Date of Award

5-2004

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Chemical Engineering

Advisor

Douglas W. Bousfield

Second Committee Member

Hemant P. Pendse

Third Committee Member

Adriaan R. P. van Heiningen

Abstract

Roll coating is a common process to deposit a thin liquid film onto a continuous web. When the web is porous, such as in paper coating or printing, some amount of the fluid is forced into the web in the nip. This removal of fluid, along with the deformation of the backing material, can cause the process to be quite complex. A better understanding of this process is needed to provide operational limits and to help provide design guidelines. A model is proposed to describe the pressure profile, rubber deformation, and absorption in a forward roll coating device. The differential equations are solved iteratively to describe the nip behavior. A simplified model is also proposed, using an average nip pressure and Darcy's law, to predict penetration in the nip. A laboratory roll coating device is used to characterize the pressure profile, the rubber deformation, and the film thickness on one roll surface. A pressure transducer is used to record the pressure profile in the nip. The film thickness on the steel roll surface and the actual gap between the rolls can be measured with the capacitance probes. Silicone oils with three different viscosities are used as test fluid. Three different base papers are used in these tests. The absorption rate of the fluid depends on the base paper properties, fluid properties and the nip dynamics. The proposed models compare well with the experimental results and predict the dependence on viscosity, load and paper permeability. However, the speed dependency is not predicted well by the models. This may be due to issues related to other forces that are generated at higher speeds. The rolling nip device, which resembles actual presses conditions, is used to measure the ink tack dynamics at time scales less than one second. As repeated contact is made with the paper, the most negative part of the measured pressure pulse increases to a maximum before decreasing. The nip pressure plays a role on the ink setting rate. Both coated and uncoated papers were used in the experiments, but newsprint grades did not show a negative pressure peak at the nip exit. The results of the micro-tack tester agree qualitatively to the results of the rolling nip device, but the new device is closer to actual printing. Several industrial paper coating formulations, with different misting behavior, are used in the device to characterize the sub-ambient pressure at the nip exit. The negative pressure correlates well with the misting tendency for the given coating formulations. Potentially, the device could be used to screen coating formulations for misting in the metered size press process.

Share