JPH0995663A - Composition for pressure-resistant sealing material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the subject composition composed of a perfluoroalkoxy resin and a fibrous reinforcing material at a specific ratio, having sufficiently satisfiable creep resistance under a high-pressure condition, exhibiting excellent sliding characteristics and low friction property and useful e.g. for a power steering apparatus of automobile. SOLUTION: This composition contains (A) 70-98wt.% of a perfluoroalkoxy resin e.g. a copolymer containing tetrafluoroethylene and (A1 ) a compound of the formula CF3 -(CF2 )n -OCX1 =CX2 X3 (X1 to X3 are each H or F; (n) is 0-7) or (A2 ) a compound of the formula CF3 -(CF2 )n -[OC(Rr)FCF2 ]m -OCX1 = CX2 X3 (Rr is F or CF3 ; (m) is 1-5) as essential components, a modified composition of a polytetrafluoroethylene and A1 or A2 , etc.} and (B) 2-30wt.% of a fibrous reinforcing material (e.g. glass fiber or aramid fiber).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pressure-resistant seal member composition for sealing a high-pressure fluid, an oil seal ring for an automobile power steering device, and a sealing member for an automobile power steering device. Specifically, the present invention relates to a pressure-resistant seal member composition used in a power steering device of an automobile, an oil seal ring for a power steering device of an automobile, and a seal member for a power steering device of an automobile.

[0002]

2. Description of the Related Art Generally, a tetrafluoroethylene resin (hereinafter abbreviated as "PTFE") is used as a pressure-resistant seal member such as a seal ring that seals important points such as a pump, a valve, and a cylinder that come into contact with a fluid of relatively high pressure. There is known a fluororesin composition in which a predetermined amount of glass fiber or carbon fiber is blended as a reinforcing material.

Such a PTFE resin composition is
E The original creep resistance is not sufficient, and in order to improve this physical property, a predetermined amount of fibrous reinforcing material made of glass fiber or carbon fiber is blended, and 100 kgf / cm
It can withstand a fluid pressure of about ² or less and exhibits sealing performance.

However, in order to exhibit a sealing property at a higher pressure than this, it is necessary to fill the PTFE resin composition with a large amount of fibrous reinforcing material, which impairs the low friction characteristics inherent to PTFE. .

As a composition having improved creep resistance without impairing the low friction property inherent to PTFE, PTF
A modified PTFE obtained by partially modifying the molecular structure of E, that is, a repeating unit substituted with a perfluoroalkoxy group or the like is copolymerized with a repeating unit of PTFE, and a sealing member composition in which carbon fiber is filled with this as a main component. It has been known.

[0006]

However, in the above-mentioned conventional PTFE resin-based seal member composition, 150 kg is used.
The problem is that it cannot exhibit creep resistance that withstands fluid pressure such as hydraulic pressure exceeding f / cm ² , that is, it cannot exhibit sealing performance as a seal member under such a high-pressure pressure condition. There is.

Further, as described above, 150 kgf / cm ²
In an oil seal ring for a power steering device of an automobile, which is a specific example of a seal member that requires a high pressure resistance that exceeds the above, when used under the above-mentioned pressure conditions, for example, a portion that has undergone creep deformation in the gap between the housing and the shaft There is a problem in that the phenomenon of so-called "stick-out wear" occurs by causing the stick-out to be torn off and further repeating this state, which greatly damages the seal member.

Therefore, the present invention solves the above-mentioned problems and provides a pressure-resistant seal member composition at 150 kgf / cm.
It is an object of the present invention to sufficiently satisfy creep resistance under pressure conditions of ² or more and to have low friction characteristics.

Or, by solving the same problems as described above,
It is an object of the present invention to provide an oil seal ring for a power steering device of an automobile which does not cause the phenomenon of “extrusion wear” and has a low friction coefficient and good operability.

[0010]

In order to solve the above-mentioned problems, the invention according to claim 1 provides a pressure-resistant seal comprising 70 to 98% by weight of a perfluoroalkoxy resin and 2 to 30% by weight of a fibrous reinforcing material. It was a composition for members. In the invention according to claim 2, the pressure is 140 kgf / cm.
^The lower one has a compression creep deformation rate of 0 to 7%, which is an oil seal ring for a power steering apparatus, which is made of the composition for a pressure resistant seal member according to claim 1.

