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At least 217 records · Page 12

All sources NASA NTRS DOE OSTI

NASA NTRS1975

Synthesis and development of low cost, high temperature N-arylene polybenzimidazole foam material

Polymer (and foam) studies followed two basic routes: (1) formation of high molecular weight uncyclized polyamide followed by subsequent fusion and cyclodehydration to yield NABI (foam) and (2) polymer and foam formation by reaction of diphenyl esters (or anhydrides) with the tetramine. The latter route was found much more attractive since considerable versatility in both basic polymer structure and crosslinkability is achievable. Preliminary studies on BAB, phthalic anhydride (PA), and 3, 3 (prime), 4, 4(prime) benzo pheno netetracarboxylic acid dianhydride (BTDA) as crosslinked polymer precursors were conducted. Nonmelting rigid char forming foams with densities as low as 2.7 lb/cubic ft. were achieved. The program was successful in the preparation of a potentially low cost, low density, high char yield, high temperature foam material.

Harrison, E. S.↗

NASA NTRS1977

Jupiter Probe Heatshield design

A heatshield performance assessment is made for extraterrestrial entry into the atmosphere of the planet Jupiter. A rigorous determination of the potential ablation and thermostructural failure mechanisms for a candidate carbon-phenolic heatshield is made. The thermostructural integrity of the char layer is addressed in detail with the total response phenomenology involving char layer formation, stress under aerodynamic and thermodynamic loads, and points of stress concentration under the high radiative heat rates and severe temperature gradients encountered in the Jovian atmosphere. An alternate material concept in the carbon-carbon generic material class is examined to assess the viability of this material class for application to the Jupiter Probe Heatshield.

Kratsch, K. M.↗

NASA NTRS1979

Performance characteristics of a slagging gasifier for MHD combustor systems

The performance of a two stage, coal combustor concept for magnetohydrodynamic (MHD) systems was investigated analytically. The two stage MHD combustor is comprised of an entrained flow, slagging gasifier as the first stage, and a gas phase reactor as the second stage. The first stage was modeled by assuming instantaneous coal devolatilization, and volatiles combustion and char gasification by CO2 and H2O in plug flow. The second stage combustor was modeled assuming adiabatic instantaneous gas phase reactions. Of primary interest was the dependence of char gasification efficiency on first stage particle residence time. The influence of first stage stoichiometry, heat loss, coal moisture, coal size distribution, and degree of coal devolatilization on gasifier performance and second stage exhaust temperature was determined. Performance predictions indicate that particle residence times on the order of 500 msec would be required to achieve gasification efficiencies in the range of 90 to 95 percent. The use of a finer coal size distribution significantly reduces the required gasifier residence time for acceptable levels of fuel use efficiency. Residence time requirements are also decreased by increased levels of coal devolatilization. Combustor design efforts should maximize devolatilization by minimizing mixing times associated with coal injection.

Smith, K. O.↗

NASA NTRS1978

Thermochemical and flammability properties of some thermoplastic and thermoset polymers - A review

The thermochemical and flammability properties of thermoplastic and thermoset polymers are discussed. The results of a thermogravimetric analysis of the polymers conducted on a DuPont 950 thermogravimetric analyzer using both nitrogen and air atmospheres are presented. Experimental data on smoke evolution are given, and the methodology for assessing the relative toxicity of the pyrolysis effluents is described. The values obtained from the flammability tests are compared with the stoichiometric char yield, and it is shown that the ignition tendency of the polymers is a linear function of the resin char yield

