JPH04234463A - Composition and method for treating metal with water-retentive polymer film - Google Patents

Abstract

PURPOSE: To provide a protective coating in place of chromate allowing a metal not coated with coating material to withstand even the attack of saline water spraying for 168 hr.

CONSTITUTION: A compd. for the corrosion resistance of a nonferrous metal contains an aq. emulsion of a thermosetting or thermoplastic acrylic polymer (a), a water-soluble blocked zirconium catalyst like ammonium zirconium carbonate or an alkanolamine chelate of zirconium oxide (b), a (non-toxic) volatile org. solvent mixed with water (c) and a corrosion preventing additive such as 2-mercaptobenzothiazole (MBT), sodium 2-mercaptbenzothiazole(SMB), 2- mercaptobenzimidazole, sodium 2-mercaptobenzimidazole or tolyltriazole (d).

COPYRIGHT: (C)1992,JPO

Description

[Detailed description of the invention]

0001

FIELD OF THE INVENTION This invention relates to non-chromate compositions and methods for applying protective films to metal surfaces, particularly non-ferrous metal surfaces such as aluminum. More specifically, coating such metals with acrylic polymer films from water-based emulsions renders them resistant to corrosive effects such as salt spray. Such compositions include an aqueous emulsion of an acrylic polymer; a water-soluble blocked zirconium catalyst; a non-toxic volatile organic solvent mixed with water; and a corrosion protection additive (or package). Application of a protective film (or coating) is usually followed by the application of a "finish coat" or paint. Aircraft aluminum panels, marine structures, and many other articles that encounter harsh corrosive environments are products that rely on pretreatment with a protective coating before painting.

0002

BACKGROUND OF THE INVENTION A method for obtaining salt spray resistance of unpainted metals, such as aluminum alloys with high copper content, is difficult to achieve without affecting the adhesion of the finish coat. This includes process coating with a hexavalent chromium solution and applying a wash primer based on a solvent containing insoluble chromate pigment and phosphoric acid. For example, "Encyclopedia of Chemistry" (Kirk Osmer, Vol. 9, pp. 14-17 (1952)); "Handbook of Metals" (9th edition, Vol. 5, pp. 597-600 (1952));
1982); Preparation of Metals for Painting, Samuel Spring, pp. 94-96 and 142-158, (1965).

[0003] Today, the use of such hazardous chromium salts is also becoming limiting as hazardous waste restrictions become more stringent. Therefore, there is an alternative need in the industry for protective coatings that allow unpainted metal to withstand 168 hours of salt spray attack.

SUMMARY OF THE INVENTION A formulation is provided for coating metal surfaces, such as aluminum, with a protective film of a polymer. The formulation comprises (a) an aqueous emulsion of an acrylic polymer, which can be thermoset or thermoplastic, the former being preferred, and (b) a water-soluble emulsion, such as an alkanolamine chelate of zirconium ammonium carbonate or zirconium oxide. , a blocked zirconium catalyst, and
(c) a (non-toxic) volatile organic solvent mixed with water;
(d) 2-mercaptobenzothiazole (MBT),
Sodium 2-mercaptobenzothiazole (SMB)
, 2-mercaptobenzimidazole, sodium 2-
and corrosion-inhibiting additives such as mercaptobenzimidazole, or tolyltriazole. In addition, defoamers,
Leveling agents, mold inhibitors, etc. may also be included.

The present invention also provides a method of applying a protective coating to a metal surface by contacting the metal surface with the formulation. In the method, when the acrylic polymer is thermosetting, the polymer is preferably cured. The coating method is preferably a dipping method.

DETAILED DESCRIPTION The combination of a tough acrylic polymer, a blocked zirconium water-soluble catalyst, a non-toxic organic solvent in combination with water, and a corrosion-inhibiting additive provides good resistance. It has been found that corrosive properties can be obtained. The water/solvent combination improves film formation. The zirconium catalyst renders the acrylic film water insoluble and promotes its adhesion to finishes. Corrosion resistance at metal interfaces can be improved by adding chromate inhibitors to M
Improved by replacing with non-toxic additives such as BT, SMB, 2-mercaptobenzimidazole, sodium 2-mercaptobenzimidazole, and tolyltriazole.

The present invention is particularly useful for protecting nonferrous metals such as aluminum, zinc, magnesium, copper, cadmium, nickel, and titanium, but provides improved rust protection and paint adhesion properties for steel substrates. can also be brought about. Aluminum and zinc are preferred, particularly aluminum. Aluminum includes pure aluminum and aluminum alloys, including extruded, cast, refined and sintered alloys. A preferred alloy is a high copper content aluminum alloy such as 2024T3 containing about 4% copper by weight.

