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speed of the tensile test. The higher the drawing speed, the higher the tensile strength is. This indicates that the measured IWWS may depend on the conditions in the paper machine to a significant degree. High machine speeds result in different loads compared to slower running equipment. Therefore, the visco-elastic prop- erties of the paper play an important role (Andersson and Sjo¨berg 1953; Hardacker 1970; Retulainen and Salminen 2009; Skowronski and Robertson 1986). For this reason, recent studies have used a speed of 1000 mm/s (Kouko et al. 2007; Kurki et al. 2004). However, the DIN 54514 standard refers to the straining rate from DIN ISO 1924-3:2007-06. This standard specifies a strain rate of only 100 mm/min (1.667 mm/s). This significantly lower speed was chosen to avoid an influence of the inertia of the measurement equipment during testing (Schwarz and Bechtel 2003). Nevertheless, Kurki and others did comprehensive research on wet web tensile and relaxation characteristics with extensive findings as shown above. Recent developments in pilot plant scale are able to measure additional parameters. Tanaka et al. (2009) reported about wet web rheology by means of ‘‘a wet web winder installed on a pilot paper machine’’ . This enables the measurement of dynamic stress–strain relationship
considering the strain of unrolling. Ora did a similar investigation but used rewetted reels (Ora 2012). In addition to the measurement technique itself, the assessment and correlation to the boundary conditions is of particular importance. The ‘‘failure envelope method’’ evaluates the strength and elongation values in relation with the dryness (Seth et al. 1982). This enables an assessment of the applied forces and the resulting elongation at different dryness levels on the runability of wet webs. Additionally, statistical exper- imental planning enables the calculation of the importance of the used factors. Thus, it is possible to make significantly more precise statements to explain observed phenomena and to optimize the paper production process in a better way (Belle et al. 2014b).
Integral explanatory models
As described in the sections on the nanometer, micrometer and macro levels, different chemical, physical and mechanical forces are involved in the development of paper strength during the processes of dewatering and drying. These may be complementary to each other, overlap each other or counteract one another. In the late 1950s, Brecht defined different stages of strength development (Fig. 1), which depend
Fig. 13 Overview about the initial strength development of the wet web
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