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Saturday, August 9, 2008

Aliphatic Polyamine Hardeners

The importance of the group of aliphatic polyamine hardeners with primary amine groups lies in the fact these are the hardeners used in adhesive systems capable of curing at normal or slightly elevated temperatures. The most important ones are the homologous alkylene polyamines, especially diethylenetriamine and triethylenetetramine.

The chemical mechanism involved in curing bisphenol A resin with amines (and other compounds) has been discussed by Narracott and by bruin, and been reviewed by Smith. It can be said that, at least in part, a primary amine on reacting with an epoxide becomes progressively a secondary amine and a tertiary amine; since, however, a tertiary amine is known to catalyse polymerization of epoxide groups themselves, it would appear that the curing reaction need not involve crosslinking via molecule of hardener entirely.

Indeed for amines containing both a primary and tertiary amine group, for instance the N,N-dialkyl-1,3-propylene diamines (dialkylaminopropylamines), a curing mechanism involving polymerization of some the epoxide groups may be deliberately invoked by incorporating such a quantity of amine that there are insufficient amino hydrogen to react with all the epoxide groups, curing being completed, usually at elevated temperature, by catalytic polymerization of the remaining epoxide groups. This same mechanism can operate if instead of an amine containing both primary and tertiary groups, a mixture of two amines, one containing only tertiary and the other containing only primary or primary and secondary groups, is used. Again fewer amino hydrogens than are required to react with all the epoxy groups are introduced.

In the alkaline series of polyamines, as the length of the molecule increases the hardener becomes less reactive, thus diethylenetriamine is faster curing that triethylenetetramine, and ethylenediamine is faster still, in fact too fast for most work, and not convenient to use anyway. At the other end of the series, tetraethylenepentamine needs a slightly raised temperature in order to cure satisfactorily. Certain hydroxyic compounds may have an accelerating cure satisfactorily. Certain hydroxylic compounds may have an accelerating effect on curing, for example alcohol and water, but clearly the latter is undesirable, and the former may be.