Absorbing the payback, extracting the benefits

Tom Shelley reports on latest developments in composites reinforced with natural fibres

Linseed and other plant waste-derived fibres can be used to make composite sheets and drawn into complex shapes by cutting them into shorter lengths. The fibres can also be formed into mats for sound absorption in cars and elsewhere, and also used to selectively absorb oil from water-based mixtures. There have been many attempts to do this already, as Ben Schadla-Hall, marketing manager of Eco-Mats and Eco-Composites, based in Ely, explained to Eureka at the recent ‘Tigers of Tomorrow’ event in Cambridge. In fact, we reported on the Silsoe Research Institute developed decorticator machine back in the 1990s that could take straw from linseed and other crops, and extract useful fibre. Schadla-Hall told us its machine exists in the form of a prototype and, while not as large as the Silsoe machine, is still quite big. The process begins by allowing the straw to partially rot in the fields (retting), after which the woody parts – shives/pith – and dust are removed in the machine. Looking just at linseed, about 36,000 hectares are currently under cultivation in the UK, yielding two to three tonnes of straw per hectare, of which 30-40% represents extractable fibre. The crop is also grown all over Europe and in Canada. The company buys direct from farmers, who normally consider the material a waste product. To make fibre mats into hot formable sheet, the mats have to be blended with polypropylene or polyester. “Injection moulding is a very energy-intensive process, and of course plastics use a lot of oil and are not biodegradable,” states Schadla-Hall. “Our plant fibre alternatives can be formed by press moulding, which uses much less energy. We experimented with different shapes and found that there was almost no limit to what could be made. So far, we’ve made chairs, a bed-head and, of course, internal car parts.” Eureka was shown a piece of sandwich construction, with an EPP foam centre between layers of impregnated mat, which seemed to be both stiff and strong. “In terms of performance, the plant fibres are stronger and more rigid, and less brittle, than plastic. They don’t shatter on impact and can absorb shock, giving them potential safety advantages, too,” he adds. The company says more than 50,000 tonnes of natural fibre are already being used in composites for automotive applications in Germany and Austria. These include wool and hemp, but have a significant value and cost, since they are not waste products. Potential applications for waste plant-derived fibre fabrications in cars include: side trims, flooring, parcel shelves, seat backs, door trims, air ducting and sun visors with live hinges. Under bonnet and elsewhere, they can also be used for heat and sound insulation. At present, the company mainly sells its mats for use as absorbents. They are able to absorb 12 to 15 times their own weight, whereas a standard spun bonded polypropylene mat can only absorb 8 to 9 times its own weight. The mats are made into socks and pillows, using an ultrasonically welded web of polylactide, (PLA) derived from corn starch, which is then filled with the fibre. Pointers * Waste plant stems, particularly those derived from growing linseed, can be treated to recover useful fibre, which can then be blended with polymer and hot-formed into mats. * Plant fibres are stronger, more rigid and less brittle than unreinforced plastic. They do not shatter on impact and can absorb shock * They can also provide heat and sound insulation, including use under bonnet, and be made into very effective oil and chemical spill absorbing mats and pillows