WO2007077455A1

WO2007077455A1 - Improvements in and relating to security printing

Info

Publication number: WO2007077455A1
Application number: PCT/GB2007/000030
Authority: WO
Inventors: David Malcolm Lewis; Peter Jeffrey Broadbent
Original assignee: Inovink Limited
Priority date: 2006-01-06
Filing date: 2007-01-05
Publication date: 2007-07-12
Also published as: EP1968802A1; GB0600193D0

Abstract

A composition comprising a mixture of a thermographic compound and a compound capable of undergoing a change in at least one optical property upon application of a stimulus is described. Also described are a printed image precursor comprising a substrate on at least a portion of which is coated such a composition, a method of manufacturing a printed image precursor and a method of manufacturing a printed image.

Description

IMPROVEMENTS IN AND RELATING TO SECURITY PRINTING

Field of the invention

This invention relates to compositions for use in security- printing, and methods of manufacturing image articles and image article pre-cursors .

Background to the invention

Raised printing requires raised lettering or images to be printed onto a substrate. Raised lettering can be used in many applications such as business cards and letterhead paper, Braille literature and the like, for example.

There are many methods of manufacturing raised letter printed articles and raised image printed articles, a common method of which is the use of thermographic powders, which when printed onto a substrate and subsequently heated, expand to form raised lettering or images on the substrate.

US5627578 suggests an inkjet printer system in which a substrate is passed through an inkjet printer, and a delayed drying ink applied image-wise onto the substrate. The delayed drying ink forms a tacky surface on which thermographic powder may adhere on subsequent passing of the tacky substrate through thermographic particles. The themographically coated substrate may then be heated to effect expansion of the thermographic powder to form raised images on the substrate.

The manufacture of raised letter or raised image printed articles is important, especially for use in Braille literature. There is, however, a need for raised letter literature and raised image literature to have security features incorporated therein, in order for a reader or user to know that the article upon which the image is being 5 printed is a genuine article, and not counterfeit. For example, packaging may include raised letter printing or Braille, and it may be desirable for customs officials, or state authorities to determine that the packaging is authentic and not counterfeit.

10. It would therefore be advantageous to provide compositions- suitable for printing onto a substrate, which may be manipulated to effect raised printing of text and/or images, which also include security features visible only upon application of an external stimulus; such that on

15 application of an external stimulus, a change is undertaken within the image and/or text, to allow confirmation of a genuine article.

It would also be advantageous to provide methods of manufacturing printed image precursors and printed images 20 which include both raised image areas and security features to enable detection of counterfeit images .

It is therefore an aim of preferred embodiments of the present invention to overcome or mitigate at least one problem of the prior art, whether expressed herein or not.

25 Summary of the invention

According to a first aspect of the invention, there is provided a composition comprising a mixture of a thermographic compound and a compound capable of undergoing a change in at least one optical property upon 0 application of a stimulus. Preferably the thermographic compound is a thermographic powder, or granules of thermographic material.

Thermographic powders are commonly resin powders that are capable of melting and coalescing to form a solid image. Examples of such powders include Neutral Gloss SupaFlow, SupaFlow Ultra Clear, Neutral Laser Resist and Neutral SupaMatt thermographic powders, all supplied by Caslon Limited, UK.

Preferably the compound capable of undergoing a change in at least one optical property upon application of stimulus comprises a compound capable of undergoing a change in at least one of the following optical properties: light absorbance, light transmission, light reflectance, light scattering and any combination thereof. Suitably the compound capable of undergoing a change in optical property upon application of stimulus is a compound which is capable of undergoing a change to more than one optical property upon application of one or more stimuli, and may be a compound capable of undergoing a change in a first optical property upon application of a first stimulus, and capable of undergoing a change in a second optical property upon application of a second stimulus, different to the first stimulus.

Preferably the stimulus is an external stimulus.

Suitable stimuli include heat, radiation, pressure, physical movement of the composition, light, and any mixture thereof .

