US 3491011 A
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United States Patent 3,491,011 METHOD OF ELECTRODEPOSITING COATING COMPOSITIONS WITH REDUCED SOLUBLE CHROMATE IONS Louis R. Le Bras, Gibsonia, Pa., Frederick M. Loop, North Olmsted, Ohio, and Donald P. Hart, Allison Park, and Joseph E. Plasynski, Arnold, Pa., assignors to PPG Industries, Inc., Pittsburgh, Pa., a corporation of Pennnsylvania No Drawing. Filed May 4, 1965, Ser. No. 453,180 Int. Cl. C23b 13/00; B01k 5/02 US. Cl. 204-181 7 Claims ABSTRACT OF THE DISCLOSURE It has been found that soluble chromate ions have detrimental effects in electrodepositable compositions. This invention relates to aqueous chromate-containing coating compositions having reduced levels of soluble chromate ions to concentrates of electrodepositable compositions, to the method of electrodeposition and to the articles so coated. The compositions are formed by ageing a composition comprising a polycarboxylic acid vehicle resin, strontium chromate pigment and Water at a pH below 7.0, thereby reducing the soluble chromate ion concentration, preferably to a level less than about 300 parts per million.
This invention relates to a method of making electrodeposition concentrates and to the electrodepositable compositions derived from said concentrates. More particularly, this invention relates in part to a method of preparing electrodepositable compositions containing a controlled level of soluble chromate ions.
Electrodeposition is a relatively new coating technique, which, although based on well-known principles, has only recently become technically feasible through the development of electrodepositable compositions which have the desired characteristics to meet the demands placed on a modern coating material. The coatings achieved have excellent properties for many applications and electrodeposition results in a coating which does not run or Wash off during baking. Virtually any conductive substrate may be coated by electrodeposition. Most commonly employed substrates include base metals such as iron, steel, copper, zinc, brass, tin, nickel, chromium and aluminum, as well as other metals and pretreated metals. Impregnated paper or other substrates rendered conductive under the conditions employed may also be coated.
While electrodeposition is in many respects advantageous compared to ordinary application methods, problems have arisen. Examples of these problems include obtaining smooth, uniform films on substrates which are coated and also the problem of film build or film thickness and the related problem of the limitations on the voltage which might be impressed upon the system.
It has been found, for example, that when a relatively high level of soluble chromate ions exist in electrodeposition compositions, such as those present when strontium chromate is used as a corrosion-inhibitive pigment, that at least some or all of the following problems arise:
That at relatively high voltages, a pigskin pattern is found on the article being coated; and/or that there is a reduction in throwing power of the electrodeposition composition and the ruptured voltage of the electrodeposition composition is lowered. Furthermore, it is often found that the presence of soluble chromate ions reduces the wet adhesion of the electrodeposition film prior to drying, which is usually accomplished by baking. Ordinarily, a freshly-deposited film appears quite dry and feels only slightly tacky. Such a film is relatively resistant to physical distortion and is relatively water-insoluble. This water insolubility permits the film to be rinsed with water in order to remove bath dragout. Dragout consists of non-deposited paint which adheres to the coated article upon its removal from the bath. For best appearance of the deposited film, it is desirable to rinse off the dragout. It has been found that when high levels of soluble chromate ion exist in the electrodeposition bath that when the panel is rinsed in the usual manner portions of the deposited film may be removed or damaged. Yet another problem presented by high soluble chromate ion levels is that there is a high specific conductivity in the bath, causing more gasssing due to accelerated electrolysis of the water, which is the major solvent, and eventual film rupture at a relatively low voltage.
When a continuous electrodeposition process is attempted, special problems arise, since, as the paint solids are coated onto the substrates, there is a tendency to build up a concentration of cations in the electrodeposition bath and this is manifested by an increase in the pH of the bath. When this occurs, there is a subsequent deterioration in the quality of the deposited film. In addition to film degradation, the buildup in cations, derived from the neutralizing agent, leads to a bath having higher conductivity, which in turn requires higher current densities to deposit films of suitable thickness. The excess cations can be reduced and the pH lowered by the addition of coating compositions which are made from unneutralized or relatively les neutralized vehicles than those used in making the original composition. It has been found desirable to use concentrates, a composition of high solids content, and preferably one in which the vehicle has been slightly neutralized since this aids in disbursing the additional composition throughout the electrodeposition bath. Compositions made from vehicles which have up to 20 percent of their acidity neutralized have generally been employed for this purpose.
A method has now been found of producing concentrates containing chromates which in turn produce, on one hand, electrodepositable compositions when employed in electrodepositions baths yield improved properties and, on the other hand, when employed to replenish the supply of electrodepositable composition in the bath are more readily dispersible and which prolong the stability of the electrodeposition bath and which, when chromate ions are present in electrodepositable composition, control the level of soluble chromate ions.
A number of electrodepositable resins are known and can be employed to provide the electrodepositable composition of this invention. Virtually any water-soluble, water-dispersible or water-emulsifiable polycarboxylic resinous material can be electrodeposited and, if filmforming, provides a coating which may be suitable for certain purposes. Any such electrodepositable is included among those which can be employed in the present invention, even though the coating obtained may not be entirely satisfactory for certain specialized uses.
