[american institute of aeronautics and astronautics space 2005 - long beach, california ()] space...

American Institute of Aeronautics and Astronautics

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Shielding Technology and the Bush Initiative - Shielding for Lunar and Martian Missions

Sheila A. Thibeault,* John W. Wilson,† Martha S. Clowdsley‡ NASA Langley Research Center, Hampton, Virginia 23681

and

Richard L. Kiefer,§ Robert A. Orwoll,¶ Amanda Boone,# Lucy Y. Hu,** Hillary A. Huttenhower,†† Barbara H. Besal,‡‡ Meghan E. Schulz,§§ Sha Yang,¶¶ Christopher A. O’Neill,## Adriane K. Miller***

College of William and Mary, Williamsburg, Virginia 23185

Past space missions beyond the confines of Earth’s protective magnetic field have been of short duration, and protection from the effects of solar particle events was of primary concern. Aluminum alloy structures provided sufficient protection to meet requirements to prevent early radiation syndrome and were further successfully employed as protection against trapped protons in low Earth orbit (LEO). Aluminum alloys have been the materials of choice for the first 40 years of the space program. The extension of operations beyond LEO to enable routine access to other interesting regions of space will require protection from the hazards of the accumulated exposures of Galactic Cosmic Rays (GCR), and efficient fragmentation of GCR ions with minimal production of secondary particles in shielding materials is essential to protecting the astronauts. Aliphatic polymer composites are the most efficient structural materials exhibiting significantly improved radiation shielding properties, but they are limited in meeting the requirements of the service environment. Aromatic polymer composites developed mainly for high-speed aircraft applications have improved thermal-mechanical properties over aluminum alloys, as well as improved radiation shielding performance. Aliphatic/aromatic hybrid polymers are being developed to give a range of thermal-mechanical properties. The development of functionally graded structures will provide optimum solutions to multifunctional materials requirements in the near-term space program, with far-term utility.

Introduction HE International Space Station (ISS) typifies the first step in establishing space infrastructure by providing a base and transportation hub in low Earth orbit (LEO). The ISS is basically an aluminum structure providing a

well-proven technology for that historic development. Research at the NASA Langley Research Center has

* Senior Research Physicist, Advanced Materials & Processing Branch, Mail Stop 226, nonmember. † Senior Research Scientist, Computational Structures & Materials Branch, Mail Stop 388, member. ‡ Research Scientist, Computational Structures & Materials Branch, Mail Stop 388, member. § Emeritus Professor of Chemistry, Department of Chemistry, member. ¶ Professor of Chemistry, Department of Chemistry, member. # Student, Department of Chemistry, nonmember. ** Post Doctoral Research Fellow, Department of Chemistry, nonmember. †† Student, Department of Chemistry, nonmember. ‡‡ Student, Department of Chemistry, nonmember. §§ Student, Department of Chemistry, nonmember. ¶¶ Student, Department of Chemistry, nonmember. ## Student, Department of Physics, nonmember. *** Student, Department of Chemistry, nonmember.

T

Space 200530 August - 1 September 2005, Long Beach, California

AIAA 2005-6601

This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.


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established aluminum as a poor radiation shield material and even hazardous outside of LEO applications.1 It has been shown that implementation of hydrogen bearing materials is the best alternative for improved shield performance in both LEO and space operations outside the geomagnetic field.2 The best such materials with reasonable structural properties are aliphatic polymers, such as the high density polyethylene chosen to augment shielding within the ISS, and some applications of polyethylene have now been space qualified.3 In this case, basic structural loads and micrometeoroid/debris protection are provided by the aluminum structure, and the polyethylene is an added mass with the sole purpose of reducing astronaut exposure. Clearly, a multifunctional wall/support structure incorporating all these functions will reduce design mass, lower costs, and improve radiation protection.4

Plans are now being made for the next step in Earth neighborhood infrastructure development for operations beyond the confines of the geomagnetic field. The current vision is a reusable Crew Exploration Vehicle (CEV) that will adequately service ISS and be utilized for lunar operations. The CEV is augmented with a lunar landing vehicle capable of short-term habitation and a long-term surface habitation module.

