Liquid Developers for Electrophotography and Developing Method Using the Same

This invention relates to a liquid developer for electrophotography and a developing method using the same. More particularly, it is concerned with a liquid developer for electrophotography which is employed for the development step in a process for preparing a lithographic printing plate according to an electrophotographic method (hereinafter referred to as "liquid developer") and also with a process for developing an original printing plate by the use of the said liquid developer.

Hitherto, there has been employed as a lithographic printing plate the printing plate having coated a photosensitive resin thereon, but this printing plate has exhibited an xtremely lower sensitivity, as compared with silver salt-containing photosensitive materials or electrophotographic materials. Therefore, where a printing plate containing a photosensitive resin is to be exposed, there is usually effected exposure of the said printing plate having contacted therewith a silver salt type photosensitive material prepared according to photomechanical process. Recently, there has been developed a process method wherein a printing plate is prepared through direct copying of an original copy, without employing the photomechanical process step for preparing the so-called silver salt-containing lith film, namely, the silver halide photosensitive material capable of being developed with a lith developer, for the purposes of labor-saving and resources-saving.

More specifically, there is well-known a process for making a printing plate electrophotographically wherein a printing plate is prepared by direct copying of an original copy according to an electrophotographic method. For example, Japanese Patent Publications No. 17162/1962, No. 6961/1963, No. 12703/1964 and so on disclose a method for preparing a printing plate wherein a photoconductive layer is provided over a roughened aluminum plate, a toner image is formed by charging, exposing and developing according to an electrophotographic method and then the photoconductive layer in a non-image portion (namely, the portion having no toner which adheres thereto) is dissolved out with an alkali solution. Moreover, Japanese Provisional Patent Publication No. 144203/1979 discloses a method for preparing a lithographic printing plate wherein a toner image is formed on a zinc oxide photosensitive material according to an electrophotographic method, transferred onto a transfer printing plate composed of an aluminum plate having an epoxy resin layer thereon and, after heat fixing, the epoxy resin layer in a non-image portion is dissolved out with a solvent.

In these prior art techniques in which a dry developer has been used to obtain a toner image according to the electrophotographic method, there is a disadvantage in that an image resolving power is poor. Generally, where a toner image is to be formed according to an electrophotographic method, a wet development has the characteristics of a far more excellent resolving power and a superior image-reproducibility as compared with a dry development. However, any useful wet developer has not been yet proposed for the electrophotographic method involving the step wherein a toner image is formed according to the electrophotographic method and a non-image portion is dissolved away (hereinafter referred to as "etching"). Namely, in preparing a printing plate by forming a toner image through a wet development with a wet developer followed by etching, there is seen such a defect that an imaged portion (the portion having a toner which adheres thereto) also tends to be readily dissolved out and hence the image portion shows a poor etching resistance.

An object of this invention is to provide a liquid developer for producing a printing plate with a high resolving power according to an electrophotographic method and a method for development using the said liquid developer.

A further object of this invention is to provide a liquid developer having an excellent etching resistance in a process for making a printing plate electrophotographically including the etching step and a developing method using the said liquid developer.

A still further object of this invention is to provide a liquid developer having a high stability when dispersed and a method for development with the said liquid developer.

These and other objects and advantages of this invention will be apparent to those skilled in the art from the detailed description as stated hereinbelow.

According to one aspect of this invention, there is provided a new type of a liquid developer which comprises (a) a coloring agent, (b) a charge control agent and (c) one or more of a polyethylene, a polypropylene, an ethylene copolymer and a propylene copolymer in an electro-insulating liquid.

According to another aspect of this invention, there is also provided another type of a liquid developer which comprises the above-defined three components (a), (b) and (c) and further (d) a hydrocarbon resin that is soluble in a hydrocarbon solvent and contains 1,3-pentadiene in an electro-insulating liquid.

