Method for coating metal surfaces of substrates, and objects coated according to said method

Abstract

A coating, a method for coating surfaces, and the coated surfaces. The method includes providing a substrate with a cleaned metal surface; contacting and coating the metal surface with an aqueous composition having a ph of from 0.5 to 7.0 and in the form of a dispersion and/or a suspension; optionally rinsing the organic coating; and drying and/or baking the organic coating, or optionally drying the organic coating and coating same with a similar or another coating composition thereto. The composition contains a complex fluoride in a quantity of 1.1 10.sup.6 mol/l to 0.30 mol/l based on the cations. An anionic polyelectrolyte in a quantity of 0.01 to 5.0 wt % based on the total mass of the resulting mixture is added to an anionically stabilized dispersion made of film-forming polymers and/or a suspension made of film-forming inorganic particles.

Claims

1. A method for coating metal surfaces of substrates, said method comprising: providing a substrate with a cleaned metal surface; contacting and coating the metal surface with an aqueous composition to form an organic coating, wherein the organic coating is based on an ionogenic gel which binds to cations dissolved out the metal surface, the cations originating from a pretreatment stage and/or from contacting and coating the metal surface with the aqueous composition to form the organic coating, wherein the aqueous composition is a dispersion or a suspension and has a pH value ranging from 0.5 to 7.0; optionally rinsing the organic coating; and drying or baking the organic coating, or optionally drying the organic coating and coating the organic coating with another coating composition and then drying or baking; wherein the aqueous composition comprises a complex fluoride and at least one anionic polyelectrolyte, wherein the complex fluoride is selected from the group consisting of hexa- or tetrafluorides of cations of titanium, zirconium, hafnium, silicon, aluminum, and/or boron in a quantity of 1.1 10.sup.6 mol/l to 0.30 mol/l based on the cations of the complex fluoride, wherein the aqueous composition comprises the at least one anionic polyelectrolyte in a quantity of 0.01 to 5.0 wt % based on a total mass of the aqueous composition, and wherein the dispersion comprises an anionically stabilized dispersion comprising film-forming polymers or wherein the suspension comprises film-forming inorganic particles, the dispersion and the suspension each include a solid content of 2 to 40 wt % and a mean particle size of 10 to 1,000 nm, the dispersion and the suspension being stable in the pH value range of 0.5 to 7.0, and the at least one anionic polyelectrolyte is added to the dispersion and/or suspension.

2. The method according to claim 1, wherein the complex fluoride is used in an amount of 1.1 10.sup.5 mol/l to 0.15 mol/l based on the cations of the complex fluoride, wherein the aqueous composition has a pH value ranging from 1.0 to 6.0.

3. The method according to claim 1, wherein the at least one anionic polyelectrolyte is selected from the group consisting of a) polysaccharides based on glycogens, amyloses, amylopectins, calloses, agar, algins, alginates, pectins, carrageenans, celluloses, chitins, chitosans, curdlans, dextrans, fructans, collagens, gellan gum, gum arabic, starches, xanthans, tragacanth, karayan gum, tara grain meal, and glucomannans, b) anionic polyelectrolytes of natural origin based on polyamino acids, collagens, polypeptides, and lignins and c) synthetic, anionic polyelectrolytes based on polyamino acids, polyacrylic acids, polyacrylic acid copolymers, acrylamide copolymers, lignins, polyvinylsulfonic acid, polycarboxylic acids, polyphosphoric acids and polystyrenes.

4. The method according to claim 1, wherein the at least one anionic polyelectrolyte comprises or consists of at least one polysaccharide based on pectins or gellan gum.

5. The method according to claim 1, wherein the aqueous composition and/or the organic coating produced therefrom contains a mixture of at least two different anionic poly electrolytes.

6. The method according to claim 5, wherein the aqueous composition and/or the organic coating produced therefrom contains a mixture of two pectins.

7. The method according to claim 1, wherein the aqueous composition and/or the organic coating produced therefrom contains at least one anionic polysaccharide selected from those having a degree of esterification of carboxyl functions in the range of 5 to 75% based on a total number of alcohol and carboxyl groups.

