Description:
FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003

COMPLETE SPECIFICATION
[See section 10, Rule 13]

CORROSION RESISTANT QUICK DRY ORGANIC COATING, A PROCESS TO PREPARE THE COATING & IT’S APPLICATION ON ALUMINUM

HINDALCO INDUSTRIES LIMITED, A COMPANY INCORPORATED UNDER THE COMPANIES ACT, 1956, WHOSE ADDRESS IS AHURA CENTRE, 1ST FLOOR, B-WING, MAHAKALI CAVES ROAD, ANDHERI (EAST), MUMBAI-400 093, MAHARASHTRA, INDIA

THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED.

FIELD OF THE INVENTION
[1] The present invention relates to an organic coating composition, its preparation and application on heat exchangers particularly heat exchangers made of aluminum and its alloys.

BACKGROUND OF THE INVENTION
[2] Aluminum metal being infinitely recyclable material is an important material of construction across a wide spectrum of machines. Aluminum is non-toxic, has a high thermal conductivity, has excellent corrosion resistant, and can be easily cast, machined, and formed. It is also non-magnetic and non-sparking. It is the second most malleable metal and the sixth most ductile. It finds its application in a huge variety of products including cans, foils, kitchen utensils, window frames, beer kegs, gas cylinders, and aero-plane parts, automotive parts etc.

[3] Aluminum is often used as an alloy because aluminum itself is not particularly strong. Alloys with copper, manganese, magnesium, and silicon are lightweight but strong.

[4] Aluminum sheets as heat exchanging media is one of the important applications. Aluminum sheets in very thin gauges being used in heat exchanges and they are very closely spaced to improve the efficiency. These heat exchangers are being used in various environmental conditions like salty, humid, dry etc.

[5] Bare aluminum or its alloys in such severe condition can undergoes corrosion. Once the process of corrosion begins in aluminum heat exchanger the process cannot be stopped as fins are very closely spaced. This results in to reducing the usable life of heat exchanger & that of associated equipment.

[6] Therefore, for harsh environmental conditions, bare aluminum or its alloy sheets are precoated with corrosion protection coating before manufacturing the heat exchangers.

[7] Coatings used for corrosion protection of heat exchanger fins are not regular coatings. Regular coatings available in market does not provide adequate corrosion protection in thin coating layer if coating thickness increases heat exchanger efficiency decreases.

[8] Regular coatings also do not have required quick dry time & surface finish.

[9] Therefore, it is a dire need that a coating for aluminum sheets used in heat exchanger applications has ability for quick dry, self levelling, crosslinking. The coating should further have significant adhesion to aluminum, and superior corrosion resistance even for a thin gauge aluminum. Still further, the coating should be able to protect the edge trimmer blades from wear and tear and should be able to provide an attractive finish when applied by way of a topcoat. The coating should also have excellent adhesion with water-based topcoat.

[10] In India there is a dearth in the manufacture of corrosion resistant quick dry organic coating for aluminum sheets for utility in heat exchangers, nor corrosion resistant coated aluminum sheets for heat exchangers.

[11] It is therefore an object of the invention to develop a quick dry organic coating, a process to prepare the coating & it’s application on aluminum for use on aluminum sheets for heat exchanger application which overcome the aforesaid disadvantages.

OBJECTS OF THE INVENTION

[12] An object of the invention is to provide a quick dry organic coating, which gives sufficient corrosion protection, quick dry property along with good product quality in terms of surface finish.

[13] Another object of the present invention is to develop a process to prepare the said coating.

[14] Another object of the present invention is to develop method for its application on aluminum for use on aluminum sheets for heat exchanger application

[15] For achieving the above object, the invention provides quick dry organic coating composition comprises of:
a) At least one epoxy resin in the range of 19-30wt. %;
b) a crosslinker in the range of 3 -7 wt. %;
c) one or more adhesion promoters in the range of 0-6wt%;
d) catalyst in the range of 0.1-0.5 wt. %;
e) Levelling/Wetting Agent in the range of 0.1-0.5 wt. %;
f) Pigment in the range of 0-1.0 wt. %;
g) Solvent mixture in the range of 50-85 wt. %

