CA1330006C

CA1330006C - Electrographic magnetic carrier particles

Info

Publication number: CA1330006C
Application number: CA000568479A
Authority: CA
Inventors: Edward Timothy Miskinis; Bijay Shankar Saha
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1987-06-15
Filing date: 1988-06-02
Publication date: 1994-06-07
Anticipated expiration: 2011-06-07
Also published as: EP0296072A2; US4764445A; DE3882603D1; JP2612035B2; EP0296072A3; DE3882603T2; JPS6419361A; EP0296072B1

Abstract

ELECTROGRAPHIC MAGNETIC CARRIER PARTICLES

ABSTRACT OF THE DISCLOSURE
Hard ferrite magnetic carrier particles for use in two component developers together with toner particles for the development of electrostatic latent image patterns and containing from about 1 to about 5 percent by weight of lanthanum exhibit improved development efficiency.

Description

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ELECTRO~RAPHIC MAGNETIC CARRIER PARTICLES

BACKGROUND OF THE INVENTION
This invention relates to electrography ~nd more particul~rly it relates to magnetic carrier particles Pnd developers for the dry development of electrost~tic charge images.
In electrography, an electroststic chsrge image i8 formed on a dielectric surface, typicslly the surface of the photoconductive recording element. Development of this image is commonly achieved by contacting it with a two-component developer comprising a mixture of plgmented resinous particles, known as toner, and magnetically attractable particles, known as carrier. The carrier particles serve as sltes against which the non-magnetic toner particles can impinge and thereby acquire a triboelectric charge opposite to that of the electrostatic image. During contact between the electrostatic image and the developer mixture, the toner particles are stripped from the carrier particles to which they had formerly adhered (via triboelectric forces) by the relatively strong electrostatic forces a~sociated with the charge image. In this menner, the toner particles sre deposlted on ~he electro~tatic image to render it visible.
It is known in the art to apply developer compo~itions of the above type to electrostatic image~ by me~ns of a magnetic applicator which compri~es a cylindrical ~leeve of non-magnetic material having a magnetic core positioned within.
The core u~u~lly comprise~ a plurality of parallel magnetic ~trips which are arranged around the core surface to present alternative north and ~outh -` 133~6 magnetic fields. These fields pro~ect radially, through the sleeve, and serve to attract the developer composition to the sleeve outer surface to form a brushed nap. Either or both the cylindrical sleeve and the magnetic core are rotated with respect to each other to cause the developer to advance from a ~upply sump to a position in which it contacts the electrostatic imsge to be developed. After development the toner depleted carrier particles are returned to the sump for toner replenishment.
Conventionally, c~rrier particles made of soft magnetic materials have been employed to carry and deliver the toner particles to the electrost~tic image. U.S. Patents 4,546,060 and 4,473,029 teach the use of h~rd msgnetic materi~ls ss csrrier particles and an app~ratus for the development of electrostatic images utilizing ~uch hard magnetic carrier psrticles, respectively. These p~tents require th~t the carrier particles comprise a hard m~gnetic m~terial exhibiting a coercivity of at least 300 Oersteds when magnetically s~turated snd an induced magnetic moment of at least 20 EMU~gm when in an spplied magnetic fleld of 1000 Oersteds. The terms "hsrd" snd "soft" when referring to magnetic msterials have the generslly accepted meaning as indicated on psge 18 of Introduction To MaRnetic Materials by B. D. Cullity published by Addison-Wesley Publishing Company, 1972. These hsrd msgnetic csrrier materisls represent ~ great sdvance over the use of soft msgnetic csrrier m~terials in thst the speed of development is remarkably increased without experiencing deteriorstion of the image.
Speeds ~s high as four times the maximum speed utilized in the use of soft magnetic carrier particles hsve been demonstrsted.

