US3481735A - Polymeric binders for electrophotographic coating applications - Google Patents

Description

United States Patent 3,481,735 POLYMERIC BINDERS FOR ELECTROPHOTO- GRAPHIC COATING APPLICATIONS Richard B. Graver, Savage, and Stephen C. Heidecker and David D. Taft, Minneapolis, Minn., assignors, by mesne assignments, to Ashland Oil and Refining Company, a corporation of Kentucky No Drawing. Filed Feb. 23, 1966, Ser. No. 529,247 Int. Cl. G03g 7/00, /00 U.S. Cl. 961.5 18 Claims ABSTRACT OF THE DISCLOSURE acid, and (3) a vinyl monomer free of either hydroxy or carboxy substituents are disclosed as binders for use in electrophotographic coating applications.

The present invention relates to electrophotography. In one aspect, the present invention relates to the use of polymers of hydroxyalkyl esters of acrylic or methacrylic acid as binders for zinc oxide in manufacturing coated copy paper. In still another aspect, the present invention relates to the use of such coated copy paper in direct electrophotographic processes.

Electrophotography is one of several terms used to describe a reproduction or image transfer process utilizing electrical and light stimulation of conductive materials. Some common terms for commercial processes based on this technique are Electrofax, electrostatography, electrography, xerography, and elcctrographic recording. These processes all employ electromagnetic radiation and a photo-responsive member to obtain, on exposure to light, a latent electrostatic image on the photo-responsive member. Ordinarily, this latent electrostatic image is then converted into a positive image, off-set master, etc. Such tech niques areused in commercial off-set printing, computer readout, film developing, map making, long-distance copying, recording, oflice copying, and the like.

There are two general types of electrophotography which'have been commercially successful in the office and industrial copying field. These types, although using the same basic principle, differ in the type of photoresponsive member employed. These types of electrophotography are the direct process and the transfer process.

THE TRANSFER PROCESS The transfer process relies upon the photoconductive properties of amorphous selenium. This process operates in the following manner:

(1) A selenium-coated aluminum drum is given a uniform electrostatic charge by exposure to a corona discharge in the dark.

(2) A light image of the object to be copied (i.e. the object image) is then projected through a lens system in such a manner that the light strikes the charged drum, dissipating the previously-applied charge in the nonimaged area. As a result, the drum then carries an electrical charge in those areas corresponding to, for example, printed matter contained on the object being copied (e.g. a page of a book).

(3) The charged latent image is then developed by cascading an oppositely charged dry toner (or ink) across the surface of the drum. The powdered toner clings to the oppositely charged areas on the drum by electrostatic attraction. In this manner, a powder or toner image of the object being copied is formed on the selenium-coated drum.

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(4) Finally, a sheet of ordinary paper (i.e. paper not coated with a photoconductor) is placed over the powder image and given an electrostatic charge by the use of a corona discharge. The charged powder or toner is then transferred from the drum to the paper.

(5) The paper is then heated to, for example, 200 C. to fuse the powder on the surface of the paper and produce the fixed image.

THE DIRECT PROCESS The direct process utilizes the same principle as the transfer process, except that a selenium-coated drum is not used. Instead, the image is directly produced on, for example, paper. The basis for this technique is the use of coated paper or the like containing, on the surface thereof, a finely-divided photoconductive material. Typically, zinc oxide is employed as the photoconductive material and is bonded to the paper 'by the use of some suitable organic binder. Often, dyes are used to sensitize the zinc oxide. The electrical conductivity of zinc oxide shows a significant increase when subjected to light. Consequently, the zinc oxide will lose any previously applied electrical charge when exposed to light.

The direct process operates as follows:

(1) The coated paper is made light sensitive by placing a uniform electrostatic charge on the coated surface in the dark by means of a high voltage corona discharge, e.g. 4000 to 6000 volts. The coated paper is not sensitive to light until properly charged.

(2) A latent image is produced on the paper by projecting an object image onto the paper, typically employing white light in the visible range. The original electrostatic charge carried by the paper is dissipated in all areas exposed to light and is retained in the shadowed areas, thus forming a latent electrical image on the paper.

