WO1995028448A1

WO1995028448A1 - Method for preparing liquid coating compositions

Info

Publication number: WO1995028448A1
Application number: PCT/US1995/004320
Authority: WO
Inventors: Yeong-Ho Chang; Joseph Clark Jernigan; Lanney Calvin Treece
Original assignee: Eastman Chemical Company
Priority date: 1994-04-13
Filing date: 1995-04-12
Publication date: 1995-10-26

Abstract

Provided is a method for preparing a liquid coating composition which comprises: (a) forming a mixture comprising a suspension of coarse solid particles which include a curable resin and a cross-linking agent that is reactive with the curable resin, the coarse solid particles having a mean particle size of about 30 νm to 500 νm, in an aqueous liquid medium comprising water and a surfactant; and (b) milling the mixture at a temperature of up to about 40 °C to form a coating composition comprising a suspension of fine solid particles having a mean particle size of about 0.1 νm to 15 νm suspended in the aqueous liquid medium.

Description

METHOD FOR PREPARING LIQUID COATING COMPOSITIONS

The invention relates to liquid coating composi— tions, and more particularly to a method for preparing aqueous coating compositions containing curable resins and cross—linking agents that are reactive with the resins.

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, as disclosed, for example, in GB 1,572,996. The use of such solvent systems is inherently undesirable not only because of the expense of the solvents but also because of the environmental and safety problems that may be occasioned by their evaporation.

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. This pulverizing operation typically produces a powder in which the particles are characterized by irregular shape and a broad 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. As another alternative to extrusion for the mixing of the components of a powder coating composition, the resin, cross—linking agent, and other ingredients may be dissolved in a water miscible organic solvent such as a glycol ether (Cellosolve^®, for example) ; the resulting solution can be added to water with vigorous agitation to produce powder coating particles, which can be separated from the liquid phase and dried. Such a procedure is described in U.S. Patent No. 4,263,352.

PROBLEMS TO BE SOLVED BY THE INVENTION

Methods of forming powder coating compositions that utilize standard pulverization processes following extrusion subject the compositions to high temperature and produce dusty powders with broad particle size distribution. The use of organic solvents to accomplish mixing of the components of the composition is undesirable from the standpoint both of cost and environmental consequences. The method of the present invention avoids the use of high temperatures that are typically used in extrusion mixing and can lead to premature curing of the resin; it also avoids the use of organic solvents for dissolution of the resin and cross- linking agents.

SUMMARY OF THE INVENTION

The present invention provides a method for preparing a liquid coating composition which comprises; (a) forming a mixture comprising a suspension of coarse solid particles which include a curable resin and a cross—linking agent that is reactive with the curable resin, the coarse solid particles having a mean particle size of about 30 μm to 500 μm, in an aqueous liquid medium comprising water and a surfactant; and

(b) milling the mixture at a temperature of up to about 40°C to form a coating composition comprising a suspension of fine solid particles having a mean particle size of about 0.1 μra. to 15 μm suspended in the aqueous liquid medium.

ADVANTAGEOUS EFFECTS OF THE INVENTION

The method of the present invention provides useful liquid coating compositions containing fine solid particles suspended in an aqueous medium. Compositions prepared by the method of the invention, which avoids the use of high temperatures and organic solvents, can be conveniently applied to substrates by spraying, with subsequent drying and curing of the coating at relatively low temperatures.

DETAILED DESCRIPTION OF THE INVENTION

The method of the present invention provides an aqueous coating composition that comprises fine solid particles which include a curable resin and a cross- linking agent reactive with the curable resin and are suspended in an aqueous liquid medium comprising water and a surfactant. In accordance with the present invention, the curable resin is preferably 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 EP0N™ tradename of the Shell Chemical Company, the Araldite™ tradename of CIBA— 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 carboxy1) 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-l,3— propanediol, 2—ethyl—2—butyl—1,3-propanediol, 2—ethy1-2- isobutyl—1,3—propanediol, 1,3—butanediol, 1,4—butane- diol, 1,5— entanediol, 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— cyclohexanedicarboxylie, 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 dL/g determined using 0.5 g/lOOmL of a 60/40 (w/w) phenol/tetrachloroethane blend at 25°C. 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— imethyl— 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—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 glass transition temperature, T , 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.

