CA1112500A - Carrier materials of insulating and conductive particles - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Electrostatographic carrier particles having insulating and/or conductive surface area are provided by partially or completely coating carrier materials with amorphous polyamides. The carrier particles are characterized as possessing mechanical toughness superior to known coated carrier materials, obtain high degrees of coatability and coating integrity, and possess excellent triboelectric properties even at high relative humidity conditions. The carrier particles may be mixed with any suitable toner material to form developer mixtures and employed to develop electrostatic latent images.

Description

5~0 BACKGROUND OF THE INVENTION
This invention relates in general to electrophotography,and more particularly, to carrier materials useful in the development of electrostatic latent images.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known.
The basic electrostatographic process, as taught by C.F. Carlson in U.S. Patent 2,297,691, involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing ~he layer to a light and shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image by depositing on the image a finely-divided electroscopic material referred to in the art as "toner'.
The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This powder image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to the support surface as by heat. Instead of latent image formation by uniformly charging the photoconductive layer and then exposing the layer to a light and shadow image, one may form the latent image by directly charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing step.
Many methods are known for applying the electroscopic particles to the electrostatic latent image to be developed. One ~ L;25~

development method, as disclosed by E. N. Wise in U.S. Patent

2,618,522 is known as "cascade" development. In this method, a developer material comprising relatively large carrier particles having finely-divided toner particles electrostatically clinging to the surface of the carrier particles is conveyed to and rolled or cascaded across the electrostatic latent image-bearing surface.
The composition of the toner particles is so chosen as to have a triboelectric polarity opposite that of carrier particles. As the mixture cascades or rolls across the image-bearing surface, the toner particles are electrostatically deposited and secured to the charged portion of the latent image and are not deposited on the uncharged or background portions of the image. Most of the toner particles accidentally deposited in the background are removed by the rolling carrier, due apparently, to the greater electrostatic attraction between the toner and the carrier than between the toner and the discharged background. The carrier particles and unused toner particles are then recycled.
This technique is çxtremely good for the development of line copy images. The cascade development process is the most widely used commercial electrostatographic development technique. A
general purpose office copying machine incorporating this technique is described in U.S. Patent 3,009,943.
Another technique for developing electrostatic latent images is the "magnetic brush" process as disclosed, for example, in U.S. Patent 2,874,063. In this method, a developer material containing toner and magnetic carrier particles is carried by a magnet. The magnetic field of the magnet causes alignment of the magnetic carriers in a brush-like configuration. This "magnetic brush" is engaged with an electrostatic-image bearing surface 1~ 5~

and the toner particles are drawn from the brush to the electro-static image by electrostatic attraction.
In automatic reproduction equipment, it is conventional to employ as the imaging plate, a photoconductor in a conductive substrate in the form of a cylindrical drum or a flexible belt which is continuously rotated through a cycle of sequential operations including charging, exposing, developing, trans-erring and cleaning. The developer chamber is charged with developer mixture comprising carrier particles and enough toner for hundreds of reproduction cycles. Generally, the freshly charged developer mixtures contain between about 1.0 and 3.0% toner toner based upon the weight o the developer. This initial concentration provides sufficient toner for many reproduction cycles without causing undesirably high background toner deposition.
The imaging plate is usually given a uniform positive charge by means of a corona generating device connected to a suitable source of high potential as disclosed by L. E. Walkup in U.S.
Patent 2,777,957. The plate is then discharged in imagewise configuration by exposure to a light image corresponding to the original to be copies. The resultant latent image is then brought into developing configuration with the developer mixture.
The relatively high electric fiald over the imaged areas of the plate attracts the toner powder from the carrier particles whereas, ideally the unimaged areas of the plate, do not. Optimally the charged pattern on the imaged plate corresponds to the light and dark areas of the original. However, as explained in "Xerography and ~elated Processes", Dessauer and Clark, The Focal Press, New York (19652, development - ~ . .

fields of dark imaged areas which are very large compared with the thickness of the photoconductive film are confined to the edges of the images. In order to overcome this undesirable effect, a conductive surface called a "development electrode"
is placed near the metal substrate of the imaged plate, either with or without bias potential, to increase the electric field above the large uniformly charged areas to aid in solid area development and reduce background development. When such electrodes are biased, as they usually are in commercially available machines, a field is created between the plate and the electrode which accurately represents the charge density of the latent image. Ideally, such an electrode should be held in virtual contact, since both development and background suppression fields are increased with decreasing distance between the electrode a~d plate. However, it is not practical to have the development electrode in virtual contact with the plate in development processes employing developer mixtures with solid carrier particles.
After development the image is transferred to a copy support surface such as paper, by electrostatically charging the paper to cause it to attract the developed image. After image transfer, the residual toner and carrier particles are removed before the plate is reused in subsequent cycles. This is generally accomplished by imparting an opposite charge to the photoconductive surface thereby nullifying any electrostatic attraction between the surface and the particles, then rubbing the surface to physically remove all the remaining particles and exposing it to light to fully discharge the surface.

