The invention relates to a direct to metal coating composition comprising acrylic polyols and isocyanate-functional cross-linkers. More particularly, the coating composition is for direct application on metal surfaces. The invention further relates to kit of parts for preparation of the coating composition and to a method of applying the coating composition.
BACKGROUND OF THE INVENTION
The widely diverse coating field has developed a huge variety of compositions that can be applied to the surface of various metal structures to prevent corrosion and extend the service life of the substrates.
In today’s time the transportation business demands a longer lifetime and reliable service without interruptions. Limited corrosion resistance is one of the main factors determining the lifetime of a transportation means like bus. To achieve the best anticorrosive property the bus manufacturing companies are now using Al and galvanized iron (GI) substrates. The aluminium and galvanized iron (GI) substrates have poor adhesion. To overcome this problem the best practice followed is the application of 2K epoxy primer system and then 2K PU system. The low molecular bis-phenol epoxy primer system with amine curing technology is used to achieve better adhesion and anti-corrosive property. Topcoat is comprised of an acrylic polyol and isocyanate system to meet the desired aesthetic and weathering properties.
The main reason for the primer application until 1990 was to improve corrosion resistance, appearance, and chipping resistance. Most of the primers in bus body applications developed are solvent-borne. Today’s primers have to improve adhesion between the substrate and topcoat, provide chipping protection, enhance paint appearance, and, also, be environmentally compatible with emission regulations. The primer fills and smoothens minor imperfections and scratches that may be created during prior steps and by the intermediate sanding or grinding used to remove small imperfections. Hence, although the main purpose of the primer has been to act as a leveler that produces a smoother finished surface, it also provides additional protection against corrosion and maximizes adhesion with the metal substrate; thereby, the primer increases paint durability. At the same time, good adhesion of the topcoat to the primer is required to ensure minimal detraction from the visual appearance if chipping has occurred. The film thickness of the primer layer is 20 -30 µm. Robot primer painting of the exterior is accomplished after the interior priming is completed, typically using rotating bell applicators, to form a film of 25–40 µm in thickness. Subsequently, the primer is cured by baking the bus body in an oven at 60- 650C for 30-35 min.
After the mild sanding of the primer surface, the final step in the body coating process is to apply the topcoat. The topcoat contains the primary coloring pigment provides a protective coating against environmental effects, UV light degradation, and provides a smooth, unblemished, even finish. Two coats are applied including a break of short flash-off time in between. The total system is cured inside the baking oven. Typical thermal cure times and temperatures are 30 to 35 min at 60-650C respectively.
The conventional primer- topcoat system for bus body painting suffers from disadvantages like: associated with higher amount of total volatile organic content; time-consuming application procedure for two coats of primer and two coats of topcoat; more energy consumption due to two times baking process also sometimes inter-coat adhesion failure is observed between primer and topcoat.
Hence, there is a need to develop a coating system for heavy and light duty applications, commercial vehicles, and original equipment manufacturers (OEM) having advantages of reduced number of application steps thus lower application costs, time saving, energy efficient and requiring minimal surface preparation. The present disclosure is for a direct to metal coating composition that can be directly applied on the metal body with excellent adhesion, anticorrosive, water resistance and electric resistance property.
OBJECT OF THE INVENTION
An object of the present invention is to provide an improved coating composition which can be directly applied to metal surfaces.
It is a further object of the present invention to provide a direct to metal coating composition which requires minimal or no surface preparation.
It is yet another object of the present invention to provide a direct to metal coating composition with reduced number of application steps, time saving and energy efficient.
It is yet another object of the present invention to provide a solvent-borne direct to metal coating composition with 30 – 50% improvement in productivity while matching performance of epoxy systems on different metal substrates along with superior aesthetic and anticorrosive, weathering properties.
It is yet another object of the present invention to provide a solvent-borne direct to metal coating composition for bus body application.
This object and additional objects not expressly stated will become apparent upon review of the remaining disclosure.
SUMMARY OF THE INVENTION
The present invention relates to a direct to metal coating composition comprising: (a) a base coat composition, wherein the base coat composition comprises a film-forming acrylic polyol oligomer or polymer present in arrange from 25% to 65% by wt., more preferably 30% to 60% by wt. and most preferably 30% to 55% of the base coating composition and (b) a hardener composition, wherein hardener composition comprises a cross-linking agent having isocyanate functional groups present in a range from 20% to 30% by wt. of the total coating composition.
