US3493369A - Low electrical resistance varnish coatings on an insulating base - Google Patents

Description

United States Patent T 3,493,369 LOW ELECTRICAL RESISTANCE VARNISH COATINGS ON AN INSULATING BASE Thomas William Busch and Arthur Barron Kaplan, Appleton, Wis., assignors to Appleton Coated Paper Company, Appleton, Wis., a corporation of Wisconsin No Drawing. Filed Apr. 3, 1964, Ser. No. 357,289 Int. Cl. G03g 5/08; 344d 1/18; HOlb 1/02 US. Cl. 96-1.8 8 Claims ABSTRACT OF THE DISCLOSURE Electrophotographic paper bearing a thin film between 0.1 and 3 microns in thickness of low electrical resistance varnish having an electrical resistivity between 10 and 10 ohms per square, the varnish consisting of a. flexible synthetic resin, film-forming binder, the synthetic resin having a melting point of above 95 C. and a volume electrical resistivity less than 10 ohm centimeters and being soluble at room temperature in common alcohol, ester solvents and hydrocarbon solvents or mixtures of these, and the varnish further containing a light colored conductive metal in discrete particulate flake form, such as aluminum, zinc, steel, copper, bronze, nickel or silver, at 45% of metal flake by weight of the total. The flexible synthetic resin may be any one of the polyvinyl acetals, vinyl chloride-vinyl acetate-maleic anhydride copolymer, air drying long oil alkyd resin, styrenated short oil alkyd resin and A-stage phenol formaldehyde resol, and mixtures of these. The thin Varnish film is especially adapted to underlie a photoconductive zinc oxide coating and to make the photoresponsive and electrical characteristics of the zinc oxide coating less affected by changes in humidity.

This invention relates to a novel low electrical resistance varnish coating especially adapted to make a new and improved electrophotographic recording paper or plastic sheet material by simple and economical coating procedures onto a conventional insulating paper or insulating plastic film base. The pigmented varnish of this invention provides a thin dry coating confined to the surface of the base and having a thickness of between 0.05 mil and about 3 mils, the coating consisting essentially of a light colored electrically conductive metal pigment, such as aluminum flake, tin flake, copper flake, brass flake, bronze flake, zinc flake, nickel flake, chromium flake and the like, which is present in a large proportion of at least 45% to about 85% by weight, preferably more than 50%, and the entire remainder being a water-impermeable, flexible, film-forming, low electrical resistance, synthetic plastic binder material exhibiting good adhesion to the pigment, very limited penetration to the base and resistance to atack by common organic solvents as are employed in the Electrofax topcoat application, and which has unusually good adhesion.

The preferred low resistance, water-impermeable varnish coatings of the present invention which contain at least about 45% by weight of metal pigment are flexible despite the high metal content and are characterized by a lateral resistivity (ohms/ square) value lying in a narrow range between about 1X 10 to about 1X 10 which varies by less than a factor of within the humidity 3,493,369 Patented Feb. 3, 1970 range of 5 to relative humidity as will be encountered in the ambient atmosphere.

The metal pigment is from 0.1 to 3 microns in thickness, is usable in the special film forming binder in 200 mesh, 300 mesh and 400 mesh size, and is capable of being applied in a smooth continuous film as thin as 0.03 mil thickness without protruding through the surface of the thin surface coating while being confined to the surface which is especially adapted as a. primer coating. As a result of this surface location of the thin flake metal synthetic film as a surface barrier, the egress of water or migratory organic solvent or plasticizer is prevented from occurring in a direction from the topcoating through the barrier and into the insulating base by its presence in the topcoating. Also, any high humidity-based pickup of moisture by the hydrophilic insulating base, such as paper or transparent plastic, e.g., nitrocellulose or cellulose acetate, is effectively blocked in a direction starting from a location at the bottom or within the body of the hydrophilic base.

The value of this special barrier can be appreciated in the case where the varnish of the invention is used as a primer coating on a paper base which is specially prepared for a photoconductive zinc oxide topcoating, used as an electrophotographic printing plate member.

In a preferred form of binder a vinyl ether polymer is employed as the binder of the varnish, the vinyl ether made from a vinyl carboxylate polymer, such as vinyl chloride-vinyl acetate copolymer which is hydrolyzed to uncover free hydroxyl groups which are then condensed with aldehydes, e.g., formaldehyde or butyraldehyde. This binder is water impermeable and provides outstanding wetting and adhesion to the metal flakes and to the paper substrate.

