This invention relates to transparent pigments that are made of chiral birefringent liquid crystal polymers for covert security printing applications.
It is desirable to add security features to a variety of documents, articles, or product packages for authentication purposes. Holographic authentication features have been used for more than two decades and by now they are relatively easy to counterfeit. Recently optical variable pigments (OVP) were introduced to the market as a new technology for overt anti-counterfeiting applications. Among them, for example, ChromaFlair® (JDS Uniphase Flex Products Group in Santa Rosa, Calif.) and Helicone® (LCP Technology GmbH in Burghausen, Germany) are two products that are were adopted for optical security applications.
Both ChromaFlair® and Helicone® products are highly reflective in the visible spectrum and can be used for overt marking. They exhibit a unique color travel property that can be used in bank notes and other security printing, as well as for decorative applications. As the angle at which the surface is viewed is increasing, the perceived wavelength becomes shorter (“blue shift”). The Helicone® product, being a cholesteric liquid crystal polymer, has in addition a secondary security feature: it reflects and transmits opposite circular polarizations in a wavelength band corresponding to its reflective color.
UV fluorescence ink or patterned birefringent films are two examples of practical technologies that are used for covert markings. Patterned birefringent films are overlaid with a polarizer to reveal hidden images (e.g. U.S. Pat. No. 6,144,428). However, this approach requires patterning of features in an aligned polymerizable liquid crystal film and hence requires special and complex substrate coating, alignment and processing. A more recent technology invented by Karasev (U.S. Pat. No. 6,740,472) relates to the use of non-opaque latent image layer made in anisotropic polymer material which becomes visible when viewed with a polarizer. Both methods have the drawbacks of requiring complicated processes such as UV exposure through masks and/or special substrate treatment to create hidden images. Furthermore, these methods have a common drawback of material waste since only partial areas contain the hidden marks. It is very desirable to have the security medium in the form of pigments that can be simply printed by many of the existing printing technologies on a wide variety of substrates.
Non-chiral birefringent pigments technology recently disclosed by Hammond-Smith (U.S. Pat. No. 7,297,292) attempts to addresses the above problem. However, this technology is vulnerable to counterfeiting due to the availability of many other, functionally similar, birefringent pigments, such as mica or calcite pigments, or other pigments or flakes made by stretching synthetic polymer films.
Cholesteric liquid crystal materials (CLC) are an example of a chiral birefringent material that posses a periodic structure. When the period length (“pitch”) is in the 0.2-2 micrometers range, such materials posses a narrow polarized reflection band which is situated, depending on the pitch value, approximately within the 300-3000 nm wavelength range (UV-Visible-IR). Chiral birefringent materials and CLC in particular, are essentially transparent with the exception of the reflection band. In the 1980's, crosslinkable cholesteric liquid crystal materials were developed enabling the “locking-in” of their unique reflection properties. CLC polymers may be formed into pigments known also as “flakes” or “platelets”, for example, as disclosed in Faris U.S. Pat. Nos. 5,364,557, 5,599,412, and 6,338,807, which are incorporated by reference herein. Sicpa (LCP Technologies) launched the Helicone pigments, based on CLC pigments, for overt security printing applications.
Yet a covert security ink which can be detected using simple means, that is compatible with conventional printing processes, has low material waste, and is difficult to counterfeit is highly desirable.
The above-discussed and other problems and deficiencies of the prior art are overcome or alleviated by the present invention.
As described herein, security pigment systems generally comprise composite of a reflective material and a security ink comprising transparent chiral birefringent material and a transparent carrier medium. In certain embodiments, the reflective material comprises a reflective base layer, upon which the security ink may be applied. In other embodiments, the reflective material comprises reflective pigments that are printed below the security ink or are mixed with the security ink. The security ink is invisible when viewed with naked eye and becomes visible as an achromatic bright mark on a dark background when viewed with a circular polarizer. Furthermore, the security ink possesses a selective wavelength reflection features and polarized reflection features that are detectable by detection devices.
The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.
Herein disclosed are security pigment systems generally a composite of a reflective material, and a security ink comprising transparent chiral birefringent material and a transparent isotropic carrier medium. In certain embodiments, the reflective material comprises a reflective base layer, upon which the security ink may be applied. In other embodiments, the reflective material comprises reflective pigments that are mixed with the security ink. The security ink is invisible when viewed with a naked eye. However, for authentication of the article upon which the composite of a reflective material and a security ink described herein is applied, an authenticator (e.g., including but not limited to a financial institution, consumer, retailer, or manufacturer) may view the article with a circular polarizer, whereafter the security ink is visible.
