Glycidoxypropyltrimethoxysilane (GPTMS) and Bis (3-trimethoxysilyl-propyl) amine (Dynasylan 1124 ®) are provided by Evonik Industries, USA. Heloxy Modifier 65® (EEW=230) and epoxy resin (EPON 828 ®, EEW=188) are supplied with Momentive, USA. Isophorone diisocyanate (IPDI) is supplied by Bayer Material Science. Colloidal silica Ludox HSA is provided by Grace Davison. Brulin 815 GD is obtained from Brulin & company Inc. Mercaptanbenztriazol (MBT) (corrosion inhibitor) and all other chemicals and solvents are purchased from Sigma-Aldrich, and used without any modifications. Aluminum alloy AA2024-T3 panels are supplied by ACT panels LLC, USA.
EPON 828 and Heloxy Modifier 65 (7.4 g) and anhydrous ethanol (40 g) are charged to three neck flask attached
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Reaction is carried out in a 500 ml three-neck flask supported on a heating mantle and provided with a mechanical stirrer, a water condenser, a thermometer probe with an inlet for dry nitrogen gas, and an addition funnel. The calculated amount of IPDI and THF are charged to the three- neck flask under mechanical stirring and BTMSPA is added slowly from the addition funnel at a rate to maintain the reaction temperature below 50°c.
After addition of BTMSPA is complete, mixture is stirred for additional 1 hour at room temperature. The completion of the reaction is verified by FT-IR following the disappearance of peak associated with NCO groups. The obtained solution is transferred to a single-neck flask and by using a rotary vacuum evaporator THF is distillate in two steps and replaced by dry ethanol to yield around 60-65% solution. The bis-ureasil solution is transferred into an air-tight container and stored under refrigeration.
Preparation of epoxy based sol-gel
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In the first step, the panels are scrubbed with a sponge side of Scotch Brite pad dipped in a 10:1 mixture of tap water and Brulin 815GD at room temperature followed by rinsing with tap water and checked for “break-free” test by immersion in de-ionized water. The panels, then immersed 5 minutes in the tap water/ Brulin 815 GD (10:1) circulating bath are heated to 65°C, and then rinsed by 10 dunks in tap water immersion followed by spraying of low pressure de-ionized water. The panels are then immersed in a circulating de-oxidizing bath for 2 minutes, and then 10 dunks immersion in tap water following by low pressure spray of de-ionized water. Deoxidizing bath contains 35% butyl alcohol, 25% isopropyl alcohol, 18% phosphoric acid, and 22% de-ionized water (by vol.). Panels are placed vertically in a panel stacker for drying and are applied with coatings within 3 hours.
Application of sol-gel coating
All coatings are applied at room temperature (25–30˚C) using an automatic dip coater (PTL-200, MTI Corporation), at an immersion/withdrawal speed of 15–17 cm/minutes, with a residence time of 15–20 seconds. After application, the panels are placed vertically in a panel stacker for 15 minutes of air drying, followed by 30 minutes of thermal curing in an air circulating oven at 120°C. The typical dry-film thickness of coatings should be