CA2187819A1 - Method for preparing powder coating compositions - Google Patents

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 µm; (c) agglomerating the particles so as to increase their mean particle size to at least about 20 µm; (d) separating the particles from the aqueous liquid phase; and (e) drying the particles to form a powder.

Description

WO 95/28435 r~ 'c ~549 METEIOD FOR PREPARING
POWDER COATING COMPOSITIONS
. .
The invention relates to powder coating composi--5 tions, and more particularly to a method f or preparing powder coating compositions at relatively low tempera--tures .
Plastic materials u6ed in the manufacture of powder coatings are classified broadly as either ~h~ Ling or th~ Rtic In the application of i , l;~tic 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 f low together and form a smooth coating.
--ett;ng coatings, when ~ ed to coatings derived from thermoplastic compositions, generally are tougher, more resistant to solvents and detergents, have better ~Ah~ ior\ to metal substrates, and do not soften when exposed to elevated t~ LUL~S. However, the 20 curing of th~ 3etting coatings has created problems in obtaining coatings which have, in addition to the above stated desirable characteristics, good _Llllless and flexibility. Coatings prepared from ~h~ LLing powder compositions, upon the application of heat, may 25 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 30 problem has caused th~ ~s~tting 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 35 also suffer from the disadvantage of relatively poor WO 95/2~43s r~ S ~ 3 percent utilization; in some modes of application, only 60 percent or lesx 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 eYhibiting good gloss, impact strength, and resistance to solvents and -h~-micAl~:, coatings derived from tht:L -ett;ng coating compositions must possess good to excellent f 1 ~Y; h; l; ty . For ~xample, good flexibility is essential for powder coating compositions used to çoat sheet steel that is destined to be f ormed 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 ~ ~LuLes in the range of ~bout 80 to 130C, cooling the e~LLudd~e, and then chipping and grinding the resulting solid into particles of suitable size. Because the mixture is subjected to high temperatures during this process, ~L~ILCILULe 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.
3 0 other processes f or the ~JL t:ydL O Lion of powder coating compositions which do not employ extrusion to mix the ~~ ^ntS nonetheless subject them to tempera--tures sufficiently high either to cause ~JL~ LuLe curing or put substantial limitations on the curing properties 35 of the compositions . U. S . Patent No . 3, 759, 864, for ~ W095l28435 2t87819 .~ 4~
_ 3 _ example, discloses a proce6s for preparing pigmented epoxy resin particles by emulsi~ying the liquif ied '~ polymer in a continuous volatile liquid phase containing pigment at tempt:LaLulas in the range of 80--150C, 5 cooling the mixture to solidify the polymer, and removing the volatile liquid. In U. S . Patent No.
4,049,744, particles of a poly1~ydl~ y polyether resin are ~ ar~d by mixing the resin with water containing a polymeric polycarboxyli ~cid or salt at a t a~uLa 10 of at least 60C, agitatiDg the mixture to form a dispersion, and cooling to form solid polymer particles.
PROBLEM TO BE SOLVED BY THE INVENTION
With the increa6ing 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 to~ LaLuLe:s. Conventional extrusion and pulverization 20 techniques can cause ~L~ LULa cross--linking in composi--tions intended for low t~ _L~LULt: curing ~Lccesses.
The method of the invention allows the ylc:~alc~tion at t~ aLul~:6 near ambient o~ the heat--sensitive powder coating compositions required for low t~ ~ aLura cure 25 applications.
SU~ARY OF THE INVENTION
The present invention provides a method for 30 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 cllcppn~d in an aqueous liquid WO 9Sl2~43~ r~ 3 -- 4 -- =
phase comprising water and a surfactant;
(b) milling the mixture at a temperature of up to ~-about 4 o o C 80 as to reduce the mean particle size of the particles to no greater than about - -15 I~m;
(c~ agglomerating the particles 80 as to increase their mean particle size to at least about 20 ~m;
(d) separating the particles from the agueous liguid phase; and (e) drying the particles to form a powder.
ADVANTA~-T't~TJS EFFECTS OF TEIE INVT'hTTION
In the method of the present invention, ~u~ l y 20 fine particles are ~Lt:l.aled 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 25 narrowed, which results in;, ~,v~d powder flow and coating smoothness properties.
DF:TATT T~n L1~ Kl~lON OF Tn~ INV~NTION
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, 35 carboxy, hydroxy, amino, or anhydride functional groups WO gSl28435 2 1 8 7 B I q A .~ 5.1O l'~3 .

