Gravure printing processes for the production of security features made with oxidative-drying gravure inks

Abstract

The present invention relates to the field of processes for producing a security feature on a substrate by means of a rotogravure printing process comprising a step a) of inking a rotogravure-engraved printing plate with an oxidative-drying rotogravure ink, said rotogravure-engraved printing plate being at a temperature of between approximately 45 °C and approximately 85 °C; a step b) of removing any excess of the oxidative-drying rotogravure ink using a paper from a fabric cleaning system or using a polymeric cleaning cylinder and cleaning said polymeric cleaning cylinder with an alkaline aqueous cleaning solution in combination with one or more mechanical means; a step c) of transferring said oxidative-drying rotogravure ink in the form of the security feature onto the substrate;and a step d) of drying the oxidative gravure ink in the presence of air to form the security element.;

Description

Gravure printing processes for the production of security features made with oxidative-drying gravure inks The present invention relates to the field of protecting valuable documents against counterfeiting and illegal reproduction. In particular, the present invention relates to the field of oxidative drying inks suitable for gravure printing of valuable documents and gravure printing processes that use oxidative drying inks. BACKGROUND OF THE INVENTION With the constant improvement in the quality of photocopies and color printing, and in an attempt to protect security documents, such as banknotes, valuable documents or cards, transport tickets or cards, revenue stamps, and product labels, which do not have reproducible effects against counterfeiting, forgery, or illegal reproduction, the conventional practice has been to incorporate various security features into these documents. Gravure printing refers to printing methods and processes used, in particular, in the field of valuable documents. Gravure printing is known to be the most consistent and high-quality printing process for producing fine, tapering lines and is therefore the preferred printing technology for fine design in the field of security documents, especially banknotes and stamps. Specifically, one of the distinguishing features of gravure printing is that the layer thickness of the ink transferred to the substrate can vary from a few micrometers to several tens of micrometers by using correspondingly shallow or deep engravings in the gravure printing device.As mentioned earlier in this document, the layer thickness of the security features printed in rotogravure therefore allows a sufficiently high amount of material on the substrate for recognition and detection. Oxidative-drying inks are commonly used in gravure printing processes. These inks dry by oxidation in the presence of oxygen, particularly atmospheric oxygen. During the drying process, oxygen combines with one or more components of the ink vehicle, converting the ink into a semi-solid or solid state. The process can be accelerated by using driers, also known in the industry as catalysts, drying agents, desiccants, or desiccators, such as metallic salts, and / or by applying heat treatment. Oxidative-drying gravure inks can experience a so-called "transfer" problem, which is the transfer of ink from one printed sheet to the back of the next printed sheet in the stack or to the back of an endless sheet in a web. Although this problem can occur with any industrial printing process using oxidative-drying inks, the pronounced relief of gravure-printed patterns can accentuate the transfer issue. With prior art oxidative-drying gravure inks, the transfer problem has been reduced primarily through optimization of the ink formulation. WO 2020079154 could be cited as an example of prior art.There remains a need for rotogravure printing processes that use oxidative drying rotogravure for the production of security features that exhibit good drying performance and transfer characteristics to avoid transfer problems. SUMMARY Accordingly, an object of the present invention is to overcome the deficiencies of the prior art, as discussed above. This is achieved by utilizing oxidatively drying gravure inks comprising one or more polythiol compounds and one or more fusible waxes. This document describes processes for producing a security feature on a substrate by means of a gravure printing process, as well as the security features obtained therefrom, said process comprising: a step a) of inking a gravure-engraved printing plate with an oxidative-drying gravure ink, said gravure-engraved printing plate being at a printing plate temperature between approximately 45 °C and approximately 85 °C, said oxidative-drying gravure ink comprising: i) at least one oxidative drying varnish, ii) one or more dryers in a total quantity of approximately 0.01% by weight to approximately 10% by weight, iii) one or more polythiol compounds present in a total amount exceeding 0.5% by weight, preferably in a total amount of approximately 0.75% by weight to approximately 2% by weight, and (iv) one or more fusible waxes present in a total amount of approximately 1% by weight to approximately 10% by weight, the weight percentages being based on the total weight of the oxidative gravure ink; a step (b) of removing any excess oxidative gravure ink by washing using a paper or fabric washing system or using a polymeric washing cylinder and cleaning said polymeric washing cylinder with an alkaline aqueous washing solution in combination with one or more mechanical means; a step (c) of transferring said oxidative gravure ink in the form of the security feature onto the substrate; and a step d) of drying the ink for gravure by oxidative drying in the presence of air to form the security feature. DETAILED DESCRIPTION The following definitions are to be used to interpret the meaning of the terms and expressions discussed in the description and cited in the claims. As used herein, the article "un" or "una" indicates both one or more than one and does not necessarily limit its noun of reference to the singular. As used herein, the term "approximately" means that the quantity or value in question may be the designated value or some other approximately equal value. The expressions are intended to convey that similar values ​​within a range of ±5% of the stated value promote equivalent results or effects according to the invention. As used herein, the expression "and / or" or "or / and" means that all or only one of the elements in that group may be present. For example, the expression "A and / or B" will mean "only A, or only B, or both A and B." As used herein, the expression "at least" is understood to mean one or more than one, e.g., one, or two, or three. The expression "security feature" is used to indicate an image, pattern, or graphic element that can be used for authentication purposes. The present invention provides oxidative drying rotogravure inks suitable for rotogravure printing processes, in particular for rotogravure printing processes for the production of security features on valuable documents. This document also describes security features produced using the process described herein and the oxidative-drying rotogravure ink described herein, and valuable documents comprising one or more of these security features. This document also describes the uses of the security features described herein for protecting a valuable document against fraud or illegal reproduction. Gravure printing refers to a printing method and process used, in particular, in the field of printing valuable documents. In an industrial gravure printing process, a rotating steel cylinder carrying a plate engraved with a pattern or image to be printed is supplied with ink by one or more selective inking cylinders (or stencil cylinders), each selective inking cylinder being inked in at least one corresponding color. This document also describes processes for producing a security feature, as well as the security features obtained from them. The process described herein comprises step a) of inking a gravure printing plate with the oxidative-drying gravure ink described herein, said gravure printing plate being engraved at a printing plate temperature between approximately 45 °C and approximately 85 °C. Step a) of inking the gravure printing plate with the oxidative-drying gravure ink described herein onto a substrate, such as those described herein, is typically performed with a gravure plate that has areas of varying depth and / or width of engraving. The process further comprises a step b) of removing excess oxidative-drying rotogravure ink from the printing plate by washing using a paper or fabric washing system or using a polymeric washing cylinder and cleaning said polymeric washing cylinder with an alkaline aqueous washing solution in combination with one or more mechanical means, said washing removal step being carried out after step a).Preferably, step b) of washing away any excess oxidative-drying gravure ink is carried out by using a polymeric wash cylinder and cleaning said polymeric wash cylinder with an alkaline aqueous wash solution in combination with one or more mechanical means, wherein the wash cylinder is typically a cylinder coated with a material to which the ink readily adheres (such as, for example, polyvinyl chloride (PVC)), wherein the alkaline aqueous wash solution typically comprises caustic soda and surfactant (such as, for example, sulfated / sulfonated castor oil), and wherein the mechanical means can be one or more brushes and / or pads (for example, Scotch-Brite™ pads). The process further comprises a step (c) of transferring the oxidative gravure ink as a security feature onto the substrate, this drying step being performed after step (b). Step (c) consists of bringing the substrate, which may be in sheet or strip form, into contact with the engraved gravure printing plate so that the oxidative gravure ink is transferred under pressure from the engravings of the printing plate to the substrate to be printed, thus forming the security feature as a raised pattern on the substrate. Typically, the security features are deposited onto the substrate under very high pressure. The process further comprises a step (d) of drying the oxidative gravure ink described herein in the presence of air to form a layer or coating in the form of the security feature described herein on the substrate, said drying step (d) being carried out after step (c). The drying step (d) of the oxidative gravure ink described herein may be carried out with hot air, with