This primer coating provides a dimension-ally stable electrophotographic paper printing plate capable of many runs, e.g., 250 or more copies.

An object of the invention is to provide an improved low electrical resistance varnish coating for an insulated base adapted to underlie a photoconductive zinc oxide coating, said varnish consisting of a light colored electrically conductive metal in discrete particulate flake form, such as aluminum, zinc, steel, copper, bronze, nickel, silver or other conductive metal at a weight of the total weight of flaked metal and binder loading of metal of at least about 45% by weight in a special resinous binder of proper electrical characteristics which produces a coated layer having the proper level of lateral electrical resistivity between 10 to 10 ohm per square over a broad relative humidity range from 5 to 95 relative humidity to improve the responsiveness of the photoconductive zinc oxide coating whereby the conductivity which is needed in the so-metalized varnish on the base is obtained for the particular electrostatographic process to which the resulting electrographic paper is subjected regardless of the atmospheric relative humidity.

Still another object of the invention is to provide an improved barrier of low resistance or a primary coating for a lithographic paper printing plate which will act as a satisfactory generally water-impermeable seal coat with adequate adhesion at the interface between the primer coating and the electrophotoconductive face which will resist separation between these layers during printing using said plate.

It is unexpected that the present synthetic resins with critical amounts of metal pigment provide low resistance varnishes by air drying rather than conducting coatings since epoxy resin compositions containing silver flakes which have been applied as conducting adhesives in the bonding of metal adherents exhibit a conduction which is at least 0.0015 ohm when bonded to brass surfaces in the form of an adhesive film of 0.005 to 0.010 inch in thickness. The present resistance in the same film thickness is 1,000 times greater than the epoxy-silver flake adhesive.

In contrast to the present varnish which requires no curing treatment, the epoxy-silver flake adhesive require one hour of heating at 100 C. to set the resin and to obtain reproducible high conduction, which is expressed by the manufacturer, Emerson and Cuming, Inc., Canton, Mass., as a volume resistivity of 50 ohm-cm. at room temperature (25 C.) and at 93 F. This commercial characterization of the electrically conducting epoxy adhesive is in line with the electrical characteristics of the conductive molded plastic formed of a mineral filled phenolic resin base and a conductive carbon black filled plastic body integrally co-molded therewith and which is commercially available in the form of conductive plastic potentiometers (see Electronic Markets, Issue No. 215, June 1961), produced by the Markite Corporation, New York, N.Y.; these conductive plastic M'arkite materials have a high conduction expressed as volume resistivity of about 0.001 ohm-cm. which is about the same as the lowest value of conduction (highest resistance value) of the epoxy-silver flake adhesive. This comparison emphasizes the unique intermediate to low electrical resistance characteristics of the metal pigment-filled synthetic resin varnish of the present invention relative to silver conductive epoxy or carbonfilled phenolic plastic. The outer limits of lateral resistivity of the metal-filled varnish of the present invention, from 10+ to ohm per square, is a resistance of from 100 times up to about 10,000 times the resistance value of the carbon-loaded phenolic resin of the prior art or of the silver flake-loaded epoxy resin of the prior art. It is surprising that the amount of metal used in the present varnish does not impart a much lower resistivity value, whether based upon volume or lateral measurement. Essentially, these lateral or surface resistance limits of the present invention define an intermediate area of lateral resistivity in a dielectric consisting of a thin film provided by air drying varnish which cannot be produced, as far as the inventors know, in any other way, in order to localize the resistance barrier at the surface of the insulating base.

This is an important technical achievement if one considers that the most common substrate, dry paper, has :a volume resistivity of 10 ohm-cm.; but after this dry paper has absorbed only 3% of water, the volume resistivity drops to 10 ohm-cm. The measurement of resistivity of paper depends upon the humidity of the atmosphere and this same dependency is observed with most hydrophilic plastics, all cellulosic plastics, all polyester plastics, all vinyl carboxylic plastics, etc., in short, with substantially all polar plastics whether thermoset or thermoplastic.

The most surprising characteristic and the most important advantage of the present invention is the relative constancy of the lateral resistivity of the moisture barrier, thin film coating under wide extremes of relative humidity. It is believed that this low coefficient of resistivity with humidity change is based partly upon the polar characteristic of the synthetic resin, partly upon its relatively low volume resistivity value of 10 ohm-cm., and partly upon the relatively low proportion of crystalline phase and high proportion of amorphous phase in the structure of the film-forming binder of the metallized barrier. The substantially amorphous character of the water-resistant polar polymer binder is evidenced by the high degree of flexibility, the high degree of solubility in common organic solvents such as butyl alcohol, ethanol, isopropanol, ethyl acetate and the combinations of alcohol and toluene, low viscosity and low molecular weight, the high degree of elongation and the relatively high tensile strength of the polymer in the form of the free film.

