US3694202A - Paper containing electroconductive pigment and use thereof - Google Patents

Abstract

THE CONDUCTIVITY OF PAPER IS INCREASED BY INCORPORATING A CONDUCTIVE ZEOLITIC ALUMINOSILICATE WITH THE PAPER AS A COATING OR AS A FILLER. THE RESULTING PAPER IS USEFUL IN VARIOUS NONIMPACT PRINTING PROCESSES. FOR EXAMPLE. WHEN PROVIDED WITH A SURFACE COATING OF PHOTOSENSITIVE MATERIAL SUCH AS ZINC OXIDE, THE PAPER IS EMPLOYED IN DIRECT ELECTROPHOTOGRAPHIC COPYING.

Description

UnitedStates Patent O 3,694,202 PAPER CONTAINING ELECTROCONDUCTIVE PIGMENT AND USE THEREOF Edgar W. Sawyer, Jr., 22 Nottingham Road, Edison, NJ. 08817, and Frank J. Dzierzanowski, 2 Norfolk Road, Somerset, NJ. 08873 No Drawing. Filed June 5, 1970, Ser. No. 43,951 Int. Cl. G03g 7/00; D21h 3/66 US. Cl. 961.8 8 Claims ABSTRACT OF THE DISCLOSURE The conductivity of paper is increased by incorporating a conductive zeolitic aluminosilicate with the paper as a coating or as a filler. The resulting paper is useful in various nonimpact printing processes. For example, when provided with a surface coating of photosensitive material such as zinc oxide, the paper is employed in direct electrophotographic copying.

BACKGROUND OF THE INVENTION In the manufacture of direct electrophotocopying paper (such as Electrofax), a sized paper base stock is coated on both sides with a conductive layer or coating. One side of the paper is then provided with a photosensitive layer, usually a photosensitive grade of zinc oxide. Normally the photosensitive layer also contains dyes to extend the spectral sensitivity of the photosensitive layer to a desired range.

The light-sensitive coating is charged by exposed the sheet to a corona discharge. The charged sheet is imaged by exposing the sheet to radiation from various sources such as a vidio tube, projected originals, etc. The portions of the charged photosensitive surface which have been exposed to light discharge through the conductive resin layer; the portions which are not exposed to the light retain their charge. The exposed sheets are then treated with a toner, which comprises finely divided pigmented resin particles. Usually the toner isapplied in the form of a colloidal dispersion in mineral spirits. When treated with the toner, the charged areas on the sheet selectively pick up the pigmented resin, thereby developing the latent image. To set the pigmented resin on the sheet, the sheet is heated to drive off the solvent.

In present electrophotocopying paper the conductivity of the paper substrate is increased by coating the paper with a conductive resin, specifically a polycationic resin having a high charge density. In direct electrophotocopying paper, the conductive resin coating under the photosensitive coating functions to dissipate the electrical charge from the irradiated areas of the photosensitive coating. It also effects decay (dark decay) of the charges on the photosensitive layer.

The conductive resin coating on the reverse side of the sheet effects charge decay and it also aids in dissipating charge during irradiation. In addition, this conductive coating functions to prevent smudging in larger white discharged areas on the photosensitive coating during toner pickup. When a wet toner system (e.g., a toner containing mineral spirits) is employed, the resin coating on the reverse side of the sheet may also function as a barrier to prevent penetration of the toner solvent into the sheet.

Electrically conductive paper is also required in other nonimpact printing processes, including certain electrographic processes which do not use light energy to develop an image and processes in which static charges are not involved.

