EP3909781A1

EP3909781A1 - Laser markable articles

Info

Publication number: EP3909781A1
Application number: EP20174025.5A
Authority: EP
Inventors: Marie LEHERICEY; Guy Damen; Fabienne Goethals; Johan Loccufier
Original assignee: Agfa Gevaert NV
Current assignee: Agfa Gevaert NV
Priority date: 2020-05-12
Filing date: 2020-05-12
Publication date: 2021-11-17

Abstract

A laser markable article comprising a laser markable coating provided on a support, the coating comprising a colour forming agent, a polymeric matrix and an optional optothermal converting agent, characterized in that the Glass Transition Temperature (Tg) of the coating is 15°C or higher.

Description

Technical field of the Invention

The present invention relates to laser markable articles and to laser markable compositions wherewith such articles may be prepared.

Background art for the invention

Various substrates, for example paper, paperboard or plastics, are very often marked with information such as logos, bar codes, expiry dates or batch numbers.
Tradionally, the marking of these substrates has been achieved by various printing techniques, such as for example inkjet or thermal transfer printing.
However, for some applications, these printing techniques are more and more replaced by laser marking as laser marking is cheaper in terms of overall economics and shows performance benefits such as high speed and contact free marking, marking of substrates with uneven surfaces, creation of marks that are so small that they are invisible or nearly invisible to the human eye, and creation of marks in the substrate rather than on the substrate.
Laser marking is typically carried out by image-wise exposing a laser markable article with a laser. Typically, such a laser markable article is prepared by applying a laser markable composition on a substrate.
The laser markable composition may be applied on the substrate by inkjet printing, flexographic printing, rotogravure printing, offset printing or any other printing technique. Also, the laser markable composition may be applied on the substrate by any coating or spraying technique.
The laser markable composition may be radiation curable. With such radiation curable compositions no solvents have to be evaporated after printing. Instead, solidification of the applied composition is the result of a polymerization reaction.
A so-called colour forming agent added to the laser markable composition may form a visible colour upon laser marking. An example of such colour forming agents are leuco dyes used in combination with a developing agent.
It has been observed that colours formed upon laser marking often fade, i.e. loose their colour density as function of time. A very fast and pronounced fading may result in a disappearance of the laser markings.

Summary of the invention

It is an object of the present invention to provide a laser markable article of which the laser markings are characterized by less fading.
This object has been realised with the laser markable article as defined in claim 1.
Further objects of the invention will become apparent from the description hereinafter.

Detailed description of the invention

Definitions

Unless otherwise specified the term "alkyl" means all variants possible for each number of carbon atoms in the alkyl group i.e. methyl, ethyl, for three carbon atoms: n-propyl and isopropyl; for four carbon atoms: n-butyl, isobutyl and tertiary-butyl; for five carbon atoms: n-pentyl, 1,1-dimethyl-propyl, 2,2-dimethyl-propyl and 2-methylbutyl, etc.
Unless otherwise specified a substituted or unsubstituted alkyl group is preferably a C₁ to C₆-alkyl group.
Unless otherwise specified a substituted or unsubstituted alkenyl group is preferably a C₂ to C₆-alkenyl group.
Unless otherwise specified a substituted or unsubstituted alkynyl group is preferably a C₂ to C₆-alkynyl group.
Unless otherwise specified a substituted or unsubstituted aralkyl group is preferably a phenyl or naphthyl group including one, two, three or more C₁ to C₆-alkyl groups.
Unless otherwise specified a substituted or unsubstituted alkaryl group is preferably a C₇ to C₂₀-alkyl group including a phenyl group or naphthyl group.
Unless otherwise specified a substituted or unsubstituted aryl group is preferably a phenyl group or naphthyl group
Unless otherwise specified a substituted or unsubstituted heteroaryl group is preferably a five- or six-membered ring substituted by one, two or three oxygen atoms, nitrogen atoms, sulphur atoms, selenium atoms or combinations thereof.
The term "substituted", in e.g. substituted alkyl group means that the alkyl group may be substituted by other atoms than the atoms normally present in such a group, i.e. carbon and hydrogen. For example, a substituted alkyl group may include a halogen atom or a thiol group. An unsubstituted alkyl group contains only carbon and hydrogen atoms
Unless otherwise specified a substituted alkyl group, a substituted alkenyl group, a substituted alkynyl group, a substituted aralkyl group, a substituted alkaryl group, a substituted aryl and a substituted heteroaryl group are preferably substituted by one or more constituents selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl and tertiary-butyl, ester, amide, ether, thioether, ketone, aldehyde, sulfoxide, sulfone, sulfonate ester, sulfonamide, -CI, -Br, -I, -OH, -SH, -CN and -NO₂.

Laser marked article

The laser markable article according to the present invention comprises a laser markable coating provided on a support, the coating comprising a colour forming agent, a polymeric matrix and an optional optothermal converting agent, characterized in that the Glass Transition Temperature (Tg) of the coating, measured as described below, is 15°C or more, preferably 20°C or more, most preferably 25°C or more.
The Tg of the coating is measured using Differential Scanning Calorimetry (DSC) as illustrated in the examples.
When more than one Tg is observed for the coating, the lowest Tg is 15°C or more, preferably 20°C or more, most preferably 25°C or more.
As illustrated in the examples, it is important to take into account a measured Tg of the coating and not a theoretical Tg calculated based on the polymerizable compounds used to make the coating.
The laser markable article is preferably prepared by the method described below.
The laser markable article is preferably selected from the group consisting of a packaging, a foil, a laminate, a security document, a label, a decorative object and an RFID tag.

Method of preparing a laser markable article

The method of preparing a laser markable article according to the present invention comprises the steps of:

providing a laser markable radiation curable composition comprising at least one polymerizable compound, a colour forming agent and an optional optothermal converting agent on a support thereby forming a laser markable coating on the support;
curing the laser markable coating;
wherein the Glass Transition Temperature (Tg) of the cured coating, measured as described below, is more than 15°C.

