Printing method using particles of non-amorphous carbon allotropes

Abstract

To provide a universally applicable (lift) printing technique capable of achieving high print quality. The present invention relates to a printing method, in which a substrate to be printed is placed against an ink carrier having a liquid ink layer, the liquid ink layer is locally irradiated, the liquid ink layer comprises a liquid and carbon-containing particles X dispersed in the liquid, the carbon-containing particles X being present as carbon nanotube particles, graphite particles, and / or structural analogue particles of graphite selected from the group consisting of graphene particles, graphene oxide particles, graphite oxide particles, fullerene particles, and particles of graphite intercalation compounds.

Description

[Technical field]

[0001] The present invention relates to a printing method, a print assembly, a substrate, and the use of an ink layer. [Background technology]

[0002] A method for printing a substrate is known from US 6,241,344 B1, in which droplets of ink are thrown from an ink-coated carrier onto a substrate to be printed. To transfer (move) the ink, energy is introduced through the carrier to the ink on the carrier at the position where the substrate is to be printed. This causes a vaporization of part of the ink or of the liquid present in the ink, thus detaching the ink from the carrier. As a result of the pressure of the vaporized ink, the droplets of ink thus detached are thrown onto the substrate.

[0003] By introducing energy in a directed manner, it is possible to transfer the ink onto the substrate according to the pattern to be printed. The energy necessary to transfer the ink is introduced, for example, by a laser.

[0004] The carrier carrying the applied ink is, for example, a circulating ribbon, to which the ink is applied before the print zone by an application device. The laser is arranged inside the circulating ribbon, so that it acts on the carrier on the side opposite the ink. The application of ink to the ink carrier is achieved, for example, by a roll immersed in an ink reservoir. A printing machine of this kind is also known from US Pat. No. 5,021,808 A.

[0005] According to the teaching of this document, ink is applied from a reservoir container by means of an applicator to a circular ribbon in which a laser is disposed, by which the ink is vaporized at a predetermined position and accordingly thrown onto a substrate to be printed, the ribbon being made of a material transparent to the laser (laser-transparent material).

[0006] For selective vaporization of the ink, the circulating ribbon can be coated with an absorbent layer, in which the laser light is absorbed and converted to heat, thus vaporizing the ink at the location of exposure to the laser.

[0007] When liquid ink is transferred to an opposing substrate by a laser using the above-mentioned methods (lift printing techniques), an amorphous "dot" with many adjacent satellites (droplets) is generally produced. Furthermore, a lot of energy is required to separate the drop from the ink-carrying layer on the ink belt and then carry it in free flight onto the opposing substrate. This requires the use of very powerful lasers, especially to achieve acceptably high printing speeds, thus increasing costs and limiting possible applications.

[0008] Furthermore, it is known from US20050212888 that the efficiency of laser-induced printing can be increased by using small laser-absorbing particles. This is important in that it allows a significant increase in the productivity of the described process. One drawback of using absorbing particles is that these particles often also exhibit absorption in the visible wavelength spectrum, resulting in a more or less strong discoloration of the ink (liquid ink) that is printed.

[0009] WO 2019 / 175056 deals with (lift) printing techniques using an ink printing device that functions as a nozzleless drop ejection device for ejecting droplets of ink from an ink layer. In such processes, a problem may be that the laser-induced ejection of particles (drop flight) may involve the ejection of a large number of undesirable satellites, which reduces the quality of the print result. WO 2019 / 145300, however, relates to an alternative (lift) printing mechanism, whereby a heated bulge is formed in the ink layer, which bulge contacts the substrate, and the relative movement between the substrate and the ink carrier results in ink separation.

