Coatings 2023 , 13 , 195
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phenomenon disappears as kaolin is added to the formulations, showing how it is related to the pure latex. It must be noted how, in general, H39K 100 and H40K 100 coatings are sticky—regardless of their T g —possibly leading to blocking when processed in an industrial plant. 3.2. Cobb Test The Cobb 1800 test results are reported in Figure 3. As clearly visible, the commercial coating grades (i.e., SB-B and SA-B) seem to better perform on both considered substrates, despite SB-B performing similarly to experimental formulations on the more porous sub- strate, namely, KB.
Figure 3. Water absorptiveness (Cobb 1800) values for the uncoated (UC), commercial (SA-B and SB-B), and experimental dispersion coatings. On the other hand, experimental formulations proved the role of the kaolin filler to be detrimental to water barrier properties. Indeed, the higher the filler content, the worse the barrier. There can be at least two explanations to such behavior. A first motivation can be related to the higher kaolin content that determines a formulation closer to the critical pigment volume concentration (CPVC), which is reported to be around 43% [31]—meaning a critical pigment mass concentration of around 65%, given a kaolin density of 2.6 g/cm 3 . Therefore, increasing the filler content to 60% by weight implies a higher coating porosity, leading to a higher roughness, as in Figure 4, thus a lower film continuity. An additional reason can be the higher surface wettability of the experimental coatings compared with the commercial ones (see Figure 5). This is attributed to the more hydrophilic behavior of the experimental formulations, which is due to the hydrophilic surfaces of kaolin [32]. Consequently, kaolin decreased the water barrier of the coatings containing up to 40% by weight of kaolin by attracting and possibly providing passage to water molecules through the coating itself. Instead, for the formulations containing 60% by weight of kaolin, the factor playing a major role in worse Cobb 1800 values seemed to be the entrapment of water molecules in the coating pores. For the experimental coatings, it is possible to observe from Figure 3 how, in general, Cobb 1800 values reach a minimum with a kaolin mass content of 20%, whereas higher concentrations lead to worse properties, which was attributed to a lower latex continuity throughout the coating. Indeed, Cobb 1800 results limit the fields of application for the experimental formulations, excluding packaging for liquids and very wet content, for example, soups and porridge. Comparing the effect of the substrate, the experimental formulations generally showed worse performance on the KP substrate. Although a lower substrate porosity generally allows better film homogeneity—hence a higher barrier—latex intrinsic stickiness made its application over such a smooth substrate challenging.
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