US3389105A - Flake metal powders coated with fluorocarbon resin - Google Patents

Description

United States Patent 3,389,165 FLAKE METAL POWDERS COATED WITH FLUOROCARBON RESIN William T. llolger, Chathatn, N.J., assignor to Alcan Metal Powders, Inc., a corporation of Delaware N0 Drawing. Filed Mar. 5, 1965, Ser. No. 437,569 37 Claims. (Cl. 260Z3) ABSTRACT OF THE DISLOSURE Procedure for making flake metal powder, wherein finely divided metal is ground to flake form in a ball mill, stamping mill or the like, in the presence of a fluoro-- carbon resin as a grinding agent. The fluorocarbon resin may be polytetrafluoroethylene, fluorinated ethylene propylene, polyvinylidene fluoride or polychlorotrifluoroethylene, and may be used either alone, as the sole grinding This invention relates to flake-type metal powders and to methods of making such powders.

Metal powders in the form of minute flakes are widely used as metallic pigments in inks, paints and the like, i.e. in dispersion in a suitable film-forming vehicle to provide a protective or decorative surface-coatlng composition. Examples of such flake metal powders are the socalled gold bronze powders (the term gold bronze being employed to designate flake metal powders either of copper or of brass containing e.g. up to about zinc) and aluminum pigments.

These powders are commonly made by grinding finely divided metal, such as foil scrap or atomized powder, in a ball mill, stamping mill or the like which hammers the metal particles into the desired flake form. This grinding may be done either dry, in air or other gas, or wet in some liquid such as mineral spirits. For the preparation of gold bronze powders, dry milling is ordinarily preferred; in such operation, the finely divided metal can be blown into the mill and the produced flake powder blown out in dry form, facilitating handling and size classiiication of the product. On the other hand, wet milling is preferred for the preparation of flake aluminum powder, owing to the fact that aluminum powder (unlike gold bronze) may explode during dry grinding unless extreme caution is used.

In both wet and dry grinding operations, it is necessary to perform the grinding or milling step in the presence of a small amount of a grinding agent. Various oils, fats and the like, such as olive oil, tallow and lard oil, have been proposed for this purpose; in present-day commercial practice, fatty acids (for example stearic acid and oleic acid) are very widely employed as grinding agents, although a number of other materials are also used, e.g. zinc stearate and other derivatives and mixtures of fatty acids. The grinding agent acts to protect the metal particles during grinding, so that they are flattened into the desired flake form rather than merely being broken up, and to prevent cold welding of the particles. In addition, the grinding agent coats the particles with a thin film; this coating (which remains on the particles after grinding and is presently believed to be chemically attached to the flake surfaces, at least in part) serves to protect the flakes from corrosion or other deleterious chemical attack "ice and thereby aids in preserving the brightness or luster of the flake pigment.

The grinding agent may also have the effect of giving the flakes so-called leafing properties. In this connection it may be explained that leafing is a property whereby in a paint or ink vehicle or the like, the flakes float to the surface of the vehicle and tend to lie in fiat, parallel or overlapping relation at the surface, thus forming a more reflective and impervious coating than nonleafing flakes which are disposed in random attitudes when dispersed in a vehicle. Accordingly, leafing powders are employed when a highly metallic appearance is desired for a paint or ink, while ncn-leafing powders are used when it is desired to impart metallic luster without bare .tetailic appearance to a surface coating. Stearic acid is conventionally employed as a grinding agent to produce leafing flakes, particularly in the production of gold bronze, where leafing properties are usually desired; oleic acid is commonly used as a grinding agent to produce a nonleaflng flake product e.g. in the manufacture of aluminum flake pigments for use in automobile finishes and the like. Leafing properties, it may be further explained, are believed to be attributable to the surface coating imparted to the flakes by the grinding agent and in particular to surface tension effects produced by the: coating.

A diificulty heretofore encountered in the production of flake metal powders, and especially in the production of gold bronze using a grinding agent such as stearic acid, is that the product tends to be overground, i.e. to contain an excessive amount of super-fines; these fines tend to darken the product and give it a dirty appearance. That is to say, agents such as stearic acid appear to have a preferential effect on fine particles in the feed of divided metal, so that there is a strong tendency to produce in the mill a mixture of coarse and fine flakes which is difiicult to separate into desired grades of relatively uniformly sized flakes; by the time most of the particles are sufficiently thinned out, those that were first struck and flaked have been over-ground or broken down to a size too small to reflect light, and therefore appear dark, dulling the luster of the produced flake pigment. This tendency has heretofore been counteracted by using extreme care in the addition of the grinding agent and by attempting to continuously classify and remove the finer flakes as they are produced. Such methods, however, are necessarily expensive and only partly successful.

While the reason for this production of superfines is not fully understood, it is presently believed that conventional griding agents such as stearic acid, after adsorbing on the surface of the divided metal in the mill, may tend to migrate into micro-cracks or to affect crystal defects and thus have a disruptive effect close to the surface of the metal particles. Such near-surface effect would, of course, have greater effect on smaller particles.

