Synthetic Polymer Adhesives

5 Synthetic Polymer Adhesives HUBERT J. FABRIS GenCorp Research Division, Akron, OH, USA and WOLFGANG G. KNAUSS California Institute of Technology, Pasadena, CA, USA 5.1

INTRODUCTION

131

5.2 ADHESIVE MATERIALS 5.2.1 Nonreactive Adhesive Systems 5.2.1.1 Solvent- and water-based compositions 5.2.1.2 Hot-melt adhesives 5.2.1.3 Pressure-sensitive adhesives 5.2.2 Reactive or Thermosetting Adhesive Systems 5.2.2.1 Polyurethane adhesives 5.2.2.2 Epoxy adhesives 5.2.2.3 Acrylic adhesives 5.2.2.4 Formaldehyde andfuran resin adhesives 5.2.2.5. Unsaturated polyesters 5.2.3 Hybrid Adhesive Systems 5.2.3./ Nitrile rubber-phenolic blends 5.2.3.2 Neoprene-phenolic blends 5.2.3.3 Polyvinyl-phenolic blends 5.2.3.4 Miscellaneous phenolic blends 5.2.3.5 Epoxy h)lbrid systems 5.2.4 High-performance Adhesives

133 134 134 137 139 141 141 143 144 145 147 147 147 148 148 148 149 149

5.3 SURFACE PREPARATION

150

5.4 SURFACE ANALYSIS

152

5.5 PHYSICAL PROPERTIES OF ADHESIVES 5.5.1 Thermomechanical Behavior 5.5.2 Physical Aging of Polymers 5.5.3 Diffusion of Solvents and Water

154 158 158 158

5.6 LOAD TRANSMISSION THROUGH ADHESIVE BONDS 5.6.1 Stress Distributions in Bonded Joints 5.6.1.1 Integral bond geometries 5.6.1.2 Thermal stresses 5.6.1.3 Cracked test geometries

159 159

5.7 BOND STRENGTH DETERMINATION 5.7.1 Fracture Energy 5.7.1.1 DCB specimen 5.7.1.2 Peel test

170 170 170 172

5.8 REFERENCES

173

5.1

160

163

164

INTRODUCTION

One of the most important and far-reaching applications of polymers is their use as adhesives. Synthesis has provided us with a wide range of adhesives that allow bonding of nearly~anythingto 131

132

Generic Polymer Systems and Applications

any surface, so that significant changes in manufacturing technology are being developed. Although adhesive applications of polymers seem to be rather new developments of technology, it is interesting to note that polymers have been used for joining purposes since long before the advent of the industrial revolution. The use of animal and vegetable extracts for joining purposes and construction of weaponry is well known. Natural polymers have been used for setting arrow and lance tips and for feathering arrows;1,2 they aided in the manufacture of furniture for the Egyptians as early as 1800BC 3 and of artifacts in Middle America more than two millenia ago. 4 These bonding agents were strengthened by adding fine particulate matter such as clay 4 and soot, l a 'reinforcement' procedure still practiced today. Of greater consequence for the survival or dominance of tribes or nations was the development of using animal adhesives for the construction of high-performance bows. The American Indian of the Northwest was very familiar with the composite bow,s-7 a strong and resilient construction of wood, horn and layers of sinew, all joined with natural polymers. Probably developed by the Tartars,S the use of natural polymers (through the composite bow) has been held accountable for the extraordinary military exploits of the Turks in the later Middle Ages. 9 We see, therefore, that very old methods of producing weaponry anticipated methods in vogue today in the defense .industry through the use of composite materials. While it is true that the technologist of the 20th century still uses principles developed by our ancestors of many years ago, there has been an important development: instead of the old natural polymers, there are now available synthetic polymers with a great variety of physical properties. In addition, analytical tools are becoming available to evaluate in a rational way the strength characteristics of these polymers in bonding applications. The first man-made adhesive ever used was probably nitrocellulose; it was introduced around 1870 under the name of pyroxylin. However, it was when Baekeland developed phenol-formaldehyde resins in 1912 that synthetic adhesives entered into their first realistic phase of commercial development. It was, however, not before 1935 that, coinciding with the start of modern polymer technology, synthetic adhesives experienced their most remarkable growth. Poly(vinyl formal) was introduced during World War II to bond wood to metal in aircraft, and since then a vast number of polymeric systems including epoxies, urethanes, acrylates, neoprene and ethylene-vinyl acetate copolymers have been developed. Adhesives are now employed in virtually all industries. The US production of synthetic adhesives in 1984 equalled 2.2 billion US dollars with an annual growth rate of nearly 90~.10 The consumption in European Economic Community (EEC) countries is expected to be about equal in volume. With new market needs being continuously generated by the rapid introduction of new construction materials, adhesive technology has become one of the most dynamic fields of polymer science. Much of the knowledge regarding the efficacy of bonded joints has been developed through trial and error. The desire to extend the technology of bonding to new combinations of materials has resulted in the emergence of the science of adhesion which aims to predict bond formation and durability. The main difficulty in developing such a science is the fact that it has to cut across and contain so many diverse disciplines which are not normally taught jointly in universities that it is virtually impossible for anyone individual to master all of them adequately. At one end of this spectrum, one must consider the forces applied to the joint which give rise to stresses which tend to destroy the bond; their distributions are governed by the laws of solid mechanics. At the other end of the spectrum, the chemical aspects of synthesizing appropriate polymers which possess both intrinsic strength as well as the requisite compatibility with the chemistry of the adherends requires a concise knowledge of the molecular constitution of polymers. Both disciplines are needed to address the (time dependent) mechanical properties of adhesive polymers, including their fracture behavior. Similarly, a knowledge of the interaction of distinctly different atoms and molecules is required in order to study the region where adherend and adhesive meet. In practical situations, the adherend surfaces are exposed to molecules (water) from the manufacturing environment which compete with the polymer molecules for bonding to the adherend. Thus surface chemistry is also an important constituent of adhesion science. Moreover, preparation of the surfaces to generate the appropriate texture for locking adherend and polymer together is an, as yet, only moderately understood field. Thus it cannot be emphasized enough that, in contrast to pharmaceutical chemistry, for example, the evaluation and thus chemical design of adhesives cannot be separated from the discipline of mechanics. Associated with these various disciplines is a tremendous range of size scales, and to emphasize the difficulty of combining these disciplines we illustrate this range in Figure 1. One of the most important yet difficult interactions in adhesion science exists between the mechanical properties and the chemistry of the adhesive. For a chemist who evaluates a polymer for adhesion purposes, it is

133

Synthetic Polymer Adhesives

important to know these properties and how to determine them. Today it is believed all too often that merely joining two adherends together with a possible adhesive material and recording the 'failure load' while pulling them apart in a testing machine is sufficient to evaluate the application potential of.an adhesive. Such a view is clearly too simplistic· and the present chapter aims at elucidating the importance of certain mechanics-based evaluations besides chemistry concerns in the strength characterization of adhesives. Certainly, the relaxation or creep behavior of polymers is important for projecting the usefulness of an adhesive, because it is its underlying time dependence which, besides interfacial corrosion-type problems, determines the durability of a bond. Similarly, the fracture characteristics of adhesives measured in crack-propagation studies are an important measure of the time-dependent strength of these materials. Moreover, it is often forgotten that thermally residual stresses can detract significantly from the load-bearing capacity of a bonded joint manufactured with a 'rigid' adhesive so that the thermal expansion data of a polymer is needed for assessing the mechanical performance potential of an adhesive. In a similar way, it is important to be conversant with the effects of moisture on the mechanical behavior of these materials, both with respect to the effect which water or other solvents have on the swelling characteristics and on the time dependence of the material properties.

--Vehicle

Fastener

Structure _

dimension Adherend

Bond

I ine thickness Crock boundary displacements

Surface

roughness

Scrim

f abric

Primer

thickness Latex toughener p articles

Interphase

10 2

104 jJ.m

Figure 1 Range of size scales involved in adhesion science

Out of these concerns arise chemical and physicochemical questions that address the formulation and development of virtually all types of adhesives. We recognize that this chapter is within a work geared primarily to the needs of the chemist. Nevertheless, we feel it is important that the chemist should also be properly informed on the importance of mechanical aspects of the adhesion problem. F or this reason, we shall follow an exposition of the chemical aspects of adhesives with some features of mechanics in the hope· of providing the chemist and chemical physicist with improved understanding and tools for the evaluation of synthetic adhesives. In the following sections we shall, therefore, first consider the chemistry of these materials and then give a briefdescription of surface chemistry and preparation before addressing the more mechanical aspects of material properties that govern bond performance.

5.2 ADHESIVE MATERIALS For the present review, an adhesive will be defined as a synthetic polymeric material (or precursor) which, at least initially, must be a liquid or a tacky semisolid and can be used in a relatively thin layer that is capable of transmitting stresses between two substrates.

134

Generic Polymer Systems and Applications

Adhesives have been classified by function, physical form, application and according to their chemical composition. While all of these classifications have their advantages, none has been accepted universally.11 For this review, the chemistry-related discussion of adhesives has been organized into two main groups: nonreactive and reactive systems. These characteristics broadly dominate the choice of bonding process. Within these two classes, the chemical compositions of specific materials and systems will be briefly described, since the chemistry determines to a large degree the 'curing' (or setting) profile and performance of the adhesive bond. In discussing a complex. topic, such as that of adhesives, it is impossible to avoid minor inconsistencies. We have defined as 'reactive systems' only those systems that form a polymeric network in situ from low molecular weight linear or branched oligomeric intermediates with functional end-groups, after assembly of the adhesive joint. Polymers, even though they may (and often do) contain reactive groups along their chains that can be used for cross-linking, are arbitrarily classified as 'nonreactive systems'. The fundamental principle behind the phenomenon of adhesion is the fact that atoms and molecules attract each other if brought together closely enough. Since wetting of the substrate surface by the adhesive is a prerequisite for this close contact, it follows that the adhesive must at some time behave as a fluid before it sets and becomes a strong solid. This requirement can be met in a number of ways: (a) the solid adhesive is formed from its solution (or dispersion) by evaporation (solvent or latex cements); (b) the adhesive passes through a liquid/solid transition (hot-melt adhesive); and (c) the adhesive is produced from reactive liquid precursors in situ (thermosetting reactive systems). Exceptions to this rule are the pressure-sensitive adhesives (PSA) which do not pass through a phase change; contact and wetting relies on the compliance of the viscoelastic material with the substrate surface under relatively low pressures and its capability to undergo fast stress-relaxation to minimize elastic memory effects which would counteract the attractive forces. These materials will be discussed as a special group of nonreactive adhesive systems, even though they largely utilize the primary techniques of solution, emulsion or hot-melt application. Similarly, the newer adhesives designed for extremely high temperature performance are discussed separately and can be found at the end of the materials section, following the reactive adhesive systems. Adhesives are formulated to satisfy an enormous variety of process and performance requirements. No attempt will be made to cover the vast literature on this subject comprehensively. The review in this section will rather be restricted to a description of the principal chemical types of adhesives and their handling and performance characteristics. 5.2.1 5.2.1.1

Nonreactive Adhesive Systems Solvent- and water-based compositions

Adhesives are very frequently supplied in the form of cements, i.e. solutions primarily in volatile organic solvents or in water, or as aqueous polymer dispersions (latices). The solid contents of the products vary from 25 to 60 wt %. Cements are available in a wide range of viscosities and are normally applied by spraying, roll coating, brushing or troweling to one or both surfaces of the substrate, as in the case of contact cements. The solvent is removed nearly entirely by drying and the two surfaces are joined under light pressure. Solvent cost and environmental concerns have reduced the use of organic solution cements in industry in favor of water-based or hot-melt adhesives. Synthetic adhesives applied from organic solution include polyacrylates, poly(vinyl acetal)s, polychloroprenes, poly(vinyl chloride), polyisobutylene, poly(vinyl acetate), poly(vinylidene chloride), styrenejbutadiene rubbers, polysulfide, nitrile rubbers, polyamides, cellulose derivatives and many others. Water-soluble adhesive compositions are generally based on vinyl alcohol polymers and, where thermal and solvent resistance of the bond is required, on phenolics. Aqueous dispersions are prepared either by emulsion polymerization or by high shear dispersion of a polymer solution in water in the presence of a surfactant and under simultaneous removal of solvent. These 'water-borne adhesives' have, at the same total solids content, lower viscosities than solutions. Polyurethanes, silicones, poly(vinyl acetate), polychloroprene and acrylate are often supplied in latex form. Organosols are somewhat related to latices. They are two-phase systems with a nonvolatile organic liquid as the continuous phase (e.g. PVC plastisol). Fusion and formation of a homogeneous solid phase is achieved by heating.

Synthetic Polymer Adhesives

135

In 'contact adhesives', the adhesive solution is applied to the surfaces of both adherends. Mixtures of neoprene and nitrile rubber, compounded with tackifiers, are the main polymers in this class. The substrates are pressed together after evaporation of the solvent and immediately provide a strong bond. Delayed tack adhesives are formulated with a plasticizer that is solid at ambient temperatures. The nontacky mixtures are activated by heating above the plasticizer softening (or melt) temperature. Some of the polymers used more frequently in this polymer class will be briefly reviewed. (i)

llot-~l1~

Hot-SBR (the styrene/butadiene copolymer produced by emulsion polymerization at 50 ec) is an economically very attractive adhesive raw material. I2 The polymer is distinguished from cold-SBR (produced at 5 ec) by a broader molecular weight distribution. The lower molecular weight fraction provides better wetting, while the higher fraction gives higher cohesive strength. Solutions in hexane or toluene are used as tile cements, wall panel adhesives and for floor coverings. For higher temperature resistance, the solutions are compounded with vulcanizing agents (sulfur or phenolics). SBR is in many cases modified by copolymerization with a third carboxylic-group-carrying monomer (acrylic ·acid, methacrylic acid, itaconic acid). These carboxylated SBR latices have further improved 'quick stick' and are widely used as binders for paper, carpets or nonwoven fabrics. A special styrene/butadiene/vinylpyridine copolymer. has found extensive use as a tire-cord cement to improve the bond between rayon, nylon or polyester cord and the rubber matrix. (ii)

Nitrile rubbers

Nitrile rubbers are random copolymers of butadiene and 25 to 400/0 acrylonitrile. Nitrile rubbers are used·in polar-solvent-based formulations for textile, paper and wood bonding. 13 The materials provide joints with excellent wear, oil and water resistance. Compounding with sulfur or phenolic resins is used when greater strength retention at elevated temperatures ( ~40 ec) is desired. ( iii)

Butyl rubbers and polyisobutylene

Butyl rubbers and polyisobutyiene I4 are produced by cationic polymerization of isobutene.Butyl rubber contains a small amount of isoprene to provide double bonds for cross-linking (see, for example, structures 1 and 2).

