US3428452A - Photoconductive compositions and electrophotographic recording elements made therefrom - Google Patents

Description

United States Patent 3,428,452 PHOTOCONDUCTIVE COMPOSITIONS AND ELECTROPHOTOGRAPHIC RECORDING ELEMENTS MADE THEREFROM Edward C. Giaimo, Jr., Princeton, N.J., assignor to Radio Corporation of America, a corporation of Delaware No Drawing. Filed Jan. 18, 1965, Ser. No. 426,443 US. Cl. 961.7 Int. Cl. G03c J 74 ABSTRACT OF THE DISCLOSURE An improved photoconductive layer is applied to a substrate of an electrophotographic record from an aqueous coating mixture comprising particles of water insoluble resin-encapsulated dye-sensitized zinc oxide dispersed in an aqueous mixture comprising a resinous binding agent for said particles. The encapsulation of the dyesensitized zinc oxide prevents desorption of the water solu- 'ble light-sensitizing dyes in the aqueous coating mixtures.

This invention relates generally to photoconductive compositions which are particularly useful in electrophotography. Particularly, the invention relates to improved photoconductive layers, containing dye-sensitized photoconductors, and improved methods of making these layers from aqueous coating solutions.

By the term aqueous coating solution, as used herein, is meant a dispersion of a dye-sensitized photoconductor in water in which a resinous binding agent is either dissolved or dispersed. Although the dispersion of the dyesensitized photoconductor is not a true solution, the term solution is nevertheless herein used because this term has been adopted by the prior art.

It has been proposed to render certain photoconductive materials, such as the photoconductive layer of an electrophotographic recording element, for example, responsive to light in the visible spectrum by causing certain dyes, such as auramine, acridine orange, and rhodamine B, to be adsorbed to the photoconductive layer. Other suitable dyes are listed in US. Patent No. 3,052,540, issued to H. G. Greig on Sept. 4, 1962. Since, however, most lightsensitizing dyes in common use are very soluble in water, the photoconductive layer, a white body colored photoconductor, such as zinc oxide, has heretofore been deposited from a coating solution that comprises a lightsensitizing dye, zinc oxide, and a resin binder for the zinc oxide in a liquid vehicle other than water. Organic solvents, such as toluene and methyl ethyl ketone, for example, in which light-sensitizing dyes are substantially insoluble, have been used heretofore as the liquid vehicle for photoconductive coating solutions to prevent desorption of the dye into the liquid. Compared to aqueous coating solutions, however, organic solvent coating solutions are relatively expensive, dangerous to handle, and usually present a fire hazard in a factory or paper mill.

Where water has been used heretofore as the liquid vehicle coating solutions, the dyes desorbed by the water would tend to objectionably color the substrate of an electrophotographic recording element to which the photoconductive coating was applied, leaving uneven colored streaks. Because of this objectionable desorption of the dyes in water, an excessive quantity of dyes had to be used, resulting in an excessive waste of the dyes and uneven sensitization. Also, it has often been impossible to obtain the same degree of light sensitization of the photoconductive layers from prior art aqueous coating solutions as has been obtained from non-aqueous coating solutions because of the objectionable desorption of the dyes in water.

14 Claims 1 It is an object of the present invention to provide an Improved photoconductive layer deposited on a substrate from a relatively inexpensive and safe aqueous coating solution.

. Another object of the present invention is to provide improved methods of producing photoconductive compositions employing an aqueous solution of dye-sensitized photoconductive particles whose dyes are not easily desorbed in aqueous coating solutions.

Still another object of the present invention is to provrde improved photoconductive layers and improved methods of making them that overcome the aforementioned disadvantages of employing light-sensitizing dyes in prior art aqueous coating solutions.

Still a further object of the present invention is to provide improved photoconductive layers that may be applied, by the improved methods of the present invention, directly, safely, and economically to a web of substrate record material, such as paper, during its manufacture.

