CA1064857A - Flexible metal belt photoreceptors - Google Patents

Abstract

FLEXIBLE METAL BELT PHOTORECEPTORS

ABSTRACT OF THE DISCLOSURE
A class of durable flexible xerographic photoreceptors comprising a flexible charge-conductive metal or metal-coated base, the base surface of which is exclusive of aluminum or aluminum alloys, at least one applied inorganic photoconductive layer, and at least one intermediate blocking layer comprising aluminum oxide; said class of durable photoreceptors being obtained by applying the intermediate blocking layer at least partly in the form of positive aluminum ions obtained through a glow discharge. The steps of pretreatment of the base and the application of a photoconductive layer can also advantageously be applied partly or wholly by using a glow discharge.

Description

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This invention relates to a method or obtaining durable 1exible x~rographic photoreceptors and photoreceptors obtained in accordance with such method in which a 1exible eharge conductive metal or metal coated base having an inside surface exclusive of alumin~lm, is utilized in combination with at least one inorganic photoconductive layer and an aluminum oxide intermediate blocXing layer.
BACKGROUND_OF THE INVENTION
Photoreceptors, particularly those related to the xerographic copying, traditionally comprise a photoconductive insulating layer such as an ionizable element or alloy thereof exemplified by selenium (amorphous or trigonal) and selenium alloys such as a selenium-arsenic alloy with varying amounts of a halogen. Such materials are customarily applied in eharge-blocking contact with a supporting metal- or metal-covered charge-conductive substrate. Suitable substrates for such purpose can include, fGr instance, aluminum, steel, nickel, brass, ~ESA glass or corresponding metal-coated polymeric materials.
Functlonally speaking, photoreceptors comprising the -~;
above components are generally given a uniform electrostatic eharge, and the resulting sensitized surface exposed to an image pat$ern defined by an electromagnetic radiation, such as light.
Such impingement causes the selective dissipation of the initially applied charge leaving a positive latent electrostatic image.
The clectrostatic image is then customarily developed by applying oppositely charged electroscopic marking particles onto the ehaxge-pattern-bearing photoreceptor surface.
The above basic concept was originally described by Carlson in U.S. Patent 2,297,691, and has since been amplified .
~-2-.: - . . :

~64~3S7 and redescribed in many related patents in th~ field. Generally ~peaking, however, photoconductive layers suitable for carrying out the above unctions have a speciic resistivity of about 1~lO _ 1ol3 ohm-cm, in the absence of illumination. This s resistivity must drop at least several orders of magnitude whereover exposed to an activating radiation such as light, x-rays or similar radiation.
Photoreceptors meeting the above criteria, however, normally exhibit some loss in applied charge, even in the absence of light exposure. This phenomenon is known as "dark decay" and will vary somewhat depending upon the sensitivity of the photo-conductive material and with usage of the photoreceptor. The existence of the problem of "dark decay" is well known and has been controlled to a substantial extent by incorporation of a thin ~5 barrier layer such as a dielectric film between the base of sub-strate and the photoconductive insulating layer. U.S. Patent

2,901,348 o~ Dessauer et al utilizes a film of aluminum oxide of about 25 to 200 angstroms or a .1 - 2~ insulating resin layer, such as a polystyrene for such purpose. With some limitations, such barrier layers function to allow the photoconductive layer to support a charge of high field strength with minimal "dark decay".
When activated by illumination, however, the photoconductive layer/
barrier layer combination must become sufficiently charge-conductive to permit substantial dissipation of the applied charge in light-struck area within a short period of time.
In addition to the above-indicated electrical require-ments, it is also important that photoreceptors meet rather stringent requirements with regard to mechanical properties such as ~lexibility and durability, and also remain chemically inert.
~- 30 Such criteria becomes particularly important in modern xerographic .

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copiers operating at high speeds, where the photoreceptor is often in the form of an endless flexible belt (ref. U. S. Patent 2,691,450).
While belt-type photoreceptors have many ad~antages, there are also serious technical problems inherent in their use.
For example, high speed machine cycling conditions require very strong adhesion between the photoconductive layer and the under-lying layers. Unfortunately, however, some of the most sensi-tive and efficient photoconductive materials are relatively brittle as films and do not generally adhere well to a flexing metal base or substance or necessarily maintain a good charge blocking contact. It is particularly important that any inter-face layer between the electrically conductive base or substrate and the photoconductive layer be chem;cally stable and also strongly adherent to both since any changes in this locale will have a substantial effect on the xerographic properties of the photoreceptor.
It is an object of an aspect of the present invention to obtain improved photoreceptors suitable for high speed xero- ;
graphic copying purposes.
It is an object of an aspect of the present invention to develop a new method for successfully utilizing brittle photo- -conductive elements in a class of high speed flexible photorecep-tor belts wi'chout the need for complicated chemical pretreatment of the substrate to obtain good blocking contact and durability.
THE INVENTION
In accordance with one aspect of this invention there is provided a method for obtaining a durable xerographic photo-receptor comprising a flexible charge-conductive metal or metal-30 coated base having a base surface exclusive of aluminum or -~