The preferred resins which may be employed in the process invention comprise a reaction product or adduct of the drying oil or semi-drying oil fatty acid ester with a dicarboxylic acid or anhydride. By drying oil or semidrying oil fatty acid esters are meant esters of fatty acids which are or can be derived from drying oils or semidrying oils, or from such sources as tall oil. Such fatty acids are characterized by containing at least a portion of polyunsaturated fatty acids. Preferably, the drying oil or semi-drying oil per se is employed. Generally, drying oils are those oils which have an iodine value of above about 130, and the semi-drying oils are those which have an iodine value of about 90 to 130, as determined by method ASTMDl467-57T. Examples of such esters include linseed oil, soya oil, saifiower oil, perilla oil, tung oil, citicica oil, poppyseed oil, sunflower oil, tall oil esters, Walnut oil, dehydrated castor oil, herring oil, menhadan oil, sardine oil and the like.
Also included among such esters are those in which the esters themselves are modified with other acids, including saturated, unsaturated or aromatic acids such as butyric acid, stearic acid, linoleic acid, phthalic acid, isophthalic acid, terophthalic acid or benzoic acid, or an anhydride of such an acid. One inexpensive acid material which has been found to produce good results in many instances is resin, which is composed of chiefly abiotic acid and other resin acids. The acid modified esters are made by transesterification of the ester, as by forming a dior monoglyceride by alcoholysis, followed by esterification with the acid; they may also be obtained by reacting oil acids with a polyol and reacting the acid with the partial ester. In addition to glycerol, alcoholysis can be carried out using other polyols such as trimethylolpropane, pentaerythritol, sorbitol, and the like. If desired, the esters can also be modified with monomers such as cyclopentadiene or styrene and the modified esters produced thereby can be utilized herein. Similarly, other esters of unsaturated fatty acids, for example, those prepared by the esterification of tall oil fatty acids with polyols, are also useful.
Also included within the terms drying oil fatty acid esters and semi-drying oil fatty acid esters as set forth herein are alkyd resins prepared utilizing semi-drying or drying oils; esters of epoxides with such fatty acids, including esters of diglycidyl ethers of polyhydric compounds as well as other mono-, diand polyepoxides; semi-drying or drying oil fatty acid esters of polyols, such as butanediol, trimethylolethane, trimethylolpropane, trimethylolhexane, pentaerythritol, and the like; and semidrying or drying fatty acid esters of resinous polyols such as homopolymers or copolymers of unsaturated aliphatic alcohols, e.g., allyl alcohol or methallyl alcohol, including copolymers of such alcohols with styrene or other ethylenically unsaturated monomers or with non-oil modified alkyd resins containing free hydroxyl groups.
Any alpha, beta-ethylenically unsaturated dicarboxylic acid or anhydride can be employed to produce the reaction products described herein. These include such anhydrides as maleic anhydride, itaconic anhydride, and other similar anhydrides. Instead of the anhydride, there may also be used ethylenically unsaturated dicarboxylic acids which form anhydrides, for example, maleic acid or itaconic acid. These acids appear to function by first forming the anhydride. Fumaric acid, which does not form an anhydride, may also be utilized, although in many instances it requires more stringent conditions than the unsaturated dicarboxylic acid anhydrides or acids which form such anhydrides. Mixtures of any of the above acids or anhydrides may also be utilized. Generally speaking, h anhydride or acid employed 9Qrtt in from 4 to 12 carbon atoms, although longer chain compounds can be used if so desired.
While the exact nature of the reaction product of the acid or anhydride with the fatty acid ester is not known with certainty, it is believed that the reaction takes place by addition of the unsaturated linkage of the acid or anhydride to the carbon chain of the oil. In the case of nonconjugated double bonds, such as are present in linseed oil, the reaction may take place with the methylene group adjacent the nonconjugated double bond. In the case of oils having conjugated double bonds, such as tung oil, the reaction is probably of the Diels-Alder type.
The reaction between the acid or acid anhydride and the drying oil or semi-drying oil fatty acid ester takes place readily without the use of a catalyst and at temperatures in the range of about C. to about 300 C. or higher, with the reaction generally being carried out between about 200 C. and about 250 C.
While the reaction products can be comprised solely of adducts of the fatty acid ester and the dicarboxylic acid or anhydride, in many instances it is desirable to incorporate into the reaction product another ethylenically unsaturated monomer. The use of such monomer often produces films and coatings which are harder and more resistant to abrasion and which may have other similar desirable characteristics. For this purpose, any ethylenically unsaturated monomer can be employed. Examples of such monomers include monoolefinic and diolefinic hydrocarbons such as styrene, alpha-methyl styrene, alpha-butyl styrene, vinyl toluene, butadiene-l,3, isoprene, and the like; halogenated monoolefinic and diolefinic hydrocarbons, such as alpha-chlorostyrene, alphabromostyrene, chlorobutadiene and similar compounds; esters of organic and inorganic acids, such as vinyl acetate, vinyl propionate, vinyl 2-chlorobenzoate, methyl acrylate, ethyl methacrylate, butyl methacrylate, heptyl acrylate, decyl methacrylate, methyl croton ate, isopropenyl acetate, vinyl alpha-bromopropionate, vinyl alpha-chlorovalerate, allyl chloride, allyl cyanide, allyl bromide, allyl acetate, dimethyl itaconate, dibutyl itaconate, ethyl alpha-chloroacrylate, isopropyl alpha-bromoacrylate, decyl alphachloroacrylate, dimethyl maleate, diethyl maleate, dimethyl fumarate, diethyl fumarate, and diethyl glutaconate; organic nitriles, such as acrylonitrile, methacrylonitrile, and ethacrylonitrile; and the like.
