LASER INDUCED TRANSFER PRINTING PROCESS

Abstract

Printing process in which a substrate to be printed is disposed opposite an ink carrier having an ink layer, the ink layer being irradiated regionally by a laser beam, said layer accelerating by absorption of the laser beam in the substrate direction, wherein for laser absorption the ink layer comprises reflective particles, a solvent, and a soluble polymer, wherein the reflective particles have an aspect ratio>25, the aspect ratio being defined as the average particle size/average particle thickness.

Claims

1. Printing process comprising: disposing a substrate to be printed opposite an ink carrier including an ink layer, and regionally irradiating the ink layer with a laser beam, wherein the ink layer comprises reflective particles, a solvent, and a soluble polymer dissolved in the solvent, the reflective particles have an aspect ratio>25, the aspect ratio being defined as the average particle size/average particle thickness, and the reflective particles comprise metal or a metal-coated carrier material.

2. Printing process according to claim 1, wherein for the reflective particles P.sub.T<80+3 P.sub.S, wherein P.sub.T is the absolute value of the average particle thickness in nm and P.sub.S is the absolute value of the average particle size in μm.

3. Printing process according to claim 1, wherein the reflective particles have an average particle size in the range of 0.1-25 μm.

4. Printing process according to claim 1, the soluble polymer having a weight average (Mw) molecular weight of greater than 250,000 g/mol, where the weight average (Mw) of the molecular weight of the soluble polymer is determined according to DIN 55672-2: 2016-3.

5. Printing process according to claim 1, wherein the soluble polymer has a weight average (Mw) molecular weight of 250,000 g/mol to 2,500,000 g/mol, where the weight average (Mw) of the molecular weight of the soluble polymer is determined according to DIN 55672-2: 2016-3.

6. Printing process according to claim 5, wherein the soluble polymer has a weight average (Mw) molecular weight of 250,000 g/mol to 1,500,000 g/mol.

7. Printing process according to claim 1, wherein the proportion of the soluble polymer accounts for between 0.05 to 2 wt. %, of a total ink mixture comprised in the ink layer.

8. Printing process according to claim 7, wherein the proportion of the soluble polymer accounts for between 0.05 to 1 wt. %, of a total ink mixture comprised in the ink layer.

9. Printing process according to claim 7, wherein the proportion of the soluble polymer accounts for between 0.1 and 0.8 wt. %, of a total ink mixture comprised in the ink layer.

10. Printing process according to claim 1, wherein the soluble polymer comprises a cellulose ester, a cellulose nitrate, a cellulose ether, a polyurethane or a vinyl polymer.

11. (canceled)

12. Printing process according to claim 10, wherein the reflective particles have an L* value in the L*a*b* colour space of more than 50 and an a* and/or b* value in the L*a*b* colour space of less than +/−5.

13. Ink comprising reflective particles, solvent, and a soluble polymer dissolved in the solvent, wherein the reflective particles have an aspect ratio>25, the aspect ratio being defined as the average particle size/average particle thickness, the reflective particles comprise metal or a metal-coated carrier material, the soluble polymer comprises a cellulose ester, a cellulose nitrate, a cellulose ether, a polyurethane or a vinyl polymer, and the reflective particles have an L* value in the L*a*b* colour space of more than 50 and an a* and/or b* value in the L*a*b* colour space of less than +/−5.

14. Printing process according to claim 10, wherein the soluble polymer comprises a hydroxypropylcellulose.

15. Printing process according to claim 12, wherein the reflective particles have an a* and/or b* value in the L*a*b* colour space of less than +/−3.

16. Printing process according to claim 10, wherein the reflective particles have an L* value in the L*a*b* colour space of more than 70 and an a* and/or b* value in the L*a*b* colour space of less than +/−5.

17. Printing process according to claim 16, wherein the reflective particles have an a* and/or b* value in the L*a*b* colour space of less than +/−3.

18. Printing process according to claim 10, wherein the reflective particles have an L* value in the L*a*b* colour space of more than 80 and an a* and/or b* value in the L*a*b* colour space of less than +/−5.

19. Printing process according to claim 18, wherein the reflective particles have an a* and/or b* value in the L*a*b* colour space of less than +/−3.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The subject matter of the invention is to be elucidated in more detail below, referring to the drawing of FIG. 1.

[0040] FIG. 1 is a schematic view of a printing machine used for the process of the invention.

DETAILED DESCRIPTION OF THE DRAWING

[0041] FIG. 1 is a schematic view of one exemplary embodiment of a printing machine (1) of the invention.

[0042] The printing machine (1) comprises as ink carrier (4) a circulating ink ribbon.

