A Comparative Study on X-ray Shielding and Mechanical Properties of Natural Rubber Latex Nanocomposites Containing Bi2O3 or BaSO4: Experimental and Numerical Determination

This work experimentally determined the X-ray shielding and morphological, density, and tensile properties of sulfur-vulcanized natural rubber latex (SVNRL) nanocomposites containing varying content of nano-Bi2O3 or nano-BaSO4 from 0 to 200 phr in 100 phr increments, with modified procedures in sample preparation to overcome the insufficient strength of the samples found in other reports. The experimental X-ray shielding results, which were numerically verified using a web-based software package (XCOM), indicated that the overall X-ray attenuation abilities of the SVNRL nanocomposites generally increased with increasing filler content, with the 0.25-mm-thick SVNRL films containing 200 phr of the filler providing the highest overall X-ray shielding properties, as evidenced by the highest values of lead equivalence (Pbeq) of 0.0371 mmPb and 0.0326 mmPb in Bi2O3/SVNRL nanocomposites, and 0.0326 mmPb and 0.0257 mmPb in BaSO4/SVNRL nanocomposites, for 60 kV and 100 kV X-rays, respectively. The results also revealed that the addition of either filler increased the tensile modulus at 300% elongation (M300) and density but decreased the tensile strength and the elongation at break of the Bi2O3/SVNRL and BaSO4/SVNRL nanocomposites. In addition, the modified procedures introduced in this work enabled the developed nanocomposites to acquire sufficient mechanical and X-ray shielding properties for potential use as medical X-ray protective gloves, with the recommended content of Bi2O3 and BaSO4 being in the range of 95–140 phr and 105–120 phr, respectively (in accordance with the requirements outlined in ASTM D3578-19 and the value of Pbeq being greater than 0.02 mmPb). Consequently, based on the overall outcomes of this work, the developed Bi2O3/SVNRL and BaSO4/SVNRL nanocomposites show great potential for effective application in medical X-ray protective gloves, while the modified procedures could possibly be adopted for large-scale production.


Introduction
High-energy electromagnetic (EM) waves, especially X-rays and gamma rays, are currently utilized in various applications, including the quantification of elements, compounds, and radionuclides contained in commercial products, plants, and foods [1][2][3]; medical and varying from 0 to 100 and 200 phr, for potential use as medical X-ray protective gloves (the maximum filler content was 200 phr, based on our previous work that indicated the recommended filler content of 90-170 phr for GVNRL composites [28]). The properties of the nanocomposites investigated in this work were: X-ray shielding (based on the linear attenuation coefficient (µ), the mass attenuation coefficient (µ m ), the half value layer (HVL), and the lead equivalence (Pb eq )) and morphological, physical (density), and mechanical (tensile modulus at 300% elongation (M300), tensile strength, and elongation at break). Furthermore, to verify the reliability and correctness of the experimental results for X-ray shielding measurements, the obtained results were compared with those numerically computed using a web-based software package (XCOM [31]), to determine the recommended filler content for the attenuation of 60 kV and 100 kV X-rays, and subsequently compared to the requirements outlined in ASTM D3578-19 and the value of Pb eq > 0.02 mmPb for medical X-ray-protective gloves. The outcomes of this work do not only present new data on SVNRL nanocomposites for X-ray attenuation but also offer improved procedures for sample preparation that would be beneficial and suitable for actual production at larger scales.

Materials and Chemicals
High-ammonia natural rubber latex (HA-NRL) samples, with total solid and dry rubber content of 61.0% (ISO 124: 2014) and 60.3% (ISO 126: 2005), respectively, were supplied by the Office of Rubber Authority of Thailand (RAOT), Bangkok, Thailand. Names, contents, and the roles of chemicals used for the sample preparation are shown in Table 1. Nano-Bi 2 O 3 and nano-BaSO 4 were obtained from Shanghai Ruizheng Chemical Technology Co., Ltd. (Shanghai, China), distilled water was supplied by the Faculty of Science, Kasetsart University (Bangkok, Thailand), and other chemicals were supplied by the RAOT (Bangkok, Thailand). The images of nano-Bi 2 O 3 and nano-BaSO 4 , taken using a scanning electron microscope (SEM; Quanta 450 FEI: JSM-6610LV, Eindhoven, the Netherlands), are shown in Figure 1, indicating that the average particle sizes of nano-Bi 2 O 3 and nano-BaSO 4 were 234.9 nm and 287.6 nm, respectively, as determined using the ImageJ software version 1.50i. It should be noted that in order to improve the compatibility between the added chemicals and the NRL matrix, all chemicals used in this work (except KOH and Teric 16A16) were prepared using a stainless-steel ball mill by diluting each pure chemical with vultamol, bentonite, and distilled water for 72 h (final weight content of the chemical: vultamol: bentonite: distilled water was 50:1:1:48). It should be noted that the nanoparticles of Bi 2 O 3 and BaSO 4 were selected for this investigation due to their superior radiation-shielding and mechanical properties in the nanocomposites in comparison with those containing microparticles at the same filler content found in previous reports [32,33].

