APPLYING A PATTERN TO A SUPPORT LAYER

This invention relates to methods of manufacturing die forms for use in the formations of patterns, such as high resolution patterns utilised in security devices, and to methods of forming patterns using such die forms. The die forms and patterns formed therefrom are particularly well adapted for use in producing security devices on security documents or for later application to security documents. Typically, such security devices use patterns, especially micropatterns, to generate optical effects that are difficult to counterfeit, such as lenticular effects, moire magnification effects, and integral imaging effects.

Articles of value, and particularly security documents such as banknotes, cheques, passports, identification documents, certificates and licences, are frequently the target of counterfeiters and persons wishing to make fraudulent copies thereof and/or changes to any data contained therein. Typically such objects are provided with a number of visible security devices for checking the authenticity of the object. By "security device" we mean a feature which it is not possible to reproduce accurately by taking a visible light copy, e.g. through the use of standardly available photocopying or scanning equipment. Examples include features based on one or more patterns such as microtext, fine line patterns, latent images, Venetian blind devices, lenticular devices, moire interference devices and moire magnification devices, each of which generates a secure visual effect. Other known security devices include holograms, watermarks, embossings, perforations and the use of colour-shifting or luminescent / fluorescent inks. Common to all such devices is that the visual effect exhibited by the device is extremely difficult, or impossible, to copy using available reproduction techniques such as photocopying. Security devices exhibiting non-visible effects such as magnetic materials may also be employed.

One class of security devices are those which produce an optically variable effect, meaning that the appearance of the device is different at different angles of view. Such devices are particularly effective since direct copies (e.g. photocopies) will not produce the optically variable effect and hence can be readily distinguished from genuine devices. Optically variable effects can be generated based on various different mechanisms, including holograms and other diffractive devices, moire interference and other mechanisms relying on parallax such as Venetian blind devices, and also devices which make use of focusing elements such as lenses, including moire magnifier devices, integral imaging devices and so-called lenticular devices.

Moire magnifier devices (examples of which are described in EP-A-1695121 , WO-A-94/27254, WO-A-201 1/107782 and WO201 1/107783) make use of an array of focusing elements (such as lenses or mirrors) and a corresponding array of microimages, wherein the pitches of the focusing elements and the array of microimages and/or their relative locations are mismatched with the array of focusing elements such that a magnified version of the microimages is generated due to the moire effect. Each microimage is a complete, miniature version of the image which is ultimately observed, and the array of focusing elements acts to select and magnify a small portion of each underlying microimage, which portions are combined by the human eye such that the whole, magnified image is visualised. This mechanism is sometimes referred to as "synthetic magnification". The magnified array appears to move relative to the device upon tilting and can be configured to appear above or below the surface of the device itself. The degree of magnification depends, inter alia, on the degree of pitch mismatch and/or angular mismatch between the focusing element array and the microimage array.

Integral imaging devices are similar to moire magnifier devices in that an array of microimages is provided under a corresponding array of lenses, each microimage being a miniature version of the image to be displayed. However here there is no mismatch between the lenses and the microimages. Instead a visual effect is created by arranging for each microimage to be a view of the same object but from a different viewpoint. When the device is tilted, different ones of the images are magnified by the lenses such that the impression of a three-dimensional image is given. "Hybrid" devices also exist which combine features of moire magnification devices with those of integral imaging devices. In a "pure" moire magnification device, the microimages forming the array will generally be identical to one another. Likewise in a "pure" integral imaging device there will be no mismatch between the arrays, as described above. A "hybrid" moire magnification / integral imaging device utilises an array of microimages which differ slightly from one another, showing different views of an object, as in an integral imaging device. However, as in a moire magnification device there is a mismatch between the focusing element array and the microimage array, resulting in a synthetically magnified version of the microimage array, due to the moire effect, the magnified microimages having a three-dimensional appearance. Since the visual effect is a result of the moire effect, such hybrid devices are considered a subset of moire magnification devices for the purposes of the present disclosure. In general, therefore, the microimages provided in a moire magnification device should be substantially identical in the sense that they are either exactly the same as one another (pure moire magnifiers) or show the same object/scene but from different viewpoints (hybrid devices). Moire magnifiers, integral imaging devices and hybrid devices can all be configured to operate in just one dimension (e.g. utilising cylindrical lenses) or in two dimensions (e.g. comprising a 2D array of spherical or aspherical lenses).

Lenticular devices on the other hand do not rely upon magnification, synthetic or otherwise. An array of focusing elements, typically cylindrical lenses, overlies a corresponding array of image sections, or "slices", each of which depicts only a portion of an image which is to be displayed. Image slices from two or more different images are interleaved and, when viewed through the focusing elements, at each viewing angle, only selected image slices will be directed towards the viewer. In this way, different composite images can be viewed at different angles. However it should be appreciated that no magnification typically takes place and the resulting image which is observed will be of substantially the same size as that to which the underlying image slices are formed. Some examples of lenticular devices are described in US-A-4892336, WO-A- 201 1/051669, WO-A-201 1051670, WO-A-2012/027779 and US-B-6856462. More recently, two-dimensional lenticular devices have also been developed and examples of these are disclosed in British patent application numbers 1313362.4 and 1313363.2. Lenticular devices have the advantage that different images can be displayed at different viewing angles, giving rise to the possibility of animation and other striking visual effects which are not possible using the moire magnifier or integral imaging techniques. Security devices of the above types depend for their optical effect at least in part upon the high resolution with which the image array has been produced. For instance, in a lenticular device, each image element or "slice" making up the image array must be narrower than the pitch of the focussing element array, which may typically be no more than 100 microns, usually less. For example, if the diameter of the focusing elements is 30 microns then each image element may be around 15 microns wide or less. Alternatively for a smooth lenticular animation effect it is preferable to have as many different interleaved images as possible, typically at least five but ideally as many as thirty. In this case the size of the image elements might need to be in the range 0.1 to 6 microns. Similar considerations apply to other types of devices. For example, in moire magnifiers and integral imaging devices, each microimage must be of the same order of magnitude as one lens, or smaller. Thus, the microimage may desirably have overall dimensions of 50 microns or less. In order to provide the microimage with any detail, small line widths are required, e.g. of 15 microns or preferably less, ideally 5 microns or less.

