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Natural rubber composites as thermal neutron radiation shields
Polymer Testing 21 (2002) 513–517 www.elsevier.com/locate/polytest
Material Behaviour
Natural rubber composites as thermal neutron radiation shields II — H3BO3/NR composites S.E. Gwaily a
a,*
, H.H. Hassan b, M.M. Badawy b, M. Madani
a
National Center for Radiation Research and Technology, Nasr City, 8th Sector, P.O. Box 29, Cairo, Egypt b Physics Department, Faculty of Science, Cairo University, Cairo, Egypt Received 21 May 2001; accepted 27 September 2001
Abstract Different amounts of boric acid (H3BO3) were mixed with a conductive natural rubber (loaded with 40 phr of HAF carbon black) to get thermal neutron radiation shielding composites. It was found that the total macroscopic crosssection reaches 0.29 cm⫺1 at 30 phr of H3BO3. The dependence of thermal and electrical properties of such composites on the concentration of H3BO3 was also studied. Thermal oxidative aging was found to markedly affect the above properties. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Natural rubber composites; Thermal neutron shields; H3BO3/NR composites; Thermal properties; Electral properties; Thermal oxidative aging
1. Introduction The development of materials enabled the production of polymeric cartons and garments with resistance to neutron radiation to protect instruments and personnel. It is necessary to use a high thermal neutron cross-section material such as H3BO3 for making a shield against thermal neutrons [1–3]. In previous works, borated composites with different boron content have been studied [4–7]. The present work deals with the effect of H3BO3 concentration on the attenuation of thermal neutron radiation of such composites. Moreover, in order to know the applicability of such composites, the electrical and thermal properties were studied. The study was extended to investigate the effect of thermal oxidative aging on the same properties in order to get reasonable stability and reproducibility of the properties and consequently to indicate the best conditions for application for these composites as shielding materials.
* Corresponding author.
2. Experimental
2.1. Sample preparation
Since sample properties depend on the method of preparation it is important that samples should be made under the same conditions [8,9]. Natural rubber was mixed with 40 phr (part per hundred parts of rubber by weight) of HAF carbon black and different amounts of boric acid to get composites with different concentrations (0, 1, 3, 5, 7, 10, 15, 20 and 30 phr). Mixing was done by a laboratory two-roll mill of 300 mm length, 170 mm diameter, speed of slow roll 24 rpm and gear ratio 1.16 at 40°C for 20 minutes. The recipes of H3BO3/NR composites is illustrated in Table 1. All the above samples were vulcanized by using a heated press (Carver M-154) at 143°C and pressure 4 MPa for an optimum curing time of 20 min. The cure time was determined by a rheometer type Monsanto-R 100.
0142-9418/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 0 1 ) 0 0 1 1 7 - 9
514
S.E. Gwaily et al. / Polymer Testing 21 (2002) 513–517
Table 1 The compositions of H3BO3/NR samples Ingredients
Sample
(phr)
BA0
BA1
BA3
BA5
BA7
BA10
BC15
BC20
BC30
NR Stearic acid HAF/black-N330 Processing oil H3BO3 MBTS a PBNb Zinc oxide Sulfur
100 2 40 10 0 2 1 5 2
100 2 40 10 1 2 1 5 2
100 2 40 10 3 2 1 5 2
100 2 40 10 5 2 1 5 2
100 2 40 10 10 2 1 5 2
100 2 40 10 20 2 1 5 2
100 2 40 10 30 2 1 5 2
100 2 40 10 40 2 1 5 2
100 2 40 10 50 2 1 5 2
a b
Dibenzothiazyle disulphide. Phenyl-β-napthyl amine.
2.2. Radiation measurements 2.2.1. Source The neutron source used in this work was 241Am/Be of type NCS-R,S (USA). It was in a cylindrical capsule of type X 14, with activity 5 Ci, average energy 4.5 MeV and half-life time 458 years [10].
ground loop was used. The temperature of the test sample was varied by using a regulated non inductive electric furnace.
3. Results and discussion 3.1. Thermal neutron attenuation
2.2.2. Detection The solid state nuclear detectors used for measuring slow neutrons and γ-rays were types TLD-600 and TLD700 respectively [11,12]. To calculate the dose of slow neutrons, the γ-rays dose obtained from the TLD-700 (sensitive to γ-rays) was subtracted from that of TLD600 to eliminate the contribution of γ-dose, since the TLD-600 is sensitive to both γ-rays and slow neutrons. For the thermoluminesence measurements a Harshow 4500 (Ohio, USA) TL-readout system was used. For some applications, it is useful to know information about the thermal and electrical properties of the investigated samples.
