EP3928995A1

EP3928995A1 - Marking of articles

Info

Publication number: EP3928995A1
Application number: EP20181362.3A
Authority: EP
Inventors: Fabienne Goethals
Original assignee: Agfa NV
Current assignee: Agfa NV
Priority date: 2020-06-22
Filing date: 2020-06-22
Publication date: 2021-12-29

Abstract

A method of preparing a markable article (1) comprising the step of providing at least a first (10) colour-forming composition on a support (500) and a second (20) colour-forming composition on the support (500), the compositions capable of forming respectively a first (C1) and a second (C2) colour upon marking, in such a way that the compositions flow into each other thereby forming a colour-forming layer (100).

Description

Technical field of the Invention

The present invention relates to a method of marking articles, in particular heat-sensitive articles such as packaging and security articles.

Background art for the invention

A principal objective of security cards and also of other "security documents", such as banknotes, is that they cannot be easily modified or reproduced in such a way that the modification or reproduction is difficult to distinguish from the original.
Rainbow, also referred to as iris printing, is a special printing technique used for security documents, such as banknotes, passports, ID-cards and other security articles, preventing accurate colour separation or illegal reproduction of the documents by subtly merging colours into each other, resulting in a gradual colour change.
Conventional rainbow printing is one-dimensional. In such one-dimensional rainbow printing two or more inks of different colours are mixed along a single axial direction, transversely to the path of the substrate being printed. Printing equipment suitable for one-dimensional rainbow printing is for example disclosed in WO2010/056711 (KBA-NOTASYS). Such one-dimensional rainbow printing results in prints of which the colour varies smoothly in one-direction.
Two-dimensional rainbow printing resulting in prints wherein the colour varies in two directions is also known. For example, WO2008/099330 and WO2019/101683 (both from KBA-NOTASYS) disclose such systems. However, the methods disclosed are rather complex and may suffer from registration problems due to the use of two different printing plates.
Rainbow printing is typically carried out with offset printing technology. However, such offset rainbow printing is very cumbersome and needs a lot of technical skills of the operator. Moreover, offset printing is typically used for printing non-variable information on a document of packaging.
Laser marking is a well known technique to provide information on security documents and/or packaging.
EP-A 2648920 (Agfa Gevaert NV) discloses a laser markable security document comprising a so-called colour-forming layer including a leuco dye, a developing agent and an optothermal converting agent. A colour may be formed when the colour-forming layer is exposed with an IR laser.
EP-A 2722367 (Agfa Gevaert NV) discloses security documents capable of forming a full colour image upon laser marking. The security documents comprise three different colour-forming layers, the colour-forming layers each comprising a different leuco dye and a different infrared dye. Exposing the security document with three different IR lasers results in full colour laser marked images.
EP-A 3095825 (Agfa Gevaert NV) dislose an aqueous inkjet ink for forming a colour upon heat treatment wherein the leuco dyes are encapsulated or bonded to a polymer particle.

Summary of the invention

It is an object of the present invention to provide a method to produce rainbow patterns on a security article or packaging characterized by a reduced complexity, less registration problems and that may be used for providing variable data.
That object has been realised by the method as defined in claim 1.
Further objects of the invention will become apparent from the description hereinafter.

Brief description of drawings

Figure 1 : A schematic representation of a preferred embodiment of a method to prepare a heat-sensitive article according to the present invention.
Figure 2 : A schematic representation of an embodiment of a heat-sensitive article according to the present invention.
Figure 3 : A schematic representation of a preferred embodiment of a method of marking a heat-sensitive article according to the present invention.
Figure 4 : A schematic representation of a preferred embodiment of a heat-sensitive security card used in a method according to the present 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-methyl-butyl, 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, -Cl, -Br, -I, -OH, -SH, -CN and -NO₂.

Method of marking an article

The method of marking an article (1) according to the present invention includes the steps of:

providing a markable article (1) described below; and
marking the article thereby forming an image (200).

