RU2833478C1 - Radically curable under action of uv-led radiation inks for offset printing and printing methods - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to the field of inks radically curable under the effect of UV-LED radiation for offset printing of protected documents. Disclosed is an offset printing ink radically curable under the effect of UV-LED radiation, containing from 50 wt.% to 80 wt.% of one or more radically curable (meth)acrylate compounds, from 8 wt.% to 12 wt.% of one or more photoinitiators, from 2 wt.% to 12 wt.% of one or more amine-containing compounds, from 1 wt.% to 30 wt.% of one or more inorganic pigments and/or one or more organic pigments, from 0.5 wt.% to 5 wt.% of one or more fillers and/or diluents and additives selected from co-initiators, anti-settling agents, antifoaming agents, surfactants and other process additives, wherein viscosity of which ranges from 2.5 to 25 Pa⋅s at 40 °C and 1000 s^-1; method of printing a feature on a substrate using an offset printing process using the disclosed ink; use of one or more photoinitiators to produce the disclosed paint; a security feature consisting of a cured layer of the disclosed paint, and a security document comprising the disclosed security feature.

EFFECT: obtaining inks for offset printing, which are characterized by good curing properties, while printing features obtained with their help have low fluorescence or no fluorescence.

21 cl, 1 dwg, 8 tbl, 12 ex

Description

Field of technology to which the invention relates

[001] The present invention relates to the field of protecting protected documents from counterfeiting and illegal reproduction. In particular, the present invention relates to the field of UV-LED-radiation-curable offset printing inks and methods for producing features or patterns on protected documents.

Prerequisites creations inventions

[002] With the continuous improvement in the quality of color photocopies and printed works and in an attempt to protect security documents such as banknotes, valuable documents or cards, travel tickets or cards, tax stamps and product labels that do not have reproducible effects from counterfeiting, falsification or illegal reproduction, it has become common practice to incorporate various security features in and on these documents. Typical examples of security features include security threads, windows, fibers, confetti, foils, transfers, holograms, watermarks, security inks containing optically variable pigments, magnetic or magnetizable thin-film interference pigments, interference coated particles, thermochromic pigments, photochromic pigments, luminescent compounds, infrared absorbing compounds, ultraviolet absorbing compounds or magnetic compounds.

[003] It is known in the prior art to apply inks to documents to be protected in several printing steps, including various printing processes using low-viscosity inks, such as screen printing inks, flexographic printing inks and gravure printing inks, as well as high-viscosity or paste-like inks, such as offset printing inks, gravure printing inks and letterpress inks.

[004] Offset printing processes consist of indirect methods in which the ink is first transferred from the printing plate to the blanket cylinder and onto the substrate. Offset printing processes are either wet offset processes or dry offset processes. The advantage of wet offset processes is the difference in surface energy between the imaged area and the non-imaged area of the printing plate. The imaged area is oleophilic and the non-imaged area is hydrophilic. Thus, the oily inks used in the method tend to adhere to the imaged area and be repelled by the non-imaged area of the printing plate. Wet offset printing is typically accomplished by applying both a fountain solution (also referred to in the art as a wetting solution) and an oleophilic ink to the surface of the printing plate, allowing image areas to preferentially receive ink and non-image areas to preferentially receive the fountain solution, and then transferring the ink deposited on the image areas to the substrate via an impression cylinder. Dry offset printing processes are methods in which a blanket cylinder is coated with a metal-based photopolymer that forms reliefs that carry the designs to be printed. The ink adheres to the reliefs and is then transferred to the substrate via the impression cylinder. In this case, there is no need for a fountain solution.

[005] It is known from the prior art that offset printing is usually the first step in obtaining printed features as a background pattern for security documents. The curing of offset printing inks involves either an oxidation process (i.e. the ink contains an oxidizing drying agent that reacts with atmospheric oxygen to form free radicals that initiate the polymerization process of the ink matrix) or a radiation curing process, in particular a UV curing process (i.e. the ink contains photoinitiators that, when interacting with light, form free radicals that initiate the polymerization of the ink matrix). Oxidation curing is a slow process, whereas UV curing is almost instantaneous. Since security printing is an expensive and multi-step process that requires several intermediate curing steps, UV-radical-curing offset printing inks are generally preferred in this area. UV-radical-curing offset printing inks are applied in thin layers of approximately 0.5 µm to approximately 3 µm (in the cured state) to produce features on the documents to be protected.

[006] Offset printing inks can be applied to both sides of the substrate (simultaneous process) to print features as a background image of the documents to be protected, and the curing step can be carried out simultaneously or sequentially. Typical industrial printing speeds range from approximately 8,000 sheets per hour to approximately 12,000 sheets per hour.

[007] The failure of ink to dry and harden quickly and effectively results in overprinting. Overprinting occurs when printing ink that is not sufficiently dried or cured adheres at least partially to the upward-facing back of the printing substrate during the folding of the printing substrates as they exit the printing presses (see, for example, US 4604952). This is a particular problem during the printing of features on security documents, especially banknotes, since these documents typically contain several overlapping or partially overlapping features that are applied in subsequent steps. If a previously applied feature, such as a background image or a graphic design, is insufficiently dried or cured, not only is the entire multi-step printing process delayed, but the feature thus produced may still suffer from overprinting or marking due to overprinting on the machine during any subsequent printing or process steps.

[008] During the normal banknote printing process, offset printing inks are applied during one of the first stages of the overall multi-stage printing process, where offset printing is followed by other printing stages, including gravure printing and screen printing. If a feature or design is printed on a security document, such as a banknote, using an offset printing process, and if said feature or design suffers from poor surface curing properties, the subsequent printing stage may be delayed or over-imprinting may occur. Accordingly, it is very important that the offset printing inks are completely dried or cured before other printing stages begin to avoid any over-imprinting problems, particularly when the next stage is the gravure printing stage, since the gravure printing press applies very high pressure to the printing substrate.

[009] UV-radical-curable paints are cured by free radical mechanisms consisting of the activation of one or more photoinitiators capable of releasing free radicals upon exposure to radiation, particularly UV light, which in turn initiate polymerization to form a cured layer. Known free radical photoinitiators include, for example, acetophenones, benzophenones, alpha-aminoketones, alpha-hydroxyketones, phosphine oxides and phosphine oxide derivatives, and benzyl dimethyl ketals.

[010] UV energy is typically provided by mercury vapor lamps, particularly medium-pressure mercury vapor lamps. Mercury vapor lamps consume large amounts of energy, require efficient and expensive heat removal systems, are prone to ozone formation, and have a limited lifespan.

[011] In order to provide lower cost, lower intervention and more environmentally friendly solutions, UV LED-based lamps and systems have been developed for curing paints and coatings. Unlike medium pressure mercury lamps, whose emission bands are in the UVA, UVB and UVC ranges of the electromagnetic spectrum, UV LED lamps emit radiation in the UVA range. Moreover, modern UV LED lamps emit quasi-monochromatic radiation, i.e. they emit only one wavelength, such as 365 nm, 385 nm, 395 nm or 405 nm.

[012] The efficiency of curing a coating or paint layer under the influence of UV radiation depends, among other things, on the overlap of the emission spectrum of the radiation source used for said curing and the absorption spectrum of the photoinitiator contained in the coating or paint. Accordingly, curing under the influence of UV radiation of a coating or paint layers comprising commonly used free radical photoinitiators with UV LED lamps suffers from reduced curing efficiency as a result of poor overlap of the emission spectrum of the lamp with the absorption of commonly used free radical photoinitiators, which leads to slow or poor curing or curing defects.

[013] Photoinitiators whose absorption spectrum roughly matches the emission wavelength of currently available UV LED sources include alpha-aminoketones, thioxanthones, ketocoumarins, bis[4(dimethylamino)phenyl]methanone derivatives (also called Michler ketones), and acylphosphines. However, alpha-aminoketones and Michler ketones have recently raised health and/or environmental concerns. Ketocoumarins, which are also known to be highly fluorescent, have poor curing efficiency and are primarily experimental and/or developmental products.

[014] Acylphosphine photoinitiators are widely used for curing with UV LED sources due to their red-shifted absorption spectrum (maximum absorption wavelength approximately 370 nm). They are known to exhibit low fluorescence. However, acylphosphine photoinitiators can still suffer from slow curing, especially slow surface curing, and are known to be particularly sensitive to oxygen inhibition, making them not very effective for curing thin layers.

[015] Currently used thioxanthone derivatives have a maximum absorption wavelength of approximately 370 nm to approximately 410 nm. However, unlike acylphosphine oxides, currently used thioxanthones suffer from fluorescence.

[016] As mentioned above in this document, radical-curing UV offset printing inks are used in the field of protecting security documents, valuable documents and articles from counterfeiting and illegal reproduction for applying thin layers of printed features or designs to security documents. It is well known that luminescent security features are widely used in the field of security documents, in particular for banknotes, to impart an additional hidden security feature to said security documents, whereby the protection of said security documents from counterfeiting and illegal reproduction is based on the concept that, as a rule, special equipment and knowledge are required to detect such features. Luminescent security features include, for example, luminescent fibers, luminescent threads, luminescent patches, stripes or foils (wherein at least a part of said patches, stripes or foils is luminescent) and printed luminescent features. The specified printed features include luminescent numbering (printed by letterpress), printed patches (printed by letterpress), and luminescent features printed by offset printing, usually at the same time and on the same printing press that is used to produce the security features as a background image of the documents being protected.

[017] Since the security documents additionally comprise, in addition to the offset-printed features, luminescent security features, in particular printed luminescent security features, either on the same side as the offset-printed feature or on the opposite side, it must be said that the said offset-printed features do not themselves exhibit fluorescence, do not exhibit fluorescence through the substrate on which they are applied, and do not exhibit fluorescence upon contact between two printed security documents due to contamination, in order to avoid a negative impact on the detection of the luminescent security features by a machine and/or their recognition by a person.

[018] Thus, there remains a need for UV-LED-curable offset printing inks and methods for printing security features on protected documents at high speed (i.e., industrial speed), wherein said UV-LED-curable offset printing inks are characterized by good curing properties, after curing of said inks, they allow obtaining printed features that exhibit low or no fluorescence.

Brief description inventions

[019] Accordingly, the object of the present invention is to overcome the above-mentioned disadvantages of the prior art. This is achieved by providing UV-LED radical-curing offset printing inks, the viscosity of which is in the range of about 2.5 to about 25 Pa⋅s at 40°C and 1000 s ^-1 , for offset printing a feature on a substrate, said UV-LED radical-curing offset printing ink comprising:

i) from about 10 wt.% to about 80 wt.% of one or more radical-curing (meth)acrylate compounds;

ii) from about 4 wt.% to about 20 wt.% of one or more photoinitiators of formula (I):

(I),

where Q has the following formula (IIa), (IIb) or (IIc):

(IIc)

where n is equal to or greater than 1, R1 are identical to or different from each other and are selected from the group consisting of hydrogen and _C1 - _C3- alkyl groups, R2 are selected from the group consisting of hydrogen and _C1 - _C3 -alkyl groups, and where the sum of a+b+c is in the range from 3 to 12, and the sum of d+e+f+g is in the range from 4 to 16,

iii) from about 1 wt.% to about 15 wt.% of one or more amine-containing compounds selected from the group consisting of aminobenzoate compounds having a weight average molecular weight of at least 400 g/mol PS equiv., amine-modified polyester acrylates having a weight average molecular weight of at least 400 g/mol PS equiv., and combinations thereof, and

iv) from about 1 wt.% to about 30 wt.% of one or more inorganic pigments and/or one or more organic pigments, and

v) from about 0.5 wt.% to about 10 wt.% of one or more fillers and/or diluents,

where the mass percentage is calculated based on the total mass of the UV-LED radical-curing offset printing ink.

