Hard-elastomer impression cylinder

Abstract

The present invention relates to an imaging printing forme in the form of a cylindrical forme body having a cylindrical metal core and a hard elastomer, wherein the hard elastomer comprises 100 phr of a mixture of styrene-butadiene rubber and ethylene propylene diene rubber, and wherein the hard elastomer covers the outwardly facing surface of the metal core, and wherein the hard elastomer has a material hardness of at least 70 Shore D measured according to DIN EN ISO 868. A printing forme of this type is distinguished, in particular, by its dimensional stability in polar organic solvents, and is therefore suitable for use with solvent-based inks.

Description

[0001] title

[0002] Hard elastomer pressure cylinder

[0003] Description

[0004] The invention relates to an imaging printing form in the form of a cylindrical shaped body comprising a cylindrical metal core and a hard elastomer, wherein the printing form is particularly suitable for use in the gravure printing sector.

[0005] Hard elastomers – also known as "hard rubber" or "ebonite" – are densely cross-linked rubbers that typically contain sulfur concentrations between 30 and 50 phr (parts per hundred parts rubber). To date, only highly unsaturated rubbers (with many C=C double bonds) can be used to produce ebonite in order to achieve the high degree of cross-linking required for strength. These are typically cross-linked natural rubber products, although vulcanizates from other rubbers are also suitable.

[0006] Rubbers, e.g., rubbers based on acrylonitrile butadiene polymers (NBR), are known. Vulcanizates of natural rubber with 30-50 phr of sulfur produce ebonite with hardnesses > 45 Shore D (measured according to DIN EN ISO 868).

[0007] The intaglio printing process is a printing technique in which the elements to be reproduced are present as depressions (cells) in the printing form. The printing process is carried out by first inking the entire printing form, removing the excess ink with a squeegee or wiper so that the printing ink remains only in the depressions, and then transferring the ink to the paper.

[0008] In the industrial sector, doctor blade gravure or rotary gravure printing is particularly important, where the printing form is a gravure cylinder. This process is used, for example, for printing magazines or catalogs. The production of the image-forming (printing) surfaces of the printing form is carried out in the conventional way by first machining a soft copper layer with a Mohs scratch hardness of, for example, 2 using electromechanical engraving or laser engraving, and then engraving the print-forming cells. The softness of the copper layer is necessary to apply the engraving using the engraving tools used in the prior art without immediately wearing them out. If these tools become excessively worn, this process would no longer be economical.Subsequently, a hard chromium layer with a Mohs hardness of, for example, 8 is applied in a galvanic process to protect the engraving layer from wear caused by the squeegee during the printing process.

[0009] However, this conventional process has its disadvantages. It requires the application of two layers to the printing form, making it comparatively complex and energy-intensive. Furthermore, this process involves a galvanic process step for applying the chromium layer, which is extremely unfavorable from a health and environmental perspective. Hard chromium plating typically uses chromium(VI) trioxide, which is highly toxic during application until it is deposited. Furthermore, the proper disposal of chromium compounds is problematic from both an environmental and cost perspective. As batch sizes for print jobs become ever smaller, the disadvantages associated with this conventional process for producing printing forms no longer outweigh the benefits.

[0010] EP 3 727 868 A1 discloses an imaging printing plate that has a hard elastomer instead of a metallic coating. The elastomers used there (particularly acrylonitrile butadiene, NBR) are imageable, but exhibit poor ozone and weather resistance, as well as low-temperature flexibility. Another problem is the lack of dimensional stability when in contact with solvent-based inks.

[0011] The present invention is therefore based on the object of providing an imaging printing form in the form of a cylindrical shaped body which avoids the disadvantages of conventional metal forms described above and is nevertheless engravable (imageable), solvent-resistant and weather-resistant.

[0012] This object is achieved by the embodiments characterized in the claims.

[0013] In particular, an imaging printing form is provided in the form of a cylindrical molded body comprising a cylindrical metal core and a hard elastomer. The hard elastomer comprises 100 phr of a mixture of styrene-butadiene rubber and ethylene-propylene-diene rubber and covers the outward-facing surface of the metal core. The hard elastomer has a material hardness of at least 70 Shore D, measured according to DIN EN ISO 868.

[0014] Surprisingly, it has been shown that a rigid elastomer made from a blend of SBR and EPDM rubbers offers the optimal combination of low-temperature flexibility, weather resistance, solvent resistance, and engraving capability to replace a copper-chrome coating. The imaging printing plate is preferably a gravure printing plate.

