GB2635891A

GB2635891A - Optical security element

Info

Publication number: GB2635891A
Application number: GB2314988.3A
Authority: GB
Inventors: Holec Petr; Houha Roman; Cervenka Petr; Dvorák Robert; Têthal Tomás
Original assignee: Iqs Group A S
Current assignee: Iqs Group A S
Priority date: 2023-09-29
Filing date: 2023-09-29
Publication date: 2025-06-04
Also published as: WO2025068504A1; GB202314988D0

Abstract

A method for the manufacture of an optical security element 3 comprises providing a substrate 10 having diffractive optically active relief 20 formed on or applied to a surface (14, Fig.1) thereof; applying to said surface of the substrate a layer of a composition comprising particles of a material having a refractive index different from the refractive index of the material of the substrate, the composition being applied in the form of a sol; drying the applied sol layer to form a xerogel intermediate layer 30 comprising the said different refractive index material; and applying over the xerogel intermediate layer 30 at least one covering layer 40 for protecting the surface optical relief on the substrate against subsequent mechanical damage. The substrate material may have a refractive index in the range of from about 1.4 to about 1.6. The particles may comprise or are composed of a material with a refractive index in the range of from about 1.6 or 1.7 or 1.8 up to about 2.55 or 2.6. The particles may be composed of or comprise an inorganic material being a metal (e.g. aluminium (Al) or copper (Cu) or a metal oxide (e.g. titanium dioxide (TiO2) nanoparticles).

Description

OPTICAL SECURITY ELEMENT

TECHNICAL FIELD

This invention relates to a method for manufacturing an optical security element, such as for application onto or incorporated into various kinds of identification (ID) or security documents or articles. More particularly, though not exclusively, the invention relates to a method for manufacturing an optical security element comprising a multi-layer structure. The invention also relates to an optical security element manufactured by the method, and to an identification (ID) or security or other document or article including one or more such optical security elements.

BACKGROUND AND PRIOR ART

Optical security elements of various types are widely-known for a variety of practical security purposes involving identification, authentication or protection, and are typically applied onto a surface of a component of, or incorporated into the structure of, various such documents, items or objects, including for example ID (identification) cards, passports, visas, driving licences, credit and debit cards, banknotes, securities, certificates, tickets, branded goods, and various other items whose value or nature benefits from an added security feature. One commonly employed type of optical security element is a diffractive optically variable image device (DOVID), which is typically based on a multi-layer (that is to say, plural-layer) structure that creates different or varying visible optical effects from the security element depending on the conditions under which it is illumined or has light applied thereto. Such DOVID devices typically include at least one layer within its structure which includes diffractive optically active relief on a surface thereof.

In the stages of manufacture of such multi-layer DOVID devices, the optically active relief formed on or applied to a surface of a given layer thereof (which then becomes an "optically active layer" thereof) must generally be covered or encapsulated by some kind of covering or protective layer, before that optically active layer is united with any remaining layer(s) to form the finished security element, in order primarily to protect the surface optical relief on the optically active layer against mechanical damage during the subsequent lamination and/or other processing steps. At the same time this covering or protective layer serves to protect the surface optical relief on the optically active layer of the security element against copying or duplication, e.g. for counterfeiting or other security-breaching purposes.

In a typical manufacturing procedure for known such multi-layer DOVID devices, an optical relief structure or pattern (typically a diffractive optical relief structure or pattern) is formed in or applied to a surface of a plastics carrier or substrate, either by imprinting of a relief embossing tool directly into the surface of the body of the carrier/substrate material, or by using a known nano-imprinting lithography technique to imprint the optical relief into a polymer layer at the surface of the plastics carrier/substrate. In the use of such known technologies, the carrier/substrate materials into the surface of which the optical relief is formed or applied typically have a refractive index of around 1.5. Once the optical relief has been formed or applied on the carrier/substrate surface, a plastics covering film or printed covering layer is then applied over the optical relief surface layer and the lamination procedure is then completed, with the refractive index of such covering film or printed covering layer materials also typically being around 1.5.

However, in order to maintain the efficacy of the optical function of the surface optical relief it is generally necessary that the material of the covering film or layer should have a refractive index that is significantly different from the refractive index of the material of the carrier/substrate in whose surface the optical relief is formed or applied. Otherwise, the desired optical function of the optical security element will disappear after the covering film or layer has been applied. In practice this difference in refractive index is often usefully achieved by the introduction of a thin intermediate (i.e. third) layer in between the surface optical relief and the covering film or layer, which intermediate, third layer is itself of a material whose refractive index is significantly different from that of at least the carrier/substrate material in whose surface the optical relief is formed or applied. Such a thin intermediate layer with the different refractive index is generally applied using vacuum deposition technology, many examples of which are known in the art. Typically, metals such as aluminium (Al) or copper (Cu) or high refractive index metal oxides such as titanium dioxide or zinc sulfite are used for such intermediate layers, in order to achieve an optimum required different refractive index (which for these materials may typically be around 1.8) and to facilitate their vacuum deposition (since such materials are well suited to such a deposition technique). Thus, by application of such a high-refractive index intermediate layer before the final covering film or layer, the surface optical relief maintains and fulfils its desired optical function even after the final covering film or layer has been applied to complete the laminate structure of the security element.

However, as an industrial technique vacuum deposition requires hardware and operating conditions which are far from being energy efficient, which as well as being energy-wasteful also increases manufacturing costs. Vacuum deposition also comes with various technical limitations, especially in terms of the impossibility of depositing especially thin intermediate layers with high selectively (e.g. in complex patterns) using known vacuum deposition hardware employing known vacuum evaporation devices and/or sputtering lines.

Additionally, in the case of especially thin intermediate layers, it may also be necessary to apply an additional fixing layer thereover before the final covering film or layer is itself applied, in order to protect the thin high refractive index intermediate layer itself from damage etc before it is subjected to the subsequent final covering or lamination steps. Such a fixing layer increases manufacturing times and costs and further complicates the overall manufacturing procedure and the structure of the resulting security element produced thereby.

Thus, it is a primary object of the present invention to address the above shortcomings in the known art of manufacturing multi-layer optical security elements, especially those known manufacturing methods which rely on vacuum deposition technology to apply a high refractive index covering layer over an optical relief surface on a carrier or substrate, and to provide a new method for the manufacture of such optical security elements that is technically more versatile and reliable and economically advantageous.

SUMMARY OF THE INVENTION

Accordingly, in a first aspect the present invention provides a method for the manufacture of an optical security element, the method comprising: providing a substrate having diffractive optically active relief formed on or applied to a surface thereof; applying to said surface of the substrate a layer of a composition comprising particles of a material having a refractive index different from the refractive index of the material of the substrate, the composition being applied in the form of a sol; drying the applied sol layer to form a xerogel intermediate layer comprising the said different refractive index material; and applying over the xerogel intermediate layer at least one covering layer for protecting the surface optical relief on the substrate against subsequent mechanical damage.

In a second aspect the present invention provides an optical security element comprising: a substrate having a surface with diffractive optically active relief formed thereon or applied thereto; an intermediate layer formed on the substrate surface, the intermediate layer being in the form of a xerogel and comprising a material having a refractive index different from the refractive index of the material of the substrate; and at least one covering layer formed over the xerogel intermediate layer for protecting the surface optical relief on the substrate against mechanical damage.

Embodiments of the above-defined optical security element according to the second aspect may include optical security elements formed by embodiments of the above-defined manufacturing method according to the first aspect.

In a third aspect the present invention provides a document or article, such as an ID (identification) card, passport, visa, driving licence, credit or debit card, banknote, security, certificate, ticket, other ID or security document, or item of merchandise, including or having applied thereto one or more optical security elements according to the second aspect or any embodiment thereof.

As used herein, the term "optically active" (or linguistic equivalents) referring to the optical relief applied to the surface of the substrate means that the optical relief has or exhibits an optical function which causes or imparts to incoming light or light incident thereon a change in one or more optical properties or characteristics of the light as it interacts with the optical relief and is then outputted therefrom, e.g. by either transmission or reflection therefrom.

Such optical properties or characteristics may include a distribution or directional property or parameter of the light in question and/or any inherent optical property of the light in question, such as frequency, wavelength, colour, polarisation, intensity, energy or other physical property of the light itself.

As used herein, the term "light" is to be construed broadly and encompasses any electromagnetic radiation in any region of the electromagnetic spectrum, including not only visible light (i.e. that in the range of wavelengths of from about 380 nm up to about 750 nm) but also electromagnetic radiation in the infra-red (e.g. -750 nm -500 pm) or ultraviolet (e.g. -100 -380 nm) regions of the spectrum.

