CA1052494A - Thermosetting powder coating composition of a blend of acrylic polymers having different glass transition temperatures and a blocked polyisocyanate cross-linking agent - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A thermosetting polymer powder coating composition comprising finely divided powder particles that have a diameter of about 1-100 microns wherein the powder particles are a blend of (A) a hard acrylic polymer having a high glass transition temperature;
(B) a soft acrylic polymer having a low glass transition temperature;
(C) a blocked organic polyisocyanate;
(D) a hydroxy functional plasticizers;
and (E) catalytic amount of an organometallic compound;
the thermosetting powder coating composition is particularly useful as an exterior finish for automobile and truck bodies.

Description

~.~5Z~94 ~

BACKGROUND OF THE INVENTION
' -: ' ' ' lo Field of the Inventlon ....
This invention is related to powder coating composition~ and in particular, to a thermosett~ng polymer powder coating composition.
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2. crqD~lon ol ~e ~rlor r~ .

To curb pollution caused by conventional so1vent based coating systems, the automobile and truck manu~actur- .
ing industry intends to utilize powder coating compositions.
However~ the industry demands that these powder coating compositions be of a high qualit~.
Thermosetting powder coating composition~ of polyester resins cross-linked with polyisocyanates are well known in the art a~ shown in British Patent 15 265,634, published March l, 1972:and British Patent l,320,657, ~ ;
publ~shed June 20, 1973. In general, these polyester powder coatings do not hav~ the high level o~ outdoor durability that iB required for exterior u~e on automobile and truck bodies. Powders of acrylic polymers cross-linked with aromatic polyisocyanates are known as shown ln Labana U.S. 3,676,405, hut coatings from these powders would have poor exterior durability. In addition, the thermosetting . .
powders heretofore known in the art have a poor appearance caused by inadequate flow of the finish during baking.~:
... ..
The above defects make the compositions unacceptable for an exterior finish for automobile and truck bodies.

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SUMMARY OF THE INVENTION

The thermosetting polymer powder coatlng composition of this invention comprises ~inely divided particles that have a particle size o~ l-lOO micronæ; the particles are an intimate blend of the ~ollowing con-stituents: :
(A) 20-70% by weight of a hard acrylic :~
polymer of (1) 80-95~ by weight, based on th~
weight o~ the acrylic polymer, of methyl methacrylate, styrene or a -mixture of methyl methacrylate and styrene;
(2) 5-2,0~ by weight, based on the weight of the acrylic polymer, of a hydroxyalkyl acrylate or a hydroxyalkyl methacrylate that have 2-4 earbon atoms in the alkyl groups or a mixture of the :
above acrylate or methacrylate;

wherein the hard acrylic polymer has a glass i`
transition temperature of about 70-103C. and a weight average molecular weight of 5,000-20,000; -~
... . . . .
(B) 5-20% by weight of a soft acrylic polymer .
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(1) 30-62~ by weight, based on the weight of the soft acrylic polymer, of methyl methacrylate; styrene or a mixture of methyl methacrylate and styrene;

(2) 35-60% by weight, based on the weight o~ the so~t acrylic polymer, of an alkyl acrylate having 2-12 carbon atoms in the alkyl group, an alkyl methacrylate having 4-12 carbon atoms in the alkyl group or a mlxture of the above;

(~) 3-10% by weight of a hydroxy- .. .
alkyl methacrylate, a hydroxyalkyl acrylate each having 2-4 carbon atoms in the alkyl groups or a mixture of the above;

wherein the soft ac~ylic polymer has a glass .`.
tra~sition temperature of about -20C. to 15C. and a .
weight average molecular welght o~ about 5,000-50,000;
;: ~- ' (C) 15-30% by weight of a blocked organic :
polyisocyanate, `

''' ' .

(D) 10-3Q~ by weight o~ a hydroxy functional plasticizer ha~ing a hydroxyl number of 18~300 and a number a~erage molecular weight of about 500-6,000; and (E) a catalytic amount of an organo-metallic compound.
` ~ ' DESCRIPTION OF THE INVENTION
. . .
The thermosetting powder coating composition o~ :.
this invention has a suf~iciently high glass transition temperature to maintain a ~ree-f'lowing powder and has su~ficiently low melt viscosit~ to flow to an exceptionally ::
smooth, glossy finish. The ~inish is of a high quality and has a good appearance and good physical properties as is required for the exterior finishes of automobile and truck bodies.
- . :. -The thermosetting powder coating composition of ~
this invention has powder particles that have a diameter .-of about 1-100 microns and preferably the powder particles are 10 to 75 microns ln diameter to provlde a hlgh quallty finish.
~....... .
The particle size of the powder coating ls determlned by the dlrect measurement of the diameter of the indlvldual particles under a microscope or by a Coulter Counter Instrument.
. .

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~52~4 The powder particles of the powder coating can contain pigment or can be unpigmented to provlde a clear finish. Generally, the powder particles are pigmented and contain up to 60~ by weight, based on the welght o~ unpigmented powder coating, of pigment.
Pigmentation can be as low as 001% by ~e~ght. Any o~ the conventional inorganic pigments, metallic powder~ and flakes, organic dyes, organic pigments, and lake~ can be used.
The hard acrylic polymer and the ~o~t acrylic polymer are chosen 50 that there i9 borderline incompati-bility between the polymers. Th~s is readily observed in an electron micrograph o~ an unpigmented coalesced cured or uncured film o~ the powder wherein discrete areas of so~t acrylic polymer are noted in the hard acryllc polymer ~ilm. I~ the polymers are completely incompatible, a stable powder cannot be ~ormed and the film properties of the powder are poor.
About 20-70% ~y weight of the hard acrylic polymer ?0 is utilized in the powder coating composition. Also, blends of different hard acrylic polymers can be used. The hard acrylic polymer has a weight average molecular weight of about 5,000 to 20,000 and preferably 6,0oo to 17,000 to form a high quality powder. The hard acrylic polymer has a glass transition temperature o~ 70 to 103C. and pre~erably 80 to 102C. and a hydroxyl content of about 0.5 -3% by weight and preferably a hydroxyl content which provides good reactivity of the polymer Or l. 0-l . 8% by weight.

