Polyimide foams for aerospace vehicles∗ - CiteSeerX

High Perform. Polym.12 (2000) 1–12. Printed in the UK PII: S0954-0083(00)10143-6

Polyimide foams for aerospace vehicles∗

Erik S Weiser†, Theodore F Johnson†, Terry L St Clair†, Yoshiaki Echigo‡,Hisayasu Kaneshiro‡ and Brian W Grimsley§† 6A West Taylor Street M/S 226, NASA Langley Research Center, Hampton, VA 23681, USA‡ Unitika Ltd, Kyoto, Japan 611§ Old Dominion University, Norfolk, VA 23529, USA

Received 1 November 1999, accepted for publication 7 December 1999

Abstract. Due to a demand by the aerospace industry, NASA has begun developing the nextgeneration of polyimide foams which could be utilized to reduce vehicle weight for the X-33 andReusable Launch Vehicle (RLV) programmes. The activity at NASA Langley Research Centerfocuses on developing polyimide foam and foam structures which are made using monomericsolutions or salt solutions formed from the reaction of a dianhydride and diamine dissolved in amixture of foaming agents and alkyl alcohols. This process can produce polyimide foams withvarying properties from a large number of monomers and monomer blends. The specific densitiesof these foams can range from 0.008 g cc−1 to 0.32 g cc−1.

Polyimide foams at densities of 0.032 g cc−1 and 0.08 g cc−1 were tested for a wide rangeof physical properties. The foams demonstrated excellent thermal stability at 321◦C, a goodthermal conductivity at 25◦C of 0.03 W m−1 K−1, compressive strengths as high as 0.84 MPa at10% deflection and a limiting oxygen index of 51%. Thermomechanical cyclic testing was alsoperformed on these materials for 50 cycles at temperatures from−253◦C to 204◦C. The foamssurvived the cyclic testing without debonding or cracking. Thermal forming of the 0.032 g cc−1

foam was performed and a minimum radius curvature of 0.0711 m was achieved. The foamsexhibited excellent properties overall and are shown to be viable for use as cryogenic insulation onthe next generation RLV.

1. Introduction

Foamed plastics, also known as cellular plastics or plastic foams, have been important sinceprimitive man began to use wood, a cellular form of the polymer cellulose [1]. Cellulose isderived from the Latin word cellula, which means very small cell or room, and the polymer in itsnatural form exists mostly in its cellular form as seen in wood and straw. The first commercialcellular polymer was sponge rubber, introduced between 1910 and 1920 [1]. Cellular polymers,or foams, were first introduced in the United States and Europe in the mid-to-late 1930s [2].The term polymer foams or cellular polymers refers to a two-phase gas–solid system in whichthe solid polymer is continuous and the gaseous cells are dispersed throughout the solid [3].Polymeric materials are foamed to meet various application needs, such as weight reduction,insulation, buoyancy, energy dissipation, conveyance and comfort. These polymeric foamscan be produced by several different methods including extrusion, compression moulding,injection moulding, reaction injection moulding and solid-state methods. In the late 1960s, anew type of polymeric foam was developed by Monsanto and DuPont based on a polyimideprecursor. These new polyimide foams showed improved properties when compared to

∗ This paper is declared a work of the US Government and is not subject to copyright protection in the United States.

© 2000 US Government 1

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2 E S Weiser et al

traditional polymer foams, polyurethane or StyrofoamTM, in areas such as thermal stability,non-flammability, radiation resistance, improved toughness and reduced smoke and toxic fumegeneration [2].

Recently, the aerospace industry has had a growing need for high-performance polymerfoams for applications such as cryogenic insulation, fireproofing, energy absorbers, etc, and hasturned to polyimide foams to meet these needs. For example, the X-33, a single stage to orbit(SSTO) reusable launch vehicle (RLV) requires a lightweight foam which retains its structuralintegrity at temperatures ranging from−250◦C to 250◦C [4]. The High Speed ResearchProgramme has a need for low density core-splice material with a density of 0.50 g cc−1 anda potting material with a density of 0.56 g cc−1 [5].