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The above two mentioned U.S. patents, while generic to all hard magnetic materials having the properties set forth, prefer the hard magnetic ferrites which are compounds of barium and/or strontium such as, BaFel2Olg, SrFel2Olg and the magnetic ferrites having the formula MO-6Fe2O3, where M is barium, strontium or lead as disclosed in U.S. Patent No. 3,716,630. While these hard ferrite carrier materials represent a substantial increase in the speed with which development can be conducted in an electrostatographic apparatus, it is desired that even further improvements in this regard be made.
SUMMARY OF THE INVENTION
The present invention provides carrier particles for use in the development of electrostatic images wherein the carrier particles comprise a hard magnetic ferrite msterial having a hexagonal crystal structure, exhibiting a coercivity of at lecst 300 20 Oersteds when magnetically saturated and an induced magnetic moment of at least 20 EMU/gm when in an applied field of l000 Oersteds and containlng from about l to about 5 percent by weight of lanthanum.
The invention also contemplates an 25 electrographic developer suitable for extremely high speed copying applications without the loss of copy image quality including charged toner particles and ~ oppositely charged carrier particles as described s above. The method of developing electrostatic images 30 on a surface is also contemplated utilizing a ~ ~ two-component developer.
: DETA'I~ED'DE'SCRI~TION OF ~HE ~REFE~RED ~MBODIMENTS

As pointed out above in connection with U.S.
Patents 4,546,060 and 4,473,029 the use of "hard"
magnetic materials as carrier particles increases the ., .

1330~6 speed of development dramatically when compared with carrier particles made of "soft" magnetic particles. The preferred ferrite materials disclosed in these patents include barium, strontium and lead ferrites having the formula MO' 6Fe203 wherein M is barium, strontium or lead. These materials have a hexagonal structure. While the speed with which development can be carried out is much higher than the heretofore techniques employed, they are limited by the resistivity of the above described ferrite materials which have the necessary magnetic properties for carrying out the development method.
For example, the resistivity of strontium ferrite having the formula SrO-6Fe2O3 is approximately 10 ohm.cm. It is generally known that the resistivity of the carrier particles bears a direct result on the speed of development that can be employed.
While development speed is generally referred to in the prior art, a more meaningful term is to speak of "development efficiency". In a magnetic brush development system, development efficiency is defined as the potential difference between the photoreceptor in developed image areas before and after development divided by the potential difference between the photoreceptor and the brush prior to development times 100. Thus, for example, if the photoreceptor film voltage is -250 volts and the magnetic brush is -50 volts the potential difference is -200 volts prior to development. If, during development, the film voltage is reduced by 100 volts to -150 volts in image areas by the deposition of positively charged toner particles, the development efficiency is (-100 volts . -200 volts) x 100, which gives an efficiency of development of S0 ~ A ~

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percent. It can be readily seen that as the efficiency of the developer material increases the various parameters employed in the electrost~tographic method c~n be altered in ~ccord~nce therewith. For example, ~ the efficiency increa~es the volt~ge differenti~l prior to development can be reduced in order to deposit the s~me amount of toner in image areas ~s was previously done at the lower efficiency. The same is true with regard to the exposure energy level employed to impart the latent electro~tatic image on the photoreceptor film. The speed of the development tep of the procedure can be increased as the efficiency increases in that as the efficiency increases more toner can be deposited under the Qame conditions in a shorter period of time. Thus, higher development efficiency permits the reoptimization of the variou~ parameters employed in the electrostatic proce~s thereby resulting in savings in both energy and time.
The efficiency of development when employing the ferrite carrier~ of the prior art is llmlted by the reslstlvlty of the ferrlte materials themselves.
For example, because these materlals h~ve a re~istlvlty of approximately 10 ohm.cm the highe~t efficiency 1~ approxlmately 50 percent. However, ln order to obtain hlgh quallty copies of the orlglnal lmage, it ls necessary to maintain the high magnetic propertie~; l.e. a coercivlty of at least 300 Oersteds when magnetically saturated and an induced magnetlc moment of at least 20 EMU/gm when ln an applled field of 1000 Oersteds while at the same time increaslng the conductivity of the particles.
The invention contemplates the incorporation of an effective amount of lanthanum into the ..... .

