SECURITY DEVICE WITH MULTIPLE AUTHENTICATION FEATURES

Abstract

A security device comprising the following components: a) 30-80 wt % mono-functional LCP's, b) 0-50 wt % higher functional LCP's, preferably below 50 wt % c) 0-30 wt % liquid crystalline inert monomers, preferably below 20 wt % d) 0-50 wt % non-liquid crystalline (mono- or higher) functionalized monomers, preferably below 30 wt % e) 0-30 wt % non-liquid crystalline inert monomers, preferably below 20 wt % f) 0.01-10 wt % initiators, preferably below 2 wt % g) 0.-10 wt % inhibitors, preferably below 2 wt % h) 0.01-50 wt % additives, preferably below 20%, more preferably below 10 wt %, with the provisions that the total amount of the components is 100 wt %, characterized in that the additives h) comprise magnetic additives, such as paramagnetic, super-paramagnetic, diamagnetic or ferri-magnetic particles

Description

The present invention pertains to a security device in particular as authentication feature for prevention of counterfeiting of documents and/or products.

In order to prevent counterfeiting, there is a continuing need to secure valuable documents and/or products. Adding authentication features, which are very difficult to forge but preferably easy to inspect, to these products helps against counterfeiting. There are many authentication features known in the art. Many are based on well-known and thus non-distinct optical effects. Examples of optical authentication features are holograms, based on diffracting structures, watermarks, based on thickness differences in semi-opaque substrates and fluorescent markings. Furthermore, gaining access to the materials and technology to produce these or highly similar effects is often straightforward. The strength of authentication features produced with Polymerizable Liquid Crystals (LCP's) are the many possible optical effects, which are much less known and very difficult to imitate

Polymerizable Liquid Crystals (LCP's) are a class of materials which exhibit one or more liquid crystalline phases, such as a nematic, smectic or chiral nematic (also called cholesteric) phase, within a certain temperature range. Furthermore, LCP's can be polymerized due to reactive groups which are part of the molecule. Before polymerization, LCP's are monomers, but also after polymerization the resulting polymers are commonly referred to as LCP's. In the text, where LCP's are mentioned the monomer form is referred to; the polymer form is referred to as LCP polymer. Moreover, the skilled person is able to differentiate between the polymeric and monomeric LCP's in the context of the specification and by using his common knowledge. Polymerization of LCP's can be induced spontaneously at elevated temperatures or aided by means of suitable initiators, such as for instance photo-initiators or thermal initiators. Common examples of reactive groups are acrylates, methacrylates, epoxies, oxethanes, vinyl-ethers, styrenes and thiol-enes. Here, monomers which by means of reactive end groups have the ability to form links with two other molecules are called mono-functional, since two links are the minimum number required to form a polymer. Monomers with the ability to form links with more than two other molecules are called higher functional.

It is possible to obtain security devices by printing flakes of LCP polymers which are formulated into an ink, but these flakes are expensive and difficult to produce as well as apply. Such flakes are for instance disclosed in EP0968255. The production process of flakes consists of a number of time-consuming steps before the flakes are polymerized and broken into pieces of suitable size. Afterwards, these flakes are mixed into a transparent polymerizable binder. Due to the average size of the flakes in the binder as well as the variability of the flake geometry, the print resolution is limited. Furthermore, flakes are not suited for inkjet printing as the flakes clog the conduits and nozzles in the inkjet head. Furthermore, the optical effects are not as uniform nor as clear and striking as a single layer of LCP.

Another method to create security devices containing LCP-based authentication features is by inkjet printing of LCP's to form features with distinct optical properties. Furthermore, inkjet printing allows that each print is made unique, which is especially useful for instance when a need exists to track and trace each individual document or product or to include specific information such as biometric information. The possibility of combining LCP-based optical effects and inkjet printing is only mentioned in very few documents, which are all non-specific with regards to the actual requirements.

Creating security devices by printing LCP's with solvents has been described e.g. in EP1381520. Solvents are materials which cause the LCP's to dissolve in them and together form a solution. Furthermore, such solvents are usually meant to evaporate after processing but before polymerization, so the solvent is not contained in any significant amount in the final product. Examples of commonly employed solvents for LCP's are xylene, toluene and acetone. The problem with ink jetting of LCP's is that commercial inkjet printing equipment is unable to reproducibly print LCP's if these are dissolved in solvents. Ink jetting solvent based mixtures leads to a drying effect which is detrimental to the homogeneity of the prints, the so-called coffee stain effect. Furthermore the use of solvents can affect the substrates on which the LCP's are printed and also it can dissolve any previously printed LCP structure on the substrate before polymerization. When printing structures containing different materials stemming from different reservoirs, the use of solvent based mixtures gives rise to significant mixing of the materials on the substrate, leading to effects such as ‘colour bleeding’ or highly blurred images.

Solvents can also chemically attack parts of the print head or heads and conduits in the printing system, in particular polymer-based parts. These solvents also often evaporate at or near the nozzle, leading to clogging. Furthermore, such solvents are often harmful to the equipment because of corrugation as well as to people and to the environment due to toxicity and therefore require extensive equipment for air filtration near the printing equipment. Furthermore, heating of the printed materials to enhance evaporation and thus production speed is highly desirable, requiring heating installations within the production equipment which increase complexity and cost and also can be detrimental to the surrounding equipment as well as the substrates on which is printed.