(I) Polyisobutylene

(2) Butyl rubber, n = 5~

These polymers are soluble, in hydrocarbon and chlorinated solvents. They have low strength and exhibit considerable creep under constant load. Butyl rubbers are cured with sulfur, p-quinone dioxime or phenolic resins. Their major advantages are good adhesion to polyalkenes, low gas permeability and excellent oxidative resistance. In chlorobutyl rubber, a hydrogen of the isoprene units has been replaced by chlorine. Even though the chlorine content is only 1-1.50/0, it has a positive effect on adhesion to polar substrates. The chlorine substitution also facilitates cure and renders the polymer more compatible with, for example, natural rubber. 1 5 ( iv)

Polychloroprenes

Polychloroprenes are crystallizing polymers, similar to natural rubber. They are, however, stronger and more resistant to aging and decomposition under exposure to higher temperatures.16 The materials are compounded with t-butyl phenolic resins and used in contact adhesives. Mixtures of hexane, ketones and. toluene are generally used as solvents. The crystallization rate controls handling (open) time; the extent of crystallinity determines cohesive (bond) strength. Polychloro~ prenes with about 85% 1,4-trans content have been found satisfactory in these respects. 17 MgO is added as a curing agent for the phenolic resin and as a scavenger for traces of acid (HCl). Polychloroprene latices are· slowly displacing solvent systems from use in industrial operations. 18

136

Generic Polymer Systems and Applications

Newer chloroprene polymers for adhesives contain carboxylic groups.19 The rubbers are used as shoe cements, for laminating plastic to wood and in bonding aluminum. (v)

Homo- and co-poly(vinyl acetate)s

Vinyl acetate homo- and co-polymer latices are the basis for the widely used household glues ('white glues'). They are excellent adhesives for paper, plastics, metal foil, cloth and leather. The materials have good low temperature flexibility and are used in frozen-food packaging, for example. The adhesives resist grease, oil and hydrocarbon fluids but have low resistance to weather and water. 20 Comonomers frequently used are ethylene,21 acrylates, maleates and fumarates (to modify Tg , tack and compatibility behavior), acids (to provide redispersibility and increase adhesion) and N-hydroxymethylacrylamides (to provide cross-linking sites for cure). Systems that cross-link upon heating are available as two-component adhesives. Some poly(vinyl acetate) compositions are available as dry powders that require only addition of water before use. These compositions usually contain 10-150/0 poly(vinyl alcohol) as a protective colloid to facilitate remoisturizing of the dry material. (vi)

Poly(vinyl alcohol)

Poly(vinyl alcohol) is prepared by (partial) hydrolysis of poly(vinyl acetate) by the process indicated in equation (1). The polymer is water soluble and is compounded with glycols (as plasticizers) and aldehydes, as phenol- and amine-formaldehyde resins, for cross-linking and increased moisture resistance. Its major use is in packaging, paper coating, and as a binder for nonwoven fabrics, furniture, envelopes, etc. 20

f

COMt hy.m~ ~

HCH 2

(1)

x

Poly(vinyl acetate)

(vii)

f:CHzt Poly(vinyl alcohol)

Poly(vinyl acetal)s

Poly(vinyl acetal)s are obtained by reaction of poly(vinyl alcohol) with aldehydes, especially formaldehyde and butyraldehyde, as indicated in equation (2). The resins contain acetate as well as unreacted hydroxy groups in addition to acetal rings. The contents of the various functional groups and the molecular weight determine the polymer properties. Poly(vinyl formal) phenolic hybrids, discussed later on in more detail, are used as structural adhesives in the aircraft industry. Poly(vinyl butyral) has found its main use as the interlayer in automotive safety glass. 22

H

ReMO ~ R = H.C)M,

(2)

H Poly(vinyl alcohol)

(viii)

Poly(vinyl acetal)

Polyacrylates

Polyacrylate adhesives are predominately copolymers of acrylates and methacrylates with small amounts of the acids. Glass transition (and stiffness) of the copolymer is controlled by the type and amounts of comonomers. Adhesives with higher stiffness are used in ceramic· tile bonding and lamination of plastic films to wood, for example. The more flexible (lower Tg ) polymers, made crosslinkable by copolymerization with, for example, N-hydroxymethylacrylamide, are used in textile treatment. Both emulsions and solutions in organic solvents are in current use. A wealth of commercial products and adhesive formulations can be found in the literature. 23

Synthetic Polymer Adhesives (ix)

137

Polyurethanes

Thermoplastic polyurethanes are linear polymers. They are generally prepared by reaction of a polyester or a polyether possessing a molecular weight of about 2000, a diisocyanate and a low molecular weight glycol (e.g. 1,4-butanediol). The polymers are considered to be block copolymers in which flexible (polyester or polyether) segments alternate with rigid frequently crystalline urethane blocks, as indicated by the example in equation (3). The strength of these polymers depends on the specific domain morphology present in the solid state. The rigid urethane domains act as reinforcement and are the primary contributors to the material's high strength. 23 Thermoplastic polyurethanes are used in solution (in THF, MEK and/or toluene) either alone or blended with other polymers (e.g. nitrile rubber, PVC, etc.).24 They show excellent adhesion to rubber, plastics, metals, wood and leather. Lower cost low-modulus polyurethanes can be cross-linked by compounding in solution with a small amount of a polyisocyanate.

Polyester or polyether

Diisocyanate

Butanediol

(3)

Thermoplastic polyurethane

Aqueous dispersions are made by emulsification of special polyurethanes containing small amounts of carboxylate or sulfonate groups. A number of polyurethane lattices are available. 24 They are predominantly used as textile adhesives or to laminate foil to Kraft paper. 25 (x)

Polystyrene

Polystyrene, either in solution or as a latex, is used to bond porous surfaces, such as tiles, to wall plaster or wood. (xi)

Poly(vinyl chloride)

Poly(vinyl chloride)s, containing up to 200/0 vinyl acetate and small amounts of carboxylic monomers, are used to bond PVC to metals, paper, textiles, leather and other substrates. (xii)

Plastisols

Plastisols (emulsion poly(vinyl chloride) dispersed in a plasticizer) are low viscosity dispersions that fuse at 160-170 °C to produce tough elastic materials. They are used in the automotive industry for gap filling and adhesion, but are being slowly replaced by thermosetting (reactive) systems such as polyurethanes or epoxides.

5.2.1.2 Hot-melt adhesives 26 Any thermoplastic material that reversibly liquifies upon heating and solidifies on cooling can be used as a hot-melt adhesive. The ideal polymer is solid b((low at least 80°C, has a relatively sharp melt transition and gives a low viscosity melt with good flow characteristics and wetting properties. The main disadvantages of hot-melt adhesives are low strength and low creep resistance under load. Accordingly, these adhesives (with the exception of high temperature adhesives, discussed below) are seldom used as permanent adhesives, but rather where quick temporary holding is desired and load requirements are small. Many commodity hot-melt adhesives start losing strength above 70 °C, although up to this point shear loads of 3.5 MPa have been reported. 27 The use of hotmelt adhesives has been stimulated by environmental concerns. Hot-melt adhesives can be applied from equipment ranging from hand-held guns up to large dispensing equipment capable of laying down molten material up to 3 m wide. Special heating equipment, designed to melt on demand only the adhesive close to the pump entry-port (to reduce thermal degradation in melting polymers) is

138

Generic Polymer Systems and Applications

also available. The temperature of the heated melt has to be sufficiently above the solidification point to give enough open time, i.e. time between application and setting of the adhesive, for mating the adherends to be bonded. Alternatively, the adhesive can be applied as a bead, powder or film, cooled and later remelted for bonding. Hot-melt adhesives are often applied in the form of supported films. Two-sided films are available and may be used to join different adherends with matching adhesives. Only the most commonly used compositions are discussed below. (i)

Polyethylenes

Polyethylenes perform well on most porous adherends where low temperature flexibility is not required. They are mainly used for paper and cardboard sealing. Good bond strengths are obtained up to 70°C. (ii)

Ethylene-vinyl acetate copolymers

Ethylene-vinyl acetate copolymers (EVA), containing from 25 to 500/0 vinyl acetate, are used in adhesives. The vinyl acetate contributes tack, solubility, improved adhesion to polar surfaces and low temperature flexibility. The copolymers are produced in a wide range of molecular weights and corresponding melt indices « 10 to 5(0). Commonly, polymers with low and high molecular weights are blended to obtain the best balance between melt viscosity, wetting and material cohesive strength. The polymers are generally compounded with tackifiers (both rosin and terpene types are used), waxes (to lower melt viscosity and cost) and, at times, plasticizers (commonly hydrocarbon resins). The major use of EVA copolymers is in book binding, paper and carton sealing and for carpet seaming. Copolymers of ethylene with ethyl acrylate (about 18 wt 0/0) give better adhesion to polyalkenes and improve thermal stability. Modification of ethylene-vinyl acetate copolymers, either by copolymerization or by grafting, improves adhesion to a variety of substrates. 28 ( iii)

Polyamides

Polyamides (nylons) can be used under low load up to service temperatures of 125°C. They are not brittle above - 40 °C. Thermoplastic polyamides are produced from a variety of diacids and diamines and are available in a wide range of molecular weights and viscosities. The most frequently used polyamides in adhesives (3) are based on dimer acids, a 36-carbon dicarboxylic acid obtained by dimerization of oleic acid. 29 The presence of both hydrocarbon and polar amide groups makes these adhesives useful for bonding adherends with a broad range of surface energies. Polyamide hot-melt adhesives have found use in the furniture, textile, shoe and electronics industries.

H~N(CH2l6~C34H66C+NH(CH2)6NH2

.

amide group

•

climer acid

(3) Hot-melt polyamide, x

( iv)

~

10

Polyesters

Polyesters are prepared from a multitude of dicarboxylic acids and glycols. Mixtures of terephthalic acid with other aromatic or with aliphatic dicarboxylic acids are often used to obtain lower crystalline melting points. 30, 31 Polyesters possess high tensile strength and high elongation. They have to be dried prior to melting since water leads to chain scission which lowers the molecular weight with an accompanying reduction in adhesive strength. (v)

Reactive polyamides and polyesters

Reactive polyamide and polyester hot-melt adhesive compositions claim the advantages of hot melting (no solvent, short setting time, high cohesive strength) at lower processing temperatures but with improved thermal resistance. Epoxy cross-linked polyamides 32 as well as polyamides crosslinked with phenol- or caprolactam-blocked diisocyanates 33 have been described. In the latter case, it was possible to lower processing temperatures from 275°C to about 100 °C while increasing the thermal resistance to 220°C.

Synthetic Polymer Adhesives

(vi)

139

Foamed hot-melt adhesives

Foamed hot-melt adhesives, based on polyethylene, have been offered more recently. The foaming is achieved by introduction of an inert gas (N 2 or CO 2 ) into the melt prior to dispensing by a patented process 34 causing an increase in volume by 20-70%. Advantages claimed include longer open time, better spreading, faster set time and greater gap-filling capability, as well as reduced adhesive consumption and reduced thermal distortion of substrate. Application is primarily as gaskets and sealants. Other polymers used in hot-melt adhesives are poly(vinyl butyral) and elastomeric block copolymers, discussed in Section 5.2.1.3 on pressure-sensitive adhesives.

5.2.1.3

Pressure-sensitive adhesives

Pressure-sensitive adhesives, as stated before, have a special position among the adhesive classes since they do not change their physical state from an initial liquid to a solid after final bond formation. 35 The materials, which are permanently tacky, are most commonly supplied on one- or two-sided tape backing. The main applications are packaging, splicing, masking tape, skin bandages, double-sided carpet tapes, etc. Historically, pressure-sensitive adhesives were based on natural rubber. More recently, synthetic elastomers have also been used. More noteworthy among these synthetic elastomers are EVA, styrene/diene triblock copolymers, poly(vinyl ether)s, polyacrylates and silicones. Pressure-sensitive adhesives are generally applied from solutions, aqueous dispersions or hot melts. 36

(i)

Acrylic polymers

Acrylic polymers have recently achieved widespread use; they are white and exhibit much better resistance to aging than the old pressure-sensitive adhesives based on natural rubber, and are widely used where these two characteristics are desired. Polyacrylates have been synthesized 37 which are inherently pressure-sensitive, i.e. without the need for the addition of a tackifier. Among the functional groups introduced by copolymerization are carboxylic, hydroxyl, epoxy, allylic amide and tertiary amine groups. Reversible cross-lihking can be obtained by salt formation (reaction of carboxy groups with zinc, zirconium or titanium compounds), for example. 38 Permanent irreversible cross-links are established by the reaction of hydroxy groups with epoxide or formaldehyde resins,39 or by reaction of epoxy groups with carboxy groups, amides or amines. Amide groups containing polymers are also cross-linked with formaldehyde resins. 40 Hot-melt pressure-sensitive adhesives have been introduced which have thermally reversible cross-links of a nondisclosed nature. The material is applied to tape at 175°C. Bonds with high levels of peel strength and permanency have been reported. 41 Acrylates are used for packaging, bandages, paper and film labels, decals and a variety of specialty items. ( ii)

Triblock copolymers

Thermoplastic elastomeric triblock copolymers of the ABA type have been found to be of greatest value for pressure-sensitive adhesives. 42 The end blocks (A) in these polymers are plastic in nature with a high glass transition (or melt) temperature, while the center block (B) is rubbery. A typical thermoplastic elastomer of this kind is a two-phase system in which the plastic portions of the molecule are aggregated in domains that are dispersed through a matrix formed by the elastic (B) segments, as indicated in Figures 2(a) and (b). The plastic domains act as physical cross-links and give these materials their strength. 43 The thermal and mechanical response of joints was found to be strongly dependent on joint preparation. 44 In particular, the [styrene-b-butadiene(or isoprene)-b-styrene] polymers, prepared by anionic polymerization techniques, or their hydrogenated derivatives, poly[styrene-b-(ethylene-cobutylene)-b-styrene] and poly[styrene-b-(ethylene-co-propylene)-b-styrene], are useful in these applications. The polymers are completely soluble without mastication, but can also be processed from a hot melt. They generally need to be compounded with tackifiers. In particular, resins that are compatible with the median block (B), rather than with the high melting end-blocks (A), are useful to increase tack. Among these resins are the mixed polyalkenes, rosin esters and the polyterpenes.

