Effect of DES-NiO System on Modified Lignin and Synthesis of Lignin-Based Epoxy Resin

A new method that nickel oxide (NiO) and the deep eutectic solvent (DES) consisted of choline chloride-urea (ChCl-urea) was used to modify lignin and subsequently synthesize LGEP (lignin-based epoxy resin). The effect of DES-NiO system on lignin and as-prepared LGEP structures was investigated by using FTIR (Fourier transform infrared spectroscopy), TG-DTG (Thermo gravimetric analysis and Derivative thermo gravimetric analysis), SEM (Scanning electron microscope), XRD (X-ray Diffraction), AFM (Atomic force microscopy) and C-13-NMR (C-13-Nuclear magnetic resonance). The results demonstrated that the hydroxyl content and three-dimensional structure of lignin could effectively improve and change by using DES-NiO system. This was attributed to the ability of NiO to fracture selectively C-O bonds and hydrogen-bond interaction of DES can influence the structure of lignin. These various changes on lignin structures improved the reactivity of lignin. The synthetic experiment proven that DES-NiO system modified lignin could efficiently reacted with epoxy chloropropane (ECH) to prepare LGEP. In addition, it was found that the excess ECH was against producing LGEP due to the self-polymerization of ECH.