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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

Comparison of the weight loss and adherence of nine different polyimide films thermally aged at 315 C and 350 C in air

Thermal exposure experiments at 315 and 350 C were performed in air on nine different types of polyimides applied to thin 304 stainless steel foils. The tests were conducted to determine which polyimide was the most thermally stable and adherent when subjected to long exposure times at elevated temperatures. One polyimide designated PIC-7 was found to be more thermally stable than the others; however, it did not possess the adherent properties of PIC-2 and PIC-5. It was concluded that as far as thermal stability and adherence are concerned, five of the polyimides are more suitable for high temperature applications than the other four.

Fusaro, R. L.↗
NASA NTRS1979

Resin/fiber thermo-oxidative interactions in PMR polymide/graphite composites

The amounts of resin weight loss and fiber weight loss in four PMR-polyimide graphite fiber composites were calculated from the composite weight losses and the fiber/resin ratios of the composites after long term thermo-oxidative aging in 600 F air. The accelerating effect of graphite fiber on resin weight loss, compared to neat resin weight loss, indicated the presence of a deleterious resin/fiber thermo-oxidative interaction, presumably due to fiber impurities. Similarly, the decelerating effect of the protective matrix resin on fiber weight loss, compared to bare fiber weight loss, was also demonstrated. The amount of hydrazine-indigestible resin and the amount of loose surface graphite fiber that formed during 600 deg F exposure of the composites were quantitatively determined. The indigestible residual resin was also qualitatively studied by scanning electron microscopy.

Alston, W. B.↗
NASA NTRS1998

Salmonella infections in the absence of the major histocompatibility complex II

We examined the pathogenesis of the facultative intracellular bacterium, Salmonella typhimurium in MHCII-/-, C2D knock-out mice, and wild-type C57BL/6J mice. The MHCII knock-out shortened the kinetics of animal death and reduced the dose of S. typhimurium needed to kill mice. We measured the physiological and cytokine responses of both mouse strains after S. typhimurium injection. Animal weight loss, spleen weights, liver weights, thymus weights, and serum corticosterone concentrations were comparable after injection with several doses of bacteria. The only physiological differences observed between the two strains were observed 3 days after injection of the highest dose of bacteria tested. Serum concentrations of tumor necrosis factor alpha, interleukin-2, and interleukin-6 increased in a dose-dependent fashion irrespective of mouse MHCII expression. Therefore, even in the absence of MHCII, mice are able to mount relatively normal physiological and immunological responses. Consistent with these normal responses, an increased percentage of MHCII-/- mice, primed with a low dose of bacteria 13 days earlier, were able to survive a lethal challenge of Salmonella compared with unprimed controls. Lastly, C2D mice had significantly higher serum interleukin-10 concentrations than C57BL/6J mice 48 h after infection with all doses of S. typhimurium. C2D macrophages also secreted significantly more IL-10 and less NO and O2- after lipopolysaccharide or phorbol ester stimulation in vitro than wild-type macrophages.

Non-NASA Center↗
NASA NTRS1993

Effects of orbital exposure on RTV during the LDEF mission

Thermomechanical analysis (TMA), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) were performed on samples of RTV 511 exposed on the Long Duration Exposure Facility (LDEF) mission for 6 years in orbit and unexposed RTV 511 control samples. Slices 20- to 400-microns thick were removed from the exposed surface down to a depth of 1,500 microns through the 3-mm thick samples. The TMA and DSC results, which arise from the entire slice and not just its surface, showed no significant differences between the LDEF exposed and the control samples. TMA scans were run from ambient to 500 C; results were compared by a tabulation of the onset temperatures for flow. DSC scans were run from ambient to 600 C; no endotherms or exotherms occurred over the range observed. What appear to be glass transition temperatures were compared for the samples as a function of section depth within the sample and between the exposed and control samples. The TGA scans from 25 to 900 C, which arise from the surface of the sample initially, showed a slight increase in the top most 105-micron slice (the exposed, discolored side) in the weight loss at 600 C in oxygen. This weight loss dropped to bulk values at the next slice below the top section, a mean depth of 258 microns. The control sample also showed an increase in weight loss as the top surface was approached, but the 600 C weight losses were very inconsistent. The LDEF RTV sample appears to be mechanically undamaged, with a surface layer which oxidizes slightly faster as a result of orbital exposure.

Brower, William E., Jr.↗
NASA NTRS1972

Outgassing and contamination properties of prospective Apollo Telescope Mount materials.

