WO2004024835A1

WO2004024835A1 - Self-priming chromate free corrosion resistant coating composition and method

Info

Publication number: WO2004024835A1
Application number: PCT/IB2002/003749
Authority: WO
Inventors: Dhrubo Bhattacharya
Original assignee: Rotomac Electricals Pvt Ltd
Current assignee: Rotomac Electricals Pvt Ltd
Priority date: 2002-09-10
Filing date: 2002-09-10
Publication date: 2004-03-25
Anticipated expiration: 2005-03-10
Also published as: AU2002330698A1

Abstract

The invention discloses a self-priming rapid curing chromate free corrosion resistant coating composition based on a random polyvinyl terpolymer, a saturated oligomeric polyester resin and an alkoxy amino resin in association with a mineral acid catalyst and a blend of organic solvents wherein the said vinyl terpolymer is predominantly a polyvinyl formal with polyvinyl alcohol and polyvinyl acetate as the other two co-polymers. The composition can be applied as a clear coat or as a pigmented composition with addition of pigments on ferrous and non- ferrous metallic substrate and is particularly suitable for continuous coil coating lines. The invention also concerns a method of coating a ferrous and non-ferrous metallic substrate by applying the said coating composition on its surface and coated articles so produced.

Description

SELF-PRIMING CHROMATE FREE CORROSION RESISTANT COATING COMPOSITION AND

METHOD

FIELD OF THE INVENTION

This invention relates to a chromate free self-priming coating composition based on a polymeric material, particularly a vinyl teφolymer of polyvinyl formal, polyvinyl alcohol and polyvinyl acetate with a saturated oligomeric polyester co-resin and an alkoxy-amino resin cross-linker. This coating composition can be applied on ferrous or non- ferrous metallic substrates. The rapid curing nature coupled with high flexibility and resistance to wear, abrasion, staining, corrosion and thermal shocks makes it most ideal for continuous coil coating lines. This can be formulated both as a clear coat as well as a pigmented coating in a range of colors in a blend of volatile organic solvents. It is self-priming in nature and can be applied as a topcoat directly on the metal surface without any primer thereby making the coating method most cost effective. The absence of any primer helps to avoid toxic materials, especially chromate salts normally accompanying a primer and thereby makes the coating environment friendly.

BACKGROUND OF THE INVENTION

Pre-coated metals generated in continuous coil coating lines processing rolled stock such as cold rolled steel, hot dip galvanized steel, stainless steel, tin plated steel and aluminum, constitutes a major industrial activity today. Steel, excluding hot rolled steel, is increasingly being coated in this manner. The organic products normally employed for this puφose are based on silicones, polyesters, epoxies, urethanes, acrylics and combinations thereof, PVC plastisols and fluorocarbons. Most of these products are solvent based i.e. contain a volatile organic solvent, though some are also available as powdered resin to be applied as powder coatings. Generally these products require their specific primers as these lack the ability to adhere directly to substrates in the short oven dwell times of continuous coil coating lines.

The products used for metal pre-coating in continuous coil coating lines ranging from vinyl plastisols and fluorocarbons to other thermosetting resins as stated in prior art have a limitation in that these do not generally offer a combination of high scratch hardness with good flexibility. This combination is most desirable since the pre-coated metal sheets have to undergo fabrication involving rigorous forming and flexing operations in engineering, appliance and construction industries. Hardness of thermosetting coatings depends on the density of cross-links achieved during cure. As this density increases, the material hardens but begins to lose its flexibility. Contrarily, a coating that is inherently flexible such as plastisol, has poor thermal resistance and hardness. Polyesters have been extensively studied in this regard to obtain the desired combination of hardness and flexibility. A survey of such compositions (US Patent nos. 4,140,729, 4,734,467, 5,322,884) reveals that it is very difficult to resolve these conflicting demands in coatings on hard metallic substrates. Increasing the aromatic content and/or inorganic pigment level increases the hardness but lowers the flexibility. Increasing the molecular weight of linear aliphatic chains has the opposite effect on cured resin properties though this is restricted by requirement of resin compatibility with the amino cross-linkers. A combination of these approaches has been generally employed in polyester resin compositions.

