The incorporation of carbon nanofibres to enhance the properties of self reinforced, single polymer composites

Abstract

This paper describes the incorporation of carbon nanofibres (CNF) into polypropylene (PP) single polymer composites, materials where both the reinforcing phase and the matrix phase are PP. The CNF/PP composites were produced from an assembly of highly oriented tapes. The process of making the composites involves heating the tapes to a critical temperature such that a small fraction of the surface of each tape is melted; on cooling this recrystallises to form the matrix of the composite. The production of the composites required optimisation of three stages; incorporation of CNF into PP tapes, orientation of CNF/PP tapes by tensile drawing and hot compaction of the tapes. Results are presented to describe the research and findings in each of these key stages. Preliminary studies showed that the introduction of small amounts of carbon nanofibres (CNF) significantly improved the properties of isotropic PP. For example, 5% volume addition of CNF gave a 60% increase in the room temperature Young's modulus and a reduction of 35% in the thermal expansion coefficient. Moreover, the percentage enhancement of properties was greater at high temperatures where the stiffness of the PP is much reduced. These results can be very well understood in terms of conventional composite modelling. In unidirectional CNF/PP hot compacted composites the major improvements in mechanical behaviour are in the direction transverse to the orientation direction, where the CNF can make a proportionately greater contribution to the properties, and as shown by dynamic mechanical behaviour, this is most marked at high temperatures. Composite modelling based on uniform strain with appropriate allowance for the CNF aspect ratio predicts the behaviour extremely well. A very interesting result is that the peel strength of composites produced by hot compaction of woven CNF/PP shows a four-fold increase over woven PP composites and this is increased by another factor of two by the addition of a maleic anhydride compatibiliser. A further interesting result, of some practical significance, is that although the incorporation of CNF into PP causes voiding and some loss of molecular orientation during drawing, the hot compaction process closes and seals the voids, so that the original PP density is recovered.