EP1280944A2

EP1280944A2 - Method and device for treating contaminated coating compositions

Info

Publication number: EP1280944A2
Application number: EP01926626A
Authority: EP
Inventors: Keith S. Ritter; Michael G. Sandala; Ellor J. Van Buskirk
Original assignee: PPG Industries Ohio Inc
Current assignee: PPG Industries Ohio Inc
Priority date: 2000-05-01
Filing date: 2001-04-04
Publication date: 2003-02-05
Also published as: WO2001083853A3; BR0110678A; CA2407331A1; KR20020092462A; WO2001083853A2; AU2001253148A1

Abstract

The present invention provides a method of treating a liquid including an electrodepositable coating composition and a contaminant. The method includes contacting at least a portion of the liquid with a filtering medium containing diatomaceous earth to separate at least a portion of the contaminant from the liquid.

Description

METHOD AND DEVICE FOR TREATING CONTAMINATED COATING COMPOSITIONS

Field of the Invention

The present invention relates generally to electrodeposition processes and, more particularly, to a method and device for treating contaminated electrodepositable coating compositions to provide electrocoated substrates having good smoothness and appearance.

Background of the Invention Electrodeposition has become the primary method for applying corrosion-resistant primers m automotive applications. Advantages of electrodeposition over non- electrophoretic coating processes include increased paint utilization, which reduces raw material and waste disposal costs, as well as improved corrosion protection.

The electrodeposition process involves immersing an electroconductive substrate into an electrocoating tank containing a bath of an aqueous electrocoating composition, the substrate serving as a charged electrode m an electrical circuit comprising the electrode and an oppositely charged counter-electrode. Sufficient electrical current is applied between the electrodes to deposit a substantially continuous, adherent film of the electrocoating composition onto the surface of the electroconductive substrate.

The electrocoated substrate is then conveyed to a rinsing operation where it is rinsed with an aqueous rinsing composition. Typical rinsing operations have multiple stages which can include closed loop spray and/or dip applications such as are described below. For example, in a spray rinse application the electrocoated substrate exits the electrocoating tank and is conveyed over a rinse tank while an aqueous rinsing composition is spray applied to the electrocoated surfaces of the substrate. Excess rinsing composition is permitted to drain from the substrate into the rinse tank below. The rinsing composition is then recirculated to the spraying apparatus for subsequent spray applications. In a dip rinse application, the electrocoated substrate is conveyed into a dip tank, where it is immersed in an aqueous rinsing composition, and is subsequently conveyed through one or more spray rinse applications as described above .

Recirculating the coating or rinsing compositions is both economically and environmentally desirable. However, should either the coating or rinsing compositions become contaminated, the contaminate could adversely affect the quality and appearance of the electrodeposited coating. For example, the contaminate may lead to cratering or other unacceptable defects in the cured coating.

Therefore, it would be desirable to provide a method and device for removing contaminates from electrodeposition coating and/or rinsing compositions to provide smooth electrodeposited coatings of good appearance.

Summary of the Invention The present invention provides a method of treating a liquid comprising an electrodepositable coating composition and a contaminant. The method comprises contacting at least a portion of the liquid with a filtering medium comprising diatomaceous earth to separate at least a portion of the contaminant from the liquid. The invention also provides a method of treating a liquid comprising a topcoating composition and a contaminant, the method comprising contacting at least a portion of the liquid with a filtering medium comprising diatomaceous earth to separate at least a portion of the contaminant from the liquid.

The invention additionally provides an electrodeposition coating system comprising a coating tank containing a liquid electrodepositable coating composition and at least one rinse tank containing a liquid rinse composition. At least one of the coating composition and the rinse composition further comprises a contaminant and at least one filter device is in flow communication with at least one of the coating tank and the rinse tank. The filter device comprises diatomaceous earth. Liquid from at least one of the tanks is filtered through the filter device to remove at least a portion of the contaminant from the liquid.

The invention further provides an electrodeposition coating system comprising a liquid electrocoating bath comprising (a) a resinous material capable of deposition on a cathode to form a coating and (b) a contaminant capable of forming defects in a surface of the coating deposited from the bath. A filtering medium is in flow communication with the bath. The filtering medium comprises diatomaceous earth. Liquid from the bath is filtered through the filtering medium to remove at least a portion of the contaminant from the liquid.

