CA2014727A1 - Methods for removing solids from water-based paint systems - Google Patents

Abstract

METHODS FOR REMOVING SOLIDS FROM
WATER-BASED PAINT SYSTEMS

ABSTRACT OF THE DISCLOSURE

Methods of treating circulating water containing over-sprayed, water-borne paint, such as in a paint spray booth operation, comprise adjusting the alkalinity of the paint spray booth water to between about 20 and 600 ppm, on a calcium carbonate basis, by adding an alkalinity source thereto, adding a melamine polymer to the water, contacting the over-sprayed, water-borne paint with the alkalinity-adjusted paint spray booth water before or after addition of the melamine polymer, adding an effective amount of a flocculant to the paint spray booth water, and removing resulting sludge from the paint spray booth water.

Description

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METHODS FOR RENOVING SOLIDS FROM
WA~ER-BASED PAINT SYSTEMS

FIELD OF THE INVENTION
The present invention relates to methods of treating circulating water containing over-sprayed, water-borne paint, for example, circulating water in a w~t paint spray booth operation.

~ACKGROUND OF THE INVENTION
Automobile bodies and many industrial and consumer articles are conventionally spray painted in areas called spray booths, wherein water curtains are employed to cleanse the air of over-sprayed paint. The wash water is then treated to remove paint solids, and the treated water is recirculat~d.
A water curtain is typically created by pumping water into a trough above the over spray area. The overflow from the trough is controlled so that a uniform sheet of water falls along the length of the trough and down the booth' Q wall. Spray nozzles are also commonly used. Fine droplets of over sprayed~paint, emitted by a spray gun, contact and are captured by the wa~er curtain.
~he amount of paint contacting a wa~er curtain may change depending on a number of variables, including plant or process shutdowns, the ~ize and shape of the object being , : . :

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painted, the type of spray equipment used, the spraying and purge technique used, and the water flow and the type of paint used.
In the past, solvent-based or solvent-borne paints have commonly been employed in spray booths.
However, in response to federal regulations limiting the amount of volatile hydrocarbons, namely the solvent dilutant used in solvent-based paint, which can be emitted from a plant site, water-borne or water-based paints are now being used in spray booth operations.
The term ~wa~er-based paints", as used herein, refers to all varieties of coatings which contain in excess of approximately 10% water in the coating formulation, including, but not limited to, water-reducible alkyd and epoxy ester compositions, water-borne thermoplastic latex compositions using acrylic polymer/
copolymers, water-based latex~s of polyurethane dispersions, and blends of such compositions. As used herein, the terms ~water-based paints" and ~water-borne paints" are synonymous.
A primary treatment objective relative to water-based paints is to capture and collect finely-dispersed paint solids. Uncaptured solids tend to accumulate in the system and to settle in sludge recovery pits and in booth weirs. Such solids encourage the growth of anaerobic bacteria colonies which may result in odor problems. This treatment problem i0 aggravated with paints which are water-based, because they are more hydrophilic. In o~her words, they con~ain resins and dyes which are more compatible with water.
Other problems which severely interfere with spray booth operations occur in areas of high agitation , . . .

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~47;~'7 where foaming occurs and in areas where foam accumulates.
Foaming is caused by chemical additives, surfactants, solvents or combinations thereof. Also, finely dispersed paint solids which are not captured and removed tend to stabilize foam, which aggravates foaming problems.
Foaming generally mandates that copious amounts of defoamers be used, which results in higher operating costs. Water-based paints generally tend to cause foaming to a greater extent than solvent-based paints.
A wide variety of chemicals have been proposed as txeating agents for circulating wet spray booth waters containing overspray paint, including compositions containing polymers and amphoteric metal salts which form insoluble hydroxides at pH's greater than about 7. The use of combinations of this type are described in the following U.S. Patents: 3,861,887 to Forney; 3,9~0,986 to Gabel et al; 4,002,490 to Michalski et al; 4,130,674 to Roberts et al; and 4,440,647 to Puchalski. Further, U.S.
Patent No. 4,637,824 to Pominville discloses the use of silica es and polydiallyldialkylammonium halidPs with amphoteric metal salts, and U.S. Patent No. 4,853,132 to Merrell et al discloses ~he use of precipitates formed by the reaction of cationic polymers and salts of inorganic anions to detackify solvent-based paints. Bentonite clays, aluminum salts and zinc salts have also been used with cationic polymers.
U.S. Patent No, 4~656,059 to Mizuno et al relates to t~e use of melamine-aldehyde acid colloid solutions for treating paint in wet spray booths, and U.S.
Patent No. 4,629,572 to Leitz et al relates to the use of urea or amino triazine-aldehyde condens~tion reaction ,:

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products in combination with water-swellable clays to treat paint spray booth wastes.
Additionally, copending U.S. Patent Application Serial No. 185,720, filed April 25, 1988 by Charles A.
Faust and Joseph P. Mikne~ich, and commonly assigned, discloses a method employing melamine-formaldehyde-type polymers in conjunction with alkalinity sources and flocculant polymers to detackify solvent~based coatings captured in paint spray boo~h systems. The instant invention represents an advancement over the inventions of Mizuno and Serial No. 185,720 in that the instant inventor has discovered that particular alkalinity/melamine polymer/flocculant-based systems can be used with improved results to treat circulating paint spray booth waters containing oversprayed water-based paints.

SUMMARY OF THE INVENTION
That is, the instant inventor has discovered that melamine polymers, in conjunction with specified alkalinity sources and polymeric flocculants, applied within designated operating ranges, can be used with improved results to treat water which contains water-based paints. Such water, after being treated to capture and collect the oversprayed, water borne pain~ con~ained therein is typically recirculated in paint spray booth operations.
More particularly, thP present invention rela~es to methods of treating circulating paint spray booth water containing over sprayed water-borne paint to facilitate remoYal of over-sprayed, water-based paint from such water. The methods comprise adjusting the alkalinity of the circulating paint spray booth water being treated ~ , . . .
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between about 20 and 600 ppm, on a calcium carbonate basis, by adding an alkalinity source thereto; adding an effective amount of a melamine polymer to the water being treated; contacting over-sprayed, water-borne paint with the alkalinity-adjusted paint spray booth water before or after the addition of an effective amount of the melamine polymer to the paint spray booth water, adding an effectiva amount of a flocculant to the alkalinity and melamine polyme~-treated and paint-contained paint spray booth water, and removing resulting sludge from the paint spray booth water. The methods of the present invention are highly efficient for treating systems containing a wide variety of water-based paints. Additionally, the present methods generally produce a low-volume, flocculated, predominantly organic sludge which may be readily disposed of in land fill~ or by incineration.
These and additional advantages will be more apparent in view of the following detailed description.

The following detailed description will be more fully understood in view of the drawing which comprises a sole figure disclosing the measured transmittance of various water-borne and solvent-borne paints as described in Example 1.

DETAILED DESCRIPTION
The present invention relates to a method for treating circulating paint spray booth water containing over-sprayed, water-based paint to facilitate the removal of over-~prayed paint from the paint being treated, i.e., the water of a spray booth operation. The present method ::., ,.~: ,::: : :

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2~g~7 comprises adjusting the alkalinity of the water in the aqueous system being treated, namely circulating paint spray booth water, to between 20 to 600 ppm, on a calcium carbonate basis, and preferably, to between 50 to 400 ppm as CaCO3; adding to said water an effective amount of a melamine polymer; contacting o~er-sprayed water-borne paint with the alkalinity-ad~usted water before or after the addition of an effective amount of the melamine polymer; adding a flocculant to the water being treated after the oversprayed, water-based paint contacts the alkalinity and melamine polymer; and removing resulting sludge from the water being treated.
Before over-sprayed, water-based paint contacts paint spray booth water the alkalinity of the water in the paint spray booth system bein~ treated should be adjusted to provida a minimum alkalinity of about 400 ppm (as CaCO3) to a maximum alkalinity of about 600 ppm.
Preferably, the alkalinity should be maintained between about 50 and about 400 ppm (as CaCO3)~ and most preferably be~ween about 100 and 200 ppm (as CaCO3). These alkalinity ranges are generally critical. At hi~her alkalinity dosages, paint solids become increasingly difficult to capture, which decreases separation efficiency. At even higher dosages, the paint solids tend to sink instead of floa~. If sufficient alkalinity is not present (i.e., below about 20 ppm), the melamine polymer is ~ot as effective, resulting in very poor collection efficiency.
Additionally, the pH of the water being treated should be maintained between about 6.0 and about 8.0, prefexably between about 6.0 and about 7.5. A pH of at least about 6.0 is desirable in order to activate ~he . . . .

