Sunday, June 27, 2010

Polyvinyl Alcohol (PVA)

Many application is used of Polyvinyl Alcohol (PVA) as based material such as for glue or adhesive and for paint. 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 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 humidities 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.