METHOD FOR LASER ENGRAVING AND/OR LASER MARKING, LASER MARKED AND/OR ENGRAVED ARTICLE AND ARTICLE FOR LASER ENGRAVING AND/OR LASER MARKING

Abstract

 The invention is directed to a method for laser engraving and/or laser marking comprising the steps of:
- providing a substrate with at least one layer, wherein the at least one layer comprises a polymer and an irreversible thermochromic pigment,
- generating a carbon trace or vapor trace within or on the at least one layer by irradiating the substrate with a pulsed laser beam, such that a focus spot of the laser beam is within or on the at least one layer at a first predetermined position, and
- irradiating the substrate with the pulsed laser beam such that the focus spot of a consecutive laser pulse is within or on the at least one layer at a predetermined consecutive position and a distance between the first position and the consecutive position is less or equal to twofold a diameter of the carbon trace or vapor trace.
Furthermore, the invention is directed to a laser marked and/or engraved article produced by said method and to an article for laser engraving and/or laser marking.

Description

[0001] The invention relates to a method for laser engraving and/or laser marking.

[0002] Furthermore, the invention relates to an article for laser engraving and/or laser marking and further to a laser marked and/or engraved article produced by the method for laser engraving and/or laser marking.

[0003] Laser marking and laser engraving of polymers are rapid, and precise methods that can be integrated in processing lines for marking or engraving information such as bar codes, dates or numbers directly on the surface of polymer parts (laser marking) or within a transparent or translucent polymer part (laser engraving). Since the laser marking or laser engraving is permanent, solvent, wipe- and scratchproof, it offers a high security. Furthermore, there is no need for pretreatment before laser marking/engraving process, thus making it an attractive method.

[0004] Conventional laser marking/engraving is the result of a thermal response of the polymer. For heat induced marking/engraving, laser radiation is absorbed and transformed into heat. The absorbed energy is high enough to locally raise the temperature above the decomposition temperature of the polymer and cause thermal degradation of the polymer. The polymer thus carbonizes, leading to a dark marking. Alternatively, the absorbed energy is sufficiently high enough to cause polymer degradation to gases, which leads to the formation of foams, and gives a light marking due to light scattering.

[0005] As a variety of polymers do not easily generate markings, the outcome of the laser marking/engraving process may be improved by laser marking pigments added to the polymer. The laser marking pigments are configured to absorb the laser energy and thus enhance the laser marking/engraving outcome. However, such laser marking pigments may not only improve the marking/engraving process but may also affect the appearance of the polymer before marking or the processability of the polymer.

[0006] Furthermore, the markings generated are either dark with a color that is conceived by the human eye as black or greyish due to carbonization or light with a color that is conceived by the human eye as white or greyish due to foaming.

[0007] In order to achieve colored markings, the document FR 3 106 527 A1 proposes to use an irreversible thermochromic pigment in a laser marking process.

[0008] The object of the present invention is to improve the laser marking and/or engraving process and/or enhance the appearance of color in the laser marked and/or engraved article.

[0009] According to the invention a solution for this problem is achieved by a method for laser engraving and/or laser marking comprising the steps of:

• providing a substrate with at least one layer, wherein the at least one layer comprises a polymer and an irreversible thermochromic pigment,
• generating a carbon trace or vapor trace within or on the at least one layer by irradiating the substrate with a pulsed laser beam, such that a focus spot of the laser beam is within or on the at least one layer at a first predetermined position, and
• irradiating the substrate with the pulsed laser beam such that the focus spot of a consecutive laser pulse is within or on the at least one layer at a predetermined consecutive position and a distance between the first position and the consecutive position is less or equal to twofold a diameter of the carbon trace or vapor trace.

[0010] It has been found that the appearance of the color in a laser marking and/or engraving process can be improved by irradiating the substrate with the pulsed laser beam such that the distance between the first position and the consecutive position of the laser pulses is less or equal to twofold the diameter of the carbon trace or vapor trace.

[0011] When irradiating the at least one layer the energy of the laser beam sometimes leads to a carbonization of the polymer which is visible as greyish or black carbon trace. Some polymers however do not carbonize, but vaporize and start building a white foam, also called vapor trace.

[0012] Without being bound to a specific theory, it is believed that the placement of the consecutive focus spot on the position of the previous focus spot, or close to the position of the previous focus spot - and thus on or close to the position of the carbon or vapor trace created by the previous focus spot - leads to a disintegration of the carbon trace or vapor trace created by the previous focus spot. Thus, the appearance of the color is enhanced since the dark black or greyish appearance of the carbon trace or the white marking and/or turbidity of the vapor trace is reduced.

[0013] In other words, it was found that with two laser pulses on one and the same position or close together, the colorfulness of the laser marking and/or engraving process increases. It is assumed that with the first laser pulse there is both a carbonization or vaporization of the polymer matrix and a color change of the irreversible thermochromic pigments due to the heating of the immediate surroundings of the focus spot. It is further believed that a plasma is generated in the polymer matrix by non-linear absorption of the focused laser beam, so that the polymer carbonizes or vaporizes due to the high temperatures. By dissipation, the immediate surroundings are heated up.

[0014] With the second pulse on the same position or close to the same position, the expansion of the carbonization or vaporization and thus the blackening or whitening decreases, while thermal energy is supplied to the immediate surroundings a second time. This leads to a further color change of not yet reacted irreversible thermochromic pigments and a deepening of the coloring.

[0015] The irradiation of the substrate with the pulsed laser beam such that the focus spot of the consecutive laser pulse is within or on the at least one layer at the predetermined consecutive position and the distance between the first position and the consecutive position is less or equal to twofold a diameter of the carbon trace or vapor trace is preferably achieved by moving the substrate and/or the laser beam accordingly. More preferably, the substrate and/or the laser beam are moved in the x,y plane - i.e. in the plane of the at least one layer - such that the colored markings are also created in the x,y plane. The method however also allows to introduce colored markings at different depths z of the at least one layer, without changing the appearance above and below the markings. The z-direction is parallel or antiparallel to the direction of the normal vector of the at least one layer.

[0016] In other words, the velocity with which the laser beam is moved relative to the substrate is preferably adapted to the pulse repetition rate of the pulsed laser beam, such that the distance between the first position and the consecutive position of the focus spot of the laser beam is zero, or less or equal to twofold the diameter of the carbon trace or vapor trace. Therefore, two pulses of the pulsed laser beam are deposited on the same position or at a distance less or equal to twofold the diameter of the carbon/vapor trace.

[0017] For a third consecutive laser pulse there are preferably two options: In one preferred embodiment the irradiation of the substrate with the pulsed laser beam is such that the focus spot of the third laser pulse is within or on the at least one layer at a predetermined third position and a distance between the second (previous) position and the third position is less or equal to twofold a diameter of the carbon trace or vapor trace. Preferably, in this way the substrate can be irradiated with multiple laser pulses. In this way a continuous chain of colored markings can be achieved. This embodiment is preferably achieved by irradiating the substrate with the pulsed laser beam having a constant shot repetition rate, and moving the substrate with a constant velocity and/or moving the laser beam with a constant velocity.

[0018] In another preferred embodiment the irradiation of the substrate with the pulsed laser beam is such that the focus spot of the third laser pulse is within or on the at least one layer at a predetermined third position and a distance between the second (previous) position and the third position is greater than twofold a diameter of the carbon trace or vapor trace and such that the focus spot of a fourth laser pulse is within or on the at least one layer at a predetermined fourth position and a distance between the third (previous) position and the fourth position is less or equal to twofold a diameter of the carbon trace or vapor trace. In other words, preferably after having irradiated the substrate with two consecutive laser pulses having focus spots close together the next two consecutive laser pulses having focus spots close together are irradiated in a further distance to the first two laser pulses. In this way isolated colored markings are generated, wherein each colored marking is generated by two consecutive laser pulses having focuses spots close together. As no continuous chain of colored markings is created the weakening of the structural stability of the material is lessened. This embodiment is preferably achieved by irradiating the substrate with the pulsed laser beam having an alternating shot repetition rate, and moving the substrate with a constant velocity and/or moving the laser beam with a constant velocity. Alternatively, his embodiment is preferably achieved by irradiating the substrate with the pulsed laser beam having a constant shot repetition rate, and moving the substrate and/or the laser beam with an alternative velocity with respect to each other. The alternating shot repetition rate of the pulsed laser beam preferably means, that the time between two consecutive pulses alternates. For example, the time between the first and the second pulse is 2,5 µs, the time between the second and the third pulse is 0,5 µs, the time between the third and the fourth pulse is again 2,5 µs, the time between the fourth and the fifth pulse is again 0,5 µs, and so forth.

[0019] In the first step of the method the substrate with the at least one layer is provided, wherein the at least one layer comprises the polymer and the irreversible thermochromic pigment. In one alternative it is possible that the substrate is formed by the at least one layer - in other words in this case the substrate is the at least one layer and comprises the polymer and the thermochromic pigment. Alternatively, at least one layer comprising the polymer and the thermochromic pigment is formed on a substrate. In this case the substrate may not be a polymeric substrate, but can be of any suitable material - e.g. wood, ceramic, glass or any other solid material.

[0020] Furthermore, the irreversible thermochromic pigment is preferably predominantly present in the polymer as finely distributed particle. In contrast to dyes, which are coloring agents that are predominantly present in the polymer in a molecularly solved state, pigments are coloring agents that are not predominantly molecularly solved in the polymer. However, the thermochromic pigment may comprise a thermochromic dye, which is for example encapsulated in microcapsule, thus forming a pigment.

[0021] Thermochromic pigments can be divided into reversible and irreversible types. The first exhibit a reversible color change following a temperature variation. The change in color can be reversed through heating-cooling cycles, wherein the pigment regains its original color after cooling. In contrast, irreversible thermochromic pigments exhibit an irreversible color change based on the peak temperature of the surrounding environment. The change in color cannot be reversed upon cooling, thus providing permanent records of a past temperature increase. Preferably, the thermochromic pigment is configured to changes its color due to a change in temperature in an irreversible fashion - in other words the thermochromic pigment is an irreversible thermochromic pigment.

