FT-IR and 29Si-NMR Studies on UV-Curable DDS-MPTMS/SiO2 Hybrid Coating

Ultraviolet (UV)-curable DDS-MPTMS/SiO₂ hybrid coating was prepared by sol-gel process from tetraethylorthosilicate (TEOS), dimethyl diethylorthosilicate (DDS), and methacrylate propyl trimethoxysilicate (MPTMS). The formation of inorganic–organic hybrid network was studied by FT-IR and ²⁹Si-NMR during UV curing. It was found that the three-dimensional (3D) inorganic networks were formed mainly by condensation of TEOS, and the linear Si-O-Si bonds were also formed by condensation of DDS. The organic networks were formed by the polymerization of C[dbnd]C bonds in methacrylate units and the silanol groups were further condensed when the hybrid coating was cured by UV light. The steric hindrance of organic–inorganic hybrid networks resulted in incomplete polymerization and condensation. When the UV curing time was 240 s, the conversion degree of C[dbnd]C bonds was 81.0% and the condensation degree of silanol groups in TEOS and MPTMS was 81.8% and 91.5%, respectively.     

Structural studies of sol–gel urea/polydimethylsiloxane barrier coatings and improvement of their corrosion inhibition by addition of various alkoxysilanes

A sol–gel organic–inorganic hybrid precursor, bis[(ureapropyl)triethoxysilane]bis(propyl)-terminated-polydimethylsiloxane 1000 (PDMSU, for short), was tested as a corrosion barrier coating for AA 2024 aluminium alloy. The PDMSU coatings were prepared in either ethanol (PDMSU/EtOH) or propanol (PDMSU/PrOH) solvents. XRD measurements of xerogels showed the diffraction peak of amorphous silica domains at 21.5° and a broad peak at approximately 12.2°, which could be associated with the presence of the polyhedral silsesquioxane structural units (T² and T³) determined in our previous investigations from the ²⁹Si NMR spectra. The structure of thin coatings on AA 2024 prepared by heat-treatment at 140 °C was studied with the surface-sensitive IR reflection–absorption (IR RA) spectroscopic technique. Results revealed that in both coatings the poly(dimethylsiloxane) (PDMS) chain segments were projecting from the metal surface, however, this effect was more pronounced for the PDMSU/PrOH than for the PDMSU/EtOH coatings. Information gathered from the structural studies (IR, IR RA, ²⁹Si NMR and XRD) enabled some correlations to be drawn between the coatings’ structure and the effectiveness of the corrosion inhibition, which was assessed from the potentiodynamic and salt-spray measurements. Results showed the improved corrosion inhibition of PDMSU/PrOH coatings attributed to their denser and more compact sol–gel network and also to their higher hydrophobicity, i.e. lower surface energy determined from the contact angle measurements. Addition of various tetraalkoxysilanes and alkyltriethoxysilanes further improved the corrosion inhibition of PDMSU coatings due to more extensive cross-linking. The salt-spray tests showed that tetraethoxysilane and phenyltriethoxysilane were the most effective additives.