JPH056495B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial application field] Tetrafluoroethylene polymer (hereinafter referred to as PTFE)
) is currently being used due to its outstanding characteristics.
It has become one of the important industrial materials. Sometimes, in the field of sealing materials such as gaskets and packing, PTFE is used for its chemical resistance, heat resistance, cold resistance, low friction,
It is widely used as products in various shapes due to its excellent properties such as non-contamination, non-adhesiveness, and electrical insulation. The present invention relates to the above sealing material made of PTFE. [Prior art] The melt viscosity of normal thermoplastic resins at molding temperatures is 10 ³ to 10 ⁴ poise, but PTFE has a high viscosity of 10 ¹¹ poise even at 380°C, which is higher than its melting point (327°C), making it difficult to melt extrude. It is difficult to mold using molding methods such as molding or injection molding. Therefore, PTFE products are generally produced by a method in which PTFE powder is preformed by compression, etc., and then heated and fired to a temperature above the melting point (compression molding method,
(ram extrusion method, paste extrusion method, etc.), or the PTFE base formed in this process is further processed by cutting, cutting from the cut product, punching, stretching (roll rolling method, tension stretching method, etc.), etc. has been done. Furthermore, in the field of gasket materials, PTFE containing fillers such as glass fiber, graphite, carbon fiber, and zirconium oxide is also manufactured. [Problems to be solved by the present invention] The heat resistance temperature of PTFE is generally 260°C, but when used as a sealing material for gaskets, etc., it is required to have compression resistance and mechanical strength to withstand external forces. The maximum usable temperature is considerably lower than 260℃, depending on the structure, shape, and application. this is
This is due to the inherent property of PTFE that it tends to creep under load, and of course, the higher the temperature, the more likely it is to creep. In the field of corrosion-resistant gasket materials, improvements have been made to improve compression creep resistance, such as adding fillers or structurally combining low-creep materials with PTFE or filled PTFE, but none of them are effective at high temperatures. There are problems such as insufficient compression creep resistance and low chemical resistance. In addition, sealing materials other than gaskets, such as valve seals (ball valve seats, gate valve seats, etc.)
PTFE is also used when corrosion resistance is required for other dynamic sealing materials (Gland Packin, U Packin, V Packin, etc.), but it is also desired to improve compression creep resistance. Uniaxially stretched long sheets made by stretching and forming PTFE using the roll rolling method also have improved compression creep resistance, but the roll rolling method only allows uniaxial stretching in one rolling process, so it cannot be stretched in the high temperature range of 100℃ or higher. When compressive force is applied, the material contracts in the direction of the stretching axis in a plane perpendicular to the compressive force and stretches in the direction perpendicular to it, resulting in a substantial dimensional change. In addition, a low tension stretching method that does not involve rolling results in porous PTFE. The problem to be solved by the present invention is to improve the creep resistance of PTFE sealing materials, which are used in situations where compression resistance (sometimes compression creep resistance) and mechanical strength are required. The purpose is to improve the performance of PTFE sealing materials. [Means and effects for solving the problem] As a result of repeated studies by the present inventors to develop a PTFE molded product with excellent creep resistance and gas barrier properties, the highly biaxially oriented sheet of PTFE was found to be outstanding. The present invention was based on the discovery that it has excellent compression creep resistance and gas barrier properties. That is, the present invention is made from ultra-high molecular weight PTFE having a number average molecular weight of 5 million or more, which is stretched in at least two axial directions, and which is obtained by stretching a compacted and fired base material under pressure in a die. The material is highly oriented with an effective stretching ratio D of 3 times or more, a specific gravity of 1.8 or more, an orientation release stress of 5Kg/cm2 or ^more at 200℃, and a compression set S of 30% or less, 80 ℃, the shrinkage rate in 12 hours is less than 1.5%
It is a sealing material made of PTFE. The wall thickness of the PTFE sealing material of the present invention can be arbitrarily determined depending on the purpose, but is preferably
0.1mm thick or more, more preferably 0.25mm thick or more20
A thickness of mm or less is preferable. PTFE mentioned in the present invention refers to a homopolymer of tetrafluoroethylene (hereinafter abbreviated as TFE) or a copolymer containing TFE at 70 mol% or more, preferably 90 mol% or more, and a blend polymer consisting of these. Number average molecular weight is 5×
It is an ultra-high molecular weight substance of 10 ⁶ to 5×10 ⁷ . Particularly preferred is an ultra-high molecular weight homopolymer of TFE. Monomer components to be copolymerized with TFE include:
Perfluoroalkyl vinyl ether, hexafluoropropylene, ethylene, chlorotrifluoroethylene and the like are preferred. The standard specific gravity of molded products (hereinafter abbreviated as SSG) is generally used as a standard for the molecular weight of PTFE.
