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Ind. Eng. Chem. Res. 2001, 40, 3540-3546
Smart Polymeric Coatings for Surface Decontamination H. Neil Gray,*,† Betty Jorgensen,‡ Donald L. McClaugherty,† and Andrew Kippenberger† Department of Chemistry, The University of Texas at Tyler, 3900 University Boulevard, Tyler, Texas 75799, and Materials Science and Technology Division, Mail Stop E-549, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
Polymeric “smart” coatings have been developed that are capable of both detecting and removing hazardous nuclear and heavy metal contaminants from contaminated surfaces. These coatings consist of strippable polymeric compositions containing blends of polymers, copolymers and additives that can be brushed or sprayed onto a surface as a solution or dispersion in aqueous media. Upon drying, these coatings form strong films that can easily be peeled or stripped from the surface. When applied to a contaminated surface, these coatings display responsive behavior. Areas of contamination are indicated by a color change. As the coatings dry, the contaminants are drawn into and fixed in the polymer matrix. Subsequent removal of the coating with entrapped contaminants results in some degree of surface decontamination. Here we report the development and investigation of a smart, decontaminating coating developed for uranium and plutonium. Introduction New approaches for detecting, preventing, and remedying environmental damage are important for protection of the environment. Procedures must be developed and implemented to reduce the amount of waste produced in chemical processes, to detect the presence and/ or concentration of contaminants, and to decontaminate fouled environments. Contamination can be classified into three general types: airborne, surface, and structural. The most dangerous type is airborne contamination, because of the opportunity for inhalation and ingestion. The second most dangerous type is surface contamination. Surface contamination can be transferred to workers by casual contact and, if disturbed, can easily be made airborne. Of particular interest to nuclear facilities, and some national laboratories, are gloveboxes, walls, and tools contaminated by the radioactive isotopes of plutonium and uranium. Any effective decontamination scheme for these environments must have, as an integral part, a plan for surface decontamination. Our work has focused on the realization of such procedures via the development of “smart”, strippable, decontaminating coatings. Smart materials are materials that sense and respond in a controlled and reproducible manner to some change in their environment.1-4 Such changes in a material’s physical environment could be manifest in the form of altered temperature,5-12 electrical current,13 pressure, sound,14 or illumination.15 Alternatively, chemical stimuli such as changes in pH or the presence of some predetermined concentration of a hazardous material can represent an environmental change to which a polymer might respond.16-19 * Corresponding author: Dr. H. Neil Gray, Department of Chemistry, The University of Texas at Tyler, 3900 University Blvd., Tyler, TX 75799. Phone: (903) 566-7206. Fax: (903) 5667189. E-mail:
[email protected]. † The University of Texas at Tyler. ‡ Los Alamos National Laboratory.
Strippable polymeric coatings are polymeric solutions or dispersions that can be applied to a surface by brushing or spraying. Upon curing or drying, these coatings form strong films that can easily be peeled or stripped from the surface. Such coatings are an accepted means for the decontamination of contaminated surfaces.20-26 The effectiveness of strippable coatings has been demonstrated at several nuclear facilities, including Knolls Atomic Power Laboratory,26 Los Alamos National Laboratory, and Rocky Flats. Strippable coatings were also used at the nuclear accident sites in Chernobyl and Three Mile Island. The process by which strippable coatings remove contaminants from a surface is simple. As the coatings dry, the contaminants are drawn and fixed into the polymer matrix. Subsequent removal of the coating with entrapped contaminants results in some degree of surface decontamination. Although strippable coatings have demonstrated some success, their development is still in its infancy, and there are still many problems and limitations associated with conventional strippable coatings. One problem is that conventional coatings are often toxic themselves, containing carcinogenic solvents and chelators and other materials such as ammonia that make application unpleasant. Another major problem with conventional coatings is that they are unresponsive, i.e., they do not respond to a contaminant in an observable manner. This can lead to entire segments of stripped coating being disposed of as contaminated material even though they might be clean or have only a small portion that contains contaminant. Additionally, present commercially available strippable coatings cannot be reused or recycled, and above all other pitfalls, they do not decontaminate well with a single application. Here we present the results observed in our laboratories from the initial development and study of waterbased, contaminant-sensing, decontaminating coatings for the detection and removal of plutonium and uranium from stainless steel, glass, painted cement, aluminum, painted aluminum and nickel surfaces.
