JP5265119B2 - gasket - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To inhibit the creep of an oriented porous PTFE highly. <P>SOLUTION: This oriented porous polytetrafluoroethylene sheet has a thickness of &ge;0.5 mm and is so controlled as to have a porosity of 5-20%. The porosity is, for example, attained by the compaction treatment after orientation. The size of the sheet has, for example, a width of &ge;1 m and a length of &ge;1 m. A gasket is produced by cutting the sheet. In this gasket, a compressible section formed for adaptability is made from the oriented porous polytetrafluoroethylene of the porosity of 5-20%. <P>COPYRIGHT: (C)2008,JPO&amp;INPIT

Description

  The present invention relates to a sheet gasket useful for sealing piping and flanges of various devices.

  Conventionally, a sheet gasket has been widely used as a gasket for sealing the flange. The sheet gasket is obtained by punching various sheet members such as a joint sheet (a sheet-like consolidated body made of various rubbers, fibers and fillers) and a rubber sheet in accordance with the shape of the flange. However, these joint sheets and rubber sheets are inferior in chemical resistance and purity.

  As a gasket excellent in chemical resistance and purity, a gasket obtained by cutting out from a polytetrafluoroethylene (PTFE) sheet is known. For example, a PTFE sheet (referred to as a skived PTFE sheet) obtained by cutting a compression molded body of PTFE molding powder is widely used. A skived PTFE gasket is obtained by punching a skived PTFE sheet into a ring shape. However, the skived PTFE gasket has poor creep resistance, and is particularly difficult to use at high temperatures (for example, at a temperature of 100 ° C. or higher). Furthermore, since the skived PTFE gasket is hard, the conformability (followability) to the fine irregularities of the flange is low.

  It is known that stretched porous PTFE gaskets (hereinafter referred to as ePTFE gaskets) have improved creep resistance and excellent conformability compared to skived PTFE gaskets (Patent Documents 1 to 3, etc.). This ePTFE gasket is obtained by mixing and molding PTFE fine powder and a molding aid, removing the molding aid, stretching at a high temperature and a high speed, and firing as necessary. Since it is manufactured through the stretching process, the tensile strength in the stretching direction is improved, and as a result, the creep resistance is also improved. Further, the ePTFE gasket obtained in this way is made porous, and the porosity is, for example, about 60 to 80%. Moreover, since it is made porous, it is softened, and the familiarity with the unevenness of the flange is enhanced. However, ePTFE gaskets are flexible and have no waist. Therefore, high density ePTFE sheet gaskets with increased density and improved rigidity are commercially available ("SGM (registered trademark)" manufactured by Japan Gore-Tex Co., Ltd., "SQ-S" V "RIGID manufactured by Inertec). "," Multitex (registered trademark) -rigid "manufactured by KWO, etc.). These high-density ePTFE sheet gaskets have a porosity of about 23 to 60%, and have an appropriate rigidity without significantly impairing the conformability of the flanges.

  In addition, an ePTFE sheet in which the holes of the ePTFE are completely crushed to be substantially non-porous, and a gasket using the non-porous ePTFE sheet are also known (Patent Documents 4 to 6). However, the relationship between the porosity of ePTFE gaskets and gasket properties, such as creep resistance, sealing properties, compressibility, and restoring properties, has not been elucidated. Further, when the holes are completely crushed, they become hard like the skived PTFE gasket, so that the familiarity (followability) becomes insufficient.

