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Fig. 6 The 2D formability (%) at 23 C and 90 Cof handsheets sprayed with the different polysaccharide solutions. The paper properties were measured after unrestrained drying. The average values with standard deviations are shown. Alg alginate, C-gg cationic guar gum, Chit chitosan. The amount of added sorbitol is also shown in the x-axis legends
Alginate increased the 2D formability to a similar extent as 4% gelatin, and 4% gelatin ? cross-linker, while outperforming addition of 4% agar (Vishtal and Retulainen 2014a; Khakalo et al. 2014). However, it has also been reported that addition of 4% gelatin only increased the 2D formability by approximately 0.5% at room temperature after unrestrained drying, high- lighting some inconsistencies (Vishtal et al. 2015). With alginate, cationic guar gum or chitosan, there were no significant differences in the formability of the sheets as a function of temperature. It has previously been reported that the 2D formability of paper sprayed with agar and/or gelatin does increase with increasing temperature (Vishtal and Retulainen 2014a; Khakalo et al. 2014; Vishtal et al. 2015). In order to better understand the previously pre- sented results, the measured paper shrinkages were plotted against their corresponding strain at break values after restrained and unrestrained drying (Fig. 7). It was seen that also the handsheets dried under restraint between blotters had shrunk to some extent; between 0.1 and 1.8% 1D shrinkage was measured (Table 3). Even limited paper shrinkage during restrained drying will affect the paper proper- ties, by enhancing the strain at break and lowering the tensile stiffness to some extent. In a previously published study, it was seen that a strain at break of 6.4% could be considered the upper limit of elongation potential from strength increases alone without paper shrinkage (Strand et al. 2017). In a previously reported refining study, it was seen that the strain at break of
papers leveled off at about 5% with increasing refining energy, when the drying shrinkage of the sheets had been minimized (Strand et al. 2018). Also shrinkage during restrained drying needs to be considered and measured when dealing with strain at break values surpassing 6.5%, to ensure that the high values are not a function of unexpected drying shrinkage. Strain at break values of up to 7% after restrained drying have been reported for sheets sprayed with 4% gelatin, 4% agar ? cross-linker, but the paper shrinkage during restrained drying was not reported (Vishtal et al. 2015). When all of the measured 1D shrinkages were combined with their corresponding strain at break values, an almost linear correlation emerged. The correlation indicated that handsheets without any shrinkage should be able to strain 4.6%, which is very close to previously reported values for this pulp after 135 kWh/t of LC-refining (Strand et al. 2018). A linear trend between strain at break and drying shrinkage during unrestrained drying has previously been presented, as a function of sprayed amount of agar (Vishtal and Retulainen 2014a). The current study extends the linear correlation towards lower shrinkage values also. The 1D shrinkage after unrestrained drying was significantly larger than after restrained drying (Table 4). The 1D shrinkage of the reference sheets was 4.3%. Cationic guar gum and chitosan decreased the shrinkage slightly, and had quite limited effects on the strain at break of the sheets (Fig. 7). The tendency for cationic polysaccharides to decrease paper
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