Rubber composition for side tread

Abstract

The present disclosure provides a rubber composition for side treads, which is capable of suppressing a decrease in elongation at break over time of a rubber composition stored in an unvulcanized state, while maintaining good hardness and tensile strength, and improving low heat generation (tan) [delta] (60 DEG C)), and which has a high balance of these properties. The rubber composition is obtained by blending silica having a CTAB adsorption specific surface area of 60-100 m2 / g, carbon black having a CTAB adsorption specific surface area of 30-100 m2 / g, a sulfur-containing silane coupling agent, and a guanidine vulcanization accelerator with respect to 100 parts by mass of a diene rubber containing 35-55% by mass of an isoprene rubber and 45-65% by mass of a butadiene rubber. The amount of silica is 5-50 parts by mass, the total amount of silica and carbon black is less than 55 parts by mass, and the ratio Ma / Ms, which is the ratio of the amount of guanidine vulcanization accelerator Ma to the amount of silica Ms, is 0.02-0.08.

Description

Technical Field

[0001] The present invention relates to a rubber composition for a side tread layer mainly used for a tire side tread. Background Art

[0002] In a pneumatic tire, in order to reduce the environmental load, improvement in fuel consumption performance during driving is required. Therefore, heat generation of the rubber compositions of the respective parts constituting the pneumatic tire is to be suppressed. In recent years, in order to further improve the fuel consumption performance, for example, suppressing heat generation of the rubber composition (rubber composition for side tread) of the side tread rubber layer constituting the pneumatic tire has been studied.

[0003] As an index of the heat generation property of a rubber composition, generally, tanδ at 60°C obtained by dynamic viscoelasticity measurement (hereinafter referred to as “tanδ(60°C)”) is used. The smaller the tanδ(60°C) of the rubber composition, the smaller the heat generation property. And as a method for reducing tanδ(60°C) of the rubber composition, for example, reducing the blending amount of a filler such as carbon black, or increasing the particle size of carbon black can be cited. Alternatively, blending silica having a large particle size has also been proposed (see, for example, Patent Document 1). However, by these methods, rubber hardness may not be sufficiently obtained, and in addition, there is a concern about the influence on the tensile strength required for the rubber composition for side tread. Further, there is a concern about the deterioration over time (particularly, a decrease in elongation at break) of the rubber composition stored in an unvulcanized state. Therefore, for the rubber composition for side tread, further countermeasures are required to improve low heat generation property (tanδ(60°C)) while maintaining hardness and tensile strength well and suppressing the decrease in elongation at break over time.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2013-001889 Summary of the Invention

[0007] Problems to be Solved by the Invention

[0008] An object of the present invention is to provide a rubber composition for a side tread that can suppress a decrease in elongation at break over time of the rubber composition stored in an unvulcanized state, maintain hardness and tensile strength well at the same time, improve low heat generation property (tanδ(60°C)), and highly balance these properties in an all-round manner.

[0009] Means for Solving the Problems

[0010] The rubber composition for side tread of the present invention for achieving the above object is characterized in that, relative to a diene rubber containing 35 to 55% by mass of an isoprene rubber and 45 to 65% by mass of a butadiene rubber, silica having a CTAB adsorption specific surface area of 60 to 100 m 2 / g, carbon black having a CTAB adsorption specific surface area of 30 to 100 m 2 / g, a sulfur-containing silane coupling agent, and a guanidine-based vulcanization accelerator are compounded. Relative to 100 parts by mass of the diene rubber, the compounding amount of the silica is 5 parts by mass or more and 50 parts by mass or less, and the total compounding amount of the silica and the carbon black is less than 55 parts by mass. The ratio Ma / Ms of the compounding amount of the guanidine-based vulcanization accelerator Ma to the compounding amount of the silica Ms is 0.02 to 0.08.

[0011] Advantages of the Invention

[0012] The rubber composition for side tread of the present invention, which is composed of the above formula, can maintain hardness and tensile strength well, and while suppressing the decrease in elongation at break over time, it can improve low heat build-up, and highly balance these properties well. Specifically, by using large-particle-size silica having a CTAB adsorption specific surface area of 60 to 100 m 2 / g, tanδ(60°C) can be reduced and low heat build-up can be improved. In addition, in terms of setting the compounding ratio of large-particle-size silica and carbon black as described above, a decrease in tanδ(60°C) (improvement in low heat build-up) can be expected. On the other hand, by compounding a guanidine-based vulcanization accelerator in an appropriate amount relative to silica as a vulcanization accelerator, hardness and tensile strength can be maintained well, and a decrease in elongation at break over time can be suppressed. Through these collaborations, hardness and tensile strength can be maintained well, and while suppressing the decrease in elongation at break over time, low heat build-up can be improved, and the physical properties required for the side tread rubber layer can be exhibited well.

