BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a rubber composition, a production method therefor, and a method for evaluating the wear resistance of a rubber composition.

Description of the Background Art

A rubber composition used for a tire tread portion generally tends to provide better steering stability as the hardness thereof increases (see, for example, Japanese Laid-Open Patent Publication No. 2011-246527). In addition, it is known that as the hardness of the tire tread portion increases, the stiffness thereof increases, so that it is possible to make a turn along a curve even at a small steering angle, and a lateral force applied to the tire tread portion is reduced, resulting in improvement of the wear resistance.

However, the wear resistance does not necessarily improve when the hardness increases, and there are some exceptions. Hitherto, the wear phenomenon of rubber has been observed by various methods in order to unravel the mechanism for creation of such exceptions, but the mechanism has not fully been unraveled.

SUMMARY OF THE INVENTION

The present invention has been made in order to solve the above problem, and an object of the present invention is to provide a rubber composition having excellent wear resistance, a production method therefor, and a method by which the wear resistance of a rubber composition can be evaluated with high accuracy.

As a result of examination, the present inventors have found that a standard deviation σ_c of a maximum value Ξ_c of a correlation length Ξ_i of 10 nm to 100 µm obtained by performing curve-fitting with specific expressions on a scattering intensity curve obtained by performing X-ray scattering measurement or neutron scattering measurement in a specific region exhibits a high correlation with the wear resistance of a rubber composition, and the wear resistance tends to become more favorable as the standard deviation σ_c decreases. This is inferred to be because an ununiform network structure, in a polymer, represented by the correlation length Ξ_i serves to inhibit crack growth, and as the size of the network structure is more uniform, the number of locations on which stress is concentrated is reduced, and the crack growth inhibition effect is enhanced, so that the wear resistance improves.

As a result of further examination, the present inventors have found that particularly favorable wear resistance is achieved with a rubber composition which has a hardness of not less than 65 and in which a maximum value Ξ_c and a standard deviation σ_c satisfy a relationship of σ_c ≤ Ξ_c/2, and have arrived at the present invention. That is, the present invention is directed to a rubber composition in which a relationship between: a maximum value Ξ_c of a correlation length Ξ_i of 10 nm to 100 µm obtained by performing curve-fitting with the following (Expression 1-2) to (Expression 1-7) on a scattering intensity curve I(q) obtained by performing X-ray scattering measurement or neutron scattering measurement in a region of q represented by the following (Expression 1); and a standard deviation σ_c of the maximum value Ξ_c is σ_c ≤ Ξ_c/2, the rubber composition having a hardness of not less than 65.

[Math. 1] $$q=\frac{4\pi \sin \left(\theta /2\right)}{\lambda }$$ (θ: scattering angle, λ: a wavelength of X-rays or neutron rays)

[Math. 2] $$I_{\left(q\right)}=\int \frac{A}{1+q^2{\xi }^2}{g}_{a\left(r\right)}\mathit{dr}+{\displaystyle \sum _{i=1}^n}\int \frac{B_i}{{\left(1+q^2{\mathrm{\Xi }}_i^2\right)}^2}{g}_{\mathit{bi}\left(r\right)}\mathit{dr}$$ $$\xi <{\mathrm{\Xi }}_i$$ $$A=8\pi N_a{\rho }^2{\xi }^3$$ $$B_i=4\pi N_i{\rho }^2{\mathrm{\Xi }}_i^2$$ $$g_{a\left(r\right)}=\frac{1}{\sqrt{2\pi {\sigma }_a^2}}\mathit{exp}\left(-\frac{{\left(r-\xi \right)}^2}{2{\sigma }_a^2}\right)$$ $$g_{\mathit{bi}\left(r\right)}=\frac{1}{\sqrt{2\pi {\sigma }_i^2}}\mathit{exp}\left(-\frac{{\left(r-{\mathrm{\Xi }}_i\right)}^2}{2{\sigma }_i^2}\right)$$

• (A, B_i, ξ, Ξ_i: fitting parameters)
• (n: an integer)
• (q: the same as in (Expression 1))
• (N_a: the number of scatterers having a correlation length ξ per unit volume (number/cm³))
• (N_i: the number of scatterers having the correlation length Ξ_i per unit volume (number/cm³))
• (p: a scattering length density difference (cm^-2) or an electron density difference (electron cm^-3) between the scatterers and a deuterated solvent or air therearound)
• (ga(r): a probability density function having an average correlation length ξ and a standard deviation σ_a when a correlation length is assumed to be present in a Gaussian distribution)
• (gbi(r): a probability density function having an average correlation length Ξ_i and a standard deviation σ_i when a correlation length is assumed to be present in a Gaussian distribution)

A rubber component is preferably a polymer of one or more conjugated diene compounds.

The rubber composition is preferably used for a tire member.

Preferably, the X-ray scattering measurement is small-angle X-ray scattering measurement, and the neutron scattering measurement is small-angle neutron scattering measurement.

The X-ray scattering measurement or the neutron scattering measurement is preferably performed in a region of q that is not greater than 10 nm^-1.

The present invention is also directed to a production method for the rubber composition according to any one of claims 1 to 5, the production method comprising the step of kneading the rubber component, a sulfur donor, and a sulfur atom-containing vulcanization accelerator before the rubber component and a filler are kneaded, then adding the filler to an obtained kneaded product, and performing kneading at a kneading temperature of not lower than 120°C.

