Patent Description:
From the viewpoint of enhancing the safety of vehicles, studies have been made to enhance the braking performance and the driving performance of a tire not only on a dry road surface but on a wet road surface, a snowy or icy road surface, etc. as well. For example, a rubber composition has been disclosed (see, for example, patent document <NUM>) which can provide a tread rubber having a high braking performance on a dry road surface and also on a wet slippery road surface, such as a manhole surface which is more slippery than an asphalt road surface. The rubber composition comprises <NUM> pars by mass of a rubber component (A) comprising <NUM>% by mass or more of a natural rubber, <NUM> to <NUM> parts by mass of at least one thermoplastic resin (B) selected from a C<NUM>-based resin, a C<NUM>-C<NUM>-based resin, a C<NUM>-based resin, a terpene-based resin, a terpene-aromatic compound-based resin, a rosin-based resin, a dicyclopentadiene resin, and an alkylphenol-based resin, and <NUM> to <NUM> parts by mass of a filler (C) comprising silica, wherein the content of silica in the filler (C) is <NUM> to <NUM>% by mass.

Further, a rubber composition for a tire has been disclosed (see, for example, patent document <NUM>) which can enhance the grip performance of a tire on a dry road surface without impairing the grip performance on a wet road surface. The rubber composition comprises <NUM> parts by mass of a diene rubber component comprising <NUM> pars by mass or more of a styrene-butadiene rubber having an styrene content of <NUM> to <NUM>% by mass; a group of fillers consisting of <NUM> to <NUM> parts by mass of silica, <NUM> to <NUM> pars by mass of an inorganic agent represented by the general formula mM1•xSiOy•zH<NUM>O, and <NUM> to <NUM> pars by mass of carbon black, the total amount of the silica and the inorganic agent being <NUM> to <NUM> parts by mass, and the total amount of the silica, the inorganic agent and the carbon black being <NUM> to <NUM> parts by mass; a softener in an amount of <NUM>% by mass or more based on the total amount of the fillers; and <NUM> to <NUM> pars by mass of at least one resin having a softening point of <NUM> or less. Patent document <NUM> relates to a pneumatic tire, comprising a tread, the tread being formed from a rubber composition comprising a diene rubber. Patent document <NUM> relates to a tire, comprising a tread formed from a rubber composition. Patent document <NUM> relates to a tire rubber composition and a pneumatic tire using the tire rubber composition.

However, the rubber compositions disclosed in patent documents <NUM> and <NUM> are required to further improve the balance between an enhancement of wet grip performance and a reduction in rolling resistance.

It is therefore an object of the present invention to provide a tire having an excellent wet grip performance and a low rolling resistance, and a rubber composition which can be used to produce the tire and a vulcanized rubber having excellent handling properties, thereby solving the above problem.

A rubber composition according to the present invention comprises: a rubber component; a filler in an amount of <NUM> to <NUM> parts by mass per <NUM> parts by mass of the rubber component; and a hydrogenated resin in an amount of <NUM> to <NUM> parts by mass per <NUM> parts by mass of the rubber component, the resin having a softening point of higher than <NUM> and a weight average molecular weight, as determined in terms of polystyrene, of <NUM> to <NUM>/mol,.

In one or more embodiments, the content of the silica in the filler is <NUM>% by mass or more.

In one or more embodiments, the styrene-butadiene rubber has no modified group at the ends of the main chain.

A tire according to the present invention uses the rubber composition as set forth in the application.

According to the present invention, it is possible to provide a tire having an excellent wet grip performance and a low rolling resistance, and a rubber composition which can be used to produce the tire and a vulcanized rubber having excellent handling properties.

The rubber composition of the present invention comprises: a rubber component; a filler in an amount of <NUM> to <NUM> parts by mass per <NUM> parts by mass of the rubber component; and a hydrogenated resin in an amount of <NUM> to <NUM> parts by mass per <NUM> parts by mass of the rubber component, the resin having a softening point of higher than <NUM> and a weight average molecular weight, as determined in terms of polystyrene, of <NUM> to <NUM>/mol.

