Patterned dichroic security features

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 VIS absorber with linearly polarized absorption and/or linearly polarized emission. Such a security item can be used to generate spatially resolved areas of different colour in the visible spectral range in combination with linear polarisation, providing a very effective means for protecting documents like banknotes, identification cards, credit cards, and the like.

Description

TECHNICAL FIELD

[0001] The present invention relates to novel security items as well as to a method for producing novel security items.

BACKGROUND OF THE INVENTION

[0002] Light-polarizing elements produced from uniaxially oriented polymer films comprising dichroic dyes have been proposed as optical features for use in security applications (see for example US 3,391,479, H. O. Buzzell, P. F. Jordan, (Polaroid), "Laminations" and US 5,004,327, A. Rosén, (Svecia Antiqua Limited), "Light polarizing material in the form of sheets or of a web and a method for the manufacture of the material"). The fact that the effect of anisotropic absorption observed in such uniaxially oriented materials comprising dichroic dyes cannot be photocopied nor mimicked by common printing techniques makes such systems of interest for optical security features. However, this beneficial characteristic also hampers the production of visually attractive patterns and images displaying the desired dichroic effect. The main aspect of the present invention is thus to provide methods that yield light-polarizing color patterns in uniaxially oriented polymer films.

[0003] A patterning method for uniaxially oriented polymer films comprising dichroic photoluminescent species was published in C. Kocher et al., "Patterning of Oriented Photofunctional Polymer Systems Through Selective Photobleaching," Adv. Funct. Mater., vol. 11, p. 31 (2001). However, light-polarizing patterns that are visible in daylight without the aid of a UV-light source are sometimes more attractive, for instance in applications such as self-verifying bank notes. Attempts to produce patterned visible light-polarizing structures are scarce, and often require elaborate manufacturing techniques. A methodology that relies on epitaxial deposition of evaporated species onto alignment layers is described in A. G. León et al., "Method for fabricating pixilated, multicolor polarizing films," Appl. Optics, 2000, vol. 39, p. 4847. Another method described in W. C. Yip et al., "Photo-patterned e-wave polarizer," Displays, 2001, vol. 22, p. 27 relies on photo-aligment of an azo chromophore by employing linearly polarized light, as well as a class of photoisomerizable compounds disclosed in EP1 247 796 A2.

[0004] Methods to selectively stain oriented films of a hydrophilic polymer with solutions of dichroic dyes or iodine have been disclosed in JP58085405 and JP58049902, respectively. These methods can not be applied to hydrophobic, water-insoluble materials such as poly(ethylene), poly(propylene), poly(terphthalate) or poly(amides).

[0005] These state-of-the-art methodologies are often academic, require sophisticated equipment and the features are cumbersome to produce. The light-polarizing patterns often suffer from poor environmental stability, very often degrade if exposed to daylight for a certain time, can easily be rubbed off and are characterized by low dichroic ratios.

SUMMARY OF THE INVENTION

[0006] The objective problem underlying the present invention is therefore to provide new polarizing security items which can for example be used for the protection of security documents. It additionally relates to a method for the production of such security items.

[0007] The present invention provides a new security item in that a security element is provided having at least one segment comprising at least one latent VIS absorber with linearly polarized absorption and/or linearly polarized emission.

[0008] The expression latent VIS absorber has to be understood that this moiety is capable of being transformed into another moiety which acts as an absorber in the visible spectral range, i.e. in the range between 400nm up to 800nm. This other moiety shall be called converted VIS absorbing form. The latent VIS absorber may preferentially itself already be an absorber in the visible spectral range, but it may also be a moiety which is transparent in the visible range of the electromagnetic spectrum, but for example absorbing in the UV or in the IR region. Such a latent VIS absorber can e.g. be converted either chemically or photo-chemically into its converted VIS absorbing form.

[0009] Preferentially, also the converted VIS absorbing form shows linearly polarised absorption and/or emission.

[0010] The object of the present invention is therefore a product according to claim 1, and a process according to claim 17.

[0011] The key feature of the invention is therefore the fact that the provision of such a latent VIS absorber showing linearly polarised absorption and/or emission, which can be converted into the converted VIS absorbing form, usually showing different spectral characteristics than the latent VIS absorber, allows the provision of spatially resolved colour patterns whereby these patterns also have polarisation characteristics. Since such features cannot be easily copied and since such features can easily be verified by means of a simple polarisation filter or by means of polarised irradiation or a combination of these two methods, it provides a very effective means for securing documents like banknotes and the like.

[0012] According to a first preferred embodiment of the present invention, the at least one latent VIS absorber is applied to or embedded in a matrix, which matrix is preferentially transparent for wavelengths in the visible region. Such a matrix can for example be made of a polymer or a polymer blend. 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), polyamide (PA), polypropylene (PP), polyethyleneterephthalate (PET), polycarbonate (PC), polyvinylalcohol (PVA1), polyvinylchloride (PVC), polyurethane (PU) and mixtures thereof.

[0013] According to another preferred embodiment, such a matrix shows uniaxial orientation. This uniaxial orientation can for example have been obtained by uniaxial stretching of the matrix with the embedded latent VIS absorber, wherein preferentially the drawing ratio (1/1₀) of the stretching is more than 2, even more preferentially above or equal to 4 or 8. Such a uniaxial stretching of the matrix with the embedded latent VIS absorber leads to a molecular orientation not only of the matrix but also of the latent VIS absorber, provided that the molecular structure being used as the latent VIS absorber shows corresponding anisotropy. Typically such a security item is characterized in that the latent VIS absorber and/or its converted VIS absorbing form show a dichroic ratio of more than 2 in absorption and/or emission, preferentially a dichroic ratio of more than 5 in absorption and/or emission, and most preferentially a dichroic ratio of more that 10 or even more than 20 or 50.

