Effects of chemical treatments on hemp fibre reinforced polyester composites

Natural fibres have recently become an attractive alternative to synthetic fibres in the implementation in polymer composite structures. This is due to contemporary environmental concerns such as climate change which have caused engineers to consider renewable resources in composite structures. Inherent flaws within natural fibres in terms of their constituent contents (hemicellulose, cellulose and lignin) reduce the compatibility of these fibres with polymer matrices. Fibre surface modifications using chemical treatments have the potential to improve fibre-matrix compatibility. In this study, the effects of chemical treatments on hemp fibres and the resulted polyester matrix composite are discussed. The fibres were treated with alkali (0-10% NaOH), acetyl and silane chemicals. The structural composition and thermal decomposition of the fibres after treatments were investigated through chemical analysis, fibre morphology analysis, FTIR analysis, and TGA and DSC analyses. Fibre structure and morphology analysis showed that the amount of hemicellulose and lignin constituents decreased in the case of alkalised and acetylated fibres. Conversely, silane molecules formed couplings on fibre surfaces and there was no changes observed in terms of hemicellulose and lignin contents. Thermal analysis revealed that, due to the presence of hemicellulose constituents, the untreated fibres had lower thermal stability. The tensile properties of treated hemp fibres were measured through DMA. Treated fibres exhibited lower strengths compared to the untreated fibres due to the removal of hemicellulose and lignin binder from their cellulose surfaces. Long unidirectional fibre composites and sandwich structures were tested using tension, compression, shear, bending and impact tests. In both composite cases, 4% NaOH treatments reduced interface bonding strength and decreased composite properties compared to untreated samples. At higher concentrations (6-10% NaOH), composite properties increased as a result of greater interface bonding. The opposite results were achieved in the case of alkalised fibres that were further treated with acetyl and silane treatments. Failure analysis of tested composites was undertaken through OM and SEM. Micrographs showed results which were in agreement which the properties achieved. Treated fibres were able to effectively improve the bonding properties of composites, even though the mechanical strength of these fibres could be decreased. Thus, the use of chemical treatments on fibres can be justified as overall composite mechanical properties increased when compared to untreated cases.