CA1308214C

CA1308214C - Compositions comprising ionic resins and capped polyisocyanate mixtures containing a diphenyl-2,4'- diisocyanate and a diphenyl-4, 4'-diisocyanate

Info

Publication number: CA1308214C
Application number: CA000529499A
Authority: CA
Inventors: George W. Mauer; Gregory J. Mccollum
Original assignee: PPG Industries Inc
Current assignee: PPG Industries Ohio Inc
Priority date: 1986-02-25
Filing date: 1987-02-11
Publication date: 1992-09-29
Anticipated expiration: 2009-09-29
Also published as: EP0236050A1; JPH0448807B2; AR241539A1; AU6908687A; AU573114B2; ES2015051B3; KR930005392B1; JPS62230819A; KR870007984A; BR8700900A; DE3762869D1; US5114552A; ZA87814B; EP0236050B1

Abstract

Abstract of the Disclosure Curable electrocoating compositions and the use of these compositions in a method of electrodeposition are disclosed. The electrocoating compositions comprise an aqueous dispersion of an active hydrogen-containing ionic resin and a capped polyisocyanate in which the polyisocyanate comprises a mixture of a diphenyl-2,4'-diisocyanate, a diphenyl-4,4'-diisocyanate and optionally polyphenyl polyisocyanate in which the diphenyl-2,4'-diisocyanate comprises at least 5 percent by weight of a polyisocyanate mixture. The capped polyisocyanate mixture results in a more stable aqueous dispersion than comparable dispersions prepared with capped polyisocyanates derived from the more common diphenyl-4,4'-diisocyanate and mixtures of diphenyl-4,4'-diisocyanate with polyphenyl polyisocyanate.

Description

COMPOSITION~ COMPRISING IONIC RESINS AND
CAPPED POLYISOCYANA'rE MIXTURES CONTAINING A
DIPHENYL-2,4'-DIISOCYANATE AND A DIP~ENYL-4,4'-DIISOCYANATE

Background of the Invention 5Field of the Invention. The present invention relates to electrodeposition compositions and, more particularly, to aqueous dispersions of electrodeposition compositlons containing capped polyisocyanate curing agents.
Brief Description of ~he Prior Art: Electrodeposition lO compositions, particularly cationic electrodeposition compositions containing capped polyisocyanate curing agents, are known in the art.
T~e cationic compositions, particularly those derived from cationic epoxy-amine reaction products, provide for outstanding corrosion resistance over steel substrates and have been used extensively as 15 primers for automobile and truck bodies. A particularly desirable polyisocyanate for use in these compositions is diphenylmethane-4,4'-diisocyanate, commonly called MDI, and a mixture of diphenylmethane-4,4'-diisocyanate and polymethylene polyphenyl isocyanate, commonly referred to as crude MDI. Such polyisocyanates are relatively non-20 volatile at curing temperatures compared to other aromaticpolyisocyanates such as toluene diisocyanate, and this lack of volatility is a distinct advantage in primer applications where the volatile polyisocyanates can discoIor and impede the cure o~ the subsequently applied top coats. This problem is discussed in U.S.
25 Patent No. 4,296,010.
~ disadvantage assoc~ated wlth MDI and crude MDI is that these polylsocyanates often result in unstable electrodeposition compositions. The compositions are in the form of aqueous dispersions and it has been found that the resinous phase often sediments from the , 8Z~4 dispersion. The problem is particularly acute when the polyisocyanate is capped with a low molecular weight capping agent such as methanol or ethanol. The sediment cannot easily be redispersed and results in an electrodeposition bath which cannot be serviced adequately and 5 which does not provide for satisfactory electrodeposited coatings.
Surprisingly, it has been found that particular mixtures of a diphenyl-2,4'-diisocyanate and a diphenyl-4,4'-diisocyanate provide for capped isocyanates which, when formulated into aqueously dispersed electrocoating compositions, provide for stable disperslons and 10 satisfactory electrodeposited coatings.
Summary of the Invention In accordance with the present invention, there is provided a curable composition comprising a mixture of an active hydrogen-containing ionic resin and a capped polyisocyanate dispersed in 15 aqueous medium. The polyisocyanate comprises a mixture of a diphenyl-2,4 7 -diisocyanate, a diphenyl-4,4'-diisocyanate and optionally poly-phenyl polyisocyanate in which the diphenyl-2,4'-diisocyanate comprises at least 5 percent by weight of the polyisocyanate mixture.
The invention also provides for a method of electrodeposi-20 tion employing the curable compositions described above.
Detailed Description The compositions of the present invention contain ase~sential ingredients an ionic resin and a capped polyisocyanate.
The ionic resins for use in electrodeposition are well known 25 in the art and include both anionic resins and cationic resins with the cationic resins being preferred because of the superior corrosion resistance attainable with such resins. The ionic resins should contain active hydrogens, such as hydroxyl, primary amino, secondary amino and thiol, including mixtures thereof. The active hydrogens are 30 reactive with the capped polyisocyanates resulting in a curing reaction when the coatings are heated. Examples of cationic resins are those which contain amine salt groups such as the acid-solubilized reaction products of polyepoxides and primary or secondary amines as described in U.S. Patent No. 4,031,050 to Jerabek and U.S. Patent No.
35 3,922,253 to Jerabek et al.

