CA2001700A1 - Urethane diol extended cathodic electrodeposition resins - Google Patents

Abstract

URETHANE DIOL EXTENDED
CATHODIC ELECTRODEPOSITION RESINS

ABSTRACT OF THE DISCLOSURE
An improved electrodepositable cationic resin is disclosed. Prior art electrodepositable cationic resins are formed from polyepoxides which are chain extended with polyether or polyester polyols to internally flexibilize the resin. Our polyepoxide resin is chain extended with urethane diol which gives a resin with improved throw power and coatings with better chip resistance while maintaining other important resin characteristics.

Description

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URETHANE DIOL EXTENDED
CATHODIC ELECTRODEPOSITION RESINS

The field of art to which this invention pertains is electrodepositable epoxy resins chain extended with urethane diols containing crosslinking agents to be used in cathodic electrocoat processes.

BACKGROUND ART
The coating of electrically conductive substrates by electrodeposition is a well known and important industrial process. (For instance, electrodeposition is widely used in the automotive industry to apply primers to ~utomotive substrates).
In this process, a conductive article is immersed as one electrode in a coating composition made from an aqueous emulsion of film-forming polymer. An electric current is passed ~etween the article and a counter-electrode in electrical contact with the aqueous emulsion, until a ~esired coating is produced on the article. The article to be coated is made the cathode in the electrical circuit with the counter-electrode being the anode.
Resin compositions used in cathodic electrodeposition baths are also well ~nown in ~he art.
These reslns are typically manufactured from polyepoxide resins which have been chain extended and adducted to include a nitrogen. The nitrogen is typically introduced through reaction with an amine compound. Typically these resins are blended with a crosslin~ing agent and then salted with an acid to ~orm a water emulsion which is usually referxed to as a princ~pal emulsion.

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~oo~o The principal emulsion is combined with a pigment paste, coalescent solvents, water, and other additives to foFm the electrodeposition bath. The electrodeposition bath is placed in an insulated tank containing the anode. The article to be coated is made the cathode and is passed through the tank containing the electrodeposition bath. The thickness of the coating is a function of ~he bath characteristics, the electrical operating characteristics, the immersion tim~, and so forth.
The coated objec~ is removed from the bath after a certain period of time. The object is rinsed with deionized water and the coating is cured typically in an oven at sufficient temperature to produce ~rosslinking-Prior art of cathodic electrodepositable resin compositions, coating baths, and cathodic electrodeposition processes are disclosed in U.S.
Patent Numbers 3,922,253; 3,984,299; 4,093,594;
2Q 4,134,864; 4,137,140; 4,419,467; and 4,468,307, the disclosures of which are incorporated by reference.
An important ch,aracteristic of the electrodeposition bath is its throw power. Throw power concerns the ability of the cationic resin to coat the recessed areas and shielded portions of the cathode. Asecond important characteristic of the final coating is chip resistance. This has become increasingly important to automobile manufacturers as cars have become more aerodynamic in shape and therefore are more susceptible to being chipped by pebbles or other debris. What is nPeded is an electrodepositable resin which ha~ increased throw power and greater chip resistance.

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SUMMARY OF THE INVENTION
In accordance with the present invention, an electrodepositable resin is provided. The resin is typical of the well known aromatic polyepoxide resins except that it has been internally plasticiz~d by reacting the aromatic polyepoxide with a urethane diol which has been found to give improved throw power and chip resistance without adversely affecting other important electrodepositable resin characteristics.

DETAILED D~SCRIPTION
As discus~ed above, it is well known that most principal emulsions in electrodeposition coatings comprise an epoxy amine adduct blended with a crosslinking agent and salted with an acid in order to get a water soluble product.
The polyepoxide resins which are used in the practice of the invention are polymers having a 1,2 epoxy equivalency greater than one and preferably abou~ two, that is, polyepoxides which have on an average basis two epoxy groups per molecule. The preferred polyepoxides are polyglycidyl ethers of cyclic polyols. Particularly preferred are polygly-cidyl ethers o~ polyhydric phenols such as bisphenol A.
These polyepoxides can be produced by etherification of polyhydric phenols with epihalohydrin or dihalohydrin such as epichlorohydrin or dichlorohydrin in the presence of alkali. Examples of polyhydric phenols are 2,2-bis-(4-hydroxy-3-tertiarybutylphenyl)-propane, 1,1-bis-(4-hydroxyphenyl)ethane, 2-methyl-1,1-bis-~4-hydroxyphenyl) propane, 2,2-bis-(4-hydroxy-3-tertiarybutylphenyl~propane, bis-(2-hydroxynaphthyl) methane, 1~5-dihydroxy-3-naphthalene or ~he like~
Besides polyhydric phenols, other cyclic polyols can be used in preparing the polyglycidyl X~ 7~

