WO2016089199A1

WO2016089199A1 - Rapid stratifying compositions

Info

Publication number: WO2016089199A1
Application number: PCT/MY2015/050147
Authority: WO
Inventors: Khong Wei TEH; Kong Chin Chew
Original assignee: Becker Industrial Coatings (m) Sdn Bhd
Current assignee: Becker Industrial Coatings (m) Sdn Bhd
Priority date: 2014-12-02
Filing date: 2015-12-01
Publication date: 2016-06-09
Anticipated expiration: 2017-06-02
Also published as: MY171231A

Abstract

A self-stratifying multi-layer coating capable of rapid stratification and curing in less than 1 minute comprising of a resin system characterized in that the coating comprises of at least two incompatible resin systems wherein (a) a first resin system is adapted for use as a primer coat; (b) a second resin system is adapted for use as a top or back coat; and wherein first resin system is a polyester system or a mixture of polyester systems and the second resin system is a polyvinyl-butyral (PVB) system. The self-stratifying multi-layer coating is capable of rapid stratification in less than 15 seconds. The self-stratifying multi-layer coating provides at least a two layer coating under a single curing step and can be applied on varied substrates including galvanized steel, aluminum, plastic and wood. The said self-stratifying multi-layer coating is prepared comprising the steps of: i. providing a first resin system in a solvent or in a paint formulation; ii. providing a second resin system in a solvent or in a paint formulation; iii. adding the second resin system from step (ii) into the first resin system from step (i); and iv. stirring the mixture from step (iii) before coating onto a substrate. The said multi-layer coating is coated on a substrate in a method comprising the steps of: i. casting a layer of the said self-stratifying multi-layer coating on a substrate; ii. curing the casted substrate from step (i) to a suitable peak metal temperature; and iii. cooling the casted substrate.

Description

RAPID STRATIFYING COMPOSITIONS

FIELD OF INVENTION

The present invention relates to a self-stratifying multi-layer coating capable of rapid stratification and curing comprising of a plurality of resin systems which are incompatible to each other.

BACKGROUND ART

Existing coil coating applications are typically 2-coat systems, where a layer of primer and then subsequently a topcoat/backcoat is applied. The metal coil passes through a thermal oven after each of the primer and topcoat/backcoat application in order to cure the applied coating. Coil coating process is energy intensive, especially due to the energy needed to heat up the thermal oven and requirement of the oven for two times for 2 coat applications.

While self-stratifying compositions are known in the literature, and the concept has been proposed for application in coil coatings, none is known to be targeted for this application, or proven to be able to stratify rapidly, which is a significant requirement for coil coating application.

The state of the art practiced in the coil coating industry is by coating and curing each layer separately. The primer is first coated and cured, followed by the topcoat/backcoat in the same coating run.

The state of the art on self-stratification reported in the literature involves stratification that takes place in applications other than coil coating, usually via spray applications and requiring a relatively long time for stratifying and curing, typically in tens of minutes or hours, contrary to the need for significantly faster curing as is favourably desired in coil coating applications.

The state of the art on self-stratification in the literature has mostly utilized epoxy/acrylic resin pairs or derivatives thereof to achieve stratification; however, the state of the art in the coil coating process has not changed from being a 2-coat process. It is energy intensive in the sense that it requires the use of 2 curing ovens to cure the primer and then the topcoat/backcoat subsequently. The ovens are conventionally thermal convection ovens, but there are also near infra-red (NIR) and magnetic induction ovens.

Apart from the extensive use of energy to run the ovens (specifically thermal ovens), investment on 2 ovens in each line is also significant.

The state of the art on self-stratification composition is mainly concerning thermosetting systems that require time for solvent flash off, followed by one or more heating/baking steps which also requires a relatively long time. This process is significantly longer than that required for a coil coating process. This is because a certain amount of time is needed in order to achieve sufficient stratification in the state of the art, without which the intended performance is not likely to be achieved.