3) Development of the latent image is similar to that described for the transfer process, except that commercial equipment is available which utilizes both liquid and dry toners. In the dry method, the toner (consisting of pigmented resin and ferromagnetic particles, e.g. iron filings) is applied to the paper by a magnetic brush. As the brush moves across the paper, the charged toner is attracted to the charges of opposite polarity on the paper. In the liquid method, the pigmented resin particles are suspended in an organic liquid such as odorless mineral spirits. This liquid is then brought in contact with the paper.

(4) Two methods are utilized to permanently fix the image to the paper. With the dry toner system, the image is fixed by heat (below the charring temperature of the paper) which fuses the toner to the coated paper. Fixing the image in the liquid toner system is accomplished by heat and solvent evaporation.

Multi-color copying can be conducted by re-charging the coated copy paper for each new color and utilizing the proper color of toner. This technique is currently applied in map copying.

The binder for the zinc oxide (or other photoconductor) is extremely important to the success of the electrophotographic process. The binder should be one into which the zinc oxide (or other photoconductor) can be dispersed and-the mixture then applied to paper as a coating. The binder must not interfere with the photoconductive properties of the zinc oxide. The binder should adhere strongly to the paper and provide a flexible coating. Since the binder is ordinarily applied to a sheet of paper, it should be able to withstand 180 bends and wrinkling without cracking or chipping. The binder, when mixed with zinc oxide and applied to a paper substrate, should be capable of providing pleasing, clear, legible copies. To enable the photoconductor to accept a maximum electrostatic charge, it is desirable that the individual photoconductor particles be separately encapsulated by the binder.

For this latter purpose, the binder should be capable of properly wetting the photoconductor and firmly positioning it on the paper in the photoconductive matrix. Initially, the binder must be an extremely good insulator and prevent any significant decay of the electrostatic charge on the paper (before the photoconductive coating is exposed to the light of the object image). Additionally, the binder should not interfere with the rapid dissipation of the electrostatic charge when the coated paper is ex osed to the light of the object image. Desirably, the binder should not accept or retain any appreciable residual voltage in the exposed area. The binder should not exhibit color or decompose on aging. The binder should bond readily to the pigmented developer or toner and give rise to good printing quality. Desirably, the binder should be effective when used at very low coating weights. Additional properties which the binder should exhibit are nonyellowing tendencies, resistance to solvents (if the coated paper is to be used in combination with a liquid toner) and a resistance to flowing at the fixing or fusing temperature of the toner. Still further, the electrical properties of the coated paper should not be significantly affected by changes in humidity.

At the present time, a variety of binders have been suggested for use in manufacturing paper for direct electrophotography. In certain instances, some acrylic resins have been utilized as modifiers for the more conventional binders. However, acrylic resins have not been used to any significant extent as the primary or sole binders because of certain undesirable properties which they exhibit. For example, in the wet toner process, the use of high molecular weight acrylic resins has increased what is known as solvent holdout. The increased solvent holdout did not allow a suitable fusing of pigment particles to the surface of the exposed paper. Consequently, the pigmented particles did not adhere and smeared. The resulting copies were of poor quality.

It has now been discovered that a specific type of acrylic copolymer gives outstanding physical and electrical properties when used as a binder, even in the wet toner process. These copolymers contain (1) from 1 to 30% by weight (preferably 10 to 20% by weight) of a hydroxyalkyl acrylate or methacrylate, or mixture thereof, (2) from 1 to 15% by weight (preferably 1 to 5% by weight) of a copolymerizable a,/3-unsaturated carboxylic acid, or mixture thereof, and (3) the balance to make 100% of one or more copolymerizable monoethylenically unsaturated vinyl monomers which are devoid of hydroxyl groups. The copolymers can be prepared from these monomers by conventional techniques as shown in, for example, US. Patents 2,681,897, 3,082,184, and 3,198,850. The copolymers can be used as binders both with and without cross-linking agents such as the various aminoplasts (e.g. an melamine-formaldehyde resin) and phenolis (e.g. phenol-formaldehyde resins).

