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Cellulose (2017) 24:1759–1773
renewable composites was the use of cellulose nanofibrils as reinforcement for polymers (Lee et al. 2014a). Recently, cellulose nanofibrils (CNF) with diame- ters at the nanoscale received much attention due to their outstanding chemical and mechanical properties (Chen et al. 2010; Lee et al. 2012a; Klemm et al. 2011). They were utilized for a wide range of applications (Klemm et al. 2011), such as membranes (Mautner et al. 2014, 2015), flame-retardant and fire- protection applications (Carosio et al. 2015, 2016; Liu and Berglund 2013) and in particular for the produc- tion of composites (Blaker et al. 2011; Lee et al. 2009, 2012b c, 2014b; Nogi and Yano 2008; Lee and Bismarck 2012; Eichhorn et al. 2010; Pommet et al. 2008; Wan et al. 2006). Accordingly, numerous approaches to utilize CNF in composites have been proposed and tested. One approach to utilize the potential of CNF was to directly reinforce a soft matrix with small amounts of CNF, which resulted in improved strength of the composites provided that the affinity between matrix and cellulose fibrils was high enough. Mikkonen et al. (2011) utilized 5–15 wt% of CNF to reinforce spruce O-acetyl galactoglucomannan films while Peng et al. (2011) and Hansen et al. (2012) reinforced xylan films. Additionally, chitosan based and thermoplastic starch composites were reinforced by CNF (Tome´ et al. 2013): Addition of 10–20 wt% of CNF was sufficient to improve the thermal stability and mechanical properties of the composites, i.e. the Young’s modulus and the tensile strength improved significantly at the expense of the ductility of the composite. To manu- facture hierarchical composites, cellulose microfibers were combined with CNF (Lee et al. 2012a, 2014c), but also utilization of CNF as sole reinforcing agent was considered a possible track en route to high performance composites (Eichhorn et al. 2010). The best results have been obtained using a biomimetic approach; introducing a very high loading of CNF in a small amount of a soft polymer. While the CNF matrix provides stiffness and strength, the role of the soft polymer is to dissipate energy and hinder crack propagation, thus improving toughness. This ulti- mately aims at exceeding the mechanical properties of the individual constituents of the composite. For example, a cationic block-co-polymer was combined with highly negatively charged CNF, resulting in synergistic effects (Wang et al. 2011; Sehaqui et al.
2013). Unfortunately, this led to the removal of water from the CNF gel due to ionic interactions between cationic polymer chains and anionic fibrils resulting in fibril aggregation. CNF aggregation easily leads to defects in the composite, therefore it is very important to control the interactions between the individual fibrils and avoid aggregation (Ben´ıtez et al. 2013). Thus, approaches utilizing non-ionic interaction between a polymer matrix and cellulose have been suggested, e.g. CNF were combined with poly(ethy- lene glycol) grafted carboxymethyl cellulose (Ol- szewska et al. 2013a, b). Furthermore, bacterial cellulose (BC) was combined with hydroxyethyl cellulose (Zhou et al. 2009). Within these systems, the contact points between fibrils during film forma- tion were lubricated by the water-swollen polysac- charides, leading to the formation of strong films. Further aligning the fibrils did substantially improve the strength and stiffness of the composite in one direction (Sehaqui et al. 2012). While there have been extensive efforts to prepare thin nanopapers and composites from CNF, there are only a few reports where these nanopapers have been used to prepare (nano)paper based laminated compos- ites to utilize the CNF properties. This would be of outmost practical importance. The use of nanopapers as reinforcement for polymers was first demonstrated by Yano (Yano et al. 2005; Nakagaito and Yano 2005). Henriksson and Berglund (2007) later prepared nanopaper-composites with a water-soluble mela- mine–formaldehyde resin while Lee et al. (2012c), Ansari et al. (2014) and Aitoma¨ki et al. (2016) manufactured epoxy composites by vacuum infusion and impregnation, respectively. However, as of yet, not much research has focused on the effect of the nanopaper properties on composite properties. We hypothesize that the properties of the nanopaper reinforcement strongly affect the properties of the composite in multi-layer laminates. It would be very desirable to have a process at hand in which the mechanical properties of final multi-layer composites were defined by the mechanical performance of the nanopaper base. It was previously found that adding only 2 wt% of water-soluble polysaccharides to a suspension of CNF significantly improves the dry and wet strength of CNF nanopapers (Lucenius et al. 2014). In this study, we aimed to make use of this increased nanopaper strength for production of multi-layer, laminated paper
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