s. Spinning Count does not depend on staple length only. It also depends on fineness and processing machinery. Length is measured by hand stapling or Fibrograph for 2.5% Span Length

Cotfon Fibres

101

2.5%SL (Spun Length) means at least 2.5% of total fibers have length exceeding this value. 50% SL means at least 50% of total fibers have length exceeding this value.

Length Uniformity Length Uniformity is Calculated by 50SL x 100 / 2.5 SL

Fiber strength Fiber Strength, next important quality is tested using Pressley instrument and the value is given in Thousands of Pounds per Square inch. (1000 psi) For better accuracy, Stelometer is used and results are given in grams / Tex. Lately, strength is measured in HVI (High Value Instrument) and result is given in terms of grams/tex. Interpretation of Strength value is given below: G / texClassifica tionBelow 23Weak24-25Medi um2628Average29-30StrongAbove 31 Very Strong Strength is essential for stronger yarns and higher processing speeds. Fiber Fineness Fiber Fineness and maturity are tested in a conjunction using Micronaire Instrument. Finer Fibers give stronger yarns but amenable for more neppiness of Yarn due to lower maturity. Micronaire values vary from 2.6 to 7.5 in various varieties.

Fineness and maturity

Textile Technology

102

Usually Micronaire value is referred to evaluate fineness of Cotton and its suitability for spinning particular count of Yarn. As the value is a combined result of fineness and maturity of Cotton fiber, it cannot be interpreted, property for ascertaining its spinning Value. This value should be taken in conjunction with standard value of Calibrated Cotton value. The following table will explain that micronaire value goes up along with maturity but declines with thickness of fiber. An Egyptian variety of Cotton, three samples of High maturity. Low maturity and Medium maturity were taken and tested. Test results are given below, Here, Micronaire Value of 4.3 is higher than 3.9 of low maturity cotton Another Greek Cotton was tested and results are give below High 3.857.075.10.88 Medium 3.554.970.70.84 Low 3.255.265.80.80 Micronaire Value of 3.8 is higher than 3.2 of low maturity cotton. Another American Cotton was tested and results are as follows High 4.164.475.90.87 Medium 3.462.168.00.80 Low 2.759.856.10.67 Hence, it is essential to know what Micronaire value is good for each variety of Cotton. Maturity RatioClassification1.00 and aboveVery Mature 0.95 - 1.0Above Average 0.85 - 0.95 Mature 0.80 -

Cotton Fibres

103

0.85 Below Average Less than 0.80 immature.

Cotton Grade Cotton grade is determined by evaluating colour, leaf and ginning preparation. Higher grade cottons provide better yarn appearance and reduced process waste. Colour is determined by using Nickerson-Hunter Calorimeter. This gives values Rd (Light or Dark) and +b (Yellowness) .

How to buy cotton? Cotton buying is the most important function that will contribute to optimum profit of a Spinning Mill. Evaluation of cotton quality is generally based more on experience rather than scientific testing of characteristics only. Timing of purchase depends on comprehensive knowledge about various factors which affect the prices. Choosing the supplier for reliability of delivery schedules and ability to supply cotton within the prescribed range of various parameters which define the quality of Cotton. Bargaining for lowest price depends on the buyer'S reputation for prompt payment and accept delivery without dispute irrespective of price fluctuations. Organising the logistics for transportation of goods and payment for value of goods will improve the benefits arising out of the transaction.

Textile Technology

104

Profit depends on producting high quality Yarn to fetch high prices. Influence of quality of raw material is very important in producing quality Yarn. But, quality of yam is a compound effect of quality of raw material, skills of work-force, performance of machines,- process know-how of Technicians and management expertise. A good spinner is one who produces reasonably priced yarn of acceptable quality from reasonably priced fiber. Buying a high quality, high priced cotton does not necessarily result in high quality Yarn or high profits.

Guidelines for Buyer and seller should clearly reach correct understanding on the following factors. 1.

Country of Origin, Area of Growth, Variety, Crop year

2.

Quality - Based on sample or Description of grade as per ASTM standard or sample For grade only and specifying range of staple length, Range of Micronaire, range of Pressley value, uniformity, Percentage of short fiber, percentage of non-lint content, Tolerable level of stickiness

3.

Percentage of Sampling at destination

4.

Procedure for settling disputes on quality or fulfillment of contract obligations.

Cotton Fibres

105

5.

Responsibility regarding contamination or stickiness.

6.

Price in terms of currency, Weight and place of delivery.

7.

Shipment periods

8.

Certified shipment weights or landing Weights

9.

Tolerances for Weights and Specifications

10. Port of Shipment and port of destination, partial shipments allowed or not, transshipment allowed or not, shipments in containers or Break-bulk carriers 11. Specifications regarding age of vessels used for shipment, freight payment in advance or on delivery 12. Responsibility regarding Import & Export duties 13. Terms of Insurance cover 14. Accurate details of Seller, Buyer and Broker 15. Terms of Letter of. Credit regarding bank .negotiation, reimbursement and special conditions, if any Apart from ensuring correct terms of Contract, Buyer should ensure that purchase is made from Reliable Supplier or through a Reliable Agent. Some suppliers evade supplies under some pretext if the market goes up. Otherwise, they supply inferior quality Either way buyer suffers. By establishing long term relationship will reliable Suppliers, Buyers can have satisfaction :)f getting correct quality, timely deliveries and fair prices.

Choosing Supplier: It is good to establish long term relationship with a few

106

Textile Technology

Agents who represent reputed Trading Companies in various Cotton Exporting Countries. They usually give reliable market information on quality, prices and market trends so that buyer can take intelligent decision. As cotton is not a manufactured Commodity, it is good to buy from dependable suppliers, who will ensure supply of correct quality with a variation within acceptable limits at correct price and also deliver on due date.

Choosing Qaulity: In a market with varying market demand situation. Buyers should decide which counts of Yarn to spin. Buyer can call for samples suitable for spinning Yarn counts programmed for production. Many spinners plan to do under-spinning. For Example, cotton suitable for 44s is used for spinning 40s. Some spinners do overspinning. They buy cotton suitable for 40s and spin 44s count. But, is advisable to spin optimum count to ensure quality and also keep cost of raw material at minimum level as for as possible. Some spinners also buy 2 or more varieties and blend them for optimum spinning. For' this purpose, a good knowledge to evaluate cotton quality and co-relate with yarn properties of required specifications. Cotton buyer should develop expertise in assessing cotton quality. Machine tests must be done only to confirm manual evaluation.

Taking Right Option: It is not advisable just to look at price quoted by supplier.

Correct costing should be done to work out actual cost when the cotton arrives at Mills. Further lowest price

Cotton Fibres

107

does not always mean highest profit for buying. Profitability may be affected by anyone or more of the following factors. If the trash is higher, more waste will be produced reducing the Yarn out- turn and hence profit. If the uniformity is less, end - breakages will be more reducing productivity and profitability. If grade is poor or more immature fibers are found in cotton, the yarn appearance will be affected and Yarn will fetch lesser price in the market. If the transit period for transport of cotton is longer, then also profitability will be reduced due blocking of funds for a longer period and increased cost of Interest.

Rate of Sales Tax varies from State to State. This must be taken in to account. Hence, thorough costing should be worked out before deciding on the quoted price only. The margin of profit in spinning cotton should be calculated before deciding on The various options available depending on market conditions should be studied. The factors to be considered for taking options are as follows. Count for which demand is good in market Prices for various counts for which demand exists. Cost of manufacturing various counts. Adequacy of machinery for the selected count. Various varieties of cotton available for spinning the

Textile Technology

108

selected count. Profit margin for each count using different varieties. Price quoted by different Agents for same variety of selected cotton. Reliability of supplier for quality and timely delivery.

Cost Consideration: Apart from the price quoted by the seller, other incidental costs must be taken into consideration before buying. a)

Duration for goods to reach Buyer's god own from the seller's Warehouse. If the duration is longer, buyer will incur higher interest charges.

b)

Cost of Transportation and taxes.

Resolution of differences If any discrepancy arises in the quality, weight and delivery periods, sellers should be willing to resolve the differences amicably and quickly. In case the matter is referred to Arbitrator, the award of the Arbitrator must be immediately enforced.

Bench Marks for Easy Reference It is better if quality bench marks are established for

different varieties so that buying decisions are easy for buyers Following standards have been found to be appropriate for Strict Middling Grade Cotton of staple 1.3/32".

Cotton Fibres

109

1.Staple Length ( 2.5% Spun Length) - Minimum 1.08" or 27.4 mm 2.Micronaire : Minimum 3.8, Maximum4.6 Variation within bulk sample should not be more than 0.1 3.Colour : Rd not less than 75 not more than 10 4.Nep Content: Less than 150 per gram 5.Strength : More than 30 grams/tex 6.Length Uniformity Ratio: Not less than 85% 7.Elongation : More than 8% 8.Short Fiber Content: Less than 5% 9.5eed Count Fragments: Less than 15 per grams 1.Commercial Bench marks can be given as follows: 1.

Price Competitiveness

2.

Price Stability

3.

Easy Availability throughout year

4.

Uniform Classing and Grading system

5.

Even- running Cotton in all Characteristics

6.

Reliable deliveries or Respect for sanctity of contract.

Quality Evaluation: The need for quality evaluation is for following purposes a)

To get optimum quality at lowest price.

b)

To decide whether cotton bought will can be processed to spin Yarn of desired specifications.

c)

To check the quality of sample cotton with quality of delivered cotton.

d)

To decide about correct machine settings and speeds for processing the cotton

e)

To estimate profitability of purchase decisions.

110

Textile Technology

Knowing the cotton properties is only half the battle for profits. It needs expertise to know how to get best of its value. Currently popular instrument called HVI gives ready information on various parameters to make correct purchase decisions. If may not be possible to get all the desired qualities in one variety or one lot of Cotton. In such case, an intelligent decision to select best combination of different varieties or lots to get desired Yam quality is necessary to get optimum yarn quality at optimum cost. If correct evaluation is made, profits are large. Hence, evaluation of quality is essential for optimum profit making and also make the customers happy with supply of correct quality of Yarn.

Expert dassers can manage to achieve reasonable level of correct evaluation. Now, with availability of better instruments, it is better to check qualities to make sure that desired quality of cotton is procured. These details should give cotton buyer reasonable guidance to make correct evaluation of cotton quality and ensure its suitability for producing required quality of yarn.

Quality Evaluation Chracteristics Co-relation to Yarn. 1.

Staple Length Spinning Potential

2.

Fiber Strength Yarn strength, less Breakages

3.

Fineness Finer Spinning Potential

4.

Maturity dyeing

Yarn Strength and even ness, better

111

Cotton Fibres

5.

Non-Lint.content (Trash) Reduced Waste

6.

Uniformity Ratio Better productivity and Evenness

7.

Elongation Less end Breakages

8.

Friction Cohesiveness

9.

Class

Yarn Appearance

10. Stickiness quality

Spinning problem by lapping & Dyeing

11. Grey ValueYarn lustre 12. Yellowness Yarn Appearance 13. Neppiness Yarn neppiness 14. Moisture Content 8.5% moisture content optimum for spinning at 65%

4 Cotton Mixing and Blowroom Operations COTTON MIXING

Cotton is a hygroscopic material, hence it easily adopts to the atmospheric airconditions. Air temperature inside the mxing and blowroom area should be more than 25 degree centigrade and the relative humidity(RH%) should be around 45 to 60 %, because high moisture in the fibre leads to poor cleaning and dryness in the fibre leads to fibre damages which ultimately reduces the spinnability of cotton. Cotton is a natural fibre. The following properties vary very much between bales (between fibres) fibre micronaire fibre length fibre strength fibre color fibre maturity Out of these, fibre micronaire, color, maturity and the origin of growth results in dye absorption variation. There fore it is a good practice to check the maturity . color and micronaire of all the bales and to maintain the

Cotton Mixing and Blowroom Operations

113

following to avoid dye pick up variation and barre in the finished fabric. Bale Management In a particular lot •

Micronaire range of the cotton bales used should be same for all the mixings of a lot

•

Micronaire average of the cotton bales used should be same for all the mixings of a lot

•

Range of color of cotton bales used should be same for all the mixings of a lot

•

Average of color of cotton bales used should be same for all the mixings of a lot

•

Range of matutrity coefficient of cotton bales used should be same for all mixings of a lot

•

Average of maturity coefficient of cotton bales used should be same for all mixings of a lot

In practice people do not consider maturity coefficient since Micronaire variation and maturity variation are related to each other for a particular cotton. It the cotton received is from different ginners, it is better to maintain the percentage of cotton from different ginners throught the lot, even though the type of cotton is same. It is p.ot advisable to mix the yarn made of out of

two different shipments of same cotton. For example , the first shipment of west african cotton is in january and the second shipment is in march, it is not advisable to mix the yarn made out of these two different shipments. If there is no shadevariation after dyeing, then it can be mixed.

Textile Technology

114

According to me, stack mixing is the best way of doing the mixing compared to using automatic bale openers which picks up the material from 40 to 70 bales depending on the length of the machine and bale size, provided stack mixing is done perfectly. Improper stack mixing will lead to BARRE or SHADE VARIATION problem. Stack mixing with Bale opener takes care of short term blending and two mixers in series takes care of long term blending. Why? •

Tuft sizes can be as low as 10 grams and it is the best way of opening the material(nep creation will be less, care has to be taken to reduce recyling in the inclined lattice) contaminations can be removed before mixing is made

•

The raw material gets acclamatised to the required temp and R.H.%, since it is allowed to stay in the room for more than 24 hours and the fibre is opened, the fibre gets conditioned well.

Disadvan tages •

more labour is required

•

more space is required

•

mixing may not be 100% homogeneous( can be overcome by installing double mixers)

If automatic bale opening machine is used the bales should be arranged as follows let us assume that there are five different micronaires and five different colors in the mixing, 50 bales are used in the mxing. 5 to 10 groups .should be made by grouping the bales in a mixing so that each group will have average micronaire and average

Cotton Mixing and Blowroom Operations

115

color as that of the overall mixing. The position of a bale for micronaire and color should be fixed for the group and it should repeat in the same order for all the groups It is always advisable to use a mixing with very low Micronaire range.Preferably .6 to 1.0 . Because •

It is easy to optimise the process parameters in blow room and cards

•

drafting faults will be less

•

dyed cloth appearance will be better because of uniform dye pickup etc

It is advisable to use single cotton in a mixing , provided the length, strength micronaire, maturity coefficient and . trash content of the cotton will be suitable for producing the required counts. Automatic bale opener is a must if more than two cottons are used in the mixing, to avoid BARRE or SHADE VARIAnON problem. It is better to avoid using the following cottons

•

cottons with inseparable trash (very small size), even though the trash % is less

•

sticky cotton (with honey dew or sugar)

•

cotton with low maturity co-efficient

Stickiness of cotton consist~ of two major causes. Honeydew from Whiteflies and aphids and high level of natural plant sugars. The problems with the randomly distributed honey dew contamination often results in costly proudction interruptions and requires immediate action often as severe as discontinuing the use of contaminated cottons.An effective way to control cotton stickiness in processing is to blend sticky and non-sticky cotton. Sticky cotton percentage should be less than 25%.

116

Textile Technology

BLOWROOM OPERATIONS

Basic operations in the blowroom: 1.

opening

2.

cleaning

3.

mixing or blending

4.

micro dust removal

5.

uniform feed to the carding machine

6.

Recycling the waste

Blow room installations consists of a sequence of different machines to carry out the above said operations.Moreover Since the tuft size of cotton becomes smaller and smaller, the required intensities of processing necessitates different machine configuration. Technological operation Opening in blowroom means opening into small flocks. Technological operation of opening means the volume of the flock is increased while the number of fibres remains constant. i.e. the specific density of the material is reduced. The larger the dirt particle, the better they can be removed Since almost every blowroom machine can shatter particles, as far as possible a lot of impurities should be eliminated at the start of the process.Opening should be followed immediately by cleaning, if possible in the same machine. The higher the degree of opening, the higher the degree of cleaning. A very high cleaning effect is almost

Cotton Mixing and Blowroom Operations

117

always purchased at the cost of a high fibre loss. Higher roller speeds give a better cleaning effect but also more stress on the fibre. Cleaning is made more difficult if the impurities of dirty cotton are distributed through a larger quantity of material by mxing with clean cotton. The cleaning efficiency is strongly dependent on the TRASH %. It is also affected by the size of the particle and stickyness of cotton. Therefore cleaning efficiency can be different for different cottons with the same trash %. There is a new concept called CLEANING RESISTANCE. Different cottons have different cleaning resistance. If cotton is opened well in the opening process, cleaning becomes easier because opened cotton has more surface area, therefore cleaning is more efficient If automatic bale opener is used, the tuft size should be as small as possible and the machine stop time should be reduced to the minimum level possible If Manual Bale openers are used, the tuft size fed to the feed lattice should be as small as possible Due to machine harvesting, cotton contains more and more impurities, which furthermore are shattered by hard ginning. Therefore cleaning is always an important basic operation. In cleaning, it is necessary to release the adhesion of the impurities to the fibres and to give hte particles an opportunity to separate from the stock. The former is achieved mostly by picking of flocks, the latter is achieved by leading the flocks over a grid. Using Inclined spiked lattice for opening cotton in the intial stages is always a better way of opening the

Textile Technology

118

cotton with minimum damages. Ofcourse the production is less with such type of machines. But one should bear in mind that if material is recyled more in the lattice, neps may increase. Traditional methods use more number of machines to open and clean natural fibres. Mechanical action on fibres ca uses some deterioration on yarn quality, particularly in terms of neps. Moreover it is true that the staple length of cotton can be significantly shortened. Intensive opening in the initial machines like Bale breaker and blending machines means that shorter overall cleaning lines are adequate. In a beating operation, the flocks are subjected to a sudden strong blow. The inertia of the impurities accelerated to a high speed, is substantially greater than that of the opened flocks due to the low air resistance of the impurities. The latter are hurled against the grid and because of their small size, pass between the grid bars into the waste box, while the flocks continue around the periphery of the rotating beater. By using a much shorter machine sequence, fibres with better elastic properties and improved spinnability can be produced. Air streams are often used in the latest machine sequence, to separate fibres from trash particles by buoyancy differences rather than beating the material against a series of grid bars. There are three types of feeding apparatus in the blowroom opening machines 1.

two feed rollers( clamped)

2.

feed roller and a feed table

Cotton Mixing and Blowroom Operations

3.

119

a feed roller and pedals

Two feed roller arrangements gives the best forwarding motion, but unfortunately results in greatest clamping distance between the cylinders and the beating element feed roller and pedal arrangement gives secure clamping throughout the width and a small clamping distance,. which is very critical for an opening machine In a feed roller and table arrangement, the clamping distance can be made very small. This gives intensive opening, but clamping over the whole width is poor, because the roller presses only on the highest points of the web. Thin places in the web can be dragged out of hte web as a clump by the beaters Honeydew(sugar) or stickiness in cotton affect the process very badly. Beacause of that production and quality is affected. Particles stick to metal surfaces, and it gets aggreavated with heat and pressure. These deposits change the surface characteristics which directly affects the quality and running behavior. There are chemicals which can be sprayed to split up the sugar drops to achieve better distribution. But this system should use water solutions which is not recommeded due to various reasons. It is better to control the climate inside the department when sticky cotton is used. Low temperature (around 22 degree celcius) and low humidity (45% RH). This requires an expensive air conditioning set up. • The easiest way to process sticky cotton is to mix with good cotton and to process through two blending machines with 6 and 8 doublings and to install machines which will seggregate a heavier particles by buoyanccy differences.

Textile Technology

120

General factors which affect the degree of opening, cleaning and fibre loss are, 1.

thickness of the feed web

2.

density of the feed web

3.

fibre coherence

4.

fibre alignment

5.

size of the flocks in the feed (flock size may be same but density is different)

6.

the type of opening device

7.

speed of the opening device

8.

degree of penetration

9.

type of feed (loose or clamped)

10. distance between feed and opening device 11. type of opening device 12. type of clothing 13. point density of clothing 14. arrangement of pins, needles, teeth 15. speeds of the opening devices 16. throughput speed of material 17. type of grid bars 18. area of the grid surface 19. grid settings 20. airflow through the grid 21. condition of pre-opening 22. quantity of material processed, 23. position of the machine in the machine sequence

Cotton Mixing and Blowroo11l Operations

121

24. feeding quantity variation to the beater 25. ambient R.H.% 26. ambient teperature Cotton contains very little dust before ginning. Dust is therefore caused by working of the material on the machine. New dust is being created through shattering of impurities and smashing and rubbing of fibres. However removal of dust is not simple. Dust particles are very light and therefore float with the cotton in the transport stream. Furthermore the particles adhere quite strongly to the fibres. If they are to be eliminated they are to be rubbed off. The main elimination points for adhering dust therefore, are those points in the process at which high fibre/metal friction or high fibre/fibre friction is produced. Removal of finest particles of contaminants and fibre fragments can be accomplished by releasing the dust into the air, like by turning the material over, and then removing the dust-contaminated air. Release of dust into the air occurs whereever the raw material is rolled, beaten or thrown about.Accordingly the air at such positions is sucked away. Perforated drums, stationary perforated drums, , stationary combs etc. are some instruments used to remove dust.

5 Carding "Card is the heart of the spinning mill" and "Well carded is half spun" are two proverbs of the experts. These proverbs inform the immense significance of carding in the spinning process.High production in carding to economise the process leads to reduction in yarn quality. Higher the production, the more sensitive becomes the carding operation and the greater. danger of a negative influence on quality.The technological changes that has taken place in the process of carding is remarkable. Latest machines achieve the production rate of 60 - 100 kgs / hr, which used to be 5 - 10 kgs / hr, upto 1970.

THE PURPOSE OF CARDING 1.

To open the flocks into individual fibres

2.

cleaning or elimination of impurities

3.

reduction of neps

4.

elimination of dust

Carding

5.

elimination of short fibres

6.

fibre blending

7.

fibre orientation or alignment

8.

sliver formation

123

TECHNOLOGICAL POINTS OF CARDING There are two types of feeding to the cards 1. feeding material in the form of scutcher lap

2. flock feed system (flocks are transported pneumatically) lap feeding 1. linear density of the lap is very good and it is easier to maintain(uniformity)

2. the whole installation is very flexible 3. deviations in card output will be nil, as laps can be rejected 4. autolevellers are not required, hence investment cost and maintenace cost is less 5. transportation of lap needs more manual efforts( more labour) 6. lap run out is an additional source of fault, as it should be replaced by a new lap 7. more good fibre loss during lap change 8. more load on the taker-in, as laps are heavily compressed flock feeding 1. high performance in carding due to high degree of openness of feed web

124

Textile Technology

2. labour requirement is less due to no lap transportaion and lap change in cards 3. flock feeding is the only solution for high prouduction cards 4. linear density of the web fed to the card is not as good as lap 5. installation is not felxible 6. autoleveller is a must, hence investment cost and maintenance cost is more -

Type of flock feed(chute feed) there are two basic concepts of flock feed 1. one piece chute without an opening device

2. two piece chute with an opening system 2.

one piece chute is simple, economical and requires little maintenance

3.

two piece chute is complex, expensive, but delivers a uniform batt.

4.

One piece chut is a closed system, i.e.excess flock returns to the distrbutor, if too much material is present, neps can be increased

5.

one piece chute is not flexible to run different mixings

6.

layout restrictions are more with one piece chute A feeding device is a must to feed the web to the Taker-in region and it should perform the following tasks 1. to clamp the batt securely throughout its width 2. to grip the fibres tightly without slippage during the action of taker-in

Carding

125

3. to present the fibres in such a manner that opening can be carried out gently The divertor nose(sharp or round) and the length of the nose(guide surface) have a significant influence on quality and quantity of waste removed. Shart nose divertor avoids fibre slippage but the opening action is not gentle. If the length of the guide surface is too short, the fibres can escape the action of the taker-in. They are scraped off by the mote knives and are lost in the waste receiver. Feed roller clothed with sawtooth is always better , because it gives good batt retention. Thus the opening effect of the taker-in is more as it is in combing Rieter has devloped a "unidirectional feed system" where the two feed devices(feed roller and feed plate are oppositely arranged when compared with the conventional system. i.e. the cylinder is located below and the plate is pressed against the cylinder by spring force. Owing to the direction of feed roller, the fibre batt runs downwards without diversion directly into the teeth of the taker-in(licker-in) which results in gentle fibre treatment. This helps to reduce faults in the yam. The purpose of the taker-in is to pluck finely opened flocks out of the feed batt, to lead them over the dirt eliminating parts like mote knives, combing segment and waste plates, and then to deliver the fibres to the main cylinder. In high production cards the rotational speed ranges from 700-1400 The treatment for opening and cleaning imparted by Taker-in is very intensive, but unfortunately not very

Textile Technology

126

gentle. Remember that around 60% of the fibres fed to the main cylinder is in the form of individual fibres. The circumferential speed of Taker-in is around 13 to 15 m/ sec and the draft is more than 1000.lt clearly shows that fibre gets deteriorated at this opening point. Only the degree of deterioration can be controlled by adjusting the following: 1. the thickness of the batt

2. the degree of openness of the rawmaterial 3. the degree of orientation of the fibres 4. the aggressiveness of the clothing 5. the distance between the devices 6. the rotational velocity of the taker-in 7. the material throughput Latest TRUTZSCHLER cards work with three lickerins compared to one liker-in. The first one is constructed as needle roll. This results in very gentle opening and an extremely long clothing life for this roll. The other two rollers are with finer clothing and higher speeds, which results in feeding more %of individual fibres and smallest tufts compared to single lickerin, to the main cylinder. This allows the maing cylinder to go high in speeds and reduce the load on cylinder and flat tops. There by higher productivity is achieved with good quality. But the performance may vary for different materials and different waste levels. between the taker-in and main cylinder , the clothings are in the doffing disposition. It exerts an influence on the sliver quality and also on the

Carding

127

improvement in fibres longitudinal orientation that occurs here. The effect depends on the draft between main cylinder and taker-in.The draft between main cylinder and taker-in should be slightly more than 2.0.

