GB2457911A

GB2457911A - Producing an optically variable security device

Info

Publication number: GB2457911A
Application number: GB0803622A
Authority: GB
Inventors: Adam Lister; Lawrence George Commander
Original assignee: De la Rue International Ltd
Current assignee: De la Rue International Ltd
Priority date: 2008-02-27
Filing date: 2008-02-27
Publication date: 2009-09-02
Anticipated expiration: 2028-02-27
Also published as: DE102009010770A1; GB0803622D0; GB2457911B

Abstract

The invention addresses the problem of counterfeiting of documents by photocopying and other imaging systems. A security device consisting of a liquid crystal layer on a security document is modified in selected regions so that the angular dependent colour reflection of the liquid crystal layer is changed in those regions. This is achieved by applying a transparent polymeric resin to one or more regions of one side of a cholesteric liquid crystal layer to form an intermediate structure; heating the intermediate structure at a temperature so that the resin penetrates the liquid crystal layer and increases the pitch of its helical structure in the regions thereby changing the angular dependent coloured reflection of the liquid crystal layer in the regions and subsequently curing the structure.

Description

-1-2457911

IMPROVED METHOD FOR PRODUCING AN OPTICALLY VARIABLE

SECURITY DEVICE

The present invention relates to an improved method for the production of an optically variable security device utilising liquid crystal materials and the resulting security device.

The increasing popularity of colour photocopiers and other imaging systems and the improving technical quality of colour photocopies has led to an increase in the counterfeiting of banknotes, passports and identification cards and the like. There is, therefore, a need to add additional authenticating or security features to existing security features. Steps have already been taken to introduce optically variable features into substrates used in such documentation that cannot be reproduced by a photocopier. There is also a demand to introduce features which are discernible by the naked eye but which are "invisible" to, or viewed differently, by a photocopier.

Since a photocopying process typically involves scattering high-energy light off an original document containing the image to be copied, one solution would be to incorporate one or more features into the document which have a different perception in reflected and transmitted light, an example being watermarks and enhancements thereof.

It is known that certain liquid crystal materials exhibit a difference in colour when viewed in transmission and reflection, as well as an angularly dependent coloured reflection. Liquid crystal materials have been incorporated into security documents, identification cards and security elements with a view to creating distinctive optical characteristics. EP-A- 0435029 is concerned with a data carrier, such as an identification card, which comprises a liquid crystal polymer layer or film in the data carrier. The liquid crystal polymer is solid at room temperature and is typically held within a laminate structure. The intention is that the liquid crystal layer, which is applied to a black background, will demonstrate a high degree of colour purity in the reflected spectrum for all viewing angles. Automatic testing for verification of authenticity is described using the wavelength and polarization properties of the reflected light in a single combined measurement. This has the disadvantage of being optically complex using a single absolute reflective measurement requiring a uniform liquid crystal

area on a black background.

AtJ-A-488, 652 is also concerned with preventing counterfeit copies by introducing a distinctive optically-variable feature into a transparent window security element. This document discloses the use of a liquid crystal "ink" laminated between two layers of plastic sheet. The liquid crystal is coated on a black background so that only the reflected wavelengths of light are seen as a colour. The security feature is primarily provided by thermochromic liquid crystal materials, which have the characteristic of changing colour with variation in temperature.

Cholesteric liquid crystals have certain unique properties in the chiral nematic phase. It is the chiral nematic phase which produces an angularly dependent coloured reflection and a difference in colour when viewed in either transmission or reflection. Cholesteric liquid crystals form a helical structure which reflects circularly polarised light over a narrow band of wavelengths. The wavelength is a function of the pitch of the helical structure which is formed by alignment within the liquid crystal material. An example of such a structure is depicted in Figure 1 with the cholesteric helical axis in the direction of the arrow X. The reflection wavelength can be tuned by appropriate choice of chemical composition of the liquid crystal. The materials can be chosen to be temperature sensitive or insensitive. Both haridednesses of circularly polarised light can be reflected by choice of the correct materials and thus high reflectivities at specific wavelengths can be achieved with double layers of liquid crystals. The wavelength of reflected light is also dependent on the angle of incidence, which results in a colour change perceived by the viewer as the device is tilted (see Figure 2).

On a dark background, only the reflective effect is observed, since little light is being transmitted from

behind. When the dark background is removed or is

otherwise not present and the device is viewed in transmission, the intensity of the transmitted colour saturates the reflective colour. Of the light which is not reflected, a small proportion is absorbed and the remainder is transmitted through the liquid crystal material. When correctly configured there is a dramatic change between the transmitted colour in the direction of arrow Y and reflected colour in the direction of arrow Z (see Figure 3) . The region on either side of the liquid crystal layer in Figure 3 is a transparent polymer or glass. To achieve this effect, the area of the substrate which is occupied by the liquid crystal must be transparent or translucent. The transmitted and reflected colours are complementary, for example, a green reflected colour produces a magenta transmitted colour.

Liquid crystal materials can be incorporated into security devices either as a film, as for example in WO-A-03061980, or in the form of an ink as a liquid crystal pigment in an organic binder, as for example in EP-A- 1156934. The advantage of a liquid crystal ink is that it can be applied using conventional printing processes and therefore it is relatively straightforward to apply the liquid crystal material in the form of a design. However the colour purity, brightness and sharpness of the observed colour and colour-shift are significantly degraded for a pigmented liquid crystal ink compared to a liquid crystal film. This degradation is due to the variability in alignment of the cholesteric helical axis between the individual liquid crystal pigments compared to the uniform alignment of the liquid crystal film.

