GB2096176A

GB2096176A - Process for producing controlled density metal bodies

Info

Publication number: GB2096176A
Application number: GB8209311A
Authority: GB
Original assignee: National Standard Co
Current assignee: National Standard Co
Priority date: 1981-04-01
Filing date: 1982-03-30
Publication date: 1982-10-13
Also published as: DE3211718A1; JPS57177902A

Abstract

Porous metal bodies of controlled density are produced by initially forming green bodies from a slurry comprised of at least one particulate metal compound capable of undergoing reduction to its free metal state wherein the ultimate density of the metal product is established through control of the precursor metal compound particle size and temperature conditions during reduction and sintering of the green bodies. In a preferred embodiment, the particulate metal compound includes a metal oxide in an aqueous slurry that is spray dried to form green oxide spheroids which are substantially simultaneously reduced and sintered to produce porous metal spheroids that are particularly useful as a carrier vehicle in electrostatographic developer compositions.

Description

SPECIFICATION Process for producing controlled density metal bodies The present invention generally relates to the field of technology involving the production of metal bodies. More specifically, the invention is directed to an improved process for making porous metal particles wherein the density of the particles can be varied within a wide range through control of process operating conditions.

Description of the prior art The production of metal and metal oxide particles having spherical, flake, irregular or other such configurations is well documented in the prior art. For example, metal oxide particles of substantially spherical configuration can be produced by spray drying a slurry mixture comprising fine oxide particles through nozzles or centrifugal heads. Similarly, fine metal particles can also be produced through atomization by passing a stream of liquid metal through the cross-fire of either a liquid or gaseous stream to thereby produce, respectively, rounded and irregular shaped particles. Such fine metal particles are useful in many industrial applications, particularly the electrostatic photocopying field.

Photocopying systems generally form and develop images on the surfaces of photoconductive materials by electrostatic means. This essentially comprises placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow image to dissipate a charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image. This is achieved by depositing a finely divided electroscopic material known as "toner" on the image, with the toner being attracted to those areas of the layer which retain a charge, thereby forming a toner or powder image corresponding to the electrostatic latent image. This powder image is then transferred to a support surface, such as paper, on which it is permanently affixed by the application of heat and/or pressure.Instead of latent image formation by uniformly charging the photoconductive layer and then exposing the layer to a light and shadow image, it is possible to form the latent image by directly charging the layer and image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing step.

The formation of the powder image is achieved through the use of a suitable developer composition comprising the toner powder and a carrier vehicle. The toner basically consists of carbon black in a polymer binder, while the carrier may comprise particulate iron or ferrite powder of various configurations. It has been recognized by the prior art that the irregular shaped carrier particles do not exhibit good flow properties through an electrographic developer system.

Likewise, solid spherical iron particles having smooth surfaces do produce fine line images and good solid area reproduction, but such particles are characterized by relatively high density and exhibit undesirable toner throw-off and difficult toner replenishment characteristics. Moreover, smooth solid metal spheroids also have a relatively low apparent volume so that, for optimum results, it is generally necessary to utilize about twice the weight level of these particles in comparison to the weight of more conventional magnetic carriers such as iron filings and other particles having a much lower density and much higher apparent volume.

The charge on the toner is generated by natural triboelectric phenomena and depends primarily upon the composition of the toner, the composition of the materials which the toner contacts and the intimacy of the contact. This triboelectric transfer of charges between materials in contact with one another is often referred to as surface electrification and depends upon the relative work functions of the materials brought into contact. When the materials involved are conductive, the charges are rapidly neutralized. When the materials are nonconductive, the imbalance of charges is retained and the oppositely charged materials attract each other. Therefore, a stable resistance is essential for toner particles.Although triboelectric forces are difficult to control and fully understand, they are essential to the toning operations in all photocopying apparatus and therefore are critical in formulating toners and the materials that come into contact therewith. The carrier particles serve to provide a degree of control to the developing operation since the travel of the combined toner and carrier can be more easily controlled that that of the smaller toner particle itself.

Balancing the electrostatic attraction between toner and carrier is essential and is accomplished to a large extent by properly formulating the toner and carrier exterior. Too great an attraction of carrier to toner results in a lack of toning intensity because the latent image is not able to pull sufficient toner from the carrier. Too little attraction results in dirty background areas because the carrier too easily gives up toner and is not able to scavenge toner from the background or uncharged areas. In addition, if the attractive forces between toner and carrier are not uniform and stable over their lifetime, uneven toning results and the toner-carrier blend tends to be selectively fraction during use.

