US3135695A - Liquid developers for electrostatic photography - Google Patents

Description

June 2, 1964 w. c. YORK 3,135,695

LIQUID DEVELOPERS FOR ELEC'IROSTA'JSIC PHOTOGRAPHY Filed Feb. 20, 1961 Pigment Particle Encupsulufing aromatic soluble resin envelope Diffuse Negative Layer in Currier Liquid WILL/AM 6. YORK 1N VEN TOR.

A TTOR/VEYS United States Patent 3,135,695 LIQUE DEVELQPERS FQR ELECTRGSTATIC PHOTOGRAPHY William C. Yorlr, Rochester, Nfl Z, assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey Filed Feb. 29, 1961, Ser. No. 9%,494 3 Claims. (Cl. 252-62.l)

This invention relates to Xerography and more par ticularly to a novel composition of matter for developing a visible image from a latent electrostatic image existing as a charged pattern on xerographic plates.

Processes for producing a visible image from a latent electrostatic image by developing with an electrostatically attractable dry powder having optical density are well known and have been described in nurnerous US. patents including the Carlson Patents 2,297,691, issued October 6, 1942, and 2,357,809, issued September 12, 1944. To develop a latent electrostatic image, the dry granular electroscopic powders have in the past ordinarily been cascaded over latent electroscopic images or brought in the form of powder clouds into intimate contact with the charge image to be rendered visible. Dry developers of tlu's type have been described in such US. patents as Wise 2,618,- 552, issued November 18, 1952, and Landigran 2,753,308, issued July 3, 1956. More recently it has been proposed to develop latent electrostatic images using liquid media rather than the above-described dry processes. Straughan in [1.8. 2,899,335, issued August 11, 1959, for example, proposed the development of an electrostatic image using a liquid developer in which the charged solid developing particles are carried into contact with the electrostatic image in a liquid medium instead of in air or a gaseous medium as in the aforesaid Carlson patents, for example. Another type of charged pigment-suspending liquid developer of electrostatic images has been described by Metcalfe and Wright in Australian Patent 212,315, patented December 6, 1956. Liquid xerographic developers for the most part, however, have encountered a difiiculty in stability in that they tend to clump on standing and in many cases the charged pigment particle tends to lose charge, exhibits a charge reversal, or a change in pigment-tobinder ratio, etc. The present invention overcomes, very unexpectedly, each of these persistent dir culties by a relatively simple process such that charged pigment-suspending liquid developers are stable beyond 800 hours.

It is, therefore, a primary objective of the present in vention to provide liquid electrostatic developers which have improved stability. Another object of the present invention will be to provide a process for preparing the stable liquid electrostatic developers of the present invention. A further object is to provide a process for developing an electrostatic image to a visible image using an electrostatic developer of the present invention. ther objects will become apparent from a reading of the specification and appended claims.

The objects of the present invention are accomplished by surrounding individual xerographic type pigment particles with a suitable resinous-binder envelope, treating the pigment-binder combination with a small amount of an alkyl-aryl compound before suspending the combination in a liquid aliphatic carrier. The resin used to enclose the pigment particles of the present invention can be any of the well known resins which are soluble in aromatic solvents, but insoluble, or at least only slightly soluble, in aliphatic solvents. The liquid aliphatic carrier according to the present invention must have a low dielectric constant and a resistivity of at least about ohm centimeters to preclude the possibility of discharging the latent electrostatic image. The low dielectric constant is needed so that the energy in the image field is not dissipated in aligning and moving highly polar molecules. If the pigment-binder combination, i.e., toner, of the present invention is dispersed in the liquid aliphatic carrier without first treating it with an alkyl-aryl compound, the system coagulates badly. Where the pigment-binder combination is prepared, treated and suspended in an aliphatic carrier according to the present-invention, liquid electrostatic developers have been prepared which are stable beyond 800 hours without apparent deleterious efiect. While I do not wish to be bound by any specific theory of operation, I believe that the alkyl-aryl compound used as a treating means for the pigment-binder combination becomes oriented so that the aryl group is directed inward towards the aromatic soluble binder and the aliphatic tail is directed outward forming a liquid bridge between the aromatic soluble binder and the aliphatic carrier. In any event the liquid electrostatic developers of the present invention remain stable over very long periods with the binder remaining as a discrete envelope surrounding the pi ment particle and without the binder diffusing into the carrier liquid. Where such stability of the binder in liquid developers of this type is not obtained there results a number of deleterious effects including an undesirable shift in the pigment to binder ratio with progressive use and age of the developer.

