Cellulose nanofibril-based hybrid coatings with enhanced moisture barrier properties

Abstract

Cellulose nanomaterials have garnered significant attention as the next generation of environmentally friendly packaging materials because of their abundance, biodegradability, low density, superior mechanical properties, and excellent oxygen barrier characteristics. However, due to their hydrophilic nature, CNMs exhibit poor water barrier properties at high humidity conditions, which limits their potential applications. Our previous research has successfully incorporated CNF with CMC and coated the CNF/CMC coating on molded pulp trays for food packaging, but the barrier properties of CNF/CMC were weakened under high humidity conditions due to the hydrophilic nature of the coating and plasticization effect of the water molecules during the permeation process. In this study, we enhanced the water barrier properties of CNF-based coatings on molded pulp trays by chemically modifying the CNF through crosslinking with polyamide-epichlorohydrin (PAE), the incorporation of Cloisite-Na⁺ nano-clay, and the addition of polyvinyl alcohol (PVA). This formulation further improved the water vapor transmission rate (WVTR) in both wet-cup and dry-cup conditions, showing 40.5% in wet WVTR and 89.2% in dry WVTR values compared to unmodified CNF/CMC coatings. The chemical modification also helped enhance oxygen barrier performance, in which OP decreased from 6.48 × 10⁻¹⁵ cm³[STP] cm cm⁻² s⁻¹ Pa⁻¹ to 2.31 × 10⁻¹⁵ cm³[STP] cm cm⁻² s⁻¹ Pa⁻¹. A reduction in Cobb value from 137 ± 9 g m⁻² to 56.3 ± 4.4 g m⁻² was also observed. The formulated CNF-coated MP trays maintained the same #12 oil and grease resistance level as the unformulated ones. Mechanical testing proved that the formulated CNF coated tray samples showed 23.3% increase in ultimate tensile strength, 96.7% increase in strain at failure, but 37.6% decrease in Young's modulus. These results demonstrate that our chemically modified CNF coatings offer a promising sustainable alternative to conventional synthetic packaging materials, particularly for food packaging applications requiring enhanced barrier properties at high humidity conditions.