The problem of eflectively dealing with variations in atmospheric moisture or ambient humidity in order to improve operation of the photographic recording process has long occupied the attention of the skilled workers in the prior art. One of the earliest practical solutions was proposed by Coberly in US. Patent No. 1,455,428 which proposed adding a body of hygroscopic material in the form of a coating to the hydrophilic resin support (celluloid) in order to eliminate the accumulation of static charges on the film base. Lithium chloride was the hygroscopic material suggested by Coberly and it was dissolved in alcohol and supplied to the substrate as the coating. This same concept is used in Uber et al., US. Patent No. 3,116,147, which incorporates a hygroscopic agent into the photoconductive zinc oxide top layer of an electrophotographic recording paper in order to eliminate the undesirable poor recording characteristics exhibited at low humidity.

This concept of achieving consistent reproducibility on an insulating base, whether it be paper or plastic, has stimulated the suggestion, noted in Doggett et al., U.S. Patent No. 3,110, 621, that conductive materials be employed, such as carbon black, a simple inorganic salt electrolyte, sodium chloride, an electrolyte polymer (polymerized vinyl benzyl ammonium chloride), or metal fibers, all of these materials to be uniformly distributed throughout the insulating base in order to completely irrevocably alter the resistivity of the insulating base. Insulating paper is thus made conductive by Doggett et al. by lowering the volume resistivity from 10 ohm-cm. to a much lower value corresponding to a lateral resistivity of 8X10 ohms per square.

It is a fundamental precept of sound insulation engineering practice dealing with the technology of composite dielectrics, for example, paper treated with oil, wax or resin, that as one increases the thickness of the higher dielectric constant material without increasing the overall thickness of the total assembly (as by compacting under pressure) one increases the dielectric stress on the lower dielectric constant material. (Clark, Insulating Materials for Design and Engineering Practice, John Wiley & Sons, Inc., 1962, page 13.)

The conventional impregnant for paper, as stated by Clark, whether it be mineral oil or a synthetic resin invariably has a dielectric constant ranging from about 2 for mineral oil to a maximum of about 3.5 for the synthetic resins.

It is, therefore, obvious that a great economy of expensive organic material is obtained if the applied insula tion can be used in the form of a varnish coating confined to the surface of the hydrophilic paper or cellulosic substrate.

By using a thinner film of the resin, there results an increased dielectric strength in volts per mil (see Clark, page 13). Thus, the greater proportion of the voltage stress under DC applied potential is initially localized in the low dielectric constant material, but, due to the greater conductivity of this low dielectric material, the potential stress changes rapidly. As a result of this change, the potential difference concentrates in the material of higher dielectric constant for DC applied potential. This concentration is not the same with AC voltage wherein the imbalance of voltage stress is inversely proportional to the dielectric constants of the respective materials.

It is quite unusual to discover an air drying varnish of the synthetic resin type capable of surface film application which achieves such economy of expensive binder material while exhibiting a level of resistivity intermediate between conducting silver or carbon filled resin values and conventional commercial insulating resin values, the commercial insulating resin values being generally greater than 10 ohm-cm.

. For the purposes of the present invention, the insulatmg resins, which serve as the insulating base herein can be any one of the following:

The dry paper which is used throughout the industry as a base for varnish, shellac, asphalt and the like, has a volume resistivity of ohm-cm. and a dielectric constant at 60 c.p.s. of about 5 to 5.5.

The following table demonstrates the dielectric properties of the insulating base materials, tested at 25 C., to which the thin surface film of metal pigment synthetic binder at intermediate resistivity value is applied, the data source being Manufacturers Data and derived tabulations in Clark, Insulating Materials for Design and Engineering Practice, Wiley & Sons, New York, 1962.

TABLE l'.

terial by virtue of adjusting the viscosity (resin solids content); and in every case of solvent application 'by conventional methods, such as roller coating, the solvent is completely evaporated and the varnish film is substantially dry within about one hour. The resulting primer film is Water-impermeable and acts as a barrier to the migration of any soluble materials from the base into the topcoat. The barrier also prevents migration of any soluble materials from the topcoat into the base.