As is apparent from the above discussion and the examples set forth, which, of course, do not include all of the ethylenically unsaturated monomers which may be employed, any such monomer can be utilized. The preferred class of monomers can be described by the formula:
where R and R are hydrogen or alkyl, R; is hydrogen, alkyl or carboxyalkyl and R is cyano, aryl, alkyl, alkenyl, aralkyl, alkaryl, alkoxycarbonyl or aryloxycarbonyl. The preferred compounds are styrene, substituted styrenes, alkyl acrylates, alkyl methacrylates, diolefins and acrylonitrile.
The reaction of the fatty acid ester, the acid or anhydride and any additional monomer or monomers can be carried out concurrently, that is, with each of the components of the reaction product being mixed together and heated to reaction temperature. However, because the monomer and the acid or anhydride are often quite reactive with each other, the oil or other fatty acid ester is preferably first reacted with the acid or acid anhydride, and then this product is subsequently reacted with any ethylenically unsaturated monomer or monomers employed. For example, a reaction product of linseed oil, maleic anhydride and styrene is made by first reacting maleic anhydride with linseed oil and then reacting the maleinized oil with styrene. When the process is carried out in this manner, the reaction of the additional monomer with the initial reaction product is usually carried out at somewhat lower temperatures, usually between about 25 C. and 200 C.
The proportions of each of the components gOing to make up the reaction product are ordinarily not critical. Generally speaking, between about percent and about 45 percent by weight of the unsaturated acid or acid anhydride is reacted with from about 55 percent to about 90 percent by weight of fatty acid ester. In the presently preferred products, usually percent to percent of anhydride and 70 percent to 85 percent of oil ester are employed. If an ethylenically unsaturated monomer is incorporated in the reaction product, it is typically used in amounts between about 5 percent and about percent by weight, based upon the total weight of acid or anhydride and ester, with between 10 percent and 25 percent being used in those products preferred at present. Thus, in most instances the total composition of the reaction product may comprise from about 35 percent to about 90 percent by weight of the fatty acid ester and from about 10 percent to about 65 percent of the acid or anhydride and other monomer combined, with between about 6 percent and about percent of the acid or anhydride always present.
The products produced in the above manner are comprised of polymeric chains of moderate length. The average molecular weight of the products to be used in electrodeposition should be low enough so that its flow characteristics at high solids are maintained, but high enough to provide adequate throwing power. The desirable molecular weight levels vary with the coating composition and conditions employed. Generally those products having molecular weights of up to 10,000 or somewhat higher have given the best results.
Neutralization of these products is accomplished by reaction of all or part of the dicarboxylic anhydride groups with a base. Usually up to about half of such groups are neutralized in unesterified adducts; the partially esterified products are often neutralized to a greater extent, based on unesterified acid groups remaining.
It is preferred in certain instances that the neutralization reaction be carried out in such a manner that amido groups are attached to part of the carbonyl carbon atoms derived from the dicarboxylic acid or anhydride. By amido groups are meant trivalent nitrogen atoms attached with one valence to the carbonyl carbon atom with the other two valences being linked to hydrogen or carbon atoms in the same or different organic radicals. Amido groups are formed, for example, when the reaction with the neutralizing base is carried out with a water solution of ammonia, a primary amine or a secondary amine, or when the product is reacted with such an amine in the absence of water.
Compositions within this general class are described in copending applications, Ser. No. 222,674, filed Sept. 10, 1962, now US. 3,366,563; and Ser. No. 282,880, filed May 24, 1963, now US. 3,369,938.
Another type of electrodepositable coating composition which gives desirable results are the water-dispersible coating compositions comprising at least partially neutralized interpolymers of hydroxyalkyl esters of unsaturated carboxylic acids, unsaturated carboxylic acids and at least one other ethylenically unsaturated monomer. These are employed in the composition along with an amine-aldehyde condensation product or a polyepoxide, or both, with the interpolymer usually making from about percent to about 95 percent by weight of the resinous composition.
The acid monomer of the interpolymer is usuall acrylic acid or methacrylic acid, but other ethylenically unsaturated monocarboxylic and dicarboxylic acids, such as ethacrylic acid, crotonic acid, maleic acid, or other acids of up to about 6 carbon atoms can also be employed. The
hydroxyalkyl ester is usually hydroxyethyl or hydroxypropyl acrylate or methacrylate, but also desirable are the various hydroxyalkyl esters of the above acids having, for example, up to about 5 carbon atoms in the hydroxyalkyl radical. Monoor diesters of the dicarboxylic acids mentioned are included. Ordinarily, the acid and ester each comprise between about 1 percent and about 20 percent by weight of the interpolymer, with the remainder being made up of one or more other copolymerizable ethylenically unsaturated monomers. The most often used are the alkyl acrylates, such as ethyl acrylate; thealkyl methacrylates, such as methyl methacrylate; and the vinyl aromatic hydrocarbons, such as styrene; but others can be utilized.
The above interpolymer is at least partially neutralized by reaction with a base as described above; at least about 10 percent, and preferably 50 percent or more of the acidic groups are neutralized, and this can be carried out either before or after the incorporation of the interpolymer in the coating composition. The bases above can be used, with ammonia and amines being preferred; except when a polyepoxide is present, in which case there is preferably employed a hydroxide, such as sodium hydroxide, or if an amine, a tertiary amine.
The amine-aldehyde condensation products included in these compositions are, for example, condensation products of melamine, benzoquanamine, or urea with formaldehyde, although other amino-containing amines and amides, including triazines, diazines, triazoles, guanadines, guanamines and alkyl and aryl-substituted derivatives of such compounds can be employed, as can other aldehydes, such as acetaldehyde. The alkylol groups of the products can be etherified by reaction with an alcohol, and the products utilized can be water-soluble or organic solvent-soluble.
