US3471308A

US3471308A - Metal flake

Info

Publication number: US3471308A
Application number: US544389A
Authority: US
Inventors: Erith T Clayton
Original assignee: Tainton Co
Current assignee: Tainton Co
Priority date: 1966-04-22
Filing date: 1966-04-22
Publication date: 1969-10-07
Anticipated expiration: 1986-10-07
Also published as: DE1669099A1; GB1183379A; NL6705704A

Description

US. Cl. 106290 6 Claims ABSTRACT OF THE DISCLOSURE Finely divided metal particles, such as zinc dust particles, are flattened by mechanical flattening forces in an aqueous media to provide metal flakes. The aqueous media includes a polishing agent which is capable of exhausting itself in a thin -film over the surface of the metal particles, a metallurgical flux which is capable of dissolving metal oxides and salts without being strongly reactive with the base metal, and a Weldment inhibitor selected from the group consisting of strong nitrogenous basic ferrous metal corrosion inhibitors, aliphatic sulfur compounds, aromatic sulfur compounds, aromatic aldehydes, and aromatic ketones. 'Ihe flakes are eminently suitable in decorative and protective paints.

The present invention relates to flattened particles of metal and to methods and materials used in the fabrication thereof. More particularly, the invention relates to fabrication of small metal flakes useful in protective and decorative paints.

It is known in the art to make metal flakes, such as zinc flakes, by grinding and polishing in a ball mill or other similar apparatus capable of imparting mechanical energy to metal particles admixed with an impact material such as iron shot. This operation, which is described, for example, in US. Patent No. 1,954,462 and No. 2,080,346, is carried out in a non-aqueous medium, which consists of a solvent such as a hydrocarbon solvent, chlorinated hydrocarbon, alcohol, ketone or the like, and which dissolves water insoluble fatty lubricants such as oils, fatty acids and the like, which are necessary for the polishing and flattening of the flakes.

It is also known in the art to encase the zinc particles resulting from the above described procedures with micacious materials to protect the flakes as described, for example, in my US. Patent No. 2,528,034.

The old process described above, using volatile or organic solvents, is subject to several major disadvantages. The flammable hydrocarbon solvents such as naptha constitute a serious and dangerous fire hazard which is objected to in commercial plants. The use of non-flammable, chlorinated hydrocarbons is subject to other disadvantages. These materials are highly toxic; the vapors are dangerous; they are expensive and heavy, which results in a high cost per unit volume. Because of the high cost, these organic liquids must be recovered by distillation. Recovery by distillation is not a simple matter because there is a reaction between the finely divided zinc and the plating agents which are dissolved in the organic solvent which raise the boiling point and which retire some portion of the solvent used. This, together with unavoidable handling losses and drag-out, etc., leads to a relatively high cost for solvent use and recovery.

Other disadvantages associated with the prior art process are due to the fact that it is diflicult to make a sufficiently fine flake for commercial use. In the process of polishing and flattening the flakes, chemically clean and active surfaces are created which tend to weld or mate with the clean, reactive surfaces of other flakes with the result that larger flakes grow by accretion from smaller United States Patent See flakes and these weldments are such that the larger flakes become solid entities which can no longer be broken down into their component parts. The end result of this process has been to make it difficult or impossible to produce a really fine flake suitable for use in paints rich in the metal flake. The enlarged flakes of the earlier processes are extremely heavy and, because of their larger size, tend to sink in the paint vehicle. This problem is particularly troublesome with metals which have a tendency to weld, such as zinc, tin, cadmium and lead but is less pronounced with the noble metals, such as gold and silver, and with metal which behave similarly such as copper, brass and stainless steels.

Another disadvantage of the earlier flakes lies in the fact that when made up into paint, the clean surfaces of the flakes tend to bunch together so that the flakes tend to smear out in that paint film like cards in a sticky deck. This sticky adherence of flake to flake produces a poor brushing-out paint. Attempts to overcome this by encasing, enclosing or encapsulating the flakes with mica are successful, but such mica-coated flakes have a low coeflicient of friction which leads to the formation of brush marks in the painted film.

