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Evaluation of uni-axially expanded PTFE as a gasket material for fluid sealing applications
Materials Chemistry and Physics 70 (2001) 197–207
Evaluation of uni-axially expanded PTFE as a gasket material for fluid sealing applications James Huang a,∗ , Yuan-Haun Lee b a
b
Chemistry Department, Chung-Yuan University, Chung-Li 32023, Taiwan, ROC Graduate Institute of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan, ROC Received 2 March 2000; received in revised form 4 March 2000; accepted 16 October 2000
Abstract A new process was invented for preparing an improved polytetrafluoroethylene (PTFE) sealant material from a paste formed of PTFE resin with special plasticizer. This expanded PTFE sealant material consisted of a special node and fibrous microstructure. The preferred orientation for mechanical strength of the fibrous structure was parallel to the direction of calendering. The expanded PTFE material was investigated for its mechanical properties, including creep relaxation, compressibility, recovery, torque retention, and gas permeability, as defined by the ASTM and DIN standards. The sealing performance of the material was further characterized by the room temperature tightness test established by the Pressure Vessel Research Council (PVRC). The test results suggest that uni-axially expanded PTFE is soft and compressible, with excellent sealability. It is strong, possessing good resistance against creep and cold-flow. These favorable properties of expanded PTFE, as compared to traditional PTFE-based gasket types, make it a promising gasket material for fluid sealing applications. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Expanded PTFE; Creep relaxation; Torque retention; Gas permeability
1. Introduction In today’s typical chemical process plants, a gasketed joint is only a small component of a large piping system, but its function is by no means trivial. In fact, it serves as the most important “gate-keeper” for controlling fugitive fluid emission to keep the environment clean and safe. The terms joint, jointing and gasket are often used synonymously to describe static seals cut or fabricated from compressible flat sheet material, in preference to the term “seal”. Academic definition would restrict the term “jointing” to describe the sheet material, when the actual seal cut from it is then either a joint or gasket (synonymous) [1]. Practically, other forms of materials such as ropes may very well qualify for gasketing functions. An effective jointing material offers tighter sealing and long-term performance, thus reducing environmental concerns, labor costs, material losses, and possible down time associated with system failures or frequent replacements of less reliable gaskets. Asbestos-based material was once the choice for many of fluid sealing applications, but its hazardous nature to the ∗ Corresponding author. Tel.: +886-4-358-0021; fax: +886-4-359-6963. E-mail address: [email protected] (J. Huang).
health of general public has been confirmed, resulting in a ban on the use of the material, and a large scale of search by the fluid sealing industry for replacements of asbestos-based gaskets. Although the development of plastics has introduced a wide range of materials to the market, very few have been found readily applicable as static flat seals. This is due to the limited temperature range over which they can be reliably used, together with comparatively poor resilience and, in some cases, a tendency to flow under load. A major exception to this is polytetrafluoroethylene (PTFE) which has such a remarkable combination of properties that it takes a special place in the sealing of aggressive and toxic fluids (Figs. 1 and 2) [2–4]. The molecular structure shows that the carbon backbone of PTFE is totally covered by fluorine, thus hindering the access of other aggressive media that may destroy the backbone bonds. Additionally, the bond between carbon (C) and fluorine (F) is extremely strong compared with others, so the replacement of the fluorine is practically excluded. This is why PTFE has such good chemical stability. There are only a few ways to destroy PTFE: (i) chemically by molecular fluorine (F) or other fluorinating media and by molten alkalis and (ii) physically by heating over 400◦ C (750◦ F), and by high energy radiation (i.e. nuclear radiation). Both of these mechanisms attack and shorten the
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Fig. 1. The manufacturing process for PTFE.
chain length of PTFE to release PTFE monomers. These examples show the limitations of PTFE-based gaskets. In summary, PTFE has the following properties [2–4]. • It does not burn. • It is stable to light. • It does not absorb water, it is hydrophobic. It is physiologically safe. • It is insoluble in all solvents, even at increased temperature. • It is inert to all media except melted alkali metal, fluorine at liquid and gas states. • It has excellent electrical insulating capacity, even at high humidity. • Its thermal conductivity is low at about 6×10−4 cal cm−1 s−1 ◦ C−1 . • Due to the highly symmetric molecular structure, it has high crystallinity of 90–95%. All properties mentioned so far should make PTFE a very good gasket material, but unfortunately, there are two unfavorable properties that very much to its disadvantage: (i) low strength and (ii) high creep, caused by a transition point at 19◦ C. Because the number of ambient or low temperature gasket applications is limited, the usage of this material for gaskets is limited [4]. For this reason, there have been countless trials to improve strength and creep by incorporating into the material inorganic fillers like glass fiber, carbon fiber, precipitated silica, graphite, carbon black, and bronze
Fig. 2. Schematic presentation of the PTFE molecular helix [3].
powder. Success was rather limited, because nothing really changed the basic properties of PTFE, and the slightly improved creep resistance was obtained at the price of reduced chemical compatibility. In this study, a new process has been developed to improve the strength of PTFE and reduce its creep [5]. The resultant product is a uni-axially expanded PTFE material which is intended for use in fluid sealing applications, i.e. as a gasket or sealant material. Expanded PTFE is essentially a new class of PTFE-based material, possessing properties significantly different from all other traditional PTFE materials, such as skived virgin PTFE and filled PTFE. The properties of the material related to gasketing functions have been investigated extensively by various gasket characterization techniques, as summarized below. 1.1. Gasket characterizations The sealing capability of a material remains the most critical issue deciding whether or not the material can be effectively used for gasketing functions. For any typical fluid sealing application, the two major forces competing to affect the sealability of a gasket are the gasket compressive stress and internal fluid pressure. Generally, higher the stress, lower the leakage; the leak rate, however, increases with increasing fluid pressure. The actual joint leakage is, thus the net effect of these two factors, and the ability of a gasket to maintain the applied compressive stress is essential to the control of joint leakage. As a result, many gasket characterization techniques center around determination of mechanical properties of a potential gasket material corresponding to various compressive conditions. These test methods include those described in ASTM and DIN standards. For example, specifications required by gasket users typically include information on compressibility, recovery and creep relaxation of a gasket material (i.e. ASTM F36 and F38, DIN 52913). These measurements are related to how a material responds mechanically to compression and decompression, either under short-term or long-term conditions. Unfortunately, these quantities cannot be used to correlate with the actual sealing performance of a gasket, as no fluid is involved with the tests of these properties. Rather, this information may be more relevant to quality control issues of gasket production. The remedy to this deficiency is to conduct additional tests on sealability (ASTM F37), or gas permeability (DIN 3535), of the material to delineate the sealing characteristics of a gasket. However, the correlation with mechanical behavior is commonly missed, and no effect on the operating load changes such as pressurization and vibration of the piping system can be quantified. Adding to the problems of these testing methods is the fact that many testing parameters implemented in the tests may be varied, depending on the agreements between gasket users and manufacturers. This sometimes makes a direct comparison between two competing materials less obvious if test results sup-
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plied by each manufacturer were performed at different conditions. A more sophisticated testing method for gasket performance has evolved over the past 20 years, through the efforts of the Pressure Vessels Research Council (PVRC), Material Technology Institute, and Tightness Testing and Research Laboratory (TTRL) of Ecole Polytechnique, University of Montreal. This test, the room temperature tightness (ROTT) test, is a “leakage-based” testing method for characterizing physical and functional properties of a gasket, quantifying both mechanical and sealing behaviors of a gasket, simultaneously. The method not only measures gas leak rates and gasket deflection (loss of thickness) during the gasket assembly stage (i.e. initial loading or tightening), but also those during gasket decompression (unloading) to simulate operating conditions. The results will, thus more realistically reflect the leakage behavior of a gasket under actual plant conditions. Because all gaskets will be tested by the same set of test conditions, direct comparison of the leakage data between two different gaskets becomes possible, and will readily reveal the sealing performance differential of the two. In addition, bolted joint design rules can be derived from these test results to more closely account for different application conditions, an advantage not available from the current ASTM and DIN series of test methods. In this communication, we will report the test data based on the ASTM and DIN test standards as a general portrait of physical characteristics of the expanded PTFE material. The rest of the paper will be focused on presentation and utilization of the ROTT test results for characterization of gasket sealing performance. The ROTT data of the expanded PTFE will be compared with those of skived virgin PTFE and filled PTFE gasket materials.
