Rubber composition, vulcanized rubber, tire tread rubber, and tire

Abstract

The present invention addresses the problem of providing a rubber composition that has a low environmental impact and that makes it possible to improve on-ice performance of a tire. The solution to the problem is a rubber composition which comprises: a rubber component containing diene rubber; an inorganic blowing agent; and a blowing aid, and which is characterized in that the total contained amount of the inorganic blowing agent and the blowing aid is 1-20 parts by mass with respect to 100 parts by mass of the rubber component, and the mass ratio (inorganic blowing agent : blowing aid) between the inorganic blowing agent and the blowing aid is 1:1.1 to 1:3.3.

Description

[Technical Field]

[0001] The present invention relates to a rubber composition, a vulcanized rubber, a tread rubber for a tire, and a tire. [Background technology]

[0002] In order to improve the braking and driving performance of tires on snowy and icy roads (hereinafter referred to as "ice performance"), active research has been conducted, particularly on tire treads. On snowy and icy roads, a water film is easily generated due to frictional heat between the tire and the snowy and icy road surface, and this water film reduces the coefficient of friction between the tire and the snowy and icy road surface. Therefore, in order to improve tire performance on ice, it is necessary to improve the tire tread's ability to remove the water film.

[0003] One way to give a tire tread the ability to remove water films is to create microscopic drainage grooves in the tire's road surface, which can remove water films and increase the tire's coefficient of friction on icy and snowy roads. However, while this can improve the tire's performance on ice during the initial stages of use, its performance deteriorates as the tire wears. Therefore, there is a need for technology that prevents the deterioration of tire performance on ice, even as the tire wears.

[0004] Meanwhile, a technology for applying foamed rubber to a tire tread to impart water film removal capability to the tire tread is also known. Such foamed rubber generally uses a rubber composition containing a rubber component and a foaming agent, and organic foaming agents have traditionally been used as the foaming agent. In addition to conventional organic foaming agents, efforts have also been made to use inorganic foaming agents as foaming agents (e.g., Patent Document 1 below). Because inorganic foaming agents have a low environmental impact, using inorganic foaming agents extensively can provide rubber compositions with even lower environmental impact. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2017 / 068772 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the inventors have conducted studies and found that when a rubber composition using an inorganic foaming agent is used in a tire tread, the tire's performance on ice is not sufficient, and there is room for improvement in the performance on ice.

[0007] Therefore, an object of the present invention is to solve the above-mentioned problems of the conventional technology, to provide a rubber composition that has a low environmental impact and can improve the performance of tires on ice. Another object of the present invention is to provide a vulcanized rubber and a tire tread rubber that are environmentally friendly and capable of improving the performance of tires on ice, and further to provide a tire that is environmentally friendly and has excellent performance on ice. [Means for solving the problem]

[0008] The gist and configuration of the present invention to solve the above problems is as follows.

[0009] The rubber composition of the present invention is a rubber composition comprising a rubber component containing a diene rubber, an inorganic foaming agent, and a foaming aid, a total content of the inorganic foaming agent and the foaming aid is 1 to 20 parts by mass based on 100 parts by mass of the rubber component, The mass ratio of the inorganic foaming agent to the foaming assistant (inorganic foaming agent:foaming assistant) is 1:1.1 to 1:3.3.

[0010] The vulcanized rubber of the present invention is obtained by vulcanizing the above rubber composition and is characterized by having a foaming rate of 1 to 45%.

[0011] The tread rubber for a tire of the present invention is characterized by comprising the above rubber composition or the above vulcanized rubber.

[0012] The tire of the present invention is characterized by comprising the above rubber composition or the above vulcanized rubber in the tread portion. [Effects of the Invention]

[0013] According to the present invention, it is possible to provide a rubber composition that has a low environmental impact and is capable of improving the performance of tires on ice. Furthermore, according to the present invention, it is possible to provide a vulcanized rubber and a tire tread rubber that are environmentally friendly and can improve the performance of tires on ice, as well as a tire that is environmentally friendly and has excellent performance on ice. DETAILED DESCRIPTION OF THE INVENTION

[0014] The rubber composition, vulcanized rubber, tire tread rubber, and tire of the present invention will be illustrated and described in detail below based on embodiments thereof.

[0015] <Rubber composition> The rubber composition of the present invention comprises a rubber component containing a diene rubber, an inorganic foaming agent, and a foaming aid, wherein the total content of the inorganic foaming agent and the foaming aid is 1 to 20 parts by mass per 100 parts by mass of the rubber component, and the mass ratio of the inorganic foaming agent to the foaming aid (inorganic foaming agent:foaming aid) is 1:1.1 to 1:3.3.

[0016] In the rubber composition of the present invention, the inorganic foaming agent foams during vulcanization to generate bubbles (voids) in the rubber composition (vulcanized rubber), and the foaming aid accelerates the foaming reaction of the inorganic foaming agent, improving the foaming rate of the resulting vulcanized rubber. Furthermore, in the rubber composition of the present invention, the total content of the inorganic foaming agent and the foaming aid is 1 to 20 parts by mass per 100 parts by mass of the rubber component, and the mass ratio of the inorganic foaming agent to the foaming aid (inorganic foaming agent:foaming aid) is 1:1.1 to 1:3.3. This allows the rubber composition to foam sufficiently during vulcanization, improving the foaming rate of the vulcanized rubber. When the rubber composition is applied to a tire, the tire is endowed with water film removal capabilities, improving the tire's drainage performance and its performance on ice. If the total content of the inorganic foaming agent and the foaming aid is outside the above-mentioned range, or if the mass ratio of the inorganic foaming agent to the foaming aid is outside the above-mentioned range, the foaming rate of the vulcanized rubber decreases, preventing the tire's performance on ice from being sufficiently improved. Furthermore, the inorganic foaming agent is an inorganic compound and therefore has a low environmental impact (is gentle on the human body). Therefore, the rubber composition of the present invention has a low environmental impact, and when applied to tires, the tire performance on ice can be improved.

[0017] (rubber component) The rubber component of the rubber composition of the present invention contains a diene rubber. The proportion of the diene rubber in the rubber component is preferably 80% by mass or more, more preferably 90% by mass or more, and particularly preferably 100% by mass. The rubber component may contain rubbers other than the diene rubber, but is preferably composed solely of the diene rubber.

[0018] The diene rubber may be natural rubber (NR), synthetic diene rubber, or both. By applying a rubber composition containing at least one of natural rubber and synthetic diene rubber to a tire, the tire's performance on ice can be further improved.

[0019] Examples of the synthetic diene rubber include isoprene rubber (IR), styrene-butadiene rubber (SBR), butadiene rubber (BR), styrene-isoprene rubber (SIR), and chloroprene rubber (CR). Among these, isoprene rubber, styrene-butadiene rubber, and butadiene rubber are more preferred as the synthetic diene rubber. By applying a rubber composition containing at least one rubber selected from the group consisting of isoprene rubber, styrene-butadiene rubber, and butadiene rubber to a tire, the tire's performance on ice can be further improved. The diene rubber (rubber component) may be one type alone or a blend of two or more types.

