Preparation and Performance Evaluation of X-ray-Shielding Barium Sulfate Film for Medical Diagnosis Using PET Recycling and Multi-Carrier Principles

Abstract

The use of disposable containers and packaging materials has increased due to the recent COVID-19 pandemic. Thus, the generation of plastic waste is also increasing, and research on recycling such waste is being actively conducted. In this study, an X-ray-shielding film for medical diagnosis was manufactured by mixing a radiation-shielding material and a plastic waste-based polymer material and its effectiveness was evaluated. The film, which is intended as a fabric for a shielding garment, consists of barium sulfate (BaSO₄) shielding nanoparticles embedded in a matrix of polyethylene terephthalate (PET), a commonly available waste plastic material. A particle-dispersing technology, which can improve the ratio between the shielding and matrix materials while maintaining the tensile strength of the film, was studied. Therefore, to increase the content of the barium sulfate (BaSO₄) nanoparticles used as the shielding material, this multi-carrier method—under which the particles are dispersed in units of time—was developed to improve the shielding performance. Compared with the effectiveness of lead (Pb) shielding film, the 3 mm barium sulfate film developed in this study satisfies the lead equivalent of 0.150 mmPb when stacked in two layers. Therefore, a shielding film was successfully manufactured by using plastic waste as a polymer resin and barium sulfate, an eco-friendly radiation-shielding material, instead of lead.

1. Introduction

Lead is the main material used in regular protective instruments and equipment for diagnostic X-ray shielding at medical institutions. However, because of its harmfulness and weight, lead is currently being replaced by eco-friendly and lightweight materials [1,2]. Barium sulfate (BaSO₄), bismuth, and tungsten are the most frequently used eco-friendly shielding materials [3]. Numerous studies have been conducted on the shielding performance and manufacturing process technology of these materials, and products using these materials are currently being used in medical institutions [4,5].

Recent research on X-ray shielding for medical diagnosis is mainly aimed at the weight reduction and mass production of radiation-shielding fabrics, which are used in protective clothing [6]. The high weight of existing shielding suits made of lead limits the mobility of medical personnel, preventing them from performing many activities. Therefore, the lead in current shielding suits is being replaced with lightweight, eco-friendly materials such as tungsten. However, tungsten is more expensive than lead, which in turn increases the cost of shields manufactured with it. A solution to this problem, other than simply changing the shielding material, is to modify its density during the shield manufacturing process to improve shielding performance [7,8]. Additionally, for mass production, a dispersion technology for shielding materials that can support the reproducibility of films’ shielding performance is important [9,10,11].

When manufacturing equipment such as shielding sheets, the composition and density of the shielding material directly affect the shielding performance of the sheets [12]. Protective garments also require flexibility; therefore, polymer materials and shielding materials are usually mixed together to acquire the desired weight and flexibility of the protective clothing. Films, sheets, and plates are also manufactured in this way [13]. Thus, the role of the polymer in the production of protective garments is absolutely essential.

Radiation-shielding films are manufactured through extrusion molding, calendering, coating, and injection molding. Additionally, a laminate processing method is used, wherein several films are laminated together [14,15,16,17]. An important problem in this process is the low reproducibility of the shielding performance of the film, which is caused by pinholes produced in the manufacturing process [18]. Pinholes are created by the heterogeneous affinity between the polymeric and shielding materials. Solutions to remove the pinholes include improving the dispersion method for the particles of the shielding material and using a polymer material with a high affinity with the shielding material [19,20].

This study aims to improve the surface modification of the particles of the shielding material, increase even particle dispersion in the polymer matrix, remove pinholes in the shield, and maintain the reproducibility of the shielding performance of the film. Additionally, it aims to improve the economic feasibility of preparing X-ray-shielding films by using eco-friendly BaSO₄ and plastic waste for the shielding and polymeric materials, respectively.

This study proposed and developed a multi-carrier process technology, which was applied to the particle dispersion technology of barium sulfate; this determined the shielding performance of the film. This technology allows particles to move in multiple directions from a high- to a low-pressure space. Using this technology, the polymer resin and shielding material particles were repeatedly mixed at a predetermined time unit to adjust the amount of the shielding material input per unit time and blend. In this case, the polymer could be adequately arranged around the nano-sized particles of the shielding material; this could play a role in reducing the pinholes inside the sheet. In addition, this study aimed to address the reduction that the shield tensile strength exhibits as a result of the thickness adjustment, and to propose a way for improving the bonding force between the polymers and the shielding material. The performance of the manufactured shielding film was compared with that of a standard lead film, and a way to maintain reproducibility through its evaluation is suggested. Considering that waste plastic is used as a polymer in the manufacturing process, a shield against low-dose radiation can be manufactured with a relatively high economic efficiency. The shield proposed in this study is manufactured with economical and eco-friendly barium sulfate, thus offering a solution to the problems of existing lead-based shields.