The screening of nuclear radiation is necessary to ensure that personnel do not receive a dangerous amount of radiation. Fig. 1 shows the natural logarithm of the transmitted dose relative to the initial dose ln(I/Io) as a function of sample thickness (d) for H3O3/NR composites. The linear dependence obeys reasonably well the exponential relation I/Io=e−⌺totd, where ⌺tot is the probability per unit path length that any type of interaction may occur. It is
2.3. Thermal properties Thermal properties, namely thermal diffusivity a, specific heat Cp and thermal conductivity l of the composites were determined by using the flash heating technique [13,14]. The method consists mainly of flashing a short pulse of radiant energy on the front surface of a sample in the form of a disk and monitoring the temperature of its rear surface. 2.4. Electrical properties The main component in the d.c. circuit for measuring electrical conductivity was a Keithly 610 electrometer (USA). In order to get stable measurements a common
Fig. 1.
The variaton of ln(I/Io) with sample thickness.
S.E. Gwaily et al. / Polymer Testing 21 (2002) 513–517
515
conventional to express this probability in terms of the cross-section sn per nucleus for each type of interaction. When multiplied by the number of nuclei N per unit volume, the cross-section sn is converted into the macroscopic cross-section ⌺tot, where ⌺tot⫽Nsn (cm−1), and N⫽
NA r (cm−3) A
where NA is Avogadro’s number, r is the density, A is the atomic number, and ⌺tot=⌺scatter+⌺rad.capture+.... This quantity has the same significance for neutrons as the linear absorption coefficient for γ-rays. The calculated values of ⌺tot (deduced from Fig. 1) were plotted as a function of H3BO3 concentration as shown in Fig. 2. From this figure, ⌺tot increases with increasing H3BO3 content. The maximum value of ⌺tot is 0.29 cm⫺1 at 30 phr of H3BO3 and its relaxation length l(=1/⌺tot) equals 3.45 cm, whereas ⌺tot for the unloaded sample is 0.14 cm⫺1 and its l=7.14 cm. The slow neutron attenuation of sample BA30 is about 107% higher than that of the unloaded one. 3.2. Thermal properties Figs. 3a–c show the dependence of the thermal parameters (a, Cp and l) on the boric acid concentration in the natural rubber composites. No appreciable change was observed in the specific heat and thermal conductivity, while thermal diffusivity showed a marked increase with increasing boric acid concentration. In general, thermal oxidative aging results in an appreciable decrease in the values of the above thermal parameters.
Fig. 3. The dependence of the thermal parameters on boric acid concentration (phr).
3.3. The d.c. conductivity Fig. 4 shows the relation between the logarithm of the current density J and the electric field E for boric acid/natural rubber composites at different concentrations of H3BO3. This behaviour could be matched with an empirical formula in the form: w J⫽Jo sinh 2kT
Fig. 2. The relation between the total macroscopic cross-section and H3BO3 concentration (phr).
where w=aeE, Jo is a fitting parameter, k is Boltzman’s constant, T (K) is the ambient temperature, a is the average separating distance between carbon particles and e is the electronic charge. It is noticed that J increases first with increasing boric acid content up to 3 phr, after which a marked decrease
516
S.E. Gwaily et al. / Polymer Testing 21 (2002) 513–517
Fig. 4. The relation between the logarithm of current density and the electric field.
was observed in the electrical conductivity with increasing H3BO3 concentration. The calculations of the fitting parameters Jo and a show peak values at 5 phr as shown in Fig. 5. Fig. 6 shows the temperature dependence of the electrical conductivity for all the investigated samples at different H3BO3 concentrations. It is noticed that all samples obey thermal activated behaviour with small values of the activiation energy.
4. Conclusion
Fig. 6. Temperature dependence of the electrical conductivity at different H3BO3 contractions.
H3BO3/NR sample is about 107% higher than the unloaded one. 3. The thermal diffusivity showed a marked increase upon increasing H3BO3 concentration. 4. The thermal oxidative aging showed, in general, a marked decrease in the thermal parameter values. 5. The incorporation of 3 phr of H3BO3 into natural rubber matrix increases slightly the electrical conductivity after which the further increase of H3BO3 results in a monotonic decrease in dc conductivity.
It may be concluded that: 1. The maximum value of the macroscopic cross-section for thermal neutrons reaches 0.29 cm⫺1 at 30 phr of the investigated H3BO3/NR composite. 2. The slow neutron attenuation of the 30 phr
Fig. 5. The fitting parameters’ variations with H3BO3 content (phr).
