US3663404A - Treatment of electrodeposition bath - Google Patents

Abstract

This invention relates to a method of rinsing articles coated by an electrodeposition process which comprises employing as at least a portion of the rinsing agent the effluent obtained from a selective separation process such as an ultrafiltration process, while withdrawing a portion of said effluent which contains excess and/or undesirable low molecular weight species present in the electrodepositable composition, thereby, first, conserving bath material and, second, controlling the composition of the electrodepositable composition.

Description

United States Patent Loop [54] TREATMENT OF ELECTRODEPOSITION BATH [72] Inventor: Frederick M. Loop, North Olmsted, Ohio [73] Assignee: PPG Industries, Inc., Pittsburgh, Pa.

[ Notice: The portion of the term of this patent subsequent to Feb. 15, 1989, has been disclaimed.

[22] Filed: Dec. 1, 1970 [21] Appl.No.: 94,023

Related U.S. Application Data [63] Continuation-in-part of Ser. No. 881,259, Dec. 1, 1969, abandoned, and a continuation-in-part of Ser. No. 73,093, Sept. 17, 1970, which is a continuationin-part of Ser. No. 881,259, Dec. 1, 1969.

[52] U.S. Cl ..204/l81 [51] Int. Cl. ...B0lh 5/02, C23b 13/00 [58] Field of Search ..204/181 [151 3,663,404 51 *May 16, 1972 [56] References Cited UNITED STATES PATENTS 3,556,970 1/1971 Wallace et al. ..204/181 FOREIGN PATENTS OR APPLlCATlON S 1,071,458 6/1967 Great Britain ..204/1 81 Primary Examiner-Howard S. Williams Art0rneyChisholm and Spencer ABSTRACT This invention relates to a method of rinsing articles coated by an elcctrodeposition process which comprises employing as at least a portion ofthe rinsing agent the effluent obtained from a selective separation process such as an ultrafiltration process, while withdrawing a portion of said effluent which contains excess and/or undesirable low molecular weight species present in the electrodepositable composition, thereby, first, conserving bath material and, second, controlling the composition of the electrodepositable composition.

10 Claims, 3 Drawing Figures Patented May 16, 1972 2 Sheets-Sheet l INVENTOR F/QEDERICK 41.400!

ATTORNEYS FIG-3 INVENTOR FREDERICK M. 06?

ATTORNEE TREATMENT OF ELECTRODEPOSITION BATH CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-impart of application Ser. No. 881,259, filed Dec. 1, 1969, now abandoned and also a continuation-in-part of application Ser. No. 73,093, filed Sept. 17, 1970, which is, in turn, a continuationin-part of application Ser. No. 881,259, filed Dec. 1, 1969.

STATE OF THE ART Electrodeposition has become a widely commercially accepted industrial coating technique. 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 are metal substrates including 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 substances rendered conductive under the conditions of the coating process may also be employed as substrates.

in the electrodeposition process, the articles to be electrocoated are immersed in an aqueous dispersion of a solubilized, ionized, film-forming material such as a synthetic organic vehicle resin. An electric current is passed between the article to be coated, serving as an electrode, and a counterelectrode to cause deposition of a coating of the vehicle resin on the articles. The articles are then withdrawn from the bath, usually rinsed, and then the coating either air-dried or baked in the manner of a conventional finish.

In the electrodeposition process, when the article has been coated and is being withdrawn from the coating bath, a portion of the bath material which is not electrocoated to the article but merely adherent thereto, or pocketed by complexshaped articles, is withdrawn with the article. This material is commonly called dragout". This dragout is generally rinsed from the article, leaving an adherent electrocoated film which is then dried or baked in the conventional manner. This dragout represents wasted material and reduces efficiency of the system. Further, the rinse water containing the dragout comprises a waste disposal problem. Typically, in the past, the rinsing has been conducted with tap water and/or deionized water.

It has now been found that the rinsing to remove dragout of an electrodeposited article may be accomplished by employing the effluent or filtrate derived from a selective separation process such as a filtration process, for example, ultrafiltration, utilized to control the bath composition. It has been found further that this rinsing of dragout may be accomplished in such a manner that the rinse returns to the electrodeposition bath, thus conserving materials to an even greater extent. Various advantages are derived from this technique. These advantages include the following: the use of the effluent achieves economies in water use; there the rinse is conducted in a manner so that the rinse water containing dragout is returned to the bath, the use of effluent or filtrate in the rinsing process achieves removal of dragout without unduly changing or diluting the composition bath due to the addition of relatively pure water, which is the chief reason why rinsing is typically not conducted over the bath in such a manner that the rinse water is returned to the bath.

