CA1159186A

CA1159186A - Powder coating composition for automotive topcoat

Info

Publication number: CA1159186A
Application number: CA000381911A
Authority: CA
Inventors: James M. Craven
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1980-07-21
Filing date: 1981-07-16
Publication date: 1983-12-20
Also published as: EP0045040B1; ES504122A0; ES8307266A1; AU542072B2; JPH0114945B2; JPS5751760A; AU7312481A; EP0045040A3; EP0045040A2; MX157796A; US4346144A; DE3166828D1; BR8104607A

Abstract

ABSTRACT
A thermosetting acrylic powder coating compo-sition contains an acrylic copolymer of a monoethylen-ically unsaturated monomer having at least one epoxy group and at least one monoethylenically unsaturated monomer which is free of epoxy groups, a carboxy-termi-nated primary crosslinking agent, a supplementary cross-linking agent having at least one functional group which can react with either hydroxyl groups or car-boxylic acid groups, and at least one ultraviolet light stabilizer or ultraviolet light screener capable of preventing degradation of the coating by ultraviolet light. An acrylic copolymer of methyl methacrylate, n-butyl methacrylate, and glycidyl methacrylate is favored. Preferably, the composition contains a combi-nation of ultraviolet light stabilizer and ultraviolet light screener, is nonpigmented, and provides a clear, glossy, and durable film useful as the clear coat of a clear coat/color coat automobile finish.

Description

1 15~1~6 Powder Coating Composition for Automotive ~Opcoat BACXGE~OU?~D OF THE INVENTION
Much of the reseaxch and development ef~ort in the field of automotive finishes is currently directed to the search for coating compositions and methods of applying such compositions which not only will eliminate, or nearly eliminate, the release of organic solvents during heat curing but also will pro-duce at a commercially feasible cost, coatings at leastcomparable in appearance and durability to conventional coatings.
A substantial amount of current research effort is likewise directed to the development of clear coat/color coat automotive finishes. It has been found that an excellent appearance, with depth of color and with metallic glamour, can be obtained by apply-ing a transparent coat over a pigmented coat.
Unfortunately, the durability of these transparent clear coats has left much to be desired. Often, check-ing, cracking, and flaking occur after relatively short periods of exposure to weathering, necessitating costly refinishing.
One solution to the solvent emission problem has been the replacement of liquid coating materials with coating materials in the form of dry, particulate solids, commonly called "powder" coatings. These com-positions contain very low concentrations of volatile solvents, i.e., of the order of ~ percent or slightly higher, substantially less than any other paint system.
From an environmental standpoint, powder coatings have much to recommend them. Inherent in their use, however, are certain problems of production and application which have retarded the extent of their adoption. One problem occurs when powder coatings are used in conjunction with particulate metal particles, ~ , aluminum flakes. Automobiles coated with a so-called "metallic" flnish, i.e., a topcoat of enamel or lacquer in which aluminum flakes as well as conven-tional pigments have been dispersed, have found wide acceptance in the marketplace. ~or the most part, the problems incidental to employing aluminum fla~ces in conventional liquid paints have been solved through years of experimentation and use. The problems associated with the use of aluminum flakes in dry powder are far more complex, particularly where some type of pulverizing step is involved in the paint manufacturing process or where electrostatic spray techniques are used to apply the paint to a substrate. Also, although increased use of powder coatings and improved manu~
facturing methods will undoubtedly result in a reduction of the present cost of quality powder coatings, the cost of producLng such coatings in all of the colors demanded in the marketplace may continue to be prohibitive.
In view of the problems associated with colored powder coatings, particularly those containing metallic flakes, one approach has been to utilize a clear coat/color coat system wherein the transparent clear coat is composed of a nonpigmented powder coating while the color coat, often metallic, is composed of a conventional liquid paint. Such a system possesses a number of advantages. Use of a powder coating for the clear coat reduces the solvent emission level con-siderably, and, lf desired, a water-based or high-solids coating ~.a~erial can be used for the color coat in order to further reduce the total emission level. Thus, an environmentally-acceptable finish ca~ be achieved with-out sacrificing appearance or metallic glamour. Too, ~ 15'~86 tne production of non?igmented powder coatings is marXedly less comple~ and less expensive than the pro-duction of pigmented powder coatings.
Clear coat~color coat systems consisting of a powder clear coat over a conventional liquid color coat are known in the art, as shown by Camelon et al., United States Patent 3,953,644, issued April 27, 1976.
However, such systems are not reinforced against the destructive effects of outdoor weathering and are thus susceptible to the checking, cracking, and flaking which com~only beset clear coats.
Conventional ultraviolet light screeners have sometimes been added to liquid clear coats in an attempt to retard the degradation caused by weathering, as shown by La Berge, United States Patent 3,407,156, issued October 22, 196~. Many of these conventional screeners are unsuitable for use in a powder coating because they lack special requirements, e.g., the capability of being ground to small particle size, f melting readily at the standard bake temperature for powder finishes, or of having sufficient permanence to remain in the coating film during baking and subse-quent outdoor exposure. In addition, the use of conven-tional ultraviolet light screeners is in some polymer systems less than satisfactory: the durability of certain clear coats so reinforced will be increased for a short period of time, but not to the extent required for a practical automotive finish.
~mong the acrylic polvmers which provide high auality automotive finishes are those containing glycidyl methacrylate. The use of glycidyl methacrylate in powder coating compositions is known in the art, as shown by Victorius, ~nited States Pater, 4,027,066, issued May 31, 1977. These compositions, 1 15'~18~