Some of the test techniques used to evaluate the outgassing and contamination characteristics of prospective Apollo telescope mount (ATM) materials are reviewed. These include a screening test providing information on weight losses of materials, weights of their condensed outgas products, and white-light scattered by the condensed products. They also include an ultraviolet-region screening test and a comprehensive continuous in situ weight-loss test. Examples are presented of data obtained from each of the tests on various prospective ATM materials.

Poehlmann, H. C.↗
NASA NTRS1986

Specimen geometry effects on graphite/PMR-15 composites during thermo-oxidative aging

Studies were conducted to establish the effects of specimen geometry on the thermo-oxidative stability and the mechanical properties retention of unidirectional Celion 12000 graphite fiber reinforced PMR-15 polyimide composites. Weight loss, flexural strength and interlaminar shear strength are measured at isothermal aging times as long as 1639 hr at a temperature of 316 C for three different specimen geometries. It is found that the three different types of specimen surfaces exhibit different values of weight loss/unit area. The mechanical properties retention is also found to be dependent on geometry for these composites. The interlaminar shear strength decreases significantly over the complete range of aging times. The flexural strength retention starts showing geometric dependency after about 1000 hr of aging at 316C. Weight loss fluxes, associated with the three different types of exposed surfaces, are calculated and used to develop an empirical mathematical model for predicting the weight loss behavior of unidirectional composites of arbitrary geometries. Data are presented comparing experimentally determined weight loss with weight loss values predicted using the empirical model.

Bowles, K. J.↗
NASA NTRS1986

Specimen geometry effects on graphite/PMR-15 composites during thermo-oxidative aging

Studies were conducted to establish the effects of specimen geometry on the thermo-oxidative stability and the mechanical properties retention of unidirectional Celion 12000 graphite fiber reinforced PMR-15 polyimide composites. Weight loss, flexural strength and interlaminar shear strength are measured at isothermal aging times as long as 1639 hr at a temperature of 316 C for three different specimen geometries. It is found that the three different types of specimen surfaces exhibit different values of weight loss/unit area. The mechanical properties retention is also found to be dependent on geometry for these composites. The interlaminar shear strength decreases significantly over the complete range of aging times. The flexural strength retention starts showing geometric dependency after about 1000 hr of aging at 316 C. Weight loss fluxes, associated with the three different types of exposed surfaces, are calculated and used to develop an empirical mathematical model for predicting the weight loss behavior of unidirectional composites of arbitrary geometries. Data are presented comparing experimentally determined weight loss with weight loss values predicted using the empirical model.

Bowles, K. J.↗
NASA NTRS1992

Effects of orbital exposure on Halar during the LDEF mission

Thermomechanical Analysis (TMA), Differential Scanning Calorimetry (DSC), and Thermogravimetric Analysis (TGA) were performed on samples of Halar exposed on the LDEF Mission for 6 years in orbit and unexposed Halar control samples. Sections 10-100 microns thick were removed from the exposed surface down to a depth of 1,000 microns through the 3 mm thick samples. The TMA and DSC results, which arise from the entire slice and not just its surface, showed no differences between the LDEF and the control samples. TMA scans were run from ambient to 300 C; results were compared by a tabulation of the glass transition temperatures. DSC scans were run from ambient to 700 C; the enthalpy of melting was compared for the samples as a function of section depth with the sample. The TGA results, which arise from the surface of the sample initially, showed a sharp increase in the topmost 50 micron section (the exposed, discolored side) in the weight loss of 170 C in oxygen. This weight loss dropped to bulk values in the range of depth of 50-200 microns. The control sample showed only a slight increase in weight loss as the top surface was approached. The LDEF Halar sample appears to be mechanically undamaged, with a surface layer which oxidizes faster as a result of orbital exposure.

Brower, William E., Jr.↗
NASA NTRS1987

Thermo-oxidatively stable condensation polyimides containing 1,1,1-triaryl-2,2,2-trifluoroethane dianhydride and diamine monomers

Nine new condensation polyimides containing the trifluorophenylethylidene linkage were synthesized by the amic-acid route. Several other polyimides, including some with the hexafluoroisopropylidene linkage, were also prepared as controls. Amic-acid solutions were characterized by determining their inherent viscosities prior to thermal conversion into polyimide films. Glass transition temperatures (T sug g), thermogravimetric analysis (TGA), and isothermal weight loss data were obtained for the films. The films were pulverized into molding powders which, in turn, were thermally processed under pressure into neat resin discs. The discs were also characterized by T sub gs and 316 C and 371 C isothermal weight losses. The film study identified two new polyimides with T sub gs greater that 371 C and two new polyimides with low rates of weight loss. The resin discs exhibited the same overall trends in T sub g and weight loss as the respective films, however the weight loss per unit surface area was always greater, presumably due to voids or to mechanical degradation induced during preparation of the molding powders.