A disadvantage with the use of liquid crystal films in the security devices described in the prior art is that the production route requires several steps, such as preparing the liquid crystal polymer film on a carrier substrate, and then transferring the liquid crystal polymer film from the carrier substrate to the substrate of the security device. It is neither straightforward nor cost-effective to customise the base liquid crystal film for each security application.

A method of increasing the range of available colours in liquid crystal films is described in US 4,893,906, in which two or more liquid crystal coatings are overlaid to obtain new colours as a result of the colour additive properties of the liquid crystal coatings which do riot absorb light. WO-A-2005105474 describes a security device comprising two superimposed cholesteric liquid crystal layers in which the additive mixing of the colours permits a wider range of colourshift effects. In some of the embodiments in WO-A-200510546, regions exhibiting different colourshifting effects are created by a partial application of one of the liquid crystal layers in localised areas. A partial application of a liquid crystal film is not straightforward and increases significantly the complexity of the production process compared to simply applying one uniform film over a second uniform film.

Methods are described in the prior art for producing patterned cholesteric liquid crystal film, i.e regions exhibiting different angular dependent colour variation.

Typically the patterning occurs during the formation of the liquid crystal film, i.e. before or during the final polyrnerisation of the liquid crystal materials. WO-A- 0034808 relates to a method of manufacturing a patterned layer of a cholesterically ordered polymer material, in which the patterning occurs by subjecting localised regions of the cholesteric liquid crystal material to UV radiation prior to the full polymerisation and/or cross-linking of the film. EP-A-0982605 describes a method to prepare polymer cholesteric liquid crystal films with a spatial (i.e. lateral) distribution of reflection wavelengths. The films are patterned to reflect different colours in different areas by locally varying the irradiation power of the actinic radiation used for polyrnerisation. US 6,421,107 describes a process of preparing a multilayer cholesteric film comprising two or more layers of polymerized cholesteric liquid crystal material with planar orientation, with the cholesteric helix axis oriented substantially perpendicular to the plane of the layer, wherein that the cholesterjc helical pitch of adjacent layers is varied by controlled migration of a nofl-polymerizeci chiral material between the layers.

The disadvantage with patterning the liquid crystal film during formation is that it is not cost effective for short production runs, which are common in the field of security devices used for protecting secure documents.

It is therefore an object of the present invention to overcome this problem.

According to the invention there is provided a method of forming a security device comprising the steps of applying a substantially transparent polymeric resin to at least one region of one side of a cholesteric liquid crystal layer to form an intermediate structure; heating the intermediate structure to a temperature at which the resin penetrates the liquid crystal layer and increases the pitch of its helical structure in said at least one region thereby changing the angular dependant coloured reflection of the liquid crystal layer in the at least one region; and subsequently curing the structure.

The present invention overcomes the aforementioned problem because the patterning of the liquid crystal film takes lace after the liquid crystal film has been fully formed, i.e. after cross-linking and/or polymerisatjon.

This enables the liquid crystal film to be produced in advance, and later customjsecj and/or finished in the subsequent method steps. This results in economic advantages due to the industrial scale production of non-customised liquid crystal film.

According to a further aspect of the invention there is provided a security device made by the aforesaid method.

According to a further aspect of the invention there is provided a security document formed from the aforesaid security device.

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:-Figure 1 depicts chiral nematic alignment of a cholesteric liquid crystal material; Figure 2 shows how the reflection from a cholesteric liquid crystal material varies with the angle of incidence; Figure 3 depicts the transmission and reflection of light incident on a liquid crystal material; Figures 4a-4c are cross sectional side elevations of a liquid crystal film made by the method of the present invention at different stages of production; Figures 5a-5d, 6a-6d are cross sectional side elevations of a different liquid crystal films from that of Figures 4a-4c made by the method of the present invention at different stages of production; Figures 7 to 9 are cross sectional side elevations of security devices made with the liquid crystal film made according to the present invention; Figure 10 is a cross sectional side elevation of the security device of Figure 9 applied to a security substrate; and Figure 11 is plan view of a security document incorporating a security device made according to the present invention.

In the method of the present invention, the angular dependent coloured reflection of a cholesterjc liquid crystal film, which is used to form a security device, is varied in specific regions by locally increasing the helical pitch of the cholesteric liquid crystal structure through the application of a polymeric resin. The increase in helical pitch occurs as the polymeric resin penetrates into the cholesteric liquid crystal film and swells its helical structure. The penetration and swelling mechanism is facilitated by the heating of the cholesterjc liquid crystal film. Once the resin has penetrated and swollen the cholesterjc liquid crystal structure the swollen regions are subjected to liv radiation which fixes the new helical pitch in the areas exposed to the polymeric resin.

It is preferable that the polymeric resin is a liv curable resin that comprises a solvent or low molecular weight diluents/monomers which aid the penetration into and subsequent swelling of the cholesteric liquid crystal film.

For example a security device may be produced from a polymeriseci cholesteric liquid crystal film which has a helical pitch such that it exhibits a green to blue angular dependent colourshift on tilting the device away from normal incidence. A polymeric resin is then applied to the cholesterjc liquid crystal film in localised regions, the film is heated and the resin penetrates into the cholesteric liquid crystal film and swells its helical structure resulting in an expansion of its S helical pitch. During the heating stage the film is covered to stop the evaporation of the resin and solvent from the surface of the liquid crystal film. The increase in helical pitch results in a change in the reflected wavelength towards longer wavelengths, and therefore the swollen regions now exhibit a red to green angular dependent colourshjft rather than a green to blue angular dependent colourshift. The change in the helical pitch in the swollen regions is then fixed by exposure to tJV radiation. The finished security device thus exhibits two different angular dependent colourshifting regions.

