Measurement of the oxidative stability of polyethylene by differential thermal analysis

Abstract

An evaluation has been made of the experimental features critical to the evaluation of the oxidative stability of polyethylene using constant temperature DTA, with particular reference to the control and measurement of the sample temperature.The use of the DTA technique at constant temperature to measure the oxidative stability of polyethylene and other polyolefines as an alternative to the oxygen adsorption method is well established [1,2]. However, the DTA method relies heavily on the rigorous control of experimental factors in order to produce reliable and accurate results. One main method for the measurement of oxidative stability involves heating the test sample at a constant temperature of 200 +_ 1 ~ [1 ]. Hence the three essential requirements of any DTA instrument used in this test are the maintenance of the DTA cell within 1 ~ of the required constant temperature for at least the duration of the test; the accurate measurement of the sample temperature; and the capability to display this temperature with high resolution. The present investigation has been concerned with an evaluation of the parameters which are critical to the determination of the oxidative induction time (sometimes abbreviated to OIT [1]) of polyethylene. ExperimentalAll oxidative stability measurements were carried out on a Stanton Redcrofl DTA 671B. The differential temperature is measured by the plate type chromelalumel thermocouples (see Fig. 1) and a central thermocouple wire permits simultaneous measurement of the sample temperature. The samples were contained in fiat, open aluminium pans which allows good contact of the sample with the experimental atmosphere. The desired gas is introduced into the reaction chamber via a capillary. The A T and sample temperature signals were recorded on a two pen X 1, Xz/t potentiometric recorder.At 200 ~ the sample temperature chromel-alumel thermocouple output is 8.137 mV with reference to an ice junction at 0% The required resolution of sample temperature was obtained by backing off with 7.5 mV with a high precision voltage 18"