Process for production of a metal substrate containing a protective coating

Abstract

The present invention relates to a process for the production of metal substrates containing a protective coating, especially substrates based on iron, like steel, galvanized steel, or electrogalvanized steel, generally used in construction and in the automotive industry.

Description

Aqueous Primer Compositions

Ingredients:

Aliphatic urethane-acrylate resins dispersed in an aqueous phase:

• • resin, marketed by UCB under the commercial name Ucecoat DW7770®, and
  • resin, marketed by Synthopol Chemie under the commercial name Syntholuc DRB 1577®.

Acrylic resins dispersed in the aqueous phase:

• • resin, marketed by UCB under the commercial name Duroxyn Vax®, and
  • resin, marketed by Clariant under the commercial name Mowilith LDM 7170®.

Photoinitiator:

• • alpha, alpha-dimethoxy-alpha-hydroxyacetophenone, marketed by Ciba Geigy under the commercial name Darocur 1173®.

Inhibitor pigments:

• • calcium silicate, marketed by the GRACE Division under the commercial name Shieldex C303®,
  • aluminum triphosphate, marketed by Safic Alcan under the commercial name Kwhite KTC 720®kk,
  • zinc salt of an organic acid, marketed by Cognis under the commercial name Alcophor 827®.

Fluorinated surfactant:

• • solution of polyether modified with polysiloxane, marketed by Byk Chemie under the commercial name Byk 346®.

Thickener:

• • polyurethane thickener, marketed by Schwegmann under the commercial name Schwego pur 8050®.

Anti-foaming agent:

• • mixture of polysiloxane and hydrophobic particles, marketed by Byk Chemie under the commercial name BYK 028®.

Finishing composition:

• • hydroxylated polyester resin (in organic solvents)

Composition of Primer 1

Chemical nature    Amount (parts by weight)
Ucecoat DW 7770 ®  80
Duroxyn Vax ®      20
Shieldex C303 ®    10
Kwhite KTC720 ®    5
Alcophor 827 ®     0.5
Byk 346 ®          0.2
Schwego pur 8050 ® 0.3
Irgacure 500 ®     1

Composition of Primer 2:

Chemical nature    Amount (parts by weight)
Ucecoat DW 7770 ®  80
Duroxyn Vax ®      20
Shieldex C303 ®    10
Kwhite KTC720 ®    5
Alcophor 827 ®     0.5
Byk 346 ®          0.2
Schwego pur 8050 ® 0.3
Irgacure 500 ®     1

2. Measurement Methods Salt Spray Test

The anticorrosion performances of the samples are determined with the salt spray test. After deposition of the protective coating, the coated steel sheets are subjected to the salt spray test, carried out according to Standard NF X 41-002. This test consists of spraying of a 5% by weight aqueous solution of NaCl, kept at a temperature of 35° C., in the form of a mist. The sheets are regularly examined at different stages (at 24 hours, 72 hours, 144 hours and 150 hours) of exposure to the salt spray. The presence of formed zinc salts is evaluated visually according to the following scale:

• • −: no zinc salts
  • +: traces of zinc salts
  • ++: formation of zinc salts (non-negligible)+
  • +++: abundant formation of zinc salts Solvent Resistance Test

After application of the primer, and after application of a finishing coat, the resistance of the coated sheets to a solvent like methyl ethyl ketone is evaluated.

For this purpose, the following procedure is used:

• • cotton is dipped in methyl ethyl ketone,
  • the impregnated cotton is then applied to the surface of the sheet with a back-and-forth movement, and
  • the number of back-and-forth movements (or passes) carried out on the appearance of softening and detachment of the protective coating from the sheet is determined.

The higher the number of passes, the greater the degree of protection of the substrate, due to the coating.

Adherence Test

The degree of adherence of the protective coating to the surface of the metal substrate is determined as follows, according to Standard ISO 2409:

• • incisions are made, forming a square grid pattern on the surface of the protective coating (10×10 on a 1.5 cm² surface), then
  • adhesive tape with a width at least equal to the ruled surface, carrying the reference 595TR1966, furnished by the 3M company, is attached to the incised surface,
  • during peeling of the adhesive tape, the adhesive surface is observed visually to determine the presence or absence of coating flakes. ERICHSEN Test

This test consists of deformation to 90% break of the coated substrate. This deformation is done on the back of the coated substrate.

