JP7539268B2 - Rubber composition and its uses - Google Patents

Description

The present invention relates to a rubber composition and its use.

Ethylene-α-olefin rubbers, such as ethylene-propylene-non-conjugated diene copolymer rubber (EPDM) and ethylene-propylene copolymer rubber (EPR), have no unsaturated bonds in the main chain of their molecular structure, and therefore have superior heat aging resistance and weather resistance compared to commonly used conjugated diene rubbers. As a result, they are widely used in applications such as automobile parts, electric wire materials, construction and civil engineering materials, industrial parts, and modifiers for various resins (see, for example, Patent Document 1).

On the other hand, styrene-butadiene rubber (SBR) is widely used for tires for automobiles and the like. Styrene-butadiene rubber and other diene rubbers have insufficient weather resistance when used alone, so when used for long-term outdoor applications such as tires, it is common to add amine-based antioxidants or paraffin-based waxes to improve weather resistance. However, diene rubber products containing amine-based antioxidants or paraffin-based waxes may bleed out onto the surface over time, causing discoloration on the surface. Even during storage in stores, discoloration and powdering due to bleed-out may occur, resulting in a decrease in product value. For this reason, there has been a demand for improved weather resistance from the rubber components themselves.

To solve these problems, studies have been conducted to improve weather resistance by blending ethylene-propylene-diene rubber (EPDM) with styrene-butadiene rubber, but there is a problem that styrene-butadiene rubber and EPDM are prone to phase separation during thermal crosslinking, and sufficient fatigue resistance cannot be obtained.

The applicant has proposed a rubber composition containing a random copolymer rubber consisting of structural units derived from ethylene, an α-olefin, and a specific triene compound, a diene rubber, carbon black, and a vulcanizing agent (see Patent Document 2). This rubber composition is suitable for tire sidewall applications because the ethylene-α-olefin-triene random copolymer rubber exhibits a fast vulcanization rate almost equivalent to that of the diene rubber, and is less susceptible to phase separation from the diene rubber, without impairing the excellent mechanical strength properties inherent to the diene rubber.

The applicant has also discovered and proposed a rubber composition obtained by mixing a diene rubber with a composition containing a non-conjugated polyene copolymer containing structural units derived from an α-olefin and structural units derived from a non-conjugated polyene, and a softener, which is suitable for forming a tire with excellent braking performance and fuel economy (see Patent Documents 3 and 4).

Currently, the main process used in tire manufacturing is to mold an uncrosslinked composition whose main component is a diene rubber such as styrene-butadiene rubber or natural rubber into a sheet, crosslink only the surface with an electron beam to prevent sagging, assemble it into a tire shape, and then crosslink with sulfur.

Furthermore, diene rubbers such as natural rubber (NR), styrene-butadiene rubber (SBR), and butadiene rubber (BR) are known to have excellent dynamic fatigue resistance and dynamic properties, and are used as the raw rubber for automobile tires and anti-vibration rubber. However, the environment in which these rubber products are used has changed significantly in recent years, and there is a demand for improved heat aging resistance and weather resistance of rubber products. For example, weather resistance is particularly required for the treads and tire sidewalls of automobile tires. However, there has not been any rubber that retains the excellent mechanical properties, fatigue resistance, and dynamic properties of current diene rubbers, and also has good weather resistance.

Therefore, various blend rubber compositions of diene rubber, which has excellent mechanical properties, dynamic fatigue resistance, and dynamic characteristics, and ethylene/C3-20 α-olefin copolymers such as ethylene/propylene copolymer rubber (EPR), which has excellent heat aging resistance and weather resistance, have been studied. However, because the dynamic property levels of ethylene/C3-20 α-olefin copolymers and diene rubbers are different, blend rubber compositions exhibiting uniform physical properties have not been obtained in the past. The dynamic properties of automobile tires are a matter of whether or not the material will deteriorate fuel economy, and an index of this is the tan δ (loss tangent) value, with the lower the tan δ value, the better the dynamic properties.

The tire tread is the rubber part where the tire comes into contact with the road surface, and in studless tires, in addition to the wet grip performance, fuel efficiency, and long life required of regular summer tires, excellent braking performance on ice is also required.

International Publication No. 2009/072553 JP 2001-123025 A International Publication No. WO 2005/105912 International Publication No. 2005/105913

The present invention aims to provide a rubber composition that has excellent properties, such as wet grip performance, long life performance (fatigue resistance), and braking performance on ice, which are particularly required for studless tires; a rubber material for tires made of the rubber composition; a tire tread formed using the rubber material for tires; and a tire equipped with the tire tread.

The inventors of the present invention have conducted extensive research to achieve the above object. As a result, they have discovered that the above object can be achieved by using a rubber composition in which a diene rubber is blended with a specific low molecular weight ethylene-α-olefin copolymer that is not substantially co-crosslinked, and have thus completed the present invention.

The rubber composition according to the present invention is characterized in that it contains an ethylene-α-olefin copolymer (S) having structural units derived from ethylene (A) and an α-olefin (B) having 3 to 20 carbon atoms and satisfying the following requirements (i) and (ii), and a diene rubber (T), and contains the copolymer (S) in an amount of 0.5 parts by mass or more and 50 parts by mass or less per 100 parts by mass of the diene rubber (T).
(i) the molar ratio of the structural units derived from ethylene (A) to the structural units derived from an α-olefin (B) [(A)/(B)] is 40/60 to 99.9/0.1;
(ii) The Mooney viscosity at 100° C. [ML ₍₁₊₄₎ (100° C.), JIS K6300] is 5 to 100.

The present invention provides a rubber composition and its uses that can improve wet grip performance and significantly improve braking performance on ice while maintaining the long life performance (fatigue resistance) of diene rubber.

The present invention will be described in detail below.
The rubber composition according to the present invention contains a specific ethylene-α-olefin copolymer (S) and a diene rubber (T), and contains the copolymer (S) in an amount of 0.5 parts by mass or more and 50 parts by mass or less per 100 parts by mass of the diene rubber (T).

[Ethylene/α-olefin copolymer (S)]
The ethylene/α-olefin copolymer (S) (hereinafter, also simply referred to as “copolymer (S)”) has structural units derived from ethylene (A) and an α-olefin (B) having 3 to 20 carbon atoms.

Examples of α-olefins (B) having 3 to 20 carbon atoms include propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-heptene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, and 1-eicosene. Among these, α-olefins having 3 to 8 carbon atoms such as propylene, 1-butene, 1-hexene, and 1-octene are preferred, and propylene is particularly preferred. Such α-olefins are preferred because the raw material costs are relatively low, the resulting ethylene-α-olefin copolymers exhibit excellent mechanical properties, and molded articles having rubber elasticity can be obtained. These α-olefins may be used alone or in combination.

That is, the copolymer (S) contains structural units derived from at least one type of α-olefin (B) having 3 to 20 carbon atoms, and may contain structural units derived from two or more types of α-olefins (B) having 3 to 20 carbon atoms.