2. Experimental approach 2.1. Preparation of specimens To produce soft, “expanded” PTFE, the process involves the following five steps [5]. 1. PTFE powder containing a defined quantity of hydrocarbon as lubricant is ram-extruded into a solid band. 2. The hydrocarbon lubricant is evaporated. 3. The PTFE band is stretched (i.e. expanded). 4. The PTFE band is minimally sintered. 5. The final product is slowly cooled and settled. The coagulated dispersion PTFE fine powder (Aflon CD123) with a number-averaged molecular weight of about 13 × 106 was provided by Asahi Glass (Japan) as the raw material for the process. This PTFE fine powder was mixed with about 20 wt.% naphtha as the lubricant and formed into a preform at relatively low pressures (about 3.45 N mm−2 ) until the lubricant filled in the mixture. The preform of the lubricated polymer was compressed to about one-third of
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its initial volume. The resulting preform was placed in a ram-extruder device and extruded through the die at a reduction ratio of less than 100 to form the extrudate, so that the extrudate had a diameter somewhat smaller than that of the preform. At this low reduction ratio, a certain amount of fibrillation took place. In order to increase the fibrillation of the extrudate to any particular desired extent, the extrudate was then calendered. Following calendering, the lubricant was removed from the extrudate. In this study, the step of removing the lubricant included heating of the extrudate to a temperature of about 300◦ C for a time period sufficient for the lubricant to substantially evaporate from the extrudate. The dried extrudate would next be stretched, and then sintered in a sintering oven. By stretching, the PTFE became oriented in the stretch direction and softened, basically because its porosity increased. The dimensions of those bands depend on the size of the nozzle of the ram-extruder. The sizes range from a rectangular cross-section of 3 mm (width) × 1 mm (thickness) to 25 mm × 10 mm. Microstructure of this expanded PTFE band characterized by node-interconnecting fibrils was observed with the scanning electron microscope (SEM). Thermal stability of the band was measured by the differential thermal analysis (DTA) and thermal gravimetric analysis (TGA) in the temperature range 30–600◦ C with a heating rate of 10◦ C min−1 in air. 2.2. Gasket characterization methods by ASTM and DIN standards Compressibility and recovery properties were tested according to ASTM F36. This method covers determination of the short-time compressibility and recovery at room temperature of gasket materials. Each test specimen was compressed to 30 N mm−2 at 23◦ C for 1 min, in order to define the percentage loss in initial thickness (i.e. compressibility). The load was then removed to determine the percentage of thickness gain against the compressed portion. Torque retention of the band was determined with DIN 52913. The test consists of application of a predetermined load on the test gasket via a tension screw, then heating of the gasket/flange assembly to the desired temperature without internal pressure. After temperature is held for a required time period, the remaining compressive load on the test gasket can then be measured, and compared with the preset load to obtain torque retention values. In this study, the sample band was initially loaded at 23◦ C. The gasket stress as represented by surface pressure was increased incrementally (4 min interval between each pressure level) to the required 30 min. Finally, it was heated to 150◦ C with a rate of 5◦ C min−1 . The surface pressure was read after 16 h at 150◦ C, then the assembly was cooled to room temperature of 23◦ C, and kept for 8 h. The surface pressure was measured again before the band was reheated to 150◦ C for the next temperature cycle. The cycle for testing of the torque
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retention was repeated three times. The percentage of gasket stress remained was then recorded as a measure of torque retention. The same test was also performed to a temperature of 200◦ C, following the same sequence as described above. Notice that ASTM F38 for creep relaxation is very similar to the torque retention test as defined above, but is generally performed at room temperature. For example, a PTFE sample band was also tested at 30 N mm−2 and 23◦ C for 22 h according to ASTM F38. Essentially the test uses percentage loss of compressive load to gauge creep relaxation, or the thickness reduction after a prolonged period, so the result should be complementary to torque retention values. Gas permeability of expanded PTFE band was determined according to DIN 3535, with plane parallel steel plates prepared to a peak-to-valley height <2 m. A 5 mm width band was designed to seal a 70 mm diameter flange, whereby the ends of the 200 mm long band overlapped cross-shaped, so that the final total length of the band was 160 mm. Measurements of permeability to gas were performed with surface pressures of 5, 10, 20 and 30 N mm−2 and internal nitrogen gas pressures of 1, 4, 8, 16, 25 and 40 bar. Notice also that the test method described by ASTM F37 for sealability is very similar to the above standard, but the test capability of the equipment for the DIN method is considered more versatile. 2.3. ROTT test procedure and conditions The ROTT test procedure is documented in the proposed ASTM Method Draft 9 of the standard test method for gasket constants for bolted joint design. Test equipment, a room temperature hydraulic test rig, was designed by TTRL to achieve a uniform gasket-load distribution [6]. The tested sample, a 10 mm wide band of approximately, 480 mm long, was lay around a flange surface of 145 mm diameter, with the ends of the sample band overlapped to complete a circular sealing surface (final length of ∼455 mm). The nominal gasket contact area was 4550 mm2 , and gasket stresses applied were computed-based on this initial area without considering any increase in gasket area upon loading. The rig was connected to a variety of leak measurement systems, capable of detecting helium leakage to 10−8 mg s−1 (by mass spectrometry) or to higher leakage (5×10−4 mg s−1 by pressure decay, and 5 × 10−6 mg s−1 by pressure rise). To reflect the true tightness behavior of a gasket in long-term, stable conditions, not the transitory leakage behavior, the test was maintained at a constant gasket stress and fluid pressure while the leakage was measured every 15 min, until the leakage measurement reached stabilization. The representative leak rate was then recorded, and the test was continued at a different stress/fluid pressure condition. The current test procedure limits the stabilization period within 1.5–5 h, and for PTFE-based material, the dwell time is generally about 3 h or longer. The ROTT test includes two parts [6,7]: the Part A sequence represents the sealing performance of a gasket during
Fig. 3. ROTT test procedure.
initial joint tightening and gasket seating, and Part B simulates the operating conditions by performing unload–reload cycles. Normally, two tests for each gasket material are conducted to verify test consistency. The applied gasket stresses include five major levels S1–S5 (equivalent to 7, 21, 37, 53, and 69 N mm−2 ), and three intermediate levels S2.5, S3.5, and S4.5 (29, 45, and 61 N mm−2 ). The general sequence of an ROTT test is illustrated in Fig. 3, showing gasket deflection as a function of stress, with the load sequence shown by arrows. In Part A, deflection of a gasket is measured at each new level of compression stress that is higher than any previously applied stress. Leakage of a gasket is also measured with helium pressure of 27 bar at S1, and with pressures alternating between 27 and 54 bar (filled and open squares) at S2–S5. The Part A test is interrupted with unloading sequences (open triangles) at S3 (Part B1), S4 (B2) and S5 (B3). At the end of the B1 and B2 sequences, the gasket is reloaded to S3 and S4, respectively, to complete the unload–reload cycles, and then the test is continued to the next testing stress level for Part A. During these cycles, helium pressure is maintained at 54 bar. The test is completed when the B3 sequence is unloaded to S1. 3. Results and discussions 3.1. Microstructure Fig. 4 shows the morphology of the expanded PTFE band observed by SEM (Fig. 4), indicating that the product is comprised of fibrils dominantly oriented in the stretching direction, and within the remaining node fractures of the calendered PTFE band. The sketch in Fig. 5 further illustrates the distribution of the nodes and fibrils within an expanded PTFE band. Fig. 6(b)–(d) also show the SEM photomicrographs of expanded PTFE on various cross-sections as shown in Fig. 6(a). The expansion process created a highly porous level of the material, which in turn reduced the density of PTFE from
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Fig. 4. SEM image showing the structure of the expanded PTFE band by the uni-axially stretching under stretching ratio R = 250%, stretching rate r = 50% s−1 .
2.2 g cm−3 to the range of 0.45–0.55 g cm−3 . The highly fibrillated, expanded microstructure confers an improved thermal stability, compressibility, creep resistance, and sealability, as demonstrated by the test results below. 3.2. Thermal stability It has been established that PTFE has a very good thermal stability compared to other plastic materials. This is confirmed in Fig. 7 [8] that PTFE begins to suffer from a noticeable weight loss only at temperatures above 260◦ C. For general sealing applications below 260◦ C, no problems will be expected due to thermal stability factor. In addition, the weight loss due to dissociation of PTFE remains very low
Fig. 5. A plain view of a section of an expanded PTFE.
