The Absorption and Diffusion of Water in Rubber

A new differential equation of diffusion is put forward to represent the movements of water vapor through rubber in accordance with the osmotic theory of absorption. Experiments are described which, together with the evidence quoted from existing knowledge, confirm a number of predictions as to differences between the absorption of a gas and of water vapor by rubber. These predictions deal with the shapes of the absorption- and desorption-time curves between narrow and wide limits of humidity, the relative times required for absorption and desorption, and the variation of absorption and desorption periods with thickness of the rubber sheet. The results are definitely inconsistent with the simple gas-diffusion theory, which is often assumed to apply to water in rubber. It is established that the shape of the humidity-absorption curve is a dominant factor in determining time-water content relations. Except for changes over very restricted ranges of humidity, the absorption- and desorption-time curves are of different shape, and desorption is much more rapid than absorption. The periods required for absorption and desorption are greater, the higher the humidity and the higher the hygroscopicity of the rubber. The law that the time required to reach a given stage of saturation varies as the square of the linear dimensions of the sample (the shape being constant) is equally true for diffusion, according to the old and new equations.