The objective of this project was to develop an innovative media-free atmospheric plasma coating removal system for use on Department of Defense ship and vehicle platforms. Coating removal processes currently used produce large quantities of hazardous wastes such as spent blast media, wastewater, or toxic chemicals. Disposing of these waste products is a burden with high costs and intense scrutiny under environmental regulations. Additional costs are incurred to inventory, store, and handle media prior to use, and to contain and collect the waste media before disposal. Current technologies including wet or dry media blasting, mechanical sanding, laser ablation, induction heating, FlashJet™ technology, and chemical stripping have limited flexibility as a complete coating removal solution. The Laboratory for Integrated Manufacturing Science and Technology at North Carolina State University and industrial partner Atmospheric Plasma Solutions Inc. have conducted fundamental and applied research to investigate the use of atmospheric plasma as a total coating removal system. Contributing to the current research were additional partners from Naval Sea Systems Command (NAVSEA), Naval Air Systems Command (NAVAIR), and the Air Force Research Laboratory (AFRL).
The PlasmaFlux™ system uses a low pressure compressed air source and electricity to produce a special form of atmospheric pressure, air plasma, which is highly chemically activated and attacks (oxidizes) the organic components of paints and other coatings. In this project, the system was used to remove two major coating systems commonly found on Navy ships: (1) freeboard paint typically used above the waterline, and (2) antifouling paint typically used below the waterline. Optical emission spectroscopy was used to identify the presence and distribution of chemically active atomic and molecular species of oxygen and nitrogen in the complex makeup of the plasma. The research was conducted in three overlapping phases: (1) determine the capability of atmospheric plasma to remove paint, (2) develop and investigate large area plasma devices, and (3) investigate environmental and process hazards, waste mitigation, operational safety, integration, and transition.
In the course of this research effort, it was determined that an appropriate plasma source could remove topcoat and primer from metal test panels. Due to the oxidizing nature of the plasma, the iron surfaces were cleaned to atomic levels leaving a stable form of iron oxide. Substrate temperature was monitored and did not exceed 200°C (392°F) during depainting with plasma. It was also demonstrated that residual paint left by incomplete grit blasting was subsequently removed to bare metal and its oxide using plasma. The plasma treatment did not alter the surface profile created by grit blasting. Test panels depainted using plasma along with grit blasting panels for comparison were subsequently coated with new paint, then subject to performance tests including adhesion pull-off, B117 salt fog, cathodic disbondment, and alternate immersion in sea water. Some panels were repainted immediately, but one set was aged two weeks after plasma depainting, before they were recoated for performance testing. There were no statistically significant differences in any of the sample sets, which indicated that using plasma to remove paint provided similar paint adhesion performance when compared to grit blasting for paint removal.
Organic components of the paint were broken down into small molecular weight components, primarily carbon dioxide and water, as determined by mass spectroscopy. Inorganic fillers used in the paint were the primary components of solid waste as determined using optical and electron microscopy and energy dispersive x-ray analysis. Depending on the operational conditions, especially when using aggressive removal conditions upon scale-up of the plasma system, some smaller not completely broken down fragments of the coating system were also part of the waste stream.
Theoretical calculations were performed to determine the mass of gaseous and solid products that would be expected if all initial paint was converted to mineral constituents including carbon dioxide and water. Removing paint from one aircraft carrier as an example, carbon dioxide would be produced in an amount similar to that produced by five average automobiles per year. In terms of solid waste, using plasma to remove paint, approximately 40 to 60% of the original coating mass is collected, primarily due to the inorganic fillers, the remainder of which is converted to gas. These numbers were confirmed experimentally by measuring mass before and after a confined depainting experiment. When operated with free exhaust to the air, nitric oxide (NO), a nuisance gas was detected. Through design and ensuring that plasma is in contact with paint for removal, generation of nuisance gas should be minimized.
Early results of removal rates measured using a single applicator indicated that approximately eight to fourteen nozzles would be needed to achieve reasonable commercial removal rates. Scale-up in design, manufacture, and testing was performed in increments up to an eight-nozzle plasma system including the power supply. Removal rates were calculated in many experiments and found suitable for scale-up potential. A waste management system was integrated with the plasma system and preliminary operation and experimentation completed.
The PlasmaFlux™ technology is portable, can be operated by manual or automatic means, was scaled to a desired size, presented no undue occupational hazards to the tool operators, created no significant waste beyond the breakdown products of the original coating materials, did not negatively alter the steel surface, and produced acceptable repaint adhesion performance. No occupational or undue environmental hazards were identified. Measurements did not identify any undue hazards including sound, electric and magnetic field (EMF), ultraviolet (UV)/visible, and other potential operational hazards.