Printing diffraction gratings on paper and board

Abstract

Disclosed herein is a method for forming a surface relief microstructure, including the steps of A) applying a curable composition to at least a portion of the frontside of the paper substrate, B) contacting at least a portion of the curable composition with surface relief microstructure, C) curing the composition by using at least one UV lamp which is arranged on the backside of the paper substrate, and D) depositing a metallic layer, and optionally a layer of a transparent high refractive index material, on at least a portion of the cured composition, wherein the UV lamp has an emission peak in a UV-A range of 320 nm to 400 nm and additionally in a near VIS range of 400 nm to 450 nm, the curable composition comprises a photoinitiator which absorbs in the UV-A range and also in the near VIS range.

Claims

1. A method for forming a surface relief microstructure on a paper substrate, the method comprising: A) applying a curable composition to at least a portion of a frontside of a paper substrate; B) contacting at least a portion of the curable composition with a surface relief microstructure; C) curing the curable composition with a UV lamp, which is arranged on a backside of the paper substrate, to form a cured composition; and D) depositing a metallic layer, and optionally a layer of a transparent high refractive index material, on at least a portion of the cured composition, wherein: the UV lamp has an emission peak in a UV-A range of 320 nm to 400 nm and additionally in a near VIS range of 400 nm to 450 nm; the curable composition comprises a photoinitiator which absorbs in the UV-A range and also in the near VIS range; the photoinitiator is a mixture comprising a compound formula (XII): ##STR00032## R.sub.50 is unsubstituted cyclohexyl, cyclopentyl, phenyl, naphthyl or biphenylyl; or is cyclohexyl, cyclopentyl, phenyl, naphthyl or biphenylyl substituted by one or more halogen, C.sub.1-C.sub.12alkyl, C.sub.1-C.sub.12alkoxy, C.sub.1-C.sub.12alkylthio or by NR.sub.53R.sub.54; or R.sub.50 is unsubstituted C.sub.1-C.sub.20alkyl or is C.sub.1-C.sub.20alkyl which is substituted by one or more halogen, C.sub.1-C.sub.12alkoxy, C.sub.1-C.sub.12alkylthio, NR.sub.53R.sub.54 or by (CO)OC.sub.1-C.sub.24alkyl; R.sub.51 is (CO)R.sub.52; or R.sub.51 is C.sub.1-C.sub.12alkyl which is unsubstituted or substituted by one or more halogen, C.sub.1-C.sub.12alkoxy, C.sub.1-C.sub.12alkylthio, or by NR.sub.53R.sub.54; R.sub.52 and R.sub.52 independently of each other are unsubstituted cyclohexyl, cyclopentyl, phenyl, naphthyl or biphenylyl, or are cyclohexyl, cyclopentyl, phenyl, naphthyl or biphenylyl substituted by one or more halogen, C.sub.1-C.sub.4alkyl or C.sub.1-C.sub.4alkoxy; or R.sub.52 is a 5- or 6-membered heterocyclic ring comprising an S atom or N atom; R.sub.53 and R.sub.54 independently of one another are hydrogen, unsubstituted C.sub.1-C.sub.12alkyl or C.sub.1-C.sub.12alkyl substituted by one or more OH or SH wherein the alkyl chain optionally is interrupted by one to four oxygen atoms; or R.sub.53 and R.sub.54 independently of one another are C.sub.2-C.sub.12-alkenyl, cyclopentyl, cyclohexyl, benzyl or phenyl; the mixture further comprises at least one selected from the group consisting of a monoacylphosphine oxide compound, a benzophenone compound, an alpha-amino ketone compound, an alpha-hydroxyketone compound, an alpha-alkoxyketone compound, a phenyl glyoxylate compound, an oxime ester compound, and an onium salt compound; the metallic layer is deposited by applying a metal-based ink on the cured composition; the metal-based ink comprises metal pigment particles produced by physical vapor deposition, a binder, and optionally a solvent; the metal pigment particles have a thickness in the range of 5 to 50 nm; and the curing of the curable composition with the UV lamp, arranged on the backside of the paper substrate, fully cures the curable composition.

2. The method according to claim 1, wherein the UV lamp is a gallium, or iron doped medium pressure mercury lamp.

3. The method according to claim 1, wherein the mixture comprises the compound of formula (XII) and a monoacylphosphine oxide compound.

4. The method according to claim 1, wherein the curable composition comprises: (a) from 1.0 to 20.0% by weight of the photoinitiator; and (b) from 99.0 to 80.0% by weight of a resin comprising at least one polymerizable compound, wherein a sum of the photoinitiator and the resin equals 100%.

5. The method according to claim 1, wherein the contacting of the surface relief microstructure occurs by contacting the portion of the curable composition with a shim selected from the group consisting of a nickel sleeve, a nickel plate, an etched metallic drum, a laser imaged metallic drum, and other material mounted on an opaque cylinder or metal cylinder comprising an OVD image on a surface thereof.

6. The method according to claim 5, wherein the shim is a nickel plate mounted on an opaque cylinder or metal cylinder and comprising the OVD image on the surface.

7. The method according to claim 1, wherein the paper substrate is selected from the group consisting of a regular paper, a banknote paper and a synthetic paper substrate.