In this pressure-resistant seal member composition, creep resistance is improved because the perfluoroalkoxy resin has a molecular structure in which slippage between molecules is unlikely to occur. The fibrous reinforcing material added to the composition in a predetermined amount has extremely high addition efficiency in improving the creep resistance of the predetermined perfluoroalkoxy resin, and thus impairs the original sliding characteristics of PTFE. Without any change, a seal member composition having an extremely excellent pressure-resistant sealing property can be obtained.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below. The perfluoroalkoxy resin (hereinafter abbreviated as PFA) in the present invention is a chain fluororesin having a perfluoroalkoxy side chain, and is a homopolymer PTFE and a monomer containing a perfluoroalkoxy group. And a copolymer containing the tetrafluoroethylene and a comonomer that gives the above side chain as essential components.

Such PFA has a specific melt viscosity at 372 ± 1 ° C. of 1 × 10 ³ to 1 × 10 ⁶ poise.
The melt flow index is 1 to 36 g / 10.
Minutes (ASTM D3307) are preferred.

Here, as typical ones of the comonomer which can give a perfluoroalkoxy side chain, perfluoroalkoxyalkyl fluorovinyl ethers represented by (Chemical formula 1), and perfluoroalkylfluorovinyl represented by (Chemical formula 2) There are polyethers and the like. A typical commercially available brand of such a copolymer is Teflon PFA-340J, 350 manufactured by DuPont Mitsui Fluorochemicals.
Examples thereof include J, or NEOFLON PFA manufactured by Daikin Industries, Ltd., Akron PFA manufactured by Asahi Glass Co., Ltd., and the like.

[0015]

Embedded image

(In the formula, n = 0 to 7, X _{1 to} X ₃ are each a fluorine atom or a hydrogen atom)

[0017]

Embedded image

(In the formula, n = 0 to 7, m = 1 to 5, Rr is a fluorine atom or --CF ₃ , and X _{1 to} X ₃ are a fluorine atom or a hydrogen atom, respectively).

The above-mentioned polymer may be any copolymer such as an alternating copolymer, a block copolymer, a random copolymer and the like.

As the fibrous reinforcing material used in the present invention,
Examples include inorganic fibrous reinforcing materials and organic fibrous reinforcing materials. As the inorganic fibrous reinforcing material, glass fibers, carbon fibers, graphite fibers, metal fibers such as stainless steel,
Calcium silicate fiber, potassium titanate fiber, aluminum borate fiber, silicon carbide fiber, silicon nitride fiber,
Magnesium sulfate fiber, zinc oxide whisker, magnesia fiber, calcium sulfate fiber, magnesium borate fiber,
Examples thereof include titanium boride fiber, alumina fiber, tyranno fiber, zirconia fiber, sonolide fiber and the like.

Examples of the organic fibrous reinforcing material include aramid fiber, polyethylene fiber, polyester fiber, polyamide fiber, ethylene tetrafluoride fiber, polyvinyl alcohol fiber, phenol fiber, phenylene sulfide fiber, and polyimide fiber. You can

As these fibrous reinforcing materials, glass fibers are particularly preferable. The glass fibers are SiO ₂ , B ₂ O ₃ ,
Al ₂ O ₃ , CaO, Na ₂ O, K ₂ O, MgO, Fe
_{It is} obtained from an inorganic glass containing ₂ O ₃ etc., and is generally alkali-free glass (E glass), alkali-containing glass (C
Glass, A glass) or the like can be used. For example, the E glass contains about 52 to 56% by weight of SiO ₂ , and B ₂ O ₃
About 8 to 13% by weight, Al ₂ O ₃ about 12 to 16% by weight, CaO about 15 to 25% by weight, Na ₂ O or K ₂
O is more than 0 and about 1 wt% or less, and MgO is more than 0 and about 6 wt% or less. Also, its tensile strength is about 3
00-400 kgf / mm ² , about 350 kgf on average
/ Mm ² , and the elastic modulus is about 7400 to 7700 kg
Some of them are f / mm ² , etc., and they are superior overall in terms of tensile strength, elastic modulus, mass productivity, price, etc.

