METHOD FOR HARDENING INK ON A PRINTING MATERIAL

Description

invention

[0001] The present invention relates to a method for hardening ink on a printing material according to the preamble of claim 1. Technical field of the invention

[0002] The invention lies in the technical field of the graphic industry and there in particular in the area of ​​curing, especially curing with UV radiation, of liquid ink on sheet- or web-shaped printing materials, preferably made of paper, cardboard, corrugated board, plastic or composite material. State of the art

[0003] The drying and / or curing (or more generally: the treatment) of preferably liquid or pasty printing media / fluids (such as printing ink, varnish, or paint) on a substrate can be carried out in various ways, including combinations thereof: by exposure to hot air, electromagnetic radiation (e.g., UV or IR), or particle radiation (e.g., electrons), or by contacting the substrate with heated surfaces (e.g., cylinders, rollers, or belts). Water-based printing media are usually dried thermally, preferably with IR, while polymerizable printing media are usually dried with UV. Mercury vapor lamps and, increasingly, LED lamps are used as UV radiation sources. The curing of UV-curable inks can, as is known, be carried out under an inert gas atmosphere.

[0004] The curing of UV-curable inks can be carried out in several steps, for example, first a so-called pinning (pre-curing) and then a final curing: WO2019007979A1 discloses UV curing and subsequent UV curing under inert gas. EP3045477A1 discloses a similar process. EP2767407B1 also discloses a similar process, where the initial curing is a so-called pinning process. According to these documents, the final curing step in each case takes place under inert gas. EP1473341B1 discloses two UV curing steps, both under inert gas. EP2596874A1 discloses drying (not UV curing) under inert gas followed by UV curing.

[0005] US Patent 2005 / 0104946 A1 discloses a method for curing ink on a substrate, wherein three UV lamps directly adjacent to one another in the transport direction of the substrate are used. All three lamps expose the ink to oxygen-reduced air, which is supplied to the location of a "panel" of lamps via a line.

[0006] CN208392874 U and US4421784 A disclose a prior art method for hardening ink on a substrate.

[0007] In the prior art, the problem can arise that the result of UV curing of ink is insufficient (insufficient cross-linking / polymerization of the UV-curable ink) and that consequently problematic migration of low molecular weight and potentially hazardous ink components into packaging products, e.g. food, is to be expected.

[0008] While it is possible to (re)formulate the ink with higher molecular weight and therefore less migrating components, this reduces the viscosity of the ink, which can lead to problems when printing the ink.

[0009] The use of inert gas can prevent unwanted migration, as fewer (potentially migrating) inhibitors can be used in the ink in an oxygen-deprived curing environment. However, curing under inert gas is cost-intensive, partly because it requires a lot of installation space for the necessary components and the inert gas itself. Task

[0010] It is therefore an object of the invention to provide an improvement over the prior art which in particular makes it possible to sufficiently cure UV-curable inks so that an undesirable migration of low molecular weight ink components is reduced or avoided. Inventive solution

[0011] This problem is solved according to the invention by a method according to claim 1. Advantageous and therefore preferred embodiments of the invention are described in the dependent claims, the description, and the drawings.

[0012] A method according to the invention for curing ink on a substrate, wherein at least one UV-curable ink is applied in the form of drops to a substrate and is pre-cured by means of at least one first UV lamp, wherein the ink is exposed to UV radiation by means of the first UV lamp under ambient air, is characterized in that the ink is further cured by means of at least one second UV lamp of a UV dryer not under ambient air, but under inert gas, under inert gas-enriched air or under oxygen-reduced air, and that the ink is subsequently post-cured by means of at least one third UV lamp of a further UV dryer, wherein the ink is exposed to UV radiation by means of the third UV lamp under ambient air.

[0013] The invention advantageously enables UV-curable inks to be sufficiently hardened, so that unwanted migration of low molecular weight ink components is reduced or avoided.

[0014] An inert gas device is not required for the first emitter. This saves installation space and costs.

[0015] An inert gas system is also not required for the third emitter. This saves space and costs.

[0016] According to the invention, hardening takes place in three successive steps: pre-hardening or so-called pinning, further hardening and post-hardening.

[0017] Pre-curing (pinning) serves to increase the viscosity of the ink on the substrate, thereby preventing excessive bleeding, especially of the dots created by ink droplets. The viscosity is only slightly increased; the ink is not yet fully cured. This improves print quality, particularly by preventing the bleed-through of different ink colors (so-called "intercolor bleeding").