Marking is preferably carried out with heat, radiation or pressure, more preferably by heat.
In the marking step, also referred to as image formation step, a heat treatment is used to form colours. The heat treatment may be a direct heat treatment, for example using a thermal print head. Alternatively the heat treatment is an indirect heat treatment, wherein radiation such as UV or IR radiation is applied and converted by an opthothermal converting agent into heat.
In a preferred embodiment, heat is provided by means of a laser, preferably an infrared (IR) laser, more preferably a Near Infrared (NIR) laser.

Method of preparing the markable article

The method of preparing markable article (1) according to the present invention comprises the step of providing at least a first (10) colour-forming composition on a support (500) and a second (20) colour-forming composition on the support (500), the compositions capable of forming respectively a first (C1) and a second (C2) colour upon marking, on a support (500) in such a way that the compositions flow into each other thereby forming a colour-forming layer (100).
In addition to the first and second colour-forming compositions referred to above, a third colour-forming composition (30) capable of forming a third colour (C3) upon marking may be applied on the support in such a way that it flows together with the first and/or the second composition thereby forming the colour-forming layer (100).
The colour-forming compositions preferably comprise a leuco dye and a developing agent.
The colour-forming compositions may comprise one, two, three or more leuco dyes. When the colour-forming compositions each comprise one leuco dye, the leuco dyes are different from each other and are capable of forming a different colour. When the colour-forming compositions comprise more than one leuco dye, i.e. a mixture of leuco dyes, then each mixture of leuco dyes is capable of forming a different colour.
Each colour-forming composition may comprise a different developing agent. However, it is preferred that all colour-forming compositions comprise the same developing agent.
When heat is provided by means of a laser, the colour-forming compositions preferably include an optothermal converting agent. However, when a CO₂ laser is used, the colour-forming composition preferably do not contain an optothermal converting agent. The colour-forming compositions may comprise different optothermal converting agents, however it is preferred that the optothermal converting agent is the same for all colour-forming compositions.
The optothermal converting agent and the developing agent may also be added to a layer adjacent to the colour-forming layer (100). Adding the developing agent in an adjacent layer may improve the stability of the heat-sensitive layer, adding the optothermal converting agent in an adjacent layer may result in less burning of the article during marking.
In a preferred embodiment, A UV blocking layer (400) may be provided on top of the colour-forming layer (100) to improve the daylight stability of the marked image. Such an UV blocking layer may be the adjacent layer referred to above wherein an optothermal converting agent and/or a developing agent may be added.
Also, an overlay (600) may be provided on top of the colour-forming layer (100) or the optional UV blocking layer (400) to protect the marked image for example from physical damage or moisture.
The colour-forming compositions may be provided onto the support by any printing method such as intaglio printing, screen printing, flexographic printing, offset printing, inkjet printing, valve jet printing, rotogravure printing, etc. The colour-forming compositions are preferably applied on the support by inkjet printing, valve jet printing or screen printing.
The colour-forming compositions are applied in such a way that they flow in each other thereby forming a heat-sensitive colour-forming layer (100). This means that the first composition (10) and the second composition (20) will mix at their interface (15) and that the second composition (20) and the third composition (30) will mix at their interface (25).
The colour-forming compositions may be deposited in such a way that they touch each other. Alternatively, the colour-forming compositions may be deposited in such a way that there is a space between the deposited composition, as long as they flow into each other after deposition forming the heat-sensitive layer.
The compositions are preferably optimized, for example their surface energy and viscosity, to form a smooth colour-forming layer (100) after their deposition on the support.
A drying step is preferably carried out after the formation of the colour-forming layer (100).
Drying is preferably carried out at a temperature between 15 and 130°C, preferably between 25°C and 100°C, most preferably between 40 and 80 °C.
Upon drying/heating, covalent bonds can optionally be formed, resulting in better adhesion or mechanical properties. This may be realized by adding cross-linking agents or cross-linkable binders to the colour-forming or other layers, as disclosed in EP-A 19216842.5 and EP-A19216821.9 , both from Agfa NV and filed on 17-12-2019.
When radiation curable compositions are used, curing is preferably carried out after the formation of the colour-forming layer (100).
The curing step is preferably carried out with UV radiation, preferably LED UV radiation.
The 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.

