Kanazawa University researchers processed papermaking byproduct kraft lignin into (from left to right) electrospun fibers, thermally stabilized fibers, and carbon fibers. The material in the top three images was processed for 5 minutes, the material in the bottom three was processed for 10 minutes.
Photo courtesy of Kanazawa University

Carbon-fiber reinforced plastics (CFRP) are as strong as many metals but at a tiny fraction of the weight, making them the material of choice for high-performance supercars such as the Bugatti Chiron and electric vehicles (EVs) that include BMW’s i8. However, the material is expensive and can be difficult to process.

Researchers at Japan’s Kanazawa University believe they have a solution – generating carbon fibers from a waste byproduct left over when making paper.

Presented in a study recently published in Chemical Engineering Journal, researchers from Kanazawa chemically modified kraft lignin into a possible lightweight automotive structural material.

Paper mills usually burn kraft lignin, a waste byproduct of the wood pulping process, to generate power because it’s difficult to use for anything except specialized purposes. Chemically processing kraft lignin into a more useful material could have a dual environmental impact – creating a new advanced material and improving sustainability of paper production.

“We performed a chemical modification of a kraft lignin polymer known as acetylation,” says first author László Szabó. “Optimizing the extent of acetylation was critical to our research effort.”

A controlled reaction was important for optimizing kraft lignin’s ability to be compatible with another polymer called polyacrylonitrile, making it possible to prepare quality carbon fibers to create an engineered composite. If there’s too little or too much acetylation, the carbon fibers are low quality.

“Our reaction was quite mild, producing only a rather benign side product – acetone – without changing the polydispersity of the kraft lignin,” explains Kenji Takahashi, co-senior author. “We were able to mix kraft lignin with polyacrylonitrile to obtain a dope solution for electrospinning containing more compatible polymer segments and eventually fabricate quality carbon fibers.”

The researchers’ carbon fiber mats contain fine uniform fibers, without the thermal treatment lessening fiber quality. Compared to unmodified kraft lignin, the modified polymer in the fiber mat exhibited almost 3x greater mechanical strength.

“Our fibers’ mechanical performance is attributable to the tailored graphitic structure of the materials,” Szabó explains. “This outcome is owing to the improved polymer interactions leading to a more aligned polymeric network which is then subjected to the thermal treatment.”

Engineered composites are common in spacecraft, cars, and plastics. When these researchers minimize the cost of preparing their new carbon fibers, vehicles of the future could become lighter, more durable, and more fuel-efficient.

Kanazawa University https://www.kanazawa-u.ac.jp/e