1 Introduction
X-rays and γ-rays have a wide range of applications in military, medical, health, scientific, and agricultural industries [1]. However, these rays are also harmful to human health if they are not shielded properly, especially for the staff who handles nuclear materials or radioactive sources [2,3]. Traditional material structures, such as metal [4,5], concrete [6], glass [7], and polymer [8,9] have been commonly used to attenuate γ-rays and other rays, after the addition of heavy metal elements. However, there are situations that require flexible materials for shielding low-energy γ-rays, such as protective shades, clothing, gloves, and helmets. Therefore, many attempts have been made to such develop flexible materials [10-12]. Chai et al. prepared X-ray-shielding materials using W and Bi2 O3 with methyl vinyl silicone rubber (VMQ) as the matrix [13]. The VMQ composite was fabricated by a plate-vulcanizing machine using solid rubber. However, there are significant differences between solid rubber and rubber latex, such as the molding method, product properties, and applications. Rubber latex is widely used in our daily lives as a flexible material. Carboxylated nitrile butadiene rubber (XNBR) is a copolymer of butadiene, acrylonitrile, and acrylic or meth-acrylic acid. It can be vulcanized through a variety of methods to achieve an excellent performance [14-16]. Furthermore, XNBRL is a latex that can be prepared in any arbitrary shape, possessing good plasticity, elasticity, and resistance to gamma radiation aging [17].
Lead (Pb) has been commonly used at an early stage [18]. However, materials containing Pb are toxic, making post-treatment a large environmental problem. Thus, there is a tremendous need to develop materials free of Pb for γ-ray shielding. The element Bi (Z = 83, Ar = 209) is located next to Pb (Z = 82, Ar = 207) in the periodic table of elements. However, Bi is not suitable due to its low melting point (271 °C). The post-treatment of the flexible attenuation material is usually conducted using a burning method. The flexible matrix mainly degrades into CO2 and H2 O during the burning at a decomposition temperature higher than 500 °C, and the attenuation material is left behind for collection and storage. Bi2 O3 (with a melting point of 825 °C) has been used in glass for γ-ray attenuation as it is odorless, non-toxic, inexpensive, abundant, and has stable mechanical and chemical properties [19,20]. All these characteristics indicate that a Bi2 O3 /XNBR film can be used as a flexible material against low-energy γ-rays radiation.
In the present work, XNBRL and Bi2 O3 were selected as the matrix material and the attenuation material, respectively. Different concentrations of Bi2 O3 (30–70 wt.%) were dispersed in the XNBRL via physical and chemical methods. The mechanical properties (tensile strength, elongation at break, and shore hardness) and low-energy (20–100 keV) γ-ray attenuation properties of the Bi2 O3 /XNBR flexible films were investigated.
2 Experimental
2.1 Preparation of the Bi2 O3 dispersion
The purity, density and median diameter (D50 ) of the Bi2 O3 powder (Beijing Xing Rong Yuan Technology Co., Ltd.) used in these experiments were 99.9%, 8.9 g·cm-3, and 13.7 µm, respectively. The Bi2 O3 powder was pretreated by ball milling for 12 h at 250 rpm. The Bi2 O3 dispersion consisted of Bi2 O3, distilled water, and the dispersant (Nekal, BX, C18 H24 SO3 Na) with a mass ratio of 1:0.5:0.003.
2.2 Preparation of the XNBRL mixture
The XNBRL mixture consisted of XNBRL, a compounding agent dispersion, KOH, casein, and polyoxyethylene alkyl ether (Levelling Agent O, Peregal-O). XNBRL (Nipol LX552, Zeon, Japan) is a type of aqueous latex. The solid content was approximately 44 wt.%, and the remaining content of the mixture was distilled water. The mixture viscosity was approximately 30 mPa·s, and its density approximately 1.02 g·cm-3. All chemicals and reagents were used as received. The compounding agent dispersion of the XNBRL included a vulcanizer (S, sulfur), active agent (ZnO, zinc oxide), accelerator (ZDC, zinc diethyl dithiocarbamate), reinforcing filler (C, carbon black), and BX. The composition of the XNBRL mixture is presented in Table 1. The compounding agent was ball milled at 250 rpm for 12 h. The mass ratio of the compounding agent and distilled water was 1:1.5. The compounding agent dispersion was slowly added to the latex via stirring. Then, the KOH, the casein, and the Peregal-O solution were mixed into the latex. KOH was used to adjust the pH of the latex to 9–10 to enhance its chemical stability. The viscosity of the latex was controlled by adjusting the contents of the casein and the Peregal-O solution.
