RU2741481C2 - Method of producing a security element - Google Patents

Abstract

FIELD: document security.SUBSTANCE: invention relates to protective elements for security documents and to methods for their production. According to the method of manufacturing a protective element, comprising a substrate made from oriented plastic, and at least one layer of liquid crystals (13), applied and oriented on the substrate, adhesive primer (12) is applied before or after stretching the substrate in at least one direction, wherein said stretching allows bringing the substrate into an oriented state, and liquid crystals are applied by means of printing on said adhesive primer.EFFECT: invention provides high protective effect and simplifies manufacturing of protective element.20 cl, 9 dwg, 3 tbl

Description

The present invention relates to security elements for security documents and to methods for their manufacture.

More specifically, the invention relates to security elements carrying liquid crystals.

Liquid crystals, in particular cholesteric liquid crystals, are widely used in security elements because of their goniochromatic properties. To improve the visibility of the color change effect with the viewing angle, they are usually placed in front of a dark, especially black, background.

In addition, strip-shaped security features, also called security threads, are very often included in security documents such as banknotes.

These security elements typically comprise a substrate made of a thermoplastic material such as polyethylene terephthalate (PET), which can be metallized and can have demetallized zones in the form of patterns that appear in transmitted light.

The advantage is to provide these security elements with liquid crystals in order to enhance the security by creating optical effects visible to the naked eye.

However, many problems are encountered in the deposition of liquid crystals, which are currently only solved by methods that are difficult to implement, expensive and which impose certain limitations from the point of view of the device.

Thus, the known practice of applying liquid crystals in the form of a solid through a transfer process has the disadvantage of limiting the possibilities from an artistic point of view and superimposing the presence of liquid crystals in the demetallized zones. This results in a color in transmitted light that is not very attractive and the presence of liquid crystals in the demetallized zones increases the cost of protection without providing any significant enhancement from a security or artistic point of view.

The transfer process results in material waste and lamination steps that complicate production.

EP 1744904 describes a substrate consisting of a biaxially oriented PET film coated with oriented liquid crystals. This type of structure is not entirely satisfactory in terms of adhesion to the substrate.

US 2003189684 describes various methods for orienting liquid crystals.

US 20070216518 discloses multilayer structures containing liquid crystals. Overlapping liquid crystals makes structures expensive by increasing the amount of liquid crystals used. Manufacturing using a transfer process is preferred.

EP 2073986 also describes structures with superimposed liquid crystals.

JP 2009-98568 describes a security element comprising an oriented film made of plastic that can be biaxially stretched and can carry liquid crystals.

GB 2527763 discloses a sheet containing an oriented polymer film, which may contain liquid crystals, as well as an adhesive layer.

WO 2008/116796 relates to a security document containing a variety of polymer films, including an opaque film, which is axially stretched. The adhesive layers can be printed onto this film. In addition, this film may contain liquid crystals.

There is a need for further improvement of security elements containing liquid crystals and methods for their preparation, in particular, to have available security elements in which liquid crystals adhere well to the substrate and are fully utilized and thus allow the creation of a rich set of optical effects.

The invention addresses this need, and this is achieved by a method of manufacturing a security element comprising a substrate made of oriented plastic and at least one layer of liquid crystals applied and oriented on the substrate, an adhesive primer is applied before or after stretching the substrate at least in one direction, said stretching allows the substrate to be oriented, and liquid crystals are printed on said adhesive primer.

The term "adhesive primer" should be understood to mean any layer that promotes adhesion. Thus, this expression should not be construed in a limiting manner.

Thanks to the invention, the adhesion of liquid crystals is improved, and the application of liquid crystals by means of printing offers many possibilities in terms of aesthetics and the resulting optical effects.

The application of liquid crystals can be done in any pattern, in well-defined zones, which allows great flexibility in the arrangement of various attachment means within the element. You can achieve a greater variety of color-changing effects without disturbing the transparency of plaintext openings, if necessary.

The invention avoids the deposition of liquid crystals by transfer, which facilitates the implementation of the method.

Preferably, the adhesive primer is applied prior to stretching the substrate.

Also preferably, the substrate is oriented in one direction, then the primer is applied and then the substrate is stretched in the other direction.

