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

The fracture of films of one polypropylene (PP) homopolymer and three ethyl ene-propylene (EPBC) block copolymers with different ethylene content (EC) at temperatures around their glass transition (T-g) is investigated by mean s 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-sens itive 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 b elow its T-g is suppressed for the EPBC in the temperature range studied. T he variations of the EWF fracture parameters with T are consistent with tho se of a previous study in LDPE, and are explained in terms of molecular rel axation, ethylene phase content, and changes in the yield and fracture stre sses. Some limitations of the EWF method are found with the more ductile ma terials, which show the necking phenomenon with DENT specimens at the highe r temperature studied. (C) 2000 Elsevier Science Ltd. All rights reserved.