Adhesion Science

ADHESION SCIENCE

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ADHESION SCIENCE JOHN COMYN Department of Chemistry, De Montfort University and Institute of Polymer Technology and Materials Engineering, Loughborough University

C HEMI ST RY Information Services

ISBN 0-85404-543-0 A catalogue record for this book is available from the British Library

0The Royal Society of Chemistry 1997 All rights reserved. Apart from any fair dealiiig for the purposes of research or private study, or criticism or review as permitted under the terms of the U K Copyright, Designs and Patents Act, 1988, this publication may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of The Royal Society of Chemistry, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the U K , or in accordance with the terms of the licences issued by the appropriate Reproduction Rights Organization outside the U K . Enquiries concerning reproduction outside the terms stated here should be sent to The Royal Society of Chemistry at the address printed on this page. Published by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK Typeset by Vision Typesetting, Manchester Printed by Athenaeum Press Ltd, Gateshead, Tyne and Wear, UK

Preface

The use of adhesives is widespread and growing, and there are few modern artefacts, from the simple cereal box to the magnificent Boeing 747, that are without this means ofjoining. This book sets out to explain the principles which underlie adhesive bonding, and other adhesion technologies such as sealing, printing and painting. In about 25 years of researching adhesion, I have had to bear it in mind that engineers might need to understand my writings. I like engineers and have a high regard for the intellectual weight of their subject, but here I write unashamedly for chemists. The structure of this book is that some basic polymer chemistry and theories of adhesion are dealt with in Chapter 1, and Chapter 2 basically describes the technology of surface treatment. The next four chapters describe the chemistry of adhesives and adhesion promoters, followed by a chapter on the rapidly developing field of surface analysis. Physical chemistry comes to the fore in Chapter 8 with surface thermodynamics. Engineering principles appear in Chapter 9, showing how the success or failure of adhesives chemistry can be assessed by measuring joint strengths. The major limitation on the use of adhesives in engineering structures is water, which is the main concern in Chapter 10. As is appropriate for an introductory book, it is not fully referenced but has a recommended reading list. One disadvantage of this is that authors, many of whom are my friends and colleagues, are not directly acknowledged. However, any reader wishing to enquire further via the bibliography will soon find references to the original papers. No scientific book is ever written without the help of others, and in this respect I thank Dr. D. M. Brewis of Loughborough University and Mr. T. P. A. Comyn of Leeds University for reading and commenting on the manuscript. I am most grateful to CSMA Ltd. for providing the XPS spectra of brass and glass in Chapter 7. V

vi

Preface

Adhesion is no different from other areas of science in that the ultimate aim is to formulate laws. So far we have written the 1st law of adhesive bonding, which states that ‘If all else fails use bloody great nails’.

John Comyn

Contents

Chapter 1

Introduction to Adhesion and Adhesives 2 3

Basic Properties Basic Chemistry Theories of Adhesion Polymerization Glass Transition Temperature Viscoelastic Properties

4

11 14 16

Chapter 2 Surface Treatment for Adhesion, and for Abhesion

18

19 19 19 19 20 22 23 24 24 24 24

Abrasive Methods Use of Solvents Flame and Corona-discharge Etching of PTFE Etching of Metals Anodizing of Metals Wood Glass Concrete Composites Non-stick (Abhesion) Chapter 3

Primers and Coupling Agents

26

Primers Silane Coupling Agents

26 28 vii

...

Contents

Vlll

Titanates and Zirconates Coupling Agents for Wood

36 38

Chapter 4

Chemistry of Adhesives which Harden by Chemical Reaction Epoxide Adhesives Phenolic Adhesives for Metals Formaldehyde Condensate Adhesives for Wood Acrylic Adhesives Anaerobic Adhesives Cyanoacrylates Rubber Toughening of Structural Adhesives Silicones Polyurethanes Pol ysulfides High-temperature Adhesives Chapter 5 Chemistry of Adhesives which Harden without Chemical Reaction

Adhesives which Harden by Loss of Solvent Adhesives which Harden by Loss of Water Adhesives which Harden by Cooling

40 40 43 44 45 47 47 48 49 50 51 52

54

55 58 64

Chapter 6

Pressure-sensitive Adhesives

71

Chapter 7 Surface Analysis

73

X- Ray Photoelectron Spectroscopy Secondary-ion Mass Spectrometry Surface Infrared Spectroscopy Scanning Probe Microscopy

73 82 89 94

Chapter 8

Contact Angles in the Study of Adhesion Surface Tensions of Liquids The Liquid-Liquid Interface The Solid-Liquid Interface Thermodynamic Predictions of Joint Stability Practical Applications

98 99 99 102 109 113

Contents

ix

Chapter 9 Strength of Adhesive Joints

114

Single Lap Joints Peel Testing Boeing Wedge Test Tack Reporting the Results

114 120 122 122 125

Chapter I0

Adhesive Joints and the Environment

126

Antioxidants UV-stabilizers Water and Adhesives Water and Joints

126 128 129 134

Recommended Reading

144

Subject Index

147

Chapter 1

Introduction to Adhesion and Adhesives

Glues have been around for a long time; the ancient Egyptians used them in veneering the treasures of Tutankhamun and the ancient Greek word for glue is Kohha, from which we get colloid. In all centuries up to and including the 19th, glues originated from plants and animals; during the 20th century, however, synthetic chemicals have largely taken over, and the more respectable name of adhesive has been introduced. Animal glues were mostly based on mammalian collagen, which is the main protein of skin, bone and sinew, and the plant kingdom provided starches and dextrins from corn, wheat, potatoes and rice. Nowadays adhesives are used in all types of manufacture, and in many cases have displaced other means of joining. A range of adhesives (hot melt, vegetable glues and emulsions) are used in making cardboard boxes, with rarely a staple to be seen. Apart from expensive handmade shoes, footwear is now adhesively bonded using hot melt adhesives for the basic construction, natural rubber latex for linings, and solvent based polyurethanes or polychloroprenes for sole attachment. Bookbinding is by hot melt adhesives. Adhesive bonding is used increasingly in the construction of aircraft. Structural bonding began with the World War I1 De Haviland Mosquito, which was made of plywood. Modern civil aircraft are basically made of aluminium alloy, and rubber modified epoxide adhesives are increasingly used. Rubber-to-metal bonds are used for engine, transmission and exhaust mountings in automobiles and in railway bogie suspensions. Mass produced car bodies are made of spot-welded mild steel; weight and fuel consumption can be reduced with aluminium bodies, which are more difficult to spot-weld. The large-scale bonding of car bodies is a prize that 1

Chapter I

2

awaits the adhesives industry. A recent achievement was the bonding of steel rails in the new Manchester tramway. Human beings can be repaired by adhesives. This includes the use of UV-curing cements in dentistry and acrylic bond cements in orthopaedic surgery. It has been said that cyanoacrylate adhesives were used for short term repairs during the Vietnam War. Adhesives are not the only materials that must stick or adhere. Adhesion is essential for printing inks, sealants, paints and other surface coatings, and at interfaces in composite materials such as steel or textile fibres in rubber tyres and glass- or carbon-fibres in plastics. Mother nature uses adhesion rather than mechanical fasteners (nuts and bolts, nails, staples, etc.) in constructing plants and animals, and some animals are masters at the exploitation of adhesion. Here I am thinking of barnacles sticking to anything that floats in the sea and the remarkable ability of many insects to walk on ceilings. A disadvantage of adhesives as a means of joining is that they are generally weakened by water and its vapour. Also, their service temperature ranges are less than for metal fasteners (nuts, bolts, welds, staples, etc.), being limited by their glass transition temperature and chemical degradation. Advantages include their ability to join dissimilar materials and thin sheet materials, the spreading of load over a wider area, the aesthetic and aerodynamic exteriors of joints, and application by machine or robot. BASIC PROPERTIES What is an adhesive and what are its basic properties? A definition is a material which when applied t o the surfaces of materials can join them together and resist separation. The terms adherend and substrate are used for a body or material to be bonded by an adhesive. Other basic terms are shelf-life, for the time an adhesive can be stored before use, and pot-life, the maximum time between final mixing and application. Basically an adhesive must do two things: (i) It must wet the surfaces, that is it must spread and make a contact angle approaching zero, as is illustrated in Figure 1.1. Intimate contact is required between the molecules of the adhesive and the atoms and molecules in the surface. When applied the adhesive will be a liquid of relatively low viscosity. (ii) The adhesive must then harden to a cohesively strong solid. This can be by chemical reaction, loss of solvent or water, or by cooling in the case of hot melt adhesives. There is an exception to this, and

Introduction to Adhesion and Adhesives

3

Figure 1.1 Top: liquid droplets making n high and low contact angle on n j l a t , solid surfiice. Centre: high contact angle leading to 110 spreadiizg on a rough surface. Bottom: wetting on a rough surfiice.

that is pressure-sensitive adhesives which remain permanently sticky. These are the adhesives used in sticky tapes and labels.

BASIC CHEMISTRY All adhesives either contain polymers, or polymers are formed within the adhesive bond. Polymers give adhesives cohesive strength, and can be thought of as strings of beads (identical chemical units joined by single covalent bonds), which may be either linear, branched or crosslinked as illustrated in Figure 1.2. Linear and branched polymers have similar properties and it is not easy to distinguish them, and they will flow at higher temperatures and dissolve in suitable solvents. These latter properties are essential in hot melt, and solvent-based adhesives, respectively. Crosslinked polymers will not flow when heated, and may swell, but not dissolve, in solvents. All structural adhesives are crosslinked because this eliminates creep (deformation under constant load). Automotive tyres are crosslinked natural or synthetic rubber, and if they crept they would permanently deform during parking, and a rough ride would follow.

4

Chapter 1

b

Figure 1.2 Linear (top), branched (nziddle) and crosslinked (bottom) polymers.

Many adhesives contain additives that are not polymers are these include stabilizers against degradation by oxygen and UV, plasticizers which increase flexibility and lower the glass transition temperature, and powdered mineral fillers, which may reduce shrinkage on hardening, lower cost, modify flow properties before hardening and modify final mechanical properties. Other possible additives are tackifiers and silane coupling agents.

THEORIES OF ADHESION There are six theories of adhesion; physical adsorption, chemical bonding, diffusion, electrostatic, mechanical interlocking and weak boundary layer theories. As all adhesive bonds involve molecules in intimate contact, physical adsorption must always contribute.

Introduction to Adhesion and Adhesives

5

Physical Adsorption Theory Physical adsorption involves van der Waals forces across the interface. These involve attractions between permanent dipoles and induced dipoles, and are of three types. E,, is the potential energy, in a vacuum, of a pair of permanent dipoles separated by distance r at their centres and is given by equation 1.1, where pl and p2are the dipole moments, E~ is the permittivity of a vacuum, k is Boltzmann's constant and T the absolute temperature.

If a non-polar molecule is close to a dipole, then the latter will induce a dipole (pi) in the former. The induced-dipole moment is given by equation 1.2, where a is the polarizibility of the non-polar molecule and E is the electric field.

The potential energy for such an interaction is given by equation 1.3, where p 1 is the moment of the permanent dipole.

Instantaneous dipoles exist in non-polar molecules because of the fluctuating distribution of electrons. These lead to attractive forces between molecules, without which non-polar gases such as helium and argon would not be able to liquefy. The potential energy of a pair of molecules is given by equation 1.4, where a, and a, are their polarizabilities and I , and I , are their ionization potentials. Such forces have the name of dispersion forces.

The results of some calculations from the above equations are shown in Figures 1.3-1.5, where the molecules are in contact at the lowest points of the curves, i.e. r = yo. Figure 1.3 is for a pair of water molecules at 298 K; the dipole moment of water is 1.85 D (1 Debye = 3.336 x 10- 30 Cm) and 4m0 = 1.1126 x 10-'oJ-'C2m-1. The radius of a water molecule,

6

Chapter 1 rlnm

Figure 1.3 Potential energy at 298 K for dipolar attraction between two water molecules. The moiecules m e in coritact at the point 0 . tinm

0

-

7

I

E"

7

-50

LU"

-1 0 0

Figure 1.4 Potential eiteryy f o r dipole-induced dipole attractioii between water and rnethnrie molecules. The molecules are in contact at the point .

calculated from its molar mass and density is 0.19 nm. When two water molecules are in contact, E,, is - 1.12kJ mol- '. Figure 1.4 is for the interaction of water with methane (a= 2.60 x 10-30m3). The radius of the methane molecule is about 0.24 nm, and when the two molecules are in contact E,, is - 85 J mol- I , which is very much less than for a pair of water molecules. The first ionization potential of methane is 1133 kJ mol- I. When in contact, Eii is 909 J mol- (Figure 1.5). The potential energies of all these interactions are inversely propor-

7

Introduction to Adhesion and Adhesives rlnm 0

0.5

1.0

0

500

1000

Figure 1.5 Poteiztinl eriergy for induced-dipole nttractiorz between two rnethnne wolecules. The rizolecirles are i n coritact nt the point a.

tional to the 6th power of the distance of separation, meaning that the values of - E,,, - E,, and - Eii fall off rapidly with distance. Doubling the distance reduces values of - E,,, - E,, and - E,, to &th. Figures 1.3-1.5 show that the forces are only effective at less than two diameters, which means that adhesion forces will only be felt by the molecules that are actually in the topmost surface layers. The measurement of contact angles, which is described in Chapter 8, is a means of investigating adhesion by physical adsorption. These are the weakest forces that contribute to adhesive bonds, but are quite sufficient to make strong joints.

Chemical Bonding Theory The chemical bonding theory of adhesion invokes the formation of covalent, ionic or hydrogen bonds across the interface. There is some evidence that covalent bonds are formed with silane coupling agents (see Chapter 3), and it is possible that adhesives containing isocyanate groups react with active hydrogen atoms, such as hydroxyl groups, if wood or paper are the substrates. In these two examples, Si-0 bonds (strength 369 kJ mol- ') and C-0 bonds (351 kJ mol - I ) would be formed. Another possibility is the reaction of an epoxide adhesive with a surface containing amine groups (see Chapter 4) to give C-N bonds (291 kJ mol- '). The potential energy of two ions of charge zle and z,e, separated by distance r is given by equation 1.5. E + - =-

z1z2eL

4 1 1 ~ ~ ~ ~ ~

8

Chapter 1

Here E , is the relative permittivity of the medium, which is 1 in the case of a vacuum or dry air. Taking the following values of ionic radii (Na' = 0.95, A13+ = 0.50, Ti4+ = 0.68, 02-= 1.40 and C1- = 1.81 nm) strengths of ionic attractions are NaCl503, A13+02- 4290 and T i 4 + 0 25340 kJ mol- The two last energies are very high and may contribute to adhesion between metals and epoxide adhesives, and can also account for the significant contribution that carboxylic acid groups in adhesives make to metal-adhesion. A major problem with all adhesive joints is their sensitivity to water, and possible explanations for this are its high permittivity, which by equation 1.5 would give a low value of E , -,and a high surface tension. These issues are considered, respectively, in Chapters 10 and 8. Hydrogen bonds probably contribute to the attachment of postage stamps to envelopes where the adhesive (polyvinyl alcohol) and paper (cellulosefibres) both contain -OH groups. Wood is also rich in cellulose and the reactive adhesives based on formaldehyde contain hydroxyl or amine groups capable of participating in hydrogen bonds. The strengths of hydrogen bonds are mostly in the range 8-42 kJ mol- ',with those in water being at the top of this range. Hydrogen bonds involving fluorine can be stronger than this, and the strongest of all is F - * . - H-F (243 21 kJ mol - I). The strengths of Lewis acids and bases in poorly solvating solvents (usually hexane, cyclohexane or tetrachloromethane) can be obtained from their heats of reaction (-AH), which are related to EA and CA, which are empirical parameters for the acid, and EB and C,, the corresponding values for the base, by equation 1.6.

'.

E , and E, are considered to be the susceptibilities of the acid and base to undergo electrostatic interactions, and CAand C, are their susceptibilities to form covalent bonds. The heats of reaction can be measured by direct calorimetry or from shifts in IR spectra. An example of the latter is the shift in the O H stretched frequency of phenols (Av) when they react with amines in tetrachloromethane or tetrachloroethene, which is given by equation 1.7. - AH(kca1mol - ') = 0.0103 Av(cm - ')

+ 3.08

(1.7)

Some values are given in Table 1.1. They are based on a large number of measurements of -AH, with iodine EA = 1.00 and CA = 1.00 as the reference compound, in the old units of (kcal mol - ' ) l I 2 .

9

Introduction to Adhesion and Adhesives

Table 1.1 Values of E and C for some Lewis acids and bases. Compound

No. of measurements

cA

Iodine Phenol Boron trifluoride Pyridine Dimethyl formamide Dimethyl sulfoxide Benzene Ethyl acetate

39 34 5 21 4 14 5 14

1.oo 0.442 3.08

‘B

EA

EB

1.oo 4.33 7.96 6.40 2.48 2.85 0.707 1.74

1.17 1.23 1.34 0.486 0.975

Diffusion Theory The difusion theory takes the view that polymers in contact may interdiffuse, so that the initial boundary is eventually removed (see Figure 1.6). Such interdiffusion will occur only if the polymer chains are mobile (i.e.the temperature must be above the glass transition temperatures) and compatible. As most polymers, including those with very similar chemical structures such as polyethylene and polypropylene are incompatible, the theory is generally only applicable in bonding like rubbery polymers, as might occur when surfaces coated with contact adhesives are pressed together, and in the solvent-welding of thermoplastics. An example of the latter is to swell two polystyrene surfaces with butanone and then press them together. The solvent has the effect of lowering the glass transition temperature below ambient while interdiffusion takes place; it later evaporates. This is the mechanism of adhesion in making plastic model kits. The kits are made of polystyrene and the adhesive is a solution of polystyrene in an organic solvent, the main purpose of the polymer being to thicken the adhesive. There are a small number of polymer pairs made compatible by specific interactions. One pair is poly(methy1 methacrylate) and poly(viny1 chloride), which permits the possibility of interdiffusion when structural acrylic adhesives are used to bond PVC. Electrostatic Theory

The electrostatic theory originated in the proposal that if two metals are placed in contact, electrons will be transferred from one to the other so forming an electrical double layer, which gives a force of attraction. As polymers are insulators, it seems difficult to apply this theory to adhesives.

10

Chapter 1

Figure 1.6 Difusion theory of adhesion.

Mechanical Interlocking If a substrate has an irregular surface, then the adhesive may enter the irregularities prior to hardening. This simple idea gives the mechanical interlocking theory, which contributes to adhesive bonds with porous materials such as wood and textiles. An example is the use of iron-on patches for clothing. The patches contain a hot melt adhesive that, when molten, invades the textile material.

Weak Boundary Layer The weak boundary layer theory proposes that clean surfaces can give strong bonds to adhesives, but some contaminants such as rust and oils or greases give a layer which is cohesively weak. Not all contaminants will form weak boundary layers, as in some circumstances they will be dissolved by the adhesive. This is an area where acrylic structural adhesives are superior to epoxides because of their ability to dissolve oils and greases.

11

Introduction to Adhesion and Adhesives

POLYMERIZATION Polymerization is a very important matter in adhesion, in that adhesives which harden by chemical reaction d o so either by addition or condensation polymerization, and the polymers, which are a vital and frequently major component of other adhesives, are synthesized by the same processes. Condensation Polymerization In condensation polymerization, molecules react with one another because they contain chemical groups which are mutually reactive. There are at least two such groups per molecule, and if some trifunctional molecules are present then branching or crosslinking will result. Example of some functional groups used in adhesives are shown in Scheme 1.1. -OH + Alcohol

Arnine

-NCO Isocyanate

-

-NHCOOUrethane

Epoxide

2CH20H

-

Methylol

--CH20CHr Ether

+ H20 Water

Scheme 1.1

In some, but not all, cases a small molecule such as water is formed. In the case of phenolic adhesives, which cure by the last reaction in Scheme 1.1, at a temperature above 100°C, pressure must be applied to prevent the unwanted formation of steam-filled voids. Addition Polymerization Compounds containing double bonds or rings can be polymerized by addition polymerization, which at its most basic involves opening of rings or double bonds, and joining them together to make a chain. Addition polymerization is a chain reaction involving the sequential steps of initiation, propagation and termination.

12

Chapter 1

The basic steps in free-radical addition polymerization are shown in Scheme 1.2. Initiation involves the thermal or UV decomposition of an initiator (here benzoyl peroxide) and the radicals which are formed then attack a monomer molecule. In propagation many monomer molecules are now added to produce a long-chain radical. In termination two radicals react either by recombination or disproportionation. Radical lifetimes are typically a few seconds and during this time thousands of monomer units may be added.

o-coo*

oCOO-CH2-:H I

Initiation

+

CH2=CH I R

-

-CHz-CH

I

R

Propagation

wCH2-CH-CH-CH22 mCH2-CH

/

I

R

I

R

recombination

disproportionation Termination

Scheme 1.2

Alternatively, initiation can be by a redox reaction, of which a simple example is the reaction of hydrogen peroxide with iron(11)ions (Fenton's reagent; Scheme 1.3). In reactive acrylic adhesives, organic peroxides and transition metal salts that are soluble in organic compounds may be used, but the principle is the same. As direct mixing of the reactants can cause

Introduction to Adhesion and Adhesives Fe2+

+

H202

+

Fe3+

13

+

HO-

+

HO*

Scheme 1.3

explosions, at least one component must be diluted into the adhesive. Redox initiation can be employed in emulsion polymerization using water soluble peroxides such as ammonium or sodium persulfate. Polymerization and the hardening of adhesives can be directly initiated by ionizing radiation, including electron beams. In addition to free radicals, active centres can also be anions or cations, and the stability of the active centre is an important factor in controlling the mechanism of polymerization. The more stable the active centre, then the more likely is polymerization. It is well known that 'Superglue' (ethyl cyanoacrylate, see Chapter 4) polymerizes in seconds between finger ends. This is an anionic polymerization initiated by hydroxide ions, the anionic active centres being stabilized by the electron-withdrawing nitrile and ester groups. Copolymerization

If the reaction mixture in an addition polymerization contains just one monomer, then all the repeat units in the polymer will be the same, and the term homopolymer may be used to describe it. If two monomers A and B are used then both will be incorporated into the product, which is termed a Copolymer, and except in some special cases the arrangement of the A and B units in the copolymer will be random. Three monomers are used to make terpolymers. The use of extra reactants in condensation polymerization yields copolymers and terpolymers. The term copolymer is used not only in the specific sense just defined, but also as a general term for polymers with more than one repeat unit. Copolymers are useful in adhesives in that materials with intermediate properties can be obtained, and as copolymers have lower structural regularity than homopolymers, there is a tendency for them to be less crystalline and have lower melting points.

Crosslinking Most addition polymerizations occurring in adhesives involve monomers with one polymerizable C=C bond. The addition of a second monomer with two such C=C bonds leads to a crosslinked product (see for example acrylic adhesives in Chapter 4). Linear condensation polymers are formed from difunctional monomers, and the addition of a

Chapter 1

14

monomer with three or more functional groups causes crosslinking. Epoxides are a good example; they are described in Chapter 4. With both types of polymerization, viscosity rises steadily at first, but at the gel-point when there is on average one crosslink per molecule, the viscosity rises sharply. The whole polymer now becomes a single crosslinked molecule. Further crosslinking will occur beyond the gel-point.