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until at ∼360◦ C where it becomes substantial. As a result, short-term applications up to at least several days may still be an achievable goal for temperature to ∼300◦ C. However, what limits use of PTFE in these high temperature sealing applications is its mechanical strength, not thermal stability against decomposition. Specifically, a mistake that is often made with PTFE is that since it has such excellent stability at high temperatures, it is automatically assumed that its mechanical properties are also maintained at elevated temperatures. This is not the case, however, and the mechanical strength of PTFE undergoes a dramatic decline as the temperature is raised. This is similar to properties of other plastic materials. Both the tensile strength and elastic modulus of PTFE exhibit a severe temperature dependency [2,3]. Exacerbating this situation, PTFE experiences an increase in crystallinity when held at temperatures in the crystallization zone (307–327◦ C) which leads to a further decrease in tensile strength, impacting strength and resistance to cyclic fatigue [9]. For the expanded PTFE, its thermal stability was not changed by the expansion process. As demonstrated in Fig. 8, the weight loss as delineated by the upper curve from the thermal gravimetric analysis remained undetectable until at 500◦ C, indicating that either the decomposition of the expanded PTFE band was insignificant or had not occurred below 500◦ C. This confirms that short-term thermal stability of the expanded PTFE is at least as good as PTFE shown in Fig. 7. The analysis was done on a 20 mm wide expanded PTFE band. At 500◦ C, the data start to deviate from a straight line due to more severe decomposition, and the weight loss accelerates at even higher temperatures. For example, after heating for 62.5 min to reach 530◦ C, there was 5.1% decomposition, with 94.9% PTFE remaining. After 68.9 min to 590.8◦ C, there was 73.6% decomposition and 26.4% PTFE left. As illustrated in the later section in conjunction with the ROTT test results, the expanded PTFE shows excellent creep resistance compared to regular PTFE. It is due to the extra strength by the creation of fibrils in the microstructure of the expanded PTFE. In addition, these fibrils should prevent extensive re-crystallization that typical PTFE would encounter at high temperatures. As a result, the mechanical strength of the expanded PTFE should be less affected by heat than that of PTFE. The high temperature mechanical strength is expected to be significantly reduced at temperatures where some phase transition phenomenon starts to activate. For example, as shown by the DTA data in Fig. 8, the lower curve, the thermal energy of the sample stayed roughly constant, until near 300◦ C where it began to drop. A valley starts to form near 310◦ C, with a local minimum occurring at 346.7◦ C. This indicates a phase transition, and in this case, the melting of PTFE initiated near these temperatures. This will substantially decrease the creep resistance of the material due to increased viscosity. Based on this DTA result, the use of expanded PTFE should be limited to below 310◦ C to ensure that the mechanical strength is properly maintained.
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Fig. 6. SEM pictures on different cross-sections of expanded PTFE: (a) schematic illustration of an expanded porous PTFE section; (b–d) the scanning electron micrograph for the various section of expanded porous PTFE.
3.3. Compressibility and recovery As indicated previously, the ASTM F36 method for compressibility and recovery determines the short-term characteristics of a gasket material. This test is not intended for compressibility under prolonged stress applications, generally referred to as “creep” (see Section 3.4), or for recovery following such prolonged stress applications, the inverse of which is generally referred to as “compression set”. Conditions of the compressibility and recovery tests per ASTM F36 were: gasket stress at 30 N mm−2 , operating
temperature at 23◦ C, and duration of compression at 1 min. The results as shown in Table 1 suggest that regardless of sample sizes, the expanded PTFE shows uniform properties of very high compressibility and low recovery after 1 h of compression. 3.4. Torque retention and creep relaxation Torque retention test per DIN 52913 is designed to determine the torque retention capabilities of gasket products, when subjected to compression loads and operating
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the strain increases concurrently with the decay of stress”. The result of creep relaxation is loss of gasket thickness, which in turn results in loss of bolt torque, causing leakage to increase [10]. Although torque reduction may be contributed from factors other than creep relaxation, the ASTM F38 method nonetheless uses this value as an indication of creep relaxation property of a gasket. A separate test per ASTM F38 on a 10 mm wide expanded PTFE band at 23◦ C and 30 N mm−2 has been conducted, and it produced 35% of creep relaxation, or torque reduction, consistent with the results in Table 2 at higher temperatures.
Fig. 7. Weight loss of PTFE at high temperature [8].
temperatures as defined by the test procedures discussed earlier. It quantifies how much bolt-load is retained as a function of temperature after prolonged test duration. The tested sample was an expanded PTFE band of 5 mm (width) × 2 mm (thickness). The diameter of the sealing ring was 65 mm. Additional test conditions included: gasket stress at 30 N mm−2 , tested period for 16 h, and operating temperature from 23 to 150◦ C and 200◦ C. The results are shown in Table 2, indicating that the torque retention capability of the material remains fairly constant at different temperatures. Notice that torque retention and torque reduction above are complementary measures. Torque reduction from the initial bolt-load has been used to represent a gasket’s tendency to creep and relax. Creep relaxation as defined by ASTM F38 indicates “a transient stress–strain condition in which
3.5. Gas permeability The gas permeability test per DIN 3535 is designed to obtain leakage information of a gasket material. The results can then be used as a basis to compare sealing performance of different materials at the same testing conditions, although such information is not sufficient for derivation of useful bolted joint design codes, as indicated previously. Note that the apparatus used in this study is considerably more versatile than that in ASTM F37B for sealability, because various sizes of sample gaskets can be tested, and much higher internal pressure can be achieved. However, the test principles are essentially the same for the two tests. Selective test data are tabulated in Table 3. The corresponding test conditions were: gasket stress at 30 N mm−2 ; operating temperature, 23◦ C and internal fluid, nitrogen. The tested sample was the 5 mm wide expanded PTFE band, and the diameter of the sealing ring was 70 mm. It indicates that gas leakage is roughly proportional to internal pressure, but
Fig. 8. Differential thermal analysis (DTA) curve and weight loss of expanded PTFE band at elevated temperature.
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Table 1 Test results by the ASTM F36 method Sample width (mm)
Initial thickness (mm)
Thickness after 1 min (mm)
Compressibility (%)
Recovery (%)
20 14 10
7 5 3
1.5 1.1 0.8
78.5 78.0 73.3
8 7 5
Table 2 Torque retention results as measured by DIN 52913 Temperature maximum (◦ C)
Continuous times tested (h)
Torque retention (%)
Torque reduction (%)
150 200
16 12
62.7 62.4
37.3 37.6
Table 3 Gas permeability results as measured by DIN 3535 Internal pressure (bar)
Gas leak rate (ml min−1 )
16 25 40
0.04 ± 0.02 0.04 ± 0.01 0.06 ± 0.02
the detailed functional relationship remains to be determined by more detailed studies. 3.6. ROTT test results of the expanded PTFE material Figs. 9 and 10 illustrate the ROTT test data for the expanded PTFE material, in the formats of gasket deflection and leakage versus gasket stress. The tests were conducted by TTRL during the PVRC PTFE Gasket Qualification Project [11]. Notice that test 1 and test 2 (solid and dotted lines, respectively) from the two tested samples produced
similar results, indicating consistent mechanical and sealing behaviors for the material. Fig. 9 shows the deflection results, the collection of the 15 min incremental measurements during the tests. In general, there are two major factors affecting deflection of a gasket in the Part A assembly sequence: (1) the effect of initial increase in gasket stress and (2) the tendency of the material to creep, or cold-flow, under prolonged dwell time before leakage stabilizes. As shown in Fig. 9, upon initial compression, the gasket thickness decreased significantly at the S1 level, consistent with the high compressibility measured by ASTM F36. However, the rate of thickness loss became much reduced at higher stresses such as S3–S5. The extent of continuous thickness loss at each stress level (or the “plateau”) due to factor (2) gives an indication of a gasket’s ability to resist creep under compression. Narrower a plateau, better creep resistance. For the expanded PTFE, the widths of the plateaus above S3 are very narrow compared to the initial thickness of the material, suggesting an excellent resistance of the material against creep. This will become more apparent when the results are compared with the ROTT data from other PTFE-based gasket materials. For the Part B unload–reload cycles, the deflection measurements form steep curves with only small thickness increases corresponding to the stress reductions. This represents limited thickness recovery of the material after decompression, also consistent with the recovery results by ASTM F36. The overall deflection pattern from both Parts A and B
Fig. 9. ROTT deflection graph for expanded PTFE.
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Fig. 10. ROTT leakage graph for expanded PTFE.
(Fig. 9) confirms high compressibility, excellent cold-flow resistance, and low recovery of the material. Fig. 10 shows results of helium mass leak rate versus gasket stress. Each data point represents the final measurement after the leakage stabilized. It is worth noting that there are two types of leakage paths existing for a gasketed joint, the tangential leakage for fluids going through the interface between gasket and flange surfaces, and the permeation of fluids through the gasket. Due to the softness and high surface conformability of the expanded PTFE (i.e. high compressibility), a moderate compressive load should effectively reduce the tangential leakage. As a result, fluid permeation becomes the major contributor to the overall leakage of the joint. The high leak rates at low stress levels such as S1 and S2 for the expanded PTFE are, thus interpreted as a result of high remaining porosity, and hence, high permeability, of the material at these stresses. The sealing performance was quickly improved over the interval of S1–S3 levels (by over 105 times), but the rate of leakage reduction become highly limited above S3. Upon unloading (i.e. the Parts B cycles), the expanded PTFE showed a very limited recovery in thickness (Fig. 9 deflection curve). For such a porous gasket material, this limited thickness recovery should correspond to limited porosity recovery during unloading, which in turn prevents significant increase of leakage by fluid permeation, provided that the gasket remains in good contact with flange surfaces to avoid tangential leakage. This is the case for the Parts B1 and B2 cycles (Fig. 10) where the leak rate only increased by about 3–5 times when the gasket was decompressed from S3 and S4 levels to S1, a stress reduction of ∼5–8 times. For the Part B3 unloading curve, the final leakage at S1 is significantly higher than the initial leakage at S5. This sudden increase of leakage is probably due to the lack of recovery at this compressive state at which the gasket could no longer maintain a good contact with flange surfaces, causing a significant increase in the tangential leakage. However, this
phenomenon occurs only when the gasket is compressed to a very good sealablity level, a state that cannot be reached by other types of PTFE-based gaskets, as demonstrated in Section 3.7. 3.7. Comparison with other PTFE-based gasket materials In this section, the ROTT test results of the expanded PTFE will be compared with data from other traditional PTFE-based materials. These materials include a skived virgin (i.e. pure) PTFE sheet and a filled PTFE sheet, both of which were tested in the PVRC PTFE Gasket Qualification Project [11]. The former material was skived from a PTFE log sintered at high temperatures, while the latter was formed by calendering mixtures of PTFE powders and barium sulfate fillers. Notice that the filled PTFE discussed here is perceived as one of the better performers among all filled PTFE gasket materials. As indicated in Section 1, both of these materials are expected to show little mechanical strength under compressive loads.