8. The method according to claim 1, wherein the aqueous composition comprises at least one anionic polysaccharide and/or at least one further anionic polyelectrolyte selected from those having a molecular weight in the range of 500 to 1,000,000 g/mol.

9. The method according to claim 1, wherein the aqueous composition and/or the organic coating produced therefrom contains at least one anionic polysaccharide and/or at least one further anionic polyelectrolyte selected from those having a degree of amidation of carboxyl functions in the range of 1 to 50%, and a degree of epoxidation of the carboxyl functions of up to 80%.

10. The method according to claim 1, wherein the aqueous composition and/or the organic coating produced therefrom comprises a content of at least one complexing agent for metal cations or a polymer that is modified so as to complex metal cations.

11. The method according to claim 10, wherein the aqueous composition and/or the organic coating produced therefrom comprises a content of at least one complexing agent selected from those based on maleic acid, alendronic acid, itaconic acid, citraconic acid, or mesaconic acid, or the anhydrides or semi-esters of these carboxylic acids.

12. The method according to claim 1, wherein the aqueous composition and/or the organic coating produced therefrom contains at least one kind of cation selected from those based on cationically active salts selected from the group consisting of melamine salts, nitroso salts, oxonium salts, ammonium salts, salts with quaternary nitrogen cations, salts of ammonium derivatives, and metal salts of Al, B, Ba, Ca, Cr, Co, Cu, Fe, Hf, In, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Pb, Sn, Ta, Ti, V, W, Zn and/or Zr.

13. The method according to claim 12, wherein cations of Al, Cu, Fe, Mg, Ca, and/or Zn are selected as the cations that are/have been dissolved out from the metal surface and/or that are/have been added to the aqueous composition.

14. The method according to claim 1, wherein the aqueous composition and/or the organic coating produced therefrom has a content of organic particles based on polyacrylates, polyurethanes, polyepoxides, and/or hybrids thereof.

15. The method according to claim 1, wherein the aqueous composition and/or the organic coating produced therefrom has a content of at least one emulsifier.

16. The method according to claim 15, wherein the aqueous composition and/or the organic coating produced therefrom has a content of at least one emulsifier selected from those based on anionic emulsifiers.

17. The method according to claim 1, wherein the aqueous composition and/or the organic coating produced therefrom contains at least one additive selected from the group consisting of biocides, dispersing agents, film-forming auxiliary agents, acidic and/or basic agents for adjusting the pH, thickeners, and leveling agents.

18. The method according to claim 1, wherein the aqueous composition forms a coating based on an ionogenic gel having a thickness of at least 1 m.

19. The method according to claim 1, wherein the organic coating is formed in a dipping bath in 0.05 to 20 minutes and has a dry film thickness in the range of 5 to 100 m after drying.

20. The method according to claim 1 wherein the substrate has a problem when coating with an electrodeposition coating.

Description

EXAMPLE 1

(1) The substrate 1 was mixed with a mixture of 0.25% wt % based on the total amount of the resulting mixture with a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 52%, a degree of epoxidation of 0%, and a galacturonic acid content of 87%, and 0.25% wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 10%, a degree of epoxidation of 0%, and a galacturonic acid content of 85%, with 99.5 wt % of the previously described dispersion A. 2.0 g/L of 20% hexafluorozirconic acid was added to the mixture. A dry film thickness of 55 to 65 m was measured, as determined with an eddy current meter and SEM.

EXAMPLE 2

(2) Experiment 1 was repeated with a substrate 2, and a dry film thickness of 15 to 25 m was determined with SEM.

EXAMPLE 3

(3) Experiment 1 was repeated with a substrate 3, and a dry film thickness of 3 to 4 m was determined with SEM.

EXAMPLE 4

(4) The substrate 1 was mixed with a mixture of 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 52%, a degree of epoxidation of 0%, and a galacturonic acid content of 87%, and 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 10%, a degree of epoxidation of 0%, and a galacturonic acid content of 85%, with 99.5 wt % of the previously described dispersion A. 4.0 g/L of 20% hexafluorozirconic acid was added to the mixture. A dry film thickness of 63 to 67 m was measured, as determined with an eddy current meter and SEM.