an infrared source, or any combination of hot air and an infrared source to reduce the drying time of said inks. The oxidative drying gravure inks for the gravure printing processes described herein have a viscosity in the range of approximately 3 to approximately 60 Pa·s at 40 °C and 1,000 s-1, said viscosity being measured using a Haake Roto Visco RV1 rotary rheometer, using a conical plate of 20 mm diameter and a geometry of 0.5°, at a shear rate of 1,000 s-1 and a temperature of 40 °C. The oxidative gravure inks described herein comprise at least one oxidative drying varnish, as described herein, one or more driers, as described herein, one or more polythiol compounds, as described herein, one or more fusible waxes, as described herein, and, optionally, one or more compounds, additives and / or ingredients, as described herein. The oxidative gravure inks described herein comprise at least one oxidative varnish. The term "varnish" is also known in the art as resin, binder, or ink vehicle. The at least one oxidative varnish is preferably present in the oxidative gravure inks described herein in an amount of approximately 10 to approximately 90% by weight, the weight percentages being based on the total weight of the oxidative gravure ink. Oxidative drying varnishes are typically polymers comprising unsaturated fatty acid residues, saturated fatty acid residues, or mixtures thereof, as commonly known in the art. Preferably, the oxidative drying varnishes described herein comprise unsaturated fatty acid residues to ensure air-drying properties. Particularly preferred oxidative drying varnishes are resins comprising unsaturated acid groups, and even more preferred are resins comprising unsaturated carboxylic acid groups. However, the resins may also comprise saturated fatty acid residues. Preferably, the oxidative drying varnishes described herein comprise acid groups, i.e.,Oxidative drying varnishes are selected from acid-modified resins. The oxidative drying varnishes described herein may be selected from the group consisting of alkyd resins, vinyl polymers, polyurethane resins, hyperbranched resins, rosin-modified maleic resins, rosin-modified phenolic resins, rosin esters, petroleum resin-modified rosin esters, petroleum resin-modified alkyd resins, alkyd resin-modified rosin / phenol resins, alkyd resin-modified rosin esters, acrylic-modified rosin / phenol resins, acrylic-modified rosin esters, urethane-modified rosin / phenol resins, urethane-modified rosin esters, urethane-modified alkyd resins, epoxy-modified rosin / phenol resins, epoxy-modified alkyd resins, terpene resins,Nitrocellulose resins, polyolefins, polyamides, acrylic resins, and combinations or mixtures thereof. Polymers and resins are used interchangeably in this document. Saturated and unsaturated fatty acid compounds can be obtained from natural and / or artificial sources. Natural sources include animal and / or plant sources. Animal sources may include animal fat, butterfat, fish oil, lard, liver fat, tuna oil, sperm whale oil, and / or tallow. Plant sources may include oils, such as vegetable oils and / or non-vegetable oils. Examples of vegetable oils include, but are not limited to, bitter gourd, borage, marigold, canola, castor, chinawood, coconut, conifer kernel, corn, cottonseed, dried castor beans, flaxseed, grapeseed, Jacaranda mimosifolia seed, linseed oil, palm, palm kernel, peanut, pomegranate seed, rapeseed, safflower, snake gourd, soybean (bean), sunflower, wood pulp, stick, and wheat germ.Artificial sources include distilled wood pulp oil and / or chemical or biochemical synthesis methods. Suitable fatty acids also include myristoleic acid (C14H26O2, CAS No. 544-64-9), palmitoleic acid (C16H30O2, CAS No. 373-49-9), oleic acid (C18H34O2, CAS No. 112-80-1), eleostearic acid (C18H30O2, CAS No. 506-23-0), lycanic acid (C18H28O3, CAS No. 623-99-4), linoleic acid (C18H32O2, CAS No. 60-33-3), and linolenic acid (C18H30O2, CAS No. 463-40-1). , Stearidonic acid (C18H28O2, CAS No. 20290-75-9) , Arachidonic acid (C20H32O2, CAS No. 506-32-1) , Ricinoleic acid (C18H34O3, CAS No. 141-22-0) , Erucic acid (C22H42O2, CAS No. 112-86-7), gadoleic acid (C20H38O2, CAS No. 29204-02-2), clupanodonic acid (C22H34O2, CAS No. 24880-45-3), CAS No. 68378-49-4) and mixtures thereof.These fatty acids are typically used in the form of mixtures of fatty acids derived from natural or synthetic oils. The oxidative gravure inks described herein comprise one or more polythiol compounds, wherein the term "polythiol compounds" refers to chemical compounds having at least two thiol (-SH) functional groups per molecule and are also referred to in the art as polyfunctional mercapto compounds.Preferably, the one or more polythiol compounds described herein are selected from the group consisting of dithiol compounds (i.e., compounds comprising two thiol functional groups), trithiol compounds (i.e., compounds comprising three thiol functional groups), tetrathiol compounds (i.e., compounds comprising four thiol functional groups), pentathiool compounds (i.e., compounds comprising five thiol functional groups), hexathiol compounds (i.e., compounds comprising six thiol functional groups), and mixtures thereof, more preferably selected from the group consisting of dithiol compounds, trithiol compounds, tetrathiol compounds, and mixtures thereof. Preferred examples of dithiol compounds are 3-oxa-1,5-pentanedithiol (CAS No. 111-46-6), 1,5-pentanedithiol (CAS No. 928-98-3), 1,3-pentanedithiol (CAS No. 188194-74-3), 1, 6-hexaneditiol (CAS No. 1191-43-1), 4-mercapto-cyclohexaneethanethiol (CAS No. 3232-05-1), 3-mercapto-cyclohexaneethanethiol (CAS No. 17809-99-3), 2,2'-thiobis(ethanethiol) (CAS No. 3570-55-6) , 1-[(2-mercaptoethyl)thio]-2-propanethiol (CAS No. 35330-71-3) , 1,1'-thiobis[2-propanethiol] (CAS No. 35330-70-2) , 2,2'-[1,2-ethanediylbis(thio)]bis[ethanethiol] (CAS No. 25423-55-6), 2, 2'-[thiobis (2, 1-ethanediylthio) ]bis[ethanethiol] (CAS No. 60147-09-3), bis-(4-mercaptomethylphenyl) ether (CAS No. 7344-22-1), bis Ethylene glycol (3-mercaptoacetate) (part no. CAS 123-81-9, available on the market as THIOCURE® GDMA through BRUNO BOCK Chemische Fabrik GmbH & Co. KG), 3-mercapto-, 1, 1'-(1, 6-hexanediyl) ester of propanoic acid (n.CAS No. 96663-89-7); 3-mercapto-, 1, 1'-(1,4-cyclohexanediyl) ester of propanoic acid (CAS No. 2227318-91-2); 3-mercapto-, 1, 1'-[(1-methylethylidene) di-4,1-cyclohexanediyl] ester of propanoic acid (CAS No. 24293-42-3) and bis(3-mercaptopropionate) of ethylene glycol (CAS No. 22504-50-3, available on the market as THIOCURE® GDMP through BRUNO BOCK Chemische Fabrik GmbH & Co. KG). Preferably, one or more dithiol compounds are esters of mercaptopropionic acids with polyols, preferably disubstituted polyols. More preferably, one or more dithiol compounds are selected from the group consisting of ethylene glycol bis(mercaptoacetate), ethylene glycol bis(3-mercaptopropionate), and mixtures thereof, and, more preferably, at least one of the one or more dithiol compounds is ethylene glycol dimercaptoacetate or ethylene glycol bis(3-mercaptopropionate), and, still more preferably, ethylene glycol bis(3-mercaptopropionate). Preferred examples of trithiol compounds are glycerol trimercaptoacetate (CAS No. 14974-53-9), glycerol trimercaptopropionate (CAS No. 26424-84-0), trimethylolpropane trimercaptoacetate (CAS No. 10193-96-1, commercially available as THIOCURE® TMPMA through BRUNO BOCK Chemische Fabrik GmbH & Co. KG), trimethylolpropane tris-3-mercaptopropionate (CAS No. 33007-83-9, commercially available as THIOCURE® TMPM through BRUNO BOCK Chemische Fabrik GmbH & Co. KG), and ethoxylated trimethylolpropane tris-3-mercaptopropionate (CAS No. 345352-19-4, available on the market as THIOCURE® ETTMP through BRUNO BOCK Chemische Fabrik GmbH & Co. KG) and tris[2-(3-mercaptopropionyloxy)ethyl isocyanurate] (CAS No. 36196-44-8, available on the market as THIOCURE® TEMPIC through BRUNO BOCK Chemische Fabrik GmbH & Co. KG).Preferably, the one or more trithiol compounds are selected from the group consisting of trimethylopropane tris-3-mercaptopropionate, ethoxylated trimethylolpropane tris-3-mercaptopropionate, tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate and mixtures thereof and, still more preferably, tris[2-(3-mercaptopropionyloxy)ethyl isocyanurate]. Preferred examples of tetrathiol compounds are pentaerythritol tetra(mercaptoacetate) (CAS No. 10193-99-4, available on the market as THIOCURE® PETMA through BRUNO BOCK Chemische Fabrik GmbH & Co. KG), pentaerythritol tetra(3-mercaptopropionate) (CAS No. 7575-23-7, available on the market as THIOCURE® PETMP and THIOCURE® 340 through BRUNO BOCK Chemische Fabrik GmbH & Co. KG), pentaerythritol tetra(4-mercaptobutanoate) (CAS No. 916903-92-9), and polycaprolactone tetra(3-mercaptopropionate) (CAS No. 1622079-69-9, available on the market as THIOCURE® PETMP and THIOCURE® 340). market as THIOCURE® PCL4MP 1350 through BOCK Chemische Fabrik GmbH & Co. KG), more preferably, pentaerythritol tetra(2-mercaptoacetate) and pentaerythritol tetra(3-mercaptopropionate) and, even more preferably, pentaerythritol tetra(3-mercaptopropionate). A preferred example of polythiol compounds having more than four thiol (-SH) functional groups per molecule is dipentaerythritol hexa(3-mercaptopropionate) (CAS No. 25359-71-1, available on the market as THIOCURE® DiPETMP through BRUNO BOCK Chemische Fabrik GmbH & Co. KG). The oxidative gravure inks described herein preferably comprise one or more polythiol compounds selected from the group consisting of dithiol compounds, trithiol compounds, tetrathiol compounds, and mixtures thereof, and more preferably compounds such as those described above herein. According to one embodiment, at least one of the dithiol compounds is ethylene glycol bis(3-mercaptopropionate), and / or at least one of the trithiol compounds is tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate, and / or at least one of the tetrathiol compounds is pentaerythritol tetra(3-mercaptopropionate). The one or more thiol compounds are present in a total amount greater than approximately 0.5% by weight, preferably from approximately 0.75% by weight to approximately 2% by weight, more preferably from approximately 1% by weight to 2% by weight, the weight percentages being based on the total weight of the oxidative drying rotogravure ink. The oxidative gravure inks described herein comprise one or more fusible waxes. Within the context of the present invention, one or more fusible waxes refers to waxes or wax mixtures having a melting point between approximately 50°C and approximately 120°C. The one or more fusible