JP2024093595A - Rubber composition and pneumatic tire - Google Patents

Abstract

The present invention provides a pneumatic tire that is equipped with a rubber composition containing a liquid crystal polymer and having an improved balance between tan δ (-20°C) and tan δ (60°C), and a vulcanized rubber obtained using the rubber composition as a raw material, and that is capable of achieving both wet performance and low fuel consumption.
[Solution] A rubber composition containing a diene-based rubber and a liquid crystal polymer, wherein the liquid crystal polymer has a weight average molecular weight of 40,000 or more, and the ratio of tan δ (-20°C) when tan δ is measured at -20°C to tan δ (60°C) when tan δ after vulcanization is measured at 60°C (tan δ (-20°C)/tan δ (60°C)) is 2.6 or more, and a pneumatic tire comprising vulcanized rubber obtained using the rubber composition as a raw material.
[Selection diagram] None

Description

The present invention relates to a rubber composition and a pneumatic tire.

In general, tires are used in a variety of driving environments, and there is a demand for improving wet grip performance (hereinafter simply referred to as "wet performance"), which is the grip performance on wet road surfaces in the rain, for example. However, when rubber compositions are compounded with the aim of improving such wet performance, the fuel efficiency of the resulting vulcanized rubber may deteriorate, so there has been a demand for technology that can improve these in a well-balanced manner.

The following Patent Document 1 describes a technology for improving the wet performance and fuel economy of pneumatic tires by using as a raw material a rubber composition containing at least two types of diene rubber with a bimodal tan δ temperature dispersion curve as the rubber component.

In addition, the following Patent Document 2 describes a technology for improving the initial grip performance and running stability of tires by using as a raw material a rubber composition containing a predetermined amount of at least one selected from homopolymer resins and copolymer resins of aromatic vinyl compounds having a softening point of 140°C or higher.

Furthermore, the following Patent Document 3 describes a technology for improving the wet performance and fuel economy of pneumatic tires by using as a raw material a rubber composition that contains a liquid copolymer that contains an aromatic vinyl compound and a conjugated diene-containing compound as monomer units and has a weight average molecular weight (Mw) of 1,000 to 50,000.

Japanese Patent Application Laid-Open No. 08-27313 JP 2008-169295 A JP 2016-117880 A

When trying to achieve both wet performance and fuel economy in pneumatic tires, it is important to optimize the loss tangent (tan δ) of the raw rubber composition. In general, wet performance is highly dependent on the tan δ of the raw rubber composition at -20°C (hereinafter also referred to as "tan δ (-20°C)"), and the higher the tan δ (-20°C), the better the wet performance of the pneumatic tire. On the other hand, fuel economy is highly dependent on the tan δ of the raw rubber composition at 60°C (hereinafter also referred to as "tan δ (60°C)"), and the lower the tan δ (60°C), the better the fuel economy of the pneumatic tire.

However, it has been found that the technology described in the above patent document makes it difficult to increase the tan δ (-20°C) and decrease the tan δ (60°C) of the rubber composition, making it difficult to improve the wet performance and fuel economy of the final product, a pneumatic tire, in a well-balanced manner. The present invention was completed in view of the above situation, and aims to provide a pneumatic tire that is equipped with a rubber composition containing a liquid crystal polymer and has an improved balance between tan δ (-20°C) and tan δ (60°C), and a vulcanized rubber obtained from the rubber composition as a raw material, and that is capable of achieving both wet performance and fuel economy.

The above object can be achieved by the present invention as described below. That is, the present invention relates to a rubber composition (1) containing a diene rubber and a liquid crystal polymer, the liquid crystal polymer having a weight average molecular weight of 40,000 or more, and a ratio of tan δ (-20°C) when tan δ after vulcanization is measured at -20°C to tan δ (60°C) when tan δ is measured at 60°C (tan δ (-20°C)/tan δ (60°C)) of 2.6 or more.

