WO2009003937A1

WO2009003937A1 - Powder coating composition for high temperature resistant coatings

Info

Publication number: WO2009003937A1
Application number: PCT/EP2008/058260
Authority: WO
Inventors: Richard Allman; Vishwaroop Sen; Nitesh Ranjan; Nirmalya Chakravorty
Original assignee: Akzo Nobel Coatings International B.V.
Priority date: 2007-07-02
Filing date: 2008-06-27
Publication date: 2009-01-08
Also published as: CN101720345A; TW200918619A

Abstract

A powder coating composition is disclosed which comprises a resin component and a filler, wherein the resin component comprises a first silicone resin and a second silicone resin, said first and second silicone resins being characterized by having glass transition temperatures (T9) that are different by at least 5°C and / or having melt viscosities, as measured at 140°C, that are different by at least 5, preferably 10 poise.

Description

POWDER COATING COMPOSITION FOR HIGH TEMPERATURE RESISTANT COATINGS

Field of the Invention

The present invention is directed to a powder coating composition that may be cured on a substrate to produce a coating that is resistant to high temperatures. More particularly, the present invention is directed to a powder coating composition comprising at least two distinct silicone resins.

Background of the Invention

It is obviously desirable for coatings that are to be applied to ovens, boilers, heat exchangers, automotive parts, cooking elements, cooking utensils and the like to exhibit high temperature resistance. Most organic coatings are unsuitable for such applications as they tend to be rapidly consumed when exposed to air at temperatures greater than 550⁰C. This consequence lead to the development of coatings and paints that incorporated polysiloxane resins, as described in US Patent No. 5,905,104 (Eklund et al.). In the examples a mixture of siloxane resins is mentioned, viz. a mixture of Dow Corning 1 -0543 and Dow Corning Z-

6018. These resins have the following properties:

- Dow Corning 1 -0543 (now DC 220) Tg = 49°C, viscosity at 140⁰C of 9.8 poise; - Dow Corning Z-6018 Tg = 48°C, viscosity at 140⁰C of 14.1 poise.

Despite showing improved temperature resistance, coatings containing polysiloxane resins were still found to exhibit deleterious effects at the high temperatures. When polysiloxane powder coated materials are exposed to temperatures greater than 550°C, the coatings suffered loss of their constituent organic components through oxidation; the polysiloxane resin consequently shrinks rapidly which builds up stresses within the coatings. Such stresses are relieved by cracking causing the coating to peel or flake from the material. WO2004/076572 (Dupont de Nemours and Company) purports to resolve this problem by including within the polysiloxane resin at least one matrix material, preferably low melting inorganic glass, that softens and exhibit some flow in the temperature range in which the polysiloxane resin undergoes shrinkage and embrittlement.

European Patent No. 0950695 B1 (Morton) proposes an alternative solution in which the powder coating composition consists of a single silicone resin combined with titania and a filler of mica platelets and / or calcium metasilicate particles. The single silicone resin is characterized by having siloxane functionality (Si-O-H) and only minor amounts of organic moieties. It is preferred in this citation that the single polysiloxane has a degree of substitution of less than 1.5 and an -OH content of between 2.0 and 7.5 wt.% based on the weight of said polysiloxane. The limitation of the -OH content reduces the evolution of water when the polysiloxane self-cures at temperatures between 150° and 260⁰C and thus reduces the formation of defects, such as pinholes, in the coating that are caused by said water escaping. However, it is noted this powder coating composition may only be applied to substrates at a dry film thickness in the range from 1.8 to 2.2 mils (45 to 55μm).

