Moisture Sorption Kinetics Research Articles

The physical basis of moisture transport in a cured epoxy resin system

AbstractThe moisture transport characteristics of a difunctional epoxy resin cured with different amounts of metaphenylene diamine, using two cure cycles, are reported. Besides studying the kinetics of moisture sorption at 20, 50, 75, and 100°C, the investigations also included measurement of thermal expansion coefficients and dynamic mechanical transition of the dry and wet samples. The moisture sorption of the sample is shown to be related to its specific volume and hence to its fractional free volume. In the glassy state, the free volume is apparently in the form of frozen voids, and moisture sorption/desorption at this temperature is of the Langmuir type with little or no bond formation. At higher temperatures the free volume is generated predominantly through segmental motion of the α transition. The Henry's Law mode becomes operative, and the moisture can now form bonds. The possible effect of nonuniform crosslink density on moisture sorption is also considered.

Moisture Sorption Kinetics for Water-Soluble Substances IV: Studies with Mixtures of Solids

This paper extends earlier work from this laboratory concerning the sorption kinetics of water vapor on deliquescent water-soluble substances to mixtures of these solids. A theoretical model, based on heat transport control, excellently predicted a priori the rate of water uptake by a variety of binary mixtures of alkali halides and sugars. The rates for mixtures containing highly water-soluble quatemary ammonium salts, as either one or both of the components, were less successfully predicted as the combined water solubilities of the two components increased. It is concluded that water-soluble deliquescent substances, normally encountered in pharmaceutical dosage forms, rapidly form saturated aqueous solutions in the aqueous film formed as water vapor uptake proceeds, and that the water uptake rate can be predicted a priori from known and experimentally determinable parameters using the heat transport model.

Moisture Sorption Kinetics for Water-Soluble Substances II: Experimental Verification of Heat Transport Control

The rates of water sorption as a function of relative humidity for water-soluble substances exhibiting deliquescence have been measured in an atmosphere of pure water vapor. The substances studied included a series of alkali halides, choline halides, and sugars. The results were compared with a theoretical model, previously described, which relates the rate of water uptake to the transport of heat produced during the process away from the surface. Taking into account the heat of water vapor condensation, heat of solution, and heat of hydration, when hydration occurs, the model allows excellent a priori prediction of water uptake rates as a function of relative humidity.

Moisture Sorption Kinetics for Water-Soluble Substances I: Theoretical Considerations of Heat Transport Control

A model based on heat transport control was developed to describe the uptake of water on a deliquescent solid in an atmosphere of pure water vapor. The model assumes the presence of a saturated liquid film on the surface of the solid. The decrease in the vapor pressure of water over the surface, brought about by the colligative effect of solid dissolved in the liquid film, is effectively offset by the increase in temperature of the film (and the solid) caused by the heat released on condensation of the water vapor. The thermal transients die out quickly and a steady-state analysis is valid. At steady state the temperature of the liquid film (and solid) is that temperature at which the vapor pressure of water above the saturated solution is equal to the chamber pressure. Consequently, water uptake occurs at a rate that depends on the heat flux away from the surface. The water uptake rate, Wʹh, is constant at a given relative humidity and is described by an equation of the form Wʹh=(C+F) ln (RHi/RHo), where C and F are conductive and radiative coefficients, RHi the chamber relative humidity, and RHo the relative humidity at and above which continuous water uptake (deliquescence) occurs. The model contains no adjustable parameters and can thus be directly tested against experimental results.

Polymer membranes and their permeability to water vapour

The diffusion equilibria and the mass transport of water vapour through polyamide 6, 12 and through PVC films have been studied and the results analysed. The isothermal concentration dependences of the mass transport coefficients were obtained by stationary and non-stationary methods; these agreed well. Equations have been formulated for the gas phase permeability and mass transport coefficients using the partial kinetic parameters for stationary conditions. A zonal method of calculating the moisture sorption kinetics by a hygroscopic material tightly packed in a polymer film are explained by using mass transport data.

Kinetics of Moisture Sorption by Jute and its Constituents

Kinetics of Moisture Sorption by Jute and its Constituents