[0013] In the present invention, it is preferred that the butadiene rubber contains a butadiene rubber synthesized by a neodymium-based catalyst, and optionally contains a terminal-modified butadiene rubber for silica. At this time, it is preferred that the proportion of the butadiene rubber synthesized by the neodymium-based catalyst contained in the butadiene rubber is 69 to 100% by mass, the proportion of the terminal-modified butadiene rubber for silica is 0 to 31% by mass, and the proportion of the compounding amount of the terminal-modified butadiene rubber for silica relative to 100% by mass of the diene rubber is less than 20% by mass. By containing the butadiene rubber synthesized by the neodymium-based catalyst and the terminal-modified butadiene rubber for silica in this way, hardness and tensile strength can be maintained well, and while suppressing the decrease in elongation at break over time, it is beneficial to improve low heat build-up.

[0014] In the present invention, it is preferable to blend other vulcanization accelerators in addition to the guanidine-based vulcanization accelerator. The ratio Ma / Mt of the blending amount Ma of the guanidine-based vulcanization accelerator to the total amount Mt of the vulcanization accelerator composed of the guanidine-based vulcanization accelerator and other vulcanization accelerators is preferably 0.2 to 0.5. In addition, the blending amount Ma of the guanidine-based vulcanization accelerator is preferably 0.5 parts by mass or more with respect to 100 parts by mass of the diene-based rubber. By blending the guanidine-based vulcanization accelerator in such an appropriate amount, it is possible to suppress the decrease in the elongation at break over time of the rubber composition stored in the unvulcanized state, and at the same time improve the low heat generation property, and balance these properties well.

[0015] The rubber composition for side tread of the present invention can be well used for the side tread rubber layer of a tire. A tire having a side tread rubber layer formed of the rubber composition for side tread of the present invention can maintain the hardness and tensile strength required for the side tread rubber layer well due to the excellent physical properties of the rubber composition for side tread of the present invention, and can suppress the decrease in the elongation at break over time, and at the same time reduce the rolling resistance and improve the low fuel consumption performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a partial meridian cross-section of a pneumatic tire using the rubber composition for side tread of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, the configuration of the present invention will be described in detail with reference to the drawings.

[0018] As Figure 1 shown, a pneumatic tire using the rubber composition for side tread of the present invention has a tread portion 1, a pair of sidewall portions 2 disposed on both sides of the tread portion 1, and a pair of bead portions 3 disposed on the tire radial inner side of the sidewall portions 2. In Figure 1 , the symbol CL represents the tire equator. Since Figure 1 is a meridian cross-section, it is not depicted, but the tread portion 1, the sidewall portions 2, and the bead portions 3 each extend in the tire circumferential direction to form a ring, thereby constituting the basic ring structure of the pneumatic tire. Hereinafter, the description using Figure 1 is basically based on the illustrated meridian cross-sectional shape, but each tire component extends in the tire circumferential direction to form a ring.

[0019] A carcass layer 4 is provided between the left and right pair of bead portions 3. The carcass layer 4 includes a plurality of reinforcing cords extending in the tire radial direction, and the reinforcing cords are folded back from the inner side in the tire width direction to the outer side around the bead cores 5 disposed in the respective bead portions 3. In addition, a bead filler 6 is disposed on the outer periphery of the bead core 5, and the bead filler 6 is wrapped by the main body portion and the folded-back portion of the carcass layer 4. On the other hand, a plurality of layers ( Figure 1In the middle is the belt layer 7 with two layers. Each belt layer 7 includes a plurality of reinforcing cords inclined with respect to the tire circumferential direction, and is arranged in such a way that the reinforcing cords cross each other between the layers. Among these belt layers 7, the inclination angle of the reinforcing cords with respect to the tire circumferential direction is set, for example, within the range of 10° to 40°. Moreover, a belt reinforcing layer 8 (including two layers: a full-coverage layer 8a covering the entire width of the belt layer 7 and an edge-coverage layer 8b locally covering the end portions of the belt layer 7) is provided on the outer peripheral side of the belt layer 7. The belt reinforcing layer 8 includes organic fiber cords oriented along the tire circumferential direction. In the belt reinforcing layer 8, the angle of the organic fiber cords with respect to the tire circumferential direction is set to, for example, 0° to 5°.

[0020] A tread rubber layer 10 is disposed on the outer peripheral side of the carcass layer 4 of the tread portion 1, a side tread rubber layer 20 is disposed on the outer peripheral side (outer side in the tire width direction) of the carcass layer 4 of the sidewall portion 2, and a rim cushion rubber layer 30 is disposed on the outer peripheral side (outer side in the tire width direction) of the carcass layer 4 of the bead portion 3. The side-tread rubber composition of the present invention is used for the side tread rubber layer 20 of such a tire. Therefore, the basic structure of other parts is not limited to the above structure. In addition, the side tread rubber layer 20 is different from "the tread rubber layer 10 that abuts against the road surface" or "the rim cushion rubber layer 30 that contacts the rim (not shown)", and is the part with the largest bending during driving in the tire, so it is a layer that requires excellent elongation at break (flex fatigue resistance). In addition, since there is a tendency for a relatively high frequency of being damaged, it is a layer that requires excellent cut resistance.

