2.5. Sheet formation and testing
After mixing the pulp suspension and CNFs (2, 5, and 10 wt%) for 10 min, the sheets or films were made using CEM membrane filters. Filter paper was placed below the ultrafiltration membranes to provide support. Fiber slurries with approximately 0.2% consistency were added to the ultrafiltration apparatus to make sheets with a target basis weight of 60 g/m2. After de-watering, individual films were blotted and placed between filter and blotter papers. The sheets and blotter papers were placed between caul plates with a pressure of approximately 500 kPa and put in an oven at 100 °C for approximately 20 min. The samples were further dried at the same pressure without heating for 24 h prior conditioning. Finally, the samples were conditioned at 50 ± 2% relative humidity and 23 ± 1 °C according to T 402 sp-98 for at least 4 h. The diameter and thickness of the dried specimens were 9 cm and 80–110 μm, respectively. The sheets were tested for tensile index (T 494 om-01), stretch (T 494 om-01), burst index (T 403 om-02), and air resistance (T 441 om-98).