Keywords: radiation shielding, boron, polyethylene, composites, mechanical ... The mean free path of ions in ..... Domnich, V., et al., "Nanoindentation and Raman spectroscopy studies of boron carbide single crystals," Applied Physics Letters,.
Polyethylene/Boron Composites for Radiation Shielding Applications 1
9
9
1
Courtney Harrison , Eric Burgett , Nolan Hertel , and Eric Gmlke ^Department of Chemical & Materials Engineering, University of Kentucky, Lexington, KY 40506, USA ^Neely Nuclear Research Center, Georgia Institute of Technology, Atlanta, GA 30332, USA ^859)218-6503, courtnev.hamson(a),ukV-edu Abstract. Multifunctional composites made with boron are absorbers of low energy nuetrons, and could be used for structural shielding materials. Polyethylene/boron carbide composites were fabricated using conventional polymer processing techniques, and were evaluated for mechanical and radiation shielding properties. Addition of neat boron carbide (powder and nanoparticles) to an injection molding grade HPDE showed superior mechanical properties compared to neat HDPE. Radiation shielding measurements of a 2 wt% boron carbide composite were improved over those of the neat polyethylene. Keywords: radiation shielding, boron, polyethylene, composites, mechanical properties PACS: 28.41.Qb, 81, 81.05 Je, 81.05 Qk, 81.70 Bt
INTRODUCTION Radiation shielding has been an important aspect of marmed space flight for many years. The safety of the astronauts is a top priority for NASA and upcoming missions extending outside the van Allen belts to the Moon and Mars increase the risks associated with intergalactic cosmic rays and alpha particles. Alternatives such as multifunctional polymer composites incorporating inorganic additives are being investigated as potential replacements to conventional materials, such as aluminum. These composites could potentially provide structural, radiation shielding and even flame retardancy properties. Many investigators have developed polymer composites specifically for their radiation shielding properties. Low density polyethylene has been paired with hollow glass spheres (Ashton-Patton et al., 2006). The LDPE provides radiation shielding and the hollow glass spheres improve the modulus with minimum weight gain. Poly(4-methyl-lpentene) is similar to polyethylene with respect to high hydrogen content. It has been paired with carbon nanotubes for improved modulus (Clayton et al., 2006). Polystyrene has slightly lower hydrogen content and has been paired with lead dimethacrylate, which can incorporate lead directly into the polymer chains (Lin et al., 2000), and with lead oxide particles (Pavlenko et al., 2003). Epoxy paired with graphitic nanofibers has a high modulus compared to polyethylene composites, but the radiation shielding is not quite as good (Zhamu et al., 2005). High strength polyimides have been paired with boron-rich powders and whiskers (Ko et al., 1999) and (Kraus et al., 1993). Boron carbide is a material that is known for its hardness, third only to diamond and cubic boron nitride. Increased mechanical strength, high melting point, and low specific gravity are also properties that make it an attractive material (Thevenot, 1990). The applications for boron carbide materials include uses as an abrasive, in lightweight, body armor (Domnich et al., 2002), and in nuclear applications for shielding and control of nuclear reactor pellets, shapes and coatings. Boron has a high neutron absorption cross section of 767 bam and the decay products; lithium and helium, are desirable because of their short half-lives and non-radioactive characteristics (Celli et al., 2006). However, boron is brittle and difficult to produce in shapes. Boron in the form of boron carbide makes it a better alternative. The atomic number density of boron carbide is 0.11 Boron atoms/ J^ which is comparable to boron with a value of 0.14 Boron atoms/yl^ (Celli, Grazzi et al., 2006). B4C is a strong crystalline solid and is easy to process (Pierson, 1996).
