WO1995028433A1 - Powder coating compositions prepared by microfine grinding - Google Patents

Abstract

This invention provides a method for preparing a powder coating composition having a particle size of from about 0.5 to 40 microns. Such compositions are especially useful in applications where a thin film (i.e., about 0.1 to 1.0 mil) is desired as well as applications where highly thermally reactive compositions are utilized. Optionally, the small particles are then pelletized and reground in a hammer-type mill to afford a composition having a particle size of approximately 30-40 microns. The method is especially useful for compositions which are designed to cure at a temperature at or below conventional extrusion temperatures, i.e., 90-130 °C.

Description

Powder Coating Compositions Prepared by Microfine Grinding

This invention belongs to the field of thermo¬ setting powder coating compositions. In particular, this invention provides a low—cost method for blending and reducing particle size in such compositions. Plastic materials used in the manufacture of powder coatings are classified broadly as either thermosetting or thermoplastic. In the application of thermoplastic powder coatings, heat is applied to the coating on the substrate to melt the particles of the powder coating and thereby permit the particles to flow together and form a smooth coating.

Thermosetting coatings, when compared to coatings derived from thermoplastic compositions, generally are tougher, more resistant to solvents and detergents, have better adhesion to metal substrates and do not soften when exposed to elevated temperatures. However, the curing of thermosetting coatings has created problems in obtaining coatings which have, in addition to the above- stated desirable characteristics, good smoothness and flexibility. Coatings prepared from thermosetting powder compositions, upon the application of heat, may cure or set prior to forming a smooth coating, resulting in a relatively rough finish referred to as an "orange peel" surface. Such a coating surface or finish lacks the gloss and luster of coatings typically obtained from thermoplastic compositions. The "orange peel" surface problem has caused thermosetting coatings to be applied from organic solvent systems which are inherently undesirable because of the environmental and safety problems that may be occasioned by the evaporation of the solvent system. Solvent—based coating compositions also suffer from the disadvantage of relatively poor percent utilization, i.e., in some modes of application, only 60 percent or less of the solvent—based coating composition being applied contacts the article or substrate being coated. Thus, a substantial portion of solvent—based coatings can be wasted since that portion which does not contact the article or substrate being coated obviously cannot be reclaimed.

In addition to exhibiting good gloss, impact strength and resistance to solvents and chemicals, coatings derived from thermosetting coating compositions must possess good to excellent flexibility. For example, good flexibility is essential for powder coating compositions used to coat sheet (coil) steel which is destined to be formed or shaped into articles used in the manufacture of various household appliances and automobiles wherein the sheet metal is flexed or bent at various angles.

The need for cost savings through improved materials utilization encourages powder coating composition producers to prepare coatings that maintain performance at low film builds. The container coating market which utilizes thin films is an example of where performance and cost are both important ingredients for successful products. One important prerequisite for thin films (i.e., 0.1 — 1.0 mil thickness) is small particle size. Whereas particle size means of 30 — 40 μm are typical for standard 2 mil film thicknesses, and can be produced by standard grinding mills, a mean particle size of 10 μm or less may be required for thin films applications.

We have discovered a new method for preparing powder coating compositions. This method is based on a strategy that eliminates the traditional melt phase extrusion step. The method utilizes jet milling of the raw material components to produce finely divided powder that melts and reacts together during oven curing. This method is particularly useful for products that require very small particle sizes for thin film applications (e.g., can coatings). By eliminating the extrusion step, both capital and operating costs can be reduced significantly.

U.S. Patent No. 4,383,055 describes a process for preparing curable, water—dilutable coatings by conjointly comminuting resin, crosslinker, pigments, etc. to reduce particle size to <100 μm.

U.S. Patent No. 3,362,922 describes a fluidizable polyepoxide composition made by pulverizing a poly— epoxide, a benzophenone tetracarboxylic dianhydride and tin monocarboxylate in a pebble mill containing high density grinding media. The thusly prepared composition does not suffer from segregation upon standing and gives uniform coatings.

U.S. Patent No. 4,315,884 describes an in—mold coating process utilizing an adsorbed liquid catalyst on an unsaturated polyester coating composition. A catalyst solution is absorbed onto a polyester resin and ground, the remaining components of the composition are ground to the same particle size, and the two powders are mixed together. The catalyst is not exposed to high temperatures, thereby minimizing premature curing.

Japanese Patent No. 7281328 describes a thermo¬ setting powder coating based on an acrylic copolymer and bisphenol A, prepared by rolling and grinding to 120—mesh to give a coating with acceptable properties.

DE 2,656,531 describes a process for the production of homogeneous masses whereby fine particulate materials are mixed with coarser materials, either wet or dry. The materials are mixed by being fed as two separate streams into a pin disc mill. Production of coatings, paints, building blocks, etc. are described.

