Crosslinked rubber composition and method for producing crosslinked rubber composition

Abstract

A crosslinked rubber composition that includes 20-50 mass parts of a styrene-butadiene copolymer rubber, 50-80 mass parts of a diene rubber, and 20-50 mass parts of an inorganic filler, the total of the styrene-butadiene copolymer rubber and the diene rubber being 100 mass parts, the styrene-butadiene copolymer rubber having a bound styrene level of 20-55 mass%, a ratio of the styrene block level to the bound styrene level of 0.020-0.23, a 1,2-bond level of the butadiene moiety of 10-60 mass%, a Mooney viscosity at 100°C measured in accordance with JIS K6300-1 of 50-80, and a refractive index of 1.5100-1.5900, the diene rubber having a refractive index of 1.5000-1.5400, and the inorganic filler having a refractive index of 1.3700-1.5400.

Description

[Technical Field]

[0001] The present invention relates to a crosslinked rubber composition and a method for producing the crosslinked rubber composition. [Background technology]

[0002] Footwear sole materials are required to have not only functions such as abrasion resistance but also fashionability, for example, transparency to allow the design and / or color of the midsole (intermediate layer of the footwear) to be seen through, etc. Various rubber compositions have been developed for the purpose of producing such footwear sole materials, etc.

[0003] Patent Document 1 discloses a transparent composition containing a two-component rubber polymer having a predetermined refractive index difference and wet-process silica. Patent Document 2 discloses a composition combining a low-cis isoprene rubber (hereinafter also referred to as "IR") component that is substantially free of silica with another rubber component. Patent Documents 3 and 4 disclose footwear soles using block styrene butadiene rubber (hereinafter also referred to as "SBR") and / or random SBR. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-2225 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-66423 [Patent Document 3] International Publication No. 2018 / 193555 [Patent Document 4] Japanese Patent Application Laid-Open No. 63-11102 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the composition described in Patent Document 1 may have insufficient abrasion resistance and / or strength. Furthermore, the composition described in Patent Document 2 uses a combination of rubbers with different refractive indices, such as low-cis isoprene rubber and 1,2-polybutadiene, and is therefore considered to have insufficient transparency. In the examples of Patent Document 2, only crosslinked rubber compositions of low-cis isoprene rubber / 1,2-polybutadiene or low-cis isoprene rubber / butadiene are used, and no other combinations of low-cis isoprene rubber (IR) with styrene-butadiene rubber (SBR), acrylonitrile butadiene rubber (NBR), hydrogenated nitrile rubber (HNBR), urethane rubber (U), ethylene propylene rubber (EPM), ethylene propylene diene rubber (EPDM), or chloroprene rubber (CR) are disclosed. Furthermore, none of the examples in Patent Document 2 contain silica. Furthermore, the footwear sole materials described in Patent Documents 3 and 4 may also be insufficient in transparency, strength, and / or abrasion resistance.

[0006] In view of the above-mentioned problems of the prior art, an object of the present invention is to provide a crosslinked rubber composition that maintains transparency while improving strength and abrasion resistance, and a method for producing the same. [Means for solving the problem]

[0007] As a result of intensive research to solve the above problems, the present inventors have found that a crosslinked rubber composition containing a specified styrene-butadiene copolymer rubber, a diene rubber, and silica is excellent in all of transparency, strength, and abrasion resistance, and have thus completed the present invention.

[0008] That is, the present invention is as follows. [1] A crosslinked rubber composition comprising 20 to 50 parts by mass of a styrene-butadiene copolymer rubber, 50 to 80 parts by mass of a diene rubber, and 20 to 50 parts by mass of an inorganic filler, wherein the total amount of the styrene-butadiene copolymer rubber and the diene rubber is 100 parts by mass, The styrene-butadiene copolymer rubber has a bound styrene content of 20 to 55% by mass, a ratio of the styrene block content to the bound styrene content of 0.020 to 0.23, a 1,2-bond content in the butadiene moiety of 10 to 60% by mass, a Mooney viscosity at 100°C measured in accordance with JIS K6300-1 of 50 to 80, and a refractive index of 1.5100 to 1.5900, The diene rubber has a refractive index of 1.5000 to 1.5400, The inorganic filler has a refractive index of 1.3700 to 1.5400. Crosslinked rubber composition. [2] The crosslinked rubber composition according to [1], which has one peak of loss tangent (tan δ) in the temperature range of −60° C. to 40° C. in a temperature dispersion curve obtained by dynamic viscoelasticity measurement. [3] The crosslinked rubber composition according to [1] or [2], which has a refractive index of 1.4700 to 1.5500. [4] The cross-linked rubber composition according to any one of [1] to [3], wherein the inorganic filler is silica. [5] The crosslinked rubber composition according to any one of [1] to [4], which has a haze value of 15 to 40% as measured on a 3.00 mm thick sheet in accordance with JIS K7136. [6] The crosslinked rubber composition according to any one of [1] to [5], which has a hardness of 60 to 75 according to Durometer Type A measured in accordance with JIS K6253. [7] The crosslinked rubber composition according to any one of [1] to [6], which is crosslinked with one or more crosslinking agents selected from peroxides and radical crosslinking agents. [8] (A) a kneading step of kneading 20 to 50 parts by mass of a styrene-butadiene copolymer rubber having a bound styrene content of 20 to 55% by mass, a ratio of a styrene block content to the bound styrene content of 0.020 to 0.23, a 1,2-bond content in a butadiene moiety of 10 to 60% by mass, a Mooney viscosity at 100°C measured in accordance with JIS K6300-1 of 50 to 80, and a refractive index of 1.5100 to 1.5900, (B) 50 to 80 parts by mass of a diene rubber having a refractive index of 1.5000 to 1.5400, (C) 20 to 50 parts by mass of an inorganic filler having a refractive index of 1.3700 to 1.5400, and (D) 0.10 to 3.0 parts by mass of a peroxide and / or a radical crosslinking agent relative to a total 100 parts by mass of the styrene-butadiene copolymer rubber and the diene rubber; a step of molding the obtained kneaded product; A method for producing a crosslinked rubber composition, comprising: [9] In the styrene-butadiene copolymer rubber, the bound styrene content is 41 to 52% by mass, the 1,2-bond content of the butadiene portion is 18 to 37% by mass, and the styrene block content is 0.82 to 9.00% by mass. [8] A method for producing the crosslinked rubber composition according to [8].

[10] In the styrene-butadiene copolymer rubber, the amount of 1,2-bonds in the butadiene portion is 24 to 31% by mass. [9] A method for producing the crosslinked rubber composition according to [9].

[11] The method for producing a crosslinked rubber composition according to any one of [8] to

[10] , wherein (E) a liquid diene rubber is further kneaded in the kneading step. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a crosslinked rubber composition or the like that has improved strength and abrasion resistance while maintaining transparency. [Brief explanation of the drawings]

[0010] [Figure 1]FIG. 2 is a diagram showing an example of a temperature dispersion curve obtained by measuring the dynamic viscoelasticity of a crosslinked rubber composition. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, a mode for carrying out the present invention (hereinafter referred to as "the present embodiment") will be described in detail. The present embodiment is an example for explaining the present invention, and the present invention is not limited to the following embodiment. The present invention can be carried out by appropriately modifying it within the scope of its gist.

[0012] In this specification, the term "crosslinked rubber composition" refers to a composition having a structure in which at least one rubber component is crosslinked, i.e., a composition obtained by crosslinking a composition containing at least one rubber component. In this specification, the crosslinked rubber composition may be a composition having a portion derived from at least one rubber component and a portion derived from at least one crosslinking agent, i.e., a composition obtained by crosslinking a composition containing at least one rubber component with an arbitrary crosslinking agent.

[0013] In this specification, the transparency of a crosslinked rubber composition is evaluated by its haze value. A lower haze value indicates higher transparency, and a crosslinked rubber composition having a haze value of a certain level or higher is referred to as "opaque" or "semi-transparent" in this specification. The "haze" value refers to the degree of cloudiness of a transparent material, as measured in accordance with Japanese Industrial Standard JIS K7136 (or ISO 14782). In this specification, the haze value is measured using a 3.00 mm thick sheet as a measurement sample, using a test device in accordance with the above standard.

[0014] In this specification, the hardness of a crosslinked rubber composition is evaluated by Type A durometer hardness. A larger Type A durometer hardness value indicates a harder material. "Type A Durometer Hardness" (Hs) or "Shore A" refers to the hardness of a crosslinked rubber composition measured in accordance with Japanese Industrial Standard JIS K6253-3. In this specification, the Type A Durometer Hardness or Shore A value is measured using a 6.00 mm thick sheet as a measurement sample, and is the depth of penetration 3 seconds after a plunger is pressed against the test piece with a constant force.

[0015] In this specification, the strength of a crosslinked rubber composition is evaluated by its tensile strength and tear strength. The greater the tensile strength and tear strength, the higher the strength. "Tensile strength" refers to the tensile strength of a crosslinked rubber composition measured in accordance with Japanese Industrial Standard JIS K6251. In this specification, the tensile strength value is a value measured using a 2.00 mm thick sheet as a measurement sample, using a dumbbell-shaped No. 5 test piece, and converted into MPa. "Tear strength" refers to the tear strength of a crosslinked rubber composition measured in accordance with Japanese Industrial Standard JIS K6252-1. In this specification, the tear strength value is a value measured using a 2.00 mm thick sheet as a measurement sample, using an unnotched angle-shaped test piece, and converted into N / mm units.

[0016] In this specification, the abrasion resistance of a crosslinked rubber composition is evaluated by the abrasion resistance index of the crosslinked rubber composition, which is measured in accordance with Japanese Industrial Standard JIS K6264-2 (or ISO4649). A higher abrasion resistance index indicates higher abrasion resistance. In this specification, the value of the abrasion resistance index is measured in accordance with the above standard using a DIN abrasion tester conforming to the above standard.

[0017] [Crosslinked rubber composition] The crosslinked rubber composition of this embodiment is a rubber composition obtained by crosslinking a composition containing 20 to 50 parts by mass (both inclusive; the same applies throughout the specification) of styrene-butadiene copolymer rubber, 50 to 80 parts by mass of diene rubber, and 20 to 50 parts by mass of inorganic filler, wherein the total amount of the styrene-butadiene copolymer rubber and the diene rubber is 100 parts by mass.

[0018] (styrene-butadiene copolymer rubber) The styrene-butadiene copolymer rubber contained in the crosslinked rubber composition of this embodiment has a bound styrene content of 20 to 55 mass%, a ratio of the styrene block content to the bound styrene content of 0.020 to 0.23, a 1,2-bond content in the butadiene moiety of 10 to 60 mass%, a Mooney viscosity at 100°C measured in accordance with JIS K6300-1 of 50 to 80, and a refractive index of 1.5100 to 1.5900. The present inventors have found that a crosslinked rubber composition containing such a styrene-butadiene copolymer rubber in a specified ratio in addition to a specified diene rubber and inorganic filler is excellent in all of transparency, abrasion resistance, and strength.

[0019] In this specification, styrene-butadiene copolymer rubber means a copolymer of styrene monomer and butadiene monomer. Hereinafter, in the styrene-butadiene copolymer rubber, the portion derived from the styrene monomer will be referred to as the styrene portion, and the portion derived from the butadiene monomer will be referred to as the butadiene portion.

