Relation to security printing

Abstract

An article comprising a substrate which carries a material of formula (I)
M.sup.1.sub.aM.sup.2.sub.bW.sub.cO.sub.d(P(O).sub.nR.sub.m).sub.e (I)
wherein each of M.sup.1 and M.sup.2 is independently ammonium or a metal cation; a is 0.01 to 0.5; b is 0 to 0.5; c is 1; d is 2.5 to 3; e is 0.01 to 0.75; n is 1, 2 or 3; m is 1, 2 or 3; and R is an optionally substituted hydrocarbyl group.

Claims

1. An article comprising a substrate which carries a material of formula (I)
M.sup.1.sub.aM.sup.2.sub.bW.sub.cO.sub.d(P(O).sub.nR.sub.m).sub.e (I) wherein each of M.sup.1 and M.sup.2 is independently ammonium or a metal cation; a is 0.01 to 0.5; b is 0 to 0.5; c is 1; d is 2.5 to 3; e is 0.01 to 0.75; n is 1, 2 or 3; m is 1, 2 or 3; and R is an optionally substituted hydrocarbyl group.

2. An article according to claim 1 which is a banknote.

3. An article according to claim 1 wherein M.sup.1 is caesium; M.sup.2 is selected from alkali metals, tin or zinc; a is 0.1 to 0.45; b is 0 to 0.3; c is 1; d is 2.7 to 3; e is 0.01 to 0.4; n is 2 or 3; m is 1 or 2 and each R is an unsubstituted alkyl or aryl group or an alkoxy group having 1 to 30 carbon atoms.

4. An ink composition comprising a material of formula (I)
M.sup.1.sub.aM.sup.2.sub.bW.sub.cO.sub.d(P(O).sub.nR.sub.m).sub.e (I) wherein each of M.sup.1 and M.sup.2 is independently ammonium or a metal cation; a is 0.01 to 0.5; b is 0 to 0.5; c is 1; d is 2.5 to 3; e is 0.01 to 0.75; n is 1, 2 or 3; m is 1, 2 or 3; and R is an optionally substituted hydrocarbyl group.

5. A method of manufacturing an article as claimed in claim 1, the method comprising incorporating a material of formula (I) into or onto the substrate.

6. A method according to claim 5 which comprises applying an ink composition comprising a material of formula (I) onto a surface of the substrate.

7. An article according to claim 1 wherein the difference in colour (dE) between the substrate with and without the material of formula (I) is less than 2.

8. A method of providing a covert security image or a hidden coating on a banknote, comprising applying to the banknote an ink composition comprising a material of formula (I)
M.sup.1.sub.aM.sup.2.sub.bW.sub.cO.sub.d(P(O).sub.nR.sub.m).sub.e (I) wherein each of M.sup.1 and M.sup.2 is independently ammonium or a metal cation; a is 0.01 to 0.5; b is 0 to 0.5; c is 1; d is 2.5 to 3; e is 0.01 to 0.75; n is 1, 2 or 3; m is 1, 2 or 3; and R is an optionally substituted hydrocarbyl group.

9. A method of detecting a genuine article of claim 1, the method comprising measuring at a locus on the article the absorption A.sub.1, of radiation at a first wavelength .sub.1, and the absorption A.sub.2 of radiation at a second wavelength .sub.2, and calculating the ratio of A.sub.1 to A.sub.2 wherein .sub.1 and .sub.2 are in the infra-red range.

10. A method of assessing the quality of an article of claim 1, the method comprising measuring at a locus of the article which carries a coating or an image of a material of formula (I), the absorption A.sub.1, of at least one wavelength of .sub.1 of radiation in the infra-red range and comparing the absorption with a standard.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the spectra of the powder reflectance of a material according to an embodiment of the invention.

(2) FIG. 2 shows the spectra for the ink comprising the material of the prior art.

(3) FIG. 3 shows the spectra for the ink comprising the material according to an embodiment of the invention.

(4) FIG. 4 shows the visible and infrared transmittance spectrum of a material according to an embodiment of the invention.

(5) FIG. 5 shows a photograph of ink compositions according to embodiments of the invention printed on banknote paper.