Further, Patent Literature 7 discloses a gasket containing ePTFE that is substantially non-porous, that is, a porosity of 0%, and Patent Literatures 8 to 10 disclose a gasket containing high-density ePTFE. Yes. However, in these patent documents 7 to 10, non-porous ePTFE and high-density ePTFE are always combined with low-density ePTFE, and the familiarity of the gasket is achieved by using this low-density ePTFE for the compressible portion.
Japanese Patent Laid-Open No. 11-80705 (paragraph 0023, etc.) Japanese Unexamined Patent Publication No. 2003-120815 (paragraph 0031 etc.) JP 2004-003617 A (paragraph 0038, etc.) JP-T-8-505094 (Claims) JP-A-10-237203 (paragraphs 0048 to 0050) JP 11-503080 A (Claims, page 6, lines 16-18) JP-A-2004-245286 (paragraphs 0032, 0034, 0043, etc.) Japanese Patent Laid-Open No. 04-83977 (Claims) JP 05-99343 A (Claims) JP 07-217743 A (Claims)

  Since the ePTFE has been increased in strength, the creep resistance is certainly improved as compared to the skived PTFE. However, it has been found that ePTFE having voids always causes a certain degree of creep phenomenon, and this creep phenomenon cannot be improved by increasing the strength.

  The present invention has been made paying attention to the above-described circumstances, and an object thereof is to establish a technique capable of preventing the creep phenomenon to a higher degree.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have conducted an extension process for increasing the strength, although increasing the strength is inherently important for improving the creep resistance. It has been found that the voids generated by the porous structure increase the creep phenomenon. That is, the creep phenomenon is a phenomenon similar to a conventional creep (hereinafter referred to as a true creep) at first glance, but is a phenomenon that is mechanically different from a true creep, which is an apparent creep. In true creep, when tightening in the thickness direction (longitudinal direction), the thickness decreases with time, and the area increases because the material flows in the surface direction (lateral direction) (see FIG. 1). . When a gasket made of ePTFE was tightened, air was gradually discharged from the holes in the ePTFE, and the holes were crushed, resulting in compression deformation of the material. This compressive deformation is regarded as creep in that it progresses with time even if the tightening load is constant. However, in the case of ePTFE, the expansion of the area was small even after compression deformation (see FIG. 2). From this, it was found that the cold flow of the material itself did not occur so much and that it was not a true creep mechanically. The present inventor has determined that the creep phenomenon that cannot be improved even by increasing the strength is correctly based on the decrease in the pore volume that gradually progresses with time, and controls the porosity of ePTFE. As a result, it was found that apparent creep can be prevented to a high degree, and the present invention has been completed.

The sheet of the present invention is composed of stretched porous polytetrafluoroethylene having a thickness of 0.5 mm or more, and has a gist in that the porosity is controlled to 5 to 20%. The porosity is achieved, for example, by a consolidation process after stretching. In the sheet of the present invention, the compression rate (JIS R 3453) is, for example, 7% or more, and the restoration rate (JIS R 3453) is, for example, 40% or more. The sheet has a stress relaxation rate L (%) measured based on JIS R 3453 and a stress relaxation rate L ₀ (%) measured based on JIS R 3453 when the porosity is 0%. A value L _x (= (L−L ₀ ) / t) obtained by dividing the difference by the sheet thickness t (mm) is, for example, about 4 to 10% / mm.
The size of the sheet of the present invention is, for example, a width of 1 m or more and a length of 1 m or more, and a gasket can be cut out from this sheet.

The present invention also includes a gasket in which a compressible portion formed for conformability is formed of expanded porous polytetrafluoroethylene having a porosity of 5 to 20%. The compression ratio (JIS R 3453) of the compressible portion is, for example, 7% or more, the restoration ratio (JIS R 3453) is, for example, 40% or more, and the value L _x is, for example, 4-10%. / Mm or so. The thickness of the compressible portion is, for example, 0.5 mm or more.

  According to the gasket sheet of the present invention, since the porosity of the stretched porous polytetrafluoroethylene is controlled within a predetermined range, the creep phenomenon can be prevented to a higher degree while ensuring the familiarity.

  Moreover, according to the gasket sheet of the present invention, since the porosity of the stretched porous polytetrafluoroethylene is controlled within a predetermined range, it is possible to impart an appropriate rigidity and improve handling properties. Therefore, for example, it can also be used instead of an asbestos joint sheet.

