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Adhesion Science
By John Comyn
The Royal Society of Chemistry
Copyright © 1997 The Royal Society of ChemistryAll rights reserved.
ISBN: 978-0-85404-543-3
CHAPTER 1
Introduction to Adhesion and Adhesives
Glues have been around for a long time; the ancient Egyptians used them in veneering the treasures of Tutankhamun and the ancient Greek word for glue is koλλα, from which we get colloid. In all centuries up to and including the 19th, glues originated from plants and animals; during the 20th century, however, synthetic chemicals have largely taken over, and the more respectable name of adhesive has been introduced. Animal glues were mostly based on mammalian collagen, which is the main protein of skin, bone and sinew, and the plant kingdom provided starches and dextrins from corn, wheat, potatoes and rice.
Nowadays adhesives are used in all types of manufacture, and in many cases have displaced other means of joining. A range of adhesives (hot melt, vegetable glues and emulsions) are used in making cardboard boxes, with rarely a staple to be seen. Apart from expensive handmade shoes, footwear is now adhesively bonded using hot melt adhesives for the basic construction, natural rubber latex for linings, and solvent based polyurethanes or polychloroprenes for sole attachment. Bookbinding is by hot melt adhesives.
Adhesive bonding is used increasingly in the construction of aircraft. Structural bonding began with the World War II De Haviland Mosquito, which was made of plywood. Modern civil aircraft are basically made of aluminium alloy, and rubber modified epoxide adhesives are increasingly used.
Rubber-to-metal bonds are used for engine, transmission and exhaust mountings in automobiles and in railway bogie suspensions. Mass produced car bodies are made of spot-welded mild steel; weight and fuel consumption can be reduced with aluminium bodies, which are more difficult to spot-weld. The large-scale bonding of car bodies is a prize that awaits the adhesives industry. A recent achievement was the bonding of steel rails in the new Manchester tramway.
Human beings can be repaired by adhesives. This includes the use of UV-curing cements in dentistry and acrylic bond cements in orthopaedic surgery. It has been said that cyanoacrylate adhesives were used for short term repairs during the Vietnam War.
Adhesives are not the only materials that must stick or adhere. Adhesion is essential for printing inks, sealants, paints and other surface coatings, and at interfaces in composite materials such as steel or textile fibres in rubber tyres and glass- or carbon-fibres in plastics. Mother nature uses adhesion rather than mechanical fasteners (nuts and bolts, nails, staples, etc.) in constructing plants and animals, and some animals are masters at the exploitation of adhesion. Here I am thinking of barnacles sticking to anything that floats in the sea and the remarkable ability of many insects to walk on ceilings.
A disadvantage of adhesives as a means of joining is that they are generally weakened by water and its vapour. Also, their service temperature ranges are less than for metal fasteners (nuts, bolts, welds, staples, etc.), being limited by their glass transition temperature and chemical degradation. Advantages include their ability to join dissimilar materials and thin sheet materials, the spreading of load over a wider area, the aesthetic and aerodynamic exteriors of joints, and application by machine or robot.
BASIC PROPERTIES
What is an adhesive and what are its basic properties? A definition is a material which when applied to the surfaces of materials can join them together and resist separation. The terms adherend and substrate are used for a body or material to be bonded by an adhesive. Other basic terms are shelf-life, for the time an adhesive can be stored before use, and pot-life, the maximum time between final mixing and application. Basically an adhesive must do two things:
(i) It must wet the surfaces, that is it must spread and make a contact angle approaching zero, as is illustrated in Figure 1.1. Intimate contact is required between the molecules of the adhesive and the atoms and molecules in the surface. When applied the adhesive will be a liquid of relatively low viscosity.
(ii) The adhesive must then harden to a cohesively strong solid. This can be by chemical reaction, loss of solvent or water, or by cooling in the case of hot melt adhesives. There is an exception to this, and that is pressure-sensitive adhesives which remain permanently sticky. These are the adhesives used in sticky tapes and labels.
BASIC CHEMISTRY
All adhesives either contain polymers, or polymers are formed within the adhesive bond. Polymers give adhesives cohesive strength, and can be thought of as strings of beads (identical chemical units joined by single covalent bonds), which may be either linear, branched or crosslinked as illustrated in Figure 1.2.
Linear and branched polymers have similar properties and it is not easy to distinguish them, and they will flow at higher temperatures and dissolve in suitable solvents. These latter properties are essential in hot melt, and solvent-based adhesives, respectively.
Crosslinked polymers will not flow when heated, and may swell, but not dissolve, in solvents. All structural adhesives are crosslinked because this eliminates creep (deformation under constant load). Automotive tyres are crosslinked natural or synthetic rubber, and if they crept they would permanently deform during parking, and a rough ride would follow.
