Water and Oxygen Barrier Properties of All-Cellulose Nanocomposites

Hydroxypropyl cellulose (HPC) is potentially interesting as a biobased, rigid food packaging material, but its stiffness and strength are somewhat low, and its water and oxygen transport rates are too high. To improve these characteristics, we investigated nanocomposites of HPC and cellulose nanocrystals (CNCs). These high-aspect-ratio nanoparticles display high stiffness and strength, and their high crystallinity renders them virtually impermeable. Exchanging the counterions of sulfate-ester decorated CNCs with cetyltrimethylammonium ions affords particles that are dispersible in ethanol (CTA.CNC) and allows solvent casting of HPC/CTA.CNC nanocomposite films, which, even at a CTA.CNC content of 90 wt %, are highly transparent. The introduction of CTA.CNC considerably increases the Young's modulus (E_y) and upper tensile strength (σ_UTS). For example, in the nanocomposite with 90% CTA.CNC, E_y = 7.6 GPa is increased 20-fold and σ_UTS = 42.7 MPa is more than doubled in comparison to HPC, whereas the extensibility (1.1%) remains appreciable. Composites with a CTA.CNC content of 70 wt % or less show a lower water vapor permeability (6.4-9.2 × 10^-5 g μm m^-2 s^-1 Pa^-1) than the neat HPC (1.5 × 10^-4 g μm m^-2 s^-1 Pa^-1), whereas the oxygen permeability (5.6 × 10^-7-1.3 × 10^-6 cm³ μm m^-2 s^-1 Pa^-1) is reduced by 1 order of magnitude compared to HPC (3.2 × 10^-6 cm³ μm m^-2 s^-1 Pa^-1). The biobased nanocomposites retain their mechanical integrity at a relative humidity of 75% but readily disintegrate in water.