[0010] WO2021047886 proposes to use reflective particles instead of carbon black (absorbent) particles in the corresponding (lift-off technique) printing method. Such (suitable) reflective particles also have absorbing properties with respect to the laser beam. In contrast to typical carbon black absorbing particles, these reflective (absorbent) particles can be relatively neutral with respect to the colored print carried by the ink layer. Such reflective (absorbent) particles are typically metal or metal-coated carrier materials. However, these reflective particles must be added in relatively high amounts (typically in amounts of 1-10% by weight with respect to the ink used in the ink layer) to generate a sufficient (laser beam) absorbing effect. This generally causes a considerable loss of transparency of the ink. Moreover, such reflective particles typically produce a metallic luster and / or a grey haze. This limits the suitability of this type of printing technology for many applications, especially those where high turbidity (and / or gloss) is undesirable (e.g. for clear coats). Summary of the Invention [Problem to be solved by the invention]

[0011] It is therefore an object of the present invention to provide a universally applicable (lift) printing technique which is able to achieve high print quality. [Means for solving the problem]

[0012] The object of the present invention is achieved by a printing method, in which a substrate to be printed is placed opposite an ink carrier having a liquid ink layer, and the liquid ink layer is locally irradiated, the liquid ink layer includes a liquid and carbon-containing particles X dispersed in the liquid, the carbon-containing particles X being present as structural analogue particles of graphite selected from the group consisting of carbon nanotube particles, graphite particles, and / or graphene particles, graphene oxide particles, graphite oxide particles, fullerene particles, and particles of graphite intercalation compounds, and the ink layer contains 0.02 to 0.40 wt % of the carbon-containing particles X dispersed in the liquid.

[0013] The printing method according to the present invention allows the transfer of ink by lift printing. Finally, the absorbing particles provided by the present invention ensure that a high absorption performance can still be achieved without causing significant clouding of the ink, and such a high degree of absorption efficiency not only generally means energy efficiency, but also has a direct impact on the attractive appearance of the printed matter. This is particularly relevant for color, light, transparency and glossy prints, since undesirable haze can be avoided.

[0014] It is an economic advantage that only a small dosage of said absorbent particles is required for lift of the printing ink.

[0015] This is in contrast to known absorbing particles used in lift-off techniques, such as carbon black or metal particles, which are not substantially neutral with respect to the pigmented print carried by the ink layer. [Brief description of the drawings]

[0016] [Figure 1]FIG. 1 is a schematic diagram of one exemplary embodiment of a printing machine (1) that can be used to implement the method according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] According to a preferred embodiment, the printing method is carried out by an ink printing device, in which the ink layer is locally irradiated, whereby: (a) forming a thermal bulge in the ink layer which causes ink droplets to break off, thereby causing the ink printing device to function as a nozzleless drop ejector for ejecting droplets of ink from the ink layer; and / or (b) A heated bulge is formed in the ink layer, the bulge contacts the substrate, and the relative motion between the substrate and the ink carrier causes ink separation.

[0018] Nozzleless drop ejection means that according to the printing mechanism involved, ink nozzles are not used.

[0019] Typically, the ink layer is locally irradiated by a laser, more specifically by a switched laser.

[0020] Preferably, the ink carrier and the ink layer are moved parallel to each other (in particular placed on top of each other).

[0021] Typically, the substrate and the ink carrier are moved relative to one another at a speed that corresponds to at least the printing speed, and preferably at least twice the printing speed.

[0022] The ink layer contains 0.02 to 0.40% by weight of carbon-containing particles X dispersed in a liquid.

[0023] This particularly small (relative and quantitative) range of carbon-containing particles X dispersed in the liquid ensures on the one hand sufficient (laser) absorption, but on the other hand avoids undesirable haze. Furthermore, corresponding weight-% values ​​higher than about 0.4 wt.% often cause undesirable high viscosity of the ink (thickening effect should be limited in order to maintain good printability). It is also possible to use corresponding mixtures of particle X types, e.g. mixtures of carbon nanotube particles, graphite particles and graphene particles.

[0024] In general, it is advantageous (and also energy efficient) to reduce the laser power used (if possible).