A second station (Gateway) is considered in Lunar L1 to serve as a way station for future space activity with transportation services from Gateway to other near-Earth points of interest, including ISS. This next level of infrastructure will be the first opportunity to make the radical change from aluminum-based structural technologies to other materials. Preplanning in the past has emphasized inflatable polymer structures using polyurethane foams and graphite composite stiff structures.4 While this is an improved concept, the aforementioned aliphatic polymer systems provide superior shielding and superior material strength characteristics. Although polyethylene was used to augment the ISS shield design, no effort has yet developed useful multifunctional aliphatic structural systems, in spite of clear radiation shielding advantages over polyurethane based structures.1, 4

In order for astronauts to travel to and stay on Moon or Mars for extended periods of time, they must be protected from the space radiation.5 This requires habitats and space equipment to be built from materials that will also shield against radiation. Polyethylene and polypropylene are polymers that both have the empirical formula CH2. These polymers contain high hydrogen content. For practicality purposes, regolith (interplanetary dirt) could be mixed with polyethylene (or polypropylene) to create composites to be used as bricks for habitat construction. Two approaches have been investigated in the present paper. Each approach involves heating the regolith/polymer mixture in a microwave oven. The first approach heats the mixture and then compresses it to form as it cools, while the second approach compresses the specimen during both heating and cooling.

Polymer Synthesis

We are synthesizing and characterizing polymers for applications in spacecraft and extraterrestrial habitats for

shielding against galactic cosmic radiation, as well as for ancillary functions in structural units, thermal and acoustic insulation, space suits, plumbing, and micrometeoroid shielding. These polymers are hybrids in the sense that they have highly aromatic backbones, such as is found in the high-performance polymer Kapton®, and also aliphatic units, especially methyl groups, that are rich in hydrogen. The aromatic component was chosen to impart strong mechanical properties, while the aliphatic component provides more hydrogen atoms for radiation shielding. Hydrogen - with one electron per nucleon and a nucleus containing only a proton - has the greatest shielding capacity per unit mass of any element.1

Two general classes of polymers have been studied: polyimides and poly(arylene ethers). They were synthesized using standard methods. The reaction scheme for polyimide formation is shown in Fig. 1.


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Figure 1. The reaction of arylene-containing (Ar) dianhydride with the diamine H2NBRNBNH2 to form first the polyamic acid and then, with the loss of water, the polyimide. The polymerization was carried out at room temperature in the solvent 1-methyl-2-pyrrolidinone (NMP).

The polyarylene ethers were prepared as depicted in Fig. 2.

Figure 2. The reaction of a dichloro- or difluoro-benzophenone with the bisphenol HOBYBOH in the presence of potassium carbonate to form the polyarylene ether.

These reactions have been described in detail elsewhere.6,7 The monomers used in the polymerizations in this study are listed in Table I.


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Table I. Monomers.

ABBREVIATION NAME

DIANHYDRIDES FOR POLYIMIDES

PMDA 1,2,4,5-benzentetracarboxylic dianhydride

UDA 4,4΄-(4,4΄-isopropylidenediphenoxy)bisphthalic dianhydride

BTDA 1,2,3,4-butanetetracaroxylic dianhydride

DIAMINES FOR POLYIMIDES

BDA1 4,4΄-(4,4΄-isopropylidenediphenyl-1,1΄-diyldioxy)dianiline

BDA2 2,2-bis[3,5-dimethyl-4-(4-aminophenoxy)phenyl]propane

BDA3 α,α΄-bis[3,5-dimethyl-4-(4-aminophenoxy)phenyl)]-1,4-diisopropylbenzene

DACH trans-1,4-diaminocyclohexane

BENZOPHENONES FOR POLYARYLENE ETHERS

BPCl Dichlorobenzophenone

BPF Difluorobenzophenone

BISPHENOLS FOR POLYARYLENE ETHERS

BPA 2,2-bis(4-hydroxyphenyl) propane

HPB 1,4-bis[(4-hydroxyphenyl)-2-propyl]benzene

HDMPB 1,4-bis[(4-hydroxy-3,5-dimethylphenyl)-2-propyl]benzene

The moduli and the elongations at break of six polyimides are listed in Table II and compared with the modulus of a commercially available specimen of Kapton® for both ambient and elevated temperatures. All seven were studied under the same conditions. These mechanical properties of the hydrogen-rich aromatic polyimides compare favorably with those of high-performance Kapton®.


