The effect of crosslinking type on the physical properties of carboxylated acrylonitrile butadiene elastomers

Vulcanization of carboxylated nitrile rubber (XNBR) with different vulcanizing 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 crosslinking density. Ionic crosslinks gave rise to greater stress relaxation, relaxation rates, and a greater generation of heat. In the relaxation spectra, tan δ versus temperature, two transitions appeared. Those occurring at the lower temperature corresponded to the polymer Tg, while the transition occurring at the higher temperature was associated with ionic structures. The properties of the vulcanized compounds with ionic crosslinks decreased drastically after treatment with ammonia, which acts as a plasticizer of the ionic aggregates formed. The effect of ammonia disappeared on expansion in solvent, which resulted in the recovery of the original crosslinks, producing a value of vr—volume fraction of swollen rubber in equilibrium—close to the original value and the reappearance of the ionic transition.