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~, 1~3~6 crystalline l~ttice of ~ hard magnetic ferrite material hsving a hexagonal crystal structure to reduce the resistivity of the material while maintaining the magnetic properties. Thus, the resistivity of hard hexagonal ferrite materials can be reduced from approximately 10 to approximately ohm.cm without effecting the high magnetic properties of the material. While it is not the intent to be bound by any theory or mechani~m by which the resistivity of these ferrite materials ~re decreased, it is believed that the lanthanum repl~ces the barium, strontlum or lead in the ferrite structure when introduced in amounts of from l to about 5 percent by weight. Since lanthanum exists in the +3 oxidation state and these other materials (Ba, Sr and Pb) ln the +2 oxidation state the substitution of lanthanum cau~eQ the iron to revert from the +3 ~tate to the +2 oxidation ~tate to thereby maint~in charge neutrality ln the ferrite cry~tal. -~herefore, by ~d~u~ting the amount of lanthanum that ls substltuted into the ferrite cry~tal the amount of iron ln the +2 ~tate can be controlled ~nd therefore the re~istivity of the materlal i~ ln turn ad~usted.
It is pre~erred that the amount of lanthanum substituted into the crystalllne lattlce of the ~errlte be limited ~uch that only a single phase hexagonal crystalline structure is obtained. While the qusntity of lanthanum will vary somewhat depending upon the ~intering condltions utllized in the preparstion of the ferrite particle~, it has been found that the amount of lsnthanum can vary from about l to about 5 percent by weight of the ferrite material and ~till maintain the high magnetic propertie~ needed to prevent throw-off of the developer from the magnetic bru~h developer. As the qu~ntlty of lanthanum exceeds thls amount a second _7_ 1 3 ~ ~ ~ 0 ~
phase, believed to be LaFeO3 having an orthorhombic structure is formed. While the continued increase in the amount of lanthanum reduces the resistivity significantly the formation of the orthorhombic structure causes a dramatic decrease in the msgnetic properties of the ferrites which thereby cre~tes lmage quality problems. In addition the decrease in magnetic force is responsible for an increase in throw-off from the magnetic brush.

The preparation of ferrites generally and hard hexagonal ferrites (Ba, Sr or Pb) particularly are well documented in the literature. Any suitable method of making the ferrite particles may be employed such as disclosed in U.S.
Patents 3,716,630, issued 13 February 1973, 4,623,603, issued 18 November 1986, and 4,042,518, issued 16 August 1977;
European Patent Application 0 086 445, published 24 August 1973; "Spray Drying" by K. Masters, published in 1972 by CRC
Press, Cleveland, Ohio, U.S.A., pages 502-509 and "Ferromagnetic Materials", Volume 3, edited by E.P. Wohlfarth and published in 1980 by North-Holland Publishing Company, Amsterdam, New York, Oxford, pages 315 et se~. The ferrites çontaining from about 1 to about 5 percent by weight of lanthanum in accordance with this invention are prepared in a similar manner as described above by adding lanthanum oxide to the formulation. For example, if the ferrite to be prepared is strontium ferrite containing from about 1 to about 5 percent by weight of lanthanum, about 8 to 12 parts strontium carbonate, about 1 to 5 parts lanthanum oxide and 85 to 90 parts of iron oxide are mixed with a dlspersant polymer gum arabic and water as a solvent. The solvent is removed by spray drying and the resultant beads are fired at about 1200 C
to form the ferrite LaxSrl_xFel2lg has a value of from about 0.1 to about 0.4. The i.

