US3689260A - Color electrophotographic process with resin deposition for stabilization of tonor image - Google Patents

Abstract

IN AN ELECTROPHOTOGRAPHIC PROCESS FOR OBTAINING A MULTICOLOR IMAGE BY PERFORMING AN ELECTROPHOTOGRAPHIC PROCEDURE INCLUDING THE STEPS OF ELECTROSTATIC CHARGING EXPOSING, AND DEVELOPING THREE OR MORE TIMES REPEATEDLY ON A SINGLE ELECTROPHOTOGRAPHIC SENSITIVE MATERIAL, THE IMPROVEMENT CONPRISING A COLOR ELECTROPHOTOGRAPHIC PROCESS FOR ENHANCING THE MECHANICAL STRENGTH OF THE IMAGE CHARACTERIZED BY (1) PROVIDING A PHOTOCONDUCTIVE ZINC OXIDE SPECTRALLY SENSITIZED SO AS TO EXHIBIT PHOTO-RESPONSE THROUGH OUT A SUBSTANTIAL PART OF THE VISIBLE SPECTRUM, (2) PROVIDING A BINDER FOR THE ZINC OXIDE CONSISTING ESSENTIALLY OF A POLYISOCYANATE CROSS-LINKED POLYHYDROXYL RESIN WHICH FORMS A THREE-DIMENSIONAL MOLECULAR STRUCTURE, (3) CARRYING OUT THE DEVELOPMENT WITH A LIQUID DEVELOPER COMPRISING AN INSULATIVE CARIER LIQUID WITH A COMPONENT DISSOLVED THEREIN AND FINELY DIVIDED ELECTRICALLY CHARGED PARTICLES DISPERSED THEREIN ON THE SURFACE OF WHICH PART OF THE RESIN COMPONENT IS ADSORBED OR ASSOCIATED, AND (4) BRINGING THE SURFACE OF THE DEVELOPED SENSITIVE LAYER WHILE STILL WET WITH THE DEVELOPER INTO CONTACT WITH A RINSING LIQUID HAVING LESS DISSOLVING POWER FOR THE AFOREMENTIONED RESIN COMPONENT IN THE DEVELOPER THAN THAT OF THE CARRIER LIQUID THEREBY REMOVING THE DEVELOPER FROM THE NON-IMAGE AREAS AND CAUSING INSOLUBILIZATION OF THE RESIN COMPONENT WHICH DEPOSITS AROUND THE DEPOSITED TONER TO INCREASE THE MECHANICAL STRENGTH OF THE IMAGE.

Description

United States Patent Oflice 3,689,260 Patented Sept. 5, 1972 rm. Cl. cos 13/22 US. Cl. 961.2 11 Claims ABSTRACT OF THE DISCLOSURE In an electrophotographic process for obtaining a multicolor image by performing an electrophotographic procedure including the steps of electrostatic charging, exposing, and developing three or more times repeatedly on a single electrophotographic sensitive material, the improvement comprising a color electrophotographic process for enhancing the mechanical strength of the image characterized by (1) providing a photoconductive zinc oxide spectrally sensitized so as to exhibit photo-response through out a substantial part of the visible spectrum, (2) providing a binder for the zinc oxide consisting essentially of a polyisocyanate cross-linked polyhydroxyl resin which forms a three-dimensional molecular structure, (3) carrying out the development with a liquid developer comprising an insulative carrier liquid with a resin component dissolved therein and finely divided electrically charged particles dispersed therein on the surface of which part of the resin component is adsorbed or associated, and (4) bringing the surface of the developed sensitive layer while still wet with the developer into contact with a rinsing liquid having less dissolving power for the aforementioned resin component in the developer than that of the carrier liquid thereby removing the developer from the non-image areas and causing insolubilization of the resin component which deposits around the deposited toner to increase the mechanical strength of the image.

This invention relates to a color electrophotographic process based on overprint development.

At present, the product of Remak pty in Australia is available on the market as a device designed to provide multi-color image on zinc oxide electrophotographic paper by liquid development.

This process uses a metallic vat as development electrode and requires, elaborate manual operations. In this process, there is employed liquid developers having electrically charged extremely fine particles dispersed in a carrier liquid possessed of a relatively low dissolving power, i.e. high boiling aliphatic hydrocarbon, each charged particle being composed principally of a pigment particle having a minor resin component bonded to the surface thereof. The image to be obtained by using such liquid developers suffers from extremely low mechanical strength because an excessively small resin component is bonded to the pigment primarily. When drying of the developed sheet is prompted by blowing the developing agent or rinsing liquid remaining on the surface of the sheet by means of a current of air or by squeezing the sheet with squeeze rollers immediately after development, the image is readily destroyed. Therefore, it is virtually impossible to automate such drying operations by use of mechanical means. The sensitive layer which is employed in this -Remak process is prepared by combining unsensitized white zinc oxide and a cross-linked thermosetting resin binder.

On the other hand, numerous office copying machines utilizing the principle of liquid development are available on the market. All these machines employ the type of sensitive paper which is so constructed as to permit the bonding agent to be slightly softened or swollen by the carrier liquid of developing agent thereby to facilitate the fixing of image and which has sensitized ZnO powder dispersed in a thermoplastic binder. Such a layer often shows a high attenuation rate of electric potential in many nonpolar carrier liquids. Therefore, copying machines available on the market avoid this problem by using, as the carrier liquid, iso-paraffinic solvents which have particularly low dissolving powder among the nonpolar liquids, in other words, those which have extremely low values of solubility parameter.