Commercially available polyimide foams are not widely produced. However, with anincrease in interest for next generation launch vehicles, they will become more popular dueto their many beneficial physical properties such as low density, low moisture adsorption andhigh strength. Many foams are suitable for the space transportation industry, but they do notmeet all of the requirements for surviving extreme aerospace environments. Some foams withdensities as low as 0.066 g cc−1 are capable of co-curing at temperatures as high as 182◦C.Other foams can be resin transfer moulded at temperatures up to 180◦C, with a modulus ofelasticity of 0.19 MPa, tensile strength of 3.7 MPa and compressive strength of 3.6 MPa.However, these foams have a coefficient of thermal expansion comparable to aluminum andcan crack at liquid hydrogen (LH2) temperatures (−253◦C). Other foams are able to withstandtemperatures from−253◦C to 200◦C, but they do not have the required structural integrity.

The activity at NASA Langley Research Center (LaRC) focuses on developing low-density polyimide foam and foam structures which are made using monomeric solutions orsalt solutions formed from the reaction of a dianhydride and diamine dissolved in a mixtureof foaming agents and alkyl alcohol at room temperature. This process produces foamsfrom monomers such as 4,4′-oxydiphthalic anhydride (ODPA) or 3,3′,4,4′-benzophenone-tetracarboxylic dianhydride (BTDA) and 3,4′-oxydianiline (ODA) or 3,3′-diaminodiphenylsulfone (DDS), etc. Monomer blends may be used to make a variety of polyimide foamswith varying properties. Foams can be fabricated to specific densities from 0.008 g cc−1 to0.32 g cc−1.

Dianhydride

Alkyl Alcohol

Alcohol Dialkylester-Diacid

Diamine

Stable Salt at Ambient Conditions (Stablized by the Agent)Agent, Water, &

Alcohol By-Products

Polyimide Foam

Agent

Salt with Agent

Figure 1. The TEEK polyimide foam imidization reaction.



Polyimide foams for aerospace vehicles 3

Polyimide foams with a density of 0.032 g cc−1 and 0.08 g cc−1 were analysed for awide range of physical properties. Tests were based on the properties required to qualify amaterial for use as a cryogenic insulator or structural foam on next generation RLVs. Thefoams were tested at cryogenic (−253◦C to −193◦C) and flight temperatures (177◦–260◦C)for compressive and tensile strengths, thermal conductivity, liquid oxygen (LOx) compatibility,thermal and hydrolytic stability, and uniaxial tension. Thermal forming tests were alsoconducted on the 0.032 g cc−1 polyimide foam to determine the maximum radius of curvaturethe foam could sustain without any dimensional change.

The significance of structural polyimide foams can be realized with a reduction in theoverall weight of a RLV. Due to many polyimides’ high operating temperature (∼260◦C),structural polyimide foams can potentially reduce the amount of the thermal protection system(TPS) integration structure that is required on an RLV and the total amount of TPS required.The reduction in the TPS integration structure and TPS would reduce the total weight andperhaps cost to build an RLV. This would allow the maximum payload weight to increase andmake the vehicle more efficient for commercial applications.

2. Material synthesis†

NASA LaRC and Unitika Ltd have jointly developed a salt-like polyimide precursor powderwhich exhibits excellent foaming capabilities. Over 25 different polyimide foams have beenfabricated utilizing monomeric solutions or salt-like solutions [6]. The salt-like foam precursorwas synthesized by mixing monomer reactants of a dianhydride with a foaming agent inmethanol (MeOH) at room temperature. This material was converted into dimethyl ester after3 h of mixing at 60◦C. The resulting solution was then mixed with calculated amounts ofdiamine and stirred for 2 h toyield a homogeneous polyimide precursor solution. Figure 1shows a flow diagram of the imidization process necessary to form a polyimide precursorsolution. A fine powder was obtained by drying off excess solvent from the solution. Thepolyimides LaRC-IA (ODPA/3,4′ODA), BTDA/4,4′ODA and BTDA/4,4′DDS were utilized[7] for these experiments. The trade names TEEK-HH and -HL are used for the polymerLaRC-IA, TEEK-LL for BTDA/4,4′ODA, and TEEK-CL for BTDA/4,4′DDS. Figures 2–4show the chemical structures for these polyimides.

