Effect of Polymer Shell Structure of a Gamma-ray Shielding Film Prepared Using Composite Material on Shielding Performance

Abstract

Medical protective clothing should be flexible to ensure free movement of healthcare personnel. This study aimed to investigate the effects of a polymer’s physical properties on the particle composition of a shielding material, constituent component miscibility, and shielding performance. To ensure flexibility by reducing the thickness of the shielding garment, polymer-based composite materials are mainly used as shielding materials. The shielding performance varies depending on whether the polymer used is in an emulsion or powder state. In this study, we found that a shielding film manufactured through an injection process after mixing a polymer in a powder form with tungsten powder exhibited 0.95%–2.5% higher shielding performance than that manufactured using the calendering process with an emulsion polymer. The shell structure formed when using the powder polymer maintains the spacing between the particles owing to the double coating of the tungsten particles and improves their dispersion. Additionally, the primary issue when combining an emulsion polymer and shielding material, that is the aggregation between the shielding material particles and between the polymer particles, could be alleviated, resulting in improved shielding performance. We concluded that the polymer-powder mixing method contributes to the reproducibility of the process technology when manufacturing shielding films.

1. Introduction

Gamma rays used in nuclear medicine at medical institutions have a higher energy intensity than X-rays. In the field of nuclear medicine, the exposure risk is high because the aprons worn by medical personnel have a shielding capacity in the range of only 0.25–0.5 mmPb, which is the same as that of existing X-ray shielding suits [1,2]. To shield against gamma rays, it is necessary to use a shielding garment with a thicker shielding fabric or higher lead content. However, an increase in the weight and/or thickness of such protective clothing limits the movement of medical personnel, making it difficult to use in practice [3]. Therefore, it is necessary to develop a new gamma-ray shielding suit with an appropriate weight and thickness and higher shielding performance.

The shielding performance of the shielding film used in radiation-shielding clothing depends on the atomic number and density of the shielding material rather than the size or shape [4]. Therefore, with regard to process technologies used to improve the density of shielding films, studies have focused on forming a dense particle structure of the shielding material and on directly increasing the content of the shielding material relative to the mixed material [5]. Tungsten, boron, barium sulfate, bismuth, gadolinium, and antimony are some of the eco-friendly shielding materials that can help reduce the exposure risk to heavy metals and deal with the disposal issue after use [6,7]. Among eco-friendly shielding materials, tungsten, which has a high density, can help control the thickness and weight of the shield, and it can be recycled, making it the most promising candidate for replacing lead [8,9,10].

In terms of radiation-shielding garments used in medical institutions, tungsten has the best shielding performance; however, in the manufacturing process, its high melting point leads to poor workability, and it is difficult to ensure the flexibility of the shielding garment. Therefore, tungsten is used in the form of a composite material in that it is mixed with a polymer material and manufactured and used as a shield in the form of a film [11,12]. Barium sulfate and bismuth oxide, which are low-density materials, are widely used as X-ray shielding materials owing to their excellent economy [13,14]. However, because the thickness or density of the shield is directly related to the shielding material content, there is a limit to reducing the weight of the shielding suit when using these materials. Therefore, the protective clothing used in medical institutions is often made of a composite material comprising high- and low-density materials. The low-density material is mainly used to shield scattered rays, which are secondary radiation [15,16].

The shielding film used in a radiation-shielding clothing improves the shielding performance by supplementing the density through thickness control in the manufacturing process. In this case, the weight of the shielding suit can be reduced by up to 10%–15%; however, there is a limit to reducing the weight through thickness control [17]. When metals (e.g., tungsten) are employed, their miscibility with the polymer material greatly enhances the shielding performance. For optimal mixing properties between the materials, the number of voids between the particles can be reduced, ensuring the effective dispersion of the shielding material in the shielding film [18,19].