Fig. 11. ROTT deflection graph for expanded PTFE and other PTFE materials.
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Fig. 12. ROTT Part A leakage graph for expanded PTFE and other materials.
Fig. 11 compares the deflection data for the three PTFE-based gaskets. The two tests for each material produced relatively similar results (with some variations) for our comparative purpose, so we present only the results from test 1 in Fig. 11. Because the initial thickness of the tested 10 mm expanded PTFE sample was 4.7 mm, different from that of the other materials (∼3 mm), for a fair comparison, the deflection values have been normalized against the initial thickness of each material. The figure shows drastically different deflection behaviors among the three PTFE materials. In general, the expanded PTFE is highly compressible at low stresses (over 65% of the initial thickness at S1), while maintaining its thickness at high compressive loads during the dwell times. The total thickness loss between the S3 and S5 levels is just 6% of the initial thickness, this is the combination of the small compressibility and creep at these high stresses. In contrast, the other two PTFE-based materials show small initial thickness losses in response to stress increases, but large continuos thickness losses when held at constant stresses. For example, the skived and filled PTFE were compressed by only 5 and 2% at S1, and showed total deflection of 24 and 15% above S3. These results suggest that the expanded PTFE has best compressibility and creep resistance among the three compared gasket materials. For the Parts B cycles, the deflection patterns indicate that all three PTFE materials show very limited recovering capability after prolonged services, a property typical of PTFE in general. Due to complex leakage behavior for a gasket involving both unload and reload processes with fluid pressure varying between 27 and 54 bar, the leakage data for Parts A and B will be separately illustrated in Figs. 12 and 13 to avoid confusion. Fig. 12 compares the Part A leakage data of the three PTFE-based materials. Again, the two tests of each material produced relatively comparable results for our purpose, so Fig. 12 shows only the test 1 results for each material.
As illustrated in Fig. 12, the expanded PTFE produced relatively high leak rates at low stresses where the porosity of the material was still high, but it showed a significant sealability improvement with stress increase between S1 and S3, essentially becoming the tightest among all tested gaskets above ∼30 N mm−2 . The filled PTFE seals slightly better than the expanded PTFE at S1, but quickly become worse before reaching the S2 level. It also leaks more than the skived virgin PTFE until at S5 where the two perform similarly. The skived PTFE shows a relatively good sealability at S1, due to its low initial porosity. However, its leakage level does not decrease much from here, showing essentially no improvement above S2. This very limited leakage change over a wide stress range is referred to as “tightness hardening”, a significant drawback for the skived PTFE. Fig. 13 illustrates unloading leakage characteristics of the three PTFE gaskets. In this graph, the leakage of material
Fig. 13. ROTT Part B leakage graph for expanded PTFE and other materials.
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at the beginning of an unloading cycle is plotted against the final leak rate at the lowest stress of the cycle at S1. Parameter R in the figure indicates the degree of leakage recovery during decompression, and is the ratio of the final leak rate to initial leak rate. The 45◦ straight lines are contours for various values of R. The graph confirms that the expanded PTFE could achieve a much better initial sealability (∼10 times at least) than the other materials at S3 or above, that the skived PTFE shows severe tightness hardening behavior, and that the filled PTFE performs worst within the range of gasket stresses tested. Upon unloading, the majority of data from all the PTFE samples are near or below the line of R = 10, indicating a relatively small leakage increase during decompression, consistent with the low recovery for the three materials. Again, the sudden increase of leakage at the B3 cycles for the expanded PTFE is probably due to an increase in tangential leakage. However, this should not cause a major concern in terms of usability of the material, because it occurs only at a very high compressive state. At S4 and below where this problem does not exist, the achievable sealability level of the expanded PTFE is still significantly higher than any other PTFE materials discussed here. This good assembly sealability combined with the low leakage recovery upon unloading makes the expanded PTFE a good gasket product against stress variations during operation conditions. 3.8. Expanded PTFE used for fluid sealing applications As a general summary of properties of the uni-axially expanded PTFE, the material is a low density from (0.45–0.55 g cm−3 ) of PTFE, soft but strong, highly compressible with low recovery. It is chemically inert, and resistant to cold-flow and creep relaxation. It also has high thermal stability, and relatively high mechanical strength at elevated temperatures compared to the traditional PTFE materials. The improved mechanical properties of the expanded PTFE result from its microstructure (Figs. 4–6) created by the expansion process. The softness and high compressibility is a result of the large quantity of micropores, and the strength is enhanced by existence of the node-interconnecting fibrils. It is also due to this highly porous structure that the expanded PTFE can only have limited thickness recovery, but this low recovery may correspond to low leakage sensitivity of the material to bolt-load reductions, a good physical property of sealing performance. The combination of all the good physical and chemical properties leads to excellent sealing performance of the expanded PTFE for gasketing applications. High compressibility of the material is essential for proper installation of a gasket, and is critical to compensate for any flange irregularities such as nicks, non-parallelism, corrosion or variations in groove depth. This eliminates the path for tangential leakage between the gasket and flange surface. Good blot-load retention reduces the need to constantly re-torque the bolts,
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while high sealability eases the environmental concern on serious fugitive emission. Notice that due to the high porosity, the most effective sealing occurs when the voids are sufficiently closed up, and this ensures proper seating of the gasket. In addition to the excellent properties of the expanded PTFE as a gasket material, an adhesive tape can be further attached on these expanded PTFE bands, so that they can be easily attached to flange surfaces. This offers a simple solution to many sealing problems, especially when maintenance personnel are pressed for time but the correct dimension of a sheet-based gasket is not available. With this type of gasket application, no accurate dimensioning is necessary.
4. Conclusion The mechanical properties of PTFE products have been improved through the application of the expansion process. The new method used in this study includes extruding the paste through an extrusion die at a reduction ratio of less than 100, calendering the extrudate, removing the lubricant, and stretching the extrudate at a temperature greater than the melting temperature of PTFE. The product characterized by node-interconnecting fibrils was observed with SEM, and this fibrillation structure decisively results in improved mechanical strength and sealing performance of the expanded PTFE against traditional PTFE materials. These advantages of the expanded PTFE have been confirmed by the ROTT test results, making it one of the most versatile gasketing materials available today for difficult fluid sealing applications. References [1] M.W. Brown, Seals and Sealing Handbook, Elsevier, England, 1990, p. 82. [2] DuPont, The DuPont Plunkett Awards for Innovation with Teflon® , 1995. [3] J. Scheirs, Modern Fluoropolymer, Wiley, London, 1997, Chapter 1, pp. 2–3. [4] J. Latte, D. Coomber, Industrial gaskets, in: J. H. Bickford (Ed.), Gaskets and Gasketed Joints, Marcel Dekker, New York, 1997, pp. 87–122. [5] J. Huang, et al., Process for Forming an Expanded Porous Tetrafluoroethylene Polymer, US patent 5,098,625 (1992). [6] M. Derenne, J.R. Payne, L. Marchand, A. Bazergui, PVRC/MTI technology for characterizing gaskets used in bolted flanged connections, in: J. H. Bickford (Ed.), Gaskets and Gasketed Joints, Marcel Dekker, New York, 1997, pp. 137–302. [7] M. Derenne, J. R. Payne, Guide to the PVRC ROTT Test, PVRC Project, TTRL Ecole Polytechnique, University of Montreal, Montreal, Canada, 1998. [8] A.G. Hoechst, Hoechst Plastics (HOSTAFLON), 1984, Chapter 3, p. 7. [9] J. Scheirs, Modern Fluoropolymer, Wiley, London, 1997, Chapter 1, p. 31. [10] Fluid Sealing Association, Non-Metallic Gasketing Handbook, 1989. [11] PTFE Gasket Qualification Project — Final Report, TTRL Ecole Polytechnique, Montreal, Canada, 1995.