CP969, Space Technology and Applications International Forum—STAIF 2008, edited by M. S. El-Genk © 2008 American Institute of Physics 978-0-7354-0486-l/08/$23.00
484
Polyethylene is also a well known material for shielding apphcations because of its high hydrogen content. Hydrogen, with atomic number of 1 has the highest energy loss per unit mass. The mean free path of ions in hydrogen is also shorter than other materials, causing more fragmentation of the heavy ions and a lower dose delivered. A composite combining the shielding characteristics of boron carbide and polyethylene could provide superior shielding properties than neat polyethylene. Incorporating filler into a polyethylene matrix should increase the mechanical properties of the composite. A study was recently completed (Harrison et al., 2007) on boron nitride/polyethylene composites showing increased mechanical properties over neat polyethylene. Surface modifications were also done to improve adhesion between the particle and matrix which led to increased mechanical properties. This study clearly identifies the effect fillers have on mechanical properties, and also on boron filler functionalization. Space radiation shielding polymer composites based on commercial polyethylenes would have processing advantages, and could be easily fabricated into sheets for laminates, extmded into profiles or foams, and injection molded for structural parts. Fillers such as boron-rich additives could improve the shielding characteristics as well as improve the modulus. The focus of this work is to report the properties of polyethylene^oron carbide composite for such an apphcation. Polyethylene/boron carbide composites were fabricated using conventional polymer processing techniques, and were evaluated for mechanical and radiation shielding properties. The boron carbide surfaces were also functionahzed to improve interfacial adhesion.
EXPERIMENTAL MATERIALS & METHODS An injection molding grade of high density polyethylene (HDPE) was used as purchased from Dow Chemical Company (Houston, TX) grade NT 8007. The boron carbide, supphed by Aldrich Chemical Company, (St. Louis, MO) was used as received with a particle size of less than 10 \x. Silane couphng agents were purchased from Gelest, Inc. (Morrisville, PA). Composites were made by mixing HDPE with the solids in a Haake Rheomix (Waltham, MA) melt blender at 145°C for 30 minutes. The polymer was allowed to melt in the blender at temperature, and the powder samples were slowly added to the molten material with mixing. The materials was removed, allowed to cool and then crushed using a hammermill crusher into sizes of less than 0.5 mm. The crushed material pressed into thin films at 150 °C for 15 minutes using a Carver Lab Press Model C (Wabash, IN). The films were 0.002" thick and dog bone samples were cut using an ASTM D638 Type IV Die. The mechanical properties were determined on these samples using a MTS QTest 10 (Eden Prairie, MN) with a strain rate of 0.0167 sec"^ and repeated five times. The standard error for tensile modulus was 84.8 MPa. The strains at break values have a standard error of 6.7% and tensile strength has an error of 2.4 MPa. Powder samples were functionahzed using the same methods as the previous study incorporating boron nitride. Trifunctional alkoxysilane coupling agent was dispersed in water for several minutes at 60-70°C. The boron carbide was then added to the solution and allowed to mix for 20 minutes. The samples were filtered, washed, and dried in an oven for 12 hours at 1 IO°C. The coupling agents used are shown in Table 1. TABLE 1. Trimethoxysilane coupling agents. CAS Common Name Reg.#
Symbol
Structure
134000Styrylethyltrimethoxysilane 44-5
C
3069-42n-octadecyltrimethoxysilane
H
485
Neutron and Proton Attenuation Measurements For neutron attenuation purposes, the material was molded into solid plaques. These plaques were approximately 11.25 x 7.95 x 0.5 cm thick. These samples were tested against standard aluminum which is conventionally used for space radiation shielding purposes. The samples were tested at both the Los Alamos Neutron Science Center (LANSCE) Weapons Neutron Research (WNR) neutron facility as well as the Fermi National Accelerator Lab (FNAL). At LANSCE the samples were tested in the WNR beam on the 30 left flight path (FP30L). The neutron beam at FP30L was selected for neutron testing because it delivers up to 600 MeV neutrons that most closely resemble an upper atmospheric/near earth orbit neutron spectrum. At FNAL, the M02 beam hue was used to deliver 120 GeV protons to measure attenuation. To measure the shielding effectiveness, a 30 cm x 30 cm X 30 cm tank phantom was used. Absorbed dose measurements were made at various depths with several different thicknesses of shielding material and aluminum to determine the relative shielding effectiveness. A tissue equivalent ion chamber made out of A150 tissue equivalent plastic was used to measure the absorbed dose in the tank. Due to the fluctuations of these beams, beam monitors were used to normalize the results. The setups for the LANSCE measurement and FNAL measurements can be seen in Figure (la) and (lb), respectively.
(a) LANSCE Showing in the Picture the WNR Fission (b) FNAL Experiments. Chamber Labeled a. and Banjo Detector Labeled FIGURE 1. Shielding Experiment Setup
RESULTS & DISCUSSION Polyethylene^oron carbide composites are important for space radiation shielding apphcations and are advantageous because they can be structural materials while also providing a shield for radiation. Mechancial and shielding properties were determined for both the composites and also neat HDPE.
