Active energy ray-curable water-based inkjet ink and print head

Abstract

The present application relates to a viable energy ray radical curable inkjet printing ink comprising water, a radical curable di(meth)acrylate monomer, a radical curable (meth)acrylate compound, a photoinitiator of formula (I), and one or more co-initiators: wherein X + is Na + or Li + , preferably Na + . The present application also relates to a printed feature consisting of a cured ink layer, a document comprising said printed feature, a thermal inkjet printhead comprising said ink, and a method for printing a feature made from said ink on a substrate.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of inkjet inks and printheads. The present application generally relates to active energy ray curable water-based inkjet printing inks. The present application also relates to thermal inkjet printheads comprising said inks. BACKGROUND

[0002] Active energy ray free radical curable inks cure by a free radical mechanism which comprises the activation of one or more photoinitiators able to release free radicals upon the action of active energy rays, in particular ultraviolet light, which in turn initiates polymerization to form a cured layer.

[0003] UV energy is usually provided by mercury lamps, in particular by medium pressure mercury lamps. Mercury lamps require a lot of energy, need an efficient and expensive heat dissipation system, are prone to ozone formation and have a limited lifetime.

[0004] Recently, lamps and systems based on UV-LED have been developed for curing inks and coatings. In contrast to medium pressure mercury lamps which have an emission band in the UV-A, UV-B and UV-C region of the electromagnetic spectrum, UV-LED lamps emit radiation in the UV-A region. Moreover, current UV-LED lamps emit quasi-monochromatic radiation, i.e. only one wavelength is emitted, for example 365 nm, 385 nm, 395 nm or 405 nm.

[0005] Traditionally, active energy ray inks are solvent based inks; this means that each raw material is solvent soluble with all the technical consequences known in the art related to the use of solvent systems.

[0006] Solvent based inks in the inkjet field usually encounter some key issues: relatively low reliability in the inkjet printhead, flammability and health risks, chemical compatibility with the printhead materials, unpleasant odor.

[0007] When using solvent based inks, there can be migration of chemical species through the packaging. This can lead to the chemical components of the ink coming into contact and contaminating food products, leading to the ingestion of harmful components by the consumer. These issues must be considered for human health and the environment.

[0008] Therefore, there is a need in the art for a replacement of solvent based inks. To solve the problems brought by solvent based inks, water based active energy ray curable inks have been developed.

[0009] Water based inks have been developed, such as those described in US2009136680.

[0010] While it is important to solve the above mentioned problems, water based inks must present at least the same chemical, mechanical and technical properties as solvent based active energy ray inks once printed and crosslinked. More precisely, the developed inks must guarantee:

[0011] - good jetting through thermal inkjet printheads, whether single color or multiple inks printheads,

[0012] - high reliability of the ink loaded into proprietary thermal inkjet printheads,

[0013] - high compatibility of the ink with the different types of materials usually used to assemble the printheads (hydraulic glue, sponge, fibers, plastic containers, photosensitive polymers, etc.),

[0014] - good decap-time of the ink,

[0015] - short drying time of the ink on the printed media,

[0016] - high optical density,

[0017] - good adhesion once cured on different substrates (plastic, metal, paper...),

[0018] - high crosslinking density and conversion rate after irradiation,

[0019] - advantageous durability or chemical resistance (on porous and / or non-porous media),

[0020] - absence or limited presence of chemical substances migration (for example in food or pharmaceutical packaging) to support good human health and environmental sustainability,

[0021] - time / temperature curing conditions compatible with other parts of the printing tool.

[0022] The inventors have successfully formulated water-based inks which reach each of the criteria previously disclosed. The purpose of using these formulations in each field of application requires raw materials having specific requirements.

[0023] The first object of the present invention is a water-based, active energy ray free-radical curable inkjet printing ink.

[0024] The ink and the printhead comprising the ink are the result of a fine-tuning of the ingredients to achieve all the requirements of the final application.

[0025] The present invention introduces a water-based ink which, after irradiation with an active energy ray lamp, for example an LED lamp, guarantees a good adhesion on the substrate. The correct emission of the energy of the lamp allows an effective reticulation of the reactive part included in the ink formulation. The result is a printing ink with high durability, regardless of the substrate.

[0026] The innovative aspect of the present invention is the ability of the developed formulation to achieve water and solvent resistance once crosslinked. Moreover, the use of appropriate pigment dispersions in the color ink formulation avoids any discoloration due to contact with water and solvents such as ethanol.

[0027] A second object of the present invention is a printed feature consisting of a cured ink layer made of the active energy ray free radical curable inkjet printing ink.

[0028] Furthermore, the present invention relates to an article or document comprising a printing head substrate and one or more printed features according to the second object of the present invention.

[0029] A further object of the present invention is a thermal inkjet printing head comprising the active energy ray free radical curable inkjet printing ink according to the present invention.

[0030] Finally, the present invention also relates to a method of printing a printed feature on a substrate by thermal inkjet printing, the method comprising the step of applying the active energy ray free radical curable inkjet printing ink of the present invention. SUMMARY

[0031] Therefore, the object of the present invention is to overcome the drawbacks of the prior art. This is achieved by providing an active energy ray free radical curable inkjet printing ink, the ink comprising:

[0032] i) at least 55 wt% of water,

[0033] ii) about 2 wt% to about 20 wt% of a free radical curable di(meth)acrylate monomer which is a polyethylene glycol di(meth)acrylate having more than 5 oxirane groups per molecule;

[0034] iii) about 1 wt% to about 15 wt% of a free radical curable (meth)acrylate compound which is a hydroxyalkyl (meth)acrylate wherein the alkyl group is a methyl, ethyl, propyl, butyl or isobutyl group, preferably a hydroxyalkyl (meth)acrylate wherein the alkyl group is a methyl, ethyl, propyl, butyl or isobutyl group;

[0035] iv) about 1 wt% to about 5 wt% of a photoinitiator of formula (I):

[0036]

[0037]

[0038] wherein X + is Na + or Li + , preferably Na +

[0039] v) from about 0.1 wt% to about 2 wt% of one or more co-initiators selected from the group consisting of N-[3-(dimethylamine)propyl]methacrylamide and / or poly(methylhydrogenosiloxane),

[0040] The weight percentages are based on the total weight of the active energy ray free-radically curable inkjet printing ink.

[0041] Also described herein is a printed feature consisting of a cured ink layer made from the active energy ray free-radically curable inkjet printing ink described herein, and an article or document comprising a substrate and one or more printed features described herein.