Evaluation of uni-axially expanded PTFE as a gasket material for fluid sealing applications James Huang a,∗ , Yuan-Haun Lee b a
b
Chemistry Department, Chung-Yuan University, Chung-Li 32023, Taiwan, ROC Graduate Institute of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan, ROC Received 2 March 2000; received in revised form 4 March 2000; accepted 16 October 2000
Abstract A new process was invented for preparing an improved polytetrafluoroethylene (PTFE) sealant material from a paste formed of PTFE resin with special plasticizer. This expanded PTFE sealant material consisted of a special node and fibrous microstructure. The preferred orientation for mechanical strength of the fibrous structure was parallel to the direction of calendering. The expanded PTFE material was investigated for its mechanical properties, including creep relaxation, compressibility, recovery, torque retention, and gas permeability, as defined by the ASTM and DIN standards. The sealing performance of the material was further characterized by the room temperature tightness test established by the Pressure Vessel Research Council (PVRC). The test results suggest that uni-axially expanded PTFE is soft and compressible, with excellent sealability. It is strong, possessing good resistance against creep and cold-flow. These favorable properties of expanded PTFE, as compared to traditional PTFE-based gasket types, make it a promising gasket material for fluid sealing applications. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Expanded PTFE; Creep relaxation; Torque retention; Gas permeability
1. Introduction In today’s typical chemical process plants, a gasketed joint is only a small component of a large piping system, but its function is by no means trivial. In fact, it serves as the most important “gate-keeper” for controlling fugitive fluid emission to keep the environment clean and safe. The terms joint, jointing and gasket are often used synonymously to describe static seals cut or fabricated from compressible flat sheet material, in preference to the term “seal”. Academic definition would restrict the term “jointing” to describe the sheet material, when the actual seal cut from it is then either a joint or gasket (synonymous) [1]. Practically, other forms of materials such as ropes may very well qualify for gasketing functions. An effective jointing material offers tighter sealing and long-term performance, thus reducing environmental concerns, labor costs, material losses, and possible down time associated with system failures or frequent replacements of less reliable gaskets. Asbestos-based material was once the choice for many of fluid sealing applications, but its hazardous nature to the ∗ Corresponding author. Tel.: +886-4-358-0021; fax: +886-4-359-6963. E-mail address: [email protected] (J. Huang).
health of general public has been confirmed, resulting in a ban on the use of the material, and a large scale of search by the fluid sealing industry for replacements of asbestos-based gaskets. Although the development of plastics has introduced a wide range of materials to the market, very few have been found readily applicable as static flat seals. This is due to the limited temperature range over which they can be reliably used, together with comparatively poor resilience and, in some cases, a tendency to flow under load. A major exception to this is polytetrafluoroethylene (PTFE) which has such a remarkable combination of properties that it takes a special place in the sealing of aggressive and toxic fluids (Figs. 1 and 2) [2–4]. The molecular structure shows that the carbon backbone of PTFE is totally covered by fluorine, thus hindering the access of other aggressive media that may destroy the backbone bonds. Additionally, the bond between carbon (C) and fluorine (F) is extremely strong compared with others, so the replacement of the fluorine is practically excluded. This is why PTFE has such good chemical stability. There are only a few ways to destroy PTFE: (i) chemically by molecular fluorine (F) or other fluorinating media and by molten alkalis and (ii) physically by heating over 400◦ C (750◦ F), and by high energy radiation (i.e. nuclear radiation). Both of these mechanisms attack and shorten the
0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 0 ) 0 0 5 0 8 - 3
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Fig. 1. The manufacturing process for PTFE.
chain length of PTFE to release PTFE monomers. These examples show the limitations of PTFE-based gaskets. In summary, PTFE has the following properties [2–4]. • It does not burn. • It is stable to light. • It does not absorb water, it is hydrophobic. It is physiologically safe. • It is insoluble in all solvents, even at increased temperature. • It is inert to all media except melted alkali metal, fluorine at liquid and gas states. • It has excellent electrical insulating capacity, even at high humidity. • Its thermal conductivity is low at about 6×10−4 cal cm−1 s−1 ◦ C−1 . • Due to the highly symmetric molecular structure, it has high crystallinity of 90–95%. All properties mentioned so far should make PTFE a very good gasket material, but unfortunately, there are two unfavorable properties that very much to its disadvantage: (i) low strength and (ii) high creep, caused by a transition point at 19◦ C. Because the number of ambient or low temperature gasket applications is limited, the usage of this material for gaskets is limited [4]. For this reason, there have been countless trials to improve strength and creep by incorporating into the material inorganic fillers like glass fiber, carbon fiber, precipitated silica, graphite, carbon black, and bronze
Fig. 2. Schematic presentation of the PTFE molecular helix [3].
powder. Success was rather limited, because nothing really changed the basic properties of PTFE, and the slightly improved creep resistance was obtained at the price of reduced chemical compatibility. In this study, a new process has been developed to improve the strength of PTFE and reduce its creep [5]. The resultant product is a uni-axially expanded PTFE material which is intended for use in fluid sealing applications, i.e. as a gasket or sealant material. Expanded PTFE is essentially a new class of PTFE-based material, possessing properties significantly different from all other traditional PTFE materials, such as skived virgin PTFE and filled PTFE. The properties of the material related to gasketing functions have been investigated extensively by various gasket characterization techniques, as summarized below. 1.1. Gasket characterizations The sealing capability of a material remains the most critical issue deciding whether or not the material can be effectively used for gasketing functions. For any typical fluid sealing application, the two major forces competing to affect the sealability of a gasket are the gasket compressive stress and internal fluid pressure. Generally, higher the stress, lower the leakage; the leak rate, however, increases with increasing fluid pressure. The actual joint leakage is, thus the net effect of these two factors, and the ability of a gasket to maintain the applied compressive stress is essential to the control of joint leakage. As a result, many gasket characterization techniques center around determination of mechanical properties of a potential gasket material corresponding to various compressive conditions. These test methods include those described in ASTM and DIN standards. For example, specifications required by gasket users typically include information on compressibility, recovery and creep relaxation of a gasket material (i.e. ASTM F36 and F38, DIN 52913). These measurements are related to how a material responds mechanically to compression and decompression, either under short-term or long-term conditions. Unfortunately, these quantities cannot be used to correlate with the actual sealing performance of a gasket, as no fluid is involved with the tests of these properties. Rather, this information may be more relevant to quality control issues of gasket production. The remedy to this deficiency is to conduct additional tests on sealability (ASTM F37), or gas permeability (DIN 3535), of the material to delineate the sealing characteristics of a gasket. However, the correlation with mechanical behavior is commonly missed, and no effect on the operating load changes such as pressurization and vibration of the piping system can be quantified. Adding to the problems of these testing methods is the fact that many testing parameters implemented in the tests may be varied, depending on the agreements between gasket users and manufacturers. This sometimes makes a direct comparison between two competing materials less obvious if test results sup-
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plied by each manufacturer were performed at different conditions. A more sophisticated testing method for gasket performance has evolved over the past 20 years, through the efforts of the Pressure Vessels Research Council (PVRC), Material Technology Institute, and Tightness Testing and Research Laboratory (TTRL) of Ecole Polytechnique, University of Montreal. This test, the room temperature tightness (ROTT) test, is a “leakage-based” testing method for characterizing physical and functional properties of a gasket, quantifying both mechanical and sealing behaviors of a gasket, simultaneously. The method not only measures gas leak rates and gasket deflection (loss of thickness) during the gasket assembly stage (i.e. initial loading or tightening), but also those during gasket decompression (unloading) to simulate operating conditions. The results will, thus more realistically reflect the leakage behavior of a gasket under actual plant conditions. Because all gaskets will be tested by the same set of test conditions, direct comparison of the leakage data between two different gaskets becomes possible, and will readily reveal the sealing performance differential of the two. In addition, bolted joint design rules can be derived from these test results to more closely account for different application conditions, an advantage not available from the current ASTM and DIN series of test methods. In this communication, we will report the test data based on the ASTM and DIN test standards as a general portrait of physical characteristics of the expanded PTFE material. The rest of the paper will be focused on presentation and utilization of the ROTT test results for characterization of gasket sealing performance. The ROTT data of the expanded PTFE will be compared with those of skived virgin PTFE and filled PTFE gasket materials.
