# Cellulose Nanofibril (CNF) Films and Xylan from Hot Water Extracted Birch Kraft Pulps
## 3 Results and Discussion
glucuronic and hexenuronic acid groups [18]. The hot water extractions also promoted minor losses in cellulose yield (less than 10%), but the intrinsic viscosity and molar mass in the extracted bleached and unbleached pulps decreased dramatically as a consequence of extensive hydrolytic cleavage of glycosidic bonds in the cellulose fraction [29; 31]. Under similar extraction conditions (240 \({}^{\circ}\)C for 10 min), the viscosity of the unbleached pulp (UBK240.10) was 424 mL/g, higher than the 165 mL/g of the bleached pulp (BBK240.10) (Table 1). The higher viscosity of the UBK240.10 pulp may be related to the higher viscosity of the initial unbleached pulp (UBK), compared to the bleached pulp (BBK), but it is also possible that the residual lignin in the unbleached pulp had a protective effect towards the hydrolytic degradation of cellulose [32]. The aqueous hydrolyses obtained from the hot water extractions were analysed to assess the recovery of xylan-based compounds. In those hydrolyses obtained from the bleached kraft pulp, the xylose content was 9.5% and 15.1% of the initial dry mass of pulp after 2 and 10 min of extraction time, respectively (Table 2). These amounts corresponded to 60-65% of the total xylan extracted from pulp. In the hydrolysate from the unbleached kraft pulp, the xylose content was 17.4% of the initial dry mass of pulp, which corresponded to about 88% of the total xylan extracted. It is likely that xylan losses occurred because some of the xylan was entrapped within the pulp fibres and was washed away during the washing of pulp, following the extraction process. Some of the xylan might have also been converted to degradation products such as furfural, carboxylic acids and humins [33; 34]. In addition to higher xylose yields, the hot water extraction of unbleached pulp also resulted in the recovery of xylan compounds with higher molar mass (Table 2). The average weight molar mass (Mw) of the xylan from bleached pulp was only about 2 kDa, while the Mw of the xylan from unbleached pulp was about 10 kDa; the latter Mw value was in agreement with our previous studies [29; 35]. The lower molar mass of the xylan obtained from bleached pulp was probably due to the partial degradation of this carbohydrate fraction during the pulp bleaching operations. In terms of applications, the lower \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **BBK240.2** & **BBK240.10** & **UBK240.10** \\ \hline Temperature, \({}^{\circ}\)C & - & 240 & 240 & - & 240 \\ Extraction time, min & - & 2 & 10 & - & 10 \\ Pulp yield, \% & - & 86.7 & 74.9 & - & 73.2 \\ Cellulose content, \% & 70.5 & 83.5 & 92.0 & 70.9 & 88.4 \\ Xylan content, \% & 29.1 & 15.9 & 7.5 & 25.7 & 8.2 \\ Glucomannan content, \% & 0.5 & 0.6 & 0.5 & 0.2 & 0.3 \\ Lignin content, \% & - & - & - & 3.1 & 3.2 \\ Anionic charge, mmol/g & 0.05 & 0.01 & \textless{}0.01 & nd & 0.01 \\ Intrinsic viscosity, mL/g & 738 & 363 & 165 & 1288 & 424 \\ Molar mass (Mw), kDa & nd & 307 & 125 & nd & 380 \\ Polydispersity index & nd & 8.5 & 5.4 & nd & 7.8 \\ \hline \hline \end{tabular} \({}^{\text{nd}}\) not determined.  
\end{table} 
Table 1: Properties of bleached (BBK) and unbleached kraft (UBK) pulp before and after hot water extractions in a flow-through system (flow rate 400 mL/min). \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **BBK240.2** & **BBK240.10** & **UBK240.10** \\ \hline Temperature, \({}^{\circ}\)C & 240 & 240 & 240 \\ Time, min & 2 & 10 & 10 \\ Xylose, \% on dry pulp & 9.5 & 15.1 & 17.4 \\ Other sugars, \% on dry pulp & 0.2 & 0.4 & 0.2 \\ Mw, kDa & 2.0 & 1.8 & 10.0 \\ Polydispersity index & 1.6 & 1.4 & 3.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Properties of aqueous hydrolyses produced by hot water extraction of bleached (BBK) and unbleached birch kraft (UBK) pulp in a flow-through system (flow rate 400 mL/min). -polar mass xylan (\(\sim\)2 kDa) may be used for conversion to xylitol or as xylo-oligosaccharides [23].