Japanese Patent No. 62192471 claims a method for the manufacture of powder paint, composed of forming a dispersion of solvent, pigment, pigment—dispersing resin and/or resin for powder paint in a wet dispersion machine, a wet dispersion process to obtain a uniform liquid dispersion, and a vacuum—drying process, wherein said dispersion is vacuum—dried by a vacuum heating dryer having a heating tube, vacuum powder—collection chamber, vacuum device and solvent—recovery device to obtain the powder paint.

G.B. Patent No. 1,437,849 describes a method for producing a flowable particulate coating powder comprising the steps of (i) spray drying a solvent solution of at least one thermoplastic resin to produce particles having generally rounded edges, and (ii) grinding and shattering the spray dried powder particles in a fluid—energy mill to produce a coating powder in which essentially all of the particles are irregular in shape, have sharp, jagged edges, and range in size from 1 micron to 100 microns.

Figure 1 is transmission electron micrograph (TEM) image of the uncured powder in Example 1 (6500x magnification) .

Figure 2 is an image of the same powder coating after curing (6500x) .

Figure 3 is a TEM image of a conventially prepared analog of the formulation of Example 1, which has been extruded and pulverized to a particle size of about 30 microns.

Figure 4 is a TEM image of the powder coating of Figure 3 after curing. Figure 5 is a plot of heat remaining, Cal/g versus days at 40°C for the composition of Example 1. This data is more fully described below.

Figure 6 is a plot of heat remaining in Cal/g versus days at 40°C for the composition of Example 2. This data is described more fully below.

Summary of the Invention

The present invention provides a method which is useful for blending and particle size reduction in thermosetting powder coating compositions which offers a low cost alternative to traditional extrusion/grinding methodology. In this method, the powder coating composition components are mixed together, pulverized in a hammer—type mill and then jet—milled to a particle size of preferably about 1—15 microns. Optionally, the small particles are pelletized and reground in a hammer- type mill to afford a composition having a particle size of approximately 30—40 microns.

Detailed Description of the Invention

One prerequisite for conventional thin film—based powder coatings is that all components of the formula¬ tion be melt extruded prior grinding. This is done in order to mix the ingredients uniformly, so that crosslinking, melting, degassing, etc. can occur. These conventional procedures require extensive capital outlays to cover extrusion costs, in addition to associated operating and clean—up costs.

Described herein are formulations whose ingredients have not been subjected to melt phase extrusion, but are instead finely ground in the dry state; thus, extrusion, cooling and chipping is eliminated, while at the same time, the powder coating composition has been rendered sufficiently homogeneous as to provide a uniform crosslinked coating. The finely ground ingredient particles are sized so that they melt together upon heating to the appropriate temperature.

This procedure provides powders for thin film applications that are less expensive to prepare. Also, the compositions are exposed to very little heat throughout, so there will be no appreciable premature curing. Lack of such a heat history is useful for more highly reactive systems that tend to partially cure during extrusion. The method of the present invention thus also allows one to prepare more highly reactive compositions which could not be prepared using conventional extrusion/grinding methodology due to the premature crosslinking.

Thus, the present invention provides a method for blending and reducing the particle size of components in a thermosetting powder coating composition which comprises

(a) combining said components and pulverizing said composition, followed by

(b) jet—milling said composition until a mean particle size of about 0.5 to 40 microns is obtained.

In the practice of the present invention, the pulverizing step (a) is preferably carried out using a hammer mill type pulverizer, e.g., a Bantam micro- pulverizer hammer mill fitted with a 0.010" slot screen, operating at a rotor speed of about 8000—14,000 rpms.

The step (b) jet milling step is preferably carried out using a fluid energy type mill. In this regard, a Fluid Energy Model 4 Microjet Mill, a 4" mill operating at nominal values of 60—100 psi and 40—70 scfm (standard cubic feet per minute) , having a feed rate of 2 to 16 lbs./h is preferred. Alternatively, a TROST Air impact pulverizer, operating at 10—18 scfm (standard cubic feet per minute) at 80-100 psi, having a feed rate of 0.2 to 2 lbs./h can also be utilized. Other air jet mills capable of carrying out this procedure include those manufactured by Micron Powder Systems and Sturtevant. Particle size distributions for the composition ingredients of this invention preferably fall between about 0.5 and 40 microns, most preferably between 1 and 15 microns. Mean values are 2 to 20 microns, preferably 5 to 15 microns. In a further embodiment of the present invention, there is provided further processing step (c) :

(c) pelletizing said composition under pressure and pulverizing said composition until a mean particle size of about 20—50 microns is obtained.

Agglomeration pressures used for particle size adjustment in step (c) may be between about 1000 and 30,000 psi, preferably between 5000 and 25000 psi.

