OSWALD ANDRE (CH)
WO2020083794A1 | 2020-04-30 | |||
WO2020224982A1 | 2020-11-12 | |||
WO2009056401A1 | 2009-05-07 | |||
WO2010108837A1 | 2010-09-30 | |||
WO2011064162A2 | 2011-06-03 | |||
WO2013186167A2 | 2013-12-19 | |||
WO2014041121A1 | 2014-03-20 | |||
WO2014187750A1 | 2014-11-27 | |||
WO2020083794A1 | 2020-04-30 | |||
WO2020224982A1 | 2020-11-12 | |||
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CN109663932A | 2019-04-23 | |||
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EP2667990B1 | 2016-12-14 | |||
EP1791702B9 | 2011-09-14 | |||
EP19172734A | 2019-05-06 |
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Claims 1. A composition, comprising silver nanoplatelets, wherein the silver nanoplatelets are capped by a dithiocarbamate anion of formula (XX), wherein R41 is a C2-C4alkyl group, which is substituted by one, or two hydroxy groups, and R42 is a C1-C4alkyl group, or a C2-C4alkyl group, which is substituted by one, or two hydroxy groups. 2. The composition according to claim 1, wherein the composition of the dithiocarbamate capped silver nanoplatelets is prepared i) by reacting CS2 with an amine of formula R41R42NH in the presence of a composition of silver nanoplatelets; or ii) by reacting CS2 with an amine of formula R41R42NH in the presence of a composition of silver nanoplatelets and subsequent treatment with a base; or iii) by reacting CS2 with an amine of formula R41R42NH in the presence of a composition of silver nanoplatelets and a base; or iv) by reacting CS2 with an amine of formula R41R42NH in the presence of a base to obtain a compound of formula (XXI), which is then reacted with a composition of silver nanoplatelets; wherein R41 and R42 are defined in claim 1, Catk+ is a cation and k is 1, or 2. 3. The composition according to claim 1, or 2, wherein R41 is a -CH2CH2OH group, a -CH2CH(OH)CH3 group, or a -CH2CH2CH2OH group; and R42 is a -CH2CH2OH group, a -CH2CH(OH)CH3 group, or a -CH2CH2CH2OH group. 4. The composition according to claim 2, or 3, wherein the base is selected from alkali metal hydroxides, alkali metal alkoxides, amines of formula R43R44R45N and mixtures thereof, wherein R43 is a C2-C4alkyl group, which is substituted by one, or two hydroxy groups, and R44 is a C1-C4alkyl group, or a C2-C4alkyl group, which is substituted by one, or two hydroxy groups, and R45 is H, a C1-C4alkyl group, or a C2-C4alkyl group, which is substituted by one, or two hydroxy groups. 5. The composition according to any of claims 1 to 4, wherein the dithiocarbamate capped silver nanoplatelets are prepared i) by reacting CS2 with diethanolamine in the presence of a composition of silver nanoplatelets, ii) by reacting CS2 with diethanolamine in the presence of a composition of silver nanoplatelets and subsequent treatment with a base; or iii) by reacting CS2 with diethanolamine in the presence of a composition of silver nanoplatelets and a base; wherein the base is selected from alkali metal hydroxides, alkali metal alcoholates and amines of formula R43R44R45N and mixtures thereof, wherein R43 is a C2-C4alkyl group, which is substituted by one hydroxy group, R44 is a C2-C4alkyl group, which is substituted by one hydroxy group, and R45 is H, or a C2-C4alkyl group, which is substituted by one hydroxy group; iv) by reacting CS2 with diethanolamine in the presence of alkali metal alcoholates, or alkali metal hydroxides to obtain a compound of formula (XXIa), wherein Cat+ is an alkali metal cation and then reacting the compound of formula (XXIa) with a composition of silver nanoplatelets. 6. The composition according to any of claims 1 to 5, wherein the number mean diameter of the silver nanoplatelets, present in the composition, is in the range of 50 to 150 nm and the number mean thickness of the silver nanoplatelets, present in the composition, is preferably in the range of 5 to 30 nm and the mean aspect ratio of the silver nanoplatelets is preferably higher than 2.0, wherein the number mean diameter and the number mean thickness are determined by transmission electron microscopy. 7. The composition according to claim 6, wherein the highest wavelength absorption maximum of the population of all silver nanoplatelets in the composition being within the range of 560 to 800 nm, especially 600 to 800 measured in water at ca.5*10-5 M (mol/l) concentration of silver. 8. The composition according to claim 6, wherein the number mean diameter of the silver nanoplatelets is in the range of 70 to 120 nm with standard deviation being less than 50% and the number mean thickness of the silver nanoplatelets is in the range of 8 to 25 nm with standard deviation being less than 30% and the mean aspect ratio of the silver nanoplatelets is higher than 2.5, wherein the number mean diameter and the number mean thickness are determined by transmission electron microscopy. 9. The composition according to any of claims 1 to 8, wherein the silver nanoplatelets bear at least a surface stabilizing agent which is selected from polymers of formula w R 1 is H, C1-C18alkyl, phenyl, C1-C8alkylphenyl, or CH2COOH; R2, R3, R4, R5, R6 and R7 are independently of each other H, C1-C8alkyl, or phenyl; Y is O, or NR8; R8 is H, or C1-C8alkyl; k1 is an integer in the range of from 1 to 500, k2 and k3 are independently of each other 0, or integers in the range of from 1 to 250; k4 is 0, or 1, k5 is an integer in the range of from 1 to 5, and surface stabilizing agents, which are polymers, or copolymers, which are obtained by a process comprising the steps i1) polymerizing in a first step one or more ethylenically unsaturated monomers in the presence of at least one nitroxylether having the structural element N O X , wherein X represents a group having at least one carbon atom and is such that the free radical X• derived from X is capable of initiating polymerization; or i2) polymerizing in a first step one or more ethylenically unsaturated monomers in the presence of at least one stable free nitroxyl radical N O and a free radical initiator; wherein at least one monomer used in the step s ) o i2) is a C1-C6alkyl or hydroxyC1-C6alkyl ester of acrylic or methacrylic acid; and optionally ii) a second step, comprising the modification of the polymer or copolymer prepared under i1) or i2) by a transesterification reaction, an amidation, hydrolysis or anhydride modification or a combination thereof; and mixtures thereof. 10. The composition according to claim 9, wherein the silver nanoplatelets bear at least a surface stabilizing agent which is selected from copolymers represented by formula (I ) R11 and R12 are H or methyl, R13, Ra and Ra’ are independently of each other H or methyl, Rb is saturated or unsaturated, linear or branched chain alkyl with 1–22 carbon atoms, Rb’ is RA-[O-CH2-CH2-]n1-O-, R14 is -C(=O)-N-(CH2)yNR15R16, or -C(=O)-N-(CH2)yNHR15R16+An-, w A n- is an anion of a monovalent organic, or inorganic acid; y is an integer from 2 to 10; R15 is saturated or unsaturated, linear or branched chain alkyl with 1–22 carbon atoms, R16 is saturated or unsaturated, linear or branched chain alkyl with 1–22 carbon atoms, RA is saturated or unsaturated, linear or branched chain alkyl with 1–22 carbon atoms, or alkylaryl or dialkylaryl with up to 24 carbon atoms; n1 is 1 to 150, m, n and p are independently of each other integers from 1 to 200, and o is an integer from 1 to 150; and polymers of formula (Ia), wherein R1 is H, or a C1-C8alk k1 is 22 to 450, especially 22 to 150 and mixtures thereof. 