Effect of Processing on Environmental Stress Cracking Behaviour of Hips

It is well known that processing may induce changes in the mechanical properties of polymers. These changes are most commonly effects of induced thermal stresses, physical aging, frozen-in strains and molecular orientation. In this work two different high impact polystyrenes processed by thermoforming were considered. The main effect induced by thermoforming was molecular orientation. The degree of orientation was measured on the thermoformed product considered and its effect on the environmental stress cracking behaviour in sunflower oil was studied. The fracture mechanics approach was applied. It was found that in air the stress intensity factor, KIc, increases as molecular orientation increases while in the presence of the active environment the KIc is. as expected, lower but the dependence on orientation less important.