Why is wood considered a polymer

Structural analysis of lignin The secret of wood-based polymers

Editor: Christian Lüttmann

The by-product lignin from the paper industry can serve as the basis for sustainably produced plastics - but has hardly been used for this so far. Because as a natural substance, its properties are different depending on its composition. Researchers from Sweden have now examined the biopolymer with X-rays in order to better understand its material properties.

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Stockholm / Sweden - While the backbone provides stability in humans, lignin does this in plants. The biopolymer is responsible for the strength of plants and their lignification. During paper production, it is separated from the cellulose and is there as a waste product - some of it is simply burned to generate energy.

The natural substance could play a decisive role in plastics production. "Millions of tons of it are produced every year, which could be available as a continuous flow of raw materials for new products," says research director Prof. Mats Johansson from the Royal Technical University (KTH) Stockholm in Sweden. He and his team have investigated how the internal molecular structure of various lignin components is related to the material properties of the plastics made from them.

Ultrathin cellulose films

The paper from the spray can

X-ray analysis shows varying properties

The first applications of hard plastic (thermoset) based on lignin have been available for some time, but with some difficulties: the material properties often vary and have so far been difficult to control.

At the X-ray light source PETRA III of the German Electron Synchrotron DESY, the Swedish team has now examined the nanostructure of various proportions of commercially available lignin. “It has been shown that there are lignin components with larger and smaller domains,” reports lead author Marcus Jawerth from the KTH. "This has advantages depending on the application: It makes the lignin harder or softer by changing the so-called glass transition temperature at which the biopolymer takes on a viscous state."

The X-ray analysis showed, among other things, that those lignin variants in which the central benzene rings are in a T-shape are particularly stable. “The molecular structure influences the macroscopic mechanical properties,” says DESY researcher Prof. Stephan Roth. "It is the first time that this has been characterized."

Developing the by-product lignin as a raw material source

As a natural product, lignin has numerous different configurations. Further investigations should now provide a systematic overview of how various parameters influence the lignin properties. "This is extremely important in order to manufacture the materials in a reproducible manner and, above all, to predict the material properties," emphasizes Roth. "If you want to use the material industrially, you have to understand the molecular structure and correlate it with the mechanical properties."

According to KTH scientist Jawerth, up to two thirds of the lignin produced in paper production can be converted into polyester and thus serve as a raw material for the plastics industry. "Along with cellulose and chitin, lignin is one of the most common organic compounds on earth and has enormous potential to replace petroleum-based plastic raw materials," he emphasizes. "It's far too valuable to burn."

Original publication: Marcus E. Jawerth, Calvin J. Brett, Cédric Terrier, Per T. Larsson, Martin Lawoko, Stephan V. Roth, Stefan Lundmark & ​​Mats Johansson: Mechanical and Morphological Properties of Lignin-based Thermosets, Applied Polymer Materials, 2020, 2, 2, 668-676; DOI: 10.1021 / acsapm.9b01007

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