Carboxylic Elastomers

Carboxyl groups can be introduced into elastomers by adding olefinic acids to suitable emulsion systems prior to polymerization. In general, such carboxyl groups elevate the elasticity temperature range, impart superior filming properties, increase resistance to swelling by hydrocarbon solvents, and render the polymers susceptible to cross-linkage, gelation, and vulcanization by polyvalent reagents. In vulcanization recipes based on sulfur but containing polyvalent metal salts, the presence of as little as 1 per cent of a carboxylic monomer is recognizable through its influence on vulcanizate properties. Carboxylic elastomers can be cross-linked by reactions of the carboxyl groups. Salt formation with zinc oxide, without pigmentation, gives stocks having stress-strain properties equaling or surpassing those of black-pigmented sulfur-cured vulcanizates of analogous noncarboxylic polymers. Several divalent metal oxides and other salts can be used as vulcanizing agents. The metal oxide vulcanizates possess excellent stress-strain properties, but poor compression sets. Certain vulcanizates are improved by the inclusion of small amounts of organic acids. An excess of organic acid causes devulcanization. Carbon blacks function principally as loading rather than reinforcing pigments.