The present invention is also directed to a method for evaluating wear resistance of a rubber composition by X-ray scattering measurement or neutron scattering measurement, the method comprising evaluating the wear resistance of the rubber composition on the basis of a standard deviation σ_c of a maximum value Ξ_c of a correlation length Ξ_i of 10 nm to 100 µm obtained by performing curve-fitting with the above (Expression 1-2) to (Expression 1-7) on a scattering intensity curve I(q) obtained by performing the X-ray scattering measurement or the neutron scattering measurement in a region of q represented by the above (Expression 1).

According to the present invention, excellent wear resistance is achieved because of the rubber composition in which the relationship between: the maximum value Ξ_c of the correlation length Ξ_i of 10 nm to 100 µm obtained by performing curve-fitting with the above (Expression 1-2) to (Expression 1-7) on the scattering intensity curve I(q) obtained by performing X-ray scattering measurement or neutron scattering measurement in a region of q represented by the above (Expression 1); and the standard deviation σ_c of the maximum value Ξ_c is σ_c ≤ Ξ_c/2, the rubber composition having a hardness of not less than 65.

In addition, it is possible to evaluate the wear resistance of a rubber composition with high accuracy by using σ_c as an index for the wear resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a scattering intensity curve obtained by SANS measurement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a rubber composition in which a relationship between: a maximum value Ξ_c of a correlation length Ξ_i of 10 nm to 100 µm obtained by performing curve-fitting with the above (Expression 1-2) to (Expression 1-7) on a scattering intensity curve I(q) obtained by performing X-ray scattering measurement or neutron scattering measurement in a region of q represented by the above (Expression 1); and a standard deviation σ_c of the maximum value Ξ_c is σ_c ≤ Ξ_c/2, the rubber composition having a hardness of not less than 65.

As the X-ray scattering measurement, small-angle X-ray scattering (SAXS (scattering angle: normally 10 degrees or less) measurement in which a rubber composition is irradiated with X-rays and a scattering intensity is measured, can suitably be adopted. In the small-angle X-ray scattering, of X-rays that are applied to a substance and scattered, the X-rays having a small scattering angle are measured, whereby structure information of the substance is obtained, so that an ordered structure such as the microphase-separated structure of a polymeric material can be analyzed at a level of several nanometers.

In order to obtain detailed molecular structure information from the SAXS measurement, it is desirable that an X-ray scattering profile with a high S/N ratio be measurable. Thus, X-rays emitted from a synchrotron preferably have a brightness of not less than at least 10¹⁰ (photons/s/mrad²/mm²/0.1% bw). The "bw" represents the band width of the X-rays emitted from the synchrotron. Examples of such a synchrotron include the beamlines BL03XU and BL20XU at the large-scale synchrotron radiation facility SPring-8 of the Japan Synchrotron Radiation Research Institute, which is an incorporated foundation.

The brightness (photons/s/mrad²/mm²/0.1% bw) of the X-rays is preferably not less than 10¹⁰ and more preferably not less than 10¹². The upper limit thereof is not particularly limited, but the intensity of the X-rays is preferably not greater than the intensity that does not cause radiation damage.

Also, the number of photons (photons/s) in the X-rays is preferably not less than 10⁷ and more preferably not less than 10⁹. The upper limit thereof is not particularly limited, but the intensity of the X-rays is preferably not greater than the intensity that does not cause radiation damage.

As the neutron scattering measurement, small-angle neutron scattering (SANS (scattering angle: normally 10 degrees or less)) measurement in which a rubber composition is irradiated with neutron rays to measure the scattering intensity, can be suitably adopted. In the small-angle neutron scattering, of neutron rays that are applied to a substance and scattered, the neutron rays having a small scattering angle are measured, structure information of the substance is obtained, so that an ordered structure such as the microphase-separated structure of a polymeric material can be analyzed at a level of several nanometers.

In the SANS measurement, known methods that use magnetic structures or deuteration techniques can be used. In the case where a deuteration technique is used, for example, a polymeric material is swollen in a deuterated solvent, and the polymeric material in equilibrium in the deuterated solvent is then irradiated with neutron rays to measure the scattering intensity. Here, examples of deuterated solvents used to swell the polymeric material include heavy water, deuterated hexane, deuterated toluene, deuterated chloroform, deuterated methanol, deuterated DMSO ((D₃C)₂S=O), deuterated tetrahydrofuran, deuterated acetonitrile, deuterated dichloromethane, deuterated benzene, and deuterated N,N-dimethylformamide.

The neutron rays used for neutron scattering measurement such as SANS may be obtained by using, for example, the beamline SANS-J at JRR-3 of the Japan Atomic Energy Agency, which is an independent administrative agency, etc.

Similarly to the SAXS measurement, from the standpoint that a neutron scattering profile with a high S/N ratio is obtained, the neutron flux intensity of the neutron rays (neutrons/cm²/s) is preferably not less than 10³ and more preferably not less than 10⁴. The upper limit thereof is not particularly limited, but the neutron flux intensity is preferably not greater than the neutron flux intensity that does not cause radiation damage.

The X-ray or neutron scattering measurement is performed in a region of q represented by the following (Expression 1). Regarding a region of q (nm^-1), information about a more detailed molecular structure is obtained as the numeric value increases. Thus, the X-ray or neutron scattering measurement is performed in a region of q that is not greater than 10 nm^-1 and more preferably in a region of q that is not greater than 20 nm^-1.

[Math. 3] $$q=\frac{4\pi \sin \left(\theta /2\right)}{\lambda }$$ (θ: scattering angle, λ: a wavelength of X-rays or neutron rays)

The scattered X-rays in the SAXS measurement are detected by an X-ray detector, and an image is then generated by an image processor or the like using X-ray detection data from the X-ray detector.