As described above, the copolymer (S) is a copolymer having structural units derived from ethylene (A) and an α-olefin (B) having 3 to 20 carbon atoms, and satisfies the following requirements (i) and (ii):
(i) The molar ratio of the constituent units derived from ethylene (A) to the constituent units derived from an α-olefin (B) [(A)/(B)] is 40/60 to 99.9/0.1.
(ii) The Mooney viscosity at 100° C. [ML ₍₁₊₄₎ (100° C.), JIS K6300] is 5 to 100.
In this specification, "α-olefin having 3 to 20 carbon atoms" may also be referred to simply as "α-olefin".

<Requirement (i)>
Requirement (i) specifies that the molar ratio of ethylene/α-olefin in the copolymer (S) satisfies 40/60 to 99.9/0.1, and this molar ratio is preferably 40/60 to 90/10, more preferably 45/55 to 85/15, and even more preferably 50/50 to 78/22. When such a copolymer (S) is used as a raw material for a crosslinked molded article, the resulting crosslinked molded article exhibits excellent rubber elasticity and is excellent in mechanical strength and flexibility, which is preferable.

The ethylene content (content of constituent units derived from ethylene (A)) and the α-olefin content (content of constituent units derived from α-olefin (B)) in the copolymer (S) can be determined by ¹³ C-NMR.

<Requirement (ii)>
Requirement (ii) specifies that the Mooney viscosity at 100°C [ML ₍₁₊₄₎ (100°C), JIS K6300] of the copolymer (S) is 5 to 100, and the Mooney viscosity is preferably 10 to 80, more preferably 20 to 60. Component (A) having a Mooney viscosity within the above range has excellent processability.
The copolymer (S) preferably satisfies the following requirement (iii).

<Requirement (iii)>
The requirement (iii) specifies that the ratio (molecular weight distribution; Mw/Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of the copolymer (S) measured by gel permeation chromatography (GPC) is preferably in the range of 4 to 40. This molecular weight distribution (Mw/Mn) is more preferably in the range of 4 to 30, and further preferably in the range of 5 to 25.
When the copolymer (S) satisfies the requirement (iii), it contains an appropriate amount of low molecular weight components, and therefore has good processability.

The weight average molecular weight (Mw) and number average molecular weight of the copolymer (S) can be determined as polystyrene-equivalent values measured by gel permeation chromatography (GPC).
The copolymer (S) preferably satisfies the following requirement (iv).

<Requirement (iv)>
Requirement (iv) specifies that the number average molecular weight (Mn) of the copolymer (S) is 30,000 or less. The number average molecular weight (Mn) is preferably in the range of 500 to 25,000, more preferably 1,000 to 20,000.

When the copolymer (S) satisfies the requirement (iv), it contains an appropriate amount of low molecular weight components, and therefore has good processability.
The copolymer (S) has an intrinsic viscosity [η] of preferably 0.1 to 5 dL/g, more preferably 0. to 5.0 dL/g, and further preferably 0.3 to 4.0 dL/g.

The weight average molecular weight (Mw) of the copolymer (S) is preferably 100,000 to 500,000, more preferably 120,000 to 500,000, and further preferably 150,000 to 400,000.
It is preferable that the copolymer (S) satisfies both of the above-mentioned intrinsic viscosity [η] and weight average molecular weight (Mw).

[Production of ethylene/α-olefin copolymer (S)]
The copolymer (S) is a copolymer obtained by copolymerizing ethylene (A) and an α-olefin (B) having 3 to 20 carbon atoms.

The copolymer (S) may be prepared by any method as long as it satisfies the above requirements (i) and (ii), but it can be produced by a conventional method using a vanadium catalyst, a titanium catalyst, a metallocene catalyst, or the like. In particular, the solution polymerization method described in JP-A-62-121709 and the like is preferred.

[Diene rubber (T)]
As the diene rubber (T), known diene rubbers having double bonds in the main chain can be used without limitation, and these may be used alone or in combination of two or more. As the diene rubber, a polymer or copolymer rubber having a conjugated diene compound as a main monomer is preferably used. In the present invention, the diene rubber also includes natural rubber (NR) and hydrogenated rubber. As the diene rubber (T), a non-crosslinked one can usually be used, and it is desirable that the iodine value is 100 or more, preferably 200 or more, and more preferably 250 or more.

Examples of such diene rubbers (T) include natural rubber (NR), isoprene rubber (IR), styrene-butadiene rubber (SBR), butadiene rubber (BR), chloroprene rubber (CR), acrylonitrile-butadiene rubber (NBR), nitrile rubber, and hydrogenated nitrile rubber.

In the present invention, the diene rubber (T) is preferably natural rubber (NR), isoprene rubber (IR), styrene-butadiene rubber (SBR), or butadiene rubber (BR), and more preferably styrene-butadiene rubber (SBR). These diene rubbers (T) can be used alone or in combination of two or more.

As the natural rubber (NR), natural rubber standardized by the Green Book (International Quality Packaging Standard for Various Grades of Natural Rubber) can be used. As the isoprene rubber (IR), one having a specific gravity of 0.91 to 0.94 and a Mooney viscosity [ML ₁₊₄ (100°C), JIS K6300] of 30 to 120 is preferably used.

The styrene-butadiene rubber (SBR) preferably has a specific gravity of 0.91 to 0.98 and a Mooney viscosity [ML ₁₊₄ (100°C), JIS K6300] of 20 to 120. The butadiene rubber (BR) preferably has a specific gravity of 0.90 to 0.95 and a Mooney viscosity [ML ₁₊₄ (100°C), JIS K6300] of 20 to 120.

The rubber composition of the present invention contains the above-mentioned copolymer (S) and diene rubber (T) as essential components, and contains 0.5 to 50 parts by mass, preferably 1 to 40 parts by mass, and more preferably 5 to 30 parts by mass of copolymer (S) per 100 parts by mass of diene rubber (T).

The total content of the copolymer (S) and the diene rubber (T) in the rubber composition of the present invention is 3% by mass or more, preferably 5% by mass or more, and although there is no particular upper limit, it is desirable that it be 90% by mass or less. The rubber composition of the present invention is also excellent in rubber elasticity, weather resistance, and ozone resistance, and is particularly excellent in mechanical properties, weather resistance, and fatigue resistance. The rubber composition is also excellent in abrasion resistance. Therefore, by applying the rubber composition of the present invention, a tire can be obtained that has both excellent braking performance and excellent fuel efficiency performance, and is also excellent in rubber elasticity, weather resistance, and ozone resistance, and is particularly excellent in mechanical properties and fatigue resistance. In addition, a tire that is also excellent in abrasion resistance can be obtained.

<Optional ingredients>
The rubber composition of the present invention may further contain optional components such as softeners, fillers, other resin components, crosslinking agents, foaming agents, antioxidants (stabilizers), weather resistance agents, plasticizers, colorants, and various additives that are conventionally compounded in known rubber compositions, depending on the application, within the scope of the object of the present invention.