[0022] In a second step of the method the carbon trace or vapor trace is generated within or on the layer. In order to achieve this, the substrate is irradiated with a pulsed laser beam, such that the focus spot of the laser beam is within or on the layer at the first predetermined position.

[0023] In case the focus spot is within the layer, the method is normally called laser engraving, since the surface of the at least one layer is not affected and remains smooth. In this case also the carbon trace or vapor trace is formed inside the layer. As the focus spot of the laser beam is within the layer, the pulsed irradiation raises the temperature locally inside the layer.

[0024] In case the focus spot is on the at least one layer, and in particular on the surface of the at least one layer, the method is often called laser marking. The carbon trace or vapor trace is also formed on the surface of the layer and preferably also in the upper volume close to the surface of the at least one layer.

[0025] The carbon trace is formed due to the carbonization of the polymer. The vapor trace is formed due to vaporization of the polymer. Preferably, the carbon trace, which appears to the human eye as dark black or greyish, has an ellipsoid and/or spherical form. Further preferably, the vapor trace, which appears to the human eye as white foamed marking, has an ellipsoid and/or spherical form. The diameter of the carbon trace - which is in the context of this invention the largest extend of the carbon trace along one direction - can be determined by an optical microscope. More preferably the diameter of the carbon trace is in between 2 µm to 10 µm. The diameter of the vapor trace - which is in the context of this invention the largest extend of the vapor trace along one direction - can be determined by an optical microscope. More preferably the diameter of the vapor trace is in between 2 µm to 10 µm.

[0026] The enhanced temperature in the vicinity of the focus spot not only leads to the generation of the carbon trace or vapor trace, but preferably also causes the irreversible thermochromic pigment to undergo an irreversible color change.

[0027] In a next step of the method, the substrate is irradiated with the pulsed laser beam, such that the focus spot of the consecutive laser pulse is within or on the layer at the predetermined consecutive position. The predetermined consecutive position can be the same position as the previous position. Alternatively, the predetermined consecutive position can be next to the first predetermined position. The predetermined first and consecutive positions are chosen such that the distance between the first position and the consecutive position is less or equal to twofold the diameter of the carbon trace or vapor trace. It has been found that this distance leads to an improved color appearance as the dark greyish or black appearance of the carbon trace or the white markings of the vapor traces diminishes.

[0028] Preferably, the velocity with which the laser beam is moved relative to the substrate - either by moving the laser beam, or by moving the substrate, or by moving the substrate and the laser beam - is adapted to a pulse repetition rate of the pulsed laser beam, such that the distance between the first position and the consecutive position of the focus spot of the laser beam is less or equal to twofold the diameter of the carbon or vapor trace.

[0029] As the diameter of the carbon or vapor trace is preferably in between 2 µm and 10 µm, the distance between the first position and the consecutive position is preferably not more than 20 µm.

[0030] In other words, the present method does not achieve the colored markings that are visible on or within the at least one layer after performing the method, by the carbon trace or vapor trace as conventional laser marking/engraving processes do. Instead, in the present method the markings are formed by the irreversible thermochromic pigment, which undergoes a color change due to the temperature change induced by the laser beam.

[0031] By focusing the pulsed laser beam and thus selectively heating a localized volume of the at least one layer, individual, isolated colored markings are created. Preferably, the colored markings have spherical and/or elliptical shapes. The diameter of the colored markings created in and/or on the at least one layer depends on several parameters of the laser engraving and/or laser marking process, amongst others the energy of the pulsed laser beam used for irradiation, the response temperature of the irreversible thermochromic pigment, the concentration of the irreversible thermochromic pigment, and the thermal conductivity of the at least one layer and in particular of the polymer. Preferably the colored markings created by the method in and/or on the at least one layer have a diameter of about 2 µm to 150 µm in x,y-direction, preferably about 2 µm to 50 µm in x,y-direction, and more preferably about 2 µm to 10µm. In addition to the displacement of the laser beam and/or substrate in the x,y plane - i.e. in the plane of the at least one layer - and the generation of colored markings in the x,y plane, the method also allows to introduce colored markings at different depths z, without changing the appearance above and below the marking.

[0032] As already mentioned, the focus spot may be on the surface of the at least one layer, thus the colored markings may also be generated on the surface of the at least one layer. In this case the at least one layer can be optically dense and/or opaque.

[0033] However, preferably the focus spot is within the at least one layer. Thus, the colored markings is generated within the at least one layer. In order that the markings are perceived by the human eye, it is preferable that the at least one layer is transparent and/or translucent.

[0034] In this context and according to a preferred embodiment of the invention the the step of providing the substrate with the at least one layer, comprises providing a substrate with at least one transparent and/or translucent layer. Alternatively or additionally, the step of providing the substrate with the at least one layer, comprises providing a substrate with at least one layer having a light transmittance for visible light of at least 5 %, preferably at least 10 % even more preferably at least 20 %.

[0035] The light transmittance for visible light of the at least one layer is preferably determined according to the DIN 1349 (Transmisson of optical radiation; optical clear (nonscattering) media, quantities, symbols and units). Preferably, the transmittance of the at least one layer is determined with UV-VIS spectroscopy across a path length of 1 cm of the at least one layer. Further preferably, the wavelength for determining the transmittance is in between 380 nm to 780 nm.

[0036] To avoid an elliptical deformation of the marking in z-direction due to spherical aberration, the laser beam can be corrected in z-direction by depth-adjusted compensation. Without correction in the z-direction, the regions above the predetermined position are exposed to energy that decreases with distance from the focus spot, so that increasingly brighter colourings become visible with increasing distance from the focus spot.

[0037] Due to the spherical aberration without compensation, the expansion of the marks in the z-direction increases with increasing depth, i.e. becomes ellipsoidal. This can also be desirable as it allows the distance of individual points or lines in the z-direction to be increased to create a closed surface. At depths of several mm, the z-dimensions of the created colored markings are preferably > 100 µm, and preferably ≤ 200 µm.

[0038] According to a preferred embodiment, the velocity with which the laser beam is moved relative to the substrate - also called scanning rate - is in between 0,1 mm/s to 500 mm/s. These velocities have been shown to give good color marking results.

[0039] As already mentioned, the at least one layer comprises the polymer and the irreversible thermochromic pigment. Furthermore, the at least one layer may comprise a mixture of polymers and the irreversible thermochromic pigment. It is also possible that the at least on layer comprises a mixture of irreversible thermochromic pigments.

[0040] With regard to the polymer and according to a preferred embodiment of the invention the polymer is selected from the group comprising polycarbonate (PC), polyethylene terephthalate (PET), polycaprolactone (PCL), polyethylene naphthalate (PEN), polyethylene furanoate (PEF), glycolized or amorphous polyester, glycolized or amorphous polyethylene terephthalate (PET-G or A-PET), polytrimethylene terephthalate (PTT), polybutylene terephthalate (PBT), poly(methyl methacrylate) (PMMA), polypropylene (PP), polyvinyl chloride (PVC), cellulose triacetate (TCA), polyamide (PA), poly imide (PI) or polyethylene (PE), polyacetales like polyoxymethylene (POM), polystyrene (PS), polystyrene copolymers (ABS, SAN, SB), thermoplastic elastomers (TPU, TPO, SEBS) and/or mixtures thereof.

[0041] As already mentioned, it is common in conventional laser marking/engraving processes to add a laser marking pigment, which is configured to absorb the laser energy, to the polymer in order to enhance the outcome of the laser marking/engraving process. For example, typical laser marking pigments are antimony-doped tin oxide, antimony trioxide, or aluminium particles. Nanoscale metal oxides are often used, which do not scatter visible light due to their small particle size, but absorb the wavelength of the laser. Suppliers of laser marking pigment masterbatches consisting of carrier polymer and the laser marking pigment are, for example, Merck, Gabriel-Chemie, Lifocolor, Rowa Group, Engelhard, MarkLT or Budenheim. However, the addition of such laser marking pigment is not always desirable, as the laser marking pigment can change the processability of the polymer of mixtures of polymers and/or the appearance of the at least one layer and/or may be considered toxic. In addition, although the intensity of the color change reaction increases with the content of the laser marking pigment, the penetration depth of the laser radiation decreases at the same time.

[0042] In this context and according to a preferred embodiment of the invention, the at least one layer is free of a laser marking pigment, wherein the laser marking pigment is a pigment configured to absorb at a wavelength of the laser beam. Preferably, the at least one layer is free of antimony-doped tin oxide, antimony trioxide, aluminium and/or aluminium oxide particles, particles of copper and/or copper alloys, silver particles, indium-tin-oxide (ITO), indium oxide, zinc oxide, combinations of silica and/or tin oxide, borosilicate and/or fumed silica either in combination with polypyrrole and/or polythiophene or without polypyrrole and/or polythiophene and/or pearlescent pigments.

[0043] For the present method of laser engraving and/or laser marking it is not necessary that the at least one layer comprises a laser marking pigment. It is a possibility that the at least one layer only comprises the polymer or a mixture of polymers and the irreversible thermochromic pigment or a mixture of irreversible thermochromic pigments. In other words, preferably the at least one layer consists of a polymer or a mixture of polymers and the irreversible thermochromic pigment or a mixture of irreversible thermochromic pigments.

[0044] Alternatively, the at least one layer comprises a laser marking pigment, preferably in an amount of 0,01 wt% to 15 wt% based on the weight of the at least one layer. In particular with a substrate comprising multiple layers, it is preferred that only one layer of the multiple layers comprises a laser marking pigment. It is further preferred that the layer comprising the laser marking pigment is not sandwiched in between two layers that are free of a laser marking pigment and/or that the layer comprising the laser marking pigment is not sandwiched in between two layers that comprise an irreversible thermochromic pigment. Preferably the layer comprising the laser marking pigment is an outermost layer. Furthermore, the substrate is preferably irradiated such that the layer comprising the laser marking pigment is farthest away from the laser.

