Platelet-reinforced polymer matrix composites by combined gel-casting and hot-pressing. Part I: Polypropylene matrix composites

Abstract

Platelet-reinforced polymer matrix composites were fabricated by a combined gel-casting and hot-pressing method. Submicrometer-thin alumina platelets were dispersed in a highly diluted grafted maleic anhydride polypropylene solution. Upon cooling, the polymer formed a gel which trapped the platelets in their well separated positions. During subsequent solvent evaporation, the polymer–platelet gel densified and the platelets were oriented horizontally. The dried composites were hot-pressed to further improve the platelet orientation and increase the density of entanglements in the polymer. This method combines several advantages of large scale and lab-scale fabrication methods in that it is fast, simple but also versatile. Composites with platelet volume fractions up to 0.5 were easily fabricated. The maximal achieved yield strength and elastic modulus of the composites were 82% and 13 times higher, respectively, than the values of the polymer alone. The enhancement in the composites mechanical properties was caused by classical load transfer into the platelets as the crystallinity of the polymeric matrix was not affected by the platelets. Alumina platelets with an aspect ratio below the critical value allowed for the ductile platelet pull-out fracture mode enabling large plastic deformation of the composites prior to fracture. At high concentrations of platelets, the strength and stiffness decreased again and the ductility was almost lost due to out-of-plane misalignment of platelets and the increasing number and size of voids incorporated during the fabrication. The designing principles and fabrication method described in this work can potentially be extended to other types of polymers and platelets to create new composites with tailored properties.