EP3749528A1 - Method for laser-induced forward transfer using metal oxide absorber particles - Google Patents

Method for laser-induced forward transfer using metal oxide absorber particles

Info

Publication number
EP3749528A1
EP3749528A1 EP19702298.1A EP19702298A EP3749528A1 EP 3749528 A1 EP3749528 A1 EP 3749528A1 EP 19702298 A EP19702298 A EP 19702298A EP 3749528 A1 EP3749528 A1 EP 3749528A1
Authority
EP
European Patent Office
Prior art keywords
laser
printing ink
particles
substrate
absorber particles
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP19702298.1A
Other languages
German (de)
French (fr)
Inventor
Piotr Wierzchowiec
Kevin MORITZ
Volker Wilhelm
Marc Hunger
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Merck Patent GmbH
Original Assignee
Merck Patent GmbH
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Merck Patent GmbH filed Critical Merck Patent GmbH
Publication of EP3749528A1 publication Critical patent/EP3749528A1/en
Pending legal-status Critical Current

Links

Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41M—PRINTING, DUPLICATING, MARKING, OR COPYING PROCESSES; COLOUR PRINTING
    • B41M5/00—Duplicating or marking methods; Sheet materials for use therein
    • B41M5/26—Thermography ; Marking by high energetic means, e.g. laser otherwise than by burning, and characterised by the material used
    • B41M5/382—Contact thermal transfer or sublimation processes
    • B41M5/392—Additives, other than colour forming substances, dyes or pigments, e.g. sensitisers, transfer promoting agents
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
    • C23C14/04—Coating on selected surface areas, e.g. using masks
    • C23C14/048—Coating on selected surface areas, e.g. using masks using irradiation by energy or particles
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
    • C23C14/22—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the process of coating
    • C23C14/24—Vacuum evaporation
    • C23C14/28—Vacuum evaporation by wave energy or particle radiation
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41M—PRINTING, DUPLICATING, MARKING, OR COPYING PROCESSES; COLOUR PRINTING
    • B41M2205/00—Printing methods or features related to printing methods; Location or type of the layers
    • B41M2205/08—Ablative thermal transfer, i.e. the exposed transfer medium is propelled from the donor to a receptor by generation of a gas
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41M—PRINTING, DUPLICATING, MARKING, OR COPYING PROCESSES; COLOUR PRINTING
    • B41M5/00—Duplicating or marking methods; Sheet materials for use therein
    • B41M5/26—Thermography ; Marking by high energetic means, e.g. laser otherwise than by burning, and characterised by the material used
    • B41M5/40—Thermography ; Marking by high energetic means, e.g. laser otherwise than by burning, and characterised by the material used characterised by the base backcoat, intermediate, or covering layers, e.g. for thermal transfer dye-donor or dye-receiver sheets; Heat, radiation filtering or absorbing means or layers; combined with other image registration layers or compositions; Special originals for reproduction by thermography
    • B41M5/41—Base layers supports or substrates

