GB2160670A - Lithographic printing plate - Google Patents

Abstract

A lithographic printing plate which is prepared by charging an electrophotographic plate (3) comprising a photoconductive layer (2) on a conductive substrate (1), the photoconductive layer being prepared by dispersing a mixture of a phthalocyanine pigment, zinc oxide and zinc sulfide in a binder resin, exposing the electrophotographic plate (3) to radiation (4) to form an electrostatic latent image, developing the exposed electrophotographic plate using an ink receptive toner, and fixing the developed plate. <IMAGE>

Description

SPECIFICATION Lithographic printing plate This invention relates to a lithographic printing plate, as well as to a method for preparing a lithographic printing plate and an electrophotographic plate for use in the method.

With increased rationalization of office work, the use of word processors has recently become widespread. In this connection, a conventional typewriter types characters onto paper one by one, whereas a word processor stores input characters in the memory thereof as electrical signals and reads out the electrical signals to display the same in a cathode-ray tube as the characters. In order to prepare a printing plate on the basis of the electrical signals which have been stored in the memory of a word processor, a paper hard copy may be prepared using ink jet printing, thermal printing, dot-matrix printing, electrophotographic printing or the like. A zinc oxide coated master recording paper is then charged, exposed and fixed, using the hard copy as the original, whereby a printing plate is obtained.However, this process is laborious and timeconsuming, because two steps are required for preparing the hard copy and for plate making.

Also, the quality of the image on the printing plate is poor because the image must be copied onto the printing plate from the hard copy. A process has been proposed in which a photoconductive plate is exposed using laser light to form a latent image and then the latent image is developed and fixed, whereby a printing plate is obtained. In this process, an argon, helium-neon, helium-cadmium YAG, carbonic acid gas or the like laser is utilized. However, these lasers must be large in order to obtain the required output, and the power consumption thereof is too great for the amount of light output.Semiconductor lasers compare favourably with those described above in terms of microminiaturization, high efficiency, low voltage, low power consumption, being capable of effecting high speed modulation over 1 GHz in response to input electrical signals, the high reliability peculiar to semiconductors and so on. However, semiconductor lasers such as a known Al GaAs laser of wavelength 760-800 nm or 820-880 nm, have a low light output and the wavelength output is generally long in comparison with gas laser light. This is a serious disadvantage in that zinc oxide which has previously been used in this technical field is insensitive to long wavelength laser light.Photoconductive materials which are sensitive to such long wavelengths include CdS-Cu, Se-Te/Se, amorphous silicon, phthalocyanine pigments and so on, but of these only the phthalocyanine pigments can be put to practical use in view of toxicity and cost. However, phthalocyanine pigments have a small average particle diameter of 0.05y and are of low electrical resistance. In this connection, when preparing an electrophotographic plate from only a phthalocyanine pigment and a binder resin, the surface of the resulting electrophotographic plate is found to be very smooth so that fixing of toner is very difficult. For this reason, even if a lithographic printing plate is prepared from the electrophotographic plate, the toner separates from the electrophotographic plate during the printing operation, so that the electrophotographic plate is not usable.

The present invention seeks to overcome or at least mitigate the above disadvantages, for which purpose tests were conducted by mixing fillers with a binder resin and a phthalocyanine pigment to improve fixing of toner and the hydrophilic properties of the electrophotographic plate when used to prepare a lithographic printing plate. As a result of the tests using titanium oxide, calcium carbonate, cadmium carbonate, magnesium oxide, zinc oxide and zinc sulfide as the fillers, it was found that, while the surface of the resulting electrophotographic plate was rendered matt, the photosensitivity was less than, dark decay rates greater than, and charge retentivity less than, those of an electrophotographic plate comprising only phthalocyanine pigment and resin.However, it was also found that an electrophotographic plate prepared by dispersing phthalocyanine pigment, zinc oxide and zinc sulfide in a binder resin exhibited minute irregularities on the surface of the resulting photoconductive layer whilst still having an acceptable photosensitivity. It is though that this is due at least partly to the larger average particle diameter of 0.3ju, of zinc oxide and zinc sulfide when compared with phthalocyanine pigment and to the high value of electrical resistance of the electrophotographic plate. Thus zinc oxide and zinc sulfide are desirable as fillers for the electrophotographic plate of the present invention.

