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EPDM rubber composites
Author’s Accepted Manuscript Properties of lead-free gamma-ray shielding materials from metal oxide/EPDM rubber composites Worawat Poltabtim, Ekachai Kiadtisak Saenboonruang
Wimolmala, www.elsevier.com/locate/radphyschem
PII: DOI: Reference:
S0969-806X(18)30572-3 https://doi.org/10.1016/j.radphyschem.2018.08.036 RPC7987
To appear in: Radiation Physics and Chemistry Received date: 25 June 2018 Revised date: 28 August 2018 Accepted date: 29 August 2018 Cite this article as: Worawat Poltabtim, Ekachai Wimolmala and Kiadtisak Saenboonruang, Properties of lead-free gamma-ray shielding materials from metal oxide/EPDM rubber composites, Radiation Physics and Chemistry, https://doi.org/10.1016/j.radphyschem.2018.08.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Properties of lead-free gamma-ray shielding materials from metal oxide/EPDM rubber composites
Worawat Poltabtim1, Ekachai Wimolmala2* and Kiadtisak Saenboonruang1, 3**
1
Department of Applied Radiation and Isotopes, Faculty of Science, Kasetsart University, Bangkok, Thailand
2
Polymer Processing and Flow (P-PROF) Research Group,
Division of Materials Technology, School of Energy, Environment and Materials, King Mongkut’s University of Technology Thonburi, Bangkok, Thailand
3
Specialized center of Rubber and Polymer Materials for agriculture and industry (RPM), Faculty of Science, Kasetsart University, Bangkok, Thailand
__________________________________________________________________ * Corresponding authors. E-mail address: [email protected] (E. Wimolmala) ** Corresponding authors. E-mail address: [email protected] (K. Saenboonruang)
1
ABSTRACT This work investigated the gamma-ray shielding, cure characteristics, physical, and mechanical properties of ethylene propylene diene monomer (EPDM) rubber composites with additions of metal oxides (either iron (II, III) oxide; Fe3O4, tungsten (III) oxide; W2O3 or bismuth (III) oxide; Bi2O3) for potential use as flexible, durable, and lead-free gamma-ray shielding materials. The results showed that increases in the metal oxide contents from 0 to 100, 300, and 500 phr (parts per hundred of rubber), respectively, improved the gamma-ray shielding properties of the EPDM rubber composites with the highest gamma attenuation in 500-phr Bi2O3. The results also showed initial decreases in the cure times at a 100-phr metal oxide content but subsequently increased at higher contents (300 phr and 500 phr). Furthermore, while tensile modulus at 100% elongation and hardness (Shore A) kept increasing with increasing metal oxide contents, tensile strength and elongation at break behaved differently as they initially increased at 100 phr but later decreased at 300 phr and 500 phr. In addition, after the accelerated-weathering studies, the tensile modulus at 100% elongation and tensile strength of the EPDM rubber composites increased but the elongation at break and hardness (Shore A) decreased. In summary, the overall properties suggested that these developed metal oxide/EPDM rubber composites could be effectively used to replace lead-containing materials as durable gamma-ray shielding materials.
Keywords: Gamma shielding; Radiation protection; Mechanical properties; EPDM; Metal oxide; Rubber composites
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1. Introduction Gamma rays (γ) are electromagnetic radiation emitted from the radioactive decay of atomic nuclei with typical energies above 100 keV. Since the discovery of gamma rays by Paul Ulrich Villard in 1900, gamma technologies have been significantly improved due to the fast-growing numbers of applications, including gamma imaging [Zhu et al., 2005; Rhodes et al., 2011], astrophysics and astronomy research [Budney et al., 2014], Particle Induced Gamma-ray Emission (PIGE) for elemental analysis [Chhillar et al., 2014; Khandaker et al., 2007; Khandaker et al., 2013; Khandaker et al., 2014; Srivastava et al., 2014], and gamma knife radiosurgery [Kondziolka et al., 2016; Kruyt et al., 2018]. However, excessive exposure to gamma rays could affect radiation users and the general public in several ways such as nausea, vomiting, fatigue, diarrhoea, headache, skin burns, and death [Kiefer, 1990; Walsh, 2013]. In order to reduce the risks of excessive gamma exposure, sufficient installation of high quality gamma-ray shielding materials is required. In principle, materials comprised of high-atomic number (Z) and high-density elements are good candidates to be used as effective gamma shields due to their high probability of interactions and larger energy transfer with gamma rays. These requirements make pure lead (Pb) sheets1 and leadcontaining materials excellent gamma-ray shielding candidates [McCaffrey et al., 2007]. Pb is also economically accessible and available in various forms. Examples of such materials are Portland cement mixed with granulated Pb [Hosiny and El-Faramawy, 2000], PbO-SiO2 glasses [Singh et al., 2000], and Pb/natural rubber (NR) composites [Gwaily et al., 2002]. However, these materials pose serious environmental and health risks to product users as Pb is considered toxic and lacks chemical stability [Demayo et al, 1982; Seregin et al, 2001; Needleman and Landrigen, 2004]. As a result, new kinds of lead-free gamma-ray shielding materials that are safe and environmental friendly have been developed to substitute the 1
Pb has an atomic number of 82 and a density of 11.34 g/cm3.
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troublesome lead-containing materials. For instance, bismuth borate glasses and bismuth borosilicate glasses were developed as transparent gamma-ray shielding materials [Singh et al., 2002; Singh et al., 2014], while EVA/tungsten carbide (WC) and NR/iron (II, III) oxide (Fe3O4) composites were developed as elastic and lightweight materials [Soylu et al., 2015; Toyen et al., 2018]. As the needs for better and safer gamma-ray shielding materials have widened to more applications, materials with specific properties that serve particular applications are required. EPDM rubber, which is a synthetic elastomer with properties suitable to be used in outdoor applications, is one of the flexible materials that have great potential for use in radiation-shielding applications. Unlike natural rubber (NR), the distinguishing properties of EPDM rubber include great resistance to UV, ozone, heat, weathering, steam and water, and chemicals [Azo Material, 2018]. EPDM rubber also responds well to high filler and plasticizer loadings, making intended improvement to the composites possible [Usuki et al., 2002; Zheng et al., 2004; Soto-Oviedo et al., 2006]. As a result, EPDM rubber is typically used as vehicle door and window seals, appliance hoses, or electrical insulators [Dikman and Basdogan, 2008]. Some EPDM composites with the addition of specific fillers are also used in radiation-related applications. For example, EPDM/boric acid composites have been used as neutron-shielding materials and EPDM/carbon black and carbon fibre composites as electromagnetic interference (EMI) shields [Das et al., 2001; Ozdemir et al., 2016]. This work investigated properties of EPDM rubber composites that were developed to be used as flexible and lead-free gamma-ray shielding materials by introducing suitable gamma-protective fillers, namely iron (II, III) oxide (Fe3O4), tungsten (III) oxide (W2O3), or bismuth (III) oxide (Bi2O3), into the composites. The properties of interest included cure characteristics, gamma-ray shielding, physical, mechanical properties, and QUV accelerated weathering effects caused by ultraviolet (UV) and moisture.
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2. Experimental 2.1 Materials and chemicals VistalonTM 3666 EPDM rubber, which is an oil extended terpolymer grade with an ethylene content of 64 wt% and a Mooney viscosity (ML 1+4, 125oC) of 52, was used as the main matrix for this work. The vulcanizing formulations, which include all the chemicals and their suppliers, are given in Table 1.
Table 1 Material formulations of EPDM composites including chemicals and their suppliers. Ingredient
Content
Supplier
(phr)2 EPDM rubber
100 (part)
ExxonMobil Chemical Asia Pacific, (Singapore)
Zinc oxide; ZnO
5.0
Thai-Lysaght Co., Ltd., Phranakhon Si Ayutthaya, (Thailand)
Stearic acid
1.5
Imperial Industrial Chemicals Co., Ltd. Bangkok, (Thailand)
Paraffinic oil
30.0
Chemical Innovation Co., Ltd., (Thailand)
Mercaptobenzthaisoles; MBT
0.5
CMC advance Co., Ltd. Samut Sakhon, (Thailand)
Zinc dimethacrylate; ZDMA,
1.5
Idemitsu Kosan Co., Ltd., (Japan)
Tetramethylthiuram disulphide; TMTD
0.5
Zeon Advanced Polymix Co., Ltd. Rayong, (Thailand)
N-(1,3dimethylbutyl)-N’-phenyl-p-
2.0
Behn Meyer Chemical (T) Co., Ltd.,
Dymalink 643 grade
phenylenediamine; 6PPD Sulphur powder (325 mesh)
Bangkok, (Thailand) 0.7
Zeon advanced Polymix Co., Ltd. Rayong, (Thailand)
Carbon black (HAF N550)
40.0
Thai Carbon Black Public Co., Ltd. AngThong, (Thailand)
2
phr: parts per hundred of rubber
5
2.2 Gamma-protective fillers Three metal oxide fillers were used as gamma-protective fillers in this work (Fe3O4, W2O3, and Bi2O3) and were supplied by Shanghai Ruizheng Chemical Technology Co., Ltd., China. Images and their average particle sizes of Fe3O4, W2O3, and Bi2O3, recorded and analysed using a scanning electron microscope (SEM) (Quanta 450 FEI, the Netherlands) and ImageJ software version 1.50i, are shown in Fig. 1. Different contents of Fe3O4, W2O3 or Bi2O3 (0, 100, 300 to 500 phr) were mixed with the ingredients shown in Table 1.
Fig. 1. SEM images and average particle sizes of (a) Fe3O4 (60000) (b) W2O3 (600) and (c) Bi2O3 (750)
These metal oxides were selected due to their relatively high Z, high densities, high melting points, lower toxicity than Pb and availability in fine powders. Fe, W, and Bi in these metal oxides also have relatively high mass attenuation coefficients (μm), especially Bi, which has higher values than Pb, indicating great potential for Fe3O4, W2O3, and Bi2O3 to substitute Pb as gamma-protective fillers. The μm values of Fe, W, Pb and Bi at different gamma energies are shown in Table 2 [NIST, 2004].
