Cellulose (2019) 26:3473–3487
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Papermaking procedure
times. The obtained raw data was recalculated from load (N) into stress (MPa) and from extension (mm) into strain (dimensionless), in order to determine the elastic modulus of the tested samples.
Handsheets were formed on a 125 mesh wire in a 165 mm 9 165 mm laboratory sheet former. The handsheets were prepared without recirculation of white water. The aimed grammage of all of the handsheets was 100 g/m 2 . The handsheets were couched from the wire by pressing, and stored wet (solids content 26–28%) between wet blotter papers in sealed plastic bags at 5 C until further use. The polysaccharide solutions were sprayed onto the top side of the wet sheets using a commercial, high- volume, low-pressure (HVLP) gravity feed air spray gun. The top side of the wet sheets were sprayed with polysaccharide amounts corresponding to of 2 and 4% o.d. polysaccharides based on dry paper weight. A wet sheet was placed on a rigid plastic plate on top of a gravimetric scale. The targeted amount of solution was adjusted by controlling the wet weight of the sheet after spraying. Reference sheets were sprayed with an equivalent amount of cold tap water. The sprayed samples were placed on a vacuum box covered with a fabric wire; a vacuum level of approximately 5–10 kPa was applied beneath the sprayed sheet. The aim of the vacuum treatment was to remove excess water from the sheets and aid the penetration of the polysaccharides into the wet fiber network. Ten handsheets were sprayed for each trial point; five sheets were subjected to restrained drying, and five sheets were subjected to unrestrained drying. Restrained drying: the sheets treated with polysac- charide solutions were dried on a plate with a blotter, strapped into a drying frame. Unrestrained drying: the treated handsheets were dried in specially built drying frames, between two polymer forming wires with a gap of approximately 3 mm to allow for paper shrinkage during drying, while simultaneously pre- venting severe cockling and curling of the sheets. Both, restrained and unrestrained drying methods were performed under standard testing conditions (EN ISO 5269-1:2005E). None of the sheets were wet- pressed prior to drying.
Polysaccharide viscosity measurements
Solutions of the different polysaccharides were pre- pared with their previously described solvents; 1 wt% of alginate and chitosan, and 0.5% of cationic guar gum. The viscosities of the polysaccharide solutions were determined using a Paar Physica MCR (modular compact reometer) apparatus (rod: CC27, cup: TEZ 150P). The shear rate gradually increased from 0.01 to 1000 s - 1 (Fig. 2). The measured viscosity at a shear rate of 1.1 s - 1 was 259 mPas for the 1% alginate solution, 251 mPas for the 1% chitosan solution, and 668 mPas for the 0.5% cationic guar gum solution.
Refining of pulp
The low-consistency refining of the kraft pulp was performed with a ProLab TM refining station (Valmet, Finland) at A˚ bo Akademi University (Turku, Finland) according to a previously described procedure (Strand et al. 2017). The Schopper–Riegler ( SR) values of the refined pulp was measured according to ISO 5267-1, and parallel samples of the refined pulp was charac- terized using a Kajaani Fiberlab optical fiber analyzer (Metso Automation, Finland) (Table 1). The pulp was refined to 135 kWh/t, which corresponded to SR25.
Paper testing
The basis weight of the paper samples was determined according to ISO 536:1995. The thickness was deter- mined according to ISO 534:1998, and the density was determined based on the measured values of the basis
Fig. 2 The measured viscosity curves of the polysaccharides in solution
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