Mechanical and morphological properties of stabilized and non-stabilized HDPE films versus exposure time

Non W-stabilized and UV-stabilized (a UV-absorber) HDPE films were photo-oxidized in a Xenotest. The concentration of carbonyl and the UV-stabilizer, the molecular weight, the crystallinity and the density were studied versus exposure time. Furthermore, the mechanical behaviour was studied at different strain rates by tensile testing. The number of chain scissions was found equal to the number of carbonyl groups. The density of the film increased from 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 rate at which carbonyl groups are formed, was found to accelerate with exposure time. This acceleration finally resulted in profiles, which could be measured by FT-IR and verified by computer simulations. After 14 days of exposure the specimen failed in a brittle manner. An activation volume was calculated and found to decrease. It is thought that a change in molecular mobility is responsible for the lower strain at yield values. The change in molecular mobility can also explain the increase in Young's modulus with the strain rate and exposure time. The Takayanagi and the Halpin/Tsai models were used to describe the increase of modulus of elasticity.