  In addition, according to the gasket sheet of the present invention, the porosity of the stretched porous polytetrafluoroethylene is controlled to a predetermined value or less, so that the amount of tightening at the time of mounting on the seal portion can be reduced, and the working time can be reduced. Can be shortened.

  Furthermore, according to the gasket sheet of the present invention, since the porosity of the stretched porous polytetrafluoroethylene is controlled to a predetermined value or less, the restoration rate is high compared to the conventional ePTFE gasket, and the piping during operation Excellent followability to expansion and contraction. Moreover, since the porosity is controlled to a predetermined value or higher, the compression rate is higher than that of skived PTFE, and interface leakage can be reduced.

(1) ePTFE sheet TECHNICAL FIELD This invention relates to an ePTFE sheet. PTFE is used to improve chemical resistance and purity when used as a gasket. The reason for stretching is to increase the tensile strength of PTFE and reduce true creep. The expanded PTFE has a so-called node fibril structure. A node means a granular part or an island-like part made of a PTFE folded crystal, and a fibril is a PTFE drawn out in a fiber form from a node, and connects each node in a comb or spider web shape. means. In addition, in the ePTFE sheet of the present invention, as will be described later, a consolidation treatment is performed to reduce the porosity. Therefore, it is difficult to directly observe the node fibril structure, but the influence of the node fibril structure remains, and as a result, the tensile strength is high.

  The present invention is characterized in that the porosity of the ePTFE sheet is controlled to 5 to 20%. While stretching PTFE, by setting the porosity to 20% or less, both true creep and apparent creep can be reduced to a high degree, and the entire creep phenomenon consisting of both of these phenomena can be highly prevented. Furthermore, by setting the porosity to 20% or less, the sheet can have an appropriate rigidity, and handling properties can be improved. Furthermore, the amount of tightening when used as a gasket can be reduced, and the mounting work time can be shortened. In addition, the restoration rate when used as a gasket can be increased, and the followability to expansion / contraction of piping during operation is excellent. A preferable porosity is 18% or less, particularly 15% or less.

  In the present invention, the pores are not completely crushed, but a porosity of at least 5% is secured. Therefore, a predetermined compression ratio or more can be ensured, and familiarity with unevenness can be maintained. Therefore, a leakage phenomenon (interface leakage) passing between the sealing surface and the gasket when used as a gasket can be suppressed. According to JIS standards, the arithmetic average roughness (Ra) of the finished surface of the steel flange (JIS B 2220) is 3.2 to 6.3 μm, and the finished surface roughness Ra of the cast iron flange (JIS B 2239) is 3. .2 to 12.5 μm, and by setting the porosity to 5% or more, it is possible to suppress the interface leakage of the seal surface with this degree of roughness. However, the actual flange becomes rougher than the finish roughness due to corrosion and scratches associated with use. In consideration of such rough surface sealing properties, the porosity is preferably 7% or more, more preferably 10% or more.

The porosity is the apparent density D (D = W / V: unit is g / cm ³ ) obtained by measuring the mass W of the porous PTFE and the apparent volume V including the pores. The value is calculated based on the following formula (1) using the true density D _{true of} PTFE.
Porosity (%) = [1- (D / D _true )] × 100 (1)

In the present invention, the representative value (2.17 g / cm ³ ) described in Table 2.4 of “Fluorine Resin Handbook” edited by Japan Fluoropolymer Industry Association is adopted as the true density D _true . When the apparent density D exceeds 2.17 g / cm ³ , the porosity is substantially 0%.

  As described above, the apparent creep of the ePTFE sheet of the present invention is reduced by setting the porosity to 20% or less. In order to quantitatively evaluate creep, the stress relaxation rate measured based on the stress relaxation test defined in JIS R 3453 is used (this is the original stress relaxation test defined in JIS R 3453). In ASTM F-38, the stress relaxation rate is referred to as creep relaxation rate).