(6) FIG. 6 shows the infrared spectra of prints comprising 1 wt % of a material according to an embodiment of the invention.

(7) FIG. 7 shows the infrared spectra of prints comprising 3 wt % of a material according to embodiments of the invention.

(8) FIG. 8 shows the infrared spectra of prints comprising 5 wt % of a material according to embodiments of the invention.

(9) The material of formula (I) used in all aspects of the present invention may be prepared by any suitable means.

(10) Suitable the method involves the steps of: (i) admixing: (a) a source of dopant species M.sup.1 and optionally M.sup.2; (b) a source of tungsten; (ii) adding (c) an organophosphorus compound; and (iii) heating a mixture of (a), (b) and optionally (c) in a reducing atmosphere.

(11) Step (i) involves admixing a source of dopant species M.sup.1/M.sup.2 (a) and a source of tungsten (b).

(12) When metals M.sup.1 and M.sup.2 are both used these may be each provided from the same source or a different source. Thus in some embodiments component (a) may comprise a mixture of compounds.

(13) Step (ii) involves adding an organophosphorus compound (c).

(14) In some embodiments steps (i) and (ii) may be combined and the method may involve admixing components (a), (b) and (c).

(15) In some embodiments steps (i) and (ii) may be carried out separately and the source of dopant species M.sup.1/M.sup.2 (a) and source of tungsten (b) are admixed first and then further mixed with an organic phosphorus compound (c) before or after being heated in a reducing atmosphere.

(16) In some embodiments step (ii) is carried out before step (iii) and step (iii) involves heating a mixture of a source of dopant species M.sup.1/M.sup.2 (a); a source of tungsten (b) and an organophosphorus compound (c) in a reducing atmosphere.

(17) In some embodiments step (iii) is carried out before step (ii). In such embodiments step (iii) involves heating a mixture of a source of dopant species M.sup.1/M.sup.2 (a) and a source of tungsten (b) in a reducing atmosphere and step (ii) involves adding an organophosphorus compound (c) to this reduced mixture.

(18) Suitably the source of dopant species M.sup.1/M.sup.2 comprises a salt comprising a cationic species M.sup.1/M.sup.2 and an anion. Suitable salts include halides, nitrites, nitrates, sulfates, hydroxides, carbonates and oxides. Preferably the source of dopant species M.sup.1/M.sup.2 is a carbonate, hydroxide or nitrate.

(19) In preferred embodiments when M.sup.1 is caesium, the source of dopant species M.sup.1 will suitably be a caesium source. Preferred caesium sources include caesium carbonate, caesium hydroxide, and caesium nitrate.

(20) In some embodiments component (a) may comprise a mixture of two or more sources of dopant species M.sup.1/M.sup.2. In such embodiments component (a) may comprise a mixture of two or more different salts of the same species M.sup.1/M.sup.2 and/or salts comprising different species M.sup.1/M.sup.2.

(21) Any compound comprising tungsten may be used as the source of tungsten (b). Preferred sources of tungsten include sodium tungstate, tungstic acid and ammonium metatungstate.

(22) Component (b) may comprises a mixture of two or more sources of tungsten.

(23) The ratio of component (a) and component (b) used in step (i) is determined by the desired molar ratio in the material of formula (I).

(24) Typically the molar ratio of dopant species M.sup.1/M.sup.2 to tungsten is from 1:2 to 1:4, preferably approximately 1:3. The selection of an appropriate ratio of starting materials is within the competence of the person skilled in the art.

(25) In preferred embodiments step (i) is carried out in a solvent. Preferred solvents include water, water miscible alcohols, and mixtures thereof. Especially preferred solvents are water, methanol, ethanol, propanol, isopropanol, butanol and mixtures thereof.

(26) In preferred embodiments in which steps (i) and (ii) are carried out simultaneously, this combined step is preferably carried out in the presence of such a solvent.

(27) Step (ii) involves the addition of (c) an organophosphorus compound.

(28) Any organophosphorus compound can be used. By organophosphorus compound we mean a compound which includes a phosphorus atom and a hydrocarbyl group in which a carbon atom is bonded either directly or via an oxygen atom to the phosphorus atom.

