Natural selection

The green revolution has brought natural materials into contention for engineering applications. And even composites can benefit from some of the bounty from the natural world.

Chesterfield-based Composites Evolution makes woven fabrics that use both bio-based resins and natural fibres. Most of its products are based on its proprietary ‘twistless technology’, which helps it to improve the quality of the fabrics.

“Most natural fibre yarns are quite tightly twisted, making them harder to impregnate with resin,” says managing director, Brendon Weager. “We can produce a continuous roving of natural fibre that have no twist in them, and are easier to impregnate.”

The resin it uses, poly furfuryl alcohol (PFA), is derived from waste agricultural material – specifically, from the sugar cane plant.

“It’s made from what’s left over once the sugar has been extracted,” Weager explains.
The cellulose within the waste is converted into PFA, which has similar properties to phenolic resin – including its fire performance, thermal stability and chemical resistance.

PFA can be used as a less hazardous replacement for phenolic resin, notes Weager. The lack of any formaldehyde in the formulation means that there is no issue with workplace exposure to the chemical. The company is also specialist in many types of natural fibres, including hemp, jute and flax.

“Flax is the most commonly used natural fibre in Europe, because of its availability, quality and performance,” he adds.

And natural fibres have one golden advantage to help them compete against more mature technologies.
“The main benefit is weight reduction. The density of flax is around half that of glass fibre, but has similar stiffness.”

The ability to save weight means that most flax is used in the automotive industry. But there are other applications too, which Composites Evolution has embraced – particularly in sports and leisure. It has supplied its woven flax fibre to a Canadian snowboard manufacturer, which uses it with a partially bio-based epoxy resin to boost the renewable content of its boards. The flax provides more than just an eco-advantage.

“It produces quite a stiff structure, but with vibration damping,” says Weager.

Similarly, its flax fabric has been used to make more flexible surfboards, as well as very light canoes. And he sees plenty more potential.

“We are still in the early stages. There is a host of potential markets for these materials – from sport and leisure and automotive, to construction and aerospace.”

Built to last

Meanwhile, NetComposites is coordinating a pan-European research project, BioBuild, whose aim is to develop bio-based composite products for the construction industry. The project has partners from across Europe – including several industrial collaborators.

“This is not blue sky research: we are looking to get real results,” says NetComposites’ technical manager, Chris Hare, who helped draft the original project proposal.

The aim of BioBuild is to reduce the ‘embodied energy’ within buildings by 50%: this is the energy that goes into creating all the materials used in the building. The embodied energy for a hemp fibre-reinforced polyester composite is around half that of timber, a quarter of virgin aluminium and one-eighth that of bricks.
“The energy efficiency of buildings has improved so much that the embodied energy is now a target for reduction,” states Hare.

The project will use two bio-based resins (PFA, and a bio-derived polyester), reinforced with natural fibres such as flax and jute, to create four typical building products, such as an external wall panel and an internal partition kit.

A third resin – a bio-based epoxy made from cashew nut shell liquid – was rejected after going through the project’s ‘Quick Scan’ process, which assesses the likely environmental benefit of a material.

While natural fibres have some advantages over glass fibre, such as lower density, they are also more susceptible to damage by moisture or microbial attack. For this reason, the project will also focus on developing protective treatments for the fibres.

“To some extent this problem also exists for wood, so we are looking to transfer some of those treatments and apply them to natural fibre composites,” says NetComposites’ project manager Anthony Stevenson, who co-ordinates the project.

Another important aspect of the treatment is to reduce the flammability of the fibres, especially when designing products for interior use. The inherent fire resistance of PFA, for example, is reduced when it is reinforced with natural fibres.

“If you are going to build using biocomposites, the biggest challenge alongside durability is fire performance,” continues Stevenson. “Natural fibres are combustible: they provide the fuel for a fire.”

Strong performance

To date, natural fibres have been used to add aesthetics, save weight and reduce carbon footprint. But can they compete with established fibres where it really matters – in structural components? Researchers at Nottingham University say they can.

Darshil Shah and co-authors have manufactured and tested two sets of wind turbine blades – one using traditional glass fibre, the other using flax fibre.

“The perception is that natural fibres cannot compete, but we found that this was not the case,” he says. “In many cases, glass fibre is actually over-engineered.”

Although the strength of flax-reinforced blades was lower than that of glass fibre, they still exceeded the necessary safety standards.

Other than perception, the main factor preventing flax-reinforced turbine blades is the cost – around three times that of glass fibre. This, says Shah, will hamper commercial acceptance but he believes that commercialisation might come about in a different way. In developing countries Micro-turbines, which are a common source of energy, are subject to much smaller forces, so blades could be reinforced with much cheaper fibres.

“You might not even need flax, coir fibres might be strong enough, and they are very cheap,” he says.
Shah has since moved to Oxford University, as part of the silk research group – and says that natural silk has great potential as a natural fibre reinforcement.

“A single cocoon, when unravelled, can produce more than 1km of silk – and it’s a continuous fibre,” he says.
“Its particular strengths – toughness and energy absorbance – are already being used in a prototype helmet for helicopter pilots – a far cry from the soft, luxurious feel normally associated with silk.”

www.compositesevolution.com

www.netcomposites.com