WO1996037554A1

WO1996037554A1 - Autodeposition composition and process with acrylic terpolymer coating resin

Info

Publication number: WO1996037554A1
Application number: PCT/US1996/006967
Authority: WO
Inventors: Mutsumi Hirota; Takumi Honda; Mitsuyuki Koga; Osamu Kaji; Tomoji Iwawaki
Original assignee: Henkel Corporation
Priority date: 1995-05-24
Filing date: 1996-05-23
Publication date: 1996-11-28
Also published as: CN1140739A; TW380148B; JPH08311389A

Abstract

An autodepositing aqueous coating composition has a pH of 1.6 to 5.0 and comprises resin solids, acid, oxidising agent, water, anionic surfactant either as a free component or as a copolymerized component in the said resin, and other optional components, with the resin solids being dispersed in the water and being polymer resin solids principally composed of specified proportions of the following monomers: (A) methyl methacrylate and/or acrylonitrile, (B) ethyl acrylate and/or butyl acrylate, and (C) acrylic acid and/or methacrylic acid and with the anionic surfactant being present in a specified proportion based on the total amount of the said components (A), (B), and (C). Such an autodepositing composition produces on active metal surfaces contacted with it a high corrosion-resistant coating, even without any post-treatment with an aqueous hexavalent chromium solution.

Description

AUTODEPOSITION COMPOSITION AND PROCESS WITH ACRYLIC TERPOLYMER COATING RESIN

Technical Field

This invention relates to an acrylic resin-based autodepositing aqueous coating composition that upon contact with a metal surface forms a resin coating thereon. Background Art

Autodepositing aqueous coating compositions, which are acidic composi¬ tions that contain an organic film-forming resin, are able to form a resin coating on metal surfaces when brought into contact with such surfaces. Pertinent dis¬ closures in this regard are, for example, Japanese Patent Publication Numbers Sho 52-21006 [21 ,006/1977], Sho 53-15093 [15,093/1978], Sho 53-16010

[16,010/1978], and Hei 6-57811 [57,811/1994], and Japanese Patent Application ^" Laid Open [Kokai or Unexamined] Numbers Sho 56-152874 [152,874/1981] and Sho 61-246267 [246,267/1986].

A characteristic feature of autodepositing aqueous coating compositions is their ability to form resin films whose thickness or weight increases with immer¬ sion time when a clean metal surface is immersed in the coating composition. Moreover, film formation is itself achieved through the chemical activity of the coating composition overlying the metal surface (metal ions are eluted from the metal surface by etching and act on the resin particles, leading to the deposition of both resin and metal ions on the surface). As a result of this chemically-in¬ duced deposition, a resin coating can be efficiently formed on the metal surface without utilizing an external source of electromotive force, as is used in electro- deposition.

The aforesaid Japanese Patent Publication Number Sho 53-15093 dis- closes the use of a particular type of post-treatment to improve the corrosion re¬ sistance of coatings from acrylic resin-based autodepositing aqueous coating compositions. This post-treatment is run after immersion of the substrate in the coating composition and before curing of the resin film (i.e., before the initially wet resin film formed on the substrate while it is in contact with the coating com- position is caused to dry, whether by simple exposure to air or by heating) and consists of a rinse with an aqueous solution containing a hexavalent chromium- containing compound (hereinafter called an "aqueous hexavalent chromium solu¬ tion"). However, environmental considerations make it undesirable to use aque¬ ous hexavalent chromium solutions, and in the last several years this has led to increased demands to avoid the use of hexavalent chromium compounds during the coating of metals and other substrates. At the same time, autodepositing aqueous coating compositions based on conventional acrylic resin components suffer from a major drawback: in the absence of post-treatment with an aqueous hexavalent chromium solution, these compositions are unable to generate resin coatings that can fully satisfy various property requirements, particularly as con¬ cerns corrosion resistance.

Japanese Patent Application Laid Open [Kokai or Unexamined] Number Hei 5-295297 [295,297/1994] discloses an autodepositing aqueous coating com¬ position that is able to efficiently lay down a resin coating on metal surfaces and that also provides a good bath stability. The disperse resin comprising the or¬ ganic coating-forming component in this composition is a polymer comprising:

(1) 2 to 10 weight % of carboxyl-containing ethylenically unsaturated mono- mer, for example, acrylic acid monomer, and

(2) 90 to 98 weight % of ethylenically unsaturated monomer other than (1), for example, acrylate ester monomer.

This composition also contains nonionic surfactant in an amount not exceeding 12 weight % of the weight of the resin. Even with the use of this composition, however, it has been desirable to implement a post-treatment with an aqueous hexavalent chromium solution in order to form a coating with an acceptable cor¬ rosion resistance in severely corrosive environments. Disclosure of the Invention

Problems to Be Solved by the Invention A major object of the present invention is to provide a solution to the prob¬ lems described above. In more specific terms, the object of the present invention is to provide an acrylic resin-based autodepositing aqueous coating composition that is able to form a highly corrosion-resistant resin coating without requiring post-treatment with an aqueous hexavalent chromium solution.