The opening effect is directly proportional to the number of wire points per fibre. At the Taker-in perhaps 0.3 points/ fibre and at the main cylinder 1015 points / fibre.lf a given quality of yarn is required, a corresponding degree of opening at the card is needed. To increase production in carding, the number of points per unit time must also be increased. this can be 'achieved by: 1. more points per unit area(finer clothing)

2. higher roller and cylinder speeds 3. more carding surface or carding position speeds and wire population has reached the maximum, further increase will result in design and technological problems. Hence the best way is to add carding surface (stationary flats). Carding plates can be applied at 1. under the liker-in

2. between the licker-in and flats 3. between flats and doffer -

Taker-in does not deliver 100% individual fibres to main cylinder. It delivers around 70% as small flocks to main cylinder. If carding segments are not used, the load on cylinder and flats will be very high and carding action also suffers. If carding segemets are used, they ensure further opening, thinning out and primarily, spreading out and improved distributiorl

128

Textile Technology

of the flocks over the total surface area.carding segments bring the following advantages 1. improved dirt and dust elimination

2. improved disentanglement of neps 3. possibility of speed increase (production increase) 4. preservation of the clothing 5. possibility of using finer clothings on the flats and cylinder 6. better yarn quality 7. less damage to the clothing 8. cleaner clothing In an indepth analysis, all operating elements of the card were therefore checked in regard to their influence on carding intensity. It showed that the "CYLINDER-FLATS" area is by far the most effective region of the card for. 1. opening of flocks to individual fibres

2. elimination particles)

of

remaining

impurities(trash

3. elimination of short fibres( neps also removed with short fibres) 4. untangling the neps 5. dust removal 6. high degree of longitudinal orientation of the fibres The main work of the card, separation to individual fibres is done between the main cylinder and the flats

Carding

129

Only by means of this fibre separation, it is possible to eliminate the fine dirt particles and dust. When a flat enters the working zone, it gets filled up very quickly. Once it gets filled, after few seconds, thereafter, hardly any further take-up of fibres occurs, only carding. Accordingly, if a fibre bundle does not find place at the first few flats, then it <:an be- opened only ---with difficulty.lt will be rolled between the working surfaces and usually leads to nep formation In princile, the flats can be moved forwards or backwards, i.e. in the same direction as or in opposition to the cylinder.ln reverse movement, the flats come into operative relationship with the cylinder clothing on the doffer side. At this stage, the flats are in a clean condition. They then move towards the taker-in and fill up during this movement.Part of their receiving capacity is thus lost, but sufficient remains for elimination of dirt, since this step takes place where the material first enters the flats. At this position, above the taker-in, the cylinder carries the material to be cleaned into the flats. The latter take up the dirt but do not transport it through the whole machine as in the forward movement system. Instead , the dirt is immediately removed from the machine. Rieter studies show clearly that the greater part of the dirt is hurled into the first flats directly above the taker-in. Kaufmann indicates that 75% of all neps can be disentagled, and of these about 60% are in fact disentagled. Of the remaining 40% disentaglable nep

130

Textile Technology

1. 30-33% pas on with the sliver 2.5-6% are removed with the flat strips 3.2-4%are eliminated with the waste. The intensity of nep separation depends on: 1. the sharpness of the clothing 2. the space setting between the main cylinder and the flats 3. tooth density of the clothing 4. speed of the main cylinder 5. speed of the flat tops 6. direction of flats with reference to cylinder 7. the profile of the cylinder wire The arrangement of the clothing between the cylinder and the doffer is not meant for stripping action, it is for CARDING ACTION.This is the only way to obtain a condensing action and finally to form a web. It has both advantages and disadvantages. The advantage is that additional carding action is obtained here and it differs somewhat from processsing at the fIats.A disadvantage is that leading hooks and trailing hooks are formed in the fibres, beause the fibres remain caught at one end of the main cylinder(leading hook) and some times on the doffer clothing(trailing hook). There are two rules of carding 1. The fibre must enter the carding machine, be efficiently carded and taken from it in as little time as possible. 2. The fibre must be under control from entry to exit

Carding

131

Carding effect is taking place between cylinder and doffer because, either the main cylinder clothing rakes through the fibres caught in the doffer clothing, or the doffer clothing rakes thro the fibres on the main cylinder. Neps can still be disentangled here, or nonseparated fibre bundles can be opened a bit in this area and can be separated during the next passage through the flats A disadvantage of web-formation at the card is the formation of hooks. According to an investigation by morton and Yen in Manchester, it can be assumed that 1.50% of the fibres have trailing hooks 2.15% have leading hooks 3.15% have both ends hooked 4.20% without hooks Leading hooks must be presented to the comber and trailing hooks to the ring spinning frame. There must be even number of passages between card and comber and odd number between the card and ringframe.

Carding Machines -

Lmw card

-

Rieter card

-

Trutzschler card

-

Crosrol card

-

Marzoli card

6 Effects of Fibre Preparation on Instrument Readings Cotton processing machines that mechanically work the cotton fiber from bale to yarn are designed with the intent of minimizing fiber damage. Nevertheless, opening, cleaning and blending equipment shorten the staple length while increasing short fiber content and neps. Carding and combing reverse this by removing a percentage of the short fibers and neps. Drawing is thought to have a minimal effect on fiber physicals, its purpose being to improve sliver evenness and fiber orientation. With machine settings and speeds optimized, a comparison of the fiber properties of stock-in compared with stock-out provides valuable information for achieving further optimization.

Procedure Instrument used: Uster AFIS and HVI Spinlab 900B No. of bale samples: 10 bales with different mic and length were used

Effects of Fibre Preparation on Instrument Readings

133

No. of processing method: 12 different processing combinations Machineries used:

Blow room hunter hopper feeder Rieter Mono cylinder (750 rpm) Rieter ERM B5/5(850 rpm) Rieter ERM B5/5(950 rpm) Carding: Rieter C4 card with Hollingsworth Trashmaster TM2000 (100 pounds per hour, with 60 grains per yard sliver)

Details of the findings There are slight AFIS variations in the apparent fiber diameter when going from a processing stage to another. It seems that the ERMII results in a slight increase, which could be due to the removal of dead fibers in the opening line. Certainly the card also removes neps and dead fibers; however, the diameter appears to decrease slightly. There is also a significant decrease due to the drawing. These mechanical processes cannot modify the diameter. The only logical explanation is an artifact effect. In the card sliver and the drawing slivers the fibers are oriented and paralleled, this removes the crimp. The length of the electronic signal and its height are then modified giving higher length readings and lower diameter readings. The HVI micronaire values vary slightly in the opening line, perhaps due to the removal

134

Textile Technology

of some dead fibers. The carding seems to reduce the micronaire, which is not explainable. Then the drawing leads to an increase in micronaire. The theory of the micronaire instruments is based on airflow passing through a sample constituted of randomly oriented fibers. In the drawing process the fibers are made parallel, which probably leads to an easier flow of air through the cotton sample and results in an apparent higher micronaire. As the micronaire is used to calculate the beard mass (function of optical density and micronaire) for the strength test, any positive micronaire bias will lead to a negative HVI strength bias. In addition, the drawing process is similar in effect to an increase in the brushing time (or force) on the HVI combs. Taylor has shown the effect of increasing brushing force on HVI strength readings. In his experiment two sample preparations were tested, hand brushing and HVI brushing (harder brushing than by hand). The results show an increase by 1.9 g/tex when using the HVI brushing device. In our case, we think that the drawing sliver samples have a lower optic density (for a given number of fibers in the comb) than the raw cotton. This results in a lower calculated mass of the sample to be broken. As the HVI strength is calculated by dividing the force applied to break the sample by the As expected, the AFIS nep counts increase with passage of the fibers through the opening line. The Mono- cylinder increases the average nep count by 75, then the first ERM (operating at 850 rpm) by 136 and the second ERM (operating at 950 rpm) by 240; that is 451 neps in total. The card removes 540 neps and the drawing frames have no effect.

Effects of Fibre Preparation on Instnlment Readings

135

The HVI reflectance increases slightly after each cleaning stage. The drawing seems to also have an effect on the reflectance readings. This is not due to trash removal but more likely to an artifact because the paralleled fibers are not reflecting the light the same way as the randomly oriented fibers. The changes in yellowness are quite small but significant. The most important change is due to the drawing. This is, as for the reflectance, probably due to an artifact.

Combed Process Combing affects AFIS Upper Quartile Length, Mean Length, Short Fiber Content and HVI Upper Half Mean Length and Uniformity Ratio. As expected the fiber length parameters all increase when the cotton is combed, with the exception of the Short Fiber Content. The drawing also affects the length parameters; as discussed bef?re, it is probably an artifact. It is interesting to note that combing increases the length by 0.006 inch (minimum noil settings) and that the first drawing increases it by 0.027, i.e. nearly five times more. The artifact effect seems to be much more important than the real mechanical effect. The combing process seems to have no effect on the fiber diameter. The drawing, as discussed before, decreases the diameter (artifact). The HVI micronaire increases when combing is applied, mainly because the removal of short, weak and immature fibers during the combing process increases the average maturity level. As discussed before the drawing has a positive effect on micronaire (artifact effect).

Textile Technology

136

The HVI strength also increases with combing, because of the removal of short fibers. The drawing, as discussed before, increases the apparent HVI strength (artifact effect). The AFIS neps are removed during the combing process as expected (-62% for the minimum noil setting to -91% for the normal noil setting). The combing also removes trash and dust. The decrease in trash is nearly 60% for both types of settings. The decrease in dust is about 40% for the minimum noil setting and 60% for the normal noil setting. As these are removed the HVI reflectance increases as expected and the yellowness decreases. The drawing effect on both parameters is an artifact, as discussed before.

High Volume instrument System The testing of fibres was always of importance to the spinner. It has been known for a long time that the fibre characteristics have a decisive impact on the running behaviour of the production machines, as well as on the yarn quality and manufacturing costs. In spite of the fact that fibre characteristics are very important for yarn yarn proudction, the sample size for testing fibre characteristics is not big enough. This is due to the following. The labour and time involvement for the testing of a representativesample was too expensive. The results were often available much too late to take corective action. The results often depended on the operator and / or the instrument, and could therefore not be considered objective

Effects of Fibre Preparation

011

1I1stnmlent Readings

137

one failed in trying to rationally administer the flood of the rawmaterial data, to evaluate such data and to introduce the necessary corrective measures. Only recently technical achievements have made possible the development of automatic computer-controlled testing equipment. With their use, it is possible to quickly determine the more important fibre characteristics. Recent developments in HVI technology are the result of requests made by textile manufacturers for additional and more precise fibre property information. Worldwide competitive pressure on product price and product quality dictates close control of all resources used in the manufacturing process.

Following are the advantages of HVI testing the results are practically independent of the operator the results are based on large volume samples, and are therefore more significant the respective fibre data are immediately available the data are clearly arranged in summerised reports they make possible the best utilisation of rawmaterial data problems as a result of fibre material can be predicted, and corrective measures instituted before such problems can occur Cotton classification does not only mean how fine or clean, or how long a fibre is, but rather whether it meets the requirements of the finished product. To be more precise, the fibre characteristics must be classified according to a certain sequence of importance with respect to the end product and the spinning process.

138

Textile Technology

The ability to obtain complete information with single operator HVI systems further underscores the economic and useful nature of HVI testing. Two instrument companies located in the US manufacture these HVI systems. Both the systems include instruments to measure micronaire, length, length uniformity, strength, colour, trash, maturity, sugar content etc.

Length The length measure by HVI systems used by the USDA is called upper-haH-mean length. This is the average or mean length of the longest one-half of the fibres in the sample. The spinlab system uses the fibrosampler device to load the fibres on needles, the motion control system uses the Specimen Loader to capture the fibres in a pinch clamp. However the preparation of the length specimen for both systems includes - combin to straighten and parallel the fibres, and brushing to remove fibre crimp. The length measurement is then made by the instrument scanning along the length of the specimen to determine the length data. The insturments are calibrated to to read in staple length. Length measurements obtained from the instrument are considerably more repeatable than the staple length determination by the classer. In one experiment the instrument repeated the same staple length determination 44% of the time while the classer repeated this determination only 29(Yo of the time. Similarly, the instrument repeated to 1/32" on 76% of the samples, while the classer agreed on 71% of the samples to within 1/31".

Effects of Fibre Preparation

Oil

Instrument Readings

139

The precision of the HVI length measurement has been improved over the last few years. If we take the same bale of cotton used in the earlier example and repeatedly measure length with an HVI system, over two-thirds of measurements will be in a range of only about 1/32 nd of an inch: 95% of the individual readings will be within 1/32nd of an inch of the bale average. In the 77000 bales tested, the length readings were repeated within 0.02" on 71% of the bales between laboratories.

Length Uniformity The HVI system gives an indication of the fibre length distribution in the bale by use of a length uniformity index. This uniformity index is obtained by dividing the mean fibre length by the upper-half-mean length and expressing the ratio as a percent. A reading of 80% is considered average length uniformity. Higher numbers mean better length uniformity and lower numbers poorer length uniformity. A cotton with a length uniformity index of 83 and above is considered to have good length uniformity, a length uniformity inqex below 78 is considered to show poor length uniformity. Repeated measurements on a single bale of cotton show the length uniformity index measurement to have relatively low precision. About two-thIrds of the measurements will occur within one unit of length uniformity; thus a bale with an average length uniformity index of 80 would have 68% of the readings occuring between 79 and 81, and 95% of hte readings occuring between 78 and 82. This does not seem too bad until one cGnsiders that most US upland cottons will hav(;: a length uniformity reading between 75 and 85.

140

Textile Technology

Most organizations operate their HVI systems to use an average of 2 or 4 readings per bale for the length uniformity index. Using that number tests per bale, the USDA test of 77000 bales showed that laboratoriesat different locations agreed 68% of the time to within one length uniformity index unit. In some cases low length uniformity has correlated with high short fibre content. However, in general the correlations between length uniformity index and short fibre content have not been very good. One important reason why the length uniformity index is a not a very good indicator of the short fibre content has to do with the fact that the HVI systems do not measure the length of any fibres shorter than about 4mm. Another reason for the poor correlations between length uniformity index and short fibre content is that the short fibre content is related to staple length while the length uniformity index is fairly independent of staple length. As an example, the shorter staple cottons tend to contain higher amounts of short fibre than the longer staple cottons. Howeer, many short staple cottons have length uniformity index readings above 80.

Micronaire The micronaire reading given by the HVI systems is the same as has been used in the commercial marketing of cotton for almost 25 years. The repeatability of the data and the operator ease of performing the test have been improved slightly in the HVI micronaire measurement over the original instruments by elimination of the requirement of exactly weighing the test specimen. The micronaire instruments available today use

Effects of Fibre Preparation on Instrument Readings

141

microcomputers to adjust the reading for a range of test specimen sizes. The micronaire reading is considered both precise and reperable. For example, if we have a bale of cotton that has an average micronaire of 4.2 and repeatedly test samples from that bale, over two-thirds of thet micronaire readings will be between 4.1 and 4.3 and 95 %of the readings between and 4.0 and 4.4. Thus, with only one or two tests per bale we can get a very precise measure of the average micronaire of the bale. This reading is also very repeatable from laboratory to laboratory. Tn USDA approx 77000 bales were tested per day in each laboratory, micronaire measurements made in different laboratories agreed with each other within 0.1 micronaire units on 77% of the bales. The reading is influenced by both fibre maturity and fibre fineness. For a given growing area, the cotton variety generally sets the fibre fineness, and the environmental factors control or influence the fibre maturity. Thus, within a growing area the micronaire value is usually highly related to the maturity value. However, on an international scale, it cannot be known from the micronaire readings alone if cottons with different micronaire are of different fineness or if they have different maturity levels.

Strength The strength measurement made by the HVI systems is unlike the traditional laboratory measurements of Pressley and Stelometer in several important ways. First of all the test specimens are prepared in a very different manner. In the laboratory method the fibres are selected,

142

Textile Technology

combed and carefully prepared to align them in the jaw clamps. Each and every fibre spans the entire distance across the jaw surfaces and the space between the jaws. In the HVI instruments the fibres are ramdomly selected and automatically prepared for testing. They are combed to remove loose fibres and to straighten the clamped fibres, also brushed to remove crimp before testing. The mechanization of the specimen preparation techniques has resulted in a "tapered" specimen where fibre ends are found in the jaw clamp surfaces as well as in the space between the jaws. A second important difference between traditional laboratory strength measurements and HVI strength measurements is that in the laboratory measurements the mass of the broken fibres is determined by weighing the test specimen. In the HVI systems the mass is determined by the less direct methods of light absorption and resistance to air flow. The HVI strength mass measurement is further complicated by having to measure the mass at the exact point of breaks on the tapered specimen. A third Significant difference between laboratory and HVI strength measurements is the rate or speed at which the fibres are broken. The H~ II systems break the fibres about 10 times faster than the laboratory methods. Generally HVI grams per tex readings are 1 to 2 units (3 to 5%) hihger in numerical value. In some individual cases that seem to be related to variety, the differences can be as much as 6 to 8% higher. This has not caused a great deal of problems in the US, perhaps because a precedent was set many years ago when we began adjusting our Stelometer strength values about 27% to put them on Presseley level.

Effects of Fibre Preparation on Instrument Readings

143

Relative to the other HVI measurements, the strength measurement is less precise. Going back to our single bale of cotton and doing repeated measurements on the bale we shall find that 68% of the readings will be within 1 g/tex of the bale average. So if the bale has an average strength of 25 g.tex, 68% of the individual readings will be betweeen 24 and 26 g/tex, and 95% between 23 and 27 g/tex. Because of this range in the readings within a single bale, almost all HVI users make either 2 or 4 tests per bale and average the readings. When the average readings are repeated within a laboratory, the averages are repeated to within one strength unit about 80% of the time. However, when comparisons are made between laboratories the agreement on individual bales to within plus or minus 1 g/tex decreases to 55%.This decrease in strength agreement between laboratories is probably related to the difficulty of holding a constant relative humidity in the test labs. Test data indicate that 1% shift in relative humidity will shift the strength level about 1% . For example, if the relative humidity in the laboratory changes 3% ( from 63 to 66%), the strength would change about 1 g/tex ( from 24 to 25 g/tex)

Colour The measurement of cotton colour predates the measurement of micronaire, but because colour has always been an important component of dasser's grade it has not received attention as an independent fibre property. However the measurement of colour was incorporated into the very early HVI systems as one of the primary fibre properties.

144

Textile Technology

Determination of cotton colour requires the measurement of two properties, the grayness and yellowness of the fibres. The grayness is a measure of the amount of light reflected from the mass of the fibre. We call this the reflectance or Rd value. The yellowness is measured on what we call Hunter's +b scale after the man who developed it. The other scales that describe colour space (blue, red, green) are not measured becasue they are considered relatively constant for cotton. Returning once again to the measurements on our single bale, we see that repeated measurements of colour are in good agreement. For grayness or reflectance readings, 68% of the readings will be within 0.5 Rd units of the bale average, and 95% within one Rd unit for the average. As for yellowness, over two-thirds of these readings will be within on-fourth of one +b unit of the average, and 95% within one-half of one +b unit. The grayness (Rd) and yellowness (+b) measurements are related to grade through a colour chart which was developed by a USDA researcher. The USDA test of 77000 bales showed the colour readings to be the most repeateable of all data between laboratories; 87% of the bales repeated within one grayness(Rd) unit, and 85% repeated within one-half of one yellowness( +b) unit.

Trash Content The HVI systems measure trash or non-lint content by use of video camera to determine the amount of surface area of the sample that is covered with dark spots. As the camera scans the surface of the sample, the video

Effects of Fibre Preparation on Instrument Readings

145

output drops when a dark spot (presumed to be trash) is encountered. The video signal is processed by a microcomputer to determine the number of dark spots encountered (COUNT) and the per cent of the surface area covered by the dark spots (AREA). The area and count data are used in an equation to predict the amount of visible non-lint content as measured on the Shirley Analyser. The HVI trash data output is a two-digit number which gives the predicted non-lint content for that bale. For example, a trash reading of 28 would mean that the predicted Shirley Analyser visible non-lint content of that bale would be 2.8%. While the video trash instruments have been around for several years, But the data suggest that the prediction of non-lint content is accurate to about 0.75% non lint, and that the measurements are repeatable 95% of the time to within 1% non-lint content.

Short Fiber Content The measure of short-fiber content (SFC) in Motion Control's HVI systems is based on the fiber length distribution throughout the test specimen. It is not the staple length that is so important but the short fiber content which is important. It is better to prefer a lower commercial staple, but with a much lower short-fibre content. HVI systems measure length parameters of cotton samples by the fibrogram technique. The following assumptions describe the fibrogram sampling process: The fibrogram sample is taken from some population of fibres

Textile Technology

146

The probability of sampling a particular fiber is proportional to its length A sampled fiber will be held at a random point along its length A sampled fiber will project two ends away from the holding point, such that all of the ends will be parallel and aligned at the holding point. All fibers have the same uniform density The High Volume Instruments also provide empirical equations of short fibre content based on the results of cotton produced in the United States in a particular year. Short Fibre Index = 122.56 - (12.87 x UHM) - (1.22 x UI)

where UHM - Upper Half Mean Length (inches) UI - Uniformity Index Short Fibre Index = 90.34 - (37.47 x SL2) - (0.90 x UR) Where SL2 - 2.5% Span length (inches) UR - Uniformity Ratio

In typical fibrogram curve, the horizontal axis represents the lengths of the ends of sampled fibers. The vertical axis represents the percent of fiber ends in the fibrogram having that length or greater.

Measurement of Maturity and Sugar Content Near infrared analysis provides a fast, safe and easy means to measure cotton maturity, fineness and sugar content at HVI speed without the need for time consuming sample preparation or fiber blending. This technology is based on the near infrared reflectance

Effects of Fibre Preparation on Instrument Readings

147

spectroscopy principle in the wavelength range of 750 to 2500 nanometers. Differences of maturity in cotton fibers are recognized through distinctly different NIR absorbance spectra. NIR technology also allows for the measurement of sugar content by separating the absorbance characteristics of various sugars from the absorbance of cotton material. Cotton maturity is the best indicator of potential dyeing problems in cotton products. Immature fibers do not absorb dye as well as mature fibers. This results in a variety of dye-related appearance problems such as barre, reduced color yield, and white specks. Barre is an unwanted striped appearance in fabric, and is often a result of using yarns containing fibres of different maturity levels. For dyed yarn, color yield is diminished when immature fibres are used. White specks are small spots in the yarn or fabric which do not dye at all. These specks are usually attributed to neps (tangled clusters of very immature fibers) NIR maturity and dye uptake in cotton yarns have been shown to correlate highly with maturity as measured by NIR. A correlation of R=0.96 was obtained for a set of 15 cottons. In a joint study by ITT and a European research organization, 45 cottons from four continents were tested for maturity using the NIR method and (he SHIRLEY Development Fineness/ Maturity tester(FMT). For these samples, NIR and FMT maturity correlated very highly. On 15 cottons from different growth areas of the USA, NIR maturity was found to correlate with r2 = 0.9 through a method developed by the United States

148

Textile Technology

Department of Agriculture (USDA). In this method, fibres are cross-sectioned and microscopically evaluated. Sugar Content is a valid indicator of potential processing problems. Near infrared analysis, because of its adaptability to HVI, allows for screening of bales prior to use. The information serves to selected bales to avoid preparaion of cotton mixes of bales with excessive sugar content. COTTON STICKINESS consists of two major causes- honeydew form white flies and aphids and high level of natural plant sugars. Both are periodic problems which cause efficiency los.ses in yarn manufacutring The problems with the randomly distributed honeydew contamination often results in costly production interruptions and requires immediate action often as severe as discontinuing the use of contaminated cottons. Natural plant sugars are more evenly distributed and cause problems of residue build-up, lint accumulation and roll laps. Quality problems created by plant sugar stickiness are often more critical in the spinning process than the honeydew stickiness. Lint residues which accumulate on machine parts in various processes will break loose and become part of the fiber mass resulting in yarn imperfections. An effective way to control cotton stickiness in processing is to blend sticky and nonsticky cottons. Knowing the sugar content of each bale of cotton used in each mix minimizes day-to-day variations in processif).g efficiency and products more consistent yarn quality. Screening the bale inventory for sugar content prior to processing will allow the selection of mixes with good processing characteristics while also utilizing the entire bale inventory.

Effects of Fibre Preparation on Instrument Readings

149

The relationship between percent sugar content by NIR analysis and the Perkins method shows an excellent correlation of r2=0.9S. The amount of reducing material on cotton fiber in the Perkins method is determined by comparing the reducing ability of the water extract of the fiber to that of a standard reducing substance. Using the NIR method, the amount of reducing sugar in cotton is measured. The popularity of HVI testing has steadily gained since the introduction of the technology in the early 1960s. Timely, valuable information, promotion of communication, standardisation of measurements, optimization of processes, development of new products and cost control are the outstanding benefits of technology.

7 Length of Cotton Fibres The l~ngth of cotton fibres is a property of commercial value as the price is generally based on this character. To some extent it is true, as other factors being equal, longer cottons give better spinning performance than shorter ones. But the length of a cotton is an indefinite quantity, as the fibres, even in a small random bunch of a cotton, vary enormously in length. Cotton is the shortest of the common textile fibers, hence, other things being equal, it makes the most irregular yarns and fabrics. Accordingly the market pays a premium for good length. The various methods of measuring length may be classified according to whether they measure the staple length only, or other parameters work by aligning the fiber ends, e.g comb sorters, measure only length, or use the tuft for other measurements, such as strength etc The importance of fiber length to textile processing is significant. Longer fibers produce stronger yarns by

Length of Cotton Fibres

151

allowing fibers to twist around each other more times. Longer fibers can produce finer yams to allow for more valuable end products. Longer fibers also enable higher spinning speeds by reducing the amount of twist necessary to produce yam. The variability in fiber length can be explained 70-80 percent by genetics , so variety selection is very important. Fiber elongation begins at bloom and continues for about 21 days. Moisture stress during the fiber elongation period will reduce fiber length in all varieties. Starting with a variety that has better genetic potential for fiber length win minimize the probability of producing fiber length in the discount range. Severe weathering after bolls have opened can reduce fiber length because more breakage can be expected in the ginning process. Besides variety, water management and maintaining good plant-water relations is probably the most important factor affecting fiber length Length Uniformity and Short Fiber Content. Length uniformity is now part of the premium/ discount valuation of cotton. Short fibers within a process mix of cotton cannot wrap around each other and contribute little or nothing to yarn strength. Short fibers are virtually uncontrolled in the manufacturing process, indirectly causing product defaults and directly contributing to higher waste and lower manufacturing efficiency. Since short fiber content and length uniformity are derived from length, they are influenced by the same factors as length .. Length uniformity can be more influenced by environment than effective length because temperature is involved in the regulation of genes, which cause epidermal cens to differentiate into fibers. Crop

152

Textile Technology

management practices that influence where bolls are located on the plant can impact short fiber content levels. Uniform fruit retention patterns encourage better length uniformity. Disruption to the natural length distribution is most often caused by mechanical damage, so maintaining recommended moisture levels at the gin is important. Theory of Short Fiber The original theory of the fibrogram as developed by Hertel more than fifty years ago has served as the basis of all subsequent cotton length measurements. :he major assumptions Hertel made in deriving the theory of the fibrogram are embodied in the statement "The fiber is to be selected at random and every point on every fiber is equally probable." This statement translates to: A sampled fiber is held at a random point along its length. The probability of sampling a particular fiber is proportional to its length. Since the longer fibers have a greater probability of being sampled, this results in the length distribution in the fiber beard becoming biased toward the longer fibers .. Using Suter-Webb data and assuming uniform fiber fineness, it is possible to calculate the distributions for the length biased samples. To investigate the validity of the second assumption, we measured the length distribution of a few fiber samples in their original forms and of fiber beards made from these samples. The selected samples for the experiment were two staple standard cotton samples

Length of Cotton Fibres

153

(SS28 and SS40). Length measurements were performed on the samples in their original forms using standard Suter-Webb Array (SWA) methods and the Advanced Fiber Information System Length and Diameter module (AFIS-L/D) made by Zellweger Uster, Inc. In addition, the AFIS was used to measure the length distributions of fiber beards prepared using a model 192 fibrosampler with and without allowing the beards to pass over the carding section of the fibrosampler. All AFIS-L/D results are the averages of three repetitions with three thousands fibers were measured in each repetition. A comparison of the Suter-Webb array data and the AFIS data for the raw stock show good agreement between the methods with small differences characteristic of this version of AFIS. Of more importance is a comparison of the AFIS data between the raw, uncarded and carded samples. The mean lengths and length distributions as indicated by the coefficient of variation are almost identical to those in their original forms. Even if some fiber damage occurs in the AFIS, the damage would be very similar for a given sample and allow us to detect differences in the samples due to the sampling or carding process. Since the differences of the length distributions and the calculated mean lengths between fiber beards and the original fiber samples are small, this would indicate that the second assumption should be modified such that each fiber in the original sample has equal probability to be caught in forming the fiber beard. This in turn would indicate fibers are sampled in clumps rather than individually. Thus the fibrogram theory derived by Hertel should not be applied to the fiber beards prepared

154

Textile Technology

from the fibrosampler. However, his theory appears to apply to those fibe'r beards prepared from sliver by using sliver clamp. The short fiber algorithm as developed by Zellweger Uster is based on the assumption that the fibers are sampled in clumps and integrates the optical response of the fibers over the width of the lens. The first few length groups are estimated by the character of the fibrogram in the form of a quadratic since the HVI is not able to scan in front of the 0.150 in position. This allows us to calculate a complete fiber distribution from the fibrogram, This data is then treated as Suter-Webb data and various length parameters calculated including short fiber content. The cotton set with which the short fiber algorithm was originally verified at Zellweger Uster includes international cottons coliected by sales agents from around the world along with all available ICC cottons. This set of cottons was tested on two different AFIS instruments. Suter-Webb tests were performed at Zellweger Uster and at the University of Tennessee. The USDA crop samples from 1990 to 1994 were obtained from Clemson and tested on three HVls. The entire fiber distribution is obtained. This allows us to calculate not only short fiber values but also other fiber length parameters such as the upper quartile length based on the complete fibrogram rather than a small section of the fibrogram. The relationship of the upper quartile length calcuiated from Suter-Webb data. The USDA, AMS, Cotton Program has been evaluating two methods for determining short fiber content using the Zellweger Uster HVI system. The first method utilizes a short fiber index algorithm, developed

Length of Cotton Fibres

155

by Zellweger Uster, to derive a fibrogram based short fiber index measurement. This method has been under evaluation by the Cotton Program for the past two classing seasons. The accuracy of the measurement has improved during this time with the addition of a cotton calibration routine. The second method being evaluated utilizes a prediction model to derive short fiber index from the HVI measurements of length and uniformity index. This model was designed to predict the short fiber measurement provided by the HVI short fiber index algorithm. Development of the predicted short fiber index measurement began in early 1998. Final revisions to the model, followed by a preliminary evaluation were carried out during the 1998 classing season. Results indicated a strong correlation between the two HVI short fiber measurement methods. Overall reproducibility between HVls, with a tolerance of 1.0, was 75.1% for the predicted short fiber index measurement compared to 58.7% for the HVI short fiber index algorithm. Short fiber content is defined as the percentage of fibers in a sample, by weight, less than one half inch in length. Direct short fiber content measurements can be made with methods such as the Suter-Webb Array and AFIS. Although methods such as these provide useful information, testing speed is slow and the short fiber measurement accuracy is questionable. Another option for obtaining a measurement of short fiber is through the HVI system. All HVI length related measurements such as length and uniformity index are derived from the HVI length fibrogram. Similarly, information exists in the fibrogram to provide a measure of a cotton's short fiber content.