According to other aspect of this invention, there is provided a method for preparing a printing plate wherein an original printing plate having a photoconductive layer over the support or substrate thereof is charged to form an electric latent image thereon according to an electrophotographic method, the image is exposed and then developed with a liquid developer to form a toner image and, after fixing, the photoconductive layer in a non-image portion is dissolved out to prepare a printing plate, characterized in that either of the liquid developers of the present invention as defined above is used as the liquid developer for electrophotography in the above development step.

A liquid developer for electrophotography may generally comprise a coloring agent, a coating agent, a charge control agent and a liquid carrier composed of an electro-insulating liquid. As the coating agent which may be employed therein, there may be used, for example, a variety of polymeric compounds such as alkyd resins, acrylic resins, styrene group resins, vinyl chloride resins, polyamide resins, rubbery resins, cyclized rubbers, rosin derivatives and the like. However, where a developer having incorporated therein such resin is applied to an electrophotographic process comprising an etching step, there is seen a defect of a poor etching resistance. In Japanese Patent Publications No. 33777/1975, No. 29653/1976, No. 47338/1976, No. 3582/1977 and Japanese Provional Patent Publication No. 4,9445/1973, there is proposed a liquid developer containing a polyethylene, an ethylene-vinyl acetate copolymer, a polypropylene and the like. However, these prior techniques have been contemplated merely to improve the transferability when a toner image is transferred onto a transfer paper, in a conventional copying.

As explained above, the liquid developer of this invention comprises (a) a coloring agent, (b) a charge control agent and (c) one or more of a polyethylene, a polypropylene, an ethylene copolymer and a propylene copolymer in an electro-insulating liquid. As another embodiment of the present invention, the liquid developer comprises the above-defined three components (a), (b) and (c) and, additionally, (d) a hydrocarbon resin soluble in a hydrocarbon solvent and containing 1,3-pentadiene in an electro-insulating liquid.

The term "ethylene copolymer" as used herein is meant to indicate any of those copolymers of ethylene with one or more other monomers copolymerizable with ethylene, while the term "propylene copolymer" means to indicate any of those copolymers of propylene with one or more other monomers copolymerizable with propylene. As the monomer copolymerizable with ethylene or propylene, there may be mentioned, for example, methyl acrylate, ethyl acrylate, ethyl methacrylate, vinyl acetate, vinyl chloride, tetrafluoroethylene and the like.

A total amount of one or more of polyethylene, polypropylene, ethylene copolymer and propylene copolymer to be added is usually 0.01 to 5 parts by weight, preferably 0.1 to 2 parts by weight, upon 100 parts by weight of the electro-insulating liquid.

Use of the resin can provided a printing plate having an excellent resolving power and an improved etching resistance in an electrophotographic process including an etching step.

As the charge control agent which may be employed in the present invention, there may be mentioned, for example, lecitin, a metal salt of naphthenic acid, an alkylbenzenesulfonate, a dialkylnaphthalenesulfonate, a mono- or dialkylsulfosuccinate, a dialkylphosphate, linseed oil, soybean oil, an alkyd resin and the like. An amount of the agent to the added may be dependent upon the type of the said agent and hence is not always determined clearly. However, it is preferable to select the amount suitably so that electric resistance of the liquid developer may be, for example, 10.sup.9 .OMEGA.cm or higher.

As the coloring agent which may be employed in this invention, there may be employed any pigments or dyes without limitation: For instance, carbon black, copper phthalocianine, phthalocianine green, Malachite Green, Rhodamine, Victoria Blue, Oil Black and so on. An amount of the coloring agent to be added is not critical, but it is usually 0.01 to 1.00 part by weight, preferably 0.01 to 0.1 part by weight, upon 100 parts by weight of the electro-insulating liquid.

As the electro-insulating liquid which may be employed in this invention, one can use any of the electro-insulating liquids (such as aliphatic hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbons) having an electric resistance of not less than 10.sup.9 .OMEGA.cm: For instance, n-pentane, n-hexane, chlorinated paraffin, isoparaffin, kerosine and the like.

