HETEROGENEOUS NUCLEATION OF SHORT GLASS-FIBER POLYPROPYLENE COMPOSITES

The heterogeneous nucleation of polypropylene homopolymer and disconti nuous glass fibre (critical fibre length, L(c) greater-than-or-equal-t o 3 mm) reinforced polypropylene was performed by using sodium benzoat e as the nucleating agent. Isothermal crystallization of the compressi on-moulded nucleated polypropylene, non-nucleated polypropylene, nucle ated composite and non-nucleated composite was carried out over a temp erature range of 370-398 K. Comparison of the shift in the bulk crysta llization peak temperatures, as well as the rate of isothermal crystal lization of the samples, showed that the incorporation of the nucleati ng agent resulted in a significant improvement in the crystallization of the polypropylene homopolymer. The short glass fibres nucleated the polypropylene matrix to the extent that the crystallization and morph ology of the matrix differ appreciably from those of the homopolymer. Detailed Avrami kinetic analysis was carried out at 390 K for the samp les and values of the Avrami constants K and n, ranging from 2.3 x 10( -5) to 2.4 x 10(-3) and 2.01 to 1.66 for the polypropylene homopolymer to the nucleated composite, respectively, were obtained. Optical micr oscopic analysis showed spherulitic crystallization for the homopolyme r, crystallitic crystallization for the nucleated polypropylene, and m ixed crystallitic and axialitic crystallization for the discontinuous composite.