In some, possibly relatively simpler, embodiments of the invention in its various aspects, the application of the at least one covering layer over the xerogel intermediate layer may comprise applying, by a step involving lamination, over the xerogel intermediate layer at least one pre-prepared laminating covering layer in the form of a pre-existing laminating foil, film or sheet. Such a laminating covering layer may be of any suitable material conventionally used for such laminating layers in known DOVIDs, e.g. a polymer or combination of polymers, or a metal or metal oxide (or a combination of two or more thereof), or a combination of two or more of any of the foregoing materials. Any suitable known laminating conditions or parameters, e.g. of elevated temperature and/or pressure, and/or optionally using a suitable adhesive or bonding agent, may be used to effect such lamination of such one or more laminating covering layer(s) according to known techniques, and using known laminating apparatuses as used in known DOVID production.

However, in other, possibly relatively more advanced, embodiments of the invention in its various aspects, the application of the at least one covering layer over the xerogel intermediate layer may comprise applying over the xerogel intermediate layer at least one covering layer of a curable or dryable covering composition, and curing or drying the or each curable or dryable covering composition layer to form at least one cured or dried protective covering layer over the xerogel intermediate layer.

In some such embodiments as in the preceding paragraph, the or each covering composition layer applied over the xerogel intermediate layer may be a curable covering composition, or a precursor of a curable covering composition, and the method may include a step of applying that covering composition (or precursor thereof) over the xerogel intermediate layer and then curing it to form the at least one cured protective covering layer over the xerogel intermediate layer. Examples of suitable such curable covering compositions include various lacquers, varnishes or inks, many examples of which are already known in the art and widely commercially available. Some embodiments of the invention utilising such curable covering compositions will be discussed and described further hereinbelow.

However, in other such embodiments as in the preceding-but-one paragraph, the or each covering composition layer applied over the xerogel intermediate layer may be a dryable covering composition in the form of a sol, and the method may include a step of applying that sol covering composition over the xerogel intermediate layer and then drying it to form the at least one dried protective covering layer over the xerogel intermediate layer. In various practical embodiment forms such a dryable sol covering composition may be either: (i) of substantially the same basic composition, in terms of its components and their relative amounts present therein -with the exception of and excluding the particulate different refractive index material -as that used initially to form the main xerogel intermediate layer itself, or 00 of a substantially different basic composition, in terms of its components and their relative amounts present therein -with the exception of and excluding the particulate different refractive index material -from that used initially to form the main xerogel intermediate layer itself, whereby the resulting dried protective covering layer may be or form a protective second xerogel covering layer whose composition is either the same as or different from that of the basic xerogel material -with the exception of and excluding its particulate different refractive index material -that forms the main xerogel intermediate layer. Some embodiments of the invention utilising such dryable xerogel-forming covering compositions will be discussed and described further hereinbelow.

In various embodiments of the invention, the complete optical security element may be either (i) substantially transmissive, or at least partially transmissive, to light incident thereon, or (h) it may be substantially reflective, or at least partially reflective, to light incident thereon. In any given practical embodiment, this transmissive or reflective overall property may be dictated by the end use for which the optical security element is designed or the kind of item or article to which it is designed to be applied for the desired security purpose.

In embodiments of the present invention in its various aspects, the substrate used to form the base layer or carrier for the security element may be formed from a polymeric material, such as a polycarbonate or an acrylic-based polymer such as polymethyl methacrylate (PMMA). Other examples of suitable polymers may also be possible to use. The substrate polymer material may be either substantially transparent, or at least translucent, to the light for which use of the security element is designed, or alternatively it may be substantially (or at least partially) opaque to such light. A substantially transparent substrate material may be more useful when the complete optical security element (including the applied xerogel intermediate layer applied to the substrate surface) is designed to be substantially transmissive to light incident thereon. Alternatively, a substantially opaque substrate material may be adequate (although a substantially transparent substrate material may still be able to be used in this case) when the complete optical security element -or at least the applied xerogel intermediate layer applied to the substrate surface -is designed to be substantially reflective to light incident thereon.

In many embodiments of the invention the substrate material may typically have a refractive index in the approximate range of from about 1.4 to about 1.6, e.g. approximately 1.5.

In many practical embodiments of the invention, the substrate material used to form the base layer or carrier for the security element may be provided in the form of a sheet, film, foil, strip or plate, e.g. having a thickness in the approximate range of from about 1 or 5 or 10 pm up to about 200 or 250 or 300 pm, or possibly even up to as much as about 400 or 500 pm. More usually and typically a substrate thickness in the approximate range of from about 10 or 20 or 50 pm up to about 100 pm may be used, although sometimes a thickness greater than this i.e. up to around 250 or even up to around 300 or 400 pm, may be found to be suitable, e.g. depending on the intended end use of the optical security element in question. Such substrate forms may be either generally substantially rigid or alternatively they may be flexible, especially in order to allow a degree of bending or flexing to facilitate their affixation to a non-planar object or portion of an object.

In embodiments of the invention the diffractive optically active relief may be formed on or applied to at least one surface of the substrate, especially a major facial surface of the substrate. In some embodiments the relief may be formed on or applied to substantially the whole area of the surface, whereas in other embodiments the relief may be formed on or applied to just one or more portions only of the area of the surface.

In embodiments of the invention the diffractive optically active relief formed on or applied to the said surface of the substrate may comprise one or more regions with either the same or different optically active relief patterns or structures applied to the substrate surface. Such diffractive optically active relief may be either fully or partially diffractive in its optical function, and the relief features of which the relief is composed may typically have dimensions represented by: widths and/or heights of individual surface relief features from about 0.001 up to about 70 or 80 or 90 pm, optionally from about 0.001 up to about 50 or 60 or 70 pm, further optionally from about 0.005 or 0.01 up to about 5 or 10 or 20 or 30 or 40 or 50 pm; and/or depths of less than about 10 pm, optionally less than about 5 pm, further optionally less than about 3 pm.

In some embodiments of the invention, in the initial step of the method in which the optically active relief is formed on or is applied to the relevant surface of the substrate, the relief may be formed on or applied to the surface by any suitable known embossing technique, in which a "master" relief embossing tool or plate is pressed directly into the substrate surface to transfer the relief pattern directly thereto, or alternatively by using a known nano-imprinting lithography technique to imprint relief pattern into a surface layer of the substrate. Specific details of suitable embossing or nano-lithography hardware and practical steps are well-known in the art.

In the manufacturing method of the present invention, once the appropriate substrate -with the appropriate diffractive optically active relief formed on or applied to a surface thereof -has been provided, a first key step is the application to the said surface of the substrate of a layer of a composition comprising particles of a material having a refractive index which is different from the refractive index of the material of the substrate, wherein the composition is applied in the form of a sol.

In embodiments of this method, the particulate material having the different refractive index from that of the substrate material may typically have a relatively high refractive index, especially a refractive index higher than, and often significantly higher than (e.g. by at least about 0.1 or 0.2 or 0.3), that of the substrate material. In some such embodiments the particles may typically comprise or be composed of a material with a refractive index in the approximate range of from about 1.6 or 1.7 or 1.8 up to about 2.5 or 2.6 or more, more usually in the range of from about 1.7 or 1.8 up to about 2.2 or 2.3, especially in the range of from 1.9 to about 2.05. In some embodiments the particles may usefully be composed of or comprise an inorganic material being a metal; e.g. Al or Cu, or a metal oxide or sulphide, e.g. TiO2 or ZnS. Amorphous crystalline forms of such metal oxides may be especially suitable. Titanium dioxide (TiO2) may be a particularly favoured and suitable as the particulate material in many embodiments of the invention.

In many embodiments of the above method, the particle size (i.e. average particle size measured as the average diameter or width of the particles) of the particles of the material having the different refractive index from that of the substrate material may be such that they may be termed "nanoparticles" (as that term of art is understood by the skilled person), which is to say the said average particle size may be in the approximate range of from about 5 up to about 100 or 150 nm, especially for example in the approximate range of from about 10 up to about 20 or 30 or 40 or 50 nm.

In practical embodiments of the invention, the particulate material having the different refractive index may be sourced or provided for use from any suitable commercial source and in any suitable physical form. For instance, the particulate material, e.g. TiO2 nanoparticles, may be provided ready for use in the form of the dry pre-manufactured particulate material, and physically incorporated in the required amount, along with the remaining components, into the desired final sol composition to be applied to the substrate.

Such TiO2 nanoparticles may for example be prepared from a TiOSO4 precursor, such as is described in any of the following literature references: https://www. resea rchg ate. net/pu blicati on/317751307 Sol-o& synthesis of TiO2 from TiOSO4 characterization and UV photocatalytic activity fo r the degradation of 4-chlorophenol, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7912831/ https://www. semanficscholarorg/paper/Sol°/0E2T080°/093gel-synthesis-of-Ti02-from-TiOSO 4%3A-and-UV-for-Khanifc27c623896f3b4da89d214a1378e43cb571148b (whose complete contents are each implicitly incorporated herein by reference thereto). Such TiOSO4 precursor-derived nanoparticles may have better stability than other types of such nanoparticles prepared in other ways. Alternatively, the particulate material, e.g. TiO2 nanoparticles, may be sourced and provided for use in the form of a pre-prepared nanoparticles-containing coating-forming sol (or sol-gel) composition which already comprises the nanoparticles in an already surface-functionalised form. Such compositions and examples thereof will be discussed and described further hereinbelow, for instance as described in US 11,274,219 B2 (of BASF) and possibly other patents, as well as in the form of various commercially available such compositions (e.g. from BASF).