An object of the present invention is to provide new and improved procedures for making flake metal powders. Another object is to provide such procedures wherein production of excessive lines is avoided, and a product of advantageously superior brilliance thereby achieved, in a facile and convenient manner not requiring close or critical control of operating conditions. A further object is to provide such procedures, which can be used to produce either leafing or non-leafing flakes, and suitable for either Wet or dry milling operation. Yet another object is to provide procedures for making gold bronze pigments having improved properties especially with respect to brightness and freedom from fines. A still further object is to provide procedures for making flake aluminum pigments of improved characteristics. An additional object is to provide new and improved flake metal pigments.

To these and other ends, the present invention in a broad sense contemplates the use of fluorocarbon resins as grinding agents in the manufacture of flake metal powders. The term fluorocarbon resins (see Modern Plastics, Encyclopedia issue for 1965, vol. 42 No. 1A, pp. 119-120) as used herein refers to fluorinated, so-called linear polyolefins, including polytetrafluoroethylene, fluorinated ethylene propylene (tetrafluoroethylene-hexafluoropropylene copolymer), polyvinylidene fluoride, and polychlorotrifluoroethylene. It is found that these resins constitute very effective grinding agents for such purpose; i.e. when 'used in place of or in addition to a conventional grinding agent such as stearic acid or oleic acid in grinding finely divided metal as in a ball mill or stamping mill, these resins act to protect the particles so that they are reduced to the desired flake form, and are also understood, from evidence of effects produced, to provide a surface coating on the flakes which coating is thus understood to afford the effective or indeed superior protection against corrosion that has been observed with the product. References hereinbelow to the surface coating of the product of the present invention will be understood in this context.

Moreover, leafing flake powders can be made with use of these fluorocarbon resins as grinding agents. In the case of gold bronze, the fluorocarbon resin grinding agents are themselves capable of imparting good leafing properties to the produced flakes, i.e. when used alone; in other instances, as in the production of leafing aluminum pigments, the fluorocarbon resin grinding agents may be used in conjunction with a leaf-producing grinding agent such as stearic acid without interfering with the function of such leaf-producing agent. At the same time, a non-leafing aluminum pigment (i.e. a pigment having no leaf, as demonstrated by inability to be supported on the surface of xylene at room temperature, this being a desirable property for aluminum pigments for automobile finishes and the like), having superior corrosion resistance, can be produced by wet-milling aluminum with a fluorocarbon resin used alone as a grinding agent.

It is further particularly found that the use of fluorocarbon resins as grinding agents affords very significant advantages with respect to freedom of the flake product from fines. That is to say, by grinding in the presence of a fluorocarbon resin, an advantageously brilliant flake product (free of excessive fines which would darken or dirty the product) is very readily achieved, without close or critical control of operating conditions (e.g. time of milling) as has heretofore been necessary to minimize the occurrence of fines in flake metal pigments such as gold bronze produced e.g. with stearic acid. This flake product obtained with the present invention is more uniform in paiticle size than that produced by previously proposed methods, as with stearic acid, and can more readily be separated into desired grades of relatively uniformly sized flakes. Moreover, especially in the case of gold bronze, this product exhibits improved flowing properties, as when dispersed in an ink vehicle for printing or other such application.

Broadly, then, the method of the invention comprises grinding finely divided metal to flake form in the presence of a fluorocarbon resin or resins as a grinding agent. The grinding procedure may be performed in a ball mill, stamping mill or the like, i.e. in equipment as presently used for grinding flake metal powders, operated in conventional manner and with conventional steps of supplying feed of divided metal and grinding agent to the mill and withdrawing flake product therefrom. In other words, except for the use of the fluorocarbon resin as part or all of the grinding agent, the grinding operation and desired preliminary and subsequent treatments may be carried out in essentially the same manner, and with the same techniques and equipment, as in methods heretofore employed for making metal flakes using conventional grinding agents, such operation and equipment being well known to those skilled in the art.

In the method of the invention in this broad sense,

the feed of finely-divided metal may be any metal which it is desired to reduce to flake form. The fluorocarbon resin or resins employed may be used either alone or in mixture with other grinding agent or agents, e.g. stearic acid or oleic acid, it being understood that the term grinding agent as used herein broadly refers to any material having the property of protecting finely-divided metal during milling so that the particles are reduced to flake form, and suitable for use in grinding flake metal powders. More particularly, in the preferred practice of the invention the grinding agent used is one of the following fluorocarbon resins: polytetrafluoroethylene, fluorinated ethylene propylene (tetrafluoroethylene-hexafluoropropylene copolymer), polyvinylidene fluoride, and polychlorotrifluoroethylene. Whereas all these resins have been found to be effective as grinding agents, materials found to afford especially good results are the perfluorinated linear polyolefins, i.e. including polytetrafluoroethylene and fluorinated ethylene propylene; of these, it is presently particularly preferred to use polytetrafluoroethylene, the latter resin being readily commercially available at relatively low cost and, further, being highly effective in providing the advantages of the invention. Very preferably, also, the fluorocarbon resin used is supplied to the mill in finely divided form in a minor proportion based on the feed of divided metal to the mill. Thus, for example, the fluorocarbon resin may conveniently be used in dry powder form; alternatively, the fluorocarbon resin may be supplied to the mill in finely-divided form in a suspension, as with an agent or carrier appropriate for the selected type of milling.