GLASS TRANSITION TEMPERATURE The mechanical properties of polymers radically change at the glass transiton temperature (TJ;molecular motion is the underlying cause of the change. Below Tgthere is no translational or rotational motion of the atoms that make up the polymer backbone, but these motions are present above Tg.Below Tg,polymers are relatively hard, inflexible and brittle, whilst above it they are soft and flexible. The terms glassy, and rubbery or leathery are used to describe properties in the two temperature regions. Both glassy and rubbery polymers are used as adhesives, examples being the use of glassy adhesives for structural bonding in engineering and bone cements in surgery, and rubbery ones as pressure-sensitive adhesives and for bonding flexible substrates. It is unacceptable for an adhesive to pass through the glass transition during service. The theory most used to account for the glass transition is thefree volume theory. The basis is that a polymer consists of occupied volume plus free volume, with the latter increasing on thermal expansion. Once the fraction of free volume reaches a critical amount which is about 2.5%, the chain segments become mobile and the polymer enters the leathery state. The glass transition temperatures of some polymers used in adhesives are shown in Table 1.2. Polar groups in polymers increase intermolecular forces and thus reduce free volume and increase Tg.This is illustrated by the effect of replacing the C-CH, bonds in natural rubber with C-Cl bonds, as in polychloroprene, which is to increase Tgby 25 "C. In contrast, non-polar side groups tend to hold chains apart and lower T,, as is shown by series of acrylic polymers from PMMA to PBMA, for which the repeat units are shown in structure 1.1. The aromatic amine hardeners DAB and DDM for the diglycidyl ether of bisphenol-A (DGEBA) give higher glass transition temperatures than the aliphatic amines because the molecules are more rigid. DAPEE is a particularly flexible molecule giving the lowest T, of the examples shown. See structural formulae 1.2.

Introduction to Adhesion and Adhesives

15

Table 1.2 Glass transition temperatures of some polymers.

Epoxides based on DGEBA and the following hardeners (DAB) 1,3-Diaminobenzene 4,4'-Diaminodiphen ylme t hane (DD.M) Trieth ylenetetramine (TETA) Di-( 1-aminopropyl-3-ethoxy) ether (DAPEE)

161 119 99 67

Acrylic polymers Polymethacrylic acid Poly(methy1 methacrylate) Poly(ethy1 methacrylate) Poly(n-propyl methacrylate) Poly(n-butyl methacrylate)

228 105 65 35 20

(PMAA) (PMMA) (PEMA) (PPMA) (PBMA)

Rubbers Pol ychloroprene Polyisoprene (natural rubber) Poly(dimethy1 siloxane) (silicone rubber)

- 50

- 75

- 127

Polymethacrylic acid

Poly(methy1methacrylate)

Poly(ethy1methacrylate)

Poly-(n-pro$yl methacrylate)

y

-CH*-C-

3

Poly-(n-butyl methacrylate) I CO~CH~CH~CHZCH~

Chapter 1

16

DAB

DDM

H2NCH2CH2NHCH2CH2NHCH2CH2NH2

TETA H~NCH~CH~CH~OCH~CH~OCHZCH~OCH$~H~CH~NH~

DAPEE

Liquids have a relatively high free-volume, so the effect of mixing a liquid with a polymer is to lower Tg.The plasticization of poly(viny1 chloride) (PVC) is a well known example of this, where liquids such as phthalate diesters are used to convert rigid PVC, which is used for window frames and gutterings, into the material used for flexible hosepipes and footwear. Plasticization of adhesives can be by absorbed water or by additives such as tackifiers.

VISCOELASTIC PROPERTIES Polymers are described as viscoelastic in that they show a combination of the properties of a spring, and a dashpot filled with a viscous liquid (an automotive shock-absorber). A spring will deform instantaneously when loaded, and will recover fully and instantaneously when the load is removed. The deformation of a dashpot will increase with time, and it will not recover when the load is removed. A model which contains two springs and two dashpots, and which describes the qualitative behaviour of polymers and adhesives is shown in Figure 1.7. O n loading, spring B will instantly deform and dashpot A will begin to flow interminably. The response of spring C will be delayed by dashpot D. When the load is removed, spring B will recover immediately and fully; the recovery of spring C will be total but delayed by dashpot D. The deformation of dashpot D is irreversible. Clearly the properties of an adhesive that might be used in engineering will be dominated by the spring-like properties, and such adhesives are crosslinked to eliminate the viscous element. In contrast, pressuresensitive adhesives for tapes and for sticky solids such as Blu-Tak'R have

Introduction to Adhesion and Adhesives

I

17

unload

load

TIME

Figure 1.7 Four elernent model of viscoelnstic behaviour, arid its time-dependent strain.

a large viscous component, and the transverse stripes which form on clear tapes at the point where they are left to dwell on the roll is due to viscous flow.

Chapter 2

Surface Treatment for Adhesion, and for Abhesion

Inadequate or improper surface treatment is probably the main reason why adhesive bonds fail. Adhesives are not surface-selective, in that they will bond to most uncontaminated surfaces, the exceptions being surfaces which lack polar groups and are, hence, of low surface free-energy, such as the polyolefins, polytetrafluoroethylene (PTFE)and poly(dimethy1 siloxane). The theories of adhesion suggest strong bonding will be obtained by the introduction of dipoles to surfaces which increase van der Waals forces, the removal of weak boundary layers and roughening of surfaces. Particularly when bonding metals, the desired gain is not primarily to increase the strength of the newly made joint, but to increase resistance to water. Surface treatment of an adherend can cause the following: (i) remove contaminants or weak boundary layers; (ii) modify the surface chemistry by introducing new chemical groups; (iii) change the surface geometry. Contamination can take the following forms: (i) oils and greases on metals; (ii) weak or loose oxide layers on metals; (iii) mould-release agents such as silicones, fluorocarbons and waxes on polymers; (iv) additives and low molecular weight material on the surfaces of polymers, which have a tendency to expel foreign matter from the bulk to the surface. 18

Surface Treatment for Adhesion, and for Abhesion

19

ABRASIVE METHODS Abrasive methods include blasting with sand and other particulates in air, blasting with alumina in water and the use of abrasive papers and cloths. They are able to remove contamination and also roughen surfaces. A method for treating mild steel is to grit-blast and then apply a silane coupling agent (see Chapter 3).

USE OF SOLVENTS Surfaces may be cleaned by wiping with tissues soaked in solvent or by vapour degreasing. Lax wiping methods can result in the redistribution rather than the removal of contaminants. Solvents are very effective at removing oils and greases, but health and safety, and environmental considerations, weigh much against their use.

FLAME AND CORONA-DISCHARGE These methods have the common feature of exciting gas molecules, which then attack surfaces, causing chemical modification. The excited species can be ions, electrons or neutrals. In a gas-air flame the region just above the blue cone contains excited species, and polyethylene squeezy bottles are rotated in this region of a broad flame for about 1 s to make them wettable by printing inks. A corona-discharge in air at atmospheric pressure has a purple glow, and polyolefin film material used to make printed carrier-bags is treated in this way. Coronae are generated using high voltage, typically 20 kV, and high frequency (10-20 kHz). Both these treatments introduce new chemical groups such as -OH, >CO and -COOH, which are polar, to polyolefin surfaces. Coronadischarge can also be used to treat composites and metals for bonding; it is possible that with metals, oils and greases are volatilized. Newer methods include the use of plasmas and excimer lasers. Chromic acid etch baths can also be used to treat polyolefins, and similar chemical groups are introduced as in flame and corona-discharge, but the method is not used commercially because of the environmental issues and because dry methods are preferred.

ETCHING OF PTFE

PTFE is used as a coating in non-stick kitchen utensils, and surface treatment is essential if it is to be adhered to. If sodium metal is added to

20

Chapter 2

a solution of naphthalene in dry tetrahydrofuran (THF), sodium naphthalenide is formed, giving the solution a dark green colour. Only one electron is transferred from sodium to naphthalene, and the resulting naphthalenide is a radical-anion. When PTFE is immersed in such a solution, the electron is transferred to the polymer, resulting in defluorination and the formation of a carbonaceous surface. These reactions are shown in Scheme 2.1.

Sodium naphthalenide

Scheme 2.1

An alternative treatment is a solution of sodium metal in liquid ammonia. Such solutions are royal blue in colour, and contain solvated electrons, which will attack the PTFE surface. ETCHING OF METALS Although the use of abrasion or solvents can give adhesivejoints which are strong in dry conditions, this is not the case when joints are exposed to water or water vapour. This fact is illustrated by some joint strengths shown in Figure 2.1, after exposure to wet air at 97% relative humidity (r.h.). Joints prepared with an acid etch are the most durable and those with solvent degrease are least. Resistance to water is much improved if aluminium surfaces are treated, prior to bonding, by etching or anodizing. Aluminium and its alloys can be etched in chromic-sulfuric or phosphoric acids. A procedure recommended by Ciba Composites is shown below. Vapour degrease in halocarbon solvent and/or alkaline degrease, e.g. for 10 min in an aqueous solution of Turco T 5215 at 70 "C, followed by a spray rinse in clean water. Etch in chromium trioxide 250 g (or sodium dichromate 375 g), concentrated sulfuric acid 750 ml and water to 5 1, at 60-65 "C for 30 min.

21

Surface Treatment for Adhesion, and for Abhesion

0-

1

I

0

2 000

1000 Tirndh

Figure 2.1 Eflect of high humidity (97% relative humidity at 43 "C) on the strength of aluminium joints bonded with an epo.t.ide-polyamide adhesive. Surfnce treatments are Ochromic-sulfuric acid etch, 0alkalilie etch, solvent degrease, 0 phosphoric acid anodize. (After Butt and Cotter)

(c) Immerse in a tank of ambient water. (d) Spray rinse with cold water. (e) Dry in an air-circulating oven at no greater than 45°C. (f) Bonding should take place within 8 h. It is multi-stage, requires careful control and uses chemicals which are hazardous to both the person and the environment. It is only used for making expensive products such as aircraft and racing cars. Rinsing in tap water, rather than deionized or distilled water gives more durable joints. A recommended procedure for mild steel is as follows: Remove rust or millscale by brushing. Vapour degrease. Grit-blast. Etch in 4% solution of hydrofluoric acid at room temperature for 10 min. Rinse immediately in tap water. Immediately remove smut (carbon) in a bath composed of chromium trioxide 100g, concentrated sulfuric acid 57 ml and water to make 1 1 at 70 "C. This is complete within a few minutes. Rinse in running tap water. Dip in bath of propan-2-01, and then in a second bath of dry propan-2-01 before drying. This prevents rusting. Apply a primer at once.

22

Chapter 2

The following alkaline peroxide etch is recommended for titanium alloys: (a) Vapour degrease. (b) Wet blast with alumina. (c) Immerse for 20min at 65-70°C in sodium hydroxide 20g, hydrogen peroxide (100 vol) 22.5 ml and water to 11. (d) Wash in hot water for at least 10min. (e) Dry in warm air. (f) Preferably apply primer coat immediately. Etching procedures have been recommended for other metals used in engineering.

ANODIZING OF METALS Anodizing aluminium and its alloys gives the most water-durable adhesive joints, and is used by aircraft makers. Anodizing in chromic acid is favoured by European makers, and in phosphoric acid in the USA, but there are also differences in the alloys used. An advantage in using phosphoric acid is lower toxicity and easier disposal. Below is a phosphoric acid anodize procedure used in the USA: (a) Vapour degrease for 10min. (b) Immerse for 5 rnin at 40 "C in trisodium phosphate 25 g, Teepol detergent 5ml and water to make 11. (c) Anodize in phosphoric acid 60ml and water to make 11 at room temperature. Raise voltage to 1OV over 2min. and maintain for 5 min. (d) Spray rinse with cold water. (e) Dry in an air-circulating oven at no greater than 45 "C.

A chromic acid anodizing procedure recommended by Ciba Composites is: (a) As for chromic acid etching. (b) Anodize in chromium trioxide 500g and water to 101 at 40°C. Raise to 40 V over 10 rnin and maintain for 20 min. Raise to 50 V over 5 min and hold for 5 min. (c, d and e) As for chromic acid etching. (f) Bond within 4-6h. Etching and anodizing of aluminium creates a thick honeycomb

23

Surface Treatment for Adhesion, and for Abhesion

structure of aluminium oxide; the precise morphology varies with the treatment procedure. A honeycomb structure proposed for phosphoric acid anodizing is shown in Figure 2.2; there are some whiskerlike protrusions at the top of the honeycomb. Chromic acid anodizing can also be used with titanium alloys, but anodizing in sodium hydroxide solution is preferred because of the toxicity of chromium compounds.

WOOD A freshly cut wood surface is ideal for adhesive bonding because of its porosity, but care should be taken to remove sawdust. Old timber is best resawn.

T approx 100 nm

/ aluminium oxide

1

aluminium

Figure 2.2 Honeycomb structures acid.

oil

aluminium afier anodizing iri phosphoric

24

Chapter 2

The surface of glass can be readily bonded when clean and dry. Probably the largest commercial bonding of glass is in double glazing units, where the surface is cleaned with water and the sealant is most often a polysulfide. The use of silane coupling agents is recommended where there is exposure to water or humid air, as would be the case with double glazing.

CONCRETE With the introduction of 40 tonne lorries on UK roads, it has become necessary to stiffen many reinforced concrete bridges. This can be done by bonding external panels of steel or fibre reinforced composite. Concrete surfaces can be prepared for bonding by first removing loose material and exposing coarse aggregate, followed by cleaning and drying. The surface can be etched with 10-15% hydrochloric acid but only after thorough washing, so that the acid does not penetrate. Thorough rinsing with water or dilute alkali must follow. The surface must be dried so that its water content is preferably below 4%. It is considered that mechanical interlocking and physical adsorption on the high-energy surface contribute to adhesion.

COMPOSITES Modern composites mainly consist of glass or carbon fibres in an epoxide or polyester resin. The resins are applied as liquids that subsequently set, and which have adhesive properties. To prevent the resins bonding to the mould, surfaces of the latter are coated with a mould-release agent such as a silicone or fluorocarbon [see section on non-stick (abhesion)]. It is essential that these are removed from the composite before any bonding and solvents or abrasives may be used. An elegant alternative is the use of peel plies, which is a layer of material built into the face of a composite, which can be stripped away before bonding.

NON-STICK (ABHESION) With a few exceptions all materials can be bonded with adhesives. The exceptions are the polyolefins (polyethylene and polypropylene), silicones and some fluoropolymers. The features which these materials share is a low surface-energy (a topic discussed in Chapter 8); some values are given in Table 2.1. Polymers containing fluorine and silicone are prominent amongst

Surface Treatment for Adhesion, and for Abhesion

25

Table 2.1 Surface energies of non-stick materials. Polymer

Ys(mJ m - 2,

Pol yhexafluoroprop ylene Poly tetrafluoroe t h ylene Polydimet h ylsiloxane Polyet h ylene

12-17 19-24 20-24 3 3-3 6

non-stick materials, two well-known examples being the use of polytetrafluoroethylene (PTFE) in non-stick kitchen utensils, and backing papers for sticky labels, which are impregnated with poly(dimethy1 siloxane) (PDMS). Release agents in aerosol cans are based on either silicone oils or dispersions of PTFE in water. A coating of silicone oil will also bring non-stick properties because it is cohesively weak. Their use in aerosols means that release agents may drift beyond their intended target, and this is a common cause of failure for adhesive bonds. Fortunately, silicones are very sensitive to detection by secondary-ion mass spectrometry, and the high relative atomic sensitivity of F atomic core-level 1s makes it readily detectable by X-ray photoelectron spectroscopy; these matters are dealt with in Chapter 7. The C-F bond is quite polar and in fact some fluoropolymers do not have low surface-energies. However, dipoles are vectors, and in the case of PTFE their mutual opposition means that the molecules as a whole are non-polar. Silicone release papers are made by impregnation with poly(dimethy1 siloxane) with hydride and vinyl end groups, which co-react in the presence of about 5 ppm of a platinum or rhodium catalyst, as shown in Scheme 2.2. The reaction proceeds at room temperature but can be accelerated by heating. y

3

y

3

y

+

-rSi-O-SiH I I CH3 CH3

y

3

y

3

y

3

CH2=CH-Si-O--SiI I CH3 CH3

3

y

3

-Si-O-Si-CH2-CH2-Si-O-SiI I I CH3 CH3 CH3

Scheme 2.2

y

3

I

CH3

Chapter 3

Primers and Coupling Agents

PRIMERS Primers are liquids that may be applied to adherend surfaces prior to the adhesive, and reasons for their use include the following: A coating of primer applied to a freshly prepared surface serves to protect it until the bonding operation is carried out. Primers wet the surface more readily than the adhesive. This may be achieved by using, as the primer, the adhesive dissolved in a solution of much lower viscosity. Alternatively, it may be a solution of a different polymer, which after drying is easily wetted by the adhesive. It may serve to block a porous surface, thus preventing escape of the adhesive. With structural adhesive binds this is probably only important for wood and concrete. However, some penetration of the adhesive may be very desirable and viscosity can be adjusted to give optimum penetration. It can act as the vehicle for corrosion inhibitors, keeping them near to the surface where they are needed. The inhibitors which have been most commonly employed are strontium and zinc chromates, but these are being replaced because of their toxicity. The primer may be a coupling agent capable of forming chemical bonds both with the adherend surface and the adhesive. The adsorption of the primer to the substrate may be so strong that, instead of merely being physical adsorption, it has the nature of a chemical bond. This may not correspond to a chemical compound which can be isolated. Such adsorption is referred to as chemisorption to distinguish it from the reversible physical 26

Primers and Coupling Agents

27

adsorption. Primers based on chlorinated rubber seem to be chemisorbed to some oxide surfaces. Examples of some commercial primers for structural adhesives are as follows; it may be noted that they are made for specific adhesives, have recommended coating thicknesses, and generally require curing at elevated temperatures. Cyanamid BR 127. This is particularly recommended for use with FM373 toughened epoxy film adhesive. It is a modified epoxy-phenolic which can be applied by brush or spray to give a dry coating of thickness 2.5-5.0 pm, After drying in air for 30 min the primed surface is cured for 30min at 120 6 "C; it then has an indefinite shelf-life. BR 127 has corrosion inhibiting properties and protects the metal oxide layer from hydrolysis. Hysol EA 2989. This has the advantages of being water-based and not containing chromium, even though it is corrosion inhibiting. It is formulated for use with Hysol EA 9689 epoxide film adhesive. The recommended coating thickness is 5-10 pm and the primer is dried in air for at least 1 h, and then cured for 1 h at 175°C. 3M EC-2320. This is particularly recommended for AF-111, AF-126 and AF-126-2 film adhesives; it improves shear and peel strengths and environmental resistance. The primer is supplied as a solution in organic solvents and recommended coating thicknesses are 1.3-5.1 pm. It can be dried at temperatures up to 120°C and primed surfaces have a 90 day shelf-life. Some phosphonic acid compounds improve the durability, in wet conditions, of aluminium alloy bonded with epoxide adhesives. An example is nitrilotrismethylene phosphonic acid (NTMP), which is illustrated in Scheme 3.1. The metal is pretreated by chromic acid

OH

I I

-Al-

NTMP

0-Al-

I

I

I

0-Al- 0-Al-

NTMP adsorbed on aluminium oxide Scheme 3.1

28

Chapter 3

etching or phosphoric acid anodization. The inhibitors are applied from aqueous solution and act as hydration inhibitors by bonding to the surface by P-0-A1 bonds. At low concentration (about 1 ppm) only one phosphate group of the N T M P molecule is attached to the metal oxide, but all three are attached at 1OOOppm. A single molecular layer of NTMP gives the maximum improvement in durability. Because of its physical properties and ease of moulding, polypropylene is used to make car bumpers. The low surface-energy of the untreated polymer makes painting impossible, but a primer based on chlorinated polypropylene in organic solvents adheres to polypropylene with some interdiffusion. Chlorine atoms in the outer surface of the primer increase polarity and enhance paint adhesion. SILANE COUPLING AGENTS Silane coupling agents are generally considered to react chemically with both substrate and adhesive, so forming covalent bonds across the interface that are both strong and durable. They were first used to treat glass fibres before incorporation into liquid resins of polyester or epoxide to make fibreglass, where the interfacial areais large. Without them, water can diffuse along the glass-resin interface with catastrophic results.

Chemistry Silane coupling agents have the general structure R '-Si(OR2)3, where R' is a group that can react with the adhesive or liquid resin and R 2 is usually methyl or ethyl. They are normally applied to adherends from dilute solution (1-2% by volume) in water or ethanol-water and then left to drain and dry. It is important to use fresh solutions. The main advantage is not to increase the strength of newly made joints, but to improve durability in the presence of water or water vapour. Some commercially available silanes are given in structures 3.1-3.8. The R groups in APES, APMS and AAMS contain amines, which allow them to react with epoxides. G P M S will react with amine groups in epoxide adhesives. The carbon-carbon double bonds in MPMS of VMS can copolymerize with adhesives or resins, which harden by addition polymerization. Silanes react with the surface of glass by the reactions shown in Figure 3.1, where the example is APMS. The reactions are: (i) Hydrolysis of -Si(OCH,), to trisilanol -Si(OH),, which is fast. (ii) Condensation polymerization of trisilanol and surface -OH

Primers and Coupling Agents

29

NH2CH2CH2CH2Si(OCH2CH3)3 (3.1) 3-Aminopropyltriethoxysilane (APES)

NH2CH2CH2CH2Si(OCH3)3 (3.2) 3-Aminopropyltrimethoxysilane(APMS)

NH2CH2CH2NHCH2CH2CH2Si(OCH3)3

(3.3)N -(2-Aminoethyl)-aminopropyltrimethoxysilane(AAMS)

0

/ \

CH2-CH2CHOCH2CH2CH2SI(OCH3)3 (3.4)3-Glycidoxypropyltrimethoxysilane (GPMS)

O

O CH2CH2Si(OCH3)3

(3.5) (3,4-Epoxycyclohexyl)ethyltrimethoxysilane (ECMS)

CHP=CHCOOCH&H~CH~S~(OCH~)~ I

CH3 (3.6) 3-Methacrylpropyltrimethoxysilane(MPMS) CH2=CHSi(OCH3)3 (3.7) Vinyltrimethoxysilane (VMS) HSCH2CH2CH2Si(OCH3)3 (3.8) 3-Thiopropyltrimethoxysilane (TPMS)

groups to produce a polysiloxane network that is covalently bonded to the surface. This is a slower reaction. (iii) In the case of APMS an epoxide adhesive would then be applied, and amine and epoxide groups would combine together as described in Chapter 4. The result is a continuous chain of covalent bonds from the substrate to the adhesive. Factors other than covalent bonding that have been proposed to account for the effectiveness of silanes as coupling agents include the following:

(i) they improve surface wettability; (ii) the silane layer is deformable and can relieve internal stresses; (iii) interfacial bonds are broken by hydrolysis but many then reform, making the bond ductile and permitting stress relaxation. It is not easy to see how they account for improved water durability. Figure 3.1 is an oversimplification of the facts, in that silanes do not

30

Chapter 3

Glass

Figure 3.1 Reactioii of A P M S with the surface of glass.

produce ordered single layers on substrates. The actual layer is thicker than this (up to 100nm) and contains some adhesive. Nevertheless, thinner layers of silanes seem to give stronger and more durable adhesive bonds. This is illustrated by the data in Table 3.1 which show the force needed to remove glass fibres embedded to a length of 2 mm in an epoxy resin. The more dilute solutions of silanes give stronger bonds, presumably because the layer of silane is thinner. The aminosilanes are bases and the pH of the solution from which they are applied affects joint performance; p H 8 is the optimum for APMS. Glass and silanes are both compounds of silicon, and in using them together we have the inherently favourable situation of like chemical compounds together. However, silanes are generally effective in improving adhesion to metals, including aluminium, steel, cadmium, copper and zinc. Although the best results can be obtained in using silanes as primers, they can also be added to adhesives with some effect.

Performance

VMS as a comonomer in adhesives improves bond strengths to aluminium. The peel strength of low density polyethylene, which is non-polar, to the metal is 100N m - Copolymerization with butyl acrylate introduces polar groups and increases the peel strength to 700Nm-'. Copolymerization with a monomer which is a salt of a carboxylic acid raises peel strength to 1500 N m - by introducing ions.

