Infra-red spectroscopic study of lignins

Infra-red spectra from 200–4000cm⁻¹ of lignin precipitated from black liquor produced from different pulping processes of bagasse, e.g. soda, kraft, sulfite, peroxyacid and butanol, have been characterized. Peroxyacid lignins are more degraded than other lignins. However, peroxyacid lignin has a higher intensity band at 1720cm⁻¹ than other types of lignins. At the same time, the aromatic ring of lignin produced from peroxyacid pulping of bagasse undergoes severe degradation. Syringyl type of lignin is predominant in all isolated lignins. Peroxyacid and butanol lignins have lower quantities of syringyl lignin shown by the lower ratio of relative absorbance of band intensity at 1500cm⁻¹ to the band at 1600cm⁻¹ than other lignins. Kraft lignin has a broad weak band at about 630cm⁻¹ that is probably due to a C---S bond. A sharp band at 655cm⁻¹, which is due to SO₃H, is characteristic of lignosulfonate, which is precipitated from black liquor produced from sulfite pulping process of bagasse. Generally, degradation of different lignins during pulping of bagasse with different processes has the following sequence: peroxyformic > peroxyacetic > butanol-water > butanol-alkali > kraft > sulfite > soda.