[0021] The tire using the side-tread rubber composition of the present invention is preferably the above-described pneumatic tire (a tire filled with an inert gas such as air, nitrogen, or other gas inside), but may also be a non-pneumatic tire. In the case of a non-pneumatic tire, the side-tread rubber composition of the present invention can be used for the part located between the part that abuts against the road surface (the part corresponding to the tread rubber layer 10 in the pneumatic tire) and the part that abuts against the rim when mounted on the rim (the part corresponding to the rim cushion rubber layer 30 in the pneumatic tire).

[0022] In the side-tread rubber composition of the present invention, the rubber component is a diene-based rubber, and must contain an isoprene-based rubber and a butadiene rubber. In addition, as the butadiene rubber, it is preferable to use a butadiene rubber polymerized by a neodymium-based catalyst (hereinafter sometimes referred to as Nd-BR). Alternatively, it is preferable to use a combination of Nd-BR and the silica-terminally modified butadiene rubber described later. Thus, by using an isoprene-based rubber and a butadiene rubber (especially Nd-BR and the silica-terminally modified butadiene rubber), the tensile strength, low heat generation property, and fatigue resistance can be improved.

[0023] As the isoprene rubber, various natural rubbers, epoxidized natural rubbers, and various synthetic polyisoprene rubbers can be mentioned. Among these isoprene rubbers, natural rubber is particularly preferably used. The blending amount of the isoprene rubber is 35 to 55% by mass, preferably 37.5 to 52.5% by mass, and more preferably 40 to 50% by mass in 100% by mass of the diene rubber. By blending such an amount of the isoprene rubber, the tensile strength and fatigue resistance can be improved in a balanced manner. When the blending amount of the isoprene rubber is less than 35% by mass, the tensile strength decreases. When the blending amount of the isoprene rubber exceeds 55% by mass, the fatigue resistance deteriorates.

[0024] As the butadiene rubber, the butadiene rubbers usually used in the tire rubber composition can be used, such as unmodified butadiene rubber and modified butadiene rubber. However, from the viewpoint of improving low heat generation property, it is preferable to use the butadiene rubber (Nd-BR) polymerized by the neodymium-based catalyst as described above. The blending amount of the butadiene rubber is 45 to 65% by mass, preferably 47.5 to 62.5% by mass, and more preferably 50 to 60% by mass in 100% by mass of the diene rubber. By blending such an amount of the butadiene rubber, the tensile strength and fatigue resistance can be improved in a balanced manner. When the blending amount of the butadiene rubber is less than 45% by mass, the fatigue resistance decreases. When the blending amount of the butadiene rubber exceeds 65% by mass, the tensile strength deteriorates.

[0025] The Nd-BR preferably used in the present invention is a known material and is a butadiene rubber polymerized by using a neodymium-based catalyst such as only neodymium simple substance, a compound of neodymium and other metals, or an organoneodymium compound. The butadiene rubber synthesized by these neodymium-based catalysts has the characteristics of high molecular weight and sharp molecular weight distribution. As the Nd-BR, commercially available Nd-BR can also be used, and examples include Buna CB22 and Buna CB24 manufactured by Arlanxeo Corporation. Thus, since the butadiene rubber is polymerized by using a neodymium-based catalyst, the low heat generation property can be improved more effectively compared with the butadiene rubber polymerized by other catalysts (such as nickel or cobalt).

[0026] When using Nd-BR as butadiene rubber, there is no particular limitation on the specific type of Nd-BR, but it is preferably a butadiene rubber having a vinyl content of 0 to 1% by mass, more preferably 0 to 0.8% by mass, and still more preferably 0 to 0.5% by mass. Such a low vinyl content is beneficial to improving low heat generation. When the vinyl content in Nd-BR exceeds 1% by mass, the effect of improving low heat generation cannot be fully expected. In addition, the vinyl content of the butadiene rubber (Nd-BR) is measured by infrared spectroscopic analysis (Hampton method). The increase or decrease in the vinyl content in the butadiene rubber can be appropriately adjusted by a usual method such as changing the type of catalyst (the specific type of the above-mentioned neodymium-based catalyst).

[0027] When using Nd-BR as butadiene rubber, its blending amount is 69 to 100% by mass, preferably 73 to 100% by mass, and more preferably 77 to 100% by mass based on 100% by mass of the butadiene rubber. For example, when the blending amount of the butadiene rubber is 65% by mass in 100% by mass of the diene rubber, the blending amount of Nd-BR is 45 to 65% by mass, preferably 47.5 to 65% by mass, and more preferably 50 to 65% by mass based on 100% by mass of the diene rubber. By blending such an amount of Nd-BR, low heat generation can be effectively improved. When the blending amount of Nd-BR is less than 69% by mass based on 100% by mass of the butadiene rubber, the fatigue resistance decreases.