After curing, a polymeric matrix is formed from the at least one polymerizable compound. In the curing step, the at least one polymerizable compounds polymerize thereby forming the polymeric matrix.
The laser markable composition may be provided onto a support by co-extrusion or any conventional coating technique, such as dip coating, knife coating, extrusion coating, spin coating, spray coating, slide hopper coating and curtain coating.
The laser markable composition may also be provided onto a support by any printing method such as intaglio printing, screen printing, flexographic printing, offset printing, inkjet printing, rotogravure printing, etc. Using a printing method is preferred when only a part or several parts of a support have to be provided with a laser markable layer.
The laser markable composition is preferably applied by flexographic printing or inkjet printing.
The thickness of the applied radiation curable laser markable composition is preferably 50 µm or less, more preferably 20 µm or less, most preferably 10 µm or less.
The laser markable coating can be cured by exposing them to actinic radiation, such as electron beam or ultraviolet radiation.
Preferably, the laser markable coating is cured by exposing it to ultraviolet radiation, more preferably to UV LED radiation.

The radiation curable laser markable composition

The radiation curable laser markable composition according to the present invention comprises at least one polymerizable compound, a colour forming agent and an optional optothermal converting agent.
A preferred radiation curable laser markable composition includes a leuco dye and a developing agent.
The radiation curable laser markable composition may be an aqueous or a non-aqueous composition.
A preferred aqueous based composition includes encapsulated leuco dyes. Such aqueous compositions wherein the leuco dyes are encapsulated are disclosed in for example EP-A 3297837 , EP-A 3470134 and EP-A 3470135 , all from Agfa Gevaert NV.
Preferred radiation curable aqueous compositions are disclosed in EP-A 3626471 and EP-A 3626472 (both from Agfa Gevaert NV).
The radiation curable laser markable compositions are preferably non-aqueous compositions.
To optimize the coating or printing properties, and also depending on the application for which it is used, various additives may be added to the composition, such as wetting/levelling agents, rheology modifiers, colorants, adhesion promoting compounds, biocides or antioxidants.

Colour forming agent

The radiation curable laser markable composition comprises a colour forming agent, which is capable of forming a colour upon laser marking.
All known colour forming agents may be used.
A transition metal oxide, such as molybdenum trioxide, has been disclosed in WO2008/075101 (SI LTECH).
An oxyanion of a multivalent metal, such as ammonium octyl molybdate, has been disclosed in WO2002/074548 (DATALASE) and WO2007/012578 (DATALASE).
These colour forming agents are capable of forming a black colour upon laser marking.
Diacetylene compounds, such as disclosed in WO2013/014436 (DATALASE) are capable of forming multiple colours.
Preferred colour forming agents are leuco dyes used in combination with a developing agent.
Also, a combination of different colour forming agents may be used, for example to produce different colours. In WO2013/068729 (Datalase), a combination of a diacetylene compound and a leuco dye is used to produce a full colour image upon exposure to UV and IR radiation.

Leuco dye

A leuco dye is a substantially colourless compound, which may form a coloured dye upon an inter- or intra-molecular reaction. The inter- or intra-molecular reaction may be triggered by heat, preferably heat formed during exposure with an IR laser.
Examples of leuco dyes are disclosed in WO2015/165854 (AGFA GEVAERT), paragraph [069] to [093].
The laser markable composition may comprise more than one leuco dye. Using two, three or more leuco dyes may be necessary to realize a particular colour.
The leuco dyes may be encapsulated, for example in aqueous laser markable compositions as mentioned above. The leuco dyes may also be present as leuco dye dispersions in the laser markable composition.
The leuco dye may also be part of a polymeric binder, for example covalently linked to such a polymeric binder.
The leuco dyes may also include an ethylenically unsatured bond, which allows co-reacting with the polymerizable compounds of the composition disclosed below. Such polymerizable leuco dyes are disclosed in for example EP-A 3173249 (Agfa Gevaert NV), paragraph [0153], Table 1.
The amount of the leuco dye in the radiation curable laser markable composition is preferably in the range from 0.5 and 20 wt%, more preferably in the range from 1 to 10 wt%, relative to the total weight of the composition.
After applying the composition on a support, the amount of leuco dye is preferably in the range from 0.05 to 2 g/m², more preferably in the range from 0.1 to 1 g/m².

Developing agent

The radiation curable laser markable composition preferably comprises a developing agent.
A developing agent is capable of reacting with a colourless leuco dye resulting in the formation of a coloured dye upon laser marking. Typically, upon laser marking a compound is released that may react with a leuco dye thereby forming a coloured dye.
All publicly-known photo- or thermal acid generators can be used as developing agent. Thermal acid generators are for example widely used in conventional photoresist material. For more information see for example "Encyclopaedia of polymer science", 4th edition, Wiley or "Industrial Photoinitiators, A Technical Guide", CRC Press 2010.
Preferred classes of photo- and thermal acid generators are iodonium salts, sulfonium salts, ferrocenium salts, sulfonyl oximes, halomethyl triazines, halomethylarylsulfone, α-haloacetophenones, sulfonate esters, t-butyl esters, allyl substituted phenols, t-butyl carbonates, sulfate esters, phosphate esters and phosphonate esters.
Particularly preferred developing agents have a structure according to Formula (I)
wherein

R1 represent an optionally substituted alkyl group, an optionally substituted (hetero)cyclic alkyl group, an optionally substituted alkanyl group, an optionally substituted alkenyl group, an optionally substituted alkynyl group, an optionally substituted (hetero)aryl group, an optionally substituted aralkyl group, an optionally substituted alkoxy group, an optionally substituted (hetero)cyclic alkoxy group, or an optionally substituted (hetero)aryl group;
R2 represent an optionally substituted alkyl, an optionally substituted aliphatic (hetero)cyclic alkyl group or an optionally substituted aralkyl group;
R1 and R2 may represent the necessary atoms to form a ring.

Such developing agents according to Formula I and their preparation are disclosed in WO2015/091688 . A particular preferred developing agent according to Formula I is menthyl p-toluene sulfonate and has the following chemical structure.
The developing agent may also be encapsulated or be present as a dispersion in the laser markable composition. Also, the developing agent may be a part of a polymeric binder.
The developing agent may also include an ethylenically unsatured bond, which allows co-reacting with the polymerizable compounds of the composition disclosed below. Such polymerizable developing agents are disclosed in for example EP-A 3173249 (Agfa Gevaert NV), paragraph [0228], Table 9.
The amount of the developing agent in the radiation curable composition is preferably in the range from 0.5 to 25 wt%, more preferably in the range from 1 to 15 wt%, most preferably in the range from 2.5 to 10 wt%, relative to the total weight of the composition.
After applying the composition on a support, the amount of the developing agent is preferably in the range from 0.05 to 2.5 g/m², more preferably in the range from 0.10 to 1.50 g/m², most preferably in the range from 0.25 to 1.00 g/m².

