US 3795618 A
Description (OCR text may contain errors)
March 5, 1974 G. P. KASPER 3,795,618
ELECTROGRAPHIC CARRIER VEHICLE AND DEVELOPER COMPOSITION Filed March 21, 1972 NET TONER CHARGE, m/bracou/ambs gram EXPECTED flDD/T/V/TY WE/GHT CARRIE/P2 ADDED TO CARR/ER/ United States Patent @ffim 3,795,618 Patented Mar. 5, 1974 3,795,618 ELECTROGRAPHIC CARRIER VEHICLE AND DEVELOPER COMPOSITION George P. Kasper, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y. Filed Mar. 21, 1972, Ser. No. 236,614 Int. Cl. G03g 9/02 U.S. Cl. 252-621 9 Claims ABSTRACT OF THE DISCLOSURE Cross-reference is made to Kasper et al., copending patent application U .8. Ser. No. 23 6,584 filed concurrently with the present application now abandoned and entitled Electrographic Carrier Vehicle and Developer CompositionCase C.
Cross-reference is also made to McCabe, copending patent application, U.S. Ser. No. 236,765 filed concurrently with the present application and entitled Electrographic Carrier Vehicle and Developer Composition- Case B.
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, e.g. electrophotographic imaging processes and techniques, have been extensively described in both the patent and other literature, for example, U.S. Pat. Nos. 2,221,776 issued Nov. 19, 1940; 2,277,013 issued Mar. 17, 1942; 2,297,691 issued Oct. 6, 1942; 2,357,809 issued Sept. 12, 1944; 2,551,582 issued May 8, 1951; 2,85,814 issued Mar. 4, 1958; 2,833,648 issued May 6, 1958; 3,220,324 issued Nov. 30, 1965; 3,220,831 issued Nov. 30, 1965; 3,220,833 issued Nov. 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. The electrostatic latent image is then rendered visible by a development step in which the charged surface of the electrographic element is brought into contact with suitable developer mix.
One method for applying the developer mix is by the well-known magnetic-brush process. Such a process generally utilizes apparatus of the type described, for example, in U.S. Pat. No. 3,003,462 issued Oct. 10, 1961 and customarily comprises a non-magnetic rotatably mounted cylinder having fixed magentic means mounted inside. The cylinder is arranged to rotate so that part of the surface is immersed in or otherwise contacted with a supply of developer mix. The granular mass comprising the developer mix is magnetically attracted to the surface of the cylinder. As the developer mix comes within the influence of the field generated by the magnetic means within the cylinder, the particles thereof arrange themselves in bristle-like formations resembling a brush. The bristle formations that are formed by the developer mix tend to conform to the lines of magnetic flux, standing erect in the viciinty of the poles and laying 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 this material to the supply. This mode of operation assures that fresh mix is always available to the surface of the photo-conductive 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 photoconductive 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 thermoplastic resin with coarser carrier particles of a magnetic material such as iron particles, etc.
In magnetic brush development, as we llas in various other different types of electrographic development wherein dry triboelectric mixtures 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 imparting a relatively stable, high 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 toner throw-off, i.e. the amount of toner powder thrown out of the developer mix as it is mechanically agitated, e.g. in a development apparatus.
Aside from the extraneous contamination problems inherent with airborne toner dust in the apparatus, toner throw-off also leads to imaging problems such as unwanted background and scumming of the electrostatic applied thereon such that there is no substantial dissolution of one resinous layer into an adjacent resin layer; Miller, U.S. 3,632,512 issued Jan. 4, 1972 describing methods of treating and coating iron particles 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. Ser. No. 100,299 filed Dec. 21, 1970,
and now U.S. Pat. No. 3,736,257, describes a particulatecarrier vehicle bearing an improved, highly conductive overcoat. Various other literature relating, in part, to varying the surface characteristics of a particulate carrier vehicle include U.S. Pats. 2,618,551 issued Nov. 18, 1952;
2,618,552 issued Nov. 18, 1952', 2,753,308 issued July 3, 1956; 2,874,063 issued Mar. 2, 1959; 2,880,696 issued Apr. 7, 1959; 3,202,093 issued Aug. 24, 1965; 3,526,533 issued Sept. 1, 1970; 3,533,835 issued Oct. 13, 1970;- British Pat. 1,174,571 dated Dec. 17, 1969; and Canadian Pat. 835,317 issued Feb. 24, 1970.
In accordance with the present invention, it has been discovered that an effective particulate carrier vehicle for an electrographic developer is a blended carrier vehicle comprising a mixture of (a) core particles having on the outer surface thereof a resinous coating of an ionomeric a-olefin-carboxylic acid copolyrner and (b) core particles having on the outer surface thereof a resinous coating of a copolymerized blend of (1) vinylidene chloride, (2) at least one member of the group acrylonitrile, methacrylo-. nitrile, and an alkyl ester of an acrylic or methacrylic acids, and (3) at least one acidic organic compound containing 3 to about 12 carbon atoms, at least one polymerizable vinylidene moiety and at least one carboxylic acid (-COOH) or sulfonic acid (SO H) moiety.
