Coatings 2023 , 13 , 195
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Figure4. Surface morphology of KB H39K 40 (magnification: 2000 × ). The sample results are rough due to the high filler concentration.
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( b )
Figure5. Different coating wettability. ( a ) SA-B over KB: water droplets are clearly visible; ( b )H39K 80 over KB: water is evenly spread across the tested surface. 3.3. WVTR The moisture barrier property showed to strongly improve compared with uncoated paper substrates, as represented in Figure 6. From almost 350 g/(m 2 · day) WVTR, the coatings reached values strictly lower than half of UC ones. Moreover, excluding H39K 100 and H40K 100—which still showed good performance—the coatings reduced the WVTR by more than 85%, compared with UC. Despite its double original coating—which contains mineral fillers—the Cobb 1800 and WVTR barrier properties of UC KP are close to UC KB ones. On the contrary, the addition of experimental formulations significantly reduced the WVTR, achieving values down to 10–20 g/(m 2 · day) for the less porous paper. The presence of kaolin leads to improved performance, compared with pure latices; still, the results in Figure 6 do not show a significant difference among the different filler ratios for KP. Additionally, the presence of kaolin platelets allows us to achieve values similar to commercial coatings (i.e., SB-B and SA-B), strongly improving the WVTR of pure latices. Finally, the results in Figure 6 clearly show that a lower substrate porosity is essential to achieve better barrier properties due to lower coating defects and higher thickness homogeneity.
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