Short fiber-reinforced rubber composition for tires and method for producing the same

Abstract

To provide a short fiber reinforcement rubber composition suitable for tire use and its manufacturing method.SOLUTION: A manufacturing method of a short fiber reinforcement rubber composition for tires is characterized in that an ultrafine rubber composition is a rubber composition in which 0.1 to 20 pts.mass of ultrafine short fibers are added to a rubber matrix, wherein the ultrafine short fibers are synthetic fibers having a melting point of 160°C. or higher, a length of 0.1 to 5 mm and a cross-sectional diameter of 100 to 900 nm, and the ultrafine short fibers are dispersed. Furthermore, it is preferable that the ultrafine short fibers have an aspect ratio of 800 or more and 40,000 or less, the variation of the cross-sectional diameter of the ultrafine short fiber is 30CV% or less, and the ultrafine short fiber is a polyester-based polymer or polyamide-based polymer. And a sea-island composite fiber in which an island component is a fiber-forming polymer having a melting point of 160°C. or higher, and a sea component is a polymer compatible with rubber is cut into lengths of 5 mm or less, and 0.1 to 20 pts.mass are added to un-vulcanized rubber and kneaded to disperse the island components as nanofibers having a cross-sectional diameter of 100 to 900 nm.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a short fiber reinforced rubber composition for tires and a method for producing the same.

Background Art

[0002] Techniques for reinforcing rubber with short fibers to obtain a rubber composition suitable for tires are already known. For example, Patent Document 1 below proposes using a rubber composition containing 5 to 10 parts by weight of aramid short fibers with respect to 100 parts by weight of a rubber component for a pneumatic tire. However, the fiber diameter of the aramid fibers used was thick, and there were also problems with dispersibility.

[0003] Further, in Patent Document 2, by mixing aramid short fibers and aramid particles having a fiber length of 1 to 4 mm and a ratio of fiber length (L) to fiber diameter (D) (also referred to as aspect ratio, L / D) of 50 to 400, improvement in fuel efficiency of tires due to low heat generation of rubber and maintenance or improvement of wear resistance are proposed. However, the aramid fibers used had a fiber diameter of 15 μm (fiber length = 3 mm, L / D = 200), were thick, had poor dispersibility in rubber, and the effect of reducing fuel consumption was also insufficient.

[0004] In addition, in order to improve the rigidity of tires, etc., increasing the amount of carbon black compounded in rubber has also been considered. However, when a large amount of carbon black is compounded, not only does the viscosity of the unvulcanized rubber increase, resulting in poor processability and moldability, etc., but also the rubber flow becomes poor during tire vulcanization, causing problems such as air pockets. Also, attempts have been made to reduce fuel consumption by compounding silica as a filler to reduce the rolling resistance of tread rubber, but sufficient required characteristics have not yet been obtained.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0006] An object of the present invention is to solve the problems in such prior art and provide a short fiber-reinforced rubber composition suitable for tire applications and a method for producing the same.

MEANS FOR SOLVING THE PROBLEMS

[0007] The short fiber-reinforced rubber composition for tires of the present invention is a rubber composition in which 0.1 to 20 parts by mass of ultrafine short fibers are added to rubber having a matrix of ultrafine short fibers, and the ultrafine short fibers are synthetic fibers having a melting point of 160°C or higher, with a length of 0.1 to 5 mm and a cross-sectional diameter of 100 to 900 nm, and each ultrafine short fiber is dispersed.

[0008] Furthermore, it is preferable that the aspect ratio of the ultrafine short fibers is 800 or more and 40,000 or less, the variation in the cross-sectional diameter of the ultrafine short fibers is 30 CV% or less, the ultrafine short fibers are polyester-based polymers or polyamide-based polymers, and the ultrafine short fibers are arranged in one direction when formed into a sheet. Also, it is preferable that the stress at 100% elongation in the direction in which the ultrafine short fibers are arranged is 4.0 MPa or more, and the ratio (G” / G’) of the storage shear modulus (G”) to the loss shear modulus (G’) is 0.08 or less at 100°C.