EXAMPLE 5

(5) Experiment 4 was repeated with a substrate 2, and a dry film thickness of 10 to 20 m was determined with SEM.

EXAMPLE 6

(6) Experiment 4 was repeated with a substrate 3, and a dry film thickness of 4 to 5 m was determined with SEM.

EXAMPLE 7

(7) The substrate 1 was mixed with a mixture of 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 52%, a degree of epoxidation of 0%, and a galacturonic acid content of 87%, and 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 10%, a degree of epoxidation of 0%, and a galacturonic acid content of 85%, with 99.5 wt % of the previously described dispersion A. 6.0 g/L of 20% hexafluorozirconic acid was added to the mixture. A dry film thickness of 70 to 85 m was measured, as determined with an eddy current meter and SEM.

EXAMPLE 8

(8) Experiment 7 was repeated with a substrate 2, and a dry film thickness of 5 to 7 m was determined with SEM.

EXAMPLE 9

(9) Experiment 7 was repeated with a substrate 3, and a dry film thickness of 5 to 6 m was determined with SEM.

EXAMPLE 10

(10) The substrate 2 was mixed with a mixture of 0.25% wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 52%, a degree of epoxidation of 0%, and a galacturonic acid content of 87%, and 0.25% wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 10%, a degree of epoxidation of 0%, and a galacturonic acid content of 85%, with 99.5 wt % of the previously described dispersion A. 8.0 g/L of 20% hexafluorozirconic acid was added to the mixture. A dry film thickness of 5 to 10 m was measured, as determined with an eddy current meter and SEM.

EXAMPLE 11

(11) Experiment 10 was repeated with a substrate 3, and a dry film thickness of 7 to 8 m was determined with SEM.

EXAMPLE 12

(12) The substrate 3 was mixed with a mixture of 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 52%, a degree of epoxidation of 0%, and a galacturonic acid content of 87%, and 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 10%, a degree of epoxidation of 0%, and a galacturonic acid content of 85%, with 99.5 wt % of the previously described dispersion A. 10.0 g/L of 20% hexafluorozirconic acid was added to the mixture. A dry film thickness of 8 to 9 m was measured, as determined with an eddy current meter and SEM.

EXAMPLE 13

(13) The substrate 3 was mixed with a mixture of 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 52%, a degree of epoxidation of 0%, and a galacturonic acid content of 87%, and 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 10%, a degree of epoxidation of 0%, and a galacturonic acid content of 85%, with 99.5 wt % of the previously described dispersion A. 14.0 g/L of 20% hexafluorozirconic acid was added to the mixture. A dry film thickness of 16 to 21 m was measured, as determined with an eddy current meter and SEM.

EXAMPLE 14

(14) The substrate 3 was mixed with a mixture of 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 52%, a degree of epoxidation of 0%, and a galacturonic acid content of 87%, and 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 10%, a degree of epoxidation of 0%, and a galacturonic acid content of 85%, with 99.5 wt % of the previously described dispersion A. 24.0 g/L of 20% hexafluorozirconic acid was added to the mixture. A dry film thickness of 20 to 22 m was measured, as determined with an eddy current meter and SEM.

EXAMPLE 15

(15) The substrate 3 was mixed with a mixture of 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 52%, a degree of epoxidation of 0%, and a galacturonic acid content of 87%, and 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 10%, a degree of epoxidation of 0%, and a galacturonic acid content of 85%, with 99.5 wt % of the previously described dispersion A. 44.0 g/L of 20% hexafluorozirconic acid was added to the mixture. A dry film thickness of 24 m was measured, as determined with an eddy current meter and SEM.

EXAMPLE 16

(16) The substrate 1 was mixed with 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 52%, a degree of epoxidation of 0%, and a galacturonic acid content of 87%, and 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 10%, a degree of epoxidation of 0%, and a galacturonic acid content of 85%, with 99.5 wt % of the dispersion A. 1.0 g/L of 20% hexafluorotitanic acid was added to the mixture. A dry film thickness of 52 to 55 m was measured, as determined with an eddy current meter and SEM.

