JP7499660B2 - Rubber composition for studless tires and studless tire using same - Google Patents

Description

The present invention relates to a rubber composition for studless tires and a studless tire using the same. More specifically, the present invention relates to a rubber composition for studless tires that can improve performance on ice while maintaining fracture properties at a level sufficient for practical use, and a studless tire using the same.

On snowy and icy roads, the coefficient of friction is lower than on normal roads, making the roads more slippery. For this reason, numerous methods have been proposed to improve the performance of studless tires on ice (braking performance on ice). For example, one method is known in which hard foreign matter or hollow polymers are blended into the rubber, which creates microscopic irregularities on the rubber surface, thereby removing the water film that forms on the ice surface and improving friction on ice (see, for example, Patent Document 1). On the other hand, there are also methods such as using rubber that has low-temperature flexibility.

However, the above method has the problem that it is not possible to achieve both the breaking properties of the rubber and its performance on ice.

Japanese Patent Application Laid-Open No. 11-35736

The object of the present invention is therefore to provide a rubber composition for studless tires that can improve performance on ice while maintaining breaking properties at a level sufficient for practical use, and a studless tire using the same.

As a result of extensive research, the inventors discovered that a rubber composition containing a diene rubber, a white filler, and thermally expandable microcapsules, as well as a specific amount of ethylene-1-butene-ENB copolymer, could solve the above problems, and thus completed the present invention.

The present invention relates to a composition comprising, per 100 parts by mass of diene rubber,
30 to 100 parts by mass of white filler,
The rubber composition for studless tires is characterized by comprising 0.5 to 20 parts by mass of thermally expandable microcapsules and 3 to 50 parts by mass of an ethylene-1-butene-ENB copolymer that contains a structural unit derived from ethylene [A], a structural unit derived from 1-butene [B], and a structural unit derived from 5-ethylidene-2-norbornene (ENB) [C] and satisfies the following (1) to (4):
(1) the molar ratio [[A]/[B]] of the structural unit derived from ethylene [A] to the structural unit derived from 1-butene [B] is 60/40 to 80/20;
(2) The content of the structural unit derived from ENB[C] is 1.0 to 3.0 mol %, based on 100 mol % of the total of the structural units [A], [B] and [C];
(3) Mooney viscosity ML ₍₁₊₄₎ 125°C is 10 to 50;
(4) The B value represented by the following formula (i) is 1.20 or more.
B value = ([EX] + 2[Y]) / [2 x [E] x ([X] + [Y])] (i)
[Here, [E], [X] and [Y] represent the mole fractions of ethylene [A], 1-butene [B] and ENB [C], respectively, and [EX] represents the ethylene [A]-1-butene [B] dyad sequence fraction.]

The rubber composition for studless tires of the present invention is characterized by being composed of 100 parts by mass of diene rubber, 30 to 100 parts by mass of white filler, 0.5 to 20 parts by mass of thermally expandable microcapsules, and 3 to 50 parts by mass of ethylene-1-butene-ENB copolymer, and therefore can provide a rubber composition and studless tires using the same that can improve performance on ice while maintaining fracture properties at a practically sufficient level.

The present invention will be described in more detail below.

(Diene rubber)
The diene rubber used in the present invention can be any diene rubber that can be blended in a rubber composition, and examples thereof include natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene copolymer rubber (SBR), acrylonitrile-butadiene copolymer rubber (NBR), ethylene-propylene-diene terpolymer (EPDM), and the like. These may be used alone or in combination of two or more. In addition, the molecular weight and microstructure are not particularly limited, and the rubber may be terminally modified with an amine, amide, silyl, alkoxysilyl, carboxyl, hydroxyl group, or the like, or may be epoxidized.
The diene rubber referred to in the present invention does not include the ethylene-1-butene-ENB copolymer described below.

(white filler)
Specific examples of the white filler used in the present invention include silica, calcium carbonate, magnesium carbonate, talc, clay, alumina, aluminum hydroxide, titanium oxide, calcium sulfate, and the like. These may be used alone or in combination of two or more.
Of these, silica is preferred because of its better performance on ice.

Specific examples of silica include wet silica (hydrated silicic acid), dry silica (anhydrous silicic acid), calcium silicate, aluminum silicate, etc., and these may be used alone or in combination of two or more types.

