“Tire for vehicle wheels’

FIELD OF THE INVENTION

The present invention relates to a tire for vehicle wheels, in particular cars, which have excellent grip and braking performance on snow and, at the same time, on dry and wet surfaces, for use in all weather conditions and suitable for all seasons, which do not need to be replaced in seasonal changes from winter to summer and vice versa.

PRIOR ART

In the tire industry, research pursues the goal of increasing driving safety along with increasing overall performance.

Within the scope of all-seasons tires, car manufacturers require increasingly higher performance both on dry and wet, with increased grip at low temperatures without unbalancing the overall properties. Ideally, tires that perform equally well on any type of road surface, in all weather conditions and temperatures and, at the same time, characterized by reduced wear are desired. This result is very difficult to achieve.

It is known in the art that the wear and snow grip performance of tires may be improved if the glass transition temperature (Tg) and the stiffness of the tread compound are reduced, as discussed for example in WO2018050748.

Compounds with low glass transition temperatures may be obtained for example by adding plasticising oils with low Tg (typically between -50° and - 60°C) to elastomeric polymers having low Tg, such as polybutadiene rubber (BR) (Tg around -110°C), natural rubber (NR) (Tg around -65°C) or styrene butadiene rubber (SBR), with a low styrene content (for example not more than 20%, with Tg around -70°C/-50°C) or with a higher styrene content (higher than 20%, with Tg around -40°C/-20°C).

However, the lowering of the Tg of the compound involves a significant variation of the hysteretic properties of the same. Typically, as the Tg of the compound drops, the peak and the entire Tan delta curve move towards lower temperatures, with a drop in hysteresis in the temperature range of between 0°C and 10°C, a range indicative of the grip performance of the tread in the wet. The addition of plasticising oils has a further negative effect on wet traction as they bring about a reduction in hysteresis in the temperature range affecting this performance (between 0°C and 10°C of the tanδ curve in the temperature scan).

Wet performance may be increased by introducing high Tg resins capable of providing hysteresis in the selected range, but as a consequence they lead to an increase in both the compound Tg and stiffness, particularly at low temperatures responsible for performance on snow.

Therefore, the benefit in terms of less wear and better grip on snow obtained by lowering the Tg of the tread compound by adding low Tg plasticising oils is inevitably associated with worsening wet performance.

At present, therefore, the need remains to provide an elastomeric compound for tire treads characterized by good driveability on snow, reduced wear and high grip in wet and dry conditions, for applications in all-seasons tires.

SUMMARY OF THE INVENTION

The Applicant has faced the problem of extending the working temperature range in tires capable of offering optimal performance in terms of grip in any season and in any weather conditions, without penalising the features of processability, mechanical strength and wear resistance.

The Applicant has undertaken studies aimed at giving the elastomeric compounds for tires the desired properties discussed above and, after extensive experimentation has found that, by appropriately selecting the nature and quantity of the elastomeric polymers and plasticisers, it was possible to obtain improved performance.

In particular, the Applicant has surprisingly found that the partial replacement of the plasticising oil with the use of a mixture of two or more resins, each with a specific glass transition temperature (Tg), respectively low and high, in an elastomeric composition characterized by a low Tg, was able to improve the hysteresis (tanδ) in the temperature range above 0°C, in particular in the range between 0°C and 10°C indicative of the performance on wet surfaces, and in the range between 20°C and 70°C, indicative of the performance on dry surfaces, while maintaining good stiffness values (E’) at low temperatures, indicative of the performance on snow-covered roads. The present invention therefore relates to a tire for vehicle wheels comprising:

- a carcass structure, having opposite side edges associated with respective bead structures;

- optionally, a belt structure applied in a radially external position with respect to said carcass structure;

- a tread band applied in a radially external position with respect to said carcass and/or belt structure; characterized in that said tread band comprises a vulcanised elastomeric compound obtained by vulcanisation of a vulcanisable elastomeric compound made by mixing an elastomeric composition, wherein said elastomeric composition comprises

(i) 100 phr of an elastomeric polymer composition comprising, preferably consisting of: a. at least one styrene-butadiene polymer (SBR) having a Tg of from -80°C to -15°C, in an amount of from 30 to 100 phr, b. optionally, from 0 to 70 phr of at least one isoprene polymer (IR) having a Tg of from -80°C to -50°C, and c. optionally, from 0 to 50 phr of at least one butadiene polymer (BR) having a Tg of between -120°C and -80°C,