[0018] Further hardening serves to further increase the viscosity of the ink (compared to the pre-curing). The ink then adheres well to the substrate and exhibits sufficient abrasion resistance. This further hardening is preferably carried out such that more than 90% of the double bonds in the ink are converted.

[0019] Post-curing serves to bind any monomers still present in the ink, thereby reducing or preventing potential migration.

[0020] Complete hardening cannot be achieved even with three or more hardening steps. However, sufficiently good hardening results can be achieved with the hardening process according to the invention.

[0021] The second curing step (the "further curing" or alternatively referred to as the intermediate curing) – and preferably only this step – takes place under inert gas, inert gas-enriched air, or oxygen-reduced air, i.e., not under ambient air like the other two steps. This post-curing (alternatively referred to as final curing) ensures that the ink is largely, and preferably essentially completely, cured when it leaves the printing press.

[0022] Preferably, only the second curing step is carried out under inert gas (alternatively referred to as protective gas), inert gas-enriched air, or oxygen-reduced air; all other curing steps, however, are preferably carried out under untreated air, e.g., ambient air. "Under inert gas" means that preferably essentially pure inert gas, e.g., nitrogen or helium, is used. "Under inert gas-enriched air" means that preferably the proportion of inert gas, e.g., nitrogen or helium, in the air or other gas mixture used is significantly higher than in ambient air. "Under oxygen-reduced air" means that preferably the proportion of oxygen in the air or other gas mixture used is significantly lower than in ambient air.The increase or decrease is "significant" if this measure preferentially achieves that i) an inhibition of radicals generated by UV photons and photoinitiators, which is problematic with regard to migration, is avoided, and / or that ii) an insufficient and therefore problematic cross-linking with regard to migration is avoided or sufficient cross-linking is achieved, and / or that iii) the amount of (potentially migrating and therefore hazardous to health) photoinitiators in the ink can be reduced to a level that is unproblematic with regard to migration.

[0023] A device for hardening under inert gas or similar, e.g., an inert gas enclosure including inert gas supply and exhaust, can advantageously be limited to the second hardening step or its location or station. This saves installation space, components, and associated costs. Furthermore, the amount of inert gas required can be reduced.

[0024] In this way, sufficient hardening can be achieved to advantage (for industrially produced printed products of high to very high quality), especially if the printed products are further processed industrially, e.g. for the production of packaging.

[0025] In this way, it can also be advantageously achieved in the production of packaging that an undesirable (because potentially hazardous to health) migration of low molecular weight components of the ink into the packaging and into the packaged product, e.g. a foodstuff, is sufficiently reduced or avoided. Further developments of the invention

[0026] A preferred embodiment of the invention may be characterized by the use of nitrogen, or alternatively helium, as the inert gas or protective gas.

[0027] A preferred embodiment of the invention may be characterized in that the UV dryer comprises several second UV emitters, e.g. two, three or four.

[0028] A preferred embodiment of the invention is characterized by the fact that the second UV emitters are arranged in a common inert gas housing. The housing can be made of sheet metal, preferably with a shape adapted, e.g., a curvature, to the transport path.

[0029] A preferred embodiment of the invention may be characterized in that the further UV dryer comprises several third UV emitters, e.g. two, three or four.

[0030] A preferred embodiment of the invention is characterized in that the further UV dryer comprises at least two spaced-apart stations with several third or several fourth UV lamps. The substrate can be guided in the space between the stations by means of several deflection rollers, e.g., in a meandering pattern.

[0031] A preferred embodiment of the invention is characterized in that at least one second UV lamp and / or at least one third UV lamp and / or at least one fourth UV lamp cures the ink through the substrate. This embodiment is particularly advantageous for (UV-)transparent substrates. The ink is preferably applied to the front side of the substrate, and at least one lamp is arranged on the back side of the substrate, or vice versa.

[0032] A preferred embodiment of the invention may be characterized in that the substrate is moved along the at least one second UV emitter by means of a preferably heated tempering roller which supports the curing treatment.

[0033] The features of the invention, its embodiments, and its exemplary embodiments, in any combination, also represent advantageous further developments of the invention. Further developments of the invention may also include the individual features or combinations of features disclosed in the section "Technical Field of the Invention" above. Exemplary embodiments of the invention

[0034] The invention and its preferred embodiments are described in more detail below with reference to the drawings and by way of preferred exemplary embodiments. Corresponding features are designated with the same reference numerals in the figures.

