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presented. This is followed by an overview of computer simulations to predict IWWS. This section concludes with a summary of the measurement techniques to determine the strength of wet paper webs.
CMC is also an additive that is used for strength improvement. Even for wet web it improves the strength in a range of 20–25 % at 50 % dryness (Klein 2007; Myllytie 2009). This might be explained with the more uniform paper by reduced flocculation tendency that results in stronger wet webs (Linhart et al. 1987; Nazhad et al. 2000). Even a very expensive resource like chitosan is an object of research to improve IWWS. Different researchers got strength improvements [ 30% at dryness between 30 and 55 % (Hamzeh et al. 2013; Klein 2007; Laleg and Pikulik 1992, 1993b; Myllytie 2009; Pikulik 1997; Salminen 2010). The structural similarity with the 1,4-glycosidic bonds are responsi- ble for the adsorption capacity of Chitosan onto Cellulose (Klein 2007). Especially for mechanical pulp the strength improvement is attributed to crosslinking ‘‘the fibrous network via imino bonds formed between the primary amino groups of the polymer and the aldehyde groups present in mechan- ical pulp’’ (Pikulik 1997). Guar gum has also been used to improve the IWWS (Myllytie 2009; Weigl et al. 2004). Weigl et al. (2004) were able to improve the strength of about 20 % at a dryness of 30 % with 0,5 % dosage of cationic guar to a pulp for LWC paper production. The effect of guar is attributed to the hydrophilic character, the cationically and uniform charge of the polymer. Galactoglucomannan (GGM) as a by-product of softwood pulping was recently tested by Lindqvist in 2013. She was able to increase the strength with 3,5 % GGM of about 10 % at 45 % dryness. She used bleached kraft pulp for the laboratory sheets. The GGM is supposed to disperse the fibrils on the fiber that they are more outstretched. In this way, they are able to interact with other fibrils and develop the contact points for web strength (Lindqvist 2013; Lindqvist et al. 2013). Besides these renewable additives, there exist several conventional chemicals that are used in paper making. One kind of these are surface active chem- icals. They are mainly used for stabilizing the paper making process but they influence also the paper strength. For an optimal dosage it has been found that the dewatering of the sheet was improved and the dryness and thereby the IWWS at same process conditions increased (Lindqvist 2013; Retulainen and Salminen 2009). By using oleic acid and defoamer a slight decrease of IWWS was observed (Retulainen
Chemical additives
Many paper additives derived from renewable and conventional resources have been developed and tested in recent years. The most popular additive for strength enhancement is starch. The cationic starch manly used for dry strength improvement is not able to increase IWWS of paper, it works quite contrary. This results in a decreased residual tension of about 10 to 15 % at 50 % dryness (Hamzeh et al. 2013; Laleg et al. 1991; Retulainen and Salminen 2009; Salminen 2010). Figure 9 shows this strength loss described in a paper from Retulainen and Salminen 2009. The decrease is explained by steric and electrosteric repulsion that reduces the friction force between the fibers. As Fig. 9 also shows, with cationic aldehyde starch there is a strength improvement possible at dryness above 45 %. Laleg and Pikulik came some years before to a similar result (Laleg and Pikulik 1991, 1993a). In contrast to the cationic starch, the cationic aldehyde starch is proposed to form a kind of cross linking in the fiber network ‘‘with hemiacetal bonds formed between the aldehyde groups of starch and the hydroxyl groups of cellulose’’ (Pikulik 1997).
Fig. 9 The effect of two different starches on residual tension of wet web at 2 % strain (Retulainen and Salminen 2009); (Reprinted with permission of The Pulp and Paper Fundamental Research Society)
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