Briefly stated, the improved photoconductive layers of the present invention are applied to suitable subtrates from aqueous coating solutions containing particles of completely encapsulated dye-sensitized zinc oxide dispersed in an aqueous film-forming emulsion of a natural and/or synthetic resin. The aqueous resin emulsion functions as a film-forming vehicle and, after drying, functions as a resinous binding agent for the encapsulated dye-sensitized particles. Each of the encapsulated dye-sensitized particles comprises a core of a photoconductive material such as zinc oxide, at least one light-sensitizing dye adsorbed to the photoconductor, and a substantially waterproof resin encapsulating the dye photoconductor.

The particles of encapsulated dye-sensitized photoconductor can be produced, in a preferred method of the present invention, by first making a homogeneous mixture of photoconductive zinc oxide, a solution of at least one light-sensitizing dye, and a solution of a resin in a suitable solvent for the resin and then spray-drying the homogeneous mixture under conditions to form encapsulated particles of a desired average diameter.

The novel features of the present invention, both as to its organization and operation, as Well as additional objects and advantages thereof, will be more readily understood from the following detailed description.

Encapsulated dye-sensitized zinc oxide particles of the type described herein will be hereinafter referred to as EDSZO. The quantitative ratios between the zinc oxide, the dyes and the encapsulant therefor may vary within wide limits depending upon the degree of light sensitization desired and/or the coating solutions in which the EDSZO is to be included. For example, the weight ratio of dyed zinc oxide to resin encapsulant may be as small as 2:1 and as great as 7.5 :1. The weight ratio of dyed zinc oxide to the total resin in the aqueous coating solution may be as little as 2:1 and as great as 3:1. In fact, particles of EDSZO can be fused directly onto a backing substrate, without being deposited from a solution, to form a suitable photoconductive layer.

One method of preparing EDSZO in accordance with the present invention is:

EXAMPLE 1 A concentrated stock solution of a light-sensitizing dye mixture is prepared by making a solution of 0.5 g. of Brilliant Blue FCF (42090 Color Index Acid Blue 9- 2nd ed. 1957) and 0.01 g. of fluorescein in 197.5 of ethanol.

42 g. of photoconductive zinc oxide, 2 ml. of the concentrated dye mixture, and ml. of methanol are mixed in a ball mill for 2 hours. The dyed zinc oxide then is filtered and dried.

A one percent by weight solution of Pliolite SD resin in toluene is prepared.

The dried dyed zinc oxide is mixed with the one percent 'Pliolite S-SD resin solution in toluene in the proportions of 5 g. of the dried died zinc oxide for each 45 ml. of the Pliolite 'S-SD solution in a ball mill for 2 hours. The milled mixture is spread thinly on glass and the toluene is evaporated leaving dried, died zinc oxide encapsulated in the Pliolite S5D resin. The dried, encapsulated dyed zinc oxide is scraped off the glass surface and then ground in a mortar to a fine powder of particles averaging about 1 micron in diameter. The quantities of ingredients of Example 1 may vary so that the Weight ratio of the zinc oxide to the encapsulating resin may be within the range of 27.5: l.

The photoconductive zinc oxide used in this and the following examples may, for example, be either Florence Green Seal No. 8 or Photox 801, products of the New Jersey Zinc Company, Palmerton, Pa. Pliolite S-SD is a synthetic resin of the high styrene butadiene copolymer type, a product of the Goodyear Tire and Rubber Company. EDSZO made pursuant to Example 1 is photoconductive in response to wavelengths of light over the entire visible spectrum.

The encapsulating resin of the dye-sensitized zinc oxide may by the (laboratory) method of Example 1 should be hard and grindable to produce a fine powder.

Soft, as well as hard, resinous waterproofing encapsulants for the dye-sensitized zinc oxide may be used to prepare EDSZO by the following preferred (commercial) method of Example 2, employing the technique of spray drying:

EXAMPLE 2 A light-sensitizing dye, Blue Concentrate, is prepared by dissolving 1.2 g. of fiuorescein disodium salt and 3.6 g. of Brilliant Blue FCF in 100 ml. of methanol.

