Pretreating Electrodepositable Compositions

Abstract

A method of pretreating electrodepositable compositions prior to utilization of such compositions, in an electrodeposition process, which comprises subjecting a synthetic resin ionically solubilized in an aqueous medium to a selective separation process such as ultrafiltration which passes an aqueous effluent through a physical barrier while retaining the solubilized resin component. The method enables the removal of deleterious impurities and contaminants which affect coating parameters and thus provides a direct method of controlling the quality of the deposited film.

Description

BACKGROUND OF THE INVENTION

In recent years electrodeposition has become a widely commercially accepted coating technique. Coatings deposited electrically have excellent properties for many applications. Electrophoretically applied coating compositions are particularly useful when the articles to be coated are of complex and unusual designs since the structural configuration is ordinarily of little or no significance when coating by this process. Film compositions applied by electrodeposition do not run, drip, sag, or wash off during baking. Virtually any electrically conductive surface may be coated by electrodeposition. Treated and untreated metals such as iron, steel, copper, zinc, brass, tin, nickel, chromium and aluminum are among the most commonly employed substrates. Paper or other non-conductive materials coated or impregnated with substances to render them conductive under conditions of the coating may also be employed as coating substrates.

In the electrodeposition process the articles to be coated are immersed in an aqueous dispersion of a solubilized, ionized, film-forming material, such as a synthetic organic vehicle resin. The article to be coated serves as an electrode through which an electric current passes to a counter-electrode, causing the migration and the deposition of the vehicle resin on the charged article. The articles containing the deposited films are then withdrawn from the electrodeposition bath, usually rinsed and then baked in the manner of a conventional finish.

It is known that in the electrodeposition process a delicate ionic balance must be maintained in order to produce coating compositions having uniform physical and chemical characteristics. If the ion balance is not maintained, there may be a deleterious effect on both operating parameters, as well as film qualities, for example, a decrease in the efficiency of coating weight per unit of electricity consumed and a decrease in throwing power are common. Foreign ion contamination carry-over from the pretreatment section contained on and in recessed areas and angled flanges of articles to be coated have been a major source of contamination. Contaminants such as phosphates, chromates, alkaline metal ions, acids and the like have been carried over into the electrodeposition bath, but the particular pretreatment contaminants will depend somewhat on the type of pretreatment employed.

Another source of contamination is the atmospheric air over the electrodeposition bath, for upon exposure to air, CO.sub.2 is absorbed into the aqueous bath composition. The dissolved CO.sub.2 is ordinarily present in the electrodeposition bath in the form of carbonates.

Such contamination takes place during the electrodeposition process, and because of the nature of the process, the contaminants tend to accumulate and lead to increasing process difficulties. In copending application Ser. No. 814,789, filed Apr. 9, 1969, now abandoned, there is described a process in which ultrafiltration is used to control the electrodeposition bath during the electrodeposition process.

DESCRIPTION OF THE INVENTION

It has now been discovered that the paint composition used to fill or feed an electrodeposition bath is itself a source of contamination and that by subjecting at least a portion of an electrodeposition bath to a selective separation process, the contaminant content of such composition is thereby controlled. Upon addition of such compositions to the electrodeposition bath as initial fill material or as replenishment for the paint solids consumed, such treated compositions thereby provide a bath composition that when deposited produces films having unexpectedly superior and uniform properties.

Contamination of depositable compositions, prior to utilization, may be derived from various sources, for example, from the manufacturing equipment; from the raw materials employed in formulating the compositions, such as pigments and deionized water; unreacted acids and monomers; free maleic anhydride, acrylic acid or oxidized formaldehyde; and from the atmosphere over the manufacturing container, whereby the composition may absorb carbon dioxide which exists, for the most part, in the composition in the form of carbonates; chloride ions which may also be absorbed from the atmosphere if the composition is exposed to testing equipment which releases salt spray vapors into the air, or from low purity deionized water. All have a deleterious effect on bath operating parameters and physical and chemical characteristics of the deposited film. For example, rupture voltage decreases, throw power decreases, conductivity of the bath increases, and the chemical resistance of the cured film decreases.

