CATHODIC ELECTRODEPOSITION COATINGS CONTAINING ZINC HYDROXYPHOSPHITE PIGMENT

Abstract: An improved aqueous electrocoating composition having a binder of an epoxy-amine adduct and a blocked polyisocyanate crosslinking agent; wherein the improvement is the use of zinc hydroxyphosphite pigment as a corrosion inhibitive pigment and as a curing catalyst which provides and excellent cure under overbake and underbake conditions and forms a coating with excellent chip resistance and the throwing power of the electrocoating composition is significantly improved.

Description: TITLE CATHODIC ELECTRODEPOSITION COATINGS CONTAINING ZINC HYDROXYPHOSPHITE PIGMENT CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part to Serial Number 07/784,243 filed October 29, 1991. BACKGROUND OF THE INVENTION This invention is directed to a cathodic electrocoating composition and in particular to a cathodic electrocoating composition containing zinc hydroxyphosphite pigment as a corrosion inhibitive pigment and a supplementary catalyst. The coating of electrically conductive substrates by an electrodeposition process (also called an electrocoating process) is a well known and important industrial process. Electrodeposition of primers to automotive substrates is widely used in the automotive industry. In this process, a conductive article, such as an autobody or an auto part, is immersed in a bath of a coating composition containing an aqueous emulsion of film forming polymer and acts as an electrode in the electrodeposition process. An electric current is passed between the article and a counter- electrode in electrical contact with the aqueous emulsion until a desired coating is deposited on the article. In a cathodic electrocoating process, the article to be coated is the cathode and the counter-electrode is the anode. Resin compositions used in the bath of a typical cathodic electrodeposition process also are well known in the art. These resins typically are made from a polyepoxide which has been chain extended and then an adduct is formed to include amine groups in the resin. Amine groups typically are introduced through reaction of the resin with an amine compound. These resins are blended with a crosslinking agent and then neutralized with an acid to form a water emulsion which is usually referred to as a principal emulsion. The principal emulsion is combined with a pigment paste, coalescent solvents, water, and other additives to form the electrocoating SUBSTITUTE SHEET bath. The electrocoating bath is placed in an insulated tank containing the anode. The article to be coated is the cathode and is passed through the tank containing the electrodeposition bath. The thickness of the coating that is deposited on the article being electrocoated is a function of the bath characteristics, the electrical operating characteristics, the immersion time, and the like. The resulting coated article is removed from the bath after a set period of time and is rinsed with deionized water. The coating on the article is cured typically in an oven at a sufficient temperature to produce a crosslinked finish on the article. Cathodic electrocoating compositions and resin compositions, coating baths, and cathodic electrodeposition processes are disclosed in Jerabek et al U.S. Patent 3,922,253 issued November 25, 1975; Wismer et al US 4,419,467 issued December 6, 1983; Belanger US 4,137,140 issued January 30, 1979 and Wismer et al US 4,468,307 issued August 25,1984. Currently lead based pigments are used as corrosion inhibiting pigments and a variety of catalysts and combination of catalysts are used. It would be desirable to use a non-lead containing pigment that provides corrosion inhibition and catalyzes the curing reaction to form a film that has good corrosion resistance and cures to a high quality finish even when underbaked during curing and has good chip resistance in the event the film is overbaked during curing. Underbaking for a typical commercial cathodic electrocoating composition is about 10-15°C below the standard baking temperature and similarly, overbaking is about 10-15°C over the baking temperature. SUMMARY OF THE INVENTION An improved aqueous electrocoating composition having a binder of an epo-cy-amine adduct and a blocked polyisocyanate crosslinking agent; wherein the improvement is the use of zinc hydroxyphosphite pigment as a corrosion inhibitive pigment and an as a curing catalyst. DETAILED DESCRIPTION OF THE INVENTION The improved cathodic electrocoating composition of this invention uses zinc hydroxyphosphite pigment without the commonly used SUBSTITUTE SHEET lead corrosion inhibitive pigments and provides a cured coating that has excellent corrosion