GB1573222A

GB1573222A - Dielectric imaging member and imaging process therefor

Info

Publication number: GB1573222A
Application number: GB99577A
Authority: GB
Original assignee: GAF Corp
Current assignee: GAF Corp
Priority date: 1976-03-23
Filing date: 1977-01-11
Publication date: 1980-08-20
Also published as: DE2708930A1; FR2345747B1; FR2345747A1; DE2708930C2; CH604218A5; JPS52115220A; CA1094380A; NL7703132A; IT1076319B

Description

(54) DIELECTRIC IMAGING MEMBER AND IMAGING PROCESS THEREFOR (71) We, GAF CORPORATION, a corporation organized and existing under the laws of the State of Delaware, United States of America, having its main office at 140 West 51st Street, New York, New York 10020, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a dielectric imaging member and process for producing an image on a dielectric imaging member, such as a polyester film.

Images can be reproduced on a dielectric surface through various electrostatographic processes. In one such process, an electrostatic latent image is generated on the dielectric surface from a metallic electrode or pin by air ionization. Other such processes involve the transfer of an electrostatic latent image to a dielectric surface after it has been formed either on a dielectric surface or on a photoconductive surface.

In the first such process, generally referred to as electrography, electrostatic latent images are generated by character shaped electrodes or pin electrodes which are brought into close proximity to an electrically insulating surface, such as a dielectric web, supported on a base electrode. A potential is applied across the electrodes below a critical stress value.

Transfer of the character or pin configuration from the electrode to the electrically insulating web is effected by the use of a relatively low potential triggering pulse which raises the electric field above the critical stress value to produce a field discharge in the space between the electrically-insulating web and the electrode.

The discharge action gives rise to the formation of an electrostatic latent image of the character or pin on the electricallyinsulating web. Thereafter, the generated image on the web can be rendered visible by application of liquid or dry developer thereto.

Electrography is useful in many applications where it is required that a voltage signal pulse be applied directly to a dielectric receiving member, e.g., analogue oscillographs, high speed line printers and, digital plotters. Typical requirements for such systems are pulses of 700 volts in 50 to 100 microseconds.

It has been found, however, that applications of electrographic techniques to dielectric films of finite thickness, e.g., polyester films having a thickness of greater than 75 micrometers, results in a low density, diffuse image that requires relatively high voltage pulses in the millisecond range.

In the basic electrostatographic process, a uniform electrostatic charge is deposited upon a photoconductive electrically insulating layer which is thereafter exposed to a light image to selectively dissipate charge in those areas of the layer exposed to the light, thereby forming an electrostatic latent image. The image can be developed by depositing a viewable toner thereon. Alternatively, the latent image can be transferred to a dielectric surface for subsequent development. Still further, the latent image formed upon a dielectric surface either by electrographic techniques or transfer techniques as described above can, in turn, be transferred to a dielectric surface.Techniques for transferring electrostatic latent images to an electrically-insulating surface are well known in the art and have been accorded the acronym TESI (Transfer of ElectroStatic Image) see, for example, Xerography And Related Processes, Dessauer and Clark, The Focal Press (1965) pp. 405 et seq.

If desired, the latent image on the photo conductor can be developed directly and the developed areas can then be transferred by placing an electrically insulating surface over the developed photoreceptor surface and applying thereto a high potential of opposite polarity to that of the developer by, for example, corona discharge. When the electrically insulating surface is peeled away from the photoreceptor, it will carry thereon a sizable portion of the developer in image configuration. When transfer is accomplished, the developer can be fixed by fusing it to the receiving surface or by other conventional fixing means.

When the electrostatic latent image is transferred from the photoconductor surface to a dielectric surface before development, the charged photo conductive surface is brought into intimate contact with the dielectric surface to effect charge transfer or transfer can occur across an air gap by impressing a voltage between a conductive backing on the photoconductor and the opposed dielectric surface of a polarity to attract the charge on - the imaged areas to the dielectric surface.