SUMMARY OF THE INVENTION
The present invention discloses a coating composition that dries quickly and is corrosion resistant making it suitable for heat exchangers wherein the fins are coated with the claimed composition thereby increasing their productivity, improving their surface finish, and their formability.
Accordingly, the invention discloses a quick dry corrosion resistant coating composition. The composition comprises of:

a) At least one epoxy resin in the range of 15-32wt. %;
b) a crosslinker in the range of 3.5-5.5 wt. %;
c) one or more adhesion promoters in the range of 0-5wt%;
d) catalyst in the range of 0.1-0.5 wt. %;
e) Levelling/Wetting Agent in the range of 0.1-0.5 wt. %;
f) Pigment in the range of 0-1.0 wt. %;
g) Solvent mixture in the range of 50-85 wt. %
The present invention also provides a method for obtaining the quick dry corrosion resistant organic coating composition.
BRIEF DESCRIPTION OF DRAWINGS
Fig. 1: Example of an uncoated aluminum surface with white spots and delamination

Fig. 2: Example of a substrate for the coating composition of the present invention- It is a system consisting of tubes that pass through a dense fin stack that is mechanically supported by a mounting frame.

BRIEF DESCRIPTION OF THE INVENTION

[1] In describing and claiming the invention, the following terminology will be used in accordance with the definitions set forth below. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described herein. As used herein, each of the following terms has the meaning associated with it in this section. Specific and preferred values listed below for individual process parameters, substituents, and ranges are for illustration only; they do not exclude other defined values or other values falling within the preferred defined ranges.

[2] As used herein, the singular forms "a," "an," and "the" include plural reference unless the context clearly dictates otherwise.

[3] The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention

[4] As used herein, the terms “comprising” “including,” “having,” “containing,” “involving,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to.

[5] The term “Coating” as used herein refers to covering that is applied to the surface of an object, usually referred to as the substrate to prevent direct environmental contact.

[6] The term “Heat exchanger” as used herein refers to Tube Fin Heat Exchangers a system used to transfer heat between a source and a working fluid.

[7] The term “Tube Fin Heat Exchanger” (Ref Fig -2)as used herein refers to a system consist of tubes that pass through a dense fin stack that is mechanically supported by a mounting frame.

[8] The term “fin” (Ref Figure -2) as used herein refers to a heat exchanger surfaces that extend from an object to increase the rate of heat transfer to or from the environment by increasing convection.

[9] The phrase “quick dry” as used herein refers to any the ability of a coating to dry quickly (less than 20 Sec) so as to retain its primary properties of corrosion resistance along with clean and spotless surface finish.

[10] The present invention discloses a coating composition that dries quickly and is corrosion resistant making it suitable for heat exchangers wherein the fins are coated with the claimed composition thereby increasing their productivity, improving their surface finish, and their formability.

[11] The invention relates to a quick dry corrosion resistant coating composition having adequate corrosion resistant property in thin gauge, adhesion to substrate, quick dry time, appropriate surface finish for application of topcoat.

[12] Accordingly, the invention discloses a quick dry corrosion resistant coating composition. The composition comprises of:

h) At least one epoxy resin in the range of 15-32wt. %;
i) a crosslinker in the range of 3.5-5.5 wt. %;
j) one or more adhesion promoters in the range of 0-5wt%;
k) catalyst in the range of 0.1-0.5 wt. %;
l) Levelling/Wetting Agent in the range of 0.1-0.5 wt. %;
m) Pigment in the range of 0-1.0 wt. %;
n) Solvent mixture in the range of 50-85 wt. %

[13] The coating composition according to the present invention, the achieves the properties of quick dry, being corrosion resistant with good finish.
[14] The coating composition of the present invention comprises of at least one or more epoxy resin. The polyepoxide polymer may be selected from a phenolic glycidic ether, aromatic glycidic ether or cycloaliphatic polyepoxide polymer having average molecular weight in the range of 1750 to 2100 of resin per epoxy group.
[15] Typical epoxy resins well known in the art may be utilized in the coating composition of the present invention. These include Type -7, Type-8, Type-9, Type-10,
Epotec YD 907, Epotec YD017, Epotec YD017S, Epotec YD 019, Epotec YD 019S, Epotec YD 909, Epotec YD 010S, D.E.R. 667E, D.E.R. 668-20, D.E.R. 669-20, D.E.R. 669E, Lapox P-5 (ARP-17) , Lapox P-6 (ARP-19), preferably polyepoxide polymer having molecular weight 1750- 2100 of type 7 including Epotec YD 907, Epotec YD017, Epotec YD017S, D.E.R. 667E, Lapox P-5 (ARP-17).

