The Influence of Extensional Properties on the Dewatering of Coating Colours

摘要

The influence of the extensional viscosity on the dewatering rate was investigated on three coating colours, containing various water-soluble thickeners (carboxymethyl cellulose (CMC), an alkali-swellable thickener (ASE) and polyvinyl alcohol (PVA)), that had earlier shown significant differences in dewatering. Measurements were performed on aqueous solutions of the thickeners and on the complete coating colour systems. Even at the elevated concentrations investigated, the PVA solutions did behave almost Newtonian in shear as well as in extension, with extensional viscosities well below those of the CMC and ASE solutions. None of the three thickeners studied showed any strain-hardening effects. There was no indication of any contribution of extensional flow resistance during dewatering. It is concluded that the substantially slower dewatering of the PVA-based coating colour, compared to those containing CMC or ASE, is not due to an increased extensional viscosity of the aqueous phase but is due to other factors.