that can react with cross--linking ~ ul~ds to provide cured coatings.
Preferred epoxy fllnrti~nAl resins generally have a molecular weight of about 300 to 2bout 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 EPONn' trade--name of the Shell ~'h~m; CAl Company, the Araldite~
tradename of CIBA~eigy, and D.E.R. resins of the Dow ~h~m i rA 1 Company .
Curable resins which have carboxy fllnrti~nAl 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 Alftalatn' AN 720,721, 722, 744, 758 and Alftalat'Y AN 9970 and 9983 resins available from Hoechst f`~ n~e.
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 ~L~du~d using well--known polyr~n~ n~ation 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 ~ : 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 5uch glycols include ethylene glycol, propylene glycol, 1,3--prorAn~ 1, 2,4--dimethyl--2--ethylhexane--1, 3--diol, 2, 2--dimethyl--1, 3-- prnpAnP ~
2--ethyl--2--butyl--1, 3--propanediol, 2--ethyl--2--isobutyl--1, 3--35 propanediol, 1.3--butanediol, 1,4--~utanediol, 1,5--W095/2343s 2187819 r~ o~s~
-- 6 -- - =
pentanediol, 1, 6--hf~YAnf f ~rl, thiodiethanol, 1,2--, 1,3--and 1, 4--cyn1 nh f YA nf.~l i ~ Lllallo1, 2, 2, 4, 4--te LL y 1--1, 3--cyclobutanediol, 1,4--Yylylenediol, and the like.
The dicarboYylic acid constituent of the polyesters 5 may be derived from vario~6 aliphatic, alicyclic, ,lliphatic--alicyclic, and aromatic tlifArbfYylic acids containing about 4 to 10 carbon atoms or ester--forming derivatives thereof, such as dialkyl esters and~or anhydrides. Succinic, glut~ric, adipic, azelaic, 10 sebacic, fumaric, maleic, itnconic, 1,3-- and 1,4--cyr]~h~ fflifArbo-y~ylic~ phthalic, i~:orhthAlir and terephthalic are representat:ive of the dicarboYylic acids from which the diacid residues of the amorphous polyester may be derived. A minor amount, e . g ., up to 15 10 mole percent, of the glycol and~or diacid residues may be replaced with branching agents, e.g., tri--fllnr~f~f~nAl residues derived from trimethylolethane, trimethylolpropane and trimellitic anhydride.
The preferred polyesters suitable for the practice 20 of this invention have a glass transition t~ _ ~LuLe, Tg, greater than 55C, and an inherent viscosity of about 0 .15 to 0 . 4 . The polyester resin preferably comprises (1) diacid resid~s of which at least 50 mole percent are terephthal~ o or i&ophthalic acid residues, 25 (2) glycol residues of which at least 50 mole percent are derived from 2,2--dimethyl--1,3--propAn~ l (neopentyl glycol) and (3) up to 10 mole percent, based on the total moles of (2) and (3), of trimethylolpropane residues. These preferred hydroYyl functional 30 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 il~cvL~uuL,~ted 35 herein by re~erence, and Published Japanese Patent W0 95/28435 ~ l ~7~ C 1~49 Applications tRokai) 73--05,895 and 73--26,292. The most 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--5 dimethyl--1,3--prop~no~ l residues, of trimethylolpropane residues, and pos~ s~ a Tg of about 50 to 65C, 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 dI,~g measured using 0.5 g~100 mL of a 60~40 blend 10 (w~w) of phenol~tetrachloroethane at 25C.
A curable acrylic resin suitable for the practice of this invention is preferably a polymer or resin ~L~lLed by polymerization of a hydroxy--substituted monomer such as IIYdL~ Y~L11Y1 methacrylate, IIYdL~ Y~L11Y1 15 acrylate, l~ydL~ yllexyl acrylate, llydLu~cyllexyl meth--acrylate, IIYdL~Y~LO~Y1 acrylate, IIYdL~Y~LU~JY1 meth-acrylate, IIYdL~Y}JULY1 acrylate, hydroxylbutyl meth--acrylate, and the like, optionally polymerized with other ~ such as methyl acrylate, methyl meth--20 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 l~ay_l.LL and molecular weights of the resulting 25 acrylic polymers are preferably chosen 50 as to give polymers with an average functionality (the number of OEI
groups per molecule) greater than or equal to 2.
Commercially available curable hydroxy--functional acrylic polymers include Joncryl~ 800, Joncryln' 500, and 30 Neocrylm 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 35 generally have a number 2verage molecular weight of W0 95/2~435 2 1 8 ~ ~ 9 P~ 5 0 1S4~