Not only do the highly polished bright flakes tend to stick to one another and grow by accretion, but they also will adhere to other clean metal surfaces such as the surfaces of the shot used as impact medium. The more potent and effective the polishing agents used previously, the more effective and more important the effect of plating finely divided flakes to the impacting material. In weekin, week-out operation, said shot will collect appreciable accretions of metal which lowers metal utilization and limits the amount of polishing which the flake can take because the coated impact shot which becomes rougher and rougher as the surface grows does not polish as readily as the smooth, highly polished uncoated shot. The process tends to operate in a vicious circle in that, as the shot gets larger and the surface gets rougher, the process of accretion speeds up and the shot grows ever larger, robbing the charge of free or useable metal. The rougher shot must be tumbled longer and longer with the metal to achieve the proper polish, and the longer the cycle, the less the useable metal remaining in the charge when the mill is emptied. This means that it is absolutely necessary that the steel shot or other impacting medium be cleaned in acid or otherwise uncoated periodically. This is particularly true in the case of zinc metal.

The use of solvents, heavy iron shot as polishing and flattening medium, and long polishing cycles may result in the formation of colloidal or ultra-fine metal particles which cannot be polished. In the case of zinc, this colloidally-fine material, called black stuff, is wasted from the useable metal and lost to the operation.

Despite the disadvantage of the prior art set out above, zinc flake as made by this older method had a good deal of commercial merit. This was proven by the painting of large structures such as bridges across the Mississippi, large 80,000 'barrel gasoline tanks, hot input lines and the like with zinc flakes made into paints. In these applications the zinc flake paint was found to have good properties in protection against corrosion. In the majority of these earlier applications the coating was applied to roughened, sand blasted steel where the substrate had an innate and very considerable roughness. For other applications, such as the coating of cold rolled steel plate such as is used in the automotive industries, the extremely smooth and highly polished steel surfaces are evidently without essential tooth. It has been found that the relatively coarse zinc flake paints made by earlier methods did not brush out satisfactorily on such smooth, polished surfaces.

It is an object of the invention to provide improved methods and materials for fabricating metal flakes. It is a further object of the present invention to provide methods and materials for making metal flakes which have improved properties and to provide such flakes themselves. It is still a further object of the invention to provide paint compositions rich in metal flake and having improved properties. It is yet a further object of the invention to provide methods and materials for making metal flakes which are smaller than those heretofore attained. It is still a further object of the invention to provide improved and superior coating materials for protecting metal flakes to better insulate same against an aqueous environment and to provide paints made therefrom with better brushability.

These and other objects of the invention which will become apparent in view of the following detailed description thereof are achieved by utilizing an aqueous media which includes a flux and a polishing agent in a method of flaking or flattening of metal particles by the subjection thereof to mechanical forces which flatten the particles into flake form without causing substantial adhesion thereof. The present invention relates only to aqueous systems and it is therefore considered expedient to discuss several of the known problems and solution thereof in this general type of process.

If any very finely divided powder of a reactive metal, such as zinc or the like, is mixed with water, a reaction occurs "between the metal powder and the water leading to the formation of metal hydroxide and hydrogen gas. If a reactive metal is tumbled in water in a closed steel barrel for lengthy periods, highly explosive pressures may be generated which are capable of blowing up the steel mill. The formation of metal hydroxide such as zinc hydroxide occurs as a fine white powder, and this combined zinc is lost to the operation. Because the polishing of large quantities of metal powder such as zinc requires fairly lengthy processing times, the formation of this zinc hydroxide, which proceeds with the passage of time, can result in a serious loss to make the process uneconomical assuming the question of the gas generation can be overcome and adequate polishing can be made to occur. To summarize, therefore, the use of water in the manufacture of bright flake has hithertofore been regarded as diflicult or impossible because it has not been possible to control the formation of hydrogen gas nor has it been possible to polish the very fine metal powder in a water medium, nor has it been possible to prevent the formation of metal hydroxides and other degradation products of extremely fine dust used.

Some of these problems associated iwth the admixture of extremely fine powder in water are overcome in an entirely different process, namely a process for mechanically plating fine metal powders onto steel objects in an aqueous medium using certain materials which act to film the metal particles and shield them from the aqueous environment. However, since this method is primaril a method for applying metal coatings to objects, the impact media is heavily plated with metal and even before one or two runs are completed, the impactor becomes unuseable and grows to many times its original size, tying up large quantities of metal as a metallic coating on the impactors. In such mechanical plating operations, a very small amount of metal powder is used and the time cycles are quite short, in the order of one hour. On the other hand, manufacturing bright flake, large quantities of metal are used and time cycles range up to 48 hours and more. If any attempt is made, therefore, to use mechanical plating techniques with these large concentrations of metal, the tendency to coat on the iron shot becomes enormously increased and the problem is greatly aggravated to the extent that the process is not workable. This problem existed, although in a lesser degree, in the buildup of metallic coatings by mechanical plating methods and led to the substitution of the metallic impact medium by glass beads, or other non-metallic surfaces which do not accept a metal coating. It has been found, however, that glass beads cannot be used for the manufacture of flake because of the large volume of zinc powder used. The glass beads do not have suflicient weight and energy to operate effectively in a high density water slurry of metal powder.