Conventional printing techniques will generally not be adequate to achieve such high resolution. For instance, typical printing processes used to manufacture pattern elements (image arrays) for security devices include intaglio, gravure, wet lithographic printing and dry lithographic printing. The achievable resolution is limited by several factors, including the viscosity, wettability and chemistry of the ink, as well as the surface energy, unevenness and wicking ability of the substrate, all of which lead to ink spreading. With careful design and implementation, such techniques can be used to print pattern elements with a line width of between 25 pm and 50 pm. For example, with gravure or wet lithographic printing it is possible to achieve line widths down to about 15 pm. However, consistent results at this resolution are difficult to achieve and in any case this level of resolution still imposes a significant limitation on the security device. Thus while any of the above-mentioned techniques can be employed in embodiments of the present invention, higher resolution methods (i.e. suitable for achieving smaller line widths) for forming the image array would be highly desirable.

Some methods for generating patterns or micropatterns on a substrate are known from US 2009/0297805 A1 and WO 2011/102800 A1. These disclose methods of forming micropatterns in which a die form or matrix is provided whose surface comprises a plurality of recesses. The recesses are filled with a curable material, a treated substrate layer is made to cover the recesses of the matrix, the material is cured to fix it to the treated surface of the substrate layer, and the material is removed from the recesses by separating the substrate layer from the matrix. Another method of forming a micropattern is disclosed in WO 2014/070079 A1. Here it is taught that a matrix is provided whose surface comprises a plurality of recesses, the recesses are filled with a curable material, and a curable pickup layer is made to cover the recesses of the matrix. The curable pickup layer and the curable material are cured, fixing them together, and the pickup later is separated from the matrix, removing the material from the recesses. The pickup layer is, at some point during or after this process, transferred onto a substrate layer so that the pattern is provided on the substrate layer.

Whilst the above disclosed methods have been found to achieve high resolution, it would be desirable to improve the repeatability of the process.

A first aspect of the present invention provides a method of preparing a die form for use in applying a pattern to a pattern support layer, comprising the steps of: (i) providing a die form body, the die form body having an upper surface comprising an arrangement of elevations and depressions defining the pattern;

(ii) applying a soluble material to the upper surface of the die form body such that said soluble material is received in the depressions;

(iii) applying a film material to the upper surface of the die form body such that said film material covers the upper surface of the die form body, coating the elevations and the soluble material in the depressions; and

(iv) removing the soluble material by exposure to a solvent suitable for removing the soluble material;

wherein removal of the soluble material using the solvent causes the removal of the film material from the upper surface of the die form body in regions corresponding to the depressions, and not in regions corresponding to the elevations, the film material thereby forming a mask in accordance with the pattern.

The finished die form thus comprises both the die form body and the mask (formed of the film material) thereon. By making use of the depressions to control the location of a soluble material and using the soluble material to pattern an overlying film material, the mask can be accurately formed on the surface of the die form body corresponding exactly to the elevations and leaving the depressions substantially free of the masking material. One benefit of providing the die form with a mask in this way is that curable material(s) applied to the upper surface of the die form (during later use of the die form to produce a pattern) can be partially shielded from curing radiation by the mask such that the material(s) are only selectively cured (i.e. some portions will be cured and others left uncured). Some particular advantages of such selective curing will be discussed below with respect to the second aspect of the invention. However it should be appreciated that the die form made in accordance with the first aspect of the invention could be used in many diverse applications and is not limited to use in the methods described below. For example, the die form could be used as a lithographic mask for optical patterning of photosensitive material such as vesicular film or a radiation-responsive resist material. In preferred embodiments, the die form body is substantially transparent or translucent to at least a curing radiation waveband. This enables a material located on the upper surface of the die form (during later pattern formation processes) to be cured by exposure to radiation through the die form. The curing radiation waveband will depend on the material in question and could include any one or more wavelengths e.g. in the IR, visible and/or ultraviolet spectra, but in preferred examples corresponds to ultraviolet radiation. For instance, in preferred embodiments, the die form could comprise any polymer material which is transparent to ultraviolet in the wavelength range suitable for free radical or cationic UV polymerisation, which is typically within the wavelength range 200-400nm. Acrylic based polymer films are a suitable material for this application. Alternatively, the die form body could comprise glass or quartz.

In step (ii), the soluble material could be applied only into the recesses, e.g. if the soluble material is of a suitably low viscosity that it flows into those depressions leaving the elevations substantially free of the material. However in preferred examples, step (ii) comprises applying the soluble material to the upper surface of the die form body such that the soluble material coats the elevations (at least partially) and is received in the depressions, and then removing the soluble material from the elevations, preferably using a doctor blade, a wiping roller or a squeegee. Any material which can be dissolved by a suitable solvent (preferably water but alternatively an organic solvent) can be used, but in preferred cases the soluble material comprises a soluble ink. For instance, in a preferred embodiment the soluble ink is a heavily pigmented ink, which can be dissolved by application of a solvent (aqueous or otherwise), thereby impeding adhesion of the metal applied thereto to the surface of the die form. In preferred cases, the soluble material is configured such that, when coated with a thin deposited layer of film material (e.g. metal) the soluble material creates small holes or discontinuities in the metal film by virtue of the fact that the metal film (typically 15 to 30nm thick) is not thick enough to continuously over coat the pigment grains in the soluble material. When exposed to a suitable solvent (preferably water) the solvent enters through these holes, dissolving the pigment such that the overlying metal layer disbands. In order for this mechanism to operate effectively, the pigment grain dimensions in the soluble material are preferably greater than the thickness of the metal film and typically in the range 50-500nm. Examples of suitable soluble materials are disclosed in US-A-5142383, EP-A-1023499 and US-A-3935334.