[0020] When the bound styrene content of the styrene-butadiene copolymer rubber is 20 to 55 mass%, excellent transparency is obtained. From the same viewpoint, the bound styrene content is preferably 30 to 54 mass%, more preferably 35 to 53 mass%. The bound styrene content may be 41 to 52 mass%. In this specification, the term "bound styrene content" refers to the content (mass%) of styrene moieties in the styrene-butadiene copolymer rubber. That is, the bound styrene content refers to the mass ratio of styrene moieties to the total mass of styrene moieties and butadiene moieties in the styrene-butadiene copolymer rubber.

[0021] The amount of bound styrene can be calculated by measuring the ultraviolet absorption of the phenyl group of the styrene portion of the styrene-butadiene copolymer rubber. More specifically, the method described in the Examples may be used. The amount of bound styrene can be adjusted by controlling the amount of styrene monomer added during the synthesis of the styrene-butadiene copolymer rubber. The content (mass%) of the butadiene portion in the styrene-butadiene copolymer rubber is a value obtained by subtracting the value (mass%) of the bound styrene amount from 100 mass%.

[0022] When the ratio of the styrene block amount to the bound styrene amount of the styrene-butadiene copolymer rubber (hereinafter also referred to as the "styrene block amount / bound styrene amount ratio") is 0.020 to 0.23, transparency, strength, and abrasion resistance can be improved in a well-balanced manner. From the same viewpoint, the ratio of the styrene block amount / bound styrene amount is preferably 0.22 or less, and more preferably 0.20 or less. When the ratio of the styrene block amount / bound styrene amount exceeds 0.23, strength or abrasion resistance tends to decrease. Furthermore, the ratio of the styrene block amount / bound styrene amount is preferably 0.030 or more, and more preferably 0.040 or more. When the ratio of the styrene block amount / bound styrene amount is less than 0.020, transparency tends to decrease.

[0023] The amount of styrene block is not particularly limited as long as the ratio of the amount of styrene block to the amount of bound styrene falls within the above range, but may be, for example, 0.60 to 15.0 mass%, preferably 0.70 to 12.0 mass%, and may be 0.82 to 9.00 mass%.

[0024] In this specification, the "styrene block content" means the content (mass%) of a portion in which 8 or more styrene monomers are continuously bonded in a styrene-butadiene copolymer rubber, i.e., the value measured by the method described in the Examples below. Therefore, the ratio of the styrene block content / bound styrene content means the proportion of styrene portions present as styrene blocks to the total styrene portions in the styrene-butadiene copolymer rubber.

[0025] The styrene block content can be measured by a known method in which the copolymer is decomposed by the Kolthoff method (the osmium tetroxide decomposition method described in I.M. Kolthoff, et al., J. Polym. Sci. 1, 429 (1946)) and the amount of styrene blocks insoluble in methanol is analyzed. More specifically, the method described in the Examples may be used. The styrene block content can be adjusted by controlling the amount of randomizer added when polymerizing the styrene-butadiene copolymer rubber. Increasing the amount of randomizer tends to decrease the styrene block content.

[0026] When the 1,2-bond content in the butadiene portion is 10 to 60 mass%, transparency, strength, and abrasion resistance can be improved in a well-balanced manner. From the same viewpoint, the 1,2-bond content in the butadiene portion is preferably 13 to 50 mass%, more preferably 15 to 40 mass%. When the 1,2-bond content in the butadiene portion is less than 10 mass%, transparency tends to be poor. When the 1,2-bond content in the butadiene portion is more than 60 mass%, abrasion resistance and strength tend to be poor. The 1,2-bond content in the butadiene portion may be 18 to 37 mass%, or 24 to 31 mass%.

[0027] Generally, when butadiene monomers are polymerized, they can be added to a polymer by 1,2-addition or 1,4-addition. In this specification, the "1,2-bond amount of butadiene moieties" means the proportion (mass %) of butadiene moieties polymerized by 1,2-addition to the total butadiene moieties. Note that butadiene moieties polymerized by 1,2-addition exist as moieties having vinyl groups in the styrene-butadiene copolymer rubber before crosslinking, and butadiene moieties polymerized by 1,4-addition exist as moieties having double bonds in the main chain of the styrene-butadiene copolymer rubber before crosslinking.

[0028] The amount of 1,2-bonds in the butadiene moiety can be measured, for example, according to Hampton's method using an infrared spectrophotometer (RR Hampton, Analytical Chemistry, 21, 923 (1949)). More specifically, the method described in the Examples may be used. The amount of 1,2-bonds in the butadiene moiety can be adjusted by controlling the amount of vinylating agent added during polymerization of the styrene-butadiene copolymer rubber. Increasing the amount of vinylating agent added tends to increase the amount of 1,2-bonds in the butadiene moiety.

[0029] The Mooney viscosity of the styrene-butadiene copolymer rubber at 100°C measured in accordance with JIS K6300-1 (hereinafter sometimes simply referred to as "Mooney viscosity") is 50 to 80. When the Mooney viscosity is 50 or higher, the abrasion resistance and strength of the crosslinked rubber composition are improved, and when it is 80 or lower, the processability of the crosslinked rubber composition is improved. From the same viewpoint, the Mooney viscosity is preferably 55 to 78, more preferably 55 to 76. In this specification, the Mooney viscosity is measured using an L-type rotor, with ML1+4(100°C), which is the torque after 1 minute of preheating at 100°C and 4 minutes of operation. More specifically, the method described in the examples may be used.

[0030] By appropriately controlling the bound styrene content of the styrene-butadiene copolymer rubber, the 1,2-bond content of the butadiene portion, the styrene block content, the molecular weight, the coupling agent, the coupling rate, the molecular weight distribution, etc., it is possible to realize a Mooney viscosity of 50 to 80. However, as long as the Mooney viscosity is within the predetermined range, it is not intended to limit the above-mentioned values.

[0031] As reference examples, two styrene-butadiene copolymer rubbers with a Mooney viscosity of 68 are mentioned: a styrene-butadiene copolymer rubber with a bound styrene content of 45.5% by mass, a 1,2-bond content in the butadiene portion of 38.5% by mass, a styrene block content of 1.0% by mass (ratio of styrene block content / bound styrene content of 0.022), a peak top molecular weight of 274,000, and a molecular weight distribution of 1.29 (no coupling agent used); and a styrene-butadiene copolymer rubber with a bound styrene content of 27.6% by mass, a 1,2-bond content in the butadiene portion of 14.3% by mass, a styrene block content of 0.90% by mass (ratio of styrene block content / bound styrene content of 0.033), a tetrafunctional coupling agent used, a peak top molecular weight before coupling of 240,000, and a peak top molecular weight after coupling of 683,000, and a molecular weight distribution of 1.73. Increasing any one of the values ​​of the bound styrene amount, the 1,2-bond amount in the butadiene portion, the styrene block amount, the molecular weight, the coupling rate, and the molecular weight distribution tends to increase the Mooney viscosity, while decreasing any one of the values ​​of the bound styrene amount, the 1,2-bond amount in the butadiene portion, the styrene block amount, the molecular weight, the coupling rate, and the molecular weight distribution tends to increase the Mooney viscosity.

[0032] When the refractive index of the styrene-butadiene copolymer rubber is 1.5100 to 1.5900, the crosslinked rubber composition has excellent transparency. From the same viewpoint, the refractive index of the styrene-butadiene copolymer rubber is preferably 1.5200 to 1.5800, and more preferably 1.5300 to 1.5700. When the refractive index of the styrene-butadiene copolymer rubber is outside the range of 1.5100 to 1.5900, the transparency of the crosslinked rubber composition tends to be inferior.

[0033] In this specification, the refractive index of the rubber component (i.e., styrene-butadiene copolymer rubber and diene rubber) and the inorganic filler is measured in accordance with Japanese Industrial Standard JIS K0062. More specifically, the method described in the Examples may be used. The refractive index of the styrene-butadiene copolymer rubber is greatly affected by the amount of bound styrene, and can be adjusted by controlling the amount of bound styrene. For example, an increase in the amount of bound styrene tends to increase the refractive index.

[0034] It is generally known that changing the refractive index of the components contained in a crosslinked rubber composition changes the transparency of the composition as a whole. The present inventors have found that the transparency of the entire crosslinked rubber composition can be improved by controlling the refractive index of the diene rubber to a value between the refractive index of the styrene-butadiene copolymer rubber and the refractive index of the inorganic filler. Furthermore, as a result of further intensive studies, they have found that even if the refractive index of the diene rubber is not a value between the refractive index of the styrene-butadiene copolymer rubber and the refractive index of the inorganic filler, the transparency of the entire crosslinked rubber composition can be improved by setting the refractive indexes of the styrene-butadiene copolymer rubber, the inorganic filler, and the diene rubber within a predetermined range.

[0035] As described above or below, in the crosslinked rubber composition of this embodiment, the refractive index of the styrene-butadiene copolymer rubber is 1.5100 to 1.5900, the refractive index of the diene rubber is 1.5000 to 1.5400, and the refractive index of the inorganic filler is 1.3700 to 1.5400. From the above viewpoints, it is preferable that the refractive index of the diene rubber be controlled to a value between the refractive index of the styrene-butadiene copolymer rubber and the refractive index of the inorganic filler. It is more preferable that the refractive index of the diene rubber is equal to or higher than the refractive index of the inorganic filler and equal to or lower than the refractive index of the styrene-butadiene copolymer rubber. The refractive index of the diene rubber may be set within a range of ±0.0260 of the mass-weighted average of the refractive index of the inorganic filler and the refractive index of the styrene-butadiene copolymer rubber.

[0036] The peak top molecular weight of the styrene-butadiene copolymer rubber measured by GPC measurement is not particularly limited, but is, for example, 5.00×10 4 Over 90.0 x 10 4 When coupling is performed using a coupling agent during the synthesis of styrene-butadiene copolymer rubber, two peaks are observed in GPC measurement: one derived from the uncoupled copolymer rubber and one derived from the coupled copolymer rubber.

[0037] The peak top molecular weight of the peak derived from the uncoupled copolymer rubber is preferably 10.0×10 4 Over 50.0 x 10 4 or less, more preferably 13.0 × 10 4 Over 50.0 x 10 4 or less, more preferably 15.0 × 10 4 Over 30.0 x 10 4 The peak top molecular weight is 31.0 × 10 4 It may be the following: The peak top molecular weight of the peak derived from the coupled copolymer rubber is preferably 30.0×10 4 Over 80.0 x 10 4 or less, more preferably 40.0 × 10 4 Over 70.0 x 10 4 or less, more preferably 45.0 × 10 4 Over 65.0 x 10 4 The following is the result. When the peak top molecular weight is within the above range, the transparency, strength, and abrasion resistance of the crosslinked rubber composition tend to be improved in a well-balanced manner.

[0038] In this specification, the peak top molecular weight is a molecular weight determined from the position of the peak of the distribution measured by gel permeation chromatography (GPC) using polystyrene as a standard substance. More specifically, the method described in the Examples may be used as a method for measuring the peak top molecular weight. When synthesizing styrene-butadiene copolymer rubber, the peak top molecular weight of the styrene-butadiene copolymer rubber tends to be increased by reducing the amount of initiator relative to the monomer, extending the polymerization time, or coupling with a coupling agent.

