PROCESS FOR PRODUCING MECHANICALLY ROBUST COLORANT-BASED COATINGS

Description

[0001] The present invention relates to a process for producing a coating comprising at least one colorant that accumulates at interfaces.

[0002] In the printing industry, various processes are known for applying visually appealing, metallic-lustrous coatings to printed products. One common approach, for example, is the application of glossy foils using foil transfer printing. Another well-known option is the direct printing of pigment-based inks or varnishes onto the substrate, with the printed substrates subsequently cured in the printing press. One well-known printing process for this purpose is offset printing.

[0003] Printing inks and printing varnishes containing effect pigments are known in the state of the art. A basic distinction is made between printing inks and printing varnishes with floating pigments, or leafing pigments, and non-floating pigments, or non-leafing pigments. In general, printing inks and printing varnishes with leafing pigments are considered to be the class with the most visually appealing metallic effect. Silver-colored printing varnishes containing aluminum-based leafing pigments are popular. Such leafing pigments are described, for example, in the German industrial standard DIN 55923. In contrast, printing inks and printing varnishes based on non-leafing pigments are also common, but these generally exhibit a less visually appealing metallic effect and often appear matte and gray.

[0004] Although printing inks and varnishes based on leafing pigments exhibit a pronounced metallic effect, they have one important disadvantage. They are problematic in terms of their mechanical robustness, particularly with regard to scratch sensitivity and abrasion resistance. Because the leafing pigments lie close to the surface, such coatings are sensitive and usually require protection from mechanical abrasion by one or more overlying clear coats. However, applying such additional protective layers is complex and uneconomical. In contrast, coatings based on printing inks and varnishes with non-leafing pigments are generally less sensitive because the pigments are essentially evenly distributed. In these cases, additional protective layers are generally not required.

[0005] There is a need for efficient processes that can produce mechanically robust coatings with leafing pigments. In particular, the printing industry needs to be able to efficiently produce such mechanically robust coatings in printing presses.

[0006] EP 3 165 578 A1 describes a metallic offset printing ink with a mirror finish and a printing process. An overprinted layer is intended to provide abrasion resistance for the aluminum pigments.

[0007] WO 2008 / 052645 A2 describes a finishing process for a printed product and a coating unit. In this process, a liquid printing medium is applied to a carrier in a printing machine, dried there, and transferred to a substrate.

[0008] US 2018 / 0147871 A1 describes a printing process in which platelet-shaped pigments are transferred to a printing substrate to produce a monolayer and are provided with a protective layer.

[0009] The object of the present invention is to provide a method for efficiently producing mechanically robust coatings with leafing pigments. The method should be particularly suitable for use in printing machines for producing coated printed products.

[0010] Surprisingly, it has now been discovered that mechanically robust coatings with leafing pigments can be produced relatively easily by first applying a curable liquid medium containing leafing pigments to a carrier, drying it there, and then bonding it as a film to the substrate to be coated, with the side where the leafing pigments have accumulated. This procedure keeps the sensitive leafing pigments away from the outer surface of the coated substrate and protects them from the overlying cured layer.

[0011] The process according to the invention offers several advantages. It is no longer necessary to apply one or more protective layers. It is no longer necessary to ensure that the pigment-containing layer and the protective layer are compatible with each other. The process can be carried out continuously in printing presses and is compatible with common printing inks, varnishes, and substrates.

[0012] Accordingly, the present invention relates to a process for producing a coating comprising at least one interfacially enriching colorant, comprising the following steps: i) providing a curable liquid medium comprising at least one interfacially enriched colorant, ii) applying the curable liquid medium to a carrier, wherein the interfacially enriched colorant accumulates at the interface between the curable liquid medium and the ambient atmosphere, iii) at least partially curing the curable liquid medium on the carrier and iv) transferring the at least partially cured curable liquid medium from the carrier to a substrate such that the side on which the interfacially enriched colorant has accumulated on the carrier lies on the substrate.

