US2399705A - Metallic bronze paste or powder pigments and methods of manufacturing the same - Google Patents

Description

. Patented May '5 METALLIC BRONZE PASTE OR POWDER PIG- MENTS AND METHODS OF MANUFACTUR- ING THE SAME Othon Adolf Ziehl, Union,- N. 1., assi gnor to Metals Dlsintegrating Company, Inc., Elizabeth, N. J., a corporation of'New Jersey N Drawing. Application March 31, 1941,

Serial No. 386.118

1 Claim.

This invention relates to the treatment of a paste of metal flakes, particularly leaflng metal flakes, such as is used as pigment for paint, etc., whereby the effective specular reflectivity oi the metal are increased, the characteristic color of the metal revealed, and the overall luster of the flake as a leaflng flake enhanced, without serious impairment of the leaflng quality of the flake.

A paste of this kind may be prepared in various ways, for example in accordance with the directions of the U. S. patent to Hall No. 2,002,891. As set forth in that patent, particles of aluminum, for example, of convenient size, are subjected to .impact comminution in a ball mill, the particles being charged into the mill together with a liquid such as mineral spirits, carrying'in solution a leaflng agent such as stearic acid, along 'with a quantity of steel balls.

Following in general the directions of the patent, including regulation of mill temperature, speed of rotation of the mill, etc., the metal particles are flaked out and reduced to a size and familiar frosty appearance.

eration may be performed on suspensions or dispersions of varying metal content, though there are practical limits, in the direction of too little metal at one end of the range, and of too much metal at the'other end. It is my belief that the friction impressed on the flakes, among other possible results, causes wearing of! of flake surface. If that is true, the first to wear oif would be an outer coating, which may consist of debris resulting from milling, such as slimes or extremely small'particles (colloids), metal stearate adhering to the surface, metal oxides, etc., this hypothetical coating probably giving the Next, directly attached or anchored to the metal surface, lies the leaflng-fllm proper. Until this fllm is reached the effective specular reflectivity increases as the outer coating wears thinner under continued rubbing, without impairment of leaflng power. This increase in reflectivity with decrease of outer coating thickness is probably due to increased reflection from the metal surface itself, the leaflng film being of such thinness (of monomolecular order) that it does not interfere with the light waves striking the surface, the light then being reflected by the metallic surface. This would be the point when maximum specular reflectivity is reached; beyond that point,- no

. gain in brightness could be expected, but further treated, as by means of a filter press, to remove excess liquid vehicle, and the material then brought to desired paste form.

By the present invention it is proposed to treat the material, before it reaches the final The treatment which I propose to give the suspension comprises mechanically working the mass in an extruder or device equivalent for my purpose, whereby individual flakes are caused to have relatively moving frictional surface contact with each other and/or with surfaces in such device with which the flakes may be brought into contact in the operation of the device. The opfrictional effort on the surfaces of flakes would wear off the leaflng flhn itself; the'result of too prolonged treatment would, therefore, be loss of leaflng power without further increase in reflectivity, etc.

The hypothesis advanced in the foregoing seems to be supported by the fact that a paste burnished in accordance with my invention, when subjected to a washing operation as hereinafter described, shows a still further increase in reflectivity, etc.

Another possible explanation is that the metal itself and/or any coating or fllm thereon is ironed out, i. e., the flakes which are (however slightly) dished or curved rather than plane by reason of i the impact on them of the steel balls in the milling operation, are by my burnishing operation flattened or brought more nearly to the condition of specula, thus contributing to greater reflectivity. And it is in my opinion likely that the "ironing ou and the planing off (actual removal) of flake coating are simultaneously effective.

It is, of course, also possible that an effect of the operation is to polish the metal of the flake,

and/or perhaps flatten or remove wrinkles therefrom, without actual disruption or removal of non-metal surface coating, in somewhat the way that the gold-beater hammers a sheet of gold through a gold-beaters skin, without breaking the latter. More than one'of the actions suggested above as conducive to the increased luster and other visual effects may take place at the same time or successively during the operation; and on the other hand, while these are plausible or at least not unreasonable explanations of the phenomena involved, there may be another or others more in accord with fact.

In any event I have found it to be a fact that the operation results in 'a marked improvement in metallic luster and specular reflectivity and incidentally in a breaking up of agglomerates, which very probably still further increases reflectivity.

Determination of the optimum proportion of metal to liquid is important for the purposes of the invention, depending largely on the particular operating conditions and/or the mechanism eml ployed. In some cases, as where an extrusion device is used with filter cake it is advantageous, though perhaps not absolutely necessary, that the amount of liquid in the cake should be merely sufficient to bind the flakes into a stiff, coherent. but still plastic mass when compacted. Excessive liquid means difficulty or even impossibility of accomplishing the desired burnishing or luster-enhancing movement of the flakes. On the other hand, too great a proportion of the metal will cause abrasion, and, in time, loss of leafing power. In complete absence of liquid the property of flow of and in the mass is diminished to a point at which there is little likelihood that the desired movements of the flakes and the requisite frictional eflort on the surfaces thereof can take place to any useful extent.

The optimum amount of liquid in the mass depends not only upon the operating conditions and/or mechanism selected in a given case, but also upon the particular metal and the particle size, etc.,- of the flakes. A mixture of standard filter-cake composition (78% to 82% metal) when subjected to extrusion gives a product that is superior in respect of the desired visual efiects. I believethat above 90% metal content in the case of aluminum, the danger of abrasion and impairment of leafing power is progressively greater. In the case of copper flake a metal content in the neighborhood of 90% plus gives good results. (All percentages given in this specification are by weight; obviously, if some liquid other than mineral spirits is employed, the percentages given herein might have to be changed, mutatis mu tandis.)

