The present invention relates to a cross-linkable rubber composition, a method of preparing a cross- linkable rubber composition, a cross-linked rubber composition obtained by cross-linking such a rubber composition, a method of preparing a tyre and a tyre. Tread rubber is one of the important portions of a pneumatic tyre which contributes enormously to the overall performance of a tyre. A tyre has to perform well in severe weather conditions and it has to exhibit a variety of performances such as wet grip, abrasion resistance and low rolling resistance.

It is well known in rubber compounding that there is a trade-off between wet grip and rolling resistance. The tread compound can be optimized to exhibit good wet grip by using high T _g polymers like SBR but it normally results in high rolling resistance. On the other hand, tuning the rubber compound by using low T _g polymers like BR to reduce rolling resistance possibly leads to impairment in wet grip. Several types of resins are therefore introduced to increase the wet grip properties but these often have a negative effect on the rolling resistance. In order to obtain high performance tyres, all properties need to be improved simultaneously. WO 2012/152686 Al describes a tyre that includes a tread, a crown with a crown reinforcement, first and second sidewalls, two beads, and a carcass reinforcement anchored to the two beads and extending from the first sidewall to the second sidewall. The tread includes a thermoplastic elastomer that is a block copolymer, which includes at least one elastomer block and at least one thermoplastic block. A total content of the thermoplastic elastomer in the tread is within a range varying from 65 to 100 phr (parts by weight per hundred parts of elastomer).

WO 2011/061145 describes a tyre, the tread of which comprises a rubber composition comprising at least one diene elastomer, such as SBR and/or BR, a reinforcing filler, such as silica and/or carbon black, and more than 10 phr, preferably more than 15 phr, of a hydrogenated styrene thermoplastic elastomer. This composition, which exhibits a reduced viscosity in the raw state, makes it possible to obtain treads of tires exhibiting a reduced rolling resistance, while retaining a good level of wet grip.

DE 10 2016 207396 Al concerns a sulfur cross-linkable rubber mixture, particularly for a vehicle tyre. The sulphur-rubber mixture contains at least a diene rubber, silica, a blocked and/or unblocked mercaptosilanes as a silane coupling agent and a thermoplastic elastomer of the TPE-S type (styrene triblock copolymers).

Optimizing the tread compound for wet grip normally results in trade-off in rolling resistance. The present invention has the object to provide a composition for a tyre tread which has improved wet grip and good rolling resistance, without compromising on other performances.

This object is achieved by a cross-linkable rubber composition according to claim 1, a method of preparing a cross-linkable rubber composition according to claim 11, a cross-linked rubber composition according to claim 12, a method for preparing a tyre according to claim 14 and a tyre according to claim 15. Advantageous embodiments are the subject of the dependent claims. They may be combined freely unless the context clearly indicates otherwise.

Accordingly, a cross-linkable rubber composition is provided which comprises, per hundred parts by weight of rubber (phr): > 1 phr to < 100 phr of a rubber component selected from the group of styrene-butadiene rubber (SBR), polybutadiene rubber (BR), natural rubber (NR) or a mixture thereof. The composition further comprises > 1 phr to < 50 phr of a thermoplastic elastomer (TPE) having has a glass transition temperature of > -30 °C to < 10 °C (measured by DSC, according to 1SO 22768), wherein the amount of the TPE present in the composition is not considered as rubber for the purpose of determining the amounts of the composition’s individual components based on their phr value. ft has surprisingly been found that such a rubber composition, having a thermoplastic elastomer (TPE) with a comparatively high glass transition temperature T _g as an additive worked very well in broadening the tan delta curve of the cross-linked compound, indicating that the compound can perform well in a wide range of conditions and used as a rubber for a tyre tread (determined from DMA measurement according to 1SO 4664-1), without deterioration of other compound properties.

Thermoplastic elastomers (also abbreviated as“TPEs”) have a structure intermediate between thermoplastic polymers and elastomers. These are block copolymers composed of rigid thermoplastic blocks connected via flexible elastomer blocks. The thermoplastic elastomer used for the implementation of the invention is a block copolymer, the chemical nature of the thermoplastic blocks and elastomer blocks of which can vary.