In the above rubber composition (1), rubber composition (2) is preferred in which the liquid crystal polymer is a branched liquid crystal polymer having a branched structure.

In the above rubber composition (1) or (2), a rubber composition (3) is preferred in which the liquid crystal polymer is a reaction product of a mesogen group-containing compound having at least an active hydrogen group and a branched structure, and an isocyanate compound.

The present invention further relates to a pneumatic tire (4) comprising a vulcanized rubber obtained using any one of the rubber compositions (1) to (3) as a raw material.

The liquid crystal polymer contained in the rubber composition according to the present invention is set to have a weight average molecular weight of 40,000 or more, and the rubber composition according to the present invention is designed so that the ratio of tan δ (-20°C) measured at -20°C to tan δ (60°C) measured at 60°C after vulcanization (tan δ (-20°C)/tan δ (60°C)) is 2.6 or more. Therefore, a pneumatic tire having a tread portion made of vulcanized rubber of a rubber composition containing such a liquid crystal polymer has excellent both WET performance and low fuel consumption.

When the liquid crystal polymer is a branched liquid crystal polymer having a branched structure and a high molecular weight, the tan δ (60°C) of the vulcanized rubber of the rubber composition containing such a liquid crystal polymer is maintained low while the tan δ (-20°C) is increased, which is preferable because the balance between tan δ (-20°C) and tan δ (60°C) is further improved. The reason why such an effect is obtained is unclear, but the following reasons can be assumed.

Tan δ is expressed as the ratio of the loss modulus (E'') to the storage modulus (E') (tan δ = E''/E'). Since the branched liquid crystal polymer has a flexible branched structure, when strain is applied to vulcanized rubber containing the branched liquid crystal polymer, the branched structure moves and intramolecular friction occurs. For this reason, it can be estimated that the amount of energy required to deform the vulcanized rubber, that is, the loss modulus (E''), increases, and as a result, tan δ, especially tan δ (-20°C), increases. In addition, since the liquid crystal polymer contained in the rubber composition according to the present invention has a weight average molecular weight of 40,000 or more, tan δ (60°C) decreases. It is believed that these factors improve the balance between tan δ (-20°C) and tan δ (60°C) of the liquid crystal polymer.

The vulcanized rubber of the rubber composition according to the present invention has an improved balance between tan δ (-20°C) and tan δ (60°C). Therefore, for example, when a pneumatic tire is manufactured using vulcanized rubber obtained from such a rubber composition as a raw material, it has excellent wet performance and low fuel consumption.

The rubber composition according to the present invention is designed so that the ratio of tan δ (-20°C) when tan δ is measured at -20°C to tan δ (60°C) when tan δ is measured at 60°C after vulcanization (tan δ (-20°C)/tan δ (60°C)) is 2.6 or more. From the viewpoint of improving the WET performance and fuel economy of the final pneumatic retirement tire in a more balanced manner, it is preferable that the rubber composition according to the present invention is designed so that the ratio of tan δ (-20°C) when tan δ is measured at -20°C to tan δ (60°C) when tan δ is measured at 60°C after vulcanization (tan δ (-20°C)/tan δ (60°C)) is 2.9 or more.

The liquid crystal polymer contained in the rubber composition according to the present invention preferably has a weight average molecular weight of 40,000 or more, and the ratio of tan δ (-20°C) when tan δ is measured at -20°C to tan δ (60°C) when tan δ is measured at 60°C (tan δ (-20°C)/tan δ (60°C)) is 0.21 or more. From the viewpoint of improving the WET performance and fuel efficiency of the pneumatic tire having the final vulcanized rubber in a more balanced manner, it is more preferable that the liquid crystal polymer has a (tan δ (-20°C)/tan δ (60°C)) of 0.27 or more. In particular, from the viewpoint of reducing tan δ (60°C) and further improving the fuel efficiency of the pneumatic tire having the final vulcanized rubber, it is more preferable that the weight average molecular weight of the liquid crystal polymer is 100,000 or more. There is no particular limit to the upper limit of (tan δ (-20°C)/tan δ (60°C)), but an example of about 2.0 is possible. Similarly, there is no particular upper limit to the weight average molecular weight of the liquid crystal polymer, but an example would be around 330,000.