1~82:1~

Besides amine salt group-containing resins, quaternarg ammonium salt group-containing resins can also be employed. Examples of these resins are those which are formed from reacting an organic polyepoxide with a tertiary amine acid salt. Such resins are 5 described in U.S. Patent No. 4,101,486 to Bosso et al. Examples of other cationic resins are ternary sulfonium salt group-containing resins such as those described in U.S. Patent No. 4,038,232 to Bosso et al.
Specially modified cationic reslns such as those containing 10 primary amine groups formed from reacting the polyepoxides with diketi-mines containing at least one secondary amine group, for example, the methyl isobutyl diketimine of diethylenetriamine, can also be used and in fact their use is preferred. Such resins are described in U.S.
Patent No. 4,017,438 to Jerabek et al.
Modified resins such as those obtained by chain extending the polyepoxide to increase its molecular weight are also preferred in the practice of the invention. Such materials are described in U.S.
Patent No. 4,148,772 to Jerabek et al in which the polyepoxide is chain extended with a polyester polyol and in U.S. Patent No.
20 4,468,307 to Wismer e~ al in which the polyepoxide is chain extended with a particular polyether polyol. Also, chain extension such as disclosed in Canadian Patent 1,179,443 can be used.
The epoxy polymers which are used in preparing the cationic resins are polyepoxides, that is, polymers having a 1,2-epoxy equiva-25 lency greater than 1, preferably about 2 or more. Preferred are polyepoxides which are difunctional with regard to epoxy. The pre-ferred polyepoxides are polyglycidyl ethers of cyclic polyols.
Particularly preferred are polyglycidyl ethers of polyphenols such as bisphenol A.
Besides the polyglycidyl ethers of polyphenols, epoxy-containing polymers which can be used are acrylic polymers which contain epoxy groups. These polymers are formed by polymerizing an unsaturated epoxy group-containing monomer such as glycidyl acrylate or glycidyl methacrylate with one or more polymerizable ethylenically 35 unsaturated monomers. Examples of these polymers are described in U.S. Patent No. 4,001,156, column 3, line 59, to column 5, line 60 ~;

Examples of amines which can be used in preparing the polyepoxide-amine reaction products are ammonia, primary, secondary and tertiary amines and mi~tures thereof. The reaction produc~ of the 5 polyepoxides and the amines is at least partially neutralized wit~ an acid to fo~m a polyme~ic product containing amine salt and/or a qua~ernary ammonium salt grou~. Reaction conditions of polyepoxides with ami~les, examples of various amines and at least partial neutralization with ac~d are dis~losed ln U.S. Patent No. 4,260,720, column 5, line 20, 10 column 7, line 4.
With regard to the amount of organic amine and polyepoxide which are reacted with one another, the relative amounts depend on the extent of cationic base such as cationic salt group formation desired and this in turn will depend upon the molecular weight of the poly-15 mer.- The extent of cationic salt group formation and the molecular weight of the reaction product should be selected such that when the resultant cationic polymer 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 dispersible if such sedimentation 20 occurs. In some embodiments, the dispersion should additionally be of sufficient cationic character that the dispersed polymer particles will migrate towards the cathode when an electrical potential is impressed between an anode and a cathode immersed in the aqueous dispersion.
Also, the molecular weight, structure and extent of cationic salt group formation should be controlled such that the dispersed polymer will have the required flow to form a film on the substrate.
In the case of electrodeposition, to form a film on the cathode. The film should be insensitive to moisture to the extent that it will not 30 redissolve in the electrodeposition bath or be rinsed away from the coated surface after removal from the bath. In general, the cationic polymers useful in the practice of the invention will have average molecular weights (Mw) as determined by gel permeation chromatography using a polysty~ene standard of less than 100,000, more preferably 35 less than 75,000, and most preferably less than 50,000. The minimum molecular weight i~ about 500.