In the present invention, the E glass having a fiber length of about 0.01 to 0.5 mm is preferable, and the fiber diameter thereof is about 5 to 15 μm, of which about 7 to 13 μm.
m is preferred. This is because when a large fiber having a fiber diameter of more than about 15 μm or a glass fiber having a fiber length of more than about 0.5 mm is used, it is difficult to uniformly disperse it when mixing with PFA, and such a composition is used. In that case, molding becomes difficult, which is not preferable. If the fiber diameter is too thin or the fiber length is too short, mechanical strength such as compression creep may not be expected.

The carbon fiber used in the present invention is not particularly limited in its material, and may be pitch-based, PAN-based, carbon-based or graphite-based. For example, a fiber diameter of about 4 to 20 μm, Fiber length about 10-500μ
If it is m, it is suitable because it is uniformly dispersed in the perfluoroalkoxy resin composition and sufficiently reinforced.

Considering the mechanical properties such as appropriate elastic modulus and tensile strength, and the aggression to the mating material such as cylinder and piston,
The carbon fiber diameter is preferably about 5 to 14 μm on average.

Carbon fibers are produced by firing various organic polymer fibers at an average temperature of about 1000 to 3000 ° C. This structure is composed mainly of hexagonal mesh planes of carbon atoms. An example of the mesh planes arranged in parallel to the fiber axis is a PAN-based or liquid crystal pitch-based carbon fiber having high orientation and anisotropy, while the mesh planes are randomly aggregated. An example is pitch-based carbon fiber having a directionality.

Highly oriented and anisotropic carbon fibers are highly excellent in elastic modulus and tensile strength in a specific direction, and isotropic carbon fibers are relatively high in loads applied from all directions. Can bear.

Pitch-based carbon fibers are, for example, structurally amorphous isotropic pitch-based carbon fibers such as petroleum pitch, which is a by-product of petroleum refining, and a structure in a certain direction, such as a difference in optical anisotropy. An example is an anisotropic pitch-based carbon fiber.

Isotropic pitch-based carbon fibers are classified into petroleum-based, coal-based, synthetic product-based, liquefied coal-based, etc., and their raw materials are melt-spun into pitch fibers, infusibilized, and then carbonized. It is manufactured by

The liquid crystal pitch-based carbon fiber is obtained by heating pitches in an inert gas phase to bring them into a liquid crystal state at 350 to 500 ° C. and then solidifying into coke. When this is melt-spun and heated in an oxidizing atmosphere, it becomes an oxidized fiber and becomes an insoluble and infusible fiber.
It is manufactured by a method such as heating to 00 ° C. or higher.

These are, for example, an average of 4000 kgf / m.
24000 to 50000k on average from low elastic modulus of about m ²
A medium to high elastic modulus of about gf / mm ² can be selected as required, and by mixing fibers having excellent mechanical properties such as tensile strength with a predetermined resin composition, an appropriate mechanical property can be obtained. A sealing material having strength can be obtained.

The PAN-based carbon fiber can be manufactured by a method of heating and baking acrylic fiber such as polyacrytonitrile fiber. A predetermined elastic modulus can be obtained depending on the heating temperature. For example, when heated at about 1000 to 1500 ° C., the elastic modulus averages 20000 to 30000 kgf.
/ Mm ² , and the average strength is 300 to 600 kgf / mm ² . Also, the average elastic modulus is 4 by heating at about 2000 ° C.
It can also be set to 0000 to 50,000 kgf / mm ² . Therefore, the PAN-based carbon fiber is a fiber having a high tensile strength, and its strength is 100 to 600 kgf / average depending on the heating temperature.
A range of mm ² is obtained, with an average of 100 on request.
A range of up to 300 kgf / mm ² can also be manufactured. If these values are too low, the reinforcing effect for compression creep and the like cannot be expected, and if these values are too high, it is expected that the mating material such as the piston, the cylinder, and the O-ring will be attacked.

These carbon fibers are not easily affected by chemicals such as acids and alkalis and have abrasion resistance. In addition, in order to improve the adhesion between these carbon fibers or glass fibers and the perfluoroalkoxy resin, and to improve the mechanical properties of the molded body, these carbon fibers or glass fibers are surface-treated with a silane coupling agent or the like. You may give a process.