It is. SSG refers to the specific gravity of a sample formed from PTFE under certain heat treatment conditions specified in ASTM D-1457. There is a certain relationship between the SSG and number average molecular weight (n) of PTFE [RDDoban et al.:
Meeting of the Am.Chem.Soc., Atlantic City
(1956)], is expressed by the following formula ("Fluorine Resin", Nikkan Kogyo Shimbun). SSG=-0.0579logn+2.6113 In the same way, the number average molecular weight can also be determined from a stretched sheet or a PTFE base material before stretching processing. i.e. stretched sheet or PTFE
The specific gravity of the sample obtained by heat treating the substrate under the thermal conditions shown in ASTM D-1457 is made to correspond to SSG,
The number average molecular weight can be determined from the above relationship. In addition, the above PTFE is used as the main matrix resin component, and reinforcing materials include inorganic fillers such as glass fiber, carbon fiber, graphite, carbon, molybdenum disulfide, bronze, zirconium oxide, and zirconium silicate, and aromatic polyamide fibers. Filled PTFE containing at least 60% by weight of at least one organic filler such as aromatic polyester fiber is also included in the PTFE termed in the present invention. Furthermore, other thermoplastic resins and various compounding agents can be added to the PTFE in an amount of 10 parts by weight or less, if necessary. The PTFE sealing material of the present invention has the above-mentioned ultra-high molecular weight
It is composed of a sheet in which the PTFE is compressed and then fired, and then stretched in at least two axes under pressure in a die. PTFE ultra-high molecular weight material is generally a powder, which is compacted under high pressure by compression molding at room temperature, and then fired at a high temperature (generally above the melting point temperature of PTFE, preferably in the range of 340 to 400°C).
A molded product with a specific gravity of 1.8 or more, preferably 2 or more,
This is used as a substrate and stretched in at least two axes under pressure in a die. There are three types of sheet stretching: uniaxial stretching, biaxial stretching, and multiaxial stretching, and the stretched PTFE sheet used in the PTFE sealing material of the present invention is biaxially stretched or multiaxially stretched. Uniaxial stretching is generally performed by stretching in one direction or by roll rolling, and molecular orientation occurs in one direction. Biaxial stretching is performed by stretching in two directions perpendicular to each other.
The molecules are mainly oriented in two directions, the tensile direction. Multiaxial stretching is performed using the forming method described below, and is performed in all directions (360°C).
almost evenly oriented. The stretched sheet used in the PTFE sealing material of the present invention is particularly preferably multiaxially stretched. Uniaxially oriented sheets also improve compression creep properties, but substantial dimensional changes occur, making them impractical. In biaxially stretched sheets and multiaxially stretched sheets, such anisotropic dimensional changes are small and pose no practical problem. Especially in the case of multiaxially stretched sheets, there is no problem at all. In biaxial stretching, the directions of the stretching axes that are perpendicular to each other within the stretching plane (hereinafter abbreviated as "vertical and horizontal" for convenience)
Although the ratio of the stretching ratios can be set arbitrarily, it is preferable that the ratio of the stretching ratios of the vertical and horizontal stretching ratios of the stretched PTFE sheet used for the PTFE sealing material of the present invention is 1:3 to 1:1. , particularly preferably 1:1.5 to 1:1. Furthermore, in the case of multiaxial stretching, a multiaxially stretched sheet with substantially uniform stretching ratio in all directions is preferred. The multiaxially stretched sheet described herein has a ratio of the minimum value to the maximum value of the stretching ratio in each direction of 1:1 to 1:2, preferably 1:1 to 1:1.5, and more preferably 1:1 to 1. :
It is 1.2. When the ratio of the stretching ratio exceeds 1:5, it approaches uniaxial stretching, and in the temperature range exceeding 100°C
If a pressure exceeding Kg/cm ² is applied, an anisotropic dimensional change will occur even if the compression set itself shows a small value. This phenomenon is sometimes undesirable in the field of sealants. The stretched sheet used for the PTFE sealing material of the present invention has a substantial stretching ratio D of 3 times or more. Biaxial or multiaxial stretching of ultra-high molecular weight PTFE improves compression creep resistance. As a result of a detailed study on the relationship between the actual stretch ratio D and compression creep resistance, it was discovered that when the actual stretch ratio D is 3 times or more, the compression creep resistance is extremely excellent, leading to the present invention. A more preferable range is that the actual stretching ratio D is 4 times or more. In the range of the actual stretching ratio D of 5 times or less, the compression creep resistance improves in proportion to the actual stretching ratio D, and the larger the actual stretching ratio D is, the more preferable it is, and for practical purposes, a material with an actual stretching ratio D of 3 times or more is preferable. This is within a range that can be used satisfactorily. on the other hand,
When the actual stretch ratio D exceeds 5.0, the actual stretch ratio D becomes the compression creep resistance not only in the low temperature range but also in the high temperature range of 200℃ or higher and under high compression force conditions of about 500Kg/ ^cm2 . This reduces the dependence on compression creep and increases the stability of compression creep resistance. Moreover, if it exceeds 20, breakage tends to occur when attempting to stretch to the actual stretching ratio D, and stretching is essentially impossible. When considering stretching processability, the actual stretching ratio D is 3.