llke the afore~entioned powder clear coats of the art, are not reinforced against weathering. Furthermore, these compositions are ordinarily pigmentèd and contain cellulose acetate ~utyrate, useful as a dispersant for S the organic pigments commonly encountered in powder coat-ings. I^~ile cellulose acetate butyrate imparts gonio-chromatism, i.e., metallic two-tone, to a metallic colored automotive finish, it is less suitable for nonpigmented finishes. The presence of ceIlulose acetate butyrate in an acrylic system creates a faint cloudiness or haze which is of no consequence in pigmented coats but can be de-tected in nonpigmented coats and detracts rom the over-all appearance of the finish. The presence of cellulose acetate butyrate also tends ~o have a somewhat detri-mental effect upon the cold/crack resistance and theoutdoor durability of a coating.
Thus, there exists a felt need for a nonpis-mented acrylic powder coating composition which will provide a clear coat characterized by both excellent appearance and the capability to adequately withstand long periods of outdoor weathering. In particular, there is need ror an acrylic powder coating possessing the attributes of the known glycidyl methacrylate powder coatings but overcoming their deficiencies in order to provide a clear coat finish with excellent cold/crack resistance and superior clarity an~ dur~ilit~.
SU~MARY OF THE INVENTION
Thcre i5 providcd hy the present invontion a thermosetting powder coating composition of finely 30 divided particles having a particle size of about 1 to 100 microns, wherein the particles are an intimately mixed blend consisting essentially of:
A. an acrylic copolymer consisting essentially of:

1 1~91~

(1) a~out 8 to 35 percent by weight, base~: on the weight of tr,e copolymer, of a monoethylenically unsaturated monomer having at least one epoxy group, and

(2) about 65 to 92 percent by weight, based on the weight of the copolymer, of at least one mono-ethylenically unsaturated monomer which is free of epoxy groups, wherein the copolymer has a number average molecular weight of about 1,500 to 10,000, a weight average molecular weight of about 3,000 to 18,000, and a glass transi-tion temperature of about 30C to 100C;
B. a carboxy-terminated crosslinking agent in an amount sufficient to provide 0.9-1.5 carboxylic acid groups for each epoxy group originally in the copolymer;
C. a supplementary crosslinking agent having at least one functional group which is capable of reacting with either hydroxyl groups or carboxylic acid groups, said supplementary crosslinking agent being present in an amount sufficient to orovide 0.1-0.7 functional group for each epoxy group originally in the copolymer; and D. at least one agent capable of preventing the degra-dation of the coating by ultraviolet light.
DETAILED DESCRIPTION OF THE I~ENTION
The thermosetting acrylic powder coating com-position of the present invention, particularly suitable for use as the clear coat of a clear coat/color coat automotive finish, provides. coatings possessing a combi-nation of resistance to outdoor weathering, smoothness, distinctness of image, and a high level of gloss, a com-bination that has heretofore cnly been available in liquid clear coats. These properties are obtained with-out sacrifice of storase stability, hardness, or humidit~
cold crack resistance.

The powder coating composition 1s composed of an acrylic copolymer of monoethylenically unsaturated monomers, Qrimary and secondary crosslinking agents, and one or more ultraviolet light stabilizers or screeners.
It can also contain a reaction catalyst to decrease the curing time, as well as any of the various additives that are advantageously used in automotive coating compositions.
The coating composition of thls invention con-tains an acrylic polymer having a glass transition tem-perature of about 30C to 100C, and preferably 40C to 70C. This glass transition temperature results~in a storage-stable, free-flowing powder which will flow upon baking to form an exceptionally smooth and glossy finish.
The coating composition of this invention is in the form of powder particles with a particle size, or average linear dimension, of about 1 to 100 microns and preferably, to provide a high quality finish, a particle size of 10 to 75 micron~. While the powder particles will ordinarily be nonpigmented, it is acceptable, for clear coat use, to incorporate transparent particles, i.e., pigments having a refractive index the same 2S or similar to the refractive index of the film-forming constituents. Such pigments should have a particle size of about 0.015 to 50 microns and should be used in a pigment-to-powder weight ratio of about 1/10 to 1/100.
Conventional pigments, e.g., inorganic pigments, metallic powders and flakes, organic dyes, organic pigments, and lakes, may also be added, in these same weight ratios, if the coating composition is to be employed other than as the clear coat of a clear coat/color coat finish.
The acrylic copolymer utilized in the powder coating composition of this invention has a number aver-age molecular weight of about 1,500 to 10,000, preferably