waxes are 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 microcrystalline waxes, paraffin waxes, polyethylene waxes, fluorocarbon waxes, polytetrafluoroethylene waxes, Fischer-Tropsch waxes, silicone fluids, beeswax, candelilla waxes, montana waxes, carnauba waxes, rice bran waxes, and mixtures thereof.The one or more waxes are present in a total amount of approximately 1% by weight to approximately 10% by weight, preferably from approximately 1.5% by weight to approximately 8% by weight and more preferably from approximately 2% by weight to approximately 6% by weight, the weight percentages being based on the total weight of the oxidative drying rotogravure ink. Oxidative-drying security inks dry by oxidation in the presence of oxygen, particularly atmospheric oxygen. During the drying process, oxygen combines with one or more components of the ink, converting it into a solid state. The oxidative-drying gravure inks described herein comprise one or more driers (also referred to in the art as catalysts, driers, drying agents, desiccants, or desiccators) to accelerate the oxidation process. Examples of driers include inorganic or organic salts of metal(s), metal soaps of organic acids, metal complexes, and salts of metal complexes.Suitable metal salts include salts containing cobalt, calcium, copper, zinc, iron, zirconium, manganese, barium, zinc, strontium, lithium, vanadium, and potassium as cations; and halides, nitrates, sulfates, and carboxylates such as acetates, ethylhexanoates, octanoates, and naphthenates or acetoacetonates as anions, such as, for example, cobalt, manganese, and zirconium ethylhexanoates. Suitable examples of metal complexes and salts of metal complexes include compounds of manganese, vanadium, and iron (i.e., manganese complexes, salts of manganese complexes, vanadium complexes, salts of vanadium complexes, iron complexes, and salts of iron complexes).When present, the one or more driers used in the oxidative drying gravure ink described herein are preferably present in a total amount of approximately 0.01% by weight to approximately 10% by weight, preferably in a total amount of approximately 0.1% by weight to approximately 5% by weight, the weight percentages being based on the total weight of the oxidative drying gravure ink. According to one embodiment, the oxidative gravure inks described herein are white, transparent, or lightly colored inks that allow the preparation of white, transparent, or lightly colored security features. According to one embodiment, the oxidative gravure inks described herein are white inks. According to one embodiment, the oxidative gravure inks described herein are transparent inks. According to one embodiment, the oxidative gravure inks described herein are lightly colored inks.Preferably, said white, transparent or lightly colored inks comprise one or more driers selected from the group consisting of iron compounds, manganese compounds, zirconium compounds and mixtures thereof, wherein said compounds may be inorganic or organic salts of metal(s), metallic soaps of organic acids, metal complexes and salts of metal complexes, as described above. The oxidative gravure inks described herein may further comprise one or more fillers and / or thinners preferably selected from the group consisting of talcs, micas (e.g., muscovite), montmorillonites, bentonites, wollastonites, halloysites, calcined clays, China clays, carbonates (e.g., calcium carbonate, magnesium carbonate), silicates (e.g., magnesium silicate, aluminum silicate), vermiculites, amoriate silica (e.g., pyrogenic silica, precipitated silica, powdered silica), wood flours (sawdust), natural fibers, synthetic fibers (such as carbon fibers or carbon nanotubes) and mixtures thereof; preferably selected from the group consisting of talcs, micas, wollastonites, calcined clays, carbonates, amoriate silica and mixtures thereof. When present, the one or more fillers or thinners are preferably present in a total amount of approximately 0.1% by weight to approximately 50% by weight, more preferably from approximately 20% by weight to approximately 40% by weight, the weight percentages being based on the total weight of the oxidative drying rotogravure ink. The oxidative gravure inks described herein may further comprise one or more surfactants, in particular hydrophilic macromolecular surfactants, such as those described, for example, in EP 0340163 B1. The function of the optional surfactants is to assist in washing away excess ink from the printing cylinder just before it comes into contact with the substrate. This washing process is part of any high-speed industrial gravure printing process and is carried out using a fabric or paper roll ("calico") or a polymer washing cylinder and a water-based cleaning solution ("washing solution"). In this case, the optional surfactants are used to emulsify the excess ink in the cleaning solution.These surfactants can be non-ionic, anionic, or cationic, as well as hybrid ion surfactants. In the case of hydrophilic macromolecular surfactants, the functional groups are, for example, carboxylic or sulfonic acid groups, hydroxyl groups, ether groups, or primary, secondary, tertiary, or quaternary amino groups. Acid groups can be neutralized with amines, alkanolamines, or, preferably, inorganic bases or combinations thereof. Primary, secondary, and tertiary amino groups can be neutralized with inorganic or organic acids, such as sulfonic acids, formic acid, acetic acid, trifluoroacetic acid, and others. Anionic macromolecular surfactants (AMS), such as those described in EP 2014729 A1, are particularly preferred. The oxidative gravure inks described herein may be either color-constant or optically variable inks. In other words, the oxidative gravure ink may further comprise one or more coloring components selected from the group consisting of optically variable pigments, color-constant pigments, color-constant dyes, and mixtures thereof, preferably selected from the group consisting of organic color-constant pigments, inorganic color-constant pigments, and mixtures thereof. According to one aspect of the present invention, the oxidative gravure inks described herein are colorfast inks and may be white, transparent, or lightly colored, or they may be colored inks. According to one embodiment, the oxidative gravure inks described herein are colorfast white inks. According to one embodiment, the oxidative gravure inks described herein are colorfast transparent inks. According to one embodiment, the oxidative gravure inks described herein are colorfast lightly colored inks.According to one embodiment, the oxidative gravure inks described herein are constant color composition inks that preferably comprise (a) one or more dyes and / or (b) inorganic pigments, organic pigments, or mixtures thereof. Suitable dyes for inks are known in the art and are preferably selected from the group comprising reactive dyes, direct dyes, anionic dyes, cationic dyes, acid dyes, basic dyes, food coloring dyes, metal complex dyes, solvent dyes, and mixtures thereof. Typical examples of suitable dyes include, without limitation, coumarins, cyanines, oxazines, uranines, phthalocyanines, indolinocyanines, triphenylmethanes, naphthalocyanines, indonaphtha-metal dyes, anthraquinones, anthrapyridones, azo dyes, rhodamines, squaryl dyes, and croconium dyes.Typical examples of dyes suitable for the present invention include, without limitation, CI Acid Yellow 1, 3, 5, 7, 11, 17, 19, 23, 25, 29, 36, 38, 40, 42, 44, 49, 54, 59, 61, 70, 72, 73, 75, 76, 78, 79, 98, 99, 110, 111, 121, 127, 131, 135, 142, 157, 162, 164, 165, 194, 204, 236, 245; CI Direct Yellow 1, 8, 11, 12, 24, 26, 27, 33, 39, 44, 50, 58, 85, 86, 87, 88, 89, 98, 106, 107, 110, 132, 142, 144; CI Basic Yellow 13, 28, 65; CI Reactive Yellow 1, 2, 3, 4, 6, 7, 11, 12, 13, 14, 15, 16, 17, 18, 22, 23, 24, 25, 26, 27, 37, 42; CI Food Grade Yellow 3, 4; CI Acid Orange 1, 3, 7, 10, 20, 76, 142, 144; CI Basic Orange 1, 2, 59; CI Food Orange 2; CI Orange B; CIAcid Red 1, 4, 6, 8, 9, 13, 14, 18, 26, 27, 32, 35, 37, 42, 51, 52, 57, 73, 75, 77, 80, 82, 85, 87, 88, 89, 92, 94, 97, 106, 111, 114, 115, 117, 118, 119, 129, 130, 131, 133, 134, 138, 143, 145, 154, 155, 158, 168, 180, 183, 184, 186, 194, 198, 209 211, 215, 219, 221, 249, 252, 254, 262, 265, 274, 282, 289, 303, 317, 320, 321, 322, 357, 359; Basic Red IC 1, 2, 14, 28; CI Rojo Directo 1, 2, 4, 9, 11, 13, 17, 20, 23, 24, 28, 31, 33, 37, 39, 44, 46, 62, 63, 75, 79, 80, 81, 83, 84, 89, 95, 99, 113, 197, 201, 218, 220, 224, 225, 226, 227, 228, 229, 230, 231, 253; CI Reactive Red 1, 2, 3, 4, 5, 6, 7, 8, 11, 12, 13, 15, 16, 17, 19, 20, 21, 22, 23, 24, 28, 29, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 45, 46, 49, 50, 58, 59, 63, 64, 108, 180; CI Food Grade Red 1, 7, 9, 14; CIAcid Blue 1, 7, 9, 15, 20, 22, 23, 25, 27, 29, 40, 41, 43, 45, 54, 59, 60, 62, 72, 74, 78, 80, 82, 83, 90, 92, 93, 100, 102, 103, 104, 112, 113, 117, 120, 126, 127, 129, 130, 131, 138, 140, 142, 143, 151, 154, 158, 161, 166, 167, 168, 170, 171, 182, 183, 184, 187, 192, 193, 199, 203, 204, 205, 229, 234, 236, 249, 254, 285; Basic Blue CI 1, 3, 5, 7, 8, 9, 11, 55, 81; CI Blue Direct 1, 2, 6, 15, 22, 25, 41, 71, 76, 77, 78, 80, 86, 87, 90, 98, 106, 108, 120, 123, 158, 160, 163, 165, 168, 192, 193, 194, 195, 196, 199, 200, 201, 202, 203, 207, 225, 226, 236, 237, 246, 248, 249; CI Reactive Blue 1, 2, 3, 4, 5, 7, 8, 9, 13, 14, 15, 17, 18, 19, 20, 21, 25, 26, 27, 28, 29, 31, 32, 33, 34, 37, 38, 39, 40, 41, 43, 44, 46, 77; CI Food Grade Blue 1, 2; CI Acid Green 1, 3, 5, 16, 26, 104; CI Basic Green 1, 4; CI Food Grade Green 3; CI Acid Violet 9, 17, 90, 102, 121; CI Basic Violet 2, 3, 10, 11, 21; CI Acid Brown 101, 103, 165, 266, 268, 355, 357, 365, 384; CIBasic Brown 1; CI Acid Black 1, 2, 7, 24, 26, 29, 31, 48, 50, 51, 52, 58, 60, 62, 63, 64, 67, 72, 76, 77, 94, 107, 108, 109, 110, 112, 115, 118, 119, 121, 122, 131, 132, 139, 140, 155, 156, 157, 158, 159, 191, 194; CI Direct Black 17, 19, 22, 32, 39, 51, 56, 62, 71, 74, 77, 94, 105, 106, 107, 108, 112, 113, 117, 118, 132, 133, 146, 154, 168; CI Reactive Black 1, 3, 4, 5, 6, 8, 9, 10, 12, 13, 14, 18, 31; CI Food Grade Black 2; CI Yellow Solvent 19, CI Orange Solvent 45, CI Red Solvent 8, CI Green Solvent 7, CI Blue Solvent 7, CI Black Solvent 7; CI Dispersed Yellow 3, CI Dispersed Red 4, 60, CI Dispersed Blue 3 and metallic azo dyes disclosed in documents US 5,074,914, US 5,997,622, US 6,001,161, JP 02-080470, JP 62-190272 and JP 63-218766. The dyes suitable for the present invention may be infrared-absorbing dyes or luminescent dyes.When present, the one or more dyes used in the oxidative