8. The method according to claim 7, wherein the cationic polymer is a polyvinylamine.

9. The method according to claim 1, further comprising treating a paper or board with a cationic polymer on the frontside before applying the curable composition to at least a portion of the frontside of the paper substrate.

10. The method according to claim 1, wherein the mixture comprises the compound of formula (XII), a monoacylphosphine oxide compound and at least one selected from the group consisting of a benzophenone compound, an alpha-amino ketone compound, an alpha-hydroxyketone compound, an alpha-alkoxyketone compound, a phenyl glyoxylate compound, an oxime ester compound, and an onium salt compound.

11. The method of claim 1, wherein the photoinitiator is a mixture comprising bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide and at least one selected from the group consisting of 2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide, (2,4,6-trimethylbenzoyl)-2,4-dipentoxyphenylphosphine oxide, and bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide.

12. The method according to claim 1, wherein the metal pigment particles have a thickness in the range of 8 to 21 nm.

13. The method according to claim 12, wherein the metal pigment particles have an average particle diameter may be in the range of 8 to 15 microns.

14. The method according to claim 1, wherein the metal pigment particles are selected from aluminium, stainless steel, nichrome, gold, silver and platinum.

15. The method according to claim 14, wherein the metal pigment particles are aluminium pigment particles.

16. The method according to claim 1, wherein the binder comprises one or more selected from the group comprising nitro cellulose, vinyl chloride, vinyl acetate copolymers, vinyl, acrylic, urethane, polyethyleneterephthalate, terpene phenol, polyolefin, silicone, cellulose, polyamide and rosin ester resins.

17. The method according to claim 1, wherein a content of the metal pigment particles in the metal-based ink ranges from 0.1% to 6% by weight.

18. The method according to claim 1, wherein the mixture comprises the compound of formula (XII) and at least one selected from the group consisting of a benzophenone compound, an alpha-hydroxy ketone, alpha-alkoxyketone, and alpha-aminoketone compound.

19. The method according to claim 1, wherein the paper substrate is a polymer banknote substrate being an opaque substrate allowing UV light transmission and having a thickness of 12 micrometer up to 300 micrometer.

20. The method according to claim 1, wherein the metallic layer is deposited on the entire portion of the cured composition.

21. The method according to claim 1, wherein only a portion of the paper substrate is printed with the curable composition, the surface relief microstructure, and the metallic layer.

Description

(1) The method and apparatus of the present invention will now be described, by way of example only, with reference to the accompanying examples and figures, in which:

(2) FIG. 1 shows an application apparatus for transferring surface relief microstructures from a shim to a substrate.

(3) FIG. 2 is a cross-sectional schematic view of one embodiment in accordance with the present invention.

(4) Referring to the FIG. 1 a roll of paper substrate is unwound and passed through a coating station that applies an UV varnish to the top surface of the paper substrate. This coated substrate then passes to the printing/curing station, which comprises a chilled cylinder upon which is located a layer comprising a surface relief microstructure (nickel plate). The coated substrate under tension is guided by nip rollers around the cylinder and passes through two nip sections provided by the nip rollers, which are in contact with the back surface of the paper substrate. The cylinder and nip rollers are in indirect contact with each other (the substrate is located between them). As the coated substrate passes through the printing/curing station the UV varnish is imprinted with the surface relief microstructure and simultaneously cured by the action of the UV radiation, which passes through the paper substrate to cure the UV varnish on the substrate thus ensuring that the surface relief microstructure is retained in the surface of the cured varnish layer. UV lamp 1 is arranged on the backside of the coated paper substrate. It is also possible to use 2, or more UV lamps (UV lamp 2) for curing of the UV varnish.

(5) If necessary, the UV varnish on the substrate may be completely cured by irradiation with an additional UV lamp 3, which is arranged on the coated side of the paper substrate. Thereafter a metal layer, or a layer of a high refractive index material may be applied on top of the embossed and cured UV varnish layer.

(6) The layout of this apparatus is compact and highly adaptable for the inclusion of other stations and process features.

(7) In a preferred embodiment, the method of the present invention includes irradiating the varnish layer on the substrate with a UV source rich in UVA, a gallium-doped lamp in one embodiment with a power level of up to 200 W/cm. Other embodiments may use an iron-doped lamp or a different lamp high in UVA and possibly high in both UVA and UVB. Using an UV source rich in UVA has been found to have the advantage of fast curing speed.

(8) In a further aspect of the present invention the apparatus may further comprise a UV-post-curing unit with or without a heating unit, or just an IR-heating unit, or combined UV/IR, which may be especially recommended in order to support and speed up the curing of varnish systems. This post curing unit may be used when the coated substrate leaving the printing/curing unit although successfully imprinted is not full cured. The post-curing unit ensures that the coating is fully cured.

(9) The substrate may be in the form of one or more sheets or a web. The substrate is preferably an opaque substrate that enables UV light transmission with a thickness of 12 micron up to 300 micron, such as, for example, paper. The paper substrate is selected from regular paper, banknote paper, synthetic paper, or a polymer banknote. Regular paper is made from wood pulp. Banknote paper is usually made from cotton. Synthetic paper contains a large part of synthetic resin derived from petroleum as its primary material. There are three major sub-classes of synthetic paper: film synthetic paper like Teslin (PPG Industries; a microporous, highly filled, single layer, polyolefin synthetic material), or Yupo (Covert-All, Inc.; an opaque white, multi-layered bi-axially oriented polypropylene (BOPP) product); fibre synthetic paper (polymer fibres instead of wood fibres); and film laminated synthetic paper: paper/film/paper, such as, for example, Durasafe (Landquart); film/paper/film, such as, for example Hybrid banknote substrate (Giesecke & Devrient; combination of protective polyester film around a cotton fiber core).

