Preparation and Performance Study of Color-Indicating Traceable Water-Based Strippable Decontamination Coating

Zhiyu He, Ke Ran, Hongzhi Chen, Ziyuan Zhang, Yintao Li
School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China

Abstract

In the decommissioning of nuclear facilities and emergency response to nuclear accidents, existing decontamination materials mostly lack real-time monitoring capabilities for radioactive nuclides such as uranium, making it difficult to precisely control the decontamination process and immediately evaluate its effectiveness. To address this limitation, this study prepared a strippable decontamination material with "uranium" colorimetric tracing functionality. By introducing specific chemical groups, this material undergoes obvious color changes during uranium adsorption, enabling visual monitoring of uranium contamination. This paper investigates the relevant factors affecting decontamination and colorimetric performance through testing of the decontaminant's stress-strain behavior, acid-base environment, wettability, and surface spreading degree. The film-forming performance, decontamination efficiency, colorimetric sensitivity, and environmental stability of the material were systematically studied to verify its feasibility and advantages in practical applications.

Keywords: radioactive decontamination; strippable decontamination material; block copolymer; colorimetric tracing

Introduction

Nuclear facility decommissioning is a complex systematic engineering project with significant variations in decontamination strategies, technologies, and required timelines. Particularly for large nuclear facilities such as reactors and reprocessing plants, decommissioning work is typically conducted in phases over extended periods. Therefore, scientific and rational zoning of decommissioning areas is crucial. Typically, nuclear facility decommissioning zones are divided into three main areas: the decommissioning operation area where core activities such as nuclear facility and radioactive material cleanup, cutting, excavation, and decontamination are performed; the radioactive waste classification area; and the monitoring area requiring continuous surveillance. To meet operational requirements, the decommissioning operation area is further subdivided into controlled areas, supervised areas, and unrestricted areas [1] (as shown in [FIGURE:1]). When selecting decontamination methods, multiple factors must be comprehensively considered, including expected worker exposure dose, target decontamination levels and their measurability, feasibility of existing technologies to achieve target levels, contamination types, and source terms. Different decontamination technologies must be selected according to different objects and actual requirements. Consequently, radioactive decontamination materials, as the core means for removing radionuclides (such as uranium, cesium, strontium, cobalt, etc.), are critically important for environmental protection, human health, and sustainable development of the nuclear industry.

Compared with other decontamination methods, the strippable decontamination method offers advantages of simple operation, good decontamination effectiveness, low liquid waste volume, and non-corrosiveness to nuclear facilities and equipment, making it an economical and practical approach widely used in nuclear facility decommissioning [2-5]. Among these, radioactive strippable decontamination agents capable of colorimetric tracing enable construction personnel to simply and intuitively assess on-site nuclide contamination conditions during decontamination, allowing timely expansion or reduction of controlled area boundaries. Unlike the "point" detection of detectors and test kits, this type of decontaminant provides "area" detection applicable to large spaces for nuclide range, level, and type identification. In 2001, researchers from the University of Texas [6] prepared and studied decontamination agents with different polymer compositions, innovatively adding the color-responsive indicator 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (Br-PADAP) to enable different color changes for uranium and plutonium contamination, thereby expanding the performance and applications of strippable films. However, currently used indicators suffer from poor dispersibility and stability, tending to agglomerate in environmentally friendly water-based strippable decontamination agents, which adversely affects colorimetric and decontamination effectiveness. Furthermore, there has been no further investigation into the minimum detection limits and decontamination applications for such colorimetric traceable strippable decontamination materials. Based on these considerations, the core objective of this study is to prepare a strippable decontamination material with both high-efficiency decontamination capability and uranium colorimetric tracing function to meet the need for timely monitoring of contaminants and contamination levels during radioactive decommissioning processes. This material should not only exhibit efficient enrichment of uranium contamination but also visually reflect the decontamination process and effectiveness through color changes after uranium adsorption.