Primers and Coupling Agents

31

Table 3.1 Eflect of silane coupling agents on the adhesion of glass fibres to an epoxide. Change (YO)in interfacial shear strength

Silane

AAMS

APES

I % solution

5% solution

62 17

- 14 - 20

However, when the adhesive is a terpolymer of ethylene and butyl acrylate with 0.2-0.3mol% VMS, peel strengths are in the range 1800-3000Nm-'. After immersion in water at 85 "C the VMS terpolymer gave peel strengths of 9000 N m - indicating that strong, hydrolysis-resistant covalent bonds are formed between aluminium and silane. Figure 3.2 shows the strengths of some bonds of glass bonded to lead alloy with an epoxide adhesive. Some of the glass specimens had been treated with APES. Thejoints had been exposed to warm, wet air (100% r.h. at 50 "C) and it can be seen that joint strength falls to zero without the silane, but falls a moderate amount and then tend to level out when APES is used. In fact, after 96 days exposure, most of the joints without the silane had fallen apart. The Churchill Memorial Screen at Dudley in the Midlands (UK) is a large structure with glass bonded to glass with a room-temperaturecuring epoxide adhesive, Bonds had started to fail after only months of outdoor exposure. Laboratory experiments confirmed the incipient instability of the system without a coupling agent, but also gave a confident basis for the prediction of decades of life with the silane coupling agent GPMS. Spectroscopic Studies

There are many papers in the literature in which spectroscopic techniques have been used to examine the interactions of silanes with silica, glass and metal surfaces. Figure 3.3 shows the strengths of some butt joints with cylindrical mild steel adherends, bonded with an epoxide adhesive, after immersion in water at 60°C. It shows that, in this instance, G P M S is a more effective coupling agent than APES, although it would be expected that both would react with epoxide adhesives. Indeed, one of the current problems with silanes is the difficulty in identifying the most effective. Examination of the steel surfaces by SIMS

32

Chapter 3 1500

1000

500

0 0

50

I00

time I days

Figure 3.2 Failure loads of joiiits of glass bonded to lead with mi eposide adkesice or1 esposure to air at 100% r.h. arid 50 "C. uritreated glass, Oglass treated with A P E S . Numbers iridicate the iiunzber of joints, out of 10, that fell apart during exposure. (After Comyn, Groves and Saville)

showed an ion of mass 100, which corresponds to FeOSi', only when GPMS was used as the primer. With stainless steel both FeOSi' and CrOSi' ions were detected. This indicates that covalent bonds are formed of the type metal-0-Si across the interfaces. A technique for building up adsorbed multilayers of silanes on glass, aluminium or silicon involves hexadec- 15-enyltrichlorosilane (HTS) applied from a mixture of organic solvents. In an absorbed monolayer of HTS the vinyl groups are converted into CH,CH,OH by using a solution of diborane in tetrahydrofuran followed by an aqueous solution of hydrogen peroxide; a further layer of HTS can then be added. This is shown in Scheme 3.2, where the number of - CH, - groups has been reduced for clarity. Contact angles for n-hexadecane and water on glass coated with up to four layers of HTS indicated a lowering of molecular order by the progressive accumulation of defects as the layers built up. Attenuated total reflection infrared (ATR-IR) spectra showed a band at 3200-3300cm-' due to C-OH and Si-OH, so showing that the condensation reactions are not complete. Reflectance IR spectra on

Primers and Coupling Agents

33

strength I MPa 60

40

20

0

0

500

1000

1500

time I h

Figure 3.3 Eflect of silane coupling ogeim on streiigtks of eposy-inild-steel butt joints, on immersion in water at 60°C. (After Gettings and Kinloch)

Scheme 3.2

34

Chapter 3

aluminium mirrors showed increasing imperfections in the organization of the methylene chains. Only vibrations perpendicular to the mirror plane can be detected, so if the HTS molecules are perfectly aligned, C-C but not C-H bonds should be detected. With a monolayer of HTS there is a weak peak near 2900cm- owing to the C-H stretching mode; with a second layer, the increasing imperfection of the paraffin chains is indicated by a fourfold increase in the intensity of the band at 2900 cm The compound 3-aminopropyldimethylethoxysilane (ADES) has only one alkoxy group, and it may therefore react with a substrate without the complication of transfacial Si-0-Si units being formed. Diffuse-reflectanceFTIR spectra of ADES on titanium dioxide powder are shown in Figure 3.4.The difference spectrum has a weak peak at 950cm-', which is due to Si-0-Ti groups. This assignment was

',

'.

1600

1200

800

c m"

Figure 3.4 FTZR spectra of(A) TiOz (B) TiO, treated with ADES and (C) diflerence spectrum. (After Naviroj, Koenig and Ishida)

Primers and Coupling Agents

35

confirmed by its presence when isotropyltri(isosteary1)titanate was reacted with silica powder. When alumina was used with ADES, a peak at 963cm-’ was seen and assigned to Si-0-A1 groups. Inelastic electron tunnelling spectroscopy (IETS) has been used to study some silanes on aluminium oxide. The technique records vibrational spectra of an absorbed monolayer. Silanes can be applied to the oxidized metal from solution or vapour, and devices are completed by evaporation of a top electrode, which is usually of lead because of its superconductivity. The device is cooled to the temperature of liquid helium (4.2 K) to minimize thermal broadening. Most electrons ( > 99?40) pass through the device elastically but a small number excite vibrational modes. It is these that are detected and displayed as a spectrum. Both IR and Raman modes can be observed; the selection rule for IET spectroscopy is one of orientation, in that bonds which are aligned perpendicular to the surface give the most intense peaks. The IET spectrum of triethoxysilane is shown in Figure 3.5. There is a strong and sharp peak at 2191 cm-’ due to Si-H vibrations that are perpendicular to the surface, so showing that the silane molecules are

r I

e L

iii

I

0

m

iii

(0

c

z 4000 em’’ 1

2000 I

1000 I

Figure 3.5 f ET specrrum of triethoxysilane on aluminium oxide. (After Brewis, Comyn, et 01.)

600 3

36

Chapter 3

adsorbed tripod-like by the three ethoxy groups on aluminium oxide. The Si-H bending mode at 880 cm- is also strong. The IET spectrum of GPMS shows no sign of the epoxide group at 940cm-', but C=C is present at 1595cm-' and C=O at 1646cm-'; these are absent in GPMS. Possible routes to these groups are shown in Schemes 3.3 and 3.4. This serves to show that the reactions of silanes at surfaces can be quite complicated.

'

OH Enol

Keto

Scheme 3.3

-OCHz-CH=CHz

-ocH CH3 -0

CH3

or -OCH=CH-CH3

+ HOCHCH20I

CH3

Scheme 3.4

TITANATES AND ZIRCONATES Some compounds of titanium and zirconium are also used as coupling agents, mainly for filler particles in polymers. They can be used to treat aluminium alloy for adhesive bonding. Like the silanes they react with surface hydroxyl groups (Scheme 3 . 9 , but there is no condensation polymerization to produce a polymer network at the interface. R20Ti(OR')3

+

surface-OH+ su~face-OTi(OR~)~ + R20H

Scheme 3.5

Primers and Coupling Agents

37

Examples of titanate coupling agents are shown in structures 3.9-3.1 1. The first contains amine groups and would be suitable for use with epoxide adhesives. Zirconate coupling agents have very similar structures to the titanates. (CH3)2CHOTi(OCH2CH2NHCH2CH2NH2)3

(3.9) Isopropoxytri(ethylaminoethy1amino)titanate (CH3)2CHOTi(O-PO-O-PO-OC8Hl

I

7)3

I

OH OC8Ht7 (3.10) Isopropoxytri(diocty1pyrophosphate)titanate (CH3)2CHOTi(OCO- C Z C H ~ ) ~ I CH3 (3.11) Isopropoxytri(methacry1oxy)titanate

The aqueous chemistry of zirconium is complex, and one aspect is that polymerization takes place when salt solutions are diluted. There is little information about the degree of polymerization. Structures of polymers formed from ammonium zirconium carbonate and zirconium acetate are shown in structures 3.12 and 3.13. Zirconium propionate is used as an adhesion promoter for printing inks on polyolefins that have been treated by corona-discharge, and it seems that it forms hydrogen bonds with the nitrocellulose in inks. The mode of attachment to polyolefins is by functional groups on the surface displacing ligands on the zirconium polymer. Surface COOH groups seem to be most likely to d o this and the reaction is shown in Scheme 3.6.

(3.12)

,OH,

Zirconium carbonate anion OH OH OH I ,OH, I ,OH. I /

(3.13)

,OH. ‘oH-fr\oH-yoH-fr, OAc OAc OAc --t

Zirconium acetate polymer

OH OH OH I ,OH, I ,OH, I fr\oH-qr\oH-f‘\ OAc

0

OAc

+ AcO-

Scheme 3.6

38

Chapter 3

Table 3.2 Force needed to remove steel cylinder from adhesives used with paper. Force (N) Adhesive

Untreated

Water

20 PPm Zirconium acetate

Pressure-sensitive acrylic E t h ylene-vin yl-ace ta te hot me1t

264 2 36

73 220

23

-~

--

~

~ _ _ _ _ _ _ _

~~

~~~

7 -

~~

~~~

Zirconium compounds can also reduce adhesion, and this is exploited during the reclamation of waste paper, where adhesives of the many kinds can be present, causing problems by adhering to the processing machinery and producing unsightly spots on the final product. The spots are known as ‘stickies’.The first of these problems can be much reduced by adding zirconium salts during the pulping process, and this is illustrated in Table 3.2, which gives the forces needed to remove a steel cylinder from some adhesives.

COUPLING AGENTS FOR WOOD The molecule shown as structure 3.14 is a coupling agent for wood to unsaturated polyester resins. The isocyanate reacts with hydroxyl groups on the surface of wood, and the carbon-carbon double bonds copolymerize with the curing resin.

(3.14) a,a-Dimethyl-3-isopropenylbenylisocyanate

An alternative coupling agent for wood and cellulose is shown in Scheme 3.7. It is made by reacting a polyamine hardener, such as is used to harden epoxide adhesives, with 2,4,6-trichloro- 1,3,5-triazine(cyanuric chloride). Treatment of the substrate with alkali produces some alkoxide groups, which substitute to the aromatic ring by nucleophilic substitution.

Primers and Coupling Agents

CI

39

I +

+ Alkali treated cellulose Cellulose-ONa+

Scheme 3.7

Chapter 4

Chemistry of Adhesives which Harden by Chemical Reaction

EPOXIDE ADHESIVES Epoxides are the best known and most widely used structural adhesives. They are also used as matrix-resins for fibre-reinforced composites and as surface coatings. There are only a few commercial epoxide resins, but they can be mixed with a wide range of hardeners, which include amines and acid anhydrides. An advantage is that no volatiles are formed on hardening (also called curing) and shrinkage is very low; that of Redux 330 is 3.7%. A disadvantage is that they can cause skin diseases. The most commonly used epoxide resin is commonly named the diglycidyl ether of bisphenol-A (DGEBA) and it is made by reacting the sodium salt of bisphenol-A with epichlorohydrin as shown in Scheme 4.1. Some further reaction involving the opening of epoxide rings also

Sodium salt of bisphenol-A

Epichlorohydrin

Diglycidyl ether of bisphenol-A

Scheme 4.1 40

41

Chemistry of Adhesives which Harden by Chemical Reaction

occurs, so that the final product has structure 4.1, where n is ca. 0.2. The pure compound is a solid but the commercial product is, more conveniently, a liquid. The structure of another commercial resin is shown in structure 4.2. r

1

L

-In (4.1)

Commercial epoxy resin based on DGEBA

(4.2) Tetraglycidyl diarninodiphenylmethane

Both aromatic and aliphatic amines are used as hardeners, and the stoichiometry is that one epoxide ring will react with one aminehydrogen atom in a condensation polymerization. The reaction of a primary amine group with an epoxide ring is shown in Scheme 4.2. +

'CH2-CHI

OH Scheme 4.2 Reaction of primary amine with 2 epoxide groups.

Some typical aliphatic amine curing agents are triethylenetetramine (TETA),which is six-functional(structure 4.3)and 3,9-bis(aminopropyl)2,4,8,10-tet raoxaspir o [5.51undecane which is four-functional (structure 4.4). The volatility of polyamine hardeners such as TETA can be a source of irritation, and this can be reduced by reacting them with some dicarboxylic acids, as shown in Scheme 4.3. Such compounds are often known as Versamids. Epoxide adhesives with aliphatic amines can be cured at room

42

Chapter 4

(4.3) Triethylenetetramine

(4.4)

3,9-Bis(aminopropyl)-2,4,8,1O-tetraoxaspiro[5.5]undecane

Linoleic acid

Dimer diacid

YCH2-C=CH(CH2).&H3 (CH2)5CH3

Versamid hardener

Scheme 4.3

temperature or the process can be accelerated b heating. Typical c re times are 14 h at room temperature or 3 h at 80 "C. Curing with aromatic amines requires elevated temperatures, typically 2 h at 150 "C, and the cured adhesives have higher glass transition temperatures and the joints tend to be more durable. Some aromatic amine hardeners are shown in structures 4.5 and 4.6. Curing with acid anhydride hardeners is also by condensation polymerization and requires elevated temperatures. Two examples are pyromellitic dianhydride (structure 4.7) and methyl nadic anhydride (structure 4.8) where the product groups are esters. If the parent acids were used, a disadvantage is that water would be produced.

Chemistry of Adhesives which Harden by Chemical Reaction

(4.5)

(4.6)

1 ,&Diaminobenzene

43

4,4’-Diaminodiphenyl sulfone

0

0 (4.7)

0 Pyromellitic dianhydride

0

0 (4.8)

Methyl nadic anhydride

(7-methylbicyclo[2.2.l]heptd-ene-2,3-dicarboxylic anhydride)

One-part adhesives can be made with hardeners, which require elevated temperatures. Such a hardener is dicyandiamide (H,N-C(=NHbNH-CN), which has the added advantage of being insoluble in DGEBA at room temperature, dissolving when the adhesive is heated. Such adhesives are often supplied in the form of a film, which is stored in a refrigerator, and often contains a textile fabric or carrier to assist in handling the adhesive and in controlling glue-like thickness. Rapid curing epoxide adhesives employ polythiol hardeners containing CH(OH)CH,SH groups, in which the hydroxyl groups activate the thiol groups by hydrogen bonding. Such adhesives cure within a few minutes at room temperature. All the hardeners mentioned so far react with resin by condensation polymerization. Addition polymerization by ring-opening epoxide groups is initiated by tertiary amines and some complexes of boron trifluoride. The most frequently used tertiary amines are shown in structures 4.9 and 4.10. PHENOLIC ADHESIVES FOR METALS When phenol is reacted with an excess of formaldehyde under basic conditions in aqueous solution, the product, which is known as a resole,

44

Chapter 4 (CH3)2NCH2 e c H OH 2 N ( c H 3 ) 2

OH

CH2N (CH3)2

(4.9) 2-(Dimethylaminomethyl)phenol

(4.10) 2,4,6-Tris(dimethylaminomethyl)phenol

is an oligomer containing phenols bridged by ether and methylene groups, and with methylol groups substituted on the benzene rings. This is shown in Scheme 4.4.

Resole Scheme 4.4

If used as adhesives they would be heated to 130-160°C in the joint, where further condensation of methylol groups takes place to give a crosslinked polymer (see Chapter 1). To avoid the formation of voids filled with steam, joints with phenolic adhesives have to be cured under pressure, usually between heated steel platens of a hydraulic press. Because they are brittle, other polymers are added to phenolics to toughen them. These include poly(viny1 formal), poly(viny1 butyral), epoxides and nitrile rubber.

FORMALDEHYDE CONDENSATE ADHESIVES FOR WOOD Some adhesives for wood are condensates of formaldehyde with phenol and resorcinol (1,3-dihydroxybenzene). Others are condensates with either a urea or melamine (structure 4.1 l), where reaction with formaldehyde results in the replacement of amine hydrogen atoms by methylol

(4.11)

Melamine (1,3,5-triarnino-s-triazine)

Chemistry of Adhesives which Harden by Chemical Reaction

45

groups, as shown in Scheme 4.5. Tetramethylolurea has not been isolated. /NH2

NHCH2OH

o=c\

+

CH20-

O=C\

NH2

NH2

Urea

o=c,

Methylol urea

,CH,OH N ‘CH20H

NHCH20H

o=c\ NHCH20H

NHCH20H Trimethylol urea

Dimethylol urea

Scheme 4.5

All these compounds undergo condensation polymerization uia methylol groups, to give crosslinked products. The reactions take place at ambient temperatures after the addition of a catalyst. The adhesives are water-based and water is produced on curing; it is removed by migration into the wood, making these adhesives only suitable for porous adherends. ACRYLIC ADHESIVES Structural adhesives containing acrylic monomers are cured by freeradical addition polymerization at ambient temperatures. The principal monomer is methyl methacrylate (MMA) (structure 4.12), but others may be present, such as methacrylic acid (structure 4.13), to improve adhesion to metals, by forming carboxylate salts, and heat-resistance, and ethylene glycol dimethacrylate (structure 4.14) for crosslinking.

(4.12)

Methyl methacrylate

(7-43

CH2=C

I

C02H (4.13)

Methacrylic acid

(4.14)

Ethylene glycol dimethacrylate

46

Chapter 4

Often, poly(methy1 methacrylate) is also present; this has the effect of increasing viscosity and reducing odour. The formulation of a typical structural acrylic adhesive is given in Table 4.1. Chlorosulfonated polyethylene is a rubbery toughening agent. Cumene hydroperoxide and N,N-dimethylaniline are the components of a redox initiator. The adhesive would be supplied in two parts (resin and catalyst). The catalyst contains one of the initiator components, and all the other components are in the resin. Most conveniently the resin can be spread on one surface and the catalyst on the other. After being joined for about 1min the adhesive will have cured sufficiently to hold the joint together, and maximum strength will develop in about 10min. It is also possible to premix the components. The most widely used initiator system is a hydroperoxide and a condensation product of aniline and butyraldehyde, which can also generate free radicals by reacting with sulfonyl chloride groups in the toughening rubber, leading to some grafting of acrylic polymer to the rubber particles. The reactions are shown in Scheme 4.6. Et

V l P

Et

h

+

ROOH

-

Pr

+

ROO+ HO-

Et

Scheme 4.6

Cements for fixing artificial joints to human bones, and porcelain caps to teeth are also based on MMA. In the case of the latter, the dentist uses phosphoric acid to prepare the surface and dries it with cold air, and uses UV to cure the adhesive. There is a large volume decrease of 20.7% when MMA is polymerized. Such a large change could introduce significant stresses into joints, but can be reduced by adding particulate fillers. Shrinkage is also the reason why adhesives tend to have poor gap-filling properties.

Chemistry of Adhesives which Harden by Chemical Reaction

47

Table 4.1 Formulation of a structural acrylic adhesive. Component

Parts by weight

Methyl methacrylate Methacrylic acid Ethylene glycol dimethacry late Chlorosulfonated polyethylene Cumene hydroperoxide N,N-dimethylaniline

85 15

2 100

6 2

ANAEROBIC ADHESIVES Anaerobic adhesives cure in the absence of oxygen, which inhibits polymerization. They are usually based on dimethacrylates of polyethylene glycol, but end-capped polyurethanes are also used. They contain a redox free-radical initiator, and are usually supplied in air permeable polyethylene containers only partially filled, to maintain an adequate supply of oxygen. Uses include nut-locking, strengthening cylindrical fits and gasketing.

CYANOACRYLATES The molecule shown in structure 4.15 is ethyl cyanoacrylate, and because it contains two strongly electron-withdrawing groups (CN and COO) it is very susceptible to anionic polymerization. Polymerization is initiated by water, which is adsorbed on all surfaces in the atmosphere, and is complete within seconds. Another essential feature of the process is that it is also inhibited by oxygen, so that the curing reaction does not happen until the joint is closed and the supply of oxygen cut off. The actual initiating groups in water are the basic hydroxide ions (OH-). Because the surface of glass is alkaline, cyanoacrylates are packed in polyethylene rather than glass containers. Sulfur dioxide is added as a stabilizer. Methyl, n-butyl and ally1 cyanoacrylates are also used.

YN

CH2=C

I

C02C2H5

(4.15)

Ethyl cyanoacrylate

48

Chapter 4

RUBBER TOUGHENING OF STRUCTURAL ADHESIVES Many structural adhesives have rubbery polymers dissolved in them. When the adhesives cure, the rubber precipitates as droplets about 1 pm diameter, the driving force for this being the incompatibility that generally occurs between polymers. Adhesive joints break by the growth of a crack, and rubber particles act as crack stoppers. Fracture energies and impact strengths are increased. Rubbers which are used in this way include poly(viny1formal) (PVF) and poly(viny1 butyral) (PVB), each made by reacting the appropriate aldehyde with poly(viny1 alcohol) as shown in Scheme 4.7. As two neighbouring hydroxyl groups are consumed in these reactions, some isolated hydroxyl groups will remain. PVB is widely used as the interlayer in laminated glass. Chlorosulfonated polyethylene, which is used to toughen acrylic adhesives, is made by reacting polyethylene with a mixture of sulfur

+

nCH20

Polyvinylformal

Polyvinyl alcohol

Butyraldehyde Scheme 4.7

dioxide and chlorine. The product contains 1% of S and 25-43% of C1 by weight. ATBN and CTBN are acronyms for copolymers of butadiene and acrylonitrile with either amine or carboxylic end groups. About 85% of the butadiene units are in the trans-174-configuration and molar masses are typically 3500. In an epoxide adhesive the end groups will react with the resin so there will be chemical bonding at the particle-matrix interface. In an epoxy adhesive with 5% piperidine as the hardener, the

Chemistry of Adhesives which Harden by Chemical Reaction

49

fracture energy was increased from 0.18 to 3.33 kJ m - 2 and the impact strength from 6.0 to 26.2Nm-' on adding 20% by weight of CTBN. SILICONES One-part silicone adhesives are often termed room temperature vulcanizing (RTV), and consist of poly(dimethy1 si1oxane)s(PDMS) with molar masses in the range 300-1600, with acetate, ketoxime or ether end groups. These are hydrolysed by moisture from the atmosphere to form hydroxyl groups, which subsequently condense with the elimination of water, as shown in Scheme 4.8. They are best known as sealants for use in the bathroom. When the acetate terminated PDMS cures there is a smell of acetic acid. The rate of curing is controlled by water diffusion, which is slow in comparison with the chemical reactions in Scheme 4.8. There is a sharp advancing front of cured sealant, and the cured material acts as a barrier for water permeation. Any water which passes through this barrier quickly reacts with uncured sealant, and thus the barrier is thickened. y

3

y

y

3

-Si-0-Si-OCOCH3 I

I

CH3

CH3

y

y

3

y

CH3

3

y

7H3

+ CH3C02H

I

CH3

CH3

y

y

3

3

% -Si-0-Si-OH

I

CH3

I

3

-Si-O-Si-OC2H, I

3

!% -Si-O-Si-OH

3

2 -Si-0-Si-OH I CH3 CH3

-

I

I

CH3

CH3

yH3

7H3

+ C2H,0H

CH3 CH3 I

1

I

1

-Si-0-Si-0-Si-0-Sim I

CH3

I

CH3

+

H20

CH3 CH3

Scheme 4.8

The amount of water permeating a unit cross-section of the cured layer is given by equation 4.1. dnldt

= Pp/z

(44

At time t the thickness of cured adhesive is z; n is the number of moles of water, p the vapour pressure of water in the surroundings and P is the

50

Chapter 4

permeability coefficient of water in cured sealant; V is the volume of sealant which reacts with 1 mole of water. The volume of sealant which is cured is given by equation 4.2. d n = Vdn

(4.2)

Because we are dealing with a unit cross-sectional area dz = du, and equations 4.3-4.5 hold. dn = dz/V

(4-3)

(dzldt) V = Pp/z

(4.4)

zdz = VPpdt

(4.5)

Integrating equation 4.5, with the condition that z equation 4.6.

z

=(2Vfptp2

= 0 when t = 0, gives

(4.6)

The depth of cure is proportional to the square roots of time, relative humidity and permeability coefficient. Two-part silicones, which are essential for thick sections, normally contain water and are catalyzed with stannous octoate for a fast cure, or dibutyltindilaurate for a slower cure. Silicone adhesives are soft and compliant, and have good chemical and environmental resistance. Joints with silicones can operate over a wide temperature range, from about - 60 to 200 "C. The glass transition temperature is - 120 "C but because of their regular structure crystallization can occur at - 60 "C. POLYURETHANES Polyurethane adhesives are made by reacting a low molecular weight polymer containing at least two OH end groups with a diioscyanate. The polymers can be polyethers, aliphatic polyesters or polybutadiene. The basic chemical reaction is shown in Chapter 1. In two-component polyurethane adhesives the polymer and isocyanate are mixed and then applied to the adherends. Any hydroxyl groups on the surfaces (e.g. on paper, wood or glass) will possibly react with isocyanate to form covalent bonds between adhesive and substrate. One-part adhesives consist of low molecular weight, linear polymer molecules, which have isocyanate (-NCO) end groups. Water vapour from the atmosphere diffuses into the adhesive and causes the chemical

Chemistry of Adhesives which Harden by Chemical Reaction

51

reactions shown in Scheme 4.9, which join the molecules together to form larger linear molecules. However, a further reaction is that of isocyanate with urea units, and a consequence of this is that the adhesive, which was initially linear, now becomes crosslinked. Reactions 4.9a and 4 . 9 ~will have similar rates and reaction 4.9b will be about 25-50 times faster.

