Grantee Research Project Results
Final Report: Pollution Prevention and Waste Minimization in Metal Finishing (SEER)
EPA Grant Number: R827685E04Title: Pollution Prevention and Waste Minimization in Metal Finishing (SEER)
Investigators: Mazumder, Malay K. , Engelken, Robert
Institution: University of Arkansas - Little Rock , Arkansas State University - Main Campus
EPA Project Officer: Chung, Serena
Project Period: August 1, 1999 through July 31, 2001
RFA: EPSCoR (Experimental Program to Stimulate Competitive Research) (1999) RFA Text | Recipients Lists
Research Category: EPSCoR (The Experimental Program to Stimulate Competitive Research)
Objective:
In the use 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 with a primer; (2) coating the surface with polymer paints; and (3) curing the paint to obtain a durable coat over the primer. In many applications, the metals are painted with a conventional spray process using volatile organic compounds (VOCs) mixed with the paints. During the curing process, toxic VOCs are emitted. These VOCs are carcinogenic and the U.S. Environmental Protection Agency has strict regulations on their emission. Because of this serious environmental problem, most industries are switching to powder-coating systems. In the powder-coating process, dry powder is deposited on metal surfaces by electrostatic spraying, and the deposited powder layer is cured to form a durable film. The coating process is functionally and economically competitive with the organic solvent-based paint.
The overall objective of this research project was to prevent pollution and minimize waste in metal coating (precoat plating and powder-coating) processes. The specific objectives of this research project, which involved pollution prevention and waste minimization, were to:
1. Replace hexavalent chromium used for precoating aluminum by a process that is environmentally safe and economically competitive. The hexavalent chromium can be replaced with liquid solution deposition of molybdenum or tungsten, or by reactive organic conversion coating processes.
2. Improve Faraday penetration of charged powder in recessed areas, and expand powder-coating applications to areas where solvent-based coatings currently are used. The goal was to improve transfer efficiency of the powder to more than 90 percent to reduce powder wastes in powder-coating processes where the recycling of powder is not feasible. Improved efficiency would allow fast color change without significant waste of powder.
The long-term goal of this research project was to be nationally competitive through undergraduate and graduate research and academic programs in environmental science and engineering, peer-reviewed publications, seminars and presentations, workshops, demonstrations, and training.
Summary/Accomplishments (Outputs/Outcomes):
Electrodeposition of Molybdenum Oxide and Tungsten Oxide Films
The first pretreatment of the aluminum alloy surface, after an appropriate cleaning, was electrodeposition of molybdenum oxide (MoOx) or tungsten oxide (WOx) films to replace the conventional chromate pretreatment process. WOx films were electrodeposited by a standard technique involving dissolution of a slight excess of tungsten powder in 30 percent hydrogen peroxide followed by the addition of water, sulfuric acid, and sodium sulfate to yield solutions approximately 0.1 M in soluble tungstic acid-like complexes, 0.25 M in sulfuric acid, and 0.1 M in sodium sulfate. Brown MoOx films, presumably with stoichiometries close to MoO2, were electrodeposited most conveniently with basically the same apparatus and methodology from aqueous solutions of ammonium molybdate, (NH4)6Mo7O24 • 4H2O - 0.1 M, at a pH value between 4 and 6. The MoOx and WOx coatings applied on aluminum coupons were exposed to Harrison solution (0.35 percent ammonium sulfate, 0.05 percent sodium chloride in deionized water) under ambient conditions. Electrochemical impedance spectroscopy and Tafel plots were used to determine the impedance (Z) and zero current potential (Ecorr) of the coatings, respectively. The corrosion resistance of the coupons, based on electrochemical impedance measured after 18 hours of exposure in Harrison solution, can be arranged in the following descending order: (1) chromium conversion coatings; (2) MoOx coatings; (3) WOx coatings; and (4) bare aluminum. The passivity of the samples, based on Ecorr measured after 45 hours of exposure in Harrison solution, can be arranged in the following order: (1) chromium conversion coatings; (2) WOx coatings; (3) MoOx coatings; and (4) bare aluminum.
Reactive Organic Conversion Coating for Steel and Aluminum Alloys
Water-dispersible reactive organic conversion coating such as lignosulphonic acid-doped polyaniline (LIGNI-PANI) has been utilized for coating steel and high-strength aluminum alloy substrates such as AL 2024 T3. LIGNI-PANI shows excellent corrosion inhibition properties compared to other conventional solvent-borne coatings. The presence of polyaniline, higher solubility in water, and improved processability with water-soluble coatings makes LIGNO-PANI a good corrosion inhibitor for most of the metal substrates. The most probable inhibition mechanism involving the use of inherently conducting polymer (ICP)-like LIGNO-PANI is the transfer of electrons from the metal surface to the conductive polymer. This redox process forms a passive oxide layer on the surface of the metal, which inhibits further corrosion. The coating formulations were prepared by mixing water-based epoxy resin (EPI-REZ™ 3510-W-60) with a modified aliphatic amine curing agent (EPICURE™ 8535-W-50), both obtained from Resolution Performance Products. Epoxy coatings containing 0 percent and 1 percent by weight of LIGNO-PANI were applied on polished AL 2024 T3 coupons using a draw bar, and were cured at 120°C for 10 minutes. Both types of coatings were tested for corrosion resistance in 3.55 weight percent sodium chloride solution. The presence of LIGNO-PANI in the coatings significantly increased the impedance of the aluminum coupons measured at lower frequencies (1-10 mHz). The Tafel plot of scratched epoxy coating containing 1 percent LIGNO-PANI indicates a positive shift of open circuit potential (OCP) by 200 mV compared to the 0 percent LIGNO-PANI mixed epoxy coating, which maintained its OCP at -600 mV after 12 hours of immersion.