2. Instruments and Reagents

2.1 Materials

• PEA-b-PAA block copolymer
• Polyvinyl alcohol (1788)
• Polyvinylpyrrolidone
• Ethylenediaminetetraacetic acid (EDTA)
• 2-(5-bromo-2-pyridylazo)-5-(diethylamino)phenol (Br-PADAP)

2.2 Experimental Instruments

• Electric thermostatic water bath
• Electric stirrer
• Vacuum drying oven
• Universal testing machine
• Contact angle measuring instrument
• Molecular weight 40000 (from Shanghai Aladdin Reagent Co., Ltd.)
• Reagents from Chengdu Kelong Chemical Reagent Factory and Shanghai Aladdin Reagent Co., Ltd.

2.3 Preparation of "Uranium" Colorimetric Traceable Strippable Decontamination Material

1. Mechanically stir a certain amount of distilled water in a beaker, heat to 90°C, and gradually add PVA (10 wt%) with thorough mixing.
2. After all PVA is dissolved, add PVP (6 wt%) with thorough mixing and stir the mixture for 20 minutes to ensure complete dissolution.
3. Add a certain amount (3-4 g) of glycerol, 0.4 wt% EDTA, 3 × 10⁻³ wt% Br-PADAP, and a certain amount of block copolymer PEA-b-PAA, then mechanically stir for 30 minutes.

2.4 Characterization and Test Methods

1. Contact angle measurement: Apply an appropriate amount of decontamination material onto a glass slide surface and record the contact angle between the droplet and glass surface to evaluate wetting performance.
2. Tensile strength testing: Prepare test specimens according to GB/T 1040.1-20 standard and conduct tensile tests on a universal testing machine.
3. Inductively coupled plasma testing: Analyze residual simulated pollutant concentrations after decontamination as part of the decontamination testing process.
4. Color difference measurement: Analyze color differences of the cured strippable film before and after decontamination using a colorimeter, calculating color difference values through the Lab system.

3. Results and Discussion

3.1 Decontaminant Performance

2-(2-pyridylazo)-5-diethylaminophenol (PADAP) possesses a tridentate coordination site of [pyridine N]-[azo N]-[phenolic hydroxyl O] that can firmly coordinate with uranium and cause a red shift in the maximum absorption of uranium complexes, thereby deepening solution color. The aforementioned Br-PADAP introduces an electron-withdrawing bromine group at the fifth position of the pyridine ring. However, currently used indicators suffer from poor dispersibility and stability, tending to agglomerate in environmentally friendly water-based strippable decontamination materials, which adversely affects colorimetric and decontamination effectiveness. This study selected an amphiphilic diblock copolymer PEA-b-PAA, prepared through RAFT one-pot polymerization of ethyl acrylate and acrylic acid monomers, as a dispersing aid for the indicator to solve the agglomeration problem.

To investigate the influence of the functional additive amphiphilic copolymer on material properties, dispersants prepared with different monomer ratios were added at 1% mass ratio to determine the optimal ratio for performance. Since the mechanical properties of the decontaminant during curing and drying fundamentally affect its usability, comprehensive comparison and analysis of the effects on strippable coating mechanical properties were conducted to determine the optimal monomer ratio. Block copolymers with monomer ratios of 2:1, 1.5:1, and 1:1 were first selected for 1 wt% addition, and the prepared strippable films were subjected to tensile testing. As shown in FIGURE:2, the experimental groups with indicator addition generally exhibited improved elongation at break and tensile strength compared to coatings without copolymer addition, with the 1.5:1 monomer ratio copolymer showing the greatest enhancement. Combined with structural analysis of the synthesized indicator, this demonstrates that the macromolecular chains of the indicator form an appropriate crosslinking network with the substrate during film formation.

Temperature is another important factor affecting material performance during use. Therefore, the thermodynamic properties of the decontaminant were analyzed. [FIGURE:4] shows the thermogravimetric curves of indicators with different monomer ratios. As shown in FIGURE:2, the TG curves show no significant changes overall, but due to the addition of copolymers, a small amount of ethanol exists in the coating, causing weight loss to begin below 100°C. The first mass loss step (50-180°C) corresponds to removal of water, structural water, and inorganic solvents. Because these polymers are highly hydrophilic, broad melting peaks can be observed. Following the melting peaks are two main peaks related to weight loss. The second stage (200-400°C) and third stage (400-450°C) represent degradation of side groups (-OH) and structural decomposition of oligomers, respectively. Finally, the last weight loss in the 460-780°C range represents decomposition of the polymer structure, consistent with the designed structure.