EXAMPLE 17

(17) Experiment 16 was repeated with a substrate 2, and a dry film thickness of 18 to 24 m was determined with SEM.

EXAMPLE 18

(18) Experiment 16 was repeated with a substrate 3, and a dry film thickness of 6 to 7 m was determined with SEM.

EXAMPLE 19

(19) The substrate 1 was mixed with a mixture of 25% wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 52%, a degree of epoxidation of 0%, and a galacturonic acid content of 87%, and 0.25% wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 10%, a degree of epoxidation of 0%, and a galacturonic acid content of 85%, with 99.5 wt % of the dispersion A. 2.0 g/L of 20% hexafluorotitanic acid was added to the mixture. A dry film thickness of 60 to 70 m was measured, as determined with an eddy current meter and SEM.

EXAMPLE 20

(20) Experiment 19 was repeated with a substrate 2, and a dry film thickness of 20 to 22 m was determined with SEM.

EXAMPLE 21

(21) Experiment 19 was repeated with a substrate 3, and a dry film thickness of 8 to 9 m was determined with SEM.

EXAMPLE 22

(22) The substrate 1 was mixed with a mixture of 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 52%, a degree of epoxidation of 0%, and a galacturonic acid content of 87%, and 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 10%, a degree of epoxidation of 0%, and a galacturonic acid content of 85%, with 99.5 wt % of the dispersion A. 4.0 g/L of 20% hexafluorotitanic acid was added to the mixture. A dry film thickness of 67 to 73 m was measured, as determined with an eddy current meter and SEM.

EXAMPLE 23

(23) Experiment 22 was repeated with a substrate 2, and a dry film thickness of 6 to 11 m was determined with SEM.

EXAMPLE 24

(24) Experiment 22 was repeated with a substrate 3, and a dry film thickness of 8 to 10 m was determined with SEM.

EXAMPLE 25

(25) The substrate 1 was mixed with a mixture of 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 52%, a degree of epoxidation of 0%, and a galacturonic acid content of 87%, and 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 10%, a degree of epoxidation of 0%, and a galacturonic acid content of 85%, with 99.5 wt % of the dispersion A. 6.0 g/L of 20% hexafluorotitanic acid was added to the mixture. A dry film thickness of 70 to 90 m was measured, as determined with an eddy current meter and SEM.

EXAMPLE 26

(26) Experiment 25 was repeated with a substrate 2, and a dry film thickness of 6 to 12 m was determined with SEM.

EXAMPLE 27

(27) Experiment 25 was repeated with a substrate 3, and a dry film thickness of 7 to 9 m was determined with SEM.

EXAMPLE 28

(28) The substrate 3 was mixed with a mixture of 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 52%, a degree of epoxidation of 0%, and a galacturonic acid content of 87%, and 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 10%, a degree of epoxidation of 0%, and a galacturonic acid content of 85%, with 99.5 wt % of the above dispersion A. 8.0 g/L of 20% hexafluorotitanic acid was added to the mixture. A dry film thickness of 8 to 11 m was measured, as determined with an eddy current meter and SEM.

EXAMPLE 29

(29) The substrate 3 was mixed with a mixture of 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 52%, a degree of epoxidation of 0%, and a galacturonic acid content of 87%, and 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 10%, a degree of epoxidation of 0%, and a galacturonic acid content of 85%, with 99.5 wt % of the dispersion A. 10.0 g/L of 20% hexafluorotitanic acid was added to the mixture. A dry film thickness of 8 to 12 m was measured, as determined with an eddy current meter and SEM.

EXAMPLE 30

(30) The substrate 3 was mixed with a mixture of 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 52%, a degree of epoxidation of 0%, and a galacturonic acid content of 87%, and 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 10%, a degree of epoxidation of 0%, and a galacturonic acid content of 85%, with 99.5 wt % of the dispersion A. 14.0 g/L of 20% hexafluorotitanic acid was added to the mixture. A dry film thickness of 9 to 11 m was measured, as determined with an eddy current meter and SEM.

