Worldwide, paper mills and biorefineries each year produce about 70 million tonnes of lignin, the woody polymer which gives plants their rigidity, as a by-product of processing wood pulp. Nearly all of it is either burned for heat or thrown away.
Researchers at Washington University in St. Louis believe that this is a wasted opportunity when it comes to a really useful substance, and they have now developed a new type of lignin carbon fibre which replaces half of the petroleum normally required to produce one of the most costly materials in engineering.
Carbon fibre is valued because it is both light and strong and is used in a wide variety of applications, such as the chassis of Formula 1 cars, aircraft wings and the blades of wind turbines. The process of producing it is costly, and a major part of this cost is due to one component - polyacrylonitrile or PAN, a polymer derived from petroleum which makes up about half of the total cost of manufacturing carbon fibre. The team from WashU, headed by Joshua Yuan, Lucy & Stanley Lopata Professor at the university’s McKelvey School of Engineering, wanted to replace as much of that PAN as possible without compromising the fibre’s performance.
Why lignin carbon fibre needs nanotubes to work
Their approach involves replacing 50 per cent of the PAN with lignin, the renewable polymer which is recovered during paper production. Since lignin by itself results in carbon fibres that are weaker and more brittle, the group included single-walled carbon nanotubes (SWCNTs) as a form of nanoscale reinforcement. According to Yuan’s description as reported by Interesting Engineering, the nanotubes function like microscopic rebar, directing the way that the carbon crystals arrange themselves when the fibre is heat-treated and thus leading to a more ordered and aligned internal structure than has previously been achieved with lignin blends.
The manufacturing process begins by combining the nanotube template with the mixture of lignin and PAN, after which the combination is wet-spun by forcing it through small openings to produce continuous filaments. The filaments then undergo a heat treatment assisted by tension and an optimised carbonisation process, which is the high-temperature stage that removes the non-carbon atoms and leaves the fibre with its characteristic strength. The result, published in the journal Matter, is a fibre with higher crystallinity and better alignment than previous attempts to incorporate lignin into carbon fibre production.
The figures are highly promising even just on paper: there will be a 25 per cent decrease in production costs together with a major reduction in carbon emissions, since half of the petroleum-based feedstock is now replaced by a by-product that would have been burned or dumped. As carbon fibre is already used in cars, aircraft, wind turbines and sports equipment and as demand is increasing most rapidly in the aerospace and energy infrastructure sectors, a less expensive method of achieving the same level of performance could enable the material to be used in applications where it is currently too expensive to do so. The team led by Yuan is aiming to use the automotive industry as its first commercial test case, since cost pressures are greatest there and the potential weight savings from using more carbon fibre are most clearly understood.
Engineering Deep Dive: Turning Waste Wood into Structural Fibre
To understand why lignin carbon fibre is important, it helps to understand why carbon fibre is so expensive to begin with. The usual method of producing carbon fibre begins with PAN, which is drawn into thin filaments and then oxidised and carbonised at temperatures as high as about 1,500°C. During this carbonisation process all the other elements are burned off leaving only carbon atoms, which then reorganise themselves into long chains resembling graphite that extend along the length of the fibre. It is these chains that provide carbon fibre with its combination of low weight and high tensile strength. PAN is obtained from acrylonitrile, a monomer derived from petroleum, and producing acrylonitrile to the required purity for carbon fibre manufacturing is a major cost, accounting for about half of the total cost of producing the final fibre.
Lignin is chemically suitable as a substitute since, similar to PAN, it is a carbon-rich polymer which can be converted into a fibre form and then carbonised. The structural problem that researchers have had to deal with for years is that lignin has an irregular and branched molecular structure as compared to PAN which has more linear and uniform chains. When lignin is carbonised on its own, this irregularity results in carbon fibres with poor mechanical properties - they are brittle and much weaker than the corresponding PAN-based fibres - which is why previous efforts to use lignin as a replacement have only ever managed to incorporate small amounts into the total mixture.
To deal with this, the WashU team introduced order from outside the lignin by using single-walled carbon nanotubes as a structural template. These single-walled carbon nanotubes are cylindrical carbon structures about a nanometre in diameter, possessing very high tensile strength and a naturally ordered lattice structure. When dispersed throughout the lignin-PAN mixture before spinning, the nanotubes seem to affect the way carbon crystallises around them during carbonisation, causing the surrounding material to adopt a more aligned, graphite-like structure rather than the disordered one that lignin would form by itself. This is known as the “nanoscale rebar” effect: just as steel reinforcing bars provide concrete with tensile strength that it would otherwise lack, the nanotubes provide the lignin blend undergoing carbonisation with a framework to organise around.
The process known as wet spinning, which is used for forming the raw filaments, consists of pumping the polymer solution through a spinneret - a plate having many small holes - into a coagulation bath in which the filament sets. The diameter of the filament, the spinning speed, and the make-up of the coagulation bath all have an effect on the fibre’s internal structure and therefore on its final mechanical properties. The heat treatment that comes after and is assisted by tension stretches the filaments as they are being heated, causing the internal polymer chains and the carbon crystals that are now developing to align along the length of the fibre. As the strength of a fibre is largely determined by how well its internal structure is aligned with the direction of the applied load, it is at this alignment stage that most of the fibre’s eventual performance is achieved or lost.
As the team’s results show, it is during the carbonisation stage which concludes the process that the nanotube templating proves itself most clearly, since it leads to a greater amount of crystalline material with improved alignment when compared to lignin-PAN blends which do not have the nanotube reinforcement. Generally speaking, more crystalline and better-aligned carbon implies greater tensile strength and stiffness, which are the properties most important in structural applications such as aircraft skins and turbine blades.
There is also a resource-related issue. About 70 million tonnes of lignin are produced each year as a by-product of pulping and biorefining, and at present it is either burned for the purpose of providing process heat or thrown away. Redirecting even a small part of that stream towards the production of carbon fibres would reduce the demand for petroleum-based PAN while giving paper manufacturers and biorefineries a more valuable use for the material they currently regard as waste, thus creating a real circular arrangement between two already established industries.
A quarter reduction in the cost of carbon fibre without any loss in strength is a kind of advancement that could have widespread effects across a number of industries, including car doors and wind turbine blades. While the automotive manufacturing sector is the company’s first focus, aerospace and the energy infrastructure sectors will be watching as well. The following stage involves scaling up the production of lignin-based carbon fibre beyond the laboratory level and verifying that the figures remain valid outside of the carefully controlled conditions.
- Washington University in St. Louis researchers replaced 50% of petroleum-based PAN with waste lignin in carbon fibre production.
- Single-walled carbon nanotubes act as a structural template, guiding crystal alignment during carbonisation.
- The process cuts production costs by 25% and substantially lowers carbon emissions.
- Around 70 million tonnes of lignin are generated annually as paper industry waste, mostly burned or discarded.
- The research, led by Professor Joshua Yuan and published in Matter, targets automotive manufacturing first.