11. The composition according to any of claims 1 to 10, which comprises one, or more stabilizing agents selected from the group consisting of compounds of formula (IIb), wherein R atom, a halogen atom, a C1-C8alkoxy group, or a C1-C8 alkyl group, R21b is a hydrogen atom, or a group of formula -CHR24-N(R22)(R23), R22 and R23 are independently of each other a C1-C8alkyl, a hydroxyC1-C8alkyl group, or a group of formula -[(CH2CH2)-O]n2-CH2CH2-OH, wherein n2 is 1 to 5, R24 is H or C1-C8alkyl, and compounds of formula (IIc), wherein R25 can be the same, or different in each occurrence and is a hydrogen atom, a halogen atom, a C1-C18alkyl group, a C1-C18alkoxy group, or a group -C(=O)-R26, R26 is a hydrogen atom, a hydroxy group, a C1-C18alkyl group, unsubstituted or substituted aminogroup, unsubstituted or substituted phenyl group, or a C1-C18alkoxy group, and n3 is a number of 1 to 4, m3 is a number of 2 to 4, and the sum of m3 and n3 is 6. 12. The composition according to any of claims 1 to 11, which comprises at least one surface stabilizing agent selected from the group consisting of MPEG 2000 thiol (A-1, average Mn 2,000), MPEG 3000 thiol (A-2), MPEG 4000 thiol (A-3) MPEG 5000 thiol (A-4), MPEG 6000 thiol (A-5), PEG thiols (O-(2-mercaptoethyl)- poly(ethylene glycol)) having an average Mn of 2000 to 6000, such as, for example, PEG 2000 thiol (A-6, average Mn 2,000), PEG 3000 thiol (A-7), PEG 4000 thiol (A-8), PEG 5000 thiol (A-9), PEG 6000 thiol (A-10); at least one surface stabilizing agent selected from copolymers represented by formula (I integers from 1 to 200, o is an integer from 1 to 150, especially an integer from 1 to 149; and at least one stabilizing agent selected from the group consisting of compounds of formula gallate, C-6) and (lauryl gallate, C-7). 13. A process for producing the composition according to any of claims 1 to 12, comprising i) by reacting CS2 with an amine of formula R41R42NH in the presence of a composition of silver nanoplatelets; or ii) by reacting CS2 with an amine of formula R41R42NH in the presence of a composition of silver nanoplatelets and subsequent treatment with a base; or iii) by reacting CS2 with an amine of formula R41R42NH in the presence of a composition of silver nanoplatelets and a base; or iv) by reacting CS2 with an amine of formula R41R42NH in the presence of a base to obtain a compound of formula (XXI), which is then reacted with a composition of silver nan p ; d R42 are defined in claim 1, Catk+ is a cation and k is 1, or 2. 14. The process according to claim 13, wherein the composition of the silver nanoplatelets, is prepared by a process, which comprises: (a) preparing a solution (a) comprising a silver precursor, a compound of formula (I’), wherein R R2, R3, R4, R5, R6 and R7 are independently of each other H, C1-C8alkyl, or phenyl; Y is O, or NR8; R8 is H, or C1-C8alkyl; k1 is an integer in the range of from 1 to 500, k2 and k3 are independently of each other 0, or integers in the range of from 1 to 250; k4 is 0, or 1, k5 is an integer in the range of from 1 to 5; and a polymer, or copolymer which can be obtained by a process comprising the steps i1) polymerizing in a first step one or more ethylenically unsaturated monomers in the presence of at least one nitroxylether having the structural element N O X , wherein X represents a group having at least one carbon atom and is such that the free radical X• derived from X is capable of initiating polymerization; or i2) polymerizing in a first step one or more ethylenically unsaturated monomers in the presence of at least one stable free nitroxyl radical N O and a free radical initiator; wherein at least one monomer used in the step s i1) or i2) is a C1-C6 alkyl or hydroxy C1-C6 alkyl ester of acrylic or methacrylic acid; and optionally ii) a second step, comprising the modification of the polymer or copolymer prepared under i1) or i2) by a transesterification reaction, an amidation, hydrolysis or anhydride modification or a combination thereof; water, and optionally a defoamer; (b1) preparing a solution (b), comprising a reducing agent, which comprises at least one boron atom in the molecule, and water; (b2) adding solution (a) to solution (b), and adding one or more complexing agents; (c) adding a hydrogen peroxide solution in water; and (d) optionally adding a stabilization agent to the mixture obtained in step (c), thereby synthesizing the composition, comprising the silver nanoplatelets. 15. A composition, comprising silver nanoplatelets, which is obtained by the process according to claim 13, or 14. |
(isopropyl gallate, C-4) (butyl gallate, C-5), (octyl gallate, C-6) and (lauryl gallate, C-7). In another preferred embodiment of the present invention the polyhydric phenols are compounds of formula wherein R 25 is a hydrogen atom, a C 1 -C 18 alkyl group, or a group of formula-C(=O)-R 26 , wherein R 26 is a hydrogen atom, a hydroxy group, a C 1 -C 18 alkyl group, or a C 1 -C 18 alkoxy group, an unsubstituted or substituted amino group, an unsubstituted or substituted phenyl group, especially a C 1 - C18alkyl group or C 1 -C 8 alkoxy group, such as, for example, (C-8) and (C-9). An unsubstituted or substituted amino group is, for example, a group of formula -NR 27 R 28 , wherein R 27 and R 28 are independently of each other a hydrogen atom, a C 1 -C 18 alkyl group, a phenyl group, preferably a hydrogen atom, or a C 1 -C 18 alkyl group. In a particularly preferred embodiment the stabilizing agent is selected from compounds of formula (IIb), (IIc), or mixtures thereof. In a particularly preferred embodiment the silver nanoplatelets comprise one, or more surface stabilizing agents of formula (I) and one, or more surface stabilizing agents of formula (III). In addition, the silver nanoplatelets may comprise one, or more stabilizing agents of formula (IIb). The most preferred (surface) stabilizing agents (surface stabilizing agents and stabilizing agents), or combinations thereof are shown in the below table. (HOCH 2 CH 2 )2NH 2 + , Na + , K + and Cs + represent preferred cations for the dithiocarbamate anion E-1. A process for producing the composition according to the present invention, comprising the silver nanoplatelets, comprises: (a) preparing a solution (a) comprising a silver precursor, a compound of formula (I’), wherein R 1 is H, C 1 -C 18 alkyl, phenyl, C 1 -C 8 alkylphenyl, or CH 2 COOH; R 2 , R 3 , R 4 , R 5 , R 6 and R 7 are independently of each other H, C1-C8alkyl, or phenyl; Y is O, or NR 8 ; R 8 is H, or C1-C8alkyl; k1 is an integer in the