A homogeneous mixture is made of 454 g. of photoconductive zinc oxide, 2 ml. of the aforementioned Blue Concentrate, 60 g. to 240 g. (preferably 120 g.) of Pliolite S-5E (a high styrene butadiene copolymer resin) and 800 ml. of toluene. The homogeneous mixture is then spray dried in a Bowen Conical Laboratory Spray Drier (Bowen Engineering, Inc., North Branch, NJ.) under the following conditions: Feed (rate 98 cc./min., air-inlet temperature to drying chamber 208 F., air-outlet temperature of drying chamber 155 F. Indirect gas heat is used, and a concurrent flow in a two-fluid nozzle atomizer is obtained with an air pressure input to the nozzle of about 100 lb./in. gage. The aforementioned quantities in Example 2 may vary so that the ratio of zinc oxide to the resin encapsulant may vary from 2 to 75:1 by weight.

The EDSZO formed by the preferred method of Example 2 comprises substantially spherical particles of about 1 micron in diameter. Each of the particles of EDSZO comprises a core of photoconductive dyesensitized zinc oxide completely encapsulated by the waterproof resin (Pliolite S5E). The spray drying method (Example 2) of producing EDSZO" is preferred to the grinding method of Example 1 because it can be adapted to a closed cycle wherein the solvent of the encapsulating resin may be recovered, if desired. The spray drying method permits the use of a wide choice of both hard and soft encapsulating natural or synthetic resins because the operation of grinding to form a fine powder is obviated. Also, the spray drying method is a practical commercial method, resulting in uniform, substantially spherical, completely encapsulated dye-sensitized, photoconductive zinc oxide particles.

Being encapsulated in a waterprof resin, the EDSZO can now be incorporated in photoconductive coating solulions (dispersions) that have a water base. The encapsulant on each particle of EDSZO prevents desorption of the light-sensitizing dyes into the aqueous photoconductive coating solutions.

An aqueous coating (film forming) solution for forming a photoconductive layer on a suitable substrate, such as paper, may be prepared as follows:

EXAMPLE 3 PART A Parts by Ingredients Weight Supplier (grams) EDSZO (prepared in accordance 15.0 (As per Example 2).

with Ex. 2).

PART B 'Iamol #731 (25% aqueous solution) 0.3 Rohm and Haas 00., (sodium salt of polymeric carbox- Philadelphia, Pa. ylic acid, a dispersant). Emulphor EL-719 (polyoxyethyl- 0. 1 General Dyestuffs ated fatty acid, a dispersant). gorip New York, FMC KP-l40 (plastieizer, leveling 0.05 Food Machinery agent). ICIorp New York, Methocel 400 cps. (3% aqueous solu- 2. 5 Dow Chemical Co.

tion, methyl cellulose). Water 15.0

PART C Piccotex No. 131-119 (a water emul- 9. 0 Penn. Industrial sion of a styrene homolog copoly- Chemical Corp., met, a resin). Clareton, Pa. Water 15.0 FMC KP- (tributoxy ethyl phos- 0. 5 Food Machinery phate, a plasticizer and leveling Mix together the ingredients of Part B. Add Part A to Part B and mix in a ball mill for about one hour. Mix Part C ingredients together and add to the combined Parts A and B and mix by stirring to form the aqueous coating solution. Apply the coating solution to a suitable substrate, such as paper or a metal sheet, of an electrophotographic recording element, as by flow-coating, and dry the coated substrate at about C. for about 2 minutes. During the heating and drying period, the encapsulant (Pliolite S-SE) of the EDSZO and the resin (Piccotex 131-119) of the aqueous photoconductive coating solution tend to fuse together to form the photoconductive layer of the electrophotographic recording element. The quantities of the ingredients of Example 3 may be varied so that the ratio of zinc oxide to the total quantity of resin (Pliolite S-SE and Piccotex 131419) in the aqueous coating solution is within the range of 23:1 by weight. The photoconductive layer so formed is superior in its sensitivity to visible light to photoconductive layers of the prior art formed from aqueous coating solutions.