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melamine polymer. As the pH is lowered below a~out 6.0, corrosion in the system generally increases. On the other hand, a pH of greater than about 8.0 generally results in greater solids dispersion, thus creating less efficient solids capture, and causes greater foam generation.
Alkalinity can ~e added in the form of carbonates, for example, alkali metal carbonates and alkali metal bicarbonates such as sodiuim carbonate or sodium bicarbonate, NaOH, ROH, silicates and/or blends thereof. However, preferred alkalinity sources are alkali metal carbonates and silicates, with sodium carbonate, sodium bicarbonate, and mixtures thereof being especially preferred. These alkalinity sources are generally inexpensive and safe to use~ and they provide excellent flotation to the water-based paint solids when used in combina~ion with polymeric 10cculants. Thus, alkalinity, particularly carbonate alkalinity, assists in floating the flocculated paint particles to the surface of the water system being treated.
The alkalinity should be adjusted so that the water forming the curtain which contacts the over-sprayed paint or paints is in the designated alkalinity range.
Any method of adding the source of alkalinity can be used, including batch or continuous addi~ion, wi~h continuous addition being preferred.
In addition to the alkalinity requirement, an effective amount of a melamine polymer must be used. As used herein, the melamine polymer is a polymer prepared from (a), melamine or a Rubstituted melamine; and (b) a compound described by the following formulas o Rl - C - R~, 8 ;~ 7~'~

wherein R1 and R2, which may be the same or different and are selected from the group consisting of H and straight or branched Cl 4 alkyl groups. The preferred compounds of ~b) comprise aldehydes, with methanal (formaldehyde), ethanal and propanal being especially preferred; the most preferred aldehyde is formaldehyde.
The mole ratio of component (a) to component (b) should range from about 1:1 to about 1:6, with the preferred ratio being from about 1:1 to 1:3. The most preferred mole ratio is about 1 mole of melamine or a derivative thereof to about 2 to 2.5 moles of an aldehyde.
Thus, the most preferred polymer is prepared from melamine and formaldehyde with the mole ratio of melamine to formaldehyde being about 1:2 to about 1:2.5.
The instant melamine polymers are insoluble in water. They are therefore best utilized in acidic solutions wherPin the melamine polymer is stabilized in a fine colloidal state of suspension. Calgon's product CA-289, which has a pH of about 1.6 to about 2.1, is an ~xample of the preferred form. This product contains 8%
active melamine-formaldehyde polymer in an acidic aqueous solution. Any acid can be used to prepare the melamine aldehyde acid suspension, although hydrochloric acid is preferred. Also, other stabilizing agents, such as alcohols, can be used.
~ he percent by weight of active melamine polymer in a stabilized (acidic) suspension or solution should range from about 0.1% to about 20~, preferably 1% to about 15%, and most preferably about 4% to about 12%, due to cost and product stability considerations. The pH should : . . -.