aluminum alloy, at least one inorganic photoconductive layer, and at least one intermediate aluminum blocking layer; compris-- . ~ .

;4857 ing exposing a clean charge-conductive base as an electrode under vacuum to an aluminum vapor cloud containing uncharged and charge material obtained by exposing vaporized aluminum to a glow discharge to obtain the intermediate blocking layer; and coating the base and applied intermediate blocking layer with at least one inorganic photoconductive material.
In accordance with another aspect of this invention there is provided a method for obtaining a durable xerographic photoreceptor comprising a flexible charge-conductive metal or metal-coated base of one or more members selected from the group consisting of copper, brass, stainless steel, zinc, nickel or corresponding metal-coated polymeric-aluminum, or paper backed material, at least one inorganic photoconductive layer, and at least one intermediate charge blocking layer containing aluminum oxide; comprising exposing a clean substantially o~ide-free base ~ ~
as cathode to a vapor cloud containing uncharged and positively ; ~-charged aluminum ions generated by passage o~ vaporized aluminum through a glow discharge; and coating the intermediate oxide block-ing layer with at least one inorganic photoconductive material.
By way of added explanation, the foregoing and other objects of the instant invention are obtained by utilizing a new class of xerographic photoreceptors comprising a flexible charge-conductive metal or metal-coated base, ' .
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106485~7 having a base surfac~ exclusive of aluminum or aluminum alloy, at least one inorganic photoconductive layer, and at least one intermediate aluminum blocking layer.
The photoreceptor is obtained by exposing a clean charge-conductive base as an electrode under vacuum to an aluminum vapor cloud containing uncharged and charged material obtained by exposing vaporized aluminum to a glow discharge to obtain the intermediate blocking layer~ This step is followed by coating the base and applied aluminum intermediate blocking layer with at least one inorganic photoconductive material.
More specifically, the aluminum blocking layer is applied in the form of a cloud comprising positively ionized and uncharged aluminum vapor in a vacuum coater onto a sub-stantially clean oxide-free base at a pressure up to about 40 microns (mercury) and containing not less than about 1 percent by volume available oxygen. Preferably, the aluminum coating step is effected at a coater pressure of about 5 - 30 microns (mercury) containing about 1 - 10 percent by volume available oxygen.
Suitable xerographic bases for purposes of the above-described invention include, for instance, flexible charge-conductive metal or metal~coated backing such as copper, brass, steel, zinc, nickel or corresponding non-aluminum .netal coated-polymeric or even paper backed bases.
Of particular interest, in this connection, are copper, brass and nickel in the form of belts, webs or plates.
Depending upon the nature of the charge-conductive base, various chemical etching, cleaning, and degreasing steps are includable within the scope of the concept as described above, as well as various techniques for applying one or more inorganic ii7 photoconductive layers onto the pxepared aluminum-coated basc.
G~nerally speaking, metal bases, particularly those of nickel and brass, are best prepar~d for'coating with an aluminum blocking layer by cleaning with a commercial cleaner such as "Mitchel Bradford No. 14 Cleaner" or "Mobil Acid cleaner" in combination with a degreasing step and a plurality of rinsing steps using deionized water. The base can also be optionally pretreated with an acid wash solution comprising an inorganic acid such as hydrochloric acid or phosphoric acid, in the manner described, for instance, in U.S. Patent 3,907,650 of Pinsler.
One fast and very helpful cleaning step includable within ~he present concept involves using a glow discharge technique in which the washed and degreased base is exposed in a vacuum coater to positive high energy inert gaseous non-metallic ions simultaneously with at least initial exposure to an aluminum vapor cloud comprising both chaxged and uncharged aluminum particles.
This step is very generally represented in Diagram IA, - below, the arrows indicating a general random movement of uncharged inert gas and vaporized aluminum materials (M) and oriented move-ment of ions of both materials with respect to the base (cathode).
As diagrammed, the argon ions "A+" are capable of impacting the base and dislodging the less adherent condensed uncharged aluminum.
DIAGRAM IA