protection. It has also been found that by using zinc hydroxyphosphite pigment with a primary metal catalyst such as dibutyl tin oxide, dibutyl tin dilaurate or alkyl tin diacetyl acetonate an electrocoating 5 composition is formed that has excellent underbake cure and does not sacrifice chip resistance after it has been cured under overbake conditions. An unexpected advantage of the use of the zinc hydroxy phosphite pigment is that the electrocoating composition has significantly improved throwing power in comparison to conventional compositions that do not use this 0 pigment. Typical cathodic electrocoating compositions contain lead corrosion inhibitive pigments to form coating that have good corrosion resistance properties. Useful lead pigments include lead silicate, lead cyanamide, lead chromate and lead silicochromate. Due to health and 5 environmental hazards of these types of pigments, they are being replaced with other corrosion inhibitive pigments that provide equivalent or better corrosion protection. It was not known that zinc hydroxyphosphite pigment could be used in cathodic electrocoating compositions and it was not known that it could be used as a supplementary catalyst to promote crosslinking o especially at underbake temperatures. Typically zinc hydroxyphosphite pigment is used in electrocoating compositions in a pigment to binder ratio of about 0.001:1 to 0.1:1 and preferably, of about 0.003:1 to 0.03:1. In relation to the metal catalyst, the pigment is used in a weight ratio of about 1.0:1.0 to 3.0:1.0. 5 Most principal emulsions used in an electrocoating composition which are the binder of the composition comprise an aqueous emulsion of an epoxy amine adduct blended with a cross-linking agent which has been neutralized with an acid to form a water soluble product. Generally, a metal catalyst is added to a blend of the epoxy amine adduct 0 and crosslinking agent before water is added to form an emulsion. The catalyst is added in solution and mixed with the blend of epoxy amine adduct and crosslinking agent. Useful epoxy amine adducts are generally disclosed in U.S. Patent No. 4,419,467 which is incorporated herein by reference. SUBSTITUTE SHEET Preferred crosslinking agents are also well known in the prior art and are aliphatic, cycloaliphatic and aromatic isocyanates such as hexamethylene diisocyanate, cyclohexamethylene diisocyanate, toluene diisocyanate, methylene diphenyl diisocyanate and the like. These isocyanates are prereacted with a blocking agent such as oximes, alcohols, and caprolactams which block the isocyanate functionality i.e. the crosslinking functionality. Upon heating the blocking agents separate, thereby providing a reactive isocyanate group and crosslinking occurs. Isocyanate crosslinkers and blocking agents are well known in the prior art and also are disclosed in the aforementioned U.S. Patent No. 4,419,467. The cathodic binder resin of the epoxy amine adduct and the blocked isocyanate are the principal resinous ingredients in the electrocoating composition and are usually present in amounts of about 40 to 60 percent by weight of epoxy amine adduct and 60 to 40 percent by weight of blocked isocyanate. Besides the binder components described above, the electrocoating composition usually contains pigment which is incorporated into the composition in the form of a pigment 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 are added. Any of the pigment grinding vehicles that are well known in the art can be used. After grinding, the particle size of the pigment should be as small as practical, generally, the particle size is about 6-8 using a Hegman grinding gauge. Zinc hydroxyphosphite is added to the pigment paste with other pigments which include titanium dioxide, carbon black, iron oxide, clay and the like. Pigments with high surface areas and oil absorbencies should be used judiciously because these can have an undesirable affect on coalescence and flow of the electrodeposited coating. It is possible to form a pigment paste of the zinc hydroxyphosphite pigment separately and add it to the paste or to the electrocoating bath directly. The pigment to binder weight ratio of all the pigments used also is important and should be preferably less than 0.5:1, more preferably less than 0.4:1, and usually about 0.2 to 0.4:1. Higher pigment to binder weight ratios have also been found to adversely affect coalescence and flow. SUBSTITUTE SHEET The coating composition of the invention can contain optional ingredients such as wetting agents, surfactants, defoamers and the like. Examples of surfactants and wetting agents include alkyl imidazolines such as those available from Ciba-Geigy Industrial Chemicals