Summary of the Invention It has been discovered that a highly corrosion-resistant resin coating is produced. by an autodepositing aqueous coating composition that contains resin solids that are polymers of at least three particular monomer types and that are dispersed and stabilized by amounts of anionic surfactant within a specified range.

More specifically, in one of its embodiments the present invention is an autodepositing aqueous coating composition having a pH of 1.6 to 5.0 and com¬ prising, preferably consisting essentially of, or more preferably consisting of, res¬ in solids, acid, oxidizing agent, water, and anionic surfactant either as a separate component (called an '"external surfactant") and/or as a copolymerized compon¬ ent (called an "internal surfactant") in the said resin, and, optionally, one or more of nonionic surfactant molecules, metal ions, and film-forming auxiliary mole¬ cules, wherein the said resin solids are dispersed in the water and are polymers of a mixture of monomers that consists principally of the below-described com¬ ponents (A), (B), and (C), in each case expressed on the basis of the total amount of components (A), (B), and (C): (A) 30 to 80 weight % of molecules selected from the group consisting of methyl methacrylate and acrylonitrile,

(B) 10 to 60 weight % of molecules selected from the group consisting of ethyl acrylate and butyl acrylate, and

(C) 2 to 10 weight % of molecules selected from the group consisting of acryl- ic acid and methacrylic acid, and wherein total anionic surfactant, present as a separate component and/or as a copolymerized component in the said resin is present in the autodeposition composition in an amount that is from 0.2 to 5.0 weight %, based on the total weight of the residues of above-noted components (A), (B), and (C) in the poly- meric resin solids present in the composition.

Other embodiments include a process for forming an autodeposited coat¬ ing on a metal surface by contacting the surface with a composition according to the invention as described above, articles of manufacture made by such a pro¬ cess, and the like. Detailed Description of the Invention. Including Preferred Embodiments

The term "resin solids" in the acrylic resin-based autodepositing aqueous coating cpmposition according to the present invention denotes the resin solids in the bath of the polymer resin afforded by the copolymerization of components (A), (B), and (C). In the present description, the term "resin solids" does not refer simply to the total solids in this polymer resin bath, but rather denotes only the solids that constitute the resin itself. Accordingly, "resin solids" excludes such substances as polymerization initiator, pigment, filler, surfactant, and the like, and excludes even any anionic surfactant that is copolymerized into the resin itself.

The polymerization technique for preparing the subject polymer resin is not crucial as long as the composition meets the particular proportions specified for components (A), (B), and (C). However, emulsion polymerization is pre- - ferred. Also, there are no narrow restrictions on the polymerization conditions when the copolymer resin bath is prepared by emulsion polymerization, and con¬ ventional techniques can be used for this purpose. To provide a specific examp¬ le, the copolymer resin bath can be prepared by subjecting a mixture of at least water, anionic surfactant, polymerization initiator, and components (A), (B), and (C) to a copolymerization reaction at one or more temperatures in the range form 20 °C to 90 °C for a time in the range from 1 to 10 hours. The polymerization ini¬ tiator can be those polymerization initiators ordinarily employed for the polymeri¬ zation reactions of acrylic monomers, for example, ammonium persulfate, potas¬ sium persulfate, tert-butyl hydroperoxide, and the like. The polymer resin used in a composition according to this invention may contain small quantities of residues of one or more monomers other than mono¬ mers (A), (B), and (C) insofar as this does not impair the effects of the present invention and has no other ill effects. For example, residues of styrene, 2-ethyl- hexyl acrylate, or the like, may be present in total at no more than 30 weight %, preferably no more than 10 weight %, and more preferably no more than 5 weight %, these percent values being measured in each case on the basis of the total amount of residues of monomers (A), (B), and (C). The molecular weight of the copolymer resin afforded by this polymeriza¬ tion reaction is also not crucial, but the molecular weight — as measured by gel permeation chromatography in tetrahydrofuran using polystyrene or polyacrylate ester standards — preferably is from 50,000 to 1 ,000,000 and more preferably s is from 1 0,000 to 1 ,000,000.

The anionic surfactant used in the acrylic resin-based autodepositing aqueous coating composition according to the present invention is thought to support the maintenance of the mechanical stability of the dispersed resin. The following are well-suited for use as the anionic surfactant under consideration: o salts of higher fatty acids, e.g., sodium laurate, sodium oleate, and the like; salts of sulfate esters of higher alcohols, e.g., sodium lauryl sulfate, sodium oleyl sul- fate, and the like; and higher alkylarylsulfonates, e.g., sodium dodecylbenzene- sulfonate, sodium dodecyldiphenyl ether disulfonate, and the like. However, in order to reduce the coating composition instability that arises due to desorption s - of the subject surfactant from the resin particles, an even more preferred anionic surfactant is one that contains ethylenic unsaturation and is copolymerized with monomers (A), (B), and (C). This anionic surfactant is preferably actually incor¬ porated into the structure of the polymer that also contains the residues of mono¬ mers (A), (B), and (C). These ethylenically unsaturated anionic surfactants are o nonexhaustively exemplified by salts of acrylate esters, e.g., the sodium salt of the sulfate ester of methacryloyloxypolyoxyalkylenes, sodium sulfoethyl methac¬ rylate, sodium salts of acrylate capped formal co-oligomers of -0-CH₂-CH₂- and O 5 -0-CH₂-CH₂-0-P-0-CH₂-CH₂- moieties, and the like, and by allylics, e.g., sodium

OH alkyl allyl sulfosuccinate, sodium monoalkyl ester allylglycidyl ether sulfosuccin- ate, and the like. The anionic surfactant will ordinarily be added during the pre- 0 paration of the dispersed resin, but may be added at other times also, e.g., dur¬ ing preparation of the dispersed resin and also after preparation of the dispersed resin or during preparation of the coating composition.