The composition of known carrier particles has changed over the years from sand and glass used in early cascade development to the iron-based materials, such as magnetites, steels and ferrites, used in many of the more recent photocopy apparatus having various forms of a magnetic brush development. Continuing development of carrier particles has increased copy rates, with the magnetic properties of the carrier being employed to transport the carrier to and from the photoconductor. In addition to the changes and composition, the prior art has also recognized that alterations in copy performance can be realized by changing the size, shape and density of carrier particles.

It has heretofore not been possible to produce small metallic particles, particularly metal spheroids, having the controlled density and configuration characteristics deemed desirable and advantageous for use as a carrier vehicle in electrostatic developer compositions.

Summary of the invention It is an object of the present invention to provide an improved process for making metallic particles wherein the density of the final product can be varied over a wide range.

It is another object of the invention to provide an improved process for making controlled density metallic spheroids of very small size and having a substantially perfect spherical configuration.

It is still another object of the invention to provide an improved carrier vehicle for use in electrostatic developer compositions.

It is yet another object of the invention to provide an improved process for making controlled density metallic spheroids that are particularly advantageous for use as a carrier vehicle in dry electrographic developer compositions.

It is still yet another object of the invention to provide an improved developer composition for use in dry electrographic systems wherein the composition is characterized by good flow properties, low density and the ability to reproduce fine line images and good solid areas.

These and other objects of the invention are realized by first providing an aqueous slurry comprised of water and at least one reducible particulate metal compound, such as a metal oxide, with the particle size of the precursor compound being selected in accordance with the desired porosity and ultimate density of the final metal product. The solids content of the slurry is preferably maintained at a high level, but may vary to accommodate a specific forming technique being utilized. The slurry is formed into green particles, such as through spray drying or other suitable forming technique, depending upon the ultimate physical configuration desired for the final metal product.The green bodies are then subjected to a single step reduction and sintering in a furnace containing a reducing atmosphere and maintain at a selected temperature of from about 6000F to just below the melting temperature of the metal, depending upon the degree of porosity desired, to produce free metal particles. The porosity and density of the final product may also be varied through the inclusion of pore forming agents or sintering inhibitors in the initial slurry mixture.

In a preferred embodiment of the invention, the precursor metal compound comprises iron oxide particles in a binderless slurry wherein the solids content is about 3080% by weight, though preferably from 4070% by weight, with the balance being water. After wet grinding to reduce the oxide particles to a size in accordance with the ultimate density desired for the free metal product, the slurry is spray dried to produce green iron oxide spheroids which are thereafter subjected to a single step reduction and sintering within a furnace containing a reducing atmosphere. The resulting metal product comprises small metal spheroids having a uniform porosity and density, uniform size, and substantially perfect spherical configuration.Such iron spheroids produced through the practice of the invention are particularly useful as a carrier vehicle for toner particles in an electrostatic developer composition. The bulk resistivity, surface smoothness and corrosion resistance of the iron carrier vehicle can be controlled and enhanced by providing the spheroids with an initial phosphate coating which in turn improves the adhesion of a subsequently applied organic polymer coating, thereby resulting in a stable and durable carrier vehicle.

Brief description of the drawing Figure 1 is a photomicrograph depicting a, plurality of porous metal spheroids of iron produced by a preferred embodiment of the present invention; Figure 2 is a photomicrograph of a single metal spheroid of iron as depticed in Figure 1 but at greater magnification; and Figure 3 is a photomicrograph taken under an even greater degree of magnification of the surface configuration of a single metal spheroid of iron as depicted in Figure 1.

Detailed description of the preferred embodiments The precursor compounds utilized in the practice of the invention for forming the initial aqueous slurry can comprise reducible metal compounds such as the oxides of Fe, Co, Ni, Cu, Mo and W, the chlorides of Fe, Cr and Ta and the sulfides of Cu and Fe. The most significant and preferred metal compounds comprise the oxides since they are the most plentiful and are the state in which metals are most commonly found as by-products of manufacturing and in natural ore concentrates. Generally, any of the metal compounds or combinations thereof deemed suitable for carrying out the method of making high density sintered metal products as disclosed by the Mclntire et al U.S. Patent 3,671,228, the entire disclosure of which patent is herein incorporated by reference, can also be used to advantage in practicing the present invention.