[0045] In order to enhance the endurance of the at least one layer, the at least one layer may comprise a stabilizer. According to a preferred embodiment of the invention the layer preferably comprises a stabilizer or a mixture of stabilizers selected from the group comprising primary antioxidants, secondary antioxidants, antiozonant, UV absorbers, and/or hindered amines light stabilizers. Stabilizers are chemical additives which may be added to the at least one layer to inhibit or retard the degradation of the polymer or mixtures of polymers.

[0046] Antioxidants inhibit autooxidation that occurs when the polymer reacts with atmospheric oxygen. Primary antioxidants function essentially as free radical terminators and act as radical scavengers. Preferably the primary antioxidant is a sterically hindered phenol, such as BHT, or analogues thereof, or a secondary aromatic amine, such as alkylated-diphenylamine. Secondary antioxidants act to remove organic hydroperoxides formed by the reaction of primary antioxidants. Preferably the secondary antioxidant is a phosphite ester of a phenol, preferably a phosphite ester of a hindered phenol, such as Tris(2,4-di-tert-butylphenyl)phosphite.

[0047] Antiozonants prevent or slow down the degradation of the polymer caused by ozone. Preferably the antiozonant is based on p-phenylenediamine, such as N-Isopropyl-N'-phenyl-1,4-phenylenediamine.

[0048] UV absorbers absorb and dissipate the energy from UV rays as heat, typically by a reversible intramolecular proton transfer process. Thus, UV absorbers reduces the absorption of UV rays by the polymer itself and hence reduces the rate of weathering of the at least one layer. Preferably the UV absorber is based on benzotriazoles, such as Biscotrizole (2,2'-Methylenebis[6-(2H-l,2,3-benzotriazol-2-yl)-4-(2,4,4-trimethylpentan-2-yl)phenol]); hydroxyphenyl-triazines, such as Bemotrizinol (2,2'-[6-(4-Methoxyphenyl)-1,3,5-triazine-2,4-diyl]bis{5-[(2-ethylhexyl)oxy]phenol}); oxanilides, and/or benzophenones.

[0049] Hindered amine light stabilizers, also called HALS or HAS, are chemical compounds comprising an amine functional group. HALS do not absorb UV radiation, but act to inhibit degradation of the polymer by continuously and cyclically removing free radicals that are produced by photo-oxidation of the polymer Preferably the HALS are derivatives of tetramethylpiperidine.

[0050] With regard to the stabilizer or mixtures of stabilizers and according to a preferred embodiment of the invention, the at least one layer comprises the stabilizer or mixtures of stabilizers in an amount ≥ 0,001 wt % to ≤ 1 wt %, based on the weight of the at least one layer.

[0051] The stabilizer can be added to the polymer as the pure substance preferably in the above indicated concentration. Alternatively, a mixture of a polymer and a stabilizer - also called stabilizer masterbatch can be added to the polymer. In this case the amount of stabilizer masterbatch used is dependent on the concentration of the stabilizer within the stabilizer masterbatch and is preferably chosen such that the above given concentrations of stabilizers with regard to the weight of the at least one layer can be achieved.

[0052] Suppliers of such stabilizers and mixtures of stabilizers are Schäfer Additivsysteme GmbH, Chemie Handel Schneider GmbH, Nemitz GmbH, MPI Chemie BV, L. Brüggemann GmbH & Co. KG or Songwon.

[0053] As already mentioned, the at least one layer comprises the polymer and the irreversible thermochromic pigment. With regard to the latter and according to a preferred embodiment of the invention, the at least one layer comprises the irreversible thermochromic pigment in an amount ≥ 0,01 wt % to ≤ 25 wt %, based on the weight of the at least one layer. The amount of the irreversible thermochromic pigment is preferably chosen depending on a thickness of the at least one layer, on a desired opacity, on a desired color depth, and/or on a desired contrast. Dependent on the specific irreversible thermochromic pigment, the polymer, the thickness of the at least one layer, the desired opacity, the desired color depth and/or the desired contrast, the concentration of the irreversible thermochromic pigment can be adapted. Typical amounts for an extruded 200 µm thick layer are 1 wt % to 5 wt % irreversible thermochromic pigment based on the weight of the extruded layer.

[0054] Further preferably the irreversible thermochromic pigment is configured to exist in two states, wherein in the first state the irreversible thermochromic pigment absorbs electromagnetic waves in a first wavelength range and wherein in the second state the irreversible thermochromic pigment absorbs electromagnetic waves in a second wavelength range different to the first wavelength range and wherein an irreversible change from the first state to the second state is feasible by heating the thermochromic pigment above a response temperature.

[0055] Even more preferably, the irreversible thermochromic pigment is configured such that in one state it absorbs electromagnetic waves in the visible range and appears to the human eye colored, and further that it in the other state it does not absorb electromagnetic waves in the visible range and appears to the human eye transparent. In other words, the irreversible thermochromic pigment is preferably configured to exist in two states, wherein in one state the irreversible thermochromic pigment absorbs electromagnetic waves in the visible range, and wherein in the other state the irreversible thermochromic pigment does not absorb electromagnetic waves in the visible range.

[0056] In other words, the irreversible thermochromic pigment preferably either irreversibly change from colorless to colored or from colored to colorless when exposed to temperature. The former results in so-called positive marking, where colored markings in a transparent polymer matrix can be achieved, the latter results in a so-called negative marking, where a transparent marking in a colored polymer matrix can be achieved. Irreversible thermochromic pigments are offered by various manufacturers either as powder or aqueous dispersion.

[0057] According to another preferred embodiment of the invention, the irreversible thermochromic pigment has a response temperature ≥ 60 °C to ≤ 200 °C. This has shown to be a good range in order that the change of the thermochromic pigment from the first state to the second state can be achieved by the laser beam.

[0058] In this context and according to another preferred embodiment of the invention the irreversible thermochromic pigment is preferably a leuco dye encapsulated in a shell. Further preferably the irreversible thermochromic pigment is selected from the group comprising Kromagen WB Flexo Ink Magenta K170C, Kromagen WB Flexo Ink Blue K150C, Irreversible Thermochromic Ink - 150C - Green, Kromagen WB Flexo Ink Orange K60C, Thermochrom Pigmente rot (120), Thermochrom Pigmente grün (120), Thermochrom Pigmente blau (120), Irreversible Thermochromic powder black 80C and/or mixtures thereof.

[0059] According to another preferred embodiment of the invention, the irreversible thermochromic pigment has a particle size ≥ 0,01 µm to ≤ 15 µm. Such particle sizes have been shown to give good colored markings with sharp edges.

[0060] Furthermore, and according to a preferred embodiment of the invention, the step of providing the substrate with the at least one layer, preferably comprises providing a substrate with at least one layer, wherein the at least one layer comprises the irreversible thermochromic pigment exclusively in its first state. In other words, before the substrate is irradiated, the irreversible thermochromic pigment within the at least one layer is preferable in its first state. Thus, an irreversible change to its second state can be achieved by a temperature increase above the response temperature due to the irradiation of the substrate with the pulsed laser beam.

[0061] According to another preferred embodiment of the invention the step of providing the substrate with the at least one layer, wherein the at least one layer comprises the polymer and the irreversible thermochromic pigment, comprises providing a substrate with at least one layer, wherein the at least one layer comprises a polymer and an irreversible thermochromic pigment and wherein the irreversible thermochromic pigment in its first state is distributed homogenously within the at least one layer, and wherein the step of generating the carbon trace or vapor trace within the at least one layer comprises generating an inhomogeneous distribution of the thermochromic pigment in the first state or in the second state within the at least one layer. In other words, the irradiation of the substrate with the pulsed laser beam does not lead to a homogeneous color change of the at least one layer. Instead, individual, colored markings are generated.

[0062] As the markings are generated by the temperature increase and are not generated by the absorption of the pulsed laser beam by the irreversible thermochromic pigment, but rather by the absorption of the laser beam by the polymer matrix and the associated heating of the immediate surroundings around the focus spot of the laser beam to a temperature above the colour change temperature of the irreversible thermochromic pigment, there is no need to adapt the wavelength of the pulsed laser beam to the absorption spectrum of the thermochromic pigment. It has been found that the outcome of the engraving/marking is even improved if the laser beam is not absorbed by the thermochromic pigment. In this context and according to a preferred embodiment of the invention the irreversible thermochromic pigment preferably does not have an absorption maximum in a region ≥ 750 nm to ≤ 2000 nm, or in a region ≥ 1000 nm to ≤ 2000 nm.

[0063] Further preferably, the step of providing the substrate with the at least one layer comprising the polymer and the irreversible thermochromic pigment, comprises providing a substrate with at least one layer comprising the polymer and the irreversible thermochromic pigment, wherein the at least one layer has a light transmittance at the wavelength of the pulsed laser beam of more than 95%. In other words, the at least one layer is preferably transparent for the wavelength of the laser beam. Thus, neither the polymer nor the irreversible thermochromic pigment comprises a significant absorption at the wavelength of the laser beam.

[0064] The light transmittance at the wavelength of the pulsed laser beam of the at least one layer is preferably determined according to the DIN 1349 (Transmisson of optical radiation; optical clear (nonscattering) media, quantities, symbols and units). Preferably, the transmittance of the at least one layer is determined with UV-VIS spectroscopy across a path length of 1 cm of the at least one layer.

