30 августа, 2015 
Malyar The essential property of any polymer used as an adhesive is that it should have good cohesion and stick to the surfaces it joins. This can occur only if the polymer forms a film on application or at some stage during the joining process. In the case of a water suspension such as a PVA or an EVA adhesive, this involves a smooth coalescence of the polymer particles as the water evaporates. For this to occur, the polymer must be above its minimum filming temperature (MFT). Some applications lend themselves to the application of heat. However, most rely only on the evaporation of water at ambient temperature, coupled with absorbence (or ‘‘wicking’’) into the substrate, if porous.
Poly(vinyl acetate) has a glass transition temperature (GTT) of about 30°C. The GTT marks the change in properties from a material with a glasslike nature to one with rubbery properties. In particular, particles above their GTT may deform, flow, and adhere. This results in the GTT being the greatest factor in determining the minimum temperature for the formation of a coherent film. The GTT of any polymer is dependent on its structure. As the temperature rises, polymer chains vibrate under the influence of the stretching and bending motion of individual bonds. At the GTT the steric and covalent locking of the chain is overcome, allowing the molecule as a whole to bend and rotate, subject to the special constraints of neighboring polymer chains. It is clear that any internally plasticizing copolymerized monomer or external solvent or plasticizer that assists this process by making the main chain of the polymer more flexible, or eases its rotation by spacing adjacent chains of the polymer, reduces the temperature at which film formation can take place. Ethylene acts directly as a polymer backbone plasticizer. A polymer such as poly(vinyl acetate) homopolymer:
— CH2— CH — CH2— CH — CH2— CH — CH2— etc.
OAc OAc OAc
becomes the much more flexible copolymer with ethylene:
— CH2— CH — CH2— CH2— CH2— CH— CH2— etc.
OAc OAc
In the latter structure the acetate groups are too widely spaced to influence rotation.
PVA adhesives are plasticized externally by smaller molecules that space the chains and minimize chain-to-chain interactions. The classic material for this purpose is dibutyl phthalate (DBP), but any compatible solvent may be chosen. Even water itself acts as a temporary coalescing solvent to some degree. Volatile materials gradually evaporate, allowing the PVA adhesive to harden and gain tensile strength. This is particularly important if strength at high ambient temperatures is required. Conversely, materials less volatile than DBP may be chosen to ensure that a degree of flexibility is retained in the joint, even after many years at high temperatures or repeated rinsing with water. For ultimate stability over time, copolymerization with ethylene must be chosen.
Lowering of the minimum filming temperature and increasing flexibility is not the only reason for introducing other polymerizable monomers into the preparation of the adhesive. A large variety of materials are available, having in common a double bond that is either activated or may become so on the approach of a free radical. Such monomers may modify the behavior of the final polymer, conferring resistance against alkalis, or to improve adhesion in wet conditions. Chain branching to increase molecular weight may be introduced. Stability of the polymer against freezing or mechanical shear are other desirable properties that may be gained by copolymerization.
Because of the simple addition of monomer to growing chains, and because the reaction takes place in the bulk of the polymer or on the surface of polymer particles rather than in solution, molecular weight is very high. Chain growth can cease only when two free-radical-terminated chains collide, an initiator fragment adds to and eliminates a growing chain, or a chain transfer reaction takes place with a small, volatile molecule. In practice, the chances of two growing chains mutually eliminating each other by addition are very low because of their high molecular weight and consequential lack of mobility. One recent theory relies on chain transfer to monomer and subsequent desorption of the small free radical as the only significant mechanism of chain termination. Molecular weight can be increased by the addition of small amounts of monomer with more than one double bond. These are known as chain branching agents. Conversely, molecular weight is decreased by the addition of a material containing a reactive hydrogen such as и-dodecyl mercaptan (1-dodecane thiol) as a chain transfer agent. The hydrogen is readily removed from the thiol (—SH) group, giving a terminated chain on the polymer but also a —S • free radical capable of starting a new chain. Formulation factors such as the level of the free-radical initiator used and process factors such as temperature of reaction and the amount of agitation also play a part. Low levels of initiator, low temperatures, and a carefully judged degree of agitation all favor high molecular weight.
Turning to practical considerations, choice of process and formulation is all important in successful production of PVA and EVA adhesives. Some formulation examples are given in a later section, but various aspects are discussed here.
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