Creating of security devices by printing of LCP's without solvents is also known, e.g. from EP1491332. However, this document does not describe or teach specific materials or steps to enable the inkjet printing of LCP's or potential problems related to it. In particular, mixtures containing LCP's but without solvents are hardly manageable with the currently available production-ready equipment. Only with highly dedicated inkjet printing equipment is it possible to print LCP's, and only at or above 140C, limiting the scope of security devices to be produced At such temperatures however, LCP's start to polymerize spontaneously through thermal excitation of the reactive groups or the initiators in the mixture.

To be able to print a mixture with inkjet printing it is needed that the mixture which is printed is at least chemically stable for the time it remains in the inkjet head or reservoir at the temperature at which the mixture is printed, which is in the order of minutes to weeks depending on the printing speed an the printer usage.

It is the object of the present invention to overcome the limitations of the prior art by providing a security device, which can be reproducibly printed with state-of-the art equipment, with the inclusion of various additives.

It is one object of the invention to provide a security device that can easily and reproducibly be made by common printing technologies and at the same time does not show the restriction of the security devices in the prior art.

It is another object of the inventions is to show provide for new LCP based security features by adding one or more of a range of specific additives to the LCP materials. These materials can add other authenticating effects to the resulting authentication feature, thus creating a new authenticating feature.

It is another object of the invention to provide a security device that can be manufactured via printing techniques, which allows for the creation of arbitrary structures which can contain information in the form of e.g. barcodes, images, text, numerical codes.

Surprisingly, it has been found that the object of the present invention can be achieved with a security device comprising the following components:

• • a) 30-80 wt % mono-functional LCP's
  • b) 0-50 wt % higher-functional LCP's
  • c) 0-30 wt % liquid crystalline inert monomers, preferably below 20 wt %
  • d) 0-50 wt % non-liquid crystalline (mono- or higher) functionalized monomers, preferably below 30 wt %
  • e) 0-30 wt % non-liquid crystalline inert monomers, preferably below 20 wt %
  • f) 0.01-10 wt % initiators, preferably below 2 wt %
  • g) 0-10 wt % inhibitors, preferably below 2 wt %
  • h) 0.01-50 wt % additives, preferably below 20%, more preferably below 10 wt %,
    with the provisions that the total amount of the components is 100 wt %, wherein the additives h) comprise magnetic additives, such as paramagnetic, super-paramagnetic, diamagnetic or ferri-magnetic particles.

In the following, some non-limitative examples are given of suitable components.

An example of category a) is

where x is between 1 and 10.

Suitable examples from category d are

and also

as well as

Acrylates have specific advantages in that they have fast curing times as well as a wide choice in suitable initiators. However LCP's with different reactive groups are also known from literature, and could also be applied with the same method. Epoxies for instance have the specific advantages that they provide for good adhesion to many printed surfaces, exhibit relatively small amounts of polymerization shrinkage and are less affected by oxygen inhibition during polymerization. Oxethanes have properties comparable to epoxies, but typically exhibit less polymerization shrinkage. Thiol-ene based polymerization (including both thiol and -ene groups in the molecule structure) is also possible. Vinyl-ethers are also not very sensitive to oxygen inhibition and polymerize very rapidly, but only at relatively high temperatures.

A suitable example of category e) is ethylene glycol.

Suitable examples of category f) are for instance sold by Ciba-Geigy (Switzerland), under the trade names Irgacure and Darocure, and in general polymerization initiators are known in the art.

It is further preferred if components or a mixture are applied to a substrate by means of printing, such as but not limited to inkjet printing, flexography, offset printing, screen printing, micro-contact printing, micro-transfer printing, gravure printing, rotogravure printing, reel-to-reel printing.

Examples of magnetic additives are paramagnetic, super-paramagnetic, diamagnetic or ferri-magnetic particles. Such particles are typically 5 to 500 nm in size. The magnetic properties can be probed by e.g. magnetoresistive sensors. The addition of such particles enables the creation of structures that can be moved mechanically by means of magnetic fields. Such movement can give rise to e.g. altered optical, mechanical, electrical or magnetic properties of the printed structure, which can be used to authenticate the feature.

It is particularly beneficial that the printed structures are (in part) made from LCP's, since the anisotropic properties of the aligned LCP polymer matrix can enhance the magnetic and mechanical properties desired to fully exploit the magnetic properties of the print. Especially if the magnetic material is magnetically anisotropic and also aligning with regards to the LCP polymer matrix. The anisotropic magnetic properties will then be macroscopically available.

A preferred embodiment comprising a magnetuc additive is shown by the following components:

• • a) 54.5 wt % mono-functional LCP acrylate

• • b) 22 wt % di-functional LCP acrylate

• • c) 19 wt % non-reactive LC monomer K15

• • f) 1.0 wt % photo-initiator

• • g) 0.5 wt % inhibitor hydroquinone

• • h) 3 wt % of magnetic microspheres beads of mean diameter 0.9 μm diameter containing between 20 and 60% magnetite in a polystyrene/divinylbenzene matrix

Components a through h are added in a glass vial and diluted with paraxylene with a ratio of 1 (mixture):1.25 (paraxylene), and the resulting mixture is manually stirred for 5 minutes at 70° C. The resulting mixture is then fluid. This mixture is printed in a 10 μm film with the doctor blading technique on a tri-acetylcellulose film, which is rubbed with a velvet cloth to induce alignment. The solvent film is allowed to evaporate at 50° C. during 2 minutes and the resulting film is then UV-cured in a nitrogen atmosphere.

The resulting feature is birefringent and contains magnetic particles which are evenly dispersed in the polymer matrix.

