The Difference between Guaiacyl and Guaiacyl-Syringyl Lignins in their Responses to Kraft Delignification

Kraft pulping of Douglas-fir (Pseudoisuga meniesii) and sweetgum (Liquidambar styraciflua) was carried out from 9()°C to a final temperature of 170°C at a heating rate of l °C/min. At various stages of delignification, pulps were analyzed by nucleus exchange and alkaline nitrobenzene oxidation reactions to reveal the difference between kraft pulping of hardwood and softwood in dissolution and condensation of lignin building units. The most significant difference between kraft pulping of hardwood and softwood was in the formation and dissolution of Condensed units. The formation of guaiacyl-syringyl types of diphenyl methane moieties occurred already at 110°C and reached the maximum at around 150°C. These Condensed units were dissolved at higher temperatures (\u3e 160°C) and after 30 minutes at 170°C, no syringyl unit was found associated with diphenyl methane moieties in residual sweetgum lignins. At the end of pulping (∼ kappa 20). the residual sweetgum lignin consisted of 66 and 26 mol% of guaiacyl units that were associated with diphenyl methane moieties and other types of Condensed units, respectively. The formation of guaiacyl-guaiacyl types of diphenyl methane moieties during pulping of Douglas-fir occurred at 170°C and reached the maximum after 20 minutes at this temperature. After reaching its maximum, the quantity of these types of diphenyl methane moieties in residual lignins remained almost constant äs pulping proceeded. At the end of pulping (∼ kappa 30), the residual Douglas-fir lignin consisted of 54 and 37 mol% of guaiacyl units that were associated with diphenyl methane moieties and other types of Condensed units, respectively. © 1990 Walter de Gruyte