MANUFACTURING A PIGMENT

Description

[0001] The invention relates to the production of a pigment, in particular a platelet-shaped effect pigment for use in a printing ink.

[0002] Data storage media, such as valuables or identification documents, or other valuable items, such as branded goods, are often provided with security elements for security purposes. These allow the authenticity of the data storage media to be verified and also serve as protection against unauthorized reproduction. Security elements with viewing-angle-dependent effects play a special role in authenticity assurance, as these cannot be reproduced even with the most modern copying machines. These security elements are equipped with optically variable elements that convey a different image impression to the viewer at different viewing angles, for example, displaying a different color or brightness impression and / or a different graphic motif depending on the viewing angle.

[0003] Thin-film systems that use interference to create a viewing-angle-dependent color impression for the observer are known in the art. This optical effect can serve as an optically variable security element. A large-area thin-film system can be comminuted using various techniques. The size of the resulting flakes or platelets can be down to a few micrometers laterally, but the size is usually in a range of 2 µm to 100 µm. The vertical structure of a platelet is determined by the requirements of the interference layers and is generally as thin as possible, e.g., in a range of 200 nm to 800 nm. Such platelets are used, for example, in optically variable color (so-called OVI ®< color), which serves to create a security element.

[0004] Another known technique is the application of thin-film systems that create a color impression to a ferromagnetic material. This gives the pigment platelets a magnetic moment. Magnetically orientable effect pigments are available commercially, for example, under the trade name OVMI®< from SICPA (the abbreviation OVMI stands for "optically variable magnetic ink"). The pigments typically have a platelet-shaped structure and are in the form of a layered composite, often comprising two layers of optical effect coatings and an embedded magnetic layer. With regard to optical effect coatings, metallic-reflective coatings as well as color-shifting coating systems, e.g., with an absorber / dielectric / reflector structure, are possible. The embedded magnetic layer is generally not visible but is necessary for aligning the pigments.

[0005] WO 2019 / 057322 A1 describes a method for producing pigment fragments, comprising creating a first layer on a substrate, structuring the first layer, and detaching the pigment fragments from the substrate. The first layer is a crack-forming layer, so it is structured by the formation of cracks. Before the pigment fragments are detached, a pigment layer is applied to the first layer structured by cracks.

[0006] EP 2 062 947 A1 describes opaque flakes, such as pigments or light flakes used in paints, with a selected shape and / or other features to impart a covert security feature to an object. During the production of the flakes, a sheet with embossed frames is provided, on which embossed symbols or characters are located.

[0007] WO 2007 / 105001 A2 describes a method for producing particles with controlled dimensions, comprising the steps of: (i) providing a laminar substrate having a patterned surface comprising a microrelief repeating pattern comprising one or more discrete cells, each cell consisting of a bottom portion and walls; (ii) depositing organic or inorganic material on the patterned surface and in the cells to provide a thickness (T) of deposited material; (iii) detaching the deposited organic or inorganic material from the surface of the substrate; and (iv) collecting the particles formed from the organic or inorganic material;and a composition obtainable from the process comprising a plurality of particles, wherein the number (n) of particles in the composition is at least 10, wherein the particles (P) are platelets having a planar geometry which is circular or consists of a number (x) of planar (y)-sided polygons, where x is between 1 and 20 and y is at least 3;

[0008] Fundamentally, there is a need to produce well-defined pigments, i.e., pigments of a defined shape and size, in a simple manner with as few process steps as possible. If possible, the use of an additional release layer should be avoided.

[0009] The present invention is therefore based on the object of providing a process for producing well-defined pigments, i.e., pigments of a defined shape and size, which comprises only a small number of process steps. A further object of the invention is to provide pigments obtainable by the process, in particular platelet-shaped effect pigments for use in a printing ink.

[0010] This problem is solved on the basis of the combination of features defined in the independent claim.

