JPH024891B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an electrophotographic lithographic printing plate, and more particularly to an improvement in a resin binder for forming a photoconductive layer of the lithographic printing plate. An electrophotographic lithographic printing plate is an electrophotographic photoreceptor in which a photoconductive layer mainly composed of photoconductive zinc oxide and a resin binder is provided on a conductive support. It can be obtained by forming a highly oily toner image, and then treating the surface with a desensitizing liquid called an etchant to selectively make non-image areas hydrophilic. The present invention provides that the above resin binder for forming a photoconductive layer is (A)

[Formula] R ₁ ; -H or -CH ₃ R ₂ ; Alkyl group having 6 to 12 carbon atoms (B)

[Formula] R ₃ ; -CH ₃ or -C ₂ H ₅ (C) Consists of a carboxylic acid vinyl monomer having a vinyl group copolymerizable with (A) and (B), the mixing ratio of which is (A): (B):(C)=30~60:40~70:0.5~5
(wt%) and the weight average molecular weight is
It is characterized by being a copolymer having a molecular weight in the range of 100,000 to 200,000, and its purpose is to provide a lithographic printing plate with little scumming and high printing durability. Although it is essential for a printing plate to have little scumming and high printing durability, conventional electrophotographic printing plates have not yet fully satisfied these conditions. For example, if the ratio of resin binder to zinc oxide particles in the photoconductive layer is reduced, the desensitization property of the surface of the photoconductive layer will be improved and scumming will be reduced, but on the other hand, the internal cohesive force of the photoconductive layer itself will be reduced. In other words, the mechanical strength is insufficient and the printing durability is reduced. On the other hand, if the proportion of the resin binder is increased, the printing durability will improve, but the scumming will increase. It goes without saying that scumming is a phenomenon related to the desensitization properties of the photoconductive layer surface, but the desensitization properties of the photoconductive layer surface depend only on the ratio of zinc oxide and resin binder in the photoconductive layer. It has been found that it is not only influenced by the amount of water, but also greatly influenced by the type of resin binder. There are resins such as silicone resins that increase the water repellency of the surface of the photoconductive layer and reduce the desensitization properties, and certain acrylic resins can improve the desensitization properties. For example, in the Special Publication Publication No. 50-31011,
18000<Mw≦ copolymerized with (meth)acrylic monomer and other comonomers in the presence of fumaric acid
100000 and a resin binder with a Tg of 10 to 80℃,
A copolymer resin binder consisting of a (meth)acrylic monomer and a comonomer other than fumaric acid is used in combination, and in JP-A-49-126406, isobutyl methacrylate constituent units are added in an amount of 10 to 80% of the total resin amount.
The resin binder containing % by weight is also
54-20735 contains xylene or xylene-isopropyl alcohol mixture in a certain ratio of five monomers: styrene, butyl (meth)acrylate, methyl (or ethyl) (meth)acrylate, hydroxyethyl methacrylate, and (meth)acrylic acid. It is stated that a resin binder copolymerized in a solvent is effective in improving the desensitization property of the photoconductive layer. However, even if the resin binder is said to be effective in improving the desensitization property described above, when strictly evaluated, it is still unsatisfactory in terms of scumming and printing durability. In particular, when the etching processor, which has become popular in recent years, is used to desensitize the surface, the uniform background stains on the entire surface are certainly reduced, but there are still small dots of background stains scattered about. It may not be noticeable on normal manuscripts, but it becomes noticeable when using manuscripts with a lot of white areas. These dot-like background stains can sometimes be mistaken for decimal points, or can clog the halftone dot image, impairing the image quality. Conventionally, as a method for relieving background stains, there is a so-called two-pass method in which a plate that has been processed once in an etching processor is processed again in an etching processor, but this two-pass method is effective for relieving dotted background stains. It is empirically known that it also works effectively,