From the viewpoint of improving performance on ice, the silica preferably has a CTAB adsorption specific surface area of 50 to 300 m ² /g, and more preferably 90 to 200 m ² /g.
The CTAB adsorption specific surface area is a value obtained by measuring the amount of n-hexadecyltrimethylammonium bromide adsorbed on the silica surface in accordance with JIS K6217-3:2001 "Part 3: Determination of specific surface area - CTAB adsorption method."

(Thermal expansion microcapsules)
In the present invention, the thermally expandable microcapsule has a structure in which a thermally expandable substance is encapsulated in a shell material formed of a thermoplastic resin. The shell material of the thermally expandable microcapsule can be formed of a nitrile polymer.
The thermally expandable substance contained in the shell material of the microcapsule has the property of vaporizing or expanding by heat, and is, for example, at least one selected from the group consisting of hydrocarbons such as isoalkanes and normal alkanes. Examples of isoalkanes include isobutane, isopentane, 2-methylpentane, 2-methylhexane, and 2,2,4-trimethylpentane, and examples of normal alkanes include n-butane, n-propane, n-hexane, n-heptane, and n-octane. These hydrocarbons may be used alone or in combination. A preferred form of the thermally expandable substance is one in which a hydrocarbon that is gaseous at room temperature is dissolved in a hydrocarbon that is liquid at room temperature. By using such a mixture of hydrocarbons, sufficient expansion force can be obtained from the low temperature region to the high temperature region in the vulcanization molding temperature range (150°C to 190°C) of an unvulcanized tire.
Examples of such thermally expandable microcapsules include those manufactured by Expancel AB of Sweden under the trade names "EXPANCEL 091DU-80" or "EXPANCEL 092DU-120" and those manufactured by Matsumoto Yushi Seiyaku Co., Ltd. under the trade names "Matsumoto Microsphere F-85D" or "Matsumoto Microsphere F-100D".

(Ethylene-1-butene-ENB copolymer)
The rubber composition of the present invention is prepared by blending an ethylene-1-butene-ENB copolymer which contains a structural unit derived from ethylene [A], a structural unit derived from 1-butene [B], and a structural unit derived from 5-ethylidene-2-norbornene (ENB) [C] and satisfies the following (1) to (4):

(1) The molar ratio [[A]/[B]] of the structural units derived from ethylene [A] to the structural units derived from 1-butene [B] is 60/40 to 80/20. By having the molar ratio in this range, an excellent balance between rubber elasticity at low temperatures and tensile strength at room temperature is achieved.

(2) The content of structural units derived from ENB[C] is 1.0 to 3.0 mol %, with the sum of the structural units [A], [B] and [C] being 100 mol %. With the molar ratio in this range, the material has sufficient flexibility.

(3) The Mooney viscosity at 125° C., ML ₍₁₊₄₎ 125° C., is 10 to 50. When the Mooney viscosity is within the above range, component (A) has excellent processability and rubber physical properties.

(4) The B value represented by the following formula (i) is 1.20 or more.
B value = ([EX] + 2[Y]) / (2 x [E] x ([X] + [Y])) (i)
[Here, [E], [X] and [Y] represent the mole fractions of ethylene [A], 1-butene [B] and ENB [C], respectively, and [EX] represents the ethylene [A]-1-butene [B] dyad sequence fraction.]
When the B value is within the above range, the processability of the resulting rubber composition is improved.

The B value is an index showing the randomness of copolymerization monomer sequence distribution in a copolymer, and [E], [X], [Y], and [EX] in the formula (i) can be determined by measuring ¹³ C-NMR spectrum based on the reports of JC Randal [Macromolecules, 15, 353 (1982)], J. Ray [Macromolecules, 10, 773 (1977)], etc. Meanwhile, the molar amounts of the structural unit derived from ethylene [A], the structural unit derived from 1-butene [B], and the structural unit derived from ethylidene-2-norbornene (ENB) [C] in the above (1) and (2) can be determined by intensity measurement using a ¹ H-NMR spectrometer.

The ethylene/1-butene/ENB copolymer can be obtained by a conventionally known production method using a metallocene catalyst. As the metallocene catalyst and the production method using the catalyst, for example, the examples described in WO 2015/122415, particularly paragraphs [0249] to [0320] of the publication, can be adopted.

in particular,
A transition metal compound (a) represented by the following formula (a),
An ethylene-1-butene-ENB copolymer can be obtained by copolymerizing ethylene [A], 1-butene [B], and 5-ethylidene-2-norbornene (ENB) [C] in the presence of an olefin polymerization catalyst containing at least one compound (b) selected from an organometallic compound (b-1), an organoaluminum oxy compound (b-2), and a compound (b-3) that reacts with a transition metal compound (a) to form an ion pair.