Figures 4a to 4c illustrates one embodiment of the production of a security device 10 by the method of the present invention. A cholesteric liquid crystal film 11 is produced by a method known in the art, by coating a layer of cholesteric liquid crystal material onto a carrier substrate 12 and aligning it such that its helical axis is perpendicular to the substrate 12. The film 11 is then polymerised by means of heat or actinic radiation. The cholesteric liquid crystal film 11 comprises an angular dependent coloured reflection, in this case from green to blue on tilting the device such that the viewing angle moves away from normal incidence.

In step 1 of the method of the present invention (see Figure 4a) a substantially transparent polymeric resin 13 is applied to the liquid crystal film 11 in the form of a design. Preferably the polymeric resin 13 is a UV curable polymer employing free radical polymerisation. Examples of free radical systems include photo-crosslinkable acrylate-methacrylate or aromatic vinyl oligomeric resins. One example of a typical polymeric resin 13 would be a UV curable acrylate varnish as sold by Norcote under the product number 80-049. Other suitable polymeric resins 13 include ketones, acrylics, acetates and aldehydes.

It is preferable that a strong solvent is added to the UV curable polymeric resin 13 such that the solids content of the mixture is approximately 50%. For example in this embodiment the formulation of the polymeric resin comprises 50% by weight of Nor-cote and 50% by weight of methlyethlyketone (MEK). Suitable alternative solvents to MEK include acetone, ethyl acetate, ethyl acrylate, toluene and xylene.

The polymeric resin 13 is applied to the liquid crystal film 11 using any standard printing process, such as wet or dry lithographic printing, intaglio printing, letterpress printing, flexographic printing, screen printing, and/or gravure printing. Preferably the polymeric resin 13 is applied using gravure printing.

In step 2 (see Figure 4b) the structure 11,12,13 is covered by a temporary layer (not shown), such as a further polymeric film, and then maintained at a temperature which enables the printed polymeric resin 13 to penetrate into the cholesteric liquid crystal film 11.

If the structure 11,12,13 is heated, the solvent or diluents/monomers in the resin 13 swell the liquid crystal structure in the underlying "covered" regions ila accelerating the penetration by the printed polymeric resin 13. The temperature at which the structure 11, 12, 13 is held is preferably in the range of 40°C to 100°C, more preferably in the range of 6o°C to 90°C and more preferably at 80°C. The combined result of the initial swelling and penetration is to expand the helical structure and increase the helical pitch such that a red to green angular dependent colourshift is observed rather than a green to blue angular dependent colourshift.

In step 3 (see Figure 4c) the polymeric resin 13 and swollen cholesteric liquid crystal regions ha are exposed to LIV radiation to cure the resin 13 and fix the modified helical pitch. In the resultant security device 10, the liquid crystal film 11 comprises two different angular dependent colourshifting regions; a green to blue shift in the regions hib not covered by the polymeric resin 13, and a red to green shift in the (swollen)regions ha covered by the polymeric resin 13.

Figures 5a to Sd illustrate a second embodiment of the present invention. A polymer cholesteric liquid crystal film 11, exhibiting a green to blue angular dependent colourshift, is produced on a carrier substrate 12 as described with reference to Figures 4a to 4c. In step 1 (Figure 5a), a LIV curable substantially transparent polymeric resin 13 is applied all over the cholesterjc liquid crystal film 11.

In step 2 (Figure 5b) the polymeric resin 13 is LIV cured through a mask 14. The exposed areas 13a of polymeric resin 13 are therefore cured and cannot penetrate the cholesterjc liquid crystal film 11 in the subsequent method steps.

In step 3 (Figure 5c) the structure 11,12,13,14 is maintained at a temperature such that the polymeric resin 13 in the regions 13b under the mask 14 penetrates into the underlying covered regions ha of the cholesteric liquid crystal film 11, as described above, swelling the helical structure. As a result the helical pitch in those covered regions ha is increased such that a red to green angular dependent colourshift is observed rather than a green to blue angular dependent colourshift.

In step 4 (Figure 5d) the mask 14 is removed and the polymeric resin 13 and the swollen cholesterjc liquid crystal regions ha are exposed to UV radiation to cure the resin 13 and fix the modified helical pitch. In the resultant security device 10, the liquid crystal film 11 comprises two different angular dependent colourshifting regions lla,hlb; a green to blue shift in the non-masked regions lib, and a red to green shift in the masked regions ha.

Figures 6a to 6d illustrates a third embodiment of the present invention. A polymeric cholesterjc liquid crystal film 11, exhibiting a green to blue angular dependent colourshift, is produced on a carrier substrate 12 as described above. In step 1 (Figure 6a) a UV curable substantially transparent polymeric resin 13 is applied all over the cholesteric liquid crystal film 11.

In step 2 (Figure 6b) the structure is maintained at a temperature such that the polymeric resin 13 penetrates into the cholesteric liquid crystal film 1]. swelling the helical structure and increasing the helical pitch such that a red to green angular dependent colourshift is observed rather than a green to blue angular dependent -13 -colourshift. A temporary layer, such as a further polymeric film, is preferably placed over the polymeric resin to prevent the diluents/solvents evaporating during the heating process.

