The water vapour sorption behaviour of three celluloses: analysis using parallel exponential kinetics and interpretation using the Kelvin-Voigt viscoelastic model

The objective of this study was to examine the water vapour sorption behaviour of three celluloses, which were originally derived from cotton fibers, using a dynamic vapour sorption apparatus, including analyses of the sorption rate and hysteresis occurring in the isotherm run. Cotton linter, alpha-cellulose, and microcrystalline cellulose (MCC), respectively attained equilibrium moisture contents of 14.2, 20.6, and 16.9% at a relative humidity (RH) of 95%. All three cellulosic materials exhibited sorption hysteresis to varying degrees throughout the full RH range; the MCC and alpha-cellulose displayed the lowest and highest total hysteresis, respectively. The sorption kinetics were analysed in terms of the parallel exponential kinetics (PEK) model, with excellent fits to the data being obtained. The PEK data is further interpreted on the basis of two Kelvin-Voigt elements operating in series. Clear differences in behaviour were seen between the alpha-cellulose and the other two celluloses in this study. The relative importance of capillary condensation and matrix viscoelasticity with respect to sorption hysteresis is discussed.