PROTECTION OF SURFACE RELIEF STRUCTURES

FIELD OF THE INVENTION

The present invention relates to high refractive index coatings comprising metal oxide particles, to methods of their manufacture and to their use in the protection of surface relief structures. The invention is particularly, but not exclusively, applicable to the field of security devices and documents and tokens incorporating security devices.

BACKGROUND TO THE INVENTION

Relief structures, such as printed or stamped diffractive optical surface structures and similar security features, are exposed to counterfeiting by mechanical lifting when the structure is not coated and therefore exposed. The coating required to protect such structures must be of a sufficiently high refractive index compared to the substrate to which the feature is applied to maintain visibility of the feature. Such high refractive index (HRI) coatings, used for relief structures applied to a variety of substrates, should preserve all or as much of the original, intended colour, clarity and visibility of the non-coated feature as possible. Therefore, high transparency and low (or no) colour are important requirements if a diffractive effect, which is visible both in reflection and transmission, is to be achieved, especially for relief structures applied to clear substrates. To date, these requirements have been difficult to fulfil by a printed coating where good adhesion, chemical resistance and high durability are required, whilst also rendering the relief structure resistant to mechanical lifting.

Traditionally, HRI coatings based on titanium dioxide (titania) or zinc sulfide or zinc selenide have been deposited onto a surface by vacuum deposition, a technique which requires high temperatures and is expensive. Other sputtering techniques of metal layer deposition may not be easily adapted to high speed printing. Furthermore, some surface structures require a layer of high refractive index coating that is of the order of microns thick. Metallised coatings are unsuitable, as thicker coatings are expensive and can impart a highly reflective surface with noticeable colour in the coating. Thicker metallised coatings are also less robust. High refractive index coatings made of metal- containing polymer are known, but utilisation requires coating and subsequent curing at high temperature to provide a metallic coating on the substrate (Wang et al, Proceedings of SPIE, vol. 5724, 2005). This technique requires a processing temperature exceeding 100°C and often up to 200 °C, which is too high for use with a polymer substrate, and is not amenable to reel-to-reel printing processes.

Metal dioxide particles of various sizes employed in suspension or other liquid formulations are known in HRI coating technology mainly for application in electronic displays. Clarity of the coating can be improved by reducing the coat weight. However this can reduce the effectiveness of protecting the structure on certain relief structures. Metal dioxide nanopowders, in which the particles are coated with various functional groups, may be dispersed in a suitable solvent and utilised as a coating formulation. However, these materials can suffer from poor adhesion to polymer substrates in the absence of suitable additives.

The conventional method of utilising high refractive metal oxides is as an additive, mixed with a resin or carrier. This may improve adhesion; however, it also leads to a reduction in refractive index, depending on the ratio of metal oxide to resin employed. On embossed substrates, such formulations based on titanium dioxide alone yield coatings with very poor adhesion and low transparency. Adhesion can be improved by increasing the amount of resin, but refractive index is then reduced.

Whilst high refractive index polymers containing halogens are available, they are undesirable from cost and environmental standpoints.

There is therefore a need for a high refractive index coating that provides a durable coating which is resistant to mechanical lifting, transparent, of high adhesion, whilst at the same time being simple to employ.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a coating for use in the protection of a surface relief structure, said coating having a refractive index greater than the refractive index of the surface relief structure, said coating comprising particles of at least one metal oxide. In some embodiments, the coating may additionally comprise at least one transparent varnish. By varnish it is meant a material that results in a durable protective finish on and/or within the coating. It has been advantageously found that when the coating is applied to a substrate and the solvent removed, the presence of a transparent varnish provides enhanced adhesion of the coating to the substrate. Use of a transparent varnish also improves transparency of the coating. Surprisingly, it has been found that, in cases where the coating has a slight colouration, the use of a transparent varnish reduces this colour. Exemplary transparent varnishes are, but are not limited to, nitrocellulose and cellulose acetyl butyrate.

Examples of surface relief structures include, but are not limited to, printed or stamped surface relief structures.

