Laser-engravable pad printing plate

Abstract

A laser-engravable pad printing plate comprising at least (a) a metal support, (b) an adhesion layer, (c) a laser-engravable recording layer having a layer thickness of 20 m to 200 m, (d) a cover film,
characterized in that the laser-engravable recording layer (c) comprises (c1) 40 to 95 wt % of a polyvinyl alcohol, (c2) 5 to 50 wt % of an IR absorber, (c3) 0 to 30 wt % of an inorganic filler, (c4) 0 to 20 wt % of a crosslinker, and (c5) 0 to 10 wt % of further additives.

Claims

1. A laser-engravable pad printing plate comprising: (a) a metal support, (b) an adhesion layer, (c) a laser-engravable recording layer having a layer thickness of 20 m to 200 m obtained from a mixture comprising (c1) 40 to 95 wt % of a polyvinyl alcohol, (c2) 5 to 50 wt % of an IR absorber, (c3) 0 to 30 wt % of an inorganic filler, (c4) 0.1 to 20 wt % of a crosslinker, and (c5) 0 to 10 wt % of further additives, (d) a cover film, wherein the crosslinker (c4) is selected from the group consisting of polyfunctional isocyanates, mono- or polyfunctional aldehydes, polyfunctional epoxides, polyfunctional carboxylic acids and polyfunctional carboxylic anhydrides; and wherein the laser-engravable recording layer is obtained in a process comprising chemically crosslinking the crosslinker with the polyvinyl alcohol.

2. The laser-engravable pad printing plate as claimed in claim 1, wherein the laser-engravable recording layer (c) comprises as polyvinyl alcohol (c1) a partially hydrolyzed polyvinyl alcohol ester having a degree of hydrolysis of 50 to 98 mol %.

3. The laser-engravable pad printing plate as claimed in claim 1, wherein the recording layer (c) comprises as IR absorber (c2) carbon black, graphite or carbon nanoparticles.

4. The laser-engravable pad printing plate as claimed in claim 1, wherein the recording layer (c) comprises 5 to 30 wt % of an inorganic filler.

5. The laser-engravable pad printing plate as claimed in claim 1, wherein the recording layer (c) comprises an inorganic filler (c3) having a hardness of >4 Mohs.

6. The laser-engravable pad printing plate as claimed in claim 1, wherein the recording layer (c) comprises as inorganic filler (c3) a finely ground quartz whose surface has been modified with silanes.

7. The laser-engravable pad printing plate as claimed in claim 1, wherein the sum total of IR absorber (c2) and inorganic filler (c3) in the recording layer is <50 wt %.

8. The laser-engravable pad printing plate as claimed in claim 7, wherein the crosslinker is glyoxal or glutaraldehyde.

9. The laser-engravable pad printing plate as claimed in claim 1, wherein the adhesion layer is a 2-component polyurethane adhesion varnish.

10. The laser-engravable pad printing plate as claimed in claim 1, wherein the metal support (a) is a steel plate having a thickness of 50 to 300 m.

11. The laser-engravable pad printing plate as claimed in claim 1, wherein the cover film is a PET film having a mean roughness depth Rz of 0.3 to 3 m.

12. A method for producing a pad printing plate comprising: (a) a metal support, (b) an adhesion layer, (c) a laser-engravable recording layer having a layer thickness of 20 m to 200 m, (d) a PET cover film, wherein the laser-engravable recording layer (c) is made from a mixture which comprises (c1) 40 to 95 wt % of a polyvinyl alcohol, (c2) 5 to 50 wt % of an IR absorber, (c3) 0 to 30 wt % of an inorganic filler, (c4) 0.1 to 20 wt % of a crosslinker, and (c5) 0 to 10 wt % of further additives, wherein the laser-engravable recording layer is obtained by a process comprising chemically crosslinking the crosslinker with the polyvinyl alcohol; the method comprising steps (i) to (iii): (i) coating the metal support with the adhesion layer, (ii) applying the laser-engravable recording layer to the PET cover film and drying the recording layer in one or more steps, (iii) laminating the coated PET cover film onto the metal support coated with the adhesion layer; wherein the crosslinker (c4) is selected from the group consisting of polyfunctional isocyanates, mono- or polyfunctional aldehydes, polyfunctional epoxides, polyfunctional carboxylic acids and polyfunctional carboxylic anhydrides.

