Liquid Developer Containing Extender Body Particles

Abstract

Developed images with substantially no fogging in the background areas or streaking in the image areas are obtained in a liquid development system employing a liquid developer comprising an insulating liquid vehicle having dispersed therein charged toner particles and charged extender body pigment particles selected from the group consisting of calcium carbonate, aluminum hydroxide, barium sulfate, aluminum oxide, talc, silica, calcium silicate, magnesium carbonate, magnesium oxide and mixtures thereof.

Parent Case Text

This is a continuation of application Ser. No. 94,967, filed Dec. 3, 1970, now abandoned.

Description

BACKGROUND OF THE INVENTION

This invention relates to imaging systems and more particularly to liquid development systems for developing electrostatic latent images.

The formation and development of images on the surface of photoconductor material by electrostatic means is well known. The basic electrostatographic process as taught by C. F. Carlson in U.S. Pat. No. 2,297,691 involves placing a uniform electrostatic charge on a photoconductive insulating layer exposing the 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 marking 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. The powder image may then be transferred to a support surface such as paper and permanently affixed to the support by any suitable means such as heat fixing or solvent fixing. Alternatively, the powder image may be fixed to the photoconductive layer if elimination of the powder transfer step is desired. In addition, instead of latent image formation by uniform charging and followed by imagewise exposure, the latent image may be formed by directly charging the layer in image configuration. Other methods are known for applying electroscopic particles to the imaging surface. Included within this group are the "cascade" development technique disclosed by E. N. Wise in U.S. Pat. No. 2,618,552; the powder cloud development technique disclosed by C. F. Carlson in U.S. Pat. No. 2,221,776; and the magnetic brush process disclosed for example, in U.S. Pat. No. 2,874,063.

Development of an electrostatic latent image may also be achieved with liquid rather than dry developer materials. In conventional liquid development, more commonly referred to as electrophoretic development, an insulating liquid vehicle having finely divided solid material dispersed therein contacts the imaging surface in both charged and uncharged areas. Under the influence of the electric field associated with a charged image pattern the suspended particles migrate toward the charged portions of the imaging surface separating out of the insulating liquid. This electrophoretic migration of charged particles results in the deposition of the charged particles on the imaging surface in image configuration. Electrophoretic development of an electrostatic latent image may, for example, be obtained by pouring the developer over the image bearing surface, by immersing the imaging surface in a pool of the developer or by presenting the liquid developer on a smooth surface roller and moving the roller against the imaging surface. The liquid development technique has been shown to provide developed images of excellent quality and to provide particular advantages over other development methods in offering ease in handling. Liquid development systems also capable of providing high development speed, the development speed of commercial embodiments having recently reached a level of as high as about 10 centimeters per second. However, with the currently available liquid development systems, this development speed is practical only for line copy since the development of continuous tone or halftone images generally requires a much slower speed.

The liquid developers generally employed in these electrophotographic imaging processes comprises fine electrically charged particles suspended in an electrically insulating liquid. In addition to the charged particles which are generally referred to as toner, it is common to disperse or dissolve a charge controlling agent to regulate the electric charge on toner particles and a dispersing agent to obtain a stable dispersion.

A typical liquid developer employed in the production of black developed images comprises carbon black particles suspended in a liquid such as the highly insulating nonpolar organic solvents including mineral oil, benzene, heptane, cyclohexane and decylene. Typically, charge controlling agents including various resins, varnishes, nondrying oils and wetting agents may be added to provide the necessary control of charge on the toner particles. While capable of forming satisfactory images, these liquid developers exhibit various shortcomings when employed in automatic machine configurations.

Specifically, difficulties are frequently encountered due to the phenomenon of fogging. Fogging occurs when, for example, a photoconductive insulating layer is charged and exposed to a light and shadow pattern. In the background or nonimage areas, during exposure, light renders the photoconductive layer conductive and dissipates the charge. However, in these background or nonimage areas when subjecting the photoconductive insulating layer to even the strongest irradiation, a small electric charge persists in the nonimage or background areas. This small charge attracts a small amount of toner giving rise to fogging in the background areas of the developed print.

In addition, prior liquid developers may result in the formation of streaks on the image. Thus, when bringing an electrostatographic imaging surface bearing an electrostatic latent image into contact with liquid developer, a relative speed component is present between imaging surface and the liquid developer. Streaks are formed on the imaging surface in the high density portions of the image along the direction of movement of developer relative to the imaging surface.

