Recycling fibres recovered from composite materials using a fluidised bed process
This work is concerned with the characterisation and reuse of fibres recovered from end of life thermoset composites by fluidised bed thermal processing. Emphasis is placed on the properties of recovered glass fibres and their reuse in a dough moulding compound and a tissue product. The need for a recycling process is demonstrated by a survey of the market for thermoset composites with particular consideration given to the potential growth in automotive applications. A study of processes developed to recycle such materials and the effect on the structural properties of composites containing such recyclate shows that a process capable of generating recyclate of greater value is required, particularly for the case of contaminated materials. A review of investigations into the effect of prior heat treatment on the mechanical properties of glass fibres shows that their strength will be reduced by a deterioration of the fibre surface during fluidised bed thermal recovery but that their stiffness is likely to be unaffected. The implications of these effects on possible applications for the recovered fibres are described. Tests are reported on the structural properties of laminates containing heat treated glass cloth which show that there is a strength reduction which is both time and temperature dependent but that modulus is not significantly affected. Strength loss increases with up to 20 minutes heating but then remains constant. In some cases the strength loss can be reduced by resizing the cloth. Measurement of the properties of fibres recovered using the fluidised bed process shows that the recovered fibre strength decreases with increasing processing temperature. Glass and carbon fibres recovered at 450°C retain approximately 50% and 80% of their virgin strengths respectively. The Young's modulus of the recovered fibres is shown to be largely unaffected by the process for both glass and carbon fibres. Investigation of the lengths of the recovered glass fibres by image analysis techniques suggests that their distribution depends on the structure of the composite and the method of size reduction. A weighted mean fibre length of 3.5 mm to 5.5 mm is possible with the apparatus described in this thesis. The reduced reinforcement potential of the recovered fibres is confirmed by experiment. Recovered glass fibres are used to displace virgin fibres in a dough moulding compound. At replacement levels of up to 50% there is no significant effect on the mechanical properties of a compression moulded plate. Above this level, tensile, flexural and impact strengths are reduced and with complete replacement the reduction is by approximately 40%, 50% and 70% respectively. These results are confirmed on a pilot plant scale. Recovered glass fibres can be successfully incorporated with virgin fibres into a veil. Replacement of virgin fibres reduces both wet and dry strength because the recovered fibres are both shorter and weaker. The permeability of the veil is found to depend on its porosity and not its recovered fibre content. The experimental veils can be used to protect and improve the surfaces of mouldings.