It is known to employ coated and uncoated carrier beads to prepare developer mixtures. However, most currently available coated insulating carrier materials have an electrical resistanc~ which is too high to produce excellent quality solid area development. Therefore, recent efforts have been directed toward the provision of uncoated conductive carrier particles which have triboelectric and other physical properties rendering them suitable for developer use. Since these carrier particles do not require a coating to give them the proper electricals to perform in automatic electrostatographic copy machines, they are relatively easy to produce. Moreover, because of their density and triboelectric properties, the carrier particles produce prints of relatively high print density and relatively low background development levels over wide ranges of toner concentrations.
Though developer compositions comprising uncoated carrier particles have been found to initially produce prints of excellent quality, it has been observed that as toner is depleted from the developer mixture and toner concentration approaches low permissible levels, background development in-creases and print quality is reduced. This problem is especially prominent in copying machines employing magnetic brush develop-ment techniques in which the magnetic roll has a bias potential to make the roll perform as a development electrode. This problem is particularly severe with uncoated metal carrier particles. That is, when conductive carrier is employed and conditions necessary for shorting are present, the conductive carrier grounds out the development electrode and the resultant shorting is manifested by heavy deposltion of background toner due to a loss of background suppression bias potential on the development electrode. This e~fect becomes catastrophic when the concentration is low and the carrier is conductive.
PRIOR ART
A method of overcoming some of the aforementioned problems is disclosed in U.S. Patent No. 3,533,835 issued to R. J. Hagenbach et al which teaches the incorporation of finely divided electrically conductive particulate material in at least the surface of carrier substrates. The electrically conductive particulate material employed therein preferably has a volume resistivity of less than about 101 ohm-centimeters and a maximum average particle size of less than about 15 microns.
It is also disclosed that the electrically conductive particulate material is employed to alter the triboelectric properties of carrier substrates. Factors affecting the quantity of conductive particulate material to be incorporated in at least the surface of carrier particles include: the separation in the triboelectric series between the electroscopic marking particles and the carrier material; the average particle size of the conductive particulate additive; the concentration of the particulate conductive material at the surface of the carrier particle; the average diameter of the carrier particle; and the conductivity of the finely-divided particulate additive. The finely-divided conductive particulate material may be distributed only at the surface of a coated or uncoated carrier particle or uniformly distributed throughout an uncoated carrier particle or throughout the external coating of a coated carrier particle.
In view of the ascending importance of carrier particles having controlled triboelectric and conductive properties because of the good print quality they are potentially capable of producing and the limitations which developer mixtures containing carrier particles presently have, it iS~an~~ ~ of an aspect of the present invention to provide developer compositions having the advantages, but not the disadvantages, attached to those employing carrier particles.
It is an object of an aspect of the present invention to provide a developer mixture containing conductive carrier particles, especially particles fabricated from ferromagnetic metals such as nickel, steel, or ferrites which particles are much less dependent upon toner concentration for the produc-10 tion of high quality prints than heretofore thought possibleand which are eminently suitable for use in a variety of development processes.
It is an object of an aspect of this invention to provide a developer mixture containing insulating carrier particles which particles are much more resistant to carrier coating degradation than heretofore known insulating carrier particles.
It is an object of an aspect of this invention to provide a developer mixture containing carrier particles 20 having improved triboelectric properties.
It is an object of an aspect of this invention to provide developer materials having electrostatographic properties superior to those of known developer materials.

Various aspects of the invention are as follows:
In an electrostatographic carrier particle having a diameter of from between about 30 microns and about 1000 microns, said carrier particle comprising a core at least partially coat~d with an electrically insulating resin, the improvement comprising that said insulating resin is an amorphous polyamide having a yield tenslle strength in an electrostatographic : . , electrically insulating coated carrier particle having a diameter of from between about 30 microns and about 1000 microns, said carrier particle comprising a core having a substantially continuous coating of an insulating resin being present in the amount of from about 0.1 percent to about lO.0 percent by weight based on the weight of said carrier particle, wherein the improvement comprises that said insulating resin ;
is an amorphous polyamide having a yield tensile strength of between about 10,700 psi at about 73F.
and about 5,130 psi at about 200 F., a tensile modulus of between about 405,000 psi at about 73F
and about 175,000 psi at about 200F., a compressive modulus of about 339,000 psi, and a Rockwell Hardness at 73F. of between m-89 and m-93.
In an electrostatographic imaging process comprising the steps of providing an electrostato-graphic imaging member having a recording surface, forming an electrostatic latent image on said record-ing surface, and contacting said electrostatic latentimage with a developer mixture comprising finely-divided toner particles electrostatically clinging to the surface of carrier particles having a diameter of from between about 30 microns and about 1,000 microns, each of said carrier particles-comprising a core at least partially coated with an electrically insulating resin, wherein the improvement comprises that said electrically insulating resin is an amorphous poly-amide having a yield tensile strength in an electro-statographic imaging process comprising the steps ofproviding an electrostatographic imaging member having a recording surface, forming an electrostatic latent ~.
~i ~a-image on said recording surface, and contacting said electrostatic latent image with a developer mixture comprising finely-divided toner particles electro-statically clinging to the surface of electrically insulating coated carrier particles having a diameter of from between about 30 microns and about 1,000 microns, each of said carrier particles comprising a core having a substantially continuous coating of an insulating resin being present in the amount of from about 0.1 percent to about 10.0 percent by weight based on the weight of said carrier particles, wherein the improvement comprises that said insulating resin is an amorphous polyamide having a yield tensile strength of between about 10,700 psi at about 73F.
15 and about 5,130 psi at about 200 F., a tensile modulus of between about 405,000 psi at about 73F.
and about 175,000 psi at about 200 F., a compressive modulus of about 339,000 psi, and a Rockwell Hardness at 73 F. of between m-89 and m-93, whereby at least a 20 portion of said finely-divided toner particles are attracted to and deposited on said recording surface in conformance with said electrostatic latent image.