Mechanical Properties of Composites Thin film tensile tests were performed for HDPE with neat boron carbide powder and silanized boron carbide powders to determine how surface modifiers affected Young's modulus, tensile strength, and strain at break.
Young's
modulus
The addition of boron carbide filler causes a significant increase in Young's modulus. Figure 3 shows the tensile modulus values compared to increasing volume percent boron carbide filler. A 60% increase is seen from neat HDPE compared to the addition of 10 volume % boron carbide. At high levels of filler loading, particle-particle interactions influence the fracture mechanics. These interactions could also influence the error in the modulus
486
values which increase with increasing amount of filler present. The deformation of the polymer in the elastic zone decreases with increasing filler and the modulus increases (Atikler et al., 2006). This figure also shows results for boron carbide that have been surface treated with silane couphng agents. The addition of the surface modification causes a decrease in modulus values compared to those that were left untreated. All of the silane couphng agents shows similar results with silanes C and H having comparable values for all levels of filler. The surface modifications do not increase the value of the Young's modulus. 1000 T
i
900 -800
i
• i
700
600A
4
1
• Boron Carbide • Silane C A Silane H
500 t^ 0
6 8 10 12 Volume Percent FIGURE 2. Young's Modulus for Boron Carbide Composites Treated with Coupling Agents C and H. 2
4
Tensile Strength Untreated boron carbide particles initially show an increase in tensile strength to 32.3 MPa compared to 29.5 MPa for neat HDPE. However this value decreases until 5 volume % filler where the tensile strength remains steady with increasing amount of filler. These values are shown in Figure 4 along with tensile strength values for composites with couphng agent surface treatments on the boron carbide particles. Similar results are seen as with the Young's modulus. The treated composites follow the same trend as untreated samples. A decrease in tensile strength values are shown compared to those with no surface treatments. Error in the tensile strength values is minimal for untreated and treated particles. The decrease in tensile strength with increasing volume percent filler can be explained by the filler debonding from the matrix (Parsons et al., 2005). When the stress at the interface exceeds its adhesive strength, the particles debond from the matrix. The debonded particles act as holes in the matrix causing stress in matrix ligaments to increase and resulting in the matrix yielding. As the volume fraction of filler increases, the number of holes caused by debonding particles at a given stress level increases, decreasing the tensile strength. Atikler (Atikler, Basalp et al., 2006) also reported decreasing tensile strengths with increasing filler volume fractions due to particle debonding in fly ash/HDPE composites. In this case, the untreated filler appears to create a better adhesion between the particle and the matrix resulting in higher tensile strength values. 35 ^ Boron Carbide 33 • Silane C A Silane H 31 29 27 25 23 21 19 17 15 0 2 4 6 8 10 12
i
4 k
Volume Percent FIGURE 3. Tensile Strength of Boron Carbide Composites Treated with Coupling Agents C and H.
487
Matrix-Particle Morphology Increased adhesion between the particle and matrix should be seen with increased mechanical properties with the composites. In the previous study (Harrison, Weaver et al., 2007), surface modification of boron nitride particles showed superior mechanical properties. This was also evident in the SEM micrographs taken where the particle appears to be better adhered to the matrix. Boron carbide filler showed the opposite trend where surface modifications caused the mechanical properties to decrease. With a decrease in mechanical properties, it would also be expected to see poor adhesion between the particle and matrix. Fractured surface images taken by SEM are able to characterize the adhesion properties of the composites. Images of the boron carbide composites are shown in Figure 5 with increasing volume % filler. Figure 5 (a) shows the fractured surface of 1 vol % filler without surface treatments and (b) shows the same amount of filler with the addition of surface modification. These images appear very similar and no differences in number of particles can be seen. Figure 5 (c) and (d) show the fractured surfaces with 5 vol % filler for both untreated and treated, respectively. Composites made with 10 vol % filler are shown in Figure 5 (e) and (f). An increase in the number of particles present can be seen for both treated and untreated surfaces.
WDlSmm 20. OkU x 7 0 0
(a) 1 vol% B4C untreated particles.
(b) 1 vol% B4C treated particles.
(c) 5 vol% B4C untreated particles.
(d) 5 vol% B4C treated particles.
(e) 10 vol% B4C untreated particles.
(f) 10 vol% B4C treated particles.