Preferably the compound capable of undergoing a change in at least one optical property upon application of stimulus is selected from a thermochromic compound, a photochromic compound, a phosphor, a metal, a dual polarisation compound, a mica pigment or a solid disperse dye (particularly for use in mixtures with polyester-based thermographic powders) .

Thermochromic compounds include coloured pigments which decolorise to white on heating (such as Reversatherm thermochromic pigments supplied by Keystone Europe Limited) . Other thermochromic compounds include viologen and viologen derivatives, which change from colourless to deeply coloured on heating.

Suitable thermochromic compounds include mixtures of micro encapsulated chromogens (pH dependent) and organic acid, viologen derivatives and mixtures thereof, such as Chromazone (supplied by B+H - The Colour Change Company, UK) , and Reversatherm (Keystone Europe Limited) , for example.

Photochromic compounds are compounds which on exposure to light change colour, usually from colourless to deeply coloured. When the light source is removed, the colour is lost, such as Reversacol photochromic dyes supplied by

James Robinson Limited, UK.

Suitable photochromic compounds include spiroxazines, spiropyrans and naphthopyrans , such as Reversacol photochromic dyes (supplied by James Robinsons Ltd, UK) , and Photosol photochromic dyes (supplied by Pittsburgh. Plate Glass Company, USA) .

A phosphor is a compound which on excitation with UV light develops an excited triplet state and decays back to the ground state with the emission of a lower energy photon such as green long-life phosphor prepared from copper doped zinc sulphide (GL29C/L-Ll) or green long-life phosphor prepared from europium and dysprosium doped strontium aluminate (HMK63D/L-L1) supplied by Phosphor Technology Limited, for example.

Suitable phosphors include inorganic phosphors such as zinc sulphide doped with copper, or strontium aluminate doped with europium and dysprosium, and inorganic phosphors supplied by Phosphor Technology Limited.

Suitable metals include any metal capable of undergoing oxidation upon application of moisture, in order to undergo a colour change. Suitable metals include iron, copper, silver and aluminium.

Dual polarisation powders are commonly comprised of organic liquid crystal particles, having an inner helical molecular alignment, whose twisting sense causes only a reflection of left or right circular polarised light such that, when the ink is printed on a dark background, its colour, when viewed through a particular polariser, is either maintained or razed, depending on the polarity of the polariser. Additionally, the compound's helical alignment is responsible for a change in colourmetric appearance (colour transition effect between two distinctly different colours) , depending on how the printed image is viewed. Examples include the range of Chromaflux dual polarisation powders supplied by Gans Ink and Supply Company.

OVI (optically variable image) pigments generally comprise a metal oxide layer, such as titanium oxide or iron oxide, coated on a mica (generally a sheet layer silicate material) , silica flake or alumina flake substrate. Examples of OVI pigments include Iriodin pigments, Colorstream pigments, Xirallic pigments and Biflair pigments supplied by Merck Chemicals Limited, UK.

Preferably the compound capable of undergoing a change in at least one optical property upon application of a stimulus is in the form of a powder or granules .

Suitably the composition comprises the thermographic compound and the compound capable of undergoing a change in at least one optical property upon application of a stimulus, in a ratio of at least substantially 60:40 thermographic compound to compound capable of undergoing a change in at least one optical property upon application of a stimulus; more preferably a ratio of at least substantially 70:30, and most preferably a ratio of at least substantially 80:20.

In some embodiments, the composition of the present invention further comprises a material which absorbs infrared radiation.

Such a material may be present in an amount of between 5 and 20 wt%.

Materials which absorb infrared radiation and are suitable for use in the present invention include reduced indium tin oxide, the CI Pigment Green 9, Carbon black, metallic molybdenum powder, titanium nitride and zirconium nitride.

According to a second aspect of the invention, there is provided a printed image precursor comprising a substrate on at least a portion of which is coated a composition of the first aspect of the invention. Suitably the substrate is a plastics substrate. Suitably the plastics substrate comprises a plastics material selected from cellulose acetate, polyethylene, polypropylene, ' polyvinylchloride, polyurethane, a polyamide, polyester, polycarbonate and any mixture or combination thereof.