10.1021/ie010034v CCC: $20.00 © 2001 American Chemical Society Published on Web 07/13/2001
Ind. Eng. Chem. Res., Vol. 40, No. 16, 2001 3541
Experimental Section Materials. All water was doubly distilled using an automated distillation apparatus. Medium-viscosity, 88% hydrolyzed poly(vinyl alcohol) (PVA) was obtained as Elvanol 52-22 from DuPont. Poly(vinyl pyrrolidone) (PVP), MW ∼360 000, was obtained from Scientific Polymer Products, Inc. Poly(ethylene oxide) (PEO), poly(vinylamine) (PVAm), poly(ethenyl formamide) (PEF), glycerine, ethylenediaminetetraacetic acid disodium salt (EDTA), triethylenetetraminehexaacetic acid (TTHA), diethylenetriaminepentaacetic acid (DTPA), ethyleneglycol-2-(aminoethyl)tetraacetic acid (EGTA), ethanol (absolute), chlorophosphonazo-III (CP-III), and 2-(5bromo-2-pyridylazo)-5-diethylaminophenol (BrPADAP) were obtained commercially. Both uranium and plutonium were used as their oxides in 0.5 M nitric acid at the Isotope and Nuclear Chemistry Facility at Los Alamos National Laboratory. General Coating Preparation. In a 2-L beaker, 765 g of distilled water was stirred mechanically using a Teflon-bladed mechanical stirrer. To the water was added 120 g of PVA in 10-g aliquots with good stirring. After all of the PVA had dissolved, 70 g of PVP was added with good stirring. The polymer blend was stirred for 20 min to ensure proper mixing. To the polymer solution were added, in order, 40 g of glycerine, 5 g of EDTA, and 0.03 g of BrPADAP (in 1 mL of absolute ethanol), followed by 30 min of additional stirring. The orange, viscous coating mixture was defoamed by allowing it to sit in the beaker for 2 h without stirring and was then transferred to a clean polyethylene bottle for storage. Coating Preparations for the Study of Masking Agent Effectiveness. The aqueous PVA/PVP blend was prepared as described in the General Coating Preparation section above. To the polymer blend were added, in order, 40 g of glycerine, 5 g of the potential masking agent, and 0.03 g of BrPADAP (in 1 mL of absolute ethanol), followed by 30 min of additional stirring. The orange, viscous coating mixture was defoamed by allowing it to sit in the beaker for 2 h without stirring and was then transferred to a clean polyethylene bottle for storage. Surface Decontamination in the Laboratory. In a typical decontamination procedure, uranium- or plutonium-contaminated surfaces were prepared by evaporating a known amount of the corresponding oxide in 0.1 M HNO3 solution onto a variety of coupons. The coupons were weighed before and after contamination. Each contaminated coupon was analyzed via R-scintillation counting before being treated with the sensing strippable coating. All coatings were allowed to dry for at least 24 h before removal. The smart coating of the present paper displayed a vivid color change in the regions of contamination. The coatings were stripped, and the coupons were again analyzed via R-scintillation counting. Using the count rates before and after decontamination, decontamination factors (DFs) were calculated using eq 1, where R1 is the R count before decontamination and R2 is the R count after decontamination.
DF ) R1/R2
(1)
The smart coating was used to decontaminate several different types of surfaces contaminated with varying amounts of uranium. Decontamination factors (DFs) for
glass, stainless steel, aluminum, nickel, painted aluminum, and painted cement were measured. Surface Decontamination in the Field. A single technician wearing multiple layers of protective clothing and using a self-contained breathing apparatus entered the Waste Characterization, Reduction and Repackaging Facility (WCRRF) at Los Alamos National Laboratory to perform the decontamination procedure. For safety reasons, the time spent within the facility was limited to 5 min each of the two times the technician entered. Three vertical stainless steel sections (2 ft × 2 ft) were treated with the smart coating. The coating was observed using cameras located within the WCRFF. As the coated areas began to dry, color changes were observed where uranium and plutonium contamination was present. The coatings were peeled from the surfaces the following day. Swipes of the surfaces were taken in triplicate by the WCRRF personnel prior to and after decontamination and analyzed using R-scintillation counting. These measurements were used to estimate an average DF for the three treated areas. The average observed DF for the three decontaminated areas was 179. Coating Reuse. To an uncontaminated, vertical stainless steel surface was applied 100 g of the decontaminating coating. The coating was allowed to dry and was then peeled from the surface as a single sheet. The dried coating was redissolved in enough distilled water at 23 °C to provide a total solution mass of 100 g. The coating was reapplied to the stainless steel surface, allowed to dry, and again removed as a single sheet. This process was repeated a total of four times using the same coating aliquot. Results and Discussion Coating Compositions. Our coatings consist of strippable polymeric compositions containing blends of polymers, copolymers, and