PROCESS FOR PROVIDING LOW GLOSS COATINGS

Abstract

The present invention relates to a process for producing a coating from an aqueous, radiation-curable coating composition, wherein the process comprises the steps in the sequence (1) to (4): (1) applying an aqueous, radiation-curable coating composition on a surface of a substrate, (2) drying the aqueous, radiation-curable coating composition, affording an at least partially dried coating composition, (3) irradiating the at least partially dried coating composition with UV light having a wavelength s 220 nm under inert atmosphere, followed by (4) irradiating with UV light having a wavelength 300 nm or with E-beam, wherein the aqueous, radiation-curable coating composition is a dispersion comprising: (A) at least one water-dispersible polyurethane (A), wherein the polyurethane (A) has a urea group (NHCONH) concentration of at least 0.1 milli-equivalents per g of polyurethane (A) and of at most 2.6 milli-equivalents per g of polyurethane (A) and the polyurethane (A) is essentially free of radiation-curable, ethylenically unsaturated bonds, (B) at least one radiation-curable diluent (B) with a molar mass less than 750 g/mol and with an acrylate functionality of from 2 to 5, and (C) water and optionally organic solvent, whereby the optional organic solvent is present in an amount of at most 30 wt. %, based on the total amount of water and organic solvent, wherein the amount of (A) is from 30 to 85 wt. % and the amount of (B) is from 15 to 70 wt. %, based on the total amount of (A) and (B).

Claims

1. A process for producing a coating from an aqueous, radiation-curable coating composition, wherein the process comprises the steps in the sequence (1) to (4): (1) applying an aqueous, radiation-curable coating composition on a surface of a substrate, (2) drying the aqueous, radiation-curable coating composition, affording an at least partially dried coating composition, (3) irradiating the at least partially dried coating composition with UV light having a wavelength 220 nm under inert atmosphere, followed by (4) irradiating with UV light having a wavelength 300 nm or with E-beam, wherein the aqueous, radiation-curable coating composition is a dispersion comprising: (A) at least one water-dispersible polyurethane (A), wherein the polyurethane (A) has a urea group (NHCONH) concentration of at least 0.1 milli-equivalents per g of polyurethane (A) and of at most 2.6 milli-equivalents per g of polyurethane (A) and the polyurethane (A) is essentially free of radiation-curable, ethylenically unsaturated bonds, (B) at least one radiation-curable diluent (B) with a molar mass less than 750 g/mol and with an acrylate functionality of from 2 to 5, and (C) water and optionally organic solvent, whereby the optional organic solvent is present in an amount of at most 30 wt. %, based on the total amount of water and organic solvent, wherein the amount of (A) is from 30 to 85 wt. % and the amount of (B) is from 15 to 70 wt. %, based on the total amount of (A) and (B).

2. The process of claim 1, wherein the polyurethane (A) has a urea group content of at most 1.3 meq per g of (A) and of at least 0.4 meq per g of (A).

3. The process of claim 1, wherein (A) and (B) together have a z-average molecular weight M.sub.z of at least 25,000 g/mol, whereby the z-average molecular weight M.sub.z is determined as described in the description.

4. The process of claim 1, wherein at least one of the radiation-curable diluents (B) has an acrylate functionality of from 2 to 3.

5. The process of claim 1, wherein the aqueous, radiation-curable coating composition comprises at least two radiation-curable diluents (B) and the radiation-curable diluents (B) present in the aqueous, radiation-curable coating composition have an average acrylate functionality according to formula $\stackrel{\_}{f}=\frac{{.Math.}_k\frac{w_k}{M_k}{f}_k}{{.Math.}_k\frac{w_k}{M_k}}$ of from 2 to 3, in which w.sub.k is the amount of acrylate diluents (B) in g present in the aqueous, radiation curable coating composition with a molar mass M.sub.k and with an acrylate functionality f.sub.k.