(ii) 15 to 100 phr of an oil-resin mixture comprising, preferably consisting of: a. at least one low Tg resin having a glass transition temperature lower than -10°C in an amount of from 5 to 40 phr, b. at least one high Tg resin having a glass transition temperature greater than 40°C in an amount of from 5 to 20 phr, and c. at least one oil in an amount of from 5 to 40 phr, wherein the weight ratio between the total amount of said low and high Tg resins and the amount of said oil is equal to or greater than 1 :1 , preferably between 1 .5:1 and 5:1 ,

(iii) at least one reinforcing filler in an amount from 1 to 130 phr, and

(iv) at least one vulcanisation agent in an amount from 0.1 to 12 phr. In a second aspect thereof, the present invention also relates to a vulcanisable elastomeric compound obtained by mixing an elastomeric composition, wherein said elastomeric composition comprises

(i) 100 phr of an elastomeric polymer composition comprising, preferably consisting of: a. at least one styrene-butadiene polymer (SBR) having a Tg of from -80°C to -15°C, in an amount of from 30 to 100 phr, b. optionally, from 0 to 70 phr of at least one isoprene polymer (IR) having a Tg of from -80°C to -50°C, and c. optionally, from 0 to 50 phr of at least one butadiene polymer (BR) having a Tg of between -120°C and -80°C,

(ii) 15 to 100 phr of an oil-resin mixture comprising, preferably consisting of: a. at least one low Tg resin having a glass transition temperature lower than -10°C in an amount of from 5 to 40 phr, b. at least one high Tg resin having a glass transition temperature greater than 40°C in an amount of from 5 to 20 phr, and c. at least one oil in an amount of from 5 to 40 phr, wherein the weight ratio between the total amount of said low and high Tg resins and the amount of said oil is equal to or greater than 1 :1 , preferably between 1.5:1 and 5: 1 ,

(iii) at least one reinforcing filler in an amount from 1 to 130 phr, and

(iv) at least one vulcanisation agent in an amount from 0.1 to 12 phr.

DEFINITIONS

The term “elastomeric composition” means a composition comprising at least one diene elastomeric polymer and one or more additives, which by mixing and possible heating provides an elastomeric compound suitable for use in tires and components thereof.

The components of the elastomeric composition are not generally introduced simultaneously into the mixer but typically added in sequence. In particular, the vulcanisation additives, such as the vulcanisation agent and possibly the accelerant and retardant agents, are usually added in a downstream step with respect to the incorporation and processing of all the other components.

In the vulcanisable elastomeric compound, the individual components of the elastomeric composition may be altered or no longer individually traceable as they are modified, completely or in part, due to the interaction with the other components, of heat and/or mechanical processing. The term “elastomeric composition” herein is meant to include all the components that are used in the preparation of the elastomeric compound, regardless of whether they are actually present simultaneously, are introduced sequentially or are then traceable in the elastomeric compound or in the final tire.

The term “elastomeric polymer” indicates a natural or synthetic polymer which, after vulcanisation, may be stretched repeatedly at room temperature to at least twice its original length and after removal of the tensile load substantially immediately returns with force to approximately its original length (according to the definitions of the ASTM D1566-11 Standard terminology relating to Rubber).

The term “diene polymer” indicates a polymer or copolymer derived from the polymerisation of one or more different monomers, among which at least one of them is a conjugated diene (conjugated diolefin).

The term “elastomeric compound” indicates the compound obtainable by mixing and possibly heating at least one elastomeric polymer with at least one of the additives commonly used in the preparation of tire compounds.

The term “vulcanisable elastomeric compound” indicates the elastomeric compound ready for vulcanisation, obtainable by incorporation into an elastomeric compound of all the additives, including those of vulcanisation.

The term “vulcanised elastomeric compound” means the material obtainable by vulcanisation of a vulcanisable elastomeric compound.

The term “green” indicates a material, a compound, a composition, a component or a tire not yet vulcanised.

The term “vulcanisation” refers to the cross-linking reaction in a natural or synthetic rubber induced by a typically sulphur-based cross-linking agent.

The term “vulcanisation agent” indicates a product capable of transforming natural or synthetic rubber into elastic and resistant material by virtue of the formation of a three-dimensional network of inter- and intra-molecular bonds. Typical vulcanisation agents are sulphur-based compounds such as elemental sulphur, polymeric sulphur, sulphur-donor agents such as bis[(trialkoxysilyl)propyl]polysulphides, thiurams, dithiodimorpholines and caprolactam-disulphide.

