Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41M—PRINTING, DUPLICATING, MARKING, OR COPYING PROCESSES; COLOUR PRINTING
  • B41M5/00—Duplicating or marking methods; Sheet materials for use therein
  • B41M5/24—Ablative recording, e.g. by burning marks; Spark recording
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41M—PRINTING, DUPLICATING, MARKING, OR COPYING PROCESSES; COLOUR PRINTING
  • B41M5/00—Duplicating or marking methods; Sheet materials for use therein
  • B41M5/26—Thermography ; Marking by high energetic means, e.g. laser otherwise than by burning, and characterised by the material used
  • B41M5/28—Thermography ; Marking by high energetic means, e.g. laser otherwise than by burning, and characterised by the material used using thermochromic compounds or layers containing liquid crystals, microcapsules, bleachable dyes or heat- decomposable compounds, e.g. gas- liberating
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41M—PRINTING, DUPLICATING, MARKING, OR COPYING PROCESSES; COLOUR PRINTING
  • B41M5/00—Duplicating or marking methods; Sheet materials for use therein
  • B41M5/26—Thermography ; Marking by high energetic means, e.g. laser otherwise than by burning, and characterised by the material used
  • B41M5/34—Multicolour thermography
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41M—PRINTING, DUPLICATING, MARKING, OR COPYING PROCESSES; COLOUR PRINTING
  • B41M2205/00—Printing methods or features related to printing methods; Location or type of the layers
  • B41M2205/04—Direct thermal recording [DTR]
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41M—PRINTING, DUPLICATING, MARKING, OR COPYING PROCESSES; COLOUR PRINTING
  • B41M2205/00—Printing methods or features related to printing methods; Location or type of the layers
  • B41M2205/42—Multiple imaging layers
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41M—PRINTING, DUPLICATING, MARKING, OR COPYING PROCESSES; COLOUR PRINTING
  • B41M5/00—Duplicating or marking methods; Sheet materials for use therein
  • B41M5/26—Thermography ; Marking by high energetic means, e.g. laser otherwise than by burning, and characterised by the material used
  • B41M5/267—Marking of plastic artifacts, e.g. with laser
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41M—PRINTING, DUPLICATING, MARKING, OR COPYING PROCESSES; COLOUR PRINTING
  • B41M5/00—Duplicating or marking methods; Sheet materials for use therein
  • B41M5/26—Thermography ; Marking by high energetic means, e.g. laser otherwise than by burning, and characterised by the material used
  • B41M5/28—Thermography ; Marking by high energetic means, e.g. laser otherwise than by burning, and characterised by the material used using thermochromic compounds or layers containing liquid crystals, microcapsules, bleachable dyes or heat- decomposable compounds, e.g. gas- liberating
  • B41M5/282—Thermography ; Marking by high energetic means, e.g. laser otherwise than by burning, and characterised by the material used using thermochromic compounds or layers containing liquid crystals, microcapsules, bleachable dyes or heat- decomposable compounds, e.g. gas- liberating using thermochromic compounds
  • B41M5/284—Organic thermochromic compounds