Definitions

  • the present invention relates to a method for transferring a printing ink from a donor substrate to a receiving substrate by a laser-induced forward transfer process, wherein the printing ink contains metal oxide containing particles, to the use of those metal oxide containing particles as laser absorber particles in such a process as well as to a product exhibiting a printed image on a substrate, obtained by said process.
  • the laser-induced forward transfer (LIFT) process is a direct-write process which has particular advantages when compared to traditional printing processes such as silk-screen printing processes or gravure printing processes. Contrary to the latter, the laser-induced forward transfer process, similar to an inkjet printing process, allows versatile use without expensive equipment and, in particular, personalized adaptations of the printing motif are easily available. In addition, improvements in printing speed, scale and resolution of the printing process and product are highly welcome.
  • LIFT processes have been used in particular for the production of electronic, optical and sensor elements, especially for microelectronic components such as antennas, sensors and embedded circuits, but also for transferring biological materials from one substrate to another.
  • the LIFT process may be performed in several variants.
  • a printing ink layer containing laser absorbing particles is applied onto a surface of a laser transparent substrate.
  • the transparent substrate (the donor substrate) is then irradiated by a laser beam from the reverse side which does not carry the printing ink.
  • the incident laser beam propagates through the transparent carrier before the light is absorbed by the back surface of the printing ink layer.
  • the printing ink is ejected in form of a droplet from the coated surface of the laser transparent substrate and catapulted towards a receiving substrate that is arranged in close proximity to the inked donor substrate surface.
  • the energy conversion process causing the ink ejection as well as the phase transitions involved in the LIFT process is complex and affected by a large number of diverse parameters.
  • the absorber particles are contained in the printing ink, these absorber particles absorb laser energy as well and are transferred to the receiving substrate too in a certain amount.
  • a printed ink spot is available at the receiving substrate, containing at least the solidified components of the printing ink droplet containing a certain amount of the absorber particles.
  • nano-sized carbon black particles are used in the first variant as absorber particles.
  • a separate absorbing layer is arranged between the transparent donor substrate and the printing ink layer.
  • the separate layer which is called a dynamic release layer (DRL)
  • DRL dynamic release layer
  • the laser beam may be directed either through the transparent donor substrate as in variant 1 , or may be directed to the absorber layer from the printing ink coated side of the donor substrate in an acute angle relative to the inked substrate surface.
  • the components of the printing ink are transferred to the receiving substrate upon exposure of the absorbing layer to the laser beam.
  • none of the components in the printing ink need to absorb laser light and, therefore, the choice of the ink components is fundamentally unlimited.
  • this method has also a major disad- vantage, which is transfer of traces of the absorber material from the DRL to the receiving substrate together with the printing ink. This leads to unwanted material or optical effects in the resulting printed image on the receiving substrate.
  • a partly destroyed or worn-out absorber layer has to be removed from the donor substrate as well as the printing ink layer when a new printing cycle run has to be started, leading to additional material and processual efforts.
  • absorber particles are used as in the first variant of the LIFT process as described above, particles in a submicron particle size range have heretofore been used. For such particle sizes, even the transfer of metallic particles like silver particles, copper particles or gold particles is possible, which are mostly used for integrated circuits or other electronic applications.
  • the first variant of the LIFT process was not available up to now except for carbon black containing printing inks, since a lot of these pigments or dyes, especially in brighter colors, do hardly or not at all absorb laser light in frequently used wavelengths. Therefore, in order to transmit these compounds via a LIFT process, the second variant had to be used so far.
  • the first variant is technically more attractive than the second due to less technical efforts and cost.
  • the first variant would be available to the transfer of coloring pig ments or dyes which do hardly or even not absorb in the wavelength emitted by the corresponding laser apparatus.
  • the object of the present invention is to provide a LIFT process without using a dynamic release layer for the transfer of coloring organic pigments or dyes or of functional materials such as electrically conductive polymers which do not or merely slightly absorb laser light in the wavelength emitted by the corresponding laser apparatus, where the process may be executed in a simple manner at low cost.
  • the object of the present invention is to provide a particular use for common pigments.
  • a further object of the present invention is to provide a product comprising a printed image on a substrate, where the printed image exhibits an intense non-black color in high resolution or a particular function and is printed in a LIFT process without the need of a dynamic release layer.
  • the object of the present invention is solved by a method for transferring a printing ink from a laser transparent donor substrate to a receiving substrate by a laser-induced forward transfer process, whereby
  • the donor substrate exhibits a front surface coated with the printing ink and a back surface facing away from the front surface;
  • the printing ink forms a coating which has an upper side being
  • the printing ink contains absorber particles and at least one component being capable of enlarging its volume upon exposure to laser-generated energy;
  • the donor substrate is irradiated by laser energy of a particular
  • the absorber particles are micron-sized particles comprising metal oxides
  • the printing ink comprises a coloring or functional compound.
  • the object of the present invention is solved by the use of micron-sized particles comprising metal oxides in a laser-induced forward transfer process as absorber particles in an ink coating on a donor substrate.
  • the object of the present invention is also solved by a product comprising a printed image on a substrate, wherein the printed image is composed of printed spots containing micron-sized particles comprising metal oxides and a coloring or functional compound, the printed spots being produced by the method according to the process mentioned above.
  • the printing method according to the present invention is based on a standard laser-induced forward printing process (LIFT) according to the first variant as described before.
  • LIFT laser-induced forward printing process
  • a laser transparent donor substrate is coated on one of its major surfaces with a printing ink which is subject to being transferred to a receiving substrate in form of a printed image composed of printed spots.
  • the present inventors revealed that if micron-sized particles comprising metal oxides are used as absorber particles in the printing ink, these micron-sized particles are transferred together with all other ingredients of the printing ink to a receiving substrate even if the printing ink contains colored organic pigments or dyes or functional materials which do, by themselves, not or merely slightly absorb the wavelength emitted by the corresponding laser.
  • the particle size of the micron-sized particles comprising metal oxides is in the range of from 1 -50 pm.
  • the given range is the broadest range of the nominal particle size of the micron-sized particles.
  • particle sizes in the range of from 1 -20 pm may be used, more preferably in the range of from 1 - ⁇ 15 pm.
  • the particle size is regarded as being the length of the longest axis of the pigments.
  • the particle size can in principle be determined using any method for particle-size determination that is familiar to the person skilled in the art.
  • the particle size determina- tion can be carried out in a simple manner, depending on the size of the laser sensitive absorber particles, for example by direct observation and measurement of a number of individual particles in high-resolution light microscopes, but better in electron microscopes, such as the scanning electron microscope (SEM) or the high-resolution electron microscope (HRTEM), but also in the atomic force microscope (AFM), the latter in each case with appropriate image analysis software.
  • SEM scanning electron microscope
  • HRTEM high-resolution electron microscope
  • AFM atomic force microscope
  • the determination of the particle size can advantageously also be carried out using measuring instruments (for example Malvern Mastersizer 3000, APA300, Malvern Instruments Ltd., UK), which operate on the principle of laser diffraction.
  • both the particle size and also the particle-size distribution in the volume can be determined from a pigment suspension in a standard method (SOP).
  • SOP standard method
  • the last-mentioned measurement method is preferred in accordance with the present invention.
  • the micron-size particles containing metal oxides exhibit preferably an isotropic shape such as a granular or spherical shape.
  • granular comprises regular as well as irregular granules
  • spherical comprises regular spheres as well as spheroid shapes.
  • the micron-sized particles comprising metal oxides which are used as absorber particles in the printing ink layer on the laser transparent donor substrate are either composed of the respective metal oxides in total or comprise them to an extent of at least 50% by weight, preferably at least 70% by weight, based on the weight of the respective particles.
  • the corresponding metal oxide(s) may be present on a carrier particle or may be present as a mixture with other ingredients.
  • the micron-sized particles consist of the respective metal oxide(s), which means that at least 99% by weight, based on the weight of the micron-sized particles, is composed of the respective metal oxide(s).
  • the metal oxides contained in the micron-sized particles are electrically conductive or semiconducting metal oxides, either as single compounds or in a mixture or mixed oxide of two or more thereof.