A polymer material may supplement the flexibility of the shielding film. On the contrary, it may interfere with the particle dispersion of the shielding material. This phenomenon occurs during the control process of the processing technology and may occur during the mixing, thermocompression coating, or injection processes. The most representative phenomenon is the generation of pinholes due to the aggregation of the shielding and polymer materials [20]. Porous particle dispersion methods with different particle sizes and electrospinning methods used to designate the dispersed location of particles have been studied to reduce agglomeration [21,22]. Hence, fundamentally, the most important method is to improve the miscibility of the polymer and shielding material, and studies should be conducted on developing process technologies that can uniformly disperse the shielding material in the polymer instead of using the chemical bonding method, which requires additives [23,24]. The most important factor in improving the shielding performance of a protective garment is the shielding material content. In the manufacturing process of a shielding film, if the shielding material occupies a larger area than the polymer, the binding force of the polymer and the tensile strength of the shield will decrease [25]. Hence, a new dispersion technology for shielding materials is required.

In this study, a new shielding material dispersion method was developed by establishing a method to increase the shielding material content and decrease the thickness of the shielding film to improve the shielding performance. Gamma rays require a shield with a thickness or density higher than that of an X-ray shield. As the thickness or density of the manufactured shield increases, its weight also increases proportionally. Therefore, to ensure the activity of medical personnel, it is necessary to develop a lightweight shield while minimizing its thickness.

To reduce the thickness of a shield while maintaining the same shielding performance, it is necessary to increase the density by controlling the dispersion structure of the shielding material particles. This study proposes a new process technology by analyzing the difference in the shielding performance based on the polymer properties and the reasons for this difference. In addition, a method was developed for increasing the density while maintaining the tensile strength of the shield. In particular, we proposed a double coating technique that forms a shell structure on the tungsten particles via dispersion in the polymer. The polymer shell structure can help maintain a dense distribution of the tungsten particles owing to the coating of tungsten particles of the same size with multiple layers of polymer material. Manufacturing the shield this way can help prevent any tensile strength reduction of the shield when the tungsten content is high [26].

In this study, to improve the shielding performance of a gamma-ray shielding film, a polymer shell structure that can strengthen the bonding between the polymer and shielding material was imparted. The composite materials used were tungsten and polyethylene (PE). Unlike the conventional method of mixing emulsion polymers and shielding materials, we mixed the powder polymer with tungsten powder, and the differences between the two methods were compared and analyzed. To manufacture economical and mass-producible shielding films, additives, such as curing agents and bubble removers, are added to emulsion polymers. In this study, a film prepared using a thermal coating method with the conventional emulsion polymer-based shielding film and a powder polymer-based composite material were compared and analyzed. The change in the density of the shielding film with respect to the weight percentage (wt%) of tungsten in the composite material was measured, and the factors influencing the change in the shielding performance were analyzed and presented. In particular, the changes in the physical properties required for commercialization, such as the tensile and impact strengths of the shield, were compared in terms of the polymer properties. In summary, we developed a process technology that can reduce the thickness and improve the density of eco-friendly protective clothing materials that can shield against gamma rays in medical institutions.

2. Materials and Methods

The gamma-ray shields used in medical institutions are based on a theoretical background for energy attenuation. Gamma rays from a single energy source are attenuated by shielding materials according to the Beer–Lambert law, which is expressed in Equation (1) [27]. The attenuation of the transmitted gamma rays is caused by their interaction with the shielding material and is affected by the density of the shielding material, the mass attenuation coefficient, and the thickness of the shielding film.

$$I=I_0\exp \left(-\mu \rho d\right)$$

(1)

Here, $I_0$ and $I$ are the incident and transmitted intensities of the gamma rays, respectively, $\mu$ is the mass attenuation coefficient of the shielding material, $d$ is the thickness of the shield (cm), and $\rho$ is the density of the shield (g/cm³). The mass attenuation coefficient for a mixed state of the polymer and shielding materials is given by Equation (2) [28].

$${\mu }_m={\displaystyle \sum }_i{𝓌}_i{({\mu }_m)}_i$$

(2)

where ${𝓌}_i$ and ${({\mu }_m)}_i$ are the weight fraction and mass decay coefficient of the $i$-th component, respectively. Therefore, the shielding performance of a shielding film depends on the mass attenuation coefficient based on the combination of shielding materials [29]. Tungsten, which is used as a shielding material, has an atomic number of 74 and a density of 19.25 g/m³, which are sufficient to replace lead [30]. The average size of the tungsten particles used in this study was 4 µm, and they had a purity of 99.9%. A shielding film was prepared using a composite material composed of polyethylene and tungsten powders, which are polymer materials. To improve the shielding performance, a method for increasing the density by increasing the dispersing power of the internal shielding material was studied.

