US3519819A

US3519819A - Electrophotographic image receiving element with means to space said element from an image bearing surface during image transfer

Info

Publication number: US3519819A
Application number: US673544A
Authority: US
Inventors: Eugene P Gramza; Gene H Robinson
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1967-10-09
Filing date: 1967-10-09
Publication date: 1970-07-07
Anticipated expiration: 1987-07-07
Also published as: FR1585558A; BR6802919D0; DE1797232B2; CH504026A; BE721885A; DE1797232C3; DE1797232A1; GB1237386A

Description

July 7, 1 970 E E R EI'AI 3,519,819

ELECTROPHOTOGRAPHIG IMAGE RECEIVING ELEMENT WITH MEANS To-sPAcE SAID ELEMENT FROM AN IMAGE BEARING SURFACE DURING IMAGE TRANSFER Filed Oct. 9. 1967 \Y /MA 65- BEAR/N6 ELEMENT -A/R GAP PART/CULATE sPAcv/va MEANS \Q P/GME/VT B/NDEI? S UPPO/P T IMAGE-BEARING ELEMENT l\ A T 23 SPHER/CAL SPACl/VG MEANS P/GME/VT 24 I I BINDER I SUPPORT EUGENE GRAMZA GENE H. ROB/IVSO/V A INVENTORS ATTORNE Y United States Patent 3,519,819 ELECTROPI-IOTOGRAPHIC IMAGE RECEIVING ELEMENT WITH MEANS T0 SPACE SAID ELE- MENT FROM AN IMAGE BEARING SURFACE DURING IMAGE TRANSFER Eugene P. Gramza and Gene H. Robinson, Rochester,

N.Y., assignors to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey Filed Oct. 9, 1967, Ser. No. 673,544 Int. Cl. G03g 5/02 U.S. Cl. 250-65 18 Claims ABSTRACT OF THE DISCLOSURE Receiving elements having a regulated surface roughness are capable of producing electrographic images of improved quality over elements having smooth surfaces.

This invention relates to image-receiving elements and the process for using them, and particularly to receiving elements for use in electrostatic processes. I

The process of xerography as disclosed by Carlson in U.S. 2,297,691 employs an electrophotographic element comprising a support material bearing a coating of a normally insulating material whose electrical resistance varies with the amount of incident actinic radiation it receives during an imagewise exposure. The element, commonly termed a photoconductive element, is first given a uniform surface charge generally in the dark after a suitable period of dark adaptation. It is then exposed to a pattern of actinic radiation which has the effect of differentially reducing the potential of the surface charge in accordance with the relative energy contained in various parts of the radiation pattern. The differential surface charge or electrostatic latent image remaining on the electrophotographic element is then transferred to an electrophotographic receiving element. The transfer operation is Well known in the art and is typically described in U.S. Pat. 2,825,814. Usually the receiving element is a white paper coated With a thin layer of an insulating polymeric material in such a manner as to provide a smooth surface.

The transfer is generally carried out by contacting the insulating surface of the exposed photoconductive element with the surface of the electrophotographic receiving element. An electric field is established between these surfaces and the electrostatic charge is transferred to the surface of the receiving element. Since the surface is coated with an insulating polymeric material, it is trapped there. The transferred latent image is then made visible are relatively dilficult to obtain by using normal mechanical methods. If the space is too small, there can be substantial transfer of charge in the background areas thus frequently resulting in a mottled background. If the space is too large, then little or no charge will be transferred. In U.S. Pat. 2,825,814 Walkup describes a method for maintaining this spacing by grinding a plastic or resin to a relatively uniform particle size in the order of the size of spacing desired between the two members. A small quantity of the powdered material is dusted onto one of the smooth surfaces. The :second surface is then placed on top of the dusted surface and the transfer operation carried out. The disadvantage with this type of operation is that the dusted particles tend to adhere to both surfaces after the transfer operation is complete and after the surfaces are no longer in contact. Also, when the latent image is subsequently developed by the application of a toner, the final image often contains blotches due to the presence of the small particles used to maintain the spacing. Oftentimes when the two elements are separated after the transfer of a latent or developed image, theparticles move slightly if the utmost care is not taken. As a result of this movement the transferred image is frequently of inferior quality.

