Kraft lignin in polyurethanes I. Mechanical properties of polyurethanes from a kraft lignin–polyether triol–polymeric MDI system

Polyurethanes with various NCO/OH ratios and kraft lignin contents were synthesized by polymerization of kraft lignin (free of the low and high molecular weight fractions), a propylene–oxide‐based polyether triol and polymeric MDI in tetrahydrofuran solution. Films were made by solvent casting and tested with respect to swelling behavior and tensile properties. The main findings were: At low NCO/OH ratios, kraft lignin contributed effectively to the formation of the three dimensional network; under particular conditions of NCO/OH ratio and kraft lignin content, polyurethanes of considerable toughness were obtained; at high kraft lignin contents, the obtained polyurethanes were hard and brittle regardless of the NCO/OH ratio used.