GB2616465A

GB2616465A - Security document substrate and method of manufacture thereof

Info

Publication number: GB2616465A
Application number: GB2203349.2A
Authority: GB
Inventors: Commander Lawrence
Original assignee: De la Rue International Ltd
Current assignee: De la Rue International Ltd
Priority date: 2022-03-10
Filing date: 2022-03-10
Publication date: 2023-09-13
Also published as: WO2023170376A1; GB202203349D0

Abstract

A method comprises providing a fibrous substrate 201 having a sizing substance incorporated therein and/or on a surface thereof and (i) providing a casting tool 221, the casting tool having a casting structure 225 defined in a surface thereof, the casting structure comprising a first surface region having a surface nanostructure with a surface roughness of less than 1 micrometre. The method then comprises (ii) applying one or more curable materials 205 to the fibrous substrate and/or the casting structure of the casting tool; and (iii) bringing the fibrous substrate and the casting tool into contact with the one or more curable materials therebetween, thereby forming the one or more curable materials into the casting structure. During and/or after the contact, the one or more curable material(s) are cured 222 so as to form, on the fibrous substrate, a layer of one or more cured material(s) comprising a first zone having an outer surface exhibiting a surface nanostructure corresponding to the first region of the casting structure.

Description

SECURITY DOCUMENT SUBSTRATE AND METHOD OF MANUFACTURE

THEREOF

FIELD OF THE INVENTION

The invention relates to security document substrates and methods for the manufacture of such security document substrates. The security document substrates may be used to form security documents such as banknotes, passports, certificates, licences and the like.

BACKGROUND

Objects 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. 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 relief structures such as zones plates, and also devices which make use of focusing elements such as lenses, including moire magnifier devices and so-called lenticular devices.

The structure of security features and devices that generate optically variable effects are typically designed on the assumption that they will be applied to flat surfaces. Imperfections in the real surfaces to which the devices are applied can degrade the quality of the visual effects that are produced, for example by causing misalignment of optical, reflective or diffractive elements. Diffractive features such as holographic foils are particularly susceptible to this since they are typically formed as thin layers that easily conform to even small variations in the profile on which they are placed (e.g. those on the order of a few micrometres). These difficulties are particularly strongly felt when placing security features on substrates such as paper and/or textile-based materials, which typically have surfaces that exhibit significant surface roughness. The arithmetic average surface roughness Ra of such surfaces is typically at least several micrometres.

There is hence a need to reduce the detrimental effects that the textures of fibrous substrates can have on the quality of the effects of security devices arranged thereon.

SUMMARY OF INVENTION

The invention provides a method of manufacturing a security document substrate, the method comprising: providing a fibrous substrate having a sizing substance incorporated therein and/or on a surface thereof; and (i) providing a casting tool, the casting tool having a casting structure defined in a surface thereof, the casting structure comprising a first surface region having a surface nanostructure with a surface roughness of less than 1 micrometre (pm); (ii) applying one or more curable materials to the fibrous substrate and/or the casting structure of the casting tool; Op bringing the fibrous substrate and the casting tool into contact with the one or more curable materials therebetween, thereby forming the one or more curable materials into the casting structure; and (iv) during and/or after the contact, curing the one or more curable material(s) so as to form, on the fibrous substrate, a layer of one or more cured material(s) comprising a first zone having an outer surface exhibiting a surface nanostructure corresponding to the first region of the casting structure.

The surfaces of casting structures -for example those that are mounted on casting cylinders for performing so-called cast-cure processes -can be manufactured with very high precision. Whereas casting tools have conventionally been designed for the manufacture of shaped or patterned features such as optical elements, image elements and tactile features, the inventors have realised that a casting structure can be configured to produce a highly smooth surface (i.e. a surface nanostructure as defined above) which can be replicated in a layer of curable material(s) in order to prepare a smooth surface suitable for subsequent application of security feature(s).

The surface of the casting tool in the first surface region is not entirely featureless and will have a "nanostructure" defined by the variations in the surface profile across this region, as a result of which the surface in the first surface region will have a non-zero surface roughness. When the one or more curable materials are brought into contact with the fibrous substrate and the casting structure and then (at least partially) cured, the surface nanostructure will be replicated in the curable materials. The surface nanostructure of the first surface region of the casting tool will therefore be present in the outer surface of the layer of curable materials that is produced by this method. What results is a layer of cured material that has, in the first zone, a very (but not perfectly) smooth surface that is suitable for subsequent application of security features(s). The provision of a smooth surface in this way is particularly beneficial where the feature to be applied is or carries a diffractive relief structure such as a holographic surface relief or zone plate, since roughness in the surface on which these devices are placed typically disrupts the diffraction patterns that such devices are intended to produce.

In light of the above, it will be apparent that by the term "surface nanostructure", we mean that the region of the surface to which this term is applied On this case the first surface region of the surface of the casting tool) has, throughout the region, a surface roughness that is non-zero but less than 1 pm. Throughout this specification, the term "surface roughness" refers to the arithmetic average roughness, Ra, which is a well-known parameter for characterising the roughness of surfaces and can be measured using standard techniques and apparatus known in the art such as laser-based roughness meter. The surface nanostructure may have a repeating structure or may be a random structure.

The method can be characterised as a "cast-cure" process, since the one or more curable materials are formed by contact with a casting structure and cured such that the resulting cured material exhibits the surface nanostructure that is carried by the casting structure. As noted above, this method results in the structure of the parts of the casting tool that contact the curable materials being replicated in the layer of cured material(s) that is produced. The method thus differs from known processes for producing smooth surfaces such as calendering, which does not replicate the structure of the tool surface in the manufactured substrate, and instead relies on a mechanical process involving relative motion between the substrate and the calendering cylinder.

As noted above, casting structures can be manufactured to high levels of precision, which allows surfaces that are very smooth in comparison to typical fibrous substrates to be produced. Surface roughness values of well below 1 pm can be produced. Hence, in preferred embodiments, the surface roughness of the surface nanostructure of the first surface region is in the range of 1-100 nanometres (nm), preferably in the range of 10-100 nm, more preferably 10-50 nm. The surface nanostructure will be replicated in the curable material(s) as they come into contact with the casting structure. Thus, the surface nanostructure of the first zone of the of the later of one or more cured material(s) has a surface roughness of less than 1 pm, preferably in the range of 1-100 nanometres (nm), more preferably 10-100 nm, and even more preferably 10-50 nm.

