WO1995028435A1

WO1995028435A1 - Method for preparing powder coating compositions

Info

Publication number: WO1995028435A1
Application number: PCT/US1995/004549
Authority: WO
Inventors: Yeong-Ho Chang; Joseph Clark Jernigan; Lanney Calvin Treece
Original assignee: Ppg Industries, Inc.
Priority date: 1994-04-13
Filing date: 1995-04-12
Publication date: 1995-10-26
Also published as: CA2187819A1; MX9604792A; EP0755417A1

Abstract

Provided is a method for preparing a powder coating composition comprising: (a) forming a mixture comprising solid particles which include a curable resin and a crosslinking agent that is reactive with the curable resin suspended in an aqueous liquid phase comprising water and a surfactant; (b) milling the mixture at a temperature of up to about 40 °C so as to reduce the mean particle size of the particles to no greater than about 15 νm; (c) agglomerating the particles so as to increase their mean particle size to at least about 20 νm; (d) separating the particles from the aqueous liquid phase; and (e) drying the particles to form a powder.

Description

METHOD FOR PREPARING POWDER COATING COMPOSITIONS

The invention relates to powder coating composi— tions, and more particularly to a method for preparing powder coating compositions at relatively low tempera¬ tures.

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; in some modes of application, only 60 percent or less of the solvent—based coating composi¬ tion 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 steel that is destined to be formed or shaped into articles used in the manufacture of various household appliances and automobiles, in the course of which the sheet metal is flexed or bent at various angles.

Formation of a powder coating composition typically entails the dry mixing of flakes or granules of resin with the cross—linking agent and other ingredients, extruding the mixture at temperatures in the range of about 80° to 130°C, cooling the extrudate, and then chipping and grinding the resulting solid into particles of suitable size. Because the mixture is subjected to high temperatures during this process, premature curing of the resin may occur, which would affect the quality of the subsequent coating. In addition, the grinding operation may produce a powder having a wide particle size distribution. Other processes for the preparation of powder coating compositions which do not employ extrusion to mix the components nonetheless subject them to tempera¬ tures sufficiently high either to cause premature _.curing or put substantial limitations on the curing properties of the compositions. U.S. Patent No. 3,759,864, for example, discloses a process for preparing pigmented epoxy resin particles by emulsifying the liquified polymer in a continuous volatile liquid phase containing pigment at temperatures in the range of 80—150°C, cooling the mixture to solidify the polymer, and removing the volatile liquid. In U.S. Patent No. 4,049,744, particles of a polyhydroxy polyether resin are prepared by mixing the resin with water containing a polymeric polycarboxylic acid or salt at a temperature of at least 60°C, agitating the mixture to form a dispersion, and cooling to form solid polymer particles.

PROBLEM TO BE SOLVED BY THE INVENTION

With the increasing availability of coating substrates made of plastic or other heat—deformable materials, there is a growing need for powder coating compositions that can be cured at relatively low temperatures. Conventional extrusion and pulverization techniques can cause premature cross—linking in composi¬ tions intended for low temperature curing processes. The method of the invention allows the preparation at temperatures near ambient of the heat—sensitive powder coating compositions required for low temperature cure applications.

SUMMARY OF THE INVENTION

The. present invention provides a method for preparing a powder coating composition which comprises:

(a) forming a mixture comprising solid particles which include a curable resin and a cross—linking agent that is reactive with the curable resin suspended in an aqueous liquid phase comprising water and a surfactant;

(b) milling the mixture at a temperature of up to about 40°C so as to reduce the mean particle size of the particles to no greater than about

15 μm;

(c) agglomerating the particles so as to increase their mean particle size to at least about 20 μ ;

(d) separating the particles from the aqueous liquid phase; and

(e) drying the particles to form a powder.

ADVANTAGEOUS EFFECTS OF THE INVENTION

In the method of the present invention, extremely fine particles are prepared by milling at low tempera¬ ture and are then agglomerated to larger particles suitable for powder coating applications. In the course of agglomeration, the particles become increasing spherical in shape as particle size distribution is narrowed, which results in improved powder flow and coating smoothness properties.

