Abstract
A method of producing a printed product, wherein a formulation for production of a first layer as sealing layer is applied to the substrate and the formulation has at least one monomer, oligomer or prepolymer having at least one crosslinkable functional group. Subsequently, the first layer is cured and a second layer is applied at least to regions of the first layer, wherein the second layer has a coherent surface in the printed regions. Further, a printed product having a substrate, a first layer and a second layer, wherein the first layer and the second layer comprise an organic crosslinked lacquer and have a coherent surface, and wherein the first layer is transparent and has a layer thickness in the range from 1 to 10 μm. The second layer has been applied at least to regions of the first layer, such that the first layer is disposed between the substrate and the second layer.
Claims
1. A method of producing a printed product comprising the steps of: a) providing a print substrate, wherein the print substrate is a paper or paperboard, b) applying a formulation for production of a first layer as sealing layer to at least one surface of the substrate, wherein the formulation for production of the first layer comprises at least one monomer, oligomer or prepolymer having at least one crosslinkable functional group and the formulation for production of the first layer comprises coating materials from the group having the elements of isocyanate-crosslinking systems, polyurethanes, epoxy systems, acrylates, methacrylate, polyvinylethers, polyesters based on maleic acid and fumaric acid, styrene compounds and silicone acrylates, c) curing the layer applied in step b), wherein the cured first layer has a layer thickness in the range from 1 to 10 μm, and d) applying a second layer to the surface of the first layer created in step c), wherein the second layer applied in step d) has a complete surface in the printed regions.
2. The method as claimed in claim 1, wherein, prior to step a), applying a slip on at least one surface of the print substrate provided in step a).
3. The method as claimed in claim 2, wherein the substrate provided in step a) is a coated paper or a coated paperboard.
4. The method as claimed in claim 1, wherein the substrate provided in step a) is an uncoated paper or an uncoated paperboard.
5. The method as claimed in claim 1, wherein the first layer has a thickness in the range from 1 to 5 μm.
6. The method as claimed in claim 1, wherein the formulation for production of the first layer contains at least one monomer, oligomer or prepolymer having a crosslinkable group, and the sealing layer is obtained in step c) by crosslinking the functional groups.
7. The method as claimed in claim 6, wherein the deposited layer is crosslinked in step c) by UV radiation, an electron beam or thermal treatment and/or the formulation applied in step b) contains a reactive diluent as solvent.
8. The method as claimed in claim 6, wherein the layer deposited in step b) is cured and subsequently calendered before step d), wherein the cured layer is thermoplastic.
9. The method as claimed in claim 8, wherein step c) follows after step d) and, in step c), the calendered layer is crosslinked together with the layer deposited in step d).
10. The method as claimed in claim 1, wherein, in step b), the formulation for production of the first layer is applied by a flexographic printing method, a screen printing method, by intaglio printing, with a roller, by coating bar application, with a Mayer bar, with a slot die or by curtain coating to the substrate provided in step a), or the layer deposited in step b) is a full-area roll coating.
11. The method as claimed in claim 1, wherein, in step d), a formulation containing monomers, oligomers and/or prepolymers having at least one crosslinkable group is applied to the first layer by inkjet printing.
12. The method as claimed in claim 1, wherein, after step d), the layer deposited in step d) is crosslinked.
13. The method as claimed in claim 1, wherein the second layer is a matt or gloss lacquer.
14. A printed product comprising a substrate, a first layer and a second layer, wherein the first layer and the second layer comprise an organic crosslinked lacquer and have a complete surface, and wherein the first layer is transparent and has a layer thickness in the range from 1 to 10 μm, and wherein the second layer is applied at least to regions of the first layer, such that the first layer is disposed between the substrate and the second layer, wherein the second layer is a digital print that has been applied by inkjet methods, and the second layer is separated from the substrate surface by the first layer, such that the second layer has no contact with the substrate material, wherein the substrate comprises a paper or paperboard and wherein the first layer comprises an organic crosslinked lacquer from a coating material from the group having the elements of isocyanate-crosslinking systems, polyurethanes, epoxy systems, acrylates, methacrylate, polyvinylethers, polyesters based on maleic acid and fumaric acid, styrene compounds and silicone acrylates.
15. The printed product as claimed in claim 14, wherein the substrate has a binder-containing particulate coating at least on one surface, and the first layer has been applied atop the binder-containing particulate coating.
