Hydrocarbon stability of perfluorinated polyether rubbers at elevated temperatures.
Materials used in the construction of o-rings for aircraft fuel lines and hydraulic systems must perform in these aggressive chemical environments over a very wide range of temperatures and operating forces. The o-rings used in current systems are made from nitrile (NBR) rubber . Experience has shown that NBR loses its elasticity after prolonged exposure at temperatures exceeding 90[degrees]C . Furthermore, the limit of low-temperature ductility for NBR is -30[degrees]C. At temperatures approaching -30[degrees]C NBR o-rings not only lose sealing capacity and exhibit high levels of compression set, but they also become brittle and subject to cracking. Many military and commercial flights originate at locations where the ambient temperature is below -30[degrees]C. The net result is that NBR o-rings must be replaced according to a very frequent maintenance schedule. These limitations in the properties of the base rubber add significantly to aircraft maintenance costs and represent a source of potential system failure.
Recent developments in rubber technology have resulted in a number of materials that have broader temperature utility than nitrile rubber. These materials include hydrogenated nitrile rubbers (HNBR), silicones (PDMS), fluorosilicones (FS), and fluoroelastomers (FKM) . There is a significant breadth of chemistry among the various grades and types of each of these classes of rubbers. Of particular interest are perfluorinated polyethers, a class of rubber that combines the best features of fluoroelastomers with the best features of silicones [1, 4, 5]. These rubbers have many of the processing advantages of silicones, but with greater high-temperature chemical stability. Perfluorinated polyether rubbers have comparable chemical stability to fluoroelastomers, but with a much lower use temperature limit and easier processing. Finally, they are much more chemically stable than fluorosilicone rubbers.
Perfluorinated polyether (PFE) rubbers have the following basic chemical backbone structure:
[FORMULA NOT REPRODUCIBLE IN ASCII]. (1)
This structure has excellent chemical resistance due to the presence of the fluorine side groups instead of hydrogen groups that are prone to attack by aggressive chemicals. Rubbers based on this chemistry also exhibit excellent flexibility due to the presence and frequency of the [-O-] bond. A unique feature of this rubber, provided commercially by Shin-Etsu (Gunma, Japan), is the method used to provide the crosslinks. Crosslinking is accomplished through trifunctional silane groups that are added to both ends of the repeating linear backbone structure. Although there are a number of proprietary variations in the crosslinking chemistry, a typical crosslinked structure is shown below.
[FIGURE 1 OMITTED]
[FORMULA NOT REPRODUCIBLE IN ASCII], (2)
where R and R' are organic divalent groups containing fluorine atoms. This chemistry allows for a significant degree of performance flexibility in that the molecular weight of the main chain sequence can be varied, as well as the length and structure of the crosslink ties. PFE products are produced by a liquid injection molding process. This is a low-pressure process that is fast and capable of producing parts free from residual stresses. In common with silicones and fluorosilicones, PFEs do not routinely contain carbon black. While this produces a cured rubber that has a lower tensile strength than carbon black-reinforced rubbers, such as NBR and HNBR, the tensile properties are comparable to those of silicones and fluorosilicones. In o-ring applications the rubbers are not subjected to tensile forces, but compressive forces, so the magnitude of the rubber's tensile strength is not critical.
The base PFE polymers are clear to milky white in color. Since carbon black is not required the rubbers can be colored (typically green) to make them readily identifiable. This is an important feature that can prevent other o-rings from being used in situations where the PFE rings are specified for performance reasons.
METSS (Westerville, OH), in cooperation with Shin-Etsu, undertook a study to determine the performance features of PFE rubbers exposed to common aircraft hydraulic fluids and jet fuel over the operating temperature range between -40[degrees]C and 135[degrees]C. These aging temperatures were selected in accordance with the performance requirements provided by the U.S. Air Force.