140

Generic Polymer Systems and Applications (b)

(0)

1111

\1

I' II ,

II' J

t

ill

1\

/I

Styrene (A)

II

(II I

\I Ii

1111

I

Styrene (A)

Diene (B)

Soft phase (diene matrix)

Hard phase ( styrene)

Figure 2 (a) Triblock copolymer; (b) phase morphology of a thermoplastic elastomer block copolymer

Oxygen, ozone and UV stabilizers have to be added to these polymers to protect the unsaturated (B block) section of the molecule. Even though the hydrogenated versions are considerably more resistant to oxidation, they too are generally compounded with stabilizers. The adhesives are used in general-purpose tapes, duct tapes and permanent labels. Higher temperature resistance can be obtained by cross-linking with a sulfur-accelerator vulcanization system. (iii)

Butyl rubbers

Polyisobutylene, butyl rubber and chlorobutyl rubber are another family of elastomers that have found use in pressure-sensitive adhesives. 45 Butyl rubber is a copolymer of isobutene with 3% or less of isoprene for cross-linking. Polyisobutylene is a homopolymer with no unsaturation. The polymers are prepared by cationic polymerization, the molecular weights of products available ranging from 5 x 104 to 2 X 106 • Chlorobutyl rubber (4) contains less than 1.5 % of chlorine as substituent on the allylic position of the isoprene unit. Me

-EcH2~=CH~H~CH21.-f-eH2~=CHCH2±Me

Cl

Me

(4) Chlorobutyl rubber, x

Me

~

O.015n

This polymer is readily cross-linked either through the double bond (which is made more reactive by the substitution) or via the halogen. There is also a tendency to improve adhesion to polar substrates. 14 Isobutylene polymers are generally processed from solution or as hot melts. Aqueous emulsions of butyl rubber and polyisobutylene are also available. The latices are very stable and difficult to coagulate. Dried films do, however, produce the typical polyisobutylene characteristics. Plasticizers (e.g. polybutene) and resins (polyterpene) are used to control tack, cohesive strength and viscosity range. Cross-linking, to increase permanency and reduce creep, can be achieved by sulfur vulcanization systems, quinoids (dioxime), formaldehyde resins and, in the case of chlorobutyl rubber, by zinc oxide. The butyl rubber family of pressure-sensitive adhesives is used in labels and as tape adhesives. Because of their excellent low temperature flexibility, nontoxicity and light color, the materials are used in medical applications (surgical tape, oral bandages, ostomy appliances and others), freezer label adhesives and pipe wrap and electrical tapes. They were found to adhere well to nonporous low energy surfaces, such as, for example, polyethylene. (iv)

Silicone rubbers

Silicone rubbers have excellent low temperature flexibility combined with outstanding high temperature resistance. Their service temperatures range from -70°C to 250 °C. 46 Silicone rubbers

Synthetic Polymer Adhesives

141

commonly used in pressure-sensitive adhesives are the dimethylsiloxanes or their phenyl-modified versions, shown in (5), where R = Me or (up to 25 mol 0/0) Ph. The modification with phenyl groups gives materials with higher tack, low temperature flexibility (by preventing crystallization at - 40°C) and increased high temperature stability. These linear gums are blended with highly branched silicon resins - prepared by condensation of trimethyl(chloro)silane with silicic acid - to adjust tack as well as the physical properties of the adhesive.

Silicone adhesives have good adhesion to low and high energy surfaces, including such substrates as polyalkenes and fluorocarbons which are normally difficult to bond. They are available in a wide range of viscosities and properties, and a tack strength up to 15 Pa cm - 2 ha-s been realized. The pressure-sensitive adhesive can be cross-linked with free radicals (benzoyl peroxide at 150°C) and then produces bonds with a peel strength of up to 1lOON m - 1. Silicone-coated polyester tapes are used for masking in plating operations, and coated polyimide and poly(fluorocarbon) films are used in electrical insulation. ( v)

Poly (vinyl ether) s

Poly(vinyl ether)s (6) or, to a lesser degree, their copolymers (with acrylates, for example) are frequently used in pressure-sensitive adhesive applications. 47 Vinyl ether homopolymers are produced by cationic solution polymerization. Copolymerization with acrylates utilizes free-radical emulsion techniques. The materials are used in special applications and are usually mixed with other polymers. Poly(vinyl ether)s are available in a wide variety of molecular weights and viscosities and are applied either by solution or hot-melt techniques.

-feH2~H* OR

(6) R=Me, Et or Bui

5.2.2

Reactive or Thermosetting Adhesive Systems

These compositions, which are used predominantly as structural adhesives,· set by means of chemical bond formation either at ambient or elevated temperatures. Thermosetting adhesives are supplied in the form of one- or two-component systems and are generally available as solventless liquids or pastes. Single-component systems are also provided as solid tapes or films. These singlecomponent systems usually contain latent catalysts and set by application of heat, pressure, radiation energy or, in special cases, by initiation with moisture. They generally have a limited shelf life. Radiation-curable adhesives have recently gained much attention. 49 ,50 Two-component systems have a longer shelf life, but the reactive components have to be metered and mixed in appropriate equipment 51 immediately prior to application. Once mixed, the adhesives have very limited open (handling) time. The dispensing equipment for two-component reactive adhesives must, therefore, have provisions for solvent flushing to prevent setting in the mixing chamber.There is a plethora of reactive starting materials of different chemical structure available for formulation in any of the general classes of thermosetting adhesives, and only general chemical compositions and reactions can be discussed here.

5.2.2.1

Polyurethane adhesives 52

Although they contain other bonds as well, polyurethanes are characterized by the· presence of a multitude of urethane groups in their molecules. These urethane groups are obtained by reaction of hydroxy components with isocyanates as indicated in equation (4).

* A structural adhesive is sometimes defined as producing a joint in which at least one of the adherends yields before the bond fails. 48 It generally possesses a capability of withstanding high tearing shear stresses of 7 MPa or higher over extended periods of time as well as maintaining strength under often hostile environmental conditions.

142

Generic Polymer Systems and Applications

V\fV\..OH

+

Hydroxyl component

OCN'VVV\.

-----l~~

o H II I

AA.AA.O-C-NfVVV\

Isocyanate

(4)

Urethane

Typical reaction partners are (1) hydroxy-terminated (linear or branched) polyethers, polyesters or polydienes and (2) an aromatic or (if light stability is required) an aliphatic polyisocyanate (Table 1). Two-component polyurethane adhesives customarily consist of an isocyanate-terminated prepolymer, prepared from a hydroxy component (a polyol) and an excess of a diisocyanate as shown, for example, in equation (5).

o n HO.I"VV\JOH + > 2n molecular weight SQO--3000

OCNRNCO

----~~~

II

0

\I

OCNRNHCO~OCNHRNCO

(5)

Prepolymer

Table 1 Isocyanates Frequently Used in Adhesive Formulations Aromatic isocyanates

TDI

Me ONCO NCO 2,4 isomer ( ,...., 800/0)

Polymeric MDls

2,6 isomer ( ,...., 20%)

OCN

( OCN-Q-cH 2

NCO

•

nand m = 0 or 1, average NCO functionality 2-3 Aliphatic isocyanates Adduct of hexamethylene diisocyanate (3 mol) and 1,1,1-tri(hydroxymethyl)propane (1 mol)

OCN-Q-eH 2-Q-NCO

(i)

Two-component systems

In two-component urethane adhesive systems, these prepolymers are then mixed with an aromatic polyamine or a polyhydroxy compound at NCO/OH (or NH 2 ) equivalent ratios of 1.1 to 1.4. Catalysts (e.g. tertiary amines or tin compounds) are required to accelerate cure when hydroxy components are used as reactants with the prepolymer. The handling time can be adjusted from minutes to hours by selecting an appropriate reactive component as well as the type and concentration of cat.alyst. The cure rate in production is customarily increased by clamping the assembly in heated fixtures. Substrate pretreatment is generally required to obtain optimum bonding. This is normally achieved by priming with an isocyanate solution. Representative adhesive compositions can be found in a number of references. 53-57

Synthetic Polymer Adhesives ( ii)

143

One-component systems

Single-component urethane systems can either be room-temperature (moisture) cured or heat activated. In the former case, a macroglycol (polyether, polyester or polydiene) is reacted with a calculated small excess of polyisocyanate. This results in polyurethanes with molecular weights between 20000 and 40000 and with isocyanate (NCO) end-groups. These NCO groups are then available for chain extension and cross-linking with atmospheric moisture, as delineated in equations (6) and (7). The materials are stable for long periods of time if they are stored under exclusion of moisture. Moisture-cured systems are best used for porous permeable substrates which allow water to reach the adhesive and carbon dioxide (C0 2) to diffuse out of the bond as it is formed. These systems are also used for bonding of automotive windshields. 58 - 6o Chain extension

fVV\..NCO

+

~

OCNvvv

IVV'\NHCNH/'\fV\

(6)

Urea group

+ OCNlJVV

Cross-linking

~

~

JVV\NHCNHJVV\

(7)

I c=o I f\f\f\NH

Biuret group

Heat-activated one-component urethane adhesives are based on blocked isocyanates, crystalline (room-temperature immiscible) chain extenders or cross-linking agents,61, 62 encapsulated catalysts or on combinations of these three. Blocked isocyanates are reaction products ofisocyanates with, for example, phenols, oximes, caprolactam or p-dicarbonyl compounds, which at higher temperature dissociate to reform the original isocyanate group (see equation 8).63-65 Alternatively, the curing agent can be deactivated by complexing. Thus, methylenedianiline (MDA) forms an unreactive complex with NaCI that dissociates at curing temperatures to free the amine for cure with the isocyanate. 66 ,67 Novel blocking agents for alcohols have also been described recently.68

o II

RNHCB

lOO-150°C ~

RNCO

+

(8)

HB (blocking agent)

Urethanes have found wide acceptance as structural adhesives in the automotive industry for bonding plastics (especially glass-tiber-reinforced poly(ester-styrene) resins)51 and for wood bonding (particle board) where they are slowly replacing phenol- or urea-formaldehyde resins. Substrates bonded with thermoset urethane adhesives should be kept as dryas possible, since the CO 2 formed from reaction of surface water with isocyanate can give a cellular layer at the adherend/adhesive interface with a corresponding lowering of the bond strength.

5.2.2.2 l?JJoxJ' a4ihes;ves69 ,70 An epoxy resin is a material of relatively low molecular weight, containing at least two oxirane groups per molecule (7). Excellent reviews of the chemistry and uses of epoxy resins have been written by a number of authors. 69 - 73 While there exists a large number of epoxides of different chemical structure reported in the literature, those used mainly for formulating adhesives are the glycidyl ethers of condensation products of phenols with aldehydes or ketones, in particular those based on Bisphenol A [2,2-bis(4-hydroxyphenyl)propane], as illustrated in (8). Frequently, higher molecular weight hydroxy-group-containing Bisphenol A derivatives are used. These are sometimes diluted with glycidyl ethers of aliphatic C 8-...C 10 alcohols to reduce viscosities to a workable level. Available resins vary from viscous liquids (3-15 Pa s) to low melting solids~

(

H2C/O"CH~R

\

(7)

L

/0"

o-Q-{~ ~ -o-e II ~ /0" 'I ~ 'I ~ OCH CH--cH

H 2C--eHCH 2

-1 Me

2

(8) Diglycidyl ether of Bisphenol A (DGEBPA)

2

144

Generic Polymer Systems and Applications

Epoxy resins can be cured by a variety of mechanis~s,-includinghomopolymerization of oxirane groups by acidic or basic catalysis, but with few exceptions the commercially available adhesives are two-component materials containing a coreactant for epoxy groups.74 One-part adhesives can be formulated in the form of pastes, putties, tapes, fusible powders or sticks and as films. These systems use latent catalysts (for example, a complex of boron trifluoride and monoethylamine, with a melting point of about 90°C) -or coreactants [cyanoguanidine (dicyandiamide), adipodihydrazide].7s, 76 UV-curable epoxy resins have been described and may, eventually be used in adhesives. 77 Two-part epoxy adhesives are mixed with coreactant immediately before use. The coreactants (hardeners) are either amines, mercapto compounds, hydroxy compounds, polycarboxylic acids or acid anhydrides. Commonly used amines include the aliphatic polyamines (diethylenetriamine, triethylenetetramine), amidoamines (amine-terminated oligoamides based on dimer acid, for example), aromatic amines (methylenedianiline, m-phenylenediamine) and amine-terminated polyethers. With aliphatic amines, resins develop sufficient strength in 4-12 h at room temperature to permit handling. Full strength is developed over a few days. Aromatic amines cure much slower and require heat for full cure. They do, however, have better strength retention at higher temperatures. Amidoamines contribute low water absorption as well as a degree of toughening and are probably the most widely used epoxy curing agents. Acid anhydride hardeners give higher thermal stability but the cured materials tend to be brittle. Curatives carrying mercapto groups react very fast. In particular, when catalyzed with tertiary amines, they set within 10 min, even in thin bondlines ('five-minute epoxies'). Addition of carboxy-terminated butadiene-nitrile copolymers78 or powdered ABS 79 resins has been shown to render epoxy adhesives with considerable toughness. The area of toughening of epoxy adhesives was reviewed recently.80 Epoxides have good cohesive strength and exhibit excellent adhesion to a variety of substrates (metals, plastics, wood, concrete,glass, ceramics). Similar to polyurethanes, they are 100 % solid systems, but their 'cure behavior is insensitive to water. This has made specific formulations useful for underwater repair work on concrete. Low shrinkage and corresponding low bond stresses as well as high creep resistance are further advantages that have contributed to making epoxides one of the fastest growing group of structural adhesives. Major areas of application are in aerospace, automotive, electrical and' electronics construction, and consumer products. 81

5.2.2.3

Acrylic adhesives 82

This section will include materials that contain an acrylic function as the reactive entity, regardless of the mechanism of polymerization. These are the' anaerobic acrylics, the 'reactive' acrylics (second generation, surface activated, aerobic 'and 'honeymoon' adhesives) and the cyanoacrylates.