t ~ U~ e~/ ~M ~é- t 2 } _ b - t 2 A+ M~
M~ ~ - - c A ~ M A
M t~
~b~_ J~k ~ ~ L__ ~ ~d .' . (~) ' . . .
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In Diagram I~ (a) and (d) respectively represcnt the ~luminum-free ch~rge~conductive base (cathode) and the aluminum ~M)-containin~ crucible or other clonor source (anode) while axeas (b) and (c) respecti~ely represent Cathode Dark Space and Negative Glow areas of a Glow Discharge.
- In ef~ecting the cleaning and aluminum coating steps as yenerally represented in Diagram IA, it is found useful to pump ~he coater down to a pressure of about 5 x 10-5 ~'orr or better and then backfill with an inert gas such as argon or xenon, etc. to a pressure of about 5 - 40~ (mercury). At this point, the glow discharge is conveniently effected by activating high voltage filaments, electron guns, glow bars, rod-shaped electrodes, etc. (not shown in Diagram IA) or by utilizing the - conveniently positioned aluminum-filled crucible as anode and the base as a cathode at a potential up to about 5000v and preferably at about 1500 - 5000v, depending upon the type and - pressure of gases present and the type of current utilized.
Upon establishment of the glow discharge in convenient geometric position with respect to a potential aluminum vapor source and the base or substrate, an aluminum vapor cloud can be introduced into the coater by heating the crucible and the coater backfilled with air or oxygen to provide not less than about 1 percent by volume of available oxygen.
Upon completion of a period of time sufficient to layer down an aluminum coating layer of about 1000 - 5000 Angstrom thickness, having an oxidized surface of 10 - 50 Angstrom the step of coating with photoconductive matexial(s) can beginO
If convenient, however, the aluminum layer can be applied with , .

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- Application of one or more photoconductiv~ layers onto the aluminum-coated base can be convcniently effected by classical vacuum deposition techniques in a coater. Generally, this involves heating one or more crucibles containing one or more inorganic photoconductive alloys as a source o~ vaporized photo-conductive material. Suitable materials, for purposes of the present invention include, for instance, selenium, tellurium, germanium, antimony, bismuth or alloys thereof, wikh each other and at least one of arsenic and a halogen. For such purpose, the crucibles can be heated singly, in timed sequence, or in combina-tions, depending upon the type or types of photoconductive layer desired.
By way of example, a crucible temperature up to about - 350C. and preferably about 180~C. - 300~C. is found ade~uate for vaporizing selenium and most of the known selenium alloys under a pressure up to about 30 microns.
Application of the photoconductive coat can be carried out simply by shutting down the glow discharge and vaporizing the photoconductive material in the pumped down coater at a pressure of about 5 x 10-5 Torr or better. Aternatively, this coating step can be effectively dove-tailed with the preceeding - aluminum coating step by simply shutting off the aluminum vapor source, or by reducing the coater pressure to about 5 x 10-5 Torr and backfilling with about 5 - ~0 microns ~mercury) of ar~on, xenon or similar xelatively inert gas) and by introducing the desired combination o vaporized photoconductive material at least initially under glow discharge. The latter arrangement, in combination with the preceeding aluminum deposition undex glow .
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discllar~e is founcl to provid~ a ~till further improvcm~nt in durability as well as chemical stability in flexible belt- or plate-type photoreceptors utilizing bases of brass or other metals capable of reacting with selenium or selenium alloys. A
particularly useful procedure, in this connection, involves deposition of about .5 - 10 percent o~ the desired photoconductor material (i.e., about 2 - 5 minutes) followed by standard vacuum deposition at 5 x 10 5 Torr or better to obtain total photoconductor coating(s) of about 40 - 60~ or more on the substrate.
The relationship of the electrodes and other essential components of the above-described final coating steps are again generally represented in the modified form in Diagram IB as follows:
DIAGRAM IB
I t-) a-~ A M~ ~ M M ~~ ~+
~ Nz t ; 1~ M~ t ~
c l M A~ ~ M A t ¦ M / M N2 ' I / N2 d _ ~ M M
, ' ' I ~) '.
_ _.. . __ . . . .. . . . . ... . . .. .... . . ... . ..
wherein (a) and (d~ respectively, represent the oxidized aluminized substrate (cathode) and a crucible containing photo-conductive material (M) as the anode while areas (b) and (c) again respectively, represent the Cathode Dark Space and Negativé Glow areas of a Glow Discharge in an inert gas at about 5 - 15~
pressure. As previously noted, ~he glow discharge can also be created by utilization of intermediately positioned glow bars, etc. (not shown) to permit interception of vaporized ~9-..... ~ ... ~ . --' ' ' `' ' ; ~ ' . ' " ' ` '....... ::

