Lignin Functionalized by Thermally Curable Propargyl Groups as Heat-Resistant Polymeric Material

Lignin was chemically functionalized via Williamson etherification reaction with propargyl bromide to prepare a thermosetting bio-resin, propargyllignin. The optimum reaction conditions were found at 70 °C for 4 h under the catalysis of sodium hydroxide in the ethanol solution. The propargllignin resin had highly improved processing capability, evidenced by good solubility, low melting point (<100 °C) and thermal curability. Upon heating, propargyllignin resin solidified with an exotherm around 180-280 °C peaking at 240 °C, implying its thermal cure reaction via ethynyl polymerization. Compared with lignin feedstock, the propargyllignin resin showed much higher thermal properties: an onset decomposition temperature of 410 °C and a residue yield of 58 % at 800 °C (for lignin, onset decomposition temperature of 250 °C and residue yield of 34 %). Different from the lignin feedstock, propargyllignin was readily applied as curable thermosetting resin for composite matrix, and exhibited high mechanical and thermal properties.