Non-Toxic and Flexible Radiation-Shielding Composites Based on Natural Rubber Containing Elemental W Fillers for Efficient Shielding against X/γ-rays

Abstract

Conventional radiation-shielding composites usually contain lead, which results in high toxicity and poor portability. Tungsten (W) is an ideal radiation-shielding element that can replace lead due to its high atomic number and non-toxicity. In this work, radiation-shielding composites were prepared using natural rubber (NR) as a matrix and three different particle size powders, namely W, WO₃ and WC, as fillers. The results show that, for X-rays, the linear attenuation coefficient of radiation-shielding composites based on natural rubber containing WC with a particle size of 50 μm (50 WC/NR) is 27.005 cm⁻¹ at an X-ray tube voltage of 40 kV, which is more than 14 times the linear attenuation coefficient of NR. For γ-rays, a linear attenuation coefficient of 50 WC/NR achieves 8.320 cm⁻¹ at 81 keV, which is over 55 times the linear attenuation coefficient of NR. In addition, 50 WC/NR had the highest elongation at break at 548.989% and had the lowest hardness at 62 HA. In summary, 50 WC/NR can be used as an alternative to traditional radiation-shielding materials containing lead and has wide application prospects.

1. Introduction

In the 21st century, nuclear radiation leaks occurred at both the first and second nuclear power plants in Fukushima Prefecture, Japan. The environmental contamination caused by nuclear radiation leaks spreads over time, and the staff involved must access the area of the leak to deal with the contamination [1]. Moreover, with the development of nuclear science and technology, nuclear radiation has been gradually applied in aerospace, industry, agriculture, military, medical diagnosis and medical treatment [2,3,4,5,6,7], which is also increasing the risk of nuclear radiation leaks in related areas. Therefore, the prevention and shielding of nuclear radiation have aroused worldwide concerns. The two most common types of nuclear radiation are X-rays and γ-rays, which can cause great harm to the human body [8]. In order to avoid excessive nuclear radiation damage to humans, various radiation-shielding composites against X/γ-rays have been manufactured [9,10,11].

Conventional X/γ-ray-shielding composites include concrete, ceramics, glasses and polymers containing lead or lead oxide [12,13,14]. Radiation-related staff are usually equipped with radiation-protective clothing and blankets containing lead. However, these radiation-protective devices and equipment are usually toxic and heavy [15]. Typically, radiation-shielding composites with polymer matrices and high-atomic-number fillers possess a light weight, good elasticity and good processing properties [16]. The RST company in the USA developed a γ-ray-shielding composite containing tantalum fibers with a shielding efficiency of 50% under radiation conditions of 130 KeV [17]. Due to its high atomic number of 74, tungsten (W) has become an ideal elements for radiation-shielding, and many studies have used monomers or compounds containing elemental W fillers as functional components. Currently, the main W-containing substances are monolithic tungsten (W), tungsten oxide (WO₃) and tungsten carbide (WC). Chang et al. reported that the shielding properties of W/epoxy resin increase with increases in W content [18]. Kim et al. showed that the attenuation of nano-W/PE is enhanced by 75% over micro-W/PE with Ba-133 (~0.3 MeV) as a radiation source [19]. Dong et al. showed that the shielding performance of nano-tungsten trioxide (WO₃) in an epoxy-resin-based radiation-shielding composite is superior to that of micro-WO₃ [20]. Aee et al. showed that the 10WO₃-10MoO₃-80TeO₂ glass material has better shielding properties and high transparency compared to other lead-containing glasses [21]. Fine-grained tungsten carbide (WC) is usually prepared with the solid-phase reaction method [22]. Soylu et al. showed that the shielding efficiency of tungsten carbide-vinyl acetate (WC-EVA) is higher than that of lead [23]. However, the shielding properties of radiation composites containing different types of elemental W fillers vary considerably.

