Use of Additive in Aqueous Cure of Autodeposited Coatings

This invention relates to the formation of resinous coatings on metallic surfaces. More specifically, this invention relates to improved means for curing such coatings.

Autodeposition is a relatively recent development in the coating field whereby an aqueous resinous coating composition of low solids concentration (usually less than about 10%) forms a coating of high solids concentration (usually greater than about 10%) on a metallic surface immersed therein, with the coating increasing in thickness or weight the longer the time the metallic surface is immersed in the composition. Autodeposition is similar to electrodeposition, but does not require the aid of external electrical current to cause the resin particles to deposit on the metal surface. In general, autodepositing compositions are aqueous acid solutions having solid resin particles dispersed therein.

This invention relates to improved means for curing freshly formed autodeposited coatings.

Representative U.S. patents which relate to the field of autodeposition and which disclose autodepositing compositions and process steps are the following: U.S. Pat. Nos. 3,585,084, 3,592,699,and 4,373,050, each to Hall and Steinbrecher; 3,709,743; 4,180,603; 4,191,676; 4,347,172 and 4,411,937; 4,313,861; and 3,647,567, 3,795,546 and 4,030,945.

Various of the aforementioned patents disclose curing the freshly applied autodeposited coating by subjecting it to conditions under which the wet coating is dried and the resin particles are fused to render the coating continuous. This tends to improve the corrosion resistance of the coating and its adherence to the underlying metallic surface. The conditions under which the resin is cured involve subjecting the coated part to elevated temperatures for a sufficient period of time to effect drying and fusion of the coating, the specific temperatures and times being dependent on the nature of the resin comprising the coating and the mass of the coated part. Exemplary conditions include subjecting the coated part to temperatures in excess of 100.degree. C. for periods of time ranging from about 5 minutes to 30 minutes or more. The prior art discloses effecting such curing by placing the coated part in an oven or subjecting it to a stream of hot air. Aforementioned U.S. Pat. No. 4,191,676 discloses the effective use of baking the coating for a period of time until the metallic surface has reached the temperature of the environment in which it is being heated.

The invention described in the aforementioned parent applications and the present invention relate to an improved process for curing autodeposited coatings, and particularly those which can be cured at low temperatures, for example, in the range of about 20.degree. to about 120.degree. C.

Aforementioned parent applications Ser. Nos. 629,924, and now U.S. Pat. Nos. 4,562,098 and 517,133now abandoned relate to curing autodeposited coatings by subjecting them to water or steam for a relatively short period of time at a relatively low temperature. The preferred coatings which are so cured are formed from an autodepositing composition in which the particles of resin are dispersed in an aqueous acidic solution which is prepared by combining hydrofluoric acid and a soluble ferric iron-containing ingredient, most preferably ferric fluoride. The preferred resins for use in forming autodeposited coatings which are so cured comprise externally stabilized vinylidene chloride copolymers and also internally stabilized vinylidene chloride copolymers containing in excess of 50 wt. % vinylidene chloride. Most preferably, the vinylidene chloride copolymer is crystalline in nature.

Curing autodeposited coatings by subjecting them to water, preferably hot water, or steam is to be distinguished from prior art processes of the type which involve subjecting freshly-applied (uncured) autodeposited coatings to one or more steps which involve water treatment of the autodeposited coatings prior to curing them at elevated temperature such as baking the coated substrate in an oven, as described in various of the aforementioned patents. By way of background, it is noted that it is known to subject the freshly coated substrate, after it has been withdrawn from the coating bath, to steps which involve water rinsing the coating for the purpose of removing residual coating composition from the freshly-formed coating or for the purpose of improving the corrosion resistance of the coating, for example, by immersing the uncured coating into an aqueous solution of chromium-containing compounds. Water rinsing the coating after it has been subjected to the chromium-containing solution may be involved also. Such water-treating steps have not been designed to, in fact have not resulted in, the curing of the coating, as evidenced by the teachings in the aforementioned patents and by industrial practices of effecting the cure of the coating by baking at elevated temperatures.

In contrast, such curing can be effected by subjecting an uncured autodeposited coating to water or to steam as described in the aforementioned parent applications and as is described also below. There are numerous advantages which are associated with the use of this aqueous curing method, including the opportunities to realize savings in time and energy costs and capital investment.

A functional problem has been encountered, however, in certain applications involving the use of such curing process. It has been observed that autodeposited coatings cured by utilizing an aqueous medium can have poor adhesive properties. For example, after curing the coating in a bath of hot water, the coating can be peeled readily from the underlying metallic surface. This can be demonstrated by allowing the cured coating to cool to room temperature, applying tape to the cured coating, and then stripping the tape. As the tape is stripped, the film of coating is lifted from the underlying surface as it adheres to the tape. Such coatings can be readily damaged during handling and processing. The poor adhesive properties are a puzzlement inasmuch as other properties of the coating appear to be developed fully as a result of the aqueous cure.

It has been observed also that such poor adhesive properties can develop nicely into satisfactory adhesive properties by the simple expedient of exposing the coated surface to the ambiance for a period of time. The length of this time period can vary, for example, from several hours to as many as three or four days, depending on the mass of the coated surface, the thickness of the coating, relative humidity, and ambient temperature. Accordingly, the passage of time will result in coatings having excellent adhesive properties; however, there are applications where it is advantageous to have such adhesive properties developed fully at the termination of the curing process or within a short time thereafter, for example, within minutes.

In accordance with the present invention, the adhesive properties of autodeposited coatings cured by contact with an aqueous curing medium can be improved by including in the aqueous curing medium a solute. Such solute is a material which is soluble in the aqueous curing medium. The use of the solute is effective in improving the adhesion of the coating to the underlying coated substrate, including the adhesion of the initially cured coating. Examples of preferred classes of solutes are amphoteric and cationic surfactants.

Based on developmental work, it is believed that the poor adhesive properties of an initially cured coating are due to the presence of a film of water which is sandwiched between the autodeposited coating and the underlying surface of the coated substrate. It is theorized that the amount of heat present in the coated substrate is not sufficient to evaporate spontaneously or in a relatively short time such water film after curing has terminated by removal of the coated substrate from the aqueous curing medium. The adhesive properties develop fully only as the film of water evaporates through the coating, this requiring many hours or even several days, depending on many different factors such as temperature and humidity of the ambiance in which the coated part is left, the mass of the part and the thickness of the coating. This is in contrast to spontaneous or relatively quick evaporation of the water of such film through the coating as it is believed takes place in the conventional oven cure of a coating.