Nonionic surfactant is an optional component for the acrylic resin-based autodepositing aqueous coating composition according to the present invention. The nonionic surfactant is thought to stabilize the dispersed resin against salts through the formation of a hydration layer by oxyethylene moieties present in the surfactant molecules. Suitable nonionic surfactants are exemplified by polyoxy- ethylene.nonylphenyl ethers, polyoxyethylene lauryl ethers, polyoxyethylene stearyl ethers, polyoxyethylene oleyl ethers, polyethylene glycol monolaurates, polyethylene glycol monostearates, polyethylene glycol distearates, and the like. In those cases where nonionic surfactant is used, it is ordinarily added during the preparation of the dispersed resin, but may be added after preparation of the dis- persed resin or during preparation of the coating composition.

No narrow limitations apply to the acid and oxidizing agent required in a composition according to this invention, but the acid is preferably hydrofluoric acid and the oxidizing agent usually is preferably hydrogen peroxide.

The acrylic resin-based autodepositing aqueous coating composition ac- - cording to the present invention may contain metal ions as provided by a metal ion compound, and in particular may contain ferric ions as provided by a ferric compound such as ferric fluoride, e.g. It is thought that metal ions — and partic¬ ularly ferric ions — function to accelerate dissolution of the metal ions, particular¬ ly those of iron, from the metal surface by the acid, particularly when it is hydro- fluoric acid.

The coating composition may contain a film-forming aid for the purpose of promoting fusion or coalescence of the deposited resin particles by lowering the minimum film-forming temperature. The film-forming aid is exemplified by tri- alkylpentanediol isobutyrate, alkyl carbitols, and the like. The presence of a film- forming aid (also called "coalescing agent" in the art) in a composition according to this invention is generally preferred.

An acrylic resin-based autodepositing aqueous coating composition ac¬ cording to the present invention may also contain other optional components as follows: plasticizer for imparting flexibility to the produced coating: conventional plasticizers such as, for example, dibutyl phthalate, can be used here; pigment, for example, carbon black, phthalocyanine blue, phthalocyanine green, quinacridone red, Hansa yellow, and benzidine yellow; and other — e.g., the polymerization initiator used for resin preparation may be present in whatever form it occurs. The ultimately obtained coating normally will be soft and thus have a poor corrosion, resistance when component (A) is used at less than 30 weight %, based on the total of components (A), (B), and (C), of the resin solids in an acryl¬ ic resin-based autodepositing aqueous coating composition according to the present invention. When, on the other hand, component (A) is used at more than 80 weight %, the coating normally will be brittle and lack an adequate corrosion resistance, particular when subject to mechanical shocks.

The ultimately obtained coating normally will be brittle and lack an ade¬ quate corrosion resistance when component (B) is present at less than 10 weight %, and normally will be soft and have a poor corrosion resistance when (B) is present at more than 60 weight %. It is thought that components (A) and (B) provide the resin with the hydro- philicity necessary for adherence to the substrate and particle coalescence or fusion (film formation) in water. Resins whose main component forms a hydro- phobic resin, for example, components such as styrene, 2-ethylhexyl acrylate, and the like, can not normally produce coatings with an acceptable corrosion re- sistance or acceptable film-forming properties. Considerations with regard to corrosion resistance and film-formability make it preferable for component (A) to include acrylonitrile to constitute from 5 to 40 weight % of the total of (A) - (C).

The use of less than 2 weight % of component (C) normally leads to an inadequate deposition of the resin particles on the metal surface and thus leads to the formation of a thin coating that is unable to withstand most practical uses. The use of more than 10 weight % of component (C) does lead to increased res¬ in particle deposition on the metal surface, but it also produces an increasingly poor bath stability by the coating composition because it facilitates coagulation of the resin particles by metal ions. The use of more than 10 weight % of mono- mer type (C) also leads to a relatively poor alkali resistance and hence to an oft¬ en unsatisfactory corrosion resistance.

With respect to the content of anionic surfactant as the free and/or copo- lymerized component, a content of less than 0.2 weight %, based on the total of components (A), (B), and (C), leads to a diminished bath stability of the composi¬ tion. A content in excess of 5.0 weight % usually causes the obtained coating to be a thin film that is unable to withstand most practical uses. These preferred limitations on the quantity of anionic surfactant used apply to the total quantity used when the anionic surfactant is added at two or more points in time as de¬ scribed above (e.g., during preparation of the dispersed resin and also after prep¬ aration of the dispersed resin or during preparation of the coating composition). A coating composition according to the present invention should have a pH of 1.6 to 5.0. Good-quality coatings will not be normally formed when the pH lies outside this range. A preferred pH range is 2.0 to 4.0, and a more preferred pH range is 2.5 to 3.5. The pH can be adjusted with acid.

