Agenda

1. Agenda • Carbohydrate Reaction Mechanisms • Glycosidic Cleavage • Peeling • Stopping • Glucomannan Reactions • Xylan Reactions • Other Hemicellulose Reactions • Cellulose Reactions • Viscosity

2. Carbohydrate Reactions • The main alkaline reactions of carbohydrates : • Glycosidic cleavage. • Peeling. • Stopping.

3. Peeling Mechanism * The peeling reaction basically unzips the carbohydrates by removing terminal sugars one at a time. Reaction takes place from reducing end of the molecule (aldehyde). Reducing end group Stable end group

4. Peeling Mechanism * The peeling reaction basically unzips the carbohydrates by removing terminal sugars one at a time. Reaction takes place from reducing end of the molecule (aldehyde). Acids formed by peeling reaction responsible for most alkali consumption in kraft cook

5. Stopping Mechanism (C) (A) (B) * The stopping reaction stops the peeling process when an endgroup is formed which will mot peel. (will not “peel”)

6. Hydrolysis of Glycosidic Linkage • Cleavage of glycosidic bonds. • This reaction cleaves the carbohydrate in the chain instead of at the end of the chain as in the peeling reaction. This generates a new reducing end which increases the rate of peeling. • This reaction lowers the molecular weight of carbohydrates. • Glycosidic cleavage of cellulose results in loss of pulp viscosity and can lead to strength loss if too extensive

7. Loss of Glucomannans During Kraft Pulping

8. Effect of Effective Alkali on Glucomannan Loss

9. Glucomannan losses • Glucomannans are lost mainly through primary peeling. • Responsible for much of yield loss, especially in softwoods • Pulp yield can be increased by stabilizing glucomannans • Oxidize reducing end group with either polysulfide or anthraquinone

10. Loss of Xylans During Kraft Pulping

11. Effect of Effective Alkali on Xylan Loss

12. Xylan losses • Xylans are lost mainly through glycosidic cleavage (and some secondary peeling). • Dissolve as macromolecule which can re-precipitate back on to the pulp fibers if [OH-] becomes low enough – end of the cook • End group stabilization not very effective for hardwoods • Responsible for substantial yield loss in hardwoods • Presence of xylans on pulp have a significant effect on its performance • Refining is easier with xylans in the pulp • Xylans appear to inhibit bleaching

13. Cellulose Reactions During Kraft Pulping • Cellulose undergoes peeling and glycosidic cleavage reactions during kraft pulping. • Because cellulose molecules are so long, peeling reactions only cause small yield losses. • Glycosidic cleavage is more of a problem because of molecular weight losses that may cause strength problems. This reaction also increases the rate of peeling somewhat through generation of new reducing end groups. • Because cellulose molecules are so large dissolution is not an issue.

14. Pulp Viscosity • Modifying the hemicellulose content of the pulp won’t change the viscosity • Borohydride treatment inhibits primary peeling which increase glucomannan content

15. Pulp Viscosity • Pulp strength and viscosity has a complex relation • A decrease in viscosity may not correlate with pulp strength until the viscosity reaches a critical level – then look out!

16. Pulp Viscosity • Pulp strength and viscosity has a complex relation • A decrease in viscosity may not correlate with pulp strength until the viscosity reaches a critical level – then look out!

17. 180 160 140 Zero span tensile(kPa) 120 100 80 60 0 5 10 15 20 25 30 35 40 45 CED Viscosity (mPa.s) Borohydride treated Untreated Pulp Viscosity

18. Pulp Viscosity • Pulp strength and viscosity has a complex relation • A decrease in viscosity may not correlate with pulp strength until the viscosity reaches a critical level – then look out!

19. Pulp Viscosity • Pulp strength and viscosity has a complex relation • The retention of hemicelluloses can, however, reduce the strength of the pulp without any affect on the pulp’s viscosity