NO

O

O

N

O

O

O

n

Figure 2. Chemical structure for LaRC-IA (ODPA/3,4′ODA).

n

N

O

O

OCO

O

O

N

Figure 3. Chemical structure for BTDA/4,4′ODA.

† The use of trademarks or names of manufacturers in this report is for accurate reporting and does not constitute anofficial endorsement, either expressed or implied, of such products or manufacturers by NASA.



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n

N

O

O

SO2

CO

O

O

N

Figure 4. Chemical structure for BTDA/4,4′DDS.

Expandable Powder

Upper Graphite Plate

Lower Graphite Plate

Mold Chamber

Heat Plate

Heat Plate

Figure 5. Schematic diagram of the mould concept for TEEK foaming.

3. Experimental details

Four different polyimide foams were fabricated for scale-up and testing. The testing criteriawere designed to meet the necessary requirements for use on a next generation launch vehicle.These polyimide foams were tested for thermal, mechanical and flame resistant properties.Experiments such as the thermal conductivity, maximum use temperature, glass transitiontemperature and thermal stability were conducted to determine the ability of these newpolyimide foams to perform in the hostile environment of space. Mechanical testing includedcompressive strength at room, elevated and cryogenic temperatures and tensile strength at roomtemperature. Flammability experiments were performed to test the material’s resistance to anoxygen-rich environment and the foam’s resistance to prolonged exposure to extreme burntemperatures. Thermomechanical testing was performed to determine each material’s abilityto withstand flight conditions experienced during ground hold, launch and re-entry. Thermalforming tests were performed to ensure that these novel polyimide foams could be shaped tothe radius of a cryogenic tank.

3.1. Foam fabrication

The amount of polyimide foam precursor powder necessary to completely foam a desiredvolume was placed in a chamber. The mould, shown in figure 5, composed of a metal chestwith graphite plates on the top and bottom, was heated to 140◦C for 60 min using heat plateson the top and bottom. The mould was then rapidly transferred to a nitrogen convection ovenset at 300◦C and held at 300◦C for 60 min. The mould was then cooled to room temperature.At this point the foam was post cured for several hours at 200◦C to remove all trace volatiles.The foam was removed from the mould and was ready for use. Figure 6 shows the cure cyclefor the TEEK foams. Open cell content tests were performed on several of the foam specimensusing a Quantachrome Incorporated Ultrafoam 1000.

3.2. Thermal properties

Thermal testing was performed on three different polyimide foams. A Shimadzu DSC-50thermal analyser was used to measure the glass transition temperature (Tg). A heating rate of




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Air Nitrogen

60 minute Hold

60 Minute Hold

Foaming Step Imidization Step

Time (minutes)

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(°C

)

Figure 6. Cure cycle for the TEEK foams.

20◦C min−1 was used and theTg was taken at the inflection point of the endotherm. The weight-loss characteristics of the cured foam specimens were measured using a Seiko TG/DTA 220thermogravimetric analyser (TGA) at a heat-up rate of 2.5 ◦C min−1 in flowing (40 ml min−1)air. Thermal conductivity was measured for all specimens using ASTM standard C-518 attemperatures from−100◦C to 350◦C.

3.3. Mechanical properties

Several different polyimide foams were tested for flatwise tensile, tensile (dogbone) andcompressive strength at room temperature using ASTM standard D-1623-C, D-638-97 andD-3574 (E), respectively. The tensile specimens were cut to dimensions of 0.051 m×0.051 m×0.025 m and bonded to support blocks utilizing a low-temperature epoxy adhesive. Specimenswere placed in a 9000 kg Instron test stand and loaded under tension to failure. Compressionspecimens were also machined to 0.051 m× 0.051 m× 0.025 m dimensions and placed in a9000 kg Instron testing machine. Compression specimens were tested at−253◦C, 25◦C, and177◦C. At 50% of the initial thickness (50% deflection), the load and stress were determined.Load and stress values at 10% deflection were determined from the graph of the deflectionagainst load for the 50% deflection test.