| Raw material | XNBRL | S | ZnO | ZDC | C | casein | Peregal-O | KOH | BX |
|---|---|---|---|---|---|---|---|---|---|
| Dry weight (wt.%) | 100 | 0.5 | 1 | 1 | 2 | 0.1–0.5 | 0.1–0.5 | 0.1–0.3 | 0.1 |
2.3 Preparation of the Bi2 O3 /XNBR flexible films
The Bi2 O3 /XNBR mixture consisted of the Bi2 O3 dispersion (30–70 wt.%) and the XNBRL mixture. For example, the mass fractions of Bi2 O3 and XNBR in a 30 wt.% Bi2 O3 /XNBR film are 30 wt.% and 70 wt.%, respectively, normalized by their dry weight, and so on. The Bi2 O3 dispersion was added to the XNBRL mixture via stirring. Letting the Bi2 O3 /XNBRL mixture equilibrate for 48 h, the desired latex was subsequently obtained after filtration and de-foaming. The Bi2 O3 /XNBR film was then prepared by dip-molding. The dip plate, or mold, was submerged in the latex mixture for several seconds and raised to allow the film to set. After drying, the film was vulcanized in hot air in an oven (100 °C, 60 min). Afterwards, the Bi2 O3 /XNBR flexible film was ready for γ-ray attenuation. The thickness of the film was controlled by the number of dips. After a single dip, the thickness of the film was 0.1–0.3 mm. Thicker films were obtained via multiple dips.
2.4 Property characterizations
A digital viscometer (NDJ-5S, Shanghai Ping Xuan Scientific Instrument Co., Ltd.) was used to measure the viscosity of the latex at room temperature. The D50 of the Bi2 O3 particles was measured by a laser particle size analyzer (Mastersizer 2000) using deionized water as the dispersing agent, with a scanning speed of 1000 times/second. The dispersion state of Bi2 O3 in the rubber latex was analyzed using a scanning electron microscope (SEM, JSM6390 LV, Japan JEOL) and an energy dispersive spectrometer (EDS). Fourier transform infrared (FT-IR) spectra were recorded at room temperature using a Nicolet FT-IR Nexus with a 4 cm-1 resolution in the range of 4000–400 cm-1. The test mode was set to total reflection. X-ray diffraction (XRD) was performed on a TD3500 X-ray diffractometer (China DanDong TongDa) under the following conditions: Cu Kα radiation (λ=0.15406 nm) at a voltage of 35 kV. The scanning rate was 10 °/min over a range of 10–70°. Thermogravimetric analyses (TGA) were conducted by utilizing an SDT Q600 instrument (TA, USA) from 30 °C to 700 °C at 20 °C/min. The tensile properties (tensile strength and elongation at break) of the film were tested using a tensile strength tester (AI-3000, Gotech Testing Machines Inc.). The hardness of the films was measured using a shore durometer (LX-A, China Jun Ping Machinery Factory).
2.5 γ-ray attenuation measurements
Canberra's portable In Situ Object Counting System (ISOCS) passive efficiency calibration machine for low-background high-purity germanium (HPGe) gamma spectrometry was used to measure the γ-ray spectra. The probe was a coaxial germanium detector. Its sensitive area was ø 80 mm × 30 mm, and its energy resolution was 474 eV. The instrument was cooled with liquid nitrogen for more than six hours before operation. The radiation sources were 241Am (59.5 keV) and 133Ba (30.7 keV and 81.0 keV). The doses of the sources were in the millicurie range. The distance between the radiation source and the HPGe probe surface was over 25 cm, which reduced the probability of coincidence and cascading. Each sample was tested under each energy for 360 s. The peak intensity of the γ-rays was calculated as the peak area.
