US3295967A

US3295967A - Electrophotographic recording member

Info

Publication number: US3295967A
Application number: US305979A
Authority: US
Inventors: Samuel J Schoenfeld
Original assignee: Kimberly Clark Corp
Current assignee: Kimberly Clark Corp
Priority date: 1963-09-03
Filing date: 1963-09-03
Publication date: 1967-01-03
Anticipated expiration: 1984-01-03

Description

Jan. 3, 1967 5. J. SCHOENFELD 3,295,967

ELEGTROPHOTOGRAPHIC RECORDING MEMBER Filed Sept. 5, 1963 5 6 4 PHOTO SENSiTIVE COAT 2 j"-"I'f-'.'""'.'",'-'T 5 CONDUCTIVE BARRIER COAT "'JLCONDUCTIVE BACK 3 COAT FIG. I

IO b 2 I 0 lo 10B. 2 3: Z3 {5 lo LU II l I I I I I I l 0% IO 4o so so so REL. HUM.@ 72 F United States Patent 3,295,967 ELECTROPHOTOGRAPHIC RECORDING MEMBER Samuel J. Schoenfeld, Neenah, Wis., assignor to Kimberly- Clark Corporation, Neenah, Wis., a corporation of Delaware Filed Sept. 3, 1963, Ser. No. 305,979 12 Claims. (Cl. 961.5)

This invention relates to an electrophotographic recording member and a method of producing the same; more specifically, the invention relates to new and improved electrostatic recording elements made desirably electrically conductive by the incorporation of a coating between a non-metallic base of high electrical resistance and a photosensitive coating.

Electrophotographic recording members having a nonmetallic base such as sheets of paper coated with a photosensitive material are commonly employed by electrostatically charging the coating in the dark and then exposing selected portions of the charged surface to light; a latent electrostatic image is then formed on the surface of the recording member and this is subsequently developed by dusting an oppositely charged powder on the coating in the dark. The powder adheres to the areas of higher electrostatic charge, that is, the areas of the surface which were not illuminated to a high degree, and the powder does not adhere in significant quantity to the illuminated areas. The powder applied to develop the latent image is usually fusible and an essentially permanent image is achieved without the necessity of a transfer process.

Important to the attainment of adequate prints is the requirement that the areas of the charged recording element which are exposed to light be readily discharged. Incomplete discharge results in a fuzzy print, yet the recording member itself must have sufiicient electrical resistance that the recording member will hold a suitable charge in the dark for a reasonable time. Additionally, since the recording member in conventional practice becomes the print, coated material, when the base is a paper sheet, must exhibit all the qualities of a good copying aper.

The electrical conductivity of such recording members has posed a difiicult problem in the art. Customarily, at high humidities the member responds reasonably well but at low humiditiesbelow about 25% at temperatures of about 70 F.the member tends to become dry, lowering the electrical conductivity and inhibiting the discharge of lighted areas. Various expedients have been employed in the art to overcome this defect. For example, the members have been humidified before use, additionally, paper sheets have been treated to increase their conductivity. The former operation is cumbersome and the latter has involved relatively expensive compounds.

A primaryobject of this invention is to provide a novel electrophotographic member which is useful over a wide range of humidities under normal temperature conditions.

An important object of this invention is to provide on a non-metallic insulating base a conductive coating to which a photosensitive coating is to be subsequently applied to thereby provide an electrophotographic memher, and which member is useful at low relative humidities under normal temperature conditions of about 70 F.

Another object of this invention is to provide an electrophotographic member in the form of a composite which includes an electrically conductive coating on a non-metallic base and which coating resists the penetration of organic solvents upon application of an organic solventdispersed photosensitive material to the conducting coat ing.

Yet another object of the invention is to provide a novel method of producing an electrophotographic memher.