The acrylic polyols in the base coat composition are selected from the group comprising of: methyl acrylate, methyl methacrylate, acrylic acid, meth acrylic acid, hydroxyl cyclohexyl methacrylate, hydroxyethyl acrylate, and hydroxybutyl acrylate. The acrylic polyols of the present invention has number average molecular weight in range from 4000 to 10,000.
The hardener composition comprises of isocyanates selected from the group comprising of hexamethylene diisocyanate (HDI), or isocyanurates of HDI, the biurets of HDI, and or the mixtures of isocyanurates and biurets of HDI.
The direct to metal coating composition comprising base coat composition and hardener composition further comprises solvents, pigments and additives.
The solvent is selected from the group comprising of polar and non-polar solvent like n-heptane, toluene, xylene, petroleum compounds, n-hexane, isopropanol, 2-butoxy ethanol, n-butyl alcohol, ethyl acetate, n-butyl acetate, isobutyl acetate, ethylene glycol monoethyl ether acetate, dibasic ester, 3-methoxybutyl acetate, ethyl cellosolve acetate, butyl cellosolve acetate amyl acetate, butyl glycol acetate, methyl ethyl ketone and methyl isobutyl ketone. Most preferably, the solvent is a mixture of butyl acetate, butyl cellosolve acetate, long chain hydrocarbon solvents and ethyl cellosolve acetate. Range of total solvent mixture varies from 10 to 23% by wt. is used per 100% by weight of the base coating and hardener composition.
The pigment in the direct to metal coating composition comprises of spherical shaped pigments, platy pigment, colorant, corrosion inhibitor, anti-sag additive, antifoaming agents, dispersants, surfactants, pot-life extenders, UV stabilizers, adhesion promoters, wetting agents, rheology modifiers, levelling agents, anti-blocking agents, thickeners, thixotropic agents, drying agents, anti-settling agents, and levelling agents.
Spherical shaped pigment are selected from the group comprising of titanium oxide or other spherical extenders like barium sulphate, amorphous silica present in a range from 2% to 25% by wt. of the base coat composition. Platy pigments are selected from the group comprising of fillers talc, mica present in a range from 1.5% to 2% by wt. of the base coat composition. Mixture of different grades of titanium dioxide was used in the base composition for better aesthetic and adhesion property.
Colorant selected from the group comprising of titanium dioxide white, carbon black, lampblack, black iron oxide, red iron oxide, transparent red oxide, yellow iron oxide, middle chrome, transparent yellow oxide, brown iron oxide, phthalocyanine green, phthalocyanine blue, naphthol red, quinacridone red, quinacridone magenta, quinacridone violet, organic yellow, and any combination thereof and present in a range from 2 to 25% by wt. of the base coat composition, depending upon the shade.
Corrosion inhibitor pigment is selected from the group comprising of calcium ion-exchanged amorphous silica pigments, phosphosilicate zinc and Zinc free system, Oxy-Ammonium phosphate of Mg present in a range from 2% to 5% by wt. of the base coating composition. Anti-sag additive fumed silica present in a range from 0.7% to 1% by wt. of the base coat composition.
The present invention also discloses a method of coating a metal substrate comprising the steps:
- The base coat is first mixed with a hardener composition wherein the hardener composition comprises isocyanate present in an amount of 80% by wt. of the hardener composition and a thinner.
- Then the total coating composition is applied directly to the metal substrate by spray application.
The base coat composition comprising acrylic polyol reacts with isocyanate groups of hardener and forms a paint film on the metal surfaces. The painted substrate was cured in an oven at temperatures 65° C for 35 minutes.
In the method of coating the base coat composition and hardener composition are used in 4:1 in volume ratio.
The thinner is a mixture of solvents butyl acetate, xylene, long chain hydrocarbon solvent like C9 solvent, C10/ solvesso 100 ) and ethyl cellosolve acetate.
The combination of base coat, hardener and thinner provides higher value electrical resistance for electrostatic application.
DESCRIPTION OF THE INVENTION
The invention according to its various aspects is particularly pointed out and distinctly claimed in the appended claims read in view of this specification and appropriate equivalents.
It is to be noted, as used in the specification and the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a composition containing “a compound includes a mixture of two or more compounds. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise. The expression of various quantities in the terms of “% w/w” or “%” means the percentage by weight, relative to the weight of the total composition unless otherwise specified.
This invention is directed to a direct to metal coating compositions which may be directly applied to metal composition surfaces. The coating composition is usually applied in a single coat i.e. monocoat. The coating composition comprises an acrylic polyol- Isocyanate system to improve productivity, while matching the performance characteristics of epoxy systems like adhesion, water resistance, humidity resistance, exterior weathering durability, and corrosion resistance on different metal substrates as galvanized steel and aluminum and FRP substrates for bus body.
