US3540885A - Reduction of fog formation in an electrophotographic light sensitive sheet - Google Patents

Description

United States Patent "ice 3,540,885 REDUCTION OF FOG FORMATION IN AN ELEC- TROPHOTOGRAPHIC LIGHT SENSITIVE SHEET Satoru Honjo, Yasuo Tamai, and Seiji Matsumoto,

Saitama, Japan, assignors to Fuji Shashin Film Kabushiki Kaisha, Kanagawa, Japan No Drawing. Filed Jan. 27, 1967, Ser. No. 612,076 Claims priority, applicationggapan, Jan. 27, 1966,

Int. c1. Gll3g 5/00 US. Cl. 96-1 6 Claims ABSTRACT OF THE DISCLOSURE Before charging the surface of an electrophotographic light sensitive sheet for forming electrostatic latent images thereon by light exposure followed by liquid development, the layer is coated with an insulating liquid such as kerosene to reduce formation of fogs.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a new method of electrophotography which utilizes a liquid developer.

In the xerographic process an electrostatic latent image is formed on a photoconductive insulating layer and the image is then converted to a visible image by development. Here the polarity of the latent image is primarily determined by the constitution of the layer. For example, an electrostatic image of negative polarity is preferably formed on a layer which contains an n-type photoconductor, such as ZnO and CdS. Such a layer exhibits better electrophotographic properties i.e., a higher dark resistivity and a higher photoconductivity, when a negative surface charge is applied on the surface, giving a visible image of better contrast.

On the other hand, an amorphous selenium coating exhibits better characteristics when charged to positive polarity. This selectivity is attribued to a higher mobility of holes compared with the mobility of electrons in the layer.

Having only one polarity for one type of electrophotographic material causes the following disadvantage. When one wants to get positive reproductions from any arbitrary original images, positive or negative, utilizing a determined type of electrophotographic recording member fed into a copying machine, one must prepare and charge the machine with two kinds of developer. The exchange of the developers is troublesome, and inevitably is accompanied by an undesirable contamination of them.

Thus it would be highly desirable to provide an electrophotographic recording member which could be charged to any desired polarity and still show satisfactory electrophotographic properties. In that way one could always obtain positive reproductions, regardless of originals used, by changing the polarity of the corona charging and utilizing one kind of developer. Moreover, if it is desired to get a negative from a positive, one may again change the polarity of the charging.

SUMMARY OF THE INVENTION This invention provides a method by which an electrophotographic recording member can be charged in both polarities, even though the material is otherwise preferably charged to only one determined polarity. This method is characterized by utilizing a liquid developer.

This method comprises covering a photoconductive insulating layer provided on a conductive support, such as an aluminum plate or slightly conductive support, such as paper with an insulating liquid which is inactive to 3,540,885 Patented Nov. 17, 1970 the layer, then charging the liquid bearing layer electrostatically, exposing the layer to a light image thereby forming an electrostatic latent image, and developing the layer in or by applying a liquid developer which comprises charged dispersed powders and an insulating liquid medium miscible with the above liquid.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The electrophotographic recording member used in the present invention comprises a conductive or slightly conductive support and a photoconductive insulating layer provided thereon; here the support must be conductive at least at the surface on which the photoconductive layer is formed. Accordingly, an insulating film or plate having a conductive surface, such as a laminated metal layer, may also be used.

Photoconductive insulating layers are prepared by coating a mixture comprising an insulating binding material and a pulverized photoconductor, or by depositing a volatile photoconductor in vacuum. A great number of photoconductors appear in the published literature, any of which may be used for the present method. There may be provided an insulating overcoating which can transmit at least part of an active radiation to the photoconductive layer.