3,000 to 7,500, and a weight average molecular weight of about 3,000 to 18,000, preferably 6,000 to 14,000. The 1 1591~6 number average molecular weight and the weight average molecular weight of the copolymer are determlned by gel permeation chromatography, in tetrahydrofuran, using as a standard polymethyl methacrylate having a number aver-age molecular weight of 43,000 and a weight averasemolecular weight of lO0,000.
The acrylic copolvmer is prepared by conven-tional solution, emulsion, or bead polymerization techniques, and by usins conventional polymerization catalysts. Preferred is bead polyme~ization, as dis-closed in 1~7. R. Sorenson and T. W. Campbell, Preparative ~ethods of Polymer Chemistry, Interscience Publishers, New Yor~, second edition, 1968, page 254. Preparation of the copolymer is discussed in greater detail in the lS examples~ infra.
Generally, the acrylic copolymer consists of about 8 to 35 percent by weight, based on the weight of the copolymer, of a moroethylenically unsatu~rated mono-mer having at least one epoxy group and about 65 to 92 percent by weight of one or more monoethylenically un-- saturated monomers having no epoxy groups.
Preferred monoethylenically unsaturated mono-mers having at least one epoxy group are the esters of an epoxide-containing alcohol and a monoethylenically unsaturated acid. These esters have the formula H ~ 1l 11 ~R2 ~ \
C = C -- C -- O -- C -- C -- C -- R6 where P~l-R6 can be the same or different and are each hydrogen or an aliphatic or aromatic hydrocarbon radical having l to 6 carbon atoms. More preferred are esters of this formula where Rl and R4 are each hydrogen or the methyl radical and R2, R3, R5 and R6 are each hydro-sen. Ever. more preferred are es~ers Ct- this ~ormula 115!~

where R2-R6 are each hydrogen and Rl is either hydrogen or the methyl radical, i.e., glycidyl acrylate or glycidyl methacrylate. Glycidyl methacrylate is most preferred.
Preferred monoethylenically unsaturated mono-mers having no epoxy groups are styrene and esters of acrylic or methacrylic acid having the formula H ~
~C = C - C - O - R2 H

where Rl is hydrogen or the methyl radical and R2 is an aliphatic or aromatic hydrocarbon radical having 1 to 20 carbon atoms. Preferably, R2 is an aliphatic hydrocarbon - radical having 1 to 1~ carbon atoms. Two highly preferred monoethylenically unsaturated monomers having no epoxy groups are methyl methacrylate and n-butyl methacrylate.
Durable high ~uality automotive finishes are provided by powder coating compositions that contain an acrylic copolymer of methyl methacrylate, n-butyl methacrylate, and glycidyl methacrylate. A copolymer composed of about 30 to 72 percent by weight of methyl methacrylate, 20 to 45 percent by weight of n-butyl methacrylate, and 8 to 35 percent by weight of glycidyl methacrylate is effective in the coating composition of this invention, although a copolymer of 45 to 55 percent by weight of methyl methacrylate, 30 to 38 percent by weight of n-butyl methacrylate, and 12 to 20 percent by weight of glycidyl methacrylate is preferred. A highly recommended and particularly useful copolymer contains about 50 percent by weight of methyl methacrylate, 34 percent by weight of n-butyl methacrylate, and 16 per-cent by weight of glycidyl methacrylate.
The primary crosslinking agent for the acrylic copolymer is carboxy-terminated and is preferably an 1 15918~
~ g aromatic or ~liphatic carbGxylic acid having 2 to 3 carboxylic acid groups per molecule, or a mixture of monocarboxylic, dicarboxylic, or tricarboxylic acids having a average of 1.5-3.0 carboxylic acid groups per molecule. Linear aliphatic dicarboxylic acids are preferred, particularly those having the formula HOOC - (CH2)n - COOH
where n is an integer between 4 and 20 or, more prefer-ably, between 8 and 18. Most highly recommended is dodecanedioic acid. The primary crosslinking agent must be present in an amount sufficient to provide 0.9 to 1.5 carboxylic acid groups for each epoxy group in the copolymer.
A secondary crosslinking or modifying agent for the copolymer, possessing at least one functional group that is capable of reacting either ~ith hydroxyl groups or with carboxylic acid groups, is present in an amount sufficient to provide 0.1 to 0.7 functional group for each epoxy group in the copolymer. Agents capable of reacting with hydroxyl-groups are preferred over those reacting with carboxylic acid groups.
A preferred class of supplementary crosslink-ing or modifying agents that can react with hydroxyl groups consists of blocke~ isocyanates having an a~erage of 1.5 to 3.5 blocked isocyanate groups per ~olecule.
The isocyanate is preferably an aliphatic diisocyanate, a trimer of an aliphatic diisocyanate, or an adduct of an aliphatic diisocyanate with a difunctional or tri-functional aliphatic alcohol. The blocking agent is preferably caprolactam or methyl ethyl ketoxime. A
second preferred class of agents that can react with hydroxyl groups consists of alkylated melamine formalde-hyde resins and alkylated glycoluril resins.
Somewhat less preferred are supplementary cross-linking or modifying agents that can react with carboxy-lic acid groups. Of these, a preferred class consists of 1 1~91~6 compounds containing one or more epoxy groups. Particu-larly useful are hydantoin epoxides containing 2 to 4 epoxide groups, triglycidylisocyanurate, aliphatic glycidyl ethers, aliphatic glycidyl esters, cycloali-phatic epoxides, and epoxy resins derived from hydro-genated bisphenol-A and epichlorohydrin.
Preferred hydantoin eooxides are those having one of the followi~g formulas 10C ~ - CH - CH2 - N ~ N - C~2 - CH -5~2~

