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Review RSC Advances properties, especially in a marine environment where major concerns are related to biocomposites long-term durability and life time prediction.199 Within two years, the tensile strength of jute/phenolic composites can be reduced by 50% in outdoor conditions. Fibre/polyester composites suffer less from weathering effects, with a strength loss that can vary between 5-25% and more in water conditions.200,201 Moreover, structural properties of biobased materials can be affected by fungus and bacteria development. Humid conditions facili- tate the formation of fungus aer a few days of moisture exposure, as observed in the case of ax bres that improved their environmental durability with the application of Dura- lin. Duralin ax bres show reduced moisture uptake thanks to the process that transforms lignin and hemicellulose into lower molecular weight compounds to be cured into water resistant resins.202 In contrast with ax, which falls into the steam bres category together with hemp and jute, leaf bres like abaca and sisal demonstrate better suitability in seawater applications. Sisal bres perform very well in addition to glyoxal phenolic resins to form thermosets that with 30% of reinforcement by weight improved the impact strength of one order of magnitude with respect to the glyoxal-phenolic matrix. Degradation mechanisms must be avoided at different length scales and dedicated surface modications can be listed depending on the approached scale level. A hierarchical classication of possible surface treatments would help in the selection of appropriate procedural steps to transform raw materials into superior performance composites. On one hand surface treatments can be applied at system level with the help of coatings or barriers against wettability. Inspiration can be taken from the delicate case of paper, an excellent biobased material that requires control over its intrinsic hydrophilicity and porosity in order to preserve shape and mechanical function during the product lifetime. Nowadays, paper coatings are mainly fossil-oil based as in the case of polyolens (polyethylene), waxes, ethylene vinyl alcohol, polyvinylidene chloride that provide a signicant barrier against water and oxygen permeability. Examples of biopolymer lms and biobased coating substi- tutes can be found in the literature such as polysaccharides (starch and cellulose derivatives, chitosan, and alginates), proteins (casein, whey, collagen, soya, and gluten), lipids (bees and carnauba wax, and free fatty acids) and polyesters PHA and PLA.203,204 Bioinspired solutions can be achieved by adopting superhydrophobic PPS/PTFE composite coatings which have attracted research interest for their high inter- facial strength, excellent impact resistance and high thermal stability205 However, the surface morphology characterisation by SEM showed that these superhydrophobic coatings have similar macro–nano-structures to that of lotus leaf, hence classiable as biologically inspired. Another way of approaching this problem is using a melting-blend process to produce biodegradable composite lms, commonly used in the packaging engineering eld. Hybrid composites made of traditional polymer matrix (PP), biodegradable polymers (PLA) and nanollers (NANO-TiO2) were manufactured to promote biocomposite resistance to both UV light and water penetration. It has been shown that maleic anhydride (MHA) increases the interfacial compatibility between PP and PLA, showing increased crystallinity and thermal properties. Critically, PP/PLA/MHA lm composites show increased tensile strength and elastic modulus by 100% and 140% respectively, when compared to PP/PLA based lms. A 1% wt concentration of NANO-TiO2 allowed a UV transmittance that is almost zero in the PP/PLA/MHA composite. Although this approach was intended for packaging engineering, similar solutions can be transferred to other applications such as marine structures.82 At a lower level, modifying the bre surface for a better compatibility and consequential adhesion with the matrix is preferred. Chemical treatments such as alkalization, bleaching and silanes are promising techniques that would allow improved mechanical transverse strength and stiffness of biocomposites.206–208 Alkali treatments aim to increase the contact area at the interface between bre surface and polymeric matrix. Chemical treatments such as this rely on the induced interlocking mechanisms due to a rougher, but bondable bre surface. Surface treatments can act at molecular level developing an intermediate region between bres and matrix. The intermediate elastic modulus of this third phase gradually varies so that a toughening mechanism takes place on the basis of an improved interplay between matrix and bres. Limited studies have investigated the chemical bridges formed when using silanes as a com- patibiliser. The matrix/bres bonding of resin-based composites is at risk during the curing process of the resin or when heat is applied while manufacturing the composite. Moisture would eventually reach the surfaces of the bres leading to the formation of voids when it evaporates.209 However, natural bres cannot withstand high temperatures, with 180–200 C a limit above which mechanical properties loss is guaranteed. 6. Conclusions and future perspectives 6.1 Perspectives and foreseen challenges of biocomposites for marine environments Biobased polymers and composites are new classes of light- weight sustainable materials suitable for use in marine, auto- motive, aerospace and other lightweight applications. In terms of their specic application in the marine sector, understanding their long-term durability under harsh marine environments needs to be fully analysed and appropriate methods need to be developed to improve their service conditions. Reducing the environmental burden caused by the use of non-sustainable composite materials in marine sector is an emerging issue. The development and use of sustainable biobased composites as a replacement for non-renewable conventional bres reinforced glass and carbon bre composites offers a viable materials alter- native with low cost, sustainable and recyclability attributes. However, it is important to recognise that these sustainable biobased composites also present challenges including that of meeting the functional requirements, understanding © 2021 The Author(s). Published by the Royal Society of Chemistry RSC Adv., 2021, 11, 32917–32941 | 32935PDF Image | Long-term durability and ecotoxicity of biocomposites in marine environments
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