Fire and Engineering Properties of Polyimide-Aerogel Hybrid Foam

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Downloaded by UNIV OF ARIZONA on May 14, 2013 | http://pubs.acs.org Publication Date: April 27, 2009 | doi: 10.1021/bk-2009-1013.ch010

Fire and Engineering Properties of PolyimideAerogel Hybrid Foam Composites for Advanced Applications 1

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Trent M . Smith , Martha K. Williams , James E . Fesmire , Jared P. Sass , and Erik S. Weiser 2

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John F. Kennedy Space Center, Polymer Science and Technology Laboratory, NASA, Kennedy Space Center, FL 32899 John F. Kennedy Space Center, Cryogenics Test Laboratory, NASA, Kennedy Space Center, FL 32899 Langley Research Center, Advanced Materials and Processing Branch, NASA, Hampton, VA 23861 2

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N A S A has had a growing need for high-performance materials for cryogenic insulation, fireproofing, energy absorption, and other advanced applications. New monolithic aerogels are good candidates except for their inherently fragile nature. Modern commercially available aerogels are tougher, but still lack adequate structure and can become a contamination concern in certain applications. Polyimide foam materials have been used for demanding applications that require extreme thermal, chemical, and weathering stability. The combination of aerogel with a high performance polyimide foam fabricated into a hybrid material provides structure to the aerogel, reduces heat transfer, improves vibration attenuation, and retaines the excellent fire properties of the polyimide foam. Incorporation of aerogel material into TEEK-H polyimide foam provides three favorable effects: thermal conductivity is reduced by the aerogel content in the hybrid foams, Peak Heat Release remains low, and the time to Peak Heat Release increases with increasing aerogel content.

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© 2009 American Chemical Society

In Fire and Polymers V; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

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Introduction Aerogel materials have been known for many years as excellent thermal and acoustic insulators (1-6), but have suffered limited use due to their inherently fragile nature. There have been two main commercialization thrusts of aerogel. In the late 1940's Monsanto Chemical commercialized silica aerogel under the trade name Santocel®. Santocel® products, in a number of hydrophilic grades with relatively low surface areas, were used in paints, thermal insulation, and a diverse number of applications (7-13) through the 1960's. Production was stopped due to the high cost of maintenance and labor, and in conjunction with the sale of Monsanto's silica business (14). Today's commercial aerogel materials, characterized by their high surface areas and low densities, were developed along two different lines, loose fill and composite blanket type, in the late 1990's. These modern aerogels are fully hydrophobic and have been demonstrated to have the lowest thermal conductivity, at ambient pressure 101.3 kPa (760 torr), of any material in the world (15,16). Modern commercial aerogel materials are much more resilient than monolithic forms of aerogel, however many of these new forms of aerogel still lack sufficient structural integrity to be considered for use in many applications. Commercially available aerogel beads/granules and aerogel composite blanket materials are excellent thermal insulators. For example, these aerogel materials are finding their way into space vehicle applications solving specialized problems (17-19). Aerogel materials though can cause contamination concerns with aerogel dust which becomes airborne during normal installation procedures and can be liberated during system operations which include mechanical loads or vibrations. Specialized containment systems must be employed to mitigate dust contamination concerns. The dust is considered low health risk to humans because of the amorphous nature of the silica, but can be problematic in electrical systems, optical systems, and in general use. Silica based aerogel materials are expected to be inherently fire retardant, but commercially available silica aerogel materials from Cabot Corp. and Aspen Aerogels Inc. are treated to be super-hydrophobic. The treatment is normally a branched carbon-based silane such as a trimethylsilane moiety which covalently bonds with the silica. Fire performance of surface functionalized aerogel materials is expected to be good, but limited data are available and no data are available under forced combustion conditions such as cone calorimeter. New crosslinked aerogel (x-link aerogel) materials are under development at Glenn Research Center, but the process currently limits production to small sizes. The crosslinked aerogel materials are non-dusting and can be made structural (20-22). Fire properties of x-link aerogel is currently unknown. For many years, polyimide foams have been used in demanding applications that require extreme structural stability in both hot and cryogenic environments,



150 excellent chemical stability, and good long term weathering and aging characteristics (23-31), but a further reduction in thermal conductivity would be preferred. In general, state-of-the-art polyimide foam systems have a thermal conductivity around 10 mW/m-K higher than the current insulation systems used on the Space Shuttle External Tank. The Ares launch vehicle currently being designed by N A S A may require an insulation system with a higher use temperature than the current polyurethane and polyisocyanurate foams used on the Space Shuttle External Tank while maintaining the low thermal conductivity inherent in these systems. The combination of aerogel materials with a robust polyimide foam such as T E E K developed at Langley Research Center would be desirable for such demanding applications. Incorporating modern day commercial aerogel materials in a polyimide foam matrix mitigates contamination concerns of aerogel and gives structural integrity to the aerogel while the aerogel in the polyimide foam reduces the thermal conductivity. Fire properties of commercially available aerogels, T E E K polyimide foam, and TEEK-aerogel hybrid foam composites were measured to understand fire safety risks of using these materials. Mechanical and thermal conductivity data were also produced to provide selection criteria of particular advanced applications.

