For wood tissue, lignin plays an essential role. It is a complex molecule located in the cell walls that provides mechanical strength and allows for efficient water transport. It is also quite plentiful. Depending on the species, the lignin in wood can represent between 25 to 30% of the dry weight.
Typically, the lignin produced from the pulp and paper industry (known as kraft lignin) is considered a byproduct and burned to power electrical generators or provide heat. A number of commercial products have been produced from kraft lignin including cement dispersants, dust suppressant for roads and a variety of chemical products
As with the pulp and paper industry, the process to convert wood residuals into simple sugars used to make biojet fuel leaves a lignin-rich byproduct. Creating products from the lignin byproduct with high economic value is one of NARA’s five goals. Success in this area will help drive the economic viability of a wood residual to biojet and co-product industry.
NARA funds research to develop commercial products from a range of lignin sources. Research at the University of Minnesota under Simo Sarkanen, explores commercial use from purified unaltered lignin; work done at Weyerhaeuser use an impure lignin source or “lignin rich” material, left over after the simple sugars in wood residuals are removed for biofuel production, as a raw material for product formation; and kraft lignin is the material investigated in a recent publication by Washington State University researchers Jianglei Qin, Michael Wolcott and Jinwen Zhang.
To obtain a copy of Use of Polycarboxylic Acid Derived from Partially Depolymerized Lignin As a Curing Agent for Epoxy Application, click here.
Breaking lignin into smaller parts
In this paper, two important steps were accomplished. First, Jianglei Qin and his colleagues developed a process in which they “depolymerized” or broke the kraft lignin structure into smaller pieces. In doing so, they changed the properties of the kraft lignin so that it could dissolve into organic solutions. Now the “partially depolymerized lignin” (PDL) is a biomaterial that is more compatible with other materials. “This paper introduces the concept that PDLs can be used as a feedstock for the preparation of new polymer materials”, says co author Jinwin Zhang, associate professor in the Composite Materials and Engineering Center at Washington State University.
Using PDLs as a curing agent for epoxies
To prove his point, they next modified the PDL and used it as a curing agent to cure a commercial epoxy called DER353. Epoxy resins have excellent bonding properties and are used in a wide range of commercial applications. Curing agents can modify the epoxy characteristics for mechanical strength, chemical resistance and electrical capacity. As a curing agent, the modified PDL worked as well as commercially available curing agents.
PDL’s from “NARA lignin”
Using the PDL as a curing agent is just one of many possibilities for the new raw material and new techniques are being evaluated to create alternative forms of the PDLs. In a separate paper published recently in Bioresource Technology, Dr. Zhang and this team created high yields of PDLs from a different lignin source derived from wood residuals after the simple sugars were removed by enzyme hydrolysis. The lignin in this case best resembles the lignin left after the conversion of wood residuals into isobutanol and therefor resembles the lignin material anticipated in the conversion from wood to biojet fuel. Here again, the PDLs from this lignin were highly soluble in organic solvents.
To obtain a copy of Use of Polycarboxylic Acid Derived from Partially Depolymerized Lignin As a Curing Agent for Epoxy Application, click here
Having a new biomaterial like PDLs to work with opens new opportunities to create commercial products. These opportunities are not only important for creating an economically sustainable wood residual to biojet industry, but also another avenue to reduce the use of fossil fuels as a feedstock for commercially valued chemicals.