2. Materials and Methods

In diagnostic X-rays used in medical institutions, the intensity $I$ of the radiation emitted in a certain direction is provided by the Beer–Lambert law [21],

$$I=I_0{e}^{-{\mu }_t},$$

(1)

$$l={\mu }_t=\left(\frac{\mu }{\rho }\right){\rho }^{-1},$$

(2)

$${\mu }_t=\left(\frac{\mu }{\rho }\right)={\displaystyle \sum }_i{W}_i{(\frac{\mu }{\rho })}_i$$

(3)

where I₀ is the incident intensity, $\mu$ (cm⁻¹) is the linear attenuation coefficient, $\rho$ (g/cm³) is the density of the shielding material, and $\frac{\mu }{\rho }$ (cm²/g) is the mass attenuation coefficient. Here, $W_i$ is the i-th weight ratio of the elements constituting the shield. Therefore, a shielding film with a thickness $t$(cm) undergoes a change in strength owing to the interaction between the shielding material and the radiation during transmission $\left(l\right)$. To increase the shielding rate, it is reasonable to increase the content or thickness of the shielding material [22].

Because manufacturing a thin film was of the utmost importance in this study, a shielding film with a thickness of only 3 mm was prepared. Although this can slightly lower the shielding performance, the resulting film can be used in a multi-layered structure to secure flexibility. Polyethylene terephthalate (PET) resin, a common industrial thermoplastic waste, was used in this study [23]. PET is generally used for plastic bottles, which are easily obtained as waste [24]. The proposed manufacturing process comprised three steps. First, PET bottles were collected, washed, and dried. Then, they were subjected to flake and chip processing and mixed with barium sulfate. Finally, the resulting mixture was extruded to form a film [25]. PET was prepared in the form of granules through the flaking process, as shown in Figure 1.

BaSO₄ was selected as the shielding material for the manufacture of X-ray-shielding films. BaSO₄ is an eco-friendly material that is harmless to the human body. It has a density of 4.49 g/cm³ [26]. Producing films with a thickness as low as 3 mm causes limitations in the shielding material content injection; thus, a method to increase the density inside the film was considered. To increase the interaction probability between the incident radiation and the shielding material, it is necessary to maximize the contact area between the shielding material and the polymer resin by increasing the interfacial miscibility. Therefore, a film production technology that reduced the particle size of the shielding material and simultaneously promoted the stabilization of the resin and barium sulfate interface to increase the dispersion power was needed.

In this study, BaSO₄ nanoparticles acted as a physical barrier to prevent the self-aggregation of the PET resin. This was accomplished by developing a process technology that gradually injected the nanoparticles by controlling the amount of nanoparticles per time unit during the mixing process. The compounding process was continued for about 12 h, and BaSO₄ nanoparticles were injected at a percentage of up to 5 wt% per h. The temperature was set to 260–280 °C. These compounding conditions turn PET into a gel state that is excellent for mixing. Injecting BaSO₄ particles at 30–50 wt% improves the mixability compared to the existing process. As shown in Figure 2, the multi-carrier process technology can continuously increase the injection amount as one wall is formed, without the agglomeration of particles, which enables multi-faceted bonding between particles and polymers. The existing compounding process, which works by adding both the polymer and shielding materials simultaneously, results in lower polymer viscosity and surface activity. Maintaining constant viscosity of the polymer using inter-particle space pressure by constantly injecting the shielding material was effective [27]. In Figure 2a, it is difficult to move the particles due to the high viscosity of the polymer, but injecting BaSO₄ nanoparticles step by step while maintaining the compounding process lowers the viscosity of the polymer. As shown in Figure 2b,c, a result that can inject more particles can be expected. Therefore, when the mixing process is performed, BaSO₄ particles are dispersed due to the pressure difference according to the viscosity of the polymer, and particles are fixed by the polymer, thereby reducing interparticle aggregation of BaSO₄. This process can increase the dispersibility of the BaSO₄ particles and large amounts of shielding material. Figure 3 presents the internal structure of the shielding film using the multi-carrier phenomenon.