., 2~ 27 be suf~iciently low to keep the melamine aldehyde-type polymer in a fine colloidal suspension~
The molecular weight of the melamine aldehyde-type polymer is not critical. However, the preferred molecular weight ranges from about 500 to about 50,000, and the most preferred molecular weight ranges from about 500 to about 5,000. As noted above, suitable melamine aldehyde-type polymers are commercially available from Calgon Corporation, under the tradenames CA-289, WT-2511 and PK-9511. ~hese products have molecular weights of about 2,200.
An effective amount of the melamine polymer should be added to or maintained in the water being treated. The melamine polymer reacts with the alkalinity to form a fine precipitate. As used herein, the term "effective amount" refers to that amount of melamine polymer which achie~es the desired water clarity and paint solids capture after addition of flocculant to the system being treated.
The melamine polymer can be applied intermittently or continuously to the water system being treated at a prefexred dosage of from between about 0.001 to about 1.O part melamine polymer (active basis) per part paint, most preferably between about 0.01 and about 0.5 part active melamine polymer per part pain~. Since spraying is generally continuous, continuous addition of the melamine polymer is preferred. The melamine polymer may be added at any convenient location, bu~ is preferably added so as to allow the maximum concentration of the melamine polymer to contact the over-sprayed paint(s).
For example, the melamine polymer may be added to a line supplying the ~rough or other d~ce used to form the .. . .
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water curtain. Multiple points of addition may be used.
The alkalinity adjustment and melamine polymer addition steps may be carried out simultaneously, noting, however, that alkalinity adjustment occurs prior to contact between the oversprayed paint and the circulated paint spray booth water. The key factor is that the over-sprayed paint contact water within the desired alkalinity range which either contains an effective amo-lnt of a melamine polymer, or to which an effective amount of melamine polymer is added.
A significant distinction between the treatment of solvent-based paints and water-based paints is that the melamine polymer can be added to the paint spray booth water before or after addition of the water-based paint while still obtaining the desired performance. In fact, addition of the melamine polymer after the water-based paint contacts the water ko be treated often provides improved results. By contrast, for best results the melamine pol~mer should be added to the water prior to the time when the water is contacted with a solvent-based paint.
In a typical paint spray booth opera~ion, the return water from the booth generally encounters extreme turbulence. This turbulence improves the efficacy of the treatment by promoting intimate contact between the paint and the melamine polymer.
After oversprayed, water-based paint contacts alkalinity and the melamine polymer in ~he circulating water, a polymeric flocculant is added to the paint spray booth water system. The flocculant promotes the formation of a buoyant floc structure by binding the conditioned paint particles and incorporating air into the floc , ~
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structure. The resulting floating floc facilitates the removal of paint solids from the circulating water system.
According to this invention, it has been found that the type of polymeric flocculant which is used is important and that the molecular weight of the polymeric flocculant~used is important. A suitable flocculant comprises at least one nonionic polymer, at least one ~nionic polymer, at least one cationic polymer, at least one amphoteric polymer, or a mixture thereof. Nonionic polymeric flocculants are preferred. Generally, flocculants having weight average molecular weights of at least 2 x 106 are preferred. More preferably, the molecular weight should exceed about 6 x 106. Examples of flocculants include long chain polyacrylamides and long chain polymethacrylamides. The most preferred flocculants are nonionic polyacrylamides having a weight average molecular weight ranging from about 6 x 106 to about 20 x ..

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T~pical cationic polyelectrolytes which may be used as flocculants in the instant invention include but aro not limited to polyamines, polyphosphonium compounds, S polysulfonium compounds, quaternary ammonium compounds, polymers of methacryloyloxethyl trimethyl ammonium methyl sulfate (METAMS), polymers of methacrylamido propyl trimethyl ammonium chloride (MAPTAC), polymers of acryloyloxyethyl trimethyl ammonium chloride (AETAC), polymers of methacryloyloxyethyl trimethyl ammonium chloride (METAC) and polymers prepared from combinations of METAMS, MAPTAC, AETAC and/or METAC with acrylamide and/or methyacrylamide. Representative of quaternary ammonium compounds are diethyldiallyl ammonium and dimethyldiallyl ammonium polymers and salts thereof.
The preferred cationic flocculants are quaternary ammonium polymers such as polydimethyldiallyl ammonium chloride (polyDMDAAC), poly dimethyldiallyl ammonium ~romide (polyDMDAAB~, poly diethyldiallyl ammonium rhloride (polyDE~AAB), or any of the same copolymerized with acrylamide or methacrylamide. The preferred molecular weights for the quaternary ammonium polymer~ are in excess of about 2,000,000.
The most preferred cationic flocculant is a polymer comprising dimethyl diallyl ammonium chloride and acrylamide, or a homologue thereof, having a weight average molecular weight in excess of about 4,000,000.
The ratio of the nonionic moiety (for example, acrylamide or methacrylamide~ to the cationir moiety should be greater than about 1:1, on an active weight basis.