gravure ink described herein are preferably present in a total amount of approximately 1% by weight to approximately 20% by weight, the weight percentages being based on the total weight of the oxidative gravure ink. Typical examples of organic and inorganic pigments include, but are not limited to, CI Pigment Yellow 12, CI Pigment Yellow 42, CI Pigment Yellow 93, CI Pigment 109, CI Pigment Yellow 110, CI Pigment Yellow 147, CI Pigment Yellow 173, CI Pigment Orange 34, CI Pigment Orange 48, CI Pigment Orange 49, CI Pigment Orange 61, CI Pigment Orange 71, CI Pigment Orange 73, CI Pigment Red 9, CI Pigment Red 22, CI Pigment Red 23, CI Pigment Red 67, CI Pigment Red 122, CI Pigment Red 144, CI Pigment Red 146, CI Pigment Red 170, CI Pigment Red 177, CI Pigment Red 179, CI Pigment Red 185, CI Pigment Red 202, CI Pigment Red 224, CI Pigment Brown 6, CI Brown Pigment 7, CI Red Pigment 242, CI Red Pigment 254, CI Red Pigment 264, CI Brown Pigment 23, CI Blue Pigment 15, CI Blue Pigment 15:3, CI Blue Pigment 60, CI Violet Pigment 19, CI Violet Pigment 23, C.I. Violet Pigment 32, CI Violet Pigment 37, CI Green Pigment 7, CI Green Pigment 36, CI Black Pigment 7, CI Black Pigment 11, Black Pigment 31, Black Pigment 32, CI White Pigment 4, CI White Pigment 6, CI White Pigment 7, CI White Pigment 21, CI White Pigment 22, antimony yellow, lead chromate, lead chromate sulfate, lead molybdate, ultramarine blue, cobalt blue, manganese blue, chrome oxide green, hydrated chrome oxide green, cobalt green, cerium sulfide, cadmium sulfide, cadmium sulfoselenides, zinc ferrite, bismuth vanadate, Prusia blue, Mixed metallic oxides, azoic pigments, azomethine, methine, anthraquinone, phthalocyanine, perinona, perylene, dicetopyrrolopirrol, thioindigo, thiazinindigo, dioxazine, iminoisoindoline, iminoisoindolinone, quinacridona, flavantrone, indantrona, anthrapyrimidine y quinophthalone.When present, the inorganic pigments, organic pigments or mixtures thereof described herein are preferably present in a total quantity of approximately 0.1% by weight to approximately 45% by weight, the weight percentages being based on the total weight of the oxidative drying rotogravure ink. In embodiments where the oxidative gravure inks described herein are transparent inks, these inks typically do not comprise any pigments, such as those described herein. In embodiments where the oxidative gravure inks described herein are white or lightly colored inks, these inks comprise one or more pigments, preferably selected from the group consisting of CI Pigment White 4, CI Pigment White 6, CI Pigment White 7, CI Pigment White 21, and CI Pigment White 22. According to one aspect of the present invention, the oxidative-drying gravure inks described herein are optically variable inks and comprise optically variable pigments or a mixture of different optically variable pigments. The optically variable inks may further comprise one or more color-constant pigments. The optically variable inks preferably comprise optically variable pigments or a mixture of different optically variable pigments, wherein the optically variable pigments are preferably selected from the group consisting of thin-film interference pigments, coated interference pigments, cholesteric liquid crystal pigments, and mixtures thereof.When present, the optically variable pigments are preferably comprised in the oxidative-drying gravure ink described herein in a total amount of between approximately 5% by weight and approximately 40% by weight, and more preferably in a total amount of between approximately 10% by weight and approximately 35% by weight, the weight percentages being based on the total weight of the oxidative-drying gravure ink. Experts in the field are aware of suitable thin-film interference pigments that exhibit optically variable characteristics, and these are disclosed in documents US 4,705,300; US 4,705,356; US 4,721,271; US ​​5,084,351; US ​​5,214,530; US 5,281,480; US 5,383,995; US 5,569,535, US 5,571,624 and in documents related thereto. When at least a portion of the optically variable pigments consists of thin-film interference pigments, it is preferred that the thin-film interference pigments comprise a Fabr-Perot reflective / dielectric / absorbing multilayer structure and more preferably a Fabr-Perot absorbing / dielectric / reflecting / dielectric / absorbing multilayer structure, wherein the absorbing layers are partly transmitting and partly reflecting, the dielectric layers are transmitting, and the reflecting layer reflects incoming light.Preferably, the reflective layer is selected from the group consisting of metals, metal alloys, and combinations thereof, and more preferably from the group consisting of aluminum (Al), chromium (Cr), nickel (Ni), and mixtures thereof, and even more preferably from aluminum (Al). Preferably, the dielectric layers are selected independently from the group consisting of magnesium fluoride (MgF2), silicon dioxide (SiO2), and mixtures thereof, and more preferably from magnesium fluoride (MgF2). Preferably, the absorbing layers are selected independently from the group consisting of chromium (Cr), nickel (Ni), metal alloys, and mixtures thereof, and more preferably from chromium (Cr).When at least a portion of the optically variable pigments consists of thin-film interference pigments, it is particularly preferred that the thin-film interference pigments comprise a Fabr and -Perot absorbing / dielectric / reflecting / dielectric / absorbing multilayer structure consisting of a Cr / MgF2 / Al / MgF2 / Cr multilayer structure. The thin-film interference pigments described herein are typically manufactured by vacuum deposition of the required layers onto a web. After the desired number of layers has been deposited, the stack of layers is removed from the web, either by dissolving a release layer in a suitable solvent or by peeling the material off the web. The material thus obtained is then broken down into flakes, which are further processed by crushing, grinding, or any other suitable method. The resulting product consists of flat flakes with broken edges, irregular shapes, and varying aspect ratios. Suitable coated interference pigments include, without limitation, structures consisting of a substrate selected from the group consisting of metallic cores, such as titanium, silver, aluminum, copper, chromium, iron, germanium, molybdenum, tantalum, or nickel, coated with one or more layers made of metal oxides, as well as a structure consisting of a core made of synthetic or natural micas, other layered silicates (e.g., talc, kaolin, and sericite), glasses (e.g., borosilicates), silicon dioxide (SiO2), aluminum oxides (Al2O3), aluminum oxides / hydroxides (boehmite), titanium oxides (TiO2), graphites, and mixtures thereof coated with one or more layers made of metal oxides (e.g., titanium oxide, zirconium oxide, tin oxide, chromium oxide, nickel oxide, copper oxide,iron oxide and iron oxide / hydroxide). The structures described above in this document have been described, for example, in Chem. Rev. 99 (1999), G. Pfaff and P. Reynders, pages 1963-1981 and in document WO 2008 / 083894 A2. Typical examples of these coated interference pigments include, without limitation, silicon oxide cores coated with one or more layers made of titanium oxide, tin oxide and / or iron oxide; natural or synthetic mica cores coated with one or more layers made of titanium oxide, silicon oxide and / or iron oxide, in particular, mica cores coated with alternating layers made of silicon oxide and titanium oxide; borosilicate cores coated with one or more layers made of titanium oxide, silicon oxide and / or tin oxide; and titanium oxide cores coated with one or more layers made of iron oxide, iron oxide / hydroxide, chromium oxide,copper oxide, cerium oxide, aluminum oxide, silicon oxide, bismuth vanadate, nickel titanate, cobalt titanate and / or antimony-doped, fluorine-doped or indium-doped tin oxide; aluminum oxide cores coated with one or more layers made of titanium oxide and / or iron oxide. Liquid crystals in the cholesteric phase exhibit molecular order in the form of a helical superstructure perpendicular to the longitudinal axes of their molecules. This helical superstructure is the source of a periodic modulation of the refractive index throughout the liquid crystal material, resulting in the selective transmission / reflection of specific wavelengths of light (interference filter effect). Cholesteric liquid crystal polymers can be obtained by subjecting one or more crosslinkable substances (nematic compounds) with a chiral phase to a specific alignment and orientation. The particular arrangement of the helical molecular structure leads to cholesteric liquid crystal materials exhibiting the property of reflecting a component of circularly polarized light within a specific wavelength range.The hue can be adjusted, in particular, by varying selectable factors, including temperature and solvent concentration, by changing the nature of the chiral component(s), and the ratio of nematic to chiral compounds. Crosslinking under UV radiation fixes the hue in a predetermined state by establishing the desired helical shape, so that the color of the resulting cholesteric liquid crystal materials is no longer dependent on external factors such as temperature. The cholesteric liquid crystal materials can then be shaped into cholesteric liquid crystal pigments by further crumbling the polymer to the desired particle size. Examples of films and pigments made from cholesteric liquid crystal materials and their preparation are disclosed in US patents 5,211,877, 5,362,315, and 6,423.246 and in documents EP 1213338 A1; EP 1046692 A1 and EP 0601 483 A1. The oxidative gravure inks described herein may also comprise one or more machine-readable materials preferably selected from the group consisting of magnetic materials, luminescent materials, electrically conductive materials, infrared-absorbing materials and mixtures thereof. The oxidative drying rotogravure inks described herein may also include one or more forensic markers and / or one or more identifiers. The oxidative gravure inks described herein may further comprise one or more additives, including, but not limited to, compounds and materials used to adjust the physical, rheological, and chemical parameters of the composition, such as viscosity, consistency (e.g., anti-settling agents and plasticizers), foaming properties (e.g., anti-foaming and de-airing agents), UV stability (photostabilizers), adhesive properties, etc. The additives described herein may be present in the oxidative gravure inks disclosed herein in