[020] The UV-LED radical-curing offset printing inks described herein are particularly suitable for curing with UV light, preferably one or more wavelengths from about 355 nm to about 415 nm, with a UV-LED light source. The present document describes methods for printing one or more features on a substrate using an offset printing process, comprising the steps of applying a UV-LED radical-curing offset printing ink described herein using an offset printing process to form an ink layer and exposing the ink layer to UV light at a dose of at least 150 mJ/ ^cm2 , preferably at least 200 mJ/ ^cm2 , to cure said ink layer with a UV-LED light source.

[0321] Also described in this document are features consisting of a cured ink layer made of a radical-curing UV-LED offset printing ink described in this document. This document describes uses of the features described in this document for protecting a security document, valuable document or article from counterfeiting or falsification, and security documents, valuable documents or articles containing one or more printed features described in this document.

[022] Also described herein are security documents comprising a substrate described herein and one or more printed features described herein.

[023] Also described herein are uses of one or more photoinitiators described herein in an amount of from about 1 wt.% to about 20 wt.%, preferably in an amount of from about 5 wt.% to about 15 wt.%, more preferably in an amount of from about 8 wt.% to about 12 wt.%, and one or more amine-containing compounds described herein in an amount of from about 1 wt.% to about 15 wt. %, for obtaining a UV-LED radical-curing offset printing ink, the viscosity of which is in the range from about 2.5 to about 25 Pa⋅s at 40°C and 1000 s ^-1 , wherein said UV-LED radical-curing offset printing ink is suitable for printing one or more features on a security document, wherein said UV-LED radical-curing offset printing ink contains from about 10 wt. % to about 80 wt. % of radical-curing (meth)acrylate compounds, from about 1 wt. % to about 30 wt. % of one or more inorganic pigments and/or one or more organic pigments, and from about 0.5 wt. % to about 10 wt. % of one or more fillers and/or diluents, wherein the weight percentage is calculated based on the total weight of the UV-LED radical-curing offset printing ink.

Brief description drawing

Fig. 1 shows a banknote containing a substrate (1), various security features (2-9), including a feature printed using offset printing, wherein said feature covers more than 70% of the total surface of the document being protected.

Detailed description

Definitions

[024] The following definitions clarify the meaning of terms used in the description and claims.

[025] As used herein, the singular form "an" refers to one or more objects and does not necessarily limit the object to a singular.

[026] As used herein, the term "about" means that the stated amount, amount, or limit may have a specific specified value or some other value close thereto. In general, the term "about" when used with a specific value is intended to mean a range of ±5% of the value. For example, the phrase "about 100" means a range of 100 ±5, i.e., a range from 95 to 105. In general, when using the term "about", it can be expected that similar results or effects according to the present invention can be obtained within a range of ±5% of the stated value. However, a specific amount, amount, or limit supplemented with the term "about" is also intended herein to refer to the same amount, amount, or limit as is, i.e., without the addition of the term "about".

[027] In the context of this document, the term "and/or" means that either all or only one of the elements of the specified group may be present. For example, "A and/or B" would mean "only A or only B, or both A and B". In the case of "only A", the term also covers the possibility of the absence of B, i.e. "only A but not B".

[028] In the context of this document, the term “one or more” means one, two, three, four, etc.

[029] The term "comprising" in the context of the present document is non-exclusive and subject to change. Thus, for example, a paint comprising compound A may comprise other compounds in addition to A. However, the term "comprising" also covers, as does its specific embodiment, the more restrictive meanings of "consisting essentially of" and "consisting of", so that, for example, "a paint comprising A" may also (mainly) consist of compound A.

[030] When the present description refers to “preferred” embodiments/features, combinations of these “preferred” embodiments/features should also be considered as disclosed, as long as the given combination of “preferred” embodiments/features has meaning from a technical point of view.

[031] The term "ultraviolet (UV)" as used herein means radiation having a wavelength component in the UV portion of the electromagnetic spectrum, typically between 200 nm and 400 nm.

[032] The term "secure document" refers to a document that is generally protected against counterfeiting or falsification by at least one security feature. Examples of secure documents include, but are not limited to, valuable documents and valuable commercial goods.

[033] The descriptions of specific embodiments of the present invention are presented for purposes of explanation and description. They are not exhaustive and do not limit the present invention to the specific forms described, and it is obvious that many modifications and changes are possible in light of the above teaching. Illustrative embodiments have been chosen and described to better explain the principles of the present invention and its practical application, to thereby enable those skilled in the art to best utilize the present invention and the various embodiments with various modifications that are suitable for the specific application contemplated.

[034] The present invention provides UV-LED radical-curing offset printing inks for producing (printing) one or more features on a security document using an offset printing process. The present invention further provides printed features consisting of a cured ink layer formed by the UV-LED radical-curing offset printing ink described herein and security documents comprising one or more of the printed features described herein. The UV-LED radical-curing offset printing inks described herein are applied using an offset printing step, wherein said offset printing can be a wet printing process or a dry offset printing process.

[035] One or more radical-curing (meth)acrylate compounds of the UV-LED radical-curing offset printing ink described herein are cured in the presence of one or more photoinitiators described herein and one or more amine-containing compounds described herein are exposed to UV light, preferably one or more wavelengths from about 355 nm to about 415 nm, more preferably exposed to UV light at 365 nm and/or 385 nm and/or 395 nm, emitted from a UV-LED light source. As known to one skilled in the art, the UV-LED radical-curing offset printing inks described herein may also be suitable for curing with medium pressure mercury lamps.

[036] The viscosity of the UV-LED radical-curing offset printing ink described herein is in the range of from about 2.5 to about 25 Pa⋅s at 40°C and 1000 s ^-1 ; wherein the viscosity values provided herein are measured using a Haake Roto-Visco RV1 rheometer with a 2 cm 0.5° cone geometry, linear velocity increase of 0-1000 s ^-1 for 30 seconds.

[037] The UV-LED radical-curing offset printing ink described herein comprises radical-curing (meth)acrylate compounds. The radical-curing (meth)acrylate compounds described herein are present in an amount of from about 10 wt. % to about 80 wt. %, preferably from about 20 wt. % to about 80 wt. %, more preferably in an amount of from about 30 wt. % to about 80 wt. %, even more preferably from about 50 wt. % to about 80 wt. %, wherein the weight percentage is based on the total weight of the UV-LED radical-curing offset printing ink described herein.

[038] Radical curable compounds cure by free radical mechanisms consisting of the energy activation of one or more photoinitiators that release free radicals, which in turn initiate polymerization to form a layer or coating.

[039] The described radical-curing (meth)acrylate compounds preferably consist of one or more radical-curing (meth)acrylate oligomers and one or more radical-curing (meth)acrylate monomers. The term "(meth)acrylate" in the context of the present invention refers to acrylate as well as the corresponding methacrylate.

[040] The radical-curing (meth)acrylate oligomers described herein are preferably selected from the group consisting of epoxy(meth)acrylates, (meth)acrylate-modified oils, (meth)acrylate-modified epoxidized oils, polyester(meth)acrylates, aliphatic or aromatic polyurethane(meth)acrylates, polyacrylic acid (meth)acrylates, polyacrylate ester (meth)acrylates and mixtures thereof, more preferably selected from the group consisting of epoxy(meth)acrylates, polyester(meth)acrylates, aliphatic or aromatic polyurethane(meth)acrylates and mixtures thereof.

[041] The radical-curable oligomers described herein are preferably (meth)acrylate oligomers, which may be branched or substantially linear, and the (meth)acrylate functional group(s) may, respectively, be terminal groups and/or pendant groups attached to the oligomer chain. Preferably, the radical-curing oligomers are selected from the group consisting of (meth)acrylic oligomers, urethane (meth)acrylate oligomers, polyester (meth)acrylate oligomers, polyester-based (meth)acrylate oligomers, epoxy (meth)acrylate oligomers and mixtures thereof, more preferably selected from the group consisting of polyester (meth)acrylate oligomers, epoxy (meth)acrylate oligomers and mixtures thereof.

[042] Suitable examples of epoxy(meth)acrylate oligomers include, but are not limited to, aliphatic epoxy(meth)acrylate oligomers, particularly mono(meth)acrylates, di(meth)acrylates and tri(meth)acrylates, and aromatic epoxy(meth)acrylate oligomers. Suitable examples of aromatic epoxy(meth)acrylate oligomers include bisphenol-A-based (meth)acrylate oligomers such as bisphenol-A-based mono(meth)acrylates, bisphenol-A-based di(meth)acrylates and bisphenol-A-based tri(meth)acrylates, as well as alkoxylated (such as, for example, ethoxylated and propoxylated) bisphenol-A-based (meth)acrylate oligomers such as, for example, alkoxylated bisphenol-A-based mono(meth)acrylates, alkoxylated bisphenol-A-based di(meth)acrylates and alkoxylated bisphenol-A-based tri(meth)acrylates, with ethoxylated bisphenol-A-based diacrylates being particularly suitable.

[043] The radical-curing (meth)acrylate monomers described herein are preferably selected from the group consisting of mono(meth)acrylates, di(meth)acrylates, tri(meth)acrylates, tetra(meth)acrylates, penta(meth)acrylates, hexa(meth)acrylates and mixtures thereof.

[044] Preferred examples of the mono(meth)acrylates include 2-(2-ethoxyethoxy)ethyl(meth)acrylate, 2-phenoxyethyl(meth)acrylate, C12/C14 alkyl(meth)acrylate, C16/C18 alkyl(meth)acrylate, caprolactone(meth)acrylate, cyclic trimethylolpropaneformal(meth)acrylate, nonylphenol(meth)acrylate, isobornyl(meth)acrylate, isodecyl(meth)acrylate, lauryl(meth)acrylate, stearyl(meth)acrylate, octyldecyl(meth)acrylate, tridecyl(meth)acrylate, methoxypoly(ethylene glycol)(meth)acrylate, polypropylene glycol(meth)acrylate, tetrahydrofurfuryl(meth)acrylate, 1,3-butylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 3-methyl-1,5-pentanediol di(meth)acrylate, alkoxylated di(meth)acrylate, ester diol di(meth)acrylate, and mixtures thereof.

[045] Preferred examples of di(meth)acrylates include bisphenol-A-based di(meth)acrylates, alkoxylated (such as, for example, ethoxylated and propoxylated) bisphenol-A-based di(meth)acrylate, bisphenol-A-based diglycidyl ether di(meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, dipropylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, and mixtures thereof.