In phthaolocyanine pigment-resin electrophotographic plates, since the phthalocyanine pigment has been selected for its photoconductive properties, good positive charge retention is desirable. However, the present inventors have discovered that an electrophotographic plate to the photoconductive layer of which has been added only zinc oxide as the filler has poor charge retentivity and poor dark decay properties with respect to positive charge, whilst an electrophotographic plate comprising only zinc sulfide as the filler has the disadvantage that, although there is good charge retentivity and good dark decay properties with respect to positive charge, residual potential becomes a problem.

The present invention enables the provision of a lithographic printing plate which does not have the above-mentioned disadvantages of conventional zinc oxide paper.

In a first aspect, the invention provides an electrophotographic plate comprising a photoconductive layer on a conductive substrate, the photo-conductive layer being preparable by dispersing a mixture of a phthalocyanine pigment, zinc oxide and zinc sulfide in a binder resin.

In a second aspect, the invention provides a method of preparing a lithographic printing plate, which method comprises charging an electrophotographic plate in accordance with the first aspect, exposing the charged electrophotographic plate to form an electrostatic latent image, developing the exposed electrophotographic plate using an ink receptive toner, and fixing the developed electrophotographic plate to obtain a lithographic printing plate.

In a third aspect, the invention provides a lithographic printing plate whenever prepared using a method in accordance with the second aspect.

For a better understanding of the present invention, and to show how the same may be put into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a diagrammatic side view of an electrophotographic plate in accordance with the invention being charged with a corona discharge; Figure 2 is a diagrammatic side view of the electrophotographic plate of Fig. 1 being exposed to form an electrostatic latent image; Figure 3 is a diagrammatic side view of the electrophotographic plate of Fig. 1 being developed using a positively charged ink receptive toner; and Figure 4 is a diagrammatic side view of the lithographic printing plate obtained after fixing the toner.

An electrophotographic plate 3 is formed by applying a photoconductive layer 2 prepared by dispersing a mixture of a phthalocyanine pigment and a higher weight ratio than that of said pigment of zinc oxide and zinc sulfide in a binder resin on a conductive substrate 1 as shown in Fig. 1. The surface of the electrophotographic plate 3 is then charged with a corona discharge or the like using charging means. Thereafter the charged electrophotographic plate is exposed to an image using semiconductor laser light 4 modulated by an electrical signal (such as the electrical signal read out from the memory of a word processor) to form an electrostatic latent image as shown in Fig. 2.The exposed electrophotographic plate is developed by means of an ink receptive toner 5 which has been charged positively as shown in Fig. 3, and the material thus developed is then fixed as shown in Fig. 4, whereby a lithographic printing plate 6 is fabricated.

Examples of the conductive substrate used in the present invention include; materials prepared by subjecting a metallic plate or sheet made of metal such as aluminum, brass, copper or stainless steel, or a plastic sheet to vacuum deposition by the use of aluminum, chromium, palladium, metallic oxides or the like; materials prepared by giving conductivity to a plastic plate, paper or the like; plates or sheets etc. made of metallic oxides.

Phthalocyanines used in the present invention may be classified into metal-free phthalocyanines and metal-phthalocynines, and metal-free phthalocyanines are known to have various crystal forms such as a, ss, X and the like, whereas metal-phthalocyanines include copper, aluminum, vanadium and the like phthalocynines. Particularly, copper phthalocyanine is known to have also various crystal forms such as a, ss, e and the like. In case of conducting image exposure with laser light being an embodiment of the present invention, it is required to select a phthalocyanine having sensitivity with respect to light of a long wavelength (790-800 nm).For this purpose, it is preferable to use metal-free phthalocyanines, copper phthalocyanine of form, or phthalocyanine derivatives wherein the benzene nucleus of the phthaolcyanine molecule is substituted with nitro or cyano groups as a part of the components of a photoconductive material (refer to USP 4,443,528).

As zinc oxide utilized for a filler in the present invention, any of which is powdery and is commonly employed for electrophotography may be used, whilst ordinary reagent grade of zinc sulfide may be used for the present invention and which is inexpensive and non-toxic.