In embodiments of the method of the invention, the composition containing the particles of the material with the required different (especially significantly higher) refractive index may be applied in the form of a sol with any suitable chemical composition effective to form a xerogel upon gradual drying thereof, especially by gradual removal of solvent therefrom.

The chemistry of sols and their processing to form xerogels via the sol-gel process are well-known and well-discussed in the technical literature in the present art. In the context of the present invention, by "sol" is meant a stable colloidal suspension of solid particles of the different refractive index material in a continuous liquid phase or medium. Such a continuous liquid phase is referred to elsewhere herein, for convenience and simplicity, as simply a "solvent", even though in a colloidal suspension it is not strictly speaking solvating the solid particles forming the dispersed discontinuous phase but rather holding them in stable colloidal suspension. For use in embodiments of the invention, the continuous liquid or solvent phase of the sol may comprise one or more liquid substances selected from any of the following: - alcohols (especially lower alcohols, e.g. ethanol); - ketones (e.g. symmetric or asymmetric linear ketones, such as methylethylketone, or cyclic ketones, such as cyclopentanone); -water; - mixtures of any of the above-listed substances (in any suitable relative proportions).

In embodiments of the method, the composition in the form of the nanoparticles-containing sol which is applied to the substrate surface may be formulated so as to contain any appropriate or suitable relative amounts of the solid nanoparticles phase and the liquid continuous "solvent" phase. Especially suitable in many embodiments may for example be relative proportions by weight of the solid nanoparticles phase to the liquid continuous "solvent" phase which are in the approximate weight ratio range of from about 1:99 up to about 10:90, or perhaps even up to around 20:80 or 25:75 in certain other embodiments.

In various embodiments of the invention the composition in the form of the nanoparticles-containing sol which is applied to the substrate surface may be formulated appropriately so as to effect cross-linking and/or polymerisation of its relevant components upon its drying through a process comprising either or both of (i) surface-functionalisation of the nanoparticles themselves so that they themselves then form a continuous network of cross-linked such nanoparticles, and/or (ii) polymerisation (with optional cross-linking) of one or more polymerisable monomers contained in the composition so as to form a polymerised network or matrix structure which simply incorporates the nanoparticles therewithin in the finished dried xerogel product.

In either case (i) and/or (k) in the preceding paragraph, in many such embodiments of the invention the nanoparticles-containing sol composition which is applied initially to the substrate surface may additionally comprise at least one cross-linking and/or polymerisation initiator. Such initiator(s) may be included in the composition in any suitable amount, as the skilled person will be able to readily determine depending on the chemistry involved. For example, in some practical embodiments, an amount of such initiator(s) of up to around 0.1 or 0.5 or 1 % by weight of the total applied sol composition may be suitable. Examples of suitable such initiator(s) include cyano-based initiators, such as trithiocyanuric acid (to name but one), and UV-activatable initiators. However, other examples of appropriate initiators may be available in the art and possible to use, depending on the chemistry of the system. Depending on its chemical constitution, the initiator(s) may if necessary or appropriate require the added application of a source of electromagnetic radiation, especially UV (ultra-violet) light, e.g. using a suitable known UV light source, to the reacting system in order to initiate or catalyse its initiating reaction in cross-linking and/or polymerising the relevant component(s) of the composition in question being used.

Thus, in certain such embodiments of the invention the nanoparticles-containing sol composition which is applied initially to the substrate surface may optionally comprise especially in addition to an appropriate initiator component as discussed above -one or more polymerisable monomer substances. Such polymerisable monomer(s) may be included in the composition in any suitable amount, as the skilled person will be able to readily determine depending on the chemistry involved. For example, in some practical embodiments, an amount of such polymerisable monomer(s) of up to around 5 or 8 or 10 % by weight of the total applied sol composition may be suitable. Examples of suitable such monomer(s) may include various acrylic-based or polyol-based monomers, of which many examples are available in the art, such as pentaerythritol tetraacrylate and polyurethane-forming polyols. However, other examples of suitable polymerisable monomers may be available in the art.

In certain example embodiments of the above type in which the nanoparticles-containing sol composition which is applied initially to the substrate surface additionally comprises both a cross-linking and/or polymerisation initiator and a polymerisable monomer substance, these components of such a sol composition may be provided by virtue of the composition being based on a pre-prepared coating or varnish or lacquer composition, to which the nanoparticles of the material with the different refractive index are added to form the final precursor composition that is then applied in the form of a sol to the substrate surface. Such pre-prepared coating or varnish or lacquer compositions that may be suitable for use in this way include several of those available from various commercial sources, such as BASF and possibly also others. Various examples of such pre-prepared coating compositions are well-documented in the patent literature, for example in US 6,713,559 Bl, US 6,828,381 B1, US 6,872,765 B1, WO 00/53687 A2 and WO 2009/027310 Al (all of BASF) (whose complete contents are implicitly incorporated herein by reference thereto). Other such example patents detailing other such pre-prepared coating compositions may also be available and possible to utilise in a like manner.

Moreover, in some such example embodiments which employ a cross-linking and/or polymerisation initiator and a polymerisable monomer substance in the nanoparticlescontaining sol composition which is applied initially to the substrate surface, in order to adjust or control the refractive index of the finally produced dried xerogel layer, it may be possible to vary the quantities -i.e. the absolute amounts and/or the relative proportions relative to each other -of the initiator and polymerisable monomer substances that are included in the precursor sol composition. For instance, in the case of a pentaerythritol tetraacrylate + trithiocyanuric acid monomer+initiator system, by proportionally reducing or increasing the amounts of these components relative to each other in the composition, it may be possible to adjust the refractive index of the finally produced dried xerogel layer. Alternatively or additionally, by reducing the absolute amounts of both components in the composition it may even be possible to increase the refractive index of the finally produced dried xerogel layer.

Generally speaking, in many practical embodiments of the invention the composition in the form of the nanoparticles-containing sol which is applied to the substrate surface may be provided or presented for use, ready for application to the substrate surface, as a two-part system, wherein a first part (e.g. termed "part A") comprises a pre-prepared sol composition comprising all the components of the final sol composition to be applied to the substrate but with the exception of the cross-linking and/or polymerisation initiator component, and a second part (e.g. termed "part B") which comprises the cross-linking and/or polymerisation initiator component. Those parts A and B of such a two-part system may be mixed shortly before (e.g. a few minutes or up to a few hours before) application to the substrate.

In such two-part systems, it may be possible for the part A (namely the pre-prepared sol composition comprising the basic sol components, with the exception of the initiator component, and with the nanoparticles component already incorporated therein) to be provided ready for use in the form of a pre-prepared sol composition representing a commercially available composition in its own right, e.g. as a pre-prepared coating-forming nanoparticles-containing sol (or sol-gel) composition. Such pre-prepared nanoparticlescontaining coating-forming sol (or sol-gel) compositions may typically for example comprise the nanoparticles already in a surface-functionalised form. Such pre-prepared nanoparticles-containing coating-forming sol (or sol-gel) compositions that may be suitable for use in this way include some such example compositions available from various commercial sources, such as BASF and possibly also others. Various examples of such pre-prepared nanoparticles-containing coating-forming sol (or sol-gel) compositions are well-documented in the patent literature, for example in US 11,274,219 B2 (of BASF), to name just one (whose complete contents are implicitly incorporated herein by reference thereto). Other such example patents detailing other such pre-prepared nanoparticles-containing coating-forming sol (or sol-gel) compositions may also be available and possible to utilise in a like manner as "part A" components of two-part systems in such embodiments of this invention.

In alternative embodiments of the invention, as an alternative to a two-part system for providing the nanoparticles-containing sol composition ready to use, it may be possible instead, in other embodiments of the invention, to provide the sol composition ready to apply to the substrate in the form of a simple pre-mixed system containing all its appropriate and necessary components which have been mixed and formed into the required sol, using conventional techniques and apparatus, just before or shortly before, e.g. up to a few minutes or a few hours before, being presented ready for use for application to the substrate.

In practical embodiments of the method of the present invention, the composition in the form of the nanoparticles-containing sol may be applied to the substrate surface by a printing step.

Various types of printing and associated printing apparatus may be used for this purpose, such as flexographic printing, gravure printing, or even certain other known printing techniques, e.g. ink jet printing, screen printing, pad printing or intaglio printing. Generally speaking any printing technique and associated apparatus that enables coating of the requisite sol composition in fluid or liquid form on the substrate surface to the required thickness (see below) and in the required specificity and accuracy of applied region(s) may typically be employed and be suitable in practising embodiments of this invention. (For instance, typical known offset printing techniques may not be so suitable.) Specific examples of suitable or favoured printing apparatuses and techniques of the above types that may be applied to this stage of the practising of embodiments the present invention are well known and widely used in the art for applying various types of liquid composition to substrates of various kinds in the making of various multi-layer optical elements or like products.