about 500 to about 5000 and a weight average molecular weight of about lOoO to about ~10, 000 In a preferred -mhc"li- L, the resin is a glycidyl methacrylate resin containing from about 5 to about 40 weight percent GMA
5 residues, having a numbe~ average molecular weight of ~bout lOoO to about 3000 and a weight average molecular weight of about 2000 to about 8000. Commercially available resins include those available from Mitsui Toatsu ~h~m;cAl~, Inc., available under the tradename ~lmatex~ PD 6100, PD 6300, PD 7110, PD 7210, PD 7310, PD
7610, and PD 1700. Eurther 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 ~iqrlr~llres Of which are in~UL~UL~lted 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 fllnrtif~nAl resins, cross--linking ~ with epoxy groups can be ~tilized. Likewise, with an epoxy fllnrtionAl resin, an anhydride type cross--linking ~ ~_ a can be used.
Further, with hydroxy--fllnrti~nAl resins, blocked isocyanate6 can be used. Also, a carboxy functional resin may be blended with an epoxy resin, optionally in the presence of another epoxy functional ,- _ ' such as triglycidyl iso.;ycl-.uL~,te, and cured.
l es of anhydride type cross--liking include trimellitic anhydride, hPn~orhf-n~m~ tetra--carboxylic dianhydride, ,UyL. 1 1 itic dianhydride, tetra~,y.lLvy~lulalic anhydride, and the like.
In general, carboxy--functional cross--linking agents are C3--C30 alkyl, alkenyl, or alkynyl ~ '- with two or more carboxylic acid functional groups. Preferred carboxy--functional cross--linking, uul~ds can be described by the formula W095128435 2~ 818 1 9 P`~ J~-r';~4549 H02C--[ ( CH2 ) n ]--C02H ~
wherein n is an integer from 1--10. Examples of such carboxy--~unctional cross--linking agents include . 5 , _ such as ,lnr~pr~nprlioic acid, azelaic acid, adipic acid, 1,6--hPY~nPA;oic acid, succinic acid, pimelic acid, sebacic acid, and the like. Other examples of carboxy--type cross--linking ,_ ~ 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 c _ c and may be nh~inr~r9 from commercial sources or ~Le,u~lred according to pllhl; qhPr~l procedures. Upon being heated to cure coatings of the compositions, the ~ __ ' become unblocked and the isocyanate groups react with hydroYy 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 ~--caprolactam, commercially available as Huls 1530 and Cargill 2400, or toluene 2, 4--diisocyanate blocked with i--caprolactam, commercially available as Cargill 2450, and phenol--blocked polyisocyanate.
~he most readily available blockea polyisocyanate cross--linking agents or ~ _ '- are those commonly referred to as ~--caprolactam--blocked isuul.ulùl.e diisocyanate , e . g ., those described in U . S . Patent Nos .