≦D≦20, furthermore 4<D≦15, especially 5.0<D≦
A range of 10 is preferred. The actual stretching ratio D, the stretching ratio in the stretching axis direction, the ratio of the longitudinal and horizontal stretching ratios, and the multiaxial stretching described in the present invention will be explained using FIGS. 4 and 5. The left side of Figures 4 and 5 shows the test piece cut out from the PTFE stretched sheet, and the right side shows the test piece cut out from the PTFE stretched sheet.
The test piece is schematically shown after being heat-treated under the thermal conditions specified in ASTM D-1457. _A1 ,
B ₁ ,..., H ₁ indicate the positions of A ₀ , B ₀ ,..., H ₀ after heat treatment. A circle of deformation d ₀ (for example, d ₀ =50 mm) as shown in the figure is drawn on the plane of a test piece (for example, 110 mm x 110 mm x T ₀ mm thickness) cut out from a PTFE stretched sheet at room temperature. When this is heat-treated in accordance with ASTM D-1457, the orientation state of the stretched sheet is released, and the PTFE is almost restored to its state before stretching. Naturally, the circle drawn on the plane also shrinks in accordance with the orientation state. When reduced to an elliptical shape as shown in FIG. 4, the major axis direction is the vertical stretching axis, and the minor axis direction is the horizontal stretching axis. The longitudinal and transverse stretching ratios are determined by the ratio of the lengths on each stretching axis before and after recovery, that is, OC ₀ /OC ₀ (or
OC ₀ /OC ₁ or O ₀ G ₀ /C ₁ G ₁ ) and OA ₀ /OA ₁ (or OE ₀ /OE ₁ or A ₀ E ₀ /A ₁ E ₁ ).
Also, what is the ratio of vertical and horizontal stretching ratios (vertical stretching ratio):
(Transverse stretching ratio) is expressed with the longitudinal stretching ratio being 1. As shown in FIG. 5, a PTFE stretched sheet is preferably used whose stretching ratio is approximately equal in all directions in a plane. Specifically, each stretching ratio (OA ₀ /OA ₁ , OB ₀ /OB ₁ , OC ₀ /
_OC1 , _OD0 / _OD1 , _OE0 / _OE1 , _OF0 / _OF1 ,
The standard deviation σ of OG ₀ /OG ₁ , OF ₀ /OF ₁ ) is σ≦0.9
The case of . Multiaxial stretching in the present invention is considered to be a special form of biaxial stretching, and the standard deviation σ of the stretching ratio is σ≦0.5, preferably σ≦0.2.
This refers to the case where . The substantial stretching ratio D in the present invention refers to the thickness ratio before and after heat treatment in FIGS. 4 and 5. That is, D=T ₁ /T ₀ . The PTFE sealing material of the present invention is a solid body with substantially no voids, and has a specific gravity of 1.8 or more, preferably
It is 2.0 or more, more preferably 2.1 or more. Specific gravity is measured according to ASTM D-792. The PTFE sealing material of the present invention is ultra-high molecular weight PTFE.
The sealing material is composed of a stretched sheet in which the sealing materials are sufficiently intertwined, and when the sealing material is heated, an ORS indicating the degree of orientation is generated. The ORS of the PTFE sealing material of the present invention at 200°C is required to be 5 kg/cm ² or more, preferably 7 kg/cm ² or more, especially 10 kg/cm 2 or more.
Kg/cm ² or more and 40 Kg/cm ² or less is preferable. at 200℃
When the ORS is less than 5 Kg/cm ² , the improvement in compression resistance and creep properties is generally insufficient. The ORS described here is a value measured according to ASTMD-1504, and the detailed measurement method is shown in FIG. The PTFE sealing material of the present invention has a substantial stretching ratio of D
In the range where the
The compression set S in cm ² ·1 hour falls within the range shown by the following formula. S(%)=-14D+(83±10) A more preferable range is S(%)=-14D+(83±8). In the range of the actual stretching ratio D of 5 times or more, the compression set S is generally 15% or less, more preferably 12% or less, and most preferably 10% or less. The compression set S (%) not only represents the compression creep of the PTFE stretched sheet under the conditions, but also
This serves as an indicator of the reliability of compression creep resistance under conditions that are gentler than the above conditions, or of the compression creep resistance under conditions that are more severe than the above conditions. Conventionally, roll rolling and tension stretching methods have been commonly used as stretching methods.
Among the stretched sheets, it is not possible to produce simultaneously bi-stretched sheets and multiaxially stretched sheets within the preferred range.