As the X-ray detectors, for example, a two-dimensional detectors (e.g., an X-ray film, a nuclear emulsion plate, an X-ray image pickup tube, an X-ray fluorescent amplifier tube, an X-ray image intensifier, an X-ray imaging plate, a CCD for X-rays, an amorphous body for X-rays, etc.) or a one-dimensional detector (a line sensor) can be used. The X-ray detector may be appropriately selected according to the type and the state of a polymeric material to be analyzed, and the like.

Any ordinary image processor that can generate an X-ray scattering image on the basis of X-ray detection data from the X-ray detector can be appropriately used.

The SANS measurement can also be performed on the basis of the same principle as in the SAXS measurement. The scattered neutron rays are detected by a neutron ray detector, and an image is then generated by an image processor or the like using neutron ray detection data from the neutron ray detector. Here, similarly to the above, any known two-dimensional detector or one-dimensional detector can be used as the neutron ray detector, and any known image processor that can generate a neutron ray scattering image can be used as the image processor. These devices may be appropriately selected.

Then, for the rubber composition, a correlation length Ξ_i of 10 nm to 100 µm, a maximum value Ξ_c thereof, and a standard deviation σ_c of the maximum value Ξ_c can be calculated by performing measurement such as the SAXS measurement or the SANS measurement and analyzing the obtained scattering intensity curve by the following method.

First, curve-fitting is performed with the following (Expression 1-2) to (Expression 1-7) on a scattering intensity curve I(q) obtained by the SAXS measurement or the SANS measurement as shown in FIG. 1, and fitting parameters are obtained by a least-squares method.

[Math. 4] $$I_{\left(q\right)}=\int \frac{A}{1+q^2{\xi }^2}{g}_{a\left(r\right)}\mathit{dr}+{\displaystyle \sum _{i=1}^n}\int \frac{B_i}{{\left(1+q^2{\mathrm{\Xi }}_i^2\right)}^2}{g}_{\mathit{bi}\left(r\right)}\mathit{dr}$$ $$\xi <{\mathrm{\Xi }}_i$$ $$A=8\pi N_a{\rho }^2{\xi }^3$$ $$B_i=4\pi N_i{\rho }^2{\mathrm{\Xi }}_i^2$$ $$g_{a\left(r\right)}=\frac{1}{\sqrt{2\pi {\sigma }_a^2}}\mathit{exp}\left(-\frac{{\left(r-\xi \right)}^2}{2{\sigma }_a^2}\right)$$ $$g_{\mathit{bi}\left(r\right)}=\frac{1}{\sqrt{2\pi {\sigma }_i^2}}\mathit{exp}\left(-\frac{{\left(r-{\mathrm{\Xi }}_i\right)}^2}{2{\sigma }_i^2}\right)$$

• (A, B_i, ξ, Ξ_i: fitting parameters)
• (n: an integer)
• (q: the same as in (Expression 1))
• (N_a: the number of scatterers having a correlation length ξ per unit volume (number/cm³))
• (N_i: the number of scatterers having the correlation length Ξ_i per unit volume (number/cm³))
• (p: a scattering length density difference (cm^-2) or an electron density difference (electron cm^-3) between the scatterers and a deuterated solvent or air therearound)
• (ga(r): a probability density function having an average correlation length ξ and a standard deviation σ_a when a correlation length is assumed to be present in a Gaussian distribution)
• (gbi(r): a probability density function having an average correlation length Ξ_i and a standard deviation σ_i when a correlation length is assumed to be present in a Gaussian distribution)

Among the obtained fitting parameters, the correlation length ξ is presumed to correspond to the distance between crosslinking points in the polymer, and the correlation length Ξ_i is presumed to correspond to the size of an ununiform network structure in the polymer. As described above, the standard deviation σ_c of the maximum value Ξ_c of the correlation length Ξ_i has a high correlation with wear resistance, and the wear resistance becomes more favorable as σ_c decreases. Thus, it is possible to evaluate the wear resistance with high accuracy by using σ_c as an index for the wear resistance.

Regarding the above (Expression 1-7) representing the distribution of the correlation length Ξ_i, the Gaussian function is used on the assumption that Ξ_i has a normal distribution. However, a gamma function, a Weibull function, or the like may be used as another distribution function.

A rubber composition having excellent wear resistance is obtained by performing adjustment so as to satisfy a relationship of σ_c ≤ Ξ_c/2. For the reason that more favorable wear resistance is achieved, σ_c and Ξ_c preferably satisfy a relationship of σ_c ≤ Ξ_c/3 and more preferably satisfy a relationship of σ_c ≤ Ξ_c/4. In addition, Ξ_c and σ_c only need to satisfy the above relationship, but for the reason that more favorable wear resistance is achieved, Ξ_c is preferably 500 to 1800 nm and more preferably 600 to 1500 nm, and σ_c is preferably 50 to 500 nm and more preferably 100 to 300 nm.

Ξ_i, Ξ_c, and σ_c can be adjusted on the basis of the types and the blending amounts of chemicals such as a rubber component, a filler, and the like to be blended in the rubber composition.

In addition, Ξ_i, and σ_c can also be adjusted on the basis of a kneading method. For example, a rubber composition that satisfies the relationship of σ_c ≤ Ξ_c/2 can easily be prepared by a production method including a step of kneading the rubber component, a sulfur donor, and a sulfur atom-containing vulcanization accelerator before the rubber component and a filler are kneaded, then adding the filler to the obtained kneaded product, and performing kneading at a kneading temperature of not lower than 120°C.

