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Title: The processing and characterisation of recycled glass fibre composites
Author: Nagel, Ulf
ISNI:       0000 0004 5991 6830
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2016
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The widespread use of glass fibre reinforced polymers (GFRP) over the last decades has led to an increasing amount of waste and a demand for recycling solutions. Mechanical, chemical and thermal GFRP recycling processes have been developed in academia but the commercialisation of GFRP recycling processes has proven to be difficult. The value of recycled glass fibres is low because their strength is usually reduced during recycling processes. Thermal recycling processes involve the exposure of GFRP waste to elevated temperatures to degrade the polymer matrix and extract the glass fibres. The room temperature strength of the glass fibres and organic sizings are degraded during thermal recycling processes which leads to a lower composite performance when the recycled fibres are processed into composites. The main objective of this thesis was to develop composites based on thermally recycled glass fibres that can compete with as received fibre composites. Polypropylene (PP) was chosen as matrix material because of its processability and its widespread use. First, an understanding of the structure performance relationship of glass fibre PP (GF/PP) composites was established and gaps in existing literature were studied. The addition of maleic anhydride grafted PP (MAPP) to injection moulded GF/PP composites correlated with an increase of the ultimate mechanical properties (tensile strength, strain at break) but too high MAPP contents resulted in a reduction of the ultimate mechanical composite properties and the composite modulus. The optimum MAPP content was observed to depend on the glass fibre content. The composite tensile test data was used to analyse the microstructural composite properties. It was concluded that the analysis of the composite tensile data based on the Kelly-Tyson model can be used as a screening tool to detect general trends of the interfacial adhesion between glass fibres and PP matrix. However, micromechanical tests like the microbond tests are still required for more detailed studies of the interfacial adhesion because the composite tensile properties are often affected by a number of interacting factors. Glass fibres were thermally degraded before composite processing to simulate a thermal recycling process. Thermal gravimetric analysis and microbond tests showed that all investigated commercial sizings degraded at thermal recycling temperatures. The ultimate mechanical composite properties were reduced when the glass fibres were thermally degraded. The drop of the composite performance was attributed to a reduction of the glass fibre strength and a low interfacial adhesion between fibres and PP. Most of the interfacial adhesion between thermally degraded glass fibres and PP was recovered when the MAPP content was optimised. However, the composite performance was still low compared to as received glass fibre composites because of the low fibre strength. It was concluded that the post treatment of thermally recycled glass fibres should include the regeneration of the fibre strength and the reactivation of the fibre surface functionality in order to increase the reinforcement potential of the fibres. Aminopropyltriethoxy silane (APS) and sodium hydroxide (NaOH) were applied to the thermally degraded glass fibres. Approximately 70 % of the tensile strength loss of injection moulded GF/PP composites was recovered when thermally degraded glass fibres were post treated with APS. Micromechanical analysis suggested that the APS regenerated glass fibres experienced similar stresses as the as received fibres. The increase of the glass fibre stress could not solely be explained by an increase of the interfacial adhesion between fibres and matrix. It was proposed that the APS treatment also led to an increase of the maximum glass fibre strength in the injection moulded composites. Thermally degraded glass fibres tend to form a fluffy mat during the NaOH treatment and could not be processed via extrusion compounding and injection moulding. A glass mat thermoplastic (GMT) process was set up over the course of this PhD to process NaOH treated glass fibres. The treatment of the thermally degraded glass fibres with NaOH led to an increase of the composite tensile strength. The increase of the composite tensile strength was correlated with an increase of the glass fibre strength. The APS treatment of the thermally degraded glass fibres also led to an increase of GMT composite tensile strength but the increase was lower compared to the injection moulded GF/PP composites. It was speculated that the effect of APS on the strength of thermally degraded glass fibres might be gauge length dependent. The optimum treatment of thermally recycled glass fibres might therefore depend on the final fibre length in the composite. An APS treatment might be suitable for injection moulded composites with glass fibres shorter than 1 mm while the NaOH treatments might be required for composites with longer fibres. To the author’s knowledge the work presented in this thesis shows for the first time that the reinforcement potential of thermally degraded glass fibres can be regenerated without hydrogen fluoride. Thus, this work might be a step towards the development of an economically viable GFRP recycling process.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available