[0065] According to another preferred embodiment of the invention the focus spot of the pulsed laser beam has a diameter ≥ 2 µm to ≤ 10 µm and/or a thickness of the at least one layer is at least 10 µm. In order to create colored markings with sharp edges the resolution of the created colored markings should be high enough, preferably not less than 250 dpi, more preferably around 500 dpi and most preferred around 1200 dpi. In order to achieve this, it is beneficial if the focus spot of the pulsed laser beam is in between 2 µm to 10 µm, preferably in between 2 µm to 5 µm. However, a diameter of the focus spot below 2 µm complicates the generation of the colored marking. Especially for laser engraving, i.e. when the predetermined first and consecutive positions are within the at least one layer, the thickness of the at least one layer should not be less than 10 µm.

[0066] According to another preferred embodiment of the invention the step generating the carbon trace or vapor trace within the at least one layer by irradiating the substrate with the pulsed laser beam, comprises heating a localized volume within the layer at the first predetermined position to a temperature of about 60°C to 350 °C. In other words, preferably the energy deposited within the layer is enough to raise the temperature locally above the response temperature of the irreversible thermochromic pigment.

[0067] With regard to the pulsed laser beam and according to a preferred embodiment of the invention the pulsed laser beam preferably has a wavelength in the infrared region, more preferably in between 780 nm to 2500 nm, and even more preferably in between 1000 nm and 2000 nm. Particular preferably the wavelength of the pulsed laser beam is 1025 nm, 1040 nm, 1064 nm, 1310 nm, 1350 nm, 1450 nm, 1470 nm, 1550 nm, 1625 nm, or 1650 nm.

[0068] Furthermore, the pulsed laser beam is not a continuous wave but a beam of short light pulses. In this context and according to another preferred embodiment of the invention, the pulsed laser beam has a pulse length ≥ 10^-15 seconds to ≤ 10^-9 seconds, preferably ≥ 10^-15 seconds to ≤ 10^-10 seconds. More preferably the pulse length is ≥ 350 · 10^-15 seconds to ≤ 10 · 10^-12 seconds. In other words, the pulsed laser beam is preferably generated by a femtosecond laser or a picosecond laser. It was found that these pulse lengths are able to deposit enough energy in the at least one layer in order to locally increase the temperature above the response temperature of the irreversible thermochromic pigment.

[0069] According to another preferred embodiment of the invention the pulsed laser beam has a pulse repetition rate ≥ 1 kHz to ≤ 40 MHz and/or wherein the pulse repetition rate is adjusted to the movement of the substrate and/or to the movement of the laser beam such that just one pulse of the laser beam is deposited at the predetermined first position and the predetermined consecutive position. In the context of this invention the pulse repetition rate of the pulsed laser beam is the rate at which pulses of the laser beam reach the substrate. The pulse repetition rate of the pulsed laser beam may be lower than a pulse repetition rate of a laser used for generating the pulsed laser beam. For example, a pulsed laser with a shot repetition rate of 100 kHz may be used together with a beam shutter such that only every tenth pulse generated by the laser is transmitted to the substrate. Thus, in this example the substrate is irradiated with a pulsed laser beam having a pulse repetition rate of 10 kHz.

[0070] According to another preferred embodiment of the invention each pulse of the pulsed laser beam has an energy ≥ 100 nJ to ≤ 10000 nJ, preferably ≥ 150 nJ to ≤ 7000 nJ and more preferably ≥ 200 nJ to ≤ 6500 nJ. Further preferably the laser used to generate the pulsed laser beam has a nominal power of ≥ 10 W. It was found that these pulse energies are able to deposit enough energy in the at least one layer in order to locally increase the temperature above the response temperature of the irreversible thermochromic pigment.

[0071] There are different ways how the substrate comprising the at least one layer may be prepared. According to a preferred embodiment of the invention the step of providing a substrate with the at least one layer comprises preparing the at least one layer by solution casting of a mixture comprising the polymer and the irreversible thermochromic pigment, in-situ polymerization of a mixture comprising a precursor of the polymer and the thermochromic pigment and/or an extrusion process of a mixture comprising the polymer and the irreversible thermochromic pigment.

[0072] Preferably the preparation of the at least one layer does not change the state of the irreversible thermochromic pigment. In this regard and according to a preferred embodiment of the invention during the step of preparing the at least one layer a temperature of the mixture is below the response temperature of the irreversible thermochromic pigment.

[0073] Solution casting of layers is a widely used technique in preparation of packaging materials at laboratory scale because it is easy and simple. However, solution casting is normally not practicable for commercial layer production, as it is difficult to scale-up and requires several processing steps (solubilization, casting, and drying), which lead to a relatively long processing time. On the other hand, extrusion technique can be performed as a continuous process with control of temperature, size, shape, and moisture. Extrusion technique compared to solution casting provides more structured layers and allows for a better dispersion of the irreversible thermochromic pigment within the polymer. Therefore, extrusion techniques are usually preferred in industrial application.

[0074] However, due to the upper temperature limit given by the response temperature of the irreversible thermochromic pigment and in order to avoiding premature color change, not all combinations of irreversible thermochromic pigments and polymers can be processed into layers by extrusion. Preferably, for the polymer polycaprolactone (PCL), the at least one layer is preferably prepared by an extrusion process of a mixture comprising polycaprolactone and the irreversible thermochromic pigment. As PCL has a melting point of 60 °C, it can be processed by extrusion at a temperature below the response temperature of the irreversible thermochromic pigment.

[0075] As already mentioned, it is possible that the substrate forms the one layer comprising the polymer and the irreversible thermochromic pigment. Alternatively, the at least one layer comprising the polymer and the thermochromic pigment is formed on a substrate. Furthermore, there may not only be one layer but multiple layers. It is also possible that the substrate comprises in addition to the at least one layer comprising the polymer and the irreversible thermochromic pigment, an additional layer, which is free of an irreversible thermochromic pigment. It is further possible that the substrate comprises in addition to the at least one layer comprising the polymer and the irreversible thermochromic pigment, an additional layer, wherein the additional layer comprises a polymer and a laser marking pigment, wherein the laser marking pigment is a pigment configured to absorb at a wavelength of the laser beam. Preferably the layer comprising the polymer and a laser marking pigment is also free of an irreversible thermochromic pigment.

[0076] In this context and according to a preferred embodiment of the invention the step of providing the substrate with at least one layer, comprises

• providing a substrate with multiple layers, wherein each of the multiple layers comprises a polymer and an irreversible thermochromic pigment, and wherein the irreversible thermochromic pigment within at least two of the multiple layers are different to each other;
  wherein the step of generating the carbon trace or vapor trace within or on the at least one layer by irradiating the substrate with the pulsed laser beam, such that a focus spot of the laser beam is within or on the at least layer at the first predetermined position, comprises
• generating a carbon trace or vapor trace within one layer of the multiple layers, by irradiating the substrate with a pulsed laser beam, such that a focus spot of the laser beam is within the one layer at a first predetermined position; and
  wherein the step of irradiating the substrate with the pulsed laser beam such that the focus spot of the consecutive laser pulse is within or on the at least one layer at the predetermined consecutive position and the distance between the first position and the consecutive position is less or equal to twofold the diameter of the carbon trace or vapor trace, comprises
• irradiating the substrate with the pulsed laser beam such that a focus spot of a consecutive laser pulse is within the one layer at a predetermined consecutive position and a distance between the first position and the consecutive position is less or equal to twofold a diameter of the carbon trace or vapor trace.

[0077] In other words, preferably a method for laser engraving and/or laser marking is provided comprising the steps of

• providing a substrate with multiple layers, wherein each of the multiple layers comprises a polymer and an irreversible thermochromic pigment, and wherein the irreversible thermochromic pigment within at least two of the multiple layers are different to each other;
• generating a carbon trace or vapor trace within one layer of the multiple layers, by irradiating the substrate with a pulsed laser beam, such that a focus spot of the laser beam is within the one layer at a first predetermined position; and
• irradiating the substrate with the pulsed laser beam such that a focus spot of a consecutive laser pulse is within the one layer at a predetermined consecutive position and a distance between the first position and the consecutive position is less or equal to twofold a diameter of the carbon trace or vapor trace.

[0078] By having multiple layers, wherein each layer comprises an irreversible thermochromic pigment and at least two of the multiple layers comprise different irreversible thermochromic pigments, color mixtures can be generated. For example, the concept of four-color printing, where a print image is built up from four standardized colors by subtractive color mixing (Cyan, Magenta, Yellow, Key in short CMYK), or the concept of hexachrome printing can be applied to laser engraving in order to generate color markings of any color. It is also possible in case an external illumination source is used to illumine the laser engraved article to apply the concept of additive color mixing with red, green, and blue. Thus, providing a substrate with multiple layers with a different irreversible thermochromic pigment within each of the layers, enables to generate markings of different colors.

[0079] According to a preferred embodiment of the invention the multiple layers are directly attached to each other. In other words, there is preferably no layer that does not comprise a polymer and an irreversible thermochromic pigment in between two layers that comprise a polymer and an irreversible thermochromic pigment.

[0080] Further preferably when irradiating the substrate with the pulsed laser beam, the process is preferably started at the layer farthest away from the laser source. When repeating the process within a layer nearer to the laser source, the colored markings generated in the first layer do not negatively influence the propagation of the laser beam. In this regard and according to a preferred embodiment of the invention the the steps of

• generating a carbon trace or vapor trace within one layer of the multiple layers, and
• irradiating the substrate with the pulsed laser beam such that a focus spot of a consecutive laser pulse is within the one layer,

are performed multiple times within different layers of the multiple layers, starting with a layer that is farthest away from a laser source.

[0081] Particularly preferably the substrate comprises at least three layers. In this regard and according to a preferred embodiment of the invention the step of providing a substrate with at least one layer, comprises providing a substrate with at least three layers, wherein each of the three layers comprises a polymer and an irreversible thermochromic pigment, wherein the irreversible thermochromic pigments within each of the three layers are different to each other and wherein a first irreversible thermochromic pigment of a first layer is such that after having generated the carbon trace or vapor trace within the first layer, the first irreversible thermochromic pigment appears yellow or green, a second irreversible thermochromic pigment of a second layer is such that after having generated the carbon trace or vapor trace within the second layer, the second irreversible thermochromic pigment appears red or magenta, and/or a third irreversible thermochromic pigment of the third layer is such that that after having generated the carbon trace or vapor trace within the third layer, the third irreversible thermochromic pigment appears blue or cyan.