3,822,240, 4,150,211 and 4,212,962, the r~;qr lnqllres of which are inUUL ,,UOL ~ted herein by ref erence . However, the products marketed as ~--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 WO 9512l~435 2 1 8 7 8 1 9 ~ 0 Ir lg 3--isocyanatomethyl--3, 5, 5--trimethylcyclohexylisocyanate, the blocked, difunctional dimer thereof, the blocked, trifllnr~inn~l trimer thereof or a mixture of the monomeric, dimeric ancVor trimeric forms. For example, the blocked polyisocyanate ~ _ used as the cross--linking agent may be a mixture consisting primarily of the ~--caprolactam--blocked, difunctional r lc isophorone diisocyanate and the ~--eaprolactam blocked, trifunctional trimer of isuy~ one diiso.:ycln~te. The description herein of the cross--linking agents as "polyisoeyanates" refers to ~ which contain at least two isocyanate groups that are blocked with, i . e., reacted with, another: ~ , e.g., ~--caprolactam.
The reaction of the isocyanato groups with the blocking _ ' is reversible at eleYated t _ C-LULC:S~ e.g., normally about 150C, and above, at which t~ l UL~
the iso~y~ to groups are available to react with the hydroxyl groups present in the polymer to form urethane linkages .
AlternatiYely, the blocked isocyanate may be a cross--linking effective amount of an adduct of the 1,3--cliazetidine--2,4-dione dimer of is~,yl~c,L~,.,e diiso~y~ t.e ~nd a diol having the structure OCN--R1 [X--R1--NH--COO--R2--OCO--NH--Rl ] nX--R1--NCO
wherein Rl is a methylene--1,3,3--trimethyl--5--cyclohexyl diradical; R2 is a divalent aliphatic, cycloaliphatic, arzllkyl or aromatic residue of a diol; and X is a 1, 3--30 diazetidine--2,4-dionediyl 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 diazeti-l;n.o~l;one to diol is from 2:1 to 6:5, the content of free isocyanate groups in the adduct is not greater than ~ weight 35 percent, and the adduct has a molecular weight of about W0 95~2843S 2 ~ ~37 8 1 ~ r~ c49 500 to 4000 and a melting point of about 70 to 130C.
The adducts of the l, 3~1; A 7et i 1 i nc~--2, 4--dione dimer '~ of isophorone diiso.y~ Le and a diol are ~e~clLed according to the ~l oceduL eS described in U . S . Patent S 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--uy~..uL~-e trimers of isophorone diisocyanate, with diols in a ratio of reactants which gives an iso~iy~ ato:hydroxyl ratio of about 1:0.5 to 1:0.9, preferably 1:0.6 to 1:0.8. The adduct preferably has a le~lllAr weight of 1450 to 2800 and a melting point of about 85 to 120C. The preferred diol reactant is 1,4--butanediol. Such an adduct is commercially available under the name Huls BF154 0 .
The amount of the blocked diisocyanate cross--linking agent present in the compositions ~epa~ed 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 re6in. Optionally, a catalyst such as dibutyltin dilaurate (available from Aidrich Chemical Co. ) may be used to facilitate cross--linking by the polyisoeycu.~l_e ~. Based on the weight of cross--linking agent, 0 . S to 5 weight percent, preferably 1 to 2 weight percent, of the catalyst may be employed.
The powder coating compositions ~Jduced by the method of this inventlon may be prepared from the compositions described herein by dry--mixing the curable resin and the cross--linking agent along with other 3S additives commonly used in powder coating compositions WO 95/28435 ~ J,, r ~ Ir49 -- 12 -- ==
and then milling the blend to an average particle size of no more than about 15 ILm. Typical of the additives hich may be present in powder coating compositions are benzoin, flow aids or flow control agent6, sts~h; 1 i 7~5, 5 ]?igments, and dyes- The powder coating compositions prepared by the method of the invention pref erably contain f low aids, also ref erred to as f low control or leveling agents, to enhance the surface appearance of cured coating6 of the powder coating compositions. Such 10 Elow aids typically comprise acrylic polymers and are available from several suppliers, e.g., Modaflow~ from lNonsanto Company and Acronal'Y from BASF. Other flow ~ontrol agents which may be used include I~O~dL~- MFP
available from Synthron, EX 486 available from Troy 15 ChP~nir~l, BYK 360P aYailable ~rom BYK M~ll;nr~l~rodt~ and Perenol~ F--30--P available from Henkel. An example of one 6pecific flow aid is an acrylic polymer having a --lPc~ weight of about 17,000 and containing 60 mole percent 2--ethylhexyl methacrylate residues and about 20 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 ac~ v~ ce with the method of t_e present inven--25 tion, a mixture that comprises particles containing a curable resin and a cross--linking agent, and optionally ot_er additive6 as lliccllcced above, is formed by dry--mixing the ingredients using, for example, a T~nc~hP~
mixer and~or a hammer mill. The resulting blend is then 30 Sl~crPn~P/l in an aqueous liquid phase comprising water and a surfactant, or dispersing agent. The surfactant can be an ionic ~ d~ such as sodium dodecyl sulfate, or, preferably, a nonionic, _ ' such as a polyether alcohol . Suitable surf actants include Triton'Y
35 X--100 (from Union Carbide Co. ), and Surfynol~ GA and .