(29) Suitable organophosphorus compounds include hydrocarbyl phosphines, phosphate and phosphonate esters.

(30) Preferred organophosphorus compounds are compounds of formula (II):

(31) ##STR00001##
in which each of R.sup.1, R.sup.2 and R.sup.3 is independently selected from OH, H, OR.sup.4, alkyl, aryl and (OR.sup.5).sub.nOH; wherein R.sup.4 is alkyl or aryl; each R.sup.5 is independently ethyl or propyl; and n is at least 1.

(32) Preferably each of R.sup.1, R.sup.2 and R.sup.3 is independently selected from OH, OR.sup.4, alkyl and aryl.

(33) Preferably R.sup.1 is OH, R.sup.2 is OH and R.sup.3 is a group R as defined in relation to the first aspect.

(34) Most preferably R.sup.1 is OH, R.sup.2 is OH and R.sup.3 is selected from C.sub.8H.sub.17, C.sub.18H.sub.37 and phenyl.

(35) Preferred organophosphorus compounds for use herein include octyl phosphonic acid, methylphosphonic acid dimethylester, tristearylphosphate, phenyl phosphate and triphenyl phosphate and polyethylene glycol monooleyl ether phosphate.

(36) In one embodiment the source of dopant species M.sup.1/M.sup.2 (a) and the source of tungsten (b) are pre-mixed in step (i) and the source of organophosphorus (b) is added to this mixture in step (ii).

(37) Step (i) may be carried out at ambient temperature or at elevated temperatures. In some embodiments step (i) may be carried out at temperatures above 50 C., for example 60 C. or 70 C. In some embodiments step (i) may be carried out at 80 C.

(38) In embodiments in which step (ii) is carried out before step (iii) it may be carried out at ambient temperature or at elevated temperatures. In some embodiments step (ii) may be carried out at temperatures above 50 C., for example 60 C. or 70 C. In some embodiments step (ii) may be carried out at 80 C.

(39) Steps (i) and/or (ii) suitably involve agitation the mixture formed.

(40) Preferably any solvent is removed prior to step (iii). Preferably there is a step of drying the material before step (iii). Suitable methods of drying will be known to the person skilled in the art and include, for example, the use of a spray drier.

(41) Step (iii) involves heating a mixture of (a), (b) and optionally (c) in a reducing atmosphere.

(42) Suitably step (iii) involves heating a mixture of (a), (b) and optionally (c) in a reducing atmosphere at temperatures in excess of 300 C., suitably in excess of 400 C., preferably in excess of 450 C., for example at temperatures of 500 to 600 C.

(43) Step (iii) suitably involves heating a mixture of (a), (b) and optionally (c) in a gaseous environment comprising hydrogen, nitrogen, carbon monoxide or mixture thereof. In preferred embodiments step (iii) involves heating a mixture of (a), (b) and optionally (c) in a nitrogen/hydrogen atmosphere.

(44) In preferred embodiments step (iii) is carried out after step (ii) and involves heating a mixture comprising a source of dopant species M.sup.1/M.sup.2 (a), a source of tungsten (b) and an organophosphorus compound (c).

(45) In some embodiments step (iii) is carried out before step (ii) and involves heating a mixture comprising a source of dopant species M.sup.1/M.sup.2 (a) and a source of tungsten (b). In such embodiments the reduced mixture is then admixed with an organophosphorus compound (c).

(46) The reduced mixture obtained after step (iii) is suitably mixed with an organophosphorus compound (c) in the presence of a solvent in step (ii). Preferred solvents include water, water miscible alcohols (especially methanol and ethanol), and mixtures thereof.

(47) After step (ii) the product is suitably dried. In such embodiments the product may be optionally heated again in a reducing atmosphere. Preferred conditions are as described in relation to step (iii).

(48) Embodiments in which step (iii) is carried out before step (ii) allow only low amounts of organophosphorus to be incorporated, whereas embodiments in which step (iii) is carried out after step (ii) allow an almost free variation of the W/P ratio in the final product.

(49) The material obtained after step (iii) may be used directly or subjected to further treatment.

(50) In some preferred embodiments the method may involve a further step (iv) of milling the material obtained in step (iii).

(51) Preferred milling processes involve ball milling.