References [1] A.B. Chilton, J.K. Shultis, R.E. Faw, Principles of Radiation Shielding, Prentice-Hall, Englewood Cliffs, NJ, 1984. [2] J.K. Shultis, R.E. Faw, Radiation Shielding, Prentice Hall, Englewood Cliffs, NJ, 1996. [3] B.T. Price, C.C. Horten, K.T. Spinnery, Radiation Shielding, Pergamon, Elmsford, NY, 1957. [4] Y. Harada, H. Nakahara, INPADDOC, JP patent application 1-146620 (1989) (in Japanese). [5] A. El-Khatib, M. Kassem, A.A. Ezzat, J. Polym. Mater. 7 (1) (1990) 21. [6] K. Matsuda, H. Nishikowa, H. Harada, INPADOC, JP patent application, 63-716 (1988) (in Japanese). [7] W.B. Kraus, M.B. Glasgow, M.Y. Kim, D.L. Olmeijer, R.L. Kiefer, R.A. Orwoll, S.A. Thibeault, Anon-205th ACS National Meeting, Washington, USA, 273 Paper, Polymer 23 (1993). [8] ASTM Deisgnation, D415-66T, April, 1967. [9] F. Snell, L.F. Eltre (Eds.), Encyclopaedia of Industrial Chemical Analyses, Inter Science Publishers, New York, 1969. [10] B. Shleien, The Healthphysics and Radiological Health Handbook, Sainta Inc, Silver Spring, MD, 1993.
S.E. Gwaily et al. / Polymer Testing 21 (2002) 513–517
[11] S.E. Liverhant, Elementary Introduction to Nuclear Reactor Physics, John Wiley and Sons, New York, USA, 1960. [12] K. Ayyangar, A.R. Lakshmanan, B. Candra, K. Ramadas, Phys. Med. Biol. 19 (1974) 665.
517
[13] W.J. Parker, R.J.J. Jenkins, C.P. Butter, G.L. Abbolt, J. Appl. Phys. 32 (1961) 1679. [14] S.E. Gwaily, G.M. Nasr, M. M Badawy, H.H. Hassan, J. Polym. Degrad. Stab. 47 (1995) 391.
Material Behaviour
Natural rubber composites as thermal neutron radiation shields II — H3BO3/NR composites S.E. Gwaily a
a,*
, H.H. Hassan b, M.M. Badawy b, M. Madani
a
National Center for Radiation Research and Technology, Nasr City, 8th Sector, P.O. Box 29, Cairo, Egypt b Physics Department, Faculty of Science, Cairo University, Cairo, Egypt Received 21 May 2001; accepted 27 September 2001
Abstract Different amounts of boric acid (H3BO3) were mixed with a conductive natural rubber (loaded with 40 phr of HAF carbon black) to get thermal neutron radiation shielding composites. It was found that the total macroscopic crosssection reaches 0.29 cm⫺1 at 30 phr of H3BO3. The dependence of thermal and electrical properties of such composites on the concentration of H3BO3 was also studied. Thermal oxidative aging was found to markedly affect the above properties. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Natural rubber composites; Thermal neutron shields; H3BO3/NR composites; Thermal properties; Electral properties; Thermal oxidative aging
1. Introduction The development of materials enabled the production of polymeric cartons and garments with resistance to neutron radiation to protect instruments and personnel. It is necessary to use a high thermal neutron cross-section material such as H3BO3 for making a shield against thermal neutrons [1–3]. In previous works, borated composites with different boron content have been studied [4–7]. The present work deals with the effect of H3BO3 concentration on the attenuation of thermal neutron radiation of such composites. Moreover, in order to know the applicability of such composites, the electrical and thermal properties were studied. The study was extended to investigate the effect of thermal oxidative aging on the same properties in order to get reasonable stability and reproducibility of the properties and consequently to indicate the best conditions for application for these composites as shielding materials.
* Corresponding author.
2. Experimental
2.1. Sample preparation
Since sample properties depend on the method of preparation it is important that samples should be made under the same conditions [8,9]. Natural rubber was mixed with 40 phr (part per hundred parts of rubber by weight) of HAF carbon black and different amounts of boric acid to get composites with different concentrations (0, 1, 3, 5, 7, 10, 15, 20 and 30 phr). Mixing was done by a laboratory two-roll mill of 300 mm length, 170 mm diameter, speed of slow roll 24 rpm and gear ratio 1.16 at 40°C for 20 minutes. The recipes of H3BO3/NR composites is illustrated in Table 1. All the above samples were vulcanized by using a heated press (Carver M-154) at 143°C and pressure 4 MPa for an optimum curing time of 20 min. The cure time was determined by a rheometer type Monsanto-R 100.
