Hydroxymethylation of technical lignins obtained from different pretreatments for preparation of high-performance rigid polyurethane foam

Technical lignins are promising renewable resources for the production of a variety of value-added products such as polyurethane. In this study, hydroxymethylation of three technical lignins obtained from different pretreatments was conducted to increase their reactivity before application to rigid polyurethane foam synthesis. Among the hydroxymethylated lignins, the alkali-treated lignin (designated as H-A3 lignin) with a molar ratio of 1:3 (lignin to formaldehyde), strong acid-treated lignin (H-S5) with a molar ratio of 1:5, and weak acid-treated lignin (H-W4) with a molar ratio of 1:4 showed the highest char yield. Besides, the weight average molecular weights of the resulting hydroxymethylated lignins increased compared with the unmodified technical lignins. Moreover, H-A3 showed the highest thermal stability (T_d10 of 146 °C and char yield of 48 wt%) and weight average molecular weight (54,797 g mol⁻¹) among the resulting hydroxymethylated lignins. To produce value-added products from H-A3 lignin, rigid polyurethane foams were synthesized using the H-A3 lignin, polyethylene glycol 400, and polymeric diphenylmethane 4,4′-diisocyanate. Thermogravimetric analysis, Fourier transform infrared spectroscopy, and mechanical properties measurement confirmed that high-performance polyurethane foams with high compressive strength and low density were successfully synthesized from the H-A3 lignin.