CA1128238A - Cathodic electrodepositable paint - Google Patents

Abstract

f ABSTRACT OF THE DISCLOSURE A process for cathodic electrodeposition of paints wherein a layer of protective phosphate salt is initially deposited on a metal substrate followed by an amine-group-containing hydrophobic polymer which is preferably an acrylic-epoxy resin. The amounts of water and phosphoric acid in the deposited paint are very small.

Description

~ ZE~;~3~3 BACKGROUND OF T~IE INVENTION
The present invention relates to a me-thod of cathodically depositing polymeric coatings. More speci-fically, it relates to a method of depositing coatinys of acrylic polymers containing amlne ~unctionalit~ with minimum retained water and acid in the coatings.
It is known that organic coatings can be electro-deposited either on an anodicallv-charged conducting substrate or on a cathodically-charged substrate. Although most of the earlier work in electrodeposition was done with anodic deposition, that type of process has certain disadvantages. Anodic electrodeposition is normally done in a coating bath having a basic pH. The pH decreases at the surface being coated, creating conditions which, when combined with the electrolytic action of the coating bath, can cause the dissolution of substrate metal ions and their subsequent deposition in the coatings being formed. This can be a source of staining and diminished corrosion resistance. Also, electrolysis tends to attack preformed phosphate coatings. Furthermore, oxygen formed at the anodic substrate being coated can cause a variety of difficulties such as degradation of coatings by oxidation.
Electro-endoosmosis tends to expel water from anodic coatings being formed, leading to low water re-tention with about 85 to 95% solids in the coatings.
This is an advantage over cathodic coating in which this phenomenon would not be expected to be helpful. (Parts and percentages herein are by weight except where indi-cated otherwise, and the expression of a range as "X to yl~
or as "X-Y", wherein X and Y are numbers, is meant to be ~:~2~

inclusive of both. X an~ Y.) Cathodic electrodeposition has developed more slowl.y, due in part to the acidic pH needed for the ba-th.
Also, water tends to be drawn into the coatin~s and hel~
there, along with acid residues :~rom the ba-th. It is apparent that this can lead to di:f:ficulties in the coatings.
In contrast to the oxygen formed at anodes in anodic elec-trodeposition, hydrogen is formed at the cathode in cathodic electrodeposition. Even though this hydrogen can cause pinholes in coatings, it, of course, does not cause oxida-tive film degradation.
Prior to coatin~ with protective organic coatinqs, metal surfaces, particularly iron and steel, are normally given a pretreatment such as phosphatizing. U.S. Patent 870,937 - Coslett (.1907) describes a method of treating iron or steel surfaces with phosphoric acid solutions which may include iron powder or iron phosphates. In the evolutIon o~ phosphati.zing coatings for metals, particularly ferrous metals, several chemi.cal modiications of the phosphate coating have been found desirable, including the incorporation of calcium and molybdenum into the coating and post-rinsing with chromate solutions.
Processes and compositions for the cathodic electrodeposition of paints are described in U.S. Patent

2,345,543-Wohnsiedler, et al., (:1944), which uses a cati.onic melamine-formaldehyde resin, and in U.S. Patent

3,922,212-Gilchrist (.1975), among others. Gilchrist is directed to a process for supplementing the ba-th compo-siti.on with a make-up mixture of materials containing an ionizing acid that is not consumed at as fast a rate g~2~;Z3~3 as the resin. The acid is present in the make-up at lower concentrations than are used in the bath, so as not to build up the concentration of the acid in the bath.
Gilchrist uses particular aminoalcohol esters of polyca~-boxylic acids and discloses that acrylic polymers can be codeposited with zinc phosphate from solution on a cathodic substrate at low pH's such as 2.7 with phosphoric acid as the ionizing acid.
Two U.S. patents dealing with nitrogen-based lC copolymers and their cathodic electrodeposition are 3,455,806 and 3,458,420, both to Spoor, et al., (1969). Cathodic sulfonium systems are described by Wessling et al. on pages 110-127 of "Electrodeposition of Coatings," Ed. E.F.
Brewer, American Chemical Society (1973).
Electrodeposition processes have been frequently described in the literature. Two useful reviews of the technology are: "Electro painting Principles and Process Variables," Brower, Metal Finishing, September, 1976, p. 58;
and "Coatings Vpdate: Electrocoating", Americus, Pigment and Resin Technology, August, 1976, p. 17. However, neither of these articles nor any of the patents mentioned above suggest means for obtaining cathodically electrodeposited resin coatings with optimally low levels of water and acid reten-tion and high corrosion resistance.
SUMMARY OF THE INVENTION
The present invention provides a process for electrocoating with a coating composition a negatively-charged substrate immersed in a coating bath containing an aqueous dispersion of said coating composition, said bath having a cathode zone containing said substrate and an ~iLglL2~38 anode zone containing a charged anode, said substrate and said anode constituting oppositely-charged electrodes, the charged electrodes being maintained i.n electrical contac~
with each other by means of said bath, wherein said bath comprises a cati.onic fllm-forming polymer, an acidic ionizing agent, and a crosslinking agent, the improvement which comprises:
employing phosphoric acid as an acidic ionizing agent;
employin~ as a cationlc film-forming polymer a graft copolymer having a backbone portion containing secondary and/or tertiary amine functionality, said graft copolymer being stabilized in the aque-ous dispersion by a phosphate salt of the amine functionality, said backbone portion being gra:Et polymerized with hydrophobic copolymer derived from epoxy esters, said hydrophobic copolymer having a high enough concentration in the graft copolymer that the coating deposited on the substrate has at least about 83% solids content and so that the phosphoric acid concentration in the deposited coating composition is no more than about 17.5% of the phosphoric acid concentration in the bath.; and employing as the crossli.nking agent a composition which is nonreactive in the bath but reactive with said film-forming polymer at elevated temperatures.
The invention also comprises acrylic resins parti.cularly suited for use in coating compositions for the processes of the invention, including both a resin suitable for use in primer compositions and a resin suitable for . ,~-, 8~:3~3 top-coat compositions which can be used either as a single coat or applied cathodically over an electrically-conductive prlmer .
The primer resin is a graft copolymer oF an epoxide grafted onto an acrylic backbone and consisting essentially of, by weight based on the graft copolymer, about:
a. in the acrylic backbone portion: 15 to 25% o a polymer or copolymer of at least one unit selected from alkyl, aminoalkyl, and hydroxyalkyl acrylates and methacrylates, said copolymer con-taining 0.02 to 0.1 (preferably 0.02 to 0.06) equivalent of secondary and/or tertiary amine functionality and optionally 0.01 to 0.05 (.preferably Q.01 to 0.02~ e~uivalent of quater-nary ammonium functionality; and b. in the graft: 75 to 85% of a copolymer contrib-uting:
3 to 7% of a glycidyl ester of a tertiary carboxylic acid containing 7 to 9 carbon atoms, and 72 to 80% of a blend of a 55 to 60%
condensation polymer of epichlorohydrin and bisphenol-A with 15 to 20% tall oil fatty acids.
(Equivalents herein means equivalents of functionality per 100 grams of graft copolymer.~
One preferred embodiment is a graft copolymer consisting es-sentially of, by weight based on the graft copolymer, about:

~ .

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a. 17 to 21%, preferably 19%, of a copolymer contributing:
3 to 5~, preferably 4%, methyl methacryl~te,

4 to 6%, preferably 5%, butyl acr~late,, 1 to 4%, pre~erably 3%, hyclroxyethyl methacrylate, 1 to 3%, preferably 2%, dimethylamino ethyl methacrylate, and 4 to 6%, preferably 5%, t-butylamino ethyl methacrylate, graft polymerized with b. 83 to 79%, preferably 81%, of mixture of about

5% of ,1 " / \

wherein the Rl, R2 and R3 groups are saturated aliphatic chains which contain a total of 7 to 9 carbon atoms, and at least one of Rl, R2 and R3 is a methyl group, and 74 to 78%, preferably 76%, of a blend of 57 to 60%, preferably 58.5%, of a condensation polymer of 27 to 31%, preferably 29.25%, epichlorohydrin and 27 to 31%, preferably 29.25%, bisphenol-A
with 16 to 19%, preferably 17.5%, tall oil fatty acids.
The top-coat resin is a copolymer of an epoxide grafted onto an acrylic backbone which consi.sts essentially of, by weight based on the graft copolymer, about:

'~' ,. ~

3l~21~238 a. in the acrylic backbone portion: 80 to 92% o~ a polymer or copolymer of at least one unit selected from alkyl, aminoalkyl, and hydroxyalkyl acrylates and methacrylates, said copolymer containing 0.02 to 0.1 tpreferably 0.02 to 0~08) equivalent of secondary and/or tertiary amine functionality and optionally 0.01 to 0.05 ~preferably 0.01 to 0.03) equivalent of quater-nary ammonium functionality; and b. in the graft: 20 to 8% of a comonomer which is a glycidyl ester of a tertiary carboxylic acid containing 7 to 9 atoms.
One preferrèd embodiment is a graft copolymer consisting essentially of, by weight based on the graft copolymer, about:
a. 85 to 91%, preferably 89~, of a copolymer contrlb-uting:
9 to 11%, preferably 10%, methyl methacrylater 42 to 46%, preferably 44%, 2-ethyl hexyl acrylate,

6.5 to 8.5%, pr~ferably 7.5%, t-butylamino-ethyl methacrylate, 2 to 3%, preferably 2.5%, dimethylaminoethyl methacrylate, and 23 to 27%, preferably 25~, hydroxyethyl methacrylate, graft polymexized with b. 15 to 9~, preferably 11~, of .

~ ' ~2~Z~3~

,1 ~0~
R2-C~ C-O-C~I2-CH- CH2 wherein the Rl, R2 and R3 yroups are saturated aliphatic chains which contain a total of 7 to 9 carbon atoms, and at least one o~ Rl, R2 and R3 is methyl group.
DETAILED DESCRIPTION OF THE I~VENTION
The present invention provides a practical means lQ for cathodically electrodepositing first a passivating phosphate salt coating, preferably as at least a monolayer, and then electrodepositing over the phosphate a protective resin coating havin~ low water and acid retention and high resulting durability and corrosion resistance. The process of the invention can be used either on pretreated metal such as phosphatized steel or on bare metal such as steel which has been cleaned but not phosphatized. It can also be used on other metal substrates containing zinc, such as galvanized steel, as well as on aluminum and various alloys. Since the process of the invention deposits a phosphate coating underneath the polymer coating, it is less sensitive than other electrocoatin~ processes to variations in the substrate and its pretreatment. If the process o~ the invention is applied to material which has already been phosphatized, this versatility of the invention would enhance the phosphate coating. However, phosphate pretreatment is not necessary.
The invention preferably provides in the bath a water-soluble dihydrogen phosphate salt, M(H2~O4)2 3~ (M=Fe, ~n,Ca, Mg or Al). With cathodic electrodeposition of ,, .. ~, ., .