quantities and forms known in the art, including in the form of so-called nanomaterials, where at least one of the particle dimensions is in the range of 1 to 1,000 nm. The gravure printing processes described herein allow for the production of security features on the substrate described herein. The substrates described herein are preferably selected from the group consisting of papers or other fibrous materials (including woven and non-woven fibrous materials), such as cellulose, paper-containing materials, glass, metals, ceramics, plastics and polymers, metallized plastics or polymers, composite materials, and mixtures or combinations of two or more of the same. Typical paper, paper-like, or other fibrous materials are made from a variety of fibers, including, but not limited to, Manila hemp, cotton, flax, wood pulp, and mixtures thereof.As experts in the field know, cotton and cotton / linen blends are preferred in banknotes, while wood pulp is commonly used in valuable documents other than banknotes. Typical examples of plastics and polymers include polyolefins, such as polyethylene (PE) and polypropylene (PP), including biaxially oriented polypropylene (BOPP), polyamides (PA), polyesters, such as poly(ethylene terephthalate) (PET), glycol-modified polyethylene terephthalate (PETG), including poly(ethylene glycol-co-1,4-cyclohexanedimethanol terephthalate), poly(1,4-butylene terephthalate) (PBT), poly(2,6-ethylene naphthoate) (PEN), and polyvinyl chlorides (PVC).Spun olefin fibers, such as those marketed under the registered trademark Tyvek®, can also be used as a substrate. Typical examples of metallized plastics or polymers include the plastic or polymer materials described earlier in this document that have a metal continuously or discontinuously deposited on their surface. Typical examples of metals include, but are not limited to, aluminum (Al), chromium (Cr), copper (Cu), gold (Au), silver (Ag), alloys thereof, and combinations of two or more of the aforementioned metals. Metallization of the plastic or polymer materials described earlier in this document can be achieved by electrodeposition, high-vacuum coating, or sputtering.Typical examples of composite materials include, without limitation, multilayer paper structures or laminates and at least one plastic or polymer material, as described earlier herein, as well as plastic and / or polymer fibers embedded in a paper-like or fibrous material, as described earlier herein. The substrate may, of course, comprise additional additives known to a person skilled in the art, such as fillers, sizing agents, bleaches, processing aids, strengthening agents, or wet strength agents, etc. To further enhance security and resistance against counterfeiting and illegal reproduction of valuable documents, the substrate may contain watermarks, security threads, fibers, plates, luminescent compounds, windows, sheets, distinctive features, coatings, and combinations thereof. This document also describes processes for producing the oxidative gravure inks described herein and the oxidative gravure inks obtained from them. The oxidative gravure inks described herein can be prepared by mixing all the ingredients, except for one or more driers, and dispersing or grinding them using, for example, a three-roll mill, followed by the addition and mixing of one or more driers.Alternatively, the oxidative drying rotogravure inks described herein may be prepared by mixing all the ingredients, except for one or more polythiol compounds and one or more driers, and dispersing or grinding them using, for example, a three-roll mill, then adding and mixing the one or more polythiol compounds and then adding and mixing the one or more driers. This document also describes security features produced using the oxidative gravure ink described herein and manufactured using the process described herein. This document also describes the uses of these security features for protecting valuable documents against fraud or illegal reproduction. This document also describes valuable documents comprising one or more security features produced with the oxidative gravure ink described herein, such as those described herein. Preferably, the valuable document is selected from banknotes, deeds, notes, checks, receipts, revenue stamps, contracts, identity documents such as passports, identity cards, visas, driver's licenses, bank cards, credit cards, transaction cards, access documents and cards, access tickets, public transport tickets, academic diplomas, and academic degrees. More preferably, the valuable document is a banknote. The security feature described herein may further comprise one or more additional layers or coatings either beneath or above the security feature produced with the oxidative gravure ink described herein. If the adhesion between the substrate and the security feature described herein is insufficient, for example, due to the substrate material, surface irregularity, or lack of surface homogeneity, an additional layer, coating, or primer may be applied between the substrate and the security feature, as known to those skilled in the art. To increase durability through resistance to dirt, chemicals, and cleaning, and thus extend the circulation life of security documents, one or more protective layers may be applied over the one or more security features described herein. When present, these protective layers are typically made with protective varnishes that may be transparent or lightly colored or tinted and may have varying degrees of gloss. The protective varnishes may be radiation-curable compositions, heat-curable compositions, or any combination thereof. Preferably, the protective layers are made with radiation-curable compositions, most preferably UV-Vis curable. EXAMPLES The present invention is described in more detail below with reference to non-limiting Examples. The following Examples provide further details for the preparation and use of oxidative-drying gravure inks for printing a security feature on a substrate by a gravure printing process. In particular, Process Examples E1-E22 according to the present invention and Comparative Process Examples C1-C27 described below provide further details on the preparation of the security feature, the drying properties, and the color properties of the printed samples obtained by the processes described herein. Comparative Examples C28-C39 were prepared according to a comparative lithographic process using oxidative drying lithography inks described in Table 10. A. Preparation of oxidative drying gravure inks The oxidative drying rotogravure inks (I1-I23, Table 4) used to prepare the printed samples (E1-E18 and C1-C23, Table 6) were prepared using four compositions: a black pigment paste (PP), a transparent white composition (TW, Table 1), a mixture of driers (D, Table 2), and a polythiol compound (TH, Table 3). When present, fusible wax (W1-W4) was added during the production of the transparent white color compositions (TW1-TW4), as described in Table 1. When present, the polythiol compound (TH1-TH3) was added to the dispersed mixture of the transparent white color compositions (TW0-TW4) and the black pigment paste (PP), prior to the addition of the driers mixture (D1-D4), to generate the final gravure inks (I1-I23), as described in Table 4. Examples (E1-E14, Table 6) were prepared according to the process of the present invention and using inks comprising one or more fusible waxes and one or more polythiol compounds in the required amount. Comparative Examples C1, C5, C9, and C13-C15 were prepared according to a comparative process and using inks comprising one or more fusible waxes and one or more polythiol compounds. Comparative Examples C2-C4, C6-C8, C10-C12, and C16-C19 were prepared according to the process of the present invention, but using inks lacking one or more fusible waxes or one or more polythiol compounds in the required amount, or both. The Examples (E15-E18, Table 6) were prepared according to the process of the present invention and using inks comprising one or more fusible waxes and one or more polythiol compounds in the required amount. The Comparative Examples C20-C23 were prepared according to a comparative process and using inks comprising one or more fusible waxes, but lacking one or more polythiol compounds in the required amount. The oxidative gravure inks (I24-I31, see Table 8) used to prepare the printed samples (E19-E22 and C24-C27) were prepared using three compositions: an ink premix (IP, Table 7), a drier mix (D, Table 2), and a polythiol compound (TH, Table 3). When present, the polythiol compound (TH1) was added to the ink premixes (IP1-IP2), before the addition of the drier mix (D1-D2), to generate the final gravure inks (I24-I31), as described in Table 8. The Examples (E19-E22, Table 9) were prepared according to the process of the present invention and using inks comprising one or more fusible waxes and one or more polythiol compounds in the required amount. The Comparative Examples C24-C27 were prepared according to a comparative process and using inks comprising one or more fusible waxes, but lacking one or more polythiol compounds in the required amount. Black pigment paste (PP) comprising: 39.1% by weight of an alkyd urethane resin consisting of Urakyd AL210 Q55 from Synres (a soybean oil-based polyurethane-modified long-oil alkyd resin) 18.4 wt% of a phenolic resin consisting of 42.4 wt% phenol-modified rosin ester (Bremapal 2035, Kraemer) that is boiled in 42.4% stick oil (Interfat, CAS No. 8001-20-5) and then diluted with 15.2% n-dodecane (Halterman, CAS No. 112-40-3) 13.8 wt% of CI Pigment Black 7 consisting of Orion Special Black 4A (CAS No. 1333-86-4) 28.7 wt% of an inorganic filler consisting of Omya Omyalite® 50 (limestone, CAS No. 1317-65-3, particle size d50 < 2 micrometers), wherein These ingredients were first weighed and mixed together at room temperature using a SpeedMixer® (DAC 150 SP from Hauschild Engineering) for 3 minutes at 2,500 rpm; and subsequently, a crushing stage was carried out in a Bühler SDY three-roll mill in three passes with a pressure of 0, 5, 1, 1 and 1.1 MPa (5, 11 and 11 bar), respectively, to produce the black pigment paste (PP). The transparent white compositions (TW0-TW4) were prepared, firstly, by weighing the ingredients and mixing them together using a SpeedMixer® (DAC 150 SP from Hauschild Engineering) (3 minutes at 2,500 rpm) and then grinding in a Bühler SDY three-roll mill in three passes with the respective pressures of 0, 5, 1, 1 and 1, 1 MPa (5, 11 and 11 bar). Table 2 Table 3 The dark-colored oxidative drying rotogravure inks (I1-I23) described in Table 4 