In the present invention, zinc oxide/(a phthalocyanine pigment + zinc oxide + zinc sulfide + a binder resin) are within a range of 20-60 wt%, and preferably 35-50 wt%. When zinc oxide is excessive, electrophotographic properties deteriorate, whilst hydrophilic nature decreases in case of too deficient zinc oxide. Furthermore favorable results were obtained in the case where a ratio of blending phthalocyanine pigment with (zinc oxide + zinc sulfide) is 20-45 wt% and zinc sulfide/zinc oxide are in a ratio of 10-50 wt%. This is because photosensitivity with respect to semiconductor laser light is deficient in the case where phthalocyanine pigment is less than 20 wt%, whereas when exceeded 45 wt%, fixation of an ink receptive toner as well as wetting thereof with respect to water become unfavourable. Of course, the above described fact is significantly influenced by loadings of a binder resin. As to a binder ratio, favourable results were obtained in the case where binder resin/phthalocyanine pigment = 1.5-4 (weight ratio) in view of film performance, charge retentivity, and wetting to water.

Synthetic resins used for binder resin include known thermoplastic resins such as polyester, polystyrene, polyvinyl chloride, polyvinyl acetate, vinyl chloridevinyl acetate copolymer, polyvi nyl acetal, polyvinyl butyral, alkyd resin, acrylic resin, polyacrylonitrile, polycarbonate, polyketone and the like as well as known thermosetting resins such as polyurethane and epoxy resin.

In this respect, particularly favourable resins involving hydrophilic groups include polyvinyl alcohol, polyvinyl pyrrolidone, polyamide, polyacrylamide, polyvinyl acetate, polyvinyl formal, polyvinyl butyral, phenol resin and the like.

Solvents used in case of preparing the electrophotographic plate in the present invention include aromatic hydrocarbons such as benzene, toluene, xylene, chlorobenzene and the like, ketones such as acetone, methyl ethyl ketone, cyclohexanone and the like, alcohols such as methanol, ethanol, isopropanol and the like, esters such as ethyl acetate, Methyl Cellosolve and the like, halogenated hydrocarbons such as carbon tetrachloride, chloroform, dichloromethane and the like, ethers such as tetrahydrofuran, dioxane and the like as well as dimethylformamide, dimethyl sulfoxide and the like.

There are various manufacturing processes of the electrophotographic plate utilized in the present invention, one example of which is conducted in such a way that a phthalocyanine pigment, a binder resin and a solvent are dispersed by means of a ball mill, to the mixture is then added zinc oxide and zinc sulfide powder, they are further dispersed, the resulting photoconductive composition is applied on a conductive substrate, and the substrate thus applied is dried. As the method for dispersion, any known method may be practiced, for example, paint conditioner, ultrasonic dispersing and the like methods are applicable other than the above-mentioned ball mill method.Further as the coating method, a method wherein applicator, spray coater, bar coater, dip coater, doctor blade coater or the like is utilized is applicable, and these methods are properly used dependent upon viscosity, solvent, and amount of coating.

The above described electrophotographic plate can be exposed with not only visible light of 400-700 nm, but also infrared light of over 700 nm so that a conventional ordinary exposing method and light source may be used in the present invention. The most suitable exposing method is one wherein the electrophotographic plate is directly exposed with semiconductor laser light modulated by electrical signal, if such facts that photosensitivity of the present electrophotographic plate is extremely high as compared with that of conventional plate, that a sensitive wave range is 700 nm or more, and that electrical signal can directly be taken out from word processor are taken into consideration.

An ink receptive toner is selectively formed on an electrostatic latent image formed by means of image exposure upon an electrophotographic plate to form the toner image, and the resulting toner image is then heated to effect fixation. In the lithographic printing plates thus obtained, one wherein dampening water is utilized involves unevenness on the surface of the electrophotographic plate, so that fixation can be easily carried out by heating.

Ink receptive toner includes dry toner and wet toner, and either toner may be employed.

An ink receptive toner for dry developing is composed of a charge controller such as an electron receptive organic complex, a colorant such as carbon black, and a suitable resin. In two-component system, such ink receptive toner is utilized by mixing with a carrier such as iron powder, whereas no carrier is required in a single-component system so that a toner composed of magnetite, a charge controller, a colorant such as carbon black, and a suitable resin is usually employed.