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the invention, the pH rises at the cathode, creating a boundary layer perhaps 0.01 to 0.1 cm thick, in which the soluble phosphate salt convert5 to an insoluble phosphate salt, M(HP04) or M3(P04)2, which is deposikecl onto the substrate surface. Thus, the driving force for the salt deposition is a pH change and precipitation at the sub-strate surface. The same type of p~l change phenomenon also causes deposition of dense polymer coatings with the present invention. This leads to denser coatings than electrophoresis of quaternary ammonium salts wherein a concentration gradient is the main driving force.
In a specific embodiment of the invention, with the coating voltage applied while a ferrous metal substrate is being immersed into the coating bath, the phosphoric acid in the bath dissolves small amounts of ferrous ion from the substrate. The Fe(H2P04)2 salt so formed is soluble in the bath at pH levels of 3.0 or below. However, a boundary layer of increased pH quickly develops at the substrate, leading to the formation and deposition on the substrate of the insoluble salts Fe(HP04) and Fe3(P04~2, These salts give effective cor-rosion protec-tion which is enhanced by the fact that they gen-erally form a continuous layer on the substrate, usually at least a monolayer, rather than being entirely dispersed up into the overlaying subsequently-formed resin coating. In a preferred embodiment of the invention, water-soluble zinc salts are included in the bath, and they too deposit as insol-uble phosphates at the substrate. The dihydrogen phosphate of zinc, Zn(H2R04)2, is stable in the bath at a pH of 2.5 to 3.5.
The invention is useful at pH values in the range of 2.0 to 4.0, preferably 2.5 to 3Ø At a pH below about 3.0, free 3~

phosphoric acid in the bath will react with ferrous metal sub-strates to generate the soluble dihydrogen phosphate of iron, Fe(H2PO4)2. Since the solubility of the dihydrogen metal phos-phates decreases with increasing temperature, the process oE
the invention is best operated with a bath temperature of 20 to 25C. r~qhen zinc salts are dissolved in the bath, the phos-phate coating formed on the steel may likely include two min-erals as the principal constituents. These are phosphophyl-lite, Zn2Fe(~O4)2.4H2O, and hopeite, Zn3(PO4)2.4H2O.
The lack of practical success of several previous cathodic electrodeposition painting processes is due at least in part to the amount o:E water that is held in the resin coating and the acids and salts that are dissolved in that water, not readily removable from the coating. The water can lead to coating failure by various mechanisms, and the acid residues can encourage subsequent corrosion, either directly or by providing a hygroscopic material in the coating which encourages penetration of water and other corrosive agents.
In contrast to the useful effect o.f electroendo-osmos:is at the anode in anodic electrodeposition of paint which tends to expel water from an anodic coating, water is not electrically expelled from a cathodic coating and may actually be drawn into the coating by electrical forces.
However, water held in a cathodic coating can be particu-larly undesirable. To minimize such effects, the present invention provides resins with a degree of hydrophobicity and hardness or denseness of the coating which combine to expel water from the coating as the coating is formed.

The desirable efects of the invention are ., achieved by using certain hydrophobic graft copolymers containing in their backbone portions seconaary and/or tertiary amine Eunctionality. Such unctionality aids in adhesion of the resin coating to the substrate even after heating the deposited coatings to cause them to crosslink.
This is an advantage over cathodic sulfonium systems in which hydrophobicity is only developed after thermal decomposition of the sulfonium groups. Thermal decomposi-tion of sulfonium groups during crosslinking of the film would make them unavailable for enhancing adhesion of the resin coating to the substrate. Also, although quaternary ammonium salts can be present in film-forming polymers of the invention, they cannot replace the secondary or tertiary amine groups. The quaternary ammonium salts would decompose to some extent when the film is heated to cause crosslinkinq thereby losing their effectiveness in promoting adhesion to the substrate. The polymer compositions of the invention are discussed in more detail below.
In the process of the invention, although there are advantages in using live entry, in which the coa-ting voltage is applied while the articles to be coated are being immersed into the bath, it will be apparent that reduced voltage can be applied upon entry if desired for certain special effects. However, the additional electrical apparatus required for reduced voltage entry is not nor-mally necessary or desirable. It is desirable for the coated subst~ate to be removed from the bath with the coating voltage still applied or soon after it is turned off.
For operating electrocoating baths of the invention, the tank can be lined with an organic coating resistant to the acidic pH of the bath, and stainless steel or plastic piping and pump parts can be used ko minimize corrosion. However, carbon steel parts often can be used, and the Eerrous ions added to the bath by gradual dissolu--tion of the equipment could be helpful rather than harmul to the coating process. Due to its autopassivating effec-ts, phosphoric acid is less corrosive to steel than some other mineral acids at the pH levels used, so that more expensive materials of construction often are not necessary.
It has been found that common bacteria do not grow in the aqueous coating compositions of the invention.
Therefore, ordinary ultrafiltration can be used in recir-culating the bath components ~o rinse contaminants from the coated parts. Furthermore, membranes and ordinary flushed anodes may be desirable but are unnecessary.
As an alternative to flushed anodes, excess phosphoric acid build-up in the bath can be consumed by additions of zinc, ZnO, ZntOH)2, or other metals or compounds which form the dihydrogen metal phosphates in solution.
Although an uncoated tank can be used as the anode, in commercial practice one would normally use stainless steel anodes having a surface area smaller than that of the cathodic substrate which is to be coated. This gives a ~avorable current density distribution.
In the novel electrocoating ~rocess, the metal article providlng the substrate to be coated is immersed in a bath of an electrocoating cell. The bath is an aqueous dïspersion of about 2-35% by weight oE a cationic film-forming polymer at least partially neutralized with an .~y, ,.