It is believed also that the presence of a solute in the aqueous curing medium functions in a manner such that the aforementioned water film is dissipated in the presence of the aqueous curing medium by its being drawn through the autodeposited coating. The involved mechanism of dissipation is believed to be osmotic in nature, with the film of water (solvent) diffusing through the autodeposited coating into the aqueous curing medium which itself is composed of solute dissolved in water (solvent). Accordingly, one aspect of the present invention involves dissipating in the presence of the aqueous curing medium said water film which is sandwiched between the overlying autodeposited coating and the underlying surface of the coated substrate. It is believed also that certain of the solutes that can be used in the practice of the present invention may exert on the autodeposited coating a plasticizing effect which contributes to improving the adhesive properties of the initially cured coating.

Coating compositions which form autodeposited coatings of the type that can be cured in accordance with the present invention generally comprise resin-containing acidic aqueous compositions which function to attack and dissolve from a metallic surface contacted therewith metal ions in an amount sufficient to directly or indirectly cause resin particles in the region of the metallic surface to deposit thereon in a continuous fashion, that is, in a manner such that there is a buildup in the amount of resin deposited on the surface the longer the time the surface is in contact with the composition. This deposition of the resin on the metallic surface is achieved through chemical action of the coating composition on the metallic surface. The use of electricity which is necessary for the operation of electrocoating methods is not required.

The basic constituents of an autodepositing composition are water, resin solids dispersed in the aqueous medium of the composition and activator, that is, an ingredient(s) which converts the water/resin composition into one which will form on a metallic surface a resinous coating which increases in thickness or weight the longer the surface is immersed in the composition. Various types of activators or activating systems are known, for example, as reported in U.S. Pat. Nos.: 3,592,699; 3,709,743; 4,103,049; 4,347,172; and 4,373,050, the disclosures of which are incorporated herein by reference. The activating system generally comprises an acid/oxidizing system, for example: hydrogen peroxide and HF; HNO.sub.3 ; and a ferric-containing compound and HF; and other soluble metal-containing compounds (for example, silver fluoride, ferrous oxide, cupric sulfate, cobaltous nitrate, silver acetate, ferrous phosphate, chromium fluoride, cadmium fluoride, stannous fluoride, lead dioxide, and silver nitrate in an amount between about 0.025 and about 50 g/l) and an acid that can be used alone or in combination with hydrofluoric acid, and including, for example, sulfuric, hydrochloric, nitric, and phosphoric acid, and anorganic acid, including, for example, acetic, chloracetic, and trichloracetic.

The preferred activating system comprises a ferric-containing compound and hydrofluoric acid. Thus, a preferred autodepositing composition comprises a soluble ferric-containing compound in an amount equivalent to about 0.025 to about 3.5 g/l ferric iron, most preferably about 0.3 to about 1.6 g/l of ferric iron, and hydrofluoric acid in an amount sufficient to impart to the composition a pH within the range of about 1.6 to about 5.0. Examples of the aforementioned ferric-containing compounds are ferric nitrate, ferric chloride, ferric phosphate, ferric oxide, and ferric fluoride, the last mentioned being preferred.

The types of autodeposited coatings that particularly lend themselves to being cured according to the present invention are those comprising resins which develop fully their coating properties at the elevated temperatures used, that is, temperatures up to 212.degree. F. in the case of water and steam at atmospheric pressure, and higher temperatures in case of super heated steam, and which are not degraded at such temperatures. For many applications, such autodeposited coatings can be "cured" or heated to such a degree that those properties which depend upon proper bake schedule, such as, for example, corrosion resistance, adhesion, and hardness, can be readily achieved, even on massive parts, as well as relatively thin-wall parts, by the simple expedient of treating the freshly coated part with hot water or steam. It is believed that the invention will be used most widely in curing autodeposited coatings comprising resins which have a minimum film temperature (MFT) or glass transition temperature (Tg) of no greater than about 140.degree. F., preferably no greater than about 100.degree. F., and most preferably no greater than about 85.degree. F. Resins having such characteristics and appropriate for use in autodepositing compositions are known and are available commercially.

Examples of resins that can be used in autodepositing compositions to form coatings of the type that can be cured according to the present invention include vinyl-based resins, for example, acrylic and alkyl-substituted acrylic resins and resins prepared from other ethylenically unsaturated monomeric compounds. Examples of such resin are described in aforementioned U.S. Pat. Nos.3,709,743 and 4,313,861, the disclosures of which are incorporated herein by reference. Resins of the aforementioned '861 patent can be used to form autodeposited coatings that have particularly good corrosion resistant properties when treated with an aqueous solution of chromium compounds and comprise a preferred class of resins for use in the present invention. Such resins, which can be prepared in the form of a latex, are the emulsion polymerized product of: (A) about 1 to about 20 wt. %, preferably about 4 to about 14 wt. %, of methacrylic acid or about 1 to about 12 wt. %, preferably about 4 to about 10 wt. %, of acrylic acid; (B) about 75 to about 99 wt. % of at least one other polymerizable ethylenically monounsaturated monomer; (C) 0 to about 20 wt. %, of a different hydroxy group containing polymerizable vinylenically monounsaturated monomer; and (D) 0 to about 1.5 wt. %, preferably no greater than about 1.25 wt. % of a polymerizable vinylenically polyunsaturated monomer. The aforementioned '861 patent discloses that preferred resins of the aforementioned type have a particle size of less than about 0.3 micron and more preferably a particle size of 0.15 micron or less. Preferred resins are characterized also as having a (Tg) of about 35.degree. F. to about 120.degree. F., more preferably about 45.degree. F. to about 85.degree. F. This patent discloses also that preferred resins are prepared from the following monomers: as component (B) above, styrene, methyl methacrylate, butyl acrylate, ethyl acrylate, or acrylonitrile; as component (C) above, hydroxyethyl acrylate; and as component (D) above, trimethylol propane triacrylate.