Optothermal converting agent

Typically, a laser markable composition includes a so-called optothermal converting agent, which converts radiation energy into heat. In most cases infrared radiation is used for laser marking.
The optothermal converting agent preferably generates heat upon absorption of infrared (IR) radiation, more preferably near infrared (NIR) radiation.
Near infrared radiation has a wavelength between 750 and 2500 nm.
Optothermal converting agents may be an infrared radiation absorbing dye but is preferably an infrared radiation absorbing pigment, or a combination thereof.
Various infrared absorbing compounds that may be used as optothermal converting agents in laser markable compositions are disclosed.
WO2014/057018 disclose cyanine compounds that may act as optothermal converting agents.
A disadvantage of cyanine dyes maybe their daylight and temperature stability. Their poor stability also makes it difficult to use them in UV curable compositions.
An optothermal converting agent that does not contain heavy metals and that is stable is carbon black. Carbon black is disclosed as optothermal converting agent in for example WO2016/184881 (Agfa Gevaert).

Infrared radiation absorbing inorganic pigments

A preferred inorganic infrared absorber is a copper salt as disclosed in WO2005/068207 (DATALASE).
Another preferred inorganic infrared absorber is a non-stoichiometric metal salt, such as reduced indium tin oxide as disclosed in WO2007/141522 (DATALASE).
Particular preferred inorganic infrared absorbers are tungsten oxide or tungstate as disclosed in WO2009/059900 (DATALASE) and WO2015/015200 (DATALASE). A lower absorption in the visible region while having a sufficient absorption in the near infrared region is an advantage of these tungsten oxide or tungstate. A particular preferred tungsten oxide is cesium tungsten oxide (CTO).

Carbon black

The heavy metal containing pigments described above are however to be avoided from an ecological and toxicological point of view, especially in food and pharmaceutical packaging applications.
For such applications, a particularly preferred infrared radiation absorbing pigment (IR pigment) is carbon black, such as acetylene black, channel black, furnace black, lamp black, and thermal black.
Due to its light absorption in the visible region, i.e. between 400 nm and 700 nm, a too high amount of carbon black may result in an increase of the background colour of the printed laser markable composition.
For that reason, the amount of carbon black is preferably less than 10.000 ppm, more preferably less than 1000 ppm, most preferably less than 500 ppm, all relative to the total weight of the composition.
After applying the composition on a support, the amount of carbon black is preferably less than 0.1 g/m², more preferably less than 0.01 g/m², most preferably less than 0.005 g/m².
Carbon black is preferably added to the radiation curable laser markable composition as a dispersion.
The carbon black dispersion may be prepared by all commonly known dispersion methods.
A preferred method is a mechanical dispersion method including a bead milling step.
In such a method, the carbon black is mixed with a dispersion medium to obtain a suspension. That suspension is then grinded in a bead mill to obtain a stable dispersion having a particle size below 1 µm.
The particle size of the carbon particles is preferably in the range from 10 to 500 nm, more preferably in the range from 25 to 400 nm, most preferably in the range from 50 to 300 nm.
The dispersion medium preferably includes a polymerizable compound, preferably an acrylate.
The acrylate monomers are preferably selected from the group consisting of isobornylacrylate (IBOA), dipropylene glycol diacrylate (DPGDA), pentaerythritol triacrylate and 2-(2-vinyloxyethoxy)ethyl acrylate (VEEA).
To realize stable dispersions, a dispersant is preferably added to the dispersion medium. Any commonly known dispersant may be used. A preferred dispersant is a polymeric dispersant such as Efka PX4701, available from BASF.
Examples of suitable carbon blacks are Special black 250, Special black 100, Printex G, Lamp Black 101, Printex 25, Printex A, Hiblack 40B2, XPB 545 from Orion; and Raven 410, Raven 14 from Birla Carbon.

Infrared radiation absorbing dyes

An advantage of Infrared absorbing dyes (IR dyes) compared to IR pigments is their narrow absorption spectrum resulting in less absorption in the visible region. This may be of importance for the processing of transparent resin based articles where optical appearance is of importance.
A narrow absorption band is also mandatory for multicolour laser marking using multiple laser each having a different emission wavelength, as disclosed in for example EP-A 3297838 .
Any IR dye may be used, for example the IR dyes disclosed in "Near-Infrared Dyes for High Technology Applications" (ISBN 978-0-7923-5101-6).
Preferred IR dyes are polymethine dyes due to their low absorption in the visible region and their selectivity, i.e. narrow absorption peak in the infrared region. Particular preferred polymethine IR dyes are cyanine IR dyes.
Preferred IR dyes having an absorption maximum of more than 1100 nm are those disclosed in EP-A 2722367 , paragraphs [0044] to [0083] and WO2015/165854 , paragraphs [0040] to [0051].
IR dyes having an absorption maximum between 1000 nm and 1100 nm are preferably selected from the group consisting of quinoline dyes, indolenine dyes, especially a benzo[cd]indoline dye. A particularly preferred IR dye is 5-[2,5-bis[2-[1-(1-methylbutyl)-benz[cd]indol-2(1H)-ylidene]ethylidene]-cyclopentylidene]-1-butyl-3-(2-methoxy-1-methylethyl)-2,4,6(1H,3H,5H)-pyrimidinetrione (CASRN 223717-84-8) represented by the Formula IR-1, or the IR dye represented by Formula IR-2:
Both IR dyes IR-1 and IR-2 have an absorption maximum λmax around 1052 nm making them very suitable for a Nd-YAG laser having an emission wavelength of 1064 nm.
Other preferred NIR absorbing compounds are those disclosed in WO2019/007833 , paragraph [0034] to [0046]. It has been observed that these NIR absorbing compounds have a better daylight stability compared to the IR dyes described above and are therefore more suitable to be used in UV curable compositions.
A combination of different optothermal converting agents may also be used.
The amount of optothermal converting agent is preferably at least 10^-10 g/m², more preferably between 0.0001 and 0.5 g/m², most preferably between 0.0005 and 0.1 g/m².

