Abstract
Theoretical equations for quantitative analysis of the effect of fiber swelling on the capillary absorption rate of water-swellable fibers in mixed assemblies were developed. The equation for absorption rate was derived from the Poiseuille equation, whose pressure term was replaced by capillary pressure given as a Laplace equation. The key variables determining the rate were two different capillary sizes of fibrous assembly in dry and wet states. An equation for capillary size was also developed on the basis of the hydraulic radius model. The variables considered in the equation were swelling ratio, cross-sectional shape, fineness, volume density of fiber, and thickness of the assembly. For the experiment, nonwoven webs made of swellable fibers such as cotton, rayon, and superabsorbent fiber blended with hydrophilic polypropylene fiber were prepared. In the experiment, the swelling of the fiber led to a decrease in capillary size, resulting in a decrease in absorption rate. The linear relationship between the rates measured and calculated from the equation shows that the equation could be used to design fibrous assemblies for absorbent products.