When powder coatings are applied to automotive bodies in order to protect and finish the engineered product, the substrates tend to be relatively thin and to have smooth surfaces. However, in the application of coatings to materials that are required to show high temperature resistance, it is more common for the substrate surfaces to be profiled or uneven: to provide adequate corrosion protection and an (aesthetic) finish to blast cleaned steel, for example, the substrate must be coated at a sufficient dry film thickness to compensate for surface unevenness. Blasting substrates with angular grit, rounded shot, abrasive loaded sponges or high pressure water jets can typically yield profiled surfaces that can exhibit "valley to peak" distances of between 10 and 80 μm (wherein said profiles may be defined by ISO 8503). For such uneven substrates, there is found to be a practical upper limit to the dry film thickness (DFT) of the powder coating, beyond which the coating will crack and peel from the substrate. Obviously, the lower that limit, the lower the capacity of a given powder coating to compensate for enhanced blast profiles of a substrate surface.

There consequently exists a need in the art to provide a powder coating composition that shows high temperature resistance but which also may be applied to profiled substrate surfaces to provide temperature resistance and preferably corrosion resistance to said surfaces.

Summary of the Invention

In accordance with a first aspect of the invention there is provided a powder coating composition comprising a resin component and a filler, wherein the resin component comprises a first silicone resin and a second silicone resin, said first and second silicone resins being characterized by having glass transition temperatures (T₉) that are different by at least 5°C. In accordance with a second aspect of the invention there is provided a powder coating composition comprising a resin component and a filler, wherein the resin component comprises a first silicone resin and a second silicone resin, said first and second silicone resins being characterized by having melt viscosities, as measured at 140⁰C, that are different by at least 5 poise, preferablyl O poise. The different thermal properties of the two silicone resins in the powder coating composition results means that, individually, each resin would exhibit different flow behaviour at temperatures greater than 550⁰C. However, these different behaviours synergistically combine to limit shrinkage and embrittlement of a coating containing both silicone resins in this temperature range.

Preferably the first and second silicone resins are present in resin component in a ratio by weight (First Silicone: Second Silicone Resin) of between 2:1 and 1 :2. Equally, it is preferred that said first silicone resin has a T₉ in the range from 40° to 50⁰C and said second silicone resin has a T₉ in the range from 55° to 80⁰C.

Without being bound by theory, differences in the melt viscosity and / or T₉ of the first and second silicone resins can arise as a consequence of differences in the degree of branching of the polymers. In general, the more highly branched the polymer, the greater the shrinkage observed at high temperatures.

Furthermore, the first and second silicone polymers may be distinguished on the basis of their type and amount of constituent organic moieties and their -OH content (i.e. the degree of siloxane functionality).

With respect to the filler component of the composition, it is preferred that the filler is a heat resistant material with one dimension at least four times larger than the other, said filler being present in an amount between 5 and 95 wt.% based on the weight of the resin component.

In accordance with a preferred embodiment of the invention, the resin component of the powder coating further comprises between 1 and 25 wt.%, based on the weight of the resin component, of a non-silicone resin. Optimally, said non-silicone resin is an epoxy-polyester hybrid.

In accordance with a third aspect of the invention there is provided a process for coating a substrate in which the powder coating composition described above is applied to a substrate, after which said powder coating composition is subjected to a curing step, wherein said powder coating composition is applied in such a layer thickness that the dry film thickness (DFT) of the cured powder coating is at least 65 microns. Preferably, the DFT of the cured powder coating layer is between 70 and 130 microns, preferably 75 and 100 microns.

In accordance with a fourth aspect of the invention there is provided a substrate coated with a cured layer of the powder coating composition as described above, the layer having a DFT of at least 65 microns, preferably of between 70 and 130 microns and more preferably of between 75 and 100 microns.

Detailed Description of the Preferred Embodiments

Definitions

For the purpose of describing the proportions of components in the compositions of this invention, the term "resin" includes any resin or polymer per se, as well as the curing agent.

