Cellulose (2016) 23:2249–2272
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Fig. 12 Decreasing distance between fibers effected by dewatering, pressing and drying; handsheets of unrefined bleached kraft pulp (Belle et al. 2015b)
guaranteed IWWS values (Schwarz and Bechtel 2003). In spite of these shortcomings, the number of breaks caused by insufficient IWWS can be estimated and minimized.
contained within a water meniscus increases. The drying of the sheet up to a dryness of 95 %, see Fig. 12, results in dry hornification of the fibers. They look like completely flattened ribbons. The sheet is dense and the distance between the fibers is small. Fiber crossings reach a maximum contact area and bonding capability.
Measurement techniques
IWWS can be measured using a variety of methods. Brecht described an early method (Brecht and Volk 1954; Zellcheming 1966) using a force elongation device that permits the testing of wet or dry paper in accordance with the possibilities available at that time. He used this method in a variety of studies. The disadvantage of this measurement is that the sample is fixed horizontally by two weights. These weights are pressing the wet sample and this eventually results in pressing water into the testing area. This leads to a high variance of measurements. Additional methods have been developed (Alince et al. 2006; Andrews et al. 1945; Jantunen 1985; McCallum 1957; SCAN 1981, 2005; Stephens and Pearson 1970; TAPPI 1997). The German DIN standard ‘‘Testing of paper and board—Determination of the IWWS by tensile test’’ DIN 54514 2008 was issued in 2008 for the measure- ment of the IWWS. This method is suitable for the measurement of the entirety of forces that act on the sensitive paper web. However, the determination of the dryness of each sample is essential for the correct assignment of the measured forces. A prerequisite to ensure appropriate conclusions for practical applica- tions. As shown, the result of the IWWS depends strongly not only on the dryness level but also on the
Simulation
As it is complex and expensive to perform experi- ments, particularly in recent years a significant amount of research work has been carried out using simula- tions of paper machines, with the focus on the effects of various process parameter on the IWWS (Ed- vardsson and Uesaka 2009; Jantunen 1985; Kula- chenko and Uesaka 2010; Lappalainen and Kouko 2011; Lobosco 2004; Matheas et al. 2011; Salminen 2010; Zimmermann 2012). These studies have mainly focused on wire and press dewatering and on the behavior of paper in the paper machine at increasing machine speed. Experiments indicate the effects of variables like press impulse, densification, dryness, papers’ elastic modulus and release from press roll to the first open draw. Among others these have been incorporated into more or less extensive simulation calculations and black box models. In this manner, it is currently possible to predict the IWWS within certain limits and for certain machine parts. These simulations significantly simplify both the configuration of the paper machine during plan- ning and construction based on the raw material properties as well as the determination of the
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