Powder Coating
Experimental and theoretical studies were conducted to investigate the effects and contribution of different parameters involved in the powder-coating process. The parameters studied included particle-size distribution (PSD) of powder, charge-to-mass ratio distribution of powder, charging methods, and powder mass flow rate to improve transfer efficiency as well the appearance of the cured films. It was observed that a powder with a narrow PSD generates smoother thin films compared to the films produced by a powder of wider PSD. To apply a thin film with thickness ranging from 15 to 30 µm by electrostatic spraying, the polymer powder was classified to have a narrow PSD with a mean diameter (d50) less than 10 µm. This method can be used to apply the industrial grade powder paints with a wide PSD (with d50 larger than 20 µm) to be classified to remove the coarse particles with an inline cyclone classifier. The classified powder then was charged electrostatically for deposition as thin films. Because corrosion resistance of electrostatically deposited powder coatings depends on obtaining a smooth, nonporous film with uniform thickness over the surface of the entire substrate, the film was examined for voids, pinholes, and other surface defects. A significant decrease in corrosion resistance was observed in coatings as film thickness was decreased from 30 to 5 µm, primarily when the film was uneven, caused by surface defects resulting from Faraday cage and back corona effects. Both Faraday cage and back corona problems become more severe as particle size decreases in contrast to the requirements for their film coating, and, currently, there is no commercial process available to apply thin powder films electrostatically to meet these contradictory requirements. The method developed under the current project establishes the feasibility to overcome both problems. The new method involves spraying bipolarly charged powder particles and depositing them under these three electric field conditions: (1) positive field; (2) negative field; and (3) zero electric field. Because the surface resistivity of the sprayed powder governs the charge decay process and hence the onset of the back corona, the ambient relative humidity was controlled at 60 percent to enhance the charge decay process of the powder layer. Plasma treatment of the powder was an effective technique to increase the charge decay process of powder particles and, in turn, minimize the back corona effect.
Conclusions:
An environmentally safe, multilayer, and multifunctional coating for long-term protection of structural components of aircraft from corrosion was developed. The first layer, a thin film of MoOx or WOx electrodeposited on the aluminum surface, is an alternative to the current chromate conversion process. These electrodeposited metal oxides of molybdenum and tungsten provided active corrosion protection in neutral and higher pH ranges by decreasing the oxidizing threshold potential at which passive metal oxide films form. Next to the oxide film was a 20 µm-thick second primer that was applied using water-dispersible epoxy mixed with an ICP. Corrosion studies with water-based epoxy coating containing 1 percent LIGNI-PANI showed excellent corrosion inhibition to aluminum alloy substrates. The combination of the conducting polymer with the nanocrystals of the Mo, W, or cerium metal oxides in a polymer matrix would provide an effective protective coating. Thus, the two primer layers act both as corrosion inhibitive and corrosion barrier coatings. A top coat was applied on the primer-coated surface. The top coat was comprised of two thin layers (15 µm each) of polymer or a single 30 µm film applied to provide overall protection against corrosion, UV radiation, moisture penetration, and minor mechanical impacts (chip resistance). In summary, electrochemical and electrostatic processes were developed to deposit multifunctional primer and multifunctional top coats with a minimal film thickness for a low-weight, low-gloss film with high corrosion resistance.
Journal Articles on this Report : 4 Displayed | Download in RIS Format
Other project views: | All 11 publications | 4 publications in selected types | All 4 journal articles |
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Type | Citation | ||
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Biris AS, Mazumder MK, Yurteri CU, Sims RA, Snodgrass J, De S. Gloss and texture control of powder coated films. Particulate Science and Technology, 2001; 19(3): 199-217. |
R827685E04 (Final) |
not available |
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Mountain JR, Mazumder MK, Sims RA, Wankum DL, Chasser T, Pettit Jr. PH. Triboelectric charging of polymer powders in fluidization and transport processes. IEEE Transactions on Industry Applications 2001;37(3):778-783. |
R827685E04 (Final) |
not available |
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Sharma R, Trigwell S, Biris AS, Sims RA, Mazumder MK. Effect of ambient relative humidity and surface modification on the charge decay properties of polymer powders in powder coating. IEEE Transactions on Industry Applications 2002;30(1):87-95. |
R827685E04 (Final) |
not available |
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Sims RA, Mazumder MK, Liu X, Chok W, Mountain JR, Wankum DL, Petit PP, Chasser T. Electrostatic effects on first pass transfer efficiency in the application of powder coatings. IEEE Transactions on Industry Applications 2001;37(6):1610-1616. |
R827685E04 (Final) |
not available |
Supplemental Keywords:
pollution, pollution prevention, volatile organic compounds, VOCs, metals, waste, waste minimization, powder-coating processes, powder wastes, molybdenum oxide, MoOx, tungsten oxide, WOx, steel alloys, aluminum alloys, particle-size distribution, PSD, metal coating., Scientific Discipline, INTERNATIONAL COOPERATION, POLLUTANTS/TOXICS, Sustainable Industry/Business, POLLUTION PREVENTION, cleaner production/pollution prevention, waste reduction, Environmental Chemistry, Chemicals, Environmental Engineering, hexavalent chromium, cleaner production, waste minimization, clean technology, VOC removal, electrodeposition paint coating, metal finishing, coating processes, alternative metal finishingRelevant Websites:
http://www.aesf.org/ Exit
http://www.electrochem.org/ Exit
http://www.ieee.org/portal/index.jsp Exit
http://www.electrostatics.org/ Exit
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.