Based on the above characterization, the 1.5:1 monomer ratio dispersant was selected as the additive. To determine the optimal addition amount, its influence on comprehensive performance indicators such as tensile strength, peel strength, and viscosity was investigated. Peel strength is a critical property ensuring effective contact between the cured decontaminant and object surface while enabling rapid peeling during removal. The value cannot be too high, as excessive adhesion would prevent complete stripping, nor too low, as weak bonding would make it susceptible to external influences. Therefore, based on the experimental data, addition amounts of 2%, 4%, and 6% are viable candidates. Lower viscosity facilitates decontaminant flow on object surfaces, completing wetting of contaminated areas and encapsulation of pollutants, which benefits both decontamination and pollutant response. However, excessively low viscosity is detrimental to retaining active ingredients on surfaces, affecting film thickness and thus decontamination effectiveness. Consequently, 2% and 4% are candidate ratios. Combined with the tensile performance diagram for different addition amounts shown in [FIGURE:4], the 4% addition amount provides significant improvement to tensile properties while maintaining peel strength and viscosity within reasonable ranges. Therefore, considering all factors, the optimal addition amount was determined to be 4%.

3.2 Study on Factors Affecting Decontaminant Stability

The prepared decontaminant primarily characterizes and marks pollutants through color changes resulting from complexation reactions with metal ions. During this process, the pH value of the environment affects the complexation reaction and consequently influences the colorimetric performance of the indicator. The indicator shows significant color differences under acidic and alkaline conditions, primarily attributable to the acid effect. The acid effect refers to the phenomenon where the ability of ligands to participate in the main reaction decreases due to the presence of hydrogen ions in solution. In this case, both the phenolic hydroxyl coordination site of the indicator and the hydroxyl coordination sites of ethylenediaminetetraacetic acid (EDTA) are affected by the acid effect. Specifically, in acidic environments, hydrogen ions interfere with complexation reactions between ligands and target ions, thereby affecting indicator coloration. Therefore, the colorimetric performance was analyzed by examining its behavior under different pH values.

To avoid interference from other metal ions, ammonia solution was used to adjust the pH of the system. Experimental results show that upon pollutant addition, the color of the solution system changed significantly immediately, indicating prompt and strong effects on the acid-base balance. After 30 seconds of mixing, different pH ranges exhibited distinct color change characteristics. Under strongly acidic conditions (pH 1-2), no obvious color change occurred. In solutions with pH 3-5, the orange color lightened to yellow-orange. When pH was between 6-10, the color changed from orange to purple-red. Under strongly alkaline conditions (pH 12), the color shifted from bright orange to deep purple. These pH changes essentially reflect alterations in hydrogen ion concentration, which affect solution acidity and influence indicator dissociation through the acid effect.

As pH changes alter ionic equilibrium in solution, it affects the complexation process between indicator and pollutants. In strongly acidic environments, the color reaction is inhibited. In strongly alkaline conditions, complexation stability is affected. Taking trivalent cerium ions (Ce³⁺) as an example, when Ce³⁺ in pollutants complexes with the indicator, coloration occurs first. Subsequently, Ce³⁺ reacts further with ammonia to eventually form cerium hydroxide (Ce(OH)₃) precipitate. This demonstrates that under strongly alkaline conditions, the complexation reaction between indicator and pollutants is unstable, and the final product precipitates from solution, resulting in progressively more obvious color changes.

3.3 Decontamination Performance Testing

Colorimetric decontaminants require comprehensive consideration of both decontamination and colorimetric performance. Therefore, based on colorimetric performance evaluation, the decontamination performance under different experimental conditions must be investigated.

3.3.1 Decontamination Effects on Different Object Surfaces

During nuclear facility decommissioning, large amounts of radioactive pollutants primarily exist on floors and walls within structures. To investigate the decontamination performance of this strippable material in such scenarios, experiments were designed to examine decontamination efficiency on different object surfaces and under different roughness conditions.