218781~
, CT--136 ~rrOr~ Air Products Corp. ); they can be employad, singly or in combination, in amounts ranginy ~rom aoout 0 . l to 2 0 wQight percent, pre~erably about 2 tc 10 weight percent, of the liqnid.
The G~lCr~nCi ~n of par~cicles in the aqueous li~uid phase is su~jected to ~nicrofine r~illing, fc~r exa~ple, by cedia milling. ~edia ~illin~ can be con~eniently accomplished with a Netsch L~3 ~orizon~al recirculating ~ill .
Alt~rnativ~ly, following the dry--mixing ~tep described above, the blend can ~e jet ~illed ta produce very fin~, solid particles, ~hich are then ~ n~l".l in the aqueous liquid phase. ~et ~illing can be per~ormed using, for ~xample, a Trost air _mpact pulverizer.
Slurry coating composit~ ons dispersQd in w~ter that are pr~pared by ~LiXing resin pellets or qranule~ with water and crushing the mixt~re with a b~ll 31lill, pot ~nill, or crusher arQ dLsclosed ~n J~52107033A and ~P8000430,1B. Ir~ the method of the present invelltion, particles containing cur~ble resin ~n~ cros~--linking agent 2re reduced to a pa2:ticle size no greater than about 15 tLm, and preîerably no ~reater than about 8 ~, by mil:ling at a ta~p~rature OI up to about so3c. The5e Qxtre~aly f inc particles are then caused to ~J] ~
to particles havin~ a mcan si~e of at least a}:out 20 tlrl.
In the course of agg~omer~tiOn, th~ particl~s beco~ne increasingly spherical in shape as particle siz2 ~istribution is n2~rrowed.
hgsl~ - ~tion to increase the particle size can be 3~ accomplishQd by war~ing the suspen~ion of the ~inely milled partlcle3 to tempe~atures near the glass transi--tlon te~perature, Tq~ of th~ curable r~sin, from ~ou1:
40cc a~oVQ tc about 40~C bQlow the ~, pre~erably from about 10C above to about 10C be~o~r che Ty~ in the 3a presence of smal 1 amounts of sta~illzer and pro~otor ~la~e~ent P~e ~END~D S~EET