DETAILED DESCRIPTION OF THE INVENTION

The method of the invention provides a powder coating composition comprising a curable resin and a cross—linking agent reactive with the curable resin.

Preferably, the curable resin is chosen from resins used in the powder coating art which have epoxy, carboxy, hydroxy, amino, or anhydride functional groups that can react with 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 0.05 to about 0.99 epoxy groups per

100 grams of resin, i.e., 100—2000 weight per epoxy (WPE) . Such resins are widely known and include those that are commercially available under the EPON™ trade- name of the Shell Chemical Company, the Araldite™ tradename of C BA-Geigy, and D.E.R. resins of the Dow Chemical Company.

Curable resins 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 resins which have free hydroxy groups also include the polyesters as well as acrylic polymers. 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 aralkyl 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—1,3—diol, 2,2—dimethyl—1,3— propanediol, 2—ethyl—2—butyl—1,3— ropanediol, 2-ethyl—2-isobutyl—1,3— propanediol, 1.3—butanediol, 1,4—butanediol, 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 esters 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 suitable for the practice of this invention have a glass transition temperature, T_g, greater than 55°C, and an inherent viscosity of about 0.15 to 0.4. The polyester resin preferably comprises (1) diacid residues of which at least 50 mole percent are terephthalic or isophthalic 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 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, the disclosures of which are incorporated herein by reference, and Published Japanese Patent Applications (Kokai) 73-05,895 and 73-26,292. The most preferred polyester consists essentially of terephthalic acid residues, 2,2— imethyl—1,3— ropanediol 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 T_g 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 dL/g measured using 0.5 g/100 mL of a 60/40 blend (w/w) of phenol/tetrachloroethane at 25°C.

A curable acrylic resin suitable for the practice of this invention is preferably a polymer or resin prepared by polymerization of a hydroxy—substituted monomer such as hydroxyethyl methacrylate, hydroxyethyl acrylate, hydroxyhexyl acrylate, hydroxyhexyl meth¬ acrylate, hydroxypropyl acrylate, hydroxypropyl meth¬ acrylate, hydroxybutyl acrylate, hydroxylbutyl meth¬ acrylate, and the like, optionally polymerized with other monomers such as methyl acrylate, methyl meth— acrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, isobutyl acrylate, iso— butyl methacrylate, ethylhexyl acrylate, ethylhexyl methacrylate, styrene, vinyl acetate, and the like. The ratio of reagents and molecular weights of the resulting acrylic polymers 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 resins containing epoxy groups which are suitable for the practice of the present invention can also be resins comprised of residues of glycidyl meth¬ acrylate (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, the resin 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, the disclosures of which are incorporated herein by reference. The various cross—linking agents suitable for use in the present invention are well known 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. Also, a carboxy functional resin may be blended with an epoxy resin, optionally in the presence of another epoxy functional compound such as triglycidyl isocyanurate, and cured.

Examples of anhydride type cross—liking 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 H0₂C-[ (CH₂) _n]-C0₂H,

wherein n is an integer from 1—10. Examples of such carboxy—functional cross—linking agents include compounds such as dodecanedioic acid, azelaic 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 suitable for the practice of this invention are known compounds and may be obtained from commercial sources or 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 in the polymer to cross—link the polymer chains and thus cure the compositions to form tough coatings. Examples of blocked polyisocyanate cross— linking agents include those which are based on isophorone diisocyanate blocked with e—caprolactam, commercially available as Hiils 1530 and Cargill 2400, or toluene 2,4—diisocyanate blocked with e—caprolactam, commercially available as Cargill 2450, and phenol—blocked polyisocyanate.