The primers normally used for these products contain large amounts of inorganic additives and corrosion inhibiting compounds. Such additives in some cases include chrome based compounds, which are considered to be toxic and harmful. Additionally priming of metal substrate is expensive and time consuming, as this adds another operation to the coating scheme besides increasing the cost of removal of the paint during stripping operation. It also adds to the overall thickness of the coating, which may be undesirable in certain applications. Self-priming coatings with good barrier characteristics offer excellent corrosion protection in thin films. A quick curing, self- priming polyester has been proposed for coil coating (US Patent No. 4,071,578) that is quite flexible, passing a IT bend test, but offers a maximum pencil hardness of only H-. The co-pending PCT application No.PCT/IB02/01509 by the same applicant describes a self-priming coating formulation incoφorating a vinyl teφolymer containing acetyl, hydroxyl and formal groups and an oil based alkyd co-resin. This heat reactive composition though highly scratch and abrasion resistant with pencil hardness of at least 6H and extremely flexible (passes 0T bend test), will find application mainly in dark coatings as it exhibits some darkening during cure due to the unsaturated fatty acids present in the alkyd.

The inventor of the instant invention has found that a tough linear vinyl teφolymer containing formal, acetyl and hydroxyl groups, freely compatible with a saturated oligomeric linear polyester resin and capable of cross-linking with the said polyester and an alkoxy amino resin via its reactive hydroxyl groups, results in a coating having excellent flexibility (0T), adhesion and corrosion resistance with extremely high surface hardness (at least 6H). The coating has good thermal resistance, color retention and stain resistance. Moreover, this composition when applied directly to unprimed metallic substrates, requires peak metal temperature (PMT) lower by at least 50° C for cure compared to the composition of US Patent No. 4,071,578. Further, excellent flow and adhesion is seen on a variety of ferrous substrates such as cold rolled steel, galvanized steel and stainless steel (US Patent No. 4,071,578 discloses only aluminum substrate). It will be obvious to someone skilled in the art that such properties offer important advantages in a variety of applications for both light and dark colored coated metals.

OBJECTS OF THE INVENTION

The first object of the invention is to provide a quick curing self- priming polymer coating composition that is suitable for continuous coil coating lines.

The second object of the invention is to provide a polymer coating, which has high degree of flexibility and surface hardness at the same time and has excellent color retention.

The third object of the invention is to provide an environment friendly chromate free corrosion resistant coating composition. Another object of the invention is to provide a coating method for coating a ferrous or non-ferrous metallic substrate by using a primer free corrosion resistant coating composition.

Yet another object of the invention is to apply the coating composition of the invention as an under coat followed by one or more top coat(s) of polyester, epoxy, amino, alkyd and urethane coatings.

Yet another object of the invention is to provide coated articles at least one surface of which is coated by applying the coating composition according to the invention.

SUMMARY OF THE INVENTION

Accordingly the present invention provides a self-priming, rapid curing, chromate free, corrosion resistant coating composition comprising:

a) a linear vinyl teφolymer made up predominantly of polyvinyl formal with polyvinyl alcohol and polyvinyl acetate as the two other co-polymers with three randomly distributed functional groups along the vinyl backbone, comprising acetyl, formal and hydroxyl;

b) a saturated oligomeric polyester resin;

c) a monomeric or oligomeric alkoxy amino resin;

d) a mineral acid catalyst;

e) a blend of organic solvents and optionally f) one or more chromate free inorganic pigment and/or organic dyes.

The said oligomeric saturated polyester resin preferably has an average degree of polymerization varying from 2 to 5.

The said alkoxy amino resin in oligomeric form preferably has an average degree of polymerization of not more than 3.

The said coating composition may be suitably pigmented with inorganic pigments and/or organic dyes to obtain pigmented icόatings in a choice of attractive colors with low to medium gloss.