An acrylic resin-based autodepositing aqueous coating composition ac¬ cording to the present invention in general preferably has a resin solids concen- - tration from 5 to 550 grams per liter (hereinafter usually abbreviated as "g/L"), more preferably from 30 to 100 g/L, and still more preferably from 40 to 80 g/L. When hydrofluoric acid is used as the acid in the present coating composi¬ tion, its concentration in the composition preferably is from 0.4 to 5.0 g/L and more preferably is from 0.5 to 2.0 g/L. Independently, concentrations of HF suffi- cient to provide an indicated value of about 200 microamperes to about 300 mic¬ roamperes on a Lineguard® 101 meter are preferred. Use of such a meter is disclosed in United States Patent 3,329,587. When hydrogen peroxide is used as the oxidizing agent in the present coating composition, its concentration pref¬ erably is from about 0.05 to about 3.0 g/L and more preferably is from about 0.07 to about 2.0 g/L.

Nonionic surfactant is an optional component for an acrylic resin-based autodepositing aqueous coating composition according to the present invention. It is appropriately used at no more than 1.0 weight %, based on the total of co¬ polymer resin components (A), (B), and (C), and is preferably used at no more than 0.5 weight %. The use of nonionic surfactant in excess of 1.0 weight % of the total of components (A), (B), and (C) can cause failure to form a coating, or, even when a coating is formed, may cause the coating to form a thin film unable to withstand most practical uses.

Metal ions and particularly ferric ions — when added — are used in a composition according to this invention at the same concentrations as used for conventional acrylic resin-based autodepositing aqueous coating compositions. The preferred concentration is from about 1 to about 50 g/L of a ferric compound, particularly ferric fluoride.

Film-forming aid, for example, trialkylpentanediol isobutyrate, may be used in a coating composition according to this invention at the same concentrations as in conventional acrylic resin-based autodepositing aqueous coating composi- tions. Preferred concentrations are from about 0 to 10 g/L and more preferably about 0 to 5 g/L.

No narrow restrictions apply to the technique for preparing an acrylic res¬ in-based autodepositing aqueous coating composition according to the present invention using the above-described conditions, i.e., components, concentra- - tions, and the like. In fact, the composition can be prepared by simply mixing the respective components. The acrylic resin-based autodepositing aqueous coating composition according to the present invention can in practice be prepared by proceeding directly from the polymerization bath and adding to it the other essen¬ tial components to ultimately yield a composition within the limits stipulated for the present invention.

An acrylic resin-based autodepositing aqueous coating composition ac¬ cording to the present invention can be used to treat the surface of various parts and products made of iron, zinc, iron alloys, and zinc alloys and especially iron and steel parts, prominently including, for example, automotive products such as automotive sheet parts, shock absorbers, jacks, leaf springs, suspension ele¬ ments, brackets, and the like, and also furniture components such as drawer rails and the like.

The method for treating the metal surface with the coating composition ac¬ cording to the present invention is not narrowly limited, and standard application methods, for example, dipping, spraying, roll coating, etc., can be used, with dip¬ ping being preferred. Neither the treatment temperature nor treatment time are narrowly limited. Dipping, for example, is generally suitably carried out by dip- ping for 90 to 150 seconds in a composition maintained at 20 °C to 25 °C, but may be carried out at any temperature between the freezing point and boiling point of an aqueous acrylic resin-based autodeposition composition according to the invention for whatever time is required to obtain the coating thickness needed. .Application is normally followed by a water rinse and then drying to re¬ move the water. The metal surface should ordinarily be degreased and rinsed with water prior to treatment with a coating composition according to the present invention.

The add-on to the metal by the coating composition according to the pres- ent invention is not narrowly limited, but the thickness of the dried film is prefer¬ ably 5 to 40 micrometers (hereinafter usually abbreviated as "μm") and more preferably 20 to 30 μm.

The invention will be explained more specifically below with reference to working and comparative examples, in which "parts" means "parts by weight". Examples and Comparative Examples 1 - 10 of (1. Preparation of a Dispersion of Organic Film-Forming Resin. (2) Working Autodeposition Compositions, and (3. Processes of Coating Substrates Therewith

Into a mixture of non-surfactant monomers as shown in Table 1 below was mixed the amount of surfactant shown under the same column in Table 1 , 0.3 part of ammonium persulfate, and sufficient water to make a total of 500 parts. This mixture was converted by conventional emulsion polymerization, using a standard method including maintaining for 4 hours at 75 °C, to yield an organic film-forming resin dispersion having 20 % resin solids. Each resin was cooled to 40 °C and its pH was adjusted to 5 to 8 using 25 % aqueous ammonia. Then 250 grams (hereinafter usually abbreviated as "g") of each of the thus formed resin dispersions was mixed with the amount of trialkylpentanediol monoisobutyrate film-forming aid shown in the same column of Table 1 , 0.70 g of HF equivalent of aqueous hydrofluoric acid solution, 0.10 g of H₂0₂ equivalent of aqueous hydrogen peroxide solution, and sufficient deionized water to make a total of 1 liter to make Working Composition Examples 1 - 10 according to this invention and Comparative Example Compositions 1 - 10 with the characteristics shown in Table 1. Each particular autodepositing aqueous coating composition Table 1 : WORKING AND COMPARATIVE EXAMPLES AND RESULTS