Furthermore, in the tensile stretching method, as is used to make PTFE porous, the generation and increase of voids during stretching is unavoidable. The present inventors investigated a molding method for stretching a PTFE material while suppressing the generation and increase of voids, and obtained the PTFE sealing material of the present invention. The stretching method is
This is a method in which a PTFE base heated to a temperature of 150°C or higher, preferably 170°C to 340°C, is stretched under pressure in a die. In particular, in order to obtain the highly oriented sealing material of the present invention with a stretching ratio of 5 times or more, it is necessary to
The PTFE substrate is heated to a high temperature of 340°C and stretched under pressure. An outline will be explained below. Stretching under pressure in a die means stretching the PTFE base material in a compression die or an extrusion die in compression molding or extrusion molding by causing a plug flow due to compression force or extrusion force. The term "plug flow" here includes states similar to that. Further, in order to cause plug flow, it is preferable to have a lubricant present at least on the inner surface of the die and on the surface of the PTFE substrate. PTFE substrate is a molded product made from PTFE molding powder through the general process of pre-compression molding, firing, and cooling, and particularly refers to a relatively thick sheet-like product. Prior to stretching, the PTFE substrate is preheated to 150°C or higher. The preferred preheating temperature is 170
-340°C, more preferably 200-327°C. The temperature may be uniform throughout, but it may be distributed such that the surface layer is hotter and the center side is colder, or only the surface layer is above the melting point of PTFE and the inside is below the melting point. You can also do it. The temperature of the die is such that the temperature of the inner surface of the die is at least (temperature of the surface layer of the PTFE substrate - 100) degrees Celsius or higher, preferably in the range from the temperature of the surface layer of the PTFE substrate to 400 degrees Celsius, taking into consideration stretch formability and productivity. Can be selected arbitrarily. When stretching under pressure, the pressure is 10 Kg/cm ² or more, preferably 50 Kg/cm ² or more, more preferably
A high pressure of 80Kg/cm ² or more and 2000Kg/cm ² or less is applied to the PTFE substrate to stretch it. In order to have a lubricant on the inner surface of the die, in the case of compression molding, it is generally applied before molding, and in the case of extrusion molding, it is either press-fitted into the extrusion die from the outside or applied before molding. is common. As the lubricant, silicone oil, which has poor heat resistance, is suitable. When the PTFE sheet used in the PTFE sealing material of the present invention is stretched under pressure in a die, it is extremely important to cause the sheet to plug flow. In order to cause plug flow, it is preferable to perform at least one of the following operations during molding. A lubricant is present on the inner surface of the die. The surface of the PTFE base is coated with another thermoplastic resin. The thermoplastic resin preferably has a lower melting point than PTFE and/or a lower viscoelasticity than PTFE under the temperature conditions during stretching. Coating the surface of the PTFE substrate with a thin sheet of thermoplastic resin not only improves the plug flow properties of the PTFE substrate, but also prevents the lubricant from adhering to the stretched sheet when it is present on the inner surface of the die. There is an effect that can be achieved by peeling off the laminated sheet to remove the lubricant. In order to remove the lubricant, apply PTFE of the same quality to the surface of the PTFE substrate.
It may also be carried out by laminating and covering thin sheets or films. PTFE is non-adhesive to each other;
It can be easily peeled off even when heated and pressed at temperatures below the melting point of PTFE. Various resins can be used as the laminated coating sheet to improve plug flow properties and/or remove lubricant, but ultra-high molecular weight polyethylene, poly-4-methylpentene-
1. PTFE etc. can be used best. If you try to stretch the PTFE by continuing to pressurize it without plug flow, the PTFE will undergo melt fracture or brittle fracture. Furthermore, if stretching is carried out under conditions where plug flow becomes difficult, non-uniform orientation will remain, resulting in strength weaknesses within the stretched sheet. Not suitable as a sealing material. In this way, the PTFE stretched sheet stretched under pressure in the die loses its orientation even at relatively low temperatures below 100°C. For example, if it is left at 80°C for 20 hours, there will be a dimensional change of about 3% (length (shrinkage) occurs in some cases. In order to have dimensional stability in such a low temperature range, the stretched PTFE sheet that constitutes the PTFE sealing material of the present invention must have a shrinkage rate of 1.5% or less at 80°C for 12 hours. . In order to make such a PTFE stretched sheet,
It is preferable to subject the stretched PTFE sheet to a heat treatment accompanied by shrinkage at a temperature range of approximately 70°C to 150°C, preferably 80°C to 120°C. PTFE after heat treatment
The present invention also includes sealing materials obtained from stretched sheets that meet the requirements of the present invention.
The PTFE stretched sheet with improved dimensional stability in a low temperature range preferably has a shrinkage rate of 1% or less, particularly preferably 0.6% or less, at 80°C for 12 hours.