By kneading the rubber component, the sulfur donor, and the sulfur atom-containing vulcanization accelerator before the rubber component and the filler are kneaded, absorption of the sulfur donor and the sulfur atom-containing vulcanization accelerator by the filler can be prevented, so that the sulfur donor and the sulfur atom-containing vulcanization accelerator can efficiently be dispersed in the rubber component. Then, by adding the filler to the kneaded product obtained by kneading the rubber component, the sulfur donor, and the sulfur atom-containing vulcanization accelerator and performing kneading at a kneading temperature of not lower than 120°C, active sulfur is released from the sulfur donor. The active sulfur, the sulfur atom-containing vulcanization accelerator, and the rubber component react with each other to obtain a state where the entirety or a part (hereinafter, "vulcanization accelerator residue") of the sulfur atom-containing vulcanization accelerator is bonded to the rubber component, that is, a state where a pendant type structure is formed in which "-S- vulcanization accelerator residue" is bonded to the rubber component. The mechanism of this reaction is inferred to be that the released active sulfur reacts with the sulfur atoms of the sulfur atom-containing vulcanization accelerator to form a structure in which two or more sulfur atoms are bonded, and the structure portion and the double-bond portion of the rubber component react with each other. By performing kneading in the state where the pendant-type structure is formed, the vulcanization accelerator residue moves together with the rubber component. Thus, the uniformity of the dispersion state of the vulcanization accelerator residue in the entire rubber composition improves, so that the crosslink density during vulcanization can be uniformized. The kneading temperature is the actual measured temperature of the kneaded product in a kneading machine and can be measured by a noncontact type temperature sensor or the like.

The production method is characterized in that: kneading of the rubber component, the sulfur donor, and the sulfur atom-containing vulcanization accelerator is started before the filler is kneaded; and kneading is performed at a kneading temperature of not lower than 120°C after the filler is added, as described above. As long as the above requirements are met, any of the materials may be added in any step. For example, in the case where the kneading step includes two steps composed of a step X and a step F, kneading of the rubber component, the sulfur donor, and the sulfur atom-containing vulcanization accelerator may be started at the initial stage of the step X, the filler may be added and kneading may be performed at a kneading temperature of not lower than 120°C in the middle of the step X, and then the step F may be performed. In addition, in the case where the kneading step includes three steps composed of a step X, a step Y, and a step F, kneading of the rubber component, the sulfur donor, and the sulfur atom-containing vulcanization accelerator may be started in the step X, the filler may be added and kneading may be performed at a kneading temperature of not lower than 120°C in the subsequent step Y, and then step F may be performed. Moreover, in another example of the case with three steps, kneading of the rubber component, the sulfur donor, and the sulfur atom-containing vulcanization accelerator may be started at the initial stage of the step X, the filler may be added and kneading may be performed at a kneading temperature of not lower than 120°C in the middle of the step X, and then step Y and the step F may be performed; or kneading of the rubber component, the sulfur donor, and the sulfur atom-containing vulcanization accelerator may be started at the initial stage of the step X, the filler may be added in the middle of the step X, the filler may be further added and kneading may be performed at a kneading temperature of not lower than 120°C in the subsequent step Y, and then the step F may be performed. Re-milling may be performed between the respective steps.

The kneading temperature at which the rubber component, the sulfur donor, and the sulfur atom-containing vulcanization accelerator are kneaded is not particularly limited. However, from the standpoint of inhibiting progress of a crosslinking reaction caused by the sulfur donor and the sulfur atom-containing vulcanization accelerator, the kneading temperature is preferably less than 160°C and more preferably not higher than 150°C. The lower limit thereof is not particularly limited, but the kneading temperature is preferably not lower than 60°C.

The kneading time for which the rubber component, the sulfur donor, and the sulfur atom-containing vulcanization accelerator are kneaded before the filler is added to the rubber component is not particularly limited. However, from the standpoint of improvement of the dispersibility of the sulfur donor and the sulfur atom-containing vulcanization accelerator, the kneading time is, for example, not shorter than 10 seconds. The upper limit thereof is not particularly limited, but the kneading time is preferably not longer than 8 minutes.

The kneading temperature after the filler is added only needs to be not lower than 120°C. However, from the standpoint of inhibiting excessive progress of a crosslinking reaction, the kneading temperature is preferably not lower than 170°C.

In addition, the kneading time after the filler is added to the rubber component and the kneading temperature reaches 120°C is not particularly limited. However, from the standpoint of improvement of the dispersibility of the sulfur donor and the sulfur atom-containing vulcanization accelerator, the kneading time is preferably not shorter than 2 minutes. The upper limit thereof is not particularly limited, but the kneading time is preferably not longer than 10 minutes. This kneading time is the time from the time point at which the filler is added to the rubber component and the kneading temperature reaches 120°C to the time point at which all the steps of the kneading step end. For example, when the filler is added to the rubber component and the kneading temperature reaches 120°C in the step X, this kneading time is the time from this time point to the time point at which the step F (final kneading step) ends.

As the rubber component, a polymer in which one or more conjugated diene compounds such as isoprene and butadiene are used, can be used. More specifically, as the rubber component, natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene butadiene rubber (SBR), acrylonitrile butadiene rubber (NBR), chloroprene rubber (CR), isobutylene-isoprene-rubber (IIR), halogenated isobutylene-isoprene-rubber (X-IIR), styreneisoprene-butadiene rubber (SIBR), or the like can be used. One of them may be used singly, or two or more of them may be used in combination. Among them, the SBR and the BR are preferable.