Thus, for the purposes of the invention, the synthetic resin binder of the varnish must be a film former soluble in any one of the above common solvents in an amount of 5-50% of resin by weight of the solution, whereby a surface film is laid down in a thickness of about 0.05 mil to about 3 mils.

This barrier film provides an outstanding primer coating for a zinc oxide photoconductor topcoating or for an equivalent photoconductor topcoating as is used in electrophotographic recording. The finely divided photoconductor, such as zinc oxide, is suspended in a toluene or xylene solution of a resin binder, such as silicone resin or polyvinyl acetate.

The particular zinc oxide employed in the present invention may be Florence Green Seal 8, sold by the New Jersey Zinc Company, or Azo ZZZ 661, sold by the American Zinc Sales Co. Other photoconductors which can be used are: Oxides of antimony, aluminum, bismuth, cadmium, mercury, molybdenum, and lead; also iodides, selenides, sulfides or tellurides of these metals may be used, including zinc, selenium, arsenic trisulfide, lead chromate and cadmium arsenide. The silicone or polyvinyl acetate resinous vehicle is an electrically insulating binder for the finely divided photoconductor.

The combination of the present barrier coating on a paper plate bearing a zinc oxide photoconductive insulat- Dielcctric Constant Resistance (ohm-cm.)

Material 60 c.p.s 10 c.p.s. Volume Surface Density Cellulose acetate (medium hard-38.5% acetyl) 3.5-4.0 3. 5-4.0 10 -10 1. 30 Cellulose acetate butyrate (37% butyrile) 3.5-5.0 3. 5-5.0 10 1. 30 Cellulose ethyl ether 2. 5-2. 7 2.2-3.2 10 l 2000 10 1. 12-1. 16 Cellulose propionate 3.0-3. 5 3. 3-3. 5 10 1. 20 Polyethylene, high density 2. 25-2. 3 2. 25-3. 4 10 -10 2 5X10" 0.94-0.96 Polyethylene, low density 2. 3 2. 3 10 -10 3 10 0. 91-0. 9- Polypropylene 2. 1 2. 1 10 -10 0. 86-0. 90 Polystyrene 65 2. -2. 65 IO -10 1. 05-1. 07

Polyethylene terephthalate (Mylar-C) 3. 2 10 Polyvinyl chloride, rigid 3. 4 2. 8-3.0 10 -10 1. 35-1. Polyvinyl chloride, flexible 5. 8-6. 9 3. 3 4. 5 10 1. 20-1. Polymethyl methacrylate 3. 4 2. 8 10 1. 18-1. 19 Polymerized caprolactam (Nylon 6). 4. 6 4. 5 6 10 -10 1. 14

Polymer of adipic acid and hexamethylenediarnine electrical grade (Nylon 66) 4. 6 4. 5 4X10" 1. Polytetrafluoroethylene 2.0-2. 2 2. 0-2. 2 10 -10 3. 5 10 2. 10-2. 30 Polytrifiuorornonochlorethylene 2. 7 2. 5 10 -10 0 10 -10 2. 10-2. 12 Lexan polycarbonate (condensa carbonate and bisphenol-A) 3. 17 2. 96 2X10 1.20 Copolymer of acrylonitrile and methylacrylatc (Orion polymer) 3. 7 3. 4 10 1. 10

1 At relative humidity, At relative humidity., 3 At 50% relative humidity. 4 20% relative humidity, 5 At relative humidity., 5 At 20% relative humidity, 7 At 60% relative hum1d1ty., 8 At 80% relative humidity, 9 At 50% relative humidity.

It is an essential feature and a primary object of the invention to provide an intermediate to low electrical resistance metallized synthetic varnish which dries in the air at room temperature by simple solvent evaporation, the solvent being an inexpensive, non-toxic common solvent, e.g., a monohydric aliphatic alcohol such as methanol, ethanol, isopropanol, butanol, or esters such as ethyl acetate, butyl acetate, or ketones such as methyl ethyl ketone, methyl isobutyl ketone or tetrahydrofuran, or hydrocarbons such as benzene, toluene, xylene, or mixtures of the alcohol and aromatic hydrocarbons. Minor amounts of stronger solvent, such as dioxane or acetone, may be included. All of the properly chosen solvents provide dry, thin, continuous films at the surface of the base material without in any way attacking the base when the base is of resin or plasticized resin. The solvent-laid ing resin topcoating provides for delayed charge leakage in the barrier layer leading to improved electrophotographic recording. The light decay rate is hastened so that developing and copying can be carried out rapidly, uniformly and reproducibly; and there is a particular benefit produced by providing uniform charge retention in the image area.