[0042] Also described herein is a thermal inkjet printhead comprising a printhead substrate; a nozzle layer comprising a plurality of nozzles formed therethrough; a plurality of ink ejection chambers corresponding to the plurality of nozzles; a plurality of heater resistors formed on the printhead substrate and corresponding to the plurality of ink ejection chambers, each of the heater resistors being located in a different ink ejection chamber such that ejection of ink droplets through each nozzle is caused by heating of one of the heater resistors located in the corresponding ink ejection chamber; and the active energy ray free-radically curable inkjet printing ink described herein.

[0043] Also described herein is a method of printing a feature on a substrate by a thermal inkjet printing method and the feature obtained thereby, the method comprising the steps of:

[0044] a) applying the active energy ray free-radically curable inkjet printing ink described herein by thermal inkjet printing, thereby forming an ink layer, preferably, the step a) is performed with the thermal inkjet printhead described herein, and

[0045] b) exposing the ink layer to a dose of at least 150 mJ / cm 2 of active energy rays to cure the ink layer with an active energy ray source. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1A and 1B are schematic representation of a printhead cartridge compatible with the active energy ray free-radically curable inkjet printing ink of the present invention.

[0047] Figure 2A and 2B are schematic representation of a multi-ink printhead cartridge compatible with the active energy ray free-radically curable inkjet printing ink of the present invention. DETAILED DESCRIPTION

[0048] The following definitions are used to clarify the meaning of terms discussed in the specification and recited in the claims.

[0049] As used herein, the term "about" means that the amount or value discussed can be the specified value or some other value that is approximately the same as that specified. Typically, the term "about" is used in the context of a specified value to indicate that the value can vary from the specified value by a reasonable amount. As one example, the phrase "about 100" indicates the range of 90 to 110, i.e., 100 ± 10. Typically, when the term "about" is used, it is expected that a similar result or effect as according to the invention can be obtained within a range of 105% of the indicated value.

[0050] As used herein, the term "and / or" means that all the elements or only one of the elements can be present. For example, "A and / or B" shall mean "only A, or only B, or both A and B". In the case of "only A", the possibility of B not being present is also covered, i.e. "only A, but not B".

[0051] The term "comprising", as used herein, is intended to be non-exclusive and open-ended. Thus, for example, a coating composition comprising compound A can include other compounds in addition to A. However, the term "comprising" also covers the more restrictive meanings of "consisting essentially of and "consisting of as specific embodiments thereof, such that, for example, a dampening solution "comprising A, B and optional C" can also consist (essentially) of A and B, or (essentially) of A, B and C.

[0052] The term "active energy ray" relates to energy rays such as electron beams, ultraviolet rays, and infrared rays, which affect the electron orbit of the irradiated body, thereby initiating polymerization reactions as free radicals, cations, or anions, etc. "Active energy ray-curable ink" describes an ink that forms a cured film upon irradiation with these types of active energy rays.

[0053] The term "UV" (ultraviolet) as used herein is intended to mean irradiation having a wavelength component in the UV portion of the electromagnetic spectrum; typically from 200 nm to 420 nm.

[0054] The term "(meth)acrylate" in the context of the present invention refers to acrylate as well as the corresponding methacrylate. Likewise, "di(meth)acrylate" refers to diacrylate as well as the corresponding dimethacrylate.

[0055] Where the specification refers to "preferred" embodiments / features, then so long as a combination of "preferred" embodiments / features makes technical sense, combinations of these "preferred" embodiments / features shall also be considered disclosed.

[0056] The radical-curable inks described herein cure by a radical mechanism that includes the activation of energy of one or more photoinitiators that release radicals that in turn initiate polymerization.

[0057] The high amount of water in the ink prevents the strong evaporation from the nozzle that usually in solvent based systems leads to the unreliability of solvent based inks.

[0058] In an embodiment of the present application, the active energy ray free radical curable inkjet printing ink is a UV free radical curable inkjet printing ink.

[0059] In an embodiment of the present application, the active energy ray free radical curable inkjet printing ink of the present application is a LED free radical curable inkjet printing ink.

[0060] More preferably, the active energy ray free radical curable inkjet printing ink of the present application is a UV-LED free radical curable inkjet printing ink, i.e. an ink that forms a cured film upon irradiation by a LED lamp that emits ultraviolet radiation (hereinafter referred to as "UV-LED lamp").

[0061] While it is known in the art to use a large amount of reactive functions per monomer to produce printing elements with good properties, said diacrylate allows:

[0062] i) to keep the viscosity under a certain critical value to obtain a good jetting and allows to produce high quality printing elements;

[0063] ii) to limit the evaporation of the ink in the chamber of the print head during the life of the print head: the higher the evaporation, the greater the increase in viscosity.

[0064] Said di(meth)acrylate monomer having more than 5 oxirane groups per molecule allows to avoid the precipitation of some components of the ink that would make the ink unsuitable for the end use.

[0065] The free radical curable ink described herein further comprises from about 2 wt% to about 20 wt%, preferably from about 4 wt% to about 15 wt%, most preferably from about 5 wt% to about 12 wt% of a free radical curable di(meth)acrylate monomer that is a polyethylene glycol di(meth)acrylate having more than 5 oxirane groups per molecule.

[0066] Preferably, the free radical curable di(meth)acrylate monomer is a polyethylene glycol di(meth)acrylate having more than 7 oxirane groups per molecule.

[0067] More preferably, the free radical curable di(meth)acrylate monomer is a polyethylene glycol di(meth)acrylate having more than 10 oxirane groups per molecule.

[0068] In a preferred embodiment, the molecular weight of said free radical curable di(meth)acrylate monomer is comprised between about 300 g / mol and about 600 g / mol.

[0069] In the context of the present application, the best radical curable di(meth)acrylate monomer is exemplified by diacrylate (PEG diacrylate with an ethoxylation number of 10). This radical curable di(meth)acrylate monomer imparts the ability to the ink to become tacky upon evaporation of moisture without increasing its base viscosity too much.

[0070] The radical curable ink described herein further comprises from about 1 wt% to about 15 wt%, preferably from about 2 wt% to about 12 wt%, most preferably from about 3 wt% to about 9 wt% of a radical curable (meth)acrylate compound which is a (meth)acrylic acid hydroxyalkyl ester wherein the alkyl group is a methyl, ethyl, propyl, butyl or isobutyl group, preferably a (meth)acrylic acid hydroxyalkyl ester wherein the alkyl group is a methyl, ethyl, propyl, butyl or isobutyl group.