Kourtides, D. A.↗

NASA NTRS1979

A model of smoldering combustion applied to flexible polyurethane foams

Smoldering combustion, particularly in upholstery and bedding materials, has been proven a serious life hazard. The simplest representation of this hazard situation is one-dimensional downward propagation of a smolder wave against a buoyant upflow (cocurrent smolder); the configuration treated here is identical in all respects to this except for the presence of a forced flow replacing the buoyant one. The complex degradation chemistry of the polyurethanes is here reduced to the two major overall reactions of char formation and char oxidation. The model solutions, which are in reasonable agreement with experimental results, show the smolder process to be oxygen-limited, which leads to some very simple trends. More subtle behavior aspects determine actual propagation velocity, fraction of fuel consumed, and apparent equivalence ratio (all of which are variable). The self-insulating character of the smolder wave makes it viable in a wide-ranging set of conditions if the igniting stimulus is sufficiently long. These results have significant implications regarding the problem of smolder prevention or hindrance.

Ohlemiller, T. J.↗

NASA NTRS1980

Degradation of tensile and shear properties of composites exposed to fire or high temperature

The decrease in ultimate tensile strength, shear strength, tensile modulus, and shear modulus of fiber reinforced composites exposed to fire or to high temperature was investigated. A simple model was developed for calculating the mass loss of the material and the thickness of the char layer. The mass loss as well as the degradation in tensile and shear properties of Fiberite T300/1034 and Hercules AS/3501-6 graphite epoxy composites exposed to fire were measured. A correlation between the degradation in properties and the calculated mass loss and the char layer thickness was developed. A technique was proposed for predicting material damage through the use of such correlations.

Pering, G. A.↗

NASA NTRS1980

Burning characteristics and fiber retention of graphite/resin matrix composites

Graphite fiber reinforced resin matrix composites were subjected to controlled burning conditions to determine their burning characteristics and fiber retention properties. Two types of burning equipment were used. Small samples were burned with a natural gas fired torch to study the effects of fiber orientation and structural flaws such as holes and slits that were machined into the laminates. Larger laminate samples were burned in a Heat Release Rate Calorimeter. Unidirectional epoxy/graphite and polyimide/graphite composites and boron powder filled samples of each of the two composite systems were burn tested and exposed to a thermal radiation. The effects of fiber orientation, flaws, and boron filler additives to the resins were evaluated. A high char forming polyimide resin was no more effective in retaining graphite fibers than a low char forming epoxy resin when burning in air.

Bowles, K. J.↗

NASA NTRS1980

Shuttle subscale ablative nozzle tests

Recent subscale nozzle tests have identified new and promising carbon phenolic nozzle ablatives which utilize staple rayon, PAN, and pitch based carbon cloth. A 4-inch throat diameter submerged test nozzle designed for the 48-inch Jet Propulsion Laboratory char motor was used to evaluate five different designs incorporating 20 candidate ablatives. Test results indicate that several pitch and PAN-based carbon phenolic ablatives can provide erosion and char performance equivalent or superior to the present continuous rayon-based SRM ablative.

Powers, L. B.↗

NASA NTRS1981

Curing agent for polyepoxides and epoxy resins and composites cured therewith

A curing for a polyepoxide is described which contains a divalent aryl radical such as phenylene a tetravalent aryl radical such as a tetravalent benzene radical. An epoxide is cured by admixture with the curing agent. The cured epoxy product retains the usual properties of cured epoxides and, in addition, has a higher char residue after burning, on the order of 45% by weight. The higher char residue is of value in preventing release to the atmosphere of carbon fibers from carbon fiber-epoxy resin composites in the event of burning of the composite.

Serafini, T. T.↗

NASA NTRS1981

Imide modified epoxy matrix resins

High char yield epoxy using novel bisimide amines (BIA's) as curing agents with a state of the art epoxy resin was developed. Stoichiometric quantities of the epoxy resin and the BIA's were studied to determine the cure cycle required for preparation of resin specimens. The bisimide cured epoxies were designated IME's (imide modified epoxy). The physical, thermal and mechanical properties of these novel resins were determined. The levels of moisture absorption exhibited by the bisimide amine cured expoxies (IME's) were considerably lower than the state of the art epoxies. The strain-to-failure of the control resin system was improved 25% by replacement of DDS with 6F-DDS. Each BIA containing resin exhibited twice the char yield of the control resin MY 720/DDS. Graphite fiber reinforced control (C) and IME resins were fabricated and characterized. Two of the composite systems showed superior properties compared to the other Celion 6000/IME composite systems and state of the art graphite epoxy systems. The two systems exhibited excellent wet shear and flexural strengths and moduli at 300 and 350 F.