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photoconductive material.
A point to note about evaporant ionization generally is that the ionization efficiency is generally low. Only about 1 - 5 percent o~ the total amount of the vaporized material vapor is ionized in transit through a glow discharge. Because the presence of positive non-metallic ions such as nitrogen or argon, however, it is possible to displace a substantial amount o~
condensed unionized evaporant in favor of the high energy evaporant ions. Successful impact deposition, therefore, requires a balance between the removal and deposition rate so as to obtain a net coating action such that the coating comprises a larger proportion of initially ionized evaporant material. The time required to obtain an adequate thickness will depend largely on these factors.
The following examples specifically demonstrate pre-ferred embo~iments of the present invention without limiting it thereby. :
EXAMPLE I
Two nickel alloy test belts identified as T-l and T-2, and having a thickness of 4.5 mil (.0045"), a length of 10" and - a diameter of 4.75" are cleaned with a hot aqueous solution containing 10 percent by weight of "Mitchel Bradford ~o. 14 - Cleaner" and then rinsed in deionized water for about 2 minutes.
The sample belts are then mounted on,but insulated from, a rotatable mandrel in a vacuum coater about 6" away rom - grounded shuttered stainless steel crucibles equipped with -- resistive heating means and containing, respectively, aluminumat 99.8 purity and a chlorine doped selenium/arsenic alloy (2%).
- After evacuating to 5 x 10-5 Torr and backfilling the coater with 10 micron ~mercury) argon, negative 3000 volts is applied .. ~ . . .. . . . .

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to the belt for about 2 minutes to heat and clean the belt.
Thc coater i5 back~illed with up to about 4~ air and the shutter of a preheated (1300C.) aluminum~containing crucible opened.
The first crucible is then closed, the coater pressure reduced to about 5 x 10-5 Torr and backfilled with about 15 microns (mercury) of argon to re~establish the glow discharge. The shutter of a preheated stainless steel crucible (containing selenium/arsenic/chlorine alloy at 280C.3 is then opened. After about 2 minutes, the glow discharge is turned off, the coater pressure reduced to 5 x lO 5 Torr, and the coating allowed to proceed for ~0 additional minutes to o~tain about 60~ thickn~ss of photoconductive material. The belt, coater and crucibles are then permitted to return to ambient temperature and pressure and the coated belts are then tested for durability and electronic properties. The results are reported in Table I belo~.
EXAMPLE II (Control) Example I is repeated, identical nickel belts identified as T-3 and T-4. These belts are cleaned chemically, the aluminum layer being applied at 5 x 10-5 Torr without glow dis-charge and the photoconductive layer applied for 2 minutes (15~ argon atmosphere) under glow discharge followed by conven-tional vacuum deposition at 5 x 10~5-Torr to achieve comparable thickness to that of Example I. After returning to ambient conditions the belts are then tested for durability and electronic properties and the results reported in Table I below.
EXAMPLE III (Control) Example ~ is repeated using a single nickel belt (T-5), the belt being cleaned chemically as before in combination wi~h a modified sputtering with no intermediate aluminum layer. The photoconductive layer is then applied to a thickness of about 60 by conventional vacuum deposition at 5 x lO-5 Torr to a thickness of about 60~. The belt is then tested for ~urability and ' : ~' ' ' ':

~136~1~57 electronic properties as before and reported in Table I.
Ex~M~Lr~ IV
Example I is repeated using two brass test belts of about 6 inch diameter and comparable thickness identifi.ed as T-6 and T-7 and the results o~tained found comparable to that obtained wi~h nickel belts.
EXAMPLE V (Control) Example II is repeated using two brass test belts of about 6 inch diameter and comparable thickness, identified as T-8 and T-9 and the results obtained found comparable to that obtained with nickel belts.
EXAMP~E VI (Control ~
Example III is repeated using two brass test belts of about 6 inch diameter and comparable thickness identified as T-10 and T-ll and the results obtained found comparable to that obtained with nickel belts.

