Synthesis of Polyols for Expanded Rigid Polyurethanes by Microwave Liquefaction of Kraft Lignin With Bio‐Based Solvents

ABSTRACT

In this work, polyols derived from lignin microwave (MW) liquefaction are produced using different bio‐based solvents derived from azelaic acid (Emerox) to synthesize rigid polyurethane foams (rPUFs). Lignin depolymerization is carried out according to a range of suitable conditions reported in the literature, optimizing the process by investigating the effect of solvent type, solvent‐to‐glycerin ratio, catalyst type, and concentration. The results show a liquefaction yield up to 90% with a solvent mixture containing glycerin between 30 and 50% w/w. On the other hand, the nature of the catalyst used in the liquefaction poses some limitations, as the use of H ₂ SO ₄ has to be restricted to 1% w/w to limit the acidity of the derived polyols, reducing the yield, while NaOH increases the final hydroxyl number (nOH) of the product mixture, but still maintaining a very high yield (98%). The polyols obtained under optimized liquefaction conditions are compared with commercial Emerox biopolyols for rPUFs production. The different foams exhibit similar physical properties, but fire reaction, thermal stability, and compressive strength are improved using lignin‐derived polyols, although only under specific liquefaction conditions. The best results are obtained by using a lignin‐derived polyol obtained by using NaOH as a liquefaction catalyst.