Furthermore, where aqueous systems are employed in the buildup of metallic coatings, the gas generation problem can readily be controlled because of the small volume of metal powder used, the rapidity with which it is converted into a metallic coating on the objects, and the short overall time cycle. In the manufacture of metallic flake pigments, where the polishing cycles are perhaps 50 times as long or longer, the gas generation becomes a very major problem. For these and other reasons, it will be seen that the methods used in mechanical plating are not applicable to the manufacture of bright metal flake.

It has been discovered that by utilizing a flux and a polishing agent which films on the metal in an aqueous medium, bright metal flakes in very small sizes are obtainable without the evolution of gas and without weldment of the small metal flakes produced. This aqueous environment creates conditions for covering the metal particles with a film such that each is insulated from the water with which it would otherwise react, for removing oxides or metal degradation products from their surface, and for preventing weldment of one to another, to the mill, or to the impact media or any other surface with which they may come in contact.

The flux may be any of the conventional fluxes used in metallurgical processes but the less corrosive fluxes are preferred. The amount of flux is not apparently critical and is primarily a matter of economy and dependent on the dirtiness of the metal used. In general, however, a minimum of /2 to 1% by weight, based on the weight of the metal powder to be flattened, may be used although amounts well above 1% are useable. The flux generally is capable of dissolving metal oxides or salts without being strongly reactive with the base metal. For any given metal a host of such fluxes is known and each is believed operable herein. Organic acids, such as citric, tartaric, hydroxyacetic and the like are excellent as are inorganic acids such as phosphoric. The alkali salts of these acids such as zinc chloride and ammonium chloride are also suitable. In addition, ammonium hydroxide or sodium or potassium hydroxide in weak solution are suitable. In general then, the use of fluxes known to function as same for the metal in question are suitable. In view of the fact that the metal is finely divided, somewhat more diluted use than normal is preferred to prevent excessive dissolution of the base metal.

The polishing agent is a film forming material or oil which is capable of exhausting itself in a thin film on the metal particle. The film former may be water soluble or insoluble and may be used in conjunction with a detergent which aids in wetting the metal with the film former in the aqueous medium. In fact, several detergents are film formers and may be used by themselves in this capacity.

While suitable oils fall into various chemical classes, it is the physical property of the oil to exhaust itself in a thin film on the metal that is controlling. The only chemical criterion, of course, is that the film former does not destroy the metal. I have disclosed suitable film forming materials in my earlier Patents Nos. 2,698,808; 2,640,002; Re 23,861; 3,132,043; and 3,023,127. There are virtually hundreds of materials such as those disclosed therein which have the physical property of filming a clean metal surface and interposing a physical barrier between the metal surface and its environment. The ability to film the metal is apparently enhanced in all cases Where the metal is clean and the flux aids in this regard. There is, of course, some competition between the flux and film former but this is minimized because when the metal surface is not clean, the affinity between the filmer and the metal is at a minimum and the effectiveness of the flux cleaner is maximum. As soon as the metal surface is clean, the film formers aflinity therefore is increased and the film formed serves not only to insulate the clean metal from the aqueous media, but from unwanted further action by the polishing agent. The amount of polishing agent necessary to film the metal will, of course, vary on the amount and size of the metal. In general, however, a minimum amount of from about /2 to 1% by weight of the metal is believed to be necessary for coating fine dusts having a high surface/ volume relationship. Any amount of film former substantially above the amount necessary to film the metal, is operable and the maximum is largely a matter of economy. In the usual case, therefore, a generous excess is used over the theoretical minimumwhich would be the amount necessary to form a monomolecular film over the entire surface area of the product to ensure that a suitable protective film is formed.

