FORM 2
THE PATENTS ACT, 1970
(39 OF 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
A coating system for steel
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030, Maharashtra, India, an Indian Company
INVENTORS
Ghosh Susmita, Dr Jaiswal Rajendra, Patil Vijay and Dr Singal Vivek, Crompton Greaves Ltd, Advanced Material & Process Technology Centre, CG Global R&D Centre, Kanjur Marg (East), Mumbai 400042, Maharashtra, India, all Indian nationals
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the nature of this invention and the manner in which it is to be performed:

Field of the Invention
The present invention relates to a coating system comprising a primary coat comprising a nanocomposite of epoxy resin and modified clay and polyurethane based outer coat. Background
Polyurethane coatings are widely used for the protection of steel from corrosion. US 3447949 discloses a coating system for steel having a base coat comprising polyurethane resin, epoxy resin and titanium dioxide and an outer coat comprising polyurethane resin and iron oxide pigment. To improve the adhesion strength as well as the barrier properties of the coating, micaceous iron oxide is often applied as an intermediate coat to the base coat and the outer coat. However, the improvement in the adhesion strength on application of the intermediate coat is not usually high and also the impact strength, flexibility and peel strength of such coatings are low.
There is a need for a coating system for steel that has improved properties
Detailed Description
Accordingly the invention provides a coating system for steel, the coating system comprising a primary coat comprising a nanocomposite of epoxy resin and modified clay and polyurethane based outer coat.
.In one embodiment the invention provides a coating system comprising a primary coat wherein the primary coat comprises 1 to 5 % modified clay, 5 to 15% (Zn3PO4)2 and 20 to 40 % epoxy resin and a polyurethane based outer coat.

In another embodiment, the invention provides a coating system comprising a primary coat wherein the primary coat comprises 1 to 5 % modified clay, 5 to 15% (Zn3PO4)2, and 20 to 40 % epoxy resin; a polyurethane based outer coat and one or more intermediate coat(s) comprising micaceous iron oxide
In a further embodiment of the invention, there is provided steel coated with a coating system comprising a primary coat wherein the primary coat comprises 1 to 5 % modified clay, 5 to 15% (Zn3PO4)2 and 20 to 40 % epoxy resin and a polyurethane based outer coat
The protective coating of the invention comprises a primary inner coat based on a nanocomposite of epoxy resin and clay and a polyurethane based outer coat. A bisphenol A type epoxy resin is used for the formation of the nanocomposite with clay. The clays useful for the preparation of nanocomposite include montmorillonite and other clay minerals from the montmorillonite group as well as those clay minerals capable of being expanded by intercalating with an organic modifier. The clay is first modified using organic modifiers and further the nanocomposite is formed by exfoliating the clay with a bisphenol A type epoxy resin.
The resin-clay nanocomposite used as the primary coat offers several advantages including improving the barrier properties of the coating thereby avoiding penetration of water to the surface of steel. Further, the nanocomposite primary coat adheres well to the steel substrate as well as renders high impact strength, flexibility and solvent, chemical and corrosion resistance.

The coating system of the invention can be a two coat system or can comprise more than two coats. When the coating system is a two coat system, the coating system comprises a primary coat comprising 1 to 5 % modified clay, 5 to 15% (Zn3PO4)2 and 20 to 40 % epoxy resin and a secondary coat comprising polyurethane resin. The invention is further illustrated by way of the following examples
Examples:
Example 1: Preparation of modified clay
50 g of montmorillonite clay was dispersed in 2 liters of distilled water at 80°C with stirring for 1 hr. 0.05 moles of the intercalating agent, CTAB (Cetyl trimethyl ammonium bromide) was added to the solution. The resultant mixture was stirred for 4 hours. The solution was then filtered and the modified clay was washed with hot water for about 3-4 times to remove residual modifiers. The complete removal of the modifiers was confirmed by testing for halogen in the washings. The modified clay obtained was then dried in an air circulating oven at 100 ± 10 °C for 8-10 hrs and ground into a fine powder which was then passed through 200 mesh sieve.
Example 2: Preparation of nanocomposite coating composition
Sonication and high speed mixing was carried out on a mixture of bisphenol A type resin taken in 20% to 40 % by weight of the composition and montmorillonite taken in 1.0 to 5 % by weight of the resin. Sonication and high speed mixing were continued for two hours. Further, zinc phosphate pigment in 8 to 10% by weight of the composition, solvent, anti-settling agent, dispersing agent were added to the above mixture and the sonication was continued for another 15-20 minutes. Titanium dioxide was added in 10%

