Flexible Cellulose Nanofibril Composite Films with Reduced Hygroscopic Capacity

Cellulose nanofibrils (CNFs), which are generated from abundant, environmentally friendly natural plant resources, display numerous interesting properties such as outstanding mechanical strength, negligible light scattering, and low thermal expansion (Zimmermann et al., 2010). These nanofibers are usually created by mechanical fibrillation or chemical oxidation of pulp fibers. CNFs have previously been utilized in a broad range of fields, such as special printing, package, medical carriers, filtration, polymeric reinforcement, energy storage, and electronics or display substrates (Siró and Plackett, 2010). After removing excess water in cellulose nanofibril suspensions, strong, flexible, dense, and translucence films are obtainable. These free-standing cellulose nanofibril films, produced by self-accessibly of individual nanofibrils via hydrogen bonds, are exceptional and of great potential applications. These materials are considered as potential alternatives in barrier package, optical substrates, flexible electronics substrates, and battery membranes (Siró and Plackett, 2010). However, due to abundant hydrophilic groups, these films are sensitive to humidity and water (Aulin et al., 2010). Hydrogen bonds between neighboring nanofibrils tend to deteriorate when absorbing moisture, resulting in a significant destruction of above stated properties. As a result, many attempts have recently been dedicated to modify or functionally treat cellulose nanofibril films to minimize their hygroscopic absorption. There are essentially two different approaches that have been applied. One involves modifications which convert the hydrophilic groups on nanofibrils into hydrophobic ones, for example, by means of surface silylation (Andresen et al. 2006; Lu et al. 2008) and acetylation (Tingaut et al. 2010). Such chemical treatments significantly decrease the wettability of nanofibrils, and lead to improved water-resistance for the resultant films. However, the individual nanofibrils in films were largely combined by hydrogen bonds created between neighboring hydroxyls. Conversion of hydroxyls into hydrophobic groups is likely to reduce the generation of hydrogen bonds, depending on the substitution of hydrophilic groups. Another approach is the incorporation of resins with CNFs or coating waterproof polymers onto CNF films, which have been shown to reduce the amount of moisture absorption in the resulting composite (Herriksson and Berglund 2008; Qing et al. 2012), thus providing the potential to conserve such properties of CNF composites at high humidity. In the present study, flexible cellulose nanofibril phenol formaldehyde (CNF/PF) composites with reduced hygroscopic capacity were produced by either filtering solution of well-mixed CNFs and PF resin or impregnating the PF into wet CNF films. Swelling of CNF films prior to adding PF facilitated the impregnation of PF resin …