Non-Newtonian Effects on the Ribbing Instability, 1995 Coating Conference Proceedings

M. S. Carvalho, P. Dontula And L. E. Scriven

Many papermaking, coating and printing operations, such as press nips, size presses, transfer coaters, and printing trains, involve film-splitting. Except at low speeds, film-splitting flow is three-dimensional and results in more or less regular stripes, ribs, or corduroy in the machine direction. Non-Newtonian liq-uids can drastically change the nature of the three-dimensional flow. However, the mechanisms by which rheological properties influence the flow are still a matter of research. In this work, the shape of the meniscus of two different polymeric solutions at the film split between a transparent plate and a rotating roll were recorded at different gaps and roll speeds. The onset of ribbing and its wavelength were de-tected by viewing the separation line of the coated liquid on the transparent plate. These were compared with similar data obtained for a Newtonian liquid (82 % by weight glycerin/water solution). One of the polymeric solutions was extensional-thickening, poly-(ethylene oxide) in water, and the other was extensional-thinning, xanthan gum in 50/50 by volume of water/glycerin solution. The concentrations were such that both solutions had the same shear viscosity at high shear rates. Shear viscosity was measured with an RFS-II rheometer (Rheometrics) and a Couette flow fixture. The apparent extensional viscosity was measured with an RFX device (also by Rheometrics) in the opposed nozzle configuration. The results show that the onset of ribbing in both poly-meric solutions occurred at a capillary number smaller than in the case of Newtonian liquid. The reduction of the critical speed was greater for the poly-(ethylene oxide) solution, which showed an extensional thickening behavior. The wavelength of the ribbing pattern in that solution was smaller than the one observed in the xanthan solution. Moreover, the meniscus ob-served with the poly-(ethylene oxide) solution was more sharply curved.