A biuret Scheme 4.9

POLYSULFIDES

Polysulfides are primarily used as sealants and a major use is to seal the edges of double glazing units, both to hold the units together and prevent the ingress of moisture. They are made by reacting bis(2-chloroethyl formal) with sodium polysulfide as shown in Scheme 4.10, where x is about 2 and n about 20. The addition of a small quantity of trichloropropane leads to branch points, which in turn lead to crosslinking on curing. CICH2CH20CH20CH2CH2CI

+

Na2S,

-

HS(CH2CH20CH20CH2CH2S)fi

+

NaCi

Scheme 4.10

Polysulfide sealants are formulated with mineral fillers to reduce cost and to modify flow properties, phthalate plasticizers and silane coupling agents. They are two-part systems and curing agents include manganese dioxide and chromates. Curing involves oxidative coupling of -SH end groups to form -S-S-, and has a complex free-radical mechanism.

Chapter 4

52

HIGH-TEMPERATUREADHESIVES The maximum temperature at which structural adhesives can be used is limited by the glass transition temperature and chemical degradation. The upper limit for acrylic adhesives is set by the glass transition temperature of poly(methy1 methacrylate) (105 “C) and the limit for epoxides of about 200°C is due to chemical degradation. There are a number of adhesives which can operate at higher temperatures than epoxides and phenolics. These tend to be expensive and require high curing temperatures. The best known are perhaps the polyimides, which were developed by NASA in the USA. They are made by a condensation polymerization between a dianhydride and a diamine. In the example shown in Scheme 4.1 1, pyromellitic dianhydride is reacted with 1,4-diaminobenzene. The first step in the reaction gives a polyamic acid which is soluble and fusible, and it would be applied to the substrates at this stage. Curing is then at high temperature and under pressure; the resulting polyimide is insoluble and infusible. Alternatively, the water-soluble ammonium salt can be used as a tractible intermediate. Other polyimides cure by addition polymerization. These are low molecular weight polyimides with acetylene end-groups, which cure on

Pyromellitic (PMDA) dianhydride (PMDA)

/

nobenzene 1, 4-Diaminobe/ lt4-Diamii

Polyamic acid

200 “C -H20

1

Water soluble ammonium salt

35.Q0

/

\\

0

polyimide Scheme 4.1 1

Chemistry of Adhesives which Harden by Chemical Reaction

53

heating by addition polymerization, and without the elimination of water. The compounds shown as structures 4.16 and 4.17may provide the acetylenic end-groups.

Hc= 0

HC

=xY

NH2

(4.16)

0

(4.17)

Chapter 5

Chemistry of Adhesives which Harden without Chemical Reaction

An adhesive needs to be of low viscosity so that it can wet the substrate, and in the case of adhesives which are already polymerized, this can be achieved by the addition of a liquid or by melting. The polymer can be dissolved in a liquid, or suspended in water as a paste or emulsion. There is now much pressure from environmental, and health and safety regulators to reduce or eliminate the use of solvents in adhesives, and the industry is responding by developing water-based systems to replace them. There are, however, fundamental problems, one being the low rate at which water evaporates because of its high enthalpy of vaporization, which is compared with values for some common solvents in Table 5.1. A second is that water-soluble materials are essential to stabilize emulsions, and these remain in the adhesive after drying, so increasing water absorption and the sensitivity of joints to water. The water soluble materials can be ionic or non-ionic surfactants, or water soluble polymers.

Table 5.1 Enthalpy of vaporization and solubility parameters of solvents.

Water Acetone Ethyl acetate n-Hexane Tetrachloroethylene Toluene

2440 534 404 508 242 41 3 54

47.9 20.3 18.6 14.9 19.0 18.2

Chemistry of Adhesives which Harden without Chemical Reaction

55

ADHESIVES WHICH HARDEN BY LOSS OF SOLVENT Contact adhesives are probably the best known solvent-based adhesives. These are solutions of a polymer in organic solvents, which are applied to both surfaces to be bonded. Some time is allowed for the solvents to evaporate and the surfaces are then pressed together, at which point some interdiffusion of polymer chains will occur. The surfaces can also be heated to increase tack. Clear solvent-based adhesives, which are sold to the public in tubes, are often solutions of nitrile rubber (a copolymer or butadiene and acrylonitrile) in organic solvents. Prominent contact adhesives are based on neoprene (polychloroprene, poly-2-chlorobutadiene). The diene units can be incorporated into polymer chains as four different isomers, which are shown in structural formulae 5.1. The ratios of these isomers affect the ability to

Chloroprene (2-chlorobutadiene)

1,2-unit

cis -1,4-unit

(5.1)

3,4-unit

trans -1,4-unit

Isomerism in polychloroprene

crystallize. The trans-1,4 unit is the most common and AC and AD grades are made up of about 90% of them. The W grade is composed of 85% of such units and crystallizes more slowly. Neoprene adhesives have good tack, rapid development of bond strength and are resistant to oils and chemicals. The formulation of a typical neoprene contact adhesive is shown in Table 5.2. Without stabilization, polychloroprene degrades to liberate HCl, which attacks the adhesive and is obviously a threat to metallic

56

Chapter 5

Table 5.2 Polychloroprene contact adhesive. Component

Parts per hundred resin

(PW Polychloroprene Magnesium oxide Zinc oxide Antioxidant (BHT) Resins

100 4-8 5

2

-

Solvents adherends. The metal oxides act as acid acceptors; they are more effective in combination than singly. They can also act as crosslinking agents. Oxygen initiates the dehydrochlorination of polychloroprene, which is a free-radical process, and the purpose of butylated hydroxytoluene (BHT) is to scavenge radicals. Resins improve adhesion and cohesive strength. The most common are p-tert-butyl phenolics (30-50 phr), which react with MgO to give an infusible salt that improves heat resistance. Just before application, 1-2% of diisocyanate (e.g. diphenylmethane diisocyanate, DDM) can be added as a crosslinking agent. Isocyanates normally react with active hydrogen atoms; it is not clear how they react here, but crosslinking clearly takes place because the adhesive becomes insoluble. A possible mechanism involves the hydrolysis of allylic C-Cl in 1,2-units. Uses of polychloroprene adhesives include DIY contact adhesive, shoe soling, rubber dinghies and rubber-to-metal bonding.

Solubility Parameter The use of solubility parameters is a simple but not entirely reliable way to predict whether polymers and solvents are compatible, and if mixing them will result in dissolution or swelling. The solubility parameter 6 is a measure of the energy required to separate the molecules of a liquid, and is given by equation 5.1, where AHvis the enthalpy of vaporization and Vm the molar volume.

Solubility parameters of liquids can be obtained from experimental values of AHv and Vm,but the vapour phase is not accessible to polymer molecules; they can be estimated by one of a number of group contribution methods, or by measuring the swelling of a polymer in a

Chemistry of Adhesives which Harden without Chemical Reaction

57

range of solvents and taking the solubility parameter of the highest swelling solvent as being closest to that of the polymer. The basis of using solubility parameters is that substances with similar values are compatible. Improvements in prediction can be gained if comparisons are also made of hydrogen bonding and dipole moments. Values for some common solvents are given in Table 5.1, and for some polymers in Table 5.3. Group contribution methods for calculating polymer solubility parameters are associated with the names of van Krevelen, Hoy and Small. In these methods each group has a molar attraction constant ( F ) , and the solubility parameter is related to the sum of these for the repeat unit by equation 5.2. M is molar mass of the repeat unit and p is the density of the polymer. Some values of F appear in Table 5.4.

Selection of Solvents for Polychloroprene The selection of solvents affects viscosity, drying time, bond strength and cost. Trisolvent blends of aromatic hydrocarbon, aliphatic hydrocarbon and ester or ketone (typically toluene-hexane-acetone) are usually in polychloroprene adhesives, but chlorinated solvents can be used where non-flammability is required. Solvents can be selected by comparing solubility parameter and hydrogen bonding index, both of which are additive on a molar fraction basis for solvent mixtures. Figure 1.1 is a map of these two parameters, and the kidney-shaped zone indicates suitable solvents for polychloroprene. Within the inner zone clear solutions are formed, but those in the outer zone are cloudy. Table 5.3 Solubility parameters of polymers. Polymer

6 [( M Pa)']

Polytetrafluoroethylene Polyet hylene Natural rubber Polystyrene Poly(methy1 methacrylate) Poly(viny1 acetate) Polychloroprene Poly(viny1 chloride) Poly(viny1 alcohol) Cellulose

12.7 16.0- 16.4 16.2- 17.0 18.6 18.6-19.5 19.2 19.2 19.5 25.8 32.0

58

Chapter 5

Table 5.4 Group molar attraction constants.

van Krevelen -CH, -CH,-C-

Hoy

Small

420 280 140

303 269 176

438 272 57

0

66

- 190

51 1 1517 47 1

668 1399 420

634 1503 552

I

I

-C-

I

-coo-phenyl -c1 9

2-propanol

7 ethyl acetate

acetone

bumone

a

0

aniline

n-he\ane

*\

0

nitropropane

cycloheune

1s

20

25

Solubility parameter / (MPa)’’2

Figure 5.1 SoIubiliry ninp for polychloroprene.

ADHESIVES WHICH HARDEN BY LOSS OF WATER Water Solutions and Pastes Starch is cheap and plentiful, maize and corn being the main sources for adhesive use. It consists of glucose units and has linear and crosslinked

Chemistry of Adhesives which Harden without Chemical Reaction

59

components, which are termed amylose and amylopectin, respectively (structures 5.2 and 5.3). For use in adhesives it is modified by lowering the molecular weight by the following processes: hydrolysis in dilute acid; alkaline chlorination to give an anionic product; dry heat in the presence of an acid to produce dextrin; alkaline treatment in the presence of tertiary or quaternary ammonium salts, or epoxides, gives a cationic starch.

‘ 0 (5.2) Amylose component

(5.3)

Amylopectin component

Additives can include up to 10% borax (sodium tetraborate) to increase viscosity and tack, urea as a plasticizer, and urea-formaldehyde, melamine-formaldehyde or resorcinol-formaldehyde condensates to improve water-resistance. Clay (e.g. bentonite) can be used as a filler. The main uses are for bonding paper, board and textiles. Applications include corrugated board, paper bags, tube winding, wallpaper paste and remoistenable adhesives. Water-moistenable adhesives include poly(viny1 alcohol) (PVOH), which is used on postage stamps, natural gums (e.g. acacia and dextrins), and poly(viny1 acetate) (PVA) latices with a large amount of PVOH stabilizer (e.g. 15%). PVOH is the only common polymer that is not made from its monomer. The reason is that vinyl alcohol does not exist, and if encountered would undergo an enol-keto rearrangement to

60

Chapter 5

acetaldehyde, which, incidentally, can be polymerized through the C=O bond. PVOH is made by hydrolysing PVA, as shown in Scheme 5.1.

Poly(viny1 acetate)

Poly(viny1alcohol)

Scheme 5.1

Aqueous Emulsions

The ingredients for an emulsion polymerization are water, monomer(s), stabilizer and initiator. The stabilizer can be a surfactant or watersoluble polymer, the use of these being referred to as surfactant and colloid stabilization. Anioinic surfactants include sodium and potassium salts of long chain fatty acids (stearate, laurate and palmitate) and sulfates and sulfonates with a long alkyl chain (e.g. sodium dodecyl sulfate and sodium dodecylbenzene sulfonate). Cationic surfactants are less commonly used and are quaternary ammonium salts such as dodecylammonium chloride. Colloid stabilizers include polyethylene oxide, PVOH and hydroxyethylcellulose. Amounts used are 0.2-3.0 wt% based on water for ionic surfactants and 2-10% for water soluble polymers. A stirred mixture of water, monomer and surfactant will form an emulsion that consists of'the following, as illustrated in Figure 5.2, which is not to scale (surfactant ions are depicted by the tadpole-like structures, with a hydrophilic head and a hydrophobic tail): (i) micelles, which consist of some monomer molecules surrounded by surfactant; (ii) larger droplets of monomer with little or no stabilizer; (iii) a small amount of free monomer molecules; (iv) surfactant ions at the liquid surface where it lowers surface tension. When a water-soluble initiator is added, such as sodium or ammonium persulfate, hydrogen peroxide, or a redox system such as persulfate and iron(I1) salts, free radicals are formed in the aqueous phase. Their fate is to diffuse in the water and, because of their larger surface area, to enter the micelles and initiate polymerization. A widely held view is that once a second radical enters a micelle then

Chemistry of Adhesives which Harden without Chemical Reaction

61

1 01 01 01 01 01 01 01 01 01

0

(c)

Figure 5.2 Diagrams of emulsion polymerization, not to scale. (a) Aqueous emulsion of monomer and surfnctant; (b) decomposition of irtitiator to give radicals which enter micelles, and monomer diflusing ji-oin droplets to micelles; (c) polymer latex.

termination will occur instantly. This is because rate constants for termination are very high (lo6-10' dm3 mol- s- ') and micelles are very small (2-4 nm diameter). A third radical will re-initiate polymerization but the fourth will bring about termination. As monomer is consumed in the micelles it is replenished by monomer from the larger droplets diffusing through the aqueous phase. Demand for surfactant to stabilize the growing particles (as the former micelles are now termed) causes a lowering of surface tension.

62

Chapter 5

The product of emulsion polymerization is a latex of polymer particles with adsorbed stabilizer. The particle diameters are of the order of 1 pm and the water content is normally 5@-55%. Polymer latices are best known as emulsion paints that are based on PVA. Whether used as surface coatings or adhesives, they are spread on surfaces and a continuous film is formed as the water evaporates. The lowest temperature at which a continuous film can be formed is the minimum Jilm-forming temperature (MFT), which is close to the glass transition temperature. Latex adhesives can be plasticized internally by incorporating a suitable comonomer or externally by adding conventional liquid plasticizers (e.g. phthalates) to the hot latex. Perhaps the best known example is DIY wood adhesive, which is a poly(viny1 acetate) latex. Here the adhesive hardens by water migrating into pores in the wood. Phthalate plasticizer can be added to reduce brittleness. It is only suitable for indoor applications. Another example is Copydex!, which is natural rubber latex with ammonia added as a sta bilizer. Vinyl acetate is a major constituent of adhesives for bonding wood and paper. The cost of this monomer is low, but because the glass transition temperature of the homopolymer is low (32 "C but it may be lowered by co-monomers, plasticizers or water), there is a tendency to creep, which is the major disadvantage. Such adhesives are usually stabilized with PVOH with about 20% residual acetate groups. Particle sizes are in the range 0.5-2.0pm. The use of surfactants is minimized, because the consequent lowering of surface tension permits excessive penetration of the adhesive into the substrate. If the substrate is porous (e.g. ceramic tiles) a filler such as china clay or calcium carbonate can be added to retain the adhesive in the bond-line. The simplest crosslinking agent that can be added to PVA latices is glyoxal (CHOCHO). This is stable below pH 8 and only reacts in the dried film, presumably with hydroxyl groups. Acrylic adhesives using monomers from methyl to ethylhexyl acrylate give a wide range of physical properties. Figure 5.3 plots the brittle points of poly-n-alkyl acrylates and methacrylates as a function of the number of carbon atoms in the alkyl group. The brittle point initially falls as the alkyl groups increase in size, but then rises as the long side-chains begin to crystallize. Up to 5 YO N-methylolacrylamide (CH,=CH-CONH-CH,OH) can be added as a crosslinking agent, which acts by condensation of methylol groups. Latices based on polychloroprene are used as solvent-free contact adhesives. Heat-seal adhesives are based on homopolymers and copolymers of vinyl acetate but with PVOH replaced with cellulose

Chemistry of Adhesives which Harden without Chemical Reaction

loo

63

R

5

I

10

15

number of carbon atoms in alkyl group

Figure 5.3 Brittle points of acrylic

arid methncrylic Opolyniers.

ether to lower the heat-sealing temperature and reduce water sensitivity. In contact with water, adhesive bonds with emulsion adhesives may release surfactants, which will have the effect of lowering surface tension and changing the thermodynamic work of adhesion. Some latices based on copolymers of vinyl acetate have been dried to give films that were then immersed in small quantities of water. The surface tensions (yw) fell from 72.8 m N m - to values in the range 39-53 mN m - ' in the first hour and then remained fairly static. Measurements of interfacial tensions against n-hexadecane showed that the dispersion components of surface tension remained essentially constant but polar components were reduced into the range 6 2 0 mN m- '. The effect this has on the thermodynamic work of adhesion can be obtained using equation 8.29 in Chapter 8. If it is assumed that any lowering of yw will be in the polar component, the equation can be written as equation 5.3.

'

Here Wis the work of adhesion in dry surroundings, yg and ySp are the dispersive and polar components of surface free-energy of the solid substrate, and y$ is the dispersive component for water. Calculated values of Ww are plotted against yw in Figure 5.4. Here the

64

Chapter 5 60

40

20

rE

7

E

-3

0

-20

- 40

Figure 5.4 Work of’adhesion of jiue ernulsiorz adhesives to polystyrene, in water of lowered surfcrce tension. (Reprinted from J. Comyn, D. C. Blackley and L. M. Harding, J . Adhesion, 1993, 40, 163)

substrate was expanded polystyrene (yg = 25.5mJm-2 and ySp = 0.4 mJ m- 2 , and 21.8 mJ m - 2 is the value for y&. All the interfaces are most stable in pure water but Ww falls to a minimum as yw is lowered. These adhesive bonds are thus predicted to be stable as long as yw exceeds a critical value at which Ww = 0. Adhesive bonds to emulsion adhesives can thus be self-destructive in water, and this is much the case with adhesive shown by curve e in Figure 5.4, which is always unstable in water. The thermodynamic prediction was confirmed by experiments in which bonds with this adhesive rapidly disintegrated in water.

ADHESIVES WHICH HARDEN BY COOLING Hot melt adhesives are one-part systems that are applied to substrates as a hot liquid, and rapidly form an adhesive bond on cooling. Their

Chemistry of Adhesives which Harden without Chemical Reaction

65

application is readily automated. They can be used to bond paper and board, many plastics and wood but a problem with bonding metals is that the substrate conducts heat too rapidly, restricting the extent of wetting.

Ethylene Vinyl Acetate (EVA) Hot Melts EVA random copolymers containing up to 30% vinyl acetate are used, and the effect of adding VA to polyethylene is to reduce crystallinity and increase polarity. Melt viscosity is very dependent on molecular weight. Tackifiers are added to reduce viscosity and improve wetting, and include hydrocarbon (C,-C,) resins, polyterpenes and rosin esters of pentaerythritol and glycerol. Waxes can be added to lower cost and reduce viscosity. Fillers such as calcium carbonate lower cost and increase viscosity. Antioxidants are needed to protect the adhesive during application and service life. Butylated hydroxytoluene (BHT) is a popular antioxidant but it is so volatile that it can evaporate from hot melt adhesives. Less volatile antioxidants have higher molecular weights and cost more (Chapter 10). Uses include cardboard boxes, bookbinding, iron-on patches and edge-tapes on chipboard. Because of creep, books develop a memory and tend to open at the same place each time. This can be prevented by crosslinking brought about by adding a peroxide and heating, or by electron beam irradiation. Curable hot melts are under current development.

Polyamide Hot Melts Polyamide hot melt adhesives have lower melting points than polyamide plastics, and tend to employ a mixture of monomers. They have better heat resistance than EVAs but cost more; however, they give good tack without needing additives. One product can be obtained from dimer diacid (shown in Scheme 4.3 of Chapter 4), which can be polymerized with ethylene diamine. One or two numbers, which indicate the numbers of carbon atoms in repeat units, are used to specify the structures of polyamides. If there is a single number there is just one sequence of carbon atoms in the repeat unit, as in polyamide 6 (nylon 6) which is shown in structure 5.4, but where there are two numbers the first is for the diacid repeat unit and the second for the diamine. Polyamide 6-10 is the example shown in structure 5.5.

66

Chapter 5

(5.4)

Polyamide 6

(5.5) Polyamide 6-10

The polyamide terpolymers (6,6-6,6-lo), (6,6-6,12), (6,646- 12) and (6,6-9,6-12) are used for bonding textile fabrics, where they are softened by steam, but this practice also lowers their wash resistance. They have good dry cleaning resistance. The melting points of polyamides are dominated by the ability of amide groups to form hydrogen bonds. The melting points of some polyamides made from aliphatic diacids and diamines are plotted in Figure 5.5. The melting point does not change smoothly as the number of methylene groups increases, and polymers with even numbers of carbon atoms are at the peaks of the zigzag, leaving the troughs for those with odd numbers. X-ray diffraction has shown that, in the crystalline state, polyamide molecules are in the extended zigzag configuration. The molecules lie in sheets with intermolecular hydrogen bonding between -NH- and ;C=O groups. In Figure 5.6 four parallel pairs of polyamide molecules are formed, and it can be seen that only when both acid and amine units contain even numbers of carbon atoms does every -NHand C=O group participate in hydrogen bonding. In all other cases the

300

I

250

-

200

.-

I50

\

I 4

6

8

10

number of carbon atoms in diacid

Figure 5.5 Melting points of polyamides based on tetramethylenediamine and pentamethylenediamine 0.

67

Chemistry of Adhesives which Harden without Chemical Reaction / 0

(

"1

oj

:?0 O

0

NH

0

O 0

I ' I ' :"i Hi "r 1 1 1 Oi" 0

I

i" :;"

0

0

0

0

0

O 0

I

O 0

I

Oj. 0

0

'

HN

0

0

O 0

I

O

T

odd

even

odd

No. of C atoms

{ even

even

odd

odd

Max. H-bonding

all

half

half

half

peak

trough

trough

trough

Melting point

even

Figure 5.6 Hydrogen bonding in polyamides made from diacids and diamines.

maximum extent of hydrogen bonding is 50%. The hydrogen bond is much stronger than the dispersive forces between -CH,- groups, and will make a significant contribution to the enthalpy of fusion (AHf).The free energy of fusion (AG,) is given by the Gibbs equation (equation 5.4), where T is the absolute temperature and ASf the entropy of fusion.

68

Chapter 5

At the melting point Tf,AGf= 0 whence equation 5.5 holds

The asymptotic limit of the polyamide melting point, as the number of methylene groups becomes large, is the melting point of linear polyethylene (137 "C).This type of behaviour is also shown by aliphatic polyesters and polyurethanes, both of which are used as hot melt adhesives, and the general trends of their melting points are shown in Figure 5.7. The reason for the depression of melting points by polyester groups is the higher flexibility of the -C-0- linkage, giving the molten molecules a greater number of configurations and so increasing the entropy of fusion. The use of mixed monomers in polyamide hot melts increases disorder, so lowering the extent of hydrogen bonding and the melting point.

polyethylene 100

-

I

polyesters I

I

number of chain atoms in repeat unit

Figure 5.7 Smoothed melting points for linear aliphatic polymers.

I

Chemistry of Adhesives which Harden without Chemical Reaction

69

Polyester Hot Melts Polyester hot melt adhesives are based on terephthalic acid (benzene1,4-dicarboxylic acid), but other diacids such as isophthalic (benzene1,3-dicarboxylic acid), adipic (hexane-1,6-dioic acid), and azelaic (nonane-1,9-dioic acid) are used. The range of diols includes ethane diol, butane- 1,4-diol,hexane-1,6-diol,diethylene glycol and propane- 1,2-diol. Crystallinity falls as the number of diol carbon atoms increases.