The rubber material examined was SIFEL 4750 grade perfluorinated polyether (Gunma, Japan). This is a two-component material that is processed by liquid injection molding. The nominal viscosity of the mixed two components prior to curing is 4500 Pa * s. The density of the cured rubber is 2.04 g/cc. Both molded plaques and o-rings were provided by Shin-Etsu. Initial testing was performed on samples cut from molded 15 cm X 15 cm X 2 mm plaques. Final testing was conducted using size 214 o-rings.
Test fluids included JP8 and JP8 + 100 jet fuels, and hydraulic fluids MIL-PRF-83282 and MIL-PRF-87257. JP8 and JP8 + 100 are modified kerosenes . Fuel additives in JP8 and JP-8 + 100 include antioxidants and metal deactivators, and for some aircraft fuel systems an icing inhibitor and static dissipater additive. The basic difference between JP8 and JP8 + 100 is that JP8 + 100 contains an additive, designated "+ 100", that provides improved stability to the fuel at elevated temperatures. This permits higher fuel operating temperatures for advanced aircraft. The "+ 100" additive increases the thermal stability of JP-8 by 100[degrees]F and its heat capacity by 50%. The "+ 100" additive consists of 25 ppm antioxidant, 70 ppm dispersant/detergent, 3 ppm metal deactivator, and 158 ppm solvent oil.
MIL-PRF-83282 is a synthetic poly([alpha]-olefin)-based fluid having fire resistant properties. MIL-PRF-87257 is also a synthetic fire resistant hydraulic fluid that provides a lower viscosity at low temperatures.
Initial testing involved measuring the chemical resistance of the rubber in accordance with ASTM D471, "Test Method for Rubber Property--Effect of Liquids." This standard calls for immersing test specimens in the test fluid and aging the samples for a period of 3 days at the target temperature. The properties of interest include volume swell, mass absorption, hardness, tensile properties, percent extractables, compression set, and the dynamic mechanical analysis (DMA) temperature response.
The target temperatures were 107[degrees]C for aging in the jet fuels and 135[degrees]C in the hydraulic fluids. Samples were aged in the test fluids at these temperatures for 3 days. At least three replicates were included in each individual test. All of the tests were repeated several times to ensure reproducibility. The test samples used in examining the effect of aging on volume swell, mass absorption, and hardness were die-cut 19-mm diameter discs. Sample volume was determined by the Archimedes principle. The samples were blotted dry before each measurement of volume swell, mass absorption, or hardness. Hardness was measured on a Gardner Shore A hardness tester. The tensile performance was determined from die cut Type C dumbbell specimens. A Tinius-Olsen 5000 testing unit was used to measure the tensile properties of strength-at-break and elongation-at-break.
DMA analysis was performed on 6 mm wide by 7.6 cm long die-cut samples. All measurements were made at 1 Hz between -100[degrees]C and 50[degrees]C in a TA Instruments DMA 983. The same samples were also used to determine the percent extractables after fluid aging. These samples were dried under vacuum until their weight stabilized. This provided information as to whether the rubber permanently absorbed the test fluid or lost material to the test fluid during exposure.
Compression set measurements were performed at room temperature and -40[degrees]C before and after fluid aging. Compression set was determined according to ASTM D395, "Standard Test Methods for Rubber Property--Compression Set," and ASTM D1229, "Standard Test Methods for Rubber Property--Compression Set at Low Temperatures." The compression set measurements consisted of compressing die-cut 12.7 mm diameter, nominally 2 mm thick discs 25% in compression set jigs. The samples were held under compression for a period of 70 h and allowed to recover for 30 min prior to making a final determination of the degree of set. Low-temperature compression set measurements were made at -40[degrees]C within the confines of a large freezer.
Once the baseline performance of the PFE rubber was established through tests performed on samples cut from the molded plaques, the tests were repeated on size 214 o-rings. Hardness was not evaluated on the o-rings, but volume swell, mass change, and compression set were. The tensile properties of the o-rings were determined according to ASTM D1414, "Standard Test Method for Rubber O-Rings."
