Method for coating a substrate using a drop-on-demand inkjet printer

Abstract

The present invention relates to printing 2K paint formulations in a maskless manner using a valve jet printer.

Description

Technical Field

[0001] The present invention relates to a method for coating a substrate comprising the step of applying a coating composition dropwise from a print head onto the substrate, wherein the print head has a closable opening which allows or prevents the discharge of droplets of the coating composition under command from a control unit. Background Art

[0002] Industrial lacquers are often characterized by a combination of protective and decorative functions. In application areas such as the transport, furniture and flooring industries, industrial conventional lacquers can have a lacquer layer thickness of 0.01 to 0.2 mm, wherein a plurality of functional sublayers are often combined. These sublayers are usually: a primer as an adhesion promoter to the substrate, a basecoat as a layer providing the color, and a topcoat or clearcoat as the main protective layer.

[0003] There is a need for printing technology for efficiently applying industrial conventional paint formulations in the transportation, furniture and flooring industries to industrial substrates conventional in these industries.

[0004] EP3257590A1 discloses a device for maskless painting of aircraft surfaces, comprising a multi-axis robot with at least one paint applicator, wherein the applicator is configured to apply paint by means of a technique for discharging paint. The paint may be, for example, an exterior primer, a color paint, a covering paint, a decorative paint, a varnish, a functional paint or a protective paint. The technique may be an inkjet technique.

[0005] WO 2019 / 030267 A1 discloses a method for producing a three-dimensional object comprising at least a first and a second object section, wherein the first object section is generated by means of a 3D printing method directly on a second object section that is not produced by means of a 3D printing method, and wherein the first object section generated by means of the 3D printing method contains or consists of a polyurethane resin.

[0006] WO 2016 / 046134 A1 discloses a high viscosity valve jetting method, wherein a liquid is discharged through an opening in a valve jetting print head, the opening being defined with reference to certain geometric parameters, and the liquid having a discharge viscosity of 20 mPa s to 3000 mPa s.

[0007] WO 2019 / 109040 A1 describes a system for applying a coating composition to a substrate using an applicator with high application efficiency. The system includes an applicator with high application efficiency, which defines a nozzle. The coating composition includes a carrier and a binder. The coating composition has a viscosity of about 0.002 Pa*s to about 0.2 Pa*s, a pressure of about 838 kg / m 3 Up to about 1557kg / m 3The coating composition is preferably a coating composition having a density of about 0.015 N / m to about 0.05 N / m, a surface tension of about 0.0005 s to about 0.02 s, and a relaxation time of about 0.0005 s to about 0.02 s. The applicator with high application efficiency is configured to release the coating composition onto the substrate through the nozzle to form a paint layer. At least 80% of the droplets of the coating composition released from the applicator with high application efficiency contact the substrate.

[0008] US 7,927,669 B2 relates to a method for applying a coating, in particular a paint, to a surface using a device comprising a metering head, the metering head comprising at least one nozzle controllable by a control signal. The method comprises the steps of: - moving a substrate having a surface to be coated along the surface relative to the metering head and / or moving the metering head relative to the surface to be coated of the substrate, and - applying a fluid coating to the surface via the nozzle in response to at least one control signal generated by a processor. Summary of the invention

[0009] It is an object of the present invention to provide a digital printing method with which coating compositions - rather than inks - can be processed and with which well-defined painted edges can be achieved without masking of the substrate.

[0010] This object is achieved by the method of the present invention. Advantageous expansion schemes are given in the present disclosure. They can be combined arbitrarily, unless the opposite meaning is clearly seen from the context.

[0011] In a method for coating a substrate comprising the step of applying a coating composition dropwise from a nozzle onto the substrate, the nozzle has a closable opening which allows or prevents the discharge of droplets of the coating composition under command from a control unit.

[0012] It is provided that the coating composition is applied more than once on less than 30% of the surface area of ​​the substrate, a pressure of ≥1.2 bar to ≤3.5 bar acts on the coating composition when the opening of the nozzle is closed, and the opening of the nozzle has a diameter of ≥80 μm to ≤250 μm.