The use of the technique herein described is highly flexible. As previously stated, the rinsing is accomplished in such a way that the efiluent or filtrate and dragout are either collected separate from the bath or immediately returned to the bath. Likewise, since selective filtration and particularly ultrafiltration is a method of bath control, it is desirable, in order to maintain or change the composition of the bath, to intermittently or concurrently remove at least a portion of the ultrafiltrate from the system. Thus, the rinsing of dragout may intermittently or proportionately be conducted with tap water or, preferably, deionized water, either separate from the bath or in a manner so that this rinse returns to the bath. Likewise, a

mixture of ultrafiltrate and other water may be employed as the rinsing media.

In the attached drawing, an apparatus used to carry out the method of this invention is schematically illustrated. The electrodeposition bath 1 of FIG. 3 from which films are deposited uses suitable apparatus (not shown). A portion of the bath may be continuously or intermittently withdrawn through an outlet and valve 2 and passed through line 3 to a selective filter, in this case an ultrafilter 4. Here, in the ultrafiltration process, water, free counter-ions and counter-ions present as low molecular weight salts, for example, carbonates, as well as other low molecular weight species, if present, are separated from the vehicle resin, pigment and other high molecular weight components of the bath composition. The ultrafiltrate is removed from the ultrafilter, passing through line 5, through the use of valve 6, while the concentrate or retentate is returned to the bath through line 15 and valve 16. The ultrafiltrate may be directed either unidirectionally or proportionally in either an intermittent or continuous fashion to drain 7 or for use as rinse material. In the latter case, the ultrafiltrate is then passed through line 8 when it is directed through valves 9 or 13 to either a rinse station 14 for rinsing dragout in a manner that the dragout-containing rinse returns to the bath or 10 for rinsing dragout in a manner so that it does not return directly to the bath. Valves 9 and 13 likewise accommodate the intermittent or proportional use of water 11 and 12 other than ultrafiltrate for rinsing. Line 17 allows for return of ultrafiltrate to the bath if and when desired. As stated, this drawing is schematic and does not purport to describe the necessary pumping means or apparatus which are known in the art.

The selective filtration process employed in the process of the instant invention is any process which separates water from the electrodeposition bath through a physical barrier while retaining the solubilized resin components. Thus, any means may be utilized which accomplishes this purpose. Means may pass not only water but also solute of substantially lower molecular weight than the vehicle resin such as excess amine, carbonates, low molecular weight solvent and simple organic or inorganic anions and cations which may be present in the bath. Examples of means for accomplishing this separation are reverse osmosis, where water of high purity may be obtained, and ultrafiltration, as well as other means which accomplish the intended result. Because ultrafiltration is particularly useful in bath control, this means is especially preferred.

The control of an electrodeposition bath by an ultrafiltration process has been described in copending application Ser. No. 814,789, filed Apr. 9, 1969 now abandoned. In the ultrafiltration process, exceptional control of a bath composition and removel of objectional accumulated materials has been achieved by a selective filtration process, that is, a process which selectively removed low molecular weight materials from the bath composition. This selective filtration process removes excess counter-ion and thus serves as a method of conventional bath control, but in addition this method further removes other excess materials or contaminants from the bath, thus permitting more complete control over bath constituents than has heretofore been possible.

The selective filtration process is an ultrafiltration process which separates materials below a given molecular weight size from the electrodeposition bath. With properly selected membranes, this treatment does not remove any product or desirable resin from the paint in the tank but does remove anionic, cationic and non-ionic materials from the paint in a ratio proportional to their concentration in the water phase of the paint. Thus, for example, it is possible to remove amines, alkaline metal ions, phosphates, chromates, sulfates, solvents and dissolved carbon dioxide, among others.

Ultrafiltration may be defined as a method of concentrating solute while removing solvent, or selectively removing solvent and low-molecular weight solute from a significantly higher molecular weight solute. From another aspect, it is a process of separation whereby a solution containing a solute of molecular dimensions significantly greater than the solvent is depleted of solute by being forced under a hydraulic pressure gradient to flow through a suitable membrane. The first definition is the one which most fittingly describes the term ultrafiltration" as applied to an electrodeposition bath.

Ultrafiltration encompasses all membrane-moderated, pressure-activated separations wherein solvent or solvent and smaller molecules are separated from modest molecular weight macromolecules and colloids. The term ultrafiltra tion is generally broadly limited to describing separations involving solutes of molecular dimensions greater than about ten solvent molecular diameters and below the limit of resolution of the optical microscope, that is, about 0.5 micron. In the present process, water is considered the solvent.