As a practical matter, however, it has heretofore been impossible to transfer charge from a photoconductor to a thick, i.e., 75 micrometres or greater, dielectric film and subsequently develop the image to a high optical density with conventional systems.

According to one aspect of the present invention there is provided a dielectric imaging member for receiving an electrostatic latent image comprising a dielectric substrate (A) having a thickness of 75 to 175 micrometres, an electrically conductive layer (B) directly on said substrate (A) and a dielectric coating (C) having a thickness of less than 15 micrometres on the layer (B). Preferably the electrically conductive layer (B) has a resitivity of less than 101 ohmslsquare. The said dielectric substrate (A) may be a transparent dielectric film.

According to another aspect of the invention there is provided an imaging process comprising generating an electrostatic latent image on a dielectric imaging member of the invention and developing said image by contacting said image with a developer material.

Transparent polyester films are presently used as the preferred substrate material (A), for example, in micrographics, due to their strength and stability. Accordingly, reproduction processes based on the generation of an electrostatic latent image on such dielectric substrates were heretofore unavailable for use in conjunction with such preferred, but relatively thick substrates.

Q From known relations to as C = V and C is proportionate to -, where C = t capacitance of the dielectric film, Q = surface charge density, V = Surface voltage of the charge and t = film thickness, it is known that thin films will store more electrostatic charge, albeit at the expense of surface voltage.

Therefor, in order to obtain optimum image density on a dielectric imaging member comprising a dielectric substrate having a thickness of 75-175 micrometres, it has been discovered that if a second dielectric layer of less than 15 micrometres is applied to the dielectric substrate over a thin layer of a conductive agent, the capacitance of the resultant dielectric imaging member is changed relative to the charged photoconductor or an electrographic electrode to retain high charge density of an electrostatic latent image generated on the resultant dielectric imaging member. The generated image can be conventionally developed with high resolution. To the observer, however, the original dielectric imaging member remains virtually unchanged as far as transparency and thickness is concerned.

FIGURE 1 of the accompanying drawings is a diagrammatic view illustrating the process steps of one preferred form of the present invention; FIGURE 2 of the accompanying drawings is a graph illustrating the experimental results obtained by coating a conductively treated polyester film with a dielectric of various thicknesses and the developer density obtained on development of an electrostatic latent image transferred to the film, and FIGURE 3 of the accompanying drawings is a composite graph of both the surface voltage and charge density of an electrostatic latent image transferred to a dielectric coating of various thickness which is applied to the conductively treated polyester film.

Referring now to the drawings in detail, wherein like numerals indicate like elements, the process of the present invention is illustrated in FIGURE 1.

A typical 75-175 micrometres transparent dielectric substrate 10 has a thin transparent electrically conductive layer 12 applied to its surface. Conductive layer 12 has a resistivity value of less than 101 Q/ square, and preferably less than 108Q/square.

The conductive layer 12 can be any conductive material which is typically applied to paper such as quaternary ammonium salts, sulphonated polystyrenes and polyacyrlic acid salts. In addition, provided a reasonable amount of transparency is maintained, metallized films can also be employed.

A coating 14 of a dielectric resin 14, i.e.

a resin that has electrical insulating properities, is applied over the electrically conductive layer 12. The dielectric resin coating thickness is less than about 15 micrometres and preferably 4-5 micrometres and should adhere to the electrically conductive substrate.

Dielectric resins suitable for use in either or both the dielectric substrate 10 or dielectric coating 14 include polyvinyl acetates, acrylics, styrenated acrylics, polyesters, polyvinyl butyral, polycarbonates and other high dielectric resins.

In effect by virtue of introducing the electrically conductive layer 12, the capacitance of the substrate 10 is changed relative to a photoconductor 16 having a metallic substrate 18 and bearing an electrostatic latent image 20 formed thereon by conventional techniques. For the observer, however, the substrate 10 is virtually unchanged in thickness and transparency.