11 the single figure of the accompany ng drawing is shown a hypothetical orientation of an allryl-aryl compound, propyl benzene, to the aromatic soluble resin envelope surrounding a pigment particle having a positive charge. The toner, i.e., pigment particle-resinous envelope combination is, of course, according to the invention suspended in an aliphatic liquid carrier which is represented in the present drawing as a hypothetical diffuse negative layer surrounding the positively charged toner particle.

In carrying out the present invention an electrostatic latent image can be produced by any of a number of procedures recognized in the art. For example, a sensitive element may be produced by coating a support with a suitable photoconductor, such as those described in the Carlson patents cited above, and imparting a uniform charge to said sensitive element by any of the known means such as corona discharge and by rubbing as also described in the Carlson patents. Finally, the charged sensitive element is exposed to a pattern of light and shadow illumination whereby the illuminated areas are discharged and a charged image pattern remains which is developed to a visible image by flowing a developer of the present invention over the differentially charged surface. The electrostatically attractable particles carried in the insulating liquid of the developer solution are brought into contact with the electrostatic image which results in the particles being deposited on the charged areas of the sensitive element so that the charged areas are immediately rendered visible because of the optical character istics of the toner particles. Because of the highly volatile nature of the aliphatic carriers of the developer composition, fast drying results and a visible, permanent image is rapidly obtained. In preparing the liquid electrostatic developers of the present invention it is the usual procedure to mix and grind together in a ball mill or other suitable comminuting equipment a suitable pigment having optical density with a solution of a resinous material which is soluble in aromatic solvents but insoluble or only slightly soluble in aliphatic solvents. The preferred resinous binders should also be non-chalking, should'dry to a tough wear-resistant film and should have a refractive index conducive to enhance color or transparency. On the other hand suitable resinous materims should have no adverse effect on particle charge or on developer stability. Resinous materials which I have 'either negative or positive.

found suitable for use in the present stabilized liquid developers can be selected from a number of synthetic resins, modified resins, rosin esters, modified rosin esters, etc. Examples of suitable resins to which I refer are given in Table 1 below. i

TABLE I Trade name Resin type Manufacturer Epon 100 Polyethylene Shell Oil Company. Amberol 226 Phenol-formaldehyde Rohm and Haas C0.

rosin-modified. Amberol B/S-l Do. Amberol F-7 Do. Amberol F-77 Do.

modified. Amberol K12A Do. Amberol M88- Do. Amberol M93-- Do. Amberol 800 Do. Do. Do. Do. Amberlac D-96 --do Do. Cellolyn 102 Modified pentaery- Hercules Powder Co.

thritol ester of rosin. Cellolyn 104 --do D0. Lewisol 2L Maleie alkyd-modi- Do.

fied rosin ester. Lewisol 28 clo D0. Neolyn 23 Rosin derived alkyd- Do.

type resin. Neolyn 35 -do Do. Pentalyn B25 Phenol modified Do.

pentaerythrit-ol ester of rosin. Pentalyn 802A do Do. Arochem 75 Modified phenolic--.. Arzher Daniels Midland o. Arocbem 333 Do. Aroehem 335..- Do. Aroehem 337. Do. Arocbem 339- Do. Arochem 355. Do. Arochem 402 Do. Arochem 40 do Do. Arochem 520. Modified maleie Do. Aroeliem 533- Modified penta ester Do. Arochem 542- M dified maleic.- Do. Aroehem 545. Do. Aroehem 095. Do. Aroohem 650 cquer resin--- Do. Aroiene 753 Pure phenolic resin-- Do. XLH 403.--- Modified pentaery- Sherwin-Williams Co.

thritol. XL 4 -do Do. Dymal XLM 110 aleie alkyd D0. Dyrnal XLM 101.. d Do. Dymal XLM 103.- Do. Dymal XLM 106-..- Do.