In forming the initial aqueous slurry, the metal compound particles such as particles of at least one reducible metal oxide, i.e. iron oxide, are mixed with water by means of any suitable known mixing device. The slurry is characterized by both a high solids loading of the oxide particles and a viscosity low enough to permit the utilization of a suitable forming technique for converting the slurry into green oxide particles. For example, the slurry may be spray cast into oxide spheroids. By "high solids loading" is meant a solids content of about 3080% by weight of slurry, though preferably 4070%.

The viscosity of the slurry may be adjusted by the inclusion of a dispersing agent, such as the sodium salt of a polyelectrolyte (e.g. (Tamol 850 manufactured by the Rohm and Hass Company and Nuosperse 700 manufactured by Tenneco Chemicals, Inc.). Other similar dispersants can also be utilized to advantage in the practice of the invention. Alternatively, the desired viscosity of the slurry can be established by adjusting its pH value. For example, with iron oxide, when the pH is adjusted to approximately 4.0, the slurry is placed in an "isoelectric state". This state is evidenced by an immediate thinning of the mix when the proper pH is achieved.This pH value imparts a relatively low viscosity to the iron oxide slurry so that a high solids loading of approximately 4070% oxide particles by weight can be sustained without producing agglomeration of the particles or impairing spray casting of the slurry into oxide spheroids. High solids slurries of sprayable viscosities can also be obtained at pH values as high as 9.0 or higher.

In forming the aqueous slurry, a sintering inhibitor, such as AI2O3 or other nonreducible metal oxide, can be included for the purpose of imparting a desired degree of porosity in the final metal product. Porosity can also be imparted by adding pore forming agents to the slurry. Such pore forming agents can include, for example, naphthalene, paradichlorobenzene and other sublimable materials.

Prior to forming the slurry into green oxide particles, it is subjected to wet grinding, preferably in an attritor or similar comminuting device, for the purpose of reducing the oxide particles to the required size in accordance with the ultimate desired density of the final metal product. For example, the particles can be reduced small enough to result in approximately 1-20 square meters of surface area per gram of particles. It has been determined that the porosity of the ultimate metal product can be controlled through varying the particle size of the precursor metal compound, with both product porosity and density increasing with decreasing particle size.

Although obviously not limited thereto, and depending on the desired ultimate density and intended use for the metal product, at least 35% by weight of the metal compound particles should be under 10 microns in diameter. Preferably, the metal compound should have a mean particle size no greater than 6 microns, at least 25% of which do not exceed 2.5 microns since these parameters have been found to be advantageous for spray casting an iron oxide slurry into green oxide spheroids.

The wet ground slurry is then subjected to a suitable forming technique for producing green oxide particles. If metal spheroids are desired as a final product, the slurry is cast into oxide spheroids by spray drying into a heated atmosphere. Conventional equipment is employed in this spray drying step and can comprise an atomizer connected to a source of hot gas, such as nitrogen or air, to supply the energy for drying, and a device for collecting the solid oxide spheroids, such as a cyclone and/or a bag filter. Suitable spray drying systems of this type are disclosed by the Lambert U.S. Patent 3,415,642 and Bergetal U.S. Patent 3,914,181, the entire disclosures of which patents are herein incorporated by reference. The resulting oxide spheroids are sized in accordance with their intended use. Outsized rejects are recycled to the spray casting step.The selected oxide spheroids are subjected to reduction of the metal oxide to its free metal state and the resulting free metal particles are thereafter sintered in accordance with the invention as shall be hereinfter described to form metal spheroids having the desired porosity and density characteristics.

To obtain nearly full density of the metal product, the reduction and sintering of oxide bodies have heretofore generally been conducted in sequential steps wherein the bodies are first subjected to a reducing environment at one temperature and thereafter the reduced body is subjected to sintering at a higher temperature. For example, optimum reducing temperatures for iron oxide range from about 9300F to 1 2000F and preferred sintering temperatures may range from about 1 8300F to 23000F. A typical sequential reduction and sintering technique for producing extremely dense metal bodies, exceeding 90% of theoretical density of the solid metal bodies, is disclosed by the aforementioned Mclntire U.S Patent 3,671,228.