In some instances, low molecular weight resin residues, such as maleinized fatty acid residues, and the like, have also produced undesirable coating conditions and film properties. Some pigments have introduced water-soluble chromates and phosphates into the bath which likewise have deleterious effect because they tend to accumulate in the bath. Likewise, the solvent and solubilizing agent concentration can be controlled by subjecting feed composition to such a selective separation process. Various other ions, solvents, components, ingredients, and the like may be removed from the feed compositions or controlled to the extent necessary to produce the desired product.

In addition to the advantages obtained by the use of the present process as a precleaning technique, it also provides a method of concentrating low solids feed compositions, thereby aiding in control of the electrodeposition bath composition.

When subjecting a feed composition to ultrafiltration, it may be convenient to dilute the composition with deionized water to aid in handling and transfer. However, the need for prior dilution is dependent on the viscosity of the particular composition employed, the pressure, the type of ultrafilter membrane, and the flux rate desired. If dilution of the feed composition is employed, it is usually necessary to obtain an increased volume of ultrafiltrate to compensate for such dilution addition.

As mentioned above, application Ser. No. 814,789 teaches that ultrafiltration can be utilized to control an electrodeposition bath composition during the electrodeposition process. While exceptional control of a bath composition and removal of objectional materials accumulated during the process has been achieved by that process, by utilizing the ultrafiltration process prior to employment of the compositions in the electrodeposition process, in accordance with the present invention, the bath thereby is even more readily controllable, as less contaminants are introduced into the bath, and an overall process which is easily practiced yet highly effective is achieved.

The selective filtration process employed in the process of the instant invention is any process such as ultrafiltration which separates water from the electrodeposition feed stock through a physical barrier while retaining the solubilized resin components. Thus, any means may be utilized which accomplishes this purpose. Such a filter or barrier will 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 feed composition. 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 controlling or removing contaminants from feed stock compositions, or other depositable compositions prior to utilization, ultrafiltration is especially preferred.

The ultrafiltration process separates materials below a given molecular weight size from the composition. With the proper selection of membranes, the treatment does not remove any product or desirable resin from the feed stock composition, but does remove soluble anionic, cationic and nonionic materials from the paint in a ratio proportional to their concentration in the water phase of the feed stock composition.

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 thus 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 "ultrafiltration" is generally broadly limited to describing separations involving solutes of molecular dimensions below the limit of resolution of the optical microscope, that is, about 0.5 micron.

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, Volume 64, December 1968, pages 31 through 43.

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. Ultrafiltration is continually produced and collected until the retained solute concentration in the cell solution reaches the desired level, or the desired amount of solvent or solvent plus dissolved low molecular weight solute is removed. A suitable apparatus for conducting ultrafiltration is described in U.S. Pat. No. 3,494,465, which is hereby incorporated by reference.

There are two types of ultrafiltration membrane. 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, while 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 ultrafilters 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 membrane. Since a diffusive aqueous 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 solvent 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 1/10 to about 10 microns, layer 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 copolymers formed from any of the monomeric units of the above polymers, including "Polymer 360", a polysulfone copolymer.

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

Solute Molecular Weight Percent Retention __________________________________________________________________________ Raffinose 594 0 Bacitracin 1,400 20 Cytochrome C 12,400 0 Myoglobin 17,800 65 Pepsin 35,000 85 Ovalbumin 45,000 95 Albumin 67,000 100 Dextran 110 110,000 20 --------------------------------------------------------------------------- Flow Rate--ml./min.