Advantageously, the film material is substantially opaque to at least the curing radiation waveband, which as already mentioned is preferably ultraviolet radiation. The film material should also preferably be permeable to a solvent in which the soluble material dissolves. This may be as a result of film porosity, cracks therethrough or any other mechanism allowing the passage of fluid therethrough.

In particularly preferred embodiments, the film material is a metal or alloy film comprising at least one metal or alloy, preferably aluminium, copper, nickel or chrome. Such materials are beneficial since high radiation opacity can be achieved with a relatively small film thickness, thereby retaining the ability of the film to allow the passage of solvent therethrough.

In some cases, the film material may also comprise particles dispersed therethrough, such as pigment particles. This increases the permeability of the film.

The film material can be applied by any convenient technique but preferably in step (iii) the film material is applied by vacuum deposition, most preferably sputtering, resistive boat evaporation or electron beam evaporation, or chemical vapour deposition.

In step (iv) the die form can be exposed to solvent in a number of alternative ways but in preferred cases the step comprises spraying solvent onto the die form and/or immersing at least the upper surface of the die form in a volume of solvent. The nature of the solvent will depend on the type of soluble material selected in step (ii) but may comprise water or an organic solvent for instance. The configuration of the elevations and depressions will depend on the nature of the desired pattern to be produced from the die form. However as discussed above where the pattern is for use in a security device, high resolution is required and hence small pattern dimensions. Hence in preferred examples, each depression has a width in the range 0.5 to 5 microns. The depth of each depression may typically be between 1 to 10 microns, more preferably 1 to 5 microns.

Preferably the die form body is cylindrical, which lends itself particularly well to use in continuous, web-based manufacturing lines.

The first aspect of the invention further provides a die form manufactured as described above. A second aspect of the present invention provides a method of applying a pattern to a pattern support layer, comprising the steps of:

(a) providing a die form incorporating a mask, the die form having an upper surface and a lower surface, the upper surface comprising an arrangement of elevations and depressions defining the pattern, wherein the die form is substantially transparent or translucent to at least a first curing radiation in regions corresponding to the depressions, and not in regions corresponding to the elevations;

(b) applying a first curable material to the upper surface of the die form such that said first curable material coats the elevations and is received in the depressions;

(c) curing, at least partly, at least some of the first curable material in the regions of the depressions by exposure to the first curing radiation through the lower surface of the die form;

(d) removing from the upper surface of the die form substantially all of the uncured first curable material;

(e) bringing a pattern support layer in contact with the upper surface of the die form such that it covers at least some of the depressions; and (f) separating the pattern support layer from the surface of the die form such that the first curable material in the depressions is removed from said depressions and retained on the pattern support layer in accordance with the pattern.

By at least partly curing the first curable material in the depressions through the die form (step (c)), whilst those portions of the first curable material located on the elevations are masked from the curing radiation (and hence remain uncured), when step (d) is performed, the more resilient nature of the partly cured material reduces or prevents the inadvertent removal of material from the depressions. That is, the depressions remain substantially filled with the first curable material (partly cured) after step (d) with the free surface of the material being substantially flush with the level of the elevations on either side or, as discussed below, even protruding above that level. This improves the adhesion of the material in the depressions (which ultimately constitutes the pattern) to the pattern support later, since contact can be more readily achieved at each point across the full area of the material in the recess.

The (preferably cylindrical) die form advantageously comprises a die form body which is substantially transparent or translucent to at least the first curing radiation, and a film material arranged on the elevations which is substantially opaque to at least the first curing radiation, the film material defining the mask. In particularly preferred examples the die form may be one made in accordance with the first aspect of the invention. Thus the method may further comprise, before step (a), manufacturing the die form using the method disclosed above. However it is not essential to use the die form of the first aspect of the invention nor to perform these additional die manufacture steps in the second aspect of the invention. The die form utilised in the second aspect of the invention could be manufactured using alternative techniques, such as applying a masking material only to the elevations without the need for a soluble material, or by providing a die form body with a separable mask component carrying the same pattern, e.g. in the form of a sheet or sheath fitted to the outside of the die. In particularly preferred embodiments, the first curable material is a UV curable material and the first curing radiation is UV radiation. For example, UV curable polymers employing free radical or cationic UV polymerisation are suitable for use as the first curable material. Examples of free radical systems include photo-crosslinkable acrylate-methacrylate or aromatic vinyl oligomeric resins. Examples of cationic systems include cycloaliphatic epoxides. Hybrid polymer systems can also be employed combining both free radical and cationic UV polymerization. Electron beam curable materials could alternatively be used in which case the first curing radiation is an electron beam. In some preferred embodiments, the first curable material is visually transparent or at least semi- transparent, as may be particularly desirable in cases where that material is later to be used as an etch resist, discussed below. In other preferred embodiments, the first curable material is non-transparent across at least part of the visible spectrum such that it exhibits a visible colour under at least some viewing conditions. For instance, the curable material may comprise at least one substance which is not visible under illumination within the visible spectrum and emits in the visible spectrum under non-visible illumination, preferably UV or IR. In preferred examples, the curable material comprises any of: luminescent, phosphorescent, fluorescent, magnetic, thermochromic, photochromic, iridescent, metallic, optically variable or pearlescent pigments

Advantageously, in step (c) substantially all of the first curable material in each depression is at least partially cured, and preferably some of the first curable material above each depression is at least partially cured. This further reduces the risk of any of the material being inadvertently removed from the depressions during step (d). It is particularly preferable that, in step (d), upon removal of substantially all of the uncured material, portions of the first curable material remain above each depression, the portions extending above the level of the upper surface of the die form. Such protruding portions, which can be achieved by partially curing some of the curable material above (i.e. outside and directly over) the depressions during step (c), further improve the contact between the first curable material forming the pattern and the pattern support layer in step (e) and so increase the adhesion between them. Step (d) may preferably be performed using a doctor blade, a wiping roller or a squeegee, for example. It should be noted that in step (e) the pattern support layer may not be brought into direct contact with the upper surface of the die form and the first curable material, but rather there may exist one or more intermediate layers. In some preferred embodiments, the method further comprises, after step (d) and before step (e):

(d') covering the upper surface of the die form and the depressions filled with the first curable material with a second curable material;

wherein in step (e) the pattern support layer contacts the second curable material on the surface of the die form such that in step (f) the second curable material is additionally retained on the pattern support layer, the first curable material being retained on the second curable material in accordance with the pattern.