The most readily available blockeα 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, the disclosures of which are incorporated herein by reference. However, the products marketed as e—caprolactam blocked isophorone diisocyanate may consist primarily of the blocked, difunctional, monomeric isophorone diiso— cyanate, i.e., a mixture of the cis and trans isomers of 3—isocyanatomethyl—3,5,5-trimethylcyclohexylisocyanate, the blocked, difunctional dimer thereof, the blocked, trifunctional tri er thereof or a mixture of the monomeric, dimeric and/or trimeric forms. For example, the blocked polyisocyanate 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 isocyanate groups that 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 in the polymer 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

OCN-R¹[X-R¹-NH-COO-R²-OCO-NH-R¹]nX-R^ CO

wherein R¹ is a methylene—1,3,3—trimethyl—5—cyclohexy1 diradical; R² is a divalent aliphatic, cycloaliphatic, aralkyl or aromatic residue of a diol; and X is a 1,3— diazetidine—2,4— ionediyl radical, wherein the ratio of NCO to OH groups in the formation 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 No. 4,413,079, the disclosure of which is incorporated herein by reference, by reacting the diazetidine dimer of isophorone diisocyanate, preferably free of iso— cyanurate trimers of isophorone diisocyanate, with diols in a ratio of reactants which gives an isocyanato: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 Hϋls BF1540.

The amount of the blocked diisocyanate cross- linking agent present in the compositions prepared by the method of this invention can be varied to control the properties of the resulting coatings. Typically, the amount of cross—linking agent which will effectively cross—link the curable resin to produce coatings having a desirable 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 cross—linking agent and resin. Optionally, a catalyst such as dibutyltin dilaurate (available from Aldrich Chemical Co.) may be used to facilitate cross—linking by the polyisocyanate compound. Based on the weight of cross- linking agent, 0.5 to 5 weight percent, preferably 1 to 2 weight percent, of the catalyst may be employed.

The powder coating compositions produced by the method of this invention may be prepared from the compositions described herein by dry—mixing the curable resin and the cross—linking agent along with other additives commonly used in powder coating compositions and then milling the blend to an average particle size of no more than about 15 μm. Typical of the additives which may be present in powder coating compositions are benzoin, flow aids or flow control agents, stabilizers, pigments, and dyes. The powder coating compositions prepared by the method of the invention preferably contain flow aids, 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 available 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 Mallinckrodt, 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 is preferably in the range of about 0.5 to 4.0 weight percent, based on the total weight of resin and cross—linking agent.

In accordance with the method of the present inven— tion, a mixture that comprises particles containing a curable resin and a cross—linking agent, and optionally other additives as discussed above, is formed by dry- mixing the ingredients using, for example, a Henschel mixer and/or a hammer mill. The resulting blend is then suspended in an aqueous liquid phase comprising water and a surfactant, or dispersing agent. The surfactant can be an ionic compound, such as sodium dodecyl sulfate, or, preferably, a nonionic compound such as a polyether alcohol. Suitable surfactants include Triton™ X-100 (from Union Carbide Co.), and Surfynol™ GA and CT-136 (from Air Products Corp.); they can be employed, singly or in combination, in amounts ranging from about 0.1 to 20 weight percent, preferably about 2 to 10 weight percent, of the liquid. The suspension of particles in the aqueous liquid phase is subjected to microfine milling, for example, by media milling. Media milling can be conveniently accomplished with a Netsch LMZ horizontal recirculating mill. Alternatively, following the dry—mixing step described above, the blend can be jet milled to produce very fine, solid particles, which are then suspended in the aqueous liquid phase. Jet milling can be performed using, for example, a Trost air impact pulverizer. Slurry coating compositions dispersed in water that are prepared by mixing resin pellets or granules with water and crushing the mixture with a ball mill, pot mill, or crusher are disclosed in JP52107033A and JP80004341B. In the method of the present invention, particles containing curable resin and cross—linking agent are reduced to a particle size no greater than about 15 μm, and preferably no greater than about 8 μm, by milling at a temperature of up to about 40°C. These extremely fine particles are then caused to agglomerate to particles having a mean size of at least about 20 μm. In the course of agglomeration, the particles become increasingly spherical in shape as particle size distribution is narrowed.