[16] In an embodiment of the coating composition of the present invention, a mixture of two polyepoxide polymer may be utilized to achieve the average molecular weight in the range of 1750- 2100 of resin per epoxy group.

[17] In an embodiment of the coating composition of the present invention the polyepoxide polymer is selected from diglycidyl ether bisphenol A (DGEBA) or diglycidyl ether Bisphenol F (DGEBF).

[18] In an embodiment of the coating composition of the present invention, the concentration of polyepoxide polymer in the formulation is in the range of 15- 32 wt% more preferably 19-30 wt.%.

[19] In an embodiment of the coating composition of the present invention , the polyepoxide polymers are selected from a group consisting of, but not limited to unmodified, high molecular weight Bis Phenol A resin.

[20] The coating composition according to the present invention comprises of a crosslinker which forms a three-dimensional network of connected molecule on curing.

[21] Typical crosslinkers that re well known in the art and may be used in the coating composition according to the present invention include methylated melamine e.g. CYMEL 301, CYMEL 303 LF, CYMEL 304, ETERMINO 9603-80, ETERMINO 9610-100, DooriChem MM-803, DooriChem MM-803 LF, DooriChem MM- DooriChem 850/MM812, DooriChem MM-825, DooriChem MM-8257, DooriChem MM-823, DooriChem MX-40, DooriChem MX-41, DooriChem MX-1141 more preferably a methylated high imino melamine e.g. CYMEL 325, CYMEL 323, CYMEL 327, DooriChem MM-825, DooriChem MM-8257 DooriChem MM-823 .

[22] In an embodiment of the coating composition of the present invention, the crosslinker is methylated high imino melamine.

[23] In an embodiment of the coating composition of the present invention, the crosslinker is in a range of 3 - 7 wt% , more preferably 3.5-5.5 wt.% based on alloy composition.
[24] The coating composition according to the present invention comprises of at least one adhesion promoter preferably selected from adhesion promoter 1, adhesion promoter 2 and adhesion promoter 3 or a combination thereof.

[25] According to the present invention, adhesion promotor -1, in the coating composition improves the adhesion of coating film to the aluminum substrate.

[26] In an embodiment of the coating composition of the present invention, adhesion promoter 1 is selected from fatty acid modified epoxy resin ester e.g., RESYDROL AX 250w/75EP, Lapox ARES-101, Lapox ARES-102, FTC DEE-100. It is preferred in a range of 0-4 wt. % based on alloy composition.

[27] According to the present invention, adhesion promotor -2 in the coating composition improve the adhesion of coating film to the aluminum substrate.

[28] In an embodiment of the coating composition of the present invention, it is selected from Epoxy ester phosphate acid e.g., FTC PAP-110, FTC PAP-140. It is preferred in a range of 0-2.5 wt.% based on alloy composition and thickness of the sheet.

[29] According to the present invention, adhesion promotor -3 in the coating composition improves the adhesion of coating film to the aluminum substrate.

[30] In an embodiment of the coating composition of the present invention, it is selected from bifunctional organosilane e.g. 3-Glycidyloxypropyltrimethoxysilane. It is preferred in a range of 0-1 wt.% based on alloy composition and thickness of the sheet.

[31] The coating composition according to the present invention comprises a catalyst. The catalyst in the coating composition promotes the crosslinking reaction.

[32] The coating composition according to the present invention comprising at least one more than one adhesion promoter preferably ate least 2 adhesion promoters or 3 adhesion promoters, selected from adhesion promoter 1, adhesion promoter 2 and adhesion promoter 3 or a combination thereof.

[33] The coating composition according to the present invention comprising a combination of adhesion promoters in a range of 0-6 wt% of the total composition, preferably 0-5 wt% of the total composition thereof.

[34] In an embodiment of the coating composition of the present invention, the catalyst is selected from a group consisting of, but not limited to p-Toluenesulfonic acid. It is preferred in a range of 0.1-0.5 wt.% based on alloy composition and thickness of the sheet.