By the way, the addition amount of the fibrous reinforcing material to PFA is P when the total weight of the composition is 100.
FA of 70 to 98% by weight and fibrous reinforcing material of 2 to 30% by weight are preferable, and PFA of 80 to 96% by weight and fibrous reinforcing material of 4 to 20% by weight are particularly preferable. Because, if the addition amount of the fibrous reinforcing material is less than 2% by weight, the mechanical properties and wear resistance of the molded product are hardly improved, and 3
This is because if the amount is more than 0% by weight, the friction characteristics deteriorate and the rotating torque becomes large when the seal ring is attached to the rotating shaft, which is not preferable.

As long as the effects of the present invention are not impaired,
For example, various fillers other than the above may be added within a range of about 0.1 to 15% by weight. Such fillers include aromatic polyetherketone-based resins, polyetherimide-based resins, polyethersulfone-based resins, polyamide-imide-based resins, heat-resistant polyamide-based resins, phenol-based resins, aromatic-based polyester resins, etc. Inorganic powders such as organic heat-resistant polymers, graphite or metals such as zinc, aluminum, magnesium, or oxides for improving heat conduction, glass beads, silica balloons, diatomaceous earth, asbestos, inorganic powders such as magnesium carbonate, graphite, etc. Inorganic powders for improving lubricity such as carbon, mica, talc, molybdenum disulfide, tungsten disulfide, and phosphate compounds, or inorganic pigments such as iron oxide, cadmium sulfide, cadmium selenide, and carbon black, silicone oil, ester oil, Fluorine oil Waxes, such as internal lubricant additives, such as zinc stearate, can be exemplified a number of things. Molybdenum disulfide is preferable because it has relatively good lubricating characteristics.

The mixing and molding method of PFA, fibrous reinforcing material and other fillers described above can employ the mixing molding conditions which have been widely used in the past. For example, dry mixing is performed by a mixer such as a tumbler mixer or a Henschel mixer. This may be melt-mixed with a hot roll, a kneader, a Banbury mixer, a melt extruder or the like to form a molding material, for example, pellets, and this may be melt-molded into a predetermined seal member shape by an injection molding machine or the like. The conditions at this time are not particularly limited and may be the usual molding conditions for PFA. In injection molding, barrel temperature is about 3
20 to 380 ° C., mold temperature about 190 to 230 ° C., preferably about 200 to 220 ° C., injection pressure about 200 to 10
00 kgf / cm ² , preferably about 600-100
It is considered to be good to carry out at 0 kgf / cm ² . The injection molding method can easily form a complicated shape and is excellent in productivity.

The composition for pressure-resistant sealing member according to the present invention is significantly different in creep resistance from PTFE composed of a single repeating unit.
According to D-621, measurement temperature is 25 ° C, load 140k
It was set to gf / cm ² . ), The compression creep deformation rate after 24 hours is 7% or less, preferably 6% or less, and 0%
Use more than. This is because the use of the above composition having a compression creep deformation rate of more than 7% is not preferable because the creep resistance is poor and the composition is easily deformed, resulting in a large amount of "extrusion" deformation.

The mating material to which the oil seal ring formed by molding the composition for pressure-resistant sealing member according to the present invention by the above-mentioned molding method is used, the wear of the oil seal ring or the wear of the oil seal ring is caused by the hardness of the mating material. Since the mating materials are worn, it is preferable to use the mating materials that do not wear each other.

A preferable alloy material is a structural alloy steel material. Examples of the structural alloy steel materials include SCM and spheroidal graphite cast iron products (FCD). These hardness in H _R C (Rockwell hardness (C scale)) 30
-60 is preferable. As an example having this hardness, SCM
In, SCM420H and the like, the hardness is 58-62 at H _R C. The FCD includes FCD450, and the hardness thereof is 143 to 217 in HB. It should be noted that these may be subjected to a quenching treatment such as induction hardening or carburizing to improve the hardness.

An oil seal ring obtained by molding the composition for pressure-resistant sealing member according to the present invention by the above-mentioned molding method is
Generally, as shown in FIGS. 1 and 2, it may be used together with an urging body such as an O-ring 2. The oil seal ring is pressed against the shaft 4 by this urging body and maintains the sealing property. A tension ring such as an O-ring is used as the urging body. If the specification is such that the oil under pressure functions sufficiently as a biasing body, the O-ring may be omitted and the sealing performance may be maintained by the biasing force of only the hydraulic pressure. The pressure, air pressure, and hydraulic pressure applied to the oil seal ring by the urging body is 140 kg / cm.
² or more, more preferably 150 kg / cm ² or more.