More specifically, the degree of apparent creep reduction is determined by the stress relaxation rate L (%) of the ePTFE sheet and the stress relaxation rate L ₀ (%) after the porosity of the sheet is reduced to 0%. It can be quantified by the difference (L−L ₀ ). The stress relaxation rate L generally represents the entire creep phenomenon consisting of both true creep and apparent creep. However, when the porosity is 0%, there is no apparent creep, so the stress relaxation rate L ₀ when the porosity is 0% indicates true creep. Therefore, the apparent creep can be quantified by calculating the difference L _a = (L−L ₀ ). The sheet having a porosity of 0% for measuring the true stress relaxation rate L ₀ is obtained by, for example, rolling the evaluation object sheet a plurality of times at room temperature as shown in Experimental Example 1 described later. Can be prepared.

However as is apparent from the above description, L _a is proportional to the pore volume (Soraanaryou). Thus, the ePTFE sheet of the same porosity, the value of the larger thickness L _a increases. The relative merits of the apparent creep suppression, for the like can be also compared between different ePTFE sheet thicknesses, the L _a of ePTFE sheet thickness t (mm) at a value obtained by dividing L _x (see the following formula) of creep The degree of inhibition was evaluated.
L _x = L _a / t = (L−L ₀ ) / t

In the ePTFE sheet of the present invention, L _x is, for example, about 4 to 10% / mm, preferably about 5 to 9% / mm, and more preferably about 6 to 8% / mm. It can be said that apparent creep can be suppressed as the value of L _x is smaller. However, if L _x is too small, the conformability of the ePTFE sheet is lowered and the interface leakage is increased.

The L _a is, for example, about 10% -25%, preferably about 12-23%, more preferably about 15-17%. As L _a smaller apparent creep is reduced. However, L _a is too small, reduces the conformability of the ePTFE sheet, surface leakage is increased.

In the ePTFE sheet of the present invention, it is preferable that the stress relaxation rate L ₀ representing true creep is as low as possible. Since the apparent creep is reduced in the present invention, the entire creep phenomenon (L) can be greatly reduced by reducing the true creep (L ₀ ).

  The ePTFE sheet of the present invention also has appropriate compression recovery characteristics (compression rate and recovery rate) as a gasket. The compression rate is, for example, 7% or more, preferably 9% or more, and more preferably 10% or more. The higher the compression ratio, the easier it is to absorb the irregularities on the seal surface and the interface leakage can be reduced. However, if the compression rate is too high, the tightening amount at the time of mounting becomes large, and workability is lowered. Further, the rigidity of the sheet is lowered, and it becomes difficult to insert the gasket into the narrow flange gap. Therefore, the upper limit of the compression rate is, for example, about 18%, preferably about 16%, and more preferably about 15%.

  The restoration rate is, for example, 40% or more, preferably 43% or more, and more preferably 45% or more. A member to be sealed such as piping repeatedly contracts and expands due to various external factors such as external stress, thermal cycle, and pressure cycle, and can follow the contraction and expansion as the restoration rate increases.

The matrix strength of the ePTFE sheet of the present invention is, for example, 50 MPa or more, preferably 75 MPa or more, and more preferably 100 MPa or more. In the most preferable ePTFE sheet, the matrix strength is increased and the true creep (L ₀ ) is remarkably reduced. In this most preferred ePTFE sheet, the matrix strength is, for example, 125 MPa or more, preferably 150 MPa or more, and more preferably 175 MPa or more.