[0035] The drawings show: Figure 1 Schematic representation of a printing press carrying out the process; and Figure 2 Side view of the inkjet printing press.

[0036] Figure 1Figure 1 shows a schematic representation of an inkjet printing machine 1 (hereinafter referred to as "machine") during the execution of a preferred embodiment of the method according to the invention. The machine processes web-shaped substrate 2, preferably paper, cardboard, or plastic film. The substrate is fed to the machine, for example, by a roll feeder at point A and then printed with ink.

[0037] Printing takes place in a first station 3 of the machine, specifically in an inkjet printer 3 with several, preferably eight, printhead rows 4, each with several inkjet printheads for generating ink droplets (preferably corresponding to a print image). A first UV lamp 5 is preferably directly downstream of each printhead row. These lamps serve to pre-cure, i.e., to partially polymerize, the ink (the so-called pinning process). Exposure to UV radiation preferably occurs under air, e.g., ambient air with an unchanged composition, and not under an inert gas different from air. Pinning is preferably carried out with UV radiation of a wavelength of 385 nm or (for white ink) 395 nm. The substrate 2 leaves the first station at B and is conveyed to C, where it is fed to a second station 10.

[0038] The second stage comprises a UV dryer 10 with at least one, but preferably several, second UV lamps 11, e.g., two UV lamps 11a and 11b. These lamps serve for further curing, preferably for the extensive (but still not complete) curing or polymerization of the ink. This further curing of the ink does not take place under (ambient) air, but under an inert gas, inert gas-enriched air, or oxygen-reduced air, or a functionally similar gas. Nitrogen is preferably used as the inert gas; alternatively, helium or another noble gas can be used.

[0039] The UV emitters 11 can be designed, for example, as UV lamps, preferably mercury vapor lamps or alternatively UV LED emitters, and each have between 200 and 300 W / cm² (assuming a rail transport speed of about 50 m / min).

[0040] For irradiation under inert gas, an inert gas housing 12 is provided, which preferably encloses the second UV lamps 11 together. This housing can be made of sheet metal. It has an inert gas supply line 13 and an inert gas outlet 14.

[0041] It may be provided that the substrate 2 is guided along the second UV lamps 11 by means of a tempering roller 7 or its tempered surface 8, or another tempered surface 8. Tempering, in particular heating, can support the curing process. The surface temperature can preferably be between 40 and 50 °C and can preferably be controlled as a function of speed.

[0042] The substrate web 2 can then be guided through an optional flexographic printing unit 15, in which UV-curable varnish can preferably be applied. At D, the web 2 leaves the second station 10 and at E is fed to a third station 20, i.e., another UV dryer 20, and at F' optionally to a fourth station 30, i.e., yet another UV dryer 30. The third and fourth stations can be spaced apart from each other; in particular, the web 2 can have a meandering path between E' and F'.

[0043] The third station 20 comprises at least one, but preferably several, third UV lamps 21, e.g., two UV lamps 21a and 21b. These lamps serve to post-cure the ink, preferably to substantially cure or polymerize the already pre-cured and further cured (and thereby preferably largely cured) ink.

[0044] The optional fourth station 30 comprises at least one, but preferably several additional third UV lamps 31, e.g., two UV lamps 31a and 31b. These lamps also serve for post-curing. The third lamps 21 and the additional third lamps 31 can be collectively referred to as "third lamps".

[0045] The third UV emitter 21 and the optional further third UV emitters 31 may preferably be designed as mercury vapor lamps or alternatively as UV LED emitters and each have between 100 and 200 W / cm² (assuming a rail transport speed of about 50 m / min).

[0046] Post-curing in the third and optional fourth station preferably takes place under air, e.g. ambient air unchanged in its composition, and not under an inert gas different from air.

[0047] The post-curing of the ink, i.e., irradiation with UV radiation, in the third and / or fourth station can be carried out from the printed side 40 of the substrate (compare Figure 2 Irradiation can occur from the printed side or through the substrate 2 (from the unprinted reverse side). A combination (from both the printed and unprinted sides of the substrate) is also possible: for example, irradiation in the third station 20 from the printed side and in the fourth station 30 from the unprinted side. Furthermore, irradiation in the second station 10 can also occur from the unprinted side (through the substrate 2), and thus differently than shown in the figures.