According to one embodiment of the invention, there is provided an electrographic developer composition which comprises the above-described particulate carrier vehicle physically admixed with a fine toner powder. In accordance with this embodiment, a blended carrier vehicle as described above wherein the core particles are magnetically-attractable has been found to be especially useful in a magnetic-brush electrographic development process. Advantageously, in accordance with a further embodiment of the invention the blended carrier vehicle comprises a mixture of (a) magnetically attractable core particles having on the outer surface thereof a thin layer of an ionomeric a, 3-ethylenically unsaturated carboxylic acid-ethylene copolymer and (b) magnetically attractable core particles having on the outer surface thereof a thin layer of a vinylidene chloride-acrylonitrile-acrylic acid terpolymer. As explained in greater detail hereinafter, best results have been obtained in accordance with this embodiment using a carrier vehicle comprising from about 20 to about 95 percent by Weight of core particles labelled (a) immediately above and from about 80 to about 5 percent by weight of core particles labelled (b) immediately above. Such a carrier vehicle, when admixed with a' fine toner powder, provides, inter alia, the following advantages: uniform, high net electrical charge imparted to the toner powder, relatively low toner throw-off, and relatively low carrier pick-up.
As used herein, the phrases net electrical charge imparted to the toner powder or net toner charge are equivalent and are defined to be the total electrical charge imparted to a specified amount of a control toner by a specified quantity of a particular carrier vehicle. Although the phenomena by which such an electrical charge is imparted is not fully understood, it is believed due in large part to the triboelectric effect caused by the physical admixture of toner and carrier. The term toner throw-off as used herein is defined to mean the amount of toner powder thrown out of the developer mix (i.e. carrier plus toner) as it is mechanically agitated, e.g. in an electrographic development apparatus. The term carrier pickup is used herein to describe the undesirable attraction of carrier particles to the electrostatic image bearing electrographic element which occurs when the attractive force between the carrier and the element is greater than the magnetic attractive force between the carrier particles and the magnetic brush. Low carrier pickup indicates that few or no particles are present per image frame of the electrographic element. High carrier pickup means that many particles are present per image frame of the electrographic element.
FIG. 1 is a graph illustrating the synergistic increase in net toner charge obtained using an electrographic developer and carrier vehicle of the present invention.
As indicated above, the blended carrier vehicle of the present invention wherein the core particles thereof are magnetically attractable has been found to provide a synergistic increase in the net electrical charge imparted to a fine toner powder admixed therewith. That is, it might be expected that by admixing one kind of resin-coated magnetically attractable carrier vehicle with a second kind of resin-coated magnetically attractable carrier vehicle, the total net electrical charge imparted to a toner powder admixed with such a blended carrier vehicle would be additive. In other words, the total net electrical charge imparted to a toner powder by such a carrier vehicle could be expressed mathematically as:
( Q= 1q1+ zq2 xqx wherein Q equals the total net electrical charge imparted to a control toner powder; q equals the net electrical charge imparted tothe control toner powder by a particular kind of resin-coated magnetically-attractable carrier core particle; n equals the weight fraction of a particular kind of resin-coated carrier particle present in the resultant blended carrier vehicle, the sum of equal to 1.0; and x represents a positive integer equal to the number of different kinds of resin-coated magnetically attractable core particles present in the resultant blended carrier vehicle.
However, as illustrated in the graph of FIG. 1, the blended carrier vehicle of the present invention exhibits a synergistic increase in net toner charge, Q, which is substantially greater than that which would be expected based on Formula I above. FIG. 1 represents a graph illustrating the dependency of Q, net toner charge, on the variation in composition of the carrier vehicle. In FIG. 1, Q is expressed in the units micro coulombs per gram of toner and is measured experimentally as indicated in the examples hereinafter. As stated above Q is a measure of the electrical charge imparted to a control toner powder by a specified quantity of a particular carrier vehicle. The composition of carrier vehicle noted in the graph of FIG. 1 is expressed in terms of the. weight percent of carrier vehicle #2 admixed with carrier vehicle #1. Thus, the zero weight percent point on the graph corresponds to a carrier vehicle comprising 100 percent carrier vehicle #2, the 50 weight percent point on the graph corresponds to a blended carrier vehicle composed of 50 weight percent carrier #1 and 50 weight percent carrier #2, the 100 weight percent point on the graph corresponds to a carrier composed entirely of carrier #2. In the graph of FIG. 1, carrier #1 comprises magnetically-attractable core particles having a thin layer of an ionomeric resin on the outer surface thereof as described in greater detail in Example 1. Carrier #2 of FIG. 1 is composed of magnetically-attractable core particles having a thin layer of a vinylidene chloride-acrylonitrile-acrylic acid terpolymer on the outer surface thereof as described in Example 1. The dotted line of FIG. 1 represents the Expected Toner Charge one would obtain using a blended carrier vehicle of the present invention using Formula I hereinabove. The solid line of FIG. 1 represents the experimental toner charge actually measured using a blended carrier vehicle of the present invention. As indicated by the graph of FIG. 1, the experimental toner charge obtained using a blended carrier vehicle is substantially greater than that one might expect.