The invention moreover provides a method of treating a liquid electrocoating bath comprising (a) a resinous material capable of deposition on an anode or cathode to form a coating and (b) a contaminant capable of forming defects in a surface of the coating deposited from the bath. The method comprises (a) removing at least a portion of the liquid from the electrocoating bath; (b) contacting at least a portion of the removed liquid from the electrocoating bath with a filtering medium comprising diatomaceous earth to remove at least a portion of the contaminant from the liquid; and (c) returning at least a portion of the filtered liquid back into the bath.

Brief Description of the Drawings The foregoing summary, as well as the following detailed description of the preferred embodiments, will be better understood when read in conjunction with the appended drawings, in which:

Fig. 1 is a schematic diagram of a process for applying an electrodepositable coating composition to an electroconductive substrate, rinsing the coated substrate, and treating contaminated coating and/or rinsing compositions to remove the contaminate according to the present invention; and Fig. 2 is a graph of the number of craters produced on a coated substrate versus the number of times the coating composition used to make the coating was filtered through a filter device containing diatomaceous earth.

Detailed Description of the Preferred Embodiments Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term "about". Also, as used herein, the term "polymer" is meant to refer to oligomers, homopolymers and copolymers. Molecular weight quantities herein, whether Mn or Mw, are those determinable from gel permeation chromatography using polystyrene as a standard.

A method and device for treating contaminated aqueous electrodepositable coating compositions and/or rinsing compositions of the present invention will now be discussed with reference to a continuous automotive electrocoating and rinsing process, although one skilled in the art would understand that the present invention is not limited thereto but could also be used in non-continuous, e.g., semi- continuous or indexing processes, or batch processes.

Referring now to Fig. 1, in which like numerals indicate like elements throughout, there is shown a schematic diagram of an exemplary continuous electrodeposition process (indicated generally as 10) for applying an electrodepositable coating composition to an electrically conductive substrate, for rinsing the coated substrate, and for treating contaminated coating and/or rinsing compositions according to the present invention. In a continuous process, the substrate is in continuous movement along an assembly line. Useful electrically conductive substrates that can be coated include those formed from metallic materials, for example ferrous metals such as iron, steel, and alloys thereof, non-ferrous metals such as aluminum, zinc, magnesium and alloys thereof, and combinations thereof. Preferably, the substrate is formed from cold rolled steel, electrogalvamzed steel such as hot dip electrogalvamzed steel, hot dipped galvanneal, or electro zmc-iron coated steel, aluminum or magnesium. Preferably, the electrically conductive substrates are used as components to fabricate automotive vehicles, including but not limited to automobiles, trucks and tractors. The electrically conductive substrates can have any shape, but are preferably in the form of automotive body components such as bodies (frames), hoods, doors, fenders, bumpers and/or trim for automotive vehicles. A coating system incorporating the concepts of the present invention first will be discussed generally in the context of coating a metallic automobile body. One skilled in the art would understand that a coating process incorporating the present invention also is useful for coating non-automotive electrically conductive components.

Referring now to Fig. 1, m the electrodeposition portion 12 of the process 10, a liquid electrodepositable coating composition 14 is applied to a surface 16 of the electrically conductive automobile body 18 in a first step 20, for example by dipping the automobile body 18 into a container or bath 22 containing the liquid electrodepositable coating composition 14. The liquid electrodepositable coating composition 14 can be applied to the surface 16 of the automobile body 18 by any suitable amonic or cationic electrodeposition process well known to those skilled in the art.

In a cationic electrodeposition process, the liquid electrodepositable coating composition 14 is placed in contact with an electrically conductive anode 24 and an electrically conductive cathode (the electrically conductive surface 16 of the automobile body 18) . Following contact with the liquid electrodepositable coating composition 14, an adherent film 26 of the coating composition is deposited on the automobile body 18 when sufficient voltage is impressed between the electrodes. The conditions under which electrodeposition is carried out are, in general, similar to those used in electrodeposition of other coatings. The applied voltages can be varied and can be, for example, as low as 1 volt to as high as several thousand volts, but typically between 50 and 500 volts. The current density is usually between 0.5 and 15 amperes per square foot and tends to decrease during electrodeposition indicating the formation of an insulating film.