In this experiment, barium sulfate was added up to 30–50 wt%, and as the content was high, the tendency of the strength to weaken due to the problem of the bonding force between the polymers was evaluated. The shielding film was manufactured with a thermoplastic resin film molding machine as shown in Figure 4. The content of the shielding material of the prepared shielding film was increased by 20 wt% compared to the conventional method; as a result, the content of the polymer material was different compared to the same area, so the tensile strengths were also evaluated. For tensile strength, KS M ISO 37, a test method for medical X-ray protective aprons (KS P 6023: 2018), was applied [28]. The size of the specimen was 100 × 100 × 3 mm, and the tensile speed was set to 200 ± 10 mm/min using AGX-V (Shimadzu Corp., 2010, Tokyo, Japan), a multi-purpose testing machine. The final manufactured shielding film is shown in Figure 5.

The particle dispersibility of the shielding material was observed with a field emissions scanning electron microscope (FESEM; S-4800, High-Tech Corp, Hitachi, Japan) after thin film sectioning with a microtome (Leica, RM2235, Wetzlar, Germany). The thickness of the manufactured shielding films was 3 mm, and the shielding performance of the films was evaluated based on the lead equivalent. The shielding performance was tested by designing a multilayer structure of 1, 2, and 3 sheets each. The energy used in this experiment was produced by an X-ray generator (Toshiba E7239, 150 kV–500 mA, 1999, Tokyo, Japan) used in a medical institution. After ten experiments, the average value was calculated and applied, and the calibrated DosiMax plus 1 (IBA Dosimetry Corp., 2019, Schwarzenbruck, Germany) was used as the dose detector. The energy used in the radiation diagnosis area of a medical institution was used in this experiment, and the irradiation dose was 40–120 kVp based on the tube voltage. In addition, as shown in Figure 6, backscattering was eliminated by leaving a 700 mm space behind the detector, and the distance from the X-ray generator to the detector was set to 1550 mm [29]. The shielding performance evaluation of the film was expressed as the shielding rate and was calculated according to the equation ($1-\frac{W}{W_0}$) × 100, where $W$ is the dose measured when there is a shielding film between the X-ray tube and the dosimeter and $W_0$ is the exposure dose measured when there is no shielding film between the X-ray tube and the dosimeter.

3. Results

A thin shielding film that can be used in medical institutions was manufactured. As observed in Figure 7a, the existing injection process causes the aggregation of polymer materials. To remedy this, the shielding material must be uniformly disposed. In this study, several improvements were found by controlling the supply of the shielding material particles per hour as shown in Figure 7b. In addition, when the aggregation of the filler was prevented using the inner space of the shield, it could be dispersed more uniformly, as shown in Figure 7c. Therefore, this study developed and applied multi-carrier process technology using this principle.

Figure 8a shows a cross-sectional SEM image of the shielding film produced by applying the conventional mixing method. As observed, it can be confirmed that fine pinholes are generated, which can be caused by aggregation between the shielding material particles. Figure 8b shows a cross-sectional SEM image of the film produced using the proposed multi-carrier method. Compared to Figure 8a, the number of pinholes in Figure 8b is reduced, the distribution of particles is even, and the aggregation of polymers is slightly reduced. Therefore, as observed, applying the multi-carrier process technology can predict the distribution state of the particles by controlling the polymer through the position of the particles of the shielding material.

The shielding performance of the prepared shielding film was evaluated. The shielding performance of standard lead and the manufactured shielding film are compared and evaluated. The shielding performance test results of the lead film are shown in

Table 1. Shielding performance evaluation of standard lead film (0.1, 0.2, and 0.3 mm).

mmPb	Transmission Dose	60 kVp		80 kVp		100 kVp		120 kVp
		Non	Lead	Non	Lead	Non	Lead	Non	Lead
0.1	Dose (mSv)	0.342	0.0082	0.871	0.074	1.401	0.151	1.721	0.251
	Shielding rate (%)	-	97.6	-	91.5	-	89.2	-	85.4
0.2	Dose (mSv)	0.342	0.0021	0.871	0.034	1.401	0.086	1.721	0.145
	Shielding rate (%)	-	99.4	-	96.1	-	93.9	-	91.6
0.3	Dose (mSv)	0.342	0.00009	0.871	0.026	1.401	0.061	1.721	0.109
	Shielding rate (%)	-	99.9	-	97.01	-	95.64	-	93.66

Lead: Only lead film; Non: Initial dose; dose (mSv) uncertainty is ±0.002%.