21~

The cationic polymers usually will contain from 0.01 to 10, preferably from about 0.1 to 5.0, more preferably from about 0.3 to 3.0 milliequivalents of basic group, for example, cationic group, per gram of resin solids. Obviously, one must use the skill of the art to 5 couple the molecular weight with the ca~ionic group content to arrive at a satisfactory product. The polyglycidyl ethers will have molecu-lar weights of about 500 to 10,000, preferably 1000 to 5000. Acrylic polymers, on the other hand, will have molecular weights as high as 100,000, preferably 5000 to 50,000.
The active hydrogens associated with the cationic resins of the invention can be selected from any of the actlve hydrogens which are raactive with isocyanates over the temperature range of 200-400~.
(93-204C.), preferably 250-350F. (121~177C.). Typically, the active hydrogens will be those associated with hydroxyl, primary and 15 secondary amino and thiol, including mixed groups such as hydroxyl and primary amino.
~ esides cationic resins, the ionic resin can be an anionic resin. Such resins suitable for use in electrodeposition are described in U.S. Patents Nos. 3,3~6,563; 3,369,983; 3,403,088;
20 3,530,054; 3,565,781 and 3,772,227.
The capped organic polyisocyanate which is used in the inven-tion is one which is compatible with the ionic resin to form a curable electrodeposition composition and when dispersed in aqueous medium, forms a stable dispersion suitable for use in electrodeposition. By 25 stable it is meant that the aqueous dispersion at a resin solids content of about 5 to 50 percent by weight will not sediment for at least one week upon standing at 20-25~.
The organic polyisocyanate is a mixture of a diphenyl-2,4'-diisocyanate, a diphenyl-4,4'-diisocyanate and optionally polyphenyl 30 polyisocyanate in which the diphenyl-2,4'-diisocyanate comprises at least 5 percent, preferably at least 10 percent by weight of the mixture. The preferred polyisocyanate mixtures comprise from about 10 to 75 percent by weight of the 2,4'-isomer; 15 to 55 percent by weight of the 4,4'-isomer and 0 to 75, typically 10 to 50 percent by weight 35 of the polyphenyl polyisocyanate; the percentages by weight being based on total weight of the polyisocyanate mixture. It should be 13C~

mentioned that the polyisocyanate mixture typically has a small amount, that is, less than 10 percent by weight of the 2,2'-isomer.
The diphenyl diisocyanates of the mixture contaln two phenyl groups having two isocyanate groups directly bonded to nuclear carbon 5 atoms in the 2,4' position and the 4,4' position. The phenyl groups may be bonded directly to one another such as in the .structure:
OCN

NCO
(I) or bonded to one another through an intermediate linkage such as alkylene, substituted alkylene, ether, carbonyl and sulfo such as in the structure:
OCN ~ X ~ NCO

(II) where X is alkylene, substituted alkylene, ether, carbonyl and 20 sulfo. Preferably, X is alkylene or substituted alkylene such as in the structure:
Rl OCN ~ R~ NCO

(III) where Rl and R2 can be the same or different and represent hydrogen, which is preferred, and lower alkyl containing from 1 to 4 30 carbon atoms such as methyl and ethyl.
As mentioned above, the isocyanate groups are substituted in the 2,4' position and the 4,4' position. The other positions on the phenyl rings may be substituted with lower alkyl groups containing 1 to 4 carbon atoms such as methyl and lower alkoxy groups such as 35 methoxy. Usually, the other positions are unsubstituted.

The preferred diisocyanates of the mixture are diphenyl-methane-2,4'-diisocyanate and diphenylmethane-4,4'-diisocyanate because of their low cost, ready availability and good performance.
The polyphenyl polyisocyanate can be represented by the 5 structures: ~ _ _ ~
OCN ~ ~ U~ ~ NCO (I) (IV) ~ R2 ~ R2 ~ NCO (II) (V) where n is an integer of l to 4, R1 and R2 can be the same or 15 different and represent hydrogen or lower alkyl containing l to 4 carbon atoms such as methyl. Usually, the polyphenyl polyisocyanate is of structure V above and is polymethylene polyphenylisocyanate.
The capping agent is an active hydrogen-containing material which readily reacts with the isocyanate groups to form capped 20 isocyanate groups. These groups, although relatively stable at ambien~ temperatures, uncap at elevated temperature to bring about reaction with the active hydrogens associated with the ionic resin to form a cured coating. Examples of suitable capping agents include alcohols such as those which have an average molecular weight of 76 or 25 less and would include methanol, ethanol and propanol and mixtures thereof. Such capping agents are preferred because they result in low volatile weight loss on curing and in the formation of smoother films. Capping agents having an average molecular weight greater than 76 ~an be used but the particular advantages of the invention are most 30 evident with capping agents having an average molecular weight of 76 or less. Examples of other capping agents would be higher molecular welght alcohols such as monoalkylethers of ethylene glycol such as 2-ethoxyethanol, 2-butoxyethanol; alicyclic alcohols such as cyclo-f hexyl alcohol and aryl aliphatic alcohols such as benzyl alcohol.
35 Oximes such as methyl ethyl ketoxime and lactams such as epsilon-caprolactam can also be used.