As a highly effective means for enhancing the mechanical strength of the developed image thereby realizing a multi-color electrophotographic process capable of enduring rapid treatment, there is proposed a method comprising developing an electrostatic latent image formed on an electrophotographic sheet with a liquid developer containing a resinous material dissolved in the carrier liquid, said carried liquid being non-polar but having a relatively high dissolving power, then rinsing the developed sheet with a second liquid which is compatible with the carrier liquid of the liquid developer, but has a far lower dissolving power for said resinous material than the carrier liquid. In short, this method employs a less active solvent for the resinous material as the rinsing bath so that it serves to eliminate excessive toner and, at the same time, to allow the resinous material to deposit as the fixing agent on the surface of image. In this case, one may surmise that such excessive toner will be fixed on the surface, and, consequently, background will result inevitably. It has been experimentally confirmed however that the toner adhering to the surface of sensitive layer by virtue of the weak adsorbing force other than electrostatic is Washed away completely. Further, it should be noted that the net change which takes place in such rinsing procedure is not such that the resinous material originally dissolved in the carrier liquid abruptly separates out of the liquid phase throughout the developed surface. The change seems to be the enhancement of the entangling of polymer chains which have been loosely associated with the toner surface in the liquid developer, therefore, the appearance of turbidity does not usually take place.

In order to reduce such method to practical use, it is necessary that carrier liquids possessed of a relatively high dissolving power should be used in liquid developers. If this requirement is to be fulfilled, the sensitive layer is accordingly required to have a high electric charge retaining capability even in such liquids. It has been recognized empirically that, where a given sensitive layer is composed of a photoconductive power and a resin binder, if the said layer is wetted by an organic liquid of such nature as to dissolve or swell the resin hinder, the electric charge on the layer leaks extremely rapidly. Consequently, it is generally most desirable to use a binder which is cured to acquire a three-dimensional molecular net-work structure. However, such efforts involve another troublesome problem. Where a multicolor image is desired directly from a multi-color original, the sensitive layer is required to have sensitivity covering the substantial portion of the visible spectrum. To meet this requirement, it may be most desirable to employ a photoconductive element of zinc oxide having a spectral sensitivity expanded with use of spectral sensitizer. Usually, a heavy metal catalyst is often used for curing binders. Such catalysts tend to destroy many spectral sensitizer, making it frequently impossible to obtain a sensitive layer of stable performance. As typical binders, there may be cited epoxy ester resin,

alkyd resin, etc. If such binders are cured (cross-linked) in the presence of organic acid salts of cobalt, lead, and manganese as have been carried out conventionally, the decomposition of the sensitizer always proceeds during storage.

The inventors have arrived at a discovery that, as an improvement, it is most suitable for the present method of resin curing to use a polyisocyanate. This curing method does not call for any treatment at an elevated temperature. Moreover, no decomposition of sensitizing dyes is accompanied with the curing reaction.

Suitable polyhydroxyl compounds which can be combined with polyisocyanate include various types of alkyd resin epoxy esters of dehydrated castor-oil fatty acid and acrylic resins and methacrylic resins incorporated with such copolymerizable components as beta-hydroxyethylacrylate.

The layer cured in this manner exhibits extremely high electric charge retaining capability in various kinds of solvents possessed of relatively high dissolving power and therefore is highly suitable for rapid developing and during the treatments mentioned previously.

The present invention will be explained further in detail.

The present invention is an electrophotographic process for obtaining a multi-color image by repeating the steps of electrostatic charging, image exposure and development three or more times on one same electrophotographic sensitive layer, where the color electrophotographic process for enhancing the mechanical strength of the toner image is characterized by employing an electrophotographic layer comprising finely divided photoconductive zinc oxide spectrally sensitized so as to acquire photo-responsitivity throughout substantial portion of the visible spectrum and a binder comprising a polyisocyanate crosslinked polyhydroxyl resin, the crosslinking being sufiicient to form a three dimensional network structure, and, in the following individual steps of electrophotographic procedure comprising carrying out the development by' using a liquid developer prepared by suspending, in an insulative carrier liquid with a resinous material dissolved therein, finely divided electrically charged particles on the surface of which are adsorbed or associated the said resinous material, and bringing the surface of the developed sensitive layer into contact with a liquid having a sufficiently weak dissolving power for the resin component in the aforementioned developing agent as compared with that of the aforementioned carrier liquid thereby allowing the resinous material to insolubilize on the periphery or surface of the deposited toner particles.

The present invention is described in further detail 'below.

ZINC OXIDE AND SPECTRAL SENSITIZATION As is universally known, zinc oxide synthesized by the French process is suitable. Zinc oxide may be mixed with a small amount of another photoconductive material for the process of adjusting the tone reproduction performance. Examples of such materials are titanium, dioxide, cadmium sulfide, zinc sulfide, etc.

Spectral sensitization of zinc oxide is performed for the purpose of expanding the photo-response over the substantial portion of the visible spectrum. The expression the substantial portion of the visible spectrum as used herein means the necessary minimum region for the natural color reproduction.

Therefore, the photoconductor may be sensitized so as to have at least three peaks of sensitivity falling in the ranges of 400-460 m 450-570 Ill/L, and 550-750 mg, respectively.