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In a preferred embodiment where it is desired to obtain a carrier particle having insulating properties, the outer surface of coated or uncoated carrier beads is substantially completely coated with the polyamides of this invention. It has been found that such a coated carrier particle provides a carrier particle having a useful life longer than here-to-known coated insulating carrier materials. The coated insulating carrier particles of this invention are characterized as possessing mechanical toughness far superior to styrene-methacrylate-siloxane terpolymers or al~yl methacrylate-styrene copolymers;
they obtain high degrees of coatability and coating integrity;
and possess excellent triboelectric properties even at high relative humidity conditions.
In another preferred embodiment of this invention where it is desired to obtain a carrier particle having conductive properties, ferromagnetic carrier beads such as iron, steel, ferrites, and nickel are partially coated with the polyamides.
It has been round that when such ferromagnetic carrier beads are partially coated with the polyamides of this invention, that the resultant carrier particles provide a carrier material having conductive properties which enables good solid area development when employed in an electrostatographic imaging system; the conductive developer does not accumulate a net charge; and the conductive developer has a rapid charging rate.
Any suitable amorphous polyamide may be employed to completely or partially coat the carrier particles of this invention. Typical polyamides include poly-2,2,4-trimethylhexa-fra~/c ~a~
methylene terephthalamide available under the trad~n~c Trogamid T from Dynamit Nobel of America, Inc., Norwood, New Jersey, Amidel~ from Union Carbide Corporation, New York, New York;

Elvamide~ from E. I. duPont and Co., Wilmington, Delaware,condensation products of polymerized unsaturated fatty acids with aliphatic amines available under the tradename Versamids from General Mills, Inc., Chemical Div., Kankakee, Illinois;
poly-diacetone acrylamide; and poly-N, N-dimethylacrylamide, including mixtures thereof. However, the preferred polyamide for use in this invention is poly-2,2,4-trimethylhexamethylene tere-phthalamide because its mechanical toughness is comparable to Mylar~ polyester, J ~ polycarbonate, and polysulfone; it possesses satisfactory and stable triboelectric properties; and it provides a coating integrity of about 90 percent at about 0.2%
coating weight on 100 micron steel carrier cores. Further, the amorphous polyamides of this invention have a yield tensile strength of between about 10,700 psi at about 73~F and about 5,130 psi at about 200F; a tensile modulus of between about 405,000 psi at about 73F and about 175,000 psi at about 200F, both values obtained according to ASTM test method D638; a compressive modulus (modulus of elasticity) of about 339,000 psi according to ASTM test method D695; a Rockwell Hardness at 73F of between M-89 and M-93 according to ASTM test method D785; and a Taber abrasion (CS 17 wheel, 1000 g. wt.) of 21 mg/1000 cycles according to ASTM test method D1044.
As indicated, the carrier materials of the present invention may have conductive and insulating surfaces and may also be blended in the proper proportions to meet the performance requirement of any given copying and duplicating system. Conductive carrier materials are provided in accordance with this invention by providing metals such as iron, steel, nickel, zinc, aluminum, brass, copper, and the like, as well as combined for~ms of such metals, with a partial coating of .' ' . ~ - .
: ' ' ' :

the polyamides thereon. Thus, a wide variety of particulate, magnetically responsive materials the surface of which may be oxidized including materials in such forms as steel and iron particles produced by atomization of molten metal and subsequent cooling of the droplets; particles produced by grinding, milling, filing, turning etc; as well as particles of steel and iron alloys having oxidizable iron on the surface thereof such as stainless steel and iron alloys containing nickel and/or cobalt may be employed in accordance with this invention. Insulating carriers may be of any of the above-mentioned materials having a surface coating comprising the polyamides herein capable of forming a substantially continuous coating thereon. Alternatively, the insulating carrier particles may be fabricated from an insulating material with a surface polyamide coating thereon.
Further, conductive and insulating carrier particles made in accordance with this invention can be admixed in such proportions as to give a carrier composition with an overall surface resistivity of between about 104 ohm-cm and about 1015 ohm-cm.
The polyamide coating produced upon the conductive carrier particles of this invention may be any suitable thickness or weight percent. However, a polyamide layer sufficiently distributed at or near the surface to produce at least semi-conductive electrical characteristics is preferred because the carrier particles will then possess desirable triboelectric features and allow their application in electrode development systems where RC (Resista~ce-Capacitance) time constant considerations are of importance in preventing high electrical discharges between the development electrode and the photoreceptor.
Preferably the polyamide coating should be so distributed through the preparation so that the conductive carrier, where measured in the bead aggregate, will possess a volume resistivity between about 105 ohm-cm and 1012 ohm-cm. Carrier particles having a partial surface polyamide coating and conductive properties are highly desirable with respect to their use in magnetic brush development electrostatographic copying and duplicating devices. When employed with finely divided toner particles to develop electrostatic latent images in a magnetic-brush development apparatus, they have been found to provide developed images having lower background densities and higher resolution than prior known carrier materia~s. Although not wishing to be bound by any theory, it is believed that the improved results obtained are due to the conductive properties of the carrier materials. The carrier particles may vary in size and shape, however, the size of the carrier particles employed will, of course, depend upon several factors, such as the type of images ultimately developed, the machine configuration, and so forth.
Many of the foregoing and other typical carrier materials are described by L.E. Walkup in U.S. Patent 2,618,551;
B.B. Jacknow et al in U.S. Patent 3,526,533; and R.J. Hagenbach et al in U.S. Patents 3,533,835 and 3,658,500. When the carrier particles of this invention are coated with a polyamide, any suitable electrostatographic carrier coating thickness or weight may be employed. However, a carrier coating having a thickness at least sufficient to form a thin continuous film on the carrier particle is preferred when a completely coated insulating caxrier particle is desired because the carrier coating will then possess sufficient thickness to resist abrasion and prevent pinholes which adversely affect the triboelectric properties of the coated carrier particles. Generally, for cascade and magnetic brush development, the carrier coating may comprise from about 0.1 percent to about 10.0 percent by weight based on the weight of the coated carrier particles.
Preferably, the carrier coating should comprise from about 0.5 percent to about 1.0 percent by weight based on the weight of the coated carrier particles because maximum insulation, coating durability, toner impaction resistance, and copy quality are achieved.
- Toner materials which positively or negatively triboelectrify the carrier materials herein can be used in the present invention. The choice of material depends upon the charging character of the photoconductor and whether positive or reversal development is desired. Thus, to produce a positive print from a positively charged selenium photoconductive member, a carrier should be chosen which imparts a negative charge to the toner material, whereas for reversal development of the same photoconductor the carrier material should impart a positive charge to the toner. In the development of other photoconductive surfaces which accept a negative charge such as poly(vinyl carbazole) or zinc oxide, the reverse holds true. Thus, for positive development of such surfaces, the carrier material should impart a positive charge to the toner material, whereas for reversal development the carrier material should impart a negative charge to the toner material.
The carrier particles of the present invention should have an average particle diameter of between about 30 microns to about 1,000 microns, preferably between about 50 to about 500 microns. When used in magnetic brush development systems, .