- 12% by wt. of the composition and the entire mixture was ball milled for 2 hours to get the coating composition.
Example 3: Application of the coating system of the invention
A primary coat having thickness in the range of 50-70 micron of the composition prepared as in example 1 was first applied by spray coating of steel. The primary coat was dried at room temperature for 8-10 hrs. Subsequently, an outer coat having a thickness of 50-70 microns of alphatic polyurethane resin was applied. Further the outer coat was left for drying at room temperature for 8-10 hrs.
Example 4: Application of conventional coating system
A primary coat of thickness 70-80 micron, of a zinc rich epoxy primer (having zinc present to the extent of 85-90%) was applied by spraying. The primary coat was dried at room temperature for 8-10 hrs. Subsequently, an outer coat of micaceous iron oxide having thickness 100-120 micron was applied by spraying. The second coat was dried at room temperature for 8-10 hrs. Further an outer coat of aliphatic polyurethane having thickness 70-80 micron was applied by spraying and the coat was dried at room temperature for 8-10 hrs.
The adhesion strength, impact strength, flexibility, corrosion resistance, solvent and chemical resistance of the coatings of the invention as well as that of conventional coatings were measured. The adhesion strength was measured by following the ASTM D-4541 procedure. Impact strength of the coatings were determined by using the ASTM D- 2794 method. ASTM B-l 17 was used to study the corrosion resistance of the coated steel on exposure to salt spray. Solvent resistance was determined by treatment of the

coated steel with acetone. Chemical resistance was determined by continuous dipping of the coated steel in 5% H2SO4. Chemical resistance was also determined by dipping of the coated steel in 5% NaOH. The properties of the conventional coating system and of the coating system of the invention are compared in table 1.
Table 1: Comparison of properties of the coating system of the invention with the conventional three coat system

Coatings applied as in Adhesion(MPa) Impactstrength(kgm2/s2) Flexibility Corrosion resistance Solvent Resistance Chemical Resistance
OnDipping in 5% H2SO4 OnDipping in 5% NaOH
Example 3 1.5-2.0 5.74 Paint film does not crack even at 2m on the conical mandrel Rustpenetration less than 1mm after 1200 hours Paint film does not peel off on application of acetone The coatingdoes not peel off till 7days The coating does not peel off till 7 days
Example 4 1-1.5 2.2 Paint film cracks at 8mm on theconical mandrel Rustpenetration less than 1mm after 1200 hours Paint film peels off onapplication of acetone The coating does not peel off till 7 days The coating does not peel off till 7 days

From table 1, it is clear that the coating system of the invention have improved adhesion strength, impact strength, flexibility and solvent resistance as compared to the conventional three coat system. Moreover, coating system of the invention possess corrosion and chemical resistance comparable to the conventional coatings comprising three coats. Apart from this, the coating system of the invention can be used even without the intermediate micaceous iron oxide coat as required in the conventional three coat system, thereby considerably reducing the application time of the coating. Consequently, the productivity of the coatings is increased besides reducing the cost and labour of application.
The above description is illustrative only and is not limiting. The present invention is defined by the claims which follow and their full range of equivalents

We Claim
1. A coating system for steel, the coating system comprising a primary coat wherein the primary coat comprises 1 to 5 % modified clay, 5 to 15% ZnPO4 and 20 to 40 % epoxy resin and a polyurethane based outer coat.
2. The coating system as claimed in claim 1 additionally comprising one or more intermediate coat(s).
3. The coating system as claimed in claim 2 wherein the intermediate coat comprises micaceous iron oxide.

Abstract
A coating system for steel, the coating system comprising a primary coat wherein the primary coat comprises 1 to 5 % modified clay, 5 to 15% ZnPO4 and 20 to 40 % epoxy resin and a polyurethane based outer coat. The coating system additionally comprises one or more intermediate coat(s).