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The powder contains ahout 5-2Q% by weight~ The soft acrylic polymer has a weight average molecular weight of about 5,00~ to 50,000, and preferably, lO,QaO to 30gO00 to form a good quality composition. The soft acrylic polymer has a glass transition temperature of -20C. to 15C. and a hydroxyl content of about 0.5-2% by weight, and preferably,0.5-1.5% by welght for good reactivity with the isocyanate constituent.
The weight average molecular weight Or the acrylic polymers are determined by gel permeation chromo-tography using polymethyl methacrylate or polystyrene as the standard.
Both the hard and soft acrylic polymers are -prepared by conventional solution, emulsion or bead polymerization techniques and by using conventional~
polymerization catalysts.
The acrylic polymers can be prepared by conven-tional solution polymerization techniques in which the monomer constituents are blended with solvents and a polymerization catalyst and the reaction mixture is heated to 55-150C. for about 2-6 hours to form a polymer that has the aforementioned weight average molecular weight.
Typical solvents which are used to prepare the acrylic polymers are toluene, ethyl acetate, acetone, methylisobutyl ketone, methylethyl ketone, ethyl alcohol, and other esters, ethers, ketones, and alcohols, such as are conventionally used. Solvents having a boiling point below 100C. are preferred to facilitate spray drying of a formulated composition or solvent removal from a formulated 3o composition by vacuum stripping to form the powder coating composition. Preparation of a formulated composition used to ~5Z~94 form the powder coating composition is discussed hereinafter.
About 0.1-4% by weight, based on the ~eigh~ of the monomers~ of a polymerization catalyst is used to pre-pare the acrylic polymers. Typical catalysts are azo~bis-(~,gamma-dimethyl-valeronitrile), benzoyl peroxide, t-butyl peroxy pivalate, azobisisobutyronitrile and the like. Up to about 5% by weight of a chain transfer agent can be used to control the molecular weight such as dodecyl mercaptan or mercaptoethanol which is preferred to provide acrylic polymers with terminal hydroxyl groups.
The acryllc polymers can be prepared by an emulsion polymerization process as disclosed in U.S. Patent

3,790,513. The acrylic polymer can also be prepared by suspension or bead polymerization techniques as disclosed in W. R. Sorenson and T. W. Campbell, Preparative Methods of Polymer Chemistry, Interscience Publishers, New York, 2nd Ed.
1968, page 254.
~he hard acrylic polymer utilized in the novel powder coating composition of this invention contains about 80-95% by weight of methyl methacrylate or styrene or a ^
mixture of methyl methacrylate styrene, and optionally, up to 10% by weight of an alkyl methacrylate or an alkyl acrylate having 2-14 carbo~ atoms in the alkyl groups and 5-20% by weight of a hydroxy alkyl methacrylate or a hydroxy alkyl acrylate each having 2-4 carbon atoms in the alkyl groups or mixtures thereof.
One preferred hard acrylic polymer that forms a good quality composition contains 82-94% by weight of methyl methacrylate, 1-10% by weight of the alkyl methacrylate or ;~;
acrylate, 5-17% by weight of the hydroxy alkyl acrylate or 3~5249~ :~
.~ .
methacrylate. One particularly useful hard acrylic polymer that forms a durable high quality finish is of methyl methacrylate, lauryl methac~ylate, hydroxyethyl acrylate, or hydroxypropyl methacrylate. Another use~ul hard acrylic polymer is of methyl methacrylatic and hydroxypropyl meth-acrylate.
The soft acrylic used ln the powder coating composition contains about 30-62~ by weight of methyl methacrylate, styrene or a mixture of methyl methacrylate and styrene, 35-60% by weight o~ an alkyl acrylate having 2-12 carbon atom~ in the alkyl group or an alkyl methacrylate having 4-12 carbon atom~ in the alkyl group and 3-10% by weight o~ a hydroxy alkyl acrylate or a hydroxy alkyl methacrylate having 2-4 carbon atom~ in the alkyl groups. One preferred soft acrylic polymer whlch ~orms an excellent ~inlsh contains 45-60~ by weight of methyl methacrylate, 35-45~ by ~eight of an alkyl acrylate hav~ng 4-8 carbon atoms in the alkyl groupJ and 5-10% by w~ight o~ hydroxy alkyl acrylate or a hydroxy alkyl ~o methacrylate.
One particularly useful so.~t acrylic polymer cont~ins methyl methacrylate, butyl acrylate and hydroxy ethyl acrylate.

Typlcal alkyl acrylates and alkyl methacrylate~ :
ha~ing 2-14 carbon ato~s in the alkyl groups that can be used to prepare the acryllc polymer~ ~re as follows: ~
ethyl acrylate~ propyl acrylate, butyl acrylate, hexyl . ~ :
acrylate, 2-ethylhexyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, tetradecyl acrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylatet ~0 isobutyl methacrylate9 hexyl methacrylate, 2-ethylhe~yl methacrylate~ nonyl methacrylate, decyl methacrylate, lauryl methac~ylate and tetradecyl methacrylate and the llke.

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Typical hydroxy alkyl acrylates and methacrylates which can be used to prepare the acrylic polymer are 2-hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate, hydroxyethyl methacrylate, hydroxypropyl meth-acrylate, hydroxybutyl methacrylate, and the like. Hydroxy-ethyl acrylate and hydroxypropyl methacrylate are preferred.
Hydroxypropyl methacrylate, as commercially avail-able, is a mixture of 2-hydroxypropyl methacrylate and 1-methyl-2-hydroxyethyl methacrylate. One useful mixture is of 68-75% by weight of 2-hydroxypropyl methacrylate and 25-32% by weight o~ l~methyl-2-hydroxyethyl methacrylate.
Also, hydroxypropyl acrylate, as commercially available, is a mixture of 2-hydroxy-1-propyl acrylate and 1-hydroxy-2-propyl acrylate. -Optionally, the powder coating composition of this invention can contain up to 20% by weight of cellulose acetate butyrate having a viscosity of 0.005-2.0 seconds measured according to ASTM-D-1343-56 at 25C. and has butyryl content of about 30-60% by weight. Generally, 1-20% by weight of cellulose acetate butyrate is used, but to form a good quality composition, 2-10% by weight of a cellulose acetate butyrate that has a viscosity of 0.008-0.2 second and butyryl content of 50-55% is used. The cellulose acetate butyrate provides an excellent appearance to the finish prepared from the powder coating composition. Cellulose acetate also is useful as a dispersant for pigments which are used in the powder coating composition.
The powder contains about 15-30% by weight of a blocked organic polyisocyanate blocked with an organic blocking agent.

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~L~5Z~9~ :
A wide variety o~ aliphatic polyisocy~nat~s, aromatic pol~isocyanates, alicyclic pol~isocyanate6 can be used in th~s inventionD The following is a ll~st of use~ul polylsocyanates:

pro~vlene-1,2-dii.socyanateJ
butylene~ diisocyanate, butylene-113-diisocyanate, hexamethylene dilsocyanate, octamethylene diisocyanate7 nonamethylene diisocyanate, decamethylene diisocyanate, ~ diisocyanato-dodecane and the like, meta-phenylene diisocyanate, para-phenylene diisocyanate, toluene-?~4-dllsoc~anate, toluen~-2,6-diisocyanate, xylene-2,4-diisocyanate, xylene 2,6-dilsocyanate, dia.lkyl ben~ene diisocyanates, such a9 methyl-propylbenzene diisocyanate, methylethylbenz~ne diisocyanate, and the like; : :
2,2'-bi~henylene diisocyanate, 3,~'-biphenylene di~socyanate, 4j4'-biphenyIene diisocyRnate, dimethyl-4,4'-biphenylene d~isocyanate, and the like;
methylene-bi~s-(~-phen~l isocyanate), ethy~ene~bi~s-(4-phenyl isocyanate), i.soprop~lldene-bis-~4-~h~nyl i80cyanate), butylene-bls-(lL-pheny]. isocyanate), -11~
.. : .
.