(52) Suitably milling is carried out until a specific surface area/particle size is obtained.

(53) In preferred embodiments the material is ball milled until a surface area of at least 20 m.sup.2/gram is obtained.

(54) According to a third aspect of the present invention there is provided a method of manufacturing an article of the first aspect, the method comprising incorporating a material of formula (I) into or onto the substrate.

(55) Preferred features of the third aspect are as defined in relation to the first and second aspects.

(56) In some embodiments the method of the third aspect may include mixing the material of formula (I) into a raw ingredient used to make the substrate and then forming the substrate from the raw ingredient. For example the method of the third aspect may involve dosing particles of a material of formula (I) into a paper pulp or polymer pellets and then using the pulp or pellets to make the substrate.

(57) In some preferred embodiments the method of the third aspect comprises applying an ink composition comprising material of formula (I) onto a surface of the substrate.

(58) Thus the third aspect of the present invention suitably provides a method of manufacturing an article, preferably a banknote, the method comprising providing a substrate, preferably a substantially planar substrate, and applying to a surface of the substrate an ink composition comprising a material of formula (I)
M.sup.1.sub.aM.sup.2.sub.bW.sub.cO.sub.d(P(O).sub.nR.sub.m).sub.e (I)
wherein each of M.sup.1 and M.sup.2 is independently ammonium or a metal cation; a is 0.01 to 0.5; b is 0 to 0.5; c is 1; d is 2.5 to 3; e is 0.01 to 0.75; n is 1, 2 or 3; m is 1, 2 or 3; and R is an optionally substituted hydrocarbyl group.

(59) In preferred embodiments the substrate is a planar substrate, preferably a banknote. The method of the third aspect may involve applying the ink composition to one or both surfaces of the substrate. The ink composition is suitably as defined in relation to the second aspect.

(60) In some embodiments the ink composition may be coated onto the substrate by dipping, spraying, painting or rolling.

(61) In some embodiments the material of formula (I) may be incorporated into a polymer melt composition or other composition which is painted, dipped or sprayed onto the substrate.

(62) The third aspect of the present invention may comprise coating, spraying, dipping, painting or printing onto the surface of the substrate a composition comprising A material of formula (I).

(63) In some embodiments the material of formula (I) may be coated onto one surface of the substrate. In some embodiments it may be coated onto both surfaces of a planar substrate. The ink composition may be applied to some or all of the surface or surfaces of the substrate. In some embodiments an image may be applied to the substrate using a printing technique.

(64) In some preferred embodiments the method of the third aspect comprises printing a composition comprising a material of formula (I) onto a surface of the substrate.

(65) Any suitable printing technique may be used, for example offset printing (both lithographic and gravure), intaglio printing, letterpress printing, ink-jet printing and screen printing. Preferably the composition is printed by intaglio printing.

(66) Suitably the ink composition is coated or printed onto the substrate at a thickness of at least 1 micron. The thickness will depend on the printing technique used. For offset printing a thickness of 1 to 2 microns is typically used; for a varnish coating or gravure printing a thickness of 2 to 4 microns is preferred; and for intaglio printing a thickness of at least 5 microns, suitably about 8 microns and up to 40 or even up to 80 microns may be used.

(67) The ink composition may be coated or printed onto one or both sides of the substrate.

(68) In some embodiments the ink composition may be provided as an overcoat varnish.

(69) The article of the first aspect of the present invention comprises a material of formula (I) incorporated within or, preferably, applied to the surface thereof. The material of formula (I) is suitably an absorber of infra-red radiation and thus the article absorbs infra-red radiation at the locus which carries the material of formula (I).

(70) Suitably the inclusion of the material of formula (I) within or on the surface of the substrate does not substantially alter the colour of the substrate in the visible range.

(71) Suitably the difference in colour between the substrate with and without the material of formula (I) (dE, also referred to as E, dE* and E*) is less than 4, preferably less than 2, more preferably less than 1. The skilled person will appreciate that a dE of less than 1 is generally considered to be imperceptible to the human eye.

(72) Because the presence of the material of formula (I) does not substantially affect the colour of the substrate it may be used to provide a hidden or covert security feature.