Other suitable resinous materials and mixtures of such materials have been described in such materials have been described in such US. patents as Insalaco 2,891,011, June 16, 1959, and Carlson 2,940,934, June 14, 1960.

The pigments which can be used in preparing the toners of the present invention can vary Widely. For example, in Carlson Patent 2,297,691, issued October 6, 1942, there are described numerous organic and inorganic materials, including talcum powder, aluminum bronze, carbon dust, gum copal, gum sandarac, ordinary rosin, and others. The principal requirements of any of the pigments used in the present invention is that they are particulate and capable of carrying an electrostatic charge. When finely divided pigments are suspended or dispersed in a suitable liquid vehicle, as is shown, the solid particles usually acquire a charge. With some pigments, however, it has been observed that a prepoderanw of the particles acquire a charge of one sign, while with other pigments, the number of positively-charged particles about equals the number of negatively-charged particles. For optimum efficiency, it is desirable to employ a pigment in which a preponderance of the particles acquire a charge of one sign, In my investigation I have studied a number of pigments Which I have found acquire substantially a sign of one charge. Presented herein accordingly are a number of pigments which have a negative particle charge in solution and a number of pigments which acquire a positive particle charge in solution. Several of these pigments of each type are given in the table below. The carrier solution for determining the charge was cyclohexane.

Factor, Max & C0. General Dyestufi Co. Factor, Max dz Co. Sherwin-Williams Co.

Pyramide Cerise Toner RA 520 Fast Mono Green Toner G-FG 430.- Pigment Yellow LX CyB-340 Solfast Green 63100 POSITIVE PARTICLE PIGMENTS Aluminum Powder (flake) Antimony Sulfide Cuprie Sulfide--- Carbonyl Iron Lamp Black, Germantown Mapico Black Nigrosine S.S.B. No. 5.

General Aniline and Film Corp. Colombian Carbon 00.

Do. General Aniline and Film Corp.

Grassol Black Geigy 00., Inc. Monastral Fast Blue B E.CI. du Pont de Nemours and o. Dresden Blue 81451 Imperial Paper and Color Corp. Pyramid Germanium Toner RA 500 E Factor, Max 6: Co.

Selkirk Red 8.- Imperial Paper and Color Corp.

Winthrop Red X4666. Britone Red M CP974. Britone Red MCP1290 Duratint Green 48-238. C.P. Light yellow X-179 W81 Benzidine yellow Anilide Do. Sherwin-Williams C0.

D0. Factor, Max dz Co. Imperial Paper and Color Corp. Sherwin-Williams Co.

With either the negative or positive pigment particles an alkyl-aryl stabilized toner can be prepared by milling together a suitable pigment with a suitable solution of resin or mixture of such resins as set forth hereinabove. Suitable carriers for dispersing the stabilized toners comprise liquids possessing a'high dielectric resistance. In order to avoid discharge of a latent electrostatic image, a liquid with a resistivity of about at least 10 ohm centimeters is necessary. Liquids which I have found suitable include cyclohexane, carbon tetrachloride, mixed halides such as trichlorotrifluoroethane, sold under the trade name Freon 113 by E. I. du Pont de Nemours and Co., cyclopentane, n-pentane, n-hexane, etc. In preparing the stable developer compositions of the present invention, heavier oils can be used, such as kerosene, mineral oil, silicone oil, etc. When these latter mentioned liquids are used, however, they usually require heat to set the toner and drive off excess carrier.

According to the present invention the alkyl-aryl treating compound employed before dispersion of the pigmentbinder combination in a suitable carrier can be selected from the compounds represented by the following general formula 'wherein R represents at least one straight chain or branched-chain alkyl group having from 1 to about 12 carbon atoms and Z represents the non-metallic atoms necessary to complete an aromatic nucleus selected from the class comprising benzene, naphthalene, and anthracene. Typical examples of solvents of the present type comprise xylene, mesitylene, ethylbenzene, n-propylbenzene, n-butylbenzene, isobutylbenzene, n-pentylbenzene, n-hexylbenzene, n-propyl naphthalene, butyl naphthalene, propyl anthracene, butyl anthracene, and the like. Useful results were obtained using isomers as well as the normal aliphatic hydrocarbons and mixtures of each thereof. The preferred alkyl-aryl compounds are those in which the aryl substituent is benzene. Especially useful results were obtained where the aryl substituent is benzene and the alkyl group is selected from those having from 3 to 6 carbon atoms. Preparation of any of the alkyl-aryl compounds of this invention can be by any of the well known methods.

