Synthesis of biodegradable and UV shielding waterborne polyurethane based on lewis acid demethylated lignin

To achieve high-value utilization of lignin, kraft lignin (KL) extracted from eucalyptus pulping black liquor was modified by demethylation using Lewis acids AlCl 3 (AL) and HBr (HL) and used to partially substitute of polypropylene glycol (PPG) to prepare lignin-based waterborne polyurethane (WPU) emulsion by reacting with isophorone diisocyanate (IPDI). The results of 1 H NMR spectra showed that the hydroxyl content of lignin were respectively 10.13 mmol/g and 8.31 mmol/g after demethylation with AlCl 3 and HBr, which were 73.2 % and 43.1 % higher than that of KL. When 20 wt% of PPG was replaced by AL and HL, the prepared ALWPU-20 % and HLWPU-20 % emulsions had good dispersion and storage stability, with an average particle size of 400 nm and a viscosity of 35.3 Pa·s and 24.5 Pa·s, respectively. Both ALWPU-20 % and HLWPU-20 % films had good mechanical strength, thermal stability and biodegradability, and their crosslinking density, tensile strength and natural degradation rate after 100 days’ landfilling were 19.1 mmol/cm 3 and 17.2 mmol/cm 3 , 10.9 MPa and 10.3 MPa, and 19.21 % and 21.21 %, respectively. Moreover, ALWPU-20 % and HLWPU-20 % films had excellent ultraviolet (UV) shielding performance and the UV light transmittance of them was nearly zero. Compared with HL, AL had higher reactivity and ALWPU-20 % emulsion and film had better comprehensive performance.