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Table II. Moduli and percentage elongation of polyimides.

Modulus (GPa) % Elongation at break

Polyimide At 25ºC At 180ºC At 25ºC At 180ºC

PMDA/BDA1 2.4 1.5 85 130

PMDA/BDA2 2.8 1.9 7 4

PMDA/BDA3 2.9 1.8 5 3

UDA/BDA1 2.3 1.3 66 100

UDA/BDA2 2.9 1.8 4 4

UDA/BDA3 3.0 1.8 7 5

Kapton® 2.5 1.9 -- --

The thermal stabilities of six polyimides and three polyarylene ethers were assessed in thermogravimetric analysis (TGA) measurements in which the specimens were heated in a nitrogen atmosphere at 5-10ºC/min. The resulting mass losses are shown in Figs. 3 and 4. Good thermal stability is indicated. In each case, the onset of mass loss appears above 325ºC.

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Figure 3. TGA data obtained for the polyimides (a) PMDA/BDA1, PMDA/BDA2, and PMDA/BDA3; (b) UDA/BDA1, UDA/BDA2, and UDA/BDA3 film specimens in a nitrogen atmosphere.

PMDABDA1

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UDABDA1

UDABDA2

UDABDA3


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Figure 4. TGA data obtained for the polyarylene ethers BPF/BPA, BPF/HPB, and BPF/HDMPB powder specimens in a nitrogen atmosphere. The glass transition temperatures Tg for the same nine polymers were determined using differential scanning calorimetry. The values are tabulated in Table III. These are the maximum use temperatures, above which the polymer has no structural integrity. Table III. Glass transition temperatures.

Polymer Tg (ºC)

PMDA/BDA1 329

PMDA/BDA2 312

PMDA/BDA3 291

UDA/BDA1 205

UDA/BDA2 225

UDA/BDA3 234

BPF/BPA 162

BPCl/HPB 162

BPF/HDMPB 173

BPFHPB

BPFBPA

BPFHDMPB


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Four polyimides were prepared for this study in which either the diamine (PMDA/DACH) or the dianhydride (BTDA/BDA1, BTDA/BDA2, and BTDA/BDA3) is aliphatic rather than aromatic, thus raising the hydrogen content higher than the value it would have with a fully aromatic structure. In each case, the molecular weight of the polyimide was apparently too low for the material to exhibit suitable mechanical properties. Additional polymerization studies directed toward higher molecular weights are planned. Castings of seven polymers were prepared with a thickness such that their broad face had an areal density of 5 g/cm2. These were subjected to high-energy heavy-ion bombardment at the Brookhaven National Laboratory. UDA/BDA2, BPF/BPA, PMDA/BDA1, and the commercial polyimides Udel® and Noryl 731® were tested with a beam of 600 MeV/nucleon oxygen nuclei. UDA/BDA1 and the commercial polyimide Ultem® were bombarded with 1000 MeV/nucleon iron nuclei. While the interpretation of the data is incomplete, it is evident that the results will show that the ions and their energies detected on the back side of the castings vary little from one of the polymers to another, given the same bombarding particle and energy. This is consistent with ongoing theoretical calculations using NASA Langley computer codes.

Regolith/Polymer Processing

A. Specimen Fabrication with Pressure Applied after Heating Two methods were employed in the processing of regolith/polymer composites. In the first method, simulated lunar regolith was mixed with either reversed phase high-performance liquid chromatographic (HPLC) grade polyethylene, medium density polyethylene, or HPLC grade polypropylene. The simulated lunar regolith is similar to the natural material found on the surface of Moon. Each of the polymers was in a powder form. A summary of the first method is as follows:

1. Measure the regolith and polymer into the desired proportions, according to weight. Use polymer compositions of 10%, 15%, 20%, 25%, and 30%. A total mass of the mixture should be approximately 70 grams for a 2.5-inch-diameter round steel mold (Fig. 5), and approximately 40 grams for a 3.5-inch X 0.75-inch rectangular steel mold (Fig. 6).

2. Hand shake the mixture for about 5 to 6 minutes. For the HPLC grade polyethylene, mix in a ball mill mixer for 2 to 3 hours.

