Influence of TCF and ECF bleaching chemicals on softwood kraft pulp components

To obtain full brightness in TCF (Totally Chlorine Free) bleaching requires more efficient bleaching chemicals than oxygen and hydrogen peroxide, i.e. ozone, peroxyacids or other activated peroxide compounds. Radicals formed from these agents under unfavourable conditions react not only with lignin, but also with cellulose and hemi-celluloses. The main question related to the use of TCF bleaching agents is selectivity, i.e. how effectively lignin is removed from the fibres without causing-any damage to the cellulosic material. Bleaching result and pulp strength are also affected by the reaction conditions used and the location of the stages in the bleaching sequence. Besides regular analysis methods like kappa number, brightness and viscosity more sophisticated methods are needed to follow and understand the bleaching process, the ultimate aim being greater selectivity. In this study the bleaching process was followed stage by stage in terms of the molecular mass distribution of dissolved lignin and the amount and composition of dissolved carbohydrates. The results were correlated with the chemical and physical properties of the bleached softwood kraft pulp. In the TCF bleaching sequences studied oxygen, ozone, peroxyacetic acid, Caro's acid and hydrogen peroxide were used. To provide a reference, the pulp was also bleached in an ECF (Elemental Chlorine Free) sequence. The results show the superior selectivity of peroxyacid-based TCF sequences over the corresponding ozone-based TCF sequences. The analysis results indicate formation of alkali-labile carbonyl groups in the pulp during the ozone stage, leading to dissolution of carbohydrates and low viscosity values due to depolymerization. These effects were also reflected in the strength properties of TCFZ bleached pulp.