Clean and green

Composites are a key ingredient in creating lighter cars. But these materials rarely boost a car’s ‘green’ performance, as is increasingly required these days – because carbon fibre and glass fibre components often find their way straight to landfill.
“Carbon fibre composites are not particularly harsh in landfill,” says Dan Fleetcroft, engineering design director of PES Performance, a Sheffield-based design consultancy that focuses on composite materials. “There’s not much leaching – but they do not disappear.”

The company has spent time developing and testing bio-based composites in a selection of automotive applications, as a key player in the Elcomap project – along with the AMRC Composites Centre in Sheffield, and Teks UK.

Elcomap, which stands for Environmentally-friendly Lightweight Composite Materials for Aerodynamic Body Panels, used a partially bio-based composite to create large Porsche and Subaru components. They were made from an epoxy-based composite. But 30% of the epoxy resin has been replaced with cashew nut shell liquid, while the fibre is flax. Cashew nut shell liquid was originally a waste product of the cashew nut extraction process. The dark oil, obtained from within the shell, has become a popular precursor for making ‘green’ resins.

The team produced two Porsche rear panels and a larger Subaru front end using the material. The company has done lots of previous work on bespoke vehicles.

“We’ve spent more than 10 years taking weight out of road-going Porsches,” says Fleetcroft. “This gave us a good platform to benchmark what we had done in the past.”

The flax fibres were uncrimped biax – heavy GSM and large tow. The biggest challenge was the quality of the prepreg that PES received – and the wetting of the fabric, he says. One side of the fabric had good resin coating, but it did not penetrate all the way through.

Other fibres, including unidirectional and thicker biax also showed incomplete resin penetration. This meant less tack – making it harder to get it around complex geometries.

“We’re still at quite a low manufacturing volume,” he says. “As it matures, we will get better impregnated fabrics.”
Despite this, Fleetcroft says that the properties were comparable to those of glass fibre panels.

Curves and lines

The team chose to make prototype panels for high performance, niche road vehicles, and deliberately decided to go beyond traditional ‘flat aesthetic’ panels in favour of ones with more complex geometries.

The Porsche parts were a rear bonnet and boot. The boot lid was designed as an upper and lower skin, which allowed the geometry to be moulded and removed from the tooling. Threaded inserts were included in the part allowing it to be mounted to the chassis. The part ended up being 2mm thick. The biggest challenge here, says Fleetcroft, was to keep it true.

“We needed a symmetrical, balanced laminate,” he says. “We also jigged it so it would match the body and have the right geometry.”

Overall, the part was 6kg lighter than the 1mm thick steel original and the added thickness of the part ensured a higher stiffness.

“What we lost in mechanical properties we gained in the section of the skin.”

Because the component is based on epoxy it was cured in an autoclave at 120°C for two hours at 6-7bar. Fleetcroft says this is probably near the limit of what flax fibres can withstand: if the team had been using a phenolic resin, the flax could probably not have taken the curing temperature.

The Subaru part is a full front splitter. The tooling was created from glass fibre composite, with wooden stiffeners – a low cost option.

“There were challenges to ensure that when you splash the tool off the original component you capture the geometry,” says Fleetcroft.

Components were laminated using the same process as for the Porsche boot lid, and oven cured at around 90°C. The lower temperature was to take account of the materials used in the mould.

Although these are test parts Fleetcroft says that commercial components using the material cannot be far away.
“I think the starting point will be fairings, covers and internal car body parts,” he says. “Anything painted, or which can be covered in leather, is possible – because the finish is not as good as a twill weave.”

Traditionally, the move away from established materials towards bio-based grades can come with a loss of mechanical properties. But not in this case, as the material, with 30% cashew nut oil, is actually tougher than standard epoxy. The next target is a full biocomposite panel – in which epoxy has been completely replaced by cashew nut oil.

“Once you have a 100% bio-resin with plant fibre that’s something you can crush up and put in the ground,” says Fleetcroft.

Built to last

However, as is always the case with bio-derived parts, there are always worries about longevity. But if the properties can be engineered to match those of epoxy, Fleetcroft sees no reason why the material might not be used for external panels as a viable alternative to glass fibre.

And, he says, market acceptance will be more dictated by whether it can be made in an automated process, rather than properties of the material itself.

The parts on Elcomap were cured in the oven or autoclave, but PES is looking at alternative curing methods – for both biocomposites and more traditional materials.

“We’re very interested in new methods such as microwave curing,” he says. “We’ve done some early stage testing on it, using biocomposites. It’s still being explored for carbon fibre and glass fibre.”

This, he says, is an example of a lower energy curing method – as microwaving will consume far less power than an autoclave at 120°C, for example. PES is also looking at press forming as an alternative to autoclaves, he says.
For now, biocomposites is very much an immature market, but the demand from the automotive market in particular, is getting stronger.

“There are some materials already available, but it’s still a massive race to develop better fabrics, processing and resins – including fully bio-derived ones,” says Fleetcroft.

www.pes-performance.com