156

Textile Technology

The short fiber measurement provided by the HVI is technically defined as a short fiber index since the HVI is capable of only an indication of the true short fiber content. Since many of the short fibers in a sample are too short to extend from the HVI's specimen holding clamp into the optical scanning device, a direct short fiber content measurement is not possible.

HVI Short Fiber Index The addition of the Zellweger Uster HVI Short Fiber Index measurement did not require any HVI hardware modifications. Since this measurement is derived from the same fibrogram used in the determination of length and uniformity index measurements, the only change was the addition of the short fiber algorithm to the HVI's operating software. The first version of the HVI short fiber index measurement was evaluated in 1997. This early version did not use cotton standards as a basis for calibration. The calibration routine relied on hardware settings which were not successful in providing a common level of testing between multiple instruments. In 1998, a short fiber cotton calibration was developed and added to the existing strength, length and uniformity index cotton calibration routine. Short fiber index values were established on an initial set of calibration cottons using an AFIS instrument. Subsequent value establishment on replacement standards was performed by the Quality Assurance Unit on HVI's calibrated to the initial set. Results of the 1998 evaluation showed a reduction in level differences in addition to improved reproducibility between HVI systems.

Length of Cotton Fibres

157

Predicted Short Fiber Index Considerable research has shown the predictability of short fiber content from HVI measurements of length and uniformity index. The concept of predicting short fiber content from the HVI measurements of length and uniformity index was investigated in 1989. This work resulted in a first order prediction model known as the "Zeidman equation." More recent work has shown that an improved prediction model can be developed with the help of a second order prediction model. The advantage of the second order model over the first is the ability to provide accurate short fiber predictions over a wider range of fiber lengths. The Cotton Program began development of a short fiber prediction equation during the evaluations of the HVI short fiber index measurement. Several equation revisions were made as more HVI short fiber index data was collected. The data used for developing the final prediction equation came from 31,000 samples tested two times in 1998 by the Cotton Program's Quality Assurance check lot program. These samples are representative of all the major U.S. cotton growing areas and therefore have a very wide range of fiber lengths and short fiber contents. In addition, the data contained the necessary measurements of HVI length, uniformity index and short fiber index for development of a prediction equation. In order to give the proper weighting to the data,' average short fiber indexes were calculated for every combination of length and uniformity index. A total of 269 combinations of length and uniformity index along with the averaged short fiber indexes were computed.

Textile Technology

158

The regression analysis of the combination data set resulted in an R2 of 0.97 and produced the second order equation given below:

Z

= a + bX + cY + dX2 + eY2 +fXY where = Predicted Short Fiber Index

X

= HVI Length

Z

Y = Uniformity Index a d

= 384.39664 = 12.490109

b e

= -120.3791 = 0.0295697

= -6.700362 f = 1.0305676

c

Applying the equation back to the original data set resulted in favorable predicted short fiber reproducibility between the two tests made on each of the 31,000 samples. Reproducibility was 75.1% with a tolerance of 1.0 between the two predicted measurements. A reproducibility of 58.7% was calculated for the HVI short fiber index on the same test data. In order to evaluate the agreement between the predicted and HVI short fiber index measurements, reproducibility was calculated within one test of the 31,000 samples. In other words, a comparison was made between the two short fiber measurement methods within the same sample fibrogram. Variability due to between test differences is therefore eliminated. The resulting reproducibility was 77.5%. The predicted short fiber measurement provides the simplest method for obtaining HVI short fiber information. Obtaining short fiber information is simply a matter of plugging length and uniformity index measurements into the equation. Since the short fiber measurement is derived from these well established measurements, additional calibration routines and

Length of Cotton Fibres

159

calibration standards are not required. In addition, evaluations show that the predicted measurement not only agree extremely well with the HVI short fiber measurement, but is also more repeatable. Any new HVI measurement should provide meaningful information regarding its subsequent use. Good progress is being made in the HVI determination of a cotton's short fiber content. Both of the short fiber measurement methods presented in this report are showing their potential. Studies are underway in mill processing environments to assess the utility value of short fiber measurements provided by both methods. In addition, the Cotton Program plans to continue evaluating and comparing these short fiber measurement methods during the upcoming classing season.

Fibre length and yarn Quality The prediction of yarn quality based on the technological characteristics of the raw material has been improved by the use of the AFIS. Unfortunately, information about distributions of fiber properties that are measured by the AFIS is generally not used. The studies carried out at the ITC show that the AFIS length distribution is variety related. In addition, the percentages of both the shortest and the longest fibers have an important impact on yarn quality. During recent years, the Uster AFIS (Advanced Fiber Information System) has been increasingly used in the research projects carried out at the International Textile Center (ITC), Texas Tech University. The prediction of yarn quality based on the technological characteristics of the raw material has been improved by the use of the

160

Textile Technology

AFIS. The ITC has shown in the past few months the value of AFIS measurements such as the short fiber content or the standard fineness. Unfortunately, information about distributions of fiber properties that are measured by the AFIS are generally not used, because the data are not available in an electronic file. This makes the use of these data extremely unfriendly. Nevertheless, we decided to investigate the value of the distribution information with a focus on the influence of the fiber length distribution on the yarn quality. Procedures

First Experiment Fourteen USDA (United States Department of Agriculture) standards cottons were used in this first experiment. The following measurements were performed on fiber: . AFIS with 5 replications of 3,000 fibers, . Sutter Web Fiber Array with 3 replications per technician and two technicians, . Peyer AL 101 with 6 replications

Second Experiment Variety evaluation tests were performed at the ITC during the 1998-99 crop year. Eighteen U.S. Upland cotton varieties were represented. Each variety was grown in three locations and two replicated samples were taken at each location. Therefore, a total of 108 cotton samples were collected (18 x 3 x 2). The cotton fibers from each variety were processed through the Short Staple Spinning Laboratory at the ITC and were

Length of Cotton Fibres

161

made into both ring-spun (36 and 50 Ne carded, 50 Ne combed) and rotor-spun yarns (36 Ne carded). The following measurements were performed on fiber and yarn:

Fiber Tests: Zellweger Uster HVI 900A: 4 mike measurements, 4 color-grade measurements, 10 length and strength measurements. Zellweger Uster AFIS Multidata: 5 replications of 3,000 fibers

Yarn Tests: Zellweger Uster Tensorapid: 10 breaks per bobbin and 10 bobbins Zellweger Uster UT3: 400 yards per bobbin and 10 bobbins The printout from the AFIS provides us with a distribution of the length by weight. The histogram is built based upon the percentage of fibers in each of the 40 length categories, from 0 to 2.5 inches with an increment of 1/16th of an inch. In order to get a first look at the data provided on those 108 cotton samples, we limited the number of length categories to 10 by aggregating 4 categories together; therefore, the length category increment became 0.25 inch.

Third Experiment Two commercial cotton bales were selected. A very low

162

Textile Technology

amount of ELS cotton was added (2% and 5%) in order to check if the addition of a very small amount of long fibers would increase significantly the CSP. The same measurements used in the second experiment were taken on the fibers and yams.

Results The first experiment grew out of an anomaly with AFIS measurements. During the past few years, thousands of cotton samples have been analyzed at the ITC using the AFIS. Results for most of the cottons indicate a very small percentage of fibers in the length categories of 2 inches and longer. We can postulate either that those very long fibers really exist or that the AFIS over- estimates the length of the longest fibers. To investigate this, 14 USDA standard cottons were tested on the AFIS, Sutter Web Fiber Array and Peyer AL 101. Results showed that the instruments correlate very well for the shortest fiber percentages, although the levels are different. For the very short-staple cotton (staple 26), the length distributions obtained are very similar. For the short-staple cotton (staple 32), AFIS and Peyer are in good agreement, but the Array method tends to get higher percentages for the longest fibers. For the medium (staple 35) and long (staple 40) fibers, the discrepancy between instruments is clear. Neither the Peyer nor the Array showed any fibers to the longer than 2 inches, but the AFIS did indicate some of these for most of the samples. This suggests that

Length of Cotton Fibres

163

the AFIS tends to over- estimate the length of the longest fibers. One hypothesis to explain this result is that the speed of the fibers passing trough the sensing device is not constant; i.e., the longer the fiber, the higher the friction forces for the air-to-fiber interface. This could lower the speed, resulting in a longer electronic signal. Given this anomalous result with the AFIS, the question arises whether it is a useless artifact or if it has predictive power. This led to the second experiment involving 18 upland varieties grown in 3 locations with 2 field replications per location. Using the AFIS multidata, for each length category, defined, an analysis of variance was done. The length distribution by weight is variety related; this implies that breeders could modify the length distribution. The longest fibers measured with the AFIS, although a very small percentage of total fibers, are also variety related. This means that the fibers measured as too long by the AFIS cannot be dismissed as meaningless. To investigate further, we calculated the coefficients of correlation between major yam characteristics and the percentages of fibers in the different length categories. For Count Strength Product (CSP), these correlations are quite similar for all the types of yarns.ring or rotor, carded or combed. For the fibers shorter than one inch ele correlation coefficients are negative in all cases; therefore, the larger the share of these length categories, the lower the CSP. For fibers in the 1.00-to-1.25 category the correlation coefficients are still negative but are near zero. As the length categories increase above this level, the correlations become positive and large. The category

164

Textile Technology

longer than 2 inches exhibits the highest positive correlation of all. It shows that the AFIS percent of fibers longer than 2 inches is the best length parameter to predict CSP. In fact, it performs better than the HVI strength and the AFIS standard fineness. This is even more startling given that the percentage of fibers longer than 2 inches averages only 1 percent on the 108 samples tested .

The carded ring spun yarns exhibit very similar behavior. The length categories giving the best correlation coefficients with the yarn uniformity are: [0.00;0.25], [0.25;0.50] and [>2.00], with a positive correlation for the shorter fibers and a negative correlation for the longer fibers. Therefore, the higher the short fiber content, the higher is the yarn CV%; and the higher the long fiber content, the lower is the yarn CV% .. The UT3 CV% of the combed ring-spun yarn exhibits a very good correlation with the percentage of fibers longer than 2 inches and a quite poor correlation with the shorter fibers. This is logical because a large part of the shorter fibers has been removed during the combing operation. For the rotor spun yarn, the negative effect on the yarn uniformity of the shorter fibers is limited. But the fibers between 1.75 and 2 inches exhibit the highest correlation with the yarn CV%. The fibers longer than two inches give a lower correlation, probably because a part of them (the extremely long fibers) wrap around the yarn and create imperfections. This is likely related to the rotor diameter and it will be necessary to test different rotor diameters to confirm this hypothesis. The third experiment was done to obtain some confirmation of effects of the longest fibers on the yarn strength. Using

Length of Cotton Fibres

165

two commercial bales of Upland cotton, ring-spun 30 Ne yarns were made. Then very small amounts (2% and 5%) of ELS cotton fibers were mixed with the Upland cotton and also ring spun into 30 Ne yarns. The length distribution data available with the AFIS appears to contain information that is useful to both the cotton breeders and the spinners. Since the length distribution clearly appears to be variety related, it may provide a new tool for cotton breeders in their efforts to reduce short fiber content. The causes for the AFIS measuring some fibers as longer than 2 inches are not understood; nevertheless, this measurement exhibits the highest correlation with the yarn esp. For the carded ring-spun yarns, the shortest fibers and the longest fibers exhibit the highest correlation with the yarn eV%, the number of thin places, and the number of thick places. For the combed ring-spun yarns and the rotor-spun yarns, the longest fibers exhibit the highest correlation with the yarn eV%, the thin places, and the thick places. The correlation coefficients between the different length categories and the number of neps are generally low. The shortest and the longest fibers are highly correlated with the hairiness for all the types of yarns. The shortest fibers increase hairiness and the longest fibers decrease hairiness. The three shortest length categories are highly correlated with increased combing noils.

8 Cotton Stickiness Stickiness occurs when excessive sugars present on fibers are transferred to equipment and interfere with processing. Sugars may be insect- or plant-derived. Though sugars are ubiquitous in lint, they usually occur at levels that pose no processing difficulties. This details the sources and components of problem sugars on harvested lint, the processing impacts of stickiness, and strategies for avoiding or mitigating stickiness. Cottons contaminated with stickiness cause multiple problems in the spinning mills. The honeydew present on the cotton lint is able to contaminate all the mechanical instruments used in the transformation process from fiber to yarn, i.e. opening,carding, drawing, roving and spinning operations. These contaminants are mainly sugar deposits produced either by the cotton plant itself (physiological sugars) or by the feeding insects (entomological sugars), the latter being the most common source of contamination. Honeydew, when present in sufficient quantity, is the main source of sugars that can result in sticky lint.

Cotton Stickiness

167

Honeydew is excreted by certain phloem-feeding insects including such common pests of cotton as aphids and whiteflies. These insects are capable of transforming ingested sucrose into over twenty different sugars in their excreted honeydew. The major sugars in cotton insect honeydew are trehalulose, melezitose, sucrose, fructose and glucose. Another source of stickiness is free plant sugars sometimes found in immature fibers. Cotton fiber is largely cellulose that is formed from sugars synthesized by the plant. Dry, mature cotton fibers contain little free sugar, while immature cotton fibers contain glucose, fructose, sucrose, and other sugars. If immature cotton fiber is subjected to a freeze, complex sugars may be broken down to release additional simple sugars. Less commonly, oils released by crushed seed coat fragments can also result in stickiness. In this case, raffinose is the characteristic sugar. Sugars differ in their stickiness. For example, sucrose, melezitose, and trehalulose are all significantly stickier when deposited on fiber than are glucose or fructose. Further, trehalulose-contaminated fiber is stickier than fiber with an equivalent amount of melezitose. Mixtures of sugars, such as occur in honeydew, tend to be stickier than single sugars. Localized concentration of sugars like honeydew is at higher risk of causing stickiness than more evenly distributed sources like plant sugars .. Sticky cotton can reduce cotton gin output (in bales/ hr) by up to 25%. At the textile mill, excessive wear and increased maintenance of machinery may occur even with slightly sticky cotton. In severe instances mill shutdown with a thorough cleanup is required.

168

Textile Technology

Cotton aphids Aphids are slow-moving, soft-bodied insects. Adult cotton aphids are approximately 1/10 of an inch long and roughly pear shaped. They may possess wings or may be wingless. Cotton aphids have two color phases: yellowish or dark green. The cotton aphid has two projections which arise from the upper side of the abdomen. These small tubes are called cornicles and are used to excrete defensive secretions. Both the adult and immature stages (called nymphs) of the cotton aphid have stylet like mouthparts, which they use to suck juices from the host plant. Consequently, cotton aphids are sometimes referred to as plant lice The cotton aphid, Aphis gossypii, excretes honeydew rich in melezitose (ca. 30-40%). Their droplets (inset, SOX) tend to be larger than those produced by whiteflies. Whiteflies, Bemisia spp., also excrete honeydew, but as trehalulose-rich (ca. 40-50%) droplets (inset, SOX).

Stickiness Measurement 'Stickiness' is the physical process of contaminated lint adhering to equipment. The degree of stickiness depends on chemical identity, quantity, and distribution of the sugars, the ambient conditions during processingespecially humidity -and the machinery itself. Stickiness is therefore difficult to measure. Nonetheless, methods for measuring sugars on fiber have been and are being developed. These measurements may be correlated with

Cotton Stickiness

169

sticking of contaminated lint to moving machine parts. The physical and chemical attributes of the lint and sugars that are correlated with stickiness have been measured in many ways, each with differing efficiency and precision.

Reducing-sugar tests Some textile mills use reducing-sugar tests based on reduction of the cupric ion to screen for sugar contamination. These tests are relatively quick and inexpensive. However, some insect sugars are not reducing sugars, and some others are measured at different levels of efficiency by various reducing-sugar methods. Thus conventional reducing-sugar tests are best reserved for screening lint that potentially has high levels of plant sugars. In these cases and with the potassium ferricyanide (KFeCN) test, lint with reducing sugar levels below 0.3% may be processed without difficulty.

High Perfonnance Liquid Chromatography (HPLC) High Performance Liquid Chromatography (HPLC) identifies and measures both reducing and nonreducing sugars. The main sugars of insect honeydew, trehalulose (from whiteflies) and melezitose (from aphids), and of plant sugars (glucose, fructose & sucrose) are all readily identified in this test. The benefit of HPLC analysis is the identification of the source of contamination (whitefly, aphid, or plant) which may help identify specific mitigiation measures

170

Textile Technology'

Mincard method The physical interaction of all sugars on lint with equipment can be measured by several types of machines. The primary difficulty with these physical tests is in standardizing the stickiness measurement. As with chemical testing, these tests must be correlated with measures of fiber processing efficiency in order to interpret the results. One of these tests, the minicard, is a physical test that measures actual cotton stickiness of the card web passing between stainless steel delivery rollers of a miniature carding machine. Modeled after a production carding machine, the minicard must be run under strict tolerances. A '0' minicard rating indicates that no sticking was observed, while progressively higher numbers (on a 0-3 scale) indicate progressively greater amounts of sticking during the process. Cottons with high plant sugar contents evenly distributed along the fibers may fail to be measured as sticky in this test. The minicard test is slow and has been replaced as the international standard by the manual thermodetector.

High Performance Liquid Chromatography (HPLC) The Sticky Cotton Thermodetector (SCT) measures the physical sticking points transferred to aluminum sheets by a conditioned lint sample that is squeezed and heated (to 82.5°C for 12 sec.). Levels of stickiness are categorized according to the number of specks left on the two sheets of foild.Lower numbers of specks are preferable to higher numbers; however, a specific threshold over which all cotton will result in processing problems has not been defined. The SCT takes about 5 minutes to

Cotton Stickiness

171

process each sample, requires smaller initial investment costs than the minicard, is more mobile, and its results correlate well with predicted stickiness from the minicard.

The High Speed Stickiness Detector (H2SD) The High Speed Stickiness Detector (H2SD) is a quicker, automatic version of the thermodetector. The cotton sample is pressed between a heated (54°C for 30 sec.) and an unheated pressure plate. Sticky points are counted and point size distribution determined by imageprocessing computer software. Plates are automatically cleaned between samples. The H2SD is able to analyze a sample in 30 seconds.

Fiber Contamination Tester (FCT) Like the thermodetector and H2SD, the Fiber Contamination Tester (FCT) measures physical sticking points (at 65% RH). The instrument feeds a thin web between two rollers. Contamination of the rollers interrupts a laser beam, resulting in a recording. Because the cleaning and recording is automated, samples may be processed as quickly as one per 45 seconds. While there is no reliable infield method for detection of stickiness predisposition, the insects responsible for honeydew deposits can be sampled and populations measured. Not all population levels of insects lead to sticky lint; however, chronic numbers of insects, especially during boll opening or an extended season, can lead to excessive insect sugars that result in

172

Textile Technology

stickiness. In addition, field factors associated with risk of excessive plant sugars are lateness of the crop, fiber immaturity, and freezing temperatures before harvest. Stickiness Control The most efficient way now to prevent stickiness is by managing sugar sources in the field. Detailed integrated pest management plans (see references) for both aphid and whitefly. These honeydew-producing insects may be managed by avoiding conditions leading to outbreaks, carefully sampling pest populations, and using effective insecticides when populations reach predetermined thresholds. The risk of having excessive plant sugars can be minimized by harvesting mature seed cotton. This may be accomplished through plant management tactics that include: early and uniform planting, nitrogen management according to plant growth and yield goals, high first-position boll retention, and timely chemical termination and harvest. If a freeze is imminent and immature bolls are present, the use of boll-opening chemicals can greatly dimhish the problem of plant sugar contamination. All these measures work towards early harvest, before freezing conditions that contribute to excess plant sugars. When field management of sugar sources is inadequate to prevent excess accumulation of sugars, mitigation tactics may be necessary to remove excess sugars from the lint. This mitigation may be achieved through both natural and managed processes; however, the specific impact of these processes on stickiness is variable and may depend on the initial level of contamination.

Cotton Stickiness

173

Natural processes include weathering, rainfall, and degradation by microorganisms. Since sugars are water soluble, rainfall will wash some honeydew from lint. If sufficient moisture is available, bacteria and molds living on the plants will decompose many honeydew sugars. Complex sugars are broken down to simpler sugars, and the simpler sugars, given sufficient time and moisture, are further broken down to carbon dioxide and water. Unfortunately, microbial action also leads to discoloration and to a weakening of the fibers as well as heating of cotton in modules that may result in reduced seed viability and problems in ginning. Potential in-field mitigation techniques include supplemental oversprays of enzymes or water. Certain carbohydrate degrading enzymes when sprayed on sticky cotton can reduce honeydew to simpler sugars. Microbial activity on the fibers then further degrades these simpler sugars, resulting in a significant decrease in fiber stickiness. However, these enzymes require water for activity, and metering the proper amount of water for activity is a problem yet to be solved. In some areas of the world, overhead and in-canopy irrigation has been used to remove honeydew from open bolls. The frequency of this type of irrigation may be more important than the volume applied. Use of sprinklers has been limited in the Western United States, where furrow irrigation is prevalent. If stickiness is a problem while ginning, the ginning rate of honeydew contaminated cotton can be increased by increasing the heat of the drying towers to reduce humidity. The potential for stickiness can be further reduced by lint cleaning. Both of these practices, however, can result in shorter fibers.

174

Textile Technology

Conventional textile lubricants may also be used. Stickiness due to high levels of plant sugars can be reduced by storing the cotton for approximately six months. However, storage of baled cotton will not appreciably reduce stickiness from insect sugars. At the textile mill, stickiness may be managed by blending bales and by reducing humidity during carding. A lubricant in fog form may be introduced at the end of the hopper conveyor, and card crush rolls may be sprayed sparingly with a lubricant to minimize sticking.

Processing of sticky cotton In spinning mills, sticky cotton can cause serious problems. It contaminates the textile machineries like blow room , card, drawing, roving, and spinning frames. These contaminants are mainly sugar deposits produced either by the cotton plant itself (physiological sugars) or by feeding insects (entomological sugars), the latter being the most common source of stickiness. Seventeen mixes having a moderate level of stickiness were evaluated in both ring and rotor spinning. High-performance liquid chromatography tests were performed on residues collected from the textile machinery to identify the types of sugars present. It was shown that among the sugars identified on raw fiber, only trehalulose exhibits higher percentages in the residues than on the fiber. During the fibers-to-yarn transformation, the flow of lint is submitted to different friction forces; consequently, the temperature of some mechanical elements may increase significantly and affect

Cotton Stickiness

175

the thermal properties of the contaminated lint. After a sugar becomes sticky, the other sugars present on the lint, as well as other substances such as dusts, silica, etc., will stick to the lint and could cause unevenness in the flow of lint being drawn, such as lapping up on the rolls, nep-like structures, and ends-down. Therefore, the thermal properties of the five sugars identified on the contaminated fiber and on the residues collected on the textile equipment were investigated. Among the sugars tested, trehalulose is the only one having a low melting point, around 48degre C. In addition, trehalulose is highly hygroscopic. After passive conditioning of dehydrated trehalulose at 65% ± 2% relative humidity and 21 degree C ± 1degree C for 24 h, the quantity of adsorbed water at eqUilibrium was found to be approximately 17.5%. This corresponds to three molecules of water adsorbed for each molecule of trehalulose. The combination of low melting point and high hygroscopicity could be the cause of the selective accumulation of this sugar on the textile equipment. Stickiness is primarily due to sugar deposits produced either by the cotton plant itself(physiological sugars) or by feeding insects(entomological sugars) .Insects have been documented as the most common source of contamination in some studies . The analysis of honeydew from thecotton aphid and cottonwhitefly has shown that aphid honeydew contains 138.3% melezitose plus 1.1% trehalulose. whereas whitefly honeydew contains 43.8%trehalulose plus 16.8% melezitose. Otherrelative percentages may occur, depending on the environmental or feeding conditions. Furthermore, stickiness is related to the type of sugars present on the lint. The authors showed that trehalulose and sucrose,

176

Textile Technology

bothdisaccharides, were the stickiest sugars when added to clean cotton, while melezitose(trisaccharide}, glucose, and fructose (both monosaccharides) were relatively nonsticky. Previous investigations wereconducted to elucidate the factors affecting the behavior of cotton contaminated with stickiness. In textile mills, the method mainly used to reduce the impact of stickiness is blending sticky cotton with non-stickycotton . Stickiness caused by honeydew depends on the relative humidity, which is a function of both water content and air temperature, in which the contaminated cotton is processed. Stickiness measured with the thermodetector is dependent on the relative humidity. Sticky cotton (with 1.2% reducing sugarcontent), when stored in high relative humidity(70degree F, 80% relative humidity, caused moreproblems during processing than the same stickycotton stored at low relative humidity 75degree F, 55% relative humidity. However, at low relativehumidity, the fibers are more rigid and will increase the friction forces creating static electricity .. Therefore, it will require more energy to draw the lint. Stickiness has been reported to cause a build-up of residues on textile machinery, which may result in irregularities or excessive yarn breakage. When processing low to moderately contaminated cotton blends, residues will slowly build up, decreasing productivity and quality, and forcing the spinner to increase the cleaning schedule. Consequently, we decided to study the origin of the residues collected on the textile equipment after processing sticky cotton blends with low to moderate levels of contamination.

Cotton Stickiness

177

Materials and Method

Materials We selected 12 commercial bales contaminated with insect honeydew on the basis of their insect sugar (trehalulose and melezitose) content and their stickiness as measured with the high-speed stickiness detector . In addition, five non-sticky bales from one module were purchased for mixing with the contaminated cotton, so that alternative stickiness levels in the mixes could be obtained. The 12 contaminated bales were broken and layered. Ten samples per bale were taken. Each sample was tested with a high-volume instrument (Model 900 Automatic, Zellweger Uster, ) and high-performance liquid chromatography

High-Speed Stickiness Detector The high-speed stickiness detector is derived from the sticky cotton thermodetector , which was approved as a reference test by the International Textile Manufacturers Federation in 1994 . This thermomechanical method combines the effect of heat and pressure applied to a sample of cotton placed between two pieces of aluminum foil. When the temperature increases, moisture in the cotton vaporizes and is absorbed by the sticky spots, making them stick to the foil. The high-speed stickiness detector is an automated version of the sticky cotton thermodetector . Three replications were performed on each sample (10 samples per bale x three replications = 30 readings per bale).