It is further preferred that the additives h) comprise additionally or only conductive or semi-conductive additives, more preferably, conductive or semi-conductive additives that are selected from a group comprising nanometer or micrometer sized rods, flakes, spheres or otherwise suitably shaped conductive particles of metals, alloys or semiconductor-based materials, and even more preferably conductive or semi-conductive additives are selected from a group comprising semi-conductive conjugated polymers, such as polyphenylene vinylene or mixtures of polymers such as PEDOT-PSS (a mixture of Poly(3,4-ethylenedioxythiophene) and sodium poly(styrenesulfonate)), and semi-conductive liquid crystals, such as oligothiophenes, which are preferably LCP's.

A particular benefit of adding (semi-) conductive or magnetic additives to the prints is that the authentication is straightforward by means of electric and magnetic fields or currents, and the effects can be reversible enabling non-destructive authentication. Furthermore, a particular benefit of inkjet printing such structures is that these additives can be printed in varying structures, thus enabling unique and identifiable responses to electrical or magnetic fields.

Conductive additives enable the printing of electronic circuits. Such circuits can be used for instance to create optical effects which are switchable by means of electrical signals. The conducting properties of the structure itself too can be used as an authentication feature. This can be done particularly effectively if elements of electronic circuits, such as FET's, diodes or capacitors are created within the print, since these give rise to designable and clearly identifiable electronic responses. It is possible that the conductive structures are used to make switchable another, adjacent non-conductive printed structure, either of which is not necessarily but preferably applied by means of inkjet printing. Such multi-layered prints are advantageously created sequentially or concurrently, either in a single layer or in separate layers, printed either on top of or next to each other or even on opposite sides of the substrates or on multiple substrates which are assembled together after printing. Furthermore, it is possible to create structures which are conductive and contain electroluminescent or electrochromic additives, which can be addressed (made to change the optical appearance of the feature) by currents flowing through the printed structure itself. Furthermore, by supplying charges of equal or opposite sign to two electrically isolated by adjacent parts of the structure, capacitors can be formed. If such parts of the structure are able to move mechanically, such movement can give rise to e.g. altered optical, mechanical, electrical or magnetic properties of the printed structure, which can be used to authenticate the feature.

It is particularly beneficial that the printed structures are (in part) made from LCP's, since the anisotropic properties of the aligned LCP polymer matrix can enhance the electrical and mechanical properties desired to fully exploit the conductive properties of the print.

It is further preferred that the security device according to the invention comprises additionally or only additives h) in the form of photochromic pigments or dyes, thermochromic pigments or dyes, electrochromic pigments or dyes, ionochromic pigments or dyes, halochromic pigments or dyes, solvatochromic pigments or dyes, trobochromic pigments or dyes and piezochromic pigments or dyes, fluorescent of phosphorescent pigments or dyes.

It is further preferred that these optical additives show anisotropic optical properties and that they align with the LCP material in the security device. This gives the entire security device additional anisotropic optical properties.

It is preferred for inventive security device if the components or a mixture of the components are applied to a substrate by means of printing, such as but not limited to inkjet printing, flexography, offset printing, screen printing, micro-contact printing, micro-transfer printing, gravure printing, rotogravure printing, reel-to-reel printing.

In addition, it is preferred for the security device that during the manufacturing process manufacture the components are aligned by a substrate layer comprising linearly photo-polymerizable polymers, preferably by aligned multiple types of alignment through the combination of multiple aligning substrates.

Preferably these substrates contain further authentication features, such as holograms, retro-reflecting layers, interference stack reflectors, fluorescent layers, color-shifting layers or features printed by means of flakes.

It is preferred for the security device if the components from e) through h) are selected such that they do not prohibit the alignment of the liquid crystals from components a) through d).

Preferred security devices contain components from a), b) and d) that are functionalized to form a polymer structure, preferably due to possessing compatible types of functional groups, very preferable by possessing the same type of functional group.

Moreover, the inventive security device preferably comprises components a) through h) that are selected such that phase-separation is suppressed at least only before polymerization but preferably also during polymerization.

By applying such components printing is easily possible below 120° C., preferably below 100° C., very preferably 80° C., to achieve security devices with distinctive and reproducible optical effects. In particular, it is also possible to add solvents to mixtures of these components which makes the mixture applicable to various printing techniques.

Combinations of different initiators can be used which has a number of potential advantages, of which a number are now described.

Different initiators will give different mechanical properties of the mixture before and after polymerization. Using mono-functional initiators can allow for a low viscosity of the mixture. However, multifunctional initiators tend to be more viscous. The exact choice of initiators also influences e.g. the flexibility and toughness of the polymerized structure. Furthermore, the choice of photo-initiator is dependent on the presence of other absorptive additives in the mixture, which will be described below at category h). It is possible to use mixtures of initiators which in these cases too allow for a good polymerization of both the top layer of the structure as well as throughout the structure. An initiator that can overcome oxygen inhibition of the surface can be used to initiate polymerization at the surface, whereas another initiators can be used to through-cure the structure. It is also possible to choose initiators in such a way that only the top of the structure polymerizes, but the remaining part remains non-polymerized. The non-polymerized LCP is then still fluid, and can be manipulated by means of for instance electric or magnetic fields, i.e. they remain switchable.

The list of other possible additives from category h) contains, but is not limited to, the following examples.

One example of additives are molecules that can act as tracer molecules, such as e.g. DNA-molecules. These can be added in minute quantities, typically with amounts in the order of one part per million (ppm), so that they are very difficult to trace when their exact properties are not known beforehand.