Agglomeration to increase the particle size can be accomplished by warming the suspension of the finely milled particles to temperatures near the glass transi¬ tion temperature, T_g, of the curable resin, from about 40°C above to about 40°C below the T , preferably from about 10°C above to about 10°C below the T_σ, in the presence of small amounts of stabilizer and/or promoter compounds. Warming of the suspension can be maintained from about 0.25 hour to 6 hours, preferably about 0.50 hour to 2 hours. Stabilizers, or suspending agents, control particle coalescence while the suspension is being warmed and consequently affect particle size distribution. Promoters facilitate the adherence of stabilizers to the surface of the particles.

Examples of suitable stabilizers include colloidal silica such as the commercially available Ludox™ SM, Ludox™ TM, and Nalcoag™ 1060. Other useful stabilizers are titanium dioxide or fumed aluminum oxide, which are available from Degussa Corp. Examples of suitable promoters are water soluble polymeric materials such as poly(diethanolamine adipate) and poly(methylaminoethanol adipate) . Alternatively, ionic surfactants may be used as promoters. Such compounds serve a dual purpose as particle stabilizers, thus avoiding the need for addi¬ tional stabilizer compounds. Examples of suitable ionic promoters include sodium dodecyl sulfate, sodium butane— sulfonate, and the like. Other useful promoters are described in U.S. Patent No. 4,833,060, the disclosures of which are incorporated herein by reference.

Useful concentrations of solid particles range from about 1 to 50 weight percent, preferably from about 5 to 25 weight percent, of the aqueous liquid phase. Useful concentrations of stabilizer range from about 0.1 to 40 weight percent, preferably from about 0.5 to 5 weight percent, of the particulate solids. Similarly, useful concentrations of promoter range from about 0.05 to 40 weight percent, preferably from about 0.5 to 5 weight percent, of solids.

Upon agglomeration of the particles to the desired size, they can be separated from the liquid phase _by filtration and dried in air, in a vacuum oven, or in a fluidized bed. After drying, the particles preferably have a mean particle size from about 20 μm to 100 μm, more preferably from about 25 μm to 50 μm. Particle size distribution and mean particle size in compositions prepared according to the method of invention can be determined by means of a Microtrac particle size analyzer (available from Leeds & Northrup) , using a technique that entails the measurement of the amount and angle of forward scattered light from a laser beam projected through a stream of particles. In accordance with the present invention, agglomeration of the finely milled particles, their separation from the liquid phase, and their drying to a powder can also be achieved by spray drying, using techniques known in the art. For example, U.S. Patent No. 3,325,425 discloses a method for forming a dry powder from an aqueous acrylic paint composition by introducing an atomized spray into a drying chamber by means of an air stream at a temperature of 100° to 160°F and maintaining the outlet temperature between 75° and 125°F, thereby reducing the moisture content of the solid to not more than 4 weight percent before it is discharged from the chamber. U.S. Patent No. 3,950,302 discloses the formation of a powder by spray drying of an aqueous dispersion of a copolymer of vinyl ester, acrylamide, and optionally ethylene. The disclosures of U.S. Patent Nos. 3,325,425 and 3,950,302 are incorporated herein by reference.

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 μm. 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 μm and 500 μm may be produced by this method.

The compositions also may be applied using an electrostatic process wherein a powder coating composition having a particle size of less than 100 μm, preferably about 25 μm to 50 μm, 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 because of the electrical charge thereon. The coated article is heated to melt and cure the powder particles. Coatings of 40 μm to 120 μm 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 (about 121° to 205°C) , 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 within the article.

The powder coating compositions prepared according to the method of this invention may be used to coat articles of various shapes and sizes constructed of heat materials such as glass, ceramics, and metals. The compositions are especially useful for producing coatings on articles constructed of metals and metal alloys, particularly steel articles. It is possible to cure some systems at temperatures as low as 115°C, for example, compositions containing epoxy resins, anhydride cross—linking agents, and quaternary ammonium salt or hydroxide cross—linking catalysts, as taught by U.S. Patent No. 5,244,944, the disclosures of which are incorporated herein by reference. Compositions that are curable at relatively low temperatures, around 115°C for example, are useful for coating articles formed of thermoplastic and thermosetting resin compositions.