~5;~L94 hexafluorolsoprop ~,rlidene~bis- (4-phenyl isocyanate), and the llke;
2,2'-oxydiphenyl ~iisocyanate, 3,3'-oxydiphenyl diisocyanate,

4,4'-oxydiphenyl diisocyanate, and the like;
2,2'-ketodiphenyl diisocyanate, ~,3'-ketodiphenyl diisoc~anat,--, 4,4'-ketodiphenyl diisoc~anate~
~ J 2'~t~,iodiphenyl dlisocyanate, 3,3'-thiodlphenyl diisocyanate, 4,4'-thiodlphenyl dlisocyanateJ and the like;
2,2'-sulfonediphenyl ~iisocyanate, ~,3'-sulfonediphenyl diisocyanate, lfonediphenyl ~iisocyanat~, and the like;
~'-methylene~bi~!-(c~clohexyl isoc~anate), ~,~'-methylene-bis-(cyclohexyl isocyanate), 4,41-methylene-bis-(cyclohexyl isocy~nate), li~4l-ethylene-bi~-(c~clohexyl isocyanate), 4, l~t ~propylene-bis-(cyclohexyl isocyanate), biq-(para-isocyanato-cyclohexyl)sulfide, bis-(para-isocyanato-cyclohexyl)sulfone, :~
bis-(para-isocyanato-cyclohexyl)ether, bisb(para-isocYanato-cyclohexyl~diethyl silane, bis-(para-isocyanato-cyclohexyl)diphenyl silane, bis~(para~isocyanato-cyclohexyl)ethyl phosphine oxide, bls-(para-isocyanato-cyclohexyl)phenyl phosphine oxlde, bis-(para-isocyanato-cyclohexyl)N-phenyl amine, bis-(para isocyanato-cyclohexyl)N-methyl amine, 2,~-dlisocyano-pyridine, bis-(4-isocyanato-phenyl)diethyl silane, bi~-(4-isocyanato-Phenyl)diphenyl silane~
dichloro-blphenylene diisocyanate, `

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,. . .
- . . . . .
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bis-(4-isocyanato-phenyl)ethyl phosphone oxide, bis-(4-isocyanato-phenyl)phenyl phosphine oxide, bis-(4-isocyanato-phenyl)N-phenyl amine, bis-(4-isocyanato-phenyl)N-methyl amine, 3,3'-dimethyl~4,4'-diisocyanato biphenyl, 3,3'-dimethoxy-biphenylene diisocyanate 2,4-bis-(~-isocyanato-t-butyl)toluene, .
bis-(para-~-isocyanato~t-butyl-phenyl)etherg para-bis-(2-methyl-4-isocyanato-phenyl)benzene, 3,3'-diisocyanato adamantane, ~
3,3'-diisocyanato biadamantane, - -3,3'-diisocyanatoethyl-1,1'-biadamantane, -1,2-bis-(3-isocyanato-propoxy)ethane, 2,2'dimethyl propylene diisocyanate, 3-methoxy-hexamethylene diisocyanate, ~:
2,5-dimethyl heptamethylene diisocyanate, :~

5-methyl-nonamethylene diisocyanate, 1,4-diisocyanato-cyclohexane, 1,2-diisocyanato-octadecane, ~O 2,5-diisocyanato-1,3,4-oxadiazole.
OCN(CH2)3 O(CH2)2 O(CH2)3 NCO, OCN(CH2)3 S(CH2)3 NCO~ ~`
OCN(CH2)3 N(CH2)3 NCO, polymethylene polyphenyl isocyanate, isophorone diisocyanate which is 3-lsocyanatomethyl 3,5,5-trimethyl cyclohexyl isocyanate and o C - NH- R - NCO

OCN- R - N
\ C - NH~ R - NCO
O -~.

,, .......... . : ~ ' : ' , :. " ~ . , ' :

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where R2 is an alkylene group having 2~6 carbon atoms, wherein one preferred type is the biuret of hexamethylene diisocyanate which is a good cross-linking agent.
For extended outdoor weatherability and durability, aliphatic and cycloaliphatic polyisocyanates should be used and polyisocyanates containing aromatic radicals should not be utilized in the composition. Preferred polyisocyanates which provide weatherability and durability are isophorone diisocyanate and 4,4'methylene-bis-(cyclohexyl isocyanate).
As mentioned above, blocked polyisocyanates are used in the coating composition of this in~ention. The obvious reason for using blocked polyisocyanates is that the powder composition will remain stable and ~ree-~lowing for long periods of time since the isocyanate groups are in combination with a blocking agent and therefore will not react until suf~icient heat is applied to release the block-ing agent.

Any of the aforementioned polyisocyanate~
reacted with another chemical compound to ~orm a thermally unstable reaction pro~uct can be utilized. Thi3 reac~ion product ~blocked organic polyisocyanate) must be stable below 50~C. ~or long periods of time and must be compatible with the other constituents u~ed in the po~der coating composltion. However, thi~ product mu~t break do~ under moderate b~king condition~, for example, 125-200~C. to form a polyisocyanate with reactive isocyanate groups that will cross-link with other constltuent~ in the powder coating compo~ition.

Typlcal blocking agents that can be used to form the blocked organlc`polyisocyanate used ln the powder co~ting composition are, for example, phenol compounds, , .
.:

:'', ' ' " . ~ ' ' )5;2 ~
alkyl acetoacetates, ketoximes, dialkyl malonates~
caprolactam, and th~ like. Typical phenol compounds that can be used are phenol, propyl phenol, tertiary butyl phenol, nonyl phenol, other monohydric phenols~ bromo ;~
phenol, 2-chloro phenol, dichloro phenol, 2-methoxy nitrophenol and the like. Preferably, ketoximes are used as blocking agents for the blocked polyisocyanate used in this invention to form a high quality composition.
Some preferred ketoximes are, for example, acetoxime, methylethyl ketoxime, methylisobutyl ketoxime, cyclo-hexanone oxime, and the like.
One preferred blocked polyisocyanate which forms a high quality powder coating composition is 4,4'-methylene-bis-(cyclohexyl isocyanate) blocked with methylethyl ketoxime. Another is isophorone diisocyanate blocked with methylethyl ketoxime or caprolactam.
The powder coating composition o~ this invention contains about 10-30~ by weight of a hydroxy functional plasticizer or a mixture of these plasticizers. These plasticizers are cross-linked with the other constituents in the novel powder coating composition and provide a high quaIity finish. Typical hydroxy polyesters that are ~s utillzed have a weight average molecular weight of about 500-6,000 and a hydroxyl number of 18-300 and are poly-esters of an alkylene polyol and a dicarhoxylic acid or acid mixtures such as mixtures of aliphatic, alicyclic or aromatic dicarboxylic acids. Typical polyesters are as follows:
adipic acid/butanediol/ethylene glycol; phthalic anhydride/
butanediol/ethylene glycol, and adipic acid/neopentyl .