133~6 ferrite is ball milled to reduce the particle size to that generally required of carrier particles, that is, less than 100~ and preferably from about 5 to 65~m, and then permanently magnetized by sub~ecting it to an applied magnetic field of sufficient strength to magnetically saturate the particles.
The present invention comprise two types of carrier particles. The first of these carriers comprises a binder-free magnetic psrticulate material exhibiting the requisite coercivity and induced magnetic moment. This type is preferred.
The second is heterogeneous and comprises a composite of a binder and a magnetic material exhibiting the requi~ite coercivity and induced magnetic moment. The magnetic material is dispersed as discrete ~maller particles throughout the binder;
however, the resistivity of these binder type polymers must be comparable to the binderless particles in order for the above stated advantages to be observed. It may be desirable to add conductive csrbon black to the binder to insure eleGtrical contact between the ferrite particle~.
The indlvidual bits of the magnetic material should preferably be of a relatively uniform size and sufficiently smaller in diameter than the composite carrier particle to be produced. Typically, the average diameter of the magnetic material should be no more than about 20 percent of the sverage diameter of the carrier particle. Advantageously, a much lower ratio of average diameter of magnetic component to carrier can be used. Excellent results are obtained with magnetic powders of the order of S
micrometers down to 0.05 micrometer average diameter. Even finer powders can be used when the -- 133~6 _g_ degree of subdivision does not produce unwanted modifications in the magnetic properties and the amount and character of the selected binder produce satisf~ctory strength, together with other de~irable mechanical and electrical properties in the resulting carrier partlcle.
The concentration of the magnetic material can vary widely. Proportions of finely divided magnetic material, from about 20 percent by weight to about 90 percent by weight, of compo~ite carrier can be used so long as the resistivity of the particles is that repre~entative of the ferrite particles above .
The induced moment of composite carriers in a 1000 Oersted~ applied field i~ dependent on the concentration of magnetic material in the particle.
It will be appreciated, therefore, that the induced moment of the magnetic material ~hould be sufficiently greater than 20 EMU/gm to compen~ate for the effect upon Quch induced moment from dilution of the magnetic material in the binder. For example, one might find that, for a concentration of 50 weight percent magnetic material in the compos~te particles, the 1000 OerstedQ induced magnetic moment of the magnetic material ~hould be at lea~t 40 EMU/gm to achieve the minimum level of 20 EMU/gm for the compo~ite particles.
The binder material used with the finely divided magnetic material is selected to provide the required mechanical and electrical propertie~. It should (1) adhere well to the magnetic material, (2) facilitate formation of ~trong, ~mooth--Qurfaced particles and (3) preferably pos~ess ~ufficient difference in triboelectric properties from the toner particle~ with which it will be used to insure the 133~6 proper polarity and magnitude of electrostatic charge between the toner and carrier when the two are mixed.
The matrix can be orgsnic, or inorgsnic, such as a matrix compo~ed of glas~, metsl, ~ilicone resin or the like. Preferably, sn orgsnic msterial is used such as a natursl or synthetic polymeric resin or a mixture of such resins hsving sppropriste mechsnical properties. Appropriate monomers (which csn be used to prepare resins for this use) include, for exsmple, vinyl monomers such a8 alkyl scrylstes snd methscrylate~, styrene snd substituted styrenes, basic monomers such as vinyl pyridines, etc.
Copolymers prepared with these snd other vinyl monomers such a8 scidic monomers, e.g., scrylic or methscrylic acid, can be used. Such copolymers can sdvsntsgeously contain small amounts of polyfunctional monomers ~uch a8 divinylbenzene, glycol dimethacrylste, trisllyl cltrste snd the like. Condensation polymers such a8 polye~ters, poly~mides or polyc~rbonstes can al~o be employed.
PrepQrstlon of composite carrier psrticles sccording to this Invention msy involve the spplicstion of he~t to soften thermopls~tic msterisl or to hsrden thermosetting msterisl; evaporstive drying to remove liquid vehicle; the use of pres~ure, or of hest snd pressure, in molding, casting, extruding, etc., snd in cutting or shesring to shspe the c~rrler psrticles; grinding, e.g., in bsll mill to reduce csrrier msterisl to sppropriste psrticle size; snd sifting operstions to clsssify the psrticles.
According to one prepsrstion technlque, the :~: :
powdered m~gnetic msterisl is dispersed in a solution of the binder resin. The solvent msy then be ,~ ~

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ev~porated and the resulting solid mass subdivided by grinding and screening to produce carrier particles of appropriate size.
According to another technique, emulsion or suspension polymerization is used to produce uniform carrier particles of excellent smoothness and useful life.
The coercivity of a magnetic mMterial refers to the minimum external magnetic force necessary to reduce the induced magnetic moment from the rem~nence value to zero while it is held stationsry in the externsl field, and after the material has been magnetically saturated, i.e., the material has been permanently magnetized. A variety of apparatus and methods for the messurement of coercivity of the present carrier particles can be employed. For the present invention, a Princeton Applied Research Model 155 Vibrating Sample Magnetometer*, available from Princeton Applied Research Co., Princeton; N.J., is used to messure the coercivity of powder psrtlcle s~mples. The powder was mixed with a nonmsgnetic ; polymer powder (90 percent m~gnetic powder: 10 percent polymer by weight). The mixture was placed in a cspillsry tube, heated above the melting point of the polymer, snd then allowed to cool to room temperature. The filled capillary tube was then plsced in the sample holder of the magnetometer and a magnetic hysteresis loop of external field (in Oersteds) versus induced magnetism (in EMU/gm) was ~ 30 plotted. Durlng this messurement, the ssmple was ;~ exposed to sn external field of 0 to 8000 Oersteds.
The csrrier particles may be coated ln order to properly charge the toner particles of the developer. This csn be done by forming a dry mixture 3s of suitable ferrite with a small amount of powdered * Trade Mark of Princeton Applied Research Co.

