STABILIZER CONSUMPTION AND LIFETIME PREDICTION IN AGING OF CROSS-LINKED POLYETHYLENE

The thermal oxidation of beta ray-crosslinked polyethylene (XLPE) was studied in temperatures ranging from 90 to 180 degrees C. Various analytical methods were used for determination of an end of the induction period on bulk samples: gravimetry, ultimate tensile properties, density and color change. On microtome slices of similar to 30 Ccm thickness the depth distribution of oxidation products, phenolic antioxidant, density and tensile ultimate properties were followed as a function of exposure time by respectively IR and UV spectrophotometry, densitometry and Microfoil Tensile Testing. Depending on the testing method different durations of induction period (DIP) were obtained. They increase in following order: Phenol depletion < Ultimate elongation < Density < Carbonyl build-up less than or equal to Color change < Weight loss. The difference between phenol depletion DIP and carbonyl build-up DIP can be considered as negligible at temperatures higher than the melting point (about 30%) but it reachs more than 100% at T<T-m. Different kinetic regimes of phenol consumption were observed depending on the exposure temperature. A mechanistic interpretation is proposed explaining the role of the stabilizer system in initially homogeneous and later heterogeneous oxidation of the bulk material. The Arrhenius law was applied to different durations of induction period. A discontinuity appeared in the melting point region (120-130 degrees C) which is tentatively interpreted in terms of different stabilizer concentration in amorphous phase of semicrystalline material (T<T-m) and in melt material (T>T-m). On the basis of the presented complex study of polyethylene thermo-oxidation, different aspects of lifetime predictions are discussed.