Polyvinyl Alcohol (PVA)

Polyvinyl Alcohol Adhesive

Many application is used of Polyvinyl Alcohol (PVA) as based material such as for glue or adhesive and for paint such as polyvinyl acetal adhesive. PVA more prefer use as glue than PVAc (Polyvinyl Acetate) because this adhesive relative have good smell than PVAc. The polymer of vinyl alcohol is made commercially by acid hydrolysis of polyvinyl acetate; the monomer has not been isolated. According to the degree of polymerisation and degree of hydrolysis, a wide range of polymers with molecular weight up to 200,000 may be prepared, having different solubility’s in water. With 70% acetate groups “polyvinyl alcohol” is insoluble in water, whereas with less than about 3 % it is soluble in hot water but may only swell in cold water, with about 12% acetate groups the polymer is completely soluble in both hot and cold water. The degree of hydrolysis is normally between 86% to 99%.

The limited resistance to water understandably restricts the usefulness of polyvinyl content are resistance to nearly all organic liquid, the polymers with a low acetate content are resistant to nearly all organic, liquids and consequently have an important place as adhesives in contact with greases and oils. It is possible to crosslink polyvinyl alcohol by heating with a small amount of a dicarboxylic acid and so increase its water resistance, but there appears to be no information available with respect to adhesive applications.

Polyvinyl Adhesive Use

Polyvinyl alcohol is probably the most widely used water soluble synthetic resin and its chief uses are in gluing porous materials such as kather cloth and paper. Being non-toxic and having no odour, polyvinyl alcohol is another polymer that is specially useful as an adhesive in contact with food.

Polyvinyl alcohol can be used as a re-moistenable adhesive because, although at high relative humid it absorbs water, the adhesive film does not become tacky unless it is wetted properly.

Unplasticized film which has been heated at 100oC for an hour will heat seal at 160 - 200 oC. It would seem, however, that is not very different from hot melt gluing. Glycerol and ethylene glycol are suitable plasticizers. Small amounts of polyvinyl alcohol are often incorporated in other aqueous adhesive composition in order to improve film forming properties or to increase adhesion.

Hot Melt Adhesive (Polyvinyl Acetals)

Polyvinyl acetals which appear to have been first used as adhesives in the bonding of mica, gives high strengths as hot melt adhesives, and used in this way can be considered as marginal structural adhesives especially in gluing wood and metal.

The most important hot-melt application of polyvinyl butyral is in making laminated safety glass, an application in which its use is almost universal. The process consists simply of heating the assembly of glass laminae and butyral film and then cooling while maintaining pressure. A suitable type of butyral adhesive contain about 35 % triethylene di-2-ethyl butyrate as plastizer, the butyral being of high molecular weight and having approximately 70-80% acetalised hydroxyl groups and 18-19% free hydroxyl groups. The grade that is frequently used is the same as a metal-to-metal adhesive, and a typical specification, that of Butvar B-72A is included in the table. Here it is interesting to note that when all hydroxyl groups have been replaced the polymer shown no tendency to stick to glass. In this important application, polyvinyl butyral is chosen not only for its high adhesion to glass but for its transparency and stability to sunlight, and furthermore because it has the necessary resilience to hold the glass fragments together after impact.

Polyvinyl Acetal Adhesives

Polyvinyl acetal is the generic name of a group of polymers that are the product of reaction between polyvinyl alcohol and an aldehyde. In the preparation of these acetals polyvinyl acetate is used which in the course of the reaction is partially hydrolysed to the alcohol, thus the chain length of the acetate determine the chain length of the acetal. The monomer from of the vinyl acetals cannot be isolated.