Siloxane polyacrylic sol-gel coatings with alkly and perfluoroalkyl chains: Synthesis, composition, thermal properties and log-term corrosion protection

Siloxane-polyacrylic coatings were synthesized from 3-(trimethoxysilyl)propyl methacrylate, different acrylate derivatives and tetraethyl orthosilicate. Derivatives of different lengths of alkyl and perfluoroalkyl chains were characterized in terms of kinetics of copolymerization during sol synthesis, and of the coatings composition, structure, wettability, thermal properties and porosity using real-time Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, glow discharge optical emission spectroscopy and photothermal beam deflection spectrometry. The siloxane backbones are surrounded by the organic network identified by characteristic fragments. Coatings are homogeneous. The surface water contact angle is between 70 degrees and 85 degrees for the acrylic chains and 90 degrees for the perfluoroalkyl chain. The coatings thermal conductivity is ca. 0.3 W/m.K. The porosity of the coatings increases with the length of the acrylate derivative. Long-term corrosion protection of the coatings deposited on aluminium alloy AA7075-T6 was analysed using electrochemical impedance spectroscopy in 5 wt% NaCl and salt spray chamber test. The coatings containing ethyl and butyl derivatives have superior corrosion resistance accompanied by low porosity compared to long-chain derivatives. A special emphasis was given to the possibility to follow the degradation of these sol-gel coatings upon immersion in chloride solution by surface analytical tools.