Colour-forming composition

The colour-forming compositions preferably include a leuco dye, a developing agent and an optional optothermal converting agent.
The colour-forming composition may be an aqueous or 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.
The aqueous based composition may be radiation curable, preferably UV curable. Such radiation curable aqueous composition are disclosed in EP-A 3626471 and EP-A 3626472 (both from Agfa Gevaert NV).
Non-aqueous colour-forming compositions are disclosed in for example EP-A 3083261 (Agfa Gevaert NV).
The non-aqueous colour-forming compositions are preferably radiation curable, more preferably UV curable. Such radiation curable compositions preferably comprise a polymerizable compound and a photoinitiator. Such radiation curable composition may further comprise a polymerization inhibitor.

Leuco dye

The colour-forming composition preferably comprises a 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]. Preferred leuco dyes are fluoran and phthalide leuco dyes.
The colour-forming composition may comprise more than one leuco dye. Using two, three or more leuco dyes may be necessary to realize a particular colour or to achieve a better solubility of the leuco dyes.
The total amount of leuco dye in the colour-forming layer is preferably in the range from 0.01 to 2 g/m², more preferably in the range from 0.05 to 1 g/m², most preferably in the range from 0.1 to 0.75 g/m².

Developing agent

The colour-forming composition 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.
Preferred developing agents for aqueous laser markable compositions are disclosed in EP-A 3470134 (Agfa Gevaert NV), paragraph [0142] to [0149]. A particular preferred developing agent is a metal salt of salicylic acid, for example zinc salicylate. A particularly preferred colour developing agent is zinc 3,5-bis(α-methylbenzyl) salicylate.
For non-aqueous laser markable compositions, and in particular for radiation curable non-aqueous laser markable compositions, a preferred developing agents has 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 is disclosed in WO2015/091688 , paragraph [052] to [070] (Agfa Gevaert NV).
A particular preferred developing agent according to Formula I is menthyl p-toluene sulfonate and has the following chemical structure.
The amount of developing agent in the colour-forming layer is preferably in the range from 0.05 to 5 g/m², more preferably in the range from 0.1 to 3 g/m².

Optothermal converting agent

The colour-forming composition may comprise an optothermal converting agent.
The optothermal converting agent may also be present in a layer adjacent to the colour-forming layer, for example an interlayer or an UV blocking layer.
An optothermal converting agent generates heat upon absorption of radiation.
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.
The optothermal converting agents may be an infrared radiation absorbing pigment, an infrared radiation absorbing dye or a combination thereof.
The optothermal converting agent may be different for each colour-forming composition, but preferably each colour-forming composition includes the same optothermal converting layer.
It is also possible that the optothermal converting agent is present in an intermediate layer, a primer, or a top coating.
The amount of optothermal converting agent in the colour-forming layer 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.

Infrared radiation absorbing inorganic pigments

Any Near Infrared absorbing pigment may be used in the present invention.
A preferred inorganic infrared absorber is a copper salt as disclosed in WO2005/068207 (DATALASE).
Another preferred inorganic infrared absorber are non-stoichiometric metal salts, such 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

Another 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 layer comprising the carbon black.
For that reason, the amount of carbon black in the colour-forming layer is preferably less than 0.1 g/m², more preferably less than 0.01 g/m², most preferably less than 0.005 g/m².

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.
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], both from Agfa Gevaert NV.
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 Amax 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.