[35] The coating composition according to the present invention comprises a levelling cum wetting agent which promotes the levelling of coating and improves wettability of substrate.

[36] In an embodiment of the coating composition of the present invention, the levelling cum wetting agent is selected from a group consisting of, but not limited polyether modified polysiloxan e.g. BYK® -348, Borchi® Gol LA 2, Additol 6503N. It is preferred in a range of 0.1 -0.5 wt.% based on alloy composition and thickness of the sheet.

[37] The coating composition according to the present invention comprises a pigment used to provide color to the coating. It is preferred in a range of 0 -1 wt.% .

[38] In an embodiment of the coating composition of the present invention the pigment may be Phthalocyanine Blue or Carbon Black or/and Zinc Tetroxy Chromate Yellow. The selection of the pigment is based on alloy composition.

[39] The coating composition according to the present invention also comprises a mixture of at least two solvents.

[40] In an embodiment of the coating composition of the present invention, the solvent is a combination of dimethylbenzene in a range of 10 -20 wt.% and 2-Butoxyethanol in a range of 40-65% Wt.

[41] In an embodiment of the coating composition of the present invention, the solvent mixture may be of any two such solvents which are well known in the art similar to dimethylbenzene and 2-butoxyethanol. A solvent in said mixture may include a mutual solvent which is soluble both in water and oil that are well known in art. Some other well-known solvents that may be utilized for the present invention include DEG monobutyl ether, 2-ethoxyethanol and propylene glycol methyl ether.

[42] In a further aspect, the present invention also provides a method for obtaining the quick dry corrosion resistant organic coating composition.

[43] In an embodiment, the process of preparing a coating composition as per the present invention comprises the steps of
a. Mixing a suitable amount of polyepoxide polymer with the solvent mixture of dimethyl benzene and 2-butoxyethanol and stirring at high speed to completely dissolve the polyepoxide polymer;
b. a suitable amount of crosslinker to the product of (a) and mixing till a homogenous solution is obtained;
c. Mixing at least more than 1 adhesion promoters and stirring the same;
d. Adding a suitable catalyst to the solution obtained in step (c) and stirring the reaction mixture;
e. Adding at least one levelling cum wetting agent and a pigment to the product of step (d);
f. Finally adding a combination of solvents and stirring to obtain a quick dry corrosion resistant organic hydrophobic coating composition.

In an embodiment of the process of the present invention, the stirring of the reaction mixture at each step takes place for a time period in the range of 10-20min at ambient temperature.

In a further embodiment of the process of the present invention, the step of mixing the polypeptide polymer in the solvent mixture is carried out in a sequential manner for a time in the range of 2-3 hrs.

[44] In another embodiment of the process of preparing the coating composition of the present invention, the method steps comprise:

Sr. No Component
Inventive Example
At least one polyepoxide polymer 28.52
Solvent A 14.0
Solvent B 14.0
Stirring at high-speed of 1000 RPM to dissolve the polyepoxide polymer the solvent mixture.
Adding a crosslinker 5.22
mixing for 10 min.
at least one adhesion promotor 3.75
mixing for 10 min.
at least another adhesion promotor 0
mixing for 10 min.
at least one catalyst 0.41
mixing for 10 min.
at least one levelling cum wetting agent 0.16
at least one pigment 0.81
mixing for 10 min.
at least solvent 1 3.52
at least solvent 2. 29.6
mixing for 10 min.

a) Mixing at least one polyepoxide polymer in an amount in the range of 15-32 wt.%, with 10-15 wt. % of at least one solvent & 10-15 wt. % of another solvent based on the total wight of the solution.
b) Stirring at high-speed of 1000 RPM to dissolve the polyepoxide polymer the solvent mixture.
c) Adding a crosslinker in an amount in the range of 3.5-5.5 wt. % and mixing for 10 min.
d) Adding 0- 4 wt. % of at least one adhesion promotor and mixing for 10 min.
e) Adding 0-1 wt. % of at least another adhesion promotor and mixing for 10 min.
f) Adding 0.1-0.5 wt. % of at least one catalyst for cross linking reaction and mixing for 10 min.
g) Adding 0.1-0.5 wt. % of at least one levelling cum wetting agent & 0-1.5 wt.% of at least one pigment and mixing for 10 min.
h) Adding 0-10 wt. % of at least solvent 1 & 30-50 wt. % of at least solvent 2.
and mixing for 10 min.
[45] In an embodiment, the mixing step (a) is carried out for a duration in the range of 2 to 3 hours at ambient temperature.