In preparing the stabilized liquid developer solutions of the present invention the toner is first prepared by mixing a uniform dispersion of the pigment in an aromatic solution of the resinous material and thereafter mixing and milling the combination to produce a finely divided toner paste of desirable particle size. The mixing and milling may be done by various means including a combination of the steps of blending, mixing, and milling wherein the preferred method for the milling step includes a rather extensive milling period to assure uniform and fine dispersion or" the pigment in the resin. After milling, the toner is removed from the mill jar. The toner is then treated with a small amount of an alkylaryl compound such as propylbenzene. The treatment required is usually dispersing the toner in the propylbenzene and thoroughly mixing. After treatment, the dispersion is added to the aliphatic carrier of choice. Liquid electrostatic developers prepared in this manner have been checked periodically over a period beyond 800 hours without the developer showing signs of deleterious effects.

The following examples will serve to further illustrate practice of the present invention.

Example 1 Prior to addition in the above composition the Pentalyn M was digested in a 1:1 ratio in a hydrocarbon mixture sold commercially under the trade name Solvesso 100 by the Esso Standard Oil Co.

After ball milling 0.5 gram of the toner was mixed with a composition comprising 0.5 ml. of n-propylbenzene and dispersed further in 190 ml. of cyclohexane. The resulting composition was flowed over a surface bearing a latent electrostatic charge to quickly produce a clear visible image of good resolution. The image produced dried almost instantly to a high-smudge resistance. The developer of this example was tested periodically beyond 800 hours with no sign of deterioration, agglomeration, change in binder-pigment ratio, etc.

xnmple 2 An alkyl-aryl stabilized liquid developer was used to develop the latent electrostatic image on xerographic printing material after thermographic exposure according to the following procedure.

The xerographic printing material was prepared according to the disclosure of Dulmage et al. U.S. application Serial No. 25,109, filed April 27, 1960, and comprises coating a one-half mil thickness of polyethylene terephthalate taped onto a glass support with a 10 percent solution of a lower melting polyester dissolved in methylene chloride. The coating was placed on the plate at a 0.006- inch wet thickness. After drying for about 3 minutes a sheet of paper 1.5 mil thickness was rolled onto the tacky coating with a small hand roller and the combination was oven-dried at 75 C. The polyethylene terephthalate surface was then charged by passing under a negative corona wire at a potential of 10 kv. A document was laid onto the charged surface and the sandwich was given a reflex exposure under a high intensity infrared source such as commonly used in thermography. After radiation exposure the xerothermographic sheet was developed by allowing a liquid developer of the following composition to flow over the surface of the plate. The toner of negative charge adhered only to the areas from which the excellent chemical resistance) 20 Amberol 808 (a trade name of Rohm and Haas Co.

for a modified maleic rosin) 20 The Amberol 808 was dissolved in hot Solvesso 100, a hydrocarbon solvent sold by the Esso Standard Oil Co., in a 1:1 ratio before use in the above composition.

After ball-milling, 0.5 gram of the toner was dispersed in a liquid composition comprising 0.5 ml. isobutylbenbene and carbon tetrachloride.

The xerothermographic sheet developed with a negative toner of the above composition was then placed in contact with a sheet of ordinary typewriter paper and passed again under the infra-red lamp which resulted in the image from the xerothermographic sheet being transferred to the typewriter paper to produce a permanent copy thereon. Excellent well-defined images were obtained by the process of the present example with the images having a very clear background.

In preparing the stabilized electrostatic developers of the present invention it is usually required that only relatively small amounts of the alkyl-aryl stabilizing compound need be employed. Ordinarily the toner particles are treated with an alkyl-aryl compound in an amount by weight about equivalent in a ratio of from about 1:5 to 5:1 toner to alkyl-aryl compound. The toner is usually contained in the final developer composition in concentrations in the range from about 1 percent to about 20 percent by weight. Accordingly, the aliphatic carrier can make up from about 60 percent to about 98 percent by weight of the total composition.

