CA1121932A - Powder coating composition of a mixture of epoxy resins - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A powder coating composition of a mixture of epoxy resins obtained by blending three types of epoxy resins with a curing agent. One resin is of the epichloro-hydrin-bisphenol-A type. The other two resins are epichloro-hydrin-bisphenol-A epxoy resins modified with an epoxy-novolac.
Cure is obtained by using a dicyandiamide crosslinking agent.
The composition is useful as a coating for the interior of food and beverage containers. The composition is also useful as a lining in hot-water services when finely divided poly-vinylidine fluoride powder is added for hydrophobicity.

Description

1.93~:
BACKGROUND 0~ THE INV~NTI0N
Field of the Invention This invention is related to thermosetting powder coating compositions and in particular to compositions con-taining epoxy resins.
Description of the Prior Art Epoxy resin powder coating compositions are well known,as shown by U.S. Patent 3~028~251 issued April 3~ 1962 to Nagel; U.S. Patents 3~338~863 and 3~645~960 issued August 29~ 1967 and February 29~ 1972~ respectively, to Haag. However, these powder coating compositions do not meet the approval o~f the Food and Drug Adminstration (FDA) for food contact use, such as for orotective coatings for food and beverage containers. Epoxy powder compositions of Pregmon U.S. Patent 37882~064 issued May 6~ 1975 have FDA
approval for food contact use but are not adequate for use in steel containers for highly acidic foods such as tomato products or juices.
Therefore,there remains a need for a powder coating composition which is approved for food contact and which is adequate for use with acidic foods.
SUi~.MA~Y OF THE INVENTION
The thermosetting powder coating composition of this invention consists essentially of finely divided particles at least 90 percent by weight of which have a ma~imum dimension not e~ceeding 100 microns. The particles are a blend of a cornposition of:
(A) 12.50-72.95ot by weight of an epoxy resin of the formula 3o

2 --~llZ1932 / O\ CH3 OH
CH2 - CH - CU2 - ~ , ~ 0-CH2-CH-CH -O-C~3 n C ~ - 0- C.U2- CH -\~
~3 where n is sufficiently large to provide a resin with a Gardner Holdt Viscosity of L-U
measured at 40~ polymer solids in diethylene glycol n-butyl ether at 25C and having an epoxide equivalent weight of 600-1000;
(B) 20-50% by weight of an epoxy/epoxy novolac resin which is an epoxy resin of the formula of (A) modified with an epoxy novolac resin of the formula . _ _ O CH / \ 0 C'H C/H \ o-cH2-c'!-c-2 CH - - ~ .cu _ ~

where n of the epoxy resin and m of the epoxy novolac resin have values sufficiently large to provide an epoxy/epoxy novolac resin having a Gardner Holdt Viscosity of L-U measured as in (A) and having an epoxide equivalent weight of ~ Z~ ~3~
~ .

400-900;
(C) 5-30% by weight of a second epoxy/epoxy novolac resin of the formula of (B) but having values for n and m sufficiently large to provide an epoxy/epoxy novolac resin with a Gardner Holdt Viscosity of V-~l measured as in (A) and having an epoxide equivalent weight of 600-1000;
(D) 0.05-0.5% by weight of a catalyst of tri(dimethyl aminomethyl) phenol or a fatty acid salt thereof;
and (E) 2.0-7.0% by weight of dicyandiamide.
Optionally, the composition can contain 45-86%
by weight, based on the weight of (A) plus (B) plus (C), of polyvinylidene fluoride in the form of finely divided particles at least 80% by number of which have a maximum dimension not exceeding 5 microns.
~ET~ILED DESCRIPTION OF THE INVENTION