This technique has been found to further improve the bonding between the pattern elements (i.e. the first curable material in the recesses) and the pattern support layer. The second curable material is typically referred to as a tie coat.

In an alternative preferred embodiment, the method further comprises, after step (d) and before step (e):

(d") applying a second curable material to the pattern support layer;

wherein in step (e) the second curable material on the surface of the pattern support layer contacts the die form such that in step (f) the first curable material is retained on the second curable material in accordance with the pattern. Again the second curable material acts as a tie coat and improves retention of the pattern elements. In this case it is desirable that the pattern support layer carrying the second curable material is pressed against the surface of the die form with some pressure to ensure good bonding. Preferably the die form is additionally substantially transparent or translucent to a second curing radiation in regions corresponding to the depressions, and not in regions corresponding to the elevations, the method further comprising, before during or after steps (e) or (f), at least partially curing portions of the second curable material by exposure to the second curing radiation through the lower surface of the die form. In practice the first and second curing radiation may be the same, e.g. UV radiation. By exposing the second curable material (tie coat) to curing radiation through the die form, only those parts of the tie coat overlying the retained first curable material in the depressions will be (partially) cured, whilst those parts over the elevations will be masked and hence not cured. This allows for easy removal of the uncured portions such that the tie coat only remains in locations corresponding to the pattern elements. Preferably, therefore the method further comprises removing the uncured portions of the second curable material. This could be done while the first and second curable materials are still on the die form, but more preferably after step (f). This could be performed using a doctor blade or similar, or by dissolving the uncured material in a suitable solvent. Patterns formed in this way can be put to many different uses. In a particularly preferred example, the pattern support layer comprises a substrate, preferably a polymer substrate, having a metallic layer thereon (e.g. a metal or alloy layer, or a metallic ink layer) and in step (e) the metallic layer is brought into contact with the die form such that the first curable material in the depressions is retained on the metallic layer in step (f). The metallic layer is preferably substantially visually opaque. Depending on the nature of the first curable material, the metallic layer may itself provide visibility to the finished pattern, e.g. if the first curable material is transparent and used only as a resist to pattern the metallic layer as described below. Alternatively if the first curable material is coloured, the metallic layer can nonetheless improve the optical density and hence visibility of the pattern elements formed by the first curable material, which is further enhanced by the reflective nature of the metallic material. If alternatively the metallic layer is retained all over the substrate, this can also provide a strong visual contrast between the pattern elements and their surroundings. However, in particularly preferred implementations, the method, further comprises removing the portions of the metallic layer uncovered by the first curable material, preferably by etching. For instance, the transferred portions of first curable material (and the corresponding portions of the second curable material, if used) can be used as an etch resist. In this way the transferred pattern can be viewed from both sides of the pattern support layer (provided the substrate is transparent), albeit with potentially different appearances since the pattern will have the colour of the first curable material on one side, and that of the metallic layer on the other.

The second aspect of the invention further provides a patterned support layer manufactured in accordance with the above method. Also provided is a security device comprising such a patterned support layer. The patterned support layer can be utilised in many types of security device but is particularly well suited to use in those generating optically variable effects, e.g. by the combination of an image array formed by the pattern and a viewing component such as an array of lenses (or other focussing elements such as mirrors), or a masking grid.

Thus in particularly preferred embodiments, the security device further comprises a focussing element array, the patterned support layer being configured as an image array and being located in a plane spaced from that of the focussing elements by a distance substantially corresponding to a focal length of the focusing elements, such that the focusing elements exhibit a substantially focussed image of the image array.

For example, the security device may be a moire magnifier. Thus, preferably, the image array comprises a microimage array, and the pitches of the focusing element array and of the microimage array and their relative orientations are such that the focusing element array co-operates with the microimage array to generate a magnified version of the microimage array due to the moire effect. In another case the security device may be an integral imaging device. Hence, preferably, the image array comprises a microimage array, the microimages all depicting the same object from a different viewpoint, and the pitches and orientation of the focusing element array and of the microimage array are the same, such that the focusing element array co-operates with the microimage array to generate a magnified, optically-variable version of the object.

In a still further example, the security device may be a lenticular device. Hence, the image array preferably comprises a set of first image elements comprising portions of a first image, interleaved with a set of second image elements comprising portions of a second image, the focusing element array being configured such that each focusing element can direct light from a respective one of the first image elements or from a respective one of the second image elements therebetween in dependence on the viewing angle, whereby depending on the viewing angle the array of focusing elements directs light from either the set of first image elements or from the second image elements therebetween, such that as the device is tilted, the first image is displayed to the viewer at a first range of viewing angles and the second image is displayed to the viewer at a second, different range of viewing angles.

As in the first aspect of the invention, the configuration of the elevations and depressions in the die form will depend on the nature of the desired pattern to be produced. However as discussed above where the pattern is for use in a security device, high resolution is required and hence small pattern dimensions. Hence in preferred examples, each depression has a width in the range 0.5 to 5 microns. The depth of each depression may typically be between 1 to 10 microns, more preferably 1 to 5 microns.

