Patent Description:
With the constantly improving quality of color photocopies and printings and in an attempt to protect security documents such as banknotes, value documents or cards, transportation tickets or cards, tax banderols, and product labels that have no reproducible effects against counterfeiting, falsifying or illegal reproduction, it has been the conventional practice to incorporate various security means features in these documents.

Security features, e.g. for security documents, can generally be classified into "covert" security features and "overt" security features. The protection provided by covert security features relies on the concept that such features are difficult to detect, typically requiring specialized equipment and knowledge for detection, whereas "overt" security features rely on the concept of being easily detectable with the unaided human senses, e.g. such features may be visible and/or detectable via the tactile senses while still being difficult to produce and/or to copy. However, the effectiveness of overt security features depends to a great extent on their easy recognition as a security feature, because most users, and particularly those having no prior knowledge of the security features of a document or item secured therewith, will only then actually perform a security check based on said security feature if they have actual knowledge of their existence and nature.

A special role in securing value documents is played by dichroic security features exhibiting a first color upon viewing in transmitted light and a second color different from the first color upon viewing in incident light. To provide a striking effect and draw the layperson's attention, the first color and the second color must have an attractive visual appearance, such as blue, metallic yellow, magenta, and green, and a significant color contrast (for e.g.: blue/metallic yellow, green/metallic yellow, violet/metallic yellow).

Dichroic security features showing a blue color upon viewing in transmitted light and a metallic yellow color upon viewing in incident light may be obtained from silver platelets containing inks.

<CIT> describes solvent-based inks and UV radically curable inks comprising silver platelets for producing dichroic security, or decorative elements showing a gold/copper color in reflection and a blue color in transmission. Said inks contain high concentrations of silver platelets being characterized by a weight ratio between the silver platelets and the binder of <NUM> : <NUM>. The high concentration of silver platelets in the inks used for obtaining the security, or decorative element described by <CIT> is detrimental to the mechanical resistance of the produced security, or decorative element, and additionally, renders the production process of said element expensive. Further, the mechanical resistance of the security, or decorative element, described by <CIT> is impaired by the use of UV radically curable inks or solvent-based inks, which as well known to the skilled person, provide cured coatings with limited mechanical resistance. As the mechanical resistance is an essential property for security elements and the manufacturing process described by <CIT> is lengthy and rather expensive, the inks and the manufacturing process described therein are not suitable for the industrial production on value documents of dichroic security features with acceptable mechanical resistance.

International patent application publication number <CIT> describes the use of a UV curable ink containing silver platelets, a radically curable binder and an important amount of organic solvent for coating a surface of a holographic structure. The coated holographic structure shows on the embossed surface a blue color with strong chroma in transmission and a yellow color with a low chroma value in reflection. Although the UV curable ink described by <CIT> contains a lower concentration of silver platelets when compared to the UV radiation radically curable ink described by <CIT>, said ink is still not suitable for the industrial production of dichroic features on value documents because on one side the increased amount of organic solvent is not environmentally friendly and requires an additional air drying step prior to the UV-curing step, and on the other side the coatings obtained with said ink have limited mechanical resistance, as well as low chroma in reflection.

Typically, industrial printing of value documents requires high printing speeds of about <NUM>'<NUM> sheets/hour, wherein from each sheet an important number of value documents is produced. For illustrative purpose, in the field of banknotes printing, up to <NUM> value documents, each containing one or more security features, may be produced from one sheet. To be suitable for implementation on a production line, it is essential that the production process of each printable security feature present on a value document complies with the high-speed requirements of industrial printing of value documents. Therefore, a need remains for stable security inks for producing on value documents at high speed (i.e. industrial speed) dichroic security features having improved mechanical resistance and exhibiting a blue color upon viewing in transmitted light and a metallic yellow color upon viewing in incident light.

Accordingly, it is the object of the present invention to provide UV-Vis radiation cationically curable security inks and UV-Vis radiation hybrid curable security inks for producing on value documents at high speed (i.e. industrial speed) dichroic security features having improved mechanical resistance and exhibiting a blue color upon viewing in transmitted light and a metallic yellow color upon viewing in incident light. This is achieved by the UV-VIS radiation curable security ink claimed herein, wherein said ink comprises:.

A further aspect according to the present invention is directed to a process for producing a security feature for securing a value document, wherein said security feature exhibits a blue color upon viewing in transmitted light and a metallic yellow color upon viewing in incident light, said process comprising the following steps:.

The following definitions are to be used to interpret the meaning of the terms discussed in the description and recited in the claims.

As used herein, the article "a/an" indicates one as well as more than one and does not necessarily limit its referent noun to the singular.

As used herein, the term "about" means that the amount or value in question may be the specific value designated or some other value in its neighborhood. Generally, the term "about" denoting a certain value is intended to denote a range within ± <NUM>% of the value. As one example, the phrase "about <NUM>" denotes a range of <NUM> ± <NUM>, i.e. the range from <NUM> to <NUM>. Preferably, the range denoted by the term "about" denotes a range within ± <NUM>% of the value, more preferably ± <NUM> %. Generally, when the term "about" is used, it can be expected that similar results or effects according to the invention can be obtained within a range of ±<NUM>% of the indicated value.

As used herein, the term "and/or" means that either all or only one of the elements of said group may be present. For example, "A and/or B" means "only A, or only B, or both A and B". In the case of "only A", the term also covers the possibility that B is absent, i.e. "only A, but not B".

The term "comprising" as used herein is intended to be non-exclusive and open-ended. Thus, for instance a solution comprising a compound A may include other compounds besides A. However, the term "comprising" also covers, as a particular embodiment thereof, the more restrictive meanings of "consisting essentially of' and "consisting of", so that for instance "a solution comprising A, B, and optionally C" may also (essentially) consist of A, and B, or (essentially) consist of A, B, and C.

Where the present description refers to "preferred" embodiments/features, combinations of these "preferred" embodiments/features are also deemed to be disclosed as long as the specific combination of "preferred" embodiments/features is technically meaningful.

As used herein, the term "one or more" means one, two, three, four, etc..

The term "UV-Vis curable" and "UV-Vis curing" refers to radiation-curing by photo-polymerization, under the influence of an irradiation having wavelength components in the UV or in the UV and vsible part of the electromagnetic spectrum (typically <NUM> to <NUM>, preferably between <NUM> and <NUM> and more preferably between <NUM> and <NUM>).

Surprisingly, it has been found that a UV-Vis radiation curable security ink comprising:.

The security feature made with the UV-Vis radiation curable security ink claimed herein exhibits a blue color upon viewing in transmitted light i.e. in transmission. For the purposes of the present invention, viewing in transmitted light means that the security feature is illuminated from one side, for example by holding said security feature against the daylight or in front of a light source, and viewed from the opposite side. Independently of the side from which the security feature is viewed in transmitted light, a blue color is observed. For the purposes of this invention, a security feature exhibiting a blue color refers to a security feature exhibiting a blue color characterized by a chroma value C* (corresponding to a measure of the color intensity or color saturation) higher than <NUM>. An intense to very intense blue color is characterized by a chroma value C* higher than <NUM>. The chroma value C* is calculated from a* and b* values according to the CIELAB (<NUM>) color space, wherein <MAT>.

Said a* and b* values in transmitted light are measured using a Datacolor <NUM> spectrophotometer (parameters: integration sphere, diffuse illumination (pulse xenon D65) and <NUM>° viewing, analyzer SP2000 with dual <NUM> diode array for wavelength range of <NUM>-<NUM>, transmission sampling aperture size of <NUM>).

The security feature made with the UV-Vis radiation curable security ink claimed herein exhibits a metallic yellow color or gold color upon viewing in incident light i.e. in reflection. In the present patent application, the terms "metallic yellow color" and "gold color" are used interchangeably. For the purpose of the present invention, "viewing in incident light" means that the sec urity feature is illuminated from the side printed with the security ink claimed herein and viewed from the same side. For the purpose of the present invention, a security feature exhibiting a metallic yellow color or gold color refers to a security feature exhibiting a yellow color characterized by a chroma value C* (corresponding to a measure of the color intensity or color saturation) higher than <NUM> as calculated from a* and b* values according to the CIELAB (<NUM>) color space, wherein <MAT> and wherein said a* and b* values of the security feature were measured at <NUM>° to the normal with an illumination angle of <NUM>° using a goniometer (Goniospektrometer Codec WI-<NUM><NUM>&<NUM> by Phyma GmbH Austria).

The UV-Vis radiation curable security ink claimed and described herein is preferably selected from a screen-printing security ink, a rotogravure security ink, and a flexography security ink. Preferably, the UV-Vis radiation curable security ink claimed herein is characterized by a viscosity of between about <NUM> mPas and about <NUM> mPas at <NUM> measured using a Brookfield viscometer (model "DV-I Prime) equipped with a spindle S27 at <NUM> rpm, or with a spindle S21 at <NUM> rpm for measuring viscosities between <NUM> and <NUM> mPas, and a spindle S21 at <NUM> rpm for measuring viscosities equal to or lower than <NUM> mPas. The UV-Vis radiation curable screen-printing security ink claimed herein is characterized by a viscosity of between about <NUM> mPas and about <NUM> mPas at <NUM>, preferably of between about <NUM> mPas and about <NUM> mPas at <NUM>.

As known by those skilled in the art, the term rotogravure refers to a printing process which is described for example in Handbook of Print Media, Helmut Kipphan, Springer Edition, page <NUM>. Rotogravure is a printing process wherein image elements are engraved into the surface of the cylinder. The non-image areas are at a constant original level. Prior to printing, the entire printing plate (non-printing and printing elements) is inked and flooded with ink. Ink is removed from the non-image by a wiper or a blade before printing, so that ink remains only in the cells. The image is transferred from the cells to the substrate by a pressure typically in the range of <NUM> to <NUM> bars and by the adhesive forces between the substrate and the ink. The term rotogravure does not encompass intaglio printing processes (also referred in the art as engraved steel die or copper plate printing processes) which rely for example on a different type of ink.

Flexography printing processes preferably use a unit with a chambered doctor blade, an anilox roller and plate cylinder. The anilox roller advantageously has small cells whose volume and/or density determines the ink or varnish application rate. The chambered doctor blade lies against the anilox roller, filling the cells and scraping off surplus ink or varnish at the same time. The anilox roller transfers the ink to the plate cylinder which finally transfers the ink to the substrate. Plate cylinders can be made from polymeric or elastomeric materials. Polymers are mainly used as photopolymer in plates and sometimes as a seamless coating on a sleeve. Photopolymer plates are made from light-sensitive polymers that are hardened by ultraviolet (UV) light. Photopolymer plates are cut to the required size and placed in an UV light exposure unit. One side of the plate is completely exposed to UV light to harden or cure the base of the plate. The plate is then turned over, a negative of the job is mounted over the uncured side and the plate is further exposed to UV light. This hardens the plate in the image areas. The plate is then processed to remove the unhardened photopolymer from the non-image areas, which lowers the plate surface in these non-image areas. After processing, the plate is dried and given a post-exposure dose of UV light to cure the whole plate. Preparation of plate cylinders for flexography is described in <NPL>.

As well known to those skilled in the art, screen printing (also referred in the art as silkscreen printing) is a printing technique that typically uses a screen made of woven mesh to support an ink-blocking stencil. The attached stencil forms open areas of mesh that transfer ink as a sharp-edged image onto a substrate. A squeegee is moved across the screen with ink-blocking stencil, forcing ink past the threads of the woven mesh in the open areas. A significant characteristic of screen printing is that a greater thickness of the ink can be applied to the substrate than with other printing techniques. Screen-printing is therefore also preferred when ink deposits with the thickness having a value between about <NUM> to <NUM> or greater are required which cannot (easily) be achieved with other printing techniques. Generally, a screen is made of a piece of porous, finely woven fabric called mesh stretched over a frame of e.g. aluminum or wood. Currently most meshes are made of man-made materials such as synthetic or steel threads. Preferred synthetic materials are nylon or polyester threads.

In addition to screens made on the basis of a woven mesh based on synthetic or metal threads, screens have been developed out of a solid metal sheet with a grid of holes. Such screens are prepared by a process comprising of electrolytically forming a metal screen by forming in a first electrolytic bath a screen skeleton upon a matrix provided with a separating agent, stripping the formed screen skeleton from the matrix and subjecting the screen skeleton to an electrolysis in a second electrolytic bath in order to deposit metal onto said skeleton.

There are three types of screen-printing presses, namely flat-bed, cylinder and rotary screen-printing presses. Flat-bed and cylinder screen printing presses are similar in that both use a flat screen and a three-step reciprocating process to perform the printing operation. The screen is first moved into position over the substrate, the squeegee is then pressed against the mesh and drawn over the image area, and then the screen is lifted away from the substrate to complete the process. With a flat-bed press the substrate to be printed is typically positioned on a horizontal print bed that is parallel to the screen. With a cylinder press the substrate is mounted on a cylinder. Flat-bed and cylinder screen printing processes are discontinuous processes, and consequently limited in speed which is generally at maximum <NUM>/min in web or <NUM>'<NUM> sheets/hour in a sheet-fed process.

