Protecting Photoconductor Surfaces

Abstract

A method and composition for protecting the surface of photoconductors which are employed in electrophotography and xerography which comprises coating the photoconductor surface with a thin uniform layer of a polyurethane material to protect and stabilize the photoconductive properties of the photoconductor. The polyurethane layer should have a charge acceptance of at least 1,000 volts.

Description

BACKGROUND OF THE INVENTION

This invention relates to xerography and electrophotography and more particularly to a method and composition for protecting and stabilizing the photoconductive insulating materials employed in connection therewith.

In the art of xerography, it is usual to form an electrostatic latent image on a member or plate which comprises a substantially electrically conductive backing member such as, for example, a paper or metallic member having a photoconductive insulating material applied to the surface thereof. It has previously been found that a suitable electrostatic image forming member for xerographic purposes is an electrically conductive backing member, for example, metal, which may be employed in the form of a sheet, drum or belt, having applied to the surface thereof, a photoconductive material, for example, selenium, inorganic materials, such as cadmium sulfo-selenide, cadmium sulfide, zinc oxide and mixtures thereof; organic materials such as complexed poly-N-vinyl carbazole, and other like photoconductive materials useful for such purpose. These electrostatic image forming members are characterized by being capable of receiving electrostatic charge and selectively dissipating such charge when exposed to a light pattern.

In the practice of the art of xerography, employing such electrostatic image forming members as hereinabove described, it has been found that said electrostatic image forming members suffer from various disadvantages. It has been found that handling of the light sensitive surface has a tendency to cause contamination thereof and the subsequent image reproduction capabilities of the member are adversely affected. In addition, the xerographic process is inherently abrasive to the surface of the image forming members employed which results in a wearing out of the photoconductive surface requiring frequent replacement of the image forming member.

Specifically, although there are some materials which are known to possess satisfactory photoconductive properties, they have not heretofore been capable of use in xerographic processes, and especially xerographic copying machines because of their high susceptibility to the abrasive wear encountered in the operation of such machines. For example, U.S. Pat. No. 3,658,523 discloses a cadmium sulfo-selenide/zinc oxide photo-conductor composition which cannot be employed in xerographic copying machines for this reason.

Further, it has been found that superior xerographic results are obtained when a magnetic brush toning process is employed for image reproduction. However, this magnetic brush toner process is highly abrasive to the photoconductive surface of the image forming member resulting in xerographic reproductions of inferior quality and frequent replacement of the image forming member.

Attempts have been made to protect the surface of photoconductors, all with limited success. For example, overcoating of photoconductors has been suggested in Dessauer et al, U.S. Pat. No. 2,901,348; Deubner, U.S. Pat. No. 2,860,048; Kensella, U.S. Pat. No. 3,146,145 and Petruzella, U.S. Pat. No. 3,617,265. These prior art attempts have had limited success for a number of reasons. Initially, they are not universally applicable to the variety of photoconductive materials with which they are employed, either because of incompatability of the materials in their physical or chemical relationship or because inapplicability of the process employed in obtaining the desired protected photoconductor member. One manifestation of this latter disadvantage of the prior art teachings can be seen from a study of the process disclosed in U.S. Pat. No. 3,617,265 wherein a heating-quenching process is employed in preparing the desired protected photoconductor member. The application of such a process to various photoconductive materials having binders of organic resin materials, a widely employed practice, will result in a basic and detrimental alteration of the photoconductive member and its photoconductive properties.

We have now discovered a method of producing an electrostatic image forming member useful in the practice of xerography which overcomes the problems heretofore experienced with prior art image forming members which were susceptible to surface abrasion and contamination. More particularly, we have discovered a method whereby the abrasion susceptible surface of the photoconductive material employed in the electrostatic image forming member of the xerographic process may be protected which also tends to stabilize the photoconductive properties thereof.

Accordingly, it is an object of the invention to provide a composition useful in xerography wherein the photoconductive insulating material is protected from mechanical wear and abrasion.

It is a further object of the invention to provide a composition of the above character which helps to stabilize the electrical properties of the photoconductive insulating material.