Among the other agents suggested to increase the conductivity of paper for use in various nonimpact copying processes are humectants, hygroscopic salts, polyanionic ice resins, graphite and metal powders such as powdered aluminum. See an article by Vaurio and Fird, Electrically Conductive Paper for Nonimpact Printing, TAPPI 47, No. 12, 163A166A, December 1964. The humectants, hygroscopic salts and polyanionic resins generally function satisfactorily at relative humidities of 20 percent. However, at lower relative humidities they are unsatisfactory. Graphite and metallic powders function satisfactorily over an range of relative humidities since the conductivity of such material is independent of humidity, but they are objectionable for direct electrophotographic paper because of their color and their weight. Aluminum foil provides an ideal substrate with regard to conductivity regardless of relative humidities and it is obviously a barrier against the penetration of solvent. However, the shortcomings of aluminum foil as a base for the manufacture of direct electrophotocopying paper are obvious.

Thus, to date the commercially successful materials for imparting conductivity to the base sheet of direct electrophotocopying paper are certain conductive cationic resins.

The cationic conductive resins, however, have the following deficiencies:

(1) They are expensive;

(2) They must be dried on the base sheet under carefully controlled conditions. If dried insufiiciently, the coatings become tacky at high relative humidities. If overdried, the resins a relatively non-conductive at low relative humidity;

(3) The resin coating on the reverse side of the sheet is generally tacky;

(4) The resins have an offensive odor;

(5) The resins yellow on aging;

(6) The conductivity of the resins leaves something to be desired. (Typically the surface resistivity of the resincoated paper is from 2x10 to 2x10 ohm.)

THE INVENTION A general object of the invention is to provide novel conductive paper.

Another object is to increase the conductivity of paper by including in the paper, as a coating or as a filler, certain zeolitic aluminosilicate pigments.

A specific object is to provide improved composite paper for direct electrophotographic copying.

Stated briefly, the presented invention resides in the use of finely divided naturally-occurring or synthetic zeolitic aluminosilicate pigments which exhibit relatively high conductance without external excitation as fillers or coatings to increase the conductivity of cellulosic paper. Suflicient zeolitic pigment is used to provide a sheet having a surface resistivity with the range of 1x10 to 1 10 ohm at relative humidities within the range of 7 percent to percent. (A procedure for testing surface resistivity is described in ASTM D25 6-46).

The term zeolite as used herein refers to an aluminosilicate which is built up of linked $0., and A10 tetrahedra which form a negatively charged network which contains channels running therethrough. In most zeolites, water molecules occupy the channels when the zeolites are in hydrated form. The charge of the network is electrically balanced by cations which lie between the channels and are mobile in the sense that they can move from the spaces they occupy and be replaced by other cations of suitable size. In the case of a sodium aluminosilicate zeolite there is an exchangeable sodium ion for every aluminum atom in tetrahedral coordination.

One preferred class of zeolitic zeolites includes natural and synthetic hydrated crystalline zeolitic aluminosilicates containing mobile cations such as, for example, H+, NHJ, Ca++, Mg++, Na K' Li+.

Another preferred class of silicates includes synthetic amorphous precipitated hydrated zeolitic sodium alumino silicates of the type described in U.S. 2,739,073 and U.S. 2,848,346 to Bertorelli. The sodium in these zeolites may be substituted with other cations such as those listed here inabove.

In one important embodiment of the invention, a paper base sheet which is coated or filled with conductive zeolitic pigment to increase its conductivity is provided with a surface layer of photsensitive material such as zinc oxide to provide improved direct electrophotographic copying paper.

PRIOR ART British Pat. 1,092,600 describes unsuccessful attempts to impart antistatic properties to various synthetic resins by means of small amounts of natural and synthetic zeolites. The zeolites alone had no antistatic effect and cationic surface active agents had to be employed with the zeolites.

U.S. 3,063,784 to Etchison suggests reacting nylon with a specific type of clay (montmorillonite) to prevent static charge buildup and dry soiling. Montrnorillonite clay is not a zeolite within the scope of the instant invention.

DISCUSSION The electrically conductive paper base of the present invention is especially useful as an element of copying paper for the direct electrophotographic process (Electrofax). In addition to reducing the surface resistivity of the paper substrate, the zeolite pigments may also impart a desirable white opaque appearance to the copying sheets. Such as appearance is not obtainable when cationic polymers, metallic powders or graphite are employed to increase the conductivity of the paper. Furthermore, the zeolites have the advantage over cationic polymer of be ing conductive under a wider range of conditions of temperature and humidity.