It is therefore an object of this invention to provide a novel image receiving element which is capable of receiving latent and developed images having good quality from an image-bearing element.

It is another object of this invention to provide a novel process for transferring latent electrostatic images from the insulated surface of an electrostatic image-bearing element to the insulated surface of a receiving element.

It is a further object of this invention to provide a novel process for transferring developed electrostatic images from the insulated surface of an electrostatic image-bearing element to the insulated surface of a receiving element.

These and other objects of this invention are accomplished by an image-receiving element which has a controlled surface roughness. A correlation has been found to exist between the texture of the surface of the receiving element asmeasured in terms of Sheffield Smoothness Value and the quality of the transferred image obtained. Best results are obtained if the Sheffield Smoothness of the elements of the invention are about 80 to about 180 and preferably 90 to 160.

The Sheflield Smoothness Value is a measure of paper smoothness which conforms to TAPPI Standard No. T-479SM48. The equipment for making the measurement is made by the Sheffield Corp, Dayton, Ohio.

Briefly, the components of the equipment are: (1) .a

by contacting the surface with a suitable electroscopic marking material. Such marking material or toner, whether contained in an insulating liquid or on a dry carrier, can be deposited on the receiving element either in the areas where there is an electrostatic charge or in the areas where the charge is absent.

Alternatively, prior to transfer, the electrostatic latent image can be developed directly on the photoconductive element in the same manner set forth above. The developed image can be transferred to the receiving element by contacting the two surfaces and applying an electrical potential between them.

When the deposited marking material is on the surface of the receiving element representing the developed image, it can then be fixed there by known means such as heat, pressure, solvent vapor or the like.

During the transfer operation, in order to obtain good quality, background-free reproductions it is desirable to maintain a minute air gap of a few microns between the smooth surface of the receiving element and the surface of the photoconductive element. Such small spacings precision device in which the paper sample is held against a smooth glass plate under an accurately weighted precision machined head through which regulated airflows; and (2) a Modular Precisionaire Instrument which measures the flow of air across the surface of the paper sample. Data are read in numerical units from 0 (smooth) to 400 (rough).

While the elements of this invention are useful in any process wherein it is desired to transfer an electrostatic image, they are particularly useful in the transfer of images in electrophotographic systems such as xerography. According to this invention, a member bearing an electrostatic latent image such as a xerographic photoconductive element is brought into contact with the face of a receiving element. By providing a receiving element which has a surface roughness Within the above indicated limits, the size of the air gap can be very closely controlled.

The receiving elements of controlled surface roughness of this invention are prepared by coating a suitable substrate such as paper with a thin layer containing an electrically insulating, solid, film-forming polymeric binder and particulate spacing means randomly dispersed throughout the layer and embedded therein. The spacing means comprise a plurality of substantially inert filler particles such as barium sulfate, zinc oxide, titanium dioxide, zinc carbonate, calcium carbonate, polystyrene beads, glass beads and the like. Almost any inorganic or organic material is suitable for such use provided it has a softening point or melting point substantially above that at which the transfer or developing operations are performed and is inert chemically, physically, electrically, etc., under the conditions used. Each of these particles is embedded in the polymeric binder layer in such a manner that a portion of each particle protrudes above the surface of the layer. The layer itself is generally 3-6 microns thick and preferabl 4 to 5 microns. However, layers having a thickness from 1 to about microns can be employed if desired.

The size of the spacing means employed determines the size of the air gap and thus the separation between the surfaces of the photoconducting and receiving elements. Since a portion of the filler particles is embedded in the surface of the layer and a portion protrudes above the surface of the layer, only the size of the protruding portion is considered in the determination of the gap width. By the term effective diameter is meant the greatest linear dimension of the filler particle. The filler particles can have a wide variety of shapes. For example, the particles can be spherical, polyhedral, conical, cylindrical, etc. In each instance however, the effective diameter can range from 5-35 microns and preferably from 7-25 microns. Particles having larger and smaller effective diameters are also usable such as 2-50 microns.

The air gap between the surfaces typically is 2 to 30 microns and preferably 3-20 micons and thus, this much of the spacing means protrudes above the surface of hte receiving element. Air gaps of 1-40 microns are acceptable in many applications and in these instances this portion of the spacing means extends above the surface.