The state of the art on self-stratification composition is mainly focused on resin systems that may not be commonly used in coil coatings and as such may not be applicable for this application. Typical resin pairs include epoxy and hydroxylated acrylics, epoxy and fluorinated hydroxylated acrylics, epoxy and other fluorinated polyols, and derivatives or variations of epoxy resins and fluorinated acrylic polyols. These resin systems are cured with crosslinkers such as melamine-formaldehyde resins. Conventional coil coatings use polyester polyol resin systems crosslinked with melamine-formaldehyde resins for both the primer and topcoat/backcoat.

While the possibility to apply the self-stratification concept in coil coatings has been raised in the prior art, none has come up with a suitable solution, in particular the need for rapid stratification and curing in a very short duration typical of a coil coating process.

Prior arts on self-stratifying coatings appear to have little relevance to the present invention as described below:

US20120201965 (A) is directed to a multi-phase self-stratifying coating system comprising a substantially homogeneous composition of polymeric binder components that upon application and cure generates a multi-phase stratified coating wherein each phase is rich in a different polymeric binder component and wherein individual phases are separated by a diffuse interface such that the stratified coating exhibits a gradient behavior wherein a given interface exhibits the attributes of the polymeric binder component that is rich at that interface, and wherein at least one phase is rich in a polymeric binder having a telechelic resin with reactive internal groups, reactive end groups, and an alkoxide oligomer, and at least one other phase is rich in a polymeric binder having a fluorinated polymer derived from fluorinated vinyl-based monomers.

US8492001 discloses a self-stratifying coating composition comprising a base layer having a telechelic resin with reactive end groups and an alkoxide oligomer; and a top layer having a copolymer that includes at least one of an acrylate and methacrylate selected from a group consisting of a fluorinated acrylate, a fluorinated methacrylate, a fluorinated hydrocarbon copolymerized with an acrylate, a fluorinated hydrocarbon copolymerized with a methacrylate and combinations thereof, and a crosslinking agent, said base layer and said top layer formed through self-stratification.

US8299170 relates to a reactivity-based self-stratifying coating comprising: a self-stratified coating containing silicon, having two distinct layers and containing a polyol other than a polyester polyol, a silsesquioxane, a polyester polyol and a crosslinker; wherein the polyol other than a polyester polyol, silsesquioxane and polyester polyol are crosslinked and the silicon is segregated to one of the two distinct layers, and the silsesquioxane is an epoxy functional silsesquioxane having a formula (R)6(C₆H₁₁0₂)2(SiOi.5)8 and R is selected from the group consisting of hydrogen, methyl, ethyl, propyl, butyl, pentyl, hexyl, cyclopentyl, cyclohexyl, benzyl, phenyl and isomers thereof.

US8044140 generally teaches pigmented coating compositions and, more specifically, to methods of controlling the location of pigments within self-stratifying or self-layering coating compositions.

US7863375 is directed to a coating composition comprising: a polyol, a silsesquioxane, a polyurethane dendrimer and a crosslinker forming a mixture dispersed within a solvent and also relates to a process for preparing such coating compositions.

It is therefore apparent that while self-stratifying coatings are prevalent in the art and in particular, since the need for rapid stratification and curing of such self-stratifying coatings could not be met together, there is thus a longstanding need in the art to provide for self-stratifying multi-layer coating as formulation/kit comprising plurality of resins that can be applied as a whole formulation or as ingredients of the kit on varied substrates, which when applied on said substrates would rapidly self-stratify and cure under typical coil coating conditions.

SUMMARY OF THE INVENTION

A self-stratifying multi-layer coating capable of rapid stratification and curing in less than 1 minute comprising of a resin system characterized in that the coating comprises of at least two incompatible resin systems wherein (a) a first resin system is adapted for use as a primer coat; (b) a second resin system is adapted for use as a top or back coat; and wherein first resin system is a polyester system or a mixture of polyester and other resin systems and the second resin system is a resin system or a mixture of resin systems which is incompatible with the first resin system.

The self-stratifying multi-layer coating is capable of rapid stratification in less than 15 seconds. The self-stratifying multi-layer coating provides at least a two layer coating under a single curing step and can be applied on varied substrates including galvanized steel, aluminum, plastic and wood.