[0028] In the present invention, as the butadiene rubber, a silica-terminally modified butadiene rubber can also be used in combination with the above-mentioned Nd-BR. The silica-terminally modified butadiene rubber is a butadiene rubber in which both or one of its molecular terminals is modified with a functional group reactive with a silanol group on the silica surface. Examples of the functional group reactive with the silanol group include at least one selected from a polyorganosiloxanyl group, a polyorganosiloxane structure containing a hydroxyl group, an alkoxysilyl group, a hydroxyl group, an aldehyde group, a carboxyl group, an amino group, an imino group, an epoxy group, an amide group, a thiol group, and an ether group. Among them, a polyorganosiloxanyl group, a polyorganosiloxane structure containing a hydroxyl group, an alkoxysilyl group, a hydroxyl group, and an amino group are preferred. In addition, it may also be a combination of a plurality of these functional groups (for example, two such as an amino group and an alkoxysilyl group). As the butadiene rubber, by containing at least one silica-terminally modified butadiene rubber, the affinity with the large-particle-size silica described later is good, which is beneficial to maintaining the hardness and tensile strength well, suppressing the decrease in the elongation at break over time, and at the same time improving low heat generation.

[0029] When using a silica-terminally modified butadiene rubber as the butadiene rubber, its blending amount is 0 to 31% by mass, preferably 0 to 27% by mass, more preferably 0 to 23% by mass relative to 100% by mass of the butadiene rubber. However, the blending amount of the silica-terminally modified butadiene rubber is less than 20% by mass relative to 100% by mass of the diene rubber. For example, when the blending amount of the butadiene rubber is 65% by mass in 100% by mass of the diene rubber, the blending amount of the silica-terminally modified butadiene rubber is 0% by mass or more and less than 20% by mass relative to 100% by mass of the diene rubber, preferably 0 to 17.5% by mass, more preferably 0 to 15% by mass. By blending such an amount of the silica-terminally modified butadiene rubber, the effect of improving the affinity with the large-particle-size silica described below can be sufficiently ensured, which is beneficial to maintaining the hardness and tensile strength well, suppressing the decrease in the elongation at break over time, and improving the low heat generation property. When the blending amount of the silica-terminally modified butadiene rubber exceeds 31% by mass relative to 100% by mass of the butadiene rubber, the elongation at break deteriorates.

[0030] In addition to the above isoprene rubber and butadiene rubber (Nd-BR, silica-terminally modified butadiene rubber), the rubber composition for the side tread of the present invention may further contain other diene rubbers. As the other diene rubbers, rubbers that can be usually used in tire rubber compositions can be used. For example, styrene-butadiene rubber etc. can be exemplified. These other diene rubbers can be used alone or as any blend.

[0031] In the present invention, for the above diene rubber, silica and carbon black must be blended as fillers. As the silica used in the present invention, for example, wet silica, dry silica, or surface-treated silica etc. that are usually used in tire rubber compositions can be used. However, it is necessary to use silica with a CTAB adsorption specific surface area of 60 to 100 m 2 / g, preferably 65 to 95 m 2 / g, more preferably 70 to 90 m 2 / g. By using such a large-particle-size silica, tanδ(60°C) can be reduced and the low heat generation property can be improved. When the CTAB adsorption specific surface area of the silica is less than 60 m 2 / g, the tensile strength decreases. When the CTAB adsorption specific surface area of the silica exceeds 100 m 2 / g, the low heat generation property decreases. As long as the silica satisfies the above conditions, it can be appropriately selected from commercially available silica and used, or silica obtained by a usual manufacturing method can also be used.

[0032] The compounding amount Ms of silica is 5 parts by mass or more and 50 parts by mass or less, preferably 10 to 45 parts by mass, more preferably 15 to 40 parts by mass, relative to 100 parts by mass of the above-mentioned diene rubber. By compounding silica in such an appropriate amount, low heat build-up can be effectively improved. When the compounding amount Ms of silica is less than 5 parts by mass, the tensile strength decreases. When the compounding amount Ms of silica exceeds 50 parts by mass, the low heat build-up deteriorates.

[0033] As the carbon black used in the present invention, it is necessary to use carbon black having a CTAB adsorption specific surface area of 30 to 100 m 2 / g, preferably 30 to 85 m 2 / g, more preferably 30 to 70 m 2 / g, which is commonly used in tire rubber compositions. By using such carbon black, it is beneficial to maintain hardness and tensile strength well, and to improve low heat build-up while suppressing the reduction of elongation at break over time. If the CTAB adsorption specific surface area of the carbon black is less than 30 m 2 / g, the tensile strength will decrease. If the CTAB adsorption specific surface area of the carbon black exceeds 100 m 2 / g, the low heat build-up will decrease.

