Sustainability 2023 , 15 , 2850
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2.5. Preparation of Paper Sheets Laboratory paper sheets were prepared from unbeaten and refined pulp samples that had been previously disintegrated, as described in Section 2.4 Sheets of paper were formed using the Rapid–Koethen apparatus in accordance with PN-EN ISO 5269-2 (2007). Each paper sheet had the basis weight of 80 g · m − 2 (ISO 536:2012). Only sheets with base weights between 79 and 81 g · m − 2 were used for further investigation. The paper samples were conditioned at 23 ◦ C and 50% relative humidity according to ISO 187:1990 for a minimum of 24 h before examination. 2.6. Analysis of Paper Properties Contaminants on the tested paper surfaces were assessed using the Keyence VHX- 6000 microscope (Keyence, Belgium) equipped with the VH-Z20UT lens (20/200 × mag- nification). The OP-72402 adapter (ring shape) was used for sample illumination. Image analysis-based measurements of elements was performed using the microscope software. The surface texture of samples was tested under a microscope in accordance with ISO 25178:2016 Geometrical Product Specifications (GPS). Briefly, based on three-dimensional microscopic photographs of coated surface papers, roughness profiles were prepared. Specifically, roughness profiles were obtained from the surface profiles by separating the long-wave components (waviness and shape deviations) with an λ c profile filter. The λ c profile filter determines the transition from roughness to waviness, that is, random or close-to-periodic inequalities. The basic roughness parameters (Sa and Sz) describing surface microgeometry and those related to specific profile features were obtained from the roughness profile photographs. For roughness profiles, Sa represents the arithmetic mean deviation of the roughness profile along the sampling length, whilst Sz represents the maximum roughness (the maximum height of the profile indicates the absolute vertical distance between the maximum profile peak height and maximum profile valley depth along the sampling length). Furthermore, the roughness of paper surface was determined in accordance with ISO 8791-2:2013 with the TMI 58-27 Bendtsen Roughness Tester (Kontech, Lodz, Poland). Air permeability was determined according to the ISO 5636-3:2013 with the TMI 58-27 Bendtsen Roughness Tester (Kontech, Lodz, Poland). Optical parameters were determined using the X-rite Exact spectro-densitometer in accordance with ISO 2470-1:2016. The priority strength properties of papers were determined using the Zwick 005 ProLine testing machine (ZwickRoell, Ulm, Germany) coupled with the testXpert III soft- ware in accordance with ISO 1924-2:2010. The following tensile properties of paper were examined: • I B : breaking length [m] • F B : tensile force at break [N] • σ T b : width-related force at break [N · m − 1 ] • σ T W : force at break index [Nm · g − 1 ] • ε T : strain at break [%] • W T b : energy absorption [J · m − 2 ] • W T W : energy absorption index [J · g − 1 ] • E b : tensile stiffness [N · m − 1 ] • E w : tensile stiffness index [Nm · g − 1 ] • E*: Young’s modulus [MPa] Detailed statistical analysis was performed on individual research series to determine the following basic indicators: arithmetic mean, extended deviation, and percentage relative error. 3. Results and Discussion Contents of non-fiberized substances in wastepaper pulp samples are summarized in Table 1. White wastepaper, described as number 3.04 in accordance with the PN-
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