Another example of additives are surfactants, which can enhance alignment of the liquid. These surfactants can either enhance the alignment of the liquid crystalline matrix at the top of the structure, at the bottom or in the bulk or in combinations of those locations. Furthermore, these surfactants can influence the potential phase separation of the mixture or influence the mechanical properties (e.g. viscosity, surface tension) of the mixture on the substrate or can perform a combination of these three functions.

Another example of additives are chiral additives, commonly referred to as chiral dopants, which are used to induce a chiral phase in the liquid crystal structures. Such chiral structures reflect light from a specific tunable wavelength range and with a specific handedness, which can either be commonly called left- or right-handed, and the handedness is determined by the handedness of the chirality of the structure. It is noted that one or more of the LCP's from categories a) through c) can also exhibit intrinsic chiral phases, and/or induce such a phase for all the liquid crystal molecules. By making use of chiral LCP's, a chiral structure can be obtained without the use of chiral additives. Many chiral additives are known to the person skilled in the art. Preferably the chiral additives are also polymerizable, which has the advantage that the temperature dependence of the reflected wavelength is usually decreased.

Other examples of additives are pigments and dyes. Pigments and dyes can be added to give the mixture an intrinsic color by means of absorption of part of the spectrum as well as luminescence in part of the spectrum. Such intrinsic color can enhance the optical effects of the printed structure, for instance by enhancing contrast of (parts of) the printed structure.

Pigments are particles which do not dissolve molecularly in the mixture whereas dyes can be approximately molecularly dissolved. The choice between pigments and dyes is dependent on various factors. One important factor is the solubility of the dyes or the pigments, with or without the aid of a dispersant, to create a stable ink. Solutions with dyes are generally easier to process than dispersions with pigment, but the optical properties of pigments are usually more stable. Furthermore, certain optical additives are only available as pigments and not as dyes, such as di-electric stacks, whose optical effects are not based on molecular effects, but on effects on a larger scale. Another important factor is the price of pigments, which is usually higher than that of dyes.

Examples of absorbing pigments or dyes are for instance

• • Absorbing only, meaning that a specific part of the spectrum is absorbed
  • Photochromic pigments or dyes, which by excitation with light of a particular part of the spectrum reversibly change into another chemical species having a different absorption spectrum from the original chemical species. Non-reversible photochromic pigments and dyes also exist for specific purposes
  • Thermochromic pigments or dyes, which exhibit a reversible change in absorption spectrum through the application of heat (i.e. at raised or lowered temperatures). Non-reversible thermochromic pigments and dyes also exist for specific purposes
  • Electrochromic pigments or dyes, which exhibit a change in absorption spectrum through the addition of electron charges
  • Ionochromic pigments or dyes, which exhibit a change in absorption spectrum through the addition of ionic charges.
  • Halochromic pigments or dyes, which exhibit a change in absorption spectrum through changes in pH.
  • Solvatochromic pigments or dyes which exhibit a change in absorption spectrum through changes in the polarity of the solvent which is in contact with them.
  • Tribochromic pigments or dyes, which exhibit a change in absorption spectrum as a result of friction applied to them.
  • Piezochromic pigments or dyes, which exhibit a change in absorption spectrum through changes in the pressure applied to them.

Examples of luminescent pigments or dyes are for instance

• • Fluorescent pigments or dyes, which exhibit absorption of light in a particular part of the spectrum and emission in another part of the spectrum, typically at a lower wavelength, where the absorption and emission of individual photons occur subsequently but with delays of typically nano-seconds.
  • Phosphorescent pigments or dyes exhibit similar absorption and emission as fluorescent dyes, but due to a different quantum mechanical decay mechanism typically emit photons after absorption with much larger delays of up to hours or days.
  • Chemoluminescent pigments or dyes, which exhibit emission of photons as a result of chemical reactions of the pigments and dyes. Such reactions are generally non-reversible.
  • Electroluminescent pigments or dyes, which exhibit emission of photons as a result of radiative recombinations of electrons and holes within the pigments or dyes. Such radiative recombination can occur if an electric current is passed through the pigments or dyes, or alternatively of they are subjected to strong electric field capable of exciting electron-hole pairs which subsequently recombine.
  • Triboluminescent pigments or dyes, which exhibit emission of photons as a result of friction applied to them.
  • Piezoluminescent pigments or dyes, which exhibit emission of photons as a result of pressure applied to them.
  • Radioluminescent pigments or dyes, which exhibit emission of photons as a result of ionizing radiation, such as beta particles, applied to them.

There are also pigments or dyes which combine multiple optical effects within a single additive, or which in fact an effect which is related to multiple causes concurrently. Examples are

• • Thermochromic pigment capsules which change colour if heated above a certain threshold temperature. At this temperature the crystalline solvent in the capsule melts and effectively lowers the pH. This in turn causes the halochromic compound present to change its absorptive properties.