One of the classes of monomers used in preparing copolymers for use in this invention consists of the hydroxyalkyl esters of vinyl carboxylic acids. Suitable hy droxyl-containing esters are the C C hydroxyalkyl esters of acrylic and methacrylic acids, as well as mixtures thereof. Typically, the hydroxyl groups will be in the beta (i.e. 2), gamma (i.e. 3), etc., position. Ordinarily, beta-hydroxyalkyl esters of acrylic or methacrylic acids will be used. Suitable hydroxyalkyl esters are fl-hydroxyethyl acrylate and methacrylate, fi-hydroxypropyl acrylate and methacrylate, ,B-hydroxyhexyl acrylate and methacrylate, fi-hydroxydecyl acrylate and methacrylate, 12- hydroxystearyl acrylate and methacrylate, and the like. Hydroxypropyl acrylate and methacrylate, as well as mixtures thereof, are the preferred hydroxyalkyl esters. Hydroxypropyl methacrylate is especially preferred.

A second class of monomers used in preparing the copolymers consists of the copolymerizable carboxylic acids. Any copolymerizable unsaturated carboxylic acid can be used in preparing copolymers for use in the present invention. Suitable acids include maleic acid, fumaric acid, itaconic acid, citraconic acid, crotonic acid, acrylic acid, methacrylic acid, and the like. Acrylic acid, methacrylic acid, and itaco nic acid are preferred. If desired, mixtures of these acids can be used. Often, certain of these acids are present in commercially available hydroxyalkyl esters. Thus, a single commercial raw material can provide two of the three classes of monomers needed to form the copolymers used in this invention.

A third class of monomers used in preparing the copolymers consists of other mono-ethylenically unsaturated compounds which are free of hydroxyl and carboxyl groups and which are copolymerizable with the first two monomer classes hereinbefore mentioned. Other monoethylenically unsaturated compounds copolymerizable with the hydroxyl-containing and acidic monomers are the C C cyclic and acyclic esters of acrylic and methacrylic acids, acrylonitrile, methacrylonitrile, styrene, ochlorostyrene, vinyl toluene, a-methyl styrene, etc. If desired, mixtures of these vinyl monomers can be used. The hardness, flexibility, and adhesion of the copolymers to various substrates can be varied by adjusting the proportions of these various vinyl monomers. Styrene and the C -C alkyl acrylates and methacrylates, as well as mixtures thereof, are preferred.

For coating paper, the copolymers of this invention can be optionally combined with aminoplast or phenolic resins. Suitable aminoplast resins include the alkylated and non-alkylated condensates of an aldehyde with urea, N,N-ethylene urea, dicyandiamide, and aminotriazines. If water soluble condensates are preferred, as in combination with an emulsion or water-soluble copolymer, the non-alkylated or partially alkylated water soluble aminoplasts are preferred over the more fully alkylated aminoplasts. If a solution copolymer is utilized, alkylated aminoplasts soluble in the organic solvent of the copolymer are preferred. Phenol-formaldehyde resins can also be used. The amount of aminoplast or phenolic resin used in conjunction with the copolymers will usually be from 5-100%, e.g., 1050%, based on the weight of the copolymer.

In preparing coating or binder compositions for application to a suitable substrate (e.g., paper) a photoconductive material is dispersed in a solution or emulsion of the copolymer by suitable grinding techniques. Typical photoconductive materials are zinc oxide, zinc sulfide, silver chloride, mercuric sulfide, and other photoconductive materials known to the art. Zinc oxide is the preferred photoconductor. The zinc oxide is usually dispersed at a concentration of 4-16 parts by Weight of zinc oxide per one part of copolymer. This mixture is optionally combined with the aminoplast or phenolic resin. Preferably, the zinc oxide is dispersed or mixed at a level of 6 to 12 parts by weight of zinc oxide per one part of copolymer, and more preferably 8-10 parts by weight of zinc oxide per one part of binder.

If an emulsion copolymer is utilized as the binder, the zinc oxide can first be dispersed in a suitable dispersing aid. Then, this dispersant mixture can be diluted with the copolymeric emulsion in a manner conventional to the art of pigment dispersion.