[0013] It is further provided that the coating composition comprises a reactive resin, a curing agent for the resin and a solvent, in which the resin and the curing agent are present in a total proportion of ≥20% by weight, based on the total weight of the coating composition, and the coating composition has a viscosity at 20° C. of ≥20 mPa s to ≤80 mPa s, measured at a shear rate of 1000 / s according to EN ISO 3219 / A3.

[0014] By means of the method according to the invention, individual droplets of a solvent-based two-component paint formulation (2K paint formulation) can be applied to a substrate, wherein the droplets merge on the substrate to form a continuous layer. After evaporation of the solvent, a continuous coating is then obtained on the substrate. Surprisingly, it has been found that within the parameters provided according to the invention, coatings that meet the quality requirements can be obtained efficiently and economically. For this purpose, standard formulations of paints can already be used. After application, the coating can be cured by conventional methods.

[0015] The coating method comprises a closable nozzle opening as a variant of the so-called drop-on-demand inkjet printing method, in particular as a coating method by means of a valve jet printer. The nozzle opening can be opened or closed, for example, by means of a magnetically actuated plunger. The frequency of opening and closing the nozzle opening again can preferably be 800 Hz to 4000 Hz.

[0016] Of course, a plurality of such nozzles may be used. They may be arranged together on a movable print head. The control unit is usually arranged in an apparatus for the method, such as a valve jet printer, and may for example comprise a raster image processor (RIP).

[0017] The method according to the invention is suitable for being carried out as a so-called single-pass method. This means that as little substrate surface area as possible is provided with the coating composition more than once. According to the invention, it is provided that the coating composition is applied more than once to less than 30% of the substrate surface area. Preferably, less than 20% of the surface area is to be printed, particularly preferably less than 10% of the surface area is to be printed. Ideally, each surface area area of ​​the substrate to be printed is printed only once with the coating composition, except for technically unavoidable overlaps and inaccuracies.

[0018] Compared to spraying methods for applying paint, the method according to the invention differs in particular in that larger droplets are ejected. Due to the method, the droplets are precisely positioned. In addition, significantly less solvent evaporates in the short flight path of the droplets between nozzle and substrate. The solvent still in the formulation can thus ensure that the viscosity of the formulation on the substrate is reduced so that a continuous film can be formed from the individual droplets.

[0019] As regards the machine parameters, the inventors have found that a pressure of ≥1.2 bar to ≤3.5 bar should act on the coating composition when the opening of the nozzle is closed. This can be regarded as the "working pressure" of the valve jet printer and is easier to determine than the pressure when the nozzle is open during material discharge. Preferred pressures are ≥1.2 bar to ≤3 bar, particularly preferably ≥1.4 bar to ≤2.5 bar.

[0020] The inventors have further found that the opening of the nozzle should have a diameter of ≥80 μm to ≤250 μm. A preferred diameter is ≥100 μm to ≤200 μm, particularly preferably ≥120 μm to ≤150 μm.

[0021] The type of substrate is not particularly limited. The substrate may be planar or curved. The method according to the invention can also process relatively large substrates, for example those with an area greater than 1 m2. 2 , greater than 10m 2 or more than 100m 2 Suitable materials may be aluminium, steel or polymers. The substrate may be part of a finished assembly such as a machine housing, or may be present as a semi-finished product such as a strip, coil or sheet.

[0022] The coating composition comprises a reactive resin, a curing agent for the resin and a solvent and can therefore be regarded as a 2K composition. These compositions are prepared only before application, since there is only a limited processing time due to the reaction of the resin with the curing agent. The typical reaction of the resin with the curing agent is a crosslinking reaction to form covalent bonds. Of course, the composition can also contain additional components, such as reactive diluents or additives. Therefore, solid-containing coating compositions can also be processed. The d90 value of the particle size distribution of these particles is preferably ≤5μm. The coating composition can be conceived as a primer, a color paint or a covering paint.

[0023] The total proportion of resin and curing agent of ≥ 20% by weight further distinguishes the composition from inkjet inks containing binders. In such inks, the proportion of binder is significantly lower. The total proportion of resin and curing agent is preferably ≥ 30% by weight, more preferably ≥ 40% by weight, based on the total weight of the coating composition.

[0024] The viscosity set according to the invention also distinguishes the composition from binder-containing inkjet inks; in the latter, it is significantly lower. Preferred viscosities are ≥25 mPa s to ≤70 mPa s, particularly preferably ≥30 mPa s to ≤65 mPa s.