The electrophotographic recording element so produced may be processed in the usual manner for producing electrophotographic images. For example, the electrophotographic recording element is first made sensitive to light by applying an electrostatic charge to the photoconductive layer in the dark. The electrostatic charge may be applied by any means known in the art, such as by a corona discharge device. The photoconductive layer is then exposed to a light image to discharge it selectively and to provide a conductivity pattern, that is, a latent image, thereon. The latent image may be developed by applying a pigmented resin powder of suitable electrostatic charge to the exposed photoconductive layer.

Another aqueous coating solution, employing EDSZO, for coating the conductive substrate of an electrophotographic recording element with an improved U-3101 UBATOL (14% solids by weight, water suspensions and acrylato polymer resin).

80.0 U.B.S. Chemical 00.,

Cambridge, Mass.

EXAMPLE 4.Conltinued PART B Weight in grams Durez 15546 (Rosin modified 12.5

polyester, a resin of 14% solids in an alkali-soluble solution).

Ingredients Supplier Durez Plastics and Chemical Corp, North End, Tonawanda, N.Y.

Part B is prepared as follows: Add 1 part 26 B. of ammonia to 43.6 parts by weight of cold water. Stir in 7.4 parts by weight of alkali-soluble resin (Durez 15546) using moderate but thorough agitation. Heat and add 1 part by weight of ammonia (26 B.) until solution is complete. Keep the solution cool until used.

PART

Ingredients Weight in grams Polyethylene emulsion (a resin, 14% solids in an aqueous emulsion).

PART D I Weight Ingredients in Supplier grains Igepal-Qo-QQO at aqueous 0. 6 Antara Chemical Ino.,

solutlon (nonylperoxypoly New York, N .Y.

(ethyleneoxy) ethanol, a

surfactant). KP-MO (tnbutoxy ethyl phos- 0.7 Food Machinery Corp.,

phate, plasticizer and leveling agent).

Make a mixture of the ingredients of Parts A, B, and C, adding the ingredients in the order indicated and using mild agitation for blending. Mix the ingredients of Part D with those of the blended Parts A, B, and C for at least 30 minutes to form a homogeneous mixture.

Add in the proportions of 7 g. of EDSZO, 5 ml. of water, and 5 ml. of the homogeneous mixture of Parts A, B, C, and D and mix until the EDSZO particles are evenly dispersed in the mixture, thus forming the desired aqueous photoconductive coating solution. This coating solution may be used to flow-coat any suitable conductive substrate to apply the improved photoconductive layer thereto. The quantities of the ingredients of Example 4 may be varied, as desired, to provide an aqueous coating solution in which the ratio of zinc oxide to the total quantity of resin in the solution is 23:1 by Weight, (Zinc oxideztotal resin).

Another substantially aqueous photoconductive coating solution having EDSZO particles dispersed therein may be made, in accordance with the present invention as follows:

New York, N .Y.

EXAMPLE 5 Component- Percent by weight Carnauba Wax 22 Stearic acid 2 Diethanolamine 1 Water 75 The diethanolamine is dissolved in the water and heated to 200-210 F. The stearic acid is dissolved in the wax at ZOO-210 F. The hot Wax solution is poured slowly into the hot water solution While maintaining vigorous agitation. The agitation is continued until the solution becomes smooth and homogeneous. Proportionately, 5 g. of EDSZO, prepared in accordance with Example 2, is dispersed in 10 ml. of the aforementioned homogeneous solution (emulsion) to form the aqueous photoconductive coating solution. This quantities of the ingredients in Example 5 may be varied so that the ratio of zinc oxide to the total quantity of resin (carnauba wax and Pliolite S-SE) in the coating solution is from 2-3z1 by weight, (zinc oxideztotal resin in coating solution). This coating solution can be applied to any suitable substrate, as by flow-coating, as described for the aqueous photoconductive coating solution of Example 3.

