Off-white and gray autodeposition coatings

Abstract

A method for coating a metal substrate with a white, off-white or gray colored autodeposited coating comprising water, polymeric resin, HF and pigment particles comprising a core of titanium dioxide, an intermediate zirconia and/or alumina layer, and an outer organic layer, optionally the particles are treated with an anionic surfactant.

Claims

1. An article of manufacture comprising at least one active metal substrate surface having deposited thereon a uniform white, off-white or gray autodeposition cured coating deposited according to a method comprising: contacting an active metal substrate surface for 0.5 to 10 minutes, with an autodeposition bath composition to form a white, off-white or gray initially adherent coating on said surface; said autodeposition bath composition comprising: a. an aqueous solution of an autodeposition accelerator comprising an acid, in an amount such that the composition has a pH of about 1.0 to about 4.0, and at least one oxidizing agent; b. particles of a coating-forming polymeric material dispersed throughout the composition; c. a component of non-black solid pigment particles, stabilized against said acid, dispersed throughout the composition; d. an emulsifying component comprising an anionic surfactant; and, optionally, e. black pigment and/or finely divided solids suitable as fillers in the coatings to be formed from the composition;  said composition being effective to chemically attack, in the absence of an external electrical potential, the active metal surface immersed therein to dissolve therefrom metal to release ions of said metal and sufficient to cause said polymeric material and said non-black solid pigment particles to deposit uniformly on the active metal surface as the initially adherent coating which increases in weight or thickness the longer the time said surface is immersed in said composition; rinsing said initially adherent coating with a rinse comprising water; optionally, drying said initially adherent coating; and curing said initially adherent coating to form a uniform cured, white, off-white or gray coating.

2. The article of manufacture of claim 1 wherein said acid is hydrofluoric acid and said non-black solid pigment particles comprise a titanium dioxide core, a first coating of an oxide different from titanium dioxide and a second coating of an organic material.

3. The article of manufacture of claim 2 wherein said first coating of an oxide different from titanium dioxide comprises oxides that are substantially insoluble in said acid.

4. The article of manufacture of claim 2 wherein said first coating of an oxide different from titanium dioxide comprises oxides selected from Al.sub.2O.sub.3, ZrO.sub.2 and mixtures thereof.

5. The article of manufacture of claim 4 wherein said autodeposition bath composition comprises hydrolysis products of Al.sub.2O.sub.3 and/or ZrO.sub.2.

6. The article of manufacture of claim 2 wherein said second coating of an organic material comprises at least one of an anionic dispersing additive, a cationic dispersing additive or a non-ionic dispersing additive.

7. The article of manufacture of claim 2 wherein said second coating of an organic material comprises at least one of polyphosphates, epichlorohydrin resins, dicyandiamide resins, polyols and polyesters.

8. The article of manufacture of claim 1 wherein the white, off-white or gray autodeposition coating has a weight ratio of the non-black solid pigment particles to the polymeric material that ranges from 1-49% by weight.

9. The article of manufacture of claim 1 wherein the white, off-white or gray autodeposition coating has a weight ratio of the non-black solid pigment particles to the polymeric material that ranges from 5:95 to 40:60.

10. The article of manufacture of claim 1 wherein said anionic surfactant is selected from surfactants having at least one sulfate, sulphonate, phosphate, or phosphonate functional group.

11. The article of manufacture of claim 1 wherein said anionic surfactant maintains dispersion of the particles of a coating-forming polymeric material and the component of non-black solid pigment particles, such that said polymeric material and said pigment particles deposit uniformly on the active metal surface as the initially adherent coating.

12. The article of manufacture of claim 1 wherein said polymeric material is selected from the group consisting of styrene-butadiene, acrylonitrile-butadiene, polyethylene, acrylic, tetrafluoroethylene, polyvinyl chloride, urethane resins, styrene-acrylic, epoxy, and epoxy-acrylic materials.

13. The article of manufacture of claim 1 wherein said acid comprises hydrofluoric acid and the coating-forming polymeric material is selected from epoxy or epoxy-acrylic material.

14. The article of manufacture of claim 1 wherein said non-black solid pigment particles comprise a titanium dioxide core, a first coating of an oxide comprising zirconia and a second coating of an organic material.