Lead-free rubber polymers have attracted a lot of attention from researchers for their high flexibility and ease of use [24]. NR has been widely used in the production of daily necessities, medical industry production and industrial and agricultural production. Radiation-shielding composites using NR as a polymer matrix have excellent properties, such as high elasticity and a light weight [25]. Gwaily et al. studied the radiation resistance of NR containing varied concentrations of lead powder [26]. El-Khatib et al. prepared two types of radiation-shielding materials based on waste NR, i.e., Pb/W/NR and Pb/NR, and found that Pb/W/NR has a better γ-ray-shielding ability [27]. Nattha et al. found that new shielding materials based on NR/barium sulphate (BaSO₄) nanocomposites are flexible, lightweight and lead-free, with promising applications in the development of potential radiation-shielding materials [28].

Despite great efforts, the effects of types of elemental W fillers on the properties of NR-based radiation-shielding composites remain elusive. In this study, radiation-shielding composites were prepared by mechanically doping W, WO₃ and WC fillers with different particle sizes using NR as a matrix. The mechanical and shielding properties against X/γ-rays of radiation-shielding composites containing elemental W fillers were systematically analyzed. This work provides valuable insights into the influence mechanism of elemental W fillers on the properties of radiation-shielding composites, shedding light on the development of non-toxic and flexible radiation-shielding composites toward nuclear radiation.

2. Materials and Methods

2.1. Materials

Natural rubber (NR, Vietnam Dajingbei 3L) was purchased from Shanghai Fuyou International Trading Co., Ltd. (Shanghai, China). Sulfur (S), 2-mercaptobenzothiazole (M), 2,2’-dithiodibenzothiazole (DM), 1,3-diphenylguanidine (D), N-cyclohexylbenzothiazole-2-sulfenamide (CZ), anti-2,6-di-tert-butyl-4-methylphenol (antioxidant 264), zinc oxide (ZnO) and stearic acid (SA) were provided by Beijing Excellence Farhang Technology Co., Ltd. (Beijing, China). Carbon black (CB, N330) and calcium carbonate (CaCO₃) were provided by Beijing Huawei Chemical Co., Ltd. (Beijing, China). Tungsten (W), tungsten oxide (WO₃), tungsten carbide (WC) and other ingredients (industrial grade) were obtained from Beijing Research & Development Co., Ltd. (Beijing, China).

2.2. Sample Preparation

The formulation of NR and radiation-shielding composites are shown in

Table 1. Formulation of NR and radiation-shielding composites.

Samples	NR (g)	CaCO3 (g)	CB (g)	Accelerator (g)	Activator (g)	Antioxidant (g)	Fillers (g)	S (g)
NR	100	50	15	1.5	7	1	0	1.5
50 W/NR	100	50	15	1.5	7	1	100	1.5
(2~3) W/NR	100	50	15	1.5	7	1	100	1.5
50 WO3/NR	100	50	15	1.5	7	1	100	1.5
(2~3) WO3/NR	100	50	15	1.5	7	1	100	1.5
50 WC/NR	100	50	15	1.5	7	1	100	1.5
(2~3) WC/NR	100	50	15	1.5	7	1	100	1.5

. The samples were processed in three stages: plasticization, mixing and flat vulcanization. In the first stage, 100 g of elastic NR was converted into plastic NR using an internal mixer. In the second stage, 100 g of plasticized NR, 50 g of CaCO₃, 15 g of CB, 1.5 g of accelerator (a mixture of M, DM, D and CZ), 7 g of activator (a mixture of ZnO and SA), 1 g of antioxidant 264 and 100 g of fillers (no fillers were added to the blank group) were added sequentially to the internal mixer and mixed for 10 min to obtain unvulcanized rubber. An amount of 1.5 g of S was added to unvulcanized rubber through a roll mill. In particular, for the same type of elemental W fillers, two particle sizes of fillers were intermixed into the NR. For example, 50W/NR indicates that 50 μm W fillers were incorporated into NR, and (2~3) W/NR indicates that (2~3) μm W fillers were incorporated into NR. In the third stage, unvulcanized rubber was vulcanized using the curing press at 143 °C and 20 MPa, and the plate vulcanization time was t₉₀ (obtained by testing with a Rubber Processing Analyser). The resulting samples had a length of 12.8 mm and a width of 10.8 mm. The thickness of samples for this experiment was above 1 mm. We tested the homogeneity of our samples at four sampling points, and the results show that the relative deviations of the samples’ thicknesses are all within ±3%.

