Adsorption of cationic starch onto peroxide-bleached TMP fibres -: Influence of interfering wood substances

A substantial portion of cationic starch added to added mechanical pulp may be consumed by dissolved and colloidal wood substances /1,2,3,4/. The purpose of this work was to clarify the influence of reducing the concentration of interfering wood substances in peroxide-bleached TMP (thermomechanical pulp) on the adsorption of cationic starch onto the fibres. The first part of the study was devoted to removal of dispersed and dissolved wood substances with the aid of a highly charged cationic chemical (a fixative) prior to addition of cationic starch. The addition levels were chosen to represent those used at paper mills. In the second part, pulp washing was carried out to reduce the amount of disturbing substances dispersed in the water and adsorbed onto the fibre surfaces. The results of the first part suggest that the amount of fixative added needs to be adjusted according to the amounts of colloidally dispersed lipophilic extractives, since these are aggregated by cationic starch. The results indicate that cationic starch was adsorbed onto the fibres when colloidal lipophilic extractives were completely aggregated by the fixative. In the second part, the adsorption properties of a washed peroxide-bleached TMP were characterised and compared with those of an unwashed pulp. The experiments were performed at 20 and 60 degrees C and at different salt concentrations. The adsorption of cationic starch was substantially reduced as a result of pulp washing. Both an increase in adsorption temperature and a slight increase in NaCl concentration increased adsorption onto the fibres.