  • Suitable materials for the electrically conductive or semiconducting metal oxides are, in particular, doped metal oxides.
  • the metal oxides are preferably tin oxide and/or titanium oxide.
  • the said metal oxides are present in a doped form in the micron-sized absorber particles, where the doping may take place with gallium, aluminium, indium, thallium,
  • doping element consists of metal atoms it goes without saying that main metal oxide component and doping element do not comprise the same metal.
  • Individual dopants of those mentioned, but also combinations thereof, may be present in the conductive or semiconducting metal oxide. Aluminium, indium, tellurium, fluorine, tungsten, niobium, tin and/or antimony are preferably employed for doping the metal oxides.
  • the proportion of dopants in the metal oxides can be 0.1 to 30% by weight, and is preferably in the range from 2 to 15% by weight.
  • the conductive or semiconducting metal oxide employed is antimony-doped tin oxide, antimony- and tellurium-doped tin oxide, tungsten-doped tin oxide, fluorine-doped tin oxide or niobium-doped titanium dioxide, where antimony-doped tin oxide and niobium doped titanium dioxide is most preferred.
  • the tin to antimony ratio in this preferred combination can be 4:1 to 100:1 , the ratio preferably being 8:1 to 50:1. Lower antimony contents adversely affect the conductivity, whereas higher antimony contents increase the absorption by the pigments in the visible wavelength region and reduce the transparency of the micron-sized absorber particles.
  • the percent molar proportion of niobium is advantageously in the range of from 0.05 to 15%, based on the molar mass of titanium, and in particular from 0.1 to 10%.
  • titanium dioxide with tiny oxygen deficits may be used as well as electrically semiconducting metal oxide, i.e. T1O2- X with 0.001 ⁇ x ⁇ 0.05.
  • the electrically conductive metal oxides may be present in the respective particles in combination with one or more further, colorless, dielectric compounds to the weight percentage mentioned above (i.e. at most 50% by weight, in particular at most 30% by weight, based on the weight of the respective particles).
  • the particle size and the material composition of the respective micron-sized particles containing metal oxides is adjusted in such a way that the resulting particles appear colorless or close to colorless as a powder or in a coating application.
  • the smallest possible particle size still allowing the desired laser absorption level shall be chosen.
  • micron-sized particles comprising the respective metal oxides according to the present invention, when combined in a printing ink coating with coloring means such as colored pigments or colored dyes or with functional materials such as electrically conductive polymers which are per se transferable when applied in a usual LIFT process using a dynamic release layer, but are not in a condition to absorb enough laser energy in order to be transferred at exposure to laser irradiation on their own, heavily support the vaporization of the said compounds to such an extent that, in case of colored pigments or colored dyes, chroma and color intensity achievable in the printing spots on the receiving substrate rise significantly due to a large transferred amount of these coloring means.
  • the transfer of functional materials is available which would otherwise not be transferable without dynamic release layer (DRL) on their own.
  • DRL dynamic release layer
  • the laser absorbing micron-sized particles used according to the present invention are transferred together with the coloring or functional compound and appear colorless or almost colorless on the receiving substrate. Therefore, the viewer observes, in the case of coloring materials, merely the highly chromatic printing spot which is colored in the color as expected from the respective coloring means, but exhibits a much higher chroma in comparison to a common LIFT process without DRL where only the respective coloring means is present in the printing ink on the donor substrate.
  • the concentration of the micron-sized particles containing metal oxides in the printing ink is in the range of from 0.25 to 20 % by weight, based on the weight of the printing ink.
  • the concentration of the respective coloring or functional compound in the printing ink is in the range of from 5 to 20% by weight, based on the weight of the printing ink.
  • the concentration of the coloring compound determines the maximum possible chroma on the resulting printed spot on the receiving substrate and is, therefore, chosen according to the respective require- ments. It goes without saying that a concentration of a dye of 5% by weight exhibiting a red absorption color leads to a reddish printed spot only, whereas the same dye in a concentration of 10% by weight may lead to a highly chromatic red printed spot. Concentrations of more than 20% by weight are not of advantage.
  • coloring compound that may be used in combination with the micron-sized metal oxide comprising particles in the present process
  • all coloring pigments or dyes which may be used in a usual LIFT process using a dynamic release layer are appropriate.
  • the coloring compounds absorb any emitted laser wavelength at all.
  • the absorption of laser light emitted in the IR-region, especially at 1064 nm is not necessary.
  • the micron-sized particles which preferably contain at least one electrically conductive or semiconducting metal oxide are transferred together with the coloring compounds to the receiving substrate without difficulties.
  • micron-sized absorber particles according to the present invention are hardly observable in the resulting printed spot since they are colorless or almost colorless (i.e. they do not or almost not absorb light in the visible wavelength range).
  • functional compounds such as elec- trically conductive polymers which do not absorb in a commonly used laser emitted wavelength, e.g. in the IR-region of 1064 nm, may also be transferred from a printing ink layer to a receiving substrate without the presence of any dynamic release layer, if the printing ink layer contains the micron-sized metal oxide containing absorber particles of the present invention in combination with the respective functional compounds and a vaporizable compound.
  • the printing ink which is coated onto the front surface of the donor substrate may contain all ingredients commonly used in usual printing inks, namely binders, solvents,
  • antifoaming agents surface active ingredients, anti-sagging agents, dispersing agents, levelling agents, coupling agents, corrosion inhibitors, rheology modifiers, fire redundants, stabilizers, catalysts or masking agents.
  • the viscosity of the printing ink has to be adapted to the kind of the donor substrate and the coating process which is used for coating the donor substrate front surface with the printing ink.
  • the adjustment of the viscosity of the printing ink does also play a role with respect to the vaporization of the fluids in the present LIFT process.
  • the coating layer formed by the printing ink on the front surface of the donor substrate exhibits a thickness in the range of from 20 to 90 pm, especially in the range of from 30 to 80 pm, and in particular of from 40 to 70 pm.
  • the front face of the donor substrate is coated with the printing ink containing the micron-sized metal oxide containing absorber particles and at least one component which is capable of rapidly enlarging its volume upon exposure to laser- generated energy, which is in most cases a solvent in the printing ink or the binder component or parts thereof.
  • the coloring or functional component may or may not be in a position to enlarge its volume upon laser irradiation. At least, some of the ingredients of the printing ink convert to fluids upon transfer of the laser energy.
  • the back surface of the donor substrate facing away from the front face is irradiated by laser energy of a particular wavelength at a certain point of the back surface, whereupon the laser energy is transferred through the laser-transparent donor substrate and absorbed by the absorber particles in the ink being located at the area on the inked surface where the laser beam hits the donor substrate at the back surface.
  • the absorber particles transfer the laser energy (or at least parts thereof) to the component being capable of rapidly enlarging its volume and a volume expansion of a certain amount of this component surrounding the absorber particles takes place, so that small droplets containing a certain volume of the liquid printing ink components and some micron-sized absorber particles are detached from the front face of the donor substrate.
  • the receiving substrate of the printing apparatus Adjacent to the printed front face of the donor substrate, but not in physical contact therewith, the receiving substrate of the printing apparatus is arranged.
  • the gap between the upper side of the printing ink coating and the surface of the receiving substrate is as narrow as possible and is in the range of from 1 to 100 pm, preferably in the range of from 5 to 20 pm.
  • the droplets being detached from the front face of the donor substrate hit the receiving substrate, forming a printed spot here.
  • the printed spot contains the solidified fluids as well as the micron-sized absorber particles.
  • a printed image may be received on the receiving substrate which is composed of a plurality of the printed spots described above.
  • a colored printed image or a printed image exhibiting a particular function such as electrical conductivity may be provided on the receiving substrate.
  • the color of the printed image if observable, corres- ponds to the color of the coloring component contained in the printing ink coating layer, whereas the color of the micron-sized absorber particles, if any, is hardly observable at the printed image.
  • the front face of the donor substrate is fully covered by the printing ink and the printing ink coating layer exhibits the same physical thickness at each point of the front face of the donor substrate.
  • the donor substrate is composed of a material which is highly transparent at least to the laser wavelength emitted by the laser source. Such a substrate will be named“transparent” or“laser transparent” in the following.
  • the donor substrate may be composed of glass, quartz or any synthetic material, e.g. any polymeric material, fulfilling the said requirement. Often, these materials are transparent to visible light too.
  • the donor substrate may be in form of a plate, a sheet or a flexible film or ribbon and may be arranged on or around a printing plate or printing cylinder or be part of any other printing assembly known in the art.