Softeners As the softeners, those which are conventionally compounded in rubbers are widely used.
in particular,
Petroleum-based softeners such as paraffin-based process oil, naphthene-based process oil, and aromatic process oil;
Synthetic oil-based softeners;
Co-oligomers of ethylene and α-olefins;
Paraffin wax;
Liquid paraffin;
White Oil;
Petrolatum;
Coal tar-based softeners such as coal tar and coal tar pitch;
Vegetable oil-based softeners such as castor oil, cottonseed oil, linseed oil, rapeseed oil, coconut oil, palm oil, soybean oil, peanut oil, Japan wax, rosin, pine oil, dipentene, pine tar, and tall oil;
Subs (factices) such as black subs, white subs, candy subs, etc.;
Waxes such as beeswax, carnauba wax, and lanolin;
fatty acids and fatty acid salts such as ricinoleic acid, palmitic acid, myristic acid, barium stearate, calcium stearate, magnesium stearate, zinc stearate, zinc laurate, etc.;
Ester-based plasticizers such as dioctyl phthalate, dioctyl adipate, and dioctyl sebacate;
Coumarone-indene resin;
Phenol formaldehyde resin;
Terpene-phenolic resins;
Polyterpene resins;
Examples of the resin include petroleum-based hydrocarbon resins such as synthetic polyterpene resins, aromatic hydrocarbon resins, aliphatic hydrocarbon resins, aliphatic cyclic hydrocarbon resins, aliphatic/alicyclic petroleum resins, aliphatic/aromatic petroleum resins, hydrogenated modified alicyclic hydrocarbon resins, hydrogenated hydrocarbon resins, liquid polybutene, liquid polybutadiene, and atactic polypropylene.

Among these, petroleum-based softeners, phenol-formaldehyde resins, and petroleum-based hydrocarbon resins are preferred, with petroleum-based softeners and petroleum-based hydrocarbon resins being more preferred, and petroleum-based softeners being particularly preferred.

Among the petroleum-based softeners, petroleum-based process oils are preferred, among which aromatic process oils, paraffinic process oils, naphthenic process oils and the like are more preferred, with aromatic process oils being particularly preferred. Furthermore, among the petroleum-based hydrocarbon resins, aliphatic cyclic hydrocarbon resins are preferred.
Among these softeners, aromatic process oils are particularly preferred.
The softeners may be used alone or in combination of two or more.

The content of the softener in the rubber composition of the present invention is 0.1 to 300 parts by mass, preferably 1 to 250 parts by mass, and more preferably 5 to 200 parts by mass, per 100 parts by mass of copolymer (S). Within the above range, the rubber composition has excellent moldability such as extrusion moldability, press moldability, injection moldability, and roll processability, which is preferable.

Filler The filler is not particularly limited, but inorganic fillers are preferred because they improve the mechanical strength, such as tensile strength, tear strength, and abrasion resistance, of the rubber composition.

Examples of the inorganic filler include carbon black such as SRF, GPF, FEF, MAF, HAF, ISAF, SAF, FT, and MT; surface-treated carbon black obtained by surface-treating these carbon blacks with a silane coupling agent or the like; and white fillers such as silica, activated calcium carbonate, light calcium carbonate, heavy calcium carbonate, fine talc powder, talc, fine silicic acid powder, and clay.
The fillers may be used alone or in combination of two or more.

The content of the filler in the rubber composition of the present invention is 1 to 300 parts by mass, preferably 5 to 250 parts by mass, and more preferably 10 to 200 parts by mass, per 100 parts by mass of the diene rubber (T). Within the above range, the rubber composition has excellent kneadability and processability, and the rubber molded article has excellent mechanical properties and compression set, which is preferable. In addition, a crosslinked molded article with improved mechanical properties such as tensile strength, tear strength, and abrasion resistance can be obtained, the hardness of the crosslinked molded article can be increased without impairing other physical properties, and the manufacturing cost of the crosslinked molded article can be reduced.

Other Resin Components The rubber composition of the present invention may contain other resin components than the copolymer (S) as necessary. Such other resin components are not particularly limited, but are preferably polyolefin resins.

When the rubber composition of the present invention contains a polyolefin resin, the product hardness can be adjusted and the compound viscosity at the processing temperature can be reduced, thereby further improving processability. In addition, it can be handled as a thermoplastic elastomer, which is preferable because it provides a wider range of handling and kneading techniques.

As the polyolefin resin, a polyolefin resin having a number average molecular weight of 10,000 or more in terms of standard polystyrene measured by GPC is preferably used.
Examples of polyolefin resins include α-olefin homopolymers and α-olefin copolymers. Examples of α-olefin homopolymers include polyethylene and polypropylene, and examples of α-olefin copolymers include ethylene-C3-C20 α-olefin copolymers (however, different from the copolymer (S)) and ethylene-C3-C20 α-olefin-non-conjugated polyene copolymers. Examples of ethylene-C3-C20 α-olefin copolymers include ethylene-propylene rubber (EPR), propylene-ethylene rubber (PER), ethylene-butene rubber (EBR), ethylene-octene rubber (EOR), and the like.

Examples of copolymers of ethylene, an α-olefin having 3 to 20 carbon atoms, and a non-conjugated polyene include ethylene-propylene terpolymer (EPT) and ethylene-butene terpolymer (EBT).

Of these, preferred polyolefin resins are polyethylene, ethylene-α-olefin copolymer, and polypropylene.
The polyolefin resins may be used alone or in combination of two or more kinds.

When the rubber composition of the present invention contains a polyolefin resin, the content of the polyolefin resin is 1 to 100 parts by mass, preferably 5 to 80 parts by mass, and more preferably 10 to 50 parts by mass, per 100 parts by mass of the aforementioned copolymer (S). Within the above range, the hardness of the molded article formed from the rubber composition can be adjusted, and the compound viscosity at the processing temperature can be reduced, thereby further improving processability. In addition, it can be handled as a thermoplastic elastomer, which is preferable because it allows for a wide range of handling and kneading techniques.

Crosslinking Agent The rubber composition according to the present invention is a crosslinkable composition, and by crosslinking it, a crosslinked molded article according to the present invention, which will be described later, can be produced. Crosslinking may be performed by heating or the like using a crosslinking agent, or by radiation crosslinking, which involves irradiating the rubber with radiation such as electron beams, X-rays, γ-rays, α-rays, and β-rays. Among radiation crosslinking, electron beam crosslinking is preferred.

The crosslinked molded article according to the present invention is preferably produced by crosslinking using radiation crosslinking, particularly electron beam crosslinking, and in this case, the rubber composition does not need to contain a crosslinking agent.
In addition, when the rubber composition is crosslinked by heating, the rubber composition preferably contains a crosslinking agent. The crosslinking agents may be used alone or in combination of two or more.

Examples of crosslinking agents include crosslinking agents that are commonly used when crosslinking rubber, such as sulfur-based compounds, organic peroxides, phenolic resins, hydrosilicone compounds, amino resins, quinones or derivatives thereof, amine compounds, azo compounds, epoxy compounds, and isocyanates. Among these crosslinking agents, sulfur-based compounds, organic peroxides, and phenolic resins are preferred.

When the crosslinking agent is an organic peroxide, specific examples include dicumyl peroxide, di-tert-butyl peroxide, 2,5-di-(tert-butylperoxy)hexane, 2,5-dimethyl-2,5-di-(tert-butylperoxy)hexane, 2,5-dimethyl-2,5-di-(tert-butylperoxy)hexyne-3, 1,3-bis(tert-butylperoxyisopropyl)benzene, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, n-butyl-4,4-bis(tert-butylperoxy)valerate, benzoyl peroxide, p-chlorobenzoyl peroxide, 2,4-dichlorobenzoyl peroxide, tert-butylperoxybenzoate, tert-butylperoxyisopropyl carbonate, diacetyl peroxide, lauroyl peroxide, tert-butylcumyl peroxide, etc.