[0014] According to another preferred embodiment the security item is characterised in that the latent absorber coloured in its pristine form (latent VIS absorber), wherein the colour of the pristine form differs from the one of the converted VIS absorbing form. If both forms are coloured in the visible range, particularly interesting optical effects can be achieved when the security item is either observed through a polarizer or is irradiated using linearly polarised light. This particularly so, if the latent VIS absorber is at least partially converted to its converted VIS absorbing form, wherein preferentially the VIS absorbing form is present in a spatially resolved manner. Preferentially, adjacent patterns of the two forms are produced leading to a very distinct optical effect, in particular if more than one latent VIS absorber showing different colours in the pristine form and/or in the converted VIS absorbing form are employed, and if the different substances are arranged in a spatially resolved manner.

[0015] Another embodiment, which leads to different effects if viewed under polarisation filters with different orientation or if irradiated with different polarisation directions can be achieved, if different layers with different directions of polarisation are provided adjacent to each other. This can for example be done by providing at least two layers comprising latent VIS absorber, wherein the latent VIS absorbers are at least partially converted to their converted VIS absorbing forms in a spatially resolved manner, and wherein these layers comprise different latent VIS absorbers and/or show different orientation of the linear polarization, wherein preferentially there is two layers with orthogonal directions of polarization. The two patterns of the two layers can be arranged to match each other to e.g. give rise to a flip flop effect when viewed under a rotating polarizer.

[0016] Typically a security item according to the present invention is in a form selected from the group consisting of fibres, threads, strips, films, sheets, layers, tapes, plates, discs, chips and/or combinations thereof.

[0017] The conversion from the latent VIS absorber to the converted VIS absorbing form is possible using various routes. For example this conversion can be achieved in a spatially resolved manner by using irradiation (for example UV, but also X-ray and the like as possible) or chemical treatment, preferentially by locally modifying the pH value, or a combination of these methods. Postprocessing by for example applying heat or irradiation in another spectral range might be advantageous for stabilizing the system and for stopping the conversion process definitely.

[0018] Typically the molecular structure used for the latent VIS absorber or for the converted VIS absorbing form is a dichroic rod-like molecule, the transition dipole moment of which preferentially substantially coincides with the geometrical long axis of the molecule. Such a molecular structure allows the uniaxial alignment of these molecules within a matrix of a polymer which is drawn in one specific direction. The molecules can then give rise to an anisotropic absorption of the incident light consequently giving rise to the polarisation effects. The conversion between the two forms can be made possible by providing particular groups that can be cleaved off chemically or photo-chemically. One possibility is to provide the latent VIS absorber with at least one photo-labile and/or acid-labile chromogenic leaving group.

[0019] More specifically, such a system can for example be given in that the latent VIS absorber and/or its converted VIS absorbing form is a derivative of Sudan Red B, or Dihydroxynaphthol, in particular (4-Phenylazo-phenylazo) derivatives thereof, or a mixture thereof, wherein these compounds are provided with at least one photo-labile and/or acid-labile chromogenic leaving group. Even more specifically, molecular structures enabling the described functions can be given in that the latent VIS absorber is a Benzoic acid or a Boc derivative of Sudan Red B, of 1-(4-Phenylazo-phenylazo)-naphthalene-2,6-diol, of 6-alkyloxy-1-(4-Phenylazo-phenylazo)-naphthalene-2,6-diol (with the alkyl-group preferentially in the range of C1-C20), in particular of 6-dodecyloxy-1-(4-Phenylazo-phenylazo)-naphthalene-2,6-diol, of 4-(4-Phenylazo-phenylazo)-naphthalene-1,3-diol, or of 6-alkyloxy-4-(4-Phenylazo-phenylazo)-naphthalene-1,3-diol (with the alkyl-group preferentially in the range of C1-C20), in particular of 6-dodecyloxy-4-(4-Phenylazo-phenylazo)-naphthalene-1,3-diol.

[0020] Additional preferred embodiments of the present invention are described in the dependent claims.

[0021] The present invention also relates to a method of producing security items as they have been described above. The method is basically characterised in that an object is provided with at least one security element having at least one segment comprising at least one latent VIS absorber with linearly polarized absorption and/or linearly polarized emission. Preferentially, again, also the converted VIS absorbing form shows linearly polarised absorption and/or linearly polarised emission, wherein the spectral properties of the two forms are different from each other to allow the desired effects.

[0022] A preferred embodiment of the method according to the invention is given in that the security element is produced by melt processing a polymeric substance or blend with a latent VIS absorber, forming a corresponding object like a fibre or film, and by subsequently drawing the object in one direction, preferentially using a draw ratio above 2, preferentially between 2 and 10, most preferentially between 4 and 8. The latent VIS is preferentially at least partially converted to its converted VIS absorbing form by means of a chemical or photochemical process, if need be assisted by elevated temperature or followed by heat treatment, wherein preferably the conversion is carried out in a spatially resolved manner.

[0023] The conversion between the two forms can be achieved by using a photographic process, a lithographic process, a screen printing process, an inkjet printing process or a laser printing process. It can involve the chemical, preferentially acid-induced, or photo-chemical elimination of an element of the latent VIS absorber, preferentially by means of a photo-acid generator (PAG) and subsequent stabilization by evaporation of the photo-acid generator.

[0024] Further preferred embodiments of the method according to the invention are described in the dependent claims.

[0025] To sum up, the present invention provides systems and methodologies that yield patterned, light-polarizing features of increased mechanical and environmental stability.

[0026] In one aspect, the present invention provides a process for making uniaxially oriented polymer films displaying light-polarizing colour patterns by selectively converting uniaxially aligned chemical substances comprised in the oriented substrate into dichroic dyes aligned uniaxially within the oriented substrate.

[0027] In another aspect, the present invention provides a process for making uniaxially oriented polymer films displaying light-polarizing colour patterns by e.g. selectively converting uniaxially aligned dichroic pH-sensitive dyes from their deprotonated form into a differently coloured, protonated form by use of preferentially photochemically released acidic species, e.g. by employing so-called photo acid generators (PAG's).

[0028] In still another aspect, the present invention provides a process for making uniaxially oriented polymer films displaying light-polarizing colour patterns by selectively converting uniaxially aligned dichroic pH-sensitive dyes from their protonated form into a differently coloured, deprotonated form by use of preferentially photochemically released basic species, e.g. by employing so-called photo base generators.