EXAMPLE 1

Three galvanized steel sheets are degreased by immersion for 5 to 10 seconds in a bath containing 3.75% by weight of Chemetall Parco 305E®, relative to the total weight of the bath. The pH of the bath is 13 to 14 and the temperature 50° C. to 60° C.

The sheets are then rinsed with cold demineralized water, then dried in a drying cabinet at 125° C. for 4 minutes.

The degreased, rinsed and dried sheets are then subjected to activation treatment by immersion in a bath of composition AFZ1 for one minute. The plates are then dried in a drying cabinet for 5 minutes at 125° C.

In the five minutes following drying, the primer composition 1 is then applied, thus forming a moist primer film on the surface of the sheets about 10-12 μm thick.

The sheets coated with moist primer film are subjected to heat treatment (“flash off”) for 2 minute at 125° C., then crosslinking via radiation with UV radiation. A crosslinked coating with a thickness of about 5-6 μm is obtained.

The salt spray test is conducted on the sheets, along with the adherence test, the solvent resistance test and the “ERICHSEN” test.

The coated sheets are then subjected to finishing treatment. This consists of applying a finishing composition of the polyester/melamine type, conventionally used in the paint industry. This finishing composition is applied by means of a laboratory rule, then crosslinked by firing at 240° C. for 30 s.

After finishing treatment, the solvent resistance test, adherence test and “ERICHSEN” test are performed again.

The results obtained in these different tests are summarized in Table 1.

COMPARATIVE EXAMPLE C1

As a comparison relative to example 1, the same protective coating as described in example 1 is applied to the galvanized steel sheets, proceeding in the same manner as described in example 1, except for the immersion step in the bath of composition AFZ 1 based on fluorozirconic acid, which was omitted.

In the same manner as in example 1, the salt spray test is carried out on the sheets so coated (only before application of the finishing coat), along with the adherence test, the solvent resistance test and the “ERICHSEN” test.

The results obtained in these different tests are also summarized in Table 1.

	TABLE 1
				“ERICHSEN” test
		Solvent	Adherence	of deformation
	Salt spray test	resistance test	test	resistance
Example 1	without	No oxidation	90	2	0
	finishing
	with		>100	0	0-1
	finishing
Comparative	without	Oxidation	90	2	3
example 1	finishing
	with		>100	5	1
	finishing

Comparison of the results of the salt spray test of examples 1 and C1 shows the presence of incipient oxidation when the specimen was not subjected to initial activation treatment (example C1). This is illustrated by examples 1 to 3, in different stages of exposure to the salt spray (at 24 hours, 72 hours and 144 hours, respectively). These figures show that the sheets of comparative example 1 have white longitudinal traces (called “bleeding”) that correspond to the formation of zinc chloride and zinc hydroxide (white rust), whereas the sheets of example 1 do not show them. The resistance to salt spray is therefore better when the sheets have been subjected to activation treatment with fluorozirconic acid prior to deposition of the primer composition.

In addition, the adherence and behavior in the “ERICHSEN” deformation test are significantly better with initial activation treatment. In this case (example 1), no delamination of the coating is observed. This is illustrated in FIG. 4. The sheets of FIG. 4 each show a lower part and an upper part: the lower part contains the primer film and the finishing coat, whereas the upper part contains only the primer film. FIG. 4 shows that at the deformation site, there is no longer any coating for the sheet of comparative example 1, both for the upper part (with finishing coating), or for the lower part of the sheet (without finishing coating). On the other hand, in the case of example 1, the coating persists (for the lower part and upper part with the finishing coat). As a result, activation treatment with fluorozirconic acid improves the adherence of the protective coating to the metal substrate.

EXAMPLE 2

Galvanized steel sheets are degreased by immersion for 5 seconds in a bath containing 3.25% by weight of Chemetall Parco 305E®, relative to the total weight of the bath. The pH of the bath is 13 to 14 and the temperature 50° C.

The sheets are then rinsed with cold demineralized water, then dried at 125° C. for 4 minutes.

The degreased, rinsed and dried sheets are then subjected to activation treatment by immersion in a bath of composition AFZ1 for 1 minute.

The plates are then dried in a drying cabinet for five minutes at 125° C.

In the five minutes following drying, primer composition 2 is then applied, thus forming a primer film on the surface of the sheet, having a thickness of about 10-12 μm.