WO95l28435 2 1 878 1 ~ 1~/, S ~49 -. Warming of the sllCpqn~i~In can be rqintAin~q~
from about 0.25 hour to 6 hours, preferably about 0.50 hour to 2 hours. StAhili~ors, or s~ p~nllin~ agents, control particle coAl~scqn~e while the sll~:pQn~;on is 5 being warmed and consequently affect particle size distribution. Promoters f acilitate the adherence of 8t~hi 1; ~orS to the surface of the particles.
r 1~R of suitable stAb; l; ~rs include colloidal silica such as the commercially available Ludox~ 5U, 10 ~udoxn' TU, and Nalcoag'Y 1060. Other useful stAh; l; ~qrS
are titanium dioxide or fumed aluminum oxide, which are available from Degussa Corp. r 1~: of suitable promoters are water soluble polymeric materials such as poly ( ~1 i eth A n-- 1 Am; n~ adipate) and po ly (methylaminoethanol 15 adipate). Alternatively, ionic surfactants may be used as promoters. Such _ ~ serve a dual purpose as particle stAh; l; "rS~ thus avoiding the need for addi--tional stAh; l; ~.or ~ , _ '~. Examples of suitable ionic promoters include sodium dodecyl sulfate, sodium butane--20 sulfonate, and the like. Other useful promoters are described in U.S. Patent No. 4,833,060, the ~ clos~res of which are incuL~uL~ted herein by reference.
Useful cc,l~cel~ iOns of solid particles range from about 1 to 50 weight percent, preferably from about 5 to 25 25 weight percent, of the aqueous liquid phase. Useful cu~ce~LL~tions 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 e ullcerl~L~tions of promoter range from about 0. 05 to 40 30 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 35 flll;(~i7~q~ bed. After drying, the particles preferably 218~'8~9 W0 95/2843s r~ ' C 1'49 have a mean particle size from about 20 ~m to loO ~m, more preferably from about 25 ,um to 50 ,um. Particle size distribution and mean particle size in compositions ~ t ~ared according to the method of invention can be 5 det~rmin-~d by means of a Microtrac particle size analyzer (available from Leeds & Northrup), using a technique that entails the mea~u~ ~ L 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 p~wder can also be achieved by spray drying, using techniques known in the art. For example, U. 5 . Patent No. 3,325,425 fl;~clo5~c 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 t' ~ -tuLe of 100 to 160F
and maintaining the outlet t C~LUL~ between 75 and 20 125F, 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 f ormation of a powder by spray drying of an aqueous dispersion of a copolymer of vinyl ester, 25 acrylamide, and optionally ethylene. The disclosures of U.S. Patent Nos. 3,325,425 and 3,950,302 are in~u,~uLil~ed herein by reference.
The powder coating compositions may be deposited on various metallic and non '~ ., e.g., th' , l~ctiC
30 or th~ - ~t composite, substrates by known techniques for powder deposition, such as by means of a powder gun, by electrostatic deposition, or by deposition from a fl~ g:l bed. In fluidized bed sintering, a preheated article is immersed into a suspension of the powder 35 coating in air. The particle size of the powder coating W0 95/28435 2 1 8 7 ~ 1 9 ~ C 1l4s4~
.

compositlon normally is in the range of 60 to 300 ,tLm.
The powder is maintained in suspension by passing air through a porous bottom of the f ~ ed bed chamber.
The articles to be coated are preheated to about 250 to 400F (about 121 to 205C) and then brought into contact with the f 1 1l; rl; ~ bed of the powder coating composition. The contact time depends on the th;rknP~c of the coating that is to be produced and typically is from 1 to 12 seconds. The t~ u e of the ~.u~,.LL~Ite being coated causes the powder to flow and thus fuse together to form a smooth, uniform, continuous uncratered coating. The t~ Lu t: of the preheated ~Irticle 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 th;~knP~: between 200 ,um and 500 ,um may be ~. ~Jduced by this method .
The compositions also may be applied using an elt:- L..,DL~lLic process wherein a powder coating 20 composition having a particle size of less than 100 ,um, preferably about 25 ,um to 50 ~ILm~ is blown by means of t:~ ed alr 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 25 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 ~Lm to 120 ,~Lm th; ~knPç:s may be obtained.
3 0 Another method of applying the powder coating compositions is the ele.:L.u~L~tic 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 el~-_L.- ,LclLic charge such as 50 Wo 9S/28435 2 1 87 ~1 ~ 9 r~ 549 to lO0 kV. The article to be coated, either heated, e.g., 250 to 400F (about 121 to 205C), or cold, is exposed briefly to the flllirli 7Pd powder. The coated article then can be heated to effect cross--linking if 5 the article was not preheated to a t ~ ~ LUL a suf f iciently high to cure the coating upon contact of - the coating particles within the article.
The powder coating compositions ~L_~.alc:d according to the method of this invention may be used to coat 10 articles of various shapes and sizes c.u~LLu~;Led of heat materials such as glass, ceramics, and metals. The compositions are ~cpPci~lly useful for producing coatings on articles cul.~LLuuLed of metals and metal alloys, particularly steel articles. It is possible to cure some systems at t~ _ ~LUL~S as low as 115C, for example, compositions containing epoxy resins, anhydride cross--linking agents, and quaternary i llm salt or hydroxide cross--linking catalysts, as taught by U. S .
Patent No. 5,244,944, the ~lic~losllres of which are ill~uL~uL~ted herein by reference. Compositions that are curable at relatively low temp~L~LuL.~" around 115C for example, are useful for coating articles formed of th, ~ ctic and th, ~Ling resin compositions.
Further F~ ~1PS 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 ~ngin~rS, Dearborn. (1987).
Powder coating compositions prepared by the method of the present invention are preferably applied to a coating substrate by means of el~LLu:,L~tic spraying, using dy~uaL~Lus such as a Ransburg corona type gun.
Following spraying, the substrate is heated at temperatures in the range of about 115 to 2000C for 35 periods of about 5 minutes to 30 minutes. Cure of the WO95l28435 ~ 2 1 878 1 9 P~ S49 .