The first surface region of the casting structure surface may define (e.g. consist of) a single, contiguous area, or could comprise multiple separate areas (across each of which the surface roughness is less than 1 pm) that are not contiguous with one another. In the latter case, the separate areas together constitute the first surface region. Because the first zone of the cured material layer in the manufactured substrate will often be intended to receive a security feature that is visible to the naked eye, it is preferred that the first surface region includes at least one laterally contiguous area with sufficient lateral dimensions to be easily visible to the naked eye. Therefore, in preferred implementations, the first surface region has dimensions such that the first zone of the layer of one or more cured material(s) defines a laterally continuous region having a minimum lateral dimension of at least 200 pm, preferably at least 500 pm, even more preferably at least 1mm. By "minimum lateral dimension", we mean the smallest dimension of the laterally contiguous region in any direction. For example, in the case of an ellipse, the "minimum lateral dimension" is the width of the ellipse along the minor axis. In the case of an oblong, it is the length of the smaller side of the oblong. The first zone could for example define an patch having an area in the range of 1 to 16 centimetres squared (cm2) or a stripe having a width in the range of 5 to 40 millimetres (mm).

In some preferred embodiments, the first surface region comprises areas of different relative height, each area having lateral dimensions of at least 1 pm, preferably at least 200 pm, and preferably wherein the areas of different relative height define indicia such as a graphic, an alphanumerical character or a code. Since each of the areas of different relative height are formed by parts of the first surface region, which carries the surface nanostructure defined above, the surface nanostructure will be present in each of the areas. By areas of "different relative height" it is meant that the areas are of different heights relative to one another, such that these parts of the casting structure form corresponding different areas of different height in the cured material(s) that the casting structure is used to form in step (Hi). The resulting areas of different height in the first zone of the cured material layer (hereinafter referred to as "first zone areas") may later be used to selectively apply a security feature such as a diffractive foil to the cured material layer: for example, a security feature could be applied to one or more first zone areas that are raised relative to the surrounding parts of the layer of cured material(s) and not elsewhere. These surrounding parts of the cured material layer could be other parts of the first zone and/or parts of the cured material layer that are not part of the first zone, if the first zone does not extend across the whole cured material layer In preferred embodiments the first surface region of the casting structure is configured such that the outer surface of the layer of cured materials is substantially flat at each location in the first zone. The casting structure itself will not necessarily be flat -for example, in the case of a casting structure formed in the surface of a cylinder, the casting structure will be curved but may be configured to form flat features in the curable material(s) that it is used to form. The "substantially flat" surface of the cured material layer will exhibit the surface nanostructure defined above and is therefore not entirely featureless, but on scales greater than that of the surface nanostructure (e.g. greater than 1 pm) will not exhibit any curvature or elevations/depressions. In these embodiments, the first zone may still comprise first zone areas of different heights as defined above, provided that within each first zone area the outer surface is substantially flat in the sense just described.

In some embodiments, the casting structure comprises solely the first surface region. In some embodiments, the casting structure comprises a second surface region having a surface structure which defines one or more casting surface features with dimensions of at least 1 pm. By "dimensions of at least 1 pm" we mean that the features have a height or depth (i.e. the dimension in the direction perpendicular to the immediately surrounding parts of the casting structure surface) and/or one or more lateral dimensions greater than 1 pm. Like the first surface region, the second surface region could comprise several areas that are not contiguous with one another, or a single laterally contiguous area. The first and second surface regions may be laterally separate (e.g. spaced apart or abutting). The first surface region and second surface region could be interspersed (e.g. "interlaced") with one another, such that some parts of one region are laterally separated from one another by the other region or parts thereof. In preferred embodiments, the casting surface features are arranged such that the one or more of the casting surface features form, in the one or more curable materials between the fibrous substrate and the casting tool in step (iii), an optical device, wherein preferably the optical device comprises an array of focusing features. The optical device could be a Fresnel-type lens, a caustic structure or an array of micro-lenses of such as are typically incorporated in moire or lenticular devices, for example. In other examples the optical device could comprise an array of refractive structures such as micro-prisms.

In further envisaged embodiments, the casting surface features may define raised elements that provide a predetermined tactile effect to a user as they run their finger over the raised features. In yet further embodiments, the raised elements may define image elements of an image or indicia.

As noted above, the first zone of the cured material layer in the security document substrate that is produced by the method is particularly suitable for subsequent application of a security feature. Thus, advantageously, the method may further comprise applying a security feature to (e.g. at least) the first zone of the layer of one or more cured materials.

Preferably, the security feature is or comprises a security device. In some embodiments, the security feature is a security device, preferably a diffractive or micro-optical device. In such embodiments, the security device is formed directly on the layer of one or more cured materials.

In preferred embodiments, the security feature is a security article carrying a security device. Typically, the security article is in the form of a security stripe, a security patch or a foil carrying a security device. Such a security article typically comprises a polymeric substrate (e.g. biaxially oriented polypropylene, BOPP, or polyethylene terephthalate, PET) having a surface roughness much less than that of the fibrous substrate (e.g. less than 100nm). A preferred security article is a holographic foil.

Typically, the security device is a diffractive relief structure such as a holographic relief structure or a zone plate. However, the invention is applicable to a number of different security devices, including metallic security devices, magnetic security devices, an optically variable effect generating surface relief structure (such as a diffractive device, a caustic device, one or more focusing elements, one or more reflective elements, or one or more refractive elements), a colour-shifting device, one or more sub-wavelength elements (such as plasmonic elements), one or more dispersive elements, one or more retro-reflective elements, and one or more coloured elements. Preferably, the security device is an optically variable security device, meaning that it exhibits different effects dependent on viewing angle.

The security feature can be applied to the layer of one or more cured material(s) using any suitable technique known in the art (e.g. embossing, printing, cast-curing, adhesive, hot/cold stamping).