The "hydrogenated resin having a softening point of higher than <NUM> and a weight average molecular weight, as determined in terms of polystyrene, of <NUM> to <NUM>/mol" will be sometimes also referred to as "the hydrogenated resin of the present invention".

The conventional technique uses an inorganic filler, such as silica, to enhance wet grip performance. However, such a filler has the problem of increased energy loss. A method has been studied which involves blending of rubbers having different glass transition temperatures (Tg) to provide a tire tread rubber composition having an excellent balance between the wet grip performance and the low rolling resistance performance of a tire without impairing the wear resistance of the tire. However, such a rubber composition is insufficient in the low loss performance, the wear resistance, etc..

Further, various methods, including the use of a thermoplastic resin in an increased amount in a rubber composition, the use of an increased content of silica in a filler, and the use of a modified polymer for high dispersion of silica, have been studied in recent years. However, such methods have a limitation in solving the trade-off problem. There is also a problem of a decrease in the elastic modulus of a rubber especially in a technique which uses a large amount of a softener including a thermoplastic resin.

On the other hand, owing to the above-described features of the rubber composition of the present invention, a vulcanized rubber obtained from the composition has excellent handling properties, and a tire obtained from the composition has an excellent wet grip performance and a low rolling resistance.

Though the reasons for this are not fully elucidated yet, the following is conceivable.

The hydrogenated resin of the present invention has a low molecular weight, in particular, an average molecular weight, as determined in terms of polystyrene, of <NUM>/mol or less, and therefore is compatible with the rubber component. Furthermore, since the hydrogenated resin of the present invention has a high softening point, it will reinforce the rubber component and reduce the rolling resistance of a tire without significantly reducing or increasing the elastic modulus of a vulcanized rubber. In addition, because of the compatibility with the rubber component, the hydrogenated resin of the present invention can provide a tire with a flexibility necessary to grip a road surface, thereby enhancing the grip performance of the tire on a slippery road surface, namely the wet grip performance.

Further, the inclusion of the hydrogenated resin of the present invention in the rubber composition enables control of the elastic modulus (E') of a vulcanized rubber within a range which is not too high and not too low, leading to excellent handling properties of the vulcanized rubber.

The above may be the reasons why a tire, obtained from the rubber composition of the present invention, has an excellent wet grip performance and a low rolling resistance.

The rubber composition and the tire according to the present invention will now be described in detail.

The rubber composition of the present invention contains a rubber component. The rubber component comprises a styrene-butadiene rubber and a rubber having an isoprene skeleton.

Otherwise, there is no particular limitation on the rubber component. Examples of usable rubbers include natural rubbers (NRs), and synthetic rubbers such as a styrene-butadiene rubber (SBR), a butadiene rubber (BR), an isoprene rubber (IR), a chloroprene rubber, a butyl rubber (IIR), a butyl halide rubber, an ethylene-propylene rubber (EPM, EPDM), a fluorine-containing rubber, a silicone rubber, and a urethane rubber. Among them, a natural rubber (NR) and a diene rubber such as a styrene-butadiene rubber (SBR), a butadiene rubber (BR), or an isoprene rubber (IR) are preferably used.

An unmodified rubber is preferably used as the rubber component.

The rubber component preferably contains no modified styrene-butadiene rubber. A styrene-butadiene rubber preferably has no modified group at a molecular end(s) (especially at the ends of the main chain).

Either a single rubber or a mixture of two or more rubbers may be used as the rubber component.

The rubber component comprises a rubber having an isoprene skeleton from the viewpoint of increasing the mechanical strength, the wear resistance, the tear strength, etc. of a vulcanized rubber. Further, the rubber component comprises a styrene-butadiene rubber from the viewpoint of enhancing the wet grip performance of a tire and reducing the rolling resistance of the tire.

The inclusion of a rubber having an isoprene skeleton in the rubber component can increase the breaking strength of a vulcanized rubber. This makes it possible to reduce the rolling resistance of a tire. In addition, the vulcanized rubber is excellent in wear resistance, tear strength, etc..

