True density of swellable substances at different relative humidities : a new approach to its determination

The true density of hydroxypropyl methylcellulose (HPMC), hydroxyethyl methylcellulose (HEMC), and sodium carboxymethylcellulose (NaCMC) varies with the water content. To determine the true density at equilibrium at different relative humidities, the difference in the strength of binding of water to the polymer, accompanied by a change in density of the water content, is used. Firstly, the adsorption isotherm for water and then the density at some low water content are determined, at a point, where i) water is bound sufficiently tightly not to be driven off by the gas stream during the measuring procedure as proven by experiment, and ii) already some of the water is loosely bound and therefore approaches the density of 1. Secondly, the true density at any higher relative humidity may me calculated by assigning a density of 1.000 to this additional water and accounting for the additional water content.