The term “vulcanisation accelerant” means a compound capable of decreasing the duration of the vulcanisation process and/or the operating temperature, such as sulphenamides, thiazoles, dithiophosphates, dithiocarbamates, guanidines, as well as sulphur donors such as thiurams.

The term “vulcanisation activating agent” indicates a product capable of further facilitating the vulcanisation, making it happen in shorter times and possibly at lower temperatures. An example of activating agent is the stearic acid-zinc oxide system.

The term “vulcanisation retardant” indicates a product capable of delaying the onset of the vulcanisation reaction and/or suppressing undesired secondary reactions, for example N-(cyclohexylthio)phthalimide (CTP).

The term “vulcanisation package” is meant to indicate the vulcanisation agent and one or more vulcanisation additives selected from among vulcanisation activating agents, accelerants and retardants.

The term “reinforcing filler” is meant to refer to a reinforcing material typically used in the sector to improve the mechanical properties of tire rubbers, preferably selected from among carbon black, conventional silica, such as silica from sand precipitated with strong acids, preferably amorphous, diatomaceous earth, calcium carbonate, titanium dioxide, talc, alumina, aluminosilicates, kaolin, silicate fibres and mixtures thereof.

The term “white filler” is meant to refer to a conventional reinforcing material used in the sector selected from among conventional silica and silicates, such as sepiolite, paligorskite also known as attapulgite, montmorillonite, alloisite and the like, possibly modified by acid treatment and/or derivatised. Typically, white fillers have surface hydroxyl groups.

The term “mixing step (1 )” indicates the step of the preparation process of the elastomeric compound in which one or more additives may be incorporated by mixing and possibly heating, except for the vulcanisation agent which is fed in step (2). The mixing step (1 ) is also referred to as “non-productive step”. In the preparation of a compound there may be several “non-productive” mixing steps which may be indicated with 1 a, 1 b, etc. The term “mixing step (2)” indicates the next step of the preparation process of the elastomeric compound in which the vulcanisation agent and, possibly, the other additives of the vulcanisation package are introduced into the elastomeric compound obtained from step (1 ), and mixed in the material, at controlled temperature, generally at a compound temperature lower than 120°C, so as to provide the vulcanisable elastomeric compound. The mixing step (2) is also referred to as “productive step”.

For the purposes of the present description and the following claims, the term “phr” (acronym for parts per hundreds of rubber) indicates the parts by weight of a given elastomeric compound component per 100 parts by weight of the elastomeric polymer, considered net of any extension oils.

Unless otherwise indicated, all the percentages are expressed as percentages by weight.

The elastomeric composition used in the tire tread according to the present invention comprises 100 phr of an elastomeric polymer composition comprising, preferably consisting of at least one styrene-butadiene polymer (SBR) with a Tg of between -80°C and -15°C , preferably between -65°C and -25°C, more preferably between -55°C and -35°C in an amount ranging from 30 to 100 phr, optionally, from 0 to 70 phr of at least one isoprene polymer (IR) with a Tg of between -80°C and -50°C, preferably between -75°C and -55°C, and optionally, from 0 to 50 phr of at least one butadiene polymer (BR) with a Tg of between -120°C and -80°C, preferably between -115°C and -85°C.

The glass transition temperature Tg of elastomeric polymers may be advantageously measured using a differential scanning calorimeter (DSC) according to methods well known to those skilled in the art [ISO 22768 “Rubber, green - Determination of the glass transition temperature by differential scanning calorimetry (DSC)”].

In the present context, styrene-butadiene (SBR) polymer is intended as a copolymer comprising monomer units of styrene and butadiene, with a percentage by weight of styrene preferably in the range from 10% to 55%, more preferably from 20% to 45%, and a weight percentage of vinyl (with respect to butadiene) preferably in the range from 10% to 70%, more preferably from 15% to 65%.

The styrene-butadiene polymer may contain, in addition to the styrene units and the butadiene units, a small amount, for example, equal to or lower than 5% by weight, of additional monomer units such as isoprene, dimethylbutadiene, pentadiene, methylstyrene, ethylstyrene, divinylbenzene and diisopropenylbenzene.

Advantageously, the styrene-butadiene polymer has a Tg ranging from - 50°C to -40°C.

Advantageously, the elastomeric composition used in the tire tread according to the present invention comprises from 40 to 70 phr of at least one styrene-butadiene (SBR) polymer.

Preferably, the styrene-butadiene polymer is a random polymer.

Preferably, the styrene-butadiene polymer may have a weight average molecular weight of between 100,000 and 2,000,000 g/mol, preferably between 150,000 and 1 ,000,000, more preferably between 200,000 and 600,000 g/mol.