13. A method for producing a pad printing clich from a laser-engravable pad printing plate as defined in claim 12, further comprising steps (iv) to (vi): (iv) removing the PET cover film from the pad printing plate, (v) engraving the depressions into the laser-engravable recording layer by means of an IR laser, (vi) cleaning the laser-engraved pad printing clich by rinsing with a solvent.

14. A method for printing a substrate by the pad printing process with a pad printing clich obtainable by the method of claim 13, further comprising steps (vii) to (x): (vii) fastening the pad printing clich in the pad printing machine, (viii) inking the pad printing clich with a solvent-based pad printing ink, (ix) stripping off the excess printing ink by means of a doctor blade, (x) transferring the printing ink by means of a rubber pad onto the substrate to be printed.

15. A laser-engravable pad printing plate comprising: (e) a metal support, (f) an adhesion layer, (g) a laser-engravable recording layer having a layer thickness of 20 m to 200 m obtained from a mixture comprising (c6) 40 to 95 wt % of a polyvinyl alcohol, (c7) 5 to 50 wt % of an IR absorber, (c8) 0 to 30 wt % of an inorganic filler, (c9) 0.1 to 20 wt % of a crosslinker, and (c10) 0 to 10 wt % of further additives, (h) a cover film, wherein the crosslinker (c4) is selected from the group consisting of mono- or polyfunctional aldehydes, polyfunctional epoxides, and polyfunctional carboxylic anhydrides; wherein the laser-engravable recording layer is obtained by a process comprising chemically crosslinking the crosslinker with the polyvinyl alcohol.

Description

EXAMPLES

(1) Production of Pad Printing Plates

Example 1

(2) A tin-plated steel plate 240 m thick was coated with a 2-component polyurethane adhesion varnish (2K PU topcoat GM60-6203 from BASF and Desmodur L67MPA/X from Bayer as curing agent in a ratio of 2:1) in a curtain coater. Following application of the adhesion varnish, the plate was baked at 250 C. for 1 minute. The average coat weight of the adhesion varnish was 15 m.

(3) In parallel to this, a PET film of medium roughness (Melinex 383, layer thickness 125 mm, available from Dupont-Teijin) was coated with a laser-engravable recording layer. The composition of the recording layer is reproduced in the table below.

(4) TABLE-US-00001 Fraction solids Component Function Manufacturer (wt %) Alcotex 72.5 binder Kuraray 63.00 Carbon black IR absorber Lanxess 27.75 (Pigment Black 7) Syloid ED3 filler Degussa 8.99 Capstone FS-30 flow control Dupont 0.26 assistant Total 100

(5) The components of the recording layer were dissolved in water/n-propanol in a ratio of 3:1 (solids content 16.3 wt %) and dispersed in a ball mill for 3 h. The solution was applied by the metering roller application process on a coating line with double applicator system. In the first applicator system, a dry film thickness of 10 m was applied; in the second applicator system, a dry film thickness of 20 m. The web speed was 10 m/min and the length of the drying tunnel was about 12 m, hence resulting in a drying time of 72 seconds. Drying was accomplished by heated circulating air in a countercurrent process. The maximum temperature of the circulating air in the drier was 145 C. The PET film coated with the recording layer was subsequently laminated onto the coated steel plate. n-Propanol was used as a laminating assistant. The pad printing plates were thereafter stored at room temperature for 2 days and then processed further.

Example 2

(6) Procedure as per example 1, but without inorganic filler in the recording layer.

