Thermal oxidation of blends of polypropylene and polyamide 6/66. Effect of inhibition

Abstract

The thermal oxidation of the blends of isotactic polypropylene (PP) and mixed copolyamide PA6/66 has been studied in a wide range of compositions: 0, 5, 10, 20, 30, 40, 50 and 100 wt % PA6/66. Data obtained by differential scanning calorimetry (DSC) and FTIR indicate the immiscibility of two polymer phases in the blends. The melting enthalpy of mixtures is increased during oxidation that is explained by chemi-crystallization of PP under PP oxidation. PA6/66 component is found to retard the growth of PP enthalpy during PP oxidation. According to DSC data on the PP/PA6/66 blends in air, PA6/66 phase greatly enhances the oxidation onset temperature of the PP/PA6/66 blends thus indicating the improved thermal stability of new material. The oxygen consumption of the PP/PA6/66 blends at 130°C proceeds with a large oxidation induction time (OIT) which is increased with increase in PA6/66 content. After OIT is ended, a constant oxidation rate is established, which is characteristic of the autoxidation mechanism. Kinetic analysis of the thermal oxidation of the PP/PA6/66 blends was carried out on the base of two models of polyolefin autooxidation with hydroperoxide decomposition via the 1st or the 2nd order reaction. The both models were found to describe the experimental kinetic curves of the oxygen consumption. Two possible explanations of the inhibition of PP oxidation by PA6/66 phase are discussed. The migration of active radicals from the PP phase to the PA6/66 phase with their transformation into low-active PA6/66 radicals may be responsible for inhibition and an increase in OIT of PP. Another mechanism of OIT increase may be related with a change in the structure of the PP/PA6/66 blends depending on PA6/66 content.