Hard elastomer for gravure printing

Abstract

The present invention relates to a hard elastomer comprising at least one rubber, at least one filler, at least one resin and 5 to 50 phr sulphur, wherein the hard elastomer has a material hardness of at least 70 Shore D measured in accordance with DIN EN ISO 868. A hard elastomer of this kind is characterised in particular by its material homogeneity and is extremely well suited for use as an image-providing printing forme.

Description

[0001] title

[0002] Hard elastomer for gravure printing

[0003] Description

[0004] The invention relates to a hard elastomer which is particularly suitable as an elastomer coating 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 of other rubbers, such as rubbers based on acrylonitrile butadiene polymers (NBR), are also known. Vulcanizates of natural rubber with 30 to 50 phr of sulfur produce ebonite with hardnesses > 45 Shore D (measured according to DIN EN ISO 868).

[0006] EP 3 727 868 A1 discloses an application of a hard elastomer with a hardness of more than 40 Shore D as an imaging printing form in gravure printing. The image is created by electromechanically engraving so-called cells into the material. The arrangement and size of the cells determine the final printed image. Electromechanical engraving creates cells with a depth of, for example, 45 to 55 μm. The main problem with this application of the hard elastomer is the larger sulfur efflorescence, some of which is the size of the cells. This material inhomogeneity negatively impacts the mechanical properties of the hard elastomer and reduces the quality of the resulting printed image.

[0007] The present invention is therefore based on the object of providing a hard elastomer that avoids the above-described disadvantages of conventional hard elastomers. This object is achieved by the embodiments characterized in the claims.

[0008] In particular, a hard elastomer is provided which comprises at least one rubber, at least one filler, at least one resin, and from 5 to 50 phr of sulfur. According to the invention, the hard elastomer has a material hardness of at least 70 Shore D, measured according to DIN EN ISO 868.

[0009] Surprisingly, it was shown that the material homogeneity of the hard elastomer could be reduced by adding a resin. In this way, the material hardness required for the hard elastomer could be achieved even with smaller amounts of sulfur. The resulting hard elastomer exhibited significantly fewer defects, such as sulfur blooms, when examined microscopically.

[0010] 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 points. Despite the different conditions, the Shore hardnesses can be converted into one another due to a non-linear relationship. According to this, for example, 50 Shore A corresponds to approximately 10 Shore D and 75 Shore A to approximately 20 Shore D. The competent person is aware that the amount of sulfur fundamentally influences the crosslinking density and thus the properties of the vulcanizates (crosslinked elastomers). Recipes for rubber compounds are usually based on 100 parts of rubber and are given in phr (= parts per hundred parts rubber). Sulfur dosages of up to about 7 phr produce vulcanizates with high elongation and elasticity but low hardness. The higher the amount of sulfur used, the higher the crosslinking density of the elastomer.As the cross-linking density increases, elongation and elasticity decrease while hardness increases.

[0011] Conventional rigid elastomer products require sulfur amounts between 30 and 45 phr. For the rigid elastomers of the present invention, sulfur amounts between 5 and 50 phr are suitable to achieve the required degree of crosslinking. 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. Due to the resin, the high crosslinking density can also be achieved with smaller amounts of sulfur, so that the rigid elastomer according to the invention achieves a hardness of at least 70 Shore D even with a lower sulfur dosage than is possible for conventional rigid elastomers.

[0012] The crosslinking reaction of the inventive rigid elastomer can be carried out using conventional sulfur crosslinking systems. Elemental sulfur, present in the form of ss rings, is typically used. Known accelerators such as sulfenamides, benzothiazoles, and thiurams, as well as activators (e.g., stearic acid and zinc oxide) can also be added to the conventional system.

[0013] 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).

[0014] The rigid elastomer of the present invention further comprises at least one rubber. In preferred embodiments, the rigid elastomer according to the present invention comprises 30 to 50 wt. % of at least one rubber, based on the total mass of the rigid elastomer. The rubber can be selected from the group consisting of ethylene-propylene copolymer (EPM) and ethylene-propylene-diene copolymer (EPDM) and nitrile rubber (NBR) and (partially) hydrogenated nitrile rubber (HNBR) and natural rubber (NR) and styrene-butadiene rubber (SBR) and solution-polymerized styrene-butadiene rubber (SSBR) and emulsion-polymerized styrene-butadiene rubber (ESBR) and isoprene rubber (IR) and butyl rubber (HR) and butadiene rubber (BR) and ethylene-vinyl acetate rubber (EVA) and silicone rubber (MQ, VMQ, PVMQ, FVMQ) and polyurethane (PU) and polynorbene rubber (PNR) and trans- Polyoctenamer rubber (TOR) and polyester urethane rubber (AU) and polyurethane rubber (EU).In preferred embodiments, at least one rubber is selected from the list consisting of nitrile butadiene rubber (NBR), natural rubber (NR), styrene butadiene rubber (SBR), and ethylene propylene diene rubber (EPDM). Particularly preferred rubbers are styrene butadiene rubber (SBR) and ethylene propylene diene rubber (EPDM).