[046] Preferred examples of tri(meth)acrylates include trimethylolpropane tri(meth)acrylates, alkoxylated (such as, for example, ethoxylated and propoxylated) trimethylolpropane tri(meth)acrylates, alkoxylated (such as, for example, ethoxylated and propoxylated) glycerol tri(meth)acrylates, pentaerythritol tri(meth)acrylates, alkoxylated pentaerythritol tri(meth)acrylates, alkoxylated (such as, for example, ethoxylated and propoxylated) pentaerythritol tri(meth)acrylates, and mixtures thereof.

[047] Preferred examples of tetra(meth)acrylates include ditrimethylolpropane tetra(meth)acrylates, pentaerythritol tetra(meth)acrylates, alkoxylated (such as, for example, ethoxylated and propoxylated) pentaerythritol tetra(meth)acrylates and mixtures thereof, preferably selected from the group consisting of ditrimethylolpropane tetra(meth)acrylates, alkoxylated pentaerythritol tetra(meth)acrylates and mixtures thereof.

[048] For embodiments in which the UV-LED radical-curing offset printing ink described herein is applied using a dry offset printing process, the UV-LED radical-curing offset printing ink described herein comprising the (meth)acrylate compounds described herein may further comprise one or more vinyl ethers and/or ethoxylated equivalents thereof. Examples of preferred vinyl ethers include methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, ethylhexyl vinyl ether, octadecyl vinyl ether, dodecyl vinyl ether, isopropyl vinyl ether, tert-butyl vinyl ether, tert-amyl vinyl ether, cyclohexyl vinyl ether, cyclohexanedimethanol monovinyl ether, cyclohexanedimethanol divinyl ether, 4-(vinyloxymethyl)cyclohexyl methyl benzoate, phenyl vinyl ether, methylphenyl vinyl ether, methoxyphenyl vinyl ether, 2-chloroethyl vinyl ether, 2-hydroxyethyl vinyl ether, 4-hydroxybutyl vinyl ether, 1,6-hexanediol monovinyl ether, 3-amino-1-propanol vinyl ether, ethylene glycol divinyl ether, ethylene glycol monovinyl ether, 1,4-butanediol divinyl ether, 1,6-hexanediol divinyl ether, 4-(vinyloxy)butyl benzoate, bis[4-(vinyloxy)butyl]adipate, bis[4-(vinyloxy)butyl]succinate, bis[4-(vinyloxymethyl)cyclohexylmethyl]glutarate, 4-(vinyloxy)butyl stearate, trimethylolpropane trivinyl ether, propylene carbonate propenyl ether, diethylene glycol monovinyl ether, diethylene glycol divinyl ether, ethylene glycol butyl vinyl ether, dipropylene glycol divinyl ether, triethylene glycol divinyl ether ether, triethylene glycol methyl vinyl ether, triethylene glycol monobutyl ether, tetraethylene glycol divinyl ether, poly(tetrahydrofuran) divinyl ether, polyethylene glycol methyl vinyl ether-520, pluriol-E200 divinyl ether, tris[4-(vinyloxy)butyl] trimellitate, bis[4-(vinyloxy)butyl]1,6-hexanediyl biscarbamate, 1,4-bis(2-vinyloxyethoxy)benzene, 2,2-bis(4-vinyloxyethoxyphenyl)propane, bis[4-(vinyloxy)methyl]cyclohexyl]methyl] terephthalate, bis[4-(vinyloxy)methyl]cyclohexyl]methyl]isophthalate, bis[4-(vinyloxy)butyl](4-methyl-1,3-phenylene)biscarbamate and bis[4-vinyloxy)butyl](methylenedi-4,1-phenylene)biscarbamate. Suitable vinyl ethers are sold by BASF under the trade names EVE, IBVE, DDVE, ODVE, BDDVE, DVE-2, DVE-3, CHVE, CHDM-di, HBVE. When present, one or more vinyl ethers and/or their ethoxylated equivalents are present in an amount of from about 1 wt. % to about 5 wt. %, the weight percentage being based on the total weight of the UV-LED radical-curing offset ink.

[049] The UV-LED radical-curing offset printing ink described herein comprises one or more photoinitiators described herein, wherein said one or more photoinitiators consist of thioxanthone derivatives linked to the residue via an ester linkage at position 2, i.e. the UV-LED radical-curing offset printing ink described herein comprises one or more photoinitiators of formula (I):

(I)

where Q has the following formula (IIa), (IIb) or (IIc):

(IIc)

where n is greater than 1, R1 are identical to or different from each other and are selected from the group consisting of hydrogen and _C1 - _C3 -alkyl groups (i.e. methyl (Me, _-CH3 ), ethyl (Et, _-CH2CH3 ), ₁ -propyl (n-Pr, n- _propyl , _-CH2CH2CH3 ), 2-propyl ( _i -Pr, iso-propyl, -CH( _CH3 ) ₂ )), R2 are selected from the group consisting of hydrogen and _C1 - _C3 -alkyl groups (i.e. methyl (Me, _-CH3 ), ethyl (Et, _-CH2CH3 ), 1-propyl (n-Pr, n-propyl, _-CH2CH2CH3 ), 2 _{- propyl} (i-Pr, iso- _propyl , -CH(CH ₃ ) ₂ )), and where the sum of a+b+c is in the range from 3 to 12, and the sum of d+e+f+g is in the range from 4 to 16.

[050] The weight average molecular weight (MW) of one or more photoinitiators of formula (I) is preferably greater than or equal to about 900 g/mol PS equiv, said weight average molecular weights being determined by GPC (gel permeation chromatography) according to OECD test method 118, in which a Malvern Viskotek GPCmax is used, and in which a calibration curve (log(molecular weight) = f(retention volume)) is established using six polystyrene standards (with molecular weights in the range from 472 to 512000 g/mol). The apparatus is equipped with an isocratic pump, a degasser, an autosampler and a TDA 302 triple detector consisting of a differential refractometer, a viscometer and a dual angle light scattering detector (7° and 90°). For this particular measurement, only the differential refractometer is used. Two Viskotek TM4008L columns (column length 30.0 cm, internal diameter 8.0 mm) were connected in series. The stationary phase consisted of a styrene-divinylbenzene copolymer with a particle size of 6 μm and a maximum pore size of 3000 Å. During the measurements, the temperature was fixed at 35 °C and the samples contained 10 mg/mL of the product to be analyzed dissolved in THF (Acros, 99.9%, anhydrous). As described in the examples below, the samples were independently injected at a flow rate of 1 mL/min. The molecular weight of the compound was calculated from the chromatogram as the weight average molecular weight equivalent to polystyrene (PS eq. M _W ) with a confidence level of 95% and the average of three measurements of the same solution according to the following formula:

where H _i is the detector signal level from the baseline for the retention volume V _i , M _i is the molecular weight of the polymer fraction at the retention volume V _i and n is the number of data points. Omnisec 5.12, supplied with the device, is used as the software.

[051] According to a preferred embodiment, the UV-LED radical-curing offset printing ink described herein comprises one or more photoinitiators of formula (I), wherein n is 1, 2, or 3, and wherein Q has the formula (IIb), wherein the sum of a+b+c is in the range of 3 to 12. According to a more preferred embodiment, the UV-LED radical-curing offset printing ink described herein comprises one or more photoinitiators of formula (I), wherein n is 1, 2, or 3, and wherein Q has the formula (IIb), wherein the sum of a+b+c is in the range of 3 to 12, and R1 are the same and are ethyl groups.

[052] According to an even more preferred embodiment, the UV-LED radical-curable offset printing ink described herein comprises one or more photoinitiators of formula (I), wherein n is 1 (see formula IIb-1), 2 (see formula IIb-2), or 3 (see formula IIb-3), and wherein Q is of formula (IIb), wherein the sum of a+b+c is in the range from 3 to 12, and R1 are the same and are ethylene groups, and R2 is an ethyl group:

[053] Particularly suitable photoinitiators of formula (I) are marketed by Rahn under the trade name Genopol ^® TX-2.

[054] One or more photoinitiators described herein are present in an amount of from about 4 wt.% to about 20 wt.%, preferably in an amount of from about 5 wt.% to about 15 wt.%, more preferably in an amount of from about 8 wt.% to about 12 wt.%, wherein the weight percentage is based on the total weight of the UV-LED radical-curable offset printing ink.

[055] It should also be noted that the present invention also encompasses compounds in which one or more atoms are replaced by an isotopic variant, such as one or more hydrogen atoms may be replaced by ² H or ³ H, and/or one or more carbon atoms may be replaced by ¹⁴ C or ¹³ C.

[056] The UV-LED radical-curing offset printing ink described herein comprises from about 1 wt.% to about 15 wt.%, preferably from about 2 wt.% to about 12 wt.%, of one or more amine-containing compounds selected from the group consisting of aminobenzoate compounds having a weight average molecular weight of at least 400 g/mol PS equiv, amine-modified polyester acrylates having a weight average molecular weight of at least 400 g/mol PS equiv, and combinations thereof, wherein the weight average molecular weight is measured as described herein, wherein the weight percentage is calculated based on the total weight of the UV-LED radical-curing offset printing ink. These amine-containing compounds behave as hydrogen donors. However, it is known from the prior art that, due to their relatively high hydrophilicity, the said compounds have a negative effect on the properties of UV-curable inks when using wet offset printing processes (i.e., amines tend to make the normally oleophilic ink hydrophilic, which hydrophilic ink, in turn, contaminates the non-image area of the printing plate).

[057] The weight average molecular weight of one or more aminobenzoate compounds described herein is at least 400 g/mol eq PS, preferably at least 700 g/mol eq PS, and more preferably at least 900 g/mol eq PS.

[058] According to one embodiment, the one or more aminobenzoate compounds described herein comprise one or more tertiary amines, preferably one or more 4-dialkylaminobenzoate groups, more preferably one or more 4-dimethylaminobenzoate groups. The one or more aminobenzoate compounds described herein are preferably polyether-based compounds derived from alkoxylated (such as, for example, ethoxylated and propoxylated) glycerols or alkoxylated (such as, for example, ethoxylated and propoxylated) pentaerythritols. Suitable examples of the one or more aminobenzoate compounds have the following formula (III):

(III)

where m is greater than 1, preferably in the range from 1 to 4, and where the sum of h+i+j+k is in the range from 3 to 12.

[059] Particularly suitable aminobenzoate compounds are marketed by Lambson under the trade name SpeedCure 7040.

[060] The weight average molecular weight of the one or more amine-modified polyester acrylates described herein is at least 400 g/mol PS equiv, preferably at least 700 g/mol PS equiv, and more preferably at least 900 g/mol PS equiv. Preferably, the one or more amine-modified polyester acrylates described herein are characterized by a high viscosity (such as a viscosity value above 500 mPa s, preferably above 1000 mPa s at 20-25°C) and a high acrylate functionality (e.g. the number of acrylate groups per molecule, in particular an acrylate functionality above 2, preferably above 3). According to one embodiment, one or more amine-modified polyester acrylates described herein are prepared by reacting a small proportion (approximately 2-15 wt%) of the acrylate double bonds of the polyester acrylate with an amine (Radiation curing, P. Glöckner, Vincentz, 2008, pp. 66-67). Said amines are preferably primary or secondary alkyl or cycloalkyl amines or aliphatic cyclic secondary amines such as pyrrolidine, piperidine or piperazine. Particularly suitable polyester acrylates have an acrylate functionality of at least 3. Suitable examples include ethoxylated or propoxylated glycerol triacrylate, ethoxylated or propoxylated trimethylolpropane triacrylate, ethoxylated or propoxylated pentaerythritol tetraacrylate, etc. Suitable acrylates based on amine-modified polyester are sold by Allnex under the trade name Ebecryl ^® 80), by Rahn under the trade name Genomer* 3457 and 3480, by IGM Resins under the trade name Photomer ^® 5662, and by DSM Resins under the trade name Neorad™ P-82 and Agisyn™ 701.