Acid Scavenger

The colour-forming compositions or an UV-blocking layer may contain one or more acid scavengers.
Acid scavengers include organic or inorganic bases. Examples of the inorganic bases include hydroxides of alkali metals or alkaline earth metals; secondary or tertiary phosphates, borates, carbonates; quinolinates and metaborates of alkali metals or alkaline earth metals; a combination of zinc hydroxide or zinc oxide and a chelating agent (e.g., sodium picolinate); hydrotalcite such as Hycite 713 from Clariant; ammonium hydroxide; hydroxides of quaternary alkylammoniums; and hydroxides of other metals. Examples of the organic bases include aliphatic amines (e.g., trialkylamines, hydroxylamines and aliphatic polyamines); aromatic amines (e.g., N-alkyl-substituted aromatic amines, N-hydroxylalkyl-substituted aromatic amines and bis[p-(dialkylamino)phenyl]-methanes), heterocyclic amines, amidines, cyclic amidines, guanidines and cyclic guanidines.
Other preferred acid scavangers are HALS compounds. Example of suitable HALS include Tinuvin^™ 292, Tinuvin^™ 123, Tinuvin^™ 1198, Tinuvin^™ 1198 L, Tinuvin^™ 144, Tinuvin^™ 152, Tinuvin^™ 292, Tinuvin^™ 292 HP, Tinuvin^™ 5100, Tinuvin^™ 622 SF, Tinuvin^™ 770 DF, Chimassorb^™ 2020 FDL, Chimassorb^™ 944 LD from BASF; Hostavin 3051, Hostavin 3050, Hostavin N 30, Hostavin N321, Hostavin N 845 PP, Hostavin PR 31 from Clariant.
Further examples of acid scavengers are salts of weak organic acids such as carboxylates (e.g. calcium stearate).
A preferred acid scavenger is an organic base, more preferably an amine.
A particular preferred acid scavenger is an organic base having a pKb of less than 7.
It has been observed that acid scavengers may improve the storage stability of the markable article.

UV absorbers

The markable article may also comprise an UV-absorber. The UV-absorber may be present in a colour-forming layer or may also be present in another layer. An UV-absorber is preferably present in an UV blocking layer.
Examples of suitable UV-absorbers include 2-hydroxyphenyl-benzophenones (BP) such as Chimassorb^™ 81 and Chimassorb^™ 90 from BASF; 2-(2-hydroxyphenyl)-benzotriazoles (BTZ) such as Tinuvin^™ 109, Tinuvin^™ 1130, Tinuvin^™ 171, Tinuvin^™ 326, Tinuvin^™ 328, Tinuvin^™ 384-2, Tinuvin^™ 99-2, Tinuvin^™ 900, Tinuvin^™ 928, Tinuvin^™ Carboprotect^™, Tinuvin^™ 360, Tinuvin^™ 1130, Tinuvin^™ 327, Tinuvin^™ 350, Tinuvin^™ 234 from BASF, Mixxim^™ BB/100 from FAIRMOUNT, Chiguard 5530 from Chitec; 2-hydroxy-phenyl-s-triazines (HPT) such as Tinuvin^™ 460, Tinuvin^™ 400, Tinuvin^™ 405, Tinuvin^™ 477, Tinuvin^™ 479, Tinuvin^™ 1577 ED, Tinuvin^™ 1600 from BASF, 2-(2,4-dihydroxyphenyl)-4,6-bis-(2,4-dimethylphenyl)-s-triazine (CASRN1668-53-7) from Capot Chemical Ltd and 4-[4,6-bis(2-methyl-phenoxy)-1,3,5-triazin-2-yl]-1,3-benzenediol (CASRN13413-61-1); titanium dioxide such as Solasorb 100F from from Croda Chemicals; zink oxide such as Solasorb 200F from Croda Chemicals; benzoxazines such as Cyasorb UV-3638 F, CYASORB^™ UV-1164 from CYTEC; and oxamides such as Sanduvor VSU from Clariant.
Preferred UV absorbers have in the wavelength region between 300 and 400 nm a maximum absorption above 330 nm, more preferably above 350 nm.
Particular preferred UV absorbers are hydroxyphenyl benzotriazoles and 2-hydroxyphenyl-s-triazines having a maximum absorption above 350 nm in the wavelength region 300 - 400 nm.
It has been observed that the presence of UV absorbers may improve the daylight stability of the laser marked image.

Polvmerizable compound

A radiation curable colour-forming composition preferably comprises at least one polymerizable compound. The composition may comprise one, two, three or more different polymerizable compounds.
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.
Preferred monomers and oligomers are those listed in paragraphs [0103] to [0126] of EP-A 1911814 (Agfa NV).
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.
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; 1,10 decanediol diacrylate; dipropylene glycol diacrylate (DPGDA); ethoxylated [4] bisphenol diacrylate and urethane acrylate oligomer.
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

A radiation curable colour-forming composition preferably contains a photoinitiator. The initiator typically initiates the polymerization reaction. The photo-initiator 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, a radiation curable colour-forming 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 colour-forming composition.

UV blocking layer

A UV blocking layer (400) may be provided on the colour-forming layer (100).
The UV blocking layer includes one or more UV absorbers as described above.