The powder coating compositions of this invention may be thermoplastic or thermosetting in nature. Thermosetting powder coating compositions are generally comprised of one or more curable polymers and one or more crosslinking agents along with other additives typically used in the industry, and optionally a pigment. Thermosetting powder coatings may be those based on crosslinkers such as blocked polyisocyanate, epoxy, triglycidylisocyanurate, B—hydroxyalkyl amide such as Bis(N,N'—dihydroxyethy1)adipamide, aminoplasts such as those based on glycoluril or those based on melamine. Matching resins for thermosetting powder coatings includes polyester and acrylic resins possessing hydroxyl functionality. Alternatively, thermosetting resins based on carboxyl functional resins may be crosslinked with epoxy resins or epoxy—functional crosslinkers such as tri— glycidyl isocyanurate.

Preferably, the curable polymer component is chosen from the known resins used in the powder coating art which have epoxy, carboxy, hydroxy, or anhydride functional groups which can be reacted with known cross- linking compounds to provide cured coatings.

Preferred epoxy functional resins generally have a molecular weight of about 300 to about 4000, and have approximately .05 to about .99, epoxy groups per lOOg of resin(i.e., 100—2000 weight per epoxy (WPE) ) . Such resins are widely known and are commercially—available under the EPON^® tradename of the Shell Chemical Company, the ARALDITE^® tradename of CIBA—Geigy, and D.E.R. resins of the Dow Chemical Company.

Curable polymers which have carboxy functional groups include polyesters. Such polyesters preferably have a molecular weight of about 500 to about 5000 and an acid number of about 35—75. Commercially available examples of such resins include ^®ALFTALAT AN 720, 721, 722, 744, 758, and ^®ALFTALAT AN 9970 and 9983 resins available from Hoechst Celanese.

Curable polymers which have free hydroxy groups also include the polyesters and acrylics. Hydroxy- functional polyesters and acrylic polymers preferably have a hydroxyl number from about 30 to about 60 (mg KOH/g polymer) .

The polyesters as described herein may be produced using well—known polycondensation procedures employing an excess of glycol (or acid) to obtain a polymer having the specified hydroxyl (or carboxyl) number. The glycol residues of the polyester component may be derived from a wide variety and number of aliphatic, alicyclic and alicyclic—aromatic glycols or diols containing from 2 to about 10 carbon atoms. Examples of such glycols include ethylene glycol, propylene glycol, 1,3—propanediol, 2,4— dimethyl-2-ethylhexane-l,3—diol, 2,2—dimethyl-1,3- propanediol, 2—ethy1-2—butyl—1,3—propanediol, 2—ethyl—2- isobutyl-1, 3—propanediol, 1,3—butanediol, 1,4—butane- diol, 1,5—pentanediol, 1, 6—hexanediol, thiodiethanol, 1,2—, 1,3— and 1,4—cyclohexanedimethanol, 2,2,4,4— tetramethyl—1,3—cyclobutanediol, 1,4—xylylenediol and the like. The dicarboxylic acid constituent of the polyesters may be derived from various aliphatic, alicyclic, aliphatic—alicyclic and aromatic dicarboxylic acids containing about 4 to 10 carbon atoms or ester—forming derivatives thereof such as dialkyl ester and/or anhydrides. Succinic, glutaric, adipic, azelaic, sebacic, fumaric, maleic, itaconic, 1,3— and 1,4— cyclohexanedicarboxylic, phthalic, isophthalic and terephthalic are representative of the dicarboxylic acids from which the diacid residues of the amorphous polyester may be derived. A minor amount, e.g., up to 10 mole percent, of the glycol and/or diacid residues may be replaced with branching agents, e.g., tri— functional residues derived from trimethylolethane, trimethylolpropane and trimellitic anhydride. The preferred polyesters of the composition provided by this invention have a Tg greater than 55°C, and an inherent viscosity of about 0.15 to 0.4 g/dL measured at 25°C using 0.5 g polymer per 100 L of a solvent consisting of 60 parts by weight phenol and 40 parts by weight tetrachloroethane. The polyester component preferably is comprised of (1) diacid residues of which at least 50 mole percent are terephthalic acid residues, (2) glycol residues of which at least 50 mole percent are derived from 2,2—dimethyl—1,3—propanediol (neopentyl glycol) and (3) up to 10 mole percent, based on the total moles of (2) and (3) , of trimethylolpropane residues. These preferred hydroxyl functional polyesters are commercially available, e.g., under the names AZS 50 Resin, RUCOTE 107 and CARGILL Resin 3000, and/or can be prepared according to the procedures described in U.S. Patent Nos. 3,296,211, 3,842,021, 4,124,570 and 4,264,751 incorporated herein by reference, and Published Japanese Patent Applications (Kokai) 73-05,895 and 73—26,292. A preferred polyester consists essentially of terephthalic acid residues, 2,2— dimethyl—1,3—propanediol residues and up to 10 mole percent, based on the total moles of 2,2—dimethyl—1,3— propanediol residues, of trimethylolpropane residues, and possesses a Tg of about 50° to 65°C, a hydroxyl number of about 35 to 60, an acid number of less than 10 and an inherent viscosity of about 0.1 to 0.25 g/dL.