Photochromic fluorescent dyes are dyes which exhibit fluorescence only after the molecule has absorbed photons from a part of the spectrum which it does not absorb in its subsequent fluorescent state. This effect which is concurrently photochromic and fluorescent, i.e. due to the first absorption not only the absorptive properties of the molecule changes (photochromism) but also the molecule subsequently exhibits fluorescence or a change in its fluorescent properties

A preferred embodiment comprising a phosphorescent additive is shown by the following components:

a) 53.5 wt % mono-functional LCP acrylate

b) 21.5 wt % di-functional LCP acrylate

c) 18.5 wt % non-reactive LC monomer K15

f) 1.0 wt % photo-initiator

g) 0.5 wt % inhibitor hydroquinone

h) 5 wt % of phosphorescent pigment PPSB-03, from RiskReactor, California, USA,

Components a through h are added in a glass vial and diluted with paraxylene with a ratio of 1 (mixture):1.25 (paraxylene), and the resulting mixture is magnetically stirred for 5 minutes at 70° C. The resulting mixture is then fluid. This mixture is printed in a 40 μm film with the doctor Blade technique on a tri-acetylcellulose film, which is rubbed with a velvet cloth to induce alignment. The solvent film is allowed to evaporate at 50° C. during 2 minutes and the resulting film is then UV-cured in a nitrogen atmosphere.

The resulting feature is birefringent and contains phosphorescent particles which are evenly dispersed in the polymer matrix.

When the resulting feature is viewed between crossed polarizers, the birefringence is clearly visible. When the alignment axis of the feature is parallel to either polarizer, the feature is dark between the polarizers. When the alignment axis of the feature is at 45 degrees to both polarizers, the feature is bright between the crossed polarizers. When the feature is placed under a UV light, the phosphorescent particles lights up bright orange. When the UV light is turned off, the bright orange glow remains for about half a minute.

A preferred embodiment comprising a photochromic additive is shown by the following components:

a) 53.5 wt % mono-functional LCP acrylate

b) 21.5 wt % di-functional LCP acrylate

c) 18.5 wt % non-reactive LC monomer K15

f) 1.0 wt % photo-initiator

g) 0.5 wt % inhibitor hydroquinone

h) 5 wt % of photochromic blue pigment (IT9 014) from MUTR, Herts, UK

Components a through g are added in a glass vial and diluted with paraxylene with a ratio of 1 (mixture):1.25 (paraxylene), and the resulting mixture is magnetically stirred for 5 minutes at 70° C. The resulting mixture is then fluid. Component h is then added, dissolved in ethanol with a ratio of 1 (component h):10 (ethanol). The mixture is then vigorously stirred magnetically for 30 seconds. This mixture is printed in a 40 μm film with the doctor Blade technique on a tri-acetyl cellulose film, which is rubbed with a velvet cloth to induce alignment. The solvent film is allowed to evaporate at 50° C. during 2 minutes and the resulting film is then UV-cured in a nitrogen atmosphere.

The resulting feature has the pigment dispersed throughout the polymer matrix. The resulting feature is birefringent, but due to the dispersed pigment it is also scattering. The birefringence is visually inspected between crossed polarizers. When the alignment axis of the feature is parallel to either polarizer, the feature is dark but the scattering due to the pigments gives rise to some transmission of light. When the alignment axis of the feature is at 45 degrees to both polarizers, the feature is bright between the crossed polarizers. The resulting feature has a yellow colour, when the resulting feature is placed in a UV-light for 30 seconds, the colour turns to blue. When the feature is no longer in the UV-light it retains the blue colour for a few minutes and then returns to yellow again.

A preferred embodiment comprising a thermochromic additive is shown by the following components:

A preferred embodiment are the following components:

a) 53.5 wt % mono-functional LCP acrylate

b) 21.5 wt % di-functional LCP acrylate

c) 18.5 wt % non-reactive LC monomer K15

f) 1.0 wt % photo-initiator

g) 0.5 wt % inhibitor hydroquinone

h) 5 wt % of thermochromic pigment IT9 007 “magenta” from MUTR, Herts, UK

Components a through g are added in a glass vial and diluted with paraxylene with a ratio of 1 (mixture):1.25 (paraxylene), and the resulting mixture is magnetically stirred for 5 minutes at 70° C. The resulting mixture is then fluid. Component h is then added, dissolved in ethanol with a ratio of 1 (component h):10 (ethanol). The mixture is then vigorously stirred magnetically for 30 seconds. This mixture is printed in a 40 μm film with the doctor blading technique on a tri-acetylcellulose film, which is rubbed with a velvet cloth to induce alignment. The solvent film is allowed to evaporate at 50° C. during 2 minutes and the resulting film is then UV-cured in a nitrogen atmosphere.

The resulting feature has the pigment dispersed throughout the polymer matrix. The resulting feature is birefringent, but due to the dispersed pigment it is also scattering. The birefringence is visually inspected between crossed polarizers. When the alignment axis of the feature is parallel to either polarizer, the feature is dark but the scattering due to the pigments gives rise to some transmission of light. When the alignment axis of the feature is at 45 degrees to both polarizers, the feature is bright between the crossed polarizers. When the resulting feature is at room temperature it has a red colour, when the feature is heated to 50° C. the red colour disappears. When the feature is cooled down to room temperature the red colour re-appears.

A preferred embodiment comprising a fluorescent additive and magnetic particles is shown by the following components:

• • a) 53.3 wt % mono-functional LCP acrylate

• • b) 21.6 wt % di-functional LCP acrylate

• • c) 18.6 wt % non-reactive LC monomer K15

• • f) 1.0 wt % photo-initiator

• • g) 0.5 wt % inhibitor hydroquinone

h1) 3 wt % of magnetic microspheres beads of mean diameter 0.9 μm diameter containing between 20 and 60% magnetite in a polystyrene/divinylbenzene matrix
h2) 2% DFSB-K44-65 fluorescent dye from RiskReactor, California, USA.

