L. Audouin et al., STABILIZER CONSUMPTION AND LIFETIME PREDICTION IN AGING OF CROSS-LINKED POLYETHYLENE, Die Angewandte makromolekulare Chemie, 232, 1995, pp. 1-12
The thermal oxidation of beta ray-crosslinked polyethylene (XLPE) was
studied in temperatures ranging from 90 to 180 degrees C. Various anal
ytical methods were used for determination of an end of the induction
period on bulk samples: gravimetry, ultimate tensile properties, densi
ty and color change. On microtome slices of similar to 30 Ccm thicknes
s 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, densitomet
ry and Microfoil Tensile Testing. Depending on the testing method diff
erent 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 meltin
g point (about 30%) but it reachs more than 100% at T<T-m. Different k
inetic regimes of phenol consumption were observed depending on the ex
posure temperature. A mechanistic interpretation is proposed explainin
g the role of the stabilizer system in initially homogeneous and later
heterogeneous oxidation of the bulk material. The Arrhenius law was a
pplied to different durations of induction period. A discontinuity app
eared in the melting point region (120-130 degrees C) which is tentati
vely interpreted in terms of different stabilizer concentration in amo
rphous phase of semicrystalline material (T<T-m) and in melt material
(T>T-m). On the basis of the presented complex study of polyethylene t
hermo-oxidation, different aspects of lifetime predictions are discuss
ed.