2.3. Characterization

Minimum elastic torque (ML), maximum elastic torque (MH), vulcanization characteristic parameters t_s1 (indicating the time corresponding to a 0.1 N·m increase in minimum torque) and t_s2 (indicating the time corresponding to a 0.2 N·m increase in minimum torque), scorch time (t₁₀) and positive vulcanization time (t₉₀) for unvulcanized rubber were tested with a Rubber Processing Analyser (Premier RPA 93001, Alpha Technologies, Columbus, OH, USA).

A scanning electron microscope (SEM, Gemini 300, Oberkochen, Germany) was used to characterize the microscopic morphology of samples.

The thermal weight loss curve TG of samples was tested under an air atmosphere at a temperature range of 30 °C to 100 °C with a temperature rise rate of 10 °C/min. The thermogravimetric analyzer was simultaneously a thermal analyzer (STA 2500 Regulus, Selb, Germany).

The tensile strength and elongation at break of the samples were measured using an electronic universal testing machine (WDT-W-20A, Shanghai Rutting Education Technology Co Ltd, Shanghai, China) according to the ASTM standard testing method. Samples were pressed into a dumbbell shape, and the gripper speed of the electronic universal testing machine was set at 500 mm/min. The hardness of samples was measured with a Shore A durometer according to the standard ASTM D2240 testing method. When testing hardness, samples were stacked with a total thickness of greater than 6 mm [29].

According to the standard GBZ/T 147-2002 “Determination of the attenuation properties of X-rays protective materials”, a portable lanthanum bromide γ-spectrometer (ORETC digiBASE-E, Ametek Trading (Shanghai) Ltd, Shanghai, China) was used to measure the X-ray dose (K₀) for the no-shielding samples and the X-ray dose (K₁) for the shielding samples. The schematic diagram of the X-ray-shielding properties test is shown in Figure 1.

A portable high-purity germanium γ-spectrometer (Detecticve-100T, Ametek Trading (Shanghai) Ltd, Shanghai, China) was used to test the γ-ray-shielding properties of the samples. Am-241 and Ba-133 were used as the radioactive sources in a lead chamber with a circular opening of 4 cm in diameter on the side facing the detector. These two radionuclides have energies of 59 keV and 81 keV, respectively. The radioactive sources were placed 20 cm from the samples, and the samples were placed 3 cm from the portable high-purity germanium γ-spectrometer. The schematic diagram of the γ-ray-shielding properties test is shown in Figure 2.

3. Results and Discussion

3.1. SEM Analysis of Composites

SEM images of the radiation-shielding composites are shown in Figure 3. The black background is NR, and the light white particles are the fillers. As shown in Figure 3a,c,e, the 50 μm fillers were clearly visible in the NR. As shown in Figure 3b,d,f, the (2~3) μm fillers were not easily found in the NR. However, as can be seen from the red boxes in Figure 3d,f, NR containing (2~3) WO₃ and (2~3) WC had more visible particles, even though the particle size of these fillers was only (2~3) μm. The main reason is that (2~3) WO₃ and (2~3) WC were agglomerated in the NR, forming larger particles. Overall, however, both the 50 μm fillers and (2~3) μm fillers were evenly distributed in the NR and theoretically would not have a great effect on the mechanical properties of the composites.

3.2. Processing Properties of Composites

The Rubber Processing Analyser can be used to analyze the processing properties of rubber. ML is mainly used to evaluate the flow of rubber in the mold cavity, viz., as the ML becomes smaller, the flow and processing properties of the rubber become better. MH is used to characterize the shear modulus or hardness of the rubber and is related to the elongation stress or crosslink density of the rubber, viz., as the MH becomes higher, the elongation stress and crosslink density also become higher. t_s1 and t_s2 are key parameters for measuring the vulcanization speed of rubber, viz., as t_s1 and t_s2 become lower, the vulcanization speed becomes faster. t₁₀ is the scorch time as measured by the Rubber Processing Analyser, viz., as the t₁₀ becomes higher, the processing safety of the rubber also becomes higher. t₉₀ is the process positive vulcanization time as measured by the Rubber Processing Analyser and can be used as the actual vulcanization time for unvulcanized rubber when the thickness of the sample is less than 6 mm. The processing properties of NR and the radiation-shielding composites are shown in

Table 2. Processing properties of NR and radiation-shielding composites.