Applying the primer prior to stretching the substrate allows the primer to accompany the stretching of the substrate and thus further improves the orientation of the liquid crystals.

The substrate preferably contains PET, but other materials can be used.

The primer may require crosslinking prior to the application of liquid crystals, particularly when it contains monomers. Thus, the method may include the step of cross-linking the adhesive primer, preferably after stretching the substrate.

The adhesive primer may contain a polyolefin (or polyalkene), preferably polyethylene, polyurethane, polyester, polyester, polycarbonate or polyacryl or a copolymer thereof, and may preferably contain polyacrylic.

The adhesive primer may contain, in particular prior to application, at least one olefinic (or alkene), urethane, ester, ether, carbonate or acrylic monomer, preferably an acrylic monomer.

The application of the primer can be carried out by any means, in particular by printing, coating, spraying or coextrusion.

Liquid crystal printing can be done by any means, and preferably by screen printing, photo-engraving or flexography. Liquid crystals are applied after stretching the substrate.

The method may include printing on an adhesion primer two different inks, at least one of which contains liquid crystals, more preferably both of which contain liquid crystals, the liquid crystals are preferably cholesterol.

These two inks can be applied without overlapping; the amount applied is thus limited only to the amount that is suitable.

The reflection haze effect (hereinafter "haze") of the cholesteric liquid crystal layer may be less than that of a reference security element without an adhesive primer layer. Reflection clouding is responsible for the milky appearance associated with low intensity light scattering along the main reflection, which corresponds to reflection in the specular direction. Measurement can be performed in accordance with ASTM D 4039-09 (approved in 2015) or ISO 13803: 2014. This allows the visibility of the goniochromatic effect over a wider angular range and hence background masking over a wider angular range.

A dark background can be provided under the liquid crystal layer, in particular on the opposite side of the substrate. Alternatively, a dark background can be provided on the liquid crystal layer, in particular on the same side of the substrate. This dark background can be obtained by printing a dye or (absorbing or interference) pigment. Alternatively, the dark background is obtained by metallization, in particular in a vacuum or by electrochemical method, or by any other method of deposition of a metal, metal oxide or metal oxide salt. The dark background can also be applied with an additional substrate, preferably containing PET, attached to the substrate by any suitable means, in particular by adhesion lamination. Where necessary, the additional substrate is preferably attached to the substrate on the side of the liquid crystal layer.

Preferably, the dark background is at least partially superimposed on the liquid crystal layer.

The dark background is preferably placed in the register with respect to the liquid crystal printing; in particular, a dark background can be superimposed on liquid crystals.

A dark background can be obtained before or by applying liquid crystals.

The dark background can have a transmission of less than 80% and is preferably opaque.

The adhesive primer is preferably transparent.

Another object of the invention, in accordance with another aspect thereof, is a security element for a security document, in particular obtained by a method according to the invention as defined above, comprising:

a. a backing made of stretched thermoplastic material,

b. an adhesive primer on the substrate surface,

c. at least one layer of liquid crystals, in particular cholesteric liquid crystals, printed on the adhesive primer.

The security element can comprise printing of at least two liquid crystal inks, which preferably do not overlap.

The adhesion of the liquid crystal layer to the substrate, measured according to the Tesa test described below, is preferably 2.

Preferably, the liquid crystal layer is in the form of patterns, in particular alphanumeric patterns.

Another aspect of the invention is a method for authenticating a security element according to the invention, comprising examining the element to determine if a goniochromatic effect is observed for angles of incidence greater than or equal to 70 °, and the step of obtaining a piece of information regarding authenticity based on at least said study.

The authentication method may include examining the element through a polarizing filter to reveal the orientation of the substrate and adhesion primer, and the step of obtaining a piece of information regarding authenticity based on at least said examination.