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they must obviously be fabricated of or at least contain a magnetic material such as iron, steel, nickel, cobalt or ferromagnetic oxides. If development systems such as cascade development or "shell" development systems as disclosed in U.S. Patent 3,503,776 are employed, the carrier material need not have ferromagnetic properties. In this case, a carrier material such as zinc which provides excellent triboelectri-fication of toner for development of selenium photoconductors, is suitably employed.
Developer mixtures of the present invention are particularly useful in reproduction systems employing a development electrode and magnetic brush development rolls.
In practical use, magnetic rolls with a bias potential perform the function of both the magnetic brush and the development electrode. In a typical situation in which a selenium photoconductor is developed by a plurality of magnetic rolls bearing a developer mixture comprising between about 0.75 and

3~ of a pigmented polymeric toner (e.g., a 10~ carbon black dispersion in a polymeric matrix comprising a blend of a styrene-n-butyl methacrylate copolymer with poly(vinyl butyral), the photoconductor bears a positive charge of about 800 volts in dark image areas and about 100 volts in non-imaged areas.
The development electrode, i.e., the magnetic brushes, is biased with a potential of about 200 volts. The spacing between the development electrode and the photoconductive surface is sufficient to prevent any shorting out between the development electrode and the plate, but not so large as to have the developer mixture outside of the development electric field of the charged plate. Generally, development electrode-photoconductor spacings of about 0.04 to 0.12 inches suffice to prevent un-due damage to the photoconductor surface while bringing the development electrode in sufficient proximity to the photo-conductor surface. Using carrier beads having at least a partially conductive surface in accordance with the present invention, it is noted that the density of developed dark image areas is much greater with respect to background density than when carrier beads having an insulating surface are used in otherwise identical conditions. It is theorized that this is due to the formation of conductivé paths which effectively extend the development electrode closer to the photoconductor surace thereby effectively increasing both external development and background suppression fields. This effect is absent when insulating carrier beads are substituted under identical operating conditions. Moreover, as the toner concentration decreases from the initial level of above about 2% minimum levels for acceptable print density to about 0.75~ of the developer mixture, this effect is hardly diminished. Thus, the carrier materials of the present invention are capable of providing excellent print quality, i.e., high print density and low background development, over wide toner concentration ranges.
Contrary to the relatively wide toner concentration latitude available with the partially polyamide coated conductive carrier materials of the present invention, developer mixtures containing only uncoated conductive carrier particles, e.g., nickel carrier beads, produce prints of widely varied quality, depending upon whether toner concentration is at the high or low end of the usable toner concentration range, and whether the photoconductor surface has any flaws through which shorting between the development electrode and the conductive .

backing, may occur. Under ideal conditions, the photoconductor surface is free of any imperfections; however, in practice, photoconductor surfaces frequently are scratched through frictional contact of developer material with machine parts.
A fresh developer mixture containing at least about 2~
toner and only completely uncoated conductive carrier beads having an average particle diameter of about 100 microns, produces prints of excellent quality. However, as toner concentration decreases, the density of developed areas begins to sharply decrease while the level o~ background development sharply increases. It is theorized that this effect is due to the lengthening of conductive paths and the consequent increased chance of shorting between the development electrode and the conductive layer of the electro~
statographic plate, since the toner, which is normally not conductive, acts as an insulator to interrupt conductive paths when toner concentration is sufficiently high; whereas it does not, when toner concentration decreases. Thus, in developer mixtures based upon completely uncoated conductive carrier beads, toner concentration of a used developer mixture may be sufficiently high to produce acceptable print density under ideal conditions, but too low to prevent shorting out between the development electrode and the grounded photoconductive surface. However, with the carrier materials of this invention the conductive properties of the carrier materials never reach a conduction level as to cause shorting due to the limited conductivity of the carrier particles.
By employing carrier particles having a partially conductive surface and toner particles having a good triboelectric relationship with respect to each other, the overall development capability of the developer mixture can be greatly increased over that of either an insulating or completely conductive carrier particle alone. The apparent mechanism for this beneficial effect is that sufficient conductive paths are formed through the partially conductive carrier-to-partially conductive carrier contacts. If these partially conductive paths are long enough and numerous enough, a virtual electrode (at the potential of the development electrode) is formed in the region between the photoconductor and the development electrode. Since the distance from the photoconductor surface to the development electrode is effectively decreased, the electric fields which control, to a great extent, the deposition of toner, are increased. Increasing the development field increases the density of the developed image. Increasing the background suppression field reduces the development of background. To gain this effect the proportion of conductive surface areas of the carrier particles to insulating surface areas of the carrier particles should be great enough so that conductive paths of sufficient length and number are formed to increase the electric field. To obtain the foregoing results, between about 10~ and about 90% of the surface area of the carrier materials of the present invention should be of the type having a conductive surface. The optimum proportion of conductive to insulating carrier surface area should be such that when the carrier particles are mixed with the minimum amount of toner which produces prints of acceptable density, the maximum length of uninterrupted conductive paths is less than the spacing between the development electrode and the photoconductor surface. In cases where the development electrode-photoconductor spacing is in the above-noted range, it has been .