(73) In some embodiments the substrate may be printed with two paired inks. Suitably in such embodiments a first portion of the substrate is printed with a first ink composition and a second portion of the substrate is coated with a second ink composition wherein the second ink composition has all of the same components as the first ink composition except that it further comprises a material of formula (I).

(74) Thus the paired inks are suitably identical except for the inclusion of the material of formula (I).

(75) Suitably the colour difference dE between the first portion of the substrate printed with the first ink composition of the paired inks and the second portion of the substrate printed with the second ink composition of the paired inks is less than 4, the dE is less than 2, more preferably less than 1.

(76) The article, especially when a banknote, may include one or more further security features, for example a UV image/phosphor, an NIR-absorbing image, a holographic feature, a watermark, a thread, a magnetic image, windows, a colour shift/colour change image or a microprinting feature. Further features will also be known to the person skilled in the art.

(77) According to a fourth aspect of the present invention there is provided the use of a material of formula (I)
M.sup.1.sub.aM.sup.2.sub.bW.sub.cO.sub.d(P(O).sub.nR.sub.m).sub.e (I)
to provide a covert security image or a hidden coating on a banknote, wherein each of M.sup.1 and M.sup.2 is independently ammonium or a metal cation; a is 0.01 to 0.5; b is 0 to 0.5; c is 1; d is 2.5 to 3; e is 0.01 to 0.75; n is 1, 2 or 3; m is 1, 2 or 3; and R is an optionally substituted hydrocarbyl group.

(78) The materials of formula (I) for use in the invention are particularly advantageous for use as covert security features because they have high absorption in the infra-red region but low absorbance in the visible region of the electromagnetic spectrum. Thus the materials of formula (I) used in the invention suitably are not strongly coloured.

(79) The variable infra-red absorption can be used in an authentication method.

(80) According to a fifth aspect of the present invention there is provided a method of detecting a genuine article of the first aspect, the method comprising measuring at a locus on the article the absorption A.sub.1 of radiation at a first wavelength .sub.1.

(81) Suitably the absorption A.sub.1 of radiation at wavelength .sub.1 is compared with a standard and if it matches the standard the article can be determined to be genuine.

(82) In some embodiments the method may further involve measure at the locus on the article the absorption A.sub.2 of radiation at a second wavelength .sub.2, and calculating the ratio of A.sub.1 to A.sub.2 wherein .sub.1 and .sub.2 are in the infra-red range.

(83) Suitably the ratio of A.sub.1 to A.sub.2 is compared with a standard and if it matches the standard the article can be determined to be genuine.

(84) In some embodiments the method of the fifth aspect of the present invention may involve measuring the absorbance of radiation at more than two wavelengths. The measurement of further absorbancies allows further ratios to be calculated, providing a higher degree of certainty that an article is authentic.

(85) Preferably the article of the first aspect is a banknote. Preferably it has a coating comprising material of formula (I) or is printed with an image comprising material of formula (I). Suitably the coating or image comprising the material of formula (I) is durable to heat, light, water, chemicals and mechanical impact, abrasion and wear and tear. Suitably the coating or image is durable to laundering and to UV light from sunlight.

(86) However all banknotes and other articles will become worn over time, and the quality of the coating or image may deteriorate. As the absorption of the coating/image depends on the concentration of the material of formula (I) this may change as a banknote is worn. Thus measurement of the absorption at a locus of the banknote or other article may provide an indication of the quality of the article. The present invention may therefore provide a screening method to detect inferior banknotes or other articles.

(87) According to a sixth aspect of the present invention there is provided a method of assessing the quality of an article of the first aspect, the method comprising measuring at a locus of the article which carries a material of formula (I), the absorption A.sub.1 of at least one wavelength of .sub.1 of radiation in the infra-red range and comparing the absorption with a known standard.

(88) Suitably the method of the sixth aspect involves further measuring at the locus of the article which carries a coating or image comprising a material of formula (I), the absorption A.sub.2 of a second wavelength .sub.2 of radiation in the infra-red range, calculating the ratio of A.sub.1 to A.sub.2 and comparing this with a known standard.