The principles of ultrafiltration and filters are discussed in a chapter entitled Ultrafiltration" in the Spring, 1968, volume of ADVANCES IN SEPARATIONS AND PURIFICATIONS, E. S. Perry, Editor, John Wiley & Sons, New York, as well as in CHEMICAL ENGINEERING PROGRESS, Vol. 64, Dec., 1968, pages 31 through 43, which are hereby incorporated by reference.

The basic ultrafiltration process is relatively simple. Solution to be ultrafiltered is confined under pressure, utilizing, for example, either a compressed gas or liquid pump in a cell, in contact with an appropriate filtration membrane supported on a porous support. Any membrane or filter having chemical integrity to the system being separated and having the desired separation characteristic may be employed. Preferably, the contents of the cell should be subjected to at least moderate agitation to avoid accumulation of the retained solute on the membrane surface with the attendant binding of the membrane. Ultrafiltrate is continually produced and collected until the retained solute concentration in the cell solution reaches the desired level, or the desired amount of solvent plus dissolved low molecular weight solute is removed. A suitable apparatus for conducting ultrafiltration is described in U.S. Pat. No. 3,495,465, which is hereby incorporated by reference.

There are two types of ultrafiltration membranes. One is the microporous ultrafilter, which is a filter in the traditional sense, that is, a rigid, highly voided structure containing interconnected random pores of extremely small average size. Through such a structure, solvent (in the case of electrodeposition, water) flows essentially viscously under a hydraulic pressure gradient, the flow rate proportional to the pressure difference, dissolved solutes, to the extent that their hydrated molecule dimensions are smaller than the smallest pores within the structure, will pass through, little impeded by the matrix. Larger size molecules, on the other hand, will become trapped therein or upon the external surface of the membrane and will thereby be retained. Since the microporous ultra-filters are inherently susceptible to internal plugging or fouling by solute molecules whose dimensions lie within the pore size distribution of the filter, it is preferred to employ for a specific solute a microporous ultrafilter whose mean pore size is significantly smaller than the dimensions of the solute particle being retained.

In contrast, the diffusive ultrafilter is a gel membrane through which both solvent and solutes are transported by molecular diffusion under the action of a concentration or activity gradient. In such a structure, solute and solvent migration occurs via random thermal movements of molecules within and between the chain segments comprising the polymer network. Membranes prepared from highly hydrophilic polymers which swell to eliminate standard water are the most useful diffusive aqueous ultrafiltration membranes. Since a diffusive ultrafilter contains no pores in the conventional sense and since concentration within the membrane of any solute retained by the membrane is low and time-independent, such a filter is not plugged by retained solute, that is, there is no decline in solbent permeability with time at a constant pressure. This property is particularly important for a continuous concentration or separation operation. Both types of filters are known in the art.

The presently preferred ultrafilter is an anisotropic membrane structure such as illustrated in FIG. 1. This structure consists of an extremely thin, about one-tenth to about ten micron layers of a homogeneous polymer 1 supported upon a thicker layer of a microporous open-celled sponge 2, that is, a layer of about 20 microns to about 1 millimeter, although this dimension is not critical. If desired, this membrane can be further supported by a fibrous sheet, for example, paper, to provide greater strength and durability. These membranes are used with a thin film or skin side exposed to the high pressure solution. The support provided to the skin by the spongy substrate is adequate to prevent film rupture.

Membranes useful in the process are items of commerce and can be obtained by several methods. One general method is described in Belgian Pat. No. 721,058. This patent describes a process which, in summary, comprises (a) forming a casting dope of the polymer in an organic solvent, (b) forming a film of the casting dope, and (c) preferentially contacting one side of said film with a diluent having high compatibility with the casting dope to effect precipitation of the polymer immediately upon coating the cast film with the diluent.

The choice of a specific chemical composition for the membrane is determined to a large extent by its resistance to the chemical environment. Membranes can be typically prepared from thermoplastic polymers such as polyvinyl chloride, polyacrylonitrile, polysulfones, poly(methyl methacrylate), polycarbonates, poly(n-butyl methacrylate as well as a large group of polymers formed from any of the monomeric units of the above polymers, including Polymer 360, a polysulfone copolymer. Cellulosic materials such as cellulose acetate may also be employed as membrane polymers.

Some examples of specific anisotropic membranes operable in the process of the invention include Diaflow membrane ultrafilter PM-30, the membrane chemical composition of which is a polysulfone copolymer, Polymer 360, and which has the following permeability characteristics:

Solute Retention Characteristics The membrane is chemically-resistant to acids (l-lCl, H H PO,,, all concentrates, alkalies, high phosphate buffer, and solutions of common salts as well as concentrated urea and quanadine hydrochloride. The membrane is solvent-resistant to alcohol, acetone and dioxane. The membrane is not solvent-resistant to dimethyl formamide or dimethyl sulfoxide. This membrane is hereinafter referred to as Membrane A.