Suitable electrodepositable coating compositions may be supplied as two components. For example: (1) a clear resin feed, which can include one or more film-forming materials (ionic electrodepositable resins), crosslinking material (s), and any additional water-dispersible, non- pigmented components; and (2) a pigment paste, which can include one or more pigments, a water-dispersible grind resin which can be the same film-forming material as in the clear resin feed or a chemically different film-forming material such as those discussed below, and, optionally, additives such as wetting or dispersing aids. Electrodeposition bath components (1) and (2) are dispersed in an aqueous medium which can include an admixture of water with coalescing solvents.

The electrodepositable coating composition can comprise one or more film-forming materials and crosslinking materials. Suitable film-forming materials include epoxy- functional film-forming materials, polyurethane film-forming materials, and acrylic film-forming materials. The amount of film-forming material in the electrodepositable composition generally ranges from about 50 to about 95 weight percent on a basis of total weight solids of the electrodepositable composition.

Suitable epoxy-functional materials may contain one or more epoxy or oxirane group in the molecule, such as di- or polyglycidyl ethers of polyhydric alcohols. Preferably, the epoxy-functional material contains at least two epoxy groups per molecule. Useful polyglycidyl ethers of polyhydric alcohols can be formed by reacting epihalohydrins, such as epichlorohydrin, with polyhydric alcohols, such as dihydric alcohols, in the presence of an alkali condensation and dehydrohalogenation catalyst such as sodium hydroxide or potassium hydroxide.

Suitable epoxy-functional materials can have an epoxy equivalent weight ranging from about 100 to about 2000, as measured by titration with perchloric acid using methyl violet as an indicator. Useful polyepoxides are disclosed in U.S. Patent No. 5,820,987 at column 4, line 52 through column 6, line 59, which is incorporated herein by reference. Examples of suitable commercially available epoxy-functional materials are EPON® 828 and 880 epoxy resins, which are epoxy functional polyglycidyl ethers of bisphenol A prepared from bisphenol A and epichlorohydrin and are commercially available from Shell Chemical Company.

The epoxy-functional material can be reacted with amines to form cationic salt groups, such as primary or secondary amines which can be acidified after reaction with the epoxy groups to form amine salt groups or tertiary amines which can be acidified prior to reaction with the epoxy groups and which after reaction with the epoxy groups form quaternary ammonium salt groups. Other useful cationic salt group formers include sulfides .

Suitable acrylic-functional materials include polymers derived from alkyl esters of acrylic acid and methacrylic acid such as are disclosed in U.S. Patent Nos. 3,455,806 and 3,928,157, which are incorporated herein by reference. Examples of film-forming resins suitable for anionic electrodeposition include base-solubilized, carboxylic acid- containing polymers such as the reaction product or adduct of a drying oil or semi-drying fatty acid ester with a dicarboxylic acid or anhydride; and the reaction product of a fatty acid ester, unsaturated acid or anhydride and any additional unsaturated modifying materials which are further reacted with polyol. Also suitable are at least partially neutralized interpolymers of hydroxy-alkyl esters of unsaturated carboxylic acids, unsaturated carboxylic acid and at least one other ethylenically unsaturated monomer. Other suitable electrodepositable resins comprise an alkyd- aminoplast vehicle, i.e., a vehicle containing an alkyd resin and an amine-aldehyde resin or mixed esters of a resinous polyol. These compositions are described in detail in U.S. Patent No. 3,749,657 at column 9, lines 1-75 and column 10, lines 1-13, all of which are herein incorporated by reference. Other acid functional polymers can also be used such as phosphatized polyepoxide or phosphatized acrylic polymers which are well known to those skilled in the art.

