WATER VAPOR BARRIER PROPERTIES OF COATED AND FILLED MICROFIBRILLATED CELLULOSE COMPOSITE FILMS

Microfibrillated celluloses (MFCs) have mechanical properties sufficient for packaging applications, but lack in comparison to petroleum-based plastics in water vapor barrier properties. These properties can be modified by the use of mineral fillers, added within the film structure, or waxes, as surface coatings. In this investigation it was found that addition of fillers resulted in films with lower densities but also lower water vapor transmission rates. This was hypothesized to be due to decreased water vapor solubility in the films. Associated transport phenomena were described by the Knudsen model for diffusion but due to the limited incorporation of chemical factors in the model, accurate prediction of pore diameters for filled films was not possible. Modeling the filled-films with Fick’s equation, however, takes into account chemical differences, as observed by the calculated tortuosity values. Remarkably, coating with beeswax, paraffin, and cooked starch resulted in films with water vapor transmission rates lower than those for low density polyethylene. These coatings were modeled with a three-layer model which determined that coatings were more effective in reducing WVTR.