Development and characteristics of a fully recycled CF/PP composite

Abstract: The increasing industrial use of carbon fibre in e.g. aircraft and wind turbines calls for strategies for their recovery and possible reuse. In addition, tremendous amount of energy is needed to be able to manufacture pristine carbon fibres. In this work, an engineering composite material was manufactured from recyclates. Processing scrap from PURE was extensively studied in terms of its stability and processability as a thermoplastic matrix material. Polypropylene scrap material was reprocessed into a film by press forming and introduced into a stack of carbon fibre preforms made from recycled carbon fibres recovered via a pyrolysis process from aircraft structures. The preform stack was heated and the composite material was manufactured by press forming. A challenging issue in this work was to achieve the desired distribution of the recovered carbon fibres in the fibre preforms. Here, a paper making method was employed to distribute the recovered carbon fibres randomly in the plane. It is well known that the fibre/PP interface properties are often the weakest link in the composite performance. Several modifications, including the addition of maleic anhydride grafted polypropylene (MAPP), are often used. MAPP improves the interface bonding between the fibre and polymer matrix by two simultaneous reactions. Firstly, the long molecular chain is responsible for chain entanglements and co-crystallisation with the non-polar PP matrix. These entanglements provide mechanical integrity to the host material. Secondly, the anhydride groups chemically interact with the functional groups on the fibre surface. The addition of MAPP has been found to improve the interface and increase the stiffness and strength of the composite. Inelastic mechanical behaviour in tension of a recycled polypropylene (rPP) matrix and a rPP matrix with addition of 10% of maleic anhydride grafted polypropylene (rPP+MAPP) was characterised and compared. The time dependent response was decomposed into nonlinear viscoelastic and viscoplastic parts and each of them quantified. It was found that the elastic properties of the rPP matrix did not degrade during loading. The addition of MAPP to the rPP matrix did not change the mechanical properties of the material. A non-linear material model was developed and the involved parameters (stress dependent functions) were identified. The model was then validated in a stress controlled test at a constant stress rate. The inelastic and time dependent behaviour of the MAPP modified composite material in tension was analysed. A series of quasistatic tensile and creep tests were performed to identify the material model, which accounts for: a) damage related stiffness reduction, b) development of stress and time dependent irreversible strains described as viscoplasticity, c) nonlinear viscoelastic behaviour. Fibre length distribution was investigated before and after composite manufacturing process to investigate the influence of the processing conditions on the fibre degradation. The quality of the manufactured novel, fully recycled, composite material regarding void content and fibre orientation was examined by microscopy.

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