... . ..
The coating composition of this invention has a powder particle size such that at least 90 percent by weignt of the particles have a maximum dimension not exceeding 100 microns. Preferably, the particles of binder component, designated herein to be -the epoxy and epoxy/epoxy novolac resin, are 10-74 microns in maximum dimension and more preferably 20-55 microns.
About 12.5-72~9% by weight of the powder coating composition is an epoxy resin of the epichlorohydrin-bisphenol-A type of ihe formula ~3 C~2 - C:' - C:~2 o _ ~ CH3 0-CH2-CH-C~ t ~
CH3 / \
C ~ ~ - 0- CH2 CH - C~?

where n is sufficiently large to provide a resin with a Gardner Holdt Viscosity of L-U
and having an epoxide equivalent weight of 600-1000. As used in this specificat~on, Gardner Holdt Viscosity is based on measurements taken at 40%
polymer solids in diethylene glycol n-butyl ether at 25C. The epoxide equivalent weight is the weight in grams of resin that contains one gram equivalent of epoxide.
In one preferred resin which forms a high-quality coating in combination with the other constituents of the present invention~ the Gardner Holdt Viscosity is N-R and the epoxide equivalent weight is 700-800.
About 20-50% by weight of an epoxy/epoxy novolac resin is used in the composition. Such a resin is an epichlorohydrin-bisphenol-A resin of the formula shown above modified with an epoxy novolac resin of the formula 93~:

o o o \ /\ /\
O-CH -CH-CH O-CH -CH-CH O-CH~-CH-CH2 C~I2 - ~ ~ CH2 - m where n of the epoxy resin and m of the epoxy novolac resin have values sufficiently large to provide an epoxy/epoxy novolac resin having a Gardner Holdt Viscosity of L-U and an epoxide equivalent weight of 400-900.
Epoxy/epoxy novolac resins are formed by modifying an epichlorohydrin-bisphenol-A resin with an epoxy novolac resin of the type shown in U.S. Patent 2,658,8~5 of G. D'Alelio, issued 1953 Nov. 10. It is believed that the modification is either direct crosslinking of the two resins or indirect crosslinking through reaction of both resins with bisphenol-A.
By varying the ratio of epichlorohydrin-bisphenol-A epoxy to epoxy novolac, the viscosity and epoxide equival-ent weight can be varied and epoxy/epoxy novolac resins of different reactivity can be provided.
About 5-30% by weight of a second epoxy/epoxy novolac resin is used in the composition. This resin has the same general formula as the above epoxy/epoxy novolac but has a Gardner Holdt Viscosity of V-~l and an epoxide equivalent weight of 600-1000~
A curing catalyst in the amount of 0.05-0.50%
by weight is used in the composition. Preferably, about 0.01-0.4% by weight of the ca~alyst is used. The catalyst is tri(dimethyl aminomethyl)phenol or its fatty acid salts.

Typical fat-ty acid salts that can be used are tri(dimethyl aminomethyl)phenol caprate, tri(dimethyl aminomethyl) caprylate, tri(dimethyl aminomethyl)phenol isodecanoate, tri(dimethyl aminomethyl)phenol linoleate, tri(dimethyl aminomethyl)phenol napthenate, tri(dimethyl aminomethyl) phenol neodecanoate, tri(dimethyl aminomethyl)phenol octoate, tri(dimethyl aminomethyl)phenol 2-ethyl hexoate, ~ri(dimethyl aminomethyl)phenol oleate, tri(dimethyl aminomethyl)phenol palmitate, tri(dimethyl aminomethyl)phenol resinate, tri-(dimethyl aminomethyl)phenol resinoleate, tri(dimethyl aminomethyl)phenol soyate, tri(dimethyl aminomethyl)phenol stearate, tri(dimethyl aminomethyl)phenol tallateand the like~
One preferred catalyst which forms a high quality product is 2,4,6-tri(dimethyl aminomethyl)phenol.
The composition contains 2.0-7.0% by weight of dicyandiamide curing agent. To improve the handling characteristics of the dicyandiamide, the compound can be blended with finely divided silica. For example, a blend in a ratioof 95/5 of dicyandiamide to silica can be used.
Flow control agents can be added to the powder coating composition of this invention in amounts of about 0.01-1.0% by weight. Typical flow control agents are polyacrylates, such as poly-2-ethylhexyl acrylate, finely divided ethyl cellulose, and siloxanes, such as dimethyl polysiloxanes or methyl phenyl polysiloxane.
To improve the handling properties of the powder coating composition and to prevent caking, finely divided silica in the amounts of 0.05-0.5% by weight can be added.
The silica can be blended with any of the ~poxy resins or as mentioned before with the dicyandiamide.

