WO2019154980A1

WO2019154980A1 - Method for laser-induced forward transfer using effect pigments

Info

Publication number: WO2019154980A1
Application number: PCT/EP2019/053118
Authority: WO
Inventors: Piotr Wierzchowiec; Kevin MORITZ; Volker Wilhelm; Marc Hunger
Original assignee: Merck Patent Gmbh
Priority date: 2018-02-09
Filing date: 2019-02-08
Publication date: 2019-08-15
Also published as: EP3749529A1

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 flaky effect pigments, to the use of those flaky effect pigments as laser absorber particles in such a process as well as to a product exhibiting a printed colored image on a substrate which is produced by said process.

Description

Method for laser-induced forward transfer using effect pigments

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 disadvantage, 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. Nevertheless, it would be of advantage if other pigments exhibiting a much larger particle size would be transferable by a LIFT process as well. For example, printing inks containing flaky effect pigments may be used so far only in traditional printing processes using printing forms which have costly to be adapted to the size of the flaky effect pigments. For the largest particle sizes which go beyond 200 pm, none of the traditional printing processes is useful. It goes without saying that digital printing processes would be desirable for the printing of flaky effect pigment containing printing inks as well, but the common inkjet process is so far not useful in an industrial scale, since the large effect pigments clog the common printing nozzles and adapted ink jet printing apparatuses are not yet available. The object of the present invention is to provide a digital printing process being applicable for printing inks pigmented with flaky effect pigments exhibiting a particle size up to more than 200 pm, where the process may be executed in a simple manner at low cost, leading to printed images exhibiting intense colors and/or gloss with high resolution.

Furthermore, the object of the present invention is to provide a particular use for common flaky effect pigments. In addition, a further object of the present invention is to provide a product comprising a printed colored image exhibiting a high resolution on a substrate, where the printed colored image contains flaky effect pigments exhibiting a particle size of up to more than 200 pm, being printed in a digital printing process.

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 non-metallic flaky effect pigments.

Furthermore, the object of the present invention is solved by the use of non- metallic flaky effect pigments 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 colored image on a substrate, wherein the printed colored image is composed of printed spots containing non-metallic flaky effect pigments, 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 non-metallic flaky effect pigments are used as absorber particles in the printing ink, these flaky effect pigments may be transferred together with the vaporizable part of the printing ink to a receiving substrate although they exhibit particle sizes of up to 200 pm and beyond.

In particular, the particle size of the non-metallic flaky effect pigments may be in the range of from 2-350 pm. The given range is the broadest range of the nominal particle size of the flaky effect pigments. It goes without saying that flaky effect pigments which are usually used in different application media exhibit particle sizes which belong to different particle size ranges in the broad range mentioned above. To this end, the effect pigments are usually sieved in order to fit in several particle size fractions. For instance, very fine particles exhibit a particle size of < 15 pm, fine particles exhibit sizes of 5-20 pm, the sizes useful for most applications are those of 10-60 pm, large particles exhibit sizes of 10-130 pm and extra large particles belong to the range of from 40-350 pm (where, in each case, at least 90% by vol. of the particles fit in the named range).

According to the present invention, all fractions of different particle sizes falling within the broad range mentioned above may be used.

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 pigments, for example by direct observation and measure- ment of a number of individual particles in high-resolution light micros- copes, but better in electron microscopes, such as the scanning electron microscope (SEM) or the high-resolution electron microscope (FIRTEM), 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.

According to the present invention, the non-metallic flaky effect pigments which are used as absorber particles in the printing ink layer on the laser transparent donor substrate are composed of a flaky, transparent, dielectric carrier particle having at least one metal oxide layer thereon.

In the sense of the present invention, the term“flaky” is taken to mean a flat structure which, with its top and bottom side, has two surfaces approxi- mately parallel to one another whose length and width dimension repre- sents the largest dimension of the pigment. The separation between the said surfaces, which represents the thickness of the flake, has, by contrast, a smaller dimension.

The length and width dimension of all said carrier particles for the pigments according to the invention is in the range from 2 to 350 pm, as disclosed for the flaky effect pigments already. It also represents the value which is usually referred to as particle size of the carrier particles. The thickness of the carrier particles is generally between 0.05 and 5 pm, preferably from 0.1 to 4.5 pm and particularly preferably from 0.2 to 1 pm.

The carrier particles have an aspect ratio (ratio of length to thickness) of at least 2, preferably of at least 10 and particularly preferably of at least 50.

Thickness and aspect ratio mentioned for the carrier particles are also valid for the flaky non-metallic effect pigments according to the present invention, since the coating layer(s) on the carrier particles measure merely some hundreds of nanometers and do, thus, not alter the respective values to a big extent.