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acidic material. Preferably phosphoric acid is used in an amount of from 60% of that required for stoichiometric reaction o the first hydrogen of -the trivalent acirl wi-th all of the available amine group bonds in the polymer to an excess of 120~ of stoichiometric. The use of less than about 60~ of the stoichiometric amount of phosphoric acid can lead to instability in the bath. ~ore than 120%, even as high as 270% or higher, can sometimes be tolerated, even though a low pH limit of 2.2 to 2.5 is approached as more free phos-phoric acid is present. In the presence of the acid, thefilm-forming polymer forms cations in the bath.
The metal article is connected to the negative side of a direct current (D.C.) power source to become the cathode of the cell. A voltage of about 1 to 500 volts is passed through the cell for the full dwell time of the article in the bath, about 0.01 to 5 minutes, and a coating of the cationic polymer is deposited. When the coating reaches the desired thickness, the article is removed from the bath. Preferably, the article is rinsed with water or with filtrate taken from the process to remove excess coating. Then the article is dried at ambient temperatures or baked for about 5 to 40 minutes at about 100 to 300C.
to give a finished coating about 0.1 to 5 mils thick.
Typical efficiencies of about 30 mg film solids de~osited per coulomb of electricity are obtained.
The current density used in the electrocoating cell generally does not exceed 1.85 amperes/cm2 (0.3 amperes/in2) o anode surface which is immersed in the bath, and it is preferable to use lower current densities.
In the deposition of the cationic film-forming polymer, `u~`

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voltages of 5 to 400 Eor 0.25 to 2 minutes are preferred to form a high quality finish.
Coating compositions of the present invention can contain piyments. The pigments are normall~ ad~ed to the composition in the usual manner by forming a mi~l base or pigment dispersion with the pigment and the aforementioned cationic film-forming polymer or another water-dispersible polymer or surfactant. This mill base is then blended with additional film-forming constituents and the organic solvents. When the mill base is subse-quently acidified and dispersed in water, the polymers tend to wrap themselves around the pigments. This has the effect of preventing destabilization of the dispersion or other undesirable effects that could come rom using a basic pigment such as TiO2 in an acidic dispersion. Pig-ments stable in acidic media can be used, such as the surface-treated Tio2 pigments of U.S. ~atent 3,941,603 -Schmidt (19761. Other pigments that could be used include metallic oxides such as zinc oxides, iron oxides, and the like, me~al flakes such as aluminum flake, metal powders, mica flakes with and without surface treatment such as with titania and carbon black, chromates such as lead chromates, sulfates, carbon black, silica, talc, aluminum silicates including china clay and finely divided kaolin, organic pigments and soluble organic dyes.
Aside from cathodic electrodeposition, the novel coating compositions of the present invention can also be applied by any conventional method such as spraying, electrostatic spraying, dipping, brushing, flowcoating and the like. Reaction of the amine groups of the polymer ' `.,' t ~Z~Z~8 with phosphoric acid is generally not necessary when the coating composition is to be used for purposes other than electrodeposition. Organic thermally decomposable acids, such as formic acid, can be used to obtain.water solubility for such purposes. The coating would then be baked for about 5 to 40 minutes at about 175 to 200C to give coatings of about 0.1~ mils thickness. When applied by cathodic electrodeposition, coating compositions of the invention are preferably applied to gi~e dried thicknesses of about 0.8-1.2 mils.
A crosslinking agent which can be water dispersed along with the film-forming constituent is used in the novel composition. Based on the proportions of solids in the bath, which are roughly equal to the proportions of solids in the film, about 60 to 95%, preferably about 70%, of cationic film-forming polymer are used along with about 5 to 40%, preferably about 30%r of crosslinking agent.
Typical crosslinking agents that can be used with the invention are melamine formaldehyde, alkylated melamine-formaldehyde resins such as hexakis-(methoxymethyl) melamine and partially-methylated melamine formaldehyde resins, butylated melamine formaldehyde resins, methylated urea-formaldehyde r~sinsi urea-formaldehyde resins, phenol-formaldehyde and the like. One particularly useful cross- ;
linking agent which forms a high quality product with the cationic polymers is a benzoguanamine-formaldehyde resinO
A preferred benzoguanamine formaldehyde resin is XM 1125*
produced by American Cyanamid Co., an acidic self-catalyzed crosslinking agent with an acid number of 25 to.32.
When the novel compositions of this invention are * denotes trade mark used as primers over metals including treated and untreated steel, aluminum and other metals, conven-tional acrylic enamels, acrylic dispersion enamels an~ other coaking com-positions can be applied directly as topcoats ove~ such primers. ~crylic lacquers, acrylic dispersion lac~uers, and acrylic powder coatings can be applied over the novel compositions, but a suitable intermediate coat such as a sealer can be used to improve adhesion of the lacquer or powder topcoat to the primer.
The glycidyl ester used in both the primer and topcoat compositions and the optional epoxy-fatty acid constituents used in the primer composition contribute sufficient hydrophobicity to the polymer so that the elec-trodeposited film contains at least about 83% solids, and preferably 85 to 95% solids. Although such high solids levels are not uncommon for anodically deposited coatings, they are not readily achieved in cathodic electrodeposition because of the amount of water usually entrapped. The phosphoric acid concentration of the electrodeposited film is in the range of 10 to 15% of the concentration of phos-phoric acid in the bath. This is on the order of about 0.05% of the electrodeposited film itself. These figures apply to the film as electrodeposited, before drying and baking. The amine functionality in the film causes some small phosphate concentration in the film but retained ~ water will deleteriously increase the phosphoric acia -~ content. ~mpirical tests have shown that 20 to 25% of the concentration of phosphoric acid in the bath being present in the film is an undesirable level, causing diminished corrosion resistance, blistering, and other unde~irable ~2~Z~

