Abstract
Films formed from cellulose nanofibrils (CNFs) are known to be good barrier materials against oxygen, but they lose this feature once placed in humid conditions. To tackle this issue, we applied an optimized pressing condition under elevated temperature to increase the films’ density and improve their barrier performance. Furthermore, a water barrier coating was employed on the surfaces to control the moisture uptake at high relative humidity (RH). Neat self-standing films of CNF with the basis weight of 70 g/m2 were made through a filtration technique and pressed for 1 hour at 130 °C. The resulting nanostructures were covered on both sides using a water-borne barrier layer. Hot-pressing resulted in a significant reduction in oxygen transmission rate (OTR) values, from 516.7 to 3.6 (cm3/(m2·day)) and to some degree, helped preserve the reduced oxygen transmission at high relative humidity. Introducing 35 g/m2 of latex coating layer on both sides limited the films’ swelling at 90% RH for about 4 h and maintained the OTR at the same level. A finite element model was used to predict the dynamic uptake of water into the systems. The model was found to over-predict the rate of water uptake for uncoated samples but gave the correct order of magnitude results for samples that were coated. The obtained data confirmed the positive effect of hot-pressing combined with coating to produce a film with low oxygen transmission rate and potential to maintain its oxygen barrier feature at high relative humidity.
Highlights
Cellulose nanomaterials (CNs) can form stable self-standing films with promising potentials in packaging applications
Aside from self-standing films, surface coating by cellulose nanofibrils (CNF) onto plastic or paper as a layer has great potential; the deposition of cellulose nanomaterials on many commercially available polymers can create an impermeable surface against gases [6,7]
It is demonstrated that the paths of gas diffusion within the nanofibrils are in the form of elongated cavities with a cross-sectional size as narrow as ~0.31 nm, and a thin layer of nanocellulose on poly lactic acid (PLA) could give rise to an impervious surface for O2, CO2 and N2 [8]
Summary
Cellulose nanomaterials (CNs) can form stable self-standing films with promising potentials in packaging applications. The hydrophilic nanocellulose can absorb the water molecules, causing the cellulosic fibers to swell and disrupt the chain linkages, damaging the airtight network to a significant extent [13] This points to the sensitivity of nanocellulose barrier properties to moisture and it is critical to better comprehend how the rate of oxygen permeation within a nanocellulose layer can be altered by the presence of water being absorbed. One common approach for reducing the water permeation is through coating layer(s) of water-resistant polymers, such as high-density poly ethylene (HDPE), polypropylene (PP), oriented polyethylene terephthalate (OPET) and biaxially oriented polypropylene (BOPP) [4,20] These plastics are known to be non-biodegradable, nonrenewable and as a result, problematic for the environment. The model took into account the moisture dependence of the diffusion coefficient
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