Making cars out of petroleum residue

A new method could turn the cheap byproducts of petroleum refining into high-value, ultralight structural materials made of carbon fibre for use in cars, aircraft, and spacecraft.

Materials made from carbon fibre are valuable because they are exceptionally strong while remaining lightweight. However, they are more expensive to produce than the steel or aluminium used in construction of vehicles and aircraft, and their use has been limited to niche applications – such as aerospace applications – so far.

Now, researchers have come up with a way to make carbon fibre out of an ultra-cheap source: pitch. The heavy, gloopy waste material left over after the refinement of petroleum, pitch is currently used for low-value applications (such as in asphalt) or simply treated as waste.

Using petroleum residue to manufacture carbon fibre would make it much cheaper to produce, and this could benefit not only out-of-this-world applications like spacecraft, but potentially replace significantly heavier materials in standard vehicles closer to home.

The new study is published in Science Advances.

The carbon fibre produced from this method has advantages other than lower cost of manufacture over traditional carbon fibre, which are typically made from polyacrylonitrile. One is that it can have compression strength – meaning that it could be used in applications where it was load bearing.

The research began as a way to produce cheaper materials to lower the overall weight of cars and make them more efficient and to reduce their fuel consumption.

Composites made from carbon fibre have only been used in a few very expensive vehicle models so far, because those of the quality needed for automotive use currently cost at least $22–27 per kilogram. Those for specialised use in spacecraft components are even more expensive – up to several hundred dollars per kg.

The study team worked in collaboration with researchers at Oak Ridge National Laboratory, US, who brought expertise in manufacturing carbon fibres under a variety of conditions – from the lab scale all the way up to pilot-plant scale.

“Pitch is incredibly messy,” says Dr Nicola Ferralis, a research scientist in the Department of Materials Science and Engineering at Massachussets Institute of Technology (MIT) in the U.S. It’s a hodgepodge of mixed heavy hydrocarbons, and “that’s actually what makes it beautiful in a way, because there’s so much chemistry that can be exploited.”

Industrial materials generally need to have very consistent properties. Because pitch isn’t a uniform mixture of molecules, using it for fabrication is a complex process that leads to inconsistency in the final products’ properties.

The researchers used computer simulations of the dynamics between molecules in the pitch (the way that bonds form and crosslink between them) to develop a way of predicting how different processing conditions could affect the resulting properties of the fibre.

They identified the key chemical and processing parameters that determine and control the formation of pitch-based carbon fibres.

“We were able to reproduce the results with such startling accuracy,” explains lead author Asmita Jana, a graduate student at MIT. “To the point where companies could take those graphs and be able to predict characteristics such as density and elastic modulus of the fibres.”

Their results showed that by adjusting starting conditions in the production process, carbon fibres could be made that were not only strong in tension (pulling), but also in compression.

This opens up entirely new possibilities for use. Conventional fibre composites need to be made into a cloth and laid out in precise, detailed patterns to compensate for a usual lack of compressive strength. And because of the incredibly cheap starting material, the researchers estimate that their method can produce carbon fibre at a cost as low as approximately $7 per kg.

“The new route we’re developing is not just a cost effect,” says Ferralis. “It might open up new applications, and it doesn’t have to be vehicles.”