[0043] The ink ribbon is coated homogeneously and over its full area with ink (2) by the inking unit (8). The ink ribbon subsequently moves in the arrow direction to the printing nip (10). The ink carrier (4) is distanced by a gap from the substrate (6) to be printed. Preferably the width of the gap is adjustable and/or is regulated continuously. This can be done by means, for example, of adaptable distancing rolls (5).

[0044] In the printing nip (10), using a laser scanner (11), a laser beam (3) is focused through the ink carrier (4), which is permeable to the laser light, into the ink (2). The local and targeted heating of parts of the ink (2) by means of the laser beam (3) causes explosive vaporization of a small region of the ink (2), and so a part of the printing ink (2) is transferred from the ink ribbon onto the opposite substrate (6).

[0045] The ink ribbon, controlled by the distancing rolls (5) and the deflection rollers (7), subsequently moves back in the direction of the inking unit (8). On contact between inking unit (8) and the ink ribbon, the ink (2) consumed is replenished.

[0046] The excess ink (2) in the inking unit (8) is collected in the ink trough (9) at the bottom and is added continuously in repetition to the printing operation.

[0047] Measurement Methods

[0048] Average particle size: The particle size distribution is measured by laser scattering granulometry using a Helos/BR Multirange (Sympatec) apparatus according to the manufacturer indications and in accordance to ISO 13320-1. The particles are dissolved in isopropanol under stirring before measuring the particle size distribution. The particle size function is calculated in the Fraunhofer-approximation as a volume weighted cumulative frequency distribution of equivalent spheres. The median value d50 means that 50% of the measured particles are below this value (in a volume-averaged distribution). The d50 value is taken as the average particle size.

[0049] Average particle thickness: The particle diameter is determined using a reflective electron microscope (REM). A resin customarily used in electron microscopy, for example TEMPFIX (Gerhard Neubauer Chemikalien, D-48031 Munster, Germany), is applied to a sample plate and heated to softening on a hotplate. Subsequently, the sample plate is taken from the hotplate and the sample to be measured is scattered onto the softened resin. In the measurement of the thickness, the azimuthal angle α of the pigment is estimated relative to a plane normal to the surface and allowed for when evaluating the thickness according to the formula H.sub.eff=H.sub.mes/cos α.

[0050] The cumulative frequency curve was plotted from the H.sub.eff values with the aid of the relative frequencies of occurrence. At least about 100 particles are counted and the average value of H.sub.eff is taken as the average particle thickness.

[0051] L*a*b values: The values in L*a*b* colour space are determined using a DTM 1045® spectrophotometer at an angle between 15 and 25°.

[0052] Molecular weight: The weight average (Mw) of the molecular weight of the soluble polymer is determined according to DIN 55672-2: 2016-3.

EXAMPLES

[0053] Various ink compositions were prepared comprising different reflective aluminium particles. Using the process described in U.S. Pat. No. 6,241,344 B1 the transfer of the aluminium particles to a substrate was measured. The ink compositions were prepared by mixing the following ingredients:

[0054] 43 wt. % of ethanol

[0055] 32 wt. % of methoxypropanol

[0056] 10 wt. % of a 3% solution of klucel in water

[0057] 1.5 wt. % of ethylcellulose

[0058] 1.5 wt. % of polyvinylbutyral

[0059] 12 wt. % of a reflective aluminium pigment

[0060] The results are presented in table 2.

TABLE-US-00002 TABLE 2 Average Amount of Average particle pigment particle size thickness Aspect transferred to Example P.sub.s (μm) P.sub.t (nm); (μm) ratio substrate (mg) Ex 1 9.95 40; 0.04 249 28.10 Ex 2 10.90 90; 0.09 121 16.10 Ex 3 9.73 130; 0.13  75 7.24 Ex 4 2.45 30; 0.03 82 37.6

[0061] To test the effect of the molecular weight of the soluble polymer on the splash behaviour, various experiments were performed. In the ink compositions, 3-Ethoxy-1-propanol was used as solvent, in combination with reflective aluminium particles. The experiments have been summarized in Table 3.

[0062] These experiments clearly show that a soluble polymer with a Mw>100,000 significantly reduces the splashing.

TABLE-US-00003 TABLE 3 Cellulose total scattered Example ether Mw proportion splashs % Ex. 5* No polymer  100% Ex. 6* 100.000 1% ca. 90% Ex. 7 300.000 1% ca. 70% Ex. 8 800.000 1% ca. 10% Ex. 9 1.200.000 1% ca. 10% *Comparative Example

LIST OF REFERENCE NUMERALS RELATING TO FIG. 1

[0063] 1. Printing machine [0064] 2. Ink [0065] 3. Laser beam [0066] 4. Ink carrier [0067] 5. Distancing roll [0068] 6. Substrate [0069] 7. Deflection roller [0070] 8. Inking unit [0071] 9. Ink trough [0072] 10. Printing nip [0073] 11. Laser scanner