US2663636A

US2663636A - Electrophotographic plate and method of producing same

Info

Publication number: US2663636A
Application number: US95374A
Authority: US
Inventors: Arthur E Middleton
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1949-05-25
Filing date: 1949-05-25
Publication date: 1953-12-22
Anticipated expiration: 1970-12-22
Also published as: DE872427C; CH290324A; GB693112A; FR1018549A

Description

Patented Dec. 22, 1953 ELECTROPHOTOGRAPHIC PLATE AND METHOD OF PRODUCING SAME Arthur E. Middleton, Columbus, Ohio, assignor, by mesne assignments, to The Haloid Company, Rochester, N. Y., a corporation of New York No Drawing. Application May 25, 1949, Serial No. 95,374

13 Claims.

This invention relates to electrophotographic plates and to a method of producing such plates. More particularly, the invention relates to a composition of matter including a photoconducting insulating material and to an electrophotographic plate containing a coating of such composition. Specifically, the invention relates to a method of producing a composition of matter containing a photoconducting insulating material and to a method of making an electrophotographic plate cnated with such composition.

In the electrophotographicprocess, a base plate of low electrical resistance having aphotocom ductin insulatingcoating thereon is electrostati cally charged in the dark. The charged coating is then exposed to light through a negative photographic film, positive film or mask, or otherwise to a light image, to receive a latent electrostatic image. If the plate hasbeen properly prepared,

the charges leak off rapidly to the base plate in proportion to the intensity of light to which any given area is exposed, and after such exposure, powder is dusted on the coating in the dark and adheres to the areas where the electrostatic charges remain, forming a powder image correspondin to the electrostatic image. The powder image can then be transferred to a sheet of transfer material resulting in a positive or negative print as the case may be, having excellent detail and quality.

One object of the invention is to improve the a photoconductive characteristics or the plate and insure proper functioning of the plate when exposed, this being accomplished by producing a composition of matter containing a photoconducting insulating material which can be applied as a coating on a base plate in a practical and economical manner, and which adheres firmly thereto and acts successfully as a photoconducting insulating layer.

Another purpose of the invention is to provide a method for coating a base plate at room temperatures with a composition containing a photoconduoting insulating material, and which will adhere permanently and securely to the base plate and act efficiently as a photoconductin insulating layer.

A further object of the invention is to provide a composition of matter containing a ph'otoconducting insulating material and which will adhere readily and permanently to a base plate and act as a photoconducting insulating layer.

An additional purpose of the invention is to provide an improved and more efficient electrophotographic plate having on its surface an adherent coating comprising a, composition containing a photoconducting insulating material, and capable of acting as a photoconducting insulating layer.

It has now been found that an efiicient electrophotographic plate can be obtained by spraying a base plate at room temperature with a photoconducting insulating composition prepared by intimately mixing and grinding together a photoconducting insulating material, a high electrical resistance binder, and a solvent. On evaporation of the solvent, the composition forms a successful adherent layer on the plate that will hold electrostatic charges in the dark and rapidly lose them on exposure to light.

In general, the invention comprises intimately mixing and grinding together in a ball mill or other suitable fashion a photoconducting insulati'ng material such as pure amorphous selenium with a solvent and a binder material having an electrical volume resistivity of about 10 to about 10 ohms-cm, for a period of time sufficient to insure intimate mixture and reduction of the size of the particles. At the end of the milling period additional solvent may be added and stirred into the mixture suificiently to render it sprayable. The resulting composition can readily be sprayed at room temperatures onto a clean, unheated base plate. After the solvent has evaporated from the composition, the coating can be electrostatically charged and used in the electrophotographic process.

The selenium must be of the amorphous type and free of impurities such as copper, iron, lead and bismuth which appear to aiTect adversely its ability to hold electrostatic charges by decreasing its electrical resistance so that the charges leak off rapidly in the dark and powder images cannot be obtained. It is preferred to use amorphous A. R. Q. selenium (ammonia reduced in quartz from SeOz) in pellet form, from about 1 6-" to A size, as manufactured by the Canadian Copper Refiners Compan Limited, Montreal East, Ontario, Canada, and containing lessthan about 20 parts per million of impurities.