According to another aspect of the invention, there is provided a coating for use in the protection of a surface relief structure, said coating forming all or part of the surface relief structure, said coating comprising particles of at least one metal oxide.

According to a further aspect of the present invention, there is provided a dispersion comprising particles of one or more metal oxides, at least one organic solvent and at least one transparent varnish. In one embodiment of this aspect of the invention, the dispersion provides a coating for use in the protection of a surface relief structure. In a further embodiment, the coating may itself form all or part of the surface relief structure. Exemplary transparent varnishes are, but are not limited to, nitrocellulose and cellulose acetyl butyrate.

In a preferred embodiment of any of the aforementioned aspects of the invention, the surface relief structure is a diffractive optical surface structure. The surface structure may be printed or stamped or may be formed by any other method known in the art. Advantageously, the refractive index of the coatings is sufficiently greater than that of the surface structure, so as to preserve the diffractive effect of the diffractive optical surface structure. Exemplary diffractive optical surface structures that may benefit from the coating compositions of the present invention include, but are not limited to, diffraction gratings, holograms and diffractive optical elements. In a further embodiment of any of the aforementioned aspects of the invention, the surface relief structure is a non-diffractive structure, examples of which are a sub wavelength structure, a lens, a microlens array or a waveguide.

It will be appreciated that the skilled artisan would be able to select a particular coating that matches the requirements of the surface relief structure in question. Preferably the concentration of metal particles is sufficiently high so as to produce a refractive index shift, so that the optical effect of the surface relief structure is substantially unaffected or even enhanced.

In a yet further embodiment, the coating of the present invention has a refractive index of greater than 1 .0, preferably greater than 1 .4, more preferably greater than 1 .7, even more preferably greater than 1 .8.

Preferably, the primary crystallite size of the metal oxide particles is between 1 and 100 nm. More preferably, the primary crystallite size of the metal oxide particles is between 5 and 25 nm.

Advantageously, the coatings of the present invention when utilised as an ink may be printed as is, without the requirement of a resin or carrier. When employed as a protective layer over a relief structure, the coatings of the present invention offer high transparency and low colour and are resistant to mechanical lifting. The optical effect of the surface relief structure is maintained whilst removing relief, so that contact copying is not possible. As such, the visibility and brilliance of the relief structure is preserved. The coatings also provide high adhesion.

In one embodiment of any of the aforementioned aspects of the invention, the metal oxide is titanium oxide or zirconium oxide or mixtures thereof. A preferred metal oxide is titanium oxide.

In a further embodiment of any of the aforementioned aspects of the invention, the metal oxide particles are coated with an inorganic and/or organic coating. A preferred inorganic coating is an oxide, non-limiting examples of which are alumina, silica and zirconia or mixtures thereof.

Non-limiting examples of preferred organic coatings are polyols, esters, siloxanes, silanes, organosilicon compounds and carboxylic acids and mixtures thereof. A particularly preferred organic coating is a fatty carboxylic acid. In a yet further embodiment of any of the aforementioned aspects of the invention, the coating or dispersion additionally comprises at least one dispersing aid. Preferably, the dispersing aid is a neutral, anionic or cationic polymer or copolymer. Exemplary dispersing aids include alkylolammonium salts of a polymer or copolymer, polyhydroxystearic acids and polycaprolactone polyols.

In one embodiment, the dispersing aid may comprise a hyperdispersant.

In a still further embodiment of any of the aforementioned aspects of the invention, the coating or dispersion additionally comprises at least one binder. Preferred binders are organic binders. More preferred binders are organic polymers such as those based on ketone or aldehyde polymers.

In a yet still further embodiment of the aforementioned aspects of the invention, the metal oxide particles are doped with a further metal oxide.

Non-limiting examples of suitable organic solvents are ketones, esters, glycols and glycol ethers and mixtures thereof.

In an alternate embodiment of any of the aforementioned aspects of the present invention, the coating or dispersion may additionally comprise a radiation- curable resin, for example a resin which is curable by actinic radiation such as UV radiation, X-rays or electron beams.