It is also advantageous to provide a region with an extremely low sensitivity to impart a safelight compatibility which is a usual practice in silver halide color printing procedure. The spectral sensitivity performance is determined from the spectral absorption properties of standard originals. Generally, it is a common practice to impart peaks of photoresponse around 430440 m 520 4 540 mg, and 650-700 m Such peaks are desired to be as sharp as possible. For this purpose, one may resort to the sensitization-due to J-band absorption associated with dye aggregate which is frequently made use of in the art of silver halide photography.

As sensitizing dyes, there are used various kinds such as cyanine, merocyanine, hemicyanine, hemioxonol, Xanthene, sulfo phthaleine, triphenylmethane, etc. The amount of such dye to be added varies with the purpose of use and the treatment of sensitizing material. Where a print of high quality is required, the sensitizer is desired to be washed off or decolorized the layer after development. Frequently, the decoloration is accomplished by using a solvent having an organic acid or inorganic alkali dissolved therein. In this case, the composition of the washing bath must be selected so as not to exert any adverse effect upon the sensitive layer, the toner, the fixing agent for toner (the insolubilized resinous material), and the electroconductive resin layer formed underneath the sensitive layer.

It, as in this case, the sensitive layer is cured with polyisocyanate, it becomes less susceptible to various kinds of organic solvents, the expending, the freedom of selection of the composition for washing. This offers a major advantage from the standpoint of practical use.

Where the amount of sensitizer added surpasses part based on parts of zinc oxide, it is desirable to carry out decoloration. An electrophotographic coating with a high sensitivity for use in, for example, printing through large magnification enlarging must be incorporated with sensitizing five times as much as the above cited concentration.

Where the amount of sensitizer is less than part, the resulting coating looks weakly colored. Depending on the hue, therefore, the treatment of declaration may be unnecessary.

BINDER A fairly detailed description has already been made as to the binder. To be more specific, desirable binders are those obtained by cross-linking, with polyisocyanate compound, such alkyd resins possessed of hydroxyl group capable of reacting with isocyanate group, epoxyesters of dehydrated caster-oil fatty acid, or vinyl copolymers containing primary hydroxyl groups.

As alkyl resins, there can be used almost all kinds which are modified with drying oils, nondrying oils, styrene, acrylester, and phenol resins, etc.

The important requirement is that the acid compound thereof has an aromatic nucleus. Since aliphatic polyesters have high dependence of resistance upon humidity, they can hardly be used for practical purpose. As aromatic acids, it is desirable to use phthalic acid and isophthalic acid. The acid content is desired to be 10-50% based on the total weight of resins. The hydroxyl value is desired to range from 10 to 100. Desired epoxyesters are those derived from dehydrated caster-oil fatty acids.

AS a series of particularly desirable compounds, there may be cited those which have acrylester or methacrylester as the principal ingredient and which have hydroxyethylacrylate, hydroxyethylmethacrylate, allyl alcohol, hydroxyethylacrylamide and other similar monomers represented by the following general formula copolymerized with methacrylester, etc.

R CHz=( (h-CHzOH where:

R=H or CH The compounds of this series have their compositions varied widely and can be copolymerized with acrylonitrile and styrene, etc.

Examples of commercial products include Aron SP-200l and -2002 made by Toa Gosei Chemical Industry Co., Ltd.

When isocyanate is used for cross-linking, the reaction is retarded under an acidic atmosphere. Thus, use of resins with high acid values will necessitate a longer curing period. Specifically, it is desired to use resins having acid values less than 25.

Desirable polyisocyanates are those which have low volatility at normal room temperature. Typical examples are condensation products of l-mol trimethylolpropane and 3-mol tolylene-diisocyanate. As commercial products, there maybe cited Desmodule L made by Bayer, Coronate L made by Nippon Polyurethane Industry, and Takenate D-l02. made by Takeda Chemical Industries, Ltd., etc.

Condensation product resulting from 3 mol of xylenediisocyanate and 1 mol of trirnethylol propane may also be employed.

Polyisocyanate compounds of the aliphatic family such as hexamethylenediisocyanate can naturally be used. These polyisocyanate compounds react with alkyd or epoxyester at low temperatures and therefore need not be roasted at particularly high temperatures. Thus, they are free from the danger of destroying the base paper or other similar material or spectral sensitizers. Polyisocyanate is desired to be incorporated in an amount corresponding to 1.2 times up to several times (close to times) equivalent to the hydroxyl group present in the resin (preferably more than 2 times up to about 9 times). If it is incorporated in an aquivalent amount, then the curved layer shows a relatively rapid attenuation of electric charge within a nonpolar liquid and therefore fails to meet the purpose of this invention.

Where there is employed a dry developing methodcascade, magnetic brush, or aerosol method, for example, fully satisfactory properties can be obtained by incorporating polyisocyanate in an amount 1.2-1.5 times as great as the equivalent amount. (In this case, the only problem to be involved is the dark attenuation in the air.)

However, where it is desired to use liquid development, use of a larger amount of isocyanate is required.

Where there is used a styrene-modified alkyd resin (having a hydroxy value of 50), for example, the dark attenuation property is purified kerosene or Decalin becomes comparable to that in the air only when polyisocyanate is used in an amount more than two times as great as the equivalent amount. When the added amount is less than the level just mentioned, the attenuation in the liquid becomes rapid, though the attenuation property in the arr may be satisfactory. If there is used a liquid developer which has an extremely rapid developing speed or a high toner concentration, the amount of isocyanate added by amounts on the order of 1.2-2 times as great as the equivalent amount will be allowable.

