The effect of polymer and surfactant adsorption on the colloidal stability and rheology of kaolin dispersions

We have investigated the effect of different water-soluble polymers and surfactants, commonly used as additives in paper coating formulations, on the rheology and the colloidal stability of concentrated kaolin dispersions. The adsorption of the different additives on kaolin with, and without, preadsorbed sodium polyacrylate (NaPAA) were studied at high pH. Highly negatively charged polymers like sodium polyacrylate and sodium carboxymethyl cellulose (NaCMC) polymers adsorb to a low extent on kaolin, while the adsorption of the uncharged polyvinyl alcohol (PVA) polymer is considerably higher. If the kaolin particles are predispersed with NaPAA, the adsorbed amount of negatively charged substances decreases considerably, probably due to competitive adsorption on the edges of kaolin. However, the adsorption of uncharged substances, like PVA or non-ionic surfactant, is almost unaffected by the presence of NaPAA at the surface. Addition of polymers or surfactants to kaolin dispersions influences the stability and rheology of the dispersions dramatically. Negatively charged substances such as NaPAA, NaCMC and anionic surfactant were found to stabilise the kaolin particles effectively through electrostatic repulsion, resulting in low relative viscosities for high solid content dispersions. However. for systems with PVA or non-ionic surfactant, NaPAA has to be added first in order to obtain efficient stabilisation. The relative viscosity of a kaolin slurry dispersed with NaPAA was always found to increase as a second polymer or surfactant was added.