0.25% Pepsin (35,000 mw) Membrane Pressure Distilled In Distilled Diameter psi Water Water (55 psi) __________________________________________________________________________ 25 mm. 55 8.6 1.1 150 mm. 55 350.0 46.0 __________________________________________________________________________

The membrane is chemically resistant to acids (HCl, H.sub.2 SO.sub.4, H.sub.3 PO.sub.4, all concentrates), alkalis, 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 dimethylformamide 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 Reten- (gal./sq.ft./day at Solute Weight tion 30 psi, 1.0% solute) __________________________________________________________________________ Cytochrome C 12,000 50 100 .alpha. Chymotrypsinagen 24,000 90 22 Ovalbumin 45,000 100 45 __________________________________________________________________________

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 Reten- (gal./sq.ft./day at Solute Weight tion 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 is 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.

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:

where C.sub.io is the initial solute concentration, C.sub.i is the final solute concentration of the retentate, V.sub.s is the volume of solute delivered to the cell (or the volume of the filtrate collected), and V.sub.o 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 be readily 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 to a selective separation process before it has been employed in a coating process. Initially such compositions are usually rich in contaminants from the air, manufacturing equipment, raw materials and the like. Thus, in order to preclean such compositions, an ultrafilter, preferably a diffusive membrane ultrafilter, is selected to retain the solubilized vehicle resin while passing water and low molecular weight solute, especially those with a molecular weight below about 1,000 and preferably below about 500. As previously indicated, the filters discriminate as to molecular size rather than actual molecular weight, thus these molecular 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 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 150 psi, preferably between about 25 and 75 psi. 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), the preferred flux rate being 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 parallel to the filter surface.

The retained solutes comprising the vehicle resin are then employed to fill or replenish an 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.

A number of electrodepositable resins are known and can be pretreated in accordance with this invention. Virtually any water-soluble, water-dispersible or water-emulsifiable vehicle resin in an aqueous medium can be electrodeposited and, if film-Forming, provides coatings which may be suitable for certain purposes. The present invention is applicable to any such material.

Presently, the most widely used electrodeposition vehicle resins are ionically solubilized, synthetic resin vehicles. Numerous such resins are described in U.S. Pat. Nos. 3,230,162; 3,441,489; 3,422,044; 3,403,088; 3,369,983; 3,366,563; 3,516,913 and 3,518,212. They include alkyd resins; modified or unmodified adducts of drying oil or semi-drying oil fatty acid esters with a dicarboxylic acid or anhydride, such as maleic anhydride adducts of linseed oil, soybean oil, or the like, modified in some cases with monomers such as styrene or a polyol; acrylic polymers, such as acid-containing interpolymers of acrylic monomers, in many cases including a hydroxyalkyl ester; mixed partial esters of fatty acids with resinous polyols, such as polyols derived from epoxy resins or sytrene-allyl alcohol copolymers; and others, including certain phenolic resins, hydrocarbon resins, etc. Aminoplast resins, usually made from condensation of melamine, urea, benzoquanamine or the like with formaldehyde and etherified with an alcohol such as methanol, butanol, hexanol or a mixture of alcohols, are also useful, especially in combination with hydroxyl-containing alkyd or acrylic resins.

In order to produce an electrodepositable composition from polycarboxylic acid resins, it is necessary to at least partially neutralize the acid groups present with a base in order to disperse the resin in the aqueous electrodeposition bath. Inorganic bases such as metal hydroxides, especially potassium hydroxide, can be used, as can ammonia or organic bases such as amines. Water-soluble amines are often preferred. Commonly used amines include ethylamine, diethylamine, triethylamine, diethanolamine, and the like.

Other base-solubilized polyacids which may be employed as electrodeposition vehicles include those taught in U.S. Pat. No. 3,392,165, 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, vehicle resins which deposit on the cathode. These include polybases solubilized by means of an acid, for example, an amine-terminated polyamide or an acrylic polymer solubilized with acetic acid. Other cationic polymers include reaction products of polyepoxides with amino-substituted boron esters and reaction products of polyepoxides with hydroxyl or carboxyl-containing amines; many such products are described in copending applications Ser. Nos. 772,366 (now abandoned) and 772,353, (U.S. Pat. No. 3,619,398) both filed Oct. 28, 1968, and Ser. Nos. 840,847 and 840,848, both filed July 10, 1969 and now both abandoned.

