Study on the preparation of epoxy resin materials from nano-lignin polyols

Formaldehyde attacks the ortho phenolic hydroxyl groups on the p-hydroxyphenyl and guaiac-based structures in the lignin, shedding the hydrogen atom on the ortho-position carbon and grafting hydroxymethyl group to obtain the hydroxymethylated modified lignin (HMKL). The infrared spectroscopy, hydrogen NMR spectroscopy, thermogravimetric (TG), and derivative thermogravimetric (DTG) analysis show that the hydroxymethylation modification of alkali lignin is successful. Then, lignin nanoparticles (LNPs) are prepared by the acid precipitation self-assembly method in ethylene glycol solution of hydroxymethylated modified lignin. The particle size and shape of lignin nanoparticles are determined by scanning electron microscopy and dynamic light scattering (DLS) analysis. In the liquefaction agent prepared from polyethylene glycol 400 and glycerol, the graft polyol is liquefied under methanesulfonic acid. The Fourier transform infrared spectroscopy, hydrogen NMR, and thermogravimetric analysis indicate the successful formation of nano-lignin polyols. In addition, epoxy resin is synthesized by two-step epoxidation of nano-lignin polyols including ring-opening and closed-loop epoxidation in an alkaline environment. The prepared epoxy resin from nano-lignin polyol exhibits excellent performance and wide application prospects. The optimum process conditions are the mass fraction of LNP-P is 19.98 %, the amount of catalyst (NaOH) is 20.10 %, the reaction time is 6.99 h, the solid content of the epoxy resin is 57.05 %, the viscosity is 615.58 mPa.s, the tensile strength is 44.80 MPa, the elastic modulus is 2.77 GPa.