~ s~
glycol/ethylene glycol, adipic acid/neopentyl glycol, adipic acid/azelaic acid/isophthalic acid/ethylene glycol azelaic acid/isophthalic acid/ethylene glocol and tri-methylol propane/phthalic anhydride/hexhydrophthàlic anhydride. Hydroxyl terminated polycaprolactones can also be used as plasticizers.
About o-6% of other non-reactive monomeric and polymeric plasticizers can be used in the powder coating composition of this invention. Phthalate ester plasticizers, in particular, the alkyl and cycloalkyl ester phthalates in which the alkyl group has 2-10 carbon atoms can be used such as diethyl phthalate, dibutyl phthalate, didecyl phthalate, butylbenzyl phthalate, dicyclohexyl phthalate, and mixtures thereof. Other esters such as tricresyl phosphate, diethyl adipate and sucrose benzoate can also be used. -Non-reactive polyeæter resin~ can also be used ~6 plastiCizers~ Typical polyesters are, for ex~mple alkylene glycol esters o~ adiplc and benzoic acid such a~
c~ ethylene glycol adipate benzoate, nevpentyl glycol adipate benzoate, ethylene ~lycol adipate benzoate phthalate and the like. Plasticizers o~ oil free or oil modi~led alkyd reæins .
and polyesters and epoxidized soya be~n oil can also be used.

Generally, the composition is formulated such that the molar ratio of NCO groups to hydroxyl groups of all the constituents used in the composition is about 0.9/1 to about 1.3/1. Preferably, to form a composition in which sub-stantially all of the groups react, a 1/1 to 1.2/1 ratio is used.
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:' : ; ' . ~ ', ' . ' , : ' ~ , '~
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~ 5Z~g4 A catalytic amount of an organometalllc compound is used in the powder coating composition to accelerate .
cross-linking and curing of the ~inish a~ter application and baking. Generally, about 0.02-2.0~ by weight of the organometallic compound is used. Typically useful compounds are as follows: the organo tin cataly~t, such as dibutyl tin dilaurate, dibutyl tin di-2-ethylhexoate; ~tannous octoate, sta-nnous oleate and the like; zinc naphthenateg cobalt naphthenate, vanadium acetyl acetonate~ zirconium acetyl acetonate, acetyl acetonate titanate and the like.
A silicone resin can be added to the powder coating composition of this invention to enhance flow and reduce cratering. Any of the conventional silicone resins ~:
or oils in amounts of 0.01-2% by weight can be used ~or this purpose.
Generally~ the powder coating is pigmented and the following are examples of the great variety of pigments which can be used in the powder coating composition:
metallic oxlde, preferably titanium dioxide, zinc oxide, iron oxide, and the like, metallic flake, such as aluminum .
: flake, metallic powders, metal hydroxides, "Afflair"
pigments, for example, mica flake coated with titanium dioxide, sulfides, sulfates, carbonates, carbon black, silica, talc, china clay, and other extender pigments~
organic pigments and lakes. - ;

Preparation of the_Novel Powder Coating Composition To prepare the novel powder coating composition of this invention, the acrylic polymer solutions, beads or emulsions are blended with blocked polyisocyanate, hydroxy functional plastici~ers and optionally, cellulose ' . ' .~ ..
. :. . . .

~2~9~L
acetate bytyrate and any other plasticizers and the organo-metallic compound along with pigments, pigment dispersions and other additives such as the aforementioned silicone resin and the like. The resulting mixture is then spray dried to form a powder and the powder then can be further dried in a vacuum oven ~or 6-36 hours. The powder then is passed through a sieve with openings of about 100 microns but generally a 53 micron sieve is used to remove large particles.
In another technique to prepare the powder coating, the above prepared mixture is charged into a vacuum extruder or mixer. The extruder is operated under a vacuum of about 22-25 inches of mercury, and a temperature of about 60 to 90C. and the solvent or water is removed from the composi-tion and a 100% dried product or extrudate is produced. The extrudate or dried product is then ground to a powder using conventional grinding equipment, for example, a pin disc ~`
mill, a fluid energy mill or a hammer mill can be used.
After grinding, the powder is passed through a sieve to remove large particles. `
A two-roll mill is another technique that can be used to prepare the powder composition. Pigment chips or pigment dispersion, the acrylic polymer dispersions or solutions blocked polyisocyanate, plasticizer and, optionally, cellulose acetate butyrate and organometallic compound and any other additives are charged into a two-roll mill and ;
milled together. The resulting composition is then ground and passed through a sieve to form the powder coating composition.

' ' ' ~ ' ''' 1a35;~494 The pigment dispersions utilized in powder coating compositions of this invention are prepared by conventional techniques æuch as sand grinding, pebble milling, ball milling and the like o~ the acrylic polymer solution or dispersion with the above pigments.
Another method for preparing a pigment dispersion is a two roll miIl. It is used to prepare pigment chips which are then formed into a pigment disper~ionO
Generally, the pigments and one o~ the acrylic polymers or cellulose acetate butyrate along with volatile non-solvents for the polymer and plastlcizer are blended together and then the mixture is pl~ced on a two-roll mill and the mixture is thoroughly milled to disperse the pigments and ~orm pigment chips. The~e chips are then blended with a ~olvent to ~orm a pigment dispersion which is utillzed as lndicated above.
Optionally, about 0.1-2~0~ by weight of ~lnely divided siliea or ~ e treated silica c~n be blended wlth the novel powder coating composition o~ thl~ invention to eliminate c~king of the powder and improve it~ h~ndling and ~pr~ying properties.

. m~ powder coating compo~ltlon o~ thiB in-ventlon i8 then applied to a metalJ glass, pla~tlc or ~lbeP
reln~orced plastic sub~trate by electrostatlc spraying techniques or by uælng a ~luldlzed bed or an electro~tatic fluldlzed bed. Pre~erably, electro~tatlc Rpraylng i~
utllized ln which a voltage Or 20 ko 100 kllo~olts ia ~pplled . . .

19- :

~5~94 to the gun. ~he composition is applied in several passes and then baked at 120-180C. for 15-45 minutes. The result-ing film is about 0.5-6 mils thick, but preferably, 2-3 mil films are used to provide a good quality finish.
Preferably, the coating composition of this invention is applied over a suitably treated and primed metal substrate or a primed and sealed metal substrate.
Typical alkyd primers or epox~ primers pigmented with iron oxide, carbon black, titanium dioxide and the like can be used. Conven~ional epoxy sealers can also be used. ~he novel composition can be used directly over galvanized or phosphatized steel to form a durable coating. An electrically conductiv~ carbon black piginent pre~erably is added to the primer or to the se~ler to make the surface conductive and promote uniform deposition o~ the powder while spraying.
One use~ul sealer composition is pigmented with carbon black pigment and uses a hydroxypropyl methacryIate acrylic resin as a 3-(2-methylacryloxy ethyl)-2~2-spirocyclohexyl oxazolidine acrylic resin.

Electrodepo~lted primer~ can be used on th~ metal ~ubstrate. Typical electrodepo lted prlmer compo~ltions have as the ~llm-~orming constltuents about 50-95% by wel~ht Or ~ :
a carboxyllc acld polymer having an acld number o~ 5-200 and correspondlngly abouk 5-50% by ~eight Or a cro~s-llnXlng agent.

Th~ followln~ ar~ examples Or carboxyllc poly~er~
us~d in these prl~er compositlon8; maleinized dryln6 o~

- 20 _ - ,: ' .' ,.,. . :

49~
whlch are the reactlon produc~s of malelc anhydride and a drylng oil such as llnseed oil, ~ehydrated castor oll, tung oll, soya bean oll and the like; alkyd resln~ whlch are the reaction products Or a polyhydrlc alcohol and a polybaslc acld such as, dryln~ oll ~atty acids; esteri~ied epoxy resins such as, an epoxy-hydroxy polyether resln esterlrled wlth conventlonal drylng oil ~atty acl~ which can be ~urther acldlrled wit~ maleinlzed drylng oils; acrylic polymer~;
polyesters; trlmellltlc anhydrlde alkyd resins; s~yrene/
allyl alcohol copolymer~ reacte~ wlth a carboxyllc acld con~tituent and the like.

