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Review RSC Advances moisture absorption. This phenomenon can pose a serious issue for natural bre composites to be used in marine appli- cations. In order to get a better understanding of the loss of properties, the interfacial degradation mechanism induced by water has been directly tested by Le Duigou et al.30 who studied the apparent interfacial shear strength of a single ax bre embedded in epoxy micro droplets. These authors found that for short immersion durations, the loss of mechanical proper- ties was reversible, but that for longer durations the loss was permanent, with the degradation involving the dissolution of bre constituents and at long immersion times the damage of microstructures. Espert et al.31 have also reported the same loss of adhesion between bre and matrix on a wood bre/ polypropylene composite. With Scanning Electron Microscopy (SEM) images, the authors showed the appearance of voids and the disappearance of bre constituents aer ageing. Thwe and Liao32 observed similar behaviour with a bamboo polypropylene composite with a reduction of tensile strength and modulus as well as a degradation of both the bre and the interfacial adhesion. Another reported degradation mechanism of bio- composites inducing loss of mechanical properties with ageing is the swelling of the bres. Badia et al.33 have shown that the swelling of the sisal bres in a PHBV matrix caused cracks and bre debonding. In the literature most of the studies have involved tensile testing. However, in marine applications such as boat hulls the mechanical response to impact is crucial. This mechanical property has been studied by Papa et al. on ax polypropylene composites aer ageing.34 The 44 mm thick composite samples were immersed for a week at 70 C. The authors evaluated the short beam shear strength and the ex- ural strength, together with the maximal load and the pene- tration energy from the response of the material to a low velocity impact. This section highlighted the critical role of the interface region in obtaining enhanced mechanical properties. There- fore, there is a need to develop new solutions that are capable of addressing the issues covering different marine conditions. 2.2 Ageing characteristics under different service conditions Composites can be used in marine environment in various applications. Some of them can be used for several decades, such as canoes or boats for example (see Section 3 of this paper). To simulate the marine conditions, composite samples are subjected to hygroscopic ageing in laboratories. It is well known that polymers and composites absorb water because of the difference in chemical potential between the material and the environment. In the case of biocomposites made with natural bres, the water absorption is even larger than with classic glass bre composites.35 In fact, both glass and carbon bres absorb little water whereas natural bres such as ax can absorb more than 15% by weight.35 This behaviour can be explained by the bre composition.36 Weighing of samples to measure water absorption is one of the characterisation techniques used to monitor the moisture gain behaviour during ageing.37,38 The preferred ageing method is to immerse the material in the medium of the application, i.e., seawater; however, this is not always possible in the laboratory environment and in many studies, samples are immersed in distilled water or tap water. The inuence of the nature of the water (presence of mineral salts, presence of ions, impurities, etc.) can impact water absorption in biocomposite materials. As an example, Deroin ́e et al.39 have performed a comparative accelerated ageing study of polylactide (PLA) between distilled and seawater. The PLA samples were immersed in distilled water, renewed every week, at 25, 30, 40 and 50 C and in naturally ltered and constantly renewed seawater at 25 and 40 C. The water diffusion was shown to be faster in distilled water due to the presence of mineral salts. Hygroscopic studies are sometimes preferred to immersion studies. In this case, leaching from the biocomposite into water does not occur. C ́elino et al.40 studied the water diffusion in 4 different natural bres (hemp, ax, jute and sisal) when samples were either immersed at room temperature or placed in an environmental chamber at 80% relative humidity and 23 C. They observed that all bres have similar water diffusion behaviour in the same environment but their behaviour differs from one environment to the other. R ́equil ́e et al.41 have performed hygroscopic studies on hemp/epoxy with amine hardener composite. Samples were in environmental chambers at 9, 33, 75 and 98% RH at 23 C. They found that the moisture sorption was mainly controlled by the bres, despite a non-negligible hydro- philic behaviour of the matrix, and that at high humidity level the large increase of the water uptake in the composite correlates with a similar behaviour in the natural bres. Under hydrothermal ageing, the rate of moisture gain, also known as diffusion coefficient, is greatly inuenced by the temperature and humidity. In order to get moisture absorption percentage up to the saturation level experimentally at room temperature, it oen requires long durations. In order to reduce this time, accelerated ageing conditions are used. Moreover, to estimate the long-term behaviour, accelerated ageing tests are mandatory. The most common way to perform accelerated ageing tests is by increasing the temperature and the relative humidity.15 The effect of the temperature has been studied by Le Duigou et al.42 on ax/poly-L-lactide (PLLA) composite immersed in seawater at 4, 20, 40, 60 and 80 C. The composting of PLLA has a temperature close to 58 C. They showed that ageing close to the glass transition temperature leads to a very rapid degrada- tion related to hydrolysis of the matrix, not representative of what happens in the sea. The temperature has a strong inu- ence on the water uptake, as was also conrmed by Badia et al.33 on poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV)/sisal biocomposites. The choice of the accelerating temperature has to be performed with care depending on the material, the application and the service environment. Natural exposure studies on biocomposite are needed to correlate accelerate ageing in the laboratory with natural ageing in service. For example, Le Duigou et al.,36 performed a 2 year study on injec- tion moulded ax/PLLA composites. Samples were immersed in the sea at 5 m depth and were periodically removed to be characterised. It was found that biocomposites have high water uptake content in both laboratory and sea environments, with similar loss in modulus and strength when plotted as a function of weight gain. The work carried out by Fulco et al.43 on the © 2021 The Author(s). Published by the Royal Society of Chemistry RSC Adv., 2021, 11, 32917–32941 | 32919PDF Image | Long-term durability and ecotoxicity of biocomposites in marine environments
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