In addition, the liquid developer of the present invention may further include other natural or synthetic polymeric compounds, if required.

As the 1,3-pentadiene-containing hydrocarbon resin which is to be employed in another embodiment of the liquid developer of the present invention, there may be mentioned, for example, those resins as described in Japanese Patent Publication No. 29951/1975, the disclosure of which is incorporated herein as reference. Illustrative and commercially available examples of such resin may include, for example, Quintone C-100, C-200, D-200, U-200 (they are manufactured by Nippon Zeon Co., Ltd., Japan) and the like. An amount of the resin to be added is usually 0.01 to 5 parts by weight, preferably 0.1 to 2 parts by weight, upon 100 parts by weight of the electro-insulating liquid. It is to be noted that addition of such a resin can unexpectedly improve dispersibility of toner grains and stability of the dispersion containing toner grains remarkably.

The liquid developer of the present invention may be prepared, for instance, according to one preferable embodiment wherein the above-mentioned components (a), (b) and (c) are added to the electro-insulating liquid with or without the above additional component (d) and the resulting mixture is homogeneously dispersed by means of any conventional techniques such as a ball mill, a sand mill, a colloid mill, a supersonic dispersion method and the like.

The original printing plate to which this process is to be applied has a photoconductive layer on a support and the said photoconductive layer contains an photoconductor. As the photoconductor, there may be employed one or more of any inorganic photoconductors, organic photoconductors and organic photoconductive pigments. The photoconductor can be dissolved or suspended in a solution containing a natural or synthetic polymeric substance and the resultant solution or suspension applied onto an conductive support in a known manner per se to prepare an original printing plate. The original printing plate thus prepared is charged and exposed according to a conventional electrophotographic process, developed with the liquid developer of the present invention, fixed and then the non-image portion or the portion having no toner adhered thereon is subjected to etching with a solvent to prepare a printing plate. As the light source for exposure in the above-mentioned process stage, there may be applied xenon lamp, halogen lamp, fluorescent lamp, tungsten lamp, laser ray, e.g., semi-conductor laser, Ar ion laser, He-Ne laser and the like. As the solvent for etching, there may be optionally selected any of those solvents, depending upon the sort of the photoconductor contained in a photoconductive layer or the binder resin.

Where the liquid developer of the present invention is applied, an imaged portion or the portion having a toner which adheres thereto exhibits an extremely high resistance to the etching solvent during the etching step without any imaged portion dissolved out. Further, the resulting printing plate has a high resolving power and an excellent image reproducibility.

This invention will be more fully illustrated by way of the following Examples, but these Examples are not construed to be limiting the scope of this invention. All parts are by weight unless otherwise indicated.

A composition having the above formulation was dispersed and homogenized at ordinary temperature by means of a supersonic dispersion mixer. The resulting photosensitive liquid was coated onto a roughened aluminum plate with a film thickness of 5.mu. to prepare a printing original plate.

On the other hand, the above mixture was dispersed in a porcelain ball mill for 24 hours and one part of the so obtained dispersion was added to 10 parts of Isopar G while stirring to produce a liquid developer.

Then, the printing original plate thus prepared was charged with negative by means of a corona discharger, a reflection positive was used as an original copy and fluorescent lamp (20 W.times.10) as a light source and then image exposure was made through lens system to for an electrostatic latent image, which was developed with the above-mentioned liquid developer. After fixing with heating, the plate was dipped into an alkali aqueous solution consisting of sodium silicate and sodium hydroxide to accomplish etching of the photoconductive layer in a non-imaged portion, namely, the portion having no toner adhered, thereby providing a printing plate. This printing plate has an excellent resolving power as demonstrated in the following Table 1.

Following the same procedures as in Example 1 except that the above mixture was used, there was prepared a liquid developer.

Then, the same procedure as in Example 1 was repeated except that the same original printing plate as in Example 1 was charged with positive by means of a corona discharger to form a printing plate. This plate has an excellent resolving power as demonstrated in the following Table 1.