Alston, William B.↗
NASA NTRS1989

The 3F condensation polyimides: Review and update

Nine new condensation polyimides containing the phenyltrifluoroethylidene (3F) linkage were synthesized by the amic-acid route. Several other polyimides, including some with hexafluoroisopropylidene (6F) linkage, were also prepared as controls. Amic-acid solutions were characterized by determining their inherent viscosities prior to thermal conversion into polyimide films. Glass transition temperatures (T sub g), thermogravimetric analysis (TGA), and isothermal weight loss data (at 316, 371, and 371 C under 0.5 MPa air pressure) were obtained for the films. The films were pulverized into molding powders which, in turn, were thermally processed under pressure into neat resin disks. The disks were also characterized by T sub g's and 316 and 371 C isothermal weight losses. The film study identified two new polyimides with T sub g's greater than 371 C and two new polyimides with low rates of weight loss. The resin disks exhibited the same overall trends in T sub g and weight loss as the respective films, however the weight loss per unit surface area was always greater, presumably due to molecular degradation induced during preparation of the molding powders. The overall results indicate that polyimides containing the 3F linkage have T sub g's and thermo-oxidative stability comparable to polyimides containing the 6F group. Alternate technology was also shown by the synthesis of two new polyalkyl substituted 3F diamines and five more new 3F polymers. Their potential as photoresists was demonstrated by T sub g advancement after ultraviolet exposure. Last, four U.S. patents on 3F monomers and polymers were issued and up to eight more are pending.

Alston, William B.↗
NASA NTRS1994

Thermo-Oxidative Stability of Graphite/PMR-15 Composites: Effect of Fiber Surface Modification on Composite Shear Properties

Experiments were conducted to establish a correlation between the weight loss of a polyimide (PMR- 15) matrix and graphite fibers and the in-plane shear properties of their unidirectional composites subjected to different isothermal aging times up to 1000 hr at 316 C. The role of fiber surface treatment on the composite degradation during the thermo-oxidative aging was investigated by using A4 graphite fibers with three surface modifications: untreated (AU-4), surface treated (AS-4), and surface treated and sized with an epoxy-compatible sizing (AS-4G). The weight loss of the matrix fibers, and composites was determined during the aging. The effect of thermal aging was seen in all the fiber samples in terms of weight loss and reduction in fiber diameter. Calculated values of weight loss fluxes for different surfaces of rectangular unidirectional composite plates showed that the largest weight loss occurred at those cut surfaces where fibers were perpendicular to the surface. Consequently, the largest amount of damage was also noted on these cut surfaces. Optical observation of the neat matrix and composite plates subjected to different aging times revealed that the degradation (such as matrix microcracking and void growth) occurred in a thin surface layer near the specimen edges. The in-plane shear modulus of the composites was unaffected by the fiber surface treatment and the thermal aging. The shear strength of the composites with the untreated fibers was the lowest and it decreased with aging. A fracture surface examination of the composites with untreated fibers suggested that the weak interface allowed the oxidation reaction to proceed along the interface and thus expose the inner material to further oxidation. The results indicated that the fiber-matrix interface affected the composite degradation process during its thermal aging and that the the weak interface accelerated the composite degradation.

Madhukar, Madhu S.↗
NASA NTRS1977

Interaction of NaCl/g/ and HCl/g/ with condensed Na2SO4

Na2SO4(l)-NaCl(g) interactions were studied at a total pressure of one atmosphere of air or oxygen for various temperatures of Na2SO4(l) and for various partial pressures of NaCl(g) and H2O(g). Mass spectrometric sampling techniques were used to identify and monitor gas phase species. Continuous recording thermomicrogravimetric measurements were conducted to determine condensed phase weight change rates. Experimental measurements were supplemented with thermodynamic calculations. Numerous experiments were performed at sample temperatures of 900 and 1000 C with 300 ppm NaCl(g). In these experiments, the reproducibility of the Na2SO4 vaporization weight loss rate and initial weight gain upon addition of NaCl(g) were found to be satisfactory. It was found that the addition of NaCl(g) to air flowing over Na2SO4(l) at 900 and 1000 C enhances the rate of weight loss of the Na2SO4(l). This enhancement increases when H2O(g) is also added to the air flow.