[46] In an embodiment, step (c) to (h) are carried out at an ambient temperature.

[47] The quick dry corrosion resistant coating composition of the present invention provides good corrosion resistant property of coated fins, quick dry time thus increases fins productivity, good fins surface finish & good fins formability.

[48] In an aspect of the invention, the coating composition of the present invention is applied on the aluminum surface by methods well known in the art. The aluminum sheet is cleaned with acid degreasing solution then dried and or applied with pretreatment chemicals for improved protection from corrosion. The quick dry organic coating of the present invention is then applied with three roller applicators on the aluminum sheet. The coated sheet is passed through preheated oven at 225-255 0C for 12- 30 seconds. Water quenching or air quenching is conducted such that the coated sheet cools down to room temperature attaining the desired properties.

WORKING EXAMPLES
PREPARATION OF QUICK DRY CORROSION RESISTANT COATING
[49] Mixing of 19-30 wt. % of at least one polyepoxide polymer with 10-15 wt. % of at least solvent 1 & 10-15 wt. % of at least solvent 2.
Stirring at High Speed to dissolve the polyepoxide polymer in solvent. Stir continuously until polymer is completely dissolved. Add 3.5-5.5 wt. % of at least one crosslinker and mix for 10 min. Add 0- 4 wt. % of at least one adhesion promotor 1 or 2 and mix for 10 min. Add 0-1 wt. % of at least another adhesion promotor 3 and mix for 10 min. Add 0.1-0.5 wt. % of at least one catalyst for cross linking reaction and mix for 10 min. Add 0.1-0.5 wt. % of at least one levelling cum wetting agent & 0-1.5 wt.% of at least one pigment and mix for 10 min. Add 10 wt. % of at least solvent 1 & 30-50 wt. % of at least solvent 2and mix for 10 min.

The coating composition in accordance with the present invention have been prepared as inventive examples 1-3.

TABLE 1
Sr. No. Material Details Inventive example 1
Wt.% Inventive example 2
Wt.% Inventive example 3
Wt.%
1 Polyepoxide Polymer
(1750- 2100 weight of resin per epoxy group) 28.52 19.14 19.14
2 Crosslinker
(Methylated high imino melamine) 5.22 3.50 3.50
3 Adhesion Promotor -1
(Fatty acid modified epoxy resin ester) 3.75 2.52 0.00
4 Adhesion Promotor -2
(Epoxy ester phosphate acid) 0.00 0.00 2.52
5 Adhesion Promotor -3
bifunctional organosilane 0.00 0.66 0.66
6 Catalyst
p-Toluenesulfonic acid 0.41 0.27 0.27
7 Levelling cum wetting agent
polyether modified polysiloxan 0.16 0.11 0.11
8 Pigment
0.81 0.00 0.00
9 Solvent -1
Dimethylbenzene 17.52 11.76 11.76
10 Solvent -2
2-Butoxyethanol
43.60 62.06 62.06

Table 1 provides the properties attained by the inventive examples. The coating composition in accordance to the present invention are observed to be corrosion resistant , quick drying ability, The coating compositions as per the present invention i.e., inventive examples 1, 2 and 3 gives good results in terms good corrosion resistant property of coated fins, quick dry time thus increases fins productivity, good fins surface finish & good fins formability.

Test 1: The coating thickness as measured above the substrate is the Dry Film Thickness (DFT). The present invention can be applied with a specific coating thickness such that the optimum anti-corrosive property is achieved within 20 seconds. This is possible as quick dry of the coating within 20 seconds is achieved by the coating composition of the present invention.

Test 2: PMT (Part Metal Temperature) is envisaged by the present invention to be in the optimum level. A high PMT will adversely affect the surface finish due to inducing change in the color of the coating. A lower PMT will result in uncured coating with poor anti-corrosive property. PMT also gives an indication as the hydrophobic nature of the coating.