(i) Anaerobic adhesives Anaerobic adhesiv,es 83, 84 are single-component low-viscosity liquid acrylics which cure at ambient temperature by a free-radical mechanism, but only in contact with metal surfaces and in the absence of air. Their main use is. for locking threaded parts,8S where they give prevailing torque many times that obtainable with lock-nuts and lock-screws, in pipe fitting, in situ gaskets and other applications for retaining and seating metal parts. Early adhesive compositions contained hydroperoxide and relied primarily on the inhibiting effect of oxyge,n for their storage stability. They were stored in air-permeable thin-walled polyethylene containers. 86 These materials were slow curing (1-2 h at 25°C), but substantial progress has been made since to overcome this deficiency. Powerful accelerator systems for peroxide decomposition (such as tertiary amines 87 ,-88 or hydrazide derivatives 89 ) were developed. These accelerators, however, required more efficient stabilizers. 90 - 93 Most anaerobic adhesives presently iil use are represented by (9).87,94 These original formulations were used as sealants rather than adhesives. Current formulations, useful as structural adhesives, contain minor amounts of carboxylic groups in the polymer (from copolymerization with acrylic,

?i

~

H 2 C=CCO(CH 2 CH 2 0\CC-CH 2

I

Me

\:

')" I

Me

(9) Poly(ethylene glycol) bismethacrylate, n = 3 or 4

Synthetic Polymer Adhesives

145

methacrylic, maleic or itaconic acids) to improve strength, as well as special monomers (e.g urethane methacrylates) to increase flexibility and toughness. 95 - 97 Anaerobic adhesives are now available which can withstand continuous exposure up to 230°C. (ii)

Reactive acrylics

Reactive acrylics (second generation or modified acrylics, 'honeymoon' adhesives) are two-part 'no-mix' adhesives which consist of: (a) acrylic monomers, (polyfunctional, as in anaerobic acrylics, plus substantial amounts of monofunctional methacrylates); (b) an elastomer (chlorosulfonated polyethylene,98 nitrile rubbers 99 or urethane elastomers 100 ); (c) an initiator (commonly a hydroperoxide 101); and (d) an amine-aldehyde (prominently aniline-n-butyraldehyde 98 ,lOl) condensation product as activator. The activator is applied as a primer to the substrate surface by dipping, spraying or brushing. Saccharin, when added to the activator system, has been found to give three to fqur times higher lap shear values. 102 Recent formulations (called aerobic acrylics) contain perester, a soluble transition metal salt and a tautomeric acid (maleic acid). The formulations have little odor and, when activated with aniline-n-butyraldehyde resins, are relatively insensitive to oxygen inhibition and cure through areas of greater air exposure than other reactive systems. 103 Another recent,development in this field are adhesives that can be cured by radiant energy rather than by peroxides. UV radiation in combination with photoinitiators49 , 50,104,105 and electronbeam activation 49 , 50, 106 have been described. Formulations are available that can be cured in a matter of seconds. Acrylic adhesives have been used in many applications, among them bonding of metals, (where the high tolerance of reactive acrylates to oil contamination is of great value 107 , 108) as well as of plastics and glass. ( iii)

Cyanoacrylates

Cyanoacrylates,109, 110 unlike the anaerobic and reactive acrylics discussed so far, polymerize by an anionic mechanism. These 'wonder' adheSIves set at room temperature very fast, without any catalyst, when in contact with slightly basic surfaces containing absorbed water, and form very strong bonds to metal, plastics, rubber and ceramics. Lap shear strengths of 13-14 MPa have been observed on metals. On acidic surfaces (e.g. wood), curing must be accelerated by amines or ammonia. Small amounts (0.1 to 0.3 0/0) of certain acidic compounds, e.g. maleic or itaconic anhydride 11l or acetic acid,112, 113 have been found to promote adhesion to metals. The most frequently used monomers are methyl and ethyl 2-cyanoacrylate (10).114

o

/~OMe

(or OEt)

H 2 C=C

"C=N

(10) Methyl (or ethyl) 2-cyanoacrylate

A thickener (usually polyacrylate), a plasticizer and stabilizers (hydroquinone and S02) are generally added to avoid formation of starved joints, improve flexibility and to prevent polymerization in the container. Cyanoacrylates polymerize to a linear (not truly thermoset) material which, although resistant to organic solvents 'of low to medium polarity, has poor heat or moisture durability. These adhesives join most materials with the possible exception of polyethylene, polypropylene and Teflon. Rigorous surface cleaning is, however, required to obtain adequate bonding. Careful handling is necessary, since cyanoacrylates form strong bonds to skin. Newer formulations contain a cross-linker to improve heat resistance beyond 80°C 115, 116 and tougheners to reduce brittleness. 117 Because of their capability to join so many-dissimilar adherends together, cyanoacrylates are widely used, in particular to bond metal or plastics to rubber.

5.2.2.4

Formaldehyde andfuran resin'adhesives l18 , 119

This section will include the condensation products of formaldehyde with phenol, resorcinol, melamine and urea, as well as the polycondensation products of 2-furaldehyde and furfuryl alcohol. PS7-F

146 (i)

Generic Polymer Systems and Applications Phenol-formaldehyde resins

Phenol-formaldehyde condensation products are one of the lowest cost adhesives for structural purposes. The condensation reaction can be carried out under basic or acidic conditions. The former case leads to 'resols', while acidic condensation produces 'novolacs', shown in (II) and (12), respectively.

(11) Resol

(12) Novolac

Resols contain hydroxymethyl groups which further condense at 13O-200°C, giving a threedimensional network with formation of methylene ether and methylene bridges. Novolacs, on the other hand, are thermoplastic materials that require the addition of a formaldehyde donorparaformaldehyde; hexamethylenetriamine ('Hexa') or trioxane-for cross-linking. 119 Acid-catalyzed adhesive formulations of the resol type cure in 3-6 h at 20°C. Phenol-formaldehyde resins bond well to rubber, leather and wood. Their largest use by far is in· wood bonding, including plywood. 120 They are available in the form of powders, aqueous dispersions or solutions in alcohol or acetone, for example. Heat-acti.vated resols are also available as tapes. Hybrids (with epoxies, poly(vinyl acetal), neoprene or methanes) have been developed which give good adhesion to metals as well (see below). Substantial quantities of phenolic resins are used as tackifiers in contact and pressure-sensitive adhesives and as cross-linking agents for nitrile rubber or poly(vinyl butyral). (ii)

Resorcinol-formaldehyde resins

Resorcinol~formaldehyde resins 121 are generally applied in two parts: a novolac-type resin dissolved in alcohol/water plus a formaldehyde donor. Since resorcinol is considerably more reactive than phenol, no acid catalyst is needed. Curing takes place at 20-45°C in 3-5 h. The major use for the resin is in wood bonding.

(iii)

Amino-formaldehyde resins

Amino-formaldehyde resins 122 encompass both melamine and urea resins. Melamine (13)~ the trimer of cyanamide, as well as urea (H 2 NCONH 2 ) react with formaldehyde to give hydroxymethyl compounds (14 and IS). The hydroxymethylmelamines are only stable in strongly alkaline aqueous solutions. Their reaction with alcohols gives alkyl ethers, which are soluble in organic solvents and are stable. If heated· under slightly acidic conditions, these materials lose alcohol and form internuclear bridges by the reactions illustrated in equation (9). Curing of hydroxymethyl urea takes place by loss of water at elevated temperatures (5-10 min at 120°C and a pressure of about 1.5 MPa). Two-component urea-formaldehyde resins are also available. These systems are mixed with an acidic catalyst [e.g. NH4,Cl or (NH 4)2 804] and can be cured at room temperature in 2-4 h.

?i

HOCH 2 NHCNHCH 2 0H

(13) Melamine

(14) Hexa(hydroxymethyl)melamine

(15) Di(hydroxymethyl)urea

Synthetic Polymer Adhesives

147

(9)

The urea resins are lower in cost than the melamines but have poor resistance to boiling water. Amino resins are used in furniture assembly and veneering, in production of hardwood plywood, as sand core binders, as adhesive in waterproofed corrugated board and in building and construction applications.

(iv)

Furan resins

Furan resins 123 are based on 2-furaldehyde (16) and are derived from waste vegetable matter. They are used as thermosetting adhesives in their own right, as well as modifiers for ureaformaldehyde resins, to improve gap filling and craze resistance. 2-Furaldehyde can also be converted to furfuryl alcohol. which itself can be thermoset by acids or reacted with aldehydes and ketones to· give polymerizable intermediates. The resins are dark in color, give strong chemically resistant bonds and penetrate porous surfaces well. Bonds can be exposed continuously to 150°C without deterioration. The materials are used as bonding agents for floor compositions, cements for tank lining and as binders for ablative materials in rockets and missiles and for carbon or graphite products.

OCHO

°

(16) 2-Furaldehyde

5.2.2.5

Unsaturated polyesters 124

Unsaturated (thermosetting) polyesters are primarily those based on fumaric acids and propylene glycol (17). Dissolved in styrene, they are used as adhesives for bonding fiber-reinforced polyesters and for repair of fiberglass automobile bodies, boats, masonry work, etc. Materials in which the styrene is replaced with ethylene glycol diallyl dicarbonate (18) are used as transparent optical adhesives. 125

H

t

t

o ° OCCH=CHCOCH2~:. II

1\

OH

°II

°II

CH2=CHCH20COCH2CH20COCH2CH=CH2 (18) Ethylene glycol diallyl dicarbonate

(17)n=~6

5.2.3 Hybrid Adhesive Systems There are a number of hybrid systems (interpenetrating or semi-interpenetrating networks,AB cross-linked resins) available which were developed to satisfy special processing and performance criteria. These adhesives· consist generally of: (a) a high molecular weight polymer (elastomer or plastic), contributing ductility and peel strength; (b) a cross-linking (or thermosetting) resin, that either.reacts with active sites of the polymer (a), or forms an independent network; and (c) a curing agent or catalyst for the thermosetting resin (b). The thermosetting resins are either phenolics or epoxies. The former produce volatiles during cure and require high clamping pressure, while epoxy resins need only contact pressure.

5.2.3.1

Nitrile rubber-phenolic blends l26 , 127

Nitrile rubbers contain three different reactive sites: double bonds, nitrile groups and sometimes carboxylic groups (from copolymerization with small amounts of acrylic or methacrylic acids). The

148

Generic Polymer Systems and Applications

hydroxymethyl groups of a resol can react with either of these sites under formation of, respectively, an ester, a chromone or an imino ether (Scheme 1). Polyfunctionality in the 'resolleads to crosslinking. Cure conditions are 15Q-260°C for 30 min or less under bonding pressures of 1-2 MPa. Bonds have low creepb~low 90°C and show good fatigue and impact resistance.

nitrile rubber

OH

IVV\CHvvv

I

~JC~-CH20C

'V~

~H

Scheme I

5.2.3.2

Neoprene-phenolic blends 12s , 129

Phenolic resins blended with neoprene rubber give adhesives with good thermal stability (service temperatures reach up to 95°C) and toughness. Correlations have been made between the structure of the phenolic resin and adhesive properties, especially tack strength and peel strength. 130 The systems are on the market as solutions (in toluene or MEK) or as films. The adhesive films generally cure at 9Q-250°C in 15-30 min, although formulations are available that set at room temperature. They are used to bond metal, wood and plastics.

5.2.3.3

Polyvinyl-phet;'olic blends 131

Poly(vinyl formal)s or butyrals, cross-linked with phenolic resins (resols), have found most use in metal to metal bonding. Cure takes place via the free hydroxyl (OH) groups of the vinyl polymer and requires high pressures (1-3 MPa) and temperatures of 150°C. The cure can be accelerated by acid. Durability of bonds below service temperature of about 100 °C (the decomposition temperature of acetal resins) is good. 132

5.2.3.4 Miscellaneous phenolic blends Urethane-phenolic systems give good bonds to metal. 133 Excellent adhesion to wood and metals was obtained with a, resol from p-cresol and formaldehyde and a polymeric aromatic isocyanate (polymericMDI).129_ . Acrylic-~henolic adhesives are based on acrylate latexes with a phenolic resin added as a tackifier. 13 The adhesives have good metal adhesion, good tack and good shear strength at elevated temperatures. Nylon-phenolic (or resorcinol) resin blends were found to give good adhesion to metal and glass. 134

149

Synthetic Polymer Adhesives

5.2.3.5

Epoxy hybrid systems

Phenolic-epoxy blends give adhesive bonds with good long-term service life at temperatures up to 250°C, and are primarily used for military applications. The strength retention of AI bonds under humid or dry atmospheric conditions is excellent (75 % unstressed, 80-85% at 20% strain).132, 136, 137 Peel and impact strength are, however, generally poor. The materials, available in liquid, tape or film form, tend to gel if stored at room temperature and have to be refrigerated. Polysulfide-epoxy two-part systems consist of a liquid polysulfide with mercapto end-groups and an epoxy resin. The materials, once mixed, cure at ambient temperature and the bonds have excellent low temperature ~exibility (to below -100°C). The maximum service temperature is 70-80°C. 138 Principal use is in outside bonding of concrete (floors, runways, bridges, etc.), metals, glass, wood, rubber and plastics. Nylon-epoxy adhesives 139 are primarily used as film and tape adhesives, although two-part pastes are also available. The pastes cure over 2 to 3 days at room temperature, or at higher temperature in less than 4 h. The combinations have been reported to give the highest peel strength of all structural adhesives. Up to 26000 N m- 1 have been obtained. Fatigue and ductility, even at sub-zero temperatures, are excellent. Disadvantages are low creep resistance and a nearly 800/0 loss of shear bond strength under humid conditions.