In any case, the final binder or coating compositions will usually have a solids content (i.e., non-volatile content) of from 4070%, preferably 50-60% (the remainder being solvent or water).

In order to render the dry, cured photoconductive coatings more receptive to a charge and a photoconductive effect, suitable dyes such as rose bengal, methylene blue, rhodamine B, dibromo-fluorescein, and various cyanine dyes can be included in the binder or coating compositions. Usually a mixture of dyes is added, incorporating complementary colors so that the dried coatings have a pleasing, off-white appearance. These techniques and others similar to the art of electrophotographic binder compositions can be applied to the coating or binder compositions of this invention.

The coating compositions, containing the zinc oxide, the copolymer, solvent or water, and optionally an aminoplast or phenolic resin, are then applied to a suitable substrate (usually paper, although metal, foil, etc., can be used) in any suitable fashion such as by brushing, spraying, dipping, roller coating, or the like to give a coated paper having an average of from 5-50 pounds, e.g., 8-30 pounds of deposited dry coating per 3000 square feet of coated paper surface. The wet coated paper can be air dried at room temperature or dried by baking, e.g., baked at 200350 F. The preferred drying temperature will depend on whether or not an aminoplast or phenolic resin is present. Without an aminoplast or phenolic resin, a curing temperature in the range of 150250 F., preferably l90210 F. will ordinarily be used. In the presence of an aminoplast or phenolic resin, higher temperatures are usually required to obtain maximum benefits. The addition of small portions (0.1 to 1%) of an acidic catalyst such as toluene sulfonic acid or phthalic acid serves to lower the temperature required for proper curing.

The present invention will be further understood by reference to the following specific examples which include a preferred embodiment. Unless otherwise indicated, all parts and percentages are by weight.

Example 1 16 parts of methyl methacrylate, 26.6 parts of styrene, 19.9 parts of butyl methacrylate, 19.9 parts of butyl acrylate, and 37.4 parts of a commercially available xylene solution containing 40% of hydroxypropyl methacrylate and 6% of methacrylic acid were mixed together. A mixture of 1.25 parts of azo-bis-isobutyronitrile and 0.2 part of di-tertiary butyl peroxide was then added to the mixture of monomers. The resulting mixture was then sparged with nitrogen and then added dropwise during a 2 /2 hour period at 250 F. to a solution of xylene and n-butanol contained in a 2-liter, three-necked, glass flask equipped with an agitator, condenser, nitrogen inlet, thermometer, and addition tube. During this addition, the xylene/n-butanol solution (which had been carefully sparged with nitrogen) was agitated. Including the time required for addition, the reaction mixture was heated at 250 F. for 8% hours. The resulting mixture was then cooled. This mixture had a viscosity of 6 stokes and an acid value of 12 at a non-volatile content of 49.9%. Of the 50.1% volatile (i.e., solvent) portion of this mixture, 90% was xylene and 10% was n-butanol.

A 250 gram portion of zinc oxide (American Zinc Sales AZOZZZ661) was then added to 50 grams of the copolymer solution just prepared to give a weight ratio of zinc oxide to copolymer of about 10: 1. The zincoxide was dispersed in the solution by agitation for 5 minutes in a Hamilton-Beach mixer. The dispersion was then diluted with toluene to a 58% concentration of pigment and resin (i.e., 42% solvent).

Next, the following mixture of dyes was added to sensitize the zinc oxide/copolymer composition:

0.086 gram of a 1% solution in methanol of Hidacid Eosine (Hilton-Davis) 0.986 gram of sodium fluorescein (1% solution in methanol) 0.357 gram of Alphazurine 2G (Allied Chemical) (1% solution in methanol) 0.571 gram of Alizarine Cyanine Green GWA (Gen.

Aniline & Film) (1% solution in methanol) The resulting coating or binder composition was then applied to paper at a rate sufficient to give 22, 15, 10, and 8 lb. of dry coating/ 3000 sq. feet, using the appropriate coating rod for this purpose. The wet, coated sheets were baked at 120 F. for 5 minutes. Then the dry, tack-free coated sheets were allowed to stand in the dark for 24 hours at a humidity of 50% at 77 F.