The invention also provides a method of coating ferrous or non- ferrous metal substrates by applying the composition according to invention on the surface of the said substrate in desired thickness and curing the same.

The invention further provides a coated article comprising a ferrous or a non-ferrous metallic substrate of which at least one surface is coated with the coating composition according to the invention.

DETAILED DISCRIPTION OF THE INVENTION

A linear vinyl teφolymer, namely polyvinyl formal, polyvinyl alcohol and polyvinyl acetate with three functional groups randomly distributed along the vinyl backbone offers cross-linking sites through the hydroxyl groups. The density of cross-links may be controlled by the number and placement of hydroxyl groups on the vinyl backbone of polymer. This polymer possesses a rare combination of mechanical, thermal, chemical and dielectric properties. The functional groups along with its vinyl backbone confer to this polymer the properties of adhesion, toughness, chemical inertness and heat stability while the long linear chains contributes to the outstanding flexibility. The spatial structure of this thermoplastic material helps to form a closely packed molecular structure, which in turn provides excellent barrier characteristics when coated on a wide array of substrates. The hydroxyl groups are fully accessible for cross-linking and this makes the thermoplastic polymer heat curable in the presence of a mineral acid. The chain length distribution of a poly disperse polymer is made such as to permit film formation, migration by diffusion to the substrate and development of the required cohesive strength in the coating. Specifically, the teφolymer used in the invention has weight average molecular weight ranging between 15,000 and 80,000 preferably between 20,000 and 50,000. The content of polyvinyl alcohol, polyvinyl acetate and polyvinyl formal of the vinyl teφolymer used for the invention is 6-15%, 9-15% and 70-84% respectively by weight and preferably 6.0-7.5%, 10-13% and 80-83% respectively by weight.

The said teφolymer useable in the composition may be produced by simultaneous hydrolysis and formalization of polyvinyl acetate in acetic acid media. For this puφose polyvinyl acetate of the required molecular weight (28,000-140,000) and of low to very low branching frequency is dissolved in acetic acid and formalin (37% formaldehyde aqueous solution) at room temperature. Dilute sulfuric acid (N/10 normality) is added to this solution with agitation. The contents are well stirred, heated to 75°C and maintained at this condition for 20-24 hours. The whole process is conducted in a homogenous solution state. By regulating quantities of acetic acid, water and formaldehyde, the required composition of the functional groups of the vinyl backbone viz. acetyl, hydroxyl and formal may be obtained. Typically for one part of polyvinyl acetate, 1.65 parts of acetic acid, 0.55 part of water and 0.45 part of formalin (37% formaldehyde solution in water) is used to obtain a composition comprising 6.0 - 6.5 % polyvinyl alcohol, 11.0-12.0 % polyvinyl acetate and 81.5-83.0 % polyvinyl formal. The extent of reaction is determined by the percentage of hydroxyl and acetyl groups in the extracted polymer and the reaction is terminated at the desired point by neutralizing the acid catalyst with a dilute alkali. The polymer is next precipitated from solution by adding water as non-solvent, washed and dried. The simultaneous hydrolysis and formalization reactions prevent the development of blocky sequences on vinyl chain in a homogenous media and thus helps to get a random teφolymer. The T_g of the teφolymer so produced ranges between 100 - 115°C.

The oligomeric polyester resin used, has predominantly hydroxyls as the terminal group. The alkoxy amino resin used is a low molecular weight melamine formaldehyde resin which is methylated by etherifying its methylol groups with methanol to increase its stability and solvency in organic media. It contains heat reactive methoxy - methyl and methylol groups. Upon cure in the presence of acid the pendant hydroxyls of the vinyl teφolymer and the terminal hydroxyls of the polyester react with the melamine resin to yield a lightly cross-linked three dimensional network which provides the outstanding hardness, toughness, chemical inertness, thermal and corrosion resistance as well as gloss of the coating.

