Study on the demethylation of enzymatic hydrolysis lignin and the properties of lignin–epoxy resin blends

To increase hydroxyl content in lignin, demethylation of enzymatic hydrolysis lignin (DML) was conducted using 1-dodecanethiol as nucleophilic reagent and sodium methoxide as catalyst in this work. Under alkaline conditions, the demethylated lignin–epoxy resin (DML-EP) was synthesized by demethylated lignin reacted with epichlorohydrin (ECH). The optimum conditions for the synthesis of DML-EP were: 80 °C, 2.5 h, molar ratio of ECH and NaOH to phenolic hydroxyl of 7.5 and 1.2, respectively. FTIR spectrum showed the epoxy group appeared in the product. Then, the lignin-based epoxy composites were produced by cast molding method. Tensile strength test reflects that DML-EP-reinforced epoxy composites have an obvious improvement in tensile strength and elongation of about 27 and 34% compared to the pure epoxy resin with 5% content. Scanning electron microscopy images showed the fractured surface of DML-EP/epoxy composites was rough. Rheological analysis showed that the cured DML-EP/E-51 epoxy resin composites had better elasticity and bonding effect. TGA experiment showed the thermal stability of DML-EP/epoxy composites was also improved. Non-isothermal method was used to analyze the curing kinetics of E-51 resin and DML-EP/E-51 composites, and the curing apparent activation energy was calculated using the Kissinger and Ozawa methods.