Rl~¢ ~-R2 C~2 Cf CX2 N~ / -CH2-1H-CH2-N\ ~-CH2C~-CH2, ~CH
~ CH

9~o C ~ - CH - CX2 - N \ ~ N CH2 i ~ 2 where Rl and R2 are aliphatic h~rdroca~bon radicals hav-ing 1 to 10 carbon atoms.
Preferred aliphatic glycidyl ethers are those having the formula where R is an aliphatic hydrocarbon radical havins 1 to 20 carbon atoms~.

1 1 591~

Preferred aliphatic glycidyl esters are those having the formula ~0 ~ O
CH2 ~ CH - CH2 - O - C - R
where R is an aliphatic hydrocarbon radical having l to 20 carbonatoms.
Preferred cycloaliphatic epoxides are those having either of the following formulas lo R
C - O - CH ~ O

O O
O ~ CH2 - O - C ~ R - C ~ O - CH2- ~ o - where R is a bivalent aromatic hydrocarbon radical having 6 to lO carbon atoms or a bivalent aliphatic hydrocarbon radical having 2 to 20 carbon atoms.
Preferred epoxy resins derived from hydrogen-ated bisphenol-A and epichlorohydrin are those having the for~la CH2-CH-CH2-O~C-~O-C~{2-CH-C~2-~--l~C 30 0-CH2-CH-CH2 C~3 n CH3 where n is an integer from 0 to 10.
Another preferred class of supplementary crosslinking or modifying agents that react with car-boxylic acid groups consists of disubstituted oxazolinesand trisubstituted oxazolines. Preferred are those with the formula ~ ~ R - ~ ~

where R is a bivalent aiiphatic hydrocarbon radical 1 15~186 having 2 to 20 carbon atoms or a bivalent aromatic hydrocarbon radical having 6 ~o 10 carbon atoms.
Still another 2referred class of supplementary crosslinking or modifying agents that react with carbo~ylic acid groups consists of difunctional, trifunctional, or tetrafunctional ~-h~droxyal~ylamides, of the type dis-closed in Swift et al., U.S. Patent 4,076,917, issued February 21, 1978.
The powder coating composition of the present invention is fortified with at least one ultraviolet light stabilizer or screener to prevent degradation of the resultant finish by ultraviolet light. Highly pre-ferred for this purpose is a combination of an ultra-violet screener and a hindered amine light stabilizer.
The powder coatin~ composition contains about 0.3 to 6 percent by weight, based on the total weight of the co~position, of this combination, prefera~ly about 0.5 to 2 percent by weight of a hindered amine light stabilizer and about 1 to 3 percent by weight of an ultraviolet screener. ~oth the ultraviolet screener and the hindered amine light stabilizer should have a weight average molecular weight greater than 300 and a particle size of less than 40 microns. Preferred ultraviolet screeners are 2-(o-hydroxyphenyl)benzotriazoles, nickel chelates, _-hydroxybenzophenones, or phenyl salicylates.
Most preferred are the 2-(o-hydroxyphenyl)benzotriazol-es.
The hindered amine li~ht stabilizer can be either mono-meric or polymeric, although the latter are preferred from a permanence standpoint.
Hindered amine light stabilizers provide greater efficiency at economical use levels than, for instance, nickel organics and benzophenones. It has been estimated that as much as four times longer product life can be expected from products reinforced with the hindered amines than from products reinforced with con-ventional ultraviolet screeners and stabilizers.