[0082] For CMYK, the substrate preferably comprises an additional fourth layer, wherein the additional fourth layer preferably comprises a polymer and a laser marking pigment, wherein the laser marking pigment is a pigment configured to absorb at a wavelength of the laser beam. Particular preferably the fourth layer is an outer layer. Furthermore, when using a substrate that comprising a layer with a laser marking pigment, the substrate is preferably irradiated such that irradiations starts with the layer comprising the laser marking pigment and such that this layer is farthest away from the laser source.

[0083] With regard to preparing the multiple layers different options are possible. For example, solution casting, in-situ polymerization, extrusion processes and/or coextrusion processes can be applied.

[0084] According to a preferred embodiment of the invention the step of providing a substrate with multiple layers, wherein each of the multiple layers comprises a polymer and an irreversible thermochromic pigment, and wherein the irreversible thermochromic pigment within at least two of the multiple layers are different to each other, comprises

• casting a first layer by solution casting of a mixture comprising a polymer and a first irreversible thermochromic pigment,
• drying the first casted layer, and
• casting a second layer on the dried first layer by solution casting of a mixture comprising a polymer and a second irreversible thermochromic pigment, wherein the first irreversible thermochromic pigment is different from the second irreversible thermochromic pigment.

[0085] Alternatively to solution casting, in-situ polymerization is possible. According to a preferred embodiment of the invention the step of providing a substrate with multiple layers, wherein each of the multiple layers comprises a polymer and an irreversible thermochromic pigment, and wherein the irreversible thermochromic pigment within at least two of the multiple layers are different to each other, comprises

• providing a first layer by in-situ polymerization of a mixture comprising a precursor of a polymer and a first irreversible thermochromic pigment,
• providing a second layer on the first layer by in-situ polymerization of a mixture comprising a precursor of a polymer and a second irreversible thermochromic pigment on the first layer, wherein the first irreversible thermochromic pigment is different from the second irreversible thermochromic pigment.

[0086] Further alternatively an extrusion process is possible. According to a preferred embodiment of the invention the step of providing a substrate with multiple layers, wherein each of the multiple layers comprises a polymer and an irreversible thermochromic pigment, and wherein the irreversible thermochromic pigment within at least two of the multiple layers are different to each other, comprises

• providing a first layer by an extrusion process of a mixture comprising a polymer and a first irreversible thermochromic pigment,
• providing a second layer by an extrusion process of a mixture comprising a polymer and a second irreversible thermochromic pigment, wherein the first irreversible thermochromic pigment is different from the second irreversible thermochromic pigment, and
• binding the first and second layer by lamination.

[0087] Further alternatively a coextrusion process is possible, where the individual layers are extruded at the same time and the individual layers adhere to each other and are bonded together by placing the molten layers on top of each other. According to a preferred embodiment of the invention the step of providing a substrate with multiple layers, wherein each of the multiple layers comprises a polymer and an irreversible thermochromic pigment, and wherein the irreversible thermochromic pigment within at least two of the multiple layers are different to each other, comprises

• providing a first layer by an extrusion process of a mixture comprising a polymer and a first irreversible thermochromic pigment,
• providing a second layer by an extrusion process of a mixture comprising a polymer and a second irreversible thermochromic pigment, wherein the first irreversible thermochromic pigment is different from the second irreversible thermochromic pigment, and
• binding the first and second layer by heat necessary for the extrusion of the first and second layer.

[0088] Furthermore, the invention relates to a laser marked and/or engraved article produced by the above method. Since the carbon traces or vapor traces are diminished during the marking and/or engraving process the laser marked and/or engraved article shows brighter color markings. Further technical features and advantages are evident for the person skilled in the art from the description of the method for laser engraving and/or laser marking and the description of the examples.

[0089] Furthermore, the invention relates to an article for laser engraving and/or laser marking comprising a substrate with at least three layers, wherein each of the three layers comprises a polymer and an irreversible thermochromic pigment, wherein the irreversible thermochromic pigments within each of the three layers are different to each other and wherein

a first irreversible thermochromic pigment of a first layer is such that after having generated a carbon trace or vapor trace within the first layer by a pulsed laser beam, the first irreversible thermochromic pigment appears yellow or green,

a second irreversible thermochromic pigment of a second layer is such that after having generated a carbon trace or vapor trace within the second layer by a pulsed laser beam, the second irreversible thermochromic pigment appears red or magenta, and/or

a third irreversible thermochromic pigment of the third layer is such that that after having generated a carbon trace or vapor trace within the third layer by a pulsed laser beam, the third irreversible thermochromic pigment appears blue or cyan.

[0090] The article for laser marking and/or engraving allows to generate a colored marking with basically any color by the concept of additive or subtractive color mixing. Preferably, the three layers are directly attached to each other. Further preferably, the article comprises a fourth layer, wherein the fourth layer is not sandwiched in between the first to third layer and/or wherein the fourth layer is an outermost layer. Preferably, the fourth layer comprises a polymer and a laser marking pigment, wherein the laser marking pigment is a pigment configured to absorb at a wavelength in between 780 nm to 2500 nm, and more preferably in between 1000 nm and 2000 nm.

[0091] Further technical features and advantages of the article for laser marking and/or engraving are evident for the person skilled in the art from the description of the method for laser engraving and/or laser marking, from the description of the laser marked and/or engraved article and/or from the description of the examples.

Examples

[0092] The invention will further be described without limiting effect by reference to the following examples.

[0093] In the following the preparation of different substrates with at least one layer, wherein the at least one layer comprises a polymer and an irreversible thermochromic pigment, is described. In examples 1 to 16 the layer or layers are prepared by solution casting (summarized in table 4), in examples 17 to 20, the layer or layers are prepared by in-situ polymerization (summarized in table 5) and in examples 21 to 25 the layer or layers are prepared by extrusion (summarized in table 6).

A. Layer preparation by solution casting technique and solvent evaporation

[0094] In a first step, a clear solution with 20 wt % of a polymer in a solvent is prepared. For this purpose, the polymer is added stepwise to the heated solvent and stirred until the polymer has dissolved completely. Afterwards the solution is cooled to room temperature and filtered. The polymer solutions prepared are given in table 1:

Table 1: Polymer solutions
	Polymer	Type; Supplier	Solvent	mixing temperature
1.1	Ethyl cellulose	ET 200; Kremer Pigmente GmbH & Co. KG	70% toluene, 30% ethanol 95%	60°C
1.2	Condensation resins of urea with aliphatic aldehydes	Laropal A 81; BASF SE	mixture of aromatic and non-aromatic solvents	60°C
1.3	Poly methyl methacrylate	PMMA, Mw ~ 120,000 g/mol; Sigma-Aldrich	Tetrahydrofuran	50°C
1.4	Poly butylene methacrylate	Plexisol® P 550-40; Kremer Pigmente GmbH & Co. KG	Special petrol 100/125	50°C
1.5	Poly vinylbutyral	Polyvinylbutyral 30 H; Kremer Pigmente GmbH & Co. KG	Ethanol, 95%	60°C
1.6	Poly epsilon caprolactone	Capa 6400; Ingevity	Toluene	60°C
1.7	Poly carbonate	Makrolon 2405; Covestro AG	Dioxane	60°C
1.8	Polyvinyl alcohol	Mowiflex C 17; Kuraray	Water	80°C

[0095] In a next step an irreversible thermochromic pigment is mixed with the polymer solution. Some irreversible thermochromic pigments are received from the supplier as powder, other as dispersion or concentrate (thickened slurry).

[0096] The following irreversible thermochromic pigments and solvents have been used:

Table 2: Pigments
	Irreversible thermo-chromic pigment	First state → second state; Supplier	Received from supplier as	response temperature
a	Kromagen WB Flexo Ink Magenta K170C	Colorless → magenta; Lawrence Industries Ltd.	Water, 40% solid	170°C
b	Kromagen WB Flexo Ink Blue K150C	Colorless → blue; Lawrence Industries Ltd.	Water, 40% solid	150°C
c	Irreversible Thermochromic Ink - 150C - Green	Colorless → green; NNC New Prismatic Enterprises & Co.	Water, 40% solid	150°C
d	Kromagen WB Flexo Ink Orange K60C	Colorless → orange; Lawrence Industries Ltd.	Water, 40% solid	60°C
e	Thermochrom Pigmente rot (120)	Colorless → red; Sintal Chemie GmbH	powder	120°C
f	Thermochrom Pigmente grün (120)	Colorless → green; Sintal Chemie GmbH	powder	120°C
g	Thermochrom Pigmente blau (120)	Colorless → blue; Sintal Chemie GmbH	powder	120°C
h	Irreversible Thermochromic powder black 80C	black → colorless; New Color Chemical Co.,Limited.	powder	310°C

[0097] The irreversible thermochromic pigments have been mixed with the polymer solution according to table 3 under intensive agitation at room temperature. For some polymer/pigment combination (mixtures 1, 2, 5, and 8) water or another solvent miscible with the polymer solution was added to the pigment as received from the supplier before mixing with the polymer solution. All solvents were received from Sigma-Aldrich and used as supplied without further purification.

