ASPECTS OF RECYCLING OF CARBON FIBRE REINFORCED POLYETHERETHERKETONE (CF/PEEK)

Abstract

This study focused on the recycling of the carbon fibre reinforced polyetheretherketone (CF/PEEK) composite, as it is an expensive material worth recycling. In its virgin form, CF/PEEK has excellent mechanical properties. Thus it is hoped that even after recycling, the extent of degradation to its mechanical properties will still be acceptable and the recycled material can be reused in non-structural applications. In general, the study involved three areas: 1) Identification of the most effective method to recycle CF/PEEK (chemical matrix digestion and comminution methods were tried among the more popular methods). 2) Characterization of the recycled material and 3) correlating the experimental results with a simple mathematical model, the Cox Shear-Lag model. Mechanical testing such as tensile and flexural tests were conducted. Differential Scanning Calorimetry (DSC) was used to investigate the recycled material's crystallinity and also its transition temperature. Other techniques such as Scanning Electron Microscopy (SEM) and Image Analysis were used to study the microstructurc of the recycled material. From the experiments conducted, the chemical digestion method was found to be an unsuitable method for recovering the PEEK and carbon fibre because it used large quantity of corrosive sulphuric acid and the yield was low. Furthermore the method separated the fibre from its matrix and these will only be useful as a feedstock if they were mixed with virgin PEEK material. The comminution method was found to be favourable especially using injection moulding method to reprocess the recycled material. In the comminution method, fibre damage from the granulation process was observed. Amount of damage inflicted at the fibre's ends seems to be independent of the fibre length. Using the granulated material as feedstock, injection moulding was preferred over compression moulding. This was because in injection moulding, though fibre breakage might be high, the ability to align the fibre in the flow direction had significantly improved its mechanical properties compared to the compression moulded specimens. With the presence of voids and random fibre orientation in the compression moulded specimens, mechanical properties were compromised. But given the simplicity and convenience of compression moulding, this method of reprocessing can still be attractive for products which are anisotropic and do not require high mechanical properties. Simple models such as the Cox shear-lag and the Halpin-Tsai models were used to predict the tensile modulus of the recycled material. Using these models, effects from fibre length degradation, fibre orientation and interfocial degradation were quantified. Though the critical length of the composite was exceeded by the fibres from the recycled material and together with the excellent alignment of fibres, the full theoretical strength of the composite was not realised. This was to be expected, as the material used was recycled. The lower mechanical properties were probably due to the degradation in the interfacial strength between the fibres and the matrix. By isolating the effects of fibre length and orientation, it was observed that the influence of the interface limited the tensile modulus to just 50% of that estimated using the Cox's model that assumed perfect bonding at the interface. The influences of processing variables, in this case the injection pressure, on the mechanical properties of the recycled CF/PEEK were studied. It was found that generally an increase in injection pressure led to better mechanical properties. The higher injection pressure led to better fibre alignment and higher fibre length degradation. From the measurement of the fibre pulled out, the interfacial strength was observed to improve with higher injection pressure.