Effect of ZnO particle size on the curing of carboxylated NBR and carboxylated SBR

A carboxylated nitrile rubber (XNBR) and a carboxylated SBR (XSBR) were mixed with zinc oxide particles of different specific surface areas (S, 35 m(2)/g; M, 3.5 m(2)/g; L, 0.5 m(2)/g) and cure behavior at 165 degreesC studied using oscillating disc rheometry. Without added zinc oxide, both raw rubbers slowly stiffen over many hours of heating. This is probably due to condensation of carboxyl groups to form anhydride crosslinks. XNBR compositions containing the finely divided S crosslink much more rapidly. Full cure is reached after about 10 minutes of heating. Cure rate decreases markedly as the specific surface area of the ZnO decreases. A composition containing M at twice stoichiometry requires about an hour to cure well, while with L, about 10 hours are required. In contrast, curing of the XSBR depends little on the specific surface area of the ZnO, either with S or L, curing is essentially complete after 30 minutes. After simply mixing ZnO into either rubber, it remains as a dispersed particulate. With XNBR, curing appears to be controlled by the rate of dissolution and diffusion of ZnO, while, with XSBR, reaction is not diffusion limited and may be confined to regions near particle surfaces.