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 aerylate, hydroxyhexyl aerylate, hydroxyhexyl meth— acrylate, hydroxypropyl aerylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxylbutyl methacrylate, 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, isobutyl methacrylate, eth lhexyl 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 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, the resin is a glycidyl meth¬ acrylate 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, pyro ellitic 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]-CO₂H,

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 chainβ 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 Hϋls 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 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, 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—trimethyleyclohexylisocyanate, 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 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]^-R^NCO

wherein R¹ is a methylene—1,3,3—trimethy1—5—cyclohexyl diradical; R² is a divalent aliphatic, cycloaliphatic, aralkyl or aromatic residue of a diol; and X is a 1,3— 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 diazetidinedione to diol is from 2:1 to 6:5, the content of free isocyanate groups in the adduct is riot 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 disclosures of which are 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 as 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 850 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 tn 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 (DBTDL) 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 liquid coating compositions produced by the method of this invention may be prepared from the compositions described herein by first mixing, in the dry state or in an aqueous suspension, the curable resin and the cross—linking agent along with other additives commonly used in powder coating compositions. Typical of the additives which may be present in these compositions are benzoin, flow aids or flow control agents, stabilizers, pigments, and dyes. The 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 liquid 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 invention, the coarse solid particles included in the mixture have a mean particle size in the range of about 30 μm to 500 μm, preferably about 100 μm to 300 μm. Pulverization to form the coarse particles can be accomplished using any of several types of mills such as, for example, a Henschel mixer and/or a hammer mill. Alternatively, the coarse grinding may be carried out by agitating an aqueous suspension of the solids in a blender. 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.

The resulting coarse particles, suspended in an aqueous liquid phase comprising water and a surfactant, or dispersing agent, are milled at a temperature of up to about 40°C to a mean particle size of about 0.1 μm to 15 μm, preferably about 0.5 μ to 5 μ . This fine grinding may be carried out, for example, by media milling. Media milling can be conveniently accomplished with a Netsch LMZ horizontal recirculating mill.

Alternatively, the dry coarse particles obtained as described above can be jet milled to produce very fine particles, which are then dispersed in the aqueous liquid phase. Jet milling can be performed using a Trost air impact pulverizer.

The surfactant can be ionic, for example, 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.).

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 surfactant range from about 0.1 to 20 weight percent, preferably from about 2 to 10 weight percent, of the liquid phase.

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 measure¬ ment of the amount and angle of forward scattered light from a laser beam projected through a stream of particles.

The liquid coating compositions prepared according to the method of this invention may be used to coat articles of various shapes and sizes constructed of 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.

Liquid coating compositions prepared by the method of the present invention are preferably applied to a coating substrate by means of a spray gun typically used with water—based paint compositions. Following spraying, water can be evaporated by air drying at 25— 35°C for about 30 min. to 2h. After removal of water, 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 coatings can be determined by standard test procedure ASTM 4752—87 and reported as MEK (Methyl Ethyl Ketone) solvent resistance. In this procedure, 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. Pi 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 liquid 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 Hϋls 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; 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 — Application of liquid coating composition to substrate

A steel panel of dimensions 3 in x 9 in (7.7 cm x 23 cm) was coated with the composition prepared as described in Example 1, using a conventional liquid coating spray gun. The coating was air dried at room temperature for 10 min, then cured for 20 min in an oven at 375°F (191°C) . The cured coating withstood 200 double rubs in the MEK solvent resistance test without 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 modifications can be effected within the spirit and scope of the invention.

Claims

ClaimsWe Claim:

1. A method of preparing a liquid coating composition which comprises:

(a) forming a mixture comprising a suspension of coarse solid particles which include a curable resin and a cross—linking agent that is reactive with said curable resin, said coarse solid particles having a mean particle size of about 30 μm to 500 μm, in an aqueous liquid medium comprising water and a surfactant; and

(b) milling said mixture at a temperature of up to about 40°C to form a coating composition comprising a suspension of fine solid particles having a mean particle size of about 0.1 μm to 15 μm suspended in said aqueous liquid medium.

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 carboxysubstituted polyester or polyether, or a hydroxy—, epoxy—, amino—, or carboxy—substituted acrylic or methacrylic polymer.

4. A method according to Claim 1 or 2 wherein said crosslinking agent is a polyisocyanate, a blocked polyisocyanate, a carboxylic anhydride, or an epoxy—substituted compound.

5. A method according to Claim 1 or 2 wherein said curable resin is an epoxy—substituted polyester or polyether and said cross—linking agent is a carboxylic anhydride.

6. A method according to any one of Claims 1 to 5 wherein said fine particles are about 1 to 50 weight percent of said aqueous liquid medium, and wherein said coarse solid particles have a mean particle size of about 100 μm to 300 μm.

7. A method according to Claim 6 wherein said fine particles are about 5 to 25 weight percent of said aqueous liquid medium and wherein said fine solid particles have a mean particle size of about 0.5 μm to 5 μm.

8. A method according to Claim 6 or 7 wherein said surfactant is a nonionic compound.

9. A method according to Claim 8 wherein said surfactant is a polyether alcohol.

10. A method according to Claim 6 or 7 wherein said surfactant is an ionic compound.