Properties of lignin–polyurethane films prepared by casting method

Abstract

A polyurethane elastomer made from a 1:5:6 mole ratio of poly(ethyleneadipate), ethylene glycol, and 4,4′-diphenylmethanediisocyanate was blended with flax soda lignin to form dimethylformamide-cast films containing between 4.2 and 23.2 wt.% lignin. The spectral, mechanical and thermal properties of this new type of blends were investigated in an attempt to establish their applicative potential. Films containing more than 9.3 wt.% lignin were heterogeneous, without relevance from the standpoint of potential applications, and were not tested for mechanical or thermal stability properties. The thermal degradation range of polyurethane and blends is quite similar, but the presence of lignin accelerates decomposition at lower temperatures. At 4.2 wt.% lignin, blends lost almost one half more weight until 300 °C, and almost twice at 9.3 wt.% lignin than pure polyurethane. The presence of 9.3 wt.% lignin also lowers the main degradation intensity corresponding to the polyurethane soft segment with about 120 °C (from 360 to 235–245 °C). The lignin concentration and polyurethane soft segment properties were shown to be the key degradation factors. The tensile strength increased up to 370%, toughness up to 470% and elongation at break up to 160% at blends compared to the pure polyurethane film. By increasing the strain rate from 4.6 to 120 mm/min the blends behavior generally improves, whereas the polyurethane elastomer is detrimental affected. Lignin contents up to about 10 wt.% were adequate to increase the polyurethane strength in detriment of elasticity. However, the better thermo-mechanical properties were found for the 4.2 wt.% lignin-containing blend.