The said self-stratifying multi-layer coating is prepared in a process comprising the steps of:

i. providing a first resin system in a solvent or in a paint formulation; ii. providing a second resin system in a solvent or in a paint formulation; iii. adding the second resin system from step (ii) into the first resin system from step (i) ; and

iv. stirring the mixture from step (iii) before coating onto a substrate.

The said multi-layer coating is coated on a substrate in a method comprising the steps of: i. casting a layer of the said self-stratifying multi-layer coating on a substrate; ii. curing the casted substrate from step (i) to a suitable peak metal temperature; and

iii. cooling the casted substrate. BRIEF DESCRIPTION OF FIGURES

Figure 1 illustrates an optical microscopy image illustrating the separation achieved between the polyvinyl-butyral (PVB) and polyester components of the paint as described in Example 1 .

Figure 2 illustrates an optical microscopy image illustrating the 3-layer separation achieved between the incompatible polyester components of the paint as described in Example 4.

DETAILED DESCRIPTION OF THE INVENTION

Detailed description of preferred embodiments of the present invention is disclosed herein. It should be understood, however, that the embodiments are merely exemplary of the present invention, which may be embodied in various forms. Therefore, the details disclosed herein are not to be interpreted as limiting, but merely as the basis for the claim and for teaching one skilled in the art of the invention. The numerical data or ranges used in the specification are not to be construed as limiting. The terms 'resin' and 'resin systems' are understood to also mean 'paint' and 'paint systems' and vice versa.

The present invention provides for a self-stratifying multi-layer coating as formulation comprising plurality of resins or resin systems involving at least two incompatible resins or resin systems suitable for self-stratification as compared to the individual coatings from the same incompatible resins or resin systems coated and cured one after the other in two separate coating and curing steps.

More specifically, the present invention provides for said self-stratifying multi-layer coating as formulation which when applied as a whole formulation aids in the achievement of at least two layer coating under a single coating application and single curing step, and which when applied as the ingredients of the kit even as layer-by-layer aids in the achievement of multi-layer coating that are simultaneously curable. Advantageously also, said self-stratifying multi-layer-coating as formulation comprising said at least two incompatible resins or resin systems are capable of rapid stratification and curing in less than 1 minute under coil coating conditions and achieves rapid stratification in less than 15 seconds.

The present invention and its various embodiments are described hereunder in greater details, in the non-limiting examples, figures and tables below, and should never be construed to limit the scope of the invention.

Suitable systems for a self-stratifying coating system can be thus selected from pairs of resin systems or resin mixtures that are known to be incompatible. Incompatible resin systems can be understood to be those that do not mix well into a stable, single homogeneous system, and will therefore separate into individual resin layers over time. Incompatible resin systems or resin mixtures may also be derived from predictive methods such as those from the calculations of Hansen solubility parameters of different resin systems, or their surface tensions. Systems suitable for coil coating application must achieve adequate stratification in a short time, typically in less than 15 seconds, preferably in less than 10 seconds.

Systems suitable for coil coating application must also achieve adequate curing upon coating and stratification, typically in less than 60 seconds dwell time in the curing oven.

The preferred system in this invention is a polyvinyl butyral (PVB)-polyester system in which the polyester system (the first system) will form the primer coat while the PVB system (the second system) will form the top or back coat layer.

The PVB system composition typically includes the PVB resin, solvents, catalyst, pigments and extenders. Optionally, crosslinkers, plasticisers, co-binders and additives can also be added.

Suitable PVB resin for the invention can be selected from commercially available PVB resins which include but not limited to the Mowital® range from Kuraray Co. Ltd. such as B20H, B30H, B16S, B30HH, B60H, and B60HH; the Butvar® range from Eastman Chemical Co. such as B72, B76, B90 and B98; the S-Lec™ ranges from Sekisui Chemical Co. Ltd. such as BM-1 , BM-2, BM-5 and BH-3; ranges from Changchun Petrochemical Co. Ltd. such as B-05HX, B-06HX, B- 08X, and B-04X; and mixtures thereof.