Conversely, rotary screen presses are designed for continuous, high speed printing. The screens used on rotary screen presses are for instance thin metal cylinders that are usually obtained using the electroforming method described hereabove or made of woven steel threads. The open-ended cylinders are capped at both ends and fitted into blocks at the side of the press. During printing, ink is pumped into one end of the cylinder so that a fresh supply is constantly maintained. The squeegee is fixed inside the rotating screen and squeegee pressure is maintained and adjusted to allow a good and constant print quality. The advantage of rotary screen presses is the speed which can reach easily <NUM>/min in web or <NUM>'<NUM> sheets/hour in a sheet-fed process.

Screen printing is further described for example in <NPL>, in<NPL> and in <NPL>.

More preferably the UV-VIS radiation curable security ink claimed and described herein is a screen-printing security ink. Such UV-Vis radiation curable screen-printing security ink is particularly useful for the industrial manufacturing of dichroic security features on value documents because it enables printing at very high-speed of dichroic security features having large thicknesses of at least about <NUM>.

The UV-VIS radiation curable ink claimed and described herein contains a) from about <NUM> wt-% to about <NUM> wt-%, preferably from about <NUM> wt-% to about <NUM> wt-%, more preferably from about <NUM> wt-% to about <NUM> wt-%, of silver nanoplatelets having a mean diameter in the range of <NUM> to <NUM> with a standard deviation of less than <NUM>%, a mean thickness in the range of <NUM> to <NUM> with a standard deviation of less than <NUM>%, and a mean aspect ratio higher than <NUM>, wherein the mean diameter is determined by transmission electron microscopy and the mean thickness is determined by transmission electron microscopy, and wherein the silver nanoplatelets bear a surface stabilizing agent of general formula (I)
<CHM>
wherein.

The silver nanoplatelets described herein bearing the surface stabilizing agent of general formula (I) are readily dispersible in the vehicle of the UV-Vis radiation curable security ink claimed herein. Upon printing, the silver nanoplatelets described herein migrate from the mass of the ink layer obtained with the UV-Vis radiation curable security ink claimed herein at the interface between the ink layer and air and at the interface between the ink layer and the substrate and align themselves to form a thin layer of silver nanoplatelets at said interfaces, thereby leading to the expedient development of the metallic yellow color observed in incident light. This property of the UV-Vis radiation curable security ink claimed herein is particularly advantageous because on one side, the time required for the development of the metallic yellow color is compatible with the high-speed requirements of industrial printing of value documents, and on the other side, it enables production of dichroic security features with inks containing amounts of silver nanoplatelets as low as <NUM> wt-%, which drastically reduces the production costs, especially for dichroic security features having a large thickness of at least about <NUM>. Depending on the thickness of the dichroic security feature to be produced and the composition of the ink vehicle, the amount of the silver nanoplatelets in the UV-Vis radiation curable security ink can be adjusted so that the metallic yellow color in reflected light is rapidly developed without impacting the hue and chroma of the blue color in transmitted light.

The silver nanoplatelets contained by the UV-Vis may be in the form of disks, regular hexagons, triangles, especially equilateral triangles, and truncated triangles, especially truncated equilateral triangles, or mixtures thereof. They are preferably in the form of disks, truncated triangles, hexagons, or mixtures thereof.

The mean diameter of the silver nanoplatelets is in the range of <NUM> to <NUM>, preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>, with a standard deviation of less than <NUM>%, preferably less than <NUM>%. The diameter of a silver nanoplatelet is the longest dimension of said silver nanoplatelet and corresponds to the maximum dimension of said silver nanoplatelet when oriented parallel to the plane of a transmission electron microscopy (TEM) image. As used herein, the term "mean diameter of the silver nanoplatelets" refers to the mean diameter determined by transmission electron microscopy (TEM) using Fiji image analysis software based on the measurement of at least <NUM> randomly selected silver nanoplatelets oriented parallel to the plane of a transmission electron microscopy image (TEM), wherein the diameter of a silver nanoplatelet is the maximum dimension of said silver nanoplatelet oriented parallel to the plane of a transmission electron microscopy image (TEM). TEM analysis was conducted using an EM <NUM> instrument from ZEISS in bright field mode at an e-beam acceleration voltage of 100kV. A dispersion of silver nanoplatelets in isopropanol at a suitable concentration, preferably lower than <NUM> wt-%, was used for conducting the TEM analysis.

The mean thickness of the silver nanoplatelets is in the range of <NUM> to <NUM>, preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>, with a standard deviation of less than <NUM>%, preferably less than <NUM>%. The thickness of a silver nanoplatelet is the shortest dimension of said nanoplatelet and corresponds to the maximum thickness of said silver nanoplatelet. As used herein, the term "mean thickness of the silver nanoplatelets" refers to the mean thickness determined by transmission electron microscopy (TEM) based on the manual measurement of at least <NUM> randomly selected silver nanoplatelets oriented perpendicular to the plane of the TEM image, wherein the thickness of the silver nanoplatelet is the maximum thickness of said silver nanoplatelet. TEM analysis was conducted using an EM <NUM> instrument from ZEISS in bright field mode at an e-beam acceleration voltage of 100kV. A dispersion of silver nanoplatelets in isopropanol at a suitable concentration, preferably lower than <NUM> wt-%, was used for conducting the TEM analysis.

The mean aspect ratio of the silver nanoplatelets (defined as the ratio between the mean diameter and the mean thickness) is larger than <NUM>, preferably larger than <NUM> and more preferably larger than <NUM>.

Preferably, the mean diameter of the silver nanoplatelets is in the range of <NUM> to <NUM> with the standard deviation being less than <NUM>%, the mean thickness of said silver nanoplatelets is in the range of <NUM> to <NUM> with the standard delation being less than <NUM>% and the mean aspect ratio of said silver nanoplatelets is higher than <NUM>.

The silver nanoplatelets used in the UV-Vis radiation curable ink described herein are characterized by a highest wavelength absorption maximum of between <NUM> and <NUM>, preferably <NUM> and <NUM>, most preferably <NUM> to <NUM>. The highest wavelength absorption maximum was measured in water at ca. <NUM>*<NUM>-<NUM> M (mol/l) concentration of silver using a Varian Cary <NUM> UV-Visible spectrophotometer. The absorption maximum has a full width at half maximum (FWHM) value in the range of <NUM> to <NUM>, preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>. The molar extinction coefficient of the silver nanoplatelets, measured at the highest wavelength absorption maximum, is higher than <NUM>(cm*molAg), especially higher than <NUM>/(cm*molAg), very especially higher than <NUM>/(cm*molAg).

The silver nanoplatelets contained by the UV-Vis radiation curable ink claimed herein bear a surface stabilizing agent of general formula (I)
<CHM>
wherein the residue RA is a C<NUM>-C<NUM>alkyl group substituted with a hydroxy group; the residue RB is selected from a C<NUM>-C<NUM>alkyl group, and a C<NUM>-C<NUM>alkyl group substituted with a hydroxy group; and Cat+ is an ammonium cation of general formula +NH<NUM>RCRD, wherein the residue RC is a C<NUM>-C<NUM>alkyl group substituted with a hydroxy group; and the residue RD is selected from a C<NUM>-C<NUM>alkyl group, and a C<NUM>-C<NUM>alkyl group substituted with a hydroxy group. Without being bound by the theory, it is believed that the surface stabilizing agent of general formula (I) besides preventing the agglomeration and sedimentation of the silver nanoplatelets in the security ink claimed herein, aids in promoting migration of the silver nanoplatelets from the mass of the ink layer obtained with the security ink claimed herein at the interface between the ink layer and air and at the interface between the ink layer and the substrate.

The surface stabilizing agent of general formula (I) may be present in an amount from about <NUM>% to about <NUM>%, preferably from about <NUM>% to about <NUM>%, and more preferably in an amount of <NUM>%, of the weight percent (wt-%) of the silver nanoplatelets.

The term "C<NUM>-C<NUM>alkyl group" as used herein refers to a saturated linear or branched-chain monovalent hydrocarbon radical of one to four carbon atoms (C<NUM>-C<NUM>). Examples of C<NUM>-C<NUM>alkyl groups include methyl (Me, -CH<NUM>), ethyl (Et, -CH<NUM>CH<NUM>), <NUM>-propyl (n-Pr, n-propyl, -CH<NUM>CH<NUM>CH<NUM>), <NUM>-propyl (i-Pr, iso-propyl, -CH(CH<NUM>)<NUM>), <NUM>-butyl (n-Bu, n-butyl, -CH<NUM>CH<NUM>CH<NUM>CH<NUM>), <NUM>-methyl-<NUM>-propyl (i-Bu, i-butyl, -CH<NUM>CH(CH<NUM>)<NUM>), <NUM>-butyl (s-Bu, s-butyl, -CH(CH<NUM>)CH<NUM>CH<NUM>) and <NUM>-methyl-<NUM>-propyl (t-Bu, t-butyl, -C(CH<NUM>)<NUM>).

The term "C<NUM>-C<NUM>alkyl group substituted with a hydroxy group" refers to a linear or branched alkyl group having two to four carbon atoms, which is substituted by a hydroxy group (-OH). The C<NUM>-C<NUM>alkyl group may be substituted by one or two hydroxy groups.

In general formula (I), the residue RA may be a C<NUM>-C<NUM>alkyl group substituted with two hydroxy groups and the residue RB may be a C<NUM>-C<NUM> alkyl group.

In a preferred embodiment according to the present invention, the residues RA and RB are independently of each other a C<NUM>-C<NUM>alkyl group substituted with a hydroxy group, preferably one hydroxy group. Thus, in an embodiment according to the present invention the residues RA and RB are independently of each other selected from the group consisting of: -CH<NUM>CH<NUM>OH, -CH<NUM>CH(OH)CH<NUM>, -CH<NUM>CH<NUM>CH<NUM>OH, -CH(CH<NUM>)(CH<NUM>OH), -CH<NUM>CH(OH)CH<NUM>CH<NUM>, -CH<NUM>CH<NUM>CH(OH)CH<NUM> -CH<NUM>CH<NUM>CH<NUM>CH<NUM>OH, -CH(CH<NUM>)CH(OH)CH<NUM>, -CH(CH<NUM>OH)CH<NUM>CH<NUM>, -CH(CH<NUM>)CH<NUM>CH<NUM>OH, -CH<NUM>CH(CH<NUM>OH)CH<NUM>, -CH<NUM>C(CH<NUM>)(OH)CH<NUM>, -CH<NUM>CH(CH<NUM>)CH<NUM>(OH), -CH<NUM>C(OH)(CH<NUM>)<NUM>, -CH<NUM>C(CH<NUM>)(CH<NUM>OH), more preferably selected from the group consisting of: -CH<NUM>CH<NUM>OH, -CH<NUM>CH(OH)CH<NUM>, and -CH<NUM>CH<NUM>CH<NUM>OH. The residues RA and RB may be the identical, or may be different.

In general formula (I), the residue RC may be a C<NUM>-C<NUM>alkyl group substituted with two hydroxy groups and the residue RD may be a C<NUM>-C<NUM> alkyl group.

In a preferred embodiment according to the present invention, the residues RC and RD are independently of each other a C<NUM>-C<NUM>alkyl group substituted with a hydroxy group, preferably one hydroxy group. Thus, in an embodiment according to the present invention the residues RC and RD are independently of each other selected from the group consisting of: -CH<NUM>CH<NUM>OH, -CH<NUM>CH(OH)CH<NUM>, -CH<NUM>CH<NUM>CH<NUM>OH, -CH(CH<NUM>)(CH<NUM>OH), -CH<NUM>CH(OH)CH<NUM>CH<NUM>, -CH<NUM>CH<NUM>CH(OH)CH<NUM>, -CH<NUM>CH<NUM>CH<NUM>CH<NUM>OH, -CH(CH<NUM>)CH(OH)CH<NUM>, -CH(CH<NUM>OH)CH<NUM>CH<NUM>, -CH(CH<NUM>)CH<NUM>CH<NUM>OH, -CH<NUM>CH(CH<NUM>OH)CH<NUM>, -CH<NUM>C(CH<NUM>)(OH)CH<NUM>, -CH<NUM>CH(CH<NUM>)CH<NUM>(OH), -CH<NUM>C(OH)(CH<NUM>)<NUM>, -CH<NUM>C(CH<NUM>)(CH<NUM>OH), more preferably selected from the group consisting of: -CH<NUM>CH<NUM>OH, -CH<NUM>CH(OH)CH<NUM>, and -CH<NUM>CH<NUM>CH<NUM>OH. The residues RC and RD may be the identical, or may be different.

Preferably, in general formula (I) the residues RA, RB, RC and RD are independently of each other a C<NUM>-C<NUM>alkyl group substituted with one hydroxy group. More preferably, in general formula (I) the residues RA, RB, RC and RD are independently of each other selected from the group consisting of: -CH<NUM>CH<NUM>OH, -CH<NUM>CH(OH)CH<NUM>, and -CH<NUM>CH<NUM>CH<NUM>OH. Even more preferably, in general formula (I) the residues RA, RB, RC and RD represent -CH<NUM>CH<NUM>OH.