[0025] Usually, the ink layer contains 10 to 99% by weight, preferably 15 to 95% by weight, and more preferably 50 to 90% by weight of liquid.

[0026] Generally preferred are carbon nanotube particles, graphite particles, and / or particles of structural analogs of graphite selected from the group consisting of graphene particles, graphene oxide particles, and graphite oxide particles.

[0027] More preferred are particles of structural analogues of graphite selected from the group consisting of carbon nanotube particles, graphite particles, and / or graphene particles.

[0028] Typically, at least 50% by weight of all carbon-containing particles dispersed in the liquid are present as carbon nanotube particles, graphite particles and / or graphene particles.

[0029] Most preferred are particles of graphite and / or structural analogues of graphite selected from the group consisting of graphene particles.

[0030] Typically, particles X were used that have a relatively small size (approximately <5 μm) and a relatively narrow size distribution (particle size distribution).

[0031] When transparency is not required, relatively large particles X having a relatively broad particle size distribution can also be used.

[0032] Preferably, the liquid contains or consists of water and / or an organic solvent.

[0033] According to a particular embodiment, the liquid ink contains a free-radically polymerizable organic solvent (reactive thinner).

[0034] Typically, however, the ink layer contains a polymer dispersed and / or dissolved in a liquid.

[0035] In many cases, the ink layer comprises (as a rheology modifier) ​​a soluble polymer having a weight average (Mw) molecular weight of more than 250,000 g / mol, where the weight average (Mw) of the molecular weight of the soluble polymer is determined according to DIN 55672-2:2016-3 (wherein N,N-dimethylacetamide is used as the dissolution solvent).

[0036] The proportion of such soluble polymers, according to one embodiment of the present invention, is between 0.05 and 2% by weight of the total ink mixture.

[0037] Such soluble polymers used according to one preferred embodiment of the present invention include cellulose esters, cellulose nitrates, cellulose ethers, and more specifically, hydroxypropyl cellulose, polyurethanes, or vinyl polymers. In particular, hydroxypropyl cellulose, in other words, cellulose ethers in which some of the hydroxyl groups are bonded to hydroxypropyl groups as ethers, are considered to be particularly suitable for the effects of the present invention.

[0038] However, it is possible to use conventional (organic or inorganic) binders, preferably conventional polymeric binders.

[0039] According to a particular embodiment, the ink layer contains a pigment.

[0040] According to a preferred embodiment, the ink layer contains less than 0.4% by weight of carbon black, and preferably contains no carbon black.

[0041] In many cases, the ink layer contains 0.1 to 80.0% by weight, preferably 1.0 to 20.0% by weight, of non-carbon particles, preferably present as pigment particles, having a diameter of 3 to 300 micrometers (as measured by laser diffraction according to DIN ISO 13220, 2020 edition).

[0042] Such large particles cannot be printed by conventional jetting methods (nozzles will become clogged), but can be printed by the lift printing technique of the present invention.

[0043] The present invention further relates to a print assembly including the ink layer described above.

[0044] Furthermore, the present invention relates to a substrate printed with the above-mentioned printing method.

[0045] Furthermore, the present invention relates to the use of the above-mentioned ink layer in a lift printing process.

[0046] The present invention will now be further described with reference to examples.

[0047] Implemented (Lift) Print Method: A printing apparatus according to FIG. 1 was used.

[0048] FIG. 1 is a schematic diagram of one exemplary embodiment of a printing machine (1) that can be used to implement the method according to the invention.

[0049] The printing machine (printing device) (1) is equipped with a circulating ink ribbon as an ink carrier (4).

[0050] The ink ribbon is coated with ink (2) evenly and over its entire area by an inking unit (8). The ink ribbon then moves in the direction of the arrow towards a print nip (10). The ink carrier (4) is spaced apart from the substrate (6) to be printed. Preferably, the width of the space is adjustable and / or continuously controlled. This can be done, for example, by means of an adaptable spacing roll (5).