  • the receiving substrate may be composed of several materials and may be in form of a plate, a sheet, a flexible film or a compact shaped body, as the case may be. Contrary to the donor substrate, the receiving substrate may be composed of paper, wall paper, metal, glass, wood, stone, ceramic materials, polymer materials etc.
  • the receiving substrate is not necessarily transparent. To the contrary, it is of advantage if the receiving substrate is semi-transparent or even opaque and may be colored as well. The diminished transparency of the receiving substrate as well as a color, if present, may enlarge the visibility of the coloring effects of the coloring components or of the functional components contained in the printed spots on the receiving substrate.
  • substrates colored in background colors such as white, red, blue, green etc.
  • even deep grey or black colored substrates may be of advantage.
  • the receiving substrate is coated with a colored primer layer or is intrinsically colored by color pigments distributed in the receiving color surface material is not of importance.
  • Nd:YAG lasers neodymium doped yttrium aluminium garnet lasers
  • YV04 lasers yttrium vanadate lasers
  • 1064 nm fibre lasers are preferably those which emit light of
  • a Nd:YAG laser emitting at 1064 nm is the most preferred laser apparatus type in the process according to the present invention.
  • the laser is a pulsed near infrared laser.
  • the fibre laser, the Nd:YAG laser and the YV04 laser mentioned above belong to this class of lasers.
  • the laser shall be pulsed with a pulse duration ranging from nano to femto seconds.
  • Corresponding lasers which can be used in the process according to the invention are commercially available. It goes without saying that the lasers are advantageously steered by computer software programs.
  • the laser energy provided by the laser source must be strong and focussed enough in order to being able to provide the requested energy from the back side of the donor substrate, passing the donor substrate and being capable to be absorbed by the absorber particles in the printing ink containing layer on the inked surface of the donor substrate.
  • the laser energy provided by the LIFT printing apparatuses which are available in the market already is sufficient in order to fulfil the requirements of the present process. Therefore, established printing apparatuses using the LIFT process according to the first variant (without DRL) may be used in a standard manner for the process according to the present invention as well.
  • the laser energy absorbed by the absorber particles used in the process of the present invention is strong enough in order to transfer energy to the vaporizable components contained in the printing ink to be readily evaporized in the contact zone of the laser beam, and the transferred laser energy is also strong enough for detaching the absorber particles themselves from the inked donor substrate surface and being transferred together with the vaporized and fluidized printing ink
  • the present invention does also relate to the use of micron-sized particles comprising metal oxides in a laser-induced forward transfer process as absorber particles in an ink coating on a donor substrate.
  • the metal oxide comprised in the micron-sized absorber particles is preferably an electrically conductive or semiconducting metal oxide, mixed metal oxide or metal oxide mixture and is advantageously composed of doped metal oxides or of T1O2- X as described in detail before.
  • the present invention does also relate to a product exhibiting a printed image on a substrate, wherein the printed image is composed of printed spots produced by the method as described before.
  • the printed image is composed of only one or of a plurality of printed spots, whereby the printed spots contain at least the micron-sized absorber particles containing metal oxides and the solidified compounds obtained by vaporization and solidification of the vaporizable and fluidizable compounds of the printing ink on the receiving substrate.
  • the printed spots contain colorless or almost colorless micron-sized metal oxide containing absorber particles as described before as well as the amount of the coloring or functional compounds which were contained in the printing ink coating on the donor substrate prior to the transfer.
  • the micron-sized absorber particles act as transfer means for the coloring compounds or for the functional material in the printing ink. They enhance the transferred amount of the latter or even enable the transfer, so that color intensity and chroma of the colors visible in the printed spots rises to a great extent or the transfer of functional materials becomes possible at all. Even a small amount of merely 0.25% by weight content of the respective micron-sized absorber particles in the printing ink leads to a distinct improvement of the optical characteristics of the corresponding printed spots or allows the transfer of functional materials without the need of any dynamic release layer.
  • the printed spots exhibit in most cases the shape of a regular or irregular dot having some micrometers diameter each. Regularly, a plurality of these spots forms the respective printed image, like in usual printing processes.
  • the shape of the printed image does not play any role in the present invention and might be any desired shape, ranging from figures, numbers, lines, regular or irregular patterns to all kinds of motifs being printable with common printing processes.
  • printing of fine lines of merely some micrometers line width as well as of more complex patterns is possible.
  • the substrate being part of the product of the present invention is advantageously selected from the group consisting of paper, wall paper, glass, plastic films, plastic bodies, metal films, metal bodies, ceramic bodies and sheets or bodies being composed of at least two different of these materials.
  • all substrates capable of being printed by a common LIFT process may be used here as well.
  • the substrate being printed with the printed image corresponds to the receiving substrate in the printing process explained above. It may be printed while being part of the product already (e.g. for packaging materials) or may be assembled with different parts of the products at any time after the printing process is executed (e.g. a printed insert of an automotive part).
  • the respective product is a decorative element or a functional element in a commercial printing product, a part of a vehicle, a part of a plane, a part of a train, an architectural part, a consumer electronics product, a solar cell component, a packaging material, a security product, a textile product or a leather product.
  • a commercial printing product is to mean e.g. a magazine, a flyer, a poster, a brochure, a newspaper, a furniture decorative paper, a floor covering, a wall covering, a gift wrapping paper, a carrier bag or a decorative foil, to name only a few.
  • the present printing process is a digital printing process allowing printing inks containing coloring pigments, coloring dyes or functional materials such as electrically conductive polymers to be printed in a common LIFT process without any dynamic release layer in an industrial scale.
  • These coloring or functional materials not being capable to absorb enough laser energy to being transferable in a common LIFT process not using a DRL may, thus, now transferred in such a process without the technical and costly efforts which are necessary when a DRL is necessary. Therefore, the present process provides a convenient and easily available digital printing method for several optical and/or functional applications.
  • Fig .1 shows the working mechanism of the process according to the
  • present invention in a general manner, representing: (1 ) the laser beam, (2) the donor substrate, (3) the printing ink coating layer, (4) the gap between printing ink coating layer surface and receiving substrate surface and (5) the receiving substrate
  • Fig.2 shows the test scheme for finding the best laser mode of action
  • Fig.3 shows the test pattern applied to the printing tests of all kinds of colored dyes used in the examples according to the present invention
  • Fig. 4 shows the result of a printed test pattern using a red dye on a white colored receiving substrate according to example 1
  • a printing ink comprising 10 % of a colored compound [Folco FS-10 218 (red) or Folco FS-10 120 (yellow)], 0-20% of a micron-sized absorber particle according to the present invention (Iriotec® 9230, > 99% Sb doped Sn0 2 , particle size ⁇ 10 pm (90%), product of Merck KGaA), 1.6% of an antifoaming agent (Follmann Entschaumer 5280), 1.2% of a wetting agent (Follmann Netzsch 5293) and a variable amount of a commercially available printing ink binder, adding to 100% in each case (Follmann FS-10 931 ) is prepared by mixing the ingredients.
  • a colored compound [Folco FS-10 218 (red) or Folco FS-10 120 (yellow)
  • a micron-sized absorber particle according to the present invention Iriotec® 9230, > 99% Sb doped Sn0 2 , particle size ⁇ 10 pm
  • a glass plate having a thickness of about 1 mm is fully coated with the printing ink using a silk-screen printing process on a surface area of 100x70 mm.
  • the thickness of the printing ink layer is about 45 pm.
  • the glass plate coated with the printing ink is arranged onto a white colored paper sheet (Papyrus LuxoSatin 250 g/m 2 white) with metal shims of 50 pm, forming a 5 pm gap between the surface of the printing ink and the receiving paper surface.
  • a white colored paper sheet Paperyrus LuxoSatin 250 g/m 2 white
  • metal shims 50 pm, forming a 5 pm gap between the surface of the printing ink and the receiving paper surface.
  • Each printing ink composition is printed with two different patterns (*. bmp- format, 95x65 mm, 300 ppi).
  • the corresponding laser (Trumpf VectorMark VMC-5) is used in several different frequency and power modes, see Fig. 2
  • the laser mode exhibiting the best result is used for printing a second pattern according to Fig. 3. According to the latter, line width and accuracy of patterned shapes including text fields may be easily observed.
  • the relative content of the micron-sized metal oxide comprising absorber particle is chosen to be 0%, 0.25%, 1 %, 5%, 10% and 20%.
  • the number of test fields in Fig. 2 which are filled with the red or yellow color is more than doubled in comparison to the concentration of 0% absorber.
  • concentration in the limits as described above (0.25 to 20%)
  • the number of test fields which are fully covered with colors rises continuously, while chroma and distinctness of the test fields rise over a broad laser parameter range.