Components a through h2 are added in a glass vial and diluted with paraxylene with a ratio of 1 (mixture):1.25 (paraxylene), and the resulting mixture is manually stirred for 5 minutes at 70° C. The resulting mixture is then fluid. This mixture is printed in a 20 μm film with the doctor blading technique on a tri-acetylcellulose film, which is rubbed with a velvet cloth to induce alignment. The solvent film is allowed to evaporate at 50° C. during 2 minutes and the resulting film is then UV-cured in a nitrogen atmosphere.

The resulting feature is birefringent and contains magnetic particles which are evenly dispersed in the polymer matrix. The birefringence is visually inspected between crossed polarizers. When the alignment axis of the feature is parallel to either polarizer, the feature is dark. When the alignment axis of the feature is at 45 degrees to both polarizers, the feature is bright between the crossed polarizers. Under a microscope the dispersed magnetic particles are clearly visible. When placed under a UV-lightsource it shows bright yellow fluorescence. This fluorescence is isotropic in emission and absorption.

Pigments and dyes can exhibit anisotropic optical properties, depending on their molecular orientation. If anisotropic dye molecules align to a significant degree within the LCP matrix, typically parallel or perpendicular to the LCP alignment, typically caused by a distinct anisotropic molecular shape, these molecules can exhibit their anisotropic optical properties collectively, leading to distinctive optical effects, which remain after polymerization of the LCP matrix. This effect is commonly known as dichroism or pleochroism. Pigments can also exhibit dichroic effects if the particles as such have anisotropic optical properties. However, such properties are difficult to exploit since for a collective effect all pigments have to be effectively aligned in the direction of their inherent anisotropy.

It is possible to create features which exhibit fluorescent dichroism in absorption but not in emission or vice versa. This effect can be achieved for instance by using two fluorescent molecular species, one of which absorbs and emits essentially non-dichroic and the other essentially dichroic. By choosing both species in such a way that the absorbed photon-energy is transferred to the other species, such effects can be obtained. Also, fluorescent molecules can exhibit different degrees of dichroism in absorption and emission, but the effect with using multiple suitably chosen species is in general more pronounced.

A preferred embodiment comprising an anisotropic fluorescent additive is shown by the following components:

• • a) 54.5 wt % mono-functional LCP acrylate

• • b) 22 wt % di-functional LCP acrylate

• • c) 19 wt % non-reactive LC monomer K15

• • f) 1.0 wt % photo-initiator

• • g) 0.5 wt % inhibitor hydroquinone

• • h) 3 wt % of an anisotropic fluorescent dye DFSB-K82 from RiskReactor, California, USA.

Components a through h are added in a glass vial and diluted with paraxylene with a ratio of 1 (mixture):1.25 (paraxylene), and the resulting mixture is magnetically stirred for 15 minutes at 70° C. The resulting mixture is then fluid and clear. This mixture is printed with printed in a 10 μm film with a doctor blading technique on a tri-acetylcellulose film, which is rubbed with a velvet cloth to induce alignment. The solvent film is allowed to evaporate at 50° C. during 2 minutes and the resulting film is then UV-cured in a nitrogen atmosphere. The resulting film is birefringent. When placed under a UV-lightsource it shows bright yellow fluorescence. This fluorescence is anisotropic in emission: when the emission is inspected visually through a polarizing filter the emission is high for one polarization direction and low for the orthogonal polarization direction. The difference between the high and low emission is optically very distinctive, giving the security feature a striking effect.

When using the same mixture described above, only with different fluorescent dyes, we have also obtained layers that show:

• • isotropic fluorescence in both emission and absorption with dye DFSB-K44-65 from RiskReactor, California, USA,
  • anisotropic fluorescence in both emission and absorption with dye DFSB-K61.

The anisotropy of the emission is inspected optically by viewing the emission through a polarizing filter which is rotated, while the UV-source is not polarized. The anisotropy of the absorption is inspected optically: a UV-polarizing filter is placed in front of the UV source and the sample is then rotated, while the emission is inspected.

UV-absorbing pigments and dyes or pigments can serve several specific purposes. Such UV-protecting pigments and dyes can be present in the printed mixture or applied over the printed structure after curing by another printing step, preferably by means of flexography or offset printing. Also other application methods can be employed, such as bar-coating, doctor blading, spraying or by applying a UV-absorbing substrate on top of the printed substrate.

As these pigments or dyes absorb UV light, they can protect the printed layer, or the substance underneath this layer, from harmful UV-radiation which can lead to degradation of the (mechanical) properties of the structures, such as brittling. During the UV-curing of the printed layer, UV-absorbers can also be used to prevent the deeper parts of the layer of being polymerized, thus allowing for a non-polymerized layer to exist, whereas the top layer is solidified during polymerization. Also, such a non-polymerized layer is not created but there is formed a gradient in the structure if specific components of the mixture diffuse towards or away from the higher polymerized regions during polymerization. Such gradients create new optical effects. For instance, a gradient in the amount of chiral dopant leads to structures after polymerization which exhibit a gradient in the chiral pitch, thus reflecting light over a greater wavelength range than a single pitched structure would. This effect is known as a broadband cholesteric mirror.

Again it has to be emphasized that the additives mentioned above can either be present together or separately in the mixture. It goes without saying, however, that the total amount of components forming the polymerizable mixture is always 100 wt %.