With respect to the silicone resins of this invention, the degree of substitution is herein defined as the average number of substituent organic groups per silicon atom and is the summation of the mole percent multiplied by the number of substituents for each ingredient. This calculation is further described in "Silicones in Protective Coatings", by Lawrence H. Brown (in Treatise on Coatings Vol. 1 , Part III, "Film-Forming Compositions" pp. 513-563, R. R. Meyers and J. S. Long eds. Marcel Dekker, Inc. New York, 1972).

As used herein, the "glass transition temperature" or T₉ of any polymer may be calculated as described by Fox in Bull. Amer. Physics. Soc, 1 , 3, page 123 (1956). The T₉ can also be measured experimentally using differential scanning calohmetry (at a rate of heating 20° C per minute, wherein the T₉ is taken at the midpoint of the inflection). Unless otherwise indicated, the stated T₉ as used herein refers to the calculated T₉.

The normative term "high temperature" is used herein to indicate that the cured, powder coating compositions of the present invention are intended to withstand temperatures at which most organic components, including the organic moieties of the silicone resin, burn away. It is desirable that the cured, powder coatings of the present invention withstand temperatures of at least 550⁰C. The Powder Coating Composition

Although the first and second silicone resins of the present invention are to be characterized by distinct T_g's and/or melt viscosity, these two silicone resins should both be solid at room temperature and both have a glass transition temperature (T₉) greater than 45°C. This lower limit of T₉ is necessary to prevent undue blocking (or sintering) of a coating powder.

The organic moieties of the first and second silicone resins are aryl and / or short chain (Ci to C₅) alkyl. It is known that for good heat resistance, methyl, ethyl and phenyl groups are desirable organic moieties, phenyl groups particularly so as the greater the number of phenyl groups the higher the heat resistance provided. Consequently, the first and second silicone resins compositions should preferably include methyl, ethyl, phenyl, dimethyl, diphenyl, methylphenyl and phenylpropyl organic moieties and their mixtures.

More preferably, both silicone resins of the present invention comprise random mixtures of methyl and phenyl groups, dimethyl siloxane and diphenyl siloxane groups, or phenylmethylsiloxane groups, wherein the ratio of phenyl to methyl groups is 0.5 to 1.5:1 , more preferably 0.7:1 to 1.1 :1. In any event, it is desired that the first and second silicone resins have a degree of organic substitution of

1.5 or less, preferably between about 1 and about 1.5.

The first and second silicone resins self-condense at high end-use temperatures which thus requires silanol functionality (Si — O — H). Both silicone resins should have a condensable hydroxyl content of from 2 to 7 wt. %, more preferably from 3 to 5 wt. % but slight variations from these ranges may be tolerated depending on any catalyst present. The condensable hydroxyl content should not be too high to prevent water outgassing during curing of the coating powder. On the other hand, the lower limit of the condensable hydroxyl content range is important because below this the coating powder will cure too slowly to be suitable for commercial applications. The first and second silicone resins of the present invention should preferably contain less than 0.2 wt.% of organic solvents, preferably less than 0.1 wt.%. However, most commercial silicone resins contain some residual organic solvent as a consequence of the process of silicone resin synthesis. Such organic solvent tends to be internally trapped within the silicone resin and is generally not removed when the silicone resin is melt blended with other components to form a coating powder composition. Accordingly, it may be necessary to substantially remove such residual organic solvent. This is accomplished by melting the silicone resin and removing solvent from the molten resin by sparging with an inert gas or by vacuum.

It is crucial to the present invention that the first and second silicone resin according to the present invention are characterized by having distinct T_g's and / or distinct melt viscosities as measured at 150⁰C. The differences in these properties may be achieved by employing silicone resins which are distinct in at least one of: the degree of polymeric branching; the type of organic moieties and the degree of substitution; and, the condensable hydroxyl content. Given this, it is preferred that at least one silicone resin has viscosity of between 5 and 100 poise at 140° C, preferably between 20 and 50 poise in order to ensure that resin imparts appropriate melt-flow on the molten coating powder at the temperatures at which the coating powder is fused and cured. Furthermore, it is preferred that at least one of said first and second silicone resins has a glass transition temperature (T₉) greater than 55°C, preferably greater than 60° C.