?~

The capped polyisocyanate can be used in two similar ways.
The polyisocyanate can be fully block~d, that is, no free isGcyanate groups remain, and then combined with the ionic polymer to form a two-component resin. Alternately, the polyisocyanate can be partially 5 capped so that there are remaining reactive isocyanate groups. The partially capped polyisocyanate can then be reacted with the active hydrogens in the polymer backbone under conditions which will not uncap the blocked isocyanate groups. This reaction makes the iso~yanate part of the polymer molecule and a one-component resin.
10 ~lso, mixtures of fully capped and integral capped isocyanate can be used.
Whether present as a separate component or integral with the active hydrogen-containing ionlc resin, the capped polyisocyanate is present in an amount sufficient to impart complete cure to the compo-15 sition. Typically, the capped polyisocyanate is present with anisocyanate:active hydrogen equivalent ratio of 0.05 to 1.5:1, prefer-ably 0.1 to 1.3:1. The active hydrogens are those associated with the ionic resin and other non-volatile active hydrogen-containing materials which may be present in the composition, for exampleJ as 20 modifying materials.
The ionic resin and the capped polyisocyanate curing agent are used for electrodeposition in the form of an aqueous dispersion.
The term "dispersion" is believed to be a two-phase transparent, translucent or opaque resinous system in which the organic resin is 25 the dispersed phase and the water is the continuous phase. The average particle diameter of the resinous phase is less than 10 and preferably less than 5 microns. The concentration of the resinous phase (ionic resin and capped polyisocyanate curing agent and any other optional organic materials) in the aqueous med~um is usually at 30 least 0.5, usually from 0.5 to 50 percent by weight based on total weight of the aqueous dispersion. For use as an electrodeposition bath, concentrations of 5 to 30 percent by weight are typical.
Besides water, other solvents such as organic solvents may be present in the composition. These solvents usually assist in 35 coalescing when the composition is applied to a substrate to form a film~ Useful coalescing solvents include hydrocarbons, alcohols, esters, ethers and ketones. The prsferred coalescing solvents include alcohols, polyols and ketones. Specific coalescing solvents include isopropanol~ butanol, 2-ethylhexanol, isophorone, 4-methoxypentanone, ethylene and propylene glycol, the monoethyl, monobutyl and monohexyl 5 ethers of ethylene glycol and methyl isobutyl ketone. The amount of coalescing solvent is not unduly critical and is generally between about 0.01 and 40 percent, preferably about 0.05 to about 25 percent by weight based on total weight of the aqueous medium.
Usually, a pigment composition and various additives such as 10 surfactants or wetting agents are optionally included in the disper-sion~ The pigment compositions may be of the conventional types comprising, for example, iron oxide, lead oxide, strontium chromate, carbon black, titanium dioxide, talc, barium sulfate, as well as color pigments such as cadmium yellow, cadmium red, chromium yellow and the 15 like. The pigment content of the dispersion is usually expressed as the pigment-to-resin ratio. In the practice of the invention, the pigment-to-resin ratio is usually within the range of 0.02 to 1:1.
The other additives mentioned above are usually in the dispersion in amounts of about 0.01 to 10 percent by weight based on total weight of 20 resin solids.
~ cure catalyst is usually employed in the compositions and examples include organotin compounds such as dibutyltin dilaurate and dibutyltin oxide. The catalyst is usually present in amounts of about 0.1 to 5 percent by weight based on total weight of resin solids.
For use in electrodeposition, the aqueous dispersions are placed in contact with an electrically conductive anode and an electrically conductive cathode in which the surface to be coated being determined by the charge on the ionic resin. Following contac~
with ~he aqueous dispersîon, an adherent film of the coating composi-30 tion is deposited on one of the electrodes and a sufficient voltage is impressed between the electrodes. The conditions under which the electrodeposition is carried out are, in general, similar to those used in electrodeposition of other types of coatings. The applied voltage may be varied and can be, for example, as low as 1 volt to as 35 high as several thousand volts, but is typically between 5 and 500 volts. The current density is usually between about 0.5 ampere and 15 :~3~