[0015] For the purposes of this invention, a "hard elastomer" is a cross-linked, rubber-containing mixture with a hardness of at least 70 Shore D, preferably at least 80 Shore D, most preferably 85 Shore D, measured according to DIN EN ISO 868. The hard elastomer within the meaning of the invention has a maximum hardness of 98 Shore D, also measured according to DIN EN ISO 868. Shore hardness is understood to mean the resistance to penetration by a body of a specific shape under a defined spring force. While conventional elastomers have a hardness in the range of 10 to 90 Shore A, the hardness of test specimens made of hard elastomers and plastics is evaluated according to Shore D. In the Shore D test method, the indenter consists of a slightly rounded conical tip. The indenter travel is 2.50 mm and is divided into 100 Shore units. The Shore D device has no preload; the spring force is 0 to 44.5 N (CD = 17.8 N / mm).The samples must be at least 6 mm thick and have a diameter of more than 35 mm. The test is carried out at at least three different locations. Despite the different conditions, the Shore hardnesses can be converted into one another based on a non-linear relationship known to the expert. For example, 50 Shore A corresponds approximately to 10 Shore D, and 75 Shore A corresponds approximately to 20 Shore D.

[0016] Solvents in the sense of the present invention are liquids that can dissolve and / or dilute solids, in particular polymers, without chemical reactions occurring between the solvent, the substance to be dissolved, and the dissolved substance. In particular, solvents are understood to be liquids based on alcohols, amines, ketones, esters, and mixtures thereof. Surprisingly, compared to known hard elastomer-coated printing plates, particularly good solvent resistance in polar solvents such as esters, alcohols, and ketones was observed. This particularly good solvent resistance is particularly evident in the fact that the material absorbs very little to no solvent into its structure (swells) and remains essentially dimensionally stable upon contact with such solvents.

[0017] According to the invention, the imaging printing form is in the form of a cylindrical shaped body. "Cylindrical" in the context of this invention refers to the geometry of the resulting object. From a geometric perspective, the entire surface of a cylinder is composed of two circular surfaces (end faces) and a peripheral surface (body), with the body being arranged between the two end faces. In preferred embodiments, the metal core within the meaning of this invention has a body that forms a cavity, so that a surface is formed inside the hollow steel core and outside the hollow steel core. According to the invention, "surface" refers to the outward-facing surface. The end faces of the metal core can be completely or partially closed, or completely open.Preferably, the end faces are completely or partially closed, so that the cavity formed in some embodiments does not come into contact, or only partially comes into contact, with the environment outside the hollow metal core. The end faces can be provided with axes to which the cylindrical shaped body is attached. The end faces essentially define the diameter of the cylindrical shaped body. In preferred embodiments, the end faces have a diameter of 20 mm to 2000 mm.

[0018] The hard elastomer covers the outward-facing surface of the metal core. The hard elastomer can cover the entire barrel homogeneously and completely, so that the outward-facing surface of the imaging printing form is formed exclusively by the hard elastomer. According to the invention, the hard elastomer layer does not extend beyond the outer surface of a cylinder (barrel), so the end faces are not covered with the hard elastomer.

[0019] In principle, any conventional method can be used to produce the printing form according to the invention. For example, a cylindrical molded body is first provided. The molded body can be made of any suitable material. The molded body can, for example, comprise steel and / or plastic. Preferably, the molded body is a cylindrical steel core, particularly preferably a cylindrical hollow steel core. The hard elastomer can be coated on the molded body so that the hard elastomer covers the cylindrical metal core. This can be done using conventional methods. The molded body can be coated with a hard elastomer or a hard elastomer mixture, for example, using a calender plate that is wound around the outer surface of the molded body. Alternatively, it is also possible for the hard elastomer or the hard elastomer mixture to be extruded onto the molded body.It is also possible to coat the molded body by strip coating or by doctoring / dipping from a solution of the hard elastomer or the hard elastomer mixture. The coating is applied with a layer thickness such that the required or desired final layer thickness can be achieved after vulcanization and grinding. The hard elastomer layer is preferably evenly distributed so that the layer thickness of the hard elastomer does not differ by more than 1 μm across the entire coated surface of the metal core.