range of from 1 to 500, k2 and k3 are independently of each other 0, or integers in the range of from 1 to 250; k4 is 0, or 1, k5 is an integer in the range of from 1 to 5; and a polymer, or copolymer which can be obtained by a process comprising the steps i1) polymerizing in a first step one or more ethylenically unsaturated monomers in the presence of at least one nitroxylether having the structural element wherein X represents a group having at least one carbon atom and is such that the free radical X• derived from X is capable of initiating polymerization; or i2) polymerizing in a first step one or more ethylenically unsaturated monomers in the presence of at least one stable free nitroxyl radical and a free radical initiator; wherein at least one monomer used in the steps i1) or i2) is a C 1 -C 6 alkyl or hydroxy C 1 -C 6 alkyl ester of acrylic or methacrylic acid; and optionally ii) a second step, comprising the modification of the polymer or copolymer prepared under i1) or i2) by a transesterification reaction, an amidation, hydrolysis or anhydride modification or a combination thereof; water, and optionally a defoamer; (b1) preparing a solution (b), comprising a reducing agent, which comprises at least one boron atom in the molecule, and water; (b2) adding solution (a) to solution (b), and adding one or more complexing agents; (c) adding a hydrogen peroxide solution in water; and (d) optionally adding a stabilization agent to the mixture obtained in step (c), thereby synthesizing the composition, comprising the silver nanoplatelets. The silver precursor is preferably a silver(I) compound, selected from the group consisting of AgNO3; AgClO4; Ag2SO4; AgCl, AgF, AgOH; Ag2O; AgBF4; AgIO3; AgPF6; R 200 CO2Ag, R 200 SO3Ag, wherein R 200 is unsubstituted or substituted C 1 -C 18 alkyl, unsubstituted or substituted C5-C8cycloalkyl, unsubstituted or substituted C7-C18aralkyl, unsubstituted or substituted C6-C18aryl or unsubstituted or substituted C2-C18heteroaryl; Ag salts of dicarboxylic, tricarboxylic, polycarboxylic acids, polysulfonic acids, P-containing acids and mixtures thereof. More preferably, the silver precursor is selected 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. Silver nitrate is most preferred. 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. Nonlimiting 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 penta acetic acid (DTPA); propylene diamine tetracetic acid (PDT A); 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 (Na3MGDA) 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 1488, 1495, 3062, 7447, 800, 8030, 805, 8050, 810, 815N, 822, 825, 830, 835, 840, 842, 843, 845, 855, 860, 883, K 3, K 7, K 8, N, Antifoam SE-15 from Sigma, Struktol SB-2080 and the like. The amount of the defoamer is in the range of from 0.00001 % to 5% by weight based on total weight of reaction mixture prior to hydrogen peroxide addition, preferably from 0.0001 % to 3% and more preferably from 0.001 % to 2 % by weight. The defoamer can be added to any of the solutions (a) and (b). Preferably, the reaction of silver nanoplatelets formation is carried out at a total silver concentration of >1% w/w after combining the first solution with the second solution. Preferably, the reaction of silver nanoplatelets formation is carried out by gradually adding the silver precursor solution into reducing agent solution, whereas the temperature of both solutions is in the range of -3 to 40°C and the gradual addition is completed within 15 minutes to 24 h time. The silver nanoplatelets can be isolated by known methods such as decantation, filtration, (ultra)centrifugation, reversible or irreversible agglomeration, phase transfer with organic solvent and combinations thereof. The silver nanoplatelets may be obtained after isolation as a wet paste or dispersion in water. The silver nanoplatelets content in the final preparation of said particles may be up to about 99% by weight, based on the total weight of the preparation, preferably between 5 to 99% by weight, more preferably 5 to 90% by weight. A preferred aspect of the present invention relates to a method which comprises further a step e), wherein the dithiocarbamate capped silver nanoplatelets are prepared i) by reacting CS2 with an amine of formula R 41 R 42 NH in the presence of a composition of silver nanoplatelets; or ii) by reacting CS2 with an amine of formula R 41 R 42 NH in the presence of a composition of silver nanoplatelets and subsequent treatment with a base; or iii) by reacting CS2 with an amine of formula R 41 R 42 NH in the presence of a composition of silver nanoplatelets and a base; or iv) by reacting CS2 with an amine of formula R 41 R 42 NH in the presence of a base to obtain a compound of formula (XXI), which is then reacted with a composition of silver nanoplatelets; wherein R 41 and R 42 are defined above, or below, Cat k+ is a cation and k is 1, or 2. In case of ii) and iii) the base is preferably selected from alkali or alkaline earth metal hydroxides, alkali or alkaline earth metal alkoxides, amines and quaternary ammonium hydroxides and mixtures thereof. In case of iv) the base is preferably selected from alkali or alkaline earth metal hydroxides, alkali or alkaline earth metal alkoxides and mixtures thereof. Preferred examples of bases are NaOH, KOH and CsOH, NaOCH 2 CH 3 , KOCH 2 CH 3 , CsOCH 2 CH 3 , and mixtures thereof. More preferably, the base is selected from alkali metal hydroxides, such as, for example, NaOH, KOH and CsOH, alkali metal alkoxides, such as, for example, NaOCH 2 CH 3 , KOCH 2 CH 3 and CsOCH 2 CH 3 , amines of formula R 43 R 44 R 45 N, especially (HOCH 2 CH 2 )2NH and (HOCH 2 CH 2 )3N, and mixtures thereof. With respect to the compound of formula (XXI) the same preferences apply for R 41 and R 42 as in case of formula (XX) and Cat k+ is selected from alkali metal cations, alkaline earth metal cations and R 43 R 44 R 45 NH + , such as, for example, (HOCH 2 CH 2 )2NH 2 + and (HOCH 2 CH 2 )3NH + , and mixtures thereof and is preferably selected from alkali metal cations, alkaline earth metal cations and mixtures thereof. Cat k+ is more preferably selected from alkali metal cations, most preferred from Cs + , Rb + , K + and Na + and mixtures thereof. Examples of compounds of formula (XXI) are compound (21b), (21b) and (21c). Compounds (21a) and (21b) are more stable and are preferred. In reaction sequences iv) it is preferred that the compound of formula (XXI) is reacted with the silver nanoplatelets immediately after preparation The in-situ synthesis of the dithiocarbamate anions in the presence of the silver nanoplatelets, i.e. reaction sequences i), ii) and iii) are more preferred. In reaction sequence i) carbon disulfide or its solution in a solvent and an amine of formula R 41 R 42 NH or its solution in a solvent are successively added to an Ag nanoplatelets dispersion under inert gas atmosphere at 0-30°C. The reaction mixture is stirred for 5 minutes to 24 h at 0-30°C. In reaction sequence ii) carbon