As an ink receptive toner for wet developing, one prepared by dispersing coabon black, a resin, a charge controller and the like into any of isoparaffin petroleum solvents is desirably used.

As a resin for ink receptive toners, it is required that such resin has high insulating properties and a comparatively low mocleular weight, and that either such resin combines chemically with a photoconductive layer at the time of thermal fixation, or such resin is compatible with a dispersing resin in the photoconductive layer. After fixation, since a printing ink adheres to the ink receptive toner, a resin accepted to the ink composition is particularly preferable, and resins suitable for this purpose include phenol resin, polyester resin, copolymer of styrene and maleic anhydride, polyamide resin, drying oil alkyd and the like.

The present invention will be more fully described by way of the following exmaples.

Example 1 A semiconductor laser beam printer LBP-10 manufactured by Canon Co., Ltd. was reorganized as follows. First, a floppy disk reader and an interface circuit were attached to the input side so that it made possible that electrical signal of characters or the like which had been inputted through a keyboard and stored in the floppy disk is read out and put into the LBP-10.

Furthermore a photosensitive drum was replaced by an aluminum drum, at the same time, grooves were defined on the aluminum drum, and two hooks were mounted on the grooves. An electrophotographic plate was wound around the drum in such that before and behind the plate could be secured by means of these hooks. Further, in the printer, a positively charging means and a liquid developing means were adjusted to be only functional.

Thereafter 80 g of copper phthalocyanine and 1.0 g of tetranitro copper phthalocyanine were dissolved in 1,000 g of 98% sulfuric acid while sufficiently agitating them. The dissolved liquid was admixed with 101 of water to separate out a composition of copper phthalocyanine and tetranitro copper phthalocyanine, then the composition was filtered, washed with water, and dried at 120 under reduced pressure. 50 g of the composition (A) thus obtained was blended with 50 g of e from copper phthalocyanine (Lionol Blue ER manufactured by Toyo Ink Mfg. Co., Ltd.), and the mixture was dispersed into 5 kg of methanol to obtain a holmogeneous mixed dispersion. Thereafter the dispersion was filtered, and dried at 1 20 C under reduced pressure to prepare a mixed composition (B).

In accordance with the following formulation, a photoconductive composition was prepared.

Mixed Composition (B) 10 g Silicone Resin (KR-211, trade name, manufactured by Shinetsu Chemical Industry Co., Ltd., solid content: 70 wt%) 33.4g Acrylic Resin (Aron S 1001, trade name, manufactured by Toa Gosei Chemical Industry Co., Ltd., solid content: 50 wt%) 5.2 g Toluene 101.4 g The above compositions were dispersed by means of a pocelain ball mill 48 hours, then, to the dispersion obtained were added 30g of photoconductive zinc oxide (SAZEX 2000, trade name, manufactured by Sakai Chemical Industry Co., Ltd.) and 3.3 g of reagent since sulfide (manufactured by Kanto Chemical Industry Co., Ltd.), and the mixture was further dispersed in the ball mill for 6 hours.Then, the resulting mixed liquor was again diluted with toluene to reduce the viscosity thereof, the diluted liquor was roll coated on a laminated aluminum foil composed of an aluminum foil having a thickness of 1 ii and a polyester film of 75joe so as to have a dry film thickness of 8 L, and the coated foil was dired at 50"C for 8 hours thereby obtaining an electrophotographic plate for electrophotography.The electrophotographic plate sample thus obtained was subjected to corona discharge in + 5.7 KV, with a corona gap of 10 mm and at a charging speed of 10 m/min., and in this case, the potential at 1.0 second after ceasing the discharge was defined as "initial surface potential'' and ''photosensitivity' ' was similarly defined in such that the sample was exposed with monochromatic light of 800 nm in an intensity of light of 2yW/cm2 at 1.0 second after the discharge, and an amount (J/cm2) of light energy required for reducing the initial surface potential by 50% was considered to be the sensitivity. Further a half-life period of initial surface potential in dark room was defined as "dark decay time" (sec.).

