bioresources. com
PEER-REVIEWED REVIEW ARTICLE
Despite the significant growth in the number of publications related to nanocellulose fibrils during the latest decade, it is notable how little emphasis has been given on the upscaling of nanocellulose production and the associated economic aspects. Recently, extensive reviews on these challenges for both cellulose micro- and nanofibrils (De Assis et al. 2018) and CNC (de Assis et al. 2017) were published. Several technical challenges in the behavior of NFC should be resolved prior to implementing industrial production. Some products are already produced by applying NFC as raw material (De Assis et al. 2018). Accordingly, Lindström and Aulin (2014) articulated a few practical challenges that might limit the implementation of nanocellulose in packaging applications. One key challenge is to blend hydrophilic nanocellulose with hydrophobic matrices: until today, most advancements are related to demonstrations of surfactant and emulsion-based systems to improve nanocellulose dispersion in a matrix and achieve improved overall properties. Another key challenge relates to the prevention of unwanted hornification and shrinkage, an issue that has not been fully addressed to date. In terms of conventional pulp, the extent of hornification relates to the content of hemicelluloses (Östlund et al. 2010), which suggests that hemicelluloses influence the hornification of NFC. A third key challenge relates to the tendency of nanocellulose to absorb moisture from air, given its hygroscopic nature, which tends to compromise other advantageous properties. This issue can be addressed by applying nanocellulose in the form of dense, layered structures within which a high density of hydrogen bonding and the tight packing reduces related interactions. Likewise, combining nanocellulose with other materials, such as PLA or lignin can reduce its hygroscopic nature. A fourth key challenge relates to the drying of nanocellulose, as it typically exhibits high water-holding capacity and high swelling ability. In addition, one challenge relating to upscaling, also pointed out by Lindström and Aulin (2014) is the inherently high viscosity of NFC, which might affect pumping and transport (Hubbe et al. 2017; Kumar 2018). Furthermore, according to Kangas (2014), there are a few more issues related to the industrial implementation of nanocellulose. The possibilities of nanocellulose are indicated by the variety of potential applications, for example, to replace oil-based products in packaging, such as polyethylene and polypropylene (Piringer and Baner 2000), and their ability to add new functionalities, such as electroconductivity and printability (Guo 2017; Kumar 2018). A current drawback is the uncertainty in the costs and production scale. Despite these factors, the interest continues to grow at an accelerated pace (Kangas 2014). Some recent attempts to accelerate the use of nanocelluloses for the manufacture of continuous films include a laboratory-scale coating (Kumar 2018) and a pilot-scale SUTCO machine to produce surface-treated nanocellulose films (Peresin et al. 2012), to name only a few. Hemicelluloses Hemicelluloses can be found in plant cell walls between cellulose microfibrils. Hemicelluloses have been separated from wood and various agro-based materials (Albertsson et al. 2011; Mikkonen and Tenkanen 2012; Laine et al. 2013). The content and the composition of hemicelluloses are dependent on the origin and location in the plant (Albertsson et al. 2011; Mikkonen and Tenkanen 2012). Depending on the species, wood contains 20 wt% to 30 wt% of hemicelluloses: hardwood contains slightly more hemicelluloses than softwood. The hemicelluloses in hardwood are mainly xylans, while a lesser amount consist of glucomannans. The main hemicelluloses in softwood, on the
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Helanto et al. (2019). “ Bio-based barriers ,” B io R esources 14(2), Pg #s to be added.
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