In addition, the SBR is preferably modified SBR in which the main chain thereof is modified by a nitrogen-containing compound, and at least one terminal thereof is modified by a terminal modifier. Examples of the nitrogen-containing compound include 3- or 4-(2-azetidinoethyl)styrene, 3- or 4-(2-pyrrolidinoethyl)styrene, 3- or 4-(2-piperidinoethyl)styrene, and 3- or 4-(2-hexamethyleneiminoethyl)styrene. Among them, 3- or 4-(2-pyrrolidinoethyl)styrene is preferable. Moreover, the terminal modifier is preferably a silicon-containing compound, and examples thereof include 3-(N,N-dimethylaminopropyl)trimethoxysilane, 3-(N,N-diethylaminopropyl)trimethoxysilane, 3-(N,N-dimethylaminopropyl)triethoxysilane, 3-(N,N-diethylaminopropyl)triethoxysilane, 3-glycidoxypropyltrimethoxysilane, 2-(4-pyridylethyl)triethoxysilane, N-(3-triethoxysilylpropyl)-4,5-dihydroimidazole, and silicon tetrachloride. Among them, 3-(N,N-dimethylaminopropyl)trimethoxysilane is preferable.

In the rubber composition of the present invention, the amount of the SBR in 100% by mass of the rubber component is preferably not less than 40% by mass and more preferably not less than 60% by mass, and is preferably not greater than 95% by mass and more preferably not greater than 85% by mass.

In the rubber composition of the present invention, the amount of the BR in 100% by mass of the rubber component is preferably not less than 5% by mass and more preferably not less than 15% by mass, and is preferably not greater than 60% by mass and more preferably not greater than 40% by mass.

As the filler, a reinforcing filler such as silica and carbon black which are generally used in the tire field can be used.

In the rubber composition of the present invention, the amount of silica with respect to 100 parts by mass of the rubber component is preferably not less than 30 parts by mass and more preferably not less than 60 parts by mass, and is preferably not greater than 120 parts by mass and more preferably not greater than 90 parts by mass.

In the rubber composition of the present invention, the amount of carbon black with respect to 100 parts by mass of the rubber component is preferably not less than 1 part by mass and more preferably not less than 3 parts by mass, and is preferably not greater than 20 parts by mass and more preferably not greater than 10 parts by mass.

The sulfur donor is elemental sulfur or a sulfur compound that releases active sulfur under a vulcanization condition (e.g., 150°C, 1.5 MPa) or at a temperature lower than 150°C and under a pressure lower than 1.5 MPa. In other words, the sulfur donor is a compound that generally functions as a vulcanizing agent under a vulcanization condition (e.g., 150°C, 1.5 MPa) or at a temperature lower than 150°C and under a pressure lower than 1.5 MPa. The released active sulfur forms a part of the above-described pendant type structure.

As the sulfur donor, elemental sulfur and/or the above-described sulfur compound that releases active sulfur can be used. Examples of the elemental sulfur include powdery sulfur, precipitated sulfur, colloidal sulfur, surface-treated sulfur, and insoluble sulfur.

When the elemental sulfur is blended in an excessive amount as the sulfur donor, a vulcanization reaction may excessively progress in the kneading step. Thus, in the rubber composition of the present invention, in the case where the elemental sulfur is used as the sulfur donor, the amount of the elemental sulfur to be added before the rubber component and the filler are kneaded, is preferably not greater than 0.1 parts by mass with respect to 100 parts by mass of the rubber component (the total amount of the rubber component to be used in all the steps). In addition, the amount is preferably not less than 0.05 parts by mass from the standpoint of breaking strength.

Examples of the sulfur compound that serves as the sulfur donor include a polymeric polysulfide represented by -(-M-S-C-)_n-, and a compound that has a structure -S_n- (n ≥ 2) in which two or more sulfur atoms are each bonded by single bond and that releases active sulfur. Examples of the compound include alkylphenol disulfide, morpholine disulfide, thiuram-based vulcanization accelerators having -S_n- (n ≥ 2) (tetramethylthiuram disulfide (TMTD), tetraethylthiuram disulfide (TETD), tetrabutylthiuram disulfide (TBTD), dipentamethylenethiuram tetrasulfide (DPTT), etc.), 2-(4'-morpholinodithio)benzothiazole (MDB), and polysulfide type silane coupling agents (e.g., Si69 manufactured by Evonik Industries Ag (bis(3-triethoxysilylpropyl)tetrasulfide)). One of them may be used singly, or two or more of them may be used in combination. Among them, thiuram-based vulcanization accelerators having -S_n- (n ≥ 2) are preferable, and dipentamethylenethiuram tetrasulfide (DPTT) is more preferable.

In the rubber composition of the present invention, in the case where the sulfur compound is used as the sulfur donor, the amount of the sulfur compound to be added before the rubber component and the filler are kneaded is preferably not less than 0.1 parts by mass and more preferably not less than 0.2 parts by mass, with respect to 100 parts by mass of the rubber component (the total amount of the rubber component to be used in all the steps), for the reason of accelerating formation of the pendant type structure. In addition, from the standpoint of inhibiting gelling during kneading, the amount is preferably not greater than 5 parts by mass, more preferably not greater than 3 parts by mass, and further preferably not greater than 2 parts by mass.