Samples	ML (N·m)	MH (N·m)	ts1 (min)	ts2 (min)	t10 (min)	t90 (min)
NR	1.62	27.30	0.62	0.64	0.95	2.35
50 W/NR	1.48	29.32	0.66	0.69	1.00	2.29
(2–3) W/NR	1.10	29.59	0.66	0.68	1.01	2.57
50 WO3/NR	1.87	29.75	1.10	1.17	1.97	4.15
(2–3) WO3/NR	1.33	31.77	0.83	0.87	1.41	3.09
50 WC/NR	1.40	30.69	0.69	0.72	1.06	2.72
(2–3) WC/NR	1.49	31.02	0.69	0.72	1.09	2.82

.

It can be seen from Table 2 that composites with the addition of elemental W fillers had smaller ML than NR, except for 50WO₃/NR, indicating that the addition of elemental W fillers was beneficial to the processing of composites. The ML of (2~3) W/NR was 1.10 N·m, which had the best processing performance, and it was easier to mold and could more easily eat powders in later processing than the other samples. The MH value of all composites with elemental W fillers was larger than that of NR. The MH of (2~3) WO₃/NR was 31.77 N·m, indicating that it had higher elongation stress and crosslink density. The t_s1 and t_s2 of composites with the addition of elemental W fillers were larger than those of NR, indicating that the vulcanization speed of composites slowed down with the addition of functional fillers, and the vulcanization time t₉₀ of composites increased. The main reason is that the addition of inorganic fillers to NR prevented the cross-linking of molecular chains of NR, ultimately leading to longer vulcanization time for composites. However, the increase in botht t₁₀ and t_90he of composites suggests that the addition of elemental W fillers improved the safety of processing composites, but they also increased the vulcanization time of the composites.

3.3. Thermal Stability of Composites

As shown in Figure 4a,b, when the temperature reached 199.6 °C, the rate of decomposition of NR gradually increased. At 30 °C–298.7 °C, the mass loss of NR was approximately 3.7%. At 298.7 °C–581.4 °C, NR started to burn itself in this stage. Decomposition was fastest when temperature reached 364.9 °C and slowed down at 400.2 °C–427.1 °C. In this temperature range, the mass loss of NR was about 51.6%. At 581.4 °C–780.1 °C, CaCO₃ in NR decomposed into CO₂, and CB started to burn, with a mass loss of approximately 13.1% of NR at this temperature range. At 780.1 °C–1000 °C, only inorganic residues remained in NR, with no further change in mass and a percentage of residues of 31.6%.

The initial decomposition temperature of T_i indicates the temperature corresponding to 5% weight loss of the composite; therefore, the magnitude of the initial decomposition temperature was used in this study to characterize the thermal stability of composites [30]. As shown in Figure 4a, the Ti of the radiation-shielding composite ranged from 342.5 °C to 353.3 °C. The thermal stability of the radiation-shielding composites containing different kinds of fillers did not differ much.

In addition, radiation-shielding composites containing different particle sizes of W and WC showed a curve section with a mass increase after 500.2 °C due to the formation of new fillers during the heating process. The main reason for this is the reaction of the two particle sizes of W and WC with oxygen in the air, forming oxides of the elemental W fillers. However, composites containing different particle sizes of WO₃ did not show an increase in mass at temperatures below 500.2 °C. The different particle sizes of WO₃ did not react with oxygen, and their properties remained essentially stable. The composites containing WO₃ did not produce new fillers during heating, which indicates that only WO₃ did not react with oxygen in the air. In general, NR containing elemental W fillers met the basic requirements for radiation-shielding composites in terms of thermal stability, with fillers in composites remaining stable at higher temperatures.

3.4. Mechanical Properties of Composites

The values for the mechanical properties of NR and radiation-shielding composites are shown in

Table 3. Mechanical properties of NR and radiation-shielding composites.