Chapter 6

Pressure-sensitive Adhesives

Pressure-sensitive adhesives remain permanently sticky and are familiar from their use in adhesive tapes and labels. The adhesives are mainly based on natural rubber, styrene-butadiene block and random copolymers, and acrylics. Plasticized PVC and polyethylene are common tape materials. One face is coated with a primer or tie-layer so that the adhesive will stick permanently, and the other has a release coat so that it will part with the adhesive when the tape is unrolled. The most commonly used release material is a copolymer of vinyl alcohol and vinyl octadecyl carbamate (Scheme 6. l), made by reacting PVOH with octadecyl isocyanate.

Scheme 6.1

Other release coats are based on vinyl behenate (CH2=CH-COO-C2,H43) or stearyl acrylate. In each case the long, non-polar hydrocarbon chains will give a surface of low energy. Sticky labels are supplied on backing paper impregnated with crosslinked poly(dimethy1 siloxane), which has a low surface-energy. Table 6.1 gives the formulation for a masking tape adhesive, based on natural rubber, which is cheap. The block copolymers are three block styrene-diene-styrene types with 15-30% styrene. Hydrogenation of the diene units improves oxidative stability. The styrene and diene units form separate phases. The rubbery diene phase is continuous, and the glassy polystyrene phase forms dispersed droplets, which improve cohesive properties of the adhesives. Styrene butadiene random copolymers are produced by emulsion polymerization. 70

71

Pressure-sensitive Adhesives

Acrylics have superior resistance to UV and oxygen but are more expensive. Properties can be adjusted by copolymerization, which is in emulsion. The formulation of a general purpose grade is given in Table 6.2, and a high tack grade, containing much hydrogenated rosin ester, in Table 6.3. Tackifiers include terpene resins, which are oligomers of a- and P-pinenes, petroleum resins and rosin esters. The chemical structures of the first two groups are not well understood. Rosin acid is obtained from pine trees and is a mixture of abietic acid (see structures 6.1) and isomers with repositioned C=C bonds. It is esterified with pentaerythritol [C(CH,OH),] and glycerol (CH,OH-CHOH-CH,OH).

Table 6.1 Masking tape adhesive. Coriipoiierit

Parts per hundred resiiz (PW

Natural rubber Polyterpene Calcium carbonate Resole phenolic resin An tioxidan t (BHT) Solvent (Hexane-toluene, 70: 30)

100 41 58 51 2 450

Table 6.2 Monomers in general purpose acrylic pressure-sensitive adhesive. Component

Parts b y weight

2-Eth ylhex yl acryla te Vinyl acetate Acrylic acid N-methylol acrylamide

75 20 4 1

Table 6.3 Monomers and tack@er in high tack acrylic pressuresensitive adhesive. Component

Parts b y weight

2-Ethylhexyl acryla te Acrylic acid Hydrogenated rosin ester

95.5 4.5 50.0

~

72

Chapter 6

One of the most familiar sticky substances is Blu-Tack@,which is a product of Bostik. It is basically a linear rubbery polymer, such as polyisobutene, with particulate fillers added to modify its viscoelastic properties. It behaves as a pressure-sensitiveadhesive.

@

@

COOH

COOH

Abietic acid

Dehydroabietic acid

&

@

--

COOH

COOH

Neoabietic acid

Pimaric acid

flH @ @ COOH

COOH

lsopimaric acid

Levopimaric acid

COOH

Palustric acid 6.1

Chapter 7

Surface Analysis

Instrumentation for chemistry always seems to develop at a robust pace, and two techniques which have, in recent years, had a notable impact on the study of adhesion are X-ray photoelectron spectroscopy and secondary-ion mass spectrometry. Infrared spectrophotometry in reflectance modes has been available for a longer time, and has been made more useful by the arrival of Fourier transform instruments, Acronyms abound in surface analysis and those for the above techniques are XPS, SIMS and FTIR, respectively. Others which can give useful information about surfaces are AES (Auger electron spectroscopy), IETS (inelastic electron tunnelling spectroscopy) and SERS (surface-enhanced Raman spectroscopy). X-RAY PHOTOELECTRON SPECTROSCOPY XPS has also been named electron spectroscopy for chemical analysis (ESCA), and it involves placing the sample in a high-vacuum chamber and irradiating it with soft X-rays. This causes electrons from atomic core-levels to be ejected into the vacuum; their kinetic energy E , is related to binding energy E , by equation 7.1 where hv is the quantum energy of the X-rays and $ is the work-function.

Valenceelectrons are also emitted, but these are much weaker than the core emissions and more difficult to interpret. X-ray sources are usually aluminium (AlKa of 1486.6eV and linewidth 0.85 eV) or magnesium (MgKa of 1253.6 eV and linewidth 0.7 eV). The X-rays can damage the sample and care is needed to minimize this with samples containing 73

74

Chapter 7

covalently bonded chlorine and bromine. With insulators there can also be problems with charging of the surface. The spot-size gets smaller with each new instrument and is now about 50 pm square. It is usual first to obtain a survey spectrum, which detects all elements in the surface except hydrogen, which has no core-electrons, followed where appropriate by high-resolution examination of selected elements. Binding energies are specific to core-levels, and this gives XPS its ability for elemental analysis. Values for some elements are shown in Table 7.1; the bench mark for binding energies is carbon 1s in polyethylene, which is 285.0 eV. Peak heights or areas are proportional to the numbers of atoms in the sampling zone, but the proportionality constant, which is empirical and is known as the relative atomic sensitivity, varies. Some values are shown in Table 7.1, where the bench mark is F 1s = 1.00. In high resolution spectra it can be seen that binding energies may vary with chemical bonding, and that in appropriate cases there may be more than one peak. An example is the case of aluminium where the 2p binding energy for the metal is 72.85eV, shifting to 73.85eV for aluminium oxide and to 74.3 eV for the hydroxide. In elemental silicon the 2p binding energy is 99.7 eV, shifting to 103.4 eV in silica, 102.4eV in poly(dimethy1 siloxane) and 102.95eV in soda-glass. These differences are small and very much less than the chemical shifts in nuclear magnetic resonance, so that it might not be easy to say whether silicon in a surface is due to glass from the adherend or reinforcing fibres, silica as a filler in the adhesive, silicone as a contaminant or the

Table 7.1 Binding energies and relative atomic sensitivities. Element

Core-level

E,(eV)

Relative atomic sensitivity

C 0 F Na

1s 1s 1s 1s 2P 2P 2P 2P 2P 2P 3d

285 530 690 1072 73 102

0.25 0.66 1.oo 2.3 0.18 0.27 1.8 3.0 6.3 4.8 4.3

A1

Si Ti Fe

cu Zn Sn

459

707 932 1021 485

75

Surface Analysis benzene ring

285

290

binding energy / eV

Figure 7.1 High resolution X P S carbon I s spectrum of polyethylene terephthalate.

presence of a silane coupling agent. This could be resolved by the use of SIMS. The high resolution carbon 1s spectrum of polyethylene terephthalate is shown in Figure 7.1, where the major peak is due to carbon atoms in the benzene ring and the two lesser peaks to methylene and -COO-. Electronegative atoms attract electrons from their neighbours, so causing chemical shifts. Examples of this are shown in structure 7.1, where the electronegative atoms are F and 0,and chemical shifts for C 1s are measured in eV, relative to -CH,- in polyethylene. CF3-CHz-CHz-

7.0 1.2

0.2

-CH2-CO-CH20.5

2.1

-CH2-CH(OH)-CH2-

1.6

0.2

-CH2-C02H 0.5

0.7 4.7

(7.1)

N 1s has chemical shifts similar in magnitude to those of C Is, but those for 0 1s are smaller. It is common practice to separate overlapping peaks by curve-fitting with the use of a computer.

Surface Sensitivity Although X-rays penetrate deep into the sample, only electrons generated close to the surface escape with their original energy intact. The sampling depth is about three times the mean free path of the

76

Chapter 7 Si02

30' 50'

70' 90' 95

1

I

100

105

J

110

binding energy I eV

Figure 7.2 High resolutioi? X P S silicon 2 p spectra of silicon with u 2.5 mz layer of silicon dioxide, nt various take of angles.

electrons, i.e. 0.15-10 nm for polymers. Surface sensitivity can be increased by varying the take off angle for the electrons; the spectrum being more surface-sensitive at low angles. Figure 7.2 shows the high resolution Si2p spectra of Si coated with a 2.5nm layer of SiO,. The oxide peak is more prominent at low angles. Depth profiling can be done by ion-etching (sputtering) using Ar' or Xe', with a current of about 200 FA cm- etching at about 25 nm per min; the rate of etching may not be uniform over the sample. p, d and f Core Electrons

Spin-orbit (i-j) coupling of p, d and f orbitals leads to doublet XPS peaks; details of the splits are given in Table 7.2. The separation of the peaks can be so small in some cases that the

Table 7.2 Doublets from p,d and f core levels. Level

j Values

Relative area

1 -

-

2

1 2 9 3 2

1:2

3 2 7 5 2

2: 3 3:4

5 2, 1 2

Surface Analysis

298 8

294 E

77

290 E

286 0

282 8

278 8

Binding energy ( e V )

Figure 7.3 High resolution X P S spectrum of dried Jilin froin an einulsion adhesioe. (Reprinted from J. Comyn, D. C. Blackley and L. M. Harding, In?. J . Adhesion Adhesioes, 1992, 12, 263. With kind permission from Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington OX5 1GB)

doublet is not resolved. Some separations are C12p = 1.6, S 2p = 1.2, Si2p = 0.6 and P 2 p = 0.9eV. Figure 7.3 shows a high resolution spectrum for the dried film from an emulsion adhesive; it shows doublets for K 2 p and curve fitting for C 1s.

Shake-up Satellites

Loss of core electrons leads to reorganization of the valence electrons, which may be excited (shaken up) to an unfilled core level. Energy for this is taken from ejected electrons and this leads to a small peak of lower energy, known as a shake-up satellite. Conjugated and, particularly, aromatic systems show this phenomenon. Intensities of satellite peaks have intensities up to 5 or 10% of the primary peak and are separated from it by 6-7eV. In aromatic systems the satellite is due to a z-z* transition. It is illustrated by the high resolution C 1s spectrum of polystyrene shown in Figure 7.4. Derivatization or Tagging Because chemical shifts with XPS are small it may not be easy to tell, from high-resolution spectra, what chemical groups are actually present. One solution is to use chemical reagents to modify specific groups, and to introduce a new element. Some examples are given in Schemes 7.1-7.7.

Chapter 7

78

290

295

285

binding energy I eV

Figure 7.4 High resolution X P S carbon I s spectrum of polystyrene, showing n shake-up satellite.

In reaction 7.1, one >CO group is replaced by five F atoms, which has a much higher relative atomic sensitivity than C Is, leading to a 20-fold increase in sensitivity.

‘CO

/

+

C6F5-NH-NH2

-

‘C=N-NH-C~F, /

Perfluorophenylhydrazine

Scheme 7.1 Tagging reaction forcarbonvl groups

+ (CFsC0)20-

-OH

CF3COr

Trifluoroacetic anhydride -OH

+ C3F+OCI

-

C3F7COr

+

HCI

Perfluorobutanoyl chloride -OH

+ (acac)2Ti(OPi)2

-

-o~i(o~i)(acac),

Diisopropyl titanuim diacetylacetate -OH

+ CIC6F4NCO

-

CICGF4NHCOr

Chlorophenylisocyanate

Scheme 7.2 Tagging reactions for hydroxyl groups

Surface Analysis

+

-SH

ASNO3

79

-

-s-Agi

Silver nitrate Scheme 7.3 Tagging reaction for thiols

+ CGF5CHO

-NH2

-

CcF5CH=N-

Perfluorobenzaldehyde Scheme 7.4 Tagging reaction for amine groups

+ SO2

-C-O-OH

-

-C-O-S03H

Scheme 7.5 Tagging reaction for hydroperoxid8 groups

+

-C=C-

Br2

-

-CBr--CBr-

(in CC14in dark) Scheme 7.6 Tagging reaction for carbon-carbon double bonds

+ NaOH

-C02H

-CO2H

+ C2F5OTI

-

-

-C02Na

-CO2TI

Thallium ethoxide

-COPH

+ NR3

-

-C02NR3

Tertiary amine Scheme 7.7 Tagging reaction for carboxylic acids

80

Chapter 7

Surface Treatment of Polyolefins Polyethylene and polypropylene are difficult to bond because they lack polar groups and are of low surface-energy. Commercial methods of treating them for print-adhesion are by flame and corona-discharge. Figure 7.5 shows high resolution spectra of the C 1s and 0 1s regions of corona-treated low density polyethylene, showing that the untreated surface contains no oxygen. Treatment leads to the appearance of 0 1s peaks, and a shoulder on the C 1s peak, which is due to chemical shift on bonding to oxygen. Figure 7.6 shows spectra of flame-treated low density polyethylene, showing the incorporation of both oxygen and nitrogen. Here the curve-fitting indicates that there are two nitrogen and two oxygen species, and three new carbon species.

m

291

207

203

537

533

(evl

Figure 7.5 High resolution X P S curbori is and oxygen Is spectra of low density polyethylene afer corona-discharge treatment in air for 0, 8 or 30s. (Reprinted from D. M. Brewis and D. Briggs, Polymer, 1981, 22, 7. With kind permission from Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington OX5 1GB)

81

Surface Analysis

104

400

c.V

Figure 7.6 High 1.eso1ution X P S cnrbon I s , osygen I s mid nitrogen I s spectra of uritreated arid Jlaine treated low density polyethylene. (Reprinted from D. M. Brewis, D. Briggs and M. B. Konieczko, J . Mater. Sci.,1979, 14, 1344. With kind permission from Chapman and Hall)

Glass

Figure 7.7 shows a spectrum of float-glass after solvent cleaning with heptane. Sodium, silicon, tin and oxygen, which are components of glass are present, but there is also a considerable amount of carbon. Carbon contamination is a common feature on glass and metal surfaces exposed to the atmosphere, but the cause is not clear; is it adsorbed carbon dioxide or hydrocarbon fuel? The chemical analysis calculated from Figure 7.7, and also after lightly etching with Ar', is shown in Table 7.3. Etching removes much of the carbon contamination and exposes the glass.

Brass Figure 7.8 shows an XPS spectrum of brass after light Ar+-etching,and Table 7.4 gives the composition of this and a solvent-wiped surface. Here

82

Chapter 7

1105 7

885.7

665 7 Binding Energy

445 7 (eV)

225 7

5 7

Figure 7.7 X P S survey spectrum of glass. (Kindly provided by CSMA Ltd, Manchester, UK)