The method of the invention embraces both dry-milling and wet-milling operation; i.e. the feed of metal particles /ith the fluorocarbon resin may be ground either dry, or wet in the presence of a liquid wet-milling vehicle (commonly an organic liquid) such as mineral spirits. Suitable liquid wet-milling vehicles are those convention ally employed for wet-milling flake metal powders, such vehicles being well-known in the art. The proportion of grinding agent used, while not critical, may vary in accordance with the milling conditions employed. Thus in dry-milling or flake metal powder, a convenient range of proportions for the supplied fluorocarbon grinding agent (based on the feed of divided metal) is between about and about 4% by weight, a presently preferred range being from about /2 to about 2%; in wet milling, a somewhat larger proportion of fluorocarbon resin may be used, e.g. about 1% up to about 10% by weight (preferably about 2% to about 6%), since much of the grinding agent remains in suspension in the liquid used in the wet milling.

The product of the described method is a flake metal powder having a surface coating of the fluorocarbon resin used as grinding agent. This product may be either leafing or nonleafing in character, and may be subjected to further finishing operations if desired; for example, it may be polished, in accordance with procedures heretofore known, to improve its brilliance and leafing properties.

While, as stated, in a broad sense the method of the invention is applicable generally to the preparation of flake metal powders, it is found to be especially advanta geous for the manufacture of gold bronze powders, and accordingly the latter application constitutes an important specific aspect of the invention. In this aspect, the invention contemplates grinding to flake form a finely divided gold bronze feed (copper, or brass-i.e. copper-zinc alloy-containing e.g. up to about 30% zinc), in the presence of a fluorocarbon resin grinding agent, as described above, in finely divided form. As indicated above, this grinding operation may be performed with conventional equipment, such as a ball mill or stamping mill, preferably under dry-milling conditions to facilitate handling and size classification of the product.

Thus, in an exemplary instance of such operation, incorporating presently preferred features, a feed of metal powder of an appropriate alloy to make gold bronze pigment, of for example minus 100 mesh (Tyler scale) particle size, is continuously blown into a ball mill, together with polytetrafluoroethylene powder in a proportion conveniently ranging between about /a% and about 2% by weight (eg, a proportion of about 1%) based on the weight of metal to be flaked. As the mill is continuously rotated to grind the feed of metal to flake form, flaked powder is continuously blown out of the mill and classified as to size in any appropriate manner, e.g. with a conventional screen or air classifier. The desired size grade of flakes (e.g. flakes of -325 mesh particle size) is removed as the product, and the larger flakes are returned tothe mill for further grinding.

The gold bronze flake pigment thereby produced is found to be of superior brilliance, i.e. containing an advantageously lower proportion of fines than gold bronze made for example with stearic acid as a grinding agent; of more uniform size than heretofore obtainable; and more readily classifiable into uniform size grades. In addition, with the present method a -gold-bronze product can be achieved having a significantly higher apparent density than the stearic acid product. That is to say, whereas gold bronze ground with stearic acid in a ball mill may have an apparent density of about 12 to 14 g./in. or up to about 16 g./in. or above when ground in a stamping mill, the product of the present method may have an apparent density ringing up to about 24 g./in. Because of this greater apparent density, the gold bronze flake produced by the present method exhibits desirably superior flowing properties when dispersed in an ink or like vehicle; that is to say, this pigment is more free-flowing and hence more readily transferred (in ink) from printing rolls to paper than the gold bronze products made with stearic acid. It is further found that with the described operation there can be produced a flake having good leafing properties.

These desirable properties are achieved by the present method without the necessity for such close or critical control of operating conditions as has heretofore been necessary in making gold bronze, e.g. with stearic acid to avoid excessive fines in the product. Further, the invention provides these advantages even when the feed is socalled rich bronze feed (containing above about 25% zinc) which has heretofore been particularly difficult to grind to flake form without production of excessive fines.

The product of the above-described example of operation is a gold-bronze flake having thereon a surface coating of polytetrafluoroethylene, effective to provide leafing in typical surface-coating vehicles.