When an organic peroxide is used as the crosslinking agent, the rubber composition of the present invention preferably contains the following crosslinking aid.
When an organic peroxide is used as a crosslinking agent, examples of the crosslinking aid that the rubber composition preferably contains include sulfur; quinone dioxime crosslinking aids such as p-quinone dioxime; acrylic crosslinking aids such as ethylene glycol dimethacrylate and trimethylolpropane trimethacrylate; allyl crosslinking aids such as diallyl phthalate and triallyl isocyanurate; other maleimide crosslinking aids; and divinylbenzene. The amount of the crosslinking aid is usually 0.5 to 10 moles, preferably 0.5 to 7 moles, and more preferably 1 to 5 moles, per mole of the organic peroxide. It is also desirable that the amount of the crosslinking aid is 0.5 to 2 moles, preferably 0.5 to 1.5 moles, and more preferably approximately equimolar, per mole of the organic peroxide.

When the crosslinking agent is a sulfur-based compound, specific examples include sulfur, sulfur chloride, sulfur dichloride, morpholine disulfide, alkylphenol disulfide, tetramethylthiuram disulfide, selenium dithiocarbamate, etc.

When the crosslinking agent is a sulfur-based compound, the amount of the sulfur-based compound is usually 0.2 to 15 parts by mass, preferably 0.5 to 7.0 parts by mass, and more preferably 0.7 to 5.0 parts by mass, per 100 parts by mass of the diene rubber (T). When the amount of the sulfur-based compound is within the above range, it is preferable because the rubber composition exhibits excellent crosslinking properties without blooming on the surface of the obtained rubber molded article.

When a sulfur-based compound is used as the crosslinking agent, the rubber composition of the present invention preferably contains the following crosslinking aid.
When a sulfur-based compound is used as the crosslinking agent, examples of the crosslinking aid that the rubber composition preferably contains include zinc oxide, zinc oxide, etc. The amount of the crosslinking aid to be blended is usually 1 to 20 parts by mass based on 100 parts by mass of the copolymer (S).
When a sulfur-based compound is used as the crosslinking agent, it is desirable to use sulfur in combination with a vulcanization accelerator.

Specific examples of vulcanization accelerators include thiazole-based agents such as N-cyclohexyl-2-benzothiazole sulfenamide (CBS) (e.g., "Noccela NS" (trade name; manufactured by Ouchi Shinko Co., Ltd.)), N-oxydiethylene-2-benzothiazole sulfenamide, N,N'-diisopropyl-2-benzothiazole sulfenamide, 2-mercaptobenzothiazole (e.g., "Suncerer M" (trade name; manufactured by Sanshin Chemical Industry Co., Ltd.)), 2-(4-morpholinodithio)benzothiazole (e.g., "Noccela MDB-P" (trade name; manufactured by Sanshin Chemical Industry Co., Ltd.)), 2-(2,4-dinitrophenyl)mercaptobenzothiazole, 2-(2,6-diethyl-4-morpholinothio)benzothiazole, and dibenzothiazyl disulfide; guanidine-based agents such as diphenylguanidine, triphenylguanidine, and diorthotolylguanidine; acetaldehyde- Aldehyde amines such as aniline condensates and butylaldehyde-aniline condensates; imidazolines such as 2-mercaptoimidazoline; thioureas such as diethylthiourea and dibutylthiourea; thiuram compounds such as tetramethylthiuram monosulfide and tetramethylthiuram disulfide (TMTD) (for example, "Noccela TT" (trade name; manufactured by Ouchi Shinko Co., Ltd.)); zinc dimethyldithiocarbamate, diethyldithiocarbamate Examples include dithioacid salts such as zinc dibutyldithiocarbamate (ZnBDC) (for example, "Suncerer Bz" (trade name; manufactured by Sanshin Chemical Industry Co., Ltd.) and tellurium diethyldithiocarbamate); thiourea-based salts such as ethylenethiourea, N,N'-diethylthiourea, and N,N'-dibutylthiourea; xanthates such as zinc dibutylxatogenate; and zinc oxide (for example, "Zinc Oxide Type 2" (trade name; manufactured by Hakusui Tech Co., Ltd.).

The amount of these vulcanization accelerators is usually 0.1 to 20 parts by mass, preferably 0.2 to 15 parts by mass, and more preferably 0.5 to 10 parts by mass, per 100 parts by mass of copolymer (S). When the amount of vulcanization accelerator is within the above range, it is preferable because the resulting rubber molded article shows excellent crosslinking properties without blooming on the surface.

The vulcanization aid can be appropriately selected depending on the application, and can be used alone or in a mixture of two or more kinds. Specific examples of vulcanization aids include magnesium oxide and zinc oxide (for example, zinc oxide such as "Zinc Oxide Type 2" (product name; manufactured by Hakusui Tech Co., Ltd.)). The amount of vulcanization aid is usually 1 to 20 parts by mass per 100 parts by mass of copolymer (S).

The rubber composition of the present invention may contain a foaming agent. When the rubber composition of the present invention contains a foaming agent, it usually contains the crosslinking agent as well. By using a rubber composition containing a crosslinking agent and a foaming agent, the rubber composition can be crosslinked and foamed to obtain a foam.

Examples of foaming agents include inorganic foaming agents such as sodium bicarbonate and sodium carbonate; nitroso compounds such as N,N'-dinitrosopentamethylenetetramine and N,N'-dinitrosoterephthalamide; azo compounds such as azodicarbonamide (ADCA) and azobisisobutyronitrile; hydrazide compounds such as benzenesulfonylhydrazide and p,p'-oxybis(benzenesulfonylhydrazide) (OBSH); and organic foaming agents such as azide compounds such as calcium azide and 4,4'-diphenyldisulfonyl azide. ADCA and OBSH are preferred as foaming agents. The foaming agents may be used alone or in combination of two or more.

When the rubber composition of the present invention contains a foaming agent, the amount of the foaming agent is usually 0.2 to 30 parts by mass, preferably 0.5 to 25 parts by mass, and more preferably 0.5 to 20 parts by mass, per 100 parts by mass of copolymer (S).

When the rubber composition of the present invention contains a foaming agent, it may further contain a foaming assistant as necessary. The foaming assistant has the effects of lowering the decomposition temperature of the foaming agent, accelerating the decomposition, and homogenizing the bubbles.
Examples of such foaming assistants include organic acids such as salicylic acid, phthalic acid, stearic acid, oxalic acid, and citric acid, and salts thereof, and urea and its derivatives.

When the rubber composition of the present invention contains a foaming aid, the amount of the foaming aid is usually 0.2 to 30 parts by mass, preferably 0.5 to 25 parts by mass, and more preferably 0.5 to 20 parts by mass, per 100 parts by mass of copolymer (S).

