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(11) | EP 0 573 988 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Process for producing support for lithographic printing plates |
| (57) A process for producing a support for lithographic printing plates is disclosed.
The process comprises contacting a rough surface with the surface of an aluminum plate
under pressure to transfer the roughness onto the surface of the aluminum plate, wherein
the transferring is repeatedly carried out at least 3 times. |
FIELD OF THE INVENTION
[0001] The present invention relates to a process for producing a support for lithographic printing plates and, more particularly, to a process for producing a support for lithographic printing plates composed of a surface-grained aluminum plate and suitable for offset printing.
BACKGROUND OF THE INVENTION
[0002] Hitherto, aluminum plates have been widely used as supports for lithographic printing plates. The surface of the aluminum plate is usually grained for improving the adhesion to a light-sensitive layer formed thereon, and also for improving the water retention property of the non-image portions (the region receiving dampening water and repelling an oily ink as well as the exposed or uncovered region of the support) of a lithographic printing plate produced using the support.
[0003] The above-described graining treatment is an essential step in the preparation of the support for lithographic printing plates and requires a skilled technique. Graining is generally classified as a mechanical graining method or an electric graining method. Examples of the mechanical graining method are a ball graining method, a wire graining method and a brush graining method.
[0004] In the case of ball graining, there are many factors, for example, the quality of balls, the kind of abrasives, the amount of water at graining, etc., which require skill in making the proper selection. Also, the graining work cannot be carried out continuously, thereby the graining must be carried out one by one. Also, in the case of wire graining, the degree of graining obtained is generally non-uniform. Brush graining is an improved method as compared with the above two graining methods. Furthermore, since uniform graining is obtained through brush graining and continuous treatment is possible, brush graining is suitable for mass production.
[0005] However, by the mechanical methods described above, it is difficult to obtain a satisfactory support for lithographic printing plates.
[0006] In general, it is known that, when the aluminum support has a high surface roughness, the water retention property is better. And in preparing a lithographic printing plate having a good water retention property or for facilitating printing, it is preferable for the surface of the aluminum support to have as uniform a roughness as possible. As a method for obtaining such a preferred uniform surface roughness, attention has been directed to the electrochemical graining method.
[0007] In the electrochemical graining method, an aluminum plate having a uniformly grained surface is obtained if various conditions such as the composition of the electrolyte, the temperature, the electrolytic condition, etc., are maintained constant. But the range of applicable electrolytic conditions is very narrow and, hence, it is very difficult to have all of the conditions satisfy the desired ranges for the electrolysis. Also, since the electrochemical graining requires a large amount of electric power, such graining is disadvantageous from an economical standpoint. Furthermore, in electrochemical graining, there is the disadvantage that a large amount of aluminum ions is accumulated in the electrolyte during electrolysis, and the labor and chemical cost for the treatment of the waste liquid is high.
[0008] Also, methods for forming an uneven surface on the aluminum plate include a method of using a rolling roller having an inversion graining surface as disclosed in JP-A-55-74898 (the term "JP-A", as used herein, means an "unexamined published Japanese patent application"). However, in this method, there is the disadvantage that it is very difficult to form a fine inversion grain surface (suitable for the support) on the roller used for the inversion graining.
[0009] JP-A-60-36195 and JP-A-60-36196 disclose a method comprising forming elliptical concaved portions having an average long axis of from 10 to 140 µm and an average short axis of from 7 to 80 µm on a support and, thereafter, chemically or electrochemically forming a fine unevenness of from 1 to 10 µm. Also, JP-A-60-203496 discloses an aluminum plate prepared by transferring an unevenness having an average diameter of from 10 to 100 µm to the surface of the aluminum plate by means of a roller having the embossed surface and, thereafter, subjecting the aluminum plate to a chemical etching treatment and an electrochemical etching treatment.
[0010] Various rolling rollers each having an uneven surface have been proposed for producing a support for lithographic printing plates by bringing the uneven surface into contact with the surface of an aluminum plate under pressure and forming an uneven surface on the aluminum plate by transferring the unevenness. However, it is difficult to precisely form on the surface of the roller a sufficiently uniform and fine unevenness suitable for the support of a lithographic printing plate. Even when the working precision for forming fine unevenness is increased, the cylindricality of the transfer roller is gradually changed while carrying out the transfer. Accordingly, when the fine unevenness on the surface of a transfer roller is transferred onto the surface of an aluminum plate, the draft must be adjusted to high, thereby causing a large problem for controlling the change in the thickness, the elongation percentage, etc., of the aluminum plate.