16. The printed product as claimed in claim 15, wherein the substrate comprises a coated paper, or a coated paperboard.
17. The printed product as claimed in claim 14, wherein the first layer has a layer thickness in the range from 1 to 5 μm.
18. The printed product as claimed in claim 14, wherein the second layer has been applied in a laterally structured manner atop the first layer.
19. The printed product as claimed in claim 14, wherein the second layer has a complete and homogeneous surface.
20. The printed product as claimed in claim 14, wherein the printed product has a pinhole density <30/cm.sup.2, preferably <1/cm.sup.2.
21. The printed product as claimed in claim 14, wherein the second layer has been crosslinked.
22. The printed product as claimed in claim 14, wherein the first layer and the second layer differ in their level of gloss, wherein the second layer preferably has a higher level of gloss than the first layer.
23. The printed product as claimed in claim 22, wherein the first layer contains inorganic or organic particles.
24. The printed product as claimed in claim 14, wherein the first layer comprises a calendering lacquer.
25. The printed product as claimed in claim 14, wherein the first layer has a layer thickness in the range from 2 to 3 μm.
26. The method as claimed in claim 1, wherein the first layer has a thickness in the range from 2 to 3 μm.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0052] The invention is described in detail with reference to working examples and with reference to FIGS. 1 to 14. The figures show:
[0053] FIG. 1 a schematic diagram of the surface of a coated paper,
[0054] FIG. 2 a schematic diagram of an applied primer layer on the paper shown in FIG. 1,
[0055] FIG. 3 a schematic diagram of the primer layer applied in FIG. 2 after the drying process,
[0056] FIG. 4 a schematic diagram of a lacquer layer applied by inkjet methods in the coated paper shown in FIG. 3,
[0057] FIG. 5 a schematic diagram of a lacquer layer applied by inkjet methods in the coated paper shown in FIG. 2,
[0058] FIG. 6 a schematic diagram of a working example of the invention with a coated paper as substrate,
[0059] FIG. 7 a schematic diagram of a lacquer layer applied atop an uncoated paper,
[0060] FIG. 8 a schematic diagram of a working example of the invention with an uncoated paper as substrate,
[0061] FIG. 9 a schematic diagram of one embodiment of the sealing layer of the invention, in which the sealing layer has been calendered,
[0062] FIG. 10 a schematic diagram of one embodiment in which a lacquer layer has been applied to a calendered sealing layer,
[0063] FIG. 11 a schematic diagram of a working example with a calendered sealing layer and a lacquer layer comprising VMP color pigments,
[0064] FIG. 12 a microscope image using coaxial incident light of a coated paper lacquered by means of inkjet printing,
[0065] FIG. 13 a microscope image using coaxial incident light of a coated paper lacquered by means of inkjet printing as a working example with a sealing layer of thickness 2.5 μm, and
[0066] FIG. 14 a microscope image using coaxial incident light of a coated paper lacquered by means of inkjet printing as a working example with a sealing layer of thickness 4.5 μm.
DETAILED DESCRIPTION
[0067] FIG. 1 shows a schematic of the surface of a coated paper 1. The paper surface 2 has been coated here with what is called a slip 3. The slip 3 here comprises particulate inorganic fillers 4 that are deposited on the paper surface 2 and are held together by an organic binder layer 5. The paper surface 2 here is not smooth but has unevenness. Moreover, on account of the irregular form of the inorganic fillers 4, what are called undercuts 6 are formed, which likewise cannot be filled by the binder 5.
[0068] FIG. 2 shows a schematic of the coated paper shown in FIG. 1, which has been provided with a primer coating composition 15. This can be effected, for example, by a flexographic printing method, a screen printing method, by intaglio printing, with a roll or by coating bar application or curtain coating. Alternatively, the coating may also be a slot die coating or a Mayer bar coating. The primer layer 15 has not yet dried and consists of a formulation that undergoes a certain shrinkage in volume in the course of drying/curing. The as yet undried layer covers the undercuts 6.
[0069] FIG. 3 shows the corresponding primer layer 14 that has been obtained by drying of the coating composition 15 shown in FIG. 2. As a result of the drying process and the associated loss of volume of the coating composition 15, the substrate surface is no longer fully covered by the primer 14 obtained by drying of the coating composition 15. In addition, the drying process also reduces the layer thickness of the primer layer 14 above the undercuts 6, as a result of which the film no longer fully covers the undercuts.