15. The article of manufacture of claim 1 wherein said autodeposition accelerator comprises hydrofluoric acid, ferric cations and hydrogen peroxide, said composition providing an oxidation-reduction potential, measured by potential of a platinum or other inert metal electrode in contact with the composition, that is at least 150 mV more oxidizing than a standard hydrogen electrode.

16. An intermediate article of manufacture comprising: a. an active metal substrate surface; b. a uniform white, off-white or gray, initially adherent, uncured autodeposition coating deposited on the active metal substrate surface, dried thereon, and comprising a coating-forming polymeric material having TiO.sub.2 particles dispersed throughout; and optionally further comprising at least one of black pigment, oxides Al, oxides of Zr, hydrolysis products of oxides of Ti, hydrolysis products of oxides of Al. and hydrolysis products of oxides of Zr.

17. An article of manufacture comprising: a. an active metal substrate surface; b. a cured uniform white, off-white or gray, autodeposition coating chemically adhered to the active metal substrate surface; said coating comprising a cured coating-forming polymeric material and TiO.sub.2 particles dispersed throughout; and optionally further comprising at least one of black pigment, oxides Al, oxides of Zr, hydrolysis products of oxides of Ti, hydrolysis products of oxides of Al. and hydrolysis products of oxides of Zr.

18. The article of manufacture of claim 17 wherein the TiO.sub.2 particles dispersed throughout the cured coating are sourced from modified TiO.sub.2 particles coated with an inorganic layer of a second material comprising alumina, zirconia or mixtures thereof.

19. The article of manufacture of claim 18 wherein the cured uniform white, off-white or gray, autodeposition coating comprises at least one of oxides Al and oxides of Zr.

20. The article of manufacture of claim 19 wherein the at least one of oxides Al and oxides of Zr is sourced from hydrolysis of the modified TiO.sub.2 particles.

21. The article of manufacture of claim 18 wherein the alumina, zirconia or mixtures thereof of the inorganic layer has undergone hydrolysis such that the second material on the TiO.sub.2 particles in the cured autodeposition coating is present in a reduced amount relative to the modified TiO.sub.2 particles.

22. The article of manufacture of claim 16 wherein the cured uniform white, off-white or gray, autodeposition coating has a cross hatch adhesion according to ASTM D 3359-02 of 5B.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 depicts a cross-sectional view of one type of titanium dioxide pigment useful in the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

(2) Applicants have developed a white, off-white or gray colored autodeposition coating suitable for use as a stand alone coating or as a primer for general industrial application that comprises titanium dioxide pigment particles either alone or in combination with black pigment, such as carbon black. The instability of autodeposition concentrates and baths using ordinary titanium dioxide pigment particles has been overcome by developing a new autodepositing bath composition having titanium dioxide particles that have been stabilized against the bath.

(3) The first difficulty encountered in making a stable titanium dioxide autodeposition coating bath was the instability of the titanium dioxide particle in the acidic environment of the autodeposition bath. In the absence of any additional outer coating layer, such as an organic layer, deposited on titanium dioxide particles, the acidity of the autodeposition bath will dissolve or hydrolyze the titanium dioxide. In particular hydrofluoric acid will react with titanium dioxide to generate fluorotitanic acid.

(4) Autodeposition baths contain an accelerator component that desirably comprises ferric cations, hydrofluoric acid, and hydrogen peroxide. In a working composition according to the invention, independently for each constituent: the concentration of ferric cations preferably is at least, with increasing preference in the order given, 0.5, 0.8, or 1.0 g/l and independently preferably is not more than, with increasing preference in the order given, 2.95, 2.90, 2.85, 2.80, or 2.75 g/l; the concentration of fluorine in anions preferably is at least, with increasing preference in the order given, 0.5, 0.8, 1.0, 1.2, 1.4, 1.50, 1.55, 1.60, 1.80, 2.0 g/l and independently preferably is not more than, with increasing preference in the order given, 20, 17, 15, 13, 12, 11, 10, 7.0, 5.0, or 4.0 g/l; and the amount of hydrogen peroxide added to freshly prepared working composition preferably is at least, with increasing preference in the order given, 0.050, 0.10, 0.20, 0.30, or 0.40 g/l and independently preferably is not more than, with increasing preference in the order given, 2.1, 1.8, 1.5, 1.2, 1.00, 0.90, or 0.80 g/l.

