Electrophotographic Reversal Development Process Employing A Pre-toner

Abstract

A reversal development process which comprises, after uniformly charging a photoconductive insulating coating with a charge of one polarity, applying a finely divided pre-toner having a white or off-white appearance and a charge of the same polarity as that of the uniform charge whereby the pre-toner deposits at the charge-deficient areas in the coating, then subjecting the thus treated coating to image exposure to form an electro-static latent image, and finally developing the latent image by applying a finely divided toner differently colored from the pre-toner, the toner having a charge of the same polarity as that of the latent image.

Description

BRIEF DESCRIPTION OF THE INVENTION

This invention relates to a reversal development method in electrophotography, and more particularly to one which can provide electrophotographic prints with reduced background or black spots which are referred to as pin hole spots.

An optically positive toner image results by uniformly charging an electrofax sheet material in a subdued light, then exposing said charged material to an optically negative image of light and shadow to form an electrostatic latent image, and then by developing the latent image by the application of toner having the same polarity of charge as that of the latent image.

In the case where the photoconductive coating is made of a homogeneous mixture comprising a finely divided powder photoconductor and a resinous binder, the coating has microscopic irregularities which will not be present in an organic photoconductive layer or amorphous selenium layer having a uniform structure in molecular order. Such irregularities cause local electrical breakdown when the coating is subjected to corona charging, and at such areas where the impinged corona ions undergo breakdown discharge the charged density will be nearly zero or greatly lower than the surroundings.

Such pin hole areas in the coating will be developed as black spots in a white background in reversal development in which toner predominantly deposits at lower charge density areas.

Such black spots detrimentally affect on the image quality since they are conspicuous in the highlight regions of a resulting electrophotographic print.

The invention provides a reversal development method which overcomes the above-cited shortcomings, and is characterized by that a photoconductive insulating coating is imparted an electrostatic charge of one polarity, supplied with finely divided powder of the same color as that of the photoconductive insulating coating and of the same polarity whereby the powder particles deposit at the charge deficient pin hole areas in the charged coating, then subjected to image exposure to form an electrostatic latent image, and finally developed with colored finely divided marking powder also having the same polarity of charge as that of the latent image. The white or off-white powder will be referred to as pre-toner while the marking powder as toner.

The essential feature of the present invention resides in that pre-toner is allowed to deposit or mask the charge-deficient spots formed during charging of the coating in order that such spots will no longer attract toner in the subsequent image development. It should be emphasized that in the present inventions the pre-toner is applied onto the coating prior to the image exposure.

Application of pre-toner after image exposure is far less effective for the present purpose. The reason is as follows; prior to image exposure deposition of pre-toner will proceed only at the charge deficient spots while, on the other hand, application of pre-toner after image exposure will result in deposition of the pre-toner not only at the charge-deficient spots but also at areas where charge density changes abruptly. This will make it impossible to produce a toner image of desirable quality since the toner will deposit with a reduced deposition density at those areas.

Suitable particles for pre-toner in the present invention include colorless or pale colored, finely divided natural or synthetic polymers, white or off-white pigments, mixtures of polymer and pigment particles, or similar pigments encapsulated with polymeric materials, etc. Suitable polymers are ethyl cellulose, nitrocellulose, triacetylcellulose, diacetylcellulose, polyvinylacetate, polyvinylalcohol, gelatin polymethylmethacrylate, polyvinylchloride, polycarbonate, etc., while examples of pigments are barium sulfate, calcium carbonate, kaolin, aluminum hydroxide, zinc oxide, titanium dioxide, zinc sulfide, lead white, etc.

As for the compositions of toner, those well known for those skilled in the art by already disclosed literatures or patent may be applicable for the present invention, including those for dry development comprising carbon black and resinous material or those for liquid development comprising pigment particles associated with resinous materials absorbed on the surface of the particles.

Though the present invention is effective when combined with or applied on a photoconductive coating comprising a finely divided photoconductor and resinous binder, it is also applicable to a homogeneous layer comprising organic photoconductor or amorphous selenium.

EXAMPLE I.

A photoconductive insulating coating was provided by vacuum depositing amorphous selenium on an aluminum plate. This coating exhibited pin-hole like charge-deficient spots in repeated use.

Such spots became noticeable when the coating was charged to a high potential.

The plate was charged by corona to +1000 volts, and then immersed in a dispersion comprising kerosene as dispersant in which a small quantity of linseed oil was dissolved as a dispersing-agent and finely divided ethylcellulose of 1 to 10 micron diameter which had a positive polarity of charge. After about a 10 second immersion, the plate was pulled out and then subjected to an image exposure utilizing a negative transparency.

The exposed plate was then developed with a liquid developer which had been prepared by dispering a blended mixture of carbon black and ethyl cellulose in kerosene in which was dissolved a small quantity of linseed oil. The particle diameter of the pigmented resin toner fell in the range between 1 and 10 microns.

Since the toner had a positive charge, reversal development resulted.