Preparation of SVNRL Mixture
NRL was mechanically stirred using an automatic top stirrer (Eurostar 60 digital, IKA, Bangkok, Thailand) at a rotation speed of 300 rpm for 60 min. Then, all chemicals listed in Table 1 (except nano-Bi2O3 and nano-BaSO4) were consecutively added to the stirred NRL (from top to bottom order), with a 2 min interval between each chemical, and the stirring was continued for another 60 min. Then, the NRL mixture was stored in a closed container at room temperature for 72 h before the addition of the nano-Bi2O3 or nano-BaSO4 to the NRL mixture. The mixture was continuously stirred for another 60 min and kept in a closed container for further use. It should be noted that this step (adding nano-Bi2O3/nano-BaSO4 after the pre-vulcanization process of 72 h) was different from our previous work [28]. This procedure was modified to reduce the effects of nano-Bi2O3/nano-BaSO4 on obstructing the functionality of the main activators and accelerators during vulcanization, which helped the SVNRL mixture to achieve a higher degree of curing that could potentially improve the overall mechanical properties of the nanocomposites [28].

Preparation of SVNRL Mixture
NRL was mechanically stirred using an automatic top stirrer (Eurostar 60 digital, IKA, Bangkok, Thailand) at a rotation speed of 300 rpm for 60 min. Then, all chemicals listed in Table 1 (except nano-Bi 2 O 3 and nano-BaSO 4 ) were consecutively added to the stirred NRL (from top to bottom order), with a 2 min interval between each chemical, and the stirring was continued for another 60 min. Then, the NRL mixture was stored in a closed container at room temperature for 72 h before the addition of the nano-Bi 2 O 3 or nano-BaSO 4 to the NRL mixture. The mixture was continuously stirred for another 60 min and kept in a closed container for further use. It should be noted that this step (adding nano-Bi 2 O 3 /nano-BaSO 4 after the pre-vulcanization process of 72 h) was different from our previous work [28]. This procedure was modified to reduce the effects of nano-Bi 2 O 3 /nano-BaSO 4 on obstructing the functionality of the main activators and accelerators during vulcanization, which helped the SVNRL mixture to achieve a higher degree of curing that could potentially improve the overall mechanical properties of the nanocomposites [28].

Preparation of Nano-Bi 2 O 3 /SVNRL and Nano-BaSO 4 /SVNRL Gloves
The procedure to prepare nano-Bi 2 O 3 /SVNRL and nano-BaSO 4 /SVNRL gloves followed the steps outlined in our previous work [28]. In summary, after thorough washing, the ceramic molds were oven-dried at 70 • C for 40 min, dipped in a 35% coagulant consisting of Ca(NO 3 ) 2 , Teric 16A16, 50% CaCO 3 , and distilled water (RAOT, Bangkok, Thailand) with the final weight content of 35.0:0.1:5.0:59.9, respectively, for 5 sec, and oven-dried again at 70 • C for 2 min. Then, the dried molds were dipped in the nano-Bi 2 O 3 /SVNRL or nano-BaSO 4 /SVNRL mixture for 40 sec, carefully flicked and rotated (at least 3 times), and oven-dried at 70 • C for 5 min. The molds were dipped in 70 • C distilled water for 5 min to rinse off all remaining chemicals and dried again at 100 • C for 40 min. The nano-Bi 2 O 3 /SVNRL or nano-BaSO 4 /SVNRL gloves were peeled off the molds and processed using chlorination to remove any powder that remained on the surface of the gloves [28]. Figure 2 shows images of the prepared nano-Bi 2 O 3 /SVNRL and nano-BaSO 4 /SVNRL samples containing 200 phr of the fillers, which clearly indicate smooth and uniform surfaces, while the colors of the nano-Bi 2 O 3 /SVNRL and nano-BaSO 4 /SVNRL samples were yellow and white, respectively (the same as the colors of the nano-Bi 2 O 3 and nano-BaSO 4 particles).
cessed using chlorination to remove any powder that remained on the surface of th gloves [28]. Figure 2 shows images of the prepared nano-Bi2O3/SVNRL and nan BaSO4/SVNRL samples containing 200 phr of the fillers, which clearly indicate smoo and uniform surfaces, while the colors of the nano-Bi2O3/SVNRL and nano-BaSO4/SVNR samples were yellow and white, respectively (the same as the colors of the nano-Bi2O3 an nano-BaSO4 particles).