This shrinkage rate is the average value of the length shrinkage rates in each direction on the surface of the stretched sheet, and is the value obtained when the stretched sheet is placed on a smooth solid and allowed to shrink freely. Generally, the maximum temperature during transportation and storage is about 80°C, and if the shrinkage rate is 1.5% or less at 80°C for 12 hours, it can generally be used. The PTFE stretched sheet obtained by stretching the PTFE base material under pressure in a die can be further punched and cut into the PTFE sealing material of the present invention such as gaskets and packing. Furthermore, when the PTFE base material is stretched under pressure in a die, it can be simultaneously formed into the shape of a sealing material such as a gasket. In this case, it is necessary that the entire PTFE base material be stretched uniformly, and sufficient measures such as coating the inner surface of the die with a lubricant must be taken. Therefore, it is easier to obtain a uniformly stretched product by forming a large stretched sheet and processing the stretched sheet to obtain the PTFE sealing material of the present invention. This will be explained below with reference to the drawings. Figure 1 shows the process of stretching a PTFE substrate by compression molding. Figure 2 shows a PTFE substrate laminated with PTFE before stretching. FIG. 3 is a schematic diagram showing the structure of a compression die suitable for carrying out the present invention. 4 and 5 show the actual stretching ratio D of the PTFE stretched sheet constituting the PTFE sealing material of the present invention,
This section shows how to determine the stretching ratio, etc. 6 and 7 show stretching by ram extrusion. FIG. 8 shows an apparatus for measuring compression set S. FIG. 9 shows the ORS measuring device. FIG. 10 shows the relationship between the actual stretching ratio D and the compression set S. In FIG. 1, at least the inner surface 2 of the compression die 1 is preset at a temperature of the surface layer of the PTFE substrate (-100).
Heat to any temperature above ℃. The PTFE substrate adjusted to the above-mentioned temperature is placed between the heated compression dies 1 (1-1). At this time, it is preferable that a lubricant be present on the inner surface 2 of the compression die. compress it
PTFE base material 3 is plug-flowed and stretched (1-
2) After cooling to about 700°C or less while applying compressive force, die 1 is opened and the stretched PTFE 4 is taken out. Used as a PTFE base when manufacturing thin-walled expanded PTFE, when manufacturing multiple sheets of expanded PTFE in one press to improve productivity, or when wanting to easily remove lubricant adhering to molded products after pressing. As shown in FIG. 2, it is preferable to take the form of a laminate of PTFE and/or other thermoplastic resins. When using a laminated PTFE base, each layer is peeled off after stretch forming. If the temperature of the PTFE substrate is below the melting point, the laminated interface between PTFE is
Easy to peel off after stretching. Peeling at the lamination interface with other thermoplastic resins is caused by laminated PTFE during stretch molding.
There is no problem at all as long as the temperature of the substrate is below the melting point of the thermoplastic resin, preferably below (melting point -30)C. If the thermoplastic resin is a resin such as ultra-high molecular weight polyethylene whose viscosity decreases little even when the melting point is exceeded, it can be used satisfactorily even above the melting point. In FIG. 3, the die plate 10 of the compression press
The cold/heat die plate 11 is connected via the heat insulating material 13 to
is fixed, and the cold/heat die plate 11 is provided with cold/heat medium holes 12 through which cold/heat medium flows, and the temperature is constantly regulated. A die 14 is attached to the cold/hot die plate 11. Prior to molding, the die 14 is heated to an arbitrary temperature of (temperature of the surface layer of the PTFE substrate -100°C) or higher. PTFE base 17
is also preheated to a temperature of 150°C or higher, preferably 170-340°C. Further, it is preferable to apply a lubricant to the inner surface of the die in advance. The PTFE substrate 17 is compressed and stretched. At this time, the thickness at the time of formation is adjusted using spacers 15 so that the actual stretching ratio is 2 to 5 times. Further, after cooling in the compressed state, the oriented sheet is taken out. At this time, the cooling rate is adjusted by using the heat insulating plate 18 and the cold/heat die plate 11.
It is carried out by. In FIG. 3, if the die 14 and the cold/hot die plate 11 have a curved shape such as a spherical shape, the molded product will also have a curved shape accordingly. FIG. 6 shows an apparatus for extruding PTFE sheets. In FIG. 6, a PTFE base material is put into a ram extrusion molding machine 26 consisting of a prismatic heating cylinder 24 with a square cross section and a square ram 25, and extruded into a die 27 while being heated by the ram 25.
