Coatings 2025 , 15 , 214
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3.2. Coat Weights and Coating Thicknesses The weight of the different layers of the coat is presented in Figure 2. The function of the bottom layer of PLAX was to provide a smooth closed surface and thus to prevent the penetration of low viscous bioORMOCER ® into the paper, enabling better coating hold-out, while the top PLAX layer protects the bioORMOCER ® layer and provides a water vapour barrier. A difference of the coat weights of the first PLAX layer was observed between substrates A and S, as approximately twice as much of coating was applied on the substrate A compared to substrate S. Following factors were identified to affect the coat weights: (1) surface roughness (Base papers A has rougher surface than S, 2.7 μ mvs1.5 μ m); (2) coating method (rod coater meters more coating on rougher surface); (3) changes in sus- pension during the process (agglomeration, foaming, evaporation) and (4) swelling of base paper causing uneven surface for coating application. The coat weights of bioORMOCER ® show that it was applied uniformly on both substrates using the reverse gravure coating method. The surfaces of substrates were smoothened with PLAX precoating, providing even coatability for the bioORMOCER ® layer. The cross-cut images (Figure 3) display that the bioORMOCER ® coating layer of A-POP and S-POP adheres well with PLAX. The foaming problem of PLAX was detected in the SEM-image of A-POP. Air cavities cause deviations in coatings, which deteriorate the uniformity of coating quality and thus affect barrier performance. The issue could be resolved by collapsing the air bubbles either by vacuum or by using defoamers. In an industrial process, the latter is preferable. The overall coating quality of base papers after three coating layers was demonstrated by testing the air permeabilities of A-POP and S-POP. Both of them were fully covered as they showed air permeabilities of 0.003 μ m/Pa · s, which is the minimum value that the device can detect.
Figure2. Coat weights and thicknesses of coating layers of A-POP and S-POP.
Figure3. SEM cross-cut images of the samples ( a ) A-POP and ( b ) S-PO with 5000x magnification.
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