Investigation on the thermoformability of polyolefin blends by hot tensile and rheological tests

Due to desirable performance/cost properties, polypropylene (PP) is a suitable alternative for acrylonitrile butadiene styrene, polyvinyl chloride and polystyrene in packaging and automotive thermoformed articles. Because of low elasticity and poor melt strength, thermoforming of PP is difficult. In this study, the effect of blending of PP with high-density polyethylene (HDPE) and ethylene propylene diene monomer (EPDM) is presented. Ternary blends of PP/HDPE/EPDM were prepared using a laboratory co-rotating twin-screw extruder. Three levels of HDPE (10, 20, 30 phr) and EPDM (5, 10, 15 phr) and 10 phr of calcium carbonate were incorporated into the PP matrix. Thermoformability of the samples was investigated by hot tensile and rheological tests. The effects of blending and processing conditions, such as sheet temperature and strain rate, were studied. The stress-strain behavior showed that the ternary blending improved thermoformability of PP. In all blends, elastic modulus, load at yield and the post yield modulus decreased in comparison with original PP. Increasing temperature and crosshead speed (strain rate) had significant effects on the hot tensile properties. Rheological properties of the blends were studied using a rheometer. It was found that blends had higher elastic share and lower tan δ, hence more elasticity relative to conventional PP. All of the results confirmed that blending with HDPE and EPDM improved the thermoformability of PP.