Samples	Tensile Strength (MPa)	Elongation at Break (100%)	Shore Hardness (HA)
NR	15.575 ± 0.739	551.523 ± 19.909	57
50 W/NR	11.184 ± 0.481	519.285 ± 7.440	62
(2~3) W/NR	10.889 ± 2.484	508.819 ± 77.506	62
50 WO3/NR	6.068 ± 1.127	408.406 ± 44.032	71
(2~3) WO3/NR	6.955 ± 0.315	450.500 ± 32.565	68
50 WC/NR	10.558 ± 0.398	548.989 ± 2.772	62
(2~3) WC/NR	9.998 ± 0.131	481.223 ± 32.702	65

, and Figure 5 provides a direct comparison of the differences between the mechanical properties of the different samples. As shown in Figure 5a, the tensile strength and elongation at break of NR were 15.575 MPa and 551.523%, respectively. After the addition of fillers, the tensile strength and elongation at break of NR radiation shielding decreased compared to NR. The main reason is that inorganic fillers were incorporated into the molecular chains of NR, and fillers led to stress concentrations in the composites when they were subjected to tensile forces. Ultimately, composites were vulnerable to damage in localized areas [27,28,29]. Among radiation shielding, 50W/NR had the maximum tensile strength of up to 11.484 MPa and elongation at break of up to 519.285%. In addition, 50 WC/NR had the maximum elongation at break of up to 548.989% and tensile strength of up to 10.558 MPa. The main reason is that the high density of W and WC resulted in a small volume share of NR for the same mass conditions, which had little effect on the mechanical properties of the radiation-shielding composites. It can be seen that 50 W and 50 WC added to NR had better mechanical properties.

For radiation-shielding composites containing different particle sizes but the same type of fillers, the tensile strength and elongation at break of 50 W/NR and 50 WC/NR were greater than those of the corresponding (2~3) W/NR and (2~3) WC/NR, respectively. However, the tensile strength and elongation at break of (2~3) WO₃/NR were greater than those of 50 WO₃/NR. For both W and WC, the tensile properties of the radiation-shielding composites were worse with the addition of (2~3) μm of fillers. The main reason may be that, as the particle size of the fillers became smaller, the fillers’ agglomeration became more serious, resulting in the formation of filler particles with a larger diameter. In contrast, for WO₃, the tensile properties of radiation-shielding composites were good with the addition of (2~3) μm fillers. The main reason may be that the O atoms in WO₃ formed chemical interactions with NR molecules, allowing (2~3) WO₃ to bond more tightly with NR molecules and allowing fewer defects to appear, hence the good tensile properties of the radiation-shielding composites containing WO₃ with a small particle size. Therefore, the particle size and type of different fillers had an effect on the mechanical properties of radiation-shielding composites.

As shown in Figure 5b, the hardness of NR was 57 HA, and the hardness of the radiation-shielding composites with elemental W fillers were higher than that of NR. The main reason for this is that the fillers containing elemental W were dispersed into the NR, forming a tight three-dimensional mesh structure. As a result, the external pressure was more easily dispersed within the NR, thus increasing the hardness of the radiation-shielding composites [31].

For radiation-shielding composites containing the same type but different particle sizes, they did not differ much in hardness. Of these, 50 W/NR, (2–3) W/NR and 50 WC/NR had a hardness of 62 HA, which was closest to that of NR, so radiation-shielding composites containing W or WC fillers would be softer. Moreover, 50 WO₃/NR and (2–3) WO₃/NR had a hardness of 72 HA and 68 HA, respectively. Therefore, the radiation-shielding composites containing WO₃ were harder than NR, and they would not be suitable as radiation-shielding composites. The reason for this phenomenon is that the density of WO₃ was less than that of the corresponding particle sizes of W and WC, so the radiation-shielding composites containing WO₃ fillers had a greater volume share of inorganic fillers, resulting in greater hardness.