The ~ollowing are typ~cal cross-linking agent~
u~ed wlth the above carbox~llc polymer~ ~o form~conv~ntlonal prlmers and primers that can be electrodeposlted: melamine rormaldehyde, alkyla~ed melamlne rormaldehyde, urea rormaldehyde, benzoguanamlne formaldehyde, toluene sul~on-amide resln~; one pre~erred cross-llnklng agent 19 hexa-(methoxymethyl)melamlne. Other cross-linking agents such as, amlne~ and other compatlble hydroxyl termlnated compounds ca~
àl~o be uscd.

The ~ollowing Example~ lllu~trate the inventlon.
All qu2ntltl~s are on a welght basls unles~ otherwlse indl~atcd.

~ 21 -.: - ............................ ~ . :.. . .
. .

~5Z4~

EX~MPLE 1 Soft Acrylic Polymer Solution A

A so~t acrylic polymer solution is prep~red as follows:
Grams Portion 1 Methyl methacrylate monomer 290.00 Butyl acrylate monomer 446900 2-Hydroxyethyl acrylate monomer 34~00 Methylethyl ketone 270.00 ~
Lauryl mercaptan 8.70 ;~-Portion 2 Azo-bis-isobutyronitrile 0.28 Methylethyl ketone 20.00 ;;~
Total1068098 ;
: ' . .
The constituents o~ Portion 1 are mixed together.
Portion 1 is charged into a polymerization vessel equipped with a thermometer~ stirrer, re~lux condenser, addition ~unnel and a heating mantle and the lngredlents are heated to 98C. and then Portion 2 is mixed and then added to the polymerization vessel to ~orm a reaction mixture.
A monomer solution and a solvent catalyst solution are prepared as ~ollows:
Grams Monomer Solution Methyl methacrylate monomer 52~.0 Butyl acrylate monomer 240.0 -2-Hydroxy ethyl acrylate monomer 61,0 `
Lauryl mercaptan 829.7 . .. .......

' ' ' "' ' ' ' ' .' ' ' . ,:,', .
. . , ~ , 9~L

Grams : .
Solvent Catalyst Solution Methylethyl ketone 48.o Toluene 201,0 ~
Azo-bis-isobutyronitrile 11.1 _ - .
: .
Total 260.1 The above prepared monomer solution and solvent catalyst solution are added to the reaction mixture over - .
the following time periods while the reaction mixture is 10 maintained during these addition periods at about 90C. ;;.
In each case, the monomer solution and the solvent catalyst solution are premixed before addition to the reaction mixture. The following table shows a time.over which each Or the additions o~ monomer and solvent catalyst solution ..
is made.
ISolvent Catalyst Time Monomer Solution Solution (Minutesl (Cubic Centimeters) (Cubic Centimeters) 2.5-15 92 ~2 15-30 110 36 .
3-45 110 25 `-~ ' 45_60 110 20 ;
60-go 220 67 ~20-150 . o 65.1 .

Afker the above ingredients are added, the reaction mixture is maintained at about 85C. for an additional 15 minutes and then diluted with 751 grams o~
acetone7 The resulting polymer solution has a solids ~0 content of 54.3~. `.

- .
, .

~SZ9~9~ ::

The resulting polymer has a methyl methacrylate/ ;~
butyl acrylate/hydroxyethyl acrylate weight ratio of 51/4~/6. The polymer has a number average molecular weight o~ 17,000 and a weight average molecular weight of about 35,000 determined by gel permeation chromotography using polymethyl methacrylate as a starldard and a calculated glass transition temperature of 9C.
.
A Hard Acrylic Polymer Solution B
' ,"
A hard acrylic polymer solution is prepared a~
~0 ~ollows:
Gram ;~
Portion 1 Methy~ methacrylate monomer 857.00 Hydroxypropyl methacrylate monomer 110.00 (68-75% of 2-hydroxypropyl methacrylate and 25-32% of 1-methyl-2-hydroxyethyl methacrylate) Lauryl methacrylate monomer 45.40 Acetone 134.00 -Methylethyl ketone 491.00 2-Mercapto ethanol 26.90 Portion 2 :
Methylethyl ketone 20.00 Azo-bis-isobutyronitrile 0.41 Total1684.71 The constituents of Portion 1 are mixed together and Portion 1 is charged into a polymerization vessel equipped as a~ove. Portion 2 is then mixed and added to the polymeriæation vessel and heated to about 85C~ to orm a reaction mixture.

_ ~,'4 -.
~.

~5;~

The follow1ng monomer and catalyst feed solutions are prepared by blending the constituents together:

~rams Feed Solutlon 1 Methyl methacrylate monomer 152.00 Hydroxypropyl methacryla~e monomer (described 19.40 . above) Lauryl methacrylate monomer 7.80 Azo-bi~ obutronttrile 1.52 Total 18G.72 Feed Solution 2 Methyl methacrylate monomer 24800 Hydroxypropyl methacrylate monomer (described 30O5 Lauryl methacrylate ~onomer above) 13.0 Acetone 1200 Azo-bi~-isobutyronltrile 2.8 Total306.3 :
FBed Soluti~
~ .
Methyl methacrylate monomer 137~0 Hydroxypropyl methacrylate monomer (described 14.1 above) Lauryl methac~ylate monomer 5.9 Azo-bis-isobutyronitrile 16.0 Total 173.~

Feed Solutlon 1 is charged into the polymerization vessel at a feed rate o~ about 3.5 cUbic centi~eter~ p~r -:
minute over about a 60-minute perlod while malntainlng the reactlon mixture at about 85C. Feed Solut~on 2 i6 then ~5~ 9~ :
added at a rate of about 7,3 cubic centimeter~ per minute over the ne~t 60 minutes while ma~ntaining the te~perature of the reaction mixture~ at about 82C. Feed solution 3 is then added at a rate of about 209 cubic centimeter~ per minute over the next 65 mlnutes while maintaining the reactlon mixture at about 81C, A~ter ~eed Solution 3 is added, the reaction mlxture i5 held at about 78C. for `: ~:
20 minute~ and then cooled to room temperature and dlluted :
wlth a mlxtur0 of 400 gram~ of acetone and 225 o~ methyl-ethyl ketone. The rç~ultlng polymer solution has a solids content of 57~5~
The polymer has a methyl metacrylate/lauryl methacrylate/hydroxypropyl meth~crylate weight ratio of 85/L~,5/10.5, The polymer has a number average molecular weight of 7000, a weight average molecular welght of 13 000 determlned by gel permeation chro~otography using ~`
polymethyl metacrylate a~ a ~tandard and a calculated gla98 transltion temperature of 101C, A black mill base i9 prepared as follo~s:
Gram~ .

