Dewatering of Coating Dispersions - Model Experiments and Analysis, 1999 Coating Fundamentals Symposium Proceedings

Sven Lohmander, Mark Martinez, Lucyna Lason, Mikael Rigdahl, Tie-Qiang Li

The dewatering of coating colours is an important phenomenon that to a large extent governs the runnability of in-dustrial coaters. In this work, the dewatering behaviour of model coating dispersions was investigated under static conditions in the laboratory. Using monodisperse spherical polystyrene particles at a 48 % solids content initially, filtration experiments were performed at a constant pressure of 50 kPa. In most cases, an anionic water-soluble

polymer, either carboxymethyl cellulose (CMC) or polyacrylamide (PAM), was added to the dispersions, in different

amounts, and the volume of filtrate collected was used to estimate a Darcian permeability. The results indicate that the ratio between the theoretically estimated permeability, based upon the Kozeny-Carman relationship, and the measured permeability decreased with increasing viscosity of the liquid phase. The filtration process was monitored

non-invasively using a dynamic magnetic resonance imaging (MRI) technique based upon a multi-shot spiral read-out.

It was demonstrated that there is a concentration gradient of particles in the formed filter cake, which is not acc-ounted for in the traditional filtration theory used in coating technology. The concentration of particles was highest at the filter membrane and was found to decrease almost linearly towards the top of the filter cake. Good agreement was found between a literature model describing filtration with a compressible filter cake and the concentration gradients

measured by MRI. Finally, it is widely recognised that water-soluble polymers sometimes give rise to con-siderably

retarded dewatering rates owing to the stretching of single macromolecules and, if the concentration of polymer is sufficiently high, of the network formed. However, under the conditions applied in this work, this effect could only be observed with the PAM, which had a significantly higher molecular weight than the CMC-grades used.