[0025] In the method according to the invention it is further provided that the substrate does not have a temporarily placed mask on which the coating composition is applied. Therefore, the method is a maskless method. The inventors have found that the method does not require masking on the substrate, because the edge sharpness of the printed image achieved with the coating composition is already satisfactory. If, for example, colored stripes or text are to be applied to a substrate that has already been painted in the background color, this can lead to a considerable saving of time and costs.

[0026] The process can be carried out so that the desired wet paint layer thickness, based on the amount of paint used, is produced with a material yield of >90%, preferably >95%, very particularly preferably 98%, since there is no overspray and no significant evaporation of solvents or components on the way to the substrate.

[0027] The process can furthermore be carried out so that the desired dry paint film thickness is as low as 1 / 1.5 times, preferably 1 / 1.7 times and very particularly 1 / 2 times the wet paint film thickness.

[0028] The method can further be carried out so that the applied solvent-based lacquer loses less than 20%, preferably less than 10%, very particularly less than 5% of its solvent on the way to the substrate, so that the necessary solvent evaporation process can take place in a controlled manner on the substrate, thereby resulting in less environmental burden and improved surface quality compared to conventional spraying methods.

[0029] The lacquer layers obtainable by the process according to the invention can have a dry lacquer layer thickness (dry film thickness, DFT) of 0.015 to 0.080 mm, preferably 0.02 to 0.06 mm, very particularly preferably 0.025 to 0.05 mm after a single run (single printing pass).

[0030] In one embodiment, the following parameters are selected so that the index K calculated according to the following formula is ≥ 0.4 to ≤ 4

[0031]

[0032] in:

[0033] R: Spatial resolution of application of the coating composition, expressed in dots / 2.54 cm

[0034] p: pressure acting on the coating composition when the nozzle opening is closed, expressed in bar

[0035] d: diameter of the closable opening of the nozzle, expressed in μm

[0036] S: proportion of non-volatile components of the coating composition, expressed in wt % based on the total weight of the coating composition, according to ISO 3251, 120 minutes, 100°C

[0037] η: viscosity at 20° C., measured at a shear rate of 1000 / s in accordance with EN ISO 3219 / A3, expressed in mPa s. C: dynamic contact angle, determined in accordance with DIN EN ISO 19403-6 as the advancing angle of the coating composition on the substrate, expressed in degrees.

[0038] This empirically determined formula originates from a statistical analysis of test results carried out by the inventors and allows, when some parameters are known, an optimal adjustment of the remaining parameters. Thus, for example, given known machine parameters (resolution in dpi, pressure, nozzle diameter) and a solids content predetermined by the lacquer recipe, the viscosity or contact angle can be brought into the target range by adding solvent. For the calculation of the index K, the physical units are neglected. Only the values ​​of the parameters involved are important.

[0039] In another embodiment, the coating composition is applied dropwise to the substrate by a plurality of nozzles, and each nozzle has a closable opening that allows or prevents discharge of droplets of the coating composition under command from a control unit independently of other closable openings of other nozzles.

[0040] In another embodiment, the spatial resolution of the discharge of the coating composition is ≥30 dots / 2.54 cm to ≤150 dots / 2.54 cm. This is a description of the resolution conventionally expressed in dots / inch (dpi) in the printing industry. A preferred resolution is ≥40 dots / 2.54 cm to ≤130 dots, particularly preferably ≥50 dots / 2.54 cm to ≤120 dots / 2.54 cm.

[0041] In another embodiment, in the coating composition, the solvent is selected from: water, n-hexane, isohexane, cyclohexane, n-heptane, isoheptane, n-octane, isooctane, testbenzin, xylene, solvent naphtha, propanol, n-butanol, isobutanol, butyl glycol, butyl diethylene glycol, ethylene glycol, diethylene glycol, butyl acetate, ethyl acetate, 2-butoxyethyl acetate, 1-methoxy-2-propyl acetate, butanone, acetone, 2-heptanone, 2,4-pentanedione, 2-pentanone, ethyl-3-ethoxy propionate, 1,2,4-trimethylbenzene, 4-methylpentan-2-one or a mixture of at least two of the above solvents.

