Sorption of Water Vapor in Jute Fibers

Sorption kinetics of the jute—water-vapor system have been followed gravimetrically using a sorption balance under controlled temperature and humidity conditions and absorption isotherms determined from the equilibrium water ' contents. The sorption kinetics are dependent on the initial water content of the jute sample and the humidity range of the sorption step and can be related to the various regions of the sigmoidal-shaped isotherm. At low water contents, corresponding to the initial steep portion of the isotherm, sorption can be described as a diffusion process in accordance with Fick's laws. Above 2.5% water content, only the initial portion of the uptake, to a quasi-equilibrium, occurs by a rate-controlling diffusion process, and a slow second stage of uptake is attributed to a stress relaxation mechanism. For a large step change in concentration no separate second stage is observed, presumably because stress relaxation occurs rapidly. At high humidities (>80% RH) absorption and desorption processes are non-Fickian over the whole range. On absorption a transient overshoot above the equilibrium water content value is explained as a consequence of the break down of the hydrogen-bonded structure of the fiber followed by the reformation of the interchain network to a new stable state. Similarities between the data obtained for the jute-water system and other polymer-penetrant systems are pointed out.