[0020] The diene rubber (rubber component) may be an unmodified rubber or a modified rubber. When the diene rubber is modified, the diene rubber is preferably modified with at least one selected from the group consisting of a hydrocarbyloxysilane compound represented by the following general formula (I), a hydrocarbyloxysilane compound represented by the following general formula (II), a hydrocarbyloxysilane compound represented by the following general formula (III), a coupling agent represented by the following general formula (IV), a coupling agent represented by the following general formula (V), a lithioamine represented by the following general formula (VI), and vinylpyridine.

[0021] [ka] In the above general formula (I), q1+q2=3 (wherein q1 is an integer of 0 to 2, and q2 is an integer of 1 to 3). R 11 is a divalent aliphatic or alicyclic hydrocarbon group having 1 to 20 carbon atoms or a divalent aromatic hydrocarbon group having 6 to 18 carbon atoms. R 12 and R 13 are each independently a hydrolyzable group, a monovalent aliphatic or alicyclic hydrocarbon group having 1 to 20 carbon atoms, or a monovalent aromatic hydrocarbon group having 6 to 18 carbon atoms. R 14represents a monovalent aliphatic or alicyclic hydrocarbon group having 1 to 20 carbon atoms or a monovalent aromatic hydrocarbon group having 6 to 18 carbon atoms, and when q1 is 2, may be the same or different. R 15 represents a monovalent aliphatic or alicyclic hydrocarbon group having 1 to 20 carbon atoms or a monovalent aromatic hydrocarbon group having 6 to 18 carbon atoms, and when q2 is 2 or greater, may be the same or different.

[0022] [ka] In the above general formula (II), r1+r2=3 (wherein r1 is an integer of 1 to 3, and r2 is an integer of 0 to 2). R 21 is a divalent aliphatic or alicyclic hydrocarbon group having 1 to 20 carbon atoms or a divalent aromatic hydrocarbon group having 6 to 18 carbon atoms. R 22 represents a dimethylaminomethyl group, a dimethylaminoethyl group, a diethylaminomethyl group, a diethylaminoethyl group, a methylsilyl(methyl)aminomethyl group, a methylsilyl(methyl)aminoethyl group, a methylsilyl(ethyl)aminomethyl group, a methylsilyl(ethyl)aminoethyl group, a dimethylsilylaminomethyl group, a monovalent aliphatic or alicyclic hydrocarbon group having 1 to 20 carbon atoms, or a monovalent aromatic hydrocarbon group having 6 to 18 carbon atoms, and when r1 is 2 or more, they may be the same or different. R 23 represents a hydrocarbyloxy group having 1 to 20 carbon atoms, a monovalent aliphatic or alicyclic hydrocarbon group having 1 to 20 carbon atoms, or a monovalent aromatic hydrocarbon group having 6 to 18 carbon atoms, and when r2 is 2, may be the same or different.

[0023] [ka] In the above general formula (III), A 3is a monovalent group having at least one functional group selected from (thio)epoxy, (thio)isocyanate, (thio)ketone, (thio)aldehyde, imine, amide, isocyanuric acid trihydrocarbyl ester, (thio)carboxylic acid ester, metal salt of (thio)carboxylic acid, carboxylic acid anhydride, carboxylic acid halide, and dihydrocarbyl carbonate ester. Here, "(thio)epoxy" refers to epoxy and thioepoxy, "(thio)isocyanate" refers to isocyanate and thioisocyanate, "(thio)ketone" refers to ketone and thioketone, "(thio)aldehyde" refers to aldehyde and thioaldehyde, "(thio)carboxylic acid ester" refers to carboxylic acid ester and thiocarboxylic acid ester, and "metal salt of (thio)carboxylic acid" refers to metal salt of carboxylic acid and metal salt of thiocarboxylic acid. R 31 is a single bond or a divalent inert hydrocarbon group, and the divalent inert hydrocarbon group preferably has 1 to 20 carbon atoms. R 32 and R 33 each independently represents a monovalent aliphatic hydrocarbon group having 1 to 20 carbon atoms or a monovalent aromatic hydrocarbon group having 6 to 18 carbon atoms, n is an integer of 0 to 2, and R 32 If there are multiple R 32 may be the same or different, OR 33 If there are multiple, multiple OR 33 may be the same or different. Furthermore, the molecule of the hydrocarbyloxysilane compound represented by general formula (III) does not contain an active proton or an onium salt.

[0024] In the general formula (III), A 3 Among the functional groups in the above, imines include ketimines, aldimines, and amidines, and (thio)carboxylic acid esters include unsaturated carboxylic acid esters such as acrylates and methacrylates. Examples of metals in metal salts of (thio)carboxylic acids include alkali metals, alkaline earth metals, Al, Sn, and Zn. R 31Among these, preferred divalent inert hydrocarbon groups are alkylene groups having 1 to 20 carbon atoms. The alkylene groups may be linear, branched, or cyclic, with linear groups being particularly preferred. Examples of linear alkylene groups include methylene, ethylene, trimethylene, tetramethylene, pentamethylene, hexamethylene, octamethylene, decamethylene, and dodecamethylene. R 32 and R 33 Examples of the alkyl group include an alkyl group having 1 to 20 carbon atoms, an alkenyl group having 2 to 18 carbon atoms, an aryl group having 6 to 18 carbon atoms, and an aralkyl group having 7 to 18 carbon atoms. Here, the alkyl group and alkenyl group may be linear, branched, or cyclic, and examples thereof include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, a pentyl group, a hexyl group, an octyl group, a decyl group, a dodecyl group, a cyclopentyl group, a cyclohexyl group, a vinyl group, a propenyl group, an allyl group, a hexenyl group, an octenyl group, a cyclopentenyl group, and a cyclohexenyl group. The aryl group may have a substituent such as a lower alkyl group on the aromatic ring, and examples thereof include a phenyl group, a tolyl group, a xylyl group, and a naphthyl group. Furthermore, the aralkyl group may have a substituent such as a lower alkyl group on the aromatic ring, examples of which include a benzyl group, a phenethyl group, and a naphthylmethyl group. n is an integer of 0 to 2, preferably 0, and the molecule must be free of active protons and onium salts.