[0008] The water-soluble acrylic emulsion usually has a molecular weight of about 500,000, preferably about 100,000 to 5,000.
It has a molecular weight of up to 0.00. Emulsions generally constitute about 30-90%, usually about 55-70% by weight of the formulation. For example, about 41-47% acrylic polymer, 0-2% anionic emulsifier such as dimethylaminoethanol, and 51-58% water as a substrate;
Molecular weight exceeds 200,000 and pH is 7.5-9.9
Satisfactory results are obtained with this emulsion. Such emulsions are commercially available, for example Rohm &
RHOPLEX A, a product of Haas Company
C1803 (thermosetting), RHOPLEX AC1561
(thermosetting), RHOPLEX WL91 (thermoplastic) and RHOPLEX WL96 (thermoplastic). RHOPLEX AC1561 is a preferred emulsion. Because the acrylic polymer contains hydroxy-functional groups resulting from the polymerization of hydroxy-functional acrylate and methacrylate monomers,
This is because self-crosslinking occurs at high temperatures of 150 to 300°C. Such emulsions also provide resistance to spotting (or detachment) by solvents and do not release formaldehyde upon curing.

Satisfactory results have also been obtained with 35-60% acrylic/styrene copolymer, 1-4% ammonium hydroxide, 40-64% water, 0-3% polypropylene glycol, and 0-3% octyl. It is also obtained by thermoplastic acrylic emulsions based on phenoxy-polyethoxyethanol with a molecular weight of more than 200,000 and a pH of 8.3-9.0. Such emulsions include, for example, the Johnson Wax products JONCRYL80, JONCRYL537 and JONCRYL537.
It is commercially available as NCRYL554. A combination of thermoplastic acrylic emulsions has been found to be useful, as shown below.

Zirconium ammonium carbonate (AZC)
Alternatively, blocked ionic crosslinking catalysts such as alkanolamine chelates of zirconium oxide are useful. This is because they become active only when the ammonium or amine moiety evaporates at the metal surface. Coatings prepared without these catalysts showed poor finish adhesion. For example, 20%
An aqueous solution of AZC containing an equal amount of zirconium oxide gave satisfactory results, and it was found that Magnesium E
It is commercially available as the product BACOTE 20 from Lectron and contains 45-50% 1-propanol and 55-50% 1-propanol.
50% alcoholic solution of alkanolamine chelate. Such a solution is manufactured by DuPont's product TYZ.
It is also commercially available as OR212. The catalyst solution is approximately 1
It is used in relatively small amounts, such as ~5%, usually about 1.6-4.5%. Such chelate catalysts are preferred when increased bulk stability is desired, such as for long periods of time at 43°C (110F).

Although various volatile organic solvents can be used in combination with water, those that do not increase the acrylic latex particles in the bulk are preferred to prevent changes in viscosity during storage or transport. Typically, propyl alcohol, isopropyl alcohol, glycol ethers such as diethylene glycol monoethyl ether (DGME) and ethylene glycol monopropyl ether (EGME), n-methylpyrrolidone, and mixtures thereof are preferred organic solvents. The solvent acts as a thickening agent as the acrylic emulsion particles coalesce upon evaporation of water on the metal surface. Tap water or deionized water can be used, but deionized water is preferred. The solvent is
Generally, it will constitute about 3% to about 20% of the formulation, more typically about 6% to about 10%. Water generally comprises about 8 to about 55% of the formulation, more typically about 17 to about 33%.

Preferred corrosion inhibitor additives include MBT, triethanolamine phosphate (TEP) and optionally 2
, 2'-methylenebis(4-methyl-6-t-butylphenol) (MBMT). SMB, MBT, 2-
Mercaptobenzimidazole and sodium 2-mercaptobenzimidazole inhibit copper corrosion and
EP chelates copper ions and other alloying elements in other high-strength aluminum alloys, and the MBMT antioxidant aids in oxygen retention and protection of the dried film from UV degradation during exposure to salt spray. do. Corrosion inhibitor additives are generally used in small amounts, such as from about 0.3 to about 3.0%, more typically from about 0.5 to about 1.2%.