Particularly preferred classes of resins for use in forming autodeposited coatings which can be cured in accordance with the present invention comprise externally stabilized vinylidene chloride copolymers and internally stabilized vinylidene chloride copolymers containing in excess of 50 wt. % vinylidene chloride. Autodepositing compositions including such resins are described in U.S. patent application Ser. No. 06/723,677 filed Apr. 16, 1985 and entitled "Vinylidene Chloride Resin in Autodeposition", the disclosure of which is incorporated herein by reference. Of these two classes of resins, the internally stabilized copolymers are preferred. Most preferably each of the aforementioned resin classes is crystalline in nature. Autodeposited coatings prepared from vinylidene chloride-containing copolymers can have extremely high resistance to corrosion without being treated with an aqueous solution of chromium compounds or other post treatment steps designed to improve the corrosion resistance of autodeposited coatings.

As disclosed in said application, internally stabilized polymers or resins include as part of their chemical structure a surfactant group which functions to maintain polymer particles or resin solids in a dispersed state in an aqueous medium, this being the function also performed by an "external surfactant", that is, by a material which has surface-active properties and which is adsorbed on the surface of resin solids, such as those in colloidal dispersion. As is known, the presence of an external surfactant tends to increase the water sensitivity of coatings formed from aqueous resin dispersions containing the same and to adversely affect desired properties of the coatings. The presence of undue amounts of surfactant in autodepositing compositions can lead to problems, as described in U.S. Pat. No. 4,191,676, the disclosure of which is incorporated herein by reference, particularly as regards its description respecting surfactants and amounts thereof in autodepositing compositions. As discussed in this patent, the presence of an undue amount of surfactant in autodepositing compositions can deter the build-up of resin particles on the metallic surface being coated. In addition, the presence of undue amounts of surfactant can also adversely affect desired coating properties, for example, corrosion resistant properties. An advantage of internally stabilized vinylidene chloride-containing polymers is that stable aqueous dispersions, including acidic aqueous dispersions of the type comprising autodepositing compositions, can be prepared without utilizing external surfactants. (It is noted that there is a tendency in the literature to use interchangeably the following terms in connection with describing surface active materials which are used in polymerization processes for preparing polymers of the type to which the present invention relates: surfactant, wetting agent, emulsifier or emulsifying agent and dispersing agent. As used herein, the term "surfactant" is intended to be synonymous with the aforementioned.) Various types of internally stabilized vinylidene chloride-containing polymers are known and species thereof are available commercially.

Various surfactants which function to maintain polymeric particles in dispersed state in aqueous medium include organic compounds which contain ionizable groups in which the anionic group is bound to the principal organic moiety of the compound, with the cationic group being a constituent such as, for example, hydrogen, an alkali metal, and ammonium. Speaking generally, exemplary anionic groups of widely used surfactants contain sulfur or phosphorous, for example, in the form of sulfates, thiosulfates, sulfonates, sulfinates, sulfaminates, phosphates, pyrophosphates and phosphonates. Such surfactants comprise inorganic ionizable groups linked to an organic moiety.

Althouoh various ways may be used to introduce into the molecular structure of the vinvlidene chloride resin such ionizable groups, it is believed that the most widely used method for preparing such resins will involve reacting vinylidene chloride with a monomeric surfactant and optionally one or more other monomers. In such reaction, the monomeric surfactant comprises a material which is polymerizable with monomeric vinylidene chloride or with a monomeric material which is polymerizable with monomeric vinylidene chloride and which is ionizable in the reaction mixture and in the acidic aqueous medium comprising autodepositing compositions.

With respect to particular resins that can be used in the practice of the present invention, a preferred class can be prepared by copolymerizing (A) vinylidene chloride monomer with (B) monomers such as methacrylic acid, methyl methacrylate, acrylonitrile, and vinyl chloride and (C) a water soluble ionic material such as sodium sulfoethyl methacrylate. Although the constituents comprising the above-desired resin can vary over a relatively wide range, in general the resin will comprise the polymerized constituents in the following amounts:

(1) about 45 to about 99 weiqht percent based on the total weight of monomers used of vinylidene chloride monomer;

(2) about 0.5 to about 30 weiqht percent based the total weight of (1) and (2) of a second relatively more hydrophilic ethylenically unsaturated monomeric material wherein such monomeric material has a solubility in both the water phase and the oil phase of the polymer latex of at least 1 weight percent at the temperature of polymerization; and

(3) from about 0.1 to about 5 weight percent based on the total weight of other monomers of an ionic significantly water-soluble material which is copolymerizable with (2) and is selected from the group of sulfonic acids and their salts having the formula:

Examples of resins prepared from such monomers are disclosed in U.S. Pat. No. 3,617,368. As disclosed in this patent, the radical "R" is selected from the group consisting of vinyl and substituted vinyl, for example, alkyl-substituted vinyl; the symbol "Z" represents a difunctional linking group which will activate the double bond in the vinyl group; --Q-- is a divalent hydrocarbon having its valence bonds on different carbon atoms; and the symbol "M.sup.+ " represents a cation.

Exemplary of preferred hydrophilic monomers of (2) above, particularly when used in conjunction with monomeric vinylidene chloride are: methacrylic acid and methyl methacrylate. Other monomers which may be advantageously employed include the hydroxyethyl and propyl acrylates, hydroxyethyl-methacrylate, ethyl hexylacrylate, acrylic acid, acrylonitrile, methacrylonitrile, acrylamide, and the lower alkyl and dialkylacrylamides, acrolein, methylvinyl ketone, and vinyl acetate.

Examples of the difunctional linking group (Z) which will activate the double bond present in the vinyl group include groups of the structure: ##STR1## and the like. The alkyl group is preferably alkyl of 1 to 8 carbon atoms, especially methyl, ethyl or propyl. Examples of the aforementioned divalent hydrocarbon having its valence bonds on different carbon atoms include alkylene and arylene divalent hydrocarbon radicals. Although the alkylene (CH.sub.2) group can contain up to about 20 carbon atoms, it will generally have 1 to about 8 carbon atoms.

The solubility of the defined copolymerizable ionic material as described herein is strongly influenced by the cation M.sup.+. Exemplary cations are the free acids, alkali metal salts, ammonium and amine salts and sulfonium and quaternary ammonium salts. Preferred are the free acids, alkali metal salts, particularly sodium and potassium, and ammonium salts.

It is further noted that, with one of the ions above, and the usual choices for R and Z, the solubility of the monomer depends on Q. As indicated, this group can be either aliphatic or aromatic and its size will determine the hydrophilic/hydrophobic balance in the molecule, that is, if Q is relatively small, the monomer is water soluble, but as Q becomes progressively larger the surface activity of such monomer increases until it becomes a soap and ultimately a water insoluble wax. It is to be understood, however, that the limiting size of Q depends on R, Z, and M.sup.+. As exemplary of the above, it has been found that sodium sulfoalkyl methacrylate of the formula: ##STR2## wherein n is 2, is a highly acceptable copolymerizable ionic material for use in the present invention.