[0013] In a preferred embodiment, the at least one interfacially enriching colorant accumulates in the curable liquid medium due to its lower density compared to the curable liquid medium and / or its surface properties at the interface between the curable liquid medium and the ambient atmosphere. The ambient atmosphere is understood to mean the surrounding air atmosphere, for example, in a printing shop.

[0014] In a particularly preferred embodiment, the colorant that accumulates at interfaces is surface-active due to its surface properties. The surface properties of the colorant that accumulates at interfaces can also be modified by functionalization, for example, by coating it with surface-active compounds such as stearic acid. Different surface energies play a particularly important role in the accumulation of the colorant at interfaces due to its surface properties.

[0015] In a further preferred embodiment, the at least one colorant accumulating at interfaces is at least one mechanically sensitive pigment. This includes, in particular, pigments that can be damaged or destroyed by a relatively low external force. An example of such a relatively low external force is the human hand touching a cardboard packaging provided with a coating according to the invention, for example, a cosmetic product. In contrast, purely thermally sensitive pigments play a rather minor role.

[0016] In a further preferred embodiment, the at least one colorant that accumulates at interfaces is at least one effect pigment. The at least one colorant that accumulates at interfaces is particularly preferably at least one inorganic pigment, most preferably at least one inorganic metallic effect pigment.

[0017] In a further preferred embodiment, the at least one colorant that accumulates at interfaces is at least one leafing pigment.

[0018] In a further preferred embodiment, the curable liquid medium is substantially transparent. As a result, the product obtained in the process according to the invention retains the visually appealing effect of the colorant used even through the comparatively thick layer of the cured, curable liquid medium.

[0019] In a further preferred embodiment, the process according to the invention is carried out using a printing press, in particular in at least one printing unit of a printing press, at least one coating unit of a printing press, or in combinations thereof. In this embodiment, the carrier is preferably a cylinder of the printing press or a plate in the printing press, particularly preferably a cylinder of the printing press. In this embodiment, a UV-curable printing ink or a UV-curable printing varnish is preferably applied to the cylinder of the printing press, which is mentioned as preferred, using a chambered doctor blade with an anilox roller or a roller unit. The UV-curable printing ink or the UV-curable printing varnish can also be applied by means of one or more nozzles by spraying it onto the cylinder of the printing press.The UV-curable printing ink or UV-curable printing varnish is then cured on the preferred cylinder of the printing press by irradiation with UV radiation, so that an at least sufficiently cured, transferable film is obtained. This at least partially cured film is then transferred in the printing nip of the printing unit or coating unit of the printing press from the cylinder used as a carrier to the substrate, preferably paper, cardboard, or plastic film. To improve the adhesion of the at least partially cured liquid curable medium, which is transferred to the substrate as a film, it is possible to apply an auxiliary adhesive layer to the substrate or to the cured liquid curable medium before transferring the film, for example by spraying it using one or more nozzles or by contacting it with another printing press cylinder to transfer the adhesive from an adhesive reservoir.

[0020] The process according to the invention is preferably carried out using a printing press, whereby, in principle, all known printing processes can be used. Preferred printing processes are web-fed printing processes, digital printing processes, pad printing, indirect gravure printing, indirect screen printing, indirect electrophotography with liquid or dry toner, indirect spray coating processes, and combinations thereof.

[0021] In the preferred embodiment, in which the process is carried out using a printing press, the curable liquid medium can be applied to an imaged or non-imaged substrate. Accordingly, the substrate can also be a spot varnish plate or an imaged printing plate.

[0022] In the preferred embodiment in which the process is carried out using a printing machine, a water-based UV varnish can also be used as the curable liquid medium, which is at least partially cured in step iii) by the action of heat and which is finally cured by means of UV radiation after transfer to the substrate.

[0023] It is also possible to successively produce several layers of different cured, curable liquid media with different interfacial colorants in the printing press. The printing press is preferably equipped with several printing units arranged one behind the other, with the method according to the invention being carried out in each of the printing units, preferably with a different interfacial colorant. In further printing units, as is known in principle in the prior art, additional printing inks or printing varnishes that do not contain interfacial colorants can also be printed.