Polymerizable compound

The laser markable composition preferably comprises at least one polymerizable compound. The composition may comprise one, two, three or more different polymerizable compounds.
The polymerizable compounds polymerize upon curing thereby forming a polymeric matrix. The type of this polymeric matrix has a major influence on the Tg of the cured coating obtained after the curing step. Therefore also the type, the molecular weight and the relative amount of the polymerizable compounds that forms the polymeric matrix has an influence on the Tg of the cured coating.
The polymerizable compounds may be monomers, oligomers or prepolymers.
The polymerizable compounds may be diluted or dispersed, for example in water.
The polymerizable compounds may be free radical polymerizable compounds or cationic polymerizable compounds.
Cationic polymerization is superior in effectiveness due to lack of inhibition of the polymerization by oxygen. However it is expensive and slow, especially under conditions of high relative humidity. If cationic polymerization is used, it is preferred to use an epoxy compound together with an oxetane compound to increase the rate of polymerization.
Preferred monomers and oligomeranderss are those listed in paragraphs [0103] to [0126] of EP-A 1911814 .
Radical polymerization is the preferred polymerization process. Preferred free radical polymerizable compounds include at least one acrylate or methacrylate group or at least one acrylamide or methacrylamide group as polymerizable group, referred to herein as (meth)acrylate or (meth)acrylamide monomers, oligomers or prepolymers. Due to their higher reactivity, particularly preferred polymerizable compounds are acrylate monomers, oligomers or prepolymers.
Other preferred monomers, oligomers or prepolymers are N-vinylamides, such as N-vinylcaprolactam and acryloylmorpholine.
Particularly preferred (meth)acrylate monomers, oligomers or prepolymers are selected from the group consisting of tricyclodecanedimethanol diacrylate (TCDDMDA), isobornyl acrylate (IBOA), ethoxylated [4] bisphenol A diacrylate and 1,10 decanediol diacrylate.
The total amount of polymerizable compounds is preferably at least 50 wt%, more preferably at least 70 wt%, most preferably at least 80 wt%, relative to the total weight of the composition.

Photoinitiator

The radiation curable laser markable composition preferably contains a photoinitiator. The initiator typically initiates the polymerization reaction. The photoinitiator may be a Norrish type I initiator, a Norrish type II initiator or a photo-acid generator, but is preferably a Norrish type I initiator, a Norrish type II initiator or a combination thereof.
A preferred Norrish type I-initiator is selected from the group consisting of benzoinethers, benzil ketals, α,α-dialkoxyacetophenones, α-hydroxyalkylphenones, α-aminoalkylphenones, acylphosphine oxides, acylphosphine sulphides, α-haloketones, α-halosulfones and α-halophenylglyoxalates.
A preferred Norrish type II-initiator is selected from the group consisting of benzophenones, thioxanthones, 1,2-diketones and anthraquinones.
Suitable photo-initiators are disclosed in CRIVELLO, J.V., et al. VOLUME III: Photoinitiators for Free Radical Cationic & Anionic Photopolymerization. 2nd edition. Edited by BRADLEY, G.. London,UK: John Wiley and Sons Ltd, 1998. p.287-294 .
In order to increase the photosensitivity further, the radiation curable composition may additionally contain co-initiators.
A preferred co-initiator is selected from the group consisting of an aliphatic amine, an aromatic amine and a thiol. Tertiary amines, heterocyclic thiols and 4-dialkylamino-benzoic acid are particularly preferred as co-initiator.
The most preferred co-initiators are aminobenzoates for reason of shelf-life stability of the radiation curable composition.
A preferred amount of photoinitiator is 0.3 - 20 wt% of the total weight of the radiation curable composition, more preferably 1 - 15 wt% of the total weight of the radiation curable composition.
The amount of co-initiator or co-initiators is preferably from 0.1 to 20.0 wt%, more preferably from 1.0 to 10.0 wt%, based in each case on the total weight of the radiation curable composition.

Polymerization Inhibitors

For improving the shelf-life, the radiation curable laser markable composition may contain a polymerization inhibitor. Suitable polymerization inhibitors include phenol type antioxidants, hindered amine light stabilizers, phosphor type antioxidants, hydroquinone monomethyl ether commonly used in (meth)acrylate monomers, and hydroquinone, t-butylcatechol, pyrogallol may also be used.
Suitable commercial inhibitors are, for example, Sumilizer™ GA-80, Sumilizer™ GM and Sumilizer™ GS produced by Sumitomo Chemical Co. Ltd.; Genorad™ 16, Genorad™ 18 and Genorad™ 20 from Rahn AG; Irgastab™ UV10 and Irgastab™ UV22, Tinuvin™ 460 and CGS20 from Ciba Specialty Chemicals; Floorstab™ UV range (UV-1, UV-2, UV-5 and UV-8) from Kromachem Ltd, Additol™ S range (S100, S110, S120 and S130) from Cytec Surface Specialties.
Since excessive addition of these polymerization inhibitors will lower the sensitivity to curing, it is preferred that the amount capable of preventing polymerization is determined prior to blending. The amount of a polymerization inhibitor is preferably lower than 2 wt% of the total radiation curable laser markable composition.

Inorganic filler

The laser markable composition preferably comprises at least 1 wt% of an inorganic filler, relative to the total weight of the composition.
Examples of inorganic fillers that may be used are selected from the group consisting of calciumcarbonate, clays, alumina trihydrate, talc, mica, and calcium sulfate.
Preferably, an inorganic nanofiller is used to obtain optimal transparency of the laser markable composition. A preferred nanofiller is nanosilica.
Nanosilica as referred to herein consist of amorphous silicon dioxide particles having a nano-particle size.
To obtain optimal transparency of the laser markable composition the particle size of the nanosilica is preferably in the range from 5 to 250 nm, more preferably in the range from 7.5 to 100 nm, most preferably in the range from 10 to 50 nm.
Preferably dispersions of nanosilica in acrylate monomers are used. Such commercially available dispersions are for example the Nanocryl® nanosilica dispersions available from Evonik.
The amount of the inorganic filler is preferably in the range from 1 to 15 wt%, more preferably in the range from 2 to 10 wt%, most preferably in the range from 2.5 and 7.5 wt%, all relative to the total weight of the composition.
After applying the composition on a support, the amount of the inorganic filler is preferably in the range from 0.1 to 1.5 g/m², more preferably in the range from 0.2 to 1 g/m², most preferably in the range from 0.25 to 0.75 g/m².