Alternatively, part or all of the polyester resins described above may be replaced with a semi—crystalline resin. Coatings having such semi—crystalline resins generally possess good powder storage stability, good balance of hardness and flexibility, excellent smoothness and high gloss. Examples of such compositions can be found in U.S. Patent 4,859,760, incorporated herein by reference. The semi—crystalline polyesters are preferably comprised of a major portion of terephthalic acid, 1,4—cyclohexanedicarboxylic acid (CHDA) or a linear diacid such as adipic acid, and a glycol such as 1,4—butanediol, 1,6—hexanediol, 1,10-decanediol, 1,12-dodecanediol or a combination thereof. The semi—crystalline polyester resin have number average molecular weight Mn of from about 1,500 to about 10,000, preferably from about 2,000 to 6,000 and a glass transition temperature, Tg, of about 50 C or below, preferably 30 C or below. Further it is preferred that such resins have a melting point of from about 70° to about 150° C, preferably from 90° to 140°C with a heat of fusion (second heating cycle of DSC) greater than 5 cal/g-°C, preferably greater than 8 cal/g-°C.

The semi—crystalline polyesters may have a hydroxyl or carboxyl number of from about 20 to 100, preferably from about 30 to about 80, for crosslinking. The ratio of the semi—crystalline polyester to amorphous polyester is dependent on characteristics of each polyester component, crosslinker, pigment loading and final objectives of the coating desired.

The acrylic polymer component is preferably a polymer or resin prepared by polymerization of a hydroxyl—bearing monomer such as hydroxyethyl methacrylate, hydroxyethyl acrylate, hydroxyhexyl acrylate, hydroxyhexyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxylbutyl methacrylate and the like optionally polymerized with other monomers such as methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, isobutyl acrylate, isobutyl methacrylate, ethylhexyl acrylate, ethylhexyl methacrylate, styrene, vinyl acetate, and the like. The ratio of reagents and molecular weights of the resulting acrylic polymer are preferably chosen so as to give polymers with an average functionality (the number of OH groups per molecule) greater than or equal to 2. Commercially—available curable hydroxy-functional acrylic polymers include JONCRYL 800, JONCRYL 500, and NEOCRYL LE-800. Curable polymers which have epoxy groups can also be resins comprised of residues of glycidyl methacrylate(GMA) and/or glycidyl acrylate. Such resins generally have a number average molecular weight of about 500 to about 5000, and a weight average molecular weight of about 1000 to about 10,000. In a preferred embodiment of the present invention, the curable polymer component is a glycidyl methacrylate resin containing from about 5 to about 40 weight percent GMA residues, having a number average molecular weight of about 1000 to about 3000, and a weight average molecular weight of about 2000 to about 8000. Commercially—available resins include those available from Mitsui Toatsu Chemicals, Inc., available under the tradename ALMATEX^® PD 6100, PD 6300, PD 7110, PD 7210, PD 7310, PD 7610, and

PD 1700. Further examples of such resins include those described in U.S. Patent Nos. 4,042,645; 4,091,024; 4,346,144; and 4,499,239; incorporated herein by reference. The various cross—linking compounds suitable for use in the present invention are well—known to one of ordinary skill in the art of powder coatings. For example, with carboxy functional resins, cross—linking compounds with epoxy groups can be utilized. Likewise, with an epoxy functional resin, an anhydride type cross- linking compound can be used. Further, with hydroxy- functional resins, blocked isocyanates can be used. As will be shown below, a carboxy functional resin may be blended with an epoxy resin and, optionally, in the presence of another epoxy functional compound such as triglicidyl isocyanurate, and cured.

Examples of anhydride type cross-linking compounds include trimellitic anhydride, benzophenone tetra— carboxylic dianhydride, pyromellitic dianhydride, tetrahydrophthalic anhydride, and the like. In general, carboxy—functional cross—linking agents are C₃—C₃₀ alkyl, alkenyl, or alkynyl compounds with two or more carboxylic acid functional groups. Preferred carboxy—functional cross—linking compounds can be described by the formula

H₂OC-[(CH₂)_n]-C0₂H,

wherein n is an integer of 1—10. Examples of such carboxy-functional cross—linking agents include polycarboxy alkyl compounds such as dodecanedioic acid, azeleic acid, adipic acid, 1,6—hexanedioic acid, succinic acid, pimelic acid, sebacic acid, and the like. Other examples of carboxy—type cross—linking compounds include maleic acid, citric acid, itaconic acid, aconitic acid, and the like.