As used herein “galvanized iron or steel” as used herein refers to iron or steel coated with zinc.
As used herein “metal” as used herein refers to metals such as aluminium, manganese, nickel, chromium, molybdenum, vanadium, tungsten and cobalt.
As used herein “steel” as used herein refers includes, but is not limited to, cold rolled steel, alloys of iron with carbon or metals such as manganese, nickel, copper, chromium, molybdenum, vanadium, tungsten and cobalt.
As used herein, the term “corrosion inhibiting pigments” refers to particles which, when included in a coating composition that is deposited upon a substrate, act to provide a coating that resists or, in some cases, even prevents, the alteration or degradation of the substrate, such as by a chemical or electrochemical oxidizing process, including rust in iron containing substrates and degradative oxides in aluminium substrates.
As used herein “two-component” refers to the number of solutions and/or dispersions, which are mixed together to provide a curable coating composition. Up to the point of mixing, neither of the individual components alone provides such a curable coating composition.
As used herein, the term “curing” means subjecting to conditions effective for chemically transforming or chemically transforming under such conditions.
As used herein, the term “curing temperature” means a degree of hotness or coldness effective for chemically transforming the invention ambient temperature curable composition to the invention cross-linked polyurethane.
The current invention provides a formulation comprising a two-component coating composition containing: (a) a film-forming component comprising acrylic polyol system named as a base and (b) a hardener composition, wherein hardener composition comprises a cross-linking agent having isocyanate functional groups along with a silane additive which provides better adhesion between paint film and a metal substrate. A thinner is used to achieve the desired performance properties like drying property of wet paint film, levelling property and aesthetic look of cured paint film. The combination of the composition of base, hardener and thinner provides superior bonding to metal substrate, water resistance, humidity resistance, exterior weathering durability and higher electric resistance property.
The acrylic polyol resin is the main resin in the base coat composition of the present invention. The acrylic polyols resin used in the present invention is Macrynal acrylic polyols from Allnex. The acrylic polyol resin is preferably having a number average molecular weight of 4000 to 10,000. Acrylic polymers and oligomers suitable for use in present invention are a combination of acrylic acid, meth acrylic acid, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, butyl acrylate, Butyl methacrylate, isobutyl acrylate, isobutyl methacrylate, hexyl acrylate, hexyl methacrylate and the like, and it may also include hydroxyl derivatives of acrylic polymers and oligomers like hydroxyl cyclohexyl methacrylate , hydroxyethyl acrylate, hydroxyl propyl acrylate, and hydroxybutyl acrylate.
The acrylic polyol resin is used for improving the physical properties of the coating film such as the smoothness and gloss of the coating film, the moisture resistance and the adhesion of the material, and is preferably contained in 25 to 65% by weight of the base coating composition, more preferably from 30 to 60% by weight of the base coating composition and most preferably from 30% to 55% by weight of the base coating composition.
The cross-linking agent preferably used for curing the acrylic polyol resin is a polyisocyanate which can react with a hydroxyl group in the resins and can be cured at room temperature or hardened. Polyisocyanate suitable for use in present invention are selected from the group comprising of lysine diisocyanate, hexamethylene diisocyanate, trimethylhexane diisocyanate, and like aliphatic diisocyanate compounds; hydrogenated xylylene diisocyanate, isophorone diisocyanate, methylcyclohexane-2,4-diisocyanate, methylcyclohexane-2,6-diisocyanate, 4,4'-methylene bis(cyclohexylisocyanate), 1,3-(isocyanatomethyl)cyclohexane, and like alicyclic diisocyanate compounds; tolylene diisocyanate, xylylene diisocyanate, diphenylmethane diisocyanate, and like aromatic diisocyanate compounds; organic polyisocyanates, such as lysine triisocyanate and like tri- or higher polyisocyanates; and cyclopolymers (e.g., isocyanurate), isocyanurates of HDI, biuret adducts of HDI, etc., of such organic polyisocyanates. Such polyisocyanate compounds can be used singly, or in a combination of two or more. The coating composition comprises isocyanates in a range from 20% to 30% of by wt. of the total coating composition. Preferably, the isocyanate used in the present invention is hexamethylene diisocyanate.
