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the grammages of base papers from the grammages of the coated samples. The coating thicknesses were determined using crosscut images obtained from scanning an electron microscope (SEM; Merlin, Carl Zeiss, Germany). 2.3.2. Surface Roughness and Air Permeability Measurements Surface roughness was measured with a PPS-tester by L&W (ABB Ltd.; Zürich, Switzer- land), applying the standard ISO 9791-4 to compare the surface properties of base papers. Air permeability was tested to analyse how closed the coating structure is using an L&W Air Permeance Tester (ABB Ltd.) (ISO 5636:5). 2.3.3. Imaging of Surface and Cross-Section The coating quality was analysed by imaging the surface and cross-section of the samples using scanning electron microscopy (SEM; Merlin, Carl Zeiss). Acceleration voltage of 2 kV was used. The samples were prepared by freeze-cracking with liquid nitrogen. Prior to the imaging, a thin gold layer was sputtered on the samples. 2.3.4. Moisture and Mineral Oil Barrier Measurements Water vapour transmission rates (WVTR) were tested using the gravimetric method by applying standard ISO 2528:2017. The measurements were conducted in two conditions: 23 ◦ C, 50% RH and 23 ◦ C, 80% RH. The samples were cut using a circular sample mallet die punch (diameter 75 mm, Thwing–Albert Instrument Company) and conditioned in the particular condition for at least 12 h. One test set up contained cups (EZ-Cup Vapometer cup, Thwing–Albert Instrument Company), two neoprene gaskets and dry calcium chloride powder (Thermo Scientific Chemicals). A test cup was filled with CaCl 2 , the sample was sealed between two gaskets placing coated side upwards and tightened with a screwable lid. The test set up was kept in a conditioning chamber (CTC256, Memmert GmbH; Schwabach, Germany) for 24 h. WVTR values (g/m 2 /day) were calculated by weighing the increase in mass caused by the moisture that was diffused through the sample and absorbed by the salt, as an average of three parallel tests. Heptane transmission rate (HVTR) was determined using a similar method, where CaCl 2 was replaced with n-heptane, and the heptane diffusion was monitored as the mass decreased over time. The method is described by Miettinen et al. (2015) [46]. 2.3.5. Oxygen Barrier Measurement Oxygen transmission rates (OTR) were measured using OXTRAN 2/22 by Mocon (Brooklyn Park, MN, USA) (complies with the standard ASTM D3985). A 5 cm × 5 cm sample was cut and placed between two aluminium masks with a test area of 5 cm 2 . Masking is used to prevent the side leakage of gases, as the paper edges are not coated or sealed. Two conditions were used: 23 ◦ C, 0% RH and 23 ◦ C, 50% RH. The accuracy was six adjacent test points within 1%, and one cycle lasted 30 min. The sensors were flushed (ReZero) for 30 min every six cycles. The results (cc/m 2 /day) were reported as an average of two parallel measurements. 2.3.6. Grease Barrier Measurement Grease resistance was tested using the KIT-test method (ISO 16532-2). The method includes 12 solutions of castor oil and two solvents, n-heptane and toluene. The solutions are ranked from 1 to 12, where 1 is the mildest one (only castor oil) and 12 is the most aggressive (mixture of n-heptane and toluene). One droplet of solution is dropped on the sample and wiped off after 15 s. The KIT value refers to the highest ranked solution that does not leave any stain on the surface. The KIT value (1–12) was reported as an average of 3 parallel tests.
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