ethers of cyclic polyol derivatives. Examples of other cyclic polyols would be alicyclic polyols, particularly cycloaliphatic polyols, such as 1,2-cyclohexanediol;
1,4-cyclohexanediol, 1,2-bis(hydroxymethyl)cyclohexane, 1,3-bis-(hydroxymethyl)cyclohexane and hydrogenated bisphenol A.
The polyepoxides have molecular weights of at least 200 and preferably within the range of 200 to 2000, and more preferably about 340 to 2000.
To be useful as an electrocoat, the polyepoxide must be chain extended by an intèrnal flexibilizer. The internal flexibilizer enhances flow and coalescence and increases rupture v~ltage of the composition. Currently, internal flexibilization is usually accomplished by chain extending the polyepoxide with a polyether or a polyester polyol.
It has been found that substituting a urethane diol for the polyether polyol or polyester polyol results in both improved throw power and better chip resistance. These improvements are realized while maintaining the flow and coalescence characteristics of a polyepoxide resin chain extended with a polyether polyol or polyester polyol.
The urethane diol is formed in one of two ways: (1) mixing diisocyanate and polyol; or (23 mixing alkylene carbona~e and diamine. The reac~ion condition for the diisocyanate-polyol route is to add diisocyanate to polyol at 25-100C for about two hours.
Prefered diisocyanates are aliphatic diisocyantes such as hydrogenated methylene diphenyl diisocyanate.
Prefered polyols are diols such as butane diol, hexane diol t ethoxylated bisphenol A and so forth. The pre~ered eguivalsnt ratio of diisocyanate ~o ~olyol is 1:2.

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The prefered method for making the urethane diol is the alkylene carbonate-diamine route. In this method alkylene carbonate is added to a diamine at 25-100~C for about three hours. Prefered alkylene carbonates are ethylene carbonate and propylene carbonate. The prefered diamines are hexamethylene diamine and 2-methyl pentane diamine. The prefered equivalent ratio of alkylene carbonate to diamine is ~ his method is disclosed in U.S. Patent lo No.3,248,373. The urethane diol molecular weight should be between 250-2000 and most preferably 270-600~
Urethane diol is commercially available from Xing Industries under the tradename ~K-Flex UD
320-100~.
In addition, the urethane diol can be chain extended by reacting it with an alkylene oxide to make a polyether urethane diol.
The urethane diol chain extended polyepoxide is reacted with a cationic group former, ~or example, an amine.
The amines used to adduct the epoxy resin are monoamines, particularly secondary amines with primary hydroxyl groups. When you react the secondary amine containing the primary hydroxyl group with the terminal epoxide groups in the polyepoxide ~he result is the amine epoxy adduct in which the amine has become ter~iary and contains a primary hydroxyl group.
Typical amines that can be used in the invention are methyl ethanol amine, diethanolamine, and so forth.
Our preferred amine is diethanol amine.
Mixtur~s of the various amines described above can be used. ~he reaction of the secondary amine with the polyepoxide resin takes place upon mixing the amine with the product. The reaction can be conducted neat, or, optionally in the presence of ~uitable - 5 ~

