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

The thermooxidative aging of extruded ribbons of linear polyethylene c rosslinked 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 dist ribution into the sample thickness, and other previously reported meth ods (density, ultimate tensile properties, carbonyl index, color chang e 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 or der: 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 pr ediction for exposures at lower temperatures. The Arrhenius plots of l ifetime display a discontinuity at the melting point. This is tentativ ely interpreted in terms of stabilizer concentration changes in the am orphous phase, but the process is complicated by the occurrence of an exudation process occurring in the solid state and responsible for los s of about 50% of the initial stabilizer.