Processing of lignin in urea–zinc chloride deep-eutectic solvent and its use as a filler in a phenol-formaldehyde resin

Abstract

The main goal of our research deals with a new greener and more efficient lignin modification method to optimize its structural performance as a phenol-formaldehyde resin filler. Through the appropriate increase in the phenolic hydroxyl content, added value as a raw material was intended. For that, a series of mixtures of zinc chloride–urea with different molar ratios was prepared to obtain deep-eutectic solvents (DESs). Two heating methods (oil bath and microwave heating) were compared and optimized. Microwave helps in reducing the preparation time but to the detriment of temperature control. On the other hand, heating with an oil bath provided better temperature control and homogeneity of the mixture. The preferable molar ratio of ZnCl₂–urea was 3 : 10 (T_g = −26.3 °C). The structural changes of the pretreated lignin samples were investigated by Fourier transformed infrared and X-ray photoelectron spectroscopies, scanning electron microscopy, induced coupled plasma and X-ray diffraction. Thermogravimetric analysis demonstrated significant differences in the thermal behavior of the recovered lignin as a result of DES treatment. The weight of lignin recovered was 4 times that of original lignin, indicating that the structure of lignin was transformed through the integration of Zn. The integration of Zn enhanced the thermal stability and enhanced lignin's reactivity towards phenol-formaldehyde resin formation. Phenol-formaldehyde resin containing the recovered lignin exhibited lower thermocuring temperatures and better thermostability than those without filler. To the best of our knowledge, this is the first report on the investigation of the structural transformations of lignin through its dissolution in urea–ZnCl₂ DES and subsequent use as filler for phenol formaldehyde resin synthesis.