[0020] The imaging printing form features a rigid elastomer. The rigid elastomer compound comprises a cross-linked (vulcanized) rubber. Cross-linking can, in principle, be achieved using any cross-linking system known to a person skilled in the art. The sulfur cross-linking system has proven particularly advantageous for rigid elastomers. It is known to the person skilled in the art that the amount of sulfur fundamentally influences the cross-linking density and thus the properties of the vulcanizates (cross-linked elastomers). Typically, recipes for rubber compounds are based on 100 parts of rubber and are expressed in phr (= parts per hundred parts rubber).

[0021] Sulfur dosages up to approximately 7 phr produce vulcanizates with high elongation and elasticity, as well as low hardness. The higher the sulfur content, the higher the crosslink density of the elastomer. As the crosslink density increases, elongation and elasticity decrease, while the hardness of the material increases.

[0022] In preferred embodiments of the present invention, the molded body comprises a hard elastomer which additionally comprises at least 30 phr of a resin. A resin within the meaning of the present invention can include all resins known to a person skilled in the art, either alone or in a combination of at least two resins. Preference is given to the use of formaldehyde resin, alkylphenol resin, or alkylphenol-formaldehyde resin, particularly resins containing tert-butyl radicals or tert-octyl radicals on the phenol ring. The use of a para-tert-butylphenol-formaldehyde resin has proven particularly suitable, in particular a para-tert-butylphenol-formaldehyde resin with a preferred methylol content of 8 to 15% and a particularly preferred methylol content of 10 to 14%.

[0023] In particularly preferred embodiments, the rigid elastomer comprises from 30 to 85 phr, in particular from 55 to 75 phr, of a resin. Based on the total mass of the rigid elastomer, the amount of resin according to the invention is above 15 wt.%, in particular from 15 to 25 wt.%. Higher amounts lead to embrittlement of the rigid elastomer, while rigid elastomers with smaller amounts of resin do not achieve the desired hardness. With smaller amounts of resin, the material hardness cannot reach a hardness of over 70 Shore D without the addition of larger amounts of sulfur.

[0024] Rigid elastomers require sulfur amounts between 30 and 55 phr to achieve a high degree of crosslinking. The addition of the resin makes it possible to reduce the sulfur amounts while still achieving the high degree of crosslinking required for hardness. The rigid elastomer preferably additionally comprises 5 to 30 phr of sulfur. Converted to the total mass of the rigid elastomer according to the invention, the sulfur amount is between 1 and 15 wt.%. In advantageous embodiments, the rigid elastomer comprises 5 to 30 phr, in particular 5 to 15 phr, of sulfur.

[0025] The crosslinking reaction of the rigid elastomer can be achieved using conventional sulfur crosslinking systems. Elemental sulfur, present in the form of ss rings, is typically used. Known additives, such as accelerators, can also be added to the conventional system. Examples of such additives include sulfenamides, benzothiazoles, and thiurams, as well as activators (e.g., stearic acid and zinc oxide).

[0026] The rigid elastomers may additionally comprise thermoplastic polymers. Such thermoplastic polymers may be, for example, acrylonitrile butadiene styrene (ABS), polyamides (PA), polylactide (PLA), polymethyl methacrylate (PMMA), polycarbonate (PC), polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), polystyrene (PS), polyetheretherketone (PEEK), polyvinyl chloride (PVC), or combinations thereof. In preferred embodiments, the amount of the optionally added thermoplastic polymer is selected such that the thermoplastic polymer has no effect on the material hardness of the rigid elastomer. In particularly preferred embodiments, the rigid elastomer does not comprise any thermoplastic polymers (>1 wt. %, based on the total mass of the rigid elastomer).

[0027] The imaging printing form according to the present invention comprises a hard elastomer comprising 100 phr of a mixture of styrene-butadiene rubber (SBR) and ethylene-propylene-diene rubber (EPDM). In preferred embodiments, the hard elastomer comprises 30 to 50 wt. % of a mixture of SBR and EPDM, based on the total mass of the hard elastomer. Surprisingly, it has been found that a blend of SBR and EPDM exhibits particularly good solvent resistance in solvents based on esters, alcohols, and ketones compared to NBR or SBR-based hard elastomers. Hard elastomers comprising EPDM / SBR blends in a mixing ratio of 1:1 to 1:5, preferably 1:2 to 1:4, based on the mass weight of SBR to EPDM, have proven particularly advantageous in the above-mentioned solvents.

[0028] The rigid elastomer is preferably electrically conductive. Electrical conductivity is induced by the addition of conductive carbon blacks, graphite and graphite oxides, carbon nanotubes (CNTs), metals, and their compounds, especially iron compounds.