(10) The term paper substrate also comprises polymer banknotes, such as, for example, Guardian (Securency; biaxially-oriented polypropylene (BOPP) core with white basecoat applied by gravure printing).

(11) In a specific embodiment of the invention the paper or board has been treated with a cationic polymer on the frontside before applying a curable composition (varnish) to at least a portion of the frontside of the paper substrate.

(12) Treating in the context of the instant invention comprises all suitable means for applying the polymer solution to the surface of the paper substrate; in particular printing or coating.

(13) The cationic polymers utilized in the present invention for treating the paper include repeating amine units that are capable of forming cationic amine salts. The amine group-containing cationic polymer may be a homopolymer or a copolymer. The homopolymer or copolymer may be either in the base form, or partially, or wholly, in the cationic amine salt form. Such cationic polymers are, for example, described in US 2008/0318150 on page 3 to 4.

(14) Preferably the cationic polymer is a polyvinylamine, which is preferably hydrolysed to at least 90%.

(15) Polyvinylamine or partially or fully hydrolysed polyvinylformamide are obtainable by polymerisation of N-vinylformamide and subsequent hydrolysis and elimination of the formyl groups to obtain amine groups. The degree of hydrolysis may range from 1% to 100%, preferably 50% and more preferably 90%. Particularly preferred is a fully hydrolysed poylvinylformamide.

(16) The preparation of N-vinylformamide polymers and the subsequent hydrolysis is, for example, extensively described in U.S. Pat. No. 6,132,558, col. 2, line 36 to col. 5, line 25. Polyvinylamine and partially or fully hydrolysed polyvinylformamide are commercially available under the trade names Catiofast und Polymin from BASF SE.

(17) For example the average molecular weight of these polymers M.sub.w is from 20 000 to 2 000 000 g/mol, for instance from 50 000 to 1 000 000, in particular from 100 000 to 500 000 g/mol.

(18) For example the polyvinylamine contains 0.1 to 22 milliequivalent (meq), for instance 5 bis 18 meq cationic groups per gramm polyvinylamine. The polyvinylamine polymers are typically in the form of a dispersion or solution, for example with a solid content from 10% to 40%, for instance from 15% to 30% and preferably from 20% to 25%. They are usually applied to the paper or board from such solutions or dispersions.

(19) The amount applied of the above mentioned polymer solution is, for example 2 to 20 g, for instance 2 to 15 g and preferably 4 to 12 g per m.sup.2 paper substrate. The polymer solution is subsequently dried by means of an infra red dryer and/or a hot air dryer.

(20) It is also possible to apply together with the cationic polymer further natural polymers such as starch, in particular amylopectine. The amount admixed to the cationic polymer is typically from 5% to 50% based on the weight of the cationic polymer.

(21) The forming of an optically variable image on the substrate may comprise depositing a curable composition (varnish) on at least a portion of the substrate. The composition, generally a coating or lacquer may be deposited by means of offset, gravure, flexographic, ink jet and screen process printing, or other coating methods, but is preferably deposited by means of gravure or flexographic printing.

(22) The curable lacquer is cured by ultraviolet (U.V.) light. The lacquers exposed to UV radiation used in the present invention are required to reach a solidified stage when they separate again from the imaging shim in order to keep the record in their upper layer of the sub-microscopic, holographic diffraction grating image or pattern (OVI).

(23) Diffraction requires that the medium the grating is made of and the media bordering the grating have a difference in optical index. The larger this difference is, the brighter the diffraction will appear. To create highest diffraction, full reflective materials such as metals like aluminum, copper or gold, are thin film coated onto the surface of the grating. Alternately, the grating is coated with a thin film of transparent material having a high refractive index (HRI).

(24) The metallic layer, or the layer of the transparent high reflective index material can be deposited by physical vapour deposition, but are preferably formed by depositing a metallic ink, or an ink of a transparent high reflective index material on the cured composition.

(25) The metallic ink comprise preferably any one or more selected from the group comprising aluminium, stainless steel, nichrome, gold, silver, platinum and copper.

(26) A paper substrate is printed with an ultra violet curable composition on its lower surface. An optically variable image (OVI) is cast into the surface of the composition with a shim having the OVI thereon. The OVI is imparted into the composition and instantly cured via an UV lamp disposed at the upper surface of the paper substrate at normal processing speeds. The OVI is a facsimile of the image on the shim. Metallic ink is printed over the OVI and causes the optically variable device or other lens or engraved structure to become light reflective. Further colours can be subsequently conventionally printed in-line at normal printing process speeds.

(27) The shim is selected from the group consisting of a nickel sleeve; a nickel plate; an etched, or laser imaged metallic drum, or other materials mounted on an opaque cylinder or metal cylinder containing the OVD image on the surface.

(28) Most preferred, the shim is a nickel plate mounted on an opaque cylinder or metal cylinder and containing the OVD image on the surface (nickel shim).

