FIELD OF THE INVENTION: This invention relates to an anti-corrosive sol-gel coating composition for metal surfaces and the coated surfaces obtained using the same.

This invention further relates to an anti-corrosive sol-gel coating composition for metal surfaces particularly on zinc or zinc alloys coated steel substrate, where the coating is a combination of organosilanes, phosphate compound, organic solvent, dimineralized water and an acid catalyst.

BACKGROUND OF THE INVENTION:

Chromate conversion coating has been applied on galvanized sheets to prevent the white rust formation. But the chromate and chromate containing compounds are highly toxic, so extensive research work is being carried out to avoid the use of hexavalent chromium. Major efforts are concentrated around the development of chrome free coating by advanced coating technology such as sol-gel, thermal spray, physical and chemical vapor deposition techniques, plasma assisted deposition, ion-beam-assisted deposition etc. Amongst these, the sol-gel is an expanding technology favorably for thin film coating.

US Patent No. 6579472, describe a sol-gel formulation consisting of Zn (IV) propoxide and 3-glycidoxypropyl trimethoxysilane along with triethyle phosphate inhibitors. Furthermore, US Patent No. 5091468 describes a typical sol formulation which consists of 3- glycidoxypropyl trimethoxysilane, methyl trimethoxy silane and aluminium dihydrogen phosphate. However, these sol-gel formulations were discarded in the initial study.

A large amount of sol-gel research is based on silicon precursors because of their balance of reactivity and ease of handling, as well as their ready availability. The development of organic-inorganic hybrid silane coatings gives many advantages over pure inorganic or organic coating. The inorganic component gives better mechanical properties and incorporation of an organic component into the inorganic materials open a wide range of possibilities for functionalizing and tailoring of the final material. However, the need exists in the industry to provide a sol-gel coating which is manufactured form simple starting materials.

OBJECTS OF THE INVENTION;

It is therefore an object of this invention to propose an anti-corrosive sol gel coating composition for metal surfaces, which has superior corrosion resistance.

It is a further object of this invention to propose an anti-corrosive sol gel coating composition for metal surfaces, which provides an improved adhesion layer for subsequent surface coating.

These and other objects and advantages of the invention will be apparent from the ensuring description.

DESCRIPTION OF THE INVENTION;

According to this invention is provided a sol-gel coating composition for metal surfaces, comprising an anti-corrosion sol-gel coating composition for metal surfaces comprising: a) a first organosilane compound having at least one epoxy group and at least one hydrolysable alkoxy group; b) a second organosilane compound having no epoxy group and two or more hydrolysable alkoxy group. c) a corrosion inhibiting compound having phosphate functionality. d) a polar solvent is selected from the group consisting of ethanol, methanol, n-propanol, isopropanol, n-butanol, isobutanol, and mixture thereof, e) an acid selected from acetic acid, citric acid, phosphoric acid, hydrochloric acid and mixture thereof, f) water

In accordance with this invention is provided a superior barrier film on the metal surface particularly the zinc and zinc alloy steel which in turn have significant corrosion resistance properties without the need of addition anti-corrosive layer. Furthermore, the sol-gel coating is also compatible with the top polymer coating to give extra protection from the environment.

The substrates used in the present invention are zinc and zinc alloy coated steel sheet commercially known as galvanized and galvannealed respectively. The galvanized is a pure zinc coating whereas the galvanneled is a zinc-iron alloy which contains about 10% iron in coating. The coating is a combination of organosilanes, phosphate compound, organic solvent, demineralized water and acid catalyst.

The first organosilane component is selected from compounds having at least one epoxy group and at least one hydrolysable alkoxy group. The organosilane compound is selected from 2-(3,4 epoxy cyclohexyl) ethyl trimethoxysilane, γ-glycidoxypropyl trimethoxysilane, γ-glycidoxypropyl methyldimethoxysilane, γ-glycidoxypropyl dimethylmethoxysilane, γ-glycidoxypropyl triethoxysilane, γ-glycidoxypropyl methyldiethoxysilane, γ-glycidoxypropyl dimethylethoxy silane , β-(3,4-epoxycyclohexyl)ethylmethyldimethoxysilane, β-(3,4-epoxycyclohexyl)ethyldimethylmethoxysilane, β-(3,4-epoxycyclohexyl)ethyltrietoxysilane, β-(3,4-epoxycyclohexyl)ethylmethyldiethoxysilane, β-(3,4-epoxycyclohexyl)ethyldimethylethoxysilane and mixtures thereof. The preferred compound is 2 -(3, 4 epoxycyclohexyl) ethyl trimethoxysilane. The first organosilane is present in the composition in about 30 wt% to about 50 wt% of the composition.