Table 7.3 X P S analysis (atomic %) of glass. ~

~~~~~

Element

Heptane rinse

Ar

0 C Si Sn Na ME Ca

42.3 38.7 13.5 2.8 2.7 0.0 0.0

50.4 20.3 21.2 4.3

+

etch

1.8 0.9 1.o

the effect of etching is to reduce the amount of carbon contamination and to remove some of the initial metal surface, which is rich in Zn. Some of the elements in the surfaces of brass and glass show more than one peak; it is general feature that the number of peaks will increase with atomic number.

SECONDARY-ION MASS SPECTROMETRY In this technique, ions of Ar+, Xe' or Ga' of energy about 4 keV are directed at the sample in ultrahigh vacuum. The surface is sputtered into the vacuum. Fragments can contain one or more atoms; most are neutral but 1-2% are changed and these are analysed by a mass spectrometer; the positive and negative ions are analysed in separate scans. A variant is to bombard the surface with fast atoms, usually Ar,

83

Surface AnnIysis

.

lr .i _ ^. I

EC-0 0

660 0

Binding Energy

220 0

440 0

0 0

(eV)

Figure 7.8 X P S survey spectrum of brass. (Kindly provided by CSMA Ltd, Manchester, UK)

Table 7.4 X P S analysis (atomic YO)of brass. Element

Heptnne rinse

A r + etch

C 0 Zn

55.4 32.2 8.0 4.3

18.1 25.7 28.5 27.8

cu

in what is termed fast atom bombardment mass spectrometry FABMS. In dynamic SIMS the surface is etched away and depth profile information is obtained. In studying adhesion, static SIMS is mostly used, and here the primary-ion current is very low (1 nA cm - 2, so there is little damage to the surface and data are obtained from the top few molecular layers. A problem with insulating samples is the build up of surface charge, but this can be reduced by flooding the surface with low energy electrons (about 700eV). The spot size is of the order of 1 pm and decreases with each new instrument; this means that the surface can be scanned (TV like) whilst tuned to a single mass number and an image produced. Mass Spectrometers Two types of mass spectrometers are used in SIMS. The quadrupole filter consists of four parallel rods with a variable DC superimposed on a

84

Chapter 7

m/z

43.0

43.1

43.0

43.1

43.2

43.0

43.1

Figure 7.9 High resolution time-ofjlight S I M S spectra of,fiom leji to right, polypropylene (a), untreated (b) and (c) corona-discharge treated. (Kindly provided by Dr J. S . Hammond of Physical Electronics Inc.)

radiofrequency AC. The filter scans through the mass range. Time of flight (TOF) mass spectrometers analyse all masses at once, have greater sensitivity and can deal with much higher masses, The latest TOF instruments can measure masses of ions to the 3rd or 4th decimal place. This permits the precise identification of ions, and separation of peaks at the same nominal mass number. Figure 7.9 shows spectra of untreated polypropylene, and also after corona-discharge and flame treatment. The untreated polymer has a single peak at 43.0558, but two peaks occur after treatment. The peaks are C,H: (m/z = 43.0558) and C,H,O+ (m/ z = 43.0188). Peak Height There is no simple relationship between ion current and the numbers of parent species in the surface, and in some cases small quantities of some species such as sodium give strong spectral peaks. Nevertheless, if the amount of a chemical substance which gives rise to a particular ion increases, then the intensity of the peak with respect to its neighbours will also increase. The abscissa is the mass-to-charge ratio (m/z) expressed in atomic mass units (Daltons).

SIMS Spectra of Polyethylene and Polypropylene These polymers give very similar SIMS spectra. Positive ion spectra in the range 0-150amu are given in Figure 7.10. They show one series of

85

Surface Analysis

1

1

1

1

1

1

1

1

1

1

1

I

1

I

l

l

Figure 7.10 Positive ion S I M S spectra of(a) low density polyethylene and (b) of polypropylene. (Reprinted from D. Briggs, Sur$ Interface Anal., 1988, 9, 391. Reprinted by permission of John Wiley & Sons Ltd)

86

Chapter 7

ions C,Hin+ with n = 0-8 (e.g. CH3-CH+-CH3 and isomers) and another series C,H&-, with n = 1-8 (e.g. CH3-CH=CHz and isomers). These series of ions generally appear in the SIMS spectra of vinyl polymers from fragmentation of the backbone. The peak of high intensity at 69' for polypropylene has been assigned to the ion shown in structure 7.2.

CH3T7-CH3+ (7.2)

SIMS Spectra of Polydimethylsiloxane Poly(dimethy1 siloxane) (PDMS) is widely used as a release-agent and often encountered as a contaminant on surfaces involved in adhesive bonding. It is easily detected by SIMS from its characteristic fragmentation pattern; however, the peaks are particularly strong so that a surface giving a spectrum seemingly dominated by PDMS may in fact only contain a few per cent. The actual amount of Si may of course be measured by XPS. The positive ion spectrum of poly(dimethy1siloxane) (PDMS) has a series of linear ions and another due to cyclic ions, which are shown in structures 7.3 and 7.4, respectively.Ions which appear in the negative spectrum are CH,SiO-, 59; SO,, 60; CH,SiO,, 75; C H SiOSiOZ, 1 19; (CH ,), SiOSiO2, 149; (CH3)3 Si0Si(CH 3)2 0Si 0r, 223.

,

/

CH3

\+

(7.4)

n 0 1 2

295

Stable Isotopes Where there is more than one stable isotope, these will be seen in the SIMS spectrum. Perhaps the best known example is chlorine where the

87

Surface Analysis

isotopes and their abundances are 35Cl 75.77% and 37Cl 24.23%. Chlorinated polymers show peaks due to these in the negative ion spectrum with heights in proportion to the abundances. Other elements with multiple stable isotopes are: 28Si 92.2%, 29Si 4.6%, 30Si 3.1% 39K 93.2%, 41K6.7%. 64Zn 48.6%, 66Zn 27.9%, 67Zn 4.1%, 68Zn l8.8%, 70Zn 0.6%. Figure 7.11 shows part of the negative ion spectrum of a degraded polychloroprene adhesive that originally contained zinc oxide. It has peaks at m/z = 169-, 171-, 173- and 175-, which are due to ZnC1,. The 169- peak is formed from the most abundant isotope of each element, and the 171- peak can be due to 66Zn35C1; or 64Zn35C1,37C1-.Other combinations give the peaks at 173- and 175-. Zn' was not seen in the positive ion spectrum. Contaminated Polyurethane Figure 7.12 shows part of the positive ion spectra of a contaminated polyurethane that was difficult to adhere to, and the spectra of two possible contaminants, namely stearamide and ethylene bis(stearamide). Stearamide shows a peak at 284' due to the protonated molecule, and ethylene bis(stearamide) gives fragments at 282' and 3 10' formed by cleavage of -CH,-NHbonds. This positively identifies the latter compound as the contaminant.

i

:oc

-I

I17\

120

140 160 A c c c i c !.'.jss U n i t s

180

Figure 7.1 1 Negatioe ion S I M S spectrum of degraded polychloroprene adhesive.

200

88

Chapter 7

282

Figure 7.12 Positive ion S l M S spectra of(a) a polyurethane, (b) ethylene bis(srearamide)and (c)stearamide. (Reprinted from D. Briggs, Surf: Interface Anal., 1988, 9, 391. Reprinted by permission of John Wiley & Sons Ltd)

Pentaerythritol Rosin Ester Pentaerythritol rosin ester (PERE) is used as a tackifier in pressuresensitive and hot melt adhesives, and the amounts used are quite large, often equalling the weight of the polymers in some formulations. Other substances which are used as tackifiers include terpene resins, which

Surface Analysis

89

are oligomers of 01- and P-pienes, petroleum resins, and esters with glycerol of rosin acid and hydrogenated rosin acid; the chemical structures of the first two groups of compounds are not well understood. PERE is prepared from pentaerythritol [C(CH,OH),] and rosin acid, which is a mixture of abietic acid and some isomers, with the C-C bonds in different positions. Each -CH,-OH group in pentaerythritol can react with the -COOH group in rosin acid, making it possible to obtain mono-, di-, tri- and tetraesters. SIMS spectra using a time-offlight analyser and GA' primary ions are shown in Figures 7.13 and 7.14. Major peaks in the positive ion spectrum (Figure 7.13) can be assigned as follows; 27, 29, 41, 43, 55, 57, 69 and 71 are due to hydrocarbon ions C,,Hit,+ and C,,Hin-1, which appear in the spectrum of polyethylene, 77' and 91 ' are due to C 6 H l and C,HT while that at 43' can be assigned to CH,-CO+ and 55' to CH,=CH-CO'. The 105' peak could be due to C,H,-CH+-CH,, but perhaps a more likely assignment is CH,=CH-(CH=CH),-CH-CH with n = 2. Other ions in this series appear at 79' ( n = 1) and 131' ( n = 3). Assignments of some other ions in the positive and negative spectra are shown in reaction schemes 7.8 and 7.9; most of these have a number of possible isomers.

,

+

SURFACE INFRARED SPECTROSCOPY Techniques which have been established for some time are attenuated total reflectance (ATR) spectroscopy and multiple internal reflectance (MIR). The difference between these two techniques is that ATR involves one reflection and MIR many, e.8. 5, 9 or 25 in commercially available attachments (see Figure 7.15). The sample is placed in contact with a prism of either germanium or thallium bromide iodide, which can be placed in either a dispersive or FT spectrophotomer. The sampling depth is given by the Harrick Equation: d, =

LO

2nn [sin 28 - { n,/n

12 ]

Here d, is the distance below the surface at which the electric vector has fallen to l/e of its value at the surface, 8 is the angle of incidence, n , is the refractive index of the sample and n2 that of the prism (n, = 2.4 for TlBrI and 4.0 for Ge). Except for the wavelength ho,all the parameters in the Harrick equation have values roughly in the range 0.5-5, which means

90

Chapter 7 35

I

3a

25 20 1

6

15

I

10

91

77

I

1

5

0

40

20

60

100

80

1

1 128 4Ooo-

Y

30001 lmb

173

2000

100 0

120 ~

140

160

180

200 1

1400

I

I

257

llo00 2 I Z

800

2 U

600

24 1

-

400

A .

A _

6

Atomic mass units

Figure 7.13 Positive ion S I M S spectrum of pentaerythritol rosin ester. (Reprinted from J. Comyn, Int. J. Adhesion Adhesioes, 1995, 15, 9. With kind permission from Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington OX5 1GB)

91

Suvface Analysis 2500

2000

-

r

I

t

30

400 300 U v)

40

50

70

60

80

90

100

-

-

i

30 1

-

a

I . . . -.-,

I

..

I

-

I

I

92

Chapter 7

Abietic acid

239

257

-2H

2

24 1

r

+ or

L

.d'

185

185

1'

173 Scheme 7.8

Surface Analysis

Abietic acid

93

1

301

Scheme 7.9

that d, is about A,. The sampling depth is thus about 2-50 pm, which is many thousands of molecular layers; it is thus much less surface-sensitive than XPS and SIMS. It is important that the sample makes good optical contact with the prism, and for this reason it is easier to work with soft materials. Where polymers are filled with carbon black no spectrum is obtained due to strong absorption. Other methods of presenting samples to the beam in Fourier transform instruments are diffuse reflectance (DRIFT) and

/

Figure 7.15 Attenuated total rejectnnce ( A T R ) and multiple internal reflectance (MIR).

Chapter 7

94

reflectance-absorption (RAIR). In DRIFT, the sample is a powder that is surrounded by a set of mirrors, which collect the scattered beam. RAIR involves reflecting the beam from a polished surface at a low angle. Great advantage is given by FTIR by the incorporation of a computer. This includes multiple scanning to improve the signal-to-noise ratio, and the ability to produce difference spectra.

Surface Treatment of Polyethylene Figure 7.16 shows the MIR spectra of low density polyethylene, after treatment with chromic acid at 70 "C for periods up to 60 min. IR peaks due to stretching modes develop at about 3200cm- ' (-OH), 1700cm- ' (C=O) and 1200 and 1000cm- ' (C-0), showing that -OH, -C=O and -COOH groups are formed at the polymer surface. The minimum level of treatment is sufficient to render the surface suitable for adhesive bonding. The sampling depth of MIR is so large that it can only detect changes after they have penetrated quite deep into the sample. The application of surface IR spectroscopy to silane coupling agents is dealt with in Chapter 3. SCANNING PROBE MICROSCOPY

The latest surface characterization techniques to have an impact have the collective name of scanning probe microscopy. Resolution is very

I

I

w

I

I

2000

3803

I

1600

Wavenumber

I

1

I

1400

1m

1 m

I

800

(cm

Figure 7.16 M I R spectra of low density polyethylene treated in chrornic acid at 70 "C. (Reproduced with permission from M. B. Konieczko, PhD thesis, Leicester Polytechnic, 1979)

95

Surface Analysis

high, allowing the detection of single atoms and molecules, and threedimensional images of surfaces are obtained. In atomic force microscopy (AFM) a probe tip located on a cantilever (Figure 7.17) maps surface contours and forces, movement of the tip being measured by a laser beam reflected to a four-section photodetector. Distances less than 0.1 nm and forces of 1 nN can be measured. Lateral force microscopy measures frictional and other lateral interactions with the tip. In tunnelling probe microscopy there is a gap between the probe and sample, through which electrons tunnel. The sample must be an electrical conductor, and the tunnel current depends on the applied voltage and the size of the gap, to which it is particularly sensitive (Figure 7.17). Probes may be pyramids or pyramids topped with a fibre. With scanning thermal microscopy the fibre is a platinum wire about 0.2 mm long. In the constant temperature mode the probe acts as a resistive heater that responds to changes in thermal conductivity, and in the

laser beam detector

1

7

Figure 7.17 Scanning probe microscopy by force probe (top)and tunnelling probe (bottom).

96

Chapter 7

constant current mode it acts as a resistance thermometer, measuring the surface temperature. Some images are shown in Plates 7.1 to 7.3, The first of these is a thermal image of a PVC polybutadiene blend; the bright areas are PVC, which has the lower thermal conductivity. The concept of characterizing surfaces and surface profileometry using a diamond stylus is not new. Advantages offered by scanning

Plate 7.1

probe nzicroscopy iiz rherimil inode of (i P VC polj,butridiene b 1e II d. (Reproduced with permission from D r Hubert Pollock and the Topo Met r ix Corporation) Sciirirtiiig

Plate 7.2 Atomic force microscopy imuge of a poly(n-butyl methacrylate) lutes. (Reproduced with permission from Michigan Molecular Institute and the TopoMetrix Corporation)

Surface Analysis

97

Plate 7.3 Atomic force rnicroscopy iiiznge of window of the ‘Challengei.’ space slz u t t 1e. (Reproduced with permission from NASA and the TopoMetrix Corporation)

probe microscopy are increased resolution, three-dimensional mapping and a variety of detection modes. Surfaces can also be characterized by optical and electron microscopies.

Chapter 8

Contact Angles in the Study of Adhesion

It has been recognized for some time that there is a relationship between the contact angles which liquids make with surfaces and the strength of adhesive bonds to the surfaces. It is illustrated in Figure 8.1 for the strengths of lap joints in polyethylene bonded with the epoxide adhesive

. B

20

60

40

80

contact angle / degrees

Figure 8.1 Variation of strength of lap joints in polyethylene bonded with an epoxide adhesive, with the contact angle of water against polyethylene. (After DeBruyne)

98

Contact Angles in the Study of Adhesion

99

AralditeE. It can be seen that the strongest adhesive bonds are obtained when contact angles for water are low. The polyethylene was etched for various times in chromic acid.

SURFACE TENSION OF LIQUIDS The surface tension of a liquid can be obtained by measuring the force needed to remove a metal ring from the surface of the liquid, the ring-pull method. A correction factor has to be applied to the measured force; it is a function of the radius of the ring, the radius of the wire and the volume of liquid raised above the surface. For liquids, surface tension and surface free-energy are numerically the same, but have different units, which are usually mN m - ' and mJ m-2. T H E LIQUID-LIQUID INTERFACE Spreading of One Liquid on Another It is important that adhesives spread on substrates, and in the case of liquids spreading on liquids, spreading depends on their surface tensions. If a drop of liquid B is placed on the surface on liquid A, the change in Gibbs free energy accompanying a change in the area covered by the drop, at constant temperature and pressure, is given by equation 8.1.

Here A , is the surface area of liquid A,AB is the surface area of liquid B and A,, is the area of the interface. The spreading is illustrated in Figure 8.2 If B increases in area it is at the expense of A, and more interface is formed, i.e.

The partial differentials are surface or interfacial free-energies, e.g.

Hence equation 8.1 can be written as

100

Chapter 8

Figure 8.2 Spreading of liquid B on liquid A.

dG/dAA is known as the spreading coefficient of B and A, i.e.

A positive value of SA,B is the condition for spreading. Some values of spreading coefficients for liquids on water at 20°C (y, = 72.8mJm-2) are compared with observed spreading behaviour in Table 8.1.

Measurement of Interfacial Tension The ring-pull method can be used to measure the interfacial tension between two immiscible liquids such as water and an alkane. The procedure is to float a layer of alkane on water and place the ring at the interface. When a lifting force is applied the ring rises above the interface and draws the meniscus with it. Because of bouyancy a large volume of water is raised above the interface and quite a deep layer of alkane is needed to contain it; the correction factor is now larger than in measuring surface tension of liquids.

Table 8.1 Spreading coefficients of liquids on water. y,(mJ m P 2 ) yAB(mJm P 2 ) S,,,(mJ m-2) Spreading behaviour

n-Hexadecane 30.0 n-Octane 21.8 n-Octanol 27.5

52.1 50.8 8.5

- 9.3

0.2 36.8

Will not spread Just spreads Strong tendency to spread

Contact Angles in the Study of Adhesion

101

Interfacial Tension Between Two Non-polar Liquids In Figure 8.3 some molecules of liquid 1 are lying upon some molecules of liquid 2; the surface tension of the liquids are y1 and y2, respectively and both are non-polar so only dispersion forces will be acting across the interface. The force by which the molecule marked A is attracted to its own kind is the surface tension of liquid 1, but what is the force which attracts it to liquid 2? Fowkes considers that it is the geometric mean of the two surface tensions (equation 8.7), and Wu considers it to be the harmonic mean (equations 8.8 and 8.9). Interfacial attraction

= (y1y2)+

(8.7)

V Y , + l/Y2

(8.8)

1

Interfacial attraction Interfacial attraction

=

=

2yly2/(yl

+ y2)

t 8.9)

In the geometric mean case, the net force acting on the molecule of liquid 1 is y1 - (y1y2)) and by a similar argument the force acting on a molecule ofliquid 2 at the interface is y2 - (y1y2)*.Hence the total force across the interface (ylz) is the sum of these, as given by equation 8.10. y12 = Y 1

+ Y2 - 2(Y,Y2)'

LIQUID 1

LIQUID 2

Figure 8.3 Forces acting on a rnolecule at a liquirl-liquid interface.

(8.10)

102

Chapter 8

Interfacial Tension Between Two Polar Liquids The surface tension of a polar liquid is the sum of dispersive and polar components, so equations 8.11 and 8.12 apply to polar liquids 1 and 2, respectively. Y1 Y2

+ Y’l = Y; + Y’; = Y;

(8.1 1) (8.12)

Equation 8.10 can be modified to give the interfacial tension between two polar liquids and the result is equation 8.13. (8.13)

Interfacial Tension Between n-Hexadecane and Water If liquid 1 is n-hexadecane, which is non-polar, then y; is zero and y1 = y;; equation 8.13 then becomes equation 8.14. (8.14) This allows the determination of yt, which is the dispersive component of the surface tension of water, from a knowledge of the interfacial tension between n-hexadecane and water. THE LIQUID-SOLID INTERFACE

Measurement of Contact Angles All adhesive bonds are made by placing an adhesive, which is a liquid at the time of wetting, on a solid substrate. If a drop of liquid is placed on a flat, horizontal solid surface, it will make a contact angle 8 (Figure 8.4) with the surface. If the contact angle is zero the liquid is said to wet the surface fully. Small droplets (a few pl) are used to minimize distortion due to gravity. Contact angles can be measured by several methods including:

(i) Direct measurement by viewing through a microscope with a goniometer eyepiece. (ii) Measuring height (h)and radius (r) of the base of a drop, using a microscope or by projecting an image on a screen, followed by use of equation 8.15. tan(0/2) = h/r

(8.15)

103

Contact Angles in the Study of Adhesion

t.

1

Figure 8.4 Contact angles of liquids OIZ n solid s u r f x e (top), and advancing and receding comet angles (bottom).

(iii) The Wilhelmy plate method in which a plate of the test solid is suspended from a microbalance, and partially immersed in the liquid. The method can be adapted to measure contact angles on fibres. The measured force is given by equation 8.16, where X is the length of the contact between the plate and the liquid and yLis the surface tension of the liquid. Force

=

weight of plate

+ Xy,cosO

-

bouyancy in liquid (8.16)

A plot of force against immersion depth for a single cycle of immersion and removal, which permits the measurement of both advancing and receding contact angles is shown in Figure 8.5.

Contact Angle Hysteresis The latter phenomenon described above is known as contact angle hysteresis, which is due to surface heterogeneity caused by roughness, or patchy composition such as might occur on the surface of a block copolymer. It can be measured on droplets made to grow or shrink by the addition or removal of liquid, as illustrated in Figure 8.4.

Forces Between a Solid Surface and a Liquid Drop The forces acting at the periphery of a droplet making a contact angle 8 with a solid surface are illustrated in Figure 8.6, and are related by Young’s equation (equation 8.17). Ysv = YSL

+ YLvcose + *e

(8.17)

-

104 Force

receding

out

T XYLCOS

0,

Chapter 8

in

w

f

advancing

0 Immersion depth

Figure 8.5 Force agaiiist immersion depth for n Willzeliizy plate. YLV

? Ysv

+ Figure 8.6 Forces acting at the circunference of a liquid drop on surface.

N

solid

Here yLv is the surface tension of the liquid in equilibrium with its vapour and ysv is the corresponding value for that of the solid. The spreading pressure is ne;this is usually small and is often neglected. The interfacial tension between solid and liquid is ysL. The surface energies of both phases are the sum of dispersion (d) and polar (p) components, as given by equations 8.18 and 8.19.

+ YL Ysv = Y: + Ysp

YLV

=: Y

(8.18) (8.19)

105

Contact Angles in the Study of Adhesion

Equation 8.13 can be transposed to give the interfacial free-energy ysL between liquid and solids; equation 8.20 is the result. It contains geometric mean terms for both the dispersion and polar interfacial attractions. It is assumed that these can be treated independently, and that the polar-dispersion interactions can be neglected. These attractions are van der Waals forces and the equations considered here will only apply to systems where the mechanism of adhesion is physical adsorption.

Equations 8.17 and 8.20 can be combined to give equation 8.21, in which the spreading pressure is neglected.

This means that if yLv(1 + cos 0)/2(y3* is plotted against (yurt)*,the graph should be linear with an intercept (y!)’ and slope (y!j’)*, thus permitting the determination of the polar and dispersive components of the surface free-energy of the solid. An example is given in Figure 8.7 for

0

I

I

I

0-5

1.0

1-5

Figure 8.7 Plot based on equation 8.21 for an emulsion adhesive.

106

Chapter 8 r

I

1

1

0-

Figure 8.8 Plot based on eqtintion 8.21 for ziiic stenrate.

dried films of an emulsion adhesive based on a copolymer of vinyl acetate and butyl acrylate. Here yz = 6.4 f 2.1 m J m - 2 and y{ = 38.5 & 6.3 mJ m-2. Figure 8.8 is for the release-agent zinc stearate, which was pressed into discs; here y! = 22.4 f m J m - 2 and y{ = 0.06 0.05 mJ m-2, showing that it is the non-polar alkyl groups which dominate the surface, rather than the polar zinc carboxylate units. Wu considers that harmonic means give more consistent results for interactions between low energy systems (such as liquids and adhesives on polymers), while geometric means are more appropriate for high energy systems (such as adhesives on metals). Equation 8.22 is the harmonic equivalent of equation 8.21, but the fact that there is no simple way to plot this equation may account for the greater popularity of the geometric mean approach. YLVU

+ c o w = 4YzY3cY: + Y 3 + 4rs”rms”+ re>

(8.22)

Test-liquids Table 8.2 contains a list of some test-liquids for which the values of the polar and dispersive contributions to surface free-energy are available. The liquids are arranged in order of (yuyt)* as this is the abscissa of plots based on equation 8.21.

Spreading Pressure Adsorption of vapour on a solid surface will change the surface

107

Contact Angles in the Study of Adhesion

Table 8.2 Test-liquids for contact angle measurements.

Water Glycerol Ethane diol Formamide Ethanol Dimethyl sulfoxide 2-Ethoxyethanol Dimethyl formamide Tricresyl phosphate Trichlorobiphenyl Pyridine n-Hexadecane

72.8 63.4 48.3 58.2 22.4 43.54 28.6 37.30 40.9 45.3 38.00 27.6

21.8 f 0.7 51.0 37.0 & 4 26.4 29.3 19.0 39.5 & 7 18.7 17.0 5.4 34.86 8.68 23.6 5.0 32.42 4.88 39.2 & 4 1.7 44+6 1.3 37.16 0.84 27.6

1.529 & 0.035 0.845 & 0.11 0.805 f 0.14 0.688 & 0.19 0.563 0.499 0.460 0.388 0.208 0.172 0.150 0.0

free-energy of the solid. This will be greatest when the contact angle is low, i.e. when the liquid has a high affinity for the solid. The lowering of surface free-energy is known as the spreading pressure, ze,and is given by equation 8.23. (8.23) Here ys is the surface free-energy of the solid in a vacuum and ySVap is that when in equilibrium with the saturated vapour. The term IE,, is usually negligible when 0 > 10'. Spreading pressure can be measured by vapour adsorption using equation 8.24. PO

PO

r d p = RTS F d l n p

ze = 0

(8.24)

0

Here p is vapour pressure, po is the saturated vapour pressure, r is the number of moles adsorbed per unit area, and p is the chemical potential of the adsorbate. Some values of spreading pressure are given in Table 8.3. Surface Energies of Adhesives and Substrates Table 8.4 gives surface energies for a range of materials, arranged in order of increasing surface-energy. All metals in common use have oxide coats, which are of high surface-energy; however, the important dividing line in Table 8.4 is that between polyethylene and poly(methy1 meth-

108

Chapter 8

Table 8.3 Spreading pressures of liquids on solids. ~~~

Liquid

Solid

T("C)

e(O)

x,(mJ m-2)

n-Hexane n-Octane Water (33212 Hexadecane Hexane

PTFE PTFE PE PE PE PE

25 25 20 20 20 20

12 26 94 52 0 0

3.28 4.9 0 0 7.6 14.5

Table 8.4 Surface energy parameters of solid surfaces. ~~

y$mJ m - 2 )

Solid

ySp(mJ m P 2 )

ys(mJ m - 2 )

Dificult to b o d Pol ytetrafluoroethylene Polypropylene Polyethylene

18.6 30.2 33.2

0.5 0.0 0.0

19.1 30.2 33.2

Poly(methy1 methacrylate) Nylon 66 Poly(viny1 chloride) Polystyrene Rubber modified epoxide Amine-cured epoxide