In addition to the vehicle resin, there may be present in the electrodepositable compositions any desired pigment or pigment composition, including practically 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 or surface-active agent, if any, are ground together in a portion of the vehicle, or alone in an aqueous medium, to make a paste and this is blended with the vehicle to produce a coating composition.

In many instances, it is preferred to add to the electrodeposition bath certain additives to aid dispersibility, viscosity and/or film quality, such as a nonionic modifier or solvent. There may also be included additives such as anti-oxidants, wetting agents, anti-foaming agents, fungicides, bactericides, and the like.

In formulating the coating compositions, ordinary tap water may be employed, but where such water contains a relatively high level of metals and cations, deionized water, i.e., water from which free ions have been removed by the passage through ion-exchange resins, is preferably employed.

The invention is further described in conjunction with the following examples, which are to be construed as illustrative rather than limiting. All parts and percentages in the examples and throughout the specification are by weight unless otherwise stated.

EXAMPLE I

The vehicle resin in this example was a maleinized tall oil fatty acid ester of a styrene-allyl alcohol copolymer (Shell X-450) of 1,150 molecular weight and an average hydroxyl functionality per mole of 5.2, comprising 38.5 percent of the copolymer, 55.5 percent tall oil fatty acid, and 6.0 percent maleic anhydride, having an acid value of 40.6 and a viscosity of 120,000 centipoises at 100 percent solids. It was employed with a black pigment paste made as follows: --------------------------------------------------------------------------- Black Paste A

Parts by Weight __________________________________________________________________________ Anthracite (pigmentary) 62.5 Basic lead silicate 25.0 Manganese dioxide 6.25 Strontium chromate 6.25 __________________________________________________________________________

The above pigments were ground in a 20 percent maleinized linseed oil solution, employing 16.5 percent diethylamine as a solubilizing agent and 1.0 percent cresylic acid as an anti-oxidant.

The electrodepositable composition was made from the following:

Parts by Weight __________________________________________________________________________ Vehicle resin (above) 1368.0 Sorbitan monolaurate 129.5 (wetting agent) Hexakis-(methoxymethyl)melamine 64.0 Cellosolve 68.5 Black Paste A 1025.0 Deionized water 1790.0 Aqueous KOH solution 420.0 (15 percent solids content by weight) __________________________________________________________________________

This composition had the following characteristics:

Solids Content Pigment-to- (Percent) pH Binder Ratio MEQ*/100 gm. Solids __________________________________________________________________________ 46.25 8.95 0.23/1.0 69.0 *Milliequivalents of base

Of the above composition, 1,000 parts were reduced with 1,000 parts of deionized water and the reduced composition was subjected to ultrafiltration for 13 hours, utilizing "Membrane B," as hereinabove described, at 50 psi.

During the 13 hours of ultrafiltration, 1,000 parts of ultrafiltrate were collected; analysis of the ultrafiltrate showed the following:

Solids content (percent) 1.15 CO.sub.2 (ppm) 236 CrO.sub.4 (ppm) 25 Ethyl Cellosolve (percent) 0.84 Diethylamine (percent) 0.03 Nitrogen (percent) 0.12

Electrodeposition of the thus treated composition is carried out easily and efficiently, without encountering many of the problems of unsatisfactory coating parameters and poor film properties often found with corresponding untreated compositions. For example, it had higher rupture voltage (420 versus 380), lower conductivity (1,350 versus 1,700), and after 10 days stirring, the ultrafiltered paint was still stable, while non-ultrafiltered paint had a rough, patchy, poor appearance. Succeeding stability tests showed numerous problems with the non-ultrafiltered paint when compared with the ultrafiltered system.

