CA1069745A

CA1069745A - Photoconductive layer containing dispersed elastomeric material derived from a heterogenous copolymer

Info

Publication number: CA1069745A
Application number: CA247,018A
Authority: CA
Inventors: John M. Pochan; Richard G. Crystal; James M. O'reilly
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1975-03-26
Filing date: 1976-03-01
Publication date: 1980-01-15
Also published as: FR2305759B1; DE2608781A1; FR2305759A1; GB1543763A; NL7603154A; JPS51133027A

Abstract

CONTROLLED MORPHOLOGY PHOTORECEPTOR
ABSTRACT OF THE DISCLOSURE
A method of making a photoconductive layer which comprises providing a heterogeneous copolymer which includes a matrix phase (I) (preferably 50 percent or less) and a dispersion phase (II) (preferably 50 percent or more); adding to a liquid which is a nonsolvent for the dispersion phase and a solvent for the continuous phase; adding finely divided photoconductive particles to said liquid such that they are dispersed in said continuous phase, and coating said solution onto a supporting substrate and allowing said coating to dry to form a photoconductive layer having a morphology characterized by a major portion of a dispersed material suspended in a substantially continuous photoconductive or electrically active phase.

Description

.~
BAC~GROUND OF THE INVENTION
This invention relates to xerography and more specifically to a novel photosensitive member and method of making such a member.
The art of xerography involves the use of a photo-sensitive element or plate containing a photoconductive insulating layer which is usually first uniformly electro-statically charged in order to sensitize its surface. The plate is then exposed to an image of activating electromagnetic radiation such as light, x-ray, or the like, which selectively dissipates the charge in the exposed areas of the photocon-ductive insulator while leaving behind a latent electrostatic image in the non-exposed areas. This latent electrostatic image may then be developed and made visible by depositing finely divided electroscopic marking particles on the surface of the photoconductive layer. This concept was originally dis-closed by Carlson in U. S. Patent 2,297,691, and is further amplified and described by many related patents in the field.
One type of photoconductor used in xerography is illustrated by U. S. Patent 3,121,006 to Middleton and Reynolds which describes a number of binder layers comprising finely divided particles of photoconductive inorganic compound dis-persed in an organic electrically insulating resin binder. It has been found that structures of the Middleton et al type must use a substantially continuous particle-to-particle contact for the photoconductive material throughout the layer in order to-permit the charge dissipation required for cyclic operations.
With uniform photoc~nductor dispersions, relatively high volume concentrations of photoconductor, up to about 50 percent or more by volume, is usually necessary in order to obtain sufficient ;~ , 1~69745 photoconductor particle-to-particle contact for rapid discharge.
It has been found, however, that high photoconductor loadings in binder layers of this type result in the physical continuity of the resin being destroyed and thereby significantly reduce the mechanical properties of the binder layer. Layers with high photoconductor loadings are often characterized by a brittle binder layer having little or no flexibility. On the other hand, when the photoconductor concentration is reduced apprec-iably below about 50 percent by volume, the discharge rate is reduced, making high speed cyclic or repeated imaging difficult or almost impossible.
In ~. S. Patent 3,787,2b8 to R. N. Jones, the above problems of high photoconductor loading are obviated by a novel xerographic photoreceptor which includes a binder layer which comprises photoconductive particles dispersed in a controlled geometry in an insulating resin matrix. More specifically, substantially all of the photoconductive particles, which are present in a concentration of 1 to 25 percent by volume, are in the form of a plurality of continuous photoconductive paths through the thickness of the binder layer.
- The present invention is directed to improving the above methods noted in the prior art through the novel morphol-ogy of block copolymers, and to provide a novel photoconductive or charge transport layer having a controlled geometry analogous to that set forth in the 3,787,208 patent.
SUMMARY OF THE INVENTION
In accordance with one aspect of this invention there is provided a method of making a photoconductive layer which comprises: (a) providing a heterogeneous copolymer which includes a matrix phase and a dispersion phase, said phases com-prising on the one hand an insulating resin or a glassy or ~ _3_ ~0~9745 crystalline material and on the other hand an elastomeric material; (b) dissolving the copolymer in a solvent; (c) selectively precipitating the dispersion phase; (d) dispersing a photoconductive material in the matrix phase; and (e) coating said solution onto a supporting substrate and allowing said coating to dry to form a photoconductive layer having a rphology characterized by the dispersed phase being dispersed in a substantially continuous photoconductive phase.
In accordance with another aspect of this invention there is provided a photoconductive layer comprising a dis-persed elastomeric material as a dispersion phase contained within a substantially continuous photoconductive phase which comprises an oryanic matrix material comprising an insulating resin or glassy or crystalline material which contains a material selected from the group which consists essentially of a particulate photoconductive material, an electrically active material, a dye sensitizer, and mixtures thereof, said dis-persion phase and said matrix material being derived from a heterogeneous copolymer.
In accordance with another aspect of this invention there is provided a method of making a photoconductive layer which comprises: (a) providing a heterogeneous copolymer which includes a matrix phase and a dispersion phase, said phases comprising on the one hand an insulating resin or a glassy or crystalline material and on the other hand an elastomeric material; (b) dissolving the matrix phase in a solvent; (c) dispersing a photoconductive material in the matrix phase; and (d) coating said solution onto a supporting substrate and allowing said coating to dry to form a photo-conductive layer having a morphology characterized by the dis-persed phase being dispersed in a substantially continuous photoconductive phase.