When it is judged that dotted background smudges greatly affect the value of printed matter, the double pass method is often adopted. However, it is good if the etching processor can be operated independently and the two-pass method is possible, but if the etching processor and the plate-making machine, or the etching processor and the printing machine, or the plate-making machine and the etching processor are Automatic plate making and printing systems with an integrated printing press are becoming widespread, and in these systems, it is practically difficult to run the etching processor twice. The present invention has been made in view of these circumstances, and provides a lithographic printing plate with excellent fat-insensitivity that does not cause not only uniform background smudges on the entire surface but also dotted smudges even after one pass through an etching processor. One purpose is to provide. Furthermore, another object of the present invention is to provide a lithographic printing plate having high printing durability. The objects of the present invention were achieved by using a copolymer having the composition and molecular weight described above as a resin binder for a photoconductive layer. Monomer (A) is at least one monomer selected from alcohols having 6 to 12 carbon atoms, preferably acrylic esters or methacrylic esters of cyclohexanol, n-hexanol, z-ethylhexyl alcohol, or lauryl alcohol. However, if this monomer component is less than 30% of the copolymer weight, the desired high desensitizing property cannot be obtained, and if it exceeds 60%, it will be difficult to obtain the desired high desensitization property, although it also depends on the molecular weight, during wet development. The coating film tends to swell and soften, causing problems such as adhesion to the squeezing roll and image deterioration. The monomer (B) is methyl methacrylate or ethyl methacrylate, and if this monomer component is less than 40% of the total weight of the copolymer, the coating film will swell and become soft, resulting in adhesion to the squeezing roll or deterioration of the image. If it exceeds 70%, the desired high desensitization property cannot be obtained. Monomer (C) is a carboxylic acid having a vinyl group that can be copolymerized with monomers (A) and (B), and includes acrylic acid, methacrylic acid, itaconic acid, maleic acid,
Examples include fumaric acid and crotonic acid. If this monomer component is less than 0.5% of the total weight of the copolymer, it will have insufficient affinity for zinc oxide particles and it will be difficult to disperse the paint, and if it exceeds 5%, it will be mixed with zinc oxide particles to create a paint. When this happens, the viscosity of the paint increases significantly, and there are problems such as not only being unable to achieve stable coating, but also the sensitivity becoming slower and the desensitization property becoming worse. The weight average molecular weight is 100,000 to 200,000, and if it is less than 100,000, the monomer (A) component is 50 to 60.
%, the coating film becomes loose during wet development, moisture resistance during imaging becomes insufficient, and printing durability decreases. On the other hand, if it exceeds 200,000, the viscosity of the paint increases and stable coating becomes difficult. This is particularly noticeable in areas containing a large amount of monomer (B). One of the characteristics of the resin binder of the present invention is that the monomer
It is in (A). As is well known, acrylic acid ester monomers and methacrylic acid ester monomers are monomers obtained by the ester reaction of acrylic acid or methacrylic acid with alcohol, and among the industrially available monomers, methyl (meth)acrylate , ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, n-hexyl (meth)acrylate, cyclohexyl (meth)acrylate, z Ethylhexyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate,
There are a wide variety of them, including benzyl (meth)acrylate, z-hydroxyethyl methacrylate, hydroxypropyl methacrylate, glycidyl methacrylate, dimethylaminoethyl methacrylate, and tert-butylaminoethyl methacrylate. However, monomers (A) that can actually be used in the present invention are those having 6 to 12 carbon atoms such as cyclohexyl (meth)acrylate, n-hexyl (meth)acrylate, z-ethylhexyl (meth)acrylate, and lauryl (meth)acrylate. alkyl (meth)acrylates. Monomers other than these may have insufficient desensitization properties even within the above-mentioned content and molecular weight ranges, and even if desensitization properties are sufficient, the coating film resistance during wet development may be insufficient. There are problems such as deterioration of imaging characteristics in a hot or humid atmosphere. The monomer (A) of the present invention has a large alcohol residue compared to monomers that are widely used as structural units for resin binders for forming photoconductive layers, so the second-order transition point of the polymer is low and it can be used as a carrier for wet development. It has the property of being easily dissolved or swollen in liquids (petroleum solvents such as indecane). Due to these properties, the monomer (A) of the present invention cannot be used in conventional photoconductive layer-forming resin binders as a component of a blending resin or within a range