The electrodepositable compositions can also include a polyepoxide, which can be any epoxide compound or mixture with an epoxy functionality of greater than 1.0. Numerous such polyepoxides are known and are described in patents such as US. Patents Nos. 2,467,171; 2,615,007; 2,716,123; 2,786,067; 3,030,336; 3,053,855; and 3,075,999. Included are polyglycidyl ethers of polyphenols, such as bisphenol A, or aliphatic polyhydric alcohols, such as 1,4-butanediol; polyglycidyl esters of polycarboxylic acids, such as diglycidyl adipate; and polyepoxides from the epoxidation of unsaturated alicylic compounds, such as 3,4-epoxy-6-methylcyclohexylmethyl- 3,4-epoxy-6-methylcyclohexanecarboxylate.'
Electrodepositable compositions comprising the above interpolymers and an amine-aldehyde resin or a polyepoxide, or both, are more fully describedin copending application Serial No. 368,394, filed May 18, 1964, now
Still another electrodepositable composition of desirable properties comprises an alkyd-amine vehicle, that -is, a vehicle containing an alkyd resin and an aminealdehyde resin. A number of these are known in the art and may be employed. Preferred are water-dispersible alkyds such as those in which a conventional alkyd (such as a glyceryl phthalate resin), which may be modified with drying oil fatty acids, is made with a high acid number (e.g., 50 to 70) and solubilized with ammonia or an amine, or those in which a surface active agent, such as a polyalkylene glycol (e.g. Carbowax), is incorporated. High acid number alkyds are also made by employing a tricarboxylic acid, such as trimellitic acid or anhydride, along with a polyol in making the alkyd.
The above alkyds are combined with an aminealdehyde resin, such as those described hereinabove. Preferred are water-soluble condensation products of melamine or a similar triazine with formaldehyde with subsequent reaction with an alkanol. An example of such a product is hexakis (methoxymethyl)melamine.
The alkyd-amine compositions are dispersed in water and they ordinarily contain from about 10 percent to about 50 percent by weight of amine resin based on the total resinous components.
Examples of compositions of this class are described in US. Patents Nos. 2,852,475; 2,852,476; and 2,853,459.
The neutralization and solubilization of the above vehicles is accomplished by the use of a base. Inorganic bases such as metal hydroxides or, more desirably, ammonia can be used for this purpose, as can organic bases, particularly amines. Among the preferred class of neutralizing bases are ammonia and any basic amine. Examples of such amine are primary and secondary amines including alkyl amines, such as methylamine, ethylamine, propylamine, butylamine, amylamine, dimethylamine, diethylamine, dipropylamine, dibutylamine, and N-methylbutylamine; cycloalkyl amines, such as cyclohexylamine; unsaturated amines, such as allylamine, 1,Z-dimethylpentenylamine and pyrrole; aryl amines, such as aniline; aralkyl amines such as benzylamine and phenethylamine; alkaryl amines, such as m-toluidine; cyclic amines, such as morpholine, pyrrolidine and piperidine; diamines, such as hydrazine, methylhydrazine, 2,3-toluenediamine, ethylenediamine, 1,2-naphthalenediamine and piperazine; and substituted amines, such as histamine, hydroxylamine, ethanolamine, and diethanolamine.
It has been found advantageous in many instances to effect part of the neutralization with certain solid amines, notably amino-alkyl-alkanediols, such as, for example, 2-methyl-2-amino-1,3-propanediol, 2-ethyl-2-amino-1,3- propanediol or 2-methyl-2-amino-1,4-butanediol. The films produced when a small amount of such amines are employed are considerably harder and often have improved water resistance. However, preferably not more than about 4 per cent by weight of the resinous components of these solid amines are utilized, since they are relatively expensive and greater amounts do not further improve the fllms properties and may even slightly decrease its water resistance.
The electrodepositable coating compositions of the instant invention comprise the above vehicles, containing a strontium chromate-containing pigment composition. The pigment composition, in addition to strontium chromate, may be of any conventional type, comprising, for example, iron oxides, lead oxides, carbon black, titanium dioxide, talc, barium sulfate and the like, as well as combinations of these and similar pigments. Color pigments such as cadmium yellow, cadmium red, phthalocyanine blue, chromic yellow, toluidine red, hydrated iron oxide and the like may be included if desired. Better results with pigmented compositions are attained if the weight ratio of pigment solids to vehicle solids is not higher than about 1.5 to 1, and preferably not higher than about 1 to 1.
There may also be included in the coating composition, if desired, additives such as antioxidants, for example, orthoamyl phenol or cresol (the commercial mixture of isomeric cresols is satisfactory). It is found especially advantageous to include such antioxidants in coating compositions which are used in baths which may be exposed to atmospheric oxygen at elevated temperatures and with violent agitation over extended periods of time.
In formulating the coating composition, ordinary tap water may be employed. However, such water may contain a relatively high level of metals and cations; while not rendering the process in operative, the use of water containing these cations may result in variations in the properties of the bath when used for electrodeposition. Thus, it is often desirable to utilize deionized water, i.e., water from which free ions have been removed as by passage through an ion exchange resin, in making up the coating compositions of the invention.