2. Experimental approach 2.1. Preparation of specimens To produce soft, “expanded” PTFE, the process involves the following five steps [5]. 1. PTFE powder containing a defined quantity of hydrocarbon as lubricant is ram-extruded into a solid band. 2. The hydrocarbon lubricant is evaporated. 3. The PTFE band is stretched (i.e. expanded). 4. The PTFE band is minimally sintered. 5. The final product is slowly cooled and settled. The coagulated dispersion PTFE fine powder (Aflon CD123) with a number-averaged molecular weight of about 13 × 106 was provided by Asahi Glass (Japan) as the raw material for the process. This PTFE fine powder was mixed with about 20 wt.% naphtha as the lubricant and formed into a preform at relatively low pressures (about 3.45 N mm−2 ) until the lubricant filled in the mixture. The preform of the lubricated polymer was compressed to about one-third of
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its initial volume. The resulting preform was placed in a ram-extruder device and extruded through the die at a reduction ratio of less than 100 to form the extrudate, so that the extrudate had a diameter somewhat smaller than that of the preform. At this low reduction ratio, a certain amount of fibrillation took place. In order to increase the fibrillation of the extrudate to any particular desired extent, the extrudate was then calendered. Following calendering, the lubricant was removed from the extrudate. In this study, the step of removing the lubricant included heating of the extrudate to a temperature of about 300◦ C for a time period sufficient for the lubricant to substantially evaporate from the extrudate. The dried extrudate would next be stretched, and then sintered in a sintering oven. By stretching, the PTFE became oriented in the stretch direction and softened, basically because its porosity increased. The dimensions of those bands depend on the size of the nozzle of the ram-extruder. The sizes range from a rectangular cross-section of 3 mm (width) × 1 mm (thickness) to 25 mm × 10 mm. Microstructure of this expanded PTFE band characterized by node-interconnecting fibrils was observed with the scanning electron microscope (SEM). Thermal stability of the band was measured by the differential thermal analysis (DTA) and thermal gravimetric analysis (TGA) in the temperature range 30–600◦ C with a heating rate of 10◦ C min−1 in air. 2.2. Gasket characterization methods by ASTM and DIN standards Compressibility and recovery properties were tested according to ASTM F36. This method covers determination of the short-time compressibility and recovery at room temperature of gasket materials. Each test specimen was compressed to 30 N mm−2 at 23◦ C for 1 min, in order to define the percentage loss in initial thickness (i.e. compressibility). The load was then removed to determine the percentage of thickness gain against the compressed portion. Torque retention of the band was determined with DIN 52913. The test consists of application of a predetermined load on the test gasket via a tension screw, then heating of the gasket/flange assembly to the desired temperature without internal pressure. After temperature is held for a required time period, the remaining compressive load on the test gasket can then be measured, and compared with the preset load to obtain torque retention values. In this study, the sample band was initially loaded at 23◦ C. The gasket stress as represented by surface pressure was increased incrementally (4 min interval between each pressure level) to the required 30 min. Finally, it was heated to 150◦ C with a rate of 5◦ C min−1 . The surface pressure was read after 16 h at 150◦ C, then the assembly was cooled to room temperature of 23◦ C, and kept for 8 h. The surface pressure was measured again before the band was reheated to 150◦ C for the next temperature cycle. The cycle for testing of the torque
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retention was repeated three times. The percentage of gasket stress remained was then recorded as a measure of torque retention. The same test was also performed to a temperature of 200◦ C, following the same sequence as described above. Notice that ASTM F38 for creep relaxation is very similar to the torque retention test as defined above, but is generally performed at room temperature. For example, a PTFE sample band was also tested at 30 N mm−2 and 23◦ C for 22 h according to ASTM F38. Essentially the test uses percentage loss of compressive load to gauge creep relaxation, or the thickness reduction after a prolonged period, so the result should be complementary to torque retention values. Gas permeability of expanded PTFE band was determined according to DIN 3535, with plane parallel steel plates prepared to a peak-to-valley height <2 m. A 5 mm width band was designed to seal a 70 mm diameter flange, whereby the ends of the 200 mm long band overlapped cross-shaped, so that the final total length of the band was 160 mm. Measurements of permeability to gas were performed with surface pressures of 5, 10, 20 and 30 N mm−2 and internal nitrogen gas pressures of 1, 4, 8, 16, 25 and 40 bar. Notice also that the test method described by ASTM F37 for sealability is very similar to the above standard, but the test capability of the equipment for the DIN method is considered more versatile. 2.3. ROTT test procedure and conditions The ROTT test procedure is documented in the proposed ASTM Method Draft 9 of the standard test method for gasket constants for bolted joint design. Test equipment, a room temperature hydraulic test rig, was designed by TTRL to achieve a uniform gasket-load distribution [6]. The tested sample, a 10 mm wide band of approximately, 480 mm long, was lay around a flange surface of 145 mm diameter, with the ends of the sample band overlapped to complete a circular sealing surface (final length of ∼455 mm). The nominal gasket contact area was 4550 mm2 , and gasket stresses applied were computed-based on this initial area without considering any increase in gasket area upon loading. The rig was connected to a variety of leak measurement systems, capable of detecting helium leakage to 10−8 mg s−1 (by mass spectrometry) or to higher leakage (5×10−4 mg s−1 by pressure decay, and 5 × 10−6 mg s−1 by pressure rise). To reflect the true tightness behavior of a gasket in long-term, stable conditions, not the transitory leakage behavior, the test was maintained at a constant gasket stress and fluid pressure while the leakage was measured every 15 min, until the leakage measurement reached stabilization. The representative leak rate was then recorded, and the test was continued at a different stress/fluid pressure condition. The current test procedure limits the stabilization period within 1.5–5 h, and for PTFE-based material, the dwell time is generally about 3 h or longer. The ROTT test includes two parts [6,7]: the Part A sequence represents the sealing performance of a gasket during
Fig. 3. ROTT test procedure.
initial joint tightening and gasket seating, and Part B simulates the operating conditions by performing unload–reload cycles. Normally, two tests for each gasket material are conducted to verify test consistency. The applied gasket stresses include five major levels S1–S5 (equivalent to 7, 21, 37, 53, and 69 N mm−2 ), and three intermediate levels S2.5, S3.5, and S4.5 (29, 45, and 61 N mm−2 ). The general sequence of an ROTT test is illustrated in Fig. 3, showing gasket deflection as a function of stress, with the load sequence shown by arrows. In Part A, deflection of a gasket is measured at each new level of compression stress that is higher than any previously applied stress. Leakage of a gasket is also measured with helium pressure of 27 bar at S1, and with pressures alternating between 27 and 54 bar (filled and open squares) at S2–S5. The Part A test is interrupted with unloading sequences (open triangles) at S3 (Part B1), S4 (B2) and S5 (B3). At the end of the B1 and B2 sequences, the gasket is reloaded to S3 and S4, respectively, to complete the unload–reload cycles, and then the test is continued to the next testing stress level for Part A. During these cycles, helium pressure is maintained at 54 bar. The test is completed when the B3 sequence is unloaded to S1. 3. Results and discussions 3.1. Microstructure Fig. 4 shows the morphology of the expanded PTFE band observed by SEM (Fig. 4), indicating that the product is comprised of fibrils dominantly oriented in the stretching direction, and within the remaining node fractures of the calendered PTFE band. The sketch in Fig. 5 further illustrates the distribution of the nodes and fibrils within an expanded PTFE band. Fig. 6(b)–(d) also show the SEM photomicrographs of expanded PTFE on various cross-sections as shown in Fig. 6(a). The expansion process created a highly porous level of the material, which in turn reduced the density of PTFE from
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Fig. 4. SEM image showing the structure of the expanded PTFE band by the uni-axially stretching under stretching ratio R = 250%, stretching rate r = 50% s−1 .
2.2 g cm−3 to the range of 0.45–0.55 g cm−3 . The highly fibrillated, expanded microstructure confers an improved thermal stability, compressibility, creep resistance, and sealability, as demonstrated by the test results below. 3.2. Thermal stability It has been established that PTFE has a very good thermal stability compared to other plastic materials. This is confirmed in Fig. 7 [8] that PTFE begins to suffer from a noticeable weight loss only at temperatures above 260◦ C. For general sealing applications below 260◦ C, no problems will be expected due to thermal stability factor. In addition, the weight loss due to dissociation of PTFE remains very low
Fig. 5. A plain view of a section of an expanded PTFE.