[0039] The molecular weight distribution (ratio of weight average molecular weight to number average molecular weight) of the styrene-butadiene copolymer rubber is not particularly limited, and is, for example, 1.03 to 2.50, preferably 1.10 to 2.00. In this specification, the molecular weight distribution is calculated using the weight average molecular weight measured by GPC measurement method and the number average molecular weight measured by GPC measurement method. The molecular weight distribution can be adjusted by appropriately changing the polymerization conditions.

[0040] The styrene-butadiene copolymer rubber may be coupled with a coupling agent (polymerization coupling). When coupled, the coupling rate is not particularly limited and may be, for example, 0.10% to 95%, 5.0% to 90%, 10% to 88%, 30% to 85%, or 50% to 83%. The coupling rate can be determined from the peak area of ​​the peak derived from the uncoupled copolymer rubber and the peak derived from the coupled copolymer rubber detected by GPC measurement. As the coupling agent, for example, the coupling agents described below may be used.

[0041] The styrene-butadiene copolymer rubber as described above may be synthesized, for example, by polymerizing a styrene monomer and a butadiene monomer in an appropriate solvent with an appropriate polymerization initiator. The styrene monomer and the butadiene monomer may be mixed at once or in multiple batches. The polymerization initiation temperature (i.e., the temperature when the polymerization initiator is added), the polymerization peak temperature (i.e., the highest temperature reached in the polymerization process), the polymerization time, etc. can be appropriately adjusted depending on the desired properties of the styrene-butadiene copolymer rubber.

[0042] The polymerization initiation temperature is not particularly limited and is, for example, 40°C or higher and 80°C or lower, preferably 45°C or higher and 70°C or lower, and more preferably 50°C or higher and 65°C or lower. The polymerization peak temperature is not particularly limited and is, for example, 50°C or higher and 100°C or lower, preferably 60°C or higher and 95°C or lower, and more preferably 70°C or higher and 90°C or lower. The polymerization time is not particularly limited and is, for example, 30 seconds to 30 minutes after the polymerization peak temperature is reached, preferably 45 seconds to 15 minutes after the polymerization peak temperature is reached, and more preferably 1.0 minute to 5.0 minutes after the polymerization peak temperature is reached. The polymerization reaction is terminated by adding a polymerization terminator such as methanol or a coupling agent as described below.

[0043] The solvent used in the polymerization may be any inert hydrocarbon solvent conventionally used in the synthesis of styrene-butadiene copolymers. Non-limiting examples of solvents include linear and branched hydrocarbons such as pentane, hexane, heptane, and octane, as well as their alkyl-substituted derivatives; alicyclic hydrocarbons such as cyclopentane, cyclohexane, and cycloheptane, as well as their alkyl-substituted derivatives; aromatic hydrocarbons such as benzene, naphthalene, toluene, and xylene, as well as their alkyl-substituted derivatives; and hydrogenated aromatic hydrocarbons such as tetralin and decalin, as well as their alkyl-substituted derivatives. These solvents may be used alone or in combination.

[0044] The polymerization initiator used in the polymerization may be any polymerization initiator (e.g., radical polymerization initiator, living polymerization initiator, etc.) conventionally used in the synthesis of styrene-butadiene copolymers. It is preferable to use a lithium polymerization initiator, as this tends to result in a styrene-butadiene copolymer rubber with less residue and contribute to high transparency. As the lithium polymerization initiator, for example, an organolithium compound can be used, such as an organolithium compound mono- to tetra-substituted with a hydrocarbon group having 1 to 20 carbon atoms, preferably a hydrocarbon group having 2 to 8 carbon atoms. Emulsion polymerization is not preferred from the viewpoint of improving transparency.

[0045] Non-limiting examples of organolithium compounds include alkyllithium (methyllithium, ethyllithium, propyllithium, n-butyllithium, s-butyllithium, t-butyllithium, etc.), aryllithium (phenyllithium, tolyllithium, etc.), alkenyllithium (vinyllithium, propenyllithium, etc.), and alkylenelithium (tetramethylenelithium, pentamethylenelithium, etc.). Among these, it is preferable to use alkyllithium as the polymerization initiator, and it is more preferable to use n-butyllithium. These organolithium compounds may be used alone or in combination of two or more.

[0046] In the polymerization process of the styrene-butadiene copolymer rubber, an additive may be added to the reaction system to control the amount of styrene blocks and / or the amount of 1,2-bonds in the butadiene moiety. Such additives include a vinylating agent and a randomizing agent. In the polymerization process of the styrene-butadiene copolymer rubber, it is preferable to add at least a vinylating agent.

[0047] Non-limiting examples of vinylating agents include diethyl ether, ethylene glycol dimethyl ether, ethylene glycol di-n-butyl ether, ethylene glycol n-butyl-t-butyl ether, ethylene glycol di-t-butyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetrahydrofuran, α-methoxytetrahydrofuran, 2-methoxymethyltetrahydrofuran, dioxane, 1,2-dimethoxybenzene, triethylamine, N,N,N',N'-tetramethylethylenediamine, and 2,2'-ditetrahydrofurylpropane, etc. These vinylating agents may be used alone or in combination of two or more. When the amount of vinylating agent added is large, it not only functions to increase the amount of 1,2-bonds in the butadiene moiety but also functions as a randomizing agent to reduce the amount of styrene blocks.

[0048] Non-limiting examples of randomizing agents include potassium-t-amyl alkoxide and potassium-t-butyl alkoxide, etc. These randomizing agents tend to have the effect of reducing the amount of styrene blocks without affecting the amount of 1,2-bonds in the butadiene moiety.

[0049] When a vinylating agent is added, its amount is not particularly limited, but the ratio relative to the amount of polymerization initiator added may be adjusted. The amount of vinylating agent added may be, for example, 0.050 to 1.0 mol, preferably 0.10 to 0.80 mol, and more preferably 0.12 to 0.70 mol, per 1.0 mol of polymerization initiator added. The amount of vinylating agent added may be 0.040 mol or more per 1.0 mol of polymerization initiator added. Furthermore, when a randomizer is added, its amount is not particularly limited, but the ratio relative to the amount of polymerization initiator added may be adjusted. The amount of randomizer added may be, for example, 0.010 to 0.50 mol, preferably 0.020 to 0.30 mol, and more preferably 0.030 to 0.10 mol, relative to 1.0 mol of polymerization initiator added.

[0050] Furthermore, when terminating the polymerization reaction of the styrene-butadiene copolymer rubber, the styrene-butadiene copolymer rubber may be coupled or modified by using a coupling agent and / or a modifier instead of a polymerization terminator such as methanol. Therefore, the styrene-butadiene copolymer rubber of this embodiment may be a modified polymer rubber whose terminals are modified with a modifier.

[0051] Examples of the modifying agent include compounds having one or more functional groups selected from the group consisting of an amino group, an amide group, an alkoxysilyl group, an isocyanate group, an imino group, an imidazole group, a urea group, an ether group, an epoxy group, a carbonyl group, a carboxyl group, a hydroxyl group, a nitrile group, and a pyridyl group. Examples of coupling agents include compounds having two or more functional groups that can react with radicals, such as isocyanate groups, isothiocyanate groups, isocyanuric acid groups, vinyl groups, epoxy groups, amino groups, (meth)acrylic groups, halogen groups, and mercapto groups. Among these, it is preferable to use a coupling agent having two or more epoxy groups, and it is more preferable to use a coupling agent having two or more epoxy groups and a nitrogen atom. In addition, when the coupling agent has a modifying group, the coupling agent also functions as a modifying agent.

[0052] Non-limiting examples of coupling agents that also function as modifiers include 3-(N,N-dimethylamino)propyltrimethoxysilane, 3-(N,N-diethylamino)propyltrimethoxysilane, 3-(N,N-dimethylamino)propyltriethoxysilane, 3-(N,N-diethylamino)propyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane, 2-(4-pyridylethyl)triethoxysilane, N-(3-triethoxysilylpropyl)-4,5-dihydroimidazole, 1,3-bis(N,N'-diglycidylaminomethyl)cyclohexane, and silicon tetrachloride. The above coupling agents and / or modifying agents may be used either individually or in combination of two or more.

[0053] (diene rubber) The diene rubber contained in the crosslinked rubber composition of this embodiment is not particularly limited as long as it has a refractive index of 1.5000 to 1.5400. In this specification, "diene rubber" means a rubber having at least a portion derived from a diene compound monomer, that is, a rubber obtained by polymerizing at least a diene compound monomer. Here, the diene compound monomer constituting the diene rubber may be a conjugated diene compound or a non-conjugated diene compound.

[0054] The diene rubber is preferably derived from a conjugated diene compound monomer, and may be, for example, butadiene rubber (BR), isoprene rubber (IR), acrylonitrile butadiene rubber (NBR), or any grade of natural rubber (NR). Alternatively, the diene rubber may not contain monomer units having an aromatic ring and an unsaturated bond, such as styrene, and may consist only of a portion derived from an aliphatic monomer. The butadiene rubber may be 1,2-polybutadiene rubber or 1,4-polybutadiene rubber. The above diene rubbers may be used alone or in combination of two or more.

[0055] Diene rubbers may contain cis and trans stereoisomers, for example, when the main chain contains a double bond. The physical properties of diene rubbers can vary depending on the cis and trans content. The cis content can be measured, for example, by infrared spectroscopy in accordance with Japanese Industrial Standard JIS K6230 (or ISO 4650).

[0056] The degree of cis content varies depending on the type of polymer, but for example, in the case of polybutadiene, a cis content of about 20 to 40% is generally referred to as "low cis (low cis, low cis)," a cis content of about 94 to 99% is generally referred to as "high cis (high cis, high cis)," and anything in between is referred to as "medium cis (medium cis)." In addition, in the case of polyisoprene, for example, a cis content of about 90 to 95%, more typically about 90 to 94%, and even more typically about 90 to 92% is generally referred to as "low cis," while one with a cis content of more than about 95%, more typically about 98% to about 99%, is generally referred to as "high cis," and anything in between is referred to as "medium cis."

[0057] From the viewpoint of obtaining a crosslinked rubber composition having higher transparency or excellent strength and abrasion resistance, it is preferable to use at least one low-cis, medium-cis, or high-cis polybutadiene. The diene rubber is more preferably 1,4-polybutadiene rubber, even more preferably medium-cis or high-cis 1,4-polybutadiene rubber, and even more preferably high-cis 1,4-polybutadiene rubber.

[0058] The refractive index of the diene rubber is in the range of 1.5000 to 1.5400. This improves the transparency of the crosslinked rubber composition. The refractive index of the diene rubber is preferably 1.5050 to 1.5350, more preferably 1.5100 to 1.5300, and may be 1.5200 to 1.5250. If the refractive index of the diene rubber is out of the range of 1.5000 to 1.5400, the crosslinked rubber composition tends to have poor transparency. The refractive index of the diene rubber may be adjusted appropriately based on the refractive index of the styrene-butadiene copolymer rubber and the refractive index of the inorganic filler.