The radiation-curable resin is preferably a transparent or translucent ink formed from a clear resin material. Such a transparent or translucent ink is particularly suitable for printing light-transmissive security elements such as numerical-type DOEs and lens structures.

In one particularly preferred embodiment, the transparent or translucent ink preferably comprises an acrylic-based, UV-curable, clear, embossable lacquer or coating.

Such UV-curable lacquers can be obtained from various manufacturers, including Kingfisher Ink Limited, product ultraviolet type UVF-203 or similar. Alternatively, the radiation-curable, embossable coatings may be based on other compounds, e.g. nitro-cellulose.

According to a further aspect of the present invention, there is provided a method of protecting a surface relief structure said method comprising the following steps: (a) providing a dispersion comprising metal oxide particles in an organic solvent according to any of the aforementioned aspects of the invention,

(b) applying said dispersion to a surface relief structure, and

(c) removing the solvent so as to form a coating,

wherein the coating has a refractive index greater than the refractive index of the surface relief structure. The surface relief structure may be printed or stamped or formed by any other method known in the art.

In one embodiment of this aspect of the invention, the method further comprises the step of:

(d) applying one or more coats of transparent varnish to the so-formed coating.

The transparent varnish may comprise one or more UV-curable components.

Additionally or alternatively, the dispersion may itself further comprise at least one transparent varnish. Additionally or alternatively, the dispersion may also further comprise one or more binders.

In a further embodiment, the dispersion may further comprise one or more radiation-curable resins, for example a resin which is curable by actinic radiation such as UV radiation, X-rays or electron beams. The radiation-curable resin is preferably a transparent or translucent ink formed from a clear resin material. In one particularly preferred embodiment, the transparent or translucent ink preferably comprises an acrylic-based, UV-curable, clear lacquer or coating.

In one embodiment, the surface relief structure is a diffractive optical surface structure.

In a further aspect of the invention, the application of the dispersion to a surface or substrate may itself form all or part of the surface relief structure.

In a yet further aspect of the present invention, the coatings or dispersions may find use as a printing ink, utilising any method of printing well known in the art. Advantageously, such coatings or dispersions may be utilised in reel-to-reel printing, obviating the need for high temperature sputtering.

In a still yet further aspect of the present invention, there is provided the use of a coating or dispersion according to any of the aforementioned aspects and embodiments of the invention in the protection of surface relief structures. In a preferred embodiment the surface relief structure is a diffractive optical surface structure.

In another aspect of the present invention, there is provided a security device including a surface relief structure and a coating in accordance with any one of the aforementioned embodiments.

In a further aspect of the present invention, there is provided a security device comprising a surface relief structure protected by a coating in accordance with any one of the aforementioned methods.

In a yet further aspect of the present invention, there is provided a security document or token including a security device in accordance with any one of the aforementioned embodiments.

Throughout this specification, use of the terms 'comprises' or 'comprising' or grammatical variations thereon shall be taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof not specifically mentioned.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

Security Document

As used herein, the term 'security document' includes all types of documents and tokens of value and identification documents including, but not limited to the following: items of currency such as banknotes and coins, credit cards, cheques, passports, identity cards, securities and share certificates, driver's licenses, deeds of title, travel documents such as airline and train tickets, entrance cards and tickets, birth, death and marriage certificates, and academic transcripts.

The invention is particularly, but not exclusively, applicable to security documents such as banknotes or identification documents such as identity cards or passports formed from a substrate to which one or more layers of printing are applied.

Substrate

As used herein, the term 'substrate' refers to the base material from which the security document or token is formed. The base material may be paper or other fibrous material such as cellulose; a plastic or polymeric material including but not limited to polypropylene (PP), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), polyethylene terephthalate (PET); or a composite material of two or more materials, such as a laminate of paper and at least one plastic material, or of two or more polymeric materials.

