2262
Cellulose (2016) 23:2249–2272
Wet pressing
Several authors have addressed the subject of pressing work and compacting in relationship to the develop- ment of strength (Brecht 1947; Clos et al. 1994; Edvardsson and Uesaka 2009; Guldenberg and Sch- warz 2004; Hua et al. 2011; Kurki et al. 1997; Lobosco 2004; Mardon 1961; Paulapuro 2001; Pye et al. 1965; Stephens and Pearson 1970; Washburn and Buchanan 1964). To summarize the results of these studies, the press work is an external force that causes elastic and plastic flow of the fibers. It can be assumed it supports the already formed capillary and surface forces and it overcomes possible steric or electrostatic repulsive forces. This leads to more closed pores, closer fiber to fiber proximity, a denser web and increased tensile strength of the sheet (Maloney et al. 1997). Additionally, the press dewatering increases the dryness of the paper. This inspired Shallhorn to improve Page’s method of calculating the tensile strength of wet webs. The increased dryness after the press enhances the ability to separate the paper web from the press felt or press roll into the first open draw. One possibility to increase the dryness after press is to increase the temperature of the sheet during pressing (Back and Andersson 1993; Jantunen 1985). But the higher temperature leads to less ‘‘work of straining and both elastically and plastically adsorbed energy’’ of the wet paper web at constant dryness (Kouko et al. 2014). It is explained by softening of the wet fibers via heating. A number of studies have investigated the separa- tion of the web from the press roll into a free open draw, attempting to support this process using chem- ical additives (Edvardsson and Uesaka 2009; Ha¨ttich 2000; Mardon 1961, 1976; Oliver 1982; Pikulik 1997; Sutman 2011). All of these optimizations lead indi- rectly to an increase in the IWWS by increasing the dryness. Figure 12 shows what happens in the z-direction of paper during dewatering, pressing and drying. At 20 % dryness there is much space between the fibers and especially at fiber crossings a water film with resulting capillary forces are imaginable. Further dewatering and pressing leads to a compacted sheet at 45 % dryness. This results in elastic and plastic flow and to force induced conformability. Some fibers are wet hornificated. The mechanic force leads to more contact points and a higher proportion of fiber surfaces
Fig. 11 Sheet structure of beaten softwood pulp at SR 30 and 20 % dryness
Furthermore, the sheet structure and the fiber orientation is largely determined by the condition in the headbox and forming section. The structure in the three dimensions x, y, and z have a major impact on the size of bonds, their distribution in the network and the conformability. This develops frictional connec- tions and entanglement (Ora 2012; Ora and Maloney 2013; Salminen 2010). Figures 10 and 11 show the sheet structure at 20 % dryness for unbeaten and beaten softwood pulp, respectively. Even these labo- ratory sheets show the entanglement of the fibers at this stage of dewatering that leads to the frictional connections and entanglement. Sheet forming is a crucial step in the papermaking process. In this regard the jet to wire ratio has also an important effect on IWWS, because with this param- eter the fiber orientation in-or cross machine direction (MD/CD) is adjusted. Increased fiber orientation results in higher MD tensile in wet webs (Kouko et al. 2007; Ora 2012; Salminen 2010). In recent years for many paper mills the problem area of the first open draw has moved from web transfer to press to web transfer to the drying section. Only for some specialty paper machines the open draw after the forming section is still existent. Especially for their machines, the most important factor to obtain high strength is to achieve the highest possible dryness level at the end of the forming section. Chemical additives and plant adjustments can be used to increase dryness at this point.
123
Made with FlippingBook Digital Proposal Creator