Reference is made to U.S. 3,052,539 to Greig and U.S. 3,121,008 to Jones et al. for details as to the preparation of photoconductive insulating layers of zinc oxide and insulating resin binder for use in the preparation of direct electrophotographie paper. The photoconductive zinc oxide layer is normally sensitized with a dye such as one of the dyes enumerated in U.S. 3,121,008 to Jones et al. or U.S. 3,052,040 to Greig.

Paper base which has been rendered electrically conductive by means of the presence of zeolitic pigments, in accordance with the present invention, may also be employed in any of the nonimpact printing methods described in the article by Vaurio and Fird (supra). For example, the zeolites may be used in Timemark and Teledeltos type papers. In such papers an image is formed by causing an electrical discharge from a wire stylus to pierce an insulating film and expose the electrically conductive paper. This printing method may be called electrography. When used in such paper the insulating film may have an intense color to provide a contrast with the white image when the zeolite is exposed. The zeolites may be used in similar paper in which the electrically conductive base sheet carries a white nonconductive coating capable of storing a latent image formed as an electrostatic charge (e.g., Videograph electrography paper) As another example, the conductive paper may be employed as the image receiving substrate for printing by xerography, such as the process described in U.S. 3,121,- 006 to Middleton et al. Sheets coated on one side (nonima-ge receiving side) with zeolitic pigments are useful for indirect Xerox copying since the sheets will be free from static charges prior to, during and after image reception.

The crystalline aluminosilicate zeolites used in carrying out this invention are electrically conductive without external excitation over a wide range of relative humiditiese.g., from percent to 85 percent RH.

4 Generally, useful aluminosilicate zeolites are defined by the formula:

Mz o Z wherein M is cation such as hydrogen, ammonium, alkali metal, alkaline earth metal and combinations thereof; N is hydrogen or sodium; n is the valence of 'M; x and y are positive numbers which vary with the species of zeolite and fall within a definite range for each species. as usually varies from 2 to 15 and y from 1 to 10.

A preferred class of zeolites has a rigid, three-dimensional crystalline framework of SiO and A10 tetrahedra with pores of uniform diameter within the range of 3 to 15 angstrom units. These zeolites are known in the art as zeolitic molecular sieves and have SiO /Al -O ratios (x values) from 2 to 6. Species are designated by letters, e.g., Type A (U.S. 2,882,243), Type X (U.S. 2,882,244), Type Y (U.S. 3,130,007). To be useful in the practice of the invention the sieves must be in hydrated form, e.g., N is hydrogen and the 1 values range from 2 to 9, and the sieves must contain mobile, ion-exchangeable cations. A so-called decationized sieze (see U.S. 3,130,006) is not suitable.

Also within the scope of the invention are natural crystalline zeolites and their synthetic counterparts. Examples an analcite, erionite, cha'bazite, faujasite (similar to synthetic type X and Y), natrolite, thomsonite, laumonite, scolecite, edingtonite, phillipsite, gmelinite, levynite, mordenite, etc. Synthetic counterparts of those natural zeolite may be employed. Excellent results have been obtained with synthetic sodalite of the approximate formula Other conductive crystalline hydrated zeolites containing exchangeable cations are the clay minerals attapulgite and sepiolite. The pure minerals may be used or the clay may be employed after removing grit and gross impurities.

The precipitated zeolitic pigments of Bertorelli Pat. 2,739,073 consist essentially of oxides of alkali metal, aluminum and silicon in a molar ratio of SiO /Na O of at least 4/1 and have a molar ratio of Na O to A1 0 of about 1/ 1. The zeolites are obtained in extremely small particle sizes (finer than /2 micron) by reacting at low concentrations dilute aqueous solutions of alkali silicate and an aluminum salt of a mineral acid. The pigments obtained contain about 10 percent water (after being dried at C.). To be useful in practice of this invention, at least a substantial proportion of this combined water must be present with the zeolite. Thus the precipitated amorphous zeolites should not be overdried.