178

Textile Technology

Spinning Trials Opening, carding, drawing, roving, ring spinning, and rotor spinning machines used were all industrial equipment. In the ring spinning trial, the yarns were spun to a 19.68 x 10-6 kg m-l (19.68-tex or 30 English number) count. Fourteen spindles were used for each mix spun, and each mix was run for 72 h. For the open-end spinning trials, the yarn produced was 26.84 x 10-6 kg m1 (26.84-tex or 22 English number); 10 positions were used, and each mix was run for 20 h. We ran preliminary tests on ring spinning before testing the mixes. A 13.6 kg sample of lint from each bale was carded and drawn. If noticeable problems occurred at the draw frame, the process was stopped. If not, the drawing slivers were transformed into roving. If noticeable problems occurred at the roving frame, the process was stopped. If not, the roving was transformed into yarn at the ring spinning frame. If noticeable problems occurred at the ring spinning frame, the process was stopped. If not, 45.4 kg of lint was processed for the large-scale test. If noticeable problems occurred at any step of the process, the cotton was mixed with 50% nonsticky cotton and the process was repeated. This procedure was used for 17 large-scale tests. Four bales were spun without mixing the lint with the non-sticky cotton. Four bales were spun after mixing the lint with 50% non-sticky cotton. Four bales were spun after mixing the lint with 75% nonsticky cotton. Three bales were spun after mixing the lint with 87.5% non-sticky cotton. Finally, two baleswere spun after mixing the lint with 93.75% nonsticky cotton. Card slivers, flat wastes, draw frame residues, and sticky

Cotton Stickiness

179

deposits collected at the end of each test on the rotor spinning and ring spinning frames were analyzed by high-performance liquid chromatography. These tests quantify the amount of sugars, expressed as a percentage of total sugars present. In addition, high-speed stickiness detector measurements were made on card slivers. After each spinning test was completed, the opening line and the card were purged by processing a non-contaminated cotton, then all the equipment was washed with wet fabric and thoroughly dried.

High-Performance Liquid Chromatography on Sticky Deposits Residues on textile equipment were collected using wet wipes. Each wipe was identified, placed into a. plastic bag, and frozen. After the spinning trials, sugars were extracted from the wipes using 20 mL of 18.2- megohm water. High-performance liquid chromatography tests were performed following the same procedure used for the bale samples. Three replications were performed on each sample. The results for each sugar were expressed as a percentage of total sugars identified.

Dust Test Dust was collected from 20 rotors after a 4-h run. The spinning equipment for this test was an Elitex BD200M , because it has no auto-cleaning devices to remove dust. Collected dust was frozen. We extracted the sugars from the dust using 20 mL of 18.2-megohm water. Highperformance liquid chromatography tests were performed following the same procedure used for the

180

Textile Technology

bale samples. Three replications were performed on each sample. The results for each sugar were expressed as a percentage of total sugars identified.

Water Adsorption The selected sugars were fructose, glucose, sucrose, trehalulose, and melezitose. Trehalulose was obtained from Cornell University; the other sugars were from Sigma Chemical Company (St. Louis, MO). The sugars first were dehydrated at room temperature under vacuum for 48 h. They were weighed immediately in tightly closed weighing containers in a controlled atmosphere (65% ± 2% relative humidity, 21degreeC ± 1degreeC. Recorded weight, mO (dry weight), at time, to = 0, was used for calculation of weight-gain. Since the . stickiness tests were done at 65% ± 2% relative humidity and 21degree C ± 1degreeC, the open containers containing sugar samples were stored at these conditions and weighed (weight mt) over time until the weight stabilized (14 wk). The percentage of adsorbed water on each sugar was then calculated as [(mt - mO)/mO] x 100 and plotted against time.

Differential Scanning Calorimetry The differential scanning calorimetry technique is widely used to examine and characterize substances. The principle of this method is based on measuring the heat flux between the sample and a reference while the temperature is rising. The sample and the reference are deposited into two different pans and heated at the same rate. In this work, the reference was an empty pan. The

Cotton Stickiness

181

analysis of the differential scanning calorimetry profiles indicates the thermal properties of the substances being tested; specific values such as melting point and decomposition point are obtained. The differential scanning calorimetry profiles were recorded by heating at the rate of 5degreeeC min-1 between 25degreeC and 250degreeC.

Scanning Electron Microscope Following the processing of the 17 mixes, yarn neps were identified and collected. The samples were mounted in the stub and coated with a layer of gold by means of thermal evaporation in a vacuum coating unit. They were then examined in the scanning electron microscope using an accelerating voltage of 20 KV. Sucrose is virtually the only sugar in the phloem sap of the cotton plant . Insects produce trehalulose and melezitose by isomerization and polymerization of sucrose; neither of these sugars occurs in the cotton plant . Therefore, their presence on cotton lint demonstrates honeydew contamination. Stickiness can cause a build-up of residues on the textile machinery, which may result in irregularities or excessive yarn breakage. When cotton is very sticky, it cannot be processed through the card; however, with low to moderate stickiness levels, yarn can generally be produced. For this reason we decided to work with mixes having a very moderate level of stickiness so that residue would build-up slowly on the textile equipment. Performing the spinning test this way is more representative of industrial practice. Indeed, a spinner will not run a very, or even moderately, sticky blend. Rather, the spinner will mix the sticky cotton in

182

Textile Technology

such a way that no short-term effect will be noticed. Nevertheless, residues will build up over time and translate into a slow decrease in productivity and quality, forcing the spinner to increase the cleaning schedule. In this chapter, we present only the results of the study on the composition of residues found on the textile equipment after processing of sticky cotton blends. Stickiness caused by honeydew contamination has been reported to cause residue build-up on textile machinery, which may cause subsequent irregularities or yarn breakage. We evaluated 17 mixes having a moderate level of stickiness. In both ring and rotor spinning, trehalulose content had the tendency to increase in the residues collected on the equipment while the other sugars did not. The study of the thermal properties of the identified sugars present on contaminated lint shows that among the selected sugars, trehalulose has the lowest melting point 48 degree C . It begins to melt as 'soon as the temperature starts rising. Therefore, any increase in the temperature of the textile processing equipment will first affect trehalulose. In addition, trehalulose is highly hygroscopic. The combination of high hygroscopicity and low melting point could explain the higher concentration of trehalulose in the residues collected on the textile equipment than on the original fiber.

9 White Specks The term 'white specks' describes a condition in dyed cotton fabric where small nep-like forms can be seen as white or, more accurately, lighter shade specks on the surface of the finished fabric. This condition can exist in both woven and knitted goods. White specks are undyed spots on dyed fabric, and are commonly caused by neps. According to the American Society for Testing and Materials, a nep is "a tightly tangled knot-like mass of unorganized fibers." This is to be differentiated from a mote, which is another impurity found in cotton consisting of a seed fragment encompassed by cotton fibers. Many researchers have studied neps over the decades, including types, formation, effects, and solutions. Watson classified neps into two groups, mechanical and biological, and observed that mechanical neps are similar to the classical ASTM definition, where they are formed from mechanical actions on the fibers. They also reported that fibers with low micronaire values tend to form mechanical neps because the fibers are finer and

184

Textile Technology

less mature, and are therefore less rigid. Biological neps are clumps of very immature fibers that can be found in seed cotton before mechanical processing has occurred. They also reported that fibers with low micronaire values (finer and immature fibers) tend to form mechanical neps because of the weak, poorly developed, less rigid fibers. Goynes et al. reported that because of low cellulose content of the undeveloped, flat, ribbon-like fibers, clumps of these fibers do not accept dye. Therefore, when a fabric is dyed, the mechanical and biological neps' formed by fine or immature fibers create undyed spots in the finished fabric. These undyed spots are known as white specks. Quality of a finished garment is determined by, among other things, the number of imperfections contained within the fabric. The more imperfections found in the cloth, the less value can be added to the product by the manufacturer. Since uniform surface color is a desirable aspect for fabrics, the inclusion of white specks is detrimental to fabric quality. White specks are a result of neps being included in the raw cotton product supplied to a processor or result from ensuing mechanical treatment. The process of counting neps is very tedious and time consuming. Since the late 1930s, neps were counted manually using a back light or a black background. Even today, neps are being counted manually relying on visual inspection. The manual counting is not only a timeconsuming process, but also inconsistent and prone to error because it is very subjective. Recently, many studies focused on automatic counting of neps and white specks. The Advanced Fiber Information System (AFIS) module has been used by

White Specks

185

many researchers to detect and count seed coat neps. Mor introduced a fiber contamination tester that detects sticky deposits by an electro-optical device and evaluates nonsticky parameters such as neps, trash, and seed coat fragment using an image processing system. Bel-Berger et al. used three different image processing hardware systems in analyzing the number and area of white specks found in dyed fabric. The area of white specks was calculated in terms of number of pixels. The white speck counter developed consists of three major components: an illumination chamber, fabric transport mechanism, and image processing hardware and software. The illumination chamber is needed to provide uniform illumination on the fabric surface by blocking ambient light and furnishing a consistent light source. An aluminum roller is mounted on each side of the chamber so that a roll of dyed fabric can be mounted on a roller on one side of the chamber, and transported through the chamber to the other side onto the second roller. An image of the fabric is then captured by a blackand-white camera. Image analysis software counts the number of white specks in the image and measures the area of individual white specks. The appearance of white specks on dyed and finished fabrics continues to be a sporadic and periodic problem for dyers, knitters, and spinners. Perhaps the most troubling aspect of this problem is the fact that its presence is not usually known until the fabric is dyed and finished. The severity of the occurrence can range from barely noticeable to rendering the material useless as first-quality goods. White specks are not normally visible in bleached or greige state goods.

186

Textile Tec1tnology

White specks are actually small clusters of immature fibers (often "fused" together) which lie on the surface of the dyed fabric. Because these fibers are immature, orunderdeveloped, their cell walls contain relatively little cellulosic material. This condition causes the fiber to take on a very flat and ribbon-like form. It is this flat form that, when seen on the surface of a dyed fabric, reflects light more efficiently than the surrounding fibers. This high reflectivity is perceived by the eye as being lighter in shade or, in some situations, as white specks. All cottons (different varieties and bales) contain some amount of immature fibers. They are a natural product of the plant's developmental physiology. It is only when these immature and underdeveloped fibers reach certain concentrations in a bale, or group of bales, that the problem of white specks becomes an issue. Depending on their form and/or concentration level, immature fibers mayor may not actually produce a white speck occurrence. This is part of the dilemma facing yarn manufacturers ... There is no way of absolutely predicting (or avoiding) a white speck outbreak. With that said, there are some general rules of thumb (based on empirical data and actual experience) that a spinner can follow in order to lower the probability of producing yarns which contain potential white specks. 1.

For any given growth area and! or variety, higher micronaire values are less likely than lower micronaire values for producing white specks.

2.

Higher maturity ratios are less likely than lower maturity ratios for producing white specks.

3.

Stripper harvested cottons are more likely to produce white specks than- spindle picked cottons. This is

White Specks

187

largely due to the tendency of the stripper to harvest bolls that are not fully mature. 4.

Removing more waste through cleaning and carding (especially under the lickerin and fine openers) can minimize, or make an unacceptable situation acceptable. This suggestion should also imply that the introduction of reclaimed waste is a high risk activity for introducing white specks.

5.

Maintaining a higher nep reduction factor on all cards can be very effective in minimizing white speck problems

Problems with white specks can be completely avoided or eliminated. This is especially true if the concentration of white speck producing material is high enough, but even a severe problem can be improved by their implementation. The real crux of this very costly and frustrating situation is that there is no definitive or quantitative means of identifying, absolutely, the potential for this problem before it actually appears in dyed fabric. With all the indicators available to fiber users, none offer the ability to positively warn of a white speck outbreak. For this reason, many spinners, knitters, and dyers will perform sample dyeings on a given lot of yam, which has been knitted into a small amount of fabric expressly for that purpose. Since it should be clear at this point that complete avoidance is not possible, then it should also follow that the responsibility for white speck occurrences is very difficult to assign to one party in the production chain. If the conditions are shown to be favorable for white specks to appear, all parties must communicate quickly so that

188

Textile Technology

alternative processing and production decisions can be made in a timely manner. Using known white speck containing fabrics for only bleached whites is one possible recourse. There are also many dyes that do a better, or worse, job of actually covering the problem. Dye selection (and shade choices) alone can prove to be a very effective means of dealing with this serious issue. Caustification of white speck infected fabrics has also shown to be quite successful. There is no one, single best answer to this very frustrating issue. But, with the understanding and cooperation of all those involved, there may be found some fair compromises that could very well turn an unacceptable situation into one of shared acceptability. Dyeing imperfections that appear as white specks on cotton fabrics that have been dyed deep shades are a major problem in the textile industry. The presence of these imperfections in raw cotton is not evident since they only show up after dyeing. Processing through fabric dyeing results in both time and product losses when white specks are present. Approaches for eliminating or minimizing the problem include plant breeding, changes in growing and harvesting procedures, and additional finishing during dyeing. None of these provide immediate cost free solutions. A method of screening samples for dye defect potential before processing would allow mills to divert affected cotton batches to non-problem products. In this paper, a simple light microscopy process is described for screening undyed fabrics, yarns, and sliver. This darkfield procedure discriminates between common fiber tangle neps that are not dye resistant, and those that

White Specks

189

consist of bundles of extremely thin-walled fibers that will not dye. The nature and effect of undyed white defects in cotton fabrics has been extensively investigated . These defects have been confirmed to be bundles of extremely immature (undeveloped) fibers that come in with ginned cotton, and though some are removed in cleaning processes many are carried through processing to the final fabric, and become apparent on dyeing. Processing can affect apparent size and number of defects. Growing location and conditions can influence the number of defective fiber bundles in a harvested lot. Variety also is responsible for amount of defects in a lot. Breeding programs caf\ possibly decrease the pre-disposition for production :"of motes, which in turn produces undeveloped fibers. Effects of environmental conditions in open fields are difficult to control. It is possible to adjust dye formulations for variations in overall bulk maturity, but it is difficult to achieve even dyeing when concentrated areas of undeveloped fibers are present in lots of otherwise average maturity. Therefore, the most immediate solution to the problem would seem to be a system for predetermining presence of large quantities of undyeable materials. If such tests could be developed for incoming lots, then those with high speck potential could be rejected, or at least diverted to non-problem uses. Detection even at the yarn or pre-dyed fabric level would at least prevent use of white speck goods for dark textiles: Therefore, we attempted to develop a method to "see" white specks in undyed cotton.

Samples used in the study were from a series of cottons especially grown for white speck studies. They

190

Textile Technology

were grown under irrigated conditions in a field in the San Joaquin Valley in California, and included a commercial Delta Upland (DP-90), a Mississippi hybrid (ST-825), and two Acalas (EA-C 30, early maturing; and EA-C 32, a Prema). Each sample was available as bale cotton; sliver, yam, undyed fabric, and dyed fabric. For microscopical examination, a wide field stereo zoom light microscope equipped with substage darkfield illumination was used. Observations were made at 1020X magnification. Identified defects were marked, cut from the sample, and prepared for examination at higher magnification using a scanning electron microscope (SEM). When textile fabrics are examined using surface lighting with a stereo light microscope, it" is possible to see the weave of the fabric, outlines of fibers within yams, and both contamination and fiber defects on the fabric surface. Contamination defects such as seed coat fragments, and leaf and bract materials' appear very dark, and can easily be segregated from fiber defects. One type of fiber defect consists of individual fibers that have been tangled with other fibers during processing. These defects do not normally cause dye defects. Another type of fiber defect is caused by bundles of undeveloped fibers. Both of these fiber defects can be seen using surface lighting. However, it is not possible to distinguish them as two different types using surface illumination, so determinations of white speck potential cannot be made from such opservations. If the fabrics are examined using dark field lighting, there is an obvious difference in the two types of fiber defects. Dark field illumination is accomplished using a substage lighting system, and a field stop that blocks the

White Specks

191

path of the light beam that normally is projected through the sample. This system forms a hollow cone of light that travels around the field stop. A ground glass or filter system can be used to decrease the intensity of the light. With small objects scattered on a clear field, the field appears black, and the sample appears self illuminated because the light observed is that transmitted to the objective lens by the sample itself. Thus the nature of the sample determines the brightness of the object in the observed field. If samples containing fiber defects are examined first using surface illumination to show presence of the defect, then the lighting is switched to darkfield, an immediate differencev can be seen in the fabric image. Yarns appear with bright edges because they are thinner at the edge, and more light is transmitted. Differences can also be seen in thick and thin yarns because of the amount of light that passes through them. Thick, non-fiber neps (usually plant parts) appear completely dark, and those containing only thin areas of seedcoat may appear gold or orange. Of greater significance, differences can be seen between fiber neps. Tangled fiber neps blend into the yarn and are hardly seen, but defects formed from clumped, undeveloped fibers appear as a shadow on, or in the yarn. The tangled network of the thin-walled fibers can be seen. This difference is subtle, and careful observation is required to become familiar with the differences in appearance. However, switching back and forth between surface and darkfield, subsurface lighting shows the obvious differences in tangled fiber neps and undeveloped fiber neps. To verify that defects identified as undeveloped fiber clumps were actually the same

192

Textile Technology

undyeable white defects that were found in dyed fabrics, the defects identified by dark field microscopy were cut from the fabric and examined using scanning electron microscopy. Results of these examinations showed that all examined defects were composed of undeveloped fiber clumps. Although detection of white speck potential of fabrics is of great significance because it would prevent dyeing of fabrics that would be unusable, detection of these defects at earlier stages of processing would be of even greater value. Therefore, a procedure was devised for examination of yarns using darkfield illumination. Yarns are more difficult to examine at low magnifications than are fabrics. Even in samples of high white speck content, an individual defect may only be found once in a 36 inch length of yarn. Therefore, yarns must be moved rapidly through the viewing field because a large portion of the yarn has no defect. This was accomplished by locating a spindle containing yarn on the left side of the microscope stage, pulling the yarn across the stage so that it was visible through the binoculars, and rolling the examined yarn onto a dowel attached to the right side of the stage. Turning the dowel pulls yarn from the spindle, across the stage and reroUs it onto the dowel while the yarn is being observed. When a defect is detected, that section of the yarn can be clipped and prepared for examination by SEM. As with defects found in fabrics, those cut from yarns and examined by SEM were also identical to white speck defects on dyed fabrics. Similar examinations were made on cotton in the sliver form. Sliver was flattened and thinned so that light could pass through. A cast aluminum plate with a 2 in2

White Specks

193

opening was placed over the sample to maintain the proper location of the examined area. Dark defects detected by darkfield microscopy were removed and examined by SEM and were shown to be the expected bundles of undeveloped fibers. Of the four specifically grown cotton varieties, the EA-C 32 sample was found to have the highest number of white defects as shown by image analysis, and EA-C 30 the lowest . In developing the darkfield procedure for dye defect surveys, these two samples were compared. Results indicated significantly more defects in the EA-C 32 sample than in the EA-C 30, which is consistent with data from Image analysis on dyed fabrics. This method provides a means of determining presence of dye defects in undyed cotton. Fabrics with high and low defect counts can be distinguished. However, standardization of the method would require a sufficient number of samples from different sources to be examined to determine a thresh hold level of defects that would make fiber lots unusable for dark shade dyeing.

10 Fibre Dynamics Over the last 30 years numerous developments have taken place with the cotton card. The production rate has risen by a factor of 5 with the main rotating components running at significantly higher speeds. Triple taker-in rollers and modified feed systems are in use, additional carding segments are fitted for more effective fibre opening, and improved wire clothing profiles have been developed for a better carding action. Advances in electronics have provided much improved monitoring and process control. Most of these developments have resulted in enhanced cleaning of cotton fibres, reduced neppiness of the card web and better sliver uniformity. Despite the various improvements made to the card a commonly held view is that more is known about the cleaning processes on the card than about the carding process itself. For instance, modern cards can achieve an overall cleaning efficiency of 95%. It is well established that the cleaning efficiency of modern taker-

Fibre Dynamics

195

in systems is a round 30%, that the cylinder/flats action with the latest wire clothing profiles gives 90% cleaning efficiency and that effective cleaning is associated with lower neps in the card web . However, even though the nep content and the sliver Uster CV% are used as quality measures of carding performance they are not satisfactory indicators for anticipating yarn quality. This is because some fibre arrangements in the sliver may lead to nep formation and imperfections during up-stream drafting processes . In addition to the removal of trash and neps, important aspects of the carding process in relation to yarn quality and spinning performance are the degree of fibre individualisation, the fibre extent and the fibre hook configurations in the sliver. With regard to these factors, increased production rate can reduce cardulg quality. It is therefore of importance that a better understanding is established of the effect that carding actions have on such quality parameters, particularly at high production rates. The most widely accepted view of how fibres are distributed within the card under steady-state conditions. Reported studies into the fundamentals of the carding process have largely been concerned with how the principal working components of the card affect this distribution of fibre mass and interact with the mass to achieve:trash and nep removal from cottons; the disentangling of the fibre mass into indi';idual fibres, with minimal fibre breakage; and the alignment of the fibres to give a sliver suitable for drafting in down stream processes. These actions occur at the interface of the card components within the three zones indicated. This paper

196

Textile Technology

therefore gives a critical review of published research on the: mechanisms by which the fibre mass is broken down into individual fibres, mechanisms of fibre transfer between the component parts of the card effect of the saw-tooth wire geometry on these actions

Separation and CleaniIlS' .of the Input Fibre Mass The taker-in has effectively a combing action , which results in the breakdown of the tufts, consituting the fed fibre mass, into single fibres and smaller size tufts (tuflets), and in the liberation of trash particles ejected from the mass flow by the mote knives positioned below the taker-in. To effectively breakdown the fibre mass feed into tuftlets with minimal fibre breakage, the taker-in wire has to be coarse, with a low number of points per unit area (4.2 to 6.2 pcm-2) and not too acute an angle of rake. The objective is to obtain gentle opening of the fibre mass feed and easy transfer of the tuftlets to the cylinder. Angles of 800 - 850 are used for short and medium length cottons to give effective opening and cleaning. For longer cottons and synthetics, a 900 or negative rake may be needed to facilitate g-~ntler opening and satisfactory fibre transfer to prevent lapping of the taker-in . Fibres, usually very short fibres, which are not adequately held by the teeth or present in the interspaces of the clothing are ejected causing fibre loss. However, it is the mote knives that govern the amount of fibre to trash (Le. lint) in the extracted waste. Experimenting with

Fibre Dynamics

197

the settings of two mote knives below the taker-in, Hodgson found that the absence of the knives greatly increased the lint content with little increase in trash. With the knives present, the best setting was that which gave the least waste since increasing the amount of waste did not improve cleaning. Artzt found that irrespective of teeth density and tooth angle the waste increased with taker-in speed but the increase was attributed to higher lint content. It is reasona1Jle to assume that the smaller the tuftlet

size and the greater the mass ratio of individual fibres to tuftlets the better the cleaning effect of the taker-in. Supanekar and Nerurkar suggest that the takerin breaks down the fibre feed into tuftlets of various sizes and mass, conforming to a normal frequency distribution. In the case of cotton, some tuftlets may consist of only fibres whilst others will contain seed or trash particles embedded among the fibres, these tuftlets constituting the heavier end of the distribution curve. Thus, the mean of the distribution would depend on the trash content of the material, as well as on the production rate, the takerin speed and the wire clothing specification. However, the authors did not report any data to support their ideas. Little detailed information has yet been published on the mass variation of tuftlets or on the relative proportion of discrete fibres to tuftlets resulting from the combing action of the taker-in. Nittsu using photographic techniques studied the effect of process variables on tuftlet size. It was found that the total number of tuftlets decreases the closer the feed plate setting, the lower the feed rate, the smaller the steeper rake of the saw-tooth clothing and the higher the lickerin speed. Since th licker-in opens the batt into both

198

Textile Technology

tuftlets and individual fibres, a decrease in the total number of tuftlets suggests an increase in the mass of individual fibres. Liefeld calculated estimates of the opened fibre mass at various stages through the blowroom and gives a value of 50mg for tuftlets on the taker-in. Mills claims that the calculated optimum number of fibre per tooth is one, and that this should be maintained at increased production rates by increasing the taker-in speed. There is, however, the question of fibre damage at high taker-in speeds. Honold and Brown found no fibre damage occurred at speeds of up to 600 r/min. Krylov reports the absence of fibre breakage at speeds up to 1,380 r/min, and Artzt's work shows taker-in speeds to have a negligible effect on fibre shortening and subsequently on yarn strength. In all cases cotton fibres of 26.5- 30.2 mm (2.5% span length) and 3.8 - 4.9 micronaire were processed. The level of fibre breakage, however, would seem to depend on production rate and the batt fringe setting to the licker-in. High production rates achieved by increased sliver counts and a close setting of the batt fringe result in significant fibre breakage.No fundamental studies have been reported on the forces involved in the fibre-wire interaction of revolvingflat card components. However, Li and etal report a simulated study of fibre-withdrawal forces for wool in high-speed roller- clearer cards. Although impact forces could cause damage , it was found that card component speeds had no significant effect on the withdrawal-force, and that fibre configuration and entanglement were the important factors. The importance of producing small size tuftlets is evident form the various components fitted in the

Fibre Dynamics

199

fibreopening zone on modern short-staple cards. Sawtooth wire covered plates, termed combing segments, fitted below the taker-in or built into the taker-in screen are claimed to give improved trash removal. Reportedly , the stationary flats fitted between the taker-in and the revolving flats provide extra opening of the tuftlets transferred to the cylinder from the taker-in. They also act as a barrier to large, hard, trash particles such as seed coats, protecting the wire of the revolving flats from damage, particularly at high cylinder speeds. This enables finer wire to be used for the revolving flats and thereby improves the cleaning effect of the interaction between cylinder and revolving flats. The chances are also reduced of longer length fibres becoming deeply embedded in the revolving flats to become part of the flat strips. These attachments are widely accepted by the industry as beneficial, particularly at high production speed. However, there is no published systematic study of their effectiveness in reducing tuft size, and the effect of stationary flats on the recycling layer, .Q2, is unknown. The little information that is available attempts to illustrate the effectiveness of these components on yarn quality, but there is no evidence of analytical rigour in the way the data were obtained. Fig 3. shows the effect of the combing segment and the stationary flats on dust deposits in rotor spinning and on the imperfections in several types of ring spun yarn. The values for the effect of stationary flats above the doffer, but this will be considered in a later section. It would appear that the added components in the taker-in region might well reduce the dust deposit in the rotor, but the results showing improvements in yarn quality are not convincing, and in all cases the stationary flats above the doffer appear the most effective. Leifeld reports that

.

200

Textile Technology

the cylinder - revolving flats carding action occurs when the fibre mass delivered to the cylinder is in a highly opened state. Tandem cards are said to give a high standard of carding with low nep and trash levels in the card web. This is because a uniform web of almost discrete fibres is fed to the second cylinder of the tandem card and closer revolving flat settings with higher cylinder speeds can be used . Single taker-in systems, even with combing segments and stationary flats, cannot give as high a degree of opening. However, Leifeld reports that a triple taker-in system facilitates high taker-in speeds and, fitted to a single-cylinder card, feeds a uniform web of d;screte fibres to the cylinder, thereby offering a more costeffective process than the tandem card, but no comparative data for the two types of card are given. Although it may be reasoned that a triple taker-in action should improve nep removal, it is of importance to compare the web qualities with regard to dust and trash content, the level and type of fibre hooks, and the degree of fibre parallelism since these greatly influence yarn quality. Contradicting the triple taker-in approach, Mills states that the fibrous material fed to the card should not be broken down into individual fibres by the taker-in system. This is because the fibres would remain largely disoriented with a high proportion of them lying transversely to the direction of mass flow when transferred to the cylinder and subsequently to the revolving flats. This can result in fibre loss during transfer to the cylinder and an unevenness of the fibre mass across the cylinder width, causing neps to be formed and degrading the carding action between the cylinder and the revolving flats.