SHORT DESCRIPTION OF THE FIGURES

[0029] In the accompanying drawings preferred embodiments of the invention are shown in which:

Fig. 1

gives the synthetic route and activation mechanism of the chromogenic dyes, exemplified for the photo-labile chromogenic dye 2Bz and the acid-labile chromogenic dye 2Boc. The compounds 1Bz, 1Boc, 3Bz and 3Boc are obtained accordingly;

Fig. 2

gives structures of the dichroic chromogenic azo-dyes used in the examples; R=Bz represents photo-labile chromogenic dyes, structures with R=Boc are acid-labile systems and structures with R=OH are the red coloured equivalents released upon adequate treatment;

Fig. 3

shows the conversion of 2Bz comprised in an LLDPE film upon irradiation with UV light at 254 nm; the inset shows the increase of the absorption corresponding to the released species 2OH (measured at 530 nm); dots are measured absorption values, the curve represents a first order kinetics fit;

Fig. 4

shows the dichroic effect generated by two perpendicularly stacked stretched LLDPE film's comprising the dichroic, chromogenic dye 2Boc imprinted with the patterned in a photolithographic methods;

Fig. 5

shows the polarised absorption spectra of 3Boc comprised in the stretched LLDPE film before conversion (thin line) and after acid-catalyzed conversion to 3OH (bold line); and

Fig. 6

shows the polarisation effect displayed by stretched LLDPE comprising the dichroic, chromogenic dye 3Boc imprinted with the pattern in a photolithographic method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODYMENTS

[0030] The technology involves chromogenic dyes based on ortho-phenylazonaphthol-dyes (R. Kuhn, F. Bär, Ann., 1935, 516, 143) that were derivatized with cleavable protective groups. These caged species exist in an exclusive azo-configuration but upon cleavage of the caging group can release the respective tautomerizing ortho-phenylazonaphthol-dye (V. F. Traven, A. M. Tsygankova, B. L. Stepanov, Chem. Abstr., 1985, 103, 143344w; V. Bek á rek, K. Rothschein, P. Vete

ník, M.Ve

e

a, Tetrahedron Lett., 1968, 34, 3711), which is characterized by an absorption band that is dramatically shifted to a longer wavelength regime when compared to the absorption band of its parent counterpart.

[0031] One possible chromogenic compound releases Sudan Red B, a common azo-dye that is characterized by a long molecular axis ('rigid-rod'-type molecule) that coincides with the molecule's transition dipole moment. Sudan Red B has already been used as active component in light-polarizing polymer sheets exhibiting appreciable dichroism (Y.

[0032] Dirix, T. A. Tervoort, C. Bastiaansen, Macromolecules, 1995, 28, 486). However, we established that uniaxially drawn films containing chromogenic derivatives of Sudan Red B (1Bz and 1Boc, see Fig. 2) displayed only moderate dichroism, which can ultimately be attributed to the fact that the peripheral labile protective groups have a detrimental effect on the molecule's aspect ratio and, consequently, the orientability of the dye molecule. Triggered by the need for patterned, visible light-polarizing systems, we embarked upon developing a number of chromogenic dyes that are structurally similar to Sudan Red B, but are characterized by an extended long molecular axis. By the introduction of long aliphatic substituents, we attempted to counteract the noted detrimental effect of the peripheral protective group located at a most unfavorable position regarding the orientability of the dye molecule, and, simultaneously, to increase the compatibility of the dye with a polyolefin host.

Results and Discussion

Synthesis of dichroic chromogenic compounds

[0033] Ortho-hydroxynaphthol-dyes closely related to Sudan Red B bearing an additional alkoxy-substituent were synthesized. As indicated above, the principal purpose of this substituent was to extend the molecule's long axis. Thus, 2,6-dihydroxynaphthol and 2,4-dihydroxynaphthol were reacted with diazotized p-aminoazobenzene resulting 1-(4-phenylazo-phenylazo)-naphthalene-2,6-diol (2a) and 4-(4-phenylazo-phenylazo)-naphthalene-1,3-diol (3a), respectively (Figure 1, which displays the synthetic route and the activation mechanism of the present chromogenic dyes, here exemplarily shown for the photo-labile chromogenic dye 3Bz and the acid-labile chromogenic dye 3Boc. The compounds 2Bz and 2Boc are obtained accordingly, by employing 2,4-dihydroxynaphthol in the azo-coupling reaction). The dihydroxy-naphthols were chosen for the azo-coupling reaction (H. Zollinger, Azo and Diazo Chemistry, Interscience Publishers, Inc., New York, 1961) to preferentially yield one single product which predominantly exists in the hydrazone form (E. F. Saad, E. A. Hamed, A. El-Faham, Spectroscopy Lett., 1996, 29, 1047; A. Ly

ka, Z. Vrba, M. Vrba, Dyes Pigments, 2000, 47, 45) and thus allowing for a strong chromogenic behavior.

[0034] The dihydroxynaphthol-dyes 2a and 3a were subsequently etherified with one equivalent of bromododecane. Column chromatography yielded the alkyloxy-substituted o-hydroxynaphthol diazo-dyes, which were subsequently converted to their chromogenic equivalents by introducing either the photo-labile benzoyl (Bz) group or the acid-labile tert-butoxycarbonyl (t-Boc) group. The beneficial use of the benzoyl group as photo-labile leaving group was recently demonstrated (C. Kocher, C. Weder, P. Smith, J. Mater. Chem., 2003, 13, 9), while the t-Boc group has long been employed as protecting group in peptide synthesis and proved useful as cleavable group for different chromogenic systems (C. Kocher, P. Smith, C. Weder, J. Mater. Chem., 2002, 12, 2620). Thus, the present dichroic chromogenic dyes 2Bz, 2Boc, 3Bz and 3Boc as well as 1Bz and 1Boc derived from the commercially available dye Sudan Red B were synthesized. The structures of the discussed chromophores are shown in Fig. 2 (Figure 2 displays the structures of the present dichroic chromogenic azo-dyes. Structures with R=Bz represent photo-labile chromogenic dyes, structures with R=Boc are acid-labile systems and structures with R=OH are the red colored equivalents released upon adequate treatment).