In the present invention, in order to improve the balance between tan δ (-20°C) and tan δ (60°C) of the vulcanized rubber to a higher level, it is preferable that the liquid crystal polymer has a transition temperature (Ti) from the liquid crystal phase to the isotropic phase of 20°C or less, and it is more preferable that Ti is 5°C or less. Also, in the present invention, it is preferable that the liquid crystal polymer is a liquid crystal polyurethane.

In the present invention, the liquid crystal polymer is preferably a branched liquid crystal polymer having a branched structure. Branched liquid crystal polymers having a branched structure tend to have a large tan δ (-20°C). Therefore, when a branched liquid crystal polymer having a branched structure is used, the WET performance of the pneumatic tire having the finally obtained vulcanized rubber is further improved, which is preferable. Such a branched liquid crystal polymer may be (i) a branched liquid crystal polyurethane which is a reaction product between a mesogen group-containing compound having at least an active hydrogen group and a branched structure and an isocyanate compound, or (ii) a branched liquid crystal polyurethane which is a reaction product between a mesogen group-containing compound having at least an active hydrogen group and a branched isocyanate compound having a branched structure. Each of the components will be described below.

The mesogen group-containing compound (i) can be, for example, a compound in which an alkylene oxide having 4 or more carbon atoms is added to a compound represented by the following general formula (1) (hereinafter, also referred to as an "oxide adduct compound").

In the above general formula (1), X is an active hydrogen group, R ₁ is a single bond forming a part of an adjacent bonding group, -N=N-, -CO-, -CO-O-, or -CH=N-, R ₂ is a single bond forming a part of an adjacent bonding group, or -O-, and R ₃ is a single bond forming a part of an adjacent bonding group, or an alkylene group having 1 to 20 carbon atoms. However, this does not include cases where R ₂ is -O- and R ₃ is a single bond forming a part of an adjacent bonding group. In addition, the "single bond forming a part of an adjacent bonding group" means a state in which the single bond is shared with a part of an adjacent bonding group. For example, in the above general formula (1), when R ₁ is a single bond forming a part of an adjacent bonding group, R ₁ , which is a single bond, is shared with the benzene rings on both sides, and forms a biphenyl structure together with the benzene rings on both sides. Examples of X include OH, SH, NH ₂ , COOH, and secondary amines.

The liquid crystal polyurethane polymer produced using an oxide compound as a mesogen group-containing compound can easily be designed to have a transition temperature (Ti) from the liquid crystal phase to the isotropic phase in a desired temperature range. In the present invention, the amount of alkylene oxide having 4 or more carbon atoms is adjusted to 2 to 20 moles, preferably 4 to 16 moles, per mole of the compound represented by general formula (1).

In particular, in the present invention, the following compound (also referred to as "BH6BOn") is obtained by adding 1,2-butylene oxide to BH6, which is a compound represented by general formula (1) in which _R1 is a single bond that is a part of an adjacent bonding group, _R2 is -O-, and _R3 is an alkylene group having 6 carbon atoms:

The following compound is formed by adding 1,2-epoxyhexane to BH6 (also called "BH6HOn");

The following compound is the addition of isobutylene oxide to BH6 (also referred to as "BH6iBOn");
can be preferably used.

The amount of the mesogen group-containing compound, particularly the oxide addition compound, is adjusted to 20-95% by mass, preferably 30-85% by mass, of the total raw materials for the liquid crystal polymer. If the amount of the mesogen group-containing compound, particularly the oxide addition compound, is less than 20% by mass, the resulting polymer will be less likely to exhibit liquid crystallinity. If the amount of the mesogen group-containing compound, particularly the oxide addition compound, is more than 95% by mass, polymerization by the urethane reaction will be insufficient.

