Polyurethanes from Alcell® lignin fractions obtained by sequential solvent extraction

Lignin-derived polyurethane films were synthesized by solution casting using three fractions of distinct molecular weight and chemical functionality from Alcell® lignin. A three-component system of lignin fraction, polyethylene glycol (PEG) of MW=400 g mol−1 and polymeric methyl-diisocyanate (MDI) was used. For each lignin fraction, the effects of varying the NCO/OH ratio and lignin content on the crosslink density, ultimate tensile properties and dielectric strength of the polyurethanes were studied. The crosslink density was found to increase with molecular weight of lignin. Strong polyurethanes were produced with the high molecular weight fraction of Alcell® lignin, but the polyurethanes from the medium molecular weight fraction appeared to be tougher and more flexible. For both the medium and high molecular weight fractions of Alcell® lignin used, polyurethanes produced with lignin contents above 35 wt% were found to be too brittle and glassy to be tested. A maximum of 18 wt% of the low molecular weight lignin fraction could be used to prepare polyurethanes. Any higher compositions attempted produced polyurethanes which were too brittle to be tested. For all three fractions, the dielectric constant of the polyurethanes, decreased with increasing lignin content or NCO/OH ratio.