A particularly preferred first silicone resin, which can be used without flaking is SILRES® 604 available from Wacker Chemie. This resin has a reactive hydroxyl content of between 3.5 and 7 %, a T₉ in the range of 55° to 80⁰C, and a melt viscosity at 140⁰C of 1.03 Pa. s, which corresponds to a melt viscosity at 140°C of 10.3 poise (1 Pa.s ~ 10 poise).

A particularly preferred second silicone resin, which can also be used without flaking is DC-233 available from Dow Corning. This resin has a reactive hydroxyl content of 6 %, a T₉ of 45°C and a melt viscosity at 140⁰C of 2.13 Pa. s, which corresponds to a melt viscosity at 140⁰C of 21.3 poise.

In accordance with this invention, fillers are employed to reinforce silicone coatings. As described in EP-A-O 950 695 A1 (Morton) suitable heat resistant fillers are characterized by having one dimension at least four times larger than another would provide useful reinforcement. In the same manner as described in that teaching, fillers comprising glass, metal fibers, metal flakes, mineral fibers, micas and calcium metasilicate, and which conform to this dimension requirement could therefore be included in the powder coating compositions of this invention.

In accordance with a preferred embodiment of the invention, the filler comprises fibres of an aluminium, silicon and magnesium mixed metal oxide. Preferably these particular fibers are included in the powder coating composition in an amount between 20 and 60 wt.%, based on the weight of the resin component. Equally preferably, the filler may comprise a blend of these fibers with mica platelets.

It is important to ensure that any filler is dispersed homogeneously throughout the powder coating composition that composition may be prepared with and comprise a suitable dispersant. Herein it is preferred that the powder coating composition comprises between 0.5 and 2 wt.% of a dispersant, said dispersant preferably comprising polyvinyl butyral.

In accordance with a preferred embodiment of this invention, the powder coating compositions comprise from 1 to 25 wt.%, based on the weight of the resin component, of a non-silicone resin. It is further preferred that this non- silicone resin is an epoxy-polyester hybrid. As known in the art, polyester-epoxy hybrids comprise both epoxy resins and carboxyl terminated polyester resins and may also comprise a catalyst to drive the curing reaction. In this invention it is preferred that that the powder coating compositions are based on a mixture of such polyester and epoxy resins in polyester/epoxy ratio between 80/20 and 50/50.

Suitable polyester resins for use in said polyester-epoxy hybrids should have an acid number of less than 12, preferably less than 5. Said polyester resins should also be characterized by a hydroxyl number in the range from about 20 to about 50 mg KOH/g polymer.

The weight average molecular weight (M_w) of the polyester resin may range from about 1 ,000 to about 40,000, preferably between about 1 ,500 and about

10,000. The hydroxyl functionality of the resin, i.e. the average number of hydroxyl groups present in each molecule of the resin, is 2 or more and preferably 2.2 or more, and more preferably 3.5 or more. The upper limit of hydroxyl functionality, a molecular function, should correspond to the upper limit of hydroxyl number, a molecular weight function.

It is preferred that the T₉ of the polyester resin be higher than 50° C, preferably higher than 55°C, in order to prevent blocking in a powder composition containing said polyester resin.

The polyester resins included with the powder coating composition of the present invention may be made from aromatic and/or saturated aliphatic acids and polyols using methods that are well established in this technical field. The reactants may be heated - optionally in the presence of a catalyst such as p- toluene sulfonic acid - to a temperature in the range of from about 135°C to 220⁰C while being sparged with a stream of inert gas to remove water as it forms. Vacuum or an azeotrope-forming solvent may be used at the appropriate temperature to assist the removal of water. Examples of aliphatic polycarboxylic acids include succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, diglycolic acid, 1 ,12-dodecanoic acid, tetrapropenyl succinic acid, maleic acid, fumaric acid, itaconic acid and malic acid. Examples of aromatic polycarboxylic acids are phthalic acid and its anhydride, isophthalic acid, benzophenone dicarboxylic acid, diphenic acid, 4,4-dicarboxydiphenyl ether, 2,5-pyridine dicarboxylic acid and trimellitic acid. Among the suitable polyols are ethylene glycol, 1 ,3-propylene glycol, diethylene glycol, neopentyl glycol and trimethylolpropane. Mixtures the acids and of the polyols may be used.