Properties of the Organic Film-Forming Resins Working Example Numbers

1 2 3 4 5

% by Weight of Non-Surfactant Monomers Polymerized to Make the Resin

Methyl Methacrylate 28 25 25 30 30

Acrylonitrile 30 5 30 35 35

Ethyl Acrylate 20 40 20 20 20

Butyl Acrylate 20 20 15 10 10

Styrene 0 0 0 0 0

2-Ethylhexyl Acrylate 0 0 0 0 0

Acrylic Acid 0 0 0 5 2

Methacrylic Acid 2 10 10 0 3

Surfactant in Weight %

Anionic Surfactant A2.0 A0.2 A2.0 A2.0 A2.0

Nonionic Surfactant 0.0 0.0 0.0 0.0 0.0

Theoretical Tg, °C 30 10 39 45 50

Other Characteristics of the Coating Compositions

Concentration of Film-Forming Aid in g/L 4.0 0.0 4.5 3.2 3.5

PH 3.3 3.2 3.3 3.5 3.5

Stability Rating + + + + + + + + + +

Coating Performance Achieved

Film Thickness, μm 20 27 25 23 23

Adherence a 100 100 100 100 100 b 100 100 100 100 100

Corrosion Resistance in Millimeters 5 5 4 5 4

This table is continued on the next page Table 1: WORKING AND COMPARATIVE EXAMPLES AND RESULTS

Properties of the Organic Film-Forming Resins Working Example Vumbers

6 7 8 9 10

% by Weight of Non-Surfactant Monomers Polymerized to Make the Resin

Methyl Methacrylate 30 40 30 30 30

Acrylonitrile 40 40 35 35 35

Ethyl Acrylate 0 10 20 20 20

Butyl Acrylate 25 0 10 10 10

Styrene 0 0 0 0 0

2-Ethylhexyl Acrylate 0 0 0 0 0

Acrylic Acid 0 0 0 0 0

Methacrylic Acid 5 10 5 5 5

Surfactant in Weight %

Anionic Surfactant A2.0 A5.0 B2.0 C2.0 A2.0

Nonionic Surfactant 0.0 0.0 0.0 0.0 1.0

Theoretical Tg, °C 47 86 50 50 50

Other Characteristics of the Coating Compositions

Concentration of Film-Forming Aid in g L 3.7 3.6 3.2 3.2 3.2

PH 3.0 3.1 3.0 3.1 2.9

Stability Rating + + + + + + + + +

Coating Performance Achieved

Film Thickness, μm 22 24 22 22 20

Adherence a 100 100 100 100 100 b 100 100 100 100* 100

Corrosion Resistance in Millimeters 5 4 4 5 5

This table is continued on the next page ... Table 1 : WORKING AND COMPARATIVE EXAMPLES AND RESULTS

Properties of the Organic Film-Forming Resins Comparative Exampl e Numbers

1 2 3 4 5

% by Weight of Non-Surfactant Monomers Polymerized to Make the Resin

Methyl Methacrylate 25 25 29 25 25

Acrylonitrile 30 25 25 30 30

Ethyl Acrylate 20 20 20 20 20

Butyl Acrylate 15 15 15 15 15

Styrene 0 0 0 0 0

2-Ethylhexyl Acrylate 0 0 0 0 0

Acrylic Acid 0 15 1 0 0

Methacrylic Acid 10 0 0 10 10

Surfactant in Weight %

Anionic Surfactant A2.0 A2.0 A2.0 A6.0 A0.1

Nonionic Surfactant 2.0 0.0 0.0 0.0 0.0

Theoretical Tg, °C 39 40 70 39 39

Other Characteristics of the Coating Compositions

Concentration of Film-Forming Aid in g/L 4.5 3.7 7.0 4.5 4.5 pH 3.5 3.3 3.2 3.2 3.3

Stability Rating + + + + + + + X

Coating Performance Achieved

Film Thickness, μm 12 25 13 13 27

Adherence a 100 100 100 100 100 b 100 50 100 80 100

Corrosion Resistance in Millimeters X X X X 7

This table is continued on the next page ... Table 1: WORKING AND COMPARATIVE EXAMPLES AND RESULTS

Properties of the Organic Film-Forming Resins Comparative Exampl e Numbers

6 7 8 9 10

% by Weight of Non-Surfactant Monomers Polymerized to Make the Resin

Methyl Methacrylate 85 10 50 0 0

Acrylonitrile 0 5 0 30 30

Ethyl Acrylate 10 40 0 0 0

Butyl Acrylate 0 40 0 0 0

Styrene 0 0 25 45 45

2-Ethylhexyl Acrylate 0 0 20 20 20

Acrylic Acid 0 0 0 0 0

Methacrylic Acid 5 5 5 5 5

Surfactant in Weight %

Anionic Surfactant A2.0 A2.0 A2.0 A2.0 A2.0

Nonionic Surfactant 0.0 0.0 0.0 0.0 0.0

Theoretical Tg, °C 87 -20 39 38 38

Other Characteristics of the Coating Compositions

Concentration of Film-Forming Aid in g/L 8.0 2.0 3.0 3.0 3.0 pH 2.9 3.4 3.3 3.2 3.2