The invention accordingly comprises a composition of matter possessing the characteristics, properties, and the relation of components which will be exemplified in the composition hereinafter described, and the scope of the invention will be indicated in the claims.

SUMMARY OF THE INVENTION

In general, our invention comprises the application to the photoconductive surface of an electrostatic image forming member, of a thin uniform coating of polyurethane to provide positive protection against abrasion and contamination. Specifically, we have found that the photoconductive surface of electrostatic image forming members can be protected from abrasion and contamination by the application thereto of a thin, uniform coating of a polyurethane.

The polyurethane coating which may be satisfactorily employed in the practice of this invention must have a very high resistance to abrasion. In addition, the polyurethane must have low surface leakage properties as indicated by a high dielectric strength and surface resistance, so that the applied electrical charges will not be dissipated by bypassing of the underlying photoconductive material. The total surface leakage of the polyurethanes which are useful in the practice of this invention may be determined in the same manner as is done for photoconductors, i.e., a thin coating of the polyurethane, about 1 mil or less, is tested for charge acceptance. We have found that when the polyurethane is tested by being charged in a Victoreen Electrostatic Paper Analyzer the polyurethane must accept a charge equivalent to at least 1,000 volts/mil of thickness and preferably at least 1,500 volts/mil, to yield satisfactory results hereunder.

DETAILED DESCRIPTION OF THE INVENTION

The polyurethane coating must be inert to the photoconductive material upon which it is to be applied and must have good adhesion properties which will permit its permanent bonding to the photoconductive surface on which it is applied. The adhesion properties of the polyurethane must provide a uniform coating and help prevent air pockets or other surface irregularities which could interfere with the photoconductive properties of the image forming member. In addition to the foregoing, the polyurethane coating must have fast air drying properties to permit facile coating thereof on the photoconductive surface. In the practice of this invention we have found that a polyurethane capable of being cured by solvent evaporation provides satisfactory results.

The electrically conductive backing member which may be employed in the electrostatic latent image forming member useful in the practice of the instant invention may be comprised of any material that has been previously found to be satisfactory in the practice of xerography. Included among the electrically conductive materials which may be employed in the practice of this invention are metals, for example, aluminum or brass, conductive paper, graphitized Mylar, metallized Mylar and other like material.

The photoconductive insulating materials which may be satisfactorily employed in the practice of this invention are those photoconductive materials which have heretofore been so employed in the practice of xerography and which may be satisfactorily applied on the electrically conductive backing materials. Among the photoconductive materials which may be employed in the practice of the instant invention are such materials as selenium, cadmium sulfo-selenide, cadmium sulfide, zinc oxide, poly-N-vinyl carbazole and other like materials. In particular, we have found that a mixture of cadmium sulfo-selenide/zinc oxide is both protected and electrically stabilized by a thin, uniform coating of polyurethane.

The polyurethane protective coating employed in the practice of this invention must have the physical properties set forth hereinabove. In addition, we have found that satisfactory results are obtained when the polyurethane coating employed is possessed of a charge acceptance of at least 1,000 volts/mil of thickness. In the preferred practice of the instant invention, we have found that most satisfactory results are obtained when a polyurethane having a charge acceptance of at least 1,500 volts/mil is employed. The successful practice of this invention is dependent upon the characteristics and properties of the polyurethane employed and although many polyurethanes were tested it was unexpectedly found that only the polyurethanes possessing the specific properties set forth hereinabove provided satisfactory results.

The polyurethane protective coating must be applied to the photoconductive surface in such a manner as to avoid adversely affecting the photoconductive properties thereof. We have found that the thickness of the polyurethane coating must be controlled to avoid masking the photoconductive response to the underlying photoconductive material, while at the same time providing a coating which is thick enough to provide the required protection. We have found that satisfactory results are obtained when the coating is applied in a uniform thickness of from about 0.02 to about 0.1 mils; and preferably when the coating was applied uniformly in a thickness of from about 0.04 to about 0.08 mils.