[061] The amount of one or more amine-containing compounds described herein (in particular one or more aminobenzoate compounds described herein and one or more amine-modified polyester acrylates described herein) and one or more photoinitiators of formula (I) described herein is preferably selected such that the molar ratio of the nitrogen of the one or more amine-containing compounds described herein (in particular one or more aminobenzoate compounds described herein and one or more amine-modified polyester acrylates described herein) to the thioxanthone moiety of the one or more photoinitiators of formula (I) described herein is preferably in the range of from about 0.1 to about 1, more preferably from about 0.2 to about 0.8, and even more preferably from about 0.4 to about 0.6. The preferred ranges described herein are particularly suitable for embodiments in which the UV-LED radical-curing offset printing ink described herein is applied using a wet offset printing process.

[062] The UV-LED radical-curing offset printing ink described herein further comprises one or more inorganic pigments and/or one or more organic pigments described herein, wherein said one or more inorganic pigments and/or one or more organic pigments described herein are present in an amount of from about 1 wt. % to about 30 wt. %, wherein the weight percentage is based on the total weight of the UV-LED radical-curing offset printing ink.

[063] Typical examples of organic and inorganic pigments include, but are not limited to, C.I. Pigment Yellow 12, C.I. Pigment Yellow 42, C.I. Pigment Yellow 93, C.I. Pigment Yellow 110, C.I. Pigment Yellow 147, C.I. Pigment Yellow 173, C.I. Pigment Orange 34, C.I. Pigment Orange 48, C.I. Pigment Orange 49, C.I. Pigment Orange 61, C.I. Pigment Orange 71, C.I. Pigment Orange 73, C.I. Pigment Red 9, C.I. Pigment Red 22, C.I. Pigment Red 23, C.I. Pigment Red 67, C.I. Pigment Red 122, C.I. Pigment Red 144, C.I. Pigment Red 146, C.I. Pigment Red 170, C.I. Pigment Red 177, C.I. Pigment Red 179, C.I. Pigment Red 185, C.I. Pigment Red 202, C.I. Pigment Red 224, C.I. Pigment Brown 6, C.I. Pigment Brown 7, C.I. Pigment Red 242, C.I. Pigment Red 254, C.I. Pigment Red 264, C.I. Pigment Brown 23, C.I. Pigment Blue 15, C.I. Pigment Blue 15:3, C.I. Pigment Blue 60, C.I. Pigment Violet 19, C.I. Pigment Violet 23, C.I. Pigment Violet 32, C.I. Pigment Violet 37, C.I. Pigment Green 7, C.I. Pigment Green 36, C.I. Pigment Black 7, C.I. Pigment Black 11, C.I. Pigment White 4, C.I. Pigment White 6, C.I. Pigment White 7, C.I. Pigment White 21, C.I. Pigment White 22, Antimony Yellow, Lead Chromate, Lead Chromate Sulfate, Lead Molybdate, Ultramarine Blue, Cobalt Blue, Manganese Blue, Chromium Oxide Green, Chromium Oxide Green Hydrated, Cobalt Green, Cerium Sulfide, Cadmium Sulfide, Cadmium Sulfoselenides, Zinc Ferrite, bismuth vanadate, Prussian blue, mixed metal oxides, azo pigments, azomethine, methine, anthraquinone, phthalocyanine, perinone, perylene, diketopyrrolopyrrole pigments, thioindigo pigments, thiazine indigo pigments, dioxazine, iminoisoindoline, iminoisoindolinone, quinacridone, flavanthrone, indanthrone, anthrapyrimidine and quinophthalone pigments. When present, the inorganic pigments, organic pigments, or mixtures thereof described herein are preferably present in an amount of from about 0.1 wt.% to about 45 wt.%, the weight percentage being based on the total weight radical-curing UV-LED offset printing ink.

[064] According to a preferred embodiment, the particle size of the one or more inorganic pigments described herein and the one or more organic pigments described herein is independently less than or equal to 5 μm, preferably less than or equal to 3 μm, when measured using a fineness measuring instrument (such as an Erichsen grindometer model 232, scale 0-15 μm) according to the ISO 1524:2013 standard. The fineness measurement test is carried out directly on the ready-to-use paint, i.e., the paint that contains all the ingredients and has been prepared and ground as described for Examples E1-E2 and Comparative Examples C1-C4 described later herein.

[065] The UV-LED radical-curing offset printing ink described herein further comprises one or more fillers and/or diluents in an amount of from about 0.5 wt.% to about 10 wt.%, preferably from about 0.5 wt.% to about 5 wt.%, wherein the weight percentage is calculated based on the total weight of the UV-LED radical-curing offset printing ink. Preferably, one or more fillers and/or diluents are selected from the group consisting of carbon fibers, talcs, micas (muscovites), wollastonites, clays (calcined clays and white clays), kaolins, carbonates (e.g. calcium carbonate, sodium aluminum carbonate), silicates (e.g. magnesium silicate, aluminum silicate), sulfates (e.g. magnesium sulfate, barium sulfate), titanates (e.g. potassium titanate), aluminum oxide hydrates, silicon dioxide (also including colloidal silicon dioxides), montmorillonites, graphites, anatases, rutiles, bentonites, vermiculites, zinc white, zinc sulfides, wood flour, quartz flour, natural fibers, synthetic fibers and combinations thereof. More preferably, one or more fillers and/or diluents are selected from the group consisting of carbon fibers, talcs, micas, wollastonites, clays, kaolins, carbonates, silicates, sulfates, titanates, aluminum oxide hydrates, silicon dioxide, montmorillonites, graphites, bentonites, vermiculites, wood flour, quartz flour, natural fibers, synthetic fibers, and combinations thereof. Even more preferably, one or more fillers and/or diluents are selected from the group consisting of carbonates (e.g., calcium carbonate, sodium aluminum carbonate), silicon dioxides, talcs, clays, and mixtures thereof.

[066] The UV-LED radical-curing offset printing ink described herein may further comprise one or more waxes, preferably selected from the group consisting of synthetic waxes, petroleum waxes and natural waxes. Preferably, the one or more waxes are selected from the group consisting of amide waxes, erucamide waxes, paraffin waxes, polyethylene waxes, polypropylene waxes, fluorocarbon waxes, polytetrafluoroethylene waxes, Fischer-Tropsch waxes, silicone fluids, beeswaxes, candelilla waxes, montan waxes, carnauba waxes and mixtures thereof, more preferably selected from the group consisting of paraffin waxes, polyethylene waxes, fluorocarbon waxes, polytetrafluoroethylene waxes, carnauba waxes and mixtures thereof. If present, one or more waxes are preferably present in an amount of from about 0.1 to about 5 wt.%, wherein the weight percentage is calculated based on the total weight of the UV-LED radical-curing offset printing ink.

[067] The UV-LED radical-curable offset printing ink described herein may further comprise one or more phosphine oxides as coinitiators. Suitable examples of phosphine oxides include, but are not limited to, 2,4,6-trimethylbenzoyldiphenylphosphine oxide; ethyl (2,4,6-trimethylbenzoyl)phenylphosphinate; phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide; bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide; substituted acylphosphine oxides sold as Speedcure XKm by Lambson; a mixture of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide; a mixture of diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide and 2-hydroxy-2-methylpropiophenone, a mixture of phenyl bis (2,4,6-trimethylbenzoyl) phosphine oxide and 2-hydroxy-2-methylpropiophenone; and a mixture of ethyl (2,4,6-trimethylbenzoyl) phenyl phosphinate and 2-hydroxy-2-methylpropiophenone. When present, the UV-LED radical-curable offset printing inks preferably comprise from about 6 wt. % to about 25 wt. %, preferably from about 10 wt. % to about 20 wt. % of a combination of one or more photoinitiators of formula (I) described herein and one or more phosphine oxides described herein, wherein the weight percentage is calculated based on the total weight of the UV-LED radical-curing offset printing ink.

[068] The UV-LED radical-curing offset printing ink described herein may further comprise one or more machine-readable materials selected from the group consisting of magnetic materials, IR-absorbing materials, and mixtures thereof. In the context of the present document, the term "machine-readable material" refers to a material that is characterized by at least one distinctive property that is detectable by a device or machine, such as, for example, a CCD or CMOS sensor, a magnetic detector (when the machine-readable materials are characterized by magnetic properties) or an IR camera (when the machine-readable materials are characterized by IR absorption properties), and which can be contained in a security feature made by a radical-curing UV-LED offset printing ink described in this document to provide a method for authenticating said security feature by using specific equipment for detecting and/or authenticating it. One or more machine-readable materials described in this document are present in an amount of from about 1 wt. % to about 60 wt. %, preferably from about 5 wt. % to about 40 wt. %, where the mass percentage is calculated based on the total mass of UV-LED radical-curing offset printing ink.

[069] Magnetic materials are characterized by specific, detectable magnetic properties of the ferromagnetic or ferrimagnetic type and include permanent magnetic materials (magnetically hard materials with a coercive force Hc > 1000 A/m) and magnetizable materials (magnetically soft materials with a coercive force Hc <= 1000 A/m according to IEC60404-1 (2000)). Typical examples of magnetic materials include iron, nickel, cobalt, manganese and their _magnetic alloys, carbonyl iron, chromium dioxide _CrO2 , magnetic iron oxides (e.g. _Fe2O3 ; _Fe3O4 ), magnetic ferrites M(II)Fe( _III ) _{2O4 and hexaferrites M(II)Fe(III)12O19, magnetic garnets M(III)3Fe(III)5O12 ( such as yttrium iron garnet Y3Fe5O12 ) and their magnetic} isostructural substitution products _and permanently magnetized particles (e.g. _{CoFe2O4 )} . Magnetic pigment particles comprising a magnetic core material which is surrounded (coated) by at least one layer of another material, such as those described in WO 2010/115986 A2, may also be used for the present invention. When present, one or more magnetic materials are preferably present in an amount of from about 5 to about 60 wt.%, wherein the weight percentage is calculated based on the total weight of the UV-LED radical-curing offset printing ink.

