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PEER-REVIEWED REVIEW ARTICLE
nanocellulose, while facilitating other superior barrier properties (Lindström and Aulin 2014). As is typical with hydroxyl group-abundant biopolymers, nanocellulose exhibits low water-resistance and high water vapor permeability (Hubbe et al. 2017). Principally, crystallinity is beneficial in terms of barrier properties, because it is more difficult for molecules to penetrate the crystalline areas (Siró and Plackett 2010). As such, crystallinity is beneficial in terms of water resistance and water vapor barrier. Moreover, heat treatment improves wet strength, rendering the film denser, possibly due to the aggregation of adjacent cellulose chains, and less porous, which is beneficial in preventing leakage (Österberg et al. 2013; Hubbe et al. 2017). Sharma et al. (2014) showed that when heating films of nanofibrillated cellulose (NFC) for 3 h at 175 °C, the water vapor permeability was reduced by 50% (Nair et al. 2014), whereas Xia et al. (2018) reported a ten-fold decrease in WVTR while comparing 3 h post-treated TEMPO- oxidized nanofibrillated cellulose (TONFC) films to untreated TONFC films. Likewise, for a treated film OTR values of 0.007 and 0.584 mL.μm.kPa -1 m -2 day -1 were measured at room temperature and relative humidity (RH) of 50% and 80%, respectively. The result for RH 80% is 100 times lower than most plastic films, such as PET or PVC. The results are in line with Österberg et al. (2013) who indicated OTR values that improved by hot- pressing NFC films. Feng et al. (2015) reported variations in the properties of bacterial cellulose (BC), depending on the drying method used. This was highlighted by a water- holding capacity obtained by freeze-drying half that obtained by oven-drying, with values of 6000% and 12,000%, respectively. Likewise, while comparing NFC with CNC, Peng et al. (2013) noticed differences in crystallinity and thermal stability from different drying methods. As a result, for NFC, spray-drying displayed the highest combination, in terms of thermal stability and crystallinity, whereas the conclusion was ambiguous for CNC. According to Xia et al. (2018), the barrier-enhancing effects of heat on NFC are explained by both increase in crystallinity and reduction in porosity. The increase in crystallinity by heat treatment was also found by Peng et al. (2013), who discussed various drying methods for NFC. The higher crystallinity leads to lower oxygen permeability, while water vapor permeability is reduced simply by increased material density. The larger the length-to-width ratio and the surface area of fibrils, the higher the fibril entanglement and the longer the path for molecules need to travel through the barrier material (Dufresne 2012). The mechanical fibrillation is used to manufacture micro- or nano-fibrils, which influences the barrier properties by affecting fibril dimensions (Kangas 2014). The mechanical fibrillation step involves a few alternative disintegration methods, as presented in Fig. 3. The influence on barrier properties correlate with the reduction of length-to-width ratio and surface area of the fibrils during the mechanical fibrillation (Dufresne 2012; Kangas 2014). As expected, this reduction is greater as the number of steps or passes, is increased (Siró and Plackett 2010). However, mechanical fibrillation also tends to decrease crystallinity (Siró and Plackett 2010), which is a drawback when aiming for superior barrier properties. Nanocellulose and reinforced composites Nanocellulose possesses a high capacity for interacting with fillers when blended in a polymer matrix, leading to exceptional mechanical strength. These properties are enhanced by the high aspect ratio of nanofibers, as well as the inherent reactivity of cellulose. Given suitable structural conditions in the blend, one form of interaction of NFC with a filler is by generating a rigid percolation network. The percolation network
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Helanto et al. (2019). “ Bio-based barriers ,” B io R esources 14(2), Pg #s to be added.
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