Flow rheology of light foams generated from aqueous solutions of polyvinyl alcohol

Recent studies have shown that foam-assisted application of additives into a wet web has advantages over the conventional way of adding the chemicals into the pulp suspension before forming, e.g., increased mechanical retention as well as high dosage giving increased wet strength without impairing the sheet uniformity. To engineer processes utilizing this new technology, the complex flow behavior of applied foams must be quantified. At the minimum, the foam viscosity and the slip velocity at the solid surfaces need to be known to build practical models that can be used in analyzing and upscaling unit processes of foam-assisted application.

In this study, the rheological behavior was quantified for foams having polyvinyl alcohol (PVOH), a widely used strength-aid chemical, as the surfactant. The foam density was varied between 100 g/l and 300 g/l, and the concentration of the PVOH solution between 0.5% and 6.0%. The foams were generated with a commercial foam generator and the rheological properties of the foams were measured by using a horizontal pipe bank. At the outlet from the generator, the volumetric flow rate, the absolute pressure, and the bubble size distribution of the foam were measured. In the pipe section the viscous pressure gradient and the slip velocity were measured after which the foam was discharged to ambient air pressure. The viscosity and the dynamic surface tension of the PVOH solutions were quantified with commercial laboratory devices.

In the viscosity analysis, the apparent shear rate calculated from the volumetric flow rate and the resulting apparent viscosity were translated to real material viscosity data by applying the Weissenberg-Rabinowitsch correction. The results indicated that PVOH foams can be described with high accuracy as shear-thinning power-law fluids where the detailed behavior depends on the foam density and the PVOH concentration. Slip flow, as usual, increased with increasing wall shear stress, but was dependent also on the PVOH concentration, the air content, and the bubble size. For both the foam viscosity and the slip flow, a correlation was found that described the quantitative behavior of all the studied foams with a good accuracy.