[0025] Examples of the hydrocarbyloxysilane compound represented by the general formula (III) include (thio)epoxy group-containing hydrocarbyloxysilane compounds such as 2-glycidoxyethyltrimethoxysilane, 2-glycidoxyethyltriethoxysilane, (2-glycidoxyethyl)methyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane, (3-glycidoxypropyl)methyldimethoxysilane, and 2-(3,4-epoxycyclohexyl) Preferred examples include ethyltrimethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltriethoxysilane, 2-(3,4-epoxycyclohexyl)ethyl(methyl)dimethoxysilane, 2-(3,4-epoxycyclohexyl)trimethoxysilane, and compounds in which the epoxy groups are replaced with thioepoxy groups. Of these, 3-glycidoxypropyltrimethoxysilane and 2-(3,4-epoxycyclohexyl)trimethoxysilane are particularly suitable. Furthermore, examples of imine group-containing hydrocarbyloxycyanide compounds include N-(1,3-dimethylbutylidene)-3-(triethoxysilyl)-1-propanamine, N-(1-methylethylidene)-3-(triethoxysilyl)-1-propanamine, N-ethylidene-3-(triethoxysilyl)-1-propanamine, N-(1-methylpropylidene)-3-(triethoxysilyl)-1-propanamine, N-(4-N,N-dimethylaminobenzylidene)-3-(triethoxysilyl)-1-propanamine, N-(cyclohex ... Preferred examples of the triethoxysilyl compounds include N-(1-methylpropylidene)-3-(triethoxysilyl)-1-propanamine and the trimethoxysilyl compounds, methyldiethoxysilyl compounds, ethyldiethoxysilyl compounds, methyldimethoxysilyl compounds, and ethyldimethoxysilyl compounds corresponding to these triethoxysilyl compounds. Among these, N-(1-methylpropylidene)-3-(triethoxysilyl)-1-propanamine and N-(1,3-dimethylbutylidene)-3-(triethoxysilyl)-1-propanamine are particularly preferred.

[0026] [ka] In the above general formula (IV), R 41 , R 42 and R 43 each independently represents a single bond or an alkylene group having 1 to 20 carbon atoms. R 44 , R 45 , R 46 , R 47 and R 49 are each independently an alkyl group having 1 to 20 carbon atoms. R 48 and R 51 are each independently an alkylene group having 1 to 20 carbon atoms. R 50 represents an alkyl group or a trialkylsilyl group having 1 to 20 carbon atoms. m represents an integer of 1 to 3; p represents 1 or 2; R 41 ~R 51 When there are a plurality of i, m, and p, they are independent of each other, and i, j, and k are each independently an integer of 0 to 6, with the proviso that (i+j+k) is an integer of 3 to 10. A 4 represents a hydrocarbon group having 1 to 20 carbon atoms, or an organic group having at least one atom selected from the group consisting of oxygen, nitrogen, silicon, sulfur and phosphorus atoms, and having no active hydrogen.

[0027] Here, the coupling agent represented by the general formula (IV) is preferably at least one selected from the group consisting of tetrakis[3-(2,2-dimethoxy-1-aza-2-silacyclopentane)propyl]-1,3-propanediamine, tetrakis(3-trimethoxysilylpropyl)-1,3-propanediamine, and tetrakis(3-trimethoxysilylpropyl)-1,3-bisaminomethylcyclohexane.

[0028] (R 5 ) a ZX b (V) In the above general formula (V), Z is tin or silicon, and X is chlorine or bromine. (R 5 ) is selected from the group consisting of alkyl having 1 to 20 carbon atoms, cycloalkyl having 3 to 20 carbon atoms, aryl having 6 to 20 carbon atoms, and aralkyl having 7 to 20 carbon atoms, wherein (R 5 Specific examples of the alkyl group include a methyl group, an ethyl group, an n-butyl group, a neophyl group, a cyclohexyl group, an n-octyl group, and a 2-ethylhexyl group. a is 0 to 3, and b is 1 to 4, where a+b=4.

[0029] The coupling agent represented by the general formula (V) includes tin tetrachloride, (R 5 )SnCl3, (R 5 )2SnCl2, (R 5 )3SnCl and the like are preferred, and among these, tin tetrachloride is particularly preferred.

[0030] (AM)Li(Q) y (VI) In the above general formula (VI), y is 0 or 0.5 to 3, (Q) is a solubilizing component selected from the group consisting of hydrocarbons, ethers, amines, or mixtures thereof, and (AM) is a compound represented by the following formula (VII): [ka] [In formula (VII), R 71 and R 72 each independently represents an alkyl, cycloalkyl, or aralkyl group having 1 to 12 carbon atoms.] or a group represented by the following formula (VIII): [ka] [In formula (VIII), R 81represents alkylene having 3 to 16 methylene groups, linear or branched alkyl having 1 to 12 carbon atoms, substituted alkylene having a cycloalkyl, bicycloalkyl, aryl or aralkyl as a substituent, oxydiethylene or an N-alkylamino-alkylene group.

[0031] The presence of Q of the general formula (VI) above makes the lithioamine soluble in hydrocarbon solvents. Q also includes dienyl or vinyl aromatic polymers or copolymers having a degree of polymerization of 3 to about 300 polymerization units. These polymers and copolymers include polybutadiene, polystyrene, polyisoprene, and copolymers thereof. Other examples of Q include polar ligands such as tetrahydrofuran (THF), tetramethylethylenediamine (TMEDA), etc.

[0032] The lithioamine represented by the general formula (VI) can also be a mixture with an organic alkali metal. The organic alkali metal is preferably a compound represented by the general formula: (R 91 )M, (R 92 )OM, (R 93 )C(O)OM, (R 94 )(R 95 )NM and (R 96 )SO3M, wherein (R 91 ), (R 92 ), (R 93 ), (R 94 ), (R 95 ) and (R 96 Each of the radicals M is selected from the group consisting of alkyl, cycloalkyl, alkenyl, aryl, and phenyl having from about 1 to about 12 carbon atoms. The metal component M is selected from the group consisting of Na, K, Rb, and Cs. Preferably, M is Na or K. The mixture may also contain the organic alkali metal, preferably in a mixing ratio of about 0.5 to about 0.02 equivalents per equivalent of lithium in the lithioamine.

[0033] In addition, a chelating agent can be used to prevent non-uniform polymerization in the mixture of the lithioamine and the organic alkali metal. Useful chelating agents include, for example, tetramethylethylenediamine (TMEDA), oxolanyl cyclic acetals, and cyclic oligomeric oxolanyl alkanes. Cyclic oligomeric oxolanyl alkanes are particularly preferred, and a specific example is 2,2-bis(tetrahydrofuryl)propane.

[0034] Examples of the vinylpyridine include 2-vinylpyridine and 4-vinylpyridine.

[0035] Among the various modifiers mentioned above, N,N-bis(trimethylsilyl)-3-[diethoxy(methyl)silyl]propylamine, N-(1,3-dimethylbutylidene)-3-triethoxysilyl-1-propanamine, 3-glycidoxypropyltrimethoxysilane, tetrakis(3-trimethoxysilylpropyl)-1,3-propanediamine, tin tetrachloride, a reaction product of hexamethyleneimine with n-butyllithium, 4-vinylpyridine, and 2-vinylpyridine are preferred.

[0036] Furthermore, the rubber composition of the present invention more preferably contains, as the styrene-butadiene rubber, a styrene-butadiene rubber having a styrene bond content of 15% by mass or less. By applying a rubber composition containing a styrene-butadiene rubber having a styrene bond content of 15% by mass or less to a tire, the tire's performance on ice can be significantly improved. In this specification, the styrene bond amount of styrene-butadiene rubber is 1 It can be determined from the integral ratio of the H-NMR spectrum.