as " Amine C", acetylenic alcohols available from Air Products and Chemicals as "Surfynol 104". These optional ingredients, when present, constitute from about 0.1 to 2.0 percent by weight of binder solids of the composition. Optionally, plasticizers can be used to promote flow. Examples of useful plasticizers are high boiling water immiscible materials such as ethylene or propylene oxide adducts of nonyl phenols or bisphenol A. Plasticizers are usually used at levels of about 0.1 to 15 percent by weight binder solids. The electrocoating composition of this invention is an aqueous dispersion. The term "dispersion" as used within the context of this invention is believed to be a two-phase translucent or opaque aqueous resinous binder system in which the binder is in the dispersed phase and water the continuous phase. The average particle size diameter of the binder phase is about 0.1 to 10 microns, preferably, less than 5 microns. The concentration of the binder in the aqueous medium in general is not critical, but ordinarily the major portion of the aqueous dispersion is water. The aqueous dispersion usually contains from about 3 to 50 percent preferably 5 to 40 percent by weight binder solids. Aqueous binder concentrates which are to be further diluted with water when added to an electrocoating bath, generally have a range of binder solids of 10 to 30 percent weight. Besides water, the aqueous medium generally contains a coalescing solvent or solvents. Useful coalescing solvents include hydrocarbons, alcohols, polyols, esters, ethers and ketones. The preferred coalescing solvents include alcohols, polyols and ketones. Specific coalescing solvents include monobutyl and monohexyl ethers of ethylene glycol, and phenyl ether of propylene glycol. The amount of coalescing solvent is not unduly critical and is generally between about 0.1 to 15 percent by weight, preferably about 0.5 to 5 percent by weight based on total weight of the binder solids. The following example illustrates the invention. All parts and percentages are on a weight basis unless otherwise indicated. SUBSTITUTE SHEET EXAMPLE 1 Preparation of Epoxy Amine Adduct The following ingredients were charged into a suitable reaction vessel: 1478 parts Epon 828® (epoxy resin of diglycidyl ether of Bisphenol A from Shell Chemical Company having an epoxy equivalent weight of 188); 533 parts ethoxylated Bisphenol A having a hydroxy equivalent weight of 247; 421 parts of Bisphenol A; and 121 parts xylene. The resulting reaction mixture was heated to 160°C under a nitrogen blanket and held at this temperature for 1 hour. 5.1 parts dimethyl benzyl amine were added and the mixture was held at 147°C until a 1150 epoxy equivalent weight was obtained. The reaction mixture was cooled to 98°C and 168 parts of diketimine (reaction product of diethylenetriamine and methyl isobutyl ketone at 72.7% solids) and 143 parts of methyl ethanol amine were added. The resulting mixture was held at 120°C for 1 hour and then 727 parts of methyl isobutyl ketone were added. The resulting resin solution had a non-volatile content of75%. Pr r i n f r z Total 564.0 The quaternizing agent was prepared by adding dimethylethanol amine to the blocked isocyanate solution in a suitable reaction vessel at ambient temperature. An exothermic reaction occurs and the reaction mixture was stirred for one hour and held at a temperature of 80°C. Lactic acid solution was added followed by the addition of SUBSTITUTE SHEET 2-butoxyethanol. The reaction mixture was stirred for an additional hour at 65°C to form the quatemizing agent. Pr arati n of Pi ment Grindin Vehicle Epon 829 ® and Bisphenol A were charged under a nitrogen atmosphere to a suitable reaction vessel and heated to 150-160°C to initiate an exothermic reaction. The reaction was allowed to continue for one hour. The reaction mixture was cooled to 120°C and the blocked isocyanate solution was added. The temperature of the reaction mixture was held at 110-120°C for one hour, followed by the addition of 2-butoxyethanol. The reaction mixture then was cooled to 85-90°C, homogenized and then the deionized water was added followed by the addition of the Quatemizing agent. The reaction mixture was held at about 80-85°C until an acid value of about 1 was obtained. The resulting mixture had a solids content of about 58%. Preparation of Emulsion Parts by Weight Epoxy amine adduct (prepared above) 840 Crosslinking Resin Solution 1 Downol PPH ® (from Dow Chemical Co.) 10 SUBSTITUTE SHEET Surfactant 2 15 Deionized water 1349 Lactic acid 3f Total 2778 1 Crosslinking resin solution is formed from half-capping toluene diisocyanate (80/202,4/2,6 isomer mixture) with 2-he-_yloxy ethanol and reacting the resulting product with trimethylol propane in a 3:1 molar ratio in methyl isobutyl ketone to form a 70% solids solution. 