Effects of various acid acceptors upon stress relaxation and set in epichlorohydrin elastomers

Epichlorohydrin (ECO) elastomers are used as seals and gaskets. Therefore, it is desirable to minimise compression stress relaxation and compression set. These properties are strongly influenced by the extent and nature of crosslinks. Compounding ingredients that interfere with crosslinking will profoundly affect stress relaxation and set. Acid acceptors are essential for ECO elastomers because they impart heat stability and neutralise hydrochloric acid. The present paper describes and discusses the way in which various acid acceptors affect compression stress relaxation and set in ECO elastomers. Lead based and non-lead based acid acceptors were studied. The lead based acid acceptors were litharge, red lead, and dibasic lead phosphite-dibasic lead phthalate. The non-lead based types comprised zinc oxide, magnesium oxide, and calcium oxide. Results appear to indicate that at similar crosslink concentrations, ECO vulcanisates containing lead based acid acceptors give lower rates of compression stress relaxation and lower set at ambient temperatures than do the non-lead based acid acceptors. This feature may be due to the variable stiffening action of the various metal chlorides that are produced during vulcanisation. As regards high temperature compression set, the lead based acid acceptors gave lower set values than did the non-lead based counterparts. The high temperature compression set of ECO containing zinc oxide was extremely high, probably because oxide becomes a strong Lewis acid and thus rapidly promotes elastomer breakdown.