Further, the selection of R and Z is governed by the reactivity needed and the selection of Q is usually determined by the reaction used to attach the sulfonic acid to the base monomer (or vice versa).

Processes for preparing latexes containing resins of the aforementioned type are known, such latexes being commercially available and being referred to herein as "self-stabilizing latexes", that is, latexes, the polymeric particles of which contain in the polymer molecule functional groups that are effective in maintaining the polymeric particles dispersed in the aqueous phase of the latex. As mentioned above, such latexes do not require the presence of an external surfactant to maintain the particles in their dispersed state. Latexes of this type generally have a surface tension very close to that of water (about 72 dynes/cm). It has been observed that autodepositing compositions containing such latexes form coatings which build up at a relatively fast rate.

A preferred embodiment of this invention comprises the use of vinylidene chloride-containing latexes in which a water soluble ionic material such as, for example, sodium sulfoethyl methacrylate is copolymerized with the comonomers comprising the copolymer. Sodium sulfoethyl methacrylate is particularly effective for use with monomeric vinylidene chloride and the relatively hydrophilic monomers methyl methacrylate or methacrylic acid when used in the amounts and in the manner described above.

Particularly preferred latexes for use in this invention are latexes with about 35 to about 60 weight % solids comprising a polymeric composition prepared by emulsion polymerization of vinylidene chloride with one or more comonomers selected from the group consisting of vinyl chloride, acrylic acid, a lower alkyl acrylate (such as methyl acrylate, ethyl acrylate, butyl acrylate), methacrylic acid, methyl methacrylate, acrylonitrile, methacrylonitrile, acrylamide, and methacrylamide and stabilized with sulfonic acid or sulfonic acid salt of the formula:

wherein R represents vinyl or lower alkyl-substituted vinyl, Z represents one of the functional groups: ##STR3## n is an integer from 1 to 20 (preferably 1 to 6) and M.sup.+ is hydrogen or an alkali metal cation, preferably sodium or potassium.

A subgroup of preferred polymers are those having at least about 50% by weight of vinylidene chloride, but less than about 70%, and about 5 to about 35% vinyl chloride, and about 5 to about 20% of a vinyl compound selected from the group consisting of acrylic acid, methyl acrylate, ethyl acrylate, butyl acrylate, methacrylic acid, methyl methacrylate, acrylonitrile, methacrylonitrile, acrylamide and methacrylamide, and combinations thereof, and about 1 to about 3% by weight of sulfoethyl methacrylate.

A particularly preferred group of latexes, however, are latexes containing about 30 to about 70 weight % of solids formed by emulsion polymerization of about 50 to about 99% vinylidene chloride based on total weight of polymer and about 0.1 to about 5% by weight of sulfoethyl methacrylate, with optionally other comonomers selected from the group consisting of vinyl chloride, acrylic and methacrylic monomers such as acrylonitriles, acrylamides, methacrylamides and mixtures thereof in amounts between about 5 and about 50% by weight, and substantially free of unpolymerized surfactant or protective colloid.

Among other preferred subclasses of resin for use in this invention are dispersions of copolymers of about 50 to about 90% by weight vinylidene chloride, about 5 to about 30% by weight of butyl acrylate and about 1 to about 2% by weight of sulfoethyl methacrylate based on the total weight of polymer. Another preferred subclass of polymers are the latexes of vinylidene chloride-containing polymers internally stabilized with sulfoethyl methacrylate and free of surfactant, and including optionally vinyl chloride and one or more acrylic comonomers.

Another preferred vinylidene chloride-containing copolymer is one comprising about 15 to about 20 weight % vinyl chloride, about 2 to about 5 weight % butyl acrylate, about 3 to about 10 weight % acrylonitrile, about 1 to about 2 weight % sulfoethyl methacrylate. This particular copolymer will have less than 70% by weight vinylidene chloride copolymer based upon total weight of comonomers (including.the sulfoethyl methacrylate) used in the emulsion polymerization.

In its most preferred form, the present invention comprises the curing of coatings formed from internally stabilized vinylidene chloride-containing resins of the type used in Example 1 reported hereinbelow. Such resins are of relatively high crystallinity. Exemplary crystalline resins are described in U.S. Pat. No. 3,922,451 and aforementioned U.S. Pat. No. 3,617,368. Generally speaking, crystalline vinylidene chloride-containing resins comprise a relatively high proportion of vinylidene chloride, for example, at least about 80 wt. % thereof.

With respect to preferred externally stabilized vinylidene chloride-containing resins, they contain a high proportion of vinylidene chloride, that is, in excess of 50 wt. %. The amount of vinylidene chloride comprising the resin should be less than 100 wt. % for the reason that the pure homopolymer of vinylidene chloride is thermally unstable. Examples of monomers that can be copolymerized with vinylidene chloride to form a thermally stable copolymer include one or more of vinyl chloride, acrylic acid, methacrylic acid, methylacrylate, methyl methacrylate, ethylacrylate, butylacrylate, acrylonitrile, methacrylonitrile, acrylamide and methacrylamide. A few examples of externally stabilized vinylidene chloride polymers that can be used in the practice of the present invention are the following: (A) about 50 to about 90 wt. % vinylidene chloride, about 5 to about 20 wt. % acrylonitrile and about 5 to about 20 wt. % butyl acrylate; (B) about 60 to about 76 wt. % vinylidene chloride, about 4 to 10 wt. % ethylhexyl acrylate or methacrylate, and about 1 to about 4 wt. % acrylic acid; and (C) 66 wt. % vinylidene chloride and 34 wt. % vinyl chloride. In preferred form, the externally stabilized vinylidene chloride-containing resin is crystalline in nature.

Externally stabilized resins of the above type are prepared typically by emulsion polymerization utilizing a sufficient amount of surfactant to maintain the resulting resin particles in a dispersed state in the aqueous medium of the reaction mixture. The nature of this resin type is that the structure of the polymer molecule requires that surfactant be present to maintain the colloidal dispersion of the resulting latex, the surfactant being adsorbed on the surface of the resin particles.