Commercially available polyesters useful in the powder coating composition of the present invention may comprise: P-1407 (Twin Hill); Uralac® P6505 (DSM); Morkote® 98HT (Rohm and Haas); Crylcoat® 820 (UCB); Alftalat® AN 745 (Solutia); Rucote® 625 (Bayer); and, Sparkle® SP400 (Sun Polymers).

Epoxy resins for use in the epoxy-polyester hybrids are exemplified by, but not limited to, those produced by the reaction of epichlorohydrin and a bisphenol, e.g., bisphenol A and bisphenol F. The low melt viscosities of these resins facilitate the extrusion of them in admixture with other components of the powder coating at, preferably, below 200⁰C. Suitable epoxy resins should also have a melt viscosity in the range from 2 to 20 poise at 150⁰C and preferably from 3 to 10 poise. Commercially available epoxy resins which are preferred for the purposes of this invention are the bisphenol A epoxies sold under the trademarks ARALDITE® GT-7004, GT-7071 , GT-7072, GT-6259 (Huntsman LLC) EPON® 1001 and 2042 (Shell Chemicals, Inc.).

It is known that zinc particulates may be added to powder coating compositions to impart corrosion resistance to the underlying substrate. Herein it is preferred that the powder coating composition comprises from 1 to 50 wt.%, based on the weight of the composition, of at least one of zinc dust or zinc flakes. In a preferred embodiment zinc dust is added to improved the micro crack resistance, in particular when the coating is exposed to temperatures above the melting point of zinc (419°C).

The composition preferably further comprises zinc salts, such as zinc octoate, zinc acetylacetonate or zinc neodecanoate, in a total amount from 0.1 wt.% to 2.0 wt. %, based on the weight of the powder coating composition. These salts - or alternatives such as dibutyl tin dilaurate and stannous octoate - catalyze the auto-condensation of the silicone resins thereby reducing the gel time thereof.

Flow control and leveling additives may be present in the powder coating compositions in an amount between 2 and 10 wt.%, based on the weight of the composition. Such flow control agents, which enhance the compositions melt- flow properties and assist in eliminating surface defects, typically include acrylics and fluorine based polymers. Examples of commercially available flow control agents include: Resiflow® P-67, Resiflow® P-200 and Clearflow® (all available from Estron Chemical Inc., Calvert City, KY); BYK® 361 and BYK® 300 from YK Chemie (Wallingford, CONN); and, Mondaflow® 2000 from Monsanto (St. Louis, MO).

In one embodiment of the invention, it is preferred that the flow and leveling additives include acrylic resins, especially difunctional acrylic resins and more particularly acrylic resins having glycidyl and hydroxyl functionality and having an epoxide equivalent weight (EEW) of greater than 300, such as Fine Clad ® A 241 (available from Reichold Inc.).

