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Friday, November 14, 2008

Versamid Complex Polymer

The Versamid are complex polymeric substances having relatively high melocular weights in properties to their amine content. From this it follows that the amount required for curing is large and the proportion not very critical. This is confirmed by Floyd, Peerman and Wirrcoff in reporting on versamid/epoxy resin system, including their reaction mechanism. Niermann and Gunther considered that the best high temperature adhesion is given by stoichiometrc proportion combined with curing at 100 – 150 oC, a fact that should not be taken as inconsistent with the general belief and experience that within sensible limits the ratio of Versamid to epoxy is not at all critical. According to the epoxy value of the resin, the stoichiometric proportion may vary from less than one half the weight of epoxy resin to more than an equal weight.

Versamids, having a long carbon chain between functional groups, increase the flexiblity of an epoxy adhesive and therefore increase peel strength. A remarkable flexible adhesive can be made by substantially increasing the amount of Versamid above that needed to produce satisfactory curing, but in this case chemical resistance is reduced. Although flexibilishing an adhesive strength (depending to some extent on the type of test), the hot strength is reduced the cohesive strength of the glue suffers.

Although the Versamid are capable of curing bisphenol A resins at room temperature the degree of crosslinking is not high and adhesive strength is increased by curing at a higher temperature, for example at 60oC for an hour; alternatively, the strength of glued joint that have been cured initially at room temperature can be increased by post curing. The rate of curing can be accelerated by adding one of the more reactive amines such as triethylenetetramine, or tertiary amines, or some organic acids. N,N-dimethyl-1,3-propylene diamine is a particularly useful accelerator. Phenols, and certain compounds made by the Mannich reaction also accelerate Versamid curing, but one common example, namely tris-(dimethylaminomethyl) phenol, is reported to reduce adhesive strength by 20-30 %.