Cellulose (2016) 23:2249–2272
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Fig. 3 ‘‘Comparison of the interaction force-distance curves between a cellulose sphere and the cellulose I, II and amorphous cellulose surfaces at pH 3.5 and in 0.1 mM aqueous NaCl solution. The interaction with the cellulose I sample ( red ) is characterized by an electrostatic repulsion, while van der Waals forces and steric interactions predominate with the cellulose II sample ( black ) and amorphous cellulose sample ( grey ), respectively’’ [Reprinted with permission of Eriksson (2006)]
Fig. 5 Fiber–fiber interactions of refined NBSK (SR 30) fibers in a paper sheet at 45 % dryness
fibrils bridge the gap due to interactions between fibers and fibrils.
Electrostatic forces
A share of the strength developed at low dryness levels may be attributed to acid/base interactions (Gardner et al. 2008; Lindstro¨m 1980; Wa˚gberg and Annergren 1997; Williams 1983). As a general rule, carboxyl groups and/or sulfonic acid groups are fixed at lignocellulosic fibers (Sjo¨stro¨m 1989). Therefore, the influence of metallic ions in aqueous solutions must be taken into account. The micro and macro fibrils of the fibers react similarly to polyelectrolytes (Caseri 2009). In water with low conductivity, dispersed fines and fibrils stretch into the surrounding water. High conductivity levels force the fibrils to retract from the surrounding suspension onto the attachment points on the fibers. Both parameters, pH and salt content, have a direct influence on the electrostatic potential and swelling behavior of the fibers. Because of that, these parameters have an indirect effect on the strength formation in both, wet and dry paper (Grignon and Scallan 1980; Nelson and Kalkipsakis 1964a, b; Scallan 1983; Scallan and Grignon 1979). These phenomena can be explained with DLVO theory, named after Derjaguin, Landau, Vervey and Overbeek (Derjaguin 1954; Derjaguin and Landau 1941; Israe- lachvili 2006b; Pelton 1993; Vervey and Overbeek
Fig. 4 Uneven fiber surface of a never dried unrefined Nordic bleached softwood kraft (NBSK) fiber in a paper sheet at 20 % dryness
However, if instead of considering the fiber as a smooth, well-formed unit, its actual shape is consid- ered with numerous micro and macro fibrils, espe- cially if it is refined, the formation of van der Waals forces is quite conceivable. In the middle of the image in Fig. 5, two beaten fibers arranged in parallel are bound via fibril bundles from the S1 layer. These
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