[0037] (inorganic foaming agent) The rubber composition of the present invention contains an inorganic foaming agent. The inorganic foaming agent is an inorganic compound that has a low environmental impact. The inorganic foaming agent foams (by heating) during vulcanization of the rubber composition, thereby generating bubbles (voids) in the rubber composition (vulcanized rubber).

[0038] Examples of the inorganic foaming agent include carbonates and bicarbonates (hydrogen carbonates), and among these, hydrogen carbonates are preferred. Examples of the carbonates include ammonium carbonate, sodium carbonate, potassium carbonate, etc. Examples of the bicarbonates include ammonium bicarbonate, sodium bicarbonate, potassium bicarbonate, etc. Among these, ammonium bicarbonate and sodium bicarbonate are preferred as the inorganic foaming agent, in which case foaming of the inorganic foaming agent is further promoted, the foaming rate of the vulcanized rubber is further improved, and the performance of the tire on ice can be further improved. The inorganic foaming agents may be used alone or in combination of two or more.

[0039] Sodium bicarbonate (NaHCO3) is particularly preferred as the inorganic foaming agent. When the rubber composition contains sodium bicarbonate, foaming due to the sodium bicarbonate is promoted, the foaming rate of the vulcanized rubber is further improved, and when the rubber composition is applied to a tire, the tire's performance on ice can be further improved.

[0040] From the viewpoint of the foaming rate of the vulcanized rubber and the on-ice performance of the tire, the content of the inorganic foaming agent is preferably in the range of 1 to 12 parts by mass, more preferably in the range of 1 to 10 parts by mass, even more preferably in the range of 2 to 7 parts by mass, and particularly preferably in the range of 4 to 6.5 parts by mass, relative to 100 parts by mass of the rubber component.

[0041] (foaming aid) The rubber composition of the present invention contains a foaming aid that has the effect of accelerating the foaming reaction of the inorganic foaming agent during vulcanization of the rubber composition, thereby improving the foaming rate of the resulting vulcanized rubber.

[0042] Examples of the foaming aid include urea, zinc stearate, zinc benzenesulfinate, zinc oxide, etc., and among these, urea is particularly preferred. When the rubber composition contains urea, foaming of the inorganic foaming agent is further promoted, and the foaming rate of the vulcanized rubber is further improved. When the rubber composition contains urea, the tire performance on ice can be further improved. The urea may be subjected to a treatment such as oil treatment. Treatment such as oil treatment can make the urea hydrophobic, thereby improving the dispersibility of the urea in the rubber component. The oil used for the oil treatment is not particularly limited, and various oils can be used. The foaming aids may be used alone or in combination of two or more.

[0043] In the rubber composition of the present invention, the total content of the inorganic foaming agent and the foaming aid is 1 to 20 parts by mass per 100 parts by mass of the rubber component. If the total content of the inorganic foaming agent and the foaming aid is less than 1 part by mass, the rubber composition does not foam sufficiently during vulcanization, resulting in a reduced foaming rate of the vulcanized rubber. On the other hand, if the total content of the inorganic foaming agent and the foaming aid exceeds 20 parts by mass, the foaming rate also decreases. From the viewpoint of the foaming rate of the vulcanized rubber, the total content of the inorganic foaming agent and the foaming aid is preferably 5 parts by mass or more, more preferably 7 parts by mass or more, per 100 parts by mass of the rubber component, and preferably 20 parts by mass or less, more preferably 19 parts by mass or less, per 100 parts by mass of the rubber component.

[0044] In the rubber composition of the present invention, the mass ratio of the inorganic foaming agent to the foaming aid (inorganic foaming agent:foaming aid) is 1:1.1 to 1:3.3. If the mass ratio (inorganic foaming agent:foaming aid) is less than 1:1.1, the rubber composition does not foam sufficiently during vulcanization, resulting in a reduced foaming rate of the vulcanized rubber. On the other hand, if the mass ratio (inorganic foaming agent:foaming aid) exceeds 1:3.3, the foaming rate also decreases. From the viewpoint of the foaming rate of the vulcanized rubber, the mass ratio of the inorganic foaming agent to the foaming aid (inorganic foaming agent:foaming aid) is preferably 1:1.2 or more, and more preferably 1:1.3 or more. From the viewpoint of the foaming rate of the vulcanized rubber, the mass ratio of the inorganic foaming agent to the foaming aid (inorganic foaming agent:foaming aid) is preferably 1:3.2 or less, more preferably 1:3.1 or less, even more preferably 1:2.9 or less, still more preferably 1:2.7 or less, even more preferably 1:2.5 or less, and particularly preferably 1:2.3 or less.

[0045] The content of the foaming aid is preferably in the range of 4 to 14 parts by mass, more preferably 6 to 14 parts by mass, per 100 parts by mass of the rubber component, from the viewpoint of the foaming rate of the vulcanized rubber and the performance of the tire on ice.

[0046] (short fibers) The rubber composition of the present invention preferably further contains short fibers, and more preferably contains hydrophilic short fibers. Here, the hydrophilic short fibers refer to short fibers having a contact angle with water of 5 to 80°. The contact angle of the hydrophilic short fibers with water can be determined by preparing a test piece by molding a hydrophilic resin used as a raw material for the hydrophilic short fibers into a smooth plate shape, dropping water onto the surface of the test piece using an automatic contact angle meter DM-301 manufactured by Kyowa Interface Science Co., Ltd. under conditions of 25°C and a relative humidity of 55%, and then observing the test piece from directly beside it immediately, and measuring the angle formed by a straight line formed by the surface of the test piece and a tangent to the surface of the water drop.

[0047] When the rubber composition contains short fibers, the gas generated from the inorganic foaming agent during vulcanization penetrates into the short fibers, forming voids (air bubbles) in the vulcanized rubber obtained by vulcanizing the rubber composition, the voids having a shape corresponding to the shape of the short fibers. When such vulcanized rubber is used in a tire tread, the voids present in the vulcanized rubber function as drainage channels as the tire wears, providing the tire with excellent water film removal ability and further improving the tire's performance on ice.

[0048] Furthermore, when the rubber composition contains hydrophilic short fibers, the gas generated from the inorganic foaming agent during vulcanization penetrates into the hydrophilic short fibers, forming voids (air bubbles) having shapes corresponding to the shapes of the hydrophilic short fibers, and the walls of the voids are covered with a resin derived from the hydrophilic short fibers, making them hydrophilic. Therefore, when a tire is manufactured using a rubber composition containing hydrophilic short fibers in a tread, the walls of the voids are exposed to the tread surface during use, improving the affinity with water and enabling the voids to actively absorb water, thereby imparting excellent water film removal ability and drainage properties to the tire and significantly improving the tire's performance on ice. Furthermore, when the rubber composition contains hydrophilic short fibers, the length of the voids (air bubbles) in the vulcanized rubber obtained by vulcanizing the rubber composition becomes even longer, further improving performance on ice.