[0009] Another method for producing the short fiber-reinforced rubber composition for tires of the present invention is to cut sea-island composite fibers, in which the island component is a fiber-forming polymer having a melting point of 160 °C or higher and the sea component is a polymer compatible with rubber, to a length of 5 mm or less and add 0.1 to 20 parts by mass to unvulcanized rubber and knead, thereby dispersing the island component as nanofibers having a cross-sectional diameter of 100 to 900 nm. Furthermore, it is preferable that the island component is a polyester-based polymer or a polyamide-based polymer, and the sea component is a polyolefin-based polymer. [Effect of the Invention]

[0010] According to the present invention, it is possible to provide a short fiber reinforced rubber composition suitable for tire applications and a method for producing the same. [Embodiments for Carrying Out the Invention]

[0011] The short fiber reinforced rubber composition for tires of the present invention is a rubber composition in which 0.1 to 20 parts by mass of ultra-fine short fibers are added to a matrix, and the ultra-fine short fibers are synthetic fibers having a melting point of 160°C or higher, a length of 0.1 to 5 mm, and a cross-sectional diameter of 100 to 900 nm, and each ultra-fine short fiber is dispersed.

[0012] And as the ultra-fine short fibers used in the short fiber reinforced rubber composition for tires of the present invention, it is essential that the fiber diameter is 100 to 900 nm, and more preferably, the fiber diameter is 400 to 700 nm. If the fiber diameter is less than 100 nm, a rubber composition with sufficient rigidity cannot be obtained, and if it exceeds 900 nm, the abrasion resistance of the rubber composition deteriorates.

[0013] And by using such nanofiber short fibers having a diameter of 900 nm or less, the composite of the short fibers and the matrix rubber exhibits the behavior of a close contact continuum as if there is no interface due to intermolecular forces (van der Waals forces).

[0014] Therefore, even with the addition of a small amount of short fibers, the finally obtained short fiber rubber composite exhibits a high stress against deformation and has high rigidity. Also, tanδ is small from room temperature to 100°C, and the energy loss due to the conversion of kinetic energy during running into thermal energy is small, exhibiting an excellent effect of low fuel consumption.

[0015] Furthermore, it is preferable that such nanofiber short fibers have fine irregularities, and they are more excellent in low friction properties. The tire finally obtained from the rubber composition of the present invention has a moderately small friction coefficient, excellent abrasion resistance, and more excellent low fuel consumption properties.

[0016] Furthermore, in the present invention, since short fibers for reinforcing nanofibers are used, the adhesion between the rubber and the fibers is high, and there is an effect that an RFL (resorcinol-formalin-latex) adhesive usually used to improve adhesion can be omitted. By omitting the treatment agent and drying process to be used, there is also an effect of reducing the generation of harmful substances and energy loss.

[0017] In addition, the CV% value representing the variation in the diameter of the ultrafine short fibers is preferably 30 CV% or less. Here, "CV%" is a numerical value obtained by dividing the standard deviation of the diameter by the average value. Furthermore, it is preferably 0 to 25 CV%, particularly preferably 0 to 15 CV%. A low CV value means less variation in fineness, making it easier to mix more homogeneously and making it less likely for the ultrafine short fibers to entangle with each other.

[0018] In addition, the length of the ultrafine short fibers must be in the range of 0.1 to 5 mm, more preferably 0.1 to 3.0 mm, and even more preferably in the range of 0.5 to 2.0 mm. When the fiber length is less than 0.1 mm, a rubber composition with sufficient rigidity cannot be obtained, and when it becomes too long, the processability and moldability of the rubber tend to deteriorate. Also, like the fiber diameter, it is preferable that the length of the ultrafine short fibers has little variation.

[0019] The aspect ratio, which is the ratio of the length to the diameter of the ultrafine short fibers, is preferably 800 or more and 40,000 or less. Furthermore, it is preferably in the range of 1000 to 20,000, particularly preferably in the range of 1200 to 8000.

[0020] By being fibrous rather than granular in this way and further having a preferable aspect ratio, an increase in the viscosity of the unvulcanized rubber is suppressed, and the processability and moldability are improved. Also, the flow of the rubber during vulcanization is good, and air pockets and the like are less likely to occur. Also, for this reason, it becomes easy to increase the addition amount.

[0021] The ultra-fine short fibers used in the present invention need to be synthetic fibers with a melting point of 160°C or higher. If the melting point is too low, the fibers will soften or dissolve during molding with rubber, reducing the effect of the reinforcing fibers. In addition, it is difficult to uniformly disperse the fibers inside the rubber molded product, and the physical properties of the resulting short fiber-reinforced rubber composition tend to deteriorate. The melting point of the polymer constituting the ultra-fine short fibers is preferably further in the range of 200 to 400°C. Synthetic fibers having such a melting point can be made into ultra-fine short fibers having the above-mentioned uniform fiber diameter and length using dry spinning or the like.