A natural rubber (NR) and a polyisoprene rubber (IR) are examples of the rubber having an isoprene skeleton.

The content of the rubber having an isoprene skeleton in the rubber component is preferably <NUM> to <NUM>% by mass, more preferably <NUM> to <NUM>% by mass, and even more preferably <NUM> to <NUM>% by mass from the viewpoint of reducing the rolling resistance of a tire and increasing the wear resistance, the tear strength, etc. of a vulcanized rubber.

The rubber composition of the present invention contains a filler in an amount of <NUM> to <NUM> parts by mass per <NUM> parts by mass of the rubber component.

Reinforcement of a tire is insufficient when the content of the filler in the rubber composition is less than <NUM> parts by mass per <NUM> parts by mass of the rubber component, while when it exceeds <NUM> parts by mass, the rolling resistance of the tire cannot be reduced and the elastic modulus of the tire is too high, and therefore the wet grip performance of the tire is poor.

The content of the filler in the rubber composition is preferably <NUM> parts by mass or more, more preferably <NUM> parts by mass or more, and even more preferably <NUM> parts by mass or more per <NUM> parts by mass of the rubber component from the viewpoint of enhancing the wet grip performance. The content of the filler in the rubber composition is preferably <NUM> parts by mass or less, more preferably <NUM> parts by mass or less, and even more preferably <NUM> parts by mass or less per <NUM> parts by mass of the rubber component from the viewpoint of reducing the rolling resistance.

The filler comprises silica. Otherwise, there is no particular limitation on the filler. Examples of the filler include inorganic fillers such as silica, clay, talc, calcium carbonate, and aluminum hydroxide; and carbon black. Among them, silica and carbon black are preferred. The filler comprises at least silica.

Examples of the silica include wet method silica (hydrous silicic acid), dry method silica (anhydrous silicic acid), calcium silicate, and aluminum silicate. Among them, wet method silica is preferred. These silicas may be used singly or in a combination of two or more.

The silica preferably has a nitrogen adsorption specific surface area (BET method) of <NUM><NUM>/g or more and less than <NUM><NUM>/g.

When the nitrogen adsorption specific surface area (BET method) of the silica is <NUM><NUM>/g or more, a tire can be adequately reinforced and the rolling resistance can be reduced. When the nitrogen adsorption specific surface area (BET method) of the silica is less than <NUM><NUM>/g, the elastic modulus of a tire is not too high and the tire has an excellent wet grip performance.

The nitrogen adsorption specific surface area (BET method) of the silica is more preferably <NUM><NUM>/g or more, even more preferably <NUM><NUM>/g or more from the viewpoint of reducing the rolling resistance of a tire and increasing the wear resistance, the tear strength, etc. of the tire. The nitrogen adsorption specific surface area (BET method) of the silica is more preferably <NUM><NUM>/g or less, even more preferably <NUM><NUM>/g or less, and still more preferably <NUM><NUM>/g or less from the viewpoint of enhancing the wet grip performance.

The content of the silica in the filler is preferably <NUM>% by mass or more, more preferably <NUM>% by mass or more, and even more preferably <NUM>% by mass or more from the viewpoint of increasing the mechanical strength of a tire and reducing the rolling resistance.

The content of the silica in the rubber composition is preferably <NUM> parts by mass or more, more preferably <NUM> parts by mass or more, and even more preferably <NUM> parts by mass or more per <NUM> parts by mass of the rubber component from the viewpoint of increasing the mechanical strength of a tire and enhancing the wet grip performance. The content of the silica in the rubber composition is preferably <NUM> parts by mass or less, more preferably <NUM> parts by mass or less, and even more preferably <NUM> parts by mass or less per <NUM> parts by mass of the rubber component from the viewpoint of reducing the rolling resistance.

Preferably, the filler further contains carbon black. The carbon black is preferably contained in such an amount as to allow <NUM>% by mass or more of silica to be contained in the filler.

The carbon black reinforces a vulcanized rubber and increases the wear resistance of the vulcanized rubber.