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Table 2 Mass attenuation coefficient (μm) of Fe, W, Pb and Bi at different gamma energies Mass attenuation coefficient; μm (cm2/g) Element
Z
Gamma energy Gamma energy Gamma energy E = 0.1 MeV
E = 1.0 MeV
E = 10.0 MeV
Fe
26
0.372
0.059
0.029
W
74
4.438
0.062
0.047
Pb
82
5.549
0.071
0.049
Bi
83
5.739
0.072
0.050
2.3 Preparation of EPDM composites The EPDM composites were prepared using two steps: mastication and compounding. The EPDM was masticated on a laboratory two-roll mill (Yong Fong Machinery Co., Ltd., Samutsakhon, Thailand) for 5 min. The masticated EPDM was then compounded with the prepared chemicals given in Table 1 and metal oxide fillers with their contents varying from 0 to 100, 300 and 500 phr, respectively. The compounding process was carried out on the two-roll mill for 25 min to ensure a uniform distribution of the fillers. The resultant EPDM rubber composites were then kept at 25-30oC and 50% humidity prior to use.
2.4 Characterizations 2.4.1 Cure characteristics The scorch time (ts1), cure time at 90% (tc90), and torque differences of the EPDM compounds were evaluated using an oscillating die rheometer (ODR GT 70-70-S2, GOTECH Testing Machine, Inc., Taichung, Taiwan) whose procedures followed ASTM D2048-11 standard testing at a test temperature of 170oC. The determined cure time was used for vulcanizing the EPDM composites sheet in a hot compression moulding at 170 kg/cm2 using
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2 mm thick and 5 mm thick moulds with the dimensions of 15 cm 15 cm for every individual compound.
2.4.2 Mechanical properties The tensile properties tensile modulus, tensile strength, and elongation at break of the EPDM rubber composites were investigated using a universal testing machine (Autograph AG-I 5kN, Shimadzu, Tokyo, Japan), according to ASTM D412-06 standard testing. It should be noted that for comparative purposes, the tensile modulus in this work was reported at 100% elongation at which the minimum elongation of the hardest rubber compound could be stretched. The tensile testing speed used for all specimens was 500 mm/min. For hardness measurement, all EPDM rubber composites were tested according to ASTM D2240-05 using a hardness durometer (Shore A) (Teclock GS-719G, Japan).
2.4.3 Specific gravity and morphology studies For all EPDM rubber composites, the specific gravity was measured following the procedures of ASTM D297-06 by comparative measurement of each sample mass in air and water (Gibitre DenS-i-Meter, USA). A scanning electron microscope (SEM) (Quanta 450 FEI: JSM-6610LV, the Netherlands) at a 10-kV accelerating voltage with back-scattered electrons (BSE) was used to study the morphology and the distribution of fillers in EPDM rubber composites. It should be noted that the samples were coated with gold using a sputter coater (Quorum SC7620: Mini Sputter Coater/ Glow Discharge System, England) at a power voltage of 10 kV and a current of 10 mA for 120 sec before performing SEM.
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2.4.4 QUV accelerated weathering test For the studies of accelerated weathering effects caused by ultraviolet (UV) and moisture, the EPDM rubber composites were placed in a QUV accelerated-weathering tester (Q-Lab, USA) under a UV cyclic aging condition, which followed the standard testing method of ASTM G154-12, for a duration of 96 hr. A complete UV cyclic aging cycle consisted of (i) UV radiation conditions of 0.77 W/m2/nm at 60oC for 8 hr and (ii) moisture condensation conditions of 50oC for 4 hr. After the aging procedures were completed, all vulcanizates were mechanically tested to determine their level of mechanical degradation caused by UV radiation and moisture.
2.4.5 Gamma shielding properties The gamma shielding properties were measured using the setup shown in Fig. 2. The radioactive sources used in this measurement were 1-μCi 137Cs and 1-μCi 60Co point sources, which emit 662-keV and 1.25-MeV (the average of 1.173 MeV and 1.332 MeV) gamma rays, respectively.
Fig. 2. Setup for gamma shielding measurement
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The measurement was carried out inside a 6 cm thick lead cylinder. Samples of 15 cm 15 cm of EPDM rubber composites with thicknesses varying from 5 mm to 80 mm in 5 mm increments were placed between a gamma source and a gamma detector that were separated by 10 cm. The gamma source used in this test was kept in a 3-cm thick lead collimator with a 1-mm pinhole exit to minimize build-up effects that could increase the transmitted gamma rays caused by secondary or re-scattered gamma. The NaI gamma detector (Ortec 905-4, USA) used in the measurement had a circular active area of 10 cm in diameter and was powered by a high voltage power supply (Canberra 3102D, USA). Its output was sent through a series of amplifiers (Ortec 590A, USA) and a counter/timer (Canberra 2071A, USA) for signal processing and counting. The gamma attenuation properties of the shielding samples could be described by Eq. 1-3,
(1) (2) (3)
where I, I0, μl, μm, HVL, and x are the intensity of transmitted gamma rays, the intensity of incident gamma rays, the linear attenuation coefficient, the mass attenuation coefficient, the half value layer, and the thickness of the shielding material, respectively. Three independent 1-minute tests were performed and average I and I0 were recorded for each sample and thickness in order to calculate μl, μm, and HVL. The detailed definitions of μl, μm, and HVL and calculation procedures using a graph of the ratio I/I0 and x can be found in Toyen et al. (2018). For comparison purposes, pure lead sheets were also tested
10
under the same testing procedures and setup to measure the corresponding values of μl, μm, and HVL. 3. Results and discussion 3.1 Cure characteristics The results of the cure characteristics and torque differences of the EPDM rubber composites are shown in Table 3. The results showed that the scorch time of EPDM rubber composites decreased with increasing contents of Fe3O4, W2O3, and Bi2O3 except for 500-phr Fe3O4, which had a similar scorch time as pristine EPDM rubber. The decrease in scorch times could be explained by the fact that the increasing contents of metal oxide powders increased the frictional heat in the EPDM compound during the ODR testing process leading to the faster commencement of the curing process [Salaeh et al, 2011]. However, the cure times of the EPDM rubber composites behaved differently as the values initially decreased as metal oxide powder was introduced into the mixtures but later increased as the metal oxide content increased to 300 phr and 500 phr, respectively. This behaviour was observed because, with an appropriate amount of metal oxides in the EPDM rubber composites, the metal oxides could act as co-activators during the chemical vulcanizing process, leading to an initial decrease in cure times. However, as the contents of metal oxide further increased, they inversely obstructed the action of the main activators and accelerators, resulting in prolonged cure times [Ninyong et al., 2017; Toyen et al., 2018]. It should be noted that the EPDM composites with 500-phr Fe3O4 had exceptionally high scorch and cure times. This could have been due to Fe3O4 having the lowest density compared to W2O3 and Bi2O3, which led to a higher volume with the Fe3O4 powders, resulting in greater obstruction of the actions of activators and accelerators. The torque differences shown in Table 3, which could indirectly refer to the crosslink density of the composites [El-Sabbagh and Yehia, 2007], increased with increasing contents
11
of Fe3O4, W2O3, and Bi2O3. This increase in the torque difference could be due to that these metal oxides acted as co-activators during the chemical vulcanizing process. Another possible explanation of the increases in the torque differences was the much higher rigidity of metal oxides than EPDM particles, resulting in higher viscosity of the composites when more metal oxide was added during the compounding process. Another interesting result was that Fe3O4/EPDM rubber composites had higher torque differences than W2O3/EPDM and Bi2O3/EPDM rubber composites for the same content because the Fe3O4 has much smaller particle sizes (see Fig. 1 ) than the other two metal oxides, leading to better particle dispersion and higher frictional heat in addition to being coactivators.
Table 3. Scorch time, cure time and torque differences of EPDM rubber composites with addition of either Fe3O4, W2O3, or Bi2O3 Cure characteristics of EPDM rubber composites (± represent the standard deviation of the average values)
Metal oxide type
Fe3O4
W2O3
Bi2O3
Metal oxide
Scorch time,
Cure time,
Torque differences
content (phr)
(ts1) (min:sec)
(tc90) (min:sec)
(dN m)
0
1:36 (±0:01)
9:31 (±1:48)
27.0 (±0.5)
100
1:16 (±0:01)
8:02 (±0:16)
33.0 (±0.4)
300
1:07 (±0:01)
9:51 (±0:17)
44.1 (±0.2)
500
1:31 (±0:04)
17:48 (±0:36)
54.5 (±1.5)
0
1:36 (±0:01)
9:31 (±1:48)
27.0 (±0.5)
100
1:31 (±0:01)
8:28 (±0:03)
31.4 (±0.2)
300
1:12 (±0:01)
9:48 (±1:41)
39.7 (±0.4)
500
1:12 (±0:01)
11:08 (±0:11)
42.8 (±0.2)
0
1:36 (±0:01)
9:31 (±1:48)
27.0 (±0.5)
100
1:39 (±0:03)
7:42 (±0:08)
27.5 (±0.2)
12
300
1:25 (±0:01)
9:15(±0:06)
30.8 (±0.1)
500
1:11 (±0:02)
10:50 (±0:10)
33.7 (±0.5)
3.2 Mechanical properties
Fig. 3. Mechanical properties of EPDM rubber composites with varying contents of metal oxides before and after QUV accelerated weathering test: (a) tensile modulus at 100% elongation and (b) hardness (shore A). (Error bars indicate standard deviations of the average values.)
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Fig. 4. Mechanical properties of EPDM rubber composites with varying contents of metal oxides before and QUV after accelerated weathering test: (a) tensile strength and (b) elongation at break. (Error bars indicate standard deviations of the average values.)
As shown in Fig. 3, the tensile modulus at 100% elongation and hardness (Shore A) increased with increasing Fe3O4, W2O3, and Bi2O3 contents with greater effects on Fe3O4/EPDM rubber composites. The almost linear increases in the tensile modulus at 100% elongation and hardness (Shore A) could have been due to the high rigidity of the metal oxide powders used in this work, which increased the torque differences/crosslink densities of the composites that resulted in restriction of the crosslink joints of the inter-crosslink chains and an increase in the overall rigidity of the EPDM rubber composites [Zhao et al., 2011]. However, different types of behaviour by the EPDM rubber composites were seen in tensile strength and elongation at break as shown in Fig. 4. Their values initially increased when 100-phr metal oxides were introduced to the EPDM rubber composites; however, the values decreased as more metal oxide content was added. These results were observed because, with appropriate metal oxide addition, the metal oxides could act as co-activators, leading to a higher crosslink density and higher tensile strength/elongation at break. However, as more metal oxides were added to the composites, filler-filler interactions replaced more favourable rubber-filler interactions, leading to an agglomeration of metal oxide particles and worse particle dispersion. The reduction in the tensile strength and elongation at break could be explained by the morphology of the EPDM rubber composites shown in Fig. 5, where Fig. 5(a) shows EPDM rubber composites without metal oxides and Fig. 5(b) -5(d) show EPDM rubber composites with the addition of metal oxides. Fig. 5b clearly shows better filler dispersion of Fe3O4 than for W2O3 and Bi2O3 in Fig. 5(c) and 5(d), due to much the smaller particle sizes of Fe3O4 (0.3
14
µm). This better dispersion as well as the higher surface area of Fe3O4 particles led to better tensile properties in the Fe3O4/EPDM rubber composites. Fig. 5(c) and 5(d) also showed poor interfacial compatibility between W2O3, and Bi2O3 particles and the EPDM matrix, resulting in visible voids inside the composites.