X-Ray Shielding Properties
The X-ray shielding properties of the Bi2O3/SVNRL and BaSO4/SVNRL nanocomp sites were investigated at the Secondary Standard Dosimetry Laboratory (SSDL), the O fice of Atoms for Peace (OAP), Bangkok, Thailand. The X-ray shielding parameters of i terest were the X-ray transmission ratio (I/I0), the linear attenuation coefficient (µ), th mass attenuation coefficient (µm), the half value layer (HVL), and the lead equivalen (Pbeq), with their relationships shown in Equations (1)-(4): where I0 is the intensity of incident X-rays, I is the intensity of transmitted X-rays, x is th thickness, µ is the linear attenuation coefficient, µm is the mass attenuation coefficient, ρ the density, HVL is the half value layer, Pbeq is the lead equivalence, and µPb is the line attenuation coefficient of a pure lead sheet. For Equation (4), the values of µPb were 63.0 cm −1 and 25.99 cm −1 for the X-ray energies of 45 keV and 80 keV, respectively, numerical determined using the XCOM software (National Institute of Standards and Technolog Gaithersburg, MD, USA). These were the average values of incident X-rays emitted fro an X-ray tube with supplied voltages of 60 and 100 kV, respectively, in our setup. The X rays were collimated using a 1 mm pinhole to achieve a narrow-beam setup and pointe directly to the center of the 0.25-mm-thick SVNRL nanocomposites. The transmitted X rays were detected and counted using a free air ionization chamber (Korea Research I stitute of Standards and Science, KRISS; Daejon, Korea) that was powered by a high-vol age power unit (Keithley 247, Cleveland, OH, USA) and connected to an electromet

X-ray Shielding Properties
The X-ray shielding properties of the Bi 2 O 3 /SVNRL and BaSO 4 /SVNRL nanocomposites were investigated at the Secondary Standard Dosimetry Laboratory (SSDL), the Office of Atoms for Peace (OAP), Bangkok, Thailand. The X-ray shielding parameters of interest were the X-ray transmission ratio (I/I 0 ), the linear attenuation coefficient (µ), the mass attenuation coefficient (µ m ), the half value layer (HVL), and the lead equivalence (Pb eq ), with their relationships shown in Equations (1)-(4): Pb eq = µx µ Pb (4) where I 0 is the intensity of incident X-rays, I is the intensity of transmitted X-rays, x is the thickness, µ is the linear attenuation coefficient, µ m is the mass attenuation coefficient, ρ is the density, HVL is the half value layer, Pb eq is the lead equivalence, and µ Pb is the linear attenuation coefficient of a pure lead sheet. For Equation (4), the values of µ Pb were 63.06 cm −1 and 25.99 cm −1 for the X-ray energies of 45 keV and 80 keV, respectively, numerically determined using the XCOM software (National Institute of Standards and Technology, Gaithersburg, MD, USA). These were the average values of incident X-rays emitted from an X-ray tube with supplied voltages of 60 and 100 kV, respectively, in our setup. The X-rays were collimated using a 1 mm pinhole to achieve a narrow-beam setup and pointed directly to the center of the 0.25-mm-thick SVNRL nanocomposites. The transmitted X-rays were detected and counted using a free air ionization chamber (Korea Research Institute of Standards and Science, KRISS; Daejon, Korea) that was powered by a high-voltage power unit (Keithley 247, Cleveland, OH, USA) and connected to an electrometer (Keithley 6517B, Cleveland, OH, USA). The X-rays used in this work were controlled by an X-ray system (YXLON MGC41, Hudson, NY, USA) and the energies were selected based on ISO 4037-1:2019. The schematic setup for the X-ray shielding measurement is shown in Figure 3 [25].
(Keithley 6517B, Cleveland, OH, USA). The X-rays used in this work were controlled an X-ray system (YXLON MGC41, Hudson, NY, USA) and the energies were select based on ISO 4037-1:2019. The schematic setup for the X-ray shielding measurement shown in Figure 3 [25]. To verify the correctness and reliability of the experimental results, the numeric determination based on the XCOM software was conducted at X-ray energies of 45 ke and 80 keV and the results were compared with those obtained experimentally [34]. N tably, since XCOM provided only the value of µm, the density (ρ) for each formulatio which was used for the calculation of µ, HVL, and Pbeq, was theoretically determined u ing Equation (5): where ρNR (ρF) is the density of NR (radiation-protective filler) and CNR (CF) is the conte of the NR (radiation-protective filler).

Morphology and Density Measurement
The morphology, the dispersion of nano-Bi2O3 and BaSO4 particles, and the dispe sion of Bi and Ba elements in the SVNRL composites were determined using scanni electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX; Quanta 4 FEI: JSM-6610LV, Eindhoven, the Netherlands). All samples were coated with gold usi a sputter coater (Quorum SC7620: Mini Sputter Coater/Glow Discharge System, Laug ton, UK) prior to the SEM-EDX images being taken.
The density of each sample was determined using a densitometer (MH-300A, Shan hai, China) with a precision of 0.001 g/cm 3 . The determination was carried out based the Archimedes principle [35].

