Curing with Polycarboxylic Acids

The reaction of an epoxy group with a carboxyl group can be described by the following simplified scheme 

For catalysis by bases, addition esterification is the first and the fastes reaction. It can be followed by slower polyetherification reaction, if epoxy groups are in excess, or by condensation esterification (even slower), if carboxyl groups are in excess. This mechanism was verified on a model system caproic acid — phenylglycidyl ether 

1 The polyetherification of epoxide groups catalyzed by tertiary amines is characterized by formation of polymers of low degree of polymerization, a part of which is nitrogen-free (cf. Berger and Lohse, Section 4.5). 

transesterification causes branching as well as chain scission Let us consider the branching statistics for the case of a polycarboxylic acid and polyepoxide with groups of independent reactivity. The use of the statistical method is justified because neither the substitution effect nor the initiated chain growth are operative. If only addition esterification occurs (and indeed this reaction can be selectively accelerated by special catalysts the statistics is analogous to the polyamine-polyepoxide addition and even simpler because the substitution effect is here absent. The pgf s for the number of bonds issuing from polyacid (C) and polyepoxide (E) units read  

In the literature, two additional reactions following addition esterification have been treated using the cascade theory the addition esterification followed by polyetherification with epoxide groups in excess (a reaction used for crosslinking of carboxyl terminated polydienes) and addition esterification followed by transesterification. Transesterification often interferes wherever hydroxyester groups are formed, for example, in synthesis of linear oligomeric polyesters from diepoxide and acids. As has been explained before, polyetherification is an initiated reaction and, therefore, the statistical treatment offerend in Refs. should be revised. Below we show the treatment of transesterification for a system composed of a diepoxide and a dicar boxylic acid. 

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Curing of epoxy resins by polycarboxylic acids and cyclic anhydrides is also important in applications, but it is much less understood due to more complex reaction mechanism. Also, the statistical treatment is less developed and partly requires a revision. In this section, the statistics of curing of epoxy resins with polycarboxylic acids and cyclic anhydrides is discussed. 

Permanent-press fabrics are often cured with TV-methy-lol compounds, such as 4.18, and tend to release formaldehyde after the cure. (Formaldehyde [a carcinogen] from various formaldehyde resins is a problem in some homes.115) These can be replaced with polycarboxylic acids, such as the 1,2,3,4-butanetetracarboxylic acid (4.19), which cure via the cyclic anhydride.116 Both dyeing and durable-press treatments can be combined in a single step, if desired.117... 

Acceleration of epoxy cures by carboxylic acids is frequently accomplished by addition of a base catalyst. Cure of epoxy resins by polymeric fatty acids (molecular weight 500-900) has been accelerated by the formation of fatty acid/melamine soaps (53). DGEBA-resin-based adhesives have also been cured with piperazine/polycarboxylic acid (e.g., succinic acid) salts (54). These adhesives showed rapid heat cures (23 min at 120°C) and good room temperature latency (shelf life of 60-65 days). The rate of cure in these adhesives was faster than comparable adhesives having dicy... 

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