Dorr-Oliver XPA membrane, the membrane chemical composition of which is Dynel (an acrylonitrile-vinyl chloride copolymer) and which has the following permeability characteristics:

Flux Molecular Percent gal./sq.ft./day at This membrane is hereinafter referred to as Membrane B.

Dorr-Oliver BPA type membrane, the membrane chemical composition of which is phenoxy resin (polyhydroxy ether), and which has the following permeability characteristics:

Flux Molecular Percent (gal./sq.ft./day at Solute Weight Retention 30 psi, 1.0% solute) Cytochrome C 12,600 50 30 This membrane is hereinafter referred to as Membrane C The microporous ultrafilters are generally isotropic structures, thus flow and retention properties are independent of flow direction. It is preferred to use an ultrafilter which is anisotropic in its microporous membrane structure, FIG. 2. In such a membrane, the pore size increases rapidly from one face to the other. When the fine-textured side 4 is used in contact with the feed solution, this filter is less susceptible to plugging since a particle which penetrates the topmost layer cannot become trapped in the membrane because of the larger pore size 5 in the substrate.

The process of the invention may be operated as either a batch or a continuous process. In batch selective filtration or batch ultrafiltration, a finite amount of material is placed in a cell which is pressurized. A solvent and lower molecular weight solutes are passed through the membrane. Agitation is provided by a stirrer, for example, a magnetic stirrer. Obviously this system is best used for small batches of material. In a process requiring continuous separation, a continuous selective filtration process if preferred. Using this technique, material is continuously recirculated under pressure against a membrane or series of membranes through interconnecting flow channels, for example, spiral flow channels.

Likewise, the ultrafiltration process may be conducted as either a concentration process or a diafiltration process. Concentration involves removing solvent and low molecular weight solute from an increasingly concentrated retentate. Filtration flow rate will decrease as the viscosity of the concentrate increases. Diafiltration, on the other hand, is a constant volume process whereby the starting material is connected to a reservoir of pure solvent, both of which are placed under pressure simultaneously. Once filtration begins, the pressure source is shut off in the filtration cell and thus, as the filtrate is removed, an equal volume of new solvent is introduced into the filtration cell to maintain the pressure balance. The configuration of the filter may also vary widely and is not limiting to the operation of the process. The filter or membrane may, for example, be in the form of a sheet, tubes or hollow fiber bundles, among other configurations.

Under ideal conditions, selected low molecular weight solutes would be filtered as readily as solvent and their concentration in the filtrate is equal to that in the retentate. Thus, for example, if a material is concentrated to equal volumes of filtrate and retentate, the concentration of low molecular weight solute in each would be the same.

Using diafiltration, retentate solute concentration is not constant and the mathematical relationship is as follows:

MGOIG) i/ 0) where C is the initial solute concentration, C, is the final solute concentration of the retentate, V, is the volume of solute delivered to the cell (or the volume of the filtrate collected), and V, is the initial solution volume (which remains constant).

Electrodepositable compositions, while referred to as solubilized. in fact are considered a complex solution, dispersion or suspension or combination of one or more of these classes in water, which acts as an electrolyte under the influence of an electric current. While, no doubt, in some circumstances the vehicle resin is in solution, it is clear that in some instances and perhaps in most the vehicle resin is a dispersion which may be called a molecular dispersion of molecular size between a colloidal suspension and a true solution.

The typical industrial electrodepositable composition also contains pigments, crosslinking resins and other adjuvants which are frequently combined with the vehicle resin in a chemical and a physical relationship. For example, the pigments are usually ground in a resin medium and are thus wetted with the vehicle resin. As can readily be appreciated then, an electrodepositable composition is complex in terms of the freedom or availability with respect to removal of a component or in terms of the apparent molecular size of a given vehicle component.

As applied to the process of this invention, ultrafiltration comprises subjecting an electrodepositable composition, especially after it has been employed in a coating process or aged, which inherently causes contaminants and other low molecular weight materials to accumulate in the bath, such as metal pretreatment chemicals, water, absorbed CO (either dissolved, or more likely, combined as an aminic salt or carbonate), neutralizing agent, organic solvent and ions such as forrnate, chromate, phosphate, chloride and sulfate, for example, to an ultrafiltration process employing an ultrafilter, preferably a difiusive membrane ultrafilter selected to retain the solubilized vehicle resin while passing water and low molecular weight solute, especially those with a molecular weight below a out 500. As previously indicated, the filters discriminate as to molecular size rather than actual molecular weight, thus, these molecule weights merely establish an order of magnitude rather than a distinct molecular weight cut-off. Likewise, as previously indicated, the retained solutes may, in fact, be colloidal dispersions or molecular dispersions rather than true solutes.

