US3403023A - Dye sensitization of zinc oxide photoconductors - Google Patents

Description

United States Patent 3,403,023 DYE SENSITIZATION 0F ZINC OXIDE PHOTOCONDUCTORS Hugh R. Carrington, Binghamton, and Frank M. Gaesser,

Jr., Endicott, N.Y., assignors to GAF Corporation, a

corporation of Delaware No Drawing. Filed May 14, 1965, Ser. No. 455,971 12 Claims. (Cl. 961.7)

ABSTRACT OF THE DISCLOSURE Sensitization of photoconductive zinc oxide by the utilization of dye blends having dye components capable of absorbing for each of the primary colors, and including components designed to overcome the tendency of the background to yellow.

The present invention pertains to zinc oxide photoconductive materials for use in electrophotographic printing and more particularly to such materials which are panchromatically dye sensitized zinc oxide-binder coatings having the desi ed speed, balance, background and storage stability.

The commonly practiced form of electrophotographic printing uses as a photosensitive element a thin zinc oxide photoconductive insulating layer coated on a suitably conductive base sheet.

In this 'form of electrophotography, it is customary to deposit: an electrostatic charge on the surface of the zinc oxide insulating layer while the backing layer is either connected to ground or covered with an approximately equivalent charge of opposite polarity.

The zinc oxide insulating layer is then illuminated by a light and shadow pattern which represents the image to be reproduced. Where illuminated the hitherto insulating photoconductive layer becomes electrically semi-conductive and therefore any previously deposited electrostatic surface charge is dissipated conductively in consonance with the actinic radiation which each element of area receives. In this way an electrostatic latent image is formed, the greatest surface charge density remaining in unexposed areas and the least in maximally illuminated portions of the photoconductive element.

The latent image is then rendered visible by development with electrostatically charged pigment particles or toners which may be applied to the image as powder clouds, as suspensions in insulating liquids of low di electric constant or the like. The toner adheres differentially to charged and discharged areas thereby rendering them visibly different. The visible image may be fixed to the backing member by fusion or may be transferred to another sheet to which it is fused.

Zinc oxide is almost universally used as the photoconductor in such electrophotographic printing processes, particularly where the powder image is fused to the base of the photoconductive recording element. Zinc oxide, however, has one particular property which mitigates against its most efiicient use for the intended purpose and that is its restricted or limited spectral response which is confined essentially to the ultraviolet. Because of such limitation, best results are only secured where U.V. radiation is available. Use of white light containing but a minimum amount of U.V. very materially reduces the speed of response of the photoconductor during exposure.

This defect has been recognized and efforts have been made to cope with it by dye sensitizing the zinc oxide to make it responsive to light rays in the visible spectrum. This is accomplished by the employment of an organic dye or dye mixture incorporated with the zinc oxide and having the ability of absorbing radiant energy in the ice visible spectrum and transferring the absorbed energy to the photoconductor. Various dyes and dye blends designed to accomplish this end are mentioned in Greig, U.S.P. 3,052,540 dated Sept. 4, 1962, and Sugarman et al., U.S.P. 3,051,569 dated Aug. 28, 1962.

It is represented in the prior art that the dye blends employed will not only sensitize in the visible spectrum but will give when mixed with zinc oxide a mixture which when viewed by reflected light will have a white or offwhite appearance. It is also represented that the dye blends will have several peaks in different regions of the visible spectrum.

It goes without question that the contributions made by the prior art have improved the response of the zinc oxide to visible radiation and have expanded employment of the zinc oxide insulating materials to permit more effective image discharge by visible radiation. Unfortunately, however, the improvements have not been completely satisfactory in meeting the prerequisites of an electrophotographic zinc oxide binder coating as regards speed, balance and background and storage stability.

It would appear self-evident that the ultimate in dye blends for panchromatic sensitization of the zinc oxide should be one in which there is present a dye for each primary color and which absorbs for only one primary color, i.e., a blend of a pure cyan, magenta and yellow. Such a dye blend when mixed with zinc oxide and the mixture viewed by reflected radiation would appear as white. However as is well known, such ideal dyes are the most difficult things to come by. Thus there are few dyes of the desired color or shade which do not have overlapping absorptions in the visible spectrum. Typically most red dyes have a blue or yellow cast and absorb not only green but red and blue as well; again green dyes usually have a yellow cast thus causing absorption of red and blue. Since economy of operation dictates for the most part the utilization of dyes available on the open market, the selection of a dye blend having optimum characteristics forces the operator to Work within very narrow confines.