5.2.4 High-performance Adhesives 140 , 141 Epoxy adhesives have for a long time been the choice for structural bonding where thermal resistance was required. Even the most stable epoxy adhesives, however, can only be used at service temperatures that do not exceed about 180°C. 142 This stability falls considerably short of the needs of modern aircraft, space vehicles or missiles and electronic equipment, for example, where the adhesive joint is required to resist deformation and thermal degradation when exposed to temperatures as high as 500°C for minutes or 250°C for hundreds ofhours. 141 In order to satisfy this need for thermal stability, a number of highly resonance-stabilized polyaromatic polymers were developed in recent years. Most notable among those are the poly(benzimidazole)s143,144 the poly(phenylquinoxaline)s141 , 145 and, most importantly, the polyimides.146-148 Poly(benzimidazole)s, although having outstanding high temperat~re stability, were never used commercially as adhesives because of severe processing problems.. Polyquinoxalines and poly(phenylquinoxaline)s perform similarly to poly(benzimidazole)s, but again their high cost and difficult processing have kept these materials from becoming commercial products. Polyimides are prepared from aromatic tetracarboxylic dianhydrides and aromatic diamines, as illustrated in Scheme 2. Early polyimides, such as that shown, were intractable and insoluble, and had to be processed as the· soluble poly(amic acid) intermediate. Ring closure was then accomplished by heating the assembled joint under pressure (about 0.7 MPa) for 2 h, followed by a postcure of about 16 h at 300 °C. Polyimides that can be processed as thermoplastics have since been

p (or m) phenylenediamine

Pyromellitic dianhydride

Poly(amic acid)

polyimide

Schemel

150

Generic Polymer Systems and Applications

developed.149-151 NASA's LARC 2 or LARC TPI (19) is a thermoplastic polyimide based on benzophenone derivatives. The polymer melts and is processed at 350°C. Overlap shear with anodized titanium at 230 °c is still 500/0 of its room-temperature value (14.8 MPa vs. 29.7 MPa at room temperature).

o

,0

0

N~W-oJf_' o

0

(19) LARe TPI

An alternative way of avoiding the high temperature time-consuming cyclization step and curing without the development of volatiles (water in the cyclization) is to prepare oligomers which have the stable cyclic imide structure preformed and can be cured via unsaturated end-groups, capable to undergo addition reactions. In particular, a nadic (norbornene) terminated imide of molecular weight 1300 was prepared and cured by a heat-induced reaction. 149 Even these materials (e.g. NASA-Langley's LARC 13, 20), after an initial cure under pressure (0.5 MPa), still need heating to 300°C for more than 6 h to develop ultimate strength. These 'addition-type' polyimides have, however, somewhat reduced long-term thermal stability.

o

o

~N~CH2

II~~-

o

0

f" -

~ o

(20) LARe 13

In contrast to epoxy adhesives, polyimides require the use of high pressure and high temperature equipment to obtain satisfactory bonds. Although considerable research effort continues in this area, there is presently no single application requiring large quantities of these polyaromatic high temperature. specialty adhesives.

5.3 SURFACE PREPARATION Adhesive strength is determined by interactions between the adhesive and the adherend at the contact interface. The nature of the adherend surface, and therefore its preparation prior to bonding, is one of the most crucjal steps in the bonding process. The choice of surface preparation has to be made .such that bond failure takes place predominantly either throughout the adhesive layer (cohesive failure) or through the adherend (substrate failure), rather than at the adherend/adhesive interface (adhesive failure). The type and sequence of preparation steps depend on the nature of the substrate and its surface contamination, the bond strength required and the expected service life. Cleaning of the surface to be bonded is mandatory to obtain adequate bond strength. This serves to remove any foreign matter that may have accumulated on the surface. Cleaning with detergents or solvents as well 'as mechanical treatments, from mild abrasion to sand blasting, have been used on metals as well as plastics, even though adhesive compositions have been developed (see, for example, acrylics or epoxies for specific substrates) that give adequate adhesion even without any treatment. Metals in particular are cleaned by vapor degreasing. 15 3 A chemical treatment often follows this surface cleaning step. In metals, this chemical treatment is generally an etching process, i.e. a treatment of the metal with strong acids (sulfuric, phosphoric, chromic, hydrochloric, nitric and hydrofluoric acids). Etching recipes for metals can be found in the literature.153-155 Polymers are often difficult to bono because, in addition to surface contamination and the presence of weak boundary layers, they inherently have lower surface energies and in many cases are chemically inert. Surface preparation of polymeric materials depends greatly on the type of surface to be bonded, and a variety of treatments for specific polymers to increase surface energy and modify surface chemical composition as well as surface topology and morphology have been devised. 156 Because of

Synthetic Polymer Adhesives

151

space restrictions we will, in this review, only describe the treatment of two of the most difficult to bond plastics, the fluoropolymers and the polyalkenes. Both of these classes have extraordinarily low surface energy and do not wet with ease. Fluorocarbon polymers [poly(tetrafluoroethylene), poly(trifluorochloroethylene), or fluorinated polyalkenes] have been subjected to sodium etching or plasma treatment to remedy this deficiency. Sodium etching involves immersion of the polymer into sodium in ammonia (or a sodium naphthalene solution) for .1 to 5 min. The treatment removes fluorine atoms and introduces unsaturation. While the treatment does render these materials readily bondable (increases in surface energy of up to 1500/0 have been reported 157 ), the ·process is extremely impractical. Plasma treatment, either glow discharge under low pressure (cold plasma) or corona discharge at atmospheric pressure (hybrid plasma), is one of the most effective techniques to improve bondability. Surface modification of polymers proceeds by a free-radical mechanism and generally involves chain scissioning, ablation, surface cross-linking and generation of polar groups.158 By way of example, the lap shear bond strength of a Teflon/epoxy bond was increased by a factor of four if the Teflon was exposed to neon cold plasma. 159 One of the oldest and still frequently used commercial treatments of polyethylene and polypropylene is etching with chromic acid. This is predominantly done to increase bondability prior to metal plating. 160, 161 Other treatments that are often used are contact with an oxidizing flame (0.01 to 0.1 S)162,163 or a blast of hot air. 164 Both treatments cause free-radical-induced oxidation and formation of polar groups with concomitantly increased wettability. Plasma is also very effective for increasing the surface energy of polyalkenes. 158 Very short (5-10 s) exposure to RF helium plasma increased the lap shear of a polyethylene/epoxy bond from ~ 3 to 20 MPa. 159 As indicated in Figure 3, corona discharge was found to be vastly superior to surface ozonation. 165 1.8

(a)

46

0

~

...

E

c

>-

'"0

38

~

30

-. I

~.

3

./

+If) '"0 C 0

Q)

30

Time

Figure 3

c ~

./ ~.

10

2

Corona

CJ'l

Ozone

~

\O

1.4

.r::.

O2 Corona

u

•

( b)

0 (L

100

300

Ozone

0.6

-.--. ~-

0.2 I

3

10

/30

100

300

Time

Effect of oxygen corona or ozone treatment on (a) surface tension and (b) bond strength of low density polyethylene (units of time are s for corona and min for ozone)

Introduction of small amounts (0.1 to 10 mol 0/0) of specific functional groups into the polymer has been found to increase adhesive strength. 166 For instance, radiation grafting of monomers containing sulfonic, carboxylic or cyanic groups onto polypropylene surfaces substantially increased adhesion to aluminum, steel and copper. 167 Considerable strides have been made in preparing metal surfaces for structural bonding. Starting in the 1940s with bonding wood to metal in the aircraft industry, the US Forest Products Laboratory (in Madison, Wisconsin) developed methods of surface etching 168 ,169 as well as added anodizing treatments. 170, 171 Different treatments are compared in refs. 172 and 173; intended initially to alter the chemical constitution of the surface, it evolved gradually that a major component of the improvement achieved with these surface conditioning methods was to change the mechanical interlock of the adhesive to the metal. A major role in this interlock is played by the surface oxide layer and its porous structure. In the particular case of aluminum, under the influence of moisture, the oxide layer transforms to a hydroxide with a morphological change and a reduction in strength. These results are well reviewed in ref. 174, which also points out the beneficial effects of certain organic acid additions to reduce the susceptibility of oxides to moisture attack. A comparison of some of these surface treatments in terms of long-term durability are reported in ref. 175. A summary of surface treatments and structural performance of bonds on aluminum alloys is presented in ref. 176. Surface priming is a widely used technique to improve bond performance. 15s Primers are normally very dilute solutions of specific materials (polymers, monomers, etc.) in volatile solvents.

152

Generic Polymer Systems and Applications

The primer solutions are sprayed, brushed or wiped on the surface and, after drying, leave a thin film (0.2 to 5 x 10- 2 mm) which may serve to protect the surface (e.g. inhibit corrosion of metals), to provide a barrier coat between adhesive and adherend to prevent undesired reaction and to modify the surface to improve wetting. The solvent used for polymer surfaces is generally chosen to provide a degree of swelling and molecular interpenetration of the adherend surface. Coupling agents are often added to primers to promote primary bonding between substrate and adhesive. These coupling agents are compounds of relatively low molecular weight that contain, within the same molecule, two different functional groups that are reactive with the adherend and adhesive, respectively. The formed primary bonds reduce slippage on the substrate/adhesive interface during bond failure and increase fracture energy of the adhesive joint substantially. Examples of frequently used primers/coupling agents are polyisocyanates 177 for urethane adhesives on substrates that may contain isocyanate reactive groups at or near the surface; or silanes,1 78 'containing appropriate reactive groups for bond formation, on glass surfaces. This is of particular interest in improving glass-matrix interaction in glass-fiber-reinforced composites. Typically, these silanes belong to the class represented by the formula in (21). In this formula, X is an aliphatic or aromatic radical that contains functional groups that are capable of undergoing chemical reaction with the polymer matrix or the adhesive used for bonding. Frequently used silanes are: aminosilanes, e.g. (EtO)3SiCH2CH2CH2NH2 for bonding with epoxy, isocyanate or formaldehyde resins; epoxy silanes, e.g. (MeO)3Si(CH2)30CH2CH=CH2, for bonding with epoxy and amine/formaldehyde resins; and unsaturated silanes, e.g. (EtO)3SiCH=CH2, for bonding to polyester/styrene resins. RO"" RO-Si-X

/

RO

(21) Reactive silane derivative

Chromium complexes 179 ,180 or titanates 18 1, 182 are another type of coupling agent often used for glass/matrix bonding. Titanates 183 and zirconates 184 are also used to improve bonding of polymer matrices to particulate mineral fillers. Priming of polyalkenes with zirconates was found to increase the adhesion of printing inks. 185

5.4

SURFACE ANALYSIS

Reference has been made earlier in this chapter to the fact that surface characteristics are a decisive factor in determining the strength of an adhesive bond. Various instrumental analytical techniques have been developed and employed to study the chemical composition of surfaces. 186, 187 Many of these techniques require high vacuum and are useful only for substrates not giving off any volatiles under these conditions. This generally excludes most polymer surfaces. While a detailed discussion of the analytical methods utilized for the characterization of surfaces is beyond the scope of this chapter, their main features are listed in Table 2. Electron spectroscopy for chemical analysis (ESCA) or X-ray photoelectron spectroscopy (XPA) is the most useful spectroscopic method for surface analysis. Irradiation of the sample surface by a monochromatic X-ray beam causes ejection of core-level electrons from atoms. The kinetic energy of the ejected electrons is measured. Not only is this energy specific for the irradiated atom and allows its identification, but slight shifts in the energy peak positions (chemical shifts) are a function of the distribution of valence electrons and furnish information concerning the bonding states of these atoms. The sampling depth depends on the X-ray wavelength. Depth profiling can be achieved by using X-ray sources of different energies. Depths from 1.5 to 30 nm can be explored. Fourier transform infrared spectroscopy (FTIR) is most commonly used in its multiple internal reflection mode (i.e. as attenuated total reflection - ATR). The penetration depth of IR radiation is about 10 2 to 10 3 times larger than that of ESCA. In that respect, it cannot be considered strictly a surface analysis technique, and surface features are generally obscured by bulk composition. A more surface-sensitive IR technique" is diffusive reflective Fourier transform IR spectroscopy (DRIFT). This technique employs a KBr powder overlayer over films or fibers to reduce penetration depth. 203 Photoacoustic spectroscopy (PAS) is particularly suited for rough surfaces, where it is difficult to obtain meaningful ESCA or IR spectra. The technique was found to be more surface sensitive than ATR,204 and excellent results were obtained on polymer surfaces. 205 ,206

~

~

Table 2 Methods for Surface Analysis Method

Incident Beam/ Emitted Beam

Depth of Sampling (nm)

Electron spectroscopy for chemical analysis (ESCA, XPS) Attenuated total reflection Fourier transform IR spectroscopy (ATR-FTIR, DRIFT, PAS) Ion scattering spectroscopy (ISS)

X-ray (1-10 KeV)/ e-(200--104 eV)

1-2

Photons/photons

200--3000

Ions (1-3 KeV)/ rebound ions

<1

Secondary ion mass spectroscopy (SIMS, SSIMS)

Ions (0.3-10 KeV)/sputtered surface ions (up to 20eV)

<1

Auger electron spectroscopy (AES)

e- (0.1-5 KeV)/ e- (20-2000 eV)

Surface Damage

Pressure (mmHg)b

Ref

E,Q,C

None

10- 6 -10- 10

178

Q,C

None

Diameter of Information Obtained a Sample

1-3mm

t.'.)

179, 180

~

;s ~

::s~

r;. ~

~

c

2

E,Q 10-300 J.l.m

0.1-1 mm

E

E,Q,C

Sputtering damage (inorganics only Sputtering damage (SSIMS also used for polymers) Extensive on 10- 9 -10- 10 polymers (inorganics only)

13-17, 181-185 18, 19, 186, 187

~ ~ ~

"""i

~

~

::s~

t:'J ;. ~

t:'J

20-24, 185, 189-192

a E, identification of chemical elements (qualitative); Q, quantitative determination of chemical elements in the sample; C, identification of oxidative and chemical bonding states of chemical elements. b 1 mmHg = 133.322 Pa.

~

Ul W

154

Generic Polymer Systems and Applications

Surface enhanced Raman spectroscopy (SERS) was found to be a promising tool for the study of polymer surfaces. Penetration depth can be adjusted to less than 2 nm. 207 The bombardment of the sample surface with primary ions of high energy in dynamic secondary ion mass spectroscopy (SIMS) leads to a rapid destruction of surfaces and is useless for polymeric materials. It is used only for depth profile studies in inorganics. The lower primary ion current used in static secondary ion mass spectroscopy (SSIMS) gives a monolayer lifetime of several hours and can be utilized for the characterization of organic surfaces. Because of their insulating properties, electrical charges can build up in polymers during primary ion beam bombardment. These charges can alter the peak intensities in the secondary ion mass spectrum and give false results. The problem is eliminated if the sample is bombarded with neutral atoms as in fast atom bombardment mass spectroscopy (FABMS). This technique, which has a sensitivity of about ten times that of ESCA, provides qualitative information concerning the element~l composition of 1 nm thick surface layers. 208 High-resolution solid-state nuclear magnetic resonance (NMR) spectroscopy can be used to identify surface. species. The technique gives qualitative information on chemical surface structure and molecular interactions on interfaces. 209 The second factor affecting wetting, mechanical interlocking and therefore adhesive bond strength is surface topography. 174 This, too, is a function of processing conditions, surface treatment, surface migration, weathering and oxidation. The methods of greatest use in this respect, as well as in the study of fracture surfaces to gain valuable information concerning the loci of failure, include light microscopy,210 including Hoffman modulation contrast microscopy,211 but especially scanning electron microscopy (SEM).212,213 SEM, in particular low voltage SEM, has the advantage over optical microscopy of much higher magnification and resolution coupled with about 1000 times the depth of field of even the best optical computer-automated photometric systems. No surface coating is required in low voltage SEM and surface damage is minimal.