In practice, a portion of the electrodepositable composition may be continuously or intermittently removed from the electrodeposition bath and passed under pressure created by a pressurized gas or by means of pressure applied to the contained fluid in contact with the ultrafilter. Obviously, if desired, the egress side of the filter may be maintained at a reduced pressure to create the pressure difference.

The pressures necessary are not severe. The maximum pressure, in part, depends on the strength of the filter. The minimum pressure is that pressure required to force water and low molecular weight solute through the filter at a measurable rate. With the presently preferred membranes, the operating pressures are between about 10 and p.s.i., preferably between about 25 and 75 p.s.i. Under most circumstances, the ultrafilter should have an initial flux rate, measured with the composition to be treated of at least about 3 gal./sq.ft./day (24 hours) and preferably at least about 4.5 gal./sq.ft./day.

As previously indicated, the bath composition should be in motion at the face of the filter to prevent the retained solute from impeding the flow through the filter. This may be accomplished by mechanized stirring or by fluid flow with a force vector to the filter surface.

The retained solutes comprising the vehicle resin are then returned to the electrodeposition bath. If desired, the concentrate may be reconstituted by the addition of water either before entry to the bath or by adding water directly to the bath.

If there are present in the bath desirable materials which, because of their molecular size, are removed in the ultrafiltration process, these may likewise be returned to the bath either directly to the retained solute before entry to the bath, in the makeup feed as required, or independently.

A number of electrodepositable resins are known and can be employed to provide the electrodepositable compositions which may be utilized within the scope of this invention. Virtually any water-soluble, water-dispersible or water-emulsifiable polyacid or polybasic resinous material can be electrodeposited and, if film-forming, provides coatings which may be suitable for certain purposes. Any such electrodepositable composition is included among those which can be employed in the present invention, even though the coating obtained might not be entirely satisfactory for certain specialized uses.

Presently, the most widely used electrodeposition vehicle resins are synthetic polycarboxylic acid resinous materials. Numerous such resins are described in U.S. Pat. Nos. 3,441,489; 3,422,044; 3,403,088; 3,369,983 and 3,366,563, which all include 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 semi-drying oil fatty acid esters are meant esters of fatty acids which are or can be derived from drying oils or semi-drying 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.

Also included among such esters are those in which the esters themselves are modified with other acids, including saturated, unsaturated or aromatic acids or an anhydride thereof. The acid-modified esters are made by trans-esterification 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 the other polyols such as trimethylol propane, 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 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 semi-drying or drying fatty acid esters of resinous polyols such as homopolymers or copolymers of unsaturated aliphatic alcohols, e.g., ally] 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 the above acids or anhydrides may also be utilized. Generally speaking, the anhydride or acid employed contains from four to 12 carbon atoms, although longer chain compounds can be used if so desired.

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.

As shown in the art, it is preferred that in certain instances 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.

Compositions within this general class are described in U.S. Pat. Nos. 3,366,563 and 3,369,983.

Another vehicle comprises the fatty acid ester, unsaturated acid or anhydride reaction products and any additional unsaturated modifying materials (as described above) which are further reacted with the polyol.

Essentially any polyol can be employed, but diols are preferred. When higher polyols, such as trimethylolpropane, glycerol, pentaerythritol, and the like are utilized, they are employed in small amounts, or in conjunction with the diol, or in the presence of a monohydric alcohol, and are used with adducts having a relatively low proportion of acidic component. Water-insoluble diols are often preferable, and especially desirable water-dispersed compositions for electrodeposition are obtained using 2,2-bis(4-hydroxycyclohexyl)propane which has given the best results), neopentyl glycol, 1,1- isopropylidene-bis(p-phenyleneoxy)di-2-propanol, and similar diols.

The proportions of the polyol and ester-anhydride adduct which are employed depend upon various factors, but are in general limited only by the need to avoid gelation of the product. The total functionality of the reactants is a guide to determining the optimum proportions to be employed, and in most instances should not be greater than about 2.

In many instances, only part of the anhydride groups of the adduct, e.g., about 10 percent, are reacted with the polyol. Of those anhydride groups reacted, it is preferred that only one of the carboxyl groups is esterified in each instance.

The product contains a substantial part of the original acidity derived from the dicarboxylic acid or anhydride; ordinarily the product should have an acid number of at least about 20. To provide a water-dispersed product, such as is used in electrodeposition processes, at least part of the remaining acidic groups are neutralized by reaction of the partially esterified product with a base.