6. The process of claim 1, wherein 100 wt. % of the radiation-curable diluents (B) is selected from the group consisting of: di(trimethylolpropane) tetra-acrylate (di-TMPTA) with the corresponding molecular formula C.sub.24H.sub.34O.sub.9 and its corresponding molar mass of 467 g/mol, di(trimethylolpropane) tri-acrylate (di-TMP3A) with the corresponding molecular formula C.sub.21H.sub.32O.sub.8 and its corresponding molar mass of 412 g/mol, di(trimethylolpropane) tri-acrylate comprising alkoxy groups, glyceryl propoxy triacrylate (GPTA) with the corresponding molecular formula C.sub.21H.sub.32O.sub.9 and its corresponding molar mass of 428 g/mol, glyceryl propoxy triacrylate comprising additional alkoxy groups, pentaerythritol tetra-acrylate (PET4A) with the corresponding molecular formula C.sub.17H.sub.20O.sub.8 and its corresponding molar mass of 352 g/mol, pentaerythritol tetra-acrylate comprising alkoxy groups, pentaerythritol tri-acrylate (PET3A) with the corresponding molecular formula C.sub.14H.sub.18O.sub.7 and its corresponding molar mass of 298 g/mol, pentaerythritol tri-acrylate comprising alkoxy groups, trimethylolpropane triacrylate (TMPTA) with the corresponding molecular formula C.sub.15H.sub.20O.sub.6 and its corresponding molar mass of 296 g/mol, trimethylolpropane triacrylate comprising alkoxy groups, dipropyleneglycol diacrylate (DPGDA) with the corresponding molecular formula C.sub.12H.sub.18O.sub.5 and its corresponding molar mass of 242 g/mol, dipropyleneglycol diacrylate comprising additional alkoxy groups, and any mixture thereof.

7. The process according of claim 1, wherein 100 wt. % of the radiation-curable diluents (B) is a mixture of (1) one or more of di(trimethylolpropane) tetra-acrylate (di-TMPTA), pentaerythritol tetra-acrylate (PET4A) with the corresponding molecular formula C.sub.17H.sub.20O.sub.8 and its corresponding molar mass of 352 g/mol and/er pentaerythritol tetra-acrylate comprising alkoxy groups, and (2) one or more of glyceryl propoxy triacrylate (GPTA), glyceryl propoxy triacrylate comprising additional alkoxy groups, dipropyleneglycol diacrylate (DPGDA) and dipropyleneglycol diacrylate comprising additional alkoxy groups.

8. The process of claim 1, wherein the amount of (A) is from 40 to 80 wt. % and the amount of (B) is from 20 to 60 wt. %, based on the total amount of (A) and (B).

9. The process of claim 1, wherein the summed amount of (A) and (B) is from 25 to 35 wt. %, based on the entire weight of the aqueous, radiation-curable coating composition.

10. The process of claim 1, wherein the organic solvent is present in an amount of at most 1 wt. %, wherein the amount of organic solvent is given based on the total amount of water and organic solvent present in the aqueous, radiation-curable coating composition.

11. The process of claim 1, wherein the polyurethane (A) is the reaction product of at least the following components: (A1) At least one polyisocyanate, (A2) At least one isocyanate-reactive compound that contains at least one salt group which is capable to render the polyurethane (A) dispersible in water and/or at least one functional group that can be converted into a salt group which is capable to render the polyurethane (A) dispersible in water, (A3) Optionally at least one isocyanate-reactive compound containing at least one non-ionic group which is capable to render the polyurethane (A) dispersible in water, (A4) At least one isocyanate-reactive polyol other than (A2) and (A3) having an OH number of from 25 to 225 mg KOH/g solids, (A5) Optionally at least one isocyanate-reactive polyol other than (A2) and (A3) having an OH number higher than 225 mg KOH/g solids and lower than 1850 mg KOH/g solids, and (A6) Water and/or at least one nitrogen containing chain extender compound.

12. The process of claim 11, wherein the isocyanate-reactive polyol(s) (A4) has an OH number of from 45 to 125 mg KOH/g solids.

13. The process of claim 11, wherein the polyurethane (A) comprises the isocyanate-reactive polyols (A4) in an amount of from 35 to 65 wt. %, based on the total amounts of compounds (A1) to (A6).

14. The process of claim 1, wherein the polyurethane (A) comprises at least one nitrogen containing chain extender with a NH.sub.x (wherein x is 1 or 2) functionality of 2.

15. The process of claim 1, wherein the polyurethane (A) has an acid value of from 15 to 30 mg KOH/g solids of the polyurethane (A).

16. The process of claim 1, wherein the polyurethane (A) has a radiation-curable, ethylenically unsaturated bond concentration of less than 0.01 meq per g of polyurethane (A).

17. The process of claim 1, wherein the aqueous, radiation-curable coating composition is a dispersion consisting of (A), (B) and (C) having a z-average particle size is from 20 to 1000 nm, whereby the z-average particle size is determined with the method as described in the description.

18. The process of claim 1, wherein the coating has a dry thickness of at least 2 micron, and of at most 100 micron.

19. The process of claim 1, wherein the irradiating in step (3) is effected by excimer UV lamps with UV light having a wavelength 120 nm.