I have found that a sheet very suitable in all other characteristics and which is useful in the low humidity rangesdown to at least 15% relative humidity at about 70 F.may be produced by incorporating in an intermediate coating, between the non-metallic base and the photosensitive upper layer, a coating containing relatively specific components. These components include hydrated silicic acid and a hygroscopic hydrated inorganic salt. The coating containing these components is well bonded both to the base and the upper photosensitive layer and is not materially subject to deterioration in the practice of the printing process, or storage, before or after the printing. Y

The coating composition which forms the intermediate stratum is preferably an aqueous mineral coating composition of the type essentially common to the paper coating art but modified to include the noted specific components and to attain desired properties. In this latter connection I prefer to usually employ in such compositions a relatively high binder content, about 50% to 70% by weight of the solids of the composition, to assure against powdering, solvent penetration, and lack of binder strength; most suitably, the binder content is 55% to 60% of the total solids and may be any conventional binder such as starch, casein, or other protein.

Further, in the practice of the invention, I prefer to form the hydrated silicic acid in the com-position in situ by employing an excess of an acidic reacting hygroscopic hydrated inorganic salt; however, if desired, the silicic acid may be precipitated from a silicate as the gel by other components such as methyl alcohol, mineral acids and amines, for example, methylamine. In any event, the pH is further lowered beyond that sufficient to provide for the hydrated gelatinous silicic acid formation by the addition of the hydrated inorganic salt.

I have found that, while a pH of the coating composition as low as 4 may be utilized, it is preferable to maintain the pH of the coating composition as applied to the paper to between about 7.2 and 7.8. The upper limit of pH, I have found to be about 8.2 in order to secure adequate gelation and formation of the hydrated silicic acid. While a pH of the final composition on the acid side is useful, difiiculty may be had with the storage of such papers over a long period of time.

The non-metallic base and the photosensitive coating may be materials commonly employed in the art. Preferably, the base is paper as already noted, but it may be cloth, leather and the like. Suitably, the photosensitive coating is a photoconductive zinc oxide dispersed in a binder material of high electrical resistance, higher than that of the non-metallic base; other photosensitive components such as zinc sulfide, cadmium sulfide, and various selenides and oxides, particularly those applied from organic solvent systems, are useful.

In general, I prefer to mix the complete coating composition minus the hydrated inorganic salt; then the salt is added as a solution to gel the silicate; and this salt is added in sufiicient quantity to reduce the pH to at least 8.0 and preferably to between 7.2 and 7.8. Where a gellant other than the inorganic hydrated salt is employed, the gellant is added first to give the composition a pH of about 8.2, and after gelation takes place, the hydrated salt is mixed in. The same final pH ranges then hold for the completed composition.

The invention will be more fully understood by referenceto the following detailed examples and accompanying drawings wherein:

FIG. 1 represents a sectional view of an electrophotographic member in accordance with the invention; and

FIG. 2 is a graph illustrating resistance characteristics versus humidity of a conductive sheet prepared in accordance with the invention.

Example 1 A paper base comprised of about 55% by weight bleached sulfite and 45% kraft having a mixed Canadian standard freeness of about 350 and a basis weight of about 44 pounds (25 X 38 x 500 sheets) and a lateral resistivity in excess of ohms per square at RH, 70 F., was coated with the following composition:

Water suflicient to make solids of about 40%.

The above noted composition was formulated by combining the clay, starch and the hexametaphospha-te in slurry form with the water and cooked at a temperature of 190 F. The cooking time was 30 minutes. While the slurry was hot, the sodium silicate was added while mixing in a ribbon-type kneader. A Z-bar kneader or Lightnin mixer has also been found suitable. To the mixed slurry the magnesium chloride solution was then added slowly. During this addition there is a marked thickening of the batch and a tendency to appear grainy. This graininess disappears as the batch is mixed and the components are thoroughly interspersed. Subsequent to the incorporation of the magnesium chloride solution the soap is added dry. The viscosity of the resulting coating composition is approximately 60 as measured on the Brookfield viscometer with a No. 6 spindle at 100 r.p.m. at 50 C. The pH of the composition was about 7:5.

The composition was applied initially to one side of the paper sheet and the dry coating weight was about four pounds per ream (25 x 38 x 500 sheets). While application of a coating to the second side is not necessary for this purpose of achieving electrical conductivity, a coating on the second side is desired to avoid any curling of the final pro-duct. Accordingly, a coating of about the same weight and composition in this instance is applied to the second side of the sheet. The coating composition is applied to the sheet by conventional roll coating procedure and thereafter the sheet is dried and supercalendered.

