Natural Rubber Composites for Paper Coating Applications ^†

Natural rubbers are characterized by extremely high molecular weight that might be beneficial in the formation of a protective barrier layer on paper substrates, providing good cohesive properties but limited adhesion to the substrate. In parallel, the low glass transition temperature of natural rubber might give the opportunity for good sealability, in contrast with severe problems of tack. Therefore, natural rubbers can be good candidates to serve as an alternative ecological binder in paper coatings for water and grease barrier resistance. In order to tune the surface properties of the paper coating, the effect of different fillers in natural rubber coatings are evaluated on rheological, thermo–mechanical and surface properties. The fillers are selected according to common practice for the paper industry, including talc, kaolinite clay and a type of organic nanoparticle, which are all added in the range of 5 to 20 wt.-%. Depending on the selected natural rubber, the dispersibility range (i.e., dispersive and distributive mixing) of the fillers in the latex phase highly varies and filler/matrix interactions are the strongest for nanoparticle fillers. An optimum selection of viscosity range allows us to obtain homogeneous mixtures without the need of surface modification of the additives. After bar-coating natural rubber latex composites on paper substrates, the drying properties of the composite coatings are followed by spectroscopy, illustrating the influences of selected additives on the vulcanization process. In particular, the latter most efficiently improves in the presence of nanoparticle fillers and highly increases the coating hydrophobicity in parallel, reducing the adhesive tack surface properties, as predicted from calculated work of adhesion.