EXAMPLE 31

(31) The substrate 3 was mixed with a mixture of 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 52%, a degree of epoxidation of 0%, and a galacturonic acid content of 87%, and 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 10%, a degree of epoxidation of 0%, and a galacturonic acid content of 85%, with 99.5 wt % of the above dispersion A. 24.0 g/L of 20% hexafluorotitanic acid was added to the mixture. A dry film thickness of 12 to 17 m was measured, as determined with an eddy current meter and SEM.

EXAMPLE 32

(32) The substrate 3 was mixed with a mixture of 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 52%, a degree of epoxidation of 0%, and a galacturonic acid content of 87%, and 0.25 wt %, based on the total amount of the resulting mixture, of a pectin having a molecular weight of about 70,000 g/mol, a degree of amidation of 0%, a degree of esterification of 10%, a degree of epoxidation of 0%, and a galacturonic acid content of 85%, with 99.5 wt % of the above dispersion A. 44.0 g/L of 20% hexafluorotitanic acid was added to the mixture. A dry film thickness of 16 to 24 m was measured, as determined with an eddy current meter and SEM.

EXAMPLE 33

(33) The substrate 1 was mixed with a mixture of 0.5 wt %, based on the total amount of the resulting mixture, of a chitosan having a degree of diacetylation between 65% and 85% dissolved in 1% acetic acid, and with 99.5 wt % of the dispersion A. 2.8 g/L of 20% hexafluorozirconic acid was added to the mixture. A dry film thickness of 4 to 6 m was measured, as determined with an eddy current meter and SEM.

EXAMPLE 34

(34) The substrate 1 was mixed with a mixture of 0.5% wt %, based on the total amount of the resulting mixture, of a chitosan having a degree of diacetylation between 75% and 85% dissolved in 1% acetic acid, and with 99.5 wt % of the dispersion B. 2.4 g/L of 20% hexafluorozirconic acid was added to the mixture. A dry film thickness of 45 to 50 m was measured, as determined with an eddy current meter and SEM.

EXAMPLE 35

(35) Experiment 35 was repeated with a substrate 3, and a dry film thickness of 3 to 4 m was determined with SEM.

EXAMPLE 33

(36) The substrate 1 was mixed with a mixture of 0.5 wt %, based on the total amount of the resulting mixture, of a gellan gum having a molecular weight of about 70,000 g/mol and a low acyl content with 99.5 wt % of the above dispersion A. 2.0 g/L of 20% hexafluorozirconic acid was added to the mixture. A dry film thickness of 5 to 6 m was measured, as determined with an eddy current meter and SEM.

EXAMPLE 34

(37) Experiment 33 was repeated with a substrate 2, and a dry film thickness of 7 to 8 m was determined with SEM.

EXAMPLE 35

(38) Experiment 33 was repeated with a substrate 3, and a dry film thickness of 7 to 8 m was determined with SEM.

COMPARATIVE EXAMPLE 1

(39) The substrate 1 was coated with the dispersion A. A dry film thickness was not determined by SEM.

COMPARATIVE EXAMPLE 2

(40) The substrate 2 was coated with the dispersion A. A dry film thickness was not determined by SEM.

COMPARATIVE EXAMPLE 3

(41) The substrate 3 was coated with the dispersion A. A dry film thickness was not determined by SEM.

COMPARATIVE EXAMPLE 4

(42) The coating of the substrate 1 with the polyelectrolytes referred to in the description of the invention, without mixture with the dispersion A, resulted in a dry film thickness of 300 to 500 nm.

COMPARATIVE EXAMPLE 5

(43) The coating of the substrate 2 with the polyelectrolytes referred to in the description of the invention, without mixture with the dispersion A, resulted in a dry film thickness of 300 to 500 nm.

COMPARATIVE EXAMPLE 6

(44) The coating of the substrate 3 with the polyelectrolytes referred to in the description of the invention, without mixture with the dispersion A, resulted in a dry film thickness of 300 to 500 nm.

(45) The microscope images consistently show a homogeneous layer formation, indicating a reliable, self-regulating, and readily controllable coating method.