Preferably, the security feature is applied to the first zone using an adhesive having an adhesive surface structure that is presented to the layer of one or more cured materials, and wherein the surface nanostructure of the first zone is configured to complement the adhesive surface structure. In this way, the casting structure can be designed to advantageously optimise the application of the security feature to the security document substrate, for example increasing durability. Other methods of applying the security feature are envisaged however, such as hot or cold stamping (e.g. in the case of security articles).

Typically, the lateral dimensions of the first zone are greater than the lateral dimensions of the security feature. This advantageously allows for coarser registration tolerances when applying the security feature as compared to if the first zone and the security feature had substantially the same lateral dimensions.

In some instance, the relative lateral dimensions of the first zone and the security feature are such that the region of first zone adjacent to the security feature has dimensions such that it is not visible to the naked eye (e.g. less than 200 pm), dependent on the achievable registration. In other embodiments, the region of the first zone adjacent the security feature may have larger dimensions (e.g. up to 1mm or greater).

As noted above, the method can be characterised as a "cast-cure" process. In such processes, the pressure ("nip pressure") that is applied between the substrate and the casting structure in order to form the curable material(s) is typically much lower than in many other processes to which fibrous substrates may be subjected, e.g. calendering and most printing processes (e.g. intaglio printing). Preferably the pressure between the fibrous substrate and the surface of the die (e.g. between the fibrous substrate and the casting structure) during step (iii) is less than 2000 pascals (Pa), preferably less than 1000 Pa. Typically this pressure will be greater than 100 Pa. These pressures are much less than those used in mechanical processes for providing a smooth layer (e.g. calendering). This advantageously reduces damage to surrounding features on the substrate, and also reduces stretching of the fibrous substrate. Furthermore, the cast-cure process does not required heating to high temperatures (some typical cast-cure processes are performed at about 50 degrees Celsius, for example), further reducing adverse effects on the remainder of the substrate.

Various examples of curable materials suitable for implementing the method of the invention will be described later. Preferably, curing the one or more curable materials in step (v) comprises irradiating the one or more curable materials with electromagnetic radiation, preferably ultraviolet (UV), visible or infrared radiation, and/or electron beam radiation.

The invention also provides a security document substrate comprising: a fibrous substrate having a sizing substance incorporated therein and/or on a surface thereof; and a layer of one or more cured materials disposed on the substrate, wherein the layer of one or more cured materials comprises a first zone in which the layer of one or more cured materials has an outer surface exhibiting a surface nanostructure with a surface roughness of less than 1 micrometre (pm).

This substrate can be manufactured in accordance with the method described above, and provides the advantages described above.

Preferably the surface roughness of the surface nanostructure is in the range of 1-100 nm, preferably 10-100 nm, more preferably 10-50 nm. This can be achieved by forming the surface nanostructure of the cured material layer with a suitable casting structure that carries the surface nanostructure. The surface roughness of the surface nanostructure is (e.g. significantly) less than the surface roughness of the fibrous substrate adjacent to the layer of the one or more cured material(s).

In preferred embodiments the first zone defines a laterally continuous region having a minimum lateral dimension of at least 200 pm, preferably at least 500 pm, even more preferably at least 1mm. As discussed previously, the provision of regions that are sufficiently large so as to be easily visible to the naked eye is advantageous where the substrate carries, or is intended to carry, a visible security feature applied in the first zone.

Preferably the first zone comprises one or more first zone areas of different heights relative to the fibrous substrate, each first zone area having lateral dimensions of at least 1 pm, preferably at least 200 pm, and preferably wherein the first zone areas of different relative height define indicia such as a graphic, an alphanumerical character or a code. As noted above, this can enable selective application of a security feature to the substrate.

In preferred embodiments the outer surface of the layer of cured materials is substantially flat at each location in the first zone. As noted above, the "substantially flat" surface of the cured material layer will exhibit the surface nanostructure defined above and is therefore not entirely featureless, but on scales greater than that of the surface nanostructure (e.g. greater than 1 pm) will not exhibit any curvature or elevations/depressions. In these embodiments, the first zone may still comprise first zone areas of different heights as defined above, provided that within each first zone area the outer surface is substantially flat in the sense just described.

The thickness of the layer of cured material(s) is preferably great enough so substantially completely "fill" the undulations in the fibrous substrate to thereby present the surface nanostructure for subsequent application of a feature. Thus, typically, the layer of one or more cured material(s) in the first zone has a thickness of between 3 and 20 pm. A suitable amount of curable material is used in the method process to provide the desired thickness. In embodiments in which the first zone comprises one or more first zone areas of different height relative to the fibrous substrate, the thickness ("height") of the cured material in the region of the raised portions may be greater than 20 pm, for example up to 200 pm, preferably up to 100 pm.

The fibrous substrate of the invention preferably comprises paper and/or textiles. Fibrous substrates of this kind are suitable for the manufacture of various security 10 documents, examples of which have been listed above.

In some embodiments, the layer of one or more cured materials is substantially transparent to visible light. However, in some preferred embodiments, the layer of one or more cured materials comprises a pigment, preferably an opacifying substance. The layer of cured material(s) can undesirably reduce the opacity of the underlying fibrous substrate (e.g. by removing air from it). Providing a pigment such as an opacifying substance (e.g. a white pigment such as Ti02) can offset this.

The layer of one or more cured materials may comprise a radiation-responsive substance, preferably a fluorescent substance. By "radiation-responsive substance" we mean any substance that interacts with an input radiation to produce a detectable output -for example fluorescence, phosphorescence, or Raman scattering (whereby the substance modifies the wavelength of the input radiation in a predictable manner). The radiation-responsive substance may be arranged in accordance with a pattern, e.g. one that carries encoded data.