[070] Infrared (IR) absorbing materials, i.e. materials that absorb in the near infrared (NIR) range of the electromagnetic spectrum, most often in the wavelength range from 700 nm to 2500 nm, are widely known and are used as marking materials in security applications to provide an additional, hidden, protective element to printed documents, which helps to establish their authenticity. For example, security features having IR absorbing properties have been incorporated into banknotes for use by automatic banknote processing equipment in the banking and retail industries (ATMs, vending machines, etc.) to recognize a banknote of a certain denomination and certify its authenticity, in particular, to distinguish it from replicas produced on color copiers. IR-absorbing materials include IR-absorbing inorganic materials, glasses containing significant amounts of IR-absorbing atoms or ions or substances exhibiting IR absorption as a by-product, IR-absorbing organic materials, and IR-absorbing organometallic materials (complexes of a cation(s) with an organic ligand(s), wherein either a single cation and/or a single ligand or both in combination have IR-absorbing properties). Typical examples of IR-absorbing materials include, but are not limited to, carbon black, quinone diimmonium or aminium salts, polymethines (e.g. cyanines, squarenes, croconenes), phthalocyanine or naphthalocyanine type compounds (IR-absorbing pi-system), dithiolenes, quaterrylene diimides, metal (e.g. transition metals or lanthanides), phosphates, lanthanum hexaboride, indium tin oxide, antimony tin oxide in nanoparticle form, and doped tin(IV) oxide (the associative part of the SnO ₄ crystal). IR-absorbing materials comprising a transition element compound whose infrared absorption is a consequence of electron transitions within the d-shell of transition element atoms or ions, such as those described in WO 2007/060133 A2, may also be used in the present invention. When present, one or more IR-absorbing materials are preferably present in an amount of from about 1 to about 40 wt.%, the weight percentage being based on the total weight of the UV-LED radical-curable offset printing ink.

[071] The UV-LED radical-curing offset printing ink described herein may further comprise one or more markers and/or marks.

[072] The UV-LED radical-curing offset printing ink described herein may further comprise one or more UV stabilizers to stabilize said ink, in particular during storage. Typical examples of suitable UV stabilizers include, but are not limited to, hydroquinone, monomethyl ether of hydroquinone, 4-tert-butylcatechol, 4-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol (BHT), pyrogallol, phenothiazine (PTZ), 2,4-diazabicyclo[2.2.2]octane (DABCO), copper(II) salts (such as, for example, copper(II) phenoxide, copper(II) acetylacetonate, copper(II) gluconate, copper(II) tartrate, copper(II) acetate, copper(II) carbamate, copper(II) thiocarbamate, copper(II) dithiocarbamate or copper(II) dimethyldithiocarbamate), copper(I) salts (such as, for example, copper(I) chloride or copper(I) acetate), tris[N-(hydroxyl-κO)-N-(nitroso-κO)benzenelaminato]-aluminum, and any mixtures thereof. When present, one or more UV stabilizers described herein are present in the UV-LED radical-curing offset printing ink in an amount of from about 0.1 wt.% to about 5 wt.%, preferably in an amount of from about 0.5 wt.% to about 2 wt.%, the weight percentage being based on the total weight of the UV-LED radical-curing offset printing ink described herein.

[073] As is known to those skilled in the art, the UV-LED radical-curing offset printing ink described herein may further comprise one or more solvents and/or diluents.

[074] The UV-LED radical-curing offset printing ink described herein may further comprise one or more inert resins (i.e., resins that do not participate in the polymerization reaction). The inert resins may be used to adjust the viscosity of the radical-curing offset printing ink described herein, to lower the glass transition temperature of the ink layer produced with the radical-curing offset printing ink described herein, or to increase the adhesion of the ink layer produced with the radical-curing offset printing ink described herein. The one or more inert resins are preferably selected from the group consisting of hydrocarbons (such as, for example, styrene-based hydrocarbon resins), acrylics (such as, for example, acrylic copolymers), styrene allyl alcohols, phenolic resins, rosin-modified resins, ketone resins, alkyd resins and mixtures thereof. When present, the one or more inert resins described herein are present in the UV-LED-curable wet-offset ink described herein in an amount of from about 0.1 wt. % to about 10 wt. %, preferably in an amount of from about 0.5 wt. % to about 2 wt. %, the weight percentage being based on the total weight of the UV-LED-curable wet-offset ink described herein.

[075] The UV-LED-curable offset ink described herein may further comprise additives that include, but are not limited to, one or more of the following components, as well as combinations thereof: coinitiators, anti-settling agents, defoamers, surfactants, and other processing aids known in the art of offset inks. The additives described herein may be present in the UV-LED-curable wet offset inks described herein in amounts and in forms known in the art, including in the form of so-called nanomaterials in which at least one of the particle sizes is in the range of 1-1000 nm.

[076] UV-LED-curable offset printing inks can be applied using the offset printing processes described herein to print features on security documents at high speed, wherein said UV-LED-curable offset printing inks have good curing properties and, after curing, are capable of producing printed features, wherein said printed features exhibit low or no fluorescence. Advantageously, this allows ink manufacturers to take advantage of this low or no fluorescence to produce inks with low or no fluorescence and printed features with low or no fluorescence. Also described herein are compositions comprising a UV-LED-curable offset printing ink described herein and one or more luminescent materials. Taking advantage of the low or no fluorescence of UV-LED radical-curing offset inks, this allows for the creation of compositions comprising one or more luminescent materials as described herein, wherein the luminescence characteristics of said compositions have been deliberately and freely selected, since no masking or interference occurs between the components of the UV-LED radical-curing offset ink and the incorporated luminescent materials. In other words, UV-LED radical-curing offset inks can be used to create compositions and security features with targeted fluorescent properties (i.e., fluorescent properties that are predictable and unchangeable components/ingredients of the UV-LED radical-curing offset ink itself). In other words, the combination in the UV-LED-radical-curable offset printing inks described herein of one or more radical-curable (meth)acrylate compounds described herein, one or more photoinitiators of formula (I) described herein, one or more amine-containing compounds selected from the group consisting of aminobenzoate compounds, amine-modified polyester acrylates and combinations thereof described herein, one or more inorganic pigments and/or one or more organic pigments described herein, one or more fillers and/or diluents and optional additives described herein, per se, allows the production of inks with low or no fluorescence and printed features with low or no fluorescence or allows the production of compositions with one or more luminescent compounds and printed security features with low fluorescence or absence thereof with controlled and desired luminescent properties by incorporating luminescent materials described herein, wherein the luminescent properties of said luminescent compounds are substantially unaltered or masked by the UV-LED-radical-curable offset printing inks described herein. Moreover, and due to the low or absence of fluorescence of the UV-LED-radical-curable offset printing inks, the amount of one or more luminescent materials described herein can be reduced, thereby reducing manufacturing and production costs.

[077] Representative examples of luminescent materials include, but are not limited to, inorganic pigments (such as inorganic host crystals or glasses doped with luminescent ions), organic and organometallic (complexes of luminescent ion(s) with organic ligand(s)) materials. When present, one or more luminescent materials that are inorganic pigments are preferably present in the UV-LED-radical-curable offset printing ink composition described herein in an amount of from about 1 to about 40 weight percent, the weight percent being based on the total weight of the composition described herein. When present, one or more luminescent materials that are organic compounds are preferably present in the composition comprising the UV-LED-radical-curable offset printing ink described herein in an amount of from about 1 to about 20 wt. %, wherein the weight percentage is calculated based on the total weight of the composition described herein. The luminescent compounds can absorb certain types of energy applied to them and, therefore, emit at least a portion of this absorbed energy as electromagnetic radiation. The luminescent compounds are detected by exposing them to a certain wavelength of light and analyzing the emitted light. Down-conversion luminescent compounds absorb electromagnetic radiation at a higher frequency (shorter wavelength) and at least partially re-emit it at a lower frequency (longer wavelength). Up-conversion luminescent compounds absorb electromagnetic radiation at a lower frequency and at least partially re-emit it at a higher frequency. Light emission by luminescent materials occurs due to excited states of atoms or molecules. The radioactive decay of such excited states has a characteristic decay time, which depends on the material and can vary in the range from 10 ^-9 seconds up to several hours. Both fluorescent and phosphorescent compounds are suitable for the present invention. In the case of phosphorescent compounds, the measurement of the decay characteristics can also be performed and used as a machine-readable feature. Luminescent compounds in the form of a pigment have been widely used in paints (see documents US 6,565,770, WO 2008/033059 A2 and WO 2008/092522 A1). Examples of luminescent compounds include, but are not limited to, sulfides, oxysulfides, phosphates, vanadates, etc. of non-luminescent cations doped with at least one luminescent cation selected from the group consisting of a transition metal and rare earth ions; complexes of rare earth oxysulfides and rare earth metals, such as those described in WO 2009/005733 A2 or US 7,108,742. Examples of inorganic compound materials include, but are not limited to, La ₂ O ₂ S:Eu, ZnSiO ₄ :Mn, and YVO ₄ :Nd.

[078] The UV-LED radical-curable offset printing ink described herein is typically prepared by a process comprising the step of dispersing, mixing and/or grinding all of the ingredients described herein, i.e., one or more photoinitiators of formula (I) described herein, one or more amine-containing compounds described herein, one or more inorganic pigments and/or one or more organic pigments described herein, one or more fillers and/or diluents described herein, one or more waxes described herein, if present, and one or more additives, if present, in the presence of the (meth)acrylate compounds described herein, thereby forming paste-like compositions. One or more photoinitiators described herein may be added to the paint either during the dispersion/mixing/grinding step of all other ingredients, or may be added at the last stage.

[079] Also disclosed herein are intaglio printing inks (also referred to in the art as "engraved steel die" or "engraved copper plate" printing inks) and letterpress printing inks comprising the ingredients described herein, in particular comprising i) one or more photoinitiators of formula (I) described herein, and ii) one or more amine-containing compounds selected from the group consisting of aminobenzoate compounds having a weight average molecular weight of at least 400 g/mol PS equiv, amine-modified polyester acrylates having a weight average molecular weight of at least 400 g/mol PS equiv, and combinations thereof.

[080] As described herein, a method for producing a printed feature described herein comprises the step of a) applying a radical-curing UV-LED offset printing ink described herein by offset printing to form an ink layer, and b) exposing the ink layer to UV light at a dose of at least 150 mJ/ ^cm2 , preferably at least 200 mJ/ ^cm2 , to cure said ink layer with a UV-LED radiation source. As further described herein, the dose can be measured using a UV Power Puck® II radiometer from EIT, Inc., USA.

[081] The UV-LED radical-curing offset printing inks described herein are applied by an offset printing step, wherein said offset printing may be a wet printing process or a dry offset printing process.

[082] Also described herein are methods in which a UV-LED radical-curing offset printing ink described herein is applied by the offset printing process described herein to both sides of a substrate described herein to form layers of ink on both sides, i.e., the same UV-LED radical-curing offset printing ink is used on both sides. Also described herein are methods in which two UV-LED radical-curing offset printing inks described herein are applied by the offset printing process described herein to both sides of a substrate described herein to form layers of ink on both sides, i.e., two different UV-LED radical-curing offset printing inks are used on both sides.

[083] Preferably, step b) described herein consists of exposing the paint layer to one or more wavelengths in the range of 355 to 415 nm. Typically, commercially available UV LED light sources use one or more wavelengths such as, for example, 365 nm, 385 nm, 395 nm, and 405 nm. According to one embodiment, step b) described herein consists of exposing the paint layer to one wavelength in the range of 355 to 415 nm.

[084] The method described in this document is particularly suitable for producing one or more printed features on a substrate that is suitable as a substrate for a security document. The one or more printed features described in this document may be continuous or discontinuous.