To prevent agglomeration and sedimentation of the silver nanoplatelets upon storage, the silver nanoplatelets may bear on their surface further surface stabilizing agents.

In a preferred embodiment, the silver nanoplatelets bear on their surface a further surface stabilizing agent of general formula (II)
<CHM>
wherein.

The surface stabilizing agent of general formula (II) has preferably an average molecular weight (Mn) of from <NUM> to <NUM> [g/mol], and more preferably from <NUM> to <NUM> [g/mol], most preferably from <NUM> to <NUM> [g/mol].

If the surface stabilizing agent of formula (I) comprises, for example, ethylene oxide units (EO) and propylene oxide units (PO), the order of (EO) and (PO) may be fixed (block copolymers), or may not be fixed (random copolymers).

Preferably, in general formula (II), R<NUM> is H, or C<NUM>-C<NUM>alkyl, R<NUM>, R<NUM>, R<NUM>, R<NUM>, R<NUM> and R<NUM> are independently of each other H, CH<NUM>, or C<NUM>H<NUM>, k1 is an integer in the range of from <NUM> to <NUM>, k2 and k3 are independently of each other <NUM>, or integers in the range of from <NUM> to <NUM>, k4 is <NUM>, or <NUM>, and k5 is an integer in the range of from <NUM> to <NUM>. More preferably, in general formula (II) R<NUM> is H, or C<NUM>-C<NUM>alkyl, R<NUM>, R<NUM>, R<NUM>, R<NUM>, R<NUM> and R<NUM> are independently of each other H, or CH<NUM>, k1 is an integer in the range of from <NUM> to <NUM>, k2 and k3 are independently of each other <NUM>, or integers in the range of from <NUM> to <NUM>, k4 is <NUM>, k5 is an integer in the range of from <NUM> to <NUM>.

The most preferred surface stabilizing agent of general formula (II) has the general formula (II-a)
<CHM>
wherein.

The preferred surface stabilizing agents of general formula (II) are derived from MPEG thiols (polyethylene glycol) methyl ether thiols) having an average molecular weight (Mn) of <NUM> to <NUM>, such as, for example, MPEG <NUM> thiol, MPEG <NUM> thiol, MPEG <NUM> thiol, MPEG <NUM> thiol, MPEG <NUM> thiol, PEG thiols (O-(<NUM>-mercaptoethyl)-poly(ethylene glycol)) having an average Mn of <NUM> to <NUM>, such as, for example, PEG <NUM> thiol, PEG <NUM> thiol, PEG <NUM> thiol, PEG <NUM> thiol, PEG <NUM> thiol.

The silver nanoplatelets contained by the security ink may further bear a surface stabilizing agent which is a polymer, or copolymer described in <CIT>, which can be obtained by a process comprising the steps:.

The monomer in step i-<NUM>) or i-<NUM>) is preferably selected from <NUM>-vinyl-pyridine or pyridinium-ion, <NUM>-vinyl-pyridine or pyridinium-ion, <NUM>-vinyl-imidazole or imidazolinium-ion, or a compound of formula CH<NUM>=C(Ra)-(C=Z)-Rb, wherein.

The second step ii) is preferably a transesterification reaction. In step ii) the alcohol is preferably an ethoxylate of formula Rc-[O-CH<NUM>-CH<NUM>-]c-OH, wherein Rc is saturated or unsaturated, linear or branched chain alkyl with <NUM> - <NUM> carbon atoms, or alkylaryl or dialkylaryl with up to <NUM> carbon atoms and c is <NUM> to <NUM>.

Preferably, step i-<NUM>) or i-<NUM>) is carried out twice and a block copolymer is obtained wherein in the first or second radical polymerization step the monomer or monomer mixture contains <NUM> to <NUM>% by weight, based on total monomers, of a C<NUM>-C<NUM> alkyl ester of acrylic or methacrylic acid and in the second or first radical polymerization step respectively, the ethylenically unsaturated monomer or monomer mixture contains at least a monomer without primary or secondary ester bond.

In the first polymerization step, the monomer or monomer mixture contains from <NUM> to <NUM>% by weight based on total monomers of a C<NUM>-C<NUM>alkyl ester of acrylic or methacrylic acid (first monomer) and in the second polymerization step the ethylenically unsaturated monomer or monomer mixture comprises <NUM>-vinyl-pyridine or pyridinium-ion, <NUM>-vinyl-pyridine or pyridinium-ion, wnyl-imidazole or imidazolinium-ion, <NUM>-dimethylaminoethylacrylamide, <NUM>-dimethylaminoethylmethacrylamide, or corresponding ammonium ion, <NUM>-dimethylaminopropylacrylamide, or corresponding ammonium ion, or <NUM>-dimethylaminopropylmethacrylamide, or corresponding ammonium ion (second monomer).

Preferably, the nitroxylether has the following structure
<CHM>.

The surface stabilization agent is preferably a copolymer which can be obtained by a process comprising the steps:.

Preferably the surface stabilizing agent obtained via the process described herein is a copolymer of the following formula (III)
<CHM>
wherein.

Surface stabilizing agents of general formula (III) have been described in the international patent application publication number <CIT>.

A preferred surface stabilizing agent of general formula (III) is a compound of general formula (III-a)
<CHM>
wherein.

Examples of preferred copolymers to be used as surface stabilizing agents are the copolymers described in Example A3 and Example A6 of <CIT>.

To improve the stability of optical properties of the silver nanoplatelets upon storage or heat exposure, said silver nanoplatelets may bear a further surface stabilizing agent of general formula (IV)
<CHM>
wherein.

Examples of compounds of formula (IV) include, but are not limited to:
<CHM>
<CHM>
<CHM>.

A dispersion of silver nanoplatelets to be used for preparing the UV-Vis radiation curable security ink claimed herein may be obtained by using the method comprising the following steps:.

The silver precursor is a silver(I) compound selected from the group consisting of: AgNO<NUM>; AgClO<NUM>; Ag<NUM>SO<NUM>; AgCI; AgF; AgOH; Ag<NUM>O; AgBF<NUM>; AgIO<NUM>; AgPF<NUM>; R<NUM>CO<NUM>Ag, R<NUM>SO<NUM>Ag, wherein R<NUM> is unsubstituted or substituted C<NUM>-C<NUM>alkyl, unsubstituted or substituted Cs-Cscycloalkyl, unsubstituted or substituted C<NUM>-C<NUM>aralkyl, unsubstituted or substituted C<NUM>-C<NUM>aryl or unsubstituted or substituted C<NUM>-C<NUM>heteroaryl; Ag salts of dicarboxylic, tricarboxylic, polycarboxylic acids, polysulfonic acids, P-containing acids and mixtures thereof, preferably from the group consisting of: silver nitrate, silver acetate, silver perchlorate, silver methanesulfonate, silver benzenesulfonate, silver toluenesulfonate silver trifluoromethanesulfonate, silver sulfate, silver fluoride and mixtures thereof, and more preferably is silver nitrate.

The reducing agent is selected from the group consisting of alkali, or alkaline earth metal borohydrides, such as sodium borohydride, alkali, or alkaline earth metal acyloxyborohydrides, such as sodium triacetoxyborohydride, alkali, or alkaline earth metal alkoxy- or aryloxyborohydrides, such as sodium trimethoxyborohydride, aryloxyboranes, such as catecholborane, and amine-borane complexes, such as diethylaniline borane, tert-butylamine borane, morpholine borane, dimethylamine borane, triethylamine borane, pyridine borane, ammonia borane and mixtures thereof. Sodium borohydride is most preferred.

The one or more complexing agents are selected from the group of chlor-containing compounds, which are capable to liberate chloride ions under reaction conditions, such as metal chlorides, alkyl or aryl ammonium chlorides, phosphonium chlorides; primary or secondary amines and corresponding ammonium salts, such as methyl amine or dimethylamine; ammonia and corresponding ammonium salts; and aminocarboxylic acids and their salts, such as ethylenediaminetetraacetic acid.

Non limiting examples of complexing agents include ammonia, methylamine, dimethylamine, ethylamine, ethylenediamine, diethylenetriamine, ethylene-diamine-tetraacetic acid (EDTA); ethylenediamine N,N'-disuccinic acid (EDDS); methyl glycine diacetic acid (MGDA); diethylene triamine pentaacetic acid (DTPA); propylene diamine tetracetic acid (PDTA); glutamic acid N,N-diacetic acid (N,N-dicarboxymethyl glutamic acid tetrasodium salt (GLDA); nitrilotriacetic acid (NTA), and any salts thereof; N-hydroxyethylethylenediaminetri-acetic acid (HEDTA), triethylenetetraaminehexaacetic acid (TTHA), N-hydroxyethyliminodiacetic acid (HEIDA), dihydroxyethylglycine (DHEG), ethylenediaminetetrapropionic acid (EDTP) and derivatives thereof, such as, for example, trisodium salt of methylglycinediacetic acid (Na<NUM>MGDA) and tetrasodium salt of EDTA.

The defoamer is a compound or composition, capable to suppress foam formation in the reaction mixture, such as, for example, commercially available TEGO® Foamex <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, 815N, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, K <NUM>, K <NUM>, K <NUM>, N, Antifoam SE-<NUM> from Sigma, Struktol SB-<NUM> and the like. The amount of the defoamer is in the range of from <NUM> % to <NUM>% by weight based on total weight of reaction mixture prior to hydrogen peroxide addition, preferably from <NUM> % to <NUM>% and more preferably from <NUM> % to <NUM> % by weight.

The defoamer can be added to the solution prepared at step <NUM>) and/or to the solution prepared at step <NUM>).

The reaction of silver nanoplatelets formation is carried out by gradually adding the silver precursor solution to the reducing agent solution, whereas the temperature of both solutions is in the range of -<NUM> to <NUM> and the gradual addition is completed within <NUM> minutes to <NUM> time.

The silver nanoplatelets obtained at step <NUM>) and/or <NUM>) can be submitted to further purification and/or isolation methods, such as decantation, (ultra)filtration, (ultra)centrifugation, reversible or irreversible agglomeration, phase transfer with organic solvent, and combinations thereof. The dispersion of silver nanoplatelets may contain up to about <NUM> wt-% silver nanoplatelets, preferably from <NUM> wt-% to <NUM> wt-% silver nanoplatelets, more preferably from <NUM> wt-% to <NUM> wt-% silver nanoplatelets, the wt-% being based on the total weight of the dispersion.

Starting from the silver nanoplatelets obtained by purification and/or isolation, the silver nanoplatelets bearing the surface stabilizing agent of general formula (I) can be prepared by:.

Silver nanoplatelets bearing a dithiocarbamate of general formula (I), wherein RA is identical with RC and RB is identical with RD can be obtained starting from the silver nanoplatelets subjected to purification and/or isolation methods:.

The silver nanoplatelets described herein are disclosed by the <CIT>.

The UV-Vis radiation curable security ink claimed herein contains b) from about <NUM> wt-% to about <NUM> wt-% of either a cycloaliphatic epoxide, or a mixture of a cycloaliphatic epoxide and one or more UV-Vis radiation curable compounds. The one or more UV-Vis radiation curable compounds may comprise one or more cationically curable monomers, and/or one or more radically curable monomers and/or oligomers. If the one or more UV-Vis radiation curable compounds comprise one or more radically curable monomers and/or oligomers, then the UV-Vis radiation curable security ink claimed herein further comprises g) one or more free radical photoinitiators. Thus, the present invention is directed to a UV-Vis radiation curable security ink for producing a security feature exhibiting a blue color upon viewing in transmitted light and a metallic yellow color upon viewing in incident light, wherein said ink comprises:.

with the proviso that if the security ink comprises b-<NUM>) or b-<NUM>), the security ink further comprises g) one or more free radical photoinitiators; the weight percents being based on the total weight of the UV-Vis radiation curable security ink.

A preferred embodiment according to the present invention is directed to a UV-Vis radiation cationically curable security ink (i.e. an ink containing exclusively cationically curable monomers and no radically curable monomers/oligomers) for producing a security feature exhibiting a blue color upon viewing in transmitted light and a metallic yellow color upon viewing in incident light, wherein said ink comprises:.