l 15~

DesDite their good light stabilization characteristics, hindered amine light stabilizers are nevertheless more effective in many polymer systems when used in conjunc-tion with certain conventional ultraviolet screeners.
The combination of a hindere~ amine light stabilizer and an o-hydroxyphenyl benzotriazole is preferred for use in the coating compositions of the present invention, and is especially preferred in those compositions con-taining an acrylic copolymer of methyl methacrylate, n-butyl methacrylate, and glycidyl methacrylate. An apparent synergistic effect resulting from the combina-tion of the hindered amine light stabilizer and the o-hydro~yphenyl benzotriazole imparts exceptional dura-bility to the clear powder coat of this invention.
The coating composition of this invention may contain a reaction catalyst to decrease the curing time. Preferred catalysts include tin catalysts, e.g., stannous octoate, for blocked isocyanate systems, and strong acids, ~ ~ , p-toluenesulfonic acid, for systems containing dimethoxymethyldiethoxy~.ethylglycoluril, hexa-methoxymethy~x~amlne, and other melamine-formaldehyde resins. Strong acid catalysts are preferably blocked with an epoxide or an amine.
The catalysts will ordinarily be present in an amount up to about 2 percent by weight, based on the weight of the film,fonming blend.
The coating composition may also contain cer-tain other additives that are typically incorporated into powder coating compositions. Particularly recom-mended are antipopping agents, which allow volatiles to gently escape from the film during baking, and flow con-trol agents, which prevent cratering of the finish.
Benzoin is the highly preferred antipopping agent and is present in an amount ranging from about 0.05 percent by weight to 1.0 percent by weight, based on the weight of the total powder composition. The flow control agent is present in an amount ranging from about 0.05 percent by weight to 5.0 percent by weight.
One preferred flow control agent is alkylene oxide 1 l~gl86 modified dimethyl polvsiloxane fluid. Other useful flow control aqents include those disclosed in Labana et al., United States Patents 4,091,048 and 4,091,049, issued May 23, 1978, e.g., acrylic polymers such as polylauryl s acrylate, polybutyl acrylate, poly(2-ethylhexyl acrylate), polylauryl methacrylate, and polyisodecyl methacrylate, and fluorinated polymers such as the esters of poly-ethylene glycol or polypropylene glycol, and fluorinated fatty acids.
The powder coating composition of this inven-tion can be applied directly to a metal, glass, plastic, or fiber-reinforced plastic substrate or to one which has been primed and/or sealed in a conventional manner An electrically conductive carbon black pigment may be lS added to the primer or sealer to make the surface con-ductive and to promote uniform deposition of the powder during spraying. Application of the powdex can be by electrostatic spraying or by use of a fluidized bed.
Preferred is electrostatic spraying wherein a negative charge of 20 to 100 kilovolts is applied to the spray gun. The powder composition can be applied either in one pass or in several passes to provide a film thick-ness, after cure, of about 0.5 to S mils. Preferably, to provide a high ~uality finish of reasonable cost, the thickness of the clear powder coat is about 1.2 to 2 mils and, more preferably, 1.4 to 2 mils.
The substrate to be coated can, optionally, be preheated prior to the application of the powder to promote more uniform powder deposition. Upon applica-tion of the powder, the powder-coated substrate is baked at 250F to 3S0F for 20 to 60 minutes.
Preferred for the powder coating composition of the present invention is a staged bake, wherein the powder-coated substrates are baked for 10 minutes at 250F, another 10 minutes at 300F, and then for 30 minutes at 350F.