The polyester resin may be made from saturated carboxylic dibasic acids and anhydrides and combinations thereof. A variety of aliphatic acids such as adipic, succinic or sebacic acid and aromatic acids such as phthalic, isophthalic or terephthalic acid may be used. Alcohols used for polyesterification may be selected from diols such as ethylene glycol, diethylene glycol, triethylene glycol, neopentyl glycol, propylene glycol and cyclohexane dimethanol or a combination thereof. The resin is produced by esterification to high conversion, a mix of an excess of alcohols over acids/ anhydrides using a catalyst such as triphenyl phosphite and removing the water of condensation through an azeotropic solvent such as toluene. The polyester solids are obtained in solution in the azeotropic solvent. The polyester resin chosen in this invention was made from adipic and isophthalic acids and propylene and neopentyl glycols using triphenyl phosphite catalyst with toluene as azeotropic solvent and contains 82% solids.

The amino formaldehyde resin is obtained by the condensation reaction of formaldehyde and a poly functional amine with the former used in excess in the presence of an alkali catalyst and the product is butylated or methylated after condensation with the corresponding alcohol to provide the corresponding alkoxy resin. Methoxy melamine resin with approximately 96 % solids was used for the tests of the present invention. The heat curable clear coat may be formulated by dissolving a ternary system comprising of one part of the polyester resin with 1.5 to 2.5 parts of the vinyl teφolymer with the methoxy melamine resin amounting to 10 to 20 % of the total weight of the vinyl polymer and polyester resin, together with a mineral acid catalyst in one or more organic solvents. The basic attributes of the cured coating remains unaffected by increasing the proportion of the vinyl polymer vis-a-vis polyester resin, from 1.5 to 2.5 but due to the high molecular weight of the polymer, viscosity of the composition increases dramatically. The same effect is observed on increasing the molecular weight of the vinyl polymer. The preferred proportion of the vinyl polymer is 2 parts per part of polyester, and its weight average molecular weight range is chosen between 20,000 to 50,000 to obtain the best balance of properties. Increasing the melamine resin content decreases the curing time of the composition and also increases the hardness of the cured coating but adversely affects the flexibility. The most preferred level of melamine cross-linker is found to be 15 % of the total weight of polymer and polyester.

Components of the solvent blend may be selected from aliphatic alcohols such as methanol, ethanol, isopropanol, butanol and aromatics such as toluene, xylene. Other solvents such as methyl/butyl cellosolve, cellosolve acetate, diacetone alcohol, methyl ethyl ketone, methyl isobutyl ketone, dichloroethane and solvent naphtha may also be used as co-solvents/diluents. Phosphoric acid, para toluene sulfonic acid may be used as the acid catalyst for this formulation. This heat curable clear coat system may be pigmented to impart color in low to medium gloss to coated substrates. Titanium dioxide may be added for opacity and colored inorganic pigments and/or organic dyes may be used for color. The pigment binding power of the polymer-resin system is quite high, however, low levels of such addition up to a maximum of 40% of the polymer and resin content using titanium dioxide for a white coating, is suggested for optimum flexibility and chemical resistance. This ensures that the passivating nature of the cured film and its barrier characteristics are maintained with high flexibility. Blister resistance is obtained even with a low porosity of the film due to the passivation achieved on the metal interface. A range of corrosion inhibiting pigments may be selected comprising of inorganic and organic pigments which offer passive inhibition, for example, zinc molybdate and other molybdates, zinc phosphate, mica, tolyltriazole, complex organotitanate and other organic inhibitors which operate by passive inhibition. The incoφoration of corrosion inhibiting pigments improves further the corrosion resistance of the coating. The pigments can be incoφorated in the coating by first forming a mill base by conventional sand grinding or ball milling techniques, a concentrated solution of the polymer and resin of concentration 45-50% in the organic solvent blend, together with the pigments, and then blended with the remaining portion of solvents by high speed stirring to obtain a final solids content of 30-40 %. This coating, with or without pigments, dries by solvent evaporation on substrates such as cold rolled steel, hot dip galvanized steel, aluminum etc. when sprayed or flow coated on these surfaces. The rate of evaporation of the solvents especially at the cure temperatures used in metal pre-coat lines with peak metal temperatures ranging from 210- 250° C may be adjusted by the choice of the solvents from the ones enumerated earlier. Specifically, a blend of xylene and butanol in a 70:30 weight ratio was used for this invention. A preferred blend of solvent is 46.5 to 55.0 parts of xylene with 19.9 to 23.5 parts of butanol by weight.Cure time in the heated oven varied from 90 to 20 seconds depending on the temperature and may be controlled by the acid catalyst level between 5 and 10 % of the vinyl teφolymer content. Preferably the total solid content of the composition is 20-40% by weight.