After completing what were essentially static tests, the o-rings were examined for their dynamic response in a compression stress relaxation device [7-9]. The unit used by METSS was fabricated by Akron Rubber Development Labs specifically to handle both fluid environmental exposures and cyclic thermal behavior. While the procedure is simple in principle and described in all rheology texts, the physical apparatus required to perform the actual experiments is quite challenging. Consequently, there are very few reports on the combined environmental and cyclic thermal responses of elastomers.
The basic procedure is to compress the o-rings 25% of their initial thickness at room temperature and then to monitor the compressing force over time under various conditions. In the compression stress relaxation device the samples are placed on a thermoelectric plate located at the bottom of a 1-cm deep reservoir. The device is capable of measuring six o-rings at a time. Each one is independently compressed and monitored for compressive force. Figure 1 shows the arrangement of six experimental o-rings in the device's reservoir, which also serves as its environmental chamber. A 5-mm diameter platen mechanically compresses the rings. The platens are attached through a long shaft to strain gages that monitor the compressive force continuously as a function of time. The shaft connecting the platens to the strain gage must be chemically resistant to the environmental fluid and capable of withstanding the operating temperature.
To determine low-temperature performance, unaged and fluid-aged samples were examined at -40[degrees]C for 48 h, after which they were allowed to return to room temperature. Prior to compression stress relaxation testing the samples had been aged for 3 days in JP8 + 100 jet fuel at 107[degrees]C, and 3 days in MIL-PRF-83282 hydraulic fluid at 135[degrees]C.
In a second set of compression stress relaxation (CSR) experiments, sample o-rings were tested in both air and each test fluid over a cyclic sequence of temperatures. By monitoring the sealing force starting with an unaged o-ring and stepping through a high-temperature and low-temperature exposure, actual service conditions were simulated. The test protocol followed is presented in Table 1. The same tests were also performed on a standard, nitrile rubber sample.
Tables 2-5 summarize the basic response of PFE molded slabs to aging in the four test fluids. Table 2 presents data on the changes in volume, mass, and hardness after 3 days exposure at the target elevated temperatures. The data in Fig. 2 and Table 2 clearly show that the PFE rubber is highly resistant to JP8, JP8 + 100, MIL-PRF-83282, and MIL-87257 after 3 days continuous exposure at elevated temperatures. The changes in volume, mass absorption, and hardness change are quite small. In comparison, the NBR rubber sample showed greater volume swell, higher mass absorption, and a larger reduction in hardness in direct comparisons to PFE when aged in JP8 and MIL-PRF-83282. Similar results would be expected after aging in JP8 + 100 and MIL-PRF-87257, as these are chemically similar to JP-8 and MIL-PRF-83282.
Table 3 shows the effect of high-temperature fluid aging on the tensile properties of PFE and NBR. The tensile properties of PFE changed very little after 3 days exposure at 107[degrees]C, while the NBR sample lost over one-third of its initial strength and elongation-at-break. While exposure at 135[degrees]C in the hydraulic fluids was harsher than exposure in the jet fuel at 107[degrees]C, the PFE samples retained over 75% of their initial properties. In contrast, the NBR sample lost over 40% of its initial strength under comparable exposure conditions.
[FIGURE 2 OMITTED]
The results of the DMA experiments are summarized in Table 4 for the PFE samples. These data are intended to characterize low-temperature mobility and define low-temperature transitions. DMA measures the dynamic modulus of materials over a range of temperatures, providing a quick and easy method to generate information that can be used to evaluate low-temperature performance. The existence of low-temperature transitions can be related directly to low-temperature flexibility, mechanical hysteresis, and resilience. Departure from the high modulus behavior exhibited by materials below their glass transition temperature to the rubbery plateau modulus characteristic of elastomers occurs over a range of temperatures, with the glass transition temperature ([T.sub.g]) being the mid-point in this transition. The onset of the glass transition region is associated with the transformation from brittle-to-ductile behavior when examining a material that is heated from a low temperature to a high temperature, and, therefore, provides a measure of the material's ability to function adequately at a given operational temperature. The use of this temperature as an indicator of low-temperature performance in rubbers has been substantiated by Thomas . Thomas showed that the low-temperature sealing ability of a variety of fluoroelastomers was maintained down to ~14[degrees]C below the glass transition temperature. The data generated for this program show that the temperature associated with the onset of the glass transition region was an average of 10[degrees]C below the glass transition temperature, consistent with Thomas' results.