[085] According to an embodiment, one or more features described herein are used as background features or designs on a substrate or security document (either on one or both sides) for subsequent printing or treatment with other inks or security features. This means that on top of one or more features printed using the offset printing process described herein with the UV-LED radical-curing offset printing ink described herein, additional features are printed or applied in one or more additional print runs or applications, and the one or more features printed using the offset printing process described herein and the additional features at least partially overlap. In Fig. 1 shows the front side of a security document, in particular a banknote, which comprises a non-fluorescent substrate (1) for producing fiduciary money, such as a cellulose-based substrate or a polymer substrate, a background pattern (10) consisting of a printed feature obtained by applying and curing the UV-LED radical-curing offset printing inks described in this document using the offset printing step described in this document, and additional security elements (2-9). Said security elements (2-9) include, among other things, a serial number (2) printed using letterpress printing, a security thread (3), a fluorescent or phosphorescent patch (4), a security feature (5) printed using intaglio printing, a transparent window (6), a numeral (7), fluorescent security threads (8) and a printed luminescent security element (9). Fluorescent fibers (8) are incorporated into the substrate during its preparation, and the fluorescent or phosphorescent patch (4) can either be applied in a separate process or printed in a subsequent step. The printed luminescent security element (9) can be based on the composition described in this document and comprising a radical-curing under the influence of UV-LED radiation offset printing ink described in this document and one or more luminescent materials described in this document, and can be applied by offset printing during the same printing step as the background pattern made with the radical-curing under the influence of UV-LED radiation offset printing inks described in this document.

[086] According to one embodiment, the total surface area of one or more printed features described herein and consisting of a cured ink layer formed by the UV-LED radical-curing offset printing ink described herein (i.e., the addition of the surface area of all one or more printed features described herein) is greater than or equal to about 50%, preferably greater than or equal to about 60%, even more preferably greater than or equal to about 70%, wherein the % is calculated based on the total surface area of the substrate on which said one or more printed features are present. The one or more printed features may be applied to one or both sides of the substrate described herein. When both sides of the substrate comprise one or more of the printed features described herein, the percentage described herein, i.e. greater than or equal to about 50%, preferably greater than or equal to about 60%, even more preferably greater than or equal to about 70%, may be the same for both sides or may be different for both sides.

[087] Typical examples of substrates include, but are not limited to, fiber-based substrates, preferably substrates based on cellulose fibers, such as paper, paper-containing materials, polymer-based substrates, composite materials (e.g., substrates obtained by laminating layers of paper and polymer films), metals or metallized materials, glasses, ceramics, and combinations thereof. Typical examples of polymer-based substrates are substrates made of ethylene- or propylene-based homo- and copolymers such as polypropylene (PP) and polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), and polyethylene terephthalate (PET). As is known to those skilled in the art, polymer-based substrates do not exhibit fluorescence. Typical examples of composite materials include, but are not limited to, multilayer structures (e.g., laminates) of at least one paper layer and at least one polymer film, including polymers such as those described herein above, as well as paper-like substrates based on mixtures of cellulose fibers and synthetic polymer fibers. In one preferred embodiment, the features are printed on a substrate selected from offset paper and fiduciary paper. Offset paper is made from wood pulp pulp with properties that make the paper suitable for offset printing, including dimensional stability, curl resistance, high surface strength, a surface free of foreign particles, and a high level of resistance to moisture penetration. Typically, the basis weight of offset paper is from 30 g / m ² to 250 g / m ² , preferably from 50 g / m ² to 150 g / m ² .

[088] Fiduciary paper (also referred to in the art as security paper) is made from lignin-free cellulose that contains cotton. Compared to offset papers, additional properties of fiduciary papers include increased mechanical resistance (especially tear and wear resistance), resistance to contamination, and protection against degradation by microorganisms (bacteria, viruses, and fungi). The mechanical resistance of fiduciary papers can be increased by introducing synthetic fibers into the paper (cotton-based) pulp, and the contamination-preventing properties can be improved by coating or printing with a contamination-preventing polymer layer prior to printing or application of the banknote features. Typically, biocide treatment is combined with contamination-preventing treatment. As a rule, the basis weight of paper for the production of fiduciary money is from 50 to 150 g/ ^m2 , preferably from 80 to 120 g/ ^m2 .

[089] In addition, the use of fiduciary paper instead of offset paper adds an additional element of security against counterfeiting, as fiduciary paper is produced on special papermaking machines that are only available to security paper manufacturers, and the supply chain is secured to prevent fiduciary paper from being passed on to counterfeiters. Unlike ordinary writing paper, fiduciary paper does not contain optical brighteners. These optical brighteners are used to give ordinary writing paper a whiter, brighter appearance, but at the same time exhibit a strong blue fluorescence when exposed to UV light, whereas fiduciary paper remains comparatively dark when exposed to the same radiation. Paper without optical brighteners is not accessible outside the security area, meaning that bright blue fluorescence under UV light is already a sign of counterfeiting (Optical Document Security, 3rd ed., 2005, Artech House, 3.2.5 p. 71).

[090] According to one embodiment, the substrate described herein is a non-fluorescent substrate, preferably a non-fluorescent polymer-based substrate or fiduciary paper without an optical brightener.

[091] The term "security document" refers to a document that has value, making it potentially susceptible to counterfeiting or unauthorized reproduction, and that is typically protected against counterfeiting or tampering by one or more features. Examples of security documents include, but are not limited to, documents of value and valuable commercial goods. Typical examples of security documents include, but are not limited to, banknotes, legal documents, tickets, checks, vouchers, revenue stamps and excise stamps, agreements and the like, identity documents such as passports, identification cards, visas, bank cards, credit cards, transaction cards, access documents, security marks, entrance tickets, transport tickets or documents entitling to travel on public transport, and the like.

[092] The term "valuable commercial item" refers to a packaging material, particularly for the pharmaceutical, cosmetic, electronic or food industries, which may contain one or more features to ensure the authenticity of the contents of the package, such as genuine drugs. An example of such packaging material includes, but is not limited to, labels such as authentication product labels, tax stamps, tamper-evident labels and seals. The security document described herein may further comprise one or more additional layers or coatings either under or over the printed feature described herein. In case of insufficient adhesion between the substrate and the printed feature described in this document and consisting of a cured ink layer made of a radical-curing UV-LED offset printing ink described in this document, for example due to the substrate material, surface unevenness or surface heterogeneity, an additional layer, coating or primer can be applied between the substrate and the printed feature, as is known to those skilled in the art.

[093] In order to further increase the level of protection and security against counterfeiting and illegal reproduction of protected documents, the substrate may contain watermarks, security threads, fibers, confetti, luminescent compounds, windows, foil, transfer images, coatings and combinations thereof.

[094] The substrate described herein onto which the UV-LED radical-curing offset printing ink described herein is applied may comprise a portion of the security document itself, or alternatively, the UV-LED radical-curing offset printing ink described herein is applied to an auxiliary substrate, such as, for example, a security thread, security strip, foil, transfer image, or label, and then transferred to the security document in a separate step.

[095] Also described herein are uses of one or more photoinitiators described herein for producing a UV-LED radical-curing offset printing ink described herein, wherein said UV-LED radical-curing offset printing ink is suitable for printing one or more features on a security document.

Example

[096] The present invention will now be described in more detail with reference to non-limiting examples. The examples then disclose in more detail the preparation of UV-LED-radical-curable printing inks and the use of photoinitiators and amine-containing compounds according to the present invention, as well as comparative data.

Photoinitiators And amino-containing connections

Table 1A

Concentration thioxanthone fragment V photoinitiators P1-P3: definition With with help ED-XRF

[097] The molar concentration of the thioxanthone moiety was determined by ED-XRF (Spectro XEPOS) using the sulfur atom signal. For each of the photoinitiators P1-P3 from Table 1A, three 50 mL solutions were prepared at a concentration of 2 mg/mL of the corresponding photoinitiator in acetonitrile (Sigma-Aldrich, 99.9%). 5 mL samples were taken from each solution and increasing amounts of 5 mg/mL Genocure ITX (Rahn, 99.3% according to the certificate of analysis) in acetonitrile were added. Each sample was made up to 10 mL with acetonitrile. The following solutions were prepared and are presented in Table 1B.

Table 1B

[098] Each sample was independently subjected to ED-XRF measurement (Spectro XEPOS) and the spectrum was recorded. A blank measurement (pure acetonitrile) was obtained from all spectra. For each sample series (triple measurement), the measured fluorescence intensity at 2.31 keV (peak S Kα1) was plotted as a function of the concentration of added ITX and a linear regression was performed. The x-intercept of the regression line indicates the concentration of thioxanthone moiety present at 0 in each sample. The mean values (average of three measurements) are presented in Table 1A. The corresponding mean value was used to determine the concentration of thioxanthone in each of the photoinitiators P1-P3 in mmol/g and to calculate the amount of photoinitiator P1-P3 to be added to obtain the Examples and Comparative Examples.

Table 1C

Concentration nitrogen V amino-containing connections S1-S3: definition potentiometric titration

[099] The molar concentration of nitrogen in the amine-containing compounds S1-S3 from Table 1C was obtained by potentiometric titration with perchloric acid. For each compound S1-S3, three 50 mL samples were prepared at a concentration of 3 mg/mL in dichloromethane (Acros, 99.99%). 5 mL of each sample was transferred to a volumetric flask and made up to 50 mL with dichloromethane. These solutions were titrated with 0.01 M perchloric acid in glacial acetic acid, prepared by mixing 9 parts glacial acetic acid (Sigma-Aldrich, 99%) and 1 part perchloric acid solution (Sigma-Aldrich, 0.1 mol/L in glacial acetic acid). The titration was performed on a Metrohm Titrando 905 equipped with an 826 conductivity meter and a Solvotrode electrode for non-aqueous solutions (saturated LiCl in ethanol). The inflection point was determined graphically. The average values (in mmol/g) of three measurements are presented in Table 1C.

Measurement medium-weight molecular masses

[0100] The weight average molecular weight of the photoinitiators P1 and P3 and the amine-containing compounds S2 and S3 were independently determined by GPC (gel permeation chromatography) according to the method described below (based on OECD test method 118):

A Malvern Viskotek GPCmax was used. The apparatus was equipped with an isocratic pump, a degasser, an autosampler and a TDA 302 triple detector consisting of a differential refractometer, a viscometer and a dual angle light scattering detector (7° and 90°). For this particular measurement, only the differential refractometer was used. A calibration curve (log(molecular weight) = f(retention volume)) was constructed using six polystyrene standards (with molecular weights ranging from 472 to 512000 g/mol). Two Viskotek TM4008L columns (column length 30.0 cm, internal diameter 8.0 mm) were connected in series with UV LEDs. The stationary phase consisted of styrene-divinylbenzene copolymer with a particle size of 6 μm and a maximum pore size of 3000 Å. During the measurement, the temperature was fixed at 35°C. The analyzed samples contained 10 mg/mL of the studied compounds dissolved in THF (Acros, 99.9%, anhydrous), introduced at a rate of 1 mL/min. The molecular weight of the compounds was calculated from the chromatogram as the average molecular weight equivalent to polystyrene (PS eq. MW), with a confidence level of 95% and the average value of three measurements of the same solution according to the following formula:

where H _i is the detector signal level from the baseline for the retention volume V _i , M _i is the molecular weight of the compound fraction at the retention volume V _i and n is the number of data points. Omnisec 5.12 supplied with the device was used as the software.