In step 3 (Figure 6c) selective regions of the polymeric resin 13 and swollen wholly covered layer of liquid crystal film ha are exposed to UV radiation through a mask 14 in order to cure the resin 13 and fix the modified helical pitch.

In step 4 (Figure 6d) the mask 14 is removed and the structure 11, 12, 13 is reheated to allow the swollen, covered regions ha of the cholesterjc liquid crystal film 11 to return to their non-swollen state as the diluents and small molecular weight species diffuse out from the surface of the cholesterjc liquid crystal film 11. A second exposure to UV radiation is then preferably carried out to cure the previously uncured regions of polymeric resin 13 such that no penetration of the resin 13 into the cholesteric liquid crystal film 11 can occur during the lifetime of the security device 10. In the resultant security device 10, the liquid crystal film 11 comprises two different angular dependent colourshifting regions ila, lib which are inverted from the example in Figures 5a to 5d; i.e. a green to blue shift in the masked regions ha, and a red to green shift in the non-masked regions hib.

In a preferred embodiment of the present invention the method is carried out as a continuous reel to reel process. The structure may be heated by passing the material through an in-line heater or by placing the finished web in an oven at the end of the process. If the web is placed in an oven at the end of the process care must be taken in winding up the web if there are exposed regions of uncured polymeric resin 13 which may cause the web to adhere to itself, i.e. block. The method illustrated in Figures 5a to 5d may be modified, by changing step 4, to overcome such a potential blocking issues. In this modification the second exposure to UV radiation is carried out through the carrier substrate 12 with the mask 14 still in place. The structure 11,12,13,14 is then wound into a web and the presence of the mask 14 prevents the polymeric resin 13 from sticking to the adjacent film 11 in the web. The structure 11,12,13,14 is then reheated to allow the covered regions ha of the cholesteric liquid crystal film 11 to return to their non-swollen state and the mask 14 is then removed.

In a further alternative embodiment, and again referring to the method illustrated in Figures 5a-5d, the mask 14 may be applied to the opposite side of the carrier substrate 12 to the liquid crystal film 11. In this case the first exposure to (JV radiation is carried out through the carrier substrate 12 and the second exposure to tJV radiation is carried out from the same side of the polymeric resin 13. The advantage of this embodiment is that the mask 14 never needs to be removed in order to visualise the final security feature.

Preferably the resulting colourshifting regions ha, hib form designs. The designs generated by the printing of the polymeric resin 13 or the use of the mask 14 are preferably in the form of images such as patterns, symbols and alphanumeric characters and combinations thereof. The designs can be defined by patterns comprising solid or discontinuous regions which may include for example line patterns, fine filigree line patterns, dot structures and geometric patterns.

Possible characters include those from non-Roman scripts of which examples include but are not limited to, Chinese, Japanese, Sariskrit and Arabic.

The security device 10 made by the method of the present invention can be viewed in reflection or transmission. If the device 10 is intended to be viewed in reflection, then it is preferable to have an additional dark absorbing layer present under the cholesterjc liquid crystal film 11. The dark absorbing layer may be applied to either side of the carrier substrate 12 for the structures shown in Figures 4 to 6 or, after following the methods illustrated in Figures 4 to 6, the cholesteric liquid crystal film 11 may be subsequently removed from the carrier substrate 12 and transferred onto a second substrate that comprises a dark absorbing layer. The absorbing layer may also be applied in the form of a design such that when viewed in reflection the first and second colourshifting regions ha, hib will only be apparent in the regions of the cholesteric liquid crystal film 11 above the dark layer absorbing layer.

Whilst the use of a black, or very dark, absorbing layer may give rise to the most strong colourshift effects, other effects may be generated by the use of an absorbing layer of other colours or a combination of colours, giving rise to differing apparent colourshift colours. The use of different coloured absorbing layers enables the number of optically variable regions to be increased further. The absorbing layer of the present invention may comprise a pigmented ink or coating or alternatively a non-pigmented absorbing dark dye can be used.

The security device 10 can be incorporated into secure documents in any of the conventional formats known in the prior art, for example as patches, foils, stripes, strips or threads. The security device 10 can be arranged either wholly on the surface of the document, as in the case of a stripe or patch, or can be visible only partly on the surface of the document in the form of a windowed security thread. Security threads are now present in many of the world's currencies as well as vouchers, passports, travellers' cheques and other documents. In many cases the thread is provided in a partially embedded or windowed fashion where the thread appears to weave in and out of the paper and is visible in windows in one or both surfaces of the document. One method for producing paper with so-called windowed threads can be found in EP-A- 0059056. EP-A-0860298 and WO-A-03095188 describe different approaches for the embedding of wider partially exposed threads into a paper substrate. Wide threads, typically having a width of 2 to 6mm, are particularly useful as the additional exposed thread surface area allows for better use of optically variable devices, such as that produced by the present invention.

The security device 10 may be incorporated into a secure document such that regions of the device 10 are viewable from both sides of the document. Methods of incorporating a security device so that it is viewable from both sides of the document are described in EP-A- 1141480 and WO-A-3054297 In the method described in EP-A-1141480 one side of the device is wholly exposed at one surface of the document in which it is partially embedded, and partially exposed in windows at the other surface of the substrate.

In the case of a stripe or patch, the security device 10 is prefabricated on a carrier strip and transferred to a substrate in a subsequent working step.