The solvent used in the present invention is used for maintaining the thickness of a coating film and improving the workability of coating, and comprises of any one or more of polar and non-polar solvent such as n-heptane, toluene, xylene, petroleum compounds, n-hexane, isopropanol, 2-butoxy ethanol, n-butyl alcohol, ethyl acetate, n-butyl acetate, isobutyl acetate, ethylene glycol monoethyl ether acetate, dibasic ester, 3-methoxybutyl acetate, ethyl cellosolve acetate, butyl cellosolve acetate amyl acetate, butyl glycol acetate, methyl ethyl ketone and methyl isobutyl ketone. A mixture of the nonpolar solvent and the polar solvent may be appropriately selected depending on the characteristics of the resin and the rate of volatilization. There is no particular limitation on the amount of the solvent to be used, but if the non-polar solvent is used excessively, there is a problem of low adhesion and durability deterioration in the whole physical properties if it is used in too small amount. Preferably, 10-23% by weight of the total solvent is used per 100% by weight of the base coating and hardener composition.
The direct to metal coating composition may comprise pigment combination comprising: (a) at least one substantially spherical pigment; (b) at least one plate shaped pigment. Two or more pigments having different pigment morphologies are incorporated to achieve the desired solid level along with anti-sag property. These pigments can be colorant pigments, filler pigments, extender pigments, or combinations of them. The pigment particles with different morphologies enable high loading and exhibit physical compatibility and a closer space-packing effect. Combinations of different extenders having different morphologies, such as spherical, acicular, and platy pigments, allows for increased levels of pigment and improved packing of the pigment, making the coating less porous, and in turn improving barrier qualities such as water resistance. Pigments like calcium carbonates, calcium phosphosilicates can fill in the voids created by spherical pigments (like barium sulfate and TiO2). Plate shaped pigments like fine talc and mica powder have a high aspect ratio, and for this reason, such pigments are believed to impart anti-sag properties also. In some cases, platy pigments are one of the causes of adhesion improvement.
The direct to metal coating composition comprises about 2 % to 30 wt. % of at least one spherical shaped pigment particles (such as titanium oxide, barium sulphate), most preferably 2-25 % by weight depending upon the shade of the topcoat. Platy filler pigment particles (mica or talc) are present in a range from 2% - 5 % by weight, most preferably 1.5% - 2 % by weight of platy pigments, depending upon the desired gloss level of the final cured film.
Optionally, the direct to metal coating composition may further comprise from 2% to 30% by weight, most preferably from 2% to 25% by weight of at least one colorant depending upon the desired shade. The colorants may be of organic or inorganic origin. Preferably, the colorants are inorganic coloured pigments. Suitable examples of the colorants are selected from titanium dioxide white, carbon black, lampblack, black iron oxide, red iron oxide, transparent red oxide, yellow iron oxide, transparent yellow oxide, middle chreom, brown iron oxide, phthalocyanine green, phthalocyanine blue, naphthol red, quinacridone red, quinacridone magenta, quinacridone violet, organic yellow, and any combination thereof.
In certain embodiments, the present invention is directed to coating compositions that comprise at least one corrosion inhibiting pigments which acts to inhibit or reduce corrosion, such as a non-toxic environmentally-friendly micronized cation-containing pigments such as calcium ion-exchanged amorphous silica pigments. Phosphosilicate chemistry with zinc and zinc free system, both system are suitable for use in the present invention, most preferably Calcium strontium zinc phosphosilicate or Oxy-Ammonium phosphate of Mg with their required dosage, preferably 2- 10% by weight, more preferably 2 - 5 % by weight of the coating composition. The corrosion inhibiting pigments also helps on increasing film strength, prevent rust creepage and under film corrosion at defected areas, retards process of corrosion and provides cathodic and/or anodic passivation of the metal surface.
In certain embodiments, the direct to metal coating composition of the present invention may comprise fumed silica, also known as pyrogenic silica. Fumed silica consists of microscopic droplets of amorphous silica fused into branched, chainlike, three-dimensional secondary particles which then agglomerate into tertiary particles. The resulting powder has an extremely low bulk density and high surface area. Its three-dimensional structure results in viscosity increasing, thixotropic behaviour. Fumed silica is used as anti-sag additive in this composition. The most preferable dosage of the anti-sagging additive is 0.1% to 5% by weight, preferably from 0.5% to 3% by weight, and more preferably from 0.7% to 1% by weight of the coating composition.