3.4. Flammability

Several tests were performed to determine the feasibility of using these materials withinan oxygen-rich, flammable environment. The first test performed was the limiting oxygenindex test which determines a materials resistance to instantaneous combustion in an oxygen-rich environment. The test was performed using ASTM standard D-2863. The second testperformed was the vertical burn test, FAR 25.853(a), which tests a materials resistance to smokegeneration and burning. The specimens had dimensions of 0.051 m×0.305 m×0.025 m andwere placed in a fixture vertically above an igniter. The igniter was lit and burned for 25± 5 sat a temperature of 1093◦C. The burn length must be less than 0.152 m for the specimen topass the test.



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Figure 7. Location of the cryogenic propellant tanks for Lockheed–Martin’s X-33 and the simulatedcircumferential hoop load.

Cross-Sectionof fixture

Structuralwall

Convectiveheaterchamber

Cryogenicinsulation

LN2inlet

LN2inlet

LHe/LN2inlet

Cryogenicchambers

Supportingframework

Loadintroductionstructure

Finned stripheater

Fan

Heaterchimney

GN2inlet

NRA 8-21 Panel #2Cryogenic foam

LaRC TEEK-HHªFiber optic sensor

ThermocoupleStrain gage lead

Gr-Ep (IM7/977-2)

Figure 8. Schematic diagram and an assembled view of the test fixture for the uniaxial tensiontest.

3.5. Uniaxial tension tests

Combined cyclic thermal and mechanical tests of cryogenic tank wall concepts simulatingring and stringer tanks were performed on two flat 0.30 m× 0.60 m panel specimens. A flatspecimen closely approximates a tank wall due to its large radius. These tests were developedfrom earlier tests of a cryogenic insulation tile developed for the advanced launch system (ALS)[8]. The purpose of the tests was to simulate both the thermal and mechanical loads experiencedin an RLV mission from launch to orbit and to re-entry. The cryogenic tanks in an RLV, shownin figure 7, must endure biaxial tension loads associated with internal pressurization as well asmaximum thermal and mechanical flight loads. However, for these tests, the only mechanicalload applied was a uniaxial tension load simulating circumferential pressure loading, shown infigure 7. This combined cyclic, thermal-mechanical test verified the durability of the cryogenic




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Tension loadCryogenic sideHeater side

Figure 9. Typical thermal-mechanical cycle for a LH tank specimen in the uniaxial tension test.

Metal Plate

TEEK-LL Foam

Metal Cylinder

Vacuum Bag

Metal Plate

TEEK-LL Foam

Metal Cylinder

Vacuum Bag

Figure 10. Vacuum-bagged specimen before and after thermal forming was performed.

insulation when subjected to cyclic mission-profile conditions, the bond line integrity betweenthe cryogenic insulation and the structure and the performance of cryogenic tank fabricationtechnologies on a small scale.

Specialized test fixtures [9] have been developed that allow a test specimen to be thermallycycled according to pre-defined temperature profiles between a minimum temperature of−253◦C and a maximum temperature of 372◦C. Figure 8 shows a specimen mounted inthe fixture with the cryogenic chambers mounted on the surface of the inner tank wall ofthe specimen and a convective heating chamber adjacent to the external surface of the foaminsulation. Tension load and temperatures for the cryogenic and high-temperature chambersare independently controlled in a test cycle. A typical cycle lasts 30–50 min. An example ofa thermal-mechanical load profile for a LH2 tank specimen is shown in figure 9. This figuredisplays the complex tension loading and temperature profiles on the high-temperature sideand cryogenic side of the panel over a period of time.



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3.6. Thermal forming

A 0.095 m× 0.229 m× 0.0254 m TEEK-LL foam specimen was vacuum bagged over a0.0445 m radius cylinder, shown in figure 10. The vacuum-bagged specimen was placed in anoven and heated to 188◦C where 5′′ Hg of vacuum was applied to shape the foam specimenaround the metal cylinder. The specimen was held under vacuum for 1 h at 188◦C and thencooled.

