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calculated values are represented in Figure 5b, where it can be noticed that the predicted adhesion varies for the different coating types. The theoretical adhesion is highest for the pure natural rubber and is lower in the presence of filler materials: with increasing filler concentrations, the adhesion gradually decreases in the presence of kaolinite fillers and SMI nanoparticles, while the adhesion increases in the presence of talc particles. This can indeed be related to the hydrophobic effect of the kaolinite and SMI nanoparticles and the hydrophilic effect of the talc particles. The results of experimental adhesion measurements from a loop test with contact between similar rubber composite materials is represented in Figure 5c, and confirm the trends from theoretical calculations. From that, it is mainly revealed that the tendency for adhesion of rubber composite coatings can be predicted from water contact angle measurements and is steered by the hydrophobicity of the coated surface. 4. Conclusions Different macro- and nanoscale fillers can be homogeneously mixed with a natural rubber latex and applied as a paper coating. The use of hydrophobic nanoparticles shows the most interactions with the latex through interactions with the molecular side chains of the poly-isoprene, while the nanoparticles also provide the highest hydrophobicity and reduced tendency for self-adhesion (tack). Author Contributions: Experimental design, D.S., P.S. and F.D.; sample preparation, F.D.; experimental characterization P.S.; data processing, P.S. and F.D.; manuscript writing, P.S., F.D. and D.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by VLAIO HBC 2017.0310_BioBAR. Conflicts of Interest: The authors declare no conflict of interest. References 1. Panrat, K.; Boonme, P.; Taweepreda, W.; Pichayakorn, W. Formulations of natural rubber latex as film former for pharmaceutical coating. Proc. Chem. 2012 , 4 , 322–327. 2. Jianprasert, A.; Monvisade, P.; Yamaguchi, M. Combinatino of tung oil and natural rubber latex in PVA as water based coatings for paperboard application. MATEC Web Conf. 2015 , 30 , 03010. 3. Wang, H.; Easteal, A.; Edmonds, N. Prevulcanized natural rubber latex/modified lignin dispersion for water vapour barrier coatings on paperboard packaging. Adv. Mater. Res. 2008 , 47–50 , 93–96. 4. Das, D.; Datta, M.; Chavan, R.B.; Datta, S.K. Coating of jute with natural rubber. J. App. Polym. Sci. 2005 , 98 , 484–489. 5. Samyn, P.; Deconinck, M.; Schoukens, G.; Stanssens, D.; Vonck, L.; Van den Abbeele, H. Synthesis and characterization of imidized poly(styrene-maleic anhydride) organic nanoparticles in stable aqueous dispersion. Polym. Adv. Technol. 2012 , 23 , 311–325.
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