Study on the thermal stability of stabilized and unstabilized low-density polyethylene films

The oxidation and thermal stability of low-density polyethylene (LDPE) films (stabilized and unstabilized) used as greenhouse covering have been investigated. A mixture of kaolin, UV absorber, hindered amine light stabilizers (HALS) and heavy metals was used to stabilize LDPE films; these films were also compared with unstabilized LDPE films. Both films were exposed to natural weathering in Laghouat, Algeria. The oxidation resistance of unstabilized specimens for different ageing periods was determined by measuring the oxidation induction time (OIT) at 180 °C. It has been found that the highest OIT value was obtained for the unaged film and the values decline with ageing time. On the other hand, the stabilized specimens showed a significant stability under the conditions of OIT determination experiments. Furthermore, the oxidation resistance of these samples was evaluated by measuring the oxidation onset temperature (OOT). The results obtained showed that the OOT of the stabilized films ranges from 219 to 236 °C. Finally, a comparative analysis of the thermal stability results of both films was conducted. The activation energy of the photodegradation process of the both films was calculated by using TGA thermograms following the isoconversional approach. The results revealed that the stability of the stabilized specimens diminishes with ageing time and that the stabilizer additives improve the thermal stability of LDPE films. The influence of the photooxidation on the degree of crystallinity (based on DSC and WAXS techniques) of both films was also reported. Generally, there is a strong correlation between TGA, OIT, OOT, UV and FTIR results.