EXAMPLE II

A white electrodepositable composition was produced utilizing the following interpolymer, resin solution and paste: --------------------------------------------------------------------------- Interpolymer A

Parts by Weight __________________________________________________________________________ Hydroxyethyl methacrylate 8.7 Styrene 219.0 Methacrylic acid 131.0 Butyl acrylate 517.0 Di-tertiary butyl peroxide 13.2 Cumene hydroperoxide 8.8 Butyl Cellosolve 306.0

Interpolymer A had the following characteristics:

Solids content (percent by weight) 74.4 Viscosity (centipoises) 90,000 Acid number 66.7 Resin Solution B __________________________________________________________________________ Interpolymer A 100.0 Ethoxymethoxymethyl melamine* 18.5 (XM-1116) Aqueous KOH solution (45 percent 5.3 solids content by weight) Deionized water 184.2 *Ratio of ethoxy to methoxy groups about 60/40

Resin solution B was a thin clear fluid at 30 percent solids content by weight. --------------------------------------------------------------------------- White Paste C

Parts by Weight __________________________________________________________________________ Resin Solution B 208.0 Aqueous KOH solution (45 percent 5.3 solids content by weight) Titanium dioxide 370.0 Deionized water 100.0 __________________________________________________________________________

The above composition was ground in a steel ball mill of the type utilized widely in the art to produce a Hegman grind gauge reading of 7.5. The ground White Paste C had the following properties:

Pigment Solids Resin Solids Content (percent) Content (percent) pH __________________________________________________________________________ 47.2 11.8 8.9

the electrodepositable composition had the following composition:

Parts by Weight __________________________________________________________________________ Acrylic Interpolymer A 584 Ethoxymethoxymethyl melamine* 108 Aqueous KOH solution (45 percent 31 solids by weight) Deionized water 869 White Paste C 508 *Same as in Resin Solution B

the composition had the following characteristics:

Pigment-to-binder ratio 0.4/1.0 pH 8.0 Conductivity, .mu.mhos/cm., 75.degree. F. 4080 MEQ/100 grams total 16.2 Total solids content (percent) 41.8 Nitrogen content (percent) 1.16

The above composition was divided into two portions designated Composition D and Composition E. Composition D was not ultrafiltered, while Composition E was treated as follows:

To Composition E sufficient deionized water as added to reduce the solids content to about 20 percent and this composition was then subjected to ultrafiltration, utilizing Membrane B above, at 50 psi for 10 hours. The collected ultrafiltrate and resulting concentrate had the following characteristics:

Ultrafiltrate Concentrate __________________________________________________________________________ pH 7.8 8.0 Conductivity, .mu.mhos/cm., 75.degree. F. 780.0 3890.0 MEQ/100 grams total 0.80 14.2 Total solids content (percent) 0.79 41.7 Nitrogen content (percent) 0.14 1.03 Butyl Cellosolve (percent) 2.31 -- __________________________________________________________________________

To the concentrate (894 parts) there were added 6.6 parts of ethoxymethoxymethyl melamine in 20.7 parts of butyl Cellosolve and the mixture was agitated for 2 hours, then mixed with 2,879 parts of deionized water, the said melamine and Cellosolve were added to replace equivalent amounts extracted by ultrafiltration. The treated composition had the following characteristics:

Conductivity, .mu.mhos/cm., 75.degree. F. 1040 MEQ/100 grams solids 34.5 MEQ/100 grams total 3.56 Solids content (percent) 10.30 pH 8.40

composition D was not subjected to ultrafiltration; it was reduced to coating solids content by the addition of 2,891 parts of deionized water, and had the following characteristics:

Conductivity 1170 MEQ/100 grams solids 36.2 MEQ/100 grams total 3.6 Solids content (percent) 9.95 pH 8.45

each of Composition D and E were electrodeposited on various substrates to give a 1 mil thick film using comparable conditions, and each coating baked at 350.degree. F. for 20 minutes. Generally, Composition E had improved gloss and smoothness when compared to Composition D.