Security Device or Feature

As used herein, the term 'security device or feature' includes any one of a large number of security devices, elements or features intended to protect the security document or token from counterfeiting, copying, alteration or tampering. Security devices or features may be provided in or on the substrate of the security document or in or on one or more layers applied to the base substrate, and may take a wide variety of forms, such as security threads embedded in layers of the security document; security inks such as fluorescent, luminescent and phosphorescent inks, metallic inks, iridescent inks, photochromic, thermochromic, hydrochromic or piezochromic inks; printed and embossed features, including relief structures; interference layers; liquid crystal devices; lenses and lenticular structures; optically variable devices (OVDs) such as diffractive devices including diffraction gratings, holograms and diffractive optical elements (DOEs).

Diffractive Optical Elements (DOEs)

As used herein, the term 'diffractive optical element' refers to a numerical- type diffractive optical element (DOE). Numerical-type diffractive optical elements (DOEs) rely on the mapping of complex data that reconstruct in the far field (or reconstruction plane) a two-dimensional intensity pattern. Thus, when substantially collimated light, e.g. from a point light source or a laser, is incident upon the DOE, an interference pattern is generated that produces a projected image in the reconstruction plane that is visible when a suitable viewing surface is located in the reconstruction plane, or when the DOE is viewed in transmission at the reconstruction plane. The transformation between the two planes can be approximated by a fast Fourier transform (FFT). Thus, complex data including amplitude and phase information has to be physically encoded in the micro- structure of the DOE. This DOE data can be calculated by performing an inverse FFT transformation of the desired reconstruction (i.e. the desired intensity pattern in the far field).

DOEs are sometimes referred to as computer-generated holograms, but they differ from other types of holograms, such as rainbow holograms, Fresnel holograms and volume reflection holograms.

It will now be convenient to describe the invention with reference to particular embodiments and examples. These embodiments and examples are illustrative only and should not be construed as limiting upon the scope of the invention. It will be understood that variations upon the described invention as would be apparent to the skilled addressee are within the scope of the invention. Similarly, the present invention is capable of finding application in areas that are not explicitly recited in this document and the fact that some applications are not specifically described should not be considered as a limitation on the overall applicability of the invention.

The coating or dispersion of the present invention may be prepared by dispersing particles of a metal oxide in a suitable organic solvent. Preferably, the coating or dispersion comprises 1 to 60% by weight of metal oxide, more preferably 2 to 40% by weight, most preferably 2 to 20% by weight. A particularly preferred metal oxide is titanium oxide.

The particle size of the metal oxide is important to producing high refractive index (HRI) coatings. Preferably, the primary crystallite size of the metal oxide is between 1 and 100 nm, more preferably between 5 and 25 nm. It will be appreciated that the measured particle size may not reflect this primary crystallite size and will be dependent on the extent of particle surface treatment and on the presence or otherwise of additives that may influence primary crystallite agglomeration. Further, the skilled artisan will appreciate that different particle size measurement techniques may yield different results depending on the nature of the sample under analysis.

Preferred metal oxides are titanium oxide, zirconium oxide, zinc oxide, tin oxide and cerium oxide.

Any suitable organic solvent may be used to disperse the metal oxide.

Preferred organic solvents are ketones, esters, glycols and glycol ethers and mixtures thereof.

The coating or dispersion of the present invention may also comprise at least one binder material, preferably an organic binder material. Organic binder materials are materials that can function as a viscosity modifier, have film forming properties, can add mechanical strength to films or coatings that are formed therewith, or some combination thereof. Generally, the at least one organic binder has a minimal solubility in polar solvents, and/or high boiling point solvents. Generally, the at least one organic binder material is compatible with other solvents used to form the coating or dispersion. Also, the at least one organic binder is compatible with the dispersion so that a homogenous solution is created and maintained when combined with the dispersion.

In one embodiment, organic binder materials that are utilised include high molecular weight polymers. Exemplary materials include, but are not limited to polyethylene oxide (PEO), polyvinyl alcohol (PVA), or polyacrylic acid (PAA). In one embodiment, the organic binder is an alkyl cellulose ether. Examples of alkyl cellulose ethers include, but are not limited to methyl cellulose, hydroxypropyl methyl cellulose and derivatives of hydroxyethyl cellulose. In one embodiment, the at least one organic binder is present in the coating or dispersion from 0.5 to 20 % by weight. In another embodiment, the at least one organic binder is present in the coating or dispersion from 0.5 to 15 % by weight. In yet another embodiment the at least one organic binder is present in the coating or dispersion from 0.5 to 12 % by weight.

