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strength value of 63 MPa (Table 2). The elastic modulus of the alginate film was also quite high, with a value of 2 GPa. However, the strain at break of the alginate film was only 7%. Other studies have reported that the tensile strength of films made from sodium alginate ? 40% glycerol was 12 MPa, their elastic modulus roughly 170 MPa, and their strain at break just below 30% (Yoo and Krochta 2011). It is known that there are quite large variations between available alginate sources and/or qualities; there are variations in the ratio between mannosyluronic and gulosy- luronic units, as well as the length of each block depending on source (Lee and Mooney 2012). These model films from alginate had quite extraordinarily high strength and stiffness values. Chitosan gave significantly weaker model films than alginate, and the elastic modulus was also much lower (Table 2). However, the strain at break of the chitosan films was 55%. Additionally, the stress–strain curves of the chitosan films revealed a distinctive initial stiffness of the films prior to the plastic region of the curves (Fig. 3). The chitosan films could be strained almost seven times further than the corre- sponding alginate films. Cationic guar gum gave even weaker model films with extremely low elastic modulus (Table 2). The strain at break of films from cationic guar gum was quite high, much like with chitosan. The cationic guar gum films lacked the initial stiffness of chitosan. These results indicated that there is an inverse relationship between tensile strength and strain at break; stronger polysaccharide films tend to have lower strain at break values. It is known that the mechanical properties of model films from polysac- charides are heavily influenced by the properties of the compounds and their packing possibilities, as well as the amount and type of plasticizer (Arvanitoyannis and Biliaderis 1999; Mikkonen et al. 2009; Yoo and Krochta 2011). After these initial results, the main
focus of the main study was to determine whether or not the characteristic tensile properties of the different polysaccharides were transferable to a fiber network. The applications were performed as spray additions of the polysaccharides in solution onto pre-formed, wet handsheets.
Spray addition of polysaccharides onto wet handsheets
Wet handsheets were sprayed with the different polysaccharide solutions to dosage levels of 2 or 4 wt%. The polysaccharides were alginate, chitosan and cationic guar gum. Sorbitol, 20 and 30% calcu- lated on polysaccharide dry weight, was added to the solutions before spraying as softener. The tensile index of the handsheets after restrained drying were plotted as a function of their density (Fig. 4). The tensile index of the sprayed reference sheet was 57.2 Nm/g. All of the polysaccharides increased the tensile index and density of the sheets after restrained drying (Table 3). Cationic guar gum increased the tensile index by 7.2–11.0 Nm/g, while chitosan resulted in increases of 5.7–19.1 Nm/g. Alginate resulted in larger increases in tensile index, between 17.5 and 30.9 Nm/g, reaching a maximum value of 88.1 Nm/g with 2% ? 30% sorbitol. The overall increases in tensile index after restrained drying followed a similar pattern as the results from the tensile tests of the polysaccharide model films; alginate formed the strongest films, while films from chitosan and cationic guar gum were significantly weaker. In previous studies, spray addition of 4% gelatin ? cross-linker or spray addition of 4% gelatin, 4% agar ? cross-linker was required in order to surpass 80 Nm/g after restrained drying (Vishtal et al. 2015). The tensile index of the handsheets after unre- strained drying were plotted as a function of their
Table 2 The determined values of tensile strength, elastic modulus, and strain at break of model films from the polysaccharides
Polysaccharide film
Tensile strength (MPa)
E-modulus (MPa)
Strain at break (%)
Alginate ? 50% sorbitol
63
2045
7
Chitosan ? 50% sorbitol
25
203
55
Cationic guar gum ? 50% sorbitol
18
67
47
50% sorbitol was added as softener
123
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