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.