US2919247A - Tripartite developer for electrostatic images - Google Patents

Description

United States TRIPARTITE DEVELOPER FOR ELECTROSTATIC IMAGES No Drawing. Application December 23, 1954 Serial No. 477,380

10 Claims. (Cl. 25262.1)

This invention relates to improved mixtures for developing electrostatic and electrophotographic images, and more particularly to a tripartite developer mixture adapted to enhance the reproduction of half tone, continuous tone and other dark-area copy by cascade development.

In the electrophotographic process, the photoconductive layer of a xerographic plate is first charged electrostatically, the charged surface then being exposed under light to the subject to be copied, thereby forming a latent electrostatic image. The latent image is developed by subjecting the image layer to a cascading operation of developer powder to produce a powder image, which is thereafter transferred and aflixed to paper or other transfer medium.

Commercial developer materials are presently known which consist of two major components; namely, electroscopic powder in combination with a carrier. Developers of this type are constituted by a dry mixture of loose, movable particles of finely divided electrostatically-attractable and pigmented powder, generally called toner powder, and separate granular carrier material. The granular material is composed of loose particles, each of which includes a core, such as a glass bead, which is encased in a triboelectric resinous coating. To effect adherence of the resinous casing to the core, an intermediate bonding layer, for example, a liquid resin which contains a hydrochloric acid catalyst, is first applied to the core particles.

The casing and powder particles have a triboelectric relationship of opposing polarity, the powder particles thereby being electrostatically charged through triboelectric action by mixing with the granular carrier material to adhere movably to the surface of the carrier granules. Hence, the powder particles are attractable by an electrostatically-charged insulating layer; whereas the casing of the carrier which is correspondingly charged to opposite polarity, is adapted to attract the charged powder and to remove them from uncharged areas of the electrostatic latent image surface when cascaded thereacross. Suitable two-component developer mixtures are more fully disclosed in the Patents 2,618,551 and 2,618,552, issued on November 18, 1952, and in Patent 2,638,416, issuedMay 12, 1953.

5 When making use of conventional two-component developers in normal cascade development, it is found that large dark areas are fully developed near their edges but that the development is quite inadequate at a distance of inch or so in from their edges. This phenomenon, which is called the halo eifect, arises from the fact that the electrical field immediately above the plate is intense only in those areas where there is a gradient of electrical potential along the surface of the xerographic plate. However, in large dark areas, where there is little or no potential gradient along the surface of the plate, the strength of the electrical field which attracts toner particles is practically zero.

Another drawback encountered during development with commercial developer mixtures is the adhesion of carrier or developer powder to the xerographic plate. This adhesion is customarily referred to as blocking, which blocking action interferes with proper development. When commercial developer is used with conventional xerographic machines, it becomes necessary to overcome blocking by tapping the plate sharply to dislodge any carrier which clings to the plate after development.

Accordingly, it is the primary object of the present invention to provide a novel and improved developer, whereby large dark areas, half tones and continuous-tone images are more completely developed in the cascading development process. It is a further object of the invention to provide a developer which minimizes blocking effects.

More particularly, it is an object of the invention to provide a tripartite developer constituted by a conventional two-component developer in conjunction with an additive which imparts superior performance characteristics to the resultant mixture. Generally stated, existing xerographic developers are rendered more effective when modified by the incorporation of certain electrically conductive additives. Iron, cobaltic oxide, stannous oxide, zinc and ferromauganese in powdered form are among the most effective additives discovered.

A preferred form of toner for use in conjunction with an additive in accordance with the invention is that known commercially as the A-l toner. A toner of this type consists of particles of pigmenting material encased in or surrounded by an insulation material which acquires by contact with the granular carrier material an electrostatic charge having a polarity opposite to that acquired by the granular material and opposite to that of the photoconductive insulating layer of the plate. This toner is in the particle size range of from 0.1 to 20 microns and the granular carrier material is in the size range of from 30 to 200 mesh.

The coloring material may be carbon or other suitable pigments and the insulating material may be a rosinmodified phenol-formaldehyde resin, such as known commercially as Amberol F-71, manufactured by Rohm & Haas Company, The Resinous Products Division, Washington Square, Philadelphia 5, Pennsylvania, or asphaltum, or other suitable material.

The pigmented electroscopic powder is prepared by first micronizing the resin material, such as Amberol P -71, after which it is mixed with approximately 5% by weight of carbon black or other pigmenting material and the mixture ball-milled for about four hours in a ceramic jar with stone pellets. The mixture is then heated to a temperature of about 300 F. or to flowing viscosity and mixed for five minutes in order to encase the pigmenting particles with the Amberol F71. The mass is then permitted to cool, after which it is broken into small chunks and again micronized.