The present invention has discovered that very fine metallic particles of controlled density can be obtained by conducting reduction and sintering of the green particles as a single step wherein both reduction and sintering are achieved substantially simultaneously. This single step technique has been found to permit the ability to control and impart a uniform degree of porposity in the final metal product, with extremely desirable results being realized when applied to the production of very small metal spheroids of less than about 250 microns. For example, metal spheroids of iron produced in accordance with the invention have a theoretical density of less than about 6.70 grams per cubic centimeter, which is less than about 85% of the theoretical density.Preferred bulk or apparent densities of such iron spheroids for the invention are on the order of about 1.0 to 4.5 grams per cubic centimeter, though preferably 1.5 to 4.0 grams per cubic centimeter. In practicing the single step reduction and sintering technique of the invention wherein the final product comprises iron spheroids,the spray cast green oxide spheroids are passed into a furnace maintained at a temperature within the range of about 6000F to just below the melting temperature of the metal, though preferably within a range of 15000Fto 21000F. The furnace is provided with a suitable reducing atmosphere, such as hydrogen, within which the green oxide spheroids are substantially simultaneously reduced and sintered into free iron spheroids having both uniform porosity and substantially perfect spherical configuration.The ultimate density of the spheroids is determined by the degree of porosity imparted thereto during the simultaneous reduction and sintering step. It has been discovered that the porosity of the final product can be effectively controlled by varying the furnace temperature, with the degree of ultimate product porosity decreasing with increasing temperature. In addition to hydrogen, other reducing gases, such as dissociated ammonia, can also be used during the single stage furnacing step. As previously indicated, the degree of porosity can also be controlled through the addition of sublimable pore forming agents or sintering inhibitors in the form of nonreducible oxides to the precursor slurry.

The following examples will illustrate the method of the present invention in a preferred mode thereof wherein aqueous slurries of iron oxide are spray cast into green oxide spheroids which are subsequently reduced and sintered to produce porous iron spheroids. It it to be understood that these examples are merely illustrative and are not intended to limit the invention to the specific illustrations hereinafter set forth: Example 1 A quantity of Fe304 was ground for 2-1/2 hours in a Union Process Company "30-S Attritor" until an oxide powder was obtained. An adequate slurry containing 2% of a dispersant such as Nuosperse 700, was prepared from the ground oxide. The amount of oxide added was sufficient to provide 61.7% solids. After the pH was adjusted to 9.9, the slurry had a viscosity of 4625 cps.The slurry was then dried by spraying through a Spraying Systems Company TC-5 nozzle under a pressure of 30 psi, at an inlet temperature of 6620F and an outlet temperature of 4250F to produce a spray-cast product having the following average particle sizes (U.S.

Standard 19.6% +50 45.6% -50 to +70 33.0% -70 After removing +50 and -70 mesh material from the spray-cast product, the remainder was reduced by heating in a furnace at 1 5000F for 1-1/2 hours in an atmosphere of dissociated ammonia. The resulting product was a metal powder having an average particle size (U.S.

Standard Mesh) in the range of -60 to + 100 and a bulk density of 1.7 gram/cc.

In the manner similar to that described in detail in Example 1, above, other slurries were prepared under varying conditions. The details and results are set forth in outline form in the examples which follow.

Example 2 Material: Fe,O, Solids: 61.7% Viscosity: 4625 cps.

Dispersant: 2% Nuosperse 700 pH: 9.9 Grind: 2-1/2 hrs. in Union Process Co. 30-S Attritor Spray conditions: 30 psi.

Oxide powder size (U.S. Standard Mesh): 1.4%+50, 33.6%-50+70, 64%-70 Furnacing conditions: 14500 F for 1.5 hours in an atmosphere of dissociated ammonia Metal powder product size (U.S. Standard Mesh):-60+100 (+50 mesh and -70 mesh material removed before furnacing).

Bulk Density of Metal Powder: 1.45 g/cc Example 3 Material: Fe2O3 Solids: 58% Viscosity: 4375 cps.

Dispersant: 0.5% Nuosperse 700 pH: 10.2 Grind: 30 min. in Union Process Co. 1-S Attritor Spray Conditions: Spraying Systems Co. TC-6 nozzle, 50 psi.