Antioxidant: The rubber composition according to the present invention preferably contains an antioxidant in order to extend the material life. Examples of such antioxidants include:
Aromatic secondary amine stabilizers such as phenylnaphthylamine, 4,4'-(α,α-dimethylbenzyl)diphenylamine, and N,N'-di-2-naphthyl-p-phenylenediamine;
Phenolic stabilizers such as 2,6-di-t-butyl-4-methylphenol and tetrakis-[methylene-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate]methane;
Thioether-based stabilizers such as bis[2-methyl-4-(3-n-alkylthiopropionyloxy)-5-t-butylphenyl]sulfide; benzimidazole-based stabilizers such as 2-mercaptobenzimidazole;
Dithiocarbamate stabilizers such as nickel dibutyldithiocarbamate;
and quinoline-based stabilizers such as polymers of 2,2,4-trimethyl-1,2-dihydroquinoline. These may be used alone or in combination of two or more kinds.

Processing aid The rubber composition of the present invention may contain a processing aid. As the processing aid, a wide variety of processing aids that are generally compounded in rubber as processing aids can be used. Specific examples include ricinoleic acid, stearic acid, palmitic acid, lauric acid, barium stearate, zinc stearate, calcium stearate, and esters. Among these, stearic acid is preferred.

When the rubber composition of the present invention contains a processing aid, the amount of the processing aid is usually 0.1 to 10 parts by mass, preferably 0.5 to 8 parts by mass, and more preferably 1 to 6 parts by mass, per 100 parts by mass of copolymer (S). If the amount is within the above range, there is no bloom on the surface of the resulting rubber composition, and furthermore, crosslinking inhibition does not occur when the rubber composition is crosslinked, which is preferable. In addition, a rubber composition containing a processing aid is preferable because it has excellent moldability, such as extrusion moldability, press moldability, and injection moldability, and roll processability.

Surfactant The rubber composition of the present invention may contain a surfactant. Examples of the surfactant include amines such as di-n-butylamine, dicyclohexylamine, monoethanolamine, triethanolamine, "Acting B" (manufactured by Yoshitomi Pharmaceutical Co., Ltd.), and "Acting SL" (manufactured by Yoshitomi Pharmaceutical Co., Ltd.), polyethylene glycol, diethylene glycol, lecithin, triallyl trimellitate, zinc compounds of aliphatic and aromatic carboxylic acids (e.g., "Struktol activator 73", "Struktol IB 531", and "Struktol FA541" manufactured by Schill & Seilacher), "ZEONET ZP" (manufactured by Zeon Corporation), octadecyltrimethylammonium bromide, synthetic hydrotalcite, and special quaternary ammonium compounds (e.g., "Lipoguard 2HT-F" (manufactured by Lion Akzo Co., Ltd.).

When the rubber composition of the present invention contains a surfactant, the amount of the surfactant is usually 0.2 to 10 parts by mass, preferably 0.3 to 5 parts by mass, and more preferably 0.5 to 4 parts by mass, per 100 parts by mass of the copolymer (S). The surfactant can be appropriately selected depending on the application, and can be used alone or in combination of two or more types.

Antiaging agent The rubber composition of the present invention may contain an antiaging agent. When the rubber composition of the present invention contains an antiaging agent, it is possible to extend the life of the product obtained from the composition. As the antiaging agent, a conventionally known antiaging agent, such as an amine-based antiaging agent, a phenol-based antiaging agent, or a sulfur-based antiaging agent, can be used.

Specific examples of anti-aging agents include aromatic secondary amine-based anti-aging agents such as phenylbutylamine and N,N-di-2-naphthyl-p-phenylenediamine, phenol-based anti-aging agents such as dibutylhydroxytoluene and tetrakis[methylene(3,5-di-t-butyl-4-hydroxy)hydrocinnamate]methane, thioether-based anti-aging agents such as bis[2-methyl-4-(3-n-alkylthiopropionyloxy)-5-t-butylphenyl]sulfide, dithiocarbamate-based anti-aging agents such as nickel dibutyldithiocarbamate, and sulfur-based anti-aging agents such as 2-mercaptobenzoylimidazole, zinc salt of 2-mercaptobenzimidazole, dilaurylthiodipropionate, and distearylthiodipropionate.

When the rubber composition of the present invention contains an anti-aging agent, the amount of the anti-aging agent is usually 0.01 to 10 parts by mass, preferably 0.02 to 7 parts by mass, and more preferably 0.03 to 5 parts by mass, per 100 parts by mass of copolymer (S). If the amount is within the above range, the molded article obtained from the rubber composition of the present invention has excellent heat aging resistance, which is preferable.

Other Additives The rubber composition of the present invention may further contain other additives, such as a heat stabilizer, a weather stabilizer, an antistatic agent, a colorant, a lubricant, and a thickener.

In addition, the rubber composition according to the present invention can contain various optional components without any particular restrictions as described above, but the copolymer (S) and diene rubber (T) constituting the rubber composition have good compatibility, and when a rubber composition containing them is used, a crosslinked molded article can be produced without phase separation. Moreover, since the copolymer (S) imparts excellent weather resistance to the obtained crosslinked molded article, a crosslinked molded article with excellent weather resistance can be easily obtained even when the content of weather resistance agents, antioxidants, etc. in the rubber composition is suppressed. Therefore, the content of additives such as weather resistance agents and antioxidants can be suitably suppressed, which is economical and prevents deterioration of the quality of the crosslinked molded article due to bleed-out.

<Method of preparing rubber composition>
The rubber composition of the present invention is a rubber composition containing the above-mentioned copolymer (S) and diene rubber (T), and preferably contains components such as a crosslinking agent, carbon black, a white filler, a silane coupling agent, etc. For example, a rubber composition containing 0.2 to 15 parts by mass of a crosslinking agent, 5 to 100 parts by mass of carbon black, 5 to 150 parts by mass of a white filler, and 0.2 to 10 parts by mass of a silane coupling agent relative to 100 parts by mass of the diene rubber (T) can be mentioned, but the preparation method thereof is not particularly limited.

Examples of methods for preparing the rubber composition include a method of mixing each component contained in the rubber composition using a conventionally known kneading machine such as a mixer, kneader, or roll, or further a continuous kneading machine such as a twin-screw extruder, and a method of preparing a solution in which each component contained in the rubber composition is dissolved or dispersed, and then removing the solvent.
The rubber composition according to the present invention can be prepared by simultaneously or successively blending the copolymer (S), the diene rubber (T), and, if necessary, any optional components.

The method for preparing the rubber composition is not particularly limited, and a general method for preparing rubber compounds can be used without any particular restrictions. For example, when the rubber composition of the present invention contains optional components, at least a portion of the optional components may be mixed in advance with the copolymer (S) or the diene rubber (T) and then the remaining optional components may be blended. Alternatively, the optional components may be added and blended after blending the copolymer (S) and the diene rubber (T).

For example, the copolymer (S), diene rubber (T), and other components to be mixed as required are kneaded for 3 to 10 minutes at a temperature of 80 to 170°C using an internal mixer such as a Banbury mixer, kneader, or intermix, and then a crosslinking agent and, if necessary, a crosslinking accelerator, crosslinking assistant, foaming agent, etc. are added as required, and the mixture is kneaded for 5 to 30 minutes at a roll temperature of 40 to 80°C using rolls such as open rolls or a kneader, and then separated out. In this way, a rubber composition in the form of a ribbon or sheet is usually obtained. When the kneading temperature in the internal mixers is low, the crosslinking agent, crosslinking accelerator, foaming agent, etc. can also be kneaded simultaneously.

