METHOD FOR COATING A SUBSTRATE WITH A DROP-ON-DEMAND PRINTER

Abstract

The invention relates to a 2K coating formulation that is printed using a valve jet printer.

Claims

1. A process for coating a substrate comprising: dropletwise application of a coating composition from a nozzle onto a substrate, wherein the nozzle has a closable opening which allows or prevents discharge of a droplet of the coating composition under instructions from a control unit, wherein the coating composition is applied more than once to less than 30% of a surface area of the substrate, wherein a pressure of ≥1.2 bar to ≤3.5 bar acts upon the coating composition when the opening of the nozzle is closed, wherein the opening of the nozzle has a diameter of ≥80 μm to ≤250 μm, wherein the coating composition comprises a resin, a hardener for the resin and a solvent, wherein a reactive resin and a hardener are altogether present in the coating composition in a proportion of ≥20% by weight based on a total weight of the coating composition, and wherein the coating composition has a viscosity at 20° C. measured according to DIN EN ISO 3219/A3 at a shear rate of 1000/s of ≥20 mPa.Math.s to ≤80 mPa.Math.s.

2. The process as claimed in claim 1, wherein the following parameters are chosen such that according to the following formula the calculated index K≥is 0.4 to ≤4: $K=\frac{{\left(\frac{R}{50}\right)}^2{\left(\frac{p}{2}\right)}^{2.5}{\left(\frac{d}{120}\right)}^2\left(\frac{S}{45}\right)}{{\left(\frac{\eta }{50}\right)}^3}$ where: R: spatial resolution of application of coating composition in dots per 2.54 cm p: pressure acting on coating composition when opening of nozzle is closed in bar d: diameter of closable opening of nozzle in μm S: proportion of nonvolatile constituents of coating composition in % by weight based on total weight of coating composition according to ISO 3251, 120 min, 100° C. η: viscosity at 20° C. measured according to EN ISO 3219/A3 at a shear rate of 1000/s in mPa.Math.s.

3. The process as claimed in claim 1, wherein the dropletwise application of the coating composition onto the substrate is carried out through a plurality of nozzles and wherein each of the nozzles has a closable opening which independently of other closable openings of other nozzles allows or prevents discharge of a droplet of the coating composition under instructions from the control unit.

4. The process as claimed in claim 1, wherein the spatial resolution of application of the coating composition is ≥30 dots per 2.54 cm to ≤150 dots per 2.54 cm.

5. The process as claimed in claim 1, wherein the solvent comprises water, n-hexane, isohexane, cyclohexane, n-heptane, isoheptane, n-octane, isooctane, mineral spirits, xylene, solvent naphtha, propanol, n-butanol, isobutanol, butyl glycol, butyl diglycol, ethylene glycol, diethyl glycol, butyl acetate, ethyl acetate, 2-butoxyethyl acetate, 1-methoxy-2-propyl acetate, butanone, acetone, 2-heptanone, 2,4-pentanedione, 2-pentanone, ethyl-3-ethoxypropionate, 1,2,4-trimethylbenzene, 4-methylpentan-2-one, or a mixture of at least two of the abovementioned solvents.

6. The process as claimed in claim 1, wherein the coating composition has a pot life of ≥30 minutes to ≤480 minutes, wherein the pot life is defined as the time until doubling of the viscosity measured at 23° C. according to DIN EN ISO 3219/A.

7. The process as claimed in claim 1, wherein the resin is an epoxy resin and the hardener is a polymerization catalyst, a primary amine, a cyclic anhydride, a polyphenol, a thiol, or a mixture of at least two of the abovementioned compounds.

8. The process as claimed in claim 1, wherein the resin is a polyol, a polyamine, an amino alcohol, or a mixture of at least two of the abovementioned compounds and the hardener is a blocked or unblocked polyisocyanate.

9. The process as claimed in claim 8, wherein in the coating composition the NCO index is ≥0.8 to ≤1.5.

Description

[0054] The present invention is more particularly elucidated using the examples which follow without, however, being limited thereto.

[0055] Methods

[0056] Dry Film Thickness (DFT):

[0057] For measurement on nonmagnetic substrates (for example aluminum) the eddy current method is employed (DIN EN ISO 2360). It is based on the alteration to the magnetic field of an electromagnet brought about by eddy currents in the electrically conductive substrate. This magnetic field alteration depends on the dry film thickness of the coating.

[0058] Film thickness measuring devices with integrated probes or handheld interchangeable probes are obtainable for example from the manufacturers Helmut Fischer, Erichsen, BYK Gardner, Elcometer or TQC.

[0059] To determine dry film thickness, 9 individual measurements were performed at different points on the surface to be tested and an average thereof was taken.

[0060] Visual assessment: Surface quality was visually evaluated using a scale of −5 to +5: −5 is too thick and wrinkled, +5 is striped or not printed. Values of −1 to 1 are regarded as good paint qualities.

