Security paper and other security items

Abstract

 A novel security item is proposed which is characterised by at least one security element having at least one segment comprising at least one latent UV or IR absorber (III). A latent absorber is a compound or composition capable of liberating a UV absorber on demand. Preferentially, the latent absorber is converted to its absorbing form in a spatially resolved manner (III) thus providing the desired information as latent image. Information imprinted within a substrate containing said absorbers is invisible to the naked eye. Moreover, the information is also invisible under UV irradiation. Only if a photoluminescent screen or background is used under UV irradiation, the "imprinted" information becomes apparant as a shadow image on the photoluminescent screen, since, in the areas where the latent absorber has been converted to the UV absorber, the UV light is being absorbed and does not reach the photoluminiscent device.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to a security item, as well as to a method for producing such security item. More specifically, it relates to a security item that can be imprinted with specific information. It generally applies to articles whose counterfeiting is to be made difficult or impossible and/or whose authenticity and/or validity is to be identified.

BACKGROUND OF THE INVENTION

[0002] There is a growing need for articles for identification or so-called security features which cannot easily be tampered with or reproduced by an unauthorized person. Examples of such articles are photoluminescent inks, fibers and watermarks, holograms, metal threads and the like. Such articles are commonly employed to protect security documents such as bank notes, cheques, passports, credit cards, stock certificates, tickets etc. from unauthorized reproduction. It is desirable to imprint specific, unique information in such an article, e.g. the name of the passport-holder, the serial number of a bank note, the number and expiration date of a credit card, the date of birth on a driver's licence or the validity date of a ticket.

[0003] In state-of-the-art security features are contained in or applied on the substrate - usually paper or plastic - of the document and hamper the unauthorized reproduction of the document. They can usually not individually be imprinted with specific information. Such information is - as for example on a passport - imprinted on the substrate of the document, but not within a security feature. While such security features can efficiently hamper unauthorized reproduction of such a substrate, they can, however, not prevent that the information on such an authentic document is altered, e.g. the name of a passport holder or the date of birth on a driver license is changed.

[0004] Thus, efforts are being made to invariably combine the security document and the information contained therein. Such efforts include, e.g., inkjet-printing of a halftone shadow-image of the passport holder's photograph over the entire document or laminating of the document with a protective layer that can not be removed without destroying document and/or protective layer.

[0005] In this view, security features that can be imprinted with specific information, e.g. the passport holder's name or photograph offer an enhanced level of security when compared with security articles of prior art.

[0006] British Patent Specification GB 1123274 discloses an identification document having embedded therein superposed layers of a fluorescent material and an ultraviolet screening agent. The layer of ultraviolet screening agent has a code formed therein so that when the document is illuminated with ultraviolet radiation, the code can be seen. The code is invisible in light containing only small amounts of ultraviolet radiation (e.g. daylight).

[0007] United States Patent Specification US 5,005,873 discloses an article for identification having embedded therein superposed layers of at least two different fluorescent materials and, in one preferred embodiment, visual identification data is provided by a pattern of a UV absorbing agent.

[0008] British Patent Specification GB 2300596 discloses a security feature comprising a transparent or translucent substrate and two fluorescent components and a UV light absorber preferably incorporated in a layer between each of the fluorescent components, thus providing a composite which has different appearance in reflected and transmitted UV light.

[0009] United States Patent Specification US 5,360,235 discloses an optical marking system with coating materials transparent in daylight and having the property of absorbing ultraviolet light, applied to a portion of a surface which is a good ultraviolet reflector. This object appears unmarked when observed by the naked eye, but when observed through a device, the different UV absorbing/reflecting markings become apparent. In one preferred embodiment, the ultraviolet absorbing material is a ultraviolet absorbing benzophenone.

[0010] A major disadvantage to all of the articles disclosed in the above documents of prior art is that the employed UV screening means bearing the information can not individually be imprinted with information. While common printing techniques like screen-printing could be employed for a series of documents containing exactly the same information, unique imprinting of single articles is not feasible.

SUMMARY OF THE INVENTION

[0011] It is therefore the general object of the present invention to provide an improved security item which allows individual imprinting of information.

[0012] This object is achieved by providing at least one security element having at least one segment comprising at least one latent UV or IR absorber.

[0013] Upon appropriate treatment, e.g. in a chemical, thermal or lithographic process (e.g. photo-lithography, electron beam lithography, X ray lithography, spatially resolved heat- or micro wave treatment), said latent absorber can be converted into an actual absorber (alternatively referred to as "the latent absorber's active form" or "the absorber" in the following). Since both the latent absorber and the liberated absorber itself are preferentially colorless and transparent throughout the visible spectrum, information imprinted within a substrate containing said absorbers is invisible to the naked eye. Moreover, the information is also invisible under UV or IR irradiation. Only if a photoluminescent screen or background is used under UV or IR irradiation, the imprinted data becomes apparent as a shadow image on the photoluminescent screen, since, in the areas where the latent absorber has been converted to the absorber, the light is being absorbed and does not reach the photoluminescent device. In case of infrared irradiation, a screen has to be used which allows the conversion of infrared radiation into the visible range. Preferentially however, the latent absorber is a latent UV absorber allowing for a much easier detection, as a corresponding photoluminescent device simply has to convert ultraviolet irradiation into the visible range.

[0014] According to a first preferred embodiment of the present invention, the security element is in a form selected from the group consisting of fibres, threats, strips, films, sheets, layers, tapes, plates, discs, chips and/or combinations thereof. Thus the actual security element can have a multitude of forms.

[0015] According to another preferred embodiment of the present invention, the at least one latent absorber is applied to or embedded in a matrix. Preferentially, this matrix is transparent for wavelengths in the visible region and/or in the spectral region which can potentially be absorbed by the latent absorber, i.e. by the latent absorbers active form.