(5) Preferably, an accelerator component is selected which is sufficient in strength and amount to impart to the autodeposition composition an oxidation-reduction potential, measured by the potential of a platinum or other inert metal electrode in contact with the autodepositing liquid composition, that is, with increasing preference in the order given, at least 150, 175, 200, 225, or 250 mV more oxidizing than a standard hydrogen electrode and independently preferably is, with increasing preference in the order given, not more than 550, 525, 500, 475, or 450 mV more oxidizing than a standard hydrogen electrode. Desirably the accelerator component also comprises a source of hydrogen cations, i.e. acid, in an amount sufficient to impart to the autodeposition bath a pH that is at least, with increasing preference in the order given, 1.0, 1.4, 1.6, 1.8, or 2.0 and independently preferably is not more than, with increasing preference in the order given, 3.8, 3.6, 3.2, 3.0, 2.8, or 2.6.

(6) Typically titanium dioxide particles are stabilized with alumina and silica. Silica is unstable in HF, as to some extent is alumina in HF. Thus far, silica coated particles have not proven to be stable against HF; desirably, the coatings on the titanium dioxide particles comprise, in increasing order of preference, less than 30, 20, 10, 5, 4, 3, 2, 1 wt % silica. In testing titanium dioxide particles coated with alumina, aluminum leached from the titanium dioxide particles into the autodeposition bath to form aluminum fluoride. The aluminum leaching phenomenon was found to be correctable by increasing the amount of HF in the bath. In additional testing, zirconia either alone or combined with alumina was found to protect the titanium dioxide from the acidic environment of the autodeposition bath.

(7) A second hurdle to producing a stable, white, off-white or gray autodeposition bath was maintaining titanium dioxide suspended in the aqueous autodeposition concentrates and baths, and ensuring the desired amount of deposition of both the polymer and the pigment particles. Desirably, in one embodiment there is relatively equal deposition between polymeric particles and pigment particles.

(8) It was found that hydrophobicity/hydrophilicity of the titanium dioxide particles and hydrophobicity/hydrophilicity of the polymeric particles, relative to each other are variables in controlling whether titanium dioxide particles remain dispersed in the bath and deposit adherently on the active metal substrate to achieve the desired color.

(9) The type of surfactant used and composition of the surfactant used for the polymeric particles affects the relative compatibility, thus the relative deposition rate, between polymeric and pigment particles. Without being bound by a single theory, it is hypothesized that hydrophobic polymeric particles used in conjunction with hydrophilic titanium dioxide particles caused surfactant in the autodeposition bath to migrate toward the emulsion particles leaving the titanium dioxide pigment deprived of surfactant and eventually causing the titanium dioxide particle to settle out of dispersion. Selection of a more hydrophilic surfactant results in more surfactant being available to aid in dispersing the pigment particles. Desirably, there is relatively equal partitioning of surfactant between the two particles and an equilibrium condition is maintained such that the dispersion of polymer particles and pigment particles is stable in the absence of active metal.

(10) In some embodiments, the emulsifying component of anionic surfactant is sufficient to provide adequate stability to the anionically stabilized polymer particles and to the non-white solid pigment particles. Generally, the polymer particles in an autodeposition bath are anionically stabilized. This polymer stabilization may be achieved, as is known in the art, by either incorporating an anionic surfactant into the polymer or by adding anionic surfactant to the polymer emulsion. In some cases, both means are used. This emulsifying component comprising anionic surfactant may be sufficient to achieve stable dispersion and adequate deposition of the solid pigment particles in addition to the polymer particles.

(11) In other embodiments, a second stabilizing surfactant may be included to further contribute to stable dispersion and adequate deposition of the solid pigment particles.

(12) The surfactant package, meaning the emulsifying component and any second stabilizing surfactant different from the emulsifying component, should be selected and should be present in sufficient concentration to emulsify or disperse the polymer particles and disperse the pigment particles in the autodeposition composition so that no separation or segregation of bulk phases that is perceptible with normal unaided human vision occurs during storage at 25° C. for at least 24 hours after preparation of the autodeposition composition, in the absence of contact of the autodeposition composition with any active metal.

