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Robertson 1959). In general, the beating of fibers is a non-specific process during which both the fiber morphology and the particle size distribution are modified in many respects (Banavath et al. 2011; Laitinen et al. 2014). Beating is partly able to reverse hornification and results in better fiber swelling, even with virgin fibers, with all the positive effects on IWWS. The mechanical energy put into the process affects the internal and external fibrillation and enables more contact points for van der Waals bonds in the wet state and more H-bonds in dry state. The change in fiber morphology can be measured as fiber length, coarseness, fines and some other values. For calculation the IWWS as shown by Page (1993) and Shallhorn (2002) only fiber length and coarseness are significant values. In summary, beating has a strong influence on the fibrous material blend and the fiber morphology, which in turn have a decisive impact on the IWWS.
dihydrate; AC: chemical treatment of the sample with hydrochloric acid SEM studies by the author show a very large irregularity of the fiber surface of industrially manu- factured pulp, caused by fragments of the S1 and S2 fiber wall (Belle et al. 2015a).
Fiber morphology
The fiber morphology is related to the fiber structure. These are primarily given by the wood and pulping type, as well as by beating. In addition to the fiber length, additional fiber characteristics can be calculated by means of image analysis procedures. Generally, when describing fibrous suspensions, the following terminol- ogy is used: fiber length, fiber width, kink, curl, coarseness, fibrils, fines and broken fibers (Page et al. 1985; Saren et al. 2013; Weihs and Wa¨tzig 2007). Studies of unbleached pine sulfate pulp have shown that an increase in fiber length contributes to the IWWS (Seth 1995). This paper further demonstrates that an increase in fiber coarseness will decrease the IWWS. Another investigation on various recovered paper fibers about the influence of beating show that the strength index can be increased from 2.5 to 3.3 Nm/g at a dryness level of 50 % (Klein 2007). After the fiber analysis, Klein attributes the increase in strength to the larger specific surface of the fiber material after the beating process. Thus, a certain extent of beating can increase the IWWS due to the higher portion of fines and the creation of additional surface area on the fibers. The curl determines the maximum elongation of the paper up to breakage, and can therefore be a direct indicator of the stability of the paper web in the first open draw (Brecht and Erfurt 1959a; Rance 1954). The more pronounced the curl is, the more the fibers can be elongated under tensile load before the fiber structure breaks (Seth et al. 1984; Tejado and van de Ven 2010).
Fibrils and fines
Fines are in general defined as particle that passes the 100 or 200 mesh wire of e.g. a Bauer McNett fiber classifier and consist of cellulose, hemicelluloses, lignin and extractives (Retulainen et al. 1999). As a rule 0.2 mm is the size that is considered as fines in optical fiber analyzers. Several scientist, amongst others, worked on the topic of using or generating fibrils and fines to enhance the IWWS of paper (Brecht and Erfurt 1959b; Corson and Lobben 1980; Lindqvist 2013; Myllytie et al. 2009; Pye et al. 1965; Retulainen and Salminen 2009; Salminen 2010; Washburn and Buchanan 1964). Fibrils and fibrillar fines are sup- posed to increase the IWWS by better conformability that leads to improved frictional connection. Salminen (2010) reported that wet web tensile is dependent on quality and amount of fines. An ‘‘addition of heavily refined kraft pulp with a high amount of fines to wood containing paper grades could increase the residual tension of wet web significantly … ’’ (Salminen 2010). Lindqvist worked with a harsh and a gently refining strategies (Lindqvist 2013). Pulp that was gently refined generated fines without decreasing the fiber length. The resulting fibers were more flexible and lead to increased wet tensile strength and residual tension. It is concluded that the internal and external fibrillation are critical for the residual tension. The movement of the fibrils and fines during sheet forming
Beating
Many publications have studied the development of paper strength by beating, but only a small number of these studies have focused on the influence of beating on the IWWS (Belle et al. 2016; Bhardwaj et al. 2000; Brecht and Erfurt 1959a; da Silva et al. 1982; Gurnagul and Seth 1997; Kibblewhite and Brookes 1975; Koskenhely et al. 2011; Lindqvist et al. 2011;
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