amperes per square foot and tends to decrease during electrodeposition indicating the formation of an insulating film.
The coating composition of the present invention can be applied to a variety of electroconductive substrates especially metal 5 such as steel, aluminum, copper, magnesium and the like, but also including metallized plastic and conductive carbon-coated materials.
After the coating has been appl~ed by electrocoating, it is cured usually by baking at elevated temperatures. The compositions of the present invention, after being electrodeposited, have been found 10 to cure completely at temperatures over the range of about 275-375F.
(135-191C.), preferably 300-350F. (149-177C.) in about 20 to 45 minutes. Evidence of cure is seen by the solvent resistance of the cured coating, specifically its resistance to acetone. Cured coatings prepared with compositions done by the method of the invention can 15 withstand 100 acetone double rubs without rubbing through the coating to the substrate. An acetone double rub is a rub back and forth across the same area of the coating with an acetone-saturated cloth using normal hand pressure.
Besides electrodeposition, the composition can be applied by 20 other techniques such as spraying, dipping and roll coating. In these applications, they can be applied to both electroconductive and non-electroconductive substrates such as plastics, wood and ceramics.
Curing can be at the times and temperatures mentioned above.
Illustrating the invention are the following examples which, 25 however, are not to be considered as limiting the invention to their details. All parts and percentages in the examples as well as throughout the specification are by welght unless otherwise indicated.
EXAMPLES
The following examples show the preparation of curable 30 electrocoating compositions comprising an active hydrogen-containing cationic resin and various capped polyisocyanate crosslinking agents.
Certain of the compositions were cathodically electrodeposited.
Example 1 A cationic electrodeposition resin was prepared from the 35 following mixture of ingredients:

h~

;~ Ingredients Parts by Weight (grams~

Bisphenol A-ethylene o~ide adduct (1/6 molar ratio) 213.5 Bisphenol A 173 Methyl isobutyl ketone 51.8 Benzyldimethylamine (catalyst) 0.84 Benzyldimethylamine 2.19 Crosslinker 1037 Diketimine 65.4 N-methylethanolamine 57.4 1Epoxy resin solution made from reacting epichlorohydrln and bisphenol A ha~ing an epoxy equivalent of about 188, comme~cially available from Shell Chemical Company.
The capped polyisocyanate crosslinker was prep~red from the following mixture of ingredients:
Ingredients Parts by Weight (grams) Polyisocyanate 1330 Methanol 320 Methyl isobutyl ketone 707 Dibutyltin dilaurate 1.65 aMixture of about 12 percent by weight diphenyl-methane-2,4'-diisocyanate; about 35 percent by weight diphenylmethane-4,4'-diisocyanate and about 53 percent by weight polymethylene polyphenyl isocyanate; the percentage by weight being based on total weight of polyisocyanate. The polyisocyanate was obtained from Mobay Chemical Company as nONDUR MRS.
The polyisocyanate, methyl isobutyl ketone and dibutyltin dilaurate 30 were charged to a reaction flask and heated under a nitrogen atmos-phere to 40C. The methanol was added slowly while holding the temperature between 50 and 60C. The temperature was held at 60C.
until no isocyanate was detectabIe by I~fra Red analysis (IR). In the event IR indicated the presence of unreacted NCO, small amounts of 35 2-butoxyethanol were added to remove it.

~r,~

~3~

3Diketimine derived from diethylene triamine and methyl isobutyl ketone (73 percent solids in methyl isobutyl ketone).
The EPON 828, bisphenol A-ethylene oxide adduct, bisphenol A
and methyl isobutyl ketone were charged to a reaction vessel and 5 heated under a nltrogen atmosphere to 140C. The first portion of the benzyldimethylamine was added and the reaction mixture allowed to exotherm to about 185C. and refluxed under reduced pressure tv remove azeotropically any water present. The reaction mixture was cooled to 160C., held for ~ hour, cooled further to 145C. and the second 10 portion of benzyldimethylamine added. The reaction mixture was held at 145C. until a reduced Gardner-Holdt vlscosity (50 percent resin solids in 2~methoxypropanol) of P-Q was obtained. The reaction mixture was cooled to 135C. followed by the addition of the diketimine and N-methylethanolamine. The reaction mixture was cooled 15 to 118C. and held for l~ hours followed by the addition of the crosslinker.
The resinous reaction mixture (1700 grams) was then dis-persed in aqueous medium by adding it to a mixture of 43.3 par~s of 88 percent aqueous lactic acid, 21.9 grams of a surfactant (described 20 below~ and 2190 grams of deionized water.
The surfactant was a cationic surfactant prepared by blend-ing 120 parts of alkyl imidazoline commercially available from Geigy Industrial Chemicals as GEIGY ~MINE~C, 120 parts by weight of an acetylenic alcohol commercially available from Air Products and 25 Chemicals Inc. as SURFYNOL 104, 120 parts by weight of 2-butoxy-ethanol, 221 parts by weight of deionized water and 19 parts by weight of glacial acetic acid.
The dispersion was further thinned with 875 grams of deionized water and vacuum stripped to remove organic solvent to give 30 a dispersion having a solids content of 37.6 percent. The crosslinker comprised about 40 percent by weight of the resin solids. The dispersion was stable with no sedimentation and remained stable for 3 weeks upon standing at 21C. with no indication of sedimentation.
Example 2 A dispersed cationic electrodeposition resin similar to that of Example 1 was prepared with the exception that 2-butoxyethanol was ~rc~ k 13~