Table 3: Mixture of polymer and irreversible thermochromic pigment
Mixture	Polymer Solution i.)	Irreversible thermochromic Pigment
		Kind of	Dosage wt% ii)	Form of dosage
1	1.1	e	4%	20 wt% in water/ethanol 50/50
2	1.2	e	4%	20 wt% in mixture of aromatic and aliphatic solvents
3	1.3	a	2%	as received from supplier
4	1.3	c	2%	as received from supplier
5	1.4	e	3%	20 wt% in Special Petrol 20/100
6	1.5	a	3%	as received from supplier
7	1.5	c	3%	as received from supplier
8	1.6	e	8%	20 wt% in toluene
9	1.7	b	3%	as received from supplier
10	1.7	c	3%	as received from supplier
11	1.8	a	4%	as received from supplier
12	1.8	b	4%	as received from supplier
13	1.8	d	4%	as received from supplier

i.) As described above: 20 wt % polymer solution ii.) Dosage in form of dosage to the 20 wt % polymer solution

Single-layer: Examples 1 - 13

[0098] Single layers have been prepared by casting the mixtures given in table 3 into a petri dish. A Petri dish 120 × 120mm² is first degreased and cleaned using neodisher^® LaboClean FLA (3mL/L in water) and subsequently using neodisher^® N (2mL/L in water) (both from neo disher). The Petri dish is rinsed with clear water and dried.

[0099] To prepare the sample 1 - 13 the well-stirred mixture of the thermochromic pigment in the respective polymer solution according to table 3 is poured slowly to the Petri dish so that no air bubbles are formed. The mixture in the Petri dish is placed on a horizontal surface, slowly heated to 60°C (exception mixture 13; T = 50°C) and kept at this temperature for 24h so that most of the solvent has evaporated and a solid film is formed. The temperature is then increased to 80°C (exception mixture 13; T kept at 50°C) and the film in the Petri dish is dried until it reaches a constant weight. After cooling to room temperature, the film is carefully removed from the Petri dish.

Multi-layer: Examples 14 - 16

[0100] To produce multilayers, the first layer is produced and dried as described under single layer. To produce the second layer, the mixture of the second layer is poured onto the dried first layer and dried quickly without forming bubbles. The fast drying of the second layer prevents the previous layer from completely dissolving again. By dissolving the upper most surface of the first layer again, the two layers are firmly bonded together. For the third layer, process is repeated as for the second layer.

Table 4 summarizes the samples prepared by solution casting.

Table 4: Samples prepared by solution casting
Sample	Mixture	Thickness [µm]
1	1	60
2	2	120
3	3	150
4	4	150
5	5	120
6	6	100
7	7	120
8	8	30
9	9	120
10	10	90
11	11	60
12	12	60
13	13	60
Multilayer sample 14	1. layer: mixture 3	1. layer: 97
	2. layer: mixture 4	2. layer: 103
Multilayer sample 15	1. layer: mixture 9	1. layer: 187
	2. layer: mixture 10	2. layer: 93
Multilayer sample 16	1. layer: mixture 11	1. layer: 50
	2. layer: mixture 12	2. layer: 53
	3. layer: mixture 13	3. layer: 56

B. Film manufacturing by in-situ polymerisation: PMMA

[0101] A Petri dish 120 × 120mm² is first degreased and cleaned using neodisher^® LaboClean FLA (3mL/L in water) and subsequently using neodisher^® N (2mL/L in water) (both from neo disher). The Petri dish is rinsed with clear water and dried.

[0102] A well-stirred and thus homogeneous mixture of thermochromic pigment in methyl methacrylate (Sigma-Aldrich, used as received), 2.5 wt.% PEROXAN BP powder 50 W (Pultex GmbH) and 0.05% PERGAQUICK A200 (Pultex GmbH) (each based on 100% methyl methacrylate) is added slowly to this Petri dish so that no air bubbles are formed.

[0103] The mixture in the Petri dish is placed on a horizontal surface, slowly heated to 60°C and kept at this temperature for 120 min. The mixture is then cooled to room temperature in the Petri dish and the film thus prepared is carefully removed.

[0104] Multi-layer films are prepared by casting the next layer on top of the polymerized previous one. Preparation and curing of the next layers are the same as for the first layer. The multi-layer films are removed from the Petri dish together after all the intended layers have been cast.

Table 5 summarizes the samples prepared by in situ polymerization.

Table 5: Samples prepared by in situ polymerization
Sample	Thermochromic pigment (see table 2)	Dosage of pigment in wt%, based on 100% methyl methacrylate	Film thickness [µm]	Comment
17	e	3	98	Clear film
18	f	2	105	Clear film
19	g	1	290	Slightly hazy film
Multilayer sample	1. layer: e	1. layer: 2	1. layer: 195	slightly hazy film
	2. layer: f	2. layer: 2	2. layer: 195
20	3. layer: g	3. layer: 2	3. layer: 204
21	h	3	50	Colored film

C. Film manufacturing by extrusion: Poly-s-caprolactone

[0105] Poly-s-caprolactone layers with irreversible thermochromic pigments were prepared by using a co-rotating twin-screw extruder (Haake Process 11; Thermo Fisher Scientific Inc., Karlsruhe, Germany) with a screw diameter of 11 mm and length of 440 mm. Poly-s-caprolactone pellets (Capa 6400) and irreversible thermochromic pigments were mixed and fed into the feeder. To ensure homogeneous distribution of the thermochromic pigments in the Poly-s-caprolactone, pellets were extruded and pelletized three times before they were extruded in the form of a sheet. The strand die (2.0 mm diameter) and sheet die (3-mm width × 0,5 mm height) was used for producing pellets and films, respectively. The speed of the feeder and the screw was 5 rpm and 50 rpm, respectively. The temperature profile was varied from 65 °C at the feeder to 75 °C at the die.

[0106] The extruded films are laminated by heat and pressure using a heating press machine (Ocean Science Co., Uiwang, Korea). Compression was carried out between mirror polished stainless steel plates for 2 min at 40 MPa and 59 °C. The heated plates were removed from the machine and immediately cooled to RT. Alternatively, multi-layer films can also be extruded directly and cast using appropriate tools.

[0107] Multi-layer film laminates with different thermochromic pigments were produced on the press using the above method. The individual film layers were placed on top of each other accordingly and bonded together as described by pressure and temperature.

Table 6 summarizes the samples prepared by extrusion.

Table 6: Samples prepared by extrusion
Sample	Thermochromic pigment (see table 2	Dosage of pigment in wt%, based on 100% poly-ε-caprolactone	Film thickness [µm]	Comment
22	e	2	198	Slightly hazy film
23	f	2	183	Slightly hazy film
24	g	2	201	Slightly hazy film
Multilayer sample 25	1. layer: e	1. layer: 2	1. layer: 192	Total thickness: 572 µm hazy film
	2. layer: f	2. layer: 2	2. layer: 180
	3. layer: g	3. layer: 2	3. layer: 200

D. Laser engraving of the film samples 1-25

[0108] To engrave the produced samples the "Satsuma HP" laser from Amplitude (wavelength λ = 1030 nm, pulse duration 350 fs < τ < 10 ps, repetition rate 1 kHz < f_Rep < 40 MHz) was used in combination with the Mitutoyo Plan APO NIR optics (scale 5 - 20x, numerical aperture NA 0.4). The scanner for deflection and positioning of laser beams in the working plane used is the ScanLab "intelliSCANde14". All markings have been created in a depth that corresponds to half the layer thickness. For engraving the films are fixed in a frame. The pulse distance is calculated according to pulse distance

. Lines are formed by color markings executed one after the other in one direction (preferably x-direction). The term line is thus synonymous with a linear carbon or vapor trace. Areas are correspondingly formed by several lines next to each other or on top of each other at different depths. Increasing the pulse energy leads to a larger z-expansion of the modification to a small extent, only. The process results are not influenced by the scanning rate for the process parameters given. This also applies to the maximum used scanning rate of 200 mm/s.

Table 7: Laser engraving parameters and results
PMMA (samples 3, 4, 14, and 17 to 21) does not show carbonization and black coloring when engraved by laser, but foaming with white marking and/or turbidity, in other words a vapor trace.
Poly-e-caprolactone (samples 36 to 39) shows carbonization and a greyish coloring due to carbonization when engraved by laser.
Trial	Sample	Pulse Repetition Rate [kHz]	Scanning Rate [mm/s]	Pulse Distance [µm]	Diameter Carbon Trace or vapor trace [µm]	Pulse Energy [nJ]	Result
1	1	5	20	4	2	350	from 1 to 3 decreasing blackness of the carbon trace and increasing color strength (red)
2	1	5	10	2	2	350
3	1	5	5	1	2	350
4	2	1	20	20	5	500	5 shows significant higher color strength (red) and lower blackness than 4
5	2	5	20	4	5	500
6	3	1	20	20	10	600	7 shows significant higher color strength (magenta) and lower turbidity than 6
7	3	4	20	5	10	600
8	4	1	20	20	10	600	9 shows significant higher color strength (green) and lower turbidity than 8
9	4	4	20	5	10	600
10	5	5	5	1	2	450	red line comparable to trial 3; higher pulse energy has no significant influence on the result
11	5	5	5	1	2	450	several lines next to each other at a distance (y-direction) of 10 were inserted and result in a horizontal red area
12	6	4	20	5	10	750	magenta line comparable to trial 7; higher pulse energy has no significant influence on the result
13a	6	4	20	5	10	750	several lines next to each other at a distance (z-direction) of 25 µm were inserted and result in a vertical magenta area

