Mechanically Strong and Fast‐Curing Kraft Lignin‐Based Polyurethane Foam

ABSTRACT The creation of biobased rigid polyurethane from nonfood resources such as lignin has garnered significant interest, especially concerning the biorefinery concept. However, few studies have investigated virgin lignin as a polyol source for polyurethane foam and the effects of single factors, such as the lignin solvent, lignin concentration, and lignin:isocyanate ratio, on the properties of polyurethane. High‐density rigid foam (~500 kg/m3) was successfully formed by all the solvents except sodium hydroxide, which was confirmed via Fourier transform infrared (FTIR) spectroscopy and pyrolysis‐gas chromatography–mass spectrometry (Py‐GCMS). The highest to lowest modulus of elasticity (MOE) and modulus of rupture (MOR) values for the different lignin solvents were as follows: tetrahydrofuran (THF) (402.25 and 13.24 N/mm2) > dimethyl formamide (DMF) (46 and 1.69 N/mm2) > dimethyl sulfoxide (DMSO) (29 and 1.39 N/mm2). DMSO was selected as the optimal solvent because of the considerable curing time (26.67 s) needed to produce rigid foam. A higher lignin concentration resulted in better physical, mechanical, and thermal properties of the foam to some extent. Moreover, the opposite occurred with increasing isocyanate ratio in the system. These findings can serve as a basis for optimizing the utilization of lignin as a bioresource in the manufacture of rigid polyurethane foam with more specific uses.