Suitable polyester primer systems are commercially available, such as the BeckryPrim range from Becker Industrial Coatings, Malaysia, and also from other paint formulators known in the coil coating industry such as the Optima® range from Akzo Nobel.

Suitable crosslinkers for the PVB resin include but not limited to melamine-formaldehyde resins such as Cymel® 303, Cymel® 325 and Cymel® 370 from Allnex, Luwipal® 066 and Luwipal® 072 from BASF; polyisocyanate resins such as Desmodur® BL3175 and Desmodur® BL3475 from Bayer MaterialScience, Tolonate™ D2 from Perstorp; tetraethyl orthosilicate (TEOS) from Momentive, phenolic resins, epoxy resins, and dialdehydes, or mixtures thereof. Preferred crosslinkers are polyisocyanates, TEOS and melamine formaldehyde resins. If the crosslinker selected is a melamine formaldehyde resin, the preferred PVB:melamine ratio in the system is between 60:40 to 95:5 by weight solids. If the crosslinker selected is a polyisocyanate, the preferred OH:NCO ratio in the system is between 1 :1 and 13:1 . The ratios for the PVB:melamine ratio and OH:NCO ratio in the system were calculated based on the steps and examples as described further in the description later.

Suitable catalysts for the PVB resin include but not limited to phosphoric acid, blocked and unblocked p-toluenesulfonic acid (PTSA) and other sulfonic acid derivatives available from King Industries Inc, dibutyltindilaurate (DBTDL), metal carboxylates or chelates from King Industries Inc, or mixtures thereof. The catalyst used is preferably blocked phosphoric acid, blocked PTSA or DBTDL.

Suitable plasticizers for the PVB resin include but not limited to triethylene glycol di-2- ethylhexanoate (3GO), dibutylsebacate (DBS), tetraethylene glycol di-2-heptanoate (4G7), phthalates such as butyl benzyl phthalate, and mixtures thereof.

Suitable solvents for the PVB resin include xylene, toluene, ethanol, butanol, butyl glycol and other solvents (including mixtures) that are found to be compatible, which can also be inferred from determinations of their Hansen solubility parameters.

Suitable co-binders include but not limited to epoxy resins (aromatic, aliphatic and cycloaliphatic) such as Epikote™ 828 from Momentive and Uvacure® 1500 from Allnex; polycaprolactone such as Capa™ 3050 from Perstorp, and polycarbonate diol such as Desmophen® 2716 from Bayer MaterialScience.

Suitable additives to be added into the PVB resin include, but not limited to, flow and levelling agents, defoamers, rheology modifiers, thermal stabilisers, UV stabilisers, pigment wetting agents and pigment dispersants.

The process for preparing the PVB paint is similar to the typical process known in the paint industry.

The incompatible paint systems can be mixed together prior to the coating process via conventional stirring systems typically used in the paint industry.

The self-stratifying system can be coated or applied via conventional coating processes such as brushing, spraying, roller coating and curtain coating. The preferred method of this invention is roller coating, typical of the method known in the coil coating industry. Depending on the constituents used in the PVB formulation and the primer formulation, such as the type of crosslinker, catalyst, solvents and additive, the self-stratifying system of this invention can also be applied via other coating methods such as brushing and spraying, and cured in a non-rapid way (> 1 minute) either with the application of heat, or air-dried (ambient- cure).

The self-stratifying coating of this invention is preferably cured thermally in a conventional thermal convection oven, a near infra-red (NIR) oven, or a magnetic induction oven; methods known in the coil coating industry. The peak metal temperature range for curing the paint under coil coating conditions is between 160-240 °C, preferably between 170-210°C.

Depending on the primer system used, the self-stratifying system of this invention can be coated on various substrates such as galvanized steel, aluminium, plastic and wood.

The PVB paint system can also be used in conventional layer-by-layer applications where the primer is separately coated first, followed by the PVB layer. In this case, the methods of application and curing of the paint can be similar to those mentioned previously. Such systems are also suitable for coil coatings.

