Over 300 million tonnes of plastic and billions of tonnes of biomass are produced every year. The biomass comes about naturally through photosynthesis. While around 4 to 12 million tonnes of plastic end up in the sea¹, only a fraction of the billions of tonnes of biomass is used, usually either as building material or feedstock for the chemical industry. However, biomass has huge potential for value-added products that is far from being exhausted. Companies, universities and government departments are increasingly seeking to reduce plastic consumption and find renewable alternatives. Only one percent of all plastic products worldwide is made from renewable raw materials. The potential is much greater: from a technical viewpoint, eco-plastics have the potential to replace oil-based plastics.

Cellulose is still the most important constituent of wood used in the industrial production of paper. Lignin is a by-product of paper production and is almost exclusively combusted and used for energy production. However, in order to improve raw material utilisation cascades, focus needs to be put on the use of lignin in value-added products, rather than combusting it. The expectation is that the repeated utilisation of one and the same raw material and a circular economy without waste has ecological and economic benefits. The German Federal Ministry of Food and Agriculture (BMEL) promotes pioneering work on the development of methods that allow the production of products from biomass. In the effort to shift from a fuel-based to a biobased future, researchers are examining plants and their ingredients for their suitability as industrial raw materials. The BMEL funds the development of a lignocellulose biorefinery based on beech and poplar wood in Leuna.

The Leuna Research Centre was set up to close the gap between basic research and industry. The centre develops concepts and methods entirely using wood. In addition to cellulose and hemicellulose, lignin is now also coming to the fore as important biomass resource. Pure lignin is recovered using a pulping technique known as organosolv that generates a dark brown powder. This powder can be used for further research and development. Every year, about 50 million tonnes of lignin accumulate as a by-product of the paper industry. Plants produce around 20 billion tonnes of lignin every year. "Many things can be made from lignin in the laboratory, but there are not many industrial products yet,” says Prof. Dr. Marie-Pierre Laborie who occupies the Chair of Forest Biomaterials at the University of Freiburg. Laborie is researching methods that can be used to combine lignin with cellulose into a composite that can be further processed in a liquid state. The environmental idea behind this is that producing lignin composites leads to a CO₂ sink. It is better to keep the CO₂ bound for some time in the composite material, rather than burning the lignin and immediately releasing the CO₂.

Lignin is not a uniform substance and therefore not easy to process. It is made up of a group of different macromolecules with a very complex structure. “Tree lignin is a heterogeneous molecule that can adapt to different environmental conditions and adopt different chemical structures,” Laborie says. “That’s an advantage, especially for the tree.” In the laboratory, however, variability reduces predictability. Raw materials with exactly defined and unchanging characteristics are normally preferred. This is the case with artificial polymers derived from petroleum oil, but not with natural products. Moreover, polymers made of lignin have a high level of contamination and sulphur, which is why there are currently only a few approaches geared towards producing lignin-based polymers.

Over the past few decades, German forests have undergone a conversion from coniferous to hardwood forests, especially beech. There is still quite a high demand for industrial beech utilisation concepts,” says Laborie who is part of the Bioeconomy Research Programme Baden-Württemberg run by the Baden-Württemberg government that specifically addresses biogas, lignocellulose and microalgae issues. The lignin powder which Laborie uses for her research comes from Leuna and is derived from beech. “Our predominant interest is to give beech lignin a value,” said Laborie. The organosolv method involves pretreating lignin with ethanol, which isolates fractions of lignin from the wood in as pure a state as possible. “This makes the lignin a bit more controllable in terms of structure, heterogeneity and molecular weight,” said the researcher. The availability of pure lignin fractions makes it easier for the researchers to control the reactivity of the wayward molecule. But it still remains problematic.

Lignin is suitable for producing biomaterials since it contains significantly more carbon than oxygen. This highly complex material is very stable and resistant to pressure as well as bacterial and fungal degradation. Lignin can be combined with natural fibres and natural additives into a fibre composite material (ARBOFORM®). It is even possible to produce formulations containing conventional polymers (ARBOBLEND®). Laborie's idea is to synthesise a largely nature-based material and recombine lignin with cellulose in a similar way to wood, something that has been happening for millions of years. LIGNOSIT is a scientific concept in which the two wood components are brought into a liquid phase and subsequently dried, resulting in a firm composite with good mechanical and physical characteristics.

The liquid intermediate phase could ideally be used for 3D printing. The biggest challenge is to achieve a viscoelastic consistency that allows the material to be pressed through the printer nozzles. Moreover, two substances of different solubility that do not mix voluntarily must be brought together in water. In this initially liquid-crystalline ratio of the two components, the cellulose nanocrystals provide the framework that facilitates the orientation of lignin molecules along the crystalline nanocrystals. "If the solvent is removed or another one used, or if the temperature changes, the liquid becomes a flexible and solid material," Laborie says, "then we can give it a shape." The hope is that the uniformly defined orientation of the cellulose nanocrystals will help harness the material. The question is, up to what proportion of lignin allows the composite to remain in the liquid-crystalline phase in which it can be processed? Initial results show that a nanocomposite involving the model molecule vanillin remains together with the cellulose in the desired organisation. The mediator between lignin and cellulose is not yet a completely natural product, but a first step towards bioplastics has been taken.

In future, platform chemicals will increasingly be produced from raw materials containing lignocellulose. Scientists are also thinking of using carbon fibres for lignin applications, which are of major interest for automotive, aircraft and bicycle construction. Carbon fibres must have a minimum amount of carbon, which is abundant in lignin. Of course, it is preferable for these fibres to be produced from renewable raw materials. Because it is ecologically safe, lignin can be an excellent alternative to phenols and formaldehydes in fillers and binders.

References

¹ Jambeck et al.: Plastic waste inputs from land into the ocean, Science 13 Feb 2015, Vol. 347, Issue 6223, pp. 768-771, DOI: 10.1126/science.1260352