Other grades and types of selenium as furnished commercially can be used if purified and converted into the amorphous state. To obtain essentially purified amorphous selenium from the impure forms, it is the usual practice to first free it of the copper, iron, lead or bismuth by distillation and then to melt down the required amount of selenium in a container exposed to the atmosphere by heating to a temperature of from 250 C. to-300" C., and preferably about 250 C. The

low-boiling point hydrocarbon solvent, and like the binder should not introduce impurities which would lower the electrical resistance of the coating. The solvent should preferably be one that is suitable for the particular binder empl since the solvent must not react with the binder material to a 3; fierized product or similar substances which cannot be sprayed. The solvent should not affect the adhesiveness of the binder or its electrical resistance, nor should it adversely affect the pure amorphous selenium. Part of the solvent is added to the selenium binder in the mill to provide good grinding viscosity and the balance may be added after milling to increase the fluidity of resulting mixture. The total amount of solv used is from about to about 3 parts by volume of the sol vent to about 1 part by volume of the binder and this proportion has resulted in readily sprayable compositions. It is preferred, however, to use from about 0.8 to about 1.4: parts per volume of solvent to about 1 part by volume of binder in the composition. Suitable solvents for use with the binders disc osed herein are toluene, Cellosolve acetate, ;.y no, gasoline, fimsco F, benzene, methyl iso-butyl get-one or mixtures thereof.

Cellosolve acetate is an organic chemical whose formula is CzHsOCHzCHzOOCCHs. Amsco F is an aromatic petroleum solvent manufactured by American Mineral spirits Company, Chicago, 111., with the following approximate characteristics: specific gravity (60 F.) 0.8028, 77 per cent aromatics, refractive index of 1.4905, mixed aniline point of 86 F., flash point 130 F., and Kouri butanol value of 70. When less solvent is present, it may be necessary to warm the composition slightly in order to spray it. The temperature of the composition should, however, not exceed 50 C. in order to obviate the production of the hexagonal form of selenium.

The base material or plate should have an electrical resistance less than either the binder material or the amorphous selenium so that it will act as a ground when the electrostatically charged coating is exposed to light. The base plate should be at room temperature and at least not above about 50 (3., while being sprayed with the photoconducting insulating composition, in order to prevent formation of any crystals of hexagonal selenium in the coating which would reduce or destroy its electrophotographic usefulness. Materials having electrical resistivities of less than 10 ohms-cm. are satisfactory as a plate for the purposes of the invention, although it is preferred to use materials having an electrical resistivity of less than about 10 ohms-cm.

The base plate is generally flat, square or rectangular but can be of any desired shape. The surface of the plate should be cleaned before coating with the selenium composition in order to remove grease and other dirt which might prevent firm adherence of the coating to the base plate. Such cleaning can be accomplished by washing the plate with any suitable alkali cleaner, or with a hydrocarbon solvent, followed by rinsing. Any gross surface irregularities should be removed by grinding or polishing, although it is not necessary to polish the plate until it has mirror-like reflectivity. Acceptable materials for the base plate have been found to be aluminum,

brass, glass, aluminum-coated glass, stainless steel, nickel, steel, bronze, copper, engravers copper, engravers zinc, and grained lithographic zinc, although other materials having electrical resistances similar to the aforementioned can also be used as base plates to receive the composition coated thereon.

It is not necessary to add the selenium to the binder in the mill in the form of finely divided particles, but it can be added in the form of pellets of about 1 5" to diameter. The selenium, binder and solvent should be thoroughly mixed and ground up and as a result the selenium is reduced in size and thoroughly dispersed throughout the hinder or binder and solvent so that no surface irregularities or masses of unmixed particles appear in the coating on low magnification. It is preferred to mix and grind, or mill, the mass until the particles in the composition have a size less than about 50 microns, although compositions having particles up to about .0002 inch in size have proven satisfactory. Instead of adding the amorphous selenium in pellet or pulverized form to the mill, followed by separate additions of binder and solvent, a solution of the binder in the solvent can be added to the selenium in the mill. It is only essential that enough solvent be present during milling to give good grinding viscosity.

Any commercially available mixing and grinding machinery can be utilized for this step, although it is desirable to use a ball mill, and at room temperature. The mill is not heated or cooled, although while the room temperature is about C., it is believed that the composition adjacent the balls may have a temperature of about C. due to frictional heat developed by the balls during grinding. Ceramic, glass, or steel balls can be used in the mill. Steel balls, however, have been found best as they produce a better mixing and grinding action. The time of mixing can vary from 3 to 72 hours, although it has been found preferable to mix for about 10 to 20 hours to reduce the particles to approximately microns in size. The mixing equipment should also be thoroughly cleaned before mixing to prevent the introduction of any impurities which might adversely affect the resulting selenium composition.