3.5. X-ray-Shielding Properties of Composites

The attenuation properties of X-rays can be measured by the attenuation efficiency (AE₁), the linear attenuation coefficient (μ₁) and the mass attenuation coefficient (μ_m1), which can be expressed simply by the following equation.

$$A{E}_1 = (K_0 - K_1)/K_0$$

(1)

$$K_1 = K_0{e}^{-{\mu }_{1}d} = K_0{e}^{-{\mu }_{m1}\rho d}$$

(2)

where K₀ and K₁ are X-ray dose for the no-shielding composites and the X-ray dose for the shielding composites, respectively, ρ is the density of composites, and d is the thickness of composites. μ₁ is the linear attenuation coefficient, which depends on the energy (in this case, the tube voltage), the effective atomic number and the density ρ. μ_m1 is the mass attenuation coefficient, which depends on the energy and the effective atomic number [32].

The X-ray attenuation efficiencies of the NR and radiation-shielding composites for different tube voltage conditions are shown in Figure 6. As can be seen from Figure 6, the attenuation efficiency of NR at different tube voltages was much lower than those of the radiation-shielding composites. The (2~3) W/NR had the highest attenuation efficiency at different tube voltages, which reached (4–10) times that of NR. In order to exclude the influence of the thickness and density of the composites, we compared the linear and mass attenuation coefficients of NR and radiation-shielding composites at different tube voltage conditions.

Table 4. X-ray linear attenuation coefficient of NR and radiation-shielding composites.

X-ray Linear Attenuation Coefficient (cm−1)	NR	50 W/NR	(2~3) W/NR	50 WO3/NR	(2~3) WO3/NR	50 WC/NR	(2~3) WC/NR
40	1.814 ± 0.103	18.699 ± 0.243	23.758 ± 0.266	18.899 ± 0.407	18.397 ± 0.091	27.005 ± 0.308	22.711 ± 0.113
60	0.951 ± 0.000	9.034 ± 0.113	10.255 ± 0.015	8.163 ± 0.209	7.976 ± 0.079	12.056 ± 0.268	9.976 ± 0.029
80	0.503 ± 0.029	5.305 ± 0.168	5.912 ± 0.138	4.727 ± 0.107	4.737 ± 0.052	7.088 ± 0.111	5.660 ± 0.044
100	0.261 ± 0.047	6.781 ± 0.075	7.603 ± 0.084	5.956 ± 0.103	5.823 ± 0.083	8.833 ± 0.098	7.290 ± 0.081
120	0.254 ± 0.009	5.182 ± 0.082	5.677 ± 0.071	4.340 ± 0.076	4.272 ± 0.015	6.578 ± 0.105	5.450 ± 0.052
150	0.240 ± 0.000	3.916 ± 0.060	4.232 ± 0.075	3.265 ± 0.058	3.201 ± 0.000	4.961 ± 0.064	4.071 ± 0.010

and

Table 5. X-ray mass attenuation coefficient of NR and radiation-shielding composites.

X-ray Mass Attenuation Coefficient (cm2/g)	NR	50 W/NR	(2~3) W/NR	50 WO3/NR	(2~3) WO3/NR	50 WC/NR	(2~3) WC/NR
40	0.959 ± 0.054	7.593 ± 0.099	10.189 ± 0.114	8.510 ± 0.183	8.376 ± 0.041	10.466 ± 0.119	9.011 ± 0.045
60	0.503 ± 0.000	3.668 ± 0.046	4.398 ± 0.006	3.676 ± 0.094	3.631 ± 0.036	4.672 ± 0.104	3.958 ± 0.011
80	0.266 ± 0.016	2.154 ± 0.068	2.535 ± 0.059	2.129 ± 0.048	2.156 ± 0.024	2.747 ± 0.043	2.246 ± 0.018
100	0.138 ± 0.025	2.753 ± 0.031	3.261 ± 0.036	2.682 ± 0.046	2.651 ± 0.038	3.423 ± 0.038	2.892 ± 0.032
120	0.135 ± 0.005	2.104 ± 0.033	2.435 ± 0.030	1.955 ± 0.034	1.945 ± 0.007	2.549 ± 0.041	2.163 ± 0.021
150	0.127 ± 0.000	1.590 ± 0.024	1.815 ± 0.032	1.470 ± 0.026	1.457 ± 0.000	1.923 ± 0.025	1.615 ± 0.004