In some embodiments, the layer of one or more cured materials comprises solely the first zone. In some embodiments, the layer of one or more cured materials may further comprise a second zone having a surface structure which defines one or more structure features having dimensions of at least 1 pm. As noted above with reference to the method of the invention described previously, the second zone could comprise a plurality of areas laterally spaced from one another. The first zone and the second zone could be interspersed with one another. The one or more structure features preferably comprise raised elements that are raised relative to the outer surface of the layer or one or more cured materials in the first zone. Additionally and/or alternatively the one or more structure features preferably comprise features that define an optical device, wherein preferably the features defining the optical device comprise an array of focusing features. The provision of raised elements and/or features defining an optical device has the advantages described previously with reference to the method.

The first zone and the second zone may be formed as a continuous patch of the one or more curable materials. The two zones could alternatively be formed in different patches that are laterally spaced from one another. Each of the two zones could also be present on multiple patches of cured material since, as explained previously, the first and second zones are not necessarily single, continuous areas and could each be formed of multiple areas that are laterally spaced from one another. However, the first zone and the second zone are formed during the same manufacturing (e.g. cast curing) step.

The invention also provides a security document comprising the security document substrate described above. The security document or security document substrate preferably comprises a security feature applied to the outer surface of the layer or one or more cured materials in (e.g. at least) the first zone.

As discussed above with reference to the method of the present invention, the security feature may be a security device or a security article carrying a security device. Preferably the security device is diffractive or micro-optical device. Typically, the security article is a security stripe, a security patch or a foil.

The invention also provides a method of manufacturing a security document, the method comprising: providing the security document substrate defined above; and applying a security feature to at least the first region of the layer of one or more cured materials.

As discussed above, the security feature may be a security device or a security article carrying a security device. Preferably the security device is diffractive or micro-optical device.

BRIEF DESCRIPTION OF DRAWINGS

Examples of methods, security documents and security articles in accordance with embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figures 1(a) and 1(b) shows schematically apparatus suitable for performing methods in accordance with embodiments of the invention; Figures 2(a) to 2(d) show various stages of the manufacture of a security documents substrate in accordance with a first embodiment of the invention; Figures 3(a) to 3(d) show various stages of the manufacture of a security documents substrate in accordance with a second embodiment of the invention; Figures 4(a) to 4(d) show various stages of the manufacture of a security documents substrate in accordance with a third embodiment of the invention; Figures 5(a) and 5(b) show a first example of a security document incorporating 20 the security document substrate of Figure 2(d) in accordance with an embodiment of the invention; Figures 6(a) and 6(b) show a second example of a security document in accordance with an embodiment of the invention; and Figure 6(c) shows a modified version of the security document of Figures 6(a) and 25 6(b).

DETAILED DESCRIPTION

An example of a suitable cast-cure process for performing methods in accordance with embodiments of the invention will be described with reference to Figures 1(a) and 1(b) hereto. The process is shown as applied to a fibrous substrate 201 (in this example a paper substrate 201) having a sizing substance incorporate therein and/or on a surface thereof. During the paper manufacturing process for a security document such as a banknote, the paper is surface sized at a size press after a first drying process. The size press forces the sizing resin into the pores of the paper substrate and results in a thin coating of the sizing resin on the paper substrate. Preferably, traditional sizing resins such as polyvinylalcohol (PVOH) or gelatin are used as functionally these are generally the most successful. There are, however, many other chemicals which can be used such as starch or emulsion based polymers. Alternatively the sizing resins can be an aqueous polymer dispersion containing particles or solids of polyurethane resins, polyether-urethane resins, and/or urethane-acrylic resins as described for example in W02008/054581. Surface sizing improves water resistance, print receptivity and overall durability of the paper substrate.

Where the sizing substrate is provided on a surface of the fibrous substrate 201, it is typically present (e.g. in the form of a layer) on the surface to which the curable material(s) are to be applied (in this case the surface visible in Figure 1(b)). Figure 1(a) depicts the apparatus from a side view, and Figure 1(b) shows the fibrous substrate 201 in a perspective view, the manufacturing apparatus itself being removed for clarity.

A curable material 205 is first applied to the fibrous substrate 201 (typically in the form of a web or a sheet) using an application module 210 which here comprises a patterned print cylinder 211 which is supplied with the curable material from a doctor chamber 213 via an intermediate roller 212. In this example, the surface of the cylinder 211 is arranged such that the curable material 205 is laid down only on selected regions 202 thereof, which results in the curable material 205 forming discrete patches on the fibrous substrate 201 (e.g. corresponding to respective security documents of the web or sheet). The size, shape and location of the patches can be selected by control of the print process, e.g. through appropriate configuration of the pattern on cylinder 211. However, in other cases, an all over coating method could be used, e.g. if the surface relief structure is to be formed all over the fibrous substrate 201. The curable material 205 is applied to the substrate 201 in an uncured (or at least not fully cured) state and therefore may be fluid or a formable solid.

The fibrous substrate 201 is then conveyed to a casting module 220 which here comprises a casting tool 221 in the form of a cylinder carrying a casting structure 225 defining a surface nanostructure which is to be cast into the curable material 205. As each region 202 of curable material 205 comes into contact with the cylinder 221, the curable material 205 fills a corresponding region of the casting structure, forming the surface of the curable material into the shape defined by the casting structure 225. The cylinder 221 may be configured such that the casting structure 225 is only provided at regions corresponding to shape and position of the first regions 202 of curable material 205.

Having been formed into the desired surface relief structure, the curable material 205 is cured by exposing it to appropriate curing energy such as radiation R from a source 222. This preferably takes place while the curable material 205 is in contact with the casting structure 225 although if the material is already sufficiently viscous this could be performed after separation. In the example shown, the material is irradiated through the substrate 201 (e.g. the paper substrate is sufficiently transparent to the curing radiation for the curing to take place) although the source 222 could alternatively be positioned above the substrate 201, e.g. inside cylinder 221 if the cylinder is formed from a suitable transparent material such as quartz.

In an alternative embodiment, the curable material 205 could be applied directly onto the casting structure 225 rather than on to the substrate 201. This could be done in an all-over or patternwise manner.

In some embodiments, the curable material 205 (or one or more of the curable materials, if multiple curable materials are used) may include a pigment such as an opacifying material (such as Ti02) in order to offset transparentisation of the substrate due to the manufacturing of the cured material layer.