At the end of the milling period, additional solvent is added to the mixture and the mill operated for a short time to remove portions of the mixture adhering to the walls and balls of the mill and to enable it to be readily poured. This insures sufficient fluidity so that the composition can be easily sprayed, and provides a complete dispersion of particles in the solvent carrier or a homogenous solution or mixture. More than per cent recovery of materials is thereby made possible, and while the mixture at the end of the milling period actually contains a dispersion of selenium in the binder and can be used to coat plates to provide a photoconducting insulating layer thereon, is apparent that additional solvent is indicated as a means to facilitate spraying or coating of the plates.

Commercially available paint-spraying equipment can be used to spray the composition onto the base plate which is at room temperature. The

. paint-spraying mechanism should be thoroughly cleaned prior to use to prevent introduction of harmful impurities into the coating mixture. It should also provide means for heating the composition when not sufliciently fluid. The equipment should be flexible and provided with adjusting means to enable the operator to readily produce on the base plate coatings having thicknesses of from .0003 to .002 inch. Thicker coatings can be applied by longer spraying if desired, although thicker coatings increase the exposure 7 time:fartherelectrophotographic plate. If the coating is too thin, pinholes are apt to occur so that a continuousfilrn is not produced. Thus, coatings as thin as .0003 inch represent about the practical lower'limitfor spraying. It will be obviousto those skilled in the art that other methods besides spraying can be utilized to produce coatings of the composition on the base plate, as for example,. the composition can be applied by means of a brush or draw blade, by dipping, or by roller coating.

The following examples will serve to illustrate the invention with more particularity to those skilled. in the art:

Example 1 .200 grams of pelletized amorphous, black pure selenium, 4414 grams of Parlon, 19.2 grams of Rezyl and '90 grams of toluene were placed in a quart (Bacilli') size steel ball mill filled with inch steel balls up to about the half-way mark and milled for about fourteen hours. After mixinc; about 120 grams of additional toluene was added to the mixture to render it sprayable and the resulting red composition placed in a paint sprayer and sprayed onto a clean, fiat unheated aluminum plate. As soon as the plate had been coated with a thin film of the composition about .0005 inch thick, the spraying was stopped and the coating allowed to dry by evaporation of the solvent. The hard, dry coating was completely adherent to the base plate. The plate was then charged electrostatically and used in the electrophotog'raphic process, resulting in prints having excellent detail and no background.

Example 2 Component: Amount, gms.

Amorphous selenium 100 Vinylite VYHH 15.2 Vinylite VMCH 15.2 Methyl isobutyl ketone 60 Toluene 60 The ingredientswere charged to a pint-size glass jar approximately half filled with inch glass balls and ground for approximately 20 hours. After milling, the mixture was drained from the jar and thinned to spray viscosity with a 50:50 mixture of toluene and methyl isobutyl ketone.

Example 3 Component: Amount, gms.

Vitreous selenium 100 Polystrene 23.8

The components were charged to a pint-size glass jar filled approm'mately half full with inch glass balls, sumcient xylene being added to give grinding viscosity (50-100 seconds No. 4 Ford cup), and then ground for approximately 20 hours. After milling the mixture was drained from the mill and thinned with xylene to spraying viscosity.

Example 4 Component: Amount, gms.

Vitreous selenium 1G0 Acryloid A10 89.2

The components were charged to a pint-size glass jar filled approximately half full with /2 inch glass balls, thinned to suitable grinding viscosity with Cellosolve acetate, and ground for '72 hours. The mixture was then drained from the mill and thinned to spraying viscosity.

Example 5 Component: Amount, gms.

Vitreous seleniurnrn; o Acryloid B-72 31.3 Toluene 100 The selenium, acryloid and toluene were charged to a steel ball mill filled approximately half full with /2 inch steel balls and ground for 24 hours. The composition was then drained from the mill and thinned with additional toluene to spraying viscosity.

Example 6 Component: Amount, gms.

Amorphous selenium Silicone (DC-804) 45.8

The ingredients were charged to a steel ball mill approximately half filled with ,4 inch steel balls, toluene was then added to give suitable grinding viscosity, and the composition was ground for 42 hours. The resulting material was then drained from the mill and thinned with toluene to spraying viscosity.