[0042] In another embodiment, the coating composition has a pot life of ≥30 minutes to ≤480 minutes, wherein the pot life is defined as the time until the viscosity doubles according to DIN EN ISO 3219 / A at 23° C. The pot life is preferably ≥60 minutes to ≤240 minutes.

[0043] In another embodiment, the resin is an epoxy resin and the curing agent is a polymerization catalyst, a primary amine, a cyclic anhydride, a polyphenol, a thiol, or a mixture of at least two of the foregoing compounds.

[0044] Epoxy resins and epoxy curing agents form epoxy resin adhesives as a reactive mixture, which cure by polyaddition reaction. During curing, a high molecular weight three-dimensional network is formed by crosslinking reactions from the usually low-viscosity or low-molecular-weight monomers and oligomeric components of the adhesive. The network nodes are formed by the reaction of the functional groups of the resin with the curing agent. Epoxy resins that come into consideration include, in particular, those based on glycidyl ethers, glycidyl esters, glycidyl amines, alicyclic epoxides and glycidyl isocyanurate.

[0045] Examples of curing agents are 1,3-diaminobenzene, diethylenetriamine, 4,4'-methylenebis(cyclohexylamine) and hexahydrophthalic anhydride.

[0046] In another embodiment, the resin is a polyol, a polyamine, an amino alcohol, or a mixture of at least two of the foregoing compounds, and the curing agent is a blocked or unblocked polyisocyanate.

[0047] Suitable polyols are the polyether polyols, polyester polyols, polycarbonate polyols, polyesteramide polyols, polyamide polyols and epoxy resin polyols known from polyurethane chemistry, and their reaction products with CO2 and polyacrylate polyols.

[0048] Examples of polyamines are 3-amino-1-methylaminopropane, 3-amino-1-ethylaminopropane, 3-amino-1-cyclohexylaminopropane or 3-amino-1-methylaminobutane. Examples of amino alcohols are N-aminoethylethanolamine, ethanolamine, 3-aminopropanol, neopentanolamine or diethanolamine.

[0049] Examples of polyisocyanates are polyisocyanates or polyisocyanate mixtures having exclusively aliphatically and / or cycloaliphatically bonded isocyanate groups having an (average) NCO functionality of 2.0 to 5.0 and a viscosity of 10 to 2000 mPas at 23° C. Suitable polyisocyanates are in particular those based on isophorone diisocyanate, hexamethylene diisocyanate, bis(4-isocyanatocyclohexyl)methane and ω,ω′-diisocyanato-1,3-dimethylcyclohexane (H6XDI).

[0050] The diisocyanates listed can optionally be used as such, or derivatives of the diisocyanates can be used. Suitable derivatives include polyisocyanates containing biuret, isocyanurate, uretdione, urethane, iminooxadiazinedione, oxadiazinetriaone, carbodiimide, acylurea or allophanate groups. Particularly preferred are monomer-poor paint polyisocyanates having structural units selected from isophorone diisocyanate (IPDI), hexamethylene diisocyanate (HDI), 1,4-diisocyanatocyclohexane or bis(4-isocyanatocyclohexyl)methane.

[0051] The polyisocyanate component may optionally be hydrophilically modified. Water-soluble or water-dispersible polyisocyanates are obtainable, for example, by modification with carboxylate groups, sulfonate groups and / or polyethylene oxide groups and / or polyethylene oxide / polypropylene oxide groups.

[0052] Suitable blocking agents for polyisocyanates are, for example, monoalcohols, such as oximes, for example acetone oxime, methyl ethyl ketone oxime, cyclohexanone oxime, lactams, for example ε-caprolactam, phenols, amines, for example diisopropylamine or dibutylamine, dimethylpyrazole or triazole, and also dimethyl malonate, diethyl malonate or dibutyl malonate.

[0053] Particularly preferred are coating compositions comprising isocyanurates of 1,6-HDI ("HDI trimer") and polyacrylate polyols. Butyl acetate (BA), 1-methoxy-2-propyl acetate (MPA), solvent naphtha and mixtures thereof can be used for viscosity adjustment. Silicone additives can be added to influence the surface activity.

[0054] In a further embodiment, in the coating composition, the NCO index is ≧0.8 to ≦1.5, preferably ≧0.9 to ≦1.3, particularly preferably ≧1 to ≦1.2.