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resin, e.g., 0.05 to 3.0 weight percent re~in, and heating the m~xture to fuse the resln. Such a low concentration of resin will form a thin or discontinuous layer of resin on the ferrite psrticle~.
Since the presence of lanthanum in the ferrite is intended to improve conductivity of carrier particles, the layer of resin on the carrier particles should be thin enough that the ma~s of p~rticles remains conductive. Preferably the resin layer is discontinuou~; ~pot~ of bare ferrite on each particle provide conductive contact.
Various resin materials c~n be employed as a coa~ing on the "hsrd" magnetic c~rrier p~rticles.
Example~ include tho~e described in U.S. Patent Nos.

3,79S,617 is~ued March S, 1974, to J. McC~be, 3,79S,618 issued March 5, 1974, to G. Ka~per, ~nd 4,076,8S7 to G. Ka~per. The choice of re~in will depend upon its triboelectric relationship with the intended toner. For use with toner~ which are de~ired to be po~itively ch~rged, preferred re~in~
for the c~rrier co~ting include fluoroc~rbon polymer~
~uch ag poly(tetrafluoroethylene), poly(vinylidene fluoride) ~nd poly(vinylidene fluoride-co-tetra-fluoroethylene).
The developer is formed by mixing theparticle~ with toner particle~ in a Quitable concentrstion. Within developer~ of the invention, high concentration~ of toner c~n be employed.
Accordingly, the pre~ent developer prefersbly contain~ from about 70 to 99 weight percent carrier ~nd about 30 to 1 weight percent toner ba~ed on the total weight of the developer; most preferably, ~uch concentration i~ from ~bout 75 to 99 weight percent c~rrier and from ~bout ~5 to 1 weight percent toner.

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The toner component of the invention can be a powdered resin which is optionally colored. It normally is prepared by compounding a resin with a colorant, i.e., a dye or pigment, and any other desired addenda. If a developed image of low opacity is desired, no colorant need be added. Normally, however, a colorant is included and it can, in principle, be any of the materisls mentioned in Colour Index, published in 1956 by Textile Chemists and Colorists, Lowell, Massachusetts, U.S.A., Vols. I and II, 2nd Edition. Carbon black is especially useful. The amount of colorant can vary over a wide range, e.g., from 3 to 20 weight percent of the polymer. Combinations of colorants may be used.
The mixture is heated and milled to disperse the colorant and other addenda in the resin. The mass is cooled, crushed into lumps and finely ground. The re~ulting toner particles range in diameter from 0.5 to 25 micrometers with an average size of 1 to 16 micrometers. Preferably,-~the Average particle size ratio of carrier to toner lies within the range from about 15:1 to about 1:1. However, carrier-to-toner average particle size ratios of as high as 50:1 are also useful.
The toner resin can be selected from a wide variety of materials, including both natural and synthetic resins and modified natural resins, as disclosed, for example, in the patent to Kasper et al., U.S. Patent No. 4,076,857 issued February 28, 1978. Especially useful are the crosslinked polymers disclosed in the patent to Jadwin et al., U.S. Patent No. 3,938,992 issued February 17, 1976, and the patent to Sadamatsu et al., U.S. Patent No. 3,941,898 issued March 2, 1976. The crosslinked or noncrosslinked copolymers of styrene or lower alkyl styrenes with acrylic monomers such as alkyl ` r ~
; A