Useful crosslinking materials comprise blocked or unblocked polyisocyanates including as aromatic diisocyanates such as p-phenylene diisocyanate, 4 , 4' -diphenylmethane diisocyanate and 2,4- or 2,6-toluene diisocyanate; aliphatic diisocyanates such as 1, 4-tetramethylene diisocyanate and 1,6- hexamethylene diisocyanate or oligomers or trimers thereof; and cycloaliphatic diisocyanates such as isophorone diisocyanate and 4 , 4 ' -methylene-bis (cyclohexyl isocyanate) . Examples of suitable blocking agents for the polyisocyanates include lower aliphatic alcohols such as methanol, oximes such as methyl ethyl ketoxime and lactams such as caprolactam. The amount of the crosslinking material in the electrodepositable coating composition generally ranges from about 5 to about 50 weight percent on a basis of total resin solids weight of the electrodepositable coating composition. Generally, the electrodepositable coating composition also comprises one or more pigments which can be incorporated in the form of a paste; water-dispersible, non- pigmented components such as surfactants and wetting agents; catalysts; film build additives; flatting agents; defoamers; microgels; pH control additives; and volatile materials such as water, organic solvents and low molecular weight acids, such as are described in U.S. Patent No. 5,820,987 at column 9, line 13 through column 10, line 27 (incorporated by reference herein) .

Suitable pigments include iron oxides, strontium chromate, carbon black, coal dust, titanium dioxide, talc, barium sulfate, cadmium yellow, cadmium red, chromium yellow and the like. The pigment content of the dispersion is usually expressed as a pigment-to-resin ratio. When pigment is employed, the pigment-to-resin ratio preferably is within the range of about 0.02 to 1:1. The other additives mentioned above are usually in the dispersion in amounts of about 0.01 to about 10 percent by weight, preferably about 0.1 to about 3 percent by weight based on weight of resin solids .

Useful solvents included in the coating composition, in addition to any provided with other coating components, include coalescing solvents such as hydrocarbons, alcohols, esters, ethers, glycol ethers and ketones . Preferred coalescing solvents include alcohols, polyols, ethers and ketones. Non-limiting examples of suitable solvents include isopropanol, butanol, 2-ethylhexanol, isophorone, 4-methoxy-2- pentanone, ethylene glycol, propylene glycol and the monoethyl, monobutyl and monohexyl ethers of ethylene glycol. Generally the amount of coalescing solvent ranges from 0.01 and 25 weight percent, and preferably from 0.05 to 5 percent by weight based on total weight of the electrodepositable coating composition. Other useful electrodepositable coating compositions are disclosed in U.S. Patent Nos. 4,891,111; 4,933,056 and 5,760,107, which are incorporated herein by reference.

Suitable electrodepositable compositions preferably are m the form of an aqueous dispersion. As used herein, "dispersion" means a two-phase transparent, translucent or opaque resinous system in which the resin is m the dispersed phase and the water is in the continuous phase. The average particle size of the resinous phase is generally less than 1.0 and usually less than 0.5 microns, preferably less than 0.15 micron.

The concentration of the resinous phase in the aqueous medium is at least 1 and usually ranges from 2 to 60 percent by weight based on total weight of the aqueous dispersion (electrodeposition bath) . When the electrodepositable compositions are in the form of resin concentrates, they generally have a resin solids content of 20 to 60 percent by weight based on weight of the aqueous dispersion.

The thickness of the electrodepositable coating applied to the substrate can vary based upon such factors as the type of substrate and intended use of the substrate, i.e., the environment in which the substrate is to be placed and the nature of the contacting materials. Generally, the thickness of the electrodepositable coating applied to the substrate ranges from about 5 to about 50 micrometers, and more preferably about 12 to about 40 micrometers.

The coating composition n the bath 22 can be recycled m conventional manner, such as by a recycling system 13 having a pump that prevents the solids of the coating composition from settling to the bottom of the bath 22. Further, the temperature of the electrodepositable coating composition may be controlled by use of a heat exchanger 28 in flow communication with the bath 22 in any conventional manner, such as through pipes or conduits. Before or during operation, the electrodeposition coating in the bath 22 may become contaminated with one or more materials which could adversely impact upon the quality of the deposited and cured coating. Such materials are referred to generally herein as "contaminate (s) " . Examples of such contaminates include silicone, and petroleum based lubricants, such as forming lubricants and mill oils.