Generally, the attenuation property in the liquid for practical use is required to equal or approximate that of the composition of the dispersed layer in the developing agent. Measurement of attenuation is difficult in highly volatile solvents from the practical point of view. Thus, there are used solvents which have suitable dissolving power and are dilficult to volatilize. Ordinarily there are used Decalin, kerosene, and tetralin which have been purified to the extent of showing a specific resistance of about 10 (2cm. Such a solvent is applied uniformly over an electrically charged sensitive layer so as to form a thin liquid film thereon. Then, the degree of electric charge attenuation is measured.

Experiences to date have demonstrated satisfactory correlation between the attenuation property determined in the manner mentioned and the actual performance observed in liquid development. In case where rapid leakage of electric charge occurs in kerosene, any developing agent which uses kerosene or other carrier liquid having stronger dissolving power than kerosene fails to give an image of high density.

Noteworthy is a fact that, since actual liquid developing agents include resins, ionic impurities derived from a pigment, and active agents, etc. in addition to pure Decalin or kerosene, the electric charge on the sensitive layer attenuates more rapidly in the developing agent than in pure solvent, etc. Therefore, the sensitive layer is desired to be of such type that the attenuation in pure Decalin |(showing a volume resistance of about 10 9cm.) is less than (or more than 40% in terms of residual ratio) for 30 second immersion. Particularly desirable level of attenuation is less than 30% (more than 70% in terms of residual ratio residue).

Since the present invention uses, as a carrier liquid, a nonpolar solvent having a resistively strong dissolving power as already described and will also be explained in further detail later on, the attenuation property of the sensitive layer in the liquid is particularly important. As pointed out earlier, the sensitive layer which uses a thermoplastic resin shows rapid attenuation in Decalin and kerosene. To cite examples of such properties in reproducing sensitive papers available on the market, the attenuation of electric charge is about 7% of the initial electric potential (93% in terms of residual ratio) after one minutes standing in the air and 5-6% after one minutes immersion in an isoparaflin is solvent (such as isoper H made by Esso Standard Oil 00.), whereas it is as high as 36% (64% in terms of residual ratio) after one minutes immersion in refined kerosene. After one minutes immersion in refined Decalin, the attenuation is 65% (35% in terms of residual ratio).

The inventors have found it necessary to use a solvent equivalent to or stronger than kerosene. This means that the speed of attenuation in kerosene must be at least substantially the same as that in the air.

The cured sensitive layer of the composition mentioned above has been found to meet this requirement sufliciently. It is added in this connection that the sensitive layer which uses, as the binder, an alkyd resin or epoxy ester reisn cured by oxygen in the presence of an organic acid salt of heavy metal such as cobalt or lead as the catalyst suffers from rapid attenuation of electric potential in a stronger solvent and therefore can hardly give satisfactory reproduction of continuous tone image.

The slow attenuation in the liquid of liquid developing process is an indispersible requirement for the reproduction of continuous tone image for the following reason. Development proceeds preferentially in the zone having high field strength with or without development electrode. In case where a large area of uniform charge density is present in the middle of a substantially charge-free area, development process from the edge of the changed area. As the edge portion is neutralized as a consequence of toner deposition, the portion of high field strength moves gradually inward. So, development similarly moves inward. Through this process, a large area is finally developed uniformly. However, if electric charge escapes rapidly in the course of development, there will result an image suffering from strong edge effect.

Where repulsion development (reversal development) is desired to be performed, the latent image is developed with a toner having the same polarity of change as the latent image while a bias voltage is applied to the development electrode to keep substantially zero the electric field above the maximum charge density area in the latent image where the toner deposition is desired.

If, the electric charge attenuates rapidly in the liquid, the optimal bias voltage varies with time and thus must be controlled following the attenuation. Such demand complicates the apparatus and gives rise to various problems from the standpoint of practical operation. If the attenuation in the liquid is quite slow, such troubles are eliminated.

Even if there is required adjustment of voltage at all, the width of adjustment is small.

Another characteristic of the sensitive material of this invention is the fact that the tone reproduction performance is prominent. This can be demonstrated by using a liquid developer containing therein toner particles having diameters in the range of less than 1 1. that is, 0.1-0.5}L.

v of characteristic curve in the range of 1.5-2 or more, the average gamma of 0.7-1.8. Moreover, the maximum The determination of average gamma, v, is made above-cited unit length so as to tangentially toughen the the two lines as When the electrically charged sensitive plate is exposed to light projected through a photographic optical wedge, the electrophotographic sensitive plate cured by the conentionally known technique shows the average gamma,

7, of characteristic curve in the range of 1.5-2 or more, whereas the sensitive plate of the present invention shows the average gamma of 0.7*l.8. Moreover, the maximum density is about 2, probably because of the advantage of attenuation in the liquid.

The determination of average gamma, 7, is made according to the practice prevailing in the photographic paper industry which comprises plotting the reflective density on the ordinate and log (amount of exposure) along the abscsisa from a developed optical wedge whereby the length of unit density and that of log 10 are taken equal, drawing two parallel lines separated by A of the above-cited unit length so as to tangentially toughen the plotted characteristic curve, and designating the slope of the two lines as 'y.

In the ordinarily used range, the ratio of the inorganic photoconductive powder to the binder is 100 parts of the former to 50 parts to parts of the latter. According to a special film forming method, a highly insulative film may be obtained by using the latter in the amount of 1 part to 0.5 part.