The blocked polyisocyanate compounds of the compositions of this invention are known compounds and can be obtained from commercial sources or may be prepared according to published procedures. Upon being heated to cure coatings of the compositions, the compounds become unblocked and the isocyanate groups react with hydroxy groups present on the amorphous polyester to cross—link the polymer chains and thus cure the compositions to form tough coatings. Examples of the blocked polyisocyanate cross—linking component include those which are based on isophorone diisocyanate blocked with e—caprolactam, commercially available as Hύls 1530 and Cargill 2400, or toluene 2,4—diisocyanate blocked with e—caprolactam, commercially available as Cargill 2450, and phenol—blocked hexamethylene diiso¬ cyanate.

The most readily-available, and thus the preferred, blocked polyisocyanate cross—linking agents or compounds are those commonly referred to as e—caprolactam-blocked isophorone diisocyanate, e.g., those described in U.S. Patent Nos. 3,822,240, 4,150,211 and 4,212,962, incorporated herein by reference. However, the products marketed as e—caprolactam—blocked isophorone diiso— cyanate may consist primarily of the blocked, difunctional, monomeric isophorone diisocyanate, i.e., a mixture of the cis and trans isomers of 3—isocyanato— methyl—3,5,5—trimethylcyclohexylisocyanate, the blocked, difunctional dimer thereof, the blocked, trifunctional trimer thereof or a mixture of the monomeric, dimeric and/or trimeric forms. For example, the blocked poly¬ isocyanate compound used as the cross—linking agent may be a mixture consisting primarily of the e—caprolactam- blocked, difunctional, monomeric isophorone diisocyanate and the e—caprolactam—blocked, trifunctional trimer of isophorone diisocyanate. The description herein of the cross—linking agents as "polyisocyanates" refers to compounds which contain at least two isocyanato groups which are blocked with, i.e., reacted with, another compound, e.g., e—caprolactam. The reaction of the isocyanato groups with the blocking compound is reversible at elevated temperatures, e.g., normally about 150°C, and above, at which temperature the isocyanato groups are available to react with the hydroxyl groups present on the free hydroxy groups of the polyester to form urethane linkages.

Alternatively, the blocked isocyanate may be a cross—linking effective amount of an adduct of the 1,3— diazetidine—2,4—dione dimer of isophorone diisocyanate and a diol having the structure

0CN-R¹4x-R¹-NH-C-O-R²-O-C-NH-R¹lx-R¹-NC0

wherein R¹ is a divalent 1—methylene—1,3,3—trimethyl—5- cyclohexyl radical, i.e., a radical having the structure

R² is a divalent aliphatic, cycloaliphatic, araliphatic or aromatic residue of a diol; and

X is a 1,3—diazetidine—2,4—dionediyl radical, i.e. a radical having the structure

-<>

wherein the ratio of NCO to OH groups in the forma— tion of the adduct is about 1:0.5 to 1:0.9, the mole ratio of diazetidinedione to diol is from 2:1 to 6:5, the content of free isocyanate groups in the adduct is not greater than 8 weight percent and the adduct has a molecular weight of about 500 to 4000 and a melting point of about 70 to 130°C.

The adducts of the 1,3—diazetidine—2,4—dione dimer of isophorone diisocyanate and a diol are prepared according to the procedures described in U.S. Patent 4,413,079, incorporated herein by reference, by reacting the diazetidine dimer of isophorone diisocyanate, preferably free of isocyanurate trimers of isophorone diisocyanate, with diols in a ratio of reactants which gives as isocyanto:hydroxyl ratio of about 1:0.5 to 1:0.9, preferably 1:0.6 to 1:0.8. The adduct preferably has a molecular weight of 1450 to 2800 and a melting point of about 85 to 120°C. The preferred diol reactant is 1,4—butanediol. Such an adduct is commercially available under the name Hulε BF1540. The amount of the blocked diisocyanate cross- linking compound present in the compositions of this invention can be varied depending on several factors such as those mentioned hereinabove relative to the amount of curable polymer components which are utilized. Typically, the amount of cross—linking compound which will effectively cross—link the polymers to produce coatings having a good combination of properties is in the range of about 5 to 30 weight percent, preferably 15 to 25 weight percent, based on the total weight of curable polymer and crosslinker components.