In such embodiments, the composition in the form of the nanoparticles-containing sol which is applied to the substrate surface may therefore have a viscosity that renders it especially suitable for application by printing. Suitable such viscosities of the composition may for example be in the approximate range of from about 1 up to about 20 mPa.s (at room temperature, i.e. 20°C). In practice, an optimum viscosity of a prepared sol composition ready for application to the substrate surface by printing may be attained by appropriate adjustment of the relative amount of the liquid ("solvent") phase relative to the amount of solid phase therein.

In many practical embodiments of the method, the composition in the form of the nanoparticles-containing sol may be applied to the substrate surface to form an initial layer thereof, prior to any drying, with an average thickness in the approximate range of from about 10 or 20 or 30 nm up to about 100 or 200 or 300 or 500 or 1000 nm, or even up to as much as about 1.5 or 2 pm. The thickness of the applied nanoparticles-containing sol layer may be approximately or substantially uniform over the area or areas of the optical relief on the substrate surface over which it is applied, or its thickness may vary thereover, e.g. by up to around 5 or 10 or 15 or 20 or 30 % of the overall average (or alternatively the minimum or maximum) thickness thereof.

In practising embodiments of the invention, the nanoparticles-containing sol composition may be applied to the surface of the substrate either (i) substantially over the whole area of that surface, or (ii) more usually, in just one or more portions or regions of that surface which are collectively less than the whole of the surface, wherein, when there are plural such portions/regions having the composition applied thereto, at least one or more of those portions/regions may be either separate from the remainder thereof or integral or joined with or connected to one or more other portions/regions thereof. In many such practical embodiments, therefore, the nanoparticles-containing sol composition may be applied to the surface of the substrate in any desired pattern or design, possibly a complex pattern or design comprising any desired number of portions in any desired relative physical or geometrical arrangement or configuration. Such versatility may be especially useful for the manufacture of optical security elements with particularly complex or unique designs, which may enhance their security-imparting function.

In the manufacturing method of the present invention, a next key step is the drying of the applied sol layer to form a xerogel intermediate layer comprising the said different refractive index material therewithin, especially distributed therewithin as either primary or secondary (i.e. agglomerated) particles or as a more extensive network of the different refractive index material in solid form within the xerogel structure.

In many practical embodiments of the method, this drying step may be carried out at atmospheric pressure and at either room temperature (i.e. approx. 20 °C) or alternatively, if desired or appropriate, a moderately elevated temperature of from about 25 °C up to about or 80 or 100 or 120 or 140 °C. Further optionally, drying in a moving flow of air (e.g. warm or hot air) or with the aid of a source of infra-red radiation may even be used. Further optionally still, a suitable solvent-extraction apparatus may even be employed to assist in the drying step by evaporation or removal of solvent from the applied sol layer.

The drying step may be carried out for any duration of time that is appropriate for the production of the required xerogel layer with the optimum desired physical properties once it is dried. Drying times of anywhere from a period of several minutes (e.g. from about 5 or 10 or 15 up to about 60 minutes) up to any number of hours (e.g. from about 1 or 2 or 3 up to about 6 or 12 or 18 or 24 hours), or possibly even several days (e.g. from 1 or 2 up to 4 or 5 or 6 days) may be suitable for various embodiment security elements, depending on the precise materials making up their multi-layer structure and the degree and rate of drying of the applied sol composition that is required to form the desired xerogel layer.

Generally speaking, the drying conditions, and in particular the rate of drying, may be selected and controlled so as to facilitate an optimum conversion of the applied "wet" solbased composition layer on the substrate into the required dried xerogel layer, by virtue of the applied wet composition undergoing the well-known sol-gel transformative process in which its internal structure evolves via a gel-like diphasic system to eventually become a solid, especially mesoporous, microstructure, and in so doing undergoes a significant amount of shrinkage and densification as it dries.

Because of the nature of the material of the substrate on whose surface the sol-based composition is initially applied and is then subjected to the drying step, any elevated temperature at which the drying step is carried out should in practice be kept suitably and sufficiently below the melting point or softening temperature of the substrate material. This is especially so that the diffractive optical relief applied to the substrate surface does not itself undergo damage or degradation during the drying step.

In general chemical terms, a xerogel is a solid, porous material resulting from the slow drying of a gel under non-supercritical conditions of temperature and pressure, with unconstrained shrinkage. Typically the xerogel is reduced in volume by a factor of around 5 or 10 compared with the original wet gel from which it is derived. In the context of embodiments of the present invention such drying may generally be carried out under atmospheric pressure and at or around room temperature, or alternatively at moderately elevated temperatures above room temperature if desired or appropriate. Xerogels are generally characterised by having a high shrinkage ratio, high porosity (e.g. typically around 15 -50 vol%), large specific surface area (e.g. typically around 150 -900 m2/g), small pore size (e.g. typically around 1 -10 nm), and high thermal stability.

In some embodiments of the method of the invention, the drying step may be followed by an additional step of fixing or stabilising the formed xerogel layer by curing or cross-linking (or further curing or further cross-linking) of the solid material itself of the formed xerogel layer or by polymerisation of a monomer component included initially in the nanoparticlescontaining sol composition which is applied initially to the substrate surface. In this way, a degree of cross-linking formed between united or agglomerated particles inside the xerogel structure, or the formation of a discrete polymer network or matrix within the xerogel structure, may serve to further unify, fix and strengthen the resulting dried/cured xerogel structure in the intermediate layer that is finally formed on the substrate surface. It may also serve to improve the integrity of the resulting dried xerogel intermediate layer and also enhance its adhesion to the underlying optical relief on the substrate surface.

Once the drying step (and any subsequent fixing/stabilising step) has/have been carried out on the nanoparticles-containing sol composition which has been initially applied to the substrate surface, the resulting dried (and optionally fixed/stabilised) xerogel layer may have a resulting thickness typically in the approximate range of from about 10 nm up to about 1 pm. The (still relatively high) refractive index of the dried (and optionally fixed/stabilised) xerogel layer may typically be in the approximate range of from about 1.7 to about 2.2, especially from about 1.9 or 1.95 to about 2.0 or 2.05.

It is a special and useful feature of many embodiments of the present invention that as the applied sol layer dries, so the removal of solvent therefrom causes it to contract and thus its thickness to significantly reduce. Because of this, it may be possible to produce, very usefully, a resulting final (i.e. dried) xerogel layer (i.e. with the characteristic significantly different (usually significantly higher) refractive index compared with that of the substrate material) that is unusually or especially thin and accurate in its geometric shape and/or configuration. In this manner especially accurate and complex shapes or designs of the final xerogel layer may be possible to produce. This may be especially useful in the manufacture of optical security elements within the scope of the invention which have with particularly complex or unique designs, which may enhance their security-imparting function.

In the manufacturing method of the present invention, a next key step is the applying over the xerogel intermediate layer at least one covering layer for protecting the surface optical relief on the substrate against subsequent mechanical damage.

As already mentioned earlier, in some, possibly relatively simpler, embodiment forms the application of the at least one covering layer over the xerogel intermediate layer may comprise simply laminating over the xerogel intermediate layer at least one pre-prepared laminating covering layer in the form of a pre-existing laminating foil, film or sheet, e.g. of one or a combination of polymeric materials, a metal or metal oxide (or a combination of two or more thereof), or a combination of two or more of any of the foregoing materials. Any suitable known laminating conditions or parameters, e.g. of elevated temperature and/or pressure, and/or optionally using a suitable adhesive or bonding agent, may be used to effect such lamination of such one or more laminating covering layer(s) according to known techniques, and using known laminating apparatuses as used in known DOVID production.

However, as also already mentioned earlier, in other, possibly more advanced, embodiment forms the application of the at least one covering layer over the xerogel intermediate layer may comprise applying over the xerogel intermediate layer at least one covering layer of a curable or dryable covering composition, and curing or drying the or each curable or dryable covering composition layer to form at least one cured or dried protective covering layer over the xerogel intermediate layer.

In some such curable/dryable covering composition/layer embodiments, the or each covering composition layer applied over the xerogel intermediate layer may be a curable covering composition or a precursor thereof, and the method may include a step of applying that covering composition (or precursor thereof) over the xerogel intermediate layer and then curing it to form the at least one cured protective covering layer over the xerogel intermediate layer. Examples of suitable such curable covering compositions include various lacquers, varnishes or inks, many examples of which are already known in the art and widely commercially available. Examples of suitable known such lacquers, varnishes or inks include those available from companies such as BASF, Akzo-Nobel, lnkron, Pixelligent, Azteca, and possibly also others.

However, in other such curable/dryable covering composition/layer embodiments, the or each covering composition layer applied over the xerogel intermediate layer may be a dryable covering composition in the form of a sol, and the method may include a step of applying that sol covering composition over the xerogel intermediate layer and then drying it to form the at least one dried protective covering layer over the xerogel intermediate layer. Such a dryable sol covering composition may be either: (i) of substantially the same basic composition, in terms of its components and their relative amounts present therein -with the exception of and excluding the particulate different refractive index material -as that used initially to form the main xerogel intermediate layer, or (ii) of a substantially different basic composition, in terms of its components and their relative amounts present therein -with the exception of and excluding the particulate different refractive index material -from that used initially to form the main xerogel intermediate layer, whereby the resulting dried protective covering layer may be or form a protective second xerogel covering layer whose composition is either the same as or different from that of the basic xerogel material -with the exception of and excluding its particulate different refractive index material -that forms the main xerogel intermediate layer.