When such contamination occurs, the coating composition should be cleaned, i.e., the contaminate (s) removed. In the practice of the present invention, this cleaning of the coating composition is accomplished using a filter device 100 in flow communication with the bath 22, e.g., by conduits or pipes. The filter device 100 includes a housing with an at least partially hollow interior chamber. A filtering medium, such as activated carbon, a conventional flatting agent, e.g., polyester flatting agent, or most preferably comprising diatomaceous earth, is removably placed within the housing chamber. Diatomaceous earth suitable for the practice of the invention s commercially available from Celite Corporation as Hyflo Super-Cel diatomaceous earth. Diatomaceous earth is a soft, bulky, solid material typically comprising about 88 weight percent silica and is composed of the skeletons of small prehistoric aquatic plants related to algae, as discussed in Hawley's Condensed Chemical Dictionary at page 365 (12^th ED. 1993), herein incorporated by reference. Although not to be considered as limiting, the diatomaceous earth filtering medium can be supported on a support, e.g., a wire or metal mesh screen in the housing. Containment material, such as conventional filter paper or conventional filter cloth, e.g., polypropylene filter cloth, or 10 micron filter cloth commercially available from American Felt & Filter Co., can be used to prevent the filtering material from being moved out of the housing during filtering of the contaminated coating composition. The amount of diatomaceous earth in the filter device 100 is preferably less than about 1 weight percent based upon the total weight of the coating composition in the bath to be cleaned, more preferably less than about 0.1 weight percent, and even more preferably about 0.01 weight percent to about 0.1 weight percent. In the practice of the invention, should the coating composition in the bath 22 be contaminated with a contaminate that adversely impacts upon the quality of the deposited coating, at least some of the contaminated coating composition can be removed from the bath 22, e.g., by a pump, and contacted with, e.g., flows through, the diatomaceous earth filtering medium in the filter device 100 to remove at least a portion of the contaminate from the coating composition. This filtered coating composition can then be reintroduced back into the bath 22 in conventional manner, such as through conduits or pipes. After a selected period of time, which is determined by the type and amount of contamination, the diatomaceous earth filtering medium in the filter device 100 can be removed and replaced with fresh diatomaceous earth. For example but not to be considered as limiting, the filtering material can be replaced after about 1 to about 100 turnovers, preferably after about 1 to about 50 turnovers, more preferably after about 1 to about 25 turnovers, and even more preferably after about 1 to about 10 turnovers. As discussed more fully in the Examples below, by "turnover" is meant the time for pumping, filtering, and reintroducing a volume of material from the bath 22 which is equivalent to the total starting volume of coating composition in the bath 22. During this cleaning operation, a portion of the coating composition from the bath 22 is being removed, preferably constantly removed, filtered, and recycled back into the remaining coating composition in the bath 22.

The electrodepositable coating composition from the bath also may be in flow communication with a conventional ultrafiltration system 30 to remove soluble impurities (ultrafiltrate 32) and the filtered material recycled to the electrodeposition bath 22. In the ultrafiltration system 30, the coating composition flows over a membrane permeable to water and small particles, e.g., less than about 1,000 Mw, such as salts. The "permeate" 32, i.e., the portion of the coating composition which passes through the membrane, can be used m further rinsing operations and a portion of the permeate, e.g., about 20 weight percent, may be discarded. The "non-permeate" portion of the coating composition is directed back into the bath 22, e.g., through one or more conduits or pipes.

During normal operating conditions when the electrocoating composition has not been contaminated, the filter device 100 can be bypassed so that the coating composition simply flows through the ultrafiltration device 30 and back into the bath 22. For example, the filter device 100 may be isolated from the flow of the coating composition from the bath 22 by isolation valves 102 and 104 or in any other conventional manner. When the coating material is to be passed through the filter device 100, the isolation valves 102 and 104 can be opened and a throttling or bypass valve 105 m the main line can be closed or throttled down to ensure flow of at least a portion of the coating composition through the filter device 100.

A second electrodepositable coating may also be applied upon the surface of the dried electrocoat after the first electrocoat is rinsed as described below. The second electrodepositable coating can be applied m a manner similar to that discussed above for depositing the first electrodepositable coating. The second electrodepositable coating can be the same or different from the first electrodepositable coating. For example, the individual components of the second electrodepositable coating, such as film-forming material, can vary or the amounts of each component can vary, as desired. Suitable components for the second electrodepositable coating include those discussed above as suitable for the first electrodepositable coating. Preferably, the first electrodepositable coating comprises an epoxy-functional film- forming material and polyisocyanate crosslinking material to provide corrosion resistance and the second electrodepositable coating comprises an acrylic or polyurethane, preferably polyurethane, film-forming material and polyisocyanate crosslinking material to provide chip resistance from impacts by stones and road debris as well as resistance to ultraviolet light that can cause photodegradation and loss of adhesion of the coating to the substrate.