Various oily materials, such as mineral oils, oily hydrocarbons, oily organic acids, amines and amides, quaternary ammonium compounds, silicone oils and others may be used. The organic acids are particularly attractive in that, in addition to satisfying the requirements of the film former, they may also qualify as a flux, thus obviating the need for the use of a separate flux material. Long chain fatty acids, both saturated and unsaturated, are particularly suitable in this respect as well as several various derivatives thereof. Fatty acids having at least six carbon atoms are suitable and of these, isostearic acid is excellent. The alkyl esters of these acids, particularly the lower alkyl esters thereof, such as butyl oleate and butyl stearate are suitable. Additionally, fatty acid esters of lower acids, such as butyl acetate, are useful. Similarly, fatty amines and amides of C and higher acids are effective. The organic oils, such as linseed oil, tung oil, palm oil, safllower oil, corn oil, etc., which are glyceryl esters of the higher fatty acids are also suitable. In addition, oily quaternary ammonium compounds, such as alkyl methyl ammonium chloride, are suitable as are oily aromatic materials such as safrole. Still another useful class of materials includes oily synthetic resins such as polyvinyl acetate in a finely dispersed state. In short, the lubricants function is apparently entirely physical and any material capable of exhausting itself on the surface of the metal, either with or without the aid of a surfactant, is apparently suitable. In addition to this physical function, the oil may act as a flux and it is to be understood that, as used herein the term flux includes a film former which acts additionally as a flux.

Where the metals to be flattened are prone to adhere to one another, a weldment inhibitor is used in the aqueous medium to prevent adhesion. The inhibitor which prevents the adhesion or weldment of flakes to one another or to the impact media or other surfaces may be any of several materials which are capable of exhausting themselves into a tough film on the metal. It is to be noted that whereas base metals having low recrystallization temperatures like zinc, cadmium, lead, and tin have a very pronounced tendency to cold weld, with noble metals such as gold and silver, and with metals which behave nobly such as titanium and stainless steel, the tendency is much less pronounced. However, the inhibitors do reduce the tendency of the flakes of all of the metals to cold weld.

There are various materials which form protective conversion coatings by reaction with the surface of the metal. Hexavalent chromium, for example, functions in this manner to produce a zinc chromate finish on zinc particles. Chrome salts, however, attack zin and other metals and are thus not suitable as inhibitors in view of the fine size of the metal flakes. Antimony trichloride and other materials which similarly attack the metal to form a conversion coating are therefore not suited as inhibitors in the processing of fine particles.

Inhibitors which are suitable are the various materials which are substantially inert to the base metal and are capable of forming tough tenacious films on the metal which is not partable under the process conditions. A number of suitable materials are known which possess such properties. A class of inhibitor which has been found to be suitable and which does not attack even the reactive metals such as zinc, consists of an organic material having a hydrocarbon moiety and a polar or ionizable moiety. Any of the strong nitrogenous organic basic inhibitors disclosed in U.S. Patent No. 2,217,921 are considered applicable and particularly when used in conjunction with a water soluble high molecular weight polyethylene glycol having a molecular weight of at least 15,000, as disclosed in U.S. Patent No. 3,141,780.

Other inhibitors which are suitable include aliphatic and aromatic sulfur compounds such as thiourea, o-tolylthiourea, p-tolylthiourea, phenyl thiourea, p-thiocresol, thiophenol and aromatic aldehydes and ketones such as rn-toluylaldehyde and butyrophenone.

The inhibitors are relatively powerful and as little as A% or even less by weight of the metal powder may be suitable. In some cases, as for example in the case of a noble metal, the inhibitor can be omitted altogether and the film former relied upon to prevent weldment. Where the metal tends to weld, however, some inhibitor is required. While a little of the inhibitor goes a long way, excess amounts seem to do no harm and, accordingly, amounts substantially beyond the bare minimum necessary are conveniently employed to ensure that the end result is obtained.

If a reactive metal flake is placed in hostile environment which will attack it, the flake must be protected by application of a tenacious and continuous film which will serve as a barrier between the flake and the environment. These films fall into two classifications: partable films and non-partable films. Whether a film is partable or not will depend to some extent on the disruptive forces available for flattening and polishing the flake. Such films are essential for the production of brightly polished metal flakes having smooth surfaces. If the flakes can cold-weld together and the film is non-partable or creates a surface hostile to the cold-welding process, cold-welding does not take place. It is a function of the inhibitor to prevent such cold-welding and to prevent the cold-welding of particle to particle or particle to impact material. In the case of those metal powders which do not exhibit the tendency to plate or cold-Weld together, the requirement for the inhibitor is minimized but the requirement for the partable film remains as this lubricates and assists in the polishing and flaking operation.

In order to make flattened and polished metal flakes, the metal powder is subjected to mechanical action in the presence of the aqueous media which contains the oil and flux and optional weldment inhibitor. This is conveniently achieved by subjecting the particles to the mechanical action of impacting media, such as metal shot, in a ball mill of the type mentioned in my aforementioned patents.