<Crosslinked Molded Article>
The crosslinked molded article of the present invention is obtained by crosslinking the above-mentioned rubber composition of the present invention. Note that a mold may or may not be used during crosslinking. When a mold is not used, the rubber composition is usually molded and crosslinked continuously.

Examples of methods for crosslinking a rubber composition include (a) a method in which a rubber composition containing a crosslinking agent is preformed into a desired shape, usually by a molding method such as extrusion molding, press molding, or injection molding, or by roll processing, and then heated while molding or by introducing the molded product into a crosslinking tank, and (b) a method in which a rubber composition containing a crosslinking agent is preformed in the same manner as in (a), and then irradiated with an electron beam.

In method (a), a crosslinking reaction occurs due to the crosslinking agent in the rubber composition when heated, resulting in a crosslinked product. In method (b), a crosslinking reaction occurs due to an electron beam, resulting in a crosslinked product. In method (b), the preformed rubber composition is usually irradiated with an electron beam having an energy of 0.1 to 10 MeV so that the absorbed dose of the rubber composition is usually 0.5 to 36 Mrad, preferably 0.5 to 20 Mrad, and more preferably 1 to 10 Mrad.

In addition, the crosslinking of the rubber composition can be carried out by preforming the uncrosslinked rubber composition into a desired shape by various molding methods using a molding machine such as an extrusion molding machine, a calendar roll, a press, an injection molding machine, or a transfer molding machine, and then either simultaneously with molding or by introducing the molded product into a crosslinking tank and heating it, or by irradiating it with radiation such as electron beams, X-rays, gamma rays, alpha rays, and beta rays. As a molding or preforming method, a known molding method for molding into a desired shape by extrusion molding, injection molding, inflation molding, blow molding, extrusion blow molding, press molding, vacuum molding, calendar molding, and foam molding can be appropriately adopted. In addition, when the crosslinked molded product is a foam, it can be produced by foam molding an uncrosslinked rubber composition containing a foaming agent, followed by crosslinking by electron beam irradiation or heating, or by proceeding with crosslinking simultaneously with foam molding. Furthermore, the step of crosslinking the rubber composition may be carried out by combining crosslinking by heating and electron beam crosslinking.

When crosslinking the above rubber composition by heating, it is usually preferable to use a rubber composition containing a crosslinking agent such as sulfur, a sulfur-based compound, or peroxide, and heat the rubber composition for 1 to 30 minutes at a temperature of 150 to 270°C using a crosslinking bath with a heating form such as hot air, a glass bead fluidized bed, UHF (ultra-high frequency electromagnetic waves), steam, or an LCM (molten salt bath). Sulfur crosslinking or peroxide crosslinking has the advantage that no special equipment is required for the crosslinking process, and has therefore been widely used in the crosslinking process of rubber compositions.

When crosslinking is performed by electron beam crosslinking, it is usually preferable to use a rubber composition that does not contain a crosslinking agent, and irradiate the preformed rubber composition with electron beams to produce a crosslinked molded product. Crosslinking by electron beam irradiation can be performed without using a crosslinking agent, and has the advantage of generating less volatile matter during the crosslinking process.

The manufacture of crosslinked molded products involving a crosslinking step by electron beam irradiation can be carried out, for example, as follows. First, using a mixer such as a Banbury mixer, the copolymer (S), diene rubber (T), and various additives and crosslinking aids, etc., as necessary, are kneaded for 3 to 10 minutes at a temperature of 80 to 170 ° C., and then using rolls such as open rolls, the mixture is kneaded for 5 to 30 minutes at a roll temperature of 40 to 80 ° C., and then separated to prepare a ribbon-shaped or sheet-shaped rubber composition, or the rubber composition is prepared by blending each component in a container, etc. The rubber composition thus prepared is either molded into a desired shape as it is in a sheet form, or by an extrusion molding machine, calendar roll, injection molding machine, or press, or extruded from an extruder into a strand shape and crushed into pellets using a cutter, etc., and irradiated with an electron beam. Alternatively, a powder of the copolymer (S) and diene rubber (T) impregnated with a compound such as a crosslinking aid may be directly irradiated with an electron beam to prepare a crosslinked product of the rubber composition. Electron beam irradiation is typically performed with an energy of 0.1 to 10 MeV (megaelectronvolts), preferably 0.3 to 5 MeV, so that the absorbed dose is typically 0.5 to 100 kGy (kilograys), preferably 0.5 to 70 kGy.

Gamma ray irradiation has a higher permeability to rubber compositions than electron beam irradiation, and in particular when irradiating rubber compositions in pellet form, direct irradiation with a small amount of radiation is sufficient to crosslink the interior of the pellets. Gamma ray irradiation can be performed so that the amount of gamma ray irradiation on the rubber composition is usually 0.1 to 50 kGy, preferably 0.3 to 50 kGy.

The degree of crosslinking of a crosslinked molded body can be expressed as the gel fraction. Usually, the gel fraction of a crosslinked body is 1 to 80%. However, in the crosslinked molded body of the present invention, the degree of crosslinking is not limited to this range, and even in a crosslinked body with a low degree of crosslinking, which shows a gel fraction of less than 10%, particularly less than 0.5%, the same effect of excellent surface appearance can be obtained as with the crosslinked molded body of the present invention with a high degree of crosslinking.

The crosslinked molded article according to the present invention can be used without restriction in various products having rubber properties. The crosslinked molded article according to the present invention may constitute at least a part of the product, and it is also preferable that the whole product is constituted by the crosslinked molded article according to the present invention. It is also preferable that the crosslinked molded article according to the present invention constitutes at least a part of the product, and that the product is a laminate or composite. Examples of laminates include multilayer laminates having two or more layers, at least one of which is the crosslinked molded article according to the present invention, and examples of such forms include multilayer films and sheets, multilayer containers, multilayer tubes, and multilayer coating film laminates contained as one component of water-based paints.

The cross-linked molded article according to the present invention has particularly excellent weather resistance, and can therefore be suitably used for applications requiring long-term outdoor use, such as tires and electric wire covering materials, and can be particularly suitably used for tire components that constitute at least a portion of various types of tires.

Examples of tire components include tire innerliners, tire innertubes, tire flaps, tire shoulders, tire beads, tire treads, and tire sidewalls. Among these, the tire tread and tire sidewalls can be suitably used, and the tire treads can be particularly suitably used.

The crosslinked molded article of the present invention retains the excellent mechanical strength and fatigue resistance (long life performance) inherent to diene rubber, and also exhibits excellent wet grip performance and braking performance on ice. Tire components such as tire treads and tire sidewalls that use the crosslinked molded article of the present invention have excellent weather resistance and dynamic fatigue resistance properties.

<Foam>
The foam of the present invention can be obtained by crosslinking and foaming the rubber composition of the present invention containing the foaming agent described above.

The rubber composition contains a foaming agent, so when the rubber composition is heated, the crosslinking reaction by the crosslinking agent occurs and the foaming agent decomposes to generate carbon dioxide gas and nitrogen gas. This results in a foam with a cell structure.

<Applications>
The rubber composition of the present invention is extremely excellent in low-temperature properties, mechanical properties, moldability such as extrusion moldability, press moldability, and injection moldability, and roll processability, and from the rubber composition of the present invention, a molded article having excellent low-temperature properties (flexibility at low temperatures, rubber elasticity, etc.), mechanical properties, etc. can be suitably obtained.