After conveying from the electrocoating bath 22, the electrocoated automobile body 34 is exposed to air to permit excess electrodeposited coating composition to drain from the interior cavities and surfaces of the automobile body 18. The electrocoated automobile body 34 is then conveyed to a rinsing process 35 for removing excess electrodepositable coating from the automobile body 34. The rinsing process can include one or more spray and/or dip rinsing operations, as desired. Preferably, the electrocoated automobile body 34 is conveyed over a spray rinsing tank 36 where a first rinsing composition 38 is spray applied to the coated surfaces 40 of the electrocoated automobile body 34. The excess spray composition is permitted to drain into the rinse tank 36 below for recirculation, by a recirculation system 110, and subsequent spray reapplication. A filter device 100 of the invention containing diatomaceous earth filtering medium can be in flow communication with the recirculation system 110 as described above to remove contaminates from the rinsing composition should the rinsing composition become contaminated in a manner similar to that which is described above for treating the electrodepositable coating composition..

The rinsing composition, since it is typically recycled, also can comprise minor amounts of an electrodepositable coating composition such as is described above. The spray rinse step can be followed by a dip rinse step in which the electrocoated automobile body 34 is conveyed to a rinse dip tank 42 and immersed in the aqueous rinsing composition 44 contained therein. The electrocoated automobile body 34 is then conveyed out of the rinse dip tank 42 and the excess rinsing composition is permitted to drain back into the tank for reuse. The aqueous rinsing composition 44 used in the dip rinse can have the same or different components from the first rinsing composition 38 discussed above, but preferably has the same components as the first rinsing composition 38.

The dip rinse step may be followed by one or more spray applied rinsing steps as the electrocoated automobile body 34 is conveyed over subsequent spray rinsing tanks 46, 48, and aqueous rinsing compositions 50, 52 are spray applied as described above. Preferably, the drainage period for each rinsing step is at least one minute so that there is no standing water from the final rinsing composition. The temperature of the air during the drainage period preferably ranges from about 10°C to about 40°C.

The present invention will be described further by reference to the following examples. The following examples are merely illustrative of the invention and are not intended to be limiting. Unless otherwise indicated, all parts and percentages in the following examples, as well as throughout the specification, are by weight.

EXAMPLE 1 This example illustrates the use of diatomaceous earth to filter a contaminated electrodepositable polyurethane coating composition .

A filter housing or cell was made from a cylindrical, one-inch diameter PVC (polyvinylchloride) pipe union commercially available from Plotkin Brothers Supply Co., Inc. of Pittsburgh, Pennsylvania. When the outer ring of the union was unscrewed, it provided access to a cavity, the bottom portion of which had the capacity to contain about 10 cubic centimeters of diatomaceous earth filtering medium filter media . A stainless steel screen (approximately 0.024 inch diameter stainless steel wire, 15 mesh) , followed by a circular piece of 10 micron filter cloth (So~Clean 10 micron Filter Bag from American Felt & Filter Co., Newburgh, New York) was placed into the filter cell to retain 2.18 grams of diatomaceous earth filtering medium.

