Recycling high performance carbon fibre reinforced polymers using sub- and supercritical fluids

Keith, Matthew James (2020). Recycling high performance carbon fibre reinforced polymers using sub- and supercritical fluids. University of Birmingham. Ph.D.

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Abstract

The research presented in this thesis considers the solvolytic recycling of high performance composite materials using an acetone / water solvent mixture, supplied both neat and in conjunction with alkaline and weak-Lewis acids. The neat solvent was capable of recovering clean fibres within 120 and 90 min when heated to 320 and 330˚C respectively. The alkaline catalysts did not accelerate the decomposition of the matrix due to a reaction with acetone thus eliminating one of the solvents from the system. ZnCl2, MgCl2 and AlCl3 all facilitated a reduction in the process temperature necessary for the complete elimination of the resin of 40˚C.

A kinetic study of all solvent systems was carried out using two different approaches: a conventional, first order rate equation and a shrinking core model (SCM). Both models were successfully fitted to the experimental data and were able to predict the decomposition of the resin to within ± 10% of the measured value. ZnCl2 appears to be the most effective catalyst, reducing the activation energy, EA (kJ mol-1), of the degradation reaction by up to 29% compared to the neat solvent mixture.

Characterisation of carbon fibres demonstrated that at temperatures of less than 330˚C, there was no statistically significant change in the tensile strength or modulus. Higher process temperatures, and the presence of AlCl3, caused a reduction in strength of up to 12.4%. ZnCl2, however, caused an increase in both the tensile strength and modulus of up to 22%. Analysis of the fibre surface showed a reduction in the relative abundance of oxygen for all conditions investigated which may correspond to the loss of the sizing.

Fourier transform infrared spectroscopy (FTIR) of the dried organic products demonstrated that both the neat solvent at temperatures in excess of 320˚C and the metal chloride systems successfully cleaved the C-N bonds within the epoxy resin. These products were further analysed with gas chromatography-mass spectrometry (GC-MS) which demonstrated that the resin was predominantly decomposed to benzene and phenolic derivatives.

The findings discussed throughout this work describe and analyse a novel CFRP recycling process. The use of chloride catalysts facilitated a significant reduction in the operating temperature required while characterisation of the recovered fibres demonstrated that, under certain conditions, there is no change in the mechanical properties. Furthermore, analysis of the organic resinous products revealed the presence of a mixture of potentially useful compounds, which may have further use in a new polymer matrix. It is hoped, therefore, that this research contributes towards the circular economy through the effective recycling of CFRPs.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):