Radical curing under impact UV LED radiation paints For offset seals, containing colored pigments And printed signs, received from them (C1-C4 And E1-E2)

Table 2A

Radical curing under impact UV LED radiation paints For offset seals, containing colored pigments And fluorescent pigments, And printed signs, received from them (C5-C8 And E3-E4)

Table 2B

[0101] The UV-LED radical-curing offset printing inks from Table 2A were prepared to evaluate the effectiveness of photoinitiators P1-P4 (Table 1A) and amine synergists S1-S3 (Table 1C) in curing layers printed with the inks, and to evaluate their effect on the residual fluorescence of the printed and cured layers.

[0102] The UV-LED radical-curing offset printing inks from Table 2B were prepared to evaluate the effectiveness of photoinitiators P1, P3 and P4 (Table 1A) and amine synergists S2 (Table 1C) in curing layers printed with the inks and to evaluate their effect on the measured luminescence of printed and cured layers containing either a fluorescent pigment (RADGLO VSFX02 UV GREEN) or a phosphorescent pigment (LUMILUX GREEN CD 140 FS 52155 (Honeywell)). The objective was to determine whether the residual fluorescence of the printed and cured layers due to the presence of the photoinitiator (P1, P3 and P4, Table 1A) affects the luminescence signals of the luminescent pigments.

[0103] The UV-LED radical-curing offset printing inks from Tables 2A-B were independently prepared by mixing the ingredients listed in Tables 2A-B, except for photoinitiators P1-P4, using a SpeedMixer ^™ (DAC 150 SP CM31 from Hauschild Engineering) at room temperature. The resulting pastes were independently milled in a SDY300 three-roll mill in three passes (first pass at 5 bar, second and third passes at 11 bar).

[0104] The photoinitiators P1-P4 were added independently to the pastes obtained in this way and the paints obtained in this way were mixed in a SpeedMixer ^™ (DAC 150 SP CM31 from Hauschild Engineering) at 2500 rpm for three minutes at room temperature, ground in a Loher mill (3 x 50 rpm with a mass of 7.5 kg) and mixed again with the SpeedMixer ^™ for three minutes.

[0105] The viscosity values of the UV-LED radical-curing offset printing inks from Tables 2A-B were independently measured at 40˚C and 1000 s ^-1 on a Haake Roto-Visco RV1 rheometer with a 2 cm 0.5° cone geometry, 0-1000 s ^-1 ramp rate for 30 seconds, and are presented in Tables 2A-B.

Efficiency curing

[0106] The UV-LED radical-curing offset printing inks from Tables 2A-B were independently printed as a feature (4.5 cm x 23 cm) on a polymer substrate for the production of fiduciary money (Guardian ^TM , CCL Secure) using under pressure of 1000 N (T = 22°C, relative humidity = 54%).

[0107] The obtained printed layers were independently cured with a UV-LED UV lamp (LUV3 385 nm, IST) at a dose of approximately 200 mJ/cm ² . The irradiation dose was determined with the following details:

The irradiation source (LUV3 385 nm, IST) was turned on. A Power Puck ^® II radiometer (EIT, Inc., USA) was placed on the conveyor belt of the irradiation device designed to receive the irradiated samples. The Power Puck ^® , equipped with four filters (UVA, UVB, UVC and UVA2), was irradiated with the irradiation source at different belt speeds. Only the dose received in the UVA2 region was saved. The conveyor belt speed for obtaining an irradiation of approximately 200 mJ/ ^cm2 was determined to be approximately 36 m/min (the average of two measured values).

[0108] The printed features were cured at a speed of 36 m/min (corresponding to a dose of approximately 200 mJ/ ^cm2 ) using a UV-LED UV lamp (LUV3 385 nm, IST). The exact number of printed and cured layers was calculated for each sample by weighing the substrate before and after printing and curing. The printed and cured layer weight of all printed features was 1 g/ ^m2 ± 3%.

[0109] For each sample, a curing test was performed by placing a piece of blank substrate (i.e., unprinted substrate) on the face of the substrate carrying the printed and cured layer and subjecting the assembly thus formed to a backpressure of 3.4 bar at 65°C using an ORMAG Intaglio Proof Press. The substrate carrying the printed and cured layer and the blank substrate were separated and the optical density of the blank substrate was checked for ink transfer (re-embossing). Two blanks were obtained to determine the two possible extreme values of the curing behavior. The first blank for determining the maximum efficiency (100% curing efficiency) was obtained by measuring the optical density of the substrate sample without printing, while the minimum efficiency (0% curing efficiency) was determined by measuring the optical density of the sample obtained by backpressure of a completely uncured ink layer (without passing under the UV-LED lamp) immediately after printing. The maximum optical density (corresponding to 0% curing efficiency) was defined as OD _max , while the minimum optical density (corresponding to 100% curing efficiency) was defined as OD _min . The actual curing efficiency (curing efficiency for each sample in %) was defined as OD, and calculated as

[0110] Table 3 provides the results of the curing efficiency of the printed layer obtained from the UV-LED radical-curing offset printing inks from Tables 2A-B.

Table 3

[0111] As shown in Table 3, comparative offset printing ink 4, 7 and 8 (C4, C7 and C8) containing phosphine oxide as a photoinitiator suffered from very poor curing performance.

[0112] As shown in Table 3, Comparative Offset Printing Ink 3 (C3) containing a photoinitiator in the form of thioxanthone of formula (I) having a weight average molecular weight (MW) of more than 900 g/mol PS equiv, and also containing a benzoate compound as an amine-containing compound having a molecular weight of less than 400 g/mol PS equiv (specifically, a molecular weight of 277.4, as stated by the supplier), suffered from average curing performance.

[0113] As shown in Table 3, comparative offset printing inks 1, 2, 5 and 6 (C1, C2, C5 and C6), i.e. inks containing a photoinitiator in the form of a thioxanthone of formula (I) other than formula (I), the weight-average molecular weight (MW) of which is less than 900 g/mol equiv. PS, and containing an acrylate compound based on an amine-modified polyester as an amine-containing compound, the molecular weight of which is more than 400 g/mol equiv. PS (in particular about 4000 g/mol PS equiv.), showed average to good curing performance, whereas the offset printing inks according to the present invention (E1-E4), i.e. inks containing a photoinitiator in the form of a thioxanthone of formula (I) whose weight average molecular weight (MW) is more than 900 g/mol PS equiv. and containing either an acrylate compound based on an amine-modified polyester as the amine-containing compound whose molecular weight is more than 400 g/mol PS equiv. (in particular about 4000 g/mol PS equiv.), or a benzoate compound as the amine-containing compound whose molecular weight is more than 400 g/mol PS equiv. (in particular about 1000 g/mol PS equiv.), showed good curing performance.

Measurements fluorescence

[0114] The UV-LED radical-curing offset printing inks from Tables 2A-B were independently printed as a printed feature (4.5 cm x 23 cm) on a cotton fiduciary backing (BNP paper from Louisenthal, 100 g/ ^m2 ) on at 800 N. The resulting printed layers were independently cured with a UV-LED UV lamp (LUV3 385 nm, IST) at a dose of 200 mJ/ ^cm2 (36 m/min). The exact number of printed and cured layers was calculated for each sample by weighing the substrate before and after printing and curing. The printed and cured layer weight of all printed features was 2 g/ ^m2 ± 3%.

[0115] The fluorescence of the printed and cured layers made with the inks from Tables 2A-B was evaluated using the method described later in this document. The results of the fluorescence tests are presented in Tables 4A-B.

Fluorescence facial sides (C1-C8 And E1-E4)

[0116] The residual fluorescence of the printed and cured layers was assessed using a Fluorolog II (Spex) at 254 nm and 365 nm using the following parameters:

- Detector: R928/0115/0381

- Angle: 30°

- Position: front side

- Excitation slit: 2 nm (254 nm) and 1.2 nm (365 nm)

- Covered wavelength: 400-700nm (1nm step)

- Detection slit: 1 nm (254 nm) and 0.6 nm (365 nm).

[0117] First, the fluorescence spectrum of a piece of blank substrate (i.e., substrate without printing) was measured. Then, the fluorescence spectrum of the printed and cured layers was recorded. From the obtained spectrum, the maximum fluorescence intensity was determined, the fluorescence of the blank substrate was calculated at the same wavelength, and the obtained value was reported with the corresponding wavelength. This corresponds to the maximum fluorescence of the front side (printed side).

[0118] The luminescence of the face of the printed and cured layers made with the inks from Table 2B was evaluated to determine the effect of the residual fluorescence of the printed and cured layer on the intrinsic luminescence intensity of the added luminescent compound. A CAMAG 4 UV cabinet (equipped with two 254 nm and 366 nm UV tubes, 8 W each) was used to observe the perceived color of the printed and cured security features with the naked eye. The measured luminescence intensity and the observed color are presented in Table 4B.

Fluorescence reverse sides (C1-C4 And E1-E2)

[0119] Each of the printed and cured layers was placed on a piece of blank substrate (i.e., unprinted substrate) of equal size such that the printed surface (the “front side” surface in Table 4) was in contact with the blank substrate. Both pieces were held between two glass plates (20 cm x 15 cm x 3 mm), and the assembly was placed horizontally in an oven at 60 °C and 50% relative humidity for 72 hours. Two fluorescence measurements were then performed as described herein:

fluorescence of the reverse side of the printed layer: the result (the average of three measurements at three different locations on the reverse side) is marked as “reverse side of the printed sample” in Table 4;

fluorescence of the back side of the unprinted sample: the result (the average of three measurements at three different locations on the back side) is labeled as "back side of unprinted sample" in Table 4 and indicates the transfer of fluorescent compounds from the printed layer of one sheet to the back side of the sheet lying on it.

[0120] In all cases, the fluorescence spectrum of the blank substrate (i.e., the substrate without printing) was first measured, and the result was determined from the fluorescence of the printed and cured layers.

Table 4A

a) absolute values are given in photons/second; values in brackets refer to the fluorescence intensity of comparative example C2 (ITX) used as a control (cf. = 100%),

b) when excited at 365 nm.

Table 4B

a) absolute values are given in photons/second; values in brackets refer to the luminescence intensity of comparative example C5 relative to C6 (photoinitiator P1), used as a control (ctr. = 100%)

[0121] As shown in Table 4A, the printed feature produced by comparative offset printing ink 4 (C4) containing phosphine oxide as a photoinitiator exhibited very low fluorescence on the front and back sides.

[0122] As shown in Table 4A, the printed feature produced by comparative offset printing ink 3 (C3) containing a photoinitiator in the form of thioxanthone of formula (I) and also containing a benzoate compound as an amine-containing compound, the molecular weight of which is less than 400 g/mol eq. PS (in particular, a molecular weight of 277.4, as stated by the supplier), exhibited low fluorescence.

[0123] As shown in Table 4A, the printed feature produced by comparative offset printing inks 1 and 2 (C1 and C2), i.e., inks containing a photoinitiator in the form of a thioxanthone of formula (I) other than formula (I), the weight average molecular weight (MW) of which is less than 900 g/mol PS equiv, and containing an acrylate compound based on an amine-modified polyester as an amine-containing compound, the molecular weight of which is more than 400 g/mol PS equiv (in particular, approximately 4000 g/mol PS equiv), showed strong fluorescence on the front side, which makes it unsuitable. As shown in Table 4A, the printed feature produced by comparative offset printing ink 2 (C2), i.e., an ink containing a photoinitiator in the form of a thioxanthone of formula (I) other than formula (I), the weight-average molecular weight (MW) of which is less than 900 g/mol PS equiv, and containing an acrylate compound based on an amine-modified polyester as an amine-containing compound, the molecular weight of which is more than 400 g/mol PS equiv), exhibited strong fluorescence not only on the front and back sides of the printed feature but also on the back side of the blank substrate (non-printed substrate).