The polyurethane coating may be applied in any manner which is known and convenient to the skilled worker, for example, spraying, painting, Mayer rod, doctor blade or reverse roller applicators, which will provide a uniform coating of the polyurethane in the required thickness. In addition, care must be exercised in the use of solvents employed in the application of the polyurethane coating to the photoconductive surface so as to avoid interaction of the solvents with the underlying photoconductive materials or binders which may have been employed in connection therewith. Satisfactory solvents which we have found to be employable in connection with the polyurethane coating material of this invention include such solvents as isopropanol, cellosolve acetate and methyl ethyl ketone, although other solvents may be employed as may be determined by the worker skilled in the art.

The effects of mechanical wear and burnishing have been found to be particularly pronounced with the use of a mixed pigment photoconductor system of cadmium sulfo-selenide and photoconductive zinc oxide. As set forth in copending application Ser. No. 134,730 and assigned to the assignee of this application, the photoconductor composition should comprise a binder having a mixed pigment therein of from 20 percent to 70 percent of cadmium sulfo-selenide and from 30 percent to 80 percent zinc oxide by weight of total pigment. The mole fraction ratio of selenium to selenium plus sulfur in the cadmium sulfo-selenide should be from 0.05 to 0.7 e.g. where n equals the number of atoms of sulfur and selenium the ratio (n(Se)/n(S)+n(Se)) is from 0.05 to 0.7.

When a photoconductive insulating composition of cadmium sulfo-selenide/zinc oxide is used in a xerographic machine without a top coating there are substantial changes in the electrical properties of the photoconductor which reduce its useful life below commercially acceptable levels. With a thin top coating of a polyurethane having a charge acceptance of at least 1,000 volts per mil of thickness, however, the photoconductor is commercially usable.

Table A shows the necessity for particularly protecting the surface of a cadmium sulfo-selenide/zinc oxide photoconductor from burnishing. The effects of burnishing were simulated by rubbing the surfaces with cotton.

Table A indicates that the mixed CdSSe/ZnO photoconductor shows a significant reduction in acceptance voltage as compared to either of the constituents when burnished.

TABLE A ______________________________________ Acceptance Voltage Range (Volts) ______________________________________ Sample Before After Burnishing Burnishing ______________________________________ CdSSe 720 - 790 640 - 700 ZnO 195 - 270 240 - 255 CdSSe/ZnO mixture 610 - 640 160 - 195 ______________________________________

It is indicated from the above data that the CdSSe and ZnO particles interact when the surface is abraded.

Table B below illustrates the effects of changing the pigment to binder ratio (P/B) and CdSSe/ZnO ratio.

TABLE B ______________________________________ Fraction of Acceptance Voltage Remaining After Burnishing ______________________________________ CdSSe/ZnO P/B = 3:1 P/B = 6:1 P/B = 9:1 ______________________________________ 100/0 0.85 0.91 0.91 75/25 0.79 0.83 0.86 50/50 0.85 0.74 0.72 25/75 0.84 0.50 0.43 10/90 0.92 0.69 0.45 ______________________________________

Table C below illustrates the mechanism of burnishing by simulating particle interaction by crushing the dry pigment powders before formulating.

TABLE C __________________________________________________________________________ Acceptance Voltage Speed Range Sample Range (Volts) (fcs) __________________________________________________________________________ CdSSe 625 - 720 0.24 - 0.32 Crushed CdSSe 615 - 630 0.33 - 0.43 CdSSe + ZnO 575 - 580 0.13 - 0.19 Crushed CdSSe + ZnO 590 - 630 0.19 - 0.26 CdSSe + Crushed ZnO 510 - 555 0.10 - 0.17 Crushed CdSSe + Crushed ZnO 485 - 540 0.16 - 0.27 CdSSe and ZnO Crushed Together 325 - 340 0.07 - 0.12 __________________________________________________________________________

It is indicated from Table C that simple mechanical action alone on the CdSSe does not account for the significant differences observed, as is the case when the much harder ZnO particles abrade the surface of the CdSSe particles in the mixed pigment system. Lubricants such as diphenylamine and clay added to the mixture have somewhat helped resistance to burnishing by reducing the interaction of the particles, but top coating with a thin layer of polyurethane provides more effective protection. In the above example there was no evidence of increased dark decay as a result of crushing. This is very evident in burnished coatings and probably operates by electrostatic charge injection into the binder surface as a result of friction. A thin uniform top coating of polyurethane in accordance with the present invention provides positive protection against such effects.