LIQUID DEVELOPING AGENT Numerous pieces of literature and patent specifications have already been published concerning the manufacturing processes for liquid developers. Generally speaking, a standard liquid developer comprises a finely divided toner carrying a suitable amount of electric charge and a carrier liquid serving as the dispersing medium. For use in the present invention, the toner particle diameter should be less than 1ft. This means that the particle size of toner to be deposited on the surface of latent image must not exceed 1 as the unit. A toner whose particle size exceeds this limit is liable to come olf the layer surface readily during the rinsing operation. Further, the electrophotographic characteristics of such toner are not satisfactory in many points and therefore make it extremely difiicult to obtain desirable results. In addition, the liquid developer is required to have a resin dissolved in the carrier liquid thereof. This is frequently referred to as fixing agent. As the carrier liquid, there is generally selectcd a highly insulative nonpolar liquid. Since the resin is required to be soluble in such liquid, it is restricted to a considerable extent. Frequently there are selected oligomers of small molecular weight. Suitable materials include condensation resins such as long-oil type alkyd resins, particularly alkyd resins comprising isophthalic acid epoxyester resins, vegetable oils, rosin-modified phenolformaldehyde resins, and xylene-formaldehyde resins. Occasionally, there may be used vinyl polymers such as polybutylmethacrylate, styrenebutadiene copolymer, and other styrene copolymers. Resins of large molecular weights are likely to form a strong film upon rinsing and therefore are difficult to use.

The resins mentioned above are soluble in cyclohexane, tetralin, Decalin and other alicyclic hydrocarbons, aliphatic hydrocarbons (including intermixtures thereof), and these solvents incorporated therein with a small amount of stronger solvents (such as chlorinated hydrocarbons, ketones, and aromatic hydrocarbons). Liquid developers are prepared by dispersing a finely-divided organic or inorganic pigment in carrier liquids containing such resins. There exists an opinion that the dissolved resin is strongly adsorbed on the surface of such pigment par ticle and behaves to control the electric charge of the pigment. However, it is difficult to observe a phenomenon which strongly supports such View. The major part of the resin is believed to be dissolved uniformly on the liquid phase side.

To take an example, the elcctrophoretic performance of a developer containing a pigment having a strongtendency to be negatively charged will be considered. When lead chromate, which tends to assume a negative charge, is thoroughly blended with a long-oil alkyd resin to form a paste and the paste is dispersed in cyclohexane, the dispersed particles are charged positive due to the charge controlling capability of the alkyd resin. This fact supports that the alkyd is adsorbed on the surface of lead chromate. After a prolonged standing, however, the resin layer formed on the surface of resin seems to slowly dissolve out and give rise to a negative toner. From this, it can be concluded that the pigment particle will eventually have its surface exposed so far as the carrier liquid is capable of dissolving the resin.

Liquid developers may be prepared by a method based on another principle by using two or more kinds of resin, one of which is soluble in the carrier liquid and the other kinds are insoluble and consequently remain permanently deposited on the surface of the pigment. The soluble component and the insoluble component must be miscible with each other. The soluble component serves to ensure a stable dispersion of pigment. Such mixture of resin is blended with a pigment into a paste. When this paste is dissolved in the carrier liquid, the consequent dissolution of the soluble component permits the pigment in the paste to disperse maintaining its original dispersion state in the paste.

At this point, the insoluble component remains on the surface of pigment. Thus, the capability of the insoluble component to control the electrophoretic property is allowed to work stably for a long period.

Liquid developers of still another type use two or more kinds of resins components, which are both soluble in the carrier liquid but which have different degrees of solubility to other solvents.

When the liquid developing agent of the first or third type is applied to the surface of latent image, the toner flows and deposits on the surface of latent image in accordance with the change distribution. Since the toner is always accompanied with an excess carrier liquid it forms an image containing therein a homogeneously dissolved resin component. In the case of liquid developing agent of the second type, the pigment enclosed with resin deposits on the image portion and, at the same time, the developed image contains a small amount of the dissolved resin.

For the present invention, it is an important requirement that the resin component should remain dissolved in the carrier liquid for any case.

In the subsequent step of rinsing, there must occur insolubilization of resin. Therefore, the carrier liquid is required to have a composition capable of offering strong dissolving power for resins. If the carrier liquid is a paraffin solvent or fluorochlorinated hydrocarbon, then there will be no room for selecting a rinsing liquid having a still weaker dissolving power. Consequently, the object of enhancing the mechanical strength of image by rinsing cannot be accomplished. The carrier liquid is required to have the level of dissolving power surpassing that of kerosene and preferably substantially the same as that of cyclohexane. If cyclohexane is used alone, the evapora tion rate is too high and, consequently, there are involved various difficulties from the practical point of view. Thus, there is used a solvent having a suitable evaporation rate or a mixture of a solvent of weaker dissolving power with 9 a solvent with a stronger dissolving power in order to suitably control the evaporation rate.

For example, there are used kerosene, Decalin, mixture of cyclohexane with kerosene, mixture of Decalin with kerosene, mixture of isooetane with cyclohexane, cyclohexance containing about 3-10% of vegetable oil, Decalin containing 3-10% of vegetable oil, kerosene containing 3-15 of tetralin, isoparaffin containing -30% of tetralin, and kerosene containing 2-8% of toluene. Depending on the requirements of dissolving power and evaporation rate more complicated mixtures may be used. Generally, it is desirable to use carrier liquids having such compositions as will give a solubility parameter of from 7.5 to about 8.8 per the room temperature. The values approximately correspond to kauri-butanol values of 33 8 Regarding concentrations or the amounts of such resinous materials which are dissolved in the carrier liquid of the developer liquid, and which are insolubilized by the application of washing liquid, 0.3 to 30 parts by weight of such material, and more preferably 0.5 to 10 parts by Weight, may be employed per 1 part by weight of the dispersed particles in the developer.