A film of an emulsion polymer-based composite material, which is the conventional method, and a film of a powder polymer-based composite material were prepared and analyzed for comparison. The physical properties of the shielding film produced with respect to the change in the content of tungsten as the shielding material were analyzed. In particular, the changes were compared by analyzing the tensile strength, which, in existing methods, is reduced. The cross-section of the shielding film was analyzed using a field-emission scanning electron microscope (FE-SEM, Hitachi, S-4800, Tokyo, Japan) to compare the coating state of the tungsten particles when using an emulsion polymer and a powder polymer [31].

Shielding film samples were prepared with tungsten contents of 80, 85, 90, and 95 wt%, and the amount of polymer added in the emulsion and powder forms was set the same and then compared. The tungsten particles were subjected to a pretreatment before being incorporated into the final mixtures, which were then processed through compression molding to produce the shielding films [32]. Aside from the formulation of the polymer used, the shielding films were fabricated in accordance with established manufacturing processes. The density was analyzed using the ASTM D 792-20 Test method A and the tensile strength using INSTRON 5969 to determine the change in the physical properties of the produced shielding film; the tensile speed was set to 5 mm/min in accordance with ASTM D638-14 [33]. The impact strength was tested at a temperature of 23 °C in accordance with the ASTM D 256-10 method A; complete fracture type C with an energy of 6 J [34]. The thickness of the shielding film was measured using a microgauge tester (O-21, Mitutoyo, Tokyo, Japan, 2011), and the thicknesses of the emulsion and powder polymer shielding films were compared.

For a comparative evaluation of the gamma-ray shielding performance of the shielding film, the dose of the radioactive source Co-60 (1 μCi (37 kBq)), date of manufacture (11 October 2021) was measured using a radioactivity-measuring instrument (RaySafe 442, FLUKE Biomedical, Billdal, Sweden, 2021). Figure 1 shows the measurement method. In order to eliminate the effect of external sources when measuring the radioactive source count rate of Co-60, a lead brick with a thickness of 5 cm was used to shield five sides of the detector, and the background count rate was measured. In order to reduce the measurement error of the count rate, the background count rate was measured once again after removing the source, and the count rate measurement was conducted for 1 min. Subsequently, a radioactive source was placed inside, and the count rate was measured for 1 min. The prepared shielding film was then placed in the opening above the block, and the count rate was measured again for 1 min. The count rate with all the manufactured shielding films were measured five times under the same conditions, and the average value was used. The degree at which gamma rays are absorbed while passing through the shielding film can be derived as a count rate, and this was calculated as the gamma-ray attenuation rate, as shown in Equation (3), to evaluate the shielding performance of the shielding film [35].

$$\mathrm{Attenuation} \mathrm{rate}=\left[1-\left(\frac{net counting rate 2}{net counting rate 1}\right)\right]\times 100\%$$

(3)

where the net counting rate is the measured radioactive source count rate subtracted by the background count rate.

3. Results

Eight emulsion and powder polymer films were prepared with different tungsten contents. The manufactured shielding film was in the form of a square with a side length of 300 mm, and the thickness was manufactured based on 3 mm. Figure 2 shows the manufactured shielding films.

Table 1. Comparison of the properties of shielding films in terms of the tungsten content and polymer properties.

wt%	Density (g/cm³)		Tensile Strength (MPa)		Impact Strength (J/m)		Thickness (mm)
	E-P	P-P	E-P	P-P	E-P	P-P	E-P	P-P
80	9.8 ± 0.05	12.2 ± 0.05	50.4	50.1	41.8	42.6	3.01	2.69
85	10.2 ± 0.03	13.6 ± 0.03	48.5	48.4	43.5	43.5	3.01	2.70
90	10.6 ± 0.01	13.1 ± 0.01	43.2	46.2	39.5	41.2	3.00	2.72
95	11.2 ± 0.01	14.5 ± 0.01	40.9	45.6	38.4	40.9	3.00	2.74

E-P: emulsion polymer; P-P: powder polymer.

shows the physical properties of the shielding films. The reproducibility of the shielding films was confirmed by repeating the same manufacturing process three times. Overall, the shielding film produced by the injection process employing powdered polymer exhibited higher tensile strength and density. The film thickness exhibited a difference in the range of 0.26–0.32 mm depending on the tungsten content. The shielding film using powdered polymer had better thickness compression than the emulsion polymer shielding film produced by the calendering process technology. In addition, the polymer was well dispersed between the tungsten particles inside the powder polymer-based shielding film owing to this thickness difference.