[0071] The radical curable ink described herein further comprises from about 1 wt% to about 5 wt%, preferably from about 1.5 wt% to about 4.5 wt%, most preferably from about 2.2 wt% to about 3.8 wt% of a photoinitiator of formula (I)

[0072]

[0073] wherein X + is Na + or Li + , preferably Na + .

[0074] The photoinitiator allows the ink of the present application to cure properly without having to use excessive amounts of active radiation energy.

[0075] In a preferred embodiment, the photoinitiator of formula (I) is BAPO-ONa.

[0076]

[0077] The photoinitiator of formula (I) can be introduced into the composition according to the present application at a lower concentration than the photoinitiators of the prior art, which in turn reduces the risk of migration of unbound photoinitiator or photoinitiator decomposition products, even in the case where a low migration potential photoinitiator is employed.

[0078] This is particularly advantageous for UV-inkjet compositions because a relatively high concentration of photoinitiator is generally required to help overcome the effects of oxygen inhibition, which is a unique problem associated with the UV curing of inkjet compositions in air. It is quite common for UV inkjet compositions to contain 8% w / w or more of a photoinitiator blend to achieve the required UV curing response.

[0079] In particular, any photoinitiator used in the composition according to the present application preferably exhibits a migration of less than 10 ppb.

[0080] The migration potential of a given photoinitiator is measured according to the method described in the EFSA Note for guidance FCM evaluation 2008.08.07.

[0081] The migration potential of a given photoinitiator is measured at 60°C.

[0082] The free radical curable ink described herein further comprises from about 0.1 wt% to about 2 wt%, preferably from about 0.2 wt% to about 1.5 wt%, most preferably from about 0.2 wt% to about 1.2 wt% of one or more co-initiators selected from the group consisting of N-[3-(dimethylamine)propyl]methacrylamide and / or poly(methylhydrogenosiloxane).

[0083] The co-initiators selected from the group consisting of N-[3-(dimethylamine)propyl]methacrylamide and / or poly(methylhydrogenosiloxane) impart sufficient cross-linking to the ink, which in turn gives the cured ink sufficient water resistance.

[0084] In embodiments of the present application, the active energy ray free radical curable inkjet printing ink further comprises from about 1.0 wt% to about 15 wt%, preferably from about 2 wt% to about 12 wt%, most preferably from about 3 wt% to about 10 wt% of a colorant, the weight percent being based on the total weight of the active energy ray free radical curable inkjet printing ink.

[0085] The colorant described herein includes pigments and / or dyes.

[0086] The color ink formulations described herein (i.e. containing one or more colorants, i.e. one or more pigments and / or dyes) can be used to print images and / or colored text on different types of materials, ensuring excellent durability over time on the printed support.

[0087] Dyes include, but are not limited to, azo dyes, anthraquinone dyes, xanthene dyes, azine dyes, and combinations thereof, among others. The organic pigment can be one pigment or a combination of pigments, such as Pigment Yellow Numbers 12, 13, 14, 17, 74, 83, 114, 126, 127, 174, 188; Pigment Red Numbers 2, 22, 23, 48:1, 48:2, 52, 52:1, 53, 57:1, 112, 122, 166, 170, 184, 202, 266, 269; Pigment Orange Numbers 5, 16, 34, 36; Pigment Blue Numbers 15, 15:3, 15:4; Pigment Violet Numbers 3, 23, 27; and / or Pigment Green Number 7. The inorganic pigment can be one of the following non-limiting pigments: iron oxide group, titanium dioxide group, chromium oxide group, ferric ammonium ferrocyanide group, iron oxide black group, Pigment Black Number 7, and / or Pigment White Numbers 6 and 7. Other organic and inorganic pigments and dyes can also be employed, as well as combinations to achieve the desired color.

[0088] The colorant is preferably dispersed in a mixture comprising one or more mono(meth)acrylate monomers and / or one or more di(meth)acrylate monomers and / or one or more tri(meth)acrylate monomers prior to its incorporation into the ink.

[0089] Optionally, the active energy ray free radical curable inkjet printing ink lacking a colorant can be used to print images and / or text, and can optionally be used as a cover to protect the images or text printed by the color or black ink.

[0090] In embodiments, the active energy ray free radical curable inkjet printing ink further comprises from about 0.05 wt% to about 2 wt%, preferably from about 0.1 wt% to about 1.8 wt%, most preferably from about 0.15 wt% to about 1.5 wt% of a non-ionic fluorinated surfactant, the weight percent being based on the total weight of the active energy ray free radical curable inkjet printing ink.

[0091] The addition of the non-ionic fluorinated surfactant reduces the surface tension of the ink, which in turn allows the ink to properly spread on the surface of the substrate to be printed.

[0092] Preferably, the non-ionic fluorinated surfactant is a non-ionic high molecular ethoxylated fluorinated surfactant and / or a non-ionic high molecular acrylic fluorinated surfactant.

[0093] More preferably, the non-ionic fluorinated surfactant is selected from the group comprising Hexafor 672 (MAFLON) and Hexafor 644-D (MAFLON).

[0094] In embodiments, the active energy ray free radical curable inkjet printing ink further comprises from about 1 wt% to about 5 wt%, preferably from about 1.5 wt% to about 4.5 wt%, most preferably from about 2.2 wt% to about 3.8 wt% of one or more free radical curable oligomers having a molecular weight of at least 80 g / mol, the weight percent based on the total weight of the active energy ray free radical curable inkjet printing ink.

[0095] The free radical curable oligomers having a molecular weight of at least 80 g / mol improve the curing and resistance of the ink.

[0096] Preferably, the free radical curable oligomers having a molecular weight of at least 80 g / mol are selected from the group consisting of tri(meth)acrylate oligomers, tetra(meth)acrylate oligomers, hexa(meth)acrylate oligomers, and mixtures thereof.

[0097] More preferably, the free radical curable oligomers having a molecular weight of at least 80 g / mol are one or more hexa(meth)acrylate oligomers having a molecular weight of at least 80 g / mol.

[0098] In even more preferred embodiments, the free radical curable oligomers having a molecular weight of at least 80 g / mol are Photomer Aqua 6903 (IGM).