disulfide or its solution in a solvent and an amine of formula R 41 R 42 NH or its solution in a solvent are successively added to an Ag nanoplatelets dispersion under inert gas atmosphere at 0-30°C. The reaction mixture is stirred for 5 minutes to 24 h at 0-30°C and then a solution of a base, such as, for example, cesium hydroxide, potassium ethoxide or diethanolamine, in a solvent is added and the reaction mixture is stirred for 5 min to 24 h at 0-30°. In reaction sequence iii) carbon disulfide or its solution in a solvent and a mixture of amine of formula R 41 R 42 NH and a base, such as, for example, cesium hydroxide, potassium ethoxide or diethanolamine, or their solution in a solvent, are successively added to an Ag nanoplatelets dispersion under inert gas atmosphere at 0-30°C and the reaction mixture is stirred for 5 min to 24 h at 0-30°. The amine of formula R 41 R 42 NH and the base may also be added separately in parallel. Examples of suitable solvents are water, ethanol, isopropanol, ethyl-3-ethoxypropionate and 1-methoxy-2-propanol, or mixtures thereof. A preferred aspect of the present invention relates to a method which comprises further a step f), wherein the dispersion of the silver nanoplatelets is concentrated and/or water is replaced at least partially with an organic solvent. Examples of suitable organic solvents are ethanol, isopropanol, ethyl acetate, ethyl-3-ethoxypropionate and 1-methoxy-2-propanol, or mixtures thereof, optionally with water. The present application is also directed to compositions, comprising silver nanoplatelets, which are obtained by above-described process. The compositions of the present invention may be used in coatings, or printing inks, which are described for example, in WO2020/224982. The suitability of dithiocarbamate capped silver nanoplatelets for use in security inks for producing security features for securing value documents, which exhibit a first color upon viewing in transmitted light and a second color different from the first color upon viewing in incident light is described in two European patent applications both entitled "UV-VIS radiation curable security inks for producing dichroic security features" filed by SICPA HOLDING SA on November 10, 2020. Part of the European patent applications of SICPA HOLDING SA is outlined below in excerpts for reference purpose only. The 1 st European patent application of SICPA HOLDING SA describes UV-VIS radiation curable security inks, wherein said inks comprise: a) from about 7.5 wt-% to about 20 wt-% of silver nanoplatelets having a mean diameter in the range of 50 to 150 nm with a standard deviation of less than 60%, a mean thickness in the range of 5 to 30 nm with a standard deviation of less than 50%, and a mean aspect ratio higher than 2.0, 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 (XXI), wherein the residue R 41 is a C 2 -C 4 alkyl group substituted with a hydroxy group; the residue R 42 is selected from a C1-C4 alkyl group, and a C 2 -C 4 alkyl group substituted with a hydroxy group; and Cat k+ is a cation selected from the group consisting of Na + , K + , Rb + and Cs + ; b) a perfluoropolyether surfactant functionalized with at least a hydroxy group; c) from about 3 wt-% to about 12 wt-% of a polyvinyl chloride copolymer containing at least 69 wt-% of vinyl chloride; d) d1) from about 25 wt-% to about 55 wt-% of a cycloaliphatic epoxide, and from about 1 wt-% to about 10 wt-% of a cationic photoinitiator; or d2) from about 30 wt-% to about 65 wt-% of a mixture of a cycloaliphatic epoxide and a radically curable compound, from about 1 wt-% to about 6 wt-% of a cationic photoinitiator, and from about 1 wt-% to about 6 wt-% of a free radical photoinitiator; and optionally e) a cationically curable compound selected from the group consisting of: e1) a vinyl ether having two vinyloxy residues in an amount lower than 50% of the weight percent (wt-%) of the cycloaliphatic epoxide of d); e2) a vinyl ether having one vinyloxy residue in an amount lower than about 5 wt-%; e3) an epoxide other than a cycloaliphatic epoxide in an amount lower than about 10 wt-%; e4) an oxetane having two oxetanyl residues in an amount lower than about 20 wt-%; e5) an oxetane having one oxetanyl residue in an amount lower than about 3.5 wt-%; and e6) a mixture of e1) and/or e2) and/or e3) and/or e4) and/or e5); the weight percents being based on the total weight of the UV-Vis radiation curable security ink; and 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: A) printing, preferably by screen printing, rotogravure, or flexography, the UV-Vis radiation curable security ink claimed and described herein on a transparent or partially transparent region of a substrate of a value document to provide an ink layer; and B) UV-Vis curing the ink layer obtained at step A) to form the security feature. The combination of the ditiocarbamate capped silver nanoplatelets of the present invention and the specific ink vehicle described in the European patent application of SICPA HOLDING SA allows expedient migration of the silver nanoplatelets contained in an ink layer obtained by printing the security ink from the mass of the ink layer at the interface between the ink layer and air and at the interface between the ink layer and the substrate and alignment at said interfaces to form thin reflective layers, thereby producing independently of the thickness of the printed ink layer the metallic yellow color in reflection and the blue color in transmission. The expedient development of the metallic yellow color in reflection and of the blue color in transmission cannot be achieved with the inks descried in the prior art. The cationically curable binder or hybrid curable binder contained by the UV- Vis radiation curable security ink provides the dichroic security feature obtained from said ink with a high mechanical resistance. The UV-Vis radiation curable ink described therein has outstanding shelf stability. A UV-Vis radiation cationically curable security ink may comprise: a) from about 7.5 wt-% to about 20 wt-% of silver nanoplatelets having a mean diameter in the range of 50 to 150 nm with a standard deviation of less than 60%, a mean thickness in the range of 5 to 30 nm with a standard deviation of less than 50%, and a mean aspect ratio higher than 2.0, 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) (XXI), wherein the residue R 41 is a C 2 -C 4 alkyl group substituted with a hydroxy group; the residue R 42 is selected from a C1-C4 alkyl group, and a C 2 -C 4 alkyl group substituted with a hydroxy group; and Cat k+ is a cation selected from the group consisting of Na + , K + , Rb + and Cs + ; b) a perfluoropolyether surfactant functionalized with at least a hydroxy group; c) from about 3 wt-% to about 12 wt-% of a polyvinyl chloride copolymer containing at least 69 wt-% of vinyl chloride; d1) from about 25 wt-% to about 55 wt-% of a cycloaliphatic epoxide, and from about 1 wt-% to about 10 wt-% of a cationic photoinitiator; and