used as the capping agent for the polyisocyanate. The capped polylsocyanate was prepared as generally describsd in Example 1 from the following mixture of ingredlents:
Ingred_ent~s Parts by Wsight (grams?
Polyisocyanate as used in Example 1 79~
2-Butoxyethanol 708 Methyl isobutyl ketone 643.8 Dibutyltin dilaurate 2.51 The aqueous dlspersion of the cationic resin was prepared as 10 generally described in Example 1. After vacuum stripping, the dispersion had a solids content of 36.7 percent. The dispersion was stable with no sedimentation and remained stable for 2 weeks upon star.ding at 21C. with no indication of sedimentation.
Comparative Example 3 A dispersed cationic resin similar to that of Example 1 was prepared with the exception that the capped polyisocyanate was based on a polyisocyanate mixture containing small amounts of diphenyl-methane-2,4'-diisocyanate. The capped polyisocyanate was prepared as generally described in Example 1 from the following ingredients:
20 Ingredients Parts by Weight (grams) Polyisocyanate 1330 Methanol 320 Methyl isobutyl ketone 707 Dibutyltin dilaurate 1.65 Mixture of about 3 percent by weight diphenylmethane-2,4'-diisocyanate; about 51 percen~ by weight diphenylmethane-4,4'-diisocy-anate and about 46 percent by weight polymethylene polyphenyl isocyanate; the percentage by weight being based on total weight of polyisocyanate. The polyisocyana~e was obtained from Mobay Chemical 30 Co. as MONDUR~MR. The procedure for preparing the crosslinker was as generally described in Example 1.
The aqueous dispersion of the catlonic resin was prepared as generally described in Example 1~ After vacuum strippingJ the disper-sion had a solids content of 37.1 percent. The dispersion was not 35 stable in that immediately after vacuum stripping the resin began to sediment from the dispersion resulting quickly in a layer of sludge forming at the bottom o f the container.

fr~

Comparative Example 4 A dlspersed cationic resin similar to Example 2 was prepared with ~he exception that the capped polyisocyanate was based on a polyisocyanate in Comparative Example 3. The capped polyisocyanate 5 was prepared as generally described in Example 1 from the following ingredients:
In~redients Parts by Weight (grams) Polyisocyanate as used in Comparative Example 3 798 2-Butoxyethanol 708 Methyl isobutyl ketone 643.8 Dibutyltin dilaurate 2.51 The aqueous dispersion of the cationic resin was prepared as generally described in Example 1. After vacuum stripping, the disper-15 sion had a solids content of 36.3 percent. The dispersion was not stable in that a few days after stripping, the resin began to sediment from the dispersion resulting quickly in a layer of sludge forming on the bottom of the container.
Comparative Example 5 A dispersed cationic resin similar to that of Example 1 was prepared with the exception that the capped polyisocyanate was a mixed methanol-ethanol capped diphenylmethane-4,4'-diisocyanate and the capped polyisocyanate was present in the dispersion at about 30 percent by weight based on weight of resin solids.
The cationic resin was prepared as generally described in Example 1 from the following ingredients:

Ingredients Parts by Weight (grams) Bisphenol A~ethylene oxide adduct (l/6 molar ratio) 213.5 Bisphenol A 173 Methyl isobutyl ketone 51.8 Benzyldimethylamine 0.84 ~enzyldimethylamine 2.19 Crosslinker 667 10 Diketimine 67 N methylethanolamine 57.4 The crosslinker was prepared from the following ingredients:
Ingredients Parts by Weigh~ (~rams) Diphenylmethane-4,4'-diisocyanate 500.0 15 Methanol 49.7 Ethanol 107.5 Methyl isobutyl ketone 281.0 Dibutyltin dilaurate 0.66 MONDUR M available from Mobay Chemic~l Co.
The polyisocyanate, methyl isobutyl ketone and dibutyltin dilaurate were charged to a reaction vessel and heated under a nitro-gen atmosphere to 40C. The methanol and ethanol were blended together and added slowly to the reaction mixture while holding at 60C. The reaction mixture solidified wi~h 35 ml of the 25 methanol/ethanol blend left to be added. The reaction mixture was melted at about 100C. and the remainder of the methanol and ethanol added. The reaction mixture was held between 115-122C. until no isocyanate was detectable by IR.
The aqueous dispersion of the cationic resin was prepared as 30 generally described in Example 1. The dispersion was not stable in that prior to vacuum stripping, the resin began to sediment from the dlspersion. After stripping, the dispersion was completely flocculated.
Example S
A dispersed cationic resin similar to that of Comparative Example 5 was prepared with the exception that the polyisocyanate was ~3~3~21~