Other materials suitable as substrates include textile materials such as cotton, linen, flax, hemp, jute, wool and any mixture thereof, for example.

Also preferred as substrates are paper, card or any mixture thereof, metals, rubber, ceramics, composite materials, carbon fibre materials, glass, and any mixtures or combinations thereof, for example.

The substrate may be a rigid substrate or a flexible substrate. Suitably the substrate is a substantially planar substrate, and is preferably a sheet substrate. The sheet substrate may be a flexible sheet substrate, or may be rigid.

Preferably the composition of the first aspect of the invention is coated on the substrate to a concentration of at least 2 gdm^"3, more preferably at least 5 gdm^"3, and most preferably at least 10 gdm^"3.

According to a third aspect of the invention, there is provided a method of manufacturing a printed image precursor, the method comprising the steps of:

(a) providing a substrate;

(b) coating at least a portion of the substrate with an agent capable of adhering solids; and (c) coating at least a portion of the substrate with a composition of the first aspect of the invention.

Suitably the substrate is as described for the second aspect of the invention.

Suitably the agent capable of adhering solid particles is an agent capable of adhering solid particles when wetted with a solvent, more preferably an aqueous medium. Suitably the agent capable of adhering solid particles when wetted comprises a water-soluble agent. Preferably different areas of the coating of the agent are differentially wetted with an aqueous medium in step (b) . Thus, when differentially wetted, the coating of water- soluble agent effects differential adhesive strength across different regions of the water-soluble agent, and therefore may selectively adhere varying concentrations of the composition of the first aspect of the invention on different regions of the substrate. The water-soluble agent may be differentially wetted by applying different amounts of aqueous medium containing the water-soluble agent to different areas of the coating of water-soluble agent. Alternatively or additionally, the water-soluble agent may be differentially wetted by coating the water- soluble agent on the substrate at different concentrations in an aqueous medium over different areas of the substrate. Alternatively or additionally, the water- soluble agent may be coated onto the substrate, dried, and then differentially wetted by applying different amounts of aqueous medium to different areas of the coating of water-soluble agent. Preferably the agent capable of adhering solids comprises a saccharide, a hydroxyl acid, an amine oxide, a urea, a glycol or water-soluble polymer.

Saccharides are particularly useful water-soluble agents capable of adhering solids. Saccharides may be applied in solution, to create a "tacky" adhesive surface coating able to adhere a wide range of solid particles. Saccharides also have the property of being cross-linkable to further bind the saccharide to the substrate and any solid particles to the saccharide. Saccharides are cheap and abundant, and do not interfere with the chemical mechanisms of many of the target compositions of the first aspect of the invention. Saccharides may also contribute to coloured images, as many saccharides, if heated above certain temperatures, oxidise and turn brown or black.

Preferred saccharides include monosaccharides, disaccharides, oligosaccharides and polysaccharides.

Preferred monosaccharides include glucose, fructose, tagatose and galactose.

Preferred disaccharides include sucrose, mannose and lactose.

Preferred polysaccharides include starch, cellulose, hydroxyalkylcelluloses , carboxyalkylcelluloses , cyclodextrins, chitosan and derivatives thereof.

Suitable water-soluble polymers include polyvinyl alcohol, polyvinylpyrrolidone, polyvinylacetate, polyacrylates , alginates, guar gum, locust bean gum, polymeric surfactants and polyols, cross-linkable water-soluble polymers, whether self cross-linkable or cross-linkable via an external stimulus, such as heat, light, chemical curing, or the like, for example.

The agent capable of adhering solids may be coated on the substrate in solution and the solvent may be partially removed from the coating to effect wetting of the agent, and thus activate the adhesive properties of areas of the coating to solids. Preferably the solvent is partially removed in varying amounts from different areas of coating to effect differential wetting of different areas of the coating, and thus effect different adhesive strengths to solids across different areas of the coating.

Suitably the agent capable of adhering solids is coated onto the substrate in solution, and preferably the agent is a water-soluble agent and is coated onto the substrate in aqueous solution. Suitably the agent capable of adhering solids is printed onto the substrate using inkjet printing, roller printing, lithographic printing or gravure printing, but especially preferred is inkjet printing.