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until at ∼360◦ C where it becomes substantial. As a result, short-term applications up to at least several days may still be an achievable goal for temperature to ∼300◦ C. However, what limits use of PTFE in these high temperature sealing applications is its mechanical strength, not thermal stability against decomposition. Specifically, a mistake that is often made with PTFE is that since it has such excellent stability at high temperatures, it is automatically assumed that its mechanical properties are also maintained at elevated temperatures. This is not the case, however, and the mechanical strength of PTFE undergoes a dramatic decline as the temperature is raised. This is similar to properties of other plastic materials. Both the tensile strength and elastic modulus of PTFE exhibit a severe temperature dependency [2,3]. Exacerbating this situation, PTFE experiences an increase in crystallinity when held at temperatures in the crystallization zone (307–327◦ C) which leads to a further decrease in tensile strength, impacting strength and resistance to cyclic fatigue [9]. For the expanded PTFE, its thermal stability was not changed by the expansion process. As demonstrated in Fig. 8, the weight loss as delineated by the upper curve from the thermal gravimetric analysis remained undetectable until at 500◦ C, indicating that either the decomposition of the expanded PTFE band was insignificant or had not occurred below 500◦ C. This confirms that short-term thermal stability of the expanded PTFE is at least as good as PTFE shown in Fig. 7. The analysis was done on a 20 mm wide expanded PTFE band. At 500◦ C, the data start to deviate from a straight line due to more severe decomposition, and the weight loss accelerates at even higher temperatures. For example, after heating for 62.5 min to reach 530◦ C, there was 5.1% decomposition, with 94.9% PTFE remaining. After 68.9 min to 590.8◦ C, there was 73.6% decomposition and 26.4% PTFE left. As illustrated in the later section in conjunction with the ROTT test results, the expanded PTFE shows excellent creep resistance compared to regular PTFE. It is due to the extra strength by the creation of fibrils in the microstructure of the expanded PTFE. In addition, these fibrils should prevent extensive re-crystallization that typical PTFE would encounter at high temperatures. As a result, the mechanical strength of the expanded PTFE should be less affected by heat than that of PTFE. The high temperature mechanical strength is expected to be significantly reduced at temperatures where some phase transition phenomenon starts to activate. For example, as shown by the DTA data in Fig. 8, the lower curve, the thermal energy of the sample stayed roughly constant, until near 300◦ C where it began to drop. A valley starts to form near 310◦ C, with a local minimum occurring at 346.7◦ C. This indicates a phase transition, and in this case, the melting of PTFE initiated near these temperatures. This will substantially decrease the creep resistance of the material due to increased viscosity. Based on this DTA result, the use of expanded PTFE should be limited to below 310◦ C to ensure that the mechanical strength is properly maintained.
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Fig. 6. SEM pictures on different cross-sections of expanded PTFE: (a) schematic illustration of an expanded porous PTFE section; (b–d) the scanning electron micrograph for the various section of expanded porous PTFE.
3.3. Compressibility and recovery As indicated previously, the ASTM F36 method for compressibility and recovery determines the short-term characteristics of a gasket material. This test is not intended for compressibility under prolonged stress applications, generally referred to as “creep” (see Section 3.4), or for recovery following such prolonged stress applications, the inverse of which is generally referred to as “compression set”. Conditions of the compressibility and recovery tests per ASTM F36 were: gasket stress at 30 N mm−2 , operating
temperature at 23◦ C, and duration of compression at 1 min. The results as shown in Table 1 suggest that regardless of sample sizes, the expanded PTFE shows uniform properties of very high compressibility and low recovery after 1 h of compression. 3.4. Torque retention and creep relaxation Torque retention test per DIN 52913 is designed to determine the torque retention capabilities of gasket products, when subjected to compression loads and operating
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the strain increases concurrently with the decay of stress”. The result of creep relaxation is loss of gasket thickness, which in turn results in loss of bolt torque, causing leakage to increase [10]. Although torque reduction may be contributed from factors other than creep relaxation, the ASTM F38 method nonetheless uses this value as an indication of creep relaxation property of a gasket. A separate test per ASTM F38 on a 10 mm wide expanded PTFE band at 23◦ C and 30 N mm−2 has been conducted, and it produced 35% of creep relaxation, or torque reduction, consistent with the results in Table 2 at higher temperatures.
Fig. 7. Weight loss of PTFE at high temperature [8].
temperatures as defined by the test procedures discussed earlier. It quantifies how much bolt-load is retained as a function of temperature after prolonged test duration. The tested sample was an expanded PTFE band of 5 mm (width) × 2 mm (thickness). The diameter of the sealing ring was 65 mm. Additional test conditions included: gasket stress at 30 N mm−2 , tested period for 16 h, and operating temperature from 23 to 150◦ C and 200◦ C. The results are shown in Table 2, indicating that the torque retention capability of the material remains fairly constant at different temperatures. Notice that torque retention and torque reduction above are complementary measures. Torque reduction from the initial bolt-load has been used to represent a gasket’s tendency to creep and relax. Creep relaxation as defined by ASTM F38 indicates “a transient stress–strain condition in which
3.5. Gas permeability The gas permeability test per DIN 3535 is designed to obtain leakage information of a gasket material. The results can then be used as a basis to compare sealing performance of different materials at the same testing conditions, although such information is not sufficient for derivation of useful bolted joint design codes, as indicated previously. Note that the apparatus used in this study is considerably more versatile than that in ASTM F37B for sealability, because various sizes of sample gaskets can be tested, and much higher internal pressure can be achieved. However, the test principles are essentially the same for the two tests. Selective test data are tabulated in Table 3. The corresponding test conditions were: gasket stress at 30 N mm−2 ; operating temperature, 23◦ C and internal fluid, nitrogen. The tested sample was the 5 mm wide expanded PTFE band, and the diameter of the sealing ring was 70 mm. It indicates that gas leakage is roughly proportional to internal pressure, but
Fig. 8. Differential thermal analysis (DTA) curve and weight loss of expanded PTFE band at elevated temperature.
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Table 1 Test results by the ASTM F36 method Sample width (mm)
Initial thickness (mm)
Thickness after 1 min (mm)
Compressibility (%)
Recovery (%)
20 14 10
7 5 3
1.5 1.1 0.8
78.5 78.0 73.3
8 7 5
Table 2 Torque retention results as measured by DIN 52913 Temperature maximum (◦ C)
Continuous times tested (h)
Torque retention (%)
Torque reduction (%)
150 200
16 12
62.7 62.4
37.3 37.6
Table 3 Gas permeability results as measured by DIN 3535 Internal pressure (bar)
Gas leak rate (ml min−1 )
16 25 40
0.04 ± 0.02 0.04 ± 0.01 0.06 ± 0.02
the detailed functional relationship remains to be determined by more detailed studies. 3.6. ROTT test results of the expanded PTFE material Figs. 9 and 10 illustrate the ROTT test data for the expanded PTFE material, in the formats of gasket deflection and leakage versus gasket stress. The tests were conducted by TTRL during the PVRC PTFE Gasket Qualification Project [11]. Notice that test 1 and test 2 (solid and dotted lines, respectively) from the two tested samples produced
similar results, indicating consistent mechanical and sealing behaviors for the material. Fig. 9 shows the deflection results, the collection of the 15 min incremental measurements during the tests. In general, there are two major factors affecting deflection of a gasket in the Part A assembly sequence: (1) the effect of initial increase in gasket stress and (2) the tendency of the material to creep, or cold-flow, under prolonged dwell time before leakage stabilizes. As shown in Fig. 9, upon initial compression, the gasket thickness decreased significantly at the S1 level, consistent with the high compressibility measured by ASTM F36. However, the rate of thickness loss became much reduced at higher stresses such as S3–S5. The extent of continuous thickness loss at each stress level (or the “plateau”) due to factor (2) gives an indication of a gasket’s ability to resist creep under compression. Narrower a plateau, better creep resistance. For the expanded PTFE, the widths of the plateaus above S3 are very narrow compared to the initial thickness of the material, suggesting an excellent resistance of the material against creep. This will become more apparent when the results are compared with the ROTT data from other PTFE-based gasket materials. For the Part B unload–reload cycles, the deflection measurements form steep curves with only small thickness increases corresponding to the stress reductions. This represents limited thickness recovery of the material after decompression, also consistent with the recovery results by ASTM F36. The overall deflection pattern from both Parts A and B
Fig. 9. ROTT deflection graph for expanded PTFE.
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Fig. 10. ROTT leakage graph for expanded PTFE.
(Fig. 9) confirms high compressibility, excellent cold-flow resistance, and low recovery of the material. Fig. 10 shows results of helium mass leak rate versus gasket stress. Each data point represents the final measurement after the leakage stabilized. It is worth noting that there are two types of leakage paths existing for a gasketed joint, the tangential leakage for fluids going through the interface between gasket and flange surfaces, and the permeation of fluids through the gasket. Due to the softness and high surface conformability of the expanded PTFE (i.e. high compressibility), a moderate compressive load should effectively reduce the tangential leakage. As a result, fluid permeation becomes the major contributor to the overall leakage of the joint. The high leak rates at low stress levels such as S1 and S2 for the expanded PTFE are, thus interpreted as a result of high remaining porosity, and hence, high permeability, of the material at these stresses. The sealing performance was quickly improved over the interval of S1–S3 levels (by over 105 times), but the rate of leakage reduction become highly limited above S3. Upon unloading (i.e. the Parts B cycles), the expanded PTFE showed a very limited recovery in thickness (Fig. 9 deflection curve). For such a porous gasket material, this limited thickness recovery should correspond to limited porosity recovery during unloading, which in turn prevents significant increase of leakage by fluid permeation, provided that the gasket remains in good contact with flange surfaces to avoid tangential leakage. This is the case for the Parts B1 and B2 cycles (Fig. 10) where the leak rate only increased by about 3–5 times when the gasket was decompressed from S3 and S4 levels to S1, a stress reduction of ∼5–8 times. For the Part B3 unloading curve, the final leakage at S1 is significantly higher than the initial leakage at S5. This sudden increase of leakage is probably due to the lack of recovery at this compressive state at which the gasket could no longer maintain a good contact with flange surfaces, causing a significant increase in the tangential leakage. However, this
phenomenon occurs only when the gasket is compressed to a very good sealablity level, a state that cannot be reached by other types of PTFE-based gaskets, as demonstrated in Section 3.7. 3.7. Comparison with other PTFE-based gasket materials In this section, the ROTT test results of the expanded PTFE will be compared with data from other traditional PTFE-based materials. These materials include a skived virgin (i.e. pure) PTFE sheet and a filled PTFE sheet, both of which were tested in the PVRC PTFE Gasket Qualification Project [11]. The former material was skived from a PTFE log sintered at high temperatures, while the latter was formed by calendering mixtures of PTFE powders and barium sulfate fillers. Notice that the filled PTFE discussed here is perceived as one of the better performers among all filled PTFE gasket materials. As indicated in Section 1, both of these materials are expected to show little mechanical strength under compressive loads.
