They may be sustainable, but how good are flax and jute for the engineer?

You may think that the use of natural fibres like flax or jute in composite parts is all about ticking a box for environmental credentials. In many cases, you'd be right, but increasingly natural materials are finding applications where they can offer some distinct advantages.

Products on the market have been as diverse as surfboards, car door cards, boats and even motorsport aerofoils. It is still early days, of course, and both suppliers and users are still exploring just how to fully exploit the potential.

Fibres from plant stock include flax, hemp, bamboo and sisal. Fibres are typically processed into yarns and the development of low/zero twist yarns and fibre treatments has been a significant step in the improvement of the mechanical performance. These yarns are then usually processed into traditional continuous fibre forms including various unidirectional woven's, non-crimp bi-axials and spread tow fabrics all commercially available.

Neil Appleton, chair of Composites UK biocomposites subgroup, says: "The supply chain is now robust and the properties can be comparable. Normally with composites one of the key properties are the mechanicals, but with the plant fibres I've worked - in pure structural terms - they are good, but not as good. However, they do have other benefits like toughness.

"There are some challenges like durability data, life cycle assessment, accountability and education, and we are looking to collate understanding of the material to make it easier for people to use them where appropriate. The opportunities are the same way as conventional composites."

One company that is keen to lead the way is Chesterfield based Composites Evolution, which was spun out of research and development by Net Composites in 2009. It develops biocomposite fibres that can be used with almost any existing composite processes. It want to make the switch as easy as possible and has fibres compatible with liquid moulding processes, hand lay-up, vacuum infusion or resin transfer moulding, resin injection and also materials that can be prepreged.

"In most cases we have a material to suit an existing process," says managing director, Dr Brendon Weager. "The performance levels of our flax fibres are similar to glass fibres and have similar stiffness and strength. But, the fibres are actually lighter as well as being environmentally friendly."

Natural fibres are normally short and discontinuous and these are traditionally spun to get them in continuous yarns. However this twists the fibres, and for a composite material this stops the resin from properly wetting out properly and impregnate the yarn.

Composite Evolution has developed the ability to convert natural fibres in to aligned forms that provide both a good reinforcement and also allow proper impregnation of the resin. From here it has been able to develop its product range using a number of different weaves, tapes and fibre options using the same base technology to produce, 'aligned twisted fibres'.

Also available are co-mingle systems comprising of a fibre weave that is combined with a thermoplastic aimed at the high production rates of the automotive industry. These can be heated and then thermoformed or stamped formed very quickly. The polypropylene inside the yarn melts when heated and forms the binder or matrix of the material, acting as the resin system.

Most Flax fibres are sourced from French and Belgium, and only a tiny proportion goes to technical applications, while most goes to the production of clothing. Flax comes from the same plant used to make linen for clothes and while the finer fibres are separated out, flax for technical applications tends to be the lower grade fibres that have previously been discarded.

"It is European fibre and is high quality and probably the best performing natural fibre there is," says Dr Weager. "But it is more expensive than glass fibre and it sits in between glass and carbon as a price point.

"This is why we also carry a range of Jute materials, an Indian product and it is substantially cheaper than flax. In that case you can get most of the performance of flax – most of it not all of it – for a price that is equal to glass fibre. So you can do a direct replacement of glass fibre without any real effect to cost."

Flax also yields another interesting, and potentially useful property in its ability to dampen vibrations. Some recent projects for automotive applications show that it is able to reduce noise and harshness inside the car.

"Carbon fibre is a very brittle harsh material and is very resonant," says Dr Weager. "It's used to make loud speakers and pianos. It is getting increasingly used on cars, and is also getting thinner and that means more noise ultimately. By using some of this material, with carbon fibre, you can reduce this down."

Composites Evolution says one of its main target markets is the replacement of glass fibre. However, it has also seen a lot of interest in using its flax fibres in combination with carbon fibre. This can be less about mechanical performance and more about the aesthetic properties of carbon fibre. It is not unusual for products to use carbon fibre on the outside and glass on the inside, so it looks like carbon but is a lower cost. However, this increases the weight.

"Flax fibres are lighter than glass fibres so it can be used almost as a lightweight bulking agent," says Dr Weager. "One of the main differences, however, is you need slightly more resin as the fibre is a bit more absorbent than glass or carbon. You can also put a polyester gel coat finish on it with a flax fibre laminate underneath which results in an excellent finish in its own right."

Also keen to establish the potential of composites is material specialists Performance Engineered Solutions (PES). It has recently completed a project in conjunction with Teks UK, and the University of Sheffield Advanced Manufacturing Research Centre.

It received a £50,000 grant from the Niche Vehicle Network to evaluate the potential use of biocomposite materials in the production of future automotive vehicle bodywork panels. The project called ELCOMAP (Environmentally Friendly Lightweight Composite Materials for Aerodynamic Body Panels) took existing parts from standard OEM vehicles and set about producing the same part from a biocomposite to evaluate any difference.

The tests were carried out on a flax/cashew nut shell liquid (CNSL) biocomposite, on a flax/low-cost epoxy (LCE), and glass/LCE. AMRC also ran a project concerning moisture loss of flax under varying heating conditions to better prepare the flax for resin impregnation. A test panel of flax/CSNL was also experimentally cured in a microwave to explore the possibilities of alternative curing methods.

The results were the glass/low-cost epoxy showed much higher mechanical strength. However, the flax composites are much closer when it comes to modulus, especially once normalised.
The significantly lower density of the biocomposite meant that for certain structural applications, the biocomposite would have a superior stiffness/mass ratio.

When compared directly, the flax/CNSL shows properties very similar to those of the flax/LCE which suggests no negative impact with the inclusion of CNSL. Not only this, but the higher interlaminar shear strength displayed by the flax/CNSL suggests that the bioresin is actually an improvement.

Further research into the treatment of the flax fibres could improve the fibre/resin interface, thus increasing mechanical strength and reducing the gap between the properties of biocomposites and glass fibre composites.

While biocomposites are unlikely Ito be used as structural components of the next generation of aircraft, it is likely that the automotive industry will increasingly take advantage of them as a replacement of any glass fibre parts. And, it is still relatively early days with many projects and developments continually yielding new results.

Justin Cunningham

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My area of interest in flax is the use of flax fibres in composites
I would be happy to discuss further

Patrick L O’Brien B.Sc (NUI) M.Sc (NUI) MPhil (Ulster University) FCIWEM (UK) CEnv (UK) CSci (UK) CWEM (UK) MIChemE (UK) MCIWM (UK) M.I.Biol.I(Irl)
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Comment Patrick L O'Brien , 16/01/2018

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