Fibre Structure Preservation during Carbon Fibre Reinforced Polymer Recycling – Characterisation and Parameter Prediction Model of the Recycling Process
In 2021, 24,000 tonnes of carbon fibre reinforced polymer (CFRP) waste worth USD 630 million was either landfilled or incinerated globally. The amount of CFRP waste is estimated to increase as a result of the ever-rising market demand. The CFRP market demand is due to the remarkable properties of high specific strength and stiffness, which can provide considerable weight savings compared to steel and aluminium, largely contributing to reducing fuel usage and gas emissions. However, CFRP is an expensive material primarily due to the energy-intensive production process, making it a high-valued material. Yet, if CFRP becomes a waste at any time during the material’s life cycle, its value is instantaneously diminished. Thus it makes sense to recycle the valuable material, except that carbon fibre recovery is a sophisticated process. Thermal recycling of CFRP waste has emerged as a technologically mature process to recirculate carbon fibre into the supply chain. In addition to the lack of sizing, the thermally recovered carbon fibres lose their material value for two other reasons: (a) short, discontinuous recovered carbon fibres in a fluffy form and (b) resin residue on the fibre surface and fibre degradation during thermal recycling. This study addressed the two problems by demonstrating the effectiveness of fibre structure preservation during recycling, using the optimised furnace process parameters predicted by a novel mathematical model. The recovered fibre structure after thermal recycling has the fibre orientation, alignment, and length, well preserved and intact, ready for remanufacturing. The process of recycling and remanufacturing was repeated multiple times, and the mechanical tests on the remanufactured parts suggest that fibre maintains most of its mechanical properties during thermal recycling except for the impact of sizing removal. The strength and stiffness of the remanufactured composites from preserved fibre structure were at par – if not better – with the properties reported in the literature for composites manufactured from long-aligned recovered fibres. However, to attain better mechanical properties of the remanufactured composite, achieving a better volume fraction is also an essential factor. In almost all CFRP composites, the reinforcement structure is compressed at some stage of the manufacturing process, increasing fibre volume fraction and enhancing the composite’s mechanical properties. However, compaction changes fibre architecture affecting its permeability, thus influencing resin flow. During thermal recycling, the sizing and stitching are burnt off, along with some fibre degradation; thus, changing the way fibres interact with each other in the recovered structure, compared to the virgin fibre structure. Minimum changes in compaction and permeability behaviour between virgin and recovered fibre structures were observed for unidirectional fibres in a 0/90 staking sequence. However, not all CFRP parts are flat; therefore, the change to fibre architecture of curved parts undergoing thermal recycling is also an important aspect to incorporate while considering fibre structure preservation. Large CFRP waste parts with smaller curvatures (near flat shapes) can be cut into the required sizes and recycled with fibre structured preserved. However, the curved part would flatten out during recycling, referred to as ‘shape manipulation’ in this research. The experimental results suggest that the best flattening of multi-layered shape manipulation was experienced with the unidirectional fibres with a 0/90 stacking. Fibre structure preservation has, thus, proved to be an efficient way to preserve material value, yet, it addresses just one part of the problem. Resin residue and fibre degradation during thermal recycling also negatively impact the remanufactured composite part properties. The results from the experiments, supported by the literature, also emphasise the importance of carefully selecting the furnace process parameters to obtain a better quality of the recovered fibre. The selection of furnace process parameters to recover the best quality fibre is a complex task as it depends on many factors extending outside of the composite itself. Thus, to cater for a multi-factor problem, a set of MATLAB® codes was written to model heat transfer and material degradation, providing the ideal thermal recycling process parameters for recycling CFRP composites. The ideal furnace process parameters ensure that the recovered carbon fibres are least thermally degraded while the maximum amount of resin has been removed. The research concludes that fibre structure preservation during thermal recycling with optimised process parameters can directly replace long virgin carbon fibre in the manufacture of products with structural value, enabling environmental as well as economic gains.
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