It is essential that the charge retention in the image area be of uniform density to prevent charge concentration at the edges of the image which can lead to an image outline rather than a solid image area.

It appears that, with the simple varnish coating method of the present invention, the proper resistance path with uniform charge retention in the image area is invariably achieved upon exposure and development. This is based exclusively upon the novel varnish of intermediate to coating is confined to the surface of the paper base ma- 75 low electrical resistivity which operates independent of humidity changes. The barrier varnish thus assures a uniform charge density throughout the image area without charge concentration at the periphery of the image in a manner that could not be foreseen.

Of course, one might have provided a very highly conductive metal film by a vacuum metallizing process, but such film is completely conductive and not resistive, as is the present barrier. Further, the metal film does not bond as well as the present varnish to the zinc oxide insulating binder topcoating. Indeed, the present varnishes in certain preferred embodiments constitute certain outstanding primers for the topcoating, whether used as a printing plate or as a recording sheet. The plates and sheets produce copies of greatly improved quality at far less cost than could ever be foreseen from a cost analysis of the vacuum .metallizing processes of the prior art.

It is emphasized that metal flake pigment is an essential element of the metallized varnish combination. Improved imaging is not had by introducing clay filler with a volume resistivity value of 10 -10 ohm-cm. The result is not achieved if carbon black is introduced in an amount to produce a resistivity value of 103 ohm-cm. The achievement of this intermediate resistivity value of the present invention is completely unexpected; and it appears to be due to the combination of the special binder with critical proportions of flaked metal pigment. Oleoresinous varnishes are not suitable; baking varnishes are not suitable. Neither shellac or asphalt are useful. Hydrocarbon-based varnishes are not useful. Only the polar polymers having relatively high tensile strength, great flexibility, strong adhesion to substrate and metal flake pigments and good solubility in common solvents can be used. These polymers comprise polyvinyl ethers, the vinyl ester polymers and copolymers, including those having free carboxylic ethers, cellulose nitrate, the vinylidene chloride polymers and copolymers, the phenol-formaldehyde resins, certain acrylonitrile resins and the alkyd resins. These resins may be modified with only minor proportions (up to of aminoplasts, alcohol soluble urea-formaldehyde resins, alcohol soluble melamine-formaldehyde resins and alcohol soluble aniline-formaldehyde resins.

Tables 11 and III which follow show the essential binders; any one of Which may be used, and these binders are shown in the order of their preference. All of the binders produce outstanding results; but the polyvinyl ethers, especially the acetal, butyral and formal, are the most versatile and can be used with every insulating base disclosed herein or with their equivalents.

TABLE III Moisture Tensile absorption Dielectric strength (percent, strength Synthetic resin vehicle (psi. 24 hour Elongation (volts pet for hinder X10- immersion) (percent) mil) Polyvinyl butyral,

plastieized 1. 5-3 2 150-450 Polyvinyl butyral (XYSG) rigid. 4-8 2 5-60 Polyvinyl formal 10 1. 3-2 5-200 3 1 Polyvinyl acetat. 10 1-2 15-75 \inyl choride-vinyl acetate-maleic anhydride copolymer (low molecular weight) 5-8 1-2 Vinyl chloride-vinyl acetate copolyzner (low molecular weight) 1. 2-5. 0 0. 5-1. 5 Polyvinyl acetate. 1. 5-4. 2 2-3 200-300 150-300 Ihcool-formaldehyde,

A-stage resole 6-0 0. 1-0. 5 400-500 Styrenated Short Oil alkyd varnish 0. 5-1. 0 Up to 6 600-2, 000 Alkyd air drying glyptal base varnish. 6 0. 5-1. 0 Up to 6 600-2, 000 vinylidene chloridevinyl chloride copolymer (Saran) 7-15 0. 1 2040 500-2, 500 Cellulose nitrate varnish 5. 6-8 1. 0 25-45 1, 000-1, 200 Diallyl phthalate varnish 6-8 0 1-0. 5 500 1 Depend on thickness 01 film. 2 In very thin film. 3 Based upon tr isted pair measurement.

The foregoing binders combined with critical proportion of metal flake pigment in volatile alcohol solvent or its equivalent, as pointed out above, adjusted to coating viscosity, constitute the examples of the present invention.