The metals which can be polished and flattened according to the invention include zinc and metals below zinc in the electromotive series such as brass, bronze, aluminum bronze, etc. and exclude aluminum and other metals above zinc in the series. The metals which can be flattened according to the invention fall into two classesthose which tend to stick or weld together and those which do not. The former class includes metals having low recrystallization temperatures such as zinc, tin, cadmium and lead which have recrystallization temperatures of about room temperature or below or from about 50 F. to F. The latter class includes metals having a high recrystallization temperature such as stainless steel, copper, copper alloys such as brass, bronze and aluminum bronze which have recrystallization temperatures above room or over about 100 F.

The principal use for the flattened and polished particles according to the invention is in decorative and protective paints. Accordingly, the final size and size distribution of the materials is usually significant and these factors, in turn, are largely dependent on the size of the starting material. With metals which do not tend to stick, the size of the starting material is not too important because a grinding aid can be employed during the process so that larger pieces formed by flattening are reduced to a size suitable for paints. With sticky materials, however, it is preferable to start with fine materials because even under the best of circumstances there is some weldment and the use of a smaller size starting material will result in the recovery of a larger yield of particles of a suitable small size.

The impact media should be dense in order to flatten the dust into flake form in a relatively short time. For this reason, metal impactors are preferred. Suitable impactors are spheres such as iron shot and the like. The spheres are massive with respect to the dust and may be conveniently A3" or less in diameter and larger. Commercial scale runs have been effected with iron shot of from A" to /1" diameter.

The amount of impact media can be varied widely and this variation can be used to effectuate certain results with respect to the configuration of the end product.

For example, if one hundred pounds of Zinc dust, together with all the necessary chemicals for the process, were tumbled with one hundred pounds of iron shot of approximately Ms diameter in a tumbling barrel, the iron shot would be more or less imbedded in zinc dust, and its freedom of movement would be retarded by the mass of this dust, which would act to cushion the blows required for flaking. The flaking operation would proceed slowly and the flakes would tend to be only lightly flattened, resulting in a more cubical type of flake. To reduce the whole hundred pounds of zinc dust to a bright, polished cubical type of flake might require, let us say, 6 days or 144 hours, assuming this were done in a small barrel of say 6" diameter. If the operation were done in a barrel of say 4 feet diameter, the time would be reduced because of the greater pressures, the higher temperatures generated, and the longer length of the slide or active zone of the barrel.

If, however, instead of one hundred pounds of zinc dust, one pound of zinc dust was selected and used with the same one hundred pounds of iron shot, all the other conditions remaining exactly the same, the flaking and polishing operation would proceed very much faster so that a very highly polished, flat flake might be produced in say 1.44 hours of tumbling time. At the end of 144 hours of tumbling time, the production would be (neglecting losses and downtime of the barrel) one hundred pounds of finished flake in each instance.

There would be a substantial difference in the characteristics of the flake produced by these two procedures. The one pound charges would result in a very flat, very highly polished, but very coarse flake. This coarseness or enlargement of the flake would be the result of cold welding of one flake on to another and continuing the process until dozens or hundreds of flakes would be cold welded and cohered into a single flake, which would constitute a united mass of metal which could under no circumstances be reduced to its original components. In the other case, there would be only slight cold welding due to the reduced severity and frequency of impact, and the flake would be much closer to the original fineness of the zinc dust. The flakes would be less flattened and less bright, and there would be a greater diversity of particle size which is highly desirable in a paint pigment. In the case of the one pound of zinc loading, the internal friction of the revolving charge will be quite low, and the round steel balls will roll on one another almost as if no zinc were present. In a major percentage of the impacts there may be only a single flake between two rubbing or colliding steel balls. Under these circumstances, the probability of the flake cold welding to the surface of one or the other of the steel 8 balls is inordinately high. Once cold welded to the ball it becomes a part of it and cannot be removed, so that the flake is lost to the operation.

Exactly the same mechanism may explain the enlargement of individual flakes. If the flake does not cold weld to the impacting ball, it will be flattened and rolled out into a very thin flake of maximum surface area. If two or more particles are caught in an impact they may be cold welded one to another to produce a larger flake or alternatively, a flake previously flattened may at a later impact be caught with a round, unflattened particle of zinc dust which will be rolled out and cold welded onto the surface of the previously flattened flake.

