GB2075209A - Carrier particles for electro-photographic developers - Google Patents

Abstract

Electrophotographic composite carrier particles comprise abrasive magnetic particles embedded in a polymeric matrix. During use in admixture with a toner in an electro-photographic process, the carrier particles are abraded to expose a previously unexposed, uncontaminated, surface which will maintain a constant tribo-electric interaction and thus ensure an optimum copy quality over an extended developer life. The magnetic particles are iron, nickel, cobalt, soft ferrite, hard ferrite, ferrisoferric oxide and or nickel oxide. <IMAGE>

Description

SPECIFICATION Magnetic carrier particles for electrophotography This invention relates in general to electrophotographic imaging processes and, in particular, to magnetically responsive particles suitable for use in reprographic machines which employ magnetic brush development.

In the electrophotographic process, an electrical charge is placed uniformly on the surface of a photoconductor. Image-wise exposure of the photoconductor with light causes the charge to be dissipated in the light areas, while the dark areas retain the charge, thus forming a "latent" photographic copy of the original.

The latent image on the photoconductor is made visible by the use of small black particles called toner. A number of methods are known for applying the toner. One of these is known as cascade development and is described in U.S. Pat. No. 2,618,552 to Wise. Another method, known as the magnetic brush process, is described in U.S. Pat. No. 2,874,063 to Greig.

The cascade and magnetic brush development processes utilize a two component developer material. The developer consists of carrier particles admixed with small toner particles. When a mixture of carrier and toner particles is agitated, an exchange of electrons occurs between the surfaces of the components, giving rise to the well known triboelectric charge, and the toner and carrier particles acquire opposite, but equal, charges. When a suitably agitated developer mixture is placed in contact with the latent electrostatic image on a photoconductor, the toner is attracted from the carrier to the photoconductor surface, thus forming a visible image.

In principle, any pair of dissimilar materials will produce a triboelectric charge when agitated. However, the special structural and functional features of the current typical electrophotographic copiers have necessitated the development of complex toner/carrier compositions in order to obtain satisfactory operation of conventional systems. Nonetheless, these prior art compositions are deficient in a number of respects.

To more fully understand the deficiencies of prior art developer mixtures, it is useful to first review the physico-chemical properties of commercial toner particles used in electrophotographic copiers. Toners typically comprise a resin system (of one or more components), coloring agents such as dyes, pigments and carbon blacks, and one or more additives which, although present in small amounts, serve as modifiers of the basic toner properties.

In order to produce acceptable copy quality, among other things particles must possess the correct triboelectric charge, a high degree of blackness, small size, and thermal qualities which permit the toner to be permanently fixed to the copy paper upon application of heat.

The correct triboelectric charge is obtained by selection of a polymeric resin or resins having the ability to either donate or accept electrons from the desired carrier material. The high degree of blackness is obtained, in general, by blending into the polymer resin a relatively high concentration of carbon black. Toners made by this procedure are well known to give rise to dusting in a copy machine. This dust, which contains micro-sized carbon particles, adheres to critical copier parts such as the photoconductor surface and the cleaning brush, giving rise to an undesirable film of material on these components. This film gradually increases in thickness until a point is reached at which copy quality is impaired. This situation can only be rectified by a costly service call which involves cleaning and/or replacement of the critical parts.

In general, current manufacturing techniques for toner particles are designed to produce particles in a size range of 5-30 microns. While fine particles, below 10 microns, will improve optical resolution on a copy, their utilization will tend to produce filming problems of the type involved with the use of carbon black. Since minimization of service calls is of prime importance in the copier business, toner particles below the 10 micron level are usually removed from a toner particle mass by a process called classification. However, this method is inherently expensive and is inefficient in that a certain amount of the larger particles are also removed. Thus, particle classification methods are only practical for those few situations where a relatively expensive toner is commercially acceptable.Moreover, since large toner particles are subject to breakdown during copier machine operation, even classified toners will ultimately produce undesirable filming during extended operations.

Most of the common copy machines now in use employ a heat method to fuse or fix the toner to the copy paper, thus rendering the image smear-free. The successful accomplishment of heat fusing requires a toner resin which has a relatively low melting point and which will flow easily in the semi-solid state. These physical requirements, however, are directly contrary to the physical material properties called for in minimizing filming. Until the advent of the present invention, formulations have been developed to produce the best balance between undesirable filming and desirable fusing characteristics. As a result, certain resins which had good low temperature fusing characteristics were rejected because they produce an unacceptable level of filming.