(13) Anionic surfactants are generally preferred, although amphoteric as well as nonionic surfactants may also be utilized. Anionic surfactants useful in the present invention include anionic surfactant having sulfate, sulphonate, phosphate, or phosphonate end groups. In one embodiment, the anionic surfactant used for coating the titanium dioxide pigment particles and stabilizing the polymer particles is selected from alkoxylated ether sulfates, such as by way of non-limiting example, Polystep B-40; Rhodapex™ CO-128, -433, and -436 and Texapon™ E-12 and K-12.

(14) Modified TiO.sub.2 particles have been successfully tested in autodeposition baths for decomposition and ability to deposit on the metal substrate. Desirably, the modified titanium dioxide particles are sufficiently stable in acidic conditions generally found in the autodeposition bath; and, in a particular embodiment, are sufficiently stable in the presence of HF, such that the solid pigment particles remain available for deposition on the metal substrates placed in the autodeposition bath. It is believed that the particles do not hydrolyze, however levels of hydrolysis of the pigment particles which do not interfere with deposition and bath stability are acceptable.

(15) Performance properties (chemical, photochemical and physical characteristics) are determined principally by at least the following features of pigment particles: the particle size of the pigment and the chemical composition of the surface of the pigment particles. The chemical composition of the surface of the TiO.sub.2 particles is modified by coating them with an inorganic layer of a second material different from titanium dioxide.

(16) Many commercial grades of TiO.sub.2 have inorganic and in some cases organic surface treatments, not all of which are suitable for use in autodeposition baths. Inorganic surface modifiers most often used for titanium dioxide are precipitated coatings of alumina and silica. Applicants' research has shown that, for use in autodeposition baths, it is desirable that the first layer comprise an inorganic material having low solubility in the acid used in the autodeposition bath, generally this is HF. Preferably, the inorganic material is substantially insoluble in the acid used in the autodeposition bath. As used herein, “substantially insoluble” means that the material has a solubility at ambient temperature of less than, in order of increasing preference, 0.5, 0.25, 0.1, 0.075, 0.05, 0.025, 0.01, 0.0075, 0.005, 0.0025, or 0.001 g/100 ml of the acid. Preferably, the inorganic layer is a metal oxide, such as by way of non-limiting example zirconia and/or alumina.

(17) Once coated with a first layer, i.e. the inorganic layer, the pigment particles are coated with second layer that is an organic layer. Alternatively, the pigment particles may be simultaneously contacted with the inorganic and organic materials thereby forming a first inorganic layer and a second outer organic layer; or a composite coating may be formed.

(18) Generally, components of the organic layer are selected from substances, known to those of skill in the pigment art, which are useful in dispersing solid pigment in an aqueous liquid medium, such as by way of non-limiting example, anionic additives, cationic additives and non-ionic additives that assist in dispersing the particles without interfering with deposition of the particles on metallic substrates during the autodeposition process.

(19) Suitable anionic additives include for example polyacrylates or polyphosphates and the like; cationic additives include for example cationized polyacrylates or polymethacrylates, such as quaternary dimethylaminoethyl methacrylates or melamine-formaldehyde resins, epichlorhydrin resins, dicyandiamide resins and the like. Non-ionic additives include polyols and/or polyesters and the like. A number of other suitable coatings for pigments are recited in U.S. Pat. No. 3,825,438, incorporated herein by reference.

(20) FIG. 1 is a cross-sectional view of a pigment particle useful in the invention. In this embodiment, pigment particles comprise a core of TiO.sub.2 (1) coated with a mixed zirconia/alumina layer (2) followed by an outer organic layer (3), see FIG. 1. Optionally, the coated TiO.sub.2 pigment may be subsequently treated with anionic surfactant that forms a third outer layer (4). Desirably the anionic surfactant is based on sulfate chemistry, that is has sulfate functional groups, to promote uniform deposition and consumption rate of the pigment particles to that of the polymer emulsion particles.

(21) The inner first inorganic layer and second organic layer surface treatment of the TiO.sub.2 core in conjunction with the post treatment with anionic surfactant provides a stable and depositable TiO.sub.2 pigment slurry in the acidic HF containing autodeposition baths. With the successful incorporation of the modified TiO.sub.2 particles into the autodeposition bath, the autodeposition coating process provides coatings in the traditional black, white, off-white, and in shades of gray. The color of the coating is a function of the amount of conventional black pigment used in conjunction with the titanium dioxide pigment.