Test 3: MEK Rub: The test consists of scratch the surface with a gauze soaked with methylethylketone until failure of the film occurs. The present invention achieves a high >50 value uniformly in the test. A lower value of MEK rub is reflective of insufficient cross linking. The values achieved by the coating composition of the present invention, results in high anti-corrosive property.

Test 4, 5, 6 and 7: The bend test cross hatch test, the Ericsson cupping Tape and the Impact test all relate to adhesion properties. Failure in any of these three tests is a result of insufficient adhesion.

Test 8: Copper Accelerated Acetic Acid Salt Spray Test is a corrosion test. Failure in the test is reflective of poor anti-corrosive property.

Test 9: Salt Spray Test: The test is conducted to test the corrosion resistance of a coating composition. It is an accelerated means of testing the ability of surface coatings to withstand atmospheric corrosion. The time that samples can resist against corrosion is the criterion used to understand test sample durability. The coating composition of the present invention achieves >2000 hrs anti corrosion in stringent conditions of the test.

Autoclave test:

Table -2
Sr. No. Property Standard Method Details Inventive example
1 Inventive example
2 Inventive example
3
1 DFT dry film thickness test (GSM) Gravimetry 0.75-1 0.75-1 0.75-1
2 PMT part metal temperature (12-20 Sec) 0C Exp 225-255 225-255 225-255
3 MEK Rub ASTM D4752
Solvent Resistant Rub Test ASTM D4752 >50 >50 >50
4 Bend Test – Tape Adhesion ASTM D4145 Pass Pass Pass
5 Cross hatch test – Tape Adhesion (visual) ASTM D3359 Pass Pass Pass
6 Ericsson Cupping- Tape adhesion ISO 20482 Pass Pass Pass
7 Impact Test - Tape adhesion ASTM E23 Pass Pass Pass
8 Accelerated CuSO4 Corrosion ASTM B117 Pass Pass Pass
9 Salt Spray Test ASTM D1193 ASTM D1193 > 2000 Hrs. > 2000 Hrs. > 2000 Hrs.
10. Observations No delamination
Autoclave test pass No delamination
Autoclave test pass No delamination
Autoclave test pass

The coating composition of the present invention has been found to have a combination of all the essential properties of a hydrophobic, quick dry, anti-corrosive coating composition.
Coating compositions with compositions that differed in the components as well as their wt% in the coating composition were considered to evaluation of the desired properties. The specific compositions of the comparative samples are reproduced in Table 3. The results of the evaluation are presented in Table 4.

Table 3

Table 4:

Inference from the results:
The results establish that the coating composition of the present invention are able to reflect all the desirable properties as required for coating a heat exchanger which is preferably made of aluminum or aluminum alloys.
Comparative study of the coating compositions with different compositions show that compositions that are beyond the scope of the claimed composition fail to complete the spray test in a significant manner whereas the coating composition of the present invention withstands more than 2000 hrs of the test thereby establishing its durability and resistance to corrosion.

The coating of the present invention simultaneously achieves the desired results in DFT test, PMT test and has a high MEK Rub value which indicates that the coating composition has a high level of cross linking which increase its anti-corrosive property.

Comparative composition example 1 (S-1):
The composition had only 1 adhesion promoter namely the fatty acid modified epoxy resin. Also, a different crosslinker was included. Evaluation of the coating composition properties were done. It was observed that S-1 did not achieve any significant corrosion resistant properties as the results for the Salt Spray Test and the Accelerated copper sulphate test were not significant enough to be compared. Also, S-1 clearly shows insufficient crosslinking as the MEK Rub test value achieved is very less.

Comparative composition examples 2 (S-2): S-2 incudes aminopropyltriethoxysilane as an adhesion promoter along with fatty acid modified epoxy ester resin. Evaluation of the properties of the coating composition was done. S-2 does not achieve requisite crosslinking as reflected from the low value obtained in the MEK Rub test. Also, no significant anti-corrosive properties were achieved. The surface finish was not acceptable due to white posts observed on the material in the form of pits.

Comparative composition example 3(S-3): S-3 composition has a high wt% of the polyepoxide polymer and a low wt% of crosslinker. Evaluation of the coating composition properties were done. It was observed that S-3 did not show any anti-corrosive properties as the Salt Spray Test and the Accelerated copper sulphate Test had insignificant results. Further, the surface finish was affected by delamination and occurrence of white spots.