Exposure in the test fluids lowered the temperature at which the PFE rubber became brittle. This is shown by ~6[degrees]C reductions in both [T.sub.o] and [T.sub.g]. This suggests that the fluids provided a degree of plasticization to the rubber. By way of comparison, NBR has a [T.sub.g] of -8.0[degrees]C and an onset of ductility temperature of -23[degrees]C.
The very low levels of material extracted from the PFE (Table 5) demonstrate that the polymer had been fully crosslinked and that there were no residual low molecular weight species within the rubber that were removed by the test fluids.
The room temperature and -40[degrees]C compression set values for PFE are presented in Table 6. They are very low, indicative of excellent polymer resilience. Aging in the test fluids at elevated temperatures increased the compression set somewhat, but even the samples aged in JP8 + 100, which has the highest value of compression set, has a set well within desired limits. The relatively low values of compression set at -40[degrees]C are considered good within the aircraft industry. Aging in the test fluids prior to compressing the samples at -40[degrees]C resulted in an increase in compression set in all cases. These low values are consistent with the low onset of ductility temperature, [T.sub.o], recorded from DMA data.
The geometrical differences between o-rings and molded slabs (and the discs, bars, and tensile specimens cut from the slabs) are sufficient to warrant testing of molded o-rings. This is particularly true for compression set measurements. Because of geometrical differences, compression set values obtained from discs cannot be compared directly to those obtained from o-rings.
The data presented in Fig. 2 show the same relative stability of PFE in the four test fluids at elevated temperatures and the greater degree of deterioration in properties experienced by the NBR o-rings that was observed from testing die-cut samples from molded slabs. The volume changes and mass changes were all very small for the PFE o-rings.
Similar relative responses were obtained for tensile properties. These data, presented in Table 7, show very little change in properties of the aged PFE o-rings, while the aged NBR o-rings lost considerably more strength and ductility. The effect of heated MIL-PRF-87257 hydraulic fluid was particularly severe for the NBR o-rings.
Compression set data are presented in Table 8 for the o-ring samples. The PFE o-rings exhibited very low sets at room temperature, just as observed from the die-cut discs. For the most part, the compression set of the o-rings was of a comparable magnitude to that of the slabs. The compression set values for the NBR o-rings were significantly higher in all cases. Even at -40[degrees]C the compression set of the PFE o-rings was not only reasonable, but better than that of the NBR o-rings at room temperature. The compression set of the PFE o-rings at -54[degrees]C showed the anticipated increase in set with decreasing temperature, but were also reasonable, confirming that PFE has a significant degree of ductility and resilience at -54[degrees]C. The NBR o-rings all exhibited excessive set at both -40[degrees]C and -54[degrees]C
Compression Stress Relaxation Testing
As discussed in the Experimental section, two sets of CSR measurements were conducted to determine the best way to use CSR testing to analyze the performance of the materials subjected to a fixed degree of compression. In the first set of CSR experiments the test samples consisted of unaged o-rings, o-rings that had previously been aged for 3 days in MIL-PRF-83282 at 130[degrees]C, and o-rings that had been aged for 3 days in JP8+100 jet fuel at 107[degrees]C. The o-rings were not aged under compression. After aging, they were placed in the CSR device and compressed to 25% deflection at room temperature, then cooled to -40[degrees]C at a controlled linear rate over a period of 1 h. CSR measurements were taken for 48 h at -40[degrees]C prior to reheating the samples to room temperature. The CSR measurements were continued at room temperature for an additional 48 h to evaluate compression set recovery. The CSR data for this set of experiments are presented in Figs. 3-5 for unaged, hydraulic fluid aged, and jet fuel aged samples, respectively. Duplicate size 214 NBR and PFE o-rings were used in each test. Individual replicate data are presented in the figures. The CSR data are normalized with respect to the initial sealing force exerted by the o-rings under 25% deflection at room temperature.