a mixture of about 55 percent by weight diphenylmethane-2,4'-diisocyanats and about 45 percent by weight diphenylmethane-4,4'-diisocyanate. The capped polyisocyanate was prepared as generally described in Comparative Example 5 from the following mixture of 5 ingredients:
In~redients Parts by Weight (grams) Polyisocyanate 500 Methanol 49.7 Ethanol 107.5 Methyl isobutyl ketone 281.0 Dibutyltin dilaurate 0.66 1Mixture of about 55 percent by weight diphenylmethane-2,4'-diisocyanate and about 45 percent by weight diphenylmethane-4,4'-diiso-cyanate; the percentage by weight being based on total weight of 15 polyisocyanate.
The polyisocyanate, methyl isobutyl ketone and dibutyltin dilaurate were charged to a reaction vessel and heated under a nitrogen atmosphere to 40C. The methanol and ethanol were blended together and added to the reaction mixture at a temperature to keep 20 the reaction mixture fluid. After all the methanol/ethanol blend was added, the reaction mixture was held at 90C. until no isocyanate was detectable by IR.
The aqueous dispersion of the cationic resin was prepared as generally described in Example 1. After vacuum stripping, the 25 dispersion had a solids content of 34.9 percent. The dispersion was stable with no sedimentation and remained stable for 2 weeks upon standing at 21C. with no indication of sedimentation.
An untreated steel panel was cathodically electrodeposited in the dispersion at 24C., at a voltage of 180 volts for 2 minutes to 30 give a continuous coating having a thickness of 1.2 mils. The coating was cured at 170C. for 30 minutes to give a hard, solvent-resistant coating.
A summary of the results of Examples 1-6 is presented in Table I below.

8~ ~

o o r~
a ~
o ~ ~~
~,~, U ~ ~ .0,D
d ~ ~ ~ 4v u .~
O ~ v 4 Rd i~ u ¢
~D
to ~1 ,~
a ad ~ ~ ~d.C ~
1~1 ~a ,¢ u 4 O .~ ,= r H D~ td u 1 a)I ~ u a~ S~ ~ ¢
,~
~ u~
E~ ~
o o :1 ~:r ¢ 4~ a u~ a 44 a a ~ 4~
O ~ ~d u ,C ~ u d u u u~ ~ u t~ ~d u c~
O O O ~rl U O `rl d O
~rl tq u~ o V ~rl ~ ~1 ~q ~rl ~I P~ rl ~ ~ ~ a r~ ~ d O ~ ~
O _ _ ,~ I _ ,~ ~ _ _ I I O c~ l O - l I
d ~ a d td ~' d I ~ ~

4 4 a~ c) ,C 4 ~ v 4 al ~ ~ 'I ~I~1 ~ ,1 ~I d ~: P~ ~ V ~ v ~d 4 h P~
u~ 3 ~ ~ x~ a O ~ ~1 ~~ ~ ~q u, ~ ~ o ~ rl o r ~ ~ ~ ~ ~ c æ ~
O ~ u~ ~ Cd ~ _I ~ ~ ~r1 U~ V~

~1 :
~ . ~
x o ~ z

Claims

1. An aqueous dispersion composition comprising a mixture of an active hydrogen-containing ionic resin derived from at least one epoxy functional reactant and a fully capped polyisocyanate dispersed in aqueous medium for which the capping agent is a low molecular weight alcohol having an average molecular weight no greater than 76, and in which the polyisocyanate comprises a mixture of a diphenyl-2,4'-diisocyanate and a diphenyl-4,4'-diisocyanate in which the diphenyl-2,4'-diisocyanate comprises at least 5 percent by weight of the polyisocyanate mixture, such that said aqueous dispersion composition is physically stable for at least two weeks at a temperature of at least about 21°C.

2. The composition of Claim 1 in which the active hydrogen-containing ionic resin is a cationic resin.

3. The composition of Claim 1 in which the diphenyl-2,4'-diisocyanate and the diphenyl-4,4'-diisocyanate are diphenylalkane diisocyanates.

4. The composition of Claim 3 in which the diphenylalkane-2,4'-diisocyanate and the diphenylalkane-4,4'-diisocyanate are diphenylmethane-2,4'-diisocyanate and diphenylmethane-4,4'-diisocyanate.