[0061] Viscosity:

[0062] Determination of viscosity is carried out according to DIN EN ISO 3219/A3 and was performed with an Anton Paar MCR301 rheometer. Anton Paar measuring beaker: Cylinder geometry, measuring beaker diameter 28.92 mm, cylinder diameter 26.66 mm, annular gap length 40 mm, measurement temperature 20° C., shear rate of 1 to 1500 l/s in 55×5 s=275 s and of 1500 l/s to 1 in 275 s.

[0063] Materials

[0064] Component A:

[0065] Acrylate-containing polyol Setalux® D A 665 (BA/X) from Allnex Resins Germany GmbH/Allnex Netherlands BV.

[0066] BYK® 331 (10% in methoxypropyl acetate) from BYK-Chemie is a silicone-containing surface additive for solvent-containing, solvent-free and aqueous paints and printing inks

[0067] Component B:

[0068] Polyisocyanate: Employed was an HDI trimer DESMODUR ULTRA N 3390 BA/SN (NCO functionality >3) having an NCO content of 19.6% by weight (according to ISO 11909) and a nonvolatile content of 90% (according to ISO 3251, 120 min, 100° C.) from Covestro AG. The viscosity is about 550 mPa.Math.s at 23° C. (DIN EN ISO 3219/A3).

[0069] The solvent used was butyl acetate or 1-methoxy-2-propyl acetate (MPA)/solvent naphtha 100 (1:1).

[0070] The substrates were aluminum sheets. These were first cleaned with ethyl acetate.

[0071] The raw materials were used without further purification or pretreatment unless otherwise stated.

[0072] Formulations

[0073] To produce component A the acrylate-containing polyol or blends of both polyols were diluted with BA or MPA/SN and the surface additive homogeneously mixed. The hardener component was employed with the polyisocyanate in the ratio NCO:OH=1.0. The solids content of the mixture was about 50% by weight and was optionally further diluted. At a solids content of 50% the application viscosity at 23° C. (ISO cup 5 mm DIN EN ISO 2431) was about 30 s.

TABLE-US-00001 A B Formulation [g] [g] Component A SETALUX D A 665 BA/X 49.06 49.06 BYK 331 0.50 0.50 BUTYL ACETATE 32.04 43.70 Component B DESMODUR ULTRA N 3390 BA/SN 18.40 18.40 Sum 100.00 111.66

[0074] Printing Parameters

[0075] The experiments were performed with a ChromoJET TT Version 2.0 tabletop printer from Zimmer Austria. The premixed paint formulations were filled into 300 mL printing tanks 10 min after mixing. A pressure controller was used to establish an operating pressure of 1 to 3.5 bar. The system was initially washed through several times with the paint formulation and subsequently printing was carried out on the cleaned substrates. The printing surface area was 130*70 mm The carriage speed was 0.6 m/s. A printing head having 9 nozzle groups with a nozzle diameter of 100 μm, 120 μm or 150 μm was used. After printing was complete the system was washed through with ethyl acetate.

[0076] After a flash-off time of 10 min at room temperature, the hardening of the applied paints is carried out in the oven at 140° C. for 25 min.

[0077] Results marked with * are according to the invention.

TABLE-US-00002 No. 1A 1B 2B* Description Automotive Automotive Automotive clearcoat clearcoat clearcoat diluted diluted Formulation A B B Viscosity [mPas] at 120 33 33 1000/s at 23° C. Solids [%] 50 44 44 Printer parameters Nozzle diameter [μm] 100 100 100 Pressure [bar] 3 3 1.5 Resolution x/y [dpi] 76 76 50 DFT n.a. 94 28 Visual assessment 5 −3 1 Index K 0.36 15.04 2.36

[0078] The following examples are hypothetical examples. The formulations were not printed. It is expected that Example No. 4 will yield a striped surface and that Example No. 5 and No. 6 will deliver a satisfactory printed image.

TABLE-US-00003 No. 3B 4B* 5B* Description Automotive Automotive Automotive clearcoat clearcoat clearcoat diluted diluted diluted Formulation B B B Viscosity [mPas] at 33 33 33 1000/s at 23° C. Solids 44 44 44 Printer parameters Nozzle diameter [μm] 100 150 120 Pressure [bar] 1 1.5 1.5 Resolution x/y [dpi] 25 50 76 DFT n.a. n.a. n.a. Visual assessment striped good good Index K 0.10 2.59 3.83

[0079] The “index K” shown in the tables was calculated according to the previously elucidated formula.

[00002]$K=\frac{{\left(\frac{R}{50}\right)}^2{\left(\frac{p}{2}\right)}^{2.5}{\left(\frac{d}{120}\right)}^2\left(\frac{S}{45}\right)}{{\left(\frac{\eta }{50}\right)}^3}$

[0080] The index formed from the defined threshold values according to the invention results in a parameter range which surprisingly affords desired surface qualities and printing qualities/thicknesses for the printed industrial paint qualities on the valvejet printer.