Also, the following additional examples are given to show details of the best methods in providing the new varnish coatings of intermediate to low resistivity.

EXAMPLE I The base sheet or web used is of a suitable known type of book paper grade of raw stock especially used for electrostatographic papers and planographic printing plates.

The paper web Was coated by roller coating, in two groups of products (1) on one side and (2) on both sides, with about one pound dry weight per 3000 square feet of low resistance varnish coating composition having the following formulation, the coating being dried by the solvent evaporation method. The dry coating may be rehumidified with steam in a steam box if desired.

Wet Dry TABLE II Vinyl butryal XYHL binder 8.8 8. 8 Particulate aluminum powder (MD 6090) 11. 8 11.8 Resis- Isopropyl alcohol, 00% 59. 7 Dielectric Constant hntivit Butyl alcohol 19. 7 0 -cm. Material 60 cps. 10 c.p.s. Volume Density 100.0 20.6 Polyvinyl butyral P Xvsc r 10 1.1-1.2 In the above formulation the vinyl butyral resin is disgggiggh iggl 6- 53 solved in a blend of the isopropyl and butyl alcohol by Vinyl chloride-vinyl slowly dusting the resin into the solvent with good agitaj gi g gg fig ag tion. The resin solution is slowly added to the aluminum to iec t i is tre gtpu- 4.5 1. as powder and dispersed well. The final mix is adjusted with my c a blend of but 1 and iso ro l lcoh l t o t f acetate copolymer y p P a o o a Vlsc S1 y 0 (low molecular 35 seconds as determined wlth a No. 3 Zahn cup. gig g gz -g 3:3 l In the foregoing formulation the preferred form 0f Phenol formaldehyde, the aluminum metal owder is MD 6090, this rade bein At 1 50-60 4550 10 10 118120 g g gEfg; {fis 0.5 micron average particle size, 95% through 325 mesh, vglyptplbasielvartishn 5.8-6.2 4. 5-5.3 10 m 1. ,1,3 0.5% oleic acid added for leafing and very low in oxide myieneciorl evinyl chloride coat ng and available commercially from Metals Disintecopo ymer aran 1 1. 5-,? g 8. mg om an 1221 e I Celluldsenitr te 4 0-4 5 3 H O 5 04 6 1 5 p 1 f th my 1 utyra X BL 18 a bin er or e metal powder varnish 1. 0-1. 5 6. 0-6. 5 2 10 -10 1. 4O Diallylphthalate and provides a subbing layer.

Vams 101M015 The above blend of isopropyl and butyl alcohol was xarnih fillm valtue. at chosen for rapid, efficient and economical solvent evapo- 2 t 50 a re ative umi i y. ration 3 At 60 C. At 60 (land 10 0.115. The referred photoconduction topcoatm WhlCh ad- 8 "esistivity insulatwn gmde' heres to the varnish primer coating is prepared as follows:

A mixture is prepared of the following materials: 65 grams of a 60% solution of a silicone resin dissolved in xylene, such as GE SR-SZ marketed by the General Electric Company, Silicone Products Division, Waterford, N.Y., 200 grams of toluene and 100 grams of CP white zinc oxide. This liquid mixture is ball-milled for about 3 hours and then applied to the surface of the primer coated backing. The doubly coated paper base, hereinafter referred to as the printing plate, is then dried by solvent evaporation.

The coated products were tested as recording paper and as printing plates. Excellent recordings were made.

Approximately 1 pound of coating per 3000 square feet provides a satisfactory conductivity of polyvinyl butyral with a value lying between 1 l0 to 1 1O ohms per square over a wide range of relative humidity, e.g., from 5% to 100% relative humidity.

The proportion of powdered photoconductor to vehicle in the final coating may vary over a wide range, e.g., 50% of photoconductor to 50% to 10% of vehicle, the optimum proportion depending upon the nature of the photoconductor, the nature of the vehicle and the results desired.

The vehicle for the photoconductive coating may in clude any natural or synthetic resin or wax, for example, silicone resins, cellulose esters, polystyrene or shellac. Mixtures of vehicles may be used.

Polyvinyl butyral may be mixed with 5 to of nonmigrating plasticizer along with the solvent alcohol to form a colloid or plastic mass. A typical plasticizer is triethylene glycol di-2-ethylbutyrate.

