Media SummaryThe properties of natural-fibre reinforced-plastic (NFRP) composites based upon epoxy polymer matrices containing silica nanoparticles and rubber microparticles have been studied using (a) unidirectional flax-fibres or (b) regeneratedcellulose fibres in the architecture of a plain-woven fabric. They were manufactured using a low-cost resin-infusion method. The values of toughness of the NFRP composites were remarkably high, being typically about 75% higher than for the corresponding, traditional, glass-fibre composites. The reasons for the very good performance of the NFRPs were identified and a quantitative model developed. AbstractIn the present paper the morphology and toughness of a range of bulk epoxy polymers, which incorporate a second-phase of well dispersed silica nanoparticles and/or rubber microparticles, have been determined. Secondly, the macro-properties of natural-fibre reinforced-plastic (NFRP) composites based upon these epoxy polymers have been ascertained, using (a) unidirectional flax-fibres or (b) regenerated-cellulose fibres in the architecture of a plain-woven fabric. Thirdly, the toughening mechanisms which are induced in these materials by the presence of the silica nanoparticles, the rubber microparticles and the natural fibres have been identified. Finally, the values of the toughness of the bulk epoxy polymers and corresponding NFRPs have been quantitatively modelled. The increased toughness recorded for the bulk epoxy polymer due to the presence of the silica nanoparticles and/or rubber microparticles was indeed typically transferred to the NFRP composites when using such epoxies as the matrices for the fibres. Thus, the important role that may be played by modifications to the epoxy matrices in order to increase the toughness of the composites was very clearly demonstrated by these results. However, notwithstanding, the toughening mechanisms induced by the fibres were essentially responsible for the very high toughnesses of the NFRP composites, compared with the bulk epoxy polymers. The modelling studies successfully predicted the values of toughness of the bulk epoxy polymers and of the NFRP composites. These studies also quantified the extent to which each toughening mechanism, induced by the second-phase nano-and micro-particles and the natural fibres, contributed to the overall values of toughness of the materials. Keywords:Fracture; Modelling; Nanocomposites; Natural-fibre Composites IntroductionHigh-performance polymer-matrix composites (PMCs) typically employ epoxy resins as the matrix for continuous fibres. When cured, epoxy resins are highly-crosslinked thermosetting polymers which exhibit good elevated temperature resistance and low creep. However, their high crosslink density causes them to have a poor resistance to the initiation and growth of cracks. The addition of a second-phase, which is well dispersed, can significantly increase the toughness of thermoset polymers [1][2][3][4][5][6][7][8][9]. Thus, to achieve a relatively tough epoxy-polymer matrix, an ...

Flax fibres and cellulose fibres were used to manufacture composites with particle-modified epoxy matrices in order to develop 'green' composites which possess relatively high values of interlaminar fracture energy, G c . The flax used had a unidirectional architecture of continuous yarns spun from short, interlocked fibres. The regenerated cellulose consisted of continuous and nontwisted pure cellulose fibres in a plain-woven architecture. The natural-fibre-reinforced-polymer (NFRP) composites employed an anhydride-cured diglycidyl ether of bisphenol-A epoxy as the matrix. The epoxy polymeric matrix was modified with (a) silica nanoparticles, (b) rubber microparticles, and (c) a combination of both of these types of particles to give a hybrid-toughened epoxy matrix. The composites were manufactured via a resin infusion under flexible-tooling (RIFT) process. Preliminary studies on the NFRP composites manufactured using the initial-RIFT process clearly showed the deleterious effect that moisture present in the natural fibres had upon the properties of the NFRP composites, since the trapped water cannot escape from the composite panel. Hence, an optimised-RIFT process was developed whereby the natural fibres were dried in a fan oven prior to being employed in the RIFT process. This reduced the water content of the fibres from around 9 to 10 wt% to about 1 wt%. Significant improvements in the physical and mechanical properties were recorded for the NFRP composites manufactured using this optimised-RIFT process. Indeed, in particular, very dramatic improvements in the G c of the NFRP composites were measured, especially when the epoxy polymeric matrix was modified using the silica nanoparticles and/or rubber microparticles. For example, a steady-state propagation value of G c of about 1935 J/m 2 was measured for the flax-fibre composite with the hybrid epoxy matrix, compared to values of 1110 and 535 J/m 2 for the flax-fibre and glass-fibre composites based on the unmodified (i.e., the 'control') epoxy matrix, respectively.

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