The zeolites of Bertorelli patent U.S. 2,848,346 are obtained by a modification of the precipitation process of U.S. 2,739,073 wherein the zeolite is precipitated in the presence of pre-precipitated silica. The pigments have a Na O/Al O mole ratio of about 1 and a Si-O /Na O mole ratio within the range of about 4 to 14. These zeolites contain about 10 percent water when dried at 110 C. and must contain water of composition when employed in the practice of this invention.

The zeolitic molecular sieves and synthetic zeolites are normally produced in sodium form. Naturally occurring zeolites usually contain sodium, calcium or barium or combinations thereof or with magnesium as the principle exchangeable cations. These natural zeolites or the synthetic zeolites may be ion-exchanged before use.

The zeolites should be free from grit (particles plus 325 mesh Tyler). The particles are preferably finer than 10 microns (equivalent spherical diameter). Particles within the range of about 0.3 to 5 microns are especially desirable.

In an embodiment of the invention, the zeolite is incorporated with the paper as a filler. This may be accomplished by the conventional method of adding the finely divided zeolite particles to cellulose pulp and forming the mixture into a sheet by means such as a Fourdrinier screen. Filler retention aids such as alum may be employed. Alternatively, the zeolite can be incorporated by spraying a slurry containing the zeolite onto the paper Web at or near the wet end of a papermaking machine.

When employed as filler the zeolite is generally present in amount within the range of about percent to percent, based on the dry weight of the paper. When used in amount less than about 10 percent, the effect of the zeolite may be minimal or insufficient. When used in amount in excess of 15 percent, certain properties of the paper such as strength may be adversely affected. The filled paper is normally sized, dried, calendered, etc.

To produce zeolite-filled direct electrophotographic copying paper, a photosensitive coating (e.g., a coating described in Greig patent U.S. 3,052,539) is applied to one side of the zeolite-filled paper base. Preferably a barrier coating, e.g. a coating of starch or polyvinyl alcohol, is applied to the reverse side.

It is also within the scope of the invention to include the zeolite in paper as a coating. This coating may be applied to uncoated paper or the coating may be superimposed on a coating of clay or the like. When used as a coating, it may be desirable to dilute or blend the zeolite with kaolin clay or calcium carbonate because coating compositions containing such mixtures exhibit better coating characteristics than do compositions containing a zeolite as the sole pigment.

The conductive pigment may be incorporated with the paper as a coating on one side or both sides of the paper sheet. The coating is normally applied to the sheet as an aqueous composition containing conventional coating adhesives (such as starches, e.g., oxidized, chemically modified, etc., protein, latex, starch-latex, protein-latex, etc.).

It is within the scope of the invention to provide sheets containing a coated layer of electrically conductive zeolitic pigment underlying a photosensitive coating and a barrier coating, e.g., starch or polyvinyl alcohol, coated on the underside (reverse side) of the sheet. It is also within the scope of the invention to coat both sides of paper sheet with conductive zeolitic pigment, apply a photosensitive layer of zinc oxide over one of the coatings and, if necessary, apply a barrier coating over the conductive coating on the underside.

The use of conductive white or oil-white zeolitic aluminosilicate pigments, in accordance with the present invention, represents an improvement over the use of polycationic resins in many respects. The coated sheets may have better color and the sheets will not yellow with age. The conductivity of the paper sheets may be increased at a fraction of the cost entailed when using the resins. An important advantage of using zeolites to decrease the electrical resistivity of paper is that sheets containing conductive zeolitic pigments have appreciably better conductivity at low relative humidity than those treated with cationic resins. Problems encountered when cationic resins are overdried or underdried are obviated. Furthermore, sheets formulated with the zeolitic silicate pigments are free from objectionable odor and are nontoxic.

