Cellulose nanofibril (CNF) films have been extensively explored as gas barrier films since the early days of CNF research. CNF films typically exhibit low oxygen permeability under dry conditions, but as the moisture content increases with increasing relative humidity, the permeably increases significantly. Unfortunately, due to the hygroscopic nature of cellulose, films made thereof do not exhibit any technically relevant moisture barrier properties. This work explores two approaches to impart improved tolerance towards moisture and better barrier properties: one nanocomposite approach and one nanocoating approach. In the first approach, we use cationic CNFs in combination with amphoteric gibbsite nanoplatelets. By mixing the two particles under non-alkaline conditions, where both components carry a net cationic charge, a well-dispersed dispersion is formed. Upon vacuum filtration of this dispersion, the good dispersion is maintained in the ultimately dry film, resulting in a tortuous diffusion path for gas molecules. After film formation, the CNF films were post-treated in alkaline water, reversing the charge of the gibbsite nanoplatelets to anionic net charge, which improves the interaction between the CNFs and the platelets. This charge reversal results in a significantly reduced interaction with water, measured as film swelling in water, and a lower oxygen permeability under high relative humidity (18 ml mm m-2 kPa-1 24h-1). Furthermore, the charge reversal had a positive effect on the mechanical properties, and the films formed had a tensile strength of 190 MPa, a Young’s modulus of 12 GPa and a strain-at-break of 6%. In the second approach, aimed mainly to impart a water vapour barrier, but also improved oxygen barrier properties, ultrathin layers of parylene and/or aluminum were deposited by chemical vapour deposition. Deposition of an aluminum layer as thin as 20 nm on a 20 µm CNF film was enough to lower the water vapour transmission rate from &gt;2000 g m-2 24h-1 down to about 100 g m-2 24h-1. Furthermore, by combining a 40 nm aluminium layer with an 80 nm layer of parylene, the measured water vapour transmission rate was as low as 3 g m-2 24h-1. This means that with the inclusion of a nanometre-thin layer that constitutes less than about one percent of the material, the CNF film is turned into a vapour barrier on par with or even better than conventional 20–25 µm layers of polymers such as polypropylene, polyethylene or polyethylene terephthalate.

Nanocomposite and nanocoating approaches for improved gas barrier properties of cellulose nanofibril films

ACS Spring 2022

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