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MOLECULAR WEIGHT EFFECTS OF THE SOFT SEGMENT ON THE ULTIMATE PROPERTIES OF LIGNIN-DERIVED POLYURETHANES

Polyurethane (PU) films were prepared by solution casting using a three-component system, namely a novel semi-rigid solvolytic lignin, soft segment and a co-monomer. In the present study, the effects of varying the chain length of polyethylene glycol (as the soft segment) were investigated to produce lignin-based polyurethanes with variable thermal and mechanical properties. An important objective was to incorporate as much lignin as possible. The polyethylene glycol (PEG) studied included 5 different molecular weights (200, 400, 1000, 1500 and 2000 g/mol). The polyurethane films, prepared by solvent casting, were evaluated for crosslink density, and ultimate mechanical and thermal properties. Results showed that the films derived from the PEG 200 were either too weak or brittle to be tested. It was found that the PEG (400, 1000, and 1500) are better choices for producing polyurethanes from the solvolysis lignin studied. Crosslink densities of PU films using the 400, 1000 and 1500 were determined to be in the range of 0.8–2.6 mmol/cm3, which is a lower range than those of films from PEG 2000, namely 2.4–2.8 mmol/cm3. Also, the ultimate tensile strength decreased from about 50 MPa at high lignin content for PEG 400, 1000 and 1500 to about 18 MPa for PEG 2000 at low content of lignin. Ultimate strain also decreased from the 30.9–62.7% range for the PEG 400 to 1500 series at low lignin content down to about 4% for PEG 2000 at high lignin content. Young's Modulus varied from a high of 2 GPa (PEG 400, lignin content = 30 wt%) down to 0.6 GPa (PEG 2000, lignin content = 20 wt%). The glass transition temperature was found to decrease from 108°C to about 45°C with increasing molecular weight of PEG for a lignin content of 30 wt% and an NCO/OH of 1.2. The data are consistent with the percolation theory approach to network formation, as well as the notion that the network itself consists of relatively large and stiff islands, each comprising many branch points, held together by a soft and pliable matrix.