Fig. 11. ROTT deflection graph for expanded PTFE and other PTFE materials.
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Fig. 12. ROTT Part A leakage graph for expanded PTFE and other materials.
Fig. 11 compares the deflection data for the three PTFE-based gaskets. The two tests for each material produced relatively similar results (with some variations) for our comparative purpose, so we present only the results from test 1 in Fig. 11. Because the initial thickness of the tested 10 mm expanded PTFE sample was 4.7 mm, different from that of the other materials (∼3 mm), for a fair comparison, the deflection values have been normalized against the initial thickness of each material. The figure shows drastically different deflection behaviors among the three PTFE materials. In general, the expanded PTFE is highly compressible at low stresses (over 65% of the initial thickness at S1), while maintaining its thickness at high compressive loads during the dwell times. The total thickness loss between the S3 and S5 levels is just 6% of the initial thickness, this is the combination of the small compressibility and creep at these high stresses. In contrast, the other two PTFE-based materials show small initial thickness losses in response to stress increases, but large continuos thickness losses when held at constant stresses. For example, the skived and filled PTFE were compressed by only 5 and 2% at S1, and showed total deflection of 24 and 15% above S3. These results suggest that the expanded PTFE has best compressibility and creep resistance among the three compared gasket materials. For the Parts B cycles, the deflection patterns indicate that all three PTFE materials show very limited recovering capability after prolonged services, a property typical of PTFE in general. Due to complex leakage behavior for a gasket involving both unload and reload processes with fluid pressure varying between 27 and 54 bar, the leakage data for Parts A and B will be separately illustrated in Figs. 12 and 13 to avoid confusion. Fig. 12 compares the Part A leakage data of the three PTFE-based materials. Again, the two tests of each material produced relatively comparable results for our purpose, so Fig. 12 shows only the test 1 results for each material.
As illustrated in Fig. 12, the expanded PTFE produced relatively high leak rates at low stresses where the porosity of the material was still high, but it showed a significant sealability improvement with stress increase between S1 and S3, essentially becoming the tightest among all tested gaskets above ∼30 N mm−2 . The filled PTFE seals slightly better than the expanded PTFE at S1, but quickly become worse before reaching the S2 level. It also leaks more than the skived virgin PTFE until at S5 where the two perform similarly. The skived PTFE shows a relatively good sealability at S1, due to its low initial porosity. However, its leakage level does not decrease much from here, showing essentially no improvement above S2. This very limited leakage change over a wide stress range is referred to as “tightness hardening”, a significant drawback for the skived PTFE. Fig. 13 illustrates unloading leakage characteristics of the three PTFE gaskets. In this graph, the leakage of material
Fig. 13. ROTT Part B leakage graph for expanded PTFE and other materials.
J. Huang, Y.-H. Lee / Materials Chemistry and Physics 70 (2001) 197–207
at the beginning of an unloading cycle is plotted against the final leak rate at the lowest stress of the cycle at S1. Parameter R in the figure indicates the degree of leakage recovery during decompression, and is the ratio of the final leak rate to initial leak rate. The 45◦ straight lines are contours for various values of R. The graph confirms that the expanded PTFE could achieve a much better initial sealability (∼10 times at least) than the other materials at S3 or above, that the skived PTFE shows severe tightness hardening behavior, and that the filled PTFE performs worst within the range of gasket stresses tested. Upon unloading, the majority of data from all the PTFE samples are near or below the line of R = 10, indicating a relatively small leakage increase during decompression, consistent with the low recovery for the three materials. Again, the sudden increase of leakage at the B3 cycles for the expanded PTFE is probably due to an increase in tangential leakage. However, this should not cause a major concern in terms of usability of the material, because it occurs only at a very high compressive state. At S4 and below where this problem does not exist, the achievable sealability level of the expanded PTFE is still significantly higher than any other PTFE materials discussed here. This good assembly sealability combined with the low leakage recovery upon unloading makes the expanded PTFE a good gasket product against stress variations during operation conditions. 3.8. Expanded PTFE used for fluid sealing applications As a general summary of properties of the uni-axially expanded PTFE, the material is a low density from (0.45–0.55 g cm−3 ) of PTFE, soft but strong, highly compressible with low recovery. It is chemically inert, and resistant to cold-flow and creep relaxation. It also has high thermal stability, and relatively high mechanical strength at elevated temperatures compared to the traditional PTFE materials. The improved mechanical properties of the expanded PTFE result from its microstructure (Figs. 4–6) created by the expansion process. The softness and high compressibility is a result of the large quantity of micropores, and the strength is enhanced by existence of the node-interconnecting fibrils. It is also due to this highly porous structure that the expanded PTFE can only have limited thickness recovery, but this low recovery may correspond to low leakage sensitivity of the material to bolt-load reductions, a good physical property of sealing performance. The combination of all the good physical and chemical properties leads to excellent sealing performance of the expanded PTFE for gasketing applications. High compressibility of the material is essential for proper installation of a gasket, and is critical to compensate for any flange irregularities such as nicks, non-parallelism, corrosion or variations in groove depth. This eliminates the path for tangential leakage between the gasket and flange surface. Good blot-load retention reduces the need to constantly re-torque the bolts,
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while high sealability eases the environmental concern on serious fugitive emission. Notice that due to the high porosity, the most effective sealing occurs when the voids are sufficiently closed up, and this ensures proper seating of the gasket. In addition to the excellent properties of the expanded PTFE as a gasket material, an adhesive tape can be further attached on these expanded PTFE bands, so that they can be easily attached to flange surfaces. This offers a simple solution to many sealing problems, especially when maintenance personnel are pressed for time but the correct dimension of a sheet-based gasket is not available. With this type of gasket application, no accurate dimensioning is necessary.
4. Conclusion The mechanical properties of PTFE products have been improved through the application of the expansion process. The new method used in this study includes extruding the paste through an extrusion die at a reduction ratio of less than 100, calendering the extrudate, removing the lubricant, and stretching the extrudate at a temperature greater than the melting temperature of PTFE. The product characterized by node-interconnecting fibrils was observed with SEM, and this fibrillation structure decisively results in improved mechanical strength and sealing performance of the expanded PTFE against traditional PTFE materials. These advantages of the expanded PTFE have been confirmed by the ROTT test results, making it one of the most versatile gasketing materials available today for difficult fluid sealing applications. References [1] M.W. Brown, Seals and Sealing Handbook, Elsevier, England, 1990, p. 82. [2] DuPont, The DuPont Plunkett Awards for Innovation with Teflon® , 1995. [3] J. Scheirs, Modern Fluoropolymer, Wiley, London, 1997, Chapter 1, pp. 2–3. [4] J. Latte, D. Coomber, Industrial gaskets, in: J. H. Bickford (Ed.), Gaskets and Gasketed Joints, Marcel Dekker, New York, 1997, pp. 87–122. [5] J. Huang, et al., Process for Forming an Expanded Porous Tetrafluoroethylene Polymer, US patent 5,098,625 (1992). [6] M. Derenne, J.R. Payne, L. Marchand, A. Bazergui, PVRC/MTI technology for characterizing gaskets used in bolted flanged connections, in: J. H. Bickford (Ed.), Gaskets and Gasketed Joints, Marcel Dekker, New York, 1997, pp. 137–302. [7] M. Derenne, J. R. Payne, Guide to the PVRC ROTT Test, PVRC Project, TTRL Ecole Polytechnique, University of Montreal, Montreal, Canada, 1998. [8] A.G. Hoechst, Hoechst Plastics (HOSTAFLON), 1984, Chapter 3, p. 7. [9] J. Scheirs, Modern Fluoropolymer, Wiley, London, 1997, Chapter 1, p. 31. [10] Fluid Sealing Association, Non-Metallic Gasketing Handbook, 1989. [11] PTFE Gasket Qualification Project — Final Report, TTRL Ecole Polytechnique, Montreal, Canada, 1995.