d 1 8 6 ~ e ~-ese~t inve~.tion will be more fully under-s~ood '-om ~he followins illustr ,ive e~m~les, whe~ein all a~anti~ies, percentages, and ratios are on a weight basis unless otherwise indicated.
EX~PLE 1 ?re~aration of Acrvlic Co~ol~-mer Methyl metnacrylate mcnomer 50.00 n-Butvl methacrylate monomer 34.00 Glvcidvl methacrylate monomer 16.00 T.~later 100 . 00 10 Lauryl merca?tan 4.25 Vazo~ 52 polymerization initiator 0.75 (registered trademark of E. I. du Pont de Nemours and Company) Acrysol*A-3 surfactant (available from0.75 ~ohm and Haas Company)(25% solution in water) 205.75 The acrylic copolymer is prepared using bead poly~erization techniques by stirrins the above compon-ents at about 175F to 190F for one hour. The copoly-mer is isolated by filtration, washed with water, and dried.
Preparation of Powder Coating Composition Acrylic copolymer 82.33 Dodecanedioic acid ~6 micron average 9.67 particle size) blocked isocyanate derived from Hylene@ W 8.00 orsanic isocyanate (registered ~rademark of E. I. du Pont de ~emours and Company) wherein 48% of the NCO has been reacted ~ith trimethylolpropane and ,he remaining NCO has been reacted ~ith capro'actam Benzoin (6 micron average particle size)0.50 Alkylene oxide modified dimethyl 0.50 polysiloxane fluid 30 St2nnous octoate 0.10 CGL-900*o-hydroxyphenylbenzotriazole ultra-2.00 violet absorber (available from Ciba-C-eigv Cor~oration) (7 micron average ?a~ticle size) Tinuvin*622 hindered amine light stabilizer 1.00 (zvailable from Ciba-Geigy Corpora-ion) (10 micron averase particle size) 104.00 * denotes trade mark 1 15~186 The above components are mixed for 45 minutes in a p~anetary mixer, then blended for 10 minutes at 200~ on a 2-roll mill. The resulting chips are ground in a pin mill, and the resulting powder is then jet-sieved through a 270 mesh screen.
The powder clear coating composti~on thus pre-pared is electrostatically sprayed over a color-coated automotive substrate and baked for 10 minutes at 250F, 10 minutes at 300F, and 30 minutes at 350F.
The resultant coating exhibits high gloss, a smooth finish, and excellent resistance to weathering.
EXA~LE 2 Acrylic copol~mer (as prepared in Example 1) 83.57 Dodecanedioic acid (6 micron average 12.55 ~article size) 15 Hylene~ W organic isocyanate (registered3.78 trademark of E. I. du Pont de ~lemours and Company) capped with 2 moles of methyl ethyl ketoxime Benzoin (6 micron average particle size)0.50 Alkylene oxide modi~ied dimethyl 0.50 polysiloxane fluid 20 Stannous octoate 0.02 CGL-900 (7 micron average particle size)2.00 Tinuvin 622 (10 micron average particle size) 1.00 104.02 Preparation and application of this powder coating composition follow the procedures outlined in Example 1, and comparable results are obtained. The same holds true for the following examples, except where otherwise indicated.
EXAMP~E 3 Acrylic copolYmer (as prepared in Example 1) 35.12 Dodecanedioic acid (6 micron average 12.77 particle size) Dimethoxymethyldiethoxymethyl glycoluril2.11 Benzoin (6 micron avera~e particle size)0.50 Alkylene oxide modi~ied dimethyl 0.50 polysiloxane fluid p-Toluenesulfonic acid 0.50 35 Isopropyl alcohol 2.00 ~ 159~3~

Cycloaliphatic diepoxide 1.50 ( O~C - O - CH2~0) CGL-900 (7 micron averaae particle size)2.00 Tinuvin 144 hindered amine light stabilizer 1.00 (available from Ciba-Geigy Corporation) (S micron average particle size) 108,00 ~he p-toluenesulfonic acid is first dissolved in the isopropyl alcohol, after which the cycloaliphatic diepoxide is added, and the resultant mixture is allowed to stand for 1 hour. The remaining components are then added and mixed, as in Example 1.
EXAMPL~ 4 Acrylic copolymer (as prepared in Example 1) 85.43 lS Dodecanedioic acid (6 micron average 12.82 particle size) Triglycidylisocyanurate (6 micron average 1.75 particle size) Benzoin (6 micron averaae particle size)0.50 Alkylene oxide modified dimethyl 0.50 polysiloxane fluid CGL-900 (7 micron average particle size)2.00 Tinuvin 144 (5 micron average particle size) 1.00 104.00 EXAMPLE S
Acrylic copolymer ~as prepared in Example 1) 84.75 Dodecanedioic acid (6 micron average12.71 particle size) Hydantoin diepoxide 2.54 O~ o~ /o\ ~

t~ J
o (6 micron averaae particle size) Benzoin (6 micron average particle size~ 0.50 Alkylene oxide modified dimet~yl0.50 polysiloxane fl~id CGL-900 (7 micron average particle size) 2.00 Tinuvin 622 (10 micron average particle size) 1.00 104.00 l 1 5 ~ 6 Acrylic copolymer (as prepared in Example 1) 85.30 Dodecanedioic acid (6 micron average 12.79 particle size) m-Phenylenebis(oxazoline) (6 micron 1.91 average particle size) Benzoin (5 micron average ~article size) 0.50 Alkylene oxide modified dimethyl 0.50 polysiloxane fluid CGL-900 (7 micron average particle size) 2.00 Tinuvin 144 (5 micron average particle size) 1.00 104.00 EXA~LE 7 Acrylic copolymer (as pre~ared in Example 1) 84.96 Dodecanedioic acid (6 micron average12.74 particle size) Hydroxyalkylamide 2.29 ((HO - CH - CH~ C(CHZ)4CH lH2 ~ C'A - OH) ) Benzoin (6 micron average particle size) 0.50 Alkylene oxide modified dimethyl 0.50 polysiloxane fluid CGL-900 (7 micron average particle size) 2.00 20 Tinuvin 144 (5 micron average particle size) 1.00 1~3.9 EXA~æLE 8 Acrylic copolymer (as prepared in Example 1) 82.71 Dodecanedioic acid (6 micron average12.41 particle size) 25 Aliphatic glycidyl ether 4.89 - ~0\

where R is mostly C12 and C14 al1~cyl /
30 Benzoin (6 micron average particle size) 0.50 Alkylene oxide modified dimethyl 0.50 polysiloxane fluid CGL-900 (7 micron average particle size) - 2.00 Tinuvin 144 (5 micron average particle size) 1.00 104.00 1 159181~