Fibre Dynamics

201

It is claimed that good carding requires a thin,

uniformly distributed sheet of well-opened tuftlets fed to the cylinder from the taker-in. Fujino reports results that would appear to confirm the view that as the level of opening increases through faster taker-in speed, the degree of fibre parallelism on transfer to the cylinder decreases. The nep level in the card web was, however, observed to decrease noticeably with increased taker-in speed. This was attributed to the reduced speed ratio of the cylinder and taker-in. Artzt found that reducing the takerin/ cylinder draft ratio from 2.4 to 1.4 caused yarn imperfections to increase. In contrast to these findings Harrison states that increasing taker-in speed did not affect the nep level in the card web, the exception being for low micronaire cottons. The apparent contradictions in these results suggest that a better understanding of the transfer mechanism may be needed which takes into account fibre properties. Fibre Mass Transfer to Cylinder Two contrasting views have been reported on the mechanism of fibre transfer. Oxley suggests that the fibre mass on the taker-in is ejected between the cylinder wire and the back plate. Whereas Varga believes that the fibre mass is stripped from the taker-in in the following way. In the feed to the card, tufts and fibres lie randomly and by the action of the taker-in are brought into length-wise orientation in the direction of the roller rotation. The trailing ends of newly formed tuftlets protrude above the taker-in wire and are easily stripped by the cylinder wire clothing. This implies that the transfer involves a reversal of the leading and trailing ends of the fibres. Further orientation and parallelism of the fibre mass is thought to occur during the transfer onto the cylinder. No experimental work has been published which specifically involves a study of the transfer of fibres from the taker-in

202

Textile Technology

to the cylinder. Therefore it has yet to be established whether at the interface, the cylinder, which has the faster surface speed, strips the fibre mass with its clothing or the taker-in, through the action of centrifugal forces, ejects the tuftlets and single fibres onto the cylinder, or a combination of both occurs. It is also of interest to determine if the airflow in the region assists the fibre mass transfer. Whatever the case, the fibre mass is likely to be subjected to an uncontrolled drafting effect, which could introduce irregularities in the mass flow. In the carding zone, it is the interaction of the fibre mass and the wire-teeth clothing of cylinder and flats that fully individualises the fibres and gives parallelism to the fibre mass flow. In considering how fibres enter and are individualised in the carding zone, Oxley suggests that tuftlets are not strongly held on the cylinder clothing because the tooth angle faces the direction of cylinder rotation. They are, thus, easily removed and more firmly held by the opposing teeth of the ilats. It is therefore assumed that as a flat enters the carding zone it becomes almost fully loaded with fibres, the airflow within the region assisting the fibre mass transfer. Having been stripped of fibre mass, subsequent following areas of the cylinder wire clothing move past the fully loaded flat and proceed to comb fibres from the flat, carrying them towards the doffer. The action of combing causes the fibres to be hooked around the cylinder wire points and prevents them from being easily removed by other flats. Debar and Watson's experiments of the movement of radioactive tracer fibres through a miniature card showed that some fibres caught by the flats were often only removed by the cylinder-wire clothing after many revolutions of the cylinder. Varga reports an alternative

Fibre Dynamics

203

view to Oxley's, stating that two types of action occur at the cylinder-flats interface. First, a carding action where the upper layer of a tuftlet or a loosely opened fibre group is caught and held by the flats whilst simultaneously the bottom layer is sheared away by the fast moving cylinder surface. This action causes the top to hang from the flats and to contact subsequent parts of the cylinder wire surface resulting in the 'second action which is combing, where the wire clothing of the cylinder hooks single or a small group of fibres and combs them from the top layer. A second flat catches the bottom layer on the cylinder and the actions are repeated. In this way tuftlets or groups of fibres are separated into individual fibres. By making abrupt changes in the colour of the fibre mass fed to the card, Oxley demonstrated that tuftlets from the load on a given flat are carried forward by the cylinder clothing and separated into individual fibres over a small number of preceding flats, typically 4. It was concluded that the interchange of fibres between cylinder and flats does not occur over the full carding zone. Sengupta ] made measurements of the carding/combing forces and showed that essentially these actions were on average confined to the first ten working flats. A study by Hodgson showed that moving in the direction of the cylinder rotation, a given flat acquires two-thirds of its final load directly it comes into position over the cylinder. The load then increases exponentially with time, reaching nine-tenths of the final value within 6-8 minutes. Completion of the load takes place slowly during the remainder of the working time.With flats moving in the reverse direction the load first increases rapidly with time and then slows until the flat is about to leave the working area. Here it encounters the fibre layer

204

Textile Technology

being transported on the cylinder surface from the takerin. The flat receives a sudden addition of fibre mass to become fully loaded, and, in agreement with other results , the load weighs more than for the forward direction of motion. Contrary to Oxley's conclusions, it was found that 30% of the final load on a given flat resulted from fibre interchange between flats and cylinder over the full carding zone. It may be reasoned that the number of flats involved

in separating a tuftlet depends on the tuftlet size, the mass flow rate and the flat setting. Large tuftlets will be pressed into the cylinder wire during the carding action, whereas small tuftlets will be more easily carded and will remain at the top of the cylinder wire teeth. The larger the tuftet, the higher the production rate and the closer the flat settings, the greater the number flats involved in the separation of a given tuftlet. Bogdan reports that flats tend to load quickly at the beginning of their cycle of contact with the cylinder. This, however, is only a partial loading, since the fibre mass tends to resist more fibres entering the space but, in the case of cotton, not the leaf and trash particles present. Analysis of the trash in cotton flat strips showed that initially the percentage of trash in a given flat strip is low and increases slowly during the first 10 minutes of carding, then remains at almost a constant value. The final percentage depends on the trash content of the cotton. For a fixed production rate, the amount of flat strips was found to be directly proportional to the flat speed, but provided the speed was such that the working time was not less than 10 minutes, both the weight and composition of the flat strips remained approximately constant. Feil claims that a high degree of air turbulence exists in the flat/cylinder zone. A combination of

Fibre Dynamics

205

centrifugal forces, mechanical contact with the flat wire and air turbulence causes the trailing ends of fibres attached individually to the cylinder clothing to vibrate and shake loose trash and dust particles. Short fibres which cannot adequately cling to the cylinder clothing will also be shaken free, and along with impurities become part of the flat strips. Fibres that are deeply embedded in the flats, and cannot be reached by the cylinder wires become flat strips. For this reason the closeness of the flats setting to the cylinder is important. It may be assumed that closer flats/ cylinder setting and faster cylinder speeds will give more effective carding and combing actions as described by Varga and thereby improve web quality through reduced neps and trash. Cylinder diameters vary and Karasev showed mathematically that for a given cylinder rotational speed the carding power will be greater for a larger cylinder diameter with a higher number of working flats. However, because of lower mechanical stresses, smaller cylinders can be rotated at higher speeds than larger cylinders. The above advantage is therefore reduced the higher the speed of the smaller cylinder. Artzt studying the influence of card clothing parameters and cylinder speeds on yarn imperfections, report that the teeth density of the flats and cylinder, and the speed of the cylinder must prevent tuftlets lying within the spiral pitch of the cylind.er clothing. If this occurs the tuftlets generally become the thick pJaces in the yarn. It was found that high teeth densities and low cylinder speeds were as effective as lower teeth densities and high cylinder speed. High teeth densities with high cylinder speeds did not give effective carding, but no reason was reported for this.

",

206

~"

Textile Technology

Since the action of the cylinder in this region is to individualise fibres, the wire clothing on the cylinder has a steeper rake and a higher point density than the wire clothing of the flats. Thus, with closer settings and higher cylinder speeds greater forces may be involved and may result in fibre breakage. However, the work of Li indicates that the withdrawal forces needed to separate an entangled fibre mass was largely dependent on the density of the fibre mass and the contact angle fibres made with the wire clothing, than on the machine speeds. Van Alphen reports that increasing cylinder speed causes more fibre breakage than increasing taker-in speed and that this is reflected in the yarn properties. Rotor yarn tenacity was reduced by up to 5% with increasing cylinder speeds between 480 -600 r / min. Whereas ring yarns showed a 5% reduction for speeds between 260 380 r/min and 10% at 600 r/min. The higher sensitivity of ring yarns to fibre breakage was attributed to the negative effect of short fibres during roller drafting. Krylov reports that no fibre shortening was observed for cylinder speeds up to 380 r/min. It may be reasoned that the smaller the tuftlets and

the more parallel fibres in tuftlets are to the direction of mass flow the lower the probability of fibre breakage. Honold attributes fibre damage to the cylinder/flat interaction and suggested that the degree of damage depends on the size of the tuftlets entering the working area; the smaller the tuftlets, the closer the setting that can be used and the lower the fibre breakage . Hodgson's work showed fibre length is also an important factor. For cottons, fibre breakage was only found to have occurred when the staple length was greater than 25mm. Increasing the flat speed appears to

Fibre Dynamics

207

have no effect on fibre breakage. However, the amount of flat strips increased proportionally with the flat speed and the mean fibre length of the strips increased significantly. This means that faster flat speeds result in larger amounts of useable fibre in the waste. Interestingly, when carding cottons, immature fibres were not readily found in the flat strips. The coarser rigid fibres seem more easily retained by the flats. The effectiveness of the carding and combing actions within the cylinder/flats area is, inter alia, dependent on the quantity of fibre mass on the cylinder, and this includes the recycling layer, Q2. It is of interest therefore to consider how the Q2 is formed during fibre transfer from cylinder to doffer, and its importance to the card web quality. Varga reports that the action of fibre mass transfer to the doffer is similar to the transfer at the input to the cylinder-flats zone. The regions above and below the line of closest approach of the cylinder to the doffer (i.e. the setting line) are important to the mechani.sm of fibre mass transfer and the transfer coefficient, K. The two regions may be termed the top and bottom co-operation arcs or top and bottom zones. Simpson claims that transfer can occur in both zones and that the particular region in which transfer actually occurs influences the fibre configuration and the nep level of the card web, although cylinder-flats action is more important in reducing neps. Which zone transfer occurs in is dependent on the cylinder-doffer surface speed ratio, C/ D. For high C/Ds, transfer occurs in the top zone and results in a larger number of trailing than leading hook fibres and a low nep level. The reverse occurs when transfer takes place in the bottom zone owing to lower C/Ds. Simpson does not however say at what C/D value

208

Textile Technology

transfer changes from one zone to the other. Although reference is made to other authors who have proposed a mechanism for fibre transfer in the top zone, no mechanism or experimental evidence is given to support the idea of fibre transfer in the bottom zone. Lauber and Wulfhorst used laser-doppler anemometry and highspeed cine photography to study fibre behaviour in the bottom zone, i.e. up tollO mm below the setting line. Their findings showed no evidence of fibre transfer within the bottom zone. Since Morton and Summers' work in 1949 other researchers have confirmed that the values given in the five classes of fibre configuration observed in slivers. It is of interest to note that the hooked lengths are greater for leading than trailing hooks. Although, the calendar draft can be used to change the relative proportions, Gosh and Bhaduri showed that the method of removing the web from the doffer does not influence the propensity of any class of configuration. It is the mechanism of transfer that is seen as principally responsible for the shape fibres have in the sliver. Several studies have been reported on the fibremass-transfer mechanism. A number used tracer fibres with one end of a fibre dyed a different colour from the other. The reported findings suggest that fibre mass transfer occurs by fibres acting independently and not as a web of fibres. Observations showed that prior to transfer, neariy 70% of fibres on the cylinder had leading hooks, only 9% had trailing hooks. On transfer the relative proportions changed. Half the number observed underwent reversals, with greater than 70% changing their configurations [e.g. leading hooks becoming trailing hooks]. Of those that transferred without reversals ca 90% did so with a change of configuration.

Fibre Dynamics

209

Ghosh and Bhaduri report that tracer fibres were noted generally to go around with the cylinder for several revolutions before being transferred by the doffer. On occasions transfer only happened when the cylinder speed was increased. Debar and Watson's work with radioactive viscose tracer fibres showed that a fibre on the cylinder wire passes the doffer up to a maximum 20 times before being removed by the doffer, sometimes interchanging several times between the cylinder and flats, during the 20 revolutions on the cylinder. Hodgson found that cotton fibres make between 10 and 25 cylinder revolutions before being removed by the doffer. With the continuity of fibre mass flow through the card, this means that the doffer web is built up over many cylinder revolutions and that the recycling layer, Q2, is comprised of multiple fractional layers of the fibre mass transferred from taker-in to cylinder during these cylinder revolutions . A proposed hypothesis for the mechanism of fibre transfer. Here the trailing ends of fibres are lifted from the cylinder surface by centrifugal forces and become hooked around the teeth of the doffer clothing. The frictional drag of the doffer clothing eventually removes these fibre from the cylinder clothing. This mechanism only explains the formation, without reversal, of trailing hooks in the doffer web. However, the importance to fibre transfer of the relative angles and tooth lengths of the cylinder and doffer is self evident. Baturin developed equations that showed the importance of tooth angle and teeth density of the cylinder and doffer wires to the value of K and thereby Q2. Other investigators have reported experimental data that verify Baturin's equations. It was found that the more acute the working angle of the doffer wire compared to the cylinder wire, the higher the

210

Textile Technology

value of K, and the lower Q2, and that higher teeth densities on the doffer increased K. These findings would tend to suggest that the proposed mechanism is a principal action by which fibres are removed from the cylinder. However, this mechanism of fibre transfer does not explain the change of fibre configuration with reversals and the formation of leading hooks in the doffer web. It also does not explain how fibres forming the recycling layer, Q2, are subsequently removed, even though an input layer of fibre mass is added to Q2 each time it passes the taker-in. The above studies did not take account of the degree of fibre parallelism on the cylinder prior to transfer, nor the number of fibres per tooth on the cylinder and consequently the likelihood of fibre interaction during transfer. Fujino and Itani used a microscopic technique to observe the orientation of fibres on the cylinder surface above the taker-in and just before the doffer, and in the doffer web. They found that fibres showed the highest degree of parallelism when on the cylinder surface just above the doffer. The degree of parallelism decreases on transfer to the doHer, and further· deteriorates when the web is removed from the doffer to form the sliver, even though the calendar draft helps to maintain some degree of parallelism. Grimshaw and others report the use of fixed flats just before the cylinder / doffer top transfer zone, to improve fibre parallelism in the card web.; up to 20% reduction in fibre hooks and 25% improvement in fibre parallelism were obtained in the card web, resulting in improved yarn properties. The fixed flats in this region are more effective in improving yarn properties compared with the fixed flats above the taker-in. The action of the flats fitted above the doffer is not fully understood. It is assumed

Fibre Dynamics

211

that they tend to lift the fibres to the tip of the cylinder wire for more effective transfer to the doffer, particularly at high cylinder speed. Lauber and Wolfhorst , Kamogawa, report that in this region aerodynamic forces affect the parallelism of the fibres and the way they are transferred to the doffer. However, no details are given. Owing to the higher speed and larger diameter of the cylinder, it is assumed that during transfer in the top zone the fibres are more substantially affected by the flow of air transported with the cylinder's than by the doffer's wire clothing. High-speed photographs showed that in the bottom zone the main flow of fibre mass was with the doffer at close to the doffer speed, even when the fibres were just below the cylinderdoffer setting line. However, some fibres were seen to be free of both the doffer and cylinder and tended to move with the air currents and eventually with the motion of the cylinder surface. From the above discussion, it can be seen that work is still needed to establish a more detailed understanding of fibre mass transfer between the cylinder and doffer. The results of such work may also help in better explaining how fibres remain on the cylinder to form the recycling layer Q2. Varga suggests that with fibre transfer in the top zone, the thicker layer of web on the doffer surface protrudes above the doffer wire and into the gap setting between doffer and cylinder. The faster moving cylinder wire clothing combs through the doffer web and thereby pulls fibres back onto the cylinder surface. De Swann showed that fibres can be readily transferred f!."Om the doffer to the cylinder as well as from cylinder to doffer. In Hodgson's study, changing cylinder/doffer setting affected the neppiness of the web but did not affect K,

212

Textile Technology

which seems to contradict Varga's view. Baturin and Simpson however showed that K will increase if the region of interaction between the cylinder and doffer is reduced by decreasing the doffer or the cylinder diameter and this tend to supports Varga's suggestion for a combing and robbing action of the cylinder. It is reasonable to assume that the combing action could lead to fibres in Class II and IV, but there is still no verified explanation of how fibres in Classes I, III, and V are formed, with and without reversals. Much of the research on the cylinder / doffer interaction concerns the effect of machine variables on the size of Q2 (or the operational layer, Qo), on the web quality and changes to the relative proportions of the classified configurations, and on ultimately the yarn quality. Sing and Swani developed a Markovian model for the carding process in order to determine the probabilities of fibre transfer between cylinder and flats and cylinder and doffer, taking into account the recycling of fibres. It was shown that the times spent by a fibre on the cylinder, Tr, and in the flats/ cylinder region, Td, are given by:

= 1 / K and Td = Tr . Pf K = Ql / Qo and Pf = Qf /

Tr Where

Qo

Reported values for K would seem to vary between 0.2% to 20% , depending on doffer and cylinder speeds, on the relative profiles of the saw-tooth wire clothing, and on the sliver count. Simpson suggests that fibre properties are also of importance, in that there is a tendency for low micronaire cottons to give higher cylinder loading and for fibres with low shear friction and good compression recovery to result in higher K values. No physical

213

Fibre Dynamics

explanation is given for these findings and no other studies are reported on the effect of fibre properties. Further work is therefore needed in this area. A popular view is that a low fibre mass entering the cylinder / flats interface, i.e. a low fibre load on the cylinder, results in better quality carding. This would seem to imply that the higher the value of K the better the carding since less fibre mass is recycling to be added to the mass transferred from the taker-in. However, there are several ways of increasing K and not all of them result in improved carding quality. Baturin reports an alternative approach to the above in which the following expression was derived for the number of cycles, Np, under steady state conditions that fibres on the cylinder clothing make pass the flats before being removed by the doffer: Np

=1

+ Vc/KVd

Where K is the transfer coefficient Vc and Vd are cylinder and doffer surface speeds (m/min). Since this gives the number of times the recycling fibre mass is subjected to the carding action, it may be a better indication than Pf of the importance of Q2. From the expression, Np decreases when K increases by increasing doffer speed. Web quality decreases when Np decreases with doffer speed, even though the cylinder load decreases and a high number of cylinder teeth per fibre is obtained. The last two parameters are usually taken as indicative of good carding. The effect of increased doffer speed and sliver count on web quality and there is a consistent trend which suggests that increasing the

214

Textile Technology

production rate by increasing the sliver count, instead of doffer speed, gives better web quality. With regard to sliver irregularity, several investigators report theoretical and experimental studies showing that increasing the recycling layer, Q2, reduces the short-term irregularity. Karasev attempted to show experimentally the importance of Q2 by removing it during carding using a suction extractor. It was found that without Q2 a large proportion of tr.e fibre mass transferred from the takerin became embedded into the empty teeth of the cylinder clothing. Only the larger tuftlets and groups of individual fibres would then be subjected to the carding and combing actions. Hence,. there is a greater chance of small groups of entangled fibres being removed by the doffer. Q2 therefore acts as a support to new layers of fibre mass being transferred form the taker-in, keeping the new fibre mass at the tips of the cylinder wire teeth and thereby promoting the interaction of tuftlets with the flats and cylinder clothing. This idea, however, does not facilitate an explanation of the mechanism by which fibres leave the recycling layer to form part of the doffer web, Ql . Gupta suggest that the rotating cylinder could be considered as a large centrifuge that would cause fibres, impurities and seed fragments to migrate to the cylinder periphery and thereby make contact with the flats clothing and, presumably, the doffer teeth. However, no experimental verification of this hypothesis is reported. Many of the authors have reported the effect of machine variables on fibre configurations within the card sliver and several have related yarn properties to the observed configurations. Generally it was found that for a fixed sliver count increasing the carding rate by increasing the doffer

Fibre Dynamics

215

speed, increased the number of minority hooks and reduced the number of majority hooks, irrespective of cylinder speed. However, for a given doffer speed, increased cylinder speed gave the reverse trend for minority hooks, but no clear trend for majority hooks. Baturin and Brown showed that increased cylinder speed decreases cylinder load owing to the effect of centrifugal forces and Simpson showed that increased cylinder speed also increased minority hooks and decreased majority hooks. Bhaudri reports that when the fibres are forced nearer the surface of the cylinder teeth, either by increasing the fibre load or increasing the centrifugal force on the cylinder, the proportion of minority hooks increases. Simpson found that there was a direct relation between yarn imperfections and increased occurrence of minority hooks and that spinning end breakage rates and yarn imperfection increased with increased card production speed owing to minority hooks. Gosh and Simpson found that heavier slivers had fewer minority hooks. However, the increased draft needed to process the heavier slivers into yarn led to increased yarn imperfections. Conclusions 1.

The taker-in action separates the fed fibre mass into tuftlets and individual fibres. Although it is reported that the taker-in action gives a normal mass distribution of tuftlet sizes, this is speculation. Little research has been reported on the effect of taker-in parameters, fibre properties and the blowroom process on tuftlet size distribution and on the relative proportions of tuftlets to individual fibres.

216

Textile Technology

2.

The perceived benefits of combing segments built into the taker-in under-screen and of stationary flats fitted before and after the revolving flats are well known, but only limited experimental findings have been reported to support the use of these attachments. There are conflicting views qn the benefits of triple taker-in systems, concerning whether the fibre opening by such systems would give a high misalignment of fibres to the direction of mass flow during transfer to the cylinder and degrade the subsequent carding action. A better understanding is therefore required of the fibre mass transfer from taker-in to cylinder, since the surface speed ratio of these components is seen as a key factor in the proper functioning of high production cards.

3.

The cylinder-flats and cylinder-doffer interactions have been well researched. Published findings show that each flat acquires two-thirds of its load at the beginning of its cycle of contact with the cylinder, and that separation of a given tuftlet occurs over a few flats. With regard to clothing parameters and cylinder speed, high teeth densities and lower cylinder speeds gave similar results to the converse arrangement. However, a high teeth density and cylinder speed did not give effective carding. Results showed that high cylinder speeds caused more fibre breakage than hi~h taker-in speed.

4.

A high cylinder to doffer speed reduces cylinder load, gives a higher K value and a better web quality. Increasing doffer speed was also found to increase K, but the web quality deteriorated. The reported mechanism of fibre transfer from cylinder to doffer

Fibre Dynamics

217

does not adequately explain the effect of the cylinder- doffer speed ratio, or the various reported changes in fibre configuration during transfer. Further work is therefore still needed in this area.

11 Roving Frame and Draw Frame Roving machine is complicated, liable to faults, causes defects, adds to production costs and delivers a product that is sensitive in both winding and unwinding. This machine is forced to use by the spinner for the following two reasons. 1.

Sliver is thick, untwisted strand that tends to be hairy and to create fly. The draft needed toconvert this is around 300 to 500. Drafting arrangements of ringframes are not capable of processing this strand in a single drafting operation to create a yarn that meets all the normal demands on such yarns.

2.

Drawframe cans represent the worst conceivable mode of transport and presentation of feed material tothe ring spinning frame.

TASKS OF ROVING FRAME: 1.

Attenuation- drafting the sliver into roving

Roving Frame and Draw Frame

2.

twisting the drafted strand

3.

winding the twisted roving on a bobbin

219

Fibre to fibre cohesion is less· for combed slivers. Rollers in the creel can easily create false drafts. Care must be taken to ensure that the slivers are passed to the drafting arrangement without disturbance. Therefore, a perfect drive to the creel rollers is very important. The drafting arrangement drafts the material with a draft between 5 and lS.The delivered strand is too thin to hold itself together at the exit of the front bottom roller. Bobbin and flyer are driven separately, so that winding of the twisted strand is carried out by running the bobbin at a higher peripheral speed than the flyer. The bobbin rail is moving up and down continuously, so that the coils must be wound closely and parallel to one another to ensure that as much as material is wound on the bobbin. Since the diameter of the packages increases with each layer, the length of the roving per coil also will increase. Therefore the speed of movement of bobbin rail must be reduced by a small amount after each completed layer Length delivered by the front roller is always constant. Owing to the increase in the diameter of the package for every up and down movement, the peripheral speed of package should keep on changing to maintain the same difference in peripheral speeds between pakcage and flyer. I

There are two types of drafting systems.

220

Textile Technology

1.

3/3 drafting system

2.

4/4 drafting system

In general 3/3 drafting system is used, but for higher draft applications 4/4 drafting system is used. The draft often has limits not only at the upper limit (15 to 20), but also at lower limit. It is around 5 for cotton and 6 for synthetic fibres. If drafts below these lower limits are attempted, then the fibre masses to be moved are too large, the drafting resistance becomes too high and the drafting operation is difficult to control. It is advisable to keep the break draft(predarft) as

low as possible, because lower breakdraft always improves roving evenness. In general two condensers are used in the drafting arrangement. The purpose of this condensers is to bring the fibre strands together. It is difficult to control, Spread fibre masses in the drafting zone and they cause unevenness. In addion, a widely spread strand leaving the drafting arrangement leads to high fly levels and to high hairiness in the roving. The size of condensers should be selected according to the volume of the fibre sliver. Flyer inserts twist. Each flyer rotaion creates one turn in the roving. Twist per unit length of roving depends upon the delivery rate. Turns per metre = (flyer rpm)/(delivery speed (m/ min» Higher levels of roving twist, therefore, always represent production losses in Roving frame and possible

Roving Frame and Draw Frame

221

draft problems in the ring spinning machine. But very low twist levels will cause false drafts and roving breaks in the roving frame. Centrifugal tension is created at the bobbin surface as the layers are being wound and is created by the rotation of the package. Each coil of roving can be considered as a high-speed rotating hool of roving on which centrifugal tension increases with increasing diameter of the package. centrifugal tension in the roving is proportional to the square of the winding surface velocity.In this context, ccentrifugal force acts in such a manner as to lift the top roving strand from the surface of the package so that the radial forces within the strand that hold the fibres together are reduced and the roving can be stressed to the point of rupture. Breaks of this type may occur at the winding-on Point of the presser or in strands that have just been wound on the top surface of the package. This phenomenon is known as "bobbinbursting" This phenomenon will be prominent if the twist per inch is less or the spindle speed is extremely high when the bobbin is big. Apart from inserting twist, the flyer has to lead the very sensitive strand from the flyer top to thepackage without introducing false drafts. Latest flyers have a very smooth guide tube set into one flyer legand the other flyer leg serves to balance the flyer. The strand is completely protected against air flows and the roving is no longer pressed with considerable force against the metal of the leg, as it is inthe previous designs. Frictional resistance is considerably reduced, so that the strand can be pulled throughwith much less force.

222

Textile Technology

False twisters are used on the flyers to add false twist when the roving is being twisted between the front roller and the flyer. Because of this additional twist, the roving is strongly twisted and this reduces thebreakage rate. Spinning triangle is also reduced which will reduce the fibre fly and lap formation on the front bottom roller. Because of the false twister, the roving becomes compact which helps to increase the length wound on the bobbin. This compactness helps to increase the flyer speed also. Roving strength is a major factor in determining winding limitations. It must be high enough for the fibresto hold together in a cohesive strand and low enough for satisfactory drafting at the spinning machine. The factors affecting roving strength are as follows: •

the length, fineness, and parallelisation of fibres

•

the amount of twist and compactness of the roving

•

the uniformity of twist and linear density.

BUILDER MOTION MACHINE This device has to perform the following tasks 1.

to shift the belt according to the bobbin diameter increase

2.

to reverse the bobbin rail direction at top and bottom

3.

to shorten the lift after each layer to form tapered ends

Shifting of the belt is under the control of the ratchet wheel. The ratchet wheel is permitted to rotateby a half

Roving Frame and Draw Frame

223

tooth. The bobbin diameter increases more or less rapidly depending upon roving hank. The beltmust be shifted through corresponding steps. The amount of shifting, which depends upon the thickness of the roving, is modified by replacement of the ratchet wheel or by other gears.If a ratchet wheel with fewer teeth is inserted, then the belt is shifted through larger steps, i.e. it moves more rapidly, and vice versa. To form a package, the layer must be laid next to its neighbours. For that the lay-on point must continually be moved. The shift of the winding point is effected by moving the bobbin rail. This raising and lowering isdone by rails.5ince the package diamter is steadily increasing, the lift speed must be reduced by a small amount after each completed layer. During winding of a package, the ratchet is rotated at every change-over. Reversal of the bobbin layer occurs little earlier for every reversal.This gives a conitnuous reduction in the lift of the rail. Thus bobbins are built with taper.

DRAW FRAME

Tasks of Draw frame 1.

Through doubling the slivers are made even

2.

doubling results in homogenization(blending)

3.

through draft fibres get parallelised

4.

hooks created in the card are straightened

5.

through the suction, intensive dust removal is achieved

224

6.

Textile Technology

autoleveller maintains absolute sliver fineness

Quality of the drawframe sliver determines the yarn quality. Drawing is the final process of quality improvement in the spinning mill .

