CA1072801A - Electrophotographic carrier of ferromagnetic material coated with copolymer containing electrically conductive particles - Google Patents

Abstract

ELECTROGRAPHIC CARRIER VEHICLE AND
DEVELOPER COMPOSITION - CASE C
Abstract of the Disclosure An electrographic carrier vehicle and developer composition containing said carrier vehicle are described.
The carrier vehicle is composed of magnetically attractable core particles having a metal ion-linked ?-olefin-carboxylic acid copolymer and finely divided electrically conductive particles coated thereon.

Description

2~01 This invention relates to electrography and to a particulate carrier vehicle and a dry electrographic developer composition containing such a carrier vehicle useful in the development of latent electrostatic charge images.
Electrographic imaging and developing processes, eOg., electrophotographic imaging processes and techniques, have - been extensively described in both the patent and other literature, for example, U.S. Patent Nos. 2,221,776 issued November 19, 1940;
2,277,013 issued March 17, 1942; 2,297,691 issued October 6, 19~2;
2,357,809 issued September 12, 1944; 2,551,582 issued May 8, 1951;
2,825,~14 issued March 4, 1958, 2,833,648 issued May 6, 1958;

3~220~324 issued November 30, 1965; 3,220~831 issued November 30, 1965; 3~220~833 issued November 30, 1965 and many others. Generally these processes have in common the steps of forming a latent electrostatic charge image on an insulating electrographic element~ me electrostatic latent image is then rendered visible by a development step in which the charged surface of the photo-conductive element is brou~ht into contact with suitable developer ; mix.
One method for applying the deYeloper mix is by the well-known magnetic-brush process. Such a process generally utilizes apparatus of the type described, for example, in U.S.
Patent No. 3,003~462 issued October 10, 1961 and customarily comprises a non-magnetic rotatably mounted cylinder having fixed magnetic means mounted inside. me cylinder is arranged to rotate so that part of the surface is immersed in or otherwise contacted with a supply of developer mix. The granula~ mass comprising the developer mix is magnetically attracted to the surface of the cyllnder. As the developer mix comes within the influence of the field genera~ed by the magnetic means within the cylinder~
the particles thereof arrange themselves in bristle-like forma~
tions resembling a brush~ The bristle formations that are formed by the developer mix tend to con~orm to the lines of magnetic flux, standing erect in the vicinity of the poles and laying -?-~7~

substantially flat when said mix is outside the environment of the magnetic poles. Within one revolution the continually rotating cylinder picks up developer mix from a supply source and returns part or all of ehis material to the supply. This mode of operation assures that fresh mix is always available to the surface of the photoconductive element at its point of contact with the brush. 'In a typical rotational cycle, the roller performs the successive steps of developer mix pickup, brush formation, brush contact with the electrographic element, e.g., a photocon-ductive element~ brush collapse and finally mix release.
In magnetic-brush development of electrostatic images the developer is commonly a triboelectric mixture of fine toner powder comprised, for example, of a dyed or pigmented thermo-plastic resin with coarser carrier particles of a magnetic material such as iron particles, etc.
In magnetic brush development as well as in v~rious other different types of electrographic development wherein a dry triboelectric mixture of a particulate carrier vehicle and a fine toner powder are utilized, it is advantageous to modify the surface properties of the particulate carrier vehicle so that a high net electrical charge is imparted to the toner powder. Particulate carrier particles capable of lmparting a relatively stable, hiBh net electrical charge to the toner powder aid in improving retention of the toner to the carrier vehicle and thereby aid in reducing the amount of undesired l~toner throw-off", i.e. the amount of toner powder thro~n out of the developer mix as it is agitated in an electrographic devel-opment apparatus. Aside from the extraneous contamination problems inherent with airborne toner dust in the a~paratus, l~toner throw~offi' also leads to imaging problems such as unwanted background and scumming of the electro~tatic image-bearing element. - ~
,~, .. .

~Ll)7~8~1 A variety of methods and materials for modifying the surface properties of particulate electrographic carrier vehicles have been proposed. See, for example, Miller, U.S.
3,547,822 issued December 15, 1970, describing certain carboxylated resins useful for coa~ing a particulate carrier - vehicle; ~iller Belgina Patent 746,111 dated April 30, 1970 which describes magnetic carrier particles having two or more coatings of a resinou~ material applied thereon such that there is no substantial dissolution of one resinous layer into an ad~acent resin layer; Miller, UéS. 3632,512 issued January 4, 1972~ describing me~hods of treating and coating iron part:icles utilizing an acid wash followed by controlled drying and, if desired, a subsequent coating operation to .-produce a particulate iron carrier vehicle having improved surface characteristics~ Miller, U.S. 3,736,257 issued May 29, 1973, describes a particulate carrier vehicle bearing an improved, highly conductive overcoat. ~arious other ~.
literature relating, in part, to varyin~ the surface characteristics of a particulate carrier vehicle include U.S. Patents 2,618,551 issued November 18, 1952; 2,618,552 issued November 18, 1952; 2,753,308 issued July 3, 1956;
2,874~063 issued February 17, 1959; 2,880,696 issued April 7, 1959; 3,202,093 issued August 24, 1965; 3,526,533 issued September 1, 1970; 3,533,835 issued October 13~ 1970;
British Patent 1,174,571 dated December 17, 1969; and Canadian Patent 835,317 issued February 24, 19700 In accordance with the present invention~ it has been ` discovered that core particles having on the outer surface ; thereof a resinous coating of an ionomeric carboxylic acid --~- 30 ~-olefin copolymer provide an effective particulate carrier :
..
vehicle for an electrographic developer. .
According to one embod~ment of the imvention, the resinous ionomeric coating i5 provided as an outer coating for magnetically-attractable core particlesa The resultant ionomeric~
::' _4-, . ., ' : ` , - :