.932 The silica is preferably ~umed silica. This ls a very fine, amorphous silica formed fxom silica spheres having an average diameter of 7-14 milimicrons. A suitable substance for this purpose is commercially available under the trademark Cab-O-Sil~ registered to Cabot Corporation.
One particularly preferred composition that forms a high quality interior coating for steel containers is of (A) 33.6-55.9~ by weight of an epoxy resin having the aformentioned formula wherein the resin has a Gardner Holdt Viscosity of N-R and an epoxide equivalent weight of 700-800;
(B) 35-45% by weight of an epoxy/epoxy novolac resin described previously where n and m are sufficiently large to provide a Gardner Holdt Viscosity of O-S and an epoxide equivalent weight of 500-700;
(C) 5-15% by weight of a second epoxy/epoxy novolac resin described previously where n and m are sufficiently large to provide a Gardner Holdt Viscosity of W-~ and an epoxide equivalent weight of 700-900;
(D) 0.1-0.4% by weight of a catalyst of tri(dimethyl aminomethyl~ phenol; and (E) 4. o-6 . o% by weight of dicyandiamide.
Another useful composition that for~s a high quality finish on the interior of steel containers is of ~A) ~8.6-60.9% by weight of the epoxy resin ~12~L~3~

described in the above preferred compositions;
(B) 20.0-30.0% by weight of the epoxy/epoxy novolac resin described in the above preferred composition, (C) 15.0-35.0% by weight of a second epoxy/epoxy novolac resin described in the above preferred composition, (D) 0.1-0.4% by weight of a catalyst of tri(dimethyl aminome-thyl) phenol and (E) 4.0-6.0% by weight of dicyandiamide.
The composition can be pigmented or unpigmented but is usually pigmented and contains pigments in a pigment/
binder ratio of 2/100 to 80/100. Any of the conventional inorganic or organic pigments, filler pigments, or dyes can be used. Examples of the great variety of usable pigments are as follows: metallic oxides, such as titanium dioxide, zinc oxide, iron oxide and the like; metallic flakes such as aluminum flake, metallic powders; metal hydroxides, AFFLAIR*
pigments such as mica flake coated with titanium dioxide;
sulfides; sulfates; carbonates, carbon black; silica; talc, china clay; and other extender pigments.
However, if the powder coating composition is to be in contact with foods only those pigments and additions having Food and Drug Administration approval are used.
One method for forming the powder coating com-position of this invention is to blend the components together and then to pass the mixture through a conventional extruder.
The extrudate can then be reduced to a powder using conventional grinding equipment. After grinding, the powder is passed through a sieve to remove particle larger than * denotes trade mark ....6,.