Each of the aqueous coating solutions described in Examples 3, 4, and 5 is essentially a dispersion of EDSZO in an aqueous solution (mixture, dispersion, or emulsion) of a resinous binding agent for the EDSZO. The aqueous coating solutions described in Examples 3, 4, and 5 differ from each other primarily in the type of resinous binding agent (natural and/or synthetic) used and in the type of wetting agents, leveling agents, and surfactants employed. Since the photoconductive dyesensitized zinc oxide herein described is encapsulated, the EDSZO may be dispersed in a large number of aqueous mixtures of resinous binding agents to provide an improved aqueous coating solution wherein desorption of the light-sensitizing dye into the Water vehicle of the coating solution is substantially prevented.

From the foregoing description, it can be seen that there has been provided improved photoconductive layers that can be deposited on suitable substrates from improved aqueous coating solutions. Although, to the naked eye, the photoconductive layers produced by the improved photoconductive coating solutions and methods herein described are somewhat similar in appearance to those of the prior art, they are superior in their lightresponsive characteristics to any derived from the aqueous coating solutions and methods of the prior art. Hence, the improved photoconductive layers can be described best by their method of manufacture.

Although the improved products and methods have been described with a limited number of examples, variations in the products and methods, all. coming within the spirit of the invention, will, no doubt, readily suggest themselves to those skilled in the art. Hence, it is desired that the foregoing shall the considered. as illustrative and not limiting.

What is claimed is:

1. An aqueous coating solution for forming a photoconductive layer on a substrate, said coating solution comprising particles of a resin-encapsulated dye-sensitized photoconductive zinc oxide dispersed in an aqueous mixture comprising a resinous binding agent, the sensitizing dye of said particles being water soluble, and the encapsulating resin of said particles being water insoluble.

2. An aqueous coating solution for forming a photoconductive layer on a substrate, said coating solution comprising particles of a resin-encapsulated dye-sensitized zinc oxide dispersed in an aqueous mixture comprising a resinous binding agent, the ratio of zinc oxide to the total quantity of the resin of said encapsulant and said binding agent in said coating solution being within a range of about 2 to 3:1 by weight, the sensitizing dye of said particles being water soluble, and the encapsulating resin of said particles being water insoluble.

3. An aqueous coating solution for forming a photoconductive layer on a substrate of an electrophotographic recording element, said coating solution comprising particles of encapsulated dye-sensitized zinc oxide dispersed in an aqueous mixture comprising a resinous binding agent, each of said particles comprising a core of photoconductive zinc oxide,

at least one water soluble light-sensitizing dye adsorbed to said core, and

a substantially water insoluble resin encapsulating said dyed core.

4. An aqueous coating solution for forming a photoconductive layer of an electrophotographic recording element, said coating solution comprising particles of encapsulated dye-sensitized zinc oxide dispersed in an aqueous mixture comprising a resin, each of said particles comprising a core of photoconductive zinc oxide,

at least one water soluble light-sensitizing dye adsorbed to said core, and

a substantially water insoluble resin encapsulating said dyed core, each of said particles having a diameter of about 1 micron.

5. The method of storming a photoconductive layer on a substrate of an electrophotographic recording element comprising the steps of mixing together zinc oxide particles, a water soluble light-sensitizing dye, a resin substantially insoluble in water, and a solvent for said resin, whereby to dye-sensitize said zinc oxide particles and coat the sensitized particles with resin,

drying the coated particles to provide resin-encapsulated, dye-sensitized, photoconductive zinc oxide particles,

dispersing said encapsulated particles in water to form an aqueous coating solution, and

