THERMOOXIDATIVE AGING OF CROSS-LINKED LINEAR POLYETHYLENE - STABILIZER CONSUMPTION AND LIFETIME PREDICTION

The thermooxidative aging of extruded ribbons of linear polyethylene crosslinked by irradiation was studied in the temperature interval 110-180-degrees-C. The end of the induction period was detected by various methods, including a UV spectrophotometric determination of phenolic antioxidant made on microtomed sections in order to establish its distribution into the sample thickness, and other previously reported methods (density, ultimate tensile properties, carbonyl index, color change and weight loss). The kinetic curves of phenol depletion display the earliest rate change associated with the end of the induction period. The other properties begin to vary after a delay, in the following order: Ultimate elongation < density < color change and carbonyl groups < weight loss. A mechanistic interpretation of this delay is proposed. Its duration can be considered negligible at temperatures higher than the melting point, but it can induce noticeable errors of lifetime prediction for exposures at lower temperatures. The Arrhenius plots of lifetime display a discontinuity at the melting point. This is tentatively interpreted in terms of stabilizer concentration changes in the amorphous phase, but the process is complicated by the occurrence of an exudation process occurring in the solid state and responsible for loss of about 50% of the initial stabilizer.