[0099] In embodiments, the hydroxyalkyl (meth)acrylate of iii) is a hydroxyalkyl (meth)acrylate monomer wherein the alkyl group is a methyl, ethyl, propyl, butyl, or isobutyl group, preferably a 4-hydroxyalkyl (meth)acrylate monomer wherein the alkyl group is a methyl, ethyl, propyl, butyl, or isobutyl group, more preferably a 4-hydroxybutyl (meth)acrylate monomer.

[0100] In embodiments, the active energy ray free radical curable inkjet printing ink further comprises from about 0.1 wt% to about 3 wt%, preferably from about 0.15 wt% to about 2.25 wt%, most preferably from about 0.2 wt% to about 1.5 wt% of a second photoinitiator, the weight percent based on the total weight of the active energy ray free radical curable inkjet printing ink.

[0101] The presence of the second photoinitiator improves the curing of the ink.

[0102] Preferably, the second photoinitiator comprises one or more thioxanthone compounds having a molecular weight of less than 400 g / mol, preferably 2-isopropylthioxanthone, 4-isopropylthioxanthone, 2,4-dimethylthioxanthone, 2,4-diethylthioxanthone, 2,4-diisopropylthioxanthone, 2-chlorothioxanthone, 2-chloro-4-isopropoxythioxanthone, and mixtures thereof, the weight percent based on the total weight of the active energy ray free radical curable inkjet printing ink.

[0103] More preferably, the second photoinitiator is 2-isopropylthioxanthone.

[0104] Preferably, the active energy ray radical curable inkjet printing ink has a viscosity in the range of about 0.5 centipoise to about 10 centipoise at 25°C.

[0105] The person skilled in the art is well versed in methods that can be used to measure the viscosity of a fluid. For the purpose of example, and without wishing to be bound by this example, the viscosity of the ink can be measured with a RHEOLOGICA VISCOTECH- (RD312) following the manufacturer's instructions.

[0106] Preferably, the active energy ray radical curable inkjet printing ink has a viscosity in the range of about 1 centipoise to about 9 centipoise, more preferably about 2 centipoise to 8 centipoise, measured at 25°C.

[0107] The viscosity and the amount of water of the ink impart the reliability of the print head required during the lifetime of the print head; this means that:

[0108] - the print head will not exhibit significant nozzle failure throughout its shelf life,

[0109] - the print head will not have significant "decap failure" during printing pauses, even beyond 3 minutes

[0110] - the print head will work properly at the frequency usually required in applications such as card printing, coding and marking.

[0111] The formulation must contain only water-soluble or dispersible raw materials (monomers, photoinitiators, surfactants, co-initiators, etc.) in order to be stable and printable through thermal inkjet print heads.

[0112] The compositions according to the present application can also contain other components that enable them to perform their intended purpose. These components include, but are not limited to: stabilizers, wetting aids, slip agents, inert resins such as acrylic polymers, antifoams, fillers, rheological aids, amine synergists, etc.

[0113] The radical curable inks described herein can also contain one or more additional surfactants to ensure proper substrate wetting, reducing the surface tension of the ink.

[0114] In a preferred embodiment of the present application, any component used in the compositions according to the present application preferably exhibits a migration of less than 10 ppb.

[0115] The migration potential of a given component is measured according to the method described in the EFSA Guidance Notes on the EC Commission Regulation (EC) No 1935 / 2004 on materials and articles intended to come into contact with foodstuffs - Guidance on the evaluation of the migration of components from FCMs 2008.08.07.

[0116] The migration potential of a given component is measured at 60°C.

[0117] Another aspect of the present application is a printed feature consisting of a cured ink layer made of the above described active energy ray free radical curable inkjet printing ink.

[0118] The present application also relates to an article or document comprising a substrate and one or more printed features as described above.

[0119] Typical examples of substrates include, but are not limited to, fibrous substrates, preferably cellulose fiber based substrates, such as paper, paper containing materials, polymeric substrates, composite materials (e.g. substrates obtained by lamination of paper layers and polymeric films), metals or metallized materials (e.g. aluminum), silicon, ceramics, glass, ceramic articles and combinations thereof. Typical examples of polymeric substrates are substrates made of ethylene or propylene based homo- and copolymers such as polypropylene (PP) and polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), polyamide (PA), polymethyl methacrylate (PMMA) and polyethylene terephthalate (PET).

[0120] Preferably, the substrate of the article or document is a paper containing material, a polymeric material, a composite material, a metal, a glass, a ceramic or any combination thereof.

[0121] Another aspect of the present application is a thermal inkjet printhead comprising a printhead substrate; a nozzle layer comprising a plurality of nozzles formed therethrough; a plurality of ink ejection chambers corresponding to the plurality of nozzles; a plurality of heater resistors formed on the printhead substrate and corresponding to the plurality of ink ejection chambers, each of the heater resistors being located in a different ink ejection chamber such that ink droplet ejection through each nozzle is caused by heating of one of the heater resistors located in the corresponding ink ejection chamber; and the active energy ray free radical curable inkjet printing ink according to the present application.

[0122] Thanks to the above described printhead and to the chemical properties of the developed ink, it is possible to well meet market requirements in terms of number of printable substrates, printing speed, flexibility of the printable area and reliability of the system.

[0123] Moreover, as described above, thanks to the presence of a high amount of water in the ink, the system has an intrinsic high reliability which prevents the strong evaporation from the nozzles usually present in solvent based systems.

[0124] The reactivity of the system, together with the above described chemical-physical properties of the ink, and the properties of the proprietary printhead, provide a complete system which meets the high requirements of printing needs of the market.

[0125] The present application also relates to a method for printing a feature on a substrate by a thermal inkjet printing process, comprising the following steps:

[0126] a) applying the active energy ray free radically curable inkjet printing ink according to the present application by thermal inkjet printing, thereby forming an ink layer, and

[0127] b) exposing the ink layer to a dose of at least 150 mJ / cm 2 of active energy rays to cure the ink layer with the active energy ray source.

[0128] Preferably, step a) of the method is performed with a thermal inkjet printhead as described above.

[0129] Preferably, the active energy ray source of step b) is a UV-LED source.

[0130] Preferably, step b) of the method consists of exposing the ink layer to one or more wavelengths between about 380 nm and about 420 nm. Typically, commercially available UV-LED light sources use one or more wavelengths, such as 365 nm, 385 nm, 395 nm and 405 nm.

[0131] Preferably, the speed range of the method is comprised between about 0 m / min and about 60 m / min. The speed range is measured at ambient temperature.

[0132] Preferably, the drying time range of the method is comprised between about 0.02 seconds and about 1 second, more preferably between about 0.07 seconds and about 0.44 seconds.