The second aspect of the invention further provides a security article comprising a security device as described above, wherein the security articles is preferably a security thread, strip, patch, label or insert. Also provided is a security document comprising a security device or a security article each as described above, wherein the security document is preferably a banknote, passport, ID card, licence, cheque, visa, stamp or certificate. Examples of methods in accordance with the first and second aspects of the present invention will now be described with reference to the accompanying drawings, in which:

Figure 1 schematically shows an exemplary die form body for use in a first embodiment of a method of manufacturing a die form, in cross section;

Figures 2(a) to (d) show selected steps of the method of manufacturing a die form according to the first embodiment;

Figure 3 schematically shows exemplary apparatus suitable for implementing a first embodiment of a method of applying a pattern to a pattern support layer;

Figures 4(a) to (c) show selected steps of the method of applying a pattern to a pattern support layer according to the first embodiment;

Figures 5(a) to (c) show further steps of the method of applying a pattern to a pattern support layer according to the first embodiment;

Figures 6 and 7 schematically show two exemplary apparatus suitable for implementing further embodiments of a method of applying a pattern to a pattern support layer;

Figures 8(a) to (c) show selected steps of the method of applying a pattern to a pattern support layer according to a second embodiment;

Figure 9 shows a cross-section through an embodiment of a security article or security document comprising a security device manufactured in accordance with an embodiment of the invention; and

Figure 10 to 12 show three exemplary security documents comprising security devices in accordance with embodiments of the invention. Preferred techniques for the manufacture of a die form will first be described with reference to Figures 1 and 2. Subsequently, preferred methods of applying patterns to pattern support layers will be described with reference to Figures 3 to 8. It will be appreciated that whilst, in the examples given, the described methods of applying the patterns make use of the die form described with reference to Figures 1 and 2, this is not essential (but is preferred).

Figure 1 shows schematically a die form body 1 suitable for use in embodiments of the invention. Here, the die form body 1 is substantially cylindrical and the Figure shows a cross section along its long axis. This is preferred since the shape lends itself well to use in continuous, web-based processes as will be discussed with reference to Figures 3 to 8. However, alternative die form bodies could be substantially planar, or could take the form of a flexible belt for example. The die form body is substantially transparent or translucent to radiation at least in a waveband corresponding to that of curing radiation which will subsequently be used to cure patterning material as discussed in relation to Figures 3 onward. Many curable materials are curable under ultraviolet radiation and hence typically the die form body 1 will be selected to be transparent to at least ultraviolet radiation. For instance, the die form body 1 could comprise any polymer material which is transparent to ultraviolet in the wavelength range suitable for free radical or cationic UV polymerisation, this is typically within the wavelength range 200-400nm. Acrylic based polymer films are a suitable material for this application. Alternatively the die form body 1 may be formed from glass or quartz, in which case the body 1 will typically also be substantially transparent across the visible waveband.

The die form body 1 has an upper surface 1 a, here the outer curved surface of the cylinder, and an inner surface 1 b which here defines an interior cavity within the die form body 1. This is preferably used to house a radiation source such as a UV lamp (not shown in Figure 1 ), so that in use material applied to the upper surface 1 a can be exposed to radiation through the die form body.

The upper surface 1 a of the die form body is provided with an arrangement of elevations 2 and intervening depressions (recesses) 3, which together define a pattern P which is ultimately to be transferred to another surface, such as a pattern support layer. The material elements ultimately transferred on to that other surface corresponds to the shape of the depressions 3 and will be spaced from other such elements by areas corresponding to the elevations 2. The surface relief (depressions 3 and elevations 2) can be formed by any suitable method depending on the material from which the die form body 1 is made, e.g. engraving, etching etc.

Figures 2(a) to (d) show selected steps of a first embodiment of a method for manufacturing a die form which can be performed on a die form body 1 such as that shown in Figure 1. In each of Figures 2(a) to (d), the die form body 1 is illustrated as planar but it will be appreciated that in practice it may be curved, e.g. forming the surface of a cylinder, as discussed above. Figure 2(a) shows more clearly the arrangement of elevations 2 and depressions 3 forming surface 1 a. The shape and dimensions of the depressions 3 will depend on the nature of the pattern ultimately to be formed, but in typical security element applications will have widths w (e.g. line widths) of the order of 0.5 to 5 microns. The depth d of each depression may typically be between 1 to 10 microns, more preferably 1 to 5 microns.

In the next step, illustrated in Figure 2(b), the depressions 3 are substantially filled with a soluble material 5. Substantially no such material is applied to the elevations 2. The soluble material 5 can be applied to the depressions by first coating it all over the surface 1 a and then removing it from the elevations 2, e.g. using a doctor blade, wiping roller or squeegee, for instance. Alternatively if the dimensions of the depressions 3 permit and the viscosity of the soluble material is sufficiently low, it may flow into the depressions 3 upon application without the need for a removal step. Once the soluble material 5 is dry (which may or may not require an active drying step, e.g. heating), a film material 7 is applied to the surface 1 a of the die form body 1 , over both the elevations 2 and the depressions 3, as shown in Figure 2(c). The film material 7 is preferably substantially opaque to the curing radiation to be used in later processes, e.g. UV radiation as mentioned above.

Preferably, the film 7 is a metal or alloy layer, such as aluminium, copper, nickel or chrome. Such materials are preferred since a relatively thin layer of the material typically achieves high opacity. The film 7 can be applied by any convenient technique but is preferably provided by vacuum deposition, e.g. sputtering, resistive boat evaporation or electron beam evaporation, or chemical vapour deposition.

The film material 7 is configured such that it remains permeable to a solvent (typically a fluid) in which the soluble material 5 will dissolve. Soluble materials 5 suitable for use in the presently disclosed methods include soluble inks such as a heavily pigmented ink and corresponding solvents include water or organic solvents. Examples of suitable soluble materials are disclosed in US-A- 5142383, EP-A-1023499 and US-A-3935334. Typically a metal or alloy film 7 of thickness 20 to 100 microns (preferably 20 to 30 microns) will retain sufficient permeability either due to cracks through the film and/or to boundaries between grains in the microstructure. However in some cases it may be appropriate to enhance the permeability of the film 7 by adding a dispersion of particles such as a pigment to the film material.