noted that the surface area of carrier particles having a conductive surface should be about 25 percent to about 75 percent of the total carrier particles of the developer mixture.
Any suitable well known toner material may be employed with the carrier materials of this invention. Typical toner materials include gum copal, gum sandarac, rosin, cumaroneindene resin, asphaltum, gilsonite, phenolformaldehyde resins, rosin modified phenolformaldehyde resins, methacrylic resins, polystyrene resins, polypropylene resins, epoxy resins, polyethylene resins, polyester resins, and mixtures thereof. The particular toner material to be employed obviously depends upon the separation of the toner particles from the carrier materials in the triboelectric series and should be sufficient to cause the toner particles to electrostatically cling to the carrier surface. Among the patents describing electroscopic toner compositions are U.S. Patent 2,659,670 to Copley; U.S.
Patent 2,753,308 to Landrigan; U.S. Patent 3,079,342 to Insalaco; U.S. Patent Reissue 25,136 to Carlson and U.S. Patent 2,788,288 to Rheinfrank et al. These toners generally have an average particle diameter between about 1 and 30 microns.
Any suitable colorant such as a pigment or dye may be employed to color the toner particles. Toner colorants are well known and include, for example, carbon black, nigrosine dye, aniline blue, Calco Oil Blue, chrome yellow, ultramarine blue, Quinoline Yellow, methylene blue chloride, Monastral Blue, Malachite Green Ozalate, lampblack, Rose Bengal, Monastral Red, Sudan Black BM, and mixtures thereof.
The pigment or dye should be present in a quantity sufficient to render it highly colored so that it will form a clearly visible image on a recording member. Preferably, the pigment is employed in an amount from about 3 percent to about 20 percent by weight based on the total weight of the colored toner because high quality images are obtained. If the toner colorant employed is a dye, substantially smaller quanitities of colorant may be used.
Any suitable conventional toner concentration may be employed with the carrier materials of this invention. Typical toner concentrations for development systems include about 1 part toner with about 10 to about 200 parts by weight of carrier.
The carrier materials of the instant invention may be mixed with finely divided toner particles and employed to develop electrostatic latent images on any suitable electrostatic latent image-bearing surface including conventional photoconductive surfaces. Typical inorganic photoconductor materials include:
sulfur, selenium, zinc sulfide, zinc oxide, zinc cadmium sulfide, zinc magnesium oxide, cadmium selenide, zinc silicate, calcium strontium sulfide, cadmium sulfide, mercuric iodide, mercuric oxide, mercuric sulfide, indium tri-sulfide, gallium selenide arsenic disulfide, arsenic trisulfide, arsenic triselenide, antimony trisulfide, cadmium sulfoselenide, and mixtures thereof.
Typical organic photoconductors include: quinacridone pigments, phthalocyanine pigments, triphenylamine, 2,4-bis(r,r'-diethylamino-phenol)-1,3,4-oxadiazole, N-isopropylcarbazole, triphenylpyrrole,

4,5-diphenylimidazolidinone, 1,5-dicyanonaphthalene, 1,4-dicyanonaphthalene, aminophthalodinitrile, nitrophthalo-dinitrile, 1,2,5,6-tetra-azecyclooctatetraene-(2,4,6,8), 2-mercaptobenzothiazole-2-phenyl-4-diphenylidene-oxazolone, 6-hydroxy-2,3-di(p-methoxyphenyl)-benzofuran~e, 4-dimethylamino-benzylidene-benzhydrazide, 2-benzylidene-aminocarbazole, polyvinyl carbazole, (2-nitrobenzylidene)-p-bromoaniline, 2,4-diphenyl-quinazoline, 1,2,4-triazine, 1,5-diphenyl-3-methyl-pyrazoline,2-(~'-dimethylamino phenyl)-benzoxazole, 3-amine-carbazole, and mixtures thereof. Representative patents in which photoconductive materials are disclosed include U.S.
Patents 2,803,542 to Ullrich, U.S. Patent 3,121,007 to Middleton, and U.S. Patent 3,151,982 to Corrsin.
The carrier materials of this invention provide numerous advantages when employed to develop electrostatic latent images. For example, when mixed with a toner material the resultant developer composition is found to greatly reduce the development electrode effect of a biased magnetic brush and there is no opportunity for strong transient currents occuring to damage photoreceptor surfaces or produce copy defects. Thus, it is not possible to short circuit and draw heavy currents between development electrodes and the photoreceptor through the electrical path offered by these carrier materials.
Further, the conductive carrier particles herein provide excellent solid area development; there is no net charge accumulation resulting therefrom when employed in a developer mixture; and the developer mixture has a rapid triboelectric charging rate. Further still, this invention enables the use of low cost irregular magnetic carrier core materials such as sponge iron which is very difficult to completely coat, yet provides a carrier material having excellent electro-statographic properties. Therefore, developer compositions employing these carrier materials are found to provide lower background densities, higher image resolutions, and greatly improved overall print qualities. Further, a wide range of xerographic properties ma~ be derived from these carrier materials since xerographic properties may be controlled based ~L~h~