The security device 10 can be applied to the substrate using an adhesive layer, which is applied either to the security device 10 or the surface of the substrate to which the device 10 is to be applied. After transfer, the carrier strip is removed leaving the security device 10 exposed. Alternatively the carrier strip can be left in place to provide an outer protective layer.

The security device 10 produced using the method of the present invention may be used in many different applications, for example by attachment to objects of value. Preferably, the security devices 10 are adhered to, or substantially contained within, a security document. Such security documents include banknotes, cheques, passports, identity cards, certificates of authenticity, fiscal stamps and other documents for securing value or personal identity.

Security devices comprising liquid crystal materials are inherently machine-readable due to the polarisation properties and wavelength selectivity of the liquid crystal materials. The machine readable-aspect of the present security device 10 can be extended further by the introduction of detectable materials in the existing liquid crystal film 11 or absorbing layers or by the

introduction of separate machine-readable layers.

Detectable materials that react to an external stimulus include, but are riot limited to, fluorescent, phosphorescent, infrared absorbing, thermochromjc, photochromic, magnetic, electrochromic, conductive and piezochromjc materials.

In one preferred embodiment, the pigment in the separate absorbing layers is machine-readable, for example carbon black, to produce a machine-readable, conducting or IR absorbing layer. Alternatively it may be a magnetic material, such as magnetite, to produce a machine-readable magnetic layer.

It will be further understood by those skilled in the art that the security device 10 may be used in combination with existing approaches for the manufacture of security thread. Examples of suitable methods and constructions that can be used include, but are not limited to, those cited within WO-A-03061980, EP-A- 0516790, WO-A-9825236, and WO-A-9928852.

Figure 7 illustrates how the security device 10 can be combined with demetallised characters for application as a windowed security thread. The method requires a metallised film comprising a substantially clear polymeric film 16, of PET or the like, which has an opaque layer of metal 17 on a first side thereof. A suitable pre-metallised film is metalljsed MELINEX S film from DuPont of preferably 19p.m thickness. The metal layer 17 is printed with a resist 18 which contains a black or dark dye or pigment. Suitable resists include the dye BASE Neozapon X51 or the pigment (well dispersed) "Carbon Black 7" mixed into a material with both good adhesion to metal and caustic resistance.

The printed metallised film 16,17,18 is then partially dernetallised, according to a known demetallisation process using a caustic wash which removes the metal in the regions not printed with the resist 18. The remaining regions of metal 17 coated with resist 18 provide a black layer which is visible when the demetallised film 16, 17,18 is viewed from its first side (along arrow Y) interspersed with clear regions 19. The shiny metal of the remaining regions of metal 17 are only visible from an opposite side of the demetalljsed film 16,17,18 (along arrow X). The resist 18 may be printed in the form of the indicja such as words, numerals, patterns and the like; in which case the resulting indicia will be positively metallised, with the metal 17 still covered by the dark or black resist 18.

Alternatively the resist 18 may be printed so as to form indicia negatively, in which case the resulting indicia will be provided by the demetallised clear regions 19. The indicia, however formed, are clearly visible from both sides, especially in transmitted light, due to the contrast between the clear regions 19 where the metal has been removed and the remaining opaque regions of metal 17.

A cholesterjc liquid crystal film 11 is then coated, transferred or laminated to the demetalljsed film 16,17,18. In the example in Figure 7 the polymeric resin 13 is applied to the cholesteric liquid crystal film 1].

prior to the transfer to the demetalljsed film 16,17,18 using any of the processes illustrated in Figures 4 to 6.

In this case the modified cholesteric liquid crystal film 11 and the layer of polymeric resin 13 is removed from the first carrier substrate 12 and transferred or laminated onto the demetallised film 16,17,18 with the final device structure being illustrated in Figure 7.

Alternatively the polymeric resin 13 could be applied after the transfer of the liquid crystal film 1]. to the demetallised film 16,17,18 using any of the methods described with reference to Figures 4 to 6. An adhesive layer 20 may be applied to the outer surfaces of the device to improve adherence to the substrate.

The security device 10 illustrated in Figure 7 exhibits two visually contrasting security characteristics The device 10 comprises the optical effects of the cholesterjc liquid crystal film 11, when the finished substrate is viewed in reflection from the first side (along arrow Y); and a metallic shiny partial coating when viewed from the other side (along arrow X).

When viewed in reflection from the first side, the authenticator observes two different colourshifting regions ha, lib, preferably arranged in the form of a design. If the transferred cholesteric liquid crystal film 11 has the same structure as that shown in Figure 6d, then the first colourshifting region ha will exhibit a green-blue colourshift and the second colourshifting region llb will exhibit a red-green colourshift. The second colourshifting region lib may be arranged in the form of a repeating design of alphanumeric characters, for example the numeral "50" symbol, such that on tilting away from normal incidence the authenticator observes the numeral "50" changing from red to green and the

background changing from green to blue.

Additionally clear positive or negative indicia, defined by the black resist 18, can be seen in transmission from either side. In the example shown the clear indicia are only coincident with the first colourshifting region ha. However they can also be coincident with the second colourshifting region lib as long as they do not detract from the visibility of the design. The example shown in Figure 7 is particularly advantageous when used in a security device 10 that is vjewable from both side of the substrate or document in which it is incorporated. For example the security device may be incorporated into a secure document using the methods described in EP-A-1141480 or WO-A--03054297.

Figure 8 illustrates a machine-readable version of the security device 10 illustrated in Figure 7. The security device 10 comprises a metallised PET layer 16 demetalhised with a suitable design including tramlines of metal 17 left along each edge of the device 10. As described with reference to Figure 7 a black resist 18 is used during the demetallisation process.