2 Surfactant is a mixture of 120 parts Amine C from Ciba-Geigy, 120 parts acetylenic alcohol, commercially available as Surfynol "104", 120 parts of 2-butoxy ethanol and 221 parts of deionized water and 19 parts glacial acetic acid. The epoxy amine adduct was thoroughly mixed with the crosslinking resin solution, Downol PPH ® , lactic acid and the surfactant. Deionized water was added under agitation to form a dispersion. The resulting dispersion has a solids content of 36% and apH of 6.8. Preparation of Pigment Pastes A. B. & C Pigment Grinding Vehicle (prepared above) Deionized water Carbon black pigment Aluminum silicate pigment Lead silicate pigment Dibutyl tin oxide Titanium dioxide pigment Zinc Hydroxyphosphite pigment Total 1000 1000 1000 For each pigment paste A, B, & C, the above constituents were charged into a conventional sand mill and ground to a No.7 Hegman fineness. SUBSTITUTE SHEET Preparation of an Electrocoating Baths A-C Parts by Weight Bath No.l Emulsion (prepared above) 1620 Pigment Paste A (prepared above) 329 Deionized water 1751 Total 3700 Electrocoating bath A was prepared by blending the above ingredients together. Electrocoating Baths B and C were prepared using the same constituents in the above amounts except for Bath B, Pigment Paste B was used instead of Pigment Paste A and for Bath C Pigment Paste C was used instead of Pigment Paste A Three sets of two zinc phosphate coated cold-rolled steel panels each were cathodically electrocoated in each bath at 275 volts for 2 minutes at a bath temperature of 28°C. One set of each of the coated panels was bake at 165°C for 17 minutes, the second set of panels was baked at 182°C for 17 minutes and the third set was baked at 200°C for 17 minutes. One set of panels from each bath was tested for solvent resistance. The panels were double rubbed with a cloth soaked in methyl ethyl ketone and the number of rubs were counted before there was damage to the coating. The results are as follows: The second set of panels from each bath was coated with an acrylic enamel topcoat. The topcoats were cured at 130°C for 17 minutes to form a topcoat about 1.8 mils thick. The chip resistance of each of the panels was measured using a standard gravelometer. The results are determined as the percentage of paint area lost after being subjected to the gravelometer and are as follows: SUBSTITUTE SHEET Each of the panels from the third set of panels from each bath was scribed through the coating and into the metal and each panel was subjected to 20 cycles of GM scab corrosion test. A cycle of the test consisted of soaking a panel in an ambient temperature 5% salt solution for 24 hours. The panel was dried and placed in a relative humidity cabinet at 60°C and 85% RH. A hot cold cycle was incorporated into cycles 1, 6, 11 and 16 in which the panel was heated to 60°C and cooled to -9°C. After 20 cycles, each panel was blown off with compressed air and scraped to remove loose paint. The creep was measured in millimeter i.e., the distance the paint flaked away from the scribe mark on the panel. The above results show that zinc hydroxy phosphite pigment which was present in Bath C but not in baths A & B provides a finish that has excellent cure under underbake conditions (165°C bake) as shown by the solvent resistance test and chip resistance test and is a good corrosion inhibitor as shown by the scab corrosion test. EXAMPLE 2 Electrocoating baths D, E. & F were prepared and the throwing power of each of these baths was measured Preparation of Flex Emulsion Additive A flex emulsion additive was prepared by charging 2322 parts by weight of Jeffamine ® D-2000 (a polyoxypropylene diamine having a SUBSTITUTE SHEET molecular weight of about 1992 from Texaco Company) to a reaction vessel under a nitrogen atmosphere and heated to 90°C, followed by the addition of a solution of 859 parts by weight of Εpon 1001" (polyglycidyl ether of bisphenol A having an epoxy equivalent of 500 from Shell Chemical Company) in 345 parts by weight of 2-butoxyethanol. The resulting reaction mixture was dispersed by combining 68 parts by weight of acetic acid and 5354 parts of deionized water. Preparation of Emulsion Parts by Weight Epoxy amine adduct (prepared in Example 1) 1110 Crosslinking Resin Solution (described in Example 1) 624 Downol PPH ® (from Dow Chemical Co.) 13 Surfactant (described in Example 1) 20 Deionized water 1884 Lactic acid 48 Total 3700 The epoxy amine adduct was thoroughly mixed with the crosslinking resin solution, Downol PPH ® , lactic acid and the