Experimental Materials Polyimide-aerogel hybrid foams were made from 4,4'-oxydiphthalic anhydride / 3,4'-oxydianiline (TEEK-H) friable balloons from Unitika Ltd., with combinations of Spaceloft® and Cryogel® aerogel blanket ( A B ) from Aspen Aerogels Inc., and/or Nanogel® aerogel beads from Cabot Corp. Cryogel aerogel blanket is produced by a solution-gelation (sol-gel) process and is a composite of hydrophobic aerogel within a polymeric fiber matrix; the drying step to remove the solvent is a supercritical process. Nanogel® beads, nominally 1-mm in diameter, are also produced by a sol-gel process but with an ambient pressure drying step. Friable balloons are partially cured T E E K - H balloons or powder that contain some blowing agent. When processed, these balloons form polyimide foams of various open/closed cell content and densities. Open/closed cell content and densities can be targeted by varying the processing parameters.


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Fabrication of Composites T E E K aerogel bead composites (TEEK X X % aero) were prepared by mixing the appropriate amount of Nanogel® beads with T E E K friable balloons followed by placing the mixture into a mold and curing in a convection oven at 200°C for 2 hours. The aerogel beads are uniformly distributed in the resulting foam composite. The T E E K aerogel blanket (TEEK A B X layer) composites were made in a similar manner: a) Half of the T E E K friable balloons were disbursed evenly across the mold surface, b) The aerogel blanket was placed in the center of the mold, c) The remaining T E E K friable balloons were poured over the blanket and finally, d) The foam composite was cured at 200°C for 2 hours. The foam composite with the pocket of aerogel beads was made in a similar manner as the aerogel blanket composites by substituting a bed of aerogel beads in place of the blanket. The combined T E E K aerogel bead and blanket composites were made by substituting a balloons/beads mixture in place of the pure balloons. The diagonal strips designation refers to the aerogel blanket being cut into strips and placed diagonally across the mold instead of a single blanket.

Test Methods Thermal conductivity was measured using a Netzsch Lambda 2300 heat flow meter per test method A S T M C518. This instrument is a comparative heat flow meter that uses a standard reference material for calibration. Our tests were performed at ambient pressure with boundary temperatures of 34°C (warm) a n d l 3 ° C (cold). Fire performance was measured at an irradiance of 50 kW/m on a CSI cone calorimeter per a modified A S T M El354 (non-standard thickness and limited number of specimens). A l l specimens were 4" (0.102 m) wide and 4" (0.102 m) long. The virgin polyimide foam specimens and polyimide-aerogel hybrid foam specimens were 0.50" (0.013 m) thick, the Pyrogel® blanket was 0.24" (0.006 m)thick, and the Cryogel® blanket was 0.39" (0.010 m) thick rather than the standard 1" (0.025 m) and only two specimens of each material were tested. Cryogel blanket was two strips placed together in an aluminum foil boat. Aerogel beads and granules were 1" (0.025 m) thick and were poured into an aluminum foil boat. A l l aerogel materials were tested unrestrained and all aerogel foam hybrid composites were tested in a restrained configuration. T E E K polyimide foam was tested both restrained and unrestrained. Samples were restrained with a metal grid that fits inside a sample housing which fits over the normal sample holder. The grid surface area of 0.00088 m was substracted from the sample surface area. 2

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Vibration attenuation measurements were accomplished by sandwiching 1.25" (0.032 m) x 6" (0.152 m) x 6" (0.152 m) foam specimens between aluminum plates and striking with an instrumented impact hammer. Accelerometers strategically placed on the opposite aluminum plate recorded the vibration that was transmitted through the foam. Mechanical properties were measured in an Instron 4507 tensile test machine by compressing 1" (0.025 m) x 1" (0.025 m) x 1" (0.025 m) foam specimens to greater than 75% strain at a rate of 0.257min (6.35 mm/min).