[0049] The hydrophilic resin used as the raw material for the hydrophilic short fibers includes a resin having a hydrophilic group in the molecule. Specifically, it is preferably a resin containing an oxygen atom, a nitrogen atom, or a sulfur atom, more preferably a resin containing at least one functional group selected from the group consisting of -OH, -COOH, -OCOR (R is an alkyl group), -NH2, -NCO, and -SH, and even more preferably a resin containing at least one functional group selected from the group consisting of -OH, -COOH, -NH2, and -NCO.

[0050] Various resins can be used as the raw material for the short fibers. Specific examples of hydrophilic resins used as the raw material for the hydrophilic short fibers include ethylene-vinyl alcohol copolymers, vinyl alcohol homopolymers, poly(meth)acrylic acid or its esters, polyethylene glycol, carboxyvinyl copolymers, styrene-maleic acid copolymers, polyvinylpyrrolidone, vinylpyrrolidone-vinyl acetate copolymers, and mercaptoethanol. Among these, ethylene-vinyl alcohol copolymers, vinyl alcohol homopolymers, and poly(meth)acrylic acid are preferred, with ethylene-vinyl alcohol copolymers being particularly preferred.

[0051] The hydrophilic short fibers may be coated on their surfaces with a low-melting-point resin that has affinity for the rubber component and preferably has a melting point lower than the maximum vulcanization temperature of the rubber composition. By forming such a coating layer, the hydrophilic short fibers' affinity for water is effectively maintained while the coating layer has good affinity with the rubber component, improving the dispersibility of the short fibers in the rubber component. Furthermore, the low-melting-point resin melts during vulcanization to form a fluid coating layer, which contributes to adhesion between the rubber component and the hydrophilic short fibers, thereby easily achieving a tire with good drainage and durability. The thickness of the coating layer varies depending on the amount and average diameter of the hydrophilic short fibers, but is typically 0.001 to 10 μm, preferably 0.001 to 5 μm. The melting point of the low-melting-point resin used in the coating layer is preferably lower than the maximum vulcanization temperature of the rubber composition. The maximum vulcanization temperature refers to the maximum temperature reached by the rubber composition during vulcanization. For example, in the case of mold vulcanization, this refers to the maximum temperature reached by the rubber composition from the time the rubber composition enters the mold until it leaves the mold and cools. This maximum vulcanization temperature can be measured, for example, by embedding a thermocouple in the rubber composition. There is no particular upper limit for the melting point of the low-melting-point resin, but it is preferably selected taking the above points into consideration. Generally, it is preferably at least 10°C lower than the maximum vulcanization temperature of the rubber composition, and more preferably at least 20°C lower. The industrial vulcanization temperature of rubber compositions is generally at most about 190°C. For example, if the maximum vulcanization temperature is set to 190°C, the melting point of the low-melting-point resin is usually selected in the range below 190°C, preferably 180°C or lower, and more preferably 170°C or lower. The low-melting-point resin is preferably a polyolefin resin, and examples thereof include polyethylene, polypropylene, polybutene, polystyrene, ethylene-propylene copolymer, ethylene-methacrylic acid copolymer, ethylene-ethyl acrylate copolymer, ethylene-propylene-diene terpolymer, ethylene-vinyl acetate copolymer, and ionomer resins thereof.

[0052] The short fibers preferably have an average length of 0.1 to 50 mm, more preferably 1 to 7 mm, and an average diameter of 1 μm to 2 mm, more preferably 5 μm to 0.5 mm. When the average length and average diameter are within the above ranges, there is no risk of the short fibers becoming entangled more than necessary, and good dispersibility can be ensured.

[0053] The short fibers may be selected from the fibrous resins disclosed in WO 2018 / 207472, that is, short fiber resins (flat resins) in which the ratio A / B of the length A of a cross section in the major axis direction in a cross section perpendicular to the major axis direction to the length B of a cross section in the minor axis direction perpendicular to the major axis direction is greater than 1. As long as A / B is greater than 1, the shape, area, and length in the major axis direction of the short fiber resin (flat resin) are not particularly limited. The cross-sectional shape of the flat resin perpendicular to the long axis direction may be elliptical, triangular, rectangular, polygonal, or irregular, but from the viewpoint of improving the water absorption capacity of the vulcanized rubber, it is preferably elliptical or rectangular, and more preferably elliptical. The cross-sectional area of ​​the flat resin perpendicular to the major axis direction is preferably 0.000001 to 0.5 mm2 in average, from the viewpoint of further improving the water absorption of the vulcanized rubber. 2 It is preferable that the thickness is 0.00002 to 0.2 mm. 2 It is more preferable that the average length C of the flat resin in the major axis direction is 0.1 to 500 mm, and more preferably 0.1 to 7 mm. When the cross-sectional area and major axis length of the flat resin are within the above ranges, the water absorption capacity of the vulcanized rubber is further improved, and the short fiber resin is less likely to become entangled with each other more than necessary, facilitating good dispersion within the rubber composition. The average area of ​​the cross section perpendicular to the major axis direction of the flat resin and the average length C of the flat resin in the major axis direction are average values ​​for 100 randomly selected resins. Furthermore, the lengths A, B, and C of the flat resin can be measured by observing the resin using an optical microscope at 20 to 400 times magnification. From the viewpoint of increasing the water absorption capacity of the vulcanized rubber, the A / B ratio is preferably 1.5 or more, and more preferably 2.0 or more. There is no particular upper limit to the A / B ratio, but the A / B ratio is preferably 10 or less, and from the viewpoint of increasing the water absorption capacity of the vulcanized rubber, it is more preferably 5 or less. The length A of the flat resin particles is preferably 0.001 to 2 mm, and more preferably 0.005 to 0.5 mm, as an average value for 100 resin particles, from the viewpoint of increasing the water absorption capacity of the vulcanized rubber. The ratio C / A of the length of the flat resin in the major axis direction to the length A of the major axis of the cross section of the flat resin is usually 10 to 4,000, and preferably 50 to 2,000.

[0054] The content of the short fibers is preferably in the range of 0.1 to 100 parts by mass, more preferably in the range of 1 to 50 parts by mass, per 100 parts by mass of the rubber component. By keeping the content of the short fibers within the above range, a good balance between the performance on ice and the wear resistance of the tire can be achieved.

[0055] (organic acid) The rubber composition of the present invention further has an SP value of 9.15 to 16.0 (cal / cm 3 ) 1 / 2 The organic acid has the effect of improving the foaming rate of the vulcanized rubber by balancing the rate of the decomposition and foaming reaction of the inorganic foaming agent and the rate of the vulcanization reaction of the rubber composition during vulcanization of the rubber composition. By compounding the organic acid in the rubber composition, the workability of the rubber composition is maintained good, while the decomposition and foaming reaction of the inorganic foaming agent is promoted, thereby balancing the rate of the decomposition and foaming reaction and the rate of the vulcanization reaction of the rubber composition, thereby improving the foaming rate of the vulcanized rubber. Furthermore, by applying the organic acid to a tire, the tire's performance on ice can be further improved.