4. Results

4.1. Foam fabrication

Visual inspection of all of the foam samples indicated excellent quality. Cross sectional viewsindicated a nearly uniform distribution of cellular structure. The centreline cellular structureappeared to be of a slightly lower density on a minority of the samples tested. Foam specimensat a density of 0.032 g cc−1 showed the most non-uniformity of the two densities tested.Close-cell content measurements of the polyimide foam indicated that approximately 68% ofthe volume was open-celled.

4.2. Thermal properties

All of the polymers used in this study were aromatic polyimides which exhibit excellent stabilityup to 320◦C. The specimens’ glass transition temperatures shown in table 1 ranged from 237◦Cto 321◦C. The TEEK-HH and -HL foams exhibited the lowest glass transition temperatures(237◦C), while the TEEK-LL (300◦C) and TEEK-CL (321◦C) had higherTg’s. The percentweight loss at 204◦C was measured on the samples as shown in table 1. Both TEEK-HH and-HL showed good thermal stability with weight loss values of 0.6% and 1.07%, respectively.TEEK-LL had the best thermal stability with no appreciable weight loss at 204◦C. TEEK-CLhad excellent thermal stability with a weight loss of 0.5%. However, it had a significant amountof moisture absorption (∼1.5%), while the TEEK-HH, -HL, and -LL had little water absorption(∼0.25%). The water absorption was subtracted from the reported TGA data. Figure 11 showsthermal conductivity data for the polyimide foams from−100◦C to 350◦C.

The conductivity data for the TEEK-HH, TEEK-HL and TEEK-CL foams exhibit goodvalues at room temperature and show decreasing values at sub-ambient temperatures, whichwould indicate that these foams will be good cryogenic insulators. The cryogenic temperaturesare−253◦C for hydrogen and−193◦C for LOx. The outer surface of the exterior insulated

Table 1. Thermal properties for TEEK polyimide foams.

Property Test method TEEK-HH TEEK-HL TEEK-LL TEEK-CL

Density ASTM 0.08 g cc−1 0.032 g cc−1 0.032 g cc−1 0.032 g cc−1

D-3574 (A)Thermal TGA 0.6% 1.07% 0.0% 0.5%stability Percent weight loss

at 204◦CGlass DSC 237◦C 237◦C 300◦C 321◦CtransitiontemperatureThermal ASTM C-518 0.031 W m−1 K−1 0.029 W m−1 K−1 0.044 W m−1 K−1 0.03 W m−1 K−1

conductivity at 24◦C




0

0.00025

0.0005

0.00075

0.001

0.00125

The

rmal

Con

duct

ivity

(W

/cm

*K)

-200 -100 0 100 200 300 400

Temperature (°C)

TEEK-CL

TEEK-HL

TEEK-HH

Figure 11. Thermal conductivity data for the TEEK foams.

cryogenic tanks must be at a temperature high enough to prevent air liquefaction, frost build-upor the condensation of moisture. All of the foams have conductivity data which demonstratethat the tank would be sufficiently insulated to prevent frost build-up and air liquefaction with arelatively small thickness (<0.025 m). The elevated thermal conductivities are not as efficientas the room temperature and sub-ambient values. This indicates that these foams are bettersuited for cryogenic insulation than elevated temperature insulation. However, in the case ofnext generation launch vehicles, TPS will be the main high-temperature insulator and the foamwill provide adequate protection of the cryotank wall.