The coating composition of the instant invention may be applied to ferrous and non-ferrous substrate including cold-rolled steel, hot dip galvanized steel, stainless steel, tin plated steel, aluminum and other non-ferrous substrate in thin films of dry film thickness (DFT) ranging from 5-30 micrometers preferably 10-20 micrometers. This self-priming composition is particularly suited to metal pre-coating process in coil coating lines because of its quick curing nature. It can be formulated as free flowing one pack liquid that can be stored below 25°C for 6 (six) months.

The following examples of compositions are illustrative of the invention and are not intended to limit the scope of the invention as defined by the appended claims.

The polyvinyl teφolymer as used in these examples is prepared by the process as described earlier with the weight content of polyvinyl alcohol, polyvinyl acetate and polyvinyl formal at 6.0-7.5%, 10-13% and

80-83% respectively and a molecular weight between 25,000 and 30,000. Example 1 - Clear Coat

A clear coat composition may be prepared from the listed ingredients in the following range of weight ratios:

A specific clear coat composition was prepared by incoφorating the ingredients taken in the following weight percentages for testing:

The clear coat is prepared by dissolving the vinyl polymer and the polyester and melamine resins in a mix of xylene and butanol (70:30 parts by weight). The catalyst is then added to this solution. The cold rolled steel panels used were degreased and pickled in dilute hydrochloric acid prior to application. The aluminum panels were chrome treated. The composition was applied and cured on aluminum, CR steel and galvanized steel panels in thickness ranging from 10-20 micrometers. The cured coatings exhibited good weatherability, resistance to water immersion, humidity and heat when exposed to 120°C for a sustained period of 10 hours. The flexibility was excellent (0T) and the scratch hardness was 5H or higher. The solvent resistance to MEK (Methyl Ethyl Ketone) was excellent with the material passing 100 double rubs. The coated panels also showed excellent resistance to staining, detergents, alkaline media and boiling water and good resistance to acidic conditions.

Example 2 - Pigmented Coating

A pigmented white coating composition according to the present invention can be prepared from the listed ingredients taken in the following range of weight ratios:

A specific pigmented white coating composition was prepared from the listed ingredients in the following weight percentages for testing:

The vinyl polymer, polyester and melamine resins are all dissolved in a mix of xylene and butanol (70:30 parts by weight) to obtain a concentrated solution of about 35% by weight. This solution is then milled with the pigment and catalyst for six hours and finally mixed under agitation with the balance solvents to obtain the desired viscosity and solids level.

The white coating was applied on substrates as in earlier example. The thickness ranged from 10 - 20 micrometers. These panels showed very good weatherability and color retention, resistance to boiling water and staining. Flexibility obtained was excellent (0T) and scratch hardness was found to exceed 6H. Detergent, acid, alkali and MEK resistance showed the same behavior as reported earlier for Example 1. Further, there was no deterioration in mechanical and chemical properties on prolonged exposure of cured panels to 10 hours at 150°C.

Other colors can be incoφorated by adding colored inorganic pigments/organic dyes which offer corrosion protection by passivation and titanium dioxide if necessary in doses of 0-40% by weight of the total polymer and resin content in solvent bearing compositions.

The non-volatile components of the coating composition of this invention including the polymer, resin and pigments can range from 20- 45 % and is typically between 30-40% by weight of the total composition.