As the anisotropic optical properties of the LCP's in the matrix before and after polymerization are dependent on their alignment, the preferred substrates on which is printed induce or do not interfere with the alignment of the LCP's so as to obtain the desired optical effect. Commonly used substrates are rubbed polyimide, as well as rubbed tri-acetyl-cellulose, polyethylene terephthalate, polyethylene or polypropylene. Rubbing causes planar aligning properties for these substrates.

Other substrates, also substrates causing other types of homogenous alignment, are known in the art as well. Common types of alignment are e.g. planar, homeotropic and tilted alignment.

LPP (Linearly Photopolymerizable Polymers) layers can also be used as alignment layers. LPP allows for the patterning of the alignment layer by means of polarized light and thus multi-domain patterning of the alignment layer. Furthermore it is possible to use self-assembled mono-layers (SAM's) as alignment layers, which can easily be applied in a pattern by e.g. printing. Combinations of for instance SAM's and LPP or tri-acetyl cellulose layers allow for an increased control over the alignment of the LCP's in the azimuthal and polar direction.

Next to the aligning properties of the surfaces, the choice in substrates also determines the interactions between the mixture and the substrate. These interactions can be used to create additional (optical) effects. E.g. the use of hydrophobic of hydrophilic (chemically) patterned surfaces allows for print confinement and thus a higher print resolution and more striking optical effects. A geometrically patterned surface can also be used to confine printed ink. Confinement can lead to printed structures with more controlled geometries, leading to better defined properties which are beneficial for authentication purposes. Such chemical or geometrical patterning of the substrates can be achieved by means of printing, but also other techniques such as for instance embossing, rubbing and lithography.

The optical properties of the employed substrates influence the overall properties of the security feature. Such substrates can be combined, i.e. stacked on top of each other creating a multi-layered security feature, or a security feature created on a stack of substrates each having particularly beneficial properties. Dependent on the preferred optical effects, the substrates can be transparent, absorbing in any range of wavelengths, scattering or reflecting or can comprise patterns of these effects. The substrates can also have other optical properties. Examples are the ability to transmit only one polarization, as is the case with polarization films which transmit only one linear polarization, or the ability to reflect only one polarization, e.g. cholesteric films only reflect one handedness of light. Furthermore the substrates can change the polarization of transmitted or reflected light, as is the case with for instance retarder films and half wave plates.

The substrates can also contain other authentication features. Examples are holograms, retro-reflecting layers, interference stack reflectors, fluorescent layers, color-shifting layers or features printed by means of flakes. It is also possible to add layers containing other authentication features on top of the LCP polymer structures, via e.g. lamination.

It is preferred that the as-produced features are created such that they can be applied as tamper evident labels to products or documents. Such labels have properties which render the intact removal of the labels very difficult. Such properties could be poor mechanical integrity, for instance features which have low toughness, i.e. low resistance to tearing. Furthermore, the features upon removal can leave behind clear traces of its previous presence, for instance by means of rupture-sensitive ink particles.

It is also preferred that the features can easily be applied to the documents and products. Such application can for instance be by means of hot-embossing or by creating self-adhesive features.

Printing of LCP's is a very flexible technique, which allows for many novel optical designs, for instance by using multiple inks which allows for the concurrent printing of inks with different optical properties. Of course it is also possible to print on top of or underneath layers printed via different printing technique's, such as flexography, laser printing, offset printing, screen printing, micro-contact printing, micro-transfer printing, intaglio printing, gravure printing, rotogravure printing, reel-to-reel printing, and thermal transfer printing.

Such printing could be done in series or parallel depending on the printing equipment design. All these options enable a great host of embodiments with specific uses for specific purposes. In the following paragraph a few of the options are mentioned, although this is not an exhaustive list of the possibilities of printing LCP's.

The combination of normal black or colored inks with LCP inks, can allow for the inclusion of overt features on top of normal printed information by printing retarding structures on top of an image or text printed with regular ink on a reflection substrate. It can also allow for the enhancement or decrease of contrast by contrast by printing cholesteric LCP mixtures on top of an image.

By using inks with identical apparent color but with different reflective properties, such as left-hand and right-hand reflecting cholesteric LCP's, hidden information can be printed which can only be revealed using a polarization sensitive device or machine reader. The combination of color-shifting non-liquid crystal inks and color shifting LCP ink creates the identical effect, as one ink has a uniform reflection and the other is polarization selective. The combination of inks with various additives, but with the same optical properties also allows for hidden information to be printed.

Printing several layers on top of each other can enhance reflectivity or enhance the colors printed.

Self-authenticating structures can be printing by using e.g. structured polarizers and a LC structure on a reflective surface, which can be folded together to create additional effects. This can also recover encrypted data.

As is already clear from the above descriptions, it is a particular advantage of inkjet printing of LCP's that it is possible to create authentication features with (multiple) overt, covert, forensic and biometric levels of security, which will be explained below. Also, it is possible to use one of multiple options or combinations of options in each level by suitably choosing (combinations of additives such as pigments and dyes, substrates, printing procedures and designs of the layers present in the authentication features.

Features which incorporate levels of security that are directly apparent (in particular visible) to anyone without the use of additional devices, like the color-shifting effect, which can be seen if a printed chiral nematic structure is tilted, are known as overt. Any overt security feature should be easily recognizable, but very difficult to counterfeit. Other examples are multi-color prints of liquid crystal color-shifting inks.