[0059] The peak top molecular weight of the diene rubber measured by GPC measurement is not particularly limited, and may be, for example, 10.0×10 4 Over 200 x 10 4 The peak top molecular weight of the diene rubber is preferably 20.0 × 10 or less. 4 Over 100 x 10 4 or less, more preferably 30.0 × 104 Over 70.0 x 10 4 When the peak top molecular weight is within the above range, the transparency, strength, and abrasion resistance of the crosslinked rubber composition tend to be improved in a well-balanced manner. The molecular weight distribution of the diene rubber is not particularly limited, and is, for example, from 1.100 to 5.500, and preferably from 1.500 to 4.000.

[0060] The butadiene rubber as described above may be obtained by purchasing a commercially available product, or may be produced by a conventionally known method.

[0061] (inorganic filler) The inorganic filler contained in the crosslinked rubber composition of this embodiment is not particularly limited as long as it has a refractive index of 1.3700 to 1.5400. From the viewpoint of improving the transparency of the crosslinked rubber composition, the refractive index of the inorganic filler is preferably 1.3900 to 1.5200, and more preferably 1.4000 to 1.5000. If the refractive index of the inorganic filler is outside the range of 1.3700 to 1.5400, the transparency of the crosslinked rubber composition tends to decrease.

[0062] In the crosslinked rubber composition, the inorganic filler is preferably sufficiently dispersed. From the viewpoint of improving dispersibility, the particle size of the inorganic filler is preferably small. Furthermore, if the affinity between the inorganic filler and the rubber component is poor, aggregates of the inorganic filler (primary particles) called secondary particles may occur in the crosslinked rubber composition. If such secondary particles occur and aggregates with a size equal to or larger than the wavelength of visible light are present, the crosslinked rubber composition tends to become opaque due to the difference in refractive index with the region where the inorganic filler is not present. Therefore, by using an inorganic filler with a small primary particle size and kneading under appropriate kneading conditions, a crosslinked rubber composition with excellent transparency tends to be obtained.

[0063] From the above viewpoint, the average primary particle diameter of the inorganic filler is preferably 50 nm or less, more preferably 40 nm or less, and even more preferably 30 nm or less. The above average primary particle diameter is particularly suitable when the inorganic filler is silica. The lower limit of the average primary particle diameter of the inorganic filler is not particularly limited, and may be, for example, 1.0 nm, 5.0 nm, or 10 nm. The average primary particle diameter of the inorganic filler can be measured by observing the inorganic filler with a scanning electron microscope (SEM) before adding it to the crosslinked rubber composition or in the crosslinked rubber composition and calculating the circle-equivalent diameter. The average value is an arithmetic mean value obtained by observing 10 or more inorganic fillers.

[0064] Alternatively, the specific surface area may be used as an index of the smallness of the inorganic filler. The specific surface area of ​​the inorganic filler is, for example, 80.0 m 2 / g or more, preferably 100m 2 / g or more. The specific surface area of ​​the inorganic filler may be measured by a conventionally known method, for example, by the BET method.

[0065] From the viewpoint of improving the affinity between the inorganic filler and the rubber component, it is preferable to add a substance for improving the affinity between the inorganic filler and the rubber component, such as a silane coupling agent, to the crosslinked rubber composition or to the surface of the inorganic filler. Details of the silane coupling agent will be described later.

[0066] Non-limiting examples of inorganic fillers include silica (dry silica, wet silica, colloidal silica), and synthetic silicate-based white carbon. Surface-hydrophobized silica and mixtures of silica and inorganic fillers other than silica may also be used. The above inorganic fillers may be used alone or in combination of two or more. Silica is preferred as the inorganic filler, and dry silica is more preferred.

[0067] The inorganic fillers as described above may be commercially available or may be produced by a conventionally known method.

[0068] (Content of each ingredient) In the crosslinked rubber composition of this embodiment, the total amount of the styrene-butadiene copolymer rubber and the diene rubber is 100 parts by mass, and the inorganic filler is contained in an amount of 20 to 50 parts by mass. Furthermore, out of the total amount of 100 parts by mass of the styrene-butadiene copolymer rubber and the diene rubber, the content of the styrene-butadiene copolymer rubber is 20 to 50 parts by mass, and the content of the diene rubber is 50 to 80 parts by mass. By including the components in such a compounding ratio, the crosslinked rubber composition has a well-balanced improvement in transparency, abrasion resistance, and strength.

[0069] The content of the styrene-butadiene copolymer rubber is preferably 22 to 45 parts by mass, more preferably 25 to 40 parts by mass, within the above range. If the content of the styrene-butadiene copolymer rubber is less than 20 parts by mass, transparency tends to decrease. On the other hand, if the content of the styrene-butadiene copolymer rubber exceeds 50 parts by mass, abrasion resistance and strength tend to decrease.

[0070] The content of the diene rubber in the above range is preferably 55 to 78 parts by mass, more preferably 60 to 75 parts by mass. If the content of the diene rubber exceeds 80 parts by mass, transparency tends to decrease. If the content of the diene rubber is less than 50 parts by mass, abrasion resistance and strength tend to decrease.

[0071] The content of the inorganic filler is preferably 23 to 45 parts by mass, more preferably 25 to 40 parts by mass, within the above range. If the content of the inorganic filler is less than 20 parts by mass, transparency tends to decrease. If the content of the inorganic filler is more than 50 parts by mass, abrasion resistance and strength tend to decrease.

[0072] (Crosslinking agent) The crosslinked rubber composition of this embodiment is a crosslinked rubber composition containing at least the above-mentioned styrene-butadiene copolymer rubber, diene rubber, and inorganic filler in the above-mentioned amounts, i.e., a crosslinked rubber composition obtained by crosslinking a composition containing these components. The crosslinked rubber composition of this embodiment contains a structure in which the styrene-butadiene copolymer rubber, diene rubber, and inorganic filler are crosslinked. Such crosslinking may be caused by a crosslinking agent.

[0073] The crosslinking agent is not particularly limited, and any known crosslinking agent used for crosslinking rubber compositions may be used. The crosslinking agent is preferably one or more of a peroxide and a radical crosslinking agent.

[0074] Non-limiting examples of peroxides include dicumyl peroxide, benzoyl peroxide, di-t-hexyl peroxide, t-butylcumyl peroxide, diisobutyryl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, 1,1-bis(t-butylperoxy)-3,5,5-trimethylcyclohexane, di(2-t-butylperoxyisopropyl)benzene, cumyl peroxyneodecanoate, di-n-propyl peroxydicarbonate, diisopropyl peroxydicarbonate, di-sec-butyl peroxydicarbonate, 1,1,3,3-tetramethylbutyl peroxyneodecanoate, di(4-t-butylcyclohexyl)peroxydicarbonate, di(2-ethylhexyl)peroxydicarbonate, t-hexylperoxyneodecanoate, and t-butylperoxyneodecanoate. Noate, t-butyl peroxyneoheptanoate, t-hexyl peroxypivalate, t-butyl peroxypivalate, di(3,5,5-trimethylhexanoyl) peroxide, dilauroyl peroxide, 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate, disuccinic acid peroxide, 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane, t-hexyl peroxide -oxy-2-ethylhexanoate, di(4-methylbenzoyl) peroxide, t-butylperoxy-2-ethylhexanoate, di(3-methylbenzoyl) peroxide, benzoyl(3-methylbenzoyl) peroxide, dibenzoyl peroxide, 1,1-di(t-butylperoxy)-2-methylcyclohexane, 1,1-di(t-hexylperoxy)-3,5,5-trimethylcyclohexane, 1.1-di(t-hexylperoxy)cyclohexane, 1,1-di(t-butylperoxy)cyclohexane, 2,2-di(4,4-di-(t-butylperoxy)cyclohexyl)propane, t-hexylperoxyisopropyl monocarbonate, t-butylperoxymaleic acid, t-butylperoxy-3,5,5-trimethylhexanoate, t-butylperoxylaurate, t-butylperoxyisopropyl monocarbonate, t-butylperoxy 2-ethylhexyl monocarbonate, t-hexylperoxybenzoate, 2,5-dimethyl-2,5-di(benzoylperoxy) )hexane, t-butylperoxyacetate, 2,2-di-(t-butylperoxy)butane, t-butylperoxybenzoate, n-butyl-4,4-di-(t-butylperoxy)valerate, di(2-t-butylperoxyisopropyl)benzene, di-t-butylperoxide, p-menthane hydroperoxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3, diisopropylbenzene hydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide, cumene hydroperoxide, and t-butyl hydroperoxide.

[0075] Non-limiting examples of radical crosslinkers include, for example, ethylene glycol methacrylate (EGDMA), trimethylolpropane trimethacrylate, triallyl isocyanurate, triallyl cyanurate, diethylene glycol diacrylate, and neophenylene glycol diacrylate.

[0076] Among these crosslinking agents, peroxides are more preferred from the viewpoint of causing less contamination to the product. From the viewpoint of reducing odor and leaving less residue, the crosslinking agents are more preferably dicumyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, di(2-t-butylperoxyisopropyl)benzene, and 1,1-di(t-butylperoxy)cyclohexane.

[0077] The above crosslinking agents may be used alone or in combination of two or more.

[0078] The content of the crosslinking agent in the crosslinked rubber composition of this embodiment is not particularly limited, and is, for example, 0.10 to 10 parts by mass, preferably 0.20 to 8.0 parts by mass, and more preferably 0.50 to 6.0 parts by mass, relative to 100 parts by mass of the styrene-butadiene copolymer rubber and the diene rubber in total. That is, the crosslinked rubber composition of this embodiment is produced by adding, for example, 0.10 to 10 parts by mass, preferably 0.20 to 8.0 parts by mass, and more preferably 0.50 to 6.0 parts by mass of the crosslinking agent relative to 100 parts by mass of the styrene-butadiene copolymer rubber and the diene rubber in total.

[0079] (Other ingredients) The crosslinked rubber composition of this embodiment may contain components other than the styrene-butadiene copolymer rubber, diene rubber, inorganic filler, and crosslinking agent. For example, the crosslinked rubber composition preferably contains a silane coupling agent that improves the affinity between the inorganic filler and the rubber component. According to this embodiment, the dispersibility of the inorganic filler and the adhesion to the rubber component are improved, and the abrasion resistance, transparency, and strength of the crosslinked rubber composition tend to be further improved.

[0080] Non-limiting examples of silane coupling agents include alkoxysilane compounds such as tetraethoxysilane, vinyltrimethoxysilane, vinyltris(2-methoxyethoxy)silane, 3-aminopropyltriethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, bis-[3-(triethoxysilyl)-propyl]tetrasulfide, bis-[3-(triethoxysilyl)-propyl]disulfide, and triethoxysilylpropyl-methacrylate-monosulfide. The silane coupling agent is preferably a polysiloxane containing a vinyl group and an alkoxy group, more preferably a polysiloxane containing a vinyl group and an ethoxy group or a methoxy group, and even more preferably vinyltrimethoxysilane or vinyltris(2-methoxyethoxy)silane. The above silane coupling agents may be used alone or in combination of two or more.