Because of the capacitative change provided by electrically conductive layer 12 and dielectric coating 14, it has been found that the image 20 on the surface of photoconductor 16 can be transferred to the dielectric coating 14 on substrate 10 by bringing the photoconductor surface 16 into intimate contact with the surface of dielectric coating 14. The transferred electrostatic latent image 20' on dielectric coating 14 will have sufficiently high charge density to permit development by immersion of the dielectric imaging member in a bath of liquid developer 22 or by other conventional development techniques.

It should be understood that the process disclosed is not limited to charge transfer from a photoconductor. Any surface bearing an electrostatic latent image thereon, e.g., a dielectric surface which is corona charged and has a portion of the charge dissipated to form an electrostatic latent image or upon which an electrostatic latent image is formed by electrographic techniques can be transferred to the dielectric imaging member of this invention. Further, utilization of the present invention permits image transfer to any electrically insulative surface of a thickness which normally would have too small an electrical capacity to hold enough charge to produce sufficient development.

The dielectric imaging member of the present invention can also be imaged by conventional electrographic techniques to provide sharp, dense images of the electrode configuration at reduced voltages and at significantly shorter pulse times.

The following examples further illustrate preferred embodiments of the present invention. These examples are included herein for illustrative purposes only. Unless otherwise stated, all percentages and parts are by weight.

Example 1 Dielectric imaging members were prepared by coating a polyester substrate ("Melinex" 505 preprimed available from ICI) having a thickness of 100 micrometres with a conductive layer of sulphonated polystyrene obtained from National Starch.

"Melinex" is a Trade Mark. The surface conductivity was 107 ohm-cm. The resulting conductive substrate was overcoated with a dielectric coating of styrenated acrylic resin from DeSoto, Inc. in thicknesses ranging from 1.4 to 14.2 micrometres. Thickness measurements were made using a recording spectrophotometer and by mechanical means. An electrostatic latent image was transferred to the resulting dielectric imaging member by intimate contact with an electrostatic latent imagebearing photoconductive plate which itself had been charged by a -5500 V corona.

The 24 micrometre thick photo conductive plate had an initial surface voltage of 600 volts. The photoconductor composition was a charge transfer complex of polyvinyl carbazole and trinitrofluorenone, of the type disclosed in U.S. Patent Specification No.

3,484,237. The electrostatic latent image formed on the dielectric imaging member was developed by immersion in a liquid developer composition as disclosed in U.S.

Patent Specification 3,542,682.

Measurements and calculations were made to determine the charge and voltage transferred to the dielectric imaging member from the photoconductive plate. The resulting measured voltage (Monroe Electrometer), calculated voltage, calculated charge and toner density are shown graphically as a function of thickness in the graphs of FIGURES 2 and 3 of the accompanying drawings.

It was found that as the thickness of the dielectric coating increased, the transferred voltage increased while the amount of transferred charge decreased. This follows from the principle of conservation of total charge which is that charge initially on the photoconductor is, on contact, shared between the photo conductor and the dielectric coating. As the dielectric coating thickness is increased, its capacity to hold charge is decreased relative to the photoconductor.

Optimum results of developed image density were observed when the thickness of the dielectric overcoating was in the range of 1.4-10 micrometres with maximum density occurring at a thickness of approximately 4-5 micrometres.

Example 2 A high resolution microimaging system is provided when the dielectric imaging mem ber of the present invention is used to receive an electrostatic latent image from a polyvinyl carbazole photo conductor sensitized with trinitrofluorenone. It has been found that an electrostatic positive charge image formed on such photoconductor can be transferred to the dielectric imaging member of the present invention with no externally applied voltage while maintaining the original resolution to greater than 100 line pars per millimetre when developed.

The preferred microimaging system of the present invention as described above provides increased resolution by enabling the photoconductor to intimately contact the surface of the dielectric imaging member.