The characteristic properties of samples thus measured were as indicated in the column of Example 1 of the following Table 1, and the sensitivity thereof exhibited a satisfactory value.

The resulting electrophotographic plate was mounted on the hooks in the laser printer and at the same time, said aluminum drum was grounded. Then, electrical signal was put into the laser printer from the floppy disk reader to operate the printer, the electrophotographic plate was exposed to an image by means of a semiconductor laser after charging the material, and then the exposed plate was reversedly developed in accordance with liquid developing method by the use of a liquid developer IP-2 Set (positive type ink receptive toner) manufactured by Iwasaki Tsushinki Co., Ltd. Thereafter, the resulting electrophotographic plate was removed from the aluminum drum, whereby a lithographic printing plate was fabricated.

Then, the surface of the printing plate was damped with a dampeing aqueous solution containing gum arabic and phosphoric acid, the damped printing plate was dired, then the printing plate was applied to the printing drum of an offset printing machine 2800 CD manufactured by Ryobi Co. by means of a doublecoated adhesive type. In this offset printing machine, printing was conducted on wood free papers under a printing pressure of 0.30 mm by employing offset process ink G set (jetblack) manufactured by Toyo Ink Mfg. Co., Ltd. and dampening water (4000 revolutions/hour). In starting the printing, printed matters were lighter in color, but the density in its solid portion was also improved to 1.4 or more, when exceeded 30 pieces of printed matters so that favourable printed matters were obtained (a total of 10,000 papers was printed).

Furthermore background exposure was effected by reversing ON-OFF relationship of electrical signal in the laser printer, the laser printer was operated by means of the floppy disk reader, the resulting electrophotographic plate was (positively) charged, the charged plate was then subjected to non-image portion exposure by means of a semiconductor laser, and thereafter the electrophotographic plate thus exposed was normally developed in accordance with liquid developing method by using a liquid developer Lithoset SN 7037A (negative type ink receptive toner) manufactured by Hunt Co. Then, the electrophotographic plate was removed from the aluminum drum, whereby a lithographic printing plate was obtained.

The resulting printing plate was processed by the same manner as that mentioned above, and the processed printing plate was attached to the offset printing machine. In printing, the same favourable printed matters as those of the above case were obtained.

Table 1

Example No. 1 2 3 4 Initial Surface 550 410 410 520 Potential (V) Dark Decay 14.8 14.4 10.2 20.4 Time (sex.) Photosensitivity 2.2 3.6 2.7 3.4 ( J/cm2) Examples 2-4 The electrophotographic plate prepared by the same manner as that of Example 1 except that the pigments enumerated in Table 2 were utilized as a phthalocyanine pigment and 20 g of polyvinyl butyral (S-lec-BLS, trade name, manufactured by Sekisui Chemical Industry Co., Ltd.) was used as the resin had characteristic properties as indicated in Table 1.Any of lithographic printing plates fabricated from the electrophotographic plates as described above was excellent as in the case of Example 1.

Table 2

Example Phthalocyanine Pigment 2 E form copper phthalocyanine (Heliogen Blue L-6700F, trade name, manufactured by BASF A.G.) a a form metal-free phthalocyanine (Heliogen Blue L-7560, trade name, manufactured by BASF A.G.) 4 Phthalocyanine composition (A) in 4 Example 1 Example 5 By exmploying the same phthalocyanine composition (A) as that of Example 1, a photoconductive composition was prepared in accordance with the following formulation:: Composition (A) 159 Silicone Resin (KR-211 manufactured by Shinetsu Chemical Industry Co., Ltd., solid content: 70 wt%) 7.5 9 (Solid: 5.25 g) Butyral Resin (S-lec-BLS manufactured by Sekisui Chemical Co., Ltd.). 21 g Methyl Alcohol 1169 Methyl Ethyl Ketone 54 9 Cellosolve Acetate 65 9 The above compositions were dispersed in a porcelain ball mill for 48 hours, to the dispersion obtained were then added 45 g of photoconductive zinc oxide (SAZEX 2000 manufactured by Saki Chemical Industry Co., Ltd.) and 5 g of reagent zinc sulfide (manufactured by Kanto Chemical Industry Co., Ltd.), and the mixture was further dispersed in the ball mill for 6 hours.

