Water vapor and gas barrier properties of cellulose-based materials

Abstract

Mass transport properties in polymeric materials are of fundamental importance for different applications, such as food and pharmaceutical packaging, membrane-based gas and liquid separations, etc. In this thesis, adsorption equilibria, structure and mass transport properties (water vapor, CO2 and oxygen) of modified papers, nanofibrillated cellulose (NFC) and regenerated cellulose films, derived from different sources, were investigated to evaluate their potential as food and pharmaceutical packaging materials. The gravimetric-IGA system was used for measuring water vapor mass transfer rates (WVPR‘s), structure and permeabilities (WVP‘s) of the samples. Water vapor adsorption equilibria (isotherms) were also measured using the Belsorp instrument, while the vapor diffusivities for cellulose-based films were determined from the uptake rate measurements, at a wide range of relative humidities, using appropriate diffusion models. The diffusivities were found to depend on the moisture content of the samples. The effective diffusion coefficients of modified paper samples (e.g., PLA, PHBV coated papers), NFC and regenerated cellulose films are 1-2 orders of magnitude lower in comparison to the unmodified paper sample (made from bleached Kraft pulp), depending on the moisture content. Water vapor transmission rates (WVTRs) of regenerated cellulose and NFC films were found to be close to each other, but lower compared to unmodified paper for all relative humidity gradients. WVPs of the NFC and regenerated cellulose films were found to be moisture concentration dependent. Their values are relatively low up to RH=25%, but then show a rapid increase above 25 %. This behavior is attributed to an increase in the solubility and surface diffusivities of water vapor in the film at the higher moisture contents according to the solution-diffusion theory. Among the investigated paper samples, PHBV and PLA/polyhedral oligomeric silsesquioxane (POSS)-bentonite modified papers showed higher mass transfer resistance to water vapor and the gases (CO2 and O2) investigated in this study. The barrier properties are enhanced by the addition of fillers and surface coating onto cellulose based paper; however, surface coating is more effective in increasing the barrier properties of the modified papers. In the further study, the mechanisms of water vapor transport through the cellulose films were also determined from the uptake rate measurements. The results showed that the external surface resistance (surface barrier) and/or polymer chain relaxation rate were a more dominant mechanism for mass transfer in the NFC and regenerated cellulose films compared to unmodified paper. Mass transfer for the unmodified paper was found to be consistent with internal diffusion control, except at the high moisture contents. However, at high moisture content of the films, both external surface resistance and diffusion process equally control the mass transport mechanism for all investigated samples. Finally, theoretical modeling of parallel pore-surface diffusion of water vapor transmission rates through the paper sample and regenerated cellulose film are developed from experimentally determined mass transfer parameters. The results showed that the moisture transport dynamics can be well analyzed using parallel pore-surface diffusion for the cellulose-based materials.