Examples of suitable such dryable sol covering compositions may include not only the same basic sol compositions forming the base of the main sol composition (in combination with the additional particulate different refractive index material component) applied to the substrate and forming the main xerogel intermediate layer, but also other known sol or sol-gel compositions known in the art for forming various coating compositions and coating layers on a variety of substrates, of which many practical examples are available and widely documented in the patent and other literature. Some specific examples thereof include those described in the following published patents/applications: US6713559B1, US6828381B1, US6872765B1, W00053687A2, W02009027310A1 (all of BASF), as well as others.

In many such embodiments of the method, this applying of the curable/dryable covering composition may be carried out by a printing technique, especially by a printing technique and using printing apparatus that are the same or similar to that/those used for applying the nanoparticles-containing sol composition initially to the substrate surface in the initial application step -as discussed and defined/described hereinabove. Thus, for applying the curable/dryable covering composition layer(s) over the formed xerogel intermediate layer, various types of printing and associated printed apparatus may be used for this purpose, such as flexographic printing, gravure printing, or even certain other known printing techniques, e.g. ink jet printing, screen printing, pad printing or intaglio printing. Specific examples of suitable or favoured printing apparatuses and techniques of the above types that may be applied to this stage of the practising of embodiments the present invention are well known and widely used in the art for applying various types of liquid composition to substrates of various kinds in the making of various multi-layer optical elements or like products.

In such embodiments, the curable/dryable covering composition which is applied to the xerogel layer may therefore have a viscosity that renders it especially suitable for application by printing. Suitable such viscosities of the curable/dryable covering composition may for example -especially in the case of e.g. flexo printing -be in the approximate range of from about 1 up to about 20 mPa.s (at room temperature, i.e. 20°C). These viscosities may be especially suitable in the case of flexographic printing, but other printing techniques may possibly require somewhat different optimum viscosities within their own respective favoured ranges. As or where required, the viscosity of any proposed printable curable/dryable covering composition may be tailored, or modified or adjusted by appropriate dilution of the composition in question, such as by addition of an appropriate amount of extra water or an additional amount of an alcohol already present -all of which viscosity-adjustment techniques are well understood by persons skilled in the art. Thus, in practice, an optimum viscosity of a prepared curable/dryable covering composition ready for application to the xerogel layer by printing may be attained by any suitable and appropriate adjustment of the relative amount(s) of one or more of its constituents, especially any solvent species therein, as the skilled person will readily know or be able to deduce.

In some such embodiments of the method, the application and curing/drying of the at least one layer of a curable/dryable covering composition may involve either (i) the applying over the xerogel intermediate layer of just one layer only of the curable/dryable covering composition, and then curing or drying (as appropriate) that layer to form a single cured or dried (as the case may be) covering layer over the xerogel intermediate layer, or (U) the applying over the xerogel intermediate layer of a plurality of layers of the curable/dryable covering composition in a sequential manner, with each such layer of the curable/dryable covering composition being cured or dried (as appropriate) immediately after its application, to form a respective cured or dried (as the case may be) covering layer over the xerogel intermediate layer, before the next such layer of the curable/dryable covering composition is applied. Thus, in the latter case of a plurality of such covering layers being applied and cured/dried, an overall multi-layer covering arrangement may be built up to any desired total thickness and/or configuration over the applied xerogel layer therebeneath.

Accordingly, in many practical embodiments of the method, the or each layer of the curable/dryable covering composition which is/are to form the final covering layer(s) may be applied, prior to any curing/drying, in an individual and/or a collective thickness in the approximate range of from about 10 or 20 or 30 nm up to about 100 or 200 or 300 or 500 or 1000 nm, or even up to as much as about 1.5 or 2 or 3 or 4 or 5 or 10 or 15 or 20 or 30 or 40 or 50 or 100 pm (especially any of the latter group of maximum thickness values in the case of a single such covering layer).

In some embodiments the or each layer of the curable/dryable covering composition which is/are to form the final covering layer(s) may be applied over the xerogel intermediate layer such as to have a thickness at any given point or region on the surface thereof which is at least as much as the thickness of the xerogel layer at that point or region. Alternatively, the or each layer of the curable/dryable covering composition which is/are to form the final covering layer(s) may be applied over the xerogel intermediate layer such as to have a thickness at any given point or region on the surface thereof which is substantially greater than, e.g. greater by at least 5 or 10 or 20 or 30 or 40 or 50 or 60 or 70 or 80 or 90 or 100 % of, the thickness of the xerogel layer at that point or region.

The thickness of the or each layer of the curable/dryable covering composition which is/are to form the final covering layer(s) may be approximately or substantially uniform over the area or areas of the xerogel layer over which it is applied, or its thickness may vary thereover.

In particular, in some embodiment forms the applied and cured/dried covering layer or layers may form a complete overall covering layer over the xerogel intermediate layer which either: (i) substantially duplicates, copies or is geometrically similar to the surface profile shape and/or configuration of the xerogel layer therebeneath, and/or the overall thickness of the complete overall covering layer is approximately or substantially uniform over the area or areas of the xerogel layer over which it is applied, or (ii) varies in its thickness and substantially fills in the valleys, troughs, grooves, indentations or inwardly-extending spaces, gaps or voids in the xerogel layer therebeneath, and presents a substantially flat or planar final exposed face to the final cured covering layer(s) arrangement on the opposite facial side of the final optical security element from the substrate.

In the application of the or each layer of the curable/dryable covering composition, it may be cured or dried (as appropriate), or its curing/drying may be initiated, by the application of any appropriate means effective to cause that curing or drying (as appropriate) of the composition in question. Thus, on the one hand, in some such embodiments of the invention, a curable covering composition may be UV-curable, i.e. curable by the application of, or exposure of the respective applied layer to, ultra-violet light, especially either during or immediately (or shortly) after its application. Any suitable known UV light source or apparatus may be used for this purpose. Alternatively in other such embodiments, some other means of curing, or initiation of curing, may be used, e.g. heat. On the other hand, in some other such embodiments of the invention, a dryable covering composition may be dryable by the simple application of heat, or application of an ambient or warmed or heated gaseous atmosphere (e.g. air or nitrogen), possibly at reduced-pressure, to the composition once it has been applied.

Thus, in many practical embodiments of the invention the covering composition may comprise a curable covering composition, which may for example be provided and applied in the form of a curable precursor composition for forming a varnish or lacquer upon its curing. Such a varnish or lacquer may in particular be a substantially light-transparent varnish or lacquer. Thus, in many embodiments the final cured covering layer(s) formed over the xerogel intermediate layer may be substantially transparent to light with which the optical security element is designed to be usable.

In many embodiments of the invention, the applied -i.e. the cured or dried, or even simply laminated (in such simper embodiments) -material forming the or each protective covering layer may typically have a refractive index in the approximate range of from about 1 or 1.1 up to about 1.3, especially a refractive index in the region of around 1.2. Such a relatively low refractive index of the final protective covering layer may be especially useful for achieving higher differences between the refractive indices of the various layers of the overall optical element structure, thereby potentially making it easier to obtain improved, brighter optical functions of the optical security element.

In practical embodiments of the method, the curable/dryable covering composition may be applied in its one or more layers over the xerogel intermediate layer so as to substantially completely cover the xerogel intermediate layer therebeneath, i.e. to substantially completely cover all the various portions or regions of the xerogel intermediate layer wherever they may be located on the substrate surface. In other words, in practical embodiments of the method, the covering composition which is applied to form the final covering layer(s) may be applied to the already-formed xerogel intermediate layer over at least substantially the whole area of that xerogel intermediate layer, whereby substantially the whole of, or substantially all of the various portions or regions of, that xerogel intermediate layer is/are covered by the final cured/dried covering composition layer(s) once it/they has/have been finally cured/dried. This is to ensure that substantially no portions or regions of the xerogel intermediate layer are left uncovered by the cured/dried covering composition layer(s).

Of course, in certain practical embodiments it may be that the applied curable/dryable covering composition layer(s) extend(s) over an overall area, or over an overall collection of a plurality of individual areas, that is or are collectively greater in area extent than that/those area(s) of the substrate having the xerogel layer formed thereon. In this case, therefore, one or more portions or regions (especially minor portions/regions) of the final cured/dried covering layer(s) may extend such as to contact or overlap with one or more portions or regions (especially minor portions/regions) of the underlying optical relief on the substrate surface which does/do not have part of the xerogel layer formed thereon.