The γ-rays attenuation equation can be calculated using Beer-Lambert’s law:[21]
where I(d) and I0 are the peak intensity of the γ-rays with and without the Bi2 O3 /XNBR film between the radiation source and the HPGe probe, respectively. d is the thickness of the Bi2 O3 /XNBR film. µ is the linear attenuation coefficient of the Bi2 O3 /XNBR film, whichcan be obtained by measuring I0, I(d), and d.
For a given material, if µ is obtained by equation (1), its attenuation efficiency (AE) with an arbitrary thickness of d can be evaluated by equation (2) [22]:
For a given material, if an attenuation efficiency for the actual working conditions is required, the requisite d of the material can be determined by Eq. (2).
3 Results and Discussion
3.1 Dispersion state of Bi2 O3 in the XNBR
Obtaining a uniform and stable dispersion of Bi2 O3 in the XNBRL is a challenging because of the density inhomogeneity. Adding Bi2 O3 powder directly into XNBRL leads to uneven dispersion, particle agglomeration, and sedimentation problems. The Bi2 O3 powders must be well dispersed before being added into the XNBRL. The well-dispersed Bi2 O3 was realized by milling. The granular shape of the Bi2 O3 particles does not change after being milled. As shown in Fig. 1, after being milled for 12 h, the D50 of the Bi2 O3 particles decreased from 13.7 µm to 8.1 µm, and their specific surface area increased from 0.488 m2·g-1 to 0.763 m2·g-1. A larger specific surface area corresponded to a better dispersity. This revealed that milling was beneficial to the dispersal of Bi2 O3 particles in water.
Without BX, the Bi2 O3 powder settled within 1 hour (upper left in Fig. 2a). With BX, the sedimentation of the Bi2 O3 powder was negligible within 1 hour (upper right in Fig. 2a). The dispersion state of Bi2 O3 in the water directly influenced the dispersion state of Bi2 O3 in the XNBRL. A well-dispersed Bi2 O3 dispersion may not lead to a well-dispersed state of Bi2 O3 in the XNBRL. However, an under-dispersed Bi2 O3 dispersion leaded to an under-dispersed state of Bi2 O3 in the XNBRL (lower left in Fig. 2a).
The molecular formula of Bi2 O3, BX, and XNBR are shown in Fig. 2b. The FT-IR spectra of Bi2 O3 and BX are shown in Fig. 2c. The broad peak at 3455 cm−1 (peak 1) was attributed to the associate hydrogen bond O—H stretching. The O—H was formed by the interaction of O in the Bi2 O3 with H in the H2 O. The peak at 400—1200 cm−1 was attributed to the Bi—O deformation (Bi2 O3 ) [23]. The peak at 3068 cm−1 (peak 2) was attributed to the C—H stretching (naphthalene). The peak at 1680 cm−1 (peak 3) was attributed to the C=C stretching (naphthalene). The peak at 1380 cm−1 (peak 4) was attributed to the C—H deformation (—C4 H9 ). The peak at 845 cm−1 (peak 5) was attributed to C—C stretching. The peak at 650 cm−1 (peak 6) was attributed to the —SO3 Na deformation. After adding 0.3% BX into Bi2 O3, the FT-IR spectra of Bi2 O3 were not clear, as they were obscured by the FT-IR spectra of BX. This suggests that the affinity interaction between BX and Bi2 O3 was significant (Fig. 2c). The BX contains a hydrophilic (—SO3 Na) and a lipophilic group (—C18 H24 ). As a commonly used surfactant, the hydrophilic property of the BX can increase the dispersal of the filler particles in water [24].