[0011] Further developments of the invention are the subject of the dependent claims. Summary of the invention

[0012] Process for producing pigments of defined shape and defined size according to claim 1. Detailed description of the preferred embodiments

[0013] The present invention is based on the idea of ​​providing a process for producing pigments of defined shape and size, for which the use of an additional release layer is not necessary. The pigments to be produced can optionally have a surface structure. The invention is based on the use of an embossing lacquer that cures using UV radiation and is also removable with an aqueous solution and therefore serves as a release layer. To carry out the process according to the invention, a carrier substrate, e.g. a polyethylene terephthalate (PET) substrate, is expediently used. The carrier substrate is optionally subjected to a printing pretreatment such as a corona treatment. The carrier substrate is then coated with an embossing lacquer that cures using UV radiation.Defined structures that determine the size and shape of the pigments to be produced can be embossed into the UV embossing varnish. In addition to the embossed predetermined breaking points that define the pigment size and shape, additional fine structures that determine the surface structure of the pigments to be produced can optionally be embossed. These fine structures can change the physical, particularly optical, properties of the pigments or serve as forensic features. The fine structures can be, in particular, nanostructures, hologram structures such as hologram gratings, or microstructures such as a micromirror relief. Typically, the height of these surface structures is smaller than that of the embossed sol-fracture structures. The fine structures embossed into the embossing varnish are transferred to the pigments to be produced.The forensic feature is, in particular, a forensic feature that is not visible to the naked eye but is only visible to the observer under a microscope, and is preferably in the form of characters, a pattern or a code.

[0014] After the embossing step of the embossing lacquer, or simultaneously with the embossing step, the embossing lacquer is UV-cured. The embossing lacquer layer obtained after UV curing is characterized by its ability to be removed with an aqueous solution and, in particular, by its ability to be water-soluble or at least water-swellable. Regarding the water removability of the embossing lacquer layer, it is advantageous if the embossing lacquer layer is not fully cured. The degree of curing of the embossing lacquer layer can be influenced during the embossing process using the following parameters: Type of UV lamp, e.g., conventional, particularly iron-doped, medium-pressure mercury lamp or UV-LED lamp; quantity of a suitable photoinitiator, whereby the quantity may be wavelength-dependent; temperature input or heat exposure during the embossing process; possible addition of chain transfer agents, e.g., thiols, see, for example, document EP 3 230 795 B1.

[0015] Suitable raw materials for the UV-curing embossing lacquer are prepolymers and / or reactive diluents such as polyethylene glycol diacrylate, ethoxylated trimethylolpropane triacrylate, acryloylmorpholine (see, for example, document EP 3 230 795 B1), and other acrylate-based (co)polymers. The raw materials can, in particular, be water-soluble. A photoinitiator, e.g., 2-hydroxy-2-methyl-1-phenylpropanone, bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, ethyl(2,4,6-trimethylbenzoyl)phenylphosphinate, or mixtures of two or more of the aforementioned substances, is advantageously added to the raw materials, with additives optionally being added. The embossing lacquer layer cured by UV radiation is then metallized.

[0016] The metallization can, in particular, be a reflective metallic layer, preferably an Al layer or a Ag layer. Furthermore, the metallization can be a color-shifting thin-film system, in particular a thin-film system with an absorber / dielectric / reflector structure, which can optionally contain a magnetic layer for spatially aligning the effect pigment using an external magnetic field. A preferred thin-film system comprises the layer sequence absorber / dielectric / reflector / dielectric / absorber. Another preferred thin-film system comprises the layer sequence absorber / dielectric / reflector / magnetic layer / reflector / dielectric / absorber.

[0017] The elements Cr, Al, and Ti are particularly suitable as absorbers, with a layer thickness of less than 20 nm being preferred. SiO 2 , ZnS, and MgF 2 are particularly suitable as dielectrics, with a layer thickness of less than 600 nm and greater than 50 nm being preferred. Al is particularly suitable as reflectors, with a layer thickness of more than 15 nm being preferred. FeSi, Fe, or Ni, for example, are suitable as magnetic layers, with a layer thickness of less than 1 µm and greater than 50 nm being preferred.

[0018] To provide a platelet-shaped effect pigment that exhibits the same optically variable color-shift effect on both sides, it is preferred to use a color-shifting thin-film structure based on a symmetrical interference layer structure, which has the following layer sequence: absorbing layer – dielectric layer – reflective layer – dielectric layer – absorbing layer. A Cr layer, for example, is suitable as the absorbing layer. For the dielectric layer, an SiO 2 layer or a ZnS layer, for example, can be selected. For the reflective layer, an Al layer is expediently selected.

