Production of carbon nanofibre reinforced polypropylene tapes for self-reinforced polypropylene composites

Nanoscale fillers offer the potential for significant enhancement of a range of polymer properties, as they are available in a wide variety of shapes and mechanical properties. Filler types range from single walled carbon nanotubes, (large aspect ratios, >1000 and moduli approaching 1TPa), to more readily available clays and mineral fillers. The objective of this project has been to study the effects of adding commercially available Carbon Nanofibres (CNF) to the properties of Polypropylene (PP). Hot Compaction, a process developed at the University of Leeds, utilises high modulus, highly oriented elements to form thick section, homogeneous sheets without the need to introduce a second phase of different chemical composition. These ‘single polymer’ composites are produced by selective surface melting on the surface of the oriented elements; on cooling, this molten material re-crystallises to form a matrix phase and bind the oriented elements together [1]. Blending of PP-CNF has been achieved using a twin screw extruder, with processing conditions optimised for maximum tensile mechanical properties, specifically Young’s modulus. Scanning electron microscopy (SEM) image analysis and dynamic light scattering (DLS) have been used in order to study the structure-property relationship of the nanocomposite material. The PP/CNF nanocomposite material has then been drawn into oriented tapes, both on a small-scale using an Instron and continuously on a draw frame. The change in density of these tapes has been used to determine the void fraction in the drawn tape. Importantly, it has also been shown that hot compaction of these tapes reseals these voids. Continuously drawn tapes showed that it is necessary to have a higher draw ratio PP/CNF nanocomposite tape than pure PP tape to match the drawn tape modulus. The drawn nanocomposite tapes have then been hot compacted to form self-reinforced composite sheets, optimising the process to achieve maximum tensile mechanical properties. A recent development of the hot compaction process [2] has been to introduce thin films interleaved between the woven tape layers, in order to improve the peel force required to peel the woven layers apart. This is a crucial parameter in self-reinforced single polymer composites since without interleaved films, since the interlayer region is only a few microns thick. The mechanical properties and, most importantly, peel strength of hot compacted composites produced from drawn PP/CNF and PP tapes, both with and without interleaved PP and PP/CNF films, will be reported. 1. Ward, I.M. and P.J. Hine, The science and technology of hot compaction. Polymer, 2004. 45 1423-1437. 2. Hine, P.J., R.H. Olley, and I.M. Ward, The use of interleaved films for optimising the production and properties of hot compacted, self reinforced polymer composites. Composites Science and Technology, 2008. 68 1413.