In the course of the formulation of the composition, the magnesium chloride reacts with the highly alkaline sodium silicate to produce a hydrated silicic acid and this silica gel readily retains water. The magnesium chloride in the above formulation is itself added in sutficient amount to attain the pH of 7.5 in order that magnesium chloride hydrate is present in the final product. The dried web is supercalendered at low pressures sufiicient to attain a gloss of about 30 to simply smooth the coating on the sheet; the hydrated silicic acid and magnesium chloride hydrate are substantially unaffected thereby. I prefer low supercalender pressures to avoid forced impregnation of the sheet and to achieve a relatively stiff sheet which I have found more useful in the copying machines. The lateral resistance of this conductively coated sheet was 1.8 x 10 ohms/sq. at 50% RH at 70 F. and 5.2 x 10 ohms/sq. at 10% RH at 70 F.

A second coating comprising a dispersion of a photosensitive component in an insulating binder is then prepared as follows:

Parts by weight, lbs. 0

The above mixture is ground in a ball mill for several hours and is then ready for application as the upper layer coat. This composition is applied by rod coat procedure to the conductive coating on one side of the sheet and dried. The thickness of the photoconductive layer is about 0.45 mil.

The silicone resin employed is a film-forming commercial product marketed as GE Silicone Resin SR-82, a product of the General Electric Company of Pittsfield, Massachusetts. The zinc oxide is a French Process Phctox 801, a product of New Jersey Zinc Sales Company, Inc., of New York, NY.

Paper produced in accordance with this example was then subjected to an electrostatic charge in the dark and subsequently exposed to visible light radiation to effect discharge.

The following table illustrates the apparent surface voltage after charging, a dark decay period and after exposure to 3% foot candles for various lengths of time at 10% and 50% RH at 70 F.

Seconds V0 1 1 2 10% RH 676 541 91 73 21 0 50% RH 540 540 88 62 5. 9 0

1 Vn=apparent surface voltage after charge. 1 V =apparent surface voltage after 6.25 seconds dark decay.

As will be noted, the dark decay rate is small though slightly greater at 10% RH; also, the light decay results are entirely acceptable.

Additionally, sheets of the electrostatic recording medium were subject to regular printing operations and the prints were found to be excellent for their intended purpose.

Example 2 A paper base having a furnish as described in connection with Example 1 and a base weight of about 46 pounds (25 x 38 x 500 sheets) was coated with the following formulation:

Water addition 575 In the formulation of the above composition clay, starch, soap and water are cooked in conventional manner at about 185 F. for about 20 minutes. The mixture is then cooled to approximately F. and the sodium silicatae solution added. When the sodium silicate has been well mixed, the aluminum nitrate in solution is added slowly with continuous agitation. Again during the addition the mass thickens considerably and gel formation in the form of silicic acid takes place. With continued agitation the composition breaks down to yield a smooth coating composition easily applied by roll, rod or blade. Such composition has a solids content of about 30%, a Brookfield viscosity (same conditions as Example 1) of 54.7, and a pH of about 7.0.

The web is coated on both sides, dried and supercalendered, as described in Example 1. The coating weight in this instance was about 3 pounds per ream per side. The electrical resistance of the coated sheet at 14% relative humidity at 75 F. was 8.2 ohms/square, measured laterally. This sheet was then coated on one side with a zinc oxide coating as described in connection with Example 1 and was found thoroughly satisfactory in use in conventional electrostatic copying equipment.

In the above described electrostatic recording members the base sheet cocated with the electrically conductive layer serves as an excellent barrier to prevent penetration of the solvent of the zinc oxide coating during application of this latter coating.

The functioning of the hydrates apparently is as follows: as the humidity becomes lower, the hydrated silicic acid which is electrically conductive tends to give up a part of its water and in turn takes water from the hydrated inorganic salt. Since the latter salt is hygroscopic even at these low humidities, it takes water from the atmosphere to maintain its essentially hydrated condition. It is to be noted that the hydrated inorganic salt in the crystal form is not itself electrically conductive and does not contribute to the electrical conductivity of the sheet in any significant manner at higher relative humidities.

Example 3 The following coating composition has been found useful as a back coating for the second side of the sheet opposite the photoconductive coating. It is formulated and applied in the same manner as previously described. The utility is as a conductive coating as well as a backing coating.