3~

110 microns. Preferably, a seive which eliminates particles of maximum dimension greater than 74 microns is used.
In another aspect of the present invention, it has been found that the composition has excellent hydro-phobicity, such that it is very useful as a lininy for hot water tanks, when about 45-86% by weight, based on the weight of the binder components, of polyvinylidene fluoride is added. The polyvinylidene fluoride can be of the type des-cribed in U.S. Patent 2,435,537 of T. Ford, issued 1948 February 03. Although most useful for coating the interiors of water heater tanks, this composition is also useful to line boilers, pipe and drum interiors, industrial containers, or any surface which contacts water at elevated tPmperatures.
Preferably, the polyvinylidene fluoride is in particulate form. The particles can have a maximum dimension up to 12-15 microns with 80% by number, but most preferably 95% by number, having a maximum dimension of 5 microns or less. A suitable substance to use is Kynar~, sold by Pennwalt Chemical Company.
Polyvinylidene fluoride is ordinarily present in this composition in an amount equal to 45-86%, preferably 50-75% and most preferably 57-65%, of the weight of the binder. A preferred composition which forms a high quality lining for the interior of a hot water tank is of (A) 25.0-35.0% by weight of an epichlorohydrin-bisphenol-A epoxy resin of the aforementioned formula wherein the resin has a Gardner Holdt Viscosity of N-R and an epoxide equivalent weight of 700-800;
(B) 20.0-30.0~ by weight of an epoxy/epoxy ~"~

novolac resin as described previously where n and m are sufficiently large to provide a Gardner Holdt Viscosity of 0-S and an epoxide equivalent weight of 500-700;
(C) 4.0-8.5% by weight of a second epoxy/epoxy novolac resin described previously where n and m are sufficiently large to provide a Gardner Holdt Viscosity of W-~ and an epoxide equivalent weight of 700-900;
(D) 0.06-0.12% by weight of a catalyst of tri(dimethyl aminomethyl) phenol;
(E) 1.5-3.5% by weight of dicyandiamide; and (F) 33.0-41.5% by weight of polyvinylidene fluoride.
The powder coating composition containing polyvinyli-dene fluoride can be formed in much the same way as previously described. However, it has been found that when the powder composition is to contain more than 20% by weight of the fluoropolymer, the powder composition can be more easily formed when the excess over 20% is post-added by dry blending.
That is, ln such a situation, the composition which is ex-truded contains no more than 20% by weight, based on total composition weight, of polyvinylidene fluoride. Aft~r the extrudate is reduced to powder form, as previously described, an amount of particulate polyvinylidene fluoride which is sufficient to raise the concentration to the desired level is added to, and thoroughly mixed with, the powder.
The powder coating compositions of this invention can be applied to a metal, glass, plastic, or a fiber-rein-forced plastic substrate by electrostatic spraying or by i~ ~, 1~L2~3Z

using a fluidized bed which ean be eleetrostatic. Prefer-ably, eleetrostatic spraying is used in which a voltage of 20-100 kilowatts is applied to the spray gun. The eomposit-ion ean be applied in one pass or several passes to provide variable thicknesses, after eure, of 1.5-4.0 milsl preferably 1.8-3.0 mils. After the applieation of the powder, the eoated article is heated at 180-235C for 6-15 minutes to fuse and to eure the powder partieles into a substantially eontinuous uniform eoating.
Since the binder components of the present invention are more fluid than the polyvinylidene fluoride at the extrusion and cure temperatures, the cured eoating substant-ially approximates a continuous phase of erosslinked epoxy and epoxy/epoxy novolae resins having polyvinyldene fluoride dispersed throughout.
The powder eoating eomposltion ean be applied direetly to untreated metals sueh as aluminum or steel.
In one particular applieation, the powder is applied directly to the exterior or the interior of steel containers which are used for aqueous acidic foods and beverages, sueh as tomato produets like pastes, soups, juiees, and catsup.
The coating eomposition ean also be applied over a suitably treated or primed metal substrate. Typieal conventional alkyd primers or epoxy primers pigmented with iron oxide, earbon black, titanium dioxide and the like ean be used. Electrodeposited primers ean also be used.
Also, the eomposition ean be used direetly over galvanized phosphatized steel to form a durable coating. An elee-trically conductive carbon black pigment can be added 'llZ~1932 ~o the primer to make the surface conductive ~nd to pro20te uniform deposition of the ?owder while spraying.
The following Examples illustrate the invention.
All quantities are on a weight basis unless otherwise in-dicated.
EXAMPLE I
The following ingredients are mixed together to form powder coating composition A.

