Depolymerization of renewable resources—lignin by sodium hydroxide as a catalyst and its applications to epoxy resin

ABSTRACT Alkali lignin was successfully depolymerized into polyols with high hydroxyl number via direct hydrolysis using sodium hydroxide (NaOH) as a catalyst, without any organic solvent agent. Hydrolysis of lignin can produce a multitude of high‐value products via alkali‐catalyzed cleavage. This process usually gives good results with respect to the yield of phenols. Through this method, the numbers of the hydroxymethyl and phenolic hydroxyl groups of lignin had been dramatically increased, reaching 2.11%, nearly four times higher than that in the original one. Meanwhile, we added the same amounts (20 wt %) of different depolymerization of lignin (DL) into epoxy resin (EP), and the results showed that DL could not only increase the decomposition temperature of EP, but also remarkably improve its mechanical properties. The optimum reaction time was 1.5 h, the reaction temperature was 250°C, and the optimum sodium hydroxide concentration was 15 wt % for depolymerizing lignin. The mechanical and thermal properties of cured lignin‐based epoxy resin (LEP) were compared with cured neat EP. The cured DL‐based epoxy resin (DLEP) showed the highest adhesive shear strength up to 2.66 MPa, which displayed 122% of the adhesive shear strength of EP. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132 , 42176.