As discussed above, the method of the present invention can be characterised as a "cast-cure" process. Typically the pressure between the fibrous substrate 201 and the casting structure 225 while both are in contact with the curable material 205 therebetween is less than 2000 pascals (Pa), preferably less than 1000 Pa. Typically this pressure will be greater than 100 Pa.

Suitable apparatus, materials and methods for forming the surface nanostructures and (where provided) surface microstructures disclosed herein, both of which we will refer to as "surface relief structures" in this discussion, are further described in WO-A-2018/153840 and WO-A-2017/009616. In particular, the surface relief structures can be formed by the in-line casting devices detailed in WO-A2018/153840 (e.g. that designated 80 in Figure 4 thereof), using an embossing tool 85 carrying an appropriately designed casting structure from which can be cast the desired relief structure. Similarly, the cast-curing apparatuses and methods disclosed in section 2.1 of WO-A-2017/009616 (e.g. in Figures 4 to 8 thereof) can also be used to form the presently disclosed relief structures, by replacing the relief 225 carried on casting tool 220 with an appropriate casting structure from which can be cast the desired surface relief structures.

Whichever casting apparatus is used, the curable material(s) from which the surface relief structure(s) are cast may be applied either directly to the tool carrying the desired casting structure (e.g. to the embossing tool 85 of WO-A2018/153840 or to the casting tool 220 of WO-A-2017/009616), or the curable material(s) may be applied directly to the substrate on which the relief structure is to be formed, and then brought into contact with the casting structure (e.g. by impressing the tool carrying the casting structure onto the deposited curable material). Both options are described in the aforementioned documents. In some cases, the curable materials(s) may be provided to both the substrate surface and the tool carrying the desired casting structure.

Suitable curable materials are disclosed in WO-A-2017/009616, section 2.1. UV-curable materials are most preferred. Curing of the material(s) may take place while the casting structure is in contact with the curable material, against the substrate.

In all of the above methods, the curable material in which the surface relief structure(s) are formed can be of various different compositions. The curable material is preferably radiation-curable and may comprise a resin which may typically be of one of two types, namely: a) Free radical cure resins, which are typically unsaturated resins or monomers, pre-polymers, oligomers etc. containing vinyl or acrylate unsaturation for example and which cross-link through use of a photo initiator activated by the radiation source employed e.g. UV.

b) Cationic cure resins, in which ring opening (e.g. epoxy types) is effected using photo initiators or catalysts which generate ionic entities under the radiation source employed e.g. UV. The ring opening is followed by intermolecular cross-linking.

The radiation used to effect curing will typically be UV radiation but could comprise electron beam, visible, or even infra-red or higher wavelength radiation, depending upon the material, its absorbance and the process used. Examples of suitable curable materials include UV curable acrylic based clear embossing lacquers, or those based on other compounds such as nitro-cellulose. A suitable UV curable lacquer is the product UVF-203 from Kingfisher Ink Limited or photopolymer NOA61 available from Norland Products. Inc, New Jersey.

Figures 2(a) to 2(d) illustrate steps of a method of manufacturing a security document substrate in accordance with a first embodiment of the invention. This method can be performed using the apparatus illustrated in Figure 1(a).

A fibrous substrate 201 is provided, shown here in a cross-sectional view such that the plane of the substrate lies perpendicular to the page. A sizing substance (not shown) is incorporated in the fibrous substrate 201 and/or arranged on a surface thereof. The fibrous substrate 201 may be provided as a web, such that the method is performed as a web-based process, or could be provided in an alternative form such as one or more sheets, in which case the method may be implemented as a sheet-fed process. The fibrous substrate will typically comprise a material such as paper or textiles. The surfaces of such fibrous substrates typically exhibit arithmetic average surface roughness (Ra) values much greater than polymer substrates, often greater than 1 pm. This is illustrated by the surface 201a in Figure 2(a) (not to scale).

In this example, as shown in Figure 2(a), one or more curable materials 205 are applied to a surface 201a of the fibrous substrate 201. This can be performed by the patterned print cylinder 211 described above, which produces discrete patches of the curable material 205 as shown in Figure 1(a). The patch of curable material 205 shown in Figure 2(a) corresponds to one of the patches 205 illustrate in Figures 1(a) and 1(b). Examples of suitable curable materials, e.g. UV-curable resins, are discussed above.

A casting structure 225 is brought into contact with the curable material(s) 205 as shown in Figures 2(b) and 2(c). In the region of the casting structure 225 that is illustrated in this example, the surface 225a of the casting structure 225 has a "surface nanostructure" in which the surface roughness of the surface 225a is less than 1 pm. As noted above, "surface roughness" here refers to the arithmetic average roughness Pa of the region in question. In one dimension, this parameter may be computed by the following expression: 1 IL R = 0 lz(x)I dx where x is a coordinate in the plane of the surface, z(x) is the is the deviation in height of the surface profile from a notional centre line at position x, and L is the dimension of the region in the x direction over which Pa is being computed. The computation can be extended to a two-dimensional surface region simply by integrating over the two dimensions and substituting the area of the region (e.g. L2, in the case of a square region with side length L) in question for the divisor L in the expression above.

In preferred implementations, the surface nanostructure is such that the surface 225a is as smooth as possible in this region, for example with Ra in the range of 1-100 nanometres (nm), preferably in the range of 10-100 nm, more preferably 10-50 nm. The part of the surface 225a shown constitutes a first surface region in which the surface 225a has the surface nanostructure, and other parts of the casting structure 225 outside of the region shown may have features that do not constitute a nanostructure. Examples of such surface regions will be described below with reference to Figures 3(a) to 3(d) and 4(a) to 4(d).

As pressure is applied between the substrate 201 and the surface 225a of the casting structure 225, the curable material(s) 205 form a layer which carries on its outer surface (i.e. the surface in contact with the casting structure 225) the surface nanostructure of the surface 225a. The curable material(s) 205 are cured by irradiating them with a curing radiation R, which in this example is provided while the curable material(s) 205 are in contact with the casting structure 225. Once the curable material(s) 225 are at least partly cured, the casting structure 225 is removed, leaving a substantially flat layer of cured material(s) 206 with an outer surface 206a which exhibits the surface nanostructure that is carried by the casting structure 225. The product of these steps, shown in Figure 2(d), is a security document substrate in which the layer of cured material(s) 206 exhibits the surface nanostructure carried by the casting structure 225. The thickness, t, (e.g. mean average thickness) of the layer 206 is great enough to substantially completely fill the peaks and troughs of the fibrous substrate 201. Typical thicknesses for the layer 206 are between 3 and 20 pm.