The pigmented electroscopic powder is then in condition for mixing with a granular carrier such as polymerized methyl methacrylate, having a melting point of approximately 257 F., known commercially as Lucite and manufactured by E. I. du Pont de Nemours & Company, Wilmington, Delaware, of other material either conducting or insulating, provided the particles of granular material when brought in close contact with the electroscopic powder particles acquire a charge having an opposite polarity to that of the electroscopic,

powder particles, such that the electroscopic powder particles adhere to and surround the granular carrier particles. The granular carrier material is selected so that the particles acquire a charge having the same polarity as that of the photoconductive insulating layer of the plate on which the electrostatic image is produced, and an electrical attractionfor the electroscopic powder particles considerably less than that of the charged areas of the plate and somewhat greater than the discharged areas of the plate.

It has been discovered that the presence of a small quantity of one of several electrically-conductive powders to a standard toner, such as A-l toner, modifies the characteristics of the developer combinationwhereby large dark areas, half tone and continuous tone images are much more completely developed in cascade operation than in the absence of the additive. Moreover, it has been found that the blocking action which accompanics the use of existing developers is minimized by the incorporation of an additive therein; hence, the need to tap the plate sharply to remove particles adhering thereto is obviated or substantially reduced.

The following additives in the powdered state have been found to afford substantial benefits when mixed with a standard two-component developer in the ratios hereinafter disclosed:

1. Iron 2. Cobaltic oxide Stannous oxide Zinc . Ferromanganese Copper Cupric carbonate, basic (Bakers Analyzed Chemical Reagent, lot 81045) 8. Zinc carbonate 9. Manganese carbonate Cupric oxide Lead acetate Zirconium Nickel carbonate The above-listed materials are all effective additives and give nearly the same results. Any selection from these materials must therefore be based on other considerations, such as availability, cost and toxicity.

It has been determined that the optimum relationship of the three components constituting the developer mixture is as follows: 75 grams of carrier material, 0.6 gram of toner, and 1.3 grams of powdered additive. The volumetric equivalent of this relationship is five parts of toner to one part of additive powder. The additive particles are all in powder form and are preferably of a size that will pass through a 270 mesh screen. The selected additive is mixed thoroughly with the standard developer to form the tripartite developer in accordance with the invention.

A lesser ratio of additive to toner may also be used with beneficial results; however, when this ratio is reduced to one part additive to ten parts of toner, the results are appreciably poorer with respect to dark area coverage than with the above-defined optimum relationship. On the other hand, the use of more additive than established by optimum relationship will also alford good dark area coverage, but when the ratio is raised to a point at which there is one part additive to three parts toner, dark streaks are produced on the background areas of the image. too much additive causes excessive deposition of toner in background areas, while too little additive lessens the desired effect.

All of the above-listed additives possess the following common properties:

First, they are good electrical conductors, as compared to materials which are ineffective as additives. For example, the resistivity (ohms-centimeterx of five of the above-listed additives in powdered form is as follows: Iron (2.20), stannous oxide (0.14), zinc (12.7), ferromang'anese (142), copper (12.7). On the other hand, many of the materials which are inefiec- It will be evident therefore that tive as an additive have a relatively high resistivity. For example, magnesium selenite which has a resistivity of l2,lO0.O 10 is ineffective as an additive. Similarly, polyethylene in powder form has a resistivity of 28,40O.O 10

Second, the triboelectric relationship of the abovelisted additives with respect to the carrier of the developer as measured by an electrometer or similar means is strongly negative.

Third, the nature of the above-listed additives with respect to the toner material is such as to resist or inhibit coating of the additive particles by toner particles. As presently understood, the theory which accounts for the effectiveness of the selected additives in improving the development of dark areas is based on the assumption that the additive, which is electrically conductive and has a strongly negative triboelectric relationship with respect to the carrier, functions by discharging small randomly distributed portions oflarge dark areas, or by preventing carrier particles from rials (not listed supra) having relatively good conductivity fail to operate efiectively as an additive for the purpose intended. For example, barium chloride, a White crystalline powder of good conductivity, whenmixed with A-l toner and thereafter examined under a microscope will exhibit a coated appearance, the white particles of chloride being coated with fine particles of toner. Silver chloride will exhibit the same coated appearance when similarly prepared.

It appears therefore that some materials, while good conductors, are subject to a coating action by the toner particles, which action results in the formation of an insulating sheath effectively rendering the additive nonconductive: on the other hand, the selected additives, listed supra, resist or inhibit a coating action and remain conductive, whereby they function to change the distribution of electrical charge on the surface of the xerographic plate.

In summary, the additives in accordance with the invention are materials in powder form (a) possessing relatively good electrical conductivity, (b) having a tribm electric relationship to the carrier component of the developer which is strongly negative, and (c) exhibiting substantial freedom from a coating action by the.

toner component of the developer, whereby the conductivity characteristic is maintained in the tripartite mixture.

While there have been disclosed specific additives for improving dark area coverage in the development of electrostatic images, it is to be understood that other materials which possess the required combination of properties fall within the scope of the invention, and it is intended therefore in the appended claims to cover all such materials.