(89) The method of the sixth aspect may be used to provide an automatic screening method. For example the infra-red absorption A.sub.1 and A.sub.2 of banknotes can be measured and compared with a standard in an automated system. Banknotes which conform closely with the standard ratio can be redistributed whereas those on which the image has deteriorated can be withdrawn from circulation.

(90) The invention will now be further described with reference to the following non-limiting examples. Examples 1 to 7 describe the synthesis of materials of formula (I).

EXAMPLE 1

(91) A clean reactor is filled with 100 kg DI-water and 18 kg caesium carbonate are dissolved with stirring. After the dissolution 82 kg of ammonium metatungstate are added with stirring at room temperature. Finally 45 kg of diphenylphosphate are added with stirring. Stirring is continued for 3 hours. After 3 hours the solution has turned turbid and the resulting slurry is dried with a spray drier. The resulting powder is transferred into saggars and is heated in an electric kiln under a nitrogen/hydrogen atmosphere at 600 C. for 2 hours. The resulting deep blue powder is dispersed in 200 l of DI water and ball milled until a surface area of at least 20 m.sup.2/gr is reached.

(92) The resulting dispersion is spray dried again and results in 86 kg of blue powder.

EXAMPLE 2

(93) A clean reactor is filled with 100 kg DI-water and 18 kg caesium carbonate are dissolved with stirring. After the dissolution 82 kg of ammonium metatungstate are added with stirring at room temperature. Finally 22 kg of phenylphosphonic acid are added with stirring. Stirring is continued for 3 hours. After 3 hours the solution has turned turbid and the resulting slurry is dried with a spray drier. The resulting powder is transferred into saggars and is heated in an electric kiln under a nitrogen/hydrogen atmosphere at 600 C. for 2 hours. The resulting deep blue powder is dispersed in 200 l of DI water and ball milled until a surface area of at least 20 m.sup.2/gr is reached.

(94) The resulting dispersion is spray dried again and results in 80 kg of blue powder.

EXAMPLE 3

(95) A clean reactor is filled with 100 kg DI-water and 18 kg caesium carbonate are dissolved with stirring. After the dissolution 82 kg of ammonium metatungstate are added with stirring at room temperature. Finally 35 kg of methylphosphonic acid dimethylester, dissolved in methanol are added with stirring. Stirring is continued for 3 hours. After 3 hours the solution has turned turbid and the resulting slurry is dried with a spray drier. The resulting powder is transferred into saggars and is heated in an electric kiln under a nitrogen/hydrogen atmosphere at 600 C. for 2 hours. The resulting deep blue powder is dispersed in 200 l of DI water and ball milled until a surface area of at least 20 m.sup.2/gr is reached.

(96) The resulting dispersion is spray dried again and results in 80 kg of blue powder.

EXAMPLE 4

(97) A clean reactor is filled with 100 kg DI-water and 18 kg caesium carbonate are dissolved with stirring. After the dissolution 82 kg of ammonium metatungstate are added with stirring at room temperature. Finally 30 kg of 85% octylphosphonic acid dissolved in an water/ethanol mixture are added with stirring. Stirring is continued for 3 hours. After 3 hours the solution has turned turbid and the resulting slurry is dried with a spray drier. The resulting powder is transferred into saggars and is heated in an electric kiln under a nitrogen/hydrogen atmosphere at 600 C. for 2 hours. The resulting deep blue powder is dispersed in 200 l of DI water and ball milled until a surface area of at least 20 m.sup.2/gr is reached.

(98) The resulting dispersion is spray dried again and results in 85 kg of blue powder.

EXAMPLE 5

(99) A clean reactor is filled with 150 kg DI-water and 82 kg of tungstic acid are added with stirring at room temperature. 22 kg of caesium carbonate are added with stirring. Stirring is continued for 3 hours at elevated temperatures of 80 C. The turbid dispersion is filtered and the filter cake is dried at 105 C. for 16 hours. After cooling to room temperature the yellow orange powder is dispersed in water again and 26 kg of phenylphosphonic acid are added and the mixture is continued to be stirred another 5 hours. The dispersion is filtered again, the filter cake dried again at 105 C. for 16 hours. The dry filter cake is crushed and heated in an electric kiln under a nitrogen/hydrogen atmosphere at 500 C. for 4 hours. The resulting deep blue powder is dispersed in 200 l of DI water and ball milled until a surface area of at least 20 m.sup.2/gr is reached.