Examples of surfactants (emulsifiers) that can be used to prepare such latexes are: sodium dodecylbenzene sulfonate, alkyl sulfates, sodium dioctyl sulfosuccinate, alkylphenolicethoxylate sulfonates, sodium dodecyldiphenyl oxide disulfonate, sodium oleoyl isopropanolamide sulfosuccinate, and sodium lauryl sulfate. These surfactants or emulsifiers are exemplary only; accordingly, it should be understood that the practice of this invention is not limited to latexes containing the aforementioned surfactants. For example, there can be used any anionic surfactant which will lower the interfacial tension between the monomeric reactants and water sufficiently to result in the formation of stable colloidal dispersions of the monomers in the water and, in addition, is stable in autodepositing compositions formulated therefrom. It is noted further that the activating system (for example, acid and oxidizer) of the autodepositing composition functions to dissolve from the metallic substrate positively charged cations which cause the negatively charged latex polymer particles to autodeposit on the metallic surface. The anionic surfactant should be a material which functions accordingly. Mixtures of two or more suitable anionic surfactants may be used.

For reasons mentioned above, and as explained in aforementioned U.S. Pat. No. 4,191,676, the surfactant concentration of externally stabilized latexes should be relatively low so that the aqueous phase of the autodepositing composition has a surfactant concentration below the critical micelle concentration and preferably below the surfactant concentration which corresponds to the inflection joint on a graph of surface tension versus the logarithm of surfactant concentration in the composition, as referred to in the aforementioned '676 patent. Accordingly, it is preferred that the latex containing the externally stabilized resin be prepared by emulsion polymerization with a very low concentration of surfactant. In the present state of the art, this is best achieved by seed polymerization or semicontinuous polymerization as opposed to batch emulsion polymerization. In the seed polymerization process or semicontinuous polymerization, the amount of surfactant can be limited by adding surfactant and monomer in such a manner that the monomer continues to polymerize with particles already present rather than forming new particles. This gives greater uniformity of particle size distribution and also gives good control of the total amount of surfactant in the latex. The stability of the colloidal latex can be partially attributed to charged polymer end groups whose provenance is the polymerization initiator.

In general, such latexes will have a relatively high surface tension, that is, at least about 40 dynes/cm. Such latexes can be used in the practice of the present invention, and preferably, there are used such latexes that have a surface tension of about 55 to 70 dynes/cm. In particular, such latexes in which there is no protective colloid are preferred class for use in the present invention.

Polymers having a vinylidene chloride content of between about 50 wt. % and about 90 wt. % or higher, based upon the total weight of polymer, can be used. When vinyl chloride is employed as one of the co-monomers, the vinylidene chloride content can be less than about 70 wt. %. The total chloride content, however, is preferably 50 wt. % or more based on total polymer weight. Generally speaking, the internally stabilized latexes which are the preferred class of latexes according to this invention can be used at even lower chloride contents, thereby enabling the inclusion of other copolymers to enhance the desirable polymer characteristics such as hardness, gloss, solvent resistance and the like, in addition to corrosion resistance attributable to reduced vapor permeability.

Latexes for use in the practice of the present invention are available commercially. Examples of such latexes are the Saran latexes such as, for example, Saran 143 and Saran 112 available from Dow Chemical Co., the Serfene latexes available from Morton Chemical, and the Haloflex latexes such as, for example Haloflex 202 available from Imperial Chemicals Industries.

If desired, the autodepositing composition can be prepared from two or more latexes containing different resins. Such a composition will comprise a blend of the vinylidene chloride-containing polymers described above, typically in an amount of about 50 to about 95% of the total resin solids, and one or more other latexes including, for example, styrene-butadiene resins, poly(vinyl chlorides), acrylic resins and the like.

The amount of the resin comprising the coating composition for practicing the present invention can vary over a wide range. The lower concentration limit of the resin particles in the composition is dictated by the amount of resin needed to provide sufficient material to form a resinous coating. The upper limit is dictated by the amount of resin particles which can be dispersed in the acidic aqueous composition. In general, the higher the amount of resin particles in the composition, the heavier the coating formed, other factors being the same. Although coating compositions can be formulated with a range of about 5 to about 550 g/l of resin solids, the amount of the resin solids will tend to vary depending on the other ingredients comprising the composition and also on the specific latex or resin used. For many applications, good results can be achieved utilizing about 50 to about 100 g/l of resin solids in the composition.

Optional ingredients can be added to the composition as desired. For example, it is believed that the present invention will be used most widely in curing pigmented autodeposited coatings. For this purpose, suitable pigments can be included in the composition. Examples of pigments that can be used are carbon black, phthalocyanine blue, phthalocyanine green, quinacridone red, benzidene yellow and titanium dioxide. The pigment should be added to the composition in an amount which imparts to the coating the desired color and/or the desired depth or degree of hue. It should be understood that the specific amount used will be governed by the specific pigment used and the color of coating desired. Excellent results have been achieved by using an aqueous dispersion in an amount such that the composition contains about 0.2 to about 3 g of furnace black/100 g of resin solids.

Many pigments are available in aqueous dispersions which may include surfactants or dispersing agents for maintaining the pigment particles in dispersed state. When utilizing such pigment dispersions in the composition, they should be selected so that the surfactant concentration in the aqueous phase of the composition is below the CMC, preferably below the surfactant concentration which corresponds to the inflection point on a graph of surface tension versus the logarithm of surfactant concentration in the composition. The surfactant should preferably be selected from those indicated above with respect to the preparation of the externally stabilized latexes. Suitable pigmented compositions are illustrated in examples herein.

Colored coatings can also be cured in accordance with the present invention. They can be formed from compositions which include a dye, examples of which include rhodamine derived dyes, methyl violiet, safranine, anthraquinone derived dyes, nigrosine, and alizarin cyanine green. These are but a few examples of dyes that can be used.

Examples of other additives that may be used in the autodepositing composition are those generally known to be used in formulating paint compositions, for example, UV stabilizers, viscosity modifiers, etc.

If a surfactant is added to the composition, either as a component of the latex, or with a pigment dispersion, or with other ingredients or additives, the total amounf of surfactant in the aqueous phase of the composition should be maintained below the CMC. Preferably, the aqueous phase of the composition contains little or no surfactant.

In case a surfactant is utilized, the preferred surfactants are the anionic surfactants. Examples of suitable anionic surfactants are the alkyl, alkyl/aryl or naphthalene sulfonates, for example, sodium dioctylsulfosuccinate and sodium dodecylbenzene sulfonate.