In order to provide improved adhesion of the powder coating composition to specific substrates, the composition may also comprise adhesion promoters in an amount between 0.1 and 1 wt. %, based on the total weight of the composition. As known in the art, adhesion promoters are characterized by having pendant or free functional or polar groups - such as carboxyl, anhydride, hydroxyl, halogen, cyano, amido or sulphonate groups - or by having an inherent adherent property or by being of relatively small molecular size. In this invention it is preferred to use a polymeric adhesion promoter and suitable polymers include: Primacor ® 5990 (available from Dow Chemicals); Surlyn ® 1855 and Nucrel ® 403 or 410 (available from DuPont); Hyvis 30 (available from BP Chemicals); Lithene N4 6000 (available from Doverstrand Ltd); and, Soarnol D (EVAL resin available from British Trades & Shippers). Degassing agents can also be used in the powder coating compositions of the present invention in an amount between 0.1 and 5 wt.%, based on the weight of the composition. Such degassing agents facilitate the release of gases during the curing process. Examples of commercially available degassing agents include: Benzoin available from Well Worth Medicines; and, Uraflow® B available from GCA Chemical Corporation (Brandenton, FLA).

The powder coating compositions may also preferably comprise a dry-flow additive in an amount from 0.05 to 1.0 wt.%, based on the total weight of the composition. Examples of such additives include fumed silica, aluminium oxide and mixtures thereof.

In addition to those components described above the powder coating compositions may comprise other conventional additives. These include: pigments; gloss-modifying additives; cratehng agents; cure agents; textuhzers; surfactants; biocides; and, organic plasticizers. Colorants or pigments useful in the powders of the present invention may include carbon black, such as 9875 Black available from Engelhard Corporation (Ohio), metal flakes, and heat resistant pigments, such as the various iron oxide pigments and mixed metal oxide pigments. The amount of colorant or pigment may range up to 20 parts per hundred resin by weight (phr), and preferably ranges from 0.1 to 15 phr, more preferably from 0.5 to 10 phr.

Preparation of the Powder Coating Composition

The powder coating compositions of the present invention, which are solid particulate film-forming mixtures, are prepared by conventional manufacturing techniques used in the powder coating industry. Typically, the components of the powder coating composition will be dry blended together, melt mixed in an extruder at a temperature sufficient to melt the two constituent resins (preferably at temperatures below 200⁰C) and then extruded. The extrudate is then cooled to a solid, broken up and ground into a fine powder.

Where dry-blending and extrusion could potentially damage certain components of a powder composition, or equally where certain abrasive components could damage blenders and extruders, it may be necessary to add such components to the formed powder.

Application of the Powder Coating Composition

The powder coating compositions are most often applied by spraying, particularly electrostatic spraying, or by the use of a fluidized bed. The powder coating compositions can be applied in a single sweep or in several passes to provide a film of the desired thickness after cure. The powder coating compositions of this invention may be applied to a variety of substrates including metallic and non-metallic substrates.

Following their application to a given thickness, the coated substrate is typically heated to a temperature between 120⁰C and 260°C for a period of 1 to 60 minutes to melt the composition, causing it to flow but also to cure to form a cross-linked matrix that is bound to the substrate. Preferably the coated substrate is heated to a temperature between 200⁰C and 250°C for a period of 20 to 40 minutes. In an alternative to this process, the powder coating compositions may be at least partially melted and cured by application to a pre- heated substrate; depending on the degree of curing the powder may be further heated after application.

The present invention is further illustrated by, but not limited to, the following example. Examples

Raw Materials

Silres-604: A hydroxyl-functional methylphenyl polysiloxane resin sold by Wacker Chemie. This resin has a reactive hydroxyl content of between 3.5 and 7 %, a Tg in the range of 55 to 80⁰C, and a melt viscosity at 140⁰C of of 10.3 poise.

DC233: A methlyphenyl silicone resin sold by Dow Corning. This resin has a reactive hydroxyl content of 6 %, a T₉ of 45°C and a melt viscosity at 140⁰C of 21.3 poise.

P-1407: Acid polyester hardener available from Twin Hill Paints P. Ltd (India)

Araldite® GT-7004: Solid, medium molecular weight Epoxy resin based on Bisphenol A available from Hunstman LLC.

Lanco TF-1780: PTFE-modified polyethylene, micronized wax available from Lubhzol Advanced Materials, Inc. .