The ionomeric materials useful as polymeric coatings for the carrier particles of the invention comprise ionic copolymers of (a) an a-olefin having the general formula RCH=CH where R is a radical selected from the class consisting of hydrogen and alkyl radicals having from 1 to about 8 carbon atoms and (b) an cc,,8-ethylenically unsaturated carboxylic acid having from 3 to about =8 carbon atoms, said copolymers having from 10% to about of the carboxylic acid groups ionized with metal ions. The ionomers are typically formed by neutralization of a base u-olefin-carboxylic acid copolymer with an ionizable metal compound. The unsaturated carboxylic acid content of the copolymer is typically from about 0.2 to 25 mole percent based on the a-olefin-acid base copoly mer, and the a-olefin content of the copolymer is at least 50 mole percent based on the m-olefin-acid base copolymer. Especially useful in the invention are ionomeric, carboxylic acid-ot-olefin copolymers having a free acid content less than about 2 milliequivalents in 1 N NaOH per gram of the ionomeric copolymer based on the dry weight of the ionomer. Typically, the carboxylic acid groups contained in the ionomeric materials useful in the invention are randomly distributed 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:
C -CHz-(CHCH:) -CHg H-(CHCH3) L .I wherein M+ to M+ is a metallic ion and n is an integer within the range of 1 to about 20 or more. 7
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 usedincludiug complexed and uncomplexed metal ions.
When using the uncomplexed metal ions, the valence of the ion corresponds to the valence of the metal. These metal ions are 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 copoly mers requires only one ionized valence state, it will be apparent that such complexed metal ions are equally well suited in the present invention. The term metal ion having one or more ionized valence states means a metal ion having the general formula Me' X 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 valence 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 valence are complexed and one is readily ionized. Such compounds are in particular the mixed 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 mono-, diand trivalent ions of metals in Groups I, II, III, IV-A and VIII of the periodic Table of Elements (see page 392, Handbook of Chemistry and Physics, Chemical Rubber Publishing Co., 37th ed.). Suitable monovalent metal ions are Na+, K+, Li+, Cs+,'Ag+, Hg+ and Cu+. Suitable divalent metal ions are Be+ Mg+ Ca+ Sr, Ba, Cu, Cd, Hg+ Sn+ Pb, Fe, Co Ni+ and Zn. Suitable trivalent metal ions are M, Sc, Fe+ and Y+ The quantity of ions employed or the degree of neutralization will differ with the degree of solid property change and the degree of melt property change desired in the resultant ionomeric material. 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 neutralize at least 50 percent of the acid groups. The degree of neutralization may be measured by several techniques. Thus, infrared analysis may be employed and the degree of neutralization 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 the carboxylic acid in the base polymer up to 90 percent neutralizations. Small excess quantities of the crosslinking agent are necessary to carry the neutralization to completion. However, large excess quantities of the crosslinking agent do not add to the properties of the ionic copolymer, since once 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 about 2 milliequivalents of 1 N NaOH per gram of the ionomeric copolymer 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 throw-oil in a magnetic brush development apparatus.
The a-olefin polymers employed in the formation of the base copolymers are a-olefins which have the general formula RCH=CH where R is either a hydrogen or an alkyl group having preferably 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-l, etc. Although polymers of olefins having higher carbon numbers can be employed in the present invention, 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 mol percent.
The second component employed in the formation of the base copolymer comprises an u,fi-ethylenically unsaturated carboxylic acid group 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 to the carboxyl groups, it can be considered an acid for present purposes because of its chemical reactivity being that of an acid similarly, other a,/3-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 ion-linked (i.e. ionomeric) materials useful in the present invention may be found in Rees, US. Pat. 3,264,272 issued Aug. 2, 1966, incorporated herein by reference thereto.
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 particles of the ionomer dispersed therein. Typically, a surfactant is also present to aid in the formation and stabilization of the aqueous ionomeric dispersion. Typical aqueous-alcoholic ionomer dispersions have a solids content ranging from about 10% 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 micron, generally from 0.1 to 0.6 micron. The pH of the ionomeric dispersion is typically basic in character varying from about 7.0 to about 12. The dispersions are typically applied to the core particles to be coated at a temperature within the range of from roughly 25 C. to about C. When so applied, a thin 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 C. to develop optimum film strength and to dry the coating. The melt viscosity of a typical dry ionomeric coating on a carrier core particle is on the order of about 7 X10 poise as measured at 150 C. at a shear rate of 300 sec.
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 continuous manner or in a batch manner. The coating may also be sprayed on as a 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 0.001 to about 3% by weight based on the total weight of the carrier particle. For example, on sponge iron core particles it appears that dry ionomeric coatings in the range of about 0.05 to about 0.60 percent by weight based on the total weight of the coated carrier particle are desirable. Especially good results have been obtained on sponge iron particles utilizing ionomeric coatings comprising 0.15 to 0.30 percent by Weight of the coated carrier particle. Although more or less of the ionomeric coating may be utilized if desired, it appears that, for example, on sponge iron particle levels below about 0.05 percent start to exhibit an increase in toner throw-oft while levels above about 0.60 percent do not appear to provide as good development of solid area images as is otherwise obtainable. 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 non-ionomeric copolymeric coating materials useful in the blended carrier vehicle of the invention are copolymerized blends of vinylidene chloride; at least one member of the group acrylonitrile, methacrylonitrile, and an alkyl ester of an acrylic or methacrylic acids having from 1-18 carbon atoms in the alkyl group; and at least one unsaturated organic acid containing 2 to about 12 carbon atoms, at least one polymerizable vinylidene moiety and at least one carboxylic acid (COOH) or sulfonic acid (SO H) moiety.
