How surfactants affect trialkoxysilanes in aqueous emulsions

The study of trialkoxysilanes for inhibition of corrosion is a growing field due to the push for environmentally friendly alternatives to the chromium(VI) conversion coating. To use these molecules effectively they require activation. Activation of these molecules occurs on contact with water via hydrolysis of alkoxy bonds to form hydroxyl bonds by a reaction with water. Unfortunately, the activated molecules are prone to self condensation, leading to loss of activity, especially in high concentrations of water. One possible option is the use of surfactants, which were expected to increase the aqueous stability of alkoxysilanes. This study examines the possibility of using surfactants to stabilise trialkoxysilanes in water was tested. A preliminary set of experiments assessed basic stability of several trialkoxysilane emulsions (18 surfactant and 5 trialkoxysilanes) and measured the film resistances on zinc. The emulsion characteristics were evaluated by dynamic light scattering (DLS). The films were evaluated by scanning electron microscopy (SEM), atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS). The pilot study informed the methods for the more comprehensive study. A comprehensive study comparing the effects of system ageing on the films produced from by 16 different trialkoxysilane emulsions/mixtures and the respective films were evaluated by SEM and EIS. Films were produced from emulsions of different ageing times (1 day, 7 days and 21 days) to see how the addition of surfactants changed the film characteristics. The emulsions were made from one surfactant and one silane. The silanes used were GPS, VTES, and MPTMS. The surfactants used were Pluronic F108, Pluronic L35 and SDS. The performance of these films were compared to those from just trialkoxysilane and water. Visual changes in the emulsions were compared to the film characteristics. The reactions occurring in the emulsions were examined by following hydrolysis and condensation of silanes in water. Hydrolysis rate constants of 23 trialkoxysilane mixtures were determined by with Gas Chromatography Mass Spectroscopy (GC-MS) and the condensation of 3-Glycidoxypropyltrimethoxysilane (GPS) in 7 different surfactant mixtures was determined by silicon nuclear magnetic resonance (NMR) spectroscopy. Emulsion observations showed any increase in trialkoxysilane emulsion stability was dependent on both the individual surfactant and the concentration of the surfactant. Dynamic light scattering results showed that the trialkoxysilane emulsions were very complex and that monomodal systems were generally more stable. In addition, the measurement of the film resistances showed that over extended periods some trialkoxysilane systems produced better films without the incorporation of surfactants. The films made from 3-mercaptopropyltrimethoxysilane (MPTMS) and vinyltriethoxysilane (VTES) without surfactant gave better corrosion resistance than the films made from these silanes with surfactant incorporation. The corrosion resistance of the films decreased as the emulsions aged for the trialkoxysilane and water emulsions. The introduction of surfactants increased the usable period of the some trialkoxysilane emulsions to over three weeks. The emulsion consisting of 3-mercaptopropyltrimethoxysilane (MPTMS) and sodium dodecyl sulphate (SDS) produced a film at 7 days of emulsion aging ageing had a resistance over 200 times the resistance of the uncoated zinc substrate. This combination of trialkoxysilane and surfactant also produced the film with the highest resistance at 21 days. This indicates that the emulsion MPTMS in water with 2% SDS shows promise as a protective coating. The kinetics data showed that the rates of hydrolysis were greatly affected by the addition of surfactants. The addition of SDS increased the hydrolysis rate of VTES by as much as 500 times of the rate without the surfactant, whilst the addition of Pluronic F108 decreased the rate by 70%. The condensation results showed subtle differences between the condensation of GPS in water and when a surfactant was present. These differences showed that the surfactants were changing the condensation mechanistic pathway towards a condensed siloxane network. The concentration of uncondensed species 24 hours after the reaction started was higher in the emulsion containing the Pluronic F108 (90%) than the concentration (77%) in the emulsion GPS in water, however this higher concentration did not lead to a more corrosion resistive film. This study showed that surfactants can stabilise aqueous trialkoxysilane systems and that aqueous trialkoxysilane emulsions can produce corrosion resistive films. However, the stable emulsions did not produce corrosion resistive films. The emulsion MPTMS in water with 2% SDS has stability issues that would need to be addressed if the emulsion was to be used commercially