The pressure necessary to cause relative frictional movement of the particles within the mass, and/or to cause movement of translation of the mass, depends upon the resistance encountered in bringing about such movements, and will be determined and regulatably applied as the practical needs of the operation require. Thus, if it appears that a certain pressure results inless than the desired improvement, or if there is impairment of leafing power, pressure conditions may be changed accordingly. Approximate determination of pressure may be accomplished in various ways, and visually indicated, or even recorded.

Merely static pressure has no beneficial effect upon the flakes; there must be fairly .intimate frictional contact between relatively moving flake surfaces, and/or between flakes and the surfaces (other than flake surfaces) with which the mass as such has operative pressure contact. In all this, the liquid phase is one factor, determining are caused to exert is another factor. And the resistance of the mass, as such, to translatory movement, it there be such movement, is another factor. All these factors must be coordinated.

In some of my work, using an extruder with filter cake, I have found that the higher the metallic reflectivity obtained, the greater the frictional heat developed within the mass during the application of the invention. The degree of heat thus generated is, within limits, a good qualitative indicator of the burnishing effect, regardless of the means employed to accomplish the burnishing; and I may utilize this fact as a control in the practice of the process. In any event, the temperature to which the mass is subjected as a result of the burnishing operation will not be permitted to go to the point at which decomposition or impairment of the leafing film takes place.

Any device that will produce the desired effect on the mass may be used. Cost and practicability are the deciding factors. If the process is made a continuous one, the rate of feed and especially the resistance met in the path of the moving mass are factors to be considered. An extrusion machine, for example one of the screw conveyor type, is very effective for the purposes of the invention where the starting material is filter-cake, a relatively dry and stiff but still plastic material, and has the advantage of being adaptable to continuity of operation. It has fair capacity, the rate of lamellar flow and extrusion can be controlled to some degree, and the pressure on the mass may be kept substantially persistent, though not of uniform degree, throughout the mass during the extrusion operation. More than one extrusion operation may be performed on the material if found desirable. If batch operation be preferred to continuous, movement of the mass through the extrusion plate may be caused by the applied pressure of a, plunger or piston. The size, number and cross-sectional shape (circular, oblong, annular, etc.) of the perforations in the end-plate, through which the forward part of the mass issues under the pressure of the following mass, and the thickness of this end-plate (signifying the length of the perforations) and whether the perforations are of uniform cross-section or diminish in cross-section forwardly, mainly determine the pressure on and within the mass, in connection of course with whatever means is used to propel the mass.

- The pressure may also be positively affected in the case of the screw-conveyor extruder by using a variable pitch screw; and/or by actually tapertrasted with the bodily movement of the mass as a whole. With relatively small exit or extrusion holes the maximum number of flakes is caused to have frictional contact, flake with flake and/or flake with machine surfaces, as the mass is squeezed into narrow exit paths. A smaller number of holes of the same size will increase the back-pressure, but mere pressure, unless it is accompanied by the described frictional contact effective on the flakes, will not develop luster.

Increase of temperature observed with higher R. P. M. of the screw is in part probably due to diminished radiation, but the total work input is the same. The increase in specular reflectivity observed where a smaller number of holes is employed, as contrasted with -a. larger number of holes of the same size, is mainly due to greater impressed friction on the flakes.

A suitable kneading apparatus should accomplish the desired result, working on filter cake, for example.

I also contemplate the possibility of achieving the desired efi'ect by passing material of suitable concentration through a set of rolls with difierential speeds to eifect the desired burnishing; also by pressing the material between a set of parallel disks rotating at diflerential speeds.

After the burnishing operation the mass may be adjusted to any desired metal content, in the case of aluminum, for example, 65%. connection Ziehl U. S. Patent No. 2,144,953.) In the case of copper, the final concentration may be 85% metal, or even higher.

The burnished flake in cake form may be converted into a dry flake powder by the usual means, as by drying in air at room temperature, or in a vacuum at elevated temperatures.

If it transpires that as a result 01' the burnishing operation there is undesirable separation as between the liquid and solid phases, this may be controlled or compensated for by the addition of suitable stiffening agents; or by beating air or CO: into the mass, etc.

The following example will illustrate the practical application of the invention:

(See in this Extrusion, operating on aluminum filter cake:

Diameter of holes: 0.075

Thickness of end-plate: 0.285

Sum of cross-sectional areas of apertures=25% of total area of end-plate.

Sufficient pressure to push the 80% aluminum cake through these perforations. In the case 01' copper flake a cake containing around metal will be extruded.

The extruded cake, whether aluminum or copper, may be adjusted directly to the metal content desired in the final paste product or it may be subjected to an intermediate washing operation by suspending the flakes in mineral spirits in suflicient quantity to form, a liquid slurry, say, in the case of aluminum, 1 lb. of metal in /2 gal. mineral spirits. This slurry is then filtered. The filter cake may then be adjusted to the desired metal content.

I claim:

The method of increasing the specular reflectivity of metal flakes, which comprises .ballmilling metal particles in a liquid made up of a thinner and a leaflng agent, whereby the particles are brought to flake form and furnished with a leaflng film, discharging the mixture of metal and liquid from the mill and removing liquid to such an extent that the metal content of the mass is over about 80%, whereby the mass is capable of lamellar flow, and subjecting the mass to mechanical working involving lamellar flow and surface friction of flake on flake, by forcing said mass through the apertures of an extrusion plate, the apertures of said plate having a length more than twice their diameter, whereby individual flakes 0f the mass are caused to move mainly flat-wise over each other under friction and pressure, which are controlled between limits.

OTHON ADOLF ZIEHL.