The tension ring is made of a polymer having an appropriate elasticity, examples of which include natural rubber, styrene butadiene rubber, butyl rubber, ethylene propylene rubber, chloroprene rubber, nitrile rubber and urethane rubber. Examples thereof include rubber, epichlorohydrin rubber, acrylic rubber, silicon rubber, and fluorine rubber, and acrylic rubber is particularly preferable from the viewpoint of heat resistance and oil resistance.

The elasticity of rubber is Hs in terms of hardness of rubber.
If it is about 60 to 100, preferably about 70 to 90, an appropriate elastic force can be obtained, which is preferable.

The oil seal ring 1 is shown in FIG.
As shown in, the C surface and the R surface may be provided on the sliding surface with the shaft 4. The C surface is a surface whose chamfered portion has a substantially flat surface, and the R surface is a surface whose chamfered portion has a curved surface. When the C surface and the R surface are provided, the width of the sliding surface in the axial direction 4 or the radial dimension thereof is preferably about 10 to 40%, more preferably about 15 to 30%. The chamfered shape is preferably tapered or arcuate in cross section.

When the chamfered shape is not provided, the space 10
Although the internal pressure is applied as a force to push the oil seal ring 1 in the direction of the shaft 4, when the chamfered shape is provided, the pressure is also applied to the chamfered shape, and the oil seal ring 1 is pushed up. Therefore, the force for pushing the oil seal ring 1 in the direction of the shaft 4 is offset, and the force for pushing the oil seal ring 1 on the shaft 4 is only the force by the biasing body, and this force can be easily controlled. is expected.

The sealing member or the surface roughness of at least one sliding surface of the mating member, R _{ma x,} the R _a, evaluation method defined in JIS etc. R _z, away below about 3～25Myuemu, Preferably about 8 μm or less, more preferably about 3
μm or less. This is because when the surface roughness exceeds the above-mentioned predetermined range, the sliding surface is often scratched, which is considered to be caused by friction.

Since it takes a long time to finish the sealing member or the surface of the mating member, it may be inefficient or affected by the formation of the transfer film of the resin material, which may affect the wear. It is presumed that the range may be about 3 to 8 μm or less if the specifications and conditions are not met.

Further, the seal member of the present invention can be applied without limitation to the power steering device, and for example, a rack and pinion type including a rack and a pinion gear, an integral type including a ball nut or a ball screw and a sector, and Use as a seal member of each type of power steering device such as semi-integral type and linkage type that activates the ring mechanism by operating the hydraulic power cylinder to push and pull the internal power piston. You can

Further, it may be used for a vehicle that steers not only the front wheels but also the four wheels, and may be used for a power steering device for various automobiles such as passenger cars, trucks and industrial vehicles such as forklifts.

[0049]

EXAMPLES The raw materials used in Examples and Comparative Examples are collectively shown below. In addition, the abbreviations are shown in [], and all the blending ratios in the table are shown by weight%. (1) Perfluoroalkoxy resin [PFA] Teflon PFA manufactured by Mitsui DuPont Fluorochemicals
340-J [PFA-1] (2) Perfluoroalkoxy resin [PFA] manufactured by Mitsui DuPont Fluorochemicals: Teflon PFA
340-J pulverized into powder with a particle size of 30 μm [P
FA-2] (3) Glass fiber [GF] Made by Asahi Fiber Glass Co., Ltd .: Milled fiber (fiber diameter 1
3 μm) (4) Carbon fiber [CF] Toray: Milled fiber trading card MDL-30 (5) Tetrafluoroethylene resin [PTFE] Daikin Industries, Ltd .: Polyflon M-15 [PTFE-
2].

[Examples 1 to 3 and Comparative Examples 1 and 2] The raw materials of Example 1 were pellets of PFA alone, and Examples 2 and 3 were dry mixed at the ratios shown in Table 1, and then biaxially melted. Supplied to an extruder (PCM-30 manufactured by Ikegai Iron & Steel Co., Ltd.)
Diameter 3 while melt-kneading at screw speed of 150 rpm
It was extruded from a strand die having 5 mm holes, and the extruded strand was continuously cut to form pellets. The obtained pellets are injected into a molding machine (barrel temperature 320 to 38).
0 ℃, mold temperature 210 ℃, injection pressure 800kg / c
m ² ), and from the molded body φ48 mm × φ52
A ring of mm × 2 mm was processed. The physical properties of the molded product were determined by the methods described below, and the obtained results are shown in Table 1.

