2-
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 550ºC. 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.).
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.

2A
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 260°C 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 55um).
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 urn
(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.

3
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 that are different by at least 5°C softening points and /
or having melt viscosities, as measured at 140°C, that are different by at least
10 centipoise (cps). 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 550°C.
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 softening point in the
range from 40° to 50°C and said second silicone resin has a softening point in
the range from 55° to 80°C.
Without being bound by theory, differences in the melt viscosity and / or
softening point 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

4
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.
ln 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.
ln accordance with a second 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 third 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.

5
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 Tg of any polymer may be
calculated as described by Fox in Bull. Amer. Physics. Soc, 1, 3, page 123
(1956). The Tg can also be measured experimentally using differential scanning
calorimetry (at a rate of heating 20° C per minute, wherein the Tg is taken at the
midpoint of the inflection). Unless otherwise indicated, the stated Tg as used
herein refers to the calculated Tg.
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, bum away. It is desirable that the cured, powder coatings
of the present invention withstand temperatures of at least 550°C.
The Powder Coating Composition
Although the first and second silicone resins of the present invention are to be
characterized by distinct softening points and melt viscosity, these two silicone

6
resins should both be solid at room temperature and both have a glass
transition temperature (Tg) greater than 45°C. This lower limit of Tg 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 (C1 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 0.1wt.%.
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

7
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 softening
points and / or distinct melt viscosities as measured at 150°C. 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 500 and 10,000 centipoises (cps) at 150° C, preferably between 2000
and 5000 cps 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 (Tg) 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 Tg of approximately 52°C, a
softening range of 55° to 80°C and a melt viscosity at 150°C of 15 to 30 Pa.s.

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 Tg of 45°C and a melt viscosity at 150°C of 0.172
Pa.s.

In accordance with this invention, fillers are employed to reinforce silicone
coatings. As described in EP-A-0 950 695 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, 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, however, the filler
comprises a blend of mica platelets with fibers of an aluminium, silicon and
magnesium mixed metal oxide.
lt 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 (Mw) 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

9
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 Tg 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
220°C 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).

10
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 200°C. Suitable epoxy resins should also
have a melt viscosity in the range from 200 to 2000 centipoises (cps) at 150°C
and preferably from 300 to 1000 cps. 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.
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 agents may be present in the powder coating compositions in an
amount up to 3 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).

11
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:
adhesion promoters; pigments; flow and leveling additives; gloss-modifying
additives; cratering agents; cure agents; texturizers; 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.

12
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 200°C) 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 120°C 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 200°C 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-

13
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.
Example
Raw Materials
10
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 of 55° to 80°C and a melt viscosity at 150°C of 15 to 30 Pa.s.
DC233: A methlyphenyl silicone resin sold by Dow Corning. This resin has a
reactive hydroxyl content of 6 %, a Tg of 45°C and a melt viscosity at 150°C of
0.172 Pa.s.
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
Lubrizol Advanced Materials, Inc. .
Resiflow® P-67: Flow control agent available from Estron Chemical Inc.,
Calvert City, KY
Benzoin: Degassing agent available from Well Worth Medicines
9875 Black: Carbon black colourant available from Engelhard Corporation,
Ohio.

14
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.
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.

15
TABLE 1

NO. Component Quantity (% byweight)
1 SILRES-604 17.5
2 DC-233 17.5
3 P-1407 5.6
4 ARALDITE GT-7004 2.4
5 LANCO TF-1780 1.5
6 RESIFLOW -67 1.2
7 BENZOIN 0.3
8 ENGELHARD-9875 5.0
9 ZINC DUST 34.0
10 STANDART AT 3.0
11 COATFORCECF-10 8.0
12 MOWITAL B-30H 1.0
13 MICA 1240 2.8
14 AF ADDITIVE 0.2
5The powder was applied to a number of steel panels as single coat using an
electrostatic pistol to achieve a film thickness of between 70 and 100um. The
powder was then cured for 30 minutes at 230OC.
Tests Performed
10
Using a high pressure hose at close range, 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
15deviation of 15 microns.
The applied powder coating composition was cured by heating the substrate to
230°C and maintaining said temperature for 30 minutes.
20Four 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.

16
Table 2

Panel Number TemperatureTreatment Observation Pass / Fail (inaccordance withISO 09227)
1 Panel heated to 550°Cfor1 hour and then waterquenched. This wasrepeated 3 times. No peel off or cracks. Pass
2 Panel heated to 600°Cfor 10 minutes andthen water quenched.This was repeated 5times. No peel off or cracks Pass
3 Panel heated to 500°Cfor5 hours and then aircooled. This wasrepeated 3 times. No peel off or cracks Pass
4 Panel heated to 550°Cfor 24 hours. No peel off or cracks Pass

17
WE CLAIM;
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 (Tg) that are different by at least 5°C
and / or having melt viscosities, as measured at 140°C, that are different by
at least 10 centipoise (cps).
2. The powder coating composition according to claim 1, wherein said first
silicone resin has a softening point in the range from 40° to 50°C and said
second silicone resin has a softening point in the range from 55° to 80°C.
3. The powder coating composition according to claim 1 or claim 2, 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.
4. The powder coating composition according to any one of claims 1 to 3,
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.
5. The powder coating composition according to any one of claims 1 to 4,
wherein the resin component further comprises between 1 and 25 wt.%,
based on the weight of the resin component, of a non-silicone resin.
6. The powder coating composition according to claim 5, wherein the non-
silicone resin is selected from the group consisting of and an epoxy-
polyester hybrid.

18
7. The powder coating composition according to claim 6, wherein said non-
silicone resin is an epoxy-polyester hybrid which...
8. 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.
9. The process according to claim 8, wherein the DFT of the cured powder
coating layer is between 70 and 130 microns, preferably 75 and 100
microns.
10.A substrate coated with a cured layer of the powder coating composition as
described in any one of claims 1 to 7, the layer having a DFT of at least 65
microns, preferably of between 70 and 130 microns and more preferably of
between 75 and 90 microns.
11. The coated substrate according to claim 10, wherein said layer has a DFT
of between 65 and 90 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.
12. The coated substrate according to claim 11, 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 550°C followed by water
quenching.

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 (Tg) that are different by at least 5°C
and / or having melt viscosities, as measured at 140°C, that are different by at
least 10 centipoise (cps).