Resiflow® P-67: Flow control agent available from Estron Chemical Inc., Calvert City, KY

Fine Clad ® A 241 : Acrylic resin having glycidyl and hydroxyl functionality and having an epoxide equivalent weight (EEW) of greater than 300 available from Reichold Inc.

Primacor ® 5990: Ethylene acrylic acid (EEA) copolymer available from Dow Chemicals.

Benzoin: Degassing agent available from Well Worth Medicines 9875 Black: Carbon black colourant available from Engelhard Corporation, Ohio.

Zinc Dust: Superfine grade available from Transpek Silox Industry Ltd.

Standart® AT: Zinc flakes available from Eckart Effect Pigments.

Coatforce CF10: A synthetically engineered aluminium, magnesium and silicon mixed metal oxide provided by Lapinus Fibres.

Mica 1240: A dry milled muscovite available from 20 Microns

Mowital B-3OH: Polyvinyl butyral provided by Kuraray.

Example 1

Preparation of the Powder Coating Compositions

A powder coating was prepared by blending the components 1 to 13 provided in Table 1. Said blended material was then passed through a twin-screw extruder, which served to melt and further mix the materials. The extrudate was solidified by passing it between chilled rollers after which it fragmented into flakes. The flakes were then mixed with the additive (component 14) and ground through a mill. The resulting powder was passed through an 80-mesh sieve to remove coarse particles.

TABLE 1

Tests Performed

Using a high pressure hose at close range, a number of steel panels were blasted using shot of the S280 grade. Using the Elcometer 233 digital surface profile gauge and in accordance with the test method of ASTM D4417 B, the average peak to valley value of the steel panels was found to be 67 microns at a standard deviation of 15 microns.

The powder coating composition of Table 1 was applied to the steel panels as a single coat using an electrostatic pistol to achieve a film thickness of between 70 and 100μm. The applied powder coating composition was cured by heating the substrate to 230⁰C and maintaining said temperature for 30 minutes.

Four panels were then exposed to different temperature regimes as shown in Table 2. Each panel was then subjected to 500 hours hot neutral salt spray in accordance with the procedure of ISO 09227. The results of these tests are also illustrated in Table 2.

Table 2

Example 2 Preparation of the Powder Coating Compositions

A powder coating was prepared by blending the components 1 to 12 provided in Table 3. Said blended material was then passed through a twin-screw extruder, which served to melt and further mix the materials. The extrudate was solidified by passing it between chilled rollers after which it fragmented into flakes. The flakes were then ground through a mill. The resulting powder was passed through an 80-mesh sieve to remove coarse particles. TABLE 3

The powder thus formed was found to have a T₉ of 56°C.

Tests Performed

Using a high pressure hose at close range, a number of steel panels were blasted using alumina grits of 120 to 210 microns grade (available from Algrain Products P Ltd). The profile of the surface was determined to be in the range of 30 to 40 microns using the Elcometer 233 digital surface profile gauge.

The powder composition of Table 3 was applied to the steel panels as a single coat using an electrostatic pistol. The applied powder coating composition was cured by heating the substrate to 230⁰C and maintaining said temperature for 30 minutes. The dry film thickness (DFT) of the coating on each of the panels was between 80 and 100μm. Six panels were then exposed to different temperature regimes as shown in Table 4. Each panel was then subjected to 500 hours hot neutral salt spray in accordance with the procedure of ISO 09227. Undercreep of the applied coating was evaluated in accordance with the procedure of ASTM B-117. The results of these tests are also illustrated in Table 4.

Table 4

Since micro cracks are absent in this test for exposure to temperatures above 419°C (the melting point of zinc), it was concluded that the addition of zinc dust to the formulations leads to less or no micro cracking and hence a better corrosion resistance at longer times for systems exposed to high temperatures.

Claims

1. A powder coating composition comprising a resin component and a filler, wherein the resin component comprises a first silicone resin and a second silicone resin, said first and second silicone resins being characterized by having glass transition temperatures (T₉) that are different by at least 5°C.