The electrophotographic receiving elements of this invention can contain a substance which will give the element a desired color such as white, green, red, blue, black, etc. This substance is generally a pigment which is present in the coated layer in an amount sufficient to impart a uniform pigmentation to the viewer. However, if the color of the element is not critical, then the presence of a coloring substance is not necessary. If the desired color is white, then the coloring substance can be any white pigment such as particles of barium sulfate, zinc oxide, titanium dioxide, zinc carbonate, calcium carbonate or bentonite. The size of the pigmenting particles can vary from 0.1 to 1.0 micron and usually from 0.2 to 0.5 micron. Ideally, particles having an effective diameter of 0.3 micron are preferred.

The binder can be any electrically insulating, solid, film-forming polymer. Such polymers include (a) polyvinylbutyral,

(b) poly(bis-phenol A) carbonate,

(0) polystyrene,

(d) a polyester of terephthalic acid and a mixture of ethylene glycol (1 part by weight) and 2,2-bis[4-(B-hydroxyethoxy)phenyl]propane (9 parts by weight),

(e) polyvinylformal,

(f) a copolymer of vinylchloride and acrylonitrile,

(g) a copolymer of vinylchloride and vinyl acetate, and

(h) poly(4,4-[2-norbornylidene]-diphenyl carbonate).

A wide variety of substrates or supports can be used for the present receiving elements, but the preferred one is paper. The paper can have a clay subbing on one side but beneficial results are obtained if both sides are subbed. If both sides are subbed, then the paper absorbs less of the developer and drying time and costs are substantially reduced. It is also preferable to nip-size one side of the paper with a conducting resin such as polyvinylbenzyl-ammonium chloride. While this treatment of the paper is not absolutely necessary, if it is omitted oftentimes the developed image will print through onto the back of the paper. Other suitable substrates include polyethylene-coated paper, glass, polystyrene, cellulose acetate and polyethylene terephthalate.

Thus far, the invention has been set forth in general terms. It will now be described illustratively in terms of the drawings.

FIG. 1 is a diagrammatic view of an electrophotographic receiving element according to one embodiment of this invention.

FIG. 2 is a diagrammatic view of an electrophotographic receiving element according to another embodiment of this invention. In both figures the illustrations are diagrammataic, largely because certain key members and certain important spacings are extremely thin and small.

In FIG. 1 is shown an image-bearing element 11 having either a latent electrostatic image or a developed image on the surface thereof. Spaced therefrom by a minute air gap 12 of a few microns is a receiving element to which the image on the surface of the image-bearing element is to be transferred. The receiving element itself contains a support 14 such as paper. Coated on the support is a thin layer of an electrically insulating, solid, film-forming, polymeric binder 15. Dispersed substantially uniformly throughout the layer are very small pigmenting particles 16. The pigmenting particles are not necessary unless it is desired to impart a particular color to the receiving element. The layer also contains particulate spacing means 13. This is generally a plurality of substantially inert filler particles randomly dispersed throughout the layer and embedded therein. It is the protrusion of these particles above the layer which imparts roughness or an uneven texture to the surface. The size of the individual particles controls the width of the minute air gap. Since a portion of the particle is embedded in the binder, it is firmly held in position during the image transfer process. Preferably, at least 4 microns of each of the particles is embedded in the surface to ensure that they will not become dislodged during the transfer operation. Thus, upon separation of the elements a good quality image is obtained with no background density and n0 blotching. The particles are, of course, larger than the thickness of the layer.

In order for the transfer operation to occur, the imagebearing element and the receiving element are brought into close proximity of each other in a face-to-face relationship. The two are separated only by a space equal to the height which the spacing means protrudes above the surface of the layer. The only points of contact are where the spacing means touches the image-bearing element. both elements are mounted on conducting supports (not shown). A source of electrical potential is applied to the conducting supports to establish an electric field between them. The developed or latent electrostatic image on the surface of the image-bearing element is caused to leave and be deposited on the surface of the receiving element as a result of the electric field. Since the layer of the receiving sheet contains a binder which is an insulator, the charge representing the image is trapped there. The width of the air gap is 2 to about 30 microns. Best transfers are obtained if the air gap is 3-20 microns, however, gaps from 1 to 40 microns are also operable.