[0056] The SP value of the organic acid is 9.15 (cal / cm 3 ) 1 / 2 When the SP value of the organic acid is 16.0 (cal / cm 3 ) 1 / 2When the content is less than this, the adhesiveness of the rubber composition containing the organic acid can be reduced, and the rubber composition can be prevented from adhering to manufacturing equipment such as rolls during the production of the rubber composition, thereby improving the workability of the rubber composition. The SP value of the organic acid is 10.5 to 14.3 (cal / cm 3 ) 1 / 2 The SP value of the organic acid is preferably 10.5 (cal / cm 3 ) 1 / 2 When the SP value of the organic acid is 14.3 (cal / cm 3 ), the effect of promoting the decomposition of the inorganic foaming agent is further increased. 3 ) 1 / 2 When the content is not more than this, the adhesiveness of the rubber composition containing the organic acid can be further reduced, and the workability of the rubber composition can be further improved. Stearic acid, which is commonly used as a vulcanization aid for rubber compositions, has an SP value of 9.12 (cal / cm 3 ) 1 / 2 and the effect of promoting the decomposition of the inorganic foaming agent is low. In this specification, the SP value (solubility parameter) of an organic acid is calculated according to the Fedors method.

[0057] The organic acid may be any of a monocarboxylic acid, a dicarboxylic acid, a tricarboxylic acid, etc., and may be aliphatic or aromatic. Furthermore, it may have a functional group other than a carboxyl group, such as a hydroxyl group, a ketone group, or an ethylenically unsaturated group. The organic acid is preferably an aromatic acid having an aromatic ring (aromatic), and more preferably a monocarboxylic acid. When the organic acid has an aromatic ring, the adhesion of the rubber composition can be further reduced, the workability of the rubber composition is further improved, and the rubber composition is further less likely to adhere to manufacturing equipment such as rolls.

[0058] Examples of the aliphatic monocarboxylic acid include palmitic acid. Examples of the aliphatic dicarboxylic acid include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, and sebacic acid. Examples of the aromatic monocarboxylic acid include benzoic acid and salicylic acid. Examples of the aromatic dicarboxylic acid include phthalic acid. Examples of organic acids having a functional group other than a carboxyl group include tartaric acid, malic acid, maleic acid, glycolic acid, and α-ketoglutaric acid. The organic acids may be used alone or in combination of two or more.

[0059] Benzoic acid is particularly preferred as the organic acid. When benzoic acid is blended into the rubber composition, the adhesion of the rubber composition can be further reduced, the workability of the rubber composition is further improved, and the rubber composition becomes even less susceptible to adhesion to manufacturing equipment such as rolls.

[0060] From the viewpoints of workability of the rubber composition, foaming rate of the vulcanized rubber, and performance of the tire on ice, the content of the organic acid is preferably in the range of 0.1 to 7 parts by mass, more preferably in the range of 1.5 to 7 parts by mass, and particularly preferably in the range of 3 to 7 parts by mass, per 100 parts by mass of the rubber component.

[0061] When the rubber composition of the present invention contains an organic acid, the total content of the inorganic foaming agent and the organic acid is preferably 3 parts by mass or more and less than 15 parts by mass, more preferably 5 parts by mass or more and less than 15 parts by mass, and particularly preferably 7 parts by mass or more and less than 15 parts by mass, per 100 parts by mass of the rubber component, from the viewpoints of the foaming rate of the vulcanized rubber and the on-ice performance of the tire.

[0062] When the rubber composition of the present invention contains an organic acid, the mass ratio of the inorganic foaming agent to the organic acid (inorganic foaming agent:organic acid) is preferably 1:0.5 to 1:1.5, and more preferably 1:0.7 to 1:1.3, from the viewpoints of the foaming rate of the vulcanized rubber and the performance of the tire on ice.

[0063] When the rubber composition of the present invention contains an organic acid, the total content of the inorganic foaming agent, the foaming aid, and the organic acid is preferably in the range of 5 to 40 parts by mass, more preferably in the range of 9 to 35 parts by mass, and particularly preferably in the range of 12 to 31 parts by mass, per 100 parts by mass of the rubber component, from the viewpoints of the foaming rate of the vulcanized rubber and the performance of the tire on ice.

[0064] Furthermore, when the rubber composition of the present invention contains an organic acid, the mass ratio of the inorganic foaming agent, the foaming aid, and the organic acid (inorganic foaming agent:foaming aid:organic acid) is preferably 1:1.1:0.3 to 1:3.3:2, more preferably 1:1.1:0.6 to 1:3.3:1.4, and further preferably 1:1.3:0.7 to 1:2.7:1.3, from the viewpoints of the foaming rate of the vulcanized rubber and the performance of the tire on ice.

[0065] (Other ingredients) In addition to the rubber component, inorganic foaming agent, foaming aid, short fiber, and organic acid described above, the rubber composition of the present invention may contain compounding agents commonly used in the rubber industry, such as fillers, softeners, stearic acid, antioxidants, zinc oxide (zinc white), vulcanization accelerators, and vulcanizing agents, which may be appropriately selected and compounded within ranges that do not impair the object of the present invention. Commercially available products can be suitably used as these compounding agents.

[0066] Examples of the filler include carbon black, silica, etc. These fillers may be used alone or in combination of two or more. The content of the filler is not particularly limited, but is preferably in the range of 10 to 150 parts by mass, more preferably 20 to 100 parts by mass, per 100 parts by mass of the rubber component.

[0067] The vulcanizing agent may be sulfur, etc. The content of the vulcanizing agent is preferably in the range of 0.1 to 10 parts by mass, more preferably 1 to 4 parts by mass, in terms of sulfur content, per 100 parts by mass of the rubber component.

[0068] Examples of the vulcanization accelerator include thiazole-based vulcanization accelerators and guanidine-based vulcanization accelerators. These vulcanization accelerators may be used alone or in combination of two or more. The content of the vulcanization accelerator is preferably in the range of 0.1 to 5 parts by mass, more preferably 0.2 to 3 parts by mass, per 100 parts by mass of the rubber component.

[0069] The rubber composition of the present invention can be produced by, for example, blending and kneading the rubber component with the inorganic foaming agent, the foaming aid, and various compounding ingredients appropriately selected as necessary using a Banbury mixer, a roll, or the like, followed by heating and extrusion, etc.

[0070] The rubber composition of the present invention can be used for various rubber products including tires, and is particularly suitable as a tire tread rubber.

[0071] <Vulcanized rubber> The vulcanized rubber of the present invention is obtained by vulcanizing the above rubber composition and is characterized by having a foaming ratio of 1 to 45%. Because the vulcanized rubber of the present invention is obtained by vulcanizing the above rubber composition, it has a low environmental impact and, when applied to tires, can improve the tire's performance on ice. When the foaming rate of the vulcanized rubber is 1% or more, the effect of improving the tire's performance on ice can be sufficiently obtained, and when the foaming rate of the vulcanized rubber is 45% or less, the wear resistance of the tire can be sufficiently ensured when it is applied to a tire.