[0022] More specific examples of the polymer constituting the synthetic fiber include, for example, polyester-based polymers, polyamide-based polymers, polyolefin-based polymers, etc., and particularly polyester-based polymers or polyamide-based polymers are preferred. These resin components can be used alone or in combination.

[0023] Preferred examples of the polyester-based polymer include polyethylene terephthalate, polyethylene naphthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, polycyclohexane terephthalate, and their copolymers. Preferred examples of the polyamide-based polymer include nylon 6, nylon 66, nylon 610, nylon 11, nylon 410, etc. In addition, as the polyolefin-based polymer, an isotactic polymer can be given as a preferred example. Other polymers such as polysulfone, polyimide, polyketones, and polyarylate are also preferably used. By using such polymers, short fibers with excellent dispersibility can be obtained.

[0024] Such ultrafine short fibers made of synthetic fibers have a lower specific gravity than conventional inorganic rubber additives such as various fillers like carbon black and silica, and are excellent in the reinforcing effect per weight ratio. In recent years, due to fuel efficiency improvement, reduction of tire weight has been demanded. However, the short fiber-reinforced rubber composition of the present invention can achieve weight reduction of the tire while maintaining rigidity. Further, by using it for the tread rubber of a tire, etc., reduction of rolling resistance and reduction of fuel consumption during automobile driving by the tire can be achieved.

[0025] The rubber component which is the matrix for adding such ultrafine short fibers is not particularly limited, but it is more preferable to use various diene rubbers generally used in tire rubber compositions. For example, natural rubber, isoprene rubber, styrene-butadiene copolymer rubber, butadiene rubber, styrene-isoprene copolymer rubber, butadiene-isoprene copolymer rubber, etc. may be mentioned, and these may be used alone or in combination of two or more. Particularly for tires, it is preferable that the rubber as the matrix contains at least one of natural rubber (NR) or styrene-butadiene rubber (SBR) as the main component.

[0026] Furthermore, carbon black, petroleum resin, and other compounding agents used in the conventional rubber industry described later, for example, oil, zinc oxide, stearic acid, various anti-aging agents, sulfur, vulcanization accelerators, etc. can be appropriately compounded.

[0027] ​More specifically, in addition to the above-mentioned main rubber component and short fibers for reinforcing rubber for tires, components and additives generally used in the production of rubber compositions for tires can be blended and added in the amounts and formulations normally used as necessary. Specific examples of the components and additives include, for example, process oils (paraffin-based process oils, naphthene-based process oils, aromatic-based process oils), vulcanizing agents (sulfur, sulfur chloride compounds, organic sulfur compounds, etc.), vulcanization accelerators (guanidine-based, aldehyde-amine-based, aldehyde-ammonia-based, thiazole-based, sulfenamide-based, thiourea-based, thiuram-based, dithiocarbamate-based, zandtate-based compounds, etc.), crosslinking agents (organic peroxide compounds, radical generators such as azo compounds, oxime compounds, nitroso compounds, polyamine compounds, etc.), reinforcing agents (high styrene resin, phenol-formaldehyde resin, etc.), antioxidants or anti-aging agents (amine derivatives such as diphenylamine-based and p-phenylenediamine-based, quinoline derivatives, hydroquinone derivatives, monophenols, diphenols, thiobisphenols, hindered phenols, phosphite esters, etc.), waxes, stearic acid, zinc oxide, softening agents, silica, and other fillers, plasticizers, etc. As necessary, compounding agents commonly used in the normal rubber industry, such as fillers, coupling agents, softening agents, anti-aging agents, vulcanizing agents, vulcanization accelerators, and vulcanization accelerator aids, can be appropriately compounded.

[0028] In the short fiber-reinforced rubber composition for tires of the present invention, the content of the ultra-fine short fibers is 0.1 to 20 parts by mass with respect to 100 parts by mass of the rubber component, and more preferably, the addition amount is 1 to 14 parts by mass. If the content is too small, the effect of a rubber composition with sufficient rigidity cannot be obtained, and if it is too large, the processability and moldability of the rubber deteriorate.