(29) In an alternative embodiment a paper substrate is printed conventionally with a number of coloured inks. Using, for example, a Cerutti R950 printer (available from Cerrutti UK Long Hanborough Oxon.). The substrate is then printed with an ultra violet curable composition on the surface of the printed paper substrate. An OVI is cast into the surface of the composition with a shim, especially a nickel shim having the OVI thereon, the OVI is imparted into the composition and instantly cured via a UV lamp at normal processing speeds, becoming a facsimile of the image disposed on the nickel shim. A metallic ink is printed over the OVI and causes the optically variable device (OVD) to become light reflective.

(30) In an alternative embodiment, an UV primer (varnish), which is applied to the substrate and when exposed to the UV light source is pre-cured. The pre-curing is not complete but stable enough to have received the diffraction pattern or array of sub-microscopic images. The pre-cured coating is then exposed to an additional UV light source and totally cured. In said embodiment alternatively to the UV primers of the free radical type cationic systems can be used.

(31) Cationic epoxy based chemistry may offer additional benefits, such as, for example, low shrinkage on curing, good flexibility, low odour in the formulation and cured film. Low toxicity and skin irritation, no oxygen inhibition, improved gas barrier properties, good electrical properties, high chemical and solvent resistance and lower viscosity of the resins could aid printability.

(32) A conventional printing press rotogravure, UV flexographic or similar can have an extra station added, this being an embossing station. The substrate is first embossed (first station), then printed using a specifically formulated metallic ink to produce the metallised effect. Conventional printing can also be carried out on the same press. As the metallic ink is formulated like a normal ink, conventional printing methods can be utilised. The printing of the metallic ink can be anywhere in the line; it does not have to come directly after embossing. If an encoder for example an indexing machine which marks the sheet or web so that the mark can be recognised by the print operator is placed in the embossing area and the embossing head has specified areas of imagery, then register to print can be achieved. Printing of the metallic ink can be solid, semi translucent etc, with the resulting effect being that in one pass of the printing press metallising, semi-metallising, de-metallising and normal printing of colours in or not in register can be achieved. The specifically formulated metallic ink can be printed on either side of the film, however generally this will be carried out on the embossed side, to encapsulate the holographic embossed image/pattern so that it remains intact, should it come into contact with any filling agents such as liquids, grease, solvents, lacquers, inks or any other surface contaminants or foreign bodies of any kind.

(33) Alternately, the OVD is coated with a thin film of transparent material having a high refractive index (HRI). Examples are transparent polymers having greater refractive index than the hologram forming layer (=ca. 1.50), such as, for example, PEI (polyetherimide; =1.65-1.77) PEEK (polyetheretherketone; =1.66-1.67), and polysulfones (=1.63-1.65). In addition, extrinsic high refractive index polymers result of the incorporation of high refractive index materials, especially nanoparticles into conventional polymers or intrinsic high refractive index polymers.

(34) The transparent high reflective index material is preferably selected from nanoparticles of polymethylmethacrylat (PMMA), ZnS, ZnO, Si, Sb.sub.2S.sub.3, Fe.sub.7O.sub.3, PbO, PbS, ZnSe, CdS, TiO.sub.2, PbCl.sub.2, CeO.sub.2, Ta.sub.2O.sub.5, ZnO, CdO, and Nd.sub.2O.sub.3, wherein nanoparticles of PMMA, nanoparticles of TiO.sub.2 and platelets of ZnS are preferred. Substrates coated with a transparent HRI coating are often used for security applications such as identification or access cards, where it is desired that information positioned behind the hologram remains visible to the unaided eye.

(35) The OVD of the present invention may either comprise a metallic layer, or layer of the transparent high reflective index material on the cured embossed varnish or a layer of the transparent high reflective index material on the cured embossed varnish and a metallic layer on the layer of the transparent high reflective index material.

(36) The metallic ink may be applied to the substrate by means of conventional printing press such as gravure, rotogravure, ilexographic, lithographic, offset, letterpress intaglio and/or screen process, or other printing process. The substrate may then be rewound for subsequent off line printing at a later stage or alternatively, the substrate may be pre-printed in line or off line or subsequently printed in line.

(37) The metal-based ink may comprise metal pigment particles, a binder and optionally a colorant, such as a pigment, or dye, wherein pigments and dyes, which can be used for coloring the UV varnish, can also be used for colouring the metal-based ink.

(38) The metal pigment particles may comprise any suitable metal. Nonlimiting examples of suitable metallic materials include aluminum, silver, copper, gold, platinum, tin, titanium, palladium, nickel, cobalt, rhodium, niobium, stainless steel, nichrome, chromium, and compounds, combinations or alloys thereof. The particles may comprise any one or more selected from the group comprising aluminium, gold, silver, platinum and copper. Preferably, the particles comprise aluminium, silver and/or copper flakes.