According to a preferred. embodiment of the invention, the aforementioned copolymeric material is a vinylidene chloride-acrylonitrile-acrylic acid terpolymer containing from about 5 to about 30% by weight of acrylonitrile, from about 2 to about 25% by weight of acrylic acid, and from about 55 to about 93% by weight vinylidene chloride.
Although copolymeric materials useful in the present invention may be used which have a composition outside the specific compositional ranges described immediately above, it has been found that copolymeric materials having an organic acid content substantially higher than 25% by weight tend to exhibit decreased environmental stability at high relative humidity.
In accordance with this invention, the magnetic core particles are coated by a liquid resin application of the above-described nonionomeric copolymer to form a resultant resinous coating which, when dried, forms a thin highly adherent coating on all or on only certain portions of the outer surface of the magnetic core particles. 'Iypi cally, sponge iron core carrier particles have a resin coating varying from about 0.01 to about 0.6 micron in thickness, preferably from about 0.01 to about 0.3 micron in thickness. No special binding agent compositions are necessary to adhere the resin coating to the magnetic core particle thus providing one of the advantages of the resin coatings useful in the present invention. Typically, the resultant carrier particle is coated with from 'about0.00l weight percent to about 3 weight percent of the copolymer. (The aforementioned weight percents are based on the resultant carrier particles bearing a dry resin overcoat.) The coatings may be applied to the core material by a variety of Well known techniques including spraying a liquid mixture containing the copolymer on the magnetic core, applying the resin by a fluidized bed coating technique, dipping the magnetic core particles into a trough filled with a liquid mixture containing the above-described copolymer, etc. The liquid application of the above-described copolymers may be from either a dispersion or solvent solutionof the copolymer material. The nonionomeric copolymer materials useful in the invention may be conveniently dissolved or dispersed in organic solvents or dispersed in water. Following the application of the liquid-form copolymer. to the magnetic core, the liquid vehicle is evaporated and the resin is allowed to dry and harden. Ingeneral, the concentration of resin in the liquid coating vehicle is usually quite low to insure that the particles do not become agglomerated during coating.
As noted, according to a preferred embodiment of the invention, the nonionomeric copolymeric materials useful in the invention are vinylidene chloride-acrylonitrileacrylic acid terpolymers. However, useful results are also believed possible by substituting one or more alkyl esters of acrylic or methacrylic acid for the acrylonitrile corn ponent of the copolymer. Typically, such esters have from l-18 carbon atoms in the alkyl group and include, for example, methyl methacrylate, ethyl methacrylate, butyl methacrylate, octyl methacrylate, n-dodecyl methacrylate, n-octadecyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, etc. Similarly, useful results are believed attainable wherein methacrylonitrile is substituted for the acrylonitrile component.
Typically, the acid content of the nonionomeric copolymeric materials useful in the invention is less than 2 millequivalents of 1 N.NaOH per gram of polymer based on the dry weight of said polymer. In the preferred embodiment of the invention using a terpolymeric coating of vinylidene chloride-acrylonitrile-acrylic acid, the acid content is within the range of from about 0.3 to about 1.0 milliequivalents of 1 N.NaOH per gram of polymer based on the dry weight of the polymer. A typical nonionomeric copolymer material useful in the present invention has a molecular weight roughly on the order of about 200,000. Nonionomeric copolymeric materials having molecular weights substantially lower or substantially higher than 200,000 may also be used.
As indicated, it'is believed that various organic acids may be substituted forthe acrylic acid monomer used in the preferred terpolymeric coating of applicants invention. Typically, such organic acids should contain from 2 to about 12 carbon atoms, at least one polymerizable vinylidene group, and at least one carboxylic o-r sulfonic acid moiety. Typical of such organic acids are itaconic acid, methacrylic acid, vinyl benzoic acid, sulfoalkyl methacrylates and sulfoalkylacrylates such as sulfoethylmethacrylate, etc."Sulfoethylmethacrylate may be represented by the following structural formula:
CH3 oomom-somr There are various known' ways of making the aforedescribed nonionomeric copolymeric materials. The monomers may be copolymerized by any known method to form the copolymers used in accordance with this invention. For example, the copolymerization may be conducted in aqueous emulsion containing a mixture of the monomers, a catalyst and activator, e.g. ammonium persulfate and meta sodium bisulfite, and an emulsifying and/or dispersing agent. Alternatively the copolymers of this invention may be prepared by polymerization of the monomeric components in bulk without added diluent, or the monomers may be reacted in appropriate organic solvent reaction media. The total catalyst-activator concentration may generally be kept within a range of about 0.01% to about 2.0 by weight of the monomer charge, and typically within a range of concentration of 0.1% to 1.0%. Improved solubility and viscosity values are reported to be obtained by conducting the polymerization 9 in the presence of mercaptans, such as ethyl mercaptan, lauryl mercaptan, tertiary dodecyl mercap'tan, etc. which are effective in reducing crosslinking in the copolymer. In general, the mercaptans may be used in concentrations of 0.1% to 5.0% by weight based on the weight of polymerizable monomers present in the charge.