PMMA (samples 3, 4, 14, and 17 to 21) does not show carbonization and black coloring when engraved by laser, but foaming with white marking and/or turbidity, in other words a vapor trace.
Poly-e-caprolactone (samples 36 to 39) shows carbonization and a greyish coloring due to carbonization when engraved by laser.
Trial	Sample	Pulse Repetition Rate [kHz]	Scanning Rate [mm/s]	Pulse Distance [µm]	Diameter Carbon Trace or vapor trace [µm]	Pulse Energy [nJ]	Result
13b	6	4	40	10	5	500	several lines next to each other at a distance (y-direction) of 20 µm were inserted and result in a horizontal greyish magenta area (RGB199/0/105; CieLab: 43,0/70,7/-0,5)
13c	6	4	40	10	5	500	several lines next to each other at a distance (y-direction) of 20 µm were inserted and result in a horizontal magenta area brighter in color compared to 13b; alternating repetition rate used to modify a spot twice; (RGB: 238/0/138; CieLab: 51,9/81,7/-6,8)
		20	40	2	5	500
14	7	4	20	5	10	600	the alternating repetition rate results in single, isolated green colored dots
		1	20	20	10
15	8	1	1	1	2	350	colorful red lines with low blackness: scanning rate has no significant influence on the result
16	8	5	5	1	2	350
17	9	4	20	5	10	1500	colorful blue line with certain remaining blackness
18	10	5	10	2	2	1200	colorful green line with certain remaining blackness
19	11	100	100	1	2	500	colorful magenta line
20	11	25	200	8	2	500	greyish magenta dots strung together like a string of pearls
21	12	150	150	1	2	500	colorful blue line
22	12	30	150	5	2	500	greyish blue dots strung together like a string of pearls
23	13	100	200	1	2	500	colorful orange line
24	14	4	20	5	10	900	Multilayer sample: parameters given from bottom to top layer (bottom layer = layer 2 engraved first); vapor trace inserted offset, so that the engraved color markings (bottom: green; top: magenta) are visible next to each other in the top view; colored lines with little residual turbidity preserved
		4	20	5	10	450
25	15	1	20	20	10	900	Multilayer sample: parameters given from bottom to top layer (bottom layer = layer 2 engraved first); carbon trace inserted offset, so that the engraved color markings (bottom: green; top: blue) are visible next to each other in the top view; bottom line: colored lines with high residual blackness preserved top line: colored lines with low residual blackness preserved
		4	20	5	10	450

Trial	Sample	Pulse Repetition Rate [kHz]	Scanning Rate [mm/s]	Pulse Distance [µm]	Diameter Carbon Trace or vapor trace [µm]	Pulse Energy [nJ]	Result
26	16	50	50	1	2	800	Multilayer sample: parameters given from bottom to top layer (bottom layer = layer 3 engraved first); carbon trace inserted offset, so that the engraved color markings (bottom: orange; middle: blue; top: magenta) are visible next to each other in the top view; bottom to top line: colorful lines with low residual blackness preserved
		50	50	1	2	600
		50	50	1	2	400
27	17	5	20	4	2	750	from 1 to 3 decreasing white vapor trace and increasing color strength (red); the color impression is good in all three trials.
28	17	5	10	2	2	750
29	17	5	5	1	2	750
30	18	100	200	2	2	1000	colorful green line
31	18	25	200	8	2	1000	milky green dots strung together like a string of pearls
32	18	40	200	5	10	1500	colorful green line
33	19	40	200	5	10 - 15	2000	Due to the slight haziness of the film, the laser beam is slightly scattered. To compensate, the laser energy was increased. The scattering leads to an irregularity and enlargement of the inserted color markings (blue).
34	20	50	50	1	2	1500	Multilayer sample: parameters given from bottom to top layer (bottom layer = layer 3 engraved first); vapor trace inserted offset, so that the engraved color markings (bottom: blue; middle: green; top: red) are visible next to each other in the top view;
		50	50	1	2	1200
		50	50	1	2	900
							Due to the slight haziness of the film, the laser beam is slightly scattered. To compensate, the laser energy was increased. The scattering leads to an irregularity and enlargement of the inserted color markings;
							bottom to top line: colorful lines
35	21	10	100	10	10	800	The color change of the thermochromic pigment is from black to colorless. The engraving of the sample is therefore from top to bottom.
							The additional foaming of the PMMA enhances the contrast.
36	22	5	20	4	2	1100	The laser engraving of all three samples results in colorful lines (red, green, blue). As the distance between the pulses decreases, the remaining greyish color decreases and the color becomes more intense.
37	23	5	10	2	2	1100
							Due to the slight haziness of the film, the laser beam is slightly scattered. To compensate, the laser energy was increased. The scattering leads to an irregularity and enlargement of the inserted color markings
38	24	5	5	1	2	1100
39	25	10	10	1	2	1500	Multilayer sample: parameters given from bottom to top layer (bottom layer = layer 3 engraved first); carbon trace inserted offset, so that the engraved color markings (bottom: blue; middle: green; top: red) are visible next to each other in the top view;
						1300
						1100
							Due to the slight haziness of the film, the laser beam is slightly scattered. To compensate, the laser energy was increased. The scattering leads to an irregularity and enlargement of the inserted color markings bottom to top line: colorful lines

[0109] With regard to trials 13b and 13c several lines next to each other were engraved to create a colored area of about 5 × 5 mm². Based on this area the color was determined.

RGB

[0110] Colors can be described in the RGB or in the Lab color space. In the examples the coloration of the samples is determined in the Lab color space and measured with a Spectrometer Konika-Minolta CM-3600A - according to the guideline of the INSTRUCTION MANUAL CM-3600A (^©2011-2013 KONICA MINOLTA, INC.). The Lab color can be converted into the RGB color.

The principle of the RGB color space

[0111] This principle is based on the three-color theory. The RGB color space works on the principle of the additive color space. This means that it reproduces the entire color range by mixing the basic colors red, green and blue.

[0112] The RGB color space can be found in all self-illuminating systems, such as monitors or television screens. All possible colors are defined by their red, green and blue components and mapped accordingly by the overlay of colored light.

The principle of the Lab color space

[0113] Unlike the RGB color space, the Lab color space is based on counter-color theory. This is based on the assumption that three separate chemical processes take place in the human retina, which always contain two opposite colors, the two opposite colors striving for balance with one another. An example pair would be the combination of blue and yellow. Lab is used, for example, for photo editing software. While the RGB color space is device-dependent, it is not the Lab color space. RGB includes - regardless of the device - all potentially possible colors, which above all enables the conversion of color definitions from one device to the other.

Convert RGB to Lab

[0114] It is important for the conversion that Lab coordinates separate brightness information L from the rest of the color information. RGB images do not have such a separation - a change in brightness therefore changes the entire color information.

Convert sRGB to Lab: http://colormine.org/convert/rgb-to-lab

Conversion Lab to sRGB: http://colormine.org/convert/lab-to-rgb

The Lab - measuring device

[0115] The Lab is measured with a - Spectrometer Konika-Minolta CM-3600A - according to the guideline of the INSTRUCTION MANUAL CM-3600A (^©2011-2013 KONICA MINOLTA, INC.).

[0116] The experiments clearly show that the introduction of colored markings by heating irreversible thermochromic pigments in a polymer matrix to temperatures above the color change point with two successive laser pulses at a distance equal to or less than twice the diameter of the carbon trace or vapor trace created by the first pulse, leads to a significant improvement in the colorfulness or color intensity and at the same time the blackness of the carbonization or the turbidity of the vapor trace of the polymer matrix, which occurs simultaneously by the laser, decreases.

Claims

1. Method for laser engraving and/or laser marking comprising the steps of:

- providing a substrate with at least one layer, wherein the at least one layer comprises a polymer and an irreversible thermochromic pigment,

- generating a carbon trace or vapor trace within or on the at least one layer by irradiating the substrate with a pulsed laser beam, such that a focus spot of the laser beam is within or on the at least one layer at a first predetermined position, and

- irradiating the substrate with the pulsed laser beam such that the focus spot of a consecutive laser pulse is within or on the at least one layer at a predetermined consecutive position and a distance between the first position and the consecutive position is less or equal to twofold a diameter of the carbon trace or vapor trace.

2. Method according to claim 1, wherein the step of providing the substrate with the at least one layer comprises providing a substrate with at least one transparent and/or translucent layer.

3. Method according to any of the previous claims, wherein the polymer is selected from the group comprising polycarbonate (PC), polyethylene terephthalate (PET), Polycaprolactone (PCL), polyethylene naphthalate (PEN), polyethylene furanoate (PEF), glycolized or amorphous polyester, glycolized or amorphous polyethylene terephthalate (PET-G or A-PET), polytrimethylene terephthalate (PTT), polybutylene terephthalate (PBT), poly(methyl methacrylate) (PMMA), polypropylene (PP), polyvinyl chloride (PVC), cellulose triacetate (TCA), polyamide (PA), poly imide (PI) or polyethylene (PE), polyacetales like polyoxymethylene (POM), polystyrene (PS), polystyrene copolymers (ABS, SAN, SB), thermoplastic elastomers (TPU, TPO, SEBS) and/or mixtures thereof.

4. Method according to any of the previous claims, wherein the at least one layer is free of a laser marking pigment, wherein the laser marking pigment is a pigment configured to absorb at a wavelength of the laser beam.

5. Method according to any of the previous claims, wherein the at least one layer comprises a stabilizer selected from the group comprising primary antioxidants, secondary antioxidants, antiozonant, UV absorbers, and/or hindered amines light stabilizers.

6. Method according to the previous claim, wherein the at least one layer comprises the stabilizer in an amount ≥ 0,001 wt % to ≤ 1 wt %, based on the weight of the at least one layer.

7. Method according to any of the previous claims, wherein the at least one layer comprises the irreversible thermochromic pigment in an amount ≥ 0,01 wt % to ≤ 25 wt %, based on the weight of the at least one layer.

8. Method according to any of the previous claims, wherein the irreversible thermochromic pigment is configured to exist in two states, wherein in the first state the irreversible thermochromic pigment absorbs electromagnetic waves in a first wavelength range, and wherein in the second state the irreversible thermochromic pigment absorbs electromagnetic waves in a second wavelength range different to the first wavelength range, and wherein an irreversible change from the first state to the second state is feasible by heating the irreversible thermochromic pigment above a response temperature.

9. Method according to any of the previous claims, wherein the irreversible thermochromic pigment is configured to exist in two states, wherein in one state the irreversible thermochromic pigment absorbs electromagnetic waves in the visible range, and wherein in the other state the irreversible thermochromic pigment does not absorb electromagnetic waves in the visible range.

10. Method according to any of the previous claims, wherein the irreversible thermochromic pigment has a response temperature ≥ 60 °C to ≤ 200 °C.