FIG. 2 is similar to FIG. 1 except that the spacing means 23 is depicted as being spherical particles rather than irregular shaped particles. The remaining components are the same; namely, image-bearing element 21, minute air gap 22, receiving element support 24, polymeric binder 25 and pigmenting particles 26.

In preparing the receiving elements of this invention, the average distance between the spacing particles is typically 50 to 200 microns, with about microns be ing optimum for most transfers but average spacings of 25 to 500 microns are also operable. The elements are useful in any process where it is desired to transfer an electrostatic image from the surface of a member to the surface of a receiving element. The elements are particularly useful, in xerographic processes where an image is reproduced by simultaneously charging and exposing a photo-conductive element. Such a process is morefully described in Ser. No. 665,911, by G. H. Robinson, filed Sept. 6, 1967, now abandoned. a

The invention is further illustrated by the following examples which include preferred embodiments thereof.

' EXAMPLE 1 The following formulation is used to prepare an insulatingmaterial suitable for coating:

75 gm.polyrneric binder of a ter-polymer of vinyl butyral (88%), vinyl alcohol (943%), vinyl acetate (2 /2%), having an approximate molecular weightof 50,000 (such as Butvar B-76, made by Shawinigan Resins Corp.)

22.75 gm.pigmenting particles of titanium dioxide having an average particle size of 0.3 micron (Titanox RA manufactured by Titanium Pigments Corp.)

2.25 gm.spacing filler particles of titanium dioxide having an average effective diameter of 7 to 25 microns (Titanox' TG manufactured by Titanium Pigments 'Corp.)

670 gm.-toluene This mixture is milled for 3 hours and 47 minutes in a Kadymill at 15 percent solids..It is then coated at a dry weight of 0.55 g. per square foot to produce a dry thickness of 4 to 5 microns on a paper which has a clay subbing on one side only. The insulating layer is coated on the subbed side. The texture of the insulator-coated side of the paper as indicated by the 'Shefiield Smoothness Value is 140. An electrostatic latent image-bearing photoconductive element is placed in a face-to-face relationship in close proximity to the receiving element in such a manner that thereis a 20-micron air gap between the surfaces of the two elements. The spacing .in this air gap is controlled by the portion of the spacing titaniumdioxide particles which extend above the surface of the receiving element. A potential of approximately 1500 volts DC, with respect to ground, is applied for approximately 2 seconds to the conducting layer of the photo-conductive element; the conducting support of the receiving element being grounded. At the end of the transfer operation, the potential is removed and the various, members separated. The receiving member, bearing the electrostatic latent image corresponding to that borne by the surface of,the

photoconductive element, is developed by immersion in a. r positive-polarity liquid developer. The resulting image is a postive-appearing reproduction displaying dense, sharp, black characters with a uniform low density background. The back side of the developed receiving element displays a negative-appearing image of the developed image on the front of the sheet.

EXAMPLE 2 An insulator-coated receiver paper is prepared exactly as described in Example 1. The uncoated side of the paper is nip sized with an 8-percent solution of a conducting resin of polyvinylbenzyl-ammonium chloride in water. The Sheffield Smoothness Value of the insulating surface. is 140. A print is made on this paper in the same manner as described in Example 1. The resultant image displays dense, sharp, black characters with a clean white background. No toner deposition is detected on the rear side of the sheet.

EXAMPLE 3 An insulator-coated receiver paper is prepared according to. the formulation set forth in Example 1, except that 25 g. of titanium dioxide pigmenting particles are used and no titanium dioxide spacing particles are used. The Sheflield Smoothness Value of this material is 40. A print is made on this paper according to Example 1.

The resultant image is low in density with an extremely high and irregular background density. This type of insulator-coated paper is very similar to the prior art papers, however, no spacing means, mechanical or otherwise, is used. It produces an unacceptable final print.

EXAMPLE 4 For purposes of comparison, Example 3 is repeated except that a small amount of polystyrene beads having an average diameter of 20 microns is dusted onto the surface of the photoconductive element as suggested in US. Pat. No. 2,825,814. These particles are used to maintain an air gap of 20 microns between the surface of the photoconductive element and the surface of the receiving element. A print is made in the usual manner as described in Example 1. The resultant image is good except that there are blotches on the paper. These blotches occur since the spacing particles are dusted on the surface of the element instead of being embedded in the surface of the receiving element.