[0034] In the present invention, silica and carbon black must be used in combination as described above. At this time, the total (Ms + Mc) of the compounding amount Ms of silica and the compounding amount Mc of carbon black is less than 55 parts by mass, preferably 20 parts by mass or more and less than 55 parts by mass, more preferably 25 to 50 parts by mass, relative to 100 parts by mass of the above-mentioned diene rubber. By compounding silica and carbon black in such an appropriate amount, hardness and tensile strength can be maintained well, the reduction of elongation at break over time can be suppressed, and low heat build-up can be improved at the same time. When the total of the compounding amounts of silica and carbon black is 55 parts by mass or more, the low heat build-up decreases. In addition, there is no particular limitation on the compounding amount of carbon black alone, and it is preferably 5 to 30 parts by mass, more preferably 5 to 25 parts by mass, relative to 100 parts by mass of the diene rubber.

[0035] Other fillers other than silica and carbon black can be compounded in the rubber composition of the present invention. As other fillers, for example, materials commonly used in tire rubber compositions such as clay, talc, calcium carbonate, mica, and aluminum hydroxide can be exemplified.

[0036] In the rubber composition for side tread of the present invention, when compounding the above-mentioned silica, a sulfur-containing silane coupling agent must be compounded. By compounding the sulfur-containing silane coupling agent, the dispersibility of silica in the diene-based rubber can be improved. Examples of the sulfur-containing silane coupling agent include bis(3-triethoxysilylpropyl)tetrasulfide, bis(3-triethoxysilylpropyl)disulfide, 3-trimethoxysilylpropylbenzothiazole tetrasulfide, γ-mercaptopropyltriethoxysilane, 3-octanoylthiopropyltriethoxysilane, etc. Among them, a silane coupling agent having a tetrasulfide bond in the molecule is particularly preferably used. The compounding amount of the silane coupling agent is preferably less than 10% by mass relative to the compounding amount of silica, and more preferably 3-9% by mass. When the compounding amount of the silane coupling agent is 10% by mass or more of the compounding amount of silica, the silane coupling agents condense with each other, and the desired hardness and strength of the rubber composition cannot be obtained.

[0037] In the rubber composition for side tread of the present invention, a vulcanization accelerator must be compounded. As the vulcanization accelerator, vulcanization accelerators commonly used in rubber compositions for tires can be used, and examples include guanidine-based vulcanization accelerators, sulfenamide-based vulcanization accelerators, and thiuram-based vulcanization accelerators. However, in the present invention, considering the compounding of the above-mentioned large-particle-size silica, a guanidine-based vulcanization accelerator must be compounded therein. That is, since the large-particle-size silica is compounded, the vulcanization rate tends to decrease, and the vulcanization can be promoted by compounding the guanidine-based vulcanization accelerator, which is beneficial to obtaining the desired rubber physical properties. Furthermore, in the present invention, by compounding the guanidine-based vulcanization accelerator, the deterioration of the rubber composition in the unvulcanized state over time (the decrease in the elongation at break over time) can be suppressed. In addition, vulcanization accelerators other than the guanidine-based vulcanization accelerator can be arbitrarily used in combination. When using two vulcanization accelerators, examples of the vulcanization accelerator combined with the guanidine-based vulcanization accelerator include sulfenamide-based vulcanization accelerators and thiuram-based vulcanization accelerators, and among them, a sulfenamide-based vulcanization accelerator can also be preferably used.

[0038] Examples of the guanidine-based vulcanization accelerator include diphenylguanidine, di-o-tolylguanidine, etc. Examples of the sulfenamide-based vulcanization accelerator include N,N-dicyclohexyl-1,3-benzothiazole-2-sulfenamide (DZ), N-cyclohexyl-2-benzothiazole sulfenamide (CZ), N-oxydiethylene-2-benzothiazole sulfenamide (OBS), N-(tert-butyl)benzothiazole-2-sulfenamide (NS), etc. Examples of the thiuram-based vulcanization accelerator include tetramethylthiuram monosulfide, tetramethylthiuram disulfide, etc.

[0039] When blending a guanidine-based vulcanization accelerator, it is preferred that the ratio Ma / Ms of the blending amount Ma of the guanidine-based vulcanization accelerator to the blending amount Ms of the above-mentioned silica is 0.02 to 0.08, preferably 0.02 to 0.07, and more preferably 0.03 to 0.06. In this way, by blending an appropriate amount of the guanidine-based vulcanization accelerator with the above-mentioned large-particle-size silica, it is beneficial to inhibit the reduction in the elongation at break of the rubber composition in the unvulcanized state over time. If the ratio Ma / Ms of the blending amount Ma of the guanidine-based vulcanization accelerator to the blending amount Ms of the silica is less than 0.02, the elongation at break may occur over time when the rubber composition is stored in the unvulcanized state. When the ratio Ma / Ms of the blending amount Ma of the guanidine-based vulcanization accelerator to the blending amount Ms of the silica exceeds 0.08, the hardness cannot be maintained well, and the low heat generation property will also deteriorate.