[0133] In embodiments of the present application, the printing frequency is higher than about 7 KHz. In preferred embodiments, the printing frequency is higher than about 8 KHz. More preferably, the printing frequency is higher than about 9 KHz.

[0134] In embodiments of the method, the ink layer made of active energy ray free radically curable inkjet printing ink is transparent, and wherein said ink is applied at least partially in the form of one or more markings on a printed feature.

[0135] The method described herein is particularly suitable for producing one or more printed features on a substrate, wherein said one or more printed features can be continuous or discontinuous.

[0136] These and other objects, advantages and features of the present application will become apparent to those skilled in the art from a reading of the detailed description of the method and formulations given hereinafter.

[0137] Examples

[0138] The present application will now be described in more detail with reference to non-limiting examples.

[0139] A. Ink according to the application

[0140] Several inks were formulated according to the instructions of the present application: the compositions E1-E5 of these inks are disclosed below.

[0141]

[0142] Table 1. Composition of the aqueous ink

[0143] Ink preparation

[0144] All the chemical compounds used for this work were commercially available and used as received, without further purification treatment.

[0145] In a glass container containing a magnetic stirrer bar, the raw materials were introduced at room temperature in the following order: monomers, water; surfactant, co-initiator, photoinitiator, dye / pigment. Subsequently, the mixture thus obtained was stirred at room temperature for 45-60'. The solution was then filtered and the filtrate was introduced inside the print head under vacuum conditions. The filtration was performed using Versapore filters with a porous size diameter between 0.3 pm and 3.0 pm. The ink thus obtained was introduced into the print head by means of an inking machine (Xynertech semi-automatic filling system).

[0146] The color inks were also prepared using the same method with the following compositions.

[0147]

[0148] Table 2. Composition of the color aqueous ink

[0149] B. Comparative inks

[0150] The inks of the present application were compared with the following inks prepared according to the prior art, in order to evaluate whether, once printed and UV-crosslinked, they reached at least the same chemical, mechanical and technical performances as solvent-based UV inks.

[0151]

[0152] Table 3. Composition of the comparative inks

[0153] C. Comparison

[0154] The inks were subjected to tests of several criteria indicative of the prerequisites expected to meet the market requirements. Among these:

[0155] - the degree of reticulation reached after the curing process must be high.

[0156] - the viscosity must be sufficiently low to ensure proper jetting of the ink.

[0157] - the UV aqueous ink formulation contains the components studied to impart to the polymer a high adhesion to a large number of printable materials.

[0158] - Mechanical durability of UV water-based inks is also mandatory; tests have been carried out to evaluate their adhesion properties on printed surfaces.

[0159]

[0160] Table 4. Comparison of the ink of the application (E) with comparative inks (C)

[0161] Printing test

[0162] The type of printhead used during the printing test was single ink printhead and multi-ink printhead. The printing test was carried out using the card printer FARGO INK 1000 and the Neopost printer system. Curing was carried out with a commercial UV lamp Phosen FJ 100 (16 W) at a distance of 4 mm, an emission wavelength of 395 nm, a window size of 2 x 7.5 cm and a belt speed of 60 m / min (for the "dynamic" printing test, color ink) and with a UV lamp developed in-house (for the "static" printing test, transparent ink). The energy values provided by irradiation have been measured by a UV-Design radiometer UV-MC Microprocessor Integrator. The degree of crosslinking of the formulation after printing and irradiation has been determined by FTIR measurements using a Nicolet spectrometer FT-IR Nexus.

[0163] General experimental procedure ("dynamic" printing test):

[0164] The single ink printhead containing the desired formulation was introduced in the Neopost printer system. The substrate was placed on a conveyor belt whose speed could be adjusted. The substrate reached the printhead station (where printing took place) and the UV lamp (where irradiation took place). Finally, the printed medium was recovered.

[0165] General experimental procedure ("static" printing test):

[0166] The multi-ink printhead containing the desired formulation was introduced in the card printer FARGO INK 1000. The card loader was loaded and warmed up to the desired temperature. Then, the card was printed and "statically" irradiated with the UV lamp. After the irradiation process, the card was ejected by the printer.

[0167] Crosslinking measurement procedure

[0168] Evaluation of the degree of material conversion measured by FTIR spectroscopy. The FTIR instrument measures the typical monomer signal. The reaction of the monomer is monitored by observing the disappearance of the infrared vibration peak associated with the acrylate functionality as a function of the UV energy dose.

[0169] Chemical resistance test procedure

[0170] The chemical resistance evaluation is performed by immersing the sample in water for 24 hours. If the printed ink does not change, the test is OK. The water resistance is KO when the cured ink is removed after immersion. If the cured ink is not removed by water but changes in other ways, the ink is considered to have non-ideal water resistance.

[0171] Viscosity method procedure

[0172] The ink viscosity is measured with the RHEOLOGIC AVISCO TECH- (RD312) tool equipped with a thermostatic bath to maintain the correct temperature of the ink during the measurement (25°C).

[0173] The viscosity measurement is performed as reported below:

[0174] - a volume of 0.925 ml of ink is deposited on the temperature-controlled plate with a graduated pipette, taking care not to create air bubbles,

[0175] - the measurement is started by the software: the head of the viscometer is lowered and, once the coupling of the rotating plate with the thermostatic base is reached, the plate starts to rotate for a few seconds,

[0176] - the ink creep resistance measurement or viscosity is expressed in mPa*s (same as centipoise).

[0177] Surface tension method procedure

[0178] The ink surface tension is measured with the KRUSS K12- (RD337) TENSIO METER tool equipped with a thermostatic bath to maintain the temperature required by the ink during the measurement.

[0179] The surface tension measurement is performed as reported below:

[0180] - the platinum plate is washed with 37% hydrochloric acid and then with deionized water,

[0181] - the platinum plate is heated using the Bunsen burner flame,

[0182] - the glass is filled with two-thirds of ink,

[0183] - the glass is inserted into its appropriate slot to keep the ink thermostatic,

[0184] - the glass is brought close enough to the platinum plate (with the appropriate knob) so that the ink surface rubs the lower limit of the plate,

[0185] - the measurement is started by the instrument,

[0186] - the surface tension measurement is reported on the tool display and is expressed in dynes / cm.

[0187] Taber test

[0188] The Taber test result refers to the number of wear cycles required for each test ink to reach the stopping point. The stopping point is reached when the initial optical density measurement decreases by 50% (ANSI INCITS 322-2008, Test Method for Card Durability).