A Masterflex peristaltic pump (Model 7518-12 L/S commercially available from Cole-Parmer Instrument Co. of Vernon Hills, Illinois) and Tygon LFL tubing were used to pump a contaminated electrocoating composition (discussed in detail below) from a two liter bath through the filter medium, i.e., diatomaceous earth, in the cell and back into the electrocoat bath, with turnover rates varying from 8 to 18 minutes per turnover. By "turnover" or "turnover rate" is meant the amount of time it takes to pump a total of two liters of bath (i.e., the volume of starting material in the bath) out of the bath, through the filter cell, and back to the bath. Since the pump draws from the bath both contaminated and previously filtered material, it is expected that a large number of turnovers will be necessary to decontaminate the bath. As an example of a turnover calculation, the peristaltic pump was set on a speed setting of 1.6, which pumped 112.6 grams of coating composition from the bath into a beaker in 30 seconds. With a total amount of electrocoat composition of 2200 grams, it would take 9.8 minutes to pump 2200 grams through the system. Pumping and circulating this 2200 gram bath for 40 minutes means that 4.1 turnovers have occurred (40/9.8). Since the electrocoat bath was 12% solids and the bath density was approximately 1.0 grams per cubic centimeter, the bath weight in grams was considered to be its volume in cubic centimeters. The contaminated electrocoating composition was ED-8100 coating composition, a cationic polyurethane electrocoating composition commercially available from PPG Industries of Pittsburgh, Pennsylvania, which was contaminated during operation of an electrocoating system. The tested ED-8100 had been contaminated to the extent that after electrocoating a metal test panel (phosphated, cold rolled steel panel commercially available from ACT Laboratories, Inc. of Hillsdale, Michigan under part no. APR10739 (A) ) and baking in conventional manner (20 minutes at 350°F bake) to form a dried coating thickness of about 1.4 mils (35 microns), a 4 inch by 6 inch area of the coated panel had, by actual count, 505 "craters". A crater is a paint defect having a circular depression, often accompanied by a raised rim at the perimeter of the depression. Craters can vary widely in size, but in this case they had a diameter of about 1 to 2 millimeters Twenty-two hundred (2200) grams of this contaminated ED-8100 coating composition was circulated through a filter cell as described above that contained 2.18 grams (0.1% by weight of the material in the electrocoat bath) of diatomaceous earth (Hyflo Super-Cel diatomaceous earth commercially available from Celite Corporation) . After 60 turnovers, craters per 4 inch by 6 inch area of coated test panel were reduced from an original 505 to 140. This first portion of diatomaceous earth was removed and replaced with a second 2.2 gram portion of fresh diatomaceous earth. After another 17 turnovers, there were 49 craters per 4 inch by 6 inch area of coated test panel. This second portion of diatomaceous earth was removed and replaced with a third 2.2 gram portion of diatomaceous earth. After another eight turnovers there was only one crater on a 4 inch by 6 inch area of coated test panel. Since even fresh and uncontaminated electrocoat baths can have occasional film defects, this bath was considered to be decontaminated. Figure 2 summarizes the - 1?

results of decontamination of this electrocoat bath by filtering through diatomaceous earth.

In order to confirm the contaminate present, test panels coated with samples of contaminated and uncontaminated ED-8100 coating composition were subjected to analysis by Time of Flight Secondary Ion Spectroscopy (TOF-SIMS) . In this technique, it is known that when polydimethylsiloxane is probed with a TOF-SIMS ion beam, secondary ions are desorbed that have mass to charge ratios of 281, 221, and 207. Peaks in the TOF-SIMS spectrum with these charge to mass ratios are evidence of polydimethylsiloxane (silicone). The respective electrocoat compositions were deposited on test panels as described above to a thickness of about 1.4 mils (35 microns), and baked for between 5 and 10 minutes at about 200°F to allow craters to form without risking degradation or possible loss of contaminants. The following electrocoating compositions were analyzed: A) fresh, uncontaminated ED-8100 coating composition, B) the contaminated ED-8100 coating composition described above (uncleaned) , and C) fresh ED-8100 coating composition deliberately contaminated with 1 part per billion of DC-200 polydimethylsiloxane commercially available from Dow Corning Corporation and having a Mn of 9000. For bath A, the TOF-SIMS spectrum showed no peaks at 281, 221, or 207 mass to charge ratio. For baths B and C, distinctive peaks at 281, 221, and 207 mass to charge ratio were found. These peaks that are typical of silicone were of about the same intensity in the TOF-SIMS spectrums of B and C. Therefore, from the TOF-SIMS test results it can be concluded that Sample B was contaminated with polydimethylsiloxane or silicone.

