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Cellulose (2019) 26:1995–2012
European Fibre and Paper Research Organizations (EFPRO). Funding by the Academy of Finland, the Finnish Funding Agency for Technology and Innovation (TEKES) and the Association of European Fibre and Paper Research Organizations (EFPRO) is gratefully acknowledged. Also the financial support for the CD Laboratory from the Austrian Federal Ministry of Science, Research and Economy and the National Foundation for Research, Technology and Development is gratefully acknowledged. 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 unrest- ricted use, distribution, and reproduction in any medium, pro- vided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Open Access
The individual BSKP fiber elongation was not improved by the HC refining and a decrease in strength was observed. The influence of mechanical treatment on fiber tangential stiffness was small. The effects of the refining seemed to correlate with the number of dislocations (measured using HCl cleavage test) in fibers. With increasing number of dislocations both the average breaking stress and strain decreased. The mechanically untreated BSKP fibers already had a high number of dislocations based on the microscope images and HCl cleavage test. High consistency refining introduced additional dislocations, but low consistency refining seemed to be able heal some of them. There were clear differences between the tensile properties of thick-walled and thin-walled fibers. Also viscose fibers showed considerable variation and non- homogeneity based on their stress strain-behavior and number of HCl induced fiber cleavages. Comparison of the tensile tests of individual fibers and paper suggest that only a part of the elongation potential of individual fibers can be utilized in the elongation of paper and the fiber bonding governs not only the strength but also the elongation of fiber networks. Elongation of paper can be promoted by securing a high bonding ability of fibers and enabling shrinkage of fiber network. HC refining increased kinks and decreased the shape factor of the pulps and therefore had a major influence on the strength properties of the sheets, yet had only a minor influence on individual fibers. The results indicate that micro- compressions and individual fiber extensibility play a smaller role in sheet extensibility than has been assumed. Unrestrained drying of BSKP fiber sheets in combination with increased fiber bond strength seemed to be a promising approach for increasing the extensibility of paper. The key conclusion related to this investigation was that the increased network elongation was created by two factors. First, increased bonding improved paper strength and thus increased the strain to break. Second, the role of free shrinkage was again shown to be pivotal for sheet extensibility. Single fiber extensibil- ity was not critical and seemed not to limit the extensibility of the paper. Open access funding provided by Technical Research Centre of Finland (VTT). This work has been funded by the ExtBioNet project supported by the Academy of Finland, the ACel program of the Finnish Bioeconomy Cluster CLIC Innovation and Association of Acknowledgments
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