The number- average molecular weight (denoted Mn) of the TPE is preferably between 30 000 and 500 000 g/mol, more preferably between 40 000 and 400 000 g/mol, most preferably in a range of 50 000 to 300 000 g/mol The number-average molecular weight (Mn) of the TPE elastomer can be determined, in a known manner, by steric exclusion chromatography (SEC).

TPEs generally exhibit two glass transition temperature peaks, the lowest temperature being relative to the elastomer part of the TPE and the highest temperature being relative to the thermoplastic part of the TPE. Thus, the flexible blocks of the TPEs are defined by a T _g which is less than ambient temperature (25° C), while the rigid blocks have a T _g which is greater than 80° C. The glass transition temperatures discussed in connection with the TPEs refer to the lowest of the present T _g values, hence describing the T _g of the flexible, elastomer domains of the polymer. According to the invention the TPE has a glass transition temperature of > -30 °C to < 10 °C (measured by DSC, according to ISO 22768). This norm specifies a heating rate of 20 °C/min. Preferably the glass transition temperature T _g is > -20 °C to < -10 °C. Without wishing to be bound by theory it is believed that only TPEs with a high T _g for their elastomer domains exhibit sufficient immiscibility to the rubber polymer blend, thereby increasing wet grip without increasing the rolling resistance.

The TPEs can be provided in linear or in a branched or dendrimer form. In addition, the elastomeric part of the TPE block copolymer can contain either saturated or unsaturated blocks. For example, the TPE can be a copolymer wherein the elastomer part is saturated and the TPE comprises styrene blocks and alkylene blocks. The alkylene blocks are preferably ethylene, propylene or butylene. More preferably, this TPE elastomer is selected from the following group consisting of diblock or triblock copolymers which are linear or star-branched: styrene/ethylene/butylene (SEB), styrene/ethylene/propylene (SEP), styrene/ethylene/ethylene/propylene (SEEP), styrene/ethylene/butylene/styrene (SEBS), styrene/ethylene/propylene/styrene (SEPS), styrene/ethylene/ethylene/propylene/styrene (SEEPS), styrene/isobutylene (SIB), styrene/isobutylene/styrene (SIBS) and the mixtures of these copolymers.

Alternatively, the TPE is a copolymer wherein the elastomer part is unsaturated and the TPE comprises styrene blocks and diene blocks, these diene blocks being in particular isoprene or butadiene blocks. More preferably, this TPE elastomer is selected from the following group consisting of diblock or triblock copolymers which are linear or star-branched: styrene/butadiene (SB), styrene/isoprene (SI), styrene/butadiene/isoprene (SBI), styrene/butadiene/styrene (SBS), styrene/isoprene/styrene (SIS), styrene/butadiene/isoprene/styrene (SBIS) and the mixtures of these copolymers.

Commercial available TPEs according to the invention are SEPS, SEEPS or SEBS type sold by Kraton under the Kraton G name (e.g., G1650, G1651, G1654 and G1730 products) or Kuraray under the Septon name (e.g., Septon 2007, Septon 4033 or Septon 8004), or the elastomers of SIS type sold by Kuraray under the name Hybrar 5125 or sold by Kraton under the name D1161, or also the elastomers of linear SBS type sold by Polimeri Europa under the name Europrene SOLT 166 or of star-branched SBS type sold by Kraton under the name Dl l 84. Mention may also be made of the elastomers sold by Dexco Polymers under the Vector name (e.g., Vector 4114 or Vector 8508). Mention may be made, among multiblock TPEs, of the Vistamaxx TPE sold by Exxon; the COPE TPE sold by DSM under the Amitel name or by DuPont under the Hytrel name or by Ticona under the Riteflex name; the PEBA TPE sold by Arkema under the PEBAX name; or the TPU TPE sold by Sartomer under the name TPU 7840 or by BASF under the Elastogran name. Further mention may be made of SEBS types from Asahi Kasei such as S.O.E S1606 and S.O.E S 1611.