-3a-.

By way of added explanation, in one respect the present invention is directed to a method of making a photo-conductive layer and comprises providing a heterogeneous copolymer which includes a matrix phase (I) (preferably 50 percent or less) and a dispersion phase (II) (preferably 50 percent or more). The copolymer is dissolved in a solvent to form a solution followed by adding a liquid which is a nonsolvent for the dispersion phase and a solvent for the matrix phase. A photoconductive material is added to said solution and is dispersed in the matrix phase. The solution is then coated onto a supporting substrate and allowed to dry to form a photoconductive layer having a morphology characterized by a major portion of a dispersed material suspended in a substantially continuous photoconductive or electrically active phase.
In accordance with the invention, a photoconducting pigment may be suspended in an elastomeric copolymer which comprises a major component which is a rubber, and a minor component which may be a glassy or crystalline polymer phase at ambient temperature. The pigment is distributed in the minor component phase to form a particle contact system in which the photoconductor concentration is such that the photoconductor particles are in substantial particle-to-particle contact throughout the resulting photoconductive layer. The major component rubbery phase provides excellent mechanical properties and some reinforcement, while the .
~, ~o69745 minor phase, which contains the photoconductive particles, allows the photoconductive layer to be charged and selectively discharged to form a xerographic latent image. One way of obtaining the morphology control is to dissolve the heterogeneous copolymer in a solvent and then add a nonsolvent for the xubber dispersion phase. Just after the solution becomes turbid, indicating the rubbery phase is precipitating, the photoconductive pigment is added to the matrix solution. A film or layer of this material is then cast from the solution onto a supporting substrate, which is usually conductive, and the coating allowed to dry. The final structure is characterized by a major portion of an elastomeric or rubber phase and a minor photo-conductive portion comprising segregated domains or networks which pass through the photoconductor layer thickness. The photoconductive phase provides electronic pathways which function to effectively discharge the photoconductive layer when used in the xerographic mode. A less preferred method is to dissolve the copolymer in a single solvent which will dissolve both phases at a specified temperature, but which upon evaporation or cooling will selectively deposit the dispersion phase.
Alternatively, a solvent can be employed which will dissolve only the dispersion phase.

BRIEF DESCRIPTION OF THE DRAWINGS

_ Figure 1 schematically represents one embodiment of an imaging member of the present invention.
Figure 2 schematically represents a second embodiment of an imaging member of the present invention.

DETAILED DESCRIPTION OF THE INVEN~ION

.
The controlled morphology photoconductive layers of the present invention are used either as a photoconductive layer or -5- ~

10t;9745 charge transport layer in xerographic imaging devices.
Figure 1 illustrates one embodiment of a typical xerographic member in which these photoconductive layers are used. Imaging member 10 comprises a supporting substrate 11 overlayed with photoconductive layer 12. Layer 12 comprises at least two phases, one of which is phase 13 which is an elastomeric material, and a second phase 14 which may comprise an insulating reqin or a glassy or crystalline material. Phase 14 further con-tains photoconductive particles 15 in a concentration great enough to insure particle-to-particle contact. It can be seen from the drawing that the morphology of the structure of photo-conductive layer 12 allows for continuous electronic pathways of the photoconductive material contained in phase 14 such that there are formed pathways of photoconductive material from the top surface down to the substrate photoconductor interface. In addition, elastomeric phase 13 provides the device with a high degree of mechanical flexibility. Generally, the concentration of the elastomeric or rubber phase should exceed S0 percent by volum~ and preferably at least 60 to 70 percent by volume, the balance comprising the minor component phase which contains the photoconductive material. In some instances, however, the dispersed elastomeric phase may be present in concentrations as low as 10 percent by volume.
In general, the material used for layer 12 comprises a heterogeneous copolymer having a major component which comprises a rubber or elastomeric material, and a minor component which comprises a glassy, crystalline, polymeric phase.
Photoconductive pigment is distributed in the minor component phase to form a particle contact system in order to insure desir-able electrical characteristics for use in electrophotoglaphy.