WDlSmm £0. OkU
KSOO
FIGURE 4. SEM Micrographs of Boron Carbide in HDPE Composites
These particular images also show very similar characteristics for those samples that had surface treatments compared to those left neat. The particles also appear to still be adhered into the matrix. There are no holes on the
488
surface showing where particles have debonded from tiie matrix. However, looking closely at the fracture surfaces of the modified filler samples show different morphologies in different regions of the samples. The images identified in Figure 6 show regions where the surface appears identical to those left untreated. A different type of region is shown in Figure 6 for the same samples illustrating large agglomerates of particles present. It appears the particles have very strong particle-particle interactions and agglomerated. These agglomerates are present on the surface and are cause for premature fracture of the composites. The surface modification appears to have worked but been too strong and caused the particles to adhere to each other rather than the polyethylene matrix. The interfacial adhesion appears to coincide with mechanical properties seen for those samples left untreated.
'^^'J* (a) 1 vol% Boron Carbide.
(b) 5 vol% Boron Carbide.
(c) 10 vol% Boron Carbide
FIGURE 5. SEM Micrographs of Treated Composite Fracture Surfaces Showing Large Concentrations of Particles Present in HDPE Composites
Radiation Shielding Effectiveness The neutron and proton transmission fractions can be seen in Figures 7 and 8, respectively. The dose transmission, plotted on the y asix, is the ratio of the dose rate with the shielding material present to the dose rate with no shielding present. The faster the dose transmission goes to zero as mass thickness (x axis) increases, the better the material is at attenuating the incident radiation. The WNR neutron beam can deliver neutrons up to 600 Mev; polyethylene and boron carbide-containing materials behave similarly with a slight advantage going to the boron carbide samples. Both polyethylene and boron carbide containing polyethylene exhibited better shielding abilities than that of aluminum. The trend shown in Figure 7 shows the polyethylene composites are approaching a lower transmission factor than pure aluminum. At close to 5 g/cm^ of shielding material the polyethylene composite containing boron carbide shows a better efficiency at shielding neutrons than the neat polyethylene. With a slightly thicker composite of approximately 6 g/cm^ these values are closer. However, compared to aluminum the boron carbide composite shows a clear advantage in shielding characteristics. In the WNR and upper atmosphere high energy neutron radiation environment, the polyethylene based materials have the advantage of not producing long lived radioactive progeny further producing more radiation. The aluminum counterpart produced longer lived progeny that yielded high energy beta and photon radiation as a result of neutron captures. In the high energy proton shielding, the polyethylene compounds behaved very similar to one another. For the FNAL proton beam, the protons were at such a high energy that spallation could occur. The polyethylene material actually caused more spallation products to enter the tank causing a higher overall dose. The boron carbide containing polyethylene did not produce spallation initially, but after 6.4 g/cm^ of material, produced spallation and exhibited a better performance than polyethylene alone as seen with the increase of the boron carbide data in Figure 8. It can be seen here however, that aluminum proved to be the better shielding material for high energy protons. . For the proton beam at these energies, the increasing thickness will increase the spallation "spray" and raise the overall dose. It takes several meters of sohd steel to bring these energetic particles to rest. While the polyethylene materials exhibited similar shielding capabilities in the WNR beam, the pure polyethylene based productes were less effective than those that have the boron additives.
