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

Abstract

The invention relates to a 2K coating formulation that is printed without a mask 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 the 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, 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, and wherein the substrate comprises no temporarily applied mask onto which the coating composition is applied.

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: $K=\frac{{\left(\frac{R}{50}\right)}^2{\left(\frac{p}{2}\right)}^{2.5}{\left(\frac{d}{120}\right)}^2\left(\frac{S}{45}\right){\left(\frac{45}{C}\right)}^2}{{\left(\frac{\eta }{50}\right)}^3}$ 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 C: dynamic contact angle determined as advancing angle according to DIN EN ISO 19403-6 of coating composition on substrate in degrees.

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 in the coating composition 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.

10. The process as claimed in claim 1, wherein the coating composition is adapted to the substrate such that a dynamic contact angle, determined as the advancing angle according to DIN EN ISO 19403-6, of the coating composition on the substrate is ≥30° to ≤90°.

Description

EXAMPLES

[0056] The present invention is more particularly elucidated by the figures and examples which follow without, however, being limited thereto.

[0057] Methods

[0058] Visual Assessment:

[0059] Edge quality between coated and uncoated substrate was visually assessed on a scale of 1 to 5, wherein 1 is best. The references in the figure have the following meanings: 1: sharp edge, 2: sharp edge with slight fluctuations, 3: sharp edge with intermediate fluctuations, 4: irregular edge and 5: double edge.

[0060] Viscosity:

[0061] 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.

[0062] Materials

[0063] Component A:

[0064] Acrylate-containing polyol Setalux® D A 665 (BA/X) from Allnex Resins Germany GmbH/Allnex Netherlands BV.

[0065] 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

[0066] Component B:

[0067] 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).

[0068] The solvent used was butyl acetate (BA).

[0069] The substrates were aluminum sheets. These were first cleaned with ethyl acetate.

[0070] The raw materials were used without further purification or pretreatment unless otherwise stated.

[0071] Formulation

[0072] To produce component A the acrylate-containing polyol was diluted with BA 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 Formulation 1 [g] Component A SETALUX D A 665 BA/X 49.06 BYK 331 0.50 BUTYL ACETATE 43.70 Component B DESMODUR ULTRA N 3390 BA/SN 18.40 Sum 111.66

[0073] Digital Printing Parameters

[0074] The digital printing experiments were performed with a ChromoJET CHR-TT 118 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 2 bar. The system was initially washed through several times with the 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 8 nozzle groups with a nozzle diameter of 100 μm was used. After printing was complete the system was washed through with ethyl acetate.

[0075] The printing conditions are summarized in the table below.

TABLE-US-00002 Parameter Value Formulation 1 Viscosity [mPas] at 1000/s at 23° C. 33 Solids [%] 44 Nozzle diameter [μm] 100 Pressure [bar] 2.0 Resolution x/y [dpi] 50 Dry film thickness [μm] 36 Contact angle [°] 36 Visual assessment 1 Index K 3.1

[0076] The “index K” shown in the table was calculated according to the previously elucidated formula.

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

[0077] In the individual examples a to g the substrates having the above-described formulation 1 were coated using different coating methods. In the reported cases a subarea of the substrate was masked with adhesive tape. The hardening of all coatings was carried out at 140° C. for 30 minutes. The results of the visual edge assessment are documented in the table which follows. Corresponding micrographs of topviews of the boundary between the coating and the substrate (shown in the figures as the darker edge) are documented in FIG. 1a to FIG. 1g.

TABLE-US-00003 Doctor Digital blade printing application Spray gun application (inventive) Flash-off 10 2 10 2 2 2 10 time [min] Masking — — Yes Yes Yes — — Removal of — — After After spray After — — masking flash- application hardening off FIG. 1 . . . a b c d e f g Assessment 5 5 4 3 4 1 1