Abstract

The present invention relates to a method for transferring a printing ink from a donor substrate to a receiving substrate by a laser-induced forward transfer process, wherein the printing ink contains metal oxide containing particles, to the use of those metal oxide containing particles as laser absorber particles in such a process as well as to a product exhibiting a printed image on a substrate, obtained by said process.

Description

Method for laser-induced forward transfer using metal oxide absorber particles
The present invention relates to a method for transferring a printing ink from a donor substrate to a receiving substrate by a laser-induced forward transfer process, wherein the printing ink contains metal oxide containing particles, to the use of those metal oxide containing particles as laser absorber particles in such a process as well as to a product exhibiting a printed image on a substrate, obtained by said process.
The laser-induced forward transfer (LIFT) process is a direct-write process which has particular advantages when compared to traditional printing processes such as silk-screen printing processes or gravure printing processes. Contrary to the latter, the laser-induced forward transfer process, similar to an inkjet printing process, allows versatile use without expensive equipment and, in particular, personalized adaptations of the printing motif are easily available. In addition, improvements in printing speed, scale and resolution of the printing process and product are highly welcome.
So far, LIFT processes have been used in particular for the production of electronic, optical and sensor elements, especially for microelectronic components such as antennas, sensors and embedded circuits, but also for transferring biological materials from one substrate to another.
The LIFT process may be performed in several variants.
In a first variant, a printing ink layer containing laser absorbing particles is applied onto a surface of a laser transparent substrate. The transparent substrate (the donor substrate) is then irradiated by a laser beam from the reverse side which does not carry the printing ink. The incident laser beam propagates through the transparent carrier before the light is absorbed by the back surface of the printing ink layer. Above a specific threshold of the incoming laser energy, the printing ink is ejected in form of a droplet from the coated surface of the laser transparent substrate and catapulted towards a receiving substrate that is arranged in close proximity to the inked donor substrate surface. The energy conversion process causing the ink ejection as well as the phase transitions involved in the LIFT process is complex and affected by a large number of diverse parameters. Since the absorber particles are contained in the printing ink, these absorber particles absorb laser energy as well and are transferred to the receiving substrate too in a certain amount. By this process, a printed ink spot is available at the receiving substrate, containing at least the solidified components of the printing ink droplet containing a certain amount of the absorber particles. Usually, nano-sized carbon black particles are used in the first variant as absorber particles.
A technically useful process and apparatus to perform the LIFT process according to the first variant is disclosed in EP 1 485 255 B1.
In a second variant, no absorber particles are contained in the printing ink, but a separate absorbing layer is arranged between the transparent donor substrate and the printing ink layer. The separate layer, which is called a dynamic release layer (DRL), is usually composed of the absorber in total and does often consist of a metal layer evaporated onto the transparent donor substrate, of a layer of a polymer component that readily vaporizes upon exposure to laser irradiation, or of carbon black. In this process variant, the laser beam may be directed either through the transparent donor substrate as in variant 1 , or may be directed to the absorber layer from the printing ink coated side of the donor substrate in an acute angle relative to the inked substrate surface. The components of the printing ink, whether in form of particles and/or of a readily vaporizable material, are transferred to the receiving substrate upon exposure of the absorbing layer to the laser beam. In this case, none of the components in the printing ink need to absorb laser light and, therefore, the choice of the ink components is fundamentally unlimited. However, this method has also a major disad- vantage, which is transfer of traces of the absorber material from the DRL to the receiving substrate together with the printing ink. This leads to unwanted material or optical effects in the resulting printed image on the receiving substrate. In addition, a partly destroyed or worn-out absorber layer has to be removed from the donor substrate as well as the printing ink layer when a new printing cycle run has to be started, leading to additional material and processual efforts.
A technically useful process and apparatus to perform the LIFT process according to the second variant is disclosed in EP 1 268 211 B1.
In case that absorber particles are used as in the first variant of the LIFT process as described above, particles in a submicron particle size range have heretofore been used. For such particle sizes, even the transfer of metallic particles like silver particles, copper particles or gold particles is possible, which are mostly used for integrated circuits or other electronic applications.
Regarding the transfer of printing inks containing coloring means such as colored organic pigments or dyes, the first variant of the LIFT process was not available up to now except for carbon black containing printing inks, since a lot of these pigments or dyes, especially in brighter colors, do hardly or not at all absorb laser light in frequently used wavelengths. Therefore, in order to transmit these compounds via a LIFT process, the second variant had to be used so far.
Nevertheless, from the description of the two variants it becomes clear at the first glance that the first variant is technically more attractive than the second due to less technical efforts and cost. Thus, it would be of advan- tage if the first variant would be available to the transfer of coloring pig ments or dyes which do hardly or even not absorb in the wavelength emitted by the corresponding laser apparatus. In addition, the same applies to the transfer of functional materials such as electrically conductive polymers which do not absorb in the respective laser wavelength emitted.
The object of the present invention is to provide a LIFT process without using a dynamic release layer for the transfer of coloring organic pigments or dyes or of functional materials such as electrically conductive polymers which do not or merely slightly absorb laser light in the wavelength emitted by the corresponding laser apparatus, where the process may be executed in a simple manner at low cost.
Furthermore, the object of the present invention is to provide a particular use for common pigments.
In addition, a further object of the present invention is to provide a product comprising a printed image on a substrate, where the printed image exhibits an intense non-black color in high resolution or a particular function and is printed in a LIFT process without the need of a dynamic release layer.
The object of the present invention is solved by a method for transferring a printing ink from a laser transparent donor substrate to a receiving substrate by a laser-induced forward transfer process, whereby
- the donor substrate exhibits a front surface coated with the printing ink and a back surface facing away from the front surface;
- the printing ink forms a coating which has an upper side being
located adjacent to, but not in physical contact with the receiving substrate; - the printing ink contains absorber particles and at least one component being capable of enlarging its volume upon exposure to laser-generated energy;
- the donor substrate is irradiated by laser energy of a particular
wavelength at the back surface, whereupon the laser energy is absorbed by absorber particles in the ink and a volume expansion of the at least one component surrounding the absorber particles is caused, and
- said component is transferred to the receiving substrate along with the absorber particles, leading to a printed spot on the receiving substrate, wherein
- the absorber particles are micron-sized particles comprising metal oxides; and
- the printing ink comprises a coloring or functional compound.
Furthermore, the object of the present invention is solved by the use of micron-sized particles comprising metal oxides in a laser-induced forward transfer process as absorber particles in an ink coating on a donor substrate.
In addition, the object of the present invention is also solved by a product comprising a printed image on a substrate, wherein the printed image is composed of printed spots containing micron-sized particles comprising metal oxides and a coloring or functional compound, the printed spots being produced by the method according to the process mentioned above.
The printing method according to the present invention is based on a standard laser-induced forward printing process (LIFT) according to the first variant as described before. In this process, a laser transparent donor substrate is coated on one of its major surfaces with a printing ink which is subject to being transferred to a receiving substrate in form of a printed image composed of printed spots. Surprisingly, the present inventors revealed that if micron-sized particles comprising metal oxides are used as absorber particles in the printing ink, these micron-sized particles are transferred together with all other ingredients of the printing ink to a receiving substrate even if the printing ink contains colored organic pigments or dyes or functional materials which do, by themselves, not or merely slightly absorb the wavelength emitted by the corresponding laser.
In particular, the particle size of the micron-sized particles comprising metal oxides is in the range of from 1 -50 pm. The given range is the broadest range of the nominal particle size of the micron-sized particles.
Advantageously, particle sizes in the range of from 1 -20 pm may be used, more preferably in the range of from 1 - <15 pm. For the purposes of the present invention, the particle size is regarded as being the length of the longest axis of the pigments. The particle size can in principle be determined using any method for particle-size determination that is familiar to the person skilled in the art. The particle size determina- tion can be carried out in a simple manner, depending on the size of the laser sensitive absorber particles, for example by direct observation and measurement of a number of individual particles in high-resolution light microscopes, but better in electron microscopes, such as the scanning electron microscope (SEM) or the high-resolution electron microscope (HRTEM), but also in the atomic force microscope (AFM), the latter in each case with appropriate image analysis software. The determination of the particle size can advantageously also be carried out using measuring instruments (for example Malvern Mastersizer 3000, APA300, Malvern Instruments Ltd., UK), which operate on the principle of laser diffraction. Using these measuring instruments, both the particle size and also the particle-size distribution in the volume can be determined from a pigment suspension in a standard method (SOP). The last-mentioned measurement method is preferred in accordance with the present invention. The micron-size particles containing metal oxides exhibit preferably an isotropic shape such as a granular or spherical shape. Of course, the term “granular” comprises regular as well as irregular granules, whereas the term“spherical” comprises regular spheres as well as spheroid shapes.
According to the present invention, the micron-sized particles comprising metal oxides which are used as absorber particles in the printing ink layer on the laser transparent donor substrate are either composed of the respective metal oxides in total or comprise them to an extent of at least 50% by weight, preferably at least 70% by weight, based on the weight of the respective particles. In the latter case, the corresponding metal oxide(s) may be present on a carrier particle or may be present as a mixture with other ingredients. Advantageously, the micron-sized particles consist of the respective metal oxide(s), which means that at least 99% by weight, based on the weight of the micron-sized particles, is composed of the respective metal oxide(s).
Preferably, the metal oxides contained in the micron-sized particles are electrically conductive or semiconducting metal oxides, either as single compounds or in a mixture or mixed oxide of two or more thereof.