The core materials which can suitably be coated to form carrier particles in accordance with this invention include a variety of magnetic materials. The phrase magnetic materials as used herein encompasses a variety of magnetically attractable materials. Particularly useful materials would include ferromagnetic materials such as the metals of the first transition series, i.e., nickel, iron, cobalt,'and alloys and mixtures thereof. Other useful inaterials which exhibit a net magnetic moment are the ferrimagnetic materials. Examples of such ferrimagnetic materials would include the'ferrites, which are materials having the general formula MeO-Fe O where Me is a metal ion, as well as mix 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 phase magnetic material are particles such as those described in Miller, Canadian 835,317 issued Feb. 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 and nonionomeric polymeric resins. The core can consist of a solid particle of magnetic material or can be a nonmagnetic particle overcoat with ferromagnetic materials as described in Belgian Pat. No. 726,806, dated Mar. 14, 1969. The core can comprise rough-surfaced magnetically responsive particles; smooth-surfaced magnetically responsive particles; or a mixture of rough-surfaced and smooth-surfaced magnetically responsive particles. Particles having these varying surface properties and mixtures thereof are more fully described in Trachtenberg et al., copending patent application U.S. Ser. No. 236,724 filed concurrently herewith and entitled Electrographic Carrier Vehicle and Developer Composition-Case A.
A magnetic 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 60 microns average diameter. The size of the core particles used will, of course, depend upon several factors such as the type of images ultimately developed, desired thickness of the polymeric resin coating, etc. The phrase average diameter as used herein is not meant to imply that only perfectly uniformly dimensioned particles can be used. This phrase is used to refer to the average thick- .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 Pat. 746,109 dated Apr. 30, 1970. 1 Electrostatic developer compositions of the presen invention can be prepared by mixing from about 90 to about 99% by weight of the above-described blended carrier vehicle with from about to about 1% by weight of a suitable electroscopic toner material. i The toner material (or marking particles) useful in dry electrographic developer compositions of the present "invention are generally comprised of a resin binder and a colorant. Suitable toners can be selected from a wide variety of materials to give desired physical properties to the 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 visible image is not desired or needed, the toner may be composed solely of a colorless material, such as a resinous material, having the desired physical and triboelectric properties.
When the toner powder selected is utilized with magnetic carrier particles in a magnetic-brush development arrangement, 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 dissolving the resin, colorant and any additives in a volatile organic solvent such as dichloromethane. This solution is then sprayed through an atomizing nozzle using a substantially nonreactive gas such as nitrogen as the atomizing agent. During atomization, the volatile solvent evaporates from the airborne droplets, producing toner particles of the uniformly colored resin. The ultimate particle size is determined by varying the size of the atomizing nozzle and thepressure of the gaseous atomizing agent. Further details relating to spray-drying may be found in Carlson, U.S. Pat. 2,357,809 issued Sept. 12, 1944, and in West, U.S. Pat. 3,166,510 issued Jan. 19, 1965. conventionally, particles of a diameter between about /2 a and about 30;]. are used, with particles between about 2a and 15p. being 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-blending. This technique involves melting a powdered form 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 useful to mix or otherwise blend the resin and addend'a so as to promote the complete intermixing of these various ingredients. 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. Pat. 2,618,551, issued Nov. 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 preparing toner particles. An example of such a modified spray-freeze drying technique is described in Product Licensing Index, volume 84, April 1971. The resultant toner particles usually range in size from about /2 to about 30 The resin material used in preparing the toner can be selected from a wide variety of materials, including natural resins, modified resins and synthetic resins. Exemplary 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 for toner purposes, for example, polymers, such as certain polycarbonate resins described in U.S. patent application Ser. No. 34,557, filed May 4,
1970 and now U.S. Pat. No. 3,694,359, and in 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 pol-ystyrenes; polycondensates, e.g., polyesters, such as phthalate, terephthalic and isophthalic polyesters, maleinate resins and colophonymixed esters of higher alcohols; phenolformaldehyde resins, including modified phenol-formaldehyde condensates; aldehyde resins; ketone resins; polyamides; polyurethanes, etc. Moreover, chlorinated rubber and polyolefins, such as various polyethylenes, polypropylenes, polyisobutylones, are also suitable. Typical toner materials having incorporated therein styrene containing materials are disclosed in the following US. patents: 2,917,460, issued Dec. 15, 1959; Re. 25,136, issued Mar. 13, 1962; 2,788,- 288, issued Apr. 9, 1957; 2,638,416, issued Apr. 12, 1953; 2,618,552, issued Nov. 18, 1952 and 2,659,670, issued Nov. 17, 1953. 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 methacrylates and alkyl acrylates having up to 3 carbon atoms in the alkyl group; and, (c) 530% by weight of higher alkyl acrylates and methacrylates, e.g. alkyl methacrylates and acrylates having 6-20 or more carbon atoms in the alkyl group.