1~7281~1 coated magnetically-attractable carrier particles~ when admixed with a fine toner powder to form an electrographic developer composition, are especially useful in a magnetic-brush electro-graphic development process~
According to another embodiment of the invention the ionomeric coating of the particulate carrier vehicle may have incorporated therein finely-divided electrically-conductive particles such as carbon black particles. The incorporation of finely-divided carbon black partqcles in the ionomeric coated carrier of the invention provides a particulate carrier which, ~ -when admixed with a suitable toner powder~ provides improved solid area image development without effecting a substantial change in the net electrical charge imparted to the toner powder.
This result represents an unexpected and surprising discovery.
That is, it is generally recognized in the art that the addition of even relatively small quantities of finely-divided electrically conductive particles~ such as carbon black~ to conventional resin-coated carrier particles ordinarily substantially reduces the static triboelectric value of the! carrier coating; therefore, 20 the net electrical charge imparted to toner powder admixed with such a carrier vehicle is also substantially reduced. (See V.S.
3~533~835 at ColO 10 and Examples II-VII thereof).
The ionomeric materials useful as polymeric coatings ~or the carrier particles of the i~vention comprise ionic copolymers of (a) an ~-olefin having the general formula RCH = CH2 wherein R is a radical selected from the class of hydrogen ~ --and slkyl radicals having from 1 to about 3 carbon atoms and (b) an ~ 9~ -ethylenically unsa~urated carboxylic aeid having from 3 to about 8 carbon atoms~ said copolymers having from 10% to about 30 90% of the carboxylic acid groups ionized with metal ions. The ionomers are typically formed by llneutralization'~ of a base ~-olefin-carboxylic acid copolymer with an ionizable metal compound. The unsaturated carboxylic acid content of the copolymer ., ~ ,.' : . : ' : . - ............. . . - ~ ~ ::
- . ; -. . . - , . . , . : . . :. ~

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is typically from about 0.2 to 25 mole percent based on the Cl-olefin-acid base copolymer, and the ~ -olefin content of the copolymer is at least 50 mole percent based on the ~-olefin-acid base copolymer. Especially useful in the invention are ionomeric, carboxylic acld ~-olefin copolymers having a free acid conteh~ less than about 2 milliequivalents of lN NaOH per gram of the ionomeric copolymer based on the dry weight of the ionomer. Typically, the carboxylic acid groups contained in the ionomeric materialls useful in the invention are randomly distri-buted throughout the polymeric structure.

Ionomeric materials which have been found to provide `
especially good results according to the present invention are ionomeric carboxylic acid-ethylene copolymers having recurring structural units which are conventionally represented in the literature as follows:

~ 2 CH2)n ~CH - (CH2 ~ CH2) o~cbo M+l to M~3 0~ 0~ 0~ / H

~ H2-- C ~ (CH2 ~ CH2)n ~ CH2 -CH (CH2 2 n wherein M +1 to M+3 is a metallic ion and n is a integer within the range of 1 to about 20 or more.
As can be seen by the above structural formula, the metal ions contained in the ionomeric materials are believed to provide ionic crosslinking. Metals especially useful as the metal ions contained within the ionomeric materials are alkali metal ions including Na+, K+, Li~, and Cs~. However, a variety of other metallic ions may also be used including com-plexed and uncomplexed metal ions.
When using the uncomplexed metal ions the valence oE the ion corresponds to the valence of the metal. These metal ions are . .- ,' ' ~ ' " . . -. ' ' ' ' .' ' ' . ~ ' . ' ' ' ,.