EXAMPLE 5 This example is the same as Example 2, except that a paper is used which has a clay subbing on both sides. The resultant image obtained after developing is comparable to that described in Example 2. However, the amount of developer absorbed into the paper support is considerably less and the drying time of the final print is reduced greatly.

EXAMPLE 6' An insulator-coated receiver paper is prepared according to the formula of Example 1, except that larger titanium dioxide spacing particles are used so that the Sheffield Smoothness of the paper is 250. The imageforming procedure of Example 1 is followed and in this instance no image is transferred from the surface of the photoconductive element to the surface of the receiving paper because the air gap provided is too large for the charge to be transferred.

EXAMPLE 7 The purpose of this example is to show that the same characteristics which make for optimum charge transfer are also extremely desirable in a receiver sheet for a transfer of liquid developed xerographic images. This type of transfer of a liquid developed image is particularly useful in the production of 3-color subtractive prints by multiple transfer to a single receiving sheet. A photoconductive element bearing an electrostatic latent image is developed by pumping a liquid dispersion of cyan toner through a developing electrode as the photoconductor is moved across the electrode. The resulting visible wet image is overlaid with a sheet of coated reoeiving paper of the type described in Example 2 above and the combination is passed once rapidly beneath a negative coronawire. This operation transfers essentially of the image to the receiving sheet, leaving but a detectable residue on the photoconductor. The quality of the image is very good. The same technique using a lithographic grade press paper gives a very incomplete transfer and blotchy image. The above-described electrophotographic receiving elements are not limited in their use to receiving images from certain types of photoconductive elements; rather, the elements of this invention arecapable of receiving images from any type of photoconductive elements including those containing organic including organ0-metallic photoconductors as well as inorganic photoconductors.

\ The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected Within the spirit and scope of the invention as described hereinbefore and as defined in the appended claims.

What is claimed is:

1. An electrophotographic image receiving element comprising a paper support having coated thereon a layer having a thickness of 4 to 5 microns comprising:

(a) an electrically insulating, solid, film-forming polymeric binder,

(b) a particulate pigment substantially uniformly dispersed throughout said layer in an amount sufficient to impart whiteness thereto, the average effective diameter of said particulate pigment being 0.3 micron and (c) particulate spacing means comprising a plurality of substantially inert filler particles embedded in the polymeric binder in such a manner that said filler particles protrude 3 to 20 microns above the surface of said layer, said particles having an effective diameter of 7 to 25 microns and being randomly dispersed throughout said layer with an average spacing between particles of 100 microns.

2. The element of claim 1 wherein the pigment is selected from the group consisting of barium sulfate, zinc oxide, titanium dioxide, zinc carbonate, calcium carbonate and bentonite.

3. The element of claim 1 wherein the filler particles are selected from the group consisting of barium sulfate, zinc oxide, titanium dioxide, zinc carbonate, calcium carbonate, polystyrene beads and glass beads.

4. The element of claim 1 wherein the polymeric binder is selected from the group consisting of (a) polyvinyl butyral,

(b) poly(bis phenol A) carbonate,

(0) polystyrene,

(d) a polyester of terephthalic acid and a mixture of ethylene glycol and 2,2-bis[4-(;3 hydroxyethoxy) phenyl] propane,

(e) polyvinylformal,

(f) a copolymer of vinylchloride and acrylonitrile,

(g) a copolymer of vinylchloride and vinyl acetate,

and

(h) poly(4,4'-[2-norbornylidene]-diphenyl carbonate).

5. The element of claim 1 wherein the binder is a polymer of vinyl butyral, the pigment is titanium dioxide and the filler particles are titanium dioxide.

6. An electrophotographic image-receiving element comprising a support having coated thereon a layer comprising:

(a) an electrically insulating, solid, film-forming polymeric binder and (b) particulate spacing means comprising a plurality of substantially inert filler particles embedded in the polymeric binder in such a manner that said filler particles protrude from about 1 to about 40 microns above the surface of said layer, said particles having an effective diameter from about 2 to about 50 microns and being randomly dispersed throughout said layer with an average spacing between particles ranging from about 25 to about 500 microns.