In the exemplary method shown in Figures 2(a) to 2(d), the curable material(s) 205 were applied first to the substrate 201 before being brought into contact with the casting structure 225. However, in other embodiments, the curable material(s) 205 may be applied first to the casting structure 225, or to both the casting structure 225 and the substrate 201, before the substrate 201 and casting structure 225 are brought together. Examples of apparatus in which curable materials are applied to a casting structure before bringing the substrate into contact with the substrate are described in WO-A-2017/009616.

Figures 3(a) to 3(d) show steps of an exemplary method in accordance with a second embodiment of the invention. The steps of this exemplary method are the generally the same as those in the Figure 2 example but employ a different casting structure, which will be described below.

Like in the Figure 2 example, a fibrous substrate 301 incorporating and/or having thereon a sizing substance is provided and a curable material 305 is applied to a surface 301a of the substrate 301. A casting structure 325 is then brought into contact with the curable material(s) 305. In this example, the surface 325a of the casting structure 325 that contacts the curable material has a first surface region 331 which has a surface nanostructure of the kind described with reference to Figures 2(b) to 2(c). Additionally, the surface 325a includes a second surface region in which there is provided a surface microstructure which defines features having dimensions of at least 1 pm. In this case, the features in the second surface region 332 are recesses 327 that will produce an array of micro-lenses 307, as shown in Figure 3(d), once brought into contact with the curable material(s) 305. Here, the recesses 327 have a minimum dimension (i.e. their smallest dimension in any direction) of at least 1 pm. This minimum dimension could be the diameter of the lenses in the X direction or their height in the Z direction, for example. Focussing elements (e.g. micro-lenses) that may be used in the present invention typically have a pitch in the range of 5-100 pm, preferably 20-60 pm; a height of 5-40 pm, preferably 5-20 pm and a focal length of 5-100 pm, preferably 5-75 pm.

Like in the Figure 2 example, the resulting layer of cured material has a first zone 341 in which the surface 306a of the cured material 306 exhibits the surface nanostructure of the first surface region 331 of the casting tool. The micro-lenses 307 produced by the second surface region 332 constitute a second zone 342 of the layer of cured material 306 which exhibits the surface microstructure of the second surface region 332 (and therefore defines features having dimensions of at least 1 pm -in this case micro-lenses 307). Here, the first and second zones 341, 342 are laterally separate. A security device could incorporate image elements arranged to produce an optically variable effect when viewed through the micro-lenses 307, for example an array of image elements printed on the same side 301a of the substrate 301 to which the curable material 305 will be applied in the zone in which the micro-lenses 307 will be subsequently formed. The micro-lenses could thus form part of a security device such as a moire magnifier or a lenticular device.

Figures 4(a) to 4(d) illustrate an example of a method in accordance with a further embodiment of the invention. Like the Figures 3(a) to 3(d) example, the steps of this method are the same as those described with reference to Figures 2(a) to 2(d) but are performed using a different casting structure 425. The surface 425a of the casting structure 425 in this example has a first surface region 431 which carries a surface nanostructure as described previously. Unlike in the previous examples, the first surface region 431 comprises areas of different height due to the presence of a plurality of recesses 429 in the surface 425a of the casting structure 425.

Inside the recesses 429, the surface 425a has a different height relative to the parts of the surface 425a outside the recesses. The surface nanostructure is present in all parts of the surface 425a shown, including inside the recesses 429 (in particular the bases thereof). It is noted that the form of the surface nanostructure may differ in the regions within the recesses to the regions outside the recesses. The recesses 429 are configured to produce raised features 409 (which are raised relative to the surface 406a of the layer of cured material(s) 406) in the first zone 441 of the layer of cured materials 406 that is formed by the casting structure 425. Each the upper surface of each raised feature 409 forms a first zone area that is raised relative to the surrounding parts of the first zone 441. . Preferably, each of the raised features 409 in the first zone 441 has lateral dimensions such that it is easily discernible to the naked eye. Thus, preferably each of the raised features 409 typically has lateral dimensions of at least 200 pm, preferably at least 500 pm, even more preferably at least 1mm.

When a security feature such as a holographic foil is applied in the first zone 441, it will preferentially adhere to the tops of the raised features 409 because these parts of the cured material layer 406 will most easily contact the feature being applied (e.g. using a foiling die). The applied security feature (e.g. a holographic foil) may be arranged to register with the raised features 409. This allows the security feature to be applied in accordance with a pattern corresponding to the arrangement of the raised features 409, which could for example be arranged in accordance with indicia such as a graphic, alphanumerical character(s) or a code. The raised features may also provide a tactile effect as the user runs their finger across the applied security feature. Thus, the selective application of a security feature in this way may advantageously increase its security level.

The methods described above with reference to Figures 2(a) to 4(d) may further comprise applying a security feature to the layer of cured material(s) in at least the first zones. The application of a security feature is typically performed on a separate apparatus to that used to apply the layer of cured material(s). Such a security feature is typically in the form of a security article such as a stripe, patch or foil carrying a security device. The security article typically has a polymer substrate (e.g. PET or BOPP). In other examples, the security feature may be in the form of a security device applied directly to the layer of cured material(s). Examples of suitable security devices include a metallic security device, a magnetic security device, an optically variable effect generating surface relief structure (such as a diffractive device, a caustic device, one or more focusing elements, one or more reflective elements, or one or more refractive elements), a colour-shifting device, one or more sub-wavelength elements (such as plasmonic elements), one or more dispersive elements, one or more retro-reflective elements, and one or more coloured elements. The security device is preferably a diffractive relief structure (e.g. such that the security article is in the form of a holographic foil) or a zone plate. The surface provided by the layer of cured material(s) in the first zone is particularly suitable for subsequent application of a diffractive device, since its low roughness achieves minimal disruption of the intended diffraction pattern.

