Metal Finishing Guide Book


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The drawback of the caustic-based program is that, as paint technology has advanced, with changes to higher solids levels (primarily for reasons of environmental compliance) and catalyzed curing, this type of treatment no longer fully reacts with the components of the paint. This results in only partially killed paint, which causes most of the same problems as "live" paint. To combat this problem, caustic-based programs containing some insoluble inorganic material (such as lime) were developed. The insoluble material helps to capture some of the unkilled mass of the paint, essentially embedding it. Even these modified programs are inefficient, however, as the solids level of the paint increases past 25%. Metal Salts Programs Metal salts products primarily make use of aluminum and zinc salts combined with a source of alkalinity to form either catalyzed insoluble metal soaps (somewhat similar to the treatment with caustic) or suspended metal complexes, which can be removed by treatment with an additional polymer. The limitation of this type of treatment is that, except in the case of alkyd-based, air-cured paints, the pH control of the system is very crucial to proper operation. Fluctuations in pH level can easily cause disruption of the program, resulting in live paint and settling solids. Clay-Based Programs Clay-based products primarily represent a physical, rather than chemical, method of paint detackification. As bentonite clay absorbs water, it swells to a large irregularly surfaced material. Sticky paint particles entering the water containing this clay adhere to the outside surface and are, in turn, covered by other clay particles. This results in a large detackified mass. An amine is often fed to increase the paint's tendency to disperse prior to contact. Although the clay itself is relatively inexpensive as treatments go, large amounts of clay or clay slurry are often required to maintain good detackification. This produces voluminous amounts of sludge, as compared with other treatment types. In addition, both water and solvent are often trapped in the clay matrix, making it difficult for landfill and limiting the ability to dewater to a range of 20% to 25% maximum. Clay programs also typically have problems with foaming and biological contamination, due to the entrapment of paint and water in the clay sludge. Acid Colloid Programs The acid colloid treatments function on the principle that certain mixtures of hydrophilic (water loving) and hydrophobic (water hating) materials can form stable suspensions under acidic conditions but precipitate as associated complexes as the pH increases. There are three detackification programs currently used based on this principle: silicate amine programs, silica amine programs, and melamineformaldehyde programs. The basic principle is to feed th e product into the system at a raised alkalinity level to form the associated complex. The hydrophobic end orients onto the hydrophobic paint particle, with the hydrophilic end sticking into the water phase.This effectively allows the paint particle to be coated with a thin film of water that prevents its surface from adhering to other surfaces. Silicate Amines In this treatment, the hydrophilic portion is a polyamine and the hydrophobe is a silicate (usually sodium metasilicate). These materials are fed separately to the booth in a fixed ratio based on the paint overspray rate. (Most typically 4 to 13 parts of silicate to 1 part of polyamine and both at 5% to 15% based upon overspray.) The pH level is critical, since too high a pH can cause the complex to redissolve. 660

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