Burn, David T.
(2016)
Long discontinuous carbon fibre/polypropylene composites for high volume automotive applications.
PhD thesis, University of Nottingham.
Abstract
The volume of fibre reinforced composites is increasing within the automotive industry, as stringent emissions legislation and consumer demands for improved fuel economy are encouraging manufacturers to reduce vehicular mass. Moreover, the falling cost of carbon fibre has meant that these composites are now being considered for semi-structural and structural components for medium-volume (+50,000ppa) applications in Euro Market Segments E and F (Jaguar XF, BMW 7 Series, Mercedes S-Class).
The use of thermoplastic matrices with carbon fibre enables cycle times of less than one minute, creating opportunities for high volume manufacture of high specific stiffness components. However, the interfacial adhesion between these materials has been shown to be poor. This thesis seeks to identify whether polypropylene combined with long, discontinuous carbon fibres at high volume fractions, are suitable for high volume, semi-structural applications within the automotive industry. In particular, fibres recovered using two different recycling methods have been considered, as a potential route for reducing future material costs.
Interfacial characterisation has been performed using the microbond method to investigate the quality of adhesion between the fibre and matrix, where the effects of sizing removal and introduction of a coupling agent have been considered. Fibre surface topology and chemistry have been examined to interpret data collected from interfacial testing, in addition to fibre strength measurements to assess the validity of the microbond method for high interface strength systems. A tow coating rig has been developed to produce partially pre-impregnated carbon fibre/polypropylene tows. The continuous coated tow has been chopped and processed into random fibre composites using non-isothermal compression moulding, and mechanical properties of the moulded panels have been characterised.
The interface strength between sized and desized (pseudo-recycled) carbon fibre and unmodified polypropylene has been found to be poor. A 295% increase in interfacial shear strength (IFSS) is observed with the addition of 2wt.% maleic anhydride to the polypropylene, between the matrix and epoxy-sized carbon fibres. An increase of up to 353% in IFSS is observed for the desized fibres. These improvements can be attributed to chemical bonding as a result of esterification of hydroxyl groups on the carbon fibre surface, with anhydride functionalities of the coupling agent. Additionally, interactions occur between the nitrogen containing groups on the desized fibre surface and the anhydride carbonyl groups in the matrix. Surface roughness is not found to significantly contribute to interface strength. Good interfacial bonding has therefore been observed between polypropylene and sized carbon fibre due to the addition of a coupling agent at 2wt.%, which allows the low cost polymer to be combined with commercially available fibre.
Long, discontinuous carbon fibre/polypropylene composites have been characterised in this study at volume fractions that have not previously been reported. Mechanical property characterisation has shown linear increases in stiffness with increasing fibre volume fraction. The specific stiffness of carbon fibre/polypropylene (0.45Vf) is comparable to the carbon fibre/epoxy benchmark. A plateau is observed for both strength and impact strength above volume fractions of 0.25, due to increased void content. The specific strength of the long fibre carbon fibre/polypropylene system can be improved further to a certain extent, by optimising the processing conditions to minimise trapped air.
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