0142-9418/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 0 1 ) 0 0 1 1 7 - 9
514
S.E. Gwaily et al. / Polymer Testing 21 (2002) 513–517
Table 1 The compositions of H3BO3/NR samples Ingredients
Sample
(phr)
BA0
BA1
BA3
BA5
BA7
BA10
BC15
BC20
BC30
NR Stearic acid HAF/black-N330 Processing oil H3BO3 MBTS a PBNb Zinc oxide Sulfur
100 2 40 10 0 2 1 5 2
100 2 40 10 1 2 1 5 2
100 2 40 10 3 2 1 5 2
100 2 40 10 5 2 1 5 2
100 2 40 10 10 2 1 5 2
100 2 40 10 20 2 1 5 2
100 2 40 10 30 2 1 5 2
100 2 40 10 40 2 1 5 2
100 2 40 10 50 2 1 5 2
a b
Dibenzothiazyle disulphide. Phenyl-β-napthyl amine.
2.2. Radiation measurements 2.2.1. Source The neutron source used in this work was 241Am/Be of type NCS-R,S (USA). It was in a cylindrical capsule of type X 14, with activity 5 Ci, average energy 4.5 MeV and half-life time 458 years [10].
ground loop was used. The temperature of the test sample was varied by using a regulated non inductive electric furnace.
3. Results and discussion 3.1. Thermal neutron attenuation
2.2.2. Detection The solid state nuclear detectors used for measuring slow neutrons and γ-rays were types TLD-600 and TLD700 respectively [11,12]. To calculate the dose of slow neutrons, the γ-rays dose obtained from the TLD-700 (sensitive to γ-rays) was subtracted from that of TLD600 to eliminate the contribution of γ-dose, since the TLD-600 is sensitive to both γ-rays and slow neutrons. For the thermoluminesence measurements a Harshow 4500 (Ohio, USA) TL-readout system was used. For some applications, it is useful to know information about the thermal and electrical properties of the investigated samples.
The screening of nuclear radiation is necessary to ensure that personnel do not receive a dangerous amount of radiation. Fig. 1 shows the natural logarithm of the transmitted dose relative to the initial dose ln(I/Io) as a function of sample thickness (d) for H3O3/NR composites. The linear dependence obeys reasonably well the exponential relation I/Io=e−⌺totd, where ⌺tot is the probability per unit path length that any type of interaction may occur. It is
2.3. Thermal properties Thermal properties, namely thermal diffusivity a, specific heat Cp and thermal conductivity l of the composites were determined by using the flash heating technique [13,14]. The method consists mainly of flashing a short pulse of radiant energy on the front surface of a sample in the form of a disk and monitoring the temperature of its rear surface. 2.4. Electrical properties The main component in the d.c. circuit for measuring electrical conductivity was a Keithly 610 electrometer (USA). In order to get stable measurements a common
Fig. 1.
The variaton of ln(I/Io) with sample thickness.
S.E. Gwaily et al. / Polymer Testing 21 (2002) 513–517
515
conventional to express this probability in terms of the cross-section sn per nucleus for each type of interaction. When multiplied by the number of nuclei N per unit volume, the cross-section sn is converted into the macroscopic cross-section ⌺tot, where ⌺tot⫽Nsn (cm−1), and N⫽
NA r (cm−3) A
where NA is Avogadro’s number, r is the density, A is the atomic number, and ⌺tot=⌺scatter+⌺rad.capture+.... This quantity has the same significance for neutrons as the linear absorption coefficient for γ-rays. The calculated values of ⌺tot (deduced from Fig. 1) were plotted as a function of H3BO3 concentration as shown in Fig. 2. From this figure, ⌺tot increases with increasing H3BO3 content. The maximum value of ⌺tot is 0.29 cm⫺1 at 30 phr of H3BO3 and its relaxation length l(=1/⌺tot) equals 3.45 cm, whereas ⌺tot for the unloaded sample is 0.14 cm⫺1 and its l=7.14 cm. The slow neutron attenuation of sample BA30 is about 107% higher than that of the unloaded one. 3.2. Thermal properties Figs. 3a–c show the dependence of the thermal parameters (a, Cp and l) on the boric acid concentration in the natural rubber composites. No appreciable change was observed in the specific heat and thermal conductivity, while thermal diffusivity showed a marked increase with increasing boric acid concentration. In general, thermal oxidative aging results in an appreciable decrease in the values of the above thermal parameters.
