Method for coating a substrate with a drop-on-demand printer
The invention relates to a 2K coating formulation that is printed without a mask using a valve jet printer.
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·s to ≤80 mPa.·s,
and wherein the substrate comprises no temporarily applied mask onto which the coating composition is applied,
wherein the following parameters are chosen such that according to the following formula the calculated index K is ≥0.4 to ≤4;
K
=
(
R
5
0
)
2
(
p
2
)
2.5
(
d
120
)
2
(
S
4
5
)
(
4
5
C
)
2
(
η
50
)
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·s
C: dynamic contact angle determined as advancing angle according to DIN EN ISO 19403-6 of coating composition on substrate in degrees.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. The process as claimed in claim 7 , wherein in the coating composition the NCO index is ≥0.8 to ≤1.5.
9. 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°.