on the polyamide coated surface areas of carrier paxticles.
The following examples, other than the control examples, further define, describe, and compare preferred methods of preparing and utilizing the carrier materials of the present invention in electrostatographic applications.
Parts and percentages are by weight unless otherwise indicated.
In the following examples, the relative triboelectric values generated by contact of carrier beads with toner particles is measured by means of a Faraday Cage. The device comprises a brass cylinder having a diameter of one inch and a length of one inch. A 100-mesh screen is positioned at each end of the cylinder. The cylinder is weighed, charged with 0.5 grams of a mixture of carrier and toner particles and connected to ground through a capacitor and an electrometer connected in parallel. Dry compressed air is then blown through the brass cylinder to drive all the toner from the carrier. The charge on the capacitor is then read on the electrometer. Next, the chamber is reweighed to determine the weight loss. The resulting data is used to calculate the toner concentration and the charge in micro-coulombs per gram of toner. Since triboelectric measurements are relative, the measurements should, for comparative purposes, be conducted under substantially identical conditions. Thus, a toner comprising a styrene-n-butyl methacrylate copolymer, polyvinyl butyral, and carbon black by the method disclosed by M.A. Insalaco in Example I of U.S.
Patent 3,079,342 is used as a contact triboelectrificiation standard in all the examples. Obviously other suitable toners such as those listed above may be ~ubstituted for the toner used in the examples.

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EXAMPLE I
A control developer mixture was prepared by mixing about 6 grams of toner particles comprising a styrene-n-butyl methacrylate copolymer and carbon black with about 300 grams of carrier particles. The carrier particles comprised steel shot having an average particle size of about 100 microns which is commercially available from Nuclear Metals, Wes~ Concord, ~ass.
(Division of Whittaker Corporation). The developer mixture was employed to develop electrostatic latent images in a copying machine equipped with a magnetic brush development device. The magnetic brush development fixture comprised 1 magnetic roll, which transported developer to the electrostatic image area which had been previously formed on the flat plate photocon-ductive imaging surface of the fixture. During the test, extreme powder cloud effect was found. Background densities were approximately half of the solid area densities as measured with a Welch densitometer. Image resolution was very poor.
It was concluded that this developer mixture was unsatisfactory.
EXAMPLE II
A developer mixture was prepared by mixing about 6 grams of toner particles as in Example I with about 300 grams of carrier particles. The carrier particles were as in Example I except that the steel shot was coated with about 0.8 percent by weight, based on the weight of the carrier particles, of a coating composition comprising about 15 parts of styrene, 85 parts of methyl methacrylate, and 1 part of vinyltriethoxysilane. The developer mixture was employed to develop electrostatic latent images as in Example I. To maintain a developed image line density of no less than 1.2, which is considered acceptable, it was found that the toner concentration could not be less than about 1.22 percent of the developer mixture. At the line density of 1.2, it was found that the solid area density of the developed iamge was only 0.17. The background density of the developed images was acceptable. The relative triboelectric value of the carrier material measured by means of a Faraday Cage was about 29 micro-coulombs per gram of toner.
EXAMPLE III
A developer mixture was prepared by mixing about 6 grams of toner particles as in Example I with about 300 grams of carrier particles. The carrier particles were as in Example I except that the steel shot was coated with about 0.2 percent by weight, based on the weight of the particles, of a coating composition comprising poly-2,2,4-trimethylhexamethylene terephthalamide (Trogamid T, available from Dynamit Nobel of America Inc., Norwood, New Jersey). The carrier particles were found to have a coating integrity of about 90 percent of the surface area which is unusally high for this type of carrier core material. The developer mixture was employed to develop electrostatic latent images as in Example I. The copy quality and developer life obtained was found to be comparable to that obtained with the developer mixture of Example II. The relative triboelectric value of this carrier material measured by means of a Faraday Cage was about 25 micro-coulombs per gram of toner.
EXAMPLE IV
. .
A developer mixture was prepared by mixing about 6 grams of toner particles as in Example I with about 300 grams of carrier particles. The carrier particles were as in Example I except that the steel shot was coated with about

5~

0.4 percent by weight, based on the weight of the particles, of a coating composition comprising poly-2,2,4-trimethylhexamethylene terephthalamide (Trogamid T, available from Dynamit Nobel of America Inc., Norwood, New Jersey). The developer mixture was employed to develop electrostatic latent images as in Example I. The copy quality and developer life obtained was found to be superior to that obtained with the developer mixture of Example II. The carrier particles were found to show a considerably slower rate of copy quality degradation than those of Example II. In addition, these carrier particles consistently gave less deposition o toner particles in background areas than those of Example II. The relative triboelectric value of this carrier material measured by means of a Faraday Cage was about 35 micro-coulombs per gram of toner.
EXAMPLE V
A developer mixture was prepared by mixing about 6 grams of toner particles as in Example I with about 300 grams of carrier particles. The carrier particles were as in Example I except that the steel shot was coated with about 0.9 percent by weight, based on the weight of the particles, of a coating composition comprising poly-2,2,4-trimethylhexamethylene terephthalamide (Trogamid T, available from Dynamit Nobel of America Inc., Norwood, New Jersey). The carrier particles were found to have a coating integrity of substantially 100 percent of the carrier surface area. The developer mixture was employed to develop electrostatic latent images as in Example I. The copy quality and developer life obtained was found to be superior to that obtained with the developer mixture of Example II. The carrier particles were found to show `--~