Experiment 1 involved large-area decontamination tests on wood flooring, steel plate, and ceramic tile surfaces. The decontamination area for each plate was set at 50 cm × 50 cm, using mixed large-particle sand as contaminant particles. Decontamination effectiveness was evaluated by comparing weight changes before and after treatment. As shown in [FIGURE:5], after curing, there was noticeable color difference between contaminated and uncontaminated areas, enabling characterization of contamination composition and range through color changes. The decontaminant formed continuous films on all surfaces and could be completely stripped in one piece after curing, with no obvious particle residue remaining on the surfaces, demonstrating effective fixation of large-particle pollutants. As shown in [FIGURE:6], decontamination rates on all surfaces exceeded 99%, indicating excellent decontamination effectiveness.

Experiment 2 investigated decontamination performance on surfaces with different roughness levels. Glass surfaces with varying roughness were prepared using different grit sandpapers, and the decontaminant's performance and colorimetric capabilities were studied. As shown in [FIGURE:7], the decontaminant formed complete films on all roughness levels and could characterize surface contamination through color changes. Furthermore, [FIGURE:8] demonstrates that decontamination rates for simulated pollutants exceeded 95% on all roughness levels. These results indicate that the decontaminant can efficiently remove loose contaminants and exhibits good adaptability across different material surfaces.

3.3.2 Simulated Radionuclide Decontamination

An ICP spectrometer was used to measure potassium ion concentrations in: (C1) gauze soak solution after wiping contaminated surfaces, (C0) gauze soak solution after wiping blank uncontaminated surfaces, (C_ash) fallout soak solution, and (C_water) distilled water. These data were combined with the decontamination efficiency calculation formula for quantitative evaluation:

Decontamination efficiency (η) calculation formula:
$$
\eta = 1 - \frac{(C_1 - C_0 - C_{\text{water}}) \times V}{3.0\,\text{mg} \times 0.2\,\text{L} \times (M_0 - M_1) \times 1000}
$$

Where:
- $M_{\text{K⁺ after decontamination}} = (C_1 - C_0 - C_{\text{water}}) \times V$ (mass of K⁺ on sample after decontamination, in g)
- $M_{\text{K⁺ in fallout}} = (C_{\text{ash}} - C_{\text{water}}) \times V$ (mass of K⁺ applied to sample, in g)
- $V$ is soak solution volume (L)
- $C_1$ is K⁺ concentration in gauze soak solution after wiping contaminated surface (mg/L)
- $C_0$ is K⁺ concentration in gauze soak solution after wiping blank surface (mg/L)
- $C_{\text{water}}$ is K⁺ concentration in distilled water (mg/L)
- $C_{\text{ash}}$ is K⁺ concentration in fallout soak solution (mg/L)
- $M_1$ is filter cloth mass after fallout application (g)
- $M_0$ is filter cloth mass before fallout application (g)

As shown in [FIGURE:10], decontamination rates in three comparative experiments on different surfaces all exceeded 90%, demonstrating that the prepared decontaminant meets requirements for effective decontamination across various surfaces. To further investigate performance, optical microscopy was used to analyze pollutant encapsulation and film formation on different surfaces. Microscopic images of cured decontaminant show that pollutants were firmly fixed on all object surfaces, with relatively smooth film surfaces and consistent overall height. Maximum surface peak values were approximately 500 μm between different components. Color changes in optical microscopy images correspond to height variations, with continuous color transitions in depth maps. The absence of abrupt color changes indicates continuous film formation with uniform thickness, ensuring uniform stress distribution during peeling and preventing fracture, thereby completing decontamination through film transfer.

3.4 Colorimetric Performance Testing

The prepared decontaminant is designed to respond effectively in nuclide-contaminated environments. To thoroughly analyze the colorimetric tracing performance of the strippable decontamination material, a series of orthogonal decontamination experiments were designed and conducted based on decontamination performance evaluation.

The experiments systematically investigated the effects of different indicator addition amounts and pollutant concentrations on color development. Using orthogonal experimental design, optimal combinations of indicator dosage and pollutant concentration were efficiently screened to optimize colorimetric performance. During the experiments, real-time observation of color changes after liquid pollutant (uranyl ion standard solution) addition was performed, with dynamic changes recorded over 5 minutes. As shown in [FIGURE:12], the decontaminant produced obvious color indication within 1 minute of pollutant presence, with color changes reaching a threshold indistinguishable to the naked eye under indoor lighting after 5 minutes. The material does not produce color changes for other metal ions, providing intuitive visual feedback for real-time decontamination monitoring.