This invention is illustrated by the following examples.

EXAMPLE I

A polyurethane resin having a charge acceptance of in excess of 1,500 volts/mil of thickness (commercially available from Cargill Co. as a 30 percent solution under the designation "Cargill-X-1513-30" an aliphatic type urethane having a molecular weight of from 23,000 to 25,000) was applied to the surface of a cadmium sulfo-selenide/zinc oxide mixed pigment photoconductor in different thicknesses of from 0.04 to 0.08 mils by diluting the polyurethane to various solid concentrations before coating. The higher the percentage of solids, the thicker the coating. The control had no top coating at all. The respective photoconductor properties were measured by employment of a modified Victoreen Electrostatic Paper Analyzer and the results thereof are set forth in Table D below:

TABLE D __________________________________________________________________________ Polyurethane diluted to: Control 10% solids 20% solids 25% solids Charge Acceptance (Volts) 1st charge 910 960 1010 1170 2nd charge 845 880 960 1030 Dark Decay (max) 60 35 50 45 (volts/sec) Speed, t 1/2 0.07 0.10 0.12 0.14 (fcs) (avg.) Exposure to 60 V. 0.21 0.20 0.33 2.00 (fcs) (avg.) __________________________________________________________________________

The materials of Table D were then subjected to 2500 cycles of simulated magnetic brush burnishing and were again tested producing the results set forth in Table B below.

TABLE E ______________________________________ Polyurethane diluted: Control 10% Solids 20% Solids Charge Acceptance (Volts) 1st charge 635 900 970 2nd charge 290 890 970 Dark decay (max.) 100 110 60 (volts/sec) Speed, t 1/2 .09 .10 .11 (fcs) (avg.) Exposure to 60 V. .16 .17 .24 (fcs) (avg.) ______________________________________

EXAMPLE II

A photoconductive test belt was made using the following formulation for the photoconductor composition:

Pigments

90 gms, CdSSe (Dark Red pigment available from Ferro Corp. containing a Se/S+Se mole fraction ratio of 0.36

270 gms, ZnO (Photox 801 from New Jersey Zinc Co.)

Binder

133.4 gms 45 percent polyurethane (Estane 5715 from B. F. Goodrich) solution in methyl ethyl ketone.

Solvents

136.6 gms methyl ethyl ketone and

210.0 gms methyl isobutyl ketone.

The pigment to binder ratio was 6:1 with a solvent to binder ratio of 7:1 for a total solids content of about 50 percent, of which the pigments were in the ratio of 25 percent CdSSe and 75 percent ZnO by weight.

Because of the limited wetability of the pigments by the polyurethane binder solution the following was done to produce a smooth dispersion:

1. The 90 gms CdSSe, 136.6 gms methyl ethyl ketone, 210 gms methyl isobutyl ketone and 5 gms polyurethane/methyl ethyl ketone solution were milled for four minutes in a Kady mill. 2. The 270 gms of ZnO and 5 more gms polyurethane/methyl ethyl ketone solution were added to the above and Kady milled for 5 more minutes. 3. The remaining (123.4 gms) polyurethane/methyl ethyl ketone solution was added to the above and Kady milled for 5 more minutes. 4. The above mixture was then charged into a ball mill, milled for 30 minutes and filtered twice through cheesecloth.

The above procedure yielded a very smooth dispersion with a viscosity of about 2,000 cps after wetting out over night. After filtering again through cheesecloth the above dispersion was coated onto a metallized Mylar belt using a laboratory knife coater. After air drying for one-half hour, a top coat of Cargill-X-1513-30 polyurethane solution diluted to 11.5 percent solids was applied using the same coating technique. The resulting coating was very smooth to the touch and had an average thickness of 1.7 mils.

After air drying over night the belt was installed in a commercial xerographic copying machine and successfully made 13,000 high contrast copies.

EXAMPLE III

Another photoconductive belt was made using the same method as in Example II with the following changes.

