Effect of ozone bleaching on the fibre properties of pine and birch kraft pulp

Abstract

Pine and birch kraft pulps were bleached with a TCF sequence including an ozone stage. Different ozone charges were used to obtain pulps with different final brightnesses (between 85 and 92%). Pulps bleached with an ECF sequence or a TCF sequence in which ozone was replaced by peracetic acid were used as references. Main focus was on fibre deformation (curl and kink indexes) and how this was affected by ozone charge. The effect of fibre deformation on zero-span index and paper strength was also considered. No major differences in the degree of fibre deformation were observed directly after the ozone stage. After final bleaching with alkaline hydrogen peroxide, there was a clear trend towards greater curl and kink indexes with increasing ozone charge. A similar effect was seen in the case of zero-span index. Some straigthening of the fibres occurred in the final alkaline bleaching stage, depending on the carboxyl group content of the pulps. A connection was found between kink index and both zero-span index and tensile index of pine and birch pulps. There were also indications of a connection between fibre deformation and tear index of pine pulps.