Thermal and mechanical properties of polyurethanes derived from fractionated kraft lignin

Polyurethane (PU) sheets were prepared from fractionated kraft lignin (KL) samples having molecular weights (MW's) of 1420 and 860, polyethylene glycol having a MW of 400 (PEG 400) and diphenylmethane diisocyanate (MDI). KL contents in weight %, in a mixture of KL and PEG 400, were varied from 10 to 50%. The thermal and mechanical properties of PU's were studied by thermogravimetry (TG), differential scanning calorimetry (DSC) and tensile tests. The thermal decomposition temperatures slightly decreased with increasing KL contents. The glass transition temperatures (T-g's) increased, the temperature range of glass transition (Delta T) increased, and the heat capacity difference between the glass and rubbery states at T-g's (Delta Cp) decreased, with increasing KL contents, Tensile properties such as strength at break (sigma), elongation at break (epsilon), and Young's modulus (E) were measured at 25 degrees C. The obtained results were highly dependent on the state, i.e. the glassy state and the rubbery state. The effect of molecular weight of KL on thermal degradation and tensile properties was not prominent. However, T-g's apparently increased with decreasing molecular weights of KL when the same amounts in weight % of KL were introduced into PU molecules.