[0072] In this specification, the expansion ratio of the vulcanized rubber means the average expansion ratio Vs, and specifically means the value calculated by the following formula (1). Vs=(ρ0 / ρ1-1)×100(%) ··· (1) In equation (1), ρ1 is the density of vulcanized rubber (foam rubber) (g / cm 3 ) and ρ0 is the density of the solid phase in vulcanized rubber (foamed rubber) (g / cm 3 The density of the vulcanized rubber and the density of the solid phase of the vulcanized rubber are calculated by measuring the mass in ethanol and the mass in air. The foaming rate of the vulcanized rubber can be appropriately changed by the types and contents of the inorganic foaming agent, foaming assistant, and organic acid mentioned above.

[0073] The vulcanized rubber of the present invention preferably has voids with a length of 1000 μm or more. When the vulcanized rubber has voids with a length of 1000 μm or more, its application to a tire can further improve the tire's water film removal ability and drainage, thereby further improving the tire's performance on ice. The length of the voids in the vulcanized rubber is more preferably 1100 μm or more, even more preferably 1120 μm or more, and particularly preferably 1125 μm or more. Furthermore, although there is no particular upper limit to the length of the voids in the vulcanized rubber, the length of the voids in the vulcanized rubber is preferably 2000 μm or less from the viewpoint of ease of production.

[0074] Here, the void length of the vulcanized rubber refers to the average length of the voids in the major axis direction. In addition, in this specification, the void length of the vulcanized rubber can be calculated by observing a 7240 μm × 5430 μm field of view area on the surface of the vulcanized rubber with a microscope, binarizing the observed image, measuring the lengths of 20 to 30 voids, and calculating the average value. The length of the voids in the vulcanized rubber can be appropriately changed by adjusting the average length of the short fibers.

[0075] The vulcanized rubber of the present invention can be used for various rubber products including tires, and is particularly suitable as tire tread rubber.

[0076] <Tire tread rubber> The tire tread rubber of the present invention is characterized by comprising the above rubber composition or the above vulcanized rubber. Because the tire tread rubber of the present invention is made of the above rubber composition or vulcanized rubber, it has a low environmental impact, and when used in a tire, it can improve the tire's performance on ice. The tire tread rubber of the present invention may be applied to new tires or retread tires.

[0077] <Tires> The tire of the present invention is characterized by comprising the above-mentioned rubber composition or vulcanized rubber in the tread portion. Because the tire of the present invention comprises the above-mentioned rubber composition or vulcanized rubber in the tread portion, the tire has a low environmental impact and excellent performance on ice. The tire of the present invention is particularly useful as a winter tire such as a studless tire because of its excellent performance on ice.

[0078] Depending on the type of tire to be applied, the tire of the present invention may be obtained by molding an unvulcanized rubber composition and then vulcanizing it, or by molding a semi-vulcanized rubber that has been subjected to a pre-vulcanization step or the like and then further vulcanizing it. The tire of the present invention is preferably a pneumatic tire, and the gas to be filled into the pneumatic tire may be normal air or air with an adjusted oxygen partial pressure, or an inert gas such as nitrogen, argon, or helium. [Example]

[0079] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples in any way.

[0080] <Preparation and Evaluation of Rubber Compositions> Rubber compositions were produced using the compounding formulations shown in Tables 1 to 3 using a conventional Banbury mixer, and the workability during the production was evaluated. The rubber composition was then vulcanized in a conventional manner to obtain a vulcanized rubber. The foaming rate and void length of the resulting vulcanized rubber were measured using the methods described below, and the performance on ice was also evaluated. The results are shown in Tables 1 to 3.

[0081] (1) Workability The workability was evaluated when producing the rubber compositions having the compounding recipes shown in Tables 2 and 3. Specifically, it was evaluated whether the rubber compositions adhered to production equipment such as rolls during production of the rubber compositions. In Tables 2 and 3, "good" indicates that the rubber composition did not adhere to manufacturing equipment such as rolls and the workability was good, and "roll adhesion" indicates that the rubber composition adhered to the rolls and the workability was poor.

[0082] (2) Foaming rate The foaming rate of vulcanized rubber is calculated using the following formula (1): Vs=(ρ0 / ρ1-1)×100(%) ··· (1) The average foaming ratio Vs was calculated by the above formula and evaluated according to the following criteria. ○ (Good): When the average foaming rate Vs is 1 to 45% ×1 (bad): Average foaming rate Vs is less than 1% ×2 (bad): Average foaming rate Vs is over 45% In equation (1), ρ1 is the density of vulcanized rubber (foam rubber) (g / cm 3 ) and ρ0 is the density of the solid phase in vulcanized rubber (foamed rubber) (g / cm 3 The density of the vulcanized rubber and the density of the solid phase of the vulcanized rubber were calculated by measuring the mass in ethanol and the mass in air.

[0083] (3) Length of the gap For the rubber composition having the formulation shown in Table 1, a test piece sample was cut out from the obtained vulcanized rubber, and a 7240 μm × 5430 μm field of view on the surface of the sample was observed under a microscope. The observed image of the region was then binarized. The length of all voids observed in the binarized image in the long axis direction (the maximum length of a line segment between any two points) was measured, and the average value was calculated.

[0084] (4) Ice performance (ICEμ) A square piece of vulcanized rubber with a side length of 25 mm and a thickness of 2 mm was pressed against fixed ice at -2°C and moved back and forth. The frictional force generated was detected with a load cell, and the dynamic friction coefficient μ was calculated. In Table 1, the coefficient of dynamic friction μ of Comparative Example 1-1 is set to 100 and is expressed as an index. In Table 2, the coefficient of dynamic friction μ of Example 2-1 is set to 100, and the values ​​are expressed as an index. In Table 3, the coefficient of dynamic friction μ of Comparative Example 3-1 is set to 100 and is expressed as an index. The larger the index value, the larger the dynamic friction coefficient μ and the better the performance on ice.

[0085] [Table 1]

[0086] *1-1 NR: Natural rubber *1-2 BR: Butadiene rubber, manufactured by Nippon Zeon, high cis BR *1-3 SBR: Solution polymerization styrene-butadiene rubber, styrene bond content = 10% by mass *1-4 Carbon black: Asahi Carbon, nitrogen adsorption specific surface area = 148m 2 / g *1-5 Silica: Tosoh, nitrogen adsorption specific surface area = 222m 2 / g *1-6 Short fiber: Resin 4 (flat resin) used in Example 4 of WO 2018 / 207472, short fiber in which a coating layer made of polyethylene (manufactured by Japan Polyethylene Co., Ltd., product name "Novatec HJ360") is formed on the surface of a hydrophilic fiber made of ethylene-vinyl alcohol copolymer (manufactured by Kuraray Co., Ltd., product name "Eval F104B"), the ratio A / B of the length A of the cross section in the major axis direction in a cross section perpendicular to the major axis direction to the length B of the cross section in the minor axis direction perpendicular to the major axis direction = 2.7, the average value of length A = 0.05 mm, the average area of ​​the cross section perpendicular to the major axis direction = 0.0007 mm 2 , the average value of the longitudinal length C = 3 mm *1-7 Accelerator package: Includes vulcanization accelerator MBTS and vulcanization accelerator CZ *1-8 Other chemicals: Antioxidant 6C, including wax and resin *1-9 Inorganic foaming agent: baking soda (sodium bicarbonate), manufactured by Eiwa Chemical Industry Co., Ltd., product name "Celbon FE507" *1-10 Foaming aid: Urea, manufactured by Eiwa Chemical Industry Co., Ltd., product name "Cell Paste K5"