Covert levels of security can only be made apparent using simple devices. For instance, the birefringent nature of LCP's can be revealed using simple devices such as polarizers if choosing suitable designs. Examples are nematic structures printed on a reflecting substrate, which shows reflection or no reflection when viewed through an polarizer at 0 or 45 degrees to the alignment axis. Another example is a chiral nematic print, which reflects only one handedness of circularly polarized light. This can be discerned by a circular polarizer. Parts of the printed structure which could be left non-polymerized could be electromagnetically switched if having anisotropic dielectric constants, as is the case for many inert liquid crystals. This changes the optical properties of the structures. In use it is sometimes preferred that the covert level is not visible as a security feature (i.e. being without a combined overt level), but consists of e.g. (electro-) magnetically stored information, micro-text invisible to the eye, etc. For both optical and other covert layers it is usually possible to devise a procedure and associated device which performs a fully automatic authentication.

A more complicated level of security is the forensic level of security. This level is usually only known in detail by a selected group of users, who typically have access to expensive readout equipment. Possibilities for this level are to add tracer molecules to the printing ink, which can traced using highly advanced analysis techniques such as NMR scans. Other options are to analyze the exact ink content, or to investigate a specific size and shape of printed dots.

Finally the biometric level of security allows a feature to store unique information, typically employed for e.g. tracking and tracing or authenticating persons. Due to the flexibility of the printing processes of LCP's, it is possible to print e.g. barcodes, serial numbers, images and other information.

The availability of additives in the security device can also be used as an additional biometric level. If the additives are not evenly dispersed throughout the security device, but in a predetermined pattern, they can contain more information, which can be hidden until a proper read-out mechanism is used. This can be achieved by using different inks to create one device, which are in composition equal, except for the addition of e.g. magnetic, conductive or optical additives.

An example is the local use of magnetic additives in a barcode printed with nematic LCP inks on a planar aligning reflecting substrate. The barcode is printed with two LCP inks which are identical except that one contains magnetic additives, where the upper half is printed with the ink containing magnetic additives and the lower half is printed with the ink without the magnetic additives. The barcode can now be inspected visually using a polarizer, since the entire barcode is birefringeng. The barcode will be non visible if the polarizer has its polarization direction parallel or orthogonal to the alignment direction of the LCP, the barcode turn dark at a 45 degree angle to the alignment direction of the LCP. By using a magnetic sensor, such as a magnetoresistive sensor, the upper half can be inspected magnetically, whereas the lower half will give no magnetic signal.

Claims

1. A security device comprising the following components: with the provisions that the total amount of the components is 100 wt %, characterized in that the additives h) comprise magnetic additives, such as paramagnetic, super-paramagnetic, diamagnetic or ferri-magnetic particles.

(a) 30-80 wt % mono-functional LCP's

(b) 0-50 wt % higher functional LCP's, preferably below 50 wt %

(c) 0-30 wt % liquid crystalline inert monomers, preferably below 20 wt %

(d) 0-50 wt % non-liquid crystalline (mono- or higher) functionalized monomers, preferably below 30 wt %

(e) 0-30 wt % non-liquid crystalline inert monomers, preferably below 20 wt %

(f) 0.01-10 wt % initiators, preferably below 2 wt %

(g) 0-10 wt % inhibitors, preferably below 2 wt %

(h) 0.01-50 wt % additives, preferably below 20%, more preferably below 10 wt %,

2. The security device according claim 1 characterized in that the additives h) comprise additionally or only conductive or semi-conductive additives.

3. The security device according to claim 2, characterized in that the conductive or semi-conductive additives are selected from a group comprising nanometer or micrometer sized rods, flakes, spheres or otherwise suitably shaped conductive particles of metals, alloys or semiconductor-based materials.

4. The security device according to claim 3 characterized in that the conductive or semi-conductive additives are selected from a group comprising

semi-conductive conjugated polymers, such as polyphenylene vinylene

semi-conductive liquid crystals, such as oligothiophenes, which are preferably LCP's.

5. The security device according to claim 1 characterized in that the additives h) comprise additionally or only photochromic pigments or dyes, thermochromic pigments or dyes, electrochromic pigments or dyes, ionochromic pigments or dyes, halochromic pigments or dyes, solvatochromic pigments or dyes, trobochromic pigments or dyes and piezochromic pigments or dyes.

6. The security device according to claim 1, characterized in that the components are applied to a substrate by means of printing, such as but not limited to inkjet printing, flexography, offset printing, screen printing, micro-contact printing, micro-transfer printing, gravure printing, rotogravure printing, reel-to-reel printing.

7. The security device according to claim 1, characterized in that the components are aligned by a substrate layer comprising linearly photo-polymerizable polymers.

8. The security device according to claim 1, characterized in that the component are aligned by multiple types of alignment through the combination of multiple aligning substrates.

9. The security device according to claim 1, characterized in that the substrates contain further authentication features, such as holograms, retro-reflecting layers, interference stack reflectors, fluorescent layers, color-shifting layers or features printed by means of flakes.

10. The security device according to claim 1, characterized in that the components from e) through f) are selected such that they do not prohibit the alignment of the liquid crystals from components a) through d).

11. The security device according to claim 1, characterized in that the components from a), b) and d) are functionalized to form a polymer structure, preferably due to possessing compatible types of functional groups, very preferable by possessing the same type of functional group.

12. The security device according to claim 1, characterized in that the components a) through h) are selected such that phase-separation is suppressed at least only before polymerization but preferably also during polymerization.

13. The security device according to claim 1, characterized in that the device contains information such as e.g. text, numbers, a barcode, or an image.

14. The security device according to claim 1, characterized that the additives are in a pattern so that they contain additional, optionally hidden, information such as e.g., text, numbers, a barcode or an image.