To enhance the leafing properties as well as the brilliance of the flakes, the gold bronze product of the present method is preferably polished after grinding, as in accordance with present conventional practice for treatment of gold bronze pigments ground by methods heretofore known. Such polishing may be performed in a so-called brush polisher or other device wherein the flakes are gently rubbed so that when made into a coating wi:h an ink or paint vehicle they form a smoother and more brilliant sur-face. During this polishing operation, stearic acid or other polishing agents such as are now used may be added. Use of such conventional polishing agent (e.g. stearic acid) in the polishing step is found to improve the properties of the product, and in particular to increase the water coverage of the flake product (i.e. the area of water covered with a layer one flake thick, per unit weight of flake product). However, where a film of polishing agent is not desired on the product flake, the latter agent may be omitted in the polishing step.

Although the foregoing procedure as applied to the manufacture of gold bronze pigments has been described above with reference to the use of polytetrafluoroethylene alone as a grinding agent, it will be appreciated that other fluorocarbon resins, eg those named above, may be used in place of polytetrafluoroethylene, and similarly, that the latter or other fluorocarbon resin may be used in mixture with a conventional grinding agent such as stearic acid, in each case with the advantages set forth above. Similarly, while ball-milling operation has been described, the grinding operation may be performed in a stamping mill or other grinding equipment suitable for reducing finely divided metal to flake form.

In a further specific aspect, the invention is concerned with the production of flake aluminum pigment, and embraces the grinding of finely divided aluminum (e.g. foil scrap) to flake form in the presence of a fluorocarbon resin as described above, as grinding agent, preferably under wet-milling conditions (viz. in a liquid wet-milling vehicle such as mineral spirits or hydrocarbon solvent). It may be explained in this connection that in the con ventional grinding of flake aluminum pigments, as with stearic acid, problems are encountered due to the accumulation of dark metal superfines and a contamination (probably aluminum stearate) which together make the product darker and less leafing; to meet these problems it has been necessary either to perform an expensive distillation or other purifying treatment of the mineral spirits, or to completely discard and replace the mineral spirits at frequent intervals. Use of a fluorocarbon resin as grinding agent in accordance with the present invention is expected to eliminate or at least very materially reduce these problems, since comparatively little superfine material is produced to react with the solvent and grinding agent, and the fluorocarbon grinding agent is far less reactive than the fatty acids heretofore used.

In this regard it may be noted that grinding agents such as stearic acid heretofore used in wet-milling aluminum flake pigments dissolve in the hydrocarbon solvent used as the wet-milling vehicle; ie the vehicle acts as a solvent for the stearic acid, although in some cases not all the stearic acid may dissolve. The present fluorocarbon resin grinding agents (e.g., polytetrafluoroethylene), however, do not dissolve in the hydrocarbon solvent wet-milling vehicle, but instead remain in suspension therein, and hence the vehicle does not act as a solvent but merely serves as a carrier for the fluorocarbon resin.

The grinding of aluminum flakes by the method of the present invention may be carried out in conventional equipment and in accordance with conventional procedures for wet-milling aluminum flakes, except that a fluorocarbon resin (for example polytetrafluoroethylene) is used in place of or in addition to the grinding agents heretofore employed. The product thereby obtained is a flake aluminum pigment having a coating of the fluorocarbon resin used, e.g. a coating of polytetrafluoroethylene. This product is found to have superior resistance to corrosion, such as attack by hydrochloric acid, apparently due to the protective effect of the present coating. Either a leafing or non-leafing product may be obtained by varying process conditions; for example, a. leafing product can be made by use of supplemental grinding agents such as stearic acid in conjunction with a fluorocarbon resin or a non-leafing product can be made by omitting such agents and using the fluorocarbon resin alone as grinding agent.

Other examples of metals made in flake form and to which the present invention is applicable include gold, iron, stainless steel, nickel, tin, chromium, lead, bismuth, and various alloys of these metals.

Further features and advantages of the invention will be apparent from the following specific examples of production of flake metal powders in accordance with the present method. In these examples, percentage values (where given) of grinding agents used in milling metal powder feeds refer to the weight of grinding agent supplied to the mill, expressed as a percent of the weight of the powdered metal feed.

EXAMPLE I In a series of runs, gold bronze feed was dry-milled to flake form with polytetrafluoroethylene (TFE) powder in a steel jar 6 inches in diameter and 3 inches deep, having 3 lifter bars Mi inch square, and about half-filled with 2 to 3 kg. of polished steel balls A: to inch in diameter, by rotating the jar (about a horizontal axis) on a laboratory jar roller at about 100 rpm. The charge for each run was 100 to 200 g. of -325 mesh particle size rich gold bronze feed, grade B118 (nominal content 71.0% Cu, 28.75% Zn, 0.25% Al), and the TFE used was Halon TFE, type G-SO (a fine powder). After each run the balls were separated out and the material screened with a 325 mesh screen. The 325 mesh portion of the produced flake was taken as the product, tested for leafing and water coverage and put in a lacquer coating. Leafing was measured by a leafing test as set forth in ASTM specification No. D-267-41.