The plate was drawn from the developer and superimposed ith a sheet of ordinary paper while the surface of the plate was still wet. The assemble was subjected to negative corona from the back surface of the paper. Then the sheet was separated on which there was obtained a positive print free of pin hole spots.

EXAMPLE II.

A white photoconductive insulating coating was prepared on paper support by coating a blended mixture of photoconductive zinc oxide and insulating binder. The coating was negatively charged to about 500 volts by corona. Prior to image exposure a mixture comprising 0.5 to 1 mm diameter glass beads overcoated with a thin film of ethyl cellulose and pre-toner of 5 to 20 micron diameter vinylchloride and vinyl acetate copolymer which were adhered electrostatically to the surface of the beads was cascaded onto the surface of the coating.

Then the coating was subjected to image exposure utilizing a negative transparency. Developement was carried out by cascading developer mixture comprising the glass beads carrier described above and a pigmented resin comprising polystyrene and carbon black having a diameter of about 10 to 30 microns.

The resulting image was free of pin-hole spots in the background area.

On the contrary, a developed image obtained in the same procedures except the pre-toner application was accompanied with a great number of pin-hole spots.

EXAMPLE III.

Another sheet of the electrophotographic paper in Example II was charged -- 400 volts in subdued light, and then immersed in the following pre-toner dispersion of for 10 seconds:

Zinc oxide powder (particle diameter 0.05 to 0.5 microns) 10 parts Linseed oil 10 parts Resin varnish 5 parts Kerosene 1000 parts Cyclohexane 200 parts

The sheet was passed through a pair of insulating squeeze rollers to remove the adhering liquid.

After exposing to an optically negative image, the sheet was developed with the following developer:

Carbon black (particle diameter 0.01 to 0.1 micron 10 parts Nitrocellulose 20 parts Resin varnish 20 parts Butyl acetate 20 parts

These ingredients were blended in a ball mill jar for 50 hours, and 10 mls of the resulting mixture was dispersed in a mixture comprising 800 ml of kerosene and 200 ml of decalin under the application of ultrasonic energy.

According to this example, pre-toner application can not only expel pin-hole spots but also serve to prevent undesirable adhesion of toner due to forces other than electrostatic.

As have been described above, the present invention may be practiced by liquid or dry development. However, the present invention is more advantageously carried out by the former method of development which can provide electrophotographic prints of quite high quality having tonal rendition wherein black pin-hole spots in the highlight or background areas will fatally affect the image quality. In the application of pre-toner on the photoconductive coating, care must be taken not to greatly decrease the surface charge density, or else one will fail to obtain a toner image with sufficient contrast and density by the subsequent development operation. When the pre-toner application is carried out by liquid development, the electrical resistance of the dispersion liquid should be high enough and the liquid should have a poor affinity for the resinous binder or resinous ingredient in the photoconductive coating.

Preferable solvents may be isoparaffinic, which have poor dissolving power for many resinous materials, but more active solvents such as straight chain hydrocarbons, alicyclic hydrocarbons, decalin, or tetralin, and further, aromatic hydrocarbons, or mixtures of aliphatic and aromatic hydrocarbons may also be used if the resinous ingredient is cured.

In the case where dry development such as magnetic brush or cascade development is employed whereby the carrier particles contact with the surface of the photoconductive coating, the carrier particles should not be electrically conductive.

On the other hand, powder cloud development involves no problem of charge leakage due to pre-toner application.

Carrier particles for brush or cascade development which are suitable for use in the application of pre-toner include glass beads, natural silica sands, diamond beads, ferromagnetic iron powder, and ferromagnetic alloy powder, all of which are surface coated with insulating resinous materials.

When the present method is applied to multi-colored image formation by over-printing technique wherein charging, image exposure, and development are carried out more than twice on a single photoconductive material, pre-toner should perform similarly as the underlying photoconductive coating.

A suitable example of a pre-toner is dye-sensitized zinc oxide encapsulated by insulating resin. Generally speaking, an electrically insulating pre-toner is desirable for the following reason: The charge on the pretoner will be neutralized after deposition, draining to the ground through the insulating coating, in which case colored toner will again deposit on the pre-toner in the subsequent development and no improvement of image quality results. Fortunately, however, this is not so serious a problem since the pin-hole areas where electrical breakdown has occurred become insulating again immediately after charging.

Regarding the application of pretoner and toner, one may employ dry and/or liquid methods.

Claims

What is claimed is:

1. A reversal development process which comprises; after uniformly charging a photoconductive insulating coating with a charge of one polarity, applying a finely divided, photoconductive, pre-toner having the same color as that of said photoconductive insulating coating and a charge of the same polarity as that of said uniform charge whereby the pre-toner deposits at the charge-deficient areas in the coating, then subjecting the thus treated coating to image exposure to form an electrostatic latent image; and finally developing said latent image by applying a finely divided toner differently colored from said pre-toner, said toner having a charge of the same polarity as that of said latent image.

2. A reversal development process as in claim 1 where said photoconductive pre-toner has a white or off-white appearance.