Furthermore, prior development systems employing a roller developer dispensing device, have resulted in the formation of apparatus stains due to the friction between the rollers and the surface holding the electrostatic latent image. For example, in processes in which the liquid developer is supplied between the surface bearing the electrostatic latent image and a metal roller which functions as the developing electrode and is rolled across the imaging surface, or in imaging systems in which an electrophotographic material passes between a pair of pinch rollers while the liquid developer is supplied between the roller and the surface bearing the latent image staining of the apparatus may occur. This occurs since the metal rollers are kept in direct contact with the imaging surface, and any minute projecting portions present on the imaging surface are scraped by the metal roller permitting toner to be collected thereby forming stains.

Furthermore, the prior liquid developers are incapable of producing image tone such as that obtainable in silver halide photographic paper.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a liquid development system which overcomes the above noted deficiencies.

It is another object of this invention to provide a liquid developer which produces high density, nonstreaky developed images.

It is another object of this invention to provide a liquid development system which produces developed images with substantially no fogging.

It is another object of this invention to provide a liquid development system which is capable of producing fine grain smooth appearance developed images.

It is another object of this invention to provide a liquid development system capable of producing developed images having a tone resembling the appearance obtained with silver halide photographic paper.

The above objects and others are accomplished, generally speaking, by providing an electrostatographic imaging system of the liquid development type wherein a liquid developer comprising an insulating liquid vehicle, charged colored particles and extender body pigment particles charged to the same polarity as the charged colored particles is employed. In addition, if necessary, charge controlling agents, dispersion stabilizing agents, fixing agents and other well known materials may be suspended or dissolved in the liquid developer. Alternatively, the charge control agents, the dispersion stabilizing agents and fixing agents may be coated on the individual charged particles.

Any suitable extender body pigment particle may be employed in the practice of the present invention. By the term extender body pigment is intended to define that group of finely divided materials which will provide a color the same as or similar to the background areas of the final print surface such as either a photosensitive paper such as "Electrofax" paper or ordinary paper. In the most practical of operations, therefore, the body pigment particles will be substantially white since most commercial imaging processes produce prints of black image areas on white background areas. It is, however, to be understood that if background areas of other colors are desired appropriately colored pigments of the background areas may also be employed. Typically, when employing body pigment particles in an electrostatographic imaging system providing finished copy with white background and dark color image areas, pigments with refractive indices not greater than 1.75 may be employed. Typical specific materials include calcium carbonate, aluminum hydroxide, barium sulfate, aluminum oxide, talc, silica, calcium silicate, magnesium carbonate and magnesium oxide. It is essential that the body pigment particles do not either discolor or decompose the tone particles.

The body pigment particles employed in the practice of the present invention may be of any suitable size. Typically, the body pigment particles are within the range of from several tens of times to several tenths of the diameter of the color charged or toner particles. To provide uniformity of suspension in the liquid developer and deposition, it is preferred, however, to maintain the size of the body pigment particles within a range of several times larger to several times smaller than the toner particles. Typically, the extender pigment is of a particle size of from about 0.01 to about 5 microns. The toners employed in the practice of this invention may be of any suitable size. Typically, the toner particles do not have an average particle size exceeding about 1 micron or less than about 0.01 micron. Thus, the extender body pigment particles will generally be of about the same size as toner particles. Generally the number of extender body pigment particles may be employed in an amount within the range of from about one tenth to about ten times the amount of the charged toner particles present in the liquid developer. However, to avoid the deposition of undue quantities of extender body pigment particles in the image areas, it is generally preferred that the extender body pigment particles be present in an amount within the range of from about one quarter to about twice the amount of the charged toner particles. Optimum image density in the image areas together with substantially complete reduction of fogging in the nonimage or background areas, it is generally achieved when the number of extender body pigment particles is about the same amount as that of the charged toner particle.

The liquid developer of the present invention may be prepared in any suitable manner. Typically, the liquid developer may be prepared by mixing the several constituents of the developer together. It may also be prepared by mixing a first liquid portion containing the toner particles with an insulating liquid and a second portion containing the body pigment particles. Alternatively, a paste containing toner and body pigment particles may be dispersed within the insulating liquid.