where the secondary transition temperature does not fall below 10°C. Even if it were, it would be difficult to use it as a main constituent unit. Acrylic ester and methacrylic ester monomers, which are the main constituent units of conventional resin binders and have large alcohol residues, are at most butyl (meth)acrylate. However, the present inventors synthesized and investigated various acrylic copolymers in order to obtain a resin binder for electrophotographic printing plates that has high desensitization properties, that is, reduces background smudge. It has been found that the larger the alcohol residue of the (meth)acrylic acid ester of the combined structural unit, and the higher its content, the less background staining occurs on the electrophotographic lithographic printing plate. This was a totally unexpected and unexpected result. The monomer (A) of the present invention is based on this result. However, due consideration must be given to the resistance of the photoconductive layer during wet development due to the use of a large amount of monomer (A). The results of intensive research by the present inventors show that the wet development resistance of the photoconductive layer is not only influenced by the secondary transition point of the copolymer as previously thought, but also by its molecular weight. It's clear. The higher the molecular weight, the better the resistance. Therefore, another feature of the resin binder of the present invention is its molecular weight. In general, the lower the molecular weight of the resin binder for the photoconductive layer is, the better it is in terms of dispersibility of zinc oxide particles, stability of viscosity when used as a paint, electrostatic properties (charge amount, light attenuation properties, etc.) and image quality. However, from the viewpoint of the physical properties of the coating film, a resin binder with an unnecessarily low molecular weight cannot be used in practice. The reality is that resins with a molecular weight of about 5,000 to 20,000 are used in electrophotographic paper for office copying, and 15,000 to 100,000 in electrophotographic paper for planographic printing. The reason why the molecular weight of lithographic printing plates is higher than that for office copying is that a correspondingly higher coating strength is required. However, in the case of the resin binder containing a large amount of the monomer (A) of the present invention, the weight average molecular weight is 100,000.
Below that, the wet development resistance of the coating film will be insufficient,
Printing durability is also slightly inferior. In the case of a resin binder containing a large amount of the monomer (A) of the present invention, in order to satisfy wet development resistance and printing durability, the weight average molecular weight is
Must be greater than 100,000. This seems to be due to the fact that the monomer (A) itself has a larger molecular weight than the (meth)acrylic acid ester monomer that has conventionally been widely used. On the other hand, if the molecular weight becomes too large and exceeds 200,000, the viscosity of the coating liquid becomes high and tends to thicken, resulting in problems in stable coating. In the present invention, it goes without saying that other resin binders may be used in combination with the resin binder described above, if necessary. Examples of the present invention will be described below. Example 1 An appropriate amount of an initiator was added to n-hexyl methacrylate 55 parts by weight ethyl methacrylate 42 methacrylic acid 3 toluene 100 and solution polymerized, resulting in a copolymer having a weight average molecular weight of about 130,000. 40 parts by weight of this resin binder solution (solid content 50%) was diluted with 90 parts by weight of toluene, and photoconductive zinc oxide was added to it.
A paint for the photoconductive layer was prepared by mixing 100 parts by weight with 5 parts by weight of a 2% methanol solution of rose bengal and dispersing the mixture for 20 minutes using an ultrasonic disperser. The paint thus prepared was applied onto an electrophotographic base paper (a water-resistant material prepared for planographic printing plates) using a wire bar so that the dry coating amount was 25 g/m ² and dried. This sample was placed in a dark room at 25℃ and 60%RH.
After 24 hours of seasoning and dark adaptation, the plate was made using an electrophotographic engraving machine, Diafax Master EP-11 (manufactured by Mitsubishi Paper Industries), and a detailed image was obtained without any damage to the paint film caused by the aperture roll, and without any fogging. Obtained. This sample was then transferred to an etching converter OH-11 filled with etching liquid LOM-OH (manufactured by Mitsubishi Paper Industries).
(manufactured by Mitsubishi Paper Mills) once for desensitization treatment, and then printed using an offset printing machine Toko Model 810 (manufactured by Tokyo Aircraft Instruments).
