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RSC Advances Review can exceed by 60% that required for natural ones, as reported for the case of the glass bres.77 The degradation of natural reinforcement is also attracting the attention of materials scientists aiming to solve critical issues related to the use of biobased reinforcement and related moisture, thermal, re, and ultra UV degradation mechanisms. The intrinsic hydro- philicity of natural bres can lead to inferior interfacial bounding to the polymeric matrices, which are mostly hydro- phobic.78–81 Biodegradability is a degradation process that can be particularly inuenced by the light-degradation or photo- degradation rate of the material of interest. In general, poly- mers are destabilised by sunlight or by articial light, and UV degradation tests are performed to severely damage the mate- rials. The constantly growth of CO2 in the atmosphere is responsible for a major impact of the UV rays that arrive with a mean wave length of 280–320 nm, representing a threat not only for construction materials but even for highly organised biomaterials such as skin.82 Photodegradation of natural composites is mostly promoted by weathering periods, and outdoor exposure.83 The mechanical strength of natural bre composites can be affected as a result of photodegradation processes that act over several length scales within polymers. Critically, degradation due to oxidation acts rst on the surface because of the major exposure of the surface to oxygen. Cracks are activated by mechanical stresses within inhomogeneous regions dened by different molecular weights. Indeed, UV radiation absorbed by the polymer can start molecular chain scissions, followed by crack opening, light penetration and degradation of the global material mechanical behaviour.84–86 The increasing surface area due to the chain scission, encour- ages further degradation. Moreover, different environments present several factors that promote microbial assimilation of plastics. Water, salty water, soil and landll are environment in which UV light can activate their degradation mechanisms depending on the penetration level. For example, in landll conditions there is limited availability of UV light and oxygen given the dark and anaerobic nature of this environment.87 Previous studies addressed the UV degradation as a chain reaction which can be initiated by a prior exposure of the polymer to UV light88 and that can continue even in its absence.89,90 An environment such as water presents elements that accelerate the polymer degradability due to the joint effect of UV light, oxygen and moderate temperatures.91 Nevertheless, the seawater environment has lower concentrations of micro- organisms, therefore slows down the hydrolysis rate of poly- mers, together with the ability of these microorganisms to colonise the plastic surface.92 Natural bre polymeric compo- nents are vulnerable to UV light degradation. In particular lignin, that is the main polymeric components in natural bres, absorbs up to 80–90% of incident light. Surface oxidation, matrix crystallinity alteration and interfacial degradation compromise the mechanical properties of natural bre composites.93 Moreover, further studies addressed how water spray cycles combined with UV radiation foster photo- degradation activities leading to mechanical degradation.84,85 Research contributions towards the stabilisation of lignin within the cell wall or extracellular matrix are crucial to improve the overall biocomposite resistance to ultraviolet radiation. Moreover, additives can be included in bre reinforced poly- mers, which are subject to colour changes if exposed to UV rays. The colour reects undergoing chemical changes, beneath the surfaces of composites where the degradation of the bres is happening. Polymer coatings able to reect UV radiation can be developed by using cold plasma modication. Biocomposites suffer from oxidative degradation and antioxidants can be used to make them more competitive in terms of durability and mechanical properties. Addition of antioxidants and re retar- dants could improve the tensile, exural and impact strengths but at the expense of the wettability of the biocomposites that absorb more water.94 Most studies are conducted under accel- erated conditions in order to monitor the degradation rate. Reproducing boundary conditions similar to nature implies years of experimentation. Therefore, UV lighting can be used in conjunction with incubation at different temperatures to start polymeric chain breakdown and polymer degradation.90,95–98 3. Assessment of leaching substances from biocomposites to the marine environment 3.1 Plastic additives used in biocomposites Plastic additives are chemical compounds incorporated in the polymer matrix to polymerise, process or modify the properties of polymers.99 Common plastic additives can be categorised as: functional additives (e.g., plasticisers, ame retardants, stabil- isers and lubricants), colorants (e.g., pigments), reinforcements (e.g., glass bres, hemp bres), and lters.100 To date, 418 substances have been recognised as plastic additives and registered under the REACH (Registration, Evaluation, Autho- rization and Restriction) regulation of the European Union.101 In addition to their functional or visual role, additives are also applied to improve the performance of nal applications during the manufacturing of biocomposites and biopolymers (repre- sentative additives summarised in Table 1). For example, biopolymers like PLA and poly(3-hydroxybutyrate) (PHB) can be too hard, brittle or fragile to be used directly in nal applica- tions, but the inclusion of plasticisers improves their exi- bility.102,103 Additives used for biocomposites are usually natural- based plasticisers like polyols, citrate esters, monosaccharides, fatty acids and vegetable oils.104 These additives are used instead of conventional substances, as they have a lower potential for toxicity to the environment and organisms (Table 1). Similarly, synthetic antioxidants such as butylated hydrox- yanisole (BHA) and butylated hydroxytoluene (BHT) are usually alternated by natural-derived substances like gallic acids and a- tocopherol especially in biocomposites for packaging applica- tions.105,106 Apart from organic additives, a group of inorganic compounds are commonly included as ame retardants in biocomposite applications.107–109 The majority of substances used as additives in biocomposites have either been registered or pre-registered in the REACH regulation. Most of these substances are readily biodegradable and are not considered harmful to aquatic organisms, indicating a potential low impact 32926 | 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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