The major phase of the heterogeneous copolymer is elastomeric and would lead to improvements in the mechanical properties of the photoconductive layer. It should be understood, however, that other major phases consisting of crystalline or amorphous polymers preferably with superior mechanical properties may also be used. As defined herein the term "heterogeneous copolymer" includes block copolymers, graft copolymers and blends of homopolymers and bloc~ copolymers.
Substrate 11 is preferably made up of any suitable conductive material. Typical conductors comprise aluminum, steel, brass, aluminized Mylar or the like. The substrate may be rigid or flexible and of any convenient thickness. The substrate may also comprise a composite structure such as a thin conductive coating contained on a paper base; a plastic coated with a thin conductive layer, such as aluminum, graphite, copper iodide; or glass coated with a thin conductive coating of chromium or tin oxide. If desired, the substrate may also be substantially dielectric or electrically insulating and the device charged by techniques well known in the art for using imaging members having electrically insulating substrates.
Exemplary of suitable copolymers which may be used to form layer 12, and hence components 13 and 14 of the present invention, include without limitation, block copolymers of polybutadiene/polystyrene, polyether/polyester, polycarbonate/
silicone, polyethylene/polybutadiene, polyethylene/polybutylene, polyethylene/polyvinyl chloride (graft), hexamethylene sebacate/methyl ethyl propyl sebacate, polystyrene/polyethylene oxide, polymethyl methacrylate/polyhexyl methacrylate, polyisoprene/polymethyl methacrylate, polystyrene/silicone, polyester/urethanes ~B.F. Goodrich), polyvinyl carbazole (PVX/
silicone, PVK/polystyrene PVK/polyester, poly(~-methylstyrene)/