7. An electrophotographic receiving element comprising a support having coated thereon a layer comprising:

(a) an electrically insulating, solid, film-forming polymeric binder,

(b) a particulate pigment substantially uniformly dispersed throughout said layer, the average effective diameter of said particulate pigment ranging from about 0.1 to about 1 micron and (c) particulate spacing means comprising a plurality of substantially inert filler particles embedded in the polymeric binder in such a manner that said filler particles protrude from about 1 to about 40 microns above the surface of said layer, said particles having an effective diameter from about 2 to about 50 microns and being randomly dispersed throughout said layer with an average spacing between particles ranging from about 25 to about 500 microns.

8. The element of claim 7 wherein the layer has a thickness from 1 to about 10 microns.

9. The element of claim 7 wherein the support is paper.

10. The element of claim 9 wherein the particulate spacing means imparts a Sheffield Smoothness Value to the coated paper from about to about 180.

, 11. The element of claim 9 wherein the paper support is sized on both sides by a clay coating.

12. The element of claim 9 wherein the paper support is sized on both sides by a clay coating and subbed on one side by a conducting resin.

13. A process for transferring an electrostatic image comprising:

(a) providing an element having an electrostatic image-bearing surface,

(b) providing an electrostatic image receiving element comprising a support having coated thereon a layer comprising 1) an electrically insulating, solid, film-forming polymeric binder,

(2) a particulate pigment substantially uniformly dispersed throughout said layer, the average effective diameter of said particulate pigment ranging from about 0.1 to about 1 micron and (3) particulate spacing means comprising a plurality of substantially inert filler particles embedded in the polymeric binder in such a manner that said filler particles protrude from about 1 to about 4.0 microns above the surface of said layer, said particles having an effective diameter from about 2 to about 50' microns and being randomly dispersed throughout said layer with an average spacing between particles ranging from about 25 to about 500 microns,

(c) positioning said electrostatic image-bearing surface and the surface of said receiving element in close proximity to each other in a face-to-face relationship so that the filler particles embedded in said receiving element physically contact the electrostatic image-bearing surface thus creating a minute air gap between the two surfaces, and

(d) applying an electric field between the surface of the receiving element and the electrostatic imagebearing surface causing the electrostatic image to transfer to the surface of the receiving element.

14. The process of claim 13 wherein the inert filler particles are selected from the group consisting of barium sulfate, zinc oxide, titanium dioxide, zinc carbonate, calcium carbonate, polystyrene beads and glass beads.

15. The process of claim 13 wherein the element having the electrostatic image-bearing surface is a photoconductive element.

16. A process for transferring a developed electrostatic image comprising:

(a) providing an element having a developed electrostatic image-bearing surface,

(b) providing an electrostatic image receiving element comprising a support having coated thereon a layer comprising (1) an electrically insulating, solid, film-forming polymeric binder,

(2) a particulate pigment substantially uniformly dispersed throughout said layer, the average effective diameter of said particulate pigment ranging from about 0.1 to about 1 micron and (3) particulate spacing means comprising a plurality of substantially inert filler particles embedded in the polymeric binder in such a manner that said filler particles protrude from about 1 to about 40 microns above the surface of said layer, said particles having an effective diameter from about 2 to about 50 microns and being randomly dispersed throughout said layer with an average spacing between particles ranging from about 25 to about 500 microns,

(c) positioning said developed image-bearing surface 18. The process of claim 16 wherein the element havand the surface of said receiving element in close ing the electrostatic image-bearing surface is a photoproximity to each other in a face-to-face relationconductive element. ship so that the filler particles embedded in said red ceiving element physically contact the developed 5 References image-bearing surface thus creating a minute air UNITED STATES PATENTS gap between the two surfaces and (d) applying an electric field between the surface of u the receiving element and the developed image-bear- 3079253 2/1963 Graig ing surface causing the developed image to trans- 1O fer to the surface of the receiving element. RALPH G. NILSON, Primary Examiner 17. The process of claim 16 wherein the inert filler C E CHURCH Assistant Examiner particles are selected from the group consisting of barium sulfate, zinc oxide, titanium dioxide, zinc carbonate, US. Cl, X,R calcium carbonate, polystyrene beads and glass beads. 15 961; 250-495