The invention will become more clear upon reading the following description of non-limiting examples of its implementation and upon studying the attached drawing, in which:

- Fig. 1 is a top view of a security element according to the invention,

- Fig. 2 is a longitudinal section along II-II in FIG. one,

- Fig. 3 is an embodiment of a detail of an element,

- Fig. 4 is a view similar to FIG. 1, an embodiment,

- Fig. 5 is a longitudinal section of the element of FIG. 4,

- Fig. 6 is a view similar to FIG. 3, an embodiment of an element,

- Fig. 7 illustrates the various steps of an example of a method according to the invention,

- Fig. 8 shows an example of a security article comprising a security element in accordance with the invention, and

- Fig. 9 is a graph illustrating the "haze" measurement described below.

In the drawing, actual proportions are not always respected in the interest of clarity.

A security element 10 according to the invention comprising a substrate 11, an adhesive primer 12 and a layer of liquid crystals 13 printed onto the primer 12 is shown in FIG. 1-3.

The layer 13 is formed by two prints 13a, 13b, which do not overlap each other, of different colors. In the example of FIG. 1-3, prints 13a and 13b do not together cover the entire surface area 11a of the substrate. This leaves areas 14 around the prints that can be used by other security means, for example a layer, in particular ink, which is fluorescent or opaque.

In the example of FIG. 4-6, the prints 13a and 13b together cover the entire surface 11a of the substrate.

In order to improve the visibility of the liquid crystals, printing is preferably performed overlapping on an opaque background, preferably a dark or reflective background, such as a black background.

As an example, the presence of a dark background in the form of an opaque layer 15 on the substrate 11 on the side opposite to the liquid crystal layer 13 is illustrated in FIG. 6. The opaque layer 15 is, for example, black ink printing or metallization.

The production of the layer 15 is carried out, for example, after the application of the liquid crystal layer 13.

In one embodiment in FIG. 6, the opaque layer 15 is present on the liquid crystal layer 13 on the same side of the substrate. The security element is then observed from the side of the substrate against the liquid crystal layer 13.

Substrate

The backing is made of an extensible thermoplastic material.

Preferably, the backing is made of PET, polypropylene or polyethylene.

In order to improve the durability of the security element, the additional substrate, preferably containing PET, is attached to the substrate by any suitable means, in particular by lamination by adhesion.

The additional substrate is preferably attached to the substrate on the side of the liquid crystal layer. In one embodiment, to improve the adhesion of the sandwich thus formed, the liquid crystal layer, preferably in patterns, covers, for example, at least 50% of the surface of the substrate. The adhesion between the backing and the complementary backing is thus enhanced.

Liquid crystals

Preferably, the liquid crystals are cholesteric liquid crystals (CLC). Alternatively, the liquid crystals are nematic liquid crystals.

Numerous liquid crystal inks currently exist on the market and can be used. The ink can be a mixture of nematic and cholesteric liquid crystals, in a ratio chosen depending on the desired color variations.

Adhesive primer

The adhesion primer increases the adhesion of liquid crystals to the substrate, but does not prevent the substrate from participating, due to its internal structure, in the orientation of the liquid crystals.

The adhesive primer contains one or more polymers.

The primer can be applied in the form of monomers, in particular olefinic (or alkene), urethane, ester, ether, carbonate or acrylic, preferably acrylic, monomers, on a substrate, and this application is then followed by crosslinking, for example, by exposure to UV radiation ...

Preferably, the chemical nature of the primer is selected to have a better affinity for the liquid crystal ink used. Thus, since many liquid crystal inks are based on acrylate monomers, it is preferable to use a primer that is also based on acrylate monomers.

The amount of primer that is applied is chosen such that the thickness of the deposit remains small enough for the underlying substrate structure to remain active with respect to the orientation of the liquid crystals.

The thickness of the primer is thus preferably less than 1000 nm, more preferably less than 100 nm.

Orientation of the substrate with the primer already present above can make it possible to orient the primer and thus allow greater application of the primer.

Other protective equipment

The security element may contain, in addition to liquid crystals, any other security means selected from first, second or third level security.