3. Apply two coats of release coating agent to a 2.0-inch X 2.0-inch ceramic mold (Fig. 7), and dry with a heat gun after each application.

4. After the ceramic mold has cooled, transfer the regolith/polymer mixture to the mold and close. 5. Place the ceramic mold into the center of a microwave oven (Fig. 8) on top of the glass plate. The

glass plate is able to rotate on a roller ring. 6. Heat the mixture at the desired power until it has completely melted. Power levels of 30% - 100% are

used, as desired. Gray powder is an indication that the mixture has not completely melted. 7. During the last 8 minutes of heating the mixture, use a heat gun to heat the steel mold and its plunger.

For larger specimens, use a round 2.5-inch-diameter steel mold. For smaller specimens, use a 3.5-inch X 0.75-inch steel mold.

8. Once the mixture has completely melted, quickly scoop it into the steel mold and then plunge the mold.

9. To help compress the specimen while cooling, place lead bricks on top of the plunger. For the round steel mold, use 3 lead bricks, thus applying a pressure of 16 pounds per square inch (psi). For the rectangular 3.5-inch X 0.75-inch steel mold, use 4 lead bricks, applying a pressure of approximately 40 psi.

10. Remove the specimen from the mold after it has cooled.

The 2.0-inch X 2.0-inch ceramic mold (Fig. 7) used in this first method had a boron nitride coating. Boron nitride helped to ensure that the mixture did not stick to the mold. A releasing agent was also applied to the mold for the same reason. The inside of the ceramic mold was made with tapered walls; i.e., the walls on the inside of the mold were angled such that the top of the box was somewhat larger than the bottom. The lid was also made in the same manner so that it could fit into the mold.

In order to process the specimens, a commercial grade microwave oven (Fig. 8) was used. At 2.45 GHz, this microwave oven had an output of 1300 W. The microwave oven also featured a power level button, which allowed the power to be adjusted. Without adjusting the power level, the microwave oven automatically set itself to 100% power. To adjust the power to 40%, the power level button would have to be pressed 7 times. Before heating the mixture, the ceramic mold was placed into the center of the microwave oven onto its glass plate. The glass plate


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was positioned on a roller ring, which rotated the plate while heating. This was to ensure both continuous and uniform heating. After the time parameters for each polymer and power level were established, this procedure was repeated under constant heat.

Figure 5. The 2.5-inch-diameter round steel mold and plunger.

Figure 6. The 3.5-inch X 0.75-inch rectangular steel mold and plunger.

Figure 7. The 2.0-inch X 2.0-inch ceramic mold.


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Figure 8. Microwave oven.

Heating times varied for each composition of regolith/polymer, each type of polymer, and each power level. The

heating times established by heating the mixture and then transferring it to the steel molds are given in Tables IV, V, and VI. Table IV. Heating times for reversed phase HPLC grade polyethylene.

% PE Power Time (min) Size of steel mold Steel mold heated? 10 7 20 3.5” X 0.75” yes 15 8 17 3.5” X 0.75” yes 20 7 15 3.5” X 0.75” yes 20 7 15 3.5” X 0.75” yes 20 8 15 3.5” X 0.75” yes 20 9 15 3.5” X 0.75” yes 25 7 20 3.5” X 0.75” yes 30 5 15 2.5” diameter no 30 6 15 3.5” X 0.75” yes 30 7 15 2.5” diameter yes 30 7 20 3.5” X 0.75” yes

Table V. Heating times for medium density polyethylene.

% PE Power Time (min) Size of steel mold Steel mold heated? 10 7 25 3.5” X 0.75” yes

12.5 7 25 3.5” X 0.75” yes 15 6 12 3.5” X 0.75” yes 15 6 12 3.5” X 0.75” yes 20 4 12 2.5” diameter yes 20 4 12 2.5” diameter yes 20 7 9.5 3.5” X 0.75” yes 25 4 12 2.5” diameter no 25 6 15 3.5” X 0.75” yes 25 6 15 3.5” X 0.75” yes 30 6 15 3.5” X 0.75” yes 30 6 15 3.5” X 0.75” yes


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Table VI. Heating times for HPLC grade polypropylene.