489
0.70
B
• Aluminum • Polyethylene A Boron Carbide
0.60 0.50 0.40
1
0.30
o
a
0.20 0.10 0.00
•
•
•
•
'
•
•
•
•
'
2
•
•
•
•
'
•
3
•
•
•
'
•
•
•
•
'
4
•
5
•
•
•
'
•
•
•
•
6
'
•
•
•
•
'
•
•
•
7 :
•
'
•
•
•
•
'
•
•
10
•
•
11
Mass Thickness (g/cm2) FIGURE 6. Dose Transmission Factors for Al, HDPE, and B4C/HDPE Composites at LANSE Using WNR Neutron Beam Delivering Energies up to 600 Mev. 1.2 r 1 1.15 ^
P
1.05 r 1 \ 0.95 \ 0.9 \
0.85 0.8 0.75 0.7
• Aluminum I Polyethylene . Boron Carbide
\ \ \ ^ ^ 0
1
2
3
4
5
6
7
lv4ass Thickness (g/cm2) FIGURE 7. Dose Transmission Factors for Al, HDPE, B4C/HDPE Composites at FNAL in 120 Gev Proton Beam. Shielding effectiveness measurements show boron carbide composites are similar to aluminum. At 5.5 cm of shielding, the boron carbide composite is slightly superior to polyethylene. Boron carbide composites are advantageous over aluminum shielding because of their hghtweight characteristics. A comparison of densities can be seen in Table 2. TABLE 2. Density values for comparing boron composites to aluminum and polyethylene Material Density (kg/m ) Aluminum 2700 Polyethylene 940 PE/2wt% B4C 972
CONCLUSION Low atomic mass fillers such as boron are useful in hydrogen rich materials as shielding materials. The addition of boron carbide to high density polyethylene shows improved mechanical properties. HDPE has a tensile modulus value of 588 MPa and this value increases to 943.1 MPa at 15 vol% untreated boron carbide powder. Particles without any surface modifications show greater mechanical properties than those that have been treated. The addition of the surface treatments via coupling agents created too strong particle-particle interactions and caused the mechanical properties to decrease. This was evident in SEM micrographs where the treated composites showed
490
large agglomerates on the fracture surface. SEM micrographs of composites without surface treatments showed good adhesion between the particles and the matrix. These composites showed superior performance to other boron containing composites at various high energy neutron and proton facilities and are effective as shielding materials. This material is advantageous due to the lower density than it's conventional aluminum counterpart. In addition, the ability of the polyethylene based materials to shield these radiation fields, and not produce long lived, or high energy gamma emitting progeny is of additional merit. Future investigation is being performed on that the material be considered for a low energy neutron shielding material for shielding of neutron spectra in and around nuclear reactors.
ACKNOWLEDGMENTS The authors appreciation the support of the NASA program for space radiation shielding materials (NRA-03-OBPR07) for the project, NASA # NNM04AA60G "Synthesis and analysis of nanoparticle composites for space radiation shielding."
REFERENCES Ashton-Patton, M. M., et al., "Fonnation of low density polyethylene/hollow glass microspheres composites," Journal of NonCrystalline Solids, 352(6-7); 615-619, (2006). Atikler, U., et al., "Mechanical and morphological properties of recycled high-density polyethylene, filled with calcium carbonate and fly ash," J. Appl Polym. Sci., 102; 4460-4467, (2006). Celli, M., et al., "A new ceramic material for shielding pulsed neutron scattering instruments," Nuclar Instruments & Methods in Physics Research A, 565; 861-863, (2006). Clayton, L. M., et al., "Dispersion of single-walled carbon nanotubes in a non-polar polymer, poly(4-methyl-l-pentene)," Journal ofNanoscience and Nanotechnology, 6(8); 2520-2524, (2006). Domnich, V., et al., "Nanoindentation and Raman spectroscopy studies of boron carbide single crystals," Applied Physics Letters, 81(20); 3783-3785, (2002). Harrison, C , et al., "Polyethylene/boron nitride composites for space radiation shielding," Journal of Applied Polymer Science, Submitted, (2007). Ko, S. C , et al., "Boron containing polyimides for aerospace radiation shielding," in Materials Research Society Symposium Proceedings edited by A. F. Hepp, J. M. Prahl, T. G. Keith, S. G. Bailey and J. R. Fowler, Year, 275-280. Kraus, W. B., et al., "Boron-containing polymers for radiation shielding," Polymer Preprints (American Chemical Society, Division of Polymer Chemistry), 34(1); 592-3, (1993). Lin, Q., et al., "Synthesis, characterization and property studies of Pb2+-containing optical resins," Polymer, 41(23); 8305-8309, (2000). Parsons, E. M., et al., "Three-dimensional large-strain tensile deformation of neat and calcium carbonate-filled high-density polyethylene," Polymer, 46(7); 2257-2265, (2005). Pavlenko, V. I., et al., "Radiochemical yield of radicals under gamma irradiation of polystyrene composite material filled with modified lead oxide," Perspektivnye Materialy 5; 29-33, (2003). Pierson, H. O. (1996). Characteristics and Properties of Silicon Carbide and Boron Carbide. Handbook of Refractory Carbides and Nitrides. Noyes Publications. Thevenot, F., "Boron carbide: a comprehensive review," Journal ofEuropean Ceramic Society, 6; 205, (1990) Zhamu, A., et al., "Adhesion and mechanical property characterization of reactive graphitic nanofibers reinforced epoxy as composite matrix," in Proceedings of the American Society for Composites, Technical Conference edited by F. K. Ko, G. R. Palmese, Y. Gogotsi and A. S. d. Wang, Year, 45/1-45/10.
491