Mechanical Properties
The tensile properties, consisting of tensile modulus at 300% elongation (M300), te sile strength, and elongation at break, were determined using a universal testing machi (TM Tech, TM-G5K, Bangkok, Thailand) according to ASTM D412-06 standard testin The tensile testing speed used for all samples was 500 mm/min.

Determination of Recommended Filler Content for Medical X-Ray Protective Gloves
The determination of the recommended filler content for the production of medic X-ray protective gloves based on Bi2O3/SVNRL and BaSO4/SVNRL nanocomposites w conducted by comparing the X-ray shielding properties of the 0.25-mm-thick sampl with a minimum required Pbeq value of 0.02 mmPb, as well as their tensile properties, wi those outlined in ASTM D3578-19, which states that, for medical examination gloves, t To verify the correctness and reliability of the experimental results, the numerical determination based on the XCOM software was conducted at X-ray energies of 45 keV and 80 keV and the results were compared with those obtained experimentally [34]. Notably, since XCOM provided only the value of µ m , the density (ρ) for each formulation, which was used for the calculation of µ, HVL, and Pb eq , was theoretically determined using Equation (5): where ρ NR (ρ F ) is the density of NR (radiation-protective filler) and C NR (C F ) is the content of the NR (radiation-protective filler).

Morphology and Density Measurement
The morphology, the dispersion of nano-Bi 2 O 3 and BaSO 4 particles, and the dispersion of Bi and Ba elements in the SVNRL composites were determined using scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX; Quanta 450 FEI: JSM-6610LV, Eindhoven, The Netherlands). All samples were coated with gold using a sputter coater (Quorum SC7620: Mini Sputter Coater/Glow Discharge System, Laughton, UK) prior to the SEM-EDX images being taken.
The density of each sample was determined using a densitometer (MH-300A, Shanghai, China) with a precision of 0.001 g/cm 3 . The determination was carried out based on the Archimedes principle [35].

Mechanical Properties
The tensile properties, consisting of tensile modulus at 300% elongation (M300), tensile strength, and elongation at break, were determined using a universal testing machine (TM Tech, TM-G5K, Bangkok, Thailand) according to ASTM D412-06 standard testing. The tensile testing speed used for all samples was 500 mm/min.

Determination of Recommended Filler Content for Medical X-ray Protective Gloves
The determination of the recommended filler content for the production of medical X-ray protective gloves based on Bi 2 O 3 /SVNRL and BaSO 4 /SVNRL nanocomposites was conducted by comparing the X-ray shielding properties of the 0.25-mm-thick samples, with a minimum required Pb eq value of 0.02 mmPb, as well as their tensile properties, with those outlined in ASTM D3578-19, which states that, for medical examination gloves, the tensile strength and the elongation at break must be higher than 14 MPa and 650%, respectively [29]. Then, ranges of filler content that provided sufficient X-ray shielding and tensile properties in accordance with the above requirements could be selected and recommended for actual use.

Density
The densities of the pristine SVNRL, nano-Bi 2 O 3 /SVNRL, and nano-BaSO 4 /SVNRL composites are shown in Table 2. The results revealed that the densities of the samples increased with increasing filler content, with nano-Bi 2 O 3 /SVNRL having slightly higher densities than nano-BaSO 4 /SVNRL with the same filler content. This was due to the much higher densities of Bi 2 O 3 and BaSO 4 compared to the pristine SVNRL (ρ NR = 0.93 g/cm 3 , ρ Bi 2 O 3 = 8.9 g/cm 3 , and ρ BaSO 4 = 4.5 g/cm 3 ), leading to enhanced overall densities of the composites [36]. Notably, these density results were later used for the calculation of µ m from I/I 0 and µ (Equations (1) and (2)).  Table 3 shows the values of µ, µ m , HVL, and Pb eq of the pristine SVNRL, nano-Bi 2 O 3 /SVNRL, and nano-BaSO 4 /SVNRL composites, at the X-ray supplied voltages of 60 kV and 100 kV. The results indicated that the overall X-ray shielding abilities of the SVNRL nanocomposites increased with increasing filler content, as seen by the highest values of µ, µ m , and Pb eq , and the lowest values of HVL, observed in samples containing 200 phr of the fillers. Furthermore, Table 3 shows that the ability to attenuate X-rays of the nanocomposites was reduced at higher X-ray energies, as evidenced by the lower values of µ, µ m , and Pb eq and the higher values of HVL observed in the samples tested using 100 kV X-rays. Table 3. Linear attenuation coefficients (µ), mass attenuation coefficients (µ m ), half value layer (HVL), and lead equivalence (Pb eq ) of pristine SVNRL, nano-Bi 2 O 3 /SVNRL, and nano-BaSO 4 /SVNRL composites, at X-ray supplied voltages of 60 kV and 100 kV.