The inhibitor is added to the charge to prevent cold welding. Obviously, much greater demands are made on the inhibitor in the case of the one pound zinc loading than would be the case in the one hundred pound zinc loading. Therefore, as a practical matter, the quantity of inhibitor in the case of the low zinc loading would have to be greatly increased if exorbitant losses are to be prevented. Simultaneously with the increase in inhibitor, the flux would be reduced because this reagent promotes cold welding. The amount of filming agent or lubricant which helps to promote luster and polish, could be reduced because there would be less necessity for it in the case of the low loading of zinc. At the same time, grinding agent could advantageously be increased to promote the breakdown (i.e. fragmentation) of enlarged particles.

The above description will serve to explain the interrelationship of metal particle, impact media, load and the formulation of the flaking charge. Quite obviously, the highly enhanced requirement for labor, and expensive chemicals, like inhibitor, to say nothing of the downtime of the barrel after each pound loading, would be entirely impractical from a commercial point of view, but not necessarily from a technical point of view.

It is important to note that all of the above remarks apply to those metal powders that will cold weld together under the mill conditions prevailing. For metal powders that do not weld together, then the reverse is true and low loading of metal powder to impact medium will result in the production of ultrafine flakes of very high quality and polish. For this reason, entirely different standards are applicable, and ratios of say brass powder to impact material that would be entirely satisfactory for brass paint pigment, would be entirely unsatisfactory for zinc pigment formulation.

In any event, it can generally be said that for making paint-size particles, that is, a product which is at least minus one hundred mesh and at least 50% minus 200 mesh, the amount of impact media, preferably in the form of at least substantially spherical shot based on the amount of dust to be flattened, may be as follows:

For metals that tend to stick or weld, a weight ratio of from 10/ l to 2/1 while a ratio of from 20/1 to 0.5/1 is suitable. For metals which do not tend to weld, ratios of from 20 to 1 to 2.5/1 are preferred and ratios of from 35/1 to 1/1 are suitable.

Other materials which may be added in generally minor proportions, and all of which are optional, include defoamers, buffering compounds to maintain acid pH during the process; grinding aids to reduce the size of flakes produced; and detergents which are usefuf in distributing the film former on the metal particle.

The grinding agent is not essential in the environment but it is generally helpful when present in minor amounts as it facilitates reduction of the particle size and aids in producing a finer pigment. To be very effective in paints, the flake size should not exceed 325 mesh and should, preferably, be much finer. Having regard to the geometry of the flake, which is practically all surface, even an extremely small round particle can be converted into a flake which has substantially more surface area and consequently will not pass through a screen which the undistorted zinc particle can go through readily. This means that a grinding agent which facilitates the breakdown of flake and leads to a mixture of flake sizes aids in producing a paint of good brushability. There are a considerable number of grinding agents on the market which are used in the grinding of paint pigments and which are suitable for use in this invention. Examples of preferred form grinding agents are silicon carbide, cutting sand, alumina, and the like.

The type and character of the detergent or surface active agent is not particularly critical. A wetting surface active agent such as Tergitol from Union Carbide is suitable. Cationic and non-ionic surfactants are preferred. It is preferred to operate at an acidic pH.

The invention is further described in the examples which follow which constitute preferred embodiments thereof:

The buffering compound contains one part by weight of amine to 2% parts by weight of the acid in the mix. Tenlo 70 is an oil soluble non ionic surfactant. To approximately 1600 grams of the aqueous media were added about 10 pounds of /g" O.D. zinc-coated iron shot. The zinc coating is from previous runs. The uncoated shot is available commercially for shot blasting as No. 1110. About 3 pounds of fine zinc dust (having an average particle size of about 1-3 microns) was added to the media and the total added to a hexagonal rubber lined barrel 6 across the slats and approximately 8" high, and having a capacity of about quarts. The mill was closed and run at 67 r.p.m. for 48 hours. On opening the barrel, the zinc powder had become converted to a very fine, highly polished zinc flake. There was no gas pressure on opening the barrel. The zinc flake was washed in water by several decantations, the flake being settled between each decantation. The total zinc charged was 1590 grams. The total minus 200 mesh material recovered was 1533 grams; 1115200 mesh flakes weight 19 grams. Total useable flake recovered was 97.8% There Was no gas pressure on opening the barrel. There was no build up of zinc on the iron shot, or on the sides of the barrel. There was little or no cold welding of one flake to another.

-It is believed that the toluidine base corrosion inhibitor and filming agent provides tough non-fracturing films which together with the isostearic acid, combine in shielding the metal flake from the water environment, and thereby prevent the plating of one clean metal flake on another and the plating of the flake on the impact medium. The total recovery of 98.7% would be greatly reduced by deposition of zinc on the steel impact media were it not for the tough protective films.

