Water vapor transport properties of regenerated cellulose and nanofibrillated cellulose films

Abstract

In this study, water vapor transport properties of nano-fibrillated cellulose (NFC) and regenerated cellulose films, derived from different sources, were investigated and compared with the transport properties of unmodified paper. Gravimetric-IGA experimental system was used to measure the kinetics and water vapor permeabilities (WVPs) of the samples. Water vapor adsorption–desorption isotherms were measured for different cellulose films using Belsorp instrument. Monolayer moisture content was determined by fitting the adsorption isotherm with the Guggenheim–Anderson–De Boer equation (GAB model). The monolayer moisture content of the regenerated cellulose and NFC films were found to be more than two times larger than the monolayer moisture content obtained for unmodified paper sample (made from bleached Kraft pulp ~4.12wt%), at saturation. This indicates a large number of hydrophilic sites available on the surface of regenerated cellulose and NFC films. The water transmission rates (WVTR’s) of regenerated cellulose and NFC films were similar, but lower compared to the unmodified paper measured at a wide range of relative humidities. Furthermore, water vapor permeabilities (WVP’s) of NFC and regenerated cellulose films were found to be concentration dependent exhibiting lower values up to RH=25%, but then showing a rapidly increasing trends above RH=25%. WVP’s of the cellulose films revealed an Arrhenius type of dependency with temperature, indicating an activated process.Water vapor diffusivities were also measured from the uptake rate measurements for a wide range of relative humidities. The diffusivities generally showed dependency on the moisture content of the sample. The effective diffusion coefficients (Deff) of NFC and regenerated cellulose films are 1–2 orders of magnitude lower in comparison with the unmodified paper, depending on the moisture content. The mechanisms of water vapor transport through the cellulose films were also evaluated from the uptake rate experiments. The results showed that the external surface resistance was a more dominant mechanism to mass transfer in the NFC and regenerated cellulose films. On the other hand, water vapor uptakes on unmodified paper seem to be consistent with internal diffusion control mechanism, except at the high moisture contents. At the high relative humidity or moisture contents of the films, both the external surface resistance and diffusion control the transport in transparent cellulose films and paper samples.