RINSING O'F DEVELOPED IMAGE The developed electrophotographic material bears a uniform thin layer of the developer liquid due to surface tension, trapping to the minute structure present in the surface and other causes. Such excess toner will cause a considerable background if the liquid film is allowed to dry. When the image-carrying layer is immersed in a highly insulative liquid miscible with the carrier solvent before drying, the excess liquid which can cause background can be washed away. As a result of experiment, it has been disclosed that the image is destroyed during rinsing operation depending on the dissolving capability of the rinsing liquid to the resinous material contained in the liquid developer. In the case of half-tone images, destruction of the image is difiicult to observe because the attracting force is large between the toner and the latent image. Destruction toner of image is likewise difficult to observe where the rinsing operation is carried out with great care. However, when the toner image is of continuous tone, or when the rinsing liquid is applied to the developed surface at an increased flow rate so as to reduce the rinsing time, image destruction can be observed clearly. Further when the excessing liquid remaining on the sensitive layer is squeezed by means of squeezing rollers, the image may be tfieformed or broken and transferred onto the roller surace.

The aforementioned problems have been noted to occur when the rinsing liquid can dissolve the resin component which has been dissolved in the carrier liquid of the developer. It has been ascertained that this difficulty is overcome perfectly when the rinsing liquid exerts a poorer solubilizing activity against the dissolved resin present in the image portion. Three types of developer have been described previously. At this point, attention is made to the fact that they all contain resin components which are dissolved in the carrier liquid. If such resins remain undissolved in the course of rinsing, then highspeed rinsing and rapid squeezing can be permitted. If, in the liquid developing agents of the second type, the rinsing liquid dissolves the insoluble resin deposited on the surface of pigment particle, image destruction occurs as a natural consequence.

In fact, the resins used for the second type of developer and encapsulating the pigment particles are insoluble in many non-polar solvents (having a low value of solubility parameter) and only dissolved by those which have a stronger dissolving power than kerosene or cyclohexane. Therefore, one need not worry about the solubilization.

Practical combinations of resin and rinsing liquid will be shown afterwards in the preferred embodiments of this invention. To cite a typical example, a liquid developer is prepared by treating an organic or inorganic pigment with a varnish obtained by cooking a rosin-modified phenolformaldehyde resin together with a polymerized linseed oil or a styrene-modified alkyd resin, then dispersing the treated pigment in a carrier liquid composed principally of cyclohexane whereby the varnish or resin dissolves in the solvent. A rinsing liquid is composed of an insoparaffinic hydrocarbon a certain type of straight-chain aliphatic hydrocarbon or in a fluorochlorinated hydrocarbon, each having a solubility parameter below 7.5 or kauri-butanol value below 30, which cannot dissolve the varnish or alkyd resin.

One may fear that if solubilization of resin occurs during rinsing, then toner might also be fixed at background areas. Experiments have shown such fear to be entirely groundless. Since in the background portion, pigment particles are held on the sensitive layer by rather weak forces other than electrostatic, the toner particles are easily removed by the rinsing liquid even if the insolubilization of resin occurs. In the image portion, on the contrary pigment particles adhere fast to the surface of layer electrostatically. Upon contact with the rinsing liquid, the soluble resin component that exists between pigment particles separates out of the liquid phase to deposit the surface of pigment particles and then works to keep the particles to adhere to the surface though with a relatively weak force. This fixing of toner image is not extremely strong and the image may be readily removed by a local force exerted by rubbing with a finger tip. Yet, this is sufficient so far as the force to which the image is exposed within the apparatus is concerned. The resin which is obtained by cooking rosin-modified phenol-formaldehyde resin with polymerized linseed oil is composed of a condensation product consisting primarily of the former and the unreacted linseed oil. The former component is insoluble in hydrocarbon solvents with low KB values or in fluorochlorinated hydrocarbons and the linseed oil is soluble in almost all solvents. Accordingly, it is believed to be the former that exhibits the fixing function at the time of rinsing. Middle or long type vegetable oil-modified alkyd resins are also illustrated as the resin insolubilized by isoparaffinic solvents. Suitable vegetable oils include linseed oil, soybean oil, safflower oil, etc.

The rinsing liquid is allowed to contain therein the toner in an amount about 6 of that in the developing agent. This is because the latent image has already been developed to a great extent and will no longer attract toner. In addition, when the rinsing liquid is used repeatedly, the toner will eventually accumulate to such concentration within the liquid.

The rinsing liquid will be gradually contaminated not only with toner but with the carrier liquid of the developer making it more and more difficult to cause insolubilization of the resin component.

Thus a case must be taken in constructing or designing the processing apparatus to minimize the contamination of the developer into the rinsing bath. Besides one may use the rinsing bath with far larger volume than the developer.

When a multi-color image is to be obtained by overprint technique, a toner image which is already formed is submerged into the subsequent developing agent after the step of drying. In many cases, the carrier liquid in the second developing liquid resembles that of the first developing agent. Accordingly, there may naturally be a fear that the first image may be damaged in this sec-0nd development agent. Experiments have shown, however, that no such trouble occurs actually. This is probably because the first image has been fixed onto the sensitive layer or recording layer very strongly owing to the treatment with squeezing rollers and especially by drying.