[0035] As will be discussed in detail below, the benzoyl-caged yellow-colored chromogenic dyes 1Bz, 2Bz and 3Bz could be reverted to their respective red-colored o-hydroxy-equivalents by simple exposure to short-wavelength UV light (see Figure 1). In the case of the acid-labile chromogenic dyes, the conversion was achieved by acid-catalyzed removal of the t-Boc group (cf. Figure 1). Spatially resolved conversion in a lithographic process in this case can be achieved by employing photo acid generators (PAG's, E. Reichmanis, F. M. Houlihan, O. Nalamasu, T. X. Neenan, Chem. Mater., 1991, 3, 394), which have been developed for microlithography of chemically amplified resists for microelectronics technology. Upon irradiation, for instance in a photolithographic process, these PAG's release an acidic species, which subsequently removes the t-Boc group (cf. Figure 1).

[0036] The deprotection kinetics are highly temperature-dependent and are therefore commonly carried out in a post-exposure heat treatment which can be referred to as 'post-exposure bake'. The PAG employed here was benzenesulfonic acid phenyl ester which is also used in microlithography. The t-Boc-protected chromogenic dyes are also cleaved in absence of acid, but only at much higher temperatures well exceeding the processing temperatures employed for the production of the present LLDPE blend films. As revealed by thermal analysis, decomposition of these compounds occurs at temperatures exceeding 180 °C.

Production, conversion and analysis of dichroic chromogenic polymer-blend films

[0037] All present chromogenic dyes were melt-blended with LLDPE under common melt-processing conditions. In all cases, the processing temperature was above the melting points of the dyes, but well below their decomposition temperature. LLDPE blend films containing 1OH, 2OH, 3OH, 1Bz, 2Bz or 3Bz were processed at 180 °C; films containing the somewhat heat-sensitive compounds 1Boc, 2Boc or 3Boc were processed at 140 °C. As photo-acid generator, benzenesulfonic acid phenyl ester was employed in a concentration of 3% w/w in blend films containing the acid-labile species 1Boc, 2Boc or 3Boc. The isotropic melt-processed blend films were stretched at a temperature of up to 120 °C to a draw ratio λ of 8. Polarized absorption spectra (principal spectral absorptions) were recorded with polarized light with the wave vector parallel (p-polarized, 0°, A_¦¦) and perpendicular (s-polarized, 90°, A_┴) to the stretching direction of the blend films. From the absorption values in the absorption maximum the dichroic ratio DR_A (DR_A = A_¦¦/ A_┴) was determined. Conversion of the photo-labile chromogenic dyes 1Bz, 2Bz and 3Bz was achieved by exposure to 254 nm UV light for time intervals up to 1 h. It should be noted that the relatively long exposure times are related to the low intensity of the UV light source employed (340 µW/cm²) and the reduced transparency of the matrix material of the relatively thick film samples at the employed wavelength (254 nm). It is obvious that optimization of these conditions can reduce the required exposure times into the seconds regime. The acid-labile chromogenic dyes 1Boc, 2Boc and 3Boc were converted upon photoactivation of the PAG for 30 min with 254 nm UV light from a conventional lab-type UV lamp and subsequent post-exposure heat treatment at 100 °C for 1 h. With the PAG content employed and the film thickness of typically 30-40 µm annealing intervals of seconds to minutes were found to be sufficient for virtually complete conversion; notetheless, by heat treatment for 1 h, any remaining PAG was additionally sublimed off, rendering the device unsusceptible to repeated UV exposure or to undesired conversion in daylight. To prevent undesired shrinkage during the post-exposure heat treatment, the stretched blend films were mounted in a constraining device.

Properties of series 1

[0038] Uniaxially drawn polymer blend films stretched to a draw ratio 1/1₀ of 8 comprising 0.2% w/w of 1OH exhibited an absorption maximum centered at 520 nm and a dichroic ratio DR_A of 5.3 at the absorption maximum. Stretched blend films containing the chromogenic derivatives 1Bz or 1Boc were characterized by an absorption maximum centered at considerably shorter wavelength when compared to films with the unprotected 1OH, i.e. around 407 nm (in both cases). Therewith, these films are characterized by a yellow-orange color, while films comprising 1OH display a distinct red color. The DR_A for blend films comprising 1Bz was determined to be 2.4 and the one for films with 1Boc 2.9. Therewith, the DR_A is in both cases considerably lower than in the case of stretched films produced from blends comprising the unsubstituted dye 1OH (DR_A=5.3). As mentioned above, this finding is can be explained, since the introduced bulky protective groups have a detrimental effect on the aspect ratio of the molecule. Accordingly, the molecules are less efficiently aligned upon uniaxial deformation of the blend film.

[0039] Upon conversion of 1Bz in a stretched blend film by exposure to 254 nm UV light for up to 1 h, the characteristic absorption band of 1OH was restored. The DR_A measured for this absorption band amounted to 2.1, and is therewith, within the experimental error, comparable to the DR_A of 2.4 of the unconverted system. In the case of blend films comprising 1Boc, conversion was achieved by photoactivation of the PAG additionally contained in the film and subsequent post-exposure heat treatment. Also in this case, the characteristic absorption band of 1OH was restored, and a DR_A of 2.3 was measured in the absorption maximum, which again is similar to the DR_A of 2.9 measured for the system prior to conversion.

[0040] These data indicate that upon removal of the protective group of both 1Bz and 1Boc, the alignment of the molecule and, therewith, the macroscopically observed dichroism remained virtually unchanged. Importantly, the distinctly higher orientation normally achieved in systems containing 1OH is not obtained.

