Thermal and Mechanical Properties of Lignin Derivative–Sulfur Composites

Lignin, comprising 20%–35% of lignocellulosic biomass, is the second most abundant biopolymer after cellulose. As the bioethanol industry expands, the accumulation of lignin by-products necessitates innovative valorization strategies. This study explores the synthesis and characterization of lignin-derived composites. Specifically, the reaction of 20 wt. % lignin-derived guaiacol or syringol with 80 wt. % elemental sulfur gives composites GS₈₀ and SS₈₀, respectively. The chemical structures of composites were elucidated using GC–MS, ¹H NMR, and UV–Vis spectroscopy, revealing the formation of both SC_aryl and SC_alkyl bonds. Thermal and morphological analysis (via TGA, DSC, PXRD, and SEM-EDS) indicated SS₈₀ has higher crystallinity and thermal stability than GS₈₀, attributed to a higher degree of crosslinking and a greater content of dark sulfur. Mechanical testing showed SS₈₀ exhibits superior compressional and flexural strengths, and enhanced Young's modulus and Shore hardness, compared to GS₈₀. Notably, the mechanical strength parameters for SS₈₀ are comparable to those of C62 class bricks used in construction applications. These findings suggest that lignin-derived composites, particularly those incorporating syringol, can provide viable alternatives to traditional materials in various applications, contributing to both waste valorization and sustainable materials science.