represent the X-ray linear attenuation coefficients and mass attenuation coefficients of radiation-shielding radiation composites, Figure 7 is a graph corresponding to Table 4 and Table 5. As shown in Table 4 and Table 5 and Figure 7a,b, the linear and mass attenuation coefficients of 50 WC/NR were the highest for different X-ray tube voltage conditions. At an X-ray tube voltage of 40 kV, the linear and mass attenuation coefficients of 50 WC/NR reached 27.005 cm⁻¹ and 10.446 cm²/g, respectively. Compared to NR, the linear and mass attenuation coefficients of 50 WC/NR were more than 14 times and 10 times higher, respectively. At an X-ray tube voltage of 150 kV, the linear and mass attenuation coefficients of 50 WC/NR were 4.961 cm⁻¹ and 1.923 cm²/g. Compared to NR, the linear and mass attenuation coefficients of 50 WC/NR were more than 20 times and 15 times higher, respectively. The addition of 50 WC to NR exhibited excellent X-ray-shielding properties due to high proportion of W elemental and homogenous distribution. Therefore, considering the thickness and quality of composites, in this work, 50 WC/NR could be the best X-ray-radiation-shielding composite.

The half-value layer (HVL) and lead equivalent are important parameters for measuring the shielding performance of radiation-shielding composites, and the calculation of the HVL and lead equivalent can be found in Ref. [32].

As shown in Figure 8a, the half-value layer of 50WC/NR at 120kV was only 0.105 cm, which was lower than the half-value layer of NR/Bi₂O₃; therefore, the X-ray shielding effect of 50 WC/NR was better than that of NR/Bi₂O₃. Compared to the half-value layer of commercial glass and X-ray glass (>0.2 cm), the half-value layer of all radiation-shielding composites was much lower than the above two types of glass [33]. As shown in Figure 8b, all radiation-shielding composites had the highest lead equivalent at 100 kV, which shows that radiation-shielding composites were the most effective at shielding X-ray energy at 100 kV.

3.6. Gamma-ray-Shielding Properties of Composites

The attenuation properties of Gamma-rays (γ-rays) can be also measured by the attenuation efficiency (AE₂), the linear attenuation coefficient (μ₂) and the mass attenuation coefficient (μ_m2), which can be expressed simply by the following equation.

$$A{E}_2 = (I_0 - I)/I_0$$

(3)

$$I = I_0{e}^{-{\mu }_{2}d} = I_0{e}^{-{\mu }_{m2}\rho d}$$

(4)

where I₀ and I are the intensity of the incident and transmitted radiation, respectively [29].

The ρ and d of samples for testing γ-rays are the same as those for testing X-rays. As shown in Figure 9, the radiation-shielding composites all showed a substantial increase in γ-ray attenuation efficiency compared to that of NR. In particular, the shielding rate of (2~3) W/NR was 44.9% at 59 keV and 62.8% at 81 keV, which was the highest of all radiation-shielding composites with elemental W fillers.

The experimental and WinXCom values of the γ-ray linear and mass attenuation coefficients for NR and radiation-shielding composites are shown in

Table 6. γ-ray linear and mass attenuation coefficient of NR and radiation-shielding composites.

Samples	γ-ray Linear Attenuation Coefficient (cm−1)		γ-ray Mass Attenuation Coefficient (cm2/g)
	59 keV	81 keV	59 keV		81 keV
	Experiment	Experiment	Experiment	WinXCom	Experiment	WinXCom
NR	0.490 ± 0.120	0.149 ± 0.070	0.377 ± 0.092	0.166	0.114 ± 0.054	0.155
50 W/NR	4.287 ± 0.049	6.482 ± 0.289	1.787 ± 0.020	1.372	2.702 ± 0.120	2.899
(2–3) W/NR	4.557 ± 0.025	7.562 ± 0.005	1.844 ± 0.010	1.372	3.061 ± 0.002	2.899
50 WO3/NR	3.401 ± 0.042	5.421 ± 0.045	1.518 ± 0.019	1.135	2.420 ± 0.020	2.336
(2–3) WO3/NR	3.505 ± 0.035	5.484 ± 0.180	1.548 ± 0.015	1.135	2.421 ± 0.080	2.336
50 WC/NR	5.503 ± 0.187	8.320 ± 0.241	2.209 ± 0.075	1.302	3.340 ± 0.097	2.732
(2–3) WC/NR	4.504 ± 0.164	7.092 ± 0.401	1.818 ± 0.066	1.302	2.863 ± 0.162	2.732

. For comparison purposes, the data in Table 6 have been plotted as Figure 10.