Fig. 5. SEM images of EPDM rubber composites (a) without metal oxides, (b) with 500-phr Fe3O4, (c) with 500-phr W2O3, and (d) with 500-phr Bi2O3. All images were taken with magnification of 200.
The effects of the QUV accelerated weathering tests are shown in Fig. 3 and 4, which provide values for the tensile modulus at 100% elongation, hardness (Shore A), tensile strength, and elongation at break of the EPDM rubber composites after performing accelerated aging studies. The differences between before (solid lines) and after (dashed line)
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the aging tests could have been due to a post-curing effect caused by UV and heat, which further increased the crosslink density of the EPDM rubber composites, resulting in a higher tensile modulus at 100% elongation/tensile strength and lower elongation at break/hardness (Shore A) [Tabtim and Sombatsompop, 2014]. The small degrees of degradation/changes observed with all EPDM rubber composites could have been due to the structure of EPDM, which has excellent UV and heat-resistant properties, and also the addition of antioxidants in the EPDM rubber composites. These results are crucial as they imply that the EPDM rubber composites with the addition of metal oxides have weather and sunlight durability, making them suitable to be used in outdoor applications.
3.3 Specific gravity The specific gravity values of the EPDM rubber composites are shown in Table 4. The results indicated that the specific gravity of the EPDM rubber composites increased with the increasing contents of Fe3O4, W2O3 or Bi2O3 due to the much larger densities of the metal oxide powders than the pristine EPDM, causing the composites to be much heavier. It should be noted that the Bi2O3/EPDM rubber composites had the highest specific gravity among all EPDM rubber composites with the same content due to Bi2O3 having the highest density.
Table 4. Specific gravity of EPDM composites with additions of either Fe3O4, W2O3 or Bi2O3. Specific gravity of EPDM rubber composite Metal oxide Content (phr)
Fe3O4
W2O3
Bi2O3
0
1.01
1.01
1.01
100
1.31
1.46
1.49
300
1.98
2.19
2.25
16
500
2.39
2.73
2.89
The specific gravity values of the EPDM rubber composites in this work were similar to values reported in Toyen et al. (2018), in which the same types and contents of metal oxides were added to NR composites. 3.4 Gamma shielding properties Table 5. Linear attenuation coefficient (μl) of EPDM rubber composites with additions of either Fe3O4, W2O3 or Bi2O3 for 662-keV and 1.25-MeV gamma rays (mean ± standard deviation). Linear attenuation coefficient; μl (m-1) of EPDM rubber composites Metal oxide Content
Gamma energy E = 622 keV
Gamma energy E = 1.25 MeV
(phr)
Fe3O4
W2O3
Bi2O3
Fe3O4
W2O3
Bi2O3
0
5.3 ±1.6
5.3 ±1.6
5.3 ±1.6
4.1 ±1.2
4.1 ±1.2
4.1 ±1.2
100
7.8 ±0.7
9.6 ±0.8
10.3 ±0.9
6.3 ±1.1
5.8 ±0.7
6.4 ±0.8
300
10.9 ±1.1
14.7 ±1.0
18.6 ±1.1
8.2 ±1.3
8.6 ±0.5
10.9 ±0.9
500
12.6 ±0.7
19.2 ±1.0
24.3 ±2.2
11.3 ±2.2
11.7 ±0.5
13.3 ±1.7
Lead sheet
97.2 ±2.4
47.1 ±6.7
Table 6. Mass attenuation coefficient (μm) of EPDM rubber composites with additions of either Fe3O4, W2O3 or Bi2O3 for 662-keV and 1.25-MeV gamma rays (mean ± standard deviation). Metal oxide Content (phr) 0
Mass attenuation coefficient; μm (10-3 m2/kg) of EPDM rubber composite Gamma energy E = 622 keV
Gamma energy E = 1.25 MeV
Fe3O4
W2O3
Bi2O3
Fe3O4
W2O3
Bi2O3
5.2 ±1.5
5.2 ±1.5
5.2 ±1.5
4.1 ±1.1
4.1 ±1.1
4.1 ±1.1
17
100
6.0 ±0.5
6.6 ±0.5
6.9 ±0.6
4.8 ±0.8
4.0 ±0.5
4.3 ±0.5
300
5.5 ±0.6
6.7 ±0.5
8.3 ±0.5
4.1 ±0.7
3.9 ±0.2
4.8 ±0.4
500
5.3 ±0.3
7.0 ±0.4
8.4 ±0.8
4.7 ±0.9
4.3 ±0.2
4.6 ±0.6
Lead sheet
9.1 ±0.2
4.4 ±0.6
Table 7. Half value layer (HVL) of EPDM rubber composites with additions of either Fe3O4, W2O3 or Bi2O3 for 662-keV and 1.25-MeV gamma rays (mean ± standard deviation). Metal oxide Content
Gamma energy E = 622 keV
Gamma energy E = 1.25 MeV
(phr)
Fe3O4
W2O3
Bi2O3
Fe3O4
W2O3
Bi2O3
0
0.13 ±0.04
0.13 ±0.04
0.13 ±0.04
0.17 ±0.05
0.17 ±0.05
0.17 ±0.05
100
0.09 ±0.01
0.07 ±0.01
0.07 ±0.01
0.11 ±0.02
0.12 ±0.01
0.11 ±0.01
300
0.06 ±0.01
0.05 ±0.01
0.04 ±0.01
0.08 ±0.01
0.08 ±0.01
0.06 ±0.01
500
0.06 ±0.01
0.04 ±0.01
0.03 ±0.01
0.06 ±0.01
0.06 ±0.01
0.05 ±0.01
Lead sheet 3, 4
Half value layer; HVL (m) of EPDM rubber composite
0.013
0.014
standard deviation much less than 0.01
Tables 5-7 showed that the gamma shielding properties of the EPDM rubber composites improved with increasing Fe3O4, W2O3 or Bi2O3 contents, as seen by the increases in the values of the linear attenuation coefficients (μl) and mass attenuation coefficients (μm), and the decreases in the values of the half value layers (HVL). This improvement was due to the increase in the number of Fe, W, and Bi atoms, which were the active gamma attenuators, in the EPDM rubber composites. The effects of filler contents on gamma shielding properties could be shown by the schemes in Fig. 6, which show the EPDM rubber composites having 18
the same type of gamma protective fillers but at different contents (Fig. 6a has higher content of gamma protective fillers than Fig. 6b). Since the attenuations of gamma rays depend largely on Compton scattering and photoelectric effects between incident gamma rays and atoms of gamma protective fillers, the composites in Fig. 6a had higher probabilities of gamma interactions, leading to less transmitted gamma rays and better gamma attenuation.
Fig. 6. Schemes show gamma interactions with gamma protective fillers in EPDM rubber composites, in which (a) has higher contents than (b), leading to more gamma interactions and less gamma transmission in (a).
A comparison of Tables 5-7 indicate that the Bi2O3/EPDM rubber composites had the highest μl and μm, and the lowest HVL among all investigated composites. This was because Bi in Bi2O3 had the highest Z and μm as shown in Table 2 values among all the metal oxides used in this work, leading to better interaction with gamma rays and a higher gamma attenuating efficiency. The effects of filler types on gamma attenuation could be shown by 19
the schemes in Fig. 7, which shows the composites having the same contents but different types of gamma protective fillers and thus different μm (Fig. 7a shows the composites with the fillers having higher μm than Fig. 7b). This larger μm, represented by sizes of the gamma protective fillers, leads to more interactions between incident gamma rays and fillers’ atoms in Fig. 7a and, hence, less transmitted gamma rays. The results could be explained by the same mechanism as shown in Fig. 6
Fig. 7. Schemes show gamma interactions with different gamma protective fillers in EPDM composites, in which (a) comprises of fillers with larger values of μm as shown by larger fillers’ size than (b), leading to more gamma interactions and less gamma transmission in (a).
Another interesting result was that the ability to shield 662-keV gamma rays of all EPDM rubber composites was higher than for 1.25-MeV gamma rays for all EPDM rubber composites. This result could be explained by the fact that higher-energy gamma rays had a 20
lower probability of interactions with materials than did the lower-energy gamma rays, leading to lower numbers of interactions, less energy transfers, and more gamma rays being transmitted [Nelson and Reilly, 2018]. A comparison of the gamma shielding properties of the EPDM rubber composites with the pure lead sheets under the same setup indicated that pure lead sheets could attenuate gamma rays at both energies more efficiently than all the EPDM rubber composites for the same thickness of material. The results were expected as EPDM composites were a mixture of EPDM rubber and metal oxide powders, leading to less gamma-protective fillers than pure lead sheets at the same thickness of materials. However, when considering the same mass of materials, the EPDM composites with Bi2O3 contents of 300 phr and 500 phr had only slightly less μm than pure lead sheets at the gamma energy of 662 keV, but higher μm at the gamma energy of 1.25 MeV. This result was intriguing as it showed that the developed EPDM rubber composites have comparable gamma-ray shielding ability as pure lead sheets, posing great potential to be efficiently used in applications that require flexibility, durability, and light weight such as in personal protective wear or radioactive containers. It should be noted that the maximum metal oxide contents reported in this work were 500 phr. This was due to the increasing difficulty during mixing process of all chemicals with EPDM rubbers when metal oxide contents were greater than 500 phr. Nonetheless, the results obtained in this work were sufficient to show the differences in properties of EPDM composites caused by different types and contents of metal oxides.
Table 8. Comparison of linear attenuation coefficients in different materials. (N/A: represents data not reported and values are represented as mean ± standard deviation).