In preparing the autodepositing composition, the consituents thereof can be admixed in any suitable way, for example, as described in aforementioned U.S. Pat. No. 4,191,676.

Various steps of the overall coating process can be like those of the prior art, except as noted below. For example, cleaning of the metallic surface prior to coating and any water rinse steps effected subsequently to the cleaning step can be in accordance with the teachings of aforementioned U.S. Pat. No. 4,191,676. With respect to contacting the metallic surface with the autodepositing composition, it is believed that, for most applications, desired coating thicknesses can be obtained by immersing the metallic surface in the composition for a period of time within the range of about 30 seconds or even less to about 3 minutes. Good results have been achieved utilizing a time of immersion of not more than about 90 to about 100 seconds with compositions containing about 5 to about 10 wt. % of resin solids. However, it should be understood that longer or shorter periods of time can be used. Agitating the composition aids in maintaining it uniform and in improving the uniformity of the coatings formed. Other factors held constant, heating of the composition will result in heavier coatings. However, satisfactory results can be obtained by operating the coating process at ambient temperature.

Various of the autodeposited coatings which can be cured in accordance with the present invention, upon being immediately withdrawn from the coating composition, are initially tightly adherent to the metallic substrate. Others may require a short period of air drying, for example, about 30 seconds to about 2 minutes, to develop satisfactory adherence.

Water rinsing the coated surface after it has been withdrawn from the composition, and before significant drying takes place is effective in removing therefrom residuals such as acid and other ingredients of the composition that adhere to the coated surface. If such residuals are allowed to remain on the coated surface, they may change or adversely affect the quality of the coating. For a specific application, a determination can be made as to whether the residuals cause adverse effects which are not tolerable. If they do, they should be removed, for example, by water rinsing with tap or deionized water. Further, the residuals can be removed or rendered unreactive by treatment with an alkaline material, suitably an aqueous solution of an akaline material, for example, a solution of about 0.1 to about 2 g/l of caustic, preferably an aqueous solution of about 2 to about 15 g/l of ammonium hydroxide or an alkali metal hydroxide, particularly potassium or sodium hydroxide. (The use of such alkaline material is described in U.S. application Ser. No. 635,968, filed July 31, 1984, the disclosure of which is incorporated herein by reference as it relates to said alkaline treatment.) If removal of residuals is not necessary to the finish required, this step of removing them can be avoided.

Prior to curing the autodeposited coating in accordance with the present invention, it can be subjected to a treatment which is designed to increase the corrosion resistance of the coating. Examples of such treatments include rinsing the freshly applied coating with aqueous solutions of chromium compounds or with aqueous solutions of phosphoric acid. For example, U.S. Pat. Nos. 3,795,546 and 4,030,945 disclose methods of treating freshly formed autodeposited coatings with aqueous rinse solutions containing hexavalent chromium or aqueous solutions containing mixtures of hexavalent chromium and formaldehyde-reduced forms of hexavalent chromium to improve the corrosion resistance of the autodeposited coatings. U.S. Pat. No. 3,647,567 discloses the use of chromium-containing solutions and also the use of an aqueous solution of phosphoric acid. An advantage of autodeposited coatings formed from preferred vinylidene chloride-containing resins, as described above, is that they possess unusually high corrosion resistant properties without being so treated. However, they too may be so treated.

Following any rinse steps employed after the coated surface is withdrawn from the composition, the coating can be cured by the use of an aqueous curing medium, as described in the aforementioned parent applications. Curing of the resinous coating renders it continuous, thereby improving its resistance to corrosion and its adherence to the underlying metallic surface. Exemplary means of aqueous curing include spraying the freshly coated part with hot water, immersing the freshly coated part in hot water and exposing the freshly deposited coating to an atmosphere of steam. Utilizing such aqueous cure, heat can be transferred more quickly into the coated mass with the result that the temperatures needed for full development of coating properties are reached more quickly than they are reached when heating the coated part in air.

As mentioned above, it is believed that such aqueous cure will be used most widely in curing autodepositing coatings comprising resins which have a minimum film temperature (MFT) or glass transition temperature (Tg) of no greater than about 140.degree. F., preferably no greater than 100.degree. F., and most preferrably no greater than about 85.degree. F. Exemplary resins of the type mentioned above can have Tg's within the range of about 30.degree. F. to about 85.degree. F. They can also have an accelerated cure temperature of no greater than about 300.degree. F. With respect to resins having a Tg well below room temperature, improvements can be realized by curing with water having a temperature of at least 70.degree. F. In general, however, it is believed that most curing applications will utilize hot water, for example, water having a temperature of 100.degree. F. or more.

Resins which do not inherently have Tg's or MFT's in the desired range can be modified by the use of solvents or plasticizers in accordance with the state of the art to reduce their Tg and MFT values appropriately.

The temperature and time of treatment of the autodeposited coating will depend upon the nature of the particular resin comprising the coating. The treating conditions should be selected so that the properties of the coating are fully developed and so that the coating is not affected adversely. Exemplary conditions include treating times of about 5 seconds to about 5 minutes (although longer times can be used) at temperatures within the range of about 185.degree. F. to about 212.degree. F. (although higher temperatures can be used in the case of super heated steam). It is believed that many applications will require no more than about 2 to 3 minutes and even less time of pretreatment when using temperatures within the aforementioned range. Particularly, in the case of steam, there may be many applications in which the treating time is less than 5 seconds. Autodeposited coatings comprising vinylidene chloride-containing resins of the type described above can be properly heated or "cured" to achieve full development of coating properties by immersion of the freshly formed autodeposited coating into hot water for as little as about 5 to about 30 seconds at temperatures of about 185.degree. F. to about 212.degree. F.

Steam curing has a number of advantages over the "hot water cure" method described above. One advantage accruing to the use of a steam atmosphere for curing the freshly formed coating is that the parts need not be immersed. This is important when parts are being processed on an conveyor line. In order to immerse a part in hot water, the conveyor line must change directions as the part is carried down into a tank of hot water. Once the part has been "cured", the conveyor must then change directions again to remove the part from the hot water tank. By the use of steam to effect curing of the autodeposited coating, the conveyor simply carries the parts into a tunnel which contains the steam. There is no need for the conveyor to change directions as in an immersion process.