Suitable materials for the electrically conductive or semiconducting metal oxides are, in particular, doped metal oxides. The metal oxides are preferably tin oxide and/or titanium oxide. The said metal oxides are present in a doped form in the micron-sized absorber particles, where the doping may take place with gallium, aluminium, indium, thallium,
germanium, niobium, tin, phosphorus, arsenic, antimony, selenium, tellurium, molybdenum, tungsten and/or fluorine. In case the doping element consists of metal atoms it goes without saying that main metal oxide component and doping element do not comprise the same metal. Individual dopants of those mentioned, but also combinations thereof, may be present in the conductive or semiconducting metal oxide. Aluminium, indium, tellurium, fluorine, tungsten, niobium, tin and/or antimony are preferably employed for doping the metal oxides. The proportion of dopants in the metal oxides can be 0.1 to 30% by weight, and is preferably in the range from 2 to 15% by weight. In a particularly preferred embodiment, the conductive or semiconducting metal oxide employed is antimony-doped tin oxide, antimony- and tellurium-doped tin oxide, tungsten-doped tin oxide, fluorine-doped tin oxide or niobium-doped titanium dioxide, where antimony-doped tin oxide and niobium doped titanium dioxide is most preferred. The tin to antimony ratio in this preferred combination can be 4:1 to 100:1 , the ratio preferably being 8:1 to 50:1. Lower antimony contents adversely affect the conductivity, whereas higher antimony contents increase the absorption by the pigments in the visible wavelength region and reduce the transparency of the micron-sized absorber particles.
Regarding niobium-doped titanium dioxide, the percent molar proportion of niobium is advantageously in the range of from 0.05 to 15%, based on the molar mass of titanium, and in particular from 0.1 to 10%.
Furthermore, titanium dioxide with tiny oxygen deficits may be used as well as electrically semiconducting metal oxide, i.e. T1O2-X with 0.001 <x< 0.05.
The electrically conductive metal oxides may be present in the respective particles in combination with one or more further, colorless, dielectric compounds to the weight percentage mentioned above (i.e. at most 50% by weight, in particular at most 30% by weight, based on the weight of the respective particles).
Preferably, the particle size and the material composition of the respective micron-sized particles containing metal oxides is adjusted in such a way that the resulting particles appear colorless or close to colorless as a powder or in a coating application. To this end, the smallest possible particle size still allowing the desired laser absorption level shall be chosen. Surprisingly, it has been found that micron-sized particles comprising the respective metal oxides according to the present invention, when combined in a printing ink coating with coloring means such as colored pigments or colored dyes or with functional materials such as electrically conductive polymers which are per se transferable when applied in a usual LIFT process using a dynamic release layer, but are not in a condition to absorb enough laser energy in order to be transferred at exposure to laser irradiation on their own, heavily support the vaporization of the said compounds to such an extent that, in case of colored pigments or colored dyes, chroma and color intensity achievable in the printing spots on the receiving substrate rise significantly due to a large transferred amount of these coloring means. In all other cases, the transfer of functional materials is available which would otherwise not be transferable without dynamic release layer (DRL) on their own. The laser absorbing micron-sized particles used according to the present invention are transferred together with the coloring or functional compound and appear colorless or almost colorless on the receiving substrate. Therefore, the viewer observes, in the case of coloring materials, merely the highly chromatic printing spot which is colored in the color as expected from the respective coloring means, but exhibits a much higher chroma in comparison to a common LIFT process without DRL where only the respective coloring means is present in the printing ink on the donor substrate. The concentration of the micron-sized particles containing metal oxides in the printing ink is in the range of from 0.25 to 20 % by weight, based on the weight of the printing ink. Even at a low concentration of about 0.25% by weight, the positive influence of the micron-sized particles as absorber particles is observable, leading to a printing result exhibiting a much better chroma than without the absorber particles. Depending on an enlarging concentration of the absorber particles, the chroma of the resulting printed image rises constantly. Beyond the addition of about 20% by weight of the absorber particles, no further improvement of the printed result may be observed, since a higher content of absorber particles diminishes the maximum possible content of the coloring compound. The concentration of the respective coloring or functional compound in the printing ink is in the range of from 5 to 20% by weight, based on the weight of the printing ink. The concentration of the coloring compound, irrespective whether a coloring organic pigment or coloring dye is used, determines the maximum possible chroma on the resulting printed spot on the receiving substrate and is, therefore, chosen according to the respective require- ments. It goes without saying that a concentration of a dye of 5% by weight exhibiting a red absorption color leads to a reddish printed spot only, whereas the same dye in a concentration of 10% by weight may lead to a highly chromatic red printed spot. Concentrations of more than 20% by weight are not of advantage.
Pertaining to the coloring compound that may be used in combination with the micron-sized metal oxide comprising particles in the present process, all coloring pigments or dyes which may be used in a usual LIFT process using a dynamic release layer are appropriate. This belongs to organic pigments and dyes which are available in the market in great variations. No limitations due to the present process occur. In particular, there is no need that the coloring compounds absorb any emitted laser wavelength at all. In particular, the absorption of laser light emitted in the IR-region, especially at 1064 nm, is not necessary. The micron-sized particles which preferably contain at least one electrically conductive or semiconducting metal oxide are transferred together with the coloring compounds to the receiving substrate without difficulties. The micron-sized absorber particles according to the present invention are hardly observable in the resulting printed spot since they are colorless or almost colorless (i.e. they do not or almost not absorb light in the visible wavelength range). Similar to the coloring compounds, functional compounds such as elec- trically conductive polymers which do not absorb in a commonly used laser emitted wavelength, e.g. in the IR-region of 1064 nm, may also be transferred from a printing ink layer to a receiving substrate without the presence of any dynamic release layer, if the printing ink layer contains the micron-sized metal oxide containing absorber particles of the present invention in combination with the respective functional compounds and a vaporizable compound. Besides the micron-sized absorber particles comprising metal oxides and the coloring or functional compounds, the printing ink which is coated onto the front surface of the donor substrate may contain all ingredients commonly used in usual printing inks, namely binders, solvents,
antifoaming agents, surface active ingredients, anti-sagging agents, dispersing agents, levelling agents, coupling agents, corrosion inhibitors, rheology modifiers, fire redundants, stabilizers, catalysts or masking agents.
Except of the content of the absorber particles and of the coloring or functional compounds, commonly used printing ink vehicles which are available in the market in great variety may be used for the process of the present invention. Of course, the viscosity of the printing ink has to be adapted to the kind of the donor substrate and the coating process which is used for coating the donor substrate front surface with the printing ink. In addition, the adjustment of the viscosity of the printing ink does also play a role with respect to the vaporization of the fluids in the present LIFT process.
Regarding the coating process of the donor substrate surface, all commonly used coating or printing processes may be used which are known from the prior art to be capable to successfully coat the respective substrate surface with the printing ink in the desired thickness. Therefore, the skilled person may choose the coating or printing process which seems to fit best to the technical appliances used in the present process according to his or her common skills. The coating layer formed by the printing ink on the front surface of the donor substrate exhibits a thickness in the range of from 20 to 90 pm, especially in the range of from 30 to 80 pm, and in particular of from 40 to 70 pm. As shortly described above, in the present process, the front face of the donor substrate is coated with the printing ink containing the micron-sized metal oxide containing absorber particles and at least one component which is capable of rapidly enlarging its volume upon exposure to laser- generated energy, which is in most cases a solvent in the printing ink or the binder component or parts thereof. The coloring or functional component may or may not be in a position to enlarge its volume upon laser irradiation. At least, some of the ingredients of the printing ink convert to fluids upon transfer of the laser energy. The back surface of the donor substrate facing away from the front face is irradiated by laser energy of a particular wavelength at a certain point of the back surface, whereupon the laser energy is transferred through the laser-transparent donor substrate and absorbed by the absorber particles in the ink being located at the area on the inked surface where the laser beam hits the donor substrate at the back surface. The absorber particles transfer the laser energy (or at least parts thereof) to the component being capable of rapidly enlarging its volume and a volume expansion of a certain amount of this component surrounding the absorber particles takes place, so that small droplets containing a certain volume of the liquid printing ink components and some micron-sized absorber particles are detached from the front face of the donor substrate.
Adjacent to the printed front face of the donor substrate, but not in physical contact therewith, the receiving substrate of the printing apparatus is arranged. According to the present invention, the gap between the upper side of the printing ink coating and the surface of the receiving substrate is as narrow as possible and is in the range of from 1 to 100 pm, preferably in the range of from 5 to 20 pm. The droplets being detached from the front face of the donor substrate hit the receiving substrate, forming a printed spot here. The printed spot contains the solidified fluids as well as the micron-sized absorber particles. Correspondingly, when the process des- cribed before is repeated again and again by moving the laser beam and/or the position of the surface area hit by the laser beam and by, optionally, altering the laser parameters too, a printed image may be received on the receiving substrate which is composed of a plurality of the printed spots described above.
In this way, a colored printed image or a printed image exhibiting a particular function such as electrical conductivity may be provided on the receiving substrate. The color of the printed image, if observable, corres- ponds to the color of the coloring component contained in the printing ink coating layer, whereas the color of the micron-sized absorber particles, if any, is hardly observable at the printed image.
Advantageously, the front face of the donor substrate is fully covered by the printing ink and the printing ink coating layer exhibits the same physical thickness at each point of the front face of the donor substrate. The donor substrate is composed of a material which is highly transparent at least to the laser wavelength emitted by the laser source. Such a substrate will be named“transparent” or“laser transparent” in the following. The donor substrate may be composed of glass, quartz or any synthetic material, e.g. any polymeric material, fulfilling the said requirement. Often, these materials are transparent to visible light too. The donor substrate may be in form of a plate, a sheet or a flexible film or ribbon and may be arranged on or around a printing plate or printing cylinder or be part of any other printing assembly known in the art.