[0040] When a guanidine-based vulcanization accelerator and a vulcanization accelerator other than guanidine (other vulcanization accelerators) are contained as the vulcanization accelerator, the ratio Ma / Mt of the blending amount Ma of the guanidine-based vulcanization accelerator to the total amount Mt of the vulcanization accelerator (the total of the blending amount Ma of the guanidine-based vulcanization accelerator and the blending amount of the other vulcanization accelerator relative to 100 parts by mass of the diene rubber) is preferably 0.2 to 0.5, more preferably 0.25 to 0.5, and further preferably 0.3 to 0.5. By blending an appropriate amount of the guanidine-based vulcanization accelerator with the whole vulcanization accelerator in this way, it is beneficial to improve the low heat generation property. If the ratio Ma / Mt is less than 0.2, the elongation at break may decrease when the rubber composition is stored in the unvulcanized state. When the ratio Ma / Mt exceeds 0.5, tanδ(60°C) deteriorates, and the effect of improving the low heat generation property is limited.

[0041] Furthermore, when blending a guanidine-based vulcanization accelerator, its blending amount Ma is preferably 0.5 parts by mass or more, more preferably 0.5 to 2 parts by mass, and further preferably 0.5 to 1.5 parts by mass relative to 100 parts by mass of the diene rubber. By blending an appropriate amount of the guanidine-based vulcanization accelerator in this way, it is beneficial to inhibit the reduction in the elongation at break of the rubber composition in the unvulcanized state over time. When the blending amount Ma of the guanidine-based vulcanization accelerator is less than 0.5 parts by mass, the elongation at break may decrease when the rubber composition is stored in the unvulcanized state.

[0042] In the rubber composition for the side tread of the present invention, other compounding agents other than the above can be added. As other compounding agents, various compounding agents commonly used in tire rubber compositions such as vulcanizing or crosslinking agents, anti-aging agents, and liquid polymers can be exemplified. The compounding amounts of these compounding agents can be the usual general compounding amounts as long as they do not violate the purpose of the present invention. In addition, as a kneading machine, a usual rubber kneading machine such as a Banbury mixer, a kneader, a roll, etc. can be used.

[0043] Hereinafter, the present invention will be further described by way of examples, but the scope of the present invention is not limited to these examples.

[0044] Examples

[0045] When formulating 16 rubber compositions for side treads (Comparative Examples 1 to 6, Examples 1 to 10) having the formulations shown in Table 1, the formulation components other than the vulcanization accelerator and sulfur were weighed separately, kneaded for 5 minutes using a 1.8 L closed Banbury mixer, and the masterbatch was discharged and cooled at room temperature. Then, this masterbatch was supplied to a 1.8 L closed Banbury mixer, and the vulcanization accelerator and sulfur were added and mixed for 2 minutes to obtain each rubber composition for side treads.

[0046] In addition, the "Mc + Ms" column in Table 1 represents the total amount of the carbon black compounding amount and the silica compounding amount. The "Ma / Ms" column in Table 1 represents the ratio of the compounding amount of vulcanization accelerator 2 (guanidine-based vulcanization accelerator) to the silica compounding amount. The "Ma / Mt" column in Table 1 represents the ratio of the compounding amount of vulcanization accelerator 2 (guanidine-based vulcanization accelerator) to the total amount of vulcanization accelerators (the sum of vulcanization accelerators 1 and 2).

[0047] Using each of the obtained rubber compositions for side treads, evaluations of hardness, tensile strength, tanδ at 60 °C, and elongation at break retention rate were carried out by the methods shown below.

[0048] Hardness

[0049] Using each rubber composition for side treads, a vulcanized rubber test piece was produced by vulcanizing at 160 °C for 20 minutes using a mold of a specified shape. The hardness of each vulcanized rubber test piece was measured at a temperature of 20 °C by type A of a rubber hardness meter (durometer) according to JIS K6253. The evaluation results were shown as an index when the value of Comparative Example 1 was set as index 100 in the "Hardness" column of Table 1. The larger this index value means the higher the hardness.

[0050] Tensile strength

[0051] Using each rubber composition for side treads, a vulcanized rubber test piece was produced by vulcanizing at 160 °C for 20 minutes using a mold of a specified shape. Using these vulcanized rubber test pieces, JIS No. 3 dumbbell-shaped test pieces were cut out according to JIS K6251, and a tensile test was carried out at a tensile speed of 500 mm / minute at room temperature (20 °C), and the stress at break (tensile strength at break TB, unit: MPa) was measured. The obtained results were shown as an index when the value of Comparative Example 1 was set as index 100 in the "Tensile strength" column of Table 1. The larger this index value means the greater the tensile strength at break.

[0052] Tanδ at 60 °C

[0053] Using the rubber composition for each side tread, a vulcanized rubber test piece was produced by vulcanizing at 160 °C for 20 minutes using a mold with a specified shape. For these vulcanized rubber test pieces, a viscoelastic spectrometer manufactured by Toyo Seiki Seisakusho Co., Ltd. was used to measure tanδ at 60 °C under the conditions of an initial strain of 10%, an amplitude of ±2%, a frequency of 20 Hz, and a temperature of 60 °C. The evaluation results were shown as indices when the value of Comparative Example 1 was set as index 100 in the column of "tanδ (60 °C)" in Table 1. The smaller this value is, the more excellent the low heat generation property is.