A further advantage accruing to the use of steam for curing autodeposited coatings is that steam has a higher calorific value than does hot water. For example, a gram of steam at 212.degree. F. has a higher heat content than a gram of water at the same temperature. This provides a greater energy source for transferring energy rapidly to the freshly formed autodeposited coating. Still another advantage accruing to the use of steam is that only a relatively small amount of water need be converted to steam as opposed to raising the temperature of an entire tank of water to the operating temperature.

Post treatment steps such as rinsing the freshly formed autodeposited coating to remove therefrom residuals or rinsing in an aqueous chromium compound-containing solution and curing of the coating can be combined into one step. Thus, for example, rinsing and curing can be done simultaneously by spraying with hot water or immersing the freshly formed autodeposited coated surface into an aqueous bath. In addition, the water or steam cure process can be used in combination with heretofore known curing methods. For example, a short treating time involving aqueous cure can be used to quickly heat the coating (which surprisingly can result in drying of the coating) followed by baking or other curing methods, for example, curing by infrared radiation. However, it is believed that for most applications, curing can be effected completely utilizing the water or steam cure described above so that baking and like steps can be avoided entirely. It should be recognized that curing can be effected also by subjecting the coated surface to multiple exposures of a water bath(s) and/or steam environment(s).

It is believed also that aqueous cure will be used widely in curing coatings of the type that do not tend to crack, such as is described in aforementioned U.S. Pat. No. 4,318,944. There are types of autodeposited coatings, including those formed from vinylidene chloride-containing copolymers of the type described above, as well as others, which do not tend to crack as a result of exposure to oxygen under normal plant operating conditions. Accordingly, such coatings do not require the special treatment steps described in the aforementioned patent.

With respect to the present invention, the adhesive properties of an aqueously cured autodeposited coating can be improved by including in the aqueous curing medium a solute, that is, a material which is soluble in the aqueous curing medium at the temperature at which curing is effected. The solute should be stable under the curing conditions used and it should not affect adversely the properties of the autodeposited coating. The solute is preferably a material which does not require for disposal the use of special techniques, as are used, for example, in disposing of toxic materials such as chromium. The solute can be selected from a wide variety of materials, including organic and inorganic materials. Examples of classes of materials which can be used are surfactants, strong and weak acids and bases and salts thereof, metal oxides, and transition metal complexes, including organometallic compounds. A mixture of two or more solutes can be used.

Although developmental work appears to show that the inclusion of any solute in the aqueous curing medium tends to improve the adhesive properties of the aqueously cured coating, such work has shown also that certain classes of materials are more effective and/or more efficient in operation than others. For example, some materials can be used effectively in smaller amounts than others.

Preferred classes of materials include amphoteric and cationic surfactants, including those that contain, for example, N, S, or O in the molecule. Particularly preferred are amphoteric and cationic surfactants that are N- or S-containing, with the N-containing amphoteric and cationic surfactants being most preferred. When using an amphoteric surfactant, it is preferred that the aqueous curing medium be acidic.

Examples of classes of amphoteric surfactants that can be used are: ethoxylated alkyl phenols; alkanolamides; solfonated amine and amide compounds; and betaine derivatives. More specifically, the following are exemplary of amphoteric surfactants that can be used: cocoamidopropyl betaine; ethoxylated sulfate alkyl phenol; oleyl amidopropyl betaine; laurylamine oxide; t-butyl benzoic acid/triethanolamine; and cetyl betaine. Examples of classes of cationic surfactants that can be used are: quarternary ammonium salts; imadazolines and derivatives thereof; simple amine salts; amino amides; and amidazolines. More specifically, the following are exemplary of cationic surfactants that can be used: dimethyl distearyl ammonium chloride; polypropoxy quarternary ammonium acetate; myristyl trimethyl ammonium bromide; stearyl dimethyl benzyl ammonium chloride; and dimethyl dicoco quarternary ammonium chloride.

As mentioned above, the soluble material for use in the aqueous curing medium should not affect adversely the properties of the autodeposited coating. In this connection, it is noted that there are certain materials which, when used in the aqueous curing medium in a quantity sufficient to achieve the desired results, tend to harm t he coating. Exemplary of such materials are those which when present in relatively small amounts change significantly the pH of the aqueous curing medium. Examples of such materials are strong bases, for example, ammonium and alkali metal hydroxides, and strong acids and salts of strong acids. The aforementioned are used preferably in admixture with one or more additional solutes selected from other classes of materials, including, for example, a water soluble material which functions as a buffer.

By way of example, it is noted that the use of about 2 g/l of sodium hydroxide in an aqueous curing bath raises the pH of the bath to about 11. The use of such a bath tends to improve the adhesive properties of the coating, but to a lesser degree than generally desired for commercial applications. An increase in concentration to 5 g/l of NaOH imparts further improvements, but also to a lesser degree than generally desired for commercial applications. And, at such higher concentrations of NaOH, degradation of the coating can be encountered. By way of further example, it is noted that the use of acid in an amount to impart a pH to the aqueous curing bath of about 2 or less tends to cause a deformation of the coating. This is attributed to the evolution of hydrogen caused by acid attack of the metallic surface. The use of smaller amounts of acid tends to improve the adhesive properties of the coating to a lesser degree than generally desired for commercial applications. It is recommended that such acidic and alkaline materials be used in admixture with other solutes, for example, surfactants.

The pH of the solute-containing aqueous curing medium can vary over a wide range. The lower and upper pH limits are governed by the acidity or alkalinity that affects adversely the coating. A pH of about 3 to about 11 is recommended.

Certain solutes can comprise materials which in addition to improving the adhesive properties of the initially cured coating, improve and/or impart other desired properties to the coating, including, for example, functional and/or aesthetic properties. In this connection, there can be used, for example, an aqueous dispersion of solids or an emulsion which includes a water soluble surfactant. The surfactant constituent of the aforementioned functions to improve the adhesive properties of the coating and the solids of the aqueous dispersion or the suspended liquids particles of the emulsion can affect other properties of the coating. Examples of such properties include appearance, texture, mar resistance and impact resistance. For example, the use of a dispersion of resin such as, for example, polytetrafluroethylene or of a dispersion of wax can affect the gloss, texture and/or mar resistance of the coating.