Where there is obtained a multi-color continuous image of tone, reduction of background in each cycle of development makes a great contribution to the marked improvement of the quality of final print.

1 1 EXAMPLE 1 100 parts of photoconductive zinc oxide was dispersed in methanol. The three kinds of spectral sensitizer having the following structural formulae were added to the resultant dispersion. The amounts of these sensitizers thus added are indicated at the side of the corresponding formulae.

Part

Pigment 1. (sensitizer for blue hghtlo=cn on=o ON CHQGHZC O OH O Pigment 2. (sensitizer for green NaO- light). I

ww v Br S O 3N8 Pilgnlllent 3. (sensitizer for red -C The substantial part of methanol was removed by centrifugal separation. Then, after addition of nbutyl acetate, the remaining methanol was further removed by centrifuge. The supernatant was discarded and the resultant paste was mixed with 28 parts of styrene-modified alkyd resin varnish (nonvolatile content 50%, xylol solvent, phthalic anhydride content 21%, content of styrene 30%, hydroxyl value and acid value 5), suitably diluted with a 1:1 mixture of butyl acetate and xylol. Immediately before coating, the diluted mixture was added 8 parts of Desmodul L (a 75% ethyl acetate solution of the condensate principally of 3-mol tolylene diisocyanate and l-mol trimethylol propane, made by Bayer of West Germany) and the resultant mixture was spread to a dry thickness of 8, on the metalized surface of polyethylene terephthalate film having vacuum deposited aluminum layer. The coated was dried for 15 hours in a thermostat at C. The sensitive material thus produced showed penchromatic photoresponse. The dark attenuation property in the liquid was determined as described below.

blue filter and then developed with a liquid developer containing yellow toner. In the second cycle, exposure was made through a green filter and development was performed with a liquid containing magenta toner. Finally, exposure Was made through a red filter and development was given with a liquid developer containing cyan toner. The developing agents had the compositions shown below. After the step of development, the sensitive material was first rinsed with isopan E (KB value 27, solubility parameter 7.1), an isoparafiinic solvent made by Esso Standard Oil, to remove excessively adhering liquid developing agent and then was passed through squeezing rollers so as to be deprived of all remaining liquid. The developers used were prepared as follows:

YELLOW DEVELOPER The paste obtained by mixing the following ingredients 'was dispersed in 200 ml. of kerosene and 800 mol of cyclohexane.

Yellow pigment having the following structural formula CH; CH 200 0 on. c1 or a 011.0 I $=0 l C=O H H H ll 0 Varnish obtained by cooking rosin-modified pheuyl-iorrnaldehyde with linseed oil 800 Polymerizcd linseed oil 100 MAGENTA DEVELOPER The paste obtained by mixing the following ingredients was dispersed in 200 ml. of kerosene and 800 ml. of cyclohexane.

Brilliant carrnine 6B 200 Varnish obtained by cooking rosin-modified phenolformaldehyde resin with linseed oil 400 Polymerized linseed oil 1 3 CYAN DEVELOPING AGENT The paste consisting of the following ingredients was dispersed in the liquid mixture shown further down.

Microlish Blue 4G-T mg 200 Toluene ml Kerosene 250 Cyclohexane 700 Linseed oil 50 When the image-bearing layer was rinsed with Isopar E and subsequently passed through squeezing rollers, no damage was observed on the image at all. When squeezing was given after washing with cyclohexane or kerosene, the image was broken in the course of squeezing and a part of toner image transferred itself onto the roller surface.

The cause may be explained as follows. The hydrogenated rosin ester and rosin-modified phenol resin contained such as in Cyclolis Blue 4GT are soluble in kerosene, cyclohexane, and toluene. In the developing agent, therefore, the blue pigment is dispersed withouthaving its surface coated with another substance. The hydrogenated rosin ester and the like remain dissolved in the liquid phase. 011 the surface of the sensitive layer which has undergone the step of developing, there exist hydrogenated rosin ester and linseed oil as the components of liquid phase. These substances are dissolved out substantially completely into kerosene when they are washed in kerosene. Consequently, the image portion becomes composed of pigment only and is deprived of self-fixing 14 property. Thus, the aforementioned trouble occurs when the image is subjected to the action of squeezing. Incidentally, these resins are invariably insoluble in Isopar E. Once they are washed with it, they are insolubilized and eluted in the neighborhood of toner, enabling the image to acquire increased mechanical strength.

EXAMPLE 2 The pigments of Example 1 were used in the same amounts, while a binder was prepared with the following ingredients.

Parts Middle oil type linseed. oil-modified alkyl resin varnish (involatile content oil length phthalic anhydride content 33%, acid value 5, and hydroxyl value 20) 28 Desmodule L 8 The same color materials and resins of the developing agent as in Example 1 were used, except the carrier liquid was prepared with a mixture of 200 cc. of kerosene with 800 cc. of Decalin. Using Isopar E for washing, there were obtained satisfactory results similar to those of Example 1.

EXAMPLE 3 The same sensitizing pigments as those of Example 1 'were used, while a binder was prepared with a mixture of 20 parts of ARON 8-2001 made by Toa Gosei (involatile content 50%, hydroxyl group content 2%) with 5 parts of Desmodule L. The other conditions were the same as in Example 1.

