The influence of different chemical treatments on the mechanical properties of hemp fibre-filled polymer composites

Mayembo, Evrard

The influence of different chemical treatments on the mechanical properties of hemp fibre-filled polymer composites

Master Thesis

2021

Abstract

The fluctuation of engineering and general-purpose polymer prices, rapid exhaustion of fossil fuel world-wide reserves and heightened awareness about environment have led the research community to explore the use of natural biodegradable raw materials as substitutes for manmade resources. Natural fibres are considered as substitutes for synthetic fibres in reinforced polymer matrix composites. Increased interest has been shown in natural fibres from plants such as cotton, jute, hemp as replacements for aramid, glass, and carbon fibres. This is due to their biodegradability, low cost, low density, and satisfactory strength to weight ratio. However, they present certain disadvantages compared to synthetic fibres which include high moisture sorption rates, low durability, and weak fibre/matrix bonding strength. The poor adhesion between natural fibres and polymer matrices leads to poor mechanical properties for natural fibre reinforced composites. Improvement of the fibre/matrix interface is required to increase the mechanical properties of the natural fibre filled polymer composite In this study, the influence of selected chemical treatments on the mechanical properties of hemp-filled epoxy composites was investigated. The aim of this study was to enhance fibre/matrix interface and hence the mechanical properties of hemp yarn-reinforced epoxy composites by modifying the chemical nature of a high crystallinity hemp yarn through chemical treatments such as alkalization, silanization (3-aminopropyltriethoxysilane) and a maleic anhydride treatment. The effectiveness of the chemical treatments was assessed by means of XRD, FTIR and TGA. Density measurements of as-received yarns (1.42-1.45 g cm-3 ) were within the range reported in the literature. Crystallinity measurements revealed the astreated yarns as having high crystallinity indices (87% weft and 84.7 warp yarns). The surface treatments used increased the crystallinity index only slightly. A decision was taken to use warp yarns (UTS = 799 MPa) rather than warp yarns (UTS = 503 MPa). Silane treatment reduced the tensile strength of yarns slightly (753 MPa) while the treatment of the fibres with maleic anhydride (562 MPa) and alkali treatment (518 MPa) had a much more significant effect on ultimate tensile strength. By contrast the modulus of the treated yarns all increased compared to the as-received yarns. Silanization was confirmed by energy dispersive X-ray spectroscopy while maleation was confirmed by the presence of characteristic absorbances in FTIR spectra. TGA revealed that silanization improved fibre thermal stability while maleic anhydride treatment did the opposite, possibly due to decarboxylation reactions. Four type of fibre/matrix interfaces, based on the treated and non-treated fibres, were generated through the production of the hemp reinforced epoxy composite plates. The results showed insignificant variations in the mechanical and thermal properties compare with the as-received hemp-filled epoxy composites which showed the high mechanical properties and thermal stability. The silanization and alkalization slightly decreased the properties of their respective properties although this was deemed statistically insignificant. The maleic anhydride treatment worsened the mechanical properties significantly. Scanning electron microscopy revealed appreciable fibre-matrix debonding which is indicative of a weak fibre/matrix interface. This was postulated as a reason for the lack of any significant reinforcement of the epoxy composites by maleic anhydride treated fibres. The tensile properties were also predicted and no statistically significant differences were observed although the experimental strengths values appeared to be lower than the predicted strengths. In general, the lack of appreciable improvement in mechanical properties of as-received fibres was concluded to be due to the initially high crystallinity of the as-received fibres. This provided little scope for further alkalization to change the surface significantly as little further removal of hemicellulose and lignin could occur.

Keywords

Engineering

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