[0048] The inventive method, exemplified by the machine 1 shown, thus comprises i) UV pre-curing under air, ii) subsequent UV curing under inert gas, under inert gas-enriched air, or under oxygen-reduced air or functionally similar conditions, and finally iii) UV post-curing under air. Further upstream, intermediate, or downstream UV curing or temperature control steps are also possible.

[0049] Figure 2 shows (also schematically) a side view of machine 1. Figure 1 during the implementation of the preferred embodiment of the method according to the invention.

[0050] Opposite Figure 1 It can be seen that the web path 41 of the substrate 2 preferably meanders and can include several deflection rollers 6.

[0051] The inert gas housing 12 can be arranged in the web guidance area by means of the temperature control roller 7, as shown. It can include (inlet and outlet) orifices 9 which ensure that the inert gas supplied to the housing 12 at 13 is essentially discharged again at 14 and does not escape uncontrollably at other points.

[0052] Figure 2 This also shows that the emitters 21a and 21b of the third station 20 emit UV radiation from the printed side of the substrate, and that the emitters 31a and 31b of the fourth station 30 emit UV radiation from the unprinted side of the substrate 2 (i.e., through the substrate 2). This is a preferred, but only exemplary, configuration.

[0053] The track or rail transport between stations 3, 10 and 20, i.e. between B and C as well as between D and E, can be carried out by means of several deflection rollers 6, as well as between E' and F', i.e. between stations 20 and 30. Reference symbol list

[0054] 1 Inkjet printing machine 2 Substrate / Web 3 Inkjet printing unit / first station 4 Printhead rows / Inkjet printheads 5 First UV lamps (for pre-curing) 6 Deflection rollers 7 Tempering roller 8 Tempered surface 9 Apertures 10 UV dryer / second station 11, 11a, 11b second UV lamps (for further curing) 12 Inert gas housing 13 Inert gas supply line 14 Inert gas exhaust 15 Flexographic printing unit 20 additional UV dryers / third station 21, 21a, 21b third UV lamps (for post-curing) 30 additional UV dryer / fourth station 31, 31a , 31b additional third UV lamp (for post-curing) 40 Ink application / Printed side of the substrate 41 Path of the substrate A to F Transport path of the printing material through the machine

Claims

1. Method for curing ink on a substrate, wherein at least one UV-curable ink is applied in the form of drops to a substrate (2) and is exposed to UV radiation by means of at least one first UV lamp (5) and thereby pre-cured, wherein the ink is exposed to UV radiation by means of the first UV lamp (5) in ambient air, wherein the ink is exposed to UV radiation by means of at least one second UV lamp (11, 11a, 11b) of a UV dryer (10) not under ambient air but under inert gas, under air enriched with inert gas or under oxygen-reduced air and is thereby further cured, and wherein the ink is subsequently exposed by means of at least one third UV lamp (21, 21a, 21b, 31, 31a, 31b) of a further UV dryer (20, 30) to UV radiation and thereby post-cured, wherein the ink is exposed to UV radiation under ambient air by means of the third UV lamp (21, 21a, 21b, 31, 31a, 31b).

2. Method according to claim 1, characterized in that nitrogen is used as the inert gas.

3. Method according to any one of claims 1 or 2, characterized in that the UV dryer (10) comprises a plurality of second UV lamps (11, 11a, 11b).

4. Method according to claim 3, characterized in that the second UV lamps (11, 11a, 11b) are arranged in a common inert gas housing (12).

5. Method according to one of the claims 1 to 4, characterized in that the further UV dryer (20, 30) comprises a plurality of third UV lamps (21, 21a, 21b, 31, 31a, 31b).

6. Method according to claim 5, characterized in that the further UV dryer (20, 30) comprises a third station (20) with a plurality of third UV lamps (21, 21a, 21b) and a fourth station (30) with a plurality of further third UV lamps (31, 31a, 31b).

7. Method according to one of the claims 1 to 6, characterized in that at least one second UV lamp (11, 11a, 11b) and / or at least one third UV lamp (21, 21a, 21b) and / or at least one further third UV lamp (31, 31a, 31b) cures the ink through the substrate (2).

8. Method according to one of the claims 1 to 7, characterized in that the substrate (2) is moved along the at least one second UV lamp (11, 11a, 11b) by means of a tempering roller (7).