In certain embodiments, the coating composition may comprise further additional additives. The additional additives can be added at any suitable point during mixing operations of all the ingredients in base coat composition. Additives that are included in a coating composition include any one or more of antifoaming agents, dispersants, surfactants, pot-life extenders, UV stabilizers, adhesion promoters, wetting agents, rheology modifiers, levelling agents, anti-blocking agents, thickeners, thixotropic agents, drying agents, anti-settling agents, and corrosion inhibitors. When used, such additives may be present in any amount suitable for their intended purposes. The additives are used according to their dosage ranges from 0.2 to 5% by wt. of the coating composition. Any such additives known in the art can be used, provided there is no compatibility problems.
The method of coating a metal substrate with the direct to metal coating composition of the present invention comprises of following steps:
- The base coat is first mixed with a hardener composition wherein the hardener composition comprises isocyanate present in an amount of 80% by wt. of the hardener composition and a suitable DTM thinner.
- Then the total coating composition was applied directly to the metal substrate by spray application.
The base coat composition comprising acrylic polyol reacts with isocyanate groups of hardener and forms a paint film on the metal surfaces. The painted substrate is cured in an oven at temperatures 65° C for 35 minutes.
In the method of coating the base coat composition and hardener composition are used in 4:1 in volume ratio.
The thinner is added according to desired application viscosity for bus body application most preferably 20- 25 seconds.
The coatings of the present invention may be applied after minimal surface preparation of the surface of bus with only one application step. The coating composition of this invention can be applied directly over the surface of a metal substrate, such as sand, non-sanded galvanized steel, blasted cold rolled steel, sanded or non –sanded mild steel, aluminium, or treated metals. Various application techniques such as conventional spray, air-assisted airless or electrostatic spraying can be used for coatings applications. Since the coating composition has both the anti-corrosive properties of a primer coating and the weatherability of a topcoat, all within a single formulation, it can be applied directly to metal in the absence of a primer coat. Once a first coating has been formed on the metal surface, the coating composition may be re-applied to form a second coating layer. Thickness of the first coating is in range from 20 to 30 µ and the second coating thickness is in range from 40 to 50 µ. Total thickness of the coating is in range from 60 to 70 µ. The flash-off time is 10-12 minutes. The coating composition is then allowed to cure to form a thermoset paint film.
In order to further illustrate the invention and the advantages thereof, the following non-limiting examples are given.
Example 1
Method of formation of base coating composition
The whole mixing process for base coat composition is carried out in a Bead mill. The process comprises of following steps:
1. Before starting the batch it is ensured that the Bead Mill is cleaned and the cooling water is circulated properly to cool the bead mill.
2. The acrylic polyol resin is added into the Bead Mill. The Mill is started at slow speed first, after that polymeric dispersing agent, anti-sagging additives, pigment powders, micronized talc, solvents are added one by one under stirring. At this step the temperature of the materials should not be exceed 50°C during process.
3. After loading the above materials the rotation in bead mill is at full RPM (RPM 800-1200). Mixing in the bead mill is carried out until dispersion is less than 15 microns. Dispersion size is checked on Hegman Gauge.
4. When the dispersion is less than 10 micron, at medium speed (RPM 300-550), the materials are unloaded and transfer to high speed mixer.
5. Then silicone polymer leveling agent, Tin catalyst, UV Stabilizer, UV absorber and corrosion inhibitor were added into the mixer in the speed range 300-400 RPM.
6. After 30 minutes of mixing the materials are unloaded from the mixer.
The final mixture is the base coating composition.
