Wetting and swelling property modifications of elementary flax fibres and their effects on the Liquid Composite Molding process

Specific features of flax fibres used to manufacture composite materials

“…From the processing point of view, both the geometry and physico-chemical nature of fibres are of primary concern for bio-based composites. It may be noted that basalt fibres, which are derived from minerals but are in shape and nature very close to glass fibres, will behave very similarly to petro-sourced reinforcements based on carbon, glass, or aramid fibres [205]. Conversely, long fibres coming from crops like flax fibres, behave very differently regarding their particular fibre architecture and nature as described previously.…”

“…It has been shown on wood products [202][203] that hemicelluloses start to degrade at approximatively 200°C producing free radicals that are susceptible to create cross-linking in lignins or pectins. Treated flax fibres have thus been subjected, after drying, to a temperature of 220°C for 2 h under circulation of nitrogen to prevent oxidation and fire [200][204] [205]. Previous research on wood [202] [203] has shown that the thermal treatment induces a degradation of hemicellulose.…”

“…This may indicate an increase of the amount of structural pectins (the overall pectins are classified in structural and matrix pectins, this implies that a part of the matrix pectins have been converted into effective structural pectins that are more strongly bonded). Recent studies have proven the positive effect of this kind of treatment on the surface energy of flax fibres [205] and thus on the impregnation by mostly dispersive resins, but also on swelling of those same fibres [207]. Thermal treatment under neutral atmosphere enhances the morphological stability of fibres and allows a more regular interface with a thermoset resin [207] [205].…”

“…Recent studies have proven the positive effect of this kind of treatment on the surface energy of flax fibres [205] and thus on the impregnation by mostly dispersive resins, but also on swelling of those same fibres [207]. Thermal treatment under neutral atmosphere enhances the morphological stability of fibres and allows a more regular interface with a thermoset resin [207] [205]. Finally, atmospheric plasma can be used to modify fibres.…”

“…Disregarding any sizing effect which is of little concern for flax fibres, the wetting, flows, and permeabilities involved in processing must account, above all, for the fibre swelling due to the capacity of natural fibres to absorb liquid components in their internal structure [205,228,229]. This swelling can be observed at several scales and has a strong influence on the final part properties due to internal stresses induced by the hygro-mechanical mismatch of the fibre-matrix behavior, as described earlier in this paper.…”