[0029] Furthermore, the hard elastomer preferably comprises at least one filler.

[0030] These can be any fillers known to the expert, such as carbon black, graphite, carbon nanotubes (CNT), silica, calcium and aluminum silicates, diatomaceous earth, kaolin, limestone, zeolites, cyclodextrins, feldspar and / or talc, chalk, alumina gel, fibers (short and long fibers, glass, carbon, aramid fibers), whiskers (aluminum oxide, silicon carbide), mica, magnetite, core / shell fillers, asphalt, hard rubber dust, carbonates, sulfates, oxides and hydroxides of alkali and alkaline earth metals, Al(OH)3, polymer powder (e.g. PE or PTFE powder), factice, inorganic and organic pigments, glass beads, wood flour, nutshell flour, which can be used alone or in combination.

[0031] In preferred embodiments, the filler is selected from the list consisting of silicas, silicates, carbon black, and mixtures thereof. Carbon black, as used in the present invention, refers to carbon produced industrially by the incomplete combustion of organic compounds or by the thermal decomposition of hydrocarbons. Carbon blacks produced by the furnace or acetylene process are particularly suitable for the rigid elastomers used in this invention.

[0032] Suitable furnace soot types have a BET surface area of ​​8 to 145 m 2 / g and an OAN (oil absorption number) of 40 to 200 ml / 100g. Particularly advantageous for the purposes of this invention are the carbon black types N330, N550, and N770 (classified according to ASTM D 1765). These furnace blacks have a primary particle diameter of 30 to 70 nm and a BET surface area of ​​35 to 85 m². 2 / g and an OAN number of 70 to 135 ml / 100g. In particularly preferred embodiments, conductive carbon blacks, in particular between 2 and 10 wt. % conductive carbon blacks, based in each case on the total mass of the hard elastomer, are added to the hard elastomer. Conductive carbon blacks are carbon blacks obtained using the acetylene process. Acetylene blacks have primary particle sizes between 30 and 40 nm and a surface area of ​​approximately 65 m 2 / g and OAN values ​​between 150 and 200 ml / 100 g. They contain virtually no heteroatoms (C content > 99.7 wt%).

[0033] Alternatively or in addition to carbon black, the filler may comprise silica and / or silicates. Silicas and silicates are finely dispersed (colloidal) inorganic fillers with specific surface areas (BET) of 25 to 700 m 2 / g based on amorphous silicon dioxide. For example, the filler comprises precipitated silica and / or natural silicates. Suitable silicates are calcium carbonate (chalk), aluminum silicates (kaolin), quartz, kaolinite, and magnesium silicates. The silicon-based fillers may have a surface modification. They can be used individually or in combination. In particularly preferred embodiments, a silicon-based filler system additionally comprises carbon black.

[0034] The amount of filler used depends, among other things, on the use of the rigid elastomer, the polarity of the rubber type, the amount of resin used, and the amount of sulfur. The rigid elastomer preferably contains no less than 4 phr and no more than 235 phr of a filler. For certain applications, it is advantageous if the rigid elastomer has a non-black appearance, i.e., is based on a light-colored mixture. Such light-colored yet conductive mixtures can be obtained if the filler is formed from 1 to 170 phr of silicas and / or silicates and at least 4 phr of a conductive carbon black. Such light-colored mixtures preferably contain a filler comprising 1 to 70 phr of silica, 0 to 100 phr of natural silicates, and 4 to 35 phr of a conductive carbon black.

[0035] Black-colored rigid elastomers contain 30 to 85 phr of carbon black, especially carbon black N550. In a preferred embodiment, the rigid elastomer also comprises other additives.

[0036] The other additives are selected from the group consisting of processing aids, such as zinc oxide, zinc stearate, magnesium stearate, stearic acid, PE and / or PTFE powder, and plasticizers, such as white oils, esters, factice, and waxes, and emulsifiers, dispersants, neutralizing agents, and adhesion promoters, such as DBU or DBN and their salts, and anti-aging agents, antiozonants, flame retardants, and functional materials, such as antimicrobial additives, odor neutralizers, flavors, and lubricants, mold release agents, and anti-fouling agents and permeation-inhibiting substances, such as phyllosilicates. These additives may be present alone or in combination.

