Patent Description:
Buffer members made of rubber are incorporated into various devices and tools as members that absorb the repeated impact of operating members. In particular, buffer stoppers used in steering parts of automobiles are required to have high hardness and high durability buffer performance as rubber performance because the impact load is large.

In order to improve the impact resistance of rubber buffer materials, it is possible to increase the size and weight of the buffer materials; however, these approaches are contradictory to the reduction of space and weight in the recent development of automobiles, and it is difficult to adopt such approaches.

In addition, metal or resin materials can be used to increase the hardness. However, although these approaches are effective against high impact for a few times, their longevity against repeated impact is considered to be limited in terms of flexibility.

For the above reasons, for example, high hardness rubber materials are adopted. However, in general, as the hardness of rubber materials increases, problems are likely to occur in the processes necessary for manufacturing such as processability and formability. Therefore, if the processability is improved, the product characteristics may not be satisfied, and it is difficult to balance between the two.

Rack parallel type (RP)-electric power steering (EPS) has the following merits:.

For these reasons, items with a buffer function are required.

Patent Document <NUM> discloses a crosslinkable rubber composition for a buffer material, comprising:.

It is described that if the crosslinkable rubber composition for a buffer material does not contain even one of the three vulcanization accelerator components, extremely high impact resistance cannot be obtained, and that a filler such as carbon black can be contained. However, only furnace black is actually used.

<CIT> describes a rubber composition comprising a nitrile rubber and an epihalohydrin rubber which is crosslinked with two or more crosslinking agents selected among a sulfur-based crosslinking agent, a mercaptotriazine crosslinking agent, and a <NUM>,<NUM>-dimercaptoquinoxaline crosslinking agent to obtain a crosslinked object.

<CIT> describes a rubber composition for hoses, containing <NUM> parts by mass or more of an acrylonitrile butadiene rubber (NBR) in <NUM> parts by mass of the rubber component, while containing <NUM>-<NUM> parts by mass of a phenolic resin and <NUM>-<NUM> parts by mass of a thiuram compound serving as a vulcanization accelerator, respectively per <NUM> parts by mass of the rubber component. This rubber composition is said to be capable of achieving good adhesion to a brass-plated wire and capable of solving the problems of environmental loads; and suitable for use as an inner tube rubber for hydraulic pressure hoses.

<CIT> relates to the problem to provide a crosslinking rubber composition for a cushioning material, which can give a cushioning material having extremely excellent impact resistance while using general-purpose compounding components, and to provide a cushioning material obtained by crosslinking the rubber composition. As a solution to the problem, the document describes a crosslinking rubber composition for a cushioning material which comprises: <NUM> pbw of a rubber component comprising acrylonitrile butadiene rubber; a crosslinking accelerator containing <NUM> to <NUM> pbw of di-<NUM>-benzothiazolyl disulfide, <NUM> to <NUM> pbw of N-cyclohexyl-<NUM>-benzothiazolyl sulfonamide, and <NUM> to <NUM> pbw of tetraalkylthiuram disulfide; and <NUM> to <NUM> pbw of a crosslinking retarder. The cushioning material comprises a crosslinked product of the above rubber composition.

<CIT> relates to the problem to obtain an NBR composition which, when used as a molding used in areas in contact with various hydraulic fluids or lubricating oils in various hydraulic machines, does not form cracks on the surface or undergoes not rubber fracture, by adding sulfur and a specified vulcanization accelerator to NBR. As a solution to the problem, the document describes an NBR composition which is prepared by adding sulfur and a vulcanization accelerator comprising a C3-<NUM> alkyl thiuram or thiazole compound to NBR.

<CIT> relates to the problem to provide a nitrile rubber composition or the like having oil resistance, water proofing, high elasticity, low temperature resistance and low in flatting. As a solution to the problem, the document describes a water proofing nitrile rubber composition which comprises a rubber component, an inorganic filler and a vulcanizing agent wherein the rubber component comprises an acrylonitrile-butadiene rubber containing acrylonitrile of <NUM>-<NUM> wt. % and the inorganic filler of <NUM>-<NUM> pbw to the acrylonitrile-butadiene rubber of <NUM> pbw and the vulcanizing agent of <NUM>-<NUM> pbw to the acrylonitrile-butadiene rubber of <NUM> pbw.

An object of the present invention is to provide an NBR composition that is excellent in rubber material processability and vulcanized rubber hardness, and a buffer material using the same.

The first object of the present invention can be achieved by the NBR composition defined in claim <NUM>, preferred embodiments of which are defined in claims <NUM> and <NUM>.

The second object of the present invention is achieved by a buffer material, which is a vulcanized molded product of the above NBR.

The NBR composition according to the present invention and the buffer material using the same are excellent in rubber material processability and vulcanized rubber hardness, and makes it possible for both properties to coexist.