Stearns, C. A.↗
NASA NTRS1995

High-temperature oxidation behavior of reaction-formed silicon carbide ceramics

The oxidation behavior of reaction-formed silicon carbide (RFSC) ceramics was investigated in the temperature range of 1100 to 1400 C. The oxidation weight change was recorded by TGA; the oxidized materials were examined by light and electron microscopy, and the oxidation product by x-ray diffraction analysis (XRD). The materials exhibited initial weight loss, followed by passive weight gain (with enhanced parabolic rates, k(sub p)), and ending with a negative (logarithmic) deviation from the parabolic law. The weight loss arose from the oxidation of residual carbon, and the enhanced k(sub p) values from internal oxidation and the oxidation of residual silicon, while the logarithmic kinetics is thought to have resulted from crystallization of the oxide. The presence of a small amount of MoSi, in the RFSC material caused a further increase in the oxidation rate. The only solid oxidation product for all temperatures studied was silica.

Ogbuji, Linus U. J. T.↗
NASA NTRS1986

Pressure effects on the thermal stability of SiC fibers

Commercially available polymer derived SiC fibers were treated at temperatures from 1000 to 2200 C in vacuum and argon gas pressure of 1 and 1360 atm. Effects of gas pressure on the thermal stability of the fibers were determined through property comparison between the pressure treated fibers and vacuum treated fibers. Investigation of the thermal stability included studies of the fiber microstructure, weight loss, grain growth, and tensile strength. The 1360 atm argon gas treatment was found to shift the onset of fiber weight loss from 1200 to above 1500 C. Grain growth and tensile strength degradation were correlated with weight loss and were thus also inhibited by high pressure treatments. Additional heat treatment in 1 atm argon of the fibers initially treated at 1360 atm argon caused further weight loss and tensile strength degradation, thus indicating that high pressure inert gas conditions would be effective only in delaying fiber strength degradation. However, if the high gas pressure could be maintained throughout composite fabrication, then the composites could be processed at higher temperatures.

Jaskowiak, Martha H.↗
NASA NTRS1989

Pressure effects on the thermal stability of silicon carbide fibers

Commercially available polymer derived SiC fibers were treated at temperatures from 1000 to 2200 C in vacuum and argon gas pressure of 1 and 1360 atm. Effects of gas pressure on the thermal stability of the fibers were determined through property comparison between the pressure treated fibers and vacuum treated fibers. Investigation of the thermal stability included studies of the fiber microstructure, weight loss, grain growth, and tensile strength. The 1360 atm argon gas treatment was found to shift the onset of fiber weight loss from 1200 to above 1500 C. Grain growth and tensile strength degradation were correlated with weight loss and were thus also inhibited by high pressure treatments. Additional heat treatment in 1 atm argon of the fibers initially treated at 1360 atm argon caused further weight loss and tensile strength degradation, thus indicating that high pressure inert gas conditions would be effective only in delaying fiber strength degradation. However, if the high gas pressure could be maintained throughout composite fabrication, then the composites could be processed at higher temperatures.

Jaskowiak, Martha H.↗
NASA NTRS1977

Thermal fatigue and oxidation data of superalloys including directionally solidified eutectics

Thermal fatigue and oxidation data were obtained on 61 specimens, representing 15 discrete alloy compositions or fabricating techniques and three coating systems. Conventionally fabricated alloys included V57, MM 200, Rene 77, Rene 125, MM 246, MM 509, IN-738, IN-792 + Hf, and MM 200 + Hf. The directionally solidified alloys were MM 200, MM 200 single crystal, MM 200 bicrystal, cellular gamma/gamma' - delta) and lamellar gamma/gamma' - delta. The coatings systems included NiCrAlY on IN-738, In-792 + Hf, MM 200 DS, MM 200 DS single crystal, and cellular gamma/gamma' - delta and NiCrAlY/Pt on lamellar gamma/gamma' - delta. Crack initiation survival rates were recorded for all alloys, with and without coatings. All uncoated alloys, except MM 509, exhibited significant oxidation weight loss in 75,000 to 15,000 cycles. MM 509 specimens had weight losses only slightly higher than coated specimens through 7,500 cycles. All coated specimens had low weight loss.

Hill, V. L.↗