[0070] If a corresponding coated paper 1 is coated with a lacquer layer 7 by means of inkjet methods, channels to these defects are formed at undercuts and cavities by the incidence of the inkjet droplets. This is shown in schematic form in FIG. 4 and such that craters, called pinholes 8, form in the deposited lacquer layer 7. These constitute visually apparent defects in the lacquer layer 7, such that the substrate 1 is no longer suitable for surface finishing by means of inkjet coating.
[0071] The same effect occurs when the substrate from FIG. 1, as shown in FIG. 2, has been provided with a primer coating composition that has some undercuts again after drying, as shown in FIG. 3. After coating with a lacquer layer by means of inkjet methods, this likewise leads to formation of craters (called pinholes), as shown in FIG. 4.
[0072] FIG. 5 shows a coated paper 1 which has been provided with a primer layer 14 and then a lacquer layer 7 by means of inkjet methods. The primer layer 14 may, for example, be an aqueous primer. Here, in the drying process, previously covered defects (cf. FIG. 2) were opened up again as a result of volume shrinkage. These defects generate pinholes 8 as defects in the lacquer layer 7 applied above the primer layer.
[0073] FIG. 6 shows a schematic of a substrate coated in accordance with the invention as a first working example. The lacquer layer 7 was likewise applied by inkjet printing here, except that the surface of the coated paper 1 is fully covered by a sealing layer 9. The sealing layer 9 thus separates the surface of the coated paper 1 from the lacquer layer 7 and covers the cavities in the coated paper that are formed by undercuts 6. Because the undercuts are fully covered, the inkjet coating of the sealing layer 9 cannot form channels atop undercuts. Thus, the lacquer layer 7 does not have any craters or pinholes and is suitable for surface finishing.
[0074] For production of the embodiment of the invention shown in FIG. 6, the coated substrate 1 provided is first endowed with a coating composition. This can be effected, for example, by a flexographic printing method, a screen printing method, by intaglio printing, with a roll or by coating bar application or curtain coating. Alternatively, the coating may also be a slot die coating or a Mayer bar coating. The coating composition here contains crosslinkable functional groups.
[0075] After the coating composition has been applied to the surface of the substrate 1, the coating composition is crosslinked/cured via the crosslinkable functional groups. The crosslinkable groups here are preferably radiation-curing, such that the crosslinking in step c) can be effected with the aid of a UV lamp. In this case, during the crosslinking, there is only a very small reduction in volume of the coating, attributable predominantly to polymerization shrinkage. By virtue of the small reduction in volume during curing, unlike in the case of water-based primers for example (see FIG. 3), there is no tearing of the layer above undercuts that have not been filled completely by the primer liquid, and the cured sealing layer 9 thus has a coherent surface. The cured sealing layer 9 is thus an ideal surface for the inkjet printing process for deposition of the lacquer layer 7.
[0076] FIG. 7 shows a schematic of an uncoated paper 2 that has been provided with a lacquer layer 7. The surface of the uncoated paper 2 is uneven and porous. As a result of this porosity, a portion of the coating composition for production of the lacquer layer is absorbed by the paper in the period from the application up to the curing of the coating composition via crosslinking or loss of solvent. As a result, the layer on the surface of the substrate becomes increasingly thinner. Since the substrate surface has locally different absorption properties, the coating composition is absorbed to different degrees in the different regions of the substrate surface, such that the coating thickness varies over the substrate surface and the coating thus has a spotty appearance. The substrate 2 is thus unsuitable for surface finishing by inkjet methods.
[0077] FIG. 8 shows a second working example of a printed product of the invention, wherein the substrate, as in FIG. 3, is an uncoated paper 6. Between the lacquer layer 7 and the paper surface here too is a sealing layer 9 that seals the paper surface and has a cohesive homogeneous surface. Thus, the lacquer layer 7 also has low roughness and a homogeneous cohesive surface.
[0078] FIGS. 9 to 11 show schematics of embodiments with particularly smooth sealing layers 13. These sealing layers 17 are applied here analogously to the sealing layers 9 shown in FIGS. 6 and 8. However, the coating compositions thus supplied have greater layer thicknesses. In addition, the coating composition in these embodiments, in one working example, comprises what are called dual-cure coating formulations. These coating formulations are water-based and additionally contain radiation-curable functional groups. The application of the coating formulation atop the substrate 1, in this development of the invention, is followed by a drying step. The layer thus obtained is thermoplastic and, just like sealing layer 9 shown in FIG. 6, at least partly reflects the unevenness of the substrate surface. In order nevertheless to obtain an impervious layer with low surface roughness for the inkjet printing process, the dried layer is consolidated and smoothed by calendering. For this purpose, the layer is consolidated with a polished stainless steel calender. The calendered layer 13 thus obtained, even without further crosslinking of the functional groups, has a coherent surface with low surface roughness and sufficiently high mechanical stability for the subsequent finishing.