DRAFTING Drafting is the process of elongating a strand of fibres, with the intention of orienting the fibres in the direction of the strand and reducing its linear density.In a roller drafting system, the strand is passedthrogh a series of sets of rollers, each successive set rotating at a surface velocity greater than that of the previous set. During drafting, the fibres must be moved relative to each other as uniformly as possible by overcoming the the cohesive friction. Uniformity implies in this context that all fibres are controllably rearranged with a shift relative to each other equal to the degree of draft. In drawframe, the rollers are so rotated that their peripheral speed in the throughflow direction increases fromroller pair to roller pair, then the drawing part of the fibres, i.e.the draft, takes place. Draft is defined as the ratio of the delivered length to the feed length or the ratio of the corresponding peripheral speeds. Drawing apart of the fibres is effected by fibres being carried along with the roller surfaces. For this to occur, the fibres must move with the peripheral speed of hte rollers. This transfer of the roller speed tothe fibres represents one of the problems of drafting operation. The transfer can be effected only byfriction, but the fibre strand is fairly thick and only its outer layers have contact with the rollers, and furthermore various, non-

Roving Frame and Draw Frame

225

constant forces act on the fibres. Roller drafting adds irregularities in the strand.Lamb states that,though an irregularity causing mechanism does exist in drafting, drafting also actually reduced the strand irregularities by breaking down the fibre groups. Drafting is accompanied by doubling on the drawframe, this offsets the added irregularity. Variance(sliver out) = Variance(sliver in) + Variance(added by m/c) In Statistics, Variance is the square of standard deviation Two passages of drawing with eight ends creeled each time would produce a single sliver consisting of 64 ribbons of fibre in close contact with each other.In the ultimate product, each ribbon may be only a few fibres thick, and thus the materials of the input slivers are dispersed by the drawing process. The term doubling is also used to describe this aspect of drawing Drafting arrangement is the heart of the drawframe. The drafting arrangement should be 1.

simple

2.

stable design with smooth running of rollers

3.

able to run at higher speeds and produce high quality product

4.

flexible i.e suitable to process different materials , fibre lenths and sliver hanks

5.

able to have good fibre control 6.easy to adjust

Roller drafting causes irregularities in the drafted strand since there is incomplete control of the motionof each individual fibre or fibre group.

Textile Technology

226

The uniformity of the drafted strand is determined by l.

draft ratio

2.

roller settings

3.

material characteristics

4.

pressure exerted by the top roller

5.

hardness of top roller

6.

fluting of the bottom rollers

7.

distribution of draft between the various drafting stages

Drafting is affected by the following rawmaterial factors l.

no of fibres in the cross section

2.

fibre fineness

3.

degree of parellelisation of the fibres

4.

compactness of the fibre strand

5.

fibre cohesion which depends on

l.

surface structure

2.

crimp

3.

lubrication

4.

compression of the strand

5.

fibre length

6.

twist in the fibre

7.

distribution of fibre length

3-over-3 roller drafting arrangements with pressure bar is widely used in the modern drawframesBigger front rollers are stable and operated at lower speeds of

Roving Frame and Draw Frame

227

revolution, this necessitated pressure bararrangement for better control of fibres. Some drawframes are with 4over-3 drafting arrangement, butstrictly speaking it behaves like a 3-over-3 drafting system except for the fact that fourth roller helpsto guide the sliver directly into the delivery trumpet.

Drafting Wave Floating fibres are subject to two sets of forces acting in opposite directions. The more number of fibreswhich are moving slowly because of the contact with the back rollers. restrain the floating fibres from accelerating. The long fibres in contact with the front rollers tend to accelerate the floating fibres to the higher speed. As the floating fibres move away from the back roller, the restraining force by back roller held fibres reduces, and the front roller influence increases. At some balance point, a fibre accelerates suddenly from low to high speed. This balance point is compounded by the laws of friction, static friction being higher than dynamic friction. When onefloating fibre accelerates, the neighbouring shor fibres suddenly feel one more element tending to accelrate them and one fewer trying to restrain them. Thus there may be an avalanche effect which results in drafting wave.

Auto leveller Autoleveller is an additional device which is meant for correcting the linear density variations in the delivered

228

Textile Technology

sliver by changing either the main draft or break draft of the drafting system, according to the feed variation. There are two types of Autolevelling systems. They are •

Open loop system

•

closed loop system

Most of the drawframe autolevellers are open loop auto levellers. In open loop autolevellers , sensing is done at the feeding end and the correction is done by changing either a break draft or main draft of the drafting system. In closed loop system, sensing is at the delivery side and correction is done by changing either a break draft or main draft of the drafting system. Most of the earlier card autolevellers are closed loop autolevellers. But the latest cards have sensing at the feed rollers and as well as at the delivery calender rollers. We can say , both closed loop and open loop systems are being used in such autolevellers. Open loop system is very effective, because the correction length in open loop system is many fold lower than closed loop system. But in case of closed loop system, it is confirmed that the delivered sliver is of required linear density. In case of openloop system, since the delivered material is not checked to know whether the correction has been done or not, Sliver monitor is fixed to confirm that the delivered sliver has the required linear density. Let us discuss about an autoleveller system which is being used in most successful drawframes like RSB-951,

Roving Frame and Draw Frame

229

RSB-D-30 etc. This system is an electronic levelling system. The major componenets in the system are •

Scanning roller

•

signal converter

•

levelling CPU

•

servo drive(servo motor and servo leveller)

•

Differential gear box (Planetary gear box)

Function of the scanning roller is to measure the variation in the feed material. All slivers fed to the machine pass thro a pair of scanning rollers. One of the scanning roller is moveable. These scanning rollers are loaded either by a spring loading system or a pneumatic loading system. Pneumatic loading is alwyas better, because the pressure in kgs will be always same(consistent), irrespective of the sliver feed variation. But in the case of spring loaded, the pressure on scanning rollers may vary depending upon the feed variation. The variations in sliver mass of the incoming slviers displace the scanning roller. The distance moved by the scanning is proportional to the slvier mass fed. This displacement of scanning rollers are transformed into volatage by a signal converter and is fed to an Electronic Levelling processor. With analogue system, electronic levelling processor is a servo amplifier, but in the case of digital system, it a CPU. It is the Electronic Levelling processor which

furnishes the correct target value to the servodrive.(servo motor and servo leveller). Delivery speed of the machine and electric signal values arrived at by the slivers fed are the two important signals for the correction.

230

Textile Technology

Servo drive takes the information and is converted in such a way that servomotor RPM and direction is decided for appropriate correction. Planetary gearing (Differential gearing) with its controlled output speed drives the middle and back roller. i.e. Sliver entry of the drafting system Because the servo motor RPM and direction varies according to the feed variation, and the servo motor and servo leveller generates a control speed of the planetary gearing, the required change in main draft is accomplished, compensating for the weight variation of the sliver fed.

•

If the slivers fed are too heavy, the entry speed is reduced i.e draft increased

•

If the slivers fed are too light, the entry speed is increaed i.e. draft reduced

Delivery speed ( the front roller speed) remains constant and hence the production remains constant. Mechanical draft or nominal draft should be selected properly. Before switching on the auto leveller, gears should be selected such that, the wrapping average (linear density of sliver) should be less than plus or minus 3%.

If the feed variation indicated in the A % display of sliver fed is continuously showing more than -5% or +5%, then the mechanical draft selected is not correct. If the mechanical draft selected is correct, then the indication in A % display of sliver fed should be between -5% red lamp and 0% green lamp or +5% red lamp and 0% green lamp. In other words, green lamp(O% variation

Roving Frame and Draw Frame

231

indication) should be on atleast for 80% of the running time. Atuo leveller is meant for correcting •

continuous long term varition in the fed slvier

•

medium term variation

•

seldom occuring abnormal variations in the sliver fed due to deviations in carding and comber

•

short term variations in the sliver fed

•

variations like comber piecings

Scanning rollers should be selected properly. In some drawframes like DX7-LT OR DXA7-LT, the scanning roller is same for all sliver weights and all types of material. But in case of RSB draw frames, there are different sizes of scanning rollers. It depends on sliver weight fed and the type of material processed. Scanning roller pressure is not a constant. It depends on the material being used. It is selected so that minimum A % is achieved in the sliver. For the same material if the scanning roller pressure is changed, the linear density of the delivered material will also change. Hence enough care should be taken so that whenever the pressure is changed, the wrapping should be checked and adjusted. Following are the two important parameters for Quality Levelling •

Levelling action point ( time of correction)

•

Levelling intensity

Both feed variation sensing and correction are being done when the machine is running (continuous process) at two

232

Textile Technology

different places(i.e sensing is at one place and correction is at an other place). Hence the calculated correction should be done on the corresponding defective material. This is decided by Levelling action Point. The time required for the defective material to reach teh correction point should be known and correction should be done at the right time. Levellling action point depends upon •

break draft

•

main draft roller setting

•

delivery speed

Levelling Intensity is to decide the amount of draft change required to correct feed variation. The correlation between mass and volume for different fibres is not same. Therefore the levelling intensity .may be different for different fibres. Levelling intensity is selected based on the following trial. Wrapping of the delivered sliver should be checked with "n", "n+I", "n-I" sliver at the feeding side. The sliver weight of the delivered sliver should be same for all the three combinations or should be the minimum. This can be cheked if the sliver is checked at UT 3(uster)or premier tester 7000 for mass varia tons ( U%). If Levelling correction point and levelling intensity is selected properly, then the cuI length C.V% of I meter will be less than 0.5, if the sliver is tested in UT-3 instrument.

Advantages of Autoleveller •

All variations are corrected

Roving Frame and Draw Frame

233

•

Count C.V.% will be consistent and good, hence the yarn will be suitable for knitting

•

Off counts will be very very less in the yarn, hence off count cuts will come down drastically in autoconers

•

Thin places in the sliver, hence in the yarn will be low. Ring frame breaks will come down, hence

•

pneumfil waste will be less

•

fluff in the department will be less, therefore uster cuts will be less

•

fabric quality will be good because of lower number of fluff in the yarn

•

labour productivity will be more

•

machine productivity will be more

•

idle spindles will be less

•

RKM C.V.% will be low, because of low number of thin places.

Workability in warping and weaving will be good, because of less number of thin places and lower end breaks in spinning and winding. Production calculated will be more accurate in autoleveller drawframe compared to non autolevller drawframe. Variation in Blend percentage will be very less, if both the components are autolevelled before blending, hence fabric appearance after dyeing will be excellent. As long as the autolevelling system is set properly and all the components are working properly, the above said benefits can be achieved. Otherwise, the negative impact will be very big compared to working without autoleveller.

12 Metallic Card Clothing As Carding machine design improved in 1950's and 60's, it became apparent that card clothing was a limiting factor.Much time and effort was spent in the development of metallic card clothing. -

There are two rules of carding 1. The fibre must enter the carding machine, be efficiently carded and taken from it in as little time as possible

2. The fibre must be under control from entry to exit Control of fibres in a carding machine is the responsibilitgy of the card clothing Following are the five types of clothings used in a Carding machine 1. Cylinder wire

2. Doffer wire 3. Flat tops 4. Licker-in wire

Metallic Card Clothing

235

5. Stationary flats

Cylinder Wire The main parameters of CYLINDER Card clothing 1. Tooth depth

2. Carding angle 3. Rib width 4. Wire height 5. Tooth pitch 6. Tooth point dimensions -

Tooth depth 1. Shallowness of tooth depth reduces fibre loading and holds the fibre on the cylinder in the ideal position under the carding action of the tops. The space a fibre needs within the cylinder wire depends upon its Micronairel denier value and staple length. ould have to be reduced.

2. The recent cylinder wires have a profile called "NO SPACE FOR LOADING PROFILE"(NSL). With this new profile, the tooth depth is shallower than the standard one and the overall wire height is reudced to 2mm , which eliminates the free blade in the wire. This free blade is responsible for fibre loading. Once the fibre lodges betweent the free blade of two adjacent teeth it is difficult to remove it.Inorder to eliminate the free blade, the wire is made with a larger rib Width

236

-

Textile Technology

Front angle 1. Front angle not only affects the carding action but controls the lift of the fibre under the action of centrifugal force. The higher the cylinder speed, the lower the angle for a given fibre. Different fibres have different co-efficients of friction values which also determine the front angle of the wire.

2. If the front angle is more, then it is insufficient to overcome the centrifugal lift of the fibre created by cylinder speed. Therefore the fibre control is lost, this will result in increasing flat waste and more neps in the sliver. 3. If the front angle is less, then it will hold the fibres and create excessive recyling within the carding machine with resulting overcarding and therefore increased fibre damage and nep generation. 4. Lack of parallelisation, fibre damage, nep generation, more flat waste etc. etc., are all consequences of the wrong choice of front angle. -

Tooth pitch 1. Each fibre has a linear density determined by its diameter to length ratio. Fine fibres and long fibres necessitates more control during the carding process. This control is obtained by selecting the tooth pitch which gives the correct contact ratio of the number of teeth to fibre length.

2. Exceptionally short fibres too require more control, in this case , it is not because of the stiffness but because it is more difficult to parallelise the fibres with an open tooth pitch giving a low contact ratio.

Metallic Card Clothing

-

237

Rib thickness 1. The rib thickness of the cylinder wire controls the carding "front" and thus the carding power. Generally the finer the fibre, the finer the rib width. The number of points across the carding machine is determined by the carding machine's design, production rate and the fibre dimensions. General trend is towards finer rib thicknesses, especially for high and very low production machines.

2. Rib thickness should be selected properly, if there are too many wire points across the machine for a given cylinder speed, production rate and fibre fineness, "BLOCKAGE" takes place with disastrous results from the point of view of carding quality. In such cases, either the cylinder speed has to be increased or most likely the production rate has to be reduced to improve the sliver quality -

Point population

The population of a wire is the product of the rib thickness and tooth pitch per unit area. The general rule higher populations for higher production rates, but it is not true always. It depends upon other factors like production rate, fineness, frictional properties etc. -

Tooth point

The tooth point is important from a fibre penetration point of view. It also affects the maintenance and consistency of performance. Most of the recent cylinder wires have the smallest land or cut-to-point. Sharp points penetrate the fibre more easily and thus reduce friction, which in tum reduces wear on the wire and extends wire life.

238

-

Textile Technology

Blade thickness

Blade thickness affects the fibre penetration. The blade thickness is limited by practical considerations,but the finer the blade the better the penetration of fibres. Wires with thin blade thickness penetrate the more easily and thus reduce friction, which in turn reduces wear on the wire and extends wire life. -

Back angle

A lower back angle reduces fibre loading, but a higher value of back angle assists fibre penetration. Between the two extremes is an angle which facilitates both the reduction in loading and assists fibre penetration and at the same time gives the tooth sufficient strength to do the job for which it was designed. -

Hardness of wire

The cylinder wire needs to be hard at the tip of the tooth where the carding action takes place.The hardness is graded from the hard tip to the soft rib. High carbon alloy steel is used to manufacture a cylinder wire and it is flame hardened. Rib should not be hardened, otherwise, it will lead to mounting problems. The design or type of clothing, selected for the fibre to be carded is important,but it is fair to state that within reason, an incorrect design of clothing in perfect condition can give acceptable carding quality whereas a correct clothing design in poor condition will never give acceptable carding quality. There is no doubt that the condition of the clothings is the most important single factor affecting quality at high rates of production. Wire condition and selection of wire are considered to be the two most important factors

Metallic Card Clothing

239

which influence the performance of modern high production carding machines. The condition of the clothing may be defined as the collective ability of the individual teeth of the clothing to hold on to the fibre against the opposing carding force exerted by other teeth acting in the carding direction. For a given design of clothing the condition of the teeth determines the maximum acceptable production rate that can be achieved at the card. The speed of the main cylinder of card provides the dynamic force required to work on separating the fibres fed to the card but it is the ability of the carding teeth on the cylinder to carry the fibre forward against the opposing force offered by the teeth of the tops which determines the performance of the card. Increasing cylinder speed increases the dynamic forces acting upon the carding teeth and thus the condition of teeth becomes more important with increased speed.If the condition and design of the cylinder wire is poor, the teeth will not be able to hold onto the fibre through the carding zone, thus allowing some of the freed fibre to roll itself into nep. DoHer Wire 1.

The doffer is a collector and it needs to have a sharp tooth to pickup the condensed mass of fibres circulating on the cylinder. It also requires sufficient space between the teeth to be efficient in fibre transfer from the cylinder, consistent in the transfer rate and capable of holding the fibre under control until the doffer's stripping motion takes control.

240

Textile Technology

2.

A standard doffer wire has an overall height of approx. 4.0 rnm to facilitate the deeper tooth which must have sufficient capacity to collect all the fibre being transferred from the cylinder to meet production requirements. Heavier webs require a deeper doffer tooth with additional collecting capacity to hanndle the increased fibre mass.

3.

The doffer wire's front angle plays a very important part in releasing the fibre from the cylinder wire's influence. A smaller angle has a better chance of enabling the doffer wire's teeth to find their way under the fibres and to secure the fibre's release from the cylinder with greater efficiency. A 60 degree front angle for Doffer has been found to give the optimum performance under normal carding conditions. Too small an angle results in cloudy web and uneven sliver whilst too large an. angle results in fibre recirculation and nep generation.

4.

Having collected the fibre, it is important for the doffer to retain it until it is stripped in a controlled manner by the doffer stripping motion. The tooth depth, tooth pitch and rib width combine to create the space available for fibre retention within the doffer wire. Thus they directly influence the collecting capacity. If the space is insufficient, fibre will fill the space and any surplus fibre will be rejected. When the surplus fibre is left to recirculate on the cylinder, cylinder loading can take place. Unacceptable nep levels and fibre damage will also result. In severe cases pilling of the fibre will take place.

5.

The point of the doffer wire normally has a small land which helps to strengthn the tooth. The

Metallic Card Clothing

241

extremely small land of around 0.05 mm ensures that the doffer wire height is consistent, has no adverse effect on fibre penetration and is considred essential for efficient fibre transfer from the cylinder. The land has micropscopic striations which are created during manufacturing or grinding. The striations help to collect the fibres from the cylinder and keep them under control during the doffing process. 6.

It has been found that a cut-to-point doffer wire

penetrates the fibre better than does the landed point wire but is less likely to keep the fibre under control during the doffing process. Sometimes a cut-to-point doffer wire is accompanied by striations along one side of the tooth for this reason. Until recently 0.9mm rib thickness is standardised for doffer wire, regardless of production and fibre characteristics.This rib thickness has been found to give optimum results. However doffer wires with a 0.8mm rib thickness have been introduced for applications involving finer fibres. 7.

In general 300 to 400 PPSI(points per square inch) has been found to perform extremely well under most conditions. Doffer wire point population is limited by the wire angle and tooth geometry. Higher population for doffer does not help in improving the fibre transfer.

8.

As the production rate rises, the doffer speed also increases. The doffer is also influenced by the centrifugal force, as is the cylinder. But cylinder wire front angle can become closer to counter the effect of centrifugal force, to close the front angle on a doffer wire would reduce its collecting capacity and result in a lowering of the production rate. The solution is

242

Textile Technology

to .use the wire with striations, which will hold the fibre until the doffer is stripped. 9.

The hardness of the doffer wire is a degree lower than that of the cylinder but sufficiently hard to withstand the forces generated in doffing and the resultant wear of the wire. The reason for this slightly lower hardness requirement is the longer and slimmer tooth form of the differ wire.

10. The fibres which are not able to enter the wire will lay on top, i.e.completely out of control. There fore instead of being carded by the tops the fibres will be rolled. Similarly a fibre buried too deep within the cylinder wire will load the cylinder with fibre, weaken the carding action and limit the quantity of new fibres the cylinder can accept. Therefore, the production rate would have to be reduced.

Licker-in Wire Licker-in with its comparatively small surface area and small number of carding teeth, suffers the hardest wear of all in opening the tangled mass of material fed to it. Successful action of the Licker-in depends upon a penetrating sharp point rather than a sharp leading edge as with the cylinder wire. Therefore the licker-in wire cannot be successfully restored to optimum performance by grinding. The most satisfactory system to adopt to ensure consistent performance is to replace the licker-in wire at regular intervals before sufficient wear has taken place to affect carding quality.

Metallic Card Clothing

243

The angles most widely used are 5 degrees negative or 10 degrees. There is no evidence to suggest recommendation of a tooth pitch outside the range of 3 to 6 points per inch.

It is better to use Licker-in roller without groove. Interlocking wires are used for such type of lickerins. This avoids producing the eight precise grooves and to maintain them throughout its life. Interlocking wire is almost unbreakable and thus no threat to the cylinder, tops and doffer in the event of foreigh bodies entering the machine.

Flat Tops 1.

The flat tops are an equal and opposite carding force to the cylinder wire and it should be sharp, well maintained and of the correct design.

2.

The selection of flexible tops is very much related to the choice of cylinder wire, which in turn is related to the cylinder speed, production rate and fibre charactersitics, as previously stated.

3.

The modern top is of the semi-rigid type, having flexible foundation and sectoral wire. The points are well backed-off and side-ground co give the necessary degree of fineness. The strength of the top from a carding point of view is in the foundation and is affected by the number of plies and the type of material used. The position of the bend in the wire is determined by stress factors, at around 2:1 ratio along the length of the wire protrusion.

244

4.

Textile Technology

The modern top is made from hardened and tempered wire to increase wear resistance , thus improving the life of the flat top. Life of the cylinder wire depends upon 1. Material being processed 2. production rate 3. cylinder speed 4. settings Wear is the natural and unavoidable side effect of the work done by the vital leading edge of the metallic wire tooth in coping with the opposing forces needed to obtain the carding action which separates fibre from fibre. When the leading edge becomes rounded due to wear, there is a loss of carding power because the point condition has deteriorated to an extent where the leading edge can no longer hold on to the fibre against the carding resistance of the flats. This ultimately leads to fibres becoming rolled into nep with consequent degradation of carding quality. Therefore it is important to recognise that, due to the inevitable wear which takes place during carding, metallic wire must be reground at regular intervals with the object of correctly resharpening the leading edge of each tooth.

Grinding 1.

Wire points of cylinder have become finer and the tip is cut-to-point.Because of this new profile, it has beccome necessary to recommend a Ii ttle or no grinding of the cylinder wire following mounting.

Metallic Card Clothing

245

TSG grinding machine of GRAF(wire manufacturer) can be used to sharpen these modern wires. TSG grinding is a safe method of grinding. 2.

Before grinding, the wire should be inspected with a protable microscope to ascertain the wear. Based on this and the wire point land width, no of traverse for TSG grinding should be decided. If the width of the wire point tip is bigger and the wear out is more, the number of traverse during grinding should be more.

For a new wire, 3 or 4 traverses may be enough. But it may require 10 to 30 traverses for the last grinding before changing the wire, depending upon the maintenance of the wire.

Grinding a normal cylinder and doffer wire 1.

The first grinding of the metallic wire on the cylinder and doffer is the final and most important step leading up to providing the card with a cylinder in the best possible condition for carding well at maximum produciton rate.Grinding the lands of the teeth provides the leading edge of each tooth with the final sharpness reqauired for maximum carding power.

2.

The first grinding should be allowed to continue until at least eighty percent(for cylinder) and 100% (for doffer) of the lands of the teeth have been ground sufficient to sharpen the leading edge of the tooth.

3.

To ascertain this stage of grinding, it is necessary to stop the cylinder regularly and use a simple microscope to examine the teeth at random across and round the cylinder.

246

Textile Technology

4.

If the wire on the cylinder is of good quality and has been correctly mounted, the initial grinding period should be completed with in 20 min.

5.

It is essential to avoid over-working the wire before

taking corrective action. The regrinding cycle must be determined accurately for the conditions applying in the individual mill, by using the microscope.

6.

If regrinding is done properly, there are several

advantages 1. carding quality will remain consistent 2. There is no risk of overworking the wire 3. Time required for regrinding is very short 4. The exact condition of the clothing is known 5. The working life of the wire is likely to be longer because the points are never allowed to become worn beyond recovery 6. To obtain acceptable grinding conditions at the low grinding speed, the grindstone must always be SHARP, CLEAN and CONCENTRIC. If the grinding stone is gradually allowed to become dull and glazed through constant use, the limited cutting action available will eventually disappear, resulting in burning and hooking of the carding teeth. 7. Due to the low peripheral speed of the grindstone which has to be used, it is most important that the speed of the wire to be ground is as high as is practicable to provide a high relative speed between the grindstone surface and the cardig

Metallic Card Clothing

247

teeth'!f wire speed is low, the individual carding tooth spends too long a time in passing under the grindstone, thereby increasing the risk of hooking and burning the tooth, which is usually irreparable.

8.

With cylinder grinding, speed is no problem because the normal operating speed of the cylinder is more than sufficient. The speed of the doffer for grinding is more commonly a problem and this should be driven at a minimum speed of 250 m/min, to avoid damage when grinding the wire, the design which is particularly susceptible to hooking due to the long fine, low angled teeth needed on the doffer.

9.

The directions of rotation for metallic wire grinding are normally arranged so that the back edge of the tooth is first to pass under the grindstone. This is termed grinding "back of point"

Grinding Flat Tops 1.

Flat tops provide the opposing carding force against the cylinder wire and hence can equally effect carding quality.lt is essential to ensure that the tops are kept in good condition to maintain maximum carding power with the cylinder.

2.

For cards fitted with regrind able tops, it is good practice to regrind the flats at regular intervals thus ensuring that the conditions of the two principal carding surfaces are always complementary one to other.

13 Winding Ring spinning produces yarn in a package form called cops. Since cops from ringframes are not suitable for further processing, the winding process serves to achieve additional objectives made necessary by the requirements of the subsequent processing stages. Following are the tasks of winding process: Extraction of all disturbing yarn faults such as the short, long thick ,long thin, spinners doubles, etc Manufacture of cones having good drawing - off properties and with as long a length of yarn as possible Paraffin waxing of the yarn during the winding process Introduction into the yarn of a minimum number of knots Achievement of a high machine efficiency i.e high produciton level

Winding

249

The winding process therefore has the basic function of obtaining a larger package from several small ring bobbins. This conversion process provides one with the possibility of cutting out unwanted and problematic objectionable faults. The process of removing such objectionable faults is called as yarn ' clearing' . Practical experience has proven that winding alters the yarn structure.This phenomenon does not affect yarn evenness, but affect the following yarn properties thick places thin places neps hairiness standard deviation of hairiness If winding tension is selected properly, the following tensile properties are not affected: tenacity elongation work- to- break But excessive tension in winding will deteriarate the above said tensile properties. Changes in the yarn surface structure due to winding cannot be avoided. Since the yarn is accelerated from zero speed to 1200 or 1350 meters per min in a few milli seconds while being pulled off the bobbin, dragged across several deflection bars and eyelets, forced into a traverse motion at speed that make it invisible, and finally rolled up into a firm construction called package or cone.

250

Textile Technology

The factors that affect the yarn structure during winding include the frictional properties of the yarn itself, the bobbin geometry and the bobbin unwinding behaviour, winding speed, winding geometry as well as the number and design of the yarn / machine contact points. However, the bobbin unwinding behaviour is the major limiting factor for winding speed which also is the main reason for the above said changes in yarn structure. Most of the damage occurs at the moment when the end is detached and removed from the tight assembly of yarn layers on the bobbin and dragged along the tube at very high speeds. High speed automatic winders have frequently been blamed for causing higher nep counts but this is not a correct statement. typical nep-type imperfections, i.e shor mass defects, can be identified as tight fibre entanglements, clumps of immature or dead cotton fibres, or seed coat fragments. Naturally, such defects are not produced by the winding machine. The increase in nep counts after winding is related to the formation of loose fiber accumulations. These fibre accumulations represent a true mass defect, yet their apperance in the yarn and in the final fabric is clearly different from that of typical fibre entanglements or seed coat fragments. Some very fine and delicate yarns will result in marginal structural changes after winding. But this is not the result of mechancial stress like in winding but a natural reaction caused by the reversal of the yarn running direction. irecrronal influences are omnipresent, they become apparent in all subsequent processing stages.

Winding

251

In earlier days, knotters were used in winding machine to join two ends after cutting the fault and after chaning the ringframe bobbin . But now , splicing of the yarn ends has become quite popular and has gradually replaced knotting by way of its better appearance while at the same time retaining sufficient strength.

Waxing Process Waxing is the process which is almost exclusively used in all automatic and manual winding machines for yarns which are meant for knitting. This helps to reduce the coefficient of friction of yarns created during knitting process. Extensive tests have shown that the coefficient of friction of waxed yarn is not constant, but depends on the amount of wax on the yarn. It is proved that both too little and too much wax cause increase in coefficient of friction and thus detrioration in running efficiency on the knitting machine. The recommended wax pick up for different material are given below: Cotton and its blends - wax take-up of 1.0 to 2.0 grams per kg of yarn II.synthetics - wax take-up of 0.5 to 1.5 gram per kg of yarn lII.wool and its blends - wax takep-up of 2.0 to 3.0 grams per kg From the technical point of view, it is interesting to note that very small amounts of wax are already sufficient to give an optimal reduction in friction coefficient. If for example, we take 1 kg of 50s metric yarn, there are 50000 meters of yarn. It is quite sufficient to apply 1 gm of wax on this length of yarn, to obtain optimum reduction in friction.