Surfactant

The radiation curable laser markable composition may contain at least one surfactant. The surfactant(s) can be anionic, cationic, non-ionic, or zwitter-ionic and are usually added in a total quantity less than 5 wt%, more preferably less than 2 wt%, based on the total weight of the composition.
Preferred surfactants are selected from fluoro surfactants (such as fluorinated hydrocarbons) and/or silicone surfactants.
The silicone surfactants are preferably siloxanes and can be alkoxylated, polyester modified, polyether modified, polyether modified hydroxy functional, amine modified, epoxy modified and other modifications or combinations thereof. Preferred siloxanes are polymeric, for example polydimethylsiloxanes. Preferred commercial silicone surfactants include BYK™ 333 and BYK™ UV3510 from BYK Chemie.
Silicone surfactants are often preferred in the radiation curable laser markable composition, especially the reactive silicone surfactants, which are able to be polymerized together with the polymerizable compounds during the curing step.
Examples of useful commercial silicone surfactants are those supplied by BYK CHEMIE GMBH (including Byk™-302, 307, 310, 331, 333, 341, 345, 346, 347, 348, UV3500, UV3510 and UV3530), those supplied by TEGO CHEMIE SERVICE (including Tego Rad™ 2100, 2200N, 2250, 2300, 2500, 2600 and 2700), Ebecryl™ 1360 a polysiloxane hexaacrylate from CYTEC INDUSTRIES BV and Efka™-3000 series (including Efka™-3232 and Efka™-3883) from EFKA CHEMICALS B.V..

Support

The laser markable composition may be applied on any type of surface, for example a metallic support, a glass support, a polymeric support, or a paper support. The laser markable composition may also be applied on a textile surface.
The support may be provided with a primer to improve the adhesion between the support and the laser markable composition.
A primer containing a dye or a pigment, for example a white primer, may also be provided on the support, for example to improve the contrast of the laser marked image.
The support may be a paper support, such as plain paper or resin coated paper, e.g. polyethylene or polypropylene coated paper.
There is no real limitation on the type of paper and it includes newsprint paper, magazine paper, office paper, or wallpaper but also paper of higher grammage, usually referred to as paper boards, such as white lined chipboard, corrugated (fiber) board and packaging board.
Also, so-called synthetic papers, such as the Synaps™ synthetic papers from Agfa Gevaert, which are opaque polyethylene terephthalate sheets, may be used as support.
Suitable polymeric supports include cellulose acetate propionate or cellulose acetate butyrate, polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyamides, polycarbonates, polyimides, polyolefins, polyvinylchlorides, polyvinylacetals, polyethers, polysulfonamides, polylactide (PLA) and polyimide.
A preferred polymeric support is a biaxially stretched polyethylene terephthalate foil (PET-C foil) due to its very high durability and resistance to scratches and chemical substances.
The manufacturing of PET-C foils and supports is well-known in the art of preparing suitable supports for silver halide photographic films. For example, GB 811066 (ICI) teaches a process to produce biaxially oriented polyethylene terephthalate foils and supports.
Another preferred polymeric support includes (co)polyesters based on cyclohexyldimethanol (CHDM).
Thermoplastic polyesters containing CHDM exhibit enhanced strength, clarity, and solvent resistance. The exact properties of the polyesters vary from the high melting crystalline poly(1,4-cyclohexylenedimethylene terephthalate), PCT, to the non-crystalline copolyesters with the combination of ethylene glycol and CHDM in the backbone. The properties of these polyesters is also dependent on the cis/trans ratio of the CHDM monomer. CHDM has low melting point and reduces the degree of crystallinity of PET homopolymer, improving its processability. With improved processability, the polymer tends to degrade less to acetaldehyde and other undesirable degradation products. The copolymer with PET is known as glycol-modified polyethylene terephthalate, PETG. PETG is used in many fields, including electronics, automobiles, barrier, and medicals etc.
Another preferred polymeric support includes (co)polyesters based on 2,5-furandicarboxylic acid (FDCA). Such PEF films have, compared to standard PET films, a 10x higher oxygen barrier, a 2-3 x higher water vapor barrier, an improved mechanical strength and are fully transparent.
Other polymeric supports include copolyesters based on isosorbide, e.g. copolymers of terephtalic acid and ethylene glycol and isosorbide.
The polymeric support may be a single component extrudate or co-extrudate. Examples of suitable co-extrudates are PET/PETG and PET/PC.
There is no restriction on the shape of the support. It can be a flat sheet, such as a paper sheet or a polymeric film or it can be a three dimensional object like e.g. a plastic coffee cup.
The three dimensional object can also be a container like a bottle or a jerry-can for including e.g. oil, shampoo, insecticides, pesticides, solvents, paint thinner or other type of liquids.
The laser markable composition may also be applied on a so-called shrink foil. Such a foil shrinks tightly over whatever it is covering when heat is applied.
The most commonly used shrink foils are polyolefin foils, i.e. polyethylene or polypropylene foils. However, other shrink foils include PCV.

Packaging

The laser marking method according to the present invention is preferably used to laser mark a packaging.
Laser marking is typically used to add variable data, for example batch numbers, expiry dates, addressees, etc. on the packaging.
Preferably laser marking is carried out in-line in the packaging process.
The laser marked "image" on a packaging may comprises data, images, barcodes, QR codes, or a combination thereof.
An advantage of using laser marking in a packaging process is the ability to mark information through a wrapping foil, for example the flavour-protective foil used for cigarette packs. In such a way, variable data may be provided on the cigarette packs after the protective foil has already been provided.
Another preferred laser markable packaging is used for pharmaceutical packaging. For pharmaceutical packaging, track and trace requirements become more and more demanding to comply with the ever evolving legislation.
Another advantage of using laser marking instead of another printing technique, such as inkjet printing, is the absence of any chemicals in the marking process. Especially for pharmaceutical and food packaging, the absence of chemicals in the packaging line is a great advantage.
By selecting a proper leuco dye, or a mixture of leuco dyes, the package may be provided with data or images in any colour.
A preferred packaging is folded cardboard or corrugated cardboard laminated with paper. Such packaging is preferably used for cosmetics, pharmaceuticals, food or electronics.
Multiple colour, even full colour, images may be obtained when the packaging is provided with multiple laser markable compositions, each containing a different leuco dye and optothermal converting agent, as disclosed in EP-A2719540 (Agfa Gevaert NV) and EP-A 2719541 (Agfa Gevaert NV). Also, when using diacetylene compounds a colour forming agent, multiple colours may be realized as disclosed in WO2013/014436 (DATALASE).