(39) In a preferred embodiment of the present invention, platelet shaped transition metal particles having a longest dimension of edge length of from 15 nm to 1000 nm, preferably from 15 nm to 600 nm and particularly from 20 nm to 500 nm, and a thickness of from 2 nm to 100 nm, preferably from 2 to 40 nm and particularly from 4 to 30 nm are used. The production of the shaped transition metal particles is, for example, described in US2008/0295646, WO2004/089813, WO2006/099312, C. Xue et al., Adv. Mater. 19, 2007, 4071, WO2009056401 and WO2010/108837. The use of the platelet shaped transition metal particles for producing holograms is described in WO2011/064162. The inks comprise a total content of shaped transition metal particles of from 0.1 to 90% by weight, preferably 0.1-70% by weight based on the total weight of the ink. Preferably, the binder comprises 50% nitrocellulose in conjunction with any below mentioned resin. The ink may additionally comprise a solvent. The solvent may be ester/alcohol blends and preferably normal propyl acetate and ethanol. More preferably, the ester/alcohol blend is in a ratio of between 10:1 and 40:1, even more preferably 20:1 to 30:1. The solvent used in the metallic ink may comprise any one or more of an ester, such as n-propyl acetate, iso-propyl acetate, ethyl acetate, butyl acetate; an alcohol, such as ethyl alcohol, industrial methylated spirits, isopropyl alcohol or normal propyl alcohol; a ketone, such as methyl ethyl ketone or acetone; an aromatic hydrocarbon, such as toluene, and water.

(40) The platelet shaped (transition) metal particles may be used in combination with spherical (transition) metal particles. Alternatively, spherical (transition) metal particles having a diameter of 40 nm, especially 20 nm may be used alone.

(41) In another preferred embodiment the metal pigment is a metal pigment produced by physical vapor deposition (PVD metal pigment). The operating range of vacuum deposition may be in the range of 5 to 50 nm, the preferred thickness of the metal particles is in the range of 8 to 21 nm, Preferably, the thickness of the metal pigment particles is less than 50 nm. More preferably, the thickness of metal pigment particle is less than 35 nm. More preferably still, the thickness of pigment particle is less than 20 nm. Even more preferably still, the thickness of pigment particle is in the range 5-18 nm.

(42) The optical density may be in the range of 0.046 to 1, especially 0.09 to 0.8 as measured on the McBeth densitomiter. In another embodiment the range is 0.2 to 0.8, especially 0.5 to 0.8 as measured on the McBeth densitomiter.

(43) The metal layer may comprise aluminium, stainless steel, nichrome, gold, silver, platinum or any other metal which can be vaporised and deposited by vacuum deposition or applied by sputtering or electron beam deposition. Preferably, the metal layer comprises aluminium.

(44) The average particle diameter may be in the range of 8 to 15 microns, the preferred range being 9 to 10 microns diameter as measured by a Coulter LS130 l.a.s.e.r. diffraction granulometer.

(45) In order that the sub-microscopic or holographic diffraction grating pattern or image (OVI) is clearly visible on the first surface of a paper substrate, preferably, the aluminium or other flakes are printed in such a way as to align themselves with the contours of the sub-microscopic, holographic or other diffraction grating pattern or image surface wave length such that the flakes conform to and follow the contours of the diffraction grating. To accomplish this alignment of flakes to the contours of the diffraction grating wave length i.e. the distance between peak and peak or trough and trough of the sub-microscopic contour, the specifically formulated metallic ink preferably has a very low binder content, high pigment to binder ratio and very thin aluminium flake, preferably in the range of 9 to 10 microns, consistent to maintain good adhesion of the ink to the surface to the sub-microscopic or holographic diffraction pattern or image. The binder may comprise any one or more selected from the group comprising nitro cellulose, vinyl chloride, vinyl acetate copolymers, vinyl, acrylic, urethane, polythyleneterephthalate, terpene phenol, polyolefin, silicone, cellulose, polyamide, rosin ester resins. The preferred binder is 50% nitrocellulose (ID nitrocellulose DHL120/170 and nitrocellulose DLX30/50 supplied by Nobel Industries) 50% polyurethane (ID Neorez U335 supplied by Avecia). The solvents may be ester/alcohol blends and preferably normal propyl acetate and ethanol in a ratio of 20:1 to 30:1.

(46) The ink preferably comprises low solids, high viscosity binders. Preferably, the pigment to binder ratio is in the range of 10:1 to 1:10 by weight. More preferably, the pigment to binder ratio is by weight in the range of 6:1 to 1:6, and even more preferably 4:1 to 1:4. Most preferably the pigment to binder ratio is from 3:1 to 1:3.

(47) The metal pigment content by weight of the composition may be less than 10%. Preferably the pigment content by weight of the composition is less than 6%, more preferably in the range of 0.1% to 6%, even more preferably in the range 0.1% to 3%, more preferably still in the range 0.2% to 2% by weight. In another embodiment of the present invention the metal pigment content of the ink may be to the range of 2% to 4% by weight, and preferably 3%.

(48) An example of a metallic ink suitable for use in the methods and apparatus of the present invention is disclosed in WO05/051675, WO2005049745 and PCT/EP2009/066659.

(49) The ink comprises, as in the case of an ordinary printing ink, the metal flakes, especially aluminium flakes, a binder, an auxiliary agent, and the like.