Properties X-Ray Supplied Voltage
Pristine SVNRL The positive relationship between the filler content and X-ray shielding properties was due to the relatively larger Z values of Bi and Ba compared to the C and H in NR, as well as the higher densities of Bi 2 O 3 and BaSO 4 compared to NR. These characteristics greatly enhanced the interaction probabilities between incident X-rays and the materials through the dominant and effective photoelectric absorption, which is related to the photoelectric cross-section (σ pe ) and Z, as shown in Equation (6): where h is Planck's constant and ν is the frequency of the incident X-rays that is directly proportional to the energy, via Equation (7): As depicted in Equation (6), the addition of Bi 2 O 3 and BaSO 4 in the SVNRL matrix led to higher numbers of heavy elements (Bi and Ba) available in the composites, resulting in larger σ pe values and, hence, a better ability to attenuate incident X-rays [37]. well as the higher densities of Bi2O3 and BaSO4 compared to NR. These characteristics greatly enhanced the interaction probabilities between incident X-rays and the materials through the dominant and effective photoelectric absorption, which is related to the photoelectric cross-section (σpe) and Z, as shown in Equation (6): where h is Planck's constant and ν is the frequency of the incident X-rays that is directly proportional to the energy, via Equation (7): As depicted in Equation (6), the addition of Bi2O3 and BaSO4 in the SVNRL matrix led to higher numbers of heavy elements (Bi and Ba) available in the composites, resulting in larger σpe values and, hence, a better ability to attenuate incident X-rays [37].  Another interesting result shown in Table 3 was that nano-Bi2O3/SVNRL had slightly higher X-ray shielding properties than the nano-BaSO4/SVNRL composites at both supplied voltages. This behavior could be explained by comparing the values of µm for Bi2O3 and BaSO4, obtained from XCOM at various X-ray energies ( Figure 5), which indicated that the µm values for both Bi2O3 and BaSO4 were similar at the 45 keV X-rays (representing the average X-ray energy of those emitted from the X-ray tube with a supplied voltage of 60 kV), while Bi2O3 clearly had a higher µm than that of BaSO4 at the 80 keV X-rays (representing the average X-ray energy of those emitted from the X-ray tube with a supplied voltage of 100 kV), leading to the more pronounced enhancement in X-ray attenuation ability in nano-Bi2O3/SVNRL composites. Notably, although both Bi2O3 and BaSO4 had similar µm values at the 45 keV X-rays, the densities of nano-Bi2O3/SVNRL were greater Another interesting result shown in Table 3 was that nano-Bi 2 O 3 /SVNRL had slightly higher X-ray shielding properties than the nano-BaSO 4 /SVNRL composites at both supplied voltages. This behavior could be explained by comparing the values of µ m for Bi 2 O 3 and BaSO 4 , obtained from XCOM at various X-ray energies ( Figure 5), which indicated that the µ m values for both Bi 2 O 3 and BaSO 4 were similar at the 45 keV X-rays (representing the average X-ray energy of those emitted from the X-ray tube with a supplied voltage of 60 kV), while Bi 2 O 3 clearly had a higher µ m than that of BaSO 4 at the 80 keV X-rays (representing the average X-ray energy of those emitted from the X-ray tube with a supplied voltage of 100 kV), leading to the more pronounced enhancement in X-ray attenuation ability in nano-Bi 2 O 3 /SVNRL composites. Notably, although both Bi 2 O 3 and BaSO 4 had similar µ m values at the 45 keV X-rays, the densities of nano-Bi 2 O 3 /SVNRL were greater compared to those of nano-BaSO 4 /SVNRL (Table 2), leading to greater amplification of the overall X-ray shielding properties in nano-Bi 2 O 3 /SVNRL (determined at the same filler content). This phenomenon could also be mathematically explained using Equation (2), which implies a direct relationship between µ and ρ. Notably, the sharp increases in µ m at particular X-ray energies in Figure 5 (such as 37.4 keV and 90.5 keV) were due to the K-absorption (K-edge) and L-absorption (L-edge) of Ba and Bi (the X-ray energies that are slightly above the binding energy of the electron shell of the atoms), for which the σ pe or the interaction probabilities between incident X-rays and the compounds abruptly increased at these energies [38].
Polymers 2022, 14, 3654 9 of 16 compared to those of nano-BaSO4/SVNRL (Table 2), leading to greater amplification of the overall X-ray shielding properties in nano-Bi2O3/SVNRL (determined at the same filler content). This phenomenon could also be mathematically explained using Equation (2), which implies a direct relationship between µ and ρ. Notably, the sharp increases in µm at particular X-ray energies in Figure 5 (such as 37.4 keV and 90.5 keV) were due to the Kabsorption (K-edge) and L-absorption (L-edge) of Ba and Bi (the X-ray energies that are slightly above the binding energy of the electron shell of the atoms), for which the σpe or the interaction probabilities between incident X-rays and the compounds abruptly increased at these energies [38]. In addition, Table 3 suggests that the X-ray shielding properties of pristine SVNRL, nano-Bi2O3/SVNRL, and nano-BaSO4/SVNRL composites at the 60 kV X-rays were greater than those at the 100 kV X-rays. This behavior could be explained using Equation (6), which implies that σpe is inversely proportional to ν 3 or E 3 , resulting in less interaction probabilities with incident X-rays at higher energies [39]. The dependence of σpe could also be observed in Figure 5, which reveals overall decreases in the µm values of Bi2O3 and BaSO4 with increasing X-ray energies.
To verify the correctness and reliability of the experimental results, the µm values of pristine SVNRL, nano-Bi2O3/SVNRL, and nano-BaSO4/SVNRL composites at filler content of 100 phr and 200 phr were compared with those numerically determined using XCOM (Figure 6a). The comparison indicated strong agreement between the µm values obtained experimentally and numerically, with the percentage of difference being less than 2% for samples containing 0 and 100 phr of the fillers and being less than 7% for samples containing 200 phr of the fillers. The discrepancies between the two results could have been due to several factors, such as the fact that the experimental X-ray energies emitted from the X-ray tube were actually in spectra, with the average energies being around 45 keV and 80 keV (rather than discrete energies, as in the case of XCOM), which could cause deviations in the X-ray shielding measurements [40,41]. Nonetheless, the small percentages of difference (less than 7%) implied that the experimental results were reliable and the µm values obtained from XCOM could be further used for the prediction of µ, HVL, and Pbeq values for all filler content values in the range 0-200 phr. In addition, Table 3 suggests that the X-ray shielding properties of pristine SVNRL, nano-Bi 2 O 3 /SVNRL, and nano-BaSO 4 /SVNRL composites at the 60 kV X-rays were greater than those at the 100 kV X-rays. This behavior could be explained using Equation (6), which implies that σ pe is inversely proportional to ν 3 or E 3 , resulting in less interaction probabilities with incident X-rays at higher energies [39]. The dependence of σ pe could also be observed in Figure 5, which reveals overall decreases in the µ m values of Bi 2 O 3 and BaSO 4 with increasing X-ray energies.
To verify the correctness and reliability of the experimental results, the µ m values of pristine SVNRL, nano-Bi 2 O 3 /SVNRL, and nano-BaSO 4 /SVNRL composites at filler content of 100 phr and 200 phr were compared with those numerically determined using XCOM (Figure 6a). The comparison indicated strong agreement between the µ m values obtained experimentally and numerically, with the percentage of difference being less than 2% for samples containing 0 and 100 phr of the fillers and being less than 7% for samples containing 200 phr of the fillers. The discrepancies between the two results could have been due to several factors, such as the fact that the experimental X-ray energies emitted from the X-ray tube were actually in spectra, with the average energies being around 45 keV and 80 keV (rather than discrete energies, as in the case of XCOM), which could cause deviations in the X-ray shielding measurements [40,41]. Nonetheless, the small percentages of difference (less than 7%) implied that the experimental results were reliable and the µ m values obtained from XCOM could be further used for the prediction of µ, HVL, and Pb eq values for all filler content values in the range 0-200 phr.  Figure 6b,d, which show the numerical values of µ, HVL, and Pbeq, determined using XCOM of the Bi2O3/SVNRL and BaSO4/SVNRL composites with varying filler content from 0 to 200 phr, confirm the dependence of the X-ray shielding properties of the samples on the filler type and content, as well as the X-ray energy, shown in Table 3. For Figure  6d, the results implied that Bi2O3/SVNRL composites required less filler content to meet the minimum requirement of Pbeq being greater than 0.02 mmPb compared to those from the BaSO4/SVNRL composites (determined at the same X-ray energy). Again, these behaviors were observed due to the higher values for µm ( Figure 5) and ρ of Bi2O3 than those of BaSO4, which made the former a better X-ray attenuator than the latter [29,42]. Figure 7 shows the tensile properties, including tensile modulus at 300% elongation, tensile strength, and elongation at break, of the nano-Bi2O3/SVNRL and nano-BaSO4/SVNRL composites. The results indicated that increases in the filler content led to an increase in the tensile modulus but decreases in the tensile strength and elongation at break. The increase in tensile modulus after the addition of the fillers to SVNRL could have been due to the high rigidity of the nano-Bi2O3 and nano-BaSO4 particles, which enhanced the overall rigidity and, subsequently, the tensile modulus of the nanocomposites [43,44]. On the other hand, the addition of the nano-Bi2O3 and nano-BaSO4 particles resulted in reductions in the tensile strength and elongation at break, probably due to the  Figure 6b,d, which show the numerical values of µ, HVL, and Pb eq , determined using XCOM of the Bi 2 O 3 /SVNRL and BaSO 4 /SVNRL composites with varying filler content from 0 to 200 phr, confirm the dependence of the X-ray shielding properties of the samples on the filler type and content, as well as the X-ray energy, shown in Table 3. For Figure 6d, the results implied that Bi 2 O 3 /SVNRL composites required less filler content to meet the minimum requirement of Pb eq being greater than 0.02 mmPb compared to those from the BaSO 4 /SVNRL composites (determined at the same X-ray energy). Again, these behaviors were observed due to the higher values for µ m ( Figure 5) and ρ of Bi 2 O 3 than those of BaSO 4 , which made the former a better X-ray attenuator than the latter [29,42]. Figure 7 shows the tensile properties, including tensile modulus at 300% elongation, tensile strength, and elongation at break, of the nano-Bi 2 O 3 /SVNRL and nano-BaSO 4 /SVNRL composites. The results indicated that increases in the filler content led to an increase in the tensile modulus but decreases in the tensile strength and elongation at break. The increase in tensile modulus after the addition of the fillers to SVNRL could have been due to the high rigidity of the nano-Bi 2 O 3 and nano-BaSO 4 particles, which enhanced the overall rigidity and, subsequently, the tensile modulus of the nanocomposites [43,44]. On the other hand, the addition of the nano-Bi 2 O 3 and nano-BaSO 4 particles resulted in reductions in the tensile strength and elongation at break, probably due to the poor interfacial compatibility between the fillers and the NRL matrix (rubber-filler interactions), which led to visible voids inside the matrix [45]. Another factor that could have contributed to the decreases in the properties was the increase in filler-filler interactions at higher filler content, which resulted in higher particle agglomeration and worse particle dispersion in samples with 200 phr filler content (Figure 8c,e) than in samples with 0 and 100 phr filler content (Figure 8a,b,d) [46].