[0189] Sample preparation: PVC cards were printed at 70°C using a 16-layer printing mode (smart overlay (shingling) printing mode) and samples were prepared by UV irradiation during printing.

[0190] Friction and abrasion resistance tests were conducted using both colored and transparent inks. A rubbing resistance tester (Crock-meter) was used to evaluate abrasion resistance: a piece of cotton fabric was rubbed 100 times on the cured ink on an aluminum sheet (without adding any weight to the arm). Evaluation was based on the change in chromaticity coordinates, expressed as a ΔE value.

[0191] As a function of the desired wear resistance, the thickness of the deposit on the surface can be between 1 μm and 50 μm, and can be measured using a mechanical profilometer (TENCOR) or an optical microscope.

[0192] D. Colored ink

[0193] The colored inks (E6-E8) of the present invention were prepared and tested to evaluate their curing ability.

[0194] The minimum energy dose required to achieve a conversion rate of 70% or higher for three color inks is at least 50 mJ / cm. 2 .

[0195] The following table reports additional features:

[0196]

[0197] Table 5. Characteristics of Colored Water-Based Inks

[0198] Stain test

[0199] The stain test result refers to the color change of a sample after rubbing it 100 times on the tool arm of the rubbing fade tester with a weight of 500 grams, and is expressed as the ΔE value.

[0200] Sample preparation: Samples were prepared by printing in a 4-layer mode at 70°C using a higher ink volume (>18% by weight compared to the smart overlay printing mode) and passing the sample through a UV lamp 4 times after printing.

[0201] E. Other experiments

[0202] Further experiments were conducted to compare the ink according to the present invention with inks of the prior art.

[0203] Other phosphine oxides

[0204] Besides what has already been used 819 (phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide) is used to replace LFC 3587, with C5-C6 based on E3. The precise composition is given below.

[0205]

[0206] Table 6. Composition of water-based inks containing different phosphorus oxides

[0207] Then, E3 was tested based on the crosslinking ability of these C5 and C6 inks under UV light.

[0208] The resulting comparative ink exhibited very poor curing performance.

[0209] UV energy dose (mJ / cm 2 )]]> Cross-linking degree E3 400 ≥70％ C5 350 19％ C6 625 2％

[0210] Table 7. Comparison of inks with different properties of phosphine oxide.

[0211] The experiment was repeated using lithium phenyl-2,4,6-trimethylbenzoylphosphonate instead of LFC3587.

[0212] Lithium phenyl-2,4,6-trimethylbenzoylphosphonate (“LAP”) has the following formula:

[0213]

[0214] Raw materials C7 Water 72.63 KP-BK904 UV (Ink-Genio) 8.13 PEG diacrylate n=10 (ALDRICH) 9.76 4-hydroxybutyl acrylate (ALDRICH) 4.00 Hexafor 672 (Maflon) 1.00 Lithium phenyl-2,4,6-trimethylbenzoylphosphinate (Aldrich) 3.00 Omnirad ITX (IGM) 0.50 Poly(methylhydrogenosiloxane) (Aldrich) 0.98

[0215] Table 8. Composition of water-based inks containing LAP as a substitute for LFC3587

[0216] The degree of crosslinking of the ink after UV irradiation was completely unsatisfactory. Despite the high amount of UV energy used to achieve photocrosslinking of the ink (1000 mJ / cm²), the results were unsatisfactory. 2 However, the conversion rate of acrylate functional groups is still less than 50%.

[0217] Triethanolamine replaces the claimed co-initiator

[0218] To evaluate the performance of triethanolamine in the aqueous formulations of this invention, a new ink (C8) was prepared. This ink is similar to E5, but contains triethanolamine instead of polymethylhydrosiloxane (used in the same percentage value as in E5).

[0219] UV energy dose (mJ / cm 2 ) Cross-linking degree Water resistance E5 109 ≥70％ OK C8 602 56％ Very poor

[0220] Table 9. Comparison of co-initiators

[0221] Other thioxanone

[0222] An ink similar to E6-E8, except that 0.5 wt% of Omnipol TX (polymeric thioxanthone photoinitiator) has been used instead of 1.0 wt% of Omnirad ITX. The resulting comparative ink showed very poor curing performance.

[0223] Acrylates with less than 5 oxirane groups

[0224] In order to evaluate the possibility of using diacrylates with n values less than 5, 3 color inks containing diacrylates with MW = 258 g / mol (n = 3) were prepared.

[0225]

[0226] Table 10. Composition of inks containing acrylates with less than 5 oxirane groups

[0227] After 2 days of storage at room temperature in glass jars, the sedimentation of certain components in the inks was checked. The sedimentation of C9 (cyan) was particularly evident, that of C10 (magenta) C10 was very evident, that of C11 (yellow) was slightly visible. Sedimentation also occurred in the clear formulation C12.

[0228] Migration test experiment

[0229] The purpose of the migration experiment was to evaluate whether the molecule ITX (lower molecule in terms of molecular weight) does not migrate through the printed substrate according to the European standard.

[0230] The ITX molecule, due to its typical chemical behavior of type II photoinitiator, maintains its molecular structure after UV irradiation without any photolysis and chemical binding to the polymer macromolecule, making it easily migratable out of the cured layer.

[0231] For this reason, a deep study of its composition was carried out in order to evaluate the migration values of ITX. After 10 days of storage at 60°C of the cured samples in contact with 95% ethanol and 10% ethanol as a simulated fluid, the migration of these substrates was evaluated in the set-off condition.

[0232] The experiment was carried out according to the EFSA guidelines (EFSA guidelines - FCM evaluation guidelines note 2008.08.07).

[0233] Analytical method

[0234] This analytical method allows the check and quantification in simulated fluids by using the U-HPLC technique.

[0235] The detection limit is the total amount of photoinitiator detected in the simulated fluid, expressed in ppb. The detection limit of Omnirad ITX is 10 ppb.

[0236] Printed substrates prepared using UV lamp Phoseon FJ-100 were as follows:

[0237]

[0238] Table 11. Samples for migration test

[0239] Migration test conditions

[0240] Specific migration of photoinitiators was measured in indirect contact (blocking). For the indirect contact test, the surface of each cured sample was pressed against an unprinted substrate with 20 Kg (196 N) for 10 days at room temperature.

[0241] After blocking, the substrates were cut to the following dimensions:

[0242] Total area = 2.54 cm x 2.54 cm = 6.45 cm2 2 .