Other additives such as defoamers, leveling agents, and mold inhibitors may be used. Polyoxyethylene glycol is particularly useful because it acts as an effective defoamer and surface tension modifier to promote wetting of metal surfaces and surface smoothness. The polyoxyethylene glycol used in the examples had a molecular weight of about 2000.
PLURO from andotte Chemical
It is NIC L61. N-octanol can also be used as a defoamer if the odor of the final product is not objectionable. Mold inhibitors such as methyl parahydroxybenzoate can also be used (Kalam
METHYLPARAS, a product of Chemical
(commercially available as EPT), usually used in small amounts.

The pH range of the formulation is preferably about 7.5 to 10 to prevent premature instability.

[0015] Minimize foam and MBT corrosion inhibitor and M
To form a colloidal dispersion of BMT antioxidants, these compounds are preferably first predissolved in a volatile organic solvent layer and added dropwise to an acrylic emulsion prediluted with water.

Concentrates can be prepared by simply removing most of the water.

Application of the coating can be done by spraying, flow coating or dipping. In the example below, a clean metal panel (2024T3 aluminum) was dipped into the formulation for about 5 seconds and quickly pulled out. Drying of the surface was rapid, typically to become tack-free in less than 15 minutes. The panels of Examples 2 and 6 were dipped twice. The panels from Examples 1 and 3 were cured at a temperature of 125-150°C for about one and a half hours. Instead of heating the panel, one can simply leave it at ambient temperature for about two weeks to a month.

In the following examples, all percentages are by weight.

All Examples 1-6 formulations passed the 168 hour salt spray test (ASTM-B-117-
73) was provided.

Example 1 uses 0.5 instead of 1.1 SMB.
Repeat with MBT and increasing the amount of water from 28.89 to 29.49. The formulation provided films on metal panels that passed 168 hour and 336 hour salt spray tests. Furthermore, the requirements of these tests were met even when such panels were cured in two weeks at room temperature (i.e., about 20-25°C).

[0021] Film protection panels can be provided with a finish by well known and conventional methods to meet their intended end use.

[0022]

[Table 1]

[0023]

[Table 2]

[0024]

[Table 3]

[0025]

[Table 4]

[0026]

[Table 5]

[0027]

[Table 6]

Claims (13)

[Claims] 1. (a) from about 30 to about 90% by weight of an aqueous emulsion of a thermoplastic or thermosetting acrylic polymer.
and (b) a water-soluble blocked zirconium catalyst selected from chelates of alkanolamines of ammonium zirconium carbonate or zirconium oxide.
(c) a mixture of water and a volatile organic solvent selected from propyl alcohol, isopropyl alcohol, glycol ether, n-methylpyrrolidone, and combinations thereof, wherein the water accounts for about 8% to about 8% of the formulation. 5
(d) 2-mercaptobenzothiazole;
and about 0.3 to about 3.0 weight percent of a corrosion inhibitor additive selected from sodium 2-mercaptobenzothiazole, 2-mercaptobenzimidazole, sodium 2-mercaptobenzimidazole, and tolyltriazole. formulation for. 2. A formulation according to claim 1, wherein the metal is aluminum. 3. The formulation of claim 1, wherein the metal is a copper-containing aluminum alloy. 4. The formulation of claim 1, wherein the acrylic polymer is thermosetting. 5. The formulation of claim 4, wherein the acrylic polymer has hydroxy functionality. 6. The formulation of claim 1, wherein the acrylic polymer is thermoplastic. 7. (a) from about 30 to about 90% by weight of an aqueous emulsion of a thermoplastic or thermosetting acrylic polymer.
(b) from about 1 to about 5% by weight of a water-soluble blocked zirconium catalyst selected from alkanolamine chelates of ammonium zirconium carbonate or zirconium oxide; and (c) water and propyl alcohol, isopropyl alcohol, glycol ether. , n-methylpyrrolidone, and combinations thereof, wherein water constitutes from about 8 to about 55% by weight of the formulation and the solvent constitutes from about 3 to about 20% by weight of the formulation. and (d) a corrosion inhibitor additive selected from 2-mercaptobenzothiazole, sodium 2-mercaptobenzothiazole, 2-mercaptobenzimidazole, sodium 2-mercaptobenzimidazole, and tolyltriazole from about 0.3 to about 3. 1. A method of applying a protective coating to a metal surface comprising contacting the metal surface with a formulation comprising: 0% by weight. 8. A coated metal produced by the method of claim 7. 9. The method of claim 7, wherein the acrylic polymer is thermoset and contains hydroxy functionality. 10. The method of claim 9 further comprising the step of curing the applied coating. 11. A coated metal produced by the method of claim 10. [Claim 12]Claim 7 wherein the metal is aluminum.
the method of. 13. The method of claim 7, wherein the metal is a copper-containing aluminum alloy.