Suitably step (c) comprises coating the composition of the first aspect of the invention as solid particles or granules, and preferably substantially dry particles or granules .

Solid particles may be applied to the substrate by any suitable method in step (c) . The substrate containing a coating of agent capable of adhering solids may be immersed in the composition in step (c) , such that the solid particles of the composition adhere only to the areas of the substrate that have been coated in agent capable of adhering solids. Excess solids may be removed by agitating the substrate after step (c) , such as by shaking, tapping, and the like, or by effecting air flow across the surface of the substrate comprising the solids.

Alternatively the composition may be applied in step (c) image-wise, such as by spraying, jetting or misting, for example.

According to a fourth aspect of the invention, there is provided a method of manufacturing a printed image comprising the steps of:

(a) performing the method of the third aspect of the present invention to provide a printed image precursor; and

(b) heating the printed image precursor in order to activate the thermographic powder and produce a raised image on the substrate.

Suitably step (b) comprises passing the printed image precursor obtained in step (a) under a source of heat. Suitably step (b) comprises heating the printed image precursor obtained in step (a) to a temperature of at least 100^°C, more preferably at least 125°C, most preferably at least 150°C.

The imaged substrate may be a Braille image, a security image, such as an identity card, packaging, label or the like, for example, or may be any other suitable imaged substrate.

The length of time for which the printed image precursor is heated will depend on the time needed to activate the thermographic powder, in order to produce a raised image; and this will be determined by the type of thermographic powder present in a composition of the first aspect of the invention. However, a suitable time period for heating the printed image precursor is at least 2 seconds, more preferably at least 5 seconds, and most preferably at least 10 seconds. A suitable time range for heating is 2- 50 seconds.

In embodiments in which the composition of the first aspect includes a material which absorbs infrared radiation, step (b) of the fourth aspect may involve applying infrared radiation to the printed image precursor in order to effect heating thereof.

Examples

For a better understanding of the various aspects of the invention and to show how embodiments of the same may be put into effect, the invention will now be described by way of the following examples.

Example 1

Card, paper, polypropylene, polythene and cellulose acetate substrates were image-wise ink-jet printed with a ^Λtack' (compound capable of adhering solids when wetted) generating ink on an Apollo P-1200 ink-jet printer to yield a printed image template which demonstrated differential adhesion properties to solids when allowed to partially dry; the image template being a combination of Braille, an alpha-numeric variable code and barcode data, The 'tack' generating ink was formulated in accordance with the following recipe:

400 gdirf³ Sucrose

200 gdirf³ Ethylene glycol 0 . 5 gdπf³ Cibafast W (Ciba) pH 7 . 0

De-ionised water to 1 litre

The image template was allowed to partially dry to generate a tacky image template capable of adhering solids, and covered with a commercially available thermographic powder (supplied by Caslon Ltd) , Neutral Gloss SupaFlow 7 thermographic powder, a grade of thermographic powder specially recommended for Braille printing by Caslon, that had been pre-mixed with a commercially available black 25⁰C thermochromic powder (B+H The Colour Change Company, UK) , in a ratio of 14Og of thermochromic powder and 86Og of thermographic powder. The thermographic and thermochromic powders were thoroughly milled together prior to their application to the printed ^Λtack' template, and any non-adhered particles removed by gently tapping the card. The printed image was fixed to the substrate by passing it through a heating unit at temperatures in excess of 200⁰C for 10 seconds The resulting print achieved exhibited a series of black raised images. In particular, an area of Braille writing was achieved which was composed of a series of well defined dots; the dots being durable to touch and readily readable. The Braille dots achieved exhibited heights between 0.27-0.33mm and diameters between 1.50-1.56mm compared with a height of 0.17mm and a diameter of 1.48 for embossed printed Braille samples supplied by the Royal National Institute for the Blind (RNIB) ; the RNIB specifications requiring a height of 0.4mm and a diameter of 1.5mm. The resulting prints achieved exhibited a series of raised durable black images, which when heated above 25°C became colourless; the black image returning when the print was cooled to room temperature. Thus, a printed image was formed which was black at ambient temperatures but which included the security feature of turning colourless when heated to 25°C, demonstrating its authenticity.