optionally e) a cationically curable compound selected from the group consisting of: e1) a vinyl ether having two vinyloxy residues in an amount lower than 50% of the weight percent (wt-%) of the cycloaliphatic epoxide of d); e2) a vinyl ether having one vinyloxy residue in an amount lower than about 5 wt-%, preferably lower than or equal to about 4.1 wt-%; e3) an epoxide other than a cycloaliphatic epoxide in an amount lower than about 10 wt-%; e4) an oxetane having two oxetanyl residues in an amount lower than about 20 wt-%; e5) an oxetane having one oxetanyl residue in an amount lower than about 3.5 wt-%, preferably lower than or equal to about 3.3 wt-%; and e6) a mixture of e1) and/or e2) and/or e3) and/or e4) and/or e5); the weight percents being based on the total weight of the UV-Vis radiation curable security ink. A UV-Vis radiation hybrid curable security ink may comprise: a) from about 7.5 wt-% to about 20 wt-% of silver nanoplatelets having a mean diameter in the range of 50 to 150 nm with a standard deviation of less than 60%, a mean thickness in the range of 5 to 30 nm with a standard deviation of less than 50%, and a mean aspect ratio higher than 2.0, 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 (XXI), wherein the residue R 41 is a C 2 -C 4 alkyl group substituted with a hydroxy group; the residue R 42 is selected from a C1-C4 alkyl group, and a C 2 -C 4 alkyl group substituted with a hydroxy group; and Cat k+ is a cation selected from the group consisting of Na + , K + , Rb + and Cs + ; b) a perfluoropolyether surfactant functionalized with at least a hydroxy group; c) from about 3 wt-% to about 12 wt-% of a polyvinyl chloride copolymer containing at least 69 wt-% of vinyl chloride; d2) from about 30 wt-% to about 65 wt-% of a mixture of a cycloaliphatic epoxide and a radically curable compound, from about 1 wt-% to about 6 wt-% of a cationic photoinitiator, and from about 1 wt-% to about 6 wt-% of a free radical photoinitiator; and optionally e) a cationically curable compound selected from the group consisting of: e1) a vinyl ether having two vinyloxy residues in an amount lower than 50% of the weight percent (wt-%) of the cycloaliphatic epoxide of d); e2) a vinyl ether having one vinyloxy residue in an amount lower than about 5 wt-%, preferably lower than or equal to about 4.1 wt-%; e3) an epoxide other than a cycloaliphatic epoxide in an amount lower than about 10 wt-%; e4) an oxetane having two oxetanyl residues in an amount lower than about 20 wt-%; e5) an oxetane having one oxetanyl residue in an amount lower than about 3.5 wt-%, preferably lower than or equal to about 3.3 wt-%; and e6) a mixture of e1) and/or e2) and/or e3) and/or e4) and/or e5); the weight percents being based on the total weight of the UV-Vis radiation curable security ink. The 2 nd European patent application of SICPA HOLDING SA describes UV-VIS radiation curable security inks, wherein Cat + in formula (XXI) is an ammonium cation of general formula R C R D NH 2 + , wherein the residue R C is a C 2 -C 4 alkyl group substituted with a hydroxy group, and the residue R D is selected from a C 1 -C 4 alkyl group, or a C 2 -C 4 alkyl group substituted with a hydroxy group. Further details of the security inks are described in the two European patent applications of SICPA HOLDING SA. The following examples are intended to illustrate various aspects and features of the present invention. Examples UV-Vis spectra of dispersions were recorded on Varian Cary 50 UV-Visible spectrophotometer at such concentration of dispersions as to achieve the optical density of 0.3 to 1.5 at 1 cm optical path. TEM analysis was conducted on dispersions containing silver nanoplatelets in isopropanol using an EM 910 instrument from ZEISS, INST.109, in bright field mode at an e-beam acceleration voltage of 100kV. At least 2 representative images with scale in different magnification (5.000x, 10.000X and 20.000X) were recorded in order to characterize the dominant particle morphology for each sample. The “number mean diameter of the silver nanoplatelets” refers to the mean diameter determined by transmission electron microscopy (TEM) using Fiji image analysis software (or Image analysis software: ParticleSizer (Thorsten Wagner (2016) ij-particlesizer: ParticleSizer 1.0.9. Zenodo; 10.5281/zenodo.820296) and ImageJ version 1.53f51) based on the measurement of at least 300, especially at least 500 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 (maximal Feret diameter) oriented parallel to the plane of a transmission electron microscopy (TEM) image (recorded at magnification 20.000X). The "number mean thickness of silver nanoplatelets” refers to the mean thickness determined by transmission electron microscopy (TEM) based on the measurement of at least 50, especially of at least 300 randomly selected silver nanoplatelets oriented perpendicular to the plane of the TEM image (recorded at magnification 25.000X), wherein the thickness of the silver nanoplatelet is the maximum thickness of said silver nanoplatelet. TEM analysis was conducted on dispersions containing silver nanoplatelets in isopropanol using an EM 910 instrument from ZEISS, INST.109, in bright field mode at an e-beam acceleration voltage of 100kV. In detail, a part of the dispersion is transferred to a smooth foil. After drying the sample is embedded in Araldit®, which is cross-linked below 60°C. Ultrathin cross-sections of the embedded sample are prepared perpendicular to the foil surface. The thickness of at least 300 randomly selected silver nanoplatelets may be determined from the cross-sectional TEM images (recorded at magnification 25.000X) by fitting ellipses to the cross-sectioned particles by the software (ParticleSizer). The minor axis (the shortest diameter) of the fitted ellipse is taken as particle thickness. Synthesis Example 1 (see Example 1 of WO2020/224982) a) In a 1 L double-wall glass reactor, equipped with anchor-stirrer, 365 g of de-ionized water was cooled to +2°C.13.62 g of sodium borohydride was added, and the mixture was cooled to -1°C with stirring at 250 rounds per minute (RPM, Solution A). In a 0.5 L double-wall glass reactor, equipped with anchor-stirrer, 132 g of deionized water and 4.8 g of MPEG-5000-thiol were combined, and the mixture was stirred for 10 minutes at room temperature.72 g of the product of Example A3 of WO2006074969 was added, and the resulting mixture was stirred for another 10 minutes at room temperature for homogenization. The solution of 30.6 g of silver nitrate in 30 g of de-ionized water was added in one portion and the mixture was stirred for 10 minutes, resulting in an orange- brown viscous solution. To this solution 96 g of deionized water was added, followed by addition of 3 g of Struktol SB2080 defoamer, pre-dispersed in 36 g of de-ionized water. The resulting mixture was cooled to 0°C with stirring at 250 RPM (Solution B). After that, Solution B was dosed with a peristaltic pump at a constant rate over 2 h into Solution A under the liquid surface via a cooled (0°C) dosing tube, resulting in spherical silver nanoplatelets dispersion. During pumping, the Solution A was stirred at 250 RPM. After dosing was complete, the reaction mixture was warmed up to +5°C within 15 minutes, and a solution of 862 mg of KCl in 10 g of deionized water was added in one portion, followed by addition of 9.6 g of ethylenediaminetetraacetic acid (EDTA) in 4 equal portions with 10 minutes time intervals. After addition of the last EDTA portion, the reaction mixture was stirred for 15 minutes at +5°C, then warmed up to 35°C over 30 minutes and stirred for 1 h at this temperature. Upon this time, hydrogen evolution is completed. 