[0087] [Table 2]

[0088] *2-1 NR: Natural rubber *2-2 BR: Butadiene rubber, manufactured by Ube Industries, Ltd., "BR150L" *2-3 SBR: Styrene-butadiene rubber, manufactured by JSR Corporation, "1500" *2-4 Carbon black: Asahi Carbon Co., Ltd., "N134" *2-5 Hydrogenated fatty acid: "Stearic acid 50S" manufactured by New Japan Chemical Co., Ltd. *2-6 Anti-aging agent: Includes the product name "Nocrac 6C" manufactured by Ouchi Shinko Chemical Industry Co., Ltd., with a total blend amount of 2 parts by mass *2-7 Vulcanization accelerator: Sansera CZ manufactured by Sanshin Chemical Industry Co., Ltd. *2-8 Baking soda: Sodium bicarbonate, manufactured by Eiwa Chemical Industry Co., Ltd., "Celbon FE507" *2-9 Urea: Eiwa Chemical Industry Co., Ltd., "Cell Paste K5"

[0089] [Table 3]

[0090] *3-1 NR: Natural rubber *3-2 BR: Butadiene rubber, manufactured by Ube Industries, Ltd., "BR150L" *3-3 Modified BR: Modified butadiene rubber, manufactured by JSR Corporation, "BR500" *3-4 SBR: Styrene-butadiene rubber, manufactured by JSR Corporation, "1500" *3-5 Modified SBR: Modified styrene-butadiene rubber synthesized by the following method

[0091] (Method for synthesizing modified SBR) A cyclohexane solution of 1,3-butadiene and a cyclohexane solution of styrene were added to a dried, nitrogen-purged 800 mL pressure-resistant glass vessel so that the total weight of the mixture was 67.5 g of 1,3-butadiene and 7.5 g of styrene. 0.6 mmol of 2,2-ditetrahydrofurylpropane and 0.8 mmol of n-butyllithium were then added, and polymerization was carried out at 50 °C for 1.5 hours. At this point, the polymerization conversion rate of the polymerization reaction system reached nearly 100%, and 0.72 mmol of N,N-bis(trimethylsilyl)-3-[diethoxy(methyl)silyl]propylamine was added as a modifier, and the modification reaction was carried out at 50 °C for 30 minutes. The reaction was then terminated by adding 2 mL of a 5% by weight solution of 2,6-di-t-butyl-p-cresol (BHT) in isopropanol. The mixture was then dried in the usual manner to obtain modified SBR. The microstructure of the resulting modified SBR was measured, and the result was that the bound styrene content was 10% by mass, the vinyl bond content in the butadiene moiety was 40%, and the peak molecular weight was 200,000.

[0092] *3-6 Carbon black: Asahi Carbon Co., Ltd., "N134" *3-7 Hydrogenated fatty acid: "Stearic acid 50S" manufactured by New Japan Chemical Co., Ltd. *3-8 Anti-aging agent: "Nocrac NS-6" manufactured by Ouchi Shinko Chemical Industry Co., Ltd. *3-9 Vulcanization accelerator: Sansera CZ manufactured by Sanshin Chemical Industry Co., Ltd. *3-10 Baking soda: Sodium bicarbonate, manufactured by Eiwa Chemical Industry Co., Ltd., "Celbon FE507" *3-11 Citric acid: Kanto Chemical Co., Ltd., SP value = 16.53 (cal / cm 3 ) 1 / 2 *3-12 Benzoic acid: Fujifilm Wako Pure Chemical Industries, Ltd., SP value = 11.93 (cal / cm 3 ) 1 / 2 *3-13 Malonic acid: Kanto Chemical Co., Ltd., SP value = 14.03 (cal / cm 3 ) 1 / 2 *3-14 Urea: Eiwa Chemical Industry Co., Ltd., "Cell Paste K5"

[0093] Tables 1 to 3 show that the rubber compositions of the examples in which the total content of the inorganic foaming agent and the foaming aid is 1 to 20 parts by mass and the mass ratio of the inorganic foaming agent to the foaming aid (inorganic foaming agent:foaming aid) is 1:1.1 to 1:3.3 have excellent performance on ice. [Industrial Applicability]

[0094] The rubber composition, vulcanized rubber and tread rubber of the present invention can be used for tires, particularly studless tires, etc. The tire of the present invention is particularly useful as a studless tire.

Claims

1. A rubber composition comprising a rubber component containing a diene rubber, an inorganic foaming agent, and a foaming aid, a total content of the inorganic foaming agent and the foaming aid is 1 to 20 parts by mass per 100 parts by mass of the rubber component, The foaming aid is urea, the inorganic foaming agent is selected from ammonium bicarbonate and sodium bicarbonate; A rubber composition characterized in that the mass ratio of the inorganic foaming agent to the foaming aid (inorganic foaming agent:foaming aid) is 1:1.1 to 1:3.

3.

2. The rubber composition according to claim 1 , wherein the inorganic foaming agent is sodium bicarbonate.

3. Further, it contains hydrophilic short fibers, The hydrophilic short fibers have an average length of 1 to 7 mm, an average diameter of 5 μm to 0.5 mm, and an elliptical cross-sectional shape; the hydrophilic short fibers are made of an ethylene-vinyl alcohol copolymer, and a coating layer made of polyethylene is formed on the surfaces of the hydrophilic short fibers; The rubber composition according to claim 1 or 2.

4. The rubber composition according to any one of claims 1 to 3, wherein the diene rubber comprises at least one of a natural rubber and a synthetic diene rubber.

5. 5. The rubber composition according to claim 4, wherein the synthetic diene rubber comprises at least one selected from the group consisting of isoprene rubber, styrene-butadiene rubber, and butadiene rubber.

6. The rubber composition according to claim 5, wherein the styrene-butadiene rubber contains a styrene-butadiene rubber having a styrene bond amount of 15% by mass or less.

7. The rubber composition according to claim 5, wherein the styrene-butadiene rubber is a modified styrene-butadiene rubber, or the butadiene rubber is a modified butadiene rubber.

8. A vulcanized rubber obtained by vulcanizing the rubber composition according to any one of claims 1 to 7, characterized in that the foaming rate is 1 to 45%.

9. The vulcanized rubber according to claim 8, having voids with a length of 1000 μm or more.

10. A tread rubber for a tire, comprising the rubber composition according to any one of claims 1 to 7, or the vulcanized rubber according to claim 8 or 9.

11. A tire comprising the rubber composition according to any one of claims 1 to 7 or the vulcanized rubber according to claim 8 or 9 in a tread portion.

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