The above mixture was dispersed by means of a porcelain ball mill for 24 hours and one part of the dispersion was added with stirring to 10 parts of Isopar G containing 0.01% by weight of iron naphthenate to form a liquid developer.

Then, the same original printing plate as in Example 1 was charged with positive by means of a corona discharger, macro-photography to 16-fold area ratio was effected using a micro-film as an original copy and a commercially available photographic enlarger and the plate was developed with the aforesaid liquid developer. Subsequently, the same treatment as in Example 1 gave a printing plate having an excellent resolving power as demonstrated in the following Table 1.

The above composition was coated onto a roughened aluminum plate with a film thickness of 8.mu. to form an original printing plate.

On the other hand, the above mixture was dispersed by means of a porcelain ball mill for 24 hours and one part of the dispersion was added with stirring to 15 parts of Isopar G to prepare a liquid developer.

Then, the same treatment as in Example 2 was effected, except that the above-mentioned original printing plate was used, to give a printing plate having an excellent resolving power as demonstrated in the following Table 1.

In order to show effect of this invention more definitely, the results from the above examples are summarized in the following Table 1, with those from Comparative Examples.

The above-mentioned Comparative Examples are illustrated below.

Following the same procedure as in Example 1 except that a polystyrene type toner powder was employed, there was produced a printing plate.

Following the same procedure as in Example 1 except that the above-defined mixture was employed, there was prepared a liquid developer. Then, the same treatment as in Example 2 was effected except that the developer obtained as above was used.

Following the same procedure as in Example 3 except that the above-defined mixture was employed, there was prepared a liquid developer. Then, the same treatment as in Example 3 was effected.

Folloring the same procedure as in Example 4 except that the above-defined mixture was used, there was prepared a liquid developer. Then, the same treatment as in Example 4 was effected.

By using the same procedure and composition as in Example 1, there was prepared an original printing plate.

The above mixture was treated in the same manner as in Example 1 to prepare a liquid developer.

The printing plate was treated with the developer in the same manner as in Example 1 to form a printing plate having an excellent resolving power as demonstrated in the following Table 2.

The same procedure as in Example 1 except that the above mixture was used were repeated to prepare a liquid developer.

Then, the same procedure as in Example 1 was repeated to give a printing plate having an excellent resolving power as demonstrated in the following Table 2.

Following the same procedure as in Example 3 except that the above mixture was used, there was prepared a printing plate having an excellent resolving power as demonstrated in the following Table 2.

The original printing plate was prepared according to the same procedure as in Example 4.

The liquid developer was prepared from the above mixture according to the same procedure as in Example 4.

By using the original plate and developer in the same manner as in Example 4, there was prepared a printing plate having an excellent resolving power as demonstrated in the following Table 2.

In order to show effect of this invention more fully, the results from the above Examples are summarized in the following Table 2 with those from Comparative Examples.

The above-mentioned Comparative Examples are illustrated below.

Following the same procedure as in Example 5 except that a polystyrene type powdery toner was employed, there was prepared a printing plate.

Following the same procedure as in Example 5 except that the above mixture was employed, there was prepared a liquid developer and subsequently processing was effected in the same manner as Example 6.

Following the same procedure as in Example 7 except that the above mixture was employed, there was prepared a liquid developer and then processing was effected.

Following the same procedure as in Example 8 except that the above mixture was employed, there was prepared a liquid developer and then processing was effected.

Following the same procedure as in Example 2 except that the high molecular compounds shown below were used instead of polyethylene, there were prepared printing plates.

The results are summarized in Table 3 together with those of the Comparative Examples mentioned hereinafter.

Following the same procedure as in Example 1 except that the high molecular compounds shown below were used instead of a copolymer of ethylene and ethyl acrylate (ethylene:ethyl acrylate=72:28), there were prepared printing plates.

The results are summarized in Table 3.

Following the same procedure as in Example 2 except that the high molecular compounds shown below were used instead of polyethylene, there were prepared printing plates.

The results are summarized in Table 3.