coating said substrate with said coating solution to form said photoconductive layer thereon. 6. A method of forming a photoconductive layer on a substrate of an electrophotographic recording element, said layer comprising particles of resin-encapsulated dyesensitized zinc oxide wherein each particle comprises a core of zinc oxide, a water soluble dye adsorbed to said core, and a water soluble resin encapsulating said dyed core, said method comprising dispersing said particles of resin-encapsulated dyesensitized zinc oxide particles homogeneously in an aqueous mixture comprising a resinous binding agent, whereby to form an aqueous coating solution, the ratio of zinc oxide to the total quantity of resin encapsulant and said resinous binding agent in said coating solution being within a range of about 2 to 3:1, by weight,

applying said coating solution to said substrate to coat said substrate, whereby to form said photoconductive layer thereon, and

heating said substrate and said coating solution applied thereto to fuse said resin-encapsulated dye-sensitized zinc oxide particles with said resinous binding agent, whereby to dry said photoconductive layer.

7. A method of forming a photoconductive layer on a substrate of an electrophotographic recording element, said method comprising mixing together in a homogeneous mixture photoconductive zinc oxide particles, at least one water soluble light-sensitizing dye, a resin substantially insoluble in water, and a solvent for said resin,

drying said homogeneous mixture to form particles of resin-encapsulated, dye-sensitized photoconductive zinc oxide,

dispersing said particles of resin-encapsulated dye sensitized zinc oxide in water containing a resin binding agent to form a homogeneous aqueous coatng solution,

applying said coating solution to said substrate to coat said substrate, whereby to form said photoconductive layer thereon, and

heating said photoconductive layer to fuse said resinencapsulated particles with said resinous binding agent.

.8. A method of forming a photoconductive layer on a substrate of an electrophotographic recording element, said method comprising making a homogeneous mixture of resin-encapsulated dye sensitized zinc oxide particles dispersed in an aqueous mixture comprising a resinous binding agent, whereby to form an aqueous coating solution, flow coating said coating solution on said substrate to coat said substrate, whereby to form said photoconductive layer thereon, and drying said substrate and said photoconductive layer thereon, said particles comprising a core of photoconductive zinc oxide, at least one water soluble light-sensitizing dye adsorbed to said core, and a substantially water insoluble resin encapsulating said dyed core. 9. A method of forming a photoconductive layer on a substrate of an electrophotographic recording element, said method comprising mixing together in a homogeneous mixture photoconductive zinc oxide, at least one water soluble lightsensitizing dye, a resin substantially insoluble in water, and a solvent for said resin, spray drying said homogeneous mixture whereby to form particles of resin-encapsulated, dyed, photoconductive Zinc oxide, the ratio of said zinc oxide to said resin being in the range between 2 and 7.5 :1 by weight, dispersing said particles of resin-encapsulated dyed photoconductive zinc oxide in an aqueous mixture comprising a resinous binding agent, whereby to form a homogeneous aqueous coating solution, applying said coating solution to said substrate to coat said substrate, whereby to form said photoconductive layer thereon, and heating said photoconductive layer to fuse said resinencapsulated particles 'with said resinous binding agent. 10. A method of forming a photoconductive layer on a substrate of an electrophotoi'graphic recording element, said method comprising making a homogeneous mixture of resin encapsulated dye-sensitized zinc oxide particles dispersed in an aqueous mixture comprising a resinous binding agent, whereby to form an aqueous coating solution, flow coating said coating solution on said substrate to coat said substrate, whereby to form said photoconductive layer thereon, drying said substrate and said photoconductive layer thereon, said particles comprising a core of photoconductive zinc oxide, at least one water soluble light-sensitizing dye adsorbed to said core, and a substantially water insoluble resin encapsulating said dyed core, the ratio of zinc oxide to the total quantity of resin in said coating solution being in the range of about 2 to 3: 1, by weight. 11. A method of forming a photoconductive layer on a substrate of an electrophotographic recording element, from a substantially aqueous dispersion of particles wherein each particle comprises a core of zinc oxide, a water soluble dye adsorbed to said core, and a water insoluble resin encapsulating said dyed core, said method comprising making a homogeneous dispersion in the proportions of 7 g. of resin-encapsulated dye-sensitized zinc oxide particles, 5 ml. of water, and 5 ml. of a homogeneous mixture of g. of a 14% acrylate polymer resin water suspension, 12.5 g. of a 14% rosin modified polyester alkali-soluble solution, 7.5 g. of a 14% polyethylene aqueous emulsion, 0.6 g. of a 10% aqueous solution of nonylperoxypoly(ethyleneoxy)ethanol, and 0.7 g. of tributoxy ethyl phosphate, applying said homogeneous dispersion to said sub- 3,428,452 9 10 strate, whereby to form said photoconductive layer, 14. A method of forming a photoconductive layer on and a substrate of an electrophotographic recording element, drying said photoconductive layer. said method comprising 12. A method of forming a photoconductive layer on mixing together in a homogeneous mixture in the proa substrate of electrophotographic recording element, portions of 454 g. of photoconductive zinc oxide, 2 said method comprising making a homogeneous dispersion in the proportions 5 parts of resin encapsulated dye-sensitized zinc oxide particles, and

