Lignins: Co-Reactant in Adhesive Systems

Lignin is the natural binder found in plants that provides strength and rigidity to the cell wall thus enabling trees to grow in an upright fashion. In the acid sulfite pulping of wood, lignin is separated from the fibers as a sulfonated polymer and thus becomes available for use in many applications. The phenolic polymeric nature of lignin allows it to function as a binder in woody plants and makes it an excellent candidate for use as an extender or co-reactant in many different adhesive systems.

Historically, lignosulfonates have served as cost efficient extenders in phenol formaldehyde and urea formaldehyde resins replacing between 10-30% of the more expensive binder. This simple post-addition of lignosulfonates to resin produces a cheaper product but can also erode manufacturing properties in the production of plywood and other types of manufactured board. Improved polymer properties are obtained when lignin is used as a co-reactant in the resin system in lieu of an extender. The pattern of demand for wood products in the U.S. is being altered due to environmental concerns in the Pacific Northwest. Oriented strandboard (OSB) has begun to supplant plywood in comparable construction applications. Fortunately, this product substitution has favorable implications for the use of phenolic resins in wood adhesives. OSB is expected to continue to take market share from plywood for the near future.

An estimated 1.2 million metric tons of adhesive resin solids were used in 1994 in a variety of primary glued wood products. Sustained growth will be increasingly affected by environmental factors in North America. With increasing pressure to lower emissions of volatile organic compounds such as phenol, methanol, formaldehyde, and acetate and isocyanate monomers, alternatives to petrochemicals must be used. This bodes well for lignosulfonates, which perform effectively as co-reactants in resin systems.

Lignosulfonates are usually reacted into resin polymers through their addition to the initial stage of the cook when mainly resin monomers are present. Significant research into this area has developed a variety of processes to increase the reactivity of lignosulfonates and improve final resin properties. In a recently patented method, a lignin modified phenol formaldehyde resin was produced with up to 23% phenol replacement with no loss in resin properties.

In this process, the molecular weight of the lignosulfonates was significantly reduced and its reactivity increased through a pre-reaction with methylolated phenols.

Lignosulfonates also can be incorporated into a variety of different polymeric adhesive systems. Work has been done with melamines, polyurethanes, epoxides, polyisocyanates, polyacrylamides, and many other systems. Although current usage of lignosulfonates in these areas is limited, significant research continues.

The dispersing and crystal modification properties of lignosulfonates can be used in resin systems to inhibit crystallization and scaling in production facilities and end-use plants. This can be accomplished through additional levels of as little as 5% lignosulfonates.

Selected References
1. Lewis, N.G.; Lantzy, T.R.; Adhesives from Renewable Resources; Hemmingway, R.W.; Conner, A.H.; Branham, S.J. Eds; ACS Symposium Series 385, Am. Chem. Soc.: Washington, DC, 1987, 1-26.
2. Nimz, H.; Wood Adhesives: Chemistry and Technology; Pizzi, A.Ed.; Marcel Dekker Inc.: New York, NY, 1983, 247-288.
3. Doering, G.A.; U.S. Pat. 5,202,403, 1993.
4. Gorbaty, Larisa; Phenolic Resins , p. 580,0900, Chemical Economics Handbook, 1993.
5. Sellers, Terry, Jr.; Forest Products Laboratory, Mississippi State University, New Developments in Adhesives and Gluing in North America.