Glass transition and thermal decomposition of epoxy resins from the carboxylic acid system consisting of ester-carboxylic acid derivatives of alcoholysis lignin and ethylene glycol with various dicarboxylic acids

Alcoholysis lignin (AL) was dissolved in ethylene glycol and the obtained mixture was reacted with succinic anhydride to form a mixture of ester-carboxylic acid derivatives of AL and ethylene glycol (AL-poly(ester-carboxylic acid), ALEGPA). The obtained ALEGPA was mixed with dicarboxylic acids with different alkylene chain length such as succinic acid (alkylene chain, C2), adipic (C4) acid and sebacic acid (C8). The obtained mixture of ALEGPA and dicarboxylic acid was reacted with ethylene glycol diglycidyl ether in the presence of a catalytic amount of dimethylbenzylamine to form ester-epoxy resins. The curing reaction was carried out at 130 °C for 5 h. The molar ratio of epoxy groups to carboxylic acid groups ([EPOXY]/[AA] ratios, mol/mol) was 1.0. The ALEGPA content in the above mixture was varied from 0 to 100%. Thermal properties of epoxy resins were studied by differential scanning calorimetry (DSC) and thermogravimetry (TG). Glass transition temperatures (Tgʹs) increased with increasing ALEGPA contents, suggesting that lignin acts as a hard segment in epoxy resin networks. The values of Tgʹs of epoxy resins with dicarboxylic acids increased in the following order; epoxy resins with succinic acid (alkylene chain, C2), adipic acid (C4) and sebacic acid (C8). Thermal degradation temperatures (Tdʹs) of epoxy resins slightly decreased with increasing ALEGPA contents. The values of mass residue at 500 °C (MR500) increased with increasing AL contents in epoxy resins and also with decreasing chain lengths of dicarboxylic acids.