Zinc chloride/acetamide deep eutectic solvent-mediated fractionation of lignin produces high- and low-molecular-weight fillers for phenol-formaldehyde resins

Lignin readily dissolves in a deep eutectic solvent (DES) composed of ZnCl2/acetamide 1:3 M ratio in concentrations up to 16.7 wt %, and upon addition of water, two fractions were obtained; one composed of lignin with low-molecular weight that remained dissolved in the DES/water mixture, whereas the other fraction of lignin with higher molecular weight (regenerated lignin, RL) was regenerated by precipitation. Both the RL fraction and the whole solution of lignin in DES were used to replace part of phenol (20 wt %) in the condensation of phenol and formaldehyde to introduce biomass in the resulting resins. The lignin treatment that produced the higher yield of RL consisted of lignin/DES ratio of 1:10 (w/w) stirred for 4 h at 100 degrees C. By using C-13 NMR spectroscopy, a preferential cleavage of the S unit of lignin during DES treatment was identified. The modified phenol-formaldehyde resin (RLPF) containing the RL exhibited higher bonding strength (1.28 +/- 0.16 MPa) and shorter Sunshine gel time (557 s) than PF (587 s), showing that the RL helps to improve the polycondensation process. The resin modified with the whole solution of lignin in DES exhibited the shortest Sunshine gel time due to the effect of zinc(II) accelerating the curing process. (c) 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 48385.