Polyvinyl Acetal/phenolic Application

Where adhesive is used in some highly important application such as in an aeroplane, durability is a subject of more than usual importance. Test made in Nigeria on aluminum alloy joints glued with an acetal/phenolic adhesive (not openly exposed to weather but exposed to the tropical atmosphere), have shown no drop in strength after nearly 5 years. And in America tests made over a wide range of climates have given generally satisfactory results, but with one or two anomalies, which is not uncommon experience in such testing.

The chemical resistance of the acetal/phenolic adhesive depends on the properties of the acetal and the amount incorporated. The phenolic resin, in comparison, may be considered completely resistant to chemical attack, so the ratio of one component to the other has an influence. Although chemical resistance may be inadequate for some application, it is adequate for aircraft structures providing regard is paid to the solvents used for decreasing and removing point, and complete resistance to most activation fuels tanks. Water resistance, including salt water resistance is good to excellent, but is affected by molecular weight more in the case of the formal than the butyral. It is claimed, however, that the water resistance of polyvinyl butylral, either by itself or as a component in a two polymer adhesive, can be substantially improved by hardening it with ammonium bichromate activated by a copper salt.

As stated in the thermoplastic adhesives, a polyvinyl acetal can by prolonged heating be cross linked through a condensation reaction and made infusible; and this effect can be utilized in a two component system to increase hot strength. Even when the phenolic resin is fully cured, an increase in hot strength can be obtained by further heating because of the acetal becoming progressively infusible. An increase of 30^oC in the normal curing temperature produces a marked increase in hot strength. Unfortunately, peel strength is sacrificed as flexibility is reduced. Neglecting any effect that can be produced by prolonged curing, hot strength is adequate up to about 80 ^oC for normally stressed joints and up to 100 ^oC for lightly stressed joints.