were prepared according to the following steps: - the black pigment paste (PP), the respective transparent white color compositions (TW0-TW4) and the polythiol compound (TH1-TH3), when present, were weighed and mixed independently using a SpeedMixer® (Hauschild Engineering DAC 150 SP) at room temperature for 90 seconds at 2,500 rpm, and - The dryer mixtures (D1-D4) were added independently to the dispersed pastes obtained in the previous stage and were also mixed with the SpeedMixer® (DAC 150 SP from Hauschild Engineering) for 3 minutes at 2,500 rpm. The viscosity of the dark-colored oxidative drying gravure inks (I1-I23) was measured independently using a Haake Roto Visco RV1 rotary rheometer, using a 20 mm diameter conical plate and a geometry of 0.5°, at a shear rate of 1,000 s-1 and a temperature of 40 °C. B. Preparation of gravure printed samples (E1-E18 and C1-C23) and transfer characteristics (Tables 6A-6D) The dark-colored oxidative-drying gravure inks thus obtained (I1-I23) were applied independently by hand using an Ormag gravure proofing press. An intaglio-etched printing plate was used to prepare intaglio-printed samples (E1-E18 and C1-C23), wherein said plate comprised a set of U-shaped engravings of varying depths (from approximately 20 µm to approximately 100 µm) and widths (from approximately 60 µm to approximately 500 µm), such as to imitate a feature printed in intaglio on a banknote. The intaglio printing plate was inked independently with each of the dark-colored oxidative-drying intaglio inks using a hand-operated polymer roller (step a) ), wherein the plate had a printing plate temperature between 35 °C and 80 °C, as indicated in Tables 6A-6D. After step a) , the excess of these inks was removed by manually washing the printing plate with paper, leaving ink only on the engravings (step b) ). After step b), dark-colored oxidative drying rotogravure inks were applied independently (step c) as a security feature on a blank test sample sheet of cotton fiduciary paper (Louisenthal). After step c), the applied inks were dried independently in the presence of air to form the security feature (step d). The total residence time of the ink (including inking, washing away excess ink, and ink transfer) was approximately 15 seconds. Although dark-colored oxidative gravure inks were exposed to the engraved printing plate for approximately 15 seconds to prepare the samples (E1-E18 and C1-C23) using a laboratory gravure press, industrial processes are much shorter and typically involve exposing the oxidative gravure inks to the engraved printing plate for a time that is typically less than one second. Regarding each gravure-printed sample, six samples were printed. Immediately after the printing process, the printed substrates were stacked to form a pile with a blank test sample sheet sandwiched between them. The pile was placed between two glass plates and maintained for 24 hours at 22 °C and 50% RH (relative humidity) under a pressure of 3 kg to simulate a pile of 1,500 printed substrates. The printed substrates were then removed from the pile, and the blank test sample sheet corresponding to each printed substrate was evaluated for transfer characteristics according to the following procedure: i. An expert in the field selected a series of blank test sample sheets obtained in the same way and corresponding to different levels of transfer to design a visual scale that had values ​​between 1 and 12, where "1" corresponds to maximum transfer (i.e., all the gravure lines were transferred to the blank test sample sheet) and "12" corresponds to minimum transfer (i.e., almost no ink was transferred to the blank test sample sheet), one sample corresponding to one value on the visual scale; ii. The same series of blank test sample sheets was analyzed at the pixel level to provide a correlation curve between the visual scale values ​​and the number of pixels transferred due to transfer. Each sample was scanned using an Epson Perfection V500 scanner at 600 dpi to obtain an image that was analyzed with Photoshop CC. The absolute number of pixels transferred was determined using the histogram and expressed as the percentage of pixels transferred compared to the sample that had "1" as its visual scale value, i.e., the sample exhibiting maximum transfer; iii. A graph was established, as shown in Figure 1, with the percentage of pixels transferred (relative scale obtained in point ii.) on the xy axis and the visual scale on the y axis (circular points). A regression curve (dashed line) was established with the following equation from the experimental data shown in Figure 1: transferred (interpolated) pixels = 2.295 * 10-0.2189 * (visual value) (R2 = 0.9887) iv. According to the equation, the number of transferred pixels decreases significantly (approximately 40%) for each additional value of the visual scale. Table 5 shows the visual value, the absolute number of transferred pixels, the percentage of transferred pixels, as determined in point b), and the interpolated value obtained from the regression curve equation in point iii.; Table 5 v. The blank test sample sheets corresponding to each series of six printed samples (E1-E18 and C1-C23) were evaluated independently by comparing them with the samples on the visual scale described in point i. The values ​​provided in Tables 6A-D correspond to an average of six visual values, one for each printed sample. The number of transferred pixels was interpolated from the average visual value according to the equation shown in point iii. A "6" as a visual value corresponds to approximately 11% of the transferred pixels and indicates an industrially acceptable transfer value in the case of gravure printed samples. The transfer characteristics are provided in Tables 6A-D, where the transfer values ​​are indicated as visual values ​​ranging from 1 to 12, transferred pixels (as estimated in point ii.) and interpolated percentage of transferred pixels (using the equation in point iii.). Determining the transfer characteristics was made easy, since the oxidative drying rotogravure inks (I1-I23) were dark in color and the contrast between the white test sample sheets and the transferred pixels was high (both visually and using the scanner). An assessment of the transfer has been carried out based on the visual values ​​and the interpolated transferred pixels: - very high: a combination of a visual value greater than 8 with an interpolated percentage of transferred pixels less than 5% - Low: A combination of a visual value between 7 and 8 with an interpolated percentage of pixels transferred between 5% and 7% - acceptable: a combination of a visual value between 6 and 7 with an interpolated percentage of transferred pixels between 7% and 11% - unacceptable: a combination of a visual value less than 6 with an interpolated percentage greater than 11%. Table 6A As shown in Table 6A, the intaglio printed samples (C1-C15) obtained according to a process other than that of the present invention, namely, a process using an intaglio engraved printing plate heated to a plate temperature below 45°C, or a process using an oxidative drying intaglio ink lacking one or more fusible waxes, or a process using an oxidative drying intaglio ink lacking polythiol compounds, or a process using an oxidative drying intaglio ink lacking one or more waxes and lacking one or more polythiol compounds, exhibited unacceptable transfer characteristics and were therefore unsuitable for use in high-level applications (such as the preparation of security documents). Contrary to the Comparative Examples, the gravure-printed samples (E1-E9) obtained by the process according to the present invention, namely a process i) using a gravure-engraved printing plate heated to a printing plate temperature between approximately 45 °C and approximately 85 °C, preferably between 50 °C and 80 °C, and ii) using an oxidative-drying gravure ink comprising one or more fusible waxes and one or more polythiol compounds in a total amount exceeding 0.5% by weight, preferably between approximately 0.75% by weight and approximately 2.0% by weight, exhibited acceptable to very low transfer, making them suitable for high-end printing applications. Table 6B As shown in Table 6B, the rotogravure printed samples (C7 and C16) obtained according to a process other than that of the present invention, i.e., a process using an oxidative-drying rotogravure ink lacking one or more polythiol compounds (C7) or comprising one or more polythiol compounds in a total amount not exceeding 0.5% by weight (C16), exhibited unacceptable transfer characteristics and are therefore unsuitable for use in high-level applications (such as the preparation of security documents). Contrary to the Comparative Examples, the gravure-printed samples (E2, E10 and E11) obtained by the process according to the present invention, namely a process i) using a gravure-engraved printing plate heated to a printing plate temperature between approximately 45 °C and approximately 85 °C, preferably between 50 °C and 80 °C, and ii) using an oxidative-drying gravure ink comprising one or more fusible waxes and one or more polythiol compounds (E2, E10 and E11) in a total amount exceeding 0.5% by weight, preferably between approximately 0.75% by weight and approximately 2.0% by weight, exhibited acceptable to low transfer, making them suitable for high-end printing applications. Table 6C As shown in Table 6C, the rotogravure printed samples (C7 and C17-C19) obtained according to a process different from that of the present invention, namely a process using an oxidative-drying rotogravure ink lacking one or more polythiol compounds (C7 and C17-C19), exhibited unacceptable transfer characteristics and were therefore unsuitable for use in high-level applications (such as the preparation of security documents). Contrary to the comparative examples, the intaglio printed samples (E2, E12-E14) obtained by the process according to the present invention, namely a process i) using an intaglio engraved printing plate heated to a printing plate temperature between approximately 45 °C and approximately 85 °C, preferably between 50 °C and 80 °C, and ii) using an oxidatively drying intaglio ink comprising one or more fusible waxes and one or more polythiol compounds (E2, E12-E14), exhibited acceptable to low transfer, making them suitable for high-end printing applications. Table 6D As shown in Table 6D, the rotogravure printed samples (C20-C23) obtained according to a process different from that of the present invention, i.e., a process using an oxidative-drying rotogravure ink lacking one or more polythiol compounds, exhibited