In Comparative Examples 1 and 2, the raw materials were dry-mixed in the proportions shown in Table 2, compression-molded, and the resulting molded body was processed with an inner diameter of 48 mm, an outer diameter of 52 mm and a width of 2 mm. Got a seal ring.

Using this seal ring as a test piece, a rotary sliding test was conducted using the test apparatus shown in FIG. That is, seal ring 1 and backup O-ring 2 are F
Two pieces were attached to the CD45 housing 3 at intervals, and the shaft 4 made of SCM420H was rotated to bring the outer surface of the shaft 4 and the housing groove side surface 5 into sliding contact with each other. Further, at this time, oil was pressure-fed from an oil pressure generator (not shown) above the housing via the oil supply pipe 6, and the oil pressure was measured by the oil pressure gauge 7. Further, the oil temperature at this time was measured by the thermocouple 8, and the rotation torque was measured by the rotation torque meter 9.

As the test conditions, power steering oil (Idemitsu Kosan Co., Ltd .: Apollo Oil Dexylon) was used.
Hydraulic pressure 150 kgf / cm ² , oil temperature 80 ° C, peripheral speed 0.15
3000 cycles with 3 cycles to the right and 3 cycles to the left at m / min.
0 cycles were performed. The evaluation values of this rotary sliding test were the amount of protrusion after the end of the test (the protrusion distance of the seal ring in the axial direction: mm) and the rotational torque after the end of the test (N · m).

The creep test was conducted in accordance with ASTM D-621 at a measurement temperature of 25 ° C. and a load of 140 kgf / cm ² .

[0055]

[Table 1]

[0056]

[Table 2]

As is clear from the results shown in Tables 1 and 2, when the conventional PTFE containing glass fibers was used (Comparative Examples 1 and 2), the "protrusion amount" after the above rotary sliding test was large, The creep resistance under a hydraulic condition of 150 kgf / cm ² was not satisfactorily obtained. In addition, Comparative Examples 1 to
In 2, the rotation torque after the predetermined rotation is also large,
When used as an oil seal ring for a power steering device, it is apparent that an appropriate rotational torque value cannot be obtained.

On the other hand, in Examples 1 to 3 satisfying all the conditions, a very good result was obtained by the combination of the PFA simple substance and the fibrous reinforcing material, that is, the "protrusion wear" phenomenon did not occur. Moreover, the oil seal ring had low friction and good operability.

It should be noted that in the above table, the optimum seal ring material may be selected from the values above or below the lower limit value or below and above the upper limit value.

[0060]

EFFECT OF THE INVENTION The present invention provides a pressure-resistant sealing member containing a predetermined amount of PFA, a fibrous reinforcing material, and a filler added as necessary.
Under f / cm ² or more, further 150 kgf / cm ² or more, there is an advantage that the creep resistance is sufficiently satisfied, the sliding property is excellent, and the low friction property is provided.

Further, the oil seal ring for a power steering device made of the above pressure-resistant seal member composition does not cause the "protrusion wear" phenomenon, has a low friction coefficient almost equal to that of PTFE, and has good operability. There are advantages.

[Brief description of drawings]

FIG. 1 is a partially sectional front view illustrating a rotary sliding test device.

2 is a partially enlarged sectional view of FIG.

[Explanation of symbols] 1 seal ring 2 backup O-ring 3 housing 4 shaft 5 housing groove side surface 6 supply pipe 7 oil pressure meter 8 thermocouple 9 rotational torque meter 10 space

Claims (2)

[Claims] 1. A perfluoroalkoxy resin 70-98.
A composition for a pressure-resistant seal member, which comprises 2% by weight and 2 to 30% by weight of a fibrous reinforcing material. 2. The oil seal ring for a power steering apparatus, which comprises the composition for a pressure resistant seal member according to claim 1, having a compression creep deformation rate of 0 to 7% under a pressure of 140 kgf / cm ² .