The present invention and its various embodiments are further described below but should not be construed to limit the scope of the invention.

Step 1 : Preparation of polvvinyl-butyral (PVB) resin solution

20g of B-08X PVB resin from Changchun Petrochemical Co. Ltd. was dissolved in a mixture of 30g ethanol, 25g n-butanol and 25g xylene. The mixture was mechanically stirred at ambient temperature until all the PVB resin was dissolved.

Step 2: Preparation of PVB paint

81 g of the PVB resin solution prepared in Step 1 was mixed with 3g of Cymel® 303 melamine formaldehyde resin from Allnex. 0.4g of a phosphoric acid catalyst was added to the mixture. 9g butyl glycol and 6g of Solvesso™ 100 from ExxonMobil Chemical were then added. The mixture was stirred until homogenous at ambient temperature. 1 g of a blue colourant was added into the mixture while stirring. The mixture was then stirred for a further 30 minutes at ambient temperature.

Step 3: Preparation of first PVB-polvester self-stratifying liquid paint

50g of the PVB paint prepared in Step 2 was mixed with 50g of BeckryPrim DE211 commercial polyester chromated primer paint from Becker Industrial Coatings, Malaysia. Stirring was carried out until the mixture was homogenous under ambient temperature. Step 4: Application and curing of self-stratifying liquid paint

The prepared liquid paint was cast into a thin wet film by a stainless steel wire-wound applicator roller bar on a 0.3mm thick galvanized steel substrate. Then, it was immediately (within 15 seconds) put into an oven which was set at 340 °C for a duration of 35 seconds to achieve a peak metal temperature of 232 °C, after which the panel was removed and quenched by dipping into water and subsequently dried and cooled to ambient temperature. Step 5: Preparation of second PVB-polvester self-stratifying liquid paint

50g of the PVB paint prepared in Step 2 was mixed with 50g of BeckryPrim DE212 commercial polyester chromated primer paint from Becker Industrial Coatings, Malaysia. Stirring was carried out until the mixture was homogenous under ambient temperature. Step 6: Preparation of polyester paint

17g of Synolac® E1732 polyester resin from Arkema, 1 g of Disperbyk 174 from BYK Chemie, and 6g of Solvesso™ 100 were mixed and stirred until homogenous. 0.3g of Aerosil® R972 from Evonik was added subsequently. 31 g of Kronos® 2310 titanium dioxide from Kronos Inc., and 8g of Micro Blanc Fixe barium sulfate from Sacthleben Chemie GmbH were then dispersed in the mixture for 30 minutes. Subsequently the mixture was milled until a fineness of 7.5 on the Hegman gauge is achieved. A further amount of 8g Synolac® E1732 resin and 2g methoxypropyl acetate were then added. The mixture was stirred until homogenous and then added with 17g Synolac® E1732 resin, 5g Cymel® 303, 0.3g phosphoric acid catalyst, 2g n-butanol, 2g dibasic ester and 0.4g Dynoadd® F1 from Dynea. The final mixture was then stirred for a further 30 minutes at ambient temperature.

Step 7: Preparation of polyester-polyester self-stratifying paint and coating preparation

50g of the paint from Step 6 was mixed with 50g of BeckryPrim DE212 commercial polyester chromated primer paint. Stirring was carried out until the mixture was homogenous under ambient temperature.

Example 1 : Preparation of first PVB-polyester self-stratifying coating

The liquid paint prepared in Step 3 above was applied and cured according to Step 4 above to obtain the self-stratified coating. Example 2 (comparative reference coating): Preparation of PVB-polvester 2-laver coating by laver-bv-laver process

The primer coating of BeckryPrim DE211 was first prepared using the same process described in Step 4 above. Subsequently, the PVB paint prepared in Step 2 above was applied onto the primed substrate and the process of curing was repeated, according to the process described in Step 4 above.

Example 3: Preparation of second PVB-polvester self-stratifying coating

The liquid paint prepared in Step 5 above was applied and cured according to Step 4 above to obtain the self-stratified coating.

Example 4: Preparation of polyester-polyester self-stratifying coating

The liquid paint prepared in Step 7 above was applied and cured according to Step 4 above to obtain the self-stratified coating.

Figure 1 shows the cross-section of the self-stratifying coating as described in Example 1 , clearly indicating that separation is successfully achieved between the PVB and polyester components of the paint. The PVB forms the clear bluish top layer while the commercial primer paint forms the bottom layer.

Example 3 is another PVB-polyester system wherein the commercial primer contains a different polyester resin from that described in Example 1 . This system is also able to rapidly self- stratify under the coil coating conditions used, although not as efficiently as the system in Example 1 . Nevertheless, it indicates that it is feasible to use PVB system in combination with suitable polyester primer systems to afford self-stratification systems.

In another example to demonstrate the possibility of achieving rapid self-stratification and curing under coil coating conditions, Figure 2 shows the cross-section of Example 4, which is different from Examples 1 and 3 in that this is now a polyester-polyester system. Surprisingly, Example 4 shows that the system had self-stratified into a 3-layer system whereby one of the polyester components had sandwiched the other polyester component (the commercial primer paint) in between. While this system does not stratify into the desired 2-layer system, it nevertheless demonstrates further that it is possible to obtain rapidly self-stratifying systems from the same resin types but which are incompatible with each other.

The comparison in Table 1 shows that the self-stratified coating Example 1 has comparable properties against the layer-by-layer 2-coat reference coating, Example 2. This is achieved at a lower total coating thickness compared to the reference.

Pencil hardness is measured and graded according to the coil coating industry standard test method ECCA-T4 using pencils manufactured by Faber-Castell. T-bend is measured and graded according to the standard test method ASTM D4145. MEK resistance is measured according to the coil coating industry standard ECCA-T11 .

In the current state of the art 2-coat system in the coil coating industry, it is common to coat the primer at dry film thickness (DFT) of 2-1 O m, and the topcoat at DFT of 1 0-25 m, such that the total combined DFT of the primer and topcoat is less than 30 m. Such thicknesses can be achieved by the self-stratified coating described herein by varying the amount of the two incompatible systems in the mixture.

Table 1 : Coating properties tests

NTO = no tape off, TO = tape off, NC = no cracking The method of the present invention to provide for a self-stratified multi-coated surface allows the elimination of one coating and curing process which eventually reduces energy resources and investment in new manufacturing lines. While the present invention allows the elimination of one process step in the coil coating process by eliminating one coating and curing step, the same results in energy saving and other associated resources in running the additional coating station and curing oven. Hence new coil coating lines can be made with only one coating station and one curing oven, resulting in significant savings in investment cost and also factory space.

The present invention allows existing coil coating lines with 2 coating stations and 2 curing ovens to be able to produce pre-coated metal coils with more than 2 layers of coatings. 3- and 4- coat systems are now made feasible in such existing coating lines for new applications requiring high thickness or specific functions with the need of multilayer coatings.

More particularly, the present advancement provides for said self-stratifying multi-layer coating as formulation which when applied as a whole formulation aids in the achievement of at least two layer coating under a single coating application and single curing step, whereby when applied as the ingredients of the kit even as layer-by-layer aids in the achievement of at least two layer coating under a single curing step as the layers are simultaneously curable. The said self- stratifying multi-layer-coating formulation comprising said at least two incompatible resin systems are capable of rapid stratification and curing in less than 1 minute under coil coating conditions and achieves rapid stratification in less than 15 seconds.

The present advancement of said self-stratifying multi-layer coating can be prepared by a process comprising the steps of

i. providing a first resin system in a solvent or in a paint formulation;

ii. providing a second resin system in a solvent or in a paint formulation;

iii. adding the second resin system from step(ii) into the first resin system from step (i); and

iv. stirring the mixture from step (iii) before coating onto a substrate.