X-ray studies were made of electrophotographic plates containing the photoconducting insulating coatings described herein, and revealed that some coatings appeared to contain only amorphous selenium in the binder, others apparently contained only alpha monoclinic selenium, while still others seemed to contain both amorphous and alpha monoclinic selenium dispersed throughout the binder. All of the plates, however, produced excellent powder images when used in the electrophotographic process. This appears to indicate that coatings containing the alpha monoclinic form of selenium, and those containing both the alpha monoclinic and amorphous forms of selenium are as effective as coatings containing only amorphous selenium as the photoconducting insulating material or active ingredient.

The presence of alpha monoclinic selenium in the coatings and compositions is indicated from the fact that the black, amorphous selenium mixture turned red during milling, since alpha mono,- clinic selenium is red although this is not conclusive because very finely divided amorphous selenium is also red. It is not thoroughly understood why alpha monoclinic selenium appears in the composition after milling, although it would appear that the milling, or the other components present during milling, or a combination of these factors, favor formation of the alpha monoclinic form. In any event, however, compositions or coatings containing only alpha monoclinic selenium as the active material, or alpha monoclinic selenium in the presence of amorphous selenium, seem to produce as efiective plates as those containing essentially only amorphous selenium.

The invention teaches that electrophotographic plates can be readily produced by spraying a base plate with a composition prepared by milling amorphous selenium, a high electrical resistance binder, and a solvent, to which mixture has been added, after milling, additional solvent to render it sprayable. The finely divided particles of selenium appear to be thoroughly dispersed throughout the high electrical resistance binder and are probably coated with the binder material or substantially insulated from each other by the binder. The coated plate, or electrophot-ographic plate, on drying can be charged electrostatically and will hold electrostatic charges in the dark and rapidly dissipate them on exposure to light.

The invention is also applicable to the making of photocell and rectifier plates, and this application is intended to cover such modifications, departures, and adaptations as may come Within the scope of the following claims.

I claim:

1. An electrophotographic plate in which the photoconducting insulating coating includes at least one material of the group consisting of 2. An electrophotographic plate comprising a base plate and an adherent coating on one surface thereof, said coating comprising essentially a mixture of amorphous selenium uniformly dispersed in a binder comprising a continuous film of uniform thickness of thermoplastic synthetic resin binding together the individual particles of selenium and holding the layer on the plate, said binder having an electrical resistance higher than that of the plate and selenium.

3, An electrophotographic plate comprising a base plate and an adherent coating on one surface thereof, said coating including a mixture of alpha monoclinic selenium uniformly dispersed in a binder comprising a continuous film of uniform thickness of thermoplastic synthetic resin binding together the individual particles of selenium and holding the layer on the plate, said binder having an electrical resistance higher than that of the plate and selenium.

4. An article of manufacture consisting of a base plate and an adherent photoconducting insulating coating thereon, said coating including a mixture of equal parts by volume of substantially pure amorphous selenium uniforml dispersed in a binder comprising a continuous film of uniform 1.

thickness of thermoplastic synthetic resin binding together the individual particles of selenium and holding the layer on the plate, said binder having an electrical resistivity of from 10 to 10 ohms-cm.

5. A coated plate, the coating thereof containing essentially finely divided particles of amorphous selenium and alpha monoclinic selenium dispersed throughout a continuous phase of a binder comprising a continuous film of uniform thickness of thermoplastic synthetic resin binding together the individual particles of selenium and holding the layer onto the plate, said binder having an electrical resistivity of from 10 to 10 ohms-cm.

6, An article of manufacture comprising a base plate having an electrical resistivity less than about it) ohnis-cm. and a. photoconducting in sulating coating on the surface thereof, said coating including a mixture of from 1 t0 2 parts by volume of a binder comprising a continuous film of uniform thickness of thermoplastic synthetic resin binding together the individual particles of selenium and holding the layer onto the plate, said binder having an electrical resistivity of from to 10 ohms-cm. and 1 part by volume of at least one material of the group consisting of essentially pure amorphous selenium and alpha monoclinic selenium.

7. A composition of matter useful in the production of electrophotographic plates, including a solution of an organic, easily evaporated solvent, a binder selected from the group consisting of polystyrene, chlorinated rubber, silicone resin, acrylic resin, and vinyl-chloride-acetate resin, and at least one material of the group consisting of substantially pure amorphous selenium and alpha monoclinic selenium uniformly dispersed throughout said binder which secures together "ill the individual particles of selenium and holds.

the layer on the plate, the binder being substantial in volume and having a resistance higher tllian that of said last named material and of the p ate.

