Description:FIELD OF THE INVENTION
[001] The present invention relates to a coating composition for metal sheet or containers, more particularly to a quick drying corrosion resistant coating composition for aluminum or tin container. The present invention also relates to a process of preparing the coating composition and method of coating the same.

BACKGROUND OF THE INVENTION
[002] Aluminum has various applications due to its light weight, soft and malleable nature. Aluminum being infinitely recyclable material is important material of construction for various application. Aluminum is non-toxic, has a high thermal conductivity, has excellent corrosion resistance, 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.
[003] Aluminum 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. It is often used as an alloy because aluminum itself is not particularly strong. Alloys with copper, manganese, magnesium, and silicon are lightweight but strong.
[004] 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. Bare aluminum or its alloys in such severe condition can undergoes corrosion. Once corrosion started in aluminum heat exchanger can be corrected as fins are very closely spaced. This results in the reduction of the usable life of heat exchanger & associated equipment.
[005] Regular coatings available in market does not provide adequate corrosion protection, required quick drying time and surface finish.
[006] Owing to the foregoing problems, various coating compositions have been formulated which address the issues of coatings known in the art. Particularly, these compositions address the issues regarding the adequate adhesion, elongation, required quick dry time and surface finish. It is therefore an object of the invention to develop a quick dry organic coating, a process to prepare the coating and it’s application on aluminum for use on aluminum sheets for heat exchanger applications such as on Air Conditioner Fin.

SUMMARY OF THE INVENTION
[007] One aspect of the present invention provides a corrosion resistant coating composition comprising a polyester resin selected from saturated polyester resin, a crosslinker, an adhesion promotor, a catalyst, a levelling or a wetting agent, a pigment, and a solvent.
[008] Another aspect of the present invention relates to a process for preparing the corrosion resistant coating composition for metal sheet or container. The process for preparing the coating composition comprising the following steps: a) mixing of at least one polyester resin within a solvent and stirring at high speed to dissolve the resin in the solvent; b) stirring continuously until the polymer is completely dissolved to prepare a batch; c) adding at least one crosslinker, at least one adhesion promoter, at least one catalyst, at least one pigment wetting agent, at least one pigment and mixing for predetermined time before each addition to the batch; d) transferring the batch to bid mill and mixing in bid mill and then discharging the batch.
[009] Another aspect of the present invention relates to a method of coating metal sheet with the corrosion resistant coating composition of the present invention. The method of coating metal sheet, comprising the steps: a) degreasing of metal sheet with water and drying to remove the residual oil and smut on sheet; b) coating the metal sheet with coating composition as claimed in claims 1 to 10 using three roller applicator mechanism of appropriate thickness; c) drying the coated metal sheet in oven predetermined time; d) cooling the coated metal sheet to room temperature with chilled rolls to get final dried film coating on the metal sheet.
[010] Another aspect of the present invention relates to the use of the corrosion resistant coating composition for metal sheets or container, preferably on aluminum sheets for heat exchanger application.

BRIEF DESCRIPTION OF THE DRAWINGS
[010] Reference will be made to embodiments of the invention, examples of which may be illustrated in accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in the context of these embodiments, it should be understood that it is not intended to limit the scope of the invention to these particular embodiments.
Figure 1 demonstrates tube fin heat exchanger.
Figure 2 demonstrates salt spray results after 1000 Hrs on coated aluminum sheet having drying time of coating 20 seconds @ 235 oC. It can be seen no corrosion is observed.
Figure 3 demonstrates salt spray results after 200 Hrs on bare aluminum sheet.

