| Summary: | The processing and reuse of end-of-life composite products in an environmentally friendly manner is one of the most important challenges facing the industry and community. The development of an economically viable process for regenerating the properties of thermally recycled glass fibres (GFs) would have major technological, societal, economic and environmental impacts. The ultimate goal of this project is to enable cost-effective regeneration of the mechanical properties of GFs which have been produced from thermal recycling of end-of-life glass reinforced structural composites from automotive and wind energy applications. This work investigates the loss, and regeneration, of GF strength after thermal degradation at typical GRP recycling temperatures. The mechanical properties of APS sized and water sized (uncoated) Boron-free-E-glass fibres were first characterised using a conventional single fibre tension test. A substantial higher average strength was obtained from the APS sized fibres. Further investigation with GFs, coated with different silanes was carried out to determine any beneficial effect on GF mechanical properties. It was found that ɣ-Methacryloxypropyltrimethoxy Silane (MPS) and ɣ-Glycidoxypropyltrimethoxy Silane (GPS) prepared for 24 hours at RT and solution medium pH 5-5.5 in deionised water and ɣ-Aminopropyltriethoxy Silane prepared at 830C for 5 hours at solution medium natural pH (ET APS) in deionised water showed the higher increases in GF mechanical properties. Moreover, it was found that preparing the hydrolysed APS solution at elevated temperatures, had a beneficial effect on GF strength in comparison to a solution prepared at RT over 24 h (RT APS). Further investigation was carried out to identify any changes to the final products in solution and any structural differences between APS prepared at RT and at ET, that would lead to a conclusion about the difference in mechanical properties achieved. Using techniques such as Nuclear Magnetic Resonance (1H NMR) and Fourier Transform Infrared Spectroscopy (FTIR), several differences were identified. The results showed that ET APS contained less ethanol in solution after preparation, showing a relationship between the preparation temperature and the ethanol lost from the solution. On the other hand, the FTIR spectra indicated a higher polymerisation level of the ET APS, which suggested that a higher polymer may positively affect the mechanical performance of GFs. Hydrolysis at elevated temperature was found to be a novel and relatively easy way to prepare APS that improves the beneficial effect on GF strength. Thermal degradation of the APS sized Boron-free-E-glass fibres was also investigated across a wide range of temperatures. The results suggested that at temperatures around 3500C, the APS coating on the GFs surface starts to degrade and disappear, consequently reducing the protection provided by the APS layer. The effect of high temperature on GFs also creates cracks and flaws that may also contribute to the strength loss seen in these results which are consistent with the creation of defects for high temperature. Two other chemical treatments were investigated for their ability to regenerate the mechanical properties of the thermally conditioned GFs. An acidic treatment with hydrochloric acid (HCl) 37% v/v did produce a small increase of the average fibre strength, whilst a base treatment using concentrated sodium hydroxide (NaOH) solution at high temperature (950C) substantially improved the mechanical properties of thermal conditioned GFs, achieving up to 200% increase in fibre strength in comparison with the thermally degraded GFs. The optimum NaOH treatment conditions were further characterised in terms of treatment time, NaOH concentration and GF surface state and the effect on GF surface (i.e. OH groups on the surface) and reactions that occur with the glass. It is concluded that a number of very promising treatments with the potential to regenerate the mechanical properties of GFs recycled from composites have been identified.
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