[0051] At the print nip (10), a laser beam (3) is focused onto the ink (2) through a laser-transparent ink carrier (4) using a laser scanner (11). Localized and selective heating of portions of the ink (2) by the laser beam (3) causes explosive vaporization of small areas of the ink (2), thereby transferring (moving) portions of the print ink (2) from the ink ribbon onto a facing substrate (6).

[0052] The ink ribbon, controlled by the spacing roll (5) and the deflection roller (7), then moves back towards the inking unit (8). Upon contact between the inking unit (8) and the ink ribbon, the consumed ink (2) is replenished.

[0053] Excess ink (2) in the inking unit (8) is collected at the bottom in an ink trough (9) and is added repeatedly and continuously to the printing operation.

[0054] List of reference numbers related to Figure 1 1. Printing machine (printing device) 2. Ink 3. Laser beam 4. Ink carrier 5.Spacing roll 6. Base material 7.Deflection roller 8. Ink unit 9. Ink Trough 10.Print nip 11. Laser scanner

[0055] Print details and results: To evaluate the LASER absorption efficiency, different concentrations of carbon-containing particles were added to the clearcoat formulation (0.125-2.000 wt%) and each sample was printed onto clear PET foil according to the standard LIFT printing method described above, with a laser power of 300 W and a print speed of 2.5 m / min.

[0056] In the next step, the printed area (10 × 20 cm 2 The coverage of the laser was visually assessed under a digital optical microscope (Keyence VHX-7000). Laser absorption, and therefore ink transfer, was sufficient when no unprinted areas were visible: this means "all-over printing."

[0057] In additional preliminary tests (simulation tests), the film transparency of the samples as a function of particle concentration was evaluated by applying the films onto a transparent PET foil via a wired K-bar hand coater (tightly wound, 12 μm groove). In this way, it could be ensured that all samples had the same film thickness, a condition under which corresponding experiments were comparable to each other.

[0058] Based on the previous results of the preliminary tests, the samples were print tested by the standard LIFT method (as described above); except in the case of aluminum particles, because the transparency was already insufficient at 0.500 wt.%.

[0059] K-bar test and print results Various carbon-containing particles (and aluminum particles) were incorporated into a solvent-based clearcoat (dispersion) and applied by K-bar onto a transparent PET foil, resulting in a film with a thickness of about 6 μm after drying. This simulates the (substrate) results of the corresponding lift printing method according to the present invention (the results are comparable since this method provides the same film thickness). In other words, the film thickness after drying of the samples applied via K-bar is comparable to the printout obtained by the LIFT method.

[0060] Visual results were evaluated: the light transmittance of the corresponding films was visually evaluated (providing a range of 0 to 6, where 0 represents transparent and 6 represents opaque).

[0061] All applied dispersions were prepared by standard dispersion procedures and the quality of the obtained dispersions complies with commercial standard dispersions.

[0062] [Table 1] The weight percentages in the table above are only approximations (because they do not always add up to exactly 100); however, they at least give an indication of the weight ratio.

[0063] [Table 2]

[0064] The graphene particles used in the experiments typically consist of 8-10 aggregated ("single-ply") layers (for clarity: the layers are not aggregated according to the chemical graphite structure); the graphite particles used in the experiments (corresponding layers aggregated according to the chemical graphite structure) have an edge length of about 5 μm and a flake thickness of about 0.5 μm.

[0065] [Table 3]

[0066] Printable (or not printable) by the lift printing method described above: "Printable" means "all-over printing", and "difficult to print" means "all-over printing" is not achievable.

[0067] For example, in the case of carbon black, full surface prints could not be obtained at concentrations below 0.500% by weight, even at higher laser radiation power values.

[0068] For graphene, graphite and carbon nanotube (CNT) particles, concentrations of even 0.250 wt. % (and also below) were already sufficient to obtain full-surface prints with high transparency ("near-transparent absorbers").

