| The influence of zinc oxide particle size on the cure and mechanical properties of carboxylated butadiene-acrylonitrile rubber (XNBR) and carboxylated styrene-butadiene rubbers (XSBR) was studied. Curing XNBR with zinc oxide is diffusion-controlled. With large particles, the onset of cure was quite slow and two-step curing was observed, while with fine zinc oxide, curing was fast and normal one-step curing was seen. Upon neutralization, cured sheets became more transparent, indicating a reduction in particle size. Also, zinc oxide-cured XNBR exhibited exceptionally high strength and modulus. These enhanced properties are attributed to ionic clusters that reinforce the rubber. With sufficient zinc oxide, a tan δ peak was observed at high temperature, in addition to the glass transition. This peak, ascribed to an ionic transition, shifted to higher temperature with increased zinc oxide up to about twice stoichiometry. Smaller particles shifted the peak to higher temperature.; Contrarily, zinc oxide behaves more like filler in XSBR rather than fully effective curative. Cure rate was insensitive to zinc oxide size and concentration. Curing was fast, but only a small increase in torque occurred. Equilibrium swelling and transmittance studies indicate zinc oxide curing. Infrared spectra appear to show near complete neutralization when sufficient zinc oxide was used. In addition, dynamic mechanical measurements demonstrate that ionic aggregates form in XSBR. However, very high strength did not result. Presumably, neutralization is confined near the surface of zinc oxide particles for XSBR. This leads to a non-uniform crosslinked structure and only provides semi-reinforcement. On the other hand, a rather uniform crosslinked structure is developed throughout XNBR, giving substantial reinforcement. |