As the PTFE base, not only a single layer of PTFE but also a laminated PTFE base as shown in FIG. 2 can be used as in the case of stretch molding by compression molding. In the middle of part A of the die 27, there is a series of lubricant seeping devices in order to apply the lubricant to the interface between the surface of the PTFE substrate and the die surface. The high-pressure lubricant is guided from the lubricant introduction path 28 to the plurality of seepage ports 29, seeps out onto the surface of the PTFE base material, and applies the lubricant to the interface between the surface of the molded body and the surface of the die. The lubricant leaking port 29 is made of a material having a small slit shape or a material such as sintered metal having fine communicating holes, and the lubricant seeps out from the fine holes. Alternatively, it is also possible to apply a lubricant to the inner surface of the die by spraying or the like before molding. The PTFE substrate having the lubricant uniformly applied to the surface becomes a so-called plug flow in which the PTFE surface layer and inner core flow at approximately the same speed within the die 27. Next, in part B of the die 27, the plug flow PTFE substrate is stretched. The B portion of the die 27 has a structure in which the thickness of the PTFE substrate is reduced. Figure 7 shows the change in flow of PTFE in part B during biaxial stretching.
The PTFE substrate is simultaneously extruded biaxially in the flow direction and in the direction perpendicular to the flow direction while maintaining the plug flow, and is multiaxially stretched. The force for stretching the PTFE substrate is exerted by the extrusion force from the ram extruder 26. The multiaxially stretched PTFE substrate is cooled in the C part of the die, and then passed through die 2.
Exit 7. The biaxially stretched PTFE is taken off by rolls 30. When a laminated PTFE base is used as the PTFE base, the laminated PTFE extruded from the die 27
By peeling off the stretched sheet, the PTFE of the present invention
A stretched PTFE sheet that constitutes a manufactured sealing material is obtained. The PTFE sealing material of the present invention can be obtained by subjecting the PTFE stretched sheet formed by compression molding or extrusion molding to mechanical processing such as cutting, typified by punching or slicing, as necessary. It will be done. [Effects of the Invention] The present invention provides PTFE that has been biaxially stretched, preferably multiaxially stretched, to an actual stretching ratio D of 3 times or more,
Compression set S at 200℃・500Kg/ ^cm2・1 hour is 30
% or less, the compression creep resistance, which was one of the biggest problems with conventional PTFE sealing materials, has been significantly improved, and PTFE has even higher compressive strength. We provide manufactured sealing materials. Furthermore, the PTFE sealing material of the present invention has significantly improved gas barrier properties, tensile strength, etc. The reason for the improvement in compression creep resistance is not clear, but it is thought to be as follows. The molecules of PTFE are ultra-high molecular weight, relatively rigid and long chains, which are intricately intertwined. When this is stretched to a high degree, the molecular chains between the entanglement points become highly tensed, and each molecular chain reaches a state of approximately equal tension. It is presumed that the external force acting on the stretched sheet in this state is evenly distributed to each molecular chain, and that the phenomenon of molecular chains slipping through at entanglement points is less likely to occur, resulting in a marked improvement in compression creep resistance. . In the method of stretching under pressure in a die to obtain the stretched PTFE sheet constituting the PTFE sealing material of the present invention, there is no significant increase in void content based on the specific gravity before and after stretching.
In any case, it is presumed that the gas barrier properties are improved due to the change in morphology caused by stretching. Many of the PTFE sealing materials of the present invention have better compression creep resistance than conventional PTFE sealing materials containing fillers, even if they do not contain fillers. It has excellent compression creep resistance, especially in high temperature ranges. For this reason, no filler is added.
The range of usable conditions (temperature, seal internal pressure, etc.) as a PTFE gasket material can be greatly expanded. It is also suitable for sealing materials other than gaskets, such as V-packets, U-packets, O-rings, diagonal packings, square-shaped packings, etc. used for gland seals of low-speed/low-pressure stirrers, valves, pumps, etc. be. In addition, PTFE, which is conventionally used in combination type sealing materials,
It is also suitable for replacing parts. For example, in PTFE jacket type gaskets and spiral gaskets,
These include PTFE parts, back springs, slipper seals, etc. In addition, diaphragms and bellows as valve parts have a problem of weak strength, and the PTFE sealing material of the present invention can be suitably used here as well. It is also suitable for PTFE balls used as check valves in liquid transport devices. It is also suitable for ball valve seats in ball valves, gate valve seats in gate valves, and valve discs. The shape and dimensions of the PTFE sealing material of the present invention are arbitrarily determined depending on the application. However, for flat gaskets, the thickness is 0.2mm to 5mm, preferably 0.5mm.
It is often used in the form of a sheet with a thickness of up to 4 mm. Among the PTFE sealing materials of the present invention, those containing filler have increased dimensional stability at high temperatures and further improved compression resistance and creep properties. Furthermore, since the wear resistance is improved, depending on the usage conditions, the PTFE sealing material of the present invention containing a filler may be preferable. As described above, the PTFE sealing material of the present invention is
In applications that have conventionally been used within a limited range of usage conditions due to problems such as compression creep and gas permeation, the range of usage can be greatly expanded. [Example] The present invention will be described below with reference to Examples. First, the compression set S described in the present invention,
We will explain how to measure ORS, specific gravity, etc. 1 Measurement of compression set S Compression set S at 200℃・500Kg/cm ²・1 hour
The measurement of (%) will be explained below using FIG. The test piece 30 was cut out to ^{a size} of 50 mm x 50 mm x 1 to 3 mm from a stretched PTFE sheet constituting the PTFE sealing material of the present invention or a comparative sample. Place this on a smooth tempered glass plate 33 (160mm
×160mm×5mm ^t ). Clean the surfaces of the tempered glass plate and test piece with acetone or ethanol in advance. Furthermore, one side has a mirror finish (surface roughness 0.1S~0.6SJIS B).