Oxide Powder Size (U.S. Standard Mesh): No measurements Dimple spheres produced.

Furnacing Conditions: 20000 F for 2 hrs. in an atmosphere of dissociated ammonia Metal Powder Product Size (U.S. Standard Mesh): 7.9%+100, 43.6%-100+120, 23.3%-1 20-140, 1 3.4%-1 40+170, 1 1.7%-170 Bulk Density of Metal Powder: 3.3 g/cc.

Example 4 Material: Foe304 Solids: 60% Viscosity: 3775 cps.

Dispersant: 2% Nuosperse 700 pH: 9.9 Grind: 4 hrs. in Union Process Co. 30-S Attritor Spray Conditions: Spraying Systems Co. TC-5 nozzle, 75 psi. 6500F inlet temperature, 3000F outlet temperature Oxide Powder Size (U.S. Standard Mesh): 40.4%+80, 25.4%-80 + 100, 15%- 100+120, 10.8%-i 20+140, 6%-140+170, 3.4%-170.

Furnacing Conditions: 17000for 1 hr. in an atmosphere of dissociated ammonia Metal Powder Product Size (U.S. Standar Mesh): 20%+140, 70%-i 40+270, 10%230 (+80 mesh material removed before furnacing) Bulk Density of Metal Powder: 2.7 g/cc.

Example 5 Material: Fe304.

Solids: 55% Viscosity: 650 cps.

Dispersant: 3% Nuosperse 700 pH: 9.0 Grind: 10 min. in Union Process Co. 1-S Attritor Spray Conditions: Spraying Systems Co. TC-5 nozzle 80 psi., 61 80F inlet temperature, 4000F outlet temperature Oxide Powder Size (U.S. Standard Mesh): 27.4%+80, 27.7%-80+1 00, 20.3%-1 00+120, 12.4%--120+140, 11.9%--140+170 Furnacing Conditions: 18000for 1 hr. in an atmosphere of dissociated ammonia Metal Powder Product Size (U.S. Standard Mesh): 40%+ 140, 50%-1 40+230, 10%-230 (+100 mesh material removed before furnacing) Bulk Density of Metal Powder: 18 g/cc.

While the preferred embodiment of the invention has been described in conjunction with the production of iron spheroidswherein the precursor green oxide spheroids are formed by spray drying the slurry, it is understood that other forming techniques can be utilized to produce controlled density metal particles of other physical configurations. For example, it may be desirable to include a binding agent in the slurry mixture for adjusting viscosity at high solids loading and imparting increased green strength to the formed compound particles. In this regard, the type of binding agent used affects the rheological characteristics of the slurry and serves to determine the type of forming technique which can be employed for a given slurry composition.

An acceptable range of binder content is approximately 0.1 to 1 5% by weight of the overall slurry composition, with the preferred range being approximately 0.5 to 5.0%. Suitable binders have been found to include alginite binders made from seaweed or kelp, carboxymethylcellulose (CMC) and guar gums, such as a guar gum derivative in the form of sodium carboxymethylhydroxypropylcellu lose (CMH P) manufactured by the Stein-Hall Company.

By including or excluding a binding agent to vary the viscosity of the slurry mixture, many different forming techniques can be utilized to produce precursor green particles of almost any desired physical configuration. For example, a composition having an appropriate viscosity can be applied to and dried on rotating drums to produce a dried oxide layer which is subsequently removed and ground into green particles having a flake structure, with such flakes being thereafter reduced and sintered according to the invention to produce correspondingly shaped free metal particles.

It is therefore apparent that through variation of particle sizes and furnacing temperatures, it is now possible through the practice of the invention to produce controlled density porous metal particles, particularly metal spheroids, of different sizes and physical characteristics, depending upon the forming technique utilized and the intended application of the use for the final product.

Referring now to the photomicrograph of Fig.

1, there is shown under magnification a plurality of metal spheroids of iron produced according to the invention, with each spheroid having a diameter of approximately 100 microns. These spheroids are characterized by uniform porous structure and substantially perfect spherical configuration. As evident in the photomicrograph of Fig. 2, each spheroid of Fig 1 is made up of a multitude of free iron particles which collectively define the spheroid and its uniform porosity.The nature of the porous structure making up each spheroid of Fig. 1 is depicted in the photomicrograph of Fig. 3 wherein the porosity is specifically defined by numerous reticulated passageways, with substantially all of the passageways being in communication with each other throughout the entire body of the spheroid As previously indicated, the porous metal particles of controlled density produced by the invention, particularly iron spheroids, are especially advantageous and useful as a carrier vehicle for toner particles in electrostatic or photocopier developer compositions, specifically dry developer compositions for use in the "magnetic brush" development of latent electrostatic charge images.