(22) Autodeposition coatings comprising modified TiO.sub.2 particles deposited from autodeposition baths containing these particles have been tested and displayed corrosion resistance and physical performance similar to the black commercial counterpart. Panels coated in baths having different amounts of modified TiO.sub.2 particles coat metal substrates with a polymeric coating in uniform shades of white to gray. The resulting coated panels showed consistency in the compositions' performance in color, hiding power, and tinting strength.

(23) One embodiment of the invention provides a composition for depositing an aqueous off-white autodeposition coating comprising: modified TiO.sub.2 particles and at least one emulsion polymer. Desirably the modified TiO.sub.2 particles are provided as an aqueous slurry to aid in incorporation of the component into the composition. Optionally, the TiO.sub.2 slurry is further modified with anionic surfactant, desirably based on sulfate, sulphonate, phosphate, or phosphonate end groups. Alternatively, anionic surfactant and the slurry containing the modified TiO.sub.2 particles can be added separately to the bath. The emulsion polymer(s) can be acrylic, styrene-acrylic, epoxy, epoxy-acrylic, polyurethane dispersion, or any other water dispersible ionically stabilized polymers suitable for use in autodeposition processes, as are known to those skilled in the art.

(24) Another embodiment of the invention provides an aqueous composition suitable for depositing gray autodeposition coatings on metal substrates comprising: modified TiO.sub.2 pigment particles, black pigment particles, preferably carbon black, and at least one emulsion polymer. Desirably, both pigments are provided as aqueous slurries to aid in incorporation into the composition and the carbon black slurry is anionically stabilized. Optionally, the TiO.sub.2 slurry is further modified with anionic surfactant, desirably based on sulfate, sulphonate, phosphate, or phosphonate end groups. Alternatively, anionic surfactant and the slurry containing the modified TiO.sub.2 particles can be added separately to the bath. The emulsion polymer(s) can be acrylic, styrene-acrylic, epoxy, epoxy-acrylic, polyurethane dispersion, or any other water dispersible ionically stabilized polymers suitable for use in autodeposition processes, as are known to those skilled in the art.

(25) In another aspect of the invention, autodeposition coating baths are prepared by mixing one of the above-described autodeposition compositions with water, HF, iron and hydrogen peroxide in amounts sufficient to form an autodeposition bath wherein the percent of non-volatiles is in the range of 1-20 weight %.

(26) The practice of this invention may be further appreciated from the following working examples.

EXAMPLE 1

(27) A 35% non-volatile, off-white autodeposition coating composition concentrate was formulated as follows:

(28) To a 1.5 liter container, were added 211.65 g of deionized water, 5.63 g of anionic surfactant having 20% non-volatile (active), 786.4 g of anionically modified polymer emulsion having 42+/−1% non-volatile, and 132.6 g of TiO.sub.2 pigment slurry having 50+/−1% non-volatile. The active pigment to polymeric binder ratio was 20%. The TiO.sub.2 pigment in the slurry was described by the manufacturer as having a first inner layer of zirconia and alumina and a second outer layer of polyether polyol. The TiO.sub.2 pigment and the polymer particles remain uniformly dispersed in the concentrate.

EXAMPLE 2

(29) A 6% non-volatile, 20 ml Fe titration, off-white autodeposition coating bath was formulated using the composition of Example 1, as follows:

(30) In a 1.5 liter container, the following were combined:

(31) 5.20 g HF 1.70 g Iron powder 3.62 g Hydrogen peroxide 35% Distilled water to make 1.0 liter
The material was mixed for several minutes. 257.1 g of the composition of Example 1 was added to the 1.5 liter container slowly with agitation. Finally, sufficient distilled water to make 1.5 liters was added. The bath was mixed for one hour and bath parameters were adjusted to the following parameters under continuous agitation:

(32) TABLE-US-00001 Redox Value 275-400 mV Lineguard 101 meter reading 100-700 microamperes Total % non-volatile  1-10% Wet coating solids 20-50% Starter titration 10-40 ml Bath temperature 20-25° F. Conductivity 1,200-10,000 microsiemens
After the off-white bath was prepared and parameters optimized, metallic panels were treated according to the following method: A. Cleaning with alkaline cleaner—2 minutes B. Warm water rinsing—1 minute C. Deionized water rinsing—1 minute D. Contacting with the off-white autodeposition processing bath of Example 2—2 minutes F. Water rinsing—1 minute G. Treating with AUTOPHORETIC® Reaction Rinse E2 (commercially available from Henkel Corporation)—1 minute H. Oven curing at 53° C. for 7 minutes and at 185 ° C. for 40 minutes.
The resulting coated panels were slightly off-white in color, were uniform in color and exhibited good hiding power.