8, A composition of matterincluding a binder comprising a thermoplastic synthetic resin having an electrical resistivity of from 10 to 10 ohms-cm, an organic, easily evaporated solvent for said binder, and finely divided particles of amorphous selenium uniformly dispersed throughout said binder.

9, A composition of matter including from 1 to 2 parts by volume of a binder comprising a thermoplastic synthetic resin having an electrical resistivity of from 10 to 10 ohms-cm, from 1.6 to 2.8 parts by volume of an easily evaporated hydrocarbon solvent for said binder, and about 1 part by volume of alpha monoclinic selenium of a particle size of from 1 to 50 microns, said selenium being uniformly dispersed throughout said binder.

10. The method of producing a composition of matter which is insulating and photoconducting and adherent after being applied as a coating to a base plate, which comprises intimately mixing and grinding in a ball mill for from 3 to '72 hours substantiall pure amorphous selenium, a binder comprising a thermoplastic synthetic resin having an electrical resistivity of from 10 to 10 ohms-cm, and an organic, easily evaporated solvent.

11. In the method of producing a composition containing selenium characterized by being adherent, photoconducting and insulating after being sprayed onto a base plate and drying, the step consisting of milling in a ball mill for from 10 to 20 hours one part by volume of amorphous selenium containing not greater than about 20 parts per million of impurities and from 1 t0 2 parts by volume of a binder comprising a thermoplastic synthetic resin having an electrical resistance higher than that of the plate and selenium and selected from the group consisting of Parlon, polystyrene, silicone, acryloid, and vinylite copolymer, and an organic, easily evaporated solvent, the type of mill specified and the conditions set forth being such as not to promote crystallization of the selenium and after said milling, adding an additional amount of said solvent to the resulting mixture to provide a homogeneous mixture and to enable said mixture to be sprayed.

12 In the method of producing an electrophotographic plate, the steps comprising milling in a ball mill for from 10 to 20 hours, essentially pure amorphous selenium, an organic, easily evaporated solvent, and a binder comprising a thermoplastic synthetic resin having an electrical resistivity of from 10 to 10 ohms-0111., the type of mill specified and the conditions set forth being such as not to promote crystallization of the selenium, mixing an additional amount of said solvent in the resulting mixture to provide a homogeneous mixture and to enable said mixture to be sprayed, and then spraying the mixture containing the solvent onto a base plate, having an electrical resistivity less than about 10 ohms-cm. to provide a photoconducting insulating layer thereon.

13, In the method of producing an electrophotographic plate, the step comprising spraying a base plate at room temperature with a composition of matter including a binder comprising a thermoplastic synthetic resin having an electrical assess 11 resistivity of from 10' to 16 ohms-cm, a volatile Number hydrocarbon solvent, and at least one material of 2,168,259 the group consisting of substantially pure amor- 2,178,018 phous selenium and alpha. monoclinic selenium, 2,197,552

said. material having a particle size of from 1 to 5 2,199,104 50 microns and being dispersed throughout said 2,297,691 binder. 2,433,401

Name 1 D Gorlich .i Aug. 1, 1939 Hofmann Oct. 31, 1939 Kuymick Apr. 16, 1940 Johnson et a1 Apr. 30, 1940 Carlson Oct.- 6, 1942 Saslow Dec. 30, 194! Korn et a1 Feb. 24, 1948 Gray Oct. 25, 1949 FOREIGN PATENTS Country Date Germany fin-. 6. Sept. 18, 1942

Claims

1. AN ELECTROPHOTOGRAPHIC PLATE IN WHICH THE PHOTOCONDUCTING INSULATING COATING INCLUDES AT LEAST ONE MATERIAL OF THE GROUP CONSISTING OF AMORPHOUS SELENIUM AND ALPHA MONOCLINIC SELENIUM IN A BINDER COMPRISING A CONTINUOUS FILM OF UNIFORM THICKNESS OF THERMOPLASTIC SYNTHETIC RESIN WHICH AFFORDS AN EFFECTIVE MEDIUM WHICH UNIFORMLY DISPERSES SAID SELENIUM AND OBTAINS GOOD ADHERENCE OF THE COATING TO THE PLATE, SAID BINDER HAVING AN ELECTRICAL RESISTANCE HIGHER THAN THAT OF THE PLATE AND PHOTOCONDUCTING INSULATING MATERIAL.