What is claimed is:

1. A tripartite developer for electrostatic latent images comprising a mixture of three components, said mixture comprising a first component constituted by loose, movable particles of electrostatically-attractable toner powder, a second component constituted by granular carrier material composed of loose particles, each particle of said second component including a core and a coating bonded thereto and each particle of said second component having a triboelectr-ic relationship of opposite polarity to the toner powder, and a third component constituted by a powder composed of particles having a resistivity in powder form of no greater than.

range between said first and said second components, and said third component being added to a proper xerographic developer mixture of said first and said second component in the ratio range of from 1 part of said third component to 3 to parts of said first component.

2. A developer in accordance with claim 1 in which said third component is constituted by iron powder.

3. A tripartite developer for electrostatic latent images comprising a mixture of three components said mixture comprising a first component constituted by loose, movable particles of electrostatically-attractable toner powder, a second component constituted by granular carrier material composed of loose particles, each particle of said second component including a core and a coating bonded thereto and each particle of said second component having a triboelectric relationship of opposite polarity to the powder of said first component, and a third component constituted by a powder composed of particles having a resistivity in powder form of no greater than lead acetate, a strongly negative triboelectric relationship to said carrier particles and exhibiting freedom from coating action relative to said toner particles, said first component compriisng particles in the size range of from 0.1 to 20 microns, said second component comprising particles in the size range of from 30 to 200 mesh and said third component being of an intermediate size range between said first and said second components, and said third component being added to a proper xerographic developer mixture of said first and said second component in the ratio range of from 1 part of said third component to 3 to 10 parts of said first component.

4. A developer in accordance with claim 3 in which said third component is constituted by cobaltic oxide.

5. A developer in accordance with claim 3 in which said third component is constituted by stannous oxide.

6. A developer in accordance with claim 3 in which said third component is constituted by zinc.

7. A developer in accordance with claim 3 in which said third component is constituted by ferromanganese.

8. A developer for electrostatic latent images comprising a three component mixture, the first component comprising xerographic toner powder, the second component comprising xerographic carrier particles and the third component comprising an additive selected from the group consisting of iron, cobaltic oxide, stannous oxide, zinc, fer-romanganese, copper, basic cupric carbonate, zinc carbonate, manganese carbonate, cupric oxide, lead acetate, zirconium, and nickel carbonate, said first component comprising particles in the size range of from 0.1 to 20 microns, said second component comprising particles in the size range of from 30 to 200 mesh and said third component being of an intermediate size range between said first and said second components, and said third being added to a proper xerographic developer mixture of said first and said second component in the ratio range of from 1 part of said third component to 3 to 10 parts of said first component whereby the dark area coverage of the developer is enhanced.

9. A developer in accordance With claim 8 in which the volumetric relationship of said third component to said first component is about 5 parts of said first component to 1 part of said third component.

10. A developer in accordance with claim 9 in which the third component is of a size that will pass through a 270 mesh screen.

References Cited in the file of this patent UNITED STATES PATENTS 2,297,691 Carlson Oct. 6, 1942 2,520,651 Oswald Aug. 29, 1950 2,618,551 Walkup Nov. 18, 1952 2,618,552 Wise Nov. 18, 1952 2,638,416 Walkup May 12, 1953 2,659,670 Copley Nov. 17, 1953

Claims (1)

1. A TRIPARTITE DEVELOPER FOR ELECTROSTATIC LATENT IMAGES COMPRISING A FIRST COMPONENT CONSTITUTED BY MIXTURE COMPRISING A FIRST COMPONENT CONSTITUTED BY LOOSE, MOVABLE PARTICLES OF ELECTROSTATICALLY-ATTRACTABLE TONER POWDER, A SECOND COMPONENT CONSTITUTED BY GRANULAR CARRIER MATERIAL COMPOSED OF LOOSE PARTICLES, EACH PARTICLE OF SAID SECOND COMPONENT INCLUDING A CORE AND A COATING BONDED THERETO AND EACH PARTICLE OF SAID SECOND COMPONENT HAVING A TRIBOELECTRIC RELATIONSHIP OF OPPOSITE POLARITY TO THE TONER POWDER, AND A THIRD COMPONENT CONSTITUTED BY A POWDER COMPOSED OF PARTICLES HAVING A RESISTIVITY IN POWDEER CFORM OF NO GREATER THAN LEAD ACETATE AND HAVING A TRIBOELECTRIC RELATIONSHIP WHICH IS STRONGLY NEGATIVE RELATIVE TO SAID CARRIER MATERIAL, SAID FIRST COPONENT COMPRISING PARTICLES IN THE SIZE RANGE OF FROM 0.1 TO 20 MICRONS, SAID SECOND COMPONENT COMPRISING PARTICLES IN THE SIZE RANGE OF FROM 30 TO 200 MESH AND SAID THIRD COMPONENT BEING OF AN INTERMEDIATE SIZE RANGE BETWEEN SAID FIRST AND SAID SECOND COMPONENTS, AND SAID THIRD COMPONENT BEING ADDED TO A PROPER XEROGRAPHIC DEVELOPOER MIXTURE OF SAID FIRST AND SAID SECOND COMPONENT IN THE RATIO RANGE OF FROM 1 PART OF SAID THIRD COMPONENT TO 3 TO 10 PARTS OF SAID FIRST COMPONENT.