~L072~1 obtained from the commonly known and used metal salts. The complexed metal ions are those in which the metal is bonded to more than one type of salt group, at least one of which is ionized and at least one of which is not. Since the formation of the ionic copolymers requires only one ioniæed valence state, it will be apparent such complexed metal ions are equally well suited in the present invention. The term '~metal ion having one or more ioni~ed valence states" means a metal ion having the gene~al formula Me+nX , where n is the ionic charge and is at least one, X is a nonionized group and n~m equal the valence of the metal. The utility of complexed metal ions -employed in the formation of ionic copolymers corresponds in their ionized valences to those of the uncomplexed metal ions.
The monovalent metals are, of course, excluded but higher valent metals may be included depending on how many metal valences are complexed and how many can be ionized. The preferred complexed metal ions are those in which all but one metal valences are complexed and one is readily ionized,, Such compounds are in particular the mix~d salts of very weak acids, such as oleic and stearic acid~ with ionizable acids~ such as formic and acetic acid.
The uncomplexed metal ions which are suitable in forming the ionic copolymers used in the present invention comprise mo~o- di- and trivalent ions of metals in Groups I, II, III, IV-A a~d VIII of the Periodic Table of Elements (see page 392, I}Y b~ 9! ~b~o3J ~Y_3 ~L~}Y ~ ~ Chemical Rubber Publishing Co.~ 37th ed.). Suitable monovalent metal ions are Na+~ K+~
Li~ Cs~ Hg+ and Cu+. Suitable divalent metal ions are Be ~ Ng , Ca ~ Sr~2~ Ba+2~ CU+2 ~g~2 Sn+2 pb+2 +2 +2 Co~2, Ni+2 and Zn+ . Suitable trivalent metal ions are Al+3, Sc+3~ Fe+3 and Y+3.
The quantity of ions e~ployed or the degree of neu~

trali~ation will differ with the degree of solid property change .. . ~ , . . : , . . ~

~7;Z80~

and the degree of melt property change desired in the resultant ionomeric materialO In general, it has been reported that the concentration of the metal ion should be at least such that the metal ion neutralizes at least 10 percent of the carboxylic acid groups in order to obtain a significant change in properties.
The degree of neutralization for optimum properties will vary with the acid concentration and the molecular weight of the base copolymer. However, it is generally reported desirable to neu-tralize at least 50 percent of the acid groups. The degree of neutralization may be measured by several techniques. Thus, infrared anàlysis may be employed and the degree of neutraliza-tion calculated from the changes resulting in the absorption bands. Another method comprises the titration of a solution of the ionic copolymer with a strong base. In general, it has been reported that the added metal ion reacts stoichiometrically with teh carboxylic acid in the base polymer up to 90 percent neutralizations. Small excess ~uantities of the crosslinking agent are necessary to carry the neutrali~aeion to completion. ~`
~owever, large excess quantities of the crosslinking agent do not add to the properties of the ionic copolymer, since onca all carboxylic acid groups have been ionically crosslinked, no further crosslinks are formed. -As stated above, especially useful ionomeric materials in the present invention are those in which the resultant ionomer has a free acid content less than aboue 2 milliequivalents of lN naOH per gram of the ionomeric copvlymer based on the dry weight of the ionomer. Such ionomeric coated carrier particles have been found to impart a high net electrical charge to the toner material and produce extremely small amounts of "toner ; -thro~-of" in a magnetic brush development apparatus.
The ~-olefin polymers employed in the formation of the base copolymers are ~ -olefins which have the general formula RCH - CH2 where R is either a hydrogen or an alkyl group : '`
; -8-~7Z8~

having pre~erably from 1 to 8 carbon atoms. Thus~ suitable olefins include ethylene~ propylene, butene-l, pentene.l, hexene_l~ heptene-l~ 3-methylbutene-1~4-methylpentene-1~ etc.
Although olefins having higher carbon numbers can be employed, they are not materials which are readily obtained or available.
The concentration of the a-olefin is at least 50 mol percent in the base copolymer, and is preferably greater than 80 mol percent.
The second component employed in the formation of the base copolymer comprises an ~ ethglenically unsaturated carboxylic acid having preferably from 3 to 8 carbon atoms.
Examples of such monomers are acrylic acid, methacrylic acid, ethacrylic acid, itaconic acid, maleic acid, fumaric acid, monoesters of said dicarboxylic acids, such as methyl hydrogen maleate~ methyl hydrogen fumarate~ ethyl hydrogen fumarate and maleic anhydride. Although maleic anhydride is not a carboxylic acid in that it has no hydrogen attached ot the carboxyl groups, it can be considered an acid for pre;sent purposes because of its chemical reactivity being that of an acid. Similarly, other d~-monoethylenically unsaturated anhydrides of carboxylic acids can be employed. As indicated, the concentration of acidic monomer in the base copolymer is from 0.2 mol percent to 25 mol percent, and~ preferably, from 1 to 10 mol percent.
Greater detail concerning the ionomeric (i.e., ion- -~
linked) materials useful in the present invention may be found in Rees, U.S. Patent 3,264,272 issued August 2, 1966.
- In accordance with the present invention, ionomeric coating material is applied to a core particle in the form of an aqueous-alcoholic dispersion containing finely-divided parti-cles of the ionomer dispersed therein. Typically~ a surEactant is also present to aid in the formation and stabilization of the aqueous lonomeric dispersion. Typically, the aqueous-alcoholic lonomer dispersions ha~e a solids content ranging from about 10%
~ . ' _9~
- ., .

