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RSC Advances Review Effects of moisture absorption on the tensile strength of hemp/ UP composites. Reproduced with permission from ref. 21 [license number: 4992421301110]. to elevated temperatures. The inuence of these conditions is more prominent for natural bre composites than for glass or carbon bre composites. The following sections review some of the published work covering these aspects of natural bre composites and biocomposites. 2.4.1 Effects of moisture absorption on the mechanical properties. The effects of the temperature on the mechanical properties of hemp bre reinforced unsaturated polyester (UP) composites was investigated at room and elevated tempera- ture.21 The report highlighted rst that the presence of hemp bre reinforcement contributed to an improvement of mechanical properties of the UP matrix. The mechanical prop- erties (tensile and exural) of hemp/UP composites were signicantly inuenced by the moisture absorption at room and elevated temperatures. The saturation moisture uptake and moisture diffusion coefficient were reported to increase with the increase in temperature due to the creation of increased micro- cracks at higher temperature. The reduction in mechanical properties was attributed to weak bre matrix interfaces (dete- rioration of bre/matrix bonding) induced by the moisture ingress (Fig. 4). Furthermore, bre swelling induced by mois- ture absorption accentuates debonding. There are many re- ported works suggesting that moisture absorption decreases mechanical strength and modulus but increases ductility due to plasticisation effects. To minimise the water immersion test time, accelerated water absorption measurements at higher temperatures are usually considered. The moisture ingress at elevated temperatures can cause swelling, cracking, and further debonding resulting in a signicant loss of structural integrity. Work reported by Chang et al.62 on jute/PLA highlighted that the interfacial shear strength was signicantly reduced aer immersion in boiling water. This was attributed to the swelling as well as deterioration of the bre/matrix interface adhesion. The strength reduction evolution has been assigned to three different stages (Fig. 5). It is evident from the above discussion that moisture ingress has a negative effect on mechanical properties. Nevertheless, if available material improvement techniques are applied the negative inuence on mechanical properties from moisture attack can be minimised. 2.4.2 Techniques to improve mechanical properties and water repellence behaviour of composites. Different bre surface treatment techniques have been employed to improve the water repellence behaviour and to enhance the bre–matrix interfacial properties of natural bre composites to be used in marine applications. These various treatments promote strong interfacial bonding which is critical to achieve optimal mechanical properties. Various treatments remove impurities and hydrophilic components present on bre surfaces. Addi- tionally, these various treatments improve the wettability of bres, which in turn provides better interfacial bonding. One of the techniques used to measure the wettability bres by matrix polymers is contact angle measurement, which provides infor- mation on surface energies. In recent years hybrid techniques, based on the synergistic effects of two materials in single composites, has been Fig. 4 Fig. 5 Reduction of tensile properties of jute/PLA biocomposites after ageing 50 C (a) tensile strength against weight gain as a function of immersion time (b) effect of ageing degradation duration on stress-curves (source: B. P. Chang, A. K. Mohanty and M. Misra, RSC Adv., 2020, 10, 17955–17999). 32922 | RSC Adv., 2021, 11, 32917–32941 © 2021 The Author(s). Published by the Royal Society of ChemistryPDF Image | Long-term durability and ecotoxicity of biocomposites in marine environments
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