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16532-1:2008 [30] methodologies are different, similar latex and pigment formulations [34] coherently report how grease barrier properties are improved by kaolin addition. The commercial grades involved in this study as a reference, namely, SB-B and SA- B, have interesting properties. It seems that, compared with SA-B, SB-B provides better grease yet worse water barrier properties. Despite good WVTR, grease barrier is the main drawback for the commercial grades. The addition of kaolin is then a simple yet effective way to overcome this. As initially mentioned, the presented results were normalized over the coating thick- ness instead of dry coat grammage due to the increasing kaolin content of the experimental formulations, as kaolin-containing formulations are characterized by up to almost double coating density. Unfortunately, a higher coating density is negative when the noncellulosic content of the packaging is calculated. Indeed, comparing two coatings with the same thickness and equivalent barrier properties, kaolin-containing solutions would increase the noncellulosic content ratio of the packaging material, which is calculated by mass. Consid- ering recyclability issues, it is widely reported how the fiber content of the paper-coated material should be maximized in content to ease the recycling yield and recyclability of the substrate [9,10,38]. A higher nonfibrous content may be acceptable if the content is preserved and its shelf life guaranteed, since the content itself may have a higher relative im- pact than the packaging itself [39]. Nevertheless, in the 4evergreen alliance document [40], all of the substances used in this context (styrene-butadiene, styrene acrylate, and kaolin) are reported to be fully compatible with standard recycling paper mills. The addition of kaolin changed the aesthetic characteristics of the coated samples, especially in terms of reflectance. In Figure 10, it is possible to appreciate the aesthetic difference among experimental coating formulations.
Figure10. Different qualitative glossiness for H39K 100, H39K 80, H39K 60, and H39K 40 coatings over KB—pictures taken in the same light conditions. As the kaolin content increases, the coated surface reduces its glossiness—as visible in the lower half of the samples—giving a soft touch aesthetic aspect. Future works may investigate the consumer perception and aesthetic sensorial prop- erties with a view to packaging end-of-life, understanding whether such properties can influence the consumer behavior or suggest the right practice alongside the environmental label, strictly required for packaging. Finally, barrier properties can be influenced by the substrate porosity, as shown in this work and previously reported [19]. Lower substrate porosity allowed higher moisture and grease barrier properties for commercial coating, despite worse Cobb values, but only for experimental formulations. Both the effect of hydrophilic surfaces of kaolin and the obstacle in coating pure experimental latices influenced such behavior. A possible polymeric precoating over KB could be investigated to allow for a reduced substrate
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