As the isocyanate compound (i), for example, the diisocyanate compounds shown below can be used. Examples of diisocyanate compounds include aromatic diisocyanates such as 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 2,2'-diphenylmethane diisocyanate, 2,4'-diphenylmethane diisocyanate, 4,4'-diphenylmethane diisocyanate, 1,5-naphthalene diisocyanate, p-phenylene diisocyanate, m-phenylene diisocyanate, p-xylylene diisocyanate, and m-xylylene diisocyanate; ethylene diisocyanate; aliphatic diisocyanates such as 1,5-pentamethylene diisocyanate, 2,2,4-trimethylhexamethylene-1,6-diisocyanate, 2,4,4-trimethylhexamethylene-1,6-diisocyanate, and 1,6-hexamethylene diisocyanate; and alicyclic diisocyanates such as 1,4-cyclohexane diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, isophorone diisocyanate, and norbornane diisocyanate. The diisocyanate compounds exemplified above may be used alone or in combination.

When the total amount of the mesogen group-containing compounds constituting the liquid crystal polymer is taken as 100 parts by mass, the proportion of the isocyanate compound is preferably 5 to 70 parts by mass, and more preferably 14 to 60 parts by mass. If the amount of the isocyanate compound is less than 5 parts by mass, the polymerization by the urethane reaction becomes insufficient. On the other hand, if the proportion of the isocyanate compound exceeds 60 parts by mass, the amount of the mesogen group-containing compounds in the total raw materials of the liquid crystal polyurethane polymer becomes relatively small, and the liquid crystallinity of the liquid crystal polyurethane polymer decreases.

As the mesogen group-containing compound having an active hydrogen group of (ii), the compound represented by the general formula (1) or an oxide adduct compound can be used.

Examples of the branched isocyanate compound having the branched structure (ii) include trimethylhexamethyldiisocyanate and isophorone diisocyanate.

In the liquid crystal polymer contained in the rubber composition according to the present invention, in addition to the mesogen group-containing compound and the isocyanate compound, an active hydrogen group-containing compound may be used as a raw material for the liquid crystal polymer. Examples of the active hydrogen group-containing compound include polyol compounds and amine compounds. Examples of the polyol compound include polyether polyol, polyester polyol, polycarbonate polyol, polyester polycarbonate polyol, ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,6-hexanediol, neopentyl glycol, 1,4-cyclohexanedimethanol, 3-methyl-1,5-pentanediol, diethylene ...1,5-butanediol, 1,6-hexanediol, 1,4-butanediol, 1,5-butanediol, 1,6-butanediol, 1,5-butanediol, 1,6-butanediol, 1,5-butanediol, 1,6-butanediol, 1,5-butanediol, 1,6-butanediol, 1,5-butanediol, 1,6-butanediol, 1,5-butanediol, 1,6-butanediol, 1,5-butanediol, 1,6-butanediol, 1,5-butanediol, 1,6-butanediol, 1,5 Examples of the active hydrogen group-containing compounds include glycerol, triethylene glycol, 1,4-bis(2-hydroxyethoxy)benzene, trimethylolpropane, glycerin, 1,2,6-hexanetriol, meso-erythritol, pentaerythritol, tetramethylolcyclohexane, methyl glucoside, sorbitol, mannitol, dulcitol, sucrose, 2,2,6,6-tetrakis(hydroxymethyl)cyclohexanol, diethanolamine, N-methyldiethanolamine, and triethanolamine. Examples of the amine compounds include ethylenediamine, tolylenediamine, diphenylmethanediamine, diethylenetriamine, monoethanolamine, 2-(2-aminoethylamino)ethanol, and monopropanolamine. Each of the active hydrogen group-containing compounds listed above may be used alone or in combination.