In a first run, the jar containing about 2 kg. of balls inch to inch in diameter was charged with 100 g. of the rich gold bronze feed and 1.0 g. 'IFE powder and rotated at about 90 rpm. for 6 hours. The produced flakes were smooth and brilliant but very thick. A second run, made under identical conditions but with the milling time increased to 16 hours, produced 65 g. of 325 mesh gold bronze flakes which again showed high cleanliness (i.e., freedom from fines) and in addition exhibited good leafing properties.

In a third run, a charge of the gold bronze feed with 2% TFE powder was milled for 21 /2 hours and the produced -325 mesh flakes were then polished in a laboratory polisher for 3 hours. It was observed that this material was improved in each of the three hours of polishing, although gold bronze flakes made by conventional methods begin to break down and to deteriorate in general quality after about 1% hours of polishing by this laboratory polishing procedure. The polished product was found to have a water coverage of 2100 cm. g. and leaf In a further run, with 2750 g. of balls in the jar, a charge of 200 g. of the 325 mesh rich gold bronze feed initially mixed with 1% TFE powder was milled for hours, with addition of TFE powder during milling the total 5%. The product was polished for three hours and then had a water coverage of 1350 cmF/g.

Another run, made with 200 g. of the rich gold bronze feed milled with 1% TFE powder for 20 hours at 107 r.p.m., produced 325 mesh gold bronze flakes having a water coverage of 1100 cm. /g. and leaf 42%.

A further run, in which the rich gold bronze feed was milled with TFE powder for 22 hours, produced 40 g. of 325 mesh flakes having a water coverage of 980 cm. /g. and leaf 31%. This product, in a paint sprayed on a steel panel, showed good resistance to discoloration when heated for 2 hours at 340 F.

In a further test, gold bronze 325 mesh powder produced by milling the rich gold bronze feed with TFE powder in the rotating jar was polished (in conventional manner) with stearic acid for 4 /2 hours. This polished flake had a water coverage of 2400 cnL /g. and leaf 35-40%.

In still another run, the 325 mesh gold bronze feed was mixed with +325 mesh gold bronze flakes produced in a previous test and milled with TEE powder for 5 hours. 57 g. of 325 mesh flakes were produced, having a Water coverage of 1330 cmP/g. and leaf 30%.

By way of comparison with the foregoing tests, a series of runs was made in the same equipment using 100200 g.

. of the same rich gold bronze feed but with powdered stearic acid rather than TFE powder as a grinding agent. In one such run, 200 g. of the metal feed Were milled with stearic acid for 7 hours; the product was dark, and had no leaf. Another run, in which 200 g. of the metal feed were milled with /2 stearic acid for 8 hours, produced 72 /2 g. of 325 mesh flakes which again were dark, and had a Water coverage of 980 cm. /g. and no leaf. In a further run, wherein a charge of the rich gold bronze feed was milled for 6 hours with stearic acid, 76 g. of 325 mesh flakes were produced having a water coverage of 630 cm. g. and no leaf.

As a further comparison, a similar amount of the rich gold bronze feed was milled in the same equipment for 5 hours with no grinding agent. A dark, gritty powder was produced, containing no flakes.

EXAMPLE II Using the equipment and following the milling procedure set forth in Example I, 200 g. of the same rich gold bronze feed were milled for 14 /2 hours with /s% powdered stearic acid and 1% TFE powder. 87 g. of 325 mesh flakes were produced having a water coverage of 1680 cm. /g. and leaf 10%.

EXAMPLE III Again in the equipment of Example I, 200 g. of the described rich gold bronze feed were milled for 14 hours with 1%% powdered polychlorotrifluoroethylene (CTFE). The produced 325 mesh flakes had a water coverage of 700 cm. /g. and no leaf.

EXAMPLE IV 10 g. of the 325 mesh rich gold bronze feed were milled 11 hours in the same jar mill with 1% powdered fluorinated ethylene propylene (tetrafluoroethylcnehexafluoropropylene copolymer). The fluorinated ethylene propylene used was Liquinite P-190. 17 g. of 325 flakes were produced having a water coverage of 730 cmF/g. and leaf 22%.

EXAMPLE V g. of the same -325 mesh gold bronze feed were milled in the equipment of Example I for 8 hours with 1% powdered polyvinylidene fluoride (Kynar 401). 38 g. of -325 mesh flakes were produced having a water coverage of 670 cm. /g. and no leaf. The +325 mesh flake fraction of the product was returned to the jar with 50 g. of the -3?.5 mesh rich gold bronze feed and milled for another 11 /2 hours. The 325 mesh flake product of this further milling, after being polished for 2 hours, had a water coverage of 1020 crnF/g. and no leaf.

