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Review RSC Advances Grove49 consider the conditioning of the reinforcement bres before composite manufacture to be essential. Baley et al.50 have performed an extensive study on the drying of ax bres and its impact on the composite properties. It appears that excessive drying of the bres results in a loss of biocomposite properties. Van de Velde et al.51 have shown that boiled ax can improve composite properties. Van den Oever et al.52 show that sepa- rating ax bres into elementary bres improves properties. The matrix itself also ages and may be degraded with time. For exemple, Deroin ́e et al.24 have shown that the main abiotic degradation mechanism of immersed PHBV, a biodegradable biopolymer, is hydrolysis. Water diffusion in polymers is a complex phenomenon and it has been widely studied.53,54 Suit- able matrix selection has to include durability considerations, as well as manufacturing and end of life options. Manufacturing adjustments can also provide a way to prevent the degradation. Le Duigou et al.30 have added extra layers of PLLA on the surface of ax/PLLA composites, they showed that these extra coatings can signicantly slow down the water absorption. Van de Velde et al.51 have studied the inuence of different process parameters on press moulding of ax/polypropylene composites: bre orienta- tion, pressing temperature and time all affect the nal product. The manufacturing parameters such as temperature and pressure should be chosen carefully with respect of the bre used and the polymeric matrix to avoid degradation. It is well accepted that the bre orientation has a considerable impact on water absorption. The service conditions will also inuence ageing and degradation. Zhang et al.29 have studied the inuence of the relative humidity, and found a drop of properties of their ax/polyester composites around 70% RH. Complete immersion with accessible bres at sample edges is therefore a very aggressive test condition for bio- composite ageing. Biocomposites based on wood have been used successfully in shipbuilding for centuries, and are still employed today but if we limit the denition of biocomposites to continuous natural bre reinforced polymer composites then the marine applications are more limited. One of the rst applications was for surf and paddle boards. For example, the ECOBoard project55 started in 2012 with the aim of making suroards more sustain- able by paying more attention to the materials used and their carbon footprint. In parallel, there have been several prototype boats projects. The NAVECOMAT study (2007–2011), resulted in the manufacture of a ax/PLA canoe demonstrator (Fig. 2).56 Fig. 2 Flax/PLA canoe manufactured during the NAVECOMAT project. Other ax reinforced canoes have also been proposed in Europe, for example by Flaxland57 and Lake Constance Canoes.58 A prototype 6.5 meters racing yacht Araldite with 50% ax reinforcement and the remainder in carbon bres was developed and launched in 2010, competing in the 2011 mini- transatlantic race.59 A prototype 7 meters multihull entirely reinforced with ax bres was designed by Kairos and built by the Tricat boatyard using infusion (Fig. 3).60 It was launched in 2013 aer more than 3 years of material studies. Both multiaxial and unidirectional ax bres are used to reinforce a partially biosourced polyester matrix. Cork and balsa wood are used in the lateral and central hulls respectively. Tests performed on samples removed aer one year of navigation indicated no change or degradation in mechanical properties. Another ax bre boat project was described by Castegnaro et al.61 This used ax/epoxy in a balsa sandwich. No damage was noted aer 4 years of navigation. These results indicate that provided the matrix polymer can protect the bres from moisture then natural bre composites can be a good candidate material for marine applications. 2.4 Mechanical properties The mechanical properties of biobased composites are inu- enced by various abiotic parameters and conditions. With this in view, there has been a signicant amount of research and development work concentrated in achieving comparable mechanical performance of biobased composites with respect to conventional composites reinforced with glass bres. However, morphological structure, chemical composition and bre dimension variability can all play a contributing role in lowering mechanical properties for natural bre reinforced biobased composites. This variability can lead to further issues in sample fabrication and processing parameters. Another key issue faced by biocomposite materials is their long-term dura- bility in harsh environments. For example, natural bre rein- forced composites and biocomposites absorb moisture under various environmental conditions and this can be further accelerated by mechanical stresses, UV radiation and exposure Gwalaz, flax fibre reinforced trimaran. © 2021 The Author(s). Published by the Royal Society of Chemistry RSC Adv., 2021, 11, 32917–32941 | 32921 Fig. 3PDF Image | Long-term durability and ecotoxicity of biocomposites in marine environments
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