Properties of series 2

[0041] Films comprising the uncaged dye 2OH were characterized by a deep-red color. In comparison with blend films comprising 1OH, these films exhibited a more intense, bluish-shade red hue. The absorption maximum of 2OH in LLDPE was located at a somewhat longer wavelength than the one of 1OH, i.e. centered around 535 nm (see Table 1). This influence of an additional substituent in the 6-position of the naphthalene-system on the absorption properties is in accordance with results published on the mono-azo dye 4-phenylazo-3,7-diol (E. F. Saad, E. A. Hamed, A. El-Faham, Spectroscopy Lett., 1996, 29, 1047). (Note that according to IUPAC-nomenclature, substituent numbers in the systematic names not necessarily comply with the position numbers in the naphthalene-system. For clarity, we will in the following refer to substituent positions using the numeration of the naphthalene-system, as depicted in Figure 1)

[0042] Stretched blend films comprising the alkoxy-substituted dye 2OH exhibit a DR_A of 4.6, which is somewhat lower than the DR_A of 5.3 observed for the unsubstituted dye 1OH.

[0043] The absorption maxima of blend films comprising the caged dye 2Bz and 2Boc were found to be located at 373 and 375 nm, respectively. The hypsochromic shift upon derivatization of 2OH amounted accordingly to as much as 160 nm, and blend films containing the chromogenic derivatives 2Bz and 2Boc displayed a distinct color change from yellow to dark red upon conversion. The change of the spectral properties of an isotropic blend film comprising 0.2% of 2Bz upon exposure to UV light (254 nm) is shown in Fig. 2. The observation of a distinct isosbestic point at 473 nm indicates exclusive conversion of 2Bz into 2OH. Development was found to strictly follow first order kinetics (cf. inset in Fig. 3).

[0044] Figure 3 shows the conversion of 2Bz comprised in a LLDPE film (0.2% w/w 2Bz, thickness 100 µm) upon irradiation with UV light (254 nm). The inset shows the increase of the absorption corresponding to the released species 2OH (measured at 530 nm). Dots are measured absorption values, the curve represents a first order kinetics fit

[0045] The DR_A of blend films comprising 2Bz and 2Boc was established to be 3.4 and 3.8, respectively. Compared to the chromogenic dyes 1Bz and 1Boc that do not bear a long alkoxy chain and exhibited DR_A's of 2.4 and 2.9, respectively, the dichroism in the dodecyloxy-substituted systems was clearly enhanced. The same is true for developed films of 2Bz and 2Boc. The dichroic ratio of a developed film of 2Bz was established as 2.6, and the DR_A of a developed film of 2Boc remained at 3.8. While in patterned films obtained with chromogenic dyes of system 1 the visual dark/bright effect observed by investigating the sample through a rotating optical polarizer can be considered moderate, in the case of system 2 patterned films, the effect was readily perceived and sufficiently pronounced for the envisioned applications. Fig. 4 shows the dichroic effect generated by two perpendicularly stacked, stretched LLDPE films comprising the dichroic, chromogenic dye 2Boc (0.2% w/w 2Boc, 3% w/w PAG, 1/1₀=8) imprinted with a pattern in a photolithographic method. Red areas (dark) correspond to exposed, yellow regions (bright) to protected regions. Photographs of the object were taken without polarizer (a) as well as through an optical polarizer at different angles (b: horizontal, c: 45 deg d: vertical). Upon rotation of the polarizer, a color inversion effect was observed.

Properties of series 3

[0046] Blend films comprising the unprotected dye 3OH were characterized by an orange-red color and an absorption spectrum with a maximum at 491 nm. In comparison with the two dyes 1OH and 2OH, the absorption of 3OH is located at a somewhat shorter wavelength (see Fig. 5, which shows polarized absorption spectra of 3Boc comprised in a stretched LLDPE film (0.2% w/w 3Boc, 3% w/w PAG, 1/1₀=8) before conversion (thin line, 0°) and after acid-catalyzed conversion to 3OH (bold line, 0°). The respective dotted lines represent the absorption perpendicular to the stretching direction (90°)). This hypsochromic effect caused by a substituent in the 4-position of the naphthalene-system has already been observed for the mono-azo dye 4-phenylazo-naphthalene-1,3-diol and has been attributed to a reduced stability of the hydrazone-form due to a relatively weak hydrogen bond between the o-hydroxy group and the azo-bridge.

[0047] The absorption band of blend films comprising the protected chromogenic dyes 3Bz and 3Boc were centered around 424 and 439 nm, respectively. The bathochromic shift upon derivatization of 3OH was therewith less pronounced than in the previously discussed cases of systems 1 and 2, amounting to approximately 50 nm. Conversion of films containing the caged dyes 3Bz or 3Boc, consequently, show a color-shift from yellow to a distinct orange-red hue.

[0048] Remarkably, the DR_A displayed by stretched blend films comprising 3OH was found to be as high as 25 at a draw ratio of 8. Compared with blend films containing the unsubstituted dye 1OH, the dichroic effect was therewith dramatically improved. Consequently, upon examining a stretched sample comprising 3OH through a rotating optical polarizer, the appearance of the film changed from orange-red to almost colorless, while in the case of samples comprising 1OH or 2OH, the red shade remained apparent to some extent when observing the sample through an optical polarizer with its optical axis aligned parallel to the stretching direction of the film.

[0049] The chromogenic derivative 3Bz of 3OH in a stretched LLDPE film displayed a DR_A of only 3.3. In the case of 3Bz, the developing absorption band exhibited a DR_A of 3.7. Again, the detrimental effect of the rigid benzoyl group located in an off-center position to the long molecular axis becomes apparent. As was previously seen for both 1Boc and 2Boc, also with 3Boc a higher degree of orientation in a stretched blend film was obtained than with its benzoyl-substituted counterpart. The DR_A of stretched blend films comprising 3Boc substituted with the more flexible t-Boc-group was determined as 7.2. Upon conversion of 3Boc, the absorption band of 3OH was restored. For this absorbance band, a DR_A of 5.1 was measured. Although the dichroic ratios obtained with the latter chromogenic system seem low when compared to the DR_A of 25 obtained in stretched blend films comprising 3OH, the dichroic effect was equivalent to the effect obtained with the unsubstiuted dye Sudan Red B, and therewith sufficient for the envisioned applications. This is illustrated in Figure 6, which shows the polarization effect displayed by a stretched LLDPE film comprising the dichroic, chromogenic dye 3Boc (0.2% w/w 3Boc, 3% w/w PAG, 1/1₀=8) imprinted with a pattern in a photolithographic method. The red symbol (cross) developed upon UV exposure and subsequent heat treatment, the yellow regions (bright) were protected from UV radiation. Photographs of the object were taken through an optical polarizer with its optical axis (small arrows) aligned parallel (lower left fraction) and perpendicular (upper right fraction) to the stretching axis of the film which is aligned horizontally (large arrow).