Fig. 3. The dependence of the thermal parameters on boric acid concentration (phr).
3.3. The d.c. conductivity Fig. 4 shows the relation between the logarithm of the current density J and the electric field E for boric acid/natural rubber composites at different concentrations of H3BO3. This behaviour could be matched with an empirical formula in the form: w J⫽Jo sinh 2kT
Fig. 2. The relation between the total macroscopic cross-section and H3BO3 concentration (phr).
where w=aeE, Jo is a fitting parameter, k is Boltzman’s constant, T (K) is the ambient temperature, a is the average separating distance between carbon particles and e is the electronic charge. It is noticed that J increases first with increasing boric acid content up to 3 phr, after which a marked decrease
516
S.E. Gwaily et al. / Polymer Testing 21 (2002) 513–517
Fig. 4. The relation between the logarithm of current density and the electric field.
was observed in the electrical conductivity with increasing H3BO3 concentration. The calculations of the fitting parameters Jo and a show peak values at 5 phr as shown in Fig. 5. Fig. 6 shows the temperature dependence of the electrical conductivity for all the investigated samples at different H3BO3 concentrations. It is noticed that all samples obey thermal activated behaviour with small values of the activiation energy.
4. Conclusion
Fig. 6. Temperature dependence of the electrical conductivity at different H3BO3 contractions.
H3BO3/NR sample is about 107% higher than the unloaded one. 3. The thermal diffusivity showed a marked increase upon increasing H3BO3 concentration. 4. The thermal oxidative aging showed, in general, a marked decrease in the thermal parameter values. 5. The incorporation of 3 phr of H3BO3 into natural rubber matrix increases slightly the electrical conductivity after which the further increase of H3BO3 results in a monotonic decrease in dc conductivity.
It may be concluded that: 1. The maximum value of the macroscopic cross-section for thermal neutrons reaches 0.29 cm⫺1 at 30 phr of the investigated H3BO3/NR composite. 2. The slow neutron attenuation of the 30 phr
Fig. 5. The fitting parameters’ variations with H3BO3 content (phr).
References [1] A.B. Chilton, J.K. Shultis, R.E. Faw, Principles of Radiation Shielding, Prentice-Hall, Englewood Cliffs, NJ, 1984. [2] J.K. Shultis, R.E. Faw, Radiation Shielding, Prentice Hall, Englewood Cliffs, NJ, 1996. [3] B.T. Price, C.C. Horten, K.T. Spinnery, Radiation Shielding, Pergamon, Elmsford, NY, 1957. [4] Y. Harada, H. Nakahara, INPADDOC, JP patent application 1-146620 (1989) (in Japanese). [5] A. El-Khatib, M. Kassem, A.A. Ezzat, J. Polym. Mater. 7 (1) (1990) 21. [6] K. Matsuda, H. Nishikowa, H. Harada, INPADOC, JP patent application, 63-716 (1988) (in Japanese). [7] W.B. Kraus, M.B. Glasgow, M.Y. Kim, D.L. Olmeijer, R.L. Kiefer, R.A. Orwoll, S.A. Thibeault, Anon-205th ACS National Meeting, Washington, USA, 273 Paper, Polymer 23 (1993). [8] ASTM Deisgnation, D415-66T, April, 1967. [9] F. Snell, L.F. Eltre (Eds.), Encyclopaedia of Industrial Chemical Analyses, Inter Science Publishers, New York, 1969. [10] B. Shleien, The Healthphysics and Radiological Health Handbook, Sainta Inc, Silver Spring, MD, 1993.
S.E. Gwaily et al. / Polymer Testing 21 (2002) 513–517
[11] S.E. Liverhant, Elementary Introduction to Nuclear Reactor Physics, John Wiley and Sons, New York, USA, 1960. [12] K. Ayyangar, A.R. Lakshmanan, B. Candra, K. Ramadas, Phys. Med. Biol. 19 (1974) 665.
517
[13] W.J. Parker, R.J.J. Jenkins, C.P. Butter, G.L. Abbolt, J. Appl. Phys. 32 (1961) 1679. [14] S.E. Gwaily, G.M. Nasr, M. M Badawy, H.H. Hassan, J. Polym. Degrad. Stab. 47 (1995) 391.