Support

The compositions may be applied on any type of support, for example a metallic support, a glass support, a polymeric support, or a paper support. The compositions 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 applied layers.
The support maybe transparent or opaque.
To improve the contrast of the marked image, the support may be a white support. The support then typically comprises a white dye or pigment, for example a titanium oxide pigment. Such a white support may be obtained by providing a white primer on a support.
When the heat-sensitive article is used as laminate wherein the support becomes after lamination a top layer, for example to prepare a security document as described below, the support is preferably transparent.
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.
Preferred polymeric supports are polyvinylchloride (PVC), polycarbonate (PC) and polyethylene terephthalate (PET) supports.
A preferred PET support is a biaxially stretched polyethylene terephthalate (BOPET) foil due to its very high durability and resistance to scratches and chemical substances.
The manufacturing of such BOPET 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.
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. packaging box or a 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 heat-sensitive 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 foils.

Markable article

The markable article is preferably by the method described above.
The 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.

Packaging

Laser marking or thermal printing is typically used to add variable data, for example batch numbers, expiry dates, addressees, barcodes, etc. on the packaging. However, laser marking or thermal printing may also be used to add a combination of fixed and variable data on a packaging. Variable and/or fixed data may also include images.
The markable article described above makes it possible to mark coloured data and/or images in a rainbow pattern. The rainbow effect may be used for aesthetic reasons or to make it more difficult to imitate the data and/or images.
Marking is preferably carried out in-line in the packaging process. An advantage of laser or thermal printing marking is the possibility to add data at a very late stage of the packaging process.
Another advantage of using laser marking or thermal printing 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 an advantage.
Still another advantage of laser marking is the ability to provide the "image" with a laser on a 3D object or the mark through a transparent foil or layer.
A preferred packaging is folded cardboard or corrugated cardboard laminated with paper. Such packaging is preferably used for cosmetics, pharmaceuticals, food or electronics.
The heat-sensitive article according to the present invention may also be used for brand protection as it is difficult to counterfeit. For example, the heat-sensitive article may be a cap of a bottle of champagne, the sealing of the cap of a container, the label of a bottle of wine, etc.

Security Documents

The marking method, in particular using a laser in the image formation step, may also be used to prepare security documents, such as for example ID cards.
Preferably, the security document is prepared by laminating a laser markable article according to the present invention, optionally together with other foils or laminates, onto one or both sides of a core support.
A schematic representation of such a security document (2) is illustrated in Figure 4. The heat-sensitive article (1), also referred to as a laser markable laminate, is provided on a core support. Laser marking is carried out through the support (500). For that reason, the support is preferably transparent.
The transparent support may include UV absorbers to act as a UV blocking layer. However, a UV blocking layer may also be provided on either side of the support.
Core supports typically used to prepare security documents are disclosed in EP-A 3431304 (Agfa Gevaert NV), paragraphs [0128] to [0138]. Preferred polymeric cores are based on polycarbonate (PC), polyvinylchloride (PVC), and polyethylene terephthalate (PET).
The laser markable laminate is typically laminated on one or both sides of a core support using elevated temperatures and pressures.
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.

Rainbow marking

Rainbow printing is a widely used security feature used in for example ID cards and banknotes. By subtly merging different colours a gradual colour change is realized. Rainbow printing protects the security documents from colour separation and copying.
Such rainbow printing with offset is very cumbersome and needs a lot of technical skills of the operator.
Figure 2 illustrates how a rainbow pattern may be obtained with the method according to the present invention. Subjecting part of the heat-sensitive layer to heat by means of a laser or a thermal print head results in a rainbow pattern.
An additional advantage of the method according to the present invention is the possibility to generate variable data having a rainbow pattern. With Offset printing, which is now used to print rainbow patterns, it is not possible to print variable data.

