Coating composition for producing magnetically induced images

[0001] The present invention relates to coating compositions for producing magnetically induced images. More specifically, it relates to printing inks for the production of magnetically induced images for use on secured documents or securities or branded articles in order to protect them against counterfeiting and illegal reproduction.

BACKGROUND OF THE INVENTION [0002] Optically variable elements of various types are used as an effective security measure on secured documents and securities. Among these, optically variable printing inks (OVP®; EP-A-0227423) are particularly important optically variable copy protection agents. Optically variable printing inks (OVI®) are used to print surfaces and / or characters that show a color that varies depending on the viewing angle (= color shift).

[0003] Said copy protection paints are based on optically variable pigments (OVP); preferred types are thin film optically interference flake-shaped pigments described in US 4,705,300; US 4,705,356; US 4,721,271 and related disclosures. [0004] Other useful OVP types for optically variable ink formulations include coated interference pigments described in US 5,624,486 and US 5,607,504 and thin film cholesterol (i.e. chiral-nematic) liquid crystal pigments described in US 5,807,497 and US 5,824,733.

[0005] Optically variable printing inks, coatings and varnish products are known in the art, e.g., from EP-A0227423, US 5,279,657, or WO 95/29140. These optically variable printing inks can be used in many printing processes such as copper plate printing (intalgio printing), intaglio printing, flexographic printing or screen printing.

[0006] As is known to the skilled person, the thickness of the wet layer obtained in said printing processes may vary widely, from about 2 μm to about 50 μm, depending on the method and conditions used.

[0007] In order to obtain a strong color shift effect for an ink or optically variable coating, the optically variable pigment (OVP) preferably has a plate or flake shape, as disclosed in the prior art.

[0008] The perceived optical properties and color purity depend on the final orientation of the pigment in the cured ink layer or coating on the substrate. The randomly oriented flakes or optically variable pigment plates exhibit poor color shift and low color purity. Maximum color shift and color purity require that the flakes or plates of the optically variable pigment in the ink or coating take the same particular orientation, e.g., coplanar to the surface of the substrate.

[0009] These optical effects are further enhanced when the surface of the substrate is previously leveled by applying a primer coat. Optically variable pigment flakes can including

In this case, it is easier to lay flat, thereby increasing surface coverage, color purity and color shift.

[0010] In order to obtain coatings having optically variable pigment flakes arranged in the same position flat on the surface, an ink or coating formulation is usually used that allows the thickness of the wet layer to be reduced during the drying process to less than 10 μm. The gradual reduction of the layer thickness during the drying process forces the optically variable pigment flakes to form in a single plane parallel to the surface of the substrate, ensuring maximum coverage and color shift on the substrate.

[0011] Optically variable magnetic pigments are disclosed in WO 02/073250; US 4,838,648; EP-A686675; WO 03/00801 and US 6,838,166 as an improvement of optically variable pigments for inks for security documents, securities and banknotes; these documents are hereby incorporated as related material.

[0012] Optically variable magnetic pigments in printing inks or coatings allow the production of images, drawings and / or patterns induced magnetically by applying a suitable magnetic field, causing the local orientation of the optically variable magnetic pigment in the coating, followed by drying / curing of the latter. The result is a fixed image, drawing or pattern induced magnetically in optically variable paint.

[0013] Materials and technologies for the orientation of magnetic particles in coating compositions and printing processes have been disclosed in US 2,418,479; US 2,570, 856; US 3,791,864; DE 2006848-A; US 3,676,273; US 5,364,689; US 6,103,361; US 2004/0051297; US 2004/0009309; EP-A-710,508; WO 02/090002; WO 03/000801; WO 2005/002866 and US 2002/0160194; these documents are hereby incorporated as related material.

[0014] US 2,418,479 and US 2,570,856 disclose a method and coating composition for the magnetic orientation of metallic pigments in paint layers. These layers exhibit a strong degree of orientation and a low degree of brightness, as well as unusual properties of reflection coefficient and transparency. Metallic pigments include flakes of ferromagnetic material, preferably nickel, in amounts ranging from 0.1% to 95% by weight relative to the film-forming binder; and organic volatiles are included in the composition in amounts ranging from 50% to 70% by weight based on the total weight. A 25 mil (635 μm) wet coating is applied, and is subjected to a magnetic field to orient the metal flakes, keeping the field until the layer is dry. These documents do not apply to OVI® and disclose mainly paint compositions containing magnetic metallic pigments in the form of flakes and a coating with the effect based on them. There are no recipe rules for flake dimensions, flake concentration and coating thickness to obtain the best optical effect.

[0015] US 3,791,864 and DE 2006848-A relate to oven enamel compositions, nitrocellulose compositions and two-component compositions containing magnetic components (e.g., iron pigments shaped like lamellar or rod-shaped) for the production of magnetically oriented coatings. The documents relate to the method and process of magnetic orientation of pigments in a two-layer coating; however, the formulation aspects of the coating composition used are not considered.

[0016] US 3,676,273 discloses magnetically oriented coatings containing highly reflective nickel flakes dispersed in an acrylic binder. The amount of magnetic pigment ranges from 0.1% to 95% by weight relative to the film-forming material. Specific aspects of recipe making are not given in this document.