5.5 PHYSICAL PROPERTIES OF ADHESIVES Besides the chemical interaction of the adhesive with the adherends, the physical properties determine its performance. In this context, we should always bear in mind that performance is to be measured in terms of both short-term testing for evaluation purposes as well as in long-term applications, possibly over several years. The safe response of an adhesive bond to loads is intricately connected with the physical properties of the adhesive and the adherends. We distinguish generally between contact and flow properties that relate to the capability of the polymer to make adequate contact with the adherends, thermomechanical and mechanical properties that relate to its rigidity or.deformability and, finally, failure characteristics of the polymer. While the latter are often offered in some form of failure data extracted from a tension or osten'sible shear test, we believe that a characterization in terms of fracture parameters such as a (rate dependent) fracture energy is more appropriate. The properties relating to contact formation are primarily the viscosity during the bond formation process, be that related to high temperature or solvent staging. Furthermore, the contact angle of an adhesive in the liquid state is important as a measure of the adsorptive affinity of the adhesive to the adherend surfaces. The mechanical properties which determine the rigidity and strength of a bonded joint are time or rate sensitive in the sense that they are viscoelastic. Usually they are characterized by the relaxation modulus and/or creep compliance in tension and in shear. In principle, this characterization suffices for stress analysis purposes if only small deformations and strains are encountered to effect a linearly (visco)elastic analysis. Alternatively, the viscoelastic bulk modulus or the function for the Poisson ratio may be used to replace one of the former material functions, so that two material functions are available. In many applications it may be adequate to consider time scales such that either the glassy or short-term characteristics of the polymer are involved on the one hand, or only the long-term (elastomeric) ones on the other. In that event, it may be feasible to deal with the short- or long-term limits of the material functions, thus reducing the analysis to that of essentially an elastic material. For the long-term rubbery response, it is usually necessary to allow for nonlinear material response under large strains. In many situations, the range of temperature and time intended for an adhesive is established a priori so that a knowledge of the relaxation or creep behavior over the complete transition range is not necessary. For this reason, there are few, if any, adhesives which have been characterized over the whole time range, preference being given to either the glassy or the rubbery (or viscou~) domain.

155

Synthetic Polymer Adhesives

However, it is always necessary when dealing with such limited characterization that one identify the beginning of the transition range in order to be able to at least make a good estimate of the conditions under which the limit analysis begins to break down. The relaxation modulus is determined in principle by prescribing a sudden application of a strain to a test configuration, and measuring the stress required to maintain that strain constant with time thereafter. Figure 4 shows the relaxation modulus of several formulations of a polyurethane which are typical of cross-linked polymers. There exists high stiffness at short times - 'short time' or 'glassy' behavior - and, after passing through intermediate transition regions, relatively low values in the 'long time' or 'rubber' region result. Because of the chemical cross-linking, the stiffness for long times does not decrease further unless chemical changes are involved; it is indicative of the stiffness of the molecular network when intermolecular forces are set at a minimum.

~

a.

....--.... ~

a Solithane composition

~I~ lu

0 010

-I

-.J

Reference temperature O°C

-2

-6

-4

-2

a

2

4

6

Log lo fiaT {min}

Figure 4

Uniaxial (tension) relaxation curves for various compositions of Solithane 113, a polyurethane elastomer manufactured by Thiokol Chemical CO. 214

When chemical cross-links are absent, mechanical entanglements of the long molecule chains can act as if the chains were chemically cross-linked, as long as insufficient time is allowed to permit these entanglements to dissolve by viscous molecular flow. For this reason, the relaxation modulus possesses generally the same functional form in the glassy and transition regions; the 'rubbery plateau' appears like that for the cross-linked solid, but, depending on the molecular weight, a further decrease of the modulus occurs, with a zero value as the ultimate limit; larger molecular weights are associated with longer 'rubbery plateaus', as indicated in Figure 5. An alternative way of characterizing the viscoelastic properties of polymers is by means of the creep compliance. This characteristic is determined in tests by prescribing a suddenly applied stress (ideally applied in step fashion without inertial effects) and by recording the resulting increase in strain with time. A typical example of a shear-ereep compliance measurement is shown in Figure 6 for poly(vinyl acetate), an uncross-linked polymer often used as a component in pressure-sensitive adhesives. We note that at short times the compliance is small, commensurate with the glassy rigidity of the relaxation modulus. After the transition region the compliance reaches a plateau for some time, after which it increases with an approach to unit slope on the log-log plot. The plateau is the result of the mechanical entanglements of the molecules related to the 'rubbery plateau' in the relaxation modulus description, and the subsequent increase in the compliance signifies continued viscous flow associated with disentanglement of the molecules. For many applications it may not be necessary to characterize the creep behavior over the whole time domain. Such would be the case when an adhesive is to be used in an appropriate way only as long as it exhibits sufficient rigidity (glassy or near-glassy behavior), so that one needs to ~deF;-ttf-F;e+-lrmf+l+lin-H"e'O------the time domain within which this glass-like behavior is no longer guaranteed. An example of such a partial characterization suitable for behavior in the glassy domain is shown in Figure 7 for the adhesive FM-73. Relaxation and creep measurements can be specified in any of the following modes of deformation or stress states for analysis purposes: uniaxial tension or compression; shear; bulk or volume change.

Generic Polymer Systems and Applications

156 8 7

6

C

5

-..

4

~

~

~~

3

0\

0

-J

2

-6

-5

-4

-3

-2

-I

o

Log lo

Figure 5

t

2

3

4

5

6

(s)

Relaxation modulus of narrow distribution polystyrene at 160°C, estimated 215 from ref. 216. Numbers on curves represent viscosity-average molecular weight x 10- 4

-4

-5

-I

Z

N

-6

E

:;::

::;

-7

0\ 0

-J

Log

t (s)

Figure 6 Shear creep compliance of poly(vinyl acetate)217 (narrow molecular weight distribution) for four different specimens

Because to date only the linear theory of viscoelasticity has been established reliably, it makes sense for analysis purposes to restrict deformations for these characterizations to small strains, i.e. on the order of fractions of a percent. At the same time it is important if one anticipates use of the adhesive when such small strains are exceeded to establish the range of deformations when the material begins to follow nonlinear behavior. This determination is necessary in order to be able to estimate when a predictive failure analysis begins to break down. Analysis methods involving nonlinearly viscoelastic material behavior are not sufficiently well established at the present time to allow a brief exposition in the context of this presentation. However, a minimal determination of nonlinear behavior would be to check whether the relaxation modulus or the creep compliance is independent of stress or strain levels.

157

Synthetic Polymer Adhesives -7

(0)

-75

"I Z (\J

E

-8

;::

:::; tJ)

0

...J

-8.5

Log t

(5)

( b)

"I Z

-8

E ~

::; tJ)

0

...J

-8.5

-9---:!;-----+------±-__---1.~----L..-10

Log t

(5)

Figure 7 Creep compliance in shear for epoxy-based rubber-toughened FM-73U 218 adhesive (manufactured by Cyanamid) 18

in the near-glassy domain: (a) master creep compliance curve at 20.5°C, ( - - ) J(t) = Jo + L Ji(l - e tfti ); (b) creep i =1 compliance at several temperatures

An important property of any amorphous polymer is its glass transition temperature, and for (partially) crystalline polymers additionally the melting temperature. The glass transition temperature indicates for each polymer a relatively narrow temperature range in which the material undergoes a change from soft leathery behavior to a solid. It should always be remembered, however, that this distinction is made under deformations that occur in a time scale of a few seconds to minutes. Thus, a polymer a few degrees below the glass transition temperature will exhibit pronounced time-dependent behavior over long periods of time or under very slow deformation rates (creep), although under 'normal' deformation rates (within a few seconds or minutes) it may appear stiff and hard. In crystalline polymers, the crystal melting temperature lies above the glass transition temperature and softening of the polymer normally results from crystal melting. Large volume changes are associated with crystal formation and melting, which would lead to large cooldown stresses in bonded joints; thus, in spite of their generally excellent time-dependent properties, crystalline polymers are usually poor candidates for adhesive purposes. However, with the advance of technology in both controlling the chemistry in order to design the degree of crystallization and thus the mechanical properties (over time) as well as advances in the exploitation of mechanical properties, crystalline polymers promise to become viable materials for long-enduring adhesive applications.

158 5.5.1

Generic Polymer Systems and Applications Thermomechanical Behavior

One of the important physical properties of adhesives is the expansion/contraction behavior under temperature changes because it is associated with the development of stresses. In the vicinity of the glass transition this behavior is complex by present standards, depending to a large degree on the rate with which temperature changes occur, and whether it increases or decreases. Usually, the thermal contraction data that are recorded result from such slow temperature changes that heating or cooling generates the same data, such as shown in Figure 8. In this connection, one must note that, below the glass transition, the material is in an unstable state and that a quick ~ transition through the glass transition entails long-lasting memory of such volume changes. However, if the material has been cooled sufficiently slowly from a hot state, then heating or cooling within the glassy range may be treated, as we understand the state of affairs today, as if the material were thermoelastic. This observation appears to be true as long as one remains at least 20 of away from the glass transition temperature. A similar state of affairs prevails when the polymer is used solely in the rubbery state, though it may then become important to remember then that, in contrast to metals, the stiffness (modulus) of the polymer increases in proportion to the absolute temperature. 3

2

x

I

• x."tIX" x X .¥

o

o

10

20

30

40

50

Temperature (Ge)

Figure 8 Thermal dilatation of poly(vinyl acetate) under equilibrium conditions (heating and cooling at 4°C h -1)217

The true thermomechanical behavior is embodied in the thermal creep function 219 which replaces, for viscoelastic materials, the volume coefficient of thermal expansion of elastic solids. This function is important when one wishes to closely estimate the thermal stresses induced in passing through the glass transition temperature at rates of temperature change which cannot be considered to lead to equilibrium conditions.

5.5.2

Physical Aging of Polymers

When polymers are cooled from above the glass transition temperature, or lose solvent, they shrink, with the shrinkage continuing, depending on the initial and final temperature, over prolonged periods of time. 220 ,221 Associated with this shrinking is a continual change in the mechanical properties of the polymer in the direction of stiffer material characteristics. In keeping with the effect of thermal shrinkage just discussed, this volume change similarly establishes stresses in the polymer and along the adhesive interface which add to those induced by thermal cooldown, except that this phenomenon occurs increasingly over time. Although the volume change is small the high bulk modulus of a polymer can lead to sizable stresses, and their development over time gives rise to a seemingly time-dependent failure process.

5.5.3

Diffusion of Solvents and Water

A situation very similar to the thermally induced stresses arises for solvent-based adhesives. As the solvent diffuses out after the bond has been formed, volume shrinkage occurs which may evidence

Synthetic Polymer Adhesives

159

itself in the form of porosity. On the other hand, such mass transport may result in volume shrinkage which produces stresses very similar to those resulting from thermal cooldown. In this event the same comments apply as in the previous section. An additional problem arises when the edge of a dry bond is exposed to water or (other) solvents; diffusion into the polymer will cause swelling as well as a change in the physical properties, including polymer stiffness and strength. This swelling, which may appear to be minor, will generate stresses similar to those arising from thermal cooldown discussed above, except that these latter stresses are of opposite sign (compressive) and would thus seem beneficially to counteract the manufacturinginduced stresses. While this is true on first swelling the polymer, it is the usual experience that repeated swelling and drying of adhesive polymer tends to generate microfractures, usually in the contraction (drying or evaporative) phase because such transient stress distributions can generate high stresses which add to those already present.

5.6 LOAD TRANSMISSION THROUGH ADHESIVE BONDS The purpose of adhesives is the transmission of forces from one adherend to the other. Thus, adhesive performance is always evaluated in terms of mechanical adhesion tests in which the strength of the polymer or that of its interface with the adherends is evaluated. Usually, the criterion for adequate bond strength is defined as to whether separation occurs along the interface or not, it being believed that this interface domain constitutes the 'weak link in the chain', rather than the strength of the polymer itself. An optimally configured adhesive \vould thus be one which produces failure at the interface at the same moment that failure occurs in the polymer. We shall see that the usually observed failure along an interface may, in fact, not be indicative per se of inadequate adhesion but be the result of mechanical forces that are usually not controlled in the formation and testing of bonds. Thus interface failure may not, in general, be a true indication of 'poor' adhesion, yet could occur for a system that may indeed be already optimized. We shall return to this possibility in connection with the following discussion on other mechanical forces (e.g. thermomechanical) that can arise in adhesive bonds. Strength analysis of bonded joints usually encompasses two distinct phases, namely the analysis of stresses and deformations in the bond and the subsequent failure evaluation. The latter is always based on the former, though the criteria by which the latter is established vary: they may involve maximal stresses, maximal deformations or energy to generate new fracture surface. In this section, we shall discuss first pertinent aspects of stress analysis germane to several test geometries. Failure analysis related to energy evaluations will be discussed in the final section of this chapter. Besides loads applied to a bonded joint in tension or compression, we recognize bending moments that often act in structural applications. These moments are rarely accounted for directly in adhesive testing, even though they develop distinctly different stress distributions than the other two types. There are several test geometries that have been proposed. We list here the most prominent ones and discuss briefly those considered most useful. These are (i) lap shear geometry, with (a) thin adherends and (b) thick adherends; (ii) napkin ring geometry; (iii) scarf joint; (iv) Arcan specimen; (v) pull-out test; (vi) peel test; (vii) cracked lap shear; (viii) shear beam; and (ix) DeB (double cantilever beam) specimen (wedge test). Of these, the first five represent bond geometries normally without a pre-existing fracture or crack-like separation, while the rest incorporate such a feature in order to deal specifically with the role of fracture propagation to characterize bond strength. Each one of these tests offers a special advantage, be it particular simplicity of laboratory implementation or simplicity of 'data evaluation, though neither feature ever occurs simultaneously. For the latter reason some of these tests are useful only for comparison purposes rather than for the quantification of bond strength.

5.6.1

Stress Distributions in Bonded Joints

The distribution of stresses in bonds depends on the overall bond geometry and on the loads applied to the bonded structure. This-is true whether one deals with a 'cracked bond' or an 'integral bond' which contain, respectively, a crack-separation or not. In the following we consider typical test geometries in turn and summarize the major features of their stress distribution; because of lack of space we refer the reader to references for the finer details.