[0016] According to still another preferred embodiment of the present invention, the matrix is made of a polymer or a polymer blend, wherein preferentially the polymer is chosen from the group consisting of polyethylene (PE), in particular linear low-density polyethylene (LLDPE) or ultra-high molecular weight polyethylene (UHMW-PE), polypropylene (PP), polyethyleneterephthalate (PET), polycarbonate (PC), polyvinylalcohol (PVA1), polyvinylchloride (PVC), polyurethane (PU) and mixtures thereof. Other polymers are possible, as long as the molecular components allowing for the provision of the latent absorber can be incorporated into this matrix. Other matrix materials such as paper, transparentized paper or paper-like materials are possible. The latent absorber can be a mixture of different components, e.g. a mixture of a potentially absorbing molecule which has been chemically modified in such way as not to absorb in the UV or IR range, with another molecule capable of setting free a chemical species initiating the conversion of the potentially absorbing molecule to the latent absorbers active form by e.g. removing a particular protective group. Preferentially, the latent absorber is non-photoluminescent and colourless. If however the latent absorber is photoluminescent, there is no need for a particular photoluminescent screen to detect regions where the latent absorber has been converted to its active form since the latent absorber at the same time takes over the detection function. Such a compound therefore might, after conversion to the active form, by absorbing, directly convert the irradiated light in the UV/IR range, which is invisible to the naked eye, into visible light. Clearly, an additional photoluminescent screen can still be employed.

[0017] In cases where the absorber is photoluminescent, there is also no need for a particular photoluminescent screen to detect regions where the latent absorber has been converted to actual absorber since the absorber at the same time takes over the detection function.

[0018] According to a further preferred embodiment, the latent absorber is a latent UV absorber and the latent UV absorber is at least partially converted to its absorbing form, wherein preferentially the absorbing form is present in a spatially resolved manner.

[0019] According to still another preferred embodiment, at least one security element comprises at least one photoluminescent substance, which may preferentially be embedded in a matrix as mentioned above or which may even more preferentially be present in a separate layer. The provision of actual detection means within the security item facilitates detection of the imprinted information substantially. In such a case, no separate photoluminescent screen is necessary. Irradiation with e.g. appropriate UV light directly makes the imprinted information visible to the naked eye, as irradiation not absorbed by the converted latent absorber will be converted into the visible range by the photoluminescent substance. Optionally, a number of photoluminescent substances showing different colours in fluorescence can be employed, wherein preferentially different colours are arranged in a spatially resolved manner and/or in different layers. It is for example possible to provide particular regions like stripes of particular colour, thus increasing the inherent security of the feature and the attractive character of it.

[0020] A particularly interesting embodiment of the present invention is characterised in that there is at least one layer comprising latent UV absorber sandwiched between two layers comprising photoluminescent substance. Preferentially, the two photoluminescent layers show different colours in fluorescence. Thus, a latent image imprinted in the layer comprising the latent absorber will show up in a different colour depending on the side of the laminate from which observation occurs.

[0021] Another particularly interesting embodiment of the present invention is characterised in that there is at least two layers comprising latent UV absorber which is at least partially converted to its absorbing form in a spatially resolved manner, wherein between these two layers there is at least one layer comprising the photoluminescent substance, and wherein preferentially the information contained in the spatial arrangement of the absorbing form is different in the two layers. Such an arrangement of layers within one laminate allows to provide for an essentially transparent feature and which, if irradiated with UV or IR, displays different information depending on the side of observation. According to even more sophisticated preferred embodiment of the present invention, there is a laminate of at least three layers comprising latent UV absorber at least partially converted to its absorbing form in a spatially resolved manner, in that between the at least three layers there are layers comprising the photoluminescent substance, wherein preferentially the photoluminescent layers show different colours in fluorescence, and wherein even more preferentially the information contained in the spatial arrangement of the absorbing form is different in each of the layers comprising latent UV absorber. In a laminate like this, particular information imprinted on particular layers will appear in different colours and different superposition is depending on the side from which observation occurs. Such a laminate shows an extremely high security with respect to falsification or counterfeiting.

[0022] The security level can be increased even more by providing at least one photoluminescent substance within or on the security item which is oriented in or on a substrate such that it exhibits polarised emission and/or absorption. The visibility of the information will thus depend on the angle of the polarisation of the irradiated light and/or on the angle of a polarizing filter which is used for detection: In particular if the direction of polarisation of the irradiated light or of the polarizing filter for detection is rotated continuously, a bright/dark effect can be observed with the imprinted information. Not only the photoluminescent substance can have polarizing properties, but it is also possible to provide a latent absorber which exhibits polarised absorption in its absorbing form, and in case of a latent absorber which is photoluminescent or in the case of an absorber which is photoluminescent, exhibits polarised absorption and/or emission. Moreover, if photolytical conversion of the latent absorber to the absorber is employed, this conversion can be carried out employing polarized radiation, thus leading to preferential conversion of molecules oriented essentially parallel to the optical axis of the radiation, similarly resulting in polarized absorption.

[0023] According to still another preferred embodiment, the latent absorber is a latent UV absorber and is a derivative of 2-2-hydroxyphenylbenzotriazole, 2-hydroxybenzophenone, 2-(4,6-diphenyl-1,3,5-triazin-2-yl)-phenol, or azobenzene, or a mixture thereof.

[0024] A still further preferred embodiment of the present invention is characterised in that the security item is an object whose counterfeiting is to be made difficult or impossible and/or whose authenticity and/or validity is to be identified and/or the purpose of which is to have information contained therein in the form of areas essentially containing latent absorber and areas essentially containing absorber in a spatially resolved manner, wherein preferentially the security item is selected from the group of banknotes, checks, stocks and bonds, securities, identification cards, passports, drivers licences, admission tickets, stamps, bankcards, credit cards, packing material.

[0025] Further preferred embodiments of the present invention are described in the dependent claims.