additives that can be brushed or sprayed onto a surface as a solution or dispersion in aqueous media. Upon drying, these coatings form strong films that can easily be peeled or stripped from the surface. When applied to a contaminated surface, these coatings display responsive behavior; areas of contamination are indicated by a color change. As the coatings dry, the contaminants are drawn into and fixed within the polymer matrix. Subsequent removal of the coating with entrapped contaminants results in some degree of surface decontamination (Figure 1). A variety of coating compositions was investigated for the decontamination of uranium- and plutoniumcontaminated surfaces. The variables for the different coatings prepared and investigated were the identity and concentration of the polymeric component(s), the concentration of the plasticizer, and the identities and concentrations of both the masking agent and the indicator used. The polymer compositions studied included aqueous solutions and/or blends of PVA, PVP, PEO, PVAm, and PEF. The specific coating components and corresponding concentration ranges studied are shown in Table 1. Coating compositions containing PEO formed flexible, strong films but did not detect the contaminant well. Although an observable color change occurred soon after treating a contaminated surface with a PEO-based coating, the color quickly faded. Coating compositions containing PVAm were unable to detect the presence
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Ind. Eng. Chem. Res., Vol. 40, No. 16, 2001
Table 1. Coating Compositions Investigated for Their Potential at Sensing and Removing Uranium and Plutonium Contaminants from Surfacesa polymeric composition PVA (5-25 wt %)
indicator composition
masking chelator
plasticizer
BrPADAP (3 × 10-3 wt %)
EDTA (0-1 wt %) DTPA (0-1 wt %) EDTA (0-1 wt %) DTPA (0-1 wt %)
glycerin (0-10 wt %) glycerin (0-10 wt %) glycerin (0-5 wt %) glycerin (0-5 wt %)
CP-III (3 × 10-3 wt %) PEO (5.5 wt %)
BrPADAP (3 × 10-3 wt %) CP-III (3 × 10-3 wt %)
EDTA (0-1 wt %) EDTA (0-1 wt %)
N/A N/A
PVP (10-32 wt %)
BrPADAP (3 × 10-3 wt %)
EDTA (0-1 wt %) DTPA (0-1 wt %) EDTA (0-1 wt %) DTPA (0-1 wt %)
glycerin (0-4 wt %) glycerin (0-4 wt %) glycerin (0-4 wt %) glycerin (0-4 wt %)
EDTA (0-1 wt %) DTPA (0-1 wt %) EGTA (0-1 wt %) TTHA (0-1 wt %) EDTA (0-1 wt %) DTPA (0-1 wt %) EGTA (0-1 wt %) TTHA (0-1 wt %)
glycerin (0-4 wt %) glycerin (0-4 wt %) glycerin (0-4 wt %) glycerin (0-4 wt %) glycerin (0-4 wt %) glycerin (0-4 wt %) glycerin (0-4 wt %) glycerin (0-4 wt %)
CP-III (3 × 10-3 wt %) PVA (10-18 wt %) and PVP (2-15 wt %)
BrPADAP (3 × 10-3 wt %)
CP-III (3 × 10-3 wt %)
a
PVA (10-18 wt %) and PEO (5-8 wt %)
BrPADAP (3 × 10-3 wt %)
EDTA (0-1 wt %) DTPA (0-1 wt %)
glycerin (4 wt %) glycerin (4 wt %)
PVA (10-18 wt %) and PVAm (5-8 wt %)
BrPADAP (3 × 10-3 wt %)
EDTA (0-1 wt %) DTPA (0-1 wt %)
glycerin (4 wt %) glycerin (4 wt %)
PVA (10-15 wt %) and PEF (4-8 wt %)
BrPADAP (3 × 10-3 wt %)
EDTA (0.5 wt %)
glycerin (4 wt %)
All coating compositions were aqueous.
Figure 1. Surface decontamination using a sensing strippable coating.
of contaminant via a color change. This was likely because of the competition between the indicator and PVAm for complexation with the metal contaminant. Coating compositions containing PEF tended to have a low wet tack, which resulted in coatings that ran when painted on vertical surfaces. In addition, coatings containing PEF tended to foam severely when applied using a power sprayer. These studies indicated that, of the systems investigated, PVA, or a blend of PVA and PVP, provided the best polymer base for the coatings used in the decontamination of uranium- and plutoniumcontaminated surfaces. The polymer PVA is a nontoxic, water-soluble material that is a good film former.27 The PVA-based coatings did not require a curing cycle, as film formation occurred via simple evaporation of water. Both cold- and hotwater-soluble coatings were prepared from PVA by varying the grade, imparting greater versatility to its use as a base for our strippable coating. Coatings with
varying physical characteristics were produced by the addition of a plasticizer and other polymers. PVA was also shown to work very well with a variety of indicators in the detection of both uranium and plutonium and was the main constituent of our smart coating for the removal of these metals from surfaces. The polymer PVP is a nontoxic, water-soluble material that was added to enhance the properties of PVA. The addition of PVP to the PVA-based compositions resulted in coatings with improved wet and dry adhesion to vertical surfaces, with reduced tendency to curl on drying and/or tear on peeling and with increased grease resistance.28 Furthermore, PVP acted as a solubilizer for the colorimetric indicator BrPADAP in PVA. The preferred coating composition contained partially hydrolyzed PVA (12 wt %) and PVP (7 wt %) in water as a coating base, with the plasticizer glycerin (4 wt %), the masking agent EDTA (0.5 wt %), and the colorimetric indicator BrPADAP (3 × 10-3 wt %), shown below. This coating composition was used in all of the studies reported here.