Material
Linear attenuation coefficient (m-1)
21
Reference
Gamma energy
Gamma energy
E = 622 keV
E = 1.25 MeV
EPDM/500-phr Fe3O4
12.6 ±0.7
11.3 ±2.2
This work
EPDM/500-phr W2O3
19.2 ±1.0
11.7 ±0.5
This work
EPDM/500-phr Bi2O3
24.3 ±2.2
13.3 ±1.7
This work
NR/500-phr Fe3O4
8.0 ±1.2
4.4 ±0.6
Toyen, et al. (2018)
NR/500-phr W2O3
13.8 ±0.8
7.1 ±0.8
Toyen, et al. (2018)
NR/500-phr Bi2O3
20.4 ±1.2
10.5 ±1.0
Toyen, et al. (2018)
NR/300-phr Pb
20.0
11.0
Gwaily, et al. (2002)
NR/500-phr Pb
26.0
15.0
Gwaily, et al. (2002)
NR/400-phr Galena5
26.7
N/A
Gwaily (2002)
NR/500-phr Galena
29.5
N/A
Gwaily (2002)
5
Galena is the natural mineral form of lead (II) sulphide (PbS), the most important ore of lead.
As shown in Table 8, the developed EPDM rubber composites in this work had comparable μl values with previous reports. In particular, the EPDM rubber composites with 500-phr Bi2O3 had a slightly lower μl value than NR/500-phr Pb [Gwaily et al., 2002]. The lower μl value of EPDM/500-phr Bi2O3 could be explained by the fact that since Bi2O3 consists of Bi and O atoms, in which Bi atoms contribute approximately 89% of the molecular mass, the EPDM rubber composites with 500-phr Bi2O3 content had an actual Bi content of only approximately 445 phr, resulting in slightly lower numbers of gammaprotective fillers, that is Bi atoms. Another explanation of the differences between μl reported in this work and other work could be differences in the testing setup. For example, a smaller and longer collimator exit could reduce the numbers of re-scattered gamma rays to the detector, leading to a lower intensity of transmitted gamma rays (I) and, consequently, higher 22
μl values. This explanation was apparent when considering reports from this work and ref. [Toyen et al., 2018], which used un-collimated gamma rays. For the same 500-phr metal oxide content, the EPDM rubber composites had higher μl values than the NR composites at both gamma energies. The differences could be reasonably explained by the use of a collimator in current study. Nonetheless, the results obtained from this work clearly showed that the EPDM rubber composites had efficient gamma-ray shielding abilities and could potentially replace hazardous lead-containing materials with excellent additional properties in flexibility and durability. Furthermore, when compared EPDM/500-phr Bi2O3 composites with other gamma-ray shielding materials such as Tellurite glass, it was found that μm of EPDM/500-phr Bi2O3 was comparable with the reported values by El-Mallawany (2018), which used WinXCOM program to simulate the gamma-ray shielding properties of Tellurite glass mixed with various metal oxides. For example, EPDM/500-phr Bi2O3 had the value of 4.6×10-3 m2/kg (at gamma energy of 1.25 MeV), while 80TeO2-5TiO2-10WO3-5Nb2O5 and 90TeO2-10V2O3 had the values of 3.4 m2/kg and 3.3 m2/kg, respectively. The smaller values reported by El-Mallawany (2018) could be due to higher gamma energy used in simulation, as well as lower contents of metal oxide in Tellurite glass. Another interesting outcome that should be discussed is the correlation between gamma-ray shielding and mechanical properties of the investigated composites. The results obtained in Section 3.2 and Section 3.4 indicated that higher contents of metal oxides generally improved gamma-ray shielding but reduced overall mechanical properties, especially tensile strength and elongation at break. As a consequence, a correlation between these two properties must be thoroughly taken into account when applying the composites in actual uses such that safety of users must not be compromised in terms of both radiation and physical aspects.
23
In summary, these developed metal oxide/EPDM rubber composites could be used in various applications including shielded containers for radioactive transportation, electrical insulation in nuclear facilities, and construction parts of buildings. The composites are also safer and more environmentally friendly to users than lead-containing materials.
4. Conclusions Since lead-containing materials used in gamma-ray shielding applications pose environmental and health risks to product users, safer materials with efficient gamma-ray shielding ability are needed. This work developed flexible, lead-free, gamma-ray shielding materials based on EPDM rubber composites by introducing metal oxides (Fe3O4, W2O3 or Bi2O3) with their contents varied from 0 to 100, 300, and 500 phr. The results showed that Fe3O4, W2O3 and Bi2O3 worked effectively as active fillers to shield gamma rays with their shielding ability comparable to lead-containing materials. The results also indicated that metal oxide contents had effects on cure times of EPDM composites by initially decreased the values at 100 phr but later increased at 300 and 500 phr. Furthermore, tensile modulus at 100% elongation and hardness (Shore A) gradually increased with increasing metal oxide contents, however, similar behaviours as cure times were observed in tensile strength and elongation at break. This work also investigated the effects of QUV accelerated weathering on the possible degradation of the tensile properties in the EPDM rubber composites. The results showed that the aging tests led to increases in the tensile modulus at 100% elongation and tensile strength but to slight decreases in the elongation at break and hardness (Shore A). Hence, based on the experimental results presented in this work, it could be concluded that the developed EPDM rubber composites have great potential to be used as flexible, lead-free gamma-ray shielding materials as shown in their comparable µm with lead sheets. However, in order to improve shielding abilities of the composites while still maintaining their
24
flexibility, improved mixing procedure and revised formulation with greater metal oxide contents are encouraged in future work.
Acknowledgement We would like to thank the Kasetsart University Research and Development Institute (KURDI), Bangkok, Thailand, the National Research Council of Thailand, and the Office of the Higher Education Commission (OHEC) under the National Research University (NRU) Program for financial support. We also wish to thank the specialized Center of Rubber and Polymer Materials for Agriculture and Industry (RPM), Faculty of Science, Kasetsart University for publication support. The authors would also like to give appreciation to P r o f . Dr. Narongrit Sombatsompop at King Mongkut’s University of Technology Thonburi for his advice and suggestion during preparation of this manuscript.
References Azo Materials, 2018. Ethylene Propylene Rubbers – Properties and Applications of Ethylene Propylene Diene (EPDM) and Ethylene Propylene Copolymers (EPM). https://www.azom.com/article.aspx?ArticleID=1822 (Accessed 5 June 2018) Budnev, N.M., Astapov, I.I., Bogdanov, A.G., Boreyko, V., Buker, M., Bruckner, M., Chiavassa, A., Gafarov, A.V., Chvalaev, O.B., Gorbunov, N., 2014. TAIGA the Tunka Advanced Instrument for cosmic ray physics and gamma astronomy, J. Instrum., 9, C09021 Chhillar, S., Acharya, R., Sodaye, S., Pujari, P.K., 2014. Development of particle induced gamma-ray emission methods for nondestructive determination of isotopic
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composition of boron and its total concentration in natural and enriched samples, Anal. Chem., 86, 11167-11173. Das, N.C., Chaki, T.K., Khastgir, D., Chakraborty, A., 2001. Electromagnetic interference shielding effectiveness of conductive carbon black and carbon fiber-filled composites based on rubber and rubber blends, Adv. Polym. Technol., 20, 226-236. Demayo, A., Taylor, M.C., Taylor, K.W., Hodson, P.V., Hammond, P.B., 1982. Toxic effects of lead and lead compounds on human health, aquatic life, wildlife plants, and livestock, Crit. Rev. Environ. Control, 12, 257-305. Dikmen, E., Basdogan, I., 2008. Material characteristics of a vehicle door seal and its effect on vehicle vibrations, Vehicle Sys. Dyn., 46, 975-990. El-Mallawany, R., 2018. Chapter 2, Radiation shielding properties of Tellurite glasses, in: ElMallawany, R. (Eds.), Tellurite glass smart materials: applications in optics and beyond. Springer International Publishing Inc., Switzerland, 17-27. ISBN: 978-3-31976568-6. El-Sabbagh, S.H., Yehia, A.A., 2007. Detection of crosslink density by different methods for natural rubber blended with SBR and NBR, Egypt. J. Solids, 30, 157-173. Gwaily, S.E., 2002. Galena/(NR+SBR) rubber composites as gamma radiation shields, Polym. Test., 21, 883-887. Gwaily, S.E., Madani, M., Hassan, H.H., 2002. Lead-natural rubber composites as gamma radiation shields. II: High concentration, Polym. Compos., 23, 495-499. Hosiny, F.I., El-Faramawy, N.A., 2000. Shielding of gamma radiation by hydrated Portland cement-lead pastes, Radiat. Meas., 32, 93-99. Khandaker, M.U., Uddin, M.S., Kim, K.S., Lee, Y.S., Kim, G.N., 2007. Measurement of cross-sections for the (p, xn) reactions in natural molybdenum, Nucl. Instrum. Methods Phys. Res. B., 262 (2), 171-181.
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Khandaker, M.U., Haba, H., Kanaya, J., Otuka, N., 2013. Excitation functions of (d,x) nuclear reactions on natural titanium up to 24 MeV. Nucl. Instrum. Methods Phys. Res. B., 296, 14-21. Khandaker, M.U., Kim, K., Lee, M., Kim, G., 2014. Investigation of activation cross-sections of alpha-induced nuclear reactions on natural cadmium. Nucl. Instrum. Methods Phys. Res. B., 333, 80-91. Kiefer, J., 1990. Biological Radiation Effects, Springer-Verlag, Berlin Heidelberg. Kondziolka D., Patel A., Kano, H., Flickinger, J.C., Lunsford, L.D., 2016. Long-term outcomes after gamma knife radiosurgery for meningiomas, J. Clin. Oncol., 39, 453457. Kruyt, I.J., Verheul, J.B., Hanssens, P.E.J., Kunst, H.P.M., 2018. Gamma Knife radiosurgery for treatment of growing vestibular schwannomas in patients with neurofibromatosis Type 2: a matched cohort study with sporadic vestibular schwannomas, J. Neurosurg., 128, 49-59. McCaffrey, J.P., Shen, H., Downton, B., Mainegra-Hing, E., 2007. Radiation attenuation by lead and nonlead materials used in radiation shielding garments, Med. Phys., 34, 530537. Needleman, H.L., Landrigan, P.J., 2004. What level of lead in blood is toxic for a child?, Am. J. Public Health, 94, 8. Nelson, G., Reilly, D., 2018. Gamma-Ray Interactions with Matter. http://www.lanl.gov/orgs/n/n1/panda/00326397.pdf. (Accessed 5 June 2018) Ninyong, K., Wimolmala, E., Sombatsompop, N., Saenboonruang, K., 2017. Potential use of NR and wood/NR composites as thermal neutron shielding materials, Polym. Test., 59, 336-343. NIST Standard Reference Database 126, 2004. X-Ray mass attenuation coefficients.