[0079] The lacquer layer 7 shown in FIG. 10 was applied by inkjet atop the layer 13 that is very smooth by virtue of the calendering. It is a feature of the resultant surface quality that the substrate unevenness has been virtually completely balanced out by the calendering and hence an extremely smooth lacquer surface is formed. The curing of the lacquer layer 7 by UV radiation also results in crosslinking of the uncrosslinked radiation-curable functional groups remaining in the calendered layer 13.
[0080] FIG. 11 shows an embodiment in which the lacquer layer 16 contains metal pigments in platelet form (VMP colors). By virtue of the very smooth surface of the calendered sealing layer 13, these may be aligned parallel or at least largely parallel to the substrate surface, such that it is possible to achieve a very good mirror effect without distortion resulting from the substrate unevenness.
[0081] FIG. 12 to FIG. 14 are microscope images with 12-fold magnification with coaxial incident light of various two-dimensional coating specimens on paperboard substrates that have been applied by inkjet lacquering. The samples shown in FIGS. 12 to 13 have the same substrate 1 and the same composition of the lacquer layer 7, and differ by the pretreatment of the substrate before the inkjet lacquering for production of the lacquer layer 7.
[0082] The lacquer layer 7 was applied here in a laterally structured manner, such that the region 17 shows the untreated substrate (FIG. 12) or pretreated substrate (FIGS. 13 to 14) without lacquer layer 7. The samples shown in FIG. 12 and FIG. 13 are comparative samples here, without pretreatment of the substrate 1 in FIG. 12 prior to the lacquering operation. The samples shown in FIG. 13 and FIG. 14 are two working examples of the printed product of the invention. Here, in both cases, the lacquering operation was preceded by application of a sealing layer 9. The sample shown in FIG. 12 here has a sealing layer 9 having a layer thickness of 2.5 μm; the layer thickness of the sealing layer of the sample shown in FIG. 13 is 4.5 μm.
[0083] While the regions 17 shown in FIG. 12 have high surface roughness, the surface is smoothed by the sealing layer of the samples shown in FIG. 13 and FIG. 14. In addition, FIG. 11 to FIG. 14 shows the influence of a sealing layer on the pinhole density in the lacquer layer. The pinholes 8 are apparent in the figures as dark-colored defects in the form of dots. The pinhole density, i.e. the average number of pinholes 8 per cm.sup.2 of coating area, decreases constantly from FIG. 11 to FIG. 14. The highest pinhole density at about 2000/cm.sup.2 is possessed here by the lacquer layer 7 applied directly to the untreated substrate 1 (FIG. 11) (at a layer thickness of 8 g/m.sup.2). The pinholes 8 are created here by lack of coverage or opening of undercuts and pores during the inkjet printing operation. Even a primer layer 18 that was created by application of a corresponding aqueous coating formulation atop the substrate 1 (dry layer thickness about 1 g/m.sup.2) cannot effectively prevent the formation of pinholes 8 since the high loss of volume or mass of the coating composition in the drying process results in partial exposure of the undercuts and pores again. By contrast, the working examples shown in FIG. 12 and FIG. 13 have significantly smaller pinhole densities of 25/cm.sup.2 (FIG. 12) and <1/cm.sup.2 (FIG. 13) respectively. This is attributable to the sealing layer 9 of the invention, by which undercuts and pores in the substrate 1 are covered permanently. This advantageous effect of the sealing layer 9 is dependent on the layer thickness thereof and increases with increasing layer thickness.
REFERENCES
[0084] 1 substrate [0085] 2 paper [0086] 3 slip [0087] 4 inorganic filler [0088] 5 binder [0089] 6 undercut [0090] 7 lacquer layer [0091] 8 pinhole [0092] 9 sealing layer [0093] 13 calendered layer [0094] 14 aqueous primer [0095] 15 coating composition [0096] 16 color with VM pigments [0097] 17 uncoated region
