Degradation of polypropylene (PP) during multiple extrusions: Thermal analysis, mechanical properties and analysis of variance

The effects of repetitive extrusion cycles at different die temperatures in a single screw extruder were investigated by thermal analysis (degree of crystallinity ( χ c), heat of fusion (Δ H m), melting temperature ( T m) and the onset melting temperature ( T onset,m)) and mechanical properties (tensile and impact strength). Also, analysis of variance (ANOVA) and response surface methodology (RSM) were employed for modeling and analysis of the mechanical property results. Thermal analysis verified an increase for χ c and Δ H m with a continuous reduction in the T m and T onset,m values with increase of die temperature and/or cycles of extrusion. For mechanical properties, the observed degradation process only slightly affects the small strain properties (yield stress and modulus) and impact strength. Break properties (break stress, break strain and energy to break) were reduced significantly by the experimental conditions adopted. According to ANOVA and RSM, the degradation process was occasioned more by high shear rate (repetitive number of cycles) than by high processing temperatures.