[0017] US 5,364,689 discloses a painted product comprising non-spherical magnetic particles in a paint medium in which said magnetic particles are oriented to give a three-layer optical appearance pattern. Magnetic particles include one or more materials among nickel, cobalt, iron and their alloys. The particles have a thickness of 0.1 to 1.0 μm and a length of 10 to 100 μm. The base for the paint is selected from alkyl, polyester, acrylic, polyurethane and vinyl resins. The particles are in amounts between 1 and 60 parts per 100 parts by weight of the paint base. However, no specific rules are given for the paint base formulation.

[0018] US 6,103,361 relates to heat resistant coating compositions containing fluorinated polymers such as PFTE (polytetrafluoroethylene) and magnetizable flakes that allow magnetic development of the coating on a frying pan.

[0019] US 2004/0051297 and US 2004/0009309 relate to a method and apparatus for orienting magnetic flakes during painting or printing processes. Magnetic optically variable pigment particles are dispersed in a liquid medium of a varnish product or printing ink. A typical petal is about 20 μm long and about one μm thick. The flakes typically contain a layer of magnetic metal, such as a thin layer of ferromagnetic metal or alloy, such as cobalt, nickel or PERMALOY (typically 80% Ni, 20% Fe) and an optical interference structure, such as a dielectric absorber structure - Fabry-Perot type reflecting agent on both sides of the metallic layer. US 2004/0051297 contains comments regarding the effect of layer thickness and the type of organic carrier used on the magnetic orientation ability of pigments. However, no further details regarding the best formulation for the coating composition for use have been disclosed.

[0020] WO 02/090002 relates to methods for making coated articles with patterns by using an optically variable magnetic pigment, and coated articles. The pigment consists of reflective magnetic flakes (RMF) of the type described in WO 03/000801 "Multi-Layered Magnetic Pigments and Foils" and includes a magnetic core layer. However, no rules are given for creating recipes for coating compositions to be used.

[0021] WO 05/002866 relates to a method and means for producing a magnetically induced pattern in a coating comprising magnetic particles. Said coating preferably comprises optically variable magnetic particles. The coating composition is preferably selected from the group of liquid printing inks including screen printing inks, gravure printing inks and flexographic inks. Liquid printing inks have a low viscosity (in the range of 0.1 to 5 Pa * s in 20^aboutC) and allow easy orientation of the magnetic pigment. Drying / curing of the ink may be based on solvent or water evaporation, and on crosslinking by UV radiation or on hybrid curing mechanisms, including diluent evaporation, UV curing and other mesh formation reactions such as crosslinking reactions

EP 2 024 451 B1 by oxidation polymerization. However, none of the given paint recipes were optimized for the magnetic pattern / imprinted effect in the coating.

[0022] US 2002/0160194 relates to multilayer magnetic pigments and films. The disclosed pigment flakes can be spread in a binder to produce a coloring composition (ink) that can be applied to a wide range of objects or papers. The binding medium contains a resin or a mixture of resins and a solvent (organic solvent or water) and can be dried / cured in heat-using processes such as heat crosslinking, heat solidification or solvent evaporation by heat or by photochemical crosslinking. [0023] The printing inks and optically variable coating compositions used in the prior art are directed to exhibiting a bright color, a strong color shift, and giving a good coverage of the substrate when using as little optic variable pigment as possible. A low pigment concentration is desirable to reduce raw material costs and to obtain good printability and durability. These goals have been achieved by providing printing inks with a relatively large amount of volatile components, such as organic solvents, water or mixtures thereof, on the order of 50% by weight or more, based on the weight of the composition and a relatively small amount of non-volatile components, i.e. binding and OVP, on the order of 50% by weight or less, based on the weight of the composition.

[0024] The specific formulation ensures a reduction in the volume of the coating layer during the drying process and the correct orientation of the OVP particles in the plane of the printed substrate. This is why most OVI or coating preparations containing pigments with an optical effect are based on solvents or water, the solids content not exceeding 50%. The solids content represents the proportion of non-volatile components in the printing ink or coating layer after drying / curing processes.

[0025] However, with magnetic optically variable pigments, it has been found that this type of paint formulation when used for magnetic development of images, drawings or patterns in the printed ink layer leads to poor visual effects.

Summary of the Invention [0026] The technical problem underlying the present invention was to find coating compositions and appropriate formulation principles that are particularly suited to the magnetic orientation of an optically variable magnetic pigment (MOVP) in a printing ink or coating layer that gives an attractive visual effect. With conventional formulations suitable for OVI® printing, magnetic images transferred to the wet ink layer are remarkably reduced in resolution and contrast during the drying / curing process, due to the shrinkage of the printed paint or coating layer in the vertical direction.

[0027] The resulting printing inks should comply with standard printing requirements, such as printing speed and printing resolution, and with economic control restrictions

EP 2 024 451 B1 by reducing the amount applied. Printing technologies for use in MOVP particle printing should include flexographic printing, gravure printing, screen printing and roller coating. [0028] In accordance with the present invention, this problem is solved by means of a coating composition as defined in the appended claims.

