The performance of selected unsaturated coatings for calcium carbonate filler in polypropylene

The surface of ground calcium carbonate has been modified with several unsaturated acids and acid anhydrides of varying molecular weight. The modified calcium carbonates were then compounded with polypropylene, both in the presence and absence of dicumyl peroxide as an initiator, through twin screw extrusion to give compounds containing 75 phr of filler. Infrared analyses showed that reaction between each coating and the fillerʹs surface led to the formation of carboxylate salts. The occurrence of peroxide initiated transfer grafting between the alkenyl chains of the coatings and the polypropylene matrix during compounding was inferred from extraction studies and from physical test data. Extensive grafting was evident for the acrylic acid coated filler, but much less so for coatings with longer alkenyl chains. In the presence of peroxide the melt flow rates of each compound increased due to the reduction of molecular weight of the matrix polymer as a consequence of free radical induced grafting and degradation reactions. This led to the reduction in toughness and ductility of compounds with surface coatings that possessed alkenyl chains with fewer than 17 carbon atoms. For those with chains lengths of 17 or greater toughness was much improved due to the release of matrix constraint around the filler particles that counteracted the effects of degradation.