This smart coating exhibited different color changes for each contaminant (orange to purple for uranium and orange to red for plutonium) and was extremely effective at removing varying levels of both contaminants from different types of surfaces. Surface Decontamination. Because of the danger of working with plutonium, most of the data reported here were obtained from uranium-contaminated surfaces. Enough work was performed, however, to compare our coating with commercial coatings in terms of the
Ind. Eng. Chem. Res., Vol. 40, No. 16, 2001 3543 Table 2. Decontamination Factors for the Decontamination of a Variety of Uranium-Contaminated Coupons Using the Smart Coating surface
coating
contaminant (mg)
DF
glass glass stainless steel stainless steel stainless steel stainless steel painted Al painted Al painted cement painted cement Al Ni
smart coating smart coating smart coating smart coating smart coating ALARAa smart coating smart coating smart coating smart coating smart coating smart coating
6.7 32.4 6.4 31.3 48 28.7 17.1 41.3 15.3 38.3 17.8 9.3
1529 1395 1451 1220 813 276 completeb 959 524 418 646 487
a Alara is a latex-based commercially available strippable coating and is shown here for comparison. Note that Alara decontaminates only about 23% as well as our coating; moreover, it does not detect the presence of contaminant. b No detectable contaminant remained after decontamination.
Table 3. Decontamination of Plutonium-Contaminated Stainless Steel Coupons Using a Variety of Strippable Coatings coating used
initial radioactivity (cpm)
final radioactivity (cpm)
DF
smart coating stripcoat TLC ALARA decon
5410 11 137 14 457
36.8 2154 12 368
147 5.2 1.2
Figure 2. General uranium/plutonium decontamination process.
detection and removal plutonium from stainless steel. To measure the effectiveness of the smart coatings, a variety of uranium- and plutonium-contaminated coupons were decontaminated (Figure 2). The coupons were weighed before and after contamination. Each contaminated coupon was analyzed via R-scintillation counting and then treated with our sensing strippable coating. All coatings were allowed to dry for at least 24 h before removal. The coatings were stripped, and the coupons were again analyzed via R-scintillation counting. Using the count rates before and after decontamination, DF values were calculated using eq 1. The ability of the smart coating to decontaminate several different types of surfaces contaminated with varying amounts of uranium was studied. The results are shown in Table 2. The decontamination factors obtained for uranium on all of the surfaces studied were very high and were observed to decrease as the amount of contaminant on the surface increased. This was probably due to a conflict between the time required for the larger amounts of contaminant to permeate into the polymer and the drying time of the coatings; in other words, it is likely that the coating dried before the larger amount of uranium was drawn into it. To determine the effectiveness of our smart coating relative to that of commercially available strippable coatings for uranium decontamination, a commercial strippable coating (ALARA Decon 114B) was carried through the decontamination procedure described above. For a stainless steel coupon containing 28.7 mg of uranium contaminant, a DF of 276 was measured for the commercial coating. This value indicated that the commercial coating was only 25% as effective in uranium decontamination as the smart coating that we developed. Moreover, unlike our coating, the commercial
coating did not have the ability to detect the presence of the contaminant. The smart coating was not as effective for plutonium removal as it was for uranium (Table 3); however, it was still much more efficient than commercially available strippable coatings. Two commercially available strippable coatings, ALARA Decon 114B and StripCoat TLC, were tested for their ability to decontaminate plutoniumcontaminated stainless steel coupons using a procedure identical to that described for the smart coating. The results for these coatings are shown as the last two entries in Table 3. As can be seen from the DF values, the smart coating is far superior to the commercial coatings studied for the decontamination of plutonium from stainless steel. Field Testing of the Smart Coating for Uranium and Plutonium Decontamination. A preliminary field test of our coating was completed at the WCRRF at Los Alamos National Laboratory in Los Alamos, NM. The interior of this structure is so radioactively contaminated that technicians seldom enter. When a worker does enter, it is for very short periods of time (