SUMMARY OF THE INVENTION
[0011] An object of the present invention is, therefore, to solve the above-described problems, and to provide a process for producing a support for a lithographic printing plate having a sufficiently uniform unevenness without the need for adhering to the working precision such as the cylindrical property of the transfer roller, etc., and also without need for controlling changes in the thickness, the elongation, etc., of the aluminum plate.
[0012] The present inventors have found that in a process of bringing an uneven surface into contact with the surface of an aluminum plate under pressure to transfer the roughness onto the surface of the aluminum plate, a sufficiently uniform uneven surface for the aluminum support of lithographic printing plates can be obtained without the need for the precision of a transfer roller such as the cylindrical property, etc., and with reducing the elongation of the aluminum plate by carrying out the transfer a plurality of times. The present inventors have completed the present invention based on the above finding.
[0013] More specifically, the above-described object can be achieved by a process which comprises bringing an uneven surface into contact with the surface of an aluminum plate under pressure to form the unevenness on the surface of the aluminum plate by transfer, carrying out the transfer at least 3 times. In particular, in the process of the present invention, it is preferable to carry out the transfer at least 4 times.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Fig. 1 is a surface photograph (375 magnifications) of the aluminum plate obtained in Example 5.
[0015] Fig. 2 is a surface photograph (375 magnifications) of the aluminum plate obtained in Comparative Example 9.
DETAILED DESCRIPTION OF THE INVENTION
[0016] In the present invention, the transfer is repeatedly carried out using a substrate having a fine uneven surface which is generally formed by etching. This etching is carried out, for example, by electro spark machining, shot blast, laser, plasma etching or pattern etching using a photoresist. Also, where a fine uneven surface is formed by coating fine particles thereon, a plurality of the fine uneven patterns corresponding to the mean diameter of the fine particles are repeatedly transferred onto the aluminum plate.
[0017] A method for imparting a fine unevenness to the surface of a transfer roller is disclosed in JP-A-3-08635, JP-A-3-066404, JP-A-63-065017, etc. Also, fine grooves are formed on the surface of a roller from two directions using dies, a cutting tool, laser, etc., and a square-shaped unevenness may be formed on the surface. The surface of the roller may be further subjected to a known etching treatment to round the square-shaped unevenness thus formed. Furthermore, as a matter of course, quenching, hard chromium plating, etc., may be applied to the surface of the roller for increasing the hardness of the surface.
[0018] The surface of an aluminum plate thus grained by the transfer is preferably subjected to a chemical or electrochemical etching treatment in an aqueous solution of an acid, an alkali, or a neutral salt and then electrochemically grained in an aqueous solution of an acid or a neutral salt using an alternating electric current, a direct electric current, or a pulse direct electric current, whereby the aluminum plate becomes more suitable as a support for lithographic printing plates.
[0019] In the present invention, it is preferred that the surface of the aluminum plate is subjected to a chemical or electrochemical etching treatment after forming the rough pattern on the surface of the aluminum plate by transfer, whereby a rough pattern having an average diameter of from 4 to 20 µm (preferably from 4 to 10 µm) exists on the surface of the aluminum plate after the etching treatment. After the etching treatment, in this preferred embodiment, the surface is electrochemically grained. It is also preferred that, after the electrochemical graining treatment, the rough patterns each having an average diameter of from 1 to 3 µm are overlapped on the surface thereof after the etching treatment.
[0020] In one embodiment of the present invention, the surface roughness, formed by coating fine particles, can be obtained by coating a dispersion of fine particles having a diameter of from 0.3 to 20 µm on the surface of a substrate such as paper, a polyethylene film, or a metal roller followed by drying, or by directly coating the dispersion of the fine particles on an aluminum plate followed by drying.