A protective layer (not shown) may be applied onto the tramlines of metal 17 to prevent the metal 17 from being corroded by the magnetic layer 21, which is applied next. A suitable protective layer is VHL31534 supplied by Sun Chemical applied with coat weight of 2gsm. The protective layer may optionally be pigmented.

The magnetic material 21 is only applied over the traniljnes of metal 17 so as not to obscure the demetalhised indicia. The cholesteric liquid crystal film 11 is then applied, preferably using a transfer process.

An adhesive layer 20 may be applied to the outer surfaces of the device 10 to improve adherence to the substrate. -22 -

When a magnetic material is incorporated into the device 10, either within the absorbing layer or as a separate layer 21, the magnetic material can be applied in any design but common examples include the use of magnetic tramlines or the use of magnetic blocks to form a coded structure. Suitable magnetic materials include iron oxide pigments (Fe203 or Fe304), barium or strontium ferrites, iron, nickel, cobalt and alloys of these. In this context the term "alloy" includes materials such as Nickel:Cobalt, Iron:Alumjnium:Njckel.Cobalt and the like.

Flake Nickel materials can be used; in addition Iron flake materials are suitable. Typical nickel flakes have lateral dimensions in the range 5 to 50 microns and a thickness less than 2 microns. Typical iron flakes have lateral dimensions in the range 10 to 30 microns and a thickness less than 2 microns.

In an alternative machine-readable embodiment a transparent magnetic layer can be incorporated at any position within the structure of the device 10. Suitable transparent magnetic layers containing a distribution of particles of a magnetic material of a size and distributed in a concentration at which the magnetic layer remains transparent are described in WO-A-03091953 and WO-A-03091952.

Figure 9 is a cross-sectional view of a security device 10 produced by the method of the present invention which is suitable for application to a security substrate as a stripe. The device 10 is formed on a carrier substrate 12, which may be coated with an optional release layer (not shown), onto which is applied a cholesteric liquid crystal material forming a uniform cholesterjc liquid crystal film 11. The cholesterjc liquid crystal film 1]. can be formed on the carrier substrate 12 by coating a polymeric cholesterjc liquid crystal material and then curing to form a film or by transferring or laminating an already formed liquid crystal film 11 onto the carrier substrate 12.

In this example the cholesterjc liquid crystal film 11 exhibits a green to blue colourshjft as the sample is tilted away from viewing at normal incidence. A substantially transparent polymeric resin 13 is then applied to the cholesteric liquid crystal film 11 in the form of a design which, in this example, is a repeating pattern of the numeral "1O".The structure is then covered by a temporary layer and then heated to a temperature of approximately 80°C and the printed polymeric resin 13 penetrates into the adjacent regions ha of the cholesterjc liquid crystal film 11. As before the helical structure swells and the helical pitch increases but such that a red to green angular dependent colourshift is observed rather than a green to blue angular dependent colourshift. The polymeric resin 13 and swollen cholesterjc liquid crystal regions hlb are exposed to tJV radiation to cure the resin 13 and fix the modified helical pitch. The resultant liquid crystal film 11 then comprises two different angular dependent colourshifting regions ha, lib; a green to blue shift in the regions not printed with the polymeric resin 13, and a red to green shift in the regions printed with the polymeric resin 13. An absorbing layer 22 is then printed over the liquid crystal film 11. An adhesive layer 20 is applied to the absorbing layer 22 and the device 10 is ready to be transferred to a security substrate 23, such as banknote paper. After transfer, the carrier substrate 12 may be removed, leaving the security device 10 as the exposed layer (Figure 10). Alternatively the carrier substrate 12 can be left in situ as part of the device 10, acting as an outer protective layer.

Figure 11 is a plan view of a security device 10 of Figure 10 in the form of a stripe on the secure document 24 made from a security substrate 23. On viewing the document 24 at normal incidence, the background of the device 10 appears green and the numeral "10" appears red.

on tilting the device 10 away from normal incidence, the background switches to blue and the numeral "10" switches to green.

Following the application of the security device 10 the security substrate 23, it generally undergoes further standard security printing processes to produce the security document 24. These processes include one or more of the following; wet or dry lithographic printing, intaglio printing, letterpress printing, flexographic printing, screen-printing, and/or gravure printing. In a preferred embodiment, and to increase the effectiveness of the security device 10 against counterfeiting, the design of the security device 10 is linked to the document 24 it is protecting, by content, registration to the designs and identifying information provided on the document 24.

In the previous embodiments the cholesteric liquid crystal film 11 has been customised to create the two colourshifting regions ha, hib before the security device 10 has been incorporated into the secure substrate 23. However an advantage of the present invention is that the customisatjon process can take place after the device has been incorporated into the secure substrate 23.

For example if a security device 10 comprising a liquid crystal film 11 is applied to a substrate 23 as a surface patch or stripe, such that the liquid crystal film 11 is exposed on the top surface, then the polymeric resin 13 may be coated or printed onto the exposed film and the subsequent method steps as illustrated in Figures 4 to 6 may then be carried out.

In a preferred embodiment the application of the polymeric resin 13 takes place at the same time, and preferably using the same equipment, as the standard security printing processes.

The cholesteric liquid crystal film 11 may be further customised by overprinting or embossing either before or after it is incorporated into the security document. Further details on such customjsatjon techniques are described in GB-A-2438384 and GB-A2438383.