4.3. Mechanical properties

Mechanical testing was performed to determine whether these new polyimide foams could beused as a structural material as well as thermal insulation. In many new design concepts forfuture RLVs, the cryogenic insulation will play a larger role if the material is a load carryingmember. Flatwise tension and compression testing was performed to determine the feasibilityof these materials as structural insulation. Table 2 shows the compression and flatwise tensionresults. During the flatwise tension test, shear induced failure had to be overcome by insuringthat the specimen was loaded in pure tension. Several specimens were prepared and tested.Only the specimens at the higher density of 0.08 g cc−1 did not exhibit shear induced failure.These specimens had a mean tensile strength of 1.40 MPa. All of the specimens at the lowerdensity, 0.032 g cc−1, failed due to shearing. This can be attributed to the formation ofelongated cells, which, when under load, tend to orient in an off-axis direction and fail in shear.Additional testing was performed utilizing a dogbone specimen according to ASTM D-638-97and tensile strengths could be determined for all the specimens. The TEEK-HH, -HL, -LL and-CL had tensile strengths of 1.68 MPa, 0.28 MPa, 0.26 MPa and 0.09 MPa, respectively. Thecompression specimens performed adequately for all three polyimide foams. Measurementswere taken for both 50% deflection and 10% deflection, which was measured by reading theappropriate value of the load against deflection graph. The TEEK-HH foam had a compressionstrength of 0.84 MPa at a 10% deflection and a modulus of 6.13 MPa. The TEEK-HL, -LL,and -CL foams had values between 0.098 MPa and 0.30 MPa. Compressive strength wasalso measured at 177◦C and−253◦C. The compressive strength of the 0.08 g cc−1 foam



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Table 2. Mechanical properties of the TEEK polyimide foams.



D-3574 (A)Tensile ASTM 1.68 MPa 0.28 MPa 0.26 MPa 0.09 MPastrength D-638-97Compressive ASTM 0.84 MPa at 0.19 MPa at 0.30 MPa at 0.098 MPa atstrength D-3574 (C) 10% deflection 10% deflection 10% deflection 10% deflectionCompressive ASTM 6.13 MPa 3.89 MPa 11.03 MPa In testmodulus D-3574 (C)Compressive ASTM 0.31 MPa 0.06–0.10 MPa 0.06–0.09 MPa In teststrength at D-3574 (C) at 10% deflection at 10% deflection177◦CCompressive ASTM 0.72 MPa 0.14–0.46 MPa 0.13–0.40 MPa In teststrength at D-3574 (C) at 10% deflection 10% deflection−253◦C

Table 3. Flammability properties of the TEEK foams.



D-3574 (A)Oxygen ASTM 51% 42% 49% 46%index D-2863Vertical burn FAR 25.853(a)a Not available

Burn length 0 cm 1 cm 0 cmDripping None None None

Smoke Visual None None None Nonegeneration inspection

a The flame time applied was 25 s.

was 0.31 MPa and 0.72 MPa, respectively. The TEEK-HL and -LL had values ranging from0.06 MPa to 0.10 MPa and 0.13 MPa to 0.46 MPa, respectively.

4.4. Flammability

Resistance to an oxygen atmosphere, smoke generation and flammability are critical propertiesthat polyimide foams will be required to have if they are to be used on the next generation RLV.Table 3 shows the results from the limiting oxygen index test the vertical burn test, and thevisual inspection of gas evolution. The three polyimide foams tested for the limiting oxygenindex show excellent resistance to combustion in an oxygen-rich environment. The foamsof lower density, 0.032 g cc−1, exhibit lower values of about 42–49% as compared to thepolyimide foam at a higher density of 0.08 g cc−1 which had a limiting oxygen index of 51%.Figure 12 shows the visual effects of the vertical burn test on the three samples tested. Twoof the samples were the same polyimide foam at two densities. The largest burn length was0.013 m, while the largest value permissible for a material to pass the test was 0.152 m. Allthree foams showed excellent resistance to burning.

The test also showed that the materials were non-ignitable and would not self-combust.The TEEK-HL was the only specimen to exhibit any shrinkage due to the extreme heat.This shrinkage was determined to be less than 10% of the original thickness. Finally, duringthe visual inspection of the tests, none of these materials exhibited any smoke generation.




Figure 12. Vertical burn specimens of TEEK-LL, TEEK-HL and TEEK-HH (from left to right).

