Wednesday, April 28, 2010

Polyvinyl Acetate Adhesives

The vinyl group comprises the reactive part of a large number of compounds that produce thermoplastic polymers, most of which are extremely useful as adhesives. They include polyvinyl esters, ethers and acetals, polyvinyl alcohol and polystyrene.

Polyvinyl Acetate
The most important polyvinyl ester adhesive is polyvinyl acetate (PVA). As an adhesive, it is used in solution, in aqueous dispersion, and by hot melt technique, in one form or another it is probably the most widely used thermoplastic adhesive.

Vinyl acetate is a flammable liquid made commercially from acetylene and acetic acid in the presence of a catalyst. As the molecular weight increases the polymer ranges from a soft to a hard solid, soluble mainly in polar organic solvents; adhesive solution are commonly made up in ketones, lower alcohols and esters. As the molecular weight reaches the order of 100,000 the polymer becomes soluble in toluene.

By the most important form in which the adhesive is used is the aqueous dispersion, made by the important industrial process known as emulsion polymerisation, a process in which the liquid monomer is first emulsified in water and then polymerised by heating in the presence of a catalyst. Although loosely referred to as an emulsion, the product is not a liquid/liquid system, but mainly an aqueous dispersion of swollen solid particles. The advantage of an aqueous dispersion is not only that it uses water as the conveying medium, but also that a higher solids/viscosity ratio is possible than with a solution. Polyvinyl acetate solutions increase in viscosity as the molecular weight material is desirable because of its film strength, a solution that is fluid enough to permit easy application must necessarily be of low concentration. Emulsion polymerisation overcomes this disadvantage because the molecular weight of the dispersed polymer has little effect on the viscosity can be easily produced. The particle size of the polymer should, however, cover as wide as a range as possible.

The well-known PVA emulsion glue, with a polymer content of about 50% is a versatile industrial adhesive. After application to an adherent water is eliminated from the emulsion through evaporation and absorption by absorbent adherent, and this result in the swollen particles coalescing to form a coherent adhesive film. For satisfactory coalescence and film formation, a certain amount of plasticiser must be incorporated, of which dibutyl phthalate and butyl carbitol acetate are an example. The plasticizer is largely responsible for the main shortcoming of polyvinyl acetate emulsion adhesives, the creep under stress which takes place especially at raised temperatures. Recognising the defect, some specifications include a creep test. If the testing of a glued joint the strength obtained is dependent on the rate of loading, especially if the glue has low resistance to creep.

Creep can be reduced by adding a urea formaldehyde to a PVA emulsion; such a product is dealt with. Certain copolymers of polyvinyl acetate such as an acrylate copolymer can be emulsified and their use generally result in a more versatile adhesive with lower creep. Many PVA emulsion glues are nowadays based on copolymers, at least in part. Creep can also be reduced by adding a vinyl methyl ether/maleic an hydride copolymer.

The thickening agents such as starches, starch-ethers, methyl cellulose or sodium carboxymethyl cellulose and also anti foaming compounds are frequently added to the emulsions. The incorporation of preservatives and other additives has been comprehensively dealt with. Polyvinyl alcohol is sometimes added; it is an effective stabiliser. Although room temperatures stability is normally good, an emulsion is likely to break if the temperature falls to the freezing point of the aqueous phase. The addition of 13-17% of high molecular weight ethylene oxide polymer (a water-soluble thermoplastic resin that has not found much use as an adhesive) enables a re-wettable (solvent activated) adhesive to be formulated, presumably a rather low molecular weight polyvinyl alcohol would do the same.

Certain polyvinyl acetate emulsion adhesive can be spray dried and marketed as dry powder that can be re-dispersed by simply mixing with water. Polyvinyl acetate is not only a good adhesive for a large number of porous or fibrous materials such as paper, wood and leather, but also far many nonporous materials especially transparent plastic films such as cellophane, cellulose esters, vinyl and polyethylene terephthalate. A rather different adhesive use is in making bonded fiber fabric.

An interesting application is the use of a PVA emulsion to increase the adhesion between old and new concrete. There are two ways in which this may be done, either by coating to old concrete before applying the new cement mix, or mixing a proportion of the PVA emulsion with the new mix.

Polyvinyl acetate emulsion are reasonably flexible; nevertheless, added flexibility for applications such as bookbinding can be obtained by incorporating compatible rubber latices.

These glues are non-toxic, clean and easy to use and rapidly assume a useful degree of strength especially with highly absorbent adherents. Their heat resistance is satisfactory up to 50o – 60oC. Gap filling is not usually good but the addition of thickening agents improves it. Tensile shear strength (at the normal rate of loading used in testing glued joints) has been shown with a variety of timbers to be comparable with urea and phenolic glues, while giving for reason that are not clear – consistently less wood failure.