[0024] In a particularly preferred embodiment, the invention relates to a method for producing a coating with a printing machine, wherein the coating comprises at least one leafing pigment, comprising the following steps: i) Providing a curable printing ink or a curable printing varnish comprising at least one leafing pigment, ii) Applying the curable printing ink or the curable printing varnish to a cylinder or a plate of a printing press, wherein the leafing pigment accumulates at the interface between the curable printing ink or the curable printing varnish and the ambient atmosphere, iii) At least partially curing the curable printing ink or the curable printing varnish on the cylinder or the plate of the printing press and iv) Transferring the at least partially cured curable printing ink or the curable printing varnish from the cylinder or the plate of the printing press to a printing substrate, such that the side on which the leafing pigment has accumulated on the cylinder or the plate of the printing press lies on the printing substrate.

[0025] In this particularly preferred embodiment, paper, cardboard, and plastic film are preferred as printing substrates. The at least partial curing in step iii) is preferably carried out by means of electron beam radiation or UV radiation, particularly preferably by means of UV radiation. It can also only be initial curing (pinning) by means of UV radiation, with the printed product then being finally cured at a later time, before leaving the printing press, by irradiation with a higher dose of UV radiation. In this particularly preferred embodiment, the cylinder of the printing press or the plate of the printing press preferably has a release coating, for example a cylinder cover made of an elastomer, with the aid of which the transfer of the at least partially cured curable printing ink or curable printing varnish from the cylinder or plate to the printing substrate is facilitated.

[0026] The present invention is also suitable for the use of the process according to the invention for producing abrasion-resistant coatings comprising pigments. In a preferred embodiment, these are mechanically sensitive pigments, preferably mechanically sensitive pigments selected from the aforementioned effect pigments and their preferred forms. In a preferred embodiment, the use relates to the use in the form of a printing process in a printing press, in particular in an offset printing press.

[0027] The present invention is also suitable for producing a printed product obtained by the process according to the invention. These are preferably printed products obtained by printing on all common substrates that can be processed in printing machines, particularly preferably a printed product obtained by printing on paper, cardboard, or plastic film. Despite the use of leafing pigments, such printed products exhibit a visually appealing appearance while simultaneously being highly abrasion-resistant.

[0028] Figure 1shows a schematic of a prior art printed product with non-leafing pigments. A curable liquid medium, for example a varnish, containing non-leafing pigments (3) was printed onto a substrate (1), for example cardboard, and dried, so that the curable liquid medium hardened and a ready-to-use printed product was created in which the non-leafing pigments (3) were distributed throughout the hardened, curable liquid medium (2). Such printed products usually have no problems with regard to mechanical robustness; however, when metallic effect pigments are used as non-leafing pigments, the optical metallic effect is often not particularly pronounced.

[0029] Figure 2shows a schematic of another prior art printed product with leafing pigments. A curable liquid medium, for example a varnish, with leafing pigments (4) was printed onto a substrate (1), for example cardboard, and dried so that the curable liquid medium hardened. The leafing pigments (4) had previously accumulated on the outer surface of the curable liquid medium. A transparent protective layer (5), for example a clear varnish, was applied to the hardened curable liquid medium (2) with the leafing pigments (4) and dried. Such printed products usually have a particularly pronounced optical metallic effect when metal effect pigments are used as leafing pigments, but are comparatively complex to produce.

[0030] Figure 3shows a schematic representation of the method according to the invention. In step S1, a carrier is provided, for example a printing press cylinder. In step S2, the carrier (6) is coated with an uncured, curable liquid medium (7) containing leafing pigments (4). This step can be carried out, for example, by contacting the carrier, in particular a printing press cylinder, with another printing press cylinder, in particular a metering roller such as an anilox roller. In step S3, the leafing pigments float in the uncured liquid medium (7) and accumulate at the interface between the uncured, curable liquid medium (7) and the ambient atmosphere. In step S4, the uncured liquid medium (7) is cured, for example by irradiation with UV radiation, so that a cured, curable liquid medium (2) is obtained on the carrier (6).In step S5, the cured, curable liquid medium (2) is brought into contact with a substrate (1), for example cardboard or film, such that the side on which the leafing pigment (4) has accumulated on the carrier (6) lies on the substrate (1). Step S6 shows the unit consisting of carrier (6) with cured, curable liquid medium (2) and substrate (1). In step S7, the carrier (6) is removed, leaving the unit consisting of cured, curable liquid medium (2) and substrate (1). This is shown in step S8. The sensitive leafing pigments (4) are protected and away from the outer surface of the coated product.