unacceptable transfer characteristics and were therefore unsuitable for high-level applications (such as the preparation of security documents). Contrary to the Comparative Examples, the intaglio printed samples (E15-E18) obtained by the process according to the present invention, namely a process i) using an intaglio engraved printing plate heated to a printing plate temperature between approximately 45 °C and approximately 85 °C, preferably between 50 °C and 80 °C, and ii) using an oxidative drying intaglio ink comprising one or more drying agents, one or more fusible waxes and one or more polythiol compounds, exhibited very low transfer, making them suitable for high-end printing applications. C. Preparation of white / transparent oxidative gravure inks (E19-E22 and C24-C27) The white / transparent oxidative drying rotogravure inks (I24-I31, Table 8) used to prepare the printed samples (E19-E22 and C24-C27) were prepared using three compositions: an ink premix (IP1-IP2, Table 7) comprising a fusible wax (W1), a mixture of dryers (D1-D2, Table 2), and a polythiol compound (TH1, Table 3). When present, the polythiol compound (TH1) was added to the ink premix (IP1-IP2), prior to the addition of one of the dryer mixes (D1-D2), to generate the final rotogravure inks (I24-31), as described in Table 8. The Examples (E19-E22) were prepared according to the process of the present invention and using inks comprising one or more fusible waxes and one or more polythiol compounds in the required amount. The Comparative Examples (C24-C27) were prepared according to the process of the present invention and using inks comprising one or more fusible waxes, but lacking one or more polythiol compounds. Table 7 The ingredients described in Table 7 were first weighed and mixed together at room temperature using a SpeedMixer® (Hauschild Engineering DAC 150 SP) for 3 minutes at 2,500 rpm; subsequently, these were ground in a Bühler SDY three-roll mill in three passes with a pressure of 0, 5, 1, 1 and 1, 1 MPa (5, 11 and 11 bar), respectively, to produce the ink premixes (IP1-IP2). Table 8 The white / transparent oxidative gravure inks (I24-I31) described in Table 8 were prepared according to the following steps: - the ink premixes (IP1-IP2) and the polythiol compound (TH1), when present, were weighed and mixed independently using a SpeedMixer® (Hauschild Engineering DAC 150 SP) at room temperature for 90 seconds at 2,500 rpm, and - The dryer mixtures (D1-D2) were added independently to the dispersed pastes obtained in the previous stage and were also mixed with the SpeedMixer® (DAC 150 SP from Hauschild Engineering) for 3 minutes at 2,500 rpm. The viscosity of the white / transparent oxidative drying gravure inks (I24-I31) was measured independently using a Haake Roto Visco RV1 rotary rheometer, using a 20 mm diameter conical plate and a geometry of 0.5°, at a shear rate of 1,000 s-1 and a temperature of 40 °C. D. Preparation of gravure-printed samples (E19-E22 and C24-C27) and evaluation of color properties (Tables 9A-9B) The white / transparent oxidative drying gravure inks thus obtained (I24-I31) were independently applied onto a black / white Leneta card (Leneta Inc., opacity chart of Form 2A) using a semi-automatic laboratory coating device (K101 Control Coater, RK Print) equipped with an HC4 coating bar (nominal thickness of 36 µm). The gravure-printed samples (E19-E22 and C24-C27) were left to dry in the dark for 24 hours. After drying, their L*a*b* values, according to CIELAB (1976), were independently transferred onto the white portion of the Leneta cards using a Datacolor DC 45IR spectrophotometer (measurement geometry: 45 / 0°; spectral analyzer: patented dual-channel holographic grating. Linear arrays of 256 photodiodes were used for both reference and sample channels; light source: full-bandwidth LED illumination). These initial L*, initial a*, and initial b* values ​​are shown in Table 9. All samples were then placed in an oven (40 °C, 55% RH) for 14 days, which corresponds approximately to two months of aging at room temperature. The L*a*b* values ​​of all samples were measured again and appear as L*aged, a*aged and b*aged in Table 9.The L*, a*, b* and E* values ​​indicated in Table 9 correspond to the color variation after 14 days at 40 °C and 55% RH and were obtained by subtracting the "initial" values ​​from the "aged" values. Transparency values ​​(indicated as Tr in Table 9) were obtained by measuring the L*a*b* values ​​on the black portion of the Leneta cards before aging and comparing them with the L*a*b* values ​​measured on the white portion before aging. Tr was obtained using the following equation: A high Tr value indicates high transparency (and therefore low opacity). E. Comparison with lithographic printing The process according to the present invention, namely a gravure printing process comprising a step a) of inking a gravure-engraved printing plate with an oxidative-drying gravure ink, said gravure-engraved printing plate being at a printing plate temperature between approximately 45 °C and approximately 85 °C, said oxidative-drying gravure ink comprising i) at least one oxidative-drying varnish, ii) one or more driers in a total amount of approximately 0.01% by weight to approximately 10% by weight, iii) one or more polythiol compounds present in a total amount exceeding 0.5% by weight, preferably in a total amount of approximately 0.75% by weight and 2% by weight, and iv) one or more fusible waxes present in a total amount of approximately 1% by weight to approximately 10% by weight,It was compared to a similar process using oxidative lithography inks comprising comparable amounts of one or more fusible waxes and polythiol compounds of identical chemical formula. Oxidative lithography inks are comparable to gravure inks, as both are highly viscous inks comprising high molecular weight compounds of similar composition (such as phenolic and urethane alkyd resins) and high amounts of pigments / fillers (typically greater than 20% by weight). The oxidative drying lithography inks (OF1-OF6) were prepared by hand mixing with a spatula at room temperature the ingredients listed in Table 10, except for the dryer mixture. The resulting paste was ground in a Bühler SDY three-roll mill in three passes (the first pass at a pressure of 0.6 MPa (6 bar), and the second and third passes at a pressure of 1.2 MPa (12 bar)). The drier mixture was added to the paste, and approximately 10 g of the resulting compositions were mixed in a SpeedMixer© (Hauschild Engineering DAC 150 SP) at a speed of 2,500 rpm for three minutes at room temperature. The viscosity of the OF1 ink was measured using a Haake Roto Visco RV1 rotary rheometer with a 20 mm diameter conical plate and a 0.5° geometry, at a shear rate of 1,000 s⁻¹ and a temperature of 40 °C. To simulate a lithographic printing process, oxidative-drying lithographic printing inks (OF1-OF6) were applied independently as a pattern (4.5 cm x 23 cm) onto a blank test sample sheet of fiducial cotton substrate (Louisenthal) using a Prüfbau Multipurpose Printability Tester at a pressure of 1,000 N (T = 22 °C, relative humidity = 54%). The amount of ink applied was 1 g / m2 ± 0.05 g / m2. Two series of tests were carried out. In the first series (corresponding to C28-C33 in Table 11), the printed samples were left to dry in the dark for 24 hours at 23 °C and 55% RH before being subjected to the transfer evaluation procedure described below. A second series (corresponding to C34-39 in Table 11) was first placed in an oven at 65 °C for approximately 15 seconds and then left to dry in the dark for 24 hours before being subjected to the transfer evaluation.The process used to obtain the first series (C28-C33) was intended to imitate a classic lithographic printing process (where no heating stage is involved), whereas the process that included a heating stage used to obtain the second series (C34-C39) was more similar to the process of the present invention, where these processes used the oxidative drying lithography inks (OF1-OF6) described in Table 10 instead of oxidative drying rotogravure inks. A transfer test was performed by placing a piece of blank test sample substrate (i.e., an unprinted substrate) on the front side of the substrate bearing the printed and dried layer and subjecting the assembly thus formed to a back pressure of 0.34 MPa (3.4 bar) at 65 °C using an ORMAG Intaglio Proof Press. The substrate bearing the printed and dried layer and the blank test sample substrate were then separated, and the optical density (OD) of the blank test sample substrate was checked to assess ink transfer. The transfer characteristics are provided in Table 11 and have been evaluated according to the following equation: where DO is the value obtained, DOmax is the optical density at the maximum transfer point and corresponds to a drying efficiency of 0%, DOmin is the optical density obtained in total absence of transfer and corresponds to a drying efficiency of 100%. Two blank test samples were prepared to define the two possible extremes of the drying behavior, DOmax and DOmin. DOmax obtained by measuring a sample obtained by back-pressure from a new ink layer directly after printing, DOmin that is measured on an unprinted sample, and Optical density values ​​were obtained using a GretagMacbeth D19C densitometer (average of two measurements). In order to compare these results with the results obtained in the rotogravure printing processes as "pixels transferred in %" provided earlier in this document, this equation was modified to express the drying efficiency in % as "optical density transferred in %": Table 11 While a direct comparison between the values ​​obtained by the gravure printing process according to the Examples of the present invention (E1-E18, pixels transferred (%)) and the comparative gravure processes (C1-C23, pixels transferred (%)) with the values ​​obtained in samples produced by the lithographic printing process examples (C28-C39, optical density transferred (%)) is not relevant, given that the processes are different, relative comparisons are possible. As shown in Table 11, the addition of one or more fusible waxes and / or one or more polythiol compounds to oxidative-drying lithography inks did not affect the drying performance of these inks. In particular, the addition of one or more fusible waxes and one or more polythiol compounds led to a decrease in drying performance or did not significantly improve it.Adding a heating stage (65 °C) after lithographic printing did not significantly improve drying performance (see C38 compared to C32 or C39 compared to C33).