[0029] In the present invention, it is important that each ultra-fine fiber is dispersed in the rubber composition. Being dispersed means that each ultra-fine fiber is not in a fiber bundle or aggregated state, but the ultra-fine fibers are individually distributed. This can be easily confirmed by an electron micrograph of the cross-section or the like. Furthermore, when the ultra-fine short fibers in the rubber composition are formed into a sheet shape, it is preferable that the ultra-fine short fibers are arranged in one direction.

[0030] Also, the stress at 100% elongation in the alignment direction of the ultra-fine short fibers of the short fiber-reinforced rubber composition of the present invention is preferably 4.0 MPa or more, and more preferably in the range of 4.5 to 20 MPa. Here, the stress at 100% elongation in the alignment direction is the value measured with a dumbbell-shaped No. 3 specimen prepared in accordance with JIS K6251 "Vulcanized Rubber and Thermoplastic Rubber - Method for Determining Tensile Properties".

[0031] Furthermore, in the short fiber-reinforced rubber composition for pneumatic tire rubber reinforcement of the present invention, the tensile stress at 10% elongation in the alignment direction is preferably 1.0 MPa or more, and the tensile stress at 50% elongation in the alignment direction is preferably 3.0 MPa or more. More preferably, the tensile stress at 10% elongation is in the range of 1.2 to 10 MPa, and the tensile stress at 50% elongation is in the range of 4 to 16 MPa. The tensile strength at break is preferably 10 MPa or more, and more preferably in the range of 12 to 30 MPa. By satisfying these numerical values for the tensile stress, the rigidity of the rubber composition for pneumatic tires in particular is high, and it becomes easier to reduce the weight of the tire.

[0032] Furthermore, in the short fiber-reinforced rubber composition for tires of the present invention, the ratio (G” / G’) of the storage shear modulus (G”) to the loss shear modulus (G’) is preferably 0.08 or less at 100°C and satisfies the following (1). [G” / G’ at 100°C] - [G” / G’ at 40°C] < 0.02 (1) Furthermore, it is preferable that G” / G’ (tanδ) is in the range of 0.05 to 0.08 at 100°C and the value of the above formula (1) is in the range of -0.01 to 0.01.

[0033] More specifically, as for the physical properties of the cured product (crosslinked body) of the rubber composition for tires of the present invention after crosslinking, it is preferable that the cured product has a low internal loss tangent (tanδ). Here, the loss tangent (tanδ) is obtained by dividing the loss elastic modulus (G”) by the storage elastic modulus (G’) (tanδ = G” / G’), and is expressed as the ratio of the energy dissipated (lost) as heat during one cycle of vibration to the maximum stored energy, and is a measure of energy loss. In other words, the value of this tanδ quantifies the index of the vibration energy applied to the rubber composition being dissipated as heat. Therefore, the smaller the tanδ, the less heat is dissipated (that is, the internal heat generation is reduced and the transmission efficiency is improved).

[0034] As the short fiber reinforced rubber composition for tires of the present invention, within the temperature range in which the tire normally runs (usually a temperature range of 30 to 100 °C), preferably, the value of tanδ at 100 °C is 0.08 or less, and by making the ratio of tanδ at 100 °C to tanδ at 40 °C less than 0.02, it becomes an index of a rubber composition with low internal heat generation and low energy loss (transmission loss).

[0035] The internal loss tangent (tanδ) value of this short fiber reinforced rubber composition for tires also varies depending on the short fibers used, and the case where polyester is used for the island component and polyethylene is used for the sea component is particularly preferable. In this case, it shows a low tanδ in the temperature range of 40 to 100 °C and shows a peak at 150 °C. This indicates that it is difficult to generate heat below 100 °C. When such a rubber with suppressed heat generation is used for a tire, the rolling resistance performance is increased and the fuel efficiency is improved. Such a short fiber reinforced rubber composition for tires of the present invention has high rigidity, excellent abrasion resistance, and can reduce the rolling resistance when used in tires, making it possible to achieve low fuel consumption.