(100) The resulting dispersion is spray dried and results in 98 kg of blue powder.

EXAMPLE 6

(101) A clean reactor is filled with 150 kg DI-water and 82 kg of tungstic acid are added with stirring at room temperature. 22 kg of caesium hydroxide are added with stirring. Stirring is continued for 3 hours at elevated temperatures of 80 C. The turbid dispersion is filtered and the filter cake is dried at 105 C. for 16 hours. After cooling to room temperature the yellow orange powder is dispersed in water again and 30 kg of octylphosphonic acid in a water/ethanol mix are added and the mixture is continued to be stirred another 5 hours. The dispersion is filtered again, the filter cake dried again at 105 C. for 16 hours. The dry filter cake is crushed and heated in an electric kiln under a nitrogen/hydrogen atmosphere at 500 C. for 4 hours. The resulting deep blue powder is dispersed in 200 l of DI water and ball milled until a surface area of at least 20 m.sup.2/gr is reached.

(102) The resulting dispersion is spray dried and results in 98 kg of blue powder.

EXAMPLE 7

(103) A clean reactor is filled with 150 kg DI-water and 82 kg of tungstic acid are added with stirring at room temperature. 22 kg of caesium hydroxide are added with stirring. Stirring is continued for 3 hours at elevated temperatures of 80 C. The turbid dispersion is filtered and the filter cake is dried at 105 C. for 16 hours. After cooling to room temperature the yellow orange powder is dispersed in water again and 54 kg of distearylphosphate in ethanol are added and the mixture is continued to be stirred another 5 hours. The dispersion is filtered again, the filter cake dried again at 105 C. for 16 hours. The dry filter cake is crushed and heated in an electric kiln under a nitrogen/hydrogen atmosphere at 500 C. for 4 hours. The resulting deep blue powder is dispersed in 200 l of DI water and ball milled until a surface area of at least 20 m.sup.2/gr is reached.

(104) The resulting dispersion is spray dried and results in 98 kg of blue powder.

EXAMPLE 8

(105) The powder reflectance of the material of example 4 was recorded and is shown in FIG. 1. This spectrum shows that there is very little reflection in the range of 900-1800 nm, which means that there is a high absorption since there is no transmission in a powder sample.

EXAMPLE 9

(106) The organophosphorus tungsten oxide compound of example 4 was incorporated into a printing ink and the effect on the infrared ray absorption tested.

(107) The material of example 4 was compared with a standard commercially available caesium tungsten oxide of the prior art. The materials were blended at different levels into a non absorbing white base ink with the help of a speedmixer. The ready to use inks were applied via an orange proofer onto paper and polymer films.

(108) The prints were dried and spectroscopically analysed. FIG. 2 shows the spectra for the ink comprising the material of the prior art. FIG. 3 shows the spectra for the ink comprising the inventive material.

(109) It can be clearly seen that the organophosphorus tungsten oxide compound has a higher NIR absorption capability than the state of the art material. For example at 900 nm 5% of the prior art material resulted in an absorption of 60%, whereas the same amount of the inventive material resulted in a 75% absorption rate. At lower concentration rates the increase in NIR absorption is even bigger.

EXAMPLE 10

(110) The organophosphorus tungsten oxide compound of example 7 was dispersed in an organic aliphatic solvent at a concentration of 1 wt %. The visible and infrared transmittance spectrum of this dispersion is shown in FIG. 4.

EXAMPLE 11

(111) Ink compositions comprising 1, 3 and 5 wt % of the material of example 4 in organic solvents were prepared. These were printed onto standard printer paper and onto banknote paper. The infrared spectra of the prints were then recorded.

(112) FIG. 5 shows a photograph of the prints on banknote paper. It can be seem that they are very light in colour.

(113) FIG. 6 shows the infrared spectra of the prints comprising 1 wt % of the material of example 4.

(114) FIG. 7 shows the infrared spectra of the prints comprising 3 wt % of the material of example 4.

(115) FIG. 8 shows the infrared spectra of the prints comprising 5 wt % of the material of example 4.