Evaluation of the fracture Behavior of multilayered polypropylene sheets obtained by coextrusion

The mechanical and fracture behavior of multilayered sheets (up to three layers) is presented. The sheets were processed by plane coextrusion-calendering using two polypropylene-based materials, isotactic polypropylene homopolymer (H0), and an ethylene-propylene block copolymer (C1) with 5.5% ethylene content in weight. Combining these materials five different sheets were produced, (H0, C1, H0-C1, H0-C1-H0, and C1-H0-C1), with an overall thickness of 0.5 mm. The processing variables (temperature, extrusion rate, calendering speed, etc.) were set constant among the different structures to facilitate the evaluation of the results. The structure of the sheets was characterized with wide angle X-ray scattering (WAXS). The mechanical behavior was determined from dumbbell tensile specimens in the melt flow (MD) and transverse to the melt flow (TD) directions. The fracture parameters were determined by the essential work of fracture (EWF) technique in mode I in both MD and TD directions. The results show variations on the induced orientation of the sheets, with remarkable differences on the fracture properties. The poor fracture resistance of the homopolymer (H0) film in TD is improved through coextrusion with C1, obtaining the best results for toughness in the case of the three-layered sheets.