EXAMPLE II A polyvinyl formal varnish suitable for use as a complete substitute for the polyvinyl butyral varnish in Example I has been prepared using from the following compositions the same amount of polyvinyl formal with the same amount of Parts Polyvinyl acetate 100 Glacial acetic acid 185 Formalin (37.5% I-ICHO) 83 Conc. sulfuric acid 6.8

Polyvinyl formal is not soluble in lower alcohols, hydrocarbons, esters and most organic solvents and the best solvent for the purposes of the invention is either tolueneethanol (60-40) or toluene-methanol (75-25).

Polyvinyl acetals from long chain aldehydes such as dodecyl aldehyde and oleyl aldehyde are also useful as binders for the invention, these much softer than polyvinyl butyral and especially adapted for heavier loading with metal.

It will be appreciated that although the varnish coating may be applied on any known coating equipment in use, the roller coating method using a means for controlling the coating thickness engraved cylinder is preferred to cause uniform charge dissipation without concentration at the periphery of the image.

The same proportions of binder to aluminum pigment were used as in Example I to provide a primer coating on a flexible polyvinyl chloride base. An excellent recording was made using the topcoating of Example I.

EXAMPLE III The following formulation was also tried with successful recording and printing results using the topcoating of Example I.

10 EXAMPLE Iv Phenol formaldehyde resin In this example, the same proportions of binder and pigment as in Example I were used. Phenol formaldehyde resin varnish was prepared by the solution of A-stage resols in alcohol, using xylenol as the phenol. The films so produced can be heat hardened. The varnish film was an excellent subbing layer for the zinc oxide coating and gave a good image.

Similarly, alkyd resin varnish, of which Glyptal varnish is a widely known illustration, was applied from solution in alcohol and hydrocarbon solvents. The metallized film so produced was strongly adhesive and more flexible than the phenol-formaldehyde varnish films. The alkyd resin varnishes, modified by the presence of vegetable oils and their fatty acids, e.g., linseed, tall, tung, soybean, and dehydrated castor oils, are frequently introduced for even greater flexibility in the varnish film.

Long-oil alkyd varnishes (more than 60% oil) can be used in aliphatic hydrocarbon solvents for the air drying application.

The alkyd resin varnishes can be modified by the addition of phenolic, urea and melamine-formaldehyde resins. The addition of a phenolic resin imparts a greater resistance to water and highly humid atmospheres; urea or melamine-formaldehyde additions yield varnish films of greater hardness and fusability.

Vinyl alkyd resin mixtures in varnishes are also useful to yield tough, flexible insulating films of excellent adhesion properties, and low water transmission.

Vinylidene chloride solvent varnishes are applied by spraying and provide excellent resistance to Water, to water containing salts, and to oils used in printing inks.

The metallized cellulose nitrate varnish film possesses good mechanical properties and a good resistance to moisture.

Synthetic resin varnishes It is thus seen that a wide variety of synthetic resins can be used in the preparation of the electrically insulating metallized varnishes herein.

We claim.

1. Electrophotographic paper comprising an insulated resin-coated paper base with volume resistivity greater than about 10 ohm centimeters which is coated with a subbing coating of low electrical resistance varnish consisting essentially of a synthetic resin film-forming binder having a melting point above C. deposited from a volatile solvent mixed with a light colored conductive metal in discrete flake form of 200400 mesh and between about 0.1 to about 3 microns in thickness selected from the group consisting of aluminum, zinc, steel, copper, bronze, nickel, or silver at a weight proportion of at least about 45% of metal flake by weight of the total, the resin of said binder being selected from the group consisting of polyvinyl acetals, vinyl chloride-vinyl acetatemaleic anhydride copolymer, air drying long oil alkyd resin, styrenated short oil alkyd resin and A-stage phenol formaldehyde resol, and mixtures of the foregoing, said amount of conductive metal flake above the minimum of 45% by weight of total being adjusted so as to produce a coated metallized layer 0.05 to about 3 mils in thickness having a level of lateral electrical resistivity between 10 to 10 ohm per square over a range of from 5 to 95% relative humidity, and a topcoating of photoconductive zinc oxide in an insulating binder.

2. Electrophotographic paper as claimed in claim 1 wherein said synthetic resin is polyvinyl butyral and said metal flake is aluminum.