According to the invention, the composition comprises > 1 phr to < 100 phr of a rubber component, selected from the group of styrene -butadiene rubber (SBR), polybutadiene rubber (BR), natural rubber (NR) or a mixture thereof. The SBR rubber component may contain one type of SBR rubber or several different types. Preferably, at least one type of SBR rubber is manufactured according to the solution process (SSBR or solution SBR). Likewise, the BR rubber component may contain one type of BR rubber or several different types.

It is further understood that in formulations discussed in connection with the present invention the phr amount of all rubber components adds up to 100, whereas the amount of TPE varies from > 1 to < 50 phr as an additive. In formulations discussed in connection with the present invention the phr amount the rubber components, excluding the TPE, adds up to 100.

The cross-linkable rubber composition according to the invention comprises cross-linkable groups in the individual rubber components. They may be cross-linked (cured, vulcanised) by methods known to a skilled person in the rubber technology field.

The cross-linkable rubber compositions may be sulfur-vulcanizable and/or peroxide-vulcanizable. Other vulcanization systems may also be used. If desired, additional additives besides TPE can be added. Examples of usual additives are resins, stabilizers, antioxidants, lubricants, fillers, dyes, pigments, flame retardants, conductive fibres and reinforcing fibres.

If desired, the cross-linkable rubber composition can also comprise a coupling agent. Suitable coupling agents comprise silane compounds. Particularly suitable silane compounds comprise di- and tetrasulphides and mercaptosilanes. It is possible for the rubber composition to be provided with a conductive filler to make it at least partially conductive.

In an embodiment of the rubber composition at least one elastomer block of the TPE is selected from isoprene, butadiene, ethylene, butylene or a mixture thereof.

In another embodiment of the rubber composition the TPE is a styrene-isoprene-styrene (SIS), styrene- butadiene-styrene (SBS) or styrene-ethylene-butylene-styrene (SEBS) elastomer.

In another embodiment of the rubber composition the TPE has a styrene content of > 15 to < 70 weight-%. Preferably the TPE has a styrene content of > 18 to < 60 weight-%.

In another embodiment of the rubber composition the TPE has a Shore A hardness according to ISO 7619 of > 50 to < 90. Preferably the Shore A hardness is > 60 to < 75. In another embodiment of the rubber composition the composition comprises > 20 (preferably > 20 to < 100) phr of rubbers with a glass transition temperature of > -120 °C to < -50 °C (measured by DSC, according to ISO 22768). In particular, the composition may comprise a first styrene- butadiene rubber as a low T _g rubber and a second styrene -butadiene rubber as a high T _g rubber ( T _g measured by DSC, according to ISO 22768). A cross-linkable rubber composition according to the invention may comprise:

In another embodiment of the rubber composition the composition comprises > 5 phr to < 60 phr of a resin component. The resins may be aromatic resins or aliphatic resins. Examples for aromatic resins include C5/C9 aromatic hydrocarbon resin. Aliphatic resins are resins without aromatic structure such as polyterpene resins, or aliphatic hydrocarbon resins such as C5 or DCPD (dicyclopentadiene) resins. The resin may be one of these resins or a combination thereof.

In another embodiment of the rubber composition the resin component comprises a polyterpene resin, C5 resin, C9 resin or DCPD resin. A preferred polyterpene resin has a Mw of > 800 g/mol to < 1200 g/mol. A preferred C5 resin has a Mw of > 2000 to < 4000 g/mol. The polyterpene resin is a resin obtained by polymerizing a terpene compound, or a hydrogenated product of the resin. The terpene compound is a hydrocarbon represented by (CMT), or an oxygenous derivative thereof, whose basic structure is any of terpenes classified into monoterpenes (C10H16), sesquiterpenes (C ₁₅ H ₂₄ ), diterpenes (C ₂₀ H3 ₂ ), and the like. Examples of the compound include a-pinene, b-pinene, dipentene, limonene, myrcene, allo-ocimene, ocimene, a-phellandrene, a-terpinene, g-terpinene, terpinolene, l,8-cineole, l,4-cineole, a-terpineol, b-terpineol, and g-terpineol.