[0026] The present invention additionally relates to a method of producing security items as described above, characterised in that an object is provided with a security element which contains at least one segment comprising at least one latent UV or IR absorber. In particular, the latent absorber is a latent UV absorber and the latent UV absorber is at least partially converted to its absorbing form by means of a chemical or photochemical process, if needed assisted or followed by heat treatment, wherein preferentially the conversion is carried out in a spatially resolved manner. As methods for conversion, various processes are possible, e.g. a photographic process, a lithographic process, a screen printing process, an inkjet printing process or a laser printing process. Further preferred embodiments of the method according to the present invention are given in the dependent claims.

DESCRIPTION OF THE FIGURES

[0027] While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the invention, the invention will be better understood from the following description taken in connection with the accompanying drawings in which:

Fig. 1 shows an article according to the present invention containing latent UV absorber dispersed in a matrix;

Fig. 2 shows the spatially resolved conversion of a latent UV absorber to a UV absorber by exposure;

Fig. 3 shows an article according to the present invention comprising latent UV absorber and UV absorber in a spatially resolved manner;

Fig. 4 shows the visualization of a latent pattern contained in an article according to Fig. 3 by irradiation of a photoluminescent screen through said article with UV light;

Fig. 5 shows a laminate of two articles according to Fig. 3 with a photoluminescent middle layer containing photoluminescent species (8);

Fig. 6 shows an article according to Fig. 5 under UV irradiation viewed in reflection from the upper side;

Fig. 7 shows an article according to Fig. 5 under UV irradiation viewed in reflection from the lower side;

Fig. 8 shows an article according to Fig. 5 under UV irradiation viewed in transmission;

Fig. 9 shows an article according to Fig. 5 under UV irradiation viewed in transmission from the other side;

Fig. 10 shows an article according to the invention consisting of a laminate of three layers according to Fig. 3;

Fig. 11 shows a latent UV absorber (2-benzoylbenzophenone) that can be converted to a UV absorber (2-hydroxybenzophenone) by photocleavage;

Fig. 12 shows another latent UV absorber (2-2-benzoyl-5-methylphenylbenzotriazole) that can be converted to a UV absorber by photocleavage (2-2-hydroxy-5-methylbenzotriazole);

Fig. 13 shows another latent UV absorber (2-2-tert-butoxycarbonyloxy-5-methylphenylbenzotriazole) that can be converted to a UV absorber (2-2-hydroxy-5-methylbenzotriazole) by exposure to acidic species; and

Fig. 14 shows UV spectra of a latent UV absorber (2-2-benzoyl-5-methylphenylbenzotriazole, thin, solid line) and its corresponding UV absorber (2-2-hydroxy-5-methylbenzotriazole, thick solid line).

DETAILED DESCRIPTION OF THE INVENTION

[0028] The present invention provides an article containing a latent UV absorber. Upon appropriate treatment, e.g. in a lithographic process (photo-lithography, electron beam lithography, X ray lithography, spatially resolved heat- or micro wave treatment), said latent UV absorber can be converted into an actual UV absorber in a spatially resolved manner. Since both the latent UV absorber and the liberated UV absorber itself are essentially colorless and transparent throughout the visible spectrum, information imprinted within a substrate containing said absorbers is invisible to the naked eye. Moreover, the information is also invisible under UV irradiation. Only if a photoluminescent screen or background is used under UV irradiation, the imprinted information becomes apparent as a shadow image on the photoluminescent screen, since, in the areas where the latent absorber has been converted to the UV absorber, the UV light is being absorbed and does not reach the photoluminescent device.

[0029] Fig. 1 shows such an article I containing latent UV absorber 1 dispersed in a matrix 2. In this figure, the latent UV absorber (given as empty circles) has not been converted to the actual absorber yet.

[0030] Fig. 2 shows the actual conversion process, i.e. the selective, spatially resolved conversion of a latent UV absorber 1 to a UV absorber 6 by exposure to UV light of appropriate wavelength 3 through a photomask II, which photomask comprises the information that shall be imprinted. Converted latent UV absorber is given by circles with a cross.

[0031] Fig. 3 shows an article IV according to the present invention comprising latent UV absorber 1 and UV absorber 6 in a spatially resolved manner thus providing a latent, invisible pattern formed by zones V of latent UV absorber 1 and zones VI containing UV absorber 6.

[0032] Fig. 4 shows the visualization of a latent pattern contained in article IV according to Fig. 3 by irradiation of a photoluminescent screen VII containing photoluminescent species 8 through said article with UV light 7 of appropriate wavelength for stimulation of photoluminescence in VII. Zones IX in VII adjacent to zones VI in IV containing UV absorber 6 appear dark, while zones VIII in VII adjacent to zones V in IV containing latent UV absorber 1 show photoluminescence 9.

[0033] It is also possible to combine both the latent UV absorber and a photoluminescent substance within the same layer or in a printing ink. In areas where the latent UV absorber is converted to the UV absorber, the photoluminescence intensity will be strongly diminished, due to extinction of the excerted UV radiation and/or due to energy transfer processes from the photoluminescent compound to the UV absorber. However, in one preferred embodiment, the photoluminescent substance is provided in a separate layer, since better contrast between areas of latent and activated UV absorber can be achieved in that way. Additionally, one can imagine having both latent UV absorber and photoluminescent compound combined in one molecule. Energy transfer processes (either hopping or Förster-type) would efficiently quench the photoluminescence of the photoluminescent segment.

[0034] It is also possible to carry out the spatially resolved conversion without the aid of a photomask, e.g. by aid of a dirigible light- or laser beam.

[0035] Latent UV absorbers such as the ones disclosed herein allow for a multitude of different, personalizable security elements. One can, for example, combine the photoluminescent screen with the layer containing the imprinted, latent information and thus obtain a security element the information of which becomes immediately apparent when viewed under UV irradiation. If two such partially absorbing layers containing different imprinted images are arranged on each side of such a photoluminescent layer, a security feature is obtained which shows two different images, depending on whether the feature is viewed in transmitted or reflected UV light, or, respectively, is viewed form the one or the other side under UV irradiation.