The flaky, transparent, dielectric carrier particle is advantageously selected from the group consisting of natural mica platelets, synthetic mica platelets, talc platelets, kaolin platelets, Si0₂-platelets, AhOs-platelets, glass platelets, borosilicate platelets and mixtures of at least two of them. Preferably, natural mica platelets, synthetic mica platelets, Si0₂-platelets, A Os-platelets and borosilicate platelets are useful, in particular natural mica platelets, synthetic mica platelets and Si0₂-platelets. The flaky, transparent, dielectric carrier particles are coated with at least one layer being composed of a metal oxide, a mixed metal oxide or a metal oxide mixture. According to the present invention, all of these layers are named metal oxide layer. Two or more metal oxide layers may also be present on the transparent, dielectric carrier particles. Preferably, these metal oxide layers surround the carrier particles, leading to a continuous metal oxide outer surface layer of the flaky effect pigments.

The metal oxides, mixed metal oxides or metal oxide mixtures may be composed of materials exhibiting a high refractive index n or a low refractive index n. Preferably, at least one of the metal oxide layers is composed of a metal oxide, mixed metal oxide or metal oxide mixture exhibiting a high refractive index.

Preferably, the material for the at least one metal oxide layer is selected from the group consisting of T1O2, Fe203, Fe30₄, Cr203, CuO, Sn02, ZnO, Zr02, Sb₂0₃, S1O2, AI2O3, and mixtures or mixed oxides of at least two of them.

According to the present invention, flaky effect pigments comprising layers of T1O2, Fe₂03, Fe30₄ or of mixed oxides or oxide mixtures of T1O2 and Fe203, for example FeTi03 or Fe2TiOs, are particularly preferred, which may be present in combination with one or more layers of Sn02 and/or S1O2. Pigments comprising at least one layer of Fe₂03, Fe30₄ or of mixed oxides or oxide mixtures of T1O2 and Fe₂03 are most preferred.

The flaky effect pigments according to the present invention exhibit interference colors and, in some cases, also absorption colors which mix with the interference colors, leading to interesting color characteristics. In some cases, depending on the kind of carrier particles and the number, thickness and refractive index of the metal oxide layer(s) on the carrier particle, the flaky effect pigments may also exhibit color flops, i.e. color characteristics which depend on the viewing angle. The color

characteristics of the flaky effect pigments are transferred to the receiving substrate when the process according to the present invention is carried out, since the flaky effect pigments are transferred to the receiving substrate despite of their large particle sizes in comparison to the transfer of submicron sized carbon black particles or submicron sized metallic particles according to the prior art LIFT processes.

The flaky non-metallic effect pigments used in the present invention are available in the market in a great variety.

Astonishingly, the laser energy absorbed by the flaky effect pigments according to the process of the present invention is strong enough in order to transfer energy to the fluid 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 large flaky effect pigments themselves from the inked donor substrate surface and being transferred together with the vaporized fluids to the receiving substrate, forming a printed spot thereon. 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 may be used in a standard manner for the process according to the present invention as well.

The non-metallic flaky effect pigments are contained in the printing ink in an amount of 0.25 to 75% by weight, based on the weight of the printing ink. Preferably, effect pigment concentrations in the range of from 5 to 40% by weight, especially 12.5 to 30% by weight, and most preferred in the range of from 15 to 20% by weight, based on the weight of the printing ink in each case, are useful.

Besides the non-metallic flaky effect pigments, 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.

Advantageously, according to the present invention, the printing ink contains the non-metallic flaky effect pigments as the sole coloring means and does, especially, not contain any other coloring pigment or dye.

Except of the content of non-metallic flaky effect pigments, 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 non-metallic flaky effect pigments as 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. Therefore, the use of donor substrates carrying printing ink layers which are still in a wet, non-solidified stage, is highly preferred. 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 liquid printing ink components and some non-metallic flaky effect pigments 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 non- metallic flaky effect pigments. 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 may be provided on the receiving substrate which exhibits interference colors and/or absorption colors and/or color flops and may also exhibit glossy and sparkling effects, depending on the optical characteristics of the non-metallic flaky effect pigments used in the printing ink layer on the donor substrate. Advantageously, the front face of the donor substrate is fully covered by the printing ink and the printing ink coating 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, enlarge the visibility of the coloring effects of the non-metallic flaky effect pigments contained in the printed spots on the receiving substrate. To this end, besides substrates colored in strong background colors such as deep red, deep blue, deep 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 intrin- sically 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.

Of course, laser frequency and power have to be adapted to the

corresponding printing ink ingredients and kind of receiving substrates. Such an adaptation is part of the general knowledge and experience of the respective skilled person and may be executed without any inventive effort.

The present invention does also relate to the use of non-metallic flaky effect pigments in a laser-induced forward transfer process as absorber particles in an ink coating on a donor substrate.

All details with respect to the characteristics of the non-metallic flaky effect pigments, 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 in detail before. Insofar, these details belong to the use of the non-metallic flaky effect pigments as absorber particles in an ink coating on the donor substrate in the adapted LIFT process according to the present invention as well, 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 non- metallic flaky effect pigments and the solidified compounds obtained by solidification of the fluid compounds of the printing ink on the receiving substrate.