Results Cone Calorimeter Results Cone calorimeter results for aerogel beads manufactured by Cabot Corp. showed a peak heat release of 60 kW/m under an irradiance of 50 k W / m as shown in Figure 1. Translucent aerogel granules had a peak heat release rate of 55 kW/m under an irradiance of 50 kW/m as shown in Figure 2. The aerogel beads and granules both quickly ignited under the irradiance of 50 kW/m . The aerogel materials burned with a low flame. The peak heat release for both beads and granules occurred at approximately 18 seconds. The combustion is a result of trimethylsilane and other organosilane surface modification agents vaporizing and igniting. Aerogel beads and granules can be calcined which removes the surface modification agents and once calcined aerogel will combust as well as sand. Aerogel blanket materials manufactured by Aspen Aerogels Inc. were also tested in the cone calorimeter. Cryogel aerogel blanket 0.39" (0.010 m) thick had a peak heat release of 96 kW/m under an irradiance of 50 kW/m as shown in Figure 3. Pyrogel aerogel blanket manufactured by Aspen Aerogels Inc. is an insulating material designed for higher temperature applications and uses carbon and glass fibers instead of polymeric fibers for the aerogel particles. Pyrogel aerogel blanket 0.24" (0.006 m) thick had a peak heat release rate of 66 k W / m under an irradiance of 50 kW/m as shown in Figure 4. Both aerogel blanket materials quickly ignited under the irradiance of 50 kW/m . The peak heat release for both blankets occurred at approximately 12 seconds. Combustion of the Cryogel is a result of surface modification agents and polymer fiber. Combustion of Pyrogel is a result of organosilane surface modification agents. Cone calorimeter results for virgin T E E K polyimide foam made from friable balloons showed a low heat release rate. The first T E E K sample was run unrestrained and stress relieved under the thermal load causing increased surface 2

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In Fire and Polymers V; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009. 2

Figure 1. Heat release data of aerogel bead, Nanoget~, manufactured by Cabot Corp. under 50 kW/m irradiance.



Figure 2. Heat release data of aerogel granules, Nanoget~\ manufactured by Cabot Corp. under 50 kW/m irradiance.



Figure 3. Heat release data of 0.39" (0.010 m) thick aerogel blanket, Cryoget®, manufactured by Aspen Aerogels Inc. under 50 kW/m irradiance.


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Figure 4. Heat release data of 0.24" (0.006 m) thick aerogel blanket, Pyrogen, manufactured by Aspen Aerogels Inc. under 50 kW/m irradiance.


157 area combustion. This observed behavior correlated to the peak heat release of 86 kW/m under an irradiance of 50 kW/m for the SI sample shown in Figure 5. Subsequent foam samples and foam aerogel hybrid composites were tested restrained. The second virgin T E E K sample, S2, did not have an observable flame until 162 seconds and it burned with a low flame. The peaks for S2 before 162 seconds were cracks which formed allowing more offgassing products. The peak heat release for S2 was 25 kW/m under an irradiance of 50 kW/m around 170 seconds as shown in Figure 5. T E E K polyimide foam with 10% aerogel beads took more than two minutes to ignite under 50 kW/m irradiance. Upon ignition the foam composite burned with a low flickering flame. Both tests were terminated early because it appeared the flame was out. S2 had a peak heat release of about 30 kW/m at around 170 seconds then low level combustion around 25 kW/m as shown in Figure 6. SI had an initial peak heat release similar to S2, but a crack formed in the sample which resulted in a peak heat release of 35 kW/m just over 200 seconds as shown in Figure 6. T E E K polyimide foam with 20% aerogel beads took more than two minutes to ignite under 50 kW/m irradiance. Upon ignition the foam composite burned with a low flickering flame. The first test was terminated early because it appeared the flame was out. SI showed a peak heat release of about 25 kW/m at around 180 seconds then low level combustion around 25 kW/m as shown in Figure 7. S2 showed an initial peak heat release of about 30 kW/m around 160 seconds. A crack opened in the specimen which pushed the peak heat release to 37 kW/m at 280 second followed by low level combustion around 30 kW/m as shown in Figure 7. The peaks after 200 seconds are the foam deforming/ shrinking and exposing new surface area. T E E K polyimide foam with 25% beads took more than two minutes to ignite under 50 kW/m irradiance. Upon ignition the foam composite burned with a low flickering flame. Sample SI had a peak heat release of about 30 kW/m at around 200 seconds then low level combustion around 25 kW/m as shown in Figure 8. S2 had an initial peak heat release of 30 kW/m around 180 seconds, but then S2 deformed some while constrained pushing the peak heat release to 36 kW/m just after 300 seconds as shown in Figure 8. After fire testing aerogel beads on the surface appeared similar to before testing, but below the surface soot appeared to be deposited as shown in Figure 9, photo A . Translucent aerogel granules appeared white on the surface and appeared to have soot deposited under the surface after fire testing as shown in Figure 9, photo B . Cryogel aerogel blanket before testing was white and after fire testing appeared to have some soot deposited as shown in Figure 9, photo C. Pyrogel aerogel blanket is black and did not appear different after testing, though the color could mask soot as shown in Figure 9, photo D. No char was evident or expected. T E E K polyimide foam and TEEK-aerogel hybrid foam composites did have good char formation and aerogel beads were evident in and on the char. The 2

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Fire and Engineering Properties of Polyimide-Aerogel Hybrid Foam

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