Soap

Such composition, when applied as described in previous examples and utilized as the conductive barrier coating treatment for the photoconductive coat, has an electrical resistance of about 10 ohms/square at 14% RH at 75 F.

This latter formulation, for the purposes of enhancing its utility as a back coating, may have added thereto about 170 pounds of tack inhibitor such as Carbowax 4000 (a polyethylene glycol) to eliminate any tack in this high binder coating. Carbowax 4000 is a product of Carbide and Carbon Chemical Company and is a non-hygroscopic waxy solid of a molecular weight of about 4000 having a saybolt viscosity in seconds at 210 F. of about 375. When such component is utilized, it is preferable to add the same dry to the complete coating formulation set forth above and to knead the mass until the polyethylene glycol is dissolved. Such a coating composition has a solids content of approximately 37% and a viscosity (measured as previously described) on the Brookfield viscometer of approximately 75.

Referring now to the drawings, the numeral 1 designates a non-metallic base sheet such as paper of relatively poor electrically conducting properties in excess of 10 ohms/square. The numeral 2 indicates a conductive barrier such as has been previously described. The coating indicated at 3 is essentially a coating for the purpose of inhibiting sheet curl and may have the same constitution as coating 2 or may varying in specific composition as previously noted. The photosensitive coating is desig- 6 nated by the numeral 4 and comprises a photoconductive material indicated at 5 dispersed in an insulating binder 6. Binder 6 as is conventional in the art has a high electrical volume resistivity of about 10 to 10 ohms cm.

The paper with the intermediate layer 2 thereon, for the purposes of this invention, should have an electrical resistance of between about 10 to 10 ohms/square at relative humidities of 10-15% at about 70 F. It is to be noted that repeated tests utilizing the foregoing formulations and variations thereof as examplified by FIG. 2 of the drawings have provided sheets which meet this requirement. It is further to be noted, as is obvious, that the quantity of intermediate coating composition applied to the sheet will affect the electrical conductivity through the sheet and will, to some extent, affect the lateral conductivity. However, for application of photoconductive compositions having an organic solvent system, the minimum quantity of coating applied is an important factor in inhibiting solvent penetration. Such penetration is undesirable as it leads to inferior photoconductive coating lay. In general, coating weights per side with these aqueous mineral compositions of about 3 to 10 pounds are suitable. Additionally, the sheets should not be excessively conductive or they will fail to retain sufiicient of the charge and imperfect prints will result when used in a copying machine. I

It is my opinion that the efiiciency of the defined coating compositions and electrostatic recording members is due in large part to the cooperative relationship between the inorganic salts present and the silica gel, the silica gel functioning not only to maintain the member in suitable condition for receipt of charge and discharge, but also to prevent washout of the soluble salts during the coating application. Soluble salts present such as sodium chloride contribute in some measure to conductivity through the action of the hydrated components.

With respect to the coating composition, I may, as already noted, employ to 70% binder in the intermediate coating although I prefer about to based on the dry coating Weight. However, for backing coatings where solvent holdout is not as important, this range may be from 40% to and is preferably for economic reasons from 42% to 50% by weight on the dry coating. The backing coating is then less liable to blistering as the coating is more open and moisture escapes more readily when the paper is used in copying machines. Additionally, it is to be noted that I prefer that the silicate employed in the barrier coating composition suitably forms about 9% to 11% by weight dry 'basis, and the magnesium chloride hydrate about 11% to 13% by weight of the coating composition (dry basis), the balance being a pigment such as clay to facilitate composition drying. For a back coating composition the preferred range of silicate and hydrate are somewhat greater, about 11 to 14% and 12 to 15% (dry basis by weight) respectively, due to the decreased binder content of this coating.

It will be understood that this invention is susceptible to modification in order to adapt to different usages and conditions and, accordingly, it is desired to comprehend such modifications within the invention as may fall within the scope of the appended claims.

What is claimed is:

1. An electrophotographic member comprising a composite having a non-metallic base of high electrical resistance, a coating on said base for increasing the electrical conductivity of the composite, said coating comprising gelatinous hydrated silicic acid and a hygroscopic hydrated inorganic salt, said coating covered by a photosensitive stratum comprising a photoconductive component and an insulating binder, said component bein-g dispersed in said binder and said binder having an electrical resistance greater than that of the base.

