Improving moisture durability of flax fibre composites by using non-dry fibres

Flax fibres have good mechanical properties such as high specific modulus, making the fibres a good candidate as reinforcement materials for the application in composite materials. However, the fibre's strong hydrophilic behaviour, which leads to a high level of moisture sorption and large swelling deformation, is often considered as a limitation for composites. Despite the environmental benefits of flax fibre composites, their use and applications have been limited due to questions about their durability compared to, e.g. glass fibre composites, especially in outdoor conditions with high moisture content. This dissertation aims to improve the moisture durability of flax fibre composites by using non-dry fibre and resins with low sensitivity to moisture to take advantage of the moisture present in the fibre. The hypothesis to be tested is that, since the fibre already contains some moisture and is thus pre-swollen, the shrinking and swelling of the composite due to moisture sorption could be limited, slowing its degradation and thereby improving its moisture durability. The reductions in transverse flexural strength and modulus of composites, ranging from 14%-48% and 6%-57%, respectively or even an increase in composites' flexural properties after the cyclic ageing test, show the diverse effects of the tests on flax composites. Almost no differences in the composites' performance were found between the natural ageing under the shade and natural ageing exposed to sunlight. Additionally, the comparison among the different ageing tests showed that the water immersion test could represent the long-term durability of uncoated flax fibre composites in natural conditions. The microstructure analysis of composites supports the results of composites' flexural properties after the different ageing tests, wherein the flax/epoxy showed superior flexural properties compared to flax/polyester composites. During the long-term moisture cycling, all composites' properties seem to recover or even significantly increase until the end of the cycling test compared to unaged composites. The reason for the improvement in composites' flexural properties during the test is partially the stiffening of both polyester and epoxy resins, which occurs later in the ageing test. However, it could also be due to some stiffening and strengthening of the flax fibres. In the study, it was shown that the high-low humidity cycling of flax fibres increases the fibres' tensile properties. A mechanism similar to a hornification treatment could explain the increase in tensile properties of flax fibres with increasing cycles. Similar to the hornification treatment, besides the increase in stiffness of the fibres, cycling also reduced the fibres' hygroscopicity. The high-low humidity cycling of flax fibres supports the research results that long-term cycling at high-low humidity does not negatively affect flax fibre composite due to improvement in the mechanical properties of the fibre reinforcement. The different moisture durability tests showed that the composites made of non-dry flax fibre and resins with low sensitivity to moisture have better properties compared to composites made of dried fibre. Composites made of non-dry fibre have lower moisture sorption and degree of swelling and shrinking compared to composites made of dried fibre. The mean strength and modulus after ageing of composites made of non-dry fibre are higher than for composites made of dried fibre in both the longitudinal and transverse fibre directions. Moreover, composites made of non-dry fibre for both flax/polyester and flax/epoxy composites, after ageing have a considerably lesser area fraction of damages than composites made of dried fibre. In general, the results showed that composites made of non-dry fibre could be used for enhancing the moisture durability of composites and lessen the time and cost to produce the composites by omitting the step of drying the fibres. Moreover, the results obtained in this study suggest that flax fibre composites can be used in environmental conditions even with extreme varying humidity values, but perhaps not in applications wherein composites come in contact with liquid water for long periods, when uncoated.

Publication year:2022

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