a considerably slower rate of copy quality degradation than those of Example II. In addition, these carrier particles consistently gaye less deposition of toner particles in back-ground areas than those of Example II. The relative triboelectric value of this material measured by means of a Faraday Cage was about 43 micro-coulombs per gram of toner.
EXAMPLE ~I
A developer mixture was prepared as in Example IV
except that the toner particles comprised styrene, alkyl methacrylate, carbon black, and a quaternary ammonium salt.
The amount of toner particles and carrier particles, the carrier core, coating material and the amount of carrier coating were the same as in Example IV. This developer mixture was employed to develop electrostatic latent images as in Example I, but in a reversal mode. The copy quality and developer life obtained was found to be comparable to that obtained with the developer mixture of Example IV. The relative triboelectric of the carrier material measured by means of a Faraday Cage was about -25 micro-coulombs per gram of toner.
Although specific components, proportions and procedures have been stated in the above description of the preferred embodiments of the novel carrier system, other suitable materials, as listed above, may be used with similar results. Further, other materials and procedures may be employed to synergize, enhance or otherwise modify the novel system.
other modifications and ramifications of the present invention will appear to those skilled in the art upon the reading of a disclosure. These are intended to be included within the scope of this invention.

Claims (14)

WHAT IS CLAIMED IS:

1. In an electrostatographic carrier particle having a diameter of from between about 30 microns and about 1000 microns, said carrier particle comprising a core at least partially coated with an electrically insulating resin, the improvement comprising that said insulating resin is an amorphous polyamide having a yield tensile strength in an electrostatographic electrically insulating coated carrier particle having a diameter of from between about 30 microns and about 1000 microns, said carrier particle comprising a core having a substantially continuous coating of an insulating resin being present in the amount of from about 0.1 percent to about 10.0 percent by weight based on the weight of said carrier particle, wherein the improvement comprises that said insulating resin is an amorphous polyamide having a yield tensile strength of between about 10,700 psi at about 73°F.
and about 5,130 psi at about 200° F., a tensile modulus of between about 405,000 psi at about 73°F
and about 175,000 psi at about 200°F., a compressive modulus of about 339,000 psi, and a Rockwell Hardness at 73°F. of between m-89 and m-93.

2. In an electrostatographic electrically insulating coated carrier particle having a diameter of from between about 30 microns and about l,000 microns, said carrier particle comprising a core having a substantially continuous coating of an insulating resin being present in the amount of from about 0.1 percent to about 10.0 percent by weight based on the weight of said carrier particle, wherein the improve-ment comprises that said insulating resin is an amor-phous polyamide having a yield tensile strength of between about 10,700 psi at about 73°F. and about 5,130 psi at about 200°F., a tensile modulus of between about 405,000 psi at about 73°F. and about 175,000 psi at about 200°F., a compressive modulus of about 339,000 psi, and a Rockwell Hardness at 73°F.
of between m-89 and m-93, and said coated carrier particle is characterized as possessing improved mechanical toughness.

3. In an electrostatographic insulating coated carrier particle in accordance with claim 2 wherein said core is magnetically-responsive.

4. In an electrostatographic insulating coated carrier particle in accordance with claim 2 wherein said amorphous polyamide comprises poly-2,2,4-tri-methylhexamethylene terephthalamide.

5. In an electrostatographic conductive carrier particle having a diameter of from between about 30 microns and about 1,000 microns, said carrier particle comprising a ferromagnetic core material being partially coated with an electrically insulating resin, the improvement comprising that said insulating resin is an amorphous polyamide having a yield tensile strength of between about 10,700 psi at about 73°F.
and about 5,130 psi at about 200°F., tensile modulus of between about 405,000 psi at about 73°F. and about 175,000 psi at about 200°F., a compres-sive modulus of about 339,000 psi, and a Rockwell Hardness at 73°F. of between m-89 and m-93, and said amorphous polyamide is present over between about 10 percent and about 90 percent of the surface area of said carrier particle.

6. In an electrostatographic conductive carrier particle in accordance with claim 5 wherein said ferro-magnetic core material is selected from the group con-sisting of iron, steel, ferrites, and nickel.

7. In an electrostatographic conductive carrier particle in accordance with claim 5 wherein said amor-phous polyamide comprises poly-2,2,4-trimethylhexa-methylene terephthalamide.

8. In an electrostatographic conductive carrier particle in accordance with claim 5 wherein said amor-phous polyamide is selected from the group consisting of the condensation products of polymerized unsaturated fatty acids and aliphatic amines, poly-diacetone acrylamide, and poly-N,N-dimethylacrylamide.

9. In an electrostatographic carrier mixture comprising particles having a diameter of from between about 30 microns and about 1,000 microns, said carrier mixture comprising two distinct fractions of carrier particles wherein one fraction of said carrier particles comprises conductive particles comprising ferromagnetic core materials coated with an insulating resin over between about 10 percent and about 90 percent of the surface area of said core materials, and wherein the other fraction of said carrier particles comprises electrically insulating carrier particles com-prising carrier cores having a substantially continuous coating of said insulating resin, said coating compris-ing from about 0.1 percent to about 10.0 percent by weight based on the weight of said carrier particles, the improvement comprising that said insulating resin is an amorphous polyamide having a yield tensile strength of between about 10,700 psi at about 73°F. and about 5,130 psi at about 200°F., a tensile modulus of between about 405,000 psi at about 73°F. and about 175,000 psi at about 200°F., a compressive modulus of about 339,000 psi, and a Rockwell Hardness at 73°F. of between m-89 and m-93, and wherein said insulating carrier particles are characterized as possessing improved mechanical toughness.