surfactant. Deionized water was added under agitation to form a dispersion. The resulting dispersion has a solids content of 36% and a pH of 6.8. Pre ara ion f Pi ment Pa D E & F SUBSTITUTE SHEET Zinc Hydroxyphosphite pigment r 428 Total 1199 1199 1199 For each pigment paste D, E, and F, the above constituents were charged into a conventional sand mill and ground to a No. 7 Hegman fineness. Preparation of Electrocoating Baths D-F Parts by Weight D E F Emulsion (prepared in Example 1) 1159 1159 1159 Hex Emulsion Additive (prepared above) 131 131 131 Pigment Paste D (prepared above) 312 294 294 Pigment Paste E (prepared above) -- 18 Pigment Paste F (prepared above) - -- 18 Deionized Water 1498 1498 1498 Total 3100 3100 3100 Electrocoating baths D-F were prepared by blending the above constituents together. Zinc phosphate coated cold-rolled steel panels were cathodically electrocoated in each bath at a bath temperature of 30°C for 2 minutes at 150, 150 and 187 volts for baths D-F, respectively. A 0.95 mil thick film was deposited. The throwing power of each bath was measured according to Ford Laboratory Test Method MJ BI 20-2C wherein the distance the coating deposits on the inside of a box formed by the panels of 18"x4" is measured. The throwing power of each of the baths was as follows: The above data shows that Bath F, which contained the zinc hydroxyphosphite pigment had significantly improved throwing power in comparison to Bath D, which did not contain either the lead silicate or the SUBSTITUTE SHEET zinc hydroxyphosphite pigment and Bath E, which only contained the lead silicate pigment. Each of the above panels coated in Baths D-F were baked at about 182°C for 17 minutes and had acceptable solvent and corrosion resistance. Another set of panels was prepared as above and baked and topcoated with an acrylic enamel and the enamel was cured at 130°C for 17 minutes. Each of these panels had acceptable chip resistance as determined by a gravelometer test. SUBSTITUTE SHEET

Claims: CLAIMS 1. In a cathodic electrocoating composition, comprising an 5 aqueous carrier having dispersed therein a film forming binder comprising an epo_ty-amine adduct and a blocked polyisocyanate crosslinking agent; wherein the improvement comprises the use of zinc hydroxyphosphite pigment. 0 2. The cathodic electrocoating composition of claim 1 wherein zinc hydroxyphosphite pigment is used in a pigment to binder weight ratio of about 0.001: to 0.1:1. 3. The cathodic electrocoating composition of claim 1 5 containing a primary metal catalyst. 4. The cathodic electrocoating composition of claim 3 wherein the primary metal catalyst is dibutyl tin dilaurate, dibutyl tin oxide or a mixture thereof. 0 5. The cathodic electrocoating composition of claim 1 wherein the zinc hydroxyphosphite pigment is used in a pigment to binder weight ratio of about 0.001: to 0.1:1 and the primary metal catalyst is dibutyl tin dilaurate, dibutyl tin oxide, alkyl tin diacetyl acetonate, or mixtures thereof 5 used in a weight ratio of zinc hydroxyphosphite/ catalyst of about 1.0:1.0 to 3.0:1.0 and wherein the composition has improved throwing power. 6. In an improved method for preparing a cathodic electrocoating composition containing film forming binder comprising the o following steps in any workable order: (a) preparing an epoxy-amine adduct; (b) preparing a blocked polyisocyanate crosslinking agent; (c) blending the epoxy amine adduct with the blocked polyisocyanate crosslinking agent; SUBSTITUTE SHEET (d) neutralizing the epoxy-amine adduct with an organic acid to form an emulsion; (e) blending the emulsion with a pigment paste; and wherein the improvement comprises the use of zinc hydroxyphosphite pigment. 7. The method of claim 6 wherein zinc hydroxyphosphite pigment is used in a pigment to binder weight ratio of about 0.001: to 0.1:1. 8. The method of claim 6 wherein the cathodic electrocoating composition contains a primary metal catalyst. 9. The method of claim 6 wherein the primary metal catalyst is dibutyl tin dilaurate, dibutyl tin oxide or a mixture thereof. 10. The method of claim 9 wherein the zinc hydroxyphosphite is used in a pigment to binder weight ratio of about 0.001:1 to 0.1:1 and the primary metal catalyst is dibutyl tin dilaurate, dibutyl tin oxide or mixtures thereof used in a weight ratio of zinc hydroxyphosphite/ catalyst of about 1.0:1.0 to 3.0:1.0 and wherein the the composition has improved throwing power. SUBSTITUTE SHEET

Inventors: CHUNG DING YU, GAM ALLISA, SIKORA ROBERT JOHN, CHUNG, DING, YU, GAM, ALLISA, SIKORA, ROBERT, JOHN
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1992
Main classification: C09D5
IPC codes: C08G18, C08G59, C09D5, C09D163, C09D175