The sulfur atom-containing vulcanization accelerator refers to a vulcanization accelerator containing a sulfur atom that is bonded to another molecule by single bond. Examples of the sulfur atom-containing vulcanization accelerator include a sulfur atom-containing vulcanization accelerator that releases active sulfur, and a sulfur atom-containing vulcanization accelerator that does not release active sulfur. From the standpoint of inhibiting progress of a crosslinking reaction during kneading, a sulfur atom-containing vulcanization accelerator that does not release active sulfur (a sulfur non-releasing type sulfur atom-containing vulcanization accelerator) is preferable.

Among the sulfur atom-containing vulcanization accelerators, there are vulcanization accelerators that serve as a sulfur donor (e.g., a vulcanization accelerator containing a sulfur atom that is bonded to another molecule by single bond). Therefore, it is possible to form the pendant type structure also by blending one of the sulfur atom-containing vulcanization accelerators that serve as a sulfur donor, in a large amount, or by using two or more of the sulfur atom-containing vulcanization accelerators, in combination. However, when the sulfur atom-containing vulcanization accelerator that serves as a sulfur donor is blended in a large amount, a crosslinking reaction during kneading may excessively progress. When the sulfur atom-containing vulcanization accelerator that serves as a sulfur donor is blended in a small amount, it may be difficult to achieve the effect of uniformizing the crosslink density. Thus, the sulfur donor and the sulfur atom-containing vulcanization accelerator that are kneaded before the filler is added is preferably a combination of a sulfur donor (the sulfur atom-containing vulcanization accelerator that serves as a sulfur donor and/or a sulfur donor other than the sulfur atom-containing vulcanization accelerator) and the sulfur non-releasing type sulfur atom-containing vulcanization accelerator (a sulfur atom-containing vulcanization accelerator that does not serve as a sulfur donor).

The sulfur non-releasing type sulfur atom-containing vulcanization accelerator refers to, for example, a sulfur atom-containing vulcanization accelerator that does not release active sulfur under a vulcanization condition (e.g., 150°C, 1.5 MPa) or at a temperature lower than 150°C and under a pressure lower than 1.5 MPa. In other words, the sulfur non-releasing type sulfur atom-containing vulcanization accelerator is a sulfur atom-containing vulcanization accelerator that does not function as a vulcanizing agent under a vulcanization condition (e.g., 150°C, 1.5 MPa) or at a temperature lower than 150°C and under a pressure lower than 1.5 MPa.

Examples of the sulfur non-releasing type sulfur atom-containing vulcanization accelerator include thiazole-based vulcanization accelerators not having -S_n- (n ≥ 2) (2-mercaptobenzothiazole (MBT), the zinc salt of 2-mercaptobenzothiazole (ZnMBT), the cyclohexylamine salt of 2-mercaptobenzothiazole (CMBT), etc.), sulfenamide-based vulcanization accelerators not having -S_n- (n ≥ 2) (N-cyclohexyl-2-benzothiazolylsulfenamide (CBS), N-(tert-butyl)-2-benzothiazolesulfenamide (TBBS), N,N-dicyclohexyl-2-benzothiazolylsulfenamide, etc.), tetramethylthiuram monosulfide (TMTM), and dithiocarbamate-based vulcanization accelerators not having -S_n- (n ≥ 2) (piperidinium pentamethylenedithiocarbamate (PPDC), zinc dimethyldithiocarbamate (ZnMDC), zinc diethyldithiocarbamate (ZnEDC), zinc dibutyldithiocarbamate (ZnBDC), zinc N-ethyl-N-phenyldithiocarbamate (ZnEPDC), zinc N-pentamethylenedithiocarbamate (ZnPDC), sodium dibutyldithiocarbamate (NaBDC), copper dimethyldithiocarbamate (CuMDC), ferric dimethyldithiocarbamate (FeMDC), tellurium diethyldithiocarbamate (TeEDC), etc.). One of them may be used singly, or two or more of them may be used in combination. Among them, sulfenamide-based vulcanization accelerators not having -S_n- (n ≥ 2) are preferable, and N-(tert-butyl)-2-benzothiazolesulfenamide (TBBS) is more preferable. Di-2-benzothiazolyldisulfide (MBTS), which is a thiazole-based vulcanization accelerator, is a vulcanization accelerator that has -S_n- (n ≥ 2) and releases sulfur, but can be used as an equivalent to the sulfur non-releasing type sulfur atom-containing vulcanization accelerator, since di-2-benzothiazolyldisulfide (MBTS) does not function as a vulcanizing agent with respect to natural rubber or butadiene rubber when being blended in an ordinary amount.

In the rubber composition of the present invention, the amount of the sulfur atom-containing vulcanization accelerator to be added before the rubber component and the filler are kneaded is preferably not less than 1.0 part by mass and more preferably not less than 1.5 parts by mass, with respect to 100 parts by mass of the rubber component (the total amount of the rubber component to be used in all the steps), for the reason that a vulcanization reaction in a vulcanization step efficiently progresses. In addition, from the standpoint of inhibiting scorching or deposition on a surface, the amount is preferably not greater than 5 parts by mass and more preferably not greater than 3 parts by mass.

In the above production method, an additional sulfur donor may further be kneaded in a step other than the step before the rubber component and the filler are kneaded. By adding the additional sulfur donor, a crosslinking reaction can be caused to sufficiently progress during vulcanization while excessive progress of a crosslinking reaction during kneading is inhibited.