Increased resolution is accomplished by transferring the electrostatic latent image under conditions wherein the two surfaces are in closest proximity. By using the surprisingly smooth surfaces afforded by both the polyvinyl carbazole-trinitrofiuorenone photoconductor and dielectric imaging member of the present invention and low gap voltage, high resolution is obtained.

Accordingly, the use of a film-forming organic photoconductor such as polyvinyl carbazole is particularly advantageous.

A photoconductive plate was prepared by applying the coating composition identifield below to a finely grained aluminium substrate: Ingredients Amounts polyvinyl carbazole 9 grams tetrahydrofuran 60 ml.

2A,7-trinitro-9-fiuorenone 1 gram "Clorafine" (a chlorinated 3 grams hydrocarbon plasticizer available from Hercules Corporation) The mixture was applied by conventional means to the aluminium substrate with a resulting dry thickness of 20 micrometres.

Both the adhesion and apparent coating smoothness were enhanced by the addition of plasticizer.

The resulting photoconductive plate was charged using positive corona and imaged with tungsten light The electrostatic latent image was transferred to dielectric imaging members prepared as described in Example 1. The latent image was developed by immersion in a negative liquid developer. The resulting resolution was greater than 100 line pairs per millimetre.

Example 3 An electrographic imaging system was established by placing a dielectric imaging member prepared as described in Example 1 in contact with an earthed base electrode which was connected through a potential source to a stylus electrode. In this manner, positive high voltage (+800 V) was applied through the stylus to the dielectric imaging member for square wave pulse durations of 1.5 x 10-5 to 1 x 10-1 second. An electrostatic latent image was generated on the dielectric imaging member in the form of a circular charge pattern equivalent to the contact area of the stylus employed. The resulting electrostatic latent image was developed employing a negative liquid developer composition (Hunt negative electrostatic liquid toner available from Hunt Chemical).

Good images were thus obtained for applied pulse voltages of 450 to 800 volts at 5 x 10-5 second pulse time.

Under identical conditions, no image was obtained when a conventional polyester film having a thickness of 75 micrometres was employed. When the pulse time was increased to 0.5 second, and the applied voltage to 1000 volts, some charge in the stylus area appeared, but then only randomly.

WHAT WE CLAIM IS: 1. A dielectric imaging member for receiving an electrostatic latent image comprising a dielectric substrate (A) having a thickness of 75 to 175 micrometres, an electrically conductive layer (B) directly on said substrate (A) and a dielectric coating (C) having a thickness of less than 15 micrometres on the layer (B).

2. A dielectric imaging member according to Claim 1, wherein the electrically conductive layer (B) has a resistivity of less than 101 ohms/square.

3. A dielectric imaging member according to Claim 1 or Claim 2, wherein the said dielectric substrate (A) is a transparent, dielectric film.

4. A dielectric imaging member according to Claim 3, wherein the said dielectric substrate (A) is a material chosen from polyvinyl acetate, acrylics, styrenated acrylics, polyesters, polyvinyl butyral or polycarbonates.

5. A dielectric imaging member according to any preceding claim, wherein said dielectric coating (C) is a material chosen from - polyvinyl acetate, acrylics, styrenated acrylics, polyesters, polyvinyl butyral or polycarbonates.

6. A dielectric imaging member in accordance with any preceding claim, wherein layer (B) is rendered electrically conductive by the presence of an electrically conductive agent chosen from quaternary ammonium salts, sulphonated polystyrenes and polyacrylic acid salts.

7. A dielectric imaging member in accordance with any one of Claims 1 to 5, wherein layer (B) is a metallized electrically conductive film.