Many adhesives contain additives that are not polymers are these include stabilizers against degradation by oxygen and UV, plasticizers which increase flexibility and lower the glass transition temperature, and powdered mineral fillers, which may reduce shrinkage on hardening, lower cost, modify flow properties before hardening and modify final mechanical properties. Other possible additives are tackifiers and silane coupling agents.
THEORIES OF ADHESION
There are six theories of adhesion; physical adsorption, chemical bonding, diffusion, electrostatic, mechanical interlocking and weak boundary layer theories. As all adhesive bonds involve molecules in intimate contact, physical adsorption must always contribute.
Physical Adsorption Theory
Physical adsorption involves van der Waals forces across the interface. These involve attractions between permanent dipoles and induced dipoles, and are of three types. Epp is the potential energy, in a vacuum, of a pair of permanent dipoles separated by distance r at their centres and is given by equation 1.1, where μ1 and μ2 are the dipole moments, ε0 is the permittivity of a vacuum, k is Boltzmann's constant and T the absolute temperature.
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.1)
If a non-polar molecule is close to a dipole, then the latter will induce a dipole (μi) in the former. The induced-dipole moment is given by equation 1.2, where α is the polarizibility of the non-polar molecule and E is the electric field.
μi = αE (1.2)
The potential energy for such an interaction is given by equation 1.3, where μ1 is the moment of the permanent dipole.
Epi = -μ12α/ 4πε0r6 (1.3)
Instantaneous dipoles exist in non-polar molecules because of the fluctuating distribution of electrons. These lead to attractive forces between molecules, without which non-polar gases such as helium and argon would not be able to liquefy. The potential energy of a pair of molecules is given by equation 1.4, where α1, and α2, are their polarizabilities and I1, and I2, are their ionization potentials. Such forces have the name of dispersion forces.
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.4)
The results of some calculations from the above equations are shown in Figures 1.3-1.5, where the molecules are in contact at the lowest points of the curves, i.e. r = r0. Figure 1.3 is for a pair of water molecules at 298 K; the dipole moment of water is 1.85 D(1Debye = 3.336 x 10-30 Cm) and 4πε0 = 1.1126 x 10-10 J-1C2m-1. The radius of a water molecule, calculated from its molar mass and density is 0.19 nm. When two water molecules are in contact, Epp is – 1.12 kJ mol-1.
Figure 1.4 is for the interaction of water with methane (α = 2.60 x 10-30m3). The radius of the methane molecule is about 0.24 nm, and when the two molecules are in contact Epi is — 85 J mol-1 which is very much less than for a pair of water molecules.
The first ionization potential of methane is 1133 kJ mol-1. When in contact, Eii is 909 J mol-1 (Figure 1.5).
The potential energies of all these interactions are inversely proportional to the 6th power of the distance of separation, meaning that the values of – Epp, – Epi and – Eii fall off rapidly with distance. Doubling the distance reduces values of – Epp, – Epi and – Eii to 1/64th. Figures 1.3–1.5 show that the forces are only effective at less than two diameters, which means that adhesion forces will only be felt by the molecules that are actually in the topmost surface layers.
The measurement of contact angles, which is described in Chapter 8, is a means of investigating adhesion by physical adsorption. These are the weakest forces that contribute to adhesive bonds, but are quite sufficient to make strong joints.
Chemical Bonding Theory
The chemical bonding theory of adhesion invokes the formation of covalent, ionic or hydrogen bonds across the interface. There is some evidence that covalent bonds are formed with silane coupling agents (see Chapter 3), and it is possible that adhesives containing isocyanate groups react with active hydrogen atoms, such as hydroxyl groups, if wood or paper are the substrates. In these two examples, Si–0 bonds (strength 369 kJ mol-1) and C–0 bonds (351 kJ mol-1 would be formed. Another possibility is the reaction of an epoxide adhesive with a surface containing amine groups (see Chapter 4) to give C–N bonds (291 kJ mol-1).
The potential energy of two ions of charge z1e and z2e, separated by distance r is given by equation 1.5.
E+- = z1z2e2/ 4πε0εrr (1.5)
Here εr, is the relative permittivity of the medium, which is 1 in the case of a vacuum or dry air. Taking the following values of ionic radii (Na+ = 0.95, A13+ = 0.50, Ti4+ = 0.68, O2- = 1.40 and C1- = 1.81nm) strengths of ionic attractions are NaCl 503, A13+ O2- 4290 and Ti4+ O2- 5340 kJ mol-1. The two last energies are very high and may contribute to adhesion between metals and epoxide adhesives, and can also account for the significant contribution that carboxylic acid groups in adhesives make to metal-adhesion.
