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RSC Advances Review biopolymers and composites. The degree of biodegradability and biocompatibility of contemporary biopolymers, as well as the interaction with natural bres, are addressed hereaer, considering exclusive key factors crucial to the moisture uptake of these materials in marine applications. The goal of this review paper is to highlight benets for the industry and some key issues of biopolymers and composites materials together with their long-term durability in abiotic conditions; moreover, further recommendations on the type of biocomposites that are more suitable for marine environment will be provided and presented by critically analysing current available literature. Additionally, this work will provide an overview on biobased composites, ecotoxicological effects, related critical issues and important properties (mechanical, long-term durability and environmental) in recent marine applications. 1.1 The marine environments and their effects on service conditions The marine environment combines a number of factors, general or specic, which induce stress and pressure leading to changes in the mechanical and physical properties of natural bre reinforced and polymer matrix composites. The most obvious is exposure to liquid water, either through exposure to high rela- tive humidity or complete seawater immersion. The moisture absorption and humidity are common to many composite applications but the seawater immersion is a specic marine feature and requires particular attention. Continuous exposure to UV radiation may also degrade polymers and needs to be considered. Other environmental parameters such as temper- ature are within commonly accepted ranges, typically from around 5 C in deep sea to a maximum around 50 C at the surface. A specic marine factor is marine fouling, which involves the adhesion of marine growth to structures in shallow water. Another specic factor to consider is mechanical loading. Wave action periods are typically around 10 seconds, so marine structures can accumulate large numbers of cyclic loads leading to fatigue. Underwater applications are subjected to hydrostatic pressures, which increase with immersion depth so compres- sion behaviour is important. Lastly, the coupled effects of seawater immersion and mechanical loading require complex interactions to be considered.15–19 Material requirements for marine structures depend on where and how the materials are deployed. Overall, we consider two categories, surface vessels and underwater structures. Surface structures such as boats, buoys, and other oating systems are partly immerged and partly exposed to sunlight. In addition to these two environments an intermediate splash zone can include the worst of both, with parts of the structure continuously drying then re-wetting. This is a critical zone for metallic structures, where corrosion rates are highest20 and thus it is particularly attractive for composite solutions. For all long- term oating applications protective coatings are used, anti- fouling paints in particular and these will affect both dura- bility in service and end-of-life options. Underwater structures include a range of structures from oceanographic equipment to sub-sea offshore equipment and submarines. While composites are less widespread than for surface applications their use is increasing as renewable energy developments advance and deep-sea exploration and oil and gas exploration extend to deeper water. The design environment is more easily dened than for oating structures with pressure due to immersion depth the main parameter to consider. A typical service life for an offshore structure is 20 years so detailed knowledge of material behaviour is needed.16 2. Understanding hygrothermal ageing of biocomposites for extended service life under marine environments 2.1 Effects of seawater immersion on the mechanical properties Biocomposites are more environmentally sensitive than conventional composites.21–23 To understand the different degradation mechanisms, the effect of water on the mechanical properties of each independent component, i.e., the bres, the matrix, and the bre/matrix interphase region, as well as the biocomposite should be studied. Hydroscopic ageing of the polymers involves complex degradation phenomena. Two main degradation processes can be induced; physical ageing (matrix swelling and plasticisation) and chemical ageing (hydrolysis, oxidation).15 The changes in properties can be either reversible or permanent, oen resulting in a drop in the mechanical properties: for example, Deroin ́e et al.24 have performed a one year accelerated ageing study on poly(3-hydroxybutyrate-co-3- hydroxyvalerate) (PHBV) with tensile property characterisation. These authors observed that at a rst stage of the immersion the PHBV tends to recover its initial mechanical properties aer drying, showing a reversible phenomenon due to plasticisation. But aer one year of ageing at 40 and 50 C the behaviour evolved towards irreversible degradation with a decrease of strain and stress at break, a phenomenon correlated with hydrolysis of the polymeric matrix.24 The mechanical properties of natural bres show variable responses to environmental stresses, depending on many environmental parameters.25 Controlling these parameters is not possible, so this can lead to uncertainty in bre performance and their mechanical behav- iours. To avoid uncertainty, key design criteria can be dened. For instance, Singh Virk et al.26 have proposed the failure strain criteria. The structure of the bre is also a key parameter which inuences the moisture absorption. The materials that compose the bres have a distinct hygroscopic nature, that fosters the loss of mechanical properties, further accentuated by their composition (cellulose, hemicellulose, lignin, pectin, wax content).22,27,28 Low humidity content is generally benecial for the mechanical properties of the bres; additional humidity plays a key role in their performance, for example, at high humidity level general mechanical properties drop.29 For bio- composites, the loss of mechanical properties due to hygro- scopic ageing is mainly due to the resin–matrix interface. Dhakal et al.21 have found that the tensile and exural proper- ties of hemp/polyester composites drop signicantly due to the degradation of the bre–matrix interface as a consequence of 32918 | 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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