Environ. Sci. Technol. 2006, 40, 7925-7929
Lead Paint Removal with High-Intensity Light Pulses MICHAEL J. GRAPPERHAUS* AND RAYMOND B. SCHAEFER Phoenix Science and Technology, 27 Industrial Avenue, Chelmsford, Massachusetts 01826
This paper presents the results of an initial investigation into using high-intensity incoherent light pulses to strip paint. Measurements of light pulse characteristics, the reflectivity of different paints and initial experiments on the threshold for paint removal, and paint removal are presented, along with an approximate model consistent with experimental results. Paint removal tests include lead paint, the reduction of lead levels to below levels required for lead abatement, as well as air and light emissions measurements that are within regulatory guidelines.
Introduction Environmental regulations for air emission and waste disposal created a need for lead abatement methods that are both economically feasible and environmentally acceptable. Many architectural surfaces painted prior to 1978 have lead paint. Improved paint removal techniques are needed because “acceptable” methods such as media blasting and abrasive techniques generate a large volume of dust and debris and can damage architectural materials. Chemical strippers are labor intensive, messy, and may be toxic or lead to the generation of hazardous waste. Because of the limitations of current techniques, abatement is often accomplished by replacement or encapsulation, or interim measures are employed because of their lower cost. Photolytic paint removal is a process in which highintensity pulses of light are absorbed by a painted surface, heating the paint to vaporization, thereby removing the paint. Practical photolytic paint removal employs fast pulses of light that rapidly vaporize layers of paint to minimize heat losses to the underlying substrate. The paint ablation process is complex and in addition to vaporization, the paint can react chemically and also be affected by the intense shock waves generated by the rapid vaporization. The paint removal area is enclosed to contain the vaporized paint, which is vacuumed into a filter system. Use of light pulses to remove paint minimizes the debris generated since it does not add material during removal and only the paint material itself is collected. Systems using light pulses from flashlamps (1) or lasers (2) strip paint but are impractical for lead paint removal in homes. Attempts to use incoherent light with flashlamps have had moderate success, but low removal rates and heating of the underlying substrate are issues. A commercial system using flashlamps combines light pulses with a stream of dry ice pellets for aerospace paint stripping (2), but is too expensive and bulky for housing. Paint removal with lasers (coherent light) has been successful, but cost precludes use for lead abatement. * Corresponding author phone: 978-367-0232 x126; e-mail:
[email protected]. 10.1021/es061328g CCC: $33.50 Published on Web 11/15/2006
2006 American Chemical Society
Surface discharge (SD) lamp technology (3-5) is based on striking a plasma discharge on the outside surface of a dielectric substrate tube between two electrodes (see Figure 1). Its operation is similar to a flashlamp (i.e., high voltage pulsed discharge in xenon), but in a SD lamp, the xenon gas containment envelope is well away from the plasma discharge. This allows the SD lamp to operate at much higher pulse energies and with much shorter pulse durations than flashlamps, which is more effective at removing paint. Higher irradiance enables faster removal and shorter pulse length reduces heating of the substrate. This study examines operating conditions of the SD lamp and measures the effectiveness of a paint stripping system using SD lamp technology. Results are interpreted in the context of a simple paint removal model that explains the variation with pulse length and pulse energy.