EXAMPLE VI Grinding Milling Amount of W atcr Leaf,

Agent Time (l1r.) Product (g) Coverage Percent 1 g. TFE 6% l8 1, 050 25 1g. O'IFE 6% 33 560 None EXAMPLE VII Again using the procedure and equipment of Example I, 100 g. of -325 mesh rich pale gold bronze feed (18.75% Zn, 0.25% Al, balance Cu) were milled with /2 g. TFE powder for 6 /2 hours. 7 g. of 325 mesh flakes were screened out; the remainder of the feed was milled for 5 hours more, and another 20 /2 g. of -325 mesh flakes were then screened out. The combined 28 g. of -325 mesh flakes had a water coverage of 950 crn. /g. and 22% leaf.

9 EXAMPLE vm 100 g. of 200 mesh copper powder in mixture with 97 g. of copper flakes from a previous similar run and 2 g. of TFE powder were dry-milled for 14 hours in the jar of Example I. 41 g. of 325 mesh copper flakes were produced having a water coverage of 870 cm. /g. and leaf 19%.

EXAMPLE IX A series of dry-milling runs were made in the equipment of Example I with feeds of aluminum bronze (i.e. copper-aluminum alloys) in powder form. In each run, 100 g. of aluminum bronze powder feed and 1.0 g. of TFE powder were charged to the jar, and the 325 mesh fraction of the resultant flake was taken as product. Results, for feeds of various copper-aluminum alloys, are summarized in the following table:

50 g. of silver crystals were dry-milled with 0.5 g. TFE powder in the equipment of Example I for 5 hours. At the end of this time, only a trace of -325 mesh flakes were screened out; the product included 1 /2 g. of -l mesh +325 mesh flakes and 49 g. of +100 mesh flakes.

For purposes of comparison, 50 g. of silver crystals with 0.25 g. stearic acid powder were dry-milled in the same equipment for 1 /2 hrs. At the end of that time, 6 /2 g. of 325 mesh flake-s were screened out; the remainder (+325 mesh portion) of the material was then milled for 1 hour and 20 minutes more, and from this another /2 g. of -325 mesh flakes were screened out. Only 7.4 g. of +100 mesh material was present after this second milling period. The combined portions of '325 mesh flakes had some leaf, and a water cover of 1680 cm. g.

This comparison demonstrates the absence of superfines in material milled with TFE.

EXAMPLE XI Aluminum powder was wet-milled with TEE powder and hydrocarbon solvent in a ball mill 3 feet in diameter and 1 foot long, to produce aluminum flake pigment. In each run, 11 /4 lb. aluminum powder with 4% gal. hydrocarbon solvent were charged to the mill together with the TFE powder and milled 5 hours, washed out, screened through a 325 mesh screen, filtered and dried. In the first run, 153 g. (3%) of TFE powder was used, and the -325 mesh aluminum flake product was found to have a water coverage of 12,250 cm. /g.; in the second run, 255 g. (5%) of TFE powder was used, and the -325 mesh flake powder had a water cover of 11,900 cmF/g. Each product was tested for leafing by stirring in xylene, and exhibited no leaf in xylene. The xylene test is used when a strictly non-leafing product is desired, as some flakes which show no leaf in the leafing test referred to in Example I will show traces of leaf in xylene.

The 325 mesh flake product of each run was sprayed out in an alkyd amine vehicle on a panel and tested for staining by placing on the panel 3 drops of 5 cc. hydrochloric acid in 95 cc. of water and allowing the drops to evaporate at room temperature, this being a convenient test of tendency to spot in automobile finishes. Both products showed very little HCl staining by this test, ie less HCl staining than conventional non-leafing aluminum flakes milled with oleic acid.

10 EXAMPLE XII In the equipment of Example I, 40 g. of aluminum powder were wet-milled with 2 g. of TFE powder in mineral spirits, a total of 260 cc. of mineral spirits being added in portions during the milling period of 8 hours. The jar mill was then washed out with mineral spirits and the product filtered out. The product (tested for leafing by the test set forth in ASTM specification No. D- 480-59T) had no leaf, and had a water coverage of 5,600 cm. /g.; the lacquer coating gloss was 24, and :total reflectance 62.

EXAMPLE XIII In the same equipment, 40 g. of aluminum powder were wet-milled in mineral spirits for 5 hours with 1 g. TFE powder and 1 g. stearic acid powder, cc. of mineral spirits being introduced to the jar before grinding and 10 cc. more added during the milling operation. The product flake was washed out and filtered as before. This product (tested for leafing as in Example XII) had 62% leaf, water coverage of 9800 cm. /g.; lacquer coating gloss 32, total reflectance 65; and brushed panel gloss 44, total reflectance 75.

EXAMPLE XIV In a series of runs, rich gold bronze feed (-100 mesh atomized brass powder) Was dry-milled with fine TFE powder, to produce gold bronze flakes, in a productionscale ball-mill operated under production conditions. The mill was 3 feet in diameter by 10 feet long; it contained 5,000 lb. of diameter steel balls and was rotated continuously at 36 r.p.m., with continuous feed and product removal. In operation, air circulated through the mill removed flakes; fine flakes were delivered to a product collector, and coarse flakes were classified out and returned to the mill.