Thus, in various practical embodiments, the finally cured/dried covering layer(s) may be applied to the xerogel layer in either (i) a corresponding pattern or design to the xerogel layer, possibly a corresponding complex pattern or design comprising a corresponding desired number of portions in a corresponding desired relative physical or geometrical arrangement or configuration, or Oft a more generalised pattern or design or configured region(s) that may simply encompass the area extent of, or surround the maximum overall perimeter of, the pattern or design of the xerogel layer.

In putting embodiments of the present invention to practical use, the finally produced optical security element may be applied or affixed to a surface of, or may be incorporated into the structure of, a wide variety of documents or articles whose security, identification, authentication or protection is wanted or desirable. For example, such documents or articles, to a surface of which may be applied or affixed, or into or within whose structure may be incorporated, one or more optical security elements according to the invention or any embodiment thereof, may include any of the following: ID (identification) cards, passports, visas, driving licences, credit and debit cards, banknotes, securities, certificates, tickets, branded goods, and various other items whose value or nature benefits from an added security feature.

For the purpose of applying or affixing such one or more finally produced optical security element(s) to a surface of, or incorporating same into the structure of, such a document or article, any appropriate further step(s) involving known lamination or covering techniques and materials may be employed in order to produce the finished article.

Embodiments of the present invention may thus be effective to ameliorate or at least partially solve the above-discussed disadvantages of known multi-layer optical security elements which rely on standard vacuum deposition procedures to form their principal characteristic layers. The present invention does this by the innovative use of particular layer-forming compositions -especially which may be applied by procedures other than vacuum deposition, e.g. printing -which enable the efficient creation of required layers imparting high refractive index and also protective/covering ability, but with reduced or minimized risks of damage or other deleterious effects imparted to one or more intermediate formed layer(s) of the optical security element structure as the overall multi-layer manufacturing process progresses.

Within the scope of this specification it is envisaged that the various aspects, embodiments, examples, features and alternatives, and in particular the individual constructional or operational features thereof, set out in the preceding paragraphs, in the claims and/or in the following description and accompanying drawings, may be taken independently or in any combination of any number of same. For example, individual features described in connection with one particular embodiment are applicable to all embodiments, unless expressly stated otherwise or such features are incompatible.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the present invention in its various aspects, including in conjunction with one or more specific practical Examples, will now be described in detail, by way of example only, with reference to the accompanying drawings, in which: FIGURE 1 is a schematic cross-sectional view of the form an intermediate treated substrate takes in a first stage in the manufacture of an optical security element according to an embodiment of the invention, showing the formation of a xerogel intermediate layer on a substrate surface pre-formed with a diffractive optical relief pattern thereon; FIGURE 2 is a schematic cross-sectional view of a first embodiment of an optical security element (especially a DOVID) according to the invention, in which a final covering layer has been formed over the xerogel intermediate layer and mirroring the contour thereof so as to form a correspondingly shaped upper exposed facial surface of the finished optical security element; FIGURE 3 is a schematic cross-sectional view of a second embodiment of an optical security element (especially a DOVID) according to the invention, in which a final covering layer has been formed overthe xerogel intermediate layer to a substantially greater thickness than that of the xerogel layer and having a planar upper exposed surface forming the final upper exposed facial surface of the finished optical security element; FIGURE 4 is a face-on plan view of part of an example embodiment form of optical security element (especially a DOVID) according to another embodiment of the invention, showing an example of the kind of highly complex pattern that is possible to produce in the applications of the diffractive optical relief to the substrate and the overlying formed xerogel and final covering layers thereon, for the purpose of imparting the required security character to a document or object on whose surface or in whose structure the optical security element is applied or affixed.

DETAILED DESCRIPTION OF EMBODIMENTS

Referring firstly to FIG. 1, here there is shown schematically in cross-section an intermediate product 1 resulting from a first stage in the manufacture of an optical security element according to one or more embodiments of the invention. The final optical security element is shown as 2 in FIG. 2 and as 3 in FIG. 3, each of these optical security elements 2, 3 representing an independent embodiment form thereof within the scope of the invention. In the intermediate product 1 a substrate 10 has diffractive optically active relief 20 formed in an upper facial surface 14 thereof, and has been printed with a sol composition which has then been dried to form an intermediate layer 30 in the form of a xerogel.

The substrate 10 is a body, e.g. a sheet, film, foil, strip or plate with a thickness of e.g. around 50 to 100 pm, of polycarbonate or polymethyl methacrylate (PMMA), examples of which substantially transparent polymeric materials are widely commercially available in the art.

Thus, the substrate material has a refractive index of approximately 1.5. The upper facial surface 14 of the substrate 10 has a diffractive, or at least partially diffractive, optical relief pattern 20 formed therein, e.g. by a well-known embossing technique, in which a "master" relief embossing tool or plate is pressed directly into the substrate surface 14 to transfer the relief pattern 20 directly thereto, or alternatively by using a well-known nano-imprinting lithography technique to imprint the relief pattern 20 into the exposed surface layer of the substrate material at the upper surface 14 thereof.

Formed over the optical relief pattern 20 in the exposed surface layer 14 of the substrate 10 is an intermediate layer 30 of a high refractive index xerogel material, which is formed by the application over the optical relief pattern 20 in the substrate's exposed surface 14 of a layer of a precursor composition in the form of a sol containing nanoparticles of an inorganic high refractive index material, e.g. nanoparticles of TiO2, and the subsequent drying of that applied sol layer. The high refractive index material in the applied sol composition is TiO2, which typically has a refractive index of approximately 1.6, with an average particle size of approximately 16 nm, although particle sizes anywhere in the approximate range of about 5 to about 100 nm may be especially favoured.

In principle the sol composition which is applied to the substrate surface 14 to form the xerogel intermediate layer 30 may be formulated and provided or presented for use/application in various forms, according to various prototype systems representing different embodiments within the scope of the invention.

For instance, in a somewhat basic/simple prototype embodiment form, the various liquid components of the sol composition -namely one or more solvents as described hereinabove (forming the continuous phase) together with an initiator and optionally a polymerisable monomer (if the system is to employ same), also both as described hereinabove -may be simply mixed together along with the appropriate quantity of the appropriate TiO2 nanoparticles (as a dry commercially available product), and physically processed (e.g. by mixing and stirring or the like) according to known techniques and using known apparatuses, to form the required sol composition. Such a mixing preparation could be carried out just before or shortly before, e.g. up to a few minutes or a few hours before, being presented ready for use for application to the substrate. For use in this somewhat basic/simple prototype embodiment form, the TiO2 nanoparticles component could be e.g. amorphous crystalline TiO2 (from any of various commercial suppliers) or, possibly more desirably, TiO2 nanoparticles prepared from a TiOSO4 precursor, such as is described in any of the following literature references: https://www. resea rchg ate. net/publication/317751307 Sol-gel synthesis of TiO2 from TiOSO4 characterization and UV photocatalvtic activity fo r the degradation of 4-chlorophenol https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7912831/ https://www.semanticscholar. org/paper/Solc/oE2%80%93gel-synthesis-of-Ti02-from-TiOSO4%3A-and-UV-for-Kh anfic27c623896f3b4da89d214a1378e43cb571148b). The solvent(s) (forming the continuous phase) and the initiator component, as well as any polymerisable monomer present (if used), could each be any of those mentioned and described hereinabove.

However, in a more advanced prototype embodiment form, which for many practical embodiments may be more suitable and favourable to employ, the sol composition may be prepared and provided/presented for use by employing a two-part system. In such a two-part system a first part (termed "Part A") comprises a pre-prepared sol composition comprising all the components of the final sol composition to be applied to the substrate but with the exception of a cross-linking and/or polymerisation initiator component, and a second part (termed "Part B") comprises the cross-linking and/or polymerisation initiator component.

Those parts A and B of such a two-part system may be mixed shortly before (e.g. a few minutes or up to a few hours before) application to the substrate.

By way of example, two prototype two-part sol composition systems -each of a slightly different type -which may be suitable for practising embodiments of the invention as illustrated in the above-described drawings, are described in further detail in the Examples below:

Example 1:

This Example demonstrates the use of a two-part sol composition system based on the use of TiO2 nanoparticles in ethanol (as the primary solvent forming the continuous phase), with the TiO2 nanoparticles being derived from a TiOSO4 precursor.

Part A: This is a sol composition prepared by conventional mixing and stirring of its various components (according to well-known techniques in the art) comprising the following components (with the % values being % by weight of the Part A composition alone): (i) TiO2 nanoparticles (particle size approx. 16 nm) -5%.

-Prepared from a TiOSO4 precursor, as described in any of the following literature references: https://www.researchqate.net/publication/317751307 Sol-gel synthesis of TiO2 from TiOSO4 characterization and UV photocatal ytic activity for the degradation of 4-chlorophenol https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7912831/ https://www.semanticscholarorg/paper/Sol%E2%80%93qel-synthesis-of- Ti02-from-TiOSO4%3A-and-UV-for-Khan/fc27c623896f3b4da89d214a1378e43cb57114 8b).