8. A dielectric imaging member substantially as herein described with reference to any one of Examples 1 to 3 and/or with

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. ber of the present invention is used to receive an electrostatic latent image from a polyvinyl carbazole photo conductor sensitized with trinitrofluorenone. It has been found that an electrostatic positive charge image formed on such photoconductor can be transferred to the dielectric imaging member of the present invention with no externally applied voltage while maintaining the original resolution to greater than 100 line pars per millimetre when developed. The preferred microimaging system of the present invention as described above provides increased resolution by enabling the photoconductor to intimately contact the surface of the dielectric imaging member. Increased resolution is accomplished by transferring the electrostatic latent image under conditions wherein the two surfaces are in closest proximity. By using the surprisingly smooth surfaces afforded by both the polyvinyl carbazole-trinitrofiuorenone photoconductor and dielectric imaging member of the present invention and low gap voltage, high resolution is obtained. Accordingly, the use of a film-forming organic photoconductor such as polyvinyl carbazole is particularly advantageous. A photoconductive plate was prepared by applying the coating composition identifield below to a finely grained aluminium substrate: Ingredients Amounts polyvinyl carbazole 9 grams tetrahydrofuran 60 ml. 2A,7-trinitro-9-fiuorenone 1 gram "Clorafine" (a chlorinated 3 grams hydrocarbon plasticizer available from Hercules Corporation) The mixture was applied by conventional means to the aluminium substrate with a resulting dry thickness of 20 micrometres. Both the adhesion and apparent coating smoothness were enhanced by the addition of plasticizer. The resulting photoconductive plate was charged using positive corona and imaged with tungsten light The electrostatic latent image was transferred to dielectric imaging members prepared as described in Example 1. The latent image was developed by immersion in a negative liquid developer. The resulting resolution was greater than 100 line pairs per millimetre. Example 3 An electrographic imaging system was established by placing a dielectric imaging member prepared as described in Example 1 in contact with an earthed base electrode which was connected through a potential source to a stylus electrode. In this manner, positive high voltage (+800 V) was applied through the stylus to the dielectric imaging member for square wave pulse durations of 1.5 x 10-5 to 1 x 10-1 second. An electrostatic latent image was generated on the dielectric imaging member in the form of a circular charge pattern equivalent to the contact area of the stylus employed. The resulting electrostatic latent image was developed employing a negative liquid developer composition (Hunt negative electrostatic liquid toner available from Hunt Chemical). Good images were thus obtained for applied pulse voltages of 450 to 800 volts at 5 x 10-5 second pulse time. Under identical conditions, no image was obtained when a conventional polyester film having a thickness of 75 micrometres was employed. When the pulse time was increased to 0.5 second, and the applied voltage to 1000 volts, some charge in the stylus area appeared, but then only randomly. WHAT WE CLAIM IS:

1. A dielectric imaging member for receiving an electrostatic latent image comprising a dielectric substrate (A) having a thickness of 75 to 175 micrometres, an electrically conductive layer (B) directly on said substrate (A) and a dielectric coating (C) having a thickness of less than 15 micrometres on the layer (B).

reference to Figure 1 of the accompanying drawings.

9. An image process comprising generating an electrostatic latent image on a dielectric imaging member as defined in any one of Claims 1 to 8 and developing said image by contacting said image with a developer material.

10. An imaging process as defined in Claim 9, wherein an electrostatic latent image is formed on a photoconductive surface (D) and transferred to said dielectric imaging member upon being brought into intimate contact therewith in the absence of externally applied voltage.

11. An imaging process as defined in Claim 10, wherein the photoconductive surface (D) is a coating of a charge transfer complex of polyvinyl carbazoletrinitrofluorenone photoconductor on an electrically conductive surface.

12. An imaging process as defined in Claim 10, wherein an electrostatic latent image is formed on a dielectric surface (E) and transferred to said dielectric imaging member upon being brought into intimate contact therewith in the absence of externally applied voltage.

13. An imaging process as defined in Claim 11, wherein the electrostatic latent image is electrographically generated upon said dielectric imaging member.

14. An imaging process as defined in Claim 13, wherein the dielectric imaging member is supported on an earthed electrode which is in series connection with a potential source and an image-bearing electrode whereby when the breakdown potential of the air between the electrodes is exceeded, an electrostatic latent image is generated upon said dielectric imaging member.

15. An imaging process as defined in Claim 11 substantially as herein described and exemplified.