Exploring the application potential of incompletely soluble organosolv lignin as a macromonomer for polyurethane synthesis

• Higher homogeneity of isolated straw lignin fractions relative to original lignin.
• Reactivity of isolated wheat straw lignin fractions towards isocyanate (MDI).
• Regulation of polyurethane tensile properties using lignin originated macromonomers.
• Antioxidative and char formation effect of isolated wheat straw lignin fractions.
• Enhancing of the thermo oxidative stability of lignopolyurethanes.

A new type of organosolv wheat straw lignin, so-called Biolignin™, was studied. It is obtained according to the biomass processing in water diluted organic acid media and is not completely soluble in conventional organic solvents traditionally applied for polyurethanes (PU) synthesis. The fractionation of the lignin by sequential extraction with dichloromethane, methanol and a mixture of both solvents was performed. The total yield of the separated soluble fractions was 40%. The separated fractions were characterized, and preliminary kinetic investigations were used to reveal the reactivity of fractions obtained towards 4,4′-methylene diphenylene diisocyanate (MDI) in dioxane media. PU films were synthesized by casting the three component systems: lignin fraction – polyethylene glycol with a molecular weight of 400 g mol−1 – polymeric methyl diisocyanate prepolymerized in tetrahydrofurane solution at a constant NCO/OH ratio of 1.05. The content of lignin in PU compositions was varied in a range of 5–40%. The PU films were characterized in terms of crosslink density, tensile properties, glass transition temperature and thermal stability. The obtained results show that the isolated fractions of Biolignin™ act as a crosslinking macromonomer in the PU network. Depending on the lignin fraction type and its content in the composition, the tensile properties of PU films with a high elastic state at room temperature or the tensile properties of glassy cross-linked films were obtained. Evidence for the activity of lignin fractions as antioxidants of PU compositions and charcoal formation promoters in the case of the high temperature treatment of PU in air was obtained.