[0036] Additionally, two separate photoluminescent screens which contain photoluminescent dyes which fluoresce in .different colors can be employed instead of the single photoluminescent layer. Such a device would, if irradiated with UV light, show one image in one color on one side of the security element, and a different image in a different color on the other side of the element. For an enhanced two-color-effect, the two photoluminescent layers can additionally be separated by a supplementary UV absorbing layer which prevents undesired excitation of the layer not facing the UV light source. It is also possible to have this middle layer made from a layer containing latent UV absorber and thus providing an additional information layer. In such manner, multitudes of different combinations are feasible.

[0037] Any combination of an article containing latent UV absorbers with any other kind of features or markings visible in daylight or under ultraviolet light is to be understood as an article according to the present invention.

[0038] In any case, it is not by any means immediately obvious that the coded information is present since it is only seen under ultraviolet light in the presence of a photoluminescent substance or by aid of any other viewing device or any other device suitable for visualisation or reading out the coded information.

[0039] It is to be understood that in this specification the term 'information' is to be interpreted in its broadest sense such as to include the provision of a visually detectable feature, characteristic pattern, colour pattern, pixels, barcode, text, image, logo, photograph etc. on an article.

[0040] To illustrate the possibility of providing sophisticated laminates, Fig. 5 shows a laminate XII of two articles IV according to Fig. 3 with a photoluminescent middle layer VII containing photoluminescent species 8. Again, absorbing parts are indicated by circles with crosses.

[0041] Correspondingly, Fig. 6 shows an article XII according to Fig. 5 under UV irradiation 7 viewed in reflection from above. The pattern 'A' imprinted in layer X is visualized on VII. Regions where there is actual absorber (circles with crosses) will appear dark as no irradiated UV will reach the photoluminescent layer VII in these regions.

[0042] Fig. 7 shows an article according to Fig. 5 under UV irradiation 7 viewed in reflection from below. The latent pattern 'B' imprinted in layer XI is visualized on VII. Depending on the side of observation there will therefore be a different pattern visible.

[0043] Fig. 8 shows an article according to Fig. 5 under UV irradiation 7 viewed in transmission with irradiation from above and observation from below. The latent pattern 'A' imprinted in layer X is visualized on VII, while the pattern 'B' will not be visible as behind there is no photoluminescent element which might visualise whether UV irradiation has been absorbed or not.

[0044] Fig. 9 shows an article according to Fig. 5 under UV irradiation 7 viewed in transmission with irradiation from below and observation from above. The latent pattern 'B' imprinted in layer XI is visualized on VII.

[0045] Figures 5 to 9 therefore show that a sophisticated security feature can be obtained by a simple three layer laminate, where depending on the mode of observation (reflection, transmission) and depending on their side of observation (below, above), different patterns will appear in the visible range. The actual conversion of the latent UV absorber to the actual UV absorber can either be carried out prior or after the actual lamination process of this three-layer laminate. In case of imprinting after the lamination care has to be taken that selective imprinting takes place on one of the layers, namely the wanted one. Usually imprinting takes place from the side where the layer to be imprinted is located, thus it is e.g. in case of photochemical imprinting important to make sure that the irradiation used for the conversion process does not reach the second layer on the other side of a laminate which is not to be imprinted. This can e.g. be made sure by providing a layer between the two layers comprising the latent absorber, which prevents the light used for the conversion process to reach the backside layer. This layer may or may not be equal to the photoluminescent layer.

[0046] Fig. 10 shows an even more sophisticated article according to the invention consisting of a laminate of three layers according to Fig. 3, whereas layer X contains a latent pattern 'A', layer XI contains a latent pattern 'B' and the layer XVII contains a latent pattern 'C'. Layer X and layer XI are separated by a photoluminescent screen VII exhibiting one particular photoluminescence color, layer XI and layer XVII are separated by a photoluminescent screen XVIII also exhibiting one particular photoluminescence color which may or may not be equal to the one of the photoluminescent screen VII. Layer XI may or may not contain a latent pattern. Viewed under UV irradiation 7 from the top, a combination of the latent pattern 'A' imprinted in layer X in the photoluminescence color of screen VII and of the latent pattern 'B' imprinted in layer XI in the photoluminescence color of screen XVIII is visualized. Viewed under UV irradiation 7 from the bottom, a combination of the latent pattern 'C' imprinted in layer XVII in the photoluminescence color of XVIII and of the latent pattern 'B' imprinted in layer XI in the photoluminescence color of layer XVIII is visualized.

[0047] An article for identification as disclosed in the present patent application can be incorporated in a document as 'blank', e.g. without imprinted information. The information can later be imprinted in the article on demand. Thereby, the batch size is not crucial. It is as well possible to imprint a single, unique piece of information such as the name of a passport holder in such an article as well as it is possible to imprint the date of issue in a series of stock certificates. The imprinting can be carried out by various techniques as mentioned above, particularly advantageous combinations might be imprinting by using an inkjet printer with particular inks to introduce the agent for the conversion and to subsequently pass the substrate through a heating device such as a laminator in order to actually initiate the chemical reaction for the conversion of the latent UV absorber to the actual UV absorber. Another possibility would be to use a laser printer which already uses a combination of irradiated light and heat for the imprinting process. Slight modification of such a laser printer to irradiate in a spectral region capable of initiating the conversion process might be necessary for the use of a laser printer for conversion.

[0048] European Patent Specification EP0952467A1 discloses a composition containing latent UV absorbers for forming ocular devices. The compositions may be used to conveniently and efficiently produce ocular devices through UV initiated, free radical polymerisation. The latent absorber contained in the composition does not interfere with the UV initiation process but can - after the polymerisation step - be converted into the active UV absorber thus providing protection from harmful UV radiation potentially causing corneal damage.