The die form body 1 is then exposed to a solvent which passes through the film 7 and dissolves the soluble material 5 in the recesses 3. This can be achieved for example by spraying the surface of the die form body with the solvent (e.g. using water jets), or by immersing all or part of the die form body in a volume of the solvent (e.g. by rolling it through a shallow bath). The result, shown in Figure 2(d) is the removal not only of the soluble material 5 but also of the film material 7 in the regions of the depressions 3, leaving the film material only on the elevations 2. The film material 7 thus forms a mask defining the pattern P exactly in conformance with the arrangement of elevations 2 and depressions 3.

The so-formed die form (comprising the die form body 1 and the mask layer 7) can be used in various applications including methods of applying a pattern to a support layer, examples of which will now be described with reference to Figures 3 to 8. Figure 3 schematically illustrates a first example of apparatus suitable for applying a pattern P to a support layer 10 in accordance with a first embodiment of such a method. Continuous, web-based production methods for making such are strongly preferred in order to manufacture large volumes of consistent patterns at high speed and with high accuracy. The preferred method therefore makes use of a cylindrical die form which in this example has been formed using the method disclosed above and therefore comprises die form body 1 and mask layer 7. However, in the methods of Figure 3 to 8, other types of die form could be used instead provided they too are configured such that the elevations 2 are substantially opaque to a curing radiation, and the depressions 3 are substantially transparent or at least translucent to the same curing radiation. As before, this curing radiation is typically ultraviolet wavelengths. All of the items labelled in Figure 3 with reference numbers already discussed above correspond to those features of the die form of Figures 1 and 2 and will not be described again here.

Figures 4(a) to (c) show selected steps of a first embodiment of a method of applying a pattern P to a support layer 10 which may be implemented using the Figure 3 apparatus. As shown in Figure 4(a), a first curable material 20 is first applied to the upper surface 1 a of the die form, coating the elevations 2 and filling the depressions 3. This can be achieved using an inking roller assembly 12 to apply the first curable material 20, which may comprise a slotted die 13b configured to supply the first curable material 20 to the surface 1a of the die form via a meter roller 13a, for example. The first curable material will ultimately form the transferred pattern P on the support layer. In some cases the finished pattern of material 20 is visible (optionally after magnification) to the human eye and so advantageously the curable material 20 comprises at least one colourant which is visible under illumination within the visible spectrum. For instance, the material may carry a coloured tint or may be opaque. The colour will be provided by one or more pigments or dyes as is known in the art. Additionally or alternatively, the curable material may comprise at least one substance which is not visible under illumination within the visible spectrum and emits in the visible spectrum under non-visible illumination, preferably UV or IR. In preferred examples, the curable material comprises any of: luminescent, phosphorescent, fluorescent, magnetic, thermochromic, photochromic, iridescent, metallic, optically variable or pearlescent pigments. In other preferred embodiments, the curable material 20 is visually transparent and can be used as an etch resist to pattern a visible layer such as a metallic layer (discussed below) which then displays the visible pattern P.

UV curable polymers employing free radical or cationic UV polymerisation are suitable for use as the curable material 20. Examples of free radical systems include photo-crosslinkable acrylate-methacrylate or aromatic vinyl oligomeric resins. Examples of cationic systems include cycloaliphatic epoxides. Hybrid polymer systems can also be employed combining both free radical and cationic UV polymerization. Electron beam curable materials would also be appropriate for use in the presently disclosed methods. Electron beam formulations are similar to UV free radical systems but do not require the presence of free radicals to initiate the curing process. Instead the curing process is initiated by high energy electrons.

The first curable material 20 is then exposed to curing radiation R of the appropriate type (e.g. UV) through the mask 7, from the lower surface 1 b of the die form (also shown in Figure 4(a)). This can be achieved for example using a suitable radiation source such as a UV lamp placed in an internal cavity within the die form body 1. Since the mask 7 is substantially opaque to the curing radiation, the portions of the first curable material 20 on the elevations 2 are shielded from the radiation R and are thus not cured. Meanwhile, the first curable material 20 in the depressions 3 is exposed to the radiation R and thus is at least partly cured, depending on the power of the radiation source and the duration of exposure. Preferably, full curing does not take place so that the partly cured material remains tacky to aid later transfer of the material to the support layer 10. The result is that portions 20a of the first curable material in the regions of the depressions 3 become at least partially cured whilst portions 20b of the first curable material elsewhere remain uncured. It will be noted that in Figure 4(a) the boundary between portions 20a and 20b is denoted by a dashed line which extends above each depression 3 beyond the level of the die form (i.e. the top of film 7). This is not essential but is strongly preferred. In some preferred embodiments substantially all of the curable material 20 in any one depression 3 will be at least partially cured, in which case the boundary between portions 20a and 20b would lie substantially flush with the top level of the die form. This improves the retention of the material in the depressions 3 during subsequent steps and ultimately improves the adhesion of the material to the support layer 10 as will be discussed below. However as described further below, both of these aspects can be improved still further if the boundary of the at least partially cured material 20a protrudes above the top level of the die form at each depression (as is especially preferred), which can be achieved by controlling the dose of curing radiation applied. In the next step, the uncured portions 20b of the first curable material 20 are removed from the upper surface of the die form, as shown in Figure 4(b). This can be achieved using a doctor blade 14 as shown but alternatively a wiping roller or squeegee could be used instead. Since the at least partially cured material 20a in each of the depressions is relatively viscous compared with the uncured material 20b, the doctor blade (or other apparatus) removes the uncured material preferentially and the at least partially cured material 20a is retained in each depression 3. As such, and as shown in Figure 4(c), each depression 3 remains substantially filled with the material 20, which may indeed protrude above each depression as mentioned above (and indicated by the dashed lines in Figures 4(b) and (c)).