[0054] Elongation at break retention

[0055] The rubber composition for each side tread produced by the above method was vulcanized at 160 °C for 20 minutes using a mold with a specified shape to produce a vulcanized rubber test piece A. In addition, the rubber composition for each side tread that was left in an unvulcanized state for 30 days after being produced by the above method was vulcanized at 160 °C for 20 minutes using a mold with a specified shape to produce a vulcanized rubber test piece B. Using these vulcanized rubber test pieces A and B, JIS No. 3 dumbbell-shaped test pieces were cut out according to JIS K6251, and a tensile test was carried out at a tensile speed of 500 mm / min at room temperature (20 °C) to measure the elongation at break (elongation at break EB, unit: %) at the time of fracture for each. Then, for the rubber composition for each side tread, the ratio (unit: %) of the elongation at break measured for the vulcanized rubber test piece B to the elongation at break measured for the vulcanized rubber test piece A was calculated as an index of the elongation at break retention and shown in the column of "elongation at break retention" in Table 1. The closer this value is to 100%, the less the elongation at break decreases over time.

[0056] Table 1

[0057]

[0058] The types of raw materials used in Table 1 are as follows.

[0059] · NR: Natural rubber, STR20

[0060] · BR1: Butadiene rubber synthesized by a cobalt catalyst, Nipol BR1220 manufactured by Zeon Corporation, Japan

[0061] · BR2: Butadiene rubber synthesized by a neodymium-based catalyst, Buna CB24 manufactured by ARLANXEO

[0062] · BR3: Silica-terminally modified butadiene rubber, BR511 manufactured by JSR

[0063] · CB1: Carbon black, Shaw black N550 manufactured by Cabot Japan Co., Ltd. (CTAB adsorption specific surface area: 40 m 2 / g)

[0064] · CB2: Carbon black, Shaw black N234 manufactured by Cabot Japan Co., Ltd. (CTAB adsorption specific surface area: 115 m 2 / g)

[0065] · Silica 1: 115GR manufactured by Solvay (CTAB adsorption specific surface area: 110 m 2 / g)

[0066] · Silica 2: 1165MP manufactured by Solvay (CTAB adsorption specific surface area: 160 m 2 / g)

[0067] · Silica 3: 1085GR manufactured by Solvay (CTAB adsorption specific surface area: 80 m 2 / g)

[0068] · Silane coupling agent: Si69 manufactured by Evonik Degussa Japan Co., Ltd.

[0069] · Aromatic oil: Extract No. 4S manufactured by Showa Shell Sekiyu KK

[0070] · Antioxidant 1: Santoflex 6PPD manufactured by Flexsys

[0071] · Antioxidant 2: Nocrack 224 manufactured by Ouchi Shinsei Chemical Industry Co., Ltd.

[0072] · Wax: Paraffin wax manufactured by Ouchi Shinsei Chemical Industry Co., Ltd.

[0073] · Stearic acid: Bead stearic acid manufactured by NOF Corporation

[0074] · Zinc oxide: Zinc oxide No. 3 manufactured by Sho Do Kogyo Co., Ltd.

[0075] · Sulfur: Micron OT-20 manufactured by Shikoku Chemicals Corporation

[0076] · Vulcanization accelerator 1: Sulfenamide-based vulcanization accelerator, Nocceler CZ-G manufactured by Ouchi Shinsei Chemical Industry Co., Ltd.

[0077] · Vulcanization accelerator 2: Guanidine-based vulcanization accelerator, Perkacit DPG manufactured by Flexsys

[0078] As can be seen from Table 1, compared with Comparative Example 1, Examples 1 to 10 can improve low heat generation (tanδ at 60°C) while maintaining or improving hardness and tensile strength to an equivalent or higher level. In addition, compared with Comparative Example 1, Examples 1 to 10 can maintain the elongation at break well even after being left in the unvulcanized state for 30 days.

[0079] On the other hand, in Comparative Example 2, due to the large CTAB adsorption specific surface area of silica and the absence of guanidine-based vulcanization accelerators, low heat generation deteriorates and the retention rate of elongation at break also decreases. In Comparative Example 3, although the particle size of silica is appropriate, it does not contain guanidine-based vulcanization accelerators, so the hardness, tensile strength, and retention rate of elongation at break decrease. In Comparative Example 4, since the blending amount of guanidine-based vulcanization accelerator is relatively large with respect to the blending amount of silica and the total amount of vulcanization accelerators, low heat generation δ (at 60°C) deteriorates. In Comparative Example 5, due to the large blending amount of silica and the large total blending amount of silica and carbon black, low heat generation (tanδ (60°C)) deteriorates. In Comparative Example 6, due to the large CTAB adsorption specific surface area of carbon black, low heat generation δ (at 60°C) cannot be improved.

[0080] The present disclosure includes the following inventions.

