Introduction of Multifunctional Phenols to Lignin and Utilization of the Phenolized Lignins as Curing Agents For Epoxy Resin

Abstract

We have synthesized phenolized lignins, 2mr-L and py-L, by reacting lignin with 2-methylresorcinol (2mr) and pyrogallol (py), respectively, as the phenol analogues having multiple OH groups on an aromatic ring under acidic condition. Curing of diglycidyl ether of bisphenol A (DGEBA) by using the phenolized lignins as curing agents and 1-cyanoethyl-2- ethyl-4-methylimidazole (2E4MZ－CN) as a curing accelerator afforded BA/2mr-L and BA/py-L as the corresponding cured epoxy resins. These cured resins showed higher glass transition temperatures (T_gs) than that of the epoxy resin cured by the unmodified lignin as a curing agent (BA/L). In particular, T_g of BA/py-L was 218℃ and it was 79℃ higher than that of BA/L. The results of dynamic viscoelastic measurements of the cured resins suggested that T_gs of BA/2mr-L and BA/py-L were improved by increasing crosslink density of the cured resins, which proved the effect of introducing the phenol analogues with multiple OH groups to lignin. Thermogravimetric analysis (TGA) of BA/2mr-L and BA/py-L showed residual weights of 30% or more at 800℃ under N₂ atmosphere.