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Review RSC Advances Fig. 11 The diffusion mechanisms are illustrated: (a) micro-cracks present in resin; (b) water molecules reaching in the fibre–matrix interface, and (c) filling the hollow part of the flax fibre lumen. Diffusion phenomenon occurs also through the direction of fibres; (d) water molecules ingress by capillarity through the micro-cracks present at the fibre–matrix interface and through lumen; (e) micro-cracks present in resin and at the fibre matrix interface; (f) fibre swelling and matrix radial cracking. Reproduced with permission from ref. 68 [license number: 4992440237683]. information. Most plant bres are based on cellulose and lignin that are vulnerable at high temperatures. The thermal suscep- tibility of natural bres represents the rst limitation in bio- composites manufacturing. There have been extensive studies on how temperature affects wood products, but mostly con- cerned with pyrolysis. For example, Shen and Gu69 analysed products evolving during heating but focussed on high temperatures, above 400 C. George et al.70 describe a study on untreated (green) and treated (Duralin process, 180 C treat- ment) ax bres. The treatment increases the bre stability but above 200 C lignin degradation starts, and at higher temper- atures other polysaccharides, mainly cellulose, are oxidised and degraded. They concluded that the maximum processing temperature for ax bre composites is around 220 C. Gassan and Bledzki71 studied the thermal degradation of jute and ax bres at temperatures up to 210 C for up to 120 minutes. These authors found that temperatures below 170 C only slightly affect ax and jute bre properties, while temperatures above 170 C signicantly reduced bre tenacity and degree of poly- merisation. Chaishome et al.72 indicate that the decomposition of hemicelluloses in ax bres during thermal degradation is a factor, which will have a detrimental effect on the thermal stability of bres, particularly with slow heating rates. The second limitation on manufacturing temperature is the stability of the matrix polymer. Peelman et al.73 have provided data for several biopolymers based on PLA, polyhydroxyalkanoate (PHA), cellulose and proteins. PLA in particular has low thermal stability, and is particularly sensitive to hydrolysis.74 In a study of biocomposite manufacturing, Khanlou75 have described various degradation mechanisms, and proposed a thermo- kinetic model to account for chain scission of both ax bres and PLA at different processing temperatures. These models were then employed to predict composite tensile properties aer compression moulding. Baley et al.50 examined the inu- ence of drying on single bre and unidirectional epoxy composite properties. Drying bres for 24 hours at 105 C was shown to reduce both failure stress (44% on average) and failure strain (39%) signicantly, and these reductions were transferred to dried composites. Finally, it is interesting to note that ax bres have been shown to exhibit anti-oxidant prop- erties when used to reinforce polyethylene.76 It is evidence from the above discussion that biocomposites reinforced with cellu- lose and lignocellulose bres undergo to different thermal degradation processes in which temperature plays signicant role. The reason being these reinforcements along with their morphological structures that inuences the thermal stability. 2.7 UV degradation The challenge of replacing synthetic materials with natural materials has encouraged major R&D contributions to justify the selection of natural bres as reinforcement for polymer- based composites. In parallel, the increase in demand for natural bres such as hemp, ax, kenaf, jute and bamboo manifest a clear interest from the industry to introduce alter- natives to traditional synthetic reinforcing materials. Indeed, production and related energy consumption of synthetic bres © 2021 The Author(s). Published by the Royal Society of Chemistry RSC Adv., 2021, 11, 32917–32941 | 32925PDF Image | Long-term durability and ecotoxicity of biocomposites in marine environments
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