[0049] European Patent Specification EP 0097841A1 discloses a composition containing latent ester UV stabilizers for polycarbonate resins, permitting the cure of the resin by UV irradiation without interference by the latent UV stabilizer. Conversion of the latent stabilizer to the activate UV absorber provides a UV stabilizer for the polycarbonate.

[0050] The latent UV absorbers employed in these documents of prior art are used in a completely different field of application, i.e. not in the field of security features. The conversion to the active UV absorber does not occur photochemically and not in a spatially resolved manner. Moreover, the latent absorbers disclosed therein liberate as active UV absorber derivatives of benzophenone, which have, much in contrast to the preferred compounds disclosed in the present patent application, a comparably low extinction in the desired UV regime. Although these latent UV absorbers and their application can not - in any way - be compared to the latent UV absorbers and the field of application of the present invention, they shall be mentioned at this point with reference to the state of the art in the field of latent UV absorbers.

[0051] The invention provides latent UV absorbers for use in articles for identification and/or information storage and methods of their production.

[0052] By "UV absorber" is meant a compound or composition capable of absorbing or screening out UV radiation. The absorption properties of the UV absorber may or may not arise from internal conversion processes, such as - for example - excited state intramolecular proton transfer. Preferentially, it does arise from internal conversion processes.

[0053] By "latent UV absorber" is meant a compound or composition capable of liberating a UV absorber. With respect to the liberated UV absorber, the latent UV absorber's absorption is strongly diminished or is only present to a minimal extent or is essentially shifted out of the range of the original absorption spectrum of the UV absorber. Figure 14 shows an illustrative example. Figure 14 shows UV spectra of a latent UV absorber (2-2-benzoyl-5-methylphenylbenzotriazole, thin, solid line) and its corresponding UV absorber (2-2-hydroxy-5-methylbenzotriazole, thick solid line). It additionally shows the emission spectrum of an ordinary 'blacklight'-UV lamp (dotted line). Please note the remarkable overlap of the absorbance spectrum of the UV absorber with the emission spectrum of the UV lamp. The latent UV absorber can be converted to the UV absorber by, e.g. photocleavage or deprotection upon exposure to acidic, basic of oxidizing species. These species are preferentially photochemically released and the deprotection of the latent UV absorber can occur at elevated temperatures. Figure 13 shows an example of such an acid-labile latent UV absorber (see discussion further below).

[0054] The latent UV absorber may or may not be photoluminescent and may or may not be colorless. Preferentially, it is non-photoluminescent and colorless.

[0055] The UV absorber may or may not be photoluminescent and may or may not be colorless. Preferentially, it is non-photoluminescent and colorless.

[0056] By "UV radiation" is meant any radiation within a portion of the electromagnetic spectrum in the wavelength regime ranging from 180-500 nm.

[0057] By "protecting agent" is meant any compound or composition that reacts with the UV absorber so as to render the UV absorber substantially less effectively absorbing in the range of the original absorption spectrum of the UV absorber.

[0058] Generally, useful absorbers include, without limitation, benzophenones, benzotriazoles, triazines, substituted acrylonitriles, salicylic acid derivatives, benzoic acid derivatives, cinnamic acid derivatives, chalcone derivatives, nickel complexes, azo derivatives and mixtures and/or covalently bond combinations thereof. They can alternatively be bound to polymers, occur as polymer side groups or be part of the polymer backbone. Such compounds are disclosed in, e.g., US Patents US 3,391,110, US 5,098,445, US 4,963,160, US 3,391,110 and US 3,162,676.

[0059] Examples of benzophenones include, without limitation, 2-hydroxy benzophenones such as 2-hydroxy benzophenone, 2-hydroxy-4-(octyloxy)benzophenone, 2-hydroxy-4-acryloxy alkoxy benzophenones, 2,4-dihydroxy benzophenones, and the like, and mixtures thereof.

[0060] Examples of benzotriazoles include, without limitation, 2-2-hydroxyphenylbenzotriazole, 2-2-hydroxy-5-methylphenylbenzotriazole, 2-2-benzotriazol-2-yl)-p-cresol, 2,4-di-tert-butyl-6-(5-chlorobenzotriazol-2-yl)phenol, 2-(2-benzotriazol-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol, and the like, and mixtures thereof.

[0061] Examples of triazines include, without limitation, 2-2-hydroxy-4,6-triphenyl-1,2,3-triazine, 2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-hexyloxy-phenol and the like, and mixtures thereof.

[0062] Additional absorbers useful in the invention are azo-dyes, polymethines, and the like, and mixtures thereof.

[0063] Preferred UV absorbers are hydroxyphenylbenzotriazoles, hydroxybenzophenones and hydroxyphenyltriazines. A wide range of such UV absorbers is commercially available, e.g. from Ciba speciality chemicals under the trade names Chimassorb and Tinuvin.

[0064] Useful protecting agents, reactions of these agents with the UV absorber to render the absorber latent and reaction conditions will be readily apparent to those ordinarily skilled in the art. Some examples are given in the following.

[0065] The latent absorber undergoes regeneration or restoration of its absorbtion characteristics. The conditions for regeneration will depend on the protecting agent and absorber used. For example, in cases where the hydroxy group of a 2-hydroxy phenylbenzotriazole or a 2-hydroxy benzophenone is esterified with benzoic acid or a benzoic acid derivative, regeneration may be carried out by irradiation with high energy UV light, typically in the range of 220-400 nm, preferentially in the range of 220-340 nm, and even more preferentially within a narrow interval centered around 254 nm (see Fig. 11).