Rather peculiarly too, the lack of balance found in most zinc oxide dye sensitized systems is traceable to the cyan or green-blue dye. Most often such dye oversensitizes in the red region so that on light image exposure the red record is discharged or burned out excessively. This in turn means that in the recorded image there is a deficiency in the red response. It likewise leads to a sensitized element whose storage stability is more than a little suspect.

The problem of proper dye sensitization is further aggravated by the fact that most electrophotographic papers have a decided tendency to yellow and to produce discolored backgrounds. This deleterious effect can be overcome by use of a masking dye for yellow, i.e., blue. However, in the elimination of this headache, care must be taken not to create an ever greater one by the use of a dye which throws the sensitizing dye blend completely out of color balance.

We have now discovered that we can produce zinc oxide insulating elements for electrophtographic recording which have the following characteristics:

(1) Proper response to visible light,

(2) High visible light speed,

(3) Storage stability, and

(4) Masked yellow background.

These ends are attained by the utilization of special dye blends having dye components capable of absorbing for each of the primary colors while yielding a mixture with zinc oxide which when viewed by reflected light appears white to gray, said blend including components designed to overcome the tendency of the background to yellow.

The recording element through which we realize our objectives comprises abacking member which may be rendered conductive, as in conventional practice, coated with an insulating binder-resin having a white zinc oxide photoconductor dispersed therein. The zinc oxide is treated with a dye blend to make it absorptive to light rays of the visible spectrum to ensurethat it will have proper speed and balance when irradiated imagewise with white light. Agents are also present or provide an uncolored background.

The backing member may be any suitable paper base rendered conductive (i.e., 10 ohm cm.) by any of several means.

The zinc oxide is French process zinc oxide such as that sold by the New JerseyZinc Sales Company Incorporated of New York under the trade name Photox 801.

The resin binder may be any suitable hydrophobic insulating resin of the type used in the manufacture of electrophotographic materials. Examples of such resins are silicone, polystyrene, polyvinyl acetate and the like. We obtain particularly good results with epoxy ester resins in which the ester group is supplied by a fatty acid, i.e., stearic, or a rosin acid and abietic acid. Particularly suitable is the modified epoxy resin which is commercially available as Epidene E 168/50 from the T. F. Washburn Co., Chicago, Ill. This compound can be described as an epoxy ester polymer derived from bisphenol A, the di-(4-hydroxybenzene)-dimethylmethane, which is further modified by a fatty acid, preferably oleic acid. Epoxy polymers modified by abietic acid or stearic acid are equally suitable. Fatty acid mixtures containing oleic and linoleic acid and derived from tall oil are especially preferred.

The contemplated recording elements may be prepared by mixing a solution of an epoxy ester resin in toluene or xylene with small quantities of metallic driers, such as the naphthenates of cobalt and maganese. Typical pigment resin ratios range from 5.5 to 6.0.

- There is also added a plasticizer with the resin. For this purpose we prefer to use a petroleum-derived polymeric hydrocarbon with a fair degree of aromaticity and relatively low iodine value. A suitable plasticizer meeting these prerequisites is the product sold as Velsicol TPO #1 by Velsicol Chemical Corp., 535 5th Ave., New York, NY.

This product is a resinous, dark polymeric material of low molecular weight derived from a petroleum hydrocarbon and prepared from a mixture of styrene, a lower styrene homolog, such as a-methylstyrene and a naphthol by the use of a suitable polymerization catalyst.

To this is added the zinc oxide in typical pigment to resin ratios of from 3.5 :1 to 6.0:1. This mixture is then dispersed in any of several means to reduce agglomerates to less than 1p. average particle size. This can be accomplished by ball milling or by the use of dispersing equipment as is found throughout the paint industry. The dispersed slurry is sensitized by adding a small quantity of an alcoholic solution of the desired mixture of organic dyes.

The slurry may be coated on to the paper base by various methods using either a two-step application by an oxide or a one-step application by wire wound bar or gravure roller. After drying the coating is ready for use in the conventional xerographic or electrophotographic process.

As previously noted, the dye blend which we use absorbs for the primary colors and therefor has a magenta component, a cyan component and a yellow component. It has been ascertained that the magenta component is nicely supplanted by Rose Bengal either alone or in ad mixture with Merthiolate and Eosin Y. A better balance results with the mixture than with the Rose Bengal per se.

We use Merthiolate which is sodium ethyl mercury thiosalicylate as a catalyst. This compound is colorless but is dyed magenta by Eosin Y.