In addition, when reacting a mesogen group-containing compound having an active hydrogen group with an isocyanate compound, a urethane polymerization catalyst known to those skilled in the art may be used. Examples of such polymerization catalysts include organotin catalysts such as dibutyltin dilaurate and tin octoate, triethylenediamine and its derivatives, tertiary amine catalysts such as N-methylmorpholine, N,N,N',N'-tetramethylethylenediamine, N,N,N',N'-tetramethylhexamethylenediamine, 1,8-diazabicyclo[5,4,0]undecene-7 (DBU), bis(N,N-dimethylamino-2-ethyl)ether, and bis(2-dimethylaminoethyl)ether, carboxylate metal salt catalysts such as potassium acetate and potassium octoate, and imidazole catalysts. Among these, the use of triethylenediamine and its derivatives is preferred.

When a polyfunctional compound is incorporated into the liquid crystal polymer, the polyfunctional compound may be a compound having three or more functional groups that can react with active hydrogen groups or isocyanate groups. Specific examples include triols such as glycerin and trimethylolpropane; tetraols such as pentaerythritol; diamines such as ethylenediamine, hexamethylenediamine, 4,4'-diaminodiphenylmethane, m-phenylenediamine, and 3,3'-dichloro-4,4'diaminodiphenylmethane (MOCA); aminoalcohols such as diethanolamine, triethanolamine, and aminoethylethanolamine; and the above-mentioned trifunctional or higher isocyanate compounds.

The rubber composition according to the present invention contains at least a diene rubber and the liquid crystal polymer.

Examples of diene rubbers include natural rubber (NR), polyisoprene rubber (IR), polystyrene butadiene rubber (SBR), polybutadiene rubber (BR), chloroprene rubber (CR), and nitrile rubber (NBR). If necessary, those with modified ends (e.g., terminally modified BR, terminally modified SBR, etc.) or those modified to impart desired properties (e.g., modified NR) can also be suitably used. In addition, polybutadiene rubber (BR) synthesized using a cobalt (Co) catalyst, neodymium (Nd) catalyst, nickel (Ni) catalyst, titanium (Ti) catalyst, or lithium (Li) catalyst, as well as those synthesized using a polymerization catalyst composition containing a metallocene complex described in WO2007-129670 can also be used.

When considering the fuel economy of pneumatic tires, polystyrene butadiene rubbers having a styrene content of 10 to 40% by mass, a vinyl bond content of 10 to 70% by mass in the butadiene portion, and a cis content of 10% by mass or more are preferred, and polystyrene butadiene rubbers having a styrene content of 15 to 25% by mass, a vinyl bond content of 10 to 60% by mass in the butadiene portion, and a cis content of 20% by mass or more are particularly preferred. In addition, when the rubber composition according to the present invention is used as the tread rubber portion of a pneumatic tire, it is preferable to use non-oil-added polystyrene butadiene rubber rather than oil-added type.

In the rubber composition according to the present invention, the amount of the liquid crystal polymer is preferably designed to be 1 to 50 parts by mass when the total amount of the diene rubber is taken as 100 parts by mass, from the viewpoint of improving the balance between tan δ (-20°C) and tan δ (60°C) of the rubber composition. If the amount of the liquid crystal polymer in the rubber composition is less than 1 part by mass, it becomes difficult to improve the WET performance and fuel efficiency of the final product pneumatic tire in a well-balanced manner, and if the amount of the liquid crystal polymer in the rubber composition exceeds 50 parts by mass, the rubber properties may be impaired. In order to improve the WET performance and fuel efficiency in a well-balanced manner while maintaining the rubber properties of the final product pneumatic tire, the amount of the liquid crystal polymer is preferably 5 to 40 parts by mass, and more preferably 10 to 30 parts by mass, when the total amount of the diene rubber is taken as 100 parts by mass.

In addition to the diene rubber and liquid crystal polymer, the rubber composition of the present invention can contain various compounding agents such as carbon black, silica, silane coupling agents, antioxidants, zinc oxide, stearic acid, wax, oil and other softeners, and processing aids.

As the silica, wet silica or dry silica can be used, but it is particularly preferable to use wet silica whose main component is hydrated silicic acid.