The paper stocks employed to produce the base sheets for nonimpact printing will vary, of course, with the printing process. [Paper stocks suitable for direct electrophotographic paper containing photosensitive zinc oxide are described in an article by Joseph Savit, -A {Review of Physical, Chemical and Electrical Characteristics of Liquid Toner Conductive Base Papers, TAPPI, vol. 52, No. 10, October 1969.

Surface resistivity values for various types of paper used in nonimpact printing appear in the article by Vaurio and Fird (supra). According to data in this article, at 12 percent R. H., the surface resistivity Values for sulfite bond (Aquapel size and rosin size), kraft (impregnated, and ground wood) exceeded 3.75 10 ohm. By filling such papers with hydrated zeolites containing mobile cations (or by coating the paper with these zeolites or mixtures thereof with diluents such as kaoline clay), the surface resistivity of such paper can be decreased to values at least comparable to those obtained with commercial cationic resins.

Example I This example demonstrates the desirable electrical properties of zeolites within the scope of the invention.

Volume resistivities of cakes of sodium zeolite Y and Zeolex 20 were measured by the procedure substantially as described in US. 3,011,918 to Silvernail et al. Zeolex 20 is an amorphous precipitated hydrated sodium aluminosilicate of the type described in the Bertorelli patents. Volume resistivity of starch coatings containing these zeolites were also measured. The coatings were made by applying an aqueous coating formulation containing parts by weight zeolite to 25 parts oxidized starch to aluminum foil and drying at 250 F. for 15 minutes. Similar coatings for aluminum foil were prepared containing 25 parts by weight zeolite, 25 parts by weight of No. 1 grade coating kaolin clay as a diluent and 25 parts by weight starch. For purposes of comparison, Dow Cationic Resin ECR 34 was applied on another piece of foil. The resin is a polymer of vinylbenzyl quarternary ammonium compound of the type described in U.S. 3,011,918 (supra).

In all cases measurements were made at 156 F. on samples oven dried at 250 F. for 15 minutes and at 250 F. on samples oven dried at 250 F. for 4 hours.

The volume resistivities of cakes and coatings containing zeolite or mixtures of zeolites with a clay diluent were less than 4x10 ohm-cm. for measurements made at 158 F. The resin coating had a higher volume resistivity (158 F.) of 3.6 10 ohm-cm. At 250 F., sodium zeolite Y and coatings containing zeolite Y or Zeolex 20 without a diluent had volume resistivities below 4x10 ohm-cm. The coatings with clay diluent had volume resistivities of the order of 10 ohm-cm. The cationic resin decomposed after being heated at 250 F. for 4 hours and the resistivity at 250 F. could not be measured.

The data in this example therefore show that a synthetic crystalline zeolite (sodium zeolite Y) and a synthetic amorphous zeolite (Zeolex) were at least as effective as a commercial cationic resin in imparting electrical conductivity to paper when coated on the paper.

Example II This example illustrates the preparation of electrically conductive zeolite-filled paper.

A conventional method for filling the paper was used. Chemical fiber was pulped in water at about 1 percent solids. To portions of the pulp, Zeolex 20 and sodium zeolite Y were added, followed by addition of alum as a retention aid. The slurries were drained over a forming screen in conventional manner to form sheets. The sheets were dried and calendered. The filler contents of sheets were computed after determining the ash contents.

It was found that sheets containing 9 percent to 15 percent of the zeolites were more conductive than sheets of the same paper coated with the cationic resin used in Example 1.

Example III The procedure of Example I was repeated using Attagel 50 (fluid energy milled, degritted, crystalline hydrated attapulgite clay) as the zeolitic material. The coating contained 25 parts by weight Attagel, 75 parts by weight of No. 1 grade kaolin clay and 25 parts by weight of the starch. The volume resistivity of the coating was similar to that of coatings containing sodium zeolite Y or Zeolex 20.