[0081] The content of the silane coupling agent in the crosslinked rubber composition is not particularly limited and may be, for example, 0 to 20 parts by mass relative to 100 parts by mass of the total of the styrene-butadiene copolymer rubber and the diene rubber. Within the above range, the content of the silane coupling agent is more preferably 0.50 to 10 parts by mass.

[0082] The crosslinked rubber composition may contain a liquid rubber to improve processability. Liquid rubber is a low-molecular-weight rubber-like polymer. The crosslinked rubber composition may contain a liquid diene rubber such as liquid styrene-butadiene rubber (liquid SBR) and liquid butadiene rubber (liquid BR).

[0083] The liquid rubber (particularly liquid SBR) used preferably has a refractive index of 1.5000 to 1.5900. The refractive index of the liquid rubber (particularly liquid SBR) is more preferably 1.5100 to 1.5700, and even more preferably 1.5150 to 1.5600. When the refractive index of the liquid rubber is within the above range, the transparency of the crosslinked rubber composition tends to be maintained at a high level. It is also preferable to use a liquid rubber (particularly liquid BR) having a refractive index of 1.5000 to 1.5400. The refractive index of the liquid rubber (particularly liquid BR) is more preferably 1.5050 to 1.5350, and even more preferably 1.5100 to 1.5300. When the refractive index of the liquid rubber is within the above range, the transparency of the crosslinked rubber composition tends to be maintained at a high level.

[0084] The cis content of the liquid BR is not particularly limited, and the liquid BR may contain any of high cis, medium cis, and low cis liquid BR.

[0085] The liquid rubber is not particularly limited as long as it is liquid, but it is preferable that the peak top molecular weight measured by GPC measurement be in the range of 1,000 to 50,000. The peak top molecular weight of the liquid rubber measured by GPC measurement is preferably 4,000 to 35,000, more preferably 7,000 to 30,000. When the peak top molecular weight of the liquid rubber is within the above range, the processability and strength of the crosslinked rubber composition are further improved. Note that when a liquid rubber with a low molecular weight is used, the processability is further improved, but the strength tends to decrease.

[0086] The above liquid rubbers may be used alone or in combination of two or more.

[0087] The content of the liquid rubber in the crosslinked rubber composition is not particularly limited and may be, for example, 0.00 to 15 parts by mass relative to 100 parts by mass of the total of the styrene-butadiene copolymer rubber and the diene rubber. The content of the liquid rubber is preferably 0.10 to 10.0 parts by mass, more preferably 0.20 to 7.0 parts by mass, and even more preferably 0.30 to 4.0 parts by mass. When the content of the liquid rubber is within the above range, the processability and strength of the crosslinked rubber composition tend to be further improved.

[0088] The crosslinked rubber composition may further contain other components as long as the transparency is not significantly impaired. Examples of such components include antioxidants, colorants, modifiers, processing agents (such as fatty acids), reducing agents, oxygen scavengers, light stabilizers, pH stabilizers, surface treatment agents, heat stabilizers, colorants, fillers (such as talc and calcium carbonate), surfactants, gelling agents, UV absorbers (such as salicylic acid, benzophenone, benzotriazole, cyanoacrylate, and hindered amine), dusting agents (such as polyolefins such as polyethylene, talc, and calcium carbonate powder), and polyphosphoric acid. These components may be used alone or in combination of two or more.

[0089] Non-limiting examples of antioxidants include monophenolic, bisphenolic, polyphenolic, sulfur-based, and phosphorus-based compounds, and specific examples include Nocrac SP (manufactured by Ouchi Shinko Chemical Industry Co., Ltd.), Irganox 1076 (manufactured by BASF), Irgafos 168 (manufactured by BASF), and Irganox 1520 (manufactured by BASF).

[0090] The colorant may be used, for example, when it is desired to impart a transparent color such as clear blue, clear red, or clear green to the crosslinked rubber composition, rather than simply making it transparent. Any known colorant may be used as such a colorant, and examples thereof include color pigments, extender pigments, anti-rust pigments, and functional pigments (e.g., phthalocyanine green, titanium dioxide, iron oxide, lead suboxide, and zinc sulfide).

[0091] The crosslinked rubber composition of the present embodiment may contain one or more components selected from antioxidants, colorants, modifiers, processing agents, reducing agents, oxygen scavengers, light stabilizers, pH stabilizers, surface treatment agents, heat stabilizers, colorants, fillers, surfactants, gelling agents, UV absorbers, dusting agents, and polyphosphoric acids in a total amount of, for example, 0 to 15 parts by mass per 100 parts by mass of the styrene-butadiene copolymer rubber and diene rubber combined. When these components are contained, the total content of these components is preferably 0.10 to 10 parts by mass, more preferably 0.20 to 5.0 parts by mass, and even more preferably 0.25 to 2.0 parts by mass.

[0092] (Physical properties of crosslinked rubber composition) The refractive index of the crosslinked rubber composition of this embodiment is not particularly limited and is, for example, 1.4700 to 1.5500. From the viewpoint of excellent transparency, the refractive index of the crosslinked rubber composition is preferably 1.5000 to 1.5400, more preferably 1.5050 to 1.5350, even more preferably 1.5100 to 1.5320, and still more preferably 1.5150 to 1.5315. The refractive index of the crosslinked rubber composition can be adjusted by controlling the refractive index and content of the styrene-butadiene copolymer rubber, diene rubber, and inorganic filler.

[0093] The haze value of the crosslinked rubber composition of this embodiment, measured on a 3.00 mm thick sheet in accordance with JIS K7136, is not particularly limited and is 10 to 50%. From the viewpoint of excellent transparency, the haze value of the crosslinked rubber composition in the above range is preferably 40% or less, more preferably 38% or less, even more preferably 35% or less, and still more preferably 30% or less. The lower limit of the haze value of the crosslinked rubber composition is not particularly limited and may be, for example, 15%, 18%, or 20%. The HAZE value of the crosslinked rubber composition can be adjusted by controlling the refractive indexes of the styrene-butadiene copolymer rubber, diene rubber, and inorganic filler, as well as the magnitude relationship between them.

[0094] The hardness of the crosslinked rubber composition of the present embodiment measured in accordance with JIS K6253 by Durometer Type A is not particularly limited and may be, for example, 50 to 80. From the viewpoint of excellent abrasion resistance and strength, the hardness of the crosslinked rubber composition by Durometer Type A within the above range is preferably 60 to 75, more preferably 63 to 73, and even more preferably 65 to 70. When the hardness of the crosslinked rubber composition by Durometer Type A is 60 or more, the abrasion resistance and strength tend to be further improved, and when it is 75 or less, the tear strength and frictional force tend to be further improved. The hardness of the crosslinked rubber composition as measured by Durometer Type A can be adjusted by controlling the contents of, for example, the styrene-butadiene copolymer rubber, the diene rubber, and the inorganic filler.

[0095] The temperature dispersion curve of the crosslinked rubber composition of this embodiment, obtained by dynamic viscoelasticity measurement, preferably has one peak of loss tangent in the temperature range of -60°C to 40°C, as shown by the solid line in Figure 1. Having one peak of loss tangent means that there is one maximum value. Even if a shoulder is present, as shown by the solid line in Figure 1, the number of peaks is considered to be one. When there is one peak, it is presumed that the components contained in the crosslinked rubber composition are in a more compatible state. Furthermore, when there is one peak in the range of -60°C to 40°C, the abrasion resistance of the crosslinked rubber composition tends to be further improved. Although it is not clear why the abrasion resistance of the crosslinked rubber composition differs depending on the number and position of the peaks in the temperature dispersion curve, the inventors believe that when the components contained in the crosslinked rubber composition are more compatible, components that are prone to abrasion are more uniformly dispersed or crosslinking occurs more uniformly, thereby improving abrasion resistance. For example, by adjusting the amount of bound styrene in the styrene-butadiene copolymer rubber and / or the amount of 1,2-bonds in the butadiene moiety, it is possible to control the number and positions of peaks in the temperature dispersion curve obtained by measuring the dynamic viscoelasticity of the crosslinked rubber composition.

[0096] To control the number of peaks of the loss tangent (tanδ) in the temperature dispersion curve in the temperature range of −60°C to 40°C to one, it is preferable to control, for example, the styrene-butadiene copolymer rubber as follows. That is, the bound styrene content is preferably 41 to 52 mass%, the 1,2-bond content of the butadiene moiety is preferably 18 to 37 mass%, and the styrene block content is preferably 0.82 to 9.00 mass%. When all of the above three parameters fall within the above ranges, the number of peaks tends to be more reliably controlled to one. Furthermore, within the above ranges, the bound styrene content is preferably 42 mass% or more, more preferably 43 mass% or more, and also preferably 50 mass% or less, and even more preferably 49 mass% or less. The 1,2-bond content of the butadiene moiety is preferably 21 mass% or more, more preferably 24 mass% or more, and also preferably 34 mass% or less, and even more preferably 31 mass% or less. The amount of styrene blocks is preferably 1.00% by mass or more, more preferably 1.50% by mass, and is preferably 8.85% by mass or less, more preferably 8.60% by mass or less.

[0097] When the styrene-butadiene copolymer rubber has a bound styrene content of 20 to 55% by mass, a ratio of the styrene block content to the bound styrene content of 0.020 to 0.23, and a 1,2-bond content in the butadiene portion of 10 to 60% by mass, the peak position of the loss tangent tends to be −60° C. or higher. When the bound styrene content is 41 to 52% by mass, a 1,2-bond content in the butadiene portion of 18 to 37% by mass, and a styrene block content of 0.82 to 9.00% by mass, the peak position of the loss tangent tends to be 40° C. or lower.

[0098] The temperature dispersion curve of the crosslinked rubber composition can be obtained by dynamic viscoelasticity measurement. The measurement conditions may be a strain of 1% and a frequency of 10 Hz. More specifically, the temperature dispersion curve can be obtained by the method described in the Examples.

[0099] [Method of producing crosslinked rubber composition] The crosslinked rubber composition can be produced by adding and kneading a styrene-butadiene copolymer rubber, a diene rubber, an inorganic filler, a crosslinking agent, and optionally other components in an appropriate compounding ratio. More specifically, for example, the following method may be used.

[0100] The method for producing the crosslinked rubber composition of this embodiment includes the steps of: kneading (A) 20 to 50 parts by mass of a styrene-butadiene copolymer rubber having a bound styrene content of 20 to 55% by mass, a ratio of the styrene block content to the bound styrene content of 0.020 to 0.23, a 1,2-bond content in the butadiene moiety of 10 to 60% by mass, a Mooney viscosity at 100°C measured in accordance with JIS K6300-1 of 50 to 80, and a refractive index of 1.5100 to 1.5900; (B) 50 to 80 parts by mass of a diene rubber having a refractive index of 1.5000 to 1.5400; (C) 20 to 50 parts by mass of an inorganic filler having a refractive index of 1.3700 to 1.5400; and (D) 0.10 to 3.0 parts by mass of a peroxide and / or a radical crosslinking agent per 100 parts by mass of the total of the styrene-butadiene copolymer rubber and the diene rubber; and molding the resulting kneaded mixture. Here, the total amount of the 20 to 50 parts by mass of the styrene-butadiene copolymer rubber (A) and the 50 to 80 parts by mass of the diene rubber (B) is 100 parts by mass.