It can be:

- patterns that appear in transmitted light and that are formed by metallization and / or demetallization;

- dyes, luminescent pigments, interference pigments, in particular in a printed form or in a form mixed with at least one constituent layer of the security element;

- compounds, dyes and / or pigments that are photochromic or thermochromic, in particular in a printed form or in the form of mixed with at least one constituent layer of the security element;

an ultraviolet (UV) absorber, in particular in the form of a coating or in a form mixed with at least one constituent layer of the security element;

interference multilayer structure,

- refractive, birefringent or polarizing layer,

- diffraction structure,

- means producing a "moire effect", and, for example, for such an effect, it is possible to show a pattern obtained by superimposing two security means, for example by bringing a line of two security means close to each other;

- color filter;

- Automatically readable security devices having specific and measurable luminescence (e.g. fluorescence, phosphorescence), light absorption (e.g. ultraviolet, visible or infrared light), Raman scattering activities, magnetism, microwave interaction, X-ray interaction or electrical conductivity characteristics.

Protective element

The security element according to the invention is preferably a security thread, foil, patch or film for protecting variable data. It can also be a protective or resistant film or card.

In the case of a security element consisting of a security thread, said thread can be integrated like windows in security paper such as banknotes 30, as shown in FIG. 8. The security element then extends from one edge 31 to the other edge 32 of the document.

When the security element is in the form of a thread, its width is, for example, 1 to 10 mm and its thickness is 10 to 100 µm.

In the case of foil, the security element is applied by transfer to the surface of paper, film or card, for example.

The term "patch" refers to a film that does not cover the entire surface of the underlying substrate.

Preparation method

In accordance with the invention, the method comprises, as shown in FIG. 7, a step 21 of obtaining a film made of plastic, for example by extrusion. This film can be single-layer or multi-layer.

It undergoes at least one stage 22, which makes it possible to orient it in the first direction. This is, for example, the first stretch in the longitudinal direction (also called machine direction).

The adhesive primer is then applied in step 23, then the film is stretched again in a direction other than the first, for example, in the transverse direction (also called transverse direction) in step 24.

Finally, liquid crystals are printed in step 25 onto a biaxially stretched and adhesive primed film.

The orientation of the liquid crystals occurs during the evaporation of the solvent present in the ink used. This evaporation is preferably carried out by drying with a stream of air, preferably in a horizontal direction, in order to promote the orientation of the liquid crystals. Preferably, the ink is UV crosslinkable, and the crosslinking that follows the application of the ink allows this orientation to be permanently fixed.

The printing is preferably carried out with a plurality of liquid crystal inks, which are preferably applied without overlapping each other. Ink can be applied as a solid or pattern. The term "pattern" means any pattern recognizable to the naked eye, in particular, for example, one or more alphanumeric characters or an image of an object such as a person, animal, object, landscape, plant or monument. It can be a halftone bitmap pattern, which preferably appears in 3D due to the shadow effect by reproducing the halftone pattern. It can be a repeating pattern in a given direction, a bitmap pattern, or preferably a checkerboard pattern.

Printing can be done using flexography, screen printing or photo-engraving, among other printing methods that can be used.

In one embodiment of the method, the film is subjected to double machine and cross-direction stretching after the adhesive primer has been applied. This application preferably takes place after the first machine direction stretching.

Thus, it is possible to have the following sequence:

- stretching in the machine direction;

- applying a primer;

- stretching in the transverse direction;

- stretching in the machine direction.

Authentication method

Authentication of the security element according to the invention can be carried out by examining the element in such a way as to determine whether a goniochromatic effect is observed for angles of incidence greater than or equal to 70 °, this effect being associated with the presence of liquid crystals.

It is then possible to obtain a piece of information regarding authenticity based on at least said study.

Authentication may include examining the element through a polarizing filter to reveal the orientation of the substrate and adhesion primer, as well as the step of obtaining a piece of information regarding authenticity based on at least said examination.

The biaxial orientation of the substrate can be verified by analyzing the birefringence of the film in a conventional manner.

A linear polarizer such as a filter can be used, which must be placed on the security element, which is rotated 90 ° in such a way as to determine if there is a transition from a dark look to a lighter look when rotated.

Examples of

The "Tesa" test mentioned below is a layer adhesion test on a substrate in which an adhesive tape is applied to a layer in which it is to measure adhesion and adhesion to the substrate.

A value from 0 to 2 is obtained according to the proportion of the test layer that remains attached to the tape after it has been removed.

