Mechanistic Investigation Of Direct Modification Of Epoxy Coatings With Silane Agents For Corrosion Protection

Most metals, except some noble metals, are prone to corrosion in nature environment, which can result in destruction and function-loss of metallic materials. Organic coatings were widely used for metal protection against corrosion due to their low price and high processibility. Among them, epoxy resin was the one that was most widely used painting component due to its good adhesion with metal substrates and high resistance against organic solvent. However, the high water uptake of epoxy coatings restricts the application in highly humid environment. In this paper, silane agents were directly used to modify epoxy coatings, aiming to improve the protection performance by improving the barrier performance against water permeation of the coatings and increasing the adhesion between metal substrate and polymer coatings, giving a novel method to utilize the silane agents in metal corrosion control. The main contents are listed as follows:(1) Modification by direct incorporation3-glycidoxypropyltrimethoxy silane (GPTMS) were directly incorporated into epoxy/polyamide systems. Coating capacitance, which was measured by electrochemical impedance spectroscopy (EIS), was employed to calculate the amount of water uptake in epoxy coatings in aqueous solution.The water absorption in epoxy coatings was found to decrease drastically after incorporated with silane component. With the increase of silane content, water uptake of epoxy coatings passes through a minimum, corresponding to 5wt.% of silane incorporation. Differential scanning calormetry (DSC) was used to measure the glass-transition temperature (T_g) of the coatings before and after immersed in water. After water permeation T_g of pure epoxy-polyamide coatings is found to conventionally decrease. However, for silane-incorporated coatings T_g increase slightly after water penetration. The self-repairing effect due to the hydrolysis and condensation of silane agents is proposed to interpret the interesting T_g increase. For GPTMS incorporated coatings, it is found that the relative peak intensity at 1100cm^-1, corresponding to the Si-O-Si bonds, increased slightly after immersion, which was caused by the formation of Si-O-Si bonds.(2) Chemical modificationOur experimental results showed that only few silane monomers gave positive results by the above-mentioned simple direct incorporations. For example, some long-chained silanes (e.g. dodecyl-trimethoxy silane, DTMS) deteriorated the performance of the incorporated organic coatings, due to their poor compatibility with polymeric resins. To overcome this drawback, the silane agents were grafted to epoxy resin molecules by chemical modification.Two main methods were used in chemical modification,γ-amino-propyltrimethoxy silane (γ-APS) can be grafted to epoxy resins via the opening cycle reaction between the amino groups inγ-APS and the epoxide groups in epoxy resins. It was found that the water uptake of epoxy coatings decreased after modified with 1 wt.% content ofγ-APS. whereas excessive amounts of this silane monomer result in the deterioration in performance against water permeation. Similar, the T_g ofγ-APS modified coating was also found to increase slightly. EIS measurement and Machu test demonstrated that the protection performance was improved after modification.Those silanes. which cannot react with the epoxide groups, can be grafted to epoxy resin via the reaction between the alkoxy groups in silane and the hydroxy groups in epoxy resin under the catalysis of organotin compounds. FTIR spectra showed that the absorption intensity of -OH decreased and a new absorption corresponding to the Si-O-C bonds appeared after modification. The parameters related to the protection performance of the coatings, such as coating capacitance (C_c), coating resistance (R_c), double layer capacitance (C_dl) and charge transfer resistance (R_ct), were then obtained by fitting of the EIS data. The result indicated an enhancement in protectiveness of silane-modified epoxy coatings against substrate corrosion, which was characterized by higher R_ct and lower C_dl at substrate/electrolyte interface. The results of Machu test and boiling water test were in good accordance with the conclusion derived from EIS measurement. The best performance was observed for coating system modified by GPTMS. Thereby, we further investigate the corrosion performance of GPTMS-modified epoxy coatings with various content of this silane monomer. The result showed that the protection performance was improved with the increase of silane content. But when the amount of silane exceeds the stoichiometric values, only slight improvement in protectiveness can be observed compared with that modified by stoichiometric amount of GPTMS silane monomer.(3) Modification with prepolymers of silanesThe high hydrolysable activity of silane monomer is harmful for long-term protection. By controlling the reaction conditions, we synthesized the prepolymers based on silane monomers. There are residual alkoxy in prepolymer, which can also undergo hydroxylation and condensation to form Si-O-Si structure after water permeation as silane monomer does. The prepolymer has partly condensed which may reduce its hydrolysable activity, and this may guarantee a long-term protection.Two different prepolymers were synthesized, one was based on methyltrimethoxy silane (MTMS) and the other was based onγ-APS silane monomer. They were directly incorporated into the epoxy/polyamide systems aiming to enhance the protection performance. The improvement in protection performance is almost negligible for MTMS prepolymer-incorporated coatings due to the poor compatibility with polymeric resins. Whereas, forγ-APS prepolymer-incorporated coatings, the protection performance of coatings were improved significantly after incorporation. The evolution of C_c values and the dispersion coefficient (n) during the immersion revealed that the barrier performance against water permeation was improved after prepolymer incorporation. The contact angle measurement also showed thatγ-APS prepolymer incorporation improved the hydrophobicity of the epoxy coatings. Machu test and boiling water test also showed an improvement on protection properties forγ-APS prepolymer-incorporated coatings.