The coating or dispersion of the present invention may also comprise at least one dispersing aid.

In one embodiment, the at least one dispersing aid is present in the coating or dispersion from 1 to 15 % by weight. In another embodiment, the at least one dispersing aid is present in the coating or dispersion from 2 to 10 % by weight. In yet another embodiment the at least one dispersing aid is present in the coating or dispersion from 3 to 8 % by weight. Preferably the dispersing aid is an anionic or cationic polymer or copolymer. A particularly preferred dispersing aid is an alkylolammonium salt of a polymer or copolymer.

Particularly preferred coatings comprise a dispersion of metal oxide and a transparent varnish. The transparent varnish may contain one or more UV- curable components. In one embodiment, the coating of the present invention may be formed by printing as an ink and then the so-formed coating is itself coated with one or more coats of a transparent varnish. Again, the transparent varnish may contain one or more UV-curable components. In an alternate embodiment, mixtures of the dispersion of the present invention with transparent varnish may be applied to a surface and the so-formed coating may optionally be followed by one or more layers of transparent varnish.

In some applications it is advantageous to apply more than one layer of varnish.

In terms of coating thickness, it will be appreciated that these can vary widely depending on the nature of the surface relief structure in question. Effective coatings may result utilising a metal oxide coating depth of about 10 to 1000 nm, preferably 50 to 700 nm. The varnish coating may be up to 10 micron in depth or even more.

In respect of providing an effective coating that offers resistance to counterfeiting by mechanical lifting, the whole of the surface relief structure is preferably filled by the coating, including any additional varnish coatings, so as to result in a flat surface.

Excellent coatings may result by mixing 1 , 1 .5 or 2 parts of a 10 % by weight ink composition (10 % metal oxide dispersed in an organic solvent) with 1 part transparent varnish. The mixture may be printed as is and the resulting coating may be treated with one or more further varnish overcoats.

Similarly, 1 or 2 parts of a 10 % by weight ink composition in solvent with 1 or 2 parts varnish and 1 or 2 parts organic solvent may also be advantageously utilised. Successfully performing coatings have also been prepared by mixing 1 part 40 % by weight ink composition with 1 -5 parts transparent varnish of mixing 1 part 40 % by weight ink with 4 parts varnish and 2 parts organic solvent. EXAMPLES

Metal oxide dispersion

Metal oxide dispersions were obtained from a variety of sources, namely MK Impex, Ontario, Canada; NanoGram Corporation, California, USA, Chem-Well Tech Co., Ltd, South Korea and Sumitomo Osaka Cement Co. Ltd., Japan. The coating or dispersion may additionally comprise a radiation-curable resin.

In one example, titanium dioxide nanoparticles (40-45 % by weight) were dispersed in an organic solvent, typically dipropylene glycol monomethyl ether. The dispersions further comprised 1 1 -13 % by weight binder and 6-8 % by weight dispersing aid.

To these dispersions were added methyl ethyl ketone (MEK) to dilute to a concentration of about 13 % by weight (a 1 :2 dispersion in MEK was found to be advantageous) by conventional mixing methods.

Optionally, one or more transparent varnishes and/or UV-curable resins may be added to the metal oxide dispersion prior to coating.

Method of coating

The application of the coating may be performed using known commercial methods such as gravure printing, flexographic print, silk screen printing and the like.

In one embodiment, an in-line serial method occurs simultaneously with the process for producing the relief structures, that is, the high refractive index coating itself forms the relief structure.

Alternatively or additionally, the application of the coating may occur as a separate process to the production of the relief surface. The methods allow for an economically effective process for application of the coating with printing speeds of up to and greater than 120 m/min. This may be followed by further printing of one or more transparent varnish layers.