[0031] Figure 4shows schematically and in an even more simplified form the process according to the invention. First, a cured coating of a curable liquid medium (2) containing leafing pigments (4) is produced on a carrier (6). The leafing pigments have naturally accumulated at the interface between the liquid medium and the ambient atmosphere and have been fixed in this position by the curing. The carrier (6) coated in this way is brought into contact with a substrate (1) so that the surface on which the leafing pigments (4) have accumulated lies as close as possible to the substrate (1). The carrier (6) is then removed. The result is a coated product (8) in which the sensitive leafing pigments (4) are located protected inside, away from the outer surface of the coated product (8). List of reference symbols

[0032] 1Substrate 2Cured curable liquid medium 3Non-leafing pigment 4Leafing pigment 5Transparent protective layer 6Carrier 7Uncured curable liquid medium 8Coated product 9Interfacially accumulating colorant

Claims

1. Method of producing a coating provided with at least one colorant which accumulates at interfaces (9), comprising the following steps: i) Provision of a curable liquid medium (7) which comprises at least one colorant which accumulates at interfaces (9), ii) Application of the curable liquid medium (7) to a carrier (6), whereby the colorant which accumulates at interfaces (9) accumulates at the interface between the curable liquid medium (7) and the ambient atmosphere, iii) At least partial curing of the curable liquid medium (7) on the carrier (6) and iv) Transfer of the at least partially cured curable liquid medium (2) from the carrier (6) to a substrate (1), so that the side on which the colorant which accumulates at interfaces (9) has accumulated on the carrier (6) rests on the substrate (1).

2. Method according to claim 1, wherein the at least one colorant which accumulates at interfaces (9) accumulates in the curable liquid medium (7) by virtue of its lower density compared to the curable liquid medium (7) and / or its surface properties at the interface between the curable liquid medium (7) and the ambient atmosphere.

3. Method according to one of the previous claims, wherein the at least one colorant which accumulates at interfaces (9) is at least one effect pigment.

4. Method according to one of the previous claims, wherein the at least one colorant which accumulates at interfaces (9) is at least one inorganic metal effect pigment.

5. Method according to one of the previous claims, wherein the at least one colorant which accumulates at interfaces (9) is at least one leafing pigment (4).

6. Method according to one of the previous claims, wherein the curable liquid medium (7) is substantially transparent.

7. Method according to one of the previous claims, wherein the process is carried out with a printing press.

8. Method according to claim 7, wherein the carrier (6) is a cylinder of the printing press.

9. Method according to one of the previous claims, wherein the curable liquid medium (7) comprising at least one colorant which accumulates at interfaces (9) is a printing ink or a printing varnish.

10. Method according to one of the previous claims, wherein the substrate (1) is selected from paper, cardboard and plastic sheet.

11. Method according to one of the previous claims, comprising the following steps: i) Provision of a curable printing ink or a curable printing varnish comprising at least one leafing pigment (4), ii) Application of the curable printing ink or the curable printing varnish to a cylinder or a plate of a printing press, whereby the leafing pigment (4) accumulates at the interface between the curable printing ink or the curable printing varnish and the ambient atmosphere, iii) At least partial curing of the curable printing ink or the curable printing varnish on the cylinder or the plate of the printing press and iv) Transfer of the at least partially cured curable printing ink or curable printing varnish from the cylinder or the plate of the printing press to a printing material, so that the side on which the leafing pigment (4) has accumulated on the cylinder or the plate of the printing press rests on the printing material.