HMMM cured corrosion resistance waterborne ormosil coating for aluminum alloy

Due to the exceptional advantages of sol-gel process, organically modified silane (ormosil) coating has been applied to study the corrosion protection of aluminum alloy. In the present work, GPTMS/MTMS sol-gel solution was prepared by hydrolysis and condensation of 3-glycidoxypropyltrimetoxysilane (GPTMS) and methyltrimethoxysilane (MTMS) in aqueous solution of 0.05 M acetic acid in molar ratios 3: 1. To prepare the ormosil coating solutions, a crosslinking agent hexamethoxymethylmelamine (HMMM) and a blocked acid catalyst p-toluenesulphonic (p-TSA) were combined with the sol-gel solution. Aluminum substrates were dip coated and cured at 130 degrees C for 45 min. Effectiveness of HMMM as a crosslinking agent was analyzed by evaluating corrosion resistance, chemical resistance and hydrophobicity of coating using potentiodynamic polarization method, immersion test and water contact angle, respectively. UV stability of sol-gel solution, ormosil coating formulation and coated substrate was also studied using UV-vis spectroscopy and UV-weatherometer to see performance of coating in outdoor application. Thermal behavior of ormosil was characterized using TGA and DSC. Surface morphology and structural characteristics were also characterized using SEM, AFM and FIF-IR-RAS (reflection absorption spectroscopy). Pencil hardness, impact test and bend test were carried out to determine the adhesion, hardness and flexibility of coating. Results show that the coatings were uniform, transparent and crack free. Corrosion resistance and hydrophobicity of coating system with 20H-30H (20-30 wt% HMMM of total resin) of HMMM were found excellent. All the coatings have good resistance to isopropanol, toluene, 3.5 % NaCl and methyl ethyl ketone. However, it degraded in nitric acid solution. UV-vis spectroscopy and UV-weathering suggest that these coatings are stable in sunlight. The gloss retention for 25H coating was found about 83.32% without any visible pit or corrosion product. The coatings are thermally stable upto 325 degrees C and none of coatings show evidence of glass transition temperature. (c) 2007 Elsevier B.V. All rights reserved.