A silane coupling agent that has reactive activity towards diene rubber is used. Examples of silane coupling agents that can be used in the present invention include sulfide silanes such as bis(3-triethoxysilylpropyl)tetrasulfide (e.g., "Si69" manufactured by Degussa), bis(3-triethoxysilylpropyl)disulfide (e.g., "Si75" manufactured by Degussa), bis(2-triethoxysilylethyl)tetrasulfide, bis(4-triethoxysilylbutyl)disulfide, bis(3-trimethoxysilylpropyl)tetrasulfide, and bis(2-trimethoxysilylethyl)disulfide; mercaptosilanes such as γ-mercaptopropyltrimethoxysilane, γ-mercaptopropyltriethoxysilane, mercaptopropylmethyldimethoxysilane, mercaptopropyldimethylmethoxysilane, and mercaptoethyltriethoxysilane; and protected mercaptosilanes such as 3-octanoylthio-1-propyltriethoxysilane and 3-propionylthiopropyltrimethoxysilane.

Carbon black can be the type commonly used in the rubber industry, such as SAF, ISAF, HAF, FEF, and GPF, as well as conductive carbon blacks such as acetylene black and ketjen black.

As the antiaging agent, antiaging agents commonly used for rubber, such as aromatic amine antiaging agents, amine-ketone antiaging agents, monophenol antiaging agents, bisphenol antiaging agents, polyphenol antiaging agents, dithiocarbamate antiaging agents, and thiourea antiaging agents, may be used alone or in appropriate mixtures.

After the process of mixing compounding agents other than the vulcanization compounding agents, the vulcanization compounding agents are further mixed and dispersed. The vulcanization compounding agents used in the process of mixing the vulcanization compounding agents include vulcanizing agents such as sulfur and organic peroxides, vulcanization accelerators, vulcanization acceleration assistants, vulcanization retarders, etc.

The sulfur used as a sulfur-based vulcanizing agent may be any ordinary rubber sulfur, such as powdered sulfur, precipitated sulfur, insoluble sulfur, or highly dispersible sulfur.

As the vulcanization accelerator, sulfenamide-based vulcanization accelerators, thiuram-based vulcanization accelerators, thiazole-based vulcanization accelerators, thiourea-based vulcanization accelerators, guanidine-based vulcanization accelerators, dithiocarbamate-based vulcanization accelerators, and other vulcanization accelerators commonly used for rubber vulcanization may be used alone or in appropriate mixtures.

The rubber composition according to the present invention is obtained by kneading diene rubber and liquid crystal polymer, and optionally carbon black, silica, silane coupling agent, vulcanization compounding agent, antioxidant, zinc oxide, stearic acid, softeners such as wax and oil, processing aids, etc., using a kneading machine such as a Banbury mixer, kneader, or roll that is normally used in the rubber industry.

The method of compounding each of the above components is not particularly limited, and may be any of the following: a method in which the compounding components other than the vulcanization compounding agents such as the sulfur-based vulcanizing agent and the vulcanization accelerator are premixed to form a master batch, and the remaining components are added and further kneaded; a method in which each component is added in any order and kneaded; or a method in which all components are added simultaneously and kneaded.

In the present invention, a vulcanized rubber in which the liquid crystal polymer is dispersed in the diene rubber can be produced by mixing a diene rubber with a liquid crystal polymer and then vulcanizing the resulting rubber composition. Such vulcanized rubber has excellent dispersibility of the liquid crystal polymer in the diene rubber. As a result, the balance between tan δ (-20°C) and tan δ (60°C) is excellent, and the rubber properties are also excellent. Therefore, the vulcanized rubber of the rubber composition according to the present invention is particularly useful, for example, as a tread component of a pneumatic tire.

Below, we will explain examples that specifically demonstrate the configuration and effects of the present invention. The evaluation items in the examples were measured as follows.