Example 3

(7) Procedure as per example 1, but the recording layer was additionally crosslinked with glyoxal.

(8) The composition of the recording layer as per example 3 is reproduced in the table below.

(9) TABLE-US-00002 Fraction solids Component Function Manufacturer (wt %) Alcotex 72.5 binder Kuraray 61.35 Carbon black IR absorber Lanxess 27.02 (Pigment Black 7) Syloid ED3 filler Degussa 8.75 Glyoxal crosslinker BASF 2.62 Capstone FS-30 flow control Dupont 0.26 assistant Total 100

Example 4

(10) Procedure as per example 3, but Silbond 800 EST from Quarzwerke Group was used as inorganic filler.

(11) Laser Engraving and Printing Tests

(12) The cover film of the pad printing plates from examples 1 to 4 was removed.

(13) The plates were mounted onto the drum of an IR laser (Thermoflex X 48, Xeikon) and lasered with a resolution of 5080 dpi. The lasered motif comprised three different screen wedges, the resolution of the screen selected being 80 L/cm, 100 L/cm, and 120 L/cm. For each ruling, the surface coverage was varied from 70% to 90%. Surface coverage in pad printing means the percentage area removed by engraving, in comparison to the total area.

(14) The power of the laser was 30 watts. The optimum distinctness of imaging was achieved at a speed of rotation of 3.5 revolutions per second. This speed of rotation corresponds to an energy input of 15 J/cm.sup.2.

(15) The engraved clichs were subsequently mounted on a pad printing machine (from Morlock, closed blade pot). The pad printing ink used was a solvent-based pad printing ink, Marabu TPY980 (white). The ink contains hydrocarbons, ketones, and acetates as solvents. The curing agent added was 10% isocyanate curing agent H1 from Marabu. The clichs were processed with a frequency in each case of 1000 doctor blade operations per hour, and were subjected after 1 hour in each case to microscopic examination for damage/erosion, etc. As soon as initial damage, such as the absence of individual screen elements, was detectable, the test was terminated and the number of doctor blade operations was recorded.

(16) The results of the printing tests are reproduced in the table below.

(17) TABLE-US-00003 2 Example 1 (comparative) 3 4 Removal of the easy easy easy easy cover film Adhesion of the not not not not recording layer removable removable removable removable Water solubility soluble soluble insoluble insoluble of the recording layer Laser energy 14.0 14.0 14.0 14.0 (J/cm.sup.2) Depth of 28 30 31 30 engraving (m) Dimensions of 15 15 m 15 15 m 15 15 m 15 15 m raised elements at 120 L/cm and 90% surface covered Doctor blade 4000 1000 >40000 >40000 resistance Handling difficult difficult easy easy

(18) The cover film was readily removable from all the clichs. The adhesion to the varnished steel support was high. The recording layer could no longer be removed from the support. After coating and drying, the recording layer remained water-soluble in the tests without crosslinker, meaning that the layer could be dissolved with fine dispersion. The crosslinked layers, in contrast, were insoluble in water. There were no problems with the handling of the crosslinked recording layers. When the noncrosslinked plate surfaces from examples 1 and 2 were touched, in contrast, they exhibited significant fingerprints.

(19) The printing plates were engraved with a laser energy of 14 J/cm.sup.2. The depth of engraving of around 30 m was achieved in the case of all the clichs. In all of the clichs, fine elements could be imaged up to a surface coverage of 90%. The raised, fine elements were approximately square with an edge length of 15 m. No melt burr could be seen on any clich.

(20) A notable feature was the unexpectedly good doctor blade resistance of the printing plate from example 1 without chemical crosslinker, which withstood up to 4000 doctor blade operations without damage. In contrast, the doctor blade resistance of the printing plate according to example 2 (without inorganic filler) was significantly poorer. The printing plates of examples 3 and 4, in which the recording layer was additionally crosslinked chemically, had excellent doctor blade resistances. After 40 000 doctor blade operations, these clichs were still undamaged.