06101) stainless steel plate 32 (220mm x 220
mm x 6mm ^t ) between mirror surfaces. The laminate is sandwiched between hot press plates 31 (400 mm x 400 mm x 60 mm ^t ) whose temperature has been adjusted to 200°C in advance,
A compressive force corresponding to a pressure of 500 Kg/cm ² was applied to the test piece 30. Without considering the increase in area due to deformation of the test piece 30 during the test, heating and pressurization was continued for 1 hour while maintaining the initial compressive force, and then the heater power of the hot press plate 31 was turned off and water was passed through the hot press plate. It was then cooled to room temperature over about 30 minutes and taken out. Compressive force was maintained even during cooling. The thickness before and after the compression test (t ₀ , respectively) at one and at least five locations within the specimen
t ₁ ) with a micrometer (JIS B 7503 1st grade), calculate the compression set at each position as (t ₀ - t ₁ )/t ₀ × 100, take the arithmetic average of this, and Compression set S (%) described in the present invention
And so. However, if the entire structure is destroyed, the compression set S (%) is not calculated. Measurements basically followed ASTM D 621. The measurement of specific gravity is
In accordance with ASTM D 792. 2 Measurement of ORS A tensile testing machine modified as shown in Figure 9 was used. The test piece is a strip of length 80 mm and width 10 mm (thickness is arbitrary) cut out along two orthogonal stretching axes. Place the test piece in the grip. The distance between the grips at this time is 50 mm. After this, the oil bath heated to 200°C is raised so that the top of the grip is immersed in oil. The contractile force of the sample is detected by a load cell and recorded on a recorder. stretched
The shrinkage force of PTFE at 200℃ had an equilibrium value. The shrinkage force or the equilibrium value of the shrinkage force is read 5 minutes after immersing the sample in oil, and this is divided by the cross-sectional area of the sample (thickness x width) to convert it into stress. Numerically average the stress values in each stretching direction,
This was defined as the heat shrinkage stress at 200°C in the present invention. The measurements were in accordance with ASTM D 1504. 3 Measurement of specific gravity was in accordance with ASTM D 792. 4 Water vapor transmission rate (g/cm ² ·24hr) Measured according to ASTM F 372 at 38°C and 90% relative humidity. 5 Tensile strength at break and tensile elongation at break Measured according to ASTM D 638 and ASTM D 882. 6 Oxygen permeability (ml/m ² ·day · atm) Measured at 30°C according to ASTM D 1434. 7 Total light transmittance and haze Measured according to ASTM D 1003. Example 1 and Comparative Example 1 Various thicknesses formed by free baking method
PTFE base (manufactured by Nippon Valqua Industries Co., Ltd., VALFLON Sheet No. 7000, specific gravity 2.16, number average molecular weight approximately 1×
10 ⁷ ) were preheated to the temperatures shown in Table 1, and compressed using the compression press shown in Figure 3 at the temperatures shown in Table 1 to obtain stretched PTFE sheets (sample
No. 101-107). Each stretched sheet has the actual stretching ratio D shown in Table 1, ORS at 200℃, and 200℃・
It has a compression set (S) of 500Kg/cm ² . PTFE
Prior to compression molding the substrate, silicone oil (manufactured by Shin-Etsu Silicon Co., Ltd.) is applied to the inner surface of the compression die.
KF965 (10,000 cps per year at room temperature) was applied.
In addition, a 0.1 mm thick PTFE film was placed between the die and the PTFE substrate and stretched. The cold and hot die plates were kept at room temperature. Samples No. 201 and No. 202 were obtained by forming sheets with an actual stretching ratio of 3 times or less using substantially the same method as above.

[Table] The relationship between compression set S and real stretching ratio D is shown in Table 10.
Shown in the figure. The following was confirmed from Figure 10. (i) 2≦D≦5 S=-14D+(83±10) (%) (ii) D>5 S≦15 (%) Example 2 10 mm of PTFE containing about 20% by weight of glass fiber
Thick sheet (manufactured by Nippon Valqua Industries Co., Ltd., VALFLON)
No. 7010, specific gravity 2.3) was molded in the same manner as in Example 1 to obtain a stretched sheet with an actual stretching ratio of 6.5 times.
Table 1 shows the molding conditions and measurement results. The sheet has excellent compression creep resistance. Comparative Example 2 Two types of PTFE (Sample No. 203, No.