As shown in Fig. 3, the CSR data for the unaged samples demonstrates a significant decrease in elasticity (80% reduction in sealing force) for NBR control samples at -40[degrees]C, while the PFE samples demonstrated an average decrease in sealing force of ~45%. Upon heating to room temperature after low-temperature exposure, the PFE samples recovered 100% of their initial sealing force while the NBR o-rings recovered only about 90% of their initial sealing force. A similar response was demonstrated for the samples aged in hydraulic fluid (Fig. 4).
Interestingly, the response of the o-rings aged in JP8+100 (Fig. 5) was significantly different. Virtually no difference was observed in the normalized response of the NBR vs. PFE o-rings. This effect can be partially attributed to the difference in aging temperatures between the hydraulic fluid aged (130[degrees]C) and fuel aged (107[degrees]C) samples. It is also important to note that the NBR o-rings demonstrate significantly more volume swell during 3-day exposure to JP8+100 (20% for NBR vs. 6% for PFE). The significance of this is that the apparent improved low-temperature performance of the NBR o-rings can be associated with the plasticization of the rubber by the absorbed fluid. Pre-aging of the o-ring materials was useful as an initial insight to performance of the materials, but fails to take into account possible effects of aging under compressive forces. Service conditions would be more closely modeled by in situ aging under compression.
[FIGURE 3 OMITTED]
A second set of experiments was designed to more closely mimic the thermal and mechanical stresses imparted on static o-ring during service. In these experiments, NBR and PFE o-rings were compressed to 25% deflection (at room temperature) in the CSR device and then aged in situ (rather than being aged prior to putting in the test device) while under compression in the CSR unit. Thermal aging was conducted for a period of 3 days in air at 130[degrees]C, in MIL-PRF-83282 at 130[degrees]C, and in JP8+100 at 107[degrees]C. After aging, the o-rings were cooled to -30[degrees]C/-40[degrees]C to determine the low-temperature sealing capacity of the o-rings after high-temperature aging under compression. With the exception of the data presented for JP8+100, the low-temperature data presented for this experiment is at -30[degrees]C. The temperature sequencing was the same as presented in Table 1.
The response (sealing force) of the o-rings was constantly monitored during the course of the entire thermal program. The CSR data for these experiments are presented in Figs. 6-8 for o-rings aged in air, hydraulic fluid, and JP8+100, respectively. Once again, the data were normalized with respect to the initial sealing force exerted by the o-rings under 25% deflection at room temperature.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
The response of the NBR o-rings under in situ aging is significantly different than the response of the PFE o-rings. After an initial increase in sealing force due to volume expansion, the NBR o-rings exhibited a constant decrease in sealing force during high-temperature aging. In all test environments the o-rings soften and began to creep under the 25% deflection force (the o-rings are not constrained laterally). Once again, however, the NBR o-rings aged in situ in JP8+100 retained a greater percentage of their initial sealing force than the NBR o-rings aged in air and hydraulic fluid. The sealing force for the NBR o-rings aged in air continued to decrease in a linear fashion during the entire 3-day period, while the sealing force of the o-rings aged in the test fluids decreased in a more exponential manner. The difference in response in air and fluid indicates probable competing mechanisms for force retention as a function of the thermal and chemical influences. When the NBR o-rings are cooled back down to room temperature after 3 days of aging in air and hydraulic fluid, they retain only 10-25% of their initial sealing force. The NBR o-rings aged in JP8+100 retained 70-75% of their initial sealing force. When cooled further to the low-temperature extreme of -40[degrees]C, the sealing force of the NBR o-rings fell to zero in air and hydraulic fluid due to volume contraction. The samples aged in JP8+100 jet fuel retained 60% of their low-temperature sealing force but, as previously noted, the low-temperature data for this set were collected at -30[degrees]C and not -40[degrees]C.