Definitions

• the invention relates to a method for laser engraving and/or laser marking.
• 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.
• 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.
• 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.
• 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.
• 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.
• the document FR 3 106 527 A1 proposes to use an irreversible thermochromic pigment in a laser marking process.
• 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.
• a method for laser engraving and/or laser marking comprising the steps of:
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the next two consecutive laser pulses having focus spots close together are irradiated in a further distance to the first two laser pulses.
• isolated colored markings are generated, wherein each colored marking is generated by two consecutive laser pulses having focuses spots close together.
• 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.
• 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 ⁇ s, the time between the second and the third pulse is 0,5 ⁇ s, the time between the third and the fourth pulse is again 2,5 ⁇ s, the time between the fourth and the fifth pulse is again 0,5 ⁇ s, and so forth.
• the substrate with the at least one layer is provided, wherein the at least one layer comprises the polymer and the irreversible thermochromic pigment.
• 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.
• at least one layer comprising the polymer and the thermochromic pigment is formed on a substrate.
• 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.
• thermochromic pigment is preferably predominantly present in the polymer as finely distributed particle.
• 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.
• the thermochromic pigment may comprise a thermochromic dye, which is for example encapsulated in microcapsule, thus forming a pigment.
• 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.
• 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.
• the carbon trace or vapor trace is generated within or on the layer.
• 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.
• 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.
• the pulsed irradiation raises the temperature locally inside the 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.
• the carbon trace is formed due to the carbonization of the polymer.
• the vapor trace is formed due to vaporization of the polymer.
• the carbon trace which appears to the human eye as dark black or greyish, has an ellipsoid and/or spherical form.
• 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 ⁇ m to 10 ⁇ m.
• 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 ⁇ m to 10 ⁇ m.
• 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.
• 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.
• 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.
• the diameter of the carbon or vapor trace is preferably in between 2 ⁇ m and 10 ⁇ m, the distance between the first position and the consecutive position is preferably not more than 20 ⁇ m.
• 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.
• the colored markings 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.
• the colored markings 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.
• the colored markings created by the method in and/or on the at least one layer have a diameter of about 2 ⁇ m to 150 ⁇ m in x,y-direction, preferably about 2 ⁇ m to 50 ⁇ m in x,y-direction, and more preferably about 2 ⁇ m to 10 ⁇ m.
• the method also allows to introduce colored markings at different depths z, without changing the appearance above and below the marking.
• 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.
• the at least one layer can be optically dense and/or opaque.
• the focus spot is within the at least one layer.
• the colored markings is generated within the at least one layer.
• the at least one layer is transparent and/or translucent.
• 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.
• 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 %.
• 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).
• 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.
• the wavelength for determining the transmittance is in between 380 nm to 780 nm.
• 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.
• 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.
• the z-dimensions of the created colored markings are preferably > 100 ⁇ m, and preferably ⁇ 200 ⁇ m.
• 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.
• 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.
• 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.
• PC polycarbonate
• PET polyethylene terephthalate
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• ITO indium-tin-oxide
• 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.
• 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.
• 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.
• a substrate comprising multiple layers
• 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.
• the layer comprising the laser marking pigment is an outermost layer.
• the substrate is preferably irradiated such that the layer comprising the laser marking pigment is farthest away from the laser.
• the at least one layer may comprise a stabilizer.
• 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.
• 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.
• 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.
• 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.
• Antiozonants prevent or slow down the degradation of the polymer caused by ozone.
• the antiozonant is based on p -phenylenediamine, such as N-Isopropyl-N'-phenyl-1,4-phenylenediamine.
• 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.
• 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.
• 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-
• 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
• HALS are derivatives of tetramethylpiperidine.
• 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.
• the stabilizer can be added to the polymer as the pure substance preferably in the above indicated concentration.
• a mixture of a polymer and a stabilizer - also called stabilizer masterbatch can be added to the polymer.
• 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.
• the at least one layer comprises the polymer and the irreversible thermochromic pigment.
• 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.
• the concentration of the irreversible thermochromic pigment can be adapted.
• Typical amounts for an extruded 200 ⁇ m thick layer are 1 wt % to 5 wt % irreversible thermochromic pigment based on the weight of the extruded layer.
• 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.
• 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.
• 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.
• 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.
• 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.
• the irreversible thermochromic pigment is preferably a leuco dye encapsulated in a shell.
• 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 pertaining (120), Thermochrom Pigmente blau (120), Irreversible Thermochromic powder black 80C and/or mixtures thereof.
• the irreversible thermochromic pigment has a particle size ⁇ 0,01 ⁇ m to ⁇ 15 ⁇ m. Such particle sizes have been shown to give good colored markings with sharp edges.
• 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.
• the irreversible thermochromic pigment within the at least one layer is preferable in its first state.
• 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.
• 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.
• thermochromic pigment 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.
• 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.
• 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%.
• the at least one layer is preferably transparent for the wavelength of the laser beam.
• 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).
• 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.
• the focus spot of the pulsed laser beam has a diameter ⁇ 2 ⁇ m to ⁇ 10 ⁇ m and/or a thickness of the at least one layer is at least 10 ⁇ m.
• 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.
• the focus spot of the pulsed laser beam is in between 2 ⁇ m to 10 ⁇ m, preferably in between 2 ⁇ m to 5 ⁇ m.
• a diameter of the focus spot below 2 ⁇ m complicates the generation of the colored marking.
• the thickness of the at least one layer should not be less than 10 ⁇ m.
• 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.
• the energy deposited within the layer is enough to raise the temperature locally above the response temperature of the irreversible thermochromic pigment.
• 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.
• 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.
• the pulsed laser beam is not a continuous wave but a beam of short light pulses.
• 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.
• 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.
• 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.
• 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.
• 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.
• the substrate is irradiated with a pulsed laser beam having a pulse repetition rate of 10 kHz.
• 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.
• 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.
• 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.
• the preparation of the at least one layer does not change the state of the irreversible thermochromic pigment.
• a temperature of the mixture is below the response temperature of the irreversible thermochromic pigment.
• Solution casting of layers is a widely used technique in preparation of packaging materials at laboratory scale because it is easy and simple.
• 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.
• 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.
• the at least one layer is preferably prepared by an extrusion process of a mixture comprising polycaprolactone and the irreversible thermochromic pigment.
• 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.
• the substrate forms the one layer comprising the polymer and the irreversible thermochromic pigment.
• the at least one layer comprising the polymer and the thermochromic pigment is formed on a substrate.
• 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.
• 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.
• the layer comprising the polymer and a laser marking pigment is also free of an irreversible thermochromic pigment.
• the step of providing the substrate with at least one layer comprises
• 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.
• 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.
• 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.
• the multiple layers are directly attached to each other.
• the process is preferably started at the layer farthest away from the laser source.
• the colored markings generated in the first layer do not negatively influence the propagation of the laser beam.
• the substrate comprises at least three layers.
• 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.
• 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.
• the fourth layer is an outer layer.
• 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.
• 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
• 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
• 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
• 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
• 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.
• 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
• 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.
• the three layers are directly attached to each other.
• 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.
• 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.
• 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 clear solution with 20 wt % of a polymer in a solvent is prepared.
• 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.
• 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).
• 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.
• thermochromic pigments have been mixed with the polymer solution according to table 3 under intensive agitation at room temperature.
• 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
• a Petri dish 120 ⁇ 120mm 2 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.
• 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 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.
• the first layer is produced and dried as described under single 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.
• Table 4 summarizes the samples prepared by solution casting. Table 4: Samples prepared by solution casting Sample Mixture Thickness [ ⁇ m] 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.
• a Petri dish 120 ⁇ 120mm 2 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.
• 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.
• 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.
• 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 [ ⁇ m] 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
• 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., Düsseldorf, 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.
• 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.
• 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.
• 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- ⁇ -caprolactone Film thickness [ ⁇ m] 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.
• 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 shows carbonization and a greyish coloring due to carbonization when engraved by laser.
• Poly-e-caprolactone shows carbonization and a greyish coloring due to carbonization when engraved by laser.
• 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
• the scattering leads to an irregularity and enlargement of the inserted color markings bottom to top line: colorful lines
• Colors can be described in the RGB or in the Lab color space.
• 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.
• 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.
• 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 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.
• the Lab - measuring device The Lab - measuring device
• 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.).

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  • 2022-05-04 EP EP22171556.8A patent/EP4272972A1/fr active Pending

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