In addition, by using the copolymer (S) described above, the rubber composition of the present invention can produce a crosslinked product that is excellent in processability, moldability, and crosslinking properties, and that has excellent heat resistance stability, long life performance, wet grip performance, and braking performance on ice. Therefore, the crosslinked product obtained from the rubber composition of the present invention can be suitably used in applications where long-term use at high temperatures is expected, and in applications such as studless tires.

The rubber composition of the present invention and molded articles obtained from the composition, such as crosslinked bodies and foamed bodies, can be used in various applications. Specifically, they are suitable for use in rubber materials for tires, O-rings, industrial rolls, packings (e.g., condenser packings), gaskets, belts (e.g., heat-insulating belts, copier belts, conveyor belts), hoses such as automobile hoses (e.g., turbocharger hoses, water hoses, brake reservoir hoses, radiator hoses, air hoses), anti-vibration rubber, anti-vibration materials or vibration-damping materials (e.g., engine mounts, motor mounts), muffler hangers, sponges (e.g., weather strip sponges, heat-insulating sponges, protective sponges, microfoam sponges), cables (ignition cables, cab-tire cables, high-tension cables), electric wire coating materials (high-voltage electric wire coating materials, low-voltage electric wire coating materials, marine electric wire coating materials), glass run channels, colored skin materials, paper feed rolls, roofing sheets, etc. Among these, it is particularly suitable for use in interior and exterior automotive parts and in applications requiring heat resistance, and is also suitable as a rubber material for tires, such as tire treads.

[Rubber materials for tires]
The rubber material for tires according to the present invention is characterized by being made of the rubber composition of the present invention. The rubber material for tires according to the present invention achieves both excellent (on-ice) braking performance and excellent fuel economy performance, and is also excellent in rubber elasticity, weather resistance, and ozone resistance, and is particularly excellent in mechanical properties and fatigue resistance. The rubber material also has excellent wear resistance. Therefore, by applying the rubber material for tires according to the present invention, a tire can be obtained that achieves both excellent braking performance and excellent fuel economy performance, is also excellent in rubber elasticity, weather resistance, and ozone resistance, and is particularly excellent in mechanical properties, fatigue resistance (long life performance), and wet grip. Also, a tire with excellent wear resistance can be obtained.

[Tire tread]
The tire tread according to the present invention is formed by using the rubber material for tires according to the present invention. By applying the tire tread obtained by vulcanizing the rubber material for tires according to the present invention, a tire can be obtained which has both excellent (on-ice) braking performance and excellent fuel economy performance, is excellent in rubber elasticity, weather resistance, and ozone resistance, and is particularly excellent in mechanical properties, fatigue resistance (long life performance), and wet grip. In addition, a tire with excellent wear resistance can be obtained.

In the present invention, a copolymer (S) having appropriate properties according to the application can be selected and used within the range that satisfies the above-mentioned requirements. For example, a copolymer (S) having a relatively low molecular weight can be suitably used for tire tread applications.

[tire]
The tire according to the present invention includes the tire tread according to the present invention. The tire according to the present invention has both excellent (on-ice) braking performance and excellent fuel economy, and is also excellent in rubber elasticity, weather resistance, and ozone resistance, and is particularly excellent in mechanical properties, fatigue resistance (long life performance), and wet grip. The tire also has excellent wear resistance.

The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples. The methods for evaluating each characteristic in the examples and comparative examples are as follows.

<Composition of ethylene/α-olefin copolymer>
The mass fraction (mass%) of each structural unit of the ethylene-α-olefin copolymer was determined by ^13C -NMR measurement. The measurement was performed by measuring the 13C-NMR spectrum of the copolymer using an ECX400P nuclear magnetic resonance spectrometer (manufactured by JEOL Ltd.) at a measurement temperature of 120°C, a measurement solvent of orthodichlorobenzene/deuterated benzene= ⁴ /1, and an accumulation count of 8,000.

<Intrinsic viscosity>
The intrinsic viscosity [η] was measured using a fully automatic intrinsic viscometer manufactured by Rigo Co., Ltd. at a temperature of 135° C. and in decalin as a measurement solvent.

<Mooney Viscosity>
The Mooney viscosity ML ₍₁₊₄₎ 100°C was measured at 100°C using a Mooney viscometer "SMV-202" (manufactured by Shimadzu Corporation) in accordance with JIS K6300 (1994).

<Weight average molecular weight (Mw), number average molecular weight (Mn), molecular weight distribution (Mw/Mn)>
The weight average molecular weight (Mw), number average molecular weight (Mn), and molecular weight distribution (Mw/Mn) are values calculated in terms of polystyrene measured by gel permeation chromatography (GPC). The measurement apparatus and conditions are as follows: The molecular weight was calculated based on a calibration curve prepared using commercially available monodisperse polystyrene and a conversion method.

Apparatus: Gel permeation chromatograph Alliance GP2000 (Waters),
Analysis device: Empower2 (manufactured by Waters),
Column: TSKgel GMH6-HT x 2 + TSKgel GMH6-HTL x 2 (7.5 mm I.D. x 30 cm, manufactured by Tosoh Corporation),
Column temperature: 140 ° C.
Mobile phase: o-dichlorobenzene (containing 0.025% BHT),
Detector: differential refractometer (RI),
Flow rate: 1.0mL/min,
Injection volume: 400 μL,
Sampling time interval: 1 s,
Column calibration: monodisperse polystyrene (Tosoh Corporation),
Molecular weight conversion: Old method EPR conversion/calibration method taking viscosity into account.

<Physical Properties of Unvulcanized Rubber Composition>
(1) Mooney Scorch The minimum viscosity (Vm) and scorch time (t5) at 125°C were measured in accordance with JIS K6300 using a Mooney viscometer (SMV202 model, manufactured by Shimadzu Corporation) at 125°C.

(2) Vulcanization Speed Using the unvulcanized rubber compositions in the Examples and Comparative Examples, the vulcanization speed (tc90) was measured as follows under the measurement conditions of a temperature of 170° C. and a time of 20 minutes using a measuring device: MDR2000 (manufactured by ALPHA TECHNOLOGIES).
The torque change obtained under the conditions of a constant temperature and a constant shear rate was measured. The time required to reach a torque that was 90% of the difference between the maximum and minimum torque values was defined as the vulcanization rate (tc90; min).

<Hardness test (Duro-A hardness)>
In accordance with JIS K 6253, the hardness (type A durometer, HA) of the crosslinked sheet was measured using six 2 mm sheet-like rubber molded products with smooth surfaces, stacked on the flat parts to a thickness of approximately 12 mm. However, test pieces containing foreign matter, air bubbles, or scratches were not used. The dimensions of the measurement surface of the test piece were set so that measurements could be made with the tip of the indenter at a position 12 mm or more away from the end of the test piece. The lower the hardness, the better the braking performance on ice.