The amount of solute included in the aqueous curing medium should be that which is effective in improving the adhesive properties of the initially cured autodeposited coating. Such amount will tend to vary depending on many factors, including, for example, the particular solute used, the thickness of the coating, the particular resin comprising the coating, the mass of the coated substrate, and the time during which the coated surface is in contact with the solute. For any particular application, it is recommended that empirical testing be used to determine the effective amount of solute to use. For guideline purposes, it is noted that developmental work involving the evaluation of different kinds of solutes has shown that improvements in adhesion can be realized by using an amount of solute within the range of about 0.1 to 500 g/l. Preferred amounts are within the range of about 1 to about 100 g/l. It should be understood that, depending on the factors mentioned above, amount adjustments may be necessary.

Examples below are illustrative of the present invention. Comparative examples are set forth also.

Unless stated otherwise, the following steps were used sequentially in the preparation of the embodiments which are the subjects of the examples. Steel parts were first cleaned with an appropriate acid and/or alkaline cleaner, depending on the nature of the soil present on the metallic part. The cleaned part was then rinsed with water and thereafter immersed for 90 seconds in an autodepositing composition at room temperature. The autodepositing composition comprised 93 g/l of Saran 143 latex, 1.6 g/l of ferric fluoride, 1.3 g/l of HF, and 3.0 g/l of Aquablak S black pigment which contains 45% pigment solids in the aqueous dispersion. After withdrawing the uncured coated part from the autodepositing composition, it was exposed to the air for about 80 seconds and then rinsed by immersing it in water for about 60 seconds. The rinsed, uncured autodeposited coating was cured by immersing the coated part in a water bath under the temperature and time conditions indicated in the examples below. The adhesive properties of the cured coating were evaluated after the coated part was withdrawn from the water-cure bath. The evaluation took place at the time indicated in each of the examples, the time being measured from when the part was withdrawn from the water-cure bath to the beginning of the evaluation. Speaking generally, the evaluation consisted of placing a piece of transparent tape over an "X" that was scribed on the coating to the base metal, stripping the tape from the coating, and calculating the relative amount, if any, of the coating that was removed from the part by virtue of its adhering to the stripped tape. The tape is sold by 3M Co. under the designation "610 tape" and was 1" wide, the maximum width of the "X" also being about 1". The percent of coating removed from the part was calculated by dividing the area from which coating (if any) was stripped by the area covered by the tape and multiplying the quotient by 100.

In the first group of examples, steel panels, 2".times.3".times.1/4", were treated in the manner described above. Each of the uncured coatings was cured by immersing the coated panel for 30 seconds in a water-cure bath having a temperature of 180.degree. F. and containing anhydrous sodium metasilicate, as indicated in Table 1 below. A control example showing the use of a water-cure bath without the sodium metasilicate is set forth also in Table 1.

Improvements in the adhesive properties of the cured coatings as a result of the use of anhydrous sodium metasilicate are evident from the results reported in Table 1 above.

The next group of examples shows the use of a water-cure bath containing anhydrous sodium metasilicate to cure coatings formed on 1/2" diameter round bars (Example No. 3) and on 5/8" diameter round bars (Example No. 4). (The autodepositing composition used to coat the 158" bars had a ferric fluoride concentration of 3.2 g/l.) Each of the coated articles was immersed for 30 seconds in a water-cure bath that had a temperature of 185.degree.-190.degree. F. and a Na metasilicate content of 70 g/l.

The next group of examples illustrates the use of water-cure baths containing tetra sodium pyrophosphate in accordance with the present invention. The metallic parts used in this series of tests were 5/8" diameter bars. Each of the coated parts was immersed for 30 seconds in a water-cure bath having a temerature of 185.degree. F. and containing an amount of pyrophosphate as indicated in Table 3. below.

The thicknesses of the coatings formed on the 5/8" round bars which are the subjects of Nos. 5 and 7 above were 0.4-0.5 mil. Thicker coatings, varying from 0.6-0.9 mil., were formed on another group of 5/8" round bars by increasing the times the parts were immersed in the autodepositing composition. Curing of these thicker coatings showed that the use in the water-cure bath of 70 g/l of the pyrophosphate resulted in cured coatings that had satisfactory adhesive properties within 8 mins. after hot water-cure. Coatings cured in baths containing smaller amounts of the pyrophosphate required longer times to develop satisfactory adhesive properties.

The next example shows the use in the water-cure bath of sodium chloride in accordance with the present invention.

After coating a round bar having a diameter of 5/8", it was immersed in a water-cure bath that had a temperature of 190.degree. F. and contained 100 g/l of sodium chloride. Cure time was 30 seconds. After the coated bar was withdrawn from the water-cure bath, it was allowed to cool to room temperature (this took about 45 minutes), and then it was taped. After the tape was stripped, it was observed that there was some coating loss at the very smooth areas of the bar, but very little coating loss at the rough areas. Retesting after 12 hours showed no coating loss.

A preferred solute for use in the practice of the present invention is a polyamine prepared by condensing polyethylene glycol and tallow fatty amine. The next group of examples illustrates the use of such polyamine in an aqueous cure bath. The polyamine used is sold in the form of an aqueous solution (50 wt. %) by Henkel Corporation under the trademark POLYQUART H. This material is characterized as a "psuedo-cationic" surfactant which is compatible with nonionic, anionic and cationic surfactants, and which is particularly suited for use as an antistatic in hair products. Chemically, this material is referred to as "PEG 15 tallow polycondensate". The results of the use of this polyamine in varying amounts in water-cure baths having a temperature of 195.degree..+-.10.degree. F. are set forth in Table 4 below. Autodeposited coatings having the thicknesses indicated in Table 4 were present on 5/8" round bars which were immersed in the cure bath for 30 sec.

The next group of examples illustrates the use of a mixture of solutes in the practice of the present invention.

The above comprises various exemplary embodiments of the present invention in connection with its use in the aqueous cure of autodeposited coatings. Although it is believed that the present invention will be used most widely in such applications, it should be understood that it can be used also in other applications in which a film of water is sandwiched between a substrate and another type of coating, for example other types of resin and pigmented resin coatings. Although it is believed that the widest use of the present invention will entail the use of a solute-containing aqueous cure medium, the invention contemplates also initially curing the coating by use of an aqueous curing medium and then subjecting the aqueously cured coating to an aqueous solution of solute. For example, the uncured coating can be cured in an aqueous curing bath, withdrawn therefrom and then immersed in a bath of an aqueous solution of solute for a sufficient period of time to improve the adhesive properties of the cured coating.