Method for coating a surface of an electrically non-conductive substrate with powder coatings

Abstract

A method for coating a surface of an electrically non-conductive substrate with powder coatings, the method comprising the following steps: providing a substrate to be coated, pre-heating the substrate to be coated to a temperature of 40 to 140 C. in order to decrease the surface resistance of the substrate to less than 10.sup.12 ohms, preferably to within the range of 10.sup.10 to less than 10.sup.12 ohms, electrostatically coating the surface with powder coating in a single layer, which powder coating comprises a reactive system which, in particular, cures into a thermoset, curing the powder coating layer at a temperature of 170 C. or less.

Claims

1. A method of coating a surface of an electrically nonconducting substrate with powder coatings, comprising the steps of: providing a substrate to be coated, preheating the substrate to be coated to a temperature of 40 to 140 C. to lower the surface resistance of the substrate to less than 10.sup.12 ohms, single-layer, electrostatic coating of the surface with a powder coating which comprises a reactive system, and curing the powder coating at a temperature of not more than 170 C., wherein the surface to be coated is pretreated, prior to coating, via an increase in the surface tension to a level of >40 mN/m.

2. The method according to claim 1, wherein said preheating lowers the surface resistance of the substrate in the range from 10.sup.10 to less than 10.sup.12 ohms.

3. The method according to claim 1, wherein said reactive system is one curing to a thermoset.

4. The method according to claim 1, wherein the preheating takes place over a timespan of 5 sec to 90 min.

5. The method according to claim 1, wherein the substrate is preheated in an oven.

6. The method according to claim 1, wherein the pretreatment is a process selected from the group consisting of: machining, flaming, and plasma treatment.

7. The method according to claim 1, wherein the pretreatment of the surface takes place before and/or after the reheating.

8. The method according to claim 1, wherein the step of coating with powder coating takes place by a process selected from the group consisting of: spraying, fluidized-bed sintering, electrostatic fluidizing-bed technology and/or with transfer application, and combinations thereof.

9. The method according to claim 1, wherein A the step of curing the powder coating takes place by a process selected from the group consisting of: baking in a convection oven at temperatures in a range from 80 C. to 170 C. over a timespan of 1 min to 60 min, infrared irradiation (IR), irradiation by near infrared radiation (NIR), irradiation and curing by ultraviolet radiation (UV), and a combinations thereof.

10. The method according to claim 1, wherein no chemical adhesion promoters are used.

11. The method according to claim 1, wherein solvent-free powder coatings are used.

12. The method according to claim 1, wherein the powder coatings are selected from the group comprising: polyester powder coatings, epoxy powder coatings, polyester-epoxy powder coatings, polyurethane powder coatings, acrylate powder coatings, fluoropolymer powder coatings, and polyamide powder coatings.

13. The method according to claim 1, wherein the powder coatings are low-temperature powder coatings or ultra low-temperature powder coatings.

14. The method according to claim 1, wherein the powder coating comprises electrostatic additives.

15. The method according to claim 1, wherein the substrate comprises a material selected from the group comprising: glass fiber-reinforced plastic (GRP), polyamides (PA), polyurethanes (PU), polyesters, medium-density wood fiberboard (MDF), high-density fiberboard (HDF), natural wood, chipboard, wood-plastic composite (WPC), carbon fiber-reinforced plastic (CRP), glass, and ceramic.

16. The method according to claim 1, wherein the substrate comprises a material having a fraction of glass fibers or of carbon fibers of between 20% to 80%.

17. The method according to claim 1, wherein adhesion of the powder coating, during coating, is mediated only by electrostatic effects without melting of the powder coating on the substrate.

18. A method of coating a surface of an electrically nonconducting substrate with powder coatings, according to claim 1, further comprising one or more electrostatic additives.

Description

(1) FIG. 1 diagram with the temperature and the surface resistance of a substrate relative to time.

(2) FIG. 2 substrate coatings in the case of characteristic surface resistances.

(3) FIG. 3 comparison of substrate coatings with powder coating comprising electrostatic additives.

(4) FIG. 1 shows a diagram which depicts the temperature profile and the surface resistance of a substrate as a function of time. The substrate was installed in a heating oven for 10 sec for heating and then removed from the oven. The temperature of the heating oven was constant at 130 C. Depending on the thickness of the substrate, it is heated to a temperature between the ambient temperature (outside the heating oven) and the temperature of the heating oven (cf. FIG. 1, at about 44 C.). The temperature and the surface resistance of the substrate were measured at room temperature during the cooling process. The measurement values plotted are those directly after removal from the heating oven for 2 min 30 sec.

(5) As a result of the preheating of the substrate, the surface resistance was lowered to a level of about 510.sup.10 ohms at a temperature of about 44 C. On storage at room temperature, the substrate cools down to 36 C. within a minute. With this falling temperature, the surface resistance increases from about 510.sup.10 ohms to about 710.sup.11 ohms within a minute. Within this range, complete and high-quality single-layer electrostatic coatings can be obtained, since under these conditions the substrate has a sufficiently conducting surface.

