Low-temperature thermal degradation behaviour of non-wood soda lignins and spectroscopic analysis of residues

Abstract

Soda lignins from three non-wood species, namely bamboo (Bambusa bambos), giant cane (Arundo donax) and pearl millet (Pennisetum glaucum) were studied for their thermolytic degradation behaviour. TG characterization of the soda lignins were done under nitrogen atmosphere at two heating rates of 10 and 100 °C min⁻¹. After the initial moisture removal, two distinct thermal degradation events were observed. However, thermal degradation at 100 °C min⁻¹ shifted the onset of thermal degradations to higher temperatures. Both the thermal degradation of all the three lignins was found to be exothermic except for first thermal degradation of pearl millet lignin during thermal degradation at 10 °C min⁻¹ which was almost athermic. Batch thermal degradation runs on the three lignins were carried out at different temperatures for varying lengths of time and the FTIR spectra of the thermal degradation residues (TDRs) were recorded. At 200 °C, the marginal mass loss of less than 10% is attributed to moisture loss. At lower temperature of 250 °C, the intramolecular hydrogen bonding through hydroxyl group in the original lignin was broken. Partial to complete decarboxylation also took place. The aromatic structure of the lignin largely remained intact albeit with rearrangements. Short thermal degradation for 1 h at 250 °C yielded 75–80% residue for the three lignins.