Colorants useful in the practice of this 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 appropriate caking and charging properties can, of course, be prepared without the use of a colorant material where 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 used 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 (CI. 11680), Nigrosine Spirit soluble (CI. 50415) Chromogen Black ETOO (CI. 45170), Solvent Black 3 (Cl. 26150), Fuchsine N (CI. 42510), C.I. Basic Blue 9 (Cl. 52015), etc. Another useful class of colorants is comprisedof nigrosine salts such as nigrosine salts of monoand di- "functional organic acids having from about 2 to about 20 carbon atoms such as chloroacetic acid, stearic acid,
sebacic acid, lauric acid, 'azelaic acid, adipic acid, abietic acid and the like. Nigrosine salts of this" type are disclosed in Belgian Pat. 734,570, dated Aug. 14,- 1969. Carbon black also provides a useful colorant. The amount of colorant added may vary over a wide range, for example, from about 1 to about 20 percent of the weight of the resin binder. Particularly good results are obtained when the amount is from about 2 to about 10 percent. In certain instances, it may be desirable 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 generally can vary from about 1:3 to about 1.5 :1. Further information concerning these viscosity modifying agents may be found in Merrill et al., US. application Ser. No. 23,031, filed Mar. 26, 1970, and now abandoned.
The following examples are provided to further illustrate the present invention and certain advantages thereof:
EXAMPLE 1 particles toa carrier vehicle comprised solely of a nonionomeric vinylidene chloride-acrylonitrile-acrylic acid terpolymeric coated magnetically attractable core particles. As the composition of the carrier vehicle is varied, the net toner charge imparted by each of the carrier vehicles to a control toner composition is measured to illustrate the synergistic increase in toner charge obtained usin-g the blended carrier vehicles of the present invention. The results of these tests are graphically illustratedin FIG. 1.
As indicated, two types of polymeric coated magnetically attractable 'core particles are utilized in this example to form various blended carrier vehicles. The first polymeric coated carrier vehicle, noted hereinafter as Carrier No. 1, is composed of rough, irregularly-surfaced sponge iron particles sieved to an average particle size of from about to about mesh. These sponge iron particles are obtained from the Hoeganaes Corporation as Hoeganaes Ancor EH-80-150 iron powder; These sponge iron particles, after being sieved, are oxidized in air at a temperature of from about 370 to about 430 C. until a uniform, dark-gray oxide coating is obtained, The resistivity of these particles is measured to be within the range of from 5 X10 to 5 10 ohm-ems. These oxidized sponge iron particles form the magnetically attractable core particles used in Carrier No. 1. Next an ionomer resin is coated over the sponge iron particles as follows. The ionomer resin is obtained from E- I. du Pont Company under the trademark Elvax D-1070. This Elvax material is described by the Du Pont Company as a 42 weight percent aqueous dispersion of an ionomer resin, the ionomer resin being describedby Du Pont as a high molecular weight, ion-linked, acid-modified ethylene interpolymer. The Elvax aqueous dispersion has a pH of from 9-10. The Brookfield viscosity of this dispersion is 50-300 centipoise. The dispersions are known to contain an anionic surfactant. The film-forming temperature of ElvaX 'D-107.0 is reported by Du Pont as being 60 C., maximum film strength being developed at 120 C.
To form the ionomer coated magnetically attractable core particles used in the present invention, 10 grams of the 42' weight percent Elvax resin dispersion described immediately hereinabove, is added to 15. grams of distilled water to make a 16.9 weight percentElvax resin dispersion. The resultant 16.9 weight percent dispersion is then added to 600- grams of methanol to form an aqueousalcoholic ionomer dispersion. 555 grams of this aqueousalcoholic ionomeric dispersion are then added to 2500 grams ofthe oxidized sponge iron particles described immediatelyabove. The mixture of the material is then heated with mixing. on a hot plate heated to -260?-C. for about 2%. hours to evaporate the liquid. The temperature of the mix is about 80 C. during this heating operation until the final stages of heating whereupon the mixture 'rises to a temperature ofapproximately'140- to C..The resultant dried material is then sieved to obtain Carrier No.:'1,-i.e. ionomer coated particles having an average particle size within the range of from about-"120 toabout80mesh.
Thenon-ionomer coated carrier particles utilized" to form the blendedcarr'ier vehicle of the present invention are referred to hereinafter as Carrier No." 2. These terpolymer coated particlesare formedas" follows! First, a quantity-of oxidized sponge iron particles are; made as described immediately hereinabove. Then these oxidized sponge iron particles are coated with afvinylidene chloride-acrylonitrile-acrylic acid' terpolymr' containing approximately 80 weight percent of units derived from vinylidene chloride 15 weight percent 'ofiunits derived from acrylonitrile and';6 weight percent of units derived from acrylic acid. This terpolymer may be formed by conventional polymerization techniques which arejwell known.'l\lext, a 10 weight percent solution" of this terpolymeric material is made by adding 30 grams of the terpolymeric material to 135.0 grams of acetone andthen diluting this mixture with 135.0 grams of tetrahydrofuran. The terpolymer is dissolved in this organic solvent mixture by mixing the acetone, tetrahydrofuran and terpolymer ingredients.