Figure 3 shows the FE-SEM measurement results, from which the miscibility of the two materials with respect to the weight percentage of tungsten powder mixed in the emulsion polymer can be inferred. Figure 3a shows a film with a tungsten content of 80 wt%, and the polymer here is generally better dispersed than that in the films shown in Figure 3b,c. However, as shown in Figure 3b,c, the polymer and shielding materials are separated to a greater extent with the increase in the tungsten content. Therefore, when manufacturing a shielding film using the conventional method, there is a limitation in that the mixing property is insufficient when the tungsten content increases under the stirring of the emulsion polymer and shielding material.

Figure 4 shows a phenomenon in which tungsten and polymer particles agglomerate separately. The film shown is in a state where 95 wt% of the tungsten powder is added to the emulsion polymer and mixed. The tungsten particles aggregated, because of which there was no polymer coating around the tungsten particles. The shielding performance of the film produced this way was high at the point where the tungsten particles agglomerated; however, pinholes were generated at the point where the polymer particles agglomerated, which may cause a problem in terms of the reproducibility of the shielding performance.

Figure 5 shows the cross-sectional comparison results of the film produced using the calendering process with the emulsion polymer and the film produced using the injection process through heat treatment with the powder polymer. The double-shell structure of the polymer material and tungsten particles could be confirmed from the particle distribution state. As shown in Figure 5a, the emulsion polymer surrounds the tungsten particles; however, the position of the tungsten particles may change during the heat treatment, making the particle spacing unstable. As shown in Figure 5b, when using a powder polymer, the dispersion of the tungsten particles is stable as the polymer is surrounded by layers. Therefore, it can be confirmed that the tungsten particles were more stably distributed in the shielding film with the powder polymer.

Figure 6 shows the comparison results of the degree of dispersion of the internal particles in the emulsion and powder polymer-based shielding films. In Figure 6a, where the emulsion polymer is used, the polymer and tungsten particles are separately aggregated. Figure 6b shows a film produced by the injection process through heat treatment after mixing the powder polymer; no aggregation of the tungsten or polymer particles can be seen. Figure 7 shows the results of the distribution of tungsten particles and the double-shell structure of the powder polymer-based shielding film. As shown, the tungsten particle spacing is maintained uniformly owing to the polymer shell structure.

Table 2. Shielding performance evaluation of manufactured shielding films.

		Background 1	Radioactive Source Count Rate	Net Counting Rate 1	Background 2	Radioactive Source Count Rate after Shielding	Net Counting Rate 2	Attenuation Rate (%)
E-P	80	40.04	3975.36	3935.32	43.60	1190.70	1147.11	70.85
P-P						1052.27	1095.87	72.15
E-P	85	44.93	3950.77	3905.83	46.68	1049.45	1096.13	71.94
P-P						1012.34	1059.02	72.89
E-P	90	45.37	3960.40	3915.03	51.93	894.89	946.82	75.82
P-P						796.66	848.59	78.32
E-P	95	47.13	3998.91	3951.78	45.04	668.23	713.27	81.95
P-P						625.01	670.05	83.04

E-P: emulsion polymer; P-P: powder polymer. The unit is cpm (counts per minute).

shows the evaluation results of the gamma-ray shielding performance of the powder and emulsion polymer-based shielding films. Following the measurement of the amount of radioactivity on the prepared shielding film for 1 min, as expected, the higher the tungsten content, the greater the shielding effect. Figure 8 shows the trend of the gamma ray shielding performance with respect to the tungsten content of the shielding film. The films with 95 wt% of tungsten showed the highest shielding performance. Therefore, with increasing tungsten content, the shielding performance increases, and the shielding performance of the powder polymer-based shielding film is higher than that of the emulsion polymer-based shielding film.