3. An electrophotographic recording sheet comprising an insulating synthetic resinous sheet having a volume resistivity of from 10 -10 ohm centimeters measured at 25 C. coated with a coating of low electrical resistance varnish consisting essentially of a synthetic resin filmforming binder having a melting point above 95 C. de-

posited from a voltaile solvent mixed with a light colored conductive metal in discrete flake form of 200-400 mesh and between about 0.1 to about 3 microns in thickness selected from the group consisting of aluminum, zinc, steel, copper, bronze, nickel, or silver at a weight proportion of at least about 45% of metal flake by weight of the total, the resin of said binder being selected from the group consisting of polyvinyl acetals, vinyl chloride-vinyl acetate-maleic anhydride copolymer, air drying long oil alkyd resin, styrenated short oil alkyd resin and A-stage phenol formaldehyde resol, and mixtures of the foregoing, said amount of conductive metal flake above the minimum of 45% by weight of total being adjusted so as to produce a coated metallized layer 0.05 to about 3 mils in thickness having a level of lateral electrical resistivity between 10 to ohm per square over a range of from 5 to 95% relative humidity, and a top-coating of photoconductive zinc oxide in an insulating binder.

4. An electrophotographic recording sheet as claimed in claim 3 wherein said synthetic resin is polyvinyl butyral and said metal flake is aluminum.

5. An electrophotographic printing plate comprising paper backing laminated to an electrophotographic recording sheet comprising an insulating synthetic resinous sheet having a volume resistivity of from lO -IO ohm centimeters measured at 25 C. coated with a coating of low electrical resistance varnish consisting essentially of a synthetic resin film-forming binder having a melting point above 95 C. deposited from a volatile solvent mixed with a light colored conductive metal in discrete flake form of 200-400 mesh and between about 0.1 to about 3 microns in thickness selected from the group consisting of aluminum, zinc, steel, copper, bronze, nickel, or silver at a weight proportion of at least about 45% of metal flake by weight of the total, the resin of said binder being selected from the group consisting of polyvinyl acetals, vinyl chloride-vinyl acetate-maleic anhydride copolymer, air drying long oil alkyd resin, styrenated short oil alkyd resin and A-stage phenol formaldehyde resol, and mixtures of the foregoing, said amount of conductive metal flake above the minimum of 45 by weight of total being adjusted so as to produce a coated metallized layer 0.05 to about 3 mils in thickness having a level of lateral electrical resistivity between 10 to 10 ohm per square over a range of from 5 to 95% relative humidity, and a topcoating of photoconductive zinc oxide in an insulating binder.

6. A printing plating as claimed in claim 5 wherein said synthetic resin is polyvinyl butyral and said metal flake is aluminum.

7. An electrophotographic printing plate comprising an insulating resin-coated paper base with volume resistivity of greater than about 10 ohm centimeters coated with a subbing coating of low electrical resistance varnish consisting essentially of a synthetic resin film-forming binder having a melting point above C. deposited from a volatile solvent mixed with a light colored conductive metal in discrete flake form of 200-400 mesh and between about 0.1 to about 3 microns in thickness selected from the group consisting of aluminum, zinc, steel, copper, bronze, nickel, or silver at a weight proportion of at least about 45% of metal flake by weight of the total, the resin of said binder being selected from the group consisting of polyvinyl acetals, vinyl chloride-vinyl acetate-maleic anhydride copolymer, air drying long oil alkyd resin, styrenated short oil alkyd resin and A-storage phenol formaldehyde resol, and mixtures of the foregoing, said amount of conductive metal flake above the minimum of 45% by weight of total being adjusted so as to produce a coated metallized layer 0.05 to about 3 mils in thickness having a level of lateral electrical resistivity between 10 to 10 ohm per square over a range of from 5 to 95% relative humidity, and a top-coating of photoconductive zinc oxide in an insulating binder.

8. A printing plating as claimed in claim 7 wherein said synthetic resin is polyvinyl butyral and said metal flake is aluminum.

References Cited UNITED STATES PATENTS 2,374,214 4/1945 Kline et al 162138 2,880,181 3/1959 Williams 117-218 X 3,030,237 4/ 1962 Price 117--227 3,057,749 10/1962 Luzena 117160 X 3,113,022 12/1963 Cassiers et a1 96-1 3,137,666 6/1964 Lox et al. 117-160 X 3,245,833 4/1966 Trevoy 961.5 X 3,265,497 8/1966 Kosche 96-1.5 3,295,967 1/ 1967 Schoenfeld 961.5

OTHER REFERENCES Stern et al.: Conductive and Resistive Coatings, RCA Technical Notes No. 416 January 1961.

DONALD LEVY, Primary Examiner C. E. VAN HORN, Assistant Examiner US. Cl. X.R.