There is no particular limitation on the carbon black. Examples of the carbon black include carbon blacks of the types GPF, FEF, HAF, ISAF, and SAF. These carbon blacks may be used singly or in a combination of two or more.

The content of the carbon black in the rubber composition is preferably <NUM> parts by mass or more, more preferably <NUM> parts by mass or more, and even more preferably <NUM> pars by mass or more per <NUM> parts by mass of the rubber component from the viewpoint of increasing the wear resistance of a vulcanized rubber. The content of the carbon black in the rubber composition is preferably <NUM> parts by mass or less, more preferably <NUM> parts by mass or less, and even more preferably <NUM> pars by mass or less per <NUM> parts by mass of the rubber component from the viewpoint of maintaining the low-loss performance of the vulcanized rubber.

The rubber composition contains the hydrogenated resin having a softening point of higher than <NUM> and a weight average molecular weight, as determined in terms of polystyrene, of <NUM> to <NUM>/mol. The content of the hydrogenated resin in the rubber composition is <NUM> to <NUM> parts by mass per <NUM> parts by mass of the rubber component.

If the softening point of the hydrogenated resin is <NUM> or less, the resin cannot improve the wet performance of a tire. The softening point of the hydrogenated resin is preferably <NUM> or more, more preferably <NUM> or more, even more preferably <NUM> or more, still more preferably <NUM> or more, yet more preferably <NUM> or more, yet more preferably <NUM> or more, yet more preferably <NUM> or more, and yet more preferably <NUM> or more from the viewpoint of improving the wet performance. The softening point of the hydrogenated resin is preferably <NUM> or less, more preferably <NUM> or less, even more preferably <NUM> or less, still more preferably <NUM> or less, yet more preferably <NUM> or less, and yet more preferably <NUM> or less from the viewpoint of reducing the increase in tan δ not only at <NUM> but also at <NUM> which lies in a rolling range.

If the weight average molecular weight of the hydrogenated resin, as determined in terms of polystyrene, is less than <NUM>/mol, the hydrogenated resin may precipitate from a tire, resulting in a failure to fully achieve the intended effect of the hydrogenated resin. If the weight average molecular weight exceeds <NUM>/mol, the hydrogenated resin will be incompatible with the rubber component.

The weight average molecular weight of the hydrogenated resin, as determined in terms of polystyrene, is preferably <NUM>/mol or more, more preferably <NUM>/mol or more, even more preferably <NUM>/mol or more, and still more preferably <NUM>/mol or more from the viewpoint of preventing precipitation of the hydrogenated resin from a tire and preventing degradation of the appearance of the tire. The weight average molecular weight of the hydrogenated resin, as determined in terms of polystyrene, is preferably <NUM>/mol or less, more preferably <NUM>/mol or less, even more preferably <NUM>/mol or less, still more preferably <NUM>/mol or less, yet more preferably <NUM>/mol or less, and yet more preferably <NUM>/mol or less from the viewpoint of enhancing the compatibility of the hydrogenated resin with the rubber component and enhancing the effect of the hydrogenated resin. The hydrogenated resin is a high-softening point hydrocarbon resin prepared by polymerizing a mixture of C5 and higher hydrocarbon fractions, obtained by steam cracking of propane and ethane, using a Friedel-Crafts catalyst.

If the content of the hydrogenated resin in the rubber composition is less than <NUM> parts by mass per <NUM> parts by mass of the rubber component, the hydrogenated resin cannot produce the intended effect. If the content of the hydrogenated resin exceeds <NUM> parts by mass, the hydrogenated resin may precipitate from a tire, resulting in a failure to fully achieve the intended effect of the hydrogenated resin.

The content of the hydrogenated resin in the rubber composition is preferably <NUM> parts by mass or more, more preferably <NUM> parts by mass or more, and even more preferably <NUM> parts by mass or more per <NUM> parts by mass of the rubber component from the viewpoint of enhancing the effect of the hydrogenated resin. The content of the hydrogenated resin in the rubber composition is preferably <NUM> parts by mass or less, more preferably <NUM> parts by mass or less, and even more preferably <NUM> parts by mass or less per <NUM> parts by mass of the rubber component from the viewpoint of preventing precipitation of the hydrogenated resin from a tire and preventing degradation of the appearance of the tire.