An alternative preferred embodiment according to the present invention is directed to a UV-Vis radiation hybrid curable security ink (i.e. an ink comprising both cationically curable monomers and radically curable monomers/oligomers) for producing a security feature exhibiting a blue color upon viewing in transmitted light and a metallic yellow color upon viewing in incident light, wherein said ink comprises:.

the weight percents being based on the total weight of the UV-Vis radiation hybrid curable security ink. If the hybrid ink claimed herein contains b-<NUM>) from about <NUM> wt-% to about <NUM> wt-% of a mixture of a cycloaliphatic epoxide and one or more radically curable monomers and/or oligomers, the ratio between the total weight percent (wt-%) of the one or more radically curable monomers and/or oligomers and the weight percent (wt-%) of the cycloaliphatic epoxide is preferably lower than <NUM> : <NUM>, more preferably lower than <NUM> : <NUM>, and even more preferably lower than <NUM> : <NUM>. If the hybrid ink claimed herein contains b-<NUM>) from about <NUM> wt-% to about <NUM> wt-% of a mixture of a cycloaliphatic epoxide, one or more cationically curable monomers and one or more radically curable monomers and/or oligomers, the ratio between the total weight percent (wt-%) of the one or more radically curable monomers and/or oligomers and the sum of the weight percent (wt-%) of the cycloaliphatic epoxide and of the total weight percent (wt-%) of the one or more cationically curable monomers is preferably lower than <NUM> : <NUM>, more preferably lower than <NUM> : <NUM>, and even more preferably lower than <NUM> : <NUM>, and the ratio between the weight percent (wt-%) of the one or more cationically curable monomers and the weight percent (wt-%) of the cycloaliphatic epoxide is preferably lower than <NUM> : <NUM>, more preferably lower than <NUM> : <NUM> and even more preferably lower than <NUM> : <NUM>.

Advantageously, the UV-Vis radiation cationically curable security ink claimed herein and the UV-Vis radiation hybrid curable security ink claimed herein provide security features with improved mechanical resistance properties compared to the security features known in the art, which are obtained from UV radically curable inks or solvent-based inks, and particularly from UV radically curable inks or solvent-based inks containing high concentrations of silver nanoplatelets.

As well known to the skilled person, a cycloaliphatic epoxide is a cationically curable monomer containing at least a substituted or unsubstituted epoxycyclohexyl residue:
<CHM>.

Preferably, the cycloaliphatic epoxide described herein comprises at least one cyclohexane ring, and at least two epoxide groups. More preferably, the cycloaliphatic epoxide is a compound of general formula (V):
<CHM>
wherein -L- represents a single bond or a divalent group comprising one or more atoms. The cycloaliphatic epoxide of general formula (V) is optionally substituted by one or more linear or branched alkyl radicals containing from one to ten carbon atoms (such as methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl, t-butyl, hexyl, octyl, and decyl), and preferably containing from one to three carbon atoms (such as methyl, ethyl, n-propyl, and i-propyl).

In the general formula (V), the divalent group -L- may be a straight- or branched-chain alkylene group comprising from one to eighteen carbon atoms. Examples of said straight- or branched-chain alkylene group include without limitation methylene group, methylmethylene group, dimethylmethylene group, ethylene group, propylene group, and trimethylene group.

In the general formula (V), the divalent group -L- may be a divalent alicyclic hydrocarbon group or cycloalkydene group such as <NUM>,<NUM>-cyclopentylene group, <NUM>,<NUM>-cyclopentylene group, cyclopentylidene group, <NUM>,<NUM>-cyclohexylene group, <NUM>,<NUM>-cyclohexylene group, <NUM>,<NUM>-cyclohexylene group, and cyclohexylidene group.

In the general formula (V), -L- may be a divalent group comprising one or more oxygen-containing linkage groups, wherein said oxygen-containing linkage groups are selected from the group consisting of -C(=O)-, -OC(=O)O-, -C(=O)O-, and -O-. Preferably, the cycloaliphatic epoxide is a cycloaliphatic epoxide of general formula (V), wherein -L- is a divalent group comprising one or more oxygen-containing linkage groups, wherein said oxygen-containing linkage groups are selected from the group consisting of -C(=O)-, -OC(=O)O-, -C(=O)O-, and -O-, and more preferably a cycloaliphatic epoxide of general formula (V-a), (V-b), or (V-c), as defined below:
<CHM>
wherein.

Preferred cycloaliphatic epoxides of general formula (V-a) include, but are not limited to: <NUM>,<NUM>-epoxycyclohexylmethyl-<NUM>,<NUM>-epoxycyclohexanecarboxylate, <NUM>,<NUM>-epoxy-<NUM>-methyl-cyclohexylmethyl-<NUM>,<NUM>-epoxy-<NUM>-methylcyclohexanecarboxylate, <NUM>,<NUM>-epoxy-<NUM>-methyl-cyclohexylmethyl-<NUM>,<NUM>-epoxy-<NUM>-methylcyclohexanecarboxylate, and <NUM>,<NUM>-epoxy-<NUM>-methyl-cyclohexylmethyl-<NUM>,<NUM>-epoxy-<NUM>-methylcyclohexanecarboxylate.

Preferred cycloaliphatic epoxides of general formula (V-b) include, but are not limited to: bis(<NUM>,<NUM>-epoxycyclohexylmethyl)adipate, bis(<NUM>,<NUM>-epoxy-<NUM>-methylcyclohexylmethyl)adipate, bis(<NUM>,<NUM>-epoxycyclohexylmethyl)oxalate, bis(<NUM>,<NUM>-epoxycyclohexylmethyl)pimelate, and bis(<NUM>,<NUM>-epoxycyclohexylmethyl)sebacate.

A preferred cycloaliphatic epoxide of general formula (V-c) is <NUM>-(<NUM>,<NUM>-epoxycyclohexyl-<NUM>,<NUM>-spiro-<NUM>, <NUM>-epoxy)cyclohexane-meta-dioxane.

Further cycloaliphatic epoxides include a cycloaliphatic epoxide of general formula (VI-a) and a cycloaliphatic epoxide of general formula (VI-b), which are optionally substituted by one or more linear or branched alkyl groups containing from one to ten carbon atoms (such as methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl, t-butyl, hexyl, octyl, and decyl), and preferably containing from one to three carbon atoms (such as methyl, ethyl, n-propyl, and i-propyl). <CHM>
<CHM>.

The cycloaliphatic epoxides described herein may be hydroxy modified or (meth)acrylate modified. Examples are commercially available under the name Cyclomer A400 (CAS: <NUM>-<NUM>-<NUM>) and Cyclomer M100 (<NPL>) by Daicel Corp. , or TTA <NUM> and TTA16 46by TetraChem/Jiangsu.

The one or more cationically curable monomers described herein are selected from the group consisting of: vinyl ethers, propenyl ethers, cyclic ethers other than a cycloaliphatic epoxide, lactones, cyclic thioethers, vinyl thioethers, propenyl thioethers, hydroxyl-containing compounds, and mixtures thereof, preferably from the group consisting of: vinyl ethers, cyclic ethers other than a cycloaliphatic epoxide, and mixtures thereof. Cyclic ethers other than a cycloaliphatic epoxide include epoxides other than a cycloaliphatic epoxide, oxetanes and tetrahydrofuranes. Preferably, the ratio between the total weight percent (wt-%) of the one or more cationically curable monomers and the weight percent (wt-%) of the cycloaliphatic epoxide is lower than <NUM> : <NUM>, more preferably lower than <NUM> : <NUM>, most preferably lower than <NUM> : <NUM>, and especially preferably lower than <NUM> : <NUM>.

Vinyl ethers are known in the art to accelerate curing and reduce tackiness, thus limiting the risk of blocking and set-off when the printed sheets are put in stacks just after printing and curing. They also improve the physical and chemical resistance of the printed security element and enhance the flexibility of the printed and cured ink layer and its adhesion to the substrate, which is particularly advantageous for printing on plastic and polymer substrates. Vinyl ethers also help reducing the viscosity of the ink while strongly co-polymerizing with the ink vehicle. Examples of preferred vinyl ethers to be used in the security ink claimed herein include methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, n-butyl vinyl ether, iso-butyl 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, <NUM>-(vinyloxy methyl)cyclohexylmethyl benzoate, phenyl vinyl ether, methylphenyl vinyl ether, methoxyphenyl vinyl ether, <NUM>-chloroethyl vinyl ether, <NUM>-hydroxyethyl vinyl ether, <NUM>-hydroxybutyl vinyl ether, <NUM>,<NUM>-hexanediol monovinyl ether, ethylene glycol divinyl ether, ethylene glycol monovinyl ether, <NUM>, <NUM>-butanediol divinyl ether, <NUM>,<NUM>-hexanediol divinyl ether, <NUM>-(vinyloxy)butyl benzoate, bis[<NUM>-(vinyl oxy)butyl]adipate, bis[<NUM>-(vinyloxy)butyl]succinate, bis[<NUM>-(vinyloxymethyl)cyclohexylmethyl]glutarate, <NUM>-(vinyloxy)butyl stearate, trimethylolpropane trivinyl ether, propenyl ether of propylene carbonate, diethylene glycol monovinyl ether, diethylene glycol divinyl ether, ethylene glycol butylvinyl ether, dipropylene glycol divinyl ether, triethylene glycol divinyl ether, triethylene glycol methyl vinyl ether, triethylene glycol monobutyl vinylether, tetraethylene glycol divinyl ether, poly(tetrahydrofuran) divinyl ether, polyethyleneglycol-<NUM> methyl vinyl ether, pluriol-E200 divinyl ether, tris[<NUM>-(vinyloxy)butyl]trimellitate, <NUM>,<NUM>-bis(<NUM>-vinyloxyethoxy)benzene, <NUM>,<NUM>-bis(<NUM>-vinyloxyethoxyphenyl)propane, bis[<NUM>-(vinyloxy)methyl]cyclohexyl] methyl] terephthalate, bis[<NUM>-(vinyloxy)methyl]cyclohexyl]methyl] isophthalate. Suitable vinyl ethers are commercially sold by BASF under the designation EVE, IBVE, DDVE, ODVE, BDDVE, DVE-<NUM>, DVE-<NUM>, CHVE, CHDM-di, HBVE. The one or more vinyl ethers described herein may be hydroxy modified or (meth)acrylate modified (for example: VEEA, <NUM>-(<NUM>-vinyloxyethoxy)ethyl acrylate from Nippon Shokubai (CAS: <NUM>-<NUM>-<NUM>)).

Oxetanes are known in the art to accelerate curing and reduce tackiness, thus limiting the risk of blocking and set-off when the printed sheets are put in stacks just after printing and curing. They also help reducing the viscosity of the ink while strongly co-polymerizing with the ink vehicle. Preferred examples of oxetanes include trimethylene oxide, <NUM>,<NUM>-dimethyloxetane, trimethylolpropane oxetane, <NUM>-ethyl-<NUM>-hydroxymethyl oxetane, <NUM>-ethyl-<NUM>-[(<NUM>-ethylhexyloxy) methyl]oxetane, <NUM>,<NUM>-dicyclomethyl oxetane, <NUM>-ethyl-<NUM>-phenoxymethyl oxetane, bis ([<NUM>-ethyl(<NUM>-oxetanyl)]methyl) ether, <NUM>,<NUM>-bis [<NUM>-ethyl-<NUM>-oxetanyl methoxy)methyl]benzene, <NUM>,<NUM>-dimethyl-<NUM>(p-methoxy-phenyl)-oxetane, <NUM>-ethyl-[(tri-ethoxysilyl propoxy)methyl]oxetane, <NUM>,<NUM>-bis(<NUM>-ethyl-<NUM>-oxetanyl)methoxymethyl]biphenyl and <NUM>,<NUM>-dimethyl-<NUM>(p-methoxy-phenyl) oxetane. The one or more oxetanes described herein may be hydroxy modified or (meth)acrylate modified (for example: UVi-Cure S170 from Lambson (CAS: <NUM>-<NUM>-<NUM>)).

The use of epoxides in the UV-Vis radiation curable ink aids in accelerating curing and reducing tackiness, as well as in reducing the viscosity of the ink while strongly co-polymerizing with the ink vehicle. Preferred examples of an epoxide other than a cycloaliphatic epoxide as described herein include, but are not limited to, cyclohexane dimethanol diglycidylether, poly(ethyleneglycol) diglycidyl ether, poly(propyleneglycol) diglycidyl ether, butanediol diglycidyl ether, hexanediol diglycidyl ether, bisphenol-A diglycidyl ether, neopentylglycol diglycidylether, trimethylolpropane triglycidyl ether, glycerol triglycidyl ether, pentaerythritol tetraglycidyl ether, butyl glycidyl ether, p-tert-butyl phenyl glycidyl ether, hexadecyl glycidyl ether, <NUM>-ethyl-hexyl glycidyl ether, octyl glycidyl ether, decyl glycidyl ether, dodecyl glycidyl ether, tetradecyl glycidyl ether, C<NUM>/C<NUM>-alkyl glycidyl ether, C<NUM>/C<NUM>-alkyl glycidyl ether and mixtures thereof. Suitable epoxides other than a cycloaliphatic epoxide are commercially sold by EMS Griltech under the trademark Grilonit® (e.g. Grilonit® V51-<NUM> or RV <NUM>).

The radically curable monomer described herein is selected from the group consisting of mono(meth)acrylates, di(meth)acrylates, tri(meth)acrylates, tetra(meth)acrylates, and mixtures thereof, preferably from the group consisting of tri(meth)acrylates, tetra(meth)acrylates, and mixtures thereof. The term "(meth)acrylate" in the context of the present invention refers to the acrylate as well as the corresponding methacrylate.