A yellow component for the absorption may be added to the dye blend and for this purpose we may use the sodium salt of Fluorescein. However absorption for the blue and red may be obtained by a mixture of Chlorophyll and Patent Blue V with or without the Fluorescein. Furthermore, it is possible to supplant the Patent Blue V by Patent Blue A sold as Alphazurine A by National Division, Biological Stains Department, 40 Rector Street, New York. It is, of course, understood that inmodifying the blends by substituting one dye for another an adjustment in concentration must be provided for.

The dye components discussed above provide the necessary sensitization and as a matter of fact when using Patent Blue A for Patent Blue V and by varying the concentration of the Patent Blue A, provision is made for avoiding the tendency of the background to yellow. It is preferred however to effect this result by incorporation of a separate masking component such as a blue pigment or a mixture of a coupling component and a sensitized diazoamino compound which forms a blue dye on coupling. Such materials are available on the open market. Suitable materials comprise a mixture of a Naphthol AS coupling component and a stabilized diazoamino compound of a 2, S-dialkoxy-4-benzoylamino aniline. Preferably, we use Naphthol ASBG in admixture with the diazos from 2,5-dimethoxy-4-benzoylamino aniline and 2,5-diethoxy-4-benzoylamino aniline, the azo groups being stabilized by N- methyl taurine. This mixture will hereinafter be referred to as Blue Masking Dye Component. Another method for reducing the visible yellowing of background is to add an inert light-fast blue dye not in the alcoholic sensitizing solution but dissolved in a solvent common to the resin, such as toluene or xylene. For this purpose we prefer to use a dye such as Heliogen Blue Oil Soluble, a phthalocyanine type available from Antara Chemicals, New York. This solution will hereinafter be referred to as Blue Masking Dye Component 2.

As is understandable, the ratios of the various components in the sensitizing composition will not vary to any substantial degree. We have found that by utilizing the component in the following proportion per pound of zinc oxide that a recording paper meeting the full requirements of the trade can be produced:

The following examples will serve to illustrate the invention but it is understood that the invention is not limited thereto.

EXAMPLE I A toluene solution was prepared of the following components:

vGrs. Epoxy ester resin (50%) 788 Velsicol TPO #1 78.8 Cobalt Napthenate (6%) -1 1.63 Manganese Naphthenate (6%) 1.63

'Into this was slurried 2100 grs. French Process zinc oxide and the mixture dispersed on a standard pebble mill.

The slurry was then sensitized with a solution of the following composition:

Rose Bengal mgs 176.4 Chlorophyll mgs 168.0 Patent Blue V mgs 27.3 Fluorescein (Na) mgs 26.7 Cobalt acetate grs 1.78 Methanol ..cc 200 5. The sensitized dispersion was then coated on a clay coated sheet by conventional wire-wound rod technique. The coating thicknesses varied from 0.4 to 0.7 mil dry film thickness.

The product after drying was sensitive to the visible spectrum, gave a gray appearance by reflected light, had high speed and excellent balance. The usual loss of memory encountered in pre-illumination of the zinc oxide coatings was absent.

EXAMPLE II The procedure was the same as in Example I except that the following components were employed:

The product obtained was superior in color balance to that of Example I. The background moreover was noticeably free from yellow stain.

EXAMPLE III The procedure was the same as in Example I excepting that the following sensitizing solution was employed, the amounts being in terms of mg. per lb. of zinc oxide:

Merthiolate mg 4.79 Eosin Y mg 9.25 Rose Bengal mg 23.1 Chlorophyll mg 22.1 Patent Blue A mg 12.1 Methanol on 8.33

EXAMPLE IV The procedure was the same as in Example II except that a toluene solution of 2.7 mgs. Heliogen Blue Oil Soluble per 100 grs. zinc oxide, was added in place of Blue Masking Dye Component 1.

EXAMPLE V A mixture containing the followin ingredients was prepared:

Toluene l 1.25 Xylene ml 30 Epidene E168/50% solids grs 702 Cobalt naphthenate (6%) grs 1.45 Manganese naphthenate (6%) grs 1.45 Velsiool TPO #1 grs 157 Zinc oxide grs 2100 This mixture was dispersed in a ball mill and then sensitized with Methanol cc 500 Merthiolate mgs 20 Eosin Y m2s 40 Rose Bengal mas 97 Chlorophyll mgs 93 Patent Blue V mgs.... 30 Navy Blue mgs 27 The sensitized dispersion was then coated on paper, dried, exposed and developed with a liquid toner. Excellent results were obtained.

Modifications of the invention will occur to persons skilled in the art. We therefore do not intend to be limited in the patent granted except as necessitated by the appended claims.