Conclusions

[0050] We synthesized a number of chromogenic dyes capable of adopting a preferred orientation in stretched LLDPE blend films and thus yielding dichroic chromogenic systems. We chose to prepare compounds similar to chromogenic diazo-dyes additionally bearing a long alkoxy chain extending the long axis of the molecule in order to improve the orientation of the molecule in a drawn polyolefin film. With this approach, the detrimental effect on the orientation of an introduced cleavable protective group responsible for the chromogenic property of this class of molecules could be compensated. Clearly, for features displaying higher dichroic ratios and more pronounced color changes, different chromogenic dyes with cleavable groups located preferentially in-line with the long molecular axis could be synthesized. However, to our best knowledge, this is the first demonstration of dichroic color patterns produced in stretched polymer films.

Experimental

Materials

[0051] All chemicals and solvents purchased were of analytical grade. Anhydrous solvents were used for the esterification and etherification reactions (H₂O < 0.01 %). For the preparation of polymer blend films, a commercial-grade LLDPE obtained from Dow Chemicals was used (Dowlex BG 2340). Sudan Red B (1-(3-methyl-4-m-tolylazo-phenylazo)-naphthalen-2-ol, C.I. 26110) was purchased from Aldrich and used as received. All solvents were of analytical grade. Pyridine was dried by refluxing over KOH and subsequent distillation onto molecular sieves. For the preparation of polymer blend films, a commercial-grade LLDPE obtained from Dow Chemicals was used (Dowlex BG 2340).

Synthesis

[0052] Benzoic acid 1-(3-methyl-4-m-tolylazo-phenylazo)-naphthalen-2-yl ester (1Bz). 408.0 mg (1.1 mmol) of 1-(3-methyl-4-m-tolylazo-phenylazo)-naphthol were dissolved in 5 mL anhydrous pyridine. To the stirred mixture, 0.5 mL (0.7 g, 5.5 mmol) of benzoyl chloride were added. The mixture was heated to reflux and stirred for 2 h. Subsequently, the mixture was allowed to cool to room temperature, washed with water (3x 100 mL) and extracted with CH₂Cl₂ (150 mL). The organic layer was dried over MgSO₄, filtrated, and the solvents were evaporated. Purification was realized by column chromatography (CH₂Cl₂, silica gel), resulting 387.0 mg (0.8 mmol, 75%) of a brick red solid. Mp 155 °C.

[0053] Carbonic acid tert-butyl ester 1-(3-methyl-4-m-tolylazo-phenylazo)-naphthalen-2-yl ester (1Boc). 1005.0 mg (1.1 mmol) 1-(3-methyl-4-m-tolylazo-phenylazo)-naphthol were dissolved in 10 mL anhydrous pyridine. To the stirred mixture, 890.5 mg (4.1 mmol) of di-tert-butyl dicarbonate were added. The mixture was stirred for 2 h at RT and subsequently washed with water (3x 100 mL) and extracted with CH₂Cl₂ (150 mL). The organic layer was dried over MgSO₄, filtrated, and the solvents were evaporated. The resulting orange, highly viscous oil was subjected to column chromatography (CH₂Cl₂, silica gel) to yield 1024.0 mg (2.1 mmol, 81%) of the product as a brick red solid. Mp 121 °C.

[0054] 1-(4-Phenylazo-phenylazo)-naphthalene-2,6-diol (2a). Diazotation and azo-coupling of p-phenylazoaniline was performed according to standard procedures. An amount of 198.7 mg (1.01 mmol) of p-phenylazoaniline was dissolved in 50 mL acetone. 50 mL water and 10 mL of 2 M HCl were mixed and added to the stirred solution, which was subsequently cooled to 5 °C. A sodium nitrite solution was prepared by dissolving 103.7 mg (1.50 mmol) of sodium nitrite in 50 mL water and cooling the solution to 5 °C. A sufficient quantity of this solution was added to the stirred p-phenylazoaniline-solution to indicate a slight excess of nitrous acid in the iodine-starch paper test. An amount of 200.4 mg (1.25 mmol) of 2,6-dihydroxynaphthalene was dissolved in 100 mL 0.05 M NaHCO₃ and cooled to 10°C. The diazotized p-phenylazoaniline-soulition was subsequently slowly added to the basic (pH 9-10) 2,6-dihydroxynaphtol-solution, whereupon a red precipitate was formed. The pH of the mixture was kept constantly at 7.5 by adding 1 M NaOH. After completed addition of the diazotized p-phenylazoaniline-solution, the mixture was stirred for 30 minutes with the cooling bath removed. The precipitate was filtered off and dried at 80°C for several hours at reduced pressure to yield 326.5 mg (88%) of the product as red powder. The product was employed in the subsequent reaction without further purification.

[0055] 6-dodecyloxy-1-(4-phenylazo-phenylazo)-naphthalen-2-ol (2OH). An mixture of 504.0 mg (3.65 mmol) K₂CO₃ and 5 mL of dimethylformamide were heated to 70 °C and stirred for 10 min under argon. To the mixture, 302.2 mg (0.82 mmol) 2a were added and stirred for 10 min. Subsequently, 208.3 mg (0.84 mmol) of 1-bromo-dodecane were added by the aid of a syringe. The mixture was stirred for 3 h at 70 °C and subsequently cooled to room temperature and diluted with ca. 50 mL water. The solution was washed with water (3x 100 mL) and extracted with CH₂Cl₂ (150 mL). The organic layer was dried over MgSO₄, filtered, and the solvents were evaporated. The resulting solid was subjected to column chromatography (CH₂Cl₂, silica gel) to yield 242.4 mg (4.52 mmol, 55% with respect to 2a) of the respective product (most mobile fraction) as a dark red powder.