After the Bi2 O3 dispersion and XNBRL were mixed together, casein and Peregal-O solution were added as a thickener and stabilizer, respectively, to prevent the sedimentation of the Bi2 O3 particles in the XNBRL. According to Stokes law, the gravity settling velocity is proportional to the square of the particle radius and inversely proportional to the viscosity of the dispersal medium [25]. The viscosity of the latex was controlled by adjusting its casein and Peregal-O contents. The proper viscosity of an XNBRL mixture was 60–80 mPa·s. When the Bi2 O3 content was less than 50 wt.%, the sedimentation in the Bi2 O3 /XNBRL mixture was not obvious even after 5 days. When the Bi2 O3 content was 50–70 wt.%, sedimentation occurred over time. After stirring, the sedimentation was re-dispersed. For a Bi2 O3 /XNBRL mixture containing more than 70 wt.% Bi2 O3, the sedimentation problem was considerable, which needs to be solved. The preparation of a Bi2 O3 /XNBR film with a higher filler content with good stability is one of the possible directions in the future.
In summary, physical and chemical methods were used to tackle the sedimentation problem of Bi2 O3. Physical methods were used to reduce the particle radius and to increase the latex viscosity. The chemical method consisted of adding a dispersant.
SEM images and EDS images of the 50 wt.% Bi2 O3 /XNBR flexible films are shown in Fig. 3. There were no pores or cracks observed in the films. The EDS images of Bi, C, N, O, Zn, S, and K are presented in Fig. 3(b), Fig. 3(c), Fig. 3(d), Fig. 3(e), Fig. 3(f), Fig. 3(g), and Fig. 3(h), respectively. The elements Bi, C, N, O, Zn, S, and K were well-distributed in the Bi2 O3 /XNBR flexible films, indicating that the Bi2 O3 particles and other agents were well-distributed in the XNBR.
3.2 XRD of the Bi2 O3 /XNBR films
The XRD pattern of the XNBR and Bi2 O3 /XNBR flexible films is shown in Fig. 4. The raw XNBR denotes the raw material purchased without an added compounding agent. The sole peak at 19° revealed the amorphous structure of the XNBR[26]. After vulcanization, the peak amplitude weakened, and the peak position shifted to the right by 1° to 20°. The three main peaks of Bi2 O3 were observed at 27°, 33°, and 46°. The XRD pattern of Bi2 O3 was identical to the standard card (JCPDS: 41-1449) [27,28]. The used Bi2 O3 was an alpha-type monoclinic system (a = 0.585 nm, b = 0.817 nm, c = 0.751 nm). The relative peak amplitude at 20° weakened, and the relative peak amplitudes of the three main peaks of Bi2 O3 increased with the increasing Bi2 O3 content in the XNBR. After the added Bi2 O3 exceeded 70 wt.%, the XRD pattern of the Bi2 O3 /XNBR flexible films were close to the that of Bi2 O3, and the peak from XNBR was not clearly resolved.
3.3 FT-IR spectroscopy of the Bi2 O3 /XNBR films
The FT-IR spectra of the Bi2 O3 /XNBR films are shown in Fig. 5. The characteristic IR bands of the XNBR agreed with the literature data, which is illustrated as follows [29-31]: The peak at 3457 cm−1 was attributed to the O—H stretching. The peak at 2926–2920 cm−1 was attributed to the CH2 stretching (butadiene). The peak at 2851–2846 cm−1 was attributed to the CH2 stretching (acrylonitrile). The peak at 2237 cm−1 was attributed to the C≡N stretching (acrylonitrile). The positions of these three peaks did not change before and after curing. The peak at 1737–1731 cm−1 for XNBR was attributed to the C=O stretching (carboxyl). After vulcanization, the C=O stretching vibration peak was no longer detected, which demonstrated that the unsaturated C=O bond transformed into a saturated bond during the process. Hence, the curing reaction of the C=O bond was complete. The peak at 1698 cm−1 was attributed to the C=C stretching (isoprene). This peak appeared in the raw XNBR spectrum but not in the XNBR and Bi2 O3 /XNBR spectra, which demonstrated that the C=C bond (isoprene) became a saturated bond after the vulcanization. Hence, the curing reaction of the C=C bond (isoprene) was complete. The peak at 1596 cm−1 was attributed to the stretching of the zinc carboxylate salt.[32] This peak did not appear in the XNBR and Bi2 O3 /XNBR but appeared in the raw XNBR. This result also suggested that a reaction occurred between the C=O and ZnO. The peak at 1440–1437 cm−1 was attributed to C—H deformation. The peak at 1188–1183 cm−1 was attributed to C—C stretching (carboxyl). The peak at 1044–1035 cm−1 was attributed to C—C stretching. The peak at 968–966 cm−1 was attributed to H—C=C deformation (main chain, carboxyl group). The peak at 919–909 cm−1 was attributed to H—C=C deformation (side chain).