It is evident from the above that electrophotographic toner formulation involves many complex variables and, in particular, filming problems pose a substantial barrier to improvements in the overall technology level of copy machines.

The carrier particles known in prior art developer mixtures include glass beads, sand, Rochelle salt, steel shot, flake iron, nickel flake, ferrite and variations thereof. Any carrier particle size which is about at least twice the diameter of the largest toner particle in the developer mixture being used may be employed, however, optimum copy quality demands a relatively narrow size range distribution.

The principal operating characteristic by which the quality of a carrier or developer mix may be measured is the so-called developer life. After a number of machine cycles, or copies, it will be found that the developer is no longer capable of producing an acceptable image or copy. The number of copies thus produced is termed the "developer life". For example, the RBC Copier IV (distributed by Litton Industries) uses flake iron carrier particles which have a developer life of about 8-10,000 copies. Indeed, it has been found under experimental conditions that flake iron can produce a maximum developer life of about 12,000 copies in most types of conventional copying equipment.

In an attempt to improve the developer life of metal particles, several investigators have applied a thin organic polymer coating to the surface of the particles. See, for example, U.S. Patent 3,918,968 by Kukla. This procedure improves the developer fife by varying factors depending on the type of resin used, the thickness of the coating and the coating method. For example, the Xerox 3100 copier (manufactured by Xerox Corporation) uses coated spherical steel shot carrier and has a developer life of about 15,000 copies.

Although coated carriers provide superior performance compared to the bare particles, both types have one important shortcoming. It is generally accepted that the continued agitation of the developer mixture causes the toner to be ejected against the carrier surface at a high velocity. The toner particles are generally relatively softer than the carrier particles, the degree of softness being determined by the fusing characteristics discussed above. In such circumstances, it has also been found that toner is gradually filmed on to the surface of the carrier at a predictable rate based upon equipment use. As the free carrier surface gradually becomes coated by filmed toner, thetriboelectric charge of the developer mixture is diminished causing copy quality to deteriorate.

At the "developer life" point, it has been found that typically about 70 percent of the total toner in the developer mixture is tightly bound to the carrier in the form of a film which can only be removed by chemical action. It is therefore evident that both uncoated and coated carriers, as currently manufactured, have a finite developer life which represents a technological limitation with respect to improved high volume copy machines.

In addition to the filming deficiency found in current commercial developers, other problems have arisen when such developers are used in high speed copiers. As the rate of output copy is increased, the rate of introduction of developer to the photoconductor surface must be increased by a proportional degree. Under these circumstances, the high inertial forces generated by the development mechanism of the copying device cause the metal carrier particles to impact each other and the equipment structure at a relatively high velocity. Impact repetition causes work hardening, stress cracks, and other structural defects in the carrier material. Predictably, the strained particles fracture into several pieces. These fragments present a foreign surface to the toner and therefore interfere with the generation of the triboelectric charge.A more severe problem occurs when the carrier fragments escape the development housing of the copying device and cause premature failure of nearby components.

Another problem associated with high speed machines involves changes in ambient air conditions. Both relative humidity and temperature are determining factors in the magnitude of the triboelectric charge and thus the consistency of copy quality. Slow copiers produce little internal heat and the machine environment remains relatively stable. In contrast, high speed machines, such as computer output printers, must function at all hours of the day but the enormous amount of heat which is released on a continual basis cannot always be dissipated due to room ventilation restrictions. Commercial developer mixtures show a sensitivity to humidity and temperature necessitating the incorporation of complex electronic feedback controls to partially compensate for the variations.Since these devices are essentially follower mechanisms their mode of action is slow and several sub-standard copies may be produced before overall process control is re-established.

As previously mentioned, some carriers are coated with a polymer to enhance the operating characteristics. However, current manufacturing techniques for coated carriers, such as tumbling, hot melt mixing, fluidized bed coating and solvent reaction, are subject to severe deficiencies leading to significant batch-to-batch variations. Two specific problem areas are agglomeration of particles and coating efficiency.

In the prior art (see for example U.S. Patent 3,795,617 to McCabe) we are taught that agglomerates of the basic carrier particles must be avoided. One of the reasons for this is that agglomerates tend to break apart during copy machine operation causing maintenance problems.

The deposition of the polymer coating on the carrier has been generally found not to be a readily reproducible process. Even though theoretically correct weight of polymer is introduced to effect complete coverage of all the carrier surface, it has been typically found that up to 20% of the particles will have only minimal or incomplete coverage, and, more importantly, few particles will be adequately coated. The so-called prior art coated carrier therefore, of partial coating discontinuity or insufficiency is subject to undesired chipping and abrasion during copy machine operation. This is believed to account for the observation that many coated carriers are only marginal improvements over their uncoated counterparts. in view of the market trends toward automated office printing e.g., electronic mail, there is a clear need for upgraded highly reproductive coated carrier materials. Current technology of coated carriers does not meet these increased demands.