[0056] Benzoic acid 6-dodecyloxy-1-(4-phenylazo-phenylazo)-naphthalen-2-yl ester (2Bz). An amount of 106.9 mg (0.2 mmol) of 3a were dissolved in 10 mL pyridine. To the stirred mixture, 56.2 mg (0.4 mmol) benzoyl chloride were slowly added by the aid of a syringe. The mixture was stirred for 2 h at reflux and subsequently cooled to room temperature, washed with water (3x 100 mL) and extracted with CH₂Cl₂ (150 mL). The organic layer was dried over MgSO₄, filtered, and the solvents were evaporated. The resulting solid was subjected to column chromatography (CH₂Cl₂, silica gel) to quantitatively yield the respective product (most mobile fraction) as dark orange solid. Mp 123 °C.

[0057] Carbonic acid tert-butyl ester 6-dodecyloxy-1-(4-phenylazo-phenylazo)-naphthalen-2-yl ester (2Boc). An amount of 112.9 mg (0.21 mmol) of 3a were dissolved in 10 mL pyridine. To the stirred mixture, 98.5 mg (0.45 mmol) di-tert-butyl-dicarbonate were added. The mixture was stirred for 2 h at room temperature, washed with water (3x 100 mL) and extracted with CH₂Cl₂ (150 mL). The organic layer was dried over MgSO₄, filtered, and the solvents were evaporated. The resulting solid was subjected to column chromatography (CH₂Cl₂, silica gel) to quantitatively yield the respective product (most mobile fraction) as dark orange solid. Mp 93 °C.

[0058] 4-(4-Phenylazo-phenylazo)-naphthalene-1,3-diol (3a). An amount of 200.3 mg (1.02 mmol) of p-phenylazoaniline was reacted with 161.6 mg (1.01 mmol) of 2,4-dihydroxynaphthalene according to the above procedure to yield 339.0 mg (0.92 mmol, 91 %) of the product as red solid. The product was employed in the subsequent reaction without further purification.

[0059] 4-Dodecyloxy-1-(4-phenylazo-phenylazo)-naphthalen-2-ol (3OH). Reacting 305.1 mg (0.83 mmol) of 3a with 213.8 mg (0.86 mmol) of 1-bromo-dodecane in a mixture of 5 mL of dimethylformamide and 502.3 (3.63 mmol) K₂CO₃ according to the above procedure for 2OH resulted 197.0 mg (0.22 mmol, 26 %) of the product as red solid.

[0060] Benzoic acid 4-dodecyloxy-1-(4-phenylazo-phenylazo)-naphthalen-2-yl ester (3Bz). Reacting 93.9 mg (0.17 mmol) of 3OH with 38.5 mg (0.27 mmol) of benzoylchloride in 5 mL of pyridine according to the above procedure for 2Bz resulted 93.9 mg (1.47 mmol, 84% with respect to 3OH) of the product as brick-red solid. Mp 90 °C.

[0061] Carbonic acid tert-butyl ester 4-dodecyloxy-1-(4-phenylazo-phenylazo)-naphthalen-2-yl ester (3Boc). Reacting 71.0 mg (0.13 mmol) of of 3OH with 93.1 mg (0.43 mmol) of di-tert-butyl-dicarbonate in 5 mL of pyridine according to the above procedure for 2Boc resulted 65.7 mg (0.10 mmol, 78% with respect to 3OH) of the product as brick red solid. Mp 116 °C.

Preparation of poly(ethylene) blend films. Blend films containing 0.2% w/w of the various dyes in LLDPE (Dowlex BG 2340, Dow Chemicals) were produced as follows:

[0062] Blend films containing 0.2% w/w of the dyes in LLDPE (Dowlex BG 2340, Dow Chemicals) were produced from a 1% w/w master batch, which was obtained by dissolving 5 mg of the respective dye in approx. 2 mL CH₂Cl₂ and decorating 495 mg of LLDPE pellets with that solution. After evaporation of the solvent at ambient, the decorated pellets were pressed into a blend film between aluminum sheets in a hot press (Carver 2518) at a load of 8 tons and subsequently cooled in a cold press (Carver M) at the same load. For the benzoyl-caged chromogenic dyes, the processing temperature was 180 °C, for the t-Boc-caged chromogenic dyes, the processing temperature was reduced to 140 °C in order to prevent any undesired conversion of these somewhat heat-sensitive compounds. The obtained blend films were cut into pieces, which were subsequently mixed and again processed into a film. This process was repeated four times in order to obtain a homogeneous distribution of the dyes within the polymer matrix. This 1% w/w blend film was then used as a master-batch to produce a 0.2% w/w blend film, by processing 100 mg of the 1% w/w blend film with 400 mg of LLDPE pellets, according to the above method. For blend films containing 0.2% w/w of the acid-labile chromogenic dyes and 3% w/w photoacid generator (PAG), 15 mg of the PAG (dissolved in approx. 2 mL CH₂Cl₂) additionally processed into the blend film accordingly. All films obtained were homogeneously colored and had a uniform thickness of typically 100 µm.

[0063] Photoactivation of LLDPE/chromogenic dye blend films. A stretched film sample containing 0.2% w/w of the respective dye (plus 3% w/w of benzene-sulfonic acid phenyl ester as PAG in the case of the acid-labile systems) was mounted on a supporting frame and exposed to UV light of a standard lab-type UV lamp (Bioblock Scientific, center frequency 254 nm, 340 µW/cm²). Prior to exposure and after desired exposure intervals, the UV spectrum of the sample was recorded vs. a neat LLDPE film sample of the same grade and of equal thickness as reference. After irradiation, the acid-labile systems were mounted to a constraining device and subjected to a post-exposure heat treatment at 100 °C for 1 h. For the sample demonstrating the spatially resolved conversion, a blend film of 100 µm thickness containing 0.2% w/w of the photo-labile chromogenic dye 3Boc and 3% of PAG was used and exposed for 30 min to 254 nm UV light through a photomask obtained by inkjet printing of the desired image on a poly(vinylalcohol) film. Pictures of the reproduced yellow/red picture were taken with a Leica DC200 digital camera mounted on a Leica MS5 microscope equipped with an optical linear polarizer.