Other additives which may be included in the coating composition if desired include for example, wetting agents such as petroleum sulfonates, sulfated fatty amides,
esters of sodium isothionates, or alkylphenoxypolyoxyethylene alkanols, as well as driers such as the linoleates, the naphthenates, the octanates and the tallates of such metals as lead, cobalt, manganese, iron, copper and zirconium. Other additives which may be employed include antifoaming agents, suspending agents, bactericides and the like.
In electrodeposition processes employing the various coating compositions described above, an aqueous bath containing the coating composition is placed in contact with an electrically conductive anode and an electrically conductive cathode. The surface to be coated is employed as one of the electrodes. In the specific examples of compositions described above, the surface to be coated is employed as the anode. Upon the passage of electric current between the anode and the cathode, while in contact with the bath containing the coating composition, an adherent film of the coating composition is deposited. The conditions under which the electrodeposition step herein is carried out are those conventionally used in electrodeposition of coatings. The applied voltage may be varied greatly and can be, for example, as low as 1 volt or as high as several thousand volts, although typically between 50 volts and 500 volts. The current density is usually between about 0.1 ampere and 15 amperes per square foot, and is high initially and tends to decrease during the electrodeposition of a single article.
The concentration of the non-volatile components (i.e., vehicle and any pigments and the like) in the aqueous bath is not critical and relatively high levels can be employed. However, it is ordinarily desirable to use as low a concentration as gives satisfactory results, and in the cases of the above-described compositions, aqueous compositions containing as little as 1 percent by weight of non-volatile solids can be employed, while those containing between 5 percent and 20 percent by weight are preferred.
The process of this invention comprises forming a pigment paste by grinding a vehicle resin and pigment, and then, in order to obtain the proper pigment-binder ratio, adding additional resins of such a degree of neutralization that the final concentrate has a pH of below 7, thus obtaining the dispersion of high solids content which may be let down to form the electrodeposition composition or used as a concentrate to replenish the bath. The essential step of the instant invention comprises aging the concentrate thus formed at least to some extent prior to letting down the concentrate, thereby obtaining appreciable reductions in the amount of soluble chromate ions present in the electrodeposition bath.
The grinding of the pigment paste may be done by the conventional methods known to the arts, such as a ball mill, pebble mill, sand mill, or an attritor.
The vehicle used in the grinding step may be the unneutralized resin or a partially neutralized resin. Preferably, however, the grinding resin is an aqueous dispersion of neutralized resin having a pH above 7 and preferably about 9. The amount of water present in such an aqueous grind is not critical, however, commonly the resin employed in the grinding step is about 30 to 70 percent solids. The use of more water merely reduces the effective capacity of the mills and although less water may be employed, there is an increase in viscosity of the material, with the attendant problems inherent in a high-viscosity material.
The pigment-binder ratio in the grinding step is not critical; however, levels between about 3.5/1 to 7/1 are frequently employed, although other levels may be utilized.
The resin used to let down the pigment paste to obtain the desired pigment-binder ratio desired in the final electrodeposition composition should have a degree of neutralization such that the final concentrate composition will have a pH below 7 and preferably between 5.5 and 6.5.
9 Preferably the let down resin is at least neutralized to some extent to facilitate dispersion, although unneutralized resin may be employed.
If desired, the total solids of the final concentrate may be adjusted with the addition of water, preferably deionized water. Preferably, the total solids of the final concentrate is between about 50 percent to about 95 percent. At least about 1 percent water should be present in the final concentrate composition in order to promote the reduction of soluble chromate ions.
The essential step of this invention comprises aging the concentrate, under acid conditions, at a temperature and for a time sufiicient to substantially lower the level of soluble chromate ions in the composition. The time required is temperature dependent, at least to some extent. Usually, the higher the temperature, the shorter the time. For example, if the concentrate is aged at a temperature of about 140 F. to 160 F. or higher, from 12 to 24 hours is usually sufficient to reduce the soluble chromate ions to the preferred level. At room temperature, a period of a few days (usually at most about 6 to days) is necessary to reduce the soluble chromate ion level to the preferred level.
In order to eliminate the problems introduced by the presence of soluble chromate ions, it is necessary to reduce the level of soluble chromate ions to below about 300 parts per million. Preferably, the soluble chromate ion concentration is 'below about 200 parts per million.
The ability to reduce the level of soluble chromate ions in electrodeposition compositions containing strontium chromate is extremely useful. Strontium chromate is highly effective as a corrosion-inhibitive pigment, even when present in only relatively minor amounts in the pigment composition of a coating being applied to a metal surface. However, it was found that when about one percent of the deposited film was strontium chromate, the film, coated on zinc phosphate treated steel, failed to withstand 250 hours in the standard salt-spray test; which is exposure at 100 F. to a 100 percent relative humidity atmosphere of a five percent sodium chloride solution, the panel being scribed with an X to the metal below the film; A inch corrosion creepage from the scribe being considered failure.
It appears that at least about 0.3 percent strontium chromate, and preferably 0.5 percent strontium chromate, in the deposited film is necessary to provide superior salt spray resistance. However, when the level of strontium chromate in the electrodeposition bath, i.e., about 0.3 percent or preferably 0.5 percent based on total solids, is sufficiently high to deposit 0.3 percent, or preferably 0.5 percent, strontium chromate in the film, it has been found that under conventional preparatory procedures the level of strontium chromate ions is sufficiently high to produce the problems hereinabove enumerated.