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URL: https://physics.nist.gov/PhysRefData/XrayMassCoef/tab3.html (Accessed: 5 June 2018) Ozdemir, T., Akbay, I.K., Uzun, H., Reyhancan, I.A., 2016. Neutron shielding of EPDM rubber with boric acid: Mechanical, thermal properties and neutron absorption tests, Prog. Nucl. Energy, 89, 102-109. Rhodes, D.J.,Hruska, C.B., Phillips, S.W., Whaley, D.H., O’Connor, M.K., 2011. Dedicated dual-head
gamma imaging for breast
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mammographically dense breasts, Radiology, 258, 106-118. Salaeh, S., Muensit, N., Bomlai, P., Nakason, C., 2011. Ceramic/natural rubber composites: influence types of rubber and ceramic materials on curing, mechanical, morphological, and dielectric properties, J. Mater. Sci., 46, 1723-1731. Seregin, I.V., Ivanov, V.B., 2001. Physiological aspects of cadmium and lead toxic effects on higher plants, Russ. J. Plant Physiol., 48, 523-544. Singh, K., Singh, H., Sharma, V., Nathuram, R., Khanna, A., Kumar, R., Bhatti, S.S., Sahota, H.S., 2002. Gamma-ray attenuation coefficients in bismuth borate glasses, Nucl. Instrum. Meth. Phys. Res. B., 194, 1-6. Singh, K.J., Singh, N., Kaundal, R.S., Singh, K., 2008. Gamma-ray shielding and structural properties of PbO–SiO2 glasses, Nucl. Instrum. Meth. Phys. Res. B., 266, 944-948. Singh, V.P., Badiger, N.M., Chantima, N., Kaewkhao, J., 2014. Evaluation of gamma-ray exposure buildup factors and neutron shielding for bismuth borosilicate glasses, Radiat. Phys. Chem., 98, 14-21. Soto-Oviedo, M.A., Araujo, O.A., Faez, R., Rezende, M.C., Paoli, M.D., 2006. Antistatic coating and electromagnetic shielding properties of a hybrid material based on polyaniline/organoclay nanocomposite and EPDM rubber, Synth. Met., 156, 12491255.
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Soylu, H.M., Lambrecht, F.Y., Ersoz, O.A., 2015. Gamma radiation shielding efficiency of a new lead-free composite material, J. Radioanal. Nucl. Chem., 305, 529-534. Srivastava, A., Chhillar, S., Singh, D., Acharya, R., Pujari, P.K., 2014. Determination of fluorine concentrations in soil samples using proton induced gamma-ray emission, J. Radioanal. Nucl. Chem., 302, 1461-1464. Taptim, K., Sombatsompop, N., 2014. Effects of UV weathering on the mechanical and antibacterial performance of peroxide-cured silicone rubber containing biocide HPQM, J. Vinyl Addit. Techn., 20, 49-56. Toyen, D., Rittirong, A., Poltabtim, W., Saenboonruang, K., 2018. Flexible, lead free, gamma shielding materials based on natural rubber/metal oxide composites, Iranian Polym. J., 27, 33-41. Usuki, A., Tukigase, A., Kato, M., 2002. Preparation and properties of EPDM-clay hybrids, Polym., 43, 2185-2189. Walsh, L., 2013. Neutron relative biological effectiveness for solid cancer incidence in the Japanese A-bomb survivors: an analysis considering the degree of independent effects from γ-ray and neutron absorbed doses with hierarchical partitioning, Radiat. Environ. Biophys., 52, 29-36. Zhao, F., Bi, W., Zhao, S., 2001. Influence of crosslink density on mechanical properties of natural rubber vulcanizates, J. Macromol. Sci. B, 50, 1460-1469. Zheng, H., Zheng, Y., Peng, Z., Zhang, Y., 2004. Influence of clay modification on the structure and mechanical properties of EPDM/montmorillonite nanocomposites, Polym. Test., 23, 217-223. Zhu, Z., Zhao, L., Lei, J., 2005. 3D measurements in cargo inspection with a gamma-ray linear pushbroom stereo system, Proceedings of 2005 IEEE Computer Conference on Computer Vision and Pattern Recognition, San Diego, CA, USA.
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Highlights
Metal oxide/EPDM rubber composites were developed as gamma-ray shielding materials.
Bi2O3/EPDM rubber composites had the highest gamma-ray shielding properties.
500-phr Bi2O3/EPDM rubber composites had higher μm than pure Pb sheets.
Bi2O3/EPDM composites could replace commercial lead-containing shielding materials.
30
Wimolmala, www.elsevier.com/locate/radphyschem
PII: DOI: Reference:
S0969-806X(18)30572-3 https://doi.org/10.1016/j.radphyschem.2018.08.036 RPC7987
To appear in: Radiation Physics and Chemistry Received date: 25 June 2018 Revised date: 28 August 2018 Accepted date: 29 August 2018 Cite this article as: Worawat Poltabtim, Ekachai Wimolmala and Kiadtisak Saenboonruang, Properties of lead-free gamma-ray shielding materials from metal oxide/EPDM rubber composites, Radiation Physics and Chemistry, https://doi.org/10.1016/j.radphyschem.2018.08.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Properties of lead-free gamma-ray shielding materials from metal oxide/EPDM rubber composites
Worawat Poltabtim1, Ekachai Wimolmala2* and Kiadtisak Saenboonruang1, 3**
1
Department of Applied Radiation and Isotopes, Faculty of Science, Kasetsart University, Bangkok, Thailand
2
Polymer Processing and Flow (P-PROF) Research Group,
Division of Materials Technology, School of Energy, Environment and Materials, King Mongkut’s University of Technology Thonburi, Bangkok, Thailand
3
Specialized center of Rubber and Polymer Materials for agriculture and industry (RPM), Faculty of Science, Kasetsart University, Bangkok, Thailand
__________________________________________________________________ * Corresponding authors. E-mail address: [email protected] (E. Wimolmala) ** Corresponding authors. E-mail address: [email protected] (K. Saenboonruang)
1
ABSTRACT This work investigated the gamma-ray shielding, cure characteristics, physical, and mechanical properties of ethylene propylene diene monomer (EPDM) rubber composites with additions of metal oxides (either iron (II, III) oxide; Fe3O4, tungsten (III) oxide; W2O3 or bismuth (III) oxide; Bi2O3) for potential use as flexible, durable, and lead-free gamma-ray shielding materials. The results showed that increases in the metal oxide contents from 0 to 100, 300, and 500 phr (parts per hundred of rubber), respectively, improved the gamma-ray shielding properties of the EPDM rubber composites with the highest gamma attenuation in 500-phr Bi2O3. The results also showed initial decreases in the cure times at a 100-phr metal oxide content but subsequently increased at higher contents (300 phr and 500 phr). Furthermore, while tensile modulus at 100% elongation and hardness (Shore A) kept increasing with increasing metal oxide contents, tensile strength and elongation at break behaved differently as they initially increased at 100 phr but later decreased at 300 phr and 500 phr. In addition, after the accelerated-weathering studies, the tensile modulus at 100% elongation and tensile strength of the EPDM rubber composites increased but the elongation at break and hardness (Shore A) decreased. In summary, the overall properties suggested that these developed metal oxide/EPDM rubber composites could be effectively used to replace lead-containing materials as durable gamma-ray shielding materials.
Keywords: Gamma shielding; Radiation protection; Mechanical properties; EPDM; Metal oxide; Rubber composites
2
1. Introduction Gamma rays (γ) are electromagnetic radiation emitted from the radioactive decay of atomic nuclei with typical energies above 100 keV. Since the discovery of gamma rays by Paul Ulrich Villard in 1900, gamma technologies have been significantly improved due to the fast-growing numbers of applications, including gamma imaging [Zhu et al., 2005; Rhodes et al., 2011], astrophysics and astronomy research [Budney et al., 2014], Particle Induced Gamma-ray Emission (PIGE) for elemental analysis [Chhillar et al., 2014; Khandaker et al., 2007; Khandaker et al., 2013; Khandaker et al., 2014; Srivastava et al., 2014], and gamma knife radiosurgery [Kondziolka et al., 2016; Kruyt et al., 2018]. However, excessive exposure to gamma rays could affect radiation users and the general public in several ways such as nausea, vomiting, fatigue, diarrhoea, headache, skin burns, and death [Kiefer, 1990; Walsh, 2013]. In order to reduce the risks of excessive gamma exposure, sufficient installation of high quality gamma-ray shielding materials is required. In principle, materials comprised of high-atomic number (Z) and high-density elements are good candidates to be used as effective gamma shields due to their high probability of interactions and larger energy transfer with gamma rays. These requirements make pure lead (Pb) sheets1 and leadcontaining materials excellent gamma-ray shielding candidates [McCaffrey et al., 2007]. Pb is also economically accessible and available in various forms. Examples of such materials are Portland cement mixed with granulated Pb [Hosiny and El-Faramawy, 2000], PbO-SiO2 glasses [Singh et al., 2000], and Pb/natural rubber (NR) composites [Gwaily et al., 2002]. However, these materials pose serious environmental and health risks to product users as Pb is considered toxic and lacks chemical stability [Demayo et al, 1982; Seregin et al, 2001; Needleman and Landrigen, 2004]. As a result, new kinds of lead-free gamma-ray shielding materials that are safe and environmental friendly have been developed to substitute the 1
Pb has an atomic number of 82 and a density of 11.34 g/cm3.
3
troublesome lead-containing materials. For instance, bismuth borate glasses and bismuth borosilicate glasses were developed as transparent gamma-ray shielding materials [Singh et al., 2002; Singh et al., 2014], while EVA/tungsten carbide (WC) and NR/iron (II, III) oxide (Fe3O4) composites were developed as elastic and lightweight materials [Soylu et al., 2015; Toyen et al., 2018]. As the needs for better and safer gamma-ray shielding materials have widened to more applications, materials with specific properties that serve particular applications are required. EPDM rubber, which is a synthetic elastomer with properties suitable to be used in outdoor applications, is one of the flexible materials that have great potential for use in radiation-shielding applications. Unlike natural rubber (NR), the distinguishing properties of EPDM rubber include great resistance to UV, ozone, heat, weathering, steam and water, and chemicals [Azo Material, 2018]. EPDM rubber also responds well to high filler and plasticizer loadings, making intended improvement to the composites possible [Usuki et al., 2002; Zheng et al., 2004; Soto-Oviedo et al., 2006]. As a result, EPDM rubber is typically used as vehicle door and window seals, appliance hoses, or electrical insulators [Dikman and Basdogan, 2008]. Some EPDM composites with the addition of specific fillers are also used in radiation-related applications. For example, EPDM/boric acid composites have been used as neutron-shielding materials and EPDM/carbon black and carbon fibre composites as electromagnetic interference (EMI) shields [Das et al., 2001; Ozdemir et al., 2016]. This work investigated properties of EPDM rubber composites that were developed to be used as flexible and lead-free gamma-ray shielding materials by introducing suitable gamma-protective fillers, namely iron (II, III) oxide (Fe3O4), tungsten (III) oxide (W2O3), or bismuth (III) oxide (Bi2O3), into the composites. The properties of interest included cure characteristics, gamma-ray shielding, physical, mechanical properties, and QUV accelerated weathering effects caused by ultraviolet (UV) and moisture.