10. In an electrostatographic developer mixture comprising finely-divided toner particles electro-statically clinging to the surface of electrically insulating coated carrier particles having a diameter of from between about 30 microns and about 1,000 microns, each of said carrier particles comprising a core having a substantially continuous coating of an insulating resin being present in the amount of from about 0.1 percent to about 10.0 percent by weight based on the weight of said carrier particles, wherein the improvement comprises that said insulating resin is an amorphous polyamide having a yield tensile strength of between about 10,700 psi at about 73°F. and about 5,130 psi at about 200°F., a tensile modulus of between about 405,000 psi at about 73°F. and about 175,000 psi at about 200°F., a compressive modulus of about 339,000 psi, and a Rockwell Hardness at 73°F. of between m-89 and m-93, and said coated carrier particles are characterized as possessing improved mechanical toughness.

11. In an electrostatographic developer mixture comprising finely-divided toner particles electro-statically clinging to the surface of conductive carrier particles having a diameter of from between about 30 microns and about 1,000 microns, each of said carrier particles comprising a ferromagnetic core material being partially coated with an electrically insulating resin, the improvement comprising that said insulating resin is an amorphous polyamide having a yield tensile strength of between about 10,700 psi at about 73°F. and about 5,130 psi at about 200°F., a tensile modulus of between about 405,000 psi at about 73°F. and about 175,000 psi at about 200°F., a com-pressive modulus of about 339,000 psi, and a Rockwell Hardness at 73°F. of between m-89 and m-93, and said amorphous polyamide is present over between about 10 percent and about 90 percent of the surface area of said carrier particles.

12. In an electrostatographic imaging process comprising the steps of providing an electrostato-graphic imaging member having a recording surface, forming an electrostatic latent image on said record-ing surface, and contacting said electrostatic latent image with a developer mixture comprising finely-divided toner particles electrostatically clinging to the surface of carrier particles having a diameter of from between about 30 microns and about 1,000 microns, each of said carrier particles comprising a core at least partially coated with an electrically insulating resin, wherein the improvement comprises that said electrically insulating resin is an amorphous poly-amide having a yield tensile strength in an electro-statographic imaging process comprising the steps of providing an electrostatographic imaging member having a recording surface, forming an electrostatic latent image on said recording surface, and contacting said electrostatic latent image with a developer mixture comprising finely-divided toner particles electro-statically clinging to the surface of electrically insulating coated carrier particles having a diameter of from between about 30 microns and about 1,000 microns, each of said carrier particles comprising a core having a substantially continuous coating of an insulating resin being present in the amount of from about 0.1 percent to about 10.0 percent by weight based on the weight of said carrier particles, wherein the improvement comprises that said insulating resin is an amorphous polyamide having a yield tensile strength of between about 10,700 psi at about 73°F.
and about 5,130 psi at about 200° F., a tensile modulus of between about 405,000 psi at about 73°F.
and about 175,000 psi at about 200° F., a compressive modulus of about 339,000 psi, and a Rockwell Hardness at 73° F. of between m-89 and m-93, whereby at least a portion of said finely-divided toner particles are attracted to and deposited on said recording surface in conformance with said electrostatic latent image.

13. In an electrostatographic imaging process comprising the steps of providing an electrostatographic imaging member having a recording surface, forming an electrostatic latent image on said recording surface, and contacting said electrostatic latent image with a developer mixture comprising finely-divided toner particles electrostatically clinging to the surface of electrically insulating coated carrier particles having a diameter of from between about 30 microns and about 1,000 microns, each of said carrier particles comprising a core having a substantially continuous coating of an insulating resin being present in the amount of from about 0.1 percent to about 10.0 percent by weight based on the weight of said carrier particles, wherein the improvement comprises that said insulating resin is an amorphous polyamide having a yield tensile strength of between about 10,700 psi at about 73°F. and about 5,130 psi at about 200°F., a tensile modulus of between about 405,000 psi at about 73°F. and about 175,000 psi at about 200°F., a compressive modulus of about 339,000 psi, and a Rockwell Hardness at 73°F. of between m-89 and m-93, and said coated carrier particles are characterized as possessing improved mechanical tough-ness, whereby at least a portion of said finely-divided toner particles are attracted to and deposited on said recording surface in conformance with said electro-static latent image.

14. In an electrostatographic imaging process comprising the steps of providing an electrostatographic imaging member having a recording surface, forming an electrostatic latent image on said recording surface, and contacting said electrostatic latent image with a develop-er mixture comprising finely-divided toner particles electrostatically clinging to the surface of conductive carrier particles having a diameter of from between about 30 microns and about 1,000 microns, each of said carrier particles comprising a ferromagnetic core material being partially coated with an electrically insulating resin, the improvement comprising that said insulating resin is an amorphous polyamide having a yield tensile strength of between about 10,700 psi at about 73°F. and about 5,130 psi at about 200°F., a tensile modulus of between about 405,000 psi at about 73°F. and about 175,000 psi at about 200°F., a compres-sive modulus of about 339,000 psi, and a Rockwell Hardness at 73°F. of between m-89 and m-93 and said amorphous polyamide is present over between about 10 percent and about 90 percent of the surface area of each of said carrier particles, whereby at least a portion of said finely-divided toner partices are attracted to and deposited on said recording surface in conformance with said electrostatic latent image.