a~ 7_ ~69745 polyisoprene, poly(~ -methylstyrene)/polybutadiene, polycarbonate/
polybutadiene, and polyisoprene/polycarbonate. In addition to the above, various combinations of dispersions of photoconductive 8pecies and the elastomeric species may also be used provided care is maintained in selecting the materials such that the elastomeric or rubber phase may be dispersed in a nonsolvent, while the minor phase maintained in solution with photoconductive material such that the morphology set forth and illustrated in Figures 1 and 2 may be maintained.
Any conventional photoconductive or electrically active material 15 may be used in minor phase 14. Typical materials include cadmium sulfide, cadmium selenide, cadmium sulfoselenide, phthalocyanines, vitreous selenium, selenium alloys, trigonal selenium, photoconductive dyes such as Rhodam_ne B or Methylene Blue or pigments, such as phthalocyanine, substituted quinacridones (Monastral Violet and Monastral Red), anthanthrones, Indigo and Indigo dyes, Indofast Yellow, Indofast Orange, Golden Yellow RK, rhodamines, transport materials such as N-vinyl-carbazole, anthracene, polyvinyl carbazole, trinitrofluorenone, oxadiazoles, triphenyl methanes, N-isopropyl carbazoles, triarylamines, disubstituted diaryltriazoles and diaryl methanes, tric~anovinylated carbazole and tricyanovinylated indole.
In general, however, when using photoconductive particles or pigment in an electrically insulating minor phase 14, in order to maintain desirable electrical characteristics, it is essential that the photoconductive material be in substantial particle-to-particle contact. For example, when the elastomeric phase com-prises 70 percent by volume and the minor electrically insulating phase comprises 30 percent by volume, then the total photocon-ductor concentration should be 60 percent by volume of the minor phase or 18 percent by volume of the photoconductive layer in order to obtain the desired electrical properties.
In an alternative embodiment of the present invention, the novel photoconductive structure of the present invention may be used in a composite device such as a charge transport layer which overlays a thin photoconductive layer. This structure lS
illustrated in Figure 2 of the drawings by reference character 20 and comprises a supporting substrate 21, a photoinjecting layer 22 of any suitable photoconductor such as vitreous selen-~
ium, selenium alloys, trigonal selenium, cadmium sulfide in a binder, etc. The photoconductive layer is overcoated with the controlled morphology layer of the present invention already described above and comprises layer 23, having therein an elastomeric phase 24 containing a minor phase 25 which contains therein photoconductive particles 26 in particle-to-particle '-- contact. In operation, this device is imaged by uniformly charging the top copolymer layer to a uniform potential followed by exposing the image to radiation to which the top layer is substantially non-absorbent or transparent, and to which the lower photoconductive layer is substantially absorbing. Positive or negative electrical charges generated by photoconductive layer 22 move through transport layer 23 to form a latent electrostatic image. These structures are more fully described in Canadian Patent No. 960,073 issued December 31, 1974.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following examples further specifically define the present invention with respect to a method of making and imaging a novel photoreceptor member. Percentages are by weight unless otherwise indicated. The examples below are intended to illustrate various preferred embodiments of the present invention.
EXAMPLE I
A photosensitive binder layer similar to that shown in Figure 1 is made by the following method. 50 grams of a commercial elastomeric styrene-butadiene-styrene block copolymer (Rraton 1101 available from`Shell Chemical Company) was dissolved ~n 250cc of tetrahydrofuran, which is a mutual solvent for both species. An equal volume of methyl ethyl ketone, which is a preferential solvent for polystyrene, was then added.
The THF was then flash evaporated at room temperature resulting in the precipitation of the major butadiene phase (about 78 weight percent) as evidenced by the turbidity of the solution.
The methyl ethyl ketone was then removed by f~rther vacuum evaporation at room temperature, during which time the solution became increasingly viscous. Then 4.2 grams of photoconductive cadmium sulfoselenide pigment, having an average particle size of about 500 Angstroms, was then dispersed in the viscous solution by vigorous means with an ultrasonic probe. The resulting slurry wàs then cast on an aluminum substrate with a doctor blade, followed by air evaporation of the solvent. The film was vacuum dried at ambient for three hours prior to electrical property measurements which are summarized below:
White Light Illumination - 18 ~ film Negative Charge Positive Charge dv/dt = 5 x 104 ~/sec dv/dt = 4 x 104 ~/sec VO s 560 volts VO = 470 volts Vx = 40.volts Vr = 20 volts ~ ~d~ ~ar~ ' - ~o69745 - Monchromatic Illumination - ~ = 5800 2 ~ ~ 8 x 1012 Neaative Charge Positive Charqe. ""~ _ d~/dt = 170 voltsJsec-~ dv/dt = 52 volts/sec-p dark decay = 3.0 volts/sec-~ dark decay = 3.6 volts/sec-~
VO = 612 volts ~O = 550 volts EXAMPLE II
A-photosensiti~e binder layer similar to that shown in Figure 1 is made by the following method. Two (2) grams of a commercial elastomeric styrene-butadiene-styrene block copolymer (Kraton 1101 available from Shell Chemical Company) was dissolved in THF, a mutual solvent for the components of the block copolymer.
This solution was added to 20 mls. of methyl-ethyl-ketone (MEK), a non-solvent for the polybutadiene phase of the block. MEK
was added to this solution until it became turbid, indicating a precipitation of the butadiene phase of the polymer. The system was flash evaporated to approximately one-half its volume and more MEK added and another flash evaporation made until the final solution was 8 mls. of turbld viscous solution indicating a butadiene precipitated phase.
A dispersion of 2 grams polyvinyl carbazole-polybutadiene (PVK-PB) graft copolymer (70% PVK graft) was made in 20 mls. THF. This polymer is not totally soluble and can only be dispersed in THF. Then .282 grams of TNF were added to the dispersion so that a PVK/TNF sensitized system was formed. These weights yield a 10 mole percent TNF/carbazole ratio. Five (5) mls. of this dispersion was added to 5 mls. of the turbid block solution produced above. This system was then coated on a thin layer on an aluminum sheet, dried and tested in the xerographic mode. The electrical data is sur,lmarized below:

~ . .

:., ~069745 NA~ tive Charge Positive Charge , VO = 260 volts VO - 220 ~olts Vr = 125 volts Vr Z 80 volts dv/dt = 77 volts/sec. dv/dt = 79 volts/sec.
(tetrahydrofuran = THF) EXAMPLE III
To 5 mls. of the block copolymer slurry of Example II, .25 grams of trinitrofluorenone (TNF) were aaded. TNF is totally soluble in the THF. After dissolution of the TNF, the mixed system was coated on a selenium substrate, dried, and submitted for electrical evaluation. In this case, the matrix is being used as a charge transport matrix.
A plate similar to that shown in Figure 2 was made in the following manner. Electrical evaluation of this plate is tabulated below: -Ne~tive Charge Positive Charge VO = 300 volts VO = 340 volts Vr = 120 volts Vr = 100 volts dv/dt = 555 volts/sec. dv/dt = 455 volts/sec.