3.0 mL of 30% w/w solution of ammonia in water was added, followed by addition of 5.76 g of solid NaOH, and the mixture was stirred for 15 min at 35°C. Then 180 mL of 50% w/w hydrogen peroxide solution in water were dosed with a peristaltic pump at a constant rate over 4 h into the reaction mixture under the liquid surface with stirring at 250 RPM, while maintaining the temperature at 35°C. This has led to a deep blue colored dispersion of silver nanoplatelets, which was cooled to room temperature.1.23 g of compound of formula (B-3) was added, and the mixture was stirred for 1 h at room temperature. b) Isolation and purification of Ag nanoplatelets b1) Decantation 9.6 g of sodium dodecylsulfate was added to the reaction mixture and then ca.25 g 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 24 h, allowing the coagulated nanoplatelets to sediment at the bottom of the reactor. 890 g of supernatant was pumped out from the reactor with a peristaltic pump, and 890 g of deionized water was added to the reactor. The mixture in reactor was stirred for 1 h at room temperature, allowing the coagulated particles to re-disperse. b2) Decantation Ca.64 g 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 12 h, allowing the coagulated nanoplatelets to sediment at the bottom of the reactor.990 g of supernatant was pumped out from the reactor with a peristaltic pump, and 90 g of deionized water was added to the reactor. The resulting mixture was stirred for 30 minutes at room temperature, allowing the coagulated particles to re-disperse. b3) Ultrafiltration in water The resulting dispersion of Ag nanoplatelets was subjected to ultrafiltration using a Millipore Amicon 8400 stirred ultrafiltration cell. The dispersion was diluted to 400 g weight with de- ionized water and ultrafiltered to the end volume of ca.50 mL using a polyethersulfone (PES) membrane with 300 kDa cut-off value. The procedure was repeated in total 4 times to provide 60 g of Ag nanoplatelets dispersion in water. After ultrafiltration was completed, 0.17 g of compound (B-3) was added to the dispersion. Ag content 28.9% w/w; yield ca.89% based on total silver amount; Solids content (at 250°C) 33.5% w/w; Purity 86% w/w of silver based on solids content at 250°C. b4) Ultrafiltration in isopropanol The dispersion was further ultrafiltered in isopropanol.60 g of Ag nanoplatelets dispersion, obtained after ultrafiltration in water, was placed in a Millipore Amicon 8400 stirred ultrafiltration cell and diluted to 300 g weight with isopropanol. The dispersion was ultrafiltered to the volume of ca.50 mL using a polyethersulfone (PES) membrane with 500 kDa cut-off value. The procedure was repeated in total 4 times to provide 72 g of Ag nanoplatelets dispersion in isopropanol. Ag content 24.1% w/w; Solids content (at 250°C) 25.7% w/w; Purity 93.5% w/w of silver based on solids content at 250°C. The UV-Vis-NIR spectrum was recorded in water at Ag concentration of 9.8*10 -5 M. λmax = 700 nm; extinction coefficient at maximum ε=10200 L/(cm*mol Ag), FWHM = 340 nm. Reference is made to Fig.1. UV-Vis-NIR spectrum of Ag nanoplatelets from Example 1 b4). Number mean particle diameter 93±40 nm, number mean particle thickness 16±2.5 nm. Example 1. Preparation of Dispersion D1. Pre-synthesis of diethanolaminedithiocarbamate sodium salt. Dispersion in ethyl 3-ethoxypropionate a) Synthesis of diethanolaminedithiocarbamate sodium salt (sodium bis(2- hydroxyethyl)dithiocarbamate) 60 g Ethanol, 10.5 g diethanolamine and 4.0 g of sodium hydroxide granules was placed in 100 mL round-bottom flask under argon atmosphere, the mixture was stirred until dissolution of sodium hydroxide and then cooled to +2°C.7.6 g carbon disulfide was added dropwise over 1 h with stirring, maintaining the temperature of reaction mixture at 0 to +5°C. After that, the mixture (yellowish suspension) was stirred for 30 min at 0 to +5°C, then warmed up to 23°C and stirred for 1 h. b) Surface modification of Ag nanoplatelets 50 g (12.85 g solids) of Ag nanoplatelets dispersion obtained as described in Step b4) of Synthesis Example 1 was placed in a 250 mL round-bottom flask under argon atmosphere. 1.09 g of suspension (0.27 g of diethanolaminedithiocarbamate sodium salt), obtained in Step a), was added in one portion and the mixture was stirred for 24 h under argon at 23°C. c) Solvent switch To the dispersion, obtained in Step b), 15.0 g of ethyl 3-ethoxypropionate (CAS No.: 763- 69-9) was added. The resulting mixture was concentrated on rotary evaporator at 40 mbar pressure and 40°C bath temperature, till no more solvent was distilled off. The weight of the resulting dispersion was adjusted to 32.1 g by addition of ethyl 3-ethoxypropionate (corresponds to the calculated total solids content of 40.9% w/w). Example 2. Preparation of Dispersion D2. In-situ synthesis of diethanolaminedithiocarbamate potassium salt. Dispersion in ethyl 3- ethoxypropionate a) Surface modification of Ag nanoplatelets 50 g (12.85 g solids) of Ag nanoplatelets dispersion, obtained as described in Step b4) of Synthesis Example 1, was placed in a 250 mL round-bottom flask under argon atmosphere at 23°C.0.91 g of 5% w/w solution of carbon disulfide in absolute ethanol was added and the mixture was stirred for 5 min, followed by addition of 1.24 g of 5% w/w solution of diethanolamine in absolute ethanol. The mixture was stirred for 1 h at 23°C, then 0.986 g of 5% w/w solution of potassium ethoxide in absolute ethanol was added and stirring was continued for 30 min. b) Solvent switch To the dispersion, obtained in Step a), 15.0 g of ethyl 3-ethoxypropionate was added. The resulting mixture was concentrated on rotary evaporator at 40 mbar pressure and 40°C bath temperature, till no more solvent was distilled off. The weight of the resulting dispersion was adjusted to 32.1 g by addition of ethyl 3-ethoxypropionate (corresponds to the calculated total solids content of 40.4% w/w). Example 3. Preparation of Dispersion D3. Pre-synthesis of diethanolaminedithiocarbamate cesium salt. Dispersion in ethyl 3-ethoxypropionate a) Synthesis of diethanolaminedithiocarbamate cesium salt 16 g Ethanol, 2.7 g diethanolamine and 4.3 g of cesium hydroxide monohydrate was placed in 100 mL round-bottom flask under argon atmosphere, the mixture was stirred until dissolution of cesium