35.9 35.9 40.0 41.4 31.2 41.2

4.3 4.3 1.5 0.6 8.3 5.0

40.2 40.2 41.5 42.0 45.5 46.2

Oxides Si02 A1203 Fe203

78 100 107

209 538 1250

287 638 1357

acrylate), because plastics above the dividing line can only be adequately bonded after surface treatment, whilst those below it can be readily bonded. Complete Wetting of a Solid

Fox and Zisman have characterized some polymer surfaces by measuring contact angles for a series of liquids, and plotting the data in the form of cos 0 against the surface tension of the liquids. When 8 = 0 (cos 0 = 1) the liquid spreads on the surface and the surface tension of the liquid is then equal to the critical surface tension yc of the polymer. A plot is shown in Figure 8.9; it is for liquids on some fluorinated polymers; the

109

Contact Angles in the Study of Adhesion 1.0

0.S

0.6

cos

0 0.4

0.2

0.0

- 0 3

1 0

I

I

I

I

20

40

60

%,

Figure 8.9 Fos-Zisrizaii plot for.solve liquids 011 j7puor.incited poljwzers. 0 5 0 :50 tetr.c~uor.oetkylene-etlzylenecopolynzei., 0 PTFE. (After Fox and Zisman)

values of y, are obtained from the intercept with the upper abscissa. Values of critical surface tension for some polymers are given in Table 8.5. They are similar to the values of ys in Table 8.4. Critical surface tension is related to surface tension by equation 8.25, i.e. y, is equal to or less than the surface tension.

Wetting is not a reciprocal property, which means that if A spreads on B, B does not necessarily spread on A. An example of this is that a liquid epoxide resin will not spread on polyethylene, but if the resin is cured it will then be wetted by molten polyethylene. A solid can induce liquids of lower, but not higher, surface tension to wet it.

THERMODYNAMIC PREDICTIONS OF JOINT STABILITY Work of Adhesion The thermodynamic work of adhesion ( WA),that is the work required to separate a unit area of two phases in contact, is related to surface

Chapter 8

110

Table 8.5 Critical surface tensions of some polymers.

Polytetrafluoroethylene Poly(dimethy1 siloxane) Polyethylene Polystyrene Poly(methy1 methacrylate) Amine-cured epoxide Urea-formaldehyde

18.5

24 31

33 39

44 61

free-energiesby the Dupre equation. If the phases are separated in dry air then equation 8.26 applies.

But if separation is in the presence of water it takes the form of equation 8.27.

Here the subscripts A, S and W denote adhesive, substrate and water, respectively. The separation processes are illustrated in Figure 8.10. Equation 8.10 can be used to write expressions for the interfacial free energies, and on substituting these into equations 8.26 and 8.27, equations 8.28 and 8.29 are obtained.

Equation 8.28 indicates that a stronger bond will be obtained if the adhesive and substrate are matched in their surface energy components, as illustrated by the following calculations. Suppose both adherend and adhesive have yd = 20mJm-2 and yp = 2rnJmp2, then the work of adhesion in dry conditions is 44 mJ m - and in water it is 65.7 mJ m- 2 ; the higher stability in water is due to both materials being quite hydrophobic. If the values of the two parameters for the adhesive are interchanged then work of adhesion

Contact Angles in the Study of Adhesion

111

Figure 8.10 Separation of adhesive from substrate in dry air (top) and in water (bottom).

when dry is 25.3mJmW2and in water it is 31.8mJm-2, i.e. both are reduced. If the thermodynamic work of adhesion is positive then the bond is stable, and conversely a negative value indicates instability. The parameter which has created most interest in the literature is the work of adhesion in the presence of water, as this can be used to predict joint durability.

Theory and Practice Table 8.6 is based on one by Kinloch in which the work of adhesion in air and in some liquids is compared with the tendency to debond interfacially in an unstressed condition. The fact that interfacial debonding only occurs when the thermodynamic work of adhesion is

112

Chapter 8

negative is very strong evidence of the validity of thermodynamics in predicting the durability of adhesive bonds. The data in Table 8.6 for the vinylidene chloride-methyl acrylate copolymer bonded to polypropylene are due to Owens. A polypropylene sheet was coated with an aqueous dispersion containing 80 parts vinylidene chloride, 20 parts methyl acrylate and 4 of acrylic acid; the dispersion was surfactant free and the polypropylene surface had been flame treated. The resulting laminates were placed in some surfactant solutions, and to quote Owens: ‘In every case where W Aupon immersion in the liquid is negative, the coating spontaneously separated from the substrate, becoming completely detached. Where W A was positive, spontaneous separation did not occur. Where separation occurred between coating and substrate, it did so within 15 min. The films that did not show separation were left immersed for six months. At the end of this time, they still were not separated, and some effort was required to remove the coatings from the films.’ Orman and Kerr demonstrated that whilst ethanol swells and reduces the tensile strength of an epoxide adhesive based on the DGEBA and 4,4’-diaminodiphenylmethane, it has little effect on the strength of aluminium joints bonded with the same adhesive. In contrast, water has a minimal effect on the tensile strength of the adhesive, but causes large weakening of aluminium joints. The underlying reason for this is the

Table 8.6 Work of adhesion for interfaces in air and in liquids. Interface

Work of adhesion (mJm-’) Air

Epoxy/steel

29 1

Liquid 22 (ethanol)

-255 (water) - 137 (water) - 57 (water) 22-40 (water)

No Yes Yes Yes Yes No

37 (water)

No

- 166 (formamide)

Epox y/aluminium Epoxy/silica Epoxy/carbon fibre corn posi te Vinylidene chloride-me t h yl acrylate copolymer

232 178 88-90 88

Interfacial debond in liquid?

1.4 (Na n-octylsulfate soln) N o -0.9 (Na n-dodecylsulfate Yes soln) -0.8 (Na n-hexadecylsulfate Yes soln)

Contact Angles in the Study of Adhesion

113

high value of yf for water and the low value for ethanol; both liquids have similar values of 7.: Indeed the basic reasons why water is generally aggressive to adhesive bonds is that it has a very high polar component of surface free-energy, and all adhesives contain polar groups.

PRACTICAL APPLICATIONS Whereas actual measurements of contact angles are usually made in science laboratories, the principles are exploited in the water-break test and in the use of liquids of different surface tensions to assess the printability of polyolefins in plastic-bag manufacture. The water-break test is a simple method to check that a metal surface is clean. A few drops of distilled water are placed on the surface, or alternatively the surface can be drawn from water in a container. If the water does not break into droplets then the surface is suitable for bonding. Uniform wetting of the metal by water indicates that it will be similarly wetted by the adhesive. A standard test (ASTM 1982) involves wiping a polyolefin surface with a series of liquids, starting with one of low surface tension and noting the time needed for the film to break into droplets. Liquids with increasing surface tension are used until one is found which will wet the surface for just two seconds. The surface tension of the plastic then equals that of the liquid. Twenty-two mixtures of formamide and 2-ethoxyethanol are used with surface tensions in the range 30-56 mN m - '.

Chapter 9

Strengths of Adhesive Joints

The final test for any adhesive is that it should give joints which are strong and durable. Although ways do exist of assessing the quality of joints by ultrasonic non-destructive testing, the ultimate test is to measure the force or energy needed to break a joint. Standards authorities including BS, ASTM and I S 0 recommended specifications for test joints and for testing. Although I strongly support this principle and the tests themselves, I always seem to have a valid reason for some deviation. An example was joints of lead and glass bonded with an epoxide adhesive, where work with standard single lap joints was dogged by ductility or brittleness of the adherends. The solution was to make joints with a rectangle of glass bonded to a strip of lead, which was pushed off in the test. Many types of joints are available and illustrated in Figure 9.1 are single and double laps, cylindrical butts, and 90" peels. There are three principal modes of fracture and they are illustrated in Figure 9.2 Mode I is due to peel or cleavage forces. Mode I1 is a shearing mode, as is mode 111 but here shearing is in torsion around an axis instead of along a plane. All three types of stress occur in single lap joints. In general, rigid adhesives are strong in shear but weak in peel, whereas rubbery adhesives are resistant to peel but creep in shear. Rubber toughening of modern structural adhesives improves their peel strength.

SINGLE LAP JOINTS Single lap joints are easy to make and to test, but because of the distribution of stress their behaviour is actually quite complicated. They are commonly used for testing joints made from rigid adherends and adhesives. To have a reasonable chance of obtaining reproducible joint strengths, there are a large number of variables which need to be set and 114

115

Strengths of Adhesive Joints a

-g b

h

d

Figure 9.1 Standard test joints (a) Siitgle lap, (b) double lap, (c) cylindrical butt aid (d) 90" peel.

then controlled. From earlier chapters it will be clear that these include surface preparation of the adherends and mixing and application of the adhesives, but others include the following: (i) Size of the adherends and amount of overlap. (ii) Control of the thickness of the adhesive layer (also known as glue-like thickness). This can be done by the use of jigs, or by adding small glass spheres (Ballotini) or incorporating wires (fuse wire or fishing line). Commercial film adhesives may contain knitted or woven fabrics known as carriers (UK) or scrims (USA). Stronger joints are obtained with thin glue-lines; optimum practical glue-line thickness would be 0.10-0.15 mm. (iii) Conditions of cure such as time, temperature, application of pressure. Light pressure is applied to most joints (e.g.by the use of

116

Chapter 9

i

Mode I l l

Figure 9.2 Principal modes of fracture.

spring clips), but higher pressures will be needed where steam is evolved, as with phenolic adhesives for metals. (iv) Ageing ofjoints prior to testing, e.g. in ambient or hot and humid conditions. (v) Joint testing conditions are most commonly ambient temperatures and humidities and in a mechanical testing instrument. Loading can be at a constant crosshead speed, usually of a few mm per minute with single lap joints, but slipping of the adherends in the jaws can mean that the set crosshead speed is greater than the rate at which the joints are strained. In hydraulic instruments a constant loading rate (kN min - ') can be used. After testing, the mode of failure is noted. This can be by interfacial/ adhesive failure, cohesive failure of the adhesive, or failure of an adherend. These are illustrated in Figure 9.3. In some cases there is a mixture of failure modes. This information can indicate how best to strengthen joints; interfacial failure indicates that an improved surface treatment is needed, and if failure is cohesive the adhesive may need strengthening with a mineral filler. How does the strength of lap joints vary with width and overlap? A simple view might be that strength will be proportional to area but this is not the case. Wang, Ryan and Schonhorn measured the strengths of some joints in aluminium etched in chromic acid and bonded with an epoxide adhesive with an aliphatic amine hardener. Strength was proportional to joint width, but a plot of strength against overlap tended to level out as overlap increased (see Figure 9.4). Perhaps even more

117

Strengths of Adhesive Joints

I cohesive

r

r

= material

Figure 9.3 Modes of fracture of single lap j o i ~ t .

Shear strength (kN)

Overlap (mm)

Figure 9.4 Depeiidence of joint strength upon overlap for 25 inin wide single lap joints. (After Wang, Ryan and Schonhorn)

Chapter 9

118

surprising, they inserted thin discs of polypropylene (a low surfaceenergy material which is difficult to bond) into the laps to produce non-bonded regions. Their results which are shown in Figure 9.5 show that strength is independent of bonded area. The reason for this behaviour is the pattern of stress distribution within the joint. The first attempt to analyse these was by Volkersen who assumed that the stress in each adherend falls to zero at the free-end of the overlap, and hence the strain decays in a proportionate manner. Metals and composites are much stiffer than adhesives; some comparative values of Young’s modulus are 70.5 GPa for aluminium, 210 G P a for mild steel and 2-4.5GPa for epoxide adhesives, so the strain in the adherends is imposed on the more compliant adhesive. The situation is illustrated with much exaggerated strain and glue-line thickness in Figure 9.6. It can clearly be seen that greatest distortion of the adhesive, and hence greatest stress, occurs in the adhesive elements at the edges of the joint. A further complication is that the overlap region also bends, as in Figure 9.7, producing a mode I force at the ends of the adhesive layer. Goland and Reissner analysed this and some of their results for 25 x 25 mm laps bearing a load of 5780 N are shown in Figure 9.8. It can be seen that both mode I1 and mode I forces are concentrated at the ends. The mean shear stress is 8.96MPa, but this is concentrated to give a maximum of 96.5MPa very near the ends. The central region bears

Shear strength (kN)

Bonded area (cm3)

Strengths of Adhesive Joints

119

Figure 9.6 Volkersen's distortion of a single lap joint. Straiit and bond-line thickness are rizuch exaggerated.

__1 Figure 9.7 Cylindrical bending of the overlap region of a single lap joint (top) and lateral shrinkage leading to mode I11 stress (bottom).

120

Chapter 9

300

I I

I

Stress (MN m-’)

I

MODE1

200

100

0

Distance from edge of joint (mm) 0

2

4

Figure 9.8 Distributions of mode I and mode II stresses in ci 25riznz square sirlgle lap joint, subjected to n load of 5780 N . (Goland and Reissner)

virtually no load. When the joint breaks, cracks will be initiated where stresses are greatest, and they will then propagate through the joint. Adhesive is often extruded from the joint to produce a wedge-shaped fillet; these are often the location for crack initiation. Cleavage stresses can be reduced by using double lap joints. Mode I11 stresses arise in lap joints due to the narrowing of the adherends as they stretch, as shown in Figure 9.7.

PEEL TESTING Kaelble has proposed the following treatment for peeling a flexible tape from a rigid substrate, to which it had been bonded using a flexible adhesive. The peeling force P is assumed to produce a steady rate of peeling. The situation is illustrated in Figure 9.9. Kaelble’s treatment assumes that the tape is pivoted about the point 0,such that there is a cleavage force to the right of 0,and a compressive force just to the left. Equation 9.1 gives the normal stress G.

cJ=

2pexp(px)([PMc + P sin o]cos ox b

+ PM,sin ox)

(9.1)

121

Strengths of Adhesive Joints

Figure 9.9 Peeliiig of CI JJexibletape porn a rigid adherend.

where

p = (3 Y/8Eh3U)+

(94

M , is the peeling moment of P about 0, Y and E are Young’s moduli of adhesive and tape, respectively, and b is the width of the tape. The equation is that of a damped sine-wave, as shown in Figure 9.10, showing that some regions are in tension and some in compression. The amplitude of oscillations, and hence stress increases with p. Hence the reduction of stresses and consequent improvement in joint strength can be achieved by: (i) (ii) (iii) (iv)

increasing increasing increasing increasing

adhesive flexibility, i.e. reducing Y; the modulus of the tape E; tape thickness; the thickness of the adhesive.

For lap joints a thin bond-line is preferred because there will be a reduced chance of it containing a critical flaw for crack initiation, but a thick one is best for resistance to cleavage. Very thin glue-lines risk starvation and a good practical optimum thickness for lap joints is 0.1 mm. In some studies, peel strength at first increased with adhesive thickness but then generally levelled out at about 0.25mm. The variation of peel strength with angle is given by equation 9.3, where rn is the sum of cleavage moments and I is the moment of inertia of the tape section. P = rn2/2EI(1 + cos ci))

(9-3)

122

Chapter 9

-c-----Distance inside bond

Figure 9.10 Pattern of irol-ma1 stresses in n peel joint.

A criticism of the Kaelble theory is that it only deals with the statics and not the dynamics of peel. In some studies peel force was, on some occasions, proportional to the square root of peel rate, but that data could always be fitted to a linear log(force)-log(rate) plot.

BOEING WEDGE TEST So far the methods described need the use of a mechanical testing instrument; these are always expensive. The Boeing wedge test (Figure 9.11) dispenses with this. Two stiff adherends are bonded together, leaving a non-bonded section at one end; inserting a film of polyolefin or PTFE can be useful here. A metal wedge is forced into this to initiate a crack. The joint is then exposed to some hostile condition such as warm, wet air, and the increase in crack length is measured. It is particularly useful for examining the effect of surface treatments on wet-durability. Some results are given in Figure 9.12. Crack length can be measured by holding the sample up to light and using a plastic ruler. TACK Tack is the ability to bond under conditions of light pressure and short time, and can be measured by the time needed for a ball or cylinder to roll down an inclined plane coated with the adhesive, or by a probe method.

123

Strengths of Adhesive Joints

c

Figure 9.11 The Boeing wedge test.

E+CAA

GB

/x*-x-x

Q,

&

X-

E+PAA 0

I

I

I

100

200

300

Time (h)

Figure 9.12 Efect of surjiiice treatment on crack growth in Boeing wedges of clad aluminizirn alloy on ageing at 50°C and 96% r.h. Surfnce treatments are DG = ulkaline degrease, E = DG + chromic acid etch, GB = wet alumina grit-blast + DG, C A A = DG + E + chromic acid anodize, P A A = DG + E + phosphoric acid anodize. (Reprinted from P. Poole and J.F. Watts, Int. J . Adhesion Adhesives, 1985, 55, 33. With kind permission from Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington OX5 1GB)

Here a probe is lowered at a constant speed onto the adhesive coated surface, and, after a fixed dwell time, the force needed to remove it is measured. Some results obtained with a range of plastic probes are shown in Figure 9.13. It can be seen that tack force increases with the critical surface tension of the probe surface, and with dwell time (Figure 9.14). A

Chapter 9

124

0 20

0

40

critical surface tension of probe material / mNm-'

Figure 9.13 Force iieeded to reiwove plastic probes of diferent crjtictil surjiice tension from three adhesives. (After Counsell and Whitehouse)

z . e,

2

P

I

Y

5

O

L

I

0

20

1

40

60

dwell time I s

Figure 9.14 Dependence of tack force on probe dwell-time for three adhesives. (After Counsell and Whitehouse)

Strengths of Adhesive Joints

125

short time is required for the adhesive to wet the probe. Interfacial free tension is the driving force for wetting, and this is opposed by the viscosity of the adhesive.

REPORTING THE RESULTS It is best to report the strength of a lap joint as the force needed to break it in newtons, at the same time specifying the joint geometry. This is preferable to the failure stress, which, because of the complexity of stress distribution in lap joints, will vary with joint geometry. Some results obtained by a skilled worker with much experience in joint assembly and testing are given below. The adherends were of aluminium alloy, which had been degreased and etched in chromic acid, and bonded with an epoxide adhesive into 25mm square lap joints, which were cured for 3 h at 80 "C. They were tested at a crosshead speed of 6mmmin-', and all failed cohesively. Joint strengths (kN) 17.3, 18.7, 15.8, 20.4, 17.8, 20.4, 14.2, 15.8. Mean = 17.5 kN. Standard deviation = 2.2 kN or 12%. Scatter can often be reduced by preparing a large panel which can be sawn into individual test specimens. Peel testing has the advantage that a continuous record of the fluctuating peel strength can be obtained, and an indication of scatter is given by a single joint. It is usual to ignore the first and last 25mm of peeling and to give the result in force per unit width (N m - I). The mode of failure is recorded, and if a mixture occurs then the approximate percentage of each can be based on visual assessment.

Chapter 10

Adhesive Joints and the Environment

The use of adhesives can threaten the environment by pollution with solvents from surface cleaning and from adhesives, and regulatory pressures to replace them are high. There are also pressures against the use of chromium compounds both in primers and treatment baths. Adhesives can play a part in the recycling of materials. This includes selecting adhesives for labels on bottles and other containers that can be removed during washing, such as in alkaline solution at 70 "C. The use of zirconium compounds in recycling waste paper has been referred to in Chapter 3. The factors in the environment that can attack and deteriorate adhesive joints are oxygen, UV, water and salt-spray. The main problem with oxygen and UV is chemical degradation of adhesives, and the incorporation of chemical stabilizers is the solution. The problem with water is weakening of the interface, and here the solution is in surface treatment or the use of silane coupling agents. UV degradation is not usually a problem if both adherends are opaque; however, it should be borne in mind in bonding glass, and in designing joints where free edges and fillets of adhesive might be exposed to sunlight.

ANTIOXIDANTS Antioxidants may be added to pressure-sensitive, hot melt, and polychloroprene contact adhesives. Oxygen is a diradical and can initiate free-radical degradation. It can be absorbed by polymers with the formation of hydroperoxide groups (OOH), which can later decompose to free radicals; see Scheme 10.1. Primary antioxidants include hindered phenols and arylamines. Butylated hydroxytoluene (BHT) is a hindered phenol, which reacts with 126

127

Adhesive Joints and the Environment

I

-C-H

I

+

02

-

I I

-C-OOH Hydroperoxide

I I

-C-OOH

-

I

+

-C-0'

I

HO'

Scheme 10.1

free radicals according to Scheme 10.2. The hindered free-radical which is formed cannot react because of the bulky -C(CH,), units.

CH3

CH3

BHT

Scheme 10.2

BHT is a very well known antioxidant, and is a very effective radical scavenger, but it has the major drawback of being volatile. Volatility can be reduced by joining a number of hindered phenol moieties to make a larger molecule. The structures of some less volatile antioxidants are shown in structures 10.1 and 10.2.

C

4

(10.1)

2,2'-Methylenebis(4-methyl-6-t-butylphenol)

(10.2) lrganox 10IOTM(Ciba Geigy)

Hindered phenolic antioxidants

Secondary aromatic amines (Structures 10.3 and 10.4) work by supplying hydrogen atoms.

Chapter 10

128

(1 0.3) N,N -Diphenyl-l,4-diaminobenzene

Dioctyldiphenylamine

(1 0.4)

Phosphites remove hydroperoxide groups by Scheme 10.3; an example is tris(nonylpheny1)phosphite. OR’ I R1O-p

+

$00~

-

OR’ R~OH

+

I

R’O-P=O I

OR’ I

OR’.

Scheme 10.3

UV-STABILIZERS UV-absorbing stabilizers, such as 2-hydroxybenzophenone or a 2hydroxyphenylbenzotriazole, are converted into excited states on absorbing UV quanta, but then return to the ground state by transferring energy as heat to their surroundings. This is shown in Schemes 10.4 and 10.5. With 2-hydroxybenzophenones the R group can be H, CH, or C,,H,5 and this will affect compatibility with adhesives.

+uv

-heal

OR

enol

keto 2-Hydroxybenzophenonederivatives R = H, CH3, Ci2H25

Scheme 10.4

a>N+Q \

H-0

H- -0 2-Hydroxyphenylbenzotriazole

Scheme 10.5

129

Adhesive Joints and the Environment

WATER A N D ADHESIVES Water is the substance which gives the greatest problems in terms of the environmental stability of adhesive joints. The great majority of bonded structures are exposed to moist air, and if the relative humidity is high, then over a period of time the strengths of joints usually fall. Water is a problem because it is very polar and has a high value of the polar component of surface free-energy. Other common fluids such as lubricants and fuels are of low or zero polarity and do not weaken joints significantly. All polymers absorb water to some extent and although its effect on the interface is paramount, it can also alter the properites of the bulk adhesive by changing the glass transition temperature, inducing cracks or by chemical reaction. Epoxide adhesives only appear to be hydrolysed under extreme conditions. Also, experience with joints broken after exposure to water shows that it attacks the interface and not the adhesive. The water uptake properties of adhesives can be assessed by immersing films in water and recording increases in weight. The results of such an experiment for an epoxide adhesive are shown in Figure 10.1. It is plotted in the form of fractional uptake against the square root of time. The diffusion coefficient (D)can be obtained from the initial slope by using equation 10.1, and the water uptake at equilibrium from the plateau. Here M , and M e are the masses absorbed at time t and at equilibrium, and I the film thickness.

0

100

50

P r' . I o5s" m-' Figure 10.1 Kinetics of water uptake by un epoxide adhesive at 45°C. (After Brewis and Comyn)

Chapter 10

130

(10.1)

M J M , = 4(Dt/n)*/l

Values of D and M , for some structural adhesives are given in Table 10.1. Attempts have been made to synthesize epoxide adhesives with improved water resistance by replacing some hydrogen atoms by fluorine. It is perhaps the cost of such materials that has restricted commercial development. Water is expected to lower the glass transition temperature, and therefore the upper service temperature. Glass transition temperatures of wet polymers are given by the Fox equation (equation 10.2), which is based on the concept that the free volumes of components are additive. (10.2) Here Tgpand T,, are the glass transition temperatures of polymer and water and wp and w, are their weight fractions. The glass transition temperature of water is in the range - 124 to - 138 "C. Data for some epoxide adhesives are given in Table 10.2. The samples were stored in water at 25 "C and T, measured at equilibrium and also after 10 months. It can be seen that water initially lowers Tgin all cases but this rises on prolonged immersion and, in the cases of TETA and D D M hardeners, Table 10.1 Water uptake of structural adhesives.

Nitrile-phenolic

25 50 25 Vinyl-phenolic 50 F M 1000 epoxide-polyamide 1 25 50

3.3 4.7 1.8 2.3 0.075 1.1 3.2

Toughened acrylic adhesives with chlorosulfonated PE 23 23 with nitrile rubber

0.64 0.19

1.50 4.5 3.5 8.6 (20.4) (15.8) (15.5) 0.73 1.72