Examples of the polyterpene resins include terpene resins formed from the terpene compounds described above, such as a-pinene resin, b-pinene resin, limonene resin, dipentene resin, or b- pinene/limonene resin, as well as hydrogenated terpene resins prepared by hydrogenating any of the terpene resins. Preferably non-hydrogenated terpene resins are used; most preferably resins comprising limonene as a co-monomer are used. Without wishing to be bound by theory it is believed that a higher affinity of a polyterpene resin to the mid-block of an SIS-type TPE, i.e., isoprene, could be the reason for a pronounced increase in tan delta at 0 °C (wet grip indicator). The high affinity of resin could be related to the similar chemical structure of polyterpene resin and isoprene.

In another embodiment of the rubber composition comprises a first rubber with a T _g of > - 120 °C to < - 50 °C and a second rubber with a T _g of > - 50 °C to < - 10 °C. For example, the rubber component may comprise a first styrene-butadiene rubber and a second styrene -butadiene rubber with the above-mentioned glass temperature ranges.

In another embodiment of the rubber composition the composition comprises:

In another embodiment of the rubber composition the composition comprises:

The invention also relates to a method of preparing a cross-linkable rubber composition according to the invention, comprising the steps of: A) providing a rubber component selected from the group of SBR, BR, NR or a mixture thereof;

B) adding, in a separate step, an additive component comprising the TPE to the rubber component from step A;

C) adding curing agents.

Another aspect of the invention is a cross-linked rubber composition obtained by cross-linking a rubber composition according to the invention.

In an embodiment, the cross-linked rubber composition has a tan delta at 0 °C of > 0.30 to < 0.65 (determined from DMA measurements according to ISO 4664-1, frequency 10 Hz, 0.1% dynamic strain) and a tan delta at 70 °C of > 0.10 to < 0.25 (determined from DMA measurements according to ISO 4664-1, frequency 10 Hz, 6 % dynamic strain). Preferably the tan delta at 0 °C is > 0.34 to < 0.64 and the tan delta at 70 °C is > 0.13 to < 0.15.

In another embodiment of the cross-linked rubber composition its tan delta curve (determined from dynamic mechanical analysis (DMA) measurements according to ISO 4664-1, frequency 10 Hz, 0.1% dynamic strain) has a full width at half maximum (FWHM) range of > 35 °C to < 75 °C.

In another embodiment the cross-linked rubber composition has a rebound at 23 °C, determined according to ISO 4662, of > 15 % to < 25% (preferably > 16 to < 21%).

In another embodiment the cross-linked rubber composition has a rebound at 70 °C, determined according to ISO 4662, of > 50 % to < 60% (preferably > 53 to < 58%).

The present invention also relates to a method of preparing a tyre, comprising the steps of:

- providing a tyre assembly comprising a rubber composition according to the invention;

- cross-linking at least the rubber composition according to the invention in the tyre assembly.

The present invention also encompasses a tyre comprising a tyre tread, wherein the tyre tread comprises a cross-linked rubber composition according to the invention.

Examples

The invention will be further described with reference to the following examples and figure without wishing to be limited by them. FIG. 1 shows temperature-dependent DMA curves for the cured compositions Cl, C2, II and 12. FIG. 2 shows temperature-dependent DMA curves for the cured compositions C3, C4 and C5. FIG. 3 shows temperature-dependent DMA curves for the cured compositions A, B, C and Ref.

In accordance with the preceding, cross-linkable rubber compositions were prepared as described in the tables below. In a first step, all rubber components were added and mixed, followed by a second step wherein all additives were added and mixed and a last step wherein the curing package was added. Compositions Cl to C6 are comparative examples and composition II and 12 are compositions according to the invention. Unless stated otherwise, glass temperatures given were determined by DSC according to ISO 22768.

SSBR I had a styrene content of 21%, a vinyl content of 50% and a T _g of -25 °C.

SSBR II had a styrene content of 37%, a vinyl content of 42% and a T _g of -30 °C and was extended with 37.5 phr of oil.

SSBR III had a styrene content of 35%, a vinyl content of 59% and a T _g of -19 °C and was extended with 25 phr of oil.

SSBR IV had a styrene content of 35%, a vinyl content of 42% and a T _g of -25 °C and was extended with 25 phr of oil.