[0101] Here, each of the components (A) to (D) may be the same as the corresponding component in the [Crosslinked Rubber Composition]. In the styrene-butadiene copolymer rubber of component (A), it is preferable that the bound styrene content is 41 to 52 mass%, the 1,2-bond content of the butadiene moiety is 18 to 37 mass%, and the styrene block content is 0.82 to 9.00 mass%. Furthermore, it is more preferable that the 1,2-bond content of the butadiene moiety is 24 to 31 mass%.

[0102] The kneading step can be carried out using, for example, an open roll, a Banbury mixer, a kneader, a twin-screw extruder, and / or a Labo Plastomill, etc. In the kneading step, the components (A) to (D) may be kneaded all at once, but a method in which the components (A) to (C) are kneaded once, and then component (D) is added and further kneaded is preferred. Components (A) to (C) may be kneaded at a temperature of, for example, 120 to 160°C, from the viewpoint of uniformly mixing the components. Kneading after the addition of component (D) may be carried out at a temperature of, for example, 120°C or lower, from the viewpoint of suppressing side reactions. The temperature during kneading after the addition of component (D) is preferably 100°C or lower, more preferably 80°C or lower, and even more preferably 0 to 50°C.

[0103] In the kneading step, components other than components (A) to (D) may be added as appropriate. Examples of such components include those listed above as components that the crosslinked rubber composition may contain. It is preferable that the kneading step further includes kneading (E) a liquid diene rubber. By kneading component (E), the processability of the crosslinked rubber composition tends to be further improved. It is also preferable that the kneading step further includes kneading (F) a silane coupling agent. By kneading component (F), the transparency, strength, and abrasion resistance of the crosslinked rubber composition tend to be further improved. It is preferable that components (E) and (F) are kneaded together when components (A) to (D) are kneaded. The amount of each component may be the same as the amount of each component described in the description of the crosslinked rubber composition.

[0104] A crosslinked rubber composition having a desired shape can be obtained by a molding step of molding the kneaded mixture. The molding step may be, for example, a step of placing the kneaded mixture obtained in the kneading step into a press mold having an appropriate shape and heating it. The molding temperature in the molding step is not particularly limited, but is preferably 140 to 180°C, and more preferably 150 to 170°C. In the molding step, the kneaded mixture may be introduced into an injection molding machine and injection molded to obtain a crosslinked rubber composition in a desired shape.

[0105] The crosslinked rubber composition of the present embodiment can be used in any industrial application where transparency is important, including, but not limited to, shoe soles, rain gear, toys, vibration damping materials, building materials, wiring covering materials, packaging materials, and computer protective materials.

[0106] In particular, the crosslinked rubber composition of this embodiment is suitable for use in shoe soles, because it allows the production of shoe soles, tips, ornaments, and other parts that are excellent in transparency and have an excellent balance of tensile strength, tear strength, abrasion resistance, and processability. Such transparent shoe soles can improve fashionability and therefore have extremely high commercial value. The above uses are merely examples, and the uses of the crosslinked rubber composition of this embodiment are not limited to these. [Example]

[0107] The present embodiment will be described in more detail below with reference to examples and comparative examples, but the present embodiment is not limited to these examples and comparative examples.

[0108] [material] The materials used in the examples and comparative examples are listed below. The refractive index of each material was measured in advance by the method described below. (styrene-butadiene copolymer rubber) Styrene-butadiene copolymer rubber SBR1-11 manufactured by the following manufacturing examples or comparative manufacturing examples Asaprene 303 (Asahi Kasei Corporation) (diene rubber) BR1208 (LG Chem) (1,4-polybutadiene rubber, high cis, peak top molecular weight: 435200, refractive index: 1.5236) (inorganic filler) Reolosil (Tokuyama Corporation) (silica, primary particle size: 12 nm, refractive index: 1.4621) (Crosslinking agent) Percumyl D (Nihon Oil & Fats Co., Ltd.) (Dicumyl peroxide) Perhexa 25B (NOF Corporation) (2,5-dimethyl-2,5-di(t-butylperoxy)hexane) Perhexa 25B (NOF Corporation) (1,1-di(t-butylperoxy)cyclohexane) (Silane coupling agent) Dynasylan 6498 (Evonik) (a polysiloxane containing vinyl and ethoxy groups) (antioxidant) Nocrac SP (Ouchi Shinko Chemical Industry Co., Ltd.) (mono (or di, or tri) (α-methylbenzyl) phenol) (liquid diene rubber) LBR307 (Kuraray Co., Ltd.) (liquid 1,4-butadiene rubber, peak top molecular weight: 8000, refractive index: 1.5157) (randomizer) Potassium t-amyl alkoxide (vinylating agent) 2.2'-Ditetrahydrofurylpropane (Coupling agent (polymerized coupling agent)) 1,3-bis(N,N'-diglycidylaminomethyl)cyclohexane (Polymerization initiator) n-Butyllithium (stabilizer) n-Octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate

[0109] [Measurement of physical properties of styrene-butadiene copolymer rubber] The physical properties of the styrene-butadiene copolymer rubber were measured as follows.

[0110] (1) Measurement of bound styrene content The absorption spectrum of a styrene-butadiene copolymer rubber sample was measured using an ultraviolet spectrophotometer (UV-2450; Shimadzu Corporation). The amount of bound styrene was determined from the amount of ultraviolet light absorbed (around 254 nm) by the phenyl group of styrene.

[0111] (2) Measurement of styrene block content The styrene block content was measured according to the osmium tetroxide decomposition method described in I.M. Kolthoff, et al., J. Polym. Sci. 1, 429 (1946). More specifically, 0.050 g of styrene-butadiene copolymer rubber was dissolved in 10 ml of chloroform, to which 16 ml of a 69% by mass aqueous solution of tert-butyl hydroperoxide and 4.0 ml of a 0.050% by mass chloroform solution of osmium tetroxide were added. The mixture was refluxed in a 90°C bath for 12 minutes to carry out an oxidative decomposition reaction. After completion of the reaction, the reaction solution was cooled, and 200 ml of methanol was added to the reaction solution with stirring to precipitate the styrene block component, which was then filtered through a 5 μm glass filter. The styrene block content was determined by dividing the mass of the resulting product by the total mass of the styrene-butadiene copolymer rubber.

[0112] (3) Ratio of styrene block amount / bound styrene amount It was calculated from the ratio of the amount of styrene blocks measured above to the amount of bound styrene.

[0113] (4) 1,2-bond content of butadiene moiety Styrene-butadiene copolymer rubber was dissolved in carbon disulfide to prepare a measurement sample. The infrared spectrum of each sample was measured using an infrared spectrophotometer (JASCO Corporation, V-520V). The amount of 1,2-bonds in the butadiene moiety was determined from the absorbance at a predetermined wave number according to the Hampton method (method described in R.R. Hampton, Analytical Chemistry 21, 923 (1949)).

[0114] (5) Measurement of Mooney viscosity Measurement was performed using a Mooney viscometer (Shimadzu Corporation, SMV-301RT) conforming to JIS K6300-1. Using an L-type rotor, the sample was preheated at the test temperature (100°C) for 1 minute, and then the rotor was rotated at 2 rpm. The torque was measured after 4 minutes to determine the Mooney viscosity (ML). 1+4 (100℃) was measured.

[0115] (6) Measurement of peak top molecular weight GPC (Tosoh HLC-8320GPC EcoSEC instrument, three PLgel Column MiniMix-C columns. Tetrahydrofuran was used as the solvent, and the measurement conditions were a temperature of 40°C, a flow rate of 0.4 mL / min, a sample concentration of 0.1% by mass, and an injection volume of 50 μL) chromatograph was measured. A calibration curve prepared using commercially available standard monodisperse polystyrene with known molecular weights was used, and the peak top molecular weight of each sample was determined from the resulting GPC chromatogram. When the styrene-butadiene copolymer rubber was coupled with a coupling agent, a peak derived from the uncoupled styrene-butadiene copolymer rubber and a peak derived from the coupled styrene-butadiene copolymer rubber were observed. In this case, the peak top molecular weight was determined for each peak. The coupling ratio was calculated from the ratio of the peak derived from the uncoupled part to the peak derived from the coupled part.

[0116] (7) Refractive index measurement The refractive index was measured at 23° C. using an Abbe refractometer (NAR-3T) manufactured by Atago Co., Ltd. in accordance with JIS K0062. The refractive index of the inorganic filler was measured as follows. Two liquids with different refractive indices were prepared, and a small amount of inorganic filler was added to the liquid with the lower refractive index. At this time, the refractive index of the liquid and the inorganic filler is different, so the inorganic filler can be seen with the naked eye. Then, one of the liquids with a higher refractive index was added little by little. When the refractive index of the mixed liquid became equal to the refractive index of the inorganic filler, the inorganic filler could no longer be seen with the naked eye. The refractive index of the mixed liquid was measured, and this value was taken as the refractive index of the inorganic filler. In this example, the above method was used not only to measure the refractive index of the styrene-butadiene copolymer rubber and the inorganic filler, but also to measure the refractive index of other materials.

[0117] [Evaluation of physical properties of crosslinked rubber composition] (1) Type A durometer hardness Type A durometer hardness was measured in accordance with Japanese Industrial Standard JIS K6253-3 using two 3.00 mm thick sheets stacked together (total thickness of 6.00 mm) as the measurement sample. The plunger was pressed against the sheet with a constant force, and the depth of penetration three seconds later was measured as the Type A durometer hardness.

[0118] (2) Haze value The haze value was measured in accordance with Japanese Industrial Standard JIS K7136 (or ISO 14782). The crosslinked rubber composition was molded into a sheet having a thickness of 3.00 mm, and the haze value was measured using a haze meter (NDH 2000, manufactured by Nippon Denshoku Industries Co., Ltd.).

[0119] (3) Temperature dispersion curve (dynamic viscoelasticity measurement) The sample used was a crosslinked rubber composition molded into a sheet having a thickness of 3.00 mm and cut into a length of 2.5 mm and a width of 12.5 mm. Using an Anton Paar MCR102, dynamic viscoelasticity measurements were performed at a strain of 1%, a frequency of 10 Hz, and a temperature range of -100°C to 70°C at a heating rate of 3°C / min to obtain a temperature dispersion curve. The maximum values ​​were counted in the temperature range of -60°C to 40°C.

[0120] (4) Abrasion resistance Measurement was performed by a DIN abrasion test in accordance with JIS K 6264. Specifically, the wear volumes of the test pieces of the examples and comparative examples were measured in the DIN abrasion test, and the abrasion resistance index was calculated from the wear volumes to determine the abrasion resistance.

[0121] (5) Physical strength (tensile strength and tear strength) The tensile strength was measured as described above in accordance with Japanese Industrial Standard JIS K6251. The tear strength was measured as described above in accordance with Japanese Industrial Standard JIS K6252-1.