5. The composition of Claim 1 which additionally contains polymethylene polyphenyl polyisocyanate.

6. The composition of Claim 1 in which the capping agent is selected from the class consisting of methanol, ethanol and propanol.

7. The composition of Claim 1 in which the equivalent ratio of isocyanate to active hydrogen is from 0.05 to 1.5:1.

8. The composition of Claim 1 in which the ionic resin and capped polyisocyanate are dispersed in aqueous medium and are present in amounts of 0.5 to 50 percent by weight based on total weight of the aqueous dispersion.

9. The composition of Claim 2 in which the cationic resin is an at least partially acid-neutralized polyepoxide-amine reaction product.

10. The composition of Claim 1 in which the diphenyl-2,4'-diisocyanate comprises at least 10 percent by weight of the polyisocyanate mixture.

11. A method of electrocoating an electrically conductive surface serving as an electrode in an electrical circuit comprising said electrode and a counter-electrode of opposite polarity immersed in an aqueous dispersion of an electrocoating composition, comprising passing electric current between the electrodes to cause a coating to deposit on said electrode, wherein the curable electrocoating composition comprises an active hydrogen-containing ionic resin derived from at least one epoxy functional reactant and a fully capped polyisocyanate for which the capping agent is a low molecular weight alcohol having an average molecular weight no greater than 76, in which the polyisocyanate comprises a mixture of a diphenyl-2,4'-diisocyanate and a diphenyl-4, 4'-diisocyanate in which the diphenyl-2,4'-diisocyanate comprises at least 5 percent by weight of polyisocyanate mixture, such that said aqueous dispersion composition is physically stable for at least about two weeks at a temperature of at least about 21°C.

12. The method of Claim 11 in which the electrically conductive surface serving as said electrode is a cathode and the active hydrogen-containing ionic resin is a cationic resin.

13. The method of Claim 11 in which the diphenyl-2,4'-diisocyanate and the diphenyl-4,4'-diisocyanate are diphenylalkane diisocyanates.

14. The method of Claim 13 in which the diphenylalkane-2,4'-diisocyanate and the diphenylalkane-4,4'-diisocyanate are diphenylmethane-2,4'-diisocyanate and diphenylmethane-4,4'-diisocyanate.

15. The method of Claim 11 which additionally contains polymethylene polyphenyl polyisocyanate.

16. The method of Claim 11 in which the capping agent is selected from the class consisting of methanol, ethanol and propanol.

17. The method of Claim 11 in which the equivalent ratio of isocyanate to active hydrogen is from 0.05 to 1.5:1.

18. The method of Claim 11 in which the cationic resin and capped polyisocyanate are dispersed in aqueous medium and are present in amounts of 0.5 to 50 percent by weight based on total weight of the aqueous dispersion.

19. The method of Claim 12 in which the cationic resin is an at least partially acid-neutralized polyepoxide-amine reaction product.

20. The method of Claim 11 in which the diphenyl-2,4'-diisocyanate comprises at least 10 percent by weight of the polyisocyanate mixture.

21. An aqueous dispersion of a curable electrocoating composition comprising an active hydrogen-containing cationic resin derived from at least one epoxy functional reactant and a fully capped polyisocyanate dispersed in aqueous medium in which the polyisocyanate comprises a mixture of diphenylmethane-2,4'-diisocyanate, diphenylmethane-4,4'-diisocyanate, and polymethylene polyphenyl isocyanate in which the diphenylmethane-2,4'-diisocyanate comprises at least 10 percent by weight of the polyisocyanate mixture, such that said aqueous dispersion composition is physically stable for at least about two weeks at a temperature of at least about 21°C, and the capping agent is a low molecular weight alcohol having an average molecular weight no greater than 76.

22. A method of electrocoating an electrically conductive surface serving as a cathode in an electrical circuit comprising said cathode and an anode immersed in an aqueous dispersion of a curable electrocoating composition, comprising passing electric current between the cathode and the anode to cause a coating to deposit on the cathode, wherein the curable electrocoating composition comprises an active hydrogen-containing cationic resin derived from at least one epoxy functional reactant and a fully capped polyisocyanate in which the polyisocyanate comprises a mixture of diphenylmethane-2,4'-diisocyanate, diphenylmethane-4,4'-diisocyanate and polymethylene polyphenyl isocyanate in which the diphenylmethane-2,4'-diisocyanate comprises at least 10 percent by weight of the polyisocyanate mixture, such that said aqueous dispersion composition is physically stable for at least two weeks at a temperature of at least 21°C, and the capping agent is a low molecular weight alcohol having an average molecular weight no greater than 76.