Example 2
Different formulations of the base coating composition
Different coating compositions for different colors are prepared by method disclosed in Example 1
Table 1: Yellow base formulation 1
Yellow base formulation-1
Ingredients Description Trade name/ Chemical name % Weight
Resin Acrylic Polyol Macrynal polyols 55.08
Pigment Titanium dioxide 1 Crystal 128 1.94
Titanium dioxide 2 SR 2377 1.5
Yellow Pigment Middle chrome 1340 19.66
Micronized talc GT 10 1.5
Spherical Barium sulfate Barytes 2
Additives Anti-sag/ anti settling additive Aerosil R 972 0.72
Polymeric dispersing agent 1 BYK P 104 1
Polymeric dispersing agent 2 BYK 163 3.6
Silicone polymer leveling agent 1 Byk 306 0.25
silicone polymer leveling agent 2 Byk 399 0.25
Tin catalyst DBTDL (Dibutyltin dilaurate) 1
UV Stabilizer Tinuvin 292 0.75
UV absorber Thasorb 384 0.25
Solvent Medium Polar solvent 1 Butyl Acetate 3.5
Long chain hydrocarbon solvent 1 C9 solvent 2
Medium Polar solvent 2 Ethyl cellosolve acetate 2
Medium Polar solvent 3 Butyl cellosolve acetate 1
Long chain hydrocarbon solvent 2 C10 solvent 2
Total 100

Table 2: Yellow base formulation-2
Yellow base formulation- 2
Ingredients Description Trade name/ Chemical name % Weight
Resin Acrylic Polyol Macrynal polyol 35
Pigment Titanium dioxide 1 Crystal 128 2.47
Titanium dioxide 2 SR 2377 1.91
Yellow Pigment Middle chrome 1340 25
Micronized talc GT 10 2
Spherical Barium sulfate Barytes 3
Additives Anti-sag/ anti settling additive Aerosil R 972 1
Polymeric dispersing agent 1 BYK P 104 1.2
Polymeric dispersing agent 2 BYK 163 4.3
Silicone polymer leveling agent 1 byk 306 0.25
silicone polymer leveling agent 2 byk 399 0.25
Tin catalyst DBTDL (Dibutyltin dilaurate) 1
UV Stabilizer Tinuvin 292 0.75
UV absorber Thasorb 384 0.25
Solvent Medium Polar solvent 1 Butyl Acetate 5.5
Long chain hydrocarbon solvent 1 C9 solvent 4.6
Medium Polar solvent 2 Ethyl cellosolve acetate 4.5
Medium Polar solvent 3 Butyl cellosolve acetate 2
Long chain hydrocarbon solvent 2 C10 solvent 5.02
Total 100

Table 3. Yellow base formulation-3
Yellow base formulation- 3
Ingredients Description Trade name/ Chemical name % Weight
Resin Acrylic Polyol Macrynal polyol 42
Pigment Titanium dioxide 1 Crystal 128 2.47
Titanium dioxide 2 SR 2377 1.91
Yellow Pigment Middle chrome 1340 25
Micronized talc GT 10 1.5
Spherical Barium sulfate Barytes 3
Additives Anti-sag/ anti settling additive Aerosil R 972 0.72
Polymeric dispersing agent 1 BYK P 104 1.2
Polymeric dispersing agent 2 BYK 163 4.3
Silicone polymer leveling agent 1 byk 306 0.25
silicone polymer leveling agent 2 byk 399 0.25
Tin catalyst DBTDL (Dibutyltin dilaurate) 1
UV Stabilizer Tinuvin 292 0.75
UV absorber Thasorb 384 0.25
Solvent Medium Polar solvent 1 Butyl Acetate 4.2
Long chain hydrocarbon solvent 1 C9 solvent 3
Medium Polar solvent 2 Ethyl cellosolve acetate 3.2
Medium Polar solvent 3 Butyl cellosolve acetate 2
Long chain hydrocarbon solvent 2 C10 solvent 3
Total 100

Yellow base formulation-1 has been chosen because of better gloss and visual appearance compared to other two yellow formulation. So, the formulation with 53-55% by wt. resin has been selected for further study for performance analysis.
Table 4: Formulation for white base
White base formulation- 1
Ingredients Description Trade name/ Chemical name % Weight
Resin Acrylic Polyol Macrynal polyol 53
Pigment Titanium dioxide 1 Crystal 128 1
Titanium dioxide 2 SR 2377 1.5
Titanium dioxide 3 TS 6200 22
Micronized talc GT 10 1.5
Spherical Barium sulfate Barytes 1
Additives Anti-sag/ anti settling additive Aerosil R 972 0.72
Polymeric dispersing agent 1 BYK P 104 1
Polymeric dispersing agent 2 BYK 163 2.6
Silicone polymer leveling agent 1 byk 306 0.25
silicone polymer leveling agent 2 byk 399 0.25
Tin catalyst DBTDL (Dibutyltin dilaurate) 1
UV Stabilizer Tinuvin 292 0.75
UV absorber Thasorb 384 0.25
Anti- corrosive pigment Phospho silicate 3
Solvent Medium Polar solvent 1 Butyl Acetate 3.18
Long chain hydrocarbon solvent 1 C9 solvent 2
Medium Polar solvent 2 Ethyl cellosolve acetate 2
Medium Polar solvent 3 Butyl cellosolve acetate 1
Long chain hydrocarbon solvent 2 C10 solvent 2
Total 100

Example 3
Formulation for hardener and thinner
The formulation for hardener and thinner are prepared by manual mixing of all the components as defined in Table 5 and Table 6.