11. Method according to any of the previous claims, wherein the irreversible thermochromic pigment is selected from the group comprising Kromagen WB Flexo Ink Magenta K170C, Kromagen WB Flexo Ink Blue K150C, Irreversible Thermochromic Ink - 150C - Green, Kromagen WB Flexo Ink Orange K60C, Thermochrom Pigmente rot (120), Thermochrom Pigmente grün (120), Thermochrom Pigmente blau (120), Irreversible Thermochromic powder black 80C and/or mixtures thereof.

12. Method according to any of the previous claims, wherein the irreversible thermochromic pigment has a particle size ≥ 0,01 µm to ≤ 15 µm.

13. Method according any of the previous claims, wherein the step of providing the substrate with the at least one layer, comprises providing a substrate with at least one layer, wherein the at least one layer comprises the irreversible thermochromic pigment exclusively in its first state.

14. Method according to any of the previous claims, wherein the step of providing the substrate with the at least one layer, wherein the at least one layer comprises the polymer and the irreversible thermochromic pigment, comprises providing a substrate with at least one layer, wherein the at least one layer comprises a polymer and an irreversible thermochromic pigment and wherein the irreversible thermochromic pigment in its first state is distributed homogenously within the at least one layer;
and wherein the step of generating the carbon trace or vapor trace within the at least one layer comprises generating an inhomogeneous distribution of the irreversible thermochromic pigment in the first state or in the second state within the at least one layer.

15. Method according to any of the previous claims, wherein the irreversible thermochromic pigment does not have an absorption maximum in a region ≥ 1000 nm to ≤ 2000 nm or in a region ≥ 750 nm to ≤ 2000 nm.

16. Method according to any of the previous claims, wherein the step of providing the substrate with the at least one layer comprising the polymer and the irreversible thermochromic pigment, comprises providing a substrate with at least one layer comprising the polymer and the irreversible thermochromic pigment, wherein the at least one layer has a light transmittance at the wavelength of the pulsed laser beam of more than 95%.

17. Method according to any of the previous claims, wherein the focus spot of the pulsed laser beam has a diameter ≥ 2 µm to ≤ 10 µm and/or a thickness of the at least one layer is at least 10 µm.

18. Method according to any of the previous claims, wherein the step of generating the carbon trace or vapor trace within the at least one layer by irradiating the substrate with the pulsed laser beam, comprises heating a localized volume within the layer at the first predetermined position to a temperature of about 60°C to 350 °C.

19. Method according to any of the previous claims, wherein the pulsed laser beam has a wavelength in the infrared region, preferably in a region ≥ 780 nm to ≤ 2500 nm, and more preferably ≥ 1000 nm to ≤ 2000 nm, and/or wherein the pulsed laser beam has a wavelength of 1025 nm, 1040 nm, 1064 nm, 1310 nm, 1350 nm, 1450 nm, 1470 nm, 1550 nm, 1625 nm, or 1650 nm.

20. Method according to any of the previous claims, wherein the pulsed laser beam has a pulse length ≥ 10^-15 seconds to ≤ 10^-9 seconds, preferably ≥ 10^-15 seconds to ≤ 10^-10 seconds.

21. Method according to any of the previous claims, wherein the pulsed laser beam has a pulse repetition rate ≥ 1 kHz to ≤ 40 MHz and/or wherein the pulse repetition rate is adjusted to the movement of the substrate and/or to the movement of the laser beam such that just one pulse of the laser beam is deposited at the predetermined first position and the predetermined consecutive position.

22. Method according to any of the previous claims, wherein each pulse of the pulsed laser beam has an energy ≥ 100 nanojoule to ≤ 10000 nanojoule and/or wherein the laser used to generate the pulsed laser beam has a nominal power of ≥ 10 W.

23. Method according to any of the previous claims, wherein the step of irradiating the substrate with the pulsed laser beam such that the focus spot of a consecutive laser pulse is within or on the at least one layer at a predetermined consecutive position and a distance between the first position and the consecutive position is less or equal to twofold a diameter of the carbon trace or vapor trace comprises moving the substrate and the laser beam relative to each other preferably at a velocity ≥ 0,1 mm/s to ≤ 500 mm/s.

24. Method according to any of the previous claims, wherein the step of providing the substrate with the at least one layer comprises preparing the at least one layer by solution casting of a mixture comprising the polymer and the irreversible thermochromic pigment, in-situ polymerization of a mixture comprising a precursor of the polymer and the irreversible thermochromic pigment and/or an extrusion and/or coextrusion process of a mixture comprising the polymer and the irreversible thermochromic pigment.

25. Method according to the previous claim, wherein during the step of preparing the at least one layer a temperature of the mixture is below a response temperature of the irreversible thermochromic pigment.

26. Method according to any of the previous claims, wherein the step of providing the substrate with at least one layer, comprises

- providing a substrate with multiple layers, wherein each of the multiple layers comprises a polymer and an irreversible thermochromic pigment, and wherein the irreversible thermochromic pigment within at least two of the multiple layers are different to each other;
wherein the step of generating the carbon trace or vapor trace within or on the at least one layer by irradiating the substrate with the pulsed laser beam, such that a focus spot of the laser beam is within or on the at least layer at the first predetermined position, comprises

- generating a carbon trace or vapor trace within one layer of the multiple layers, by irradiating the substrate with a pulsed laser beam, such that a focus spot of the laser beam is within the one layer at a first predetermined position; and
wherein the step of irradiating the substrate with the pulsed laser beam such that the focus spot of the consecutive laser pulse is within or on the at least one layer at the predetermined consecutive position and the distance between the first position and the consecutive position is less or equal to twofold the diameter of the carbon trace or vapor trace, comprises

- irradiating the substrate with the pulsed laser beam such that a focus spot of a consecutive laser pulse is within the one layer at a predetermined consecutive position and a distance between the first position and the consecutive position is less or equal to twofold a diameter of the carbon trace or vapor trace.

27. Method according to the previous claim, wherein the multiple layers are directly attached to each other.

28. Method according to claim 26 or 27, wherein the steps of

- generating a carbon trace or vapor trace within one layer of the multiple layers, and

- irradiating the substrate with the pulsed laser beam such that a focus spot of a consecutive laser pulse is within the one layer, are performed multiple times within different layers of the multiple layers, starting with a layer that is farthest away from a laser source.

29. Method according to any of claims 26 to 28, wherein the step of providing a substrate with multiple layers, wherein each of the multiple layers comprises a polymer and an irreversible thermochromic pigment, and wherein the irreversible thermochromic pigment within at least two of the multiple layers are different to each other, comprises

- casting a first layer by solution casting of a mixture comprising a polymer and a first irreversible thermochromic pigment,

- drying the first casted layer, and

- casting a second layer on the dried first layer by solution casting of a mixture comprising a polymer and a second irreversible thermochromic pigment, wherein the first irreversible thermochromic pigment is different from the second irreversible thermochromic pigment.

30. Method according to any of claims 26 to 28, wherein the step of providing a substrate with multiple layers, wherein each of the multiple layers comprises a polymer and an irreversible thermochromic pigment, and wherein the irreversible thermochromic pigment within at least two of the multiple layers are different to each other, comprises

- providing a first layer by in-situ polymerization of a mixture comprising a precursor of a polymer and a first irreversible thermochromic pigment, and

- providing a second layer on the first layer by in-situ polymerization of a mixture comprising a precursor of a polymer and a second irreversible thermochromic pigment on the first layer, wherein the first irreversible thermochromic pigment is different from the second irreversible thermochromic pigment.

31. Method according to any of claims 26 to 28, wherein the step of providing a substrate with multiple layers, wherein each of the multiple layers comprises a polymer and an irreversible thermochromic pigment, and wherein the irreversible thermochromic pigment within at least two of the multiple layers are different to each other, comprises

- providing a first layer by an extrusion process of a mixture comprising a polymer and a first irreversible thermochromic pigment,

- providing a second layer by an extrusion process of a mixture comprising a polymer and a second irreversible thermochromic pigment, wherein the first irreversible thermochromic pigment is different from the second irreversible thermochromic pigment, and

- binding the first and second layer by lamination or binding the first and the second layer by heat necessary for the extrusion of the first and second layer.

32. Method according to any of the previous claims, wherein the step of providing a substrate with at least one layer, comprises providing a substrate with at least three layers, wherein each of the three layers comprises a polymer and an irreversible thermochromic pigment, wherein the irreversible thermochromic pigments within each of the three layers are different to each other and wherein

a first irreversible thermochromic pigment of a first layer is such that after having generated the carbon trace or vapor trace within the first layer, the first irreversible thermochromic pigment appears yellow or green,

a second irreversible thermochromic pigment of a second layer is such that after having generated the carbon trace or vapor trace within the second layer, the second irreversible thermochromic pigment appears red or magenta, and/or

a third irreversible thermochromic pigment of the third layer is such that that after having generated the carbon trace or vapor trace within the third layer, the third irreversible thermochromic pigment appears blue or cyan.

33. Laser marked and/or engraved article produced by a method according to any of the previous claims.

34. Article for laser engraving and/or laser marking comprising a substrate with at least three layers, wherein each of the three layers comprises a polymer and an irreversible thermochromic pigment, wherein the irreversible thermochromic pigments within each of the three layers are different to each other and wherein

a first irreversible thermochromic pigment of a first layer is such that after having generated a carbon trace or vapor trace within the first layer by a pulsed laser beam, the first irreversible thermochromic pigment appears yellow or green,

a second irreversible thermochromic pigment of a second layer is such that after having generated a carbon trace or vapor trace within the second layer by a pulsed laser beam, the second irreversible thermochromic pigment appears red or magenta, and/or

a third irreversible thermochromic pigment of the third layer is such that that after having generated a carbon trace or vapor trace within the third layer by a pulsed laser beam, the third irreversible thermochromic pigment appears blue or cyan.