[0029] In particular, the present invention relates to a coating composition for producing magnetically induced images according to the present invention, consisting of volatile components (S) and non-volatile components, the latter consisting of a varnish binder (I) and a magnetically orientated interference optically variable pigment (P), characterized by that the ratio of the volume of the varnish binder (V (I)) to the volume of the pigment (V (P)) is higher than 3.0, preferably higher than 4.0 and most preferably higher than 5.0, and that said coating composition is a printing ink selected from flexographic printing inks, gravure printing inks, screen printing inks and roller coating inks.

[0030] The present invention also relates to a method for producing a coating composition for producing magnetically induced images, which comprises the step of mixing the volatile components (S) and non-volatile components together, the latter consisting of a varnish binder (I) and orientable magnetically interference optically variable pigment (P), characterized by that the ratio of the volume of the varnish binder (V (I)) to the volume of the pigment (V (P)) is higher than 3.0, preferably higher than 4.0 and most preferably higher than 5.0, and that said coating composition is a printing ink selected from flexographic printing inks, intaglio printing inks, screen printing inks and roller coating inks.

[0031] In accordance with the present invention, the term "magnetic optically variable pigment (MOVP)" refers to magnetic pigment particles having the shape of plates or flakes containing an optical interference coating, as is known in the art. The special property of MOVP over OVP is that MOVP particles can be oriented by applying a magnetic field. MOVPs are therefore 'optically variable pigments that can be magnetically orientated'. The MOVPs contained in the printing ink or coating composition of the present invention consist of flat plates or flake-shaped particles selected from the group of interference pigments with a thin magnetic layer applied by vacuum coating, coated metallic interference pigments, coated interference non-metallic pigments, magnetic liquid crystal pigments as disclosed in PCT / EP2005 / 056260 and mixtures thereof. Five-layer or seven-layer interference pigments with a thin magnetic layer applied under vacuum according to US 4,838,648 and WO 02/73250 are particularly preferred.

[0032] The MOVPs used in the present invention are also characterized by their average particle size. In order to obtain saturated colors and rapid color changes, the average particle diameter (d50) should typically be in the range from 5 to 40 μm, preferably from 15 to 25 μm, and have a thickness of 0.1 to 6 μm, more preferably in 0.5 to 3 μm range.

[0033] According to the present invention, the term "volatile components" refers to components having a boiling point below 300^aboutC under normal pressure, i.e. anything that eventually evaporates after printing. The volatile components contained in the ink / coating composition may be

EP 2 024 451 B1 selected from organic solvents, water and mixtures thereof, i.e. from those typically used in the production of printing inks.

[0034] According to the present invention, the term "non-volatile components" refers to components having a boiling point of at least 300^aboutC under normal pressure, i.e., everything that remains after printing.

[0035] According to the present invention, the term "varnish binder" refers to a non-volatile portion of the ink or coating composition, except for the optically variable magnetic interference pigment. However, the paint binder may contain other pigments. Thus, the paint binder according to the present invention may contain components from the group consisting of colorless varnishes / varnishes (i.e. binders), oligomers, fillers, pigments, dyes, leveling improvers, wetting agents, surfactants, corrosion inhibitors, drying catalysts, photoinitiators, waxes, crosslinkers, non-volatile diluents or monomers.

[0036] According to the present invention, the term "volume of varnish binder" refers to the volume of the dried / cured varnish binder.

[0037] According to the present invention, by the term "drying", three different mechanisms are usually distinguished in the art. Two, mainly physical, drying processes relate to the evaporation of volatile components of the printed paint or coating, leaving their solid components in the form of resin and pigment, and the penetration / absorption of a non-volatile solvent of the paint or coating into the substrate. A third, chemical drying process, also called curing or crosslinking, relates to the transformation of a liquid composition into a solid composition by chemical polymerization, or a crosslinking reaction initiated by UV radiation, electron beam bombardment, or polymerization by oxidation (oxidative crosslinking caused by the joint action of oxygen and catalysts such like Co and Mn catalysts). One or more of these drying processes may be involved in drying the same particular ink or coating. Thus, curing is a specific embodiment of drying. "Double cure method" means the combination of physical evaporation and / or penetration of volatile constituents into the substrate with cure by UV radiation or by polymerization by oxidation or chemical polymerization initiated by a suitable additive; "UVOX" means a combination of UV cure and oxidation polymerization.

[0038] Printing technologies that are used to print MOVP particles include flexographic printing, gravure printing, screen printing and roller coating.

[0039] To achieve the printing requirements, suitable printing components are selected to apply a typical average dry film thickness in the range of 2 to 50 μm, preferably 5 (flexographic printing) to 30 μm (screen printing).

[0040] The average pigment diameter is chosen depending on the layer thickness that can be typically obtained and the technical limitations associated with the use of a given type of printing. Choosing too small pigment particles will in all cases lead to poor color shift, strong light scattering and low color saturation. This is well known to the skilled person and will be taken into account by him when choosing the right pigments.

[0041] It has been found that vertical shrinkage of the ink or coating layer must be avoided during the drying / curing process in order to prevent the flat positioning of the oriented particles

Pigment, which significantly reduces or even completely destroys the orientation effect produced by magnetization. This is achieved by providing a sufficiently thick layer of non-volatile varnish binder that remains after the volatiles have evaporated.