252

•

Textile Technology

As the original coefficients of friction of non-waxed yarns are so varied, due to different raw materials and blends, dye-stuffs, additives, twist etc, so also are the values obtained with waxed yarns. The table shows several typical examples of coefficient of friction for unwaxed and waxed yarns. Absolute comments about coefficients of friction are not possible. It depends on several factors, such as type of material, count, twist, dyeing process, yarn moisture content, atmospheric conditions etc. Kind of yarn count (Metric) friction coefficient of unwaxed yarnfriction coefficienr of waxed yarnpercentage of friction coefficient decrease % cotton, 50s combed 0.2850.14549 cotton, 40 bleachd cbd 0.300.1453 wool, 36s natural 0.330.15553 wool,36s dyed 0.320.15552 polyester 40s white 0.420.2150 Even with efficient waxing , the results in knitting can still be adversely affected, if the package of waxed yarn is subsequently handled. A typical example is conditioning of waxed packages. The conditioning causes an increase in friction coefficient, and thus a deterioration in running properties. Therefore one should not condition waxed packages. An increase in moisture content causes an increase in friction coefficient. If too-damp bobbins are creeled at the winding

machine, poor waxing results, because yarns with high moisture content take up hardly any wax. If bobbins have to be conditioned or steamed, the yarn should be allowed to stand for atleast 24 hours, so that it can return to its normal condition before winding. A further problem can arise during steaming, or any other treatment involving the application of heat to a waxed package.

Winding

253

Low yarn tension will affect the wax pickup. Dimensions and form of wax rollers will affect the wax uniformity As it is clear and is important that, if the waxed particles are to carry out their function, they must remain on the surface of the yarn. When the yarn is subjected to heat however, the wax melts and penetrates to the inside of the yarn body: it can then no longer work effectively. When choosing the wax, it is essential to consider the type of yarn and fibre, the temperature in the production area, etc., and the characteristics indicated by the wax manufacturer

Yarn faults and clearing Each yarn contains, here and there, places which deviate to quite a considerable extent from the normal yarn corsssection. These can be short thick places, long thin places , long thick places or even spinners doubles. Eventhough such events seldom occur, they represent a potential disturbance in the appearance of the fabric or can negatively influnece subsequent processing of the yarn. Short thick places are those faults which are not longer than approximately 8 cms, but have a crosssectional size approx. twice that of the yarn. These faults are relatively frequent in all spun yarns. To an extent they are the result of the rawmaterial ( vegetable matter, non-seprated fibres, etc). To a much larger extent, these faults are produced in the spinning section of the mill and are the result of spun in fly. Short thick places are easIly determinable in the yarn. In many cases, they cause disturbances in subsequent processing. Once they

\

I

254

Textile Technology

reach a certain size( cross-section and length) , and in each case accoridng to the type of yarn and its application, short thick place fults can considerably affect the appearance of the finished product. Long thick places are much more seldom-occuring than the short thick places and usually have a length longer than 40cms. In some cases, their length can even reach many meters. Their cross sectional size approx. + 40% to +100% and more with respect of the mean crosssection of the yarn. Long thick places will affect the fabric apperance. Faults like spinners doubles are difficult to determine in the yarn, with the naked eye. On the other hand, they can produce quite fatal results in the finished product. A spinners double in the warp or in yarn for circular knitting can downgrade hundreds of meters of woven, or knitted fabric. Thin places occur in two length groups. Short thin places are known as imperfections, and have a length approx. three times the mean staple length of the fibre. Their frequency is dependent on the rawmaterial and the setting of the drafting element. They are too frequent in the yarn to be extracted by means of the electronic yarn clearing. Long thin places have lengths of approx. 40cms and longer and a cross-sectional decrease with respect to the mean yarn cross-section of approx.30 to 70%. They are relatively seldom-occuring in short staple yarns, but much more frequently-occuring in long staple yarns. Long thin faults are difficult to determine in the yarn by means of the naked eye. Their effect in the finished product however, can be extremely serious. The quite extensive application of electronic yarn clearing has set new quality standards with respect to the number of faults in spun yarns.

Winding

255

It is therefore necessary to evolve a method of yarn

fault classification before clearing the faults in winding. The most important aspect is certainly the determination of the fault dimensions of cross-sectional size and length. With such a cross-section and length classification and by means of the correct choice of the class limits, the characteristic dimensions of the various fault types can be taken into consideration, then a classification system will result which is suitable primarily for satisfying the requirements of yarn clearing and yet allows, to quite a large extent, for a selection of the various types of faults. The yarn faults are classified according to their length and cross-sectional size, and this in 23 classes. The cross-sectional deviations are given +% or -% values. i.e theupper limit, respectively , lower limit with respect to the mean yarn fault cross-section is measure in %. The fault length is measured in cms. The classes and their limits are set out according to the following: Short thick place faults: 16 classes with the limits, 0.1 em, 2cm, 4cm, and 8cm for the lengths and +100%, +150%,+250%, and +400% for the cross-sectional sizes are provided. The classes are indicated Al...D4. The classes A4, B4,C4,D4 contain all those faults, according to their length, whose cross-sectional size oversteps +400%. spinners doubles: This refers to a class (with the indication E) for faults whose length oversteps 8cms and whose cross-sectional size oversteps +100 ( open to the right and upwards). Long thick place faults and thick ends: The long thin place faults are contained in 4 classes with the limits

Textile Technology

256

8 cms and 32 cms for the lengths, and -30% , -45% and -75% for the cross-sectional sizes. The classes are designated Hl" ... .I2. The classes 11 and 12 are open to the right. i.e they contain all those thin places having a size between -30 and -45%, respetively, -45% and 75% and whose lengths are longer than 32 cms. The classification of the shorter thin places is of no advantage in the analysis of the seldom-occuring faults.

Types of Electronic Yarn Clearers Electronic Yarn Clearers available in the market are principally of two types -capacitive and optical. Clearers working on the capacitive principle have 'mass'as the reference for performing its functions while optical clearers function with 'diameter' as the reference. Both have their merits and demerits and are equally popular in the textile industry. Besides the above basic difference in measuring principle, the basis of functioning of both the types of clearers are similar if not exactly same. Since most of the other textile measurements like, U% / CV%, thick and thin places etc., in various departments take into account mass as the reference parameter, the functioning of the capacitive clearer is explained in some detail in the following sections.

Functioning Principle The yarn is measured in a measuring field constituted by a set of parallely placed capacitor plates. When the yarn passes through this measuring field (between the capacitor plates), an electrical signal is produced which is

Winding

257

proportional to the change in mass per unit length of the yarn. This signal is amplified and fed to the evaluation channels of the yarn clearing installation. The number and type of evaluation channels available are dependent on the sophistication and features of the model of the clearer in use. Each of the channels reacts to the signals for the corresponding type of yarn fault. When the mass per unit length of the yarn exceeds the threshold limit set for the channel, the cutting device of the yarn clearer cuts the yarn. Yam Clearer Settings The yarn clearer has to be provided with certain basic information in order to obtain the expected results in terms of clearing objectionable faults. The following are some of them: a. Clearing Limit: The clearing limit defines the threshold level for the yarn faults, beyond which the cutter is activated to remove the yarn fault. The clearing limit consists of two setting parameters - Sensitivity and Reference Length. i. Sensitivity - This determines the activating limit for the fault cross sectional size. ii. Reference Length - This defines the length of the yarn over which the fault cross - section is to be measured. Both the above parameters can be set within a wide range of limits depending on specific yarn clearing requirements. Here, it is worth mentioning that the ' reference length' may be lower or higher than the actual ' fault length'. For a yarn fault to be cut, the mean value of the

258

Textile Technology

yarn fault cross-section has to overstep the set sensitivity for the set reference length. b. Yarn Count The setting of the yarn count provides a clearer with the basic information on the mean value of the material being processed to which the clearer compares the instantaneous yarn signals for identifying the seriousness of a fault. c. Material Number Besides the yarn count there are certain other factors which influence the capacitance signal from the measuring field like type of fibre (Polyester / Cotton / Viscose etc.) and environmental conditions like relative humidity. These factors are taken into consideration in the Material Number'. I

4.5polypropylene, poly ethylene50 to 80% RH3.5polyester50 to 80%RH2.5polyvinyl chloride50 to 80% RH. From the values given in the table it could be seen that, for water absorbent fibres like cotton, the Material Number is changed by 1 for a 15% change in Relative Humidity. A reduction in material number results in a more sensitive setting causing higher fault removal. For blended yarns, the material number is formed from the sum of the percentage components of the blend. For instance, when a 67/33 Polyester / Cotton blend is run at an RH of 65%, the Material umber should be set at (0.67 * 3.5) + (0.33 * 7.5) = 4.8. d. Winding Speed The setting of the winding speed is also very critical for accurate removal of faults. It is recommended that,

Winding

259

instead of the machine speed, the delivery speed be set by actual calculation after running the yarn for 2-3 minutes and checking the length of yarn delivered. Setting a higher speed than the actual is likely to result in higher number of cuts. Similarly a lower speed setting relative to the actual causes less cuts with some faults escaping without being cut. In most of the modern day clearers, the count, material number and speeds are monitored and automatically corrected during actual running of the yarn.

Fault Channels The various fault channels available in a latest generation yarn clearer are as follows: l.

Short Thick places

2.

Long Thick Places

3.

Long Thin Places

4.

Neps

5.

Count

6.

Splice

The availability of one or more of the above channels is dependent on the type of the yarn clearer. Most of the modern clearers have the above channels. Besides detection of the various types of faults, with latest clearers, it is also possible to detect concentration of faults in a specific length of yarn by means of alarms(cluster faults).

\

260

Textile Technology

Contamination Clearing Detection of contamination in normal yam has become a requirement in recent times due to the demands by yam buyers abroad. Therefore, some of the optical yarn clearers have an additional channel to detect the contamination in yam. This is mostly used while clearing cotton yam. The various facilities available in the yam clearers nowadays enable precise setting and removal of all objectionable faults while at the same time ensure a reasonably high level of productivity.

Splicing A high degree of yam quality is impossible through knot, as the knot itself is objectionable due to its physical dimension, appearance and problems during downstream processes. The knots are responsible for 30 to 60% of stoppages in weaving. Splicing is the ultimate method to eliminate yam faults and problems of knots and piecing. It is universally acceptable and functionally reliable. This is in spite of the fact that the tensile strength of the yam with knot is superior to that of yarn with splice. Splicing is a technique of joining two yam ends by intermingling the constituent fibres so that the joint is not significantly different in appearance and mechanical properties with respect to the parent yam. The effectiveness of splicing is primarily dependent on the tensile strength and physical appearance. Splicing satisfies the demand for knot free yarn joining: no thickening of the thread or only slight increase in its normal diameter, no great mass variation,

Winding

261

visibly unobjectionable, no mechanical obstruction, high breaking strength close to that of the basic yarn under both static and dynamic loading, almost equal elasticity in the joint and basic yarn. No extraneous material is used and hence the dye affinity is unchanged at the joint. In addition, splicing enables a higher degree of yarn clearing to be obtained on the electronic yarn clearer. Splicing technology has grown so rapidly in the recent past that automatic knotters on modern high speed winding machine are a thing of the past. Many techniques for splicing have been developed such as Electrostatic splicing, Mechanical splicing and Pneumatic splicing. Among them, pneumatic splicing is the most popular. Other methods have inherent drawbacks like limited fields of application, high cost of manufacturing, maintenance and operati~ns, improper structure and properties of yarn produced.

Pneumatic Splicing The first generation of splicing systems operated with just one stage without proceeding to trimming. The yarn ends were fed into the splicing chamber and pieced together in one operation. Short fibres, highly twisted and fine yarns could not be joined satisfactorily with such method. Latest methods of splicing process consist of two operations. During the first stage, the ends are untwisted, to achieve a near parallel dT"!'angement of fibres. In a second operation the prepared ends are laid and twisted together.

Principle of Pneumatic Splicing The splicing consists of untwisting and later re-twisting two yarn ends using air blast, i.e., first the yarn is

\

!

262

Textile Technology

opened, the fibres intermingled and later twisted in the same direction as that of the parent yarn. Splicing proceeds in two stages with two different air blasts of different intensity. The first air blast untwists and causes opening of the free ends. The untwisted fibres are then intermingled and twisted in the same direction as that of parent yarn by another air blast.

Structure of Splice Analysis of the longitudinal and transverse studies revealed that the structure of the splice comprises of three distinct regions/elements brought by wrapping, twisting and tucking / intermingling.

Wrapping The tail end of each yarn strand is tapered and terminates with few fibres. The tail end makes a good wrapping of several turns and thus prevents fraying of the splice. The fibres of the twisting yarn embrace the body of the yarn and thus acts as a belt. This in turn gives appearance to the splice.

Twisting The two yarn ends compnsmg the splice are twisted around the body of the yarn, each yarn strand twists on the body of the yarn on either side of the middle of the splice. The cross-section of this region distinctly shows the fibres of the two yarn strands separately without any intermingling of the fibres.

Winding

263

Tucking I Intermingling The middle portion of the splice is a region (2-5 mm) with no distinct order. The fibres from each yarn end intermingle in this splice zone just by tucking. The studies on quantitative contribution of splice elements showed that intermingling/tucking contributes the most to the strength of splice (52%), followed by twisting (33%) and wrapping (about 15%). The lower strength of the splice is attributed to the lower packing coefficient of the splice zone. Spliced yarn has a lower breaking elongation than normal yarn. Breaking elongation is mainly affected by intermingling. Wrapping and twisting provides mainly transverse forces. The absence of fibre migration gives lower breaking elongation to splice. Effect of Variables on the Properties of the Spliced yarn. Several studies have been conducted on the effect of various variables on the properties of the spliced yarn.

Effect of Fibre Properties and Blend Fibre properties such as torsional rigidity, breaking twist angle and coefficient of friction affect splice strength and appearance. The lower torsional rigidity and higher breaking twist angle permit better fibre intermingling. Higher coefficient of friction of fibres generates more inter-fibre friction to give a more cohesive yarn. Thus, these properties of fibre contribute to better retention of splice strength. In blended yarn, usually the addition of polyester to other fibre blend like P /W, P /e both for ring and rotor spun yarn increases splice strength.

\

!

264

Textile Technology

Effect of Yarn Fineness Several studies on cotton, polyester and wool report that coarser yarns have higher breaking strength but a moderate extension. The coarse yarn cross section contains more fibres and provides better fibre intermingling during pre-opening, hence the splice is stronger than that of finer yarns.

Effect of Yarn Twist An increase in the twist significantly increases the breaking load and elongation, even at higher pneumatic pressure. This could be due to better opening of the strands at higher pneumatic pressure. Splicing of twisted ply yarn is more complicated than single yarn due to the yarn structure having opposing twists in the single and doubled yarns. Twisted yarns also require a relatively longer time for complete opening of the yarn ends.

Effect of Different Spinning Methods Yarn produced with different spinning methods exhibit different structure and properties. Therefore, these yarns show significant differences in splice quality. The ring spun yarn lent best splicing but the potential of splicing is affected by the spinning conditions. The breaking strength percentage of ring spliced yarns to a parent yarn is 70% to 85% for cotton yarn. However, the breaking strength and extension of splice vary with fibre and yarn properties. Rotor spun yarns, due to the presence of wrapper fibres, make it difficult to untwist and the disordered structure is less ideal for splicing. The

Winding

265

breaking strength retention varies from 54% to 71% and is much lower compared to the splice of ring spun yarns. In case of friction spun yarns, the highest relative tensile strength obtained at the spliced joints can be above 80%, but a number of splicing failures occurs due to unfavourable yarn structure. The air-jet-spun (MJS) yarn and the cover spun yarn are virtually impossible to splice. Only very low tensile strengths and elongation values can be attained due to the inadequate opening of the yarn ends during preparation of the splicing. The coefficient of variation of these properties is also generally high.

Effect of Opening Pressure A study on 50/50 polyester cotton, 25 tex ring spun yarn shows a rise in tensile strength up to a certain opening pressure. However, long opening time deteriorates the strength. An increase in pressure up to 5 bar caused release of fibre tufts and fibre loss from the yarn ends in p /e blend which is due to intensive opening, but beyond this pressure, drafting and twisting in the opposite direction may also occur.

Effect of Splicing Duration With a given splicing length, when the splicing is extended for a long period of time, the breaking strength of the spliced yarn and also their strength retention over the normal value of the basic yarn increases because of increased cohesive force resulting from an increased number of wrapping coils in a given length. The effects are more pronounced at higher splicing lengths. It is

\

I

266

Textile Technology

desirable however, that splicing duration be as short as possible. The splicing duration alone has no conclusive effect on elongation properties of splice yarn. It has also been observed that, for maximum splice strength, different materials require different durations of blast. These are between 0.5 to 1.8 seconds.

Effect of Splicing Length Studies on splicing of flyer and wrap spun yarns spun with different materials, showed that regardless of the splicing material, the breaking strength and strength retention of both yarn types increase with the splicing length because of the increased binding length of the "CWo yarn ends. Elongation at break and retention of elongation of both flyer and wrap spun spliced yarns increase with the splice length. Compared to the splicing duration, the splicing length has more pronounced effect on the load-elongation properties of the spliced yarn. It can be therefore be stated that the splices made on longer lengths and for longer period of time have more uniform strength.

Comparison of Dry and Wet Splicing The comparative studies on dry and wet splicing with water showed that the breaking load retention for wet spliced yarns are significantly greater than dry spliced yarns. In fact, wet splicing is more effective for yarn made from long staple fibres and for coarse yarn. This may be due to higher packing coefficient resulting from wet splicing.

Winding

267

Effect of Splicing Chamber The factors like method and mode of air supply and pressure along with type of prism affect the splicing quality. It was observed that irregular air pressure has advantages over constant pressure for better intermingling in the splicing chamber, which varies with different staple fibres, filament yarns, and yarns with S and Z twists. It is not possible to make a general comment regarding potential of the splicing chamber due to the multiplicity of factors influencing splicing.

Assessment of Yarn Splice Quality The two important characteristics of a splice are appearance and strength. Although quality of splice can be assessed by methods like load-elongation, work of rupture, % increase in diameter and evaluation of its performance in down stream process etc., the appearance can be assessed either by simple visual assessment or by comparing with photograph of standard splice.

Characteristics of Bobbin Formation

Strectch length: It is the length of the yarn deposited on the bobbin tube during each chase (one up and down movement of ringrail ) of ring rail. The length should be around 3.5 to 5 meters. It should be shorter for coarser yarns and longer for fine yarns. Winding ratio:1t is the ratio of the length of yarn wound during the upward movement of the ring rail and the length wound during the downward movement of the ring rail.

I Textile Technology

268

Bobbin taper: The ratio of the length of the upper taper of the cop (bobbin with yarn) to the diameter of the bobbin must be 1:2 or greater.

Winding Speed It depends upon the following factors:

count type of yarn, (type of fibre, average strength and minimum strength) type and charactersitics of bobbin package taper final use of package The best winding speed is the speed which allows the highest level of production possible for a given type of yarn and type of package, and with no damage whatsoever to the yarn.(abrasion and breaks due to excessive tension)

Winding production: It depends upon the following factors: Winding speed time required by the machine to carry out one splicing operation bobbin length per bobbin( both bobbin weight and tpi to be considered, because TPI will affect the bobbin length). This decides the number of bobbin changes the number of faults in the yarn and the clearer settings, this decides the clearer cuts

Winding

269

count the number of doffs. It depends upon the doff weight. Higher the doff weight, lower the number of doffs the time taken for each doff either by the doffer or by an operator Down time due to red light. It depends upon, number of red lights, number of repeaters setting for red lights, clearer settings like off count channel, cluster setting which will result in red lights and others bobbin rejections, it depends on weak yarn, wrong gaiting, double gaiting, bobbin characteritics etc.

Winding package defects: Following are some of the package defects which will result in complaints Yarn waste in the cones. This is due to loose yarn ends that are wound on to the cone

Stitch, drop over, web: Yarn is visible on the small or on the big side of the cone either across the side , around the tube, or going back in the cone Damaged edges or broken ends on the cone: The yarn is broken on the edges or in the middle of the cone. Ring formation: The yarn runs in belt formation on to the package, because it is misguided Without transfer tail: The desired transfer tail is missing or too short

Ribbon formation: Pa ttern or ring forma tion are made by the drum when rpm are stying the same

Textile Technology

270

Displaced yarn layers: yarn layers are disturbed and are sliding towards the small diameter of the cone

Misguided yarn : The yarn is not equally guided over the hole package

Cauliflower: On the smaller side of the package, the yarn shows a wrinkle effect

Soft and Hard yarn layer: Some layer of yarn are pushed out on the small side of the cone Soft and Hard cones: Great difference in package density from one winder head to another.

\

14 Spinning Geometry From Roving bobbin to cop, the fibre strand passes through drafting arrangement, thread guide, ballooncontrol rings and traveller. These parts are arranged at various angles and distances relative to each other.The distances and angles together are referred to as the spinning geometry,has a significant influenceon the spinning opeartion and the resulting yarn. They are:

•

yarn tension

• • • •

number of end breaks

•

yarn irregularity binding-in of the fibres yarn hairiness generation of fly etc.

Spinning Triangle Twist in a yarn is generated at the traveller and travel against the direction of yarn movement to thefront roller.

I

272

Textile Technology

Twist must run back as close as possible to the nip of the rollers, but it never penetrates completely to the nip because, after leaving the rollers, the fibres first have to be diverted inwards and wrapped around each other. There is always a triangular bundle of fibres without twist at the exit of the rollers, this is called as SPINNING TRIANGLE. Most of the end breaks originate at this point. The length of thespinning triangle depends upon the spinning geometry and upon the twist level in the yarn. The top roller is always shifted 3 to 6 mm forward compared to bottom roller. This is called top rolleroverhang.This gives smoother running and smaller spinning triangle. The overhang must not be made too large, as the distance from the opening of the aprons to the roller nip line becomes too long resulting in poorer fibre control and increased yarn irregularity. Continuous variation of the operating conditions arises during winding of a cop.The result is that the tensile force exerted on yarn must be much higher during winding on the bare tube than during winding onthe full cop, because of the difference in the angle of attack of the yarn on the traveller. When the ring rail is at the upper end of its stroke, in spinning onto the tube, the yarn tension is substantially higher than when the ring rail is at its lowermost position. This can be observed easily in the balloon on anyring spinning machine. The tube and ring diameters must have a minimum ratio, between approx. 1:2 and 1:2.2, in order to ensure that the yarn tension oscillations do not become too great. Yarn tension in the balloon is the tension which finally penetrates almost to the spinning triangle and

Spinning Geometry

273

which is responsible for the greater part of the thread breaks. It is reduced to a very small degree by the deviation of the yarn at the thread guide. An equilibrium of forces must be obtained between the yarn tension and balloon tension.

RINGS & TRAVELLERS In most cases, the limit to productivity of the ring spinning machine is defined by the traveller in interdependence with the ring, and yarn. It is very important for the technologist to understand this and act on them to optimise the yarn production. The following factors should be considered •

materials of the ring traveller

•

surface charecteristics

•

the forms of both elements( ring and traveller)

•

wear resistance

•

smoothness of running

•

running-in conditions

•

fibre lubrication

TRAVELLER Traveller imparts twist to the yarn. Traveller and spindle together help to wind the yarn on the bobbin. Length wound up on the bobbin corresponds to the difference in peripheral speeds of the spindle and traveller. The difference in speed should correspond to length delivered at the front rollers. Since traveller does not have a drive on its own but is dragged along behing by the spidle.

Textile Technology

274

High contact pressure (upto 35 N/square mm)is generated between the ring and the traveller during winding, mainly due to centrifugal force. This pressure leads to generation of heat. Low mass of the traveller does not permit dissipation of the generated heat in the short time available. As a result the operating speed of the traveller is limited. Heat produced when by the ringtraveller is around 300 degree celcius. This has to be dissipated in milliseconds by traveller into the air.

Parts of a traveller Height of bow: It should be as low as possible for stable running of traveller. It should also have sufficient yarn pasage. Yarn passage: According to count spun the traveller profile to be selected with required yarn passage. Toe gap : This will vary according to traveller number and flange width of the ring Wire section: It plays an important role for yarn quality, life of traveller. Ring contact area: This area should be more, uniform, smooth and continuous for best performance. Inner width: This varies according to traveller profile and ring flange.

SALIENT FEATURES OF A TRAVELLER •

Generate less heat

Spinning Geometry

275

•

dissipate heat fastly

•

have sufficient elasticity for easy insertion and to retain its original shape after insertion

•

friction between ring and traveller should be minimal

•

it should have excellent wear resistance for longer life

•

hardness of the traveller should be less than the ring

When the spindle speed is increased, the friction work between ring and traveller (hence the build up) increases as the 3rd power of the spindle rpm. Consequently if the spindle speed is too high, the traveller sustains thermal damage and fails. This speed restriction is felt particularly when spinning cotton yarns of relatively high strength.

If the traveller speed is raised beyond normal levels , the thermal stress limit of the traveller is exceeded, a drastic change in the wear behaviour of the ring and traveller ensues. Owing to the strongly increased adhesion forces between ring and traveller, welding takes place between the two. These seizures inflict massive damage not only to the traveller but to the ring as well.Due to this unstable behaviour of the ring and traveller system the wear is atleast an order of magnitude higher than during the stable phase. The traveller temperature reaches 400 to 500 degrees celcius and the danger of the traveller annealing and failing is very great. The spinning tension is proportional •

to the friction coefficient between ring and traveller

•

to the traveller mass

•

to the square of hte traveler speed and inversely proportional

Textile Technology

276

• to the ring diameter and the angle between the connecting line from the traveller-spindle axis to the piece of yarn between the traveller and cop. In order to maintain the same friction or spinning tension with different coefficients of friction, different traveller weights must be used. The coefficient of friction is determined by the fiber lubrication and is subject to fluctuation. Dry cotton means higher coefficient of friction. For manmade fibres depending upon the manufacturer, lower to medium coefficient of friction. The coefficient of friction with fiber lubrication can vary from 0.03 and 0.15. R

= Co efficeint of friction x N

where R - traveller friction in mN N = Normal force >= (Fc x ML x V xV)/(R) Fc - centrifugal force ML - mass of the traveller in mg V - traveller speed in ml s R - radius of the ring (inside) The yarn strength is affected only little by the spinning tension. On the other hand the elongation diminishes with increasing tension, for every tensile load of the fibres lessens the residual elongation in the fibres and hence in the yarn. Increasing tension leads also to poorer Uster regularity and IPI values.

If the spinning tension is more, the spinning triangle becomes smaller. As the spinning triangle gets smaller, there is less hairiness.

Spinning Geometry

277

SHAPE OF THE TRAVELLER The traveller must be shaped to match exactly with the ring in the contact zone, so that a single contact surface, with the maximum surface area is created between ring and traveller. The bow of the traveller should be as flat as possible, in order to keep the centre of gravity low and thereby improve smoothness of running. However the flat bow must still leave adequate space for passage of the yarn. If the yarn clearance opening is too small, rubbing of the yarn on the ring leads to roughemng of the yarn, a high level of fibre loss as fly, deterioration of yarn quality and formation of melt spots in spinning of synthetic fibre yarns.

WIRE PROFILE OF THE TRAVELLER Wire profile influences both the behaviour of the traveller and certain yarn characteristics, they are

• • • •

• •

contact surface of the ring smooth running thermal transfer yarn clearance opening roughening effect hairiness

MATERIAL OF THE TRAVELLER The traveller should •

generate as little heat as possible

278

Textile Technology

•

quickly distribute the generated heat from the area where it develops over the whole volume of the traveller

•

transfer this heat rapidly to the ring and the air

•

be elastic, so that the traveller will not break as it is pushed on to the ring

•

exhibit high wear resistance

•

be less hard than the ring, because the traveller must wear out iT' use in preference to the ring.