Comparative composition example 4 (S-4): S-4 is very similar to the present invention. The composition comprises of the polyepoxide polymer as claimed in the present invention along however, has only two of the adhesion promoters namely fatty acid modified epoxy ester resin and a bifunctional organosilane as disclosed in the present invention. Evaluation of the coating composition properties were done. Anti-corrosive properties were found to be insignificant in the composition. Also, S-4 failed the autoclave test. Also, delamination was observed upto 10mm.

Comparative composition example 5 (S-5): S-5 has been prepared with a completely different polyepoxide polymer having the weight of resin per epoxy in the range of 2500-2800. Evaluation of the coating composition properties were done. Anti-corrosive properties were not comparable to that of the present invention as both the Salt Spray Test and the Accelerated copper sulphate test were failed. Further, the desired surface finish was not achieved as there was 5mm delamination from the bottom when punched.

The quick dry organic hydrophobic coating composition of the present invention is successful in achieving significant improvement in multiple properties such as the ability to dry quickly, being corrosion resistant for a long duration of time, having spotless surface finish and does not undergo delamination due to wear and tear.
, C , Claims:
1. A coating composition for a heat exchanger comprising:

A. At least one polyepoxide polymer in the range of 15-32 wt.%;
B. a crosslinker in the range of 3-7 wt. %;
C. one or more adhesion promoters in the range of 0-6wt%;
D. catalyst in the range of 0.1-0.5 wt. %;
E. Levelling cum wetting agent in the range of 0.1-0.5wt. %;
F. Pigment in the range of 0-1.0 wt. %;
G. Solvent mixture in the range of 50-85 wt. %, based on the weight of the total composition.

2. The coating composition for a heat exchanger as claimed in claim 1, wherein the polyepoxide polymer is a single polyepoxide polymer or a combination of one or more polyepoxide polymers with a combined weight of resin in the range of 1750-2100 per epoxy group.

3. The coating composition for a heat exchanger as claimed in claim 1, wherein the polyepoxide polymer is preferably in the range of 19-30 wt% of the total composition.

4. The coating composition for a heat exchanger as claimed in claim 1, wherein the polyepoxide polymer is selected from a group comprising high molecular weight bisphenol resin A.

5. The coating composition for a heat exchanger as claimed in claim 1, wherein the crosslinker is preferably in the range of 3.5 -5.5wt% of the total composition.

6. The coating composition for a heat exchanger as claimed in claim1, wherein said crosslinker is a methylated high imino melamine.

7. The coating composition for a heat exchanger as claimed in claim 1, wherein the adhesion promoters are selected from fatty acid modified epoxy esters, phosphate epoxy esters and bifunctional organosilanes or combinations thereof.

8. The coating composition for a heat exchanger as claimed in claim 1, wherein the adhesion promoters are in a range of 0-5 wt% of the total composition.

9. The coating composition for a heat exchanger as claimed in claim 1, wherein the solvent is a combination of two solvents.

10. The coating composition for a heat exchanger as claimed in claim 1, wherein the solvent is a combination of dimethyl benzene and 2-butoxyethanol.

11. A process of preparing a coating composition for a heat exchanger comprising the steps of
a. Mixing polyepoxide polymer with the solvent mixture of dimethyl benzene and 2-butoxyethanol and stirring at high speed to completely dissolve the polyepoxide polymer;
b. Adding a suitable amount of crosslinker to the product of (a) and mixing till a homogenous solution is obtained;
c. Mixing at least more than 1 adhesion promoters and stirring the same;
d. Adding a suitable catalyst to the solution obtained in step (c) and stirring the reaction mixture;
e. Adding at least one levelling cum wetting agent and a pigment to the product of step (d);
f. Finally adding a combination of solvents and stirring to obtain a quick dry corrosion resistant organic hydrophobic coating composition.
12. The process of preparing a coating composition as claimed in claim 11, wherein the stirring of the reaction mixture at each step takes place for a time period in the range of 10-20min at ambient temperature.

13. The process of preparing a coating composition as claimed in claim 11, wherein the step of mixing the polypeptide polymer in the solvent mixture is carried out in a sequential manner for a time in the range of 2-3 hrs.