The matrix strength is the apparent density D (D = W / V: unit is g / cm ³ ) obtained by measuring the tensile strength of the ePTFE sheet and the apparent volume V including the voids, and PTFE. This is a value calculated based on the following formula (2) using the true density D _true .
Matrix strength (MPa) = [Tensile strength (MPa)] × D _true / D (2)

  The thickness of the ePTFE sheet of the present invention is 0.5 mm or more, preferably 1 mm or more, more preferably 1.5 mm or more. The greater the sheet thickness, the greater the amount of irregularities absorbed on the seal surface when used as a gasket. In general, the creep phenomenon becomes more noticeable as the sheet thickness increases. However, since the creep phenomenon is reduced in the present invention, the effect of creep can be reduced as much as possible even if the sheet thickness is increased. Although the upper limit of sheet thickness is not specifically limited, For example, it is about 10 mm, Preferably it is about 5 mm, More preferably, it is about 3 mm.

  The size of the ePTFE sheet of the present invention can be appropriately set as long as the gasket can be cut out. For example, the width is 1 m or more (preferably 1.2 m or more, more preferably 1.4 m or more), and the length is 1 m. This is preferably (1.2 m or more, more preferably 1.4 m or more). The upper limit of the width and length is not particularly limited as long as it can be produced, but is, for example, 3 m or less, preferably about 2 m or less.

  The ePTFE sheet of the present invention is prepared by drawing an ePTFE film having a high porosity (for example, more than 20%, preferably 50 to 90%, more preferably 70 to 85%) by stretching according to a conventional method. Are laminated to form a sheet (hereinafter referred to as an original sheet), and then the original sheet is subjected to a consolidation treatment until a predetermined porosity is obtained. The consolidation treatment may be performed when ePTFE films are laminated to form a sheet. Since the film is once highly stretched until it has a high porosity, the sheet can be strengthened and the true creep can be reduced. The magnification at the time of stretching (stretched area magnification) is not particularly limited as long as the porosity can be achieved, but is, for example, about 10 to 300 times, preferably about 20 to 250 times, and more preferably about 50 to 200 times. . Biaxial stretching is preferable, but uniaxial stretching may be used.

The ePTFE sheet of the present invention most preferably has an increased matrix strength as described above, and a true creep (L ₀ ) is drastically reduced. This ePTFE sheet is achieved by setting the stretched area magnification at the time of manufacturing the ePTFE sheet high, preferably by setting the stretched area magnification high while slowing the stretching speed. The stretched area magnification can be set within a range where the stretched area becomes, for example, 70 times or more, and the stretching speed is, for example, 500% / second or less, preferably 200% / second or less, more preferably about 100% / second or less. It is.

  In the consolidation process, various known methods can be employed. For example, the compaction process may be performed by a press or may be rolled by a roll (calender roll or the like). Moreover, you may utilize a fluid pressure, as described in the Japanese translations of PCT publication No. 8-505094 and the Japanese translations of PCT publication No. 11-503080. A preferred method is roll rolling.

  The present invention also includes a gasket. This gasket has the same characteristics as the ePTFE sheet of the present invention in the compressible portion formed for familiarity. In particular, it is desirable that the softest part of the gasket has the same characteristics as the ePTFE sheet of the present invention.

  In the gasket of the present invention, the surface may be covered with a skived PTFE film or an ePTFE film having a porosity of 0% in order to prevent penetration leakage more highly.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Experimental Example An ePTFE sheet (“NSG64X” manufactured by Japan Gore-Tex Co., Ltd., nominal thickness 6.4 mm, nominal size 1.5 m × 1.5 m, porosity 72%) is rolled multiple times under the conditions shown in Table 1 below. Rolled and consolidated. The obtained consolidated sheet was shrunk by a compression load, and the length and width of each sheet shrunk to about 1.43 to 1.48 m.

  The thickness, apparent density, and porosity of the sheet before and after rolling were determined as follows. Further, various properties (sealing property, compression property, restoring property, stress relaxation property, matrix strength) of the consolidated sheet obtained in each experimental example were evaluated as follows.

1) Thickness A sample having a diameter of 30 mm was cut out from a sampling portion provided with 15 points in the longitudinal direction and 15 points in the lateral direction (15 × 15 = 225 points in total). Using a dial thickness gauge with a scale of 1/100 mm (manufactured by TECLOCK Co., Ltd.), the thickness of each sample was measured without applying a load other than the main body spring load, and the average value was taken as the thickness of the sheet.