Parts by Weight 1. Epichlorohydrin-Bisphenol-~
type epoxy resin having the formula described and shown in the above specification wherein n is sufficiently large to provide a resin with a Gardner Holdt Viscosity of P
and an epoxide equivalent weight of 765. 42.5 2. Epoxy/Epoxy Novolac type resin having the formula described and shown in the above speci-fication wherein n and m are sufficiently large to provide a resin with a Gardner Holdt Viscosity of 0-S and an epoxide equivalent weight of 500-575. 4 3. Epoxy/Epoxy Novolac II resin having the formula described and shown in the above specification wherein n and m are sufficiently large to prGvide a Gardner Holdt Viscosity of W-~ and an epoxide equivalent weight of 775-850.10.0

4. Flow agent(5% by weight polyethyl-hexyl acrylate and 95% by weight of the epoxy resin described as ingredient 1.) 7.5

5. Curing agent (95/5 weight ratio of dicyandiamide to finely divided silica) 4-

6. Gatalyst[2,4,6-tri(dimethyl amino-methyl) phenol] 0.15

7. Titanium dioxide pigment 50.0 _ Total 154.15 ~ 193~

The above mixture is charged into a standard three zone melt extruder in which zone 1 is at 50C, zone 2 is at 70C and zone 3 is at 100C and which is operated at 90 revolutions per minute. The resulting molten mixture is extruded and the resulting extrudate is cut into pellets and charged into a grinding mill that grinds the extrudate into a powder. The resulting powder is passed through a 140 mesh screen to remove large particles.
The powder is sprayed onto the exterior of phosphatized cold roll steel panels with an electrostatic powder gun and the panels are baked for 6 minutes at 205C
in a gas oven. The resulting finish on the panels is about 1.5-2.0 mils thick and is smooth, even, glossy, flexible and has a good appearance. The finish has a gloss measured at 60 of 96.3 and a Tukon hardness of 19.2.
Three of the above prepared panels are immersed about half way into boiling water for one hour and then removed and examined for staining, delamination, pinholes and other defects in the finish. No defects in the finish on any of the exposed portions o~ the panels were found.
After six hours in boiling water the panels were examined and no defects were found.
Adhesion of the finish is tested by scoring the finish through to the metal and attempting to remove finish from the panel with scotch tape. The width of coating, measured from the center of the score line, which delaminates is known as creepage. The creepage of the finish is minimal and therefore its adhesion to the metal is acceptable.
Three of the above finished panels are immersed in tomato paste at 60C for 2 weeks, tomato paste at 49C for 1 month and tomato paste at 38C for 6 months, respectively.

~1~1932 After each of the above time periods, the panel was removed and examined for defects, such as loss of adhesion, delaminating and staining. In each case the finish was found to be acceptable.
Powder coating composition B is prepared identically to the above coating composition except 25 parts by weight of the Epoxy/Epoxy Novolac resin and 25 parts of the Epoxy/Epoxy Novolac II resin are used. The resulting powder coating composition is applied as above to phosphatized steel substrates and baked as above. The resulting finish is about 1.5-2.0 mils thick and is smooth, even, glossy and has a good appearance. The finish has a Tukon hardness of greater than 15 knoops.
The panels are tested in boiling water and in tomato paste as above and similar results were noted. None of the finishes on the panels failed the tests but were acceptable.

Powder coating composition C is prepared using the same procedure and constituents used to prepare pcwder coating composition A of Example 1 except the following Resin is substituted for the Epoxy/Epoxy Novolac resin (ingredient 2):
Epoxy/Epoxy Novolac resin wherein n and m are sufficiently large to provide a Gardner ~Ioldt Viscosity of R to T and an epoxide equivalent weight of 700-825.
The resulting powder coating composition is applied as in Example 1 to phosphatized steel substrates and baked as above. The resulting finish is about 1.5-~L21~32 2.0 mils thick, is smooth, and glossy, even and ~lex~ble and has a good appearance.
About half of a panel is immersed in boiling water and in tomato paste as in Example 1. No defects in the finish such as stains, cracks, and pinholes ar~ noted.
EXAMPLE _ The ~ollowing ingredients are mixed together to form powder coating composition 3.