[0069] The results shown in the above table demonstrate the high efficiency of graphene, graphite and carbon nanotube particles as transparent (or nearly transparent) absorber materials for the LIFT method. When these particles are used, printing according to the invention is possible even at particle concentrations of less than 0.250% by weight. For example, a concentration of graphene particles of about 0.01% by weight may even be sufficient to obtain a full-surface print. However, when using such low particle concentrations, a correspondingly high laser radiation power would be necessary to enable the printing method. However, in many cases, such high laser radiation power should be avoided (e.g., to save energy).

[0070] Particles of non-amorphous carbon particles typically contribute to high viscosity. In particular, CNT particles (carbon nanotube particles) contribute to high viscosity (often also acting as a kind of thickener). Depending on further influencing parameters (such as the viscosity of the liquid used), it is often advantageous to limit the amount of CNT to reduce the thickening effect (inks with limited viscosity are usually easier to print).

Claims

1. 1. A printing method comprising: A substrate to be printed is placed against an ink carrier having a liquid ink layer thereon, and the liquid ink layer is locally irradiated; the liquid ink layer includes a liquid and carbon-containing particles X dispersed in the liquid; the carbon-containing particles X are present as carbon nanotube particles, graphite particles, and / or particles of structural analogues of graphite selected from the group consisting of graphene particles, graphene oxide particles, graphite oxide particles, fullerene particles, and particles of graphite intercalation compounds; the ink layer contains 0.02 to 0.40% by weight of the carbon-containing particles X dispersed in the liquid; Printing method.

2. 10. A printing method as claimed in claim 1, performed by an ink printing device, the ink layer being locally irradiated, whereby: (a) a thermal bulge is formed in the ink layer which causes ink droplets to break off, such that the ink printing device functions as a nozzleless droplet ejector for ejecting droplets of ink from the ink layer; and / or (b) forming a heat bulge in the ink layer, the bulge contacting the substrate and causing ink separation due to relative movement between the substrate and the ink carrier; Printing method.

3. 3. A printing method according to claim 1 or 2, wherein the ink layer is locally irradiated by a laser, more particularly by a switched laser.

4. A printing method according to any one of claims 1 to 3, wherein the ink carrier and the ink layer are moved parallel to each other.

5. Printing method according to any one of the preceding claims, wherein the substrate and the ink carrier are moved relative to each other at a speed which at least corresponds to the printing speed, preferably at least twice the printing speed.

6. A printing method according to any one of the preceding claims, wherein said ink layer contains 10-99% by weight of said liquid, preferably 15-95% by weight, more preferably 50-90% by weight.

7. The printing method according to any one of claims 1 to 6, wherein at least 50% by weight of all carbon-containing particles dispersed in the liquid are present as carbon nanotube particles, graphite particles and / or graphene particles.

8. A printing method according to any one of the preceding claims, wherein the liquid contains or consists of water and / or an organic solvent.

9. The printing method according to any one of claims 1 to 8, wherein the liquid ink contains a free-radically polymerizable organic solvent.

10. A printing method according to any one of the preceding claims, wherein the ink layer contains a polymer dispersed and / or dissolved in the liquid.

11. The printing method according to any one of the preceding claims, wherein the ink layer comprises a pigment.

12. Printing method according to any one of the preceding claims, wherein the ink layer contains less than 0.4% by weight of carbon black, preferably no carbon black.

13. A printing method according to any one of claims 1 to 12, comprising the steps of: The ink layer is 0.1 to 80.0% by weight, preferably 1.0 to 20.0% by weight, of non-carbon particles, preferably present as pigment particles, having a diameter of 3 to 300 micrometers (as measured by laser diffraction in accordance with DIN ISO 13220, 2020 edition). A printing method comprising the steps of:

14. A print assembly comprising an ink layer according to any one of claims 1 to 13.

15. A substrate printed by the printing method according to any one of claims 1 to 13.

16. Use of an ink layer according to any one of claims 1 to 13 in a printing method according to any one of claims 1 to 13.

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