Another deficiency of commercially available developer mixtures is related to the difficulty of blending in small amounts of additives for special purposes. For example, several copiers require the presence of about 0.2% of zinc stearate in the developer mixture. It is impossible to achieve a homogeneous blend of the stearate due to tendency of the stearate particles to agglomerate. Therefore some regions will contain a deficiency of stearate while others will contain an excess, both situations can result in uneven copy quality.

Yet another deficiency of current metal carriers is the manufacturing limitations presented by the techniques used by the steel industry. Metal powders are used in a variety of industries, in fact, electrophotography represents a very small fraction of the total consumption. Therefore copier manufacturers are forced to employ off-the-shelf powders which are mainly sold for e.g., abrasives. This means that the powders are not optimized for shape, size, surface properties relative to their use in copiers. For instance, several flake irons contain a thin layer of surface oil, the amount of which varies from batch to batch. Particle shape is another aspect which is crucial to good copy quality yet is highly variable in commercially available carrier raw materials.

Another deficiency of current metal carriers is the inability to change the bulk density and magnetic characteristics since these are fixed by the chemical structure of the metal. For example, flake iron has a bulk density of about 8g/cc and a saturated magnetization of about 200 emu/g. Neither of these values are optimum for copiers, therefore machine manufacturers have been forced to make severe compromises in mechanical design to accommodate widely available carrier particles.

A very important deficiency involved in the utilization of metal carriers is the inability to change surface resistivity. The surface resistivity of carrier particles establishes the relative ease or difficulty of passage of an electric current through a mass of carrier particles. Since the developer mixture in a copy machine environment acts like an electrical capacitor, the rate of deposition of toner onto the photoconductor surface is inversely proportional to the surface resistivity of the carrier. For example, uncoated flake iron carrier has a surface resistivity of about 106 ohm cm and this material gives very dense black images. In contrast steel shot coated with a polymer has a surface resistivity of 1 109 ohm cm giving rise to comparatively lighter images.There is a great need for materials having resistivities between the extremes for coated and coated particles.

In summary, considering both slow copiers and high speed devices, current carrier and developer compositions have definite manufacturing and performance limitations, which are preventing the development of more advanced, and reliable machines and demonstrate the need for improved developer mixtures.

We have now devised an improved electrophotographic composite carrier.

According to the invention, there is provided an electrophotographic composite carrier comprising a plurality of abrasive magnetic particles selected from iron, nickel, cobalt, soft ferrite, hard ferrite, ferriso-ferric oxide and nickel oxide powder, in the size range of from 0.5 to 100 microns, the particles being embedded in a polymer matrix and comprising from 25% to 95% by weight of the composition, the particle-containing matrix being in the form of generally spherical bodies having a diameter in the range of 10 to 400 microns, and having surfaces which, during use in an electrophotographic imaging process, will be continuously mechanically eroded to exposing new particle surfaces while maintaining the triboelectric charge of the particles and providing particle self-cleaning through auto-abrasion at a constant rate.

The invention also provides a process for preparing such a carrier, which comprises preparing a liquid mixture of a polymer and magnetic particles as defined above, and drying the mixture to form carrier particles of a size from 40 to 400 microns.

The invention further provides an electrophotographic developer for use in a magnetic brush copier, which comprises a composite carrier of the invention and a toner.

The invention also includes a method of electrophotography wherein there is used a composite carrier of the invention.

The composite carriers of the invention may also be made by preparing a slurry of magnetic particles and polymer resin in a liquid, drying the slurry to form agglomerated beads, and then screening the beads to provide the desired size distribution.

- In the composite carriers of the invention, the magnetic material is iron, nickel, cobalt, soft and hard ferrites, ferriso-ferric oxide or nickel oxide. The particle size of the magnetic material is in the range 0.5-100 microns. Optimum results are obtained with particles in the range of 0.5-25 microns. When the particle size c,f raw magnetic material is found to be too large, it may be comminuted using standard size reduction methods such as air milling, jet milling, hammer milling, colloid milling and ball milling, prior to making the composite carrier.