Polymer solution (50~ solids of a polymer of methyl methacrylate/l~uryl methacrylate/
2-hydro~y ethyl acrylate ln a weight ra~io o~ 90.4/3,3/6.3 in a golvent mixture of toluene/methylethyl ketone/aeetone~470.4 Methylethyl ketone 366.3 ~ .
Toluene 194.8 Ethylene glycol monoethylether acetate 88.5 Carbon black chip (homogeneQu~ chip of PEERLESS** 155 carbon black p-lgment/
3 cellulose acetate butyrate having a 55~ -butyryl content and a visco~ity of 0,02 ~econds/Polyme~lc Di~persant* in a welght ratio of 20/64/16) 288,o Total 1408.0 ~FoIymeric DI~érsRnt~- polymer of methylmethacrylate/butyl-acrylate (44/5 ratlo) havlng a weight avarage molecular welght of about 4 000 and 1~ reacted wl~h the biuret of hexamethylene diisocyanate and capped wlth ammonia, ** denote~ trade mark ~0524~ :
The above constituents are charged into a mixer and blended for about 1 hour to dissolve the chips. Weight loss due to vaporation of solvent is made up with a mixture of methylethyl ketone/toluene mixture in a 2/1 weight ratio.
The resulting composition is then set into an ice bath and stirred with an Eppenbach Homomixer for about 1 hour to form the mill base.
A yellow mill base is prepared as follows:

Grams Polymer Solution (described in above . .
black mill baæe~ 383.0 . `
Methylethyl ketone 317.9 Toluene 168.5 Ethylene glycol monoethylether acetate 75.~
Yellowchip (Irgazin yellow pigment/ -;
cellulose acetate butyrate, 55%
butyryl content and 0.02 second viscosity/butyl benzyl phthalate in a weight ratio of 47/48/5) . .?55.3 Total 1200.0 ... . . .
The above ingredients are stirred together ~or 30 minutes and then stirred ~or 1 hour with an Eppenbach Homomlxer to form the mill base.
A liquid composition is then prepared as ~ollo~rs:

Parts By Wei~ht So~t Acrylic Polymer Solution A
(prepared above) 110.0 Hard Acrylic Polymer Solution B
~ (prepared above) 275.0 ~ 05~494 Partæ by . ::
~ .
Polycaprolactone (hydroxyl terminated ~: :
polymer having a number average molecul~r weight of about 1250) 33.0 :

Isophorone diisocyanate blocked with caprolactam 45~0 ~:
Low molecular weieht silicone resin 0.9 Gr~en M111 Base (10% MONASTRAL* Green pigment-phthalocyanine green; 16~
~olymer o~ 90.4~ methyl metacrylate,

6.3~ 2-hydroxyethyl acrylate, 3.3~
lauryl methacrylate, 10.2~ o~ cellu- .
lose acetate butyrate having a 55 butyryl content and a 0,02 second vi~coslty; lo 1% 0~ butyl benzyl phthalate; all disRolved in 63.7% of a solvent mixture of methylethyl k~tone, toluene and ethylene glycol 20 monoethylether acetate ) 37.8 Black Mill Ba~e (prepared above) 40,5 Yellow Mlll Base (prepared above) 59.0 While Mill Base (44.70% titanium dioxide;
22.35% of a polymer of 90.4~ methyl methacrylate/6.3~ 2-hydroxyethyl acrylate/
3.3~ lauryl methacrylate d~persed in 33,95% of R ~olvent of methylethyl ketone~ 2.9 Dibutyl tin dilaurate solution (4% sollds 801ution ln toluene) 12.0 Total 616.1 The above constituent~ are thoroughly blended to-gether and raduced with acetone to a 40 ~econd viscoRity measured with a No, 1 Zahn cup at 25C. The resulting compo-sition i5 ~prayed dried by ~praying the compo~ltion with a suction-type gun at ~0 pounds per ~quare lnch pres~ure into a ~O-gallon stainle~ steel drum that is opcn at both ends .
* denote~ trade m~rk , .~ .

.

, : ','' " . ' ' '' , ~z~9~

and placed in a horizontal position. One end of the drum is covered with a 140 mesh nylon cloth to retain the powder particles and the other end of the drum with poly-ethylene having one opening therein through which the powder is sprayed. The powder is then placed into a vacuum oven for L~8 hours and dried. The powder is passed through a 270 mesh sieve to remove any large particles. The resulting powder coating composition has relatively uniform particles and the particles do not adhere to each other.
1~ The powder is then sprayed onto a six 20 guage phosphatized steel panel using a model 322 Ransburg electro-static powder gun. The powder is delivered from a reservoir to the gun by means of an air stream. The gun utilizes 50 kilovolts of the electricity to charge the powder particles. The panels then are baked and the film thickness, gloss at 20 and distinctness of image are measured for each of the panels. The results ar~ as follow.s:

~05;~9~L `

Baking Film 20 Distinctness Panel Conditions Thickness Gloss Of I~a~e _ 1 172C. for 15 minutes 2.4 mils 78 7 2 144C. for 20 `
minutes and ;
171C. for 30 minutes 2.0 mils 80 6 :. -3 144C. for 20 ~
minutes and ~ .
163C'. for 55 minutes 205 mils 82 7 :~ , 4 150C. for 15 minutes and ..
171C. for 30 :' :
minutes 3.0 mils 85 8 150~C. for 15 minutes and 171C. for 55 :
mlnutes 3.5 mils 85 8 :
~, . .
6 150C. for 20 minutes and 170C~ for 60 minutes 2.6 mils 87 8 . ~ .
.~ ~,....
A 20 gloss above 40 is acceptable and a Distinctness of Image Value 3 is acceptable.
Each of the panels have a smooth even fini6h with an excellent appearance.
, .: .
... . .

- ,~0 - ,~
. .

.:, ' ,, ~ , .
, ~ Z494 ~ .

The ~ollowing mill bases are prepared:

Parts By Wel~ht Blue Mill Base I ;
Portion 1 Phthalocyanine Blue Pigment Chip (homogeneous chip o~ 47 parts of phthalo-cyanine blue pigment3 5 parts butyl benzyl phthalate, and 48 parts of cellu-1~ lose acetate butyrate having a 55 butyryl content and a ViSCoBity of 0.02 seconds) 1000 Hard Acrylic Pclymer Solution C (55.7~ -solids of a polymer of 89~ methyl methacrylate, 3.3~ lauryl methacrylate, 6.7% of 2-hydrox~ethyl acrylate and containing l~ of 2-mercaptoethanol and having a weight average molecular weight of 11,000 and a glass transition temperature of ôlC. in an organic ?0 solvent mixture) 2016 Ethylene glycol monoethyl ether acetate500 Portion II
Methylethyl ketone 2000 Total 5516 ^:, Portion I is stirred until the chlp dissolves, then Portion II is added and thoroughly mixed with Portion I. The resulting mixture is ground in a conventional sand mill at a rate of 1 gallon per hour to uniformly disperse the pigment. The mixture is passed through the sand mill 3~ twice. The resulting mill base has a 36.7~ solids content.

.

~L~sZ494 Parts By _ Weight Aluminum Flake Mill Base . ~ . . ..
Portion I
Aluminum Paste (51.8~ aluminum ~lake in ethylene glycol monoethyl ether acetate) 96.60 Hard Acrylic Polymer Solution C
(described above) 215.03 Portion II
Hard Acrylic Polymer solution C 196.90 Total 508.53 .
Portion I is stirred for 2 hours then Portion II
is added and stirred ~or 3 hours. The resulting mill ba6e has a 50% solids content and an aluminum ~lake content of 9.8~ and a polymer content of 40.2~.