effects.
In the process of the invention, the critical concentration of phosphoric acid in khe dry film is speci-fied in terms of a percentage o~ the concentraticn o~
phosphoric acid in the bath. Equivalents of phosphoric acid, in the form of phosphates reacted with amine groups of the polymer and metal phosphate salts, are included in the term "phosphoric acid" for these purposes. Relative to the entire electrodeposited coating, the metal phosphate layer directly on the substrate will con-tribute negligible amounts of phosphate. Most of -the phosphoric acid equiv-alents will be present as free phosphoric acid or as amine salts along with the water entrapped in the film. The amount o~ phosphate ionically bound in the polymer will vary depending on the amount of amine in the polymer.
Larger amounts of amines will lead to larger amounts of bound phosphate reacted with them. Most of the ph~sphate is released from the polymer as phosphate ion as a result of pH change and electrical phenomena when the polymer is deposited on the substrate, but a variable amount remains in the film. The most reliable measurement of ~he concen-tration o~ phosphate to be deposited is as a proportion of the concentration in the bath. This is a dynamic value - which depends upon coating speed, dragout and flushing rates-. It is best averagea over a period of time as phosphoric acid is added to the bath and par-tially removed in the coatlngs.
Although the present invention uses phosphorlc acid for several reasons, most importantly to allow the production o a phosphate coating on the substrate in the ,;

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same process that produces the paint coating, other acids could be used in addition to the ~hosphoric acid Eor similar results. Acids which form water soluble salts of the desirable metals at low pH, especially such salts of zinc, iron, calcium, magnesium and aluminum, ancl which then convert to insoluble salts in the boundary layer at increased pI-I, can be useful. Oxalic, chromic, sulamic, benzoic and boric acids can have such effects. However, the deposited salts of such.acids in the absence o-E phos-phates may not have the passivating or corrosion-inhibiting effects of phosphates.
Compositions of the invention can include addi-tional adjuvants that do not materially change the basic and novel characteristics of the invention and thus are within the scope of "consisting essentially" terminology.
Some such. adjuvants are thickeners, defoamers, pi~ments, microgels, pi.gments dispersants, polymeric powders, micro-bi.oci.des, and coalescing solvents. Typi~al coalescing solvents whlch might be used at a concentration of about 0.5% of the total bath. volume are ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, and cyclohexanol.
The graft copolymers of the invention can have backhone portions of a variety of types so long as they contain the requisite amine functionality and are made adequately hydrophobic by grafting wlth epoxy copolymers.
The preferred backbone portions are acrylics, including alkyl acrylates such as methacrylics, and polymers derived ; from acryli.cs and methacrylics. Other useful backbone porti.ons include polyamines of maleinized oils, polyesters, maleini2ed polybutadiene, and epoxidized oils.

.
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Secondary amines in the backbone portion of the gra-ft copolymer can function similarl~ to tertiary amines.
Secondary amines can be provided, for instance, by reactin~
glycidyl methacrylate with ammonia to :Eorm a p~imar~ amine which is converted to a secondary amine on grafting with appropriate amounts of epoxies. It should be kept in mind that graft copolymerization to produce compositions of the invention changes secondary amines in the reactants to tertiary amines and likewise changes primary amines to secondary amines and tertiary amines to quaternary ammonium salts.
Quaternary ammonium salts would be coated onto the substrates mainly by concentration qradient effects rather than by pH changes in the narrow boundary zone which caus.e the deposition of secondary and tertiary amines. The concentration gradient effect is more gradual than the bound-ary zone effect, leading to softer, less. dense coatings in the absence of the secondary and tertiary amine groups.
Such softer coatings would be bulkier and more porous and, therefore, more conductive. This means that they would continue to build up in thickness with further electro-deposition. In contrast, the self-limiting effect of less conductive films gives coatings of more uniform thickness.
In addition to increasing the adhesion of the film to the substrate after baking, secondary and tertiary amines in the backbone portion also enhance stability o the polymer in water dispersions.
~or enhanced adhesion to substrates and dispersion stabi.lity in water, the film-forming polymer of the inven ti.on preferably contains 0.04-0.8 equivalent of tertiary - 20. -s ~ .

amine functionality. The preferred primer contains about 0.04 equivalent, and the preerred to~coat contains about 0.05 equivalent of tertiary amine functionality. The preferred secondary amine beore gra~ting is -t-butyl amin~
ethyl methacrylate, and the preferred tertiary amine is dimethylaminoethyl methacrylate.
Tertiary amines in the acrylic backbone portion before graft copolymerization which are converted to quaternary ammonium salts upon grafting serve the useful purpose of enhancing the graft copolymerization. Therefore, graft copolymers of the invention preferably contain about 0.01 to 0.05 (more preferably about 0.01 to 0.02) equivalent of quaternary ammonium functionality. However, the quater-nary ammonium functionality need not be built into the backbone portion but can be provided as an external catalyst to enhance the graft polymerization. In such a case, the somewhat deleterious effects of quaternary ammonium func-tionality in the backbone portion o electrodeposited coatings are avoided.
Although it is difficult to meaningfully quantify the softness or hardness of the resin, it is known that certain resins of the invention have a degree of hardness which is useful in combina-tion with the hydrophobicity characteristics of the resins in forcin~ water out of films to obtain the indicatea levels of retained water and acid.
The molecular weights of polymers of the invention are generally not critical. ~owever, typical average molecular weights determined by gel permeation chromato-graphy are: for the backbone portion - 12,000; for the primer graft copolymer - ll,OnO to 12,000; and for the ~12~

topcoat graft copolymer - 15,000O These figures show that typically 80 to 85% of the epoxide is grafted onto the backbone portion.
Although thoughts are expressed here'in on why and how the advantages of the invention are obtained, the invention is described by the claims and does not depend upon theories.
SpPcific examples will now be given of ~he preparation of graft copolymers of the invention and their use in cathodic electrodeposition processes of the invention.

