Carbon Fibre Reinforced PVDF and PEEK Nanocomposites

Abstract

There is currently a well-timed opportunity to create intensely improved structural materials to be used as risers in the offshore oil and gas industry where high mechanical performance along with superior resistance to chemical attack is required. Recent evidence shows that carbon nanotubes (CNTs) are the ideal reinforcement for polymer fine structures and are expected to improve the matrix modulus, which should lead to composites with much improved compression and other matrix dominated properties. By combining conventional reinforcing fibres and CNTs within thermoplastic matrices, a new class of materials with both superior mechanical, environmental, and chemical performance, as well as significantly reduced through-life costs should be possible. Different formulations of nanocomposites consisting of modified Polyvinylidene difluoride (PVDF) and modified CNTs e.g. Poly methyl methacrylate grafted carbon nanotubes (PMMA-g-CNTs) were fabricated using extrusion and injection moulding up to a maximum CNT content of 10 wt%. CNTs were well distributed within polymers as determined through optical and electron microscopy. Dynamic mechanical analysis was conducted in order to study the effect of CNTs on storage modulus of nanocomposites. The tensile, flexure and compression properties of PVDF nanocomposites were increased with increase in CNT content. Overall, PMMA-g-CNTs based PVDF nanocomposites with a 10 wt% CNT loading showed 20%, 30% and 60% improvement in tensile, compressive and flexural modulus as compared to PVDF nanocomposite containing 10 wt% CNT loading. The main objective of this research was to optimise processing conditions for fabricating ultra-inert hierarchical fibre reinforced nanocomposites. The CNT modified matrix, prepared by solution precipitation, was reinforced with carbon fibres via continuous composite line setup to manufacture hierarchical reinforced thermoplastic (Polyvinylene difluoride (PVDF) and Poly ether ether ketone (PEEK)) composites. Thermoplastic hierarchical composites containing up to 1.25 wt% CNTs demonstrated improved compression and interlaminar shear strength whereas a decrease was observed of the same when CNT content was increased up to 5 wt%. A similar trend of decline in mechanical performance at higher loadings of CNTs (>1 wt%) was observed in PEEK based hierarchical composites which indicated that matrix dominated properties were availed without compromising the quality of fibre/matrix interface at an optimum loading of CNTs (1.25 wt%) resulting in enhanced mechanical performance of hierarchical composites. However, further addition of CNTs adversely effected the fibre impregnation by nanocomposite matrix, due to processing issues such as high viscosity of nanocomposites at higher CNT contents, resulting in poor mechanical performance. Moreover, the influence of CNTs on the fracture toughness was also investigated by double cantilever beam testing. Polished cross sections of fracture surfaces of failed composites were analysed to understand how CNTs affected the damage mode. Fractographic analysis of compression and double cantilever beam (DCB) failed PVDF and PEEK hierarchical composites also showed the presence of bare/dry fibres which indicates that nanocomposite's infusion/impregnation in to carbon fibres is being compromised at higher CNT loadings.