The polyol reaction products and reaction conditions are more fully described in application Ser. No. 450,205, filed Apr. 22, 1965 now U.S. Pat. No. 3,565,781, as well as the art cited above.

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 with the interpolymer usually making from about 50 percent to about percent by weight of the resinous composition.

The acid monomer of the interpolymer is usually acrylic acid or methacrylic acid, but other ethylenically unsaturated monocarboxylic and dicarboxylic acids of up to about six 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 five carbon atoms in the hydroxyalkyl radical. Mono or diesters of the dicarboxylic acids mentioned are included. Ordinarily, the acid and ester each comprise between about one 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; the alkyl 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 amine-aldehyde condensation products included in these compositions are, for example, condensation products of melamine, benzoguanamine, or urea with formaldehyde, although other amine-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 watersoluble or organic solvent-soluble.

Electrodeposition compositions comprising the above interpolymers and an amine-aldehyde resin are more fully described in U.S. Pat. No. 3,403,088.

Still another electrodepositable composition of desirable properties comprises an alkyd-amine vehicle, that is, a vehicle containing an alkyd resin and an amine-aldehyde 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 amine-aldehyde 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 percent to about 50 percent by weight of amine resin based on the total resinous components.

Yet another electrodepositable composition of desirable properties comprises mixed esters of an unsaturated fatty acid adduct. Generally the polyols which are utilized with these resins are essentially any polyol having a molecular weight between about 500 and 5,000. Such resinous polyols include those resinous materials containing oxirane rings which can be opened in, prior to, or during the esterification reaction to provide an apparent hydroxy site. The vehicle resins are formed by reacting a portion of the hydroxyl groups of the polyol with the fatty acid, the ratio of the reactions being such that at least an average of one hydroxyl group per molecule of the polyol remains unreacted. The remaining functionality is then reacted with the unsaturated fatty acid adduct of an olefinically unsaturated dicarboxylic anhydride, such as maleic anhydride, this second esterification reaction being conducted under conditions so that esterification occurs through the anhydride ring, thereby introducing free acid groups into the molecule. Mixed acids of the class described are disclosed in Belgian Pat. No. 641,642, as well as in copending application Ser. No. 568,144, filed July 27, 1966 now abandoned.

In order to produce an electrodepositable composition, it is necessary to at least partially neutralize the acid groups present with a base in order to disperse the resin in the electrodeposition bath. Inorganic bases such as metal hydroxides, especially potassium hydroxide, can be used. There may likewise be used ammonia or organic bases, especially watersoluble amines, such as, for example, the mono-, diand trilower alkyl amines such as methylamine, ethylamine, propylamine, butylamine, dimethylamine, diethylamine, dipropylamine, dibutylamine, and m-methyl-butylamine, triethylamine, tributylamine, methyldiethylamine, dimethylbutylamine, and the like; cyclic amines such as morpholine, pyrrolidine, pipen'dine; diamines such as hydrazine, methylhydrazine, 2,3-toluene diamine, ethyl diamine and piperizine and substituted amines such as hydroxylamine, ethanolamine, diethanolamine, butanolamine, hexanolamine and methyldiethanolamine, octanolamine, diglycolamine and other polyglycol amines, triethanolamine and methylethanolamine,

n-amino-ethanolamine and methyldiethanolamine polyamines such as diethylene triamine.

There may be present in the electrodepositable composition any of the conventional types of pigments employed in the art. There is often incorporated into the pigment composition a dispersing or surface-active agent. Usually the pigment and surface-active agent, if any, are ground together in a portion of the vehicle, or alone, to make a paste and this is blended with the vehicle to produce a coating compositions.

In many instances, it is preferred to add to the bath in order to aid dispersibility, viscosity and/or film quality, a non-ionic modifier or solvent. Examples of such materials are aliphatic, naphthenic and aromatic hydrocarbons or mixtures of the same; monoand dialkyl ethers of glycols, pine oil and other solvents compatible with the resin system. The presently preferred modifier is 4-methoxy-4-methyl pentanone-2 (Pent- Oxone).

There may also be included in the coating composition, if desired, additives such as antioxidants. For example, orthoamylphenol or cresol. It is 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 agitation over extended periods of time.

Other additives which may be included in coating compositions, if desired, include, for example, wetting agents such as petroleum sulfonates, sulfated fatty amines, or their amides, esters of sodium isothionates, alkyl pheoxypolyethylene alkanols, or phosphate esters including ethoxylated alkylphenol phosphates. Other additives which may be employed include anti-foaming agents, suspending agents, bactericides, and the like.