<Dynamic viscoelasticity measurement>
The liquid crystal polymers of Reference Examples 2 to 6 and Reference Example 1 were used as measurement samples. In addition, the rubber compositions of Examples 1 to 5 and Comparative Example 1, which contain the liquid crystal polymers of Reference Examples 2 to 6 and Reference Example 1, were vulcanized by heating at 160°C for 20 minutes, and molded into a predetermined shape to prepare measurement samples.
The liquid crystal polymer samples were measured using a rheometer HAAKE MARS 40 manufactured by Thermo Fisher Scientific at a temperature rise rate of 3° C./min from −30 to 65° C. to determine tan δ (−20° C.) and tan δ (60° C.).
The rubber composition samples were also measured using a viscoelasticity tester manufactured by GABO Corporation at a temperature rise rate of 2.5° C./min from −40 to 60° C. to determine tan δ (−20° C.) and tan δ (60° C.).
The measurement conditions are as follows.
Liquid crystal polymer sample Measurement sample size: φ20×1mm
Measurement mode: Shear mode Measurement temperature: -30 to 65°C
Heating rate: 3°C/min Frequency: 10Hz
Dynamic strain: 0.02%
Rubber composition sample Measurement sample size: length 50 mm, width 3 mm, thickness 2 mm
Measurement mode: Tensile mode Measurement temperature: -40 to 60°C
Heating rate: 2.5°C/min Frequency: 10Hz
Dynamic strain: 0.15%

(Production of mesogen diol)
BH6 (124 g) as a mesogen group-containing compound having an active hydrogen group, potassium hydroxide (2 g), and toluene (500 ml) as a solvent were mixed in a reaction vessel, and propylene oxide was added as an alkylene oxide in an amount of 20 equivalents per mole of BH6, and the mixture was reacted under pressure at 120° C. for 3 hours (addition reaction). Next, oxalic acid (10 g) was added to the reaction vessel to terminate the addition reaction, and the insoluble salt in the reaction solution was removed by suction filtration. Furthermore, the toluene in the reaction solution was removed by reduced pressure distillation to produce the following compound (also referred to as "BH6POn") in which propylene oxide was added to BH6.
In the above formula, n is 7.5.

BH6 (400 g) as a mesogen group-containing compound having an active hydrogen group, potassium hydroxide (22.97 g), and toluene (2400 ml) as a solvent were mixed in a reaction vessel, and 1,2-butylene oxide was added as an alkylene oxide in an amount of 14 equivalents per mole of BH6, and the mixture was reacted at 120° C. under pressure for 3 hours (addition reaction). Next, oxalic acid (18.43 g) was added to the reaction vessel to terminate the addition reaction, and the insoluble salts in the reaction solution were removed by suction filtration. Furthermore, the toluene in the reaction solution was removed by reduced pressure distillation to produce the following BH6BOn in which 1,2-butylene oxide was added to BH6.
In the above formula, n is 4.9.

BH6 (65 g) as a mesogen group-containing compound having an active hydrogen group, potassium hydroxide (2.98 g), and xylene (312 ml) as a solvent were mixed in a reaction vessel, and 10.6 equivalents of 1,2-epoxyhexane were added as an alkylene oxide per mole of BH6, and the mixture was reacted at 140° C. under pressure for 3 hours (addition reaction). Next, oxalic acid (6 g) was added to the reaction vessel to terminate the addition reaction, and the insoluble salts in the reaction solution were removed by suction filtration. Furthermore, the xylene in the reaction solution was removed by reduced pressure distillation to produce the following BH6HOn in which 1,2-epoxyhexane was added to BH6.
In the above formula, n is 3.5.

BH6 (60 g) as a mesogen group-containing compound having an active hydrogen group, cesium hydroxide (15 g), and xylene (378 ml) as a solvent were mixed in a reaction vessel, and further, 27 equivalents of isobutylene oxide per mole of BH6 were added as an alkylene oxide, and the mixture was reacted at 140° C. under pressure for 4 hours (addition reaction). Next, oxalic acid (8 g) was added to the reaction vessel to terminate the addition reaction, and the insoluble salt in the reaction solution was removed by suction filtration. Furthermore, the xylene in the reaction solution was removed by reduced pressure distillation to produce the following BH6iBOn in which isobutylene oxide was added to BH6.
In the above formula, n is 5.

