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Pollution Prevention and Waste Minimization in Metal Finishing

	Principal Investigators: Research Team: EPA Contact Persons:		M.K. Mazumder, University of Arkansas at Little Rock R. Engelken, Arkansas State University University of Arkansas at Little Rock: Dr. M. K. Mazumder, Dr. R. A. Sims, Dr. R. Reddy, Dr. A. Shaikh, Dr. N. Grable, and Mr. D. L. Wankum University of Arkansas at Pine Bluff: Dr. F. Tendeku Arkansas State University: Dr. R. Engelken Industrial Participants: Dr. T. Chasser, PPG Industries, PA Outreach Participants: Ms. M. Donham, Central High School; Mr. D. Fuller, Parkview High School; and Dr. I. Lublinskaya, Arkansas School of Mathematics and Science. Dr. David Ferguson, Lead Staff Person, National Metal Finishing, EPA Cincinnati laboratory.

In most applications of metals from airframes to toys, the metal surface is painted for corrosion protection and for aesthetic appearance. A typical metal finishing process consists of three steps:

1. cleaning and pretreatment of the metal surface,
2. painting of the surface, and
3. curing.

Several years ago most metals were painted with a conventional spray process using volatile organic compounds (VOC) mixed with paints. During the curing process, toxic VOC are evaporated. These VOC are carcinogenic, and the EPA has strict regulations on their emission. Because of the serious environmental problem with solvent-based coating systems, most industries are switching to powder coating systems, When dry power is deposited on metal surfaces by an electrostatic process, a durable paint is formed. During the subsequent curing of the powder, this paint becomes functionally and economically competitive with organic-solvent based paint.

However, in some cases a powder coating process cannot replace a liquid-based process because of the following problems:

1. The appearance of the cured film is often not as good.
2. It takes a relatively longer time to change colors.
3. There is relatively poor penetration of powder particles into recessed areas (known as the Faraday Penetration Problem).
4. In certain powder paint applications, the weatherability is inferior.

In the area of pretreatment, most coating processes involve liquid-solution deposition. For steel surfaces, iron phosphate or zinc phosphate conversions are successfully used without much difficulty. However, for aluminum surfaces a chromate conversion is used and often involves exposure to hazardous emissions such as hexavalent chromium. The environmental problems related to chromium conversion have been recognized by the EPA for some time.

While the powder coating process is definitely more environmentally friendly and less hazardous to health, there are two environmental issues that have not been thoroughly researched and reported in the literature. One of these is associated with the generation of ozone by the negative corona processes. A typical powder coating booth may contain 10 to 30 corona guns continuously painting metal surfaces. In time, a possible toxic concentration of ozone may occur in the plant air. This factor has not been studied in detail, though the generation of ozone by the corona discharge process has been recognized for years.

The second environmental issue is particulate pollution, especially in the respirable range (ultrafine particles to particles with aerodynamic diameter of 10 micrometers.) For many years, the powder coating industry has used powders ranging from 15 to 100 micrometers in diameter. Although there are always some fines produced during the grinding process, the powder coating industry classifies the powder with respect to size so that most particles remain in the size spectrum larger than 15 micrometers in diameter. However, as powder coating becomes competitive economically, functionally, and in appearance, the trend will be to use finer powders to improve appearance and increase adhesion. As a result, these powders will have a significant size fraction within the respirable range. While the reclaimed powders are filtered by a cyclone device and then by using a cartridge filter containing filter paper, there is a high probability that fine powder can be discharged into the plant air. While most plant powders are considered nontoxic, some components (pigments, carbon black, flow agents, etc.) could be present that cause health problems with long-term exposure. Because paint powders are electrostatically charged, the lung deposition of inhaled charged particles in the respirable range could be dangerously efficient.

Three US auto manufacturers, GM, Ford, and Chrysler have formed a low emission paint consortium to apply powder coating technology to automotive applications. This research project relates to the consortium's goal in pollution prevention and waste reduction.