AbstractsEngineering

Water-based inorganic-organic hybrid sol-gel coatings were developed on aluminium. The coatings were based upon epoxy-silane, glycidoxypropyltrimethoxysilane (GPTMS) and methyltrimethoxysilane (MTMS) as inorganic silane precursors crosslinked with the organic polymer, hexamethylmethoxymelamine (HMMM). The aim of the present work was to modify the hybrid sol-gel coating to obtain hydrophobic properties which could be achieved by either lowering the surface energy or increasing the surface roughness of the coatings. Lowering of surface energy was achieved by incorporating various fluoro-alkylsilanes (FASs) with two different chain lengths, of 3 and 17 fluorine atoms. The subsequent water contact angles achieved were 100° and 110° respectively for FAS modification. As a result, a commercial fluoro emulsion (FE) which was a water based six carbon atom perfluoro polymer, was employed as co-precursor. A substantial improvement in hydrophobicity was achieved with FE modification resulting in contact angle of 118°. Further, the surface roughness of the fluorinated sol-gel coating was enhanced by incorporating various nano-particles with different size, surface area and surface chemistry, namely nano-Zinc Oxide (ZnO), hexadecyltrimethoxysilane (HDTMS) modified nano-silica, dichlorodimethylsilane (DDS) modified nano-silica and hexamethyldisilazane (HMDZ) modified nano-silica. Out of these HMDZ nano-silica modification resulted in maximum hydrophobicity with contact angle of 125° and sliding angle of 25° on aluminium surface. Furthermore an independent study was carried out on application of as developed composite coatings on chemically and mechanically roughened aluminium surfaces. It was found that contact angle increases up to 130° using techniques like chemical etching, plasma etching, sand blasting and grinding etc. The as developed nano-composite coatings when applied on other non-metallic substrate such as glass, cotton, cardboard, paper, concrete and wood, it showed excellent hydrophobicity with a contact angle of > 135°. Furthermore, all the substrates showed a sliding angle of about 15°. The as developed coatings were characterised for their chemical and morphological structures, using techniques including FTIR, XPS, SEM and AFM. The wetting properties were analysed using contact angle goniometry. The performance of the as developed coatings was evaluated by determining corrosion resistance, hardness, adhesion and scratch resistance properties. Finally, the role of nano-particles distribution was studied in detail. It was found that maximum hydrophobic properties were achieved at optimum concentration of nano-particles. Thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy of Indian Institute of Technology, Bombay, India and Monash University, Australia.