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FEATURE: Could cars be grown on trees?


Fruit grows on a date palm (Credit: iStock)
Fruit grows on a date palm (Credit: iStock)

Engineers want to replace metal, plastic and carbon fibre with more sustainable alternatives. It could change how cars are made.

Today, a top of the range Mercedes-Benz is likely to be made of lightweight metal, moulded plastic, and maybe a bit of carbon fibre and leather thrown in for good measure. But the first commercial car, made by Karl Benz in 1885, had bodywork made mostly of wood.

As cars got faster, safety became important, and so bodywork became stronger and stiffer – drivers now sit in metal cages to protect them in the event of a crash. But that has an environmental cost on top of the vehicle emissions – mining, processing and manufacturing with metal uses a lot of energy, and creates a lot of carbon emissions. Add in the oil-derived plastics that form much of the interior, and you’re looking at up to 35 tonnes of carbon dioxide emissions for a new car – in this case, a top of the range Land Rover Discovery.

But with driverless cars on the horizon that (hopefully) won’t crash, and renewed efforts to curb global warming, some researchers are trying to find a better way. In an echo of the early days of the automotive industry, they’re turning to renewable sources of material, and looking to nature.

Date palm panels

Researchers at the University of Portsmouth are working on new materials made from agricultural waste that could find uses in the automotive, marine and aerospace industries. They hope to use natural fibres to create lightweight materials. One particular source of material could be the date palm, which is grown widely around North Africa and the Middle East, with millions of tonnes a year generated in waste.

“The sustainable composite materials are produced from flax, hemp, jute and waste biomass date palm fibres to develop parts like car bumpers and door linings – mainly for non-structural components,” says Hom Nath Dhakal who is leading the research. “The team is also working towards making them suitable for structural and semi-structural applications by using hybrid techniques.”

Dhakal says that using natural plant fibres from date palms could provide farmers with extra income, as well as reducing carbon dioxide emissions as they are commonly burnt. “These lightweight alternatives could help to reduce the weight of vehicles, contributing to less fuel consumption and fewer emissions. The sustainable materials can be produced using less energy than glass and carbon fibres, and are biodegradable, therefore easier to recycle,” he continued.

However, there remain some problems with natural fibres – they are hard to integrate with other materials, and can sometimes absorb moisture. The next stage for the researchers is to attempt to address some of these problems, and create materials that can survive in tough environmental conditions.

Composites could be the answer. By combining natural fibres with glass, basalt or carbon fibre, new materials could have the best of both worlds. “The way forward for natural fibre composites to be used in structural applications would be a combination of both materials (natural fibres and synthetic fibres) with a hybrid approach,” says Dhakal. “Meeting these challenges requires further research and innovation between academic institutions and industry.”

Carbon fibre

A team from Washington State wants to create actual carbon fibre from plant waste. The strong, lightweight material is often used in planes and cars, and is usually made from a substance called polyacrylonitrile (PAN), an expensive non-renewable polymer. “PAN can contribute about half of the total cost of making carbon fibre,” says Jinxue Jiang, a postdoctoral fellow at Washington State University. "Our idea is to reduce the cost for making carbon fibre by using renewable materials, like biorefinery lignin.”

Lignin is a component of the cell walls of plants and trees – when they’re turned into paper or biofuel, it’s left behind, and is usually burnt or binned. In the past, researchers have tried to make carbon fibre from 100% lignin, but have found it too weak for use in the automotive industry. The team from Washington State instead combined lignin with PAN using a process called melt spinning. “You elevate the temperature of the polymer blend until it melts, so it can flow,” explained Jiang. “Then you spin these polymers until the fibre forms.”

The team tested a number of different ratios of lignin to PAN, and found they get away with as much as 20 to 30% lignin without sacrificing the strength of the material. They say their fibres could be used in the automotive industry and plan to take them to a car plant to test them in a real-world scenario. “If we can manage to get a fibre that can be used in the automobile industry, we will be in a good position to make biorefineries more economically viable, so they can sell what they usually would discard or burn,” says Birgitte Ahring, the lead investigator on the project. “And the products would be more sustainable and less expensive.”

These new materials are a long way from replacing energy-hungry components like concrete and metal, but research is progressing. The environmental sums add up, but it needs to make sense economically too. If researchers can get it right, and engineers buy into it, then in the future self-driving cars might just grow on trees.

In the March 2018 issue of Professional Engineering, we’ll hear from MIT scientist Seon-Yeong Kwak, who wants to create glowing plants that could replace street lighting.

Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.

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