TESTING METHOD

The numbers of tests carried out on each of the substrate were 10 and the average value has been quoted in the examples. The testing method that was applied for different physiochemical characteristics are given below:

i) Scratch hardness - A pencil hardness index was used as is common in the industry, ii) Solvent resistance - A cloth soaked with MEK was used to rub over coated surface for the prescribed number of times as per ASTM D-740. iii) Adhesion - ASTM 3359-76. iv) Flexibility - The coated substrate was folded on itself along a crease and an adhesive tape was used at the bend to examine any peel off of the coating. All panels passed this 0T bend test, which is the most rigorous examination of flexibility without any cracking or peel off along the crease, v) Resistance to boiling water - Edge protected coated panels were immersed in boiling water for 4 hours and the panels examined for blisters. vi) Chemical resistance - Resistance to chemicals was tested as per

ASTM D 1308.

Claims

1. A self-priming, rapid curing, chromate free, corrosion resistant coating composition comprising:

a) a linear vinyl teφolymer made up predominantly of polyvinyl formal with polyvinyl alcohol and polyvinyl acetate as the two other co-polymers with three randomly distributed functional groups along the vinyl backbone comprising acetyl, formal and hydroxyl;

b) a saturated oligomeric polyester resin;

c) a monomeric or oligomeric alkoxy amino resin;

d) a mineral acid catalyst;

e) a blend of organic solvents and optionally

f) one or more chromate free inorganic pigment and/or organic dyes.

2. The coating composition as claimed in claim- 1 wherein the percentages by weight of polyvinyl alcohol, polyvinyl acetate and polyvinyl formal of the vinyl teφolymer are 6-15%, 9- 15% and 70-84% respectively.

3. The coating composition as claimed in claim-2 wherein the percentages by weight of polyvinyl alcohol, polyvinyl acetate and polyvinyl formal of the vinyl teφolymer are 6.0-7.5%), 10-13% and 80-83% respectively.

4. The coating composition as claimed in claim 1, 2 or 3 wherein the weight average molecular weight of said vinyl teφolymer is 15,000 to 80,000.

5. The coating composition as claimed in claim 4 wherein weight average molecular weight of said vinyl teφolymer is 20,000 to 50,000.

6. The coating composition as claimed in claim 1 wherein a methoxy melamine formaldehyde resin is used as the alkoxy amino resin.

7. The coating composition as claimed in claim 1 or 6 wherein the said polyester resin is produced from adipic and isophthalic acids, and neopentyl and propylene glycols with triphenyl phosphite catalyst in toluene solvent.

8. The coating composition as claimed in claim 1, 6 or 7 wherein the average degree of polymerization of said polyester ranges from 2 to 5.

9. The coating composition as claimed in claim 1 wherein said alkoxy amino resin is in an oligomeric form with average degree of polymerization not more than 3.

10. The coating composition as claimed in claim 1 wherein the components of the solvent blend are selected from the group consisting of ethylene dichloride, xylene, toluene, naphtha, isopropanol, butanol, ethanol, methanol, methyl ethyl ketone

(MEK), methyl isobutyl ketone (MIBK), cellosolve, cellosolve acetate and diacetone alcohol.

11. The coating composition as claimed in claim 10 wherein 46.5-

55.0 parts of xylene and 19.9-23.5 parts of butanol by weight are used as solvent.

12 The coating composition as claimed in claim 1 wherein the content of total solids in the composition is 20-40%> by weight.

13. The coating composition as claimed in claim 1 wherein 5-10% by weight of phosphoric acid based on the total weight of the vinyl teφolymer is used as the catalyst.

14. The coating composition as claimed in claim 1 which includes 0- 40% of the polymer and resin content of one or more chromate free inorganic pigment(s) and/or organic dye(s).

15. The coating composition as claimed in claim 1 wherein other corrosion inhibiting agent(s) selected from the group consisting of zinc and other molybdates, zinc phosphate, mica, tolyltriazole, complex organic titanates and other organic inhibitors which act by passive inhibition is/are also included in the composition.

16. A coating composition as claimed in claim 1 or 15 as a pigmented composition wherein titanium dioxide is included as the inorganic pigment.

