Fecha de publicación: 27/10/2024 Fuente: Cellulose (Springer) Abstract
Cellulose nanomaterials (CNMs) have potential utilization as oxygen barrier layers when applied as the primary component of a single layer in multilayer packaging systems. One limitation of dense CNM films is their lack of durability due to their brittle nature. The durability of cellulose nanofibril (CNF) films was examined through the lens of understanding the residual stress that formed within the films during drying. A beam bending method consisting of drying CNF films on top of a flexible substrate was used to quantify residual stress. It was determined that for two common variations of CNFs (TEMPO-oxidized vs mechanically refined), the residual drying stress was on the order of 50% of the yield strength of the material at a given humidity. Larger residual stress formed within the TEMPO-oxidized CNFs as compared to the mechanically refined CNFs due to the smaller fibril diameters resultant of the TEMPO oxidation production process. The residual stress in the TEMPO-oxidized material was also more sensitive to moisture (humidity) than the mechanically refined material. Various plasticizers were examined for their efficacy in reducing residual drying stress. Triethyl citrate appears more effective in reducing residual stress than common polyols (glycerol/sorbitol), especially when subjecting the films to low humidity. Triethyl citrate in combination with a small amount of polyvinyl alcohol at a total plasticizer content of 30% of the mass of the CNFs resulted in a 75% reduction in the residual stress from the unplasticized state. In this low-stress state, standalone films (10–15 µm) can withstand creasing operations that compromise unmodified (high residual stress) films in terms of their ability to prevent oxygen transmission. Additionally, the low-stress formulation was coated onto a paper substrate resulting in enhanced durability during creasing operations prior to oxygen barrier testing.