The security ink comprising transparent chiral birefringent material may be formed by breaking films of transparent chiral birefringent material into platelets, also referred to herein as flakes or pigments (generally having major surfaces corresponding to the faces of the films, and minor surfaces, or edges). For instance, cholesteric liquid crystal polymer films may be formed into small platelets, where each flake effectively retains the optical property of the film. The cholesteric helical axis is essentially perpendicular to the flakes major surfaces. In contrast to commercial CLC pigments products, which reflect part of the visible light wavelength in the 380-780 nm range, the chiral birefringent materials herein do not reflect light within the visible wavelengths.
In particular, in certain preferred embodiments herein, the pigments are platelets of CLC polymer film in which the reflection peaks are outside of the visible of spectra, e.g. in ultraviolet (UV) or infrared (IR) regions. The birefringence of these transparent CLC platelets in the visible spectrum is significant. For instance, the birefringence may be generally greater than about 0.01, preferably greater than about 0.1.
In one embodiment, the background or base layer is a highly reflective material that upon reflection inverts the circular sense of the incident circularly polarized light. For example, highly reflective material such as metallic or dielectric films may be employed. When such base material is overlaid with a circular polarizer it appears very dark. In certain preferred embodiments, the highly reflective material as a base layer comprises of aluminum.
To detect the security ink herein, a viewing polarizer is employed in the form of a circular polarizer. Either left-handed or right-handed circular polarizer can be used for detection. The circular polarizer circularly polarizes the incident light as well as enables analysis of the reflected light. The circular polarizer is overlaid upon the surface of the article containing the security ink.
In certain embodiments described herein, the security ink can also be mixed with reflective pigments formed from reflective materials in the visible spectrum such as metallic pigments, pearlescent pigments and/or optically variable pigments (OVP). In certain preferred embodiments, a highly reflective material used to form reflective pigments comprises metallic pigments. In this embodiment, the reflective pigments act as the reflective background for the security inks. The mixture may therefore be printed on any substrate, since the reflective pigments serve as reflectors for the security ink.
In further embodiments, the mixture of reflective pigments and security ink can be arranged in a manner whereby certain parts of the article contains only reflective pigments, and the other parts contain the mixture of the security ink and the reflective pigments. This arrangement allows the authenticator to encode covert features in different positions (as, for example, in
In additional embodiments, the presence of a reflection band of the security ink in the UV or IR spectrum and/or the circular polarization nature of the reflection or transmission from this band can be used as a secondary detection feature. For instance, a spectrometer can be utilized to confirm the presence of a reflection band outside the visible wavelengths range. Alternatively, a detection system which admits a narrow wavelength range within the reflection band which is also equipped with a circular polarizer can verify the existence of the reflection band outside the visible range and that within this band the reflected light is predominantly circularly polarized. These secondary detection features provide means to distinguish CLC pigments from regular linear birefringent pigments thereby secure them from being counterfeited by the latter.
Referring now to
In one preferred embodiment as shown with respect to
As shown in
Transparent chiral birefringent pigments such as transparent CLC pigments have strong birefringence. However, due to the helical configuration of the molecules (see
We observed that due to inevitable thickness variations of any flakes, nematic flakes appear to reflect multiple discernable colors that vary with the viewing angle when viewed with circular polarizer. In contrast, we discovered that transparent CLC pigments have an achromatic bright uniform appearance under the same viewing condition over a large viewing cone even though they too have thickness variations. The above-mentioned feature and the fact that the signal brightness is peaked at large viewing angle make said CLC pigments distinguishable from the non-chiral birefringent pigments and allows for easy authentication and, at the same time, difficult to counterfeit by ubiquitous non-chiral birefringent pigments such as mica flakes.
In addition to visual detection, optical signals reflected from the transparent chiral birefringent pigments at their intrinsic reflection bands—in UV or IR—can be detected using a spectrometer as a secondary detection. The invisibility of the reflection band allows more freedom in designing custom marking at different wavelengths. Another secondary detection feature is the circular polarization nature of the reflected light in the invisible reflection band. A simple detection system for this feature consists of a circular polarizer in front of a detector which is sensitive only to wavelengths inside the invisible reflection band. A large signal is detected when the circular polarizer matches the circular nature of the light reflected by the CLC pigments while the signal is low for a polarizer of an opposite circular sense.