The coatings were then evaluated for discoloration, using the "Yellowness Index." "Yellowness Index" is measured by a spectrophotometer or with a colorometer, following ASTM Method D-1925 "Test for Yellowness Index of Plastics," 1968 Book of ASTM Stendards, Part 27, and ASTM Method E-313-69, "Indexes of Whiteness and Yellowness of Near-White, Opaque Materials," 1969 Book of ASTM Standards, Part 21. The readings were as follows:

Yellowness Composition Substrate Index __________________________________________________________________________ D Zinc phosphatized steel 4.31 E Zinc phosphatized steel 1.23 D Iron phosphatized steel 10.43 E Iron phosphatized steel 6.49 D Zinc phosphatized steel 2.68 (improved process) E Zinc phosphatized steel .33 (improved process) __________________________________________________________________________

As can readily be observed, the substrates coated with Composition E, which was subjected to ultrafiltration, had much better color retention in each instance than did those coated with Composition D, which was not subjected to ultrafiltration.

Such control of the color of the deposited coating is of particular importance in white and pastel electrodepositable compositions, where discoloration is common and renders the product unsatisfactory for use.

EXAMPLE III

In this example, there was employed a vehicle resin having the following composition and characteristics:

Parts by Weight __________________________________________________________________________ 2-Ethylhexyl acrylate 30.0 Styrene 15.0 Methyl methacrylate 40.0 Acrylic acid 6.0 Hydroxyethyl methacrylate 9.0 Solids content (percent by weight in isopropanol) 59.3 Viscosity (centipoises) 3250 Acid number 27.2 __________________________________________________________________________

In a manner similar to that in Example II, a white aqueous coating composition was produced from the above vehicle resin, using a mixed methyl and butyl ether of methylolated melamine, potassium hydroxide solubilizing agent, and titanium dioxide pigment. The coating composition had the following characteristics:

pH 7.9 Conductivity, .mu.mhos/cm., 75.degree. F. 1800 MEQ/100 grams total 7.56 Solids content (percent 41.30

A portion of this composition ("Composition F") was mixed with sufficient deionized water to reduce the solids content to about 20 percent and then subjected to ultrafiltration utilizing Membrane B at 50 psi for 8 hours. The concentrate obtained is mixed as follows:

Parts by Weight __________________________________________________________________________ Concentrate (at 40.0 percent solids) 944.0 Mixed methyl, butyl ether of methylolated melamine (90.8 percent solids content) 1.9 Isopropanol 43.4 n-Butanol 10.2 Mix for 2 hours. Deionized water 2800

Composition F then had the following characteristics:

pH 8.01 Conductivity, .mu.mhos/cm., 75.degree. F. 633 MEQ/100 grams solids 17.2 MEQ/100 total 1.72 Solids content (percent) 10.01

A second portion (920 parts) of the above coating composition ("Composition G") was not subjected to the ultrafiltration process and was reduced to coating solids content by the addition of 2,880 parts of deionized water.

Composition G then had the following characteristics:

pH 8.03 Conductivity, .mu.mhos/cm., 75.degree. F. 750 MEQ/100 grams solids 18.9 MEQ/100 grams total 1.82 Solids content (percent) 9.61

Various substrates were coated by electrodeposition using each of Compositions F and G, using voltages sufficient to obtain a cured film thickness of 1.0 mil. Due to impurities, Composition G had high current surges during the initial phases of the coating cycle and dropped off sharply, when compared to the amperage curve of Composition F.

The substrates having films deposited from Composition G (not ultrafiltered) were completely unacceptable, as they were extremely rough, discolored and had poor chemical and physical properties; however, the films produced from Composition F (ultrafiltered) produced smooth, undiscolored films having desirable chemical and physical properties.

The contrast in the films was reflected in the gloss reading on the various substrates employed.

