Essential Work of Fracture of photo-oxidized LDPE/EVA films

L. FASCE; G. CHIAVERANO; R. LACH; D. FASCE; P. FRONTINI

Ischia, Italy

Conferencia; 3rd International Conference on "Times of Polymers & Composites" TOP 2006; 2006

When plastic films are exposed to photo-oxidative conditions, the concomitant change of the primary structure of the polymer may lead to a drastic loss of the mechanical performance. Commercial LDPE/EVA were UV-C irradiated at a exposure intensity of 36 W/m2 and at 37¨¬C. The fracture performance of unirradiated films and films exposed from 2 to 16 days was investigated by means of the Essential Work of Fracture approach (EWF). Complementary studies including FTIR, DSC, TOM, uniaxial tensile and dart impact, were also carried out. Crosslinking reactions that govern photo-oxidation process during the firsts days of exposure stiffened the amorphous phase of the polymer, producing the enhancement of elastic modulus, yield stress, ultimate strength, disc strength and perforation energy, in addition to a detriment of the strain at break. An abrupt drop of tensile ultimate properties and perforation impact behaviour was observed in films irradiated longer than 5 days. This can be related to the appearance of microcracks and excessive chain scission. All DENT specimens underwent full ligament yielding prior to crack growth, but irradiation altered the crack propagation step. Stable crack growth took place by ductile tearing in unirradiated films and films exposed up to 5 days. A step-wise mechanism, which appeared the result of ductile tearing together with the coalescence and growth of microcracks, occurred in films longer irradiated. In the former case, load-displacement curves exhibited self similarity and experimental work of fracture versus ligament lengths plots appeared linear. Conversely, in the latter case, the propagation mode led to the loss of geometrical similarity, invalidating the application of the EWF approach. Irradiated films exhibited enhanced fracture parameters (we and ¥âwp) up to the appearance of microcracks, which altered the propagation behaviour, drastically diminishing the fracture energy. Crosslinking reactions that govern photo-oxidation process during the firsts days of exposure stiffened the amorphous phase of the polymer, producing the enhancement of elastic modulus, yield stress, ultimate strength, disc strength and perforation energy, in addition to a detriment of the strain at break. An abrupt drop of tensile ultimate properties and perforation impact behaviour was observed in films irradiated longer than 5 days. This can be related to the appearance of microcracks and excessive chain scission. All DENT specimens underwent full ligament yielding prior to crack growth, but irradiation altered the crack propagation step. Stable crack growth took place by ductile tearing in unirradiated films and films exposed up to 5 days. A step-wise mechanism, which appeared the result of ductile tearing together with the coalescence and growth of microcracks, occurred in films longer irradiated. In the former case, load-displacement curves exhibited self similarity and experimental work of fracture versus ligament lengths plots appeared linear. Conversely, in the latter case, the propagation mode led to the loss of geometrical similarity, invalidating the application of the EWF approach. Irradiated films exhibited enhanced fracture parameters (we and ¥âwp) up to the appearance of microcracks, which altered the propagation behaviour, drastically diminishing the fracture energy.