4
2. Experimental 2.1 Materials and chemicals VistalonTM 3666 EPDM rubber, which is an oil extended terpolymer grade with an ethylene content of 64 wt% and a Mooney viscosity (ML 1+4, 125oC) of 52, was used as the main matrix for this work. The vulcanizing formulations, which include all the chemicals and their suppliers, are given in Table 1.
Table 1 Material formulations of EPDM composites including chemicals and their suppliers. Ingredient
Content
Supplier
(phr)2 EPDM rubber
100 (part)
ExxonMobil Chemical Asia Pacific, (Singapore)
Zinc oxide; ZnO
5.0
Thai-Lysaght Co., Ltd., Phranakhon Si Ayutthaya, (Thailand)
Stearic acid
1.5
Imperial Industrial Chemicals Co., Ltd. Bangkok, (Thailand)
Paraffinic oil
30.0
Chemical Innovation Co., Ltd., (Thailand)
Mercaptobenzthaisoles; MBT
0.5
CMC advance Co., Ltd. Samut Sakhon, (Thailand)
Zinc dimethacrylate; ZDMA,
1.5
Idemitsu Kosan Co., Ltd., (Japan)
Tetramethylthiuram disulphide; TMTD
0.5
Zeon Advanced Polymix Co., Ltd. Rayong, (Thailand)
N-(1,3dimethylbutyl)-N’-phenyl-p-
2.0
Behn Meyer Chemical (T) Co., Ltd.,
Dymalink 643 grade
phenylenediamine; 6PPD Sulphur powder (325 mesh)
Bangkok, (Thailand) 0.7
Zeon advanced Polymix Co., Ltd. Rayong, (Thailand)
Carbon black (HAF N550)
40.0
Thai Carbon Black Public Co., Ltd. AngThong, (Thailand)
2
phr: parts per hundred of rubber
5
2.2 Gamma-protective fillers Three metal oxide fillers were used as gamma-protective fillers in this work (Fe3O4, W2O3, and Bi2O3) and were supplied by Shanghai Ruizheng Chemical Technology Co., Ltd., China. Images and their average particle sizes of Fe3O4, W2O3, and Bi2O3, recorded and analysed using a scanning electron microscope (SEM) (Quanta 450 FEI, the Netherlands) and ImageJ software version 1.50i, are shown in Fig. 1. Different contents of Fe3O4, W2O3 or Bi2O3 (0, 100, 300 to 500 phr) were mixed with the ingredients shown in Table 1.
Fig. 1. SEM images and average particle sizes of (a) Fe3O4 (60000) (b) W2O3 (600) and (c) Bi2O3 (750)
These metal oxides were selected due to their relatively high Z, high densities, high melting points, lower toxicity than Pb and availability in fine powders. Fe, W, and Bi in these metal oxides also have relatively high mass attenuation coefficients (μm), especially Bi, which has higher values than Pb, indicating great potential for Fe3O4, W2O3, and Bi2O3 to substitute Pb as gamma-protective fillers. The μm values of Fe, W, Pb and Bi at different gamma energies are shown in Table 2 [NIST, 2004].
6
Table 2 Mass attenuation coefficient (μm) of Fe, W, Pb and Bi at different gamma energies Mass attenuation coefficient; μm (cm2/g) Element
Z
Gamma energy Gamma energy Gamma energy E = 0.1 MeV
E = 1.0 MeV
E = 10.0 MeV
Fe
26
0.372
0.059
0.029
W
74
4.438
0.062
0.047
Pb
82
5.549
0.071
0.049
Bi
83
5.739
0.072
0.050
2.3 Preparation of EPDM composites The EPDM composites were prepared using two steps: mastication and compounding. The EPDM was masticated on a laboratory two-roll mill (Yong Fong Machinery Co., Ltd., Samutsakhon, Thailand) for 5 min. The masticated EPDM was then compounded with the prepared chemicals given in Table 1 and metal oxide fillers with their contents varying from 0 to 100, 300 and 500 phr, respectively. The compounding process was carried out on the two-roll mill for 25 min to ensure a uniform distribution of the fillers. The resultant EPDM rubber composites were then kept at 25-30oC and 50% humidity prior to use.
2.4 Characterizations 2.4.1 Cure characteristics The scorch time (ts1), cure time at 90% (tc90), and torque differences of the EPDM compounds were evaluated using an oscillating die rheometer (ODR GT 70-70-S2, GOTECH Testing Machine, Inc., Taichung, Taiwan) whose procedures followed ASTM D2048-11 standard testing at a test temperature of 170oC. The determined cure time was used for vulcanizing the EPDM composites sheet in a hot compression moulding at 170 kg/cm2 using
7
2 mm thick and 5 mm thick moulds with the dimensions of 15 cm 15 cm for every individual compound.
2.4.2 Mechanical properties The tensile properties tensile modulus, tensile strength, and elongation at break of the EPDM rubber composites were investigated using a universal testing machine (Autograph AG-I 5kN, Shimadzu, Tokyo, Japan), according to ASTM D412-06 standard testing. It should be noted that for comparative purposes, the tensile modulus in this work was reported at 100% elongation at which the minimum elongation of the hardest rubber compound could be stretched. The tensile testing speed used for all specimens was 500 mm/min. For hardness measurement, all EPDM rubber composites were tested according to ASTM D2240-05 using a hardness durometer (Shore A) (Teclock GS-719G, Japan).
2.4.3 Specific gravity and morphology studies For all EPDM rubber composites, the specific gravity was measured following the procedures of ASTM D297-06 by comparative measurement of each sample mass in air and water (Gibitre DenS-i-Meter, USA). A scanning electron microscope (SEM) (Quanta 450 FEI: JSM-6610LV, the Netherlands) at a 10-kV accelerating voltage with back-scattered electrons (BSE) was used to study the morphology and the distribution of fillers in EPDM rubber composites. It should be noted that the samples were coated with gold using a sputter coater (Quorum SC7620: Mini Sputter Coater/ Glow Discharge System, England) at a power voltage of 10 kV and a current of 10 mA for 120 sec before performing SEM.
8
2.4.4 QUV accelerated weathering test For the studies of accelerated weathering effects caused by ultraviolet (UV) and moisture, the EPDM rubber composites were placed in a QUV accelerated-weathering tester (Q-Lab, USA) under a UV cyclic aging condition, which followed the standard testing method of ASTM G154-12, for a duration of 96 hr. A complete UV cyclic aging cycle consisted of (i) UV radiation conditions of 0.77 W/m2/nm at 60oC for 8 hr and (ii) moisture condensation conditions of 50oC for 4 hr. After the aging procedures were completed, all vulcanizates were mechanically tested to determine their level of mechanical degradation caused by UV radiation and moisture.
2.4.5 Gamma shielding properties The gamma shielding properties were measured using the setup shown in Fig. 2. The radioactive sources used in this measurement were 1-μCi 137Cs and 1-μCi 60Co point sources, which emit 662-keV and 1.25-MeV (the average of 1.173 MeV and 1.332 MeV) gamma rays, respectively.
Fig. 2. Setup for gamma shielding measurement
9
The measurement was carried out inside a 6 cm thick lead cylinder. Samples of 15 cm 15 cm of EPDM rubber composites with thicknesses varying from 5 mm to 80 mm in 5 mm increments were placed between a gamma source and a gamma detector that were separated by 10 cm. The gamma source used in this test was kept in a 3-cm thick lead collimator with a 1-mm pinhole exit to minimize build-up effects that could increase the transmitted gamma rays caused by secondary or re-scattered gamma. The NaI gamma detector (Ortec 905-4, USA) used in the measurement had a circular active area of 10 cm in diameter and was powered by a high voltage power supply (Canberra 3102D, USA). Its output was sent through a series of amplifiers (Ortec 590A, USA) and a counter/timer (Canberra 2071A, USA) for signal processing and counting. The gamma attenuation properties of the shielding samples could be described by Eq. 1-3,
(1) (2) (3)
where I, I0, μl, μm, HVL, and x are the intensity of transmitted gamma rays, the intensity of incident gamma rays, the linear attenuation coefficient, the mass attenuation coefficient, the half value layer, and the thickness of the shielding material, respectively. Three independent 1-minute tests were performed and average I and I0 were recorded for each sample and thickness in order to calculate μl, μm, and HVL. The detailed definitions of μl, μm, and HVL and calculation procedures using a graph of the ratio I/I0 and x can be found in Toyen et al. (2018). For comparison purposes, pure lead sheets were also tested
10
under the same testing procedures and setup to measure the corresponding values of μl, μm, and HVL. 3. Results and discussion 3.1 Cure characteristics The results of the cure characteristics and torque differences of the EPDM rubber composites are shown in Table 3. The results showed that the scorch time of EPDM rubber composites decreased with increasing contents of Fe3O4, W2O3, and Bi2O3 except for 500-phr Fe3O4, which had a similar scorch time as pristine EPDM rubber. The decrease in scorch times could be explained by the fact that the increasing contents of metal oxide powders increased the frictional heat in the EPDM compound during the ODR testing process leading to the faster commencement of the curing process [Salaeh et al, 2011]. However, the cure times of the EPDM rubber composites behaved differently as the values initially decreased as metal oxide powder was introduced into the mixtures but later increased as the metal oxide content increased to 300 phr and 500 phr, respectively. This behaviour was observed because, with an appropriate amount of metal oxides in the EPDM rubber composites, the metal oxides could act as co-activators during the chemical vulcanizing process, leading to an initial decrease in cure times. However, as the contents of metal oxide further increased, they inversely obstructed the action of the main activators and accelerators, resulting in prolonged cure times [Ninyong et al., 2017; Toyen et al., 2018]. It should be noted that the EPDM composites with 500-phr Fe3O4 had exceptionally high scorch and cure times. This could have been due to Fe3O4 having the lowest density compared to W2O3 and Bi2O3, which led to a higher volume with the Fe3O4 powders, resulting in greater obstruction of the actions of activators and accelerators. The torque differences shown in Table 3, which could indirectly refer to the crosslink density of the composites [El-Sabbagh and Yehia, 2007], increased with increasing contents
11
of Fe3O4, W2O3, and Bi2O3. This increase in the torque difference could be due to that these metal oxides acted as co-activators during the chemical vulcanizing process. Another possible explanation of the increases in the torque differences was the much higher rigidity of metal oxides than EPDM particles, resulting in higher viscosity of the composites when more metal oxide was added during the compounding process. Another interesting result was that Fe3O4/EPDM rubber composites had higher torque differences than W2O3/EPDM and Bi2O3/EPDM rubber composites for the same content because the Fe3O4 has much smaller particle sizes (see Fig. 1 ) than the other two metal oxides, leading to better particle dispersion and higher frictional heat in addition to being coactivators.
