Cellulose
modulus of the cellulose nanofibril, which, even for random-in-plane orientation, dominates in-plane mod- ulus for paper structures (Page 1965) and composites.
film with comparable molecular orientation distribu- tion, for instance 5.2 GPa for biaxially oriented PET- films (Breil 2010). From modeling work, the H-bond- ing between nanofibrils was erroneously stated to be critical for mechanical performance (Meng et al. 2017), and such ideas were further developed in a recent review paper (Meng and Wang 2019). Although the suggested model is interesting, the statement needs correction. The ultimate strength of CNF nanopaper depends on cellulose molecular weight, indicating that it depends critically on cellulose nanofibril strength (Henriksson et al. 2008) and nanofibril length (Fukuzumi et al. 2013), but defects and local stress transfer mechanisms are also important. High hemi- cellulose content has a positive contribution to stress- strain behavior of nanopaper, which is related to interfibril bonding (Kontturi et al. 2021; Yang et al. 2021). However, in contrast to typical polymer films, even dry cellulose films (in the example above vacuum driedat 75 C for three days) will unavoidably contain water due to the finite, however brief, time it is exposed to moist air during the actual mechanical testing (Lindh et al. 2017). The large majority of the moisture is located in the interfibril region, since the fibril center is inaccessible to water (Sakurada et al. 1962). At 50% relative humidity, the moisture content of cellulose nanopaper is 8-10% (Yang et al. 2021). There is strong dependence of both modulus and yield strength on moisture content (Ben´ıtez et al. 2013; Yang et al. 2021). The yield strength in nanopaper is probably related to interfibril shearing in the inter- phase region, and is lowered by increasing moisture content, a problem that has been analyzed at a molecular scale (Sinko and Keten 2014; Zhang et al. 2021). In the dry state, no apparent yielding is observed (Yang et al. 2021). The mechanisms for low strain deformation (modulus) and plastic yielding (yield strength) in practice thus depend primarily on moisture-related effects. Thus, cellulose nanopaper does not obtain its excellent properties thanks to H-bonding but rather despite the abundant H-bonding sites on the nanofibril surfaces. On one hand, they lead to formation of dense structures through capillary forces during drying, but on the other hand H-bonding sites increase moisture sorption, even at low relative humidity, with reduced properties as consequence. The main reason for the superior modulus of nanopa- per to polymer films, however, is the high axial
The cellulose fiber and paper making
The scientific background to the excellent mechanical properties of different paper qualities has attracted substantial research interest over the years. This is partly due to the possibilities to form numerous materials from different types of fibers via water- based processing routes, and partly since the excellent inherent mechanical properties of the fibers are utilized (Bolam 1961). A large focus has been on the contact zone between the fibers and how the properties of this contact zone can be understood and manipu- lated (Lindstro¨m et al. 2005; Hirn and Schennach 2017). To a large extent, the extensive work by Nissan (Nissan 1955, 1976a, b; Nissan and Batten 1997) has dominated the view among paper scientists that hydrogen-bonding between the fibers is the major interaction responsible for the mechanical properties of the paper. This is appealing at a first glance, since cellulose and hemicelluloses, the dominating compo- nents of most delignified, papermaking fibers, contain a large number of OH-groups that are known to be able to engage in H-bonds. However, the process of rough cellulose-rich fibers approaching each other during drying and consolidation of paper in the presence of water is complex and a more elaborate description of the mechanisms behind the making and the breaking of fiber-fiber joints is needed, especially considering the very short-ranged nature of H-bond interaction and formation (Santiago Cintro´n et al. 2017). The devel- opment of new measuring techniques and the devel- opment of model materials mimicking the components of the fiber wall (Gustafsson et al. 2012; Li et al. 2021) provide new experimental information to separate the different molecular mechanisms responsible for fiber- fiber interactions, but still, there is no quantitative description of their relative influence on the formation of a strong fiber-fiber joint (Lindstro¨m et al. 2005; Hirn and Schennach 2017). The making and the breaking of a fiber-fiber joint are schematically described in Fig. 8. As the water between the fibers is evaporating the capillary pressure increases, pulling the fibers together. Under wet conditions, the delignified fiber wall is highly hydrated
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