[0066] In cases where the hydroxy group of a 2-hydroxy phenylbenzotriazole or a 2-hydroxy benzophenone is protected by a tertiary butoxycarbonyl (t-Boc) group or any other acid labile group, regeneration may be carried out by exposure to an acid and may - in some cases - require elevated temperatures. Said acid may be photochemically released; such agents capable of photochemically releasing acids are known as photo acid generators (PAG's). A range of acid-labile t-Boc protected UV absorbers and PAG's are disclosed, e.g., in European Patent Specification EP 0909656A2. The use of PAG's for the conversion process might be of advantage in particular in case the security item is to be used under daylight conditions (e.g. for banknotes), as the irradiation-frequency for the initiation of the PAG can be shifted well out of the spectral region of the sun thus avoiding undesired conversion of the latent UV absorber contained in the security feature during use. A possible irradiation frequency well out of the spectral region of the sun would for example be 254 nm. Even more advantageous are systems where increased temperatures are required to convert the latent UV absorber to the UV absorber in the presence of a preferentially photochemically generated acid. In such systems, the latent UV absorber is not converted to the UV absorber, even if some acid is unwantedly liberated during use, e.g. due to exposure to sunlight.

[0067] In cases in where a reducing agent is used to convert the C=O groups of benzophenones to C-OH groups, regeneration may be carried out by a mild oxidizing agent. Said oxidizing agent may be photochemically released.

[0068] An example of a class of latent UV absorbers are esters of 2-hydroxy-5-methylbenzotriazole. They can be obtained by reacting the hydroxy group of 2-hydroxy-5-methylbenzotriazole with a compound or composition such as acid chlorides.

[0069] An example of a latent UV absorber is 2-benzoyl-5-methylbenzotriazole, which can be obtained by reacting 2-hydroxy-5-methylbenzotriazole with benzoyl chloride in the presence of pyridine. The benzoyl ester group of this substance can be cleaved photochemically, thus returning 2-hydroxy-5-methylbenzotriazole. This reaction is shown in Fig. 12.

[0070] In another example, the esterification of 2-hydroxy-5-methylbenzotriazole can be carried out using di-tert-butyl dicarbamate as protecting agent. The tertiary butoxycarbonyl group of the obtained, latent UV absorber can be removed by acidic species, thus returning 2-hydroxy-5-methylbenzotriazole. This reaction is shown in Fig. 13.

[0071] The acidic species is preferentially photochemically released. Compounds which liberate acids upon irradiation are generally known as photo acid generators (PAG's). Examples of PAG's include benzenesulfonic acid derivatives, arylmethylsulfones, benzenesulfonates, thriphenylsulphoniumtriflates, thriphenylsulphoniumhexafluoro-antimonates, etc.

[0072] Alternatively, a protective ester group of a latent UV absorber can be saponified by a basic species, again returning the UV absorber. The basic species is preferentially photochemically released. Compounds which liberate bases upon irradiation are generally known as photo base generators (PBG's). The above examples are particularly interesting in the scope of the invention because they allow for a spatially resolved conversion of the latent UV absorber by lithographic methods. Examples of PBG's are benzhydrylammonium salts.

[0073] Alternatively, it is also possible to introduce cleaving species in a spatially resolved manner by different than lithographic means, e.g. by inkjet printing, screen-printing, etc.

[0074] While protection of the hydroxy group of a UV absorber is the most straight forward approach to obtain latent UV absorbers, it is alternatively feasible to reduce the C=O group of certain of the absorbers, especially benzophenones, to the corresponding alcohol by a reducing agent such as aluminum triisopropoxide. As another alternative, the ketals may be formed from absorbers containing ketone carbonyls. As yet another example, the absorbance spectrum of certain absorbers can be shifted outside the usual absorbance regime by chemical alterations. For example, phenylazophenol and derivatives can be esterified to shift the absorption spectrum out of the original range.

[0075] In one embodiment, the invention consists of one substrate for an identification article comprising, consisting essentially of, or consisting of a composition comprising at least one latent UV absorber.

[0076] In another embodiment, the invention provides a latent UV absorber comprising, consisting essentially of or consisting of a UV absorber altered reversibly in such way that the absorption of said UV absorber is strongly diminished or shifted outside the original absorption spectrum of the UV absorber.

[0077] The invention will be clarified further by consideration of the following non-limiting examples.

Example 1

[0078] 1135 mg (5.034 mmol) of 2-2-hydroxy-5-methylbenzotriazole were dissolved in 10 mL anhydrous pyridine. To the stirred mixture, 0.6 ml (0.932g, 6.629 mmol) of benzoyl chloride were added. The mixture was heated to reflux (140 °C) and stirred for 2 h. Subsequently, the mixture was allowed to cool to room temperature. The mixture was diluted with 30 ml CH₂Cl₂, transferred into a separation funnel and washed three times with water and extracted with CH₂Cl₂. The organic layer was dried over MgSO₄, filtrated, and the solvents were evaporated. The obtained solid was recrystallized from methanol to yield 1415 mg (4.296 mmol, 85.26%) of the product, 2-2-benzoyl-5-methylbenzotriazole, as white crystalline needles.

[0079] A blend film containing 1% w/w of 2-2-benzoyl-5-methylbenzotriazole in LLDPE (Dowlex BG 2340, Dow Chemicals) was produced by dissolving 5 mg 2-2-benzoyl-5-methylbenzotriazole in 2 ml CH₂Cl₂ and decorating 450 mg of LLDPE pellets with that solution. After evaporation of the solvent at ambient, the decorated pellets were pressed into a blend film at 180 °C in a hot press. The obtained blend film was cut into pieces. These pieces were mixed and processed into a film again. This process was repeated four times in order to obtain homogeneous distribution of the latent UV absorber within the polymer matrix.

[0080] Two pieces of this film were irradiated through photomasks showing different black-and-white images with high energy UV light from a high-pressure Hg lamp (Philipps HPR 125 W) for 1 h. The photomasks were produced by printing the respective images on ordinary transparency sheets made of poly(ethyleneterephthalate) with an ordinary office-type laser printer.