SSBR V had a styrene content of 15%, a vinyl content of 30% and a T _g of -62 °C.

BR I was a high cis (Ni catalyst) butadiene rubber with a T _g of -102 °C.

BR II was a high cis (Nd catalyst) butadiene rubber with a T _g of -105 °C.

SIS I was a styrene-isoprene-styrene block copolymer with a T _g of -13°C and a styrene content of 20 weight-%.

SIS II was a styrene-isoprene-styrene block copolymer with a T _g of -60 °C and a styrene content of 15 weight-%.

SIS III was a styrene-isoprene-styrene block copolymer with a T _g of -60 °C and a styrene content of 30 weight-%.

The polyterpene resin had a molecular weight of 1090 g/mol, a softening point of 125 ° C and a glass transition temperature of 73 °C.

Abbreviations: TESPT (tetrasulphide silane); TESPD (disulphide silane); FWHM (full width at half maximum) The table below shows the compositions of Cl to C5, II and 12. Amounts for the components are given in PHR. It should be noted that in examples II and 12 the SIS is considered part of the additive package and therefore does not contribute to total amount of rubber with respect to the phr values for the calculation of the individual amounts of the composition’s components. The inventive example II is obtained by adding 10 phr of SIS I as an additive to the comparative example Cl. Likewise, inventive example 12 is obtained by adding 10 phr of SIS I as an additive to the comparative example C2.

Comparative example C3 does not contain any SIS species. The comparative examples C4 and C5 contain SIS I as an elastomer, i.e. its amount contributes to the phr calculation. In this case, the high styrene high vinyl SSBR and high styrene medium vinyl SSBR originally present in the recipes is replaced with 15 phr of SIS I.

The series of examples“Ref’,“A”,“B” and“C” investigate the effect of different SIS types as additives.“Ref’ is without SIS addition,“A” with SIS I,“B” with SIS II and“C” with SIS III. It will become apparent that“B” and“C” are comparative examples and“A” is an example according to the invention.

The following tables show the results obtained from the cured compositions.

Rebound at 23 °C (ISO 4662) is believed to be an indicator for wet grip. Lower rebound value at 23 °C relates to an improvement in wet grip. Rebound testing at 70 °C (ISO 4662) is believed to be an indicator for rolling resistance (RR). A higher rebound value at 70 °C relates to a lower rolling resistance for a tyre whose tread comprises such a cured rubber. In a similar fashion, a higher tan d at 0 °C is related to beter wet grip, whereas a lower tan d at 70 °C is an indicator for improved rolling resistance. The tan d values were measured with a DMA test (ISO 4664-1) using the following settings:

- DMA tests -80 to 25 °C (dynamic strain 0.1% and frequency 10 Hz)

- DMA tests 25 to 80 °C (dynamic strain 6% and frequency 10 Hz)

As depicted in FIG. 1, the inventive example II reveals a slightly broader tan delta curve compared to Cl. It is believed that the presence of SIS possessing a T _g of -13 °C leads to an at least partial immiscibility of the polymers. As a result of immiscibility, there is an increase in wet grip indicator (tan d at 0 °C) and the corresponding rebound at 23 °C is slightly lower. Surprisingly, the rolling resistance (RR) indicator (tan d at 70 °C) for II did not increase but it is comparable to Cl. Like the inventive example II, 12 also reveals an improvement in wet grip without increasing rolling resistance compared to C2. As depicted in FIG. 2, the tan delta curves of C4 and C5 show a slight increase in T _g but there is no peak broadening effect. The T _g increase is attributed to the higher T _g of SIS compared to the T _g s of SSBRs which was originally replaced with. The wet grip indicator (tan d 0 °C) for C4 and C5 is comparable to C3 but the rebound at 70 °C is lower than C3 indicating an increase in RR. Clearly, the compound properties changes when SIS is used as an elastomer instead of an additive. As depicted in FIG. 3, in the comparison of different SIS additives only one type (A) shows improving wet grip without increasing the rolling resistance. This is because type A contains SIS which has a higher Tg (-l3°C) and it is believed that it creates immiscibility leading to an improvement in the wet grip.