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solvent. The reaction may be exothermic and cooling may be d2sired. However, heating to a moderate temperature, that is, within the range of 50 to 150C, may be used to hasten the reaction.
The reaction product of secondary amine with the polyepoxide resin attains its cationic character ~y at least partial neutralization with acid. Examples of suitable acids include organic and inorganic acids such as formic acid, acetic acid, lactic acid, and phosphoric acid. The extent of neutralization will depend upon the particular product involved. It is only necessary that sufficient acid be used to disperse the product in water. Typically, the amount of acid used will be sufficient to provide at least 30 percent of the ~otal theoretical neutraliæation. Exces~ acid beyond that required for lQ0 percent total theoretical neutralization can also be used.
The extent of cationic group formation of the resin should be selected such that when the resin is mixed with aqueous medium, a stable dispersion will form. A stable dispersion is one which does not settle or is one which is easily redispersible if some sedimentatisn occurs. In addition, the dispersion should be of sufficient cationic character that the dispersed resin particles will migrate towards the cathode when there is an electrical potential between the anode and cathode immersed in the aqueous d~sperslon .
In general, most of the cationic resins prepared by the process of the invention contain ~rom about 40 to 80, preferably ~rom about 50 to 70 milliequivalents of cationic group per hundred grams of resin solids.
The cationic resinous binder should prefPrably have average molecular weights, as . .

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determined by yel permeation chromatography using a polystyrene standard, of less than 10,000, more preferably less than 5,000 and most preferably less than 3,000 in order to achieve high flowability.
After forming the above described cationic resin, it is mixed with a crosslinking agent.
The crosslinking agents o~ our novel process are well known in the prior art. Typical crosslinkers are aliphatic and aromatic isocyanates such as hexamethylene diisocyanate, toluene diisocyanate, methylene diphenyl dii~ocyanate and so forth. These isocyanates can also be react~d with a polyol such as trimetholpropane to form a polyisocyanate. The isocyanate is then pre-reacted with a blocking agent such as methyl ethyl ketoxime or ethylene glycol mono butyl ether to block the isocyanate functionality (i.e., the crosslinking functionality). Upon heating the blocking agent seperates and crosslinking occurs.
The preferred crosslinking agent for our invention is toluene diisocyanate (TDI) reacted with trimethyol propane (TMP) and blocked with ethylen~ glycol mono butyl ether.
The ratio of TDI to TMP i~ about 3:1, The ethylene glycol mono butyl ether is usually added in an equivalent ratio of about 1:1 to the TDI~TMP
polyisocyanate. Reaction conditions for the above reactions are well known in the art and are disclosed in the following patents. U.S. Patent~ No. 4,031,050 and 3,~47,35~.
The cationic resin and the blocked isocyanate are the principal resinous ingredients in the electrocoating composi~ion and are usually present in amounts of about 30 to 50 percent by weight of solids.
Besides the resinous ingredients described above, the electrocoating compositions usually contain ~- - : ,: , ~ , :, .. . . , ~ , . ~ ..

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a pigment which is incorporated into the composition in the form o~ a paste. The pigment paste is prepared by grinding or dispersing a pigment into a grinding vehicle and optional ingredients such as wetting agents, surfactants and-defoamers. Pigment grinding vehicles are well known in the art. After grinding, the particle size of the pigment should be as small as practical, generally, a Hegman grinding gauge of about 6 to 8 is usually employed.
Pigments which can be employed i~ the practice of the invention include titanium dioxide, basic }ead silicate, strontium chromate, carbon black, iron oxide, clay and so forth. Pi~ments with high surface areas and oil absorbencies should be used judiciously because they can have an undesirable effect on coalescence and flow.
The pigment-to-resin weight ratio is also fairly important and should be preferably less than 0.5:1, more pre~erably less than 0.4:1, and usually about 0.2 to 0.4:1. Higher pigment-to-resin solids weight ratios have also been found to adversely affect coalescence and flow~
The coating compositions of the invention can contain optional ingredients such as wetting agents, surfactants, defoamers and so forth. Examples of surfactants and wetting agents include alkyl imidazolin~s such as those available from Ciba~~eigy Industrial Chemicals as ~Amine C,~ These optional ingredients, when present, constitute from ahout 0 to 20 peroent by weight of resin solids. Plasticizers are optional ingredients because ~hey promote flow.
Examples are high boiling water immiscible materials such as ethylene or propylene oxide adducts o~ nonyl phenols or bisphenol A. Plasticizers are usually used in amounts sf about 0 to 15 percent by weight resin solidsO