(50) With respect to the binder resin, a thermoplastic resin may be used, examples of which include, polyethylene based polymers [polyethylene (PE), ethylene-vinyl acetate copolymer (EVA), vinyl chloride-vinyl acetate copolymer, vinyl alcohol-vinyl acetate copolymer, polypropylene (PP), vinyl based polymers [poly(vinyl chloride) (PVC), poly(vinyl butyral) (PVB), poly(vinyl alcohol) (PVA), poly(vinylidene chloride) (PVdC), poly(vinyl acetate) (PVAc), poly(vinyl formal) (PVF)], polystyrene based polymers [polystyrene (PS), styrene-acrylonitrile copolymer (AS), acrylonitrile-butadiene-styrene copolymer (ABS)], acrylic based polymers [poly(methyl methacrylate) (PMMA), MMA-styrene copolymer], polycarbonate (PC), celluloses [ethyl cellulose (EC), cellulose acetate (CA), propyl cellulose (CP), cellulose acetate butyrate (CAB), cellulose nitrate (CN)], fluorin based polymers [polychlorofluoroethylene (PCTFE), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoroethylene copolymer (FEP), poly(vinylidene fluoride) (PVdF)], urethane based polymers (PU), nylons [type 6, type 66, type 610, type 11], polyesters (alkyl) [polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polycyclohexane terephthalate (PCT)], novolac type phenolic resins, or the like. In addition, thermosetting resins such as resol type phenolic resin, a urea resin, a melamine resin, a polyurethane resin, an epoxy resin, an unsaturated polyester and the like, and natural resins such as protein, gum, shellac, copal, starch and rosin may also be used.

(51) Furthermore, to the binder, a plasticizer for stabilizing the flexibility and strength of the print film and a solvent for adjusting the viscosity and drying property thereof may be added according to the needs therefor. The solvent may comprise any one or more of an ester, such as n-propyl acetate, iso-propyl acetate, ethyl acetate, butyl acetate; an alcohol, such as ethyl alcohol, industrial methylated spirits, isopropyl alcohol or normal propyl alcohol; a ketone, such as methyl ethyl ketone or acetone; an aromatic hydrocarbon, such as xylene and toluene. A solvent of a low boiling temperature of about 100 C. and a petroleum solvent of a high boiling temperature of 250 C. or higher, may be used according to the type of the printing method. An alkylbenzene or the like, for example may be used as a solvent of a low boiling temperature. Examples of solvents are ethoxypropanol, methylethylketon, methoxypropylacetate, diacetonalcohol etc.

(52) Further in addition, an auxiliary agent including a variety of reactive agents for improving drying property, viscosity, and dispersibility, may suitably be added. The auxiliary agents are to adjust the performance of the ink, and for example, a compound that improves the abrasion resistance of the ink surface and a drying agent that accelerates the drying of the ink, and the like may be employed.

(53) A photopolymerization-curable resin or an electron beam curable resin wherein a solvent is not used may also be employed as a binder resin that is a principal component of the vehicle. The examples thereof include an acrylic resin, and specific examples of acrylic monomers commercially available are shown below.

(54) A monofunctional acrylate monomer that may be used includes for example, 2-ethylhexyl acrylate, 2-ethylhexyl-EO adduct acrylate, ethoxydiethylene glycol acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxyethyl acrylate-caprolactone addduct, 2-phenoxyethyl acrylate, phenoxydiethylene glycol acrylate, nonyl phenol-EQ adduct acrylate, (nonyl phenol-EQ adduct)-caprolactone adduct acrylate, 2-hydroxy-3-phenoxypropyl acrylate, tetrahydrofurfuryl acrylate, furfuryl alcohol-caprolactone adduct acrylate, acryloyl morpholine, dicyclopentenyl acrylate, dicyclopentanyl acrylate, dicyclopentenyloxyethyl acrylate, isobornyl acrylate, (4,4-dimethyl-1,3-dioxane)-caprolactone adduct acrylate, (3-methyl-5,5-dimethyl-1,3-dioxane)-caprolactone adduct acrylate, and the like.

(55) A polyfunctional acrylate monomer that may be used includes hexanediol diacrylate, neopentyl glycol diacrylate, polyethylene glycol diacrylate, tripropylene glycol diacrylate, neopentyl glycol hydroxypivalate diacrylate, (neopentyl glycol hydroxypivalate)-caprolactone adduct diacrylate, (1,6-hexanediol diglycidyl ether)-acrylic add adduct, (hydroxypivalaldehyde-trimethylolpropane acetal) diacrylate, 2,2-bis[4-(acryloyloxydiethoxy)phenyl]propane, 2,2-bis[4-(acryloyloxydiethoxy)phenyl]methane, hydrogenated bisphenol A-ethylene oxide adduct diacrylate, tricyclodecanedimethanol diacrylate, trimethylolpropane triacrylate, pentaerithritol triacrylate, (trimethylolpropane-propylene oxide) adduct triacrylate, glycerine-propylene oxide adduct triacrylate, a mixture of dipentaerithritol hexaacrylate and pentaacrylate, esters of dipentaerithritol and lower fatty acid and acrylic acid, dipentaerithritol-caprolactone adduct acrylate, tris(acryloyloxyethyl) isocyanurate, 2-acryloyloxyethyl phosphate, and the like.

(56) Inks comprising the above resins are free of solvent and are so constituted as to polymerize in chain reaction upon irradiation by an electron beam or electromagnetic waves.

(57) With respect to inks of ultraviolet-irradiation type among these inks, a photopolymerization initiator, and depending on the needs therefor, a sensitizing agent, and auxiliary agents such as a polymerization inhibitor and a chain transfer agent, and the like may be added thereto.