Table 5: Hardener composition
Ingredients Description Trade name/ chemical name % Weight
Isocyanate prepolymer HDI pre polymer Tolonate HDT 90 80
Additive Moisture scavenger Ti Additive 0.2
Silane additive 3-Glycidoxypropyl)trimethoxysilane 7
Solvent Medium polar solvent Butyl Acetate 6.8
Non polar solvent Xylene 6
Total 100

Table 6: Thinner composition
Ingredients Description % Weight
Solvent Butyl acetate 5
Mixed Xylene 20
long chain hydrocarbon 50
Ethyl cellosolve acetate 25
Total 100.00

Example 4
Performance analysis of the coating composition
The coating composition comprising yellow base (formulation 1) and white base (formulation 1) as illustrated in Example 1 are cured and examined with following tests:
The cured film was tested for following physical/ mechanical tests in accordance with ASTM standards:
1. Adhesion Test - ASTM D-3359; 2. Pencil Hardness - ASTM D-3363; 3. Flexibility - ASTM D-522; 4. Impact Test - ASTM D- 6905; and Taber Abrasion- ASTM D-4060-19
The cured film was tested for following chemical resistance tests:
1. Acid and alkali Resistance - ASTM D-1308; 2. PU thinner resistance; 3. Lubricating oil resistance; and 4. Diesel resistance
The cured film was tested for following long term tests:
1. Salt spray Test- ASTM B-117; 2. QUV-B -ASTM D-154; 3. Water Immersion Test - ASTM D-870; and 4. Humidity Resistance - ASTM D-2247
The results of the aforesaid tests are illustrated below.
Table 7: Performance analysis test result
Parameters Description Yellow base (formulation 1) White base (formulation 1)
Appearance Dry film thickness 60-70 Micron 60-70 Micron
Gloss after overnight @ 600 90+/-5 90+/-5
Adhesion after overnight MS (Mild steel) 5B 5B
GI (Galvanized iron) 5B/4B+ 5B/4B+
Al (Aluminium) 4B+ 4B+
Adhesion after overnight (7 days ) MS 5B 5B
GI 5B/4B+ 5B/4B+
AL 4B+ 4B+
Anti corrosive property Salt pray test 550 hours on GI and 800 hours on Al , Creepage of less than 2 mm 600 hours on GI and 1000 hours on Al , Creepage of less than 2 mm
Impact resistance Direct impact (kg-cm) 60 60
Pencil hardness After 72 hours HB HB
After 15 days H/2H H/2H
Bending resistance Conical mandrel No crack No crack
Nail hardness After 72 hours Okay Okay
QUV B 500 hours ??? <3 ; gloss retention >80% ??? <3; gloss retention >80%
Water resistance 240 hours at 40 °C No blisters; no adhesion loss No blisters; no adhesion loss
Humidity resistance (400/ 87%) 500 hours No blistering/no softening No blistering/no softening
Chemical resistance Diesel resistance 30 minutes - no softening; no colour change 30 minutes - no softening; no colour change
Gear oil resistance 30 minutes - No softening; no colour change 30 minutes - No softening; no colour change
PU thinner rub test 50 rub pass 50 rub pass
PU thinner drop test Pass Pass
Abrasion resistance Should qualify 4000 Taber abrasion Pass Pass

The direct to metal coating composition passed all physical/ mechanical tests as shown in Table 7 and is suitable for bus body application and provides improvement in productivity while matching performance of epoxy systems on different metal substrates along with superior aesthetic and anticorrosive, weathering properties.
It is contemplated that equivalents and substitutions for elements set forth above may be obvious to those of ordinary skill in the art, and therefore the true scope and definition of the invention is to be as set forth in the following claims.

Claims:We claim:
1. A direct to metal coating composition comprising:
- a base coat composition, wherein the base coat composition comprises a film-forming acrylic polyol oligomer or polymer present in arrange from 25% to 65% by wt. of the coating composition; and
- a hardener composition, wherein hardener composition comprises a cross-linking agent in a range from 20% to 30% by wt. of the coating composition.
2. The direct to metal coating composition as claimed in claim 1, wherein the acrylic polyols in the base coat composition are selected from the group comprising of: methyl acrylate, methyl methacrylate, acrylic acid, meth acrylic acid, hydroxyl cyclohexyl methacrylate, hydroxyethyl acrylate, and hydroxybutyl acrylate.
3. The direct to metal coating composition as claimed in claim 1, wherein the acrylic polyols in the base coat composition has number average molecular weight in range from 4000 to 10,000.