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) .

Powder coating compositions prepared by the method of the present invention are preferably applied to a coating substrate by means of electrostatic spraying, using apparatus such as a Ransburg corona type gun. Following spraying, the substrate is heated at temperatures in the range of about 115° to 200°C for periods of about 5 minutes to 30 minutes. Cure of the 18 -

coatings is determined by standard test procedure ASTM 4752-87 and reported as MEK (Methyl Ethyl Ketone) solvent resistance. Cotton cheese cloth folded according to specification is attached to the end of a 13—ounce ball peen hammer and saturated with MEK. The hammer is attached to a motorized controller that forces a back and forth sliding action of the cloth—covered hammer ball against the coating surface. A coating capable of withstanding 200 back and forth (double) rubs without marring of the coating surface is deemed cured. The following examples further illustrate the invention.

Example 1 — Media milling of mixture for powder coating composition

The following materials in the amounts shown were mixed and ground using a hammer—type pulverizing mill:

3280 grams of Rucote™ 107, a hydroxy—substituted polyester;

720 grams of Hύls B1530, a blocked isocyanate cross—linking agent;

60 grams of dibutyltin dilaurate powder, a cross—linking catalyst;

40 grams of Modaflow™ III, a flow agent; 40 grams of benzoin, a degassing agent.

The ground mixture was dispersed in water containing 5 weight percent of an approximately 1:1 mixture of Surfynol™ CT-136 and Surfynol™ GA surfactants, both available from Air Products Corporation; the resulting suspension contained 39 weight percent solids. Particle size was reduced at ambient temperature, using a Netsch LMZ horizontal recirculating mill charged with 0.8—1.25 mm zirconium silicate media, to a range of about 0.4 μm to 15 μm, with a mean particle size of about 4 μm.

Example 2 — Formation of powder coating composition by coalescence of particles in aqueous suspension

A 3—L beaker was charged with 350 mL of the mixture prepared as described in Example 1, followed by 4.5 mL of Ludox™ TM (from DuPont) , 6 g of poly(methylamino- ethanol adipate) , and 50 mL of water. After mixing with a high speed laboratory dispersion unit, the suspension was transferred to a 1000— L flask and stirred for 60 min at 300 rpm and a temperature of 30°C. Stirring was continued while the temperature of the mixture was gradually raised over a period of 30 min to 60°C, where it was maintained for 45 min. The mixture was cooled, and the suspended solid was separated by filtration, allowed to air dry for 2 days, and then dried in a vacuum oven at 45°C, with slight air purge, for 24 hr. The resulting powder had a mean particle size of about 43 μm, with the size of 80 volume percent of the particles lying between about 11 μm and 105 μm.

Example 3 — Powder coating of substrates

Steel panels of dimensions 3 in x 9 in (7.7 cm x 23 cm) were electrostatically sprayed with the composition prepared as described in Example 2. The coatings were cured for 15 min in an oven at 375°F (191°C) . The cured coatings, which had a thickness of about 1.7 mils (43 μm) , withstood 200 double rubs in the MEK solvent resistance test without marring of the coated surface.

Example 4 — Jet milling of mixture for powder coating composition .

The following materials in the amounts shown were mixed with a laboratory blender and then milled using a Trost air impact pulverizer:

820 grams of Rucote™ 107, a hydroxy—substituted polyester; 180 grams of Hiils B1530, a blocked isocyanate cross—linking agent;

10 grams of dibutyltin dilaurate powder, a deblocking agent;

15 grams of Modaflow™ III, a flow agent;

10 grams of benzoin (available from GCA Corp.), a degassing agent.

The resulting powder contained particles in the size range of about 2 μm to 15 μm, with a mean particle size of about 8 μm.