TABLE I Light Charge Dark decay Residual Dry Coating Wt. acceptance, deca time, charge, (lbs/3000 sq. ft.) volts volts seconds volts In addition to giving excellent image quality, the copy paper (i.e. the coated sheets) exhibited excellent adhesion and outstanding mar resistance. The coated sheets also displayed unusual pre-fogging properties in that light exposure immediately prior to charge acceptance did not affect the amount of charge acceptance.

Most important was the ability of the coated sheets to provide copies of excellent contrast, image density, and spectral response especially at the low coating weights. This high quality copy was obtained, despite the relatively low charge acceptance of the coated sheet.

Examples 2 and 3 In Example 2, a mixture of 177.6 grams of methyl methacrylate, 201.6 grams of styrene, 272.4 grams of butyl methacrylate, 272.4 grams of butyl acrylate, 97.2 grams of acrylic acid, and 147.5 grams of a 96% solution of hydroxypropyl methacrylate in xylene (which also contained 4.8 grams of methacrylic acid), 24 grams of azobis-isobutyronitrile and 12 grams of di-tertiarybutyl peroxide was added to 340 grams of ethyl Cellosolve in an autoclave. The monomer/catalyst mixture was added over a two-hour period to the autoclave which was maintained at a temperature of 345 350 F. The pressure increased to 100 p.s.i.g. during the addition. Additional solvent (122 grams) was added to obtain a 70% NV solution. A booster catalyst of 3.5 grams of di-tertiarybutyl peroxide was added four hours after the monomer addition was completed. The total reaction time (including monomer addition) was ten hours. The copolymer had a final viscosity of 14 stokes at 70.2% NV. The acid value of the solution was 45.6. An 800 gram portion of this copolymeric solution was mixed with 239 grams of ethyl Cellosolve, 41.5 grams of triethyl amine, and 280.5 grams of water. The resulting copolymeric dispersion had a viscosity of 4100 cps. (Brookfield RVT viscometer using a #4 spindle at .20 r.p.m.) and a pH of 7.9. The dispersion was 52.5%

In Example 3, a similar copolymer was prepared except that the amount of acrylic acid was greater: 114 grams instead of 97.2 grams as in Example 2. The rest of the copolymer composition and the procedure were identical. The final copolymeric solution had a non-volatile content of 69.8%, a viscosity of 14.8 stokes, and an acid value of 51.7. A 468 gram portion of this copolymeric resin was mixed with 202 grams of ethyl Cellosolve, 39.4 grams triethyl amine, and 226.6 grams of water. The resulting resin dispersion had a non-volatile content of 52.3, a viscosity of 1920 cps. (#4 spindle at 100 r.p.m.), and a pH=7.9.

Next, 50 gram portions of the copolymeric dispersions of Examples 2 and 3 were mixed with zinc oxide and sensitizing dyes in the manner described in Example 1.

' The weight ratio of zinc oxide to copolymer was about 7 of dry coating/3000 sq. ft. of paper. The following electrical results were obtained:

TABLE II Dispersion of Example 2 (grams).

8 Examples 4-11 A number of additional copolymers were prepared in the manner described in Example 1. Zinc oxide was dispersed in each of these copolymers in the manner de- Dispersion of Example 3 (grams) 5 scribed in Example 1. The amount of zinc oxide was suffi- Zinc oxide (grams) 250 250 Charge acceptance 070m) clent to give a weight ratio of Zinc oxide to copolymer of (a) At 221b./3,000 sq. ft 440 460 about 1. A dye mixture, equivalent to that of Example (b) At lb./3,000 sq. ft. 360 370 1 dd 1 Dark Decaywoltsk was a ed to the 21m: oxide/copo ymer mixture to At 15/ 9 q-f 28 :3 give 100 p.p.m. of dye based on the total we1ght of zinc 10 oxide. The resulting coating compositions were then used At 22lb./3,000 sq. ft 3% 4% to coat a er at a wei ht of 22 ounds of dr coatin er (1)) At 15 lb./3,000 sq. it 3% 4% p p g p Y g p Residualvoltage (volts). 3000 sq. ft. of paper surface. The formulatlon of these t qcopolymers and their correspondin" properties are shown 15 .3 000 20 20 a (b) At 113/ Sq ft 1n Table HI which follows:

TABLE III Example Copolymer composition:

n-Butyl acrylate 20.0 20.5 20.5 18.2 20.5 18 2 23.5 24.1 n-Butyl methacrylate .1 20.0 20.5 20.5 18.2 20.5 18 2 23.5 24.1 Styrene. .3 Methyl methacrylate. 1 9 Methacrylic acid 6 Hydroxypropyl acrylate 5 Hydroxypropyl methacrylate D i-tertiarybutyl peroxide...

Azo-bis-butyronitn'le Solvent composition, percent:

Butanol.---..

Physical properties of the copolymerie solutions:

N on-volatile. Acid value (100% NV)..- Viscosity (stokes at 0.). Hydroxyl value (100% NV) Electrical properties of the coated paper Charge acceptance (volts) 2 Dark decay (volts) 3 Light decay time (seconds) Residual voltage (volts) 5 1 The hydroxypropyl methacrylate was supplied as a solution from Rohm and Haas Company. It is known to be a mixture of isomeric hydroxyalkyl acrylates. The solution contains 38.5-42.5% hydroxypropyl methacrylate, a maximum of 2.0% of higher methacrylates, a maximum of 0.3% of alkylene di-methacrylate, and about 56% of methacrylic acid.

2 Charge acceptance is the voltage from the base line to the maximum absorption of voltage.

3 Dark decay is the voltage drop in darkness over a 4.5 second period from the maximum charge acceptance to the start of the light decay.

4 Light decay is the time required for the static charge (i.e. the accepted charge) to be dissipated to 50 volts.

5 The residual voltage is the amount of static charge which is not dissipated.

EXAMPLES 12-22 TABLE IV Example copolymer composition:

n-Butyl acrylate 19.9 19.9 19.9 19.9 28.3 29.9 19.8 19.7

n-Butyl methacrylate-. 19 9 19.9 19.9 19.9 .7

t-Butyl methacrylate-. 14.9

2-ethyl hexyl acrylate.

Methyl methacrylate Methacrylic acid.-.

Styrene 0zMethyl styrene Hydroxypropyl methacrylate.

Hydroxypropyl acrylate Azo-bis-butyronitrile Solvent composition, percent ol Plti ysical properties of the copolymeric solu- N on-volatile Acid value (100% NV). Viscosity (stokes at 25 C Hydroxyl value (100% NV) Electrical properties of the coated Charge acceptance (volts) Dark decay (volts).- Light decay time (sec Residual voltage (volts) 15. 8 8. 3 l5. 2 16. 5 9. 4 9. 3 9. 4 l1. 8 11. 2 l0. 5 13. 8 22 5. 5 13. 5 8 5. 0 19. 4 5. 6 15. 0 13. 1 6. 4 l0. 9

In addition to the excellent results obtained with the use of these particular acrylic copolymers, it has been noted that the additional use of large amounts of cobalt naphthenate (e.g. 0.15% to 0.5% based on the weight of the copolymers) increases the charge acceptance and reduces the light decay time. While cobalt naphthenate is sometimes used as a drier for oxidizing resins at levels of, for example, 0.01 weight percent, its use at high levels in conjunction with a non-oxidizing copolymer is very unusual. Other known driers, e.g. lead naphthenate, do not exhibit the same degree of effectiveness in non-oxidizing resins.

Although the present invention has been described with a certain degree of particularity, it is not intended that the invention be limited to the specific materials and specific proportions which have been given for the sake of illustration. Numerous modifications and variations will appear obvious to one skilled in the art.

