INFLUENCE OF THE CROSSLINK STRUCTURE ON THE ACTIVATION ENERGY CALCULATED UNDER THERMO-OXIDATIVE CONDITIONS

Abstract

ABSTRACT A series of six carbon black reinforced brominated poly(isobutylene-co-isoprene) (BIIR) compounds has been developed varying only in cure system type: sulfur, sulfur donor, zinc oxide, peroxide, phenolic resin, and ionic. Compounds were aged from room temperature up to 115 °C, and hardness, mechanical properties, and network chain density were measured. Non-Arrhenius behavior was observed due to data curvature from 70 to 85 °C. The oxidation process was adequately described by assigning low (23–85 °C) and high (85–115 °C) temperature regimes. Heterogeneous aging due to diffusion limited oxygen (DLO) occurred for heat aging above 85 °C, and all measured responses except tensile strength were strongly affected, causing lower activation energies. The activation energy for the high temperature oxidation process is in the range of 107 to 133 kJ/mol in the following ascending order: zinc oxide, ionic, sulfur donor, sulfur, peroxide, and resin. The midpoint of the high temperature activation energies is of the same order as the BIIR and poly(isobutylene) elastomers. The low temperature activation energy is in the range of 55–60 kJ/mol and is likely due to a combination of oxidative chain scission (crosslink density loss) and crosslinking recombination (network building) reactions. Apart from the crosslink structure stability, the presence of unsaturation along the polymer chain after vulcanization affects the high temperature activation energy.