Typical of the additives which may be present in the powder coating compositions include benzoin, flow aids or flow control agents which aid the formation of a smooth, glossy surface, stabilizers, pigments and dyes. The powder coating compositions preferably contain a flow aid, also referred to as flow control or leveling agents, to enhance the surface appearance of cured coatings of the powder coating compositions. Such flow aids typically comprise acrylic polymers and are avail— able from several suppliers, e.g., MODAFLOW from

Monsanto Company and Acronal from BASF. Other flow control agents which may be used include MODAREZ MFP available from Synthron, EX 486 available from Troy Chemical, BYK 360P available from BYK Mallinkrodt and PERENOL F—30-P available from Henkel. An example of one specific flow aid is an acrylic polymer having a molecular weight of about 17,000 and containing 60 mole percent 2—ethylhexyl methacrylate residues and about 40 mole percent ethyl acrylate residues. The amount of flow aid present may preferably be in the range of about 0.5 to 4.0 weight percent, based on the total weight of the resin component, and the cross—linking agent.

The powder coating compositions may be deposited on various metallic and non—metallic (e.g., thermoplastic or thermoset composite) substrates by known techniques for powder deposition such as by means of a powder gun, by electrostatic deposition or by deposition from a fluidized bed. In fluidized bed sintering, a preheated article is immersed into a suspension of the powder coating in air. The particle size of the powder coating composition normally is in the range of 60 to 300 microns. The powder is maintained in suspension by passing air through a porous bottom of the fluidized bed chamber. The articles to be coated are preheated to about 250° to 400°F (about 121° to 205°C) and then brought into contact with the fluidized bed of the powder coating composition. The contact time depends on the thickness of the coating that is to be produced and typically is from 1 to 12 seconds. The temperature of the substrate being coated causes the powder to flow and thus fuse together to form a smooth, uniform, continuous, uncratered coating. The temperature of the preheated article also effects cross—linking of the coating composition and results in the formation of a tough coating having a good combination of properties. Coatings having a thickness between 200 and 500 microns may be produced by this method.

The compositions also may be applied using an electrostatic process wherein a powder coating composi— tion having a particle size of less than 100 microns, preferably about 15 to 50 microns, is blown by means of compressed air into an applicator in which it is charged with a voltage of 30 to 100 kV by high—voltage direct current. The charged particles then are sprayed onto the grounded article to be coated to which the particles adhere due to the electrical charge thereof. The coated article is heated to melt and cure the powder particles. Coatings of 40 to 120 microns thickness may be obtained. Another method of applying the powder coating compositions is the electrostatic fluidized bed process which is a combination of the two methods described above. For example, annular or partially annular electrodes are mounted in the air feed to a fluidized bed so as to produce an electrostatic charge such as 50 to 100 kV. The article to be coated, either heated, e.g., 250° to 400°F, or cold, is exposed briefly to the fluidized powder. The coated article then can be heated to effect cross—linking if the article was not preheated to a temperature sufficiently high to cure the coating upon contact of the coating particles with the article. The powder coating compositions of this invention may be used to coat articles of various shapes and sizes constructed of heat—resistance materials such as glass, ceramic and various metal materials. The compositions are especially useful for producing coatings on articles constructed of metals and metal alloys, particularly steel articles. As noted above, since the compositions provided by the present invention cure at a temperatures of as low as 115°C, it is also possible to coat many thermoplastic and thermosetting resin compositions with the compositions of the present invention.

As noted above, the method of the present invention is especially suited for preparing compositions useful for forming thin films. Accordingly, as a further aspect of the present invention there is provided a method for forming a thermoset powder coating having a film thickness of about 0.1 to about 1.0 mil on a substrate, which comprises the steps: (a) blending and reducing the particle size of components in a thermosetting powder coating composition which comprises

(1) combining said components and pulverizing said composition, followed by

(2) jet—milling said composition until a mean particle size of about 0.5 to 40 microns is obtained; followed by

(b) applying said composition to a substrate and subjecting said substrate to a temperature and for a time sufficient to crosslink said composition.

In a further aspect of the present invention, there is provided the further processing step (a) (3), referred to above as step (c) :

(3) pelletizing said composition under pressure and pulverizing said composition until a mean particle size of about 20—50 microns is obtained.

Further examples of formulation methods, additives, and methods of powder coating application may be found in User's Guide to Powder Coating. 2nd Ed. , Emery Miller, editor, Society of Manufacturing Engineers, Dearborn, (1987) .

Particle size of the powders dispersed in solution is determined with a Microtrac particle size analyzer. This instrument measures the particle size distribution of powders dispersed in a suitable aqueous solution. This technique is based on measurement of the amount and angle of forward scattered light from a laser beam projected through a stream of particles. Size distribu¬ tion is calculated by a computer using the appropriate theory and calculations. Jet milling of powder coating can be accomplished with a Trost impact pulverizer, TX Laboratory model. Powder coatings of this invention are electro¬ statically sprayed onto 3" x 9" treated panels. The coatings are cured in conventional ovens prior to testing.