4. The direct to metal coating composition as claimed in claim 1, wherein the base coat composition comprises of acrylic polyol resin in a range from preferably 30% to 60% by wt. and most preferably 30% to 55% by wt. of the base coat composition.
5. The direct to metal coating composition as claimed in claim 1, wherein the hardener composition comprises of isocyanates selected from the group comprising of hexamethylene diisocyanate (HDI), or isocyanurates of HDI, the biurets of HDI, and or the mixtures of isocyanurates and biurets of HDI.
6. The direct to metal coating composition as claimed in claim 1, wherein the hardener composition comprises of 80 % by wt. isocyanates.
7. The direct to metal coating composition as claimed in claim 1, wherein the base coat composition and hardener composition comprises solvents, pigments and additives.
8. The direct to metal coating composition as claimed in claim 7, wherein the solvent is selected from the group comprising of polar and non-polar solvent like n-heptane, toluene, xylene, petroleum compounds, n-hexane, isopropanol, 2-butoxy ethanol, n-butyl alcohol, ethyl acetate, n-butyl acetate, isobutyl acetate, ethylene glycol monoethyl ether acetate, dibasic ester, 3-methoxybutyl acetate, ethyl cellosolve acetate, butyl cellosolve acetate amyl acetate, butyl glycol acetate, methyl ethyl ketone, methyl isobutyl ketone, ethyl cellosolve acetate, butyl cellosolve acetate.
9. The direct to metal coating composition as claimed in claim 7, wherein the solvent is a mixture of butyl acetate, long chain hydrocarbon solvent and Ethyl cellosolve acetate and Butyl cellosolve acetate.
10. The direct to metal coating composition as claimed in claim 7, wherein the solvent is present in a range from 10 to 23% by wt. is used per 100% by weight of the base coating and hardener composition.
11. The direct to metal coating composition as claimed in claim 7, wherein the pigment is spherical shaped pigment selected from the group comprising of titanium oxide or other spherical colored pigments such as barium sulphate is present in a range from 2% to 25% by wt. of the base coat composition.
12. The direct to metal coating composition as claimed in claim 7, wherein the pigment is platy pigment selected from the group comprising of fillers talc, mica present in a range from 1.5% to 2% by wt. of the base coat composition.
13. The direct to metal coating composition as claimed in claim 7, wherein the pigment is colorant selected from the group comprising of titanium dioxide white, carbon black, lampblack, black iron oxide, red iron oxide, transparent red oxide, yellow iron oxide, transparent yellow oxide, brown iron oxide, phthalocyanine green, phthalocyanine blue, naphthol red, quinacridone red, quinacridone magenta, quinacridone violet, organic yellow, and any combination thereof and present in a range from 2 to 25% by wt. of the base coat composition.
14. The direct to metal coating composition as claimed in claim 7, wherein the pigment is corrosion inhibitor selected from the group comprising of calcium ion-exchanged amorphous silica pigments, phosphosilicate zinc and Zinc free system, Oxy-Ammonium phosphate of Mg present in a range from 2% to 5% by wt. of the base coating composition.
15. The direct to metal coating composition as claimed in claim 7, wherein the additive is anti-sag additive fumed silica with combination specialized clay present in a range from 0.7% to 1% by wt. of the base coat composition.
16. The direct to metal coating composition as claimed in claim 7, wherein the other pigments are selected from the group comprising of antifoaming agents, dispersants, surfactants, pot-life extenders, UV stabilizers, adhesion promoters, wetting agents, rheology modifiers, leveling agents, anti-blocking agents, thickeners, thixotropic agents, drying agents, anti-settling agents, and leveling agents present in a range from 0.2 to 5 % by wt. of the base coat coating composition.
17. A method of coating a metal substrate comprising the step of:
- Mixing the base coat composition with a hardener composition and thinner wherein the hardener composition comprises isocyanate present in an amount of 80% by wt. of the hardener composition;
- the total coating composition was applied directly to the metal substrate by spray application;
- the painted metal substrate was cured in an oven at temperatures 65° C for 35 minutes.
18. The method as claimed in claim 17 wherein the base coat composition and hardener composition are mixed in 4:1 in volume ratio.
19. The method as claimed in claim 17, wherein the thinner is a mixture of solvents butyl acetate, xylene, long chain hydrocarbon solvent (e.g. C9 solvent, C10/ solvesso 100) and ethyl cellosolve acetate.
20. The method as claimed in claim 17, wherein the combination of base coat, hardener and thinner provides higher electrical resistance for electrostatic application. ,