3.4 TGA of the Bi2 O3 /XNBR films
The TGA results for the XNBR and Bi2 O3 /XNBR films are shown in Fig. 6. The vertical axis is the mass loss of the sample at different temperatures. The XNBR contained 10 wt.% of a non-melting substance. The sum of the non-melting substance and the mass loss was 100 %. The Bi2 O3 did not melt below 700 degrees due to its high melting point (825 °C). Suppose the mass fraction of Bi2 O3 in the Bi2 O3 /XNBR is x, and the mass loss of the Bi2 O3 /XNBR is
The mass loss of the nominal 30 wt.% Bi2 O3 /XNBR was 63 wt.%. The Bi2 O3 content was calculated as follows: 100 %-63 %/0.9 = 30 %, indicating that the Bi2 O3 content was about 30 wt.%. And so on. The results indicate that the designed contents of Bi2 O3 in the Bi2 O3 /XNBR films are in line with the actual contents.
The decomposition temperature obtained from TGA is a measure of thermal stability [33,34]. The T10% is the temperature at which 10 % of the initial mass is lost. The decomposition temperatures of the XNBR and Bi2 O3 /XNBR films are specified in Table 2. With the increase of the Bi2 O3 content, the decomposition temperature of the Bi2 O3 /XNBR films increased. This result indicates that Bi2 O3 improves the thermal stability of XNBR.
| Sample | Degradation temperature (°C) | ||||
|---|---|---|---|---|---|
| T10% | T20% | T30% | T50% | T90% | |
| Raw XNBR | 397 | 417 | 431 | 447 | 481 |
| XNBR | 401 | 423 | 436 | 452 | 512 |
| 10 wt.% Bi2 O3 /XNBR | 404 | 428 | 440 | 456 | - |
| 20 wt.% Bi2 O3 /XNBR | 409 | 432 | 444 | 461 | - |
| 30 wt.% Bi2 O3 /XNBR | 413 | 437 | 449 | 468 | - |
| 40 wt.% Bi2 O3 /XNBR | 419 | 443 | 456 | 486 | - |
| 50 wt.% Bi2 O3 /XNBR | 422 | 446 | 462 | - | - |
| 60 wt.% Bi2 O3 /XNBR | 430 | 454 | 474 | - | - |
| 70 wt.% Bi2 O3 /XNBR | 438 | 463 | - | - | - |
| 80 wt.% Bi2 O3 /XNBR | 453 | - | - | - | - |
| 90 wt.% Bi2 O3 /XNBR | - | - | - | - | - |
3.5 Mechanical properties of the Bi2 O3 /XNBR films
The tensile strength of the un-vulcanized XNBR was 0.3 MPa (1 MPa = 10 kgf·cm-2), and its elongation at break was 3000 %. XNBR must be vulcanized for practical applications. The compounding agent is indispensable, as illustrated in Fig. 7. The XNBR film was not strong enough without the added compounding agent. The vertical ordinate in Fig. 7 shows the use of only ZDC, ZnO, C, and S as the compounding agents and the absence of C, ZnO, ZDC, and S. The tensile strength of the vulcanized XNBR was 27 MPa with the appropriate compounding agents and vulcanization conditions (100 °C, 60 min), with an elongation at break of 1283 %. These values were higher than the previously reported ones.[35]
The tensile properties of the flexible films with different Bi2 O3 contents are characterized in Fig. 8. As the Bi2 O3 content increased, the tensile strength and elongation at break of the Bi2 O3 /XNBR films decreased. This is caused by the filler-particle concentration dependence of the composite mechanical properties [36]. When the Bi2 O3 /XNBR system contained a sufficient amount of the reinforcement agent, the additional increase of the Bi2 O3 concentration did not help further enhancing the composite strength. When the Bi2 O3 content was below 70 wt.%, the minimum tensile strength of the Bi2 O3 /XNBR film was greater than 5 MPa, and the minimum elongation at break was above 500 %, which were higher values than those reported for rubber materials used for γ-ray shielding [13]. As the Bi2 O3 content increased, the tensile properties of the film degraded. The tensile properties of the Bi2 O3 /XNBR films containing more than 70 wt.% Bi2 O3 were not suitable for applications. The preparation of a Bi2 O3 /XNBR film with a higher filler content possessing good tensile properties is one of the research directions in the future.