EXAMPLE I
A black primer coating composition is prepared and used as follows:
Part I and Part II describe the two resin compounds that are graft polymerized and used with the piqment dispersions of Part III in the paint of Part IV.
Part I
This part describes the preparation of an epoxy ester for graft copolym~rization.
~ 20 The following ingredients are charged into a i rea~tion vessel equipped with a stirrer, thermometer, ¦ reflux condenser and a heating mantle to form an epoxy ¦ ester resin solution:
ortion I Partc b~ Vei~
Epoxy resin (EPON* 1001~ 1677.00 (EPON 1001 is an epo~y resin of the formula o * denotes trade mark .

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C~ C H- CH 2 ~0~ C~ o- cH 2 - cl~ - cH 2 ~ o--~
CH3 n , 3 / ~
-C _ ~ C 2 C 2 where m i5 an integer sufficiently large to provide a Gardner-Holdt viscosity at 25C of D-G measured in a 40% weight solids polymer solution using ethylene glycol monobutyl ether solvent, and the resin has an epoxide equivalent of 450-550~.
Por'tion 2 Tall oi.l fatty aci.ds5Q3.10 Benzyl trimethylamonium hydroxide 1.70 Portion 3 Ethylene glycol monoethyl ether 419.30 Portion 1 is charged into the reaction vessel, ~lanketed with nitrogen and heated to about 128 to 140C
to melt the resin. Portion 2 ïs then added, and the ingre-dients- are heated to about 150 to 160C for about 3 hours with constant agitation until the reaction mixture has an acid number of 0. 01. PortI.on 3 is added, and the ingre-dïents are cooled and ~iltered. ',"
The resulting epoxy ester resin solution has a soli.ds content of about 84%, an acid number no higher than 0.01, an epoxide equivalent o~ 130.0-1900, and a Gardner-Holdt viscosity of D-F at 25C in a 40% soli,ds polymer s-olution using ethylene glycol monoethyl ether solvent.
Pa`rt' II
This part des,cribes the preparati,on of an acrylic resin and the graft pol~merization of the epoxy ester described above onto it.
Portion l Parts of Wei~ht Isopropanol 400-00 Portion 2 Methyl methacrylate monomer100~00 Butyl acrylate 125.00 Tertbutylaminoethylene methacrylate 140.00 Dimethylaminoethyl methacrylate 40.00 Hydroxyethyl methacrylate75.00 Portion 3 Isopropanol 100.00 Methylethyl ketone 25.0 Axobisisobutyronitrile 10.00 Portion 4 Methylethyl ketone 8.00 Axobisisobutyronitrile 1.00 Portion 5 Ethylene glycol monoethyl ether 350.00 Portion 6 -Epoxy ester prepared in Part I 2300.00 Ethylene glycol monoethyl ether 350.00 CARDURA* E-10 125.00 ~glycidyl ester of epichlorohydrin reacted with versatic acid 911 produced by Shell Oil Co.) Dionized water 50.00 Portion l is charged into a reaction vessel, equipped as described abo~e, and is heated to its reflux temperature. The reaction mixture is held under nitrogen during the entire reaction. Portions 2 ~nd 3 are separately * denotes trade mark .

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-premixed and added slowly simultaneously over a 90-minute period while maintalning the reaction mixture at its re~lux temperature. The reaction is continued for an additional 60 minutes. The Portion 4 is added, and the reaction mixture is held at its re1ux temperature for an additional 30 minutes. Stripping o~ the reaction solvent is conducted simultaneously with the addition of Portion 5 which is to replace the reaction solvent~ T,~hen 533.00 parts of solvent are stripped and all of Portion 5 is added to the reaction vessel, Portion 6 is added and the temperature is brought at 115C to 117C and maintained for 4 hours with continuous agitation. At the end o~ that period the epoxy number is determined. When the epoxy e~uivalent is zero or less than ; 1 epoxy unit per 500,000 gm, the reaction is finished. The solids content is 70%, and the Gardner-Holdt viscosity at 25% reduction of solids with ethylene glycol monoethylether is U to X.
Part III
.
A black pigment dispersion is prepared as ollows:
Parts by Wei~ht Solution polymer prepared in Part II 318.00 Ethylene glycol monoethylether 84.00 Carbon black pigment 31.80 The above ingredients are premixed and charged into a conventional sandmill and ground at a rate of 30 gallons ` per minute while controlling the temperature o the mixture below 70C~ The resulting carbon black dispersion has ~` about 58% solids content.
An extender pigment dispersion using A1-silicate - 30 as the extender pigment is ~repared as follows:

.~

" ~2~3~

Parts by Wei~ht Solution polymer prepa~ed in Part II 193.00 Ethylene glycol monoethylether 142.00 Aluminum silicate 206.00 The above ingredients are premixed and charged into a conventional sand mill and ground at a rate of 30 gallons per minute while controlling the temperature of the mixture below 70C. The resulting aluminum silicate dispersion has about 63~ solids.
A water soluble phosphate salt of zinc, zinc dihydrogen phosphate, that is added to the coating com-position of above poiymer to improve its corrosion resis-- tance when cathodically electrodeposited, is prepared as follows: .