[0036] Furthermore, as a short fiber-reinforced rubber composition for tires, it can be used in various parts of tires such as treads, sidewalls, inner sidewalls, breaker cushions, base treads, tie bars, bead apexes, clinch apexes, strip apexes or breaker edge strips. More specifically, using the short fiber-reinforced rubber composition for tires of the present invention by a normal method, that is, extruding the rubber composition into a shape such as a sidewall at the unvulcanized stage, molding it in a normal manner on a tire molding machine, bonding it together with other tire members to form an unvulcanized tire, and heating and pressurizing the unvulcanized tire in a vulcanizer, a pneumatic tire can be manufactured.

[0037] Such a short fiber-reinforced rubber composition for tires of the present invention can be obtained by a method for manufacturing a short fiber-reinforced rubber composition for tires, which is another invention of the present invention. That is, a sea-island composite fiber in which the island component is a fiber-forming polymer having a melting point of 160 °C or higher and the sea component is a polymer compatible with rubber is cut into lengths of 5 mm or less and added to the unvulcanized rubber in an amount of 0.1 to 20 parts by mass and kneaded, whereby the island component is dispersed as nanofibers having a cross-sectional diameter of 100 to 900 nm. In the kneading step, the sea component of the sea-island composite fiber is compatible with the rubber.

[0038] The sea-island composite fiber is composed of an island component polymer and a sea component polymer arranged so as to surround it in its fiber cross-section. However, in the manufacturing method of the present invention, in the step of compounding this sea-island type composite short fiber with rubber as a matrix, as a result of the separation of the island component and the sea component of the sea-island type composite short fiber, the island component polymer is dispersed as ultrafine short fibers in the rubber as a matrix.

[0039] Unlike the present invention, when ultrafine short fibers themselves are added to rubber and then kneaded, the ultrafine fibers Since its diameter is on the nanometer scale and it has a large specific surface area and is prone to aggregation, it is necessary to knead it in the form of sea-island composite fibers for uniform dispersion. By such a manufacturing method, especially as a short fiber-reinforced rubber composition for pneumatic tires, it has high rigidity, excellent abrasion resistance, and an excellent reinforcing effect and abrasion resistance effect with improved low fuel consumption.

[0040] Here, the sea-island composite fiber used in the manufacturing method of the present invention is such that the island component is a fiber-forming polymer with a melting point of 160 °C or higher, and the sea component is a polymer compatible with rubber. As the fiber-forming polymer of the island component, the ultrafine short fibers used in the short fiber-reinforced rubber composition for tires described above can be used. More specifically, for example, polyester-based polymers, polyamide-based polymers, polyolefin-based polymers, etc. can be mentioned. In particular, polyester-based polymers or polyamide-based polymers are preferred. These resin components can also be used singly or in combination.

[0041] More specifically, in the case of polyester-based, polyethylene terephthalate, polyethylene naphthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, polycyclohexane terephthalate, and their copolymers, etc. are preferred. In the case of polyamide-based polymers, nylon 6, nylon 66, nylon 610, nylon 11, nylon 410, etc. are preferred. Also, in the case of polyolefin-based, isotactic can be given as a preferred example. In addition, polysulfone, polyimide, polyketones, polyarylate, etc. can be mentioned.

[0042] As such sea-island composite fibers, it is particularly preferred that the island component is a polyester-based polymer or a polyamide-based polymer. The melting point of the polymer serving as the island component is preferably 160°C or higher, more preferably in the range of 200 to 400°C. If it is less than 160°C, the island component will soften or dissolve during molding, the strength of the reinforcing fibers will decrease, it will be difficult to uniformly disperse them inside the molded product, and the applicable range of the component serving as the matrix will be narrowed.

[0043] Examples of polymers compatible with the rubber that is the sea component of the sea-island composite fiber include polyester-based, aliphatic polyamide-based, polyethylene-based, polypropylene-based, polystyrene-based, polyacrylic-based, etc. These resin components can be used alone or in combination. Among them, polyolefin-based polymers such as polyethylene, polypropylene, ethylene-propylene-butene copolymer, maleic anhydride graft copolymerized polyolefin, maleic anhydride-acrylic ester block copolymerized polyolefin, etc., vinyl compounds such as polyvinyl alcohol and its ethylene copolymer, acid-modified vinyl copolymer, etc. are preferable. As the sea component of such sea-island composite fibers, it is particularly preferable that it is a polyolefin-based polymer.