[0021] The dispersion of the fine particles may be formed by dispersing the fine particles in water or other solvents together with a viscous binder, or by dispersing the fine particles in water or other solvents. For controlling the viscosity of the dispersion, a thickener may be added to the dispersion, if desired. From the standpoint of pollution and safety, the liquid used for dispersing the fine particles is preferably mainly composed of water.
[0022] According to one embodiment of the present invention, an uneven pattern, formed on the surface of a sheet coated with fine particles, is transferred onto the surface of an aluminum plate by applying pressure. In this process, the surface of the rough sheet (which is formed by coating fine particles thereon followed by drying) is brought into contact with an aluminum plate, and the resulting assembly of the aluminum plate(s) and the sheet is passed between two rollers or nip rollers having a narrower gap than the thickness of the assembly so as to apply pressure to the assembly. The rough pattern is then transferred onto the surface of the aluminum plate (which is in contact with the surface having the fine particles).
[0023] In another process according to the present invention, fine particles are coated on the surface of a roller, and the rough pattern on the surface of the roller is then transferred onto the surface of an aluminum plate by applying pressure. In still another process of the present invention, a dispersion of fine particles is coated on an aluminum plate (which is used as the aluminum plate for the lithographic printing plate), and pressure is applied to the surface. In the latter case, after coating the dispersion of fine particles, if necessary, the coated surface may be dried.
[0024] The fine particles (which can be used in the present invention) include particles having a diameter of from 0.3 to 20 µm and, preferably, from 3 to 10 µm, and the material used for the fine particles can be alumina, sand, diamond, silicon oxide, silicon carbide, zirconia, etc., with alumina being preferred. The particle size distribution of the fine particles is preferably as uniform as possible.
[0025] In another embodiment of the present invention, fine grooves are spirally cut on the surface of a roller in two different directions using dies, a cutting tool, laser, etc., to form a square-shaped uneven pattern on the surface of the roller. In this case, the pitch and the depth of the grooves are preferably from 4 to 10 µm and from 1 to 5 µm, respectively. The angle of cutting the grooves, to the circumference direction, is preferably from 30 to 60°. Also, the angle of the grooves, in the vertical direction to the surface of the roller, is preferably from 30 to 80°.
[0026] The aluminum plate, having the uneven pattern on the surface thereof and formed by the transfer, is then preferably subjected to a chemical or electrochemical etching treatment in an aqueous solution of an acid, an alkali, or a neutral salt, whereby the aluminum plate becomes more suitable as a support for a lithographic printing plate.
[0027] The aqueous solution of an acid (an aqueous acid solution), as described above, can be an aqueous solution mainly composed of hydrochloric acid, sulfuric acid, or nitric acid. The aqueous solution of an alkali (an aqueous alkali solution) can be an aqueous solution mainly composed of sodium hydroxide, etc. The treatment time with the aqueous acid solution or the aqueous alkali solution is preferably from 5 to 120 seconds. The concentration of the aqueous solution is preferably from 1 to 40%, and the liquid temperature is preferably from 35 to 75°C. If necessary, the aluminum plate may be subjected to a cathode electrolytic cleaning treatment in an aqueous acid solution or an aqueous alkali solution.
[0028] Also, the aqueous solution of a neutral salt described above can be an aqueous solution of an alkali metal halide, an alkali metal nitrate, an alkali metal sulfate. Preferred neutral salts include sodium chloride, sodium nitrate and sodium sulfate. The pH of the aqueous neutral salt solution is preferably from 5 to 9; the concentration of the solution is preferably from 1 to 40%; and the liquid temperature is preferably from 35 to 75°C. When the aqueous neutral salt solution is used, it is necessary to carry out an electrolytic treatment using the aluminum plate as a cathode in the solution. The time for the cathode electrolytic treatment is preferably from 5 to 180 seconds.
[0029] Further, it is preferred that the aluminum plate treated with the aqueous neutral salt solution or the aqueous alkali solution is further immersed in an aqueous solution of sulfuric acid, nitric acid or hydrochloric acid for the purpose of removing smut components formed on the surface of the aluminum plate.
[0030] The aluminum plate thus treated may be further subjected to an electrochemical graining treatment. The electrochemical graining can be performed by a conventional process, for example, an electrochemical graining of an aluminum plate in an aqueous acidic solution using an alternating electric current as disclosed in JP-A-53-145701, an electrochemical graining of an aluminum plate in an aqueous acidic solution using a direct current as disclosed in JP-A-4-14094, and an electrochemical graining of an aluminum plate in an aqueous neutral salt solution as disclosed in JP-A-5-587.
