Mater. Proc. 2020 , 2 , 29
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on a Vertex 70 station (Bruker, Karlsruhe, Germany) with a diamond crystal (PIKE), collecting the spectra in a spectral range of 600–4000 cm ƺ 1 with a resolution of 4 cm ƺ 1 . A DTGS detector was installed to summarize 32 scans for 1 spectrum. The scanning electron microscopy (SEM) was done on a Tabletop TM3000 microscope (Hitachi, Krefeld, Germany) under an acceleration voltage of 15 kV and backscattered secondary electron compositional mode. The magnification of 4000× was operated under a working distance of 8400 ΐ m. The other paper surface properties were determined by static contact angle measurements of deionized water and diiodomethane, applying a sessile drop method with drop volume of 2 ΐ mL (water) and 0.8 ΐ l (CH 2 I 2 ), respectively. The adhesive properties of the coated paper surfaces were evaluated by an adhesive loop test on rubber films in contact with the coated paper of the same rubber composition, using a universal tensile tester (Schimadzu, Kyoto, Japan). A representative geometry of a film loop with width of 1 cm and length of 5 cm was clamped in between the upper dies and brought to a distance of 1 cm above the coated paper substrate that was horizontally fixed in the lower dies. The tack is characterized as the maximum force upon withdrawal of the rubber film from the coated paper surface, where experimental values of adhesive force are comparable due to the constant geometries.
3. Results
3.1. Rheological Properties The rheological properties of the coating suspensions are presented as the variation of shear viscosity as a function of shear rates over three subsequent cycles, as given in Figure 1. The curves are recorded for suspensions with different fillers relative to the native natural rubber latex with solid content 60% (note: the viscosity scale of the materials is different for most detailed representations of the values). All filler types increased the viscosity of the original rubber latex to a different extent; however, all of them showed a shear-thinning effect with decreasing viscosity as a function of shear rate. The shear thinning behavior is enhanced in the presence of the fillers in most cases. The highest viscosities are observed for kaolinite fillers with an almost linear decrease in viscosity with shear rate at the highest concentrations, while the hysteresis of the kaolinite fillers is relatively low. This indicates the presence of strong mixing interactions between the kaolinite fillers and strong interactions with the natural rubber latex. The viscosity increase for SMI nanoparticles is significant with a more pronounced shear thinning effect, as the orientation of the nanoparticles under shear may additionally influence the structure of the suspension. The viscosity effects of nanoparticles are different from the microsize kaolinite, as an increase in nanoparticle concentration involves a decrease in viscosity. Therefore, it can be concluded from a viscosity-reducing effect of the nanoparticles that shear-induced mechanisms are influencing the nanoparticle mobility in the suspension and eventually lead to the orientation effects. The effects of reorganization of nanofillers in the rubber latex is also indicated by the relatively high hysteresis observed between the first and second ramp-up sequences for all concentrations, which was not observed for kaolinite fillers. Indeed, the nanoscale particles may have stronger influence on the latex flow properties compared to microscale particles. The latter are minimized in the case of talcum fillers, where very little alterations in viscosity and shear thinning effects are observed compared to the native rubber latex. However, the hysteresis effects for talc are also more pronounced than for kaolinite as it may be expected that the talc has a platelet structure that can be affected more by orientation effects under flow, while the kaolinite particles have a rather symmetrical shape. As it is observed that the viscosity for intermediate talc concentrations of 10 wt.-% decreases and the viscosity for the high talc concentrations of 20 wt.-% increases, the possible benefits of the orientation of the platelet structures are optimized at intermediate concentrations and hindered at the highest concentrations, where the highest concentrations might eventually lead to platelet–platelet interactions rather than platelet–latex interactions. The chemical interactions between the fillers and the natural rubber latex were not further studied at this stage, but besides particle shape, they can be attributed to specific surface interactions owing to the functional groups at the surface of the fillers, size distribution of the fillers and/or variations in zeta potential. While the present aim is to provide a view on the influence of the rheological
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