# Cellulose Nanofibril (CNF) Films and Xylan from Hot Water Extracted Birch Kraft Pulps
## 1 Introduction  
Cellulose nanomaterials, which include cellulose nanofibrils (CNF), cellulose nanocrystals (CNC), and bacterial cellulose (BC), are a relatively new family of cellulosic materials with at least one dimension in the nanometre scale [1]. Both CNF and CNC are obtained from wood and plants, but CNF are prepared by mechanical fibrillation of chemical pulps (often after a suitable pre-treatment), and CNC are prepared by acid hydrolysis of highly purified cellulosic substrates (i.e., cotton, microcrystalline cellulose, filter paper). BC, on the other hand, is secreted extracellularly by various bacterial strains, most notably from the _Gluconacetobacter_ genus [1; 2]. The mechanical, hydrophilic and rheological properties of cellulose nanomaterials can be exploited in a wide variety of end-uses such as paper-making, composites, films, aerogels, foams, and food systems [1; 2; 3; 4; 5], but owing to their different origin and production process, the three types of nanocelluloses (i.e., CNF, CNC, BC) present unique structural features and properties. Currently, many research efforts are being directed towards the development of CNF films to replace oil-based plastics in packaging applications. Since cellulose-based films are rather brittle, the addition of plasticizers is often required to reach the desired workability and performance. Sorbitol is one of the most extensively used natural plasticizers and a promising candidate to replacesynthetic plasticizers in industrial operations [7]. Nonetheless, the main limitation for the industrial production of CNF films has traditionally been the high energy consumption required for the mechanical delamination of pulp fibres, as several passes (5-20) through a high-pressure homogenizer or similar device are often required to reach a sufficient degree of fibrillation [8; 9; 10]. The energy consumption, however, can be reduced by applying enzymatic or chemical pre-treatments to the cellulosic pulps. Enzymes are used to decrease the extent of hydrogen bonding by selectively hydrolysing amorphous cellulose and other polysaccharides in the fibres, while chemical pre-treatments are used to add ionic charge and promote the swelling, repulsion and separation of individual fibrils [3; 11; 12]. Chemical oxidation by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) is the most common pre-treatment applied. The TEMPO-mediated oxidation increases the reactivity of cellulose fibres towards fibrillation by selectively converting the C6 in the glucose units into carboxylate groups, thus creating electrostatic forces that aid the separation of the fibrils [13]. After TEMPO pre-treatment, the cellulosic pulps only need a few passes to obtain CNF of 4-20 nm in width, and films made from these fibrils are transparent, flexible and have high tensile strength [13]. Several forest companies (e.g., Borregaard, American Process, Stora Enso, UPM-Kymmene, Sappi, Nippon Paper) are now producing CNF at a pilot or industrial scale, and other producers are expected to join this rapidly growing market in the near future. Bleached chemical pulps used for the manufacture of graphic paper are typically selected for the preparation of CNF, which means that the lignin content in the pulps is negligible (<1%) but the hemicellulose content varies between 15-30%, depending on the botanical origin of the wood material. The hemicelluloses appear to facilitate the swelling and mechanical delamination of pulp fibres [14; 15]. However, the presence of xylan in hardwood pulps may limit the accessibility of TEMPO to cellulose during the chemical pre-treatment and thus reduce the extent of cellulose oxidation reached [16; 17]. Moreover, TEMPO-oxidation removes substantial amounts of xylan from the pulp [17], so the potential benefits of xylan towards fibre swelling and fibrillation are at least partially lost.