EY~LE 9 ~crylic co?olvmer (~s pre?ared in Example 1) 83.28 Dodecanedioic ~cid (6 micron averase 12.50 ~2_ticle size) Carcura ~ ester (slycidvl ester of an 4.22 aliphatic ~cid)(available Erom Shell Chemical Com~zny) Benzoin (6 micron average particle size) 0.50 Alkylene oxide modified dimethvl 0.50 ~olysiloxane flu~d CGL 900 (7 micron average ~article size) 2.00 Tinuvin 144 (S mic-on aver2ge pa~ticle sizè) 1.00 104.00 E ~PLE 10 Acrylic copolymer (as prepared in rx~mple 1) 85.82 Dodecanedioic acid ~6 micron average 10.08 particle size) Hexamethoxvmethylmelamine 4.09 Benzoin (6 micron average particle size) O.S0 15 AlXylene oxide modifiea dimethyl 0.50 ~olysiloxane fluid Isopropyl alcohol 0.60 ~Toluenesulfonic acid 0.10 ~ycloaliphatic diepoxide (as in Example 3) 0.30 CGL-900 (7 micron average ~article size) 2.00 CGL-079L*hindered amine liaht stabilizer 2.50 (available from Ciba-Geisy Corporation) 10~.49 The p-toluenesulfonic acid is dissolved in two-thirds (.40 parts) of the isopropyl alcohol, and the cycloaliphatic diepoxide is dissolved in the remainins 2; isopropvl alcohol. The t~o portions are then combined and allowed to stand for 1 hour before mixins with the rem2ining components.
Each of the above examples yields a trans-parent cle~r coat finish characterizec by excellent ap-pearance ~nd superior wea~hering ?roperties.

* denotes trade mark ~ .. . . . .. . . .... . .... ........ .... ... ...... .. . . .. . ..... .. .. ~ .. ..... ~ .. ..... .

Claims

The invention claimed is:

1. A thermosetting powder coating composition of finely divided particles having a particle size of about 1 to 100 microns, wherein the particles are an intimately mixed blend consisting essentially of:
A. an acrylic copolymer consisting essentially of:
(1) about 8 to 35 percent by weight, based on the weight of the copolymer, of a monoethylenically unsaturated monomer having at least one epoxy group, and (2) about 65 to 92 percent by weight, based on the weight of the copolymer, of at least one mono-ethylenically unsaturated monomer which is free of epoxy groups, wherein the copolymer has a number average molecular weight of about 1,500 to 10,000, a weight average molecular weight of about 3,000 to 18,000, and a glass transi-tion temperature of about 30°C to 100°C;
B. a carboxy-terminated crosslinking agent in an amount sufficient to provide 0.9-1.5 carboxylic acid groups for each epoxy group originally in the copolymer;
C. a supplementary crosslinking or modifying agent having at least one functional group which is capable of reacting with either hydroxyl groups or carboxylic acid groups, said supplementary cross-linking or modifying agent being present in an amount sufficient to provide 0.1-0.7 functional group for each epoxy group originally in the co-polymer; and D. at least one agent capable of preventing the degra-dation of the coating by ultraviolet light.

2. The powder coating composition of claim 1 wherein the monoethylenically unsaturated monomer having at least one epoxy group is the ester of an epoxide-containing alcohol and a monoethylenically unsaturated acid and has the formula where R1 and R4 can be the same or different and are each selected from the group consisting of hydrogen and the methyl radical, and R2, R3, R5 and R6 are hydrogen.

3. The powder coating composition of Claim 1 wherein the monoethylenically unsaturated monomer having at least one epoxy group is glycidyl methacrylate.

4. The powder coating composition of Claim 2 wherein the monoethylenically unsaturated monomer/
monomers having no epoxy groups is/are selected from or a mixture of:
A. styrene ; and B. esters of acrylic or methacrylic acid having the formula where R1 is selected from the group consisting of hydrogen and the methyl radical, and R2 is selected from the group consisting of C1-C20 aliphatic hydro-carbon radicals and C1-C20 aromatic hydrocarbon radicals.

5. The powder coating composition of Claim 4 which contains 45 to 55 percent by weight, based on the weight of the acrylic copolymer, of methyl methacrylate, 30 to 38 percent by weight of n-butyl methacrylate, and 12 to 20 percent by weight of glycidyl methacrylate.

6. The powder coating composition of Claim 4 wherein the carboxy-terminated crosslinking agent is an aliphatic dicarboxylic acid having the formula HOOC - (CH2)n - COOH
where n is an integer from 8-18.

