Nordic Pulp & Paper Research Journal Vol 29 no (1) 2014 Special Issue: Nanocellulose 156 Cellulose nanofibrils: Challenges and possibilities as a paper additive or coating material – A review Fredrik Wernersson Brodin, Øyvind Weiby Gregersen and Kristin Syverud KEYWORDS: Cellulose nanofibrils, Paper additive, Furnish, Coating, Barrier SUMMARY: Today, there is widespread scientific and commercial interest in cellulose nanofibrils (CNF). The exploration of new manufacturing methods and pre- treatments has enabled a less energy intensive production of CNF. In this review the use of CNF in paper making applications as a paper additive or coating material have been summarized and discussed. CNF can be added directly into the pulp furnish, and by using a relatively low amount of CNF together with suitable retention aids potential problems related to dewatering can be avoided. This type of CNF addition enables either decreased basis weight or increased filler content with maintained paper strength. CNF can also be applied in coating formulations. Increased surface smoothness and enhanced barrier properties are key benefits obtained by CNF containing coatings, but challenges exist such as the high water content of CNF coatings and the low moisture resistance of CNF barriers. Further research is still needed but, at least in some papermaking applications, CNF is not far from implementations on a commercial scale. ADDRESSES OF THE AUTHORS: Fredrik Wernersson Brodin 1 (fredrik.brodin@pfi.no), Øyvind Weiby Gregersen 2 (oyvind.w.gregersen@ntnu.no) and Kristin Syverud 1,2 (kristin.syverud@pfi.no) 1) PFI AS, Høgskoleringen 6b, NO-7491 Trondheim, Norway. 2) Norwegian University of Science and Technology, Department of Chemical Engineering, NO-7491 Trondheim, Norway. Corresponding author: Fredrik Wernersson Brodin Currently, an intense research activity is ongoing within the field of cellulose nanofibrils (CNF) and cellulose microfibrils (CMF). The widespread interest can be explained by a few advantageous basic properties, i.e. high availability as a renewable material, high mechanical strength, large specific surface area and high aspect ratios, barrier properties, dimensional stability, biodegradability and biocompatibility (Eichhorn et al. 2010). As a result of these properties, CNF and CMF have been suggested for a large number of application areas including foods, cosmetics, pharmaceuticals, paints, drilling muds, paper additives and paperboard barriers, medical products, nanocomposites, hygiene and absorbent products (Turbak et al. 1983; Klemm et al. 2011; Brodin, Theliander 2013). CNF and CMF, also known as nanofibrillated cellulose (NFC) and microfibrillated cellulose (MFC), are two types of cellulose nanomaterials which contain both crystalline and amorphous parts and typically have lengths longer than 1 μm. There is also another type of cellulose nanomaterial called cellulose nanocrystals (CNC). CNC elements are, however, much shorter than CNF or CMF and has therefore not the same ability to form networks as the CNF and CMF elements. Various definitions may be found in the literature, but in the proposal for the new TAPPI Standard (TAPPI standard WI 3021), CNF have widths of 5-30 nm and CMF have widths in the range between 10-100 nm. In this review, however, the name CNF will be used exclusively since there has been a lack of distinction between these two materials in many scientific papers. The cellulose nanofibrils are originally created during the biosynthesis of cellulose. The synthesis was for a long time a mystery but thanks to developments in microscopy, the enzyme complexes associated with cellulose synthesis have been revealed (Brown 2004). In vascular plants, such as trees, these enzyme complexes are called rosettes, because they have six hexagonally arranged subunits that each consists of six cellulose synthases molecules. The complex as a whole is responsible for the synthesis of one elementary fibril which has 6 x 6 = 36 glucan chains (Brown, Saxena 2000). These elementary fibrils are 3.5 nm wide (Meier 1962) and together with the hemicelluloses and lignin they are organized in the well-known and ingenious system that form the wood fibres. Herrick et al. (1983) and Turbak et al. (1983) were the first to find a method to produce CMF from wood pulp fibres by passing a dilute fibre suspension several times through a high pressure homogenizer. During fibrillation, bonds between elementary fibrils and bundles of fibrils are opened, which promotes the liberation CNF or CMF. The resulting material after fibrillation consists of fibrils varying in width from the size of the elementary fibrils of 3.5 nm and upwards. In addition to CNF or CMF, fibre fragments are usually present even after the fibrillation process (Chinga-Carrasco 2011). Various mechanical methods have been applied to obtain fibrillation including homogenization (Herrick et al. 1983), microfluidization (Zimmermann et al. 2004), microgrinding (Iwamoto et al. 2007), refining (Nakagaito, Yano 2004) or cryocrushing (Taniguchi, Okamura 1998). The energy demand of operation and quality of the resulting fibrillated material, obtained from various mechanical methods, have been evaluated and compared by Eriksen et al. (2008) and Spence et al. (2011). Mechanical pre-treatments can also be used to reduce the risk of blockage during fibrillation. The commercial interest was low for many years due to the high energy demand of fibrillation, where values between 12 000-65 000 kWh/ton has been reported (Lindström 2007; Eriksen et al. 2008; Spence et al. 2011). However, recent research has shown that the energy demand of fibrillation can be reduced by using some kind of chemical or enzymatic pre-treatment, e.g. acidic or