(58) With respect to photo-polymerization initiators, there are, (1) an initiator of direct photolysis type including an arylalkyl ketone, an oxime ketone, an acylphosphine oxide, or the like, (2) an initiator of radical polymerization reaction type including a benzophenone derivative, a thioxanthene derivative, or the like, (3) an initiator of cationic polymerization reaction type including an aryl diazonium salt, an aryl iodinium salt, an aryl sulfonium salt, and an aryl acetophenone salt, or the like, and in addition, (4) an initiator of energy transfer type, (5) an initiator of photoredox type, (6) an initiator of electron transfer type, and the like. With respect to the inks of electron beam-curable type, a photopolymerization initiator is not necessary and a resin of the same type as in the case of the ultraviolet-irradiation type inks can be used, and various kinds of auxiliary agent may be added thereto according to the needs therefor.

(59) The inks comprise a total content of metal, especially aluminum pigment of from 0.1 to 20% by weight, preferably 0.1-10% by weight based on the total weight of the ink.

(60) Preferably, the thickness of the metallic ink when deposited on a substrate is sufficiently thin as to permit the transmission of light therethrough. Preferably, when the substrate carrying the metallised image or pattern is subsequently over-laid onto printed pictures and/or text, or the substrate is pre-printed with pictures and/or text and the metallised image or pattern is deposited thereon those printed features are visible through the metallic ink coated optically variable image or device.

(61) The binder may comprise any one or more selected from the group comprising polyvinyl butyral, nitro cellulose, vinyl chloride, vinyl acetate copolymers, vinyl, acrylic, urethane, polythyleneterephthalate, terpene phenol, polyolefin, silicone, cellulose, polyamide, polyester, rosin ester resins. The preferred binder is 50% nitrocellulose (ID nitrocellulose DHL120/170 and nitrocellulose DLX30/50 supplied by Nobel Industries) 50% polyurethane (ID Neorez U335 supplied by Avecia). The solvents may be ester/alcohol blends and preferably normal propyl acetate and ethanol in a ratio of 20:1 to 30:1.

(62) The present invention is also directed to a paper product obtainable using the method of the present invention. The paper product may be a banknote, an identification document like a passport, an identification card, a drivers license, a packaging material, e.g. a label, folding carton, paper bag for pharmaceuticals, apparel, software, cosmetic, tobacco or any other product to be decorated or prone to counterfeiting or forgery; and can be used for for preventing forgery.

(63) The possibility of counterfeiting decreased further by adding thereto- or photochromic dyes, UV/IR fluorescent dyes, magnetic stripes etc. into the OVD primer or ink.

(64) Referring to FIG. 2 a paper substrate 100, UV curable lacquer 102 and holographic or other sub-microscopic diffraction grating 104 with metallic ink 106 printed over with the image viewable through the first surface 108 only.

(65) Various features and aspects of the present invention are illustrated further in the examples that follow. While these examples are presented to show one skilled in the art how to operate within the scope of this invention, they are not to serve as a limitation on the scope of the invention where such scope is only defined in the claims. Unless otherwise indicated in the following examples and elsewhere in the specification and claims, all parts and percentages are by weight, temperatures are in degrees centigrade and pressures are at or near atmospheric.

EXAMPLES

Example 1 and 2

(66) Various photinitiators are mixed with UV lacquer and are applied using a 4 micron thick wireround bar coater onto paper and hoard. The coated paper is laminated with the original shim under a pressure of 1 kg. Samples are exposed to a medium pressure mercury lamp (IST Metz GmbH, Nurtingen, DE, 150 Watt/cm/300 mm width) and a medium pressure mercury discharge lamp gallium doped (IST Metz GmbH, Nurtingen, DE, 150 Watt/cm/300 mm width) through the paper or board at different belt speeds and different lamp outputs to modify the light dose. The curing of the varnish and transfer of the OVD image is assessed. Speed is the maximum obtainable printing speed resulting in a fully cured varnish.

(67) All percentages given are % by weight.

(68) The composition of the UV lacquer is shown below:

(69) TABLE-US-00001 UV lacquer % by weight Tripropylene glycol diacrylate 1-25 (TPGDA) Dipropylene glycol diacrylate (DPGDA) 30-45 Ethoxylated trimethylol propane 10-50 triacrylate (TMEOPTA) Reactive tertiary amine 1-15

(70) TABLE-US-00002 Photoinitator No. Chemical structure 1 2 3 4 5 0

Example 1

Application on 70 Micron Thick Label Paper

(71) TABLE-US-00003 5% Photoinitiator Medium pressure mercury discharge lamp No. Dose Speed Result 1 150 W/cm 10 m/min Bad comparative 2 150 W/cm 60 m/min Medium 3 80 W/cm 80 m/min Excellent 4 80 W/cm 80 m/min Excellent

(72) TABLE-US-00004 5% Medium pressure mercury lamp gallium Photoinitiator doped No. Dose Speed Result 1 150 W/cm 30 m/min Bad comparative 2 80 W/cm 40 m/min Good 3 80 W/cm 80 m/min Excellent 4 80 W/cm 80 m/min Excellent