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_ g Curing catalysts such as tin catalysts are usually present in the composition. Examples are dibutyltin dilaurate and dibutyltin oxide. When used, they are typically present in amounts o~ abo~t 0.05 to 1 percent by w2ight tin based on weight of total resin solids.
The electrodepositable coating compositions of the present invention are dispersed in aqueous medium. The term ~dispersion~ as used within the ~0 context of the present invention is believed to be a two-phase translucent or opaque aqueous resinous system in which the resin is in the dispersed phase and water the continuous phase. The average particle size diameter ~f the resinous phase is about 0.1 to 10 microns preferably less than 5 microns. The concentration of the resinous products in the aqueous medium is, in general, not critical, but ordinarily the major portion of the a~ueous dispersion is water. The aqueous dispersion usually contains from about 3 to 50 percent typically 5 to 40 percent by weight resin solids. Fully dilu~ed electrodeposition baths generally have solids contents of about 3 to 25 percent by weight.
Besides water,~the aqueous medium may also contain a coalescing solven~. Useful coalescing solvents include hydrocaxbons, alcohols, esters, ethers and ketones. The preferred coalescing solvents include alcohols, polyols and ketones. Specific coalescing solvents include monobutyl ~nd monohexyl etbers of ethylene glycol, and phenyl ether of propylene glycol.
The amount of coalescing solvent is not unduly critical and is generally between about 0 to lS percent by weight, preferably about 0.5 to 5 percent by weighk bas~d on weight of resin solids.

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EXAMPLE T
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~Epon X28~ 1~6~ 1216.9 Xylene 60 Bisphenol A 355 355 Benzyl Dimethyl Amine 2~1 nK-Flex UD 320-100~ 288 276.5 Benzyl Dimethyl Amine 4.2 Diethanol Amine202.7 202.7 Methyl Isobutyl Ketone577.3 "Epon 828n~ a diglycidyl ether of ~isphenol A from Shell Chemical Co.) and xylene were charged to a reaction vessel and heated with nitrogen sparge to 145C. The reaction was held at reflux for about 1/2 hour to remove water. The reaction mixture was cooled to 140C and the bisphenol A and Benzyl dimethyl amine added. The reaction mixture was heated to 160~-190C
and held at this temperature for about an hour and then cooled to 120C. nK-Flex UD 320-100~ (a uretha~e diol from King Industries3 and a second portion of benzyl dimethyl amine were then added. The reaction was heated to 140C and held at 140C ~or 2~ hours until the proper weight per epoxide was obtained. The reaction mixture was cooled to 100C. Diethanol amine was then added. The reaction mixture was held at 120C
for one hour after methyl isobutyl ketone was added to reduce the non-volatiles (NV) to 75%.

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EXAMPL~ II
Wt._ _NV
BacXbone resin from Example I570.5 427.9 Capped Isocyanate Crosslinkerl 324.4 230.3 ~WP 1~ Plasticizer ~rom Rohm & Haas 20.0 20.0 ~ownol PPH" from Dow 23.0 Surfactant2 7.0 De~onized Water 1261.0 Lactic acid 22.8 1 Polyurethane crosslinker formed from half-capping toluene diisocyanate (80/20 2,~/2,6 isomer mixtuxe) with hexyl cellosolve and reacting this product with tri methylol propane in a 3:1 molar ratio. The crosslinker is present as a 71 percent solids solution in a 90/10 mixture of methyl isobutyl ]cetone and n-butanol.
2 Sur~actant is a mixture of 120 parts ~mine C;' from Ciba-Geigy, 120 parts acetylenic alcohol, commercially available as "Surfynol 104", 120 parts of 2-butyoxy ethanol and 221 parts by weight of deionized water and 19 parts glacial acetic acid.
Thoroughly mix the bac~bone resin from Example I, polyurethane crosslinker,~WP~n, nDownol PPH~, lactic acid, and surfactant. Then add the deionized water under agitation. This mixture was allowed to mix until majority of the organic ketone solvent evaporated. The dispersion has a solid con~ent of 36%.