Laser marking

In principle any laser may be used.
The laser can be a solid state laser, such as a disk laser, a Nd:Yag laser or a Fiber laser. The laser can be a gas laser, such as a He-Ne laser, a CO2-laser, or an Excimer laser.
The laser can be a semiconductor laser, such as a diode laser or a VCSEL laser. The laser can also be a liquid laser such as a 580 nm Ring Dye laser.
The laser is preferably an infrared (IR) laser.
The IR laser may be a continuous wave or a pulsed laser.
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).
The intensity of the image colours may be varied by varying the energy density per unit of length (E_d) measured at the surface of the laser markable article.
The energy density per unit of length (E_d) is dependent on the average laser power of the laser beam (P_avg) and the velocity of the laser beam (v) as described in the following Formula: $Ed [J / m] = \frac{P_{avg} [W]}{v [m / s]}$
The average laser power P_avg relates to the laser power according to the following Formula: $P_{avg} [W] = P * f * W$
wherein

P is the laser power [W];
f is the frequency [Hz] and
W is the pulse width [s]

The image maybe laser marked using a vector mode or a raster mode.
Raster and vector are different graphic file types which require different modes of laser processing. The main difference between vector and raster graphics is that raster graphics are composed of pixels, while vector graphics are composed of paths or lines.
In raster mode, laser marking uses the same type of process used by inkjet printers, where a file, the raster file, representing the image is printed line by line. The raster file is a bitmap, which means it is made up of pixels. The image is marked with a laser line by line, point by point, similar to the way in which an inkjet printer applies ink, but instead of ink being applied, the laser marks the material. This is a "bi-directional" process in which marking is done in alternating fashion in both directions.
During the line-by-line raster marking process, the speeds of the two axles are very different. The speed is high on the x axle (the axle to which the laser head is attached) and lower by comparison on the y axle.
In raster mode, the ppi parameter (pulses per inch) is important because it controls the density of the laser points.
In vector mode, the file representing the image to be marked is a graphic file consisting of vectors (lines and curves of a geometry). In vector mode laser marking, the axles move simultaneously, and more slowly than in raster mode.

Thermal printing

Heat may be applied to the heat-sensitive article by means of a so-called thermal print heads.
In principle any type of thermal print head may be used. A thermal print head comprises heating elements. The heating elements convert electrical energy into heat through the process of Joule heating. Electric current through the element encounters resistance, resulting in heating of the element. The amount of electrical energy supplied to the heating elements can be varied by varying the amount of electric current within a particular time interval and/or varying the time interval during which electric current is supplied.
The number of heating elements of a print head determines the print resolution. Typical values are 200 to 300 heating elements (dots) per inch (dpi). However, thermal printers with a resolution of 400 and 600 dpi are also available.
Thermal printers and thermal printing methods are disclosed in for example US850287 (Zinc Imaging), Datalase ( WO2020/020901 ) and US2020/016904 (Canon).wacht

Claims

A method of preparing a markable article (1) comprising the step of providing at least a first colour-forming composition (10) on a support (500) and a second colour-forming composition (20) on the support (500), the compositions capable of forming respectively a first (C1) and a second (C2) colour upon marking, in such a way that the compositions flow into each other thereby forming a colour-forming layer (100).
The method according to claim 1 wherein a third colour-forming composition (30) capable of forming a third colour (C3) upon marking is applied on the support in such a way that it forms a colour-forming layer (100) together with the first and second compositions.
The method according to claim 1 or 2 wherein the colour-forming compositions are applied on the support by inkjet printing, valve jet printing or screen printing.
The method according to any of the preceding claims wherein the colour-forming compositions include a leuco dye and a developing agent.
The method according to any of the preceding claims wherein the colour-forming compositions include an optothermal converting agent.
The method according to claim 4 or 5 wherein the leuco dye and/or optothermal converting agent are encapsulated.
The method according to any of the preceding claims wherein a UV blocking layer (400) or an overlay (600) is applied on the colour-forming layer (100).
The method 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 marking an article including a marking step wherein a markable article obtained with a method as defined in any of the preceding claims is exposed to radiation, heat or pressure thereby forming an image (200).
The method according to claim 9 wherein marking is carried out with a laser.
The method according to claim 10 wherein the laser is a near infrared (NIR) laser.
The method according to claim 9 wherein marking is carried out with a thermal print head.
The method according to any of the claims 9 to 12 wherein the image is a rainbow pattern.
The method according to claim 13 wherein the image includes variable data.
A marked article prepared with a method according to any of the claims 9 to 14.