Example H A final flake with still better total recovery was made by adding one pound of minus 24 mesh of silicon carbide to Example I. The effect of the silicon carbide, which is very abrasive, increased the percentage of minus 325 mesh in the minus 200 mesh material, but there was a slight reduction in the luster of the resulting flake.

Example III Example I was repeated using three pounds of very fine zinc powder, but with the addition of 10 grams of finely ground jasperine mica. This mica was stamped onto the flake. On opening the barrel a very beautiful flake was obtained which decanted and filtered very fast. There was no gas on opening the barrel and the recovery was excellent.

Example IV Example III was repeated, but with 25 grams of mica added. The product did not settle quite as well as previous runs, and was slower filtering. The recovered flake, however, was excellent. There was no gassing on opening the barrel.

Example V Example I was repeated, but with the addition of 25 grams of lead stearate as a film former. The purpose of this was to further the film action on the flakes to further protect them from the aqueous environment and to produce a final flake of top quality. All other conditions remain the same as in Example I. There was no gas on opening the barrel. The flake recovered was very pretty, settled very well, filtered fast, and was fine in particle size. The recovery was excellent.

about 4500 grams of No. 1110 impacting media. About 1400 grams of fine copper powder was added to the media and treated for 48 hours as in Example I.

There was no gassing on opening the barrel. All of the flakes were extremely fine, too fine for ordinary mesh analysis. Recovery appeared to be virtually The material was easily washed and filtered fast, and no difficulty was experienced in drying.

Example VII Three pounds of minus 200 mesh 70/30 brass was substituted for the copper powder in the copper flaking example above. All other conditions remain the same. The resulting brass powder was very bright, very fine and filtered readily.

Example VIH Type 316 stainless steel powder minus 325 mesh, obtained from Vanadium-Alloy Steel of Latrobe,

Pa. pounds 1 No. 1110 iron shot do 10 Tenlo 70 grinding compound cc 30 Isostearic acid 30 Polyoxyethylene inhibitor Buifering compound cc 100 Silicone defoamer drops 5 Finely ground jasperine mica "grams 25 The flaking and polishing operation lasted 48 hours. There was no gassing on opening. The stainless steel flake was pretty and well flaked. It will be noted that no corrosion inhibitor compound was used in this test because the stainless steel has no tendency for the flakes to plate on themselves or on the impact material.

As has been mentioned heretofore, particles produced according to the invention are particularly useful in decorative and protective paints. For this use the particles are quite small and are generally at least about 75% minus 100 mesh and at least about 50% minus 200 mesh. Particles of metals with low recrystallization temperatures of from 50 to 100 F. in this size range, are considered to be new and to constitute one embodiment of the present invention. The following examples demonstrate the use of the invention to make such metal-rich paints.

Example IX In this and the following examples a rubber lined octagon shaped tumbling barrel 3.0 inches in diameter Pounds Water 412 Tenlo 70 surfactant 4.27

Citric acid-polyoxyethylene oxide amide buffering flux 10.0 Silicone antifoamer 0.8 Toluidine based inhibitor 9.4

Mesh: Percent 200 99.3 250 97.8 -325 72.1 -400 60.3

Example X The following charge was added to the tumbling barrel:

Pounds Tenlo 70 8.61

Buffering flux of citric acid, and polyoxyethylene oxide amide 14.98 Silicon antifoamer 1.61 Toluidine based inhibitor 9.45 Water 346.00 Jasperine mica 20.72 No. 1110 iron shot 1500.00 Minus 325 mesh AA zinc dust 700.00

The charge was tumbled for 25 hours. The flake produced was bright and had the following analysis:

Mesh: Percent l 100 200 80 -325 40 Example XI The formulation of Example IX was followed except Mesh: Percent 200 99.4 -250 98.8 325 68.5 --400 45.9

Although the above examples specify the use of a toluidine-based weldment inhibitor, other weldment inhibitors such as those mentioned herein can be similarly employed to form tough, non-partable films on the metal flakes.

