UV-EXPOSURE OF STABILIZED AND NONSTABILIZED HDPE FILMS - PHYSICOCHEMICAL CHARACTERIZATION

Non UV-stabilized and UV-stabilized (a UV-absorber) HDPE films were photo-oxidized in a Xenotest. The concentration of carbonyl and vinyl, the consumption of the UV-stabilizer and the change of molecular weight were studied using infrared spectroscopy, UV-spectroscopy and gel permeation chromatography (GPC) respectively. The number of chain scissions was calculated and found equal to the number of carbonyl groups. Crystallinity and density were studied using a differential scanning calorimeter (DSC) and a density gradient column. The density of the film increased with exposure time from about 950 to 970 kg/m(3) and the crystallinity from 70 to 82%. The increase in density is explained by the increase in crystallinity and by oxygen uptake. The concentration of bound oxygen, thus calculated, is about nine times larger than the carbonyl concentration quantified by IR spectroscopy. Additional IR-scans were carried out with an XY-table to quantify the profiles in depth. The rate at which carbonyl and vinyl groups are formed, was found to accelerate with time. The acceleration of the oxidation process finally resulted in profiles. The shape of carbonyl profiles was successfully modelled by computer simulations. Although a small amount of UV-stabilizer was used in the stabilized HDPE, profiles showed up earlier in the non-stabilized films.