Any suitable organic polymer or polymer combinations may be used for the matrix such as polyolefins, polystyrenes, polyurethanes, polysulfones, polyvinyl chlorides, polycarbonates and polyacrylates. If desired, solid polymers may be ground to a fine size of about 5 microns and then dispersed in a liquid prior to making the composite carriers. Liquid polymers may be dissolved or dispersed in the liquid. Optimum results are obtained by using polymer emulsions, particularly aqueous emulsions. Examples of aqueous emulsions which are useful in carrying out the objects of this invention include SR202, a styrene-acrylic copolymer manufactured by Polyvinyl Chemical Industries; Rhoplex W70, an acrylic polymer manufactured by Rohm & Haas; Poly-N-90, a polyethylene polymer dispersion made by lonac Chemical.

The preparation of the mixture of matrix polymer and magnetic particles, prior to drying, may be effected using common industrial mixing equipment, for example, a Lightening Mixer, Hockmeyer Mixer or Colloid Mill. For certain copiers, it has been found that an optimum carrier particle is achieved by using two or more polymer systems. In such cases, the order of addition of the separate polymer components can be important, for two reasons. First, emulsions can undergo shock when suddenly diluted into another system, causing large polymer particles to precipitate out Second, we have also found that the separate addition of polymer enables preferentiai coating of one of the polymers on the surface of the magnetic particles.This latter process can produce two separate heterogeneous polymer phases in the final agglomerated particle which has utilityforsome applications.

Any suitable drying method may be used to remove the liquid phase from the reaction slurry, for example, pan drying, kiln drying, rotational drying, fluidized bed drying and spray drying. A spray dryer designed for either spray nozzle or disc atomization is preferred. In either technique, the equipment produces fine droplets of the input slurry material. The liquid is removed from the droplets by a heated air stream, which also effects a particle classification of the final product. The material which exits the dryer is a free flowing powder consisting of agglomerates of the magnetic material bound in a three dimensional polymer lattice.

By adjusting process parameters such as slurry viscosity, inlet'outlet drying temperature and atomization, a wide range of final product carrier powder sizes may be obtained. The carrier particles thus produced may be further size classified to suit individual applications.

Any suitable pigmented or dyed electrophotographic toner material may be employed with the carrier materials produced in accordance with the process of this invention. A developer mixture may for example be prepared by intensively blending from 0.3 to 10 parts of toner with from 90 to 99.7 parts of carrier to make 100 parts (by weight) of developer.

The wide range of formulations made possible by this invention permits the selective variations of the magnetic properties of the carrier particles within very wide limits. This is desirable from a mechanical design standpoint because it becomes possible to match selected magnetic properties of the carrier with a preferred specific design. In contrast, current uncoated or coated metal carriers have a magnetic strength which is invariant thus constraining the mechanical design into a very narrow area which is not conducive to obtaining an optimum copy quality. For example, in a high speed xerographic printer, the magnetic brush must rotate at 500 rpm to achieve a dense black copy. Conventional metal carriers have such a strong attraction to the magnetic roller that individual carrier particles are restrained from independent movement.

This in turn inhibits the free transport of toner through the developer mixture and tends to reduce the blackness of the copy. Using the instant invention, and particularly the flexibility provided in carrier particle and coating composition selection, the magnetic properties of the carrier particles can be formulated to produce the optimum copy quality.

Another feature of this invention is the wide range of triboelectric properties made possible since any type of solid polymer may be incorporated in the formulation to prepare the agglomerated carrier particle. This means that the carrier composition can be formulated to exactly match a particular toner. This in turn gives great flexibility in toner formulation particularly when it is necessary to obtain special properties such as a low melting point. Uncoated carriers have fixed triboelectric properties which severly limits the types of toners which can be used. Since many polymer resins cannot be coated on carriers for reasons of solubility or stability reasons, the choice of coating resin is restricted and the consequent range of triboelectric properties are likewise limited.It is evident, therefore, that a particularly important advantage of the instant carrier composition is their ability to replace either uncoated or coated carrier.

As an illustration, consider a copier using a 2% toner concentration in the developer mixture when it is discovered that a new, long life cleaning system, offers an optimum performance with the toner concentration reduced to 1.5%. It is well known that the triboelectric charge increases and copy blackness diminishes as the toner concentration is reduced. With conventional carrier technology, a substantial toner reformulation, involving a different resin system would be required to accommodate the new toner concentration. Usng the carriers of this invention, however, it would be possible to readjust the triboelectric charge merely by changing the proportion of one of the coating polymers in the formulation.In fact, our work shows that the charge of the instant carriers can be varied in increments as low as 0.1 micro C/g compared with an increment of 1 micro C/g for conventional carriers.