% PE Power Time (min) Size of steel mold Steel mold heated? 15 7 25 3.5” X 0.75” yes 20 7 25 3.5” X 0.75” yes 20 8 25 3.5” X 0.75” yes 20 10 25 3.5” X 0.75” yes

This first method used to make sample bricks for habitat construction on Mars or Moon produced some

promising results. For the first few specimens processed, the mold was not heated. The surfaces of these specimens were both uneven and unsmooth and sometimes cracked. Some specimens contained a bump in the center. One possible reason for this occurring was that the specimen cooled too quickly before the mold was plunged. Heating the mold was an attempt, however unsuccessful, to eliminate the bump in the center. Most specimens processed with a heated mold resulted in surfaces that were even and smooth. The best specimens were made from mixtures containing regolith and medium density polyethylene. Medium density polyethylene, when heated, flowed uniformly and produced regolith/polymer composites that were shiny, black, and smooth. The liquefied mixture, however, cooled very quickly. It was thus difficult to transfer all of the mixture from the ceramic mold to the steel mold. Specimens processed with reversed phase HPLC grade polyethylene also resulted in surfaces that were even and smooth. It was evident, however, that the polymer did not flow well. The mixture with HPLC grade polyethylene, after heating, was putty-like; thus, it was easier to transfer the mixture from the ceramic mold to the steel mold. Mixtures containing regolith and HPLC grade polypropylene resulted in specimens that were black and gray. This made it difficult to decide if all of the mixture had completely melted. The surfaces of these specimens were also smooth and even. Specimens were processed with polymer compositions of 10% - 30%. All specimens with 10% polymer and 90% regolith resulted in uneven surfaces and cracked edges. This indicated that there was not enough of the polymer to bind to all of the regolith. B. Specimen Fabrication with Pressure Applied during Heating The second method eliminated the transfer of the regolith/polymer mixture from the ceramic mold to the steel mold. This method also applied pressure to the specimen while heating. In order to do so, a 2.25-inch X 2.25-inch ceramic mold with a detachable bottom lid and plunger were fabricated (Fig. 9). For this part of the research, the simulated lunar regolith was mixed with only the reversed phase HPLC grade polyethylene. Using differential scanning calorimetry, the melting point of the polyethylene was determined to be approximately 112˚C. The second method for processing regolith/polymer specimens is as follows:

1. Measure out a 30% polyethylene and 70% regolith mixture, according to weight. The total mass should be approximately 45 grams.

2. Hand shake the mixture for approximately 5 to 6 minutes until thoroughly mixed. 3. Apply two coats of release coating agent to the top of the bottom lid, the inside of the mold, and the

plunger (Fig. 9). Dry with a heat gun after each application. 4. After the mold and its parts have cooled, place Kapton® tape around the outer edges of the bottom lid.

This is to prevent the mixture from falling into any cracks between the sides of the mold and the sides of the bottom lid. Also, cover the top surface of the bottom lid with Kapton® tape, in order to prevent the specimen from sticking to the mold.

5. Transfer the mixture to the ceramic mold. 6. Plunge the mold and place it into the center of the microwave oven on top of the glass plate (to ensure

uniform heating). 7. Fill the hollow center of the plunger with Zircon sand to compress further the specimen while heating. 8. Heat the regolith/polymer mixture on the desired level of power until the gray powder has completely

melted. 9. Remove the mold from the microwave oven and place it onto the countertop. Place a firebrick on top

of the mold. The pressure from the sand filled plunger and firebrick is about 1.77 psi + 0.02 psi. 10. Once the mold has cooled, remove the specimen.

The 2.25-inch X 2.25-inch ceramic mold (Fig. 9) utilized in this second method also had a boron nitride coating

to prevent the mixture from sticking to the inside of the mold. Because the bottom lid and plunger fit so tightly into


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the mold, a releasing agent was applied to all pieces. This was to help guarantee that the mixture, bottom lid, and plunger did not get wedged into the mold. The walls of this ceramic mold were not tapered.

Figure 9. The 2.25-inch X 2.25-inch ceramic mold with detachable bottom lid and plunger.

The time parameters established were only for composites made with reversed phase HPLC grade polyethylene.

These time parameters are given in Table VII. Table VII. Heating times for reversed phase HPLC grade polyethylene with pressure applied during heating.