Example 2

The process described in Example 1 was repeated, but in this case the black 25⁰C thermochromic powder was replaced with a thermochromic cyan 15⁰C powder supplied by B+H - The Colour Change Company, UK, to yield a series of colourless raised images at room temperature. On cooling below 15⁰C, a series of blue images were achieved, which again became colourless on heating to room temperature.

Example 3 The process described in Example 1 was repeated, but in this case the 'tack' image template was covered with a thermographic powder from Caslon Ltd that had been pre- mixed with a commercially available photochromic dye, Reversacol Plum (James Robinsons, UK) in a ratio of 1Og of photochromic powder to 99Og of thermographic powder. A series of colourless raised images were again achieved, but on exposure to UV light or bright sunshine the raised images turned a purplish-blue colour. The purplish-blue coloured images rapidly turned colourless once again on their removal from such light sources .

Example 4

The process described in Example 1 was repeated, but in this case the ^λtack' image template was covered with a thermographic powder from Caslon Ltd (Neutral Gloss SupaFlow 7 thermographic powder) that had been pre-mixed with iron powder supplied by Aldrich, UK, 14Og of iron powder and 86Og of thermographic powder. The iron print achieved could be readily verified by its attraction to a strong magnet. Iron prints of this type may also be used as a method of detecting if a frozen item has been thawed and refrozen. When frozen, an item printed with iron according to this process will display a grey printed image, but on thawing the water generated will cause the iron to rust and so result in a brown image being visible on the item.

Example 5

The process described in Example 1 was repeated, but in this case the 'tack' image template was covered with a thermographic powder from Caslon Ltd (Neutral Gloss SupaFlow 7 thermographic powder) that had been pre-mixed with a commercially available red inorganic phosphor powder, PTR610/F, (supplied by Phosphor Technology, UK) , 14Og of inorganic phosphor powder and 86Og of thermographic powder. A series of colourless raised images were again achieved, but on exposure to UV light (254nm) red phosphorescence was observed. The red coloured images immediately turned colourless on their removal from the UV light source.

Example 6 The process described in Example 1 was repeated, but in this case the ^Λtack' image template was covered with a thermographic powder from Caslon Ltd (Neutral Gloss SupaFlow 7 thermographic powder) that had been pre-mixed with a commercially available green long-life inorganic phosphor powder, GL29C/L-L1, (Phosphor Technology) in a ratio of 14Og of inorganic phosphor powder and 86Og of _{Λ c}

16

thermographic powder. A series of pale yellow raised images were achieved, but on exposure to UV light (254 or 365nm) green phosphorescence was observed. The green coloured images remained for at least 12 hours before returning to their pale yellow non-phosphorescent state on removal from the UV light source. The prints were readily readable in a darkened room after their exposure to UV light.

Example 7 The process described in Example 1 was repeated, but in this case the 'tack' image template was covered with a commercially available thermographic powder (Caslon Ltd) (Neutral Gloss SupaFlow 7 thermographic powder) that contained a commercially available mica pigment powder, 215 Rutile Red Pearl, (Merck) in a ratio of 14Og mica pigment powder to 86Og of thermographic powder. A series of durable raised images were achieved, which exhibited OVI (optically variable image) properties dependent on viewing angle; the raised images changing from red to green dependent on the viewing angle.

Example 8

The process described in Example 1 was repeated, but in this case the 'tack' image template was covered with a thermographic powder from Caslon Ltd (Neutral Gloss SupaFlow 7 thermographic powder) that had been pre-mixed with a commercially available mica security pigment powder, Colorcode D, (Merck) in a ratio of 86Og thermographic powder to 14Og mica powder. Such mica pigments are reserved for security printing and are not available for general use. Such pigments exhibit different colours when printed on differently coloured _χη

backgrounds . A series of durable raised images were achieved, which exhibited OVI properties dependent on viewing angle. In this case the image changed from purple to blue dependent on the viewing angle.