Security Documents

The laser marking method may also be used to prepare security documents, such as for example ID cards.
Typically, laser markable security documents are prepared by laminating a laser markable foil or laminate, optionally together with other foils or laminates, onto one or both sides of a core support.
Such laser markable security documents and their preparation have been disclosed in for example WO2015/091782 (Agfa Gevaert).
The laser markable laminate may be prepared by providing a laser markable composition according to the present invention on a support. The support is described above and is preferably a transparent polymeric support.
The laser markable laminate may comprise more than one laser markable layers or may comprise additional layers such as an ink receiving layer, a UV absorbing layer, intermediate layers or adhesion promoting layers.
The laser markable laminate is typically laminated on one or both sides of a core support using elevated temperatures and pressures.
Preferred core supports are disclosed in WO2014/057018 (Agfa Gevaert), paragraphs [0112] to [0015].
The lamination temperature depends on the type of core support used. For a polyester core, lamination temperatures are preferably between 120 and 140°C, while they are preferably above 150°C - 160°C for a polycarbonate core.

Laser marking

In principle any laser may be used in the laser marking step. Preferred lasers are ultraviolet (UV) and infrared (IR) lasers, infrared laser being particularly preferred.
The infrared laser may be a continuous wave or a pulsed laser.
For example a CO₂ laser, a continuous wave, high power infrared laser having emission wavelength of typically 10600 nm (10.6 micrometer) may be used.
CO₂ lasers are widely available and cheap. A disadvantage however of such a CO₂ laser is the rather long emission wavelength, limiting the resolution of the laser marked information.
To produce high resolution laser marked data, it is preferred to use a near infrared (NIR) laser having an emission wavelength between 750 and 2500, preferably between 800 and 1500 nm in the laser marking step.
A particularly preferred NIR laser is an optically pumped semiconductor laser. Optically pumped semiconductor lasers have the advantage of unique wavelength flexibility, different from any other solid-state based laser. The output wavelength can be set anywhere between about 900 nm and about 1250 nm. This allows a perfect match between the laser emission wavelength and the absorption maximum of an optothermal converting agent present in the laser markable layer.
A preferred pulsed laser is a solid state Q-switched laser. Q-switching is a technique by which a laser can be made to produce a pulsed output beam. The technique allows the production of light pulses with extremely high peak power, much higher than would be produced by the same laser if it were operating in a continuous wave (constant output) mode, Q-switching leads to much lower pulse repetition rates, much higher pulse energies, and much longer pulse durations.
Laser marking may also be carried out using a so-called Spatial Light Modulator (SLM) as disclosed in WO2012/044400 (Vardex Laser Solutions).

EXAMPLES

Materials

All materials used in the following examples were readily available from standard sources such as MERCK (Belgium) and ACROS (Belgium) unless otherwise specified.
WR is an abbreviation for WinCon-Red, a magenta leuco dye from Connect Chemicals GmbH.
CpTs is an abbreviation for cyclohexyl p-toluenesulfonate with the CAS number 953-91-3 from Chemgo.
Genocure DMHA is a photoinitiator from RAHN AG.
Omnirad 481 is a photoinitiator from IGM Resins b.v.
Speedcure TPO is a photoinitiator from Lambson Limited.
Sartomer 833S is tricyclodecanedimethanol diacrylate (TCDDMDA, a difunctional acrylic monomer from Arkema.
Photomer 4012 is isobornyl acrylate (IBOA), a monofunctional acrylic monomer from IGM.
Sartomer 508 is dipropylene glycol diacrylate, a difunctional acrylic monomer from Arkema.
Sartomer 339 is 2-phenoxyethyl acrylate, a monofunctional acrylic monomer from Arkema.
Sartomer 601E is difunctional ethoxylated 4 bisphenol A diacrylate, a difunctional acrylate from Arkema.
Sartomer 595 is difunctional 1,10 decanediol diacrylage, a difunctional acrylate from Arkema.
CN9245 is a urethane acrylate oligomer from Arkema.
CN9165 is a urethane acrylate oligomer from Arkema.
BYK-UV 3510 is a surface additive from BYK-Chemie GmbH.
CTO is an inorganic pigment of cesium tungsten oxide from Keeling & Walker Limited.
Cupferron AL is aluminum N-nitrosophenylhydroxylamine from WAKO CHEMICALS LTD.
INHIB is a mixture forming a polymerization inhibitor having a composition according to Table 1. Table 1

Component wt%

Sartomer 508 82.4

p-methoxyphenol 4.0

BHT 10.0

Cupferron AL 3.6
EFKA PX4733 is a high-molecular-weight dispersant from BASF SE.
DISP1 is a concentrated pigment dispersion prepared as follows: 100.0 g of CTO pigment powder, 100.0 g of dispersant EFKA PX4733 and 5.0g of INHIB stabilizer were mixed into 295.0 g of Photomer 4012 using a DISPERLUX™ dispenser. Stirring was continued for 30 minutes. The vessel was connected to a DynoMill-RL mill filled with 200 g of 0.4 mm yttrium stabilized zirconia beads ("high wear resistant zirconia grinding media" from TOSOH Co.). The mixture was circulated over the mill for 108 minutes with a rotation speed of 4500 t/min. During the complete milling procedure the content in the mill was cooled to keep the temperature below 60°C. After milling, the dispersion was discharged into a vessel. The resulting concentrated pigment dispersion exhibited an average particle size of 131.0nm as measured with a Malvern™ nano-S and a viscosity of 134.11 mPa.s at 20°C and at a shear rate of 10 s^-1.
OPV1 is a concentrated solution prepared as follow: 1.00 g of DISP1 and 9.00 g of Sartomer 339 were added into a 30 mL brown glass container with a plastic screw cap and stirred at 250 rpm with a magnetic stirring bar at room temperature for 3 hours.
OPV2 is a concentrated solution prepared as follow: 1.00g of DISP1 and 9.00 g of Sartomer 833S were added into a 30 mL brown glass container with a plastic screw cap and stirred at 250 rpm with a magnetic stirring bar at room temperature for 3 hours.
OPV3 is a concentrated solution prepared as follow: 1.00 g of DISP1 and 9.00 g of Sartomer 601E were added into a 30 mL brown glass container with a plastic screw cap and stirred at 250 rpm with a magnetic stirring bar at room temperature for 3 hours.
OPV4 is a concentrated solution prepared as follow: 1.00 g of DISP1 and 9.00 g of Photomer 4012 were added into a 30 mL brown glass container with a plastic screw cap and stirred at 250 rpm with a magnetic stirring bar at room temperature for 3 hours.
OPV5 is a concentrated solution prepared as follow: 1.00 g of DISP1 and 9.00 g of Sartomer 595 were added into a 30 mL brown glass container with a plastic screw cap and stirred at 250 rpm with a magnetic stirring bar at room temperature for 3 hours.