The wide range of resinous materials suitable for use in preparing the toner particles according to the present invention makes possible a wide range of processes with which stabilized developers of the present invention can be used. One such process includes the making of color prints. The instant-drying feature of the present invention with no smearing and without requiring fusion of xerographic elements developed with the liquid developers of this invention makes possible the multi-step processing necessary to achieve color prints. The fastdrying highly volatile aliphatic carriers make possible an almost continuous process for preparing multi-color prints. By use of the small toner particles available for incorporation into a liquid developer according to the present invention makes it possible to obtain improved resolution in the prints obtained. For example a resolution of about 40 lines per millimeter can be obtained on standard zinc oxide xerographic plates where a toner particle having a diametric size of about 1.0 micron is used. In processes of the present invention, various materials and compositions thereof can be used to encompass a wide variety of materials, for example, lithographic ink, gravure ink, silk screen ink, paints, artists colors, flash colors, etc. Moreover, liquid xerographic development can be used in preparing line copy, half-tone negatives or positive, continuous tone prints, etc.

In preparing the toner particles of the liquid electrostatic developers of the present invention the particle size of the pigment and the electroscopic toner should be in the range from about .1 micron to about 20 microns. Particle sizes in this range when used with standard xerographic printing plates gave the desired degree of resolution. The particle size of the toners is usually the result of the time and methods used in the ball milling process. For example, the particle size of many of the pigments can be reduced to less than 20 microns by milling for about 6 to 8 hours. For purposes where the particle size should be 10 microns or less an extended ball-milling period was usually satisfactory to produce the desired particle size. Where continuous tone copy was to be developed using liquid developers it was usu ally desired for obtaining optimum resolution that particle sizes be reduced to about 1 micron. In this case a super ball mill was required.

The invention has been described in detail with par-' ticular reference to preferred embodiments thereof, but it will be understood that variations and modifications canbeeifected within the scope and spirit of the invention as described hereinabove and as defined in the appended claims.

I claim:

l. A stabilized liquid developer for developing latent electrostatic images, said developer consisting essentially- 8 wherein R represents at least one aliphatic- -group having from 1 to 12 carbon atoms, and the dispersion of toner in alkyl-aryl solvent being further dispersed in 4 (3) a liquid carrier phatic hydrocarbon having an electrical resistivity of at least 10 ohm centimeters and a low dielectric constant, said developer containing, said toner and said alkyl-aryl solvent in a range of ratios by weight from about 1:5 to about 5:1, said toner making up from about 1 percent to about 20 percent of said developer and said liquid carrier making up from about percent to about 98 percent by Weight of the said developer. 2. The stabilized liquid developer of claim 1 for developing latent electrostatic images in which the said alkyl -aryl solvent is propylbenzene. v r

3. The stabilized liquid deveolper of claim 1 for developing latent electrostatic images in which the resinous binder is a phenol modified pentaerythritol ester of rosin acid and the aliphatic hydrocarbon carrier is cyclohexane.

References Cited in the file of this patent UNITED STATES PATENTS 2,877,133 Mayer Mar. 10, 1959 2,891,911 Mayer et al June 23, 1959 2,899,335 Straughan Aug. 11,1959 2,907,674 Metcalfe et a1. Oct. 6, 1959 consisting essentially of an ali

Claims (1)

1. A STABILIZED LIQUID DEVELOPER FOR DEVELOPING LATENT ELECTROSTATIC IMAGES, SAID DEVELOPER CONSISTING ESSENTIALLY OF: (1) A TONER OF (A) FINELY-DIVIDED ELECTROSTATICALLY ATTRACTABLE SOLID OPAQUE PIGMENT PARTICLES SURROUNDED BY AN ENVELOPE OF (B) A RESINOUS BINDER THAT IS HIGHLY SOLUBLE IN AROMATIC SOLVENTS BUT NOT MORE THAN SLIGHTLY SOLUBLE IN ALIPHATIC SOLVENTS, SAID TONER BEING DISPERSED IN (2) AN ALKYL-ARYL SOLVENT SELECTED FROM THOSE REPRESENTED BY THE FORMULA:

Images