Preferred examples of mono(meth)acrylates include <NUM>(<NUM>-ethoxyethoxy)ethyl (meth)acrylate, <NUM>-phenoxyethyl (meth)acrylate, C<NUM>/C<NUM> alkyl (meth)acrylate, C<NUM>/C<NUM> alkyl (meth)acrylate, caprolactone (meth)acrylate, cyclic trimethylolpropane formal (meth)acrylate, nonylphenol (meth)acrylate, isobornyl (meth)acrylate, isodecyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, octyldecyl (meth)acrylate, tridecyl (meth)acrylate, methoxy poly(ethylene glycol) (meth)acrylate, polypropylene glycol (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, <NUM>,<NUM>-butylene glycol di(meth)acrylate, <NUM>,<NUM>-butanediol di(meth)acrylate, <NUM>,<NUM>-hexanediol di(meth)acrylate, <NUM>-methyl-<NUM>,<NUM>-pentanedioldi(meth)acrylate, alkoxylated di(meth)acrylate, esterdiol di(meth)acrylate as well as mixtures thereof.

Preferred examples of di(meth)acrylates include bisphenol A di(meth)acrylates, alkoxylated (such as for example ethoxylated and propoxylated) bisphenol A di(meth)acrylate, bisphenol A 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, dipropyleneglycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, as well as mixtures thereof.

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, as well as mixtures thereof.

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, as well as mixtures thereof.

As used herein, the term "radically curable oligomer" refers to a radically curable (meth)acrylate oligomer that may be branched or essentially linear, and may have terminal and/or pendant (meth)acrylate functional group(s). Preferably, the radically curable oligomer is selected from the group consisting of (meth)acrylic oligomers, urethane (meth)acrylate oligomers, polyester (meth)acrylate oligomers, polyether 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.

Suitable examples of epoxy (meth)acrylate oligomer include without limitation aliphatic epoxy (meth)acrylate oligomers, in particular 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 (meth)acrylate oligomers such as bisphenol-A mono(meth)acrylates, bisphenol-A di(meth)acrylates and bisphenol-A tri(meth)acrylates as well as alkoxylated (such as for example ethoxylated and propoxylated) bisphenol-A (meth)acrylate oligomers such as for example alkoxylated bisphenol-A mono(meth)acrylates, alkoxylated bisphenol-A di(meth)acrylates and alkoxylated bisphenol-A tri(meth)acrylates, preferably alkoxylated bisphenol-A di(meth)acrylates.

The security ink claimed herein contains c) one or more cationic photoinitiators. Preferably, the amount of the one or more cationic photoinitiators in the UV-Vis radiation cationically curable security ink claimed herein (i.e. the ink containing exclusively cationically curable monomers and no radically curable monomers) is from about <NUM> wt-% to about <NUM> wt-%, preferably from about <NUM> wt-% to about <NUM> wt-%, more preferably from about <NUM> wt-% to about <NUM> wt-%, wherein the weight percent is based on the total weight of the UV-Vis radiation cationically curable ink. Preferably, the amount of the one or more cationic photoinitiators in the UV-Vis radiation hybrid curable security ink claimed herein (i.e. the ink containing both cationically curable monomers and radically curable monomers) is from <NUM> wt-% to about <NUM> wt-%, wherein the weight percent is based on the total weight of the UV-Vis radiation cationically curable ink.

The one or more cationic photoinitiators described herein (also referred in the art as photo-acid generators) are onium salts preferably selected from the group consisting of azonium salts, oxonium salts, iodonium salts, sulfonium salts and mixtures thereof, more preferably selected from the group consisting of oxonium salts, iodonium salts, sulfonium salts, and mixtures thereof, and even more preferably selected from the group consisting of sulfonium salts, iodonium salts, and mixtures thereof.

The iodonium salts described herein have a cationic moiety and an anionic moiety, wherein the anionic moiety is preferably BF<NUM>-, B(C<NUM>F<NUM>)<NUM>-, PF<NUM>-,AsF<NUM>-, SbF<NUM>- or CF<NUM>SO<NUM>-, more preferably SbF<NUM>- and wherein the cationic moiety is preferably an aromatic iodonium ion, more preferably a iodonium ion comprising two aryl groups, wherein the two aryl groups may be independently substituted by one or more alkyl groups (such as for example methyl, ethyl, isobutyl, tertbutyl, etc.), one or more alkoxy groups, one or more nitro groups, one or more halogen containing groups, one or more hydroxy groups or a combination thereof, preferably by one or more alkyl groups. Particularly suitable iodonium salts for the present invention are commercially available known under the name DEUTERON UV <NUM>, DEUTERON UV <NUM>, DEUTERON UV <NUM>, DEUTERON UV <NUM>, and DEUTERON UV <NUM>, all available from DEUTERON, OMNICAT <NUM>, OMNICAT <NUM>, and OMNICAT <NUM>, all available from IGM Resins, SpeedCure <NUM>, SpeedCure <NUM> and SpeedCure <NUM>, all available from Lambson.

The sulfonium salts described herein have a cationic moiety and an anionic moiety, wherein the anionic moiety is preferably BF<NUM>-, B(C<NUM>F<NUM>)<NUM>-, PF<NUM>-, (PF<NUM>-h(CjF2j-<NUM>)h)- (where h is an integer from <NUM> to <NUM>, and j is an integer from <NUM> to <NUM>), AsF<NUM>-, SbF<NUM>-, CF<NUM>SO<NUM>-, perfluoroalkyl sulfonate or pentafluoro-hydroxyantimonate, more preferably SbF<NUM>- and wherein the cationic moiety is preferably an aromatic sulfonium ion, more preferably a sulfonium ion comprising two or more aryl groups, wherein the two or more aryl groups may be independently substituted by one or more alkyl groups (such as for example methyl, ethyl, isobutyl, tertbutyl, etc.) one or more alkoxy groups, one or more aryloxyl groups, one or more halogen containing groups, one or more hydroxy groups or a combination thereof. Suitable examples of sulfonium ions comprising two or more aryl groups include without limitation triarylsulfonium ions, diphenyl[<NUM>-(phenylthio)phenyl] sulfonium ion, bis[<NUM>-(diphenylsulfonio)phenyl] sulfonium ion, triphenylsulfonium ions, and tris[<NUM>-(<NUM>-acetylphenyl)sulfanylphenyl] sulfonium ion. Particularly suitable examples of sulfonium salts for the present invention are commercially available under the name SpeedCure <NUM>, SpeedCure 976D, SpeedCure <NUM> and SpeedCure <NUM>, all available from Lambson, ESACURE <NUM>, OMNICAT <NUM>, OMNICAT <NUM>, OMNICAT <NUM> and OMNICAT <NUM>, all available from IGM Resins, DoubleCure <NUM>, DoubleCure <NUM> and DoubleCure <NUM>, all available from DoubleBond.

The oxonium salts described herein have a cationic moiety and an anionic moiety, wherein the anionic moiety is preferably BF<NUM>-, B(C<NUM>F<NUM>)<NUM>-, PF<NUM>-, AsF<NUM>-, SbF<NUM>- or CF<NUM>SO<NUM>-, more preferably BF<NUM>- and wherein the cationic moiety is preferably an aromatic oxonium ion, more preferably a pyrilium ion preferably substituted by one or more aryl groups, wherein the one or more aryl groups may be independently of each other substituted by one or more alkyl groups (such as for example methyl, ethyl, isobutyl, tertbutyl, etc.), one or more alkoxy groups, one or more nitro groups, one or more halogen groups, one or more hydroxy groups or a combination thereof. A particularly suitable oxonium salt for the present invention is <NUM>,<NUM>,<NUM>-triphenylpyrilium tetrafluoroborate.

Other examples of useful cationic photoinitiators can be found in standard textbooks such as "<NPL> in association with SITA Technology Limited.

Moreover, the hybrid security ink claimed herein contains g) one or more free radical photoinitiators. Preferably, the amount of the one or more free radical photoinitiators in the UV-Vis radiation hydrid curable ink described herein is from about <NUM> wt-% to about <NUM> wt-%, the percent being based on the total weight of the UV-Vis radiation hydrid curable ink.

The one or more free radical photoinitiators as used herein are preferably selected form the group consisting of hydroxyketones (e.g. alpha-hydroxyketones), alkoxyketones (e.g. alpha-alkoxyketones), acetophenones, benzophenones, ketosulfones, benzyl ketals, benzoin ethers, phosphine oxides, phenylglyoxylates, thioxanthones, and mixtures thereof, more preferably selected form the group consisting of phosphine oxides, hydroxyketones, thioxanthones and mixtures thereof.

Suitable alpha-hydroxyketones include without limitation (<NUM>-[<NUM>-(<NUM>-hydroxyethoxy)-phenyl]-<NUM>-hydroxy-<NUM>-methyl-<NUM>-propan-<NUM>-one), <NUM>-hydroxycyclohexyl phenyl ketone, <NUM>-hydroxy-<NUM>-methyl-<NUM>-phenylpropan-<NUM>-one, <NUM>-hydroxy-<NUM>-methyl-<NUM>-(<NUM>-tert-butyl)phenylpropan-<NUM>-one, <NUM>-hydroxy-<NUM>-[<NUM>-[[<NUM>-(<NUM>-hydroxy-<NUM>-methylpropanoyl)phenyl]methyl]phenyl]-<NUM>-methylpropan-<NUM>-one, <NUM>-hydroxy-<NUM>-[<NUM>-[<NUM>-(<NUM>-hydroxy-<NUM>-methylpropanoyl)phenoxy]phenyl]-<NUM>-methylpropan-<NUM>-one, and oligo[<NUM>-hydroxy-<NUM>-methyl-<NUM>-[<NUM>-(<NUM>-methylvinyl)phenyl]propanone].

Suitable acetophenones include without limitation <NUM>,<NUM>-diethoxyacetophenone, and <NUM>-methoxy-<NUM>-phenylacetophenone.

Suitable benzophenones include without limitation benzophenone, polymeric benzophenone derivatives, <NUM>-methylbenzophenone, <NUM>-methylbenzophenone, <NUM>-methylbenzophenone, <NUM>,<NUM>,<NUM>-trimethylbenzophenone, <NUM>,<NUM>'-dimethyl-<NUM>-methoxybenzophenone, <NUM>-phenylbenzophenone, <NUM>-chlorobenzophenone, methyl-<NUM>-benzoylbenzoate, <NUM>-(<NUM>-methylphenylthio)benzophenone, <NUM>-hydroxybenzophenone laurate, and a mixture of <NUM>% benzophenone and <NUM>% <NUM>-hydroxycyclohexyl phenyl ketone.

Suitable ketosulfones include without limitation <NUM>-[<NUM>-(<NUM>-benzoylphenylsulfanyl)phenyl]-<NUM>-methyl-<NUM>-(<NUM>-methylphenylsulfonyl)propan-<NUM>-one.

Suitable benzyl ketals include without limitation <NUM>,<NUM>-dimethoxy-<NUM>-phenylacetophenone.

Suitable benzoin ethers include without limitation <NUM>-ethoxy-<NUM>,<NUM>-diphenylethanone, <NUM>-isopropoxy-<NUM>,<NUM>-diphenylethanone, <NUM>-isobutoxy-<NUM>,<NUM>-diphenylethanone, <NUM>-butoxy-<NUM>,<NUM>-diphenylethanone, <NUM>,<NUM>-dimethoxy-<NUM>,<NUM>-diphenylethanone, and <NUM>,<NUM>-diethoxyacetophenone.

Suitable phosphine oxides include without limitation <NUM>,<NUM>,<NUM>-trimethylbenzoyldiphenylphosphine oxide, ethyl phenyl(<NUM>,<NUM>,<NUM>-trimethylbenzoyl)phenylphosphinate, phenylbis(<NUM>,<NUM>,<NUM>-trimethylbenzoyl)phosphine oxide, bis(<NUM>,<NUM>-dimethoxybenzoyl)-<NUM>,<NUM>,<NUM>-trimethylpentylphosphine oxide, substituted acyl-phosphine oxides, a mixture of diphenyl(<NUM>,<NUM>,<NUM>-trimethylbenzoyl)phosphine oxide and <NUM>-hydroxy-<NUM>-methylpropiophenone, a mixture of phenylbis(<NUM>,<NUM>,<NUM>-trimethylbenzoyl)phosphine oxide and <NUM>-hydroxy-<NUM>-methylpropiophenone, a mixture of ethyl(<NUM>,<NUM>,<NUM>-trimethylbenzoyl)phenylphosphinate and <NUM>-hydroxy-<NUM>-methylpropiophenone, and a mixture of phenylbis(<NUM>,<NUM>,<NUM>-trimethylbenzoyl)phosphine oxide and ethyl phenyl(<NUM>,<NUM>,<NUM>-trimethylbenzoyl)phenylphosphinate.

Suitable thioxanthones include without limitation <NUM>-methyl thioxanthone, <NUM>,<NUM>-diethylthioxanthone, <NUM>-isopropylthioxanthone, <NUM>-chloro-<NUM>-propoxythioxanthone, and polymeric thioxanthone derivatives.