Another advantage of this invention is the ability of the surface of the carrier particles to undergo a continual regeneration during use in a copier. In this manner, the surface remains relatively free of contaminants, such as toner film and thus the triboelectric characteristics remain constant over a long period. The regeneration process is believed to occur in the following manner.

The agglomerated particles, before being used in a copier, have been observed to include layers of magnetic particles finely dispersed in a three-dimensional polymer lattice. The polymer lattice is believed to be composed of a myriad of interconnecting fibers of finite strength. When the agglomerated carrier particles are subjected to agitation (as would occur in a copier), a constant rate of surface removal is observed. The removed material consists of small crystallites containing a few magnetic particles and polymer fibers. The surface of the agglomerates, have been observed, after agitation to be virtually identical in profile to unused particles. This and other evidence has led me to the conclusion that the agglomerated carrier particles are of homogeneous composition, thus the continual surface erosion during use retains constant surface characteristics and thus optimum triboelectric properties. An additional benefit resulting from the erosion of the carrier surface is that toner filming on the carrier is retarded which leads to longer developer life.

Yet another advantage of this invention is the ability of the carrier surface to remain relatively insensitive to changes in humidity and temperature, compared with conventional coated carriers. The surface of the instant carriers contains far less polymer than conventionally coated materials, by a factor of about one hundred, therefore environmental effects are minimized.

It is evident that the above principal advantages and others mentioned herein and those apparent to persons skilled in the art, constitute novel carrier/developer materials which result in substantial improvements to the overall electrophotographic process.

In order that the invention may be more fully understood, the following Examples (which include comparative data) are given by way of illustration only. All parts and percentages are by weight. The Examples, other than control Examples, illustrate various preferred embodiments of the present invention.

The following test procedures were used to evaluate the characteristics of the exemplified compositions of this invention.

The ability of the carrier/developer to acquire a suitable triboelectric charge was measured in a laboratory toning device. This equipment consists of a means of applying a positive or negative DC voltage to a metal plate and a means for bringing the carrier/developer into close proximity to the plate. The voltage at which toning takes place is indicative of the suitability of the carrier/developer for use in a copier.

The triboelectric charge to mass ratio of a carrier-toner combination is measured in the well known Faraday cell, using dry compressed air to remove the toner and a direct reading coulometerto monitor the charge.

Filming characteristics of photoconductor surfaces were monitored in a semi-quantitative manner using first a solvent wash of the surface and second a measurement of the relative blackness of the wash solution.

Example 1 1 00g. of a 70% aqueous dispersion of an iron oxide (&num;3100) made by Wright Industries was mixed with 30g. of a 35% aqueous dispersion of a styrene-acrylic copolymer made by Morton Chemical (#L-500). The mixture was vigorously stirred for 10 minutes then subjected to heat while the stirring was maintained.

Eventually agglomerates of the iron oxide and polymer were formed. After complete drying, the moisture content of the agglomerates was less than 0.5%. Microscopic examination of the agglomerates indicated that the iron oxide particles were tightly bonded in a matrix of polymer resin. The mechanical integrity of the agglomerates was verified by tumbling the agglomerates in a rotating cylinder for two hours. No fracturing of the agglomerates was observed. The agglomerates were mixed with a commercial toner in the ratio 100:1 and the resultant developer mixture was used in a laboratory electrophotographic toning device.Good toning density was observed at a voltage differential of +150V indicating that the agglomerates produced by the method of this invention provide a surface which is triboelectrically active and suitable for use in a magnetic brush copier which employs a positively charged photoconductor such as selenium.

Other examples were obtained generally utilizing this method as follows: A. The method used in Example 1 was repeated using St. Joe Lead magnetite M-25 and a styrene-acrylic polymer lonac Chemical Crilicon 642 in a 25%:75% composition respectively. Agglomerated particles were obtained after the material was heated to a dry state.

Bulk density .52 g/cc M5 7.02 emu/g Sp. gravity 1.4g/cc - B. The method used in Example 1 was repeated using St. Joe Lead magnetite M-25 and a styrene-acrylic polymer lonac Chemical Crilicon 642 in a 95%:5% composition, respectively. Agglomerated particles were obtained after material was heated to a dry state. Similar particles were obtained.

Bulk density .50 g/cc M5 57.0 emu/g Sp. gravity 2.14 g/cc C. The method used in Example 1 was repeated using St. Joe Lead magnetite M-25, Crilicon 642 and calcium carbonate in an 80%: 15%:5% composition, respectively. Similar particles were obtained.