This is due to the effect of limiting the grade of carbon black used as a component of the composition to N330 or N550, and also indicates that the intended objects can be achieved even in the absence of N-cyclohexyl-<NUM>-benzothiazylsulfenamide, which is regarded as an essential component in Patent Document <NUM>.

The present invention, which exhibits the above effects, provides high durability buffer stoppers and other vibration proof members excellent in processability and vulcanized rubber hardness.

NBR used herein has a bound acrylonitrile (AN) content of <NUM> to <NUM> wt. %, preferably <NUM> to <NUM> wt. %, more preferably <NUM> to <NUM> wt. %, and a median of ML<NUM>+<NUM> (<NUM>) of <NUM> to <NUM>, preferably <NUM> to <NUM>.

NBR may contain (meth)acrylic acid, (meth)acrylic acid lower alkyl ester, or the like by copolymerization.

Furthermore, NBR may be partially replaced and used, specifically replaced with <NUM> wt. % or less of sulfur vulcanizable synthetic rubber such as acrylic rubber or fluorine rubber.

As carbon black, N-<NUM> or N-<NUM> grade carbon black is used. N-<NUM> grade carbon black has a particle size of <NUM> to <NUM>. In addition to N-<NUM> (HAF carbon black), S-<NUM>, N-<NUM>, N-<NUM>, N-<NUM>, N-<NUM>, and the like can also be used. N-<NUM> grade carbon black has a particle size of <NUM> to <NUM>. In addition to N-<NUM> (FEF carbon black), N-<NUM>, N-<NUM>, and the like are also used.

If the carbon black is changed to other than the above, for example, in the direction of N220 grade (direction of improving abrasion resistance) other than the above, the hardness of the rubber can be increased; however, the processability of the rubber is deteriorated. If the carbon black is changed in the direction of N990 grade (direction of improving processability), the opposite direction will be obtained.

N-<NUM> or N-<NUM> grade carbon black is used at a ratio of <NUM> to <NUM> parts by weight, preferably <NUM> to <NUM> parts by weight, based on <NUM> parts by weight of NBR. If the carbon black is used at a ratio less than the above range, the vulcanized rubber hardness and <NUM>% tensile stress do not satisfy the required characteristics. In contrast, if the carbon black is used at a ratio greater than the above range, the vulcanized rubber hardness and <NUM>% tensile stress show good values; however, the processability level does not satisfy the required characteristics.

As the vulcanization retarder, a thioimide-based compound is used, such as N-(cyclohexylthio)phthalimide, N-(<NUM>-ethylhexylthio)phthalimide, N-(cyclohexylthio)maleimide, and N-(<NUM>-t-butylphenylthio)succinimide. In addition to the thioimide-based compound (which is mandatory), aromatic monocarboxylic acids, aromatic dicarboxylic acids, or acid anhydrides thereof, amide compounds, and the like may be used.

Such a vulcanization retarder is used at a ratio of <NUM> to <NUM> parts by weight, preferably <NUM> to <NUM> parts by weight, based on <NUM> parts by weight of NBR. If the vulcanization retarder is used at a ratio less than the above range, the vulcanized rubber hardness shows a good value; however, the processability level does not satisfy the required characteristics. In contrast, if the vulcanization retarder is used at a ratio greater than the above range, vulcanization is excessively inhibited, so that significant impact resistance cannot be imparted and the vulcanization properties (rubber hardness) are reduced.

The vulcanization accelerator includes a thiazole-based vulcanization accelerators, namely di-<NUM>-benzothiazyldisulfide; and a thiuram-based vulcanization accelerator, namely a tetraalkylthiuram monosulfide such as tetramethylthiuram monosulfide. That is, di-<NUM>-benzothiazyldisulfide and tetraalkylthiuram monosulfide are used as vulcanization accelerator.

The thiazole-based vulcanization accelerator is used at a ratio of <NUM> to <NUM> parts by weight, preferably <NUM> to <NUM> parts by weight, based on <NUM> parts by weight of NBR. Further, the thiuram-based vulcanization accelerator is used at a ratio of <NUM> to <NUM> parts by weight, preferably <NUM> to <NUM> parts by weight, based on <NUM> parts by weight of NBR. Both vulcanization accelerators are used in a total amount of <NUM> to <NUM> parts by weight, preferably <NUM> to <NUM> parts by weight.

If the total amount of the both vulcanization accelerators is less than the above range, vulcanization does not proceed sufficiently. In contrast, if the both vulcanization accelerators are used at a ratio greater than the above range, the hardness of the rubber can be increased; however, the processability and formability of the rubber are deteriorated. N-cyclohexyl-<NUM>-benzothiazylsulfenamide can be used as additional component of the thiazole-based vulcanization accelerator, but is not an essential component.