Although specific components and proportions have been stated in the above description of the preferred embodiments of the present invention, other modifications and ramifications of the present invention would appear to those skilled in the art upon reading the disclosure. These are also intended to be covered in the scope of this invention.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of making a photoconductive layer which comprises: (a) providing a heterogeneous copolymer which includes a matrix phase and a dispersion phase, said phases comprising on the one hand an insulating resin or a glassy or crystalline material and on the other hand an elastomeric material; (b) dissolving the copolymer in a solvent; (c) selectively precipitating the dispersion phase; (d) dispers-ing a photoconductive material in the matrix phase; and (e) coating said solution onto a supporting substrate and allow-ing said coating to dry to form a photoconductive layer having a morphology characterized by the dispersed phase being dispersed in a substantially continuous photoconduc-tive phase.

2. The method of Claim 1 in which the photoconductor is in particulate form and in a concentration that is sufficient to insure particle-to-particle contact.

3. The method of Claim 1 in which the dispersion phase comprises at least 50 percent by volume of the final photoconductive layer.

4. The method of Claim 1 in which the dispersed material comprises a polybutadiene.

5. A photoconductive layer comprising a dispersed elastomeric material as a dispersion phase contained within a substantially continuous photoconductive phase which comprises an organic matrix material comprising an insulat-ing resin or glassy or crystalline material which contains a material selected from the group which consists essentially of a particulate photoconductive material, an electrically active material, a dye sensitizer, and mixtures thereof, said dispersion phase and said matrix material being derived from a heterogeneous copolymer.

6. The member of Claim 5 in which the dispersed material comprises a polybutadiene and the photoconductive phase com-prises a polystyrene which contains finely divided photoconduc-tive particles dispersed therein.

7. The member of Claim 6 in which the photoconductive particles comprise at least one material selected from the group consisting of cadmium sulfide, cadmium selenide, cadmium sulfoselenide phthalocyanines, vitreous selenium, selenium alloys, trigonal selenium, photoconductive dyes such as Rhodamine B or Methylene Blue or pigments, such as phthalo-cyanine, substituted quinacridones (Monastral Violet and Monastral Red), anthanthrones, Indigo and Indigo dyes, Indofast Yellow, Indofast Orange, Golden Yellow RK, rhodamines, trans-port materials such as N-vinyl carbazole, anthracene, poly-vinyl carbazole, trinitrofluorenone, oxadiazoles, triphenyl methanes, N-isopropyl carbazoles, triarylamines, disubstituted diaryltriazoles and diaryl methanes, tricyanovinylated carbazole and tricyanovinylated indole in a concentration which insures particle-to-particle contact of the photo-conductive particles.

8. The structure of Claim 6 in which the photoconduc-tive layer is contained on a supporting substrate.

9. The structure of Claim 8 in which the supporting substrate comprises selenium.

10. The member of Claim 5 in which the photoconductive particles are selected from cadmium sulfoselenide, polyvinyl carbazole, and trinitrofluorenone.

11. The member of Claim 6 in which the photoconductive particles are selected from cadmium sulfoselenide, polyvinyl carbazole, and trinitrofluorenone.

12. The member of Claim 9 in which the photoconductive particles are selected from cadmium sulfoselenide, polyvinyl carbazole, and trinitrofluorenone.

13. The method of Claim 1 in which the dissolved copolymer is mixed with a solvent which selectively dissolves the matrix phase, the dispersion phase precipitated and the photoconductive material dispersed in the matrix phase.

14. A method of making a photoconductive layer which comprises: (a) providing a heterogeneous copolymer which includes a matrix phase and a dispersion phase, said phases comprising on the one hand an insulating resin or a glassy or crystalline material and on the other hand an elastomeric material; (b) dissolving the matrix phase in a solvent; (c) dispersing a photoconductive material in the matrix phase; and (d) coating said solution onto a supporting substrate and allowing said coating to dry to form a photoconductive layer having a morphology characterized by the dispersed phase being dispersed in a substantially continuous photoconductive phase.