Coatings 2023 , 13 , 195
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2.5. Moisture Permeability (Water Vapor Transmission Rate) Water vapor transmission rate (WVTR) evaluates the moisture transmission through- out a given sample in time. In the gravimetric method, the mass gain is due to a desiccant absorbing humidity while generating a water vapor pressure differential. WVTR was cal- culated from the steady slope of mass gain in time of the cup, as described in Equation (2):
24 · α A
(2)
WVTR =
where α is the steady slope (in g/h), A is the test area (in m 2 ), and 24 is the conversion factor to achieve a WVTR measured in g/(m 2 · day). WVTR was measured using cups filled with 35.0 ± 0.1 g silica gel—according to BS ISO 2528:2017 [29]—for 48–96 h, depending on the barrier performance. The coated side of the samples faced the inner side of the cups. The test area of the samples was almost 20 cm 2 . Environment conditions were 23 ± 1 ◦ Cand50 ± 2%, as in previous tests. 2.6. Grease Permeability The grease permeability test allows us to determine the resistance of a paper—or coated paper—to grease permeation through the substrate or substrate-coat system. In the test, a disk of grease (3 mm thick) was laid on the surface, and a defined weight applied onto it. The test was carried out with palm kernel oil in accordance with BS ISO 16532-1:2008 [30] to assess the show-through time. The maximum test time was set to 24 h, as defined in the previously mentioned standard. Contrary to Cobb 1800 and WVTR, higher grease permeability values represent higher barrier properties. 2.7. Scanning Electron Microscope Coated and uncoated samples were gold-sputtered and analyzed by scanning electron microscopy (SEM) using a ZEISS EVO 50 (Zeiss, Wetzlar, Germany) electron microscope. To retrieve surface morphology, 100 × , 250 × , and500 × magnifications were used, while magnifications ranging between 1000 × and5000 × were used to evaluate the cross section of the films. Images were analyzed using ImageJ v1.53e. 2.8. Spectrophotometry All the samples were analyzed with a portable Konica Minolta CM-2500d (Konica Minolta, Chiyoda, Tokyo, Japan) spectrophotometer with D65/10 ◦ color space to retrieve the specular component excluded (SCE) CIE L*a*b* color coordinates as well as 8 ◦ gloss. Δ E* (CIEDE2000) color difference against uncoated samples was calculated and evaluated as described in [1]. 2.9. Normalization The results in this work were normalized to a 7 μ m dry coat thickness, calculated over an exponential curve that passes through the uncoated substrate experimental data (Figure 1). The normalization is due to kaolin, whose density (almost 2.5 times higher than one of the commercial dispersion coatings) strongly influences the coating thickness at an equal dry coat grammage. If neat polymeric dispersion coatings were exclusively used, normalization on the dry coat grammage would have worked as well.
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