[0019] Advantageously, the coated or metallized substrate, which is particularly in the form of a film, is passed through a (possibly hot) water bath in a roll-to-roll process. In this way, the UV embossing lacquer layer, which can be removed with an aqueous solution, is dissolved or swollen, if necessary with the help of additional mechanical measures such as brushing, so that the vapor-deposited metallization is, so to speak, blown off and the well-formed pigments can be obtained. If the UV embossing lacquer is water-soluble, the pigments are separated and washed, for example by decanting. If the UV embossing lacquer is water-swellable, the pigments are expediently separated from any pieces of embossing lacquer that may be floating in the water and have detached from the carrier substrate, for example by decanting or possibly by separation using any magnetic properties of the pigments.

[0020] Advantageous formulations for the production of the UV embossing varnish are based on a mixture containing 95 wt% photomer and 5 wt% photoinitiator.

[0021] Furthermore, it is possible to use the recipes mentioned in EP 3 230 795 B1 for the production of a water-soluble UV embossing lacquer. The acrylate monomer mentioned in the document, namely acryloylmorpholine (ACMO), which can in principle be used as a lacquer raw material, tends to adhere to the embossing tool to a certain extent, possibly resulting in non-removability from the embossing tool after the embossing process. This problem can be remedied by expediently adding further additives to the UV embossing lacquer to be produced, which reduce adhesion to the embossing tool. Suitable additives include (particularly non-crosslinking) alkylphosphone derivatives, phosphoric acid derivatives, or alkoxysilanes. As a result of the reduced adhesion to the metal, not only can the release from the embossing tool be improved, but also the detachment of the metallization vapor-deposited in the next step from the embossing lacquer.The removal of the produced pigments is thus improved.

[0022] An optional, additional measure for the removal of well-defined pigments, which may optionally be provided with a surface structure, would be the use of a water-soluble primer, which is applied by printing technology beneath the UV embossing lacquer layer. Using an aqueous solution, the primer layer can be removed, causing the embossing lacquer layer to float away and disintegrate into the individual pigments. However, this measure requires an additional release layer and is therefore less preferred.

[0023] The use of a water-removable, possibly water-soluble, optionally UV-curable dual-cure thermoplastic formulation can be based, for example, on the following mixtures: 95 wt% Laromer 8983 + 5 wt% Irgacure 819 DW; or 75 wt% Laromer 8983 + 20 wt% Laromer 9005 + 5 wt% Irgacure 819 DW.

[0024] Curing or UV exposure is preferably carried out to the extent necessary for film formation and crosslinking, for favorable release and subsequent metallization, as well as for temperature resistance. Embossing into the thermoplastic can be performed prior to the UV curing step, if necessary.

[0025] Typically, film formation and the relative water resistance of an aqueous system occur after physical drying by expelling amines. Water-affine carbanions with ammonium ion as the counterion become carboxylic acid, possibly undergoing esterification. Removal is conveniently carried out with water, preferably using a basic aqueous solution, resulting in saponification and regeneration of the carbanions. Basicity can be conveniently achieved by adding NaOH, ammonia, and / or amines.

[0026] If necessary, the use of buffer solutions is possible to prevent oxidation of the metallization. (Mono- / diester) phosphoric acid buffers or alkylphosphonic acid buffers are preferred because these buffers can passivate the metal surface.

[0027] The advantage achieved by the manufacturing process according to the invention is based in particular on the fact that no additional release layer needs to be provided, thus significantly lowering costs, reducing material consumption, and requiring fewer processing steps. The UV embossing varnish used in the present invention also serves as a release layer. Furthermore, no release-enhancing additive is necessary.

[0028] The UV embossing lacquer used in the present invention is in particular designed in such a way that no consideration needs to be given to any possible susceptibility to heat, ie a specific softening temperature of the carrier substrate and embossing lacquer, because the embossing lacquer is UV-curable.