The electrodeposition compositions prepared by the method of this invention, on the other hand, can readily contain 8 percent or more strontium chromate based on the total pigment solids and yet have a soluble chromate ion level such that the composition may be electrodeposited free from the expected difiiculties. It is possible by the method of this invention to electrodeposit a polycarboxylic acid resin vehicle-based composition containing up to about 8 percent or more strontium chromate based on the total solids without encountering severe Wet adhesion problems, pigskinning, film rupture, excessive high conductivity, etc.
As indicated above, strontium chromate is deposited in the film in approximately the same proportions as it is contained in the total depositable solids of the electrodeposition bath. In actual practice, some minor deviation from this ratio may become apparent in a continuous electrodeposition process, Where a large number of articles are coated in a continuous process, the bath being replenished by the addition of concentrates. Therefore, in practice 10 from time to time minor corrections in the strontium chromate level of the bath may be necessary. However, for practical purposes, the 1 to 1 ratio is a valid approximation, the deviation being quite small.
The level of soluble chromate ion as mentioned herein is determined by a specific procedure, which, although not necessarily providing an absolute measure, does establish a scientifically significant, readily determinable standard. The procedure involves taking a sample of the electrodeposition composition to be examined and letting it down to 8 percent solids with deionized water. To 50 grams of this material is then added 50 grams of glacial acetic acid and the mixture stirred well. The mixture is then centrifuged for one half hour. The supernatent aqueous layer is then carefully decanted through a filter. The soluble chromate content of this serum is then determined in the following manner:
There is prepared milliliters of a 1000 parts per million solution of CrO using distilled water. There is then prepared solutions of 10, 25, 50 and 100 parts per million CrO concentrations by dilution of the standard solution with distilled water.
25 milliliters of the 10, 25, 50 and 100 parts per million solutions are placed in 100 milliliter beakers and there is added 1 milliliter of concentrated hydrochloric acid. These mixtures are stirred and there is then added to the mixtures 10 milliliters of a 15 percent potassium iodide solution with stirring. The beakers are allowed to stand 5 minutes and then there is added to each beaker 2 milliliters of starch solution. Each beaker is then titrated with 0.1 normal Na S O to a colorless end point.
A graph is then prepared plotting concentration of CrO in parts per million versus milliliters of thiosulfate used and the best straight line drawn through the points obtained on the graph. This straight line is used as the standard curve when analyzing the serum.
The serum is then titrated in the following manner:
23 milliliters of the filtered serum above is placed into a 100 milliliter beaker and there is then added 1 milliliter of concentrated hydrochloric acid. The mixture is stirred and there is then added 10 milliliters of 15 percent potassium iodide. The beaker is allowed to stand for five minutes and there is then added 2 milliliters of starch solution. The contents of the beaker are then titrated with 0.1 normal Na S O to a colorless end point.
Using the standard curve, the concentration of CrO in the sample is read off in accordance with the number of milliliters of sodium thiosulfate required to titrate the sample. The number obtained from the graph is multiplied by 2 to arrive at the actual concentration of CIO in the sample. The figure obtained using this procedure is the figure recited as parts per million soluble chromate ions throughout the specification.
The starch solution used in the above procedure is the same starch solution that is generally used as the standard for determining iodine value by titration. It was prepared as follows:
Approximately 2.5 grams of soluble starch powder (C H OQ for example Mallinckrodt 8188, is dispersed in sufficient cold Water to form a paste. This paste is then added to 1 liter of boiling water slowly so that the boiling is not interrupted, to form a clear solution, that is, one in which no precipitate is apparent. After all the starch solution has been added, 0.5 gram of mercuric chloride is added as a preservative. The solution is then cooled and used as needed.
The invention is further described in conjunction with the following examples, which are to be considered illustrative rather than limiting. All parts and percentages in the examples and throughout this specification are by weight unless otherwise stated.
EXAMPLE I A vehicle resin (Resin A) composition was produced by heating a 4 to 1 weight mixture of linseed oil and 1 1 maleic anhydride to 250 C. over a two hour period, and then holding this mixture at this temperature.
A solubilized resin (Resin Asolubilized) was prepared by mixing Resin A (above) with deionized water and diethylene amine to give a solution with a pH of about 7.2 and a solids content of 40 percent.
A pigment paste (Paste B) was made by grinding the following in a steel ball mill:
Parts by weight Resin Asolubilized (above) 230.0
Diethyl amine (pl-I of above mixture-8.3) 5.3 Dispersing agent (combination oil soluble sulfonate and non-ionic surfactant-Witco 912) 6.0
Parts by Weight Resin A (above) 900.0 Diethyl amine 31.73
(Stir 45 minutes). 4-n1ethoxy-4-methylpentanone-2 44. 33 Cresylic acid (dissolved in an additional 18.2 parts 4-methoxy-4-methylpentanone-2) 9.0
Paste B (above) 1375. 6 Deionized water 73.6
The above concentrate has approximately 75 percent solids content and a pH of 5.8.
The above concentrate was then split into 3 portions:
Concentrate Xfresh. Concentrate Y--aged at room temperature for 6 days. Concentrate Zaged at room temperature for 30 days.
After the above-indicated times, the concentrates were all reduced to 7.5 percent operating solids as follows:
12 EXAMPLE II The following example shows a composition as in Example I with a 25 percent increase in strontium chromate content.
