Lignin-Based Thermosets via Direct Functionalization using an Efficient and Solvent-Free Valorization Method

Since lignin is the most abundant aromatic biopolymer, there has been significant interest in identifying ways to convert lignin into useful materials. Degradation of lignin into oligomers and monomers has been a potential approach, however, involves additional processing steps and creates waste. Lignin has abundant hydroxyl groups which can be used for further functionalization to enable crosslinking through a variety of mechanisms. To enable functionalization, lignin—which is a solid material—needs to be converted into a homogeneous mixture for the functionalization reaction. Most commonly, lignin is dissolved in a solvent, functionalized, and then the solvent removed. This process creates waste. We have discovered that lignin is soluble in polyols—such as ethylene glycol, butane diol, glycerol—and the mixture of lignin and polyol can be functionalized together creating a 100% solids crosslinkable liquid resin without creating solvent waste. This approach has been demonstrated with two functionalization chemistries. First, lignin/polyol was functionalized with acetoacetate groups which can be cured using multifunctional amines at ambient conditions. This approach also lends itself to the formation of rigid foams without the use of isocyanates. Lignin/polyol can also be functionalized using methacrylate groups. These can then be cured via free radical initiated crosslinking either thermally or using UV. Due to the aromatic content of the lignin as well as the high functionality of the resins, the thermosets are very hard, with high modulus.