As for the XNBR flexible films after harsh treatment, the minimum tensile strength was above 20 MPa, and the minimum elongation at break above 900 %. After successive submersion in gasoline, 1.5 M sulfamic acid, and 10 M NaOH for 72 h at room temperature, the tensile strength and the elongation at break of the Bi2 O3 /XNBR films decreased. However, the reductions were not considerable. They were close to that of the original state without the harsh treatment. This suggested that the Bi2 O3 /XNBR films were resistant to oil, acid, and alkali. For the samples with Bi2 O3 less than 70 wt.%, after the harsh treatment, the minimum tensile strength was above 5 MPa, and the minimum elongation at break above 500 %, which were suitable values for general applications. The tensile properties of the Bi2 O3 /XNBR films containing more than 70 wt.% Bi2 O3 were not suitable for applications.
The mass variation ratio of the Bi2 O3 /XNBR films under harsh environments also demonstrated their resistivity to oil, acid, and alkali. As shown in Fig. 9, after successive submersion in gasoline, 1.5 M sulfamic acid, and 10 M NaOH for 72 h at room temperature, the mass ratio of the Bi2 O3 /XNBR flexible films varied within a very small range. Fig. 8 and Fig. 9 show that the Bi2 O3 /XNBR flexible films were also resistant to hot air.
As shown in Fig. 10, the raw XNBR film was very soft. After vulcanization, it was still a soft material with an enhanced shore hardness. As the Bi2 O3 content increased, the shore hardness of the Bi2 O3 /XNBR films also increased. The films with less than 70 wt.% Bi2 O3 were still sufficiently flexible, in comparison to latex gloves, for applications. The Bi2 O3 /XNBR films containing more than 70 wt.% Bi2 O3 were too hard for applications.
A Bi2 O3 /XNBR film with an appropriate Bi2 O3 content can be selected to satisfy the mechanical property requirements for practical applications.
3.6 Attenuation properties of the Bi2 O3 /XNBR films
The density of the Bi2 O3 /XNBR composite was calculated by Eq. (5):
where the variables m, v, ρ, and x represent the mass, volume, density, and mass fraction, respectively. Subscripts 1 and 2 represent Bi2 O3 and XNBR, respectively. Thus, m1 /m2 =x1 /x2 =ρ1 v1 /ρ2 v2, and x1 +x2 =1. The experimental values of ρ1 and ρ2 were 8.9 g·cm-3 and 1.07 g·cm-3, respectively, which were measured by Archimedes’ principle. The density of the Bi2 O3 /XNBR flexible film is shown in Fig. 11a. The experimental and calculated values were in good agreement.