[0081] Invention [1]. A rubber composition for side tread, characterized in that, relative to a diene rubber containing 35 to 55% by mass of isoprene rubber and 45 to 65% by mass of butadiene rubber, silica with a CTAB adsorption specific surface area of 60 to 100 m 2 / g, carbon black with a CTAB adsorption specific surface area of 30 to 100 m 2 / g, a sulfur-containing silane coupling agent, and a guanidine-based vulcanization accelerator are blended. Relative to 100 parts by mass of the diene rubber, the blending amount of the silica is 5 parts by mass or more and 50 parts by mass or less, and the total blending amount of the silica and the carbon black is less than 55 parts by mass. The ratio of the blending amount of the guanidine-based vulcanization accelerator Ma to the blending amount of the silica Ms, i.e., Ma / Ms, is 0.02 to 0.08.

[0082] Invention [2]. The rubber composition for side tread according to Invention [1], characterized in that the butadiene rubber contains butadiene rubber synthesized by a neodymium-based catalyst and optionally contains a butadiene rubber with a terminal modification for silica.

[0083] Invention [3]. The rubber composition for side tread according to Invention [2], characterized in that the proportion of butadiene rubber synthesized by the neodymium catalyst contained in the butadiene rubber is 69 to 100% by mass, the proportion of the silica-terminated modified butadiene rubber is 0 to 31% by mass, and the proportion of the compounding amount of the silica-terminated modified butadiene rubber relative to 100% by mass of the diene rubber is less than 20% by mass.

[0084] Invention [4]. The rubber composition for side tread according to any one of Inventions [1] to [3], characterized in that other vulcanization accelerators are further compounded in addition to the guanidine-based vulcanization accelerator, and the ratio Ma / Mt of the compounding amount Ma of the guanidine-based vulcanization accelerator to the total amount Mt of the vulcanization accelerators composed of the guanidine-based vulcanization accelerator and the other vulcanization accelerators is 0.2 to 0.5.

[0085] Invention [5]. The rubber composition for side tread according to any one of Inventions [1] to [4], characterized in that the compounding amount Ma of the guanidine-based vulcanization accelerator is 0.5 parts by mass or more relative to 100 parts by mass of the diene rubber.

[0086] Invention [6]. A tire, characterized in that it includes a side tread rubber layer formed of the rubber composition for side tread according to any one of Inventions [1] to [5].

[0087] Explanation of reference numerals

[0088] 1 Tread portion

[0089] 2 Sidewall portion

[0090] 3 Bead portion

[0091] 4 Carcass ply

[0092] 5 Bead core

[0093] 6 Bead filler

[0094] 7 Belt layer

[0095] 8 Belt cover layer

[0096] 10 Sulfur-containing silane coupling agent

[0097] 20 Side tread rubber layer

[0098] 30 Rim cushion rubber layer

[0099] CL Tire equator

Claims

1. A rubber composition for a side tread, characterized in that: A rubber having a CTAB adsorption specific surface area of ​​60 to 100 m2 is mixed with a diene rubber containing 35 to 55% by mass of an isoprene rubber and 45 to 65% by mass of a butadiene rubber. 2 / g silica, CTAB adsorption specific surface area is 30~100m 2 / g of carbon black, a sulfur-containing silane coupling agent and a guanidine vulcanization accelerator, the compounding amount of the silica is 5 parts by mass or more and 50 parts by mass or less relative to 100 parts by mass of the diene rubber, the total compounding amount of the silica and the carbon black is less than 55 parts by mass, and the ratio of the compounding amount of the guanidine vulcanization accelerator Ma to the compounding amount Ms of the silica, i.e. Ma / Ms, is 0.02 to 0.

08.

2. The rubber composition for side tread according to claim 1, characterized in that The butadiene rubber contains butadiene rubber synthesized by using a neodymium-based catalyst, and optionally contains butadiene rubber terminally modified with silica.

3. The rubber composition for side tread according to claim 2, characterized in that: The butadiene rubber synthesized by the neodymium-based catalyst contained in the butadiene rubber is 69 to 100% by mass, the terminal-modified butadiene rubber for silica is 0 to 31% by mass, and the compounding amount of the terminal-modified butadiene rubber for silica is less than 20% by mass relative to 100% by mass of the diene rubber.

4. The rubber composition for side tread according to any one of claims 1 to 3, characterized in that A vulcanization accelerator other than the guanidine vulcanization accelerator is further mixed, and the ratio of the mixing amount Ma of the guanidine vulcanization accelerator to the total amount Mt of the vulcanization accelerator consisting of the guanidine vulcanization accelerator and the other vulcanization accelerator, that is, Ma / Mt, is 0.2 to 0.

5.

5. The rubber composition for side tread according to any one of claims 1 to 4, characterized in that The compounding amount Ma of the guanidine vulcanization accelerator is 0.5 parts by mass or more relative to 100 parts by mass of the diene rubber.

6. A tire, characterized in that: A side tread rubber layer is provided which is formed from the rubber composition for side treads according to any one of claims 1 to 5.

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