This dissertation focuses on the relationship between substrate-surface chemistry (cleaning), reaction mechanism, film structure and water-barrier properties of water-based bis-amino silane and vinyl triacetoxysilane films. The work was undertaked to fulfill a SERDP (Strategic Environment Research and Development Program) requirement to understand the mechanism of corrosion protection in silane-based coatings. The majority of the work focuses on mixtures of the above two silanes. Such coatings are referred to as AV coatings.
The chemistry of neat AV mixtures was studied by 13C NMR. The reaction mechanism was found to be the exchange of the hydrogen atom on the secondary amine group with the acetoxy group on the vinyl triacetoxysilane. The chemistry of the AV water solution was also investigated by 13C and 29Si NMR.
The influence of the cleaning solution pH on the CRS surface chemistry and AV absorption was examined. The corrosion performance examination on CRS panels showed that the CRS surface is very sensitive to cleaning protocol. Optimum anti-corrosion performance was obtained after cleaning at pH ~ 9.5. The underlying mechanism for this observation is discussed.
The morphologies and water barrier properties of AV films were studied at different A/V ratios. AV film was found susceptible to water penetration. About 30 vol% water is absorbed in the film with only slight thickness increase. Most water is physically absorbed in the void space with the least amount being absorbed near the stoichiometric A/V ratio of 3/1.
Kinetic investigation of water uptake enables us to monitor water ingress providing more details on water absorption. Time-resolved D2O ingress in bis-amino silane and bis-sulfur silane film was studied by situ neutron reflectivity and Fourier transform infrared reflection-absorption spectroscopy. The absorbed water exists in two populations: one is dissolved in the polymer matrix (Henry’s mode) and the other occupies unrelaxed free volume within the polymer (Langmuir mode). The Langmuir absorption mode dominates the D2O absorption in both films. The initial stage of water diffusion of both bis-amino silane and bis-sulfur silane was Fickian. However, the deviation from Fickian behavior was observed at the intermediate stage of water ingress.