4.5. Uniaxial tension tests

Uniaxial tension panels were tested to simulate the hoop stress in a cryogenic tank. The panelsutilized the IM7/977-2 Gr–Ep(±45◦2, 90◦2, 0

◦1.5)S substrate with TEEK-HH and fibre optic

temperature and strain sensors. The foam was bonded to the Gr–Ep substrate with the room-temperature cure epoxy shown in figure 8. The panel was subjected to 50 thermomechanicalcycles. The maximum temperature of the high-temperature side (outer or upper surface offoam) was 177◦C for panel number one and increased to 207◦C for panel number two. Thetemperature on the cryogenic side (tank inner wall) was below−253◦C. After the 50 cycles,panel number one displayed no signs of degradation except for a slight darkening of the uppersurface on the foam. Panel number two was darkened and had a slight blistering on the uppersurface after 50 cycles, indicating that the foam outgassed volatiles at high temperatures.

4.6. Thermal forming

Several specimens were successfully thermo-formed to the desired 0.0445 m radius; however,upon removal from the vacuum bag, spring back occurred. The initial spring back was anincrease in the radius from 0.0445 m to 0.0635 m. A final radial measurement was taken after24 h of the unrestrained relaxation. The resultant radius was 0.0711 m. Upon visual inspection,slight buckling of the foam was noticed along the centreline due to the non-uniformity of thecellular structure. As a result of the spring back and the slight centreline buckling, more processoptimization must occur to ensure that the foam will not relax away from a tight radius. To bestutilize a thermo-formed TEEK foam one should thermal form the structure at a temperatureabove the use temperature to ensure relaxation does not occur at elevated temperatures. TheTEEK foam, however, can be thermo-formed to the required radius for an RLV cryogenic tank.



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5. Conclusions

A new class of polyimide foam has been developed using monomeric solutions from thereaction of a dianhydride and diamine dissolved in a mixture of foaming agents and alkylalcohol. These new polyimide foams exhibit excellent stability at cryogenic, ambient andelevated temperatures while maintaining high mechanical strength. The results from thematerial characterization tests performed on these polyimide foams indicate that these foamswill be excellent for use on next generation launch vehicles. The higher operating temperatureand strength of the foam can potentially reduce the weight of the entire vehicle by reducingTPS and TPS integration structures. Thermal, mechanical, combined thermal/mechanicaland flammability testing has indicated that these materials can endure the rigours of spaceand space-like environments. Flammability results indicate that these materials are wellsuited for extremely combustible environments and will maintain dimensional stability duringexposure to high temperature and flames. The uniaxial tension test exhibits the ability ofthese materials to survive simulated flight conditions for 50 cycles. Initial thermal formingtests indicate that these materials can be formed to a cryogenic tank radius and possibly totighter-radius parts such as fuel lines. Based on the results contained in this report, this newhigh-temperature, hydrophobic, low-density, structural polyimide foam will support the needsof the next generation of launch vehicles.

References

[1] Cellular materialsEncyclopedia of Polymer Science and Engineering1985 vol 3 (New York: Wiley) pp 1–59[2] NASA Contractor’s Report1994 NASA Contract No. NASW-4367, Research Triangle Institute, North Carolina[3] Cellular materialsEncyclopedia of Polymer Science and Technology1965 vol 3 (New York: Wiley) pp 80–130[4] McConnell V P 1997 NASA gets hands-on with X-33 designHigh-Performance Composites5 56–8[5] Fisher K 1997 Resin flow control is the key to RTM successHigh-Performance Composites5 34–8[6] Weiser E S, Baillif F F, Grimsley B W and Marchello J M 1998 High temperature structural foamProc. 43rd Int.

SAMPE Symp. (May 31, 1998)pp 730–44[7] Hou T H, Johnston N J and St Clair T L 1995 IM7/LARCTM–IA polyimide compositesHigh Performance

Polymers7 105–24[8] McAuliffe P S, Davis R C and Taylor A H 1986 Development of reusable, flight-weight cryogenic foam insulation

systemAmerican Institute of Aeronautics and Astronautics (AIAA) Space Systems Technology Conf. (June,1986)AIAA-1189-CP

[9] Johnson T F, Natividad R, Rivers H K and Smith R 1998 Thermal structures technology development for reusablelaunch vehicle cryogenic propellant tanksSpace Technologies and Application Int. Forum (Albuquerque, NM,January 1998)



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