The insulating organic liquid used in the present method has to be inactive to the photoconductive layer, not dissolving or strongly swelling the layer, and must have a dielectric constant less than about 5. It may be volatile or non-volatile at room temperature; however, too high an evaporation rate makes the succeeding development process difiicult. In the liquid, insulating resinous materials, waxy compounds, or other coloring matters may be dissolved or dispersed. The introduction of these materials depends on the particular purposes. Though such a liquid comes into contact with and difiuses into the carrier liquid of the liquid developer which is successively applied on the recording layer, it still remains in high concentration at the surface of the recording layer so that it can also be used to control the development conditions. Among suitable liquids used for covering a photoconductive layer are aliphatic hydrocarcarbons, polymethyl siloxanes, etc.; a small amount of polar or weakly polar liquids, such as ketones and esters can be used. Resinous materials or plasticizers may also be permitted, In general, those which are suitable as carrier liquids for the liquid developer may be used for this purpose. Accordingly those which have dielectric constants less than 5, and specific resistivities higher than 10 to 10 ohms-cm. are desirable. A too thick liquid layer on the photoconductive layer sometimes invites background densities and lowering of contrast. A very thin layer is preferred in practice, i.e., the preferable thickness lies between 0.5 and 15 microns. In the case of a commercially available ZnO electrophotographic paper, it may be moistened with a suitable insulating liquid at its surface, then contacted with a sheet of blotting paper or cloth so that almost all of the liquid is removed. When the surface of a recording member is porous, some of the applied liquid is impregnated into the layer; however, in such a case good results can still be obtained.

The eliect of such a liquid layer is clearly demonstrated in the following testing procedure. A sheet of an electrophotographic paper utilizing photoconductive ZnO is partly moistened with an insulating liquid, while the remaining area is kept untreated. This sheet is subjected to a positive corona charging in darkness, exposed to a light image, and developed with a liquid developer. The treated area gives an image of sufficient density and contrast free of background which can be compared with one obtained by negative charging, while the untreated area gives a faint image in a rather dense background.

Liquid developers used in this method must contain toner particles charged to the desired polarity, and the carrier liquids must contain such components which are miscible with the liquid coated on the recording member.

The present method can also be used when the electrophotographic member is charged to the polarity at which it exhibits better electrophotographic behavior. In this case, there is not observed such a remarkable improvement as in the case of the reverse polarity, but still the presence of an insulating liquid layer sometimes increases the contrast in the resulting visible image. Another improvement is observed when the electrophotographic member is in a light adapted state. The insulating liquid layer may be present on the photoconductive coating when the pre-exposure is carried out, or it may be introduced between the pre-exposure step and the charging step at darkness. Anyway, it is only necessary that the insulating liquid layer be present on the photoconductive layer when the layer is subjected to corona charging. Theexistence of the liquid layer proved to give far better contrast in the resulting visible image. But on a strongly fatigued coating, a liquid layer is sometimes ineffective.

The mechanism of the present method is presently difficult to explain theoretically. A probe of an electrometer closely placed on a ZnO coating covered by a thin layer of kerosene, which gave a clear image with positive surface charging, detected a surface potential of a very low value similar to an uncovered coating.

Utilizing the present method one can always obtain positive reproductions from positive and negative originals with one kind of developer by only changing polarities of charging. A liquid applying roller may be installed before the charging unit in the copying machine: the roller may be one like a moistening roller in an offset duplicating machine. In the case of ZnO paper, since the liquid application is necessary for positive charging, the roller may be moved to contact the surface of the ZnO paper only when the polarity of charging is positive. In some cases, the roller may always be kept in contact with the surface of an electrophotographic member.

The presence of an insulating liquid layer proved to decrease the frequency of spot generation in photoconductive coating due to local breakdown during charging.

EXAMPLE 1 A liquid developer was prepared by dispersing in 1000 cc. of kerosene an intimate mixture comprising 1.5 g. of carbon black (Asahi Carbon Co.), 2 g. of polyvinyl acetate, and 10 cc. of methyl ethyl ketone. The dispersed particles in this developer acquired positive charges and deposit on a negatively charged latent image. On the other hand, an electrophotographic member was prepared by coating a paper base with a homogeneous mixture comprising the following ingredients:

Parts by weight Photoconductive ZnO 100 Epoxyester of fatty acids from dehydrated caster oil (oil length 40%, epoxy resin usedEpikote Cobalt naphthenate 0.1

The member was dried after coating at 50 C. for 4 hours. Before charging it was kept in the dark for more than 12. hours to be dark-adapted. About half of the sheet was covered with kerosene (the thickness of the kerosene film was about 5 microns) the other half was left untreated. This sheet was subjected to negative corona charging of 7000 v., then exposed to a light image, and then developed with the liquid developer described above. A visible image was obtained on the whole area of the sheet, but a lower background density was observed at the treated area.