[0055] In another embodiment, the coating composition is adapted to the substrate so that the dynamic contact angle measured as the advancing angle of the coating composition on the substrate according to DIN EN ISO 19403-6 is ≥30 ° to ≤90 °. This can be achieved by varying the solvent content or by suitable additives. Outside these values, isolation of the droplets on the substrate or a reduction in the edge depiction accuracy in the printed image may result. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] exist Figure 1a to Figure 1g A corresponding micrograph of a top view of the boundary between the coating and the substrate (shown as the darker area in the figure) is recorded in . DETAILED DESCRIPTION

[0057] Example

[0058] The invention is explained in more detail by the following examples and figures, but is not limited thereto.

[0059] method

[0060] Visual Assessment:

[0061] The edge quality between the coated and uncoated substrates was visually assessed on a scale of 1 to 5, with 1 being the best value, which represents: 1: sharp edge, 2: sharp edge with slight fluctuations, 3: sharp edge with moderate fluctuations, 4: irregular edge and 5: double edge.

[0062] Viscosity:

[0063] The viscosity was determined in accordance with DIN EN ISO 3219 / A3 using an MCR301 rheometer from Anton Paar. Anton Paar measuring cup: cylinder geometry, measuring cup diameter 28.92 mm, cylinder diameter 26.66 mm, annular gap length 40 mm, measuring temperature 20° C., shear rate from 1 to 1500 1 / s in 55×5s=275s and from 1500 1 / s to 1 in 275s.

[0064] Material

[0065] Component A:

[0066] Acrylate-containing polyols DA 665 (BA / X) from Allnex Resins Germany GmbH or Allnex Netherlands BV.

[0067] BYK-Chemie (10% in methoxypropyl acetate) is a silicone-containing surface additive for solvent-based, solvent-free and water-based paints and printing inks.

[0068] Component B:

[0069] Polyisocyanate: HDI trimer DESMODUR ULTRA N 3390BA / SN (NCO functionality>3) from Covestro AG was used, with an NCO content of 19.6% by weight (according to ISO 11909) and a non-volatile content of 90% (according to ISO 3251, 120 min, 100° C.). The viscosity at 23° C. was about 550 mPa·s (DIN EN ISO 3219 / A3).

[0070] The solvent used was butyl acetate (BA).

[0071] Aluminum plates were used as substrates. They were cleaned with ethyl acetate beforehand.

[0072] Unless otherwise stated, the starting materials were used without further purification or pretreatment.

[0073] Preparations

[0074] To produce component A, the acrylate-containing polyol is diluted with BA and uniformly mixed with the surface additive. The curing agent component is used with the polyisocyanate in an NCO:OH ratio of 1.0. The solids content of the mixture is about 50% by weight and is optionally further diluted. At a solids content of 50%, the application viscosity at 23° C. (ISO cup 5 mm DIN EN ISO 2431) is about 30 s.

[0075]

[0076]

[0077] Digital printing parameters

[0078] The digital printing experiments were carried out with a tabletop printer of the ChromoJET CHR-TT 118 model from Zimmer Austria. Ten minutes after mixing, the premixed paint formulation was loaded into a 300 mL printing can. A pressure controller was used to establish a working pressure of 2 bar. The system was initially rinsed several times with the formulation and then printed on a cleaned substrate. The printing surface area was 130*70 mm. The conveying speed (Schlittengeschwindigkeit) was 0.6 m / s. A print head with 8 nozzle groups was used, wherein the nozzle diameter was 100 μm. After printing was completed, the system was fully rinsed with ethyl acetate.

[0079] The printing conditions are summarized in the table below.

[0080] parameter value Preparations 1 Viscosity at 1000 / s at 23°C [mPas] 33 solid[%] 44 Nozzle diameter [μm] 100 Pressure [bar] 2.0 Resolution x / y[dpi] 50 Dry layer thickness [μm] 36 Contact angle [°] 36 Visual Assessment 1 Eigenvalue K 3.1 .

[0081] The "characteristic value K" shown in the table is calculated according to the formula already explained previously.