The additional sulfur donor is added, for example, at the latter stage of the step X or in the step Y in which the filler is added to the rubber component and kneading is performed at a kneading temperature of not lower than 120°C, or in the step F after the filler is added to the rubber component and kneading is performed at a kneading temperature of not lower than 120°C. The additional sulfur donor may be the same type as or different from the sulfur donor kneaded before the filler is added to the rubber component, and examples of the additional sulfur donor include powdery sulfur, precipitated sulfur, colloidal sulfur, surface-treated sulfur, and insoluble sulfur.

In the rubber composition of the present invention, the amount of the additional sulfur donor is not particularly limited. However, for the reason that a vulcanization reaction efficiently progresses in the vulcanization step, the amount of the additional sulfur donor with respect to 100 parts by mass of the rubber component (the total amount of the rubber component to be used in all the steps) is preferably not less than 0.5 parts by mass and more preferably not less than 0.8 parts by mass. For the reason that the wear resistance is excellent, the amount of the additional sulfur donor is preferably not greater than 3.0 parts by mass and more preferably not greater than 2.5 parts by mass.

In adding the additional sulfur donor, an additional vulcanization accelerator may be added. Examples of the additional vulcanization accelerator include: thiuram-based disulfides, polysulfides, and the like, which are sulfur atom-containing vulcanization accelerators; and guanidine-based, aldehyde-amine-based, aldehyde-ammonia-based, imidazoline-based vulcanization accelerators, which are vulcanization accelerators not having any sulfur atom.

In the rubber composition of the present invention, the amount of the additional vulcanization accelerator is not particularly limited. However, the amount of the additional vulcanization accelerator with respect to 100 parts by mass of the rubber component (the total amount of the rubber component to be used in all the steps) is preferably not less than 0.1 parts by mass and more preferably not less than 1 part by mass. In addition, the amount is preferably not greater than 5 parts by mass and more preferably not greater than 3 parts by mass.

The rubber composition (vulcanized rubber composition) of the present invention is obtained by vulcanizing an unvulcanized rubber composition obtained through the step F, by an ordinary method.

The rubber composition of the present invention has a hardness of not less than 65. For the reason that more favorable wear resistance is achieved, the hardness is preferably not less than 68 and more preferably not less than 70. The hardness is preferably not greater than 80. In the present invention, the hardness is a value obtained by measuring a vulcanized rubber composition by a method described in Examples.

Similarly to Ξ_i, Ξ_c, and σ_c, the hardness can be adjusted on the basis of the types and the blending amounts of the chemicals such as the rubber component, the filler, and the like to be blended in the rubber composition. In addition, the hardness can also be adjusted on the basis of the kneading method. For example, a rubber composition having a hardness of not less than 65 can easily be prepared by the above-described production method (a production method including a step of kneading the rubber component, a sulfur donor, and a sulfur atom-containing vulcanization accelerator before the rubber component and a filler are kneaded, then adding the filler to the obtained kneaded product, and performing kneading at a kneading temperature of not lower than 120°C).

The rubber composition of the present invention can suitably be used for a tire member such as a tread, since the rubber composition has excellent wear resistance. In the case of producing a pneumatic tire by using the rubber composition of the present invention, the rubber composition of the present invention may be extruded in an unvulcanized state into the shape of a tread or the like, shaped on a tire forming machine by an ordinary method, and attached together with other tire members to form an unvulcanized tire (green tire), and then the unvulcanized tire may be heated and pressurized in a vulcanizer.

EXAMPLES

The present invention will be described in detail by means of examples, but the present invention is not limited to the examples.

Various chemicals used in synthesis of a monomer (1) and SBR (1) are listed below.

• Cyclohexane: manufactured by Kanto Chemical Co., Inc.
• Pyrrolidine: manufactured by Kanto Chemical Co., Inc.
• Divinylbenzene: manufactured by Sigma-Aldrich Co. LLC.
• 1.6 M solution of n-butyllithium in hexane: manufactured by Kanto Chemical Co., Inc.
• Isopropanol: manufactured by Kanto Chemical Co., Inc.
• Styrene: manufactured by Kanto Chemical Co., Inc.
• Butadiene: manufactured by Takachiho Chemical Industrial Co., Ltd.
• Tetramethylethylenediamine: manufactured by Kanto Chemical Co., Inc.
• Modifier: 3-(N,N-dimethylaminopropyl)trimethoxysilane manufactured by AZmax. Co.
• 2,6-tert-butyl-p-cresol: manufactured by Ouchi Shinko Chemical Industrial Co., Ltd.
• Methanol: manufactured by Kanto Chemical Co., Inc.

(Synthesis of Monomer (1))

50 ml of cyclohexane, 4.1 ml of pyrrolidine, and 8.9 ml of divinylbenzene were added into a sufficiently nitrogen-purged 100-ml vessel. Then, 0.7 ml of the 1.6 M solution of n-butyllithium in hexane was added into the vessel at 0°C, and the mixture was stirred. After 1 hour, isopropanol was added thereinto to terminate the reaction, and extraction and purification were performed to obtain a monomer (1).

(Synthesis of SBR (1))

600 ml of cyclohexane, 12.6 ml of styrene, 71.0 ml of butadiene, 0.06 g of the monomer (1), and 0.11 ml of tetramethylethylenediamine were added into a sufficiently nitrogen-purged 1000-ml pressure-resistant vessel. Then, 0.2 ml of the 1.6 M solution of n-butyllithium in hexane was added into the vessel at 40°C, and the mixture was stirred. After 3 hours, 0.5 ml of the modifier was added into the vessel, and the mixture was stirred. After 1 hour, 3 ml of isopropanol was added thereinto to terminate the polymerization. After 1 g of 2,6-tert-butyl-p-cresol was added to the reaction solution, reprecipitation was performed with methanol, and the resultant precipitate was dried by heating to obtain SBR (1).