[0031] After the electrochemical graining treatment, the grained aluminum plate is preferably subjected to a desmutting treatment in an aqueous acidic solution, a denaturing treatment in an aqueous alkali solution, or a denaturing treatment in an aqueous neutral salt solution using the aluminum plate as a cathode. The desmutting treatment in an aqueous acidic solution is known as described in JP-A-53-12739, etc. The denaturing treatment in an aqueous alkali solution is known as described in JP-A-56-139700, etc. The denaturing treatment in an aqueous neutral acid solution is known as described in JP-A-59-11295, etc.
[0032] Furthermore, the aluminum plate thus treated can be subjected to a conventional anodic oxidation treatment in an electrolyte containing sulfuric acid or phosphoric acid for improving the hydrophilicity, the water retention property, and the printing durability. Also, after the anodic oxidation treatment, a sealing treatment can be applied. Furthermore, the hydrophilic treatment of the aluminum plate may be carried out by immersing the aluminum plate in an aqueous solution of sodium silicate, etc.
[0033] The aluminum plate, which can be used in the present invention, includes a pure aluminum plate or an aluminum alloy plate.
[0034] The following examples are intended to illustrate the present invention but not to limit it in any way.
Example 1 and 2 and Comparative Example 1
[0036] Fine particles having a mean particle size of 4 µm were coated on the surface of a paper followed by drying, and the resulting paper was placed between aluminum plates of JIS 1050. The assembly was passed through nip rollers once (Comparative Example 1), 3 times (Example 1), and 6 times (Example 2) to transfer the rough pattern of the paper surface coated with fine particles to the surface of the aluminum plate facing the paper surface. In this case, the clearance of the nip rollers was narrower than the thickness of the assembly (paper plus two aluminum plates) by 0.05 mm. The surface of each aluminum plate after transfer was white, indicating that the whole surface of the aluminum plates was roughened. The average surface roughness was 0.33 µm, and the elongation percentage of the aluminum plate was 1 %.
[0037] When the surface of each aluminum plate was observed by a scanning type electron microscope, it was confirmed that fine roughness having an average diameter of 4 µm was formed. The aluminum plate obtained by transferring the unevenness once was insufficient to use as a support for a lithographic printing plate. However, the aluminum plate obtained by transferring the rough pattern 3 times was practical as a support, and the aluminum plate obtained by transferring the rough pattern 6 times had a uniform fine rough pattern on the surface and was quite suitable as a support for a lithographic printing plate.
Examples 3 to 6 and Comparative Examples 2 and 3
[0038] Fine particles having a mean particle size of 8 µm were coated on the surface of a plastic sheet followed by drying. The resulting plastic sheet, coated with the fine particles, was then placed between two aluminum plates of JIS 1050, and the assembly was passed through nip rollers once (Comparative Example 2), twice (Comparative Example 3), 3 times (Example 3), 4 times (Example 4), 5 times (Example 5), or 6 times (Example 6) to transfer the rough pattern of the surface of the plastic sheet formed by the fine particles onto the surface of each of the aluminum plates. In this case, the clearance of the nip rollers was narrower than the thickness of the assembly (plastic sheet plus two aluminum plates) by 0.05 mm. The surface of each aluminum plate after transfer was white, indicating that the whole surface of the aluminum plates was roughened. When the surface of each aluminum plate was observed by a scanning type electron microscope, it was confirmed that the roughness having an average diameter of 8 µm was uniformly and randomly formed on the surface.
[0039] In these cases, the aluminum plate having the rough pattern on the surface thereof by transferring 4 times, 5 times, and 6 times were quite suitable as supports for a lithographic printing plate. The aluminum plate having the rough pattern on the surface thereof by transferring 3 times was practical as a support, but the aluminum plate having the rough pattern formed by transferring once or twice was not practical as a support. The photograph of the surface of the aluminum plate obtained in Example 5 is shown in Fig. 1.
[0040] Also, the concaved portions formed on the surface of each aluminum plate had complicated forms different from an elliptical form. Furthermore, any clear orientation was not observed in the concaved portions.