2. A powder coating composition comprising a resin component and a filler, wherein the resin component comprises a first silicone resin and a second silicone resin, said first and second silicone resins being characterized by having melt viscosities, as measured at 140⁰C, that are different by at least 5 poise.

3. The powder coating composition according to claim 1 , wherein said first and second silicone resins being characterized by having melt viscosities, as measured at 140⁰C, that are different by at least 5 poise.

4. The powder coating according to claim 3, wherein the melt viscosities are different by at least 10 poise.

5. The powder coating composition according to any one of claims 1 -3, wherein said first silicone resin has a glass transition temperature in the range from 40° to 50⁰C and said second silicone resin has a glass transition temperature in the range from 55° to 80°C.

6. The powder coating composition according to any of one of claims 1 - 5, wherein said first and second silicone resins are present in said composition in a ratio by weight (First Silicone: Second Silicone Resin) of between 2:1 and 1 :2.

7. The powder coating composition according to any one of claims 1 to 6, wherein the filler is a heat resistant material with one dimension at least four times larger than the other, said filler being present in an amount between 5 and 95 wt.% based on the weight of the resin component.

8. The powder coating composition according to claim 7, wherein the filler comprises fibres of an aluminium, silicon and magnesium mixed metal oxide.

9. The powder coating composition according to claim 8, comprising between 20 and 60 wt.%, based on the weight of the resin component, of said fibers of the aluminium, silicon and magnesium mixed metal oxide.

10. The powder coating composition according to any one of claims 1 to 9, wherein the resin component further comprises between 1 and 25 wt.%, based on the weight of the resin component, of a non-silicone resin.

11. The powder coating composition according to claim 10, wherein the non- silicone resin is an epoxy-polyester hybrid.

12. The powder coating composition according to any one of claims 1 to 11 , further comprising between 0.1 and 1 wt.%, based on the total weight of the composition, of a polymeric adhesion promoter.

13. The powder coating composition according to any one of claims 1 to 12, further comprising between 2 and 10 wt.%, based on the total weight of the composition, of a flow additive comprising a difunctional acrylic resin.

14. The powder coating composition according to claim 1 , wherein said first and second silicone resins are present in said composition in a ratio by weight (First Silicone: Second Silicone Resin) of between 2:1 and 1 :2, wherein said composition comprises between 20 and 60 wt.%, based on the weight of the resin component, of a filler comprising fibers of the aluminium, silicon and magnesium mixed metal oxide, and wherein said composition further comprises between 2 and 10 wt.%, based on the total weight of the composition, of a flow additive comprising a difunctional acrylic resin.

15. The powder coating composition according to any of claims 1 - 14 comprising zinc dust.

16.A process for coating substrate wherein the powder coating composition as described in any of the preceding claims is applied to a substrate, after which said powder coating composition is subjected to a curing step, the powder coating composition being applied in such a layer thickness that the

DFT of the cured powder coating is at least 65 microns.

17. The process according to claim 16, wherein the DFT of the cured powder coating layer is between 70 and 130 microns, preferably 75 and 100 microns.

18.A substrate coated with a cured layer of the powder coating composition as described in any one of claims 1 to 15, the layer having a DFT of at least 65 microns, preferably of between 70 and 130 microns and more preferably of between 75 and 100 microns.

19. The coated substrate according to claim 18, wherein said layer has a DFT of between 65 and 100 microns, and wherein said cured coating composition meets the pass requirements of the procedure of ISO 09227 when subjected to 500 hours hot neutral salt spray.

20. The coated substrate according to claim 19, wherein said cured coating meets the pass requirement of the procedure of ISO 09227 when subjected to 500 hours hot neutral salt spray after being subjected to 3 heat cycles, each said heat cycle consisting of 1 hour at 550⁰C followed by water quenching.