Wood adhesives

Urea-formaldeh yde Melamine-formaldehyde Phenol-resorcinolformaldehyde

25 40 25 40

0.25 0.49 0.21 0.39

7.4 8.2 27.2 41.0

25 40

1.1 1.8

13.9 14.4

131

Adhesive Joints and the Environment

Table 10.2 Glass transition temperatures of epoxide adhesives based on DGEBA. Tg("C)

Hardener Dry

DAPEE TETA DAB DDM

67

99 161 119

Lowering of Tn

After initial After uptake 10 months

Measured

Fox eqn.

37 86 143 110

30 13 18 9

23 23 22 27

49 111 157 130

this rise is to above the Tgof the dry adhesive. Further crosslinking is the probable cause. In most cases the Fox equation predicts that Tgwill be lowered more than is observed. Water has a marked tendency to form droplets in polymers, and such water would not contribute to plasticization. The values of M e given in Table 10.1 are all for films of adhesives immersed in liquid water or saturated vapour. At a fixed temperature, water and its saturated vapour are in equilibrium and a consequence is that the values of D and M e for films immersed in either phase should be the same. However, if a film is immersed in unsaturated vapour it will absorb less water. The absorption isotherm for an epoxide adhesive is shown in Figure 10.2 (the abscissa is activity). It shows that, under typical ambient conditions of 50% r.h., a significant amount of water is absorbed, even though joints are not weakened under such conditions. The effect of absorbed water on the mechanical properties of cured films of some structural adhesives is given in Table 10.3.This shows that water lowers tensile strength and modulus but increases elongation at break; these parameters largely recover on drying. It is my invariable experience that when films of cured epoxide adhesives are left in water for some time, a small residue remains after evaporation of the water. The IR spectra of these residues generally resemble the adhesive; the difficulty is to decide whether the residues are due to material which has not reacted or which has been removed by h yd r oly sis. Antoon and Koenig used IR spectroscopy to follow changes in an epoxide adhesive, made from DGEBA and methyl nadic anhydride, which was immersed in water at 80 "Cfor up to 155 days. Their data are shown in Figure 10.3.

132

Chapter 10

L

0

I

I

0.5

1.C

Figure 10.2 Water absorption isotherm of the DGEBA-DAPEE epoxide adhesive at 45 "C. (Reprinted from D. M. Brewis, J. Comyn and J. L. Tegg, Polymer, 1980, 21, 134. With kind permission from Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington OX5 1GB)

Table 10.3 Efect of water on the mechanical properties of structural udhesiues. ~~~

Exposure conditions

Weight Tensile Elongation Modulus Failure gain strength at break mode (YO) (MPa) (%) (MPa)

F M I000 eposide-polyamide None 3 months at 65% r.h. 2.9 5d in water at 50°C 9.4 5 d in water at 50"C, then dry at 60°C for 2 d 3.3 DG EBAIDAPE E epoxide None 24 h in water at 100°C 24 h in water at lOO"C, then dry at 65 "C for 2 d

73 52 19

5.0 263 260

1880 623 3.0

brittle ductile rubbery

76

5.7

1980

brittle

41 24

7.1 37

1700 1020

ductile ductile

52

6.8

1560

ductile

In the early stages, hydrolysis and leaching of unreacted anhydride groups were dominant, but longer term experiments ruled out hydrolysis of the polymer network. The application of stress changed the situation quite markedly in that the intensity of the IR peak due to

133

Adhesive Joints and the Environment

I

I

40

80

t lday

Figure 10.3 Fraction of ester groups which have not been hydrolyzed n s a function of time for an anhydride cured epoxide under varying stress levels in water ( p H = 11.9) at 80 "C. Stress levels: 0 = zero, 0 = 19 M P a , A = 2 6 M P a and 0 = 54 MPa. (After Antoon and Koenig)

est E oups at 1744cm- declined due to hydrolysis. Hydrolysis is also accelerated by alkalinity. The rates of hydrolysis were interpreted by stress reducing the activation energy of hydrolysis (&). Its value is 45 kJ mol- when unstressed, and a stress of 54 MPa lowers this by 17kJ mol-'. The effect of stress upon the rate of destruction of chemical bonds is given by equation 10.3, which is a modified Arrhenius equation. Here k is the rate constant, S is stress and II is a constant.

The equation has been verified for the mechanical destruction of some polyolefins, and it fits the data of Antoon and Koenig shown in Figure 10.3, giving a value of u = 1.2 kJ mol- MPa- '. The work of Antoon and Koenig shows that the adhesive only hydrolyses under the combined influences of elevated temperature, stress and high pH. It is my

'

Chapter 10

134

view that under normal conditions structural adhesives are not hydrolysed.

WATER AND JOINTS Experimental conditions to which joints are exposed in durability tests include deserts and jungles and tropical greenhouses, and accelerated tests are carried out in chambers in laboratories at elevated temperatures. A not wholly frivolous criticism of accelerated ageing is ‘that boiling an egg never produced a chicken!’. A simple and reliable method of obtaining constant relative humidity (r.h.) is to use a container of not more than a few litres capacity, with a layer of water (for 100% r.h.) or saturated salt solution in the bottom. Relative humidities over saturated salt solutions generally vary slightly with temperature, and at 25°C some values are NaCl -75.4%; Mg(NO3),.6H,O, - 53.0%; and MgC1,-6H20, - 32.7%. Saturated salt solutions are self-buffering in that if water is added more salt will be dissolved, and loss of water vapour will result in salt precipitation; it is essential that the solutions always contain some salt crystals. General Patterns of Behaviour There are many cases in the literature of adhesive joints with metal adherends and rigid adhesives being weakened on exposure to wet surroundings, and a common feature is the shape of the strength c’ersus time plots. Strength falls most rapidly at the beginning and then slows down to a low or zero rate of weakening. The initial rate and overall fraction of strength lost will vary with the adhesive and surface treatment, and the latter is seen as the best way ofgivingjoints maximum durability to water, as previously shown in Figure 2.1. Minford exposed aluminium joints in a South American jungle for up to 12 years and observed that adhesive, type of aluminium alloy and surface treatment all affect durability. Generally, poor durability is expected for metal adherends that have just been adbraded or degreased, but aluminium gives excellent durability after etching or anodizing in chromic or phosphoric acid (see Chapter 2). Some results from laboratory experiments with aluminium joints and a range of epoxide and phenolic adhesives, exposed at 50 and 100% r.h. at 50°C, are shown in Figures 10.4 and 10.5. The data show the following features:

Adhesive Joints and the Environment

135

+10 0 W

-60 0

5Ooo

loo00

TimcIh

Figure 10.4 Strengths of alirrniniuinjoints with rnodijied epo.uide adhesive O H esposure to wet air at 50 "C. Oaged at 50% r.h. nnd 0100% r.h., Ajoiiits aged at 100% r.h. for 50001~then dried at 50% r.h. for a further 5000 h. (Reprinted from J. Comyn, D. M. Brewis and S. T. Tredwell, J. Adhesion, 1987, 21, 59; Crown Copyright)

-lo 0

1 ,

N

-40

6

0

5000

Timclh

Figure 10.5 Strengths of aluminium joints with vinyl phenolic adhesive on exposure to wet air at 50 "C. Oaged at 50% r.h. and 0100% r.h., Ajoints aged at 100% r.h. for 5000h then dried at 50% r.h. f o r a further 5000 h. (Reprinted from J. Comyn, D. M. Brewis and S. T. Tredwell, J . Adhesion, 1987, 21, 59; Crown Copyright)

136

Chapter 10

(i) At 100% r.h. joint strengths initially fall, typically by 40-60%, but then tend to stabilize. (ii) There is little or no weakening on ageing at 50% r.h. (iii) There is much recovery of strength when joints which have first been aged at 100% r.h. for 5000 h are then stored at 50% r.h. for 5000 h. These are the triangular points in Figures 10.4 and 10.5. (iv) Metal primers improve the strengths of dry and exposed joints. (v) On exposure at 100% r.h. the amount of interfacial failure increases.

In high humidity, stressed joints weaken more rapidly than unstressed ones. Times to failure for joints with mild steel and zinc-coated steel adherends are shown in Figure 10.6. It shows that all unstressed joints survived for 2.5 years and that survival decreases as stress increases. The high impact adhesive gives joints which are more resistant to the 3 kN load. Joints with a low glass transition temperature, flexible adhesive

-Average 01 all mild sleel joints

3 kN LOAD

nFlexbble Adhesive

Toughened Ad hes I ve

m

High Impact Adhesive

Joints still surviving

-44%

-

1 2 kN

LOAD

56%

-

NO LOAD

-0

0

I

2

100%

100% 100%

3

Exposure t I me (years)

Figure 10.6 Average failure times for stressed and unstressed zinc-nickel coated steel joints in a tropical environment. (Reprinted from R.E. Davis and P.A. Fay, Int. J . Adhesion Adhesives, 1993, 13, 97. With kind permission from Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington OX5 1GB)

Adhesive Joints and the Environment

137

failed by creep of the adhesive at relatively short times. Salt water is particularly damaging to metal-adhesive joints; it causes corrosion at the adhesive-metal interface. Water Diffusion into Bond-lines With impermeable metallic adherends, the diffusion of water into the adhesive layer can be calculated from diffusion coefficients. In Figure 10.7 some joint strengths are compared with the total amount of water calculated to have entered the adhesive layer. The two ordinates have been adjusted to give a good fit, but it is clear that the rate of loss of strength is controlled by water diffusion. The uptake of water by adhesive joints can be calculated as follows. Fick’s 1st law of diffusion (equation 10.4) states that the flux in the x-direction (F,) is proportional to the concentration gradient. Flux is the amount diffusing across a unit area in a unit time, F,

= -

m/ax

( 10.4)

For a small box-shaped element in Cartesian space, the sum of the fluxes across the six faces will control the build up or decay of diffusant in the element, leading to Fick’s 2nd law of diffusion (equation 10.5).

-8

+

0.4

0

-

a

0

*-

1.0

5 0,

$

c

2 ’j .c

20.2 -

-

0.5

0

c

0 .-c

0

$

0

-0

I

I

I

I

I

138

Chapter 10

Experiments are frequently designed so that diffusion is restricted to the x-direction, giving equation 10.6.

Equation 10.7 is the solution to equation 10.6 for concentrations ( C )of diffusant at points within a film or thin slab immersed in water or air at constant humidity. The term C, is the concentration at equilibrium and the faces of the film are located at x = L and x = - L.

+

X

+ l)]exp( - D(2n + 1)2~2t/4L2)

C/C, = 1 - ( 4 4

[( - 1)”/(2n n=O

cosC(2n + l)nx/2L]

(10.7)

At short times equation 10.7 takes the simpler form of equation 10.1. Integration of equation 10.7 gives equation 10.8, where M , is the weight absorbed at time t, and M e the weight absorbed at equilibrium. x

M,/M, = 1 -

[8/(2n

+ 1)2n2]exp(- D(2n + 1)2n2i/4L2)(10.8)

n=O

Equations 10.7 and 10.8 can be applied to uptake by rectangular adhesive layers in joints. If two slabs intersect at right angles, then concentrations at points within the prism of intersection are the same as in the adhesive layer. If the concentrations at points in the slabs along the x- and y-axes are C, and C, (given by appropriate forms of equation 10.7) then concentrations at points with coordinates x and y in the adhesive layer are given by equation 10.9.

Similarly, the total uptake of water by the adhesive is related to uptake in the separate slabs by equation 10.10.

The diffusion of water in adhesives is an activated process that obeys the Arrhenius equation, which means that the rate will increase quite strongly with temperature. This is illustrated by the data in Figure 10.8, which are for some butt joints in mild steel, prepared by degreasing and grit-blasting and bonded with an epoxide, and immersed in water at

139

Adhesive Joints and the Environment n

v

0

500

90°C; H20

+ 1000 1500 2000

" 2500

Time in environmenth

Figure 10.8 Weakening of butt joints on immersion in water. (After Gledhill and Kinloch)

temperatures between 20 and 90 "C. The diffusion coefficients, which reflect the rates of weakening, are given in Table 10.4. Critical Humidity

Although joints are weakened by exposure to air at high humidity, it is widely observed that joints can withstand exposure at lower humidities (e.g.50% r.h. or less) for very long periods without weakening. It is often the case that researchers leave control joints in the laboratory for sometimes up to 11 years, where the relative humidity is mostly about 50%, without weakening. There is a large amount of data of this kind. This indicates that there is a critical humidity for weakening, which implies there must also be a critical water concentration within the adhesive. An attempt to locate the critical conditions for some aluminium lap joints bonded with a epoxide adhesive based on DGEBA and 1,3diaminobenzene used sandblasting to prepare the metal with the thought that this would give joints which are sensitive to water. Figure 10.9 plots joint strength against r.h. after more than one year, showing a kink within the experimental scatter at 65% r.h., corresponding to a critical water concentration of 1.45YO.

140

Chapter 10

Table 10.4 Dijfusion coefJicients of water in epoxide adhesive DGEBA with 2,4,6-tris(dimethylaminomethyl)phenol-tri-2-ethylhexanoate.

20 40 60 90

1.40 1.60 2.20 6.90

2.43 6.50 18.1 60.7

0

50

100

Relative humidity (%)

Figure 10.9 Dependence of joint strength upon relative humidity nfrer 10080 h exposure. (Reprinted from D. M. Brewis, J. Comyn, A. K. Raval and A. J. Kinloch, Int. J. Adhesion Adhesives, 1990, 10, 247. With kind permission from Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington OX5 1GB)

Adhesive Joints and the Environment

141

Water absorption isotherms of epoxide adhesives d o not have kinks, but are usually gentle curves (e.g. Figure 10.2) and a significant amount of water is absorbed by the adhesive at about 50% r.h. However, the water uptake properties of the adhesive are not a key factor, because with time the interface will reach equilibrium with the surroundings irrespective of the intervening adhesive. Mechanisms of Weakening Recovery ofjoint strengths clearly indicates that reversible processes are involved, and the increasing amounts of interfacial failure with exposure show that interfacial phenomena are more important than the bulk properties of the adhesive. Attack at the interface may involve the displacement of adhesive by water, as predicted by the physical adsorption theory when the work of adhesion is negative, displacement of hydrogen bonds by water, or weakening of ion-pairs. Alternatively, the oxide layer itself might be subject to hydration and loss of cohesive strength. If ion-pairs contribute to the interfacial force, the latter will be given by equation 10.11. (10.11)

Here q1 and q2 are the ionic charges, E, is the permittivity of a vacuum, E , the relative permittivity of the medium and r the inter-ionic distance. Epoxides have low values of q(4-5) and other structural adhesives are probably the same; however, the value for water is about 80 at room temperature, so a small amount of water would increase E , and lower F , not to zero but to a fraction of the original value. Removal of the water would restore the strength of the joint. The variation of the relative permittivities of mixtures of water/organic solvents with composition is approximately linear, and if this is the case for water-adhesive mixtures then strength losses can be calculated. Data for some structural adhesives with aluminium, using E, for the adhesives as 5 and 80 for water are given in Table 10.5, where agreement between measured and calculated losses is quite good. The ion-pair concept allows for partial weakening of joints in the presence of water and recovery on drying. This contrasts with the physical adsorption theory, which predicts the total loss of strength as water displaces adhesive from the metal oxide, and no recovery as an adhesive in the glassy state will not have the molecular mobility to re-establish intimate molecular contact with the substrate.

142

Chapter 10

Table 10.5 Comparison of measured and calculated falls in joint strength on exposure at 100% r.h. at 50°C. Adhesive

Fall in joint strength (%)

Modified epoxide BSL 312 Epoxide nylon FM 1000 Epoxide DGEBA/DAPEE Nitrile-phenolic 1 Nitrile-phenolic 2 Vin yl-phenolic

Measured

Calculated

50 78 40 54 37 45

36 68 45 40 20 56

I

E E

Y

t! u)

?n ! L

1 0

n

P

5

4

3

2

1

0

Moles H,O to 1 mole CuSO,

Figure 10.10 Vapour pressure of water in equilibrium with hydrates of copper sulfate. (Reprinted from D. M. Brewis, J. Comyn, A. K. Raval and A. J. Kinloch, Int. J. Adhesion Adhesives, 1990, 10, 247. With kind permission from Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington OX5 1GB)

A possible explanation of critical humidity lies in salt hydration. Taking copper sulfate as an example, at 25 "C the vapour pressure of water in equilibrium with CuS04.5H,0 is 7.8 mmHg. As this salt loses water to form CuS04.3H,0 the vapour pressure remains constant, but it then falls precipitately to 5.6 mmHg. It stays constant at this value as the

Adhesive Joints and the Environment

143

trihydrate converts into the monohydrate and then falls sharply to 0.8 mmHg. It remains at this level as CuSO,.H,O dehydrates and finally falls to zero when all the water is lost. These changes are illustrated in Figure 10.10. The occurrence of a constant vapour pressure for a salt-hydrate system requires two solid phases in addition to the vapour. The vapour pressure of the system is then definite and independent of the amounts of the two hydrates. Salts, or more specifically cations which are susceptible to hydration, might occur at adhesive interfaces in a number of ways. These are that sodium chloride is a by-product in the synthesis of DGEBA, and metal surfaces are rinsed in tap water, which contains salts. What might occur is that water would first be consumed by hydration, but once hydration was complete and at a specific humidity, water would then be available to attack the interface.

Recommended Reading

Books relevant to more than one chapter R. D. Adams, J. Comyn and W. C. Wake, ‘Structural Adhesive Joints in Engineering’, 2nd Edition, Chapman & Hall, London, 1997. F. Garbassi, M. Morra and E. Ochiello, ‘Polymer Surfaces from Physics to Technology’, John Wiley & Sons, Chichester, 1994. G. C. Mays and A. R. Hutchinson, ‘Adhesives in Civil Engineering’, Cambridge University Press, Cambridge, 1992. A. J. Kinloch, ‘Adhesion and Adhesives; Science and Technology’, Chapman & Hall, London, 1987. W.C. Wake, ‘Adhesives and the Fundamentals of Adhesion’, 2nd Edition, Applied Science Publishers, London, 1982. ‘Adhesives and Sealants’, Volume 3, ‘Engineering Materials Handbook’, ASM International, 1990. ‘Durability of Structural Adhesives’, ed. A. J. Kinloch, Applied Science Publishers, London, 1983. ‘Handbook of Adhesion’, ed. D. E. Packham, Longman Scientific and Technical, Harlow, 1992. ‘Handbook of Adhesives’, 3rd Edition, ed. I. Skeist, Van Nostrand Reinhold, New York, 1990. ‘Handbook of Adhesive Technology’, ed. A. Pizzi and K.L. Mittal, Marcel Dekker, New York, 1994. ‘Surface Analysis and Pretreatment of Plastics and Metals’, ed. D. M. Brewis, Applied Science Publishers, London, 1982. ‘Wood Adhesives: Chemistry and Technology’, ed. A. Pizzi, Marcel Dekker, New York, Volume 1, 1983; Volume 2, 1989. Chapter 1 J. M. G. Cowie, ‘Polymers: Chemistry & Physics of Modern Materials’, 2nd Edition, Blackie, Glasgow, 1991. 144

Recommended Reading

145

Chapter 2

R. F. Wegman, ‘Surface Preparation Techniques for Adhesive Bonding’, N o yes Publications, Park Ridge, New Jersey, 1989. Chapter 3

J. Comyn, Chapter 8 in ‘Structural Adhesives; Developments in Resins and Primers’, ed. A. J. Kinloch, Elsevier Applied Science Publishers, London, 1986. E. P. Plueddemann, ‘Silane Coupling Agents’, Plenum Press, New York, 1982. Chapter 4

‘Chemistry and Technology of Epoxy Resins’, ed. B. Ellis, Blackie Academic and Professional, Glasgow, 1993. ‘Structural Adhesives: Chemistry and Technology’, ed. S. R. Hartshorn, Plenum Press, New York, 1986. ‘Epoxy Resins: Chemistry and Technology’, ed. C. A. May, Marcel Dekker, New York, 1988. Chapter 5

A. F. M. Barton, ‘Handbook of Solubility Parameters and Other Cohesion Parameters’, CRC Press, Boca Raton, Florida, 1983. H. Warson, ‘Polymer Emulsion Adhesives’, Solihull Chemical Services, Birmingham, 1993. ‘Wood Adhesives: Chemistry and Technology’, ed. A. Pizzi, Marcel Dekker, New York, Volume 1, 1983; Volume 2, 1989. Chapter 6

‘Handbook of Pressure Sensitive Adhesive Technology’, 2nd Edition, ed. D. Satas, Van Nostrand Reinhold, New York, 1989. Chapter 7

D. Briggs and G, Beamson, ‘High Resolution XPS of Organic Polymers; The Scienta ESCA300 Database’, John Wiley & Sons, Chichester, 1992.

Recommended Reading

146

D. Briggs, A. Brown and J.C. Vickerman, ‘Handbook of Static Secondary Ion Mass Spectrometry (SIMS)’, John Wiley & Sons, Chichester, 1989. ‘Practical Surface Analysis’, 2nd Edition, Volume 1, ‘Auger and X-ray Photoelectron Spectroscopy’, ed. D. Briggs and M. P. Seah, John Wiley & Sons, Chichester, 1990. ‘Practical Surface Analysis’, 2nd Edition, Volume 2, ‘Ion and Neutral Spectrometry’, ed. D. Briggs and M.P. Seah, John Wiley & Sons, Chichester, 1992. Chapter 8 L. H. Sharpe and H. Schonhorn, ‘Contact Angle, Wettability and Adhesion’, American Chemical Society Series, No. 43, 1964. S. Wu, ‘Polymer Interface and Adhesion’, Marcel Dekker, New York, 1982.

Chapter 10 J. Crank, ‘Mathematics of Diffusion’, 2nd Edition, Oxford University Press, Oxford, 1975.

There are three journals in the field in the English language, namely International Journal of Adhesion and Adhesives, Journal of Adhesion and Journal of Adhesion Science and Technology.

Subject index

Abhesion, 24-25 Abietic acid, 71-72 Abrasion, 19 Acid anhydrides, 42-43, 52, 131 Acrylic adhesives, 2, 10, 12, 14-15, 37,45-47,62, 70, 130 Adherend, 2 Aluminium, 1, 20-23, 27, 30-32, 34-36, 74, 112, 116, 125, 135, 137, 139 Amine hardeners, 14-16, 38, 41-43, 52, 112, 116, 131, 139 Anaerobic adhesives, 47 Animal glues, 1 Anodizing of metals, 22-23, 28, 123 Antioxidant, 65, 71, 126-128 ATBN (amine terminated butadiene nitrile copolymer), 49-50

Composites, 24 Concrete, 24, 26 Contact adhesives, 55 Contact angle, 2, 7, 32, 98-113 Copolymerization, 13, 28, 30, 38, 71 Copper, 30 Corona-discharge treatment, 19, 37, 80,84 Corrosion inhibitors, 26, 27 Coupling agents, 37, 38 Covalent bonds, 7, 29, 32, 50 Critical humidity, 139-141 Critical surface tension, 108-109, 123 Creep, 65 Crosslinking, 3, 13-14, 51, 56, 62 Cyanoacrylate adhesives, 2, 13, 47 Cyanuric chloride, 38

Boeing wedge test, 122-123 Biological adhesion, 2 Brass, 81-83 Brittle point, 62-63

Derivatization, 77-79 Dicyandiamide, 43 Diffusion of water, 129-130, 137-140 Diglycidyl ether of bisphenol-A (DGEBA), 14-15,4041, 131-132, 139-141 Dispersion forces, 63, 101, 105 Dipoles, 5, 25 Durability, 20-21, 122, 134-143

Cadmium, 30 , Carboxyl terminated butadiene nitrile copolymer (CTBN), 49-50 Cellulose, 38, 57 Chemical degradation, 2, 52 Chemisorption, 26 Churchill Memorial Screen, 3 1

Emulsion adhesives, 1, 13, 60-64, 70, 76-77,96, 105-106 147

148

Subject index

Epoxide adhesives, 1, 10,11, 14,27, 30-32,37,38, 4M3,48,52,98,

108,110,112,114,116,129-133, 142 Environment, 19,54,126-143 Etching of metals, 19-21,28,116, 123,125 Ethylene vinyl acetate (EVA), 65 Failure modes, 116-117, 125 Fillers, 4,65, 116 Flame treatment, 19,80,84,112 Fluoropolymers, 24,108-109 Fracture, 48-49, 114-1 17 Gel-point, 14 Glass, 24,28,30, 31, 74,81-82,114 Glass transition temperature, 2,4,9, 14-16, 50, 52,62,130 Glue-line thickness, 1 15 High-temperature adhesives, 52-53 Hot melt adhesives, 1, 2,38,64-69,

126 Hydrogen bonds, 7,37,43,57,66,

141 Humidity, 134 Infra-red spectroscopy, 32-35,73,

89-94,131- 133 Interfacial failure, 116 Interfacial free energy, 105 Interfacial tension, 63,lO(r102 Ionic bonds, 7,30,141-142 Inelastic electron tunnelling spectroscopy (IETS), 35-36,73 Isocyanates, 7,38,50-51, 56,70,78 Isotopes, 86-87 Joint testing, 114-125 Lap joints, 98,114-120,135 Laser treatment, 19 Lead, 31,35, 114 Lewis acids and bases, 8-9

Melting point, 13,66-68 Metal fasteners, 2 Natural rubber, 1,3,15,57,62,70-71 Nitrocellulose, 37 Oils and greases, 10,18,19,55 Oxide surfaces, 18,27,35,107-108 Oxygen, 4,47,56,71,80,126 Peel strength, 27,30-31,120-122 Phenolic adhesives, 11, 27,43-45,

130,135,142 Phosphonic acids, 27 Plasma treatment, 19 Plasticization, 16,82,130 Polar forces, 63,105,129 Polar groups, 14,18,19,30,65,80,

106 Polyamide hot melt adhesives, 65-69 Polychloroprene, 1, 14,55-58,62,

87,126 Poly(dimethylsiloxane), 15, 18,24,

25,49-50,70,74,86,110 Polyester hot melt adhesives, 68-69 Polyethylene, 19,24,25,48,57,68,

70,80,84-86,94,98,107-108, 110 Polyethylene terephthalate (PETP),

75 Polyimides, 51-52 Poly(methy1 methacrylate), 9,14-15,

46,52,57,107-108,110 Polymerization, addition, 11, 28,

52-53 Polymerization, condensation, 11,

28,43,45,52 Polypropylene, 24,80,84-86,118 Polystyrene, 9,57,64,108,110,112 Polysulfides, 24,51 Polytetrafluoroethylene (PTFE), 18,

19-20,25,57,108,110,122 Polyurethanes, 1, 11, 50-51, 68,87 Polyvinyl acetate, 59-60,62 Poly(viny1 alcohol), 57,59-60,62

Subject index Poly(viny1 chloride), 9, 16, 57, 70, 96, 108 Polyvinyl butyral, 48 Polyvinyl formal, 48 Pot-life, 2 Pressure sensitive adhesives, 3, 16, 38, 70, 72, 126 Primers, 22-28, 70 Redox reactions, 12, 46, 47, 60 Release agents, 18, 25, 106 Release papers, 25 Resole, 44,51 Rosin ester, 71, 88-91 Rinsing, 21 Rubber modified adhesives, 1, 46, 48-49, 114, 130 Rubber-to-metal bonds, 1 Salt spray, 126 Secondary ion mass spectrometry (SIMS), 31-32, 73, 75, 82-93 Scanning probe microscopy, 94 Shelf-life, 2 Silane coupling agents, 24, 28-36, 126 Silica, 35, 74, 77, 112 Sodium, 19, 84 Sodium naphthalenide, 20 Solubility parameter, 56-57 Solvents, 3, 19, 71 Solvent-based adhesives, 55-58 Spreading, 99-100 Spreading pressure, 103-104, 107 Stabilizers, 4, 55, 126 Starch, 58-59 Steel, 19, 30, 33, 38, 54, 81, 112, 136 St yrene-bu tadiene copolymers, 70 Substrate, 2 Surface contamination, 87-88 Surface energy, 24, 63, 70, 80, 99, 105-107, 110

149 Surface tension, 8, 61, 63, 99, 101- 102 Surface treatment, 18-25, 126 Tack, 65, 122-125 Tackifiers, 16, 65, 71, 88-89 Theories of adhesion chemical bonding theory, 7-9, 29, 32, 50 diffusion theory, 9, 55 electrostatic theory, 9 mechanical interlocking, 10, 24 physical adsorption theory, 4-7, 24,26, 141 Titanates, 36 Titanium, 22, 34 UV cure, 2, 12, 46 UV degradation, 4, 71, 126, 128 Van der Waals forces, 5-7, 18, 105 Vegetable glues, 1 Versamids, 4 1-42 Viscosity, 2, 14, 26, 46, 54, 65 Viscoelasticity, 16, 72 Waste paper, 38 Water based adhesives, 58-64 Water, water vapour, 2, 8, 21, 24, 28, 29, 31, 45, 49, 50, 54, 60, 63, 100, l l c l l l , 129-143 Wetting, 108-109 Wood, 8, 23, 26, 38, 44-45, 130 Work of adhesion, 63, 109-113, 141 X-ray photoelectron spectroscopy (XPS), 25, 73-82 Zinc, 30, 82 Zirconates, 36 Zirconium, 37