Blue 1~1 ~e Il Parts By . .
Portion I
Phthalocyanine Blue Chip (described aboYe) 1080 Hard Acrylic Polymer Solution B (described in Example 1) 1930 : .
Ethylene glycol monoethyl ether acetate 500 Portion II
Methylethyl ketone 189~
Total 5400 , Portion I is stirred ~or 2 hour~ Portion II i~
added and ~tirred ~or an additional 2 hoursO The resulting mixture iB pa~sed through a sand mill t~ice at a rate o~

- ~2 -~, . ' ~ 2~

1 gallon per hour to uniformly disperse the pigments and form a mill base.
.
Parts By Wei~ht Black Mill Base .:
Hard Acrylic Polymer Solution C 2352~0 (described above) Methylethyl ketone 1834.5 Toluene 974.O
Ethylene glycol monoethyl ether acetate 442.5 :
Carbon black chip (described in Example 1) 1400.0 Total 7O03~O

The above ingredients are mixed together until the chip dlssolves and then ground in a convention sand mill at a rate of 1 gallon per hour using 2 passes through the mill to form the mlll baseO
A liquid coatin~ composition is prepared as follows: -Parts By Wei~
Hard Acrylic Polymer Solution C
(described above) 187.00 Soft Acrylic Polymer Solution D (52.8%
solids of a polymer of 52% methyl methacrylate, 42% n-butyl acrylate and 6% 2-hydroxy ethyl acrylate having a weight average molecular welght of 20,000 and a calculated glass transi-tion temperature of 9C. 21.60 .
. : , , ' :
, ~5;~
Parts By Weight~

Polyester Plasticizer Solution (50%
solids in an organic solvent of a hydroxy polyester o~ ethylene glycol/azelate/
isophthalate/adipate having a hydroxyl no.
o~ 35 and a number average molecular .~:
weight o~ about 3200) 74.~ :.

Blocked polyisocyanate solution ~34.9~ -I '~ solids in methylethyl ketone o~ 4,4'-methylene-bis-(cyclohexyl isocyanate) blocked with methylethyl ketoxime) 108.9 Cellulose acetate butyrate solution (25 solids in a 1/1 methylethyl ketone/
toluene solvent of cellulose acetate butyrate having a 55~ butyryl content and a 0.02 second viscosity) 12.0 : :

Dibutyl tin dilaurate solution (5% . :~
solids in toluene~ 4.0 Silicone oil (low molecular silicone oil) o.6 Blue Mill Base I (prepared above) 46.4 Blue Mill Base II (prepared above) 18.2 ~
Black Mill Base (prepared above) 2.1 .

Aluminum Flake Mill Base (prepared above) 20,0 :~

Total 494.8 ;
:- :
The above constituents are thoroughly blended .
together and reduced with acetone to a 40 second viscosity measured with a No. 1 Zahn cup at 25C. The resulting com- .
position is sprayed-dried as in Example 1 and dried in a vacuum oven as in Example 1. The powder is then passed through a 270 mesh sieve to remove any large particles. The resulting powder coating composition has relatively uniform particles and the particles do not adhere to each other. .
The nowder is then sprayed onto phosphatized steel p~nels using the electrostatic powder gun and the application procedure of Example 1. The panels then are baked at 172C.

-~1~ _ . .

, .
.
., . . . ' ' 499~
ror 30 minutes to form a glossy, smooth, even finish that has good durability, weatherability, scratch and mar resistance and good chip resistance.

A liquid coating composition is prepared as ~ollows: i Parts By Weight Hard Acrylic Polymer Solution E
(55~7~ solids of a polymer o~ 88%
methyl methacrylate/12~ hydroxypropyl methacrylate and having a weight average molecular weight of about 6000 and a glass transition temperature of 103C. in acetone solvent) 90.50 Soft Acrylic Polymer Solution F
(52.8% solids of a polymer of 52% methyl methacrylate/42~ n-butyl acrylate/6%
2-hydroxy ethyl acrylate ha~ing a weight ~0 average molecular weight of 20~000 and a glass transition temperature of 9C
in an organic solvent) 15.67 Cellulose acetate butyrate solution (25% solids o~ in acetone of cellulose acetate butyrate having a 55% butyryl content and a 0.02 second viscosity) -47 Polyester plasticizer solution (described in Example 2) 42.77 " Blocked polyisocyanate solution ( 5b% solids solution in acetone of isophorone diisocyanate blocked with methylethyl ketoxime)45.18 Silicone oil (described in Example 2) o.48 Dibutyl tin dilaurate solution ( 5%
solids in toluene) 0.18 . . :
.:

2~94 Parts By Weight Midas Gold Mill Base (42.7% solids of 25% of Midas Gold iron oxide pig~nt, 22.~ llulose ~etate butyrate, described above, 2.5% butyl benzyl phthalate, 50% of hard acrylic polymer E, in a solvent mixture of ethylene glycol monoethyl ether acetate/
methylethyl ketone in a 1 to 4 ratio) 7.95 Magenta Mill Base (41% solids of 23.5% of Monastral Magenta pigment RT 203D
Quinacrldon type pigment, 24% cellulose acetate butyrate, described above, 2.5%
butyl benzyl phthalate, 50% of hard acrylic polymer E in a mixture of ethylene glycol monoethyl ether acetate/methylethyl ketone in a 1 to 4 ratio) 7.65 --Maroon Mill Base (48.~% solids of 23.5% of Perrindo Maroon Deep R 6424 thioindigoid pigment 24% cellulose acetate butyrate, ~-described above, 2.5% butyl benzyl ;
phthalate, 50% of hard acrylic polymer E
in a mixture of ethylene glycol monoethyl ether acetate/methylethyl ketone in a 1 to 4 ratio) 23.86 ;

Aluminum Flake Mill Base (51% of the hard acrylic polymer Ej 11.1% aluminum flake dispersed in an organic solvent mixture) _ 1.02 ~ Total 235.73 The above constituents are thoroughly blended to-gether and reduced with acetone to a 40 second viscosity measured with a No. 1 Zahn cup at 25C. The resulting composition is spray dried as in Example 1, dried in a vacuum oven and passed through a 270 mesh sieve as in Example 1. The resulting powder coating composition has relatively uniform particles and the particles do not adhere to each other ., . ,. ~:;,, ' : ' .
,, . ~
. . . . .
, '' ' '' ~' .
, ~¢95Z~94 The powder then is sprayed onto phosphatized steel panels using the electrostatic powder gun and the application procedure of Example 1. The panels then are baked at 172C. for 30 minutes to form a glossy, smooth~
even finish that has good durability, weatherability, hardness and has good mar and scratch resistance.

.' ~' .: , , , ; ' , .