4 E AM L The same sheet as described in the Example 1, half treated with kerosene, was subjected to a positive corona charging of 8000 v., and processed in a manner similar to Example 1. A visible image appeared at the area which had been covered with kerosene, while at the remaining area a very faint image was observed.

EXAMPLE 3 A similar processing as in Example 2 was repeated except the following liquid developer was used in the present example.

Cyanine green (Toyo Ink C0.)0.5 g. Kerosene200 cc.

Cyclohexane-65 cc.

In this developer dispersed cyanine green was charged negatively and was attracted to a positively charged latent image. A green positive reproduction of a positive original was obtained only at the covered area in the sheet.

EXAMPLE 4 A similar processing as in Example 1 was repeated, but in this example a pre-exposure of 100 lux from a tungsten lamp for 3 seconds was given to the half-covered sheet before charging. The development using the liquid developer described in Example 1 combined with negative charging resulted in a legible image only at the covered area.

EXAMPLE 5 EXAMPLE 6 A similar processing as in Example 2 was repeated except the covering liquid was changed to a mixture of 1 part of kerosene and 1 part of soybean oil. A reversal development occurred. A similar result was obtained by using peanut oil in place of soybean oil.

EXAMPLE 7 The electrophotographic paper described in Example 1 was homogeneously covered by a layer of kerosene 18 microns in thickness. Half of the sheet was pressed to a blotting paper and a major part of the kerosene was removed. Then it was subjected to positive corona charging with 8000 v., potential. A subsequent image exposure and development with the liquid developer described in Example 1 gave a visible image on the entire area of the sheet. But a lower background density was observed at the area where the blotting paper was pressed. From weight measurement, the thin kerosene coating proved to have about 0.7 micron thickness.

EXAMPLE 8 A photoconductive plate was prepared by depositing selenium in vacuo on an aluminum plate of 20 cm. x 30 cm. size. The thickness of selenium was about microns. This plate was covered with a layer of kerosene 4 microns thick, and then was subjected to negative corona charging with 7000 v. potential. A subsequent image exposure and development with the liquid developer described in Example 1 gave a clear visible image, which was then transferred electrostatically to a receiving sheet.

5 EXAMPLE 9 A similar procedure was repeated as in Example 8 except that the developer used was changed to that described in Example 3. A reversal development occurred giving a clear image.

What we claim is:

1. An electrophotographic method which comprises applying to a photoconductive insulating layer on a conductive or slightly conductive support an insulating liquid which does not dissolve said layer, having a dielectric constant less than S, and a specific resistivity higher than 10 cm., in a thickness of from about 0.5 micron to about 15 microns, electrostatically charging the liquidbearing photoconductive layer only after said insulating liquid is applied, exposing the layer to a light image to provide an electrostatic latent image in the layer, and developing the thus exposed layer with a liquid developer having fine toner particles dispersed in an insulating liquid medium miscible with the aforesaid insulating liquid.

2. The electrophotographic method as claimed in claim 1 wherein said insulating liquid is kerosene.

3. The electrophotographic method as claimed in claim 1 wherein said insulating liquid is a mixture of kerosene and soybean oil.

4. The electrophotographic method as claimed in claim 1 wherein said. insulating liquid is a mixture of kerosene and peanut oil.

5. The electrophotographic method of claim 1 further comprising the step of removing part of said insulating liquid from said layer prior to charging.

6. The electrophotographic method of claim 5 wherein said partial removal of said insulating liquid is accomplished by blotting said insulating liquid with an absorbent material.

References Cited UNITED STATES PATENTS 3,227,076 l/1966 Castle 101-l49.4 3,357,828 12/1967 Moe 961 3,368,894 2/1968 Matkan et a1. 96-1 3,412,242 11/1968 Giaimo 250-49.5

GEORGE F. LESMES, Primary Examiner M. B. WITTENBERG, Assistant Examiner US. Cl. X.R.