Claims

1. A process for producing a security feature on a substrate by an intaglio printing process comprising a step (a) of inking an intaglio-etched printing plate with an oxidative-drying intaglio ink, said printing plate being intaglio-etched at a printing plate temperature between approximately 45°C and approximately 85°C, said oxidative-drying intaglio ink comprising: (i) at least one oxidative-drying varnish, (ii) one or more driers in a total amount of approximately 0.01% by weight to approximately 10% by weight, (iii) one or more polythiol compounds present in a total amount greater than 0.5% by weight, preferably in a total amount of approximately 0.75% by weight to approximately 2% by weight, and (iv) one or more fusible waxes present in a total amount of approximately 1% by weight to approximately 10% by weight,the weight percentages being based on the total weight of the oxidative gravure ink; a step b) of removing any excess oxidative gravure ink by washing using a paper or fabric washing system or using a polymeric washing cylinder and cleaning said polymeric washing cylinder with an alkaline aqueous washing solution in combination with one or more mechanical means; a step c) of transferring said oxidative gravure ink in the form of the security feature onto the substrate; and a step d) of drying the oxidative gravure ink in the presence of air to form the security feature.

2. The process according to claim 1, wherein the engraved gravure printing plate has a plate temperature between approximately 50°C and approximately 80°C.

3. The process according to claim 1 or 2,wherein at least one oxidative drying varnish is present in the oxidative drying gravure ink in an amount of approximately 10 to approximately 90% by weight, the weight percentages being based on the total weight of the oxidative drying gravure ink.

4. The process according to any one of claims 1 to 3, wherein the one or more polythiol compounds are selected from the group consisting of dithiol compounds, trithiol compounds, tetrathiol compounds, and mixtures thereof.

5. The process according to claim 4, wherein at least one of the dithiol compounds is ethylene glycol bis(3-mercaptopropionate), and / or at least one of the trithiol compounds is tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate, and / or at least one of the tetrathiol compounds is pentaerythritol tetra(3-mercaptopropionate).

6. The process according to any one of claims 1 to 5,wherein one or more fusible waxes have a melting temperature between approximately 50 °C and approximately 120 °C.

7. The process according to claim 6, wherein the one or more waxes are selected from the group consisting of microcrystalline waxes, paraffin waxes, polyethylene waxes, fluorocarbon waxes, polytetrafluoroethylene waxes, Fischer-Tropsch waxes, silicone fluids, beeswax, candelilla waxes, montana waxes, carnauba waxes, rice bran waxes, and mixtures thereof.

8. The process according to any one of claims 1 to 7, wherein the one or more dryers are polyvalent salts containing cobalt, calcium, copper, zinc, iron, zirconium, manganese, barium, zinc, strontium, lithium, vanadium, and potassium as cation(s) and halides, nitrates, sulfates, carboxylates such as acetates, ethylhexanoates, octanoates, and naphthenates or acetoacetonates as anion(s).

9. The process according to claim 8,wherein the oxidative gravure ink further comprises one or more driers that are metal complexes and / or salts of metal complexes, preferably manganese complexes, manganese complex salts, vanadium complexes, vanadium complex salts, iron complexes, and iron complex salts.

10. The process according to any one of claims 1 to 9, wherein the oxidative gravure ink further comprises one or more fillers or thinners in a total amount of approximately 0.1% by weight to approximately 50% by weight, the weight percentages being based on the total weight of the oxidative gravure ink.

11. The process according to claim 10, wherein one or more fillers or diluents are selected from the group consisting of talcs, micas, montmorillonites, bentonites, wollastonites, halloysites, calcined clays, China clays, carbonates, silicates,vermiculite, silica, wood flour, natural fibers, synthetic fibers, and mixtures thereof.

12. The process according to any one of claims 1 to 11, wherein the oxidative gravure ink further comprises one or more coloring components selected from the group consisting of optically variable pigments, colorfast pigments, dyes, and mixtures thereof, preferably selected from the group consisting of organic colorfast pigments, inorganic colorfast pigments, and mixtures thereof.

13. The process according to any one of claims 1 to 12, wherein the oxidative gravure ink further comprises one or more machine-readable materials preferably selected from the group consisting of magnetic materials, luminescent materials, electrically conductive materials,Infrared-absorbing materials and mixtures thereof and / or one or more forensic identifiers.

14. The process according to any one of claims 1 to 13, wherein the substrate is selected from the group consisting of papers or other fibrous materials, paper-containing materials, plastics and polymers, metallized plastics or polymers, composite materials, and mixtures or combinations thereof.

15. A security feature manufactured by a process enumerated in any one of claims 1 to 14.

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