The trends of the friction coefficients during constant load scratch
The attitude of the coatings to be soiled under standard atmospheric conditions was tested by measuring the contact angles using water droplets and, then, by performing release and anti-soiling tests according to specific protocols. As concerns the contact angles, the polycarbonate features a contact angle of 69.2 °C, in the range of Tubastatin A material (contact angles from 30 to 90°). As-received epoxy resins are hydrophilic, as they feature similar contact angles (65.1°). The application of composite coatings increases the contact angles, turning the coating to hydrophobic. CE-GO and CE-fnSiO2 feature contact angles of 84.5 and 85.5°, respectively, which is very close to the range of hydrophobicity (contact angles from 90 to 150°). Despite the water affinity, GO and fnSiO2 increase the hydrophobicity of the epoxy resin. This result can be ascribable to the occurrence of covalent bonds between the fillers and the epoxy resin, which favor the cross-linking and cause the generation of a closer network with reduced wettability with water in agreement with Singh et al. . Similarly, CE-GNP and CE-fnGOr feature even higher contact angles of 100.6 and 111.8°, respectively. They can be thus considered in the range of hydrophobicity. The hydrophobicity is thus strongly influenced by the fillers, with GNP and reduced graphene oxide showing the highest hydrophobicity in agreement with Prolongo et al. . In this case, the hydrophobicity is the result of the intrinsic hydrophobic behavior of the graphene or graphene-derived fillers in agreement with Chang et al. . In the case of CE-fnGOr, the former mechanism is emphasized by the covalent bonds the GOr can form with the epoxy resin, thus promoting a closer network with limited wettability with water.