Like the donor substrate, the receiving substrate may be composed of several materials and may be in form of a plate, a sheet, a flexible film or a compact shaped body, as the case may be. Contrary to the donor substrate, the receiving substrate may be composed of paper, wall paper, metal, glass, wood, stone, ceramic materials, polymer materials etc. The receiving substrate is not necessarily transparent. To the contrary, it is of advantage if the receiving substrate is semi-transparent or even opaque and may be colored as well. The diminished transparency of the receiving substrate as well as a color, if present, may enlarge the visibility of the coloring effects of the coloring components or of the functional components contained in the printed spots on the receiving substrate. To this end, besides substrates colored in background colors such as white, red, blue, green etc., even deep grey or black colored substrates may be of advantage. Whether the receiving substrate is coated with a colored primer layer or is intrinsically colored by color pigments distributed in the receiving color surface material is not of importance.
With respect to the type of laser which may be used in the present process, it is of high advantage that the most common laser wavelength of 1064 nm may be used in the present process. Nd:YAG lasers (neodymium doped yttrium aluminium garnet lasers), YV04 lasers (yttrium vanadate lasers) and 1064 nm fibre lasers are preferably those which emit light of
wavelength 1064 nm. In particular, a Nd:YAG laser emitting at 1064 nm is the most preferred laser apparatus type in the process according to the present invention. Most preferably the laser is a pulsed near infrared laser. The fibre laser, the Nd:YAG laser and the YV04 laser mentioned above belong to this class of lasers. The laser shall be pulsed with a pulse duration ranging from nano to femto seconds. Corresponding lasers which can be used in the process according to the invention are commercially available. It goes without saying that the lasers are advantageously steered by computer software programs.
Of course, it is also possible to use other common layer types if the wave- length of the laser light emitted by the laser apparatus and the wavelength being absorbed by the non-metallic flaky effect pigments can be adapted to each other. Corresponding tests may be easily executed by the skilled person. Laser frequency and power have to be adapted to the corresponding printing ink ingredients and kind of receiving substrates. Such an adaption is part of the general knowledge and experience of the respective skilled person and may be executed without any inventive effort. It goes without saying that the laser energy provided by the laser source must be strong and focussed enough in order to being able to provide the requested energy from the back side of the donor substrate, passing the donor substrate and being capable to be absorbed by the absorber particles in the printing ink containing layer on the inked surface of the donor substrate. Fortunately, the laser energy provided by the LIFT printing apparatuses which are available in the market already is sufficient in order to fulfil the requirements of the present process. Therefore, established printing apparatuses using the LIFT process according to the first variant (without DRL) may be used in a standard manner for the process according to the present invention as well.
Astonishingly, the laser energy absorbed by the absorber particles used in the process of the present invention is strong enough in order to transfer energy to the vaporizable components contained in the printing ink to be readily evaporized in the contact zone of the laser beam, and the transferred laser energy is also strong enough for detaching the absorber particles themselves from the inked donor substrate surface and being transferred together with the vaporized and fluidized printing ink
components to the receiving substrate, forming a printed spot thereon.
The present invention does also relate to the use of micron-sized particles comprising metal oxides in a laser-induced forward transfer process as absorber particles in an ink coating on a donor substrate.
According to the present invention, the metal oxide comprised in the micron-sized absorber particles is preferably an electrically conductive or semiconducting metal oxide, mixed metal oxide or metal oxide mixture and is advantageously composed of doped metal oxides or of T1O2-X as described in detail before.
All other details with respect to the characteristics of the absorber particles, the composition of the inks on the donor substrate in which they are used and the adapted LIFT process according to the present invention are described before as well. Insofar, these details do also belong to the use of the micron-sized absorber particles in an ink coating on the donor substrate in the adapted LIFT process according to the present invention, so there is no need to repeat the details.
Furthermore, the present invention does also relate to a product exhibiting a printed image on a substrate, wherein the printed image is composed of printed spots produced by the method as described before. According to the present invention, the printed image is composed of only one or of a plurality of printed spots, whereby the printed spots contain at least the micron-sized absorber particles containing metal oxides and the solidified compounds obtained by vaporization and solidification of the vaporizable and fluidizable compounds of the printing ink on the receiving substrate.
In the present invention, the printed spots contain colorless or almost colorless micron-sized metal oxide containing absorber particles as described before as well as the amount of the coloring or functional compounds which were contained in the printing ink coating on the donor substrate prior to the transfer. Here, the micron-sized absorber particles act as transfer means for the coloring compounds or for the functional material in the printing ink. They enhance the transferred amount of the latter or even enable the transfer, so that color intensity and chroma of the colors visible in the printed spots rises to a great extent or the transfer of functional materials becomes possible at all. Even a small amount of merely 0.25% by weight content of the respective micron-sized absorber particles in the printing ink leads to a distinct improvement of the optical characteristics of the corresponding printed spots or allows the transfer of functional materials without the need of any dynamic release layer.
The printed spots exhibit in most cases the shape of a regular or irregular dot having some micrometers diameter each. Regularly, a plurality of these spots forms the respective printed image, like in usual printing processes. The shape of the printed image does not play any role in the present invention and might be any desired shape, ranging from figures, numbers, lines, regular or irregular patterns to all kinds of motifs being printable with common printing processes. Advantageously, printing of fine lines of merely some micrometers line width as well as of more complex patterns is possible.
The substrate being part of the product of the present invention is advantageously selected from the group consisting of paper, wall paper, glass, plastic films, plastic bodies, metal films, metal bodies, ceramic bodies and sheets or bodies being composed of at least two different of these materials. In principle, all substrates capable of being printed by a common LIFT process may be used here as well. The substrate being printed with the printed image corresponds to the receiving substrate in the printing process explained above. It may be printed while being part of the product already (e.g. for packaging materials) or may be assembled with different parts of the products at any time after the printing process is executed (e.g. a printed insert of an automotive part).
The respective product is a decorative element or a functional element in a commercial printing product, a part of a vehicle, a part of a plane, a part of a train, an architectural part, a consumer electronics product, a solar cell component, a packaging material, a security product, a textile product or a leather product. A commercial printing product is to mean e.g. a magazine, a flyer, a poster, a brochure, a newspaper, a furniture decorative paper, a floor covering, a wall covering, a gift wrapping paper, a carrier bag or a decorative foil, to name only a few. The present printing process is a digital printing process allowing printing inks containing coloring pigments, coloring dyes or functional materials such as electrically conductive polymers to be printed in a common LIFT process without any dynamic release layer in an industrial scale. These coloring or functional materials not being capable to absorb enough laser energy to being transferable in a common LIFT process not using a DRL may, thus, now transferred in such a process without the technical and costly efforts which are necessary when a DRL is necessary. Therefore, the present process provides a convenient and easily available digital printing method for several optical and/or functional applications.
Fig .1 : shows the working mechanism of the process according to the
present invention in a general manner, representing: (1 ) the laser beam, (2) the donor substrate, (3) the printing ink coating layer, (4) the gap between printing ink coating layer surface and receiving substrate surface and (5) the receiving substrate
Fig.2: shows the test scheme for finding the best laser mode of action Fig.3: shows the test pattern applied to the printing tests of all kinds of colored dyes used in the examples according to the present invention
Fig. 4: shows the result of a printed test pattern using a red dye on a white colored receiving substrate according to example 1
The examples below are intended to explain the invention, but without limiting it. The percentages indicated are percent by weight.
Example 1 :
A printing ink comprising 10 % of a colored compound [Folco FS-10 218 (red) or Folco FS-10 120 (yellow)], 0-20% of a micron-sized absorber particle according to the present invention (Iriotec® 9230, > 99% Sb doped Sn02, particle size <10 pm (90%), product of Merck KGaA), 1.6% of an antifoaming agent (Follmann Entschaumer 5280), 1.2% of a wetting agent (Follmann Netzmittel 5293) and a variable amount of a commercially available printing ink binder, adding to 100% in each case (Follmann FS-10 931 ) is prepared by mixing the ingredients.
A glass plate having a thickness of about 1 mm is fully coated with the printing ink using a silk-screen printing process on a surface area of 100x70 mm. The thickness of the printing ink layer is about 45 pm.
The glass plate coated with the printing ink is arranged onto a white colored paper sheet (Papyrus LuxoSatin 250 g/m2 white) with metal shims of 50 pm, forming a 5 pm gap between the surface of the printing ink and the receiving paper surface. Each printing ink composition is printed with two different patterns (*. bmp- format, 95x65 mm, 300 ppi). The corresponding laser (Trumpf VectorMark VMC-5) is used in several different frequency and power modes, see Fig. 2 The laser mode exhibiting the best result is used for printing a second pattern according to Fig. 3. According to the latter, line width and accuracy of patterned shapes including text fields may be easily observed.
The relative content of the micron-sized metal oxide comprising absorber particle is chosen to be 0%, 0.25%, 1 %, 5%, 10% and 20%.
All of the tested printing inks turned out to be transferable by the process described.
Regarding the red and yellow colored printing inks without any absorber particles (0% concentration), if the test scheme according to Fig. 2 is used, only a few test fields occur in weak red or yellow color. This means that without any addition of the micron-sized absorber particles according to the present invention, a transfer of said coloring dyes would be possible only in a very limited range at merely a limited number of laser settings.
With the addition of 0.25% of micron-sized absorber particles, the number of test fields in Fig. 2 which are filled with the red or yellow color is more than doubled in comparison to the concentration of 0% absorber. With rising concentration in the limits as described above (0.25 to 20%), the number of test fields which are fully covered with colors rises continuously, while chroma and distinctness of the test fields rise over a broad laser parameter range.
The test pattern according to Fig. 3 for the red colored dye (Folco FS-10 218 (red)) at a concentration of 10% of the micron-sized absorber is shown in Fig. 4.

