Structural insights of sulfoethylated kraft lignin at different drying temperatures

Sulfonated lignin has potential applications in various fields, including wastewater treatment, the textile industry, and construction. Although the reaction chemistry of sulfonated lignin has been well studied, the method of drying this material after synthesis has not been evaluated. In this study, sulfoethylated kraft lignin (SEKL) was synthesized as a representative sulfonated lignin to investigate the impact of drying temperature on its structure and properties. For SEKL, freeze-drying at –55 °C resulted in the highest charge density, water-solubility, and more uniform chemical structures. The sulfonic acid groups underwent alkylation reaction after the sample was oven-dried at 55 and 80 °C, which was accompanied by a reduction in solubility and charge density of the sample, as well as an increase in glass transition temperature. Furthermore, the sample after drying at higher temperatures of 105 and 130 °C not only exhibited the alkylation but also underwent hydrolysis of the sulfonic acid groups at the phenolic position of lignin. This change was accompanied by a further reduction in molecular weight, solubility, and charge density, as well as a decrease in the glass transition temperature. It was found that the alkylation and hydrolysis of SEKL occurred to varying degrees, and higher temperatures promoted hydrolysis. Based on the results of this work, drying temperature has a significant effect on the properties of the SEKL sample, implying that it must be taken into account when considering its application in different fields.