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to about 60% by weight. The particle size of the ionomeric material dispersed therein is reported to vary from about 0.02 to 0.6 microns~ generally from 0.1 to 0 6 microns. The pH of the ionomeric dispersion is typically basic in character varying from about 7.0 to about 12. The melt viscosity of a typical dry ionomeric coating on a carrier core particle is on the order o~
about 5 x 103 poise as measured at 150C~ at a shear rate of 300 sec. The dispersions are typically applied to the core particles to be coated at a temperature within the range of from roughly 25C. to about 85C. When so applied~ a thin highly adherent film of the ionomeric material is formed which may cover all or only certain portions of the outer surface of the core particles.
Once applied, the coating may be heated up to about 140C. to develop optimum film strength and to dry the coating. No special binding agents are necessary to adhere the ionomeric resin to the magnetic core particles thus providing one of the advantages of the invention.
The ionomer dispersion may be applied to the core particle by any known coating technique, e.g., by a fluidized bed coating process. It may be applied by passing the particulate material to be coated through a bath containing the coating composition, in a con~inuous manner or in a bat~h manner~ The coating may also be sprayed on the film, or applied manually by brushing or the like.
The amount of the ionomeric material coated on the carrier core particles may vary from about O.OQl to about 3%
by weight based on the total weight of the carrier particles.
For example, on spon~e iron core particles it appears that dry ionomeric coatings in tha range of about 0.05 to about 0.60 percent by weight ba~ed on the ~o~al weight of the coated carrier is desirable. Especially good results ha~e been obtained utili~in~
on sponge iron particles ionomeric coatings comprising 0.15 to 0~30 percent by weight of the coated carrier particleO Although more or less of the ion~meric coating may be utili7ed if desired, . ~ ......... . . .
, 1C~728C~L

it appears that, for example, on sponge iron particle levels below about 0.05 percent start to exhibit an increase in ~toner throw-off" while levels above about 0.60 percent do not appear to provide as good development of s~lid area images as is otherwise obtainable.
As indicated above, in accordance with one embodiment of the invention, finely-divided electrically conductive particles such as carbon black may be incorporated in the ionomeric coating.
A variety of different methods for imcorporating such additives may be used. ~ne useful method simply involves admixing the electrically conductive particulate additive into the ionomeric ~-dispersion prior to its application on the particulate carrier core particles. Typically, these electrically conductive ~ -particles have a particle size within the range of from about 1 to aboue 20 microns. The electrically conductive particles have a specific resistance less than about 10 ohm-cm., typically, as in the case of carbon black, less than 1 ohm-cm. ~Specific i resistance as used herein is measured at room temperatur Generally, if electrically conductive particles are incorporated in the ionomeric coating, useful amolmts of such conductive con~
ductive particles are within the range of from about 1 to about 50 parts by weight of conductive particles per 100 parts by ~ -weight of ionomer coating (based on the dry weight of the ionomeric coating).
Advantageously, the core ma~erials which can suitably be coated ~o form carrier particles in accordance with this invention 2re magnetic materials. The phrase "magnetic materials"
as used herein encompasses a ~ariety of magnetically attractable materials. Particularly useful materials would include ferro-~agnetic materials such as the metals of the first transition series, i~e., nickel, iron, cobale9 and alloys and mixtures :
:.
" . ' .: :. . : . . : : : . -~72~

~hereof. Other useful materials which exhibit a net magnetic moment are the ferrimagnetic materialsO Examples of such ferrimagnetic materials would include the ferrites, which are materials having the general formula ~eO.Fe203, where Me is a metal ion, as well as mixed ferrites, which contain more than one species of metal ion in addition to iron, and the substituted ferrites, in which another metal replaces some of the iron.
Also included in the phrase magnetic material are particles such as those described in Miller, Canadian 835,317 issued February 24~ 1970~ and which are comprised of, for example~ iron dispersed in a resin binder. Such magnetic materials are used as a core in accordance with this invention over which is coated the above-described ionomeric resin. Ths core can consist of a solid particle of magnetic material or can be a nonmagnetic particle overcoated with ferromagnetic materials as described in Belgian Patent No. 726,806, dated March 14, 1969. The core can comprise rough~surfaces magnetically responsive particles; smooth-surfaced magnetically responsive particle;
or a mixture of rough-surfaced and smooth-surfaced magnetically responsive particles. Par~icles having these varying surface properties and mi~tures thereof are more fully described in Trachtenberg et al~, U.S. 3,833,05~ issued September 24, 1974 entitled "Electrographic Carrier Vehicle and Developer Composition -- Case Allo A core can vary in size and shape with core particles having an average diameter of from about 1200 to about 30 microns. Particularly good development is obtained with core materials of from about 300 to about ~0 microns average diameter.
The size of the core particles used will, of course, depend upon seueral factors such as the type of images ultimately developed, desired thickness of the ionomeric resin coating, etc~