Example 1

The coating solutions S1 to S4 were prepared by mixing the ingredients according to Table 2 expressed in grams in 30 mL brown glass flasks with a plastic screw cap and stirred overnight at 350 rpm with a magnetic stirring bar at room temperature.

Table 2

Ingredients [g]	S1	S2	S3	S4
WR	0.25	=	=	=
CpTs	0.44	=	=	=
Photomer 4012	-	-	-	5.10
Sartomer 833S	-	4.91	-	-
Sartomer 339	5.05	-	-	-
Sartomer 601 E	-	-	7.41	-
CN9245	2.50	=	-	2.50
Genocure DMHA	0.22	0.27	=	0.20
Omnirad 481	0.22	0.27	=	0.20
Speedcure TPO	0.22	0.27	=	0.20
BYK-UV 3510	0.10	=	=	=
OPV1	1.00	-	-	-
OPV2	-	1.00	-	-
OPV3	-	-	1.00	-
OPV4	-	-	-	1.00

The solutions were subsequently coated with a spiral bar coater (from Elcometer) using an automatic film applicator (Elcometer 4340 from Elcometer) at a speed of 20mm/s on an A4 sheet of cardboard (Incada Exel HS (GC2) NI 255 g/m² 510^∗720 mm SG 450 µm) with a wet coating thickness of 10 µm. Each layer was cured right after being applied using a curing station (Aktiprint Mini 18 - 2.75 cm belt, 230 V, 50 Hz from Technigraf GmbH) with the lamp being at the second lowest position (second closest to the substrate). The coatings were cured with the speed and the number of passes according to Table 3. Table 3

S1 S2 S3 S4

Curing passes 1 1 1 4

Curing Speed (m/minute) 10 10 22 10
Each solution was coated twice: one sample was exposed to an infrared laser whereas the other one was used to do Differential Scanning Calorimetry (DSC) tests.
The infrared laser was an optically pumped semiconductor laser emitting at 1064 nm (Genesis MX 1064-10000 MTM from COHERENT) with a maximum power of 4.0 W, a spot size in X of 78.9 µm at 1/e² and a spot size in Y of 90.6 µm at 1/e². The used pattern was vector graphics.
The laser exposed samples were aged up to 14 days at room temperature (RT). The results are depicted in Table 4. The appreciation of the fading is qualitative:

0 represents no visible fading after 14 days (the colour of the laser mark didn't weaken in saturation);
1 represents a small fading after 14 days;
2 represents a strong fading after 14 days (the colour of the laser mark strongly decreased, maybe even fully disappeared).

The DSC analysis was performed on samples that were 3 days old for S1, S3 and S4 and 6 days old samples for S2 (storage at RT in a plastic map) with a Q1000 from TA Instruments.
A little amount of the coating was scraped off the substrate with a razor blade (4 to 10 mg) and transferred into an aluminium sample older (pan and lid), which was sealed non-hermetically. The analysis was carried out with the following procedure: conventional heat/cool/heat DSC from -50°C to 120°C in nitrogen flow with heating/cooling rate of 10°C/min.
The glass transition temperatures (Tg) were determined in the second heating cycle as the temperature at half height of the jump in heat capacity on the thermogram and are shown in Table 4. Table 4

S1 S2 S3 S4

Ageing at RT 2 2 0 2

Measured Tg via DSC (°C) -5 -21 35 -21
It is clear from Table 4 that less fading is observed at higher Tg's.
In Table 5 the theoretical Tg's of the individual polymerizable compounds of the solutions S1 to S4 are given (tabulated values in SART-easy, The easy SARTOMER Handbook, March 2011 version) as well as the theoretical Tg of the cured coating calculated according to Fox's equation: $\frac{1}{T_{g}} = \frac{w_{1}}{T_{g, 1}} + \frac{w_{2}}{T_{g, 2}}$
where w₁ and w₂ are the weight fractions and T_{g, 1} and T_{g, 2} are the glass transition temperatures of the polymerizable compounds 1 and 2 respectively.
By comparing the theoretical Tg of Table 5 and the measured Tg in Table 4 the importance of measuring the actual Tg of the cured layer becomes evident. Table 5

Tg [°C] S1 S2 S3 S4

Tg component 1 5 187 60 94

Tg component 2 -36 -36 / -36

Theoretical Tg coating -7 99 60 50

Measured Tg coating -5 -21 35 -21

Example 2

The coating solutions S5 to S8 were prepared by mixing the ingredients according to Table 6 expressed in grams in a 30 mL brown glass flasks with a plastic screw cap and stirred at 350 rpm overnight with a magnetic stirring bar at room temperature.

Table 6

Ingredients [g]	S5	S6	S7	S8
WR	0.25	=	=	=
CpTs	0.44	=	=	=
Photomer 4012	6.11	-	2.33	-
Sartomer 833S	-	-	5.00	-
Sartomer 339	-	-	-	4.88
Sartomer 595	-	6.34	-	-
CN9245	1.50	-	-	-
CN9165	-	-	-	2.50
Genocure DMHA	0.20	0.62	0.29	0.28
Omnirad 481	0.20	0.62	0.29	0.28
Speedcure TPO	0.20	0.62	0.29	0.28
BYK-UV3510	0.10	=	=	=
OPV1	-	-	-	1.00
OPV4	1.00	-	1.00	-
OPV5	-	1.00	-	-

The solutions were subsequently coated with a spiral bar coater (from Elcometer) using an automatic film applicator (Elcometer 4340 from Elcometer) at a speed of 20 mm/s on an A4 sheet of cardboard (Incada Exel HS (GC2) NI 255 g/m² 510^∗720 mm SG 450 µm) with a wet coating thickness of 10 µm. Each layer was cured right after being applied using a curing station (Aktiprint Mini 18 - 2.75 cm belt, 230 V, 50 Hz from Technigraf GmbH) at a speed of 10 m/min and with the lamp being at the second lowest position (second closest to the substrate). The coatings were cured with the number of passes according to Table 7. Table 7

S5 S6 S7 S8

Curing passes 10 4 5 7
Each solution was coated twice: one sample was exposed to an infrared laser whereas the other one was used to do Differential Scanning Calorimetry (DSC) tests.