The hydrogenated resin refers to a resin obtained by reduction/hydrogenation of a resin.

Resins usable as a raw material for the hydrogenated resin include a C<NUM>-based resin, a C<NUM>-C<NUM>-based resin, a C<NUM>-based resin, a terpene-based resin, a dicyclopentadiene-based resin, a terpene-aromatic compound-based resin, etc. These resins may be used singly or in a combination of two or more.

The C<NUM>-based resin includes an aliphatic petroleum resin obtained by (co)polymerization of a C<NUM> fraction obtained by thermal cracking of naphtha in the petrochemical industry.

The C<NUM> fraction generally includes olefinic hydrocarbons such as <NUM>-pentene, <NUM>-pentene, <NUM>-methyl-<NUM>-butene, <NUM>-methyl-<NUM>-butene, and <NUM>-methyl-<NUM>-butene; and diolefinic hydrocarbons such as <NUM>-methyl-<NUM>,<NUM>-butadiene, <NUM>,<NUM>-pentadiene, <NUM>,<NUM>-pentadiene, and <NUM>-methyl-<NUM>,<NUM>-butadiene. A commercially available product may be used as the C<NUM>-based resin.

The C<NUM>-C<NUM>-based resin refers to a C<NUM>-C<NUM>-based synthetic petroleum resin, for example, a solid polymer obtained by polymerizing a petroleum-derived C<NUM>-C<NUM> fraction using a Friedel-Crafts catalyst such as AlCl<NUM> or BF<NUM>. A specific example of the C<NUM>-C<NUM>-based resin is a copolymer comprising styrene, vinyl toluene, α-methyl styrene, indene, or the like as a main component.

A resin having a minor amount of a C<NUM> or higher component is preferred as the C<NUM>-C<NUM>-based resin from the viewpoint of compatibility with the rubber component. The phrase "having a minor amount of a C<NUM> or higher component" herein means that the amount of a C<NUM> or higher component in the overall amount of the resin is less than <NUM>% by mass, preferably less than <NUM>% by mass. A commercially available product may be used as the C<NUM>-C<NUM>-based resin.

A resin (C<NUM>-DCPD-based resin), which is a copolymer of a C<NUM> fraction and dicyclopentadiene (DCPD), may be used as a raw material for the hydrogenated resin.

When the amount of a dicyclopentadiene-derived component in the overall amount of the resin is <NUM>% by mass or more, the C<NUM>-DCPD-based resin is herein regarded as being included in the dicyclopentadiene resin. When the amount of a dicyclopentadiene-derived component in the overall amount of the resin is less than <NUM>% by mass, the C<NUM>-DCPD-based resin is herein regarded as being included in the C<NUM>-based resin.

The hydrogenated resin is at least one selected from the group consisting of a hydrogenated C<NUM>-based resin, a hydrogenated C<NUM>-C<NUM>-based resin, and a hydrogenated dicyclopentadiene-based resin (hydrogenated DCPD-based resin), more preferably at least one selected from the group consisting of a hydrogenated C<NUM>-based resin and a hydrogenated C<NUM>-C<NUM>-based resin, and even more preferably a hydrogenated C<NUM>-based resin from the viewpoint of enhancing the compatibility between the rubber component and the hydrogenated resin, enhancing the wet grip performance, and improving the balance between the rolling resistance and the handling properties.

The rubber composition preferably further contains a silane coupling agent. A silane coupling agent commonly used in the rubber industry can be used as the silane coupling agent.

In addition to the rubber component, the filler and the hydrogenated resin described above, the rubber composition of the present invention may optionally contain various components commonly used in the rubber industry, such as a softener, a resin other than the hydrogenated resin of the present invention, a processability improver, stearic acid, an antioxidant, zinc oxide, a vulcanization accelerator, and a vulcanizing agent as long as the object of the present invention is not impaired.