17. The coating composition as claimed in claim 1 wherein the ingredients are present in the following range of weight ratios:

Range of parts

Ingredients bv weight

Polyvinyl Teφolymer 13 - 17

Polyester Resin (100%solids 5.2-11.4 basis)

Methoxy Melamine Resin 1.8-5.7

(100%solids basis)

Xylene 46.5-55

Butanol 19.9-23.5

Ortho Phosphoric Acid (88-93%) 0.65-1.7

18. The coating composition as claimed in claim 1 or 14 wherein the ingredients are present in the following range of weight ratios:

Range of parts

Ingredients bv weight

Polyvinyl Teφolymer 15-18

Polyester Resin(100%solids 6-12

UcL l J

Methoxy Melamine Resin 2.1-6

(100%solids basis)

Xylene 39.6-67

Butanol 17-28.7

Ortho Phosphoric Acid (88-93%) 0.75-1.8

Titanium Dioxide 6.9-14.4

19. The coating composition as claimed in claim 1, 17 or 18 wherein it is formulated as a free flowing one-pack liquid.

20. A method of forming a coating on a ferrous or non-ferrous metallic substrate which comprises:

(a) degreasing the surface of said substrate on which coating is to be applied;

(b) preparing the coating composition as defined in claim 1 by dissolving mixture of the vinyl teφolymer, polyester resin, methoxy melamine resin, mineral acid catalyst with or without chromate free inorganic pigment(s) and/or organic dye(s) and/or other corrosion inhibitor(s), in a blend of organic solvents;

(c) applying said coating composition by spray coating or roller coating method to attain the desired thickness and curing coating so applied to desired hardness at a peak metal temperature of 210-250 °C for 90-20 seconds.

21. The method as claimed in claim 20 wherein said coating composition is prepared by dissolving 13-17 parts (by weight) of the polyvinyl teφolymer, 5.2-11.4 parts (by weight) of the polyester resin (100% solids basis), 1.8-5.7 parts (by weight) of the methoxy melamine resin and mineral acid catalyst in 0- 80 parts (by weight) of a blend of organic solvents selected from the group consisting of ethylene dichloride, xylene, toluene, naphtha, isopropanol, butanol, ethanol, methanol, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), cellosolve, cellosolve acetate and diacetone alcohol.

22. The method as claimed in claim 21 wherein a blend of 46.5-55 parts of xylene and 19.9-23.5 parts of butanol, by weight, are used as the organic solvents.

23. The method as claimed in claim 20 wherein 0.65-1.7 parts (by weight) of ortho phosphoric acid is used as catalyst.

24. The method as claimed in claim 20 wherein 0-40 % (by weight) of the resin and polymer content, of an inorganic pigment and/or organic dyes and other corrosion inhibiting agents other than chromates are added for preparing the said composition.

25. The method as claimed in claim 20 wherein 0-40 %> (by weight) of the resin and polymer content of titanium dioxide is added as the inorganic pigment.

26. The method as claimed in claim 20 wherein curing time is less than 90 sees.

27. The method as claimed in claim 20 wherein the curing time is less than 30 sees.

28. The method as claimed in claim 20 wherein thickness of the coating as applied is 5-30 micrometers.

29. The method as claimed in claim 20 where the thickness of the applied coating is 10-20 micrometers.

30. The method as claimed in claim 20 wherein said coating is applied directly on the surface of the said substrate as a top coat.

31 The method as claimed in claim 20, wherein the said coating is applied directly on the surface of the said substrate as an undercoat which is followed by application of top coat(s) based on one or more of resins selected from the group consisting of epoxy, urethane, alkyd, amino and polyester.

32. An article comprising:

a) a ferrous or non-ferrous metallic substrate

b) a coating applied to at least one surface of the said substrate by a method as claimed in claim 20.

33. The article as claimed in claim 32 wherein the said substrate is selected from the group comprising of cold-rolled steel, hot dip galvanized steel, stainless steel, tin plated steel, aluminum and other non-ferrous substrate.