In formulating the coating composition, ordinary tap water may be employed. However, such water may contain a relatively high level of metals and cations which, while not rendering the process inoperative, these cations may result in variations of properties of the baths when used in electrodeposition. Thus, in common practice, deionized water, i.e., water from which free ions have been removed by the passage through ion exchange resins, is invariably used to make up coating compositions of the instant invention.

In addition to the electrodepositable vehicle resins described above, there may be present in the electrodepositable composition other resinous materials which are non-carboxylic acid materials. For example, as shown above, there may be added up to about 50 percent by weight of an aminealdehyde condensation product.

Other base-solubilized polyacids which may be employed as electrodeposition vehicles include those taught in U.S. Pat. No. 3,392,165, which is incorporated herein by reference, wherein the acid groups rather than being solely polycarboxylic acid groups contain mineral acid groups such as phosphonic, sulfonic, sulfate and phosphate groups.

The process of the instant invention is equally applicable to cationic type vehicle resins, that is, polybases solubilized by means of an acid, for example, an amine-terminated polyamide or an acrylic polymer solubilized with acetic acid. Another case of such cationic polymers is described in copending application Ser. No. 772,366, filed Oct. 28, 1968 now abandoned.

In a manner similar to the anionic resins described above, the cationic resins may be formulated with adjuvants, such as pigments, solvents, surfactants, crosslinking resins, and the like.

The polyacids are anionic in nature and are dispersed or dissolved in water with alkaline materials such as amines or alkaline metal hydroxides and, when subjected to an electric current, they migrate to the anode. The polybasic resins, solubilized by acids, are cationic in character and when these resins are water-dispersed or solubilized with an acid such as acetic acid, the material deposits on the cathode under an electric current.

and

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 the specification are by weight unless otherwise stated.

EXAMPLE Percent Yellow iron oxide 1.3 Strontium chromate 6.0 Red iron oxide 0.5 Titanium dioxide 50.7 Carbon black 0.5 Lead silicate 1.0 Aluminum silicate 30.0 Silica 10.0

The electrodeposition bath contained:

Parts by Weight Vehicle Resin (above) (solids) Pigment (above) Benzoguanamine-formaldehyde resin Methylon 75108 Triisopropanol amine 4 Dimethylethanolamine Cellosolve A mixture of allyl ether of mono-, di-, and trirnethylol phenols with the trimethylol derivative predominating, having the following properties: I percent solids, viscosity (25 C.) 2000-4000 centipoises, pounds per gallon, 99 percent reactive.

The above was dispersed in sufficient deionized water to provide a 6.1 percent solids bath.

The electrodeposition bath above, while utilized in a continuous coating operation of metal parts at one turnover per six days was continuously subjected to ultrafiltration utilizing the XPA membrane described hereinabove. The membrane removed 0.54 percent per hour of the bath volume as ultrafiltrate, one-third of which was sent to drain and two-thirds of which were utilized in rinsing l:l.5 ultrafiltrate to deionized water in a manner so that the rinse water containing paint dragout was returned to the electrodeposition bath, 10 percent of the rinse water being carried out by the parts. The coating operation was conducted on a 16 hour a day basis. The remaining eight hours the entire ultrafiltrate was directed to drain. At the end of each 24 hour day, the electrodeposition bath had a consistent pH and solids properties:

By adjusting the bath feed stock to compensate for desired materials such as solvents and/or adjuvant resins removed in the ultrafiltrate. The above electrodeposition bath was operated continuously for many months without serious coating quality problems.

The amount of ultrafiltrate produced can be varied as desired; it is most readily increased by increasing the area of the filter surface employed. if desired, sufficient ultrafiltration capacity may be employed to provide sufficient ultrafiltrate to drain for bath control while, in addition, providing sufficient ultrafiltrate to the rinsing head to allow rinsing with ultrafiltrate alone, above the tank, on a continuous 24 hour/day basis.

The process of the invention provides great flexibility in both bath control and dragout conservation and typically from about 5 to about percent of the ultrafiltrate may be removed from the system during a given period, such as a day, to effect bath control, depending on ultrafiltrate volume, bath turnover rate, rinse water entering the bath, and the particular electrodepositable composition employed, among other factors.

If desired, additional rinsing of the relatively paint solidsfree part may be subsequently conducted with, for example, tap water or deionized water.

Other electrodepositable compositions, such as those hereinabove described, can be substituted for those exemplified, Likewise, various ultrafilters and method variations may be employed to obtain the improvements hereinabove described.

According to the provisions of the Patent Statutes, there are described above the invention and what are now considered 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.