<Tensile test>
According to JIS K 6251, a tensile test was performed at a measurement temperature of 23°C and a tensile speed of 500 mm/min, and the breaking strength (TB) [MPa] and breaking elongation (EB) [%] of the sheet were measured. That is, a No. 3 dumbbell test piece described in JIS K 6251 (2001) was prepared by punching out the sheet-like crosslinked molded body. Using this test piece, a tensile test was performed according to the method specified in JIS K 6251 at a measurement temperature of 25°C and a tensile speed of 500 mm/min, and the tensile stress at 25% elongation [25% modulus (M25)], the tensile stress at 100% elongation [100% modulus (M100)], the tensile stress at break (TB), and the tensile elongation at break (EB) were measured. The lower the M25, the better the fatigue resistance (long life performance) and braking performance on ice, and the lower the M100, the better the wet grip performance. Also, the higher the EB, the better the fatigue resistance (long life performance).

<Compression set (CS)>
According to JIS K 6262, a crosslinked body having a diameter of 29 mm and a height (thickness) of 12.5 mm was used as a test piece. The test piece was compressed by 25% from the height (12.5 mm) before applying a load, and was set together with the spacer in a gear oven at 70°C and heat-treated for 22 hours. The test piece was then taken out and left at room temperature for 30 minutes, after which the height of the test piece was measured and the compression set (%) was calculated using the following formula.
The 0° C. compression set was measured by placing the test piece in a thermostatic chamber at 0° C. for 22 hours. The test piece was then removed from the thermostatic chamber and allowed to stand for 30 minutes, after which the height of the test piece was measured and the compression set (%) was calculated using the following formula:
Compression set (%) = {(t0-t1)/(t0-t2)} x 100
t0: Height of the test piece before the test t1: Height of the test piece after being treated under the above conditions and left at room temperature for 30 minutes t2: Height of the test piece when attached to the measurement mold

<Tensile viscoelasticity test>
Storage modulus E': Dynamic viscoelasticity was measured under nitrogen for 1 mm sheets of vulcanized rubber obtained in the Examples and Comparative Examples using RSA-G2 manufactured by TA-Instruments. Here, the storage modulus (E') is a term constituting the complex modulus which expresses the relationship between stress and strain when a sinusoidal oscillatory strain is applied to a viscoelastic body, and is a value measured using RSA-G2 manufactured by TA-Instruments in tension mode (strain 1%) in the temperature range of -70°C to 100°C, at a heating rate of 4°C/min, and at a frequency of 10 Hz.
Tan δ: For 1 mm sheets of vulcanized rubber obtained in the Examples and Comparative Examples, dynamic viscoelasticity was measured under nitrogen using RSA-G2 manufactured by TA-Instruments. Here, tan δ (0°C, 60°C) = E"/E' is the value calculated.
The lower the E'@-20°C (E+7) and E'@0°C (E+7), the better the braking performance on ice. The higher the tan δ0°C, the better the wet grip.

[Example 1]
<Preparation of Rubber Composition and Production of Crosslinked Molded Article>
The mixture was mixed with 20 parts by mass of an ethylene-propylene copolymer (S-1) shown in Table 1 below (product name: Mitsui EPT (registered trademark) 0045, manufactured by Mitsui Chemicals, Inc.), 55 parts by mass of natural rubber (NR (RSS #3), sold by Takehara Rubber Co., Ltd.) as diene rubber, 45 parts by mass of butadiene rubber (Nipole BR1200, manufactured by Nippon Zeon Co., Ltd.), 3 parts by mass of zinc oxide (ZnO #1, zinc oxide type 2, manufactured by Hakusui Tech Co., Ltd.) as a crosslinking aid, 2 parts by mass of stearic acid (Camellia stearic acid series, manufactured by NOF Corporation) as a processing aid, 15 parts by mass of carbon black (Show Black N-339, manufactured by Showa Cabot Co., Ltd.), and 65 parts by mass of silica (Nipsil VN3, manufactured by Tosoh Silica Co., Ltd.) as a white filler. 20 parts by mass of silica, 4.5 parts by mass of a silane coupling agent (Si-69, manufactured by EVONIK Co., Ltd.), 2 parts by mass of sulfur as a crosslinking agent, 2 parts by mass of Suncerer CM (manufactured by Sanshin Chemical Co., Ltd.) and Suncerer D (manufactured by Sanshin Chemical Co., Ltd.) as vulcanization accelerators, and 1.8 parts by mass of aromatic oil (AH-16, manufactured by Idemitsu Kosan Co., Ltd.) as a softener were kneaded using a BB-4 type Banbury mixer (manufactured by Kobe Steel, Ltd.) to obtain a rubber compound. In the kneading, silica/coupling agent/polymer were masticated for 2 minutes, and then zinc oxide, stearic acid, carbon black, and aromatic oil were added and kneaded for 2 minutes. Thereafter, the ram was raised and cleaned, and kneaded for another 1 minute to obtain an unvulcanized rubber compound (A-1).

The rubber compound (A-1) was kneaded by wrapping it around an 8-inch roll (manufactured by Nippon Roll Co., Ltd.) with the front roll having a surface temperature of 50°C, the rear roll having a surface temperature of 50°C, the front roll rotating at 18 rpm, and the rear roll rotating at 16 rpm.

The mixture was kneaded by cutting it three times and rolling it six times to obtain a rubber composition in the form of a sheet having a thickness of 2.2 to 2.5 mm. The obtained rubber composition was used to evaluate the properties of the unvulcanized rubber compound (A-1). The results are shown in Table 2.

Then, a press molding machine was used to perform a pressing process at 170°C for 10 minutes to produce crosslinked body (A-2) sheets with thicknesses of 2 mm and 1 mm. The crosslinked body (A-2) sheets thus obtained were used to perform hardness tests, tensile tests, and tensile viscoelasticity tests.

Further, crosslinking was carried out at 170° C. for 15 minutes to obtain a crosslinked product (A-3) having a thickness of 12.5 mm and a diameter of 29 mm. The crosslinked product (A-3) was used to measure the compression set.
The evaluation results of the rubber composition and the crosslinked product are shown in Table 2.

[Comparative Example 1]
A rubber composition and a crosslinked molded article were produced in the same manner as in Example 1, except that the ethylene-propylene copolymer was not used, and various physical properties were measured. The results are shown in Table 2.

Claims (5)

An ethylene/α-olefin copolymer (S) having structural units derived from ethylene (A) and an α-olefin (B) having 3 to 20 carbon atoms and satisfying the following requirements (i) and (ii):
and a diene rubber (T),
The copolymer (S) is contained in an amount of 0.5 parts by mass or more and 50 parts by mass or less per 100 parts by mass of the diene rubber (T) ,
A rubber composition comprising, relative to 100 parts by mass of the diene rubber (T), 0.2 to 15 parts by mass of a crosslinking agent, 15 to 100 parts by mass of carbon black, 65 to 150 parts by mass of a white filler, and 0.2 to 10 parts by mass of a silane coupling agent :
(i) the molar ratio of the structural units derived from ethylene (A) to the structural units derived from an α-olefin (B) [(A)/(B)] is 40/60 to 99.9/0.1;
(ii) The Mooney viscosity at 100° C. [ML ₍₁₊₄₎ (100° C.), JIS K6300] is 5 to 100. The rubber composition according to claim 1, characterized in that the copolymer (S) is contained in an amount of 1 part by mass or more and 40 parts by mass or less per 100 parts by mass of the diene rubber (T). A rubber material for tires, comprising the rubber composition according to claim 1 or 2 . A tire tread formed using the rubber material for tires according to claim 3 . A tire comprising the tire tread of claim 4 .