Example 7
[0041] The aluminum plate surface roughened by transferring 6 times, as described in Example 6, was immersed in a 5% aqueous solution of sodium hydroxide for 15 seconds at 45°C to effect a chemical etching treatment until 1 g/m² of the aluminum plate was dissolved. After washing the plate with water, the aluminum plate was further immersed in a 25% aqueous solution of sulfuric acid for 15 seconds at 60°C and then washed with water. The aluminum plate was then subjected to an electrochemical graining treatment in a 1% aqueous solution of nitric acid for 6 seconds at 40°C with a rectangular alternating current at a duty ratio of 1 : 1 and a current density of 50 A/dm² using carbon as a counter electrode and washed with water. Then, the aluminum plate thus grained was subjected to an etching treatment in a 5% aqueous solution of sodium hydroxide until 0.5 g/m² of the aluminum plate was dissolved. Furthermore, the aluminum plate was immersed in a 25% aqueous solution of sulfuric acid for 120 seconds at 60°C and then washed with water. Then, the aluminum plate was further subjected to an anodic oxidation treatment in a 10% aqueous solution of sulfuric acid at 33°C using a direct current and, thereafter, washed with water.
[0042] When the surface of the aluminum plate was observed by a scanning type electron microscope, it was confirmed that a rough pattern of from 1 to 2 µm was uniformly formed in a large rough pattern of from 5 to 10 µm. The average surface roughness was 0.49 µm. When a light-sensitive layer was coated on the aluminum plate to form a lithographic printing plate, the resulting plate showed a good staining performance.
Comparative Examples 4 to 8
[0043] Fine particles were coated on the surface of a paper, and the paper coated with the fine particles was placed between two aluminum plates of JIS 1050. The assembly of the paper and the aluminum plates was then passed through nip rollers once to transfer the pattern on the surface of the paper formed by the fine particles onto the surface of the aluminum plate. In this case, the clearance of the nip rollers was narrower than the thickness of the assembly by 0.3 mm. In these comparative examples, the average diameter of the fine particles was changed to 4 µm (Comparative Example 4), 8 µm (Comparative Example 5), 15 µm (Comparative Example 6), 20 µm (Comparative Example 7), or 30 µm (Comparative Example 8). In these examples, the elongation percentage of the aluminum plate was 20%. The surface of the aluminum plate after transfer was white.
[0044] The average surface roughnesses of these aluminum plates and the results of observation of the fine rough pattern by a scanning type electron microscope are shown in Table 1 together with the results of other examples and comparative examples described above.
Table 1
| Mean Particle Size of Fine Particles (µm) | Draft (µm) | Number of Transfer | Evaluation of Fine Concaved Forms | Elongation of Aluminum Plate (%) | Average Surface Roughness (µm) | |
| Comparative Example 1 | 4 | 0.05 | 1 | x | 1 | 0.33 |
| Example 1 | 4 | 0.05 | 3 | △ | 1 | 0.33 |
| Example 2 | 4 | 0.05 | 6 | o | 1 | 0.33 |
| Comparative Example 2 | 8 | 0.1 | 1 | x | - | - |
| Comparative Example 3 | 8 | 0.1 | 2 | x | - | - |
| Example 3 | 8 | 0.1 | 3 | △ | - | - |
| Example 4 | 8 | 0.1 | 4 | o | - | - |
| Example 5 | 8 | 0.1 | 5 | o | - | 0.49 |
| Example 6 | 8 | 0.1 | 6 | o | 3 | 0.49 |
| Comparative Example 4 | 4 | 0.3 | 1 | o | 20 | 1.04 |
| Comparative Example 5 | 8 | 0.3 | 1 | o | 20 | 1.04 |
| Comparative Example 6 | 15 | 0.3 | 1 | o∼△ | 20 | 1.19 |
| Comparative Example 7 | 20 | 0.3 | 1 | △ | 20 | 2.00 |
| Comparative Example 8 | 30 | 0.3 | 1 | △∼x | 20 | 2.60 |
[0045] In Table 1, the term "Draft" means the difference between the thickness of the assembly (paper or plastic sheet plus two aluminum plates) and the clearance of the nip rollers.