[0124] As shown in Table 4A, the printed feature produced by the offset printing inks of the present invention (E1 and E2), i.e. the inks containing a photoinitiator in the form of a thioxanthone of formula (I) having a weight average molecular weight (MW) of more than 900 g/mol PS equiv and containing either an acrylate compound based on an amine-modified polyester as the amine-containing compound having a molecular weight of more than 400 g/mol PS equiv (in particular about 4000 g/mol PS equiv) or a benzoate compound as the amine-containing compound having a molecular weight of more than 400 g/mol PS equiv (in particular about 1000 g/mol PS equiv), exhibited low fluorescence on the front and back sides.

[01258] As shown in Tables 3 and 4A, the UV-LED radical-curing offset printing inks of the present invention (E1 and E2) combined good curing efficiency and produced features exhibiting low fluorescence on their front and back sides, while the comparative inks either suffered from very poor or poor curing efficiency or produced features that suffered from fluorescence.

[0126] As shown in Table 4B, the luminescent printed feature produced by comparative offset printing inks C7 and C8 (containing a phosphine oxide photoinitiator) exhibited a strong green luminescence signal of the added luminescent compounds (C7: fluorescent pigment; C8: phosphorescent pigment). In other words, the blue residual fluorescence of the printed and cured layer obtained from the ink containing the phosphine oxide photoinitiator did not adversely affect the signal of the added luminescent pigments.

[0127] As shown in Table 4B, the luminescent printed feature produced by comparative offset printing inks C5 and C6 (containing a thioxanthone with a structure different from formula (I), the weight average molecular weight (MW) of which is less than 900 g/mol PS equiv, and containing an acrylate compound based on an amine-modified polyester, the molecular weight of which is more than 400 g/mol PS equiv (in particular, approximately 4000 g/mol PS equiv)), exhibited a blue luminescence signal, essentially due to the strong residual fluorescence of the printed and cured layer. In other words, the green luminescence signal caused by the added fluorescent compounds (C5: fluorescent pigment; C6: phosphorescent pigment) was hidden by the strong residual fluorescence of the printed and cured layers, making it very difficult to detect the luminescence signal of the added fluorescent compounds.

[0128] As shown in Table 4B, the luminescent printing feature produced by the offset printing inks of the present invention (E3 and E4), i.e., the inks containing a photoinitiator in the form of a thioxanthone of formula (I) whose weight average molecular weight (MW) is less than 900 g/mol PS equiv, and containing an acrylate compound based on an amine-modified polyester whose molecular weight is more than 400 g/mol PS equiv (in particular, approximately 4000 g/mol PS equiv)), showed a strong green luminescence signal of the added luminescent compounds (E3: fluorescent pigment, E4: phosphorescent pigment). In other words, the blue residual fluorescence of printed and cured layers obtained from an ink containing a photoinitiator in the form of thioxanthone with a structure according to formula (I), the weight-average molecular weight (MW) of which is less than 900 g/mol PS equiv, did not negatively affect the signal of the added luminescent pigments.

[0129] As shown in Tables 3 and 4B, the UV-LED radical-curing offset printing inks of the present invention (E3 and E4) combined good curing efficiency and were able to produce luminescent security features exhibiting strong luminescent signals from the added luminescent (fluorescent or phosphorescent) pigments, whereas the comparative inks (C5-C8) either suffered from poor curing efficiency or provided luminescent security features that suffered from strong residual fluorescence of the printed and cured layers, making it very difficult to properly detect the actual luminescence signals of the added luminescent pigments.

Claims (45)

1. A UV-LED radical-curing offset printing ink, the viscosity of which is in the range of 2.5 to 25 Pa⋅s at 40 °C and 1000 s ^-1 , for offset printing a feature on a substrate, wherein said UV-LED radical-curing offset printing ink contains: i) from 50 to 80% by weight of one or more radical-curing (meth)acrylate compounds consisting of one or more radical-curing (meth)acrylate oligomers and one or more radical-curing (meth)acrylate monomers, wherein said radical-curing (meth)acrylate oligomers are selected from the group consisting of epoxy(meth)acrylates, (meth)acrylate-modified oils, (meth)acrylate-modified epoxidized oils, polyester(meth)acrylates, aliphatic or aromatic polyurethane(meth)acrylates, polyacrylic acid (meth)acrylates, polyacrylate ester (meth)acrylates and mixtures thereof, and said radical-curing (meth)acrylate monomers are selected from the group consisting of mono(meth)acrylates, di(meth)acrylates, tri(meth)acrylates, tetra(meth)acrylates, penta(meth)acrylates, hexa(meth)acrylates and mixtures thereof; ii) from 8 to 12% by weight of one or more photoinitiators of formula (I) (I), where Q has the following formula (IIa), (IIb) or (IIc): (IIc), where n is 1, 2 or 3, R ₁ are identical to or different from each other and are selected from the group consisting of C ₁ -C ₃ alkylene groups, R ₂ are selected from the group consisting of hydrogen and C ₁ -C ₃ alkyl groups, and where the sum of a+b+c is in the range from 3 to 12 and the sum of d+e+f+g is in the range from 4 to 16, iii) from 2 to 12 wt.% of one or more amine-containing compounds selected from the group consisting of aminobenzoate compounds having a weight average molecular weight of at least 400 g/mol PS equiv, amine-modified polyester acrylates having a weight average molecular weight of at least 400 g/mol PS equiv, and combinations thereof, and iv) from 1 to 30% by weight of one or more inorganic pigments and/or one or more organic pigments, and v) from 0.5 to 5% by weight of one or more fillers and/or diluents selected from the group consisting of carbon fibers, talcs, micas, wollastonites, clays, kaolins, carbonates, silicates, sulfates, titanates, aluminum oxide hydrates, silicon dioxide, montmorillonites, graphites, bentonites, vermiculites, wood flour, quartz flour, natural fibers, synthetic fibers and combinations thereof, and vi) additives selected from co-initiators, anti-settling agents, defoamers, surfactants and other processing aids; where the mass percentage is calculated based on the total mass of the UV-LED radical-curing offset printing ink. 2. A UV-LED-radical-curing offset printing ink according to claim 1, wherein said radical-curing (meth)acrylate oligomers are selected from the group consisting of epoxy(meth)acrylates, polyester(meth)acrylates, aliphatic or aromatic polyurethane(meth)acrylates and mixtures thereof. 3. A UV-LED radical-curing offset printing ink according to claim 1 or 2, wherein said one or more fillers and/or diluents are selected from the group consisting of carbonates, silicon dioxides, talcs, clays and mixtures thereof. 4. A UV-LED radical-curing offset printing ink according to claims 1-3, wherein at least one of the one or more photoinitiators has the formula (I) and wherein Q has the following formula (IIb-1), (IIb-2) or (IIb-3): (IIb-3), where n is 1, 2, or 3 and where the sum of a+b+c is in the range from 3 to 12. 5. A UV-LED radical-curing offset printing ink according to any one of the preceding claims, wherein the one or more amine-containing compounds are selected from the group consisting of aminobenzoate compounds having a weight-average molecular weight of at least 700 g/mol PS equiv, amine-modified polyester-based acrylates having a weight-average molecular weight of at least 700 g/mol PS equiv, and combinations thereof. 6. A UV-LED radical-curing offset printing ink according to any one of the preceding claims, wherein one or more amine-containing compounds are selected from the group consisting of aminobenzoate compounds. 7. A UV-LED-curable offset printing ink according to claim 6, wherein one or more amine-containing compounds are selected from compounds containing 4-dialkylaminobenzoate. 8. A UV-LED-curable offset printing ink according to claim 6 or 7, wherein one or more amine-containing compounds are selected from the group consisting of aminobenzoate compounds of formula (III) (III), where m ranges from 1 to 4, and where the sum of h+i+j+k ranges from 3 to 12. 9. A UV-LED radical-curing offset printing ink according to any one of the preceding claims, wherein the particle size of one or more inorganic pigments and one or more organic pigments is independently less than or equal to 5 µm when measured using a dispersion measuring device. 10. A UV-LED radical-curing offset printing ink according to any one of the preceding claims, wherein the particle size of one or more inorganic pigments and one or more organic pigments is independently less than or equal to 3 µm. 11. A UV-LED radical-curing offset printing ink according to any one of the preceding claims, further comprising one or more waxes selected from the group consisting of paraffin waxes, polyethylene waxes, fluorocarbon waxes, polytetrafluoroethylene waxes, carnauba waxes and mixtures thereof. 12. A method for printing a feature on a substrate using an offset printing process, comprising the steps of: a) applying a UV-LED radical-curing offset printing ink according to any one of claims 1 to 11 by offset printing to form an ink layer, and b) exposing the paint layer to UV light at a dose of at least 150 mJ/ ^cm2 to cure said paint layer with a UV LED light source. 13. The method according to claim 12, characterized in that step b) comprises exposing the paint layer to UV light at a dose of at least 200 mJ/ ^cm2 . 14. The method according to claim 12 or 13, characterized in that step b) consists of exposing the paint layer to one or more wavelengths in the range from 355 to 415 nm. 15. Use of one or more photoinitiators according to paragraphs. 1-3 in an amount of from 4 to 20 wt.% and one or more amine-containing compounds described in this document in an amount of from 1 to 15 wt.% for obtaining a radical-curing UV-LED offset printing ink, the viscosity of which is in the range of from 2.5 to 25 Pa⋅s at 40 °C and 1000 s ^-1 , wherein said radical-curing UV-LED offset printing ink is suitable for printing one or more features on a security document, wherein said radical-curing UV-LED offset printing ink contains from 10 to 80 wt.% of radical-curing (meth)acrylate compounds, from 1 to 30 wt.% of one or more inorganic pigments and/or one or more organic pigments and from 0.5 to 10% by weight of one or more fillers and/or diluents, wherein the weight percentage is calculated based on the total weight of the UV-LED radical-curing offset printing ink. 16. A security feature consisting of a cured layer of paint made from UV-LED-radical-curing offset printing paint according to any of paragraphs 1-11. 17. A protected document containing a substrate and one or more printed features according to paragraph 16. 18. The protected document according to paragraph 17, characterized in that the total surface area of one or more printed features is greater than or equal to 50%, wherein the % is calculated based on the total surface area of the substrate on which the said one or more printed features are presented. 19. The protected document according to paragraph 17, characterized in that the total surface area of one or more printed features is greater than or equal to 60%, wherein the % is calculated based on the total surface area of the substrate on which the said one or more printed features are presented. 20. The protected document according to paragraph 17, characterized in that the total surface area of one or more printed features is greater than or equal to 70%, wherein the % is calculated based on the total surface area of the substrate on which the said one or more printed features are presented. 21. A protected document according to any of paragraphs 17-20, characterized in that the substrate is a non-fluorescent substrate.