EXAMPLE 4 A pigment-adsorbed zinc oxide was prepared by the procedure of Example 1, except the following pigments were used in the place of those of Example 1.

As the binder, there was used what was obtained by ture, (3) carrying out the development with a liquid derigidifying 14 parts of epoxy ester (oil length 40%) of veloper comprising, an insulative carrier liquid with a dehydrated castor oil fatty acid with 6 parts of a conresin component dissolved therein and finely divided elecdensate of 3-mo1 trilene-diisocyanate and l-mol trimethtrically charged particles dispersed therein on the surface ylol propane. of which part of said resin component is adsorbed or as- As the developing agents, there were used the followsociated, and (4) bringing the surface of the developed ing three kinds: sensitive layer while still wet with the developer into YELLOW DEVELOPING AGENT contact with a rinsing liquid having less dissolving power for the aforementioned resin component in the developer The paste obtained by mixing the following ingredients than that of said carrier liquid thereby removing said was dispersed in a carrier liquid consisting of 100 ml. of 10 developer from the non-image areas and causing insolutetralin, 400 ml. of cyclohexane, and 500 ml. of Decalin. bilization of the resin component which deposits around Benzidlne yellow type yellow pigment of the following the deposited toner to increase the mechanical strength structural formula of the image.

a 01 (:1 E 250 (1:0 0:0 CHa-NH?|3CHN=N --N=N-(ilHfi-NHCH3 l O O CH3 (3H3 Varnish obtained by heating rosin-modified phenyl formaldehyde resin with linseed oil 600 Long oil type safflower oil-modified alkyd resin (oil-length 55%) 100 Polymerized linseed oil. 150

MAGENTA DEVELOPING AGENT 2. The process of claim 1 wherein said resin component The paste obtained by mixing the following ingredients developer 18 Varnish pregarlid cooinng a q was dispersed in a carrier 1i quid consisting of 100 m1. modified phenolformaldehyde resin 1n bolled hnseed oil.

of tetralin, 400 ml. of cyclohexane, and 500 ml. of Decalin. The claim} wherein Said @5111 1n the developer is long-oil type alkyd resins.

4. The process of claim 1 wherein the amount of the resin component in the developer is from 0.3 to 30 parts 3 0,- OH 0 200 by Weight to one part by weight of the dispersed particles in the developer.

on, (MM 5. The process of claim 1 wherein the solubility parameter of said carrier liquid of the developer at room tem- CH3 perature is from 7.5 to 8.8 and that of said liquid p0ssessed of sufficiently weak dissolving power is less than vtrnislialgtatiined biyl gealtilriilg rosn-ilmodified phenol 500 40 7.5. Th 1 h d h d d y pres See 6. e process of c aim 1 w erein sai 'qui possesse Polymmzed linseed on 300 of sufliciently weak dissolving power is selected from the group consisting of iso-parafiinic hydrocarbons and fluoro- CYAN DEVELOPING AGENT chlorinated hydrocarbons.

The paste obtained by mixing the following ingredients T Process of 9 1 WhCTBmPaId blnder was dispersed in a carrier liquid consisting of 100 ml. of Sensltlve layer comPIlses styrene'modlfied alkyd resin and tetralin, 400 ml. of cyclohexane, and 500 ml. of Decalin. polyisocyanate- 8. The process of claim 1 wherein said binder of light- Mg. sensitive layer comprises vinyl polymer containing hy- Phthalocyanin blue 250 droxyl group and polyisocyanate. Linseed oil-modified alkyl resin (oil length 52%) 550 9. The process of claim 8 wherein said polyisocyanate Polymerized linseed oil 200 is incorporated in an amount corresponding from 1.2 to

10 times the amount of the hydroxyl group present in the The linseed oil-modified alkyl resin was of the type vinyl polymer. insoluble in chlorofluorinated hydrocarbon type solvents 55 10. .A method as in claim 1 where after the rinsing and isoparaflin type solvents. liquid is applied, the electrophotographic sensitive mate- In this particular example, Daifuron S-2 (chlorofluorial is subjected to a squeezing treatment so that a subrinated hydrocarbons made by Daikin Industries) was stantial amount of liquid remaining on the material is used as the washing liquid. A sheet containing an image removed. of good quality could be obtained by passing the sheet 11. A process as in claim 1 where said rinsing liquid is through squeezing rollers to remove excess washing liquid. a volatile liquid free of non-volatile ingredients.

What is claimed is:

1. In an electrophotographic process for obtaining a Reference C e multi-colorimage by performing an electrophotographic UNITED STATES PATENTS procedure including the steps of electrostatic charging, exposing, and developing three or more times repeatedly 3,337,340 8/ 1967 Matkan 96-1 on a single electrophotographic sensitive material, the 3,403,019 9/ 1968 Stahly et a1. 96-1.5 improvement comprising a color electrophotographic 3,535,244 10/ 1970 Zabiak 252-62.l process for enhancing the mechanical strength of the 3,311,490 3/1967 Fauser et a1. 117-37 image characterized by (1) providing a photoconductive 3,244,516 4/ 1966 Neugebauer et al 96-1 zinc oxide spectrally sensitized so as to exhibit photoresponse throughout a substantial part of the visible spec- JOHN COOPER 111, Primary Examiner trum, (2) providing a binder for said zinc oxide consisting essentially of a polyisocyanate cross-linked polyhydroxyl resin which forms a three-dimensional molecular struc- 96-1, 1.8, 1.7; 117-37 Ly; 252-62.1