Suitable phenylglyoxylates include without limitation methyl benzoylformate, <NUM>-[<NUM>-oxo-<NUM>-phenyl-acetoxy-ethoxy]ethyl <NUM>-oxo-<NUM>-phenylacetate, and a mixture of <NUM>-[<NUM>-oxo-<NUM>-phenyl-acetoxy-ethoxy]ethyl <NUM>-oxo-<NUM>-phenylacetate and oxy-phenyl-acetic acid <NUM>-[<NUM>-hydroxy-ethoxy]-ethyl ester.

Preferably, the one or more free radical photoinitiators are phosphine oxides as described herein, and more preferably a mixture of phenylbis(<NUM>,<NUM>,<NUM>-trimethylbenzoyl)phosphine oxide and ethyl phenyl(<NUM>,<NUM>,<NUM>-trimethylbenzoyl)phenylphosphinate.

The UV-Vis radiation curable security ink described herein contains d) a perfluoropolyether surfactant functionalized with one or more functional groups selected from the group consisting of: hydroxyl, acrylate, methacrylate, and trialkoxysilyl, preferably two or more functional groups selected from the group consisting of: hydroxyl, acrylate, methacrylate, and trialkoxysilyl. Surprisingly, it has been found that the use of a perfluoropolyether functionalized with one or more, preferably two or more, functional groups selected from the group consisting of: hydroxyl, acrylate, methacrylate, and trialkoxysilyl, as surfactant in the UV-Vis radiation curable ink described herein is essential for producing security features exhibiting a metallic yellow color upon viewing in incident light. As attested for example by Table 3c and Table 4c, only UV-Vis radiation curable inks containing a perfluoropolyether surfactant functionalized with one or more functional groups selected from the group consisting of: hydroxyl, acrylate, methacrylate, and trialkoxysilyl provide security features exhibiting a metallic yellow color upon viewing in incident light. The security features produced with a UV-Vis radiation curable ink lacking a surfactant (for e.g.: ink C8), or comprising either a perfluoropolyether surfactant lacking the functional group (for e.g. inks C1), or a surfactant lacking the perfluoropolyether backbone (for e.g. inks C2 - C7) show a brown to dark brown color in reflection, which is not eye-catching for the layperson, and therefore not suitable for a dichroic security feature for securing a value document.

The perfluoropolyether surfactant functionalized with one or more functional groups selected from the group consisting of: hydroxyl, acrylate, methacrylate, and trialkoxysilyl, comprises a perfluoropolyether backbone and one or more, preferably two or more, terminal functional groups selected from the group consisting of: hydroxyl, acrylate, methacrylate and trialkoxysilyl is characterized by an average molecular weight (Mn) below about <NUM> [g/mol]. As used herein, a perfluoropolyether backbone denotes a residue of a perfluoropolyether polymer comprising randomly distributed recurring units selected from perfluoromethyleneoxy (-CF<NUM>O-) and perfluoroethyleneoxy (-CF<NUM>-CF<NUM>O-). The perfluoropolyether residue is connected to the terminal functional group directly or via a spacer selected from methylene(oxyethylene), <NUM>,<NUM>-difluoroethylene-(oxyethylene), methylene-di(oxyethylene), <NUM>,<NUM>-difluoroethylene-di(oxyethylene), methylene-tri(oxyethylene), <NUM>,<NUM>-difluoroethylene-tri(oxyethylene), methylene-tetra(oxyethylene), <NUM>,<NUM>-difluoroethylene-tetra(oxyethylene), methylene-penta(oxyethylene), <NUM>,<NUM>-difluoroethylene-penta(oxyethylene), and a linear or branched hydrocarbon group, optionally fluorinated at the carbon atom connecting the spacer to the perfluoropolyether residue, containing one or more urethane groups, or one or more amide groups, and optionally one or more cyclic moieties, including saturated cyclic moieties (such as cyclohexylene) and aromatic cyclic moieties (such as phenylene). Preferably, the perfluoropolyether surfactant is functionalized with one or more hydroxyl functional groups.

In a further preferred embodiment, the perfluoropolyether surfactant functionalized with one or more functional groups selected from the group consisting of: hydroxyl, acrylate, methacrylate, and trialkoxysilyl is a compound of general formula (VII) having an average molecular weight from about <NUM> [g/mol] to about <NUM> [g/mol]
<CHM>
wherein.

Preferably, in general formula (VII), FG<NUM> and FG<NUM> represent independently of each other -OC(O)CH=CH<NUM>, or -OC(O)C(CH<NUM>)=CH<NUM>;.

Also preferably, in general formula (VII), FG<NUM> and FG<NUM> represent -OH;.

Also preferably, in general formula (VII), FG<NUM> and FG<NUM> represent -Si(OR<NUM>)<NUM>;.

Particularly suitable examples of perfluoropolyether surfactant functionalized with one or more functional groups selected from the group consisting of: hydroxyl, acrylate, methacrylate and trialkoxysilyl for the present invention are commercially available under the name Fluorolink® E10H, Fluorolink® MD700, Fluorolink® AD1700, Fluorolink® E-series, and Fluorolink® S10 from Solvay.

The UV-Vis radiation curable security ink claimed herein contains e) from about <NUM> wt-% to about <NUM> wt-% of a polyvinyl chloride copolymer containing at least <NUM> wt-% of vinyl chloride, preferably at least <NUM> wt-% of vinyl chloride. UV-Vis radiation curable security inks containing no polyvinyl chloride copolymer provide security features with non-attractive colors, such as brown or dark brown, and low chroma value C* upon viewing in incident light and consequently, are not suitable to be used for the production of security feature showing a metallic yellow color in incident light.

It is preferred that the polyvinyl chloride copolymer contains at the most <NUM> wt-% of vinyl chloride.

Preferably, the polyvinyl chloride copolymer containing at least <NUM> wt-% of vinyl chloride is present in the security ink claimed herein in an amount from about <NUM> wt-% to about <NUM> wt-%, and most preferably from about <NUM> wt-% to about <NUM> wt-%, wherein the weight percents are based on the total weight of the UV-Vis radiation curable ink.

Preferably, the polyvinyl chloride copolymer is selected from the group consisting of vinyl chloride -vinyl acetate copolymer, vinyl chloride - hydroxyalkylacrylate copolymer, such as vinyl chloride - <NUM>-hydroxypropyl acrylate copolymer, and vinyl chloride - hydroxyalkylacrylate - Z-alkylenedioic acid, dialkyl ester copolymer, such as vinyl chloride - <NUM>-hydroxypropyl acrylate - <NUM>-butenedioic acid (Z)-, dibutyl ester copolymer. The polyvinyl chloride copolymer has preferably an average molecular weight of between <NUM>*<NUM><NUM> g/mol and about <NUM>*<NUM><NUM> g/mol as determined by size exclusion chromatography using polystyrene as standard and tetrahydrofuran as solvent. Particularly suitable examples of polyvinyl chloride copolymer for the present invention are commercially available under the name Vinnol® H14/<NUM>, Vinnol® E22/48A, Vinnol® E <NUM>/<NUM> A and Vinnol® H <NUM>/<NUM> from Wacker.

The UV-Vis radiation curable security inks claimed herein may contain f) up to about <NUM> wt-% of an organic solvent, the weight percent being based on the total weight of the UV-Vis radiation curable ink. The solvent has a boiling point higher than <NUM>. Suitable organic solvents to be used in the UV-Vis radiation curable inks described herein include without limitation: ethyl-<NUM>-ethoxypropionate, <NUM>-methoxy-<NUM>-methylethyl acetate, propylene glycol mono methyl ether, cyclopentanone, cyclohexanone, n-butanol, cyclohexanol, ethylene carbonate, propylene carbonate, butylene carbonate, and mixtures thereof.

In a preferred embodiment according to the present invention, the UV-Vis radiation curable security ink is solvent-free. The use of a solvent-free ink in an industrial printing process of value documents is of high interest because it prevents emission of volatile organic components, which typically have a negative impact on the environment and are harmful for human health.

The UV-Vis radiation curable security ink claimed herein may further comprise one or more photosensitizers in conjunction with the one or more photoinitiators described herein in order to achieve efficient curing. Suitable examples of photosensitizers are known to those skilled in the art (e.g. in Industrial Photoinitiators, W. Green, CRC Press, <NUM>, Table <NUM> p. Preferred photosensitizers are those that are able to achieve efficient and fast curing with UV-LED light sources, such as thioxanthone derivatives, anthracene derivatives and naphthalene derivatives (such as <NUM>,<NUM>-diethoxyanthracene sold as Anthracure UVS-<NUM> and <NUM>,<NUM>-dibutyloxyanthracene sold as Anthracure UVS-<NUM>, both sold by Kawasaki Kasei Chemicals Ltd) and titanocene derivatives (such as Irgacure <NUM> sold by BASF). Particularly preferred are thioxanthone derivatives, including without limitation isopropyl-thioxanthone (ITX), <NUM>-chloro-<NUM>-propoxy-thioxanthone (CPTX), <NUM>-chloro-thioxanthone (CTX) and <NUM>,<NUM>-diethyl-thioxanthone (DETX), and mixtures thereof. Alternatively, thioxanthone photosensitizers may be used in an oligomeric or polymeric form (such as Omnipol TX sold by IGM Resins, Genopol* TX-<NUM> sold by Rahn, or Speedcure <NUM> sold by Lambson). When present, the one or more photosensitizers are preferably present in an amount from about <NUM> wt-% to about <NUM> wt-%, more preferably from about <NUM> wt-% to about <NUM> wt-%, the weight percent being based on the total weight of the UV-Vis radiation curable ink.

The UV-Vis radiation curable ink claimed herein may further comprise one or more antifoaming agents in an amount of less than about <NUM> wt-%, preferably of less than about <NUM> wt-%.

Another aspect according to the present invention is directed to a process for producing a security feature for securing a value document, wherein said security feature exhibits a blue color upon viewing in transmitted light and a metallic yellow color upon viewing in incident light, said process comprising the following steps:.

The inventive manufacturing process claimed herein enables access in a single printing step to a security feature displaying a metallic yellow color in incident light and a blue color, especially an intense to very intense blue color, in transmitted light. As used herein, the term "printing" refers to any printing process suitable for printing the UV-Vis radiation curable ink described herein on a substrate of a value document. In particularly, the term "printing" refers to a printing process selected from the group consisting of: screen printing, rotogravure, flexography, pad printing, inkjet printing, and spray printing. Preferably, the UV-Vis radiation curable security ink is printed on a transparent or partially transparent region of the substrate of the value document by screen printing, rotogravure or flexography, more preferably by screen printing.

As used herein, "a transparent or partially transparent region of a substrate of a value document" refers to a region of the substrate of the value document, wherein said region is characterized by an average transmittance in the visible range of at least <NUM>%, preferably of at least <NUM>%, more preferably of at least <NUM>%. The transparent or partially transparent region of the substrate and the remaining region of the substrate may be made either of the same material, or of different materials. Elimination of one or more layers in a multilayer structure or application of a transparent or partially transparent material to an aperture in a substrate made of a material, which is different from the transparent or partially transparent material provides value documents substrates, wherein the transparent or partially transparent region of the substrate and the remaining region of the substrate are made of different materials.

Materials for value document substrates include without limitation, papers or other fibrous materials such as cellulose, paper-containing materials, plastics and polymers, composite materials, and mixtures or combinations thereof. Typical paper, paper-like or other fibrous materials are made from a variety of fibers including without limitation abaca, cotton, linen, wood pulp, and blends thereof. As well known to those skilled in the art, cotton and cotton/linen blends are preferred for banknotes, while wood pulp is commonly used in non-banknote security documents. Typical examples of plastics and polymers include polystyrene, polycarbonate, polyolefins, such as polyethylene (PE) and polypropylene (PP) including biaxially-oriented polypropylene (BOPP), polyamides (PA), polyesters such as polyethylene terephthalate) (PET), polyethylene terephthalate glycol-modified (PETG) including polyethylene glycol-co-<NUM>,<NUM>-cyclohexanedimethanol terephthalate), poly(<NUM>,<NUM>-butylene terephthalate) (PBT), and polyethylene <NUM>,<NUM>-naphthoate) (PEN), and polyvinylchlorides (PVC). Typical examples of composite materials include without limitation multilayer structures or laminates of paper and at least one plastic or polymer material, such as those described hereabove. Suitable materials for the transparent or partially transparent region of the substrate include, but are not limited to polystyrene, polycarbonate, polyolefins, such as polyethylene (PE) and polypropylene (PP) including biaxially-oriented polypropylene (BOPP), polyamides (PA), polyesters such as poly(ethylene terephthalate) (PET), polyethylene terephthalate glycol-modified (PETG) including polyethylene glycol-co-<NUM>,<NUM>-cyclohexanedimethanol terephthalate), poly(<NUM>,<NUM>-butylene terephthalate) (PBT), and polyethylene <NUM>,<NUM>-naphthoate) (PEN), and polyvinylchlorides (PVC). The transparent or partially transparent region of the substrate of the value document may carry a primer layer on the top of which the UV-Vis radiation curable ink is printed. The primer layer may be obtained by UV-Vis curing a varnish containing all the ingredients of the UV-Vis radiation curable ink described herein, with the exception of the silver nanoplatelets.