Mechanical Properties
Polymers 2022, 14, 3654 11 of 16 poor interfacial compatibility between the fillers and the NRL matrix (rubber-filler interactions), which led to visible voids inside the matrix [45]. Another factor that could have contributed to the decreases in the properties was the increase in filler-filler interactions at higher filler content, which resulted in higher particle agglomeration and worse particle dispersion in samples with 200 phr filler content (Figure 8c,e) than in samples with 0 and 100 phr filler content (Figure 8a,b,d) [46].  Figure 7 also reveals that the nano-Bi2O3/SVNRL composites had higher tensile strength and elongation at break than the nano-BaSO4/SVNRL composites, determined at the same filler content. This was mainly due to the higher density of nano-Bi2O3 particles than nano-BaSO4 particles; hence, when both fillers were added to the samples at the same weight content, less volume of nano-Bi2O3 would be actually added to the composites, resulting in fewer voids and less particle agglomeration created in the nano-Bi2O3/SVNRL composites. Nonetheless, Figure 7b,c imply that both SVNRL nanocomposites containing less than 100 phr filler had higher experimental values of tensile strength and elongation at break than those outlined in ASTM D3578-19 for medical examination gloves (greater than 14 MPa and 650%, respectively, represented as horizontal dotted lines in this figure). Notably, these mechanical results could be further considered along with the results from the X-ray shielding measurement to determine the recommended filler content that allowed the nanocomposites to satisfy all the requirements for medical X-ray protective gloves.
As mentioned in the experimental section above, the current work modified the procedure for sample preparation by postponing the addition of nano-Bi2O3 or nano-BaSO4 until after the completion of rubber vulcanization (72 h after sulfur was added to the SVNRL mixture). The effects of this improved procedure on the tensile strengths of the samples are shown in Table 4, which indicates that the current tensile strengths of the nano-Bi2O3/SVNRL composites were higher than those in a previous work for all nano-Bi2O3 contents investigated [28], especially for the 100 phr content, which showed an almost 3-fold increase in the values. This improvement in tensile strength could have been due to the postponed addition of nano-Bi2O3 reducing the obstruction effects of the filler  Figure 7 also reveals that the nano-Bi 2 O 3 /SVNRL composites had higher tensile strength and elongation at break than the nano-BaSO 4 /SVNRL composites, determined at the same filler content. This was mainly due to the higher density of nano-Bi 2 O 3 particles than nano-BaSO 4 particles; hence, when both fillers were added to the samples at the same weight content, less volume of nano-Bi 2 O 3 would be actually added to the composites, resulting in fewer voids and less particle agglomeration created in the nano-Bi 2 O 3 /SVNRL composites. Nonetheless, Figure 7b,c imply that both SVNRL nanocomposites containing less than 100 phr filler had higher experimental values of tensile strength and elongation at break than those outlined in ASTM D3578-19 for medical examination gloves (greater than 14 MPa and 650%, respectively, represented as horizontal dotted lines in this figure). Notably, these mechanical results could be further considered along with the results from the X-ray shielding measurement to determine the recommended filler content that allowed the nanocomposites to satisfy all the requirements for medical X-ray protective gloves.
As mentioned in the experimental section above, the current work modified the procedure for sample preparation by postponing the addition of nano-Bi 2 O 3 or nano-BaSO 4 until after the completion of rubber vulcanization (72 h after sulfur was added to the SVNRL mixture). The effects of this improved procedure on the tensile strengths of the samples are shown in Table 4, which indicates that the current tensile strengths of the nano-Bi 2 O 3 /SVNRL composites were higher than those in a previous work for all nano-Bi 2 O 3 contents investigated [28], especially for the 100 phr content, which showed an almost 3-fold increase in the values. This improvement in tensile strength could have been due to the postponed addition of nano-Bi 2 O 3 reducing the obstruction effects of the filler on the functionality of the main activators and accelerators, allowing higher degrees of vulcanization to occur prior to the addition of nano-Bi 2 O 3 , which consequently improved the overall strengths of the samples [26]. This outcome would be crucial for the actual production of medical X-ray protective gloves based on SVNRL as the achieved tensile strengths were greater than the strength requirement (ASTM D3578-19), which was unobtainable in the previous report.