160

Generic Polymer Systems and Applications

5.6.1.1

Integral bond geometries

The most common intention in employing the test specimens in this category is the desire to determine a failure or strength characterization in the form of a limiting stress or strain. This is worthwhile, however, only if one is able to determine the state of stress at the point where failure-fracture initiates. Thus, if the stress distribution is nonuniform, the failure.point needs to be ascertained as well as the state of stress at that point. Only in specimens where the stress state is truly homogeneous would the objective of determining a well-defined failure stress be accomplished. Under this category there is only one test that fulfills this requirement, namely the napkin ring test, which is, however, not easy to carry out. (i)

Single lap geometry

This geometry, usually employed because of its seeming simplicity, gives rise to a complicated stress distribution. While this particular test geometry is not especially desirable for that reason, we use it here as a demonstration model because it is so familiar. In order to emphasize the less desirable features of the normally encountered geometry we discuss first the thin adherend geometry~ That two-dimensional version is shown in Figure 9(a) with tension forces F applied, but with the specimen undeformed under these forces. Because the 'legs' of the specimen are offset from each other, the applied forces tend to rotate the joint section unless special guides, indicated in Figure 9(b), are used (not a useful proposition). Absence of such guides causes rotation of the lap region, as indicated in Figure 9(c), which is not eliminated if only the ends of the 'legs' are prevented from rotation by clamping in a testing machine. The amount of rotation depends on the thickness of the adherends and of that of the bondline. If the adherends are thick such that they are difficult to bend under the applied loads, the suspension linkage of the specimen in the loading machine becomes part of the test geometry; these are hardly desirable test conditions.

d y

f

I I I I

I I I

, I

F

-..e ..I

0=

I

d+t

(0)

(b)

(c)

(d)

Figure 9 Lap shear test geometry: (a) undeformed geometry; (b) rollers attempt to force a pure shear deformation; (c) rotation of test section though pin-application of force; (d) deformation with only ends restricted from lateral motion and from rotation

In Figure 10(a) we show typical distributions of the shear and normal stress, i.e. normal to the bondline at the mid-plane of the bond and at one interface. The stress distribution is rotationally symmetric with respect to the center of the bond. The distinguishing features of these distributions are the stress peaks at the bond ends which exceed the average stress (force divided by bond area) significantly. The initiation and development of failure is most certainly associated with these (analytically unbounded) peaks and not with the average shear stress. These peaks are a function of the thickness of the adherends and of the adhesive, of the length of the bondline as well· as the constraints imposed by the test machine against rotation of the specimen. Moreover, the magnitude

161

Synthetic Polymer Adhesives ( a)

on interface

y

d

Y= "2

I I

I

,.-

--

I

~ Average shear stress

Shear stress

_ _ Normal stress

~

on midline y=o

uy

Average shear stress

- ,/ ' Shear stress

I

/

/

Normal stress u y

( b)

F~-T;-II-I"------.l\F Shear stress on this interface

Figure 10 Typical shear stress distributions in shear lap test geometry (qualitative): (a) for example geometry (bond thickness shown exaggerated); (b) for end termination (spew) before deformation

of these stresses is highly dependent on the geometry of the termination of the bond, an example 'of which is given in Figure 10(b). It is clear that the use of such a highly variable stress field dependent on virtually every geometric parameter is not ideal for evaluating the efficacy of an adhesive. (ii)

Double lap shear geometry

The use of a double lap shear geometry alleviates some of the uncertaint~s of the single lap geometry, but hardly to the point of eliminating them. Figure 11 shows a dottble lap geometry in which the symmetry about the tension axis prevents the gross rotation of the 'test section' illustrated in Figure 9(c) and (d). However, because of the one-sided shear stress distribution on the outer adherend in the transition region (encircled in Figure lla) there is a tendency for local rotation which gives rise to a deformation shown (exaggerated) in Figure II(b). This deformation introduces compressive stresses in that bond termination region, which may give rise to artificially higher bond strength values. Moreover, the amount of this local bending depends on the thickness h of the outer adherends and on the gap g, being smaller the lower the ratio g/h is. (iii)

Thick adherend single lap specimen 222 ,223

The degree of the rotation of part or all of the test section in either specimen is somewhat alleviated but not eliminated by the use of thick adherends as illustrated in Figure 12. The specimen is produced by introducing two machine cuts of width w just past the adhesive, thus generating a

162

Generic Polymer Systems and Applications (0)

F/2

F F/2

(b)

h

Figure 11

J-----------..;......O--..l.::...:- -------------~-- _ - - ._·~~~ter 1 .__ ._. _.-. Double lap shear geometry reduces rotation of adherends compared to single lap specimen

~~

---------,~r------l-ri= ~----------~ Saw cut past adhesive layer

Figure 12 Thick adherend single lap geometry

consistent bond termination geometry. The stress singularities illustrated for the single lap specimen in Figure 11 is only modified and not eliminated. Adherend thicknesses on the order of a centimeter seem to be desirable for 'rigid' adherends.

(iv)

Napkin ring geometry224-229

The idea in this geometry is to generate shear stresses in a torsion test. There are two geometries for this type of test as shown in Figure 13 (a and b). The test specimen in Figure 13(a) has the advarttage, in principle, to be devoid of stress risers except possibly near the bond terminations. The major problem with this otherwise ideal test configuration is that the relatively stiff tubular adherends fix the bondline gap very firmly and thus do not allow for shrinkage in the cure and/or cooldown process. As a consequence the adhesive is under high residual stress and most often very porous. As a result the adhesive is not in a state desired for applications and thus the test usually yields erroneous results. (0) (b)

I

I

Adhesive

I

I I I I

:(0--'1'

"'- -- ... 1 ....

_-"" T

Figure 13 Napkin ring test geometries

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Synthetic Polymer Adhesives

The test configuration in Figure 13(b) is almost ideal for adhesives that deform under small strains. The state of stress is homogeneous throughout the adhesive and free of stress singularities other than those possibly introduced in connection with thermal residual stresses discussed above. In order to make this test effective for the determination of both modulus measurements as well as failure/fracture characterization it is necessary to manufacture specimens carefully. Usually, after bond formation, the specimen has to be machined to conform to closely specified dimensions. This requirement restricts use primarily to work with structural or 'rigid' adhesives. The tubular adherends must be aligned well axially and torqued similarly by a pure torsion without introducing bending type deformations along the tube axis. This is not an easy test to perform. (v)

Additional test geometries

There are further test geometries which are of interest for special purposes to study the effect of bond-normal and shear stresses on failure initiation. These tests, e.g. the scarf joint test, a modification of a butt joint and a further modification, the Arcan 230 , 231 specimen, generate both of these stresses in the mid-section of the bondline, but exhibit to varying degrees the stress risers at the bond end terminations. The Arcan geometry is reported to produce a minimum of such elevated stresses at the bond ends. (vi)

Pull-out test

It is often necessary to assess the degree of adhesion of a polymer to a fiber, wire or string (cord) as experienced, for example, in the tire industry. Figure 14(a) shows a pertinent configuration. This is a test which usually yields only qualitative information because the stress or failure analysis is forbiddingly complex when one deals with a realistic situation. Commonly, the average shear along the wire is evaluated for failure characterization. This is clearly inadequate since, as indicated in Figure 14(b), the shear stress along the wire is not only highly nonuniform but contains stress singularities at the ends. This shear distribution depends on the embedment length and on the ratio of the moduli of the fiber and of the material in which it is embedded. For a stress analysis based on small deformations see the work by Muki and Sternberg. 232 Moreover, radial stresses arise which are tensile on the pull side and compressive on the opposite side, so that the failure process starts on the pull side, and the pull-out force depends on the friction generated under the compression. Because of the fracture roughness, the latter can be significant. This problem has also been considered experimentally by Anderson and Williams. 233 (0)

(b) Support

Figure 14 Wire pull-out test

5.6.1.2

Thermal stresses

Excepting the use of pressure-sensitive adhesives, bonded joints are produced such that shrinkage of the polymer occurs. This shrinkage may be the result of cooling a hot-applied adhesive or -of solvent removal. Near the termination of bonds or at the tips of interface cracks these processes always lead to very high stresses, as illustrated in Figure 15(a), which are superposed onto those resulting from the applied loads discussed subsequently. In fact, such shrinkage stresses can be so high that bond-end cracks are generated in the bond formation process so that mechanical loading of the joint requires only the propagation of the crack for total failure to occur. In the interior of the polymer, i.e. away from the edge of the bond, the tension stresses parallel to the bond plane approach

164

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--------- .... UJ(

y=+

Figure 15

f

y=+f

Deformation and stress induced by thermal cooldown and/or solvent evaporation: (a) intact bond; (b) fracture at interface

a constant value within distances from the hond edge on the order of a few hondline thicknesses, while the shear stress and the stress normal to the hondline drop to zero in that range. The fact that this stress alleviation occurs over such a potentially short distance should not be taken as a measure of nonimportance, however, since it is nevertheless real and adds significantly to the failure process. It is needless to say that once a crack has formed under these conditions, whether from manufacturing-induced reasons or under use loading, these thermally induced sttesses will always be present as shown at the front of an interface crack in Figure 15(b) to drive the tip of the crack, unless the polymer is so viscoelastic that with time these manufacturing stresses dissipate. To estimate the magnitude of the average stresses it is useful to assess the stresses or strains generated in an ideally thermoelastic adhesive/adherend system. Let 1\lJ. be the difference between the coefficients of linear thermal expansion of the adherends and of the polymer, assuming equilibrium conditions for the latter. Let 1\ T be the temperature below the glass transition temperature of the polymer and consider a sandwich of infinite extent in which the adherends are substantially thicker and stiffer than the adhesive. Then the stress normal to the plane of the sandwich is zero and the stresses in the adhesive in the direction parallel to the plane are equal and given by (coefficient of thermal expansion for polymers is larger than for that of the adherends) (10)

where E is Young's modulus appropriate for the temperature or time range under consideration and v is Poisson's ratio ( ~ 0.3 to 0.4 for rigid polymers, '" 0.5 for elastomers). This stress level is useful for comparison with stresses due to applied loads. Alternatively, one may choose for comparison purposes a general strain level. For the idealized case just mentioned the strain state is equibiaxial and leads to a strain of (11)

5.6.1.3

Cracked test geometries

In this category we consider test configurations which allow the determination of bond or adhesive strength through geometries involving pre-existing cracks. Failure is then characterized in terms of parameters governing crack propagation. Thus one deals specifically not with the question of failure initiation, so that the characterization of failure is not affected in terms of a stress state prior to fracture/failure. Instead the determination of the stress distribution serves primarily to generate the basis for an energy analysis which becomes the vehicle for characterizing the failure process. In this section we shall deal only with a short description of the test geometries and the special stress state at the front of a crack or separation, which becomes the dominant feature in any crack propagation process. There are certain features in the stress distribution at the tips of interfacial cracks or disbonds that are common to all geometries as long as one allows that tip to be represented mathematically as a

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Synthetic Polymer Adhesives

sharp discontinuity before deformation. Most notable and important of these is the fact that as long as one considers the adhesive and adherends to be continuous at any level of size scale the stresses become unbounded at the crack tip. Within the framework of linearized elasticity, these stresses on the interface and ahead of the debond are of the form234-238 Uy

!xy

f3

1 { J2nx 1 J2nx 1 -In 2n Kk

K1Sin(Pln~) I

{-KISin(Pln~)

KCOS( PInT)}

+

2

-

I

{Ill

+ +

112

112

Ksin( Pln T)} 2

"1 } 4v k

-

vk )/(1

+

O(X 1 / 2 )

(12)

(13)

(14)

III K 2

3 (3

O(x 1 /;2)

+

plane strain, k

+

vk )

=

1,2

plane stress

(15)

where /11 and /12 are the shear moduli, Vl' V2 the Poisson ratios of the two materials and 1 some' appropriate scale length (like crack half-length). Two features become prominent as the crack tip is approached, i.e. when x ~ 0: the stresses become unbounded like the inverse square root of x, and they undergo oscillations stemming from the terms of the type log {sin[pln(x/l)]}. These oscillations also reappear in the associated displacements of the crack faces as the tip is approached and would represent that, very close to the crack tip, the materials on opposite sides would mutually interpenetrate. That result is unacceptable from a physical point of view. While such a 'solution' would appear therefore rather pointless, we note, however, that this anomalous behavior is limited to such a small domain that it is below all experimental detactability. Moreover, the oscillations appear to be largely if not solely the result of linearizing the theory of elasticity, because they do not occur when large rotations and large deformations of material elements at the crack tip are considered.239,240 In addition, material nonlinear behavior modifies the stress field in the region of most intense deformations to the point where the question of stress and displacement oscillations becomes a moot problem from an applications point of view. In addition, it turns out as shown below that the details of the stress distribution in the crack tip region become submerged in the computation of the energy release associated with failure propagation, so that for quantitative evaluations of fracture criteria it becomes more important to concentrate on the integration of the stress field over the crack tip domain for the determination of this energy. We note in passing that the oscillations vanish for the case of plane strain when both materials are incompressible, i.e. when their Poisson ratios equal 1/2. That result does not hold for the case of plane stress. However, since for most bonds a state of plane strain is indicated, this finding is useful, essentially when rubbery types of adhesives are involved. A feature that is very important in discussing the failure of interfacial bonding or fracture behavior at interfaces concerns the relative directions across the interface in which the materials separate. The mode I, II, and III nomenclature for fracture determined by the relative motion of material at the crack tip, as illustrated in Figure 16, has become standard. In this context we note that, in general, loading on a bond such that gross loads favor symmetric deformation across the bondline, the detailed deformation at the crack tip still involves both the crack tip local symmetric mode I and the shear mode II. This behavior is a consequence of the fact that the two materials across the interface possess different properties; it is explained most easily in terms of different Poisson ratios, but dual mode deformation exists also when the Poisson ratios are the same.