A pigment paste (Paste C) was made by grinding the following in a steel ball mill:
Parts by Weight Resin Asolubilized (above) 230.0
Diethylamine (pH of above mixture9.3) 5.3 Dispersing agent (combination oil soluble sulfonate and non-ionic surfactantWitco 912) 6.0
Red iron oxide 473.0 Carbon black (30% aqueous dispersion) 56.0 Strontium chromate 32.0
The above mixture was ground for 18 hours to a No. 7 reading on the Hegmann Grind Gauge. There was then added to the mill and mixed 30 minutes:
Parts by Weight Resin Asolubilized (above) 116.0 Deionized water 83.0
The above paste was then let down as follows:
Parts by Weight Resin A (above) 350.0
Cresylic acid (dissolved in an additional 7.0 parts 4-methoxy-4-methylpentanone-2) 3.5 Paste C (above) 534.9 Deionized water 28.61
The above concentrate had approximately 75 percent solids content and a pH of 5 .8.
The above concentrate was then split into 2 portions;
Concentrate M-fresh Concentrate Naged at room temperature for 30 days After the above-indicated times, the concentrates were all reduced to 7.5 percent operating solids as follows:
Parts by Weight 45 Parts by Weight Concentrate 425.0 Concentrate 212.5 Premixed: Premixed Diethylamine 7.0 Diethylamine 3 .5 Deionized water 205.0 Deionized water 102.5 Mix well and add: Mix well and add:
Deionized water 3603.0 Deionized water 1802.5
TABLE I Soluble Bath chromate temperion Baked film ature Film parts per appearance (degrees thickness million (350 F., Electrodeposition composition derived from F.) Voltage Amperes (mils) as CrOF) Wet film appearance 20 minutes) Slight orange peel.
520 Poor wet adhesion, greasy".-.
No tglnge peel.
240 Wet adhesion good 1 do l Deposited on zinc phosphate treated steel panels (Bonderite 37).
TABLE II Soluble Bath chromate temperion ature Film (parts per Baked film ap- Electrodeposition composition derived (degrees thlckness million pearanee (350 F., from F.) Voltage Amperes (mils) as CrOr') Wet film appearance 20 minutes) Concentrate M (fresh) 79 1. 5-0. 35 0. 7-0. 75 530 Poor wet adhesion, greasy Slight orange peel. Concentrate N (aged 30 days) 78 100 1.4-0. 34 65 200 Wet adhesion good, not greasy- No orange peel.
l Deposited on zinc phosphate treated panels (Bonderite 37).
13 EXAMPLE III The following example shows a composition as in Example I with a 60 percent increase in strontium chromate content.
which time a Hegmann fineness reading of at least 6 was obtained. There was then added to the mill the reaction product of 235.5 parts Resin A and 7.1 parts diethylamine. The final mixture had a pH of about 4. This mixture is Paste H.
i gi ig s f fii fi gfi made by gnndmg the 5 A portion of Paste H was let down to electrodeposition g solids of approximately 8 percent in the usual manner Parts by Weight and soluble chromate ion determined. Resin Asolubilized (above) 230.0 Parts by weight Diethylamine (pH of above mixture9.3) 5.3 10 Paste H (above) 10 Dispersing agent (combination oil soluble sulfonate Deionized water l0 and non-ionic surfactantWitco 912) 6.0 Diethylamine premixed in the above water 0.4 Red iron oxide 473.0 Deionized water 99.0 g igg z gg gg i i aqueous dlsperslon) The pH of the above composition was 8.5. The soluble chromate ion concentration was 600 parts per million. The above mixture was ground for 18 hours to a No. 7 A second portion of Paste H was heat aged at 150 F. reading on the Hegmann Grind Gauge. There was then for 16 hours and then let down as above and soluble added to the mill and mixed 30 minutes: chromate ion determined. The soluble chromate ion concentration was 330 parts per million. Resin A solubilized (above) ji i 20 Additional portions of Paste H were formulated with Deionized Water 83D varying percentages of added water and heat aged at 150 F. for 16 hours and the soluble chromate ion was T above paste was then letdown as follows: determined at the end of this time. The results were as Parts by weight follows: 9 A (fabove) 350'0 Soluble chromate ions: Percentage of water Dlethylajmme 1233 120 parts per million 25 (Sm 45 mmutes)- 100 parts per million 5.6 4 methoxy-4methylpentanone-2 17.23 180 parts Per million 28 Cresylic acid (dissolvled in an adzditional 7.0 parts 3 5 4-methoxy-4-methy pentanone- Paste D (above) 534.9 EXAMPLE V D i i d water 28 61 A vehicle resin (Resin H) was produced as follows: to a reactor equipped with water trap and inert gas above Concentrate approximately 75 percent r sparge, was added 252 parts pentaerythritol, 1400 parts 608mm andaPHO h 2 linseed oil and 137.6 parts trimethyolethane. The mix- T e a ove concentrate was t en sp it lnto portions. ture was heated to C. and .48 gram of Hthage Concentrate O-fresh. (lead oxide) Was added. The reaction mixture was then Concentrate Paged at room temperature for 24 days. heated to 238 C. and held at that temperature for one hour. The reaction mixture was cooled to 190 C. and After the above-indicated times the concentrates were all there was then added to the reactor 888 parts phthalic reduced to 7.5 percent operating solids as follows. anhydride, 274 parts dimethyolpropionic acid and 85 Parts by weight parts of xylene and the reaction mixture was then heated Concen rate 21 to 205 C. Water started to evolve at this point. The Premixed: reaction was heated at 205 C. for 2 /2 hours at which Diethylamine 3-5 time the acid value was 59.8. After an additional 30 min- Deionized water 102.5 utes at 205 C., at which time a total of 94.3 parts of Mix well and add: water had been removed, the reaction mixture was then Deionized water 1802-5 sparged to the atmosphere with inert gas for 30 minutes,
TABLE III Soluble Bath chromate temper- Film ion Baked film ature thick- (parts per appearance Electrodeposition (degree ness million (350 F., composition derived from F.) Voltage Amperes (rnils) as 0104") Wet film appearance 20 minutes) Concentrate 0 (fresh) 78 100 1.2-0. 25 95 730 Poor wet adhesion, pigskin, slightly greasy Slight orange peel. cgrlcgistgate P (Aged 77 100 1. 4-0. 4 65-- 70 260 Wet adhesion good, slightly greasy N 0 orange peel.
