Effect of ECF and TCF bleaching on the charge properties of kraft pulp

Unbleached softwood (Pinus sylvestris) kraft pulp with kappa number 25.9 was bleached using the sequences OZEP, OPZEP, ODEDED and DEDED. Both bulk and charge properties were investigated after each bleaching stage. The amount, nature, accessibility and location of charged groups were determined using adsorption of cationic polyelectrolytes, high-precision potentiometric titration and Electron Spectroscopy for Chemical Analysis (ESCA). Total charge was not markedly affected by alkali and peroxide treatments. Ozone and chlorine dioxide reduced the amount of charge considerably, whereas a slight increase was obtained when fibres were oxygen delignified. The reductions in total charge by ozone and chlorine dioxide are mainly due to the selective reaction of hexenuronic acid side groups of xylan by these electrophilic bleaching agents. On the other hand, total charge decreased gradually as bleaching proceeded, even for pulps from which hexenuronic acids had been removed. It is therefore evident that carbohydrate losses and concomitant dissolution of lignin also affect the charge on bleached kraft fibres. Generally, the development of surface charge (accessibility of high M-w polyelectrolyte) in different bleaching stages was similar to that of total charge. However, it seems that oxygen delignification increases not only the amount of charge bur also its accessibility. The selective removal of hexenuronic acids affected the surface charge much less than would be expected from the reduction in total charge. A tentative conclusion is that surface xylan contains fewer hexenuronic acid side groups than xy lan in other regions of the fibres. Detailed analysis of the potentiometric titrations showed that irrespective of whether the unbleached kraft pulp is treated with oxygen, peroxide, ozone or chlorine dioxide, the fibre charge in the pH interval 2-8 is due to the dissociation of two types of acidic group, one with pK approximate to 3.3 (uronic acids in xylan) and the other with pK approximate to 5.5 (probably carboxylic group in lignin). Since the same dissociation constants were found for unbleached kraft pulp, it is very likely that any carboxylic groups introduced into lignin and polysaccharide fractions by the oxidative treatments were also extensively removed from the pulp during bleaching. The situation was different with regard to the acidic groups (pK approximate to 5.5) present in the residual lignin after kraft cooking. While ozone removed the lignin fragments that contained the weaker acid group, the other bleaching chemicals, especially oxygen and peroxide, were inactive towards these structures. It is suggested that lignin containing carboxylic groups after kraft cooking is mainly bound to lignin-carbohydrate complexes, which are primarily located in the outer surface regions of fibres.