The pattern elements formed by the at least partially cured material 20 retained in the depressions 3 can then be transferred from the die form to a pattern support layer 10. As shown in Figure 3 this can be achieved by bringing a suitable support layer 10, such as a polymer substrate (e.g. BOPP), into contact with the die form, preferably in a wrap configuration as shown such that the support layer 10 remains in contact with the die form over a finite distance between two support rollers 15a, 15b (rather than a single point contact configuration as would occur at a nip between two rollers). Further curing of the material 20a may optionally take place while the support layer 10 is held in contact with the die form and/or after the support layer 10 has been separated from the die form, taking the pattern elements of cured material 20a with it.

Any type of support layer 10 could be used, including polymeric substrates (preferably transparent) as may form the basis of a polymer document, such as a polymer banknote, e.g. 70 micron thick BOPP, as well as thinner polymeric substrate, e.g. 30 micron thick, which would be suitable for use in security articles such as security threads, strips, patches etc. In still further examples the support layer could be non-transparent, e.g. opaque, and could be fibrous, e.g. a paper substrate.

In a particularly preferred example, as will now be described with reference to Figures 5(a) to (c), the support layer 10 is a metallised substrate. Hence the support layer 10 comprises a polymeric substrate 10a, such as BOPP, which carries on one side a metallic layer 10b which is preferably a metal layer (e.g. vapour deposited metal or allow, such as aluminium, copper, nickel or chrome) but could alternatively comprise a metallic ink. Figure 5(a) shows the surface of the die form (now shown upside-down compared to Figure 4) coming into contact with the metallised support layer 10. As already described, each depression 3 is substantially wholly filled with the at least partially cured material 20a, with the result that good contact is made between all points across the area of each pattern element 20a and the metallic layer 10b on the substrate. Upon separation of the support layer 10 from the die form, the material 20a therefore adheres well to the metallic layer 10b and thus the pattern P is retained on the support layer 10 as shown in Figure 5(b). If the material 20a in fact protrudes out of each depression 3, as illustrated by the dashed lines, contact between material 20a and the metallic layer is improved still further and the degree of retention is further enhanced. This implementation is especially preferred.

The material 20a may be fully cured either during the transfer step shown in Figure 5(a) or after separation as shown in Figure 5(b). The so-formed pattered substrate shown in Figure 5(b) can be incorporated into a security device without any further processing of the pattern if desired. However in a preferred additional step, the metallic layer 10b on the substrate is itself patterned by exposing the metallic layer 10b to a suitable etchant (e.g. sodium hydroxide if the layer is aluminium). The retained pattern elements formed of material 20a act as an etch resist and protect underlying areas of the metallic layer 10b such that after etching it remains only in the positions corresponding to the pattern elements. Thus, if the substrate 10a is visually transparent, the pattern P can be viewed from either side. If the material 20a and the metallic layer 10b are different in appearance (e.g. different colours), the appearance of the pattern will differ correspondingly when viewed from each side. For example an observer d on the side to which the elements are applied will see the pattern in the colour of material 20a, whereas an observer 0 ₂ on the opposite side will see the pattern in the colour of metallic layer 10b. This can be used to further enhance the complexity of the security device. If alternatively the material 20a is visually transparent, the pattern P will have the appearance of the metallic layer 10b from both sides. It should be noted that whilst the provision of a metallic layer 10b is preferred, this is not essential. Aside from the potential for a dual-sided pattern of different appearance, as described above, the metallic layer 10b also has the benefit of increasing the optical density of the pattern (whether or not the etching step is performed).

Figures 6 and 7 show exemplary apparatus suitable for use in variations of the above-described method. In the above example, the support layer 10 is brought directly into contact with the die form. However this is not essential and in the examples of Figures 6 and 7, an intermediate layer in the form of a tie coat 30 is introduced between the support layer 10 and the die form. The provision of such a tie coat can improve further the retention of the pattern elements 20a on to the support layer 10. Particularly where the partially cured material 20a in each depression protrudes above the level of the die form surface (as indicated by the dashed lines in previous Figures), the need for such a tie coat 30 is substantially reduced (hence the strong preference for this configuration), but nonetheless the tie coat 30 may be provided if desired. Features shown in Figures 6 and 7 using like reference numerals as in Figures 1 and 3 are the same as previously described. The first steps of each method are the same as described above with reference to Figures 4(a) to (c). In the Figure

6 embodiment, after the uncured material 20b has been removed, a second curable material 30 (ultimately forming the tie coat) is applied to the surface of the die form over the retained material 20a in the depressions 3 and over the elevations 2. This can be achieved using a second inking station 31 , e.g. comprising a slot die 32b and a meter roller 32a as shown. The second curable material is curable by a second curing radiation which may be the same as the first curing radiation (e.g. UV) or could be different provided the die form body 1 is also transparent/translucent to the second curing radiation, and the film mask

7 is also substantially opaque to the second curing radiation. Thus, any of the same curable materials mentioned above for use as the first curable material 20 could also be used as the second curable material 30. The second curable material 30 could include a visible colourant (or fluorescence etc) or could be visually transparent. The second curable material 30 could optionally be partially (but not fully) cured by exposure to radiation R through the die form at this stage which, due to the presence of mask 7 will selectively cure the material 30 as described further below. As in the previous embodiment, a support layer 10 is then brought into contact with the die form with the second curable material therebetween. The subsequent steps will be described with reference to Figure 8 below.

In the alternative embodiment shown in Figure 7, the second curable material is applied to the support layer 10 rather than to the die form, using for example a coating station 31 in the form of a slot die 10c. The nature of the second curable material 30 is the same as in the Figure 6 embodiment. In this case, an additional pressure roller 16b may be provided to apply an extra nip pressure between the support layer 10 and the die form during transfer of the pattern to aid bonding between the tie coat 30 and the pattern elements 20a.