The use of metal particles produced according to the invention in metal rich paints has been demonstrated with the zinc flakes produced by Examples IV and XI. The flakes were washed and dried and formulated into paint. The vehicle is the ester reaction product of Araldite 6084 (Ciba) and dehydrated Castor Oil Fatty Acids, all as fully described on page 2 of Ciba Technical Bulletin Epoxy Resins, Araldite 6084. This vehicle was found to give best results with standard zinc dust paints and was thus selected for comparison purposes. Five formulations of the zinc flakes of Examples IX adn XI were made up such that the dried paint film contained 72%, 84%, 88% and 92% of zinc metal. The paint was applied in one coat on mild carbon steel panels. One half of the painted steel panel was overcoated with an epoxypolyamide overcoat paint meeting the requirements of military specification MIL-C- 22750A (Wep). Both halves of the panel were diagonally scored with a blunt wide scribe making a groove through the zinc coating to the underlying steel base. These panels were then exposed in duplicate in a standard salt fog cabinet where the performance was compared with coatings of a commercially available zinc-rich epoxy paint and similar formulations made of high grade fine zinc dust that was found to give excellent results in previous tests in salt spray. The comparative tests were continued for 1000 hours during which the panels were examined at regular time intervals. Standard tests for zinc rich paints for the automotive industry are continued for only 500 hours. The test results are tabulated below. In the table, 10 is a perfect score indicating rust free. A score of 0 indicates 100% rust. In this regard it should be noted that once the score is zero, further rusting proceeds at such a rate as to not be analogous or comparable to the rusting rate from score 10 to score 0. For this reason, several panels scoring 0 were not tested further.

FLAKES RUSTING AT VARIOUS TIME INTERVALS 600 hours 750 hours 1000 hours One coat Over coat One coat Over coat One coat Over coat One coat Over coat One coat Over coat hours 410 hours Amount of zinc in paint film, percent Example IX:

8 10 5 10 l0 l0 l0 10 10 10 10 10 l0 10 10 0 0 0 0 0 0 5 5 0 10 9 9 92 1O 1O 5 Commercial, 92% 10 1O 0 H r-n- H omenoo acumen Someone that .10 pounds of finely ground white asperine mica was added and the amount of inhibitor was increased to 10.88- pounds. The charge was tumbled for 15 hours and produced bright flat flakes of lustrous polished appearance having the following Tyler screen analysis:

The tabulated information shows clearly that paint made with zinc flakes according to the present invention is far more corrosion resistant than similarly loaded commercial paint or paint similarly loaded with zinc dust. It also shows that paint made with less zinc flakes is as good as or better than other paints more heavily loaded with zinc.

There is a class of zinc pigments in which the vehicle contains chemicals such as silicates which attack the zinc and the resultant decomposition products form a cement which serves to bind the Zinc to the base. These products are all unattractive in appearance as are all zinc dust paints. The highly polished flakes of this invention do not require reaction with the vehicle to secure the necessary adherence to the substrate and integrity of the coating film.

Zinc flakes according to the invention have, of course, many uses other than in paint and the particular configuration of flakes according to the invention renders these flakes particularly advantageous and even unique Among the particularly advantageous and even unique uses of these flakes are: pipe threading greases; clean, film-free flakes for use in precipitation reactions such as the purification of zinc sulfate solutions in zinc electrowinning; and the provision of sherardized zinc coating rendered facile by the ease with which these flakes can be sherardized by heat.

What is claimed is:

1. A process of flattening and polishing finely divided metal particles comprising: providing finely divided metal particles of a metal not higher than zinc in the electromotive series in an aqueous media comprising water, at least /z% by weight, based on the weight of said metal particles, of a polishing agent comprising an oily material capable of exhausting itself in a thin film on the surface of the metal particles, at least /2% by weight, based on the weight of said metal particles, of a flux to remove metal oxides from the surface of the metal particles, and a welclment inhibitor present in an amount suflicient to prevent weldment of flakes to one another and selected from the group consisting of the reaction product of formaldehyde and o-toluidine, thiourea, Q-

tolylthiourea, p-tolylthiourea, phenyl thiourea, p-thiocresol, thiophenol, m-toluylaldehyde and butyrophenone; and subjecting the particles to mechanical flattening forces in the presence of said aqueous media to form polished flakes of said particles wtihout Weldment of said flakes to one another.

2. A process according to claim 1 wherein said flux and said polishing agent are each present in an amount of at least 1% by weight based on the weight of said metal particles.

3. A process according to claim 1 wherein said metal has a recrystallization temperature of not greater than F.

4. A process according to claim 3 wherein said weldment inhibitor comprises the reaction product of o-toluidene and formaldehyde and polyethylene glycol.

5. A process according to claim 1 wherein said metal particles are zinc particles.

6. A process according to claim 1 wherein said mechanical flattening forces are provided by tumbling said metal particles and aqueous media in the presence of metal impactor comprising spherical metal particles.

References Cited UNITED STATES PATENTS JAMES E. POER, 'Primary Examiner US. Cl. X.R. 106-291 6/1960 McAdoW 106290