From the above description of the invention, the choice of specific materials and operating conditions is deemed to be well within the scope of those skilled in the art and therefore the scope of the invention is not limited by the hereinabove mentioned illustrative materials. The extender body pigment particles may be employed with any suitable liquid developer. Typical liquid developers contain liquids of relatively high insulating value generally having a volume resistivity greater than about 10.sup.10 ohm-cm so as not to effect the electrostatic charge pattern on the insulating layer and low dielectric constants of less than about 3.4. Typical specific vehicles include hydrocarbons such as benzene, xylene, hexane, naptha, kerosene, halogenated hydrocarbons such as carbon tetrachloride, trichloroethylene and chloroform. Typical charged toner particles which may be employed with the liquid developer include, among others, charcoal, carbon black, magnesium oxide, lithopone, cadmium yellow, chrome yellow, cobalt blue, cadmium red, burnt siena, Hansa yellow, rose bengal and phthalocyanine. Typically, the charged toner particles are present in the liquid developer in an amount of from about 2 to about 20 grams per liter, and are conventionally dispersed and suspended in the liquid by stirring or agitation. Where a highly uniform stable suspension is desired, this suspension may be passed through a colloid mill. The liquid developers according to the present invention may be employed to develop electrostatic charge patterns present on any suitable imaging surface. Basically, any material capable of holding a charge pattern may be employed. Typical materials include dielectric layers, xeroprinting masters and photoconductors. A particularly preferred material for use in automatic copying machines is a photosensitive paper comprising photoconductive pigment particles in an insulating binding layer. Typically, this paper comprises zinc oxide photoconductive particles present in an insulating binding layer which is overcoated on a paper substrate. The choice of particular imaging member and particular development technique may be readily determined by one skilled in the art. For example, the photosensitive paper described above may be substituted with photoconductor materials made from cadmium sulfide, zinc sulfide, zinc selenide, cadmium selenide, titanium dioxide, phthalocyanine and polyvinyl carbazole. As previously discussed, the liquid developers according to the present invention may also contain dispersed in the insulating liquid vehicle charge control agents and suspending agents for their well known functions. The selection of the appropriate materials to perform these well known functions may be readily made by the artisan.

When employing a liquid developer comprising toner particles which are positively charged, the extender body pigment particles must also be positively charged in the insulating liquid. The use of calcium carbonate and aluminum hydroxide as the extender body pigment particles is particularly preferred when positively charged particles are to be employed since they become positively charged merely be being suspended in the insulating liquid. In order to disperse these minute particles to provide a stable suspension in the insulating liquid a resin may also be employed. Typically, from about 0.5 to about 50 parts by weight of the resin per part of the extender pigment may be employed. A portion of this resin appears to be strongly absorbed on the surface of the calcium carbonate or the aluminum hydroxide and the charge on these particles may be further stabilized depending on the specific resin employed. Typically, from about 2% to about 30% by weight of the total resin is absorbed on the surface of the extender pigment. A particularly preferred resin providing maximum stabilization of charge and suspension is a rosin modified phenol formaldehyde resin heated with linseed oil.

While the above explanation has been made principally with regard to toner particles carrying a positive charge, it is to be understood that the present invention also encompasses liquid developers containing toner particles which have been provided with a negative charge. Liquid developers containing negatively charged toner particles may be employed to develop positively charged image patterns. They may also be used as reversal developers. In both instances, the liquid developers may have extender body pigments added which are capable of being negatively charged. It has been observed that the liquid developer will exhibit the behavior of a negative toner when the amount of negatively charged toner particles present in the liquid developer is larger than that of the positively charged body pigment particles. Conversely, the developer will exhibit the behavior of a positive toner when the amount of positively charged extender pigment is greater than the negatively charged toner particles. This occurs since the negatively charged toner particles are encircled by the extender particles so the toner particles behave as positive toners.

It is possible to maintain the negative polarity charge of the liquid developer by the use of a charge controlling agent. Any suitable charge controlling agent may be employed. A typical material for controlling the charge is lecitin. It is also possible to provide the body pigment particles with a negative charge by coating the particles with a thin layer of a resin which is soluble in the carrier liquid. Any suitable resin may be employed for this purpose. Typical materials include polyvinyl chloride and copolymers containing vinyl chloride and nitrocellulose.

The liquid developer of the present invention when employed as an electrophoretic liquid developer is capable of producing images which are essentially, completely free from background fogging. In addition, the developer of this invention provides images of clear tone resembling the appearance of silver halide photographic paper. Furthermore, the liquid developer according to this invention, provides a very fine grain developed toner image. In addition, the absence of stain resulting from friction between the developer applicator roller and the imaging surface wherein a roller developing method is employed is substantially completely eliminated.

