Carbon Fibers From Lignin

Water-spun lignin precursor fibers

DITF has developed an environmentally friendly and economical process for carbon fiber production using lignin as a precursor.

TW Special Report

Carbon fibers are typically produced on an industrial scale using polyacrylonitrile (PAN). The stabilization and carbonization of the fibers takes place with long dwell times in high-temperature furnaces. This production method requires lots of energy and results in expensive finished fiber. In addition, toxic by-products must be separated during manufacturing, which is a costly and energy-intensive procedure.

A novel process developed at Ger-many-based DITF Denkendorf (German Institutes of Textile and Fiber Research) enables large energy savings in all the processing steps. Lignin replaces PAN in the production of precursor fibers, which is converted into carbon fibers in a second process step. Lignin as a starting material for carbon fiber has so far received little attention in industrial production. It is an inexpensive raw material that is available in large quantities and is a waste product during paper production.

Stabilized carbon fiber

New Ways To Carbon Fiber

The new process for producing lignin fibers is based on an aqueous solution of lignin. For this purpose, wood is separated into its components — lignin and cellulose. A sulfite digestion process enables the production of lignosulfonate, which is dissolved in water. An aqueous solution of lignin is then the starting material for spinning the fibers using a dry spinning process.

During spinning, an extruder presses the spinning mass through a nozzle into a heated spinning shaft. The resulting continuous fibers dry quickly and uniformly in the spinning shaft. Lignin fibers spun from an aqueous solution is a completely new and environmentally friendly approach, which does not require the use of solvents or toxic additives.

The following steps for the production of carbon fibers — namely stabilization in hot air and subsequent carbonization in the high-temperature furnace — are similar to those of the usual process when PAN is used as the precursor fiber. However, lignin fibers also show their advantages here because they can be stabilized particularly quickly in the oven with hot air and only require relatively low temperatures for carbonization. The energy savings in these process steps compared with PAN are around 50 percent and represent a real competitive advantage.

Carbonized continuous filament carbon

Lignin Fibers Spun From Water Offer Technical Advantages

The dry spinning process allows high spinning speeds. As a result, much more material is produced in a shorter time than is possible with PAN fibers. This is another competitive advantage, which nevertheless does not allow any compromises to be made on the quality of the lignin precursor fibers — the fibers are in fact extremely homogeneous, have smooth surfaces and no adhesions. Such structural features facilitate further processing into carbon fibers and ultimately also into fiber composites.

In summary, the precursor lignin fibers obtained in the new spinning process show clear advantages over PAN in terms of cost efficiency and environmental compatibility. The mechanical properties of the carbon fibers produced from them, on the other hand, are almost comparable— they are just as strong, resistant and light as is known from commercially available products.

Carbon fibers made from water-spun lignin fibers are likely to be of particular interest for applications in the construction and automotive sectors, which benefit greatly from cost reductions in the production process.

March/April 2023