Single Polymer Composites Based on Polypropylene: Processing and Properties

Abstract

Isotropic polymers lack sufficient strength and stiffness for many engineering applications. In order to improve these properties polymers can be filled with structural reinforcements such as glass or natural fibres. However, current major trends focus on simple monocomponent systems in an effort to reduce costs and increase recyclability. Composite systems, by definition, must employ at least two phases with different material properties. With the introduction of careful processing routes, it has been proven possible to create a fibrous, two phase composite, in which both are polypropylene. Polypropylene can be melt spun and solid state drawn to give oriented tapes, and moduli of -15GPa and tensile strengths of -550MPa can be achieved. These tapes can then be oriented into sheets, either in the form of woven fabrics or unidirectional layers. These sheets form the reinforcing phase of a single polymer composite material. Such single polymer composites based on polyolefins can be produced by using a separate matrix impregnation route, but these are limited by relatively low volume fractions of reinforcement. Previous work executed at the University of Leeds showed that polymer fibres can be welded together by selective melting of the fibre exterior, but this method is limited by a small temperature processing window. By using polypropylene tapes co-extruded with a copolymer skin, it has been shown that such tapes can be welded together at temperatures far below the melting temperature of the tapes, thus ensuring that thermal relaxation of the highly oriented polymer does not occur. The temperature processing window can be widened further by constraining fibres during heating. The optimisation of the drawing and structure of these tapes, together with an investigation of the static and dynamic mechanical properties, impact resistance and interfacial properties of composites formed from these tapes, are investigated in this thesis