hydroxide and then cooled to +2°C.1.95 g carbon disulfide was added dropwise over 1 h with stirring, maintaining the temperature of reaction mixture at 0 to +5°C. After that, the mixture (yellowish suspension) was stirred for 30 min at 0 to +5°C, then warmed up to 23°C and stirred for 1 h. b) Surface modification of Ag nanoplatelets 50 g (12.85 g solids) of Ag nanoplatelets dispersion, obtained obtained in Example 1, Step b4) of the European patent application 19172734.6, was placed in a 250 mL round-bottom flask under argon atmosphere.0.40 g of suspension (0.128 g of diethanolaminedithiocarbamate cesium salt), obtained in Step a), was added in one portion and the mixture was stirred for 24 h under argon at 23°C. c) Solvent switch To the dispersion, obtained in Step b), 15.0 g of ethyl 3-ethoxypropionate was added. The resulting mixture was concentrated on rotary evaporator at 40 mbar pressure and 40°C bath temperature, till no more solvent was distilled off. The weight of the resulting dispersion was adjusted to 32.1 g by addition of ethyl 3-ethoxypropionate (corresponds to the calculated total solids content of 40.4% w/w). Example 4. Preparation of Dispersion D4. In-situ synthesis of diethanolaminedithiocarbamate potassium salt. Dispersion in 3,4- epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate. a) Surface modification of Ag nanoplatelets 50 g (12.85 g solids) of Ag nanoplatelets dispersion, obtained as described in Step b4) of Synthesis Example 1, was placed in a 250 mL round-bottom flask under argon atmosphere at 23°C.0.91 g of 5% w/w solution of carbon disulfide in absolute ethanol was added and the mixture was stirred for 5 min, followed by addition of 1.24 g of 5% w/w solution of diethanolamine in absolute ethanol. The mixture was stirred for 1 h at 23°C, then 0.986 g of 5% w/w solution of potassium ethoxide in absolute ethanol was added and stirring was continued for 30 min. b) Solvent switch To the dispersion, obtained in Step a), 15.0 g of 3,4-epoxycyclohexylmethyl 3,4- epoxycyclohexanecarboxylate (CAS: 2386-87-0) was added. The resulting mixture was concentrated on rotary evaporator at 40 mbar pressure and 40°C bath temperature, till no more solvent was distilled off. The weight of the resulting dispersion was adjusted to 32.1 g by addition of 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (corresponds to the calculated total solids content of 40.4% w/w). Example 5. Preparation of Dispersion D5. In-situ synthesis of diethanolaminedithiocarbamate cesium salt. Dispersion in 3,4- epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate a) Surface modification of Ag nanoplatelets 50 g (12.85 g solids) of Ag nanoplatelets dispersion, obtained as described in Step b4) of Synthesis Example 1, was placed in a 250 mL round-bottom flask under argon atmosphere at 23°C.0.91 g of 5% w/w solution of carbon disulfide in absolute ethanol was added and the mixture was stirred for 5 min, followed by addition of 1.24 g of 5% w/w solution of diethanolamine in absolute ethanol. The mixture was stirred for 1 h at 23°C, then 1.97 g of 5% w/w solution of cesium hydroxide monohydrate in absolute ethanol was added and stirring was continued for 30 min. b) Solvent switch To the dispersion, obtained in Step a), 15.0 g of 3,4-epoxycyclohexylmethyl 3,4- epoxycyclohexanecarboxylate (CAS: 2386-87-0) was added. The resulting mixture was concentrated on rotary evaporator at 40 mbar pressure and 40°C bath temperature, till no more solvent was distilled off. The weight of the resulting dispersion was adjusted to 32.1 g by addition of 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (corresponds to the calculated total solids content of 40.6% w/w). Example 6. Preparation of Dispersion D6. In-situ synthesis of diethanolaminedithiocarbamate diethanolammonium salt. Dispersion in ethyl 3- ethoxypropionate a) Surface modification of Ag nanoplatelets 50 g (12.85 g solids) of Ag nanoplatelets dispersion, obtained as described in Step b4) of Synthesis Example 1, was placed in a 250 mL round-bottom flask under argon atmosphere at 23°C.2.05 g of 5% w/w solution of carbon disulfide in absolute ethanol was added and the mixture was stirred for 5 min, followed by addition of 2.77 g of 5% w/w solution of diethanolamine in absolute ethanol. The mixture was stirred for 1 h at 23°C, then 2.77 g of 5% w/w solution of diethanolamine in absolute ethanol was added and stirring was continued for 30 min. b) Solvent switch To the dispersion, obtained in Step a), 15.0 g of ethyl 3-ethoxypropionate was added. The resulting mixture was concentrated on rotary evaporator at 40 mbar pressure and 40°C bath temperature, till no more solvent was distilled off. The weight of the resulting dispersion was adjusted to 32.1 g by addition of ethyl 3-ethoxypropionate (corresponds to the calculated total solids content of 41.2% w/w). Example 7. Preparation of Dispersion D7. In-situ synthesis of diethanolaminedithiocarbamate diethanolammonium salt. Dispersion in 3,4- epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate a) Surface modification of Ag nanoplatelets 50 g (12.85 g solids) of Ag nanoplatelets dispersion, obtained as described in Step b4) of Synthesis Example 1, was placed in a 250 mL round-bottom flask under argon atmosphere at 23°C.2.05 g of 5% w/w solution of carbon disulfide in absolute ethanol was added and the mixture was stirred for 5 min, followed by addition of 2.77 g of 5% w/w solution of diethanolamine in absolute ethanol. The mixture was stirred for 1 h at 23°C, then 2.77 g of 5% w/w solution of diethanolamine in absolute ethanol was added and stirring was continued for 30 min. b) Solvent switch To the dispersion, obtained in Step a), 15.0 g of 3,4-epoxycyclohexylmethyl 3,4- epoxycyclohexanecarboxylate (CAS: 2386-87-0) was added. The resulting mixture was concentrated on rotary evaporator at 40 mbar pressure and 40°C bath temperature, till no more solvent was distilled off. The weight of the resulting dispersion was adjusted to 32.1 g by addition of 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (corresponds to the calculated total solids content of 41.2% w/w). The suitability of dithiocarbamate capped silver nanoplatelets for use in security inks for producing security features for securing value documents, which exhibit a first color upon viewing in transmitted light and a second color different from the first color upon viewing in incident light is described in two European patent applications both entitled "UV-VIS radiation curable security inks for producing dichroic security features" filed by SICPA HOLDING SA on November 10, 2020. Part of the examples of the European patent applications of SICPA HOLDING SA are outlined below in excerpts for reference purpose only. The examples E1 – E8 and below provide more details for the preparation the UV-Vis radiation curable screen printing security inks described in the European patent applications of SICPA HOLDING SA and optical properties of security features obtained therefrom.