The styrene-butadiene polymer may be prepared according to known techniques, for example as described in US2019062535, in US2019062529 or in US4547560.

In one embodiment, the styrene-butadiene polymer is prepared by solution polymerisation (S-SBR).

Typically, solution synthesis provides polymers with a narrow molecular weight distribution, fewer chain branches, higher molecular weight and higher cis-1 ,4-polybutadiene content than polymers obtainable in emulsion.

In another embodiment, the styrene-butadiene polymer is prepared by emulsion polymerisation (E-SBR).

The styrene-butadiene polymer may be a functionalised polymer, such as for example the functionalised SBRs described in US2019062535 (par. 9 - 13), in US2019062529 (par. 19 - 22) in WO2017/211876A1 (component a) or in WO2015/086039A1 .

The functional group may be introduced into the styrene-butadiene polymer by processes known in the art such as, for example, during the production of the styrene-butadiene polymer by copolymerisation with at least one corresponding functionalised monomer containing at least one ethylene unsaturation; or by subsequent modification of the styrene-butadiene polymer by grafting at least one functionalised monomer in the presence of a free radical initiator (for example, an organic peroxide). Alternatively, the functionalisation may be introduced by reaction with suitable terminating agents or coupling agents. In particular, the styrene- butadiene polymers obtained by anionic polymerisation in the presence of an organometallic initiator (in particular, an organolithium initiator) may be functionalised by reacting the residual organometallic groups derived from the initiator with suitable terminating agents or coupling agents such as, for example, amines, amides, imines, carbodiimides, alkyltin halides, substituted benzophenones, alkoxysilanes, aryloxy silanes, alkyldithiols, alkyldithiolsilanes, carboxyalkylthiols, carboxyalkylthiolsilanes, and thioglycols.

Useful examples of terminating agents or coupling agents are known in the art and described, for example in patents EP2408626, EP2271682, EP3049447A1 , EP2283046A1 , EP2895515A1 , WO2015/086039A1 and WO2017/211876A1 .

In one embodiment, the elastomeric composition used for manufacturing the tire tread according to the present invention comprises at least one styrene- butadiene polymer prepared by emulsion polymerisation (E-SBR), optionally functionalised, and at least one styrene-butadiene polymer prepared by solution polymerisation (S-SBR), optionally functionalised.

According to a preferred embodiment, the elastomeric composition used for manufacturing the tire tread according to the present invention comprises (i) at least one styrene-butadiene polymer (preferably E-SBR) with a percentage by weight of styrene preferably in the range from 20% to 45% and a percentage by weight of vinyl (with respect to butadiene) preferably in the range from 10% to 20%, and (ii) at least one styrene-butadiene polymer (preferably S-SBR) with a percentage by weight of styrene preferably in the range from 20% to 45% and a percentage by weight of vinyl (with respect to butadiene) preferably in the range from 15% to 65%.

Commercial examples of SBR polymers useful in the present invention are Tufdene E581 and E680 polymers from Ashai-Kasei (Japan), SPRINTAN SLR4602, SLR3402 and SLR4630 from Trinseo (Germany), HPR621 from JSR Corporation (Japan), BUNA SL-4518, BUNA SE 1502 and BUNA CB 22 from Arlanxeo (Germany), Europrene NEOCIS BR 60, Europrene 5543T, Europrene 1739 and Intol 1789 from Versalis (Italy), HP 755 from Japan Synthetic Rubber Co. (Japan), and NIPOL NS 522 from Zeon Co. (Japan). In the present context, isoprene polymer or isoprene rubber (IR) means a synthetic or natural elastomer obtained by 1 ,4-cis addition of isoprene. Preferably, the isoprene polymer is a natural rubber (NR). Isoprene polymers and natural rubbers are well known to those skilled in the field of tires. The isoprene polymer may optionally be functionalised with the same terminating or coupling agents described above.

Advantageously, the isoprene polymer has a Tg ranging from -70°C to - 60°C.

Advantageously, the elastomeric composition used in the tire tread according to the present invention comprises from 20 to 50 phr of at least one isoprene polymer (IR).

Commercial example of suitable isoprene polymer is SIR20 from Aneka Bumi Pratama or STR 20 from Thaiteck Rubber.

The polybutadiene polymer (BR) is a synthetic rubber obtained by polymerisation of 1 ,3-butadiene, preferably by nickel or neodymium catalysis.

The polybutadiene polymer preferably comprises a polybutadiene rubber with a cis group content greater than 90%, more preferