Introduction of Naturally Derived Phenols to Kraft Lignin and Application of the Phenolized Lignins as Curing Agents of Epoxy Resin

Abstract

4-Ethylphenol (4-ep), pyrogallol (py) and catechin (ca) as the naturally derived phenol compounds were chemically introduced into kraft lignin under acidic condition to obtain the corresponding phenolized kraft lignins 4-ep-L, py-L and caL, respectively. Diglycidyl ether of bisphenol A (DGEBA) was then cured by using the phenolized lignins as curing agents and 1-cyanoethyl-2-ethyl-4-methylimidazole (2E4MZ-CN) as a curing accelerator, and as a result, BA/4-ep-L, BA/py-L and BA/ca-L were obtained as the cured products. BA/py-L and BA/ca-L showed glass transition temperatures (T_gs) above 200 °C, which are higher than that of the epoxy resin cured by the unmodified kraft lignin as a curing agent (BA/KrL). On the other hand, T_g of BA/4-ep-L was lower than that of BA/KrL. The results of dynamic viscoelastic measurements of the cured resins suggested that T_gs of the cured resins reflected their crosslink density and that higher T_gs of BA/py-L and BA/ca-L were derived from the introduction of the naturally derived phenols having multiple OH groups. The results of this study proved that the cured epoxy resins with both high biomass ratios and heat resistance could be prepared by introducing naturally derived polyphenols into kraft lignin.