[0037] The hard elastomer layer, which covers the entire metal core, preferably a hollow steel core, can have any suitable thickness. Preferably, the coating consisting of the hard elastomer according to the invention has a thickness of at least 30 μm. In a particularly preferred embodiment, the coating consisting of the hard elastomer according to the invention has a thickness in the range from 30 μm to 30 mm. The large range from 30 μm to 30 mm allows the printing form to be used multiple times without re-rubbering. To do this, an engraving is simply ground off after use, and a new engraving is applied.

[0038] The roughness of the hard elastomer coating consisting of the hard elastomer according to the invention can be suitably adjusted by grinding and polishing, for example to an average surface roughness of less than 2 pm Rz. According to a preferred embodiment of the present invention, the surface of the coating consisting of the hard elastomer according to the invention has an average surface roughness of less than 1 pm Rz. During subsequent printing, the roughness ensures that an ink film allows the doctor blade to glide on the surface of the printing form. In advantageous embodiments, the surface (ball) of the imaging printing form covered with the hard elastomer has a print image in the form of engraved cells. In this embodiment, the surface of the cylindrical steel core is evenly and homogeneously covered with the hard elastomer, so that the engraving takes place directly on the hard elastomer.

[0039] Engraving is preferably performed by electromechanical engraving or laser engraving. In electromechanical engraving, a diamond stylus cuts cells into the printing form. If the printing form is a gravure cylinder, the diamond stylus cuts the cells into the rotating cylinder, with a sliding foot keeping the stylus at a constant distance from the cylinder. The arrangement and size of the cells determine the final printed image. Electromechanical engraving produces cells with a depth of, for example, approximately 45 to 55 μm.

[0040] Laser engraving is particularly preferred. Laser engraving creates cells with a depth of approximately 35 to 45 pm, for example, allowing imaging speeds to be increased by up to 75% compared to electromechanical engraving.

[0041] Examples

[0042] Rigid elastomers were prepared according to the formulations shown in Table 1. Rigid elastomer A was prepared according to the present invention and was used to coat a cylindrical molded article. All quantities are in phr.

[0043] Table 1

[0044] Solvent resistance

[0045] To determine the solvent resistance, vulcanized test flaps were tested in accordance with DIN ISO 1817 against the solvents primarily used in gravure printing.

[0046] The solvents ethyl acetate and methyl ethyl ketone (MEK) were used for testing. The surface is wetted on one side with the respective solvent for 24 hours at room temperature. The penetration of the solvent causes a change in the thickness of the test flap. This change in thickness is measured and expressed in mm. The results are presented in Table 3 and Table 4 below.

[0047] Table 2 (swelling in ethyl acetate) Table 3 (swelling in methyl ethyl ketone)

[0048] The results show that the inventive hard elastomer A exhibits particularly good solvent resistance compared to NBR-containing hard elastomers.

[0049] To determine engraving capability, the cell contours were created using electromechanical engraving (stippling) as an example, and microscope images of the hard elastomer cross-sections were evaluated. For electromechanical engraving, a standard resolution of 70 lines / cm with an angle of 0° and a stylus at an angle of 120° was used. This engraving resolution is considered critical and provides a wealth of information about a material's engraving capability. The microscope images were taken using reflected-light microscopy at 300-400x magnification. Hard elastomers A and B, as well as a conventional copper surface with a Vickers hardness of 210 +-5HV, were stippled at 0.1 N and evaluated. The measurements showed that both hard elastomers exhibited comparable stippling to the copper surface.

Claims

Patent claims 1. An imaging printing form in the form of a cylindrical body comprising a cylindrical metal core and a hard elastomer, wherein the hard elastomer comprises 100 phr of a mixture of styrene-butadiene rubber and ethylene-propylene-diene rubber, and wherein the hard elastomer covers the outwardly facing surface of the metal core, and wherein the hard elastomer has a material hardness of at least 70 Shore D as measured according to DIN EN ISO 868.

2. The printing form according to claim 1, wherein the hard elastomer additionally comprises 30 to 85 phr of a resin.

3. The printing form according to claim 2, wherein the resin comprises a phenol-formaldehyde resin.

4. The printing form according to any one of claims 1 to 3, wherein the hard elastomer additionally comprises 5 to 45 phr sulfur.

5. The printing form according to any one of claims 1 to 4, wherein the surface covered with the hard elastomer has a printed image in the form of engraved cups.

6. The printing form according to any one of claims 1 to 5, wherein the layer consisting of the hard elastomer has a layer thickness of at least 30 pm.

7. The printing form according to any one of claims 1 to 6, wherein the hard elastomer comprises 30 to 200 phr of at least one filler.

Images