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TABLE I
~ _ . . ~ _ , .,_ _ ____ . .......... . ...... .
Test Belt Tl T2 T3 T4 T5 Mandrel Bend Test Failure Diameter 1.0" 1.25"1.75" 1.50" 3.0"
Aged Beltl 1.0" 1.25~ 1.75"1.50" coating falls off - spontan flaking occur 2 5 Electrical Test2 VL/t 30.2 30.8 28.329.~ 18.1 D~3 ~ 5.2 5.7 3 9 1. Accelerated test in an oven at 60C. for 10 days.
2. Q-V scanner test to determine capacitive chax~e acceptance (VL/t) and dark decay. VL is capacitive potential, t is photo-conductor thickness.
3. ~/0 is dark decay 10 seconds after charging to about 2000 volts.
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Claims (15)

WHAT IS CLAIMED IS:

1. A method for obtaining a durable xerographic photoreceptor comprising a flexible charge-conductive metal or metal-coated base having an base surface exclusive of aluminum or aluminum alloy, at least one inorganic photoconductive layer, and at least one intermediate aluminum blocking layer;
comprising exposing a clean charge-conductive base as an electrode under vacuum to an aluminum vapor cloud containing uncharged and charge material obtained by exposing vaporized aluminum to a glow discharge to obtain the intermediate blocking layer; and coating the base and applied intermediate blocking layer with at least one inorganic photoconductive material

2. A method for obtaining a durable xerograhic photoreceptor comprising a flexible charge-conductive metal or metal-coated base of one or more members selected from the group consisting of copper, brass, stainless steel, zinc, nickel or corresponding metal-coated polymeric-aluminum, or paper backed material, at least one inorganic photoconductive layer, and at least one intermediate charge blocking layer containing aluminum oxide;
comprising exposing a clean substantially oxide-free base as cathode to a vapor cloud containing uncharged and positively charged aluminum ions generated by passage of vaporized aluminum through a glow discharge; and coating the intermediate oxide blocking layer with at least one inorganic photoconductive material.

3. A method of Claim 2 wherein the intermediate charge blocking layer is laid down in a coater under glow dis-charge on a clean oxide-free base at a pressure up to about 40 microns (mercury) and containing not less than about 1 percent by volume available oxygen.

4. A method of Claim 2 wherein the intermediate charge blocking layer is laid down under glow discharge onto a clean oxide-free base at a coater pressure of about 5 - 30 microns (mercury) containing about 1 - 10 percent by volume available oxygen.

5. A method of Claim 2 wherein the base is exposed to positive high energy inert gaseous non-metallie ions at least prior to exposure to the aluminum vapor cloud.

6. A method of Claim 5 wherein the base is exposed to positive high energy inert gaseous non-metallie ions prior to exposure to the aluminum vapor cloud.

7. A method of Claim 1 wherein the step of coating the intermediate aluminum blocking layer with photoconductive material is at least initially effected by exposure to a vapor cloud of inorganie photoconductive material containing both uneharged and eharged photoeonductive particles obtained by passing vaporized photoconductive material through a glow discharge.

8. A method of Claim 2 wherein the step of coating the intermediate charge blocking layer with photoconductive material is at least initially effected by exposure to a vapor cloud of inorganic photoconductive material containing both uncharged and charged photoconductive partieles obtained by passing vaporized photoconductive material through a glow discharge.

9. A method of Claim 7 wherein the base and applied intermediate blocking layer are exposed to inert gaseous non-metallic ions at least during initial application of uncharged and charged photoconductive material onto the intermediate charge blocking layer.

10. A method of Claim 9 wherein applied photoconductive material is initially vaporized under glow discharge in a coater containing up to about 30 microns of an inert gas, the bulk of the coating step being thereafter effected under a reduced pressure in the absence of a glow discharge.

11. A method of Claim 9 wherein the base and applied intermediate blocking layer are exposed to inert gaseous non-metallic ions from a glow discharge prior to and during the initial application of the photoconductive material onto the intermediate charge blocking layer.

12. A method of Claim 3 wherein the base is a flexible belt and the photoconductive material comprises selenium, tellurium, germanium, antimony, bismuth or alloys thereof with each other and at least one of arsenic and a halogen.

13. A method of Claim 12 wherein the flexible base comprises nickel, stainless steel or brass.

14. A method of Claim 5 wherein the base is a flexible belt and the photoconductive material comprises selenium, tellurium, germanium, antimony, bismuth or alloys thereof with each other and at least one of arsenic and a halogen.

15. A method of Claim 14 wherein the flexible belt comprises nickel, stainless steel or brass.