% PE Power Time (min) Plunger filled with sand during

heating? 20 4 20 no 20 4 21 yes 20 5 30 yes

The second approach to making regolith/polymer composites for habitat construction was not fully researched.

The number of composites made using this process was limited because of complications with the mold. Initially, two molds were made. However, over the course of the research, these two molds were broken; and one new mold has been mended together to make future research possible. The first two molds constructed were broken in the process of trying to remove the plunger after heating. When the mold was heated, its walls expanded. It was thus difficult to remove the plunger from the mold, without putting too much pressure on the mold, to prevent it from breaking. The same problems were experienced with the third mold. While trying to remove a 20% polypropylene and 80% regolith composite from the mold, the mold cracked in two corners, diagonal from each other. Polypropylene has a higher melting point temperature than polyethylene; thus, the composites with polypropylene were heated at a higher level of power and/or for a longer period of time. This may have contributed to the mold cracking. Another reason the mold cracked may have been that, in an effort to remove the specimen, the plunger was pressed into the mold. Another problem experienced when using these molds was that sometimes the regolith/polymer mixture smoked and/or burned, even at low levels of power. In the first mold used, one specimen made with 20% polymer and 80% regolith was heated for 15 minutes at 30% power, and it started to smoke within the last minute of heating. In the second mold, a 20% polymer and 80% regolith mixture began burning. After heating this particular mixture for 13 minutes and 12 seconds at 20% power, the mixture had not melted but was smoking. The mixture was heated an additional 2 minutes and 24 seconds and then it began to burn. It was also evident that there was a hot spot in the second mold; as a result, one section of the mixture would heat faster than the rest. Aside from problems with the molds, the composites made were not consistent from one to another. In the first composites made, the plunger by itself was the only pressure (approximately 0.08 psi) applied to the mixture while


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heating. The plunger was made with a hollow opening so that additional weight could be added. Mixtures heated with a hollow plunger, overall, resulted in specimens that were crumbly and fell apart. For composites made later, the hollow center of the plunger was filled with Zircon sand. When the plunger was filled with sand, the pressure applied to the mixture was approximately 1.77 psi. When the extra pressure was applied to the mixture, the composites made were the most promising. Some composites had shiny surfaces where the mixture had completely melted. To the contrary, other composites appeared in tact once the bottom lid was removed; however, they broke while trying to remove them from the mold. It was difficult to observe whether the mixture had completely melted. While the mixture may have appeared to be melted completely on the top of the surface, it may not have been in the center, or on the bottom. C. Thermal Analysis

Thermomechanical and thermogravimetric analysis tests were only run on specimens that had pressure applied to the mixture during both heating and cooling. Equally, tests were run only on the regolith/reversed phase HPLC grade polyethylene composites. The average softening temperatures are given in Fig. 10 and Table VIII.

Figure 10. Average softening temperatures for regolith/reversed phase HPLC grade polyethylene composites. Table VIII. Average softening temperatures for regolith/reversed phase HPLC grade polyethylene composites.

According to the thermal analysis for the regolith/reversed phase HPLC grade polyethylene composites, there was not a trend for the average softening temperatures with increasing polymer composition. The average softening temperatures were within three degrees of each other. The 100% polyethylene specimen softened very close to its melting temperature of 112°C. The average decomposition temperatures are given in Fig. 11 and Table IX.

HPLC grade polyethylene (%) Average softening temperature (˚C) 15 114.64 20 115.62

100 112.78


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Composition of HPLC Grade Polyethylene (% )

Average Decomposition Temperatures

5% Mass Loss10% Mass Loss

Figure 11. Average decomposition temperatures for regolith/reversed phase HPLC grade polyethylene composites. Table IX. Average decomposition temperatures for regolith/reversed phase HPLC grade polyethylene composites.

The average decomposition temperatures for the regolith/reversed phase HPLC grade polyethylene composites

appeared to follow a trend. The relationship was inversely proportional; i.e., as the polymer composition increased, the average decomposition temperature decreased. This trend was true for decomposition temperatures at both the 5% mass loss and the 10% mass loss.

