Fracture behaviour of polypropylene films at different temperatures: assessment of the EWF parameters

The fracture of films of one polypropylene (PP) homopolymer and three ethylene-propylene (EPBC) block copolymers with different ethylene content (EC) at temperatures around their glass transition (T-g) is investigated by means of the essential work of fracture (EWF) Method. The fracture behaviour is studied at low speed (2 mm/min) for the different materials, in the range from -40 to +70 degreesC. The materials are tested in the form of extruded films of 90 mum thickness, in a DENT geometry, after being annealed for 1 h at 120 degreesC. The homopolymer is found to be much more temperature-sensitive than the EPBC with lower EWF values at T < T-g and an opposite trend at T > T-g. The ductile-brittle transition of the homopolymer that occurs below its T-g is suppressed for the EPBC in the temperature range studied. The variations of the EWF fracture parameters with T are consistent with those of a previous study in LDPE, and are explained in terms of molecular relaxation, ethylene phase content, and changes in the yield and fracture stresses. Some limitations of the EWF method are found with the more ductile materials, which show the necking phenomenon with DENT specimens at the higher temperature studied. (C) 2000 Elsevier Science Ltd. All rights reserved.