While the exact mechanism and reasons for the improved results are not fully understood at the current time, it is presently believed that the development of images which are substantially free from background fogging is due to a preferential deposition of the extender body pigment in the background portions of the imaging surface which thereby prevent deposition and adherence of the toner in these background portions. It is further believed that the clear tone resembling the appearance of silver halide reproductions is due to the fact that the extender body pigment particles and the toner particles are deposited together and thereby increase the transparency of the toner image. The fact that finer grain toner images are capable of being achieved with the liquid developer of the present invention is believed attributable to the fact that aggregates which consist exclusively of a toner material are prevented since there is a simultaneous deposition of toner and extender pigment in the image areas which tends to suppress the formation of large aggregates which consist solely of colored toner material. The absence of staining is further believed to be attributed to the fact that the body pigment particles act as a lubricant or to the preferential deposition of the body pigment particles on the projecting portions of the imaging surface. The ability to produce images which are substantially free from streaks in a liquid development system of increased development speed has not been satisfactorily explained. It has been established, however, that with comparison of liquid developer according to the present invention and developers containing resin particles instead of extender body particles that increased development speed and the above mentioned improved image qualities are achieved.

DESCRIPTION OF PREFERRED EMBODIMENTS

The following, nonlimiting examples further define, describe and compare preferred materials, methods and techniques of the present invention. Example II is presented for comparative purposes. Unless otherwise specified, all amounts proportions and percentages are by weight.

EXAMPLE I

A photosensitive paper comprising an electroconductive paper substrate overcoated with a photoconductive layer of powdered zinc oxide in an insulating resinous binder is charged and exposed to a light and shadow pattern in conventional manner. The electrostatic latent image formed on the zinc oxide sheet is developed by passing the sheet through a bath of a liquid developer made according to the following procedure:

Paste A is obtained by ball milling the following composition for 8 hours.

______________________________________ Carbon black 4000g (particle size 0.05 - added ) Varnish obtained by heating a mixture of 5000g equal amounts of rosin-modified phenol- formaldehyde resin and linseed oil Linseed oil 1000g ______________________________________

Paste B is obtained by kneading the following composition on a three roll kneader.

______________________________________ Aluminum hydroxide 2000g (particle size 0.07 - 0.2.mu.) Varnish obtained by heating a mixture of 4000g rosin-modified phenol-formaldehyde resin and linseed oil Linseed oil 1000g ______________________________________

Both paste A and paste B contain particles having a charge of positive polarity. The liquid developer is obtained by dispersing 20 grams of paste A and 15 grams of paste B into 10 liters of "Decalin," decahydronapthalene available from E.I. duPont de Nemours and Company. The developed image on the zinc oxide sheet is substantially free of fogging in the background areas, is substantially free of streaks in the image areas and has a relatively fine grain image area. In addition, the tone achieved is similar to that obtained with silver halide photographic paper.

EXAMPLE II

The procedure of Example I is repeated except that only 20 grams of paste A is dispersed in 10 liters of "Decalin". The developed image on zinc oxide paper has a high fogging level, streaky images and a relatively coarse grain image. In addition, the tone is inferior to that obtained in the silver halide photographic paper.

EXAMPLE III

The procedure of Example I is repeated except that the liquid developer is made by dispersing 20 grams of paste A in 5 liters of "Decalin" and 15 grams of paste B in 5 liters of "Decalin". When mixed together in equal amounts a liquid developer is formed which when employed to develop an electrostatic latent image formed in the manner described in Example I produces prints of quality similar to that obtained in Example I.

EXAMPLE IV

The procedure of Example I is repeated except that development of the electrostatic latent image is accomplished with a liquid developer made by blending the following composition in a ball mill for one hour.

______________________________________ Phthalocyanine blue 400g Polymerized linseed oil 500g Linseed oil-modified alkyd resin 500g Aluminum hydroxide 200g Kerosene 300ml ______________________________________

Forty grams of this paste is then dispersed in 10 liters of kerosene while under the influence of ultrasonic wave. The toner has a stable positive charge in the carrier liquid. When used as a liquid developer, in the manner described in Example I, an image with substantially no fogging or streaking is obtained. In addition, the image is more transparent than that obtained by means of ordinary electrophotographic processing.