The phrase ~average diameter~l as used hsrein is not meant to imply that only perfectly uniformly dimensioned particles caD

-12~

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72~0~L -be used. This phrase is used to refer to the average thickness of particles when measured along several axes. Average diameter also refers to the approximate size of the openings in a standard sieve series which would just retain or just pass a giYen particle.
In addition, it may be noted that the core materials useful in ;
the present invention may be subjected to various treatments to modify their surface properties prior to being coated with the above-described terpolymer. For example, it may be desirable to wash a magnetic core in an acid-wash, rinse and subject the washed particles to controlled drying conditions so as to induce or exclude oxidation of the surface of the carrier particles. Such treatments are especially useful when the ~;
magnetic core comprises iron particles. Such treatment processes are described more fully in Belgian Patent 746,109 clated April 30, 1970.
Electrostatic developer compositions o~ the present invention can be prepared by mixing from about 90 to about 99%
by weight o~ the above-described carrier vehicle with from about 10 to about 1% by weight of a suitable electroscopic toner material.
The toner material (o~ marking particles) useful in .
dry electrographic developar compositions are generally comprised , .
of a resin binder and a colorant. Suitable toners can be selected from a wide variety of materials to give desired i physical properties to ehe developed image and the proper ... .... ..
triboelectric relationship to match the carrier particles used.

Generally~ any of the toner powders known in the art are suitable .
for mixing in the developer composition of this invention. In certain instances~ the toner may be comprised solely of colorant material without any resinous binder. In other cases, where a vis~ble image is not desired or needed, .
:

1~7Z~Ol the toner may be composed solely of a colorless material, such as a resinous material, having the desired physical and tribo-electric properties.
~ hen the toner powder selected is utilized with magnetic carrier particles in a magnetic-brush development arran8e~
ment, the toner clings to the carrier by triboelectric attraction.
The carrier particles acquire a charge of one polarity and the toner acquires a charge of the opposite polarity. Thus~ if the carrier is mixed with a resin toner which is higher in the triboelectric series, the toner normally acquires a positive charge and the carrier a negative charge.
Useful toner particles can be prepared by various methods.
Two convenient techniques for preparing these toners are spray-drying or melt-blending followed by grinding. Spray-drying involves dlssolving the resin~ colorant and any additives i~ a volatile organic solvent such as dichloromethane. This solution is then sprayed through an atomizing noz71e using a substantially non~
reactive gas such as nitrogen as the itomixing agent. During atomization, the volatile solvent evalporates from the sirborne droplets, producing toner particles of the uniformly colored resinO
The ul~imate particle si~e is determined by varying the size of the atomizing nozzle and the pressure of the gaseous atomizing agent.
Further details relating to spray-drying may be found in Carlson, U.S. Patent 2~357~ôO9 issued ~ept. 12~ 1944. Con~entionally~ particles of a diameter between about 1/2f~ and about 30~ are used~ with particles between about 2~4 and 15~ bsing preferred, although larger or smaller particles can be used where desired for particular development or image considerations.
Suitable toners can also be prepared by melt-blendingO
This technique involves meltlng a powdered fo~m of polymer or resin and mixing it with suitable colorants and additives. The resin can readily be melted or heated on compounding rolls which are also ., . . . : , .: : : : .: -- . . : . ... , : . , :::
- . : .... : - : - , . . :..... . :

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useful to mix or otherwise blend the resin and addenda so as to promote the comple~e intermixing of these various ingredi~
ents. After thorough blending~ the mixture is cooled and solidified. The resultant solid mass is then broken into small pieces and finely ground to form a free-flowing powder of toner particles~ Such a melt~blending technique is described in Walkup, U.S. Patent 2,618,551 issued November 18, 1951~ Of course, various other techniques for making toner particles may also be used. For example, certain spray-freeze drying -techniques may be modified to provide useful methods for pre-paring toner particles. An example of such a modified spray free~e drying technique is described in Product Licensing Index, Volume 84, Aprll, 19710 The resultant toner particles usually ran8e in size from about 112 to about 30 ~O
The resin material used in preparing the toner can be selected from a wide variety of materials, including natural resins, modified natural resins and synthetic resins. Exempla-ry of useful natural resins are balsam resins, colophony, and shellac. Exemplary of suitable modified natural resins are colophony-modified phenol resins and other resins listed below with a large proportion of colophony. Suitable synthetic resins are all synthetic resins known to be useful ~or toner purposes9 for example~ polymers, such as certain polycarbonate resins described in U.S. 3,694,359 issued September 26, 1972, and ln Product Licensing Index, Volume 84, April 1971, vinyl polymers and copolymers including polyvinyl chloride, polyvinylidene chloride, polyvinyl acetate, polyvinyl acetals, polyvinyl ether, polyacrylic and polymethacrylic esters~ polystyrene, including substituted polystyrenes; polycondensates~ e.g., polyes~ers~ such as phthalate9 terephthalic and isophthalic polyesters~ maleinate resins and colophony~mixed esters of higher alcohols; phenol- :
,'~