Example 9

The process described in Example 1 was repeated, but in this case the 'tack' image template was covered with a thermographic powder from Caslon Ltd (Neutral Gloss SupaFlow 7 thermographic powder) that had been pre-mixed with a commercially available dual polarisation powder, C- 1099, (Gans) in a ratio of 14Og of dual polarisation powder and 86Og of thermographic powder. Dual polarisation powders are a range of security pigments which offer OVI properties and a colour change when viewed through suitable polarising optical lenses . The dual polarisation powders are most successful when printed on a dark background. A series of durable raised images were achieved, which exhibited OVI properties dependent on viewing angle. In this case the image changed from black to green dependent on the viewing angle. On viewing through suitable polarising optical lenses, a shift in pigment colour can be observed.

Example 10

The method of example 1 was repeated but in this case an infra-red absorbing material, reduced indium tin oxide

(nano form from NanoProducts - Colorado, USA) , was included with the thermographic powder in an amount of 10% by weight. This infra-red absorbing material shows strong absorbance above llOOnm. To fix the deposited thermographic powder following the adhesion/excess powder removal process, simple passage under a low energy IR lamp (emission 1500-2000nm, available from Hereaus) for 2 seconds was sufficient. Without the IR absorber no melting/fixation occurred.

Claims

1. A composition comprising a mixture of a thermographic compound and a compound capable of undergoing a change in at least one optical property upon application of a stimulus.

2. A composition according to claim 1 wherein the thermographic compound is a thermographic powder, or granules of thermographic material .

3. A composition according to any preceding claim wherein the compound capable of undergoing a change in at least one optical property upon application of stimulus is selected from a thermochromic compound, a photochromic compound, a phosphor, a metal, a dual polarisation compound, a mica pigment and a solid disperse dye.

4. A composition according to any preceding claim wherein the compound capable of undergoing a change in at least one optical property upon application of stimulus is a thermochromic compound selected from mixtures of micro encapsulated chromogens (pH dependent) and organic acid, viologen derivatives and mixtures thereof.

5. A composition according to any preceding claim wherein the compound capable of undergoing a change in at least one optical property upon application of stimulus is a photochromic compound selected from spiroxazines, spiropyrans and naphthopyrans .

6. A composition according to any preceding claim wherein the compound capable of undergoing a change in at least one optical property upon application of stimulus is a phosphor selected from zinc sulphide doped with copper _{2 Q}

and strontium aluminate doped with europium and dysprosium.

7. A composition according to any preceding claim wherein the compound capable of undergoing a change in at least one optical property upon application of stimulus is a metal capable of undergoing oxidation on application of moisture, said oxidation resulting in a colour change.

8. A composition according to any preceding claim which further comprises a material which absorbs infrared radiation.

9. A printed image precursor comprising a substrate on at least a portion of which is coated a composition as defined in any preceding claim.

10. A method of manufacturing a printed image precursor, the method comprising the steps of:

(a) providing a substrate;

(b) coating at least a portion of the substrate with an agent capable of adhering solids,- and

(c) coating at least a portion of the substrate with a composition as defined in any of claims 1 to 8.

11. A method according to claim 10 wherein the agent capable of adhering solid particles is an agent capable of adhering solid particles when wetted with an aqueous medium.

12. A method according to claim 11 wherein different areas of the coating of the agent are differentially wetted in step (b) . _{2 χ}

13. A method according to any of claims 10 to 12 wherein the agent capable of adhering solids is selected from a saccharide, a hydroxyl acid, an amine oxide, a urea, a glycol and a water-soluble polymer.

14. A method of manufacturing a printed image comprising the steps of:

(a) performing the method of any of claims 10 to 13 to provide a printed image precursor; and

15. A method according to claim 14 wherein in step (a) the substrate is coated with a composition as defined in claim 8 and step (b) involves applying infrared radiation to the printed image precursor in order to effect heating thereof .