Heretofore known developer compositions utilized in magnetic brush development of electrostatic images commonly include a triboelectric mixture of fine toner powder comprising dyed or pigmented thermoplastic resins with magnetic carrier particles of irregularshaped materials, such as iron filings or reduced oxide particles. In the magnetic brush process, a developer mix containing the toner and the magnetic carrier particles is carried on the outside of a rotatable cylinder by magnetic means disposed within the cylinder. The magnetic field of the magnet causes alignment of the carrier particles in a brushlike configuration on the outer surface of the cylinder. This magnetic brush is engaged with an electrostatic latent imagebearing surface and the toner particles are drawn from the brush to the image by electrostatic attraction.As the cylinder rotates, developer mix is continuously replenished from a supply source, thus assuring that fresh mix is always available to the copy sheet surface at its point of contact with the magnetic brush. In a typical rotational cycle, the cylinder performs the successive steps of developer mix pickup, brush formation, brush contact with the photoconductive surface, brush collapse and finally mix release.

The prior art has recognized that the performance characteristics of carrier particles used in toner-carrier or two-component combinations and toner distribution systems can be enhanced by applying coatings to the carrier particles. Such coatings are generally of an organic polymer material and serve to control the polarity and magnitude of triboelectric charges, extend the life of the carrier, reduce the influence of changes in relative humidity on the carriertoner couple, stabilize the composition of the carrier, modify the abrasive properties of the carrier and improve the free flow of the carrier particles. Such coatings may comprise any of the various materials and applied according to the various techniques documented by the prior art as exemplified by the Hagenbach U.S. Patent 3,507,686; Miller U.S. Patent 3,632,512 and Kulka U.S.Patent 3,798,1 67, the entire disclosures of these patents being herein incorporated by reference. While the metal spheroids of the present invention can be used as uncoated carrier particles in electrostatic developer compositions, it is understood that such spheroids may also be provided with these known organic polymer carrier coatings for the purpose of altering or enhancing performance characteristics.

An important factor in extending the useful life of the carrier vehicle in a developer composition resides in maximizing adhesion of the organic coating to each of the carrier particles. Carrier particles have heretofore been often pretreated or prime-coated to improve adhesion of organic polymer coatings, thereby minimizing or preventing flaking and chipping of coatings from the particles, which condition is highly undesirable and serves to radically alter the triboelectric properties of the particles. The abrasion resistance of the carrier coating reflects its ability to retain its original triboelectric response as it is subjected to normal wear in the total developing environment.

The present invention has discovered that several advantages are realized when metal particles, such as iron spheroids of the invention, used as carrier vehicles in developer compositions are provided with an initial phosphate coating prior to applying the subsequent organic polymer coating. It was found that this initial phosphate coating served to control bulk conductivity, control the surfaces of the individual carrier particles by sealing pores, improve corrosion resistance of the metal and improve adhesion of the subsequently applied organic polymer coatings. In controlling bulk conductivity, it was discovered that such phosphate coatings can alter the surface conductivity of the metal spheroids within a range from being slightly conductive to highly insulative.Moreover, by providing this initial phosphate coating prior to applying the exterior organic polymer coating, carrier particles produced in this manner were found to be more stable and had a longer useful life.

The phosphate coating can be applied to the metal carrier particles according to substantially any of the well known compositions and techniques conventionally used in industry for treating iron or steel surfaces. Such compositions and techniques are well disclosed in the publication entitled Practical Phosphate Coatings, No. 180-11 R, published by the R. O.

Hall 8 Company, Inc. of Cleveland, Ohio, the entire disclosure of this publication being herein incorporated by reference. In applying phosphate coatings to metal spheroids of the invention, it is desirable that the coating be of the zinc phosphate microcrystalline type in order to provide a more adherent coating to the small particles. A preferred phosphate coating for this purpose is that designated MetaBond and manufactured by R. O. Hall s Company, Inc. of Cleveland, Ohio.

