Synthesis of Hybrid Organic-Inorganic Sol-Gel Coatings for Corrosion Resistance
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extreme microhardness and scratch resistance superior to ormocer coatings prepared without bohemite addition [6]. TiO 2-SiO 2 -epoxide composites, displaying excellent transparency, hardness, and wettability, have found use as contact lense materials [7]. In the present study, sol-gel derived GLYMO-TEOS ormosils have been prepared and characterized using multinuclear solid state NMR. Salt spray tests were conducted to determine the corrosion resistance behavior of GLYMO-TEOS thin films on aluminum alloy substrates as a function of hydrolysis water and organic content. A correlation between film structure and corrosion resistance behavior is presented.
EXPERIMENTAL METHODS and (GLYMO) 3-Glycidoxypropyltrimethoxysilane Materials and Reagents: tetraethoxysilane (TEOS) were used as received from Aldrich. Nitric acid was used to catalyze the hydrolysis reaction. Sol-Gel Preparations: Preparation of ormosil solutions may be summarized as follows: TEOS and GLYMO precursors were placed in a beaker with acidified water. The resultant twophase solution was vigorously stirred for 1 hour to induce mixing and initiate hydrolysis. Sols were prepared using R values (mol water/moles alkoxide) of 4, 16, and 40. Prior to spectral analysis, ormosils were allowed to dry at ambient temperature for approximately one month. Percent organics is denoted g C atoms/g Si0 2 formed. Spectral Analysis: Solid state NMR experiments were performed on a Chemagnetics CMX-II 300 MHz (7.06 T) spectrometer. Resonance frequencies for "3 C, 2 98i, and 'H were 75.6, 59.8, and 301.0 MHz. 'H-' 3C CP/MAS NMR: A Chemagnetics 300VXP-208 double resonance (H-X) probe was used for data collection. Magic angle spinning was carried out in 5 mm zirconia rotors, spinning at 4 kHz. NMR spectra were collected with a quasi-adiabatic crossolarization pulse sequence using a Is pulse delay, a 2 ms contact time, and a 4 Ras pulse width. H-2 9Si CP/MAS NMR: A Chemagnetics 300VXP-144 double resonance probe was used for data collection. Magic angle spinning was carried out in 7.5 mm silicon nitride rotors, spinning at 3 kHz. NMR spectra were collected with a quasi-adiabatic cross-polarization pulse sequence using a I s pulse delay, an 8 ms contact time, and an 8 gs pulse width. Aluminum Surface Preparation and Coating Methods: Aluminum 2024-T3 panels were polished with 400 and 600 grit silicon carbide papers and rinsed with methanol and hexanes. Ormosil thin films were prepared by dip coating using a withdrawl speed of 10 cm/s. Thin films were allowed to dry at room temperature prior to corrosion resistance testing. Accelerated Corrosion Resistance Tests: Ormosil-coated test coupons were exposed to 5% salt spray at 35 'C for 168 hours as per Mil-C-5541 and ASTM BI 17. After removal from the salt fog chamber, the samples were rinsed with distilled water to remove any residues. Epoxide Titrations: Epoxide concentrations in ormosils were determined by titrating sol solutions with HBr resulting from the reaction of HC10 4 with tetraethylammoniumbromide using crystal violet as an
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