Parts by Weight 1. The epoxy resin described as 22.06 ingredient 1 in Example 1 2. The epoxy/epoxy novolac resin described as ingredient 2 in Example 1. 20.76 -3. The epoxy/epoxy novolac II resin described as ingredient 3 in Example 1 5.19 4. Flow agent(described in Example 1) 3.89 5. Curing agent(described in Example 1) 2.08 6. Catalyst[2,4,6-Tri(dimethyl amino-methyl) phenol] 0.08 7. Titanium dioxide pigment 25.87

8. Aluminum/Cobalt Oxide Pigment 0.56

9. Kynar~ 20.0__ Total100.43 The above mixture is charged into a standard three-zone melt extruder in which zone 1 is at 55C, zone 2 is at 80C, and zone 3 is at 100-110C and which is operated at 90 revolutions per minute. The resulting molten mixture is extruded and the resulting extrudate is broken into chips and then charged into a grinding mill where it is ground to a fine powder. The powder is then passed through a 1~0 mesh screen.

~2~93~

This powder is then thoroughly mixed with Kynar~
powder in a concentration of 9Q parts ~y weight of powder to lQ parts Kynar~. The resulting powder composition is then sieved again through a 140 mesh screen.
The powder is sprayed onto the exterior of phos-phatized cold roll steel panels with an electrostatic powder gun. The panels are heated in a g~s oven for 6 minutes at 205C. The resulting finish on the panels is about 2.~-2.6 mils thick and is smooth and even with no popping or cratering.
Several of the above-prepared panels are immersed in a resin kettle of refluxing boiling water for two months.
At the end of this period, the panels are tested for adhesion, impact resistance, and extent of blistering.
Adhesion of the finish is tested as in Example 1.

The creepage is found to be approximately 0.8mm which is acceptable. The impact resistance is tested with a Gardner impact tester No. 16-1120 and the coated panel is found to have a reverse impact resistance of 100 inch-pounds. A
visual inspection is made for blistering and only slight blistering along the panel edge~ within acceptable limits3 is noted.

Claims (19)

WHAT IS CLAIMED IS

1. A thermosetting powder coating composition comprising finely divided particles at least 90 percent by weight of which have a maximum dimension not exceeding 100 microns wherein the particles consist essentially of (A) 12.50-72.95% by weight of an epoxy resin of the formula where n is sufficiently large to provide a resin with a Gardner Holdt Viscosity of L-U and having an epoxide equivalent weight of 600-1000;
(B) 20-50% by weight of a first epoxy/epoxy novolac resin which is an epoxy resin of the formula of (A) modified with an epoxy novolac resin of the formula where n of the epoxy resin and m of the epoxy novolac resin are sufficiently large to provide an epoxy/epoxy novolac resin having a Gardner Holdt Viscosity of L-U and an epoxide equivalent weight of 400-900;
(C) 5-30% by weight of a second epoxy/epoxy novolac resin of the formula of (B) but having values for n and m sufficiently large to provide an epoxy/epoxy novolac resin with a Gardner Holdt Viscosity of V-Z1 and having an epoxide equivalent weight of 600-1000;
(D) 0.05%-0.5% by weight of a catalyst of tri(dimethyl aminomethyl) phenol or fatty-acid salts thereof; and (E) 2.0-7.0% by weight of dicyanadiamide.

2. The powder coating composition of Claim 1 containing pigment in a pigment to binder ratio of 2/100 to 80/100.

3. The powder coating composition of Claim 1 wherein the particles consist essentially of (A) 33.5-55.9% by weight of said epoxy resin;
(B) 35.0-45.0% by weight of said first epoxy/epoxy novolac resin;
(C) 5.0-15.0% by weight of said second epoxy/epoxy novolac resin;
(D) 0.1-0.4% by weight of said catalyst, and (E) 4.0-6.0% by weight of dicyandiamide.