~15-. ' '. ' , : - , : . . . . - , , ~ .. . ,, .. : . .
, :~72~0~L
formaldehyde resins, includlng modified phenol-fonmaldehyde con-densates; aldehyde resins; ke~one resins; polyamides; polyurethanes, etc. Moreover, shlorinated rubber and polyolefins, such as various polye~hylenes, polypropylenes~ polyisobutylenes9 are also suitable.
Typical toner materials having incorporated therein styrene con-~aining materials are disclosed in the following U.S. Patents:
2,917,460, Solar, issued December 15, 1959; Re. 25,136, Carlson, issued March 13, 1962; 2,788,288, Rheinfrank et al, issued April 9, 1957; 2,638,416, Walkup et al, issued April 12, 1953;
2,618,552,Wise, issued November 18, 1952; and 2,659,670, Copley, issued November 17, 19530 Other useful styrene containing toner resins are copolymers prepared from a monomeric blend of (a) 40-60% by weight styrene or styrene homolog; (b) 20 50% by weight of lower alkyl acrylate and methacrylate~ e.g., alkyl meth-acrylates and alkyl acrylates having up to 3 carbon atoms in the alkyl groups; and (c) 5-30% by weight o higher alkyl acrylates and methacrylates, e.g., alkyl methacrylates and alkyl acrylates having 6-20 or more carbon atoms in the alkyl group.
; Colorants useful in the practice of el~is invention can be selected from a variety of materials such as dyestuffs or pigments.
Such materials serve to color the toner and thus render it more visible. Suitable toner materials having appropr~ate caking and charging proper~ies can, of course, be prepared without the use of a colorant material whe~e it is desired to have a developed image of low optical opacity. In those instances where it is desired to have high optical opacity, the colorants ~sed can, in principle, be selected from virtually all of the compounds mentioned in the Color Index~ Vols. I and II~ Second Edition. Included among the vast number of useful colorants would be such materials as Hansa Yellow G
(C~Io 11680)~ Nigrosine Spirit sol~ble (C.I. 504ls?, Chromogen Black ET00 (C.I~ 1~645)~ Rhodamine B (C.I~ 451703~ Solvent Black 3 --(C.I~ 26150)~ Fuchsine N (C.I. 42510)~ C.I. Basic Blue 9 (C.I. 52015)~
etc. Another useful class of colorants is Gomprised of nigrosine salts such as nigrosine salts of mono- and di~functional organic acids .
- - ~, : . , - . . . : .
., , ~ . . ..

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having from about 2 to about 20 carbon atoms such as chloroacetic acid, stearic acid, sebacic acid, lauric acid9 azelaic acid, adipic acid, abietic acid and the like. Nigrosine salts of this type are disclosed in Belgian Patent 734,570 dated August 14, 1969. Carbon black also provides a useful colorant as disrlDsed in Walkup, U.S. Patent 2,618,551 issued November 1~, 1952. The amount of colorant added may vary over a wide range, for example, from about 3 to about 20 percent of the weight of the resin binder. Particularly good results are obtained when the amount is from about 5 to about 10 percent. In certain instances, it may be desirable or preferred to omit the colorant, in which case the lower limit of concentration would be zero.
In addition, if desired, organic melt viscosity modifying agents for the resin binder may be incorporated in the toner particles. If such viscosity modifying agents are utilized, the ratio of melt viscosity modifying agent to resin binder gsnerally can vsry from about 1:3 to about 1.5:1~
Further information concerning these viscosity modifying agents may be found in Merrill et al, French Patent 2,085,119 dated November 22~ 1971.
The following examples are provided to further illustrate the pressnt invention and certain advantages thereof:

This example illustrates certain of the unique elec-trical properties of the ionomeric coated carrier particles of the present inveneion. The example illus~rates that even when relatively large amounes of finely-divided slectrically con- -ductive particulate matter such as carbon black are incorporated in the ionomeric coatings of the carrier particle, these coated 3Q carrier partlcles nevertheless impart relatively high net electrical charge eO toner particles admixed ~it~ the ionomeric coated carrier particles. In this example, a control toner com-posed of pigmented thermoplastic resin particles is used. The _17 , ~72~
carrier particles are composed of spon~e iron particles having an average diameter within the range of from 80 to 120 mesh7 These sponge iron carrier particles are obtained from the Hoeganaes Corporation under the tradename of Hoeganaes EH iron par-ticles. These sponge iron particles utilized in the example have an oxidized surface which may be obtained, for example, by treating these particles in an acid wash and drying in an oxygen-containing atmosphere as described in Belgian Patent 746,109 dated April 30, 1970. The oxidized sponge iron particles are then coated with an aqueous dispersion of an ionomeric, acid-modified ethylene copolymer. The dispersion of ionomeric material is purchased from the E. I. DuPont Company under the trademark Elvax D-1070. After coating the oxidized sponge iron particles with the aqueous dispersion of ionic material, the coated particles are dried to form the resultant carrier particles havin~ coated on the outer surface thereof a tough, thin discontinuous coating of ionomeric material. When dry, the total amount of ionomeric material contained on the surface of the carrier particles is approximately 0~15% by weight based on the total weight of the coated carrier particles. A total of five different particulate carrisr vehicles are made as described hereinabove, the only dif~erence among the five carrier vehicles being the ~ua~tity of electrically conductive particles incorporated in the ionomeric coating. Each of the 5 carriers are then mixed with an identical amount of ~he control toner to form 5 separate developer mixes~
each mix containing 5% by weight of toner. In each case the electrically conductive particles are incorporated in the resultant ionomexic coated carrier particles by admixing a suitable amount of finely-divided carbon particles in the aqueous ~-dispersion of ionomeric material prior to the coating thereof on the oxidiæed sponge iron core particle. The amount of finely-divided electrically conductive carbon particles incorporated in each of the ionomeric coated carrier vehicles utilized in this example are set ~orth immediately hereinbelow~

.
. , ~ ~ ... . :. .. ..

~7~:8~1 ~
The net toner charge, Q, imparted by each of the 5 carrier vehicles to the control toner is measured (using the technique described in Example 3 hereinafter)~
- TAsLE 1 Carbon Particles (pares by weight carbon particles per Carrier hundred parts by weight Net Toner Charge Vehicle of dry ionomeric coatin~) (microcoulombs /~ram) .

2 10 17.3 3 20 19.2 18.4 Observation of the above data collected in Table 1 -hereinabove indicates that there is no significant decrease in the trlboelectric charge imparted to a toner powder even when relatively large amounts of conductlve carbon particles are incorporated in the ionomeric coated carrier particles of the inventioD. This unique property of the ionomerlc coating carrier particles of the invention is surprising and unexpected in view ' of the teachings of the art which su~gests that by incorporating :
conductive particles in a polymer coated carrier vehicle9 the electrical charge imparted to the toner powder is substantially reduced.

Each of the above-described carrier vehicles~ i.e.
Number 1-5 are then admixed with 5% by ~e~ght of a dry pigmented thermoplastic resinous toner powder and used in a magnetic;brush ~-proces~ to develop an electrostatic charge pattern carried on an electrophotographic film. In each case good electrographic images :: . .
are obtained. Ie is observed that better development oE solid ` area images i~ obtained by utilizing carrier vehicles Numbers ~_5 ;
as described in Example I above which contain a quantity o elec-trically conductive particles incorporated in the ionomeric coated carrier vehicleO

-19~ ' ' "

72i~0~L
EXAk~L~ 3 In this example the ability of the ionomeric coated carrier vehicles of the present invention to impart a relatively high net electrical charge to a toner powder is compared to a similar carrier vehicle coated with a vinylidene chloride-acrylonitrile-acrylic acid terpolymeric coating material. In addition, the toner throw-off of these respective polymeric coated carrier vehicles is compared. The reason for choosing the vinylidene chloride-acrylonitrile-acrylic acid terpolymeric coated carrier vehicle for purposes of comparison with the ionomeric~coated carrier vehicle of the present invention is because the vinylidene chloride-acrylonitrile-acrylic acid terpolymeric mate~ial is know~ to be an extremely good polymeric coating material for carrier vehicles. In this regard, further details concern$ng the use of vinylidene chloride-acrylonitrile-acrylic acid terpoly~er coated carrier vehicles is provided in McCabe U.S~ 3,795,617 issued Narch 5, 1974 entitled ~Electrographic Carrier Vehicle and Developer Composition --Case ~ In this example a control toner composition is utilized which comprises:

poly(4,4'-isopropylidene diphenyl-alt-ethylene carbonate) 200 g.
Aerosol TR (a trademark)~bis-tridecylester of sulfosuccinic -acid sodium salt purchased from A~erican Cyanamid Co.] 2 g.
Nigrosine Base (Color Index No. 50415B) 9 g.
Docosanoic Acid 7.1 g, Car~on Black ~Sterling FT (a trademark) obtained from Cabot Corp,] 11.4 g~
Color balancing dyes 1.0 g.
In this example the net charge Q imparted to the toner powder `
is measured using a Farraday Cage in the following manner:
A weighed portion of each of the developers is placed in an iron tube that is covered at one end with a 200 mesh screen that retains all carrier particles within the tube. ~n air stream is then directed through the tube~ blowing toner particles off the carrier, hrough the 200 mesh screen at the exit and into a Farraday Cage condenser that is at an initial electrical potential ; --20- ;

~ . , .,, . :: .
: . . : ~ . :

