Metal Finishing Guide Book


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curtain, using the action of the water to "scrub" the paint solids from the water. This water is recirculated from a holding tank and continuously is cascaded down the "waterfall" wall. A similar mechanism can be used in the downdraft paint booth, which is most common in larger continuous operations, such as automotive assembly plants. The downdraft system makes use of a downward airflow, most typically through a steel floor grating, into a mobile flood sheet (much like a horizontal "waterfall") or pit of water. Downdraft systems normally contain much larger volumes of water than side-draft systems, making the treatment and conservation of this water even more critical. COMMON PAINT-RELATED WATER POLLUTANTS In the case of either side-draft or downdraft systems, the recirculated water comes into contact with a wide variety of potential pollutants from the paint overspray. Many of the materials in the paint, particularly in solvent-based formulations, are not particularly compatible in water systems. The solvents, which would include xylene, toluene, and methylene chloride, are typically not water soluble but can be water miscible (i.e., mixable). At any rate, most of these solvents are volatile and will evaporate over time to exit through the air exhaust. The organic resins making up the bulk of the paint coating are insoluble in water and tend to stay tacky if not treated with some additional material introduced into the water. If left untreated, tacky sludge can plug up recirculation pipes and pumps (as well as adhere to any and all surfaces of the booth), reducing overall efficiency. Other additives in the paint formulation, such as film-forming/wetting agents, may or may not be soluble in the water and will be present in varying degrees. Some pigments or other inorganic components, such as zinc or chromate compounds, may be partially or completely soluble in water. These inorganics, especially the zinc and chromium salts found in many primers, can pose major problems in disposal. The key to reducing or removing any of these pollutants is to find a way to either solubilize or detackify the paint solids and to collect and remove the dissolved solids (if possible). Water-based paints, unlike solvent-based formulations, dissolve or disperse readily in water. Because of this dispersibility, caused impart by the relatively small particle size of the waterborne pigments and resins, water-based paints can often be difficult to physically remove from the system. The problem then is one of solids concentration and removal, rather than detackification. PAINT DETACKIFICATION A number of methods have been developed to chemically treat the sticky organic paint sludge collecting in waterwash paint booths. Reviewing these detackification systems both historically and in terms of increasing effectiveness, they include caustic/hydroxide treatments,metal salts programs, clay-based programs, and acid colloid programs. Caustic/Hydroxide Treatments The use of caustic-based treatments (most commonly in the form of sodium hydroxide) represents the oldest chemically based treatment for detackification. These products work well with lacquers (paints cured by solvent loss), which made up many low-solids paints in the past. The basic principle of detackification is the cleavage of ester linkages found in the fatty acid-based resin by hydroxide. This results in the formation of a metal-based soap, which emulsifies any remaining solvents in the paint. The remaining material, with no solvent present, cures and hardens into a mass for "easy" removal. 659

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