Crosslinking Reactions of Carboxylic Elastomers

Carboxyl groups in elastomer molecules have been shown to contribute to the crosslinking or joining of these molecules through hydrogen bonding, salt formation, reaction with diepoxides or polyepoxides including epoxy resins, reaction with diamines or polyamines, esterification with di- or poly-hydric alcohols, reaction with carbodiimides, reaction with polyimines, and by reaction with polyisocyanates. Each of these reactions alone may produce useful crosslinkages. Or, they may be used in conjunction with crosslinking reactions operating in other parts of the molecule such as sulfur or peroxide vulcanization. This has been particularly true of salt formation, especially through the use of zinc oxide, with sulfur vulcanizarions. The carboxyl groups in some instances may be generated during the crosslinking reaction. Thus polyethylacrylate may be crosslinked with hexamethylene diamine, barium hydroxide and similar reagents. Many of the reactions of the carboxyl groups in elastomers are quite rapid. For some purposes such as surface toughening of elastomer films, this may be quite useful. For others such as the vulcanization of carboxylic elastomers with zinc oxide or with recipes containing zinc oxide, this may be objectionable because of the scorchiness of the stocks. The scorchiness of vulcanizates of carboxylic elastomers involving zinc oxide may be controlled with organic acid additives or by the use of zinc oxide coated with less reactive materials such as zinc sulfide or zinc phosphate. The speed of reaction of epoxides with carboxyl containing polymers is influenced by the structure of the epoxide. The ease with which carbodiimides induce crosslinkage is dependent upon the structure of the carbodiimide.