In certain preferred embodiments, it is desirable to index-match the visibly transparent chiral birefringent pigments to their carrier (binder) and the overcoat materials to eliminate light scattering from their interfaces to render them invisible to the naked eye. In preferred embodiments, where cholesteric liquid crystal polymers flakes are employed as the transparent chiral birefringent pigments, the birefringent properties it possesses include two different indices of refraction. The pigment carrier and the overcoat are usually optically isotropic materials, whereby each possesses only one index of refraction. Mismatch between the indices of the pigments and their carrier or overcoat lead to light scattering that may be observable by the naked eye and may render the mark visible. To minimize the light scattering it is desirable to choose materials such that the difference among the carrier, the overcoat, and pigment indices will be as small as possible. In one embodiment, the reflective background should not be mirror-like but rather have a reflective surface that has a diffusive component. By making the whole area where the invisible mark is embedded to appear diffusive, one can hide the light scattering from the pigments. On the other hand, the reflective surface should not be too diffusive as to destroy polarization of the incident light. In another preferred embodiment, a non-birefringent, light scattering material is added to the overcoat clear ink such that the background scattering is similar to the light scattering from the security ink, thus rendering the mark undetectable by the naked eye.
Referring now to
In further embodiments, and referring now to
Note that the various pigments, mixtures and the like described herein may be applied directly, or alternatively, such pigments or pigment mixtures may be formulated with suitable carrier materials and/or adhesives, as is known in the ink formulation art. For example, suitable carriers and adhesives are disclosed in U.S. Pat. Nos. 5,364,557, 5,599,412, and 6,338,807, which are incorporated by reference herein and mentioned hereinafter. Such materials may be applied, e.g., to a document, banknote or other article to be encoded or marked with a herein described pigment mixture. The adhesives and/or carriers may be present in amounts of 0% to over 99% of the material mixture, depending on various factors. Such application may be by methods of printing including, but not limited to, screen-printing, gravure, ink jet printing, roller printing, pens, crayons, brush applications, or other methods, as disclosed in aforementioned U.S. Pat. Nos. 5,364,557, 5,599,412, and 6,338,807. Further, the pigment mixtures described herein may be applied by dry printing methods and systems, e.g., as disclosed in Jiang et al. U.S. Pat. Nos. 6,515,717 issued on Feb. 4, 2003 entitled “Computer-Based System for Producing Multi-Color Multilayer Images on Substrates Using Dry Multi-Colored Cholesteric Liquid Crystal (CLC) Pigment Materials Applied to Binder Material Patterns”, and 6,387,457 issued on May 14, 2002 entitled “Method of Dry Printing and Painting”, both of which are incorporated by reference herein.
The chiral birefringent materials may include any suitable materials that reflect substantially one circular polarization state (e.g., only left-handed or right-handed circularly polarized light). Such polarization reflective materials, for example, may include pigments based on CLC materials. Preferably, such CLC materials are provided as polymeric materials. CLC polymers are crosslinked organic materials with their molecules fixed in the cholesteric phase. For example, siloxane and acrylate based CLC pigments are known. One format for such CLC polymers used herein is based on a CLC polymer film that is generally fractured into small platelets, retaining all the optical properties of the CLC film. Such materials are described, for example, in Faris U.S. Pat. Nos. 5,364,557 issued on Nov. 15, 1994 entitled “Aligned Cholesteric Liquid Crystal Inks”, 5,599,412 issued on Feb. 4, 1997 entitled “Method and Apparatus for Producing Aligned Cholesteric Liquid Crystal Inks”, and 6,338,807 issued on Jan. 15, 2002 entitled “Cholesteric Liquid Crystal [CLC] Based Coloring Media for Producing Color Effects Having Improved Brightness and Color Characteristics”, all of which are incorporated herein by reference. CLC pigments that reflect visible light are also commercially available from Sicpa under the trade name Helicone®.
Many different articles require anti-counterfeiting measures. For example, it is very desirable to prevent counterfeiting of paper products such as currency, bank notes, stock certificates, stationary, legal documents and tickets. In addition, other articles of value may require anti-counterfeiting measures such as tokens, chips, pharmaceutical products, consumer healthcare products, food, software, media (DVDs, videocassettes), consumer electronic devices, industrial electronic devices, military electronic devices, batteries, medical devices, luxury goods (e.g., designer clothing, handbags, wallets), eyewear, artwork, collectibles including sports and celebrity memorabilia, automotive parts, jewelry, wristwatches and other timepieces, tobacco products, alcoholic beverages, or any article whereby authenticity is important to the consumer, the manufacture, the retailers, or any or all of the above. Thus, all of the above articles may be protected implementing the security ink systems and methods described herein.
While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.
The present invention is related and claims priority to U.S. Provisional Patent Application, Ser. No. 61/198,458, entitled “Invisible Pigments and Ink” and filed on Nov. 6, 2008. The U.S. Provisional Patent Application is hereby incorporated by reference in its entireties.
Number | Date | Country | |
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61198458 | Nov 2008 | US |