[0064] The sample exhibiting the color-inverting symbol was made with two films containing 0.2% w/w of 2Boc and 3% of PAG according to the same procedure. The two films were imprinted in a perpendicular fashion with the same photomask and superimposed on a supporting glass substrate in a perpendicular manner. Photographs were taken as described above with the optical linear polarizer at the different angles indicated in the context of Fig. 4.

Claims

1. A security item, characterised by at least one security element having at least one segment comprising at least one latent VIS absorber with linearly polarized absorption and/or linearly polarized emission.

2. A security item according to claim 1, characterised in that the latent VIS absorber can be converted into a converted VIS absorbing form, and also the converted VIS absorbing form shows linearly polarized absorption and/or linearly polarized emission, wherein the converted VIS absorbing form has different spectral characteristics than the VIS absorber.

3. A security item according to one of the preceding claims, characterised in that the at least one latent VIS absorber is applied to or embedded in a matrix, which matrix is preferentially transparent for wavelengths in the visible region wherein preferentially also the converted VIS absorbing form shows linearly polarized absorption and/or linearly polarized emission.

4. A security item according to one of the preceding claims, characterized in that 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), polyamide (PA), 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, characterized in that the matrix shows uniaxial orientation as obtained by uniaxial stretching of the matrix with the embedded latent VIS absorber, wherein preferentially the drawing ratio (1/10)of the stretching is is more than 2, even more preferentially above or equal to 4 or 8.

6. A security item according to one of the preceding claims, characterized that the latent VIS absorber and/or its converted absorbing form show a dichroic ratio of more than 2 in absorption and/or emission, preferentially a dichroic ratio of more than 5 in absorption and/or emission, and most preferentially a dichroic ratio of more that 10 or even more than 20 or 50.

7. A security item according to one of the preceding claims, characterised in that the latent absorber is colourless or preferentially coloured in its pristine form, wherein if couloured in its pristine form the colour of the pristine form differs from the one of the converted absorbing form.

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

9. A security item according to claim 8, characterised in that more than one latent VIS absorbers showing different colours in the pristine form and/or in the converted VIS absorbing form are employed, wherein preferentially the different substances are arranged in a spatially resolved manner.

10. A security item according to one of the preceding claims, characterized in that there is at least two layers comprising latent VIS absorber, wherein the latent VIS absorbers are at least partially converted to their converted VIS absorbing forms in a spatially resolved manner, and wherein these layers comprise different latent VIS absorbers and/or show different orientation of the linear polarization, wherein preferentially there is two layers with orthogonal directions of polarization.

11. A security item according to one of the preceding claims, characterised in that the security element is in a form selected from the group consisting of fibres, threads, strips, films, sheets, layers, tapes, plates, discs, chips and/or combinations thereof.

12. A security item according to one of the preceding claims, characterised in that the latent VIS absorber can be converted in a spatially resolved manner into its converted VIS absorbing form using irradiation or chemical treatment, preferentially by locally modifying the pH value, or a combination of these methods.

13. A security item according to one of the preceding claims, characterised in that the latent VIS absorber and/or its converted VIS absorbing form is a dichroic rod-like molecule, the transition dipole moment of which preferentially substantially coincides with the geometrical long axis of the molecule.

14. A security item according to claim 13, characterised in that the latent VIS absorber is provided with at least one photo-labile and/or acid-labile chromogenic leaving group.

15. A security item according to one of the preceding claims, characterised in that the latent VIS absorber and/or its converted VIS absorbing form is a derivative of Sudan Red B, or Dihydroxynaphthol, in particular (4-Phenylazo-phenylazo) derivatives thereof, or a mixture thereof, wherein these compounds are provided with photo-labile and/or acid-labile chromogenic leaving groups.

16. A security item according to claim 15, characterised in that the latent VIS absorber is a Benzoic acid or a Boc derivative of Sudan Red B, of 1-(4-Phenylazo-phenylazo)-naphthalene-2,6-diol, of 6-alkyloxy-1-(4-Phenylazo-phenylazo)-naphthalene-2,6-diol with the alkylgroup preferably in the range of C1-C20, in particular of 6-dodecyloxy-1-(4-Phenylazo-phenylazo)-naphthalene-2,6-diol, of 4-(4-Phenylazo-phenylazo)-naphthalene-1,3-diol, or of 6-alkyloxy-4-(4-Phenylazo-phenylazo)-naphthalene-1,3-diol, with the alkylgroup preferably in the range of C1-C20, in particular of 6-dodecyloxy-4-(4-Phenylazo-phenylazo)-naphthalene-1,3 -diol.

17. 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 VIS absorber and areas essentially containing the converted VIS absorbing form 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.

18. A method of producing security items according to one of the claims 1 through 17, characterised in that an object is provided with at least one security element having at least one segment comprising at least one latent VIS absorber with linearly polarized absorption and/or linearly polarized emission.

19. A method according to claim 18, characterised in that the security element is made by melt processing a polymeric substance or blend with a latent VIS absorber, forming a corresponding object like a fibre or films, and by subsequently drawing the object in one direction, preferentially using a draw ratio between 2 and 10, preferentially between 4 and 8.

20. A method according to one of the claims 18 or 19, characterised in that the latent VIS absorber is at least partially converted to its converted VIS 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.

21. A method according to claim 20, characterised in that a photographic process, lithographic process, screen printing process, inkjet printing process or laser printing process is employed to at least partially convert the latent VIS absorber into its converted VIS absorbing form.

22. A method according to claim 21, characterised in that the conversion involves the chemical, preferentially acid-induced, or photo-chemical elimination of an element of the latent VIS absorber, preferentially by means of a photoacid generator and subsequent stabilization by evaporation of the photoacid generator.

23. A method for verification of the authenticity of security items according to one of the claims 1 through 17, characterised in that a electronic device is employed to visualize or read out the information contained in the security item.

Drawing