The thermo-oxidation of chlorinated and brominated isobutylene-co-isoprene polymers: Activation energies and reactions from room temperature to 100 °C

The activation energies of Chlorinated isobutylene-co-isoprene (CIIR) and Brominated isobutylene-co-isoprene (BIIR) were determined after thermo-oxidation in the 23-100 °C temperature range. Fourier Transform Infrared (FTIR) spectroscopy was used to follow carbonyl growth and aid in the identification of oxidative species along with 1H Nuclear Magnetic Resonance (NMR). Two thermo-oxidative processes at high (100-80 °C) and at low temperatures (60-23 °C) including a transition zone at 70 °C were identified. Halobutyl oxidation is occurring at the exomethylene microstructure and residual isoprene units during low temperature aging while the isobutylene sequences undergo oxidation at higher temperatures. Loss of bromine radical is quicker than chlorine, which is in line with their inherent bond strengths and agrees with the lower activation energy for BIIR (22 kJ/mol) compared to CIIR (52 kJ/mol) at low temperatures. Chain cleavage reactions dominate at low temperatures and a reaction pathway showing loss of halogen and the formation of macroradicals which undergo oxidation has been proposed. At higher temperatures (80-100 °C), chain crosslinking reactions become more numerous causing a substantial increase in activation energies for halobutyl in comparison to butyl rubber.