1 Deposited on zinc phosphate treated panels (Bonderite 37).
EXAMPLE IV cooled to 140 and 31 arts 4-m ho A Pigment Paste H was made by gnndmg the followmg pentanone was a dded. The fi iial a i d vall i e W 2; hyl asteelbanmln: Parts y Weight A pigment paste (Paste I) was prepared by charging Resin A unsolubflized (a n E p e heated the following into a steel canister contalnmg steel balls. to 200 F. 133.0 Partsby weight 4-methoxy-4-methylpentanone 33.4 Deionized water 292.0 Cresylic acid 2.9 Surfactant [a nonylpenolpoly(ethylenoxy) phos- Red iron oxide 254.0 phate ester-Gafac PE510] 11.25 Strontium chromate 12.1 Stir in: Carbon black 17.7 Strontium chromate 20.0 The above mixture was heated to about 200 F. placed Iron oxlde 3800 in a canister containing steel balls and shaken for 25 The above mixture was shaken on a Red Devil Shaker minutes on a Red Devil Shaker, a rapidly vibrating (a rapidly vibrating shaker, such as those commonly shaker such as those commonly used to mix paints, at used to mix paints), for 30 minutes, at which time a 15 Hegmann fineness reading of at least about 6 /2 was obtained.
The resultant pigment paste is Paste I. A concentrate (J) was prepared as follows:
Parts by weight Resin I 300.0 Cresylic acid 2.0 Paste I (above) 210.0
After two weeks aging at room temperature, the concentrate was let down as follows:
Parts by weight Deionized water 100.0 Diglycol amine 3.0 Concentrate J 100.0 Resin H 5.0 With pH above at 7.4, add:
Deionized water 572.0
The above electrodeposition composition had a percent content and has a specific conductivity at 75F. of 350. The soluble chromate ion level was 120 parts per million.
A similar electrodeposition composition was prepared without aging at a pH below 7, as follows:
A pigment paste (Paste K) was prepared by charging the following into a steel canister containing steel balls:
- Parts by weight Deionized water 292.0
Surfactant [a nonylpenolpoly(ethylenoxy) phosphate ester-Gafac PBS 10] 11.25
Strontium chromate 200 Iron oxide 380.0
Deionized water 642.8 Diglycol amine 26.0 Resin H 333.2 Cresylic acid 3.0 Deionized water 200.0
The above mixture had a pH of 7.6.
The final electrodepositable composition was prepared as follows:
Parts by weight Mixture L 600.0
Paste K 105.0 Deionized water 1400.0
At 10 percent solids, this electrodeposition composition had a specific conductivity at 75 F. of 1300 as against 350 for the previously described composition. The soluble chromate ion concentration of this composition was 360 parts per million. When the composition was aged for two weeks at room temperature no significant change was observed in the chromate ion concentration nor the specific conductivity.
Various other electrodepositable compositions such as those hereinabove described can be substituted for those of the examples. In the above and other tests the general applicability of the method herein has been shown and it has been found that good results are attained using varying compositions, electrodeposition conditions and substrates.
According to the provisions of the patent statutes, there are described above the invention and what are now considered to be its best embodiments; however, within the scope of the appended claims; it is to be understood that the invention can be practiced otherwise than as specifically described.
1. A method of coating a metal substrate which comprises passing electric current between an electrically conductive cathode in contact with an aqueous bath containing an electrodepositable composition comprising a solubilized polycarboxylic acid resin vehicle and strontium chromate pigment, said solubilized electrodeposition composition containing less than about 300 parts per million soluble chromate ion.
2. A method as in claim 1 wherein the coating on the metal substrate contains at least about 0.3 percent by .weight' of strontium chromate.
3. An article coated by the method of claim 2.
4. A method of coating a metal substrate which comprises passing electric current between an electrically conductive cathode in contact with an aqueous bath containing an electrodepositable composition comprising a vehicle which comprises a solubilized reaction product of a drying oil fatty acid ester with a member of the group consisting of alpha, beta-ethylenically unsaturated dicarboxylic acids and an anhydride of said acids, and strontium chromate pigment, said solubilized electrodeposition composition containing less than about 300 parts per million soluble chromate ion.
5. A method as in claim 4 wherein the coating on the metal substrate contains at least about 1.3 percent by weight of strontium chromate.
6. A method of coating a metal substrate which comprises passing electric current between an electrically conductive metal anode and an electrically conductive cathode in contact with an aqueous bath containing electrodepositable composition comprising a solubilized alkyd resin and strontium chormate pigment, said solubilized electrodeposition composition containing less than about 300 parts per million soluble chromate ion.
7. A method as in claim 6 wherein the coating on the metal substrate contains at least 0.3 percent by weight of strontium chormate.
References Cited UNITED STATES PATENTS 3,404,079 10/1968 Boardman 204-181 HOWARD S. WILLIAMS, Primary Examiner US. Cl. X.R. 26029.2, 37, 40