[0042] Thus, the most important is the volume ratio V (I) / V (P) of the dried / cured varnish binder (I) to the volume of the magnetic optically variable pigment (P) contained in the varnish binder. It was found that at a volume ratio V (I) / V (P) below 3.0 it was not possible to produce a satisfactory magnetically induced image in the coating of the present invention. In accordance with the present invention, the volume ratios are calculated based on experimental data and known product properties as disclosed in the detailed description.

[0043] What needs to be considered is the thickness of the dried / cured ink layer. The inventors have found that the dried / cured solid ink layer should have a thickness of not less than d50 / 3, preferably a thickness of not less than d50 / 2, in order to obtain an orientable coating layer giving a satisfactory magnetically oriented image. The d50 size is the average diameter of the optically variable magnetic pigment and is determined in a manner known in the art.

[0044] In coatings that are much thinner than d50 / 2 the orientation effect obtained is poor.

[0045] The coating composition of the present invention for producing magnetic images may be flexographic printing ink, intaglio printing ink, screen printing ink or roller ink, and may be suitably used in processes of flexographic printing, gravure printing, and screen printing or in a roller coating process.

Brief description of the drawings [0046]

Figures 1 to 3 show (a) different optical effects obtained from three example compositions and (b) obtained pigment orientations within the paint layers.

Fig. 1 shows the results obtained for UV-cured screen printing inks according to example 2a. In Fig. 1a the obtained magnetized image is shown. In Fig. 1b, a cross section of a printing ink layer on a substrate, from a scanning electron microscope (SEM), is shown.

Fig. 2 shows the results obtained for silk-screen printing inks cured by UV radiation according to example 2b. In Fig. 2a the obtained magnetized image is shown. In Fig. 2b a cross-sectional view of a layer of ink on a substrate from a scanning electron microscope (SEM) is shown.

Fig. 3 shows the results obtained for silk-screen printing inks cured by UV radiation according to example 2c. In Fig. 3a the obtained magnetized image is shown. In Fig. 3b, a cross section of a printing ink layer on a substrate, from a scanning electron microscope (SEM), is shown.

EP 2 024 451 B1

Detailed description of the invention [0047] The magnetic optically variable printing inks or coating compositions of the present invention are divided into three main components. Optically variable magnetic pigment (P), solvents or volatile components (S) (i.e., everything that eventually evaporates after printing: organic solvents, water or mixtures thereof) and varnish binder (I), (i.e., everything that remains after printing, except for pigment: i.e. non-volatile components, such as (transparent) varnishes, oligomers, fillers, pigments, dyes, leveling agents, wetting agents, surfactants, corrosion inhibitors, drying catalysts, photoinitiators, waxes, crosslinkers, non-volatile diluents or monomers). [0048] The magnetic pigment of the present invention is typically selected such that the plate-shaped particles have an average particle diameter (d50) in the range of 5 to 40 μm, more preferably a diameter in the range of 15 to 25 μm, and a thickness in the range of 0, 1 to 6 μm, more preferably in the range of 0.5 to 3 μm. [0049] The diameter of the plate-shaped particles should be understood as the mean (d50) size, determined as is known to the skilled person. Similarly, the thickness of the plate-shaped particles should be understood as the mean distance between the upper and lower surface of the plate, determined as is known to the skilled person.

[0050] It has been found by the inventors that, regardless of the process used to apply the magnetic ink to the substrate, there is a lower limit by volume ratio V (I) / V (P) of the varnish binder (I) to the magnetic optically variable pigment (P) contained in the binder paint, below which the visual effect of a magnetically oriented image, pattern or pattern disappears after drying the wet layer. This lower limit was found at a volume ratio V (I) / V (P) of 3.0. Good results are obtained with a V (I) / V (P) volume ratio higher than 4.0, preferably higher than 5.0.

[0051] In other words, there must be a sufficient volume of varnish binder (I) per volume of pigment (P) in the dry paint layer to allow pigment flakes to retain their orientation if it does not later fall in the surface of the substrate.

[0052] For calculating the volume ratio V (I) / V (P), the volume of the paint binder itself and the volume of the pigment in the paint binder must be known. This will then be outlined with reference to Example 2.

[0053] Three ink formulations according to example 2a to 2c (UV-curable inks for screen printing) served as the basis for the correlation of recipe parameters with the obtained magnetic orientation effect. For the details of these examples, please refer to the experimental section below. The recipe of example 2a has excellent magnetic orientation ability; the formulation of example 2b shows slight degradation compared to example 2a, and the formulation of example 2c shows severe degradation compared to example 2a. It can be assumed that recipes with V (I) / V (P) ratios less than 3.0 no longer show any useful effect.