- 0 denotes complete attachment to the tape and therefore zero adhesion to the substrate of the test layer;

- 1 indicates partial fastening to the strap and

- 2 denotes no adhesion to the tape and therefore better adhesion to the substrate of the test layer.

Examples 1 to 3 (comparative)

Several PET films are printed with liquid crystal ink made from a mixture of three products, namely:

cholesterol liquid crystals: 5.29 g

nematic liquid crystals, 3.11 g

photoinitiator, 1.6 g

The mixture is an acrylic-based paint containing nematic and cholesteric liquid crystals.

The selected ratio between cholesteric and nematic liquid crystals gives color; in this case, a change from blue to green-yellow is obtained with an observation angle.

The application of liquid crystals is carried out at a thickness of 3 μm, with a coating strip.

Drying with hot air blown is carried out in order to align the liquid crystals. The ink is UV crosslinked.

Liquid crystals are observed in front of a dark background.

Film Impact Test "Tesa" Goniochromatic effect ** (1) Standard biaxially oriented PET Not 0 Reference effect (2) Biaxially oriented PET Chemical attack * 2 Severe deterioration, whitish appearance due to film translucency (3) Biaxially oriented PET Plasma treatment 0 Identical to the reference effect

* Chemical attack: treatment with an acid solution with the creation of surface microroughnesses

** Goniochromatic effect: The observed parameters are as follows: clear observation of two colors, color brightness and color change angles.

Plasma treatment does not provide optimal adhesion, but the placement of the liquid crystals is correct.

PET film treated by chemical attack makes excellent adhesion possible, but the optical effect is deteriorated.

Example 4 (comparative) and 5 to 7 (according to the invention)

The liquid crystal deposition was carried out as described in examples 1 to 3 using a commercially available liquid crystal ink.

Film Impact Test "Tesa" Goniochromatic effect (1) Standard biaxially oriented PET Not 0 Reference effect (2) Biaxially oriented PET Off-line polyacrylic primer *** one Improvement: zoom in the range of the rendering angle and no rendering when sliding angle on a dark background (3) Biaxially oriented PET Polyacrylic primer applied online *** 2 Improvement: zoom in the range of the rendering angle and no rendering when sliding angle on a dark background (4) Biaxially oriented PET Copolyester primer applied online *** 2 Improvement: zoom in the range of the rendering angle and no rendering when sliding angle on a dark background

*** Off-line: application occurs after the stretching stage of the PET film. Online: Application occurs during the production of the film between at least two stages of stretching the PET film.

The goniochromatic effect is surprisingly retained for Examples 5 through 7, despite the presence of an intermediate primer layer between the biaxially oriented PET substrate, which provides positioning, and the liquid crystal layer.

The arrangement of the liquid crystals is altered by the presence of an adhesive primer. Modifying this position allows you to increase the visibility of the color shifting effect, and to make the liquid crystal layer transparent, thus limiting the visibility of the black background.

The degree of transparency can be determined by measuring the "haze" of the reflection according to ASTM D 4039-09 and ISO 13803. This is a haze measurement, i.e. milky opalescence, glossy or transparent topcoat.

On high-quality (class A) surfaces, a bright and shiny image is expected. The microstructure may be responsible for the milky appearance. This effect is called "haze". If microscopic and small defects are present on a very shiny surface, this results in low intensity light scattering along the main reflection. The predominant part is reflected in the mirror direction, the surface is thus very shiny with image transmission, but it is covered with milky and cloudy haze.

The "haze" of the liquid crystal layer on a substrate according to the invention can be less than the haze of the liquid crystal layer on untreated PET, resulting in much lower gloss than for the liquid crystal layer on a standard substrate. In addition, reducing "haze" increases the scattering of light in the liquid crystal layer and therefore allows the goniochromatic effect to be observed over a wider range of angles. Thus, unexpectedly, at "grazing" angles, the goniochromatic effect is maintained with the substrate according to the invention, while with the standard substrate, the liquid crystals are no longer visible and a dark background is observed, which is not desirable.

