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Textile reinforcements based on flax fibres for structural composite applications


Flax pre-yarns with the optimal amount of twist from Almedahl spinning mill in Finland have been used for the manufacture of warp-knitted unidirection non-crimp fabric. In these fabrics all reinforcing fibres are oriented in one direction and knitted together to into a fabric by Engtex. The results obtained so far from tests at QMUL are very encouraging and clearly show the potential of such novel fabrics. Even in the case of relatively low fibre volume fractions of around 28% for the composites tested, because of the hand lay-up manufacturing method employed, the mechanical properties are already much higher than those of current state of the art non-woven flax fabric based composites. Laminates with a fibre volume fraction of 28% exhibited strength of up to 190MPa and moduli of up to 16GPa. These values mean an increase in strength and stiffness of some 300-400% compared to non-woven flax composites. Even much higher values can be reached when higher fibre volume fractions are used as in vacuum infusion, pultrusion or resin transfer moulding processes. Since fibre volume fractions of 40-50% are typical for these processes strengths of 250 to 300Mpa seem feasible together with moduli of 20-28GPa! This result clearly shows the potential of the developed textile based composite materials as well as that the originally targeted strength value of 250Mpa is realistic and within the performance capability of the textile composites under development. Flax fibre laminates with strength values in the order of 250-300Mpa would be a major breakthrough as non-woven flax fibre composites have typical strength values of some 40-50Mpa!
Gardening devices for domestic and public planting were made by hand lay up without any processing difficulties compared to glass fibre reinforced products. By using flax fibre reinforcements, weight reductions of up to 60 % were achieved. These planting pots are often moved, in which case low weight is requested.
Successful, processing trials for the manufacture of constriction profiles via pultrusion were performed. The process ability was successfully demonstrated, by replacing the outer reinforcement layers with flax. Fibreforce uses around 250 tons of glass fibre per year with a total value of around 375.000 Euros. A successful implementation of the reinforcements developed in TEXFLAX in three selected products, i.e. FORCE 800 series, ladders and cable trays represent a total market value of around 250.000 Euros when 10% of glass is replaced by flax.
A major outcome of the project so far is the development of optimised pre-yarns for textiles for composite applications. Textile technology for natural fibres like flax has clearly a long history but all flax fabrics that are currently on the market are for pure textile applications like linen. Engineering fabrics for composite applications need very different requirements than plain textile applications and it is exactly this innovation, which has been delivered in the first part of the TEXFLAX project. The development of tailor made pre-yarns mean a major breakthrough in natural fibre processing and opens up market possibilities for the development of textile reinforcements for composites. The development of these pre-yarns for composites was one - if not the main challenge - in the first part of the project. One of the main problems that needed to be solved here was to find the optimal yarn construction for use in composites. Traditional flax yarns have been used in textiles like linen but these yarns are very fine and have a very high level of twist and are as such difficult to impregnate with polymer resin and therefore not useful for use in composites. The key to the success was to develop a yarn with a minimum amount of twist that is still processable on the machinery of the various textile producing SME’s Optimised yarns were developed for composites which exhibit a good balance in textile processing (e.g. exhibit a good dry strength which is needed to prevent the yarn from breaking during textile processes such as weaving or knitting) and composite processing (e.g. exhibit a good permeability for easy impregnation with polymer resin).
Sandwich panels for transport containers using flax in the surface layer were made. A sandwich panel consists of two rigid, thin surface layers, and a thick, light and brittle core material. This construction gives the sandwich exceptional good flexural strength in relation to its low weight. The surface layers were manufactured by vacuum injection using 6 layers of uniaxial DOS fabric. The laminates were glued onto a XPS (expanded polystyrene) core.
Traditional woven fabrics were manufacturing from the optimised pre-yarns by JOB and Jokiipin. Laminates were made by hand lay-up from these biaxial (0/90 degree) woven fabrics and tested by QMUL. The mechanical properties for laminates with a fibre volume fraction of 28% where a strength of 65Mpa and a modulus of 5GPa. Clearly these values are well below those obtained using uniaxial (0 degree) fabrics by Engtext. The main reason for this being the biaxial fibre orientation with fibre in the 0 and 90 degree orientation. Another reason is that the transfer of fibre properties into composite properties in the case of woven fabrics is generally less efficient because of the crimped structure with the fibres, which may lead to some property loss from fibre to composite. Analysis of the composite data showed that the fibre efficiency in the woven fabrics is around 80% compared to 100% for non-crimp knitted fabrics. Based on the results obtained so far it is expected that the maximum attainable properties for composites made by vacuum infusion, RTM or pultrusion in the case of typical fibre volume fractions of 40-50% are a stiffness of 10GPa and a strength of around 120MPa. These properties compare again vary favourable with current non-woven composites with strengths around 50MPa and moduli of 6GPa.
PTC has made a lid for a water accumulator tank for solar panels by using biaxial plain weave flax fabric. The manufacture of the cover lid went very well and without problems. No processing differences and problems compared to glass fibre were observed, which emphasises the good permeability of the reinforcements. Weight reduction is 10-15 wt-%. Results were implemented in the production process of the whole water accumulator tank at PTC made by vacuum infusion. The drapeabililty was good for the flax fabrics, as the tank could be made with no greater difficulties.