Example 2

Application on 250 Micron Thick Carton Board

(73) TABLE-US-00005 5% Photoinitiator Medium pressure mercury discharge lamp No. Dose Speed Result 1 150 W/cm 10 m/min Bad comparative 2 150 W/cm 20 m/min Bad 3 150 W/cm 40 m/min Medium 4 150 W/cm 60 m/min Good

(74) TABLE-US-00006 5% Medium pressure mercury lamp galium Photoinitiator doped No. Dose Speed Result 1 150 W/cm 10 m/min Bad comparative 2 150 W/cm 30 m/min Bad 3 150 W/cm 60 m/min Good 4 80 W/cm 80 m/min Excellent

Example 3 and 4

(75) Example 1 is repeated, except that UV lacquer is not applied by using a 4 micron thick wireround bar coater, but by gravure printing. The paper substrate is coated Wo with a UV lacquer in a thickness of 3 g/m.sup.2 using a gravure cylinder, it further runs on a chilled magnetic cylinder which is wrapped with a Nickel shim. The UV varnish coated paper substrate is pressed against the shim with two nip rollers. It is simultaneously embossed with the OVD structure and cured with UV light through the paper substrate. The substrate is then peeled off the magnetic cylinder and shim (Printing press: Rotova press (Rotocolor AG), maximal printing speed: 110 m/min, coated UV varnish thickness: 3 g/m.sup.2; UV lamp: GEW 200 Watt-medium pressure mercury lamp300 mm widthdichroic reflectorGEW 180 Watt medium pressure mercury lamp gallium doped, UV lamp max 180 Watt/cm). The curing of the varnish and transfer of the OVD image is assessed. Speed is the maximum obtainable printing speed resulting in a fully cured varnish.

(76) TABLE-US-00007 5% Photoinitiator Medium pressure mercury discharge lamp No. Dose Speed Result 1 180 W/cm 50 m/min Medium comparative Medium pressure mercury lamp gallium 5% Photoinitiator doped No. Dose Speed Result 4 110 W/cm 110 m/min Excellent

Example 4

Application on 250 Micron Thick Carton Board by Gravure Printing

(77) TABLE-US-00008 5% Photoinitiator Medium pressure mercury discharge lamp No. Dose Speed Result 1 180 W/cm 8 m/min Bad comparative Medium pressure mercury lamp gallium 5% Photoinitiator doped No. Dose Speed Result 4 180 W/cm 55 m/min Excellent

(78) Bad: OVD image partially transferred, no brightness, poor printing performance.

(79) Medium: OVD image partially transferred, poor brightness, limited printing performance.

(80) Good: OVD image fully transferred, good brightness, acceptable printing performance.

(81) Excellent: OVD image fully transferred, high brightness, excellent printing performance.

(82) High speed printing at minimal UV power intensity can be achieved by using a gallium doped lamp and a UV varnish formulation based on a mono, or a bisacylphosphine oxide compound on 250 micron thick paper board and 70 micron thick paper.

Example 5

(83) Substrates that tend to absorb low viscosity lacquers are pre-coated with a cationic fully hydrolyzed polyvinylamine dispersion, and are dried at 100 C. The coated dispersion blocks the substrate surface and enables the printing of any lacquer on the surface of the substrate.

(84) The polyvinylamine dispersion is characterized as follows: aqueous solution of a polymer based on viny amine and N-vinylformamide with a solid content of 20-22% as measured according to DIN EN ISO 3251 (2 h, 120 C.); pH value 7.0-9.0 as measured according to DIN 19268 (measured with the undiluted substance); dynamic viscosity 500-2500 mPa s as measured according to DIN EN ISO 2555 (RV (Spindle 3, 20 l/min).

(85) The polyamine dispersion is coated onto the surface of Xerox copy paper 80 g/m.sup.2 (Table 5.1) by means of a wire bar with a thickness of 4 micron, 6 micron or 12 micron wet film. The coated film is air dried and subsequently coated with 6 micron UV lacquer as described in example 1 and 2 by means of a wire and embossed on a shim containing OVD images, and cured under UV light through the paper.

(86) TABLE-US-00009 TABLE 5.1 of Xerox copy paper 80 g/m.sup.2, 5% Photoinitiator 5 Polyvinyle amine dispersion OVD image visibility No coating No image 4 micron wet film thickness Good 6 micron wet film thickness Excellent 12 micron wet film thickness Excellent

(87) Other tested substrates are:

(88) TABLE-US-00010 TABLE 5.2 Banknote paper 120 micron thickness 10% Photoinitiator 5 Polyvinyle amine dispersion OVD image visibility No coating No image 12 micron wet film thickness Excellent

(89) TABLE-US-00011 TABLE 5.3 Velin paper 135 micron thickness 10% Photoinitiator 5 Polyvinyle amine dispersion OVD image visibility No coating No image 12 micron wet film thickness Excellent

(90) TABLE-US-00012 TABLE 5.4 Velum paper 100 micron thickness 10% Photoinitiator 5 Polyvinyle amine dispersion OVD image visibility No coating No image 12 micron wet film thickness Excellent

(91) TABLE-US-00013 TABLE 5.5 Tax stamp paper 85 micron 5% Photoinitiaor 5 Polyvinyle amine dispersion OVD image visibility No coating No image 12 micron wet film thickness Excellent