Various chemicals used in Example and Comparative Examples are listed below.

• SBR(1): synthesized by the above method (Mw: 300 thousand)
• SBR(2): SLR6430 (Mw: 1.2 million) manufactured by the Dow Chemical Company
• BR: BR150B (Mw: 400 thousand) manufactured by Ube Industries, Ltd.
• Vulcanization accelerator: Nocceler NS (N-(tert-butyl)-2-benzothiazolesulfenamide (TBBS) manufactured by Ouchi Shinko Chemical Industrial Co., Ltd.
• Sulfur donor: Nocceler TRA (dipentamethylenethiuram tetrasulfide (DPTT)) manufactured by Ouchi Shinko Chemical Industrial Co., Ltd.
• Silica: ULTRASIL VN3 manufactured by Evonik Industries Ag
• Silane coupling agent: Si266 manufactured by Evonik Industries Ag
• Carbon black: DIABLACK I manufactured by Mitsubishi Chemical Corporation
• Oil: Diana Process AH-24 manufactured by Idemitsu Kosan Co., Ltd.
• Zinc oxide: zinc oxide manufactured by Mitsui Mining & Smelting Co., Ltd.
• Stearic acid: stearic acid "Tsubaki" manufactured by NOF Corporation
• Antioxidant: OZONONE 6C (N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine) manufactured by Seiko Chemical Co., Ltd.
• Sulfur: powdery sulfur manufactured by Tsurumi Chemical Industry Co., Ltd.

(Example and Comparative Examples)

According to the composition shown in Table 1, various chemicals shown in Step X were kneaded in a 1.7-L Banbury mixer at a discharge temperature of 100°C for 5 minutes (step X). Next, the kneaded product in the step X and various chemicals shown in Step Y were kneaded in the 1.7-L Banbury mixer at a temperature of not lower than 140°C for 30 seconds, and were further kneaded at a discharge temperature of 150°C for 3 minutes (step Y). Then, the kneaded product in the step Y and various chemicals shown in Step F were kneaded at approximately 80°C for 3 minutes by using an open roll (step F) to obtain an unvulcanized rubber composition. The obtained unvulcanized rubber composition was extruded into the shape of a tread by using an extruder having a die with a predetermined shape, and attached together with other tire members to form an unvulcanized tire, and press vulcanization was performed under a condition of 170°C for 12 minutes, to produce a test tire (tire size: 195/65R15). The obtained test tire was evaluated as described below. The results are shown in Table 1.

1. SANS Measurement Method

A plate-like sample (molded product) having a thickness of about 1 mm was swollen to equilibrium in deuterated toluene, and mounted to a sample holder. The sample was then irradiated with neutron rays at room temperature. Absolute scattering intensity curves obtained by measuring the sample at distances of 2.5 m and 10 m from the sample to a detector and by measuring the sample with a focusing lens were combined by the least squares method. These three curves were combined by fixing the scattering intensity curve obtained by measuring the sample at a distance of 2.5 m from the sample to the detector, and shifting the scattering intensity curves obtained by measuring the sample at a distance 10 m from the sample to the detector and by measuring the sample with the focusing lens. Curve-fitting was performed on the obtained scattering intensity curve I(q) by using (Expression 1-2) to (Expression 1-7), a fitting parameter Ξ_i (a correlation length of 10 nm to 100 µm (the size of the ununiform network structure of the polymer)) was obtained by a least-squares method, and a maximum value Ξ_c of the correlation length Ξ_i and a standard deviation σ_c thereof were calculated.

• (SANS device)
• SANS: the SANS measurement device attached to the beamline SANS-J at JRR-3 of the Japan Atomic Energy Agency, which is an independent administrative agency.
• (Measurement Conditions)
• Wavelength of neutron rays: 6.5 Å
• Neutron flux intensity of neutron rays: 9.9 × 10⁷ neutrons/cm²/s
• Distance from sample to detector: 2.5 m and 10 m (further, in order to obtain information at smaller angles, measurement was performed at a distance of 10 m from the sample to the detector, using the focusing lens)
• (Detector)
• Two-dimensional detector (³He two-dimensional detector and two-dimensional photomultiplier + ZnS/⁶LiF detector)

2. Hardness

The hardness of a test piece cut out from the tread portion of the test tire was measured according to the test method of JIS K6253 by using a type A durometer. The thickness of the test piece was set to 8 mm, and the measurement temperature was set to 25°C.

3. Wear Resistance on Actual Vehicle

The above test tire was mounted to a front wheel drive vehicle produced in Japan. The groove depth of the tire tread portion was measured after running for 8000 km, and a running distance when the tire groove depth was reduced by 1 mm was calculated, and indexed by the following expression. The higher the index is, the more favorable the wear resistance is. $$\left(\mathrm{Running\; distance\; when\; tire\; groove\; of\; each\; composition\; was\; reduced\; by}1\mathrm{mm}\right)/\left(\mathrm{Running\; distance\; when\; tire\; groove\; of\; Comparative\; Example}1\mathrm{was\; reduced\; by}1\mathrm{mm}\right)\times 100$$

From Table 1, it is found that σ_c and the wear resistance have a high correlation, and the wear resistance becomes more favorable as σ_c decreases. In addition, it is found that particularly favorable wear resistance is achieved in the Example which satisfies a relationship of σ_c ≤ Ξ_c/2 and has a hardness of not less than 65.