[0029] The manufacturing process according to the invention is furthermore associated with the advantage that an additional grinding process after the removal of the pigment layer is not necessary to obtain the pigments.

[0030] Further advantages of the invention are explained below using exemplary embodiments in conjunction with schematically highly simplified figures, in which a true-to-scale and true-to-proportion reproduction has been omitted in order to increase clarity.

[0031] They show: Figures 1 to 5 show a manufacturing method according to the invention according to a first embodiment; Figures 6 to 8 show a manufacturing method according to the invention according to a second embodiment. Figures 9 to 11 serve only as general illustrations and do not depict any objects according to the invention.

[0032] The Figures 1 to 5 illustrate a manufacturing method according to the invention according to a first embodiment. First, according to the Figure 1Providing a polyethylene terephthalate (PET) substrate 1. To achieve improved adhesion of the embossing varnish 2 to be produced, the carrier substrate 1 is subjected to a printing pretreatment, in this example a corona treatment. The carrier substrate 1 is then coated with an embossing varnish 2 that cures using UV radiation. The embossing varnish 2 is based on a mixture containing 84% by weight of acryloylmorpholine (ACMO), 10% by weight of 2-ethylhexyl thioglycolate, 5% by weight of 2-hydroxy-2-methyl-1-phenylpropan-1-one as a photoinitiator, and 1% polysiloxane surfactant. An additive that reduces the adhesion of the embossing varnish 2 to the embossing tool is also added to the mixture. The additive is advantageously selected from an alkylphosphone derivative, a phosphoric acid derivative, or an alkoxysilane; however, a mixture of two or more of the aforementioned substances may also be used. The embossing lacquer 2 is applied by gravure printing.

[0033] According to the Figure 2 Defined structures 3 are then embossed into the embossing lacquer 2, for example using a nickel embossing tool, wherein the embossed structures 3 are decisive for the size and shape of the pigments to be produced. According to a preferred variant, the embossing depth of the embossed structure 3 corresponds at most to the layer thickness of the pigment to be produced. Alternatively, according to a further preferred variant, the layer thickness of the pigment to be produced, i.e. the pigment material layer, is somewhat greater than the embossing depth of the embossed structure 3, wherein the resulting pigment material layer must be broken down subsequently to form the individual pigments. After the step of embossing the embossing lacquer 2, UV polymerization takes place, for which a Hg medium-pressure vapor lamp is used.

[0034] Additional fine structures can optionally be embossed into the UV embossing lacquer 2. These fine structures are crucial for the surface structure of the pigments to be produced. These fine structures can alter the physical, particularly optical, properties of the pigments or serve as forensic features. These fine structures can be, in particular, nanostructures, hologram structures such as hologram gratings, or microstructures. Typically, the height of these surface structures is smaller than that of the embossed solfraction structures.

[0035] The Figure 3 shows the embossed lacquer 2 with embossed structures in a top view. Figure 2 shows a cross-sectional view along the Figure 3 shown dashed line A-A'.

[0036] After the UV curing step of the embossing lacquer 2, the metallization step takes place, in which the embossing lacquer 2 is provided with a color-shifting thin-layer structure 4 (see Figure 4 ). The color-shifting thin-film structure 4 in the present embodiment is a symmetrical interference layer structure, which has the layer sequence absorbing layer - dielectric layer - reflective layer - dielectric layer - absorbing layer, in the example the structure Cr / SiO 2 / Al / SiO 2 / Cr. The metallized film is moved through a hot water bath in the course of a roll-to-roll process and in this way, if necessary with the help of additional mechanical measures such as brushing, the UV embossing lacquer layer 2, which can be removed with an aqueous solution, is dissolved, so that the vapor-deposited metallization 4 is blasted off and the well-formed, platelet-shaped effect pigments can be obtained (see Figure 5 ). The pigments are separated and washed by decanting.