~ 1 3 ~

acrylates or methacrylates are particul~rly useful.
Also useful ~re condens~tion polymers such ~s polyesters.
The shape of the toner can be irregular, BS
in the case of ground toners, or spherical.
Spherical particles are obtained by spray-drying a solution of the toner resin in a solvent.
Alternatively, spherical particle~ can be prepared by the polymer bead swelling technique disclosed in lo European Patent No. 3905 published September 5, 1979, to J. Ugelstad.
The toner can also contain minor components such as charge control sgents and antiblocking agents. Especially u~eful charge control agents are disclosed in U.S. Patent No. 3,893,935 and British Patent No. 1,501,065. Quaternary ammonium salt charge agents as disclosed in Research Disclosure, No. 21030, Volume 210, October, 1981 (published by Industri~l Opportunities Ltd., Homewell, Havant, H~mp~hire, PO9 lEF, Unlted Kingdom), are slso useful.
In the method of the pre~ent lnventlon, an electrostctic lmage ls brought into cont~ct with a magnetic brush comprising a rotating-magnetic core, an outer non-magnetlc shell ~nd the two-component, dry developer de~cribed above. The electrostatic image 90 developed can be formed by Q number of methods such as by imagewise photodecay of a photoreceptor, or imagewise application of a charge pattern on the surface of a dielectric recording element. When photoreceptors are employed, such BS
in high-speed electrophotographic copy dev~ces, the use of halftone ~creening to modify an electroststic image can be employed, the combination of screening with development in accordance with the method for .
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the present invention producing high- quality lmsges exhibiting high Dmax and excellent tonal range.
Representatlve screening methods including those employing photoreceptors with integral half-tone screens are disclosed in U.S. Patent No. 4,38S,823 issued May 31, 1984.
Developers including magnetic carrier particles in accordance with this invention when employed in an apparstus such as that described in U.S. Pstent 4,473,029 exhibit a dramatic increase in development efficiency when compared with a similar ferrite materi~l not containing lanthanum when operated at the same voltage differential of the magnetic bru~h snd photoconductive film. For example, when strontium ferrite carrier particles, similar in all respects except for the presence of lanthanum therein is compared with carrier partlcles containing 3.3 percent by weight of lanthanum, the efficiency of development is improved from about 50 percent to clo~e to 100 percent, all other condltions of development remaining the same. Thus, by employing the carrier particles in accordance wlth this invention, the operating conditions such a~ the voltage differential, the exposure energy employed in forming the latent electrostatic image and the speed ; of tevelopment may all be varied ln order to achieve optimum conditions and results.
The invention is further illustrated by the Following examples:
ExamPle 1 An electrographic device as described in U.S. Patent 4,473,029 is employed in this example.
The device has two electrostatic probes one before ~ the msgnetic brush development station and one ~fter ; 35 the station to measure the voltage on the ' .~ ' ;
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photoconductive film before and after development.
The carrier particles are a lanthanum-strontium ferrite, the lanthanum being present in an amount of 2.7 percent by weight. The toner employed is described in Example 1 of U.S. Patent 4,394,430, is present in a concentration of 13% based on the combined weight of the carrier and toner and charges to a value of 25 ~C/g. The photoconductive film is charged to -370 volts and the magnetic brush is maintained ~t -150 volts. After development, the charge on the photoconductive film in developed areas is -150 volts thus indicating a development efficiency of 100% (220 x 100 = 100%) .
(220 ExamPle 2 (Comparison) Example 1 is repeated with the exception that SrFel2019 is employed as the carrier material. The photoconductive surface is_charged to 475 volts in order to achieve the same DmaX as that of Example 1. All other conditions including the toner concentration and charge are the same. The voltage on the photoconductive film surface after development is 275 volts. The development efficiency is 475-275 x 100 = 61.5~.

ExamPles 3-7 Strontium ferrite carrier particles containing lanthanum in the amounts set forth in the following table are prepared in accordance with the procedure set forth above. A device employing a developer station as described in U.S. Patent 4,473,029 and a buchner funnel disposed over the magnetic brush such that the filter paper is in the same relative position as the photoreceptor is used l ,~
~ A.~

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to determine throw-off of developer during rotation of the brush. In each case, the same toner in the same concentration as set forth in Example 1 is used. The brush is rotated for each carrier for two minutes while vacuum is drawn and developer is collected on the filter paper. The data establishes that while the charge on the ~oner in each case is substantially the same, the throw-off is signlficantly higher when the limits of this invention are exceeded.
Table Wt. ~Charge on the Example Lanthanum Toner ~C/g Throw-off mg 3 3.3 25.5 1.0 4 7.9 27.1 13.6 2.7 32.3 0.2 6 4.9 35.5 0.1 7 8.2 26.1 11.0 Barium ferrite and lead containing ferrites commonly referred to as magnetoplumbite substituted with lanthanum achieve similar results when used as electrographic carrier materials.
Although the lnvention has been described in considerable detail, wlth particular reference to preferred embodiments, variations and modifications be made therein within the scope of the invention.

