# Cellulose Nanofibril (CNF) Films and Xylan from Hot Water Extracted Birch Kraft Pulps
## 3 Results and Discussion
### Properties of the CNF Films  
Films from the four CNF samples were produced by casting the dispersions on a plastic substrate. At first, the films were produced without addition of plasticizer, but after drying overnight at room conditions the films were fully wrinkled (not shown), which not only complicates the determination of mechanical properties but also hinders their utilization in the final end-product. Therefore, a second set of films was produced after addition of 30% sorbitol to the CNF dispersions, and the new films were smooth and transparent (Figure 1). The use of sorbitol in the manufacture of CNF films in large scale has been reported earlier [40]. The mechanical properties of the films were clearly affected by the hot water extraction, as the tensile strength and ductility (strain at failure) of the films made from water-extracted bleached pulps were lower than those of the film made from the original bleached kraft (BBK) pulp (Table 4). The hot water extraction, however, did not have an effect on the stiffness of the films, which remained at a similar level (~4 GPa). Interestingly, both BBK240.2 and BBK240.10 films had rather similar mechanical properties, despite their different xylan content. The strength and stiffness of the BBK film were lower than those reported in the literature for films made from TEMPO-oxidized nanocelluloses, but the ductility was considerably higher [41; 42]. Such effect could be attributed to the addition of sorbitol as plasticizer in this study. The presence of lignin in the UBK240.10 film also had a plasticizing effect on the mechanical performance, as both the strength and the stiffness decreased and the ductility increased compared to the BBK240.10 film (Table 4). Representative stress-strain curves for all films are shown in Figure 4.  
Films for food packaging should have not only good mechanical properties but also outstanding barrier properties against oxygen and water vapour to avoid spoiling of the food. The oxygen barrier properties are dependent on the RH of the environment, and thus were here measured in dry and in high (80%) RH conditions. In dry conditions, all the films acted as an excellent barrier to oxygen, with \begin{table}
\begin{tabular}{c c c c c} \hline  & **BBK** & **BBK240.2** & **BBK240.10** & **UBK240.10** \\ \hline Thickness, \(\mu\)m & \(31\pm 1\) & \(24\pm 1\) & \(21\pm 0\) & \(24\pm 0\) \\ Tensile strength, MPa & \(111\pm 9\) & \(87\pm 16\) & \(79\pm 8\) & \(68\pm 3\) \\ Young’s modulus, GPa & \(4\pm 0\) & \(4\pm 1\) & \(4\pm 0\) & \(3\pm 0\) \\ Ductility, \% & \(11\pm 2\) & \(8\pm 2\) & \(9\pm 1\) & \(12\pm 1\) \\ Oxygen permeability 1, & \(72\pm 1\) & \(66\pm 1\) & \(67\pm 2\) & \(87\pm 3\) \\ cc \(\times\)\(\mu\)m/m\({}^{2}\times\) day \(\times\) kPa & \(72\pm 1\) & \(66\pm 1\) & \(67\pm 2\) & \(87\pm 3\) \\ Water vapour permeability 2, & \(15\pm 0\) & \(13\pm 1\) & \(11\pm 0\) & \(13\pm 1\) \\ kg \(\times\)\(\mu\)m/m\({}^{2}\times\) day \(\times\) kPa & \(15\pm 0\) & \(13\pm 1\) & \(11\pm 0\) & \(13\pm 1\) \\ \hline \end{tabular} \({}^{1}\) Determined at 23 \({}^{\circ}\)C and 80% RH; 2 determined at 23 \({}^{\circ}\)C and 100/50% RH gradient.  
\end{table} 
Table 4: Properties of CNF films produced from bleached (BBK) and unbleached birch kraft (UBK) pulp after hot water extraction in a flow-through system (flow rate 400 mL/min). Figure 4: Representative stress-strain curves of films made from TEMPO-oxidized CNF. Sample names are the same as in Figures 1–3. permeability values lower than 0.01 cc x \(\upmu\)m/m\({}^{2}\) x day x kPa (not shown). At 80% RH, the oxygen permeability increased up to 66-87 cc x \(\upmu\)m/m\({}^{2}\) x day x kPa (Table 4). The oxygen barrier properties of the CNF films were somewhat improved by using water-extracted bleached pulps, while the presence of lignin in the films slightly increased the oxygen permeability. In all cases, the oxygen permeability was an order of magnitude lower than in the case of polyethylene [43], a common petroleum-based plastic used in food packaging.