4. Discussion

Functional particle manufacturing technology through polymer coating is widely used, including in the pharmaceutical industry [36]. This technology mainly originated from studies on the functional strengthening of substances through chemical and electrical reactions. For radiation shielding, a technology capable of protecting against radiation composed of low and high energies is required. The shielding performance depends on the properties of the shielding material; however, because of the limitations of economic feasibility and manufacturing process technology, the shielding performance is generally demonstrated using composite materials. Therefore, research on increasing the density of the shield through the mixing of high- and low-density shielding materials is ongoing [37]. The polymer plays an important role in ensuring the flexibility of the shielding film, and the mixing of the polymer and shielding material affects the shielding performance depending on the process technology. This is because the mixing process directly affects the dispersion of the shielding material.

In this study, a method for increasing the density and decreasing the thickness of the shielding film through a uniform dispersion of the tungsten particles was studied. The difference between emulsion and powder polymers used in the calendering and injection process technologies was investigated. Mixing micro-sized tungsten particles with the emulsion polymer can be a process technology for uniformly distributing the tungsten particles in the polymer, which is the base material. However, this mixing method cannot be controlled externally, because of the random particle arrangement. Therefore, in this study, a powder polymer was used to further improve the uniform distribution of the tungsten particles, and the particle distribution inside the shielding film was confirmed. The FE-SEM measurement results showed the formation of a shell structure in which polymers surround the tungsten particles in multiple layers. This shell structure plays an additional role in maintaining the spacing between the tungsten particles, so that the particles can be more uniformly dispersed, and the phenomenon of aggregation between the tungsten particles, which results in pinholes, could be alleviated. The pinholes inside the shielding film had to be minimized because they reduce the internal density and hinder the absorption of gamma rays. In other words, these pinholes cause the deterioration of the shielding performance of the shielding film.

Particles of metals, such as tungsten or tin, are often used in the manufacturing of existing radiation-shielding clothing. It is difficult to uniformly disperse metal particles; therefore, the stirring process is time-consuming [38]. In addition, various additives are required, resulting in loss of time and cost. As demonstrated in this study, when a film is manufactured by an injection process using a powder polymer, it is possible to manufacture various types of shields in the future, and it is considered that time and economic costs can be reduced. Even with the same amount of shielding material, the density of the shield can be further increased, and the thickness can be reduced, thereby enabling free movement when wearing the protective clothing.

Radiation-shielding garments in medical institutions are mainly worn at all times in the form of aprons, and they weigh approximately 2.8 kg or more [39]. If there is a limit on the conditions for reducing the weight, a method for improving the shielding performance of the shielding material should be considered for various process technologies. In particular, in the case of a composite material, it was confirmed through our experiment that the miscibility of each material is important and that it can affect the stability and dispersion power of the shielding material particles.

In this study, the manufacturing process technology was improved to increase the gamma-ray shielding performance of the shielding film used as a material in radiation-shielding clothing. The shielding performance was found to be better when the shielding film was produced by the injection process rather than when using the calendering process technology which employs a casting solution to produce the shielding film, and the reasons for this difference were elaborated. This study has a limitation in that there are no reports on the effects of additives. Most of the added amounts of curing agent, stabilizer, accelerator, etc. was less than 0.1%, and these had no effect on the actual film production process; the chemical analysis is insufficient [40]. In addition, because the shielding material was used only as a single tungsten material, future research is required on the combined use of tungsten and other shielding materials.

5. Conclusions

A method for improving the shielding performance using a polymer-based composite material in the process of manufacturing materials for gamma-ray shielding clothing used in medical institutions was studied. When tungsten was used as a shielding material, the difference in the shielding performance observed between the calendering process which uses an emulsion polymer and the injection process which uses a powder polymer could be attributed to the formation of a double-shell polymer structure. In addition, the shielding performance of the powder polymer-based shielding film with such a polymer shell structure improved by approximately 0.95%–2.5%. Therefore, the powder polymer had a higher miscibility with tungsten than the emulsion polymer.