Scola, D. A.↗

NASA NTRS1982

The properties of novel bisimide amine cured epoxy/Celion 6000 graphite fiber composites

The study is concerned with the further development of bisimide amine cured epoxy resins first reported by Serafini et al. (1979). Attention is given to properties of the neat resins and composites made with these novel epoxy resins and Celion 6000 graphite fibers. The use of bisimide amine hardeners to cure epoxy resins causes a significant improvement in the char-forming characteristics of the cured resin. Compared to epoxy resins cured with a conventional amine, such as 4,4'-diaminodiphenylsulfone, the bisimide cured epoxies exhibit about a twofold improvement in the aerobic char yield at 800 C. The levels of moisture absorption exhibited by the bisimide amine cured epoxies were found to be considerably lower than those exhibited by state-of-the-art epoxies (0.5 wt.% versus 2.0%).

Scola, D. A.↗

NASA NTRS1981

New high-temperature flame-resistant resin matrix for RP/C

The processing parameters of graphite composites utilizing graphite fabric and epoxy or other advanced thermoset and thermoplastic resins as matrices are discussed. The evaluated properties include anaerobic char yield, limiting oxygen index, smoke evolution, moisture absorption, and high-temperature mechanical properties. It is shown that graphite composites having the highest char yield exhibit optimum fire-resistant properties.

Kourtides, D. A.↗

NASA NTRS1983

Performance of an ablator for Space Shuttle inorbit repair in an arc-plasma airstream

An ablator patch material performed well in an arc plasma environment simulating nominal Earth entry conditions for the Space Shuttle. Ablation tests using vacuum molded cones provided data to optimize the formulation of a two part polymer system for application under space conditions. The blunt cones were made using a Teflon mold and a state of the art caulking gun. Char stability of formulations with various amounts of catalyst and diluent were investigated. The char was found to be unstable in formulations with low amounts of catalyst and high amounts of diluent. The best polymer system determined by these tests was evaluated using a half tile patch in a multiple High Temperature Reusable surface Insulation tile model. It was demonstrated that this ablator could be applied in a space environment using a state of the art caulking gun, would maintain the outer mold line of the thermal protection system during entry, and would keep the bond line temperature at the aluminum tile interface below the design limit.

Stewart, D. A.↗

NASA NTRS1983

Formulation and Characterization of Epoxy Resin Copolymer for Graphite Composites

Maximum char yield was obtained with a copolymer containing 25% mol fraction DGEBE and 75% mol fraction DGEBA (Epon 828). To achieve the high values (above 40%), a large quantity of catalyst (trimethoxyboroxine) was necessary. Although a graphite laminate 1/8" thick was successfully fabricated, the limited life of the catalyzed epoxy copolymer system precludes commercial application. Char yields of 45% can be achieved with phenolic cured epoxy systems as indicated by data generated under NAS2-10207 contract. A graphite laminate using this type of resin system was fabricated for comparison purposes. The resultant laminate was easier to process and because the graphite prepreg is more stable, the fabrication process could readily be adapted to commercial applications.