An example of the manner in which a suitable phosphate coating is applied to the porous metal spheroids in accordance with the invention is as follows: A solution is first prepared comprising water, 4% MetaBondB 51414, and 4% MetaBond Additive 51 504. The solution is heated to about 1 800 F, at which point 100 grams of iron spheroids are added to 1000 milliliters of the solution and the mixture stirred. After one minute, excess solution is poured off and the coated spheroids are rinsed with water. Excess water is poured off and the coated spheroids are dried in air at about 2000F.

In utilizing the porous metal particles produced by the invention as a carrier vehicle in electrostatic developer compositions, an important consideration is the electrical resistivity of the metal particles, particularly in comparison to known carrier particles, such as NiZn-iron oxide or ferrite type core materials. The resistivity is measured by the break-down voltage, i.e. the voltage at which the individual particles will become significantly conductive. Known copiers such as the Xerox Models 8200, 9200, 9400 and 9500 utilize ferrite type core material in their developer compositions, with the required breakdown voltage of such compositions being at least 800 volts. The break-down voltage for ferrite particles is 4000 to 5000 volts, thereby easily accommodating this minimum voltage requirement.There is no advantage of a breakdown voltage-being in excess of 800 volts, though this latter exact minimum value must be realized before proper developer operation is achieved.

The relatively pure iron particles produced by the invention are extremely conductive with only ten volts applied. However, it has been found that these particles can be rendered sufficiently resistive by coating the particles with zinc phosphate as previously described. Such coating also renders the surface of the particles nonporous and provides an ideal surface on which to coat the organic polymer, the latter coating serving to develop the necessary triboelectric properties of the particles.An example shall now be described wherein a developer composition was made utiiizing porous metal spheroids of the invention as the carrier vehicle: 1 8 gallons of distilled water were mixed with 680 ml of Rohco MetaBond t)Additive 51504 and 680 ml Rohco MetaBond51414.This solution was heated to 1 800F and stirred while 35 pounds of porous metal spheroids having a bulk density of 2.7 grams per cc and a size of -100+200 mesh, were poured into the solution.

The resulting mixture was mixed for one minute and the solution was decanted. The spheroids were rinsed with water for five minutes, after which the water was decanted. The spheroids where then dried in trays in an oven at 2200F, with the resulting phosphate coating weight being measured as 5.62%and break-down voltage being measured as 2200 volts. The phosphate-coated spheroids were then coated with an acrylic in a fluidized bed coater system, the acrylic being a polymer PW-1 OA manufactured by Polyvinyl Chemical Industries of Wilmington, Massachusetts. The polymer coated spheroids were then blended with 0.9% toner particles. The completed developer composition was then tested in a Xerox 9400 photocopier, producing excellent copies and exhibiting good service life.

The previously indicated problems inherent in heretofore known carrier vehicles used in developer compositions of magnetic brush development systems have now been overcome by using the controlled density metal particles, particularly the porous metal spheroids of the present invention as a carrier vehicle. These controlled density particles are capable of rendering a high degree of resolution through excellent reproduction of fine line images and solid areas. Unlike known carrier particles, the metal spheroids of this invention, in either coated or uncoated form, do not adhere to imaging surfaces and do not scratch reusable electrophotographic surfaces during image transfer and surface cleaning operations.When compared with known solid metal spheroid carrier particles, the porous spheroids of the invention provide a substantial economic advantage since their lower density affords increased surface area on a per weight unit basis. Further, the reticulated porous structure of each particle produces a reduced magnetic remanance when compared to the bulk material. This reduced remanance is required to prevent "chaining" or sequential attachment of carrier particles with relatively low density. Moreover, the lower density carrier mixes more easily with the toner, thus requiring the expenditure of less energy in producing the developer composition.

While the invention has been described with reference to particular embodiments and examples thereof, it will be understood that modification, substitution and the like may be resorted to without departing from the spirit of the invention or the scope of the subjoined claims.

Claims

1. A process for producing controlled density metal bodies comprising: a) providing a slurry comprised of water and at least one particulate metal compound of controlled particle size capable of undergoing reduction to its free metal state; b) forming the slurry into green metal compound bodies; and c) reducing and sintering the green metal compound bodies at a controlled temperature in the range of from about 6000F (31 50C) to just below the melting temperature of the metal to produce free metal bodies having a desired degree of porosity.