[0082]

[0083] In each of Examples a to g, a substrate was coated with Formulation 1 described above using different coating methods. In the cases shown, a sub-area of ​​the substrate was masked with tape. Curing of all coatings was carried out at 140° C. for 30 minutes. The results of the visual edge evaluation are reported in the table below. The corresponding micrographs of the top view of the boundary between the coating and the substrate (shown as the darker area in the figure) are reported in Figure 1a to Figure 1g middle.

[0084]

Claims

1. A method for coating a substrate, comprising the step of applying a coating composition dropwise from a nozzle onto the substrate, wherein the nozzle has a closable opening which, under command from a control unit, allows or prevents the discharge of droplets of the coating composition, Features applying the coating composition more than once over less than 30% of the surface area of ​​the substrate, When the opening of the nozzle is closed, a pressure of ≥1.2 bar to ≤3.5 bar acts on the coating composition, The opening of the nozzle has a diameter of ≥80 μm to ≤250 μm, The coating composition comprises a resin, a curing agent for the resin and a solvent, In the coating composition, the reactive resin and the curing agent are present in a total amount of ≥ 20% by weight, based on the total weight of the coating composition, The coating composition has a viscosity at 20° C. measured at a shear rate of 1000 / s according to EN ISO 3219 / A3 of ≥20 mPa·s to ≤80 mPa·s, The substrate does not have a temporarily placed mask to which the coating composition is applied, and, The following parameters are selected so that the calculated eigenvalue K is ≥ 0.4 to ≤ 4 according to the following formula: in: R: Spatial resolution of application of the coating composition, expressed in dots / 2.54 cm p: pressure acting on the coating composition when the nozzle opening is closed, expressed in bar d: diameter of the closable opening of the nozzle, expressed in μm S: proportion of non-volatile components of the coating composition, expressed in wt % based on the total weight of the coating composition, according to ISO 3251, 120 minutes, 100°C η: viscosity at 20° C., measured at a shear rate of 1000 / s in accordance with EN ISO 3219 / A3, expressed in mPa·s C: dynamic contact angle, determined in accordance with DIN EN ISO 19403-6 as the advancing angle of the coating composition on the substrate, expressed in degrees.

2. A method as claimed in claim 1, wherein the coating composition is applied dropwise to the substrate by a plurality of nozzles, and wherein each nozzle has a closable opening which allows or prevents the discharge of droplets of the coating composition under command from a control unit independently of other closable openings of other nozzles.

3. The method as claimed in claim 1 or 2, wherein the coating composition is applied with a spatial resolution of ≥30 dots / 2.54 cm to ≤150 dots / 2.54 cm.

4. The method of claim 1 or 2, wherein in the coating composition, the solvent is selected from: water, n-hexane, isohexane, cyclohexane, n-heptane, isoheptane, n-octane, isooctane, solvent oil, xylene, solvent naphtha, propanol, n-butanol, isobutanol, butyl glycol, butyl diethylene glycol, ethylene glycol, diethylene glycol, butyl acetate, ethyl acetate, 2-butoxyethyl acetate, 1-methoxy-2-propyl acetate, butanone, acetone, 2-heptanone, 2,4-pentanedione, 2-pentanone, ethyl-3-ethoxy propionate, 1,2,4-trimethylbenzene, 4-methylpentan-2-one or a mixture of at least two of the above solvents.

5. The method as claimed in claim 1 or 2, wherein the coating composition has a pot life of ≥30 minutes to ≤480 minutes, wherein the pot life is defined as the time until the viscosity measured at 23°C doubles according to DIN EN ISO 3219 / A.

6. The method of claim 1 or 2, wherein the resin is an epoxy resin, and the curing agent is a polymerization catalyst, a primary amine, a cyclic anhydride, a polyphenol, a thiol, or a mixture of at least two of the above curing agents.

7. The method of claim 1 or 2, wherein the resin is a polyol, a polyamine, an amino alcohol or a mixture of at least two of the above resins, and the curing agent is a blocked or unblocked polyisocyanate.

8. The method as claimed in claim 7, wherein in the coating composition, the NCO index is ≥ 0.8 to ≤ 1.

5.

9. The method according to claim 1 or 2, wherein the coating composition is adapted to the substrate such that the dynamic contact angle, determined as the advancing angle of the coating composition on the substrate in accordance with DIN EN ISO 19403-6, is ≥30° to ≤90°.

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