Jan., 1956
DIFFUSION OF WATERFROM STEADY-STATE DIFFUSION MEASUREMENTS
83
DIFFUSION OF WATER INTO UNCOATED CELLOPHANE. 11. FROM STEADY-STATE DIFFUSION MEASUREMENTS1 BY ALFREDJ. STAMM Forest Products Laboratory,2 Forest Service, U. 8.Department of Agriculture, Madison, Wisconsin Received June $0, 1966
Measurements were made of the diffusion of water through uncoated regenerated cellulose sausage casing films a t two different temperatures by the steady-state cup method with various relative humidity boundary conditions and with interna.1stirring. The need for internal stirring with permeable films is shown. Diffusion constants were calculated on the basis of linear vapor pressure gradients and parabolic bound-water gradients, the latter being experimentally justified. The diffusion constants on both bases increase exponentially with an increase in the fractional water volume. The data indicate that the diffusion is controlled by vapor pressure.
Introduction The preceding articlea gave experimental information on the diffusion of both liquid water and water vapor into uncoated regenerated cellulose film from rate of absorption and adsorption data'. This paper gives data on the same system obtained by steady-state diffusion measurements in order to further elucidate the mechanism of the movement of water in these films. Previous Findings Steady-state diffusion measurements have usually been made by the cup method and modifications there0f.~-12 The membrane is sealed over the top of a crystallizing dish containing water, a saturated salt solution, or desiccant. It is exposed to constant temperature and relative humidity conditions in an oven, humidity cabinet, or humidity room. The cups are weighed a t suitable intervals to determine the loss or gain in weight per unit of time after equilibrium is attained. In general, the test has been performedyunder adequate external circulation. Burr and the author13 have shown the need for internal stirring when the permeability is high. They devised a simple internal magnetic stirrer that increased the rate of moisture passage through Cellophane by as much as 50%. Steady-state diffusion is the simplest type of diffusion from the mathematical standpoint, as the boundary conditions are held constant throughout the measurement. After equilibrium is established, the rate of passage of the diffusingsubstance through the film becomes constant. (1) Paper presented a t the symposium on "Regenerated Cellulose .?ilms" sponsored by the Llivisions of Cellulose, Colloid. and Polymer Jhemistry, 125th National Meeting of the American Chemical Society, I