According to the present invention, the printed spots advantageously contain solely the non-metallic flaky effect pigments as coloring means. All other compounds in the printed spot should be colorless. In this way, the optical characteristics of the non-metallic flaky effect pigments may be transferred in a pure manner to the receiving substrate. Nevertheless, in case that color adaptions are desired, other coloring means may be part of the printing ink. The non-metallic flaky effect pigments may exhibit a particle size of up to 350 pm. Gloss and glittering effects on the printed spots depend on the particle size of the non-metallic flaky effect pigments, i.e. the larger the pigment size, the more impressive are these effects. In addition, the non-metallic flaky effect pigments provide, depending on their material composition and number and kind of metal oxide layers on the carrier particles, interference colors and/or absorption colors to the printed spots and may also provide optically variable color characteristics (color flops depending on the viewing or illumination angle).

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 the first digital printing process allowing printing inks containing flaky effect pigments exhibiting a particle size of up to more than 200 pm to be printed in an industrial scale in a LIFT process without the need for a dynamic release layer. Thus, the optical character- ristics of flaky effect pigments exhibiting interference colors, color flops, gloss and glitter may be used in a very fast and versatile printing process without the need for costly equipment and long lasting preparation efforts and without the fear to be diminished by components stemming from the DRL. The present printing process, thus, allows the use of printing inks containing flaky effect pigments to a much greater extent for personal and industrial use as being possible up to now.

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 non-metallic flaky effect pigments used in the examples according to the present invention

Fig. 4: shows the result of a printed test pattern using a golden effect

pigment on a red 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 17.5 % of the corresponding non-metallic flaky effect pigment, 78.2 % of a commercially available printing ink binder (Follmann FS-10 931 ), 1.5% of an antifoaming agent (Follmann

Entschaumer 5280), 0.8% of a wetting agent (Follmann Netzmittel 5293) and 2% of a solvent (ethylene glycol) 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 screen (36 L/cm or 27 L/cm) is chosen depending on the particle size of the flaky effect pigments. The thickness of the printing ink layer is about 45 pm. The glass plate coated with the printing ink is arranged onto a red colored paper sheet (Papyrus Chromolux-Color 250 g/m²) or 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. The color of the receiving substrate enlarges the contrast between the flaky effect pigments transferred and the substrate and is chosen accordingly.

Each pigment 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. Tested are 13 different non-metallic effect pigments exhibiting silver white, golden, red, green and black interference and/or absorption colors. The particle sizes vary in the range of from 1 to 200 pm with fractions in the range of from 1 -15 mm, 5-25 pm, 5-40 pm, 5-50 pm, 10-60 pm, 10-100 pm and 2-200 pm.

All effect pigments are products of Merck KGaA based on natural or synthetical mica platelets, S1O₂ platelets, AI₂O3 platelets or borosilicate platelets. The metal oxide layer(s) on the substrate carrier particles is/are of T1O2, Fe203, Fe30₄, S1O2, Sn02, either as a single layer, as several layers or as a layer containing a mixed oxide or an oxide mixture. All of the tested flaky effect pigments turned out to be transferable by the process

described. Pigments exhibiting a non-white interference color exhibit better optical results than those with silver-white interference color. Best results are achieved with flaky effect pigments exhibiting a non-white interference color as well as an absorption color.

The test pattern for a golden colored pigment (Iriodin® 305, T1O2 and Fe₂03 on mica, particle size 10-60 pm) 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

- 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, whereby

- the absorber particles are non-metallic flaky effect pigments.

2. A method according to claim 1 , wherein the non-metallic flaky effect pigments exhibit a particle size in the range of from 2-350 pm.

3. A method according to claim 1 or 2, wherein the non-metallic flaky effect pigments are composed of a flaky, transparent, dielectric carrier particle having at least one metal oxide layer thereon.

4. A method according to claim 3, wherein the flaky, transparent, dielectric carrier particle is selected from the group consisting of natural mica platelets, synthetic mica platelets, talc platelets, kaolin platelets, Si0₂-platelets, A Os-platelets, glass platelets, borosilicate platelets and mixtures of at least two of them.

5. A method according to claim 3 or 4, wherein the at least one metal oxide layer is selected from the group consisting of T1O₂, Fe₂03, Fe30₄, Cr203, CuO, Sn02, ZnO, Zr02, Sb203, S1O2, AI2O3, and mixtures or mixed oxides of at least two of them.

6. A method according to one or more of claims 1 to 5, wherein the non- metallic flaky effect pigments are contained in the printing ink in an amount of 0.25 to 75% by weight, based on the weight of the printing ink.

7. A method according to one or more of claims 1 to 6, wherein the

coating layer formed by the printing ink exhibits a thickness in the range of from 20 to 90 pm.

8. A method according to one or more of claims 1 to 7, 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.

9. A method according to one or more of claims 1 to 8, wherein the laser is a Nd:YAG laser emitting at 1064 nm.

10. A method according to one or more of claims 1 to 9, wherein the non- metallic flaky effect pigments exhibit interference and/or absorption colors and provide interference and/or absorption color to the printed spot, and where no further coloring component is present in the printing ink.

11. Use of non-metallic flaky effect pigments in a laser-induced forward transfer process as absorber particles in an ink coating on a donor substrate.

12. Product exhibiting a printed colored image on a substrate, wherein the printed colored image is composed of printed spots containing non- metallic flaky effect pigments, produced by the method according to one or more of claims 1 to 10.

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.