EX~MPLE III
Q~ATERNIZING AGENT
Wt. NV
2-Ethylhexanol half 320.0 304 capped TDI in MIBK
Dimethylethanolamine 8i.2 87~2 A~ueous Lactic Acid Solution117~6 B8.2 ~-~utoxyethanol 39~2 ~ 11 .. , ~0~7~0 - PIGMENT GRINDING VEHICLE
~Epon 829~ 720 682 Bisphenol A 289.6 289;6 2-Ethylhexanol half 406.4 386.1 capped T~I in MIBK
Quaternizing Agent (from above3 496.3 421.9 Deionized Water 71.2 2-Butoxyethanol 149.Q

The quaternizing agent was prepared by adding dimethylethanolamine to the ethylhexanol half-ca~ped Toluene diisocyanate in a suitable reaction ~essel at room temperature. The mixture exothermed and was stirred for one hour at 80C. Lactic acid was then charged followad by the addition of 2-butoxyethanol.
The reaction mixture was stirred for about one hour at 65C to form the desired quaternizing agent.
To form the pigment grinding vehicle "Epon 829~ (a diglycidyl ether of bisphenol A from Shell Chemical Co.) and Bisphenol A were charged under a nitrogen atmosphere to a suitable reaction vessel and heated to 150-160C to initiate an exothermic reaction. The reaction mixture was permitted to exotherm for one hour at 150-160C. The reaction mixture was then cooled to 120C and the 2-ethylhexanol half-capped toluene diisocyanate was added. The temperature of the reaction mixture was held at 110-120'C ~or one hour, followe~ by the addition of the 2-butoxyethanol. The reaction mixture was then cooled to 85~-90C, homogenized and then charged with water, followed by the addition of the quaternizing agent ~prepared above). The ~emperature of the reaction mixture was held at ~0-85C until an acid value of about 1 was obtained. The reaction mixture had a solids content of ~5 percent.

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ZO~1.7~0 PIGMENT P~STE
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Grind Vehicle 266.62 Deionized Water 385. oa Carbon Black 10.81 Aluminum Silicate 25.92 Lead Silicate 51.83 Basic Lead Silica Chromate 22.21 Dibutyl Tin Oxide 29~.23 Deionized Water 59.0 The above ingredients were mixed together and ground in a mill to a Hegman NG. 7 grind.

EXAMPLE IV
Wt. NV
Emulsion from Example II 344.4 12~
Pigment Paste 73.0 365 Deionized Water 385.1 A co~position was prepared by blending the above ingredients. The coating composition has a pH of 5. 4, a bath conductivity of 1620 micro Siemens. ~he zinc phosphate cold roll steel panels were cathodically electrocoated in the electrodeposition bath at 250 volts for 2 minutes at a bath temperature of 83'F. The wet ~ilms were cured at 360~F for 15 minutes. The film appearance is smooth with good corrosion resistance.
The chip resistance is better than current commercially available systèms at an equal film thickness. The gravilometer ra~ings were 7 in SAE-400 test metho~. The bath has 15.5 inches of throw power at 275 volts.

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Claims (8)

1. An improved cationic electrodeposition resin of the type wherein the resin is made by chain extending a polyepoxide with an internal plasticizer wherein the improvement comprises said internal plasticizer is a urethane diol.

2. The resin of claim 1 wherein said polyepoxide is a polymer containing about two epoxy groups per molecule.

3. The resin of claim 2 wherein said polyepoxide is a polyglycidyl ether of polyhydric phenol.

4. The resin of claim 1 wherein said urethane diol has a molecular weight between 270-600 as determined using gel permeation chromatography with polystyrene as a standard.

5. The resin of claim 4 wherein said urethane diol is made by mixing diisocyanate and polyol.

6. The resin of claim 4 wherein said urethane diol is made by mixing alkylene carbonate and diamine.

7. The resin of claim 1 wherein the urethane diol has been chain extended by alkylene oxide.

8. A method of coating an electrically conductive article with the resin of claim 1, comprising:
(1) mixing said resin with an amine to form a polyepoxy amine adduct;
(2) blending said adduct with an acid, crosslinker, and water to form a principal emulsion;
(3) adding more water and pigment paste to the principal emulsion thereby forming an electrocoat bath;

(4) immersing the article in the electrocoat bath; and (5) passing a direct current through the article.