The sheets so coated with primer film are then subjected to heat treatment (“flash off”) for 2 minutes at 125° C., then crosslinking by irradiation with UV radiation (one pass at 10 m/min under two emitters of the RPC 80 W type under nitrogen).

The salt spray test, adherence test (“ruling”), the solvent resistance test (A/R MEK) and the “ERICHSEN” of deformation resistance are then carried out on the sheets so coated.

The coated sheets with the primer film are then finally subjected to the same finishing treatment as in example 1.

A new solvent resistance test, adherence test and RXN “ERICHSEN” test of deformation resistance are then carried out.

The results obtained in the different tests are shown in Table 2.

COMPARATIVE EXAMPLE 2

As comparison relative to example 2, the same protective coating as described in example 2 is applied to the galvanized steel sheets, proceeding in the same manner as described in example 2, except for the immersion step in the bath of composition AFZ1 based on fluorozirconic acid, which was omitted.

The results of the different tests are also shown in Table 2, as for example 2¹. ¹Translator's note: sic—perhaps the French should have read “as for example 1” because the reference is to the way results were shown in the prior table, which showed the results of example 1.

	TABLE 2
				“ERICHSEN” test
		Solvent	Adherence	of deformation
	Salt spray test	resistance test	test	resistance
Example 2	without	No corrosion	15	0	0
	finishing
	with	X	>100	0	1-2
	finishing
Comparative	without	Traces of	17	0	0
example C2	finishing	white rust
	with	X	90	1	2
	finishing

Comparison of the results of the salt spray tests of examples 2 and C2 is illustrated by FIGS. 5 and 6. These figures show that the sheets of comparative example 2 have white longitudinal traces (called “bleeding”) after 140 hours in a salt spray, whereas the sheets of example 2 do not show them. The resistance to salt spray is therefore better when the sheets have been subjected to activation treatment with fluorozirconic acid prior to deposition of the primer composition.

Moreover, adherence and behavior in the deformation test (“ERICHSEN” test) of the sheets coated with a finishing coat are significantly better, when the substrate has been subjected to activation treatment.

EXAMPLE 3

Galvanized steel sheets are degreased by immersion for 5 seconds in a bath containing 3.75% by weight of Chemetall Parco 305E®, relative to the total weight of the bath. The pH of the bath is 13 to 14 and the temperature 50° C.

In the same manner as in examples 1, 2, C1 and C2, the sheets are then rinsed with cold demineralized water, then dried at 125° C. for 4 minutes.

The degreased, rinsed and dried sheets are then subjected to activation treatment by immersion in a bath of composition AFZ3 for 2 minutes.

The sheets are then dried in a drying cabinet (“flash off”) for 2 minutes at 1 25° C.

In the five minutes following drying, the primer composition 1 is then applied, thus forming on the surface of the sheet a wet primer film, having a thickness of about 10-12 μm. The sheets coated with primer are then subjected to the same finishing treatment as for example 1.

FIG. 7 shows the state of the coated sheets of example 3, after having been subjected to the salt spray test for 150 hours.

COMPARATIVE EXAMPLE 3

As a comparison relative to example 3, the same protective coating as described in example 3 is applied to the galvanized steel sheets, proceeding in the same manner as described in example 3, except for the immersion step in the bath of composition AFZ3 based on fluorozirconic acid, nitric acid and cationic resin, which was omitted.

FIG. 8 shows the state of the coated sheets of example 3, after having been subjected to the salt spray test for 150 hours.

The comparison of FIGS. 7 and 9 shows that the sheets of example 3 have significantly fewer white traces (“bleeding”) than those of comparative example 3, indicating that prior activation treatment imparts better corrosion protection to the primer film.

EXAMPLE 4

Electrogalvanized steel sheets are degreased by immersion for 5 seconds in a bath containing 3.75% by weight of Chemetall Parco 305E, relative to the total weight of the bath. As for the preceding examples, the pH is 13 to 14, and the bath temperature 50° C.

The electrogalvanized steel sheets, in the same manner as desribed in the preceding examples, are rinsed with cold demineralized water, then dried at 125° C. for 4 minutes.

The degreased, rinsed and dried sheets are then subjected to activation treatment by immersion in the bath at ambient temperature of composition AFZ2 for one minute.

The sheets are then dried in a drying cabinet (“flash off”) for 2 minutes at 125° C.