The security document substrates shown in Figures 2(d), 3(d) and 4(d) may subsequently be manufactured into security documents (for example bank notes, passports, certificates, licences and the like). For example, in the case of banknotes, the security document substrate may undergo numbering and varnishing processes before being cut into individual documents.

Figure 5(a) shows a plan view of an exemplary security document 250 incorporating the security document substrate shown in Figure 2(d). Figure 5(b) is a cross-sectional view of the same security document along the line A-A'. In this example, the layer of cured material(s) 206 is defines a substantially flat rectangular patch 206P that covers a portion of the fibrous substrate 201. The lateral dimensions of the patch 206P may be in the range of 1-16cm2. It also defines a substantially flat patch 206S in the form of a stripe, which is laterally spaced from the patch 206P and extends from one edge of the document 250 to the other along the Y direction. The width of the stripe in the X direction is preferably in the range of 5 to 40 mm. The outer surface of the layer of cured material(s) 206 exhibits a surface nanostructure as discussed herein. Applied to the upper surface 206a of the cured material layer 206 on the patch 206P is a holographic foil 231. The holographic foil 231 typically carries an adhesive layer that is used to apply the holographic foil to the upper surface 206a of the cured material layer 206, although it could be applied using any suitable technique such as hot/cold stamping. As can be seen in the plan view of Figure 5(b), the lateral dimensions of the patch 206P are greater than that of the holographic foil 231 (although not to scale). This advantageously allows for coarser (e.g. less precise) registration tolerance when applying the foil. This is particularly advantageous as the holographic foil (or other feature) is typically applied at a subsequent stage in the manufacturing process (e.g. at a foiling machine) to the provision of the layer of cured material(s). A holographic foil or stripe 232 is also applied to the stripe of cured material 206S. Holographic foil 232 can be applied in the same manner as the holographic foil 231 on the patch 206P just described with the same advantages. The width of the cured material stripe 206S is greater than that of the foil, so as to define "tramlines" on either side of the stripe.

Although Figures 5(a) and 5(b) illustrate an example security document carrying a layer of cured material in the form of a patch and a stripe, it will be appreciated that the layer of cured material may be provided in the form of only one of these, or indeed applied so as to cover a larger surface area of the substrate (including up to 100% of the document substrate).

Figures 6(a) and 6(b) show a security document 500 in accordance with a further embodiment of the invention. Figure 6(a) shows a plan view and Figures 6(b) shows a cross-section along the line B-B'. The security document 500 could be manufactured by a method in accordance with the apparatus and principles described with reference to Figures 1(a) to 4(d) above. The security document comprises a fibrous substrate 501 on one side of which is a layer of a cured material 506, which in this example is formed as a patch that does not cover the entire side of the document 500. In other embodiments, the cured material 506 could entirely cover the side of the document 500.

The layer of cured material 506 has a first zone 541 in which it exhibits a surface nanostructure having a surface roughness of less than 1 pm, though preferably the surface roughness is much less, for example in the range of 1-100 nm, preferably 10-100 nm, more preferably 10-50 nm. The layer of cured material also has a second zone 542, in which the outer surface 506a defines additional features. The surface 506a in the first zone 541 defines a plurality of raised features 515, which are arranged in the form of a concentric ring and circle as shown in Figure 6(a). These raised features 515 may be formed in the manner described above with reference to Figures 4(a) to 4(d). The second zone 542 defines an array of optical structures, in this case an array of micro-lenses 517, the extent of which is indicated by the rectangular dashed box in Figure 6(a). As described previously, a corresponding array of image elements could be provided on the substrate 501 (for example on the surface 501a on which the curable material 506 is disposed) overlapping the array of micro-lenses 517 so as to produce an optically variable effect that is visible to an observer. In other embodiments, the second zone may define an array of raised elements, for example tactile elements or elements corresponding to elements of an image.

A security feature, in this case a holographic foil 531, is disposed on the outer surface 506a of the cured material layer 506. This can be applied using any suitable foiling process known in the art such as adhesive or hot/cold stamping. It can be seen that the holographic foil 531 is present only on the raised features 515 of the first zone 541. The surface nanostructure in the first zone 541 provides surface that is smooth relative to the surface 501a of the fibrous substrate 501 on which the cured material 506 is disposed, which reduces the extent to which the diffraction pattern produced by the holographic foil 531 is disrupted by the surface roughness of the material on which it is placed.

Since security features such as holographic foils are typically intended to produce an effect that is visible to the naked eye, the parts of the first zone in which the holographic foil 531 is applied should have lateral dimensions that are sufficiently large that it can receive a feature that is visible to the naked eye. At least some of the parts of the first zone 541 in which the foil 531 is present (in this case the concentric ring and circle shown in Figure 6(a)) are laterally continuous regions each having a minimum lateral dimension (i.e. the smallest dimension of the area in question in any direction) of at least 200 pm, preferably at least 500 pm, even more preferably at least 1mm. The minimum lateral dimension of the ring in this example is its thickness dl along the radial direction and the minimum lateral dimension of the circle is its diameter d2. The first zone 541 may of course also include parts with smaller minimum dimensions.

Figure 6(c) shows a security document 550 that is a variation on the security document 500 of Figures 6(a) and 6(b). The security document has a similar construction to that described above, but in this example the layer of cured material(s) is divided into a first patch 506a defining the first zone, and a second patch 506b, in which the second zone 562 is formed. In this case the first zone 561 and second zone 562 are laterally spaced apart and not interspersed with one another Here, the holographic foil is applied to the first zone. Preferably, the lateral dimensions of the first zone are greater than the lateral dimensions of the security article to be applied thereto (as illustrated by the border of cured material surrounding the holographic foil in Figure 6(c)), allowing for coarser registration tolerances when applying the foil.

Although the above-described embodiments have been described in relation to the application of a holographic foil to the layer of cured material(s), other security features may be applied to the first zone of the layer of cured material(s), examples of which have been discussed herein.

