Cellulose (2016) 23:2249–2272
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Fig. 14 Schematic illustration of bonding layer formation process; (Reprinted with permission
of The Pulp and Paper Fundamental Research Society)
to a considerable degree on the dryness of the paper web (Brecht and Erfurt 1959a, b). In 2010, two explanatory models with three different phases of strength development were published by Tejado and Erhard (Erhard et al. 2010; Tejado and van de Ven 2010). These are expanded upon in the following paragraph based on own observations and experiments summarized in Fig. 13. At the top of the diagram the involved forces are shown. The bottom part illustrates the conditions to get the best IWWS. The arrow shows the contrariness of the needed conditions between the two phases of strength development. During the first phase, up to a dryness level of * 25 %, capillary forces can be assumed to be prevalent. These forces develop in the structure due to progressive dewatering and result in frictional connection and entanglement (Belle et al. 2014a; Kallmes et al. 1977; Kendall 2001a; Williams 1983). With further dewatering, the fibers progressively collapse (Belle et al. 2015a; Paavilainen 1993a, b; Weise et al. 1996; Weise and Paulapuro 1996), with the result that the macroscopic and mechanical interlocking and felting of fibers increases the entan- glement and frictional connections (de Oliveira et al. 2008; Tejado and van de Ven 2009; Williams 1983). Rigid, smooth fibers are best suited for this phase of strength development, as they support the capillary forces and interlocking at large distances between fibers (Belle et al. 2014a). In the second phase, at dryness levels of [ * 25% up to * 60 %, attractive van der Waals and repulsive forces occur in accordance with the DLVO theory (Derjaguin 1954; Derjaguin and Landau 1941; Israe- lachvili 2006b; Pelton 1993; Wa˚gberg and Annergren 1997). In contrast to the first phase of strength
development, in this second phase a flexible, visco- elastic, soft and coarse fiber surface is required, which stimulates the formation of larger contact areas between fibers (Kendall 2001b; Lindstro¨m et al. 2005; Nanko and Ohsawa 1989; Nilsson et al. 2000; Pelton 1993; Persson et al. 2013). Figure 14 shows the schematic illustration of bonding formation (Nanko and Ohsawa 1989). In this phase, the gel formation in the proximity of the fibers is an important parameter affecting the formation of contact points (Kibblewhite 1973; McKenzie 1984; Voyutskij 1963b; Wa˚gberg and Annergren 1997). These forces are supported by the diffusion of polymer chains of dissolved cellulose on the fiber surface and by wood polysaccharides, particularly xylan (Casey 1960; Clark 1978a; McKenzie 1984; Pelton 1993). In this phase, the distances between the fiber surfaces and fibrils are already so small that self-assembly take place. This enables the fibrils to form bonds between fibers (Belle et al. 2015a; Persson et al. 2013; Po¨nni et al. 2012; Yan and Li 2013). The dry content in the first open draw of modern paper machines is [ 40 %, even 60 % is possible if it is in the first drying section. During the third and final phase until the final dried paper is obtained, hydrogen bonds are established and reinforced by the increasing dryness level (McKenzie 1984; Nissan and Batten 1990; Wa˚gberg and Anner- gren 1997).
Conclusions
This paper discusses the knowledge obtained from several decades of research on IWWS; summarizes the newest insights about the surface interactions of fibers,
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