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. Bot h bulk and charge properties were investigated after each bleaching st age. The amount, nature, accessibility and location of charged groups were determined using adsorption of cationic polyelectrolytes, high-pr ecision potentiometric titration and Electron Spectroscopy for Chemica l Analysis (ESCA). Total charge was not markedly affected by alkali an d peroxide treatments. Ozone and chlorine dioxide reduced the amount o f charge considerably, whereas a slight increase was obtained when fib res were oxygen delignified. The reductions in total charge by ozone a nd chlorine dioxide are mainly due to the selective reaction of hexenu ronic acid side groups of xylan by these electrophilic bleaching agent s. On the other hand, total charge decreased gradually as bleaching pr oceeded, even for pulps from which hexenuronic acids had been removed. It is therefore evident that carbohydrate losses and concomitant diss olution of lignin also affect the charge on bleached kraft fibres. Gen erally, the development of surface charge (accessibility of high M-w p olyelectrolyte) in different bleaching stages was similar to that of t otal charge. However, it seems that oxygen delignification increases n ot only the amount of charge bur also its accessibility. The selective removal of hexenuronic acids affected the surface charge much less th an would be expected from the reduction in total charge. A tentative c onclusion is that surface xylan contains fewer hexenuronic acid side g roups 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 chlo rine dioxide, the fibre charge in the pH interval 2-8 is due to the di ssociation 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 (pr obably carboxylic group in lignin). Since the same dissociation consta nts were found for unbleached kraft pulp, it is very likely that any c arboxylic groups introduced into lignin and polysaccharide fractions b y the oxidative treatments were also extensively removed from the pulp during bleaching. The situation was different with regard to the acid ic groups (pK approximate to 5.5) present in the residual lignin after kraft cooking. While ozone removed the lignin fragments that containe d the weaker acid group, the other bleaching chemicals, especially oxy gen and peroxide, were inactive towards these structures. It is sugges ted that lignin containing carboxylic groups after kraft cooking is ma inly bound to lignin-carbohydrate complexes, which are primarily locat ed in the outer surface regions of fibres.