Pulsed Light Peels Away Poisonous Paint

Michael A. Greenwood

Removal of lead paint — a material that was banned in the US in the late 1970s — is difficult and dangerous work, not to mention costly. Traditional removal methods, such as sanding, heat stripping or chemical stripping, can worsen the problem by stirring up the toxic material. Ingesting even modest amounts of lead can have harmful consequences for children and animals.

Figure 1. An open shutter image shows the high-intensity discharge from this surface discharge lamp. Images reprinted with permission of Environmental Science and Technology.

Two researchers at Phoenix Science & Technology Inc. in Chelmsford, Mass., have designed a lead paint removal system for use in houses and apartments. It relies on high-intensity, <100-μs incoherent pulses of light that vaporize the paint while a rotating brush sweeps the debris into an attached vacuumlike device, where it is safely processed through a series of filters.

Early results indicate that the light-based ablation method may overcome many of the problems associated with lead removal. Michael J. Grapperhaus and Raymond B. Schaefer reported on their work in the Dec. 15 issue of Environmental Science and Technology. They claim that their technique minimizes toxic byproducts, reduces labor and could cost less than existing methods.

The photolytic system relies on a patented surface discharge lamp technology. The lamp’s xenon gas containment envelope is far enough away from the plasma discharge to allow the lamp to operate at the higher pulse energies and the shorter pulse durations needed to strip paint. The result is quicker removal and a paint substrate that is not significantly heated or damaged. The full intensity of the light is focused onto the paint with an elliptical reflector devised by the researchers. The reflector also helps remove paint from irregularly shaped areas — corners, molding and other decorative flourishes.

Figure 2. The surface discharge lamp is shown with several of its basic components: the envelope, substrate and electrodes.

During initial laboratory tests, light from the device was measured with an Ocean Optics Inc. multichannel spectrometer over the 200- to 800-nm range. The behavior of the pulsed light also was checked with a Thorlabs Inc. photodiode. To gauge the device’s safety, the air around the paint stripper was measured for lead particles at varying distances from the source. All measurement results were within the safety limits established by the Occupational Safety and Health Administration.

Figure 3. This paint sample, before (top) and after (below), was treated by the surface discharge lamp. The area treated was about 11 × 32 cm.

The researchers also measured stray ultraviolet and visible light escaping from the machine. These results were within the acceptable safety threshold as well.

In the actual paint-removal test, they used a piece of wood covered with two coats of dark green paint. The initial lead concentration was 1.7 mg/cm². After the paint was exposed to six pulses of light, new lead readings were taken from three points on the board. The levels were measured at 0.33, 0.44 and 0.44 mg/cm². The action level is 1.0 mg/cm². In a second test, on a sample of steel, a single coat of lead paint, 11 × 32 cm, was removed by 161 pulses of light. Air samples taken to measure airborne lead levels were found to have below the detection limit of 35 μg/m³.

Although high-intensity light has been used for paint removal, the results have been mixed. One method, involving flashlamps with a stream of dry-ice pellets, is suitable for aerospace applications but has proved too unwieldy for use in smaller spaces such as houses, the researchers said. Lasers also have been used effectively but are generally too expensive for housing applications.

With their initial experiments over, the researchers are planning to expand their testing with a grant from the US Department of Housing and Urban Development. “It’s a laboratory device, unproven in the field so far,” Schaefer said.

They also are working to increase the stripping efficiency of their surface discharge lamp while investigating how it can be economically produced for widespread commercial use. Future testing will be done on larger substrates and will include longer sampling times to get a better estimate of airborne lead levels.

If various regulatory requirements are met, they said, the device could be available for commercial use within two years. They also are using their surface discharge lamp for UV water treatment.

Contact: Michael J. Grapperhaus, Phoenix Science & Technology Inc., Chelmsford, Mass.;
e-mail: mgrapperhaus@phoenixsandt.com.