The infrared laser was an optically pumped semiconductor laser emitting at 1064 nm (Genesis MX 1064-10000 MTM from COHERENT) with a maximum power of 4.0W, a spot size in X of 78.9 µm at 1/e² and a spot size in Y of 90.6 µm at 1/e². The used laser settings are depicted in Table 8. The addressability is the distance between dots centre to centre and the energy density was calculated assuming no overlap according to the following formula: energy density

[J . {cm}^{- 2}] = \frac{Power [W] \times Period [s]}{Spot area [{cm}^{2}] .}

Table 8

	P1	P2	P3
Type	Vector Graphics	Vector Graphics	Vector Graphics
Size H x L [mm x mm]	6.00 x 6.02	=	=
Resolution [dpi]	1270	=	=
Repetition Y	300	=	=
Increment Y [mm]	0.02	=	=
Addressability [mm]	/	/	/
Speed [mm/s]	800	=	=
Frequency [kHz]	40.0	=	=
Pulse length [µs]	25	15	13
Power [W]	2.54	2.48	2.19
Energy density [mJ/cm2]	1129	1104	977

The laser exposed samples were aged up to 14 days at both room temperature (RT) and in a ventilated oven set at 50° C. The results are depicted in Table 9. The reflectance spectrum of each sample was measured two times with a X-Rite™ eXact spectrophotometer in the range from 400 up to 700 nm in steps of 10 nm before and after ageing. The CIEL*a*b* coordinates were determined for a 2° observer and a D50 light source. The densities were measured with the density standard ANSI A. The densities Dc, Dm, Dy and Db correspond respectively to the densities in cyan, magenta, yellow and black according to the density filters of ANSI A. The density Dm was of more interest because here the laser marks are magenta. Measurements were done for both the laser marks and the areas of the inks that were not exposed to the laser (background).
A modulated DSC analysis was performed on samples that were 7 days old for S5 and S8 and 11 days old for S6 and S7 (storage in a plastic map in a conditioned room (21°C/ 50% RH)) with a Q1000 from TA Instruments.
A little amount of the coating was scraped off the substrate with a razor blade (4 to 10 mg) and transferred into an aluminium sample older (pan and lid) that was sealed non-hermetically. The analysis was carried out with the following procedure: modulated DSC from -50°C to 200°C in nitrogen flow with underlying heating rate of 2°C/min. Superimposed onto the underlying heating, the temperature is modulated with 1 °C every 60 seconds.
The glass transition temperatures (Tg) were determined in the first heating cycle as the temperature at half height of the jump in heat capacity on the Reversing Heat Flow signal.

The measured Tg's are depicted in Table 9.

Table 9

		S5	S6	S7	S8
Laser parameter		P1	P1	P2	P3
Ageing RT	Dm fresh	0.69	0.85	0.90	1.13
	Dm after14 days	0.62	0.90	0.92	1.14
	ΔDm	-0.07	+0.05	+0.02	+0.01
Ageing 50°C	Dm fresh	0.54	0.86	1.01	0.98
	Dm after14 days	0.41	0.29	1.06	1.06
	ΔDm	-0.13	-0.57	+0.05	+0.08
Measured Tg	Tg (°C)	-1	18	38	23

It is clear from Table 9 that no fading is observed when the Tg of the coating is 15°C or higher.

Claims

A laser markable article comprising a laser markable coating provided on a support, the coating comprising a colour forming agent, a polymeric matrix and an optional optothermal converting agent, characterized in that the Glass Transition Temperature (Tg) of the coating is 15°C or higher.
The laser markable article according to claim 1 wherein the Tg is 20°C or higher.
The laser markable article according to claim 1 or 2 wherein the colour forming agent is a leuco dye in combination with a developing agent.
The laser markable article according to claim 3 wherein the developing agent has a chemical structure according to Formula I,
wherein
R1 represent an optionally substituted alkyl group, an optionally substituted (hetero)cyclic alkyl group, an optionally substituted alkanyl group, an optionally substituted alkenyl group, an optionally substituted alkynyl group, an optionally substituted (hetero)aryl group, an optionally substituted aralkyl group, an optionally substituted alkoxy group, an optionally substituted (hetero)cyclic alkoxy group, or an optionally substituted (hetero)aryl group;

R2 represent an optionally substituted alkyl, an optionally substituted aliphatic (hetero)cyclic alkyl group or an optionally substituted aralkyl group;

R1 and R2 may represent the necessary atoms to form a ring
The laser markable article according to any of the preceding claims wherein the optothermal converting agent is an infrared absorbing compound.
The laser markable article according to claim 5 wherein the infrared absorbing compound is a pigment.
The laser markable article according to claim 6 wherein the pigment is carbon black or Cesium Tungsten Oxide.
The laser markable article according to any of the preceding claims wherein the article is selected from the group consisting of a packaging, a foil, a laminate, a security document, a label, a decorative object and an RFID tag.
A method of manufacturing a laser markable article as defined in any of the claim 1 to 8 including the steps of:
- applying a laser markable composition comprising at least one polymerizable compound, a colour forming agent and an optional optothermal converting agent on a support thereby forming a laser markable coating on the support; and

- curing the laser markable coating,
wherein the Glass Transition Temperature (Tg) of the coating is 15°C or higher.
The method according to 9 wherein the at least one polymerizable compound is selected from the group consisting of tricyclodecanedimethanol diacrylate (TCDDMDA), isobornyl acrylate (IBOA), ethoxylated [4] bisphenol A diacrylate and 1,10 decanediol diacrylate.
The method according to claims 9 or 10 wherein the total amount of polymerizable compounds is at least 50 wt% relative to the total weight of the composition.
The method according to any of the claims 9 to 11 wherein the curing step is carried out with UV radiation.
The method according to any of the claims 9 to 12 wherein the laser markable composition is applied on the support by flexographic or inkjet printing.
A method of laser marking comprising the step of exposing the laser markable article as defined in any of the claims 1 to 8 with a laser.
The method of laser marking according to claim 14 wherein the laser is a Near Infrared (NIR) laser.