The present advancement of said self-stratifying multi-layer coating can be coated on a substrate by a method comprising the steps of:

i. casting a layer of the said self-stratifying multi-layer coating on a substrate;

ii. curing the casted substrate from step (i) to a suitable peak metal temperature; and iii. cooling the casted substrate.

In the present invention of the said self-stratifying multi-layer coating, the curing process of the casted substrate begins within 15 seconds of casting the self-stratifying multi-layer coating on the substrate. The casted substrate is cured in less than 60 seconds, wherein the peak metal temperature is between 160°C to 240°C, and preferably between 170°C to 210°C. The said self- stratifying multi-layer coating is casted on a substrate, wherein the substrate is cold rolled steel, or steel coated with zinc, aluminium-zinc alloy, or magnesium-aluminium-zinc alloy.

Claims

A self-stratifying multi-layer coating capable of rapid stratification and curing in less than 1 minute comprising of a resin system characterized in that the coating comprises of at least two incompatible resin systems wherein

a) a first resin system is adapted for use as a primer coat;

b) a second resin system is adapted for use as a top or back coat; and wherein first resin system is a polyester system or a mixture of polyester systems and the second resin system is a resin system or a mixture of resin systems which is incompatible with the first resin system.

A self-stratifying multi-layer coating as claimed in claim 1 wherein the second resin system is a polyvinyl butyral (PVB) system.

A self-stratifying multi-layer coating as claimed in claim 1 wherein the coating achieves rapid stratification in less than 15 seconds.

4. A self-stratifying multi-layer coating as claimed in claim 1 wherein combined thickness of the first resin system and the second resin system is between 3μιτι and 30 μιη.

5. A self-stratifying multi-layer coating as claimed in claim 1 wherein the second resin system includes polyvinyl-butyral (PVB) resin, solvents, catalyst, pigments and extenders.

6. A self-stratifying multi-layer coating as claimed in claim 5 wherein the second resin system further includes at least one crosslinker, one plasticizer, one co-binder and one additive.

7. A self-stratifying multi-layer coating as claimed in claim 6 wherein the crosslinker is a melamine-formaldehyde resin.

8. A self-stratifying multi-layer coating as claimed in claims 2 and 7 wherein the second resin system has a PVB:melamine ratio is between 60:40 to 95:5 by weight solids.

9. A self-stratifying multi-layer coating as claimed in claim 6 wherein the crosslinker is a polyisocyanate resin.

10. A self-stratifying multi-layer coating as claimed in claims 2 and 9 wherein the second resin system has a PVB-polyisocyanate resin system having a OH:NCO ratio between 1 :1 and 13:1 .

1 1 . A process for the preparation of a self-stratifying multi-layer coating as claimed in claim 1 to 10 comprising the steps of

i. providing the first resin system in a solvent or in a paint formulation;

ii. providing the second resin system in a solvent or in a paint formulation;

iii. adding the second resin system from step (ii) into the first resin system from step (i); and

iv. stirring the mixture from step (iii) before coating onto a substrate.

12. A method of coating a substrate comprising the steps of:

i. casting a layer of the self-stratifying multi-layer coating prepared in accordance to any of claims 1 to 1 1 on a substrate;

ii. curing the casted substrate from step (i) to a peak metal temperature; and

iii. cooling the casted substrate.

13. A method of coating a substrate as claimed in claim 12 wherein the curing process of the casted substrate begins within 15 seconds of casting the self-stratifying multi-layer coating on the substrate.

14. A method of coating a substrate as claimed in claim 12 wherein the casted substrate is cured in less than 60 seconds.

15. A method of coating a substrate as claimed in claim 12 wherein the peak metal temperature is between 160°C to 240 °C.

16. A method of coating a substrate as claimed in claim 12 wherein the peak metal temperature is preferably between 170°C to 210^qC.

17. A method of coating a substrate as claimed in claim 12 wherein the substrate is cold rolled steel, or steel coated with zinc, aluminium-zinc alloy, or magnesium-aluminium-zinc alloy.

18. A substrate coated with self-stratifying multi-layer coating as claimed in any of claims 1 to 10.