coatings i8 detprm;np~ by standard test ~LuceduL~ ASTM
4752--87 and reported as MEK (Methyl Ethyl Ketone) solvent resistance. Cotton cheese cloth folded according to srPc;f;cntion is attached to the end of a 13--ounce ball peen hammer and saturated with ~EK. 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 i5 deemed cured.
The following ~ 1PC further illu6trate the invention .
Example 1 -- Media milling of mixture for powder coating ~ --;tion The following materials in the amounts shown were mixed and ground u6ing a ha~nmer--type pulverizing mill:
3280 gram6 of Rucoten' 107, a hydroxy--6ubstituted polyester;
720 grams of H~ils B1530, a blocked isocyanate cross--linking agent;

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

40 grams of Modaflow'Y III, a flow agent;
40 grams of benzoin, a ~l~g ~ ;n~ agent.
The ground mixture was dispersed in water containing 5 weight percent of an approximately 1:1 mixture of Surfynol'Y CT--136 and Surfynol'Y GA surfactants, both available from Air Products Corporation; the resulting 40 suspension contained 39 weight percent solids. Part~ cle WO 95/2~435 ~ I ~ 78 ~ 9 ~ 9 size was reduced at ambient t~ c.LuLe, using a Netsch ~qZ horizontal recirculating mill charged with 0.8--1.25 mm zirconium silicate media, to a range of about o . 4 ,um to 15 ILm, with a mean particle size of .. 5 about 4 ,ILm.
Example 2 -- Formation of powder coating composition by CO~ cr~n~e of particles in aqueous susension A 3--L beaker was charged with 350 mL of the mixture prepared as described in Example 1, followed by 4.5 mL
of LudoxN TM (from DuPont), 6 g of poly(methylamino--ethanol adipate), and 350 mL of water. After mixing with a high speed laboratory dispersion unit, the ~URp~n~:i nn wa6 transferred to a 1000--mL flask and stirred for 60 min at 300 rpm and a t~ a~uLe of 30OC.
Stirring was continued while the temperature of the mixture was gradually raised over a period of 30 min to 60OC, where it was maintained for 45 min. The mixture was cooled, and the ~ d solid was separated by filtration, allowed to air dry for 2 days, and then dried in a vacuum oven at 45C, with slight air purge, f or 2 4 hr .
The resulting powder had a mean particle size of about 43 llm, with the size of 80 volume percent of the particles lying between about 11 ~Lm and 105 ,~m.
r le 3 -- Powder coatinq of substrates Steel panels of dimensions 3 in x 9 in (7.7 cm x 23 cm) were ele.:~L.,~,Latically sprayed with the composition prepared as described in Example 2. The coatings were cured for 15 min in an oven at 375F
(191C). The cured coatings, which had a thickness of about 1. 7 mils (43 ~m), withstood 200 double rubs in the W095/28435 21878~9 P~ l5~3 MEK solvent re6istance test without marring of the coated surf ace .
Example 4 -- Jet milling of mixture f or powder coating com~osition The following materials in the amounts shown were mixed with a laboratory blender and then milled using a 10 Trost air impact pulverizer:
820 grams of Rucote~ 107~ a hydroxy--substituted polyester;
180 grams of ~I~ls B1530, a blocked isocyanate - cross--linking agent;
10 grams of dibutyltin dilaurate powder, a deblocking agent;
15 grams of ~odaflow'Y III, a flow agent;
10 grams of benzoin (available from GCA Corp. ), a ~lprJ~cs;n~ agent.