Figure 8(a) shows the die form being brought into contact with the support layer 10. In the example shown the tie coat 30 is depicted as being carried initially by the die form as will be the case using the apparatus of Figure 6. However, if the arrangement of Figure 7 were used instead, the only difference is that the tie coat 30 would appear in Figure 8 on the surface of support layer 10 rather than on the die form. In this example, the support layer 10 again comprises a polymer substrate 10a with a metallic layer 10b there on, as is preferred for all the same reasons discussed previously, but again this is not essential.

The curable materials 20a and 30 are then exposed to curing radiation R through the die form. This further cures the pattern elements 20a (optionally) and also at least partially cures the second curable material (which may or may not have been partially cured on the die form before contact with the support layer 10 as mentioned above). However, the second curable material 30 is shielded from the radiation R by the mask layer 7 at each elevation 2 and thus only becomes partially cured in the region of each depression 3, i.e. aligned with the pattern elements. The portions 30a of the second curable material 30 which become at partially cured are shown in Figure 8(b), with those uncured portions being denoted as 30b.

Since the tie coat has only been selectively cured in those locations corresponding to the pattern elements 20a, the uncured portions of the second curable material can then be removed, e.g. by washing with a suitable solvent (not shown). The result is a structure similar to that shown in Figure 5(b) but with the addition of a tie coat under each pattern element 20a aiding its retention on the metallic layer 10b.

In an optional further step, shown in Figure 8(c), the metallic layer 10b can once again be patterned using the islands of first and second curable material 20a and 30a as an etch resist, resulting in the structure shown in Figure 8(b). Again this can be used as a dual sided pattern in the same way as described with reference to Figure 5(c). The first and second curable materials 20, 30 can each have a visible colour or could be transparent and used primarily as an etch resist.

Patterns formed using the above described techniques can be formed with line widths as small as 0.5 to 5 microns and are thus well adapted for use in security devices such as moire magnification devices, integral imaging devices and lenticular devices. As an example, Figure 9 schematically illustrates a cross section through a security device 50, which here is a moire magnifier. The article comprises a substrate 10 which corresponds to a portion of the support layer 10 web previously described. Pattern P (formed as described above) comprises an array of identical microimages formed with a certain pitch and orientation. On the opposite surface of the substrate 10, an array of focusing elements such as lenses 40 is provided. The pitch and orientation of the lens array is such that moire interference causes an observer Oi to perceive a magnified version of the microimages. Optionally, a protective layer 45 may be applied over the pattern P to ensure robustness. Many other types of security element can also be formed using the above described technique to form pattern elements thereof. For instance any of the security elements described in WO2013/0541 17 could be manufactured in this manner. Security devices such as these incorporating the so-produced pattern could be formed on security articles such as security threads, labels, patches and the like, for later incorporation into or application onto an object of value such as a security document. Alternatively the security device could be formed as an integral part of a security document, such as a banknote with the substrate 10 forming the basis of the document (e.g. as in the case of polymer banknotes or paper/polymer hybrid banknotes). Some examples of ways in which security devices can be incorporated into security documents are shown in Figures 10, 1 1 and 12: Figure 10 shows an embodiment of an object of value, here a document of value 100 such as a banknote, into which a security article 90 comprising inter alia a length of the patterned substrate web is incorporated. The substrate web (support layer 10) may be cut into individual security threads 90 before insertion into the security document 100 but in preferred embodiments, a reel of the patterned substrate web 10 may be fed into a paper-making process, for example, to form a web of documents which is then cut into individual documents of the appropriate size. Here, the thread 90 is incorporated as a windowed thread in between first and second plies 101 and 102 of the security document 100, at least one of the plies 101 having a series of windows 91 formed therein either during the paper-making process or subsequently (e.g. by grinding). The windows 91 thereby reveal portions of the security article 90 such that the pattern P carried by the support layer is observable through the windows 91 , optionally through a lens array or similar. Between the windows 91 , sections 92 of the thread 90 are concealed by the overlying document ply 101. Alternatively, the windowing thread could be incorporated into single ply paper and produced using the method described in EP0059056. Figure 1 1 shows an alternative embodiment of a document of value 100, in which the substrate web (support layer 10) is formed into strip articles 95 which are mounted to one side of a document substrate 101 in alignment with a window 96 which may be formed before or after application of the strip 95. The pattern P is observable through the window 96 and, depending on the construction of the substrate web from which strip 95 was constructed, it may be visible from the other side of the document 100 also. The strip 95 can be affixed to document ply 101 using an adhesive for example. As in the case of security thread 90, cutting of the substrate web into individual strips 95 may take place before or after incorporation with the document substrate 101.

Figure 12 shows a further embodiment in which the substrate web (support layer 10) has been formed into label articles 97 and affixed to a surface of a document 100. Here, the document substrate 101 may be opaque (e.g. paper), transparent or translucent (e.g. polymer substrate), or some combination thereof. For instance, the document substrate 101 could be transparent in the vicinity of the label 97 and substantially opaque elsewhere. As discussed below, label elements and/or transfer foils such as item 97 can be applied to a document in a number of ways and may not constitute the full layer structure of the substrate web once applied to the document of value 100.

A configuration such as that shown in Figure 12 can alternatively be arrived at by incorporating the security device as an integral part of security document 100. In this case the document substrate 101 is preferably a transparent polymer film and constitutes the support layer 10 in the above-described methods. The pattern P is applied directly to the substrate web using the method previously described. Other features such as a lens array may be applied to the substrate over or on the opposite side of the pattern P to form the desired optical effect. Opacifying layers may be printed across the remainder of the document surface leaving the pattern appearing in a window region.

The security devices of the current invention can optionally be made machine readable by the introduction of detectable materials in any of the layers or by the introduction of separate machine-readable layers. Detectable materials that react to an external stimulus include but are not limited to fluorescent, phosphorescent, infrared absorbing, thermochromic, photochromic, magnetic, electrochromic, conductive and piezochromic materials.