Parts bv l~el~ht ZnO(zinc oxide) 4.00 Phosphoric acid (85~) 14.00 Deionized Water 500.00 The above ingredients are mixed for 5 to 3 hours at room temperature until complete solubility of the zinc - oxide takes place. The pH of the solution is 2.6 to 3.0 Part IV
The electrocoating composition of a flated black paint is prepared as follows:
Portion l Parts by Weigh' Resin solution of Part II 320.00 Black pigment dispersion of Part III 97.00 Aluminutn silic~te pigment dispersion of Part III 440 00 Benzogu2namine formaldehyde solution (XM ~12S produced by American Cyanamid Co., ~5~ in ethylene glycol monobutyl ether) 190.00 Portion 2 Deionized water 632.00 - Phosphoric acid (85%) 22.00 Portion 3 Zinc dihydrogen phosphate 510.00 portioh 1 is added into a mixing vessel, heated to 150F and mixed for 3 hours,maintaining a temperature of 150F. Portion 2 is added into another mixing vessel mixed for 10 minu~es, and Portion 1 is added in~o Portion 2 with continuous agitation. The pigmented water - dispersion is mixed for 2 hours and diluted to about 15~ solids with deion~zed water and Portion 3 so that the concentration of zinc dihydrogen phosphate sa]t in the paint dispersion will be 450 ppm based on the total weight of the electrocoating composition.
The electrocoating composition, having a pH of 2.7 and a conductivity of 1700 micromhos, is charged into a stainless steel tank for electrodeposition. An untreated cold rolled steel panel or a phospllatized steel panel is positioned in the cènter of the tank, electrically connected to the negative side of a DC po~er source, and forms the cathode of the electrocoating cell. The tank is connected to the positive side of a DC power source and forms the anode of the cell. A direct currel~t of 150 volts is applied to the cell for 2 minutes at an ambient temperature of 20-25C, and a paint film of about 0.6 mils is deposited on the panel. The coated metal panel is removed from the ~0 electrocoating cell, washed and baked at about 160C for ') 7 ~L~;28~3~ -30 minutes. The resulting primer film has excellent adhesion to the metal substrate, is hard and has very good corrosion and saponification resistance over bare cold rolled steel and phosphatized steel. An acrylic enamel adheres to the primer film, and conventional acrylic lacquers can be applied with a conventional sealer coat over the primer to form a high quality finish.
Typical deposited films contain 90 to 95% solids and 10 to 12% of the phosphoric acid présent in the bath.
This coating composition is particularly useful for priming automobile and truck bodies by electrodeposi-tion for maximum corrosion protection over all parts of the car including areas of poor phosphate pretreaimen~ or no pretreatment at all.
EXAMPLE II
A white pigment dispersion is prepared as ~ollows:
Parts by Wei~ht Solution polymer of Part II of Ex. I 314.00 Ethyleneglycol monoethylether 137.00 Titanium Oxide 549.00 The above ingredients are premixed and charged into a conventional sand mill and ground at a rate of 30 gallons per minute while controlling the temperature of the mixture below 70C. The resulting titanium oxide dispersion has about 76~ solids content.
A white coating composition is prepared as follows:
Portion 1 Parts by ~eight Resin solution of Part II of Ex. 1 560-Ben~oguanamine formaldehyde resin solu-tion (85~ in ethylene glycol monobut~71 ether) 245.00 Titanium dioxide pigment dispersion 700 Portion 2 Phosphoric acid (85%) 30,00 Deionized water 1400.00 An electrocoating composition of lS~ solids and pH of 2.8 is prepared using Portions l and 2 and electro-coated following the procedure described in Example l.
This coating composition is useful either as a primer or as a single coat directly on metal for appliances or industrial equipment. It has good corrosion resistance and detergent resistance over bare col~ rolled steel or phosphatized steel.

Coatings prepared as in Example I give similar results.

- Portion 1 Parts by Wei~ht Isopropanol 1200.00 Portion 2 Methylene methacrylate 300.00 2-Ethylhexyl acrylate 1000.00 Tert-butylamino methyl methacrylate l~0.00 Dimethylamino ethyl methacrylate 60.00 Hydroxy ethyl methacrylate600.00 Azobis-isobutyronitrite - 40.00 Portion 3 Parts by Weight Azobis isobutyronitrile 2.00 Acetone 15.00 ~2~3~3~

Portion 4 Ethylene glycol monoethyl ether 900.00 Portion 5 CARDURA E-10 (Shell Oil Co. product~ 260.00 Portion 1 is charged into a reaction vessel equipped as described above and is heated to its re~lux temperature~ The reaction mixture is held under nitrogen during the entire reaction.
Portion 2 is separately premixed and added slowly simultaneously over a 90-minute period while maintaining ~he reaction mixture at its reflux temperature. The reaction is continued for an additional 60 minutes. Then Portion 3 is added, and the reaction mixture is held at its reflux temperature for an additional 30 minutes. Stripping of the reaction sol~ent takes place simultaneously with addition of Portion 4 which is to replace the reaction solvent.
1200.0~ parts o~ solvent are stripped, and all of Portion 4 is added to the reaction vessel. Then Portion 5 is added, and the temperature is brought to 115 to 117C where it is maintàined for ~our hours with continuous agitation. At the end of that period, the epoxy number is determined.
When it is zero or less than 1 epoxy unit per 500,000 gm, the reaction is finished. The so~ids con~en~ is 70%, and the product has a Gardner-Holdt viscosity of Z2-z4 Coatings are prepared as in Example I. Typically the coatings contain 85 to 90~ solids and 15% of the phosphoric acid present in the ba~h~
EX~MP~E IV
A white pi~ment dispersion is ~repared as follows:

.

Parts_~y Wei~ht Resin solution o Example IlI320.00 ~ Ethylene glycol monoethyl ether 140.00 ; Titanium oxide 550.00 The above inyredients are premixed and charged into a conventional sand mill, and ground at a rate of 30 gallons per minute while controlling the temperature of the mixture below 70C. The resulting titanium oxide pigment dispersion has about 76% solids content.
10. A white coating composition is prepared as follows:
Portion 1 Resin solution of Example II-I560.00 Benzoquanamine formaldehyde resin solution (85% in ethylene glycol monobutyl ether) 250.00 Titanium oxide pigment dispersion 700.00 Portion 2 .
Phosphoric acid (85~) 30.00 Deionized water 1400.00 Portion 3 Zinc dihydrogen phosphate510.00 Using Portions l, 2 and 3, an electrocoating com-position of 15~ solids and a pH of 2.7 is prepared and elec-trocoated.following the procedure described in Exam?le I.
This coating composition is particularly useful as a single coat directly on metal finishes that require good gloss and gloss retention after UV exposure, and good corrosion resistance regardless of the type and quality of pretreatment. .It also enables one to obtain a white finish withou~ the typical discoloration characteristics of an 3~

eIectrocoating finish.
Coatings prepared as in ~ample I give similar results.
The application is a division of copending Canadian Serial No. 292 037, filed 1977-11-29

Claims (14)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. As a new composition of matter, a graft copolymer comprising an epoxide grafted onto an acrylic backbone portion and consisting essentially of, by weight based on the graft copolymer, about:
a. in the acrylic backbone portion: 80 to 92% of a polymer or copolymer of at least one unit selected from alkyl, aminoalkyl and hydroxyalkyl acrylates and methacrylates with 0.02 to 0.1 equivalent of secondary and/or tertiary amine functionality; and b. in the graft: 20 to 8% of a graft comonomer which is a glycidyl ester of a tertiary carboxylic acid containing 7 to 9 atoms.

2. A graft copolymer of Claim 1 consisting essentially of, by weight based on the graft copolymer, about:
a. 85 to 91% of a copolymer contributing:
9 to 11% methyl methacrylate, 42 to 46% 2-ethyl hexyl acrylate, 6.5 to 8.5% t-butylaminoethyl methacrylate, 2 to 3% dimethylaminoethyl methacrylate, and 23 to 27% hydroxyethyl methacrylate, graft polymerized with b. 15 to 9% of wherein the R1, R2 and R3 groups are saturated aliphatic chains which contain a total of 7 to 9 carbon atoms, and at least one of R1, R2 and R3 is a methyl group.

3. A graft copolymer of Claim 2 consisting essentially of, by weight based on the graft copolymer, about:
a. 89% of a copolymer consisting essentially of:
10% methyl methacrylate, 44% 2-ethyl hexyl acrylate, 7.5% t-butylaminoethyl methacrylate, 2.5% dimethylaminoethyl methacrylate, and 25% hydroxyethyl methacrylate, graft copolymerized with b. 11% of wherein the R1, R2 and R3 groups are saturated aliphatic chains which contain a total of 7 to 9 carbon atoms, and at least one of R1, R2 and R3 is a methyl group.

4. A graft copolymer of Claim 1 also containing in said acrylic backbone portion about 0.01 to 0.05 equivalent of quaternary ammonium functionality.

5. A graft copolymer of Claim 1 containing in said acrylic backbone portion about 0.02 to 0.08 equivalent of secondary and/or tertiary amine functionality and 0.01 to 0.03 equivalent of quaternary ammonium functionality.

6. An aqueous coating composition comprising a graft copolymer of Claim 1 which has been dissolved in organic solvents, phosphoric acid reacted with the amine groups of the graft copoly-mer, and a crosslinking agent, said composition being dispersed in water.

7. An aqueous coating composition of Claim 6 containing about 2-3% by weight of the graft copolymer.

8. An aqueous coating composition of Claim 6 which has been adjusted to a pH of 2.0-4.0 by the addition of phosphoric acid.

9. An aqueous coating composition of Claim 6 in which the crosslinking agent is a benzoguanamine formaldehyde resin.

10. An aqueous coating composition of Claim 6 which also contains dissolved dihydrogen phosphate salts of one or more of zinc, iron, calcium, aluminum and magnesium.

11. An aqueous coating composition of Claim 6 which also contains titania pigments which have been added to the graft copolymer before it is dispersed in water.

12. In a process for electrocoating with paint a cathodically-charged substrate immersed in a coating bath containing an aqueous dispersion of said paint, said bath having a cathode zone containing said substrate and an anode zone containing a charged anode, the charged electrodes being maintained in electrical contact with each other by means of said bath, wherein said bath comprises a cationic film-forming polymer, an acidic ionizing agent, and a crosslinking agent, the improvement which comprises:
employing phosphoric acid as the acidic ionizing agent;
employing as the cationic film-forming polymer a graft copolymer of Claim 1; and employing as the crosslinking agent a composition which is nonreactive in the bath but reactive with said film-forming polymer at elevated temperatures to crosslink it to form a durable paint film.

13. A process of Claim 12 in which the graft copolymer consists essentially of, by weight based on the graft copolymer, about:
a. 89% of a copolymer contributing:
10% methyl methacrylate, 44% 2-ethyl hexyl acrylate, 7.5% t-butylaminoethyl methacrylate, 2.5% dimethylaminoethyl methacrylate, and 25% hydroxyethyl methacrylate, graft copolymerized with b. 11% of wherein the R1, R2 and R3 groups are saturated aliphatic chains which contain a total of 7 to 9 carbon atoms, and at least one of R1, R2 and R3 is a methyl group.

14. A process of Claim 13 in which the bath also contains dissolved dihydrogen phosphate salts of one or more of zinc, iron, calcium, aluminum and magnesium.