GB1570540A - Powder coating method for forming multilayer coatings - Google Patents

Description

(54) POWDER COATING METHOD FOR FORMING MULTILAYER COATINGS (71) We, KANSAI PAINT CO. LTD., a company of Japan, having its principal place of business at 365, Kanzaki, Amagasakishi, Hyogo-ken, Japan, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement:- This invention relates to a powder coating method of forming a multilayer coating.

A method of forming coating films on surfaces to be coated, such as metallic surfaces, using powder coating compositions has been widely employed in recent years because relatively thick coatings of several hundred microns can be easily formed in a single application. In addition, there are the advantages that no solvent is used and accidents with solvent are prevented. However, when coatings of several hundred microns in thickness are formed according to a conventional powder coating method, only films having homogeneous compositions are obtained.

That is, the composition of such coating film is uniform from the outer side exposed to the atmosphere to the innermost side in contact with the surface to be coated. When the surface of for example metal, wood or plastics material is to be protected or decorated by applying a coating film on the surface thereof two or more kinds of coating compositions are generally applied sequentially to form a multilayer coating rather than applying a single kind of coating composition either once or repeatedly to form a coating of the desired thickness.In other words, a composition having good adhesiveness and anti-corrosiveness with electrochemical effect in metal coating, is employed for the side of the coating which is to be in contact with the surface to be coated (such side of coating is hereinafter referred to as "lower side") and a coating composition having adequate properties of colour, gloss, friction resistance, photochemical stability, impermeability to chemicals, physical and chemical stability and other external factors, is employed for the side of coating to be exposed to the atmosphere (such "side" is hereinafter referred to as "upper side"). When a multilayer coating is formed using liquid coating compositions, it is necessary to use at least one kind of coating composition for each of the individual functional layers and the multilayer coating cannot be attained using a single kind of liquid coating composition.In the case of powder coating, it has also been impossible to form a multilayer coating with a single kind of coating composition in the conventional art.

However, it is known that an anticorrosive multilayer coating is formed by electrostatic coating in a single application using a mixture of at least two kinds of powdered resins which are differently charged. In this method, not only the electric charge of the resin particles but also the specific gravities must be taken into consideration. In addition, the coating is also influenced by the properties of pigments that are used together, so that it is quite difficult to regulate the state of multilayer coating formation and the method has never been widely used in practice. This method is described for instance in British Patent Specification No. 1,346,732.

According to the present invention there is provided a powder coating method of forming a multilayer coating, which method comprises applying a resin powder mixture to a surface to be coated and melting, by heating, the applied resin powder mixture to form a multilayer coating on the surface, the resin powder mixture comprising powders of at least two resin materials, two of the resin materials being such that the affinity parameter between the two resin materials is a positive number, zero, or a negative number less than 0.10 by absolute value so that the two resin materials are incompatible or of low compatibility, the difference between the surface tensions of the two resin materials at the same temperature in fused state being 0.5 dyne/cm or more and the two resin materials having different multilayer parameters such that the ratio of the larger multilayer parameter relative to the smaller multilayer parameter is 1.5 or more.

The present invention also relates to a substrate having a multilayer coating formed by a method in accordance with the first aspect.

The present invention enables the provision of an improved method for forming a multilayer coating which consists of differentiated layers preferably having respective particular functions.

The present invention further enables the provision of a method for forming multilayer coating by a single coating step without necessitating either troublesome repeated coating processes or the utilization of the difference of electrically charging properties of powdered materials.

Preferred embodiments of the present invention provide a coating method for forming a multilayer coating using powdered resin coating materials, which can be easily carried out, and is effective in practical purposes.

According to a preferred embodiment of the present invention, two or more kinds of resin material powders for coating are mixed and applied to the surface to be coated and the applied materials are fused together.

The nature, principle and details of preferred embodiments of the present invention will be more clearly apparent from the following detailed description and preferred examples of the invention.

The affinity parameter employed as an index in the present invention is a value indicating the degree of affinity between two kinds of resins. When this parameter is a positive number, zero or a negative number or less than 0.10 in absolute value, preferably a positive number, the affinity of the two kinds of resins is low and they are liable to separate, so that the multilayer coating can be easily formed.

When the affinity parameter is a negative number of more than 0.10 in absolute value, however, the formation of the multilayer coating generally becomes difficult since the affinity between the resins is large. This affinity parameter can be obtained by observing and measuring the state of a mixture of the solutions of both resins at a predetermined test condition and the affinity parameter obtained by this method is most preferable as the index for representing the degree of affinity between resins. In such a method, a mixed solvent is prepared by mixing equivalent volumes of toluene and methyl isobutyl ketone (MIBK).The soluble resins to be tested are then separately dissolved into the above mixed solution to obtain 33 wtO/, resin solutions, respectively, and the two kinds of the thus formed resin solutions, in equivalent volumes, are mixed together at 25"C. The mixture is then made even by agitating fully and the mixed resin solution of 5 cm in thickness is held to daylight and observed by the naked eye. When the mixed resin solution is completely transparent, the affinity parameter of the combination of such resins is zero or negative while if the mixed resin solution is turbid the affinity parameter is positive. The numerical value of this affinity parameter can be determined as follows.If the parameter is positive ethylene glycol monomethyl ether acetate (CH3OCH2.CH2OCOCH3) as a good solvent is added dropwise with stirring into the mixed resin solution to extinguish the turbidity by the effect of the solvent. If the parameter is zero or negative, n-hexane is added to the mixed resin solution as a poor solvent to make the solute unstable and make the solution turbid. The numerical value of the affinity parameter P is calculated according to the following equation: AD IPI= xK A+B+C+åD in which A and B are the weights (g) of the resins in the mixed solution, respectively, C is the weight (g) of solvents in the initial mixed solution, AD is the weight (g) of added solvent and K is a correction factor, that is, if P is negative, K=l, and the P is positive, K=100/45.When the value of P does not reach the minimum unit value, that in the second decimal place, P is represented as zero (0).

Where at least one of the two kinds of resins is insoluble to the above-described mixed solvent, the measurement of the affinity parameter using the above method is impossible. However, the round number of the parameter can be indirectly determined using a different method. In this different method, used for insoluble or slightly soluble resins, such as polyethylene and polyamide, straight borders of extruded or moulded resin sheets of from 400 to 500 microns in thickness are brought into contact with each other on a glass surface, are heated to a temperature at which both the resins are completely melted and are maintained as they stand at that temperature for about 30 minutes.If the affinity parameter between the resins is a positive number, zero or a negative number of less than 0.10 in absolute value, both the resins remain discontinuous forming boundary surfaces or boundary layers. If the affinity parameter is a negative number of 0.10 or more in absolute value, the compositions of the resins are continuously varied by mutual diffusion effect without forming any observable boundary surface.

When at least one of resins is coloured by dispersing a pigment such as carbon black or a dyestuff, the observation of the boundary surfaces with the naked eye becomes quite easy.

In the combination of two kinds of the resins that are used to form a multilayer coating, the affinity parameter measured in the solution state is within the abovedefined range so that the resins without any solvent are incompatible or of low compatibility. To show this, the mixed solution prepared in the above measurement of affinity parameter can be applied on a surface of a colourless transparent plate such as a glass plate so as to form a dried film of about 50 microns in thickness. The solvent is evaporated at room temperature for 24 hours to obtain a dried film on the transparent plate and the film is observed with transmitted light and diffused light. In this test, the turbidness or the separation of resins must be recognised.

When at least one of resins is insoluble to the above mixed solvent, both the resins may be regarded as incompatible or if the boundary surfaces are observed in the above melting test on sheet materials, of low compatibility. As can be seen, the above test is the same as that of the measurement of affinity parameter in respect of poor solubility resins.

In the present invention, it is also necessary that the surface tensions of two of the resins in the fused state are different from each other. This enables the resin having a lower surface tension in fused state to form the whole or the main constituent of the upper side of the multilayer coating when the resins are melted by heating. In order to form the coating having a distinct multilayer structure, it is necessary that the difference between the surface tensions of the resins at the same temperature in fused state is not less than 0.5 dyne/cm, preferably not less than 2.0 dyne/cm. When the difference of surface tensions is less than 0.5 dyne/cm, the complete separation of the resin components is not expected, especially in the upper side, and the formation of a multilayer coating having the multilayer structure becomes irregular.

The temperature of surface tension measurement is advantageously just above the highest melting point of the component resins used for the coating mixture of resin powders. Such temperature is normally the same as the heating temperature for the formation of multilayer coating by melting the component resins after they are applied to the surface to be coated. In practice, however an adequate comparison can be obtained using values of surface tension obtained at a certain temperature, for instance at 200"C, in any of resin materials.

The method for measuring the surface tension crn (dyne/cm) of molten resin is not especially restricted. For example, a surface tension can be obtained with the following equation of Neumann and Sell from the contact angle 0 between a molten resin and Teflon (tetrafluoroethylene resin) plate.

in which as is the surface tension (dyne/cm) of Teflon plate. The word "Teflon" is a Trade mark.

Approximate values are obtained by graphical solution on the assumption of as=18.6 dyne/cm.

With regard to the method for measuring the contact angle of molten resin relative to Teflon plate at a certain temperature, for example, at 2000 C, any method can be adopted so far as it determines advancing contact angles. For instance, a hemispherical resin to be tested having a diameter of 2 mm is placed on a horizontal surface of a Teflon plate with the spherical surface underside. It is then put in a thermostat at a predetermined temperature to fuse the hemispherical resin sample.

When the fused resin reaches the equilibrium, it is observed with a magnifying projector or a telescope to measure the contact angle.

When the melting temperature of the resin to be tested is too high and the surface tension cannot be calculated directly from the value of contact angle with Teflon at the predetermined temperature, the contact angle of the solid resin with water is measured at 200C to obtain the surface tension of the resin at 200 C. Thus, the surface tension at the predetermined temperature can be obtained from the above results by interpolation.

In order to separate the mixed resin powder by heating and melting, and to form a complete multilayer coating, it is necessary that the resins are attracted by the surface to be coated with different degrees.

The selective attraction of molten resin by the surface to be coated is governed by the melting temperature ranges, surface tensions, viscosities, specific gravities and so forth. The inventor of the present application has found that "multilayer parameters" (the parameters for forming multilayers) measured by the method described below can be used as the indices for representing quantitatively the degrees of attraction of molten resins by the polar surfaces of for examples metals, glass and porcelain and that the relative difficulty to form multilayer coating can be estimated most easily with the ratio of the multilayer parameters of resin components.That is, in order to form a multilayer with at least two resins, two of the resins must have different multilayer parameters such that the ratio of the larger multilayer parameter relative to the smaller multilayer parameter is not less than 1.5, and more preferably, it may be 1.8 or more, further 2.0 or more. If this ratio approaches 1 rather than 1.5, the separation of mixed resins becomes insufficient especially in the lower side of coating forming irregular and incomplete multilayer coating.

An advantageous method for measuring the multilayer parameter will now be described with reference to the single Figure of the accompanying drawing which shows a schematic cross-sectional view of a device for measuring the multilayer parameter.

Referring to the drawing, a predetermined quantity of 0.15 g of solid resin to be tested is fed into a cylindrical vessel 1 made of heat-resistant glass having an inner diameter of 12 cm. A thin tub 2 (outer diameter: 4.0 mm, inner diameter: 2.53 mm) made of Pyrex glass (a Trademark of heat-resistant glass) with the cleaned inner surface, is placed vertically and coaxially in the cylindrical vessel 1 so as to lower freely by its own gravity. The under surface of this thin tube 2 is ground perpendicularly to the axis of the tube 2 and notches 3 are formed in the under surface so as to allow the molten resin to pass through.

The whole of this measuring system is placed in a thermostat maintained at a predetermined temperature (Tc), in which the bottom surface of the cylindrical vessel 1 must be made horizontal. Thus, the resin to be measured is fused and forms a molten resin layer 4 in the cylindrical vessel 1. On melting of the resin, the thin tube 2 moves downwardly and the under surface of the thin tube 2 comes into contact with the bottom surface of the cylindrical vessel 1, at the same time, the molten resin flows into the thin tube 2. The molten resin to be tested presents not only meniscus in the thin tube 2 but also the phenomenon of what is called "creeping" to advance along the inner wall surface of the thin tube 2. The velocity of this creeping indicates the degree of attraction of the resin to be tested by the polar surface such as that of glass.

After 25 minutes from the start of heating in the thermostat, the height Ht (cm) from the upper edge of the creeping portion 5 to the bottom surface of the cylindrical vessel 1 is measured and the multilayer parameter is preferably determined by the product ht (g/cm2) of the above Ht by the density p(g/cm3) of the molten material, that is ht(g/cm2=Ht(cm)xp(g/cm3).

The temperature of measuring the multilayer parameter is preferably set to the temperature just above the maximum melting temperature of component resins used for the resin powder mixture for coating. When the measurement of the multilayer parameter of a resin by continuously heating for 25 minutes is difficult since the resin is decomposed or hardened by such treatment, the multilayer parameters of the resin can be measured at two or more points in the range from the temperature just above the melting point of the resin to a higher temperature which is free from such difficulty. The multilayer parameter at the predetermined temperature may be obtained from such parameters by extrapolation.In another method, assuming that the rising velocity of the top edge of the creeping portion 5 is even, the heating time of 25 minutes is shortened and the result is converted into the value for 25 minutes, which is generally acceptable for practical purposes.

In the case that three or more kinds of resin powders are mixed together, it is not necessary that all the combinations of each two of all the resins have the surface tensions and multilayer parameters within the above-defined ranges. When at least one of the combinations of resins satisfies the above relations, other combinations may be sufficient to meet any one of the above indices so as to form the multilayer structure of three or more layers.

When the resin powders for coating meet the above-described relations in view of affinity parameters, surface tensions of molten materials, multilayer parameters and compatibilities, the combination of resins for the method of the present invention is not especially restricted, regardless of thermoplastic resins or thermosetting resins.

When resins can be reduced to powder or fine particles, melted by heating at least temporarily, and formed into coating films by cooling, a variety of resin materials can be used for the method of the present invention taking the above-described conditions into consideration. Exemplified as such resin materials are: polyvinyl chloride, polyvinyl fluoride, polyvinylidene chloride, polyvinylidene fluoride, polystyrene, polyvinyl toluene, AS (acrylonitrile-styrene) resins, ABS (acrylonitrile - butadiene - styrene) resin, cellulose derivatives (such as cellulose acetate and cellulose acetate butyrate), polyesters (such as polyethylene terephthalate and polybutylene terephthalate), epoxy resin (such as the condensation product of bisphenol A with epichlorohydrin), phenoxy resin, polyolefins (including polyethylene, polypropylene and copolymers of olefins and vinyl esters such as vinyl acetate), acrylic resins (including polymers or copolymers of methacrylic esters such as methyl methacrylate, ethyl methacrylate, isobutyl methacrylate, 2hydroxyethyl methacrylate and glycidyl methacrylate and acrylic ester such as methyl acrylate, or copolymers of the above monomers and advantageously not more than 40% by weight of vinyl monomers such as styrene, acrylonitrile, acrylamide, N methylol acrylamide, N-vinyl pyridine, methacrylic acid and acrylic acid), ketone resin, coumaroneindene resin, polyamide, polycarbonate, urea-formaldehyde resin, melamine-formaldehyde resin, phenol formaldehyde resin, polysulphone, ionomer resin, polyimide, natural resins, (such as rosin, shellac and copal) and natural resin products, polyvinyl acetate, polyvinyl alcohol (partially or completely saponified products of polyvinyl ester) and acetalated polyvinyl alcohol (such as polyvinyl butyral and polyvinyl formal).Even when the same kind of resins are used, if they are different in their polymerization degrees, stereoregularities or other properties and they meet the above-defined conditions, they can be used for forming multilayer coatings.

The degree of the size reduction of these resins into powder may be such that the powdered materials can be well applied to the surfaces to be coated by the employed coating method such as electrostatic coating, fluidized bed immersion coating and so forth. Therefore, accurate regulation of the particle size distribution of resin powder is not required so as to form multilayer coating of the present invention, however, it is preferable that the powder resin material does not contain coarse particles larger than 300 microns, more preferably 100 microns, in diameter.

Further, when the particle size of the component resin powder having the maximum multilayer parameter is made relatively small, the formation of multilayer, especially the separation of components in the lower side of coating, can be promoted since the multilayer parameter relates to the easiness of melting. When the component resin powders are mixed together, the quantity of each resin powder should not be less than 10% by weight, preferably than 30% by weight relative to the total quantity of the resin powders so that each resin powder is melted and separated and becomes effective to form the multilayer structure. If the quantity of a resin powder is less than 10% by weight, it cannot normally take part in the formation of multilayer coating.

Into these resin materials can be dispersed various infusible powder components that are well known in the conventional powder coating art such as colouring pigments, fillers (extenders), rust preventives and metal powders. Further, liquid or fusible solid components compatible with one or respective of the above component resins such as plasticizers, curing agents, liquid or unpulverizable soft resins and fusible resin powders that do not meet the above described conditions of the invention in view of the affinity or compatibility, can also be added to the resin powder composition of the present invention. The quantity of these additives may be generally less than 10% by weight and with this addition, the formation of a multilayer coating can be promoted and the smoothness and gloss of the coating can be improved.

The mixing of two or more kinds of resins to form multilayer coatings may be done before they are pulverized.That is, the method of the present invention can be applied to the resin powder in which each particle consists of two or more kinds of resins.

When two kinds of resin powders A and B to be mixed meets the conditions of the present invention and the relation of surface tensions is aA > aB and that of multilayer parameters is hA > hB, the formed double layer structure consists of'a lower side of A and an upper side of B. However, when these relations are vA > aB and hA < hB, a triple layer structure is formed with the two kinds of resin materials in which the upper surface portion and the lower contact portion of the coating are mainly made of resin A and the intermediate layer is mainly made of resin B.

Even when three or more kinds or resin powders are used, if they meet the abovedescribed conditions, each constituent resin forms each layer of a multilayer coating.

For example, when resin powders A, B and C are mixed together and have the relations of (r,-0.5 dyne/cm, a,-a,LO.S dyne/cm and preferably UB < UC in surface tensions, and he~1.5 .5 hB and he~1.5 hA in multilayer parameters, a triple layer coating of three kinds of resins is formed, in which the upper side layer is mainly made of resin A, the intermediate layer, resin B and the lower side layer, resin C.

The distribution of the component resins in every portion of the multilayer coating formed by the method of the present invention can be confirmed and measured through various methods. For example, when each component resin is differently coloured with pigment and then it is pulverized and mixed with other resin material which is treated similarly, the formation of multilayer structure can be confirmed by observing the cross-section of cured coating film or by grinding the coating film from the upper surface and observing the change of colours. In the case that such discrimination of resins by colours is impossible, the coating film is ground and the powders of resin materials are subjected to infrared absorption spectrum analysis to obtain the composition of resins, or the exposed ground surface of the coating is subjected to infrared reflection spectrum analysis.

In accordance with a preferred embodiment of the present invention, since the multilayer coating comprises several layers that have desired properties suitable for the respective functions of constituent layers such as adhesiveness and weather resistance and the multilayer can be formed without any difficulty, the preferred embodiment is expected to contribute largely to the rationalization of coating work. The following Examples further illustrate the present invention.

Example 1 Equivalent weights of the powder (particle diameter: 5-70 microns) of polyester resin (Trademark "Desmophen &num;650", product of Farbenfabriken Bayer A.G.: Branched polyester containing hydroxyl groups and having hydroxyl content of 8% by weight and acid value of less than 4) and the powder (particle diameter: 5-70 microns) of thermosetting acrylic resin (Trademark "Acryl LR", product of BASF: Self-crosslinking acrylic copolymer in powder form having softening point of 85-900C and density of 1.07 at 20"C) were mixed togther and the mixture was applied to the surface of tin foil by electrostatic coating to form a layer of the mixture.It was melted by heating at 1700C for 25 minutes to obtain a coating film of about 150 micron thickness. both the resins were incompatible to each other and the affinity parameter between them was 0.69.

The measured values of surface tensions (200"C) and multilayer parameters (1700C) are as follows: Surface Tension Multilayer Parameter (dyne/cm) (g/cm2) Polyester resin 28.3 0.55 Thermosetting acrylic resin 26.2 0.20 Difference 2.1 Ratio ~1 2.75 The coating film was ground from its surface and three parts of ground powders in order were analyzed through infrared absorption spectrum method to determine their respective compositions. It was confirmed from the results shown in the following that the coating film was of double layer structure.

Content of Content of Thermo Depth from Surface Polyester setting Acryl (micron) (wt%) (wit%) 010 15 85 7080 47 53 140 or more 92 8 Example 2 To epoxy resin (Trademark "Epikote 1007", product of Shell Chemical Co.: Condensation product of bisphenol A with epichlorohydrin, having average molecular weight of 2900, epoxy equivalent of 1650 2050, and melting point of 125--132"C) was added 4.5 ', by weight of dicyandiamide and it was pulverized into powder (particle diameter: 5-70 microns). Poly-n-butyl methacrylate having a density of 1.08 at 20"C and an acid value of 1--2 mg KOH/g (Trademark "Plexigum P24", product of Roehm A.G., West Germany) was also pulverized likewise to obtain a resin powder particle diameter: 5-70 microns). The equivalent weights (as resin content) of both the above resin powders were mixed together and applied to the surface of tin foil by electrostatic coating method to form a layer of the mixture. It was then melted by heating at 1800C to obtain an about 150 micron thick coating. These resins were incompatible to each other and the affinity parameter between them was 0.47.The measured values of surface tensions (200"C) and multilayer parameters (1800C) are indicated in the following: Surface Tension Multilayer Parameter (dyne/cm) (g/cm2) Epoxy resin 31.8 0.35 Poly-n-butyl methacrylate 22.9 0.21 Difference 8.9 Ratio 1.67 The formed coating film was gradually ground from its surface and three parts of ground powders were analyzed by infrared absorption spectrum analysis to measure the compositions of the ground powders.Thus it was confirmed that the coating film was of double layer structure from the results of the above analysis, which are indicated in the following: Content of Content of Poly-n Depth from Surface Epoxy Resin butyl methacrylate (micron) (wt.%) (wt.%) 010 5 95 140 or more 82 18 Example 3 The resins pulverized to the particle diameter of 5-70 microns were epoxy resin (Trademark "Epikote 1007", the same as used in Example 2, hereinafter referred to as "Resin A") and polyethylene (Trademark "Hostalene", product of Farbwerke Hoechst A.G., low pressure polymerization process, average molecular weight: 10,000 20,000, hereinafter referred to as "Resin B").A mixture of the above resin powders was prepared in which the compounding ratio of Resin A: Resin B was equal parts by weight, and the mixture was applied to the surface of tin foil by electrostatic coating to form a layer of the mixture. It was then melted by heating at 1800C for 25 minutes forming an about 200 micron thick coating film on the foil. In these two kinds of resins, the resins were incompatible to each other.

In the melting test of sheet material, distinct boundaries were observed between Resin A and Resin B. Further, the measured values of surface tensions (200"C) and multilayer parameters (1800C) and the relations between these values are shown in the following table.

Surface Tension or Their Difference Multilayer Parameter (dyne/cm) or Their Ratio Resin A 31.8 0.35 g/cm2 Resin B 26.0 0.23 g/cm2 Difference 5.8 Ratio 1.52 The formed coating film was gradually ground from the surface and two parts of ground powders were tested by infrared absorption spectrum analysis to measure the compositions of them. The results are shown in the following table, from which it was understood that the formed coating film was of two layer structure.

Depth from Surface Content of Resin A Content of Resin B (micron) (wit.%) (wt.%) (F--10 3 97 190 or more 86 14 Example 4 The resins that were pulverized to the particle diameter of 5-70 microns were vinyl acetate homopolymer (Trademark "Mowilith M 30" product of Hoechst A.G.: Density was 1.17, acid value, 1--2 mg KOH/g, molecular weight, about 110,000, softening point by (DIN 1995), 105--120"C, hereinafter referred to as "Resin C" and epoxy resin (Trademark "Epikote 1004" product of Shell Chemical Co.; Condensation product of bisphenol A with epichlorohydrin having average molecular weight of 1400, epoxy equivalent of 870- 1025 and melting point of 95-1050C, hereinafter referred to as "Resin D").A mixture of the above resins was prepared in which the compounding ratio of Resin C: Resin D was 45:55 by weight, and the mixture was applied to the surface of tin foil to form a layer of the mixture by means of electrostatic coating method. It was then melted by heating at 1800C for 25 minutes forming an about 200 micron thick coating film on the foil. These resins are incompatible. The measured values of surface tensions (200"C) and multilayer parameters (1800C), and the relations of them are indicated in the following tables.

Multilayer Surface Tension Parameter (dyne/cm) (cm2) Resin C 27.3 0.22 Resin D 27.9 0.71 Difference 0.6 Ratio - 3.22 Affinity parameter of Resin C and Resin D was 1.29.

The formed coating film was gradually ground from the surface and two parts of ground powders were tested by infrared absorption spectrum analysis to obtain the data on the compositions of them. The results are shown in the following table, from which it was confirmed that the formed coating film was of double layer structure.

Depth Content Content of from Surface of Resin of Resin D (micron) (wt.%) (wt.%) 0--10 97 3 190 or more 2 98 WHAT WE CLAIM IS: 1. A powder coating method of forming a multilayer coating, which method comprises applying a resin powder mixture to a surface to be coated and melting, by heating, the applied resin powder mixture to form a multilayer coating on the surface, the resin powder mixture comprising powders of at least two resin materials, two of the resin materials being such that the affinity parameter between the two resin materials is a positive number, zero, or a negative number less than 0.10 by absolute value so that the two resin materials are incompatible or of low compatibility, the difference between the surface tensions of the two resin materials at the same temperature in fused state being 0.5 dyne/cm or more and the two resin materials having different multilayer parameters such that the ratio of the larger multilayer parameter relative to the smaller multilayer parameter is 1.5 or more.

2. A method according to Claim 1, wherein the particle diameter of the resin material powders is less than 300 microns.

3. A method according to Claim 2, wherein the particle diameter is less than 100 microns.

4. A method according to Claim 1, 2 or 3, wherein the difference between the surface tensions of the two resin materials in fused state is more than 2.0 dyne/cm.

5. A method according to any one of the preceding claims, wherein the ratio of the multilayer parameters is more than 1.8.

6. - A method according to Claim 5, wherein the ratio of the multilayer parameter is more than 2.0.

7. A method according to any one of the preceding claims, wherein the resin powder mixture is combined with one or more of colouring pigments, fillers, rust preventives, metal powders, plasticizers, curing agents, and fusible resin powders.

8. A method according to any one of the preceding claims, wherein the resin powder mixture comprises two or more of the thermoplastic or thermosetting resins polyvinyl chloride, polyvinyl fluoride, polyvinylidene chloride, polyvinylidene fluoride, polystyrene, polyvinyl toluene, acrylonitrile - styrene resin, acrylonitrile butadiene - styrene resin, cellulose derivatives, polyesters, epoxy resin, phenoxy resin, polyolefins, acrylic resin, ketone resin, coumaroneindene resin, polyamide, polycarbonate, urea formaldehyde resin, melamine formaldehyde resin, phenol - formaldehyde resin, polysulphone, ionomer resin, polyimide, natural resins, natural resin products, polyvinyl acetate, polyvinyl alcohol and acetalated polyvinyl alcohol.

9. A powder coating method for forming a multilayer coating, substantially as described in foregoing Example 1.

10. A powder coating method for forming a multilayer coating, substantially as described in foregoing Example 2.

11. A powder coating method for forming a multilayer coating, substantially as described in foregoing Example 3.

12. A powder coating method for forming a multilayer coating, substantially as described in foregoing Example 4.

13. A substrate having a multilayer coating formed by the method of any one of the preceding claims.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (13)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   Multilayer Surface Tension Parameter (dyne/cm) (cm2) Resin C 27.3 0.22 Resin D 27.9 0.71 Difference 0.6 Ratio - 3.22 Affinity parameter of Resin C and Resin D was 1.29.

   The formed coating film was gradually ground from the surface and two parts of ground powders were tested by infrared absorption spectrum analysis to obtain the data on the compositions of them. The results are shown in the following table, from which it was confirmed that the formed coating film was of double layer structure.

   Depth Content Content of from Surface of Resin of Resin D (micron) (wt.%) (wt.%) 0--10 97 3

   190 or more 2 98 WHAT WE CLAIM IS: 1. A powder coating method of forming a multilayer coating, which method comprises applying a resin powder mixture to a surface to be coated and melting, by heating, the applied resin powder mixture to form a multilayer coating on the surface, the resin powder mixture comprising powders of at least two resin materials, two of the resin materials being such that the affinity parameter between the two resin materials is a positive number, zero, or a negative number less than 0.10 by absolute value so that the two resin materials are incompatible or of low compatibility, the difference between the surface tensions of the two resin materials at the same temperature in fused state being 0.5 dyne/cm or more and the two resin materials having different multilayer parameters such that the ratio of the larger multilayer parameter relative to the smaller multilayer parameter is 1.5 or more.
2. 2. A method according to Claim 1, wherein the particle diameter of the resin material powders is less than 300 microns.
3. 3. A method according to Claim 2, wherein the particle diameter is less than 100 microns.
4. 4. A method according to Claim 1, 2 or 3, wherein the difference between the surface tensions of the two resin materials in fused state is more than 2.0 dyne/cm.
5. 5. A method according to any one of the preceding claims, wherein the ratio of the multilayer parameters is more than 1.8.
6. 6. - A method according to Claim 5, wherein the ratio of the multilayer parameter is more than 2.0.
7. 7. A method according to any one of the preceding claims, wherein the resin powder mixture is combined with one or more of colouring pigments, fillers, rust preventives, metal powders, plasticizers, curing agents, and fusible resin powders.
8. 8. A method according to any one of the preceding claims, wherein the resin powder mixture comprises two or more of the thermoplastic or thermosetting resins polyvinyl chloride, polyvinyl fluoride, polyvinylidene chloride, polyvinylidene fluoride, polystyrene, polyvinyl toluene, acrylonitrile - styrene resin, acrylonitrile butadiene - styrene resin, cellulose derivatives, polyesters, epoxy resin, phenoxy resin, polyolefins, acrylic resin, ketone resin, coumaroneindene resin, polyamide, polycarbonate, urea formaldehyde resin, melamine formaldehyde resin, phenol - formaldehyde resin, polysulphone, ionomer resin, polyimide, natural resins, natural resin products, polyvinyl acetate, polyvinyl alcohol and acetalated polyvinyl alcohol.
9. 9. A powder coating method for forming a multilayer coating, substantially as described in foregoing Example 1.
10. 10. A powder coating method for forming a multilayer coating, substantially as described in foregoing Example 2.
11. 11. A powder coating method for forming a multilayer coating, substantially as described in foregoing Example 3.
12. 12. A powder coating method for forming a multilayer coating, substantially as described in foregoing Example 4.
13. 13. A substrate having a multilayer coating formed by the method of any one of the preceding claims.