Claims

CLAIMS1. A method of manufacturing a security document substrate, the method comprising: providing a fibrous substrate having a sizing substance incorporated therein and/or on a surface thereof; and (i) providing a casting tool, the casting tool having a casting structure defined in a surface thereof, the casting structure comprising a first surface region having a surface nanostructure with a surface roughness of less than 1 micrometre (pm); (ii) applying one or more curable materials to the fibrous substrate and/or the casting structure of the casting tool; (iii) bringing the fibrous substrate and the casting tool into contact with the one or more curable materials therebetween, thereby forming the one or more curable materials into the casting structure; and (iv) during and/or after the contact, curing the one or more curable material(s) so as to form, on the fibrous substrate, a layer of one or more cured material(s) comprising a first zone having an outer surface exhibiting a surface nanostructure corresponding to the first region of the casting structure 2. The method of claim 1, where the surface roughness of the surface nanostructure of the first surface region is in the range of 1-100 nanometres (nm), preferably in the range of 10-100 nm, more preferably 10-50 nm.3. The method of any preceding claim, wherein the first surface region has dimensions such that the first zone of the layer of one or more cured material(s) defines a laterally continuous region having a minimum lateral dimension of at least 200 pm, preferably at least 500 pm, even more preferably at least 1mm.4. The method of any preceding claim, wherein the first surface region comprises areas of different relative height, each area having lateral dimensions of at least 1 pm, preferably at least 200 pm, and preferably wherein the areas of different relative height define indicia such as a graphic, an alphanumerical character or a code.5. The method of any preceding claim, wherein the first surface region of the casting structure is configured such that the outer surface of the layer of cured materials is substantially flat at each location in the first zone.6. The method of any preceding claim, wherein the casting structure comprises a second surface region having a surface structure which defines one or more casting surface features with dimensions of at least 1 pm.7. The method of claim 6, wherein the casting surface features are arranged such that the one or more of the casting surface features form, in the one or more curable materials between the fibrous substrate and the casting tool in step (iii), an optical device, wherein preferably the optical device comprises an array of focusing features.8 The method of any preceding claim, further comprising applying a security feature to at least the first zone of the layer of one or more cured materials.9. The method of claim 8, wherein the security feature is a security device, preferably a diffractive or micro-optical device.10. The method of claim 8, wherein the security feature is a security article carrying a security device, preferably a diffractive or micro-optical device.11. The method of any of claims 8 to 10, wherein the security feature is applied to the first zone using an adhesive having an adhesive surface structure that is presented to the layer of one or more cured materials, and wherein the surface nanostructure of the first zone is configured to complement the adhesive surface structure.12. The method of any preceding claim, wherein the one or more curable materials comprise a pigment, preferably an opacifying substance.13. The method of any preceding claim, wherein the one or more curable materials comprise a radiation-responsive substance, preferably a fluorescent substance.14. The method of any preceding claim, wherein the pressure between the fibrous substrate and the surface of the die during step (iii) is less than 2000 pascals (Pa), preferably less than 1000 Pa.15. The method of any preceding claim, wherein curing the one or more curable materials in step (v) comprises irradiating the one or more curable materials with electromagnetic radiation, preferably ultraviolet, visible or infrared radiation, and/or electron beam radiation.16. A security document substrate comprising: a fibrous substrate having a sizing substance incorporated therein and/or on a surface thereof; and a layer of one or more cured materials disposed on the substrate, wherein the layer of one or more cured materials comprises a first zone in which the layer of one or more cured materials has an outer surface exhibiting a surface nanostructure with a surface roughness of less than 1 micrometre (pm).17. The security document substrate of claim 16, wherein the surface roughness of the surface nanostructure is in the range of 1-100 nanometres (nm), preferably 10-100 nm, more preferably 10-50 nm.18. The security document substrate of claim 16 or claim 17, wherein the first zone defines a laterally continuous region having a minimum lateral dimension of at least 200 pm, preferably at least 500 pm, even more preferably at least 1mm.19. The security document substrate of any of claims 16 to 18, wherein the first zone comprises one or more areas of different heights relative to the fibrous substrate, each area having lateral dimensions of at least 1 pm, preferably at least 200 pm, and preferably wherein the areas of different relative height define indicia such as a graphic, an alphanumerical character or a code.20. The security document substrate of any of claims 16 to 19, where the outer surface of the layer of cured materials is substantially flat at each location in the first zone.21. The security document substrate of any of claims 16 to 20, wherein the fibrous substrate comprises paper and/or textiles.22. The security document substrate of any of claims 16 to 21, wherein the layer of one or more cured materials comprises a pigment, preferably an opacifying substance.23. The security document substrate of any of claims 16 to 22, wherein the layer of one or more cured materials comprises a radiation-responsive substance, preferably a fluorescent substance.24. The security document substrate of any of claims 16 to 23, wherein the layer of one or more cured materials further comprises a second zone having a surface structure which defines one or more structure features having dimensions of at least 1 pm.25. The security document substrate of claim 24, wherein the one or more structure features comprise features that define an optical device, wherein preferably the features defining the optical device comprise an array of focusing elements.26. The security document substrate of any of claims 16 to 25, wherein the first zone and the second zone are formed as a continuous patch of the one or more curable materials.27. A security document comprising the security document substrate of any of claims 16 to 26.28. The security document substrate of any of claims 16 to 26 or the security document of claim 27, further comprising a security feature applied to the outer surface of the layer or one or more cured materials in at least the first zone.29. The security document substrate or security document of claim 28, wherein the security feature is a security device, preferably a diffractive or micro-optical device.30. The security document substrate or security document of claim 28, wherein the security feature is a security article carrying a security device, preferably a diffractive or micro-optical device.31. The security document substrate or security document of claim 30, wherein the security article is a security stripe, a security patch or a foil.32. A method of manufacturing a security document, the method comprising: providing the security document substrate of any of claims 16 to 26; and applying a security feature to at least the first region of the layer of one or more cured materials.33. The method of claim 32, wherein the security feature is a security device, preferably a diffractive or micro-optical device.34. The method of claim 32, wherein the security feature is a security article carrying a security device, preferably a diffractive or micro-optical device.