Friday, November 25, 2011

Properties of Polyvinyl Acetal

The adhesive strength of polyvinyl acetal/phenolic adhesives cannot properly be considered without having regard to component ratio. It is, for example, commonly believed that increasing the acetal ratio (beyond some arbitrary limit) decreases tensile shear strength at elevated temperatures. To a certain extent this is true, but the presence of residual solvents, the inherent properties of the phenolic resin together with completeness of its cure, are other parameters that must be taken into consideration.

In bonding metal the tensile shear strength at room temperature on aluminum alloy, magnesium alloys, and steel including stainless steel, is among the highest that can be obtained. Heat resistance is not particularly high, but experience over many years has proved to be adequate in the bonding of brake lining, an application in which it probably improves with image. The polyvinyl acetal/P.F adhesive systems are excellent for wood and thermosetting plastics except the silicons.

In bonding glass, the polyvinyl/phenolic system have been found to give the highest strength of any adhesive, presumably if regard is paid to the P.F. ratio. But for reasons of colors such as adhesive could not be used to replace polyvinyl butyral alone in the production of laminated glass.

Fatigue strength necessarily depends to some extent on polymer ratio but in all cases it is extremely high and practically speaking, much higher than that of the metal used for the test joint. It has been shown that the fatigue life of an acetal/phenolic adhesive is greater at -57oC than at room temperature. Peel strength, however, is lower at a lower temperature, and therefore, contrary to what might be expected, fatigue strength is not directly related to peel strength.