2) Apparent density and porosity The mass of the sheet was measured, and the apparent density (g / cm ³ ) and the porosity were calculated based on the following formula. When the apparent density exceeds 2.17 g / cm ³ , it is assumed that the porosity is substantially 0%.
Apparent density (g / cm ³ ) = Mass of sheet / (Vertical length of sheet × Horizontal length of sheet × Thickness of sheet)
Porosity (%) = (1−apparent density / 2.17) × 100

3) Sealability The compacted sheet obtained in each experimental example was punched out to obtain a ring gasket having an inner diameter of 35 mm and an outer diameter of 65 mm. The obtained gasket was inserted between a pair of platens and clamped at a pressure of 20 MPa or 40 MPa. 2 MPa helium gas was supplied to the inside of the gasket, and the amount of helium gas leaking to the outside of the gasket was measured with a soap film flow meter. In addition, the sealing performance includes a platen with a smooth sealing surface (arithmetic average roughness Ra = 0.9 to 1.1 μm) and a platen with a rough sealing surface (arithmetic average roughness Ra = 7 to 9 μm). Evaluation was made in two cases, with and without the gasket.

4) Compression property and restoration property The compression rate and restoration rate of the compacted sheet obtained in each experimental example were determined according to JIS R 3453.

5) Stress relaxation characteristics The stress relaxation rate L (%) of the compacted sheet obtained in each experimental example was determined according to JIS R 3453. The apparent stress relaxation rate L _a (%) and the apparent stress relaxation rate L _x (% / mm) per unit thickness were calculated based on the following equations.
L _a = L−L ₀
L _x = (L−L ₀ ) / t
(In the formula, L ₀ represents the stress relaxation rate of the consolidated sheet of Experimental Example 1 (porosity 0%), and t represents the thickness (mm) of the consolidated sheet of each Experimental Example).

6) Matrix strength A dumbbell-shaped No. 3 test piece defined in JIS K 6251 was punched from the compacted sheet obtained in each experimental example from the vertical direction and the horizontal direction. The test piece was pulled at a tensile speed of 200 mm / min, and the load when it broke was divided by the initial cross-sectional area of the test piece to determine the tensile strength. Based on the following formula, the matrix strength in the vertical and horizontal directions was calculated, and the average value was defined as the matrix strength.
Matrix strength (MPa) = tensile strength (MPa) × 2.17 / apparent density

Table 1 shows the measurement results of sheet thickness, apparent density, and porosity.
Table 2 and FIGS. 3 to 8 show the evaluation results of sealing properties, compression properties, restoration properties, stress relaxation properties, and matrix strength.

  As apparent from Tables 1 and 2 and FIGS. 3 to 4, when the porosity is too large, the amount of leakage on the smooth surface (Ra = 0.9 to 1.1 μm) increases. On a smooth surface, the penetration leakage is more dominant than the interface leakage. If the porosity is too large, it seems that the penetration leakage cannot be prevented. On the other hand, if the porosity is too small, the amount of leakage on the rough surface (Ra = 7 to 9 μm) increases. In the rough surface, the interface leakage is more dominant than the permeation leakage, and it seems that when the porosity decreases, the conformability of the gasket decreases and this interface leakage increases. From the viewpoint of preventing permeation leakage, the smaller the porosity, the better. However, actual flanges are often close to rough surfaces due to corrosion and scratches associated with use, and interface leakage is often dominant. For this reason, it is necessary to set the porosity to a predetermined value or more.

  As apparent from Tables 1 and 2 and FIGS. 5 to 6, the sheet or gasket of the present invention in which the porosity is defined within a predetermined range has an appropriate compression rate and restoration rate.