[0046] Evaluation of fine concaved forms:
- o:
- Suitable as support for printing plate
- △:
- Practical
- x:
- Not practical
[0047] However, in these samples of Comparative Examples 4 to 8, the average surface roughnesses thereof are too large and, hence, these samples are unsuitable as a support for the lithographic printing plate. Also, in Comparative Examples 4 to 8, the thickness of each aluminum plate is changed since the elongation of aluminum for each sample is 20% and, hence, these samples demonstrate that no guarantee of quality can be made.
[0048] Also, the samples transferred once or twice in Comparative Examples 1 to 3 are undesirable as supports for lithographic printing plates due to poor fine concaved forms.
[0049] On the other hand, the samples transferred 3 times in Examples 1 and 3 are practical as a support for the lithographic printing plate, and the samples transferred 4 or more times in Examples 2 and 4 to 6 are quite suitable as a support for the lithographic printing plate.
Comparative Example 9
[0050] The surface of an aluminum plate was grained by a #8 nylon brush and a suspension of pumice stone. When the surface of the aluminum plate was observed by a scanning type electron microscope, it was confirmed that slender and deep concaved portions having a length of from 10 µm to 20 µm, which were oriented in one direction, existed non-uniformly. Also, each pit was not discretely separated.
[0051] The aluminum plate was immersed in a 5% aqueous solution of sodium hydroxide to carry out a chemical etching treatment until 1 g/m² of the aluminum plate was dissolved. After washing the plate with water, the aluminum plate was further immersed in a 25% aqueous solution of sulfuric acid for 15 seconds at 60°C and, thereafter, washed with water.
[0052] When the surface of the aluminum plate was observed by a scanning type electron microscope, it was confirmed that concaved portions having an average pitch of from 5 µm to 20 µm were non-uniformly overlapped with each other.
[0053] The aluminum plate was subjected to an electrochemical graining treatment in a 1% aqueous solution of nitric acid at 40°C with a rectangular alternating current of a duty ratio of 1 : 1 and a current density of 50 A/dm² using carbon as a counter electrode and washed with water. Then, the aluminum plate was subjected to an etching treatment in a 5% aqueous solution of sodium hydroxide until 0.5 g/m² of the aluminum plate was dissolved. The aluminum plate was further subjected to an anodic oxidation treatment in a 10% aqueous solution of sulfuric acid at 33°C using a direct current and then washed with water.
[0054] When the surface of the aluminum plate thus treated was observed by a scanning type electron microscope, it was confirmed that a rough pattern of from 1 to 2 µm was uniformly formed in a large rough pattern of from 5 to 20 µm as shown in Fig. 2.
[0055] The average surface roughness of the aluminum plate was 0.55 µm and, when a light-sensitive layer was coated on the aluminum plate and the resulting plate was used as a lithographic printing plate, the staining performance was inferior to that of Example 7.
[0056] As described above, in the process of forming roughness on the surface of an aluminum plate by transferring the press-contact of an uneven surface, by repeatedly press-transferring at least 3 times, and preferably at least 4 times, an aluminum support for a lithographic printing plate having no orientation and sufficiently uniform concaved portions having an average diameter of from 3 to 10 µm can be produced while restraining the elongation percentage of the aluminum plate.
1. A process for producing a support for a lithographic printing plate which comprises
bringing a substrate having an uneven surface into contact with the surface of an
aluminum plate under pressure, wherein said step of bringing a substrate having an
uneven surface into contact with the surface of an aluminum plate under pressure is
carried out at least 4 times.
2. The process for producing a support for a lithographic printing plate as claimed in
claim 1, wherein the substrate is a sheet having fine particles coated thereon, said
fine particles have a diameter of from 0.3 to 20 µm.
3. The process for producing a support for a lithographic printing plate as claimed in
claim 1, wherein the substrate is a roller having fine particles coated thereon, said
fine particles have a diameter of from 0.3 to 20 µm.
4. The process for producing a support for a lithographic printing plate as claimed in
claim 2, wherein said fine particles have a diameter of from 3 to 10 µm.
5. The process for producing a support for a lithographic printing plate as claimed in
claim 3, wherein said fine particles have a diameter of from 3 to 10 µm.