Chemical and thermal characteristics of lignins isolated from Siam weed stem by acetic acid and formic acid delignification

The stem of Siam weed, an invasive exotic plant, was delignified by Acetosolv and Milox processes. The lignin products, Acetosolv lignin (AL) and Milox lignin (ML), were characterized in order to investigate the chemical structures and thermal characteristics. It was found that the weed lignin was syringyl-guaiacyl (GS) type lignin. Both lignins showed similarity in elemental components, functional groups and molecular weights. However, ML had higher methoxyl and phenolic hydroxyl group contents, while AL showed more acetyl group content. UV and FTIR spectra showed that few p-coumaric acid or hydroxycinnamic acid type structures were present in both lignins. According to differential scanning calorimetry (DSC), the glass-transition temperature (Tg) of AL and ML was determined as 149.6 and 144.0 °C, respectively. Thermogravimetric analysis (TGA) showed that both lignins had the maximum rate of weight loss at 372.7 and 368.9 °C, respectively. The activation energy for the pyrolytic degradation of AL and ML were 31.84 and 34.61 kJ mol−1, respectively.