[0044] Note that the melting point of the sea component polymer is lower than that of the island component polymer, and it is preferably selected to be preferably 20°C or more lower. In order to knead with rubber and use the ultrafine short fibers made of the island component polymer as reinforcing fibers, it is preferable that the island component polymer has a higher melting point than the sea component polymer. Furthermore, it is preferable that the melting point of the sea component is below the processing temperature of the manufacturing process for obtaining a rubber composition such as kneading, and the melting point of the island component is above that processing temperature. More specifically, it is preferable that the melting point of the sea component is in the range of 100 to 140°C, particularly 120 to 135°C.

[0045] From the viewpoints of fiber formation, sea-island cross-sectional formability, product moldability, product physical properties, etc., although it is easier to produce when the melt flow rate (MFR) of the island component and sea component resins is the same, there is no particular limitation. Also, a compatibilizer for suppressing interfacial delamination, a viscosity reducer for adjusting the melt viscosity, or a resin of a third component may be included according to the purpose.

[0046] The short fibers for rubber reinforcement of the pneumatic tire of the present invention preferably have an island component area ratio of 10 to 90% and a sea component area ratio of 90 to 10%, and preferably an island component area of 40 to 80% and a sea component area of 20 to 60%.

[0047] As the manufacturing method of the sea-island type composite fiber which is the fibril of the ultrafine short fiber used in the manufacturing method of the present invention like this, a known technique as described in International Publication Patent 2005 / 095686 can be applied. Also, it is preferable that the number of islands is 100 or more in terms of uniformly dispersing ultrafine short fibers with a smaller fiber diameter. The number of island components is preferably in the range of 180 to 900.

[0048] The length of the sea-island composite fiber at the time of kneading is 5 mm or less, and more preferably in the range of 0.1 to 5 mm. The cutting method of the short fibers for rubber reinforcement of the pneumatic tire is not particularly specified, and it may be guillotine cutting, rotary cutting, or pulverizing cutting. Guillotine cutting and rotary cutting, which preferably have a narrow fiber length distribution and good productivity, are preferred.

[0049] Such a manufacturing method of the invention is a manufacturing method in which, in the state of sea-island type composite short fibers having a length of 0.1 to 5 mm composed of an island component polymer and a sea component polymer arranged so as to surround it, it is added to unvulcanized rubber and kneaded, so that the island component and the sea component of the sea-island type composite short fibers are separated and compounded. As the unvulcanized rubber, the rubber matrix used in the above short fiber-reinforced rubber composition for tires can be used.

[0050] By using such a manufacturing method, the island component polymer of the sea-island type composite short fibers is uniformly dispersed as ultrafine short fibers in the rubber that is the matrix. In the conventional method of simply adding the ultrafine short fibers as they are to the rubber and then kneading, the ultrafine short fibers themselves with a large specific surface area at the nanometer scale tend to aggregate easily and do not disperse uniformly even after kneading. However, by using sea-island composite fibers in which the sea component is a polymer compatible with the rubber, it has become possible to disperse them uniformly. By adding in the state of sea-island composite fibers, an increase in the viscosity of the unvulcanized rubber is suppressed, and the processability, moldability, etc. are improved. Furthermore, there are effects such as an improvement in the rubber flow during vulcanization in tire manufacturing, etc., and a reduction in air pockets.

[0051] Even if it simply contains ultrafine fibers, although the improvement in rigidity, etc. is achieved, by finely and uniformly dispersing as in the present invention, especially as a short fiber-reinforced rubber composition for pneumatic tires, it has become excellent in rigidity, reinforcement effect, and abrasion resistance.

Examples

[0052] The present invention will be described with reference to examples. The evaluation was conducted by the following method.

[0053] (1) Tensile strength and elongation The obtained short fiber-reinforced rubber composition was adjusted to a thickness of 0.4 mm using a mixing roll to obtain an unvulcanized rubber sheet in which the fibers were aligned in the pulling direction of the mixing roll. Then, they were stacked with their directions aligned and press-vulcanized at 150 °C for 30 minutes to obtain a vulcanized rubber sheet with a thickness of 2 mm. Further, samples for tensile measurement in the fiber axis alignment direction and the anti-alignment direction were prepared by the method described in JIS K6251 "Vulcanized Rubber and Thermoplastic Rubber - Method for Determining Tensile Properties", cut out in the dumbbell No. 3 shape, and used as tensile test samples for tensile strength measurement.