Degree of cure of the powder coating compositions prepared by this invention was based on ASTM standard designation 4752-87 Standard Test Method, MEK (Methyl ethyl ketone) solvent resistance. The procedure is based on the resistance of the coating to penetration by rubbing with MEK saturated cheesecloth. The cheese cloth is folded to specification, attached to the end of a ball peen hammer and soaked with MEK solvent. The hammer is attached to a motorized controller which forces a back and forth sliding action of the hammer ball against the coating surface. The requirement to pass the test is 200 double rubs against the coating surface without being removed.

The inherent viscosities (I.v.; dl/g) referred to herein were measured at 25°C using 0.5 g polymer per 100 mL of a solvent consisting of 60 parts by weight phenol and 40 parts by weight tetrachloroethane. Acid and hydroxyl numbers were determined by titration and are reported herein as mg of KOH consumed for each gram of polymer.

Impact strengths are determined using a Gardner Laboratory, Inc. impact tester per ASTM D 2794—84.

Pencil hardness is determined using ASTM D 3363—74. The hardness is reported as the hardest pencil which will not cut into the coating. The results are expressed according to the following scale: (softest) 6B, 5B, 4B, 3B, 2B, B, HB, F, H, 2H, 3H, 4H, 5H, 6H (hardest) .

20 and 60 degree glosses are measured using a gloss meter (Gardner Laboratory, Inc. Model GC-9095) according to ASTM-523.

The flexibility of coating is tested by bending panel back with panels inserted between the tow halves and pressurized with hydraulic jack to 10,000 psi. The coating capable of bending without cracks or popping with the least numbers of panels (X) in between the bend is said to have passed X—T bend.

Experimental Section

Example 1 — Hybrid Powder Coating

A polyester epoxy hybrid powder coating, consisting of the following ingredients:

Hoechst AN744 carboxyl polyester resin 460 g

DER 663 UH epoxy resin 460 g

ARQUAT 218—100P Quaternary ammonium salt 9 g

Stearamide 46 g Troy EX486 flow aid 12 g

Benzoin 9 g

was premixed and ground with a hammer—type pulverizing mill. The resulting particle size was 4 to 250 μm, with a mean value of 53 μm. This pre—ground sample was then jet milled to a particle size of 1 — 18 μm, with a average of 7 μm.

The powder was sprayed and cured at 325°C for 20 min. The resulting properties of the final coating (1 mil thickness) are as follows: Gloss 20/60 48/100

Impact Front/Rear 160/160

Pencil Hardness F

T bend 0 T MEK rubs 200

Figure 1 illustrates the physical state of Example 1 powder before curing and the corresponding coating after curing. Figure 2 shows a transmission electron micrograph (TEM) image of the uncured powder in Example 1 (6500x magnification) . The samples were obtained by pressing the powder together into pellets, then sectioning the pellets. The biphasic structure of the powder is clearly apparent. Figure 2 gives an image of the same powder sample after curing (6500x) . The biphasic structure of the powder coating has largely disappeared in the final cured state.

Figures 3 and 4 illustrate the conventionally prepare analog of this formulation, extruded and pulverized to a particle size of ca. 30 microns. Figure 3 shows the particles having a more or less homogeneous appearance. A homogeneous film is obtained after curing (Figure 4) , similar in appearance to the cured film in Figure 2. A common problem with catalyzed, thermally reactive powder coating formulations is their stability against unwanted chemical reactions. Powders commonly will be stored in warehouses during warm summer months. Such storage conditions may result in exposure to temperatures in excess of 35°C for days or weeks. These conditions may be sufficient to cause partial curing of such reactive powder coatings, giving rise to unwanted, excessive orange peel appearance in the final coating. Chemical stability was determined by measuring the heat of the exothermic crosslinking reaction with differential scanning calorimetry. With this technique, a sample of the powder coating is enclosed in a small pan and subjected to a controlled temperature ramp program. Heat is evolved when, for example, the enclosed material reaches the temperature sufficiently high to induce chemical reaction, such as during the curing of an epoxy powder coating. The heat content of the reaction is measured by electronically integrating the heat output of the sample in cals/g as the instrument scans through the curing point.

5 — 10 mg Samples of powder coating were tested with a Dupont Model 2100 Differential Scanning Calorimeter. Samples were subjected to heating ramps of

Samples were placed in a Gallencamp incubator oven at a temperature of 40 +/— 0.2°C. Samples were removed for the oven at various times, as indicated in Figure 5.

Figure 5 shows how the chemical stability is affected by the conditions used to process the powder coating. The graph indicates the amount of chemically reactive species remaining after various time intervals. The dark bar shows how the heat content does not significantly deteriorate during the 10 day temperature exposure for the subject powder coating. In contrast, the light grey bars reflect the heat content of the extruded, conventionally processed powder coating. During the same 10 day interval, the heat remaining decreases from 10 cal/g to 2 cal/g.

