Permeabilities to Fluid Flow Vary with Sheet Compression: Analysis and Model of Basics, 1991 Coating Conference Proceedings

During sizing, coating and drying, liquids and air or vapor flow in the sheet, and their flow is controlled by the permeabilities in-plane and in the z-direction. Estimates of how these basic flow properties vary with compression of the sheet are needed for accurate analysis of coating and drying processes. We present here two models. In the first the porespace is regular, and compression of the sheet affects permeability three ways: overlying fibers are pressed into each other andflow resistance per unit of z-distance rises accordingly; passages are narrowed by interfiber penetration and also by lateral spreading of fibers as the fibers are compressed. In the second model, the porespace is irregular and its permeabilities are found from the new Anisotropic Effective Medium Theory. The resulting predictions accord well with measurements by Macklem (1961) on woven wool felts, point up the need for comparable data on paper and board, illuminate mechanisms which alter sheet permeability, and suggest directions for refining the physical model.