2. An electrophotographic :member as claimed in claim 1 and in which the hygroscopic hydrated inorganic salt is selected from the group consisting of hydrated magnesium chloride and hydrated aluminum nitrate.

3. An electrophotographic member as claimed in claim 1 and in which the hygroscopic hydrated inorganic salt is hydrated magnesium chloride.

4. An electrophotographic member as claimed in claim 1 and in which the hygroscopic hydrated inorganic salt is hydrated aluminum nitrate.

5. An electrophotographic member comprising a paper base of high electrical resistivity, an upper layer comprising a zinc oxide photoconductor and an electrically insulating film-forming binder in Which the zinc oxide photoconductor is uniformly dispersed, said binder having an electrical resistance greater than that of the base and photoconductor, and an intermediate layer bonded to said base and said upper layer and comprising a coating having therein gelatinous hydrated silicic acid and a hygroscopic hydrated inorganic salt.

6. An eleotrophotographic member as claimed in claim 5 and in which the hygroscopic hydrated inorganic salt is selected from the group consisting of hydrated magnesium chloride and hydrated aluminum nitrate.

7. An electrophotographic member as claimed in claim 5 and in which the hygroscopic hydrated inorganic salt is hydrated magnesium chloride.

8. In the manufacture of coated paper for electrophotographic purposes wherein an overlying photosensitive stratum is applied, the step, prior to the application of the photosensitive stratum, of applying an aqueous mineral conductive coating containing gelatinous hydrated silicic acid and a hygroscopic hydrated inorganic salt, said coating composition having a pH of from 4 to about 8.0.

9. In the manufacture of coated paper for electrophotographic purposes wherein an overlying photosensitive stratum is applied, the step, prior to the application 8 of the photosensitive stratum, of applying an aqueous mineral conductive coating containing gelatinous hydrated silicic acid and a hydroscopic hydrated inorganic salt, said coating composition having a pH of from about 7.2 to 7.8.

10. In the manufacture of coated paper for electrophotographic purposes wherein an overlying photosensitive stratum is applied, the step, prior to the application of the photosensitive stratum, of applying an aqueous mineral conductive coating containing gelatinous hydrated silicic acid and a hygroscopic hydrated inorganic salt, said coating composition having a pH of from about 7.2 to 7.8 and a binder content based on the total solids of the composition of between about and by weight based on the total solids of the composition.

11. An electrophotographic member comprising a paper base of high electrical resistivity, an upper layer comprising a zinc oxide photoconductor and an electrically insulating film-forming binder in which the zinc oxide photoconductor is uniformly dispersed, said binder having an electrical resistance greater than that of the base and photoconductor, and an intermediate layer bonded to said base and said upper layer and comprising a coating having therein gelatinous hydrated silicic acid and a hy-groscopic hydrated inorganic salt, and a backing coating on the second side of the base, said backing coating also containing gelatinous hydrated silicic acid and a hygroscopic hydrated inorganic salt.

12. An electrop'hotographic member as claimed in claim 11 and in which the backing coating contains an agent to inhibit tack and blocking.

No references cited.

NORMAN G. TORCHIN, Primary Examiner.

Claims

1. AN ELECTROPHOTOGRAPHIC MEMBER COMPRISING A COMPOSITE HAVING A NON-METALLIC BASE OF HIGH ELECTRICAL RESISTANCE, A COATING ON SAID BASE FOR INCREASING THE ELECTRICAL CONDUCTIVITY OF THE COMPOSIT, SAID COATING COMPRISING GELATINOUS HYDRATED SILICIC ACID AND A HYGROSCOPIC HYDRATED INORGANIC SALT, SAID COATING COVERED BY A PHOTOSENSITIVE STRATUM COMPRISING A PHOTOCONDUCTIVE COMPONENT AND AN INSULATING BINDER, SAID COMPONENT BEING DISPERSED IN SAID BINDER AND SAID BINDER HAVING AN ELECTRICAL RESISTANCE GREATER THAN THAT OF THE BASE.