Claims

Patent Claims
1. A method for transferring a printing ink from a laser transparent donor substrate to a receiving substrate by a laser-induced forward transfer process, whereby
- the laser transparent donor substrate exhibits a front surface coated with the printing ink and a back surface facing away from the front surface;
- the printing ink forms a coating which has an upper side being located adjacent to, but not in physical contact with the receiving substrate;
- the printing ink contains absorber particles and at least one
component being capable of enlarging its volume upon exposure to laser-generated energy;
- the donor substrate is irradiated by laser energy of a particular wavelength at the back surface, whereupon the laser energy is absorbed by absorber particles in the ink and a volume expansion of a certain amount of the at least one component surrounding the absorber particles is caused, and
- said component is transferred to the receiving substrate along with the absorber particles, leading to a printed spot on the receiving substrate, whereby
- the absorber particles are micron-sized particles comprising metal oxides; and
- the printing ink comprises a coloring or functional compound.
2. A method according to claim 1 , wherein the absorber particles exhibit a particle size in the range of from 1 -50 pm.
3. A method according to claim 1 or 2, wherein absorber particles
comprise at least one electrically conductive or semiconducting metal oxide selected from the group consisting of doped tin oxide, doped titanium dioxide or Ti02-x, wherein 0.001 <x< 0.05.
4. A method according to one or more of claims 1 to 3, wherein the
absorber particles are contained in the printing ink in an amount of 0.25 to 20% by weight, based on the weight of the printing ink.
5. A method according to one or more of claims 1 to 4, wherein the
coating layer formed by the printing ink exhibits a thickness in the range of from 20 to 90 pm.
6. A method according to one or more of claims 1 to 5, wherein there exists a gap between the upper side of the printing ink coating and the receiving substrate, said gap being in the range of from 1 to 100 pm.
7. A method according to one or more of claims 1 to 6, wherein the laser is a Nd:YAG laser emitting at 1064 nm.
8. A method according to one or more of claims 1 to 7, wherein the
printing ink comprises a non-IR absorbing organic pigment or dye.
9. A method according to one or more of claims 1 to 7, wherein the
printing ink comprises a non-IR absorbing electrically conductive polymer.
10. Use of micron-sized particles comprising metal oxides in a laser- induced forward transfer process as absorber particles in an ink coating on a donor substrate.
11. Use according to claim 10, wherein the micro-sized particles comprise at least one electrically conductive or semiconducting metal oxide selected from the group consisting of doped tin oxide, doped titanium oxide or T1O2-X, wherein 0.001 <x< 0.05.
12. Product exhibiting a printed image on a substrate, wherein the printed image is composed of printed spots containing micron-sized particles comprising metal oxides and a coloring or functional compound, produced by the method according to one or more of claims 1 to 11.
13. Product according to claim 12, wherein the substrate is selected from the group consisting of paper, wall paper, glass, plastic films, plastic bodies, metal films, metal bodies, ceramic bodies, and sheets or bodies being composed of at least two different of these.
14. Product according to claim 13, which is a decorative element or a functional element in a commercial printing product, a part of a vehicle, a part of a plane, a part of a train, an architectural part, a consumer electronics product, a solar cell component, a packaging material, a security product, a textile product or a leather product.
EP19702298.1A 2018-02-09 2019-02-06 Method for laser-induced forward transfer using metal oxide absorber particles Pending EP3749528A1 (en)

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PCT/EP2019/052853 WO2019154826A1 (en) 2018-02-09 2019-02-06 Method for laser-induced forward transfer using metal oxide absorber particles

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DE102019128820A1 (en) * 2019-10-25 2021-04-29 Mayr-Melnhof Karton Ag Method and device for coating a substrate, cutting and packaging
IL309612A (en) 2021-07-02 2024-02-01 Heliosonic Gmbh Radiation induced printing method using an effect pigment mixture

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GB8410514D0 (en) * 1984-04-25 1984-05-31 Ici Plc Laser-imageable assembly
US5278023A (en) * 1992-11-16 1994-01-11 Minnesota Mining And Manufacturing Company Propellant-containing thermal transfer donor elements
WO2001072518A1 (en) 2000-03-30 2001-10-04 Aurentum Innovationstechnologien Gmbh Method of printing and corresponding print machine
DE10210146A1 (en) * 2002-03-07 2003-09-25 Aurentum Innovationstechnologi Quality printing process and printing machine, as well as a print audit for this
GB201009847D0 (en) * 2010-06-11 2010-07-21 Dzp Technologies Ltd Deposition method, apparatus, printed object and uses
KR20150088258A (en) * 2012-11-27 2015-07-31 디에스엠 아이피 어셋츠 비.브이. Process of depositing a metallic pattern on a medium

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