The above formula (a) will now be described.
M is a titanium atom, a zirconium atom or a hafnium atom.
Each R is independently a substituted aryl group in which one or more hydrogen atoms of the aryl group are substituted with an electron-donating group having a Hammett's rule substituent constant σ of −0.2 or less. When the substituted aryl group has a plurality of electron-donating groups, the respective electron-donating groups may be the same or different.

Q is selected from the same or different combinations of halogen atoms, hydrocarbon groups having 1 to 20 carbon atoms, anionic ligands and neutral ligands capable of coordinating with lone electron pairs, and is preferably a halogen atom.
j is an integer from 1 to 4, preferably 2.
Examples of the aryl group for R include a phenyl group, a 1-naphthyl group, a 2-naphthyl group, an anthracenyl group, a phenanthrenyl group, a tetracenyl group, a chrysenyl group, a pyrenyl group, an indenyl group, an azulenyl group, a pyrrolyl group, a pyridyl group, a furanyl group, and a thiophenyl group, and a phenyl group is preferable.

Electron-donating groups with a Hammett's rule substituent constant σ of -0.2 or less are defined and exemplified as follows. The Hammett's rule is an empirical rule proposed by L. P. Hammett in 1935 to quantitatively discuss the effect of substituents on the reaction or equilibrium of benzene derivatives, and is now widely recognized as valid. The substituent constants obtained by the Hammett's rule include σp when substituted at the para position of the benzene ring and σm when substituted at the meta position, and these values can be found in many general literature. For example, the literature by Hansch and Taft [Chem. Rev., 91, 165 (1991)] provides detailed descriptions of a very wide range of substituents. However, the values of σp and σm described in these literature may differ slightly depending on the literature, even for the same substituent. In this specification, in order to avoid confusion caused by such a situation, the values described in Table 1 (pp. 168-175) of the literature by Hansch and Taft [Chem. Rev., 91, 165 (1991)] are defined as the Hammett's rule substituent constants σp and σm for as many substituents as possible. In this specification, an electron-donating group with a Hammett's rule substituent constant σ of -0.2 or less means an electron-donating group with a σp of -0.2 or less when the electron-donating group is substituted at the para-position (4th position) of a phenyl group, and an electron-donating group with a σm of -0.2 or less when the electron-donating group is substituted at the meta-position (3rd position) of a phenyl group. In addition, when an electron-donating group is substituted at the ortho-position (2nd position) of a phenyl group or at any position of an aryl group other than a phenyl group, it is an electron-donating group with a σp of -0.2 or less.

Examples of electron-donating groups with a Hammett's rule substituent constant σp or σm of -0.2 or less include nitrogen-containing groups such as p-amino group (4-amino group), p-dimethylamino group (4-dimethylamino group), p-diethylamino group (4-diethylamino group), and m-diethylamino group (3-diethylamino group); oxygen-containing groups such as p-methoxy group (4-methoxy group) and p-ethoxy group (4-ethoxy group); tertiary hydrocarbon groups such as p-t-butyl group (4-t-butyl group); and silicon-containing groups such as p-trimethylsiloxy group (4-trimethylsiloxy group).

Each R is preferably a substituted phenyl group containing a group selected from a nitrogen-containing group and an oxygen-containing group as the electron-donating group.
The substituted aryl group may have another substituent selected from a hydrocarbon group having 1 to 20 carbon atoms, a silicon-containing group, a nitrogen-containing group, an oxygen-containing group, a halogen atom, and a halogen-containing group, other than an electron-donating group having a Hammett's rule substituent constant σ of −0.2 or less. When the substituted aryl group has a plurality of other substituents, the respective other substituents may be the same or different.

The sum of the Hammett's rule substituent constants σ of the electron donating group and other substituents contained in one substituted aryl group, each of which has a Hammett's rule substituent constant σ of -0.2 or less, is preferably -0.15 or less. Examples of such substituted aryl groups include m,p-dimethoxyphenyl group (3,4-dimethoxyphenyl group), p-(dimethylamino)-m-methoxyphenyl group (4-(dimethylamino)-3-methoxyphenyl group), p-(dimethylamino)-m-methylphenyl group (4-(dimethylamino)-3-methylphenyl group), p-methoxy-m-methylphenyl group (4-methoxy-3-methylphenyl group), and p-methoxy-m,m-dimethylphenyl group (4-methoxy-3,5-dimethylphenyl group).

