Thermoanalytical Studies of the Thermal Stability of High Polymers. II. The Differential Thermal Analysis and Thermogravimetric Analysis of Some Polyethylenes

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Abstract
The thermal stability of polyethylenes has been studied by differential thermal analysis and by thermogravimetric analysis in air and in a nitrogen atmosphere. The crystallinities of some polyethylenes have been evaluated from the data of differential thermal analysis. These values are smaller than the reported value. This discrepancy may be due to differences in the methods. In the differential thermal analysis curve for low-pressure polyethylene Hizex 5000 in air, two small exothermic peaks due to oxidation were observed at about 200 and at about 390°C. The endothermic peaks at 135 and at 457°C correspond to the melting and to the thermal decomposition points of polyethylene. In thermogravimetric analysis in a nitrogen atmosphere, the thermal decomposition of lowpressure polyethylene takes place over the range from 380 to about 450°C, and that of high-pressure polyethylene, from 360 to about 450°C. In the presence of air, the oxidative degradation of low-pressure and high-pressure polyethylenes occurs in three stages: the first from about 200 to 350°C, the second from about 350 to 390°C, and the third from about 390 to 455°C. It has been concluded that the thermal stability of high-pressure polyethylene is smaller than that of low-pressure polyethylene. This may be attributed to the difference in branching of commercial polyethylenes.