Preparation of a novel lignin-based film with high solid content and its physicochemical characteristics

Alkali lignin (AL) and polyethylene glycol (PEG) were polymerized with hexamethylene diisocyanate (HDI) to form a novel lignin-based polyurethane film (LPF) with high solid content under the condition of silica as leveling and dispersing agent. The -CNO/-OH molar ratio under the optimum lignin content condition on the effects of thermal stability, mechanical property, glass transition temperature (Tg), water contact angle (WCA), pencil hardness, and crosshatch adhesion of LPFs were systematically evaluated. Fourier transform infrared spectra confirmed that AL and PEG successfully reacted with HDI. The elasticity modulus (121.62–157.20 MPa), tensile strength (13.31–17.07 MPa), WCA (63.6–71.3°), and pencil hardness (2B–2 H) of LPFs increased simultaneously as the -CNO/−OH molar ratio increased. The elongation at break (465.53–326.74 %) of the LPFs gradually decreased. The change trends were probably related to the enhanced crosslinking density between HDI and polyols (AL and PEG). The maximum decomposition temperature (Tmax), Tg and heat capacity change (ΔCp) of the LPFs showed a trend of first increasing and then decreasing as result of the superfluous -CNO. Interestingly, all the LPFs appeared strong crosshatch adhesion (5B) and excellent biodegradability. The approach with short processing time and simple operation was a potential method for preparing LPFs with high solid content. The experiment will help to promote the value-added application of technical lignin, such as layer construction of coated fertilizer.