Cellulose (2019) 26:3473–3487
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cross-linker gave similar tensile index values as addition of 2% alginate. Similarly, it has been reported that spray addition of 4% gelatin increased the tensile index of paper from 59.3 to 70.3 Nm/g after wet- pressing and unrestrained drying, i.e. by 11 Nm/g (Khakalo et al. 2014). The tensile index could, once again, be increased further by addition of glutaralde- hyde as a cross-linker. As a comparison, addition of 2% alginate in the current study increased the tensile index of the papers by 18.9 Nm/g without wet- pressing, which is quite substantial. The addition of 2% alginate resulted in a similar tensile index values as sequential addition of 4% gelatin and 4% agar after unrestrained drying (Vishtal et al. 2015). Plotting the tensile index versus density of papers after restrained and unrestrained drying revealed an almost linear dependency (Fig. 4). It is known that the relative bonded area is heavily linked to paper density, and that there is a strong relationship between relative bonded area and tensile strength (Page 1969). A linear dependency have been shown to exist between the relative bonded area and sheet density using bleached softwood kraft pulps of different mixtures of spring- wood and summerwood fibers (Retulainen and Ebel- ing 1993). A linear dependency has also been shown between the determined BET-area of papers and their tensile index; the tensile index increased as the amount of unbound areas in the papers decreased (Lindstro¨m et al. 2016). Even for the current results, where the density ranges between 368 and 507 kg/m 3 due to the different additives and drying techniques, a somewhat linear dependency between tensile index and paper density could be seen. The strain at break of the handsheets were plotted as a function of their tensile stiffness (Fig. 5). After restrained drying, the strain at break of the handsheets were similar to, or lower than, the level of the reference sheets. Increases in strain at break of papers after restrained drying have previously been reported after addition of agar and gelatin; the highest reported value was approximately 7% after with of 4% gelatin, 4% agar ? cross-linker (Vishtal et al. 2015). In the current study, all of the polysaccharides resulted in increased tensile stiffness values of the handsheets after restrained drying. The largest increases in were seen with alginate, while cationic guar gum or chitosan resulted in less pronounced increases (Table 3). The highest stiffness value obtained was 5.8 kNm/g with 2 wt% alginate ? 30% sorbitol as
softener, i.e. an increase of 1.4 kNm/g from the sprayed reference. As a comparison, a tensile stiffness of 6.4 kNm/g was previously achieved with wet-end additions of carboxymethyl cellulose in combination with a cationic starch (Strand et al. 2017). The strain at break after unrestrained drying increased moderately with cationic guar gum or chitosan, while the largest increases were seen with alginate (Fig. 5). The largest strain at break value, i.e. 11.3%, was obtained with 2 wt% alginate ? 20 or 30% sorbitol (Table 4). It has been reported that 2 or 4% agar also increased the strain at break of paper by approximately 2% after unrestrained drying (Vishtal and Retulainen 2014a). Addition of 4% gelatin also increased the strain at break of papers after unre- strained drying by about 2%, while further addition of 0.5% cross-linker resulted in an increase of 4.4% (Khakalo et al. 2014). Sequential additions of 4% gelatin and 4% agar ? cross-linker were able to achieve strain at break increases of approximately 5%, but the dosage levels were quite high in this study (Vishtal et al. 2015). In the current study, all of the tensile stiffness values of the sheets were similar to, or lower than, the reference value of 2.4 kNm/g after unrestrained drying. Alginate, chitosan, or cationic guar gum had slight to no effect on the stiffness of handsheets after unrestrained drying, even though noticeable changes were seen in strain at break and tensile index. It has been previously reported that 4% gelatin increased the tensile stiffness slightly of the paper after unrestrained drying, while including a cross-linker made this effect more pronounced (Khakalo et al. 2014). Sequential addition of gelatin and agar resulted in increased stiffness values, from 75 kN/m up to approximately 90 kN/m, after unre- strained drying (Vishtal et al. 2015). It was noted that the results from the preliminary study of polysaccha- ride model films were not directly transferable to the fiber network after unrestrained drying; chitosan or cationic guar gum did not increase the strain at break significantly, while alginate instead resulted in higher values. The 2D formability strain of the handsheets after unrestrained drying was tested at 23 and 90 C. The reference sheets could strain approximately 10% before breaking in the 2D tests (Fig. 6), i.e. about 1% further than in regular tensile testing. When comparing the reference with the reference samples of previously published 2D formability tests, it was clear
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