In Q, examples of halogen atoms include fluorine, chlorine, bromine, and iodine; examples of hydrocarbon groups having 1 to 20 carbon atoms include alkyl groups having 1 to 10 carbon atoms and cycloalkyl groups having 3 to 10 carbon atoms; examples of anionic ligands include alkoxy groups, aryloxy groups, carboxylate groups, and sulfonate groups; examples of neutral ligands that can be coordinated with lone electron pairs include organic phosphorus compounds such as trimethylphosphine, triethylphosphine, triphenylphosphine, and diphenylmethylphosphine, and ether compounds such as tetrahydrofuran, diethyl ether, dioxane, and 1,2-dimethoxyethane.

An example of the transition metal compound (a) is [bis(4-methoxyphenyl)methylene(η ⁵ -cyclopentadienyl)(η ⁵ -2,3,6,7-tetramethylfluorenyl)]hafnium dichloride.

Examples of organometallic compounds (b-1) (excluding organoaluminum oxy compounds (b-2)) include organoaluminum compounds such as trialkylaluminums such as trimethylaluminum, triethylaluminum, triisobutylaluminum, and tri-n-octylaluminum, tricycloalkylaluminums, isobutylaluminum dichloride, diethylaluminum chloride, ethylaluminum dichloride, ethylaluminum sesquichloride, methylaluminum dichloride, dimethylaluminum chloride, and diisobutylaluminum hydride.

The organoaluminum oxy compound (b-2) may, for example, be a conventionally known aluminoxane.
Examples of the compound (b-3) that reacts with the transition metal compound (a) to form an ion pair include Lewis acids, ionic compounds, borane compounds, and carborane compounds described in JP-T-1-501950, JP-T-1-502036, JP-A-3-179005, JP-A-3-179006, JP-A-3-207703, JP-A-3-207704, USP-5321106, and WO 2015/122415.

The olefin polymerization catalyst may contain a carrier (c) as necessary. The carrier (c) is an inorganic or organic compound, and is a granular or fine particle solid. Among these, the inorganic compound is preferably a porous oxide, an inorganic halide, a clay, a clay mineral, or an ion-exchangeable layered compound.

The polymerization method can be either a liquid phase polymerization method such as solution (dissolution) polymerization or suspension polymerization, or a gas phase polymerization method. The polymerization temperature is usually -50 to +200°C, preferably 0 to 200°C. The polymerization pressure is usually normal pressure to 10 MPa gauge pressure, preferably normal pressure to 5 MPa gauge pressure. The polymerization reaction can be carried out in any of batch, semi-continuous, and continuous methods. Furthermore, the polymerization can be carried out in two or more stages with different reaction conditions.

The ethylene-1-butene-ENB copolymer used in the present invention has a polymethylene structure, which gives it good low-temperature flexibility.

(Carbon black)
From the viewpoint of improving the effects of the present invention, the rubber composition of the present invention preferably contains carbon black.
Specific examples of the carbon black used in the present invention include furnace carbon blacks such as SAF, ISAF, HAF, FEF, GPE, and SRF. These may be used alone or in combination of two or more.
From the viewpoint of improving performance on ice, the carbon black preferably has a nitrogen adsorption specific surface area (N ₂ SA) of 10 to 300 m ² /g, and more preferably 50 to 150 m ² /g.
The nitrogen adsorption specific surface area (N ₂ SA) is a value measured in accordance with JIS K 6217-2:2001 "Part 2: Determination of specific surface area - Nitrogen adsorption method - Single point method".

(Rubber composition blending ratio)
The rubber composition of the present invention is characterized in that it is obtained by compounding 30 to 100 parts by mass of a white filler, 0.5 to 20 parts by mass of thermally expandable microcapsules, and 3 to 50 parts by mass of an ethylene-1-butene-ENB copolymer with respect to 100 parts by mass of a diene rubber.
If the amount of white filler is less than 30 parts by mass per 100 parts by mass of diene rubber, the mechanical properties and abrasion resistance of the rubber composition will deteriorate, and conversely, if it exceeds 100 parts by mass, the low-temperature flexibility of the rubber composition will decrease and performance on ice will deteriorate.
If the amount of thermally expandable microcapsules mixed is less than 0.5 parts by mass per 100 parts by mass of diene rubber, the amount added is too small to achieve the effects of the present invention, and conversely, if it exceeds 20 parts by mass, the breaking properties will decrease.
If the amount of ethylene-1-butene-ENB copolymer is less than 3 parts by mass per 100 parts by mass of diene rubber, the amount added is too small to achieve the effects of the present invention, whereas if the amount exceeds 50 parts by mass, the break properties decrease.