Example IV Aluminum foil was coated at 50 percent solids with a ball-milled mixture containing 25 parts by weight synthetic crystalline so'dalite (3Na O.Al O .2SiO .2NaOI-I), 75 parts by weight No. 1 kaolin coating clay and 25 parts by weight cornstarch. The volume resistivity was 1.6 X 10 ohm-cm. at 5 percent R.H.

Another piece of foil was coated at 50 percent solids with a coating composition containing 100 parts by weight of synthetic sodalite (neutralized with hydrochloric acid) and 30 parts by weight cornstarch. The volume resistivity was 3.1 X 10 ohm-cm. at 5 percent R.H.

1W6 claim:

1. A photoconductive element for direct electrophotographic printing comprising an electrically conductive base sheet which consists essentially of cellulosic paper filled with finely divided particles of an electrically conductive zeolitic aluminosilicate containing mobile oations, said particles being present in amount such that said filled base sheet has a surface resistivity within the range of 1x10 to 1x10 ohm at relative humidities Within the range of 7 percent to 85 percent, and on a surface of said electrically conductive base sheet a continuous layer of photoconductive zinc oxide and an insulating film-forming organic resin binder therefor.

2. A photoconductive element for direct electrophotographic printing consisting essentially of a cellulosic paper base sheet material carrying on at least one surface thereof a continuous coating of a zeolitic aluminosilicate containing mobile cations in quantity sufficient to provide a coated base sheet having a surface resistivity within the range of 1 10 to 1X10 ohm at relative humidities within the range of 7 percent to 85 percent, and on one outer surface of said paper a continuous layer of photoconductive zinc oxide and an insulating film-forming organic resin binder therefor.

3. The photoconductive element of claim 1 wherein said zeolitic aluminosilicate is a crystalline molecular sieve having uniform pore openings within the range of 3 to 15 Angstrom units.

4. The photoconductive element of claim 1 wherein 8 said zeolitic aluminosilicate is a synthetic amorphous precipitated hydrated sodium aluminosilicate.

5. The photoconductive element of claim 1 wherein said zeolitic aluminosilicate is synthetic sodalite.

6. The photoconductive element of claim 2 wherein said zeolitic aluminosilicate is a crystalline molecular sieve having uniform pore openings within the range of 3 to 15 Angstrom units.

7. The photoconductive element of claim 2 wherein said zeolitic aluminosilicate is a synthetic amorphous precipitated hydrated sodium aluminosilicate.

8. The photoconductive element of claim 2 wherein Said zeolitic aluminosilicate is synthetic sodalite.

References Cited UNITED STATES PATENTS 3,295,967 1/ 1967 Schoenfeld 96-1.5 2,739,073 3/ 1956 B'ertorelli 106-306 X 3,266,973 8/1966 Crowley 1621 81 C 3,294,535 12/ 1966 Powers 96-49 2,918,399 12/1959 Eichmeier 162-181 C 3,063,784 11/1962 Etchison 8-1155 2,637,651 5/1953 Copley 96-1.4 X

FOREIGN PATENTS 1,092,600 11/ 1967 Great Britain.

449,713 5/1969 Japan 96-1.5

GEORGE F. LESMES, Primary Examiner R. E. MARTIN, JR., Assistant Examiner US. Cl. X.R.

, Patent No. 3,694,202 Dated September 26, 1972 Inventor) Edgar W. Sawyer, Jr. and Frank J. Dzierzanowski It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1 line 7, insert after the inventors names and addresses assignors to Engelhard Minerals & Chemicals Corporation, Township of Woodbridge, New Jersey line 32, "exposed" should read exposing Column 2 line 9, "over an" should read. over a Column 4 line 26, "an" should read are Column 5 line 75, "kaoline" should read kaolin Signed and sealed this 24th day of April 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTT'SCHALK Attesting Officer Commissioner of Patents FORM PO-1050 (10-69) USCOMM-DC 6O376-P69 u.s. GOVERNMENT PRINTING OFFICE: 1989 o-aee-su