The Table 1 to Table 2 and FIGS. 7 to 9, the stress relaxation ratio L, seeking stress relaxation rate L _a apparent by subtracting the stress relaxation rate L ₀ when the porosity 0%. The seeking apparent stress relaxation rate L _x per unit thickness by dividing the L _a sheet with a thickness t. The results of this L _a and L _x, the larger the porosity, it can be seen that the creep caused by the holes (creep apparent) increases.

When the stress relaxation rate L of Experimental Example 6 having a porosity of 18% and Experimental Example 7 having a porosity of 23% is compared, the rate of increase in stress relaxation rate [(L of Experimental Example 7−L of Experimental Example 6] ) / L in experimental example 6] is only 11.8%, when compared with example 6 in apparent stress relaxation rate L _a values of example 7, an apparent stress relaxation ratio L _a rate of increase in the [ L _a] of / experimental example 6 - (L _a of example 7 L _a of example 6) is also 43.7%.

  This apparent creep cannot be suppressed even if true creep is reduced by increasing the strength of ePTFE. Therefore, in order to reduce the apparent creep and improve the creep phenomenon, the porosity should be a predetermined value or less.

FIG. 1 is a schematic view for explaining the concept of creep. FIG. 2 is a schematic diagram for explaining the concept of apparent creep. FIG. 3 is a graph showing the relationship between the porosity of the gasket and the leak amount when the smooth surface is sealed with the gasket. FIG. 4 is a graph showing the relationship between the porosity of the gasket and the leak amount when the rough surface is sealed with the gasket. FIG. 5 is a graph showing the relationship between the porosity and compression rate of the ePTFE sheet. FIG. 6 is a graph showing the relationship between the porosity and the restoration rate of the ePTFE sheet. FIG. 7 is a graph showing the relationship between the porosity of the ePTFE sheet and the stress relaxation rate L. FIG. 8 is a graph showing the relationship between the porosity of the ePTFE sheet and the apparent stress relaxation rate (L _a = L−L ₀ ). FIG. 9 is a graph showing the relationship between the porosity of the ePTFE sheet and the apparent stress relaxation rate per unit thickness (L _x = (L−L ₀ ) / t).

Claims (12)

A gasket made of expanded porous polytetrafluoroethylene having a thickness of 0.5 mm or more and a porosity of 5 to 20%. The gasket according to claim 1, wherein the porosity is achieved by a consolidation treatment after stretching. The gasket according to claim 1 or 2, wherein a compression ratio based on JIS R 3453 is 7% or more. The gasket according to any one of claims 1 to 3, wherein a restoration rate based on JIS R 3453 is 40% or more. The difference between the stress relaxation rate L (%) measured based on JIS R 3453 and the stress relaxation rate L ₀ (%) measured based on JIS R 3453 when the porosity is 0% is the sheet thickness 5. The gasket according to claim 1, wherein a value Lx (= (L−L ₀ ) / t) divided by a thickness t (mm) is 4 to 10% / mm. The gasket according to any one of claims 1 to 5, which has a width of 1 m or more and a length of 1 m or more. Compressible portion formed for conformability is made of expanded porous polytetrafluoroethylene with a porosity of 5-20% gasket. The gasket according to claim 7 , wherein the porosity is achieved by a consolidation treatment after stretching. The gasket according to claim 7 or 8 , wherein the compressibility of the compressible portion based on JIS R 3453 is 7% or more. The gasket according to any one of claims 7 to 9 , wherein a restoration rate of the compressible portion based on JIS R 3453 is 40% or more. The gasket according to any one of claims 7 to 10 , wherein the compressible portion has a thickness of 0.5 mm or more. The stress relaxation rate L (%) of the compressible portion measured based on JIS R 3453, and the stress relaxation rate L _{0 of the} compressible portion measured based on JIS R 3453 when the porosity is 0%. the difference of the sheet thickness (%) t (mm) at a value obtained by dividing _{Lx (= (L-L 0} ) / t) is, according to one of claims 7 to 11, which is a 4 to 10% / mm gasket.

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