Figure 16 Definition of fracture modes applied to an interfacial separation: (a) mode I or opening mode; (b) mode II or inplane shear mode; (c) mode III or antiplane shearing mode

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167

(0)

( b)

Figure 18 (a) Definition of crack tip coordinates and (b) detail of tensile/compressive stress distribution in adhesive ahead of crack

where w = width of beam normal to paper plane. K. is seen to increase with the crack length I if the applied force F remains constant upon crack elongation (it remains constant only if the force F decreases such that the bending moment M = FI is constant). For this reason the crack tends to continue the separation process once fracture has started because K. and thus the stresses increase under a constant load. We call such propagation 'unstable'. Since the end deflection b of the beam in Figure 17 is given by 4F1 3

= - -3

fJ

wEh

(20)

the stress intensity factor K. for the problem can be cast in terms of this end displacement b as KI

--

fJEJ3h 3 21 2

(21)

Thus, if the end displacement is held fixed while the crack length I increases, the stresses decrease; this means that the stresses decrease upon crack growth and that more displacement needs to be applied to make the crack grow further. Such a situation leads to 'stable' or controllable growth. In order to keep the stresses at the crack tip approximately constant when the crack length increases under constant end force (i.e. during crack propagation) it is possible to change the beam thickness I along the specimen so that the ratio 1/ remains (nearly) constant. This observation has led to the design of the 'tapered' or contoured double cantilever geometry 241 shown in Figure 19. The stress analysis provided here is approximate in that the beam sections are assumed not to rotate near the. tip of the crack. This assumption is not quite realized in experiments and that fact may have to be accounted for. 242

P

h=

y

__..._-----t-----.x

~==:-.

o Figure 19 Tapered DeB test geometry

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168

The unbounded stresses at the crack tip are mitigated by the more compliant adhesive, yet these stresses are responsible for the fracture of the adhesive or the separation at one of the interfaces. Further away from the crack tip the stresses across the bondline become compressive in order to balance the moment applied by the end forces to the separating legs of the specimen; cf Figure (18b). A more detailed analysis accounting for the nonlinear response of the adhesive is given in the literature.242-244 As a result of the size scale associated with the adhesive thickness there occurs a phenomenon that is often not recognized with this test geometry; namely, that for an interface separation this geometry generates also shear stresses at the separation front, and thus gives rise to both normal or mode I as well as shear or mode II deformations. This phenomenon is illustrated in Figure 20, where it is indicated that the shear deformation results from the extension of the adherend surface - due to beam bending - relative to the adhesive. This shear deformation depends on the thickness of the adherend and of the adhesive and on the relative stiffnesses of the materials involved.

Adherend

(

/

~<....+-"lII~

Adhesive

Stiff adherend

Figure 20 Shear induced in adhesive by mode I deformation of beam adherend

( ii)

Peel test

This test is a variant of the double cantilever beam test which allows large deformations of the adherend. For pliable adhesives this test is one of the most often employed configurations. Common forms are shown in Figure 21(a-e). Often, it involves only one flexible adherend as indicated in Figure 21(b), which is peeled at a 'peel angle' which is either 90° (21a) or 180° (21 b). The adherend in this case may be either the adhesive itself-such as rubber - or in the event that the adhesive is too weak to sustain the peeling tensile force, a (nearly) inextensible flexible adherend, such as fabric, is used as a reinforcement. The stress analysis is complicated by the fact that one deals with large deformations (large local strains and rotations) at the peel front or crack tip. For this reason there exists, to date, no analytical framework for using detailed stress information in characterizing failure in this test. Instead, energetic deductions are made as discussed below. Regardless of the difficulty from the point of view of stress analysis, it is well known 245 that while the peel front or crack tip generates high tensile stresses leading to mechanical disintegration of the material there, it is followed (or led) by a region of compression; as in the double cantilever beam geometry, this distribution follows from the fact that the adherend bends at the peel front and the bending moment has to be counterbalanced by tension and compression stresses as sketched in Figure 18(b). (iii)

Cracked lap shear specimen for high strength adhesives 246

In order to examine the effect which the interaction of normal and shear loading has on the strength of structural joints it is necessary to employ specimens which generate both mode I and II deformations at the front of a disbond. Figure 22 shows the basic geometry and application of loads. The analysis for the stresses is not a trivial matter and therefore only numerical results have been presented. 246 The trade-off between mode I and II depends on the relative thickness of the two adherends (tension and lap leg) and can be readily arranged to vary from 1/2 to 2. It is advisable that the use of this geometry be accompanied by a numerical stress analysis which also incorporates the

Synthetic Polymer Adhesives

169

(0)

F

__

F~~

~F

I

I

I

I

I

....

.... '1

I

I

(e)

(d)

----J ~ ~I..L.-

t f

ds

0

F

Figure 21

Crack

-----J

-,

Peel test geometries

I

Lap leg

p....-----..-----=' § = =_ _7_ _.....~ "

Tension leg

==r=-t P

.~

0

it; t

Figure 22 Cracked lap shear geometry

determination of the energy parameter(s) as discussed below. This geometry leads always to mode I stresses that are tensile, besides mode II deformations, and thus mimics situations normally encountered in adhesive applications. (iv)

Shear beam geometry for high strength adhesives247-249

An alternative or complementary geometry for investigating the strength of adhesive bonds under shear is shown in Figure 23 which draws on the development of shear stresses on the axis of a beam under three- or four-point bending. While the mode I stresses in the cracked lap geometry are tensile they tend to be compressive in the present situation. That fact would indicate that the resistance to crack growth under shear is enhanced in this configuration, and that the resulting data would not be conservative from a design standpoint. However, it is possible to incorporate a smooth Teflon insert into the crack to reduce or eliminate this effect.

Figure 23 Shear beam geometry

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170

5.7 BOND STRENGTH DETERMINATION The most obviously desirable characteristic of an adhesive is its ability to form a strong bond between adherends. As has been discussed, there are many factors that affect the strength of an adhesive bond. To be sure, the load-carrying ability of a bond is affected by the properties discussed in Section 5.5, but to date the bonding characteristics per se can only be determined in a destructive test. We have already considered some examples of the stress analysis for the appropriate test geometries and turn now to the evaluation of these analyses for the extraction of a fracture parameter or function. In this endeavor it is desirable to choose a configuration that allows such a determination with minimum effort and maximal precision. The parameter/function which has established itself as the most useful description of bond strength is the adhesive energy, which in the case of a time or load history insensitive material is a constant or, in the event of viscoelastic components in the adhesive, a fracture energy function, depending on the rate of unbonding. The analyses leading to the determination of these quantities are, to date, invariably based on (linearly or large deformation) elastic behavior because the theoretical framework for these material behaviors is understood well to fairly well.

5.7.1

Fracture Energy

When fracture occurs and generates new surface(s) a certain amount of energy is required. This energy is a combination of the energy required to break atomic bonds along the fracture path as well as energy dissipated in the region of high stress around the tip of a crack. Because of the latter feature this energy is dependent on the history of failure progression and thus a fracture energy function which depends on the deformation history, mostly expressed through the (instantaneous) velocity, results. The experimental determination of the fracture energy involves thus the propagation of a crack in a bond geometry. If the crack follows the bond interface closely, the resulting fracture energy is termed an adhesive fracture energy, while fracture involving generation of two new surfaces in the adhesive leads to a cohesive fracture energy. One often finds that a fracture does not cleanly follow one or the other path; in particular it happens not infrequently that the fracture follows a path that leaves an extremely thin film of adhesive on the adherend, such that it may not be visible with the unaided eye. The reasons for this occurrence are not understood, and a clear distinction between cohesive and adhesive failure is then not usually made, though adhesive failure would seem to be the more appropriate description. We turn first to a consideration of the DeB specimen. 5.7.1.1

DeB specimen

We examine first the ideal case for which adherends of the geometry shown in Figure 17 are treated idealized as two beams within the context of the technical theory of bending (Euler beam theory). Under this idealization we neglect the thickness of the adhesive interlayer as being small compared to the thickness of the adherends and allow the beams to be supported with zero slope or rotation at the crack tip. The application of forces F to the beam ends as shown in Figure 17 cause the beam ends at x = 0 to separate by an amount 2£5 given F by J

4FP

= -_.

(22)

wEh 3 '

and to store a total amount of elastic strain energy in the two beams U

=

1 -2JF 2

4F 2 13

wEh 3 J 2 Eh 3 --w 413

(23)

(24)

Suppose one allows the crack length 1to increase by an infinitesimally small amount dl. This change may occur in at least two distinctly different ways: (1) the force remains constant during this crack elongation or (2) the deflection £5 may remain constant during this event. Let us first consider the case when the displacement remains constant. Since the forces F do then no work during crack extension

rwdl

dS=

=

-dU

(25)

or 1 dU

r

w dl

(26)

so that one arrives at r

=

3£5 2 Eh 3 4/4

(27)

Upon comparing this value with the stress intensity factor K I for fixed end displacement in equation (21) this may also be written as (28)

We call the quantity r the 'fracture energy' for the crack propagation process. If the crack propagates along the interface one adds the notation 'adhesive', while fracture through the adhesive relates to the 'cohesive' counterpart. In the event that interface separation occurs, the adhesive fracture energy renders a measure of the strength of the bond, while a failure through the adhesive indicates the strength of the polymer and that the interfacial bond strength is larger than the latter. When the forces are applied to the ends of the specimen instead of prescribing the end displacement, these forces will perform work as the beam ends move during the crack propagation process. The details of computing the fracture energy under these circumstances are slightly different, but the result (28) is the same, so that, in terms of the equation for F* at the point of (unstable and continuing) crack propagation, one determines r from (using equation 19 in 28 or equation 20 in 27)

r

=

(29)

So far we have considered the ideal situation where the beams are assumed not to rotate at the crack tip. In reality the beams are restrained from rotation there only by the bending in the continuous beam and by the support through the adhesive. This rotation can be taken into account, but the net result is not very different as long as the disbonded portion of the beam is about 10 or 20 times as long as the beam is thick.242-244 Also, approximation, that correct for this beam rotation have been offered for prescribed end loading F as 241 (30)

and for prescribed end displacement b as 250 £52 Eh 3 [3(1 "/"

=

4

[(I

+ 0.6h)2 + h2] + 0.6h)3 + Ih 2 ]2

(31)

Before passing to the discussion of another test geometry, we point out that use of the DeB geometry under fixed end displacements is appealing for certain aspects of durability testing. Usually referred to as the Boeing or wedge test,251 the arrangement is shown in Figure 17(c). Because the end displacement is fixed by the wedge, crack propagation is stable, i.e. the crack will propagate some distance and then come to rest. This test is usually employed to test bond strength over time in a 'corrosive' environment: insertion of a specimen into, say, a humid environment may cause debonding; the length of debonding before crack arrest occurs is then a measure of the bond quality. Moreover the final length of the crack together with the test specimen discussions and material properties will lead to a value for the (adhesive) fracture energy. Since this energy derives from a

Generic Polymer Systems and Applications

172

long-term test leading to no further crack growth it constitutes a threshold value for bond strength in the particular environment in which the test was conducted. We turn next to a variant of this geometry, namely the peel test.

5.7.1.2

Peel test

The test geometry appropriate for the peel measurement is shown in Figure 21. In the event that the adherend is mounted on an essentially inextensible cloth backing the energy required in the separation process is, generally, absorbed in two regions, one consisting of the bent domain of the adherend strip, the other in the region at the separation front where the failure process actually takes place. For the case or a pressure-sensitive adhesive Kaelble 245 has measured the adhesive force distribution indicated in Figure 24.

/

Cohesive stress

Figure 24 Cohesive stress distribution during peeling of a pressure-sensitive tape (sketched after Kaelble's results 245 )

Where such detailed measurements of the force-displacement relation cannot be achieved, it is informative to determine the energy required to cause separation. Assuming again that the adherend is backed by an inextensible (cloth) material one finds that a separation of the specimen ends ds (cf Figure 21) under a steady-state stress (J in the debonding process generates an amount of work, increment d W, given by dW

ubtnds

=

(32)

where band t are the dimensions indicated in Figure 21. The number n is related to the peel angle 245 for test geometry; for example n = 1 for Figure 17(b) and n = 2 for Figures 17(d) and (e). Since bds is the area of unbonding in the tension process, the work per unit area of unbond or the debond energy is

r

=

ndW

- -

b ds

=

nat

=

F nb

(33)

whereF is the extension force and where it is assumed that no energy is dissipated in the bent region of the adherend(s).

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173

In the event that the material is not reinforced with an inextensible material, some of the work done by the extension force is stored as elastic energy in the debonded leg(s) of the specimen. As soon as the specimen legs are debonded they become extended. We start with the fact that the work done (cf Figure 21d, e) by the force F in moving through the displacement ds is d W = F ds; we assume that the force remains constant through this debond process (steady state). That work supplies the strain energy in the material that was unstretched before debond propagation and which is now strained, as well as the energy of rupture. Let an amount of surface bo ds o be unbonded where bo is the width of the specimen before deformation; the total fracture energy is then bor ds o . The volume of material boto ds o ' measured in the undeformed state, is then deformed uniformly and acquires a strain energy per unit of undeformed volume of U o' Note that this volume may be considered to be located in the uniformly stretched portion of the peeled adherend. This strain energy may be determined in a uniaxial tension experiment which results in the stress-stretch diagram illustrated in principle in Figure 25. The stress u is defined in terms of the stretch ratio A, i.e. the ratio of deformed to undeformed specimen length. The strain energy Uo(A) is then the shaded area under the curve, considered either as a function of A or of u, Uo [ u ( A)]. Energy balance then yields (34)

Since ds = nAds o , n = 1 or 2 depending on whether the case of Figure 2I(d) or (e) applies, there results (F = ub oto) (35) r = { na A - U o[ a ( A)] }to Both terms in brackets can be determined readily from data such as that represented in Figure 25. Note that in the case of an inextensible adherend (S)A = I and U o ~ 0 so that equation (33) is recovered. Also, if n = I (the case in Figure 2Id applies) this result permits a simple interpretation; one has then (36) r = {a(A - 1) - Uo[a(A)] + alto = {Uc + alto where Uc is the energy complementary to the strain energy Uo and indicated by the cross-hatched area in Figure 25.

4

Figure 25 Typical

force~xtension

relation for a highly deformable material in uniaxial extension

These considerations fail when energy is dissipated viscously or through plastic deformation in the bending of the adherends.

5.8 REFERENCES 1. F. W. Hodge, (ed.), 'Handbook of American Indians (North of Mexico)', Bureau of American Ethnology, Bulletin No. 30, US Government Printing Office, 1907, part 1.

2. G. Hernandex De Alba, in ~Handbook of South American Indians', ed. I. H. Steward, Bureau of American Ethnology, Bulletin No. 143, US Government Printing Office, 1946, vol. 4, p. 405. 3. C. Singer, E. J. Holmgard and A. R. Hall (eds.), 'A History of Technology', Oxford University Press, New York, 1954, vol. 1, p. 695. 4. R. B. Woodbury, in 'Handbook of Middle American Indians', ed. G. R. Willey, University of Texas Press, Austin, TX, 1965, vol. 2, part 1, p. 163.

174

Generic Polymer Systems and Applications

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PS7-G