DETAILED DESCRIPTION OF THE INVENTION
[011] The present invention discloses a corrosion resistant coating composition comprising a polyester resin selected from saturated polyester resin, a crosslinker, an adhesion promotor, a catalyst, a levelling or a wetting agent, a pigment, and a solvent.
[012] In an embodiment the “composition” are formulations which are prepared as mixture, amalgam or blend for coating on a metal surface. The composition includes a homogenous or heterogeneous solution and similar formulations known to a person skilled in the art.
[013] In an embodiment the “Polyester resins” are preferably selected from synthetic resins formed by the reaction of dibasic organic acids and polyhydric alcohols which diacid functionality in unsaturated polyester resins. Polyester resins are cross-linked with a wide range of co-reactants (crosslinkers) to get desired properties.
[014] In a preferred embodiment, the polyester resins preferably have OH value between 60 to 80 (mg KOH/gm) and viscosity between 70 to 90 Poise.
[015] In another preferred embodiment, the polyester resins are saturated polyester resin selected from the group consisting of, but not limited to, VOY8040, VOY8239, VOY8008, SYNOLAC153S-60, ETROPOL1305, ESTROPOL1304 or combination thereof.
[016] In another preferred embodiment, the polyester resins are selected from VOY8040, VOY8239, VOY8008, SYNOLAC153S-60, ETROPOL1305, ESTROPOL1304
[017] In another preferred embodiment, the coating composition comprises 70 to 80 % wt/wt of the polyester resins based on the total weight of the coating composition.
[018] In an embodiment the “crosslinker” acts as a co-reactant of the Polyester resins to get desired properties and preferably selected from amine based crosslinkers preferably selected from methylated melamine or a methylated high imino melamine.
[019] In another preferred embodiment, the amine based crosslinkers methylated melamine are selected from the group consisting of, but not limited to, 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 or combination thereof.
[020] In another preferred embodiment, the methylated high imino melamine are selected from the group consisting of, but not limited to, CYMEL 325, CYMEL 323, CYMEL 327, DooriChem MM-825, DooriChem MM-8257 DooriChem MM-823 or combination thereof.
[021] In an embodiment the “Adhesion promoter” acts as an anchoring agent for cured/ dried film with metal sheet or container which results in excellent adhesion properties in severe conditions. The adhesion promoter is selected from fatty acid modified epoxy resin ester and solution of polyester alkyl ammonium salt or combination thereof.
[022] In another preferred embodiment, the fatty acid modified epoxy resin ester are selected from the group consisting of, but not limited to, FTC- PAP 110, RESYDROL AX 250w/75EP, Lapox ARES-101, Lapox ARES-102, FTC DEE-100 or combination thereof.
[023] In another preferred embodiment, the coating composition comprises 0.5 to 1.5 % wt/wt of adhesion promoters based on the total weight of the coating composition.
[024] As used herein the wetting cum levelling agent is selected from polyether modified polysiloxan preferably selected from a group consisting of, but not limited to, BYK® -348, Borchi® Gol LA 2, Additol 6503N, BYK 110, CFC-5100 or combination thereof.
[025] In an embodiment the “Catalyst” is selected from phosphoric acid derivate preferably selected from a group consisting of, but not limited to, CYCAT® 4045, CYCAT® 4107, CYCAT® 4786, CYCAT® VXK 6395 or p-Toluenesulfonic acid or combination thereof.
[026] In another preferred embodiment, the catalyst is preferably incorporated in 0.5 to 1.0 % wt/wt of the coating composition.
[027] In an embodiment the “Pigment” provides colour, ensure a suitable surface finish and improved corrosion protection and preferably selected from Zinc Chromate or Phthalocyanine blue or combination thereof.
[028] In an embodiment of the present invention the coating composition comprises a solvent selected from 2-Butoxyethanol.
[029] In another preferred embodiment, the coating composition comprises 50 to 55 % wt/wt of solvent based on the total weight of the coating composition.
[030] In another aspect, the present invention discloses a process for preparing the coating composition. The process comprising the following steps:
a) mixing of at least one polyester resin within a solvent and stirring at high speed to dissolve the resin in the solvent;
b) stirring continuously until polymer is completely dissolved to prepare a batch;
c) adding at least one crosslinker, at least one adhesion promoter, at least one catalyst, at least one pigment wetting agent, at least one pigment and mixing for predetermined time preferably at least 10 mins before each addition to the batch;
d) transferring the batch to bid mill and mixing in bid mill for 2 Hrs and then discharging the batch.

[031] In a preferred embodiment, the process comprising the following steps:
a) mixing of 70 to 80 wt/wt % of at least one polyester resin with 10 to 15 wt/wt % of solvent and stirring at high speed to dissolve the polymer in solvent;
b) stir continuously until polymer is completely dissolved to prepare a batch
c) adding to the batch 15 to 20 wt/wt % of at least one crosslinker and mixing for 10 min;
d) adding to the batch 0.5 to 1.5 wt/wt % of at least one adhesion promoter and mixing for 10 min.
e) adding to the batch 0.5 to 1.0 wt/wt % of at one catalyst and mixing for 10 min;
f) adding to the batch 0.1 to 0.2 wt/wt % of at least one pigment wetting agent and mixing for 10 min;
g) adding to the batch 0 to 1 wt/wt % of at least one pigment and mixing for 10 min;
h) transferring the batch to bid mill and mixing in bid mill for 2 Hrs and then discharging the batch.

