Making plant fibres flame-retardant might make them fly.

It looks like a sleek car dashboard, but under its smooth surface flax fibres criss-cross through a three-dimensional matrix to reinforce it, making it stronger and lighter than traditional materials.

The use of natural fibres in composite applications is gaining popularity in many areas, and particularly the automotive industry. Daimler-Benz in Germany has been using the components made from different natural-fibre composites since 1994. Flax and other natural fibres are used to make 50 Mercedes-Benz E-class components. Similarly, the seat shells and their panelling are made from the latest in natural-fibre composite technology. Toyota developed a biodegradable plastic made from starch extracted from sweet potatoes and other plants. This plastic was combined with natural fibres for use in interior parts. 

But the high moisture absorption and flammability of these composites has restricted their use in cars and other industries such as aerospace. This is why at the Council for Scientific and Industrial Research’s Port Elizabeth campus, in collaboration with Nelson Mandela Metropolitan University, we are researching strategies to make biocomposites safer and so increase the ways in which they can be used.

Traditional composites — which are materials made of at least two other materials — use synthetic fibres, such as glass, carbon and aramid, to make them hard and strong. But there are serious drawbacks: they are not biodegradable, consume a lot of energy to manufacture, and result in airbone fibres that cause respiratory problems. Airborne fibres are caused when sufficient amount of glass fibres are released into the air during manufacture, handling and aircraft fires.

So people are turning to natural fibres to avoid these problems. Flax, hemp, jute, sisal and kenaf are some of the most important natural fibres used in these composite materials, called biocomposites. They are abundant, easy to process, renewable and inexpensive.

One of the most common types of biocomposites is natural-fibre-reinforced composites, which comprise a polymer matrix and are reinforced with natural fibres. A polymer is a substance made up of a large number of smaller molecules that link together to form larger molecules. An example of a synthetic polymer is plastic. 

This reinforcement provides strength and rigidity, helping to support the structural load of the components. The matrix makes sure that the reinforcement does not move or slide out of place.

Flax, with its hip-height glossy bluish-green leaves and pale blue flowers, is a fiber crop that is grown in cooler regions of the world. Producers extract the very long fibers inside the wooden stem of the plant, and then spin and weave them into linen fabric. Fabrics are made in the same way: linen yarn is generally woven into sheets, with multiple threads interlaced both horizontally and vertically on a loom. Flax fibre is used to make interior panels in cars, and car manufacturer Mercedes has incorporated non-woven fabrics mats into the interior panels of several models, such as the Mercedes E-class. Flax was chosen as it is cheap and eco-friendly.

Kenaf, which can grow taller than a man, is another important source of fibre for composites, and has many other industrial applications. The fibres in kenaf are found in its bark and wood, and can be spun and woven to a fine, crisp, linen-like fabric and used as reinforcement for composites. 

Toyota has developed kenaf fibres for the interiors of its cars. Researchers at Universiti Malaysia Sabah, a university in Malaysia, are using kenaf fibres instead of glass fibres in front and back bumpers. 

Hemp often gets a bad rap because of its association with marijuana. In reality it has countless uses and does not have psychoactive properties. The confusion between the two arises from the fact that they both come from the same plant species. Hemp contains less than 1% THC (delta-9-tetrahydrocannabinol), the active ingredient in the marijuana plant, which contains 5% to 20%. It can grow up to 15m in four months, making it a useful crop for textiles.

The valued primary fibres that encase the hollow, woody core of the hemp stalk can be spun and woven into a fine, crisp, linen-like fabric and used as reinforcement for composites. 

At present the single largest use for hemp fibre is in cars, with various manufacturers — including Audi, BMW, Ford and Volvo — using natural fibres to create more eco-friendly and lightweight cars.

Why isn’t everyone using them? Well, because biocomposites have drawbacks of their own.

Natural fibres absorb moisture in humid climates and when they are immersed in water. The hemp fibre swells when composites are exposed to moisture. 

As the fibre swells, tiny cracks develop between the fibre and the matrix, and water molecules infiltrate the gaps between the fibres and matrix, pulling them apart and reducing the material’s strength.

And water isn’t the only concern: fire is also a problem.

Firefighters and emergency crews involved in clean-up and restoration operations after crashes have expressed concerns about the long-term effects of their exposure to the fibres that are released from burning composites. 

They also require special equipment to extinguish and handle the incinerated fibre composites. 

Fibre-reinforced composites pose a serious health hazard in fire. When composites catch alight, they release a complex mixture of gases, organic vapours (such as carbon dioxide) and particulate matter (including tiny inhalable bits of fibre). These combustion products can cause acute and delayed health problems and, in the worst cases, can be fatal.

It is possible to overcome these challenges, though. Researchers around the world have found ways to apply surface treatments and flame retardants to these biocomposites to reduce their moisture sensitivity and flammability. 

In the automotive industry, manufacturers can modify the fibre surface by treating it with chemicals to improve the adhesion between natural fibres and polymer matrices. This also decreases their ability to absorb moisture, making them stronger.

However, using flame retardants on biocomposites could expand their applications. Researchers can inhibit fire — or even suppress it entirely — by padding flax fabric with a flame retardant before weaving it into the fabric. Or they can mix a fire-inhibiting substance in with the plastic while it is still in its molten state (before it is moulded into a final product, such as the backs of aeroplane seats). 

Tri-Dung Ngo, a researcher from the National Research Council of Canada, found that composites made from untreated flax fabric were flammable while others containing treated flax fibre fabric did not burn at all. 

There is increasing interest in using natural fibres in the aerospace industry, where they are exposed to wide variations of temperature and humidity. 

This can affect the strength of the structures, which is why our work at the CSIR in collaboration with Nelson Mandela Metropolitan University is critical. 

The study of the long-term effects of temperature and humidity on the properties of natural fibres and composites, as well as on the fire retardants used to stop them from catching alight, means that these products — which are cheaper and more environmentally friendly than traditional composites — could find more applications, even in the demanding aerospace industry.

Tshepiso Princess Molaba is a MSc candidate at the Nelson Mandela Metropolitan University.

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