It was possible to obtain good printed matter without plate skipping or other image defects up to 5,000 sheets, and with no uniform stains on the entire surface or dotted background stains. The viscosity of the paint was measured using a food cup No. 4, and it was found that the paint temperature was 20°C for 18 seconds immediately after preparation.
After 24 hours of preparation, the paint temperature was 20°C and it took only 19.5 seconds, so there was very little increase in viscosity. Example 2 An appropriate amount of an initiator was added to z-ethylhexyl acrylate 50 parts by weight ethyl methacrylate 47 methacrylic acid 3 toluene 100 and solution polymerized to obtain a copolymer having a weight average molecular weight of about 180,000. Using this resin binder liquid, paint preparation, application, plate making, and printing were carried out in the same manner as in Example 1, and the same good plate-made prints as in Example 1 were obtained. However, the viscosity of the paint is 22 seconds immediately after preparation;
After 24 hours, the degree of viscosity increased to 25 seconds, which was a slight increase, but not to the extent that it deteriorated the applicability. Example 3 Z-Ethylhexyl acrylate 40 parts by weight Methyl methacrylate 57 〃 Methacrylic acid 3 〃 Toluene 80 〃 Isopropyl alcohol 20 〃 An appropriate amount of initiator was added to solution polymerization, resulting in a copolymer with a weight average molecular weight of about 110,000. . Using this resin binder liquid, paint preparation, application, plate making, and printing were carried out in the same manner as in Example 1. As a result, good plate-made products similar to those of Example 1 were obtained.
I got the print. The viscosity of the paint was 20 seconds immediately after preparation and 21 seconds 24 hours after preparation, with little increase in viscosity. Example 4 Lauryl methacrylate 30 parts by weight Methyl methacrylate 67 〃 Itaconic acid 3 〃 Xylene 80 〃 N-Butanol 20 〃 An appropriate amount of an initiator was added to solution polymerization, and a copolymer having a weight average molecular weight of about 120,000 was obtained. Using this resin binder solution, coating solution preparation, coating, plate making, and printing were carried out in the same manner as in Example 1, and the same good plate-made products and printed matter as in Example 1 were obtained. Comparative Example 1 An appropriate amount of an initiator was added to n-butyl methacrylate, 40 parts by weight, methyl methacrylate, 57, methacrylic acid, 3, toluene, 80, and isopropyl alcohol, and solution polymerized to obtain a copolymer having a weight average molecular weight of about 50,000. When this resin binder liquid was used for coating liquid preparation, application, plate making, and printing in the same manner as in Example 1, it was found that the coating film collapsed due to the squeezing roll, the coating film was damaged during printing, and the surface was uniform on the entire surface. Although no stains were observed, dotted background stains were observed in the printed matter. Comparative Example 2 A copolymer having a weight average molecular weight of about 80,000 was obtained by increasing the amount of initiator added to the monomer composition of Example 3. Using this resin binder, coating solution preparation, application, plate making, and printing were carried out in the same manner as in Example 1, and no uniform background stains or dotted background stains were observed at all, but the squeezing roller Slight deterioration of the coating film was observed due to this, and after printing 5,000 sheets, a slight coating film loss occurred on the lower side of the solid image.

Claims (1)

[Scope of Claims] 1. A lithographic printing plate using an electrophotographic photoreceptor in which a photoconductive layer consisting of photoconductive zinc oxide and a resin binder is provided on a conductive support, wherein the resin binder is ( A) [Formula] R ₁ ; -H or -CH ₃ R ₂ ; Alkyl group having 6 to 12 carbon atoms (B) [Formula] R ₃ ; -CH ₃ or -C ₂ H ₅ (C) (A) and Consists of a vinyl monomer of carboxylic acid having a vinyl group that can be copolymerized with (B), the mixing ratio of which is (A):(B):(C)=30-60:40-70:0.5-5
(wt%), and the weight average molecular weight is
An electrophotographic lithographic printing plate characterized in that it is a copolymer in the range of 100,000 to 200,000.