[0054] The experimental data determined for the base formulation of example 2 (first column) showed that the specific gravity (density) of the wet paint (Dm wet paint) is 1.24 g / cm³, (for dry paint, Dry paint 1.26 g / cm³). The density of the optically variable magnetic pigment was determined to be 2.82 g / cm³ (pigment density varies within certain limits, depending on the material ratio

EP 2 024 451 B1 dielectric (MgF2 (optical properties)) and magnetic material (Ni, Fe, Co or Ni alloy (magnetic properties)) in the pigment plate. The pigment used in this example has an experimental density of 2.82 g / cm³and the solvent density (Dowanol) is 0.967 g / cm³. Experimental densities were determined using a pycnometer. The use of a pycnometer for determining density is well known to those skilled in the art and does not need to be discussed in detail here (see ISO 1183-1: 2004).

[0055] The base formulation of wet printing ink can be approximately described as follows (including W = mass, V = volume before mixing, I = varnish binder, P = MOVP, S = solvent, D = density):

W (I) + W (S) + W (P) = Dark paint (V (I) + V (S) + V (P)) = 1.241 [g / cm³] * (V (I) + V (S) + V (P)) [cm³] [0056] Given the weight ratio of the recipe W (I) + W (S) = 0.80 g / g and W (P) = 0.20 g / g and pigment density D (P) = 2.82 g / cm³, the volume of pigment V (P) per gram of paint is calculated using the known relationship d = m / V as 0.071 cm³, and the remaining volume of wet varnish binder is 0.735 cm³. [0057] The dried / cured ink formulation can be approximately described as follows:

W (I) + W (P) = Dry paint (V (I) + V (P)) = 1.26 [g / cm³] * (V (I) + V (P)) [cm³] [0058] From the above data and solvent density D (S) = 0.967 g / cm³ (Dowanol), the amount of solvent to be evaporated can be determined and the weight of the varnish binder is 0.747 g / g of the initial formulation of the ink; the corresponding volume V (I) is 0.68 cm³; this gives a paint binder density D (I) of 1.098 g / cm³. In this example (basic formulation) the volume ratio V (I) / V (P) is thus defined as 9.58.

[0059] Referring to the examples of magnetic orientation (recipes from examples 2a to 2c), the respective volume ratios were calculated in the same way, using the determined specific gravities of MOVP and varnish binder (I) and the corresponding weight percentages:

Table 1

<td>Recipe</td><td>Example 2a</td><td>Example 2b</td><td>Example 2c</td>

<td>Varnish binder</td><td> 80</td><td> 60</td><td> 30</td>

<td>V (I)</td><td> 0,637</td><td> 0,729</td><td> 0,273</td>

<td>Pigment</td><td> 20</td><td> 20</td><td> 20</td>

<td>V (P)</td><td> 0,071</td><td> 0,071</td><td> 0,071</td>

<td>Solids content</td><td> 100</td><td> 80</td><td> 50</td>

<td>Volume ratio V (I) / V (P)</td><td> 10,27</td><td> 7,69</td><td> 3,85</td>

<td>Dry film thickness</td><td> 19</td><td> 14</td><td> 11</td>

EP 2 024 451 B1 [0060] Varnish, pigment and solids content in%; V (I) and V (P) in cm³, dry film thickness in μm; the pigments used had a d (50) of 22 μm.

[0061] Analogously to this, the volume ratio V (I) / V (P) for example 1, which was not an example according to the invention, which concerned gravure printing ink, was determined as 4.83. Density dry / cured and wet printing inks were marked as Dry paint = 1.37 g / cm³, Dark paint = 1.236 g / cm³. Pigment and solvent densities were defined as D (P) = 2.82 g / cm³, and D (S) = 0.805 g / cm³ (Ink solvent 27/29 for Shell Industrial Chemicals). 0.3 g / g pigment (P) was mixed with 0.7 g / g (varnish binder (I) + paint solvent (S)). The density of the paint binder D (I) was determined to be 1.066 g / cm³.

[0062] A further point to consider when preparing an ink formulation effectively is the thickness of the dried / cured solid coating layer. The coating layer should be thicker than d50 / 3, preferably thicker than d50 / 2, where d50 is the average diameter of the optically variable pigment magnetic flakes, determined in a manner known in the art.

[0063] Solvent- containing coatings must be correspondingly thicker after application than solvent-free coatings; considering that the thickness of the dry and solid layer after evaporation of the solvent must meet the above criteria. In the examples underlying Fig. 1 to Fig. 3, the dry solid layer thickness meets the preferred criterion d50 / 2 in all cases.

[0064] The amount of non-volatile components in the coating composition of the present invention is selected between 50% and 100% by weight based on the weight of the total composition, preferably between 80% and 100% by weight based on the weight of the total composition.

[0065] Fig. 1 relates to a coating composition having a V (I) / V (P) volume ratio of 10.3 and a maximum solids content of 100% by weight (20% MOVP), which leads to optimal magnetically induced effects (images, patterns or drawings) in the appropriate coating layer, while Fig. 3 relates to a coating composition having a V (I) / V (P) volume ratio of 3.8 and a maximum solids content of 50% by weight (20% MOVP) which still allows the creation of magnetically developed images, patterns or drawings in the coating layer, albeit low quality.

[0066] The skilled person will know that the general idea described herein is applicable to a series of ink formulations with different amounts of solids content (with different amounts of MOVP) leading to different V (I) / V (P) volume ratios.