Stability Rating + + + + + + + + + +

Coating Performance Achieved

Film Thickness, μm 23 22 23 23 23

Adherence a 100 100 100 100 100 b 100 30 90 100 100

Corrosion Resistance in Millimeters X X X X 3 was evaluated by accelerated stability testing at 40 °C for 7 days, and the results of this exposure are reported in Table 1 according to the following scale: + + = no change + = thickening only x = precipitation or separation of solid and/or production of coagulum. The following abbreviations are used in Table 1 : For an anionic surfact¬ ant, a prefixed letter A indicates that the following number applies to an amount of an acrylate ester-type polymerizable surfactant, a prefixed letter B indicates an allylic-type polymerizable surfactant, and a prefixed letter C indicates (non-po- lymerizable) sodium alkylbenzenesulfonate; the nonionic surfactant was polyoxy- ethylene nonylphenyl ether; Tg = glass-transition temperature; for adherence, ^* = edge breakage; and for corrosion resistance, x = entire surface peeled.

Because of adding the film-forming aid in the quantity shown in Table 1 , the actual minimum film-forming temperature of each coating composition that is shown in Table 1 with a higher theoretical Tg was brought to about 20 °C. Pre-cleaned cold-rolled rectangular steel panels having dimensions of 70 x 150 x 1 millimeter(s) were coated by dipping for 180 seconds into each of the particular autodepositing aqueous coating compositions of Examples 1 to 10 and Comparative Examples 1 to 9, maintained at 20 °C to 22 °C. This was followed by a water rinse, drying in an oven at 180 °C for 20 minutes, and then the evalu- ation tests described below. For Comparative Example 10, the processing was the same, except that between the water rinse and drying the rinsed but still wet panels were dipped for 60 seconds into an aqueous hexavalent chromium solu¬ tion (Palene® 60, registered trademark of Nihon Parkerizing Company, Limited) at ambient temperature. Test Methods

(1) Coating thickness

The coating thickness is reported as the average of measurements at three points (top, side, bottom) on the test panel.

(2) Film adherence (Crosshatch tape peel testing) In accordance with Japanese Industrial Standard ("JIS") K-5400, a grid of

100 squares (1 millimeter on each edge) was scribed into the test panel (before or after immersion in warm water at 40 °C for 240 hours). A tape peel was then carried out and the number of remaining squares was counted. The results ob¬ tained without immersion are reported under "a", and the results obtained with immersion are reported under "b", in Table 1. (3) Corrosion resistance A cross was scribed in the coating through to the basis metal, and the panel thus prepared was submitted to salt spray testing for 192 hours in accord¬ ance with JIS Z-2371. This was followed by a tape peel, and maximum peel width in millimeters on either side of the scribe line was measured and is report¬ ed in Table 1. (Lower values in this test are preferable.) As may be seen from a comparison of Comparative Examples 9 and 10 in Table 1, a poor corrosion resistance was obtained, as in Comparative Example 9, in the absence of a post-treatment with an aqueous hexavalent chromium so¬ lution prior to drying the coating. The compositions used in Examples 1 to 10, which were compositions according to the present invention, provided coating thicknesses of at least 20 micrometers and exhibited an excellent adherence and corrosion resistance. In contrast, Comparative Examples 1 to 9, which fell out¬ side the compositional requirements for the present invention, afforded a poor corrosion resistance in particular. Benefits of the Invention Contacting an acrylic resin-based autodepositing aqueous coating compo¬ sition according to the present invention with the surface of a metal produces thereon a highly corrosion-resistant coating even without the execution of a post-treatment with an aqueous hexavalent chromium solution.

Claims

1. An autodepositing aqueous coating composition having a pH of 1.6 to 5.0 and comprising organic polymer solids, acid, oxidizing agent, water, and anionic surfactant, wherein the said polymer solids are dispersed in the water and are a polymer of a mixture of monomers consisting essentially of:

(A) 30 to 80 weight % of molecules selected from the group consisting of methyl methacrylate and acrylonitrile,

(B) 10 to 60 weight % of molecules selected from the group consisting of ethyl acrylate and butyl acrylate, (C) 2 to 10 weight % of molecules selected from the group consisting of acrylic acid and methacrylic acid, and, optionally, one or both of:

(D) up to 5 % of ethylenically unsaturated anionic surfactant monomers; and

(E) not more than 30 weight % of other ethylenically unsaturated monomers, all percentages in the descriptions of monomer components (A) through (E) be- ^' ing percentages of the total of components (A), (B), and (C) in the mixture, the total amount of anionic surfactant present in the autodeposition composition in said coating composition being from 0.2 to 5.0 weight %, based on the total weight of the residues of above-noted monomer components (A), (B), and (C) in the polymeric solids present in the composition.