[032] In another aspect, the present invention discloses a method of coating metal sheet or container with the corrosion resistant coating composition of the present invention. Typically, the method of coating comprises the following steps:
a) degreasing of metal sheet with water and drying to remove the residual oil and smut on sheet;
b) coating the metal sheet with coating composition of the present invention using three roller applicator mechanism of appropriate thickness;
c) drying the coated metal sheet in oven for predetermined time preferably less than 15 sec;
d) cooling the coated metal sheet to room temperature with chilled rolls to get final dried film coating on the metal sheet.
[033] In a preferred embodiment, the the metal sheet or container is selected from aluminum or tin.
[034] In a preferred embodiment, the final dried film coating has a thickness of 0.5 to 1.5 GSM.
[035] In a preferred embodiment, the drying in oven is between 200 to 300 oC, preferably between 225 to 265 oC.
[036] As used herein the degreasing and drying of metal sheet is preferably done in 4% sulfuric acid solution at 45 oC. Metal sheet is then preferably rinsed with demineralized water, and preferably dried in air drying oven from 125 to150 oC.
[037] As used herein “three roller applicator” refers to three roll semi reverse and three roll full reverse applicators.
[038] As used herein “chilled rolls” refer to hollow rolls through which refringent is passed. Chilled rolls act as heat exchanger and reduce the coated metal sheet temperature to ambient conditions before coiling.
[039] Advantageously, the coating composition of the present invention exhibits adequate corrosion protection (an anti-corrosive property of > 1000 Salt Spray) and achieves the required quick drying time (<15 sec@280 oC) as well as provides the desired surface finish. Accordingly, the coating compositions are particularly useful on aluminum sheets for heat exchanger application.
EXAMPLE
[040] The following examples are illustrative of the invention but not limitative of the scope thereof.
[041] Materials and Methods
[042] Materials: VOY8040, Cymel 325, FTC- PAP 110, p-Toluenesulfonic acid, BYK 110, Zinc Chromate, Phthalocyanine blue and 2-Butoxyethanol.
[043] Method: Mixing of 70 to 80 wt/wt% of polyester resin (VOY8040) with 10 to 15 wt/wt% of 2-Butoxyethanol and stirring the solution at high speed to dissolve the polymer in solvent. Stir continuously until polymer is completely dissolved. Adding 15 to 20 wt/wt% of Cymel 325 and mixing for 10 minutes. Adding 0.5 to 1.5 wt/wt% of FTC- PAP 110 adhesion promoter and mixing for 10 minutes. Adding 0.5 to 1.0 wt/wt% of p-Toluenesulfonic acid and mixing for 10 minutes. Adding 0.1 to 0.2 wt/wt% of BYK 110 and mixing for 10 min. Adding 0 to 1 wt. % of mixture of Zinc Chromate and Phthalocyanine blue (1:1) and mixing for 10 min. Transferring the batch to bid mill and mixing in bid mill for 2 Hrs then discharge the batch. The composition of the examples is provided in Table 1 as follows:
Table 1
Examples
Sr. No Material Details Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7
1 VOY8040 80.5 80.0 78.0 70.0 70.0 70.0 70.0
2 Cymel 325 17.5 17.5 20.0 15.0 15.0 15.0 12.0
3 Component FTC- PAP 110 1.0 1.50 1.0 0.5 0.5 0.0 0.5
4 p-Toluenesulfonic acid 1.0 1.00 1.0 0.5 0.0 0.5 0.5
5 BYK 110 0.0 0.00 0.00 0.2 0.2 0.2 0.2
6 Zinc Chromate and Phthalocyanine blue (1:1) 0.0 0.00 0.00 1.0 1.0 1.0 1.0
7 2-Butoxyethanol 0.0 0.00 0.00 12.8 13.3 13.3 15.8

[044] The coating properties of the composition prepared according to present invention demonstrate adequate adhesion, elongation, demonstrates the required quick drying time and provides the desired surface finish and corrosion resistance as shown in Table 2.
Table 2
Coating Properties /Results
Sr. No. Properties Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7
1 Appearance Clear Colourless uniform coating Clear greenish blue uniform coating
2 MEK Rub >100 >100 >100 >100 >50 >100 >50
3 Bend Test after 180-degree bend test Pass Pass Pass Pass Pass Pass Pass
4 Cross hatch test Pass Pass Pass Pass Pass Pass Pass
5 Coating surface strains in Erichsen cupping tests >6 Pass >6 Pass >6 Pass >6 Pass >6 Pass >6 Pass >6 Pass
6 Impact Test Pass Pass Pass Pass Pass Pass Pass
7 Accelerated CuSO4 Corrosion Pass Pass Pass Pass Fail Fail Fail
8 Anti Corrosion test (Salt Spay test) Pass 1000 Hrs Pass 1000 Hrs Pass 1000 Hrs Pass 1000 Hrs Fail Fail Fail