Next, 37.5 grams of the above-described 10, weight percent terpolymeric solution is added to 380 grams of acetone. This further diluted terpolymer solution is then admixed with 2500 grams of the above-described oxidized sponge irn particles and mixed for 2 minutes. The resultant terpolymer solution-sponge iron particle mixture'is then heated with further mixing for about 1% hours to remove the solvents. During this heating operation, the temperature of the mixture is approximately 55 to 60 C. with the temperature, risingiduring the final stages of heating to a range of from about ,140 to 150..C. The dried coated material thus produced is then sieved to obtain Carrier No. 2., i.e. a particulatemixture of dried terpolymeric coated-magnetically attractable iron particles having an average particle size in the range of from 120 to 80 mesh..-
The final composition of Carrier No. 1 is determined to be as follows:
Weight percent Oxidized sponge iron powder 99.85 Ionomeric resin coating 0.15
The resultant composition of Carrier No. 2 is determined to be as follows:
Weight percent Oxidized sponge iron powder 99.85
Terpolymer resin coating 0.15
Using various blends of the above-described carriers, i.e. Carrier No. 1 and Carrier No. 2, the following experiments are run to demonstrate the synergistic increase in net toner charge obtained using the blended carrier vehicle of the present invention. In the following tests, a series of electrographic developer compositions are evaluated. Each electrographic developer composition is composed of 95 weight percent carrier vehicle and 5 weight percent of toner powder. A standard control toner powder s used in each of the tests. The control toner powder s composed of a polycarbonate resin of the type described in copending US. patent application Ser. No. 34,557 filed May 4, 1970 and a carbon black colorant. The particle size of the controltoner composition is approximately -15 microns.
To measure the net toner charge imparted by each of the various blended carrier vehicles a series of nine developer compositions are tested. In each case, grams of the developer composition is tested, the carrier vehicle composing 95% by weight of the developer composition. The net toner charge imparted by each of the various blended carrier vehicles tested is measured as follows: An identical amount of each of the 9 developers is placed individually in an iron tube that is covered at each end by a 200 mesh screen that retains all carrier particles within the tube. An air stream is then directed through the tube, blowing the toner particles off the carrier, through the 200 mesh screen at the exit end and into a Farraday cage condenser that is at an initial electrical potential of 0 volt as measured by an electrometer. As the triboelectrically charged toner particles settle on the walls of the cage, the electrical potential is measured by the electrometer. The potential obtained is converted to electrical charge in microcoulombs and this figure is divided by the weight in grams of each toner that has settled in the Farraday cage, thus providing the net toner charge in microcoulombs/ gram. Since the net charge is the algebraic sum of the charges on each toner particle, a higher reading is indicative of a higher degree of net charge imparted to the toner powder.
In addition, the degree of carrier pick-up and toner throw-off is observed for each of the 9 developer com- 14 positions. Each of the developer compositions exhibits relatively low toner throw-off. However, there are observable differences in toner throw-01f among the nine developer compositions. These differences are noted in Table 1 using the terms high, moderate, and low. These terms are used simply to reflect the relative differences between these nine compositions and are not to be interpreted in an absolutesense. p
Carrier pick-up is measured as follows: The image frames of the photoconductor element are examined visually. If less than about 5 carrier particles per image frame are found, the carrier pick-up is rated as being low; about 5-25 particles is rated medium; and more than about 25 particles is rated high.
,Table 1 shown hereinbelow sets forth the results obtained for each of the nine developer compositions.
TABLE 1 Blended carrier (percent by weight) Net toner charge Carrier Carrier (microcou- Toner Carrier Blend No. No. 1 No. 2 lombs/g.) throw ofi pick-up As indicated in FIG. 1, substantial improvements are obtained when the carrier vehicle comprises a combination of from about to about 5 weight percent of Carrier No. l and preferably from about 20 to 95 weight percent of Carrier No. 1, the remainder of the carrier vehicle being composed of Carrier No. 2. As indicated hereinbefore, the improvement in net toner charge is especially unexpected since a combination of the two carriers would have been expected merely to be additive in its effect on net toner charge. However, as indicated in the graph of FIG. 1 wherein net toner charge is plotted against the composition of the carrier vehicle (the data used to construct the graph of FIG. 1 is obtained from Table 1 shown above) the blended carrier vehicle of the present invention appears to exhibit a synergistic increase in net toner charge even at relatively low levels of addition of Carrier No. 2 to Carrier No. 1. This synergistic increase is expressed in the graph of FIG. 1 by the sharp rise in the experimental toner charge actually obtained even when low levels of Carrier N0. 2 relative to Carrier No. l are utilized. It is believed that this synergistic increase in net toner charge also, at least in part, accounts for the low levels of carrier pick-up and small amounts of toner throw-off which are also exhibited by the blended carrier vehicles and developer compositions of the present invention.