. Based on these results, the shielding performance of the 3 mm shielding film was analyzed. The analysis revealed that for 0.3 mmPb, low-dose shielding was found to be 99.9%.

Table 2. Comparative evaluation of shielding performance by thickness when applying a multi-layer structure of X-ray-shielding film for medical diagnosis.

SF Thickness (mm)	Transmission Dose	60 kVp		80 kVp		100 kVp		120 kVp
		Non	SF	Non	SF	Non	SF	Non	SF
3	Dose (mSv)	0.342	0.083	0.871	0.249	1.401	0.445	1.721	0.601
	Shielding rate (%)	-	75.73	-	71.41	-	68.23	-	65.07
	Lead equivalent	-	0.078	-	0.078	-	0.076	-	0.076
6	Dose (mSv)	0.342	0.054	0.871	0.221	1.401	0.413	1.721	0.624
	Shielding rate (%)	-	84.21	-	74.6	-	70.5	-	63.7
	Lead equivalent	-	0.169	-	0.155	-	0.150	-	0.139
9	Dose (mSv)	0.342	0.006	0.871	0.066	1.401	0.251	1.721	0.371
	Shielding rate (%)	-	98.2	-	92.4	-	82.08	-	78.4
	Lead equivalent	-	0.198	-	0.192	-	0.175	-	0.171

SF: Shielding film; Non: Initial dose; content of barium sulfate: all are the same at 50 wt%, and the uncertainty of dose (mSv) is ±0.002%.

presents the lead-equivalent shielding performance by evaluating the performance when multiple 3 mm shielding films are stacked in a multi-layered structure. The shielding performance was further improved in the case of a multilayer structure, and using two 3 mm shielding films led to a 0.150 mmPb at 100 kVp. The dispersion of the BaSO₄ nanoparticles was stable, and the change in the shielding performance was also linear.

Table 3. Comparative evaluation of content and shielding performance according to manufacturing process technology of X-ray-shielding film for medical diagnosis.

BaSO4 (wt%)	Transmission Dose	60 kVp		80 kVp		100 kVp		120 kVp
		Non	SF	Non	SF	Non	SF	Non	SF
30	Dose (mSv)	0.342	0.133	0.871	0.398	1.401	0.815	1.721	1.059
	Shielding rate (%)	-	61.1	-	54.3	-	41.8	-	38.4
	Lead equivalent	-	0.063	-	0.059	-	0.047	-	0.045
50	Dose (mSv)	0.342	0.083	0.871	0.249	1.401	0.445	1.721	0.601
	Shielding rate (%)	-	75.73	-	71.41	-	68.23	-	65.07
	Lead equivalent	-	0.078	-	0.078	-	0.076	-	0.076

SF: Shielding film; Non: Initial dose; SF thickness: all are the same at 3 mm, and the uncertainty of dose (mSv) is ±0.002%.

summarizes the results of the comparison of the shielding performance of the films produced with the existing and proposed processes. These results were evaluated based on the difference between the process technology and the content of BaSO₄ nanoparticles. While the thickness was the same, there was a difference in the amount of BaSO₄ injected, which led to a 26.4% difference in shielding performance at 100 kVp. Therefore, the difference in the shielding performance of the film produced by the proposed technology and that by the existing process was confirmed.

Table 4. Characteristics of X-ray-shielding film for medical diagnosis.

	Density (g/cm3)	Thermal Linear Expansion Coefficient	Tensile Modulus (MPa)
Shielding Film	14.93	≈2 × 10−4	180 ± 0.5
Lead	11.34	≈29 × 10−6	17–18

presents the tensile strengths of 180 MPa for the produced shielding film, which had no significant difference from that of the existing shielding film. This indicates commercializing the manufactured film as a piece of shielding clothing is achievable.

4. Discussion

Medical personnel are exposed to varying degrees of radiation caused by the intensity and direction of radiation [30]. Direct rays act within a certain range and direction, which only affects the patient being tested [31]. The radiation exposure of workers in medical institutions is mainly affected by scattered rays. Shielding fabrics are manufactured into shields in the form of a thin film or a flexible sheet. In addition, thick plates can also be manufactured for garments requiring special strength. In the case of radiation-shielding clothing used in medical institutions, shielding performances of a minimum of 0.25 and 0.50 mmPb based on lead equivalents at 100 kVp are required. Therefore, it is manufactured in the form of sheets wherein a polymer and a shielding material are mixed. In particular, when a sheet with a performance of ≥0.50 mmPb is manufactured to shield a high dose of radiation, rubber is preferred as the mixed material to improve dispersion because of the high content of the shielding material [32].

