Colloids and Surfaces A: Physicochemical and Engineering Aspects 139 (1998) 271–278 Role of surface charge in deposition of filler particles onto pulp fibres S ˇ tefan S ˇ uty ´ , Vlasta Luz ˇa ´kova ´* Department of Wood, Pulp and Paper, Faculty of Chemical Technology, Slovak Technical University, Radlinske ´ho 9, 812 37 Bratislava, Slovak Republic Received 7 February 1996; accepted 10 February 1998 Abstract The effect of cationic polyethylenimine (PEI ) on colloidal stability of ground calcium carbonate and its deposition on pulp fibres suspended in water was investigated. The adsorption of PEI on negatively charged CaCO 3 particles causes destabilization due to the elimination of charge. With increasing additions of PEI, restabilization of positively charged CaCO 3 takes place. Based upon these results the filler was pretreated with PEI in order to encourage its deposition on negatively charged fibres. Two levels of PEI addition were used to produce either CaCO 3 aggregates or dispersed particles. The extent of deposition, the deposition time t, and the capture efficiency a0 were established. The results indicate that positively charged filler aggregates deposit faster than dispersed particles. Alternatively, the fibres were treated with two levels of PEI, the same as that used for CaCO 3 treatment, and the deposition of CaCO 3 was observed. The deposition efficiency in this case was lower. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Adsorption; Calcium carbonate; Deposition; Kinetics; Polyethylenimine; Pulp fibres; Stability; Surface charge 1. Introduction coagulation by a charge neutralisation or hetero- flocculation by a bridging mechanism, most likely according to the type of polymer used, its molar Both the pulp fibres and most filler particles are mass, configuration, charge density and environ- naturally negatively charged in water dispersions, ment [1]. The polymer addition to a filler alone so the filler particles do not deposit on the fibres may also result in homoflocculation or a homoco- significantly. Therefore retention aids, often cat- agulation of small particles into aggregates, and ionic polyelectrolytes, are used to improve the thus affect the mechanism of deposition. The retention of particles due to electrostatic attrac- hydrodynamic forces created by rapid stirring or tions. A cationic polymer adsorbed onto the flow through a tube at high shear rates can enhance anionic surface of either pulp fibres or filler the rate of a coagulation called orthokinetic coagu- particles in a dilute dispersion can cause hetero- lation. In a simple shear flow of spherical filler particles with radius a p , and pulp fibres modelled * Corresponding author. Fax: +42 7 1393 198; e-mail: luzakova@.chtf.stuba.sk as slender spheroids with an axis ratio a/b&1, a 0927-7757/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0927-7757(98)00322-7