Gloss, System Substrate 60.degree. Meter __________________________________________________________________________ Composition G Zinc phosphatized steel 35 Composition F Zinc phosphatized steel 54 Composition G Iron phosphatized steel 50 Composition F Iron phosphatized steel 70 Composition G Zinc phosphatized steel 32 (improved process) Composition F Zinc phosphatized steel 71 (improved process) Composition G Untreated aluminum 15 Composition F Untreated aluminum 57 __________________________________________________________________________

The Yellowness Index reading of the substrates coated with Compositions F and G showed a correlation similar to that displayed by the Yellowness Index readings of the substrates coated in Example II with those from Composition G being substantially discolored.

In addition to the vehicles described in the examples, the process described provides similar advantages when used with compositions made from other vehicles, for example, alkyd resins prepared from semi-drying or drying oils, esters of epoxides with fatty acids, including esters of diglycidyl ethers of polyhydric compounds, as well as other mono-, di- and polyepoxides; mixed esters of a resinous polyol, such as resinous esters comprising mixed esters of an unsaturated fatty acid adduct. Cationic resins, i.e., resins which deposit on the cathode, can also be used in substantially the same manner using known procedures.

It is also quite feasible to omit the hexakis-(methoxymethyl)melamine, ethoxymethoxymethyl melamine or mixed methyl, butyl ether of methylolated melamine, or where such a material is desired to employ other crosslinking agents such as N,N'-dimethyl urea, benzyl urea and benzoguanamine or the like may be used instead.

The aqueous potassium hydroxide solutions employed as a neutralizing agent in the above examples an readily be substituted for other metal hydroxides such as lithium or sodium hydroxide, or with an amine, preferably water-soluble, such as with isopropanolamine, triethylamine, dimethylethanolamine, or the like. Acidic solubilizing agents are employed in the case of polybasic resins.

Also, the pigmentary components may be varied to produce a particular color or to impart inhibitive properties to the film. Color such as cadmium yellow, cadmium red, phthalocyanine blue, chrome yellow, toluidine red, hydrated iron oxide and the like may be included if desired. Other conventional type pigments employed in the art may be added, for example, iron oxide, carbon black, talc, barium sulfate and the like.

Various other components may be utilized to produce a desired effect. Such components include, for example, wetting agents, flow agents, fungicides, anti-oxidants, and the like.

In addition, it is also possible to substitute for "Membrane B" either "Membrane A" or "Membrane C" and similar type membranes with highly satisfactory results.

As described above, above, the method of the invention is particularly applicable to the treatment of the feed composition for an operating electrodeposition bath, where such feed composition is intermittently or continuously added to the bath. The feed composition can be the same as or somewhat different from the composition used to fill the bath, and when treated using a selective filtration process as described, prior to adding it to the bath, greatly reduces the control problems normally associated with operation of electrodeposition processes and maintains the improved film properties illustrated by the foregoing examples. In this embodiment of the invention, for example, a feed composition is produced as in the above examples, ultrafiltered, and added to the bath, using known procedures, as replenishment for the coating composition removed by the coating process.

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.

Claims

We claim:

1. In a method of electrodepositing a coating from an electrodeposition bath to which there is continuously or intermittently added a replenishment feed composition comprising an aqueous, ionically solubilized synthetic resin component, the improvement which comprises subjecting at least a portion of the feed composition to an ultrafiltration processes wherein an ultrafiltration membrane passes water and solute of substantially lower molecular size than said resin component, while retaining said resin component, adding the ultrafiltered feed composition to the electrodeposition bath and subsequently electrodepositing a coating from the bath.

2. A method as in claim 1 wherein the ultrafiltration process is operated at a pressure gradient between about 10 and about 150 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 said vehicle resin component comprises an ionically solubilized synthetic resin.

4. A method as in claim 3 wherein said synthetic resin is a polycarboxylic acid resin solubilized with a base.

5. A method as in claim 4 wherein the base is a water-soluble amine.

6. A method as in claim 4 wherein the base is an alkali metal hydroxide.

7. A method as in claim 3 wherein said synthetic resin comprises a polybasic resin solubilized with an acid.

8. A method as in claim 1 wherein the feed composition is diluted with water before being subjected to the ultrafiltration process.