Determination of Recommended Filler Content
To determine the recommended filler content for the actual production of medical X-ray protective gloves based on the requirements outlined in ASTM D3578-19 (tensile strength > 14 MPa) and ensuring a value of Pb eq > 0.02 mmPb, the relationships between the experimental tensile strength and Pb eq of the nano-Bi 2 O 3 /SVNRL and nano-BaSO 4 /SVNRL composites were plotted, as shown in Figure 9, with interpolation between data points. While most of the formulations investigated in this work did not simultaneously satisfy both the X-ray shielding and mechanical requirements, the samples containing approximately 95-140 phr of nano-Bi 2 O 3 and 105-120 phr of nano-BaSO 4 offered sufficient characteristics to satisfy the requirements; thus, these filler ranges could be regarded as the recommended filler content levels. Notably, the nano-Bi 2 O 3 /SVNRL composites had larger ranges of recommended filler content than the nano-BaSO 4 /SVNRL composites, which could have been due to the greater levels of X-ray attenuation ability and overall mechanical strength found in the nano-Bi 2 O 3 /SVNRL composites. In addition, these findings confirmed the useability of Bi 2 O 3 and BaSO 4 as effective fillers for radiation protection, which were also found in other shielding materials such as glasses and concrete [47][48][49][50]. on the functionality of the main activators and accelerators, allowing higher degrees of vulcanization to occur prior to the addition of nano-Bi2O3, which consequently improved the overall strengths of the samples [26]. This outcome would be crucial for the actual production of medical X-ray protective gloves based on SVNRL as the achieved tensile strengths were greater than the strength requirement (ASTM D3578-19), which was unobtainable in the previous report.