[0122] [Production of styrene-butadiene copolymer rubber] Styrene-butadiene copolymer rubber was synthesized by batch polymerization in a 10 L autoclave equipped with a stirrer and purged with nitrogen according to the formulation in Table 1-1, 1-2, 1-3 or 1-4 below (hereinafter, Tables 1-1, 1-2, 1-3 and 1-4 are collectively referred to as "Table 1"). In the following Production Examples and Comparative Production Examples, the total amount of monomers was 1200 g. Styrene monomer and 1,3-butadiene were copolymerized using n-butyllithium in cyclohexane solvent (550 phm) as a polymerization initiator.

[0123] [Manufacturing example 1 (SBR-1)]

[0124] The butadiene and styrene monomers listed in Table 1 were weighed into an autoclave, followed by the addition of 2,2'-ditetrahydrofurylpropane as a vinylating agent and potassium t-amyl alkoxide as a randomizing agent. At a polymerization initiation temperature of 50°C, n-butyllithium was added to initiate polymerization. The polymerization reaction peak temperature was 71°C. Two minutes after the temperature peak, methanol was added in an amount equal to the n-butyllithium molar amount to terminate the reaction, yielding styrene-butadiene copolymer rubber SBR-1. The bound styrene content was 44.4% by mass, and the styrene block content was 2.10% by mass.

[0125] [Manufacturing example 2 (SBR-2)] After weighing out each monomer listed in Table 1, a vinylating agent and a randomizing agent were added. At a polymerization initiation temperature of 61°C, n-butyllithium was added to initiate polymerization. The polymerization reaction peak temperature was 83°C. Two minutes after the temperature peak, 1,3-bis(N,N'-diglycidylaminomethyl)cyclohexane was added as a coupling agent in an amount 0.18 times the molar amount of n-butyllithium to terminate the reaction, yielding styrene-butadiene copolymer rubber SBR-2. The bound styrene content was 44.7% by mass, and the styrene block content was 2.60% by mass. The coupling ratio was 60.2%.

[0126] [Manufacturing example 3 (SBR-3)] As shown in Table 1, 454 g of butadiene monomer and 468 g of styrene monomer were weighed into an autoclave, followed by the addition of a vinylating agent. At a polymerization initiation temperature of 52°C, n-butyllithium was added to initiate polymerization. When the reaction temperature reached 60°C, an additional 194 g of butadiene monomer was added, as shown in Table 1. When the reaction temperature reached 64°C, an additional 84 g of styrene monomer was added, and the reaction was continued. The polymerization reaction peak temperature was 72°C. Two minutes after the temperature peak, methanol was added in an amount equal to the molar amount of n-butyllithium to terminate the reaction, yielding styrene-butadiene copolymer rubber SBR-3. The bound styrene content was 45.3% by mass, and the styrene block content was 8.50% by mass.

[0127] [Manufacturing example 4 (SBR-4)] Styrene-butadiene copolymer rubber SBR-4 was obtained in the same manner as in Production Example 3, except that instead of adding methanol to terminate the reaction, a coupling agent was added in an amount 0.23 times the molar amount of n-butyllithium to terminate the reaction. The bound styrene content was 45.7% by mass, and the styrene block content was 8.80% by mass. The coupling ratio was 80.5%.

[0128] [Manufacturing example 5 (SBR-5)] As shown in Table 1, 900 g of butadiene monomer and 300 g of styrene monomer were weighed into an autoclave, and then a vinylating agent was added. At a polymerization initiation temperature of 59°C, n-butyllithium was added to initiate polymerization. The reaction peak temperature was 81°C. Two minutes after the temperature peak, a coupling agent was added in an amount 0.18 times the molar amount of n-butyllithium to terminate the reaction, yielding styrene-butadiene copolymer rubber SBR-5. The bound styrene content was 24.7% by mass, and the styrene block content was 1.20% by mass. The coupling ratio was 60.4%.

[0129] [Manufacturing example 6 (SBR-6)] Styrene-butadiene copolymer rubber SBR-6 was obtained in the same manner as in Production Example 2, except that the amount of each component added was changed. The bound styrene content was 51.1% by mass, the styrene block content was 3.6% by mass, and the coupling ratio was 55.3%.

[0130] [Manufacturing example 7 (SBR-7)] As shown in Table 1, 405 g of butadiene monomer and 660 g of styrene monomer were weighed into an autoclave, followed by the addition of a vinylating agent and a randomizing agent. At a polymerization initiation temperature of 53°C, n-butyllithium was added to initiate polymerization. When the reaction temperature reached 63°C, an additional 135 g of butadiene monomer was added as shown in Table 1, and the reaction was continued. The reaction peak temperature was 82°C. Two minutes after the temperature peak, a coupling agent was added in an amount 0.18 times the molar amount of n-butyllithium to terminate the reaction, yielding styrene-butadiene copolymer rubber SBR-7. The bound styrene content was 53.3% by mass, and the styrene block content was 1.20% by mass. The coupling ratio was 58.1%.

[0131] [Manufacturing example 8 (SBR-8)] As shown in Table 1, 456 g of butadiene monomer and 432 g of styrene monomer were weighed into an autoclave, followed by the addition of a vinylating agent. At a polymerization initiation temperature of 53°C, n-butyllithium was added to initiate polymerization. When the reaction temperature reached 58°C, an additional 192 g of butadiene monomer was added, as shown in Table 1. When the reaction temperature reached 63°C, an additional 120 g of styrene monomer was added, as shown in Table 1, and the reaction was continued. The reaction peak temperature was 74°C. Two minutes after the temperature peak, methanol was added in an amount equal to the molar amount of n-butyllithium to terminate the reaction, yielding styrene-butadiene copolymer rubber SBR-8. The bound styrene content was 48.0% by mass, and the styrene block content was 10.4% by mass.

[0132] [Manufacturing example 9 (SBR-12)] After weighing 600 g of butadiene monomer and 540 g of styrene monomer into an autoclave, a vinylating agent and a randomizing agent were added. The amounts added were as shown in Table 1. At a polymerization initiation temperature of 52°C, n-butyllithium was added to initiate polymerization. When the reaction temperature reached 68°C, an additional 60 g of styrene monomer was added as shown in Table 1, and the reaction was continued. The reaction peak temperature was 76°C. Two minutes after the temperature peak, methanol was added in an amount equal to the molar amount of n-butyllithium to terminate the reaction, yielding styrene-butadiene copolymer rubber SBR-12. The bound styrene content was 50.2% by mass, and the styrene block content was 5.3% by mass.

[0133] [Manufacturing example 10 (SBR-13, 14, 15, 17, 19, 21, 22)] Styrene-butadiene copolymer rubbers SBR-13, 14, 15, 17, 19, 21, and 22 were obtained in the same manner as in Production Example 1, except that the amounts of each component added were changed to the values ​​shown in Table 1. The bound styrene amounts and styrene block amounts were as shown in Table 1.

[0134] [Comparative manufacturing example 1 (SBR-9)] As shown in Table 1, 470 g of butadiene monomer and 406 g of styrene monomer were weighed into an autoclave, followed by the addition of a vinylating agent. At a polymerization initiation temperature of 58°C, n-butyllithium was added to initiate polymerization. When the reaction temperature reached 63°C, an additional 178 g of butadiene monomer was added as shown in Table 1. When the reaction temperature reached 68°C, an additional 146 g of styrene monomer was added as shown in Table 1, and the reaction was continued. The reaction peak temperature was 80°C. Two minutes after the temperature peak, methanol was added in an amount equal to the molar amount of n-butyllithium to terminate the reaction, yielding styrene-butadiene copolymer rubber SBR-9. The bound styrene content was 44.8% by mass, and the styrene block content was 14.0% by mass.

[0135] [Comparative manufacturing example 2 (SBR-10)] After weighing out each monomer listed in Table 1, a vinylating agent and a randomizing agent were added. At a polymerization initiation temperature of 58°C, n-butyllithium was added to initiate polymerization. The reaction peak temperature was 83°C. Two minutes after the temperature peak, methanol was added in an amount equal to the molar amount of n-butyllithium to terminate the reaction, yielding styrene-butadiene copolymer rubber SBR-10. The bound styrene content was 60.9% by mass, and the styrene block content was 3.20% by mass.

[0136] [Comparative manufacturing example 3 (SBR-11)] After weighing out each monomer listed in Table 1, a vinylating agent and a randomizing agent were added. At a polymerization initiation temperature of 55°C, n-butyllithium was added to initiate polymerization. The reaction peak temperature was 78°C. Two minutes after the temperature peak, a coupling agent was added in an amount 0.23 times the molar amount of n-butyllithium to terminate the reaction, yielding styrene-butadiene copolymer rubber SBR-11. The bound styrene content was 15.5% by mass, and the styrene block content was 1.30% by mass. The coupling ratio was 78.6%.

[0137] [Comparative manufacturing example 4 (SBR-16)] After weighing 486 g of butadiene monomer and 552 g of styrene monomer into an autoclave, a vinylating agent and a randomizing agent were added. The amounts added were as shown in Table 1. At a polymerization initiation temperature of 58°C, n-butyllithium was added to initiate polymerization. When the reaction temperature reached 78°C, an additional 162 g of butadiene monomer was added as shown in Table 1, and the reaction was continued. The reaction peak temperature was 87°C. Two minutes after the temperature peak, methanol was added in an amount equal to the molar amount of n-butyllithium to terminate the reaction, yielding styrene-butadiene copolymer rubber SBR-16. The bound styrene content was 45.5% by mass, and the styrene block content was 0.70% by mass.

[0138] [Comparative manufacturing example 5 (SBR-18, 20)] Styrene-butadiene copolymer rubbers SBR-18 and 20 were obtained in the same manner as in Comparative Production Example 4, except that the amounts of each component added were changed to the values ​​shown in Table 1. The bound styrene amount and styrene block amount were as shown in Table 1.

[0139] In the above Production Examples and Comparative Production Examples, the conversion rate of styrene was 95% or more and the conversion rate of 1,3-butadiene was 99.8% or more after 2 minutes from the peak reaction temperature in each polymerization batch. The conversion rates were measured by gas chromatography. After the reaction was completed, 0.30 parts by mass of n-octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate was added as a stabilizer to the styrene-butadiene copolymer rubber solution obtained in each Production Example and Comparative Production Example per 100 parts by mass of the styrene-butadiene copolymer rubber, and the solvent was removed using a drum dryer (160°C), followed by drying to finish.

[0140] Table 1 shows the bound styrene amount, styrene block amount, styrene block amount / bound styrene amount ratio, 1,2-bond amount in the butadiene portion, Mooney viscosity, peak top molecular weight, coupling ratio, and refractive index of each styrene-butadiene copolymer rubber.

[0141] [Table 1-1] [Table 1-2]

[0142] [Table 1-3] [Table 1-4]

[0143] [Production and Evaluation of Crosslinked Rubber Composition] [Example 1] Styrene-butadiene copolymer rubber, diene rubber, inorganic filler, silane coupling agent, and antioxidant were compounded in the composition shown in Table 2 and kneaded using a 6-inch open roll (Kansai Roll, roll temperature 119-121°C, rotation ratio 1:1.25) at a discharge temperature of 120-130°C. A crosslinking agent was then compounded into the kneaded mixture, which was then further kneaded using a 6-inch open roll at a temperature of 30-40°C.