The amount of the white filler to be mixed is preferably 20 to 80 parts by mass based on 100 parts by mass of the diene rubber.
The amount of the thermally expandable microcapsules to be compounded is preferably 2 to 15 parts by mass based on 100 parts by mass of the diene rubber.
The amount of the ethylene-1-butene-ENB copolymer blended is preferably 15 to 50 parts by mass based on 100 parts by mass of the diene rubber.
When carbon black is compounded, the compounding amount is preferably from 5 to 80 parts by mass, and more preferably from 10 to 70 parts by mass, per 100 parts by mass of the diene rubber.

(Other ingredients)
In addition to the above-mentioned components, the rubber composition of the present invention may contain various additives that are generally incorporated in rubber compositions, such as vulcanizing or crosslinking agents, vulcanizing or crosslinking accelerators, zinc oxide, antioxidants, and plasticizers, and these additives can be kneaded in a general manner to form a composition, which can be used for vulcanization or crosslinking. The amounts of these additives may be conventional amounts, provided that they do not violate the object of the present invention.

The rubber composition of the present invention is also suitable for manufacturing pneumatic tires according to conventional manufacturing methods for pneumatic tires, and is preferably applied to treads, particularly cap treads, to make studless tires.

The present invention will be further explained below with reference to examples and comparative examples, but the present invention is not limited to the following examples. In the following examples, "parts" means "parts by mass."

(Preparation Example 1: Preparation of ethylene/1-butene/ENB copolymer)
[Physical properties of ethylene-1-butene-ENB copolymer]
<Molar amounts of structural units derived from ethylene, structural units derived from 1-butene, and structural units derived from ENB>
The molar amount was determined by intensity measurement using a ¹ H-NMR spectrometer. Details of the measurement conditions are described in WO 2015/122415.

<Mooney Viscosity>
The Mooney viscosity (ML ₍₁₊₄₎ 100° C., 125° C.) was measured using a Mooney viscometer (Shimadzu Corporation, SMV202 type) in accordance with JIS K6300 (1994).

<B value>
Using o-dichlorobenzene-d ₄ /benzene-d ₆ (4/1 [v/v]) as the measurement solvent, ¹³ C-NMR spectrum (100 MHz, JEOL ECX400P) was measured at a measurement temperature of 120° C., and calculation was made based on the following formula (i).
B value=([EX]+2[Y])/[2×[E]×([X]+[Y])] (i)
[Here, [E], [X] and [Y] represent the mole fractions of ethylene [A], 1-butene [B] and ENB [C], respectively, and [EX] represents the ethylene [A]-1-butene [B] dyad sequence fraction.]

Ethylene/1-butene/ENB copolymer According to the description of [Synthesis Example C1] of WO 2015/122415, an ethylene/1-butene/ENB copolymer having the following physical properties was obtained. The composition and physical properties are as follows.
Structural units derived from ethylene: 67.7 mol%
Structural units derived from 1-butene: 30.0 mol%
Structural units derived from ENB: 2.3 mol%
Mooney Viscosity ML ₍₁₊₄₎ 100℃: 30
Mooney Viscosity ML ₍₁₊₄₎ 125°C: 22
B value: 1.3

Standard Example, Examples 1-2, Comparative Examples 1-4
In the formulation (parts by mass) shown in Table 1, the components other than the vulcanization accelerator and sulfur were mixed in a 1.7-liter closed Banbury mixer for 5 minutes, and then the vulcanization accelerator and sulfur were added and further mixed to obtain a rubber composition. The unvulcanized rubber composition obtained was then press-vulcanized in a specified mold at 160°C for 20 minutes to obtain a vulcanized rubber test piece, and the physical properties of the vulcanized rubber test piece were measured by the test methods shown below.

Breaking elongation: According to JIS K6251, a No. 3 dumbbell-shaped sample piece was punched out from the vulcanized rubber test piece, and a tensile test was carried out at a tensile speed of 500 mm/min to measure the breaking elongation (%). The results were expressed as an index, with the standard example value being 100. The larger the index, the better the breaking elongation. An index value of 80 or more is deemed sufficient for practical use.
Performance on ice: The obtained vulcanized rubber test pieces were attached to a flat cylindrical rubber base and the coefficient of friction on ice was measured using an inside drum type ice friction tester under the following conditions: temperature -3.0°C, load 5.5 kg/ ^cm2 , drum rotation speed 25 km/h. The obtained coefficient of friction on ice was expressed as an index, with the standard example value set at 100. A higher index indicates greater frictional force on ice and better performance on ice.
The results are shown in Table 1.

