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fiber swelling and new methodologies; and interpret these findings. Various forces and factors influencing the IWWS are presented and placed in context. The effects were separated into the size ranges of nanome- ter, micrometer and macro scale. The corresponding forces and phenomena from the literature and from own work are assigned to the appropriate ranges. While summarizing the complex field of IWWS it might be reduced to a three-dimensional system: dryness, forces and conditions. On one hand the maximum level of dryness should be reached to get the utmost strength values and best pick up from wires, rolls and felts used. On the other hand, the dryness level indicates, which forces are acting while holding the wet web together. For each certain dryness level the best conditions have to be chosen to get highest tensile strength and best relaxation characteristics. For older, mainly specialty paper machines the dryness below 25 % is still relevant. In this range the conformability of the fiber network has big influence on the frictional connection of the wet web. The friction coefficient between two wet fibers and the coarseness are important parameters. Besides this, the capillary force is acting. This involves the conditions of water quality like surface tension at water-fiber surface, contact angle, density of the water and particularly the water film thickness between adjacent fibers. Thanks to the progressive dewatering this is a highly dynamic system with e.g. starting of fiber collapse and increasing sheet density. State of the art paper machines have dryness levels between 25 and 60 % at the first open draw. Based on mechanical pressure in the press section the steric repulsive force might be overcome and the number of contact points increases rapidly. At this stage a flexible, viscoelastic and smooth fiber is needed. Fines and fibrils are creating van der Waals bonds backed by the fiber water gel and beyond of this. The fiber water gel is important for wooden polysaccharides like xylan and other hemicelluloses to interdiffuse and to build up another kind of bonding via attraction and repulsive force according to the DLVO theory. Strong H-bonds and chemical bonds between fibers, fibrils and fines are unlikely because of too much remained water in the wet web and other mild reaction conditions like relatively low temperature. At the end, there is no doubt that two factors are particularly important: the dryness level of the wet
web and the fiber morphology. Chemical additives can improve the IWWS at intermediate dryness levels of approximately 30–60 %. In practice, the fiber water gel is the easiest factor to adjust by controlling the process water quality and the use of chemical additives. Furthermore, developments in measurement tech- niques, micro robotics and computed tomography promise to improve our ability to measure the existing forces and to describe the mechanisms of strength formation.
Remark
All samples and SEM images were prepared as described in ‘‘Demonstration of Strength Develop- ment in Initial Wet Paper Web using Field Emission- Scanning Electron Microscopy (FE_SEM)’’ (Belle et al. 2015a). Acknowledgments Financial support for this project (‘‘Initiale Nassfestigkeit von Papier’’ AZ 1000_11) was provided by the Bayerische Forschungsstiftung, Munich. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http:// creativecommons.org/licenses/by/4.0/), which permits unre- stricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Com- mons license, and indicate if changes were made. Open Access
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