Table 3. Scorch time, cure time and torque differences of EPDM rubber composites with addition of either Fe3O4, W2O3, or Bi2O3 Cure characteristics of EPDM rubber composites (± represent the standard deviation of the average values)
Metal oxide type
Fe3O4
W2O3
Bi2O3
Metal oxide
Scorch time,
Cure time,
Torque differences
content (phr)
(ts1) (min:sec)
(tc90) (min:sec)
(dN m)
0
1:36 (±0:01)
9:31 (±1:48)
27.0 (±0.5)
100
1:16 (±0:01)
8:02 (±0:16)
33.0 (±0.4)
300
1:07 (±0:01)
9:51 (±0:17)
44.1 (±0.2)
500
1:31 (±0:04)
17:48 (±0:36)
54.5 (±1.5)
0
1:36 (±0:01)
9:31 (±1:48)
27.0 (±0.5)
100
1:31 (±0:01)
8:28 (±0:03)
31.4 (±0.2)
300
1:12 (±0:01)
9:48 (±1:41)
39.7 (±0.4)
500
1:12 (±0:01)
11:08 (±0:11)
42.8 (±0.2)
0
1:36 (±0:01)
9:31 (±1:48)
27.0 (±0.5)
100
1:39 (±0:03)
7:42 (±0:08)
27.5 (±0.2)
12
300
1:25 (±0:01)
9:15(±0:06)
30.8 (±0.1)
500
1:11 (±0:02)
10:50 (±0:10)
33.7 (±0.5)
3.2 Mechanical properties
Fig. 3. Mechanical properties of EPDM rubber composites with varying contents of metal oxides before and after QUV accelerated weathering test: (a) tensile modulus at 100% elongation and (b) hardness (shore A). (Error bars indicate standard deviations of the average values.)
13
Fig. 4. Mechanical properties of EPDM rubber composites with varying contents of metal oxides before and QUV after accelerated weathering test: (a) tensile strength and (b) elongation at break. (Error bars indicate standard deviations of the average values.)
As shown in Fig. 3, the tensile modulus at 100% elongation and hardness (Shore A) increased with increasing Fe3O4, W2O3, and Bi2O3 contents with greater effects on Fe3O4/EPDM rubber composites. The almost linear increases in the tensile modulus at 100% elongation and hardness (Shore A) could have been due to the high rigidity of the metal oxide powders used in this work, which increased the torque differences/crosslink densities of the composites that resulted in restriction of the crosslink joints of the inter-crosslink chains and an increase in the overall rigidity of the EPDM rubber composites [Zhao et al., 2011]. However, different types of behaviour by the EPDM rubber composites were seen in tensile strength and elongation at break as shown in Fig. 4. Their values initially increased when 100-phr metal oxides were introduced to the EPDM rubber composites; however, the values decreased as more metal oxide content was added. These results were observed because, with appropriate metal oxide addition, the metal oxides could act as co-activators, leading to a higher crosslink density and higher tensile strength/elongation at break. However, as more metal oxides were added to the composites, filler-filler interactions replaced more favourable rubber-filler interactions, leading to an agglomeration of metal oxide particles and worse particle dispersion. The reduction in the tensile strength and elongation at break could be explained by the morphology of the EPDM rubber composites shown in Fig. 5, where Fig. 5(a) shows EPDM rubber composites without metal oxides and Fig. 5(b) -5(d) show EPDM rubber composites with the addition of metal oxides. Fig. 5b clearly shows better filler dispersion of Fe3O4 than for W2O3 and Bi2O3 in Fig. 5(c) and 5(d), due to much the smaller particle sizes of Fe3O4 (0.3
14
µm). This better dispersion as well as the higher surface area of Fe3O4 particles led to better tensile properties in the Fe3O4/EPDM rubber composites. Fig. 5(c) and 5(d) also showed poor interfacial compatibility between W2O3, and Bi2O3 particles and the EPDM matrix, resulting in visible voids inside the composites.
Fig. 5. SEM images of EPDM rubber composites (a) without metal oxides, (b) with 500-phr Fe3O4, (c) with 500-phr W2O3, and (d) with 500-phr Bi2O3. All images were taken with magnification of 200.
The effects of the QUV accelerated weathering tests are shown in Fig. 3 and 4, which provide values for the tensile modulus at 100% elongation, hardness (Shore A), tensile strength, and elongation at break of the EPDM rubber composites after performing accelerated aging studies. The differences between before (solid lines) and after (dashed line)
15
the aging tests could have been due to a post-curing effect caused by UV and heat, which further increased the crosslink density of the EPDM rubber composites, resulting in a higher tensile modulus at 100% elongation/tensile strength and lower elongation at break/hardness (Shore A) [Tabtim and Sombatsompop, 2014]. The small degrees of degradation/changes observed with all EPDM rubber composites could have been due to the structure of EPDM, which has excellent UV and heat-resistant properties, and also the addition of antioxidants in the EPDM rubber composites. These results are crucial as they imply that the EPDM rubber composites with the addition of metal oxides have weather and sunlight durability, making them suitable to be used in outdoor applications.
3.3 Specific gravity The specific gravity values of the EPDM rubber composites are shown in Table 4. The results indicated that the specific gravity of the EPDM rubber composites increased with the increasing contents of Fe3O4, W2O3 or Bi2O3 due to the much larger densities of the metal oxide powders than the pristine EPDM, causing the composites to be much heavier. It should be noted that the Bi2O3/EPDM rubber composites had the highest specific gravity among all EPDM rubber composites with the same content due to Bi2O3 having the highest density.
Table 4. Specific gravity of EPDM composites with additions of either Fe3O4, W2O3 or Bi2O3. Specific gravity of EPDM rubber composite Metal oxide Content (phr)
Fe3O4
W2O3
Bi2O3
0
1.01
1.01
1.01
100
1.31
1.46
1.49
300
1.98
2.19
2.25
16
500
2.39
2.73
2.89
The specific gravity values of the EPDM rubber composites in this work were similar to values reported in Toyen et al. (2018), in which the same types and contents of metal oxides were added to NR composites. 3.4 Gamma shielding properties Table 5. Linear attenuation coefficient (μl) of EPDM rubber composites with additions of either Fe3O4, W2O3 or Bi2O3 for 662-keV and 1.25-MeV gamma rays (mean ± standard deviation). Linear attenuation coefficient; μl (m-1) of EPDM rubber composites Metal oxide Content
Gamma energy E = 622 keV
Gamma energy E = 1.25 MeV
(phr)
Fe3O4
W2O3
Bi2O3
Fe3O4
W2O3
Bi2O3
0
5.3 ±1.6
5.3 ±1.6
5.3 ±1.6
4.1 ±1.2
4.1 ±1.2
4.1 ±1.2
100
7.8 ±0.7
9.6 ±0.8
10.3 ±0.9
6.3 ±1.1
5.8 ±0.7
6.4 ±0.8
300
10.9 ±1.1
14.7 ±1.0
18.6 ±1.1
8.2 ±1.3
8.6 ±0.5
10.9 ±0.9
500
12.6 ±0.7
19.2 ±1.0
24.3 ±2.2
11.3 ±2.2
11.7 ±0.5
13.3 ±1.7
Lead sheet
97.2 ±2.4
47.1 ±6.7
Table 6. Mass attenuation coefficient (μm) of EPDM rubber composites with additions of either Fe3O4, W2O3 or Bi2O3 for 662-keV and 1.25-MeV gamma rays (mean ± standard deviation). Metal oxide Content (phr) 0
Mass attenuation coefficient; μm (10-3 m2/kg) of EPDM rubber composite Gamma energy E = 622 keV
Gamma energy E = 1.25 MeV
Fe3O4
W2O3
Bi2O3
Fe3O4
W2O3
Bi2O3
5.2 ±1.5
5.2 ±1.5
5.2 ±1.5
4.1 ±1.1
4.1 ±1.1
4.1 ±1.1
17
100
6.0 ±0.5
6.6 ±0.5
6.9 ±0.6
4.8 ±0.8
4.0 ±0.5
4.3 ±0.5
300
5.5 ±0.6
6.7 ±0.5
8.3 ±0.5
4.1 ±0.7
3.9 ±0.2
4.8 ±0.4
500
5.3 ±0.3
7.0 ±0.4
8.4 ±0.8
4.7 ±0.9
4.3 ±0.2
4.6 ±0.6
Lead sheet
9.1 ±0.2
4.4 ±0.6
Table 7. Half value layer (HVL) of EPDM rubber composites with additions of either Fe3O4, W2O3 or Bi2O3 for 662-keV and 1.25-MeV gamma rays (mean ± standard deviation). Metal oxide Content
Gamma energy E = 622 keV
Gamma energy E = 1.25 MeV
(phr)
Fe3O4
W2O3
Bi2O3
Fe3O4
W2O3
Bi2O3
0
0.13 ±0.04
0.13 ±0.04
0.13 ±0.04
0.17 ±0.05
0.17 ±0.05
0.17 ±0.05
100
0.09 ±0.01
0.07 ±0.01
0.07 ±0.01
0.11 ±0.02
0.12 ±0.01
0.11 ±0.01
300
0.06 ±0.01
0.05 ±0.01
0.04 ±0.01
0.08 ±0.01
0.08 ±0.01
0.06 ±0.01
500
0.06 ±0.01
0.04 ±0.01
0.03 ±0.01
0.06 ±0.01
0.06 ±0.01
0.05 ±0.01
Lead sheet 3, 4
Half value layer; HVL (m) of EPDM rubber composite
0.013
0.014
standard deviation much less than 0.01
Tables 5-7 showed that the gamma shielding properties of the EPDM rubber composites improved with increasing Fe3O4, W2O3 or Bi2O3 contents, as seen by the increases in the values of the linear attenuation coefficients (μl) and mass attenuation coefficients (μm), and the decreases in the values of the half value layers (HVL). This improvement was due to the increase in the number of Fe, W, and Bi atoms, which were the active gamma attenuators, in the EPDM rubber composites. The effects of filler contents on gamma shielding properties could be shown by the schemes in Fig. 6, which show the EPDM rubber composites having 18
the same type of gamma protective fillers but at different contents (Fig. 6a has higher content of gamma protective fillers than Fig. 6b). Since the attenuations of gamma rays depend largely on Compton scattering and photoelectric effects between incident gamma rays and atoms of gamma protective fillers, the composites in Fig. 6a had higher probabilities of gamma interactions, leading to less transmitted gamma rays and better gamma attenuation.