Determination of Recommended Filler Content
To determine the recommended filler content for the actual production of medical Xray protective gloves based on the requirements outlined in ASTM D3578-19 (tensile strength > 14 MPa) and ensuring a value of Pbeq > 0.02 mmPb, the relationships between the experimental tensile strength and Pbeq of the nano-Bi2O3/SVNRL and nano-BaSO4/SVNRL composites were plotted, as shown in Figure 9, with interpolation between data points. While most of the formulations investigated in this work did not simultaneously satisfy both the X-ray shielding and mechanical requirements, the samples containing approximately 95-140 phr of nano-Bi2O3 and 105-120 phr of nano-BaSO4 offered sufficient characteristics to satisfy the requirements; thus, these filler ranges could be regarded as the recommended filler content levels. Notably, the nano-Bi2O3/SVNRL composites had larger ranges of recommended filler content than the nano-BaSO4/SVNRL composites, which could have been due to the greater levels of X-ray attenuation ability and overall mechanical strength found in the nano-Bi2O3/SVNRL composites. In addition, these findings confirmed the useability of Bi2O3 and BaSO4 as effective fillers for radiation protection, which were also found in other shielding materials such as glasses and concrete [47][48][49][50].

Conclusions
This work developed medical X-ray protective gloves based on nano-Bi 2 O 3 /SVNRL and nano-BaSO 4 /SVNRL composites, with varying filler content of 0 to 200 phr in 100 phr increments. The results suggested that the increases in filler content increased the values of µ, µ m , HVL, Pb eq , density, and tensile modulus at 300% elongation but decreased the tensile strength and elongation at break of the nanocomposites. The experimental results of X-ray shielding measurement were also numerically verified using XCOM, which indicated strong agreement between the two methods (less than 7% difference), implying the reliability and correctness of the results. Furthermore, after considering the X-ray shielding and mechanical properties of both composites, nano-Bi 2 O 3 /SVNRL with filler content of 95-140 phr and nano-BaSO 4 /SVNRL with filler content of 105-120 phr satisfied the minimum requirements of Pb > 0.02 mmPb and tensile strength > 14 MPa outlined in the commercial X-ray protective gloves standard and ASTM D3578-19, respectively. In addition, the modified sample preparation procedures introduced in this work resulted in improved tensile properties of the SVNRL composites (not obtainable in the previous work), potentially making the method suitable for implementation in actual large-scale production.