[0067] Table 2 shows one of the possible compilations, in addition to the many calculated recipe matrices according to the inventive idea. The ranges of volume ratios are about 0.6 and about 23.

Table 2 (UV curable silk-screen printing ink according to example 2)

<td></td><td>SC 100%</td><td>SC 90%</td><td>SC 80%</td><td>SC 50%</td>

<td>And [%]</td><td> 90</td><td> 80</td><td> 70</td><td> 40</td>

<td>P [%]</td><td> 10</td><td> 10</td><td> 10</td><td> 10</td>

<td>V (I) [cm³/ cm³]</td><td> 0,8197</td><td> 0,7286</td><td> 0,6375</td><td> 0,3643</td>

<td>V (P) [cm³/ cm³]</td><td> 0,0355</td><td> 0,0355</td><td> 0,0355</td><td> 0,0355</td>

<td>V (I) / V (P)</td><td> 23,1148</td><td> 20,5464</td><td> 17,9781</td><td> 10,2732</td>

EP 2 024 451 B1

<td colspan="5">continued</td>

<td></td><td>SC 100%</td><td>SC 90%</td><td>SC 80%</td><td>SC 50%</td>

<td>And [%]</td><td> 80</td><td> 70</td><td> 60</td><td> 30</td>

<td>P [%]</td><td> 20</td><td> 20</td><td> 20</td><td> 20</td>

<td>V (I) [cm³/ cm³]</td><td> 0,7286</td><td> 0,6375</td><td> 0,5464</td><td> 0,2732</td>

<td>V (P) [cm³/ cm³]</td><td> 0,0709</td><td> 0,0709</td><td> 0,0709</td><td> 0,0709</td>

<td>V (I) / V (P)</td><td> 10,2732</td><td> 8,9891</td><td> 7,7049</td><td> 3,8525</td>

<td>And [%]</td><td> 70</td><td> 60</td><td> 50</td><td> 20</td>

<td>P [%]</td><td> 30</td><td> 30</td><td> 30</td><td> 30</td>

<td>V (I) [cm³/ cm³]</td><td> 0,6375</td><td> 0,5464</td><td> 0,4554</td><td> 0,1821</td>

<td>V (P) [cm³/ cm³]</td><td> 0,1064</td><td> 0,1064</td><td> 0,1064</td><td> 0,1064</td>

<td>V (I) / V (P)</td><td> 5,9927</td><td> 5,1366</td><td> 4,2805</td><td> 1,7122</td>

<td>And [%]</td><td> 60</td><td> 50</td><td> 40</td><td> 10</td>

<td>P [%]</td><td> 40</td><td> 40</td><td> 40</td><td> 40</td>

<td>V (I) [cm³/ cm³]</td><td> 0,5464</td><td> 0,4554</td><td> 0,3643</td><td> 0,0911</td>

<td>V (P) [cm³/ cm³]</td><td> 0,1418</td><td> 0,1418</td><td> 0,1418</td><td> 0,1418</td>

<td>V (I) / V (P)</td><td> 3,8525</td><td> 3,2104</td><td> 2,5683</td><td> 0,6421</td>

<td colspan="5">SC solids content, I = mass of the paint binder fraction, P = mass of the pigment fraction, V (I) / V (P) volume ratio</td>

[0068] Table 2 provides an arbitrary selection based on different contents of total solids and their respective compositions. From a technical point of view, the skilled person will know that a minimum amount of pigment is required to achieve satisfactory results, and that a large amount of pigment contained in the printing ink reduces ink printability and increases costs.

[0069] Thus, Table 2 provides a selection of the appropriate coating composition with respect to solid limits as well as the volume ratio V (I) / V (P) according to the present invention. The most eye-catching examples correspond to the most preferred volume ratio V (I) / V (P) higher than 5.0 from Example 2 and are for illustrative purposes only.

[0070] Printing inks having a volume ratio below 4.0 may also be suitable for carrying out the present invention; however, the increased amount of magnetic optically variable pigment relative to the paint binder makes it much more difficult to give MOVP magnetic orientation and if the ink layer is thick, it can increase the unnecessary costs of optically variable magnetic ink.

[0071] The method of producing the ink or coating composition of the present invention for producing magnetically induced images comprises the step of mixing the volatile (S) and non-volatile components together, the latter consisting of a varnish binder (I) and an optically variable interference pigment magnetically orientated (P), characterized by that the ratio of the volume of the paint binder (V (I)) to the volume of the pigment (V (P)) is higher than 3.0, preferably higher than 4.0, more preferably higher than 5.0 and that said coating composition is a paint

EP 2 024 451 B1 selected from the group of flexographic printing inks, gravure printing inks, screen printing inks and roller coating inks.

[0072] The volatile components for the method of producing the ink or coating composition of the present invention are selected from organic solvents, water and mixtures thereof.

[0073] The present invention further relates to a method for producing a magnetically induced image, which comprises the steps of a) applying the coating composition of the present invention to the surface of a substrate, b) orienting the magnetic pigment particles in the applied coating composition of step a) by applying a magnetic field and c) curing / drying the oriented coating composition of step b) to fix the particles in an oriented position.