At start-up, the mill was charged with 200 lb. of the atomized brass powder and 810 g. of TFE powder, and run for one hour without air circulation, in accordance with usual practice for beginning operation with raw feed and :an empty mill. Air circulation, classification and product removal were then started, while the mill was continuously rotated; operation was thereafter continued for 7 /2 hours while the mill was fed at a rate of 20 lb./hr. of brass powder and 81 g./hr. of TFE powder. A total of 93 'lb. of flake product was collected. The unfinished material was left in the mill.

Operation of the mill was resumed the following day for 11 hours, with a feed rate of about 30 lb./hr. of brass powder and 60 g./hr. of TFE powder; 309 'lb. of product flake was collected. On a subsequent day, operation was resumed for 11 /2 hours with a feed rate of 30 lb./hr. of brass powder and 90 g./hr. TFE powder; 272 1b. of product flake was collected. Thereafter on the same day) operation was again resumed for 17 /2 hours with a feed rate of 30 lb./hr. brass powder and g./l1r. TFE powder; 433 lb. of product flake was collected, and 469 lb. of unfinished material was removed from the mill for use as initial charge in a subsequent run.

In the described operation, the hourly product collection rates indicate that a steady production of between 20 and 30 lb./ hr. can be maintained using the percentages of TFE milling iagent tested. The production of extremely fine material, i.e. dust or superfines, is very much less than occurs using stearic acid as a grinding agent to make a similarly fine product.

The mill product was then polished, the greater part being brush polished in 50 lb. lots, 5 hours with no polishing agent, then 5 hours more with 50 g. stearic acid added. All the material was then blended together.

Tested properties of samples of the product of the above-described production-scale run are set forth in the following table:

Particle Size, Percent Apparent Water Lot Density Leaf, Coverage +150 Mesh 325 Mesh (g./in. Percent (cmfl/g.)

Percent First Days Run 0.2 94. 2 12.8 Trace 840 Second Days Run 0. 2 94. 2 10. 7 1, 190 Third Days Rum... Trace 94. 6 11. 3 10 1,050 Final Polished Lot Trace 98. 4 20. 8 2, 450

A small amount of the product, polished diflerently (viz by dry-polishing, i.e. without polishing agent, for 18 hours, and then further polishing for 6 hours with 1 gram of stearic acid per pound of product), had an apparent density of 23.1 g./in. leaf of 70% and water cover of 2100 cm. /g., with 95.2% of the particles being of 325 mesh size.

In all the above examples of producing gold bronze flake with a fluorocarbon resin grinding agent, viz. EXarnples I, II, III, IV, V, VI, VII, VIII and XIV, the bronze flake product was desirably clean, bright and free from fines.

In each of the foregoing examples, the grinding agent used was in fine powder form. However, in other runs wherein finely divided metal was milled with /8" pellets of fluorinated ethylene propylene and with a rather coarse powder made by grinding these pellets, good flakes were produced, although rather slowly; in these runs, the pieces of fluorinated ethylene propylene seemed to be reinforced by the particles of brass beaten into them so that they did not readily spread out and serve the purpose of a grinding agent. In general, it is preferred to use the fluorocarbon resin grinding agents in finely divided form, as such form is believed to facilitate the desired thin and even spreading of the grinding agent through the feed of finely divided metal in the mill.

It is to be understood that the invention is not limited to the procedures and embodiments hereinabove specifically set forth, but may be carried out in other ways without departure from its spirit.

I claim: a

1. A method of making flake metal powder, comprising grinding finely-divided metal to flake form in the presence of a fluorocarbon resin as a grinding agent.

2. A method according to claim I, wherein said fluorocarbon resin is a perfluorinated linear polyolefln.

3. A method according to claim 2, wherein said fluorocarbon resin is polytetrafluoroethylene.

4. A method according to claim 2, wherein said fluorocarbon resin is fiuorinated ethylene propylene.

5. A method according to claim 1, wherein said fluorocarbon resin is polyvinylidene fluoride.

6. A method according to claim 1, herein said fluorocarbon resin is polychlorotrifluoroethylene.

7. A method according to claim 1, wherein said fluorocarbon resin is in finely divided form.

3. A method according to claim I, wherein the grinding step is performed in the presence of a fluorocarbon resin and another grinding agent.

9. A method according to claim 8, wherein said fluorocarbon resin is polytetrafluoroethylenc and said other grinding agent is stearic acid.

10. A method of making flake metal powder, comprising dry-milling finelydivided metal to flake form in the presence of a fluorocarbon resin as a grinding agent.

11. A method of making flake metal powder comprising charging a feed of finely-divided metal and a fluorocarbon resin in finely divided form to a ball mill, said fluorocarbon resin being present in a proportion of between about and about 4% of the weight of said feed, and dry-milling said finely-divided metal to flake form in said ball mill in the presence of said fluorocarbon resin.

