The influence of calcium carbonate and its modifications on the extrusion process and selected functional properties of polypropylene cast films

The purpose of the article was to determine the influence of calcium carbonate and its modification (with stearic acid) on selected properties of composite polypropylene films and their processing. Polypropylene composites containing, respectively, 5, 10, 20 and 30 wt% filler, were made in twin-screw extrusion process with cold granulation. Films, on the other hand, were produced in single-screw extrusion process (an extruder with a screw with diameter d = 20 mm and ratio l/d = 24) using a flat head (so-called cast films) and a chill roll type cooling and pull-back device. The pull-back rotational speed was selected so as to obtain films of a thickness of approx. 120 µm, irrespectively of the composite being processed. The assessment of the effect of the addition of applied fillers on the magnitude of the melt volume-flow rate (MVR) of polypropylene and the processing characteristic of extrusion process was made (the flow resistance and the variations in volumetric flow rate were determined). It was demonstrated how variations in the processability of polymeric material determined the correction of extrusion process setup parameters (pull-back system speed). The processability of composite films under extensional flow conditions was also determined by evaluating the extensional viscosity (ηE) of the composites. The strength values of composite films were determined under static tension conditions and in puncture tests. The effect of the content and modification of CaCO3 on the film haze and gloss was also assessed. The investigation carried out has demonstrated a direct effect of the addition of calcium carbonate and its modification on the processability and selected functional properties of composite films. The study has also proved that stearic acid is an effective modifier of processing properties, not only in shear flow conditions, but also under extensional flow conditions – smaller ηE values are exhibited by films containing modified calcium carbonate.