2. An autodepositing aqueous coating composition according to claim 1 , wherein component (A) includes both methyl methacrylate and acrylonitrile and acrylonitrile is present in an amount that is from 5 to 40 weight % of the total of monomer components (A), (B), and (C).

3. An autodepositing aqueous coating composition according to claim 2, wherein monomer component (D) is present in the mixture of monomers from which said organic polymer solids are prepared.

4. An autodepositing aqueous coating composition according to claim 1 , wherein monomer component (D) is present in the mixture of monomers from which said organic polymer solids are prepared. 5. An autodepositing aqueous coating composition according to claim 4, wherein: the pH is from 2.5 to 3.5; organic polymeric solids are present at a concentration of 40 to 80 g/L; the acid is hydrofluoric acid and is present at a concentration of 0.5 to 2.0 g/L; the oxidizing agent is hydrogen peroxide and is present at 0.07 to 2.0 g/L; and the composition optionally comprises one or more of the following additional components: up to 50 g/L of dissolved compound pro- s viding ferric ions in solution; up to 5 g/L of film-forming aid; and up to 0.

5 weight %, referred to the total amount of components (A), (B), and (C), of nonionic sur¬ factant molecules.

6. An autodepositing aqueous coating composition according to claim 3, wherein: the pH value is from 2.0 to 4.0; the organic polymeric solids are present o at a concentration of 30 to 100 g/L; the acid is hydrofluoric acid and is present at a concentration of 0.4 to 5.0 g/L; the oxidizing agent is hydrogen peroxide and is present at 0.05 to 3.0 g/L; and the composition optionally comprises one or more of the following additional components: up to 50 g/L of dissolved com¬ pound providing ferric ions in solution; up to 10 g/L of film-forming aid; and up to s - 1.0 weight %, referred to the total amount of components (A), (B), and (C), of nonionic surfactant molecules.

7. An autodepositing aqueous coating composition according to claim 2, wherein: the organic polymeric solids are present at a concentration of 5 to 550 g/L; the acid is hydrofluoric acid and is present at a concentration of 0.4 to 5.0 o g/L; the oxidizing agent is hydrogen peroxide and is present at 0.05 to 3.0 g/L; and the composition optionally comprises one or more of the following additional components: up to 50 g/L of dissolved compound providing ferric ions in solu¬ tion; up to 10 g/L of film-forming aid; and up to 1.0 weight %, referred to the total amount of components (A), (B), and (C), of nonionic surfactant molecules. 5

8. An autodepositing aqueous coating composition according to claim 1, wherein: the organic polymeric solids are present at a concentration of 5 to 550 g/L; the acid is hydrofluoric acid and is present at a concentration of 0.4 to 5.0 g/L; the oxidizing agent is hydrogen peroxide and is present at 0.3 to 3.0 g/L; and the composition optionally comprises one or more of the following additional 0 components: up to 50 g/L of dissolved compound providing ferric ions in solution; up to 10 g/L of film-forming aid; and up to 1.0 weight %, referred to the total amount of components (A), (B), and (C), of nonionic surfactant molecules.

9. A process of forming an autodeposited coating on a metal substrate, said process comprising steps of:

(I) contacting the metal substrate with an autodepositing aqueous coating composition according to claim 8 for a time sufficient to deposit on the metal substrate surface a wet film which, after discontinuance of contact between the metal substrate surface and any part of the autodepositing composition that will drain within one minute under the influence of natural gravity, remains sufficiently adherent to the surface and coherent within itself under the influence of natural gravity to move mechanically with the underlying substrate;

(II) discontinuing the contact between the metal substrate surface and the autodepositing composition that was established in step (I), and rinsing the metal substrate surface with the adherent wet film formed thereon during step (I) with an aqueous rinse liquid that does not include hexavalent chromium, so as to form a rinsed wet film adherent to its underlying metal substrate; and

(III) causing the rinsed wet film formed in step (II) to dry, so as to form on the metal substrate surface a dry and adherent autodeposited film.

10. A process of forming an autodeposited coating on a metal substrate, said process comprising steps of:

(I) contacting the metal substrate with an autodepositing aqueous coating composition according to claim 7 for a time sufficient to deposit on the metal substrate surface a wet film which, after discontinuance of contact between the metal substrate surface and any part of the autodepositing composition that will drain within one minute under the influence of natural gravity, remains sufficiently adherent to the surface and coherent within itself under the influence of natural gravity to move mechanically with the underlying substrate; (II) discontinuing the contact between the metal substrate surface and the autodepositing composition that was established in step (I), and rinsing the metal substrate surface having an adherent wet film formed thereon during step (I) with an aqueous rinse liquid that does not include hexaval¬ ent chromium, so as to form a rinsed wet film adherent to its underlying metal substrate; and (III) causing the rinsed wet film formed in step (II) to dry, so as to form on the metal substrate surface a dry and adherent autodeposited film.