[0243] Each sample was cut from the different coated foils with a cutter.

[0244] Surface / volume ratio: 0.6 cm2 / ml (according to EFSA guidelines, the surface / volume ratio is required to be between 0.5 and 2). 2

[0245] Each sample was put in a vial (20 ml) in contact with 10 ml of precisely measured simulated fluid: 95% ethanol and 10% ethanol.

[0246] The sample was completely covered and stored in a thermostatic water bath at 60°C for 10 days in the dark.

[0247] Each vial was tightly closed to avoid evaporation of the simulated fluid and was appropriately labeled.

[0248] After storage, each vial was cooled at room temperature and the filtered simulated fluid was transferred to a clean vial (20 ml).

[0249] In addition, a standard substrate (unprinted) was kept in contact with the simulated fluid under the same conditions to obtain a blank solution.

[0250] All migration tests were performed in duplicate.

[0251] Each sample of simulated fluid was analyzed by UHPLC with UV diode array and MS single quad detector.

[0252] The results are expressed in ppb in the simulated fluid.

[0253] ​The analysis of each sample was performed in triplicate.

[0254] Results of the blank sample

[0255] The results of the analysis (average of 3 samples) and the identification of the samples are reported in the following Table 12. The results are expressed in ppb content in the two simulated fluids.

[0256]

[0257] Table 12. Results of the blank sample

[0258]

[0259] Table 13. Indirect contact sample (smearing)

[0260]

[0261] Table 14. Indirect contact sample (smearing)

[0262] As a conclusion, it was found that ITX has a migration value at the acceptable limit of 10 ppb.

[0263] Furthermore, LFC3587 was not detected at values higher than the limit of detection of the analytical instrument used. This is more predictable due to the chemical behavior of this type I photoinitiator, which undergoes photolysis upon UV irradiation and the chemical by-products of the initiator remain chemically bound to the macromolecule.

[0264] The exemplary black ink formulation products (E3 and E5) are suitable for pharmaceutical and coding / labeling applications, as well as for food and beverage field, since they have passed the migration test according to European standards, explicitly.

[0265] The cured inks also show resistance to humidity, since the printed images are still readable after storage at -15°C and 4°C and after thermal cycles between these two temperatures and room temperature.

[0266] The cured inks also show high light resistance equivalent to 3 years of direct sunlight exposure (outdoor test using the sun test instrument XXL+ using a xenon lamp).

[0267] Sun test method:

[0268] The optical density of the samples was measured with a reflectance densitometer (ANSI STATUS I): Gretag Macbeth DensyEye700

[0269] The samples were exposed to a xenon lamp for 12 days, without window filter (illumination intensity: 0.35 watt / m2at 340 nm at the surface of the card 2Test chamber temperature: 50°C ± 5°C

[0270] Final optical density measurement: the final evaluation was performed by measuring the percentage loss of optical density. The following ranges of optical density loss % were considered for the outdoor exposure final evaluation.

[0271] Value of x Comment x>50％ Different colors 40％<x<50％ Strong color difference 25％<x<40％ Quite noticeable change 10％<x<25％ Color with a noticeable but smooth change 5％<x<10％ Very small color difference

[0272] Table 15. Sun test

[0273] The results of the sun test for the colored inks were all good, with OD loss % equal to or lower than 15%.

[0274] The results of the sun test for the black ink were also good, with OD loss % between 0% and 10%.

[0275] From the tests presented previously, it is evident to the person skilled in the art that the developed formulation cured with the lamp guarantees the following requirements:

[0276] - high cross-linking density,

[0277] - high conversion (percentage of covalent bond formation),

[0278] - high adhesion to printed surfaces (paper, plastic, metal),

[0279] - high chemical resistance to water and ethanol,

[0280] - high rub and wear resistance,

[0281] - high sun-test resistance.

[0282] Therefore, the developed formulation reaches the performances of solvent-based UV inks.

[0283] The exemplary formulations (black and colored) meet the above requirements, in particular the black ink formulation is also compatible with food and beverage applications.

[0284] F. Print head according to the application

[0285] The present application also relates to a printhead cartridge configured to be used with the above ink. As Figure 1AAs shown, such a printhead cartridge is made of a printhead jetting assembly 1 consisting of a printhead chip 2 glued to a flexible printed circuit 3. The chip is equipped with electrical and hydraulic components to provide energy to the ink towards the individual jetting locations, to produce ink droplets for printing. A nozzle plate is applied on the top surface of the chip to provide nozzles for ink ejection. The whole jetting assembly is in turn glued to a cartridge 4 containing an ink reservoir closed by a lid 5. In Figure 1B A suitable ink slot 6 is obtained in the cartridge body 7 as shown to allow the ink to reach the printhead chip and the microfluidic circuit through a slot 8 machined into the chip or from the edge of the chip, depending on the printhead arrangement.

[0286] In a multi-ink printhead cartridge, as Figure 2A There are of course multiple ink reservoirs and multiple ink paths towards the printhead as shown. They are hydraulically isolated from each other to prevent ink mixing. Since the cartridge is assembled from different components and materials, the joints between the components must not only ensure good adhesion but also a perfect and durable ink seal in the areas in contact with the ink. There are many ways to glue different materials: using a suitable glue has many advantages, provided that the glue can be precisely dispensed in the gluing area. For example, a suitable glue can be dispensed on the flat surface surrounding the flow channel 6 in the body to ensure its adhesion to the chip and to ensure a good seal around the lower surface of the slot 8 in the chip. In this way, the ink can flow from the reservoir to the chip without any mixing or leakage.

[0287] Moreover, the cartridge body of a multi-ink printhead requires a special manufacturing method: for example, in a three-ink cartridge with a parallel nozzle array, casting techniques do not allow obtaining a piece at once with a single molding method: more precisely, as Figure 2BAs shown, the cartridge body 7 has three ink reservoirs 9 separated by walls 10. Due to the small lateral distance between the different color nozzle arrays, it is not possible to create three separate straight ink paths, thus maintaining the necessary hydraulic characteristics and suitable structural robustness. A possible solution is to use two further parallel sliding inserts to create the required fluidic structure within the cartridge body (as described in patent EP 189622 B1). Once the casting process is completed, the extraction of the two sliding inserts leaves two windows 11 in the side surface 12 of the cartridge: these windows must be closed with suitable plugs 13, which are conveniently glued with the cartridge. The vertical axis y downwards in the figure corresponds to the ink ejection direction of the printhead. One possible way to glue the plugs is to use a glue dispensed along the flat recessed surface 14 of the window borders. This ensures a tight seal of the opening, without allowing the ink to leak out of the reservoir. The UV curable glue is not sufficient to reach the sealing purpose due to the front flange of the plug and the corresponding groove in the cartridge body. Glues with poor degree of polymerization and low adhesion and sealing properties cannot be effectively irradiated by UV radiation.