In the five minutes following drying, the primer composition 1 is then applied, thus a film forming on the surface of the sheet with a wet thickness of about 10-12 μm. The sheets coated with primer are then subjected to the same finishing treatment as for example 1.

COMPARATIVE EXAMPLE 4

As comparison, relative to example 4, the same protective coating as in described in example 4 is applied to the electrogalvanized steel sheets, proceeding in the same manner as described in example 4, except for the immersion step in the bath of composition AFZ2 based on fluorozirconic acid, which was omitted. The comparison of the results of the salt spray test (after exposure of 150 hours to salt spray) of examples 4 and C4 is illustrated in FIG. 9. This figure shows that the corrosion behavior is significantly better when prior activation treatment occurs with a composition based on the metal fluoroacid.

EXAMPLE 5

Electrogalvanized steel sheets are degreased by immersion for 5 seconds in a bath containing 3.75% by weight of Chemetall Parco 305E®, relative to the total weight of the bath. As for the preceding examples, the pH is 13 to 14, and the temperature of the bath is 50° C.

The electrogalvanized steel sheets, in the same manner as the preceding examples, are rinsed with cold demineralized water, then dried at 125° C. for 4 minutes.

The degreased, rinsed and dried sheets are then subjected to activation treatment by application with a roll coater in a bath at ambient temperature of composition AFZ2 for one minute.

The sheets are then dried in a drying cabinet (“flash off”) for 2 minutes at 125° C.

In the 5 minutes following drying, the following primer composition is then applied, thus forming a wet primer film on the surface of the sheet, having a thickness of about 6 μm:

Rom UV: SM1530/140B
Duroxyne VZX6127           18
Water                      4.3
Anti-foaming agent byk 028 0.1
TEA                        1
Schieldex C303             3.6
K wite ktc 720             1.6
Ucecoat DW7770             72
Byk 346                    0.3
Irgacure 500               1
Schewgo pur                0.4

The sheets so coated then receive a film with 20 μm dry thickness of the anticorrosion primer W780-9735 from PPG by cathode electrodeposition.

A corrosion test VDA 621-41, an alkaline degreasing test and an Erichsen test are then carried out on the sheets.

COMPARATIVE EXAMPLE C5

For comparison, relative to example 5, the same protective coating as described in example 5 is applied to the electrogalvanized steel sheets, proceeding in the same manner as described in example 5, except for the immersion step in the bath of composition AFZ2 based on fluorozirconic acid, which was omitted.

Corrosion Behavior VDA 621-415

		Example C5	Example 5
	Red rust	30%	0-1%
	White rust	60%	15%
	Degreasing behavior	Complete softening	Good behavior
		Example 5	Example C5
	Erichsen adherence	3-4	0.1

Comparison of the results shows that activation (example 5) imparts significantly better corrosion behavior, alkaline degreasing behavior and adherence to the substrate.

This can be observed in FIGS. 9 and 10.

Claims

1. A process for the production of a metal substrate, containing a protective coating, said substrate being based on iron, like steel, galvanized steel or electrogalvanized steel, or based on aluminum or its alloys, said process comprising the following steps: (a) degreasing of the substrate with an alkaline solution, this step being optional when treatment is carried out directly on a continuous galvanizing or electrogalvanizing line; (b) rinsing with water, preferably demineralized; (c) activation of the metal substrate with an activation composition based on at least one metal fluoroacid; (d) drying of the metal substrate; (e) application of an aqueous primer composition capable of being polymerized by irradiation with UV radiation or by electron bombardment, to form film of aqueous primer composition at least on 1 μm thick, preferably 3 to 5 μm thick, said primer composition containing: a polymer binder, containing: 40 to 85 % by weight, relative to the total weight of said primer composition, of at least one unsaturated polymer resin, containing at least one acrylate group and capable of being polymerized by irradiation with UV radiation or by electron bombardment; 5 to 50 % by weight, relative to the total weight of said primer composition, of at least one saturated polymer resin not containing an acrylate group; and in the case, where polymerization is performed by irradiation with UV radiation, 1 to 10 % by weight, relative to the weight of the said primer composition, of at least one photoinitiator; a pigment paste, containing: a grinding resin; 1 to 20 % by weight, relative to the total weight of said primer composition, of at least one corrosion inhibitor pigment; and optionally, at least one filler; and water, preferably demineralized, as dispersion medium of said primer composition, (f) heat treatment to eliminate the residual water of the film of aqueous primer composition, and (g) crosslinking of the film of aqueous primer composition to obtain a primer film, having a thickness of at least 1 μm, and preferably 3 to 5 pm, after crosslinking.

