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 ZnCl 2 -urea was 3 : 10 (T g ¼ À26.3 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. Phenolformaldehyde 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-ZnCl 2 DES and subsequent use as filler for phenol formaldehyde resin synthesis.

Polymer Science and Engineering Using Deep Eutectic Solvents

“…An example of this type of modification was published by Lian et al, which used zinc chloride:urea 3:10 (molar ratio) DES as both solvent and reactant for lignin modification. The zinc contained in DES was partially integrated into the lignin structure by chelation with the lignin functional groups, conferring to the modified lignin a 4-fold increased molecular weight and enhanced thermal stability [123]. Crosslinking is another possible application of DES as a reactant, as reported by Jordan et al, which used a deep eutectic mixture of hexaketocyclohexane octahydrate:urea to crosslink chitosan [124].…”

Green Processing of Lignocellulosic Biomass and Its Derivatives in Deep Eutectic Solvents

“…On the other hand, neutral DESs, such as ZnCl 2 –urea and ChCl–ZnCl 2 , seemingly favor the cleavage of methoxy moieties (rather than β‐O‐4 linkages) at the aromatic ring of lignin, which also leads to a significant increase in phenolic hydroxyl groups without considerable loss in the molecular weight of treated lignin . This kind of modified lignin, with an abundance of phenolic hydroxyl groups, could be employed as both a catalyst and filler for the synthesis of phenol formaldehyde resin, which could clearly facilitate the curing process and improve the thermostability of the resin.…”

Synthesis of Alkaline‐Soluble Cellulose Methyl Carbamate Using a Reactive Deep Eutectic Solvent

“…It should be noted that a 0.5 m solution of HCl was utilized during the washing of products because a white precipitate appeared after the addition of deionized water to the DES (Figure ). The precipitation of zinc upon the addition of water has been previously observed when a DES based on urea and ZnCl 2 was used in the dissolution of lignin …”

“…The precipitation of zinc upon the addition of water has been previously observed when aD ES based on urea and ZnCl 2 was used in the dissolution of lignin. [25] The nitrogen content of CMeCsi ncreased when al onger reaction time was used;amaximum nitrogen content of 1.37 %, corresponding to ad egree of substitution (DS) of 0.17, was obtained after 3h reaction time (Table 1). It wasa lso observed that the presence of ZnCl 2 has an important effect on the reaction, as an itrogen content of 0.26 %( DS of 0.03) was obtained using only dimethylurea (Sample 5).…”