A major problem with all adhesive joints is their sensitivity to water, and possible explanations for this are its high permittivity, which by equation 1.5 would give a low value of E+-, and a high surface tension. These issues are considered, respectively, in Chapters 10 and 8.
Hydrogen bonds probably contribute to the attachment of postage stamps to envelopes where the adhesive (polyvinyl alcohol) and paper (cellulose fibres) both contain –OH groups. Wood is also rich in cellulose and the reactive adhesives based on formaldehyde contain hydroxyl or amine groups capable of participating in hydrogen bonds. The strengths of hydrogen bonds are mostly in the range 8-42 kJ mol-1, with those in water being at the top of this range. Hydrogen bonds involving fluorine can be stronger than this, and the strongest of all is F- ··· H-F (243[+ or -]21 kJ mol-1).
The strengths of Lewis acids and bases in poorly solvating solvents (usually hexane, cyclohexane or tetrachloromethane) can be obtained from their heats of reaction (–ΔH), which are related to EA and CA, which are empirical parameters for the acid, and EB and CB, the corresponding values for the base, by equation 1.6.
-ΔH = EAEB + CACB (1.6)
EA and EB, are considered to be the susceptibilities of the acid and base to undergo electrostatic interactions, and CA and CB, are their susceptibilities to form covalent bonds. The heats of reaction can be measured by direct calorimetry or from shifts in IR spectra. An example of the latter is the shift in the OH stretched frequency of phenols (Δv) when they react with amines in tetrachloromethane or tetrachloroethene, which is given by equation 1.7.
- ΔH(kca1 mol-1) = 0.0103Δv(cm-1) +3.08 (1.7)
Some values are given in Table 1.1. They are based on a large number of measurements of –ΔH, with iodine EA = 1.00 and CA = 1.00 as the reference compound, in the old units of (kcal mol-1)1/2.
Diffusion Theory
The difusion theory takes the view that polymers in contact may interdiffuse, so that the initial boundary is eventually removed (see Figure 1.6). Such interdiffusion will occur only if the polymer chains are mobile (i.e. the temperature must be above the glass transition temperatures) and compatible. As most polymers, including those with very similar chemical structures such as polyethylene and polypropylene are incompatible, the theory is generally only applicable in bonding like rubbery polymers, as might occur when surfaces coated with contact adhesives are pressed together, and in the solvent-welding of thermoplastics. An example of the latter is to swell two polystyrene surfaces with butanone and then press them together. The solvent has the effect of lowering the glass transition temperature below ambient while interdiffusion takes place; it later evaporates. This is the mechanism of adhesion in making plastic model kits. The kits are made of polystyrene and the adhesive is a solution of polystyrene in an organic solvent, the main purpose of the polymer being to thicken the adhesive. There are a small number of polymer pairs made compatible by specific interactions. One pair is poly(methy1 methacrylate) and poly(viny1 chloride), which permits the possibility of interdiffusion when structural acrylic adhesives are used to bond PVC.
Electrostatic Theory
The electrostatic theory originated in the proposal that if two metals are placed in contact, electrons will be transferred from one to the other so forming an electrical double layer, which gives a force of attraction. As polymers are insulators, it seems difficult to apply this theory to adhesives.
Mechanical Interlocking
If a substrate has an irregular surface, then the adhesive may enter the irregularities prior to hardening. This simple idea gives the mechanical interlocking theory, which contributes to adhesive bonds with porous materials such as wood and textiles. An example is the use of iron-on patches for clothing. The patches contain a hot melt adhesive that, when molten, invades the textile material.
Weak Boundary Layer
The weak boundary layer theory proposes that clean surfaces can give strong bonds to adhesives, but some contaminants such as rust and oils or greases give a layer which is cohesively weak. Not all contaminants will form weak boundary layers, as in some circumstances they will be dissolved by the adhesive. This is an area where acrylic structural adhesives are superior to epoxides because of their ability to dissolve oils and greases.
POLYMERIZATION
Polymerization is a very important matter in adhesion, in that adhesives which harden by chemical reaction do so either by addition or condensation polymerization, and the polymers, which are a vital and frequently major component of other adhesives, are synthesized by the same processes.
Condensation Polymerization
In condensation polymerization, molecules react with one another because they contain chemical groups which are mutually reactive. There are at least two such groups per molecule, and if some trifunctional molecules are present then branching or crosslinking will result. Example of some functional groups used in adhesives are shown in Scheme 1.1.
In some, but not all, cases a small molecule such as water is formed. In the case of phenolic adhesives, which cure by the last reaction in Scheme 1.1, at a temperature above l00 °C, pressure must be applied to prevent the unwanted formation of steam-filled voids.
(Continues...)
Excerpted from Adhesion Science by John Comyn. Copyright © 1997 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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