Interactions of cationic polymers with components in thermomechanical pulp suspensions

The interactions of three cationic polymers with components in suspensions of unbleached and peroxide bleached TMP, and their isolated dissolved and colloidal substances in water, were studied. The polymers differed in their molecular mass, charge density, and degree of branching. The optimum polymer dose (OPD), which gives the most effective destabilization of colloidal turbidity-causing substances, differed with the type of polymer added. The OPD was also influenced by alkaline peroxide bleaching of the TMP, and by removal of the fibers and fines prior to polymer addition. Lipophilic wood extractives, anionic hemicelluloses, and UV-absorbing substances formed aggregates with the polymers. These aggregates could be sedimented by centrifugation. For unbleached TMP, only small differences in the OPD values were found upon addition of the different polymers to fines-and fiber-free water samples. However, when fines and fibers were present, the OPD values were higher and differed considerably with the polymer type. For peroxide bleached TMP, larger amounts of the polymers were needed to reach the OPD compared to unbleached samples. This was probably due to aggregation of the cationic polymers with anionic hemicelluloses, primarily polygalacturonic acids, known to be released into the water phase in alkaline peroxide bleaching. Enzymatic degradation of the polygalacturonic acids lowered the OPD values. The results demonstrate that there are significant differences in the interactions of different cationic polymers with the components in mechanical pulp suspensions. Such information can lead to better control of colloidal stability in papermaking.