(ii) Mixture of solvents (together forming the continuous phase), comprising: Ethanol -20 % Isopropanol -15 % Isobutanol -15 % Glycerol -2 % Water -balance (to 100 %) In some possible modifications to the above Part A composition, but still workable in accordance with these embodiments of the invention, it may be possible to vary the % amount of the TiO2 nanoparticles to lie anywhere in the approximate range of from -1 up to -10 %, and it may be possible to vary the particle size of the TiO2 nanoparticles to lie anywhere in the approximate range of from -1 nm up to -100 nm. It may also be possible to reduce the % amounts of each of the solvent components by up to about 10 or 20 or 30 or 40 or 50 % of the above values, which above-stated values may in many practical example systems represent reasonable practical upper limits on the % amounts of the various solvent (continuous phase) components that may be used to good effect.

Part B: Trithiocyanuric acid.

The above trithiocyanuric acid ("TTCA") Part B component acts as an initiator for the cross-linking (or "self-polymerisation") of the TiO2 nanoparticles in the forming of the dried xerogel layer, once the complete sol composition has been applied to the substrate and dried. The TTCA acts as a auto-catalyst in the one-step polymerisation of the trithiocyanuric acid (TTCA) to polythiocyanuric acid (PTCA), resulting in PTCA-functionalisation of the edge sites of the TiO2 nanoparticles to form a continuous, polymeric network of those particles in the finished xerogel layer.

Final sol composition: For forming the final complete sol composition which is applied to the subtrate, Part A and Part B are then mixed in a weight ratio of approx. 1000g A: 2.5g B. The mixed final complete sol composition should then be used within about 4 hours.

Application of sol composition to the substrate and its subsequent processing: This "ink" (i.e. complete two-part sol composition with incorporated TiO2 nanoparticles) can be printed on the substrate surface 14 very suitably by a conventional flexographic printing technique, using conventional flexographic printing apparatus. If necessary its viscosity can be adjusted by dilution with an appropriate amount of extra ethanol. Once the sol composition has been printed on the required regions or areas of the substrate surface 14, it is then dried using hot air -by passing the substrate through a 70°C drying tunnel (it is not necessary to use any UV or other curing/cross-linking step in this Example.

Example 2:

This Example demonstrates the use of a slightly different two-part sol composition system, still based on the use of TiO2 nanoparticles in ethanol (as the primary solvent forming the continuous phase) and with the TiO2 nanoparticles being derived from a TiOSO4 precursor, but this time including a polymerisable monomer in the sol composition.

Part A: This is the same as Part A of Example 1, but with the addition (in an amount of up to about 10 % (or, in a variation, possibly up to about 5 or 8 % instead)) of an acrylic or urethane-forming monomer -many possible specific examples of which are available to use in the art.

Part B: Trithiocyanuric acid (as Part B of Example 1).

Final sol composition: For forming the final complete sol composition which is applied to the subtrate, Part A and Part B are then mixed in a weight ratio of approx. 1000g A: 2.5g B (as per Example 1).

Application of sol composition to the substrate and its subsequent processing: This "ink" (i.e. complete two-part sol composition with incorporated TiO2 nanoparticles and polymerisable monomer) can be printed on the substrate surface 14 very suitably by a conventional flexographic printing technique, using conventional flexographic printing apparatus. If necessary its viscosity can be adjusted by dilution with an appropriate amount of extra ethanol. Once the sol composition has been printed on the required regions or areas of the substrate surface 14, it is then dried and fixed/hardened using a two-stage drying process: firstly, by drying the applied sol composition using hot air, by passing the substrate through a 70°C drying tunnel; and secondly, by subjecting the thus-dried printed xerogel-forming layer to a curing/cross-linking, i.e. hardening, step by exposure to UV light from a UV lamp (according to well-known techniques and using conventional apparatus for UV-hardening of polymerized optical element layers in the present art).

An advantage of this two-part system of Example 2 using the additional polymerisable monomer component in the applied sol composition is that the finally produced xerogel layer may be better fixed between chains of the acrylic/polyurethane polymer network/matrix, leading to the xerogel layer being stronger and more stable within its own structure, as well as being more stably united with the underlying substrate polymer surface 14.

In both Examples 1 and 2 the resulting fully dried (and optionally fixed/hardened) xerogel layer 30 may typically have a refractive index of approximately 1.9.

Thus, at this stage in the case of both Examples 1 and 2, the substrate 10 + dried xerogel layer 30 has the appearance of the intermediate product 1 as shown schematically in FIG. 1.

The next stage in the procedure towards the finally prepared optical security element in each of the Examples was the applying over the xerogel layer 30 of at least one layer of a e.g. curable protective covering composition, and then curing the or each curable protective covering composition layer to form at least one cured protective covering layer 40, 50 (FIGS. 2 & 3) over the xerogel layer 30. The or each curable protective covering composition layer is applied over the xerogel layer 30 by printing, especially using the same technique and using the same type of apparatus (i.e. flexographic printing) as the nanoparticles-containing sol composition is applied to the substrate surface 14 in the earlier step of the method for the formation of the xerogel layer 30 itself.

The or each curable protective covering composition layer may be any of various lacquers, varnishes or inks, many examples of which are already known in the art and widely commercially available. Examples of suitable known such lacquers, varnishes or inks include those available from companies such as BASF, Akzo-Nobel, lnkron, Pixelligent, Azteca, and possibly also others.

Thus, the applied layer(s) of the curable protective covering composition is/are applied over the xerogel layer 30 such as to have a thickness at any given point or region on the surface thereof which is at least as much as the thickness of the xerogel layer 30 at that point or region, and preferably even of somewhat greater thickness than that of the xerogel layer 30 at that point or region, e.g. greater by at least 5 or 10 or 20 or 30 or 40 or 50 % of the thickness of the xerogel layer at that point or region.

In some embodiment forms -such as the optical security element product 2 illustrated in FIG. 2 -the curable protective covering composition is applied over the xerogel layer 30 in a layer (especially a single layer) with a substantially uniform thickness over the area or areas of the xerogel layer 30 over which it is applied, in order to substantially duplicate, copy or be geometrically similar to the surface profile shape and configuration of the xerogel layer 30 therebeneath. Alternatively or additionally, the overall thickness of the finally cured protective covering layer 40 is substantially uniform over the area or areas of the xerogel layer 30 over which it is applied.

However, in some other embodiment forms -such as the optical security element product 3 illustrated in FIG. 3 -the curable protective covering composition is applied over the xerogel layer 30 in one or more layers which collectively are such as to form an overall cured protective covering layer 50 arrangement over the xerogel layer 30 which varies in its thickness and substantially fills in the valleys, troughs, grooves, indentations or inwardly-extending spaces, gaps or voids in the xerogel layer 30 therebeneath, and presents a substantially flat or planar final exposed upper face 60 to the final cured protective covering layer arrangement 50 on the opposite facial side of the final optical security element 3 from the substrate 10.

FIG. 4 shows in face-on plan view part of an example embodiment form of optical security element 4 (especially a DOVID) according to various embodiments of the invention -such as either of the embodiment forms shown in principle in either of FIGS. 2 and 3 -showing an example of the kind of highly complex pattern that is possible to produce in the applications of the diffractive optical relief 20 to the substrate 10 and the overlying formed xerogel 30 and final protective covering layers 40, 50 thereon, for the purpose of imparting the required security character to a document or object on whose surface or in whose structure the optical security element 4 is applied or affixed.

Throughout the description and claims of this specification, the words "comprise" and "contain" and linguistic variations of those words, for example "comprising" and "comprises", mean "including but not limited to", and are not intended to (and do not) exclude other moieties, additives, components, elements, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless expressly stated otherwise or the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless expressly stated otherwise or the context requires otherwise.

Throughout the description and claims of this specification, features, components, elements, integers, characteristics, properties, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith or expressly stated otherwise.