As the sulfur-based vulcanizing agent, powdered sulfur, sulfur flower, precipitated sulfur, colloidal sulfur, surface treated sulfur, insoluble sulfur, polymer polysulfide, or the like is used at a ratio of <NUM> to <NUM> parts by weight based on <NUM> parts by weight of NBR.

In addition to the above essential components, an acid acceptor (e.g., an oxide or hydroxide of a divalent metal, or hydrotalcite), a plasticizer, an antioxidant, and other necessary compounding agents are added, and using a closed kneading machine, such as a kneader, and an open kneading machine, such as an open roll, a composition is prepared.

The obtained NBR composition has the minimum Mooney viscosity of less than <NUM> according to JIS K6300-<NUM> corresponding to ISO <NUM>, and the scorch time T5 of <NUM> minutes or more.

The vulcanization of the NBR composition is performed at about <NUM> to <NUM> for about <NUM> to <NUM> minutes. Vulcanization molding into a buffer material is performed by any conventionally known molding method, such as an injection molding method, a compression molding method, or a transfer molding method.

The vulcanized molded product has a rubber hardness (Duro A, instant) of <NUM> or more according to JIS K6253 corresponding to ISO <NUM>, and can be effectively used as an automobile buffer material such as a stopper for electric power steering, a buffer material for nailers, a buffer material for devices equipped with hydraulic cylinders, or the like.

In particular, the stopper for electric power steering functions as a stopper when the steering wheel is fully turned, so that a large force is applied thereto. The buffer material of the present invention sufficiently works as an automobile buffer material.

The following describes the present invention with reference to Examples.

Each of the above components was kneaded with a kneader and an open roll, followed by vulcanizing them at <NUM> for <NUM> minutes.

The obtained vulcanized molded product was measured for the following items.

Minimum Mooney viscosity (VM): Measured according to JIS K6300-<NUM> corresponding to ISO <NUM>, using a Mooney viscometer (AM-<NUM>, produced by Toyo Seiki Seisaku-sho, Ltd. ) with a test temperature of <NUM> and an L type rotor.

The unit is M, and less than <NUM> is acceptable.

Scorch time (T5): Measured according to JIS K6300-<NUM> corresponding to ISO <NUM>, with the same tester under the same conditions as the VM measurement.

The unit is minute, and <NUM> minutes or more is acceptable.

Hardness (according to JIS K6253 corresponding to ISO <NUM>, Duro A, instant). <NUM> or more is acceptable.

<NUM>% tensile stress (according to JIS K <NUM> corresponding to ISO <NUM>). <NUM> MPa or more is acceptable.

Tensile strength (according to JIS K <NUM> corresponding to ISO <NUM>). <NUM> MPa or more is acceptable.

In Example <NUM>, the amount of the vulcanization retarder was changed to <NUM> part by weight, and that of the vulcanizing agent was changed to <NUM> part by weight, respectively.

In Example <NUM>, the carbon black grade was changed to N550 (particle size of <NUM> to <NUM>, iodine absorption amount of <NUM>/kg).

In Example <NUM>, the amount of the vulcanization retarder was changed to <NUM> part by weight, and that of the vulcanizing agent was changed to <NUM> parts by weight, respectively.

In Example <NUM>, the carbon black grade was changed to N220 (particle size of <NUM> to <NUM>, iodine absorption amount of <NUM>/g).

In Example <NUM>, the carbon black grade was changed to N770 (particle size of <NUM> to <NUM>, iodine absorption amount of <NUM>/g).

In Example <NUM>, the carbon black grade was changed to N990 (particle size of <NUM> to <NUM>, iodine absorption amount of <NUM>/g).

In Example <NUM>, the amount of the vulcanization retarder was changed to <NUM> parts by weight.

In Example <NUM>, the amount of the vulcanizing agent was changed to <NUM> parts by weight.

In Example <NUM>, <NUM> parts by weight of N550 was used as the carbon black grade.

The results obtained in the above Examples and Comparative Examples are shown in the following Table.

Claim 1:
An NBR composition comprising <NUM> to <NUM> parts by weight of carbon black with a carbon black grade of N330 or N550, <NUM> to <NUM> parts by weight of a vulcanization retarder, a total amount of <NUM> to <NUM> parts by weight of thiazole-based and thiuram-based vulcanization accelerators, and <NUM> to <NUM> parts by weight of a sulfur-based vulcanizing agent, based on <NUM> parts by weight of NBR,
wherein the vulcanization retarder is a thioimide-based compound,
wherein the thiazole-based vulcanization accelerator is di-<NUM>-benzothiazyl disulfide, and
wherein the thiuram-based vulcanization accelerator is tetraalkylthiuram monosulfide.