11. A process of forming an autodeposited coating on a metal substrate, said process comprising steps of:

(I) contacting the metal substrate with an autodepositing aqueous coating composition according to claim 6 for a time sufficient to deposit on the metal substrate surface a wet film which, after discontinuance of contact between the metal substrate surface and any part of the autodepositing composition that will drain within one minute under the influence of natural gravity, remains sufficiently adherent to the surface and coherent within itself under the influence of natural gravity to move mechanically with the underlying substrate;

(II) discontinuing the contact between the metal substrate surface and the autodepositing composition that was established in step (I), and rinsing the metal substrate surface with the adherent wet film formed thereon during step (I) with an aqueous rinse liquid that does not include hexaval- ent chromium, so as to form a rinsed wet film adherent to its underlying metal substrate; and

(III) causing the rinsed wet film formed in step (II) to dry, so as to form on the metal substrate surface a dry and adherent autodeposited film having a thickness from 5 to 40 μm.

12. A process of forming an autodeposited coating on a metal substrate, said process comprising steps of:

(I) contacting the metal substrate with an autodepositing aqueous coating composition according to claim 5 at a temperature from 20 to 25 °C for a time from 90 to 150 seconds to deposit on the metal substrate surface a wet film which, after discontinuance of contact between the metal sub¬ strate surface and any part of the autodepositing composition that will drain within one minute under the influence of natural gravity, remains suf- ficiently adherent to the surface and coherent within itself under the influ¬ ence of natural gravity to move mechanically with the underlying sub¬ strate; (II) discontinuing the contact between the metal substrate surface and the autodepositing composition that was established in step (I), and rinsing the metal substrate surface having an adherent wet film formed thereon during step (I) with an aqueous rinse liquid that does not include hexaval¬ ent chromium, so as to form a rinsed wet film adherent to its underlying metal substrate; and (III) causing the rinsed wet film formed in step (II) to dry, so as to form on the metal substrate surface a dry and adherent autodeposited film having a thickness from 20 to 30 μm.

13. A process of forming an autodeposited coating on a metal substrate, said process comprising steps of: (I) contacting the metal substrate with an autodepositing aqueous coating composition according to claim 4 at a temperature from 20 to 25 °C for a time from 90 to 150 seconds to deposit on the metal substrate surface a wet film which, after discontinuance of contact between the metal sub¬ strate surface and any part of the autodepositing composition that will drain within one minute under the influence of natural gravity, remains suf¬ ficiently adherent to the surface and coherent within itself under the influ¬ ence of natural gravity to move mechanically with the underlying sub¬ strate; (II) discontinuing the contact between the metal substrate surface and the autodepositing composition that was established in step (I), and rinsing the metal substrate surface having an adherent wet film formed thereon during step (I) with an aqueous rinse liquid that does not include hexaval¬ ent chromium, so as to form a rinsed wet film adherent to its underlying metal substrate; and (III) causing the rinsed wet film formed in step (II) to dry, so as to form on the metal substrate surface a dry and adherent autodeposited film having a thickness from 20 to 30 μm.

14. A process of forming an autodeposited coating on a metal substrate, said process comprising steps of:

(I) contacting the metal substrate with an autodepositing aqueous coating composition according to claim 3 for a time sufficient to deposit on the metal substrate surface a wet film which, after discontinuance of contact between the metal substrate surface and any part of the autodepositing composition that will drain within one minute under the influence of natural gravity, remains sufficiently adherent to the surface and coherent within itself under the influence of natural gravity to move mechanically with the underlying substrate;

15. A process of forming an autodeposited coating on a metal substrate, said process comprising steps of:

(I) contacting the metal substrate with an autodepositing aqueous coating composition according to claim 2 for a time sufficient to deposit on the metal substrate surface a wet film which, after discontinuance of contact between the metal substrate surface and any part of the autodepositing composition that will drain within one minute under the influence of natural gravity, remains sufficiently adherent to the surface and coherent within itself under the influence of natural gravity to move mechanically with the underlying substrate; (II) discontinuing the contact between the metal substrate surface and the autodepositing composition that was established in step (I), and rinsing the metal substrate surface having an adherent wet film formed thereon during step (I) with an aqueous rinse liquid that does not include hexaval¬ ent chromium, so as to form a rinsed wet film adherent to its underlying metal substrate; and (III) causing the rinsed wet film formed in step (II) to dry, so as to form on the metal substrate surface a dry and adherent autodeposited film.

16. A process of forming an autodeposited coating on a metal substrate, said process comprising steps of:

(I) contacting the metal substrate with an autodepositing aqueous coating composition according to claim 1 for a time sufficient to deposit on the metal substrate surface a wet film which, after discontinuance of contact between the metal substrate surface and any part of the autodepositing composition that will drain within one minute under the influence of natural gravity, remains sufficiently adherent to the surface and coherent within itself under the influence of natural gravity to move mechanically with the underlying substrate;

(II) discontinuing the contact between the metal substrate surface and the autodepositing composition that was established in step (I), and rinsing the metal substrate surface with the adherent wet film formed thereon dur¬ ing step (I) with an aqueous rinse liquid that does not include hexavalent chromium, so as to form a rinsed wet film adherent to its underlying metal substrate; and

17. A process according to any one of claims 9 to 16, wherein the metal substrate surface has been degreased and rinsed before beginning step (I).