202716 A. Preparation of Examples (E1 – E8) and printed security features thereof Description of the ingredients used for the preparation of the inks according to the present invention (E1 – E8) omposition umber) 4 .1.0]heptane-3-carboxylate (2386-87-0) 6) c acid (52408-84-1) + 77wt-% 4,4'- oro-2,3-epoxypropane, esters with acrylic acid 89-5) d, methyl esters, reduced, reaction products ecular weight 1700 [g/mol] thio)phenyl]-, (OC-6-11)- odi-4,1-phenylene)bis[diphenyl-, (OC-6-11)- 89452-37-9) in 50% propylene carbonate (108- thio)phenyl]-, (OC-6-11)- odi-4,1-phenylene)bis[diphenyl-, (OC-6-11)- 89452-37-9) in 65wt-% Oxirane, 2,2'-[1,4- phinate + 7.5wt-% phenyl bis(2,4,6- Amount surface stabilizing agent (wt-% of the Surface stabilizing agent Ag nanoplatelets) m salt m ate) 2.1 um salt a mate) 1.0 m salt m ate) 1.0 um salt a mate) 1.0 m salt m ate) 1.43 hio- 3 hio- 3 ethyl-3-ethoxypropionate (763-69-9) in ethyl-3-ethoxypropionate (763-69-9) ethyl-3-ethoxypropionate (763-69-9) in 7-oxabicyclo[4.1.0]hept-3-ylmethyl 7- 7-oxabicyclo[4.1.0]hept-3-ylmethyl 7- mate in ethyl-3-ethoxypropionate (763-69-9) mate in 7-oxabicyclo[4.1.0]hept-3-ylmethyl 7- 202716 Table 1b. Composition of the UV-Vis radiation curable screen printing inks E1 – E8 Ingredient Commercial name Amount [wt-%] E5 E6 E7 E8 7.4 6 36.8 36.8 20 31.1 16.2 16.2 7 16.8 16.8 4.5 9.1 2.5 5.9 5 11.5 11.5 11.5 3.1 3 25 25 31.3 31.3 n ic Cationic Cationic Hybrid Cationic 6 10.1 10.2 12.9 12.9 7 0 0 18.4 0 C1. Preparation of security inks (E1 – E8) and dichroic security features thereof C1a. Preparation of the security inks E1 – E8 Ingredients provided in Table 1b were independently mixed and dispersed at room temperature using a Dispermat CV-3 for 10 minutes at 2000 rpm so as to yield 50g of the inks E1 – E8. C1b. Preparation of security features 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 50µm, supplied by Pütz GmbH + Co. Folien KG) using a 160 thread/cm screen (405 mesh). The printed pattern had a size of 5 cm x 5 cm. 10 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 100 m/min to UV-Vis light under a dryer from IST Metz GmbH (two lamps: iron-doped mercury lamp 200 W/cm 2 + mercury lamp 200 W/cm 2 ), to generate security features. C1c. Results (optical properties) of security features The optical properties of each security features obtained at item C1b were independently assessed in reflection, in transmission, and visually using the three tests described below. The results are summarized in Table 1c. Reflection measurements were performed using a goniometer (Goniospektrometer Codec WI-105&5 by Phyma GmbH Austria). The L*a*b* values of the printed security features were determined at 0° to the normal with an illumination angle of 22.5° 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 (1976) color space, wherein: The C* values (reflection 22.5/0°) are displayed in Table 1c below. Transmission measurements were carried out using a Datacolor 650 spectrophotometer (parameters: integration sphere, diffuse illumination (pulse xenon D65) and 8° viewing, analyzer SP2000 with dual 256 diode array for wavelength range of 360-700nm, transmission sampling aperture size of 22mm). The C* values (transmission 8°) are displayed in Table 1c below. A visual 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: - Dark brown to brown colors with matte appearance and no metallic effect; - Gold color (i.e. metallic yellow color) with glossy appearance and metallic effect. The metallic effect appears for a chroma value C* in reflection 22.5/0° higher than about 20. A visual assessment was also carried out observing each security feature with the naked eye in transmission. The following colors have been observed: - Dull blue: the blue coloration is weak (but visible); - Blue (chroma value C* in transmission 8° higher than or equal to about 20) to deep blue (chroma value C* in transmission 8° higher than or equal to 30): the blue coloration is intense to very intense. Table 1c. Color properties of security features obtained from inks E1 – E8 As shown in Table 1c, the security features obtained from cationically curable or hybrid (cat/rad) inks E1 – E8 exhibited gold color in reflection and blue to deep blue color in transmission. C1d. Study of the stability of the UV-Vis radiation curable security ink E7 via an accelerated ageing experiment To evaluate the stability upon time of an ink according to the invention, 10 g of ink E7 (described in Table 1b) were placed in a cup (60 ml white polypropylene cup for SpeedMixer™ available at Hauschild & Co. KG), which was hermetically sealed and stored for five months at a temperature of 60 °C in a laboratory oven (Kendro Laboratory Products, T6060). The freshly prepared ink E7 was used as a comparison standard. Each month, the sample of ink E7 stored in the oven was cooled at room temperature for 6 hours, and subsequently applied on a piece of transparent polymer substrate (PET Hostaphan® RN, thickness 50µm, supplied by Pütz GmbH + Co. Folien KG) using a 160 thread/cm screen (405 mesh). The printed pattern had a size of 5 cm x 5 cm. 10 seconds after the printing step, the piece of printed substrate was cured at room temperature by two times exposure at a speed of 100 m/min to UV-Vis light under a dryer from IST Metz GmbH (two lamps: iron-doped mercury lamp 200 W/cm 2 + mercury lamp 200 W/cm 2 ), 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 1d summarizes the color in reflection and transmission and the C* values (reflection 22.5/0°) exhibited by the security features. Table 1d. Color properties of security features a) the security feature was manufactured with the freshly prepared ink E7 according to the present invention. As attested by the optical properties of security features shown in Table 1d, the ink E7 remains stable over an extended period of time at high temperature, which is an indication of outstanding shelf stability at room temperature.
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