8. Thermal Aging of Polyolefin and Effect of Pre-irradiation of γ Ray on Degradation

The temperature dependence of the thermal aging of ethylene-propylene elastomer (EPR) pure vulcanized and linear low-density polyethylene (LLDPE) was studied using various methods. The activation energy obtained by the measurement of chemiluminescence (CL) of EPR at the constant temperature ranging from 60 to 160 °C was 82.7 kJ/mol. The rate constant of thermal molecular chain scission of EPR was calculated from the chemical relaxation curves measured at constant temperature ranging from 80 to 140 °C. The activation energy was 110 kJ/mol. The total carbonyl concentration increased logarithmically with increased thermal aging from 60 to 160 °C; the activation energy of the rate constant of the carbonyl accumulation was 95.8 kJ/mol. Pre-irradiation on EPR increased the count of CL, the rate of molecular chain scission, and the rate of accumulation of C=O, but the pre-irradiation did not change the values of these activation energies. The weight change of LLDPE resulting from thermal aging was studied at constant temperatures ranging from 90 to 170 °C. Three stages were observed including induction, the weight increase, and the weight decrease period. The activation energy for the induction period was 136 kJ/mol and was 105 kJ/mol for the weight increase. The addition of antioxidant reagent increased the induction period of LLDPE in all of the temperatures, but the activation energy of the induction period was not changed.

When LLDPE was irradiated up to 320 kGy, the induction period of weight change of the sample by thermal aging was remarkably shortened. The mechanism of this result was considered to be as follows. The irradiation of LLDPE in air accumulated hydroperoxide in the samples, which initiated autoxidation of LLDPE rapidly with applying heat.