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did not appear in single SMI nanoparticles. The latter might indicate physical interactions between the SMI nanoparticles and the CH 3 side groups of the natural rubber polymer chain. In addition, an intensified broad peak over the 3200–3500 cm –1 region is most pronounced for the SMI nanoparticle fillers and less present for the kaolinite and talc fillers. This absorption band might be related to the presence of hydroxyl groups that appear to be generated through interactions between the natural rubber with the SMI nanoparticles. On the other hand, no direct changes in the -C=C- double bonds were observed due to chemical cross-linking reactions for either of the fillers. In conclusion, it can be confirmed that strongest physical interactions between the natural rubber matrix and SMI nanoparticles are observed. 3.3. Paper Coating Properties The morphology of paper surfaces with natural rubber composite coatings are illustrated in Figure 4, representing top views of the different coating compositions. The pure natural rubber coating was fully flat and covered the paper surface as a smooth polymer film. The aspect of kaolinite fillers is observed as a homogeneous and smooth distribution over the coating surface with progressively more dense coverage at the higher concentrations, while they bring good coating density and likely some topographical roughness effects. The SMI nanoparticles are homogeneously distributed within the coating, causing the creation of small micrometer-scale domains. The talc particles are much rougher and are densely present at the surface in an inhomogeneous distribution over the surface. Due to the platelet morphology of talc particles, they are randomly oriented at the surface either perpendicularly sticking out or embedded parallel to the surface.
Figure 4. SEM evaluation of natural rubber coatings on paper with different filler types and concentrations (magnification ×4000 for all images).
The results of surface properties, including wetting and adhesive properties, are summarized in bar charts of Figure 5. The static contact angle values of water and diiodomethane in Figure 5a show slight and consistent variations among the different coating types and filler concentrations, relative to the pure natural rubber coating. The contact angles remained stable on the coatings for about 15 seconds, except for the pure natural rubber, as the homogeneity and coverage of the coating was not perfect and fully continuous for the pure natural rubber coating. The exposure of paper fibers at the
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