Cellulose (2016) 23:2249–2272
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These trials show, that the addition of up to 70 % of a beaten pulp fraction results in higher IWWS values. Similar experiments show that a specific blend of refined softwood and ground wood pulp results in a higher IWWS compared with the individual compo- nents (Schwarz and Bechtel 2003). This results show that different fiber components have complementary properties that are needed for a good IWWS. The studies reviewed here demonstrate that in each indi- vidual case, an optimal fibrous material blend can be found based on chemical and physical principles.
Measurement techniques
The sheet strength comprises the strength values of the individual components of the sheet as well as the strength of the interactions between those compo- nents. A variety of methods are available to measure the strengths of individual fibers and of the interac- tions between fibers. One method involves sample preparation with drops of resin to fix the fibers (Groom et al. 2002; Yu et al. 2010) followed by measurement with special equipment. However, this method is very time consuming. A variety of other approaches have been developed to investigate fiber properties, includ- ing the ‘‘Zero Span Method’’ (Burgert et al. 2003; Derbyshire et al. 1996; Futo 1969; Malhaire et al. 2009; Michon et al. 1994; Saketi and Kallio 2011; Saketi et al. 2013). Until now, these measurements have mainly been used in wood research. Meanwhile, the latest measurement techniques enable fibers to be pulled from the fiber network to determine the strength with which it is anchored to the sheet structure (Saketi and Kallio 2011). Another possibility to get a deeper look inside the paper structure is the X-ray syn- chrotron radiation tomography (Latil et al. 2010; Marulier et al. 2012, 2015; Wernersson et al. 2014). This technique enables researcher to get a 3D-image from the fiber network that might help in the future to get more valid information about fiber contact points and their bonding mechanism.
Fig. 8 Fibrils between two unrefined fibers at 20 % dryness
is determined by the amount of water in the proximity of the fiber and by the elastic modulus that is in the range of 2–15 MPa for fibers (Myllytie 2009; Nilsson et al. 2001; Scallan and Tigerstro¨m 1992). The SEM image performed by the authors in Fig. 8 shows the space between two fibers that is bridged by fibrils (Belle et al. 2015a). The conductivity of the solvent water was below 1 l S/cm, with the result that the fibrils of the fibers were able to move freely in the surrounding water, making contact and bonding with the next fibrils. To facilitate the scientific study of the influence of individual fibrous material components on strength development, a selective approach of using enzymes to reduce the presence of micro and macro fibrils could be useful (Teeri 1997).
Fibrous material blends
The blending of fibrous material components is another possible approach to increase the IWWS. Several authors have shown that a fiber component blend of long fiber, short fiber, flour and mucilaginous substances from groundwood pulp production mainly leads to a higher IWWS compared to the values of the individual components (Back and Andersson 1993; Brecht and Erfurt 1961; Brecht and Klemm 1952). Towards the end of the 1950s, trials have shown (Brecht and Erfurt 1959a) that blending different softwood pulp fractions has an impact on IWWS.
Macro level (sheet level)
The macro level analysis below addresses a number of aspects. First, an overview of the optimization options including chemical additives and adjustments in the forming and press section of a paper machine will be
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