Under the conditions evaluated by NARA to convert post-harvest forest residuals into biojet fuel, approximately two-thirds of the woody material is left over as a byproduct. This left over material is rich in lignin, a complex polymer molecule found in plant cell walls.
The USDA-NIFA, through the NARA grant, funds research to develop high-value chemicals and materials that can be made from the lignin-rich material left after the various stages of producing bio-jet fuel from post-harvest forest residuals. Developing these high-value products is an essential part of an economically sustainable supply chain.
NARA researchers have used the lignin-rich material to make activated carbon, cement additives, resins, and feedstock chemicals with the intent to transfer this technology to private industry. As part of this effort, Simo Sarkanen and his team at the University of Minnesota are developing formulations for converting the co-product lignin into bio-plastics. Recently his team published a peer-reviewed paper, partially funded by the USDA-NIFA through NARA, describing the lignin-based plastics developed in their lab, which are distinguished by the highest lignin contents ever reported.
The authors isolated lignins from two sources. One source came from Jack pine (Pinus banksiana) and the other came from post-harvest forest residuals consisting mainly of Douglas-fir (Pseudotsuga menziesii). The Jack pine lignin was obtained by extracting into solution the lignin from wood that had been milled into a powder. The lignin from post-harvest forest residuals (termed NARA lignin) was obtained after the residuals had been pretreated and enzymatically hydrolyzed. Pretreatment and enzyme hydrolysis are processes used consecutively to liberate the simple sugars from wood so they can be fermented to produce bio-jet fuel; thus this lignin preparation is similar to the lignin that would be produced in a biorefinery envisioned in the NARA supply chain.
A fundamental difference between these two lignin samples is that sulfonic acid groups are linked to the NARA lignin due to the bisulfite-based pretreatment process used, whereas the milled wood lignin from Jack pine does not contain sulfonic acids. Lignins possessing sulfonic acid groups are commonly called lignosulfonates or ligninsulphonates.
The authors manipulated the lignin samples so that they combined to form polymeric materials or plastics without small voids in their interior domains. Manipulations of the lignin samples included introducing methyl groups onto the lignin and/or blending with other chemicals or polymers to reduce brittleness. The mechanical properties of the various lignin-based plastics were then compared to commercially available polystyrene and polyethylene, polymers used for packaging and building materials. The authors also examined the lignin-based plastics at the molecular level to see how the molecular lignin components were arranged within these new materials.
The paper describes the first comparison of polymeric materials or plastics based on softwood lignin and lignosulfonates that are either chemically methylated or not methylated. After methylation (introducing methyl groups onto) the lignin from milled Jack pine, the authors produced a bio-plastic containing 85wt% of the methylated lignin that surpassed polystyrene in strength (tensile behavior). Using 85wt% of the unmethylated (chemically unmodified) NARA lignosulfonate, the authors produced a bio-plastic with similar properties to polyethylene. From a commercial perspective, these results demonstrate that a polyethylene substitute can be made from the lignin material generated using the conversion processes optimized in the NARA project.
This work contradicts traditional thinking about what lignin polymers are really like. In the past, materials made with very high lignin content have been quite brittle. The brittleness was thought to be caused by lignin macromolecules being too rigid due to chemical crosslinking (covalent bonding) within the individual polymer chains. To the contrary, the properties of the new plastics, with very high lignin contents, reveal that numerous non-covalent bonds are instrumental in preserving material continuity through the arrangements of individual lignin components at the molecular level. This understanding sets a new course for manipulating lignin for commercial purposes.
Dr. Sarkanen and his colleges have applied for an international patent titled “Compositions Including Lignin” (PCT/US2015/020599) and a provisional patent entitled “Compositions Including Ligninsulfonate, Compositions Including Un-alkylated Lignin, and Methods of Forming” (62/215,017) are partially based on the results from this work.