(Production of Liquid Crystal Polymers of Reference Examples 2 to 6 and Reference Example 1)
1,5-pentamethylene diisocyanate (manufactured by Tokyo Chemical Industry Co., Ltd.) was added as an isocyanate compound to 100 parts by mass of BH6POn, BH6BOn, BH6HOn, or BH6iBOn in the blending ratios shown in Table 1. These were mixed at 80°C with stirring to produce the liquid crystal polymers of Reference Examples 2 to 6 and Reference Example 1.

(Production of Rubber Compositions of Examples 1 to 5 and Comparative Example 1)
The liquid crystal polymers of Reference Examples 2 to 6 and Reference Example 1 were mixed with diene rubber (SBR, product name: SL563, manufactured by JSR Corporation) using a lab mixer (product name: Labo Plastomill, manufactured by Toyo Seiki Seisakusho Co., Ltd.). The mixing procedure was as follows: first, silica (product name: Nipsil AQ, manufactured by Tosoh Silica Corporation), a silane coupling agent (bis(3-triethoxysilylpropyl)tetrasulfide, product name: Si69, manufactured by Evonik Degussa Co., Ltd.), zinc oxide (product name: zinc oxide type 1, manufactured by Mitsui Mining & Smelting Co., Ltd.), stearic acid (product name: Lunac S-20, manufactured by Kao Corporation), and an anti-aging agent (product name: Nocrac 6C, manufactured by Ouchi Shinko Chemical Industry Co., Ltd.) were mixed with the SBR in the mixing ratios shown in Table 2 in the first step. ) was added and kneaded at 160°C, then in the second step, the liquid crystal polymers of Reference Examples 2 to 6 and Reference Example 1 were added to the kneaded mixture and kneaded at 160°C, and in the third step, sulfur (rubber powder sulfur 150 mesh, manufactured by Hosoi Chemical Industry Co., Ltd.) and a vulcanization accelerator (product names: Noccela CZ (primary vulcanization accelerator), Noccela D (secondary vulcanization accelerator), both manufactured by Ouchi Shinko Chemical Industry Co., Ltd.) were added to the kneaded mixture and kneaded at 90°C, and the obtained products were used as the rubber compositions of Examples 1 to 5 and Comparative Example 1. The rubber compositions of Examples 1 to 5 and Comparative Example 1 were vulcanized by heating at 160°C for 20 minutes, molded into a predetermined shape to prepare a measurement sample, and tan δ (-20°C)/tan δ (60°C) was evaluated.

The results in Table 2 show that, due to the incorporation of a liquid crystal polymer with a well-balanced improvement in tan δ (-20°C)/tan δ (60°C), the vulcanized rubber of the rubber composition of Examples 1 to 5 has a large increase in tan δ (-20°C), but because tan δ (-20°C)/tan δ (60°C) is very large, pneumatic tires containing such vulcanized rubber have excellent both WET performance and fuel economy.

Claims (4)

A rubber composition containing a diene rubber and a liquid crystal polymer,
The liquid crystal polymer has a weight average molecular weight of 40,000 or more,
A rubber composition characterized in that the ratio of tan δ (-20°C) when tan δ after vulcanization is measured at -20°C to tan δ (60°C) when tan δ is measured at 60°C (tan δ (-20°C)/tan δ (60°C)) is 2.6 or more. The rubber composition according to claim 1, wherein the liquid crystal polymer is a branched liquid crystal polymer having a branched structure. The rubber composition according to claim 1, wherein the liquid crystal polymer is a reaction product of a mesogen group-containing compound having at least an active hydrogen group and a branched structure, and an isocyanate compound. A pneumatic tire comprising a vulcanized rubber obtained using the rubber composition according to claim 1 as a raw material.