1. A CdSSe "Maroon" pigment from Ferro Corp. containing Se in a mole ratio of 0.51 Se to Se+S was used instead of the Dark Red pigment. 2. Pigments were in the ratio 35 percent CdSSe to 65 percent ZnO. 3. The viscosity of the dispersion was about 1500 cps after 1.5 hours of ball milling.

Coating and top coating were accomplished in the same way as in Example II, poly-N-vinyl in a final coating thickness of 2.0 mils. The photoconductive belt was installed in a commercial xerographic copier and run for 2,500 copies with no change in copy quality.

EXAMPLE IV

The procedure of Example I was followed except that the photoconductive underlying material was poly-n-vinyl carbazole/trinitrofluorenone complexed organic photoconductor. The photoconductive properties of the image forming member were not adversely affected while the reuse capacity of the photoconductor was increased significantly.

EXAMPLE V

The procedure of Example I was followed except that the underlying photoconductor material employed was amorphous selenium. Equivalent results to those obtained in Example IV were experienced with the amorphous selenium photoconductor.

EXAMPLE VI

A comparative test was run to demonstrate the satisfactory results obtained with the polyurethanes of the instant invention. A polyurethane which has a charge acceptance of less than 1,000 volts/mil, (and is available from the KJ Quinn Co. under the designation "Quinn 2780") was subjected to the same procedures as set forth in Example I. The results obtained are set forth in Table F below.

TABLE F __________________________________________________________________________ After Burnishing Before Burnishing. (2500 cycles) __________________________________________________________________________ 10% solids* 15% solids* 20% Solids* 15% solids* Charge acceptance (Volts)1st charge 830 875 840 630 2nd charge 775 850 815 400 Dark Decay (max) 35 45 80 110 (volts/sec) Speed (fcs) (avg.) .09 .09 .16 .10 Exposure to 60 V. .15 .17 .30 .28 (fcs) (avg.) __________________________________________________________________________ *Polyurethane diluted to:

It will thus be seen from the above that a protective top coat both protects and stabilizes xerographic photoconductors, and particularly the mixture of cadmium sulfo-selenide and photoconductive zinc oxide as a photoconductor. It will also be seen that the top coating should have a charge acceptance of at least 1,000 volts per mil of top coat thickness and preferably 1,500 volts per mil.

The invention may be variously otherwise embodied within the scope of the appended claims.

Claims

What is claimed is:

1. A photoconductive insulating composition comprising a layer of photoconductive insulating material having a mixture of cadmium sulfoselenide and photoconductive zinc oxide for electrostatic imaging and a thin, uniform top coating of polyurethane having a thickness from 0.02 to 0.1 mil closely bonded to and substantially over the entire surface of said photoconductive insulating material.

2. The composition defined in claim 1 wherein the polyurethane coating has a charge acceptance of at least 1,000 volts/mil of thickness.

3. The composition defined in claim 1 wherein the polyurethane coating has a charge acceptance of at least 1,500 volts/mil of thickness.

4. The protective top coating defined in claim 1 wherein said polyurethane is an aliphatic type urethane applied from a solvent taken from the group consisting of isopropanol, cellosolve acetate and methylethyl ketone.

5. A xerographic plate comprising

A. an electrically conductive substrate;

B. a photoconductive electrically insulating composition adhered to said substrate, said composition comprising

1. a binder and

2. a pigment comprising a mixture of cadmium sulfo-selenide and photoconductive zinc oxide,

a. wherein the proportion of cadmium sulfo-selenide is 20 percent to 70 percent and the proportion of zinc oxide is 30 percent to 80 percent by weight of total pigment; and

C. a thin uniform top coating of polyurethane is adhered to said composition,

1. said polyurethane coating having a charge acceptance of at least 1000 volts per mil of thickness.

6. The xerographic plate defined in claim 5 wherein said binder is polyurethane.

7. The xerographic plate defined in claim 5 wherein said polyurethane coating has a thickness of from 0.02 to 0.1 mil.

8. The xerographic plate defined in claim 5 wherein the mole fraction ratio of selenium to sulfur in said cadmium sulfo-selenide is from 0.05 to 0.7.