Mechanical properties of LDPE films produced by extrusion blowing and crosslinked by electron beam

Though a number of studies dealing with the chemical and physical effects o f ionizing radiation on polyolefins have been published and reviewed since more than fifty years, the behaviour of anisotropic PE materials submitted to electron beam (EB) or gamma radiation has been scarcely studied. Polyeth ylene films made by the extrusion-blowing process present anisotropic physi cal characteristics. This feature provides them with several interesting me chanical properties. In this report, the effects of the EB treatment on suc h materials are presented. Two types of LDPE films, one polymer (sample B) being slightly more branched than the other (sample A), were supplied by En ichem. The source of accelerated electrons used was a CB 150 ElectroCurtain generator operating at 175 kV. We have chosen to conduct the experiments u nder nitrogen atmosphere, at a single dose rate and at room temperature. Th e samples were treated with various doses between 20 and 2 000 kGy. We have quantified the crosslinking by the determination of gel fraction. Gel cont ent was determined by means of a soxhlet extraction, using xylene, which is solvent for the unirradiated polymer. Samples (W-1) were exposed to reflux ing xylene close to its boiling point. The extraction time was 48 h. After being dried to constant weight, the insoluble residue (W-2) was weighed. Th e gel content was calculated as 100 x (W-2/W-1). At low dose, crosslinking is more effective for sample B. Above a dose of 100 kGy; the difference in reactivity seems to be progressively reduced to give finally gel fractions almost identical for samples A and B; the difference is from 50 % at low do se to about 5 % at high dose, due to the crosslinking of both methylene and methine groups. The evolution of mechanical properties versus radiation do se has been examined. The tensile mechanical behaviour of samples was chara cterized using an Instron machine equipped with a 1000 N load cell. The mea surements were carried out at room temperature and the pulling speed for th e determination of the Young's modulus and the ultimate properties was 2 an d 50 mm/min, respectively. The reported data were obtained by averaging the results of at least 5 tests. The effect of irradiation on mechanical prope rties is to increase ultimate tensile stress and to reduce ultimate elongat ion. The initial modulus is determined from the slope of the force elongati on curve in the limit of small strain. The decrease in Young's modulus at r oom temperature with increasing radiation dose is ascribed to the reduction in crystallinity that induces a greater effect than the crosslinking produ ced. As already mentioned, samples A and B present anisotropic properties d ue to their bi-orientation. We can use the theory applicable to composite m aterials and plot the anisotropy spectra of Young's modulus. It seems that crosslinking does not affect anisotropy. This was expected because the cros slinking takes place in the amorphous regions of the polymer, or on the cry stals fold surfaces. So, the radiation treatment cannot give rise to anisot ropic modification of mechanical properties.