20. The process of claim 1, wherein UV irradiation is applied in step (4) and the aqueous, radiation-curable coating composition comprises one or more photo-initiators.

21-22. (canceled)

Description

[0239] The gloss and the coffee, red wine and mustard resistances of the cured coatings were determined as described above. The measured gloss values are reported in Table 4 and 6. The coffee, red wine and mustard resistances are reported in Tables 4 and 5.

TABLE-US-00008 TABLE 4 Gloss values measured of the formulation not containing matting agent and cured with Excimer/UV cure; gloss values of the formulation containing matting agent and cured with the conventional UV cure process under atmospheric conditions; stain resistances measured colorimetric of the formulation not containing matting agent and cured with Excimer/UV cure and stain resistances measured colorimetric of the formulation containing matting agent and cured with the conventional UV cure process under atmospheric conditions (Conv UV atmosph) resp. cured with the conventional UV cure process under inert conditions (Conv UV inert) Examples Comparative Experiments Comparative Experiments Formulation without matting Formulation with matting Formulation with matting agent (formulations 1a-7a) agent(formulations 1b-7b) agent(formulations 1b-7b) Excimer/UV cure Conv UV Atmosp Conv UV Inert Gloss b a Gloss b a b a (GU20/ Coffee Red Wine (GU20/ Coffee Red Wine Coffee Red Wine GU60) 6 hrs 6 hrs GU60) 6 hrs 6 hrs 6 hrs 6 hrs PUD 0.1/1.1 3.5 0 0.2/2.9 13.7 13.8 15.1 7.5 Ex. 1 PUD 0.6/5.6 3.4 0.3 0.4/4.7 13.1 5.9 11.3 1.4 Ex. 2 PUD 0.3/3 8.7 0.6 0.3/3.5 16.3 12.2 13.3 5.9 Ex. 3 PUD 0/0.6 3.5 0.1 0.8/8.6 16.1 12.6 14.4 5.3 Ex. 4 PUD 0.1/0.8 9.2 0.1 0.5/5.6 20.0 7.5 17.8 8 Ex. 5 PUD 0.3/2 8.7 0.2 0.6/6.8 16.1 12.3 15.2 8.7 Ex. 6 PUD 0.3/4 9.8 0 0.6/6.1 17.9 6.2 15.5 5.5 Ex. 7

[0240] FIG. 1 shows the coated testcard obtained by the Excimer/UV curing process according to the process of the invention of the formulation 4a containing the PUD of Example 4 without matting agent (upper testcard) and the coated testcard obtained by conventional UV curing under atmospheric conditions of the formulation 4b containing the PUD of Example 4 with matting agent (bottom testcard);

[0241] from left to right: Coffee [6 hrs], Red Wine [6 hrs] and Coffee [1 hr] stains.

TABLE-US-00009 TABLE 5 Stain resistances measured colorimetric (b for coffee and mustard and a for red wine) of the formulation without matting agent and cured with Excimer/UV cure. b Coffee b Coffee a Red Wine b Mustard Formulation 1 hr 6 hrs 6 hr 6 hrs without matting Excimer/ Excimer/ Excimer/ Excimer/ agent UV cure UV cure UV cure UV cure Formulation 1a PUD Ex. 1 1.2 3.5 0 2 Formulation 2a PUD Ex. 2 1 3.4 0.3 1.2 Formulation 3a PUD Ex. 3 4.6 8.7 0.6 7.6 Formulation 4a PUD Ex. 4 0.8 3.5 0.1 4.1 Formulation 5a PUD Ex. 5 3.9 9.2 0.1 6.4 Formulation 6a PUD Ex. 6 3.1 8.7 0.2 5.9 Formulation 7a PUD Ex. 7 3 9.8 0 4.1