EXAMPLE 3

(33) A 35% non-volatile, grey autodeposition coating composition concentrate was formulated as follows:

(34) To a one-liter flask, were added 211.14 g of deionized water, 5.63 g of anionic surfactant having 20% non-volatile (active), 3.38 g of black pigment slurry having 30% non-volatile, 786.4 g of anionically modified polymer emulsion having 42+/−1% non-volatile, and 132.6 g of the TiO.sub.2 pigment slurry of Example 1 having 50+/−1% non-volatile. The active pigment to polymeric binder ratio was 20%.

(35) A 6% non-volatile, 20 ml Fe titration, gray autodeposition coating bath was formulated using the composition of Example 3, according to the procedure of Example 2.

(36) The bath was mixed for one hour and bath parameters were adjusted to the parameters of Example 2 under continuous agitation.

(37) After the gray bath was prepared and parameters optimized, metallic panels were treated according to the method of Example 2. The resulting coated panels were gray in color, were uniform in color and exhibited good hiding power.

EXAMPLE 4

(38) Gray autodeposition baths were formulated according to Example 3, at various pigment to binder ratios. The ratios varied from 5:95 pigment:binder to 50:50 pigment:binder. The physical and corrosion performance at various ratios are shown in Table 1, below:

(39) TABLE-US-00002 TABLE 1 Neutral Salt Cross hatch Solvent Pencil Spray (Total adhesion Reverse Resistance Hardness mm scribe creep Pigment:Binder ASTM D Impact M.E.K. ASTM at 504 hours) ratio 3359-02 (in. lb.) Double Rubs D3363-00 ASTM B117 Stability  5:95 5B >80 >200 >3H 1.6 Stable 10:90 5B >80 >200 >3H 1.5 Stable 17:87 5B >80 >200 >3H 2.9 Stable 30:70 5B >80 >200 >3H 1.2 Stable 40:60 5B >80 >200 >3H 1.4 Stable 50:50 5B >80 >200 >3H Not tested *Not stable *Not stable means that the TiO.sub.2 was not deposited and settled out of the autodeposition bath.

EXAMPLE 5: COMPARATIVE EXAMPLE

(40) A 35% non-volatile, gray autodeposition coating composition concentrate was formulated as follows:

(41) To 1.5 liter flask, were added 92.7 g of deionized water, 5.63 g of anionic surfactant having 20% non-volatile (active), 3.38 g of black pigment slurry having 30% non-volatile, 904.89 g of anionically modified polymer emulsion having 42+/−1% non-volatile, and 132.6 g of TiO.sub.2 pigment slurry of Example 1 having 50+/−1% non-volatile. The active pigment to polymeric binder ratio was 20%.

(42) The surfactant used to make the emulsion in this example was changed relative to the previous examples. The surfactant used had similar ionic end groups to the surfactant of Examples 1-4, but the number moles of alkoxylation was reduced making the surfactant of Example 5 less hydrophilic.

(43) A 6% non-volatile, 20 ml Fe titration, gray autodeposition coating bath was formulated using the composition of Example 5, according to the procedure of Example 2.

(44) The bath was mixed for one hour and bath parameters were adjusted to the parameters of Example 2 under continuous agitation. After the gray bath was prepared and parameters optimized, metallic panels were treated according to the method of Example 2. The resulting coated panels were brown in color indicating no pigment deposition. After aging of the autodeposition bath for a few days, the pigment was separated from the bath or destabilized. Changing bath hydrophobic/hydrophilic equilibrium between emulsion and pigment particles caused un-equal partitioning of the surfactant between the particles, depriving the pigment particles from surfactant, and therefore destabilization of the pigment particles occurred.