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Wednesday, November 2, 2011

Polyvinyl Acetal/Phenolic Resin Adhesives

The term phenolic resin means a resin derived from formaldehyde and phenol itself. The most important polyvinyl acetals are the butyral and formal. The name polyvinyl acetal is more specifically applied to the product of reaction with acetaldehyde, but this acetal is not important as a component in two polymer adhesives.

Adhesives based on a mixture of a phenolic resin and polyvinyl formal or butyral were develop during the Second World War, the formal system largely in England as the method of bonding and the adhesive known as Redux, and the butyral system largely in America under three or four proprietary names.

Example of the grade of polyvinyl butyral and polyvinyl formal generally used. The high molecular weight grade of the butyral can only be used if its high solution viscosity is acceptable. Both polyvinyl butyral and formal are soluble in ordinary phenolic resins (preferably not containing a large amount of caustic alkali) especially in the presence of a lower alcohol. Compatibility is preserved during and after curing of the phenolic resin, that is to say the acetal is not precipitated (desirable properties would be adversely affected it were), and the cured adhesive is a homogeneous film.

Polyvinyl butyral is more soluble in a phenolic resin than the formal. The formal is in fact difficult, but can more readily dissolved if the resin is diluted with ethanol or acetone - neither or which are solvents for the grade of polyvinyl formal used in these adhesive. Solution is facilitated by using the formal in the form of precipitated porous particles.

The use of homogeneous solution of acetal and phenolic resin, although sometimes simplifying application, is by no means essential. Indeed the acetal need not be deliberated at all but may, in the case of the formal, be applied separately as is done in the Reduce process. In this process particles of polyvinyl formal are "picked up" by the sticky phenolic resin which has previously been applied to the adherent - a technique that while lacking precise control over ratio of components has simplicity to recommend it.