The methyl groups of mtes contribute to hydropho

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The methyl groups of MTES contribute to hydropho- bicity of alkyd/SiO 2 hybrid coatings. The contact angle on the surface of hybrid material keeps stable at about 126 ° for (MTES) [ 15% [ 21 ]. Perfluoro-functionalities have the lowest surface tension among organic groups and are selected to prepare super- hydrophobic hybrid coatings, on account of the strong thermodynamic driving force to minimize the surface tension during the drying process of hybrid coatings. Poly (caprolactone- b -perfluoropolyether- b -caprolactone) tri-block copolymer is used to prepare hybrid coatings because of its solubility in solvents commonly used in sol–gel processes. During the drying process, self-stratifi- cation of coating components arises to form the coatings with a strong hydrophobic surface by a preferential segregation of fluorinated segments onto the surface and sol–gel moving to the bottom of the coating. The sol–gel bottom of the coating forms an interface with good adhesion between coatings and substrate. A strong hydrophobic surface is attained [ 95 ]. A commercial perfluoropolyether containing alkoxysi- lane functionalities is employed to prepare hybrid coatings by using the sol–gel process in the presence of TEOS. Contact angle analysis reveals a strong hydrophobic char- acter of the coatings and surface tension values are in the range of 14–16 mN/m [ 96 ]. 5.2 Surface roughness Surface roughness is employed to increase hydrophobic character of coating surface. Calculated by Dupre equation, contact angle of perfluorosegments on flat surface is about 110 ° . Therefore, even perfluoropolymer with the lowest surface tension is not enough to attain super-hydrophobic coatings without the help of surface roughness. The relationship between surface roughness and hydro- phobic character is demonstrated in the following study. The super-hydrophobic coating is fabricated by a mix- ture of fluoride latex and silicon sol from acidic hydrolysis and polycondensation of TEOS and MTES. The silicon sol ages for different days to obtain a great many micro-par- ticles with the diameters from 1 to 12 mm evenly disperse in the film surface. The coating is applied on phosphating film on carbon steel substrate. The surface roughness of the hybrid coating measured by JJI-B roughometer derives from a combination of micro-particles in aged sol and rough phosphating film on carbon steel substrate. Table 4 shows effect of surface roughness of the hybrid coating and phosphating film on water contact angles [ 97 ]. Water contact angles of super-hydrophobic films increase with development of surface roughness. Roughness on a hydrophobic surface renders the surface more hydrophobic. In order to explain the hydrophobic property on rough surfaces, two equations (the Wenzel equation and Cassie- Baxter equation) are used to describe the effect of surface roughness on water contact angles [ 91 ].
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