Degradation by Chemical Exposure

The mechanism responsible for the hardening of hydrogenated nitrile rubber automotive seal compounds on prolonged exposure to automatic transmission fluids at high temperature was investigated. The increase in hardness was found to result primarily from chemical attack at the acrylonitrile sites. This chemical attack first resulted in the formation of isocyanide or isonitrile ions. The proposed mechanism involved crosslinking at the acrylonitrile sites through the formation of isonitrile ions and the subsequent formation of imides. 

The isonitrile intermediate formed ionic bonds resulting in the formation of an ionomer once the concentration became sufficiently high. Ultimately these ions crosslinked to form imides in the presence of oxygen or water. It was proposed that the increase in hardness was due to the formation of the ionomer and the imides (154). 

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