[0015] One type of rubber (e.g., NBR) or several (e.g., SBR and EPDM) can be used, so that the total amount of rubber in the rigid elastomer is 100 phr. For example, a rigid elastomer can comprise 100 phr of a blend of at least two rubbers. Preferably, two or more rubbers are selected from the group consisting of styrene-butadiene rubber (SBR), ethylene-propylene-diene rubber (EPDM), and natural rubber (NR). Surprisingly, it has been shown that a blend of SBR and EPDM exhibits particularly good solvent resistance in solvents such as ethyl acetate, ethanol, methyl ethyl ketone, methyl isobutyl ketone, or propyl acetate. 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 the SBR to the EPDM, have proven to be particularly advantageous in the above-mentioned solvents.

[0016] It is essential to the invention that the rigid elastomer 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 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%.

[0017] In preferred embodiments, the rigid elastomer comprises from 30 to 85 phr, especially 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.%, especially from 15 to 25 wt.%. Higher amounts lead to embrittlement of the rigid elastomer, while rigid elastomers with lower amounts of resin do not achieve the desired hardness. With lower amounts of resin, the material hardness cannot reach a hardness of over 70 Shore D without the addition of larger amounts of sulfur.

[0018] 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.

[0019] The rigid elastomer also contains at least one filler. This can be any filler known to a person skilled in the art, 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.

[0020] In preferred embodiments, the filler is selected from the list consisting of silicas, silicates, carbon black, and mixtures thereof. Carbon blacks, as used in the present invention, are understood to be 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.

[0021] 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.

[0022] In particularly preferred embodiments, conductive carbon blacks, in particular between 2 and 10 wt.% conductive carbon blacks, based 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%). Alternatively or in addition to the carbon black, the filler comprises a silica and / or silicates. Silicas and silicates are finely dispersed (colloidal) inorganic fillers with specific surface areas (BET) of 25 to 700 m². 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.

[0023] 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.

[0024] Black-colored hard elastomers contain 30 to 85 phr of carbon black, especially carbon black N550.

[0025] In a preferred embodiment, the hard elastomer contains further additives.

[0026] 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, neutralizers, and adhesion promoters, such as DBU or DBN and their salts, and age inhibitors, 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.

[0027] The hard elastomer according to the invention is preferably suitable for use as an elastomer coating in the field of gravure printing. The gravure printing process is a printing technique in which the elements to be reproduced are present as depressions (cells) in the printing form. Printing is carried out by first inking the entire printing form, removing the excess ink with a doctor blade or wiper so that the printing ink is only in the depressions, and then transferring the printing ink to the paper. The printing form can be a printing plate or a gravure cylinder. In the industrial field, however, doctor blade gravure printing or gravure printing is particularly popular.

[0028] This is important for rotogravure printing, where the printing form is a gravure cylinder. This process is used, for example, for printing magazines or catalogs. In particularly preferred embodiments, the hard elastomer is a hard elastomer coating that extends over the entire surface of a gravure cylinder, comprising a cylindrical mold body and a cylindrical hollow metal core.

[0029] Examples

[0030] Rigid elastomers were prepared according to the recipes shown in Table 1, with Rigid Elastomers A and B being prepared according to the present invention. All amounts are in phr. Table 1

[0031] Material homogeneity

[0032] To determine the material homogeneity, microscope images were taken

[0033] Hard elastomer cross-sections and, for example, the cell contours were evaluated using EMG (stinging). This was done using reflected-light microscopy at 300 to 400x magnification. Massive sulfur crystallites are visible as black spots and mark the inhomogeneous areas. These crystallites are the size of cells, disrupting the cell contours and thus hindering high-quality imaging. Solvent resistance

[0034] To determine solvent resistance, vulcanized test flaps were tested against ethyl acetate and MEK, the solvents primarily used in gravure printing, in accordance with DIN ISO 1817. 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.

[0035] This thickness change is determined and expressed in mm. The results are shown in Table 3 and Table 4 below.

[0036] Table 2 Table 3

[0037] It can be clearly seen that the hard elastomers according to the invention have a comparable and, in the case of hard elastomer A, even better solvent resistance than the conventional hard elastomer.

Claims

Patent claims 1. A hard elastomer comprising at least one rubber and at least one filler and at least 30 phr of at least one resin and from 5 to 50 phr sulfur, whereby the hard elastomer has a material hardness of at least 70 Shore D measured according to DIN EN ISO 868.

2. The rigid elastomer of claim 1 comprising 5 to 30 phr of sulfur.

3. The rigid elastomer according to one of claims 1 or 2, comprising 30 to 85 phr, in particular 55 to 75 phr, of at least one resin.

4. The rigid elastomer according to any one of claims 1 to 3, comprising 100 phr of a mixture of at least two rubbers.

5. The hard elastomer according to any one of claims 1 to 4, wherein at least one rubber is selected from the list consisting of natural rubber (NR), styrene-butadiene rubber (SBR), ethylene-propylene-diene rubber (EPDM) and mixtures thereof.

6. The rigid elastomer according to any one of claims 1 to 5, comprising 30 to 200 phr of at least one filler.

7. The rigid elastomer according to any one of claims 1 to 6, wherein the filler is selected from the list consisting of silicas, silicates and carbon blacks.

8. The rigid elastomer of claim 7 comprising 30 to 85 phr of carbon black.

9. The rigid elastomer of claim 7, wherein the filler comprises 30 to 170 phr of silicas and / or silicates, and 4 to 35 phr of conductive carbon blacks.

10. The rigid elastomer according to any one of claims 1 to 9, wherein the resin comprises a phenol formaldehyde resin.

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