*1: NR (RSS #3)
*2: BR (Nipol BR1220 manufactured by Nippon Zeon Co., Ltd.)
*3: Carbon black (Seest KHA manufactured by Tokai Carbon Co., Ltd.)
*4: Silica (Zeosil 1165MP manufactured by Rhodia, CTAB specific surface area = 159 ^m2 /g)
*5: Silane coupling agent (Si69, bis(3-triethoxysilylpropyl)tetrasulfide, manufactured by Evonik Degussa)
*6: Oil (Extract No. 4 S manufactured by Showa Shell Sekiyu K.K.)
* 7: Thermally expandable microcapsules (Matsumoto Microsphere F-100D manufactured by Matsumoto Yushi Pharmaceutical Co., Ltd.)
*8: Foaming agent (product name: Cellular D, manufactured by Eiwa Kasei Co., Ltd.)
*9: EPDM (Mitsui Chemicals, Inc. product name Mitsui EPT4070)
*10: Ethylene / 1-butene / ENB copolymer (ethylene / 1-butene / ENB copolymer prepared as described above)
*11: Sulfur (Kinka-in oil-containing fine sulfur manufactured by Tsurumi Chemical Industry Co., Ltd.)
*12: Vulcanization accelerator CZ (Noccela CZ-G manufactured by Ouchi Shinko Chemical Industry Co., Ltd.)

As can be seen from the results in Table 1, the rubber composition of each example is composed of 100 parts by mass of diene rubber, 30 to 100 parts by mass of white filler, 0.5 to 20 parts by mass of thermally expandable microcapsules, and 3 to 50 parts by mass of ethylene-1-butene-ENB copolymer, and therefore, compared to the standard example, the breaking properties are maintained at a practically sufficient level and performance on ice is improved.
In contrast, Comparative Example 1 was an example in which neither the ethylene-1-butene-ENB copolymer nor the thermally expandable microcapsules were used, but instead a foaming agent was used, and therefore the breaking elongation was deteriorated.
Comparative Examples 2 and 3 did not contain thermally expandable microcapsules, and therefore showed little improvement in performance on ice.
Comparative Example 4 is an example in which EPDM was blended in place of the ethylene-1-butene-ENB copolymer, and as a result, the degree of improvement in performance on ice was small.

Claims (5)

Relative to 100 parts by mass of diene rubber,
30 to 100 parts by mass of white filler,
A rubber composition for studless tires, comprising 0.5 to 20 parts by mass of thermally expandable microcapsules, and 3 to 50 parts by mass of an ethylene-1-butene-ENB copolymer that contains a structural unit derived from ethylene [A], a structural unit derived from 1-butene [B], and a structural unit derived from 5-ethylidene-2-norbornene (ENB) [C] and satisfies the following (1) to (4):
(1) the molar ratio [[A]/[B]] of the structural unit derived from ethylene [A] to the structural unit derived from 1-butene [B] is 60/40 to 80/20;
(2) The content of the structural unit derived from ENB[C] is 1.0 to 3.0 mol %, based on 100 mol % of the total of the structural units [A], [B] and [C];
(3) Mooney viscosity ML ₍₁₊₄₎ 125°C is 10 to 50;
(4) The B value represented by the following formula (i) is 1.20 or more.
B value = ([EX] + 2[Y]) / (2 x [E] x ([X] + [Y])) (i)
[Here, [E], [X] and [Y] represent the mole fractions of ethylene [A], 1-butene [B] and ENB [C], respectively, and [EX] represents the ethylene [A]-1-butene [B] dyad sequence fraction.] The rubber composition for studless tires according to claim 1, characterized in that the amount of the ethylene-1-butene-ENB copolymer is 15 to 50 parts by mass per 100 parts by mass of the diene rubber. The rubber composition for studless tires according to claim 1 or 2, characterized in that the ethylene-1-butene-ENB copolymer has a polymethylene structure. The rubber composition for studless tires according to any one of claims 1 to 3, characterized in that the white filler is silica. A studless tire using the rubber composition for studless tires according to any one of claims 1 to 4.