[0144] Next, this kneaded mixture was pressed at 160°C under a pressure of about 15 MPa for about 5.5 minutes using a hot press to prepare a crosslinked rubber sheet having a thickness of 2.00 mm and a crosslinked rubber sheet having a thickness of 3.00 mm. Further, cylindrical crosslinked rubber pieces having a diameter of 16.0 mm and a thickness of 8.00 mm were prepared in the same manner.

[0145] The rubber sheets having a thickness of 2.00 mm were used as test pieces for measuring tensile strength and tear strength. The 3.00 mm thick rubber sheets were used as test specimens for measuring the haze value and type A durometer hardness. The cylindrical rubber pieces were used as test pieces for abrasion resistance tests.

[0146] [Table 2]

[0147] [Examples 2 to 8 and Comparative Examples 1, 4, 7, and 8] In Examples 2 to 8 and Comparative Examples 1, 4, 7 and 8, each crosslinked rubber sheet and crosslinked rubber piece was prepared in the same manner as in Example 1, except that the type of styrene-butadiene copolymer rubber was changed to that shown in Table 3-1 or 3-2 (hereinafter, Tables 3-1 and 3-2 are collectively referred to as "Table 3").

[0148] Comparative Example 2 In Comparative Example 2, each crosslinked rubber sheet and crosslinked rubber piece were produced in the same manner as in Comparative Example 1, except that the pressing time by the press was increased to 12 minutes.

[0149] Comparative Example 3 In Comparative Example 3, each crosslinked rubber sheet and crosslinked rubber piece were produced in the same manner as in Comparative Example 1, except that the pressing time by the press was shortened to 2 minutes.

[0150] Comparative Example 5 In Comparative Example 5, each crosslinked rubber sheet and crosslinked rubber piece were produced in the same manner as in Example 1, except that only styrene-butadiene copolymer rubber was compounded and no diene rubber was compounded.

[0151] Comparative Example 6 In Comparative Example 6, crosslinked rubber sheets and crosslinked rubber pieces were prepared in the same manner as in Example 1, except that no inorganic filler was added.

[0152] Table 3 shows the measurement results of the physical properties of the crosslinked rubber compositions of Examples 1 to 8 and Comparative Examples 1 to 8. Type A durometer hardness, haze value, number of temperature dispersion peaks, and refractive index are actual measured values. Abrasion resistance, tensile strength, and tear strength are expressed as an index, with the physical property of Comparative Example 1 set at 100. The higher the index, the better the physical property. Table 3 shows that each Example is excellent in all of type A durometer hardness, haze value, abrasion resistance, tensile strength, and tear strength, while each Comparative Example is inferior in one of the physical properties.

[0153] [Table 3-1] [Table 3-2]

[0154] [Example 9] Styrene-butadiene copolymer rubber, diene rubber, inorganic filler, silane coupling agent, and antioxidant were compounded according to the composition shown in Table 4, and kneaded for 7 minutes using a Banbury mixer (Toyo Seiki Co., Ltd., Labo Plastomill 10C100, B600) under temperature conditions of an initial setting temperature of 90°C and a discharge temperature of 140 to 150°C. Thereafter, the crosslinking agent shown in Table 4 was compounded into the kneaded mixture, and the mixture was further kneaded using a 6-inch open roll at a temperature of 30 to 40°C.

[0155] Next, this kneaded mixture was pressed at 160°C under a pressure of about 15 MPa for about 9 minutes using a hot press to prepare a cross-linked rubber sheet having a thickness of 2.00 mm and a cross-linked rubber sheet having a thickness of 3.00 mm. Further, cylindrical cross-linked rubber pieces having a diameter of 16.0 mm and a thickness of 8.00 mm were prepared in the same manner.

[0156] [Table 4]

[0157] [Examples 10 and 11 and Comparative Examples 9 and 10] In Examples 10 and 11 and Comparative Examples 9 and 10, each crosslinked rubber sheet and crosslinked rubber piece was prepared in the same manner as in Example 9, except that the type of styrene-butadiene copolymer rubber and the type and amount of crosslinking agent were changed as shown in Table 5.

[0158] [Examples 12 to 14] In Examples 12 to 14, crosslinked rubber sheets and crosslinked rubber pieces were prepared in the same manner as in Example 9, except that liquid butadiene rubber was added in the amounts shown in Table 5 during kneading in the Banbury mixer.

[0159] The measurement results of the physical properties of the crosslinked rubber compositions of Examples 9 to 14 and Comparative Examples 9 to 10 are shown in Table 5. The type A durometer hardness, haze value, number of temperature dispersion peaks, and refractive index are actual measured values. The Mooney viscosity before crosslinking, abrasion resistance, tensile strength, and tear strength are shown as index values, with the physical properties of Comparative Example 9 set at 100. The Mooney viscosity before crosslinking was measured using the same method as used to measure the Mooney viscosity of styrene-butadiene copolymer rubber.

[0160] The higher the index, the better the physical properties except for the Mooney viscosity before crosslinking, and the lower the index for the Mooney viscosity before crosslinking, the better the processability. Table 5 shows that, even in each example in which the type of crosslinking agent was changed, the type A durometer hardness, haze value, abrasion resistance, tensile strength, and tear strength were all excellent, while the abrasion resistance was poor in each comparative example. Furthermore, a comparison of Examples 9 and 12 to 14 shows that the addition of liquid butadiene rubber improved processability.

[0161] [Table 5]

[0162] [Examples 15 to 17, 20 to 24 and Comparative Examples 11 to 13] Crosslinked rubber sheets and crosslinked rubber pieces of Examples 15 to 17, 20 to 24 and Comparative Examples 11 to 13 were prepared in the same manner as in Example 1, except that the type of styrene-butadiene copolymer rubber was changed to that shown in Table 6-1 or 6-2.

[0163] [Examples 18 and 19] The crosslinked rubber sheets and crosslinked rubber pieces of Examples 18 and 19 were prepared in the same manner as in Example 1, except that the amount of styrene-butadiene copolymer rubber was changed to that shown in Table 6-1.

[0164] The measurement results of the physical properties of the crosslinked rubber compositions of Examples 15 to 24 and Comparative Examples 11 to 13 are shown in Tables 6-1 and 6-2. Type A durometer hardness, haze value, number of temperature dispersion peaks, and refractive index are actual measured values. Abrasion resistance, tensile strength, and tear strength are expressed as an index, with the physical property of Comparative Example 1 set at 100. The higher the index, the better the physical property. Tables 6-1 and 6-2 show that each Example is excellent in all of Type A durometer hardness, haze value, abrasion resistance, tensile strength, and tear strength, while each Comparative Example is inferior in one of the physical properties.

[0165] [Table 6-1] [Table 6-2]

Claims

1. A crosslinked rubber composition comprising 20 to 50 parts by mass of a styrene-butadiene copolymer rubber, 50 to 80 parts by mass of a diene rubber (excluding those corresponding to the styrene-butadiene copolymer rubber), and 20 to 50 parts by mass of an inorganic filler, wherein the total amount of the styrene-butadiene copolymer rubber and the diene rubber is 100 parts by mass, The styrene-butadiene copolymer rubber has a bound styrene content of 20 to 55% by mass, a ratio of the styrene block content to the bound styrene content of 0.020 to 0.23, a 1,2-bond content in the butadiene moiety of 10 to 60% by mass, a Mooney viscosity at 100°C measured in accordance with JIS K6300-1 of 50 to 80, and a refractive index of 1.5100 to 1.5900, The diene rubber has a refractive index of 1.5000 to 1.5400, The inorganic filler has a refractive index of 1.3700 to 1.5400, In a temperature dispersion curve obtained by dynamic viscoelasticity measurement, there is one peak of loss tangent (tan δ) in the temperature range of −60° C. to 40° C. Crosslinked rubber composition.

2. The cross-linked rubber composition according to claim 1, wherein the styrene-butadiene copolymer rubber has a bound styrene content of 41 to 52 mass%, a 1,2-bond content of the butadiene portion of 18 to 37 mass%, and a styrene block content of 0.82 to 9.00 mass%.

3. The crosslinked rubber composition according to claim 1 or 2, having a refractive index of 1.4700 to 1.5500.

4. The crosslinked rubber composition according to claim 1 or 2, wherein the inorganic filler is silica.

5. 3. The crosslinked rubber composition according to claim 1, wherein the haze value measured on a 3.00 mm thick sheet in accordance with JIS K7136 is 15 to 40%.

6. 3. The crosslinked rubber composition according to claim 1, wherein the hardness measured in accordance with JIS K6253 by durometer type A is 60 to 75.

7. The crosslinked rubber composition according to claim 1 or 2, which is crosslinked with one or more crosslinking agents selected from peroxides and radical crosslinking agents.

8. A method for producing the crosslinked rubber composition of claim 1, comprising: (A) A bound styrene content is 20 to 55% by mass, the ratio of the styrene block content to the bound styrene content is 0.020 to 0.23, and the 1,2-bond content of the butadiene moiety is 10 to 60% by mass, JIS a kneading step of kneading together (A) 20 to 50 parts by mass of a styrene-butadiene copolymer rubber having a Mooney viscosity of 50 to 80 at 100°C measured in accordance with K6300-1 and a refractive index of 1.5100 to 1.5900, (B) 50 to 80 parts by mass of a diene rubber (excluding those falling under the styrene-butadiene copolymer rubber) having a refractive index of 1.5000 to 1.5400, (C) 20 to 50 parts by mass of an inorganic filler having a refractive index of 1.3700 to 1.5400, and (D) 0.10 to 3.0 parts by mass of a peroxide and / or a radical crosslinking agent relative to a total of 100 parts by mass of the styrene-butadiene copolymer rubber and the diene rubber; a step of molding the obtained kneaded product; A manufacturing method comprising:

9. (A) A bound styrene content of 20 to 55% by mass, a ratio of the styrene block content to the bound styrene content of 0.020 to 0.23, a 1,2-bond content of the butadiene moiety of 10 to 60% by mass, JIS a kneading step of kneading together (A) 20 to 50 parts by mass of a styrene-butadiene copolymer rubber having a Mooney viscosity of 50 to 80 at 100°C measured in accordance with K6300-1 and a refractive index of 1.5100 to 1.5900, (B) 50 to 80 parts by mass of a diene rubber (excluding those falling under the styrene-butadiene copolymer rubber) having a refractive index of 1.5000 to 1.5400, (C) 20 to 50 parts by mass of an inorganic filler having a refractive index of 1.3700 to 1.5400, and (D) 0.10 to 3.0 parts by mass of a peroxide and / or a radical crosslinking agent relative to a total of 100 parts by mass of the styrene-butadiene copolymer rubber and the diene rubber; a step of molding the obtained kneaded product; Including, In the styrene-butadiene copolymer rubber, the bound styrene content is 41 to 52% by mass, the 1,2-bond content of the butadiene portion is 18 to 37% by mass, and the styrene block content is 0.82 to 9.00% by mass. A method for producing a crosslinked rubber composition.

10. In the styrene-butadiene copolymer rubber, the amount of 1,2-bonds in the butadiene portion is 24 to 31% by mass. A method for producing the crosslinked rubber composition according to claim 9.

11. The method for producing a crosslinked rubber composition according to claim 8 or 9, wherein in the kneading step, (E) a liquid diene rubber is further kneaded.

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