At step B) of the inventive manufacturing process claimed herein, the ink layer obtained at step A) is subjected to UV-Vis curing to form the security feature. As used herein, the term "UV-Vis curing" refers to radiation-curing of the ink layer by photo-polymerization, under the influence of an irradiation having wavelength components in the UV or in the UV and visible part of the electromagnetic spectrum (typically <NUM> to <NUM>, preferably between <NUM> and <NUM> and more preferably between <NUM> and <NUM>). Cationically curable monomers are cured by cationic mechanisms consisting of the activation by UV-Vis light of one or more photoinitiators, which liberate cationic species, such as acids, which in turn initiate the polymerization of the compound so as to form a cured binder. Radically curable monomers and oligomers are cured by free radical mechanisms consisting of the activation by UV-Vis light of one or more photoinitiators, which liberate free radicals which in turn initiate the polymerization process. Optionally, one or more photosensitizers may also be present. Photosensitizers are activated by one or more of the wavelengths emitted by a UV-Vis light source and reach an excited state. The excited photosensitizer either transfer energy to the one or more photoinitiators (in free-radical polymerization) or an electron (in cationic polymerization). Either process in turn initiates the polymerization process.

Preferably, step B) comprises exposure of the ink layer obtained at step A) to UV-Vis light emitted by a UV-Vis light source selected from the group consisting of: mercury lamps, preferably medium-pressure mercury lamps, UV-LED lamps, and sequences thereof. Typical sequences include the use of one or more UV-LED lamps in a first step to partially cure the UV-Vis radiation composition and one or more medium-pressure mercury lamps in a second step. Mercury lamps advantageously emit on a wide range of wavelengths in the UV-A, UV-B and UV-C range. Accordingly, there is a large selection of photoinitiators or combinations of photoinitiator/photosensitizer having an absorption spectrum matching at least one of the emission band of the mercury lamp. UV-LED have a more limited range of wavelengths, such that only a limited selection of photoinitiators or combination of photoinitiator/photosensitizer is efficient enough at industrial printing speed. On the other hand, UV-LEDs are less costly, require less energy (in particular, they need much less demanding heat dissipation systems), are not prone to ozone formation and have a much longer lifespan.

To provide the value document with soil resistance and/or to protect the security feature against physical and chemical attacks from the environment, the manufacturing process claimed herein preferably further comprises steps C) and D) conducted after step B):.

Examples of suitable curable protective varnishes to be used at step C) and/or of methods of applying said curable protective varnishes on the substrate and of curing the varnish layer are described in the international patent application publication number <CIT>, the international patent application publication number <CIT> and the international patent application publication number <CIT>.

Preferably, the value document is selected from banknotes, deeds, tickets, checks, vouchers, fiscal stamps, agreements, identity documents such as passports, identity cards, visas, driving licenses, bank cards, credit cards, transactions cards, access documents, and cards, entrance tickets, public transportation tickets, academic diploma, and academic titles. More preferably the value document is a banknote. The security ink claimed herein may be also used for producing a security feature directly on a value commercial good. The term "value commercial good" refers to packaging material, in particular for pharmaceutical, cosmetics, electronics or food industry that may be protected against counterfeiting and/or illegal reproduction in order to warrant the content of the packaging like for instance genuine drugs.

The present invention is now described in more details with reference to non-limiting examples. The examples E1 - E26 and comparative examples C1 - C8 below provide more details for the preparation the UV-Vis radiation curable screen printing security inks described herein and optical properties of security features obtained therefrom.

UV-Vis spectra of dispersions were recorded on Varian Cary <NUM> UV-Visible spectrophotometer at such concentration of dispersions as to achieve the optical density of <NUM> to <NUM> at <NUM> optical path.

TEM analysis of dispersions was performed on EM <NUM> instrument from ZEISS in bright field mode at an e-beam acceleration voltage of 100kV. At least <NUM> representative images with scale in different magnification were recorded in order to characterize the dominant particle morphology for each sample.

The mean diameter of the silver nanoplatelets was determined by transmission electron microscopy (TEM) using Fiji image analysis software based on the measurement of at least <NUM> randomly selected silver nanoplatelets oriented parallel to the plane of a transmission electron microscopy image (TEM), wherein the diameter of a silver nanoplatelet is the maximum dimension of said silver nanoplatelet oriented parallel to the plane of a transmission electron microscopy image (TEM).

The mean thickness of the silver nanoplatelets was determined by transmission electron microscopy (TEM) based on the manual measurement of at least <NUM> randomly selected silver nanoplatelets oriented perpendicular to the plane of the TEM image, wherein the thickness of the silver nanoplatelet is the maximum thickness of said silver nanoplatelet.

In a <NUM> double-wall glass reactor, equipped with anchor-stirrer, <NUM> of de-ionized water was cooled to +<NUM>. <NUM> of sodium borohydride was added, and the mixture was cooled to -<NUM> with stirring at <NUM> rounds per minute (RPM, Solution A).

In a <NUM> double-wall glass reactor, equipped with anchor-stirrer, <NUM> of deionized water and <NUM> of MPEG-<NUM>-thiol were combined, and the mixture was stirred for <NUM> minutes at room temperature. <NUM> of the product of Example A3 of <CIT> was added, and the resulting mixture was stirred for another <NUM> minutes at room temperature for homogenization. The solution of <NUM> of silver nitrate in <NUM> of de-ionized water was added in one portion and the mixture was stirred for <NUM> minutes, resulting in an orange-brown viscous solution. To this solution <NUM> of deionized water was added, followed by addition of <NUM> of Struktol SB <NUM> defoamer, pre-dispersed in <NUM> of de-ionized water. The resulting mixture was cooled to <NUM> with stirring at <NUM> RPM (Solution B).

After that, Solution B was dosed with a peristaltic pump at a constant rate over <NUM> into Solution A under the liquid surface via a cooled (<NUM>) dosing tube, resulting in spherical silver nanoplatelets dispersion. During pumping, the Solution A was stirred at <NUM> RPM.

After dosing was complete, the reaction mixture was warmed up to +<NUM> within <NUM> minutes, and a solution of <NUM> of KCI in <NUM> of deionized water was added in one portion, followed by addition of <NUM> of ethylenediaminetetraacetic acid (EDTA) in <NUM> equal portions with <NUM> minutes time intervals. After addition of the last EDTA portion, the reaction mixture was stirred for <NUM> minutes at +<NUM>, then warmed up to <NUM> over <NUM> minutes and stirred for <NUM> at this temperature. Upon this time, hydrogen evolution is completed.

<NUM> of <NUM>% w/w solution of ammonia in water was added, followed by addition of <NUM> of solid NaOH, and the mixture was stirred for <NUM> at <NUM>. Then <NUM> of <NUM>% w/w hydrogen peroxide solution in water were dosed with a peristaltic pump at a constant rate over <NUM> into the reaction mixture under the liquid surface with stirring at <NUM> RPM, while maintaining the temperature at <NUM>. This has led to a deep blue colored dispersion of silver nanoplatelets, which was cooled to room temperature. <NUM> of compound of formula
<CHM>
(mixture of CAS <NUM>-<NUM>-<NUM> and <NUM>-<NUM>-<NUM>) was added, and the mixture was stirred for <NUM> at room temperature.

<NUM> of sodium dodecylsulfate was added to the reaction mixture and then ca. <NUM> of anhydrous sodium sulfate powder was added in portions with stirring until the transmission color of the dispersion changed from blue to pink. Then the mixture was kept without stirring at room temperature for <NUM>, allowing the coagulated nanoplatelets to sediment at the bottom of the reactor.

<NUM> of supernatant was pumped out from the reactor with a peristaltic pump, and <NUM> of deionized water was added to the reactor. The mixture in reactor was stirred for <NUM> at room temperature, allowing the coagulated particles to re-disperse.

<NUM> of anhydrous sodium sulfate powder was added in portions with stirring until the transmission color of the dispersion changed from blue to yellowish-pink. Then the mixture was kept without stirring at room temperature for <NUM>, allowing the coagulated nanoplatelets to sediment at the bottom of the reactor. <NUM> of supernatant was pumped out from the reactor with a peristaltic pump, and <NUM> of deionized water was added to the reactor. The resulting mixture was stirred for <NUM> minutes at room temperature, allowing the coagulated particles to re-disperse.

The resulting dispersion of Ag nanoplatelets was subjected to ultrafiltration using a Millipore Amicon <NUM> stirred ultrafiltration cell. The dispersion was diluted to <NUM> weight with de-ionized water and ultrafiltered to the end volume of ca. <NUM> using a polyethersulfone (PES) membrane with <NUM> kDa cut-off value. The procedure was repeated in total <NUM> times to provide <NUM> of Ag nanoplatelets dispersion in water. After ultrafiltration was completed, <NUM> of compound of formula
<CHM>
(mixture of <NPL> and <NPL>) was added to the dispersion.

Ag content <NUM>% w/w; yield ca. <NUM>% based on total silver amount; Solids content (at <NUM>) <NUM>% w/w; Purity <NUM>% w/w of silver based on solids content at <NUM>.

The dispersion was further ultrafiltered in isopropanol. <NUM> of Ag nanoplatelets dispersion, obtained after ultrafiltration in water, was placed in a Millipore Amicon <NUM> stirred ultrafiltration cell and diluted to <NUM> weight with isopropanol. The dispersion was ultrafiltered to the volume of ca. <NUM> using a polyethersulfone (PES) membrane with <NUM> kDa cut-off value. The procedure was repeated in total <NUM> times to provide <NUM> of Ag nanoplatelets dispersion in isopropanol.

Ag content <NUM>% w/w; Solids content (at <NUM>) <NUM>% w/w; Purity <NUM>% w/w of silver based on solids content at <NUM>.

The UV-Vis-NIR spectrum was recorded in water at Ag concentration of <NUM>*<NUM>-<NUM> M. λmax = <NUM>; extinction coefficient at maximum ε=<NUM>/(cm*mol Ag), FWHM = <NUM>.

Mean diameter of the silver nanoplatelets <NUM>±<NUM>, mean thickness of the silver nanoplatelets 16t2.

<NUM> (<NUM> solids) of Ag nanoplatelets dispersion, obtained as described at item B-2d, was placed in a <NUM> round-bottom flask under argon atmosphere at <NUM>. <NUM> of <NUM>% w/w solution of carbon disulfide in absolute ethanol was added and the mixture was stirred for <NUM>, followed by addition of <NUM> of <NUM>% w/w solution of diethanolamine in absolute ethanol. The mixture was stirred for <NUM> at <NUM>, then <NUM> of <NUM>% w/w solution of diethanolamine in absolute ethanol was added and stirring was continued for <NUM>.

The UV-Vis-NIR spectrum was recorded in water at Ag concentration of <NUM>*<NUM>-<NUM> M. λmax = <NUM>.

To the dispersion obtained in Step a), <NUM> of ethyl <NUM>-ethoxypropionate was added. The resulting mixture was concentrated on rotary evaporator at <NUM> mbar pressure and <NUM> bath temperature, till no more solvent was distilled off. The weight of the resulting dispersion was adjusted to <NUM> by addition of ethyl <NUM>-ethoxypropionate (corresponds to the calculated total solids content of <NUM>% w/w).

The UV-Vis-NIR spectrum was recorded in water at Ag concentration of <NUM>*<NUM>-<NUM> M. λmax = <NUM>.

To the dispersion, obtained in Step a), <NUM> of <NUM>-oxabicyclo[<NUM>. <NUM>]hept-<NUM>-ylmethyl <NUM>-oxabicyclo[<NUM>. <NUM>]heptane-<NUM>-carboxylate (<NPL>) was added. The resulting mixture was concentrated on rotary evaporator at <NUM> mbar pressure and <NUM> bath temperature, till no more solvent was distilled off. The weight of the resulting dispersion was adjusted to <NUM> by addition of <NUM>-oxabicyclo[<NUM>. <NUM>]hept-<NUM>-ylmethyl <NUM>-oxabicyclo[<NUM>. <NUM>]heptane-<NUM>-carboxylate (<NPL>) (corresponds to the calculated total solids content of <NUM>% w/w).

Ingredients provided in Table 2a below were independently mixed and dispersed at room temperature using a Dispermat CV-<NUM> for <NUM> minutes at <NUM> rpm so as to yield <NUM> of the inks E1 - E8.

The UV-Vis radiation curable screen printing hybrid security inks E1 - E8 were independently applied on pieces of transparent polymer substrate (PET Hostaphan® RN, thickness <NUM>, supplied by Pütz GmbH + Co. Folien KG) using a <NUM> thread/cm screen (<NUM> mesh). The printed pattern had a size of <NUM> × <NUM>. <NUM> seconds after the printing step, the pieces of printed substrate were independently cured at room temperature by exposing them two times at a speed of <NUM>/min to UV-Vis light under a dryer from IST Metz GmbH (two lamps: iron-doped mercury lamp <NUM> W/cm<NUM>+ mercury lamp <NUM> W/cm<NUM>), to generate security features.