There is no particular limitation on a method for producing the rubber composition of the present invention. For example, the rubber composition can be produced by mixing the rubber component, silica and the hydrogenated resin of the present invention, and optionally other selected components, and subjecting the mixture to kneading, warming, extrusion, etc..

The tire (pneumatic tire) of the present invention is obtained by using the rubber composition of the present invention.

As described above, a tire obtained from the composition of the present invention has an excellent wet grip performance and a low rolling resistance. Therefore, the composition of the present invention can be advantageously used to produce a tread portion of a tire.

The tire may be produced in different manners depending on the type and the members of the tire. In particular, the tire may be produced by subjecting an unvulcanized rubber composition to a building process, and vulcanizing the resulting product. Alternatively, the tire may be produced by subjecting an unvulcanized rubber composition to a preliminary vulcanization process to obtain a semi-vulcanized rubber, subjecting it to a building process, and then subjecting the resulting product to a main vulcanization process. Besides ordinary air or air with adjusted partial pressure of oxygen, an inert gas such as nitrogen, argon, or helium can be used as a gas to be filled into the tire.

The following examples illustrate the present invention in greater detail and are not intended to limit the scope of the invention.

The rubber compositions of Examples and Comparative Examples were prepared by mixing the components shown in Tables <NUM> and <NUM> according to the formulations shown in the tables using an ordinary Banbury mixer. The maximum temperature of the mixture was set to <NUM>. The following are details of the components shown in Tables <NUM> and <NUM>.

Oil: trade name "Diana Process NH-<NUM>", manufactured by Idemitsu Kosan Co.

The softening point (Ts) and the weight average molecular weight (Mw) of each resin as a softener are shown in Table <NUM>.

For a vulcanized rubber specimen obtained by vulcanizing each of the rubber compositions at <NUM> for <NUM> minutes, a loss tangent (tan δ) was measured using a viscoelasticity measuring device under the conditions of: a dynamic strain of <NUM>%; a frequency of <NUM>; and a measurement temperature of <NUM>.

The measured values of the Examples and Comparative Examples were expressed in terms of an index (RR index) with the tan δ of the vulcanized rubber specimen of Comparative Example <NUM> as <NUM>. A lower index value indicates a lower and thus better rolling resistance.

The measured values of the Examples and Comparative Examples were expressed in terms of an index (Wet index) with the resistance value of Comparative Example <NUM> as <NUM>. A higher index value indicates a higher resistance value, thus indicating a superior wet grip performance. The allowable range is <NUM> or more.

For a vulcanized rubber specimen obtained by vulcanizing each of the rubber compositions at <NUM> for <NUM> minutes, a storage elastic modulus (E') was measured using a viscoelasticity measuring device under the conditions of: a temperature of <NUM>; a dynamic strain of <NUM>%; and a frequency of <NUM>.

An E' index was calculated based on the following equation, taking the measured value of the vulcanized rubber of Comparative Example <NUM> as <NUM>.

The handling properties of a vulcanized rubber are not good if the E' index is too low or too high. The allowable range of the index value is <NUM> to <NUM>.

The results of the examples shown in Table <NUM> indicate that the vulcanized rubbers of the rubber compositions according to the present invention have excellent handling properties, and that the use of the rubber compositions can produce a tire having an excellent wet grip performance and a low rolling resistance.

Claim 1:
A rubber composition comprising:
a rubber component;
a filler in an amount of <NUM> to <NUM> parts by mass per <NUM> parts by mass of the rubber component; and
a hydrogenated resin in an amount of <NUM> to <NUM> parts by mass per <NUM> parts by mass of the rubber component, the resin having a softening point of higher than <NUM> and a weight average molecular weight, as determined in terms of polystyrene, of <NUM> to <NUM>/mol,
wherein the hydrogenated resin is at least one selected from the group consisting of a hydrogenated C<NUM>-based resin, a hydrogenated C<NUM>-C<NUM>-based resin, and a hydrogenated dicyclopentadiene-based resin,
wherein the rubber component comprises a styrene-butadiene rubber and a rubber having an isoprene skeleton, and
wherein the filler comprises silica.