Vulcanization of carboxylated nitrile rubber (XNBR) by zinc peroxide

The vulcanization of carboxylated nitrile rubber (XNBR) with zinc peroxide, which produces ionic crosslinks, has been studied in relation to vulcaniza tion time. Vulcanized compounds present two transitions, corresponding to t he glass transition of the polymer at low temperatures and the ionic transi tion resulting from the formation of ionic aggregates. Both transitions are displaced to higher temperatures with increasing crosslink density. The io nic associations which give rise to high values of mechanical properties di sappear on exposure of the vulcanized compounds to saturated ammonia vapour . This treatment produces a decreased crosslink density resulting in the di sappearance of the ionic transition. When the action of ammonia is terminat ed by immersion in solvent followed by drying, the original crosslink densi ty is recovered and the ionic transition reappears, although at higher temp eratures. However, with increasing crosslink density, the difference betwee n the temperatures at which both transitions take place diminishes. All the se factors can be interpreted as reflecting the generation of a new and mor e compatible arrangement of the newly-appearing ionic clusters. (C) 1999 So ciety of Chemical Industry.