# Cellulose Nanofibril (CNF) Films and Xylan from Hot Water Extracted Birch Kraft Pulps
## 3 Results and Discussion
### Preparation and Quality of CNF  
The bleached kraft pulp and the water-extracted pulps were subjected to TEMPO-mediated oxidation to increase their anionic charge and thus facilitate the fibrillation process. Increasing the amount of NaClO in the reaction system during TEMPO oxidation increases the anionic charge of the pulp, but also decreases the DP of cellulose [13]. Since the viscosity (and molar mass) of the water-extracted pulps was rather low (see Table 1), the amount of NaClO added was limited to 2.5 mmol per g of pulp to avoid extensive depolymerization of the cellulose chains. With this dose of NaClO, the target anionic charge was about 0.8 mmol per g of pulp, which is typically deemed sufficient for effective fibrillation. As shown in Table 3, the anionic charge of the pulp BBK240.2 after TEMPO oxidation was only 0.5 mmol per g, but the charge of the bleached kraft pulp and the other water-extracted pulps was about 0.7-0.8 mmol per g, close to the target value. The reason for the lower anionic charge in the pulp BBK240.2 is not known. As expected, the viscosity of the pulps (and thus the DP of cellulose) dropped after TEMPO oxidation, but the drop was more significant in those pulps that had a higher initial viscosity (Table 3). It should be noted, however, that the viscosity of the TEMPO-oxidized pulps is nonetheless approximate. Additional oxidation of the pulps with NaClO\({}_{2}\) should be performed to convert small amounts of aldehydes to carboxyl groups, as the presence of aldehyde groups in the TEMPO-oxidized cellulose results in lower anionic charge, causing somewhat lower viscosity values [13]. After the TEMPO-mediated oxidation, the cellulosic pulps were fibrillated in a high-pressure microfluidizer. The resulting CNF had a viscous gel-like appearance, although the CNF dispersions from the water-extracted pulps were clearly thinner (i.e., less viscous) than the CNF from the original kraft pulp (Figure 1). Moreover, the presence of residual lignin in the unbleached (UBK) pulp gave a yellowish colour to the corresponding CNF sample. AFM images showed that the cellulose fibrils in the CNF dispersions were mostly in the nanometre range and formed an interconnected fibrillar network, although some fibril aggregates could be observed regardless of the chemical composition of the pulp (Figure 2). The roughness (RMS) values, measured from a 5 \(\times\) 5 \(\upmu\)m\({}^{2}\) area, were 5.5, 4.7, 5.8, and 4.7 nm for the CNF made from the pulps BBK, BBK240.2, BBK240.10 and UBK240.10, respectively. Overall, the nanostructural features of the CNF samples in this study were similar to those reported in the literature for TEMPO-oxidized CNF [39]. In addition to nanofibrils, SEM images revealed that considerable amounts of fibre fragments in the micrometre range were still present in the CNF dispersions (Figure 3). The number of fragments in the CNF obtained from the original kraft pulp appeared to be the lowest, but their dimensions were the largest, with some fragments measuring up to 14 \(\upmu\)m in cross-section and 300-400 \(\upmu\)m in length (Figure 3a). The fibre fragments in the CNF obtained from water-extracted pulps were roughly about 2.5 \(\upmu\)m in cross-section and 10-15 \(\upmu\)m in length, although particles up to 100 \(\upmu\)m in length were not uncommon. The higher amount of microfibers in the CNF from water-extracted pulps, and particularly in the CNF from the bleached BBK240.10 sample (Figure 3c), appears to indicate that these pulps exhibited a lower fibrillation tendency, possibly caused by their lower xylan content and consequently by stronger aggregation of cellulose fibrils [14; 15]. \begin{table}
\begin{tabular}{l c c c c} \hline  & **BBK** & **BBK240.2** & **BBK240.10** & **UBK240.10** \\ \hline Anionic charge, mmol/g & 0.7 & 0.5 & 0.8 & 0.7 \\ Intrinsic viscosity, mL/g & 176 & 139 & 85 & 122 \\ \hline \end{tabular}
\end{table}