(6) FIG. 2 shows substrate coatings in the case of characteristic surface resistances. The substrates were preheated and then coated by electrostatic powder application. The powder coating applied was subsequently cured.

(7) FIG. 2A shows a coated substrate, the substrate having not been preheated, and having a surface resistance of >10.sup.12 ohms. Following electrostatic powder application, the substrate is inadequately coated. The white areas feature powder of the coating, while in the black areas the substrate is visible without coating. A powder coating of the substrate with a surface resistance of >10.sup.12 ohms is incomplete and therefore inadequate.

(8) FIG. 2B shows a coated substrate, the substrate having undergone minimal preheating and thus having a surface resistance of 510.sup.11 ohms. Following electrostatic powder application, the substrate is almost completely coated in comparison to FIG. 2A, but nevertheless has black (uncoated) areas.

(9) FIG. 2C shows an inventively coated substrate which after preheating had a surface resistance of 10.sup.11 ohms. Under these conditions, electrostatic powder application results in a homogeneous, smooth and continuous coating on the substrate. The coating corresponds to a coating of the kind known for metals and conductive substrates.

(10) FIG. 3 shows coated components where the coatings took place under identical conditions. Component 1 has a coating of powder coating, the powder coating comprising electrostatic additives. In comparison, the coating of component 2 does not comprise any electrostatic additives. Especially at the narrow facing sides, the advantageous properties mediated by electrostatic additives are illustrated. In contrast, to component 2, component 1 has an advantageous coating. Coating modifications with electrostatic additives additionally improve the coating outcome.

(11) Further aspects of the invention relate to: A. A method for coating a surface of an electrically nonconducting substrate with powder coatings, comprising the steps of: providing a substrate to be coated, preheating the substrate to be coated, coating the surface with powder coating, curing the powder coating layer. B. The method according to A, characterized in that the preheating takes place at a temperature between 100 C. and 220 C., more particularly between 110 C. and 190 C., preferably between 120 C. and 170 C. and over a timespan of 5 min to 2 h, more particularly of 20 min to 90 min, preferably of 40 min to 75 mm. C. The method according to A or B, characterized in that prior to coating, the surface to be coated is pretreated, more particularly via an increase in the surface tension to a level of >40 mN/m, preferably of >60 mN/m, more preferably of >70 mN/m. D. The method according to C, characterized in that the pretreatment is a process selected from the group; mechanically roughening, more particularly by abrading and/or blasting, flaming, plasma treatment.

(12) E. The method according to C or E, characterized in that the pretreatment of the surface takes place before and/or after the preheating.

(13) F. The method according to any of aspects A to E, characterized in that the step of coating with powder coating takes place by means of a process selected from the following group: spraying, more particularly with a corona or tribe process, fluidized-bed sintering, electrostatic fluidizing-bed technology, in particular with transfer application, combination of the stated processes. G. The method according to any of aspects A to E, characterized in that the step of curing the powder coating layer takes place by means of a process selected from, the group: baking in a convection oven at temperatures in a range from 100 C. to 230 C., preferably from 110 C. to 190 C., more preferably from 130 C. to 160 C. over a timespan of 1 mm to 60 min, more particularly of 3 min to 40 min, preferably of 5 min to 20 min, infrared irradiation (IR), irradiation by near infrared radiation (NIR), irradiation and curing by ultraviolet radiation (UV), more particularly in combination with convective heat and/or IR radiation and/or NIR radiation for melting and/or curing the coating layer, combinations of the stated processes. H. The method according to any of aspects A to G, characterized in that no chemical adhesion promoters are used. I. The method according to any of aspects A to H, characterized in that solvent-free powder coatings are used. J. The method according to any of aspects A to I, characterized in that reactive systems are used as powder coatings. K. The method according to any of aspects A to J, characterized in that the powder coatings are selected from the group comprising: polyester powder coatings, epoxy powder coatings, polyester-epoxy powder coatings, polyurethane powder coatings, acrylate powder coatings, fluoropolymer powder coatings, polyamide powder coatings. L. The method according to any of aspects A to K, characterized in that the powder coatings are low-temperature powder coatings or ultra low-temperature powder coatings. M. The method according to any of aspects A to L, characterized in that the substrate comprises a material selected from the group comprising: glass fiber-reinforced plastic (GRP), polyamides (PA), polyurethanes (PU), polyesters, medium-density wood fiberboard (MDF), high-density fiberboard (HDF), natural wood, glass, ceramic. N. The method according to any of aspects A to M, characterized in that the substrate comprises a material having a fraction of glass fibers of between 20% to 80%, more particularly between 30% to 70%, preferably between 40% to 60%. O. The method according to any of aspects A to N, characterized in that before the substrate is coated, the substrate is at least partly degassed, more particularly substantially completely degassed, by thermal exposure. P. A substrate, more particularly a nonconducting substrate, having a coated surface produced by a method according to any of aspects A to O.