EXAMPLE 2 The above-noted developer compositions containing blended carrier vehicles 3-8 are then used to develop as electrostatic charge image produced on an electrographic element containing an organic photoconductive composition. Good developed images are obtained using blended carriers 3-7. Carrier blend 8 yields good image reproduction; however, as indicated, these developed images exhibit undesirably high carrier pick-up.
The invention has been described in detail with particular reference to certan preferred embodiments thereof, but it will be understood that variations and modifications can be effected wtihin the spirit and scope of the invention.
1. In an electrographic developing composition for use in developing electrostatic charge patterns comprising a physical mixture of magnetically-attractable carrier parti cles and electroscopic toner particles, the improvement wherein said carrier particles comprise a mixture of (a) from about 20 to about 95 percent by weight of magnetically-attractable core particles having on the outer surface thereofa resinous coating of a metal ion-linked at-olefin carboxylic acid copolymer, said a-olefin has'the formula RCH=CH where R is a radical selected from the class consisting of hydrogen and alkyl radicals having from 1 to about 8 carbon atoms, said carboxylic acid is an (1,13- ethylenically unsaturated carboxylic acid having from 3 to about 8 carbon atoms, and said copolymer has from -10% to about 90% of the carboxylic acid groups ionized with metal ions, and (b) from about 80 to about 5 percent by weight of magnetically attractable core particles having on the outer surface thereof a resinous coating of a copolymerized blend of monomers comprising (1) from about 55 to about 93 percent 'by weight of vinylidene chloride, (2) from about 5 to about 30 percent by weight of at least one member of the group acrylonitrile, methacrylonitrile, and an alkyl ester'of an acrylic or methacrylic acid, and (3) from about 2 to about 25 percent by weight of at least one acidic organic compound containing 3 to about 12 carbon atoms, said compound having at least one polymerizable vinylidene moiety and at least one carboxylic acid or sulfonic acid moiety, said blend having an acid content less than about 2 milliequivalents of 1 N.NaOH per gram of said blended based on the dry weight of said blend. 2. The invention of claim 1 wherein said resinous coating of said copolymerized blend is a vinylidene chlorideacrylonitrile-acrylic acid terpolymer.
3. The invention of claim 1 wherein said resinous coating of said copolymerized blend is a terpolymer comprising about 15% by Weight acrylonitrile, about 6% by weight acrylic acid, and about 79% by weight vinylidene chloride.
4. The invention of claim 1 wherein the magnetically attractable core particles contain a material selected from the group consisting of iron, nickel, cobalt and alloys and mixtures thereof.
5. The invention of claim 1 wherein the average size of said core particles is from about 30 to about 1200 microns.
6. The invention of claim 1 wherein the a-olefin of said metal ion-linked copolymer is ethylene.
7. An electrographic developer composition comprising a mixture of from about 1 to about 10 weight percent of electroscopic toner particles and from about 90 to about 99 weight percent of carrier particles, said carrier particles comprising a mixture of (a) from about -to about'95 percent by weight of magnetically attractable core particles having on the outer surface thereof a resinous coating of a metal ion-linked ethylene-carboxylic acid copolymer,
said carboxylic acid selected from the group consisting of a ,fi-ethylenically unsaturated acids having from about 3 to about 8 carbon atoms and wherein said copolymer has from 10 to about 90 percent of said carboxylic acid groups ionized with alkali metal ions, and (b) from about 80 to about 5 percent by weight of magnetically attractable coreparticles having on the outer surface thereof a resinous coating of a vinylidene chloride-acrylonitrileacrylic acid terpolymer, said terpolymer a copolymerized blend of monomers comprising from about 5 to about 30 percent by weight acrylonitrile, from about 2 to about 25 percent by weight acrylic acid, and from about to about 93 percent by Weight vinylidene chloride, said terpolymer having. an acid content less than about 2 milliequivalents of l N.NaOH per gram of said terpolymer based on the dry Weightof said terpolymer.
-8. The invention of claim 7 wherein the magneticallyattractable core particles are selected from the group consisting of iron and :iron alloys and wherein the average size of said core particles is within the range of from about to about 300 microns.
9. The invention of claim 7 wherein said metal ionlinked copolymer and said terpolymer comprise from about 0.001 to about 3 percent by weight of the resin coated carrier particles.
References Cited UNITED STATES PATENTS 3,669,885 6/1972 Wright et al, 252621 3,647,520 3/1972 Gos et al. 260Dig. 31 3,547,822 3/ 1972 Miller 25262.l 3,533,835 10/1970 Hagerbach et al 25262.l 3,526,533 9/1970 Jackrow et al. 252-62.1 3,264,272 8/ 1966 Rees 260Dig. 31 3,404,134 10/1968 Rees 260--Dig. 31 3,322,734 5/ 1967 Rees 260Dig. 31 2,874,063 2/1959 Czerg 252-621 2,416,060 2/ 1947 McClevy et al. 260Dig. 31 3,725,118 4/1973 Fuller, Jr 117-100M FOREIGN PATENTS 1,174,571 12/ 1969 Great Britain 252-621 NORMAN G. TORCHIN, Primary Examiner I. P. BRAMMER, Assistant Examiner us. 01. X.R. 111-100 M