, '7 Other preferred polymers comprise: a) acrylamide or methacrylamide and b~ METAMS, METAC, MAPTAC or AETAC, wherein the monomer ratio of a):b), on an active weight basis, is greater than about 1:1.
Preferred anionic flocculants are composed of polymers of 2-methacrylamido-2-methylpropylsulfonic acid and salts thereof, and copolymers of al acrylic acid and/or methacrylic acid and b) 2-acrylamido-2-methylpropylsulfonic acid and/or 2-methacrylamido-2-methylpropylsulfonic acid. Hydrolyzed poly~crylamides may also be used. The preferred anionic polyelectrolytes possess a low charge ~i.e., less than 2%) and are selected fxom the group consisting of polyacrylic acids and salts thereof, particularly sodium salts thereof, having a moleculax weight ranging from about 2 x 106 to about 20 x 106, hydrolyzed polyacrylamides having virtually any degree of hydrolysis and molecular weights ranging from about 2 x 106 to about 20 x 106, and polymers comprising a) acrylic acid or methacrylic acid and b) 2-acrylamido-2-methylpropylsulfonic acid and/or 2-methacrylamido-2-methylpropylsulfonic acid, wherein the weight ratio of a):b) ranges from about 1:99 to about 99:1, preferably 10:90 to 90:10 and most preferably 75:25, and wherein the molecular weight ranges from about 2 x 106 to about 20 to 106.
The inventor has also found that certain high molecular weight polyampholytes can be used in the instant method. Representative examples of suitable polyampholytes include pol~mers compri~ing a~ acrylic acid, methacrylic acid, 2-acrylamido-2-methyl-propylsulfonic acid or 2-me~hacrylamido-2-methylpropylsulfonic acid, alone or in combination, and b~

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dimethyldiallyl ammonium chloride, dimethyldiallyl ammonium bromide, diethyldiallyl ammonium chloride or diethyldiallyl ammonium bromide, alone or in combination, wherein the weight ratio of component a) to component b) ranges ~rom about 90:10 to about 10:90 and wherein the polyampholyte has a molecular weight in excess of about 2 x 106. The polyampholytes may also contain nonionic moieties such as acrylamide or methacrylamide.
Blends of the above listed nonionic, cationic, anionic and polyampholyte flocculants can be used, alone or in combination with amphoteric metal salts.
An effective amount of the polymeric flocculant should be added. The ef~ective amount depends upon the quantity of melamine polymer present in the system being treated. Preferably, the effective flocculant dosage will range from about .01 to about 150 parts (active basis) of the polymeric flocculant per part melamine polymer (active basis) and more preferably, 0.1 to 20 parts, on an active polymer:active polymer basis.
The function of the polymeric flocculant is two-fold: it reacts with the melamine polymer trea~ed paint solids to form a large, buoyant, easily-captured floc, and it reduces or totally elimina~es foam formation in the system by removing colloidal particulates present in the w~ter.
A requirement of the present invention is that the flocculant be added to the paint spray booth water after the over-sprayed, water-borne paint is contacted with the alkalinity ad~usted paint spray booth water and melamine polymer. Once the melamine polymer treated paint solids have been contacted with at least one polymeric flocculant, the resulting sludge is removed from the : . ~ `

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water. This removal may be facilitated by any means known in the art, including, but not limited to, air flotation and filtration.
Other additives commonly used for the treatment of water containing oversprayed paint may be used in conjunction with the instant method. For example, bentonite clays, carbon black, talc, gums starch, dextrin, lime, aluminum oxide, silica solids, and casein among other additives, may be used as additional process aids in con~unction with the primary steps of the instant method.
Additives from the class of amphoteric metal salts, including, but not limited to, alum, aluminum chloride, ferric sulfate and ferric chloride, can also be used to enhance the performance of the instant invention.

EXAMP~ES
The following examples demonstrate the instant in~ention. These examples are not intended to limit the invention in any way. Unless otherwise specified, reference to percentages or parts are by weight.

Fig. 1 shows distinctions between sol~ent-based and water-based paints. To ganerate the data shown in Fig. 1, 0.2 gram of each designated paint was added to 200 mls of deionized water. A magnetic mixer at 500 rpm was used to provide agitation. The mixer was turned off and observations were recorded aftex one (1~ hour. In Fig. 1:
BASF White is a solid colox water-borne paint;
CIL Blue is a metallic water-borne paint;
CIL Clear Coat is a solvent-borne finish coat for use over water-borne paints;

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, , ~ 7 DuPont Grey Primer is a solvent-borne primer;
and Red Topcoat is a solvent-borne, high-solids enamel paint.
In this figure, low transmittance values correlate to highly dispersed paint solids. The water-borne solids in this example remained dispersed and were non-adherent.

