Nanomaterials 2023 , 13 , 1931
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Figure2. Scheme of hand sheet production.
All sheets were characterized according to the different ISO standards after condi- tioning according to ISO 187 [79]. Density (kg/m 3 ) was determined following ISO 534, grammage (g/m 2 ) using ISO 536, and thickness ( μ m) with ISO 534 [80,81]. The determi- nation of air permeability (mL/min) was measured according to the Bendtsen method following the ISO 5636 [82]. Tensile index (kN · m/kg) was conducted according to ISO 1924 and the wet tensile index (kN · m/kg) following the ISO 3781 [83,84]. Tear resistance (mN) was determined according to ISO 1974 and SCT (short-span compression test) (kN · m/kg) following ISO 9895 [85,86]. Error bars were calculated using the standard deviation between samples with the same conditions. Paper samples were analyzed using a high-resolution SEM (Tescan Maya3-2016, TES- CAN Brno, s.r.o., Brno, Czechia). All samples were prepared by sputtering them with a 5 nm thin layer of iridium prior to imaging. The applied electron beam voltage was 3.00 kV, and the beam intensity was 1.00. To obtain images of the structures at different scales, magnifications 200 × , 1000 × , and2000 × were used. The working distance to the sample was set to approximately 8 mm. 2.2.6. Hot-Pressing Technology The paper sheets were hot-pressed in a planar pressing equipment built at Mid Sweden University. This equipment mainly contains three parts: the heating blocks, pillar stand, and compression testing machine (see Figure 3). The heating blocks have a dimension of 300 × 300 × 40 mm and have three pockets in which flat electrical heating elements, each of 500 W, are inserted. Due to the thickness of 30 mm steel between elements and the pressing surface, variations in the temperature distribution will be at a minimum. The heating blocks are, via 20 mm thermal isolating plates, mounted in pillar rack ball brushings to ensure the best alignment between the blocks. In turn, the upper pillar rack is fixed to a hydraulic MTS TM material testing machine loadcell and the lower part to the movable hydraulic piston rod. For the control of block temperature, each has a built-in thermocouple sensor connected to Eurotherm PID-type regulators which are limited to 300 ◦ C. For the control of compression loads, MTS TM RPC software is used for creating block-programmed load vs. time sequences up to 100 kN. In these experiments, the pressing time was set to 3 s, and the pressure to 3.5 mPa. All experiments were carried out at 260 ◦ C with a dry content of 65–70% on the sheets. The sheets were manually handled during the hot-pressing trials.
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