The effect of ECF and TCF bleaching on the chemical composition of soda-anthraquinone and kraft pulp surfaces

The surface chemistry of a sulfur free processed (soda-anthraquinone, soda-AQ) softwood pulp (kappa 23.1) was compared to that of a kraft pulp (kappa 21.3) prepared from the same wood. The pulps were bleached using sequences OQDPDP and OQPP. The surface coverage by lignin and extractives was determined by X-ray photoelectron spectroscopy (XPS/ESCA) after each bleaching stage. The results were compared with the coverages on fibers from which the fines and the fiber surface material had been removed. The surface lignin on soda-AQ pulp (i.e. the lignin on the surface of fibers and fines) was seven times more resistant to OQDPDP bleaching than the total pulp lignin. The surface lignin on kraft pulp was five times more resistant than the pulp lignin. The lower reactivity of the surface lignin to a large extent seems to be due to the presence of lignin-carbohydrate linkages and a lower content of phenolic hydroxyls in the surface lignin. Alkaline stages (O or P) removed surface lignin in kraft pulp quite extensively, while the D stage was less effective. In contrast, the surface lignin on soda-AQ pulp was quite resistant to oxygen delignification but was removed effectively in the D stage. The surface coverage by extractives was also affected by the different cooking methods applied. The relative concentration of extractives on the surface of the pulp increased throughout the bleaching process. This effect was more marked on kraft pulp than on soda-AQ-pulp. Surface extractives on both soda-AQ and kraft pulps were more difficult to remove by oxygen than the extractives on the lightly peeled fiber surfaces. Thus, the extractives on the unbleached and oxygen-delignified pulp surfaces are very tightly bound to other compounds on the pulp surface, e.g. via multivalent metal ions. The fully bleached ECF pulp is covered to a larger extent by extractives than by lignin, which is expected to be of importance when the fibers are used in papermaking.