In view of the above said requirements, traveller manufacturers have made efforts to improve the running properties by surface treatment. "Braecker" has developed a new process in which certain finishing components diffuse into the traveller surface and are fixed in place there. The resulting layer reduces temperature rise and increases wear resistance. Traveller mass determines the magnitude of frictional forces between the traveller and the ring, and these in turn determine the winding and balloon tension. Mass of the traveller depends upon

•

yarn count

•

yarn strength

•

spindle speed

•

material being spun

If traveller weight is too low, the bobbin becomes too soft and the cop content will be low. If it is unduly high, yarn tension will go up and will result in end breaks. If a choice is available between two traveller weights, then the heavier is normally selected, since it will give greater

Spinning Geometry

279

cop weight, smoother running of the traveller and better transfer of heat out of traveller. When the yarn runs through the traveller, some fibres are liberated. Most of these fibres float away as dust in to the atmosphere, but some remain caught on the traveller and they can accumulate and form a tuft. This will increase the mass of traveller and will result in end break because of higher yarn tension. To avoid this accumulation , traveller clearers are fixed close to the ring, so that the accumulation is prevented. They should be set as close as possible to the traveller, but without affecting its movement. Exact setting is very important. For the rings two dimensions are of primariy importance. 1.internal diameter 2. flange width. Antiwedge rings exhibit an enlarged flange inner side and is markedly flattened on it upper surface. This type of profile permitted to use travellers with a lower centre of gravity and precisely adapted bow (elliptical travellers), which in turn helped to run the machine with higher spindle speeds. Antiwedge rings and elliptical travellers belong together and can be used in combination. Low crown profle has the following advantage. Low crown ring has a flattened surface top and this gives space for the passage of the yarn so that the curvature of the traveller can also be reduced and the centre of gravity is 10wered.In comparison with antiwedge ring, the low crown ring has the advantage that the space provided for passage of the yarn is somewhat larger and that all current traveller shapes can be applied, with the exception of the elliptical traveller. The low crown ring is the most widely used ring form now.

Textile Technology

280

The ring should be tough and hard on its exterior. The running surface must have high and even hardeness in the range 800-850 vikcers. The traveller hardness should be lower (650-700 vickers), so that wear occurs mainly on the traveller, which is cheaper and easier to replace. Surface smoothness should be high, but not too high, because lubricating film can not build up if it too smooth. A good ring in operation should have the following features: •

best quality raw material

•

good, but not too high, surface smoothness

•

an even surface

•

exact roundness

•

good, even surface hardness, higher than that of the traveller • should have been run in as per ring manufacturers requirement

•

long operating life

•

correct relationship between ring and bobbin tube diameters

•

perfectly horizontal position

·it should be exactly centered relative to the spindle In reality, the traveller moves on a lubricating film which builds up itself and which consists primarily of cellulose and wax. This material arises from material abraded from the fibres.If fibre particles are caught between the ring and traveller, then at high traveller speeds and with correspondingly high centrifugal forces, the particles are partially ground to a paste of small, colourless, transparent and extremely thin platelets.

Spinning Geometry

281

The platelets are continually being replaced during working. The traveller smoothes these out to form a continuous running surface. The position, form and structure of lubricating film depends on

•

yarn fineness

•

yarn structure

• •

fibre raw material traveller mass

•

traveller speed

•

heigh of traveller bow

Modern ring and traveller combination with good fibre lubrication enable traveller speeds upto 40m/ sec.

Technological Gttideeliness •

When the ring diameter is less, balloon diameter will be small. This leads to more yarn tension. Hence use lighter travellers.

•

When the ring diamter is bigger, balloon diamter will be more. This leads to less yarn tension and the balloon touches the separator. Hence use heavier travellers

•

When the tube length is short, the yarn tension will be more. Hence use lighter travellers

•

When the tube length is long, the yarn tension will be less, hence use heavier travellers

•

When the yarn contact area and ring contact area in traveller is closer, fibre lubrication is better especially in cotton. For this use heavier travellers • When

282

Textile Technology

spindle speed is increased use lighter traveller with low bow height. At higher speeds, lighter travellers give lesser yarn tension. When low bow height travellers are used centre of gravity will be closest to the ring which aids in running of traveller. • Use lighter travellers on new rings. This is done to reduce end breakages by reducing the yarn tension. • Use heavier travellers on old rings. This is done to avoid bigger balloons • Heavier travellers reduce hairiness • When using lighter travellers, yarn stretch will be less. It helps for better yarn elongation • During running-in the endbreakage rate should be kept minimum, hence use lighter travellers. •

The shorter the balloon, the lighter the traveller to be used, the higher traveller speeds can be achieved.

•

The ring traveller, together with the yarn as a pull element, is set into motion on the ring by the rotation of the spindle. If the direction of pull deviates too much from the running direction of the traveller (spinning angle less than 30 degrees) the tension load will be too high.

Preconditions for good operating results The maximum ability of the ring/traveller system to withstand occuring stress situation during operation determines the performance limit of the ring spinning and twisting machine. Traveller wear does not only depend on traveller material; problems of heat dissipation are of crucial importance, too. The heat generated between ring and traveller must be reduced as quickly as possible to avoid local temperature in the

Spinning Geometry

283

traveller wear zones. The ability of the traveller to resist to stress is determined by several factors. Investigations regarding improvements of rings and travellers aimed at a further increase of performance should above all make sure that all other conditions with a certain influence on the spinning process are optimal. Therefore make sure that: the rings are correctly centered with regard to the spindles the yam guide eyelet is well centered with regard to the spindle the spindle bearing is in good condition, thus preventing spindle vibrations the ratio between bobbin diameter and ring diameter is correct the concentricity of the ballon control ring with regard to the spindle is correct the fibre tufts which accumulate on flange travellers are reni.ov~d by means of suitable traveller cleaners the climatic conditions (temperature and relative air humidity) are favourable for the spinning process the air in the mill is free from disturbing particles that influence efficient performance of the traveller It has to be stressed that a smooth and well run-in track

is of most importance. Concentricity of spindle, ring, yarn guide and balloon control ring Especially at high spindle speeds concentric positioning of ring, spindle, yarn eyelet and balloon

284

Textile Technology

control ring is required for keeping the ends down rate at low level. Spindles and rings must be aligned and centered absolutely parallel. Ring rails or ring holders should, therefore, be installed absolutely horizontally compared to the vertically fitted spindles. Ring and traveller form the main elements in ring spinning and twisting. They determine to a large extent performance and operating conditions of the machine. The traveller accomplishes two main tasks while running on the ring at high speeds: a)

It gives the roving supplied by the feed rollers the

necessary twist. b)

It assists in winding the yarn onto the bobbin in the

form of a cop with

a correct

tension.

During this operation the ring guides the traveller, which is essential for the perfect positioning of the yarn and the formation of the cop. The traveller is pressed against the ring track by centrifugal forces. The resulting frictional forces reduce traveller speed, which is dragged along by the passing-through yarn, and provide the yarn with the tensile forces necessary for assembling the individual fibres into the spun yarn as well as for limiting the yarn balloon. Steel travellers are hardened to a certain degree and polished to a mirror finish. They can be adapted in shape, weight and surface finish to the ring, yarn type and yarn count. Nylon travellers of standard quality (for HZ and J rings) are made of highly wear-resistant polyamide. Extremely aggressive yarns are processed with glassfibre-reinforced a Super Nylon travellers. Twisting and

285

Spinning Geometry

winding carried out by the traveller must be performed with appropriate yam tension. The ratio between spindle speed and the speed at which the yarn is supplied determines yam twist. Any change of this ratio is easily compensated by the traveller without having an influence on twisting, winding and tensioning. On flange rings, the gliding speed of travellers having a suitable shape can be as rapid as 130 ftls (88 MPH) or 40 mls (140 km/h); on- OIA-OUR coated rings the speed can to some extent reach 147 ftls (100 MPH) or 45 ml s (160 km/h) Having an average life span of 200-300 operating hours the traveller covers a distance of more than 18.000 miles (30.000 km) • a tremendous task for a small part of wire weighing only a few milligrams. These standards can even be surpassed by nylon travellers used on HZ rings, if operating conditions are favourable. These high traveller speeds involve pressures of up to 35 N/mm 2 . But even if high-quality materials with an optimum of hardness and resistance to wear are used, these standards can only be reached if - in the case of flange rings, a film of lubricating fibres is produced continuously, -in the case of HZ and J rings, a sufficient amount of lubricant is consistently provided..,., \

Spindles operating without vibrations contribute a great deal to a smooth operation of the traveller. Nonconcentric spindles and spindles not running smoothly cause constant changes in yam tension, because the traveller cannot run around the ring without being shaken.

286

Textile Technology

Vibration-free movement of ring rail and ring holder The ring rail should move smoothly without jerking. Vibrations and hard jolts at the reversing points of the ring rail disturb the operation of the traveller. Repeated changes in yarn tension cause the traveller to flutter. This results in increasing yarn breaks and in accelerated wear of ring ~and traveller. Correct ratio between bobbin diameter, bobbin length, ring diameter and spindle gauge Ratio bobbin length (H) : Inside ring diameter (D) Thread tension increases with growing bobbin length. In view of the limited thread tension, the total bobbin length should not exceed 5 times the ring diameter. Only when using balloon control rings or similar devices this value can be exceeded.

H:D=5:1 Ratio bobbin diameter (d) : Inside ring diameter (D) The bobbin diameter d is equivalent to the mean outer bobbin diameter d 1 + d 2 The following values are recommended: for spinning: d : D = 0.48 - 0.5 (a (minimum value a = 26°) for twisting d : D value a = 22°)

= 29°-30°),

= 0.44 - 0.5 (a = 27°-30°), (minimum

For light and heavy bobbins, the values for light bobbin types are decisive for calculating d : D. If the ratio d : D is reduced thread tension increases. Correct surface smoothness, i.e. optimum peak-tovalley height and evenness of the ring track .

Spinning Geometry

287

The traveller contact surfaces must be smooth and even. Only then a smooth operation of the traveller will be possible. The contacted surfaces should be clean and preferably without traces of wear. In addition, they should be designed in such a way that they offer sufficient adherence for potential lubricants (e.g. fibres, oil, grease). Once the sliding surfaces have lost their original quality, even the best ring traveller will not be able to run smoothly. For maintaining the surface of the running track in a good condition, it is very important - besides a certain degree of maintenance - to run the ring well in.

Balloon control rings The influence of balloon control rings is quite considerable, especially at long cops. A reduction of the yarn balloon is advantageous or may even be the prerequisite for optimum performance. If balloon control rings are mounted at correct distance (the yarn balloon should be restricted as long as possibleduring one lift of the ring rail) then a marked performance increase is possible. The balloon control rings are removed when sensitive materials are processed and sufficiently long separators are installed to avoid many yarn breaks and to prevent fibre fly from accumulating on the adjacent spindles.

Traveller cleaners Traveller cleaners are an excellent method for removing all fibre fly that accumulates on the outer part of C and El travellers. The traveller cleaner should have the right

288

Textile Technology

distance to the outside ring flange. A distance of about 0.5 mm between cleaner and traveller (in operating position) is recommended. When adjusting the distance between outside ring flange and cleaner, the size of the traveller should be taken into consideration.

Room climate Constant temperature and air humidity have positive effects on the operation of the traveller. Changes of the room climate, such as raised air humidity will increase wear by friction. Besides the regular exchange of air, the purity of the air is of great importance for the traveller. Any dust (also dust from unsuitable floors) or other impurities may impair traveller operation and lead to more ring/traveller wear.

Flange width and ring height Optimal operating results are reached when the ideal flange width is chosen for flange rings and the ideal ring height is obtained for self-lubricating HZ and J rings, dependent on yarn count range, yarn quality and traveller type.

Ring profile and traveller shape Determining the most favourable ring and traveller shapes is a precondition for obtaining the optimal individual performance. If ring profile and traveller shape match well, the traveller will adopt a stable position in the ring. It should have sufficient tolerance of movement, so that any obstacles which may occur

Spinning Geometry

289

especially when the machine is started are avoided. A satisfactory large yarn clearance counteracts yarn breaks and yarn damage.

Running-in of rings Normally the running-in procedure is decisive for the future positive/nega tive behaviour of the ring and the length of its service life. Every ring requires a certain degree of running-in time if it is to maintain high traveller speeds with as little ring and traveller wear as possible. During running-in the use of steel travellers without surface treatment is recommended. After the termination of the running-in process, steel travellers with surface treatment or nylon as well as bronze travellers can be used. The running-in process, beginning with the starting phase, consists of improving the initial running properties of the metallic running surface up to the optimal values by smoothing and passivation(oxidation) as soon as possible. In this way, together with fibre lubrication, constant minimum mixed friction conditions and minimum thermal stressing can be attained for the ring traveller. A careful running-in process will improve the lifetime of the rings. In order to keep the stress on the traveller as low as possible during the starting phase, it is advisable to always change the traveller in the upper third part of the cops. Further advantages are brought with the use of a traveller running-in program(reduction of the speed by about 10% for 10 to 20 minutes, only available on modern spinning machines).

290

Textile Technology

Spindle speed should be reduced atleast for the first 10 traveller changes. If final speed is higher than 32m/ sec, reduce by atleast 20%. If final speed is lower than 32m/ sec, reduce by at least 10%. New rings should not be degreased, but only rubbed over with a dry cloth. In general, the running in should be done with the same traveller type which is used for normal operation with the 10 to 20% less than normal speed. It is not advisable to do running with the same speed but with 1to 2 numbers lighter travellers than usual. The first traveller change should be carried out after 15 min The second traveller chage should take place after 30

min The third traveller change should be made after 1 to 1.5 hours. The fourth traveller change should be made after the first doff. Further traveller changes are to be made according to the manufacturers recommendations

HAIRINESS Following are the reasons for higher yarn hairiness due to ring and travellers Poorly centered spindles, anti balloon rings and yarn guides lead to inconsistent yarn tension. Rough surfaces roughen the yarn (due to damaged parts)

Spinning Geometry

291

Open anti balloon ring The clearance between ring and cop should not be too small. Traveller will cut the fibres protruding from the cop. The fibres get electrostatically charged Poor twist propagation to the spinning triangle due to lighter travellers Heavy friction of the balloon on the anti-balloon ring respectively impact on the balloon separator( due to lighter traveller) Poor ring centering Crooked tubes Yam getting roughened in narrow yam passage in the traveller Scratched up yam passages catch the yarn and roughen it (due to very high traveller running time) Friction of the yam due to very high traveller weight Rough gliding surface of the ring ( due to worn out rings)

COM-4 AND ELITE YARNS With the ComforSpin technology a new yarn with perfect yarn structure - the COM4 yarn - has been established in the market. With the help of a microscope the structure of the yams can easily be compared: The conventional ring yam shows to be far less perfect than commonly assumed. The long, protruding fibres cause a number of problems in downstream processing. COM4

Textile Technology

292

yarn shows a very compact structure with highly parallel fibres and much less disturbing hairiness. The air current created by the vacuum generated in the perforated drum condenses the fibres after the main draft. The fibres are fully controlled all the way from the nipping line after the drafting zone to the spinning triangle. An additional nip roller prevents the twist from being propagated into the condensing zone. The compacting efficiency in the condensing zone is enhanced by a specially designed and patented air guide element. Optimal interaction of the compacting ele-ments ensures complete condensation of all fibres. This results in the typical COM4 ® yarn characteristics. The ComforSpin technology allows aero-dynamic parallelization and condensation of the fibres after the main draft. The spinning triangle is thus reduced to a. minimum. The heart of ComforSpin machine is the compacting zone, consisting of the following elements: •

perforated drum

•

suction insert

•

air guide element

The directly driven perforated drum is hard to wear and resistant to fibre clinging. Inside each drum there is an exchangeable stationary suction insert with a specially shaped slot.

THE ELITE YARN The operating method of the SUESSEN EliTe Spinning System is well-known.After the fibres leave the drafting system they are condensed by an air-permeable lattice

Spinning Geometry

293

apron,which slides over an inclined suction slot.The fibres follow the outer edge of this suction slot and at the same time they perform a lateral rolling motion. Above the front bottom roller of the drafting system, the fibre band influeI).ced by high draft is spreading.In the area of the suction slot,which is covered by the lattice apron, the fibre band is condensed.Commencing from the semi-dotted clamping line of the EliTe Q Top Roller,twist is being• inserted.There is no spinning triangle. The improvement achieved is shown in Fig .The left side displays the fibre triangle at the exit of a conventional ring frame drafting system. The twist imparted by the spindle cannot flow up to the clamping line.The outer fibres spread out and are thus more highly tensioned than those on the inside. The right side of the picture does not show a spinning triangle.The yarn twist flows right up to the clamping line.The yarn is round and smooth. Since the spinning triangle is very very small, the end breaks will be very less and therefore the fly liberation will also be less. Condensing of the fibr bundle,which follows the drafting process,can already be seen as a significant development of the ring spinning technology. Condensed ring yarn is more than a speciality.ln view of its manifold advantages. It is of technological importance that the suction leve I relevant for the condensing operation is exactly the same for all spinning positions. To fulfil this criteria,individual motors combined with suction units for 6 spinning positions,have been arranged

Textile Technology

294

accordingly.This provides short air-flow distances with identical negativ pressures at all spinning points . During yarn formation all fibres are perfectly condensed and gathered parallel to each other in the compacting zone. Consequently all fibres are twisted in and contributing to the superior fibre utilisation rate compared to conventional ring yarn. The result is exceptionally low hairiness combined with higher yarn tenacity and elongation. These are the unique characteristics of these yarns.

Advatages •

higher fibre utilisation

•

higher tenacity with same twist factor, or

•

same tenacity with reduced twist factor for higher production

•

lowest hairiness (highest reduction in hairs longer than 3 mm)

•

fewer weak points

•

better imperfections (IPI) values

•

higher abrasion resistance

•

greater brilliance of colour

•

intensive dye penetration

•

no singeing

befor~

printing

Due to better utilization of fibre substance it is possible to reduce yarn twist of these Yarns, particularly of knitting yarns,by up to 20%,maintaining the yarn strength of conventional ring yarns.This increases yarn production.

Spinning Geometry

295

The ends-down rate in spinning these Yarns is reduced hy 30 to 60%,which imp~oves machine efficiency. Applying the same winding speed as with conventional ring yarns, there are less raised points in these Yarns and the increase in yarn imperfections is reduced because they have a better resistance to shifting. Higher winding speeds are therefore possible with compact yarns Yar ns . In accordance with up to 20%twist reduction in spinning compact yarns ,the twisting turns can be reduced for certain types of yarn.As a result,production of twisting frame is increased and twisting costs are reduced. Owing to the lower hairiness and higher tenacity of compact Yarns, the ends-down rate in beaming is reduced by up to 30%.Higher beamer efficiency,higher produc tion and fewer personnel for repair of ends-down in beaming are the consequence. Compact Warp yarns help to save up to 50%of sizing agent,while the running behaviour of weaving machi-nes is the same or even better. Cost can be saved in sizing and de sizing processes. Owing to the better work capacity of compact Yarns ,ends down can decreased by up to 50% in the warp and by up to 30%in the weft. Efficiency is consequently increased by 2 to 3%, production is increased and weaving costs are reduced. In practice, the average endsdown rate is reduced by 33% per 100,000 weft insertions of compact Yarns on rapier weaving machines and by 45% on air-jet weaving machines. Instead of a weft insertion of 500 -600 m/min with conventional ring yarn,700-800 m/min is possible with compact Yarns on air-jet weaving machines.

296

Textile Technology

Due to reduced Yarn hairiness,singeing can sometimes be dispensed with,or it can be carried out at a higher cloth advance speed.As a result, production costs are considerably reduced. Fibres upto 7% can be saved because singing can be avoided Dyeing and Printing Improved structure of compact Yarns and their reduced twist favours the absorption of colour pigments and chemical finishing agents.5aving of dyestuff is possible. Owing to the improved yarn strength, compact Yarns are well suited for non-iron treatment of woven fabrics. In the course of such treatment, the strength of fabrics made from conventional ring yarns can decrease by up to 25%,with frequent problems in the manufacture of clothes. compcat Yarns make up for this loss in strength.

Knitting Compact Yarns with their increased yarn strength and reduced formation of fluff permit to achieve higher machine efficiency and therefore production on knitting machines at a reduced ends-down rate,less interruptions and less fabric faults. Production costs therefore decrease. The enormously low hairiness of compact Yarns often permits to dispense with usual waxing. Considerable cost saving is achieved because of this. In knitting fibre abrasion reduced by 40% due to low hairiness. Fewer defects/ yarn breaks and better quality. Less contamination on all machines by foreign fibres. Less wear of needles, guide elements and sinkers due to less dust in the compact Yarn . Low hairiness has

Spinning Geometry

297

positive impact on loop structure. L Low pilling values get more and more important. In many cases single compact Yar.ns substitute conventional ply yarns. Waxing can be reduced or completely dispensed with. Compact Yarns are much more suitable for warp knitting than conventional ring yarns,because of their higher work capacity and lower hairiness. They are predestined to bear the high load due to numerous deflecting points with high friction in the warp knitting machine. •

Due to better embedding of fibres (including short ones)in compact Yarn,approx.6%fewer combing noils are possible.

•

Cheaper carded qualities instead of combed qualities can be spun with the Compact Spinning ystem.

•

In many cases single EliTe ® Yarns can substitute conventional ply yarns

•

new qualities can be developed, opening up a new creative scope for products

Hairiness Testing of Yarns Hairiness of yarns has been discussed for many years,but it always remained a fuzzy subject. With the advent of compact yarns and their low hairiness compared to conventional yarns, the issue of measuring hairiness and the proper interpretation of the values has become important again.Generally speaking,long hairs are undesirable, while short hairs are desirable (see picture ). The picture shown below just give a visual impression of undesirable and desirable hairiness at the edge of a cops.

298

Textile Technology

There are two major manufacturers of hairiness testing equipment on the market,and both have their advantages and disadvantages. Some detail is given below.

USTER USTER is the leading manufacturer of textile testing equipment. The USTER hairiness H is defined as follows H =total length (measured in centimeters) of all the hairs within one centimeter of yarn . (The hairiness value given by the tester at the end of the test is the average of all these values measured, that is,if 400 m have been measured,it is the average of 40,000 individual values) . The hairiness H is an average value,giving no indication of the distribution of the length of the hairs.

ZWEIGLE Zweigle is a somewhat less well known manufacturer of yarn testing equipment. Unlike USTER,the Zweigle does not give averages. The number of hairs of different lengths are counted separately, and these values are displayed on the equipment. In addition, the S3 value is given,which is defined as follows: SC3 =Sum (number of hairs 3 mm and longer) In the above example,the yarns would have different S3 values: S3yarn 1 =2 .

299

Spinning Geometry

S3yam 2 =4. A clear indication that yam 2 is "more hairy "than yam 1. The CV value of hairiness is given a histogram (graphical representation of the distribution of the hairiness) is given. The USTER H value only gives an average,which is of limited use when analyzing the hairiness of the yam. The Zweigle testing equipment gives the complete distributionof the different lengths of the hairs. The S3 value distinguishes between long and short hairiness, which is more informative than the H value.

RULES FOR OPERATION OF ELITE SPINNING MACHINES

RING

1. Elite Q Spinning Machines produce yam of supreme quality and come up to the expectations. Installation of the machine in the spinning mill EliTe Q Spinning Machines have a considerable air flow rate -a machine with .1008 spindles sucks in about 60 cubic meter of air per minute,i.e. it has the effect of a vacuum cleaner. The ambient air is sucked into machine and most of the fly and dirt contained in it is deposited on the EliTe Q Machine. Although EliTe Spinning Machines generate considerably less fly than standard ring spinning machines, they are soon covered with dust and fly if they are installed in the same room as conventional spinning machines. The fly has a negative effect on the yam in the condensing zone and the smooth running of the lattice apron. As a result,the yam is of substandard quality.

Rulel: EliTe Q Spinning Machines must be separated from conventional spinning machines.

300

Textile Technology

Spinning room conditions: The fibres in the condensing zone are exposed to the room conditions without any protection. Our recommendations on the room conditions suitable for processing cotton and manmade fibres should be followed, therefore. If the air humidity is too high, there will be a higher tendency towards roller laps. If the air is too dry,t here will be more fly. If the room temperature is too high, there will be higher friction values and premature wear. Rule 2:maximum room temperature:33.C humidity should be max ... ,5 g water/kg air for cotton min.9,O g water /kg air for cotton max .. O,O g water/kg air for synthetics min.9,O g water/kg air for synthetics 3.Position of the Eli Top in relation to the front bottom roller of the drafting system: If the setting is correct, the top edge of the suction slot in the Eli Tube is precisely set at the nip line of the delivery top roller. If the nip line cuts the slot, condensation is impaired. The hairiness of the yarn increases and the tearing strength is reduced. If the nip line is behind the slot, part of the spinning torsion may get into the condensation zone, resulting in an increased ends-down rate and damaged lattice aprons. Rule 3:The front top roller is precisely 3.5 mm offset towards the operator in relation to the front bottom roller of the drafting system. 4.Traverse mechanism: The roving must run over the slot in such a way, that, from the operator's view, the fibres move from the top right to the bottom left. If the fibres run over the slot top from the L.H. side,they make an S-shaped movement causing a certain unsteadiness in the condensing zone. This has a negative effect on the yarn values.

Spinning Geometry

301

Rule 4:The traverse mechanism for the sliver should be adjusted in such a way that the traverse motion at the front of the drafting system does not exceed 4 mm,and that the l.h.limit position of the sliver is level with the L.H .. edge of the top of the slot. 5.Cleaning the Eli Tubes and lattice aprons :Eli Tubes and lattice aprons are the most important components of the EliTe Q Condensing System. Careful maintenance is an important prerequisite for optimum yarn values. In the centre area, where the suction is active, a permanent air flow keeps the lattice aprons clean. To the left and right of this area, the lattice apron can be clogged by fine dust. With the time, this results in a considerable increase of the friction between the lattice aprons and the EliTube. If this friction is too high, erratic running of the lattice apron and substandard yarn quality is the result. Therefore,lattice aprons and Eli Tubes should be removed from the machine from time to time and cleaned. This can be done when the machine is running. The time needed per box length is 5 min. The expenditure of time necessary for changing the EliTubes with lattice aprons is about 90 minutes for a machine with .1008 spindles, which corresponds to a loss of production of 90' minutes. For yarn count Ne 40, the production loss involved is less than 370 g. The cleaning frequency varies depending on the portion of fine dust of the cotton. As an average value, 500 operating hours may be taken into account. The aprons are cleaned in a washing machine or in an ultrasonic cleanint; device. The EliTubes are cleaned using a damp piece of cloth. Damaged lattice aprons must be replaced. On EliTubes with considerable traces of wear, the inserts must be replaced.

302

Textile Technology

Rule 5:Lattice aprons and Eli Tubes must be cleaned from time to time. 6.Measures to be taken in the case of laps at the front top roller Laps may occur in the case of unsuitable room conditions or damaged or inappropriately buffed cuts, or if the fibre material used is prone to the formation of laps. Large laps may block the delivery and front rollers and damage the cot of the blocked roller. If spinning is continued with damaged cots,periodic yarn faults will be the result. Consequently, a blocked Eli Top must be replaced by a new Eli Top and repaired in the service room. For this purpose,all operators should carry a spare Eli Top with them. Rule 6:EliTops with blocked top rollers must be replaced by new top rollers. 7.Buffing the EliTe Q Top Rollers: The cots of the EliTe Q Top Rollers are subject to wear and should be buffed from time to time.The tension draft in the condensing zone -6 %as a general rule depends on the difference in diameter between the front top roller and the delivery top roller. Changed tension drafts may result in changed yarn parameters. Rule 7:Make sure that the difference in diameter of the front top roller and the delivery roller corresponds precisely to the desired tension draft. 8.Checking the partial vacuum As a general rule,continuous control of the vacuum pressure is not necessary. When the whole machine is cleaned, we recommend, however,to remove also the connecting hoses between the suction tubes and the fans and to clean them.

Spinning Geometry

303

Rule 8:Clean the connecting hoses with regular frequency. 9.Maintenance of the fans: Fans may be clogged after a time/which has a negative effect on the suction. Rule 9:The fans should be removed from the machine and cleaned once a year. lO.spinning speed: In the case of EliTe Q Spinning Machines, return on investment is not based on higher production, but on the production of yarn of supreme quality. The Suessen recommendations concerning traveller speeds and running-in speeds for rings and travellers should be followed, therefore. Not the ultimate increase in speed, but the yarn quality leads to success. Rule lO:Yarn quality is more important than quantity.

"This page is Intentionally Left Blank"