Carbohydrates in Oxygen Delignification. Part I: Changes in Cellulose Crystallinity

INTRODUCTION There has been a steady increase in the use of oxygen delignification worldwide since the first commercial installation in South Africa in 1970 [1]. The degree of delignification achieved in the oxygen stage is ultimately limited by pulp strength considerations, and averages 40% delignification industrially [2,3]. With greater degrees of delignification, the oxygen delignification process degrades the carbohydrate to the point where fibre strength loss occurs. This work describes changes that are occurring in the fibre carbohydrates during oxygen delignification. Cellulose is a polymer containing crystallites, thus displaying a paracrystalline morphology [4]. The linear cellulose molecules are linked laterally by hydrogen bonds to form linear bundles, giving rise to a crystalline structure [5]. The crystalline regions are interrupted every 60 nm with non-crystalline amorphous regions [6]. Crystalline regions may contain occasional kinks or folds in the polymer chain, called defects. The type and the number of defects varies with the type of cellulose [7]. Under oxygen-delignification conditions, there are two main cellulose degradation reactions. The first is alkali-driven rearrangement at reducing end groups to cause a peeling reaction. The second is oxidation at the glycosidic linkage within a cellulose molecule to cause chain scission and a site for further peeling reactions [8]. If the amorphous domains of cellulose are attacked, chain scission and peeling reactions occur which reduce the total amount of amorphous cellulose and therefore increase the relative degree of crystallinity. At the same time, there is also the possibility that random cleavage of the cellulose occurs in the accessible chains within the crystalline domains [9]. The chains at the outer portions of the crystallites can cleave randomly and protrude from the crystalline domain. The accessible chains can then be considered amorphous since they are not part of the crystallite. A significant increase in the total amorphous character of the cellulose would decrease the relative degree of crystallinity. In this work, the kinetics of crystallinity changes during oxygen delignification were studied for four lignin-free cellulosic substrates. A fully bleached softwood kraft pulp (FBSK) was selected as representative of the cellulose of unbleached pulp. Extracting the hemicelluloses of the former provided an almost pure cellulose sample. Cotton was selected as a naturally occurring pure cellulose with chain length similar to kraft pulp. Finally, microcrystalline cellulose represents a shortchain, highly ordered, defect-free cellulose with no other contaminant present.