The compression set S of 204) was measured and shown in Table 1. Sample No. 203 2 mm thick sheet of PTFE homopolymer (manufactured by Nippon Valqua Industries Co., Ltd., VALFLON No. 7000, specific gravity 2.1) 204 PTFE filled with approximately 20% by weight glass fiber
2mm thick sheet of homopolymer [Nippon Valqua Industries Co., Ltd., VALFLON (glass filled) No.
7010, specific gravity 2.3] Example 3 0.5 mm thick PTFE made of PTFE homopolymer
Four sheets (manufactured by Nippon Valqua Industries Co., Ltd., VALFLON Cutting Tape No. 7900) and a 4 mm thick PTFE sheet (manufactured by Nippon Valqua Industries Co., Ltd., VALFLON Sheet No. 7900).
7000) were used as PTFE sheets for laminated substrates. Each sheet was heated in a hot air circulation oven at 350° C. for about 1 hour. Each sheet became translucent as the crystals melted. Remove each sheet from the oven 4mm/0.5mm/0.5mm/0.5mm/0.5mm/4mm
This layer structure was quickly laminated to create a laminated PTFE base. When laminating, at least only the outermost layer of each sheet is crystallized in order to separate each layer after stretching. The laminated PTFE base material was compressed by the compression molding method shown in FIG.
A PTFE sheet constituting the PTFE sealing material of the present invention having a thickness of about 0.1 mm was obtained by multiaxial stretching to an actual stretching ratio of 2 times (sample No. 301, specific gravity 2.1). Sample No.
The heat shrinkage stress of 301 at 200℃ is 15Kg/cm ² .
The oxygen permeability was 1700 ml/m ² ·day · atm.
In addition, commercially available PTFE homopolymer
PTFE sheet 0.1mm thick (manufactured by Nippon Valqua Industries Co., Ltd.)
The oxygen permeability of VALFLON cutting tape No.7900) is
It was 7600ml/ ^m2・day・atm. Example 4 and Comparative Example 3 A laminated PTFE substrate with the same layer structure as Example 3 was
After heating for 15 minutes between press plates heated to a temperature of 15°C, the material was stretched to an actual stretching ratio of about 5 times by compression molding in the same manner as in Example 1, to a thickness of about 0.1 mm and 0.8 mm.
Comprising the PTFE sealing material of the present invention with a thickness of mm
A PTFE sheet was obtained (sample No. 401). The sheet had an ORS of 15 Kg/cm ² at 200°C and a specific gravity of 2.1. Similarly, after preheating the laminated PTFE substrate to 300℃,
It was molded in the same manner (sample No. 402). The sheet is
0.1mm thickness and 0.8mm thickness, stretching ratio 5 times, 200℃
The ORS was 15Kg/cm ² and the specific gravity was 2.0. Table 2 shows the water vapor permeability of the 0.1 mm thick sheets of samples No. 401 and No. 402.

[Table] R○
*Made by Nippon Valqua Co., Ltd., VALFLON Cuttape No.7900
Reference example Tensile breaking strength and tensile breaking elongation of sample No. 104,
108, 110, 201, 301, 401, 402, and 403 were measured and shown in Table 3. In addition, the total light transmittance and haze were determined by No.301, 401, 402,
403 was measured and shown in Table 3. As is clear from Table 3, the sheet constituting the PTFE sealing material of the present invention is excellent in tensile strength, total light transmittance, and haze.

【table】 [Brief explanation of drawings]

Fig. 1 is a process explanatory diagram showing the process of stretching PTFE by compression molding, Fig. 2 is an explanatory diagram showing a base material laminated with PTFE before stretching, and Fig. 3 is a compression die suitable for carrying out the present invention. FIGS. 4 and 5 are schematic diagrams showing the structure of the PTFE stretched sheet.
FIG. 6 and FIG. 7 are explanatory diagrams showing stretching by ram extrusion molding, FIG. 8 is an explanatory diagram showing a method for measuring compression set, and FIG. 9 is an explanatory diagram showing a method for measuring ORS. FIG. 10 is an explanatory diagram showing the relationship between the actual stretching ratio and the compression set.

Claims (1)

[Scope of Claims] 1 Ultra-high molecular weight tetra having a number average molecular weight of 5 million or more stretched in at least two axial directions, obtained by stretching a compacted and fired base material under pressure in a die. It is made of fluoroethylene polymer, has an effective stretching ratio D of 3 times or more, a specific gravity of 1.8 or more, is highly oriented with an orientation release stress of 5 kg/cm ² or more at 200°C, and has a compression set S of 30%. A sealing material made of tetrafluoroethylene polymer that has a shrinkage rate of 1.5% or less at 80°C for 12 hours. 2. The tetrafluoroethylene polymer sealing material according to claim 1, which contains 60% by weight or less of a filler.