We claim:

1. A storage stable electrophotographic recording element having adequate speed and balance for whitelight exposure comprising a backing member coated with finely divided photoconductive zinc oxide dispersed in an insulating resin binder, said zinc oxide being sensitized to light rays corresponding to the primary colors by a blend of organic dyes in which the component absorptive of the green rays is selected from the class consisting of Rose Bengal and mixtures thereof with Merthiolate and Eosin Y and the components absorptive of the red and blue rays are selected from the class consisting of (1) a mixture of Chlorophyll and Patent Blue V, (2) a mixture of Chlorophyll and Patent Blue A, and (3) mixtures of (1) and (2) with Fluorescein wherein the concentration of the components is adjusted so that when viewed by reflected light the sensitized zinc oxide appears white to neutral gray in color.

2. The article as defined in claim 1 wherein the blend of organic dyes includes a blue dye for masking a yellow background, said blue dye being formed in situ from a particulate mixture of a Naphthol AS coupling component and a diazo compound obtained from a 2,5-di-alkoxy- 4-benzoylamino aniline, the azo group of which is stabilized by N-methyltaurine.

3. The article as defined in claim 2 wherein the coupling component is Naphthol ASBO and the 2,5-di-alkoxy-4- benzoylamino aniline is a mixture of the 2,5-dimethoxy and 2,5-diethoxy derivative.

4. The article as defined in claim 1 wherein in addition to the blend of organic dyes is included in an oil soluble phthalocyanine dye for the masking of a yellow background.

5. The article as defined in claim 1 wherein the insulating resin-binder is an epoxy ester resin.

6. The article as defined in claim 1 wherein the insulating resin-binder is modified by a terpolymer derived from styrene, a lower styrene homolog and a naphthol.

7. A storage stable electrophotographic recording element having adequate speed and balance for white light exposure comprising a backing member coated with finely divided photoconductive zinc oxide dispersed in an insulating resin binder, said zinc oxide :having adsorbed to the surface thereof and being sensitized by a composition consisting of Rose Bengal, Chlorophyll, Patent Blue V and Fluorescein wherein the concentration of the components is adjusted so that when viewed by reflected light the sensitized zinc oxide appears white to neutral gray in color.

8. A storage stable electrophotographic recording ele-' ment having adequate speed and balance for white light exposure comprising a backing member coated with finely divided photoconductive zinc oxide dispersed in an insulating resin binder, said zinc oxide having adsorbed to the surface thereof and being sensitized by a composition consisting of Rose Bengal, Merthiolate, Eosin Y, Chlorophyll, Patent Blue V and the blue dye formed from a particulate mixture of a Naphthol AS coupling component, the diazo of a 2,5-di-alkoxy-3-benzoylamino aniline, the azo group of which is stabilized by N-methyl taurine wherein the concentration of the components is adjusted so that when viewed by reflected light the sensitized zinc oxide appears white to neutral gray in color.

9. A storage stable electrophotographic recording element as defined in claim 8 wherein the blue dye Heliogen Blue is used to mask the yellow background of the paper.

10. A storage stable electrophotographic recording element having adequate speed and balance for white light exposure comprising a backing member coated with finely divided photoconductive zinc oxide dispersed in an insulating resin binder, said zinc oxide having adsorbed to the surface thereof and being sensitized by a composition consisting of Merthiolate, Eosin Y, Rose Bengal, Chlorophyll and Patent Blue A wherein the concentration of the components is adjusted so that when viewed by reflected light the sensitized zinc oxide appears whiteto neutral gray in color.

11. A photoconductive composition comprising finely divided photoconductive zinc oxide having a spectral response to white light suspended in an electrically insulating resin binder and having incorporated therein a blend of organic dyes in which the component absorptive of the green rays is selected from the class consisting of Rose Bengal and mixtures thereof with Merthiolate and Eosin Y and the component absorptive of the red and blue rays are (1) a mixture of Chlorophyll and Patent Blue V, (2) a mixture of Chlorophyll and Patent Blue A, and (3) mixtures of (1) and (2) with Fluorescein wherein the concentration of the components is adjustedso that when viewed by reflected light the sensitized zinc-oxide appears white to neutral gray in color.-

12. The composition as defined in claim 11 wherein component, and a diazo compound from a 2,5-di-a1koxy.--

4-benzoylamino aniline, the azo group of which is stabilized by N-methyltaurine.

References Cited UNITED STATES PATENTS 3,051,569 3,052,540 '9/1962 Greig '96-1.7 3,245,706 4/1966 Cassiers et al. ;96,-1.8

NORMAN G. TORCHIN, Primary Examiner.

C. E. VAN HORN, Assistant Examiner.

8/1962 Sugarma n et a1. 96--1.7