Bulk density 1.12 g/cc M5 49.0 emu/g Sp. gravity 2.19 g/cc D. The method used in Example 1 was repeated using St. Joe Lead Magnetite M-25, Crilicon and calcium carbonate in a 60%: 15%:25% composition, respectively. Similar particles were obtained.

Bulk density .92 g/cc M5 38.8 emu/g Sp. gravity 1.65g/cc E. The method used in Example 1 was repeated using 100 micron powdered Belmont iron and Crilicon in a 85%:15% composition, respectively. Similar results were obtained for agglomerated particles and agglomerated particles of 400 microns were produced.

Bulk density 2.2 g/cc M5 136.0 emu/g Sp. gravity 7.32 g/cc F. The method used in Example 1 was repeated using St. Joe Lead magnetite M-25, Crilicon and conductive carbon in a 70%:5%:25% composition, respectively. Agglomerated particles were obtained after material was heated to a dry state.

Bulk density .84 g/cc M5 47.0 emu/g Sp. gravity 3.4 g/cc G. The method used in Example 1 was repeated using St. Joe Lead magnetite M-25, Crilicon and conductive carbon in an 85%:14.5%:0.5% composition, respectively. Agglomerated particles were obtained after material was heated to a dry state. Similar particles were obtained.

Bulk density 1.17 g/cc M5 52.0 emu/g SP. gravity 2.75 g/cc Example 2 The method of Example 1 was repeated using magnetic oxide (&num;CB75) manufactured by D.M. Steward and an ethylene vinyl acetate copolymer (Elvace 1962) made by DuPont. The oxide/polymer ratio was 5.0.

Substantially similar results were obtained as in Example 1.

Example 3 Using the method of Example 1, Wright 3100 magnetic oxide and Elvace 1961 were employed in the ratio 3.3:1 together with 0.05 parts of crystal violet dye C.l. 42555B. The resulting agglomerated carrier was mixed with a commercial toner in the ratio 50:1 and tested in the laboratory development device. Good toning density was observed at a voltage of -100V indicating that the carrier was suitable for use in a magnetic brush copier which employs a negatively charged photoconductor such as zinc oxide.

Example 4 The method of Example 1 was repeated using Wright &num;3100 magnetic oxide and a styrene-acrylic polymer lonac Chemical Crilicon 642 in the ratio of 3.3:1 together with 0.3 parts of a conductive carbon black. The powder product exhibited a toning voltage of +170 volts and a bulk resistivity of about 106 ohm cm. This value is equivalent to the resistivity value obtained with base metal particles of the same size.

Example 5 The method of Example 4 was repeated using powdered nickel oxide as the magnetic component. Similar results to Example 4 were obtained.

Example 6 The method of Example 4 was repeated using powder iron as the magnetic component. Results similar to Example 4 were obtained.

Example 7 One hundred pounds of a ferriso-ferric oxide with an average particle size often microns was dispersed in 50 pounds of tap water. To this slurry was added 40 pounds of Crilicon 642, a styrene acrylate polymer emulsion manufactured by lonac Chemical. The mixture was stirred for two hours then fed into the inlet port of a Niro pilot plant spray dryer operating at a wheel speed of 10,000 rpm and inlet temperature of 475 degree F and outlet temperature of 220 degree F. The carrier product exiting the spray dryer was observed to be particle agglomerates. Individual particle agglomerates were revealed to be generally spherical in shape.

The agglomerate surface of such particles consisted of magnetic particles embedded in a polymer matrix.

The agglomerated particle size ranged from about 60 microns to about 90 microns. The bulk resistivity of the powder was about 109 ohm cm and the saturated magnetization 70 emu/g.

Testing of the carrier in the laboratory development apparatus revealed that the material gave a good triboelectric charge with several toners of varying composition.

Example 8 Using the method and formulation described in Example 7, about 2,000 pounds of composite carrier were produced using plant-scale equipment. The agglomerated particle product had the following properties: Size distribution . . . 60 - 120 microns Material density . . . 3.2 g/cc Bulk density ..... 1.3 g/cc Saturated magnetization 76 emu/g Bulk resistivity 109 ohm cm Example 9 The procedure of Example 7 was repeated using the following formulation:: Ferisso-ferric oxide 85% Elvace 1962 (DuPont) 8% Crilicon 642 (lonac) 5% Conductive polymer 2% The product from the spray dryer had the following characteristics Size distribution 80 - 110 microns Material density 4.0 g/cc Bulk density 1.4 g/cc Saturated magnetization 75 emu/g Bulk resistivity 106 ohm cm The method of this example was repeated utilizing MPB (a product of the Indiana General division of Electronic Memories and Magnetics Corp. having an average particle size of 1-2 microns) and polyethylene Pen 35 resin were spray dried and resulted in agglomerated particles.