12. A method of making flake metal powder, comprising wet-milling finely-divided metal to flake form in the presence of a liquid wet-milling vehicle and a fluorocarbon resin as a grinding agent.

13. A method of making flake metal powder, comprising charging a feed of finely-divided metal and a fluorocarbon resin in finely divided form to a ball mill together with an organic liquid wet-milling vehicle, said fluorocarbon resin being present in a proportion of between about 1% and about 10% of the Weight of said feed, and wetmilling said finely-divided metal to flake form in said ball mill.

14. A method of making flake metal powder, comprising grinding finely-divided metal of which at least a major constituent is copper to flake form in the presence of a fluorocarbon resin as a grinding agent.

15. A method of making flake gold bronze powder, comprising grinding finely-divided brass to flake form in the presence of a fluorocarbon resin as a grinding agent.

16. A method of making flake gold bronze powder, comprising dry-milling finely-divided brass to flake form in the presence of a fluorocarbon resin in finely divided form as a grinding agent.

17. A method of making leafing flake gold bronze powcomprising charging a feed of finely-divided brass and polytetrafluoroethylcne in finely-divided powder form to a ball mill, said polytetrafluoroethylcne being present in a proportion of between about A and about 4% of the weight of said feed, and dry-milling said feed in said ball mill to reduce at least part of said finely-divided brass to flakes of -325 mesh particle size.

13. A method according to claim 17, further including the steps of withdrawing said 325 mesh brass flakes from said ball mill and polishing said withdrawn flakes.

19. A method according to ciaim 18, wherein said step of polishing the flakes inciudes the step of polishing the flakes in the presence of stearic acid.

A method making flake aluminum pigment, comprising grinding flneiy-divided aluminum to flake form in the presence of a fluorocarbon resin as a grinding agent.

21. A method of making flake aluminum pigments, comprising wet-milling finely-divided aluminum to flake form in the presence of a liquid wet-milling vehicle and a fluorocarbon resin, in finely-divided form, as a grinding agent.

22. A method according to claim 21, wherein said fluorocarbon resin is polytetrafluoroethylcnc.

23. A method of making leafing flake aluminum pigments, comprising wet-milling finely-divided aluminum to flake form in the presence of mineral spirits and stearic acid and polytetrafluoroethylene in finely-divided form as grinding agents.

A metal flake pigment comprising a flake metal powder raving a coating of a fluorocarbon resin, and prepared by grinding finely-divided metal to flake form in the presence of said fluorocarbon resin as a grinding agent.

25. A pigment as defined in claim 24, wherein said fluorocarbon resin is polytctrafluoroethylene.

A pigment as defined in claim 24, wherein said fluorocarbon resin is fluorinated ethyiene propylene.

2'7. A pigment as defined in claim 24, wherein said fluorocarbon resin is polyvinylidene fluoride.

28. A pigment as defined in claim 24, wherein said fluorocarbon resin is polychlorotrifluoroethylene.

29. A pigment as defined in claim 24, wherein said flake metal powder is copper powder.

3d. A pigment as defined in claim 29, wherein said fluorocarbon resin is polytetrafiuoroethyiene.

31. A pigment as defined in claim 24, wherein said flake metal powder is a copper-aluminum alloy.

32. A pigment as defined in claim 31, wherein said fluorocarbon resin is polytetrafiuoroethylene.

33. A pigment as defined in claim 24, wherein said flake metal powder is silver powder.

34. A pigment was defined in claim 24, wherein said flake metal powder is brass powder.

35. A pigment as defined in claim 34, wherein said fluoroearbon resin is polytetrafiuoroethylene.

36. A pigment as defined in claim 24, wherein said flake metal powder is aluminum powder.

37. A pigment as defined in claim 36, wherein said fluorocarbon resin is polytetrofluoroethylene.

References Cited UNITED STATES PATENTS Young 26023.4 Davis 1855 Castelli et a1. 241-16 Stephens et a1. 106-290 Brown et al. 106-290 Swenson 25258 Rolles et a1. 106-290 Wallen 26023.7 Owens et a1. 252--58 DONALD E. CZAJA, Primary Examiner.

R. A. WHITE, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,389,105 June 18 1968 William T. Bolger It is certified that error appears in the above identified patent and that'said Letters Patent are hereby corrected as shown below:

Column 2, line 49, "griding" should read grinding Column 5, llne 28, "ringing" should read ranging Column 8, line 2o, "10 g." should read 100 g. Column 11, line 51, "herein" should read wherein Column 12, line 44,

"A method making" should read A method of making Column 13, 11116 7, "was" should read es Signed and sealed this 4th day of November 1969.

lest:

ard M. Fletcher, Jr. WILLIAM E. SCHUYLER, JR.

Nesting Officer Commissioner of Patents