Experimental Section The SD lamp is a pulsed xenon lamp where the discharge is initiated on the surface of a quartz tube. The electrodes are annular rings around the outside of the tube. The pulse forming network consists of an initiator circuit that produces a fast high voltage pulse to initiate a uniform plasma around the center tube, and a driver circuit that delivers the majority of the energy to the plasma discharge. The discharge takes place inside a larger xenon-filled quartz envelope. Figure 1 shows the lamp configuration along with an open shutter photograph of a lamp pulse. The open shutter photograph shows that the plasma is on the substrate, well away from the envelope. The paint stripper combines the high-intensity surface discharge (SD) lamp with a reflector (5) as shown in Figure 2. The reflector is elliptical with one focus at the center of the lamp and the other focus at the paint surface. Light pulses directed to the surface are absorbed by the paint, ablating a layer, which is vacuumed into a filter system that captures the paint products (see Figure 2). The filter system has a coarse pleated filter, a fine HEPA filter, and refillable charcoal cell (for trapping residual vapors). The light pulse energy is deposited into a thin layer of paint so quickly that that the underlying surface is unaffected. A rotating brush helps to evacuate and remove residual paint products. Light measurements are made with a multichannel spectrometer with 2048 elements over the wavelength range of 200-800 nm (Ocean Optics USB2000). The measurements include the use of calibrated neutral density filters (Esco, metallic coated fused quartz) to prevent saturation from the high irradiance SD lamp pulses (6, 7). The UV spectrometer is calibrated using a pair of calibrated NIST traceable lamps, Deuterium (Oriel model 63345) for UV calibration and Quartz Tungsten Halogen (Oriel model 63358) for calibration up to 800 nm. The temporal behavior of the light output is measured with a photodiode (Thorlabs DET210 high-speed silicon detector), which can be used with or without a filter. Typically the photodiode is for examining either the entire spectrum or the UV output only, by using a UV bandpass filter (Oriel model 53330). Additional measurements include the pulse current (Pearson model 101) and voltage (Northstar PVM-5) for monitoring the electrical behavior of the lamp. The measurements are used to determine delivered power, pulse energy, pulse risetime and peak current, as well as evaluate circuit matching. The spectral reflectance of the paint samples are made with the same spectrometer as used for the lamp light output VOL. 40, NO. 24, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Surface discharge lamp (above), an open shutter photograph of the discharge (below). measurements. Both the specular and diffuse reflection coefficients are made, using a commercial reflectance probe (Ocean Optics R400-7-UV/SR) and comparing the measurement to a reflectance standards (Labsphere Spectralon diffuse reflectance standard and Acton Research Corporation model 1600-1D-FL). Testing included both the determination of the threshold for paint removal as well as paint removal measurement, for different paint colors, and SD pulse parameters. The threshold was determined by exposing the sample to a single pulse, and then inspecting the sample visually for indication of an effect. This procedure was repeated for a series of increasing voltages (i.e., pulse energies) until removal of paint was detected visually. The experiment included multiple-pulse paint stripping tests on samples as they moved past the paint stripping head on a motor driven slide (see Figure 2c). These tests utilized the effluent capture system, including the brush, to remove residual material from the surface. A Niton XRF lead paint detector was used to measure the lead concentration before and after the depainting. In addition to paint removal and measurements of lead in the painted and depainted samples, quantitative measurements of air borne lead and stray light during were made during operation of the depaint system. SKC AirLite sample pumps with air particulate filters were used for air monitoring at three locations: adjacent to the paint stripping surface (0.15 m below the paint sample), at the location of the operator (1.8 m from the paint sample), and in the middle of the room (4.9 m from the paint sample). Environmental Hazards Services, LLC, analyzed the sample cartridges for lead (NIOSH 7300). Stray light measurements were made at two locations. One set of measurements was taken at 1.2 m from the paint stripping head looking in from the side and the other was also 1.2 m from the paint stripping head, but directly over the pulse forming network looking into the air flow channel. The measured light spectrum was compared to the American Conference of Governmental Hygienists (ACGIH) threshold limit values (TLV) for ultraviolet and visible light after weighting them with the appropriate sensitivity function (8).
Results and Discussion Model for Paint Removal with Light Pulses. This section presents an approximate model for paint removal using light pulses. The model provides a means both for understanding and improving paint removal rates as well as interpreting 7926
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FIGURE 2. A reflector system focuses light from the SD lamp onto the painted surface for stripping. (a) SD lamp integrated into the reflector system, (b) schematic of the paint strip process, and (c) the laboratory paint strip test configuration. test results. The model partitions the incident light fluence (energy per unit area) into four terms: reflection, energy in a layer near the surface, vaporization energy, and heating of the vaporized material. Several factors influence the threshold and efficiency of paint removal. For long pulses, conduction of heat through the paint and into the underlying substrate increases the energy required to vaporize the paint. For efficient paint removal, the incident pulse fluence should be
well above threshold so that a large fraction of the light causes vaporization. Because the pulse is very fast (