[045] As demonstrated above, the coating compositions provides the required corrosion resistance even after 1000 Hrs on coated aluminum sheet. As demonstrated in Figure 2 salt spray results after 1000 Hrs on coated aluminum sheet having drying time of coating 20 seconds @ 235 oC no corrosion is observed. In contrast, salt spray results after 200 Hrs. on bare aluminum sheet did not provide adequate corrosion resistance (see Figure 3).
[046] Accordingly, the coating compositions are particularly useful for preparing a quick drying corrosion resistant coating for application on a metal sheet or container, preferably an aluminum sheets for heat exchanger applications such as on Air Conditioner Fin.
[047] The foregoing description of the invention has been set merely to illustrate the invention and is not intended to be limiting. While the present invention has been described with respect to certain embodiments, it will be apparent to those skilled in the art that various changes and modification may be made without departing from the scope of the invention as defined in the following claims. , Claims:We Claim:
1. A coating composition comprising:
- a polyester resin selected from saturated polyester resin,
- a crosslinker,
- an adhesion promotor,
- a catalyst,
- a levelling or a wetting agent,
- a pigment, and
- a solvent.
2. The coating composition as claimed in claim 1, wherein the composition comprises:
a) Polyester resin : 70 - 80 parts
b) Cross linker : 15 – 20 parts
c) Adhesion promotor : 0.5 – 1.5 parts
d) Catalyst : 0.5 - 1.0 parts
e) Levelling/Wetting Agent : 0.1 - 0.2 parts
f) Pigment : 0 - 1 parts
g) Solvent : 10 - 15 parts.
wherein the polyester resin is a saturated polyester resin.
3. The coating composition as claimed in claim 1, wherein the polyester resin has OH value between 60 to 80 (mg KOH/gm) and viscosity between 70 to 90 Poise.
4. The coating composition as claimed in claim 1, wherein the cross-linker is an amine based crosslinker.
5. The coating composition as claimed in claim 4, wherein the amine based crosslinker is methylated melamine.
6. The coating composition as claimed in claim 1, wherein the adhesion promoter is fatty acid modified epoxy resin ester or polyester alkyl ammonium salt or combination thereof.
7. The coating composition as claimed in claim 1, wherein the catalyst is selected from phosphoric acid derivative.
8. The coating composition as claimed in claim 1, wherein the wetting or levelling agent is selected from polyether modified polysiloxan.
9. The coating composition as claimed in claim 1, wherein the solvent is 2-Butoxyethanol.
10. The coating composition as claimed in claim 1, wherein the pigment is selected from Zinc Chromate and Phthalocyanine blue or combination thereof.
11. A process for preparing the coating composition as claimed in claims 1 to 10, comprising the following steps:
a) mixing of at least one polyester resin within a solvent and stirring at high speed to dissolve the resin in the solvent;
b) stirring continuously until the polymer is completely dissolved to prepare a batch;
c) adding at least one crosslinker, at least one adhesion promoter, at least one catalyst, at least one pigment wetting agent, at least one pigment and mixing for predetermined time before each addition to the batch;
d) transferring the batch to bid mill and mixing in bid mill and then discharging the batch.

12. A method of coating metal sheet, comprising the following steps:
a) degreasing of metal sheet with water and drying to remove the residual oil and smut on sheet;
b) coating the metal sheet with coating composition as claimed in claims 1 to 10 using three roller applicator mechanism of appropriate thickness;
c) drying the coated metal sheet in oven predetermined time;
d) cooling the coated metal sheet to room temperature with chilled rolls to get final dried film coating on the metal sheet.
13. The method as claimed in claim 12, wherein the metal sheet is selected from aluminum or tin.
14. The method as claimed in claim 12, wherein the final dried film coating has a thickness of 0.5-1.5 GSM.
15. The method as claimed in claim 12, wherein the drying in oven is between 225 to 265 oC.
16. Use of the coating composition for metal sheets or container, preferably on aluminum sheets for heat exchanger application.
Dated 02nd of February 2024
Hindalco Industries Limited
By their Agent & Attorney

(Nisha Austine)
of Khaitan & Co
Reg No IN/PA-1390