EXAMPLE 2

In a further test of the ability of diatomaceous earth to remove contaminant, e.g., silicone contaminate, from electrocoat baths, fresh ED-8100 coating composition was deliberately contaminated with 0.1 weight percent of Dow Corning Antifoam 1500 defoamer, a silicone containing defoamer. After electrocoating a phosphated steel test panel as described above with this contaminated coating composition and baking for 20 minutes at 350°F as described above to form a dried coating of about 1.4 mils (35 microns), the coated panel was heavily cratered, which means at least 100 craters per test panel. One thousand nine hundred sixty (1960) grams of this coating composition were pumped and recirculated through a cell similar to above except that a PVC union designed for two-inch diameter PVC pipe was used. The filter cavity was filled with 23.1 grams of diatomaceous earth, and the bath material recirculated through this filter. After 15 turnovers, a similarly coated and baked panel was essentially crater free, having only an occasional defect typical of uncontaminated electrocoat baths.

It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications that are within the spirit and scope of the invention, as defined by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A method of treating a liquid comprising an electrodepositable coating composition and a contaminant, the method comprising the step of:

(a) contacting at least a portion of the liquid with a filtering medium comprising diatomaceous earth to separate at least a portion of the contaminant from the liquid.

2. The method according to claim 1, wherein the coating composition comprises a resinous material.

3. The method according to claim 1, wherein the liquid is used as a coating in an electrodeposition coating process.

4. The method according to claim 1, wherein the liquid is an electrodepositable coating composition located in a coating tank.

5. The method according to claim 4, wherein the coating composition is waterborne.

6. The method according to claim 1, wherein the liquid is a rinse composition located in a rinse tank

7. The method according to claim 6, wherein the rinse composition is aqueous.

8. The method according to claim 1, wherein the liquid is in a container and the method further comprises the steps of:

(b) removing the liquid from the container before contacting the liquid with the filtering medium; and (c) adding the filtered liquid back into the container after contacting the liquid with the filtering medium.

9. The coating system according to claim 1, wherein the amount of diatomaceous earth is less than about 1 weight percent of the weight of the liquid in the container.

10. The coating system according to claim 1, wherein the amount of diatomaceous earth is less than about 0.1 weight percent of the weight of the liquid in the container.

11. A method of treating a liquid comprising a topcoating composition and a contaminant, the method comprising the step of:

12. An electrodeposition coating system, comprising: a coating tank containing a liquid electrodepositable coating composition; at least one rinse tank containing a liquid rinse composition, wherein at least one of the coating composition and the rinse composition further comprises a contaminant; and at least one filter device in flow communication with at least one of the coating tank and the rinse tank, wherein the filter device comprises diatomaceous earth, and wherein liquid from at least one of the tanks is filtered through the filter device to remove at least a portion of the contaminant from the liquid.

13. The coating system according to claim 12, wherein the coating system is an automotive coating system.

14. The coating system according to claim 12, wherein the coating system is selected from the group consisting of continuous, indexing semi-continuous, and batch electrodeposition coating systems.

15. The coating system according to claim 12, wherein the coating composition comprises a resinous material.

16. The coating system according to claim 12, wherein the coating composition is waterborne.

17. The coating system according to claim 12, wherein the liquid is an aqueous coating composition and the container is a coating tank.

18. The coating system according to claim 12, wherein the liquid is an aqueous rinse composition and the container is a rinse tank.

19. An electrodeposition coating system, comprising: a liquid electrocoating bath comprising (a) a resinous material capable of deposition on a substrate to form a coating and (b) a contaminant capable of forming defects in a surface of the coating deposited from the bath; and a filtering medium in flow communication with the bath, wherein the filtering medium comprises diatomaceous earth, and wherein liquid from the bath is filtered through the filtering medium to remove at least a portion of the contaminant from the liquid.

20. A method of treating a liquid electrocoating bath comprising (a) a resinous material capable of deposition on a substrate to form a coating and (b) a contaminant capable of forming defects in a surface of the coating deposited from the bath, the method comprising the steps of:

(a) removing at least a portion of the liquid from the electrocoating bath;

(b) contacting at least a portion of the removed liquid from the electrocoating bath with a filtering medium comprising diatomaceous earth to remove at least a portion of the contaminant from the liquid; and

(c) returning at least a portion of the filtered liquid back into the bath.

21. A method of treating a liquid comprising an electrodepositable coating composition and a contaminant, the method comprising the step of:

(a) contacting at least a portion of the liquid with a filtering medium selected from the group consisting of activated carbon, flatting agent, and diatomaceous earth to separate at least a portion of the contaminant from the liquid.