The influence of charge density of cationic starch on dissolved and colloidal material from peroxide bleached thermomechanical pulp

The interaction between anionically charged colloidal and dissolved material (anionic trash) from peroxide bleached thermomechanical pulp and cationic starches of different charge densities has been examined. The evaluated degrees of substitution, DS, were 0.015, 0.035, 0.05 and 0.08. For comparison, a starch of considerably higher degree of substitution, called fixative, was also evaluated. A higher amount of the cationic starch of DS 0.015 was needed than that of DS 0.035 to achieve complete destabilisation of colloidal material. However, lower dosages of the starches with DS 0.035 and 0.05 were needed than of the starch of DS 0.08. A lower dosage of fixative than of the common starch of DS 0.08 was also needed. The fixative recharged the particles upon an overdose. Measurements of residual turbidity and zeta potential indicate that the common starches destabilised the colloidal lipophilic extractives by interparticle bridging. The fixative starch of considerably higher charge density, DS 0.8, may have destabilised the colloids by the patch flocculation mechanism. The common cationic starches also formed complexes with dissolved pectic substances, which are anionically charged wood polymers consisting of galacturonic acid units. The amount of starch needed to achieve complete destabilisation of pectic substances decreased with increasing charge density, probably due to intensified electrostatic interaction. Still lower amounts of fixative were needed. An overdose of fixative restablized the pectic substances. (C) 1999 Elsevier Science B.V. All rights reserved.