[0081] The selectively irradiated film pieces were laminated each onto one side of a photoluminescent film made of polypropylene containing 0.05% w/w of a fluorescent dye (Uvitex OB ONE, Ciba Speciality Chemicals). When viewed under UV light from a low-pressure Hg UV lamp (black light, center frequency 365 nm), the respective latent image facing the UV light source was visible as fluorescent image.

[0082] The conversion reaction of this latent UV absorber into the UV absorber is shown in Fig. 12.

Example 2

[0083] 882.5 mg (4.452 mmol) of 2-hydroxybenzophenone were dissolved in 5 mL anhydrous pyridine. To the stirred mixture, 0.5 ml (0.777 g, 5.524 mmol) of benzoyl chloride were added. The mixture was heated to reflux (140 °C) and stirred for 2 h. Subsequently, the mixture was allowed to cool to room temperature. The mixture was transferred into a separation funnel and washed three times with water and extracted with CH₂Cl₂. The organic layer was dried over MgSO₄, filtrated, and the solvents were evaporated. The obtained colorless oil was subjected to column chromatography (CH₂Cl₂, silica gel) to yield 1346 mg (4.449 mol, 99.92 %) of the product, 2-benzoylbenzophenone, as a colorless, highly viscous substance.

[0084] Production of blend film containing 1% w/W of a 2-benzoylbenzophenone in, selective photoactivation thereof and production of a laminate containing a photoluminescent layer was carried out according to Example 1.

[0085] The conversion reaction of this latent UV absorber into the UV absorber is shown in Fig. 11.

Example 3

[0086] A blend film containing 1% w/w of 2-2-benzoyl-5-methylbenzotriazole in LLDPE was produced according to Example 1. A 2 cm x 2 cm piece of this blend film was irradiated for 20 min with light (low pressure Hg UV lamp, center frequency 254 nm) through a photomask made of steel containing square holes with 200 micron diameter, arranged in a chessboard-like pattern. After this time, the film was visually still uniformly transparent and colorless. Investigation of the film on a photoluminescent background under UV irradiation clearly revealed the hidden chessboard pattern as shadow image on the photoluminescent background.

Example 4

[0087] 1050.4 mg (4.663 mmol) of 2-2-hydroxy-5-methylbenzotriazole were dissolved in 10 mL anhydrous pyridine. To the stirred mixture, 1305.0 mg (5.979 mmol) of di-tert-butyl dicarbamate were added. The mixture was heated to reflux (140 °C) and stirred for 2 h. Subsequently, the mixture was allowed to cool to room temperature. The mixture was transferred into a separation funnel and washed three times with water and extracted with CH₂Cl₂. The organic layer was dried over MgSO₄, filtrated, and the solvents were evaporated. The obtained solid was recrystallized twice from methanol to yield 892 mg (2.745 mmol, 59 %) of the product, 2-2-tert-butoxycarbonyloxy-5-methylphenylbenzotriazole, as white crystalline flakes.

[0088] The product was found to efficiently cleave at elevated temperatures in the presence of acids to quantitatively restore 2-2-hydroxy-5-methylbenzotriazole.

[0089] The conversion reaction of this latent UV absorber into the UV absorber is shown in Fig. 13.

Example 5

[0090] 990.4 mg (10.524 mmol) of phenol were dissolved in 5 mL anhydrous pyridine. To the stirred mixture, 1.6 ml (2.203 g, 12.474 mmol) of benzenesulfonyl chloride were added. The mixture stirred for 2 h at room temperature. The mixture was transferred into a separation funnel and washed three times with water and extracted with CH₂Cl₂. The organic layer was dried over MgSO₄, filtrated, and the solvents were evaporated. The obtained colorless oil was subjected to column chromatography (CH₂Cl₂, silica gel) to yield 2148 mg (9.169 mol, 87.12 %) of the product, 2-benzoyl-benzenesulfonate, as a colorless liquid.

[0091] The product was found to efficiently cleave under UV irradiation (low pressure Hg UV lamp, center frequency 254 nm) to quantitatively restore benzenesulfonic acid. The product is thus suitable as photo acid generator.

[0092] A blend film containing 1% w/w of 2-2-benzoyl-5-methylbenzotriazole and 1% of 2-benzoyl-benzenesulfonate in LLDPE was produced according to Example 1. A 2 cm x 2 cm piece of this blend film was irradiated for 10 min with light from a 254 nm UV lamp through a photomask made from a poly(vinylalcohol) sheet with a black-and-white image printed thereon by aid of an ordinary office-type ink jet printer. Subsequently, the sample was baked for 10 min at 110 °C. After this time, the film was visually still uniformly transparent and colorless. Investigation of the film on a photoluminescent background clearly revealed the hidden image as shadow image on the photoluminescent background.

[0093] The conversion reaction of this latent, acid labile UV absorber into the UV absorber is shown in Fig. 14.

LIST OF REFERENCE NUMERALS

[0094] 

I

Article containing latent UV absorber

II

Photomask

III

Article containing latent UV absorber and UV absorber being formed thereof in a spatially resolved manner

IV

Article containing latent UV absorber and UV absorber in a spatially resolved manner

V

Zone containing latent UV absorber, essentially transparent for UV light of a specific wavelength regime

VI

Zone containing UV absorber, essentially non-transparent for UV light of a specific wavelength regime

VII

Photoluminescent screen

VIII

Photoluminescing zone

IX

Dark zone

X

Article (III) containing latent pattern 'A'

XI

Article (III) containing latent pattern 'B'

XII

Laminate of two articles (III) and photoluminescent screen (VII)

XIII

Photoluminescent image as visualization of latent image 'A' in (X) on (VII) as seen through (X)

XIV

Photoluminescent image as visualization of latent image 'B' in (XI) on (VII) as seen through (XI)

XV

Photoluminescent image as visualization of latent image 'A' in (X) on (VII) as seen through (XI)

XVI

Photoluminescent image as visualization of latent image 'A' in (X) on (VII) as seen through (X)

XVII

Article (III) containing latent pattern 'C'

XVIII

Photoluminescent screen of same or different photoluminescence color as (VII)

XIX

Superimposed photoluminescent images of latent image 'A' in (X) visualized on (VII) and latent image 'B' in (XI) visualized on (XVIII).