[0054] The above test pieces cut out from the 3rd type dumbbell are mounted on an Instron type tensile testing machine equipped with an extensometer capable of directly measuring the elongation between scoring points. Then, a tensile test is conducted at a chuck distance of 50 mm and a tensile speed of 500 mm / min, and the tensile stress M10 (MPa) at 10% elongation, the tensile stress M50 (MPa) at 50% elongation, the tensile stress M100 (MPa) at 100% elongation, the tensile strength (fracture) TB (MPa), and the elongation at break EB (%) are measured.

[0055] (2) Coefficient of friction The obtained short fiber reinforced rubber composition was press crosslinked, and a block with a cross-sectional area of 42 mm 2 and a length of 10 mm was cut out as a sample. Using a pin-on-disk type friction and wear testing machine manufactured by Orientec Co., Ltd., the coefficient of friction was measured when rotating on a disk-shaped stainless steel plate under the conditions of a load of 2 kg and a peripheral speed of 100 rpm.

[0056] (3) Loss tangent (tanδ: G” / G’) Similar to the above friction and wear test, but a sample of a block with a width of 5 mm, a thickness of 2 mm, and a length of 20 mm was prepared. This was clamped in a chuck for compression measurement, and using a viscoelasticity measuring device manufactured by Orientec Co., Ltd., the loss modulus (G”) and the storage modulus (G’) of the short fiber reinforced rubber composition were measured under the conditions of a load of 200 gf (2.0 N), a dynamic strain of 10%, a frequency of 10 Hz, and measurement temperatures of 40°C and 260°C, and the loss tangent (tanδ = G” / G’) was obtained from the value of G” / G’.

[0057] (4) Rubber hardness The rubber hardness was measured at a constant load using a rubber hardness meter ISO-DD2 (Type D durometer).

[0058] (5) Variation in single fiber fineness (CV%) The sea component was removed from the sea-island type composite fiber using a solvent, and the fiber bundle composed of ultrafine fibers made of the obtained island component polymer was observed at a magnification of 30,000 times using a transmission electron microscope (TEM). The fineness of each single fiber was measured, and the standard deviation (σ) and average ultrafine fiber diameter (r) of this fineness were calculated, and the variation (CV%) was calculated by the following formula. CV% = (standard deviation σ / average fiber diameter r) × 100 Here, when the single fiber diameter of each fiber was not a perfect circle, the average value of the major axis and the minor axis of the measured single fiber was used.

[0059] [Example 1] First, as a sea-island type composite fiber, 836 islands of polyethylene terephthalate (PET, melting point 260 °C) with an island component fiber diameter of 400 nm, a sea component of high-density polyethylene (HDPE, melting point 130 °C), a sea-island area ratio of 50:50, and a fineness of 3.3 dtex were spun. At this time, the variation of the island component was 12.6 CV%. Subsequently, the obtained fiber was taken up in a long fiber state to form a fiber bundle, further water was added, cut into 1 mm with a guillotine cutter, and vacuum dried to remove moisture to obtain sea-island type composite short fibers.

[0060] Subsequently, 6.0 parts by mass of sea-island type composite short fibers with a fiber length of 1 mm were added to 100 parts by mass of unvulcanized rubber for tires mainly composed of natural rubber, and kneaded for 10 minutes with a pressure kneader until the unvulcanized rubber for tires reached 140 °C to obtain a rubber-fiber mixture. Since HDPE, which is the sea component of the sea-island type composite short fiber, melted during kneading, the rubber matrix contained 3.0 parts by mass of PET ultrafine short fibers, which are the island components of the sea-island type composite short fiber. The obtained rubber-fiber mixture was sheeted out to a thickness of 0.4 mm using a mixing roll to prepare a short fiber-reinforced rubber composition for tires. The aspect ratio of the ultrafine short fibers was 2500, and each ultrafine short fiber was dispersed into single fibers in the rubber matrix. The measurement results are shown in Table 1.

[0061] [Examples 2 to 4] The compounding amount at the time of adding fibers to be ultrafine short fibers for rubber reinforcement was changed (Example 2: 10 parts by mass, Example 3: 14 parts by mass, Example 4: 20 parts by mass with respect to 100 parts by mass of the rubber component), and a short fiber-reinforced rubber composition for tires was produced in the same manner as in Example 1 except for this. Each ultrafine short fiber was dispersed into single fibers in the rubber matrix, and the fiber addition amounts were 5 parts by mass in Example 2, 7 parts by mass in Example 3, and 10 parts by mass in Example 4. The measurement results were described together with Table 1.