The method of the present invention can also be applied to the personaljsatjon of documents such as passports or identity cards. In such embodiments the design generated by the printed resin 13 or mask 14 could be varied for each individual card by using, for example, an inkjet printer to apply the polymeric resin 13 or the mask 14. Alternatively a spatial light modulator could be used to modulate a beam of tiV light in the x,y coordinates to pattern a nv curable polymeric resin 13.

In one embodiment a liquid crystal display (LCD) could be used as a spatial light modulator. With reference to the method described in Figures 5a-5d the LCD display replaces the mask 14. The LCD is illuminated with UV light and, depending on the alignment of the liquid crystal structure which is controlled by the application of an electric charge, certain pixels on the display will allow light through and certain pixels will not allow light to pass through. For example if a person's image in greyscale could be displayed on an LCD this image will be transferred into the polymeric resin by irradiation with tJV light and an image of the person is ultimately formed in the liquid crystal film 11. The advantage here is that the LCD is a dynamic mask i.e. the image is changed for each passport or identity card.

Claims

CLAIMS: 1. A method of forming a security device comprising the steps of applying a substantially transparent polymeric resin to at least one region of one side of a cholesteric liquid crystal layer to form an intermediate structure; maintaining the intermediate structure at a temperature such that the resin penetrates the liquid crystal layer and increases the pitch of its helical structure in said at least one region thereby changing the angular dependant coloured reflection of the liquid crystal layer in the at least one region; and subsequently curing the structure.
2. A method as claimed in claim 1. in which the resin comprises an ultraviolet curable resin.
3. A method as claimed in claim 1 or claim 2 in which the resin comprises a solvent, a low molecular weight diluent or a monomer.
4. A method as claimed in claim 3 in which the solids content of the resin is approximately 50%.
5. A method as claimed in any one of the preceding claims in which the resin employs free radical polymerisation.
6. A method as claimed in any one of the preceding claims in which the resin is applied to the liquid crystal layer by a printing process.
7. A method as claimed in any one of the preceding claims in which the curing process comprises ultraviolet irradiation.
8. A method as claimed in any one of the preceding claims in which the intermediate structure is covered during the heating step to prevent evaporation.
9. A method as claimed in claim 8 in which the intermediate structure is covered by a layer of polymeric film.
10. A method as claimed in any one of the preceding claims in which the intermediate structure is heated to a temperature lying in the range 40C to 100C.
11. A method as claimed in claim 10 in which the intermediate structure is heated to a temperature lying in the range 60C to 90CC.
12. A method as claimed in claim 11 in which the intermediate structure is heated to a temperature of approximately so c.
13. A method as Claimed in any one of the preceding claims in which the liquid crystal layer is formed by applying a liquid crystal material to a carrier substrate.
14. A method as claimed in any one of the preceding claims in which the resin is applied all over the liquid crystal layer.
15. A method as claimed in any one of the preceding claims further including the step of at least partially covering the surface of the resin with a mask having gaps therein and curing and subsequently heating the resin through the mask such that only regions exposed by said gaps are cured
16. A method as claimed in any one of claims 1 to 14 further including the step of at least partially covering the surface of the resin with a mask having gaps therein after the intermediate structure has been heated, curing the structure through the mask such that only regions exposed by said gaps are cured, and removing the mask before the structure is further subjected to ultraviolet radiation such that the uncured regions return to their original state.
17. A method as claimed in any one of the preceding claims in which the method is carried out as a Continuous reel to reel process.
18. A method as Claimed in any one of the preceding claims in which the resin is applied as indicia or a design.
19. A method as claimed in any one of the preceding claims in which the gaps in the mask form indicia or a design.
20. A method as claimed in any one of the preceding claims further comprising the application of a dark absorbing layer on an opposite side of the liquid crystal layer to that to which the resin is applied.
21. A method as claimed in any one of claims 10 to 20 further comprising the application of a dark absorbing layer to an opposite side of the carrier substrate to that to which the liquid crystal layer is applied.
22. A method as claimed in claim 20 or claim 21 in which the absorbing layer is coloured.
23. A method as claimed in claim 22 in which a plurality of different coloured absorbing layers are applied.
24. A method as claimed in any one of claims 20 to 23 in which the dark absorbing layer(s) is(are) applied in the form of a design.
25. A method as claimed in any one of claims 20 to 24 further comprising the addition of a machine detectable component to the absorbing layer(s).
26. A method as claimed in any one of the preceding claims further comprising the addition of a machine detectable component to the liquid crystal layer.
27. A method as claimed in any one of claims 10 to 26 further comprising the formation of demetallised indicia on the carrier substrate prior to the application of the liquid crystal layer.
28. A method as claimed in claim 26 or claim 27 in which the machine detectable component is any one or more of a fluorescent, phosphorescent, infrared absorbing, thermochromjc, photochromic, magnetic, electrochromjc, conductive and/or piezochromjc material.
29. A method as claimed in any one of the preceding claims further comprising a layer of adhesive to the outer surfaces of the security device.
30. A security device made by the method of any one of the preceding claims.
31. A secure substrate comprising a base substrate and a security device as claimed in claim 30.
32. A secure substrate as claimed in claim 31 in which the security device is applied to the surface of the base substrate.
33. A secure substrate as claimed in claim 31 in which the security device is at least partially embedded within the base substrate and at least partially visible at at least one surface of the substrate.
34. A security document made from the secure substrate of any one of claims 31 to 33.
35. A security document made from the secure substrate of any one of claims 31 to 34 further comprising printing on the surface of the document.
