Optimization of Lignin Precipitation with Functional Group Control for Use in Bio-Based Polyurethane Foams

Lignin is an organic polymer that can be precipitated from black liquor, a byproduct of the pulp and paper industry. Lignin has the potential to replace petroleum-derived polyols in polymers, such as polyurethanes. In order to cross-link the polymers, a reaction between an alcohol or polyol, and an isocyanate must take place. The substitution of the lignin polymer, which is a naturally occurring polyol, for the petroleum derived polyol will lead to an improvement in the sustainability of these products. In this project, 3-mercapto-1,2-propanediol was reacted with black liquor during the lignin precipitation process in order to increase the proportion of hydroxyl groups, thereby increasing lignin reactivity and integration into the polyurethane network. Previous attempts to increase hydroxyl content through demethylation using the catalyst dodecanethiol were unsuccessful; however, evidence for incorporation of the alkane chain into the lignin structure was found. This suggested the new approach to incorporate hydroxyls into the lignin polymer using a short chain mercapto-alcohol. Using previously optimized precipitation conditions of a modified Ligno-Boost procedure, the resulting lignin with an increased hydroxyl content were used to create a broad range of sturdy foams. The hydroxyl and sulfur content of the recovered lignin was determined using XPS and confirmed that the expected reaction took place. The control lignin product and reacted lignin product were mixed in various proportions with a synthetic polyol (glycerol propoxylate) and reacted with polymethylene polyphenyl isocyanate to create bio-based foams. Future experiments will evaluate the resulting foams for flexibility, tensile strength, and impact resistance in order to determine how much lignin can be added while maintaining the mechanical properties of petroleum-based polyurethane foams. Increased lignin hydroxyl content is expected to result in more rigid foams due to the increased capacity for crosslinking as well as darker in color due to the presence of lignin. These tests will show how these lignin-based foams can act similarly or better than existing products in industry such as cushions, packaging, furniture, and fiber applications.