Significant chemical stability of powder coating compositions produced by the present invention is demonstrated. Example 2 — Epoxy Powder Coating

An epoxy powder coating, consisting of the following ingredients:

DER 662 UH epoxy resin 924 g

Huls B—31 curing catalyst 56 g

Resiflow P—67 flow aid 12 g

Benzoin 10 g

was premixed and ground with a hammer—type pulverizing mill. The resulting particle size was 6 to 250 μm, with a mean value of 67 μm. This pre—ground sample was then jet milled to a particle size of 3 — 37 μm, with a average of 12 μm.

The powder was sprayed and cured at 325°C for 20 min. The resulting properties of the final coating (1 mil thickness) are as follows:

Gloss 20/60 68/96

Impact Front/Rear 160/160 Pencil Hardness F

T bend 1 T

MEK rubs 200

Figure 6 shows the amount of chemical reactivity remaining after various time intervals indicated. The dark bar shows how the heat content does not significantly deteriorate during the 10 day temperature exposure for the subject powder coating.

In contrast, the light grey bars reflect the heat content of the extruded, conventionally processed powder coating. During the same 10 day interval, the heat remaining decreases from 4 cal/g to 2 cal/g. Significant chemical stability of powder coating compositions produced by the present invention is thus demonstrated.

Example 3 — Epoxy Powder Coating

A dry blend of an epoxy powder coating was prepared consisting of the following ingredients:

DER 642 U Epoxy resin 1422 g

DEH 84 Epoxy hardner 670 g

P—67 Poly—acrylate flow modifier 25 g

Spacerite Aluminum hydrate extender 12 g

The blend was pulverized with a hammer mill and then jet milled. The jet milled sample had a particle size of 0.4 — 16 μm, with an average of 6 μm. This milled powder was then pelletized. 2 — 5 g samples of powder were pressed with a Carver Laboratory Press (Model M) using a 1.25 inch diameter die and 25,000 psi pressure. The pellets were then ground by agitation with a lab scale mixer, followed by pulverization in a hammer mill operating at 11,000 rpm. The resulting powder was then sifted through a 170 mesh screen. The resulting particle size was 29.9 microns (mean value), with 10% less than 4 microns, 90% less than 62 microns.

The powder was electrostatically sprayed and cured at 350°C for 20 min. The resulting properties of the final coating (2 mil thickness) are as follows:

Gloss 20/60 52/96

Impact Front/Rear 160/160

Pencil Hardness H

T bend 4 T MEK rubs 200

Claims

ClaimsWe Claim:

1. A method for blending and reducing the particle size of components in a thermosetting powder coating composition which comprises

(a) combining said components and pulverizing said composition, followed by

(b) jet—milling said composition until a mean particle size of about 0.5 to 40 microns is obtained.

2. The method of Claim 1, wherein step (a) is comprised of grinding said composition in a hammer- type mill.

3. The method of claim 1, wherein in step (b) , an average particle size of about 1 to about 15 microns is obtained.

4. The method of claim 1, wherein said components are comprised of one or more resins and said resin is a hydroxy—, epoxy—, or carboxy— functional polyester or polyether, or a hydroxy—, epoxy—, amino—, or carboxy—functional acrylic or methacrylic polymer.

5. The method of claim 1, wherein said components are comprised of one or more crosslinking agents and said agents are blocked polyisocyanates and epoxy— substituted compounds.

6. The method of claim 1, further comprising the step (c):

(c) pelletizing said composition under pressure and pulverizing said composition until a mean particle size of about 20—50 microns is obtained.

7. A method for forming a thermoset powder coating having a film thickness of about 0.1 to about 1.0 mil on a substrate, which comprises the steps:

(a) blending and reducing the particle size of components in a thermosetting powder coating composition which comprises

(1) combining said components and pulverizing said composition, followed by

(2) jet—milling said composition until a mean particle size of about 0.5 to 40 microns is obtained; followed by

(b) applying said composition to a substrate and subjecting said substrate to a temperature and for a time sufficient to crosslink said composition.

8. The method of Claim 7, wherein step (a) is comprised of grinding said composition in a hammer- type mill.

9. The method of claim 7, wherein in step (b) , an average particle size of about 1 to about 15 microns is obtained.

10. The method of claim 7, wherein said components are comprised of one or more resins and said resin is a hydroxy—, epoxy—, or carboxy— functional polyester or polyether, or a hydroxy—, epoxy—, amino—, or carboxy—functional acrylic or methacrylic polymer.

11. The method of claim 7, wherein said components are comprised of one or more crosslinking agents and said agents are blocked polyisocyanates and epoxy— substituted compounds.

12. The method of claim 7, further comprising the step (a) (3):

(3) pelletizing said composition under pressure and pulverizing said composition until a mean particle size of about 20—50 microns is obtained.