Concluding Remarks Four polyimides were synthesized in which either the diamine or the dianhydride was aliphatic rather than aromatic, thus raising the hydrogen content higher than that of a fully aromatic structure. Additional studies directed toward higher molecular weights are planned in order to increase the mechanical properties. Castings of seven polymers were prepared with a thickness such that they each had an areal density of 5 g/cm2. These were subjected to high-energy heavy-ion bombardment at the Brookhaven National Laboratory. While the interpretation of the data is incomplete, it is evident that the results will show that the ions and their energies detected on the back side of the castings vary little from one polymer to another, given the same incident particle and energy.

Simulated lunar regolith and aliphatic polymers were consolidated into composites using microwave energy for efficient processing. These materials would help in the construction of habitats to protect astronauts on extended missions to Mars or Moon. Two approaches have been developed to make these composites. In the first approach, a

HPLC grade polyethylene (%) Average temperature at 5%

mass loss (˚C) Average temperature at 10%

mass loss (˚C) 15 423.29 445.62 20 420.60 443.09

100 346.15 367.20


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polymer/regolith mixture was heated, quickly scooped into a steel mold, and then compressed while cooling. For this process, reversed phase HPLC grade polyethylene, medium density polyethylene, and HPLC grade polypropylene were used. Overall, the composites had smooth and shiny black surfaces. Medium density polyethylene flowed the best, of the three polymers used. In the second approach, a new mold was created so that the mixture could be compressed during both heating and cooling. Because of complications with this mold, a limited number of composites were made. Thermal analysis was only completed on polymer/regolith composites containing the reversed phase HPLC grade polyethylene and using the new mold. The thermal analysis indicated a trend for the average decomposition temperatures; as the polymer composition was increased, the average decomposition temperature decreased. For future research, more varied polymer compositions should be used to produce specimens by the one step process. More thermal and mechanical properties should be tested. Equally, specimens should be exposed to radiation to ensure their effectiveness as a radiation shield in space.

Acknowledgment

The authors thank Mr. Peter Vasquez of the NASA Langley Research Center for designing and fabricating the ceramic molds, without which the regolith/polymer processing research would not have been possible.

References 1 Wilson, J. W., Miller, J., Konradi, A., and Cucinotta, F. A. (eds.), Shielding Strategies for Human Space Exploration,

NASA CP 3360, 1997. 2 Wilson, J. W., Shinn, J. L., Tripathi, R. K., Singleterry, R. C., Clowdsley, M. S., Thibeault, S. A., Cheatwood, F. M.,

Schimmerling, W., Cucinotta, F. A., Badhwar, G. D., Noor, A. K., Kim, M.-H. Y., Badavi, F. F., Heinbockel, J. H., Miller, J., Zeitlin, C., and Heilbronn, L., “Issues in Deep Space Radiation Protection,” Acta Astronautica, Vol. 49, No. 3-10, 2001, pp. 289-312.

3 JSC Materials and Fracture Control Certification, MATL-01-084, Jan. 11, 2001. 4 Wilson, J. W., Glaessgen, E. H., Jensen, B., Orwoll, R., Saether, E., Adams, D. O., Humes, D. H., Kelliher, W. C.,

Thibeault, S. A., Anderson, B. M., Nealy, J. E., and Tripathi, R. K., “Next Generation Shielding Materials for Earth Neighborhood Infrastructure,” AIAA Space 2003 Conference, San Diego, CA, Vol. 8, No. 4, 2003.

5 Caron, R. P., “Radiation Shielding for Manned Missions to Mars,” Worchester Polytechnic Institute, 2004. 6 Hu, L. Y., Miller, A. K., Park, C. S., Plichta, K. A., Rochford, S. J., Schulz, M. E., Yang, S., and Orwoll, R. A.,

“Aliphatic/Aromatic Hybrid Polymers for Functionally Graded Radiation Shielding,” AIAA Space 2004 Conference, San Diego, CA, Vol. 9, No. 20, 2004.

7 Hu, L. Y., Yang, S., Miller, A. K., Park, C. S., Plichta, K. A., Rochford, S. J., Schulz, M. E., Orwoll, R. A., and Jensen, B. J., “Aliphatic/Aromatic Hybrid Polymers for Functionally Graded Radiation Shielding,” High Performance Polymers, accepted for publication, 2005.

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