The mass attenuation coefficient of the Bi2 O3 /XNBR was calculated by equation (6):[37,38]
where µm is the mass attenuation coefficient of the Bi2 O3 /XNBR. The variables µ and ρ are the line attenuation coefficient and density of the Bi2 O3 /XNBR, respectively. The variables µm,1 and µm,2 are the mass attenuation coefficient of Bi2 O3 and XNBR, respectively. The mass attenuation coefficients of Bi2 O3 for γ-rays can be obtained through a Monte Carlo simulation or literature database research. The mass attenuation coefficient of Bi2 O3 for 59.5 keV γ-rays was 0.42736 m2·kg-1 as simulated by the Monte Carlo method, which was consistent with the literature value [39]. Because XNBR is mainly composed of carbon, the mass attenuation coefficient of XNBR was approximately equal to that of carbon. According to the literature, the mass attenuation coefficient of carbon for 59.5 keV γ-rays is 0.017 m2·kg-1[40].
The linear attenuation coefficient of Bi2 O3 for low-energy γ-rays was slightly lower than that of PbO (Fig. 11a). However, Bi2 O3 is non-toxic, which is an advantage.
The linear attenuation coefficient of the Bi2 O3 /XNBR flexible film was calculated by Eq. (7):
The linear γ-ray attenuation coefficients of the Bi2 O3 /XNBR flexible film for energies 30.7 keV, 59.5 keV, and 81.0 keV are shown in Fig. 11b and 11c. The experimental values of µ obtained from equation (1) and the calculated values from Eq. (7) were in good agreement, and consistent with the literature [27]. Ideally, the higher the Bi2 O3 content, the greater the γ-ray attenuation. Since each sample was exposed to each energy only for a short time, there no obvious change was observed through SEM after the low-energy γ-ray attenuation test.
The attenuation efficiencies of the Bi2 O3 /XNBR flexible films, with different thicknesses and different Bi2 O3 contents, for selected γ-ray energies, were calculated by Eq. (2). Fig. 12 presents the γ-ray attenuation efficiencies of the Bi2 O3 /XNBR flexible films for energies 30.7 keV, 59.5 keV, and 81.0 keV, respectively. The attenuation efficiency of the Bi2 O3 /XNBR flexible films for low-energy (20–100 keV) γ-rays can be obtained using the same method. This investigation aids the engineering design. If an attenuation efficiency requirement is proposed, a suitable thickness and Bi2 O3 content can be used to satisfy it. For a Bi2 O3 /XNBR flexible film, the Bi2 O3 content can be calculated by measuring its density and the thickness of the film, and then, its γ-ray attenuation efficiency can be assessed.
4 Conclusion
The design, development, and investigation of Bi2 O3 /XNBR flexible films were successfully conducted. The important conclusions are summarized below:
The Bi2 O3 sedimentation problem in the XNBRL was solved by reducing the Bi2 O3 particle radius, increasing the viscosity of the latex, and adding a dispersant. The microscopy results confirmed that Bi2 O3 was well dispersed in the XNBRL.
When the Bi2 O3 content was below 70 wt.%, the tensile strength of the Bi2 O3 /XNBR film was greater than 5 MPa, and the minimum elongation at break was above 500 %. The prepared Bi2 O3 /XNBR flexible films prepared by dip-molding were resistant to oil, acid, alkali, and hot air. As the Bi2 O3 content increased, the mechanical properties of the films degraded. The mechanical properties of the Bi2 O3 /XNBR films containing more than 70 wt.% Bi2 O3 were not suitable for applications. The preparation of a Bi2 O3 /XNBR film with a higher filler content and good mechanical properties is one of the research directions in the future.
The linear attenuation coefficients of the Bi2 O3 /XNBR films experimentally obtained were in good agreement with the calculated values. The attenuation efficiencies of the Bi2 O3 /XNBR films with different thicknesses and different Bi2 O3 contents were obtained. The results demonstrated that the Bi2 O3 /XNBR flexible films have a good attenuation effect for low-energy (20–100 keV) γ-rays.
This work aids the engineering and design of the films. A Bi2 O3 /XNBR film with an appropriate Bi2 O3 content can be selected to meet the requirements on mechanical properties, thickness, and attenuation efficiency in practical applications.
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