As shown in Figure 10a, the linear attenuation coefficients of the radiation-shielding composites were all substantially higher compared to NR. At 59 keV, 50 WC/NR had the highest linear attenuation coefficient of 5.503 cm⁻¹, which was more than 11 times the linear attenuation coefficient of NR. At 81 keV, 50 WC/NR had the highest linear attenuation coefficient of 8.320 cm⁻¹, which was more than 55 times the linear attenuation coefficient of NR. Akman et al. added 20% BaTiO₃ to the polymer matrix and measured the linear attenuation coefficients of the polymer composites. The linear attenuation coefficients of the polymer composites were (1.4325 ± 0.0315) cm⁻¹ and (0.7371 ± 0.0153) cm⁻¹ at 59 keV and 80.9 keV, respectively, which were much lower than the linear attenuation coefficients of the radiation-shielding materials in this study [34].

As shown in Figure 10b, the radiation-shielding composites also showed a substantial increase in the mass attenuation coefficient compared to that of NR. At 59 keV, the highest mass attenuation coefficient for 50 WC/NR was 2.209 cm²/g, which was more than five times the mass attenuation coefficient for NR. At 81 keV, the highest mass attenuation coefficient of 50 WC/NR was 3.340 cm²/g, which was more than 30 times the mass attenuation coefficient for NR. The WinXCom program developed by Gerward et al. can calculate, for example, the mass attenuation coefficients of compounds [35]. As shown in Figure 10b, we calculated the mass attenuation coefficients of the radiation-shielding composites using WinXCom software and found that the calculated values for most of the radiation-shielding composites were essentially smaller than the experimental values. Similar to the principle of X-ray shielding, 50 WC/NR exhibited better radiation-shielding properties and compatibility with NR due to its high proportion of W elements and homogeneous distribution in the NR. Considering the linear and mass attenuation coefficient, 50 WC/NR was also the best γ-ray-radiation-shielding composite.

As shown in Figure 11a,b, the half-value layer of radiation-shielding composites containing different types of fillers decreased, and the lead equivalent increased at 81 keV compared to that at 59 keV, indicating that radiation-shielding composites containing elemental W fillers were more effective in shielding against γ-rays at 81 keV.

4. Conclusions

The above studies show that the different particle sizes of W, WO₃ and WC fillers were uniformly distributed in the natural rubber. The processing properties of the radiation-shielding composites containing different particle sizes of elemental W fillers were good, and the thermal stability of them did not differ significantly. The addition of different elemental W fillers reduced the tensile strength and elongation at break of the radiation-shielding composites and increased the hardness of the radiation-shielding composites; 50 WC/NR had excellent mechanical properties, with tensile strengths of up to 10.558 MPa, elongation at break of up to 548.989% and a hardness of just 62 HA. For X-ray shielding, the linear attenuation coefficient and mass attenuation coefficient of 50 WC/NR were as high as 27.005 cm⁻¹ and 10.446 cm²/g under the condition of a tube voltage of 40 kV, and the linear attenuation coefficient and mass attenuation coefficient were more than 14 times and 10 times higher, respectively, compared to those of NR. For γ-ray shielding, the linear attenuation coefficients of 50 WC/NR were as high as 5.503 cm⁻¹ at 59 keV and 8.320 cm⁻¹ at 81 keV. Compared with NR, the linear attenuation coefficients of 50 WC/NR were increased by 11 times and 55 times, respectively. For γ-ray shielding, the mass attenuation coefficients of 50 WC/NR were as high as 2.209 cm²/g at 59 keV and 3.340 cm²/g at 81 keV. Compared with NR, the mass attenuation coefficients of 50 WC/NR were increased by 5 times and 30 times, respectively. For X/γ-ray shielding, non-toxic and flexible 50 WC/NR had the highest linear attenuation coefficient and mass attenuation coefficient, offering the most promising application of radiation-shielding composites.