What is claimed is:

1. In the method of image reproduction by an electrophotographic process wherein a latent electrostatic image is produced on a substrate having a finely divided photoconductive material bound to the surface thereof with an organic binder, the improvement which comprises using as said binder a copolymer of:

(a) from 1 to 30% by weight of a hydroxyalkyl ester of a vinyl carboxylic acid, or a mixture thereof;

(b) from 1 to 15% by weight of a copolymerizable a,,8-unsaturated carboxylic acid, or a mixture thereof, and

(c) the balance to make 100% of at least one copolymerizable hydroxyl-free, carboxyl-free mono-ethylenically unsaturated vinyl monomer.

2. The method of claim 1 wherein said hydroxyalkyl ester is a C -C hydroxyalkyl acrylate or methacrylate, or a mixture thereof.

3. The method of claim 2 wherein said n e-unsaturated carboxylic acid is acrylic acid, methacrylic acid, itaconic acid, or a mixture thereof.

4. The method of claim 3 wherein said hydroxyl-free vinyl monomer is styrene or a C -C alkyl acrylate or methacrylate, or a mixture thereof.

5. The method of claim 4 wherein said photoconductive material is zinc oxide and the weight ratio of zinc oxide to said copolymer is from 4:1 to 16: 1.

6. The method of claim 5 wherein said substrate is paper.

7. The method of claim 6 wherein an aminoplast resin is used with said copolymer to bind said zinc oxide to the paper.

8. The method of claim 6 wherein said copolymer is the only binder present.

9. In the method of image reproduction by an electrophotographic process wherein a latent electrostatic image is produced on paper having finely divided zinc oxide bound to the surface thereof with an organic binder, the improvement which comprises using as said binder a copolymer of:

(a) from 10-20% by weight of a C -C hydroxyalkyl acrylate or methacrylate, or a mixture thereof;

(b) from 1-5% by weight of acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, crotonic acid, or a mixture thereof; and

(c) the balance to make 100% of C C cyclic or acyclic ester of acrylic or methacrylic acids, acrylonitrile, methacrylonitrile, styrene, o-chlorostyrene, vinyl toluene, a-methyl styrene, or a mixture thereof.

10. The method of claim 9 wherein (b) is acrylic acid, methacrylic acid, itaconic acid, or a mixture thereof.

11. The method of claim 10 wherein (c) is styrene C C alkyl acrylate or methacrylate, or a mixture thereof.

12. The method of claim 11 wherein (a) comprises hydroxypropyl acrylate or methacrylate, or a mixture thereof. I

13. The method of claim 12 wherein the weight ratio of zinc oxide to said copolymer is from 6:1 to 12:1.

14. As a binder composition, a mixture consisting essentially of:

(a) a copolymer of (1) from 1 to 30% by weight of a hydroxyalkyl ester of a vinyl carboxylic acid, or a mixture thereof; (2) from 1 to 15% by weight of a' copolymerizable a,,8-unsaturated carboxylic acid, or a miXture thereof, and (3) the balance to make 100% of at least one copolymerizable hydroxyl-free, carboXyl-free mouo-ethylenically unsaturated vinyl monomer;

(b) finely divided zinc oxide in a weight ratio of zinc oxide to said copolymer of from 4:1 to 16:1; and

(c) as a diluent, solvent or water, or a mixture there- 15. The composition of claim 14 wherein the nonvolatile content of said mixture is 40 to weight percent.

16. The composition of claim 15 wherein said diluent is a mixture of solvent and water.

17. The composition of claim 14 wherein said mixture also contains sensitizing dyes for said zinc oxide and contains from 10-50 weight percent aminoplast resin based on the weight of said copolymer.

18. Coated copy paper comprising paper having a cured coating on a surface thereof, said coating having been formed by applying to said paper a wet film of the binder composition of claim 14 and thereafter curing said composition on said paper, said coated copy aper having an average of from 5-50 pounds of deposited dry coating per 3000 square feet of coated paper surface.

References Cited UNITED STATES PATENTS 2,681,897 6/1954 Frazier et al 260-850 3,245,786 4/1966 Cassiers et al 96-1.8 3,331,687 7/1967 Kosche 961.5

NORMAN G. TORCHIN, Primary Examiner I. R. HIGHTOWER, Assistant Examiner US. Cl. X.R. 96--1.8; 252-501