This study was conducted with a foundationally deep interest in plastic wastes. The main components of plastic waste are polypropylene, ethylene-propylene copolymer, polystyrene, polyamide, polyethylene terephthalate, and polyvinyl chloride. Since this thermoplastic is bulky and inexpensive, it can be recycled [33]. However, in the case of thermosetting plastics, recycling is not possible. In recent years, there has been a mass production of plastic and its use has increased due to the COVID-19 pandemic. This eventually led to the problem of increased plastic waste [34].

Plastics can be recycled with eco-friendly and simple physical and mechanical methods [35]. Recycling is accomplished by collecting and washing plastic waste, converting it into flakes and granules through pulverization, and then melt processing it by drying and extrusion [36]. Therefore, if the recycled polymer resin and shielding material are mixed and processed as proposed in this study, the radiation-shielding film can be sufficiently manufactured and a shielding fabric with excellent economic efficiency can be produced.

The performance and characteristics of medical radiation shields are mostly affected by the composition of the shielding material used and the dispersion density generated when mixing with other materials [37]. Additionally, the shields should be lightweight and made of thin films, as they are mainly used for manufacturing protective clothing. Therefore, in this study a process technology was developed that increases the density in the polymer by processing the shielding material to nanosized particles. In previous studies, the content of the shielding material was approximately 5 wt% and 30 wt% for yarn and sheets, respectively [38]. This is because the properties of the particles were not considered. Currently, numerous studies are being conducted on process technologies involving the nanoparticle characteristics of tungsten particles in radiation shields [39].

The distribution of nanoparticles is determined at the processing stage, and the Brownian motion, which determines the distribution of the thermodynamic equilibrium state of these nanoparticles, tends to be inversely proportional to the viscosity [40]. Therefore, it is difficult to control the particle distribution of the shielding material because the polymer resin in the process step has a high viscosity and the processing time is very short. Rather than using thermodynamic calculations, it is necessary to study a method that can increase the binding force between the polymer and shielding material in the mixing procedure among the process technologies and apply a multi-carrier technology that mixes the nanoparticles with a time difference. Multi-carrier technology, which is used for blending the same polymer base with gradual pressure, increased the content of the shielding material by 20 wt% or more compared to the existing shielding material; the shielding performance was also improved.

The particle distribution of the shielding material inside the shield is essential; the uniform distribution of the shielding material and the amount of the shielding material input are separate issues [41]. Even when the same amount of shielding material is injected, errors may occur in shielding performance during mass production due to particle aggregation. In using shielding fabric for shielding clothing, it is important to control the amount of and uniform dispersion of the shielding material, which will increase the value of the product.

In this study, a 3 mm-thick shielding film for medical X-ray shielding was manufactured using barium sulfate, which is a cheaper alternative to shields made of plastic waste polymer and tungsten or lead, thus securing economic efficiency. In addition, because barium sulfate is an eco-friendly material, it is possible to solve environmental problems such as those caused when disposing of existing lead-based shields [42].

Furthermore, the 3 mm-thick shielding film can be used as a multi-layer structure to secure the flexibility of the shield, and when used as part of a multilayer structure, its shielding performance can be further adjusted as a function of the thickness. Additionally, it was suggested that using two films at 100 kVp for medical X-ray had the same effect that 0.150 mmPb did. Therefore, it is believed that the proposed technology in this study can support the mass production of eco-friendly and recyclable plastic waste shields.

5. Conclusions

In this study, economic efficiency was secured by manufacturing an X-ray-shielding film for medical diagnosis using waste-based PET. Nanoparticles of barium sulfate, an eco-friendly shielding material, were used as the shielding material, and multi-carrier process technology was applied to evenly disperse the particles in the polymer material. This process could increase the content of the shielding material by 20 wt% compared to that of the conventional mixing process. Laminating two shielding films together resulted in a shielding performance of 0.150 mmPb. These results show that the mass production of eco-friendly shielding films for manufacturing X-ray-shielding clothing for medical applications is achievable.