Claims (13)

The embodiments of the invention in which an ex-clusive property or privilege is claimed are defined as follows:
1. A thermosetting polymer power coating composition comprising finely divided particles having a particle size of 1-100 microns; wherein the particles consist essentially of an intimately mixed blend of A. 20-70% by weight of a hard acrylic polymer consisting essentially of 1. 80-85% by weight, based on the weight of the acrylic polymer, of methyl methacrylate, styrene or a mixture of a methyl methacrylate and styrene, 2. 5-20% by weight, based on the weight of the acrylic polymer, of a hydroxy alkyl acrylate or hydroxy alkyl methacrylate having 2-4 carbon atoms in the alkyl groups or a mixture of said acrylate and said methacrylate;
wherein the hard acrylate polymer has a glass transition temp-erature of about 70°-103°C., a weight average molecular weight of 5,000-20,000 and a hydroxyl content of about 0.5-3% by weight.
B. 5-20% by weight of a soft acrylic polymer consisting essentially of

1. 30-62% by weight, based on the weight of the soft acrylic polymer, of methyl methacrylate, styrene of a mixture of methyl methacrylate and styrene;
2. 35-60% by weight, based on the weight of the soft acrylic polymer, of an alkyl acrylate having 2-12 carbon atoms in the alkyl group, an alkyl meth-acrylate having 4-12 carbon atoms in the alkyl group or a mixture of the above;

3. 3-10% by weight, based on the weight of the soft acrylic polymer of a hydroxy alkyl acrylate, a hydroxy alkyl methacrylate having 2-4 carbon atoms in the alkyl group or a mixture of the above;
wherein the soft acrylic polymer has a glass transition temp-erature of -20° to 15°C, a weight average molecular weight of 5,000-50,000 and a hydroxyl content of about 0.5-2% by weight;
C. 15-30% by weight of a blocked organic polyisocyanate of an organic polyisocyanate blocked with an organic ketoxime blocking agent, wherein the organic polyisocy-anate is an aliphatic polyisocyanate or a cycloaliphatic polyisocyanate and the ketoxime blocked organic polyisocyan-ate is stable below 50°C. and breaks down under moderate baking conditions to provide reactive isocyanate groups;
D. 10-30% by weight of hydroxy functional pasticizer having a hydroxyl number of 18-300, and a number average molecular weight of about 500-69000 and wherein the functional plasticizer is a polyester of an alkylene glycol or a mixture of alkylene glycols and an aliphatic, alicyclic or aromatic dicarboxylic acid or a mixture of these acids;
E. a catalytic amount of an organometallic compound;
wherein the molar ratio of NCO/OH for all of the above is about 0.9/1 to about 1.3/1.

2. The powder coating composition of Claim 1 containing about 0.1-60% by weight, based on the weight of unpigmented powder coating, of pigment.

3. The powder coating composition of Claim 2 in which the hard acrylic polymer consists essentially of 82-94% by weight of methyl methacrylate, 1-10% by weight of an alkyl acrylate or an alkyl methacrylate each having 2-14 carbon atoms in the alkyl groups and 5-17% by weight of a hydroxy alkyl acrylate or a hydroxy alkyl acrylate having 2-4 carbon atoms in the alkyl groups and in which the acrylic polymer has a weight average molecular weight of about 6,000-17,000 and a glass transition temperature of 80-102°C.

4. The powder coating composition of Claim 2 in which the soft acrylic polymer consists essentially of 45-60% by weight of methyl methacrylate, 35-45% by weight of an alkyl acrylate having 4-8 carbon atoms in the alkyl group and 5-10% by weight of a hydroxy alkyl acrylate or a hydroxy alkyl methacrylate having 2-4 carbon atoms in the alkyl group and in which the acrylic polymer has a weight average molecular weight of about 10,000-30,000.

5. The powder coating composition of Claim 2 which contains, in addition, about 1-20% by weight of cellulose acetate butyrate has a butyryl content of 30-60% and a viscosity of about 0.005-2,0 seconds.

6. The powder coating composition of Claim 2 in which the hydroxy functional plasticizer is a hydroxy polyester of an alkylene polyol and a mixture of aliphatic alicyclic or aromatic dicarboxylic acids.

7. The powder coating composition of Claim 2 containing 0.02-2.0% by weight, of an organic tin catalyst.

8. The powder coating composition of Claim 2 in which (A) the hard acrylic polymer consisting essentially of 82-94% by weight of methyl metacrylate, 1-10% by weight of an alkyl acrylate or an alkyl methacrylate having 2-14 carbon atoms in the alkyl groups, 5-17% by weight of a hydroxy alkyl acrylate or a hydroxy alkyl methacrylate having 2-4 carbon atoms in the alkyl groups; and wherein the acrylic polymer has a weight average molecular weight if 6,000-17,000 and R
glass transition temperature of 80-102°C.;
(B) a soft acrylic polymer consisting essentially of 45-60% by weight of methyl methacrylate, 35-45% by weight of an alkyl acrylate having 4-8 carbon atoms in the alkyl group, and 5-10% by weight of a hydroxy alkyl methacrylate or a hydroxy alkyl acrylate having 2-4 carbon atoms in the alkyl group; and wherein the acrylic polymer has a weight average molecular weight of 10,000-30,000 and (C) a blocked organic polyisocyanate in which the organic polyisocyanate is either an aliphatic polyisocyanate or a cycloaliphatic polyisocyanate and is blocked with a ketoxime;

(D) a hydroxy functional plasticizer of a hydroxy polyester of an alkylene polyol and a mixture of aliphatic, alicyclic or aromatic dicarboxylic acids; and (E) an organo tin catalyst.

9. The powder coating composition of Claim 2 in which (A) the hard acrylic polymer consists essentially of methyl methacrylate, lauryl methacrylate, hydroxy propyl methacrylate;
(B) the soft acrylate polymer consists essentially of methyl methacrylate/butyl acrylate/hydroxy ethyl acrylate;
(C) the blocked organic polyisocyanate is isophorone diisocyanate blocked with caprolactam;
(D) the hydroxy functional plasticizer is a poly-caprolactone; and (E) the catalyst is dibutyl tin dilaurate.

10. The powder coating composition of Claim in which (A) the hard acrylic polymer consists essentially of methyl methacrylate, lauryl methacrylate and hydroxy ethyl acrylate;
(B) the soft acrylic polymer consists essentially of methyl methacrylate/butyl acrylate/hydroxy ethyl acrylate;
(C) the blocked organic polyisocyanate is 4,4' methylene-bis-(cyclohexyl isocyanate) blocked with methylethyl ketoxime;
(D) the hydroxy functional plasticizer is a hydroxy polyester of ethylene glycol/adipate/azelate/isophthalate; and (E) the catalyst is dibutyl tin dilaurate.

11. The powder coating composition of Claim 10 containing additionally 2-10% by weight of cellulose acetate butyrate.

12. The powder coating composition of Claim 2 in which:
(A) the hard acrylic polymer consists essentially of methyl methacrylate and hydroxy propyl methacrylate;
(B) the soft acrylic polymer consists essentially of methyl methacrylate/butyl acrylate/hydroxy ethyl acrylate;
(C) the blocked organic polyisocyanate is isophorone diisocyanate blocked with methylethyl ketoxime;
(D) the hydroxy functional plasticizer is a hydroxy polyester of ethylene glycol/adipate/azelate/
isophthalate; and (E) the catalyst is dibutyl tin dilaurate.

13. A metal substrate coated with a smooth, even coalesced layer of the powder coating composition of Claim 1.