XX

Superimposed photoluminescent images of latent image 'C' in (XVII) visualized on (XVIII) and latent image 'B' in (XI) visualized on (VII).

1

Latent UV absorber

2

Matrix

3

Radiation appropriate for conversion of the latent UV absorber into a UV absorber

4

Essentially non-transparent section in (II) for (3)

5

Essentially transparent section in (II) for (3)

6

UV absorber

7

UV light appropriate for stimulation of photoluminescence of (8) in (VII)

8

Photoluminescent species

9

Visible light emanating from (8) in (VII) upon excitation with (7)

10

Eye of observer

Claims

1. A security item, characterised by at least one security element having at least one segment comprising at least one latent UV or IR absorber (1).

2. A security item according to claim 1, characterised in that the security element is in a form selected from the group consisting of fibres, threats, strips, films, sheets, layers, tapes, plates, discs, chips and/or combinations thereof.

3. A security item according to one of the preceding claims, characterised in that the at least one latent absorber (1) is applied to or embedded in a matrix (2), which is preferentially transparent for wavelengths in the visible region and/or in the spectral region which can potentially be absorbed by the latent absorber (1).

4. A security item according to claim 3, characterised in that the matrix (2) is made of a polymer or a polymer blend, wherein preferentially the polymer is chosen from the group consisting of polyethylene (PE), in particular linear low-density polyethylene (LLDPE) or ultra-high molecular weight polyethylene (UHMW-PE), polypropylene (PP), polyethyleneterephthalate (PET), polycarbonate (PC), polyvinylalcohol (PVA1), polyvinylchloride (PVC), polyurethane (PU) and mixtures thereof.

5. A security item according to one of the preceding claims, characterised in that the latent absorber (1) is non-photoluminescent and colourless.

6. A security item according to one of the preceding claims, characterised in that the latent absorber is a latent UV absorber (1) and in that the latent UV absorber (1) is at least partially converted to its absorbing form, wherein preferentially the absorbing form is present in a spatially resolved manner.

7. A security item according to one of the preceding claims, characterised in that at least one security element comprises a photoluminescent substance (8), which may preferentially be embedded in a matrix according to claims 3 or 4 or which may even more preferentially be present in a separate layer (VII).

8. A security item according to claim 7, characterised in that several photoluminescent substances (8) showing different colours in fluorescence are employed, wherein preferentially the different substances are arranged in a spatially resolved manner.

9. A security item according to one of the claims 7 or 8, characterised in that there is at least one layer comprising latent UV absorber (1) sandwiched between two layers comprising photoluminescent substance (8), wherein preferentially the two photoluminescent layers show different colours in fluorescence.

10. A security item according to one of the claims 7 to 9, characterised in that there is at least two layers (X, XI) comprising latent UV absorber (1) which is at least partially converted to its absorbing form in a spatially resolved manner, wherein between these two layers (X, XI) there is at least one layer (XII) comprising the photoluminescent substance (8), and wherein preferentially the information contained in the spatial arrangement of the absorbing form is different in the two layers (X, XI).

11. A security item according to claim 10, characterised in that there is a laminate of at least three layers (X, XI, XVII) comprising latent UV absorber (1) at least partially converted to its absorbing form in a spatially resolved manner, in that between the at least three layers (X, XI, XVII) there are layers (VII, XVIII) comprising the photoluminescent substance (8), wherein preferentially the photoluminescent layers (VII, XVIII) show different colours in fluorescence, and wherein even more preferentially the information contained in the spatial arrangement of the absorbing form is different in each of the layers (X, XI, XVII) comprising latent UV absorber.

12. A security item according to one of the claims 7 to 11, characterised in that the at least one photoluminescent substance (8) exhibits polarised emission and/or absorption.

13. A security item according to one of the preceding claims, characterised in that the latent absorber (1) exhibits polarised absorption in its absorbing form, and in case of a latent absorber (1) which is photoluminescent, exhibits polarised absorption and/or emission.

14. A security item according to one of the preceding claims, characterised in that the latent absorber is a latent UV absorber and is a derivative of 2-2-hydroxyphenylbenzotriazole, 2-hydroxybenzophenone, -2-(4,6-diphenyl-1,3,5-triazin-2-yl)-phenol, or azobenzene, or a mixture thereof.

15. A security item according to one of the preceding claims, characterised in that the security item is an object whose counterfeiting is to be made difficult or impossible and/or whose authenticity and/or validity is to be identified and/or the purpose of which is to have information contained therein in the form of areas essentially containing latent absorber and areas essentially containing absorber in a spatially resolved manner, wherein preferentially the security item is selected from the group of banknotes, checks, stocks and bonds, securities, identification cards, passports, drivers licences, admission tickets, stamps, bankcards, credit cards, packing material.

16. A method of producing security items according to one of the claims 1 through 15, characterised in that an object is provided with a security element which contains at least one segment comprising at least one latent UV or IR absorber (1).

17. A method according to claim 16, characterised in that the latent absorber is a latent UV absorber (1) and in that the latent UV absorber (1) is at least partially converted to its absorbing form by means of a chemical or photochemical process, if need be assisted by elevated temperature or followed by heat treatment, wherein preferentially the conversion is carried out in a spatially resolved manner.

18. A method according to claim 17, characterised in that a photographic process, lithographic process, screen printing process, inkjet printing process or laser printing process is employed to at least partially converted the latent UV absorber into its absorbing form.

19. A method for verification of the authenticity of security items according to one of the claims 1 through 15, characterised in that an electronic device is employed to visualize or read out the information contained in the security item.

Drawing