2. A process according to claim 1 in which the slurry is spray dried into green metal compound bodies having a substantially spherical configuration.

3. A process according to claim 1 or claim 2 in which the particulate metal compound comprises about 30 to 80% by weight of the slurry.

4. A process according to any one of the preceding claims in which the slurry includes an effective amount of a dispersant to impart a desired viscosity to the slurry.

5. A process according to any one of the preceding claims in which the slurry is formed from a particulate metal compound which is a member selected from the group consisting of the oxides of Fe, Co, Ni, Cu, Mo and W, the chlorides of Fe, Cr, Cb and Ta or the sulfides of Cu and Fe, and mixtures thereof.

6. A process as claimed in any one of the preceding claims in which the slurry is formed from a metal compound which consists essentially of iron oxides.

7. A process according to claim 6 in which the free metal bodies of iron have a density of about 1.0 to 4.5 grams per cubic centimeter.

8. A process according to any one of the preceding claims in which the particle size of the particulate metal compound is controlled by wetgrinding the metal compound until the particle size has been reduced sufficiently to provide a surface area of about 1 to 20 square meters per gram.

9. A process according to any one of the preceding claims in which the particle size of the particulate metal compound is controlled to comprise at least 35% by weight of the particles under about

10 microns diameter.

1 0. A process according to any one of the preceding claims in which the particle size of the particulate metal compound is controlled to comprise particles having a mean size no greater than about 6 microns diameter, at least 25% of which do not exceed about 2.5 microns diameter.

11. A process according to any one of the preceding claims in which the slurry includes a binder.

12. A process according to any one of the preceding claims in which the slurry includes a nonreducible sintering inhibitor.

1 3. A process according to any one of the preceding claims in which the slurry includes a pore forming agent.

14. A process according to any one of the preceding claims in which the reducing and sintering of the green metal compound bodies are performed in sequential steps.

15. A process according to any one of the claims 1 to 13 in which the reducing and sintering of the green metal compound bodies are performed substantially simultaneously.

1 6. A process according to any one of the preceding claims in which the slurry comprises more than one particulate metal compound for forming free metal alloy bodies.

17. The porous free metal bodies produced by the process of any of the previous claims.

18. A porous metal body comprising a multitude of free metal particles sintered together to define substantially uniformly distributed reticulated passageways throughout the body.

1 9. A metal body according to claim 18 having a substantially spherical configuration.

20. A metal body according to claim 19 having a diameter of less than about 250 microns.

21. A metal body according to any one of claims 18 to 20 in which the metal body has an inner phosphate coating and an outer organic polymer coating on the exterior surface thereof.

22. A metal body according to any one of the claims 18 to 21 in which the metal is substantially iron.

23. A metal body according to claim 22 having a density of about 1.0 to 4.5 grams per cubic centimeter.

24. A process for producing metal bodies as claimed in claim 1 substantially as described herein in any one of the Examples.

25. An electrostatographic developer composition comprising a toner and porous metal bodies as claimed in any one of claims 18 to 23.

26. An electrostatographic developer composition comprising finely-divided toner particles electrostatically clinging to the surfaces of carrier particles in the form of porous metal spheroids having a diameter of less than about 250 microns and being less than about 85% of theoretical density.

27. A developer composition according to claim 26 in which the metal spheroids each includes an inner phosphate coating and an outer organic polymer coating on the exterior surface thereof.

28. A developer composition-according to claim 26 or claim 27 in which each metal spheroid is comprised of a multitude of free metal particles sintered together to define substantially uniformly distributed reticulated passageways throughout the spheroid.

29. A developer composition according to any one of claims 26 to 28 in which the metal spheroids are substantially iron.

30. A developer composition according to claim 29 in which the metal spheroids have a density of about 1.0 to 4.5 grams per cubic centimeter.

31. A developer composition according to any one of claims 26 to 28 in which the porous metal spheroids are formed by reducing and sintering a metal compound selected from the group consisting of the oxides of Fe, Co, Ni, Cu, Mo and W, the chlorides of Fe, Cr, Cb and Ta or the sulfides of Cu and Fe, and mixtures thereof.

32. A developer composition according to claim 30 in which the reducing and sintering of the metal compound are performed substantially simultaneously.