[0062]

Table 1

[0063] [Example 5] A short fiber-reinforced rubber composition for tires was produced in the same manner as in Example 1 except that the fiber diameter of the ultrafine short fibers for rubber reinforcement was changed from 400 nm to 700 nm, the sea-island component ratio was changed, and the fiber compounding amount at the time of addition was changed from 6.0% to 4.3%.

[0064] The sea-island composite fiber used at this time was a sea-island type composite fiber with a fineness of 5.6 dtex, in which the island component was polyethylene terephthalate (PET) with a fiber diameter of 700 nm and 836 islands, the sea component was high-density polyethylene (HDPE), and the area ratio of the island component to the sea component was 70:30, and it was cut so that the fiber length became 1 mm. Also, at this time, the variation of the island component was 9.9 CV%.

[0065] Similar to Example 1, the HDPE of the sea component was melted during kneading, and 3.0 parts by mass of PET as the island component serving as short fibers for rubber reinforcement was added to the matrix rubber. The aspect ratio of the ultrafine short fibers was 1429, and each ultrafine short fiber was dispersed into single fibers in the rubber matrix. The measurement results were described in Table 2.

[0066] [Examples 6 to 8] The tire short fiber-reinforced rubber composition was prepared in the same manner as in Example 5, except that the compounding amount of the fiber to be an ultra-fine short fiber for rubber reinforcement was changed (Example 6: 7.1 parts by mass, Example 7: 10 parts by mass, Example 8: 14.3 parts by mass with respect to 100 parts by mass of the rubber component). Each ultra-fine short fiber was dispersed into single fibers in the rubber matrix, and the fiber addition amounts were 5 parts by mass in Example 6, 7 parts by mass in Example 3, and 10 parts by mass in Example 4. The measurement results are shown in Table 2 together.

[0067] [Table 2]

[0068] [Comparative Example 1] The tire short fiber-reinforced rubber composition was prepared in the same manner as in Example 1, except that short fibers for rubber reinforcement were not used. The kneading time until the unvulcanized rubber for tires in the pressure kneader reached 140°C was 10 minutes. The measurement results are shown in Table 3.

[0069] [Comparative Example 2] The tire short fiber-reinforced rubber composition was prepared in the same manner as in Example 1, except that short fibers obtained by cutting a PET fiber cord (P952NL manufactured by Teijin Frontier Co., Ltd., fineness 1670 dtex, diameter 20 μm) into a fiber length of 1 mm with a guillotine cutter and drying were used, and 3 parts by mass were compounded with respect to 100 parts by mass of the rubber component. The aspect ratio of the short fibers was 50, and although each short fiber was dispersed in the rubber matrix, the fiber addition amount remained 3 parts by mass. The measurement results are shown in Table 3 together.

[0070] [Comparative Example 3] A fiber cord obtained by imparting 7.0% by weight of an RFL adhesive to para-aramid fiber "Technora" (T323SB manufactured by Teijin Frontier Co., Ltd., fineness 1670 dtex, 1000 filaments, diameter 12 μm) was cut into a fiber length of 1 mm with a guillotine cutter and dried, and the resulting short fibers were used. The tire short fiber-reinforced rubber composition was prepared in the same manner as in Example 1, except that 3 parts by mass were compounded with respect to 100 parts by mass of the rubber component. Even after kneading, the short fibers were not sufficiently dispersed by the RFL adhesive and remained in a partially fiber bundle state. The measurement results are shown together in Table 3.

[0071]

Table 3

Claims

1. A method for producing a short fiber-reinforced rubber composition for tires, characterized in that a sea-island composite fiber, wherein the island component is a fiber-forming polymer having a melting point of 160°C or higher and the sea component is a polymer compatible with rubber, is cut to a length of 5 mm or less and 0.1 to 20 parts by mass thereof is added to unvulcanized rubber and kneaded, thereby dispersing the island component as nanofibers having a cross-sectional diameter of 100 to 900 nm.

2. The method for producing a short fiber-reinforced rubber composition for tires according to Claim 1, wherein the island component is a polyester-based polymer or a polyamide-based polymer.

3. The method for producing a short fiber-reinforced rubber composition for tires according to Claim 1 or 2, wherein the sea component is a polyolefin-based polymer.

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