4. The powder coating composition of Claim 1 wherein the particles consist essentially of (A) 28.6-60.9% by weight of said epoxy resin, (B) 20.0-30.0% by weight of said first epoxy/epoxy novolac resin;
(C) 15.0-35.0% by weight of said second epoxy/
epoxy novolac resin.

(D) 0.1-0.4% by weight of said catalyst; and (E) 4.0-6.0% by weight of dicyandiamide.

5. The coating composition of Claim 1 wherein said epoxy resin has a Gardner Holdt Viscosity of N-R
and has an epoxide equivalent weight of 700-800.

6. The powder coating composition of Claim 5 containing 0.1-1.0% by weight of a flow-control agent of ethyl cellulose, a polyacrylate, or a siloxane

7. The powder coating composition of Claim 6 in which the flow control agent is a polyacrylate.

8. The powder coating composition of Claim 6 containing 0.05-0.5% by weight of finely divided silica.

9. A metal substrate coated with a smooth, even,coalesced layer of the powder coating composition of Claim l.

10. The coated metal substrate of Claim 9 in which the metal is steel.

11. A thermosetting powder coating composition comprising fine divided particles at least 90 percent by weight of which have a maximum dimension not exceeding 100 microns wherein the particles consist essentially of (A) 15.5-46.5% by weight of an epoxy resin of the formula where n is sufficiently large to provide a resin with a Gardner Holdt Viscosity of L-U and having an epoxide equivalent weight of 600-1000;
(B) 14.0-34.5% by weight of a first epoxy/epoxy novolac resin which is an epoxy resin of the formula of (A) modified with an epoxy novolac resin of the formula where n of the epoxy resin and m of the epoxy novolac resin are sufficiently large to provide an epoxy/epoxy novolac resin having a Gardner Holdt Viscosity of L-U and an epoxide equivalent weight of 400-900;
(C) 3.0-19.0% by weight of a second epoxy/epoxy novolac resin of the formula of (B) but having values for n and m sufficiently large to provide an epoxy/epoxy novolac resin having a Gardner Holdt Viscosity of V-Z1 and having an epoxide equivalent weight of 600-1000;
(D) 0.04-0.4% by weight of a catalyst of tri(dimethyl aminomethyl) phenol or a fatty-acid salt thereof;
(E) 1.5-5.0% by weight of dicyandiamide; and (F) 29.0-45.0% by weight of polyvinylidene fluoride.

12. The powder coating composition of Claim 11 containing pigment in a pigment to binder ratio of 2/100-80/100.

13. The powder coating composition of Claim 11 wherein the particles consist essentially of (A) 25.0-35.0% by weight of said epoxy resin;
(B) 20.0-30.0% by weight of said first epoxy/epoxy novolac resin;
(C) 4.0-8.5% by weight of said second epoxy/epoxy novolac resin;
(D) 0.06-0.12% by weight of said catalyst;
(E) 1.5-3.5% by weight of dicyandiamide; and (F) 33.0-41.5% by weight of polyvinylidene fluoride.

14. The powder coating composition of Claim 11 wherein said epoxy resin has a Gardner Holdt Viscosity of N-R and has an epoxide equivalent weight of 700-800.

15. The powder coating composition of Claim 14 containing 0.1-1.0% by weight of a flow control agent of ethyl cellulose, a polyacrylate, or a siloxane.

16. The powder coating composition of Claim 15 wherein the flow control agent is a polyacrylate.

17. The powder coating composition of Claim 15 containing n. 05-0.5% by weight of finely divided silica.

18. A metal substrate coated with a smooth, even, coalesced layer of the powder coating composition of Claim 11.

19. The coated metal substrate of Claim 18 in which the metal is steel.