~07280~
of 0 volt as measured by an electrometer. As the triboelec-trically charged toner particles settle on the walls of the cage, their electrical potential is measured by the electrometer.
The potential obtained is converted to electrical charge in micro-coulombs and this figure is divided by the weight in grams of the toner powder particles that have settled in the Farraday Cage, thus providing the net toner charge in microcoulombs per gram. The net toner charge is therefore the algebraic sum of the electrical charges on the toner particles which have settled on the walls of the case~ In addition, in this example the amount of toner throw-off is observed so as to obtain a comparison of toner thrbw-off between the ionomeric-coated carrier vehicle of the present invention and a carrier vehicle -~
having the terpolymeric resin overcoat described hereinabove. ~ ;
Toner throw-off is defined herein as the quantity of the toner powder thrown out of a particular developer mix upon mechanical agitation. Each of the developer compositions tested in this exflmple contain 5 weight percent of the above-described controlled toner powder. The remaining 95~h by wei~ht of the developer com-prises a carrier vehicle. A total of three developer mixes are tested~ me first developer mix (1~ contains a carrier vehicle ;
which is composed of oxidized sponge iron particles as described in Example 1 hereimabove having thereon a 0.3/. by weight resin coating of an ionic material purchased from the E. I. DuPont ;~
Company under the trademark Elvax D-1070 (which is described by the DuPont Company as an ion-linkedS acid-modified ethylene inter~
polymer that is moderately ion-linkedO The second developer mix (2) is similar to developer mix (1~ except that the ionomeric coating material is Elvax D-1263 (A trademark of ~he E. I. DuPont ` 30 Company which is described by DuPont as an ion-linked, acid-- modified ethylene interpolymer that is highly ion-linked~. The third developer mix (3) contains a carrier vehicle which is composed of oxidized sponge iron particles as described in Example l hereinabove coated with a 0.4% by weight of vinylidene ~21-~7z86~

chloride-acrylonitrile-acrylic acid terpolymeric resin composed of approximately 80% by weight vinylidene chloride units, 14%
by weight of acrylonitrile units, and 6% by weight of acrylic acid units. The net toner charge is measured for each of developer mixes Nos. 1 to Nos. 3 and is reported in Table 2 hereinbelow.
In addition the amount of toner throw-ofi observed is reported.

Develo~r Net Toner Char~e (microcoulombs!~ram) Toner Throw-_ff 1 14.46 Very little throw-off 2 11.45 do. - -3 16.89 do.
As illustrated in Table 2 hereinabove, the electro-graphic developer mixes of the present invention containing the ionomeric-coated carrier vehicle of the present invention impart a high net electrical charge to the toner powder and exhibit very small amounts of toner throw-off.
The invention has been described in detail wi~h particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifica~
tions can be effected within the spirit and scope of the invention.
' '' ` ' ~2~

Claims (4)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electrographic developer composition being a mixture of from about 1 to about 10 weight percent of electro-scopic toner particles and from about 90 to about 99 weight per-cent of carrier particles wherein said carrier particles comprise cores of ferro-magnetic material having a coating thereon com-prising a polymer of a metal ion-linked carboxylic acid .alpha.-olefin copolymer, said .alpha.-olefin having a formula RCH=CH2 where R is a radical selected from the group consisting of hydrogen and alkyl radicals having from 1 to about 8 carbon atoms, said carboxylic acid selected from the group consisting of .alpha.,.beta.-ethylenically unsaturated carboxylic acids having from about 3 to about 8 carbon atoms, said copolymers having from 10 to about 90 percent of the carboxylic acid groups ionized with metal atoms and finely-divided electrically conductive particles incorporated in said coating.

2. An electrographic developer composition being a mixture of from about 1 to about 10 weight percent of electro-scopic toner particles and from about 90 to about 99 weight per-cent of carrier particles wherein said carrier particles comprise cores of ferro-magnetic materials having a coating thereon com-prising a polymer comprising a metal ion-linked carboxylic acid .alpha.-olefin copolymer, said .alpha.-olefin having the formula RCH=CH2 where R is a radical selected from the group consisting of hydrogen and alkyl radicals having from 1 to about 8 carbon atoms, said carboxylic acid selected from the group consisting of .alpha.,.beta.-ethylenically unsaturated carboxylic acids having from 3 to about 8 carbon atoms, said copolymers having from 10 to about 90 percent of the carboxylic acid groups ionized with metal ions, said coating having incorporated therein from about 1 to about 50 parts by weight of electrically conductive particles per 100 parts by weight of the dry polymer coating, said conductive particles having a particle size in the range of from about 1 to about 28 microns and having a specific resistance less than about 10 ohm-cm.

3. The electrographic developer according to claim 2 wherein said conductive particles are carbon particles.

4. A carrier material comprising a plurality of bead-like particles, each having a core portion and an outer surface layer wherein the improvement comprises: said outer surface layer comprises a major portion of carboxylated polyethylene ionomer and finely divided electrically conductive particles incorporated in said coating.