[0288] As regards the electrical control, the printhead is regulated by C-MOS technology. This technology is more expensive than the one used before, but it is more powerful. The specific tools improve the logical control of the printhead and its guidance is possible with significant energy savings. The C-MOS technology allows a large degree of design freedom and allows more complex electronic integration on the chip, thus reducing space and energy consumption.

[0289] At the same time, due to the reactivity of the system, together with the above-mentioned chemical-physical characteristics of the ink, and the characteristics of the proprietary printhead, the complete system meets the high requirements of printing.

[0290] In any case, the present application cannot and should not be limited to the embodiments specifically described in this document, as other embodiments are possible. The present application should be extended to any equivalent means and to any combination of individually technically operable means.

Claims

1. An active energy ray free radical curable inkjet printing ink comprising: i) at least 55 wt% of water ii) 2 to 20 wt% of a free radical curable di(meth)acrylate monomer which is a polyethylene glycol di(meth)acrylate having 5 or more oxirane groups per molecule; iii) 1 to 15 wt% of a free radical curable (meth)acrylate compound which is a hydroxyalkyl (meth)acrylate wherein the alkyl group is a methyl, ethyl, propyl, butyl or isobutyl group; iv) 1 to 5 wt% of a photoinitiator of formula (I): ###0001### (I) wherein X + is Na + or Li + , v) 0.1 to 2 wt% of one or more co-initiators selected from the group consisting of N-[3-(dimethylamine)propyl]methacrylamide and / or poly(methylhydrogenosiloxane); the weight percentages being based on the total weight of the active energy ray free radical curable inkjet printing ink.

2. The active energy ray radical curable inkjet printing ink according to claim 1, wherein X + is Na + .

3. The active energy ray free radical curable inkjet printing ink according to claim 1 or 2, wherein the molecular weight of the free radical curable di(meth)acrylate monomer which is a polyethylene glycol di(meth)acrylate having 5 or more oxirane groups per molecule is comprised between 300 g / mol and 600 g / mol.

4. The active energy ray free radical curable inkjet printing ink according to claim 1 or 2, further comprising 1.0 to 15 wt% of a colorant, the weight percentage being based on the total weight of the active energy ray free radical curable inkjet printing ink.

5. The active energy ray free radical curable inkjet printing ink according to claim 1 or 2, further comprising 0.05 to 2 wt% of a non-ionic fluorinated surfactant, the weight percentage being based on the total weight of the active energy ray free radical curable inkjet printing ink.

6. The active energy ray free radical curable inkjet printing ink according to claim 5, wherein the non-ionic fluorinated surfactant is a non-ionic high molecular ethoxylated fluorinated surfactant and / or a non-ionic high molecular acrylic fluorinated surfactant.

7. The active energy ray free radical curable inkjet printing ink according to claim 1 or 2, wherein the hydroxyalkyl (meth)acrylate of iii) is a hydroxyalkyl (meth)acrylate monomer wherein the alkyl group is a methyl, ethyl, propyl, butyl or isobutyl group.

8. The active energy ray free radical curable inkjet printing ink according to claim 7, wherein the hydroxyalkyl (meth)acrylate monomer is a 4-hydroxyalkyl (meth)acrylate monomer wherein the alkyl group is a methyl, ethyl, propyl, butyl or isobutyl group.

9. The active energy ray free radical curable inkjet printing ink according to claim 1 or 2, further comprising 0.1 to 3 wt% of a second photoinitiator, the weight percentage being based on the total weight of the active energy ray free radical curable inkjet printing ink, the second photoinitiator comprising one or more thioxanthone compounds having a molecular weight of less than 400 g / mol.

10. The active energy ray free radical curable inkjet printing ink according to claim 9, wherein the second photoinitiator is 2-isopropylthioxanthone, 4-isopropylthioxanthone, 2,4-dimethylthioxanthone, 2,4-diethylthioxanthone, 2,4-diisopropylthioxanthone, 2-chlorothioxanthone, 2-chloro-4-isopropoxythioxanthone, and mixtures thereof.

11. The active energy ray free radical curable inkjet printing ink according to claim 1 or 2, having a viscosity in the range of 0.5 to 10 centipoise at 25°C.

12. A printed feature consisting of a cured ink layer made from the active energy ray free radical curable inkjet printing ink according to any one of claims 1 to 11.

13. An article or document comprising a substrate and one or more printed features according to claim 12.

14. The article or document according to claim 13, wherein the substrate is a paper containing material, a polymeric based material, a composite material, a metal, a glass, a ceramic, or any combination thereof.

15. A thermal inkjet printhead comprising: a printhead substrate; a nozzle layer comprising a plurality of nozzles formed therethrough; a plurality of ink ejection chambers corresponding to the plurality of nozzles; a plurality of heater resistors formed on the printhead substrate and corresponding to the plurality of ink ejection chambers, each of the heater resistors being located in a different ink ejection chamber such that ejection of ink droplets through each nozzle is caused by heating of one of the heater resistors located in the corresponding ink ejection chamber; and the active energy ray free radical curable inkjet printing ink according to any one of claims 1 to 11.

16. A method of printing a feature on a substrate by a thermal inkjet printing method, comprising the steps of: a) applying the active energy ray free radical curable inkjet printing ink according to any one of claims 1 to 11 by thermal inkjet printing, thereby forming an ink layer, and b) exposing the ink layer to a dose of at least 150 mJ / cm 2 of active energy rays to cure the ink layer with the active energy ray source.

17. The method according to claim 16, wherein step a) is performed with the thermal inkjet printhead according to claim 15.

18. The method of claim 16 or 17, wherein step b) consists of: exposing the ink layer to one or more wavelengths between 380 nm and 420 nm.

19. The method according to claim 16 or 17, wherein the ink layer made from the active energy ray free radical curable inkjet printing ink is transparent, and wherein the ink is applied at least partially in the form of one or more markings on the printed feature.

20. The method according to claim 16 or 17, wherein the ink layer is exposed to one or more wavelengths between 380 nm and 420 nm for a period of time in the range of 0.1 to 10 seconds.

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