For quantitative colorimetric evaluation, Lab color difference values were introduced as assessment criteria. Based on CIE Lab color space, this method is widely used in color measurement and quality control. According to colorimeter measurements, color differences greater than 5 are considered visible to the naked eye. However, in practical applications, color changes are often difficult to accurately distinguish when color difference values are below 10. Therefore, this study selected a color difference value of 10 as the standard for visible color difference. As shown in [FIGURE:14], most color differences achieved values above this threshold under different pollutant and indicator addition amounts.

The indicator's ability to distinguish pollutants shows significant numerical differences with increasing pollutant content. For example, with a median indicator addition amount of 0.002, the color difference values for the three pollutant content groups after decontamination were 8.06, 31.85, and 56.08, respectively—a difference of approximately 20 between groups. This demonstrates that the prepared decontaminant can effectively characterize contamination levels through color changes. Meanwhile, different indicator content groups showed small inter-group color difference gaps (mostly below 5) across different pollutant concentrations, indicating that response deviation does not occur due to indicator content variation, enabling effective pollutant characterization.

Finally, to investigate surface adaptability, colorimetric performance was studied on different material surfaces and roughness conditions. As shown in [FIGURE:15], obvious color changes were observed as the decontaminant interacted with pollutants. These changes result from complexation reactions between the indicator in the decontaminant and ionic pollutants, demonstrating that the material can detect surface contamination through color changes.

3.5 Decontamination Process Analysis

Based on the above experiments, the colorimetric decontamination process was investigated. As illustrated, when the colorimetric agent component in the decontaminant binds with pollutants, visible color changes occur, enabling characterization of surface contamination levels. Therefore, characterization of contact between decontaminant and object surfaces, as well as between cured decontaminant and pollutants, was necessary.

To deeply understand the decontamination mechanism and microscopic processes, scanning electron microscopy (SEM) was used to observe microstructural changes during actual decontamination. As shown in [FIGURE:18], during film curing, pollutants were effectively encapsulated within the formed film structure, creating a microscopic "encapsulation" that separates pollutants from contaminated surfaces. The images clearly show dense and uniform film morphology that firmly encapsulates pollutant particles, preventing their redispersion or detachment during subsequent processes.

Optical microscopy was also employed to observe changes during decontamination. As shown in [FIGURE:18], particulate pollutants were effectively encapsulated within the film structure, with dense and uniform morphology preventing redispersion or detachment. [FIGURE:19] shows optical microscopy images of film surfaces and cross-sections after decontamination on wood flooring, steel, and ceramic tiles. The decontamination material forms complete films after curing and can be stripped intact from glass surfaces in one piece. This characteristic not only proves its high efficiency in removing loose and large-area particulate contaminants but also demonstrates simple operation and easy cleanup in practical applications.

4. Conclusions

This paper investigated the colorimetric tracing performance and decontamination effectiveness under various conditions of the prepared strippable decontamination material. A series of dispersants with different monomer ratios were synthesized to study the influence of functional additives on decontaminant performance. The effects of different interface conditions on usability, decontamination performance, and curing properties were discussed. Using optical microscopy combined with wettability and contact angle changes, as well as pollutant encapsulation after curing, the colorimetric mechanism was elucidated. The flow process affecting decontaminant performance was discussed, and Lab color difference values were introduced for quantitative colorimetric evaluation. The minimum detection limit of the indicator was studied, providing important reference for sensitivity at extremely low pollutant concentrations. Experiments demonstrate that this material not only exhibits high-efficiency adsorption capacity for uranium contamination but also visually reflects the decontamination process and effectiveness through color changes after uranium adsorption.

Author Contributions: Zhiyu He and Ke Ran: research design, data analysis/interpretation, and manuscript writing; Ziyuan Zhang and Hongzhi Chen: research implementation and data collection; Yintao Li: research funding acquisition and administrative, technical, or material support.

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