[0037] Based on the Figures 6 to 8A manufacturing method according to the invention is described below according to a second embodiment, which is based on the first embodiment described above. According to the Figure 6A carrier substrate 1 is coated with the UV-curing embossing lacquer 2 described in the first exemplary embodiment. Defined structures 5 are then embossed into the embossing lacquer 2, for example using a nickel embossing tool. The embossed structures 5 are decisive for the size and shape of the pigments to be produced. In addition, additional fine structures are embossed into the embossing lacquer 2, which are decisive for the surface structure of the pigments to be produced. These fine structures change the optical properties of the pigments or serve as forensic features. The fine structures can be nanostructures, hologram structures such as hologram gratings, or microstructures. The height of these surface structures is smaller than that of the embossed solfraction structures. After the embossing step of the embossing lacquer 2, UV polymerization takes place, for which a medium-pressure mercury vapor lamp is used.After the step of UV curing of the embossing lacquer 2, the step of metallization takes place, in which the embossing lacquer 2 is provided with the color-shifting thin-film structure 6 described in the first embodiment (see . Figure 7 ). The metallized foil is moved through a hot water bath in a roll-to-roll process and, if necessary with the help of additional mechanical measures such as brushing, the UV embossing lacquer layer 2, which can be removed with an aqueous solution, is dissolved in this way, so that the vapor-deposited metallization 6 is blown off and the well-formed, platelet-shaped effect pigments can be obtained (see Figure 8 ). The pigments are separated and washed by decanting.

Claims

1. Method for producing pigments of defined shape and defined size, comprising a) providing a carrier substrate (1); b) coating the carrier substrate (1) with an embossing varnish (2) which cures by means of UV radiation and which is detachable with an aqueous solution after UV curing; c) introducing defined structures into the embossing varnish (2) by means of embossing, which structures are a determining factor for the defined shape and the defined size of the pigments to be produced; d) UV curing of the embossing varnish (2); e) coating the embossing varnish (2) with a pigment material layer (4, 6, 7); f) treating the obtained layer construction with an aqueous solution in order in this way to detach the embossing varnish (2) and to release the pigment material layer (4, 6, 7) suitable for providing individual pigments; and g) extracting the pigments obtainable from the pigment material layer (4, 6, 7), wherein optionally a mechanical stress is exerted on the pigment material layer (4, 6, 7); wherein introducing defined structures into the embossing varnish (2) by means of embossing in step c), which structures are a determining factor for the defined shape and the defined size of the pigments to be produced, is carried out such that the defined structures are suitable for producing a pigment material layer (4, 6) arranged at at least two different height levels and having thinned regions, wherein the thinned regions serve as predetermined breaking points.

2. Method according to Claim 1, wherein in step c) introducing a fine structure as an optically effective surface structure into the embossing varnish (2) by means of embossing is additionally carried out, wherein the fine structure is preferably selected from a nanostructure, a hologram structure such as a hologram grating or a microstructure such as a micromirror relief.

3. Method according to Claim 1 or 2, wherein in step c) introducing a fine structure into the embossing varnish (2) by means of embossing is additionally carried out, wherein the fine structure is a forensic feature that is not discernible to the naked eye but rather is only discernible to the observer under the microscope, wherein the forensic feature is preferably in the form of characters, in the form of a pattern or in the form of a coding.

4. Method according to any of Claims 1 to 3, wherein the embossing varnish (2) which cures by means of UV radiation and which is detachable with an aqueous solution after UV curing in step b) is based on a watersoluble raw material mixture, comprising at least one photoinitiator and prepolymers and / or reactive diluents, which are preferably selected from the group consisting of polyethylene glycol diacrylate, ethoxylated trimethylolpropane triacrylate, acryloylmorpholine and acrylate-based (co)polymers.

5. Method according to any of Claims 1 to 4, wherein the pigment material layer (4, 6, 7) in step e) is an optically effective functional layer and is preferably a reflective metallic layer such as an Al layer or an Ag layer or a colour-shifting thin-film layer system with an absorber / dielectric / reflector construction, wherein optionally an additional magnetic layer for the spatial alignment of the effect pigment by means of an external magnetic field can be included.

6. Method according to Claim 5, wherein the optically effective functional layer is a colour-shifting thin-film layer system with the layer sequence: absorbing layer - dielectric layer - reflective layer - dielectric layer - absorbing layer.

7. Method according to any of Claims 1 to 6, wherein the pigments are platelet-shaped effect pigments.