sensitized zinc oxide in an aqueous mixture comprising a resinous binding agent, whereby to form an aqueous coating solution, the total quantity of resinous binding agent in said coating solution being such as to provide, with said resin encapsulant, a ratio of 1 part of resin to about 2 to 3 parts of zinc oxide by weight,

applying said coatinlg agent to said substrate, whereby to form said photoconductive layer, and

drying said photoconductive layer.

ml. of a water soluble light-sensitizing dye comprising 1.2 g. of fluorescein disodium salt and 3.6 g. of Brilliant Blue FCF (Cl. 42090) in 100 ml. of methanol, 120 g. of a high styrene butadiene polymer resin, and 800 ml. of toluene,

ml. of about 22% wax emulsion in water, 10 drying and forming a powder of said homogeneous applying said homogeneous dispersion to said submixture, whereby to form particles of resin encapsustrate, whereby to coat it and to form said photolated dye-sensitized photoconducti've zinc oxide, the conductive layer, and ratio of said zinc oxide to said resin being between drying said photoconductive layer, each of said resin 2 and 7.5 :1 by weight,

encapsulated dye-sensitized zinc oxide particles comdispersing said particles of resin encapsulated dyeprising sensitized zinc oxide in an aqueous mixture coma core of photoconductive zinc oxide, prising a resinous binding agent, whereby to form a water soluble light-sensitizing dye adsorbed thereto, an aqueous coating solution, the total quantity of and resinous binding agent in said coating solution being a substantially water insoluble resin surrounding said such as to provide, with said resin encapsulant, a dyed core. ratio of 1 part of resin to about 2 to 3 parts of zinc 13. A method of forming a photoconductive layer on oxide, by 'weight, a substrate of an electrophotographic recording element, applying said coating agent to said substrate, whereby said method comprising to form said photoconductive layer, and

mixing together in a homogeneous mixture photocondrying said photoconductive layer.

ductive zinc oxide, at least one water-soluble lightsensitizing dye, a resin substantially insoluble in References Cited water, and a solvent for said resin, UNITED STATES PATENTS spray drying said homogeneous mixture whereby to form particles of resin encapsulated dyed photocon- 3,060,021 10/1962 GrPllg ductive zinc oxide, the ratio of said zinc oxide to 3,121,005 2/1964 lMlddleton et said resin being in the range between 2 and 7.521 by 3,198,647 8/1964 'F X 'weight, 3,212,887 10/1965 Miller et a1. 96--1.2 dispersing said particles of resin encapsulated d-ye- 3,238,149 3/1966 p 252501 FOREIGN PATENTS 888,371- 1/1962 Great Britain.

J. TRAVIS BROWN, Primary Examiner.

C. E. VAN HORN, Assistant Examiner.

US. Cl. X.R.