The effect of crosslinking type on the physical properties of carboxylatedacrylonitrile butadiene elastomers

Vulcanization of carboxylated nitrile rubber (XNBR) with different vulcaniz ing agents (i.e., zinc peroxide, sulphur, and a zinc peroxide-sulphur mixed system) was studied. Properties of the vulcanized compounds depend on the type of crosslinking produced (i.e., ionic or covalent) rather than the cro sslinking density. Ionic crosslinks gave rise to greater stress relaxation, relaxation rates, and a greater generation of heat. In the relaxation spec tra, tan delta versus temperature, two transitions appeared. Those occurrin g at the lower temperature corresponded to the polymer T-g, while the trans ition occurring at the higher temperature was associated with ionic structu res. The properties of the vulcanized compounds with ionic crosslinks decre ased drastically after treatment with ammonia, which acts as a plasticizer of the ionic aggregates formed. The effect of ammonia disappeared on expans ion in solvent, which resulted in the recovery of the original crosslinks, producing a value of u(r)-volume fraction of swollen rubber in equilibrium- close to the original value and the reappearance of the ionic transition. ( C) 1999 John Wiley & Sons, Inc.