The "haze" measured according to ISO 13803: 2014 of films (1) and (6) is 274 and 79, respectively. This measurement is shown in FIG. 9, in which the substrate to be measured 91 is illuminated in accordance with the angle α, which is, for example, 20 ° for the above standard. The measurement of "haze" is carried out using sensors 92 and corresponds to the intensity of the light rays 93, in contrast to the gloss, which corresponds to the intensity of the specular beam 94 (angle α).

In order to quantify the increase in the angular range of the goniochromatic effect, the maximum intensity in the 400-700 nm wavelength range (“intensity” in the table below) was measured at different viewing angles for illumination at an angle of 50 ° on samples (1) and (6). All intensity measurements indicated in the table below are given in conventional units corresponding to this calibration; in particular, the intensities of samples (1) and (6) were measured with the same calibration.

Angle
observation (°) Sample intensity (6) Sample intensity (1) ten 50 ten 20 100 20 thirty 200 100 40 400 400 50 400 1800 60 300 300 70 250 150 80 200 100

It should be noted that at grazing viewing angles, for example, 10 °, 20 °, 70 ° and 80 °, the intensity of sample 6 is greater than that of sample 1, which reflects the visibility over the longer angular range of the goniochromatic effect of sample 6.

Claims (23)

1. A method of manufacturing a security element containing a substrate (11) made of oriented plastic and at least one layer of liquid crystals (13) applied and oriented on the substrate, an adhesive primer (12) is applied before or after stretching the substrate at least in one direction, and the specified stretching allows you to bring the substrate into an oriented state, and the liquid crystals are applied by printing on the specified adhesion primer. 2. A method according to claim 1, wherein the adhesive primer (12) is applied prior to the step of stretching the substrate (11). 3. The method according to claim 1 or 2, wherein the support comprises polyethylene terephthalate (PET). 4. A method according to any one of claims. 1-3, also including the step of cross-linking the adhesive primer, preferably after said stretching of the substrate. 5. The method according to any one of claims. 1-4, wherein the adhesive primer comprises a polyolefin, polyurethane, polyester, polyester, polycarbonate, or polyacrylic, or a copolymer thereof. 6. The method of claim 5, wherein the adhesive primer comprises polyacryl. 7. The method of claim 4, wherein the adhesive primer contains, in particular prior to application, at least one urethane, ester, ether, carbonate or acrylic monomer. 8. The method according to any one of claims. 1-7, where liquid crystal printing is performed by screen printing, photo-engraving, or flexography. 9. The method according to any one of claims. 1-8, where liquid crystals are applied after stretching the substrate. 10. The method according to any one of claims. 1-9, comprising printing on an adhesive primer two different inks, at least one of which contains cholesteric liquid crystals, more preferably both of which contain cholesteric liquid crystals. 11. The method according to any one of claims. 1-10, where the haze effect of the cholesteric liquid crystal layer is less than the haze effect of a reference security element without an adhesive primer layer. 12. The method according to any one of claims. 1-11, where there is a dark background under the layer of liquid crystals. 13. The method according to any one of claims. 1-12, where the adhesive primer is transparent. 14. Security element (10) for a security document, in particular obtained using the method according to any one of claims. 1-13 containing: a) a substrate (11) made of stretched thermoplastic material, b) an adhesive primer (12) on the surface of the substrate, c) at least one layer of liquid crystals (13) applied to the adhesive primer by means of printing. 15. A security element according to claim 14, comprising printing at least two liquid crystal inks that are preferably not superimposed on each other. 16. A security element according to claim 14 or 15, wherein the adhesion of the liquid crystal layer to the substrate, as measured by the Tesa test, is 2. 17. The security element according to any one of paragraphs. 14-16, where the cholesteric liquid crystal layer is in the form of a pattern, in particular an alphanumeric pattern. 18. The security element according to any one of paragraphs. 14-17, also containing a dark background. 19. A method for authenticating a security element according to any one of claims. 14-18, comprising examining the element to determine if a goniochromatic effect is observed for angles of incidence greater than or equal to 70 °, and the step of obtaining a piece of information regarding authenticity based on at least said examination. 20. A method for authenticating a security element according to any one of claims. 14-18, comprising examining the element through a polarizing filter to reveal the orientation of the substrate and the adhesion primer and the step of obtaining a piece of information regarding authenticity based on at least said examination.

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