Then, the resulting mixed liquor was roll coated on a laminated aluminum foil composed of an aluminum foil having a thickness of 15ym and a polyester film of 75m so as to have a dry film thickness of 12ELm, and the coated foil was dried at 100"C for 5 minutes thereby to obtain an electrophotographic plate for electrophotography.

With respect to the sample thus obtained, electrophotographic properties were measured in accordance with the same manner as that of Example 1, and the results thereof are indicated in Table 3.

Table 3

0 0 0 0 IC V) m 0 Z o o a +I No. Coarative Exale No.

L1( U O O Scum surface X H ~ O W e Dark Decay 17.0 10.0 4.6 3.8 1.9 11.4 13.0 5.4 Time QD O O O. O ity (LjJ/c:n2) ~ O O X 7 o o o X um n ,1 7 ~ &commat; ~ \ w > > 1 \ H Z n \ \ J4 f E M: \ E A a > A Example 6 and Comparative Examples 1-6 The electrophotographic plates prepared by the same manner as that of Example 5 except that amounts of fillers of zinc oxide and zinc sulfide as well as type of fillers were varied in accordance with data enumerated in Table 4 has characteristic properties as indicated in Table 3. As is apparent from Table 3, any of Comparative Examples was inferior to Example of the present invention in their electrophotographic characteristics.

Table 4

Example. No. .Comparative Example No.

Filler 5 6 1 2 3 4 5 6 ZnO 45 35 - - - 50 25 15 ZnS 5 15 - - - - 25 35 TiO2 - - 50 - - - - - CdCO3 - - - 50 - - - - CaCO3 - - - - 50 - - - Since the present invention is constructed as described above, an electrophotographic plate can directly be exposed to image by means of, for example, the memory of a owrd processor to obtain printing plates without once making the hard copy on paper. Thus processing period of time becomes shortened, and exposure time is also reduced because of high sensitivity of the electrophotographic plate, besides a printing plate can be fabricated without deteriorating quality of image so that printed matters of high quality can be obtained by the present invention.

Claims (14)

1. An electrophotographic plate comprising a photoconductive layer on a conductive substrate, the photoconductive layer being preparable by dispersing a mixture of a phthalocyanine pigment, zinc oxide and zinc sulfide in a binder resin.

2. An electrophotographic plate according to Claim 1, wherein the phthalocyanine pigment comprises 50 wt% or less of the said mixture.

3. An electrophotographic plate according to Claim 2, wherein the phthalocyanine pigment comprises 20 to 45 wt% of the said mixture.

4. An electrophotographic plate according to any one of the preceding claims, wherein the zinc oxide comprises 20 to 60 wt% of the said photoconductive layer.

5. An electrophotographic plate according to Claim 4, wherein the zinc oxide comprises 35 to 50 wt% of the said photoconductive layer.

6. An electrophotographic plate according to any one of the preceding claims, wherein the ratio of zinc sulfide to zinc oxide is in the range of from 10 to 50 wt%.

7. A method of preparing a lithographic printing plate, which method comprises charging an electrophotographic plate in accordance with any one of the preceding claims, exposing the charged electrophotographic plate to form an electrostatic latent image, developing the exposed electrophotographic plate using an ink receptive toner, and fixing the developed electrophotographic plate to obtain a lithographic printing plate.

8. A method of preparing a lithographic printing plate according to Claim 7, wherein the exposure is effected using semiconductor laser light controlled by electrical signals.

9. A lithographic printing plate whenever prepared using a method in accordance with Claim 7 or 8.

10. An electrophotographic plate, substantially as herein before described with reference to, and as shown in, the accompanying drawing.

11. A method of preparing a lithographic printing plate, substantially as hereinbefore described with reference to the accompanying drawing.

12. A lithographic printing plate, substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.

13. An electrophotographic plate, substantially as hereinbefore described in any one of Examples 1 to 6.

14. A method of preparing a lithographic printing plate, substantially as hereinbefore described in any one of Examples 1 to 6.

1 5. A lithographic printing plate, substantially as hereinbefore described in any one of Examples 1 to 6.

1 6. Any novel feature or combination of features described herein.