36. A security document as claimed in claim 35 in which the designs formed on the security device are linked by content or registration to the printing on the document.
37. A security document as claimed in claim 34 comprising a banknote, cheque, passport, identity card, certificate of authenticity, fiscal stamp and other document for securing value or personal identity.
38. A method of forming a security device substantially as hereinbefore described with reference to and as shown in the accompanying drawings.40. A security device substantially as hereinbefore described with reference to and as shown in the accompanying drawings.Amendments to the claims have been filed as follows -33.CLAIMS: 1. A method of forming a security device comprising the steps of applying a substantially transparent polymeric resin to at least one region of one side of a cholesteric liquid crystal layer to form an intermediate structure; maintaining the intermediate structure at a temperature such that the resin penetrates the liquid crystal layer and increases the pitch of its helical structure in said at least one region thereby changing the angular dependant coloured reflection of the liquid crystal layer in the at least one region; and subsequently curing the structure.2. A method as claimed in claim 1 in which the resin comprises an ultraviolet curable resin.3. A method as claimed in claim 1 or claim 2 in which the resin comprises a solvent, a low molecular weight diluent or a monomer.* ,* 4. A method as claimed in claim 3 in which the solids * * * * ** content of the resin is approximately 50%.. 25 5. A method as claimed in any one of the preceding claims in which the resin employs free radical polymerisation. *0 *S * * * *6. A method as claimed in any one of the preceding claims in which the resin is applied to the liquid crystal layer by a printing process. I 3I7. A method as claimed in any one of the preceding claims in which the curing process comprises ultraviolet irradiation.8. A method as claimed in any one of the preceding claims in which the intermediate structure is heated to said temperature covered and during heating to prevent evaporation.9. A method as claimed in claim 8 in which the intermediate structure is covered by a layer of polymeric film.10. A method as claimed in any one of the preceding claims in which the intermediate structure is heated to a temperature lying in the range 40C to 100C.11. A method as claimed in claim 10 in which the intermediate structure is heated to a temperature lying in the range 60C to 90C.12. A method as claimed in claim 11 in which the * *I S. S * " intermediate structure is heated to a temperature of * * I approximately 80C.: *. 25 S...13. A method as claimed in any one of the preceding claims in which the liquid crystal layer is formed by applying a liquid crystal material to a carrier substrate.14. A method as claimed in any one of the preceding claims in which the resin is applied all over the liquid crystal layer. -3S15. A method as claimed in any one of the preceding claims further including the step of at least partially covering the surface of the resin with a mask having gaps therein and curing and subsequently heating the resin through the mask such that only regions exposed by said gaps are cured 16. A method as claimed in any one of claims 1 to 14 further including the step of at least partially covering the surface of the resin with a mask having gaps therein after the intermediate structure has been heated, curing the structure through the mask such that only regions exposed by said gaps are cured, and removing the mask before the structure is further subjected to ultraviolet radiation such that the uncured regions return to their original state.17. A method as claimed in any one of the preceding claims in which the method is carried out as a Continuous reel to reel process.18. A method as claimed in any one of the preceding claims in which the resin is applied as indicia or a design.19. A method as claimed in any one of the preceding claims in which the gaps in the mask form indicia or a design.20. A method as claimed in any one of the preceding claims further comprising the application of a dark absorbing layer on an opposite side of the liquid crystal layer to that to which the resin is applied.21. A method as claimed in any one of claims 10 to 20 further comprising the application of a dark absorbing layer to an opposite side of the carrier substrate to that to which the liquid crystal layer is applied.22. A method as claimed in claim 20 or claim 21 in which the absorbing layer is coloured.23. A method as claimed in claim 22 in which a plurality of different coloured absorbing layers are applied.24. A method as claimed in any one of claims 20 to 23 in which the dark absorbing layer(s) is(are) applied in the form of a design.25. A method as claimed in any one of claims 20 to 24 further comprising the addition of a machine detectable component to the absorbing layer(s).26. A method as claimed in any one of the preceding claims further comprising the addition of a machine detectable component to the liquid crystal layer.27. A method as claimed in any one of claims 10 to 26 further comprising the formation of demetallised indicia on the carrier substrate prior to the application of the liquid crystal layer.28. A method as claimed in claim 26 or claim 27 in which the machine detectable component is any one or more of a fluorescent, phosphorescent, infrared absorbing, thermochromjc, photochromic, magnetic, electrochromjc, conductive and/or piezochromjc material. 3'J.29. A method as claimed in any one of the preceding claims further comprising a layer of adhesive to the outer surfaces of the security device.30. A security device made by the method of any one of the preceding claims.31. A secure substrate comprising a base substrate and a security device as claimed in claim 30.32. A secure substrate as claimed in claim 31 in which the security device is applied to the surface of the base substrate.33. A secure substrate as claimed in claim 31 in which the security device is at least partially embedded within the base substrate and at least partially visible at at least one surface of the substrate.34. A security document made from the secure substrate of any one of claims 31 to 33.35. A security document made from the secure substrate of any one of claims 31 to 34 further comprising printing on the surface of the document.36. A security document as claimed in claim 35 in which the designs formed on the security device are linked by content or registration to the printing on the document.37. A security document as claimed in claim 34 comprising a banknote, cheque, passport, identity card, certificate of authenticity, fiscal stamp and other document for securing value or personal identity. -338. A method of forming a security device substantially as hereinbefore described with reference to and as shown in the accompanying drawings.S40. A security device substantially as hereinbefore described with reference to and as shown in the accompanying drawings.