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
The PFE o-rings exhibited a significant increase in sealing force due to thermal expansion during initial heating, followed by a slight decrease in sealing force as a function of time during high-temperature aging in all environments, with all samples continuing to exhibit more than 120% of their initial sealing force during the entire 3-day exposure periods. After the 3-day aging cycle, the PFE samples retained 60-85% of their initial sealing force after cooling to room temperature, and ~30-40% of their initial sealing force at -40[degrees]C in air and hydraulic fluid, respectively. The samples tested in JP8+100 jet fuel retained 70% of their initial sealing force at -30[degrees]C.
The in situ aging test procedure clearly demonstrated the low-temperature performance limitations of the NBR o-rings tested. The difference in CSR response during thermal aging is also significant, clearly demonstrating the potential benefit of using the PFE o-rings to replace NBR o-rings in hydraulic fluid applications, especially where low-temperature sealing requirements are critical.
The data generated in the program clearly demonstrate the ability of perfluorinated polyether rubber to withstand exposure to jet fuel and synthetic hydraulic fluids over a wide range of temperatures. Measurements of volume change, mass absorption, hardness change, change in tensile strength, and elongation-at-break confirm that the rubber is very stable during long-term exposure to the target fluids: JP8+100 jet fuel, MIL-PRF-83282 hydraulic fluid, and MIL-PRF-87257 hydraulic fluid. This is in contrast to the significantly larger changes observed in nitrile rubber subjected to the same aging conditions. Moreover, the compression set values of the perfluorinated polyether rubber were also considerably lower than those of nitrile rubber. Finally, data generated on the CSR device shows that perfluorinated polyether o-rings are able to easily maintain sealing force at temperatures down to -40[degrees]C, even after high-temperature fluid aging. The CSR device is a useful method for simultaneously examining the stress relaxation behavior of an elastomer subjected to both thermal and environmental stresses in both steady state and cyclic modes.
[FIGURE 8 OMITTED]
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D.M. Bigg, K.J. Heater, T.L. Skidmore
METSS Corp., Westerville, OH 43082
Shin-Etsu Chemical Co., Gunma, Japan
Correspondence to: D.M. Bigg, email: email@example.com
TABLE 1. Temperature sequence for compression stress relaxation testing of o-rings aged in situ. Temperature. [degrees]C Time, h Condition 25[degrees]C Needed to reach equilibrium 25% compressive strain in air 107[degrees]C for JP8 + 100 135[degrees]C for Heat to temperature hydraulic fluid linearly over 1 h 107[degrees]C for JP8 + 100 135[degrees]C for Hold for 70 h Aging condition hydraulic fluid 25[degrees]C Cool to temperature linearly over 1 h 25[degrees]C Hold for 10 h -40[degrees]C Cool to temperature linearly over 1 h -40[degrees]C Hold for 48 h 25[degrees]C Heat to temperature linearly over 1 h 25[degrees]C Hold for 1 h TABLE 2. Changes in physical properties of PFE slabs after 3 days chemical exposure at elevated temperatures. Fluid exposure JP8 @ 107[degrees]C JP8 + 100 @ 107[degrees]C Property NBR PFE PFE [DELTA]V, % 19.72 3.23 5.00 [DELTA]M, % 14.57 2.03 2.07 [DELTA]H, points -10.89 -1.11 0.67 Fluid exposure MIL-PRF-83282 @ MIL-PRF-87257 @ 135[degrees]C 135[degrees]C Property NBR PFE PFE [DELTA]V, % 7.41 2.65 1.00 [DELTA]M, % 6.36 0.36 0.71 [DELTA]H, points -2.33 -1.00 -4.78 TABLE 3. Changes in tensile properties of PFE slabs after 3 days chemical exposure. Fluid exposure JP8 @ 107[degrees]C Property NBR PFE Initial tensile strength, MPa 20.3 7.28 Initial elongation-at-break, % 884 453 Change in tensile strength, % -35.4 -3.1 Change in elongation-at-break, % -38.5 -11.5 Fluid exposure MIL-PRF-83282 @ MIL-PRF-87257 @ 135[degrees]C 135[degrees]C Property NBR PFE PFE Initial tensile strength, MPa 20.3 7.28 7.28 Initial elongation-at-break, % 884 453 453 Change in tensile strength, % -40.1 -22.9 -17.5 Change in elongation-at-break, % -46.6 -15.1 -9.6 TABLE 4. DMA response of PFE slabs after 3 days chemical exposure. Fluid exposure JP8 + 100 @ Property JP8 @ 107[degrees]C 107[degrees]C [T.sub.o], [degrees]C (unaged) -38.8 -38.8 [T.sub.g], [degrees]C (unaged) -28.3 -28.3 [T.sub.o], [degrees]C (aged) -45.5 -44.5 [T.sub.g], [degrees]C (aged) -35.7 -35.7 Fluid exposure Property MIL-PRF-83282 @ MIL-PRF-87257 @ 135[degrees]C 135[degrees]C [T.sub.o], [degrees]C (unaged) -38.8 -38.8 [T.sub.g], [degrees]C (unaged) -28.3 -28.3 [T.sub.o], [degrees]C (aged) -42.4 -50.0 [T.sub.g], [degrees]C (aged) -31.8 -39.9 TABLE 5. Percent extractables from PFE slabs after 3 days chemical exposure. Fluid exposure Property JP8 @ 107[degrees]C JP8 + 100 @ 107[degrees]C Percent extracted 0.04 0.05 Fluid exposure MIL-PRF-87257 @ Property MIL-PRF-83282 @ 135[degrees]C 135[degrees]C Percent extracted 0.05 0.02 TABLE 6. Compression set of PFE slabs in air and after 70 h chemical exposure. Fluid exposure Air @ JP8 @ JP8 + 100 @ Property 23[degrees]C 107[degrees]C 107[degrees]C Set at RT, % 5.2 8.5 12.3 Set at -40[degrees]C, % 4.6 16.1 19.2 Fluid exposure MIL-PRF-87257 @ Property MIL-PRF-83282 @ 135[degrees]C 135[degrees]C Set at RT, % 7.4 7.3 Set at -40[degrees]C, % 16.4 15.3 TABLE 7. Changes in tensile properties of PFE o-rings after 3 days chemical exposure at elevated temperatures. Fluid JP8 @ 107[degrees]C JP8 + 100 @ 107[degrees]C Property NBR PFE NBR PFE Tensile strength, MPa 27.7 6.96 18.1 6.96 Elongation-at-break, % 383 119 383 119 Change in tensile -16.4 -7.3 -34.5 -12.7 strength, % Change in -21.7 -9.8 -17.5 -2.9 elongation-at-break, % Fluid MIL-PRF-83282 @ MIL-PRF-87257 @ 135[degrees]C 135[degrees]C Property NBR PFE NBR PFE Tensile strength, MPa 18.1 6.96 18.1 6.96 Elongation-at-break, % 383 119 383 119 Change in tensile -27.9 7.21 -74.6 -2.5 strength, % Change in -26.9 -15.8 -64.9 -3.6 elongation-at-break, % TABLE 8. Compression set of PFE o-rings in air and after 70 h chemical exposure at elevated temperatures. Fluid exposure JP8 + 100 Air @ RT JP8 @ 107[degrees]C @ 107[degrees]C Property NBR PFE NBR PFE NBR PFE Set at RT, % 32.6 10.3 ND ND 90.3 5.9 Set at 125.0 25.0 106.4 34.0 110.3 25.0 -40[degrees]C, % Set at -54, % 119.4 41.2 ND ND 127.1 31.0 Fluid exposure MIL-PRF-83282 MIL-PRF-87257 @ 135[degrees]C @ 135[degrees]C Property NBR PFE NBR PFE Set at RT, % 114.8 10.3 98.0 ND Set at 123.6 17.7 123.0 ND -40[degrees]C, % Set at -54, % 120.5 30.6 117.5 32.3