Bulk density .50 g/cc M5 54.0 emu/g Good toning density was observed at low voltage differential of +150V indicating that the agglomerates produced by the method of this invention provide a surface which is triboelectrically active and suitable for use in a magnetic brush copier which employs a positively charged photoconductor such as selenium.

Example 10 The procedure of Example 7 was repeated using an iron-zinc ferrite magnetic component having a saturated magnetization of 44 emu/g as a carrier. The carrier was found to be useable in a variety of high speed copiers and printers.

Example ii The procedure of Example 7 was repeated using a finely ground ferriso-ferric oxide, i.e. particle size of 1 micron or less, as the magnetic component. The generally spherical carrier material thus formed has a relatively smoother surface than particle agglomerates of Example 7 but remain an acceptable carrier medium for copiers which have soft photoconductors.

Example 12 The procedure of Example 7 was repeated using 50% of a ferrisoferric oxide and 50% of polymer. The saturated magnetization of the carrier product was 40 emu/g. The magnetic and physical properties produced by this procedure are useful when a very soft magnetic brush is desired, for example for the reproduction of high definition photographs.

Example 13 The procedure of Example 8 on plant scale equipment was repeated using the resins shown below and acceptable carriers for specific purposes were produced: Tribo Special Ex Resin Sign Properties Found 13 Polyethylene -ve Resistance to humidity.

14 Polytetrafluoro -ve Resistance to tonerfilming ethylene Extra long carrier.

15 Polyurethane +ve Capable of retaining and releasing small amounts of additive.

The carrier as prepared in Example 7, was evaluated in the 3100 model copier manufactured by Xerox Corporation in the following manner. Two pounds of carrier were mixed with 1009 of toner manufactured by Xerox Corporation for use in the 3100 copier, thus forming a developer mixture. This mixture was used to make copies. No adjustments were made to the machine. Sample copies were removed at 100 copy intervals and were examined for signs of developer deterioration. The normal developer life of commercial coated carrier products in the Xerox 3100 copier is known to be about 15,000 copies. The material of this example showed excellent copy quality through 20,000 copies, when the test was discontinued. The copies exhibited a greater cleanliness at 20,000 copies than normally encountered at 15,000 using commercial developers.

Inspection of the selenium photoconductor drum showed the surface to be completely free of toner film.

As a comparison, this copy machine test was repeated using a commercial brand developer. After 12,000 copies a significant deterioration in copy quality was observed particularly the background contained a gray deposit of toner, indicating the developer was approaching its limit of useful performance. Inspection of the selenium photoconductor revealed the presence of a thin film of toner. At 14,000 copies the copy quality standard had fallen to a point at which replacement of the developer mixture was required.

To more fully quantify the anti-filming characteristic of the carriers produced in Examples 7 and 8 were compared to the coated carrier used by Xerox Corporation in its Model 3100 copying machine. The following experiment was devised to make this comparison.

Two grams each of the carriers from Examples 7 and 8 and Xerox coated carrier were evenly distributed over separate aluminum sheets. A 5 kg. weight was placed on top of each of the aluminum sheets. The weight and top sheet were dragged over each of the carrier and bottom sheets, then, a visual comparison was made of the abrasion that resulted on this sheet to previously tested sheets.

The carriers from Examples 7 and 8 visually demonstrated greater abrasive properties on the sheets and hence reduced the film forming effect of the carrier when compared to that of the Xerox coated carrier.

A ceramic tumbling mill was set up to evaluate the surface abrasion of the materials produced according to the instant invention. This test is designed to simulate the type of agitation which a carrier would experience in a magnetic brush copier. Based on experimental observation of several carriers in the tumbling mill test, one hour of tumbling is equivalent to about 10 hours of agitation under actuai copier conditions.

The evaluation was conducted by tumbling two pounds of carrier prepared according to Example 8 in a ceramic mill for 10 hours, i.e. the equivalent of approximately 100 hours of operation of a conventional copy machine. At periodic intervals, a small sample was removed from the mill and examined for ultra fine material. The following results were obtained.

Hours Tumbling Cumulative % Abraded Material 2 2.7 4 4.9 6 6.5 8 8.4 10 11.3 This data demonstrates abrasion or erosion of the surface of each carrier particle at a relatively constant rate and was confirmed by comparison of abraded with fresh particles. I observed that the tumbling effect was causing repeated regeneration of a fresh agglomerated particle surface.

The material from Example 9 was also subjected to the tumbling test as described above and produced the following results.