[0074] The coating method of step b), in order to apply the coating composition to the surface of the substrate, is selected from flexographic printing, gravure printing, roller coating and screen printing. These methods are well known to the skilled person.

[0075] Said printing processes allow the application of a wet layer with a thickness of about 2 μm to about 50 μm. The preferred wet coating layer has a thickness in the range of about 5 μm to about 30 μm. The resulting coating on the substrate has an average dry thickness of between 2 μm and less than or equal to 50 μm, preferably between 5 μm and less than or equal to 30 μm, more preferably between 10 μm and less than or equal to 20 μm.

[0076] The orientation step b) may be carried out either simultaneously with the coating step a) or after the coating step a). The magnetic orientation of magnetic particles is known and described in the art. With reference to it, reference should be made to documents from the state of the art cited in the introductory part of this application.

[0077] The curing / drying step c) may be carried out by physical evaporation of volatiles, curing by UV radiation, crosslinking by oxidation, chemical crosslinking, curing by electron beam bombardment or any combination thereof. This step is also known in the art and need not be described in detail here.

[0078] The present invention further relates to a coating with a magnetically induced image on the substrate which comprises a magnetically oriented pigment (P) in a cured solid paint binder (I), characterized in that the ratio of the volume of the paint binder (V (I)) to the volume of the pigment (V (P)) is higher than 3.0, preferably higher than 4.0, most preferably higher than 5.0, and that the coating layer is thicker than d50 / 3, preferably thicker than d50 / 2, where d50 is the average diameter of the flakes of an optically variable magnetic pigment. The amount of magnetically oriented pigments in the cured solid lacquer (I) ranges between 1% and 40% by weight, preferably between 5% and 30% by weight, more preferably between 10-20% by weight relative to the wet coating.

[0079] The printing ink or coating composition of the present invention can be used to produce magnetically induced images. These magnetically developed images can be used as security elements on e.g. banknotes, credit cards, access cards, security signs, securities, legal or ID documents, means of transport tickets, lottery tickets, party tickets, tax bands, threads protective labels, foils, protective strips or products for

EP 2 024 451 B1 applications with security features. Thus, the present invention also relates to the use of the coating composition disclosed herein for the above uses and security documents containing the magnetically induced image obtained with the coating composition of the present invention.

[0080] Said security element may additionally comprise additional marking means, such as infrared markers, fluorescent markers, UV markers, phosphorescent markers, magnetic markers, forensic markers and a mixture thereof.

[0081] The invention may be implemented on any type of material in the form of a printable sheet or tape, in particular on materials used for the production of banknotes, credit cards, access cards, security signs, securities, legal documents or identity documents, funds tickets transport, lottery tickets, event tickets, tax bands, security threads, stickers, foil, securing straps or products for applications with securing. The printable sheet or tape material may further comprise a single layer as well as multiple layers.

[0082] The present invention will now be further described by reference to non-limiting examples and drawings. Unless otherwise indicated, all amounts are by weight.

Example 1 is not an example of the invention: Ink for gravure printing on paper cloths [0083] In this example, the recipe for gravure printing ink on paper cloths is given.

<td>Addition product of tung oil and phenolic resin modified with maleic acid in oil mineral with high boiling point (PKWF 28/31)</td><td> 35 %</td>

<td>Fatty alkyd resin</td><td> 7,50 %</td>

<td>Alkylphenol resin modified with crude tung oil in paint solvent 27/29</td><td> 16 %</td>

<td>Polyethylene wax</td><td> 3,30 %</td>

<td>Aerosil 200 (Degussa-Huels)</td><td> 2,00 %</td>

<td>Optically variable magnetic pigment (7 layers)</td><td> 30 %</td>

<td>Solvent for printing inks 27/29 (Shell Industrial Chemicals)</td><td> 6 %</td>

<td>Cobalt octoate (11% metal)</td><td> 0,10 %</td>

<td>Manganese octoate (10% metal)</td><td> 0,10 %</td>

Example 2: Screen printing ink, UV curing [0084] In this example, the screen formulation of the screen printing ink according to the present invention is given.

<td></td><td>Recipe basic</td><td>Example 2a</td><td>Example 2b</td><td>Example 2c</td>

<td>Lacquer binder (I) in total:</td><td> 70%</td><td> 80 %</td><td> 60 %</td><td> 30 %</td>

EP 2 024 451 B1

<td>Epoxidized acrylate oligomer</td><td> 40</td><td> 45,7</td><td> 34,2</td><td> 17,1</td>

<td>Trimethylolpropane triacrylate monomer</td><td> 10</td><td> 11,5</td><td> 8,6</td><td> 4,3</td>

<td colspan="5">(continued)</td>

<td></td><td>Recipe basic</td><td>Example 2a</td><td>Example 2b</td><td>Example 2c</td>

<td>Tripropylene glycol diacrylate monomer</td><td> 10</td><td> 11,5</td><td> 8,6</td><td> 4,3</td>

<td>Genorad 16 (Rahn)</td><td> 1</td><td> 1,1</td><td> 0,9</td><td> 0,4</td>

<td>Aerosil 200 (Degussa-Huels)</td><td> 1</td><td> 1,1</td><td> 0,9</td><td> 0,4</td>