TABLE-US-00010 TABLE 6 Gloss values measured after combined excimer/UV cure and conventional UV cure under atmospheric conditions of the formulation without matting agent. Conventional UV Cure Excimer/UV cure atmospheric GU 20 GU 60 GU 20 GU 60 Formulation 1a PUD Ex. 1 0.1 1.1 60 86 Formulation 2a PUD Ex. 2 0.6 5.6 69 90 Formulation 3a PUD Ex. 3 0.3 3 70 85 Formulation 4a PUD Ex. 4 0 0.6 64 85 Formulation 5a PUD Ex. 5 0.1 0.8 66 85 Formulation 6a PUD Ex. 6 0.3 2 52 84 Formulation 7a PUD Ex. 7 0.3 4 59 88 Formulation 8 PUD CEx. 1 61 86 60 86 Formulation 9 PUD CEx. 2 67 90 61 86 Formulation 10 PUD CEx. 3 65 88 64 87 Formulation 11 PUD CEx. 4 47 82 52 81 Formulation 12 PUD CEX. 5 69 90 72 89 Formulation 13 PUD CEX. 6 66 88 68 88
Coating formulation 4c was applied on a Leneta card (2C Leneta Inc) using a 125 m wire rod applicator. The coated card was dried for 10 minutes in an oven with airspeed of 1.2 m/s at 50 C. Subsequently the so-obtained dried composition was cured according to the cure conditions described above except for the radiation dose of the Excirad 172 lamp which was 4.0 mJ/cm.sup.2 instead of 11.4 mJ/cm.sup.2. After 1 hour, a second layer of coating formulation 4c was applied on the first coating layer using a 125 m wire rod applicator without further treatment of the 1.sup.st layer (e.g. sanding), and the same drying and curing conditions were applied.

[0242] After the second layer was cured, the intercoat adhesion was tested by using a finger nail.

[0243] The fingernail was placed on the coating with a certain force as described below and moved over the coating over a distance of approximately 5 cm. When the 2.sup.nd coating layer easily peels from the 1.sup.st coating layer with a small/gentle force, the score is ; when the 2.sup.nd coating layer peels from the 1.sup.st coating layer when using a moderate force, the score is +/; when the 2.sup.nd coating layer does not peel from the 1.sup.st coating layer even when applying a high force, the score is +.

[0244] When the coating layers do not peeled with the fingernail test after applying a high force, the test is repeated with a stainless steel spatula with a non-curved blade width of 8 mm instead of the fingernail. The spatula is held at a 45 degree angle and is moved over the coating over a distance of approximately 5 cm with a certain force as described above for the fingernail test. The same score is used as for the finger nail test.

[0245] Table 7 reports the scores for the fingernail and spatula test and the measured gloss values for the cured coatings obtained from formulation 4c.

TABLE-US-00011 TABLE 7 Gloss Units Gloss Units Intercoat [20/60/85] [20/60/85] adhesion Fingernail Spatula 1.sup.st layer 2.sup.nd layer Formulation + + 0.1/0.8/6.2 0.1/10/15 4c

TABLE-US-00012 TABLE 8 Effect of layer thickness on gloss values Layer thickness wet Dry Film Gloss Unites Formulation film thickness [m] thickness [m] [20/60/85] 4a 125 35 0/0.6/3.7 4a 75 21 0.1/0.6/2.8 4a 50 14 0.1/0.6/3.2 4a 25 7 0.1/0.7/7.4 4a 125 - conventional 35 77/86/98 UV cure 4b 25 - matting agent, 7 No decent film conventional UV cure obtained

[0246] With the process of the invention, low gloss coatings can be obtained with a very low dry film thickness, even low gloss coatings with a dry film thickness of 7 micron could be obtained, while when the formulation contains matting agent (formulation 9b) and the formulation was cured with conventional UV curing, a decent film with such a low dry film thickness could not be obtained.

Comparative Experiments 7-13

[0247] A representative group of commercially available waterborne UV curable coating dispersions, as specified in Table 9, were applied on a Leneta card (2C Leneta Inc) using a 125 m wire rod applicator. The coated cards were dried for 10 minutes in an oven with airspeed of 1.2 m/s at 50 C. Subsequently the so-obtained dried composition was cured using Conventional UV cure atmospheric and Excimer/UV cure as described above. The measured gloss values are reported in Table 9. The commercially available waterborne UV curable coating compositions as indicated in Table 9 have a different composition than the aqueous, radiation-curable coating composition of the present invention.

TABLE-US-00013 TABLE 9 Gloss values measured after conventional UV cure under atmospheric conditions and combined Excimer/UV cure of a representative group of commercially available waterborne UV curable coating compositions Conventional UV Cure atmospheric Excimer/UV cure GU 20 GU 60 GU 85 GU 20 GU 60 GU 85 NeoRad UV14 70 89 97 72 90 97 NeoRad R-520 78 93 99 80 93 99 NeoRad R-540 66 90 96 70 90 96 Decovery SP-7100 71 92 98 65 91 97 Bayhydrol UV 2280 82 95 99 81 94 99 Bayhydrol UV 2282 29 64 69 30 66 72 Bayhydrol eco UV 2877 69 91 98 72 92 97

[0248] Table 9 illustrates that the representative group of commercially available, waterborne UV curable dispersions, when subjected to Excimer/UV cure, do not result in matt coatings.