Example 5 — Formation of powder coating composition by coalescence of particles in aqueous suspension

A 1000— L 3—necked flask was charged with 650 mL of water, 15 g of Degussa P—25 fumed Ti0₂, 5.6 g of poly— (methylaminoethanol adipate), and 4.3 g of Triton™ X—100 surfactant. To this mixture was added 34 g of the powder prepared as described in Example 4. The resulting suspension was stirred for 60 min at 200 rp and a temperature of 30°C. Stirring was continued as the temperature was gradually raised over a period of 30 min to 65 ±5°C, where it was maintained for 15 min. The mixture was cooled, and the suspended solid was separated by filtration, allowed to air dry overnight, and further dried by fluidization in dry air at 45°C. The resulting powder had a mean particle size of about 23 μm, with the size of 80 volume percent of the particles lying between about 8 μm and 37 μm.

Example 6 — Powder coating of substrates

Steel panels (3 in x 9 in, 7.7 cm x 23 cm) were electrostatically sprayed with the composition prepared as described in Example 5. The coatings were cured for 15 min in an oven at 375°F (191°C). The cured coatings, which had a thickness of about 1.2 mils (30 μm) with¬ stood 200 double rubs in the MEK solvent resistance test without marring of the coated surface.

Example 7 — Formation of powder coating composition by spray drying

A sample of the mixture prepared as described in Example 1 was stirred with a paddle stirrer overnight, then poured through a 40 mesh screen to remove oversized particles. The screened material was spray dried, using an Anhydro Lab 1 drying apparatus under the following conditions: inlet temperature, 60°C; outlet tempera¬ ture, 32°C; atomization pressure, 60 psi; spray rate, 45 g/min. The spray dried powder was further dried in a vacuum oven at 25°C, with slight air purge, then sifted through a 170 mesh screen.

The resulting powder had a mean particle size of about 31 μm, with the size of 90 volume percent of- the particles lying between about 11 μm and 74 μm. Example 8 — Powder coating of substrates

Steel panels (3 in x 9 in, 7.7 cm x 23 cm) were electrostatically sprayed with the composition prepared as described in Example 7. The coatings were cured for 20 min in an oven at 350°F (177°C) . The cured coatings, which had a thickness of about 2.2 mils (55 μm) , with¬ stood 200 rubs in the MEK solvent resistance test with¬ out marring of the coated surface. The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifica¬ tions can be effected within the spirit and scope of the invention.

Claims

ClaimsWe claim:

1. A method for preparing a powder coating composition which comprises:

(a) forming a mixture comprising solid particles which include a curable resin and a cross— linking agent that is reactive with said curable resin suspended in an aqueous liquid phase comprising water and a surfactant;

(b) milling said mixture at a temperature of up to about 40°C so as to reduce the mean particle size of said particles to no greater than about 15 μm;

(c) agglomerating said particles so as to increase their mean particle size to at least about 20 μm;

(d) separating said particles from said aqueous liquid phase; and

(e) drying said particles to form a powder.

2. A method according to Claim 1 wherein said mixture further comprises a pigment or a dye.

3. A method according to Claim 1 or 2 wherein said curable resin is a hydroxy—, epoxy—, amino—, or carboxy—substituted polyester or polyether, or a hydroxy—, epoxy—, amino—, or carboxy—substituted acrylic or methacrylic polymer.

4. A method according to Claim 1, 2 or 3 wherein said cross—linking agent is a polyisocyanate, a blocked polyisocyanate, or a carboxylic anhydride.

5. A method according to Claim 1 or 4 wherein said milling is carried out using a media mill.

6. A method according to Claim 1, 4, or 5 wherein said agglomerating of said particles is carried out by adding, after said milling, a suspending agent and a promoter to said mixture and then warming the resulting mixture.

7. A method according to any one of Claims 1 to 6 wherein said agglomerating, separating, and drying of said particles is carried out by spray drying.

8. A method according to Claim 7 wherein said particles are about 1 to 50 weight percent of said aqueous liquid phase and wherein the said particles after said milling have a mean particle size no greater than about 8 μm.

9. A method according to Claim 7 or 8 wherein said particles after said drying have a mean particle size of about 20 μm to 100 μm.

10. A method according to Claim 1 wherein said surfactant is a nonionic compound.