The resulting powder contained particles in the size range of about 2 ~m to 15 ,um, with a mean particle cize of about 8 ,um.
Example 5 -- Formation of powder coating composition by coa 1 PCcpnre of particles in aqueous FllcnPnci on A 1000--mL 3--necked flask was charged with 650 mL of water, 15 g of Degussa P--25 fumed Tio2, 5.6 g of poly--(methylaminoethanol adipate), and 4 . 3 g of Tritonn' X--100 surfactant. To this mixture was added 34 g of the 40 powder prepared as described in Example 4. The resulting suspension was stirred for 60 min at 200 ~pm and a t-, - .lL~lL'2 of 30C. Stirring was continued as W095/28435 21B7P~ ~ r~ 3 .

the t~ <ItuLa was gradually raised over a period of 30 min to 65 +5C, where it was maintained for 15 min.
The mixture was cooled, and the ~ L' l~d solid was separated by filtration, allowed to air drY overnight, 5 and further dried by fluidization in drY air at 45C.
The resulting powder had a mean particle size of about 23 ILm, with the size of 80 volume percent of the particles lying between about 8 ,~m and 37 ,ILm.
10 ~E~le 6 -- Powder coaf i n-r of ~l~hctrates Steel panels (3 in x 9 in, 7 . 7 cm x 23 cm) were ele~Llo,,l.~tically sprayed with the composition y- eLJa~ed as described in Example 5. The coatings were cured for 15 min in an oven at 375F (191C). The cured coatings, which had a thi~lrn~ of about 1.2 mils (30 ~m) with--stood 200 double rubs in the MEK solvent resistance test without marring of the coated surface.
20 Example 7 -- Formation of powder coating composition by sl2raY drYin~
A sample of the mixture prepared as de6cribed in Example 1 was stirred with a paddle stirrer overnight, 25 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, 60C; outlet tempera--ture, 32C; atomization pressure, 60 psi; spray rate, 30 45 g~min~ The spray dried powder was further dried in a vacuum oven at 25C, with slight air purge, then sifted through a 170 mesh screçn.
The resulting powder had a mean particle size of about 31 ~m, with the size of 9o volume percent of the 35 particles lying between about 11 ~m and 74 ,~m.

WO 95/28435 2 1 87 8 1 9 1 ~"~ c l549 r le 8 -- Powder coatina of substrates Steel panels (3 in x 9 in, 7.7 cm x 23 cm~ were ele~L- u~L~l~ically sprayed with the composition prepared 5 as described in :E:xample 7. The coatings were cured for 20 min in an oven at 350F (177C). The cured coatings, w~lich had a thi~kni s:c of about 2.2 mils (55 ~m), with--stood 200 rubs in the M13~ solvent resi6tancQ test with--out marring of the coated surf ace .
Ihe invention has been described in detail with particular reference to preferred Pmho~ c thereof, but it will be understood that variations and modif ica--tions can be effected within the spirit and scope of the invention .

Claims

Claims We 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 crosslinking 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 wherein said agglomerating is carried out by adding, after said milling, a suspending agent and a promoter to said mixture and then warming the resulting mixture to a temperature from about 40°C above to about 40°C below the Tg of said particles;
(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.

7. A method according to any one of Claims 1 to 5 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.