The second organosilane component is selected from compounds having two or more hydroly sable alkoxy groups but no epoxy group, such as tetraethyl orthosilcate, tetramethyl orthosilicate, methyltrimethoxysilane, the second ethyltrimethoxysilane and mixtures thereof, the preferred compound being tetraethyl orthosilicate. The second organosilane is present in the composition in about 5 wt% to about 25 wt% of the composition. The first organosilane and second organosilance compounds are present in a molar ratio of about 1 : 1 to about 5: 1.

The corrosion inhibiting compound is selected from a group of phosphate based compounds preferably triethyl phosphate. The corrosion inhibiting compound is present in the range of 2 wt% to about 10 wt% of the composition. The composition comprises besides the constituents mentioned hereinbefore, a solvent for efficient combination of the constituents. The solvent is a polar solvent and is selected from the group consisting of ethanol, methanol, n-propanol, isopropanol, n-butanol, isobutanol, and mixture thereof, preferably ethanol. The solvent is used in a proportion of about 25 wt % to about 70 wt% of the composition.

The composition also comprises an acid selected from acetic acid, citric acid, phosphoric acid, hydrochloric acid and mixture thereof, preferably acetic acid. The acid is present in a proportion of about 0.5 wt% to about 5 wt% of the composition.

The composition also contains water. Demineralised water is preferably used and is present in about 5 wt % to about 25 wt% of the composition.

In accordance with the process for the preparation of the composition according to the invention, the first organosilane and second organosilane are mixed to provide the mixture of silanes. The solvent and water are mixed separately, and to it, is added the corrosion inhibiting compound. This solution of the corrosion inhibiting compound is added to the mixture of organosilanes followed by dropwise addition of the acid thereto, to enhance the hydrolysis and condensation reaction. The reaction mixture is kept under stirring till the phase separation disappears and becomes a transparent and uniform sol that requires about 24-48 hours.

For the embodiment describe in this patent, the sol-gel coating is applied on the cleaned zinc or zinc alloy steel (GI) sheets by dip coating process at a speed of 25 to 30 cm/min but in plant condition the speed may achieved of 12000 cm/min and cured at 50 to 300 ⁰ C of peak metal temperature in a hot air oven. The coated samples are preserved for about four days before testing to ensure extensive crosslinking of the silane films with the substrate in ambient condition.

The invention will now be explained in greater detail with the help of the following non-limiting example.

Example 1: 2-(3,4 epoxy cyclohexyl) ethyl trimethoxysilane and Tetraethyl orthosilicate (TEOS) were mixed in 3: 1 molar ratio in a round bottom flask. Separately, water/ Ethanol was mixed in 1:3 molar ratio in a beaker and then 5.8 wt% Triethyle phosphate added compared to total silane precursors and this liquid solution was transferred into silane mixture present in the round bottom flask. Slowly, few drops of acetic acid was added into the sol to enhance the hydrolysis and compensation reaction. The whole sol was kept under stirring condition till the phase separation disappeared and become a transparent and uniform sol that required about 24-48 hours. The P ^H of the solution was around 5.5.

The sol-gel coating was applied on the cleaned galvanized (GI) samples by dip coating process at speed of 25-30 cm/min and cured at 100 ⁰ C for an hour in hot air oven. The coated samples were preserved for four days.

Example 2: SOL 1 was prepared by mixing 2-(3,4 epoxy cyclohexyl) ethyl trimethoxysilane (540ml) and TEOS (140 ml) under stirring for 1 hour. Separately, SOL 2 was prepared by adding TEP (40 ml)o ethanol (500 ml) and water (110 ml) and the mixture was stirred for 30 minutes. This SOL 2 was added to SOL 1 and slowly, few drops of acetic acid (16 ml) was added to the mixture. The mixture was allowed to age for 24 hours to obtain the sol-gel coating. The sol-gel coating was applied to a substrate by dip coating at 25-30 cm/min and cured at 80 ⁰ C for 1 hr.

Example 3: SOL 1 was prepared by mixing 2-(3,4 epoxy cyclohexyl) ethyl trimethoxysilane (540ml) and TEOS (150 ml) under stirring for 1 hour. Separately, SOL 2 was prepared by adding TEP (40 ml) to ethanol (500 ml) and water (110 ml) and the mixture was stirred for 30 minutes. This SOL 2 was added to SOL 1 and slowly, few drops of acetic acid (16 ml) was added to the mixture. The mixture was allowed to age for 48 hours to obtain the sol-gel coating. The sol-gel coating was applied to a substrate by dip coating at 25-30 cm/min and cured at 150 ⁰ C for 30 minute.

All the three formulations mentioned above, examples 1 to 3, showed almost similar performance and therefore were treated as same. The sol-gel coated and regular GI (no sol-gel coated) samples were evaluated by different tests and result was summarized in Table I. Table I: result of sol-gel coated and regular GI samples in different test conditions.