Abstract Nowadays, due to the global warming and pollution issues, sustainable materials must be considered. Composites materials can offer excellent mechanical performance with low weight, hence saving fuel. However, most of the composites systems are based on petrochemicals, and natural materials may be a better option; for example, fibre reinforcements from plant stems (bast) and bio-based resins are becoming available commercially. One of the major inconveniences of the NFRP is the moisture absorption in marine environments. This problem is usually solved increasing the fibre-matrix bonding. Commercially available fibre systems are chemically treated in order to tackle this issue, but those treatments produce environmental burdens. This thesis seeks to develop an environmentally-friendly, commercially competitive and easily performed treatment methodology for improving the NFRP mechanical properties. The proposed silane-in-hardener method, adding coupling agent to the hardener, rather than direct treatment of the fibres in solvent, can eliminate solvent use, considerably reducing environmental burdens. This new proposal also reduces process time and improves the composite mechanical performance, resulting in commercial competitiveness. The primary research question in this thesis is can sensible NFRP properties be achieved with silane-in-hardener replacing prior treatment of reinforcements? Flax fibre and epoxy resin were selected for the experimental campaign. First, flax fibre was mercerised in different immersion-time and concentrations conditions, and the resulting mechanical performance of composite systems evaluated; from the campaign the best mercerised system was selected. In a second stage, raw flax fibre and best mercerised flax fibre were silanised and resulting composite system mechanical properties evaluated. In a third stage, silane was directly added to the epoxy resin and the mechanical properties evaluated together with raw flax fibre. In the final stage, the developed silane in resin method was applied to flax/bio-epoxy system and compared with the basic system in order to evaluate its real improvement. Aitor Hernandez Michelena PhD thesis Page vii of xxiv Additionally, water immersion tests were performed to the silanised flax fibre/epoxy resin system in order to evaluate whether the moisture resistance was increased or not. From the experimental campaign, it was concluded that the fibre mercerisation process reduces the resulting composite mechanical performance. First, whenever the flax is immersed in the NaOH solution the fibre swells, impeding the flax fibre correct wetting, reducing as a result the composite mechanical performance. Second, because at long immersion-times and concentrations the flax fibre starts to degrade, reducing the composite performance. Similarly, when the flax was immersed in a silane solution, fibre swelling was also obtained, reducing mechanical performance. In contrast when the 1% w/w silane was added to the resin system, the swelling was avoided, and the objective interfacial properties enhanced, getting as a result static mechanical properties improvement. However, when the enhanced and base systems moisture ingress resistance was evaluated, the values difference was not as representative as expected.

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