Engineering plastics from lignin. III. Structure property relationships in solution cast polyurethane films^†

Lignin-based polyurethane films were synthesized by solution casting from hydroxypropyl lignin derivatives and either an aliphatic or an aromatic isocyanate. Two lignins, kraft and steam explosion lignin, and two diisocyanates, hexamethylene diisocyanate (HDI) and tolylene diisocyanate (TDI), were chosen for the study. It was found necessary to use stoichiometric excess diisocyanate in the synthesis of the thermosetting polyurethanes. This part of the series addresses the effect of synthesis variables on film properties. The study examines the effect of lignin type, of diisocyanate type, and of composition in terms of NCO to OH stoichiometry on thermal and mechanical properties. Stoichiometric NCO-excess was found to cause a more significant increase in the glass transition temperature of TDI-based films than of films made with HDI. The films swelled less with increasing NCO/OH ratio. Use of aliphatic diisocyanate (HDI) resulted in films with lower moduli as compared to aromatic diisocyanate (TDI). Kraft-lignin-based polyurethanes had slightly inferior strength characteristics (Young's modulus and tensile strength) in comparison with those derived from steam explosion lignin. Variation in the NCO/OH stoichiometry had no noticeable effect on modulus or tensile strength, but did significantly influence glass transition temperature, swelling, and strain at break. It is observed that the properties of these thermosetting polyurethanes are very sensitive to their composition. The study illustrates that materials of satisfactory performance characteristics can be engineered by proper selection of synthesis variables and modification of network architecture.