Claims

1. Carrier particles for use in the development of electrostatic latent images which comprise hard magnetic ferrite material having a single phase hexagonal crystal structure, exhibiting a coercivity of at least 300 Oersteds when magnetically saturated, and an induced magnetic moment of at least 20 EMU/gm of carrier in an applied field of 1000 Oersteds and containing from about 1 to about 5 percent by weight of lanthanum, where said particles are coated with a discontinuous resin layer.

2. The carrier particles of claim 1 wherein the hard magnetic ferrite material is strontium ferrite, barium ferrite or lead ferrite containing from about 1 to about 5 percent by weight of lanthanum.

3. The carrier particles of claim 2 wherein the hard magnetic ferrite is strontium ferrite.

4. The carrier particles of claim 2 wherein the hard magnetic ferrite is barium ferrite.

5. The carrier particles of claim 1 wherein the hard magnetic ferrite is lead ferrite.

6. The carrier particles of claim 1 having the formula LaxM1-xFe12O19 where x has a value such that lanthanum is present in an amount of about 1 to about 5 percent by weight and M is Ba, Sr or Pb.

7. The composition of claim 1 wherein lanthanum is present in an amount of from about 2 to 4.5 percent by weight.

8. A method for developing an electrostatic image comprising contacting the image with a two-component dry developer composition comprising charged toner particles and oppositely charged carrier particles according to claim 1.

9. An electrostatic two-component dry developer composition for use in the development of electrostatic latent images which comprises a mixture of charged toner particles and oppositely charged carrier particles which comprise hard magnetic ferrite material having a single phase hexagonal crystal structure, exhibiting a coercivity of at least 300 Oersteds when magnetically saturated, and an induced magnetic moment of at least 20 EMU/gm of carrier in an applied field of 1000 Oersteds and containing from about 1 to about 5 percent by weight of lanthanum.

10. The composition of claim 9 wherein the hard magnetic ferrite material is strontium ferrite, barium ferrite or lead ferrite containing from about 1 to about 5 percent by weight of lanthanum.

11. The composition of claim 10 wherein the hard magnetic ferrite is strontium ferrite.

12. The composition of claim 10 wherein the hard magnetic ferrite is barium ferrite.

13. The composition of claim 10 wherein the hard magnetic ferrite is lead ferrite.

14. The composition of claim 9 wherein said carrier particles have the formula LaxM1-xFe12O19 where x has a value such that lanthanum is present in an amount of about 1 to about 5 percent by weight and M is Ba, Sr or Pb.

15. The composition of claim 9 wherein lanthanum is present in an amount of from about 2 to 4.5 percent by weight.

16. A method for developing an electrostatic image comprising contacting the image with a two-component dry developer composition according to claim 9.

17. An electrostatic single-component dry developer for use in the development of electrostatic latent images which comprises a composite of a binder and a hard magnetic ferrite material having a single phase hexagonal crystal structure, exhibiting a coercivity of least 300 Oersteds when magnetically saturated and an induced magnetic moment of at least 20 EMU/gm of carrier in an applied field of 1000 Oersteds and containing from about 1 to about 5 percent by weight of lanthanum.

18. The developer of claim 17 wherein the hard magnetic ferrite material is strontium ferrite, barium ferrite or lead ferrite containing from about 1 to about 5 percent by weight of lanthanum.

19. The developer of claim 18 wherein the hard magnetic ferrite is strontium ferrite.

20. The developer of claim 18 wherein the hard magnetic ferrite is barium ferrite.

21. The developer of claim 18 wherein the hard magnetic ferrite is lead ferrite.

22. The developer of claim 17 wherein said carrier particles have the formula LaxM1-xFe12O19 where x has a value such that lanthanum is present in an amount of about 1 to about 5 percent by weight and M is Ba, Sr or Pb.

23. The developer of claim 17 wherein lanthanum is present in an amount of from about 2 to 4.5 percent by weight.

24. A method for developing an electrostatic image comprising contacting the image with a single-component dry developer according to claim 17. -