7. The powder coating composition of Claim 6 wherein the carboxy-terminated crosslinking agent is dodecanedioic acid.

8. The powder coating composition of Claim 6 wherein the supplementary crosslinking or modify-ing agent is a blocked isocyanate having an average of 1.5-3.5 blocked isocyanate groups per molecule wherein the isocyanate is selected from the group consisting of aliphatic diisocyanates, trimers of aliphatic diisocya-nates, and adducts of aliphatic diisocyanates with a difunctional or trifunctional aliphatic alcohol, and the blocking agent for the isocyanate is selected from the group consisting of caprolactam and methyl ethyl ketoxime.

9. The powder coating composition of Claim 6 wherein the supplementary crosslinking or modify-ing agent is selected from the group consisting of (A) alkylated melamine formaldehyde resins, (B) alkylated glycoluril resins, (C) an aliphatic glycidyl ether having the formula (D) an aliphatic glycidyl ester having the formula where R is selected from the group consisting of the C1-C20 aliphatic hydrocarbon radicals,(E) a cyclo-aliphatic epoxide having the formula , and (F) a cycloaliphatic epoxide having the formula where R is selected from the group consisting of bivalent C6-C10 aromatic hydrocarbon radicals and bivalent C2-C20 aliphatic hydrocarbon radicals.

10. The powder coating composition of Claim 6 wherein the supplementary crosslinking or modify-ing agent is selected from the group consisting of:
A. a hydantoin epoxide having the formula B. a hydantoin epoxide having the formula C. a hydantoin epoxide having the formula where R1 is selected from the group consisting of the C1-C10 aliphatic hydrocarbon radicals and R2 is selected from the group consisting of the C1-C10 aro-matic hydrocarbon radicals.

11. The powder coating composition of Claim 1 wherein the supplementary crosslinking or modify-ing agent is triglycidylisocyanurate.

12. The powder coating composition of Claim 6 wherein the supplementary crosslinking or modify-ing agent is an epoxy resin derived from hydrogenated bisphenol-A and epichlorohydrin.

13. The powder coating composition of Claim 6 wherein the supplementary crosslinking or modify-ing agent is an oxazoline having the formula where R is selected from the group consisting of bival-ent C2-C20 aliphatic hydrocarbon radicals and bivalent C6-C10 aromatic hydrocarbon radicals.

14. The powder coating composition of Claim 6 wherein the supplementary crosslinking or modifying agent is selected from the group consisting of difunc-tional .beta.-hydroxyalkylamides, trifunctional .beta.-hydroxy-alkylamides, and tetrafunctional .beta.-hydroxyalkylamides.

15. The powder coating composition of Claim 9 wherein a combination of an ultraviolet screener and a hindered amine light stabilizer is employed to prevent the degradation of the coating by ultraviolet light.

16. The powder coating composition of Claim 9 wherein the ultraviolet screener is selected from the group consisting of 2-(o-hydroxyphenyl)benzotria-zoles, nickel chelates, o-hydroxybenzophenones, and phenyl salicylates.

17. The powder coating composition of Claim 16 wherein the ultraviolet screener is a 2-(o-hydroxy-phenyl)benzotriazole.

18. The powder coating composition of Claim 16 which contains about 1 to 3 percent by weight, based on the weight of the acrylic copolymer, of the ultra-violet screener and about 0.5 to 2 percent by weight of the hindered amine light stabilizer.

19. The powder coating compositions of Claim 9 which contains 1 to 3 percent by weight, based on the weight of the acrylic copolymer, of a 2-(o-hydroxyphenyl)-benzotriazole and about 0.5 to 2 percent by weight of a hindered amine light stabilizer.

20. The powder coating compositions of Claim 8 which contains 1 to 3 percent by weight, based on the weight of the acrylic copolymer, of a 2-(o-hydroxyphenyl) benzotriazole and about 0.5-2 percent by weight of a hindered amine light stabilizer.

21. The powder coating composition of Claim 19 containing in addition to the aforementioned con-stituents up to about 2 percent by weight, based on the weight of the total powder composition, of a blocked strong acid catalyst.

22. The powder coating composition of Claim 19 containing in addition to the aforementioned con-stituents about 0.05 percent by weight to 1.0 percent by weight, based on the weight of the total powder composition, of benzoin.

23. The powder coating composition of Claim 9 containing in addition to the aforementioned con-stituents 0.05 percent by weight to 2.0 percent by weight, based on the weight of the total powder com-position, of a flow control agent.

24. The powder coating composition of Claim 23 wherein the flow control agent is alkylene oxide modified dimethyl polysiloxane fluid.

25. The powder coating composition of Claim 23 wherein the flow control agent is a fluorinated poly-mer.

26. A substrate coated with a pigmented liquid coating composition and subsequently coated with a trans-parent powder coating composition according to Claim 9 to form a clear coat/color coat finish.

27. The coated article of Claim 26 wherein the thickness of the cured powder coating composition is about 1.2 to 2 mils.