Fig. 6. Schemes show gamma interactions with gamma protective fillers in EPDM rubber composites, in which (a) has higher contents than (b), leading to more gamma interactions and less gamma transmission in (a).
A comparison of Tables 5-7 indicate that the Bi2O3/EPDM rubber composites had the highest μl and μm, and the lowest HVL among all investigated composites. This was because Bi in Bi2O3 had the highest Z and μm as shown in Table 2 values among all the metal oxides used in this work, leading to better interaction with gamma rays and a higher gamma attenuating efficiency. The effects of filler types on gamma attenuation could be shown by 19
the schemes in Fig. 7, which shows the composites having the same contents but different types of gamma protective fillers and thus different μm (Fig. 7a shows the composites with the fillers having higher μm than Fig. 7b). This larger μm, represented by sizes of the gamma protective fillers, leads to more interactions between incident gamma rays and fillers’ atoms in Fig. 7a and, hence, less transmitted gamma rays. The results could be explained by the same mechanism as shown in Fig. 6
Fig. 7. Schemes show gamma interactions with different gamma protective fillers in EPDM composites, in which (a) comprises of fillers with larger values of μm as shown by larger fillers’ size than (b), leading to more gamma interactions and less gamma transmission in (a).
Another interesting result was that the ability to shield 662-keV gamma rays of all EPDM rubber composites was higher than for 1.25-MeV gamma rays for all EPDM rubber composites. This result could be explained by the fact that higher-energy gamma rays had a 20
lower probability of interactions with materials than did the lower-energy gamma rays, leading to lower numbers of interactions, less energy transfers, and more gamma rays being transmitted [Nelson and Reilly, 2018]. A comparison of the gamma shielding properties of the EPDM rubber composites with the pure lead sheets under the same setup indicated that pure lead sheets could attenuate gamma rays at both energies more efficiently than all the EPDM rubber composites for the same thickness of material. The results were expected as EPDM composites were a mixture of EPDM rubber and metal oxide powders, leading to less gamma-protective fillers than pure lead sheets at the same thickness of materials. However, when considering the same mass of materials, the EPDM composites with Bi2O3 contents of 300 phr and 500 phr had only slightly less μm than pure lead sheets at the gamma energy of 662 keV, but higher μm at the gamma energy of 1.25 MeV. This result was intriguing as it showed that the developed EPDM rubber composites have comparable gamma-ray shielding ability as pure lead sheets, posing great potential to be efficiently used in applications that require flexibility, durability, and light weight such as in personal protective wear or radioactive containers. It should be noted that the maximum metal oxide contents reported in this work were 500 phr. This was due to the increasing difficulty during mixing process of all chemicals with EPDM rubbers when metal oxide contents were greater than 500 phr. Nonetheless, the results obtained in this work were sufficient to show the differences in properties of EPDM composites caused by different types and contents of metal oxides.
Table 8. Comparison of linear attenuation coefficients in different materials. (N/A: represents data not reported and values are represented as mean ± standard deviation).
Material
Linear attenuation coefficient (m-1)
21
Reference
Gamma energy
Gamma energy
E = 622 keV
E = 1.25 MeV
EPDM/500-phr Fe3O4
12.6 ±0.7
11.3 ±2.2
This work
EPDM/500-phr W2O3
19.2 ±1.0
11.7 ±0.5
This work
EPDM/500-phr Bi2O3
24.3 ±2.2
13.3 ±1.7
This work
NR/500-phr Fe3O4
8.0 ±1.2
4.4 ±0.6
Toyen, et al. (2018)
NR/500-phr W2O3
13.8 ±0.8
7.1 ±0.8
Toyen, et al. (2018)
NR/500-phr Bi2O3
20.4 ±1.2
10.5 ±1.0
Toyen, et al. (2018)
NR/300-phr Pb
20.0
11.0
Gwaily, et al. (2002)
NR/500-phr Pb
26.0
15.0
Gwaily, et al. (2002)
NR/400-phr Galena5
26.7
N/A
Gwaily (2002)
NR/500-phr Galena
29.5
N/A
Gwaily (2002)
5
Galena is the natural mineral form of lead (II) sulphide (PbS), the most important ore of lead.
As shown in Table 8, the developed EPDM rubber composites in this work had comparable μl values with previous reports. In particular, the EPDM rubber composites with 500-phr Bi2O3 had a slightly lower μl value than NR/500-phr Pb [Gwaily et al., 2002]. The lower μl value of EPDM/500-phr Bi2O3 could be explained by the fact that since Bi2O3 consists of Bi and O atoms, in which Bi atoms contribute approximately 89% of the molecular mass, the EPDM rubber composites with 500-phr Bi2O3 content had an actual Bi content of only approximately 445 phr, resulting in slightly lower numbers of gammaprotective fillers, that is Bi atoms. Another explanation of the differences between μl reported in this work and other work could be differences in the testing setup. For example, a smaller and longer collimator exit could reduce the numbers of re-scattered gamma rays to the detector, leading to a lower intensity of transmitted gamma rays (I) and, consequently, higher 22
μl values. This explanation was apparent when considering reports from this work and ref. [Toyen et al., 2018], which used un-collimated gamma rays. For the same 500-phr metal oxide content, the EPDM rubber composites had higher μl values than the NR composites at both gamma energies. The differences could be reasonably explained by the use of a collimator in current study. Nonetheless, the results obtained from this work clearly showed that the EPDM rubber composites had efficient gamma-ray shielding abilities and could potentially replace hazardous lead-containing materials with excellent additional properties in flexibility and durability. Furthermore, when compared EPDM/500-phr Bi2O3 composites with other gamma-ray shielding materials such as Tellurite glass, it was found that μm of EPDM/500-phr Bi2O3 was comparable with the reported values by El-Mallawany (2018), which used WinXCOM program to simulate the gamma-ray shielding properties of Tellurite glass mixed with various metal oxides. For example, EPDM/500-phr Bi2O3 had the value of 4.6×10-3 m2/kg (at gamma energy of 1.25 MeV), while 80TeO2-5TiO2-10WO3-5Nb2O5 and 90TeO2-10V2O3 had the values of 3.4 m2/kg and 3.3 m2/kg, respectively. The smaller values reported by El-Mallawany (2018) could be due to higher gamma energy used in simulation, as well as lower contents of metal oxide in Tellurite glass. Another interesting outcome that should be discussed is the correlation between gamma-ray shielding and mechanical properties of the investigated composites. The results obtained in Section 3.2 and Section 3.4 indicated that higher contents of metal oxides generally improved gamma-ray shielding but reduced overall mechanical properties, especially tensile strength and elongation at break. As a consequence, a correlation between these two properties must be thoroughly taken into account when applying the composites in actual uses such that safety of users must not be compromised in terms of both radiation and physical aspects.
23
In summary, these developed metal oxide/EPDM rubber composites could be used in various applications including shielded containers for radioactive transportation, electrical insulation in nuclear facilities, and construction parts of buildings. The composites are also safer and more environmentally friendly to users than lead-containing materials.
4. Conclusions Since lead-containing materials used in gamma-ray shielding applications pose environmental and health risks to product users, safer materials with efficient gamma-ray shielding ability are needed. This work developed flexible, lead-free, gamma-ray shielding materials based on EPDM rubber composites by introducing metal oxides (Fe3O4, W2O3 or Bi2O3) with their contents varied from 0 to 100, 300, and 500 phr. The results showed that Fe3O4, W2O3 and Bi2O3 worked effectively as active fillers to shield gamma rays with their shielding ability comparable to lead-containing materials. The results also indicated that metal oxide contents had effects on cure times of EPDM composites by initially decreased the values at 100 phr but later increased at 300 and 500 phr. Furthermore, tensile modulus at 100% elongation and hardness (Shore A) gradually increased with increasing metal oxide contents, however, similar behaviours as cure times were observed in tensile strength and elongation at break. This work also investigated the effects of QUV accelerated weathering on the possible degradation of the tensile properties in the EPDM rubber composites. The results showed that the aging tests led to increases in the tensile modulus at 100% elongation and tensile strength but to slight decreases in the elongation at break and hardness (Shore A). Hence, based on the experimental results presented in this work, it could be concluded that the developed EPDM rubber composites have great potential to be used as flexible, lead-free gamma-ray shielding materials as shown in their comparable µm with lead sheets. However, in order to improve shielding abilities of the composites while still maintaining their
24
flexibility, improved mixing procedure and revised formulation with greater metal oxide contents are encouraged in future work.
Acknowledgement We would like to thank the Kasetsart University Research and Development Institute (KURDI), Bangkok, Thailand, the National Research Council of Thailand, and the Office of the Higher Education Commission (OHEC) under the National Research University (NRU) Program for financial support. We also wish to thank the specialized Center of Rubber and Polymer Materials for Agriculture and Industry (RPM), Faculty of Science, Kasetsart University for publication support. The authors would also like to give appreciation to P r o f . Dr. Narongrit Sombatsompop at King Mongkut’s University of Technology Thonburi for his advice and suggestion during preparation of this manuscript.
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Highlights
Metal oxide/EPDM rubber composites were developed as gamma-ray shielding materials.
Bi2O3/EPDM rubber composites had the highest gamma-ray shielding properties.
500-phr Bi2O3/EPDM rubber composites had higher μm than pure Pb sheets.
Bi2O3/EPDM composites could replace commercial lead-containing shielding materials.
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