I claim:

1. A method of operating an electrodeposition process wherein the electrocoated articles are rinsed after coating comprising subjecting at least a portion of an electrodeposition bath comprising ionically solubilized synthetic resin to an ultrafiltration process wherein the ultrafiltration membrane passes an aqueous efiluent comprising water and solute of substantially lower molecular weight than the solubilized resin, while retaining said solubilized resin, and proportionately or intermittently utilizing said aqueous effluent as at least a portion of the rinsing agent in the electrodeposition process in a manner that the rinsing agent returns to the electrodeposition bath while proportionately or intermittently removing at least a portion of said effluent from the system.

2. A method as in claim 1 wherein the ultrafiltration process is operated at a pressure gradient between about 10 and about psi. and the ultrafiltration membrane has a flux rate of at least about 4.5 gallons per square foot per day.

3. A method as in claim 1 wherein the effluent is proportionately utilized as a rinsing agent while removing at least a portion of said effluent from the system.

4. A method as in claim 3 wherein the portion of the effiuent utilized as rinsing agent is admixed with additional rinsing agent.

5. A method as in claim 1 wherein the effluent is intermittently utilized as rinsing agent while intermittently removing at least a portion of said efiluent from the system.

6. A method as in claim 5 wherein the portion of the effluent utilized as rinsing agent is admixed with additional rinsing agent.

7. A method as in claim 5 wherein the ultrafiltration process is operated at a pressure gradient between about 25 and about 75 p.s.i., and the ultrafiltration membrane has a flux rate of at least 4.5 gallons per square foot per day.

8. A method of operating an electrodeposition process wherein electrically conductive substrate is electrocoated from an aqueous electrodeposition bath and subsequently rinsed with a rinsing agent to remove dragout comprising subjecting at least a portion of said aqueous electrodeposition bath comprising ionically solubilized synthetic resin, to an ultrafiltration process wherein an ultrafiltration membrane passes an aqueous effluent comprising water and solute of substantiaily lower molecular weight than the solubilized resin, while retaining said solubilized resin, and utilizing at least a portion of said effluent as at least a portion of the rinsing agent in the electrodeposition process in a manner that said rinsing agent, after use, returns to the electrodeposition bath while least 4.5 gallons per square foot per day. continuously or intermittently removing a portion of said effluent from the system.

9. A method as in claim 8 wherein the ultrafiltration process rinsing agent is operated at a pressure gradient between about 25 and about 75 p.s.i., and the ultrafiltration membrane has a flux rate of at 10. A method as in claim 8 wherein at least a portion of the effluent is continuously utilized as at least a portion of the "IKK

Claims (9)

1. 2. A method as in claim 1 wherein the ultrafiltration process is operated at a pressure gradient between about 10 and about 150 p.s.i. and the ultrafiltration membrane has a flux rate of at least about 4.5 gallons per square foot per day.
2. 3. A method as in claim 1 wherein the effluent is proportionately utilized as a rinsing agent while removing at least a portion of said effluent from the system.
3. 4. A method as in claim 3 wherein the portion of the effluent utilized as rinsing agent is admixed with additional rinsing agent.
4. 5. A method as in claim 1 wherein the effluent is intermittently utilized as rinsing agent while intermittently removing at least a portion of said effluent from the system.
5. 6. A method as in claim 5 wherein the portion of the effluent utilized as rinsing agent is admixed with additional rinsing agent.
6. 7. A method as in claim 5 wherein the ultrafiltration process is operated at a pressure gradient between about 25 and about 75 p.s.i., and the ultrafiltration membrane has a flux rate of at least 4.5 gallons per square foot per day.
7. 8. A method of operating an electrodeposition process wherein electrically conductive substrate is electrocoated from an aqueous electrodeposition bath and subsequently rinsed with a rinsing agent to remove dragout comprising subjecting at least a portion of said aqueous electrodeposition bath comprising ionically solubilized synthetic resin, to an ultrafiltration process wherein an ultrafiltration membrane passes an aqueous effluent comprising water and solute of substantially lower molecular weight than the solubilized resin, while retaining said solubilized resin, and utilizing at least a portion of said effluent as at least a portion of the rinsing agent in the electrodeposition process in a manner that said rinsing agent, after use, returns to the electrodeposition bath while continuously or intermittently removing a portion of said effluent from the system.
8. 9. A method as in claim 8 wherein the ultrafiltration process is operated at a pressure gradient between about 25 and about 75 p.s.i., and the ultrafiltration membrane has a flux rate of at least 4.5 gallons per square foot per day.
9. 10. A method as in claim 8 wherein at least a portion of the effluent is continuously utilized as at least a portion of the rinsing agent.

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