Claims

CLAIMS1. A method for the manufacture of an optical security element, the method comprising: providing a substrate having diffractive optically active relief formed on or applied to a surface thereof; applying to said surface of the substrate a layer of a composition comprising particles of a material having a refractive index different from the refractive index of the material of the substrate, the composition being applied in the form of a sol; drying the applied sol layer to form a xerogel intermediate layer comprising the said different refractive index material; and applying over the xerogel intermediate layer at least one covering layer for protecting the surface optical relief on the substrate against subsequent mechanical damage.
2. A method according to claim 1, wherein the application of the at least one covering layer over the xerogel intermediate layer comprises applying, by a step involving lamination, over the xerogel intermediate layer at least one pre-prepared laminating covering layer in the form of a pre-existing laminating foil, film or sheet.
3. A method according to claim 1, wherein the application of the at least one covering layer over the xerogel intermediate layer comprises applying over the xerogel intermediate layer at least one covering layer of a curable or dryable covering composition, and curing or drying the or each curable or dryable covering composition layer to form at least one cured or dried protective covering layer over the xerogel intermediate layer.
4. A method according to claim 3, wherein the or each covering composition layer applied over the xerogel intermediate layer is a curable covering composition, or a precursor of a curable covering composition, and the method includes a step of applying that covering composition (or precursor thereof) over the xerogel intermediate layer and then curing it to form the at least one cured protective covering layer over the xerogel intermediate layer.
5. A method according to claim 3, wherein the or each covering composition layer applied over the xerogel intermediate layer is a dryable covering composition in the form of a sol, and the method includes a step of applying that sol covering composition over the xerogel intermediate layer and then drying it to form the at least one dried protective covering layer over the xerogel intermediate layer.
6. A method according to claim 5, wherein the dryable sol covering composition is either: (i) of substantially the same basic composition, in terms of its components and their relative amounts present therein -with the exception of and excluding the particulate different refractive index material -as that used initially to form the xerogel intermediate layer itself, or (ii) of a substantially different basic composition, in terms of its components and their relative amounts present therein -with the exception of and excluding the particulate different refractive index material -from that used initially to form the xerogel intermediate layer itself, whereby the resulting dried protective covering layer is or forms a protective second xerogel covering layer whose composition is either the same as or different from that of the xerogel intermediate layer -with the exception of and excluding its particulate different refractive index material.
7. A method according to any preceding claim, wherein the complete optical security element is: either (i) substantially transmissive, or at least partially transmissive, to light incident thereon, or (ii) substantially reflective, or at least partially reflective, to light incident thereon.
8. A method according to any preceding claim, wherein the substrate material: (i) has a refractive index in the range of from about 1.4 to about 1.6; and/or 00 takes the form of a sheet, film, foil, strip or plate having a thickness in the range of from about 1 or 5 or 10 pm up to about 200 or 250 or 300 or 400 or 500 pm, especially in the range of from about 10 or 20 or 50 pm up to about 100 pm.
9. A method according to any preceding claim, wherein the diffractive optically active relief formed on or applied to the said surface of the substrate comprises one or more regions with either the same or different optically active relief patterns or structures applied to the substrate surface; optionally wherein the diffractive optically active relief is either fully or partially diffractive in its optical function; and further optionally wherein the relief features of which the relief is composed have dimensions represented by: widths and/or heights of individual surface relief features from about 0.001 up to about 70 or 80 or 90 pm, optionally from about 0.001 up to about 50 or 60 or 70 pm, and/or depths of less than about 10 pm, optionally less than about 5 pm.
10. A method according to any preceding claim, wherein the particulate material having the different refractive index from that of the substrate material has a refractive index higher than that of the substrate material by at least about 0.1 or 0.2 or 0.3.
11. A method according to claim 10, wherein the particles comprise or are composed of a material with a refractive index in the range of from about 1.6 or 1.7 or 1.8 up to about 2.5 or 2.6; optionally wherein the particles are composed of or comprise an inorganic material being a metal (optionally Al or Cu) or a metal oxide (optionally TiO2).
12. A method according to any preceding claim, wherein the average particle size of the particles of the material having the different refractive index from that of the substrate material is in the range of from about 5 up to about 100 or 150 nm, optionally in the range of from about 10 up to about 20 or 30 or 40 or 50 nm.
13. A method according to any preceding claim, wherein in the sol composition applied to the substrate a continuous liquid or solvent phase of the sol comprised one or more liquid substances selected from any of the following: alcohols; ketones; water; mixtures of any of the aforesaid substances.
14. A method according to any preceding claim, wherein the sol composition applied to the substrate is formulated such that the relative proportions by weight of the particles phase to the continuous liquid or solvent phase are in the weight ratio range of from about 1:99 up to about 10:90, optionally up to about 20:80 or 25:75.
15. A method according to any preceding claim, wherein the sol composition applied to the substrate is formulated such that cross-linking and/or polymerisation of one or more of its components is effectable upon its drying through a process comprising either or both of surface-functionalisation of the particles themselves so that they themselves then form a continuous network of cross-linked such particles, and/or (ii) polymerisation (with optional cross-linking) of one or more polymerisable monomers which is/are additionally included in the composition so as to form a polymerised network or matrix structure which incorporates the particles therewithin in the finished dried xerogel intermediate layer.
16. A method according to claim 15, wherein in either case (i) and/or (ii) the sol composition which is applied to the substrate surface additionally comprises at least one cross-linking and/or polymerisation initiator.
17. A method according to claim 16, wherein the sol composition applied to the substrate is provided or presented for use, ready for application to the substrate surface, as a two-part system, wherein a first part ("part A") thereof comprises a pre-prepared sol composition comprising all the components of the final sol composition to be applied to the substrate but with the exception of the cross-linking and/or polymerisation initiator component, and a second part ("part B") thereof comprises the cross-linking and/or polymerisation initiator component.
18. A method according to any preceding claim, wherein the composition in the form of the particles-containing sol is applied to the substrate surface by a printing step; optionally wherein the sol composition has a viscosity in the range of from about 1 up to about 20 mPa.s (at 20°C).
19. A method according to any preceding claim, wherein the composition in the form of the particles-containing sol is applied to the substrate surface to form an initial layer thereof, prior to any drying, with an average thickness in the range of from about 10 or 20 or 30 nm up to about 100 or 200 or 300 or 500 or 1000 nm.
20. A method according to any preceding claim, wherein the composition in the form of the particles-containing sol is applied to the substrate surface: either (i) substantially over the whole area of that surface, or 00 in just one or more portions or regions of that surface which are collectively less than the whole of the surface, wherein, when there are plural such portions/regions having the composition applied thereto, at least one or more of those portions/regions is/are either separate from the remainder thereof or integral or joined with or connected to one or more other portions/regions thereof.
21. A method according to any preceding claim, wherein the drying step is followed by an additional step of fixing or stabilising the formed xerogel intermediate layer by curing or cross-linking (or further curing or further cross-linking) of the solid material itself of the formed xerogel layer or by polymerisation of a monomer component additionally included initially in the particles-containing sol composition which is applied initially to the substrate surface.
22. A method according to any preceding claim, wherein once the drying step (and any subsequent fixing/stabilising step, if used) has/have been carried out on the particles-containing sol composition which has been applied to the substrate surface, the resulting xerogel intermediate layer has a refractive index in the range of from about 1.7 to about 2.2, optionally from about 1.9 or 1.95 to about 2.0 or 2.05.
23. A method according to any preceding claim, as dependent through claim 3, wherein the or each layer of the curable/dryable covering composition which is/are to form the final covering layer(s) is applied, prior to any curing/drying, in an individual and/or a collective thickness in the range of from about 10 or 20 or 30 nm up to about 100 or 200 or 300 or 500 or 1000 nm.
24. A method according to any preceding claim, as dependent through claim 3, wherein: either (i) the or each layer of the curable/dryable covering composition which is/are to form the final covering layer(s) is applied over the xerogel intermediate layer such as to have a thickness at any given point or region on the surface thereof which is at least as much as the thickness of the xerogel layer at that point or region; or (ii) the or each layer of the curable/dryable covering composition which is/are to form the final covering layer(s) is applied over the xerogel intermediate layer such as to have a thickness at any given point or region on the surface thereof which is greater than, by at least 5 or 10 or 20 or 30 or 40 or 50 or 60 or 70 or 80 or 90 or 100 % of, the thickness of the xerogel layer at that point or region.
25. A method according to any preceding claim, as dependent through claim 3, wherein the thickness of the or each layer of the curable/dryable covering composition which is/are to form the final covering layer(s): either (a) is substantially uniform over the area or areas of the xerogel intermediate layer over which it is applied; or (b) varies thereover; optionally wherein the applied and cured/dried covering layer(s) form(s) a complete overall covering layer over the xerogel intermediate layer which either: (i) substantially duplicates, copies or is geometrically similar to the surface profile shape and/or configuration of the xerogel layer therebeneath, and/or the overall thickness of the complete overall covering layer is substantially uniform over the area or areas of the xerogel intermediate layer over which it is applied, or (ii) varies in its thickness and substantially fills in the valleys, troughs, grooves, indentations or inwardly-extending spaces, gaps or voids in the xerogel intermediate layer therebeneath, and presents a substantially flat or planar final exposed face to the final cured covering layer(s) arrangement on the opposite facial side of the final optical security element from the substrate.
26. A method according to any preceding claim, wherein the applied material forming the or each protective covering layer has a refractive index in the range of from about 1 or 1.1 up to about 1.3, optionally a refractive index in the region of around 1.2.
27. An optical security element comprising: a substrate having a surface with diffractive optically active relief formed thereon or applied thereto; an intermediate layer formed on the substrate surface, the intermediate layer being in the form of a xerogel and comprising a material having a refractive index different from the refractive index of the material of the substrate; and at least one covering layer formed over the xerogel intermediate layer for protecting the surface optical relief on the substrate against mechanical damage.
28. An optical security element according to claim 27, which is an optical security element manufactured according to the method of any one of claims 1 to 26.
29. A document or article including or having applied thereto one or more optical security elements each being an optical security element according to claim 27 or claim 28.
30. A document or article according to claim 29, which is selected from any of the following: an ID (identification) card, passport, visa, driving licence, credit or debit card, banknote, security, certificate, ticket, other ID or security document, or item of merchandise.