This example shows the effect of melamine aldehyde-type polymer order-of-addition relative to the paint, and the impact of alkalinity on performance. In these tests, several parameters were measured to judge the performance of the treatment program, including water clarity (% transmittance), solids collection and the characteristics of the paint solids. Results are shown in Table 1 below. In this Table, PK-9511 is an 8% active dispexsion of melamine formaldehyde polymer available from Calgon Corporation.
In th~se tests, the alkalinity of 200 grams of deionized water was adjusted using soda ash at the concentrations shown. Twenty drops (approximately 0.45 gram) of CIL metallic blue paint was then added to the alkalinity-adjusted paint either before or after the addition of 0.83 gm of PK-9511. A magnetic s~irrer operating at about 1200 rpm pro~ided agitation. The paint and the PK-9511 were allowed to mix for 60 second~. 0.02 gm (active basi~) of a nonionic polyacrylamide flocculant was then added and ~he resulting sys~em was mixed for 30 seconds. The results reported in Table 1 were recorded fi~e (5) minutes later.

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Table 1 shows that lower alkalinity concentrations tend to produce poor solids capture and "stringy" floc formation, while at higher alkalinity levels, 10c size decreases and solids tend to sink.
Table 1 also shows that excellent performance can be obtained irrespective of order-of-addition.

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TABLE l THE EFFECT OF ORDER-OF-ADDITION
ON PERFORMANCE FOR WATER-BORNE PAINTS

Percent Tr~nsmittance Palnt Solids ~ 450 n~ Charncterlst~cs Soda A~hPR-9511 P~-9511 PR-9511 PR
pPm (~s C.aCQ3~ Added Before Added After Added Be~ore Adde 0 l9 l~ Stringy Floc St~i Poor Capture Poor 22 20 Stringy Floc Stri Poor Capture Poor 100 97 46 Good FlocGood 200 61 99 Finer FlocGood 400 100~ SinkGood 600 34 86 100% SinkFair ~, : : :
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The preceding examples are set forth to illustrate specific embodiments of the invention and are not intended to limit the scope of the compositions and methods of the present invention. Additional embodiments and advantages within the scope of the claimed invention will be apparent to one of ordinary skill in the art.

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Claims (11)

1. A method of treating circulating paint spray booth water containing oversprayed, water-borne paint, which method comprises:
(a) adjusting the alkalinity of said water to between about 20 and 600 ppm, on a calcium carbonate basis, by adding a source of alkalinity to said water;
(b) adding to said water an effective amount of a melamine polymer comprising the reaction product of a melamine or a substituted melamine and a compound represented by the formula:

wherein R1 and R2 are the same or different and are selected from the group consisting of H and straight or branched C1-4 alkyl, and wherein the mol ratio of melamine or substituted melamine to said compound ranges from about 1:1 to about 1:6;
(c) contacting over-sprayed, water-borne paint with said water, after completing step (a) and before or after completing step (b);
(d) adding an effective amount of a flocculant to said water after completing steps (a), (b) and (c); and (e) removing resulting sludge from said water.

2. A method as defined by claim 1, wherein the alkalinity of said water is adjusted to between about 50 and 400 ppm.

3. A method as defined by claim 1, wherein the pH
of said water is maintained in the range of about 6.0 to about 8Ø

4. A method as defined by claim 1, wherein the alkalinity source is selected from the group consisting of carbonates and silicates.

5. A method as defined by claim 4, wherein the alkalinity source is selected from sodium carbonate, sodium bicarbonate and mixtures thereof.

6. A method as defined by claim 1, wherein the melamine polymer comprises a melamine formaldehyde polymer.

7. A method as defined by claim 1, wherein the flocculant comprises a nonionic, anionic, cationic, or amphoteric flocculant or mixture thereof.

8. A method as defined by claim 7, wherein the flocculant is a nonionic long chain polyacrylamide or nonionic long chain polymethacrylamide flocculant.

9. A method as defined by claim 8, wherein the nonionic flocculant has a weight average molecular weight in the range of about 6 x 106 to about 2 x 106.

10. A method as defined by claim 1, wherein the melamine polymer is added to said water before the over-sprayed, water-borne paint is contacted with the paint spray booth water.

11. A method as defined by claim 1, wherein the melamine polymer is added to said water after the over-sprayed, water-borne paint is contacted with the paint spray booth water.