Hours Tumbling Cumulative%Abraded Material 2 0.8 4 2.1 6 3.3 8 4.5 10 5.4 After ten hours of tumbling, again the equivalent of 100 hours of machine operation, 5.4% of the initial material had been removed from the surface. This is equivalent to an average loss of surface thickness of about 2 microns.

A developer life study was conducted on the RBC IV copier distributed by Litton Industries. Using toner and carrier sold by Litton for the RBC IV, about 8800 copies were obtained before copy quality deteriorated, indicating the end of the useful life of the developer. This was contrasted with another study wherein one hundred parts of carrier from Example 9 was used with the RBC IV toner to form a developer material. This developer was then used to make copies using the RBC IV copier. Copy quality remained stable for 20,000 copies when the text was discontinued. The material of this invention demonstrated a significantly greater developer life than that of existing technology.

Claims (20)

1. An electrophotographic composite carrier comprising a plurality of abrasive magnetic particles selected from iron, nickel, cobalt, soft ferrite, hard ferrite, ferriso-ferric oxide and nickel oxide powder, in the size range of from 0.5 to 100 microns, the particles being embedded in a polymer matrix and comprising from 25% to 95% by weight of the composition, the particle-containing matrix being in the form of generally spherical bodies having a diameter in the range of 10 to 400 microns, and having surfaces which, during use in an electrophotographic imaging process, will be continuously mechanically eroded to exposing new particle surfaces while maintaining the triboelectric charge of the particles and providing particle self-cleaning through auto-abrasion at a constant rate.

2. A carrier according to claim 1, which, during constant auto-abrasion in an electrophotographic imaging process, effects impact abrasion and contaminant removal from photoconductive copier surfaces which it contacts.

3. A carrier according to claim 1 or 2, wherein the polymer matrix comprises from 5% to 75% by weight of the composition.

4. A carrier according to claims 1 or 2, wherein the polymer matrix includes a calcium carbonate filler material in a weight range of from 5 to 25%.

5. A carrier according to claim 1,2,3 or 4, in which the size of the magnetic particles is from 0.5 to 25 microns.

6. A carrier according to any of claims 1 to 5, in which the matrix comprises a polymer selected from styrene-acrylic copolymers, acrylic polymers, vinyl chloride polymers, vinyl acetate polymers, polyethylene, poiyurethane, polytetrafluoroethylene and ethylenevinyl chloride vinyl acetate terpolymers.

7. A carrier according to any of claims 1 to 5, in which the matrix comprises a binary or ternary combination of one or more styrene-acrylic copolymers, acrylic polymers, vinyl chloride polymers, vinyl acetate polymers, ethylene-vinyl chloride-vinyl acetate terpolymers, polyethylene polymer, polyurethane polymer, and polytetrafluoroethylene.

8. A carrier according to any of claims 1 to 7, which includes a conductive agent in a weight range of from 0.5 to 25%.

9. An electrophotographic carrier according to claim 1 substantially as herein described in any of the Examples.

10. A process for preparing an electrophotographic composite carrier as defined in claim 1, which comprises preparing a liquid mixture of a polymer and magnetic particles as defined in claim 1, and drying the mixture to form carrier particles of a size from 40 to 400 microns.

11. A process for preparing an electrophotographic composite carrier as defined in claim 1, which comprises dispersing the magnetic particles as defined in claim 1 in a water emulsion of a polymer, and spray-drying the mixture while maintaining temperatures across the dryer between 220"F and 475"F in the formation of the generally spherical composite carrier particles of size 10 to 400 microns.

12. A process according to claim 11, wherein the spray-drying entry temperature is about 475"F.

13. A process according to claim 11 or 12 wherein the spray drying exit temperature is about 220F.

14. A process according to claim 10, 11, 12 or 13, wherein the abrasive magnetic particles are separately coated with a polymer prior to mixing with the matrix polymer.

15. A process according to claim 14, wherein the particles are separately coated with more than one polymer.

16. A process for preparing an electrophotographic composite carrier as claimed in claim 1, substantially as herein described in any of the Examples.

17. An electrophotographic composite carrier material made by the process of any of claims 10 to 16.

18. An electrophotographic developer for use in a magnetic brush copier, which comprises a composite carrier as claimed in any of claims 1 to 9 and 17, together with a toner.

19. A developer according to claim 18, which contains from 90 to 99.7 parts of carrier and from 0.3 to 10 parts of toner, by weight, per 100 parts of developer.

20. A method of electrophotography wherein there is used a carrier as claimed in any of claims 1 to 9 and 17.