The optical properties of each security features obtained at item C1b were independently assessed in reflection, in transmission, and usually using the three tests described below. The results are summarized in Table 1c.

Reflection measurements were performed using a goniometer (Goniospektrometer Codec WI-<NUM><NUM>&<NUM> by Phyma GmbH Austria). The L*a*b* values of the printed security features were determined at <NUM>° to the normal with an illumination angle of <NUM>° on the side of the transparent polymer substrate that was printed. The C* values (chroma, corresponding to a measure of the color intensity or color saturation) were calculated from a* and b* values according to the CIELAB (<NUM>) color space, wherein: <MAT>.

The C* values (reflection <NUM>/<NUM>°) are displayed in Table 1c below.

Transmission measurements were carried out using a Datacolor <NUM> spectrophotometer (parameters: integration sphere, diffuse illumination (pulse xenon D65) and <NUM>° viewing, analyzer SP2000 with dual <NUM> diode array for wavelength range of <NUM>-<NUM>, transmission sampling aperture size of <NUM>). The C* values (transmission <NUM>°) are displayed in Table 1c below.

A usual assessment was carried out observing each security feature with the naked eye in reflection with a diffuse source (such as the light coming through a window without direct sun, the observer facing the wall opposite to the window). The following colors have been observed:.

A visual assessment was also carried out observing each security feature with the naked eye in transmission. The following colors have been observed:.

As shown in Table 1c, the security features obtained from inks E1 - E8 according to the invention exhibited gold color in reflection and blue to deep blue color in transmission.

As attested by Table 1c, solvent-containing hybrid security inks E1 - E7 according to the present invention and solvent-free hybrid security ink E8 according to the present invention comprising either a mixture of cycloaliphatic epoxide and radically curable monomers/oligomers, or a mixture of cycloaliphatic epoxide, radically curable monomers/oligomers and cationically curable monomers provide security features with excellent usual aspect and high chroma values C* both in reflected light and in transmitted light.

Ingredients provided in Table 2a were mixed and dispersed at room temperature using a Dispermat CV-<NUM> for <NUM> minutes at <NUM> rpm so as to yield <NUM> of each ink E9 - E15.

The UV-Vis radiation curable screen printing inks E9 - E15 were independently applied on pieces of transparent polymer substrate (PET Hostaphan® RN, thickness <NUM>, supplied by Pütz GmbH + Co. Folien KG) using a <NUM> thread/cm screen (<NUM> mesh). The printed pattern had a size of <NUM> × <NUM>. <NUM> seconds after the printing step, the pieces of printed substrate were independently cured at room temperature by exposing them two times at a speed of <NUM>/min to UV-Vis light under a dryer from IST Metz GmbH (two lamps: iron-doped mercury lamp <NUM> W/cm<NUM> + mercury lamp <NUM> W/cm<NUM>), to generate security features.

The optical properties of the security features obtained at item C2b were independently assessed in reflection, in transmission, and visually using the tests described at item C1c.

The colors in reflection and transmission and the C* values (reflection <NUM>/<NUM>° and transmission <NUM>°) exhibited by the security features prepared using the comparative inks E9 - E15 according to the present invention are displayed in Table 2c (below).

As attested by Table 2c, cationically curable solvent-free security inks E9 - E12 according to the present invention and cationically curable solvent-containing security inks E13 - E15 according to the present invention comprising either cycloaliphatic epoxide, or cycloaliphatic epoxide and other cationically curable monomers provide security features with excellent usual aspect and high chroma values C* both in reflected light and in transmitted light.

To evaluate the influence of the surfactant on the optical properties exhibited by the security feature, comparative inks C1 - C7 and inks E16 - E18 according to the invention were prepared.

Ingredients provided in Table 3a were mixed and dispersed at room temperature using a Dispermat CV-<NUM> for <NUM> minutes at <NUM> rpm so as to yield <NUM> of each ink C1 - C7 and E16 - E18.

The UV-Vis radiation curable screen printing inks C1 - C7 and E16 - E18 were independently applied on pieces of transparent polymer substrate (PET Hostaphan® RN, thickness <NUM>, supplied by Pütz GmbH + Co. Folien KG) using a <NUM> thread/cm screen (<NUM> mesh). The printed pattern had a size of <NUM> × <NUM>. <NUM> seconds after the printing step, the pieces of printed substrate were independently cured at room temperature by exposing them two times at a speed of <NUM>/min to UV-Vis light under a dryer from IST Metz GmbH (two lamps: iron-doped mercury lamp <NUM> W/cm<NUM> + mercury lamp <NUM> W/cm<NUM>), to generate security features.

The optical properties of the security features at item C3b were independently assessed in reflection, in transmission, and usually using the tests described at item C1c.

The colors in reflection and transmission and the C* values (reflection <NUM>/<NUM>° and transmission <NUM>°) exhibited by the security features prepared using the inks C1 - C7 and E16 - E18 are displayed in Table 3c below.

As shown in Table 3c the security features obtained from an ink according to the invention comprising a perfluoropolyether surfactant functionalized with hydroxyl groups (Fluorolink E10H: ink E16), (meth)acrylate groups (Fluorolink MD700: ink E17), or silane groups (Fluorolink S10: ink E18), exhibit gold color in reflection and deep blue color in transmission. By comparison, the security features obtained from inks comprising perfluoropolyether surfactants lacking said functional groups (Fluorolink F10: comparative ink C1), or surfactants lacking the perfluoropolyether chain, such as BYK <NUM> (comparative ink C2), BYK <NUM> (comparative ink C3), TEGO RAD <NUM> (comparative ink C4), TEGO RAD <NUM> (comparative ink C5), Dynasylan F8815 (comparative ink C6) and Dynasylan F8261 (comparative ink C7) exhibit a dull blue to blue color in transmission, but a dark brown to brown color with low chroma value in reflection. A dark brown to brown color with low chroma value in reflection is not eye-catching and therefore, not suitable for a dichroic security feature for securing a value document.

To evaluate the influence of the of the amount of the perfluoropolyether surfactant as described herein on the optical properties exhibited by the security feature, inks C8 and E19 - E23 according to the invention were prepared as described below.

Ingredients provided in Table 4a were mixed and dispersed at room temperature using a Dispermat CV-<NUM> for <NUM> minutes at <NUM> rpm so as to yield <NUM> of each ink C8 and E19 - E23.

The UV-Vis radiation curable screen printing inks C8 and E19 - E23 were independently applied on pieces of transparent polymer substrate (PET Hostaphan® RN, thickness <NUM>, supplied by Pütz GmbH + Co. Folien KG) using a <NUM> thread/cm screen (<NUM> mesh). The printed pattern had a size of <NUM> × <NUM>. About <NUM> seconds after the printing step, the pieces of printed substrate were independently cured at room temperature by exposing them two times at a speed of <NUM>/min to UV-Vis light under a dryer from IST Metz GmbH (two lamps: iron-doped mercury lamp <NUM> W/cm<NUM> + mercury lamp <NUM> W/cm<NUM>), to generate security features.

The optical properties of the security features obtained at item C4b were independently assessed in reflection, in transmission, and visually using the tests described at item C1c.

The colors in reflection and transmission and the C* values (reflection <NUM>/<NUM>° and transmission <NUM>°) exhibited by the security features prepared using the inks C8 and E19 - E23 are displayed in Table 4c below.

As shown in Table 4c, the use of an amount from about <NUM> wt-% to about <NUM> wt-% of a perfluoropolyether surfactant as described herein (such as Fluorolink E10H used in the inks E19 - E23) ensures that security features showing metallic yellow color with high chroma values in reflection and deep blue color in transmission are obtained (inks E19 - E23). By comparison, the security feature obtained in the experiment conducted with an ink C8 containing no surfactant exhibits a dark brown to brown color with low chroma value in reflection. Such color is not eye-catching for the layperson and cannot be used as security feature for securing a value document.

To evaluate the influence of the type of polyvinylchloride copolymer on the optical properties exhibited by the security feature, inks E1 and E24 - E26 according to the invention were prepared as described below.

Ingredients provided in Table 5a were mixed and dispersed at room temperature using a Dispermat CV-<NUM> for <NUM> minutes at <NUM> rpm so as to yield <NUM> of each ink E1 and E24 - E26.

The UV-Vis radiation curable screen printing inks E1 and E24 - E26 were independently applied on pieces of transparent polymer substrate (PET Hostaphan® RN, thickness <NUM>, supplied by Pütz GmbH + Co. Folien KG) using a <NUM> thread/cm screen (<NUM> mesh). The printed pattern had a size of <NUM> × <NUM>. <NUM> seconds after the printing step, the pieces of printed substrate were independently cured at room temperature by exposing them two times at a speed of <NUM>/min to UV-Vis light under a dryer from IST Metz GmbH (two lamps: iron-doped mercury lamp <NUM> W/cm<NUM> + mercury lamp <NUM> W/cm<NUM>), to generate security features.

The optical properties of the security features obtained at item C5b were independently assessed in reflection, in transmission, and usually using the tests described at item C1c.

The colors in reflection and transmission and the C* values (reflection <NUM>/<NUM>° and transmission <NUM>°) exhibited by the security features prepared using the inks E1 and E24 - E26 are displayed in Table 5c below.

As attested by the optical properties of security features shown in Table 5c, security inks as claimed herein comprising a polyvinyl chloride copolymer which contains at least about <NUM> wt-%, preferably at least about <NUM> wt-% of vinyl chloride, provide security feature exhibiting blue to deep blue color in reflection and a metallic yellow color in reflection.

To evaluate the stability upon time of an ink according to the invention, <NUM> of ink E2 (described in Table 1a) were placed in a cup (<NUM> white polypropylene cup for SpeedMixer™ available at Hauschild & Co. KG), which was hermetically sealed and stored for five months at a temperature of <NUM> in a laboratory oven (Kendro Laboratory Products, T6060). The freshly prepared ink E2 was used as a comparison standard. Each month, the sample of ink E2 stored in the oven was cooled at room temperature for <NUM> hours, and subsequently applied on a piece of transparent polymer substrate (PET Hostaphan® RN, thickness <NUM>, supplied by Pütz GmbH + Co. Folien KG) using a <NUM> thread/cm screen (<NUM> mesh). The printed pattern had a size of <NUM> × <NUM>. <NUM> seconds after the printing step, the piece of printed substrate was cured at room temperature by two times exposure at a speed of <NUM>/min to UV-Vis light under a dryer from IST Metz GmbH (two lamps: iron-doped mercury lamp <NUM> W/cm<NUM>+ mercury lamp <NUM> W/cm<NUM>), to generate a security feature. The optical properties of the security feature obtained each month were independently assessed in reflection, and visually using the tests described at item C1c. Table <NUM> summarizes the color in reflection and transmission and the C* values (reflection <NUM>/<NUM>°) exhibited by the security features.

Claim 1:
A UV-Vis radiation curable security ink for producing a security feature exhibiting a blue color upon viewing in transmitted light and a metallic yellow color upon viewing in incident light, wherein said ink comprises:
a) from <NUM> wt-% to <NUM> wt-% of silver nanoplatelets having a mean diameter in the range of <NUM> to <NUM> with a standard deviation of less than <NUM>%, a mean thickness in the range of <NUM> to <NUM> with a standard deviation of less than <NUM>%, and a mean aspect ratio higher than <NUM>, wherein the mean diameter is determined by transmission electron microscopy and the mean thickness is determined by transmission electron microscopy, and
wherein the silver nanoplatelets bear a surface stabilizing agent of general formula (I)
<CHM>
wherein
the residue RA is a C<NUM>-C<NUM>alkyl group substituted with a hydroxy group;
the residue RB is selected from a C<NUM>-C<NUM> alkyl group, and a C<NUM>-C<NUM>alkyl group substituted with a hydroxy group; and
Cat+ is an ammonium cation of general formula +NH<NUM>RCRD,
wherein the residue RC is a C<NUM>-C<NUM>alkyl group substituted with a hydroxy group; and
the residue RD is selected from a C<NUM>-C<NUM>alkyl group, and a C<NUM>-C<NUM>alkyl group substituted with a hydroxy group;
b) from <NUM> wt-% to <NUM> wt-% of either a cycloaliphatic epoxide, or a mixture of a cycloaliphatic epoxide and one or more UV-Vis radiation curable compounds;
c) one or more cationic photoinitiators;
d) a perfluoropolyether surfactant functionalized with one or more functional groups selected from the group consisting of: hydroxyl, acrylate, methacrylate, and trialkoxysilyl;
e) from <NUM> wt-% to <NUM> wt-% of a polyvinyl chloride copolymer containing at least <NUM> wt-% of vinyl chloride; and optionally
f) up to <NUM> wt-% of an organic solvent;
the weight percents being based on the total weight of the UV-Vis radiation curable security ink.