2. The process according to claim 1, characterized by the fact that degreasing step (a) of the substrate is carried out with an alkaline solution with a pH equal to or greater than 12, preferably about 13.

3. The process according to claim 1 or 2, characterized by the fact that the alkaline solution used in degreasing step (a) of the substrate is a solution based on potassium hydroxide or sodium hydroxide.

4. The process according to any of the claims 1 to 3, characterized by the fact that the degreasing step (a) of the substrate is carried out by immersion in a bath of said alkaline solution, whose temperature ranges from55 to 60° C.

5. The process according to any of the preceding claims, characterized by.the fact that the metal fluoroacid of the activation composition of step (c) is chosen from fluorozirconic acid, fluorotitanic acid and fluorosilicic acid.

6. The process according to claim 5, characterized by the fact that the fluoroacid is fluorozirconic acid.

7. The process according to claim 6, characterized by the fact that the fluorozirconic acid is present in an amount of 0.1 g/L to 20 g/L, relative to the weight of the activation composition.

8. The process according to claim 6 or 7, characterized by the fact that the fluorozirconic acid is associated with a resin having an epoxide structure, preferably a resin derived from polyepoxides, especially bisphenol A.

9. The process according to any of the preceding claims, characterized by the fact that the unsaturated polymer resin, containing at least one acrylate group of the primer composition of step e), is a resin, chosen from epoxy-acrylates, urethane-acrylates, polyester-acrylates, polyester-urethane-acrylates, ether acrylates, amine acrylates or their mixtures.

10. The process according to claim 9, characterized by the fact that the unsaturated polymer resin, containing at least one acrylate group, is a urethane-acrylate resin.

11. The process according to one of the preceding claims, characterized by the fact that the saturated polymer resin not containing an acrylate group of the primer composition of step e) is chosen among dispersions or emulsions of acrylic, acrylic-styrene, acrylic-urethane and polyester resin.

12. The process according to any of the preceding claims, characterized by the fact that the photoinitiator of the primer composition of step e) is chosen among benzyl ketones, dialkylacetylphenones, alpha-hydroxyalkylphenones, acylphosphine oxides and benzophenone.

13. The process according to any of the preceding claims, characterized by the fact that the grinding resin of the primer composition of step e) consists of a saturated polymer resin not containing an acrylate group of said primer composition.

14. The process according to any of the preceding claims, characterized by the fact that the inhibitor pigment or pigments of the pigment paste of the primer composition is (or are) chosen among calcium chromates, phosphates, borate, molybdates and silicates.

15. The process according to one of the preceding claims, characterized by the fact that the primer composition has a pigment concentration in CPV volume of inhibitor pigments ranging from 0.1 to 0.25, preferably 0.1 to 0.15.

16. The process according to one of the. preceding claims, characterized by the fact that the inhibitor pigment or pigments of the pigment paste of the primer composition is (or are) associated with one or more corrosion inhibitor(s) in liquid form.

17. The process according to claim 16, characterized by the fact that the corrosion inhibitor or inhibitors in liquid form is (or are) chosen among aqueous solutions of 1-(benzo-thiazol-2-ylthio)succinic acid and aqueous solutions of ethylene methacrylate phosphate.

18. The process according to any of the preceding claims, characterized by the fact that the crosslinking of step g) is carried out under a UV lamp under nitrogen with an oxygen level lower than 200 ppm.

19. The process according to any of the preceding claims, characterized by the fact that it comprises, after the crosslinking step g) of the aqueous primer composition film, a finishing step h), comprising: application of finishing composition on the primer film, followed by crosslinking of said finishing composition to obtain a finishing coat.

20. The process according to claim 19, characterized by the fact that the finishing step h) is a finishing [process-translator], comprising: application of a finishing composition based on polyester melamine or polyester urethane or polyvinyl difluoride, followed by crosslinking of said finishing composition by heating in a drying cabinet with a temperature in the range from 240-250° C., to obtain a crosslinked finishing coat.

21. The process according to claim 19, characterized by the fact that finishing of step h) is cold finishing, comprising: application of a finishing composition based on urethane-acrylate, followed by crosslinking of said finishing composition by electron bombardment, to obtain a finishing coat.