EXAMPLE V

The procedure of Example I is repeated with a liquid developer prepared by blending the following composition in a ball mill for one hour in the following manner.

______________________________________ Brilliant carmine 6B 250g Polymerized linseed oil 250g Linseed oil-modified alkyd resin 300g Precipitated barium sulfate 200g Kerosene 300ml ______________________________________

Thirty grams of this paste are dispersed while under the influence of ultrasonic wave into a mixture of 8 liters of cyclohexane and 2 liters of kerosene to provide a liquid developer capable of providing a red image. The precipitated barium sulfate is provided with positive charge in the carrier liquid. Image quality similar to that described in Example IV is achieved.

EXAMPLE VI

The procedure of Example I is repeated with the liquid developer prepared by blending the following composition in a ball mill for one hour.

______________________________________ Benzidine yellow 40g Varnish (same as in Example I) 80g Linseed oil 60g Aluminum hydroxide 15g Calcium carbonate 15g ______________________________________

Twenty grams of this paste are dispersed while under the influence of ultrasonic wave into a mixture of 8 liters of cyclohexane and 2 liters of kerosene. The particles of benzidine yellow when employed alone exhibit an unstable positive charge in the carrier liquid. Calcium carbonate shows strong positive charge in the carrier liquid and stabilized the positive charge on the benzidine yellow. Image quality similar to that achieved in Example IV is observed when the zinc oxide sheet is immersed in the liquid developer.

EXAMPLE VII

A photoconductive insulating layer comprising a paper substrate with a coating thereon of zinc oxide photoconductive particles in an insulating resinous binder is charged and exposed to a negative image. The electrostatic latent image thus formed is developed with a liquid developer prepared by blending the following composition in a ball mill.

______________________________________ Calcium carbonate 250g Vinyl chloride-vinyl acetate copolymer 125g Varnish obtained by heating rosin-modified phenol-formaldehyde resin and linseed oil 400g Ethyl acetate 3000ml ______________________________________

Thirty milliliters of the paste is dispersed while under the influence of ultrasonic wave in a carrier liquid of the following composition.

______________________________________ Cyclohexane 1500ml Kerosene 300ml Cottonseed oil 200ml ______________________________________

This developer contains negatively charged particles and is substantially transparent in appearance.

A developer containing negatively charged green toner of the following composition is prepared in the same manner.

______________________________________ Phthalocyanine green 5g Polymerized linseed oil 25g Cyclohexane 1500ml Kerosene 475ml ______________________________________

One part by volume of the green developer and one part by volume of the liquid developer containing the body pigment are mixed to obtain liquid developer, in which the toner has a stable negative charge. The negative electrostatic latent image formed on zinc oxide photosensitive paper is developed with this liquid developer to obtain a satisfactory reversal image. Streaking and fogging are barely observed on the developed image.

Although particular embodiments have been set forth using the development system and technique of this invention, these are merely intended as illustrations of the present invention. There are other systems and techniques which may be substituted for those described. Other modifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure which modifications are intended to be included within the scope of this invention.

Claims

We claim:

1. An electrostatographic liquid developer comprising an insulating liquid vehicle having a volume resistivity greater than about 10.sup.10 ohm-cm and dispersed therein charged colored toner particles and white extender body pigment particles selected from the group consisting of calcium carbonate, aluminum hydroxide, and mixtures thereof; said extender body pigment particles being present in from about one tenth to about ten times the amount of the charged colored toner particles and having a particle size of from about 0.01 to about 5 microns, said toner particles having a size of from about 0.01 to about 1 micron; said extender body pigment particles having a positive charge, both said charged toner particles and said charged body pigment particles being capable of being simultaneously deposited from said liquid developer in response to a charge of opposite polarity on the latent image areas of an imaging surface and said charged body pigment particles being capable of preferential deposition in the background portions of said imaging surface.

2. The liquid developer of claim 1 wherein said extender body pigment particles are present in from about one quarter to twice the amount of the charged toner particles.

3. The liquid developer according to claim 1 wherein said body pigment particles are calcium carbonate.

4. The liquid developer according to claim 1 wherein said body pigment particles are aluminum hydroxide.

5. The liquid developer of claim 1 wherein said extender body pigment particles have a refractive index less than about 1.75.

6. The liquid developer of claim 1 wherein said extender body pigment particles have a charge and suspension stabilizing resin absorbed on the surface.