<td>Irgacure 500 (CIBA)</td><td> 6</td><td> 6,8</td><td> 5,1</td><td> 2,6</td>

<td>Genocure EPD (Rahn)</td><td> 2</td><td> 2,3</td><td> 1,7</td><td> 0,9</td>

<td>Pigment (P) total:</td><td> 20%</td><td> 20 %</td><td> 20 %</td><td> 20 %</td>

<td>Optically variable magnetic pigment (7 layers)</td><td> 20</td><td> 20</td><td> 20</td><td> 20</td>

<td>Solvent (S) combined:</td><td> 10 %</td><td> 0 %</td><td> 20 %</td><td> 50 %</td>

<td>Dowanol PMA</td><td> 10</td><td> 0</td><td> 20</td><td> 50</td>

[0085] The recipe of example 2a was applied to security paper. The wet printing ink was magnetized before drying. The average dry film thickness was 19 μm. The solids content of the paint was 100%, the magnetic optically variable pigment (P) was 20% by weight, and the paint binder (I) was 80% by weight. The result is shown in Figs. 1a and 1b. In Fig. 1a, the resulting magnetic image is shown. In Fig. 1b is a cross section of a printing ink layer on a substrate from a scanning electron microscope (SEM). The average angle of inclination of the petals is defined as 24^about ± 12^about in relation to the level.

[0086] In Fig. 1b, "matrix" refers to a fixing material that is needed to make the cross-section. The effect resulting from magnetization is a clearly defined image.

[0087] The recipe of example 2b was applied to security paper. The wet printing ink was magnetized before drying. The average dry film thickness was 14 μm. The solids content of the paint was 80%, the magnetic optically variable pigment (P) was 20% by weight, the paint binder (I) was 60% by weight and the solvent (S) was 20% by weight . The result is shown in Figs. 2a and 2b. In Fig. 2a, the obtained magnetic image is shown. In Fig. 2b a cross-sectional view of a layer of ink on a substrate from a scanning electron microscope (SEM) is shown. The average angle of inclination of the petals is defined as 25^about ± 12^about in relation to the level. The effect created by magnetization is the image with reduced resolution, but still quite clearly visible.

[0088] The recipe of example 2c was applied to security paper. The wet printing ink was magnetized before drying. The average dry film thickness was 11 μm. The solids content of the paint was 50%, the magnetic optically variable pigment (P) was 20% by weight, the paint binder (I) was 30% by weight and the solvent (S) was 50% by weight . The result is shown in Figs. 3a and 3b. In Fig. 3a, the obtained magnetic image is shown. In Fig. 3b a cross section of the ink layer is shown

EP 2 024 451 B1 from a scanning electron microscope (SEM). The average angle of inclination of the petals is defined as 10^about ± 9^about in relation to the level. The effect resulting from magnetization is a picture with a rather poor resolution.

[0089] The recipe of example 2a showed excellent susceptibility to magnetic orientation; The formulation of example 2b showed some degradation compared to example 2a and the formulation of example 2c showed significant degradation compared to example 2a. It can be concluded that recipes with a V (I) / V (P) ratio less than 3.0 will not have any useful effect.

[0090] It can be seen from Figures 1 to 3 and Table 1 above that there is an excellent correlation between the magnetic pigment orientation sensitivity and the V (I) / V (P) volume ratio on the one hand, and between the pigment orientation susceptibility and the dry film thickness On the other hand.

Example 3: Screen printing ink, UV drying [0091] In this example, the formulation of the screen printing ink according to the present invention is given.

<td>Epoxidized acrylate oligomer</td><td> 40 %</td><td rowspan="7">AND</td>

<td>Trimethylolpropane triacrylate monomer</td><td> 10 %</td>

<td>Tripropylene glycol diacrylate monomer</td><td> 10 %</td>

<td>Genorad 16 (Rahn)</td><td> 1 %</td>

<td>Aerosil 200 (Degussa-Huels)</td><td> 1 %</td>

<td>Irgacure 500 (CIBA)</td><td> 6 %</td>

<td>Genocure EPD (Rahn)</td><td> 2 %</td>

<td colspan="3"></td>

<td>Optically variable magnetic pigment (5 layers)</td><td> 20</td><td></td>

<td colspan="3"></td>

<td>Dowanol PMA</td><td> 10 %</td><td>S</td>

Example 3: Flexographic printing ink, UV curing [0092] In this example, the formulation of the flexographic printing ink according to the present invention is given.

<td>Urethane acrylic oligomer</td><td> 40 %</td>

<td>Propoxylated glycerol triacrylate monomer</td><td> 10 %</td>

<td>Tripropylene glycol diacrylate monomer</td><td> 15 %</td>

<td>Florstab UV-1 (Kromachem)</td><td> 1 %</td>

<td>Optically variable magnetic pigment (7 layers)</td><td> 25 %</td>

<td>Aerosil 200 (Degussa-Huels)</td><td> 1 %</td>

<td>Irgacure 500 (CIBA)</td><td> 6 %</td>

<td>Genocure EPD (Rahn)</td><td> 2 %</td>

EP 2 024 451 B1