Keck, F. L.↗

NASA NTRS1986

Pocketing mechanics of SRM nozzle liner

A systems approach was adopted to study the pocketing phenomena on a solid rocket nozzle liner. The classical thermoelastic analysis was used to identify marginally strained regions on the composite liner erosion surface and at a depth coincident with the peak value of the across ply coefficient of thermal expansion. A failure criterion was introduced which included a thermal term and permitted failure assessment over the charred liner. The method was verified by satisfactory application to a reported related experiment. Liner pocketing mechanism was attributed to very localized material degradation caused during manufacturing process either by reduction of fiber strength and/or by concentration of resin volume fraction. Pocketing scenario over the degraged material was constructed with supporting formulation to predict size of fissures with respect to degraded material size and location in the liner and with burn time. Sensitivities of liner material parameters were determined to influence test programs designed to update mechanical data base of carbon cloth phenolic over the char temperature range.

Verderaime, V. S.↗

NASA NTRS1990

Polysiloxanes derived from the controlled hydrolysis of tetraethoxysilane as precursors to silica for use in ceramic processing

Synthesis, properties, and potential applications in ceramic processing for two polysiloxane silica precursors derived from the controlled hydrolysis of tetraethoxysilane (TEOS) are presented. The higher molecular weight TEOS-A is a thick adhesive liquid of viscosity 8000 to 12,000 c.p. having a SiO2 char yield of about 55 percent. The lower molecular weight TEOS-B is a more fluid liquid of viscosity 150 to 200 c.p. having a SiO2 char yield of about 52 percent. The acid catalyzed hydrolysis of TEOS to hydrated silica gel goes through a series of polysiloxane intermediates. The rate of this transition increases with the quantity of water added to the TEOS; thus, for ease of polymer isolation, the amount of water added must be carefully determined so as to produce the desired polymer in a reasonable time. The water to TEOS mole ratio falls in the narrow range of 1.05 for TEOS-A and 0.99 for TEOS-B. Further polymerization or gelation is prevented by storing at -5 C in a freezer. Both polysiloxanes thermoset to a glassy solid at 115 C. The liquid polymers are organic in nature in that they are miscible with toluene and ethanol, slightly souble in heptane, but immiscible with water. For both polymers, results on viscosity versus time are given at several temperatures and water additions. Based on these results, some examples of practical utilization of the precursors for ceramic fabrication are given.

Philipp, Warren H.↗

NASA NTRS1991

Proceedings of the 7th Semiannual Meeting of the Nozzle Initiative Industry Advisory Committee on Standardization of Carbon-Phenolic Test Methods and Specifications

The application of carbon fibers and fabrics (CF) for producing rocket nozzles is discussed. These materials which are essential for fabricating the carbon composites used in aerospace systems gasify when exposed to high temperatures and the mechanical properties of the composites degrade. The oxidation kinetics under isothermal (IC) and non-isothermal (NIC) conditions are examined and a comparison is made between the characteristics of IC and NIC oxidation. Several CF, chars, and carbon blacks were examined, including a microporous char, a graphitized rayon fabric, and several carbonized rayon fabrics. A summary is given of the advantages and drawbacks of isothermal and non-isothermal oxidation of carbons. The proceedings are assembled in the form of a roundtable discussion.

Hall, William B.↗

NASA NTRS1991

Evaluation of Kapton pyrolysis, arc tracking, and arc propagation on the Space Station Freedom (SSF) solar array Flexible Current Carrier (FCC)

Recent studies involving the use of polyimide Kapton coated wires indicate that if a momentary electrical short circuit occurs between two wires, sufficient heating of the Kapton can occur to thermally char (pyrolyze) the Kapton. Such charred Kapton has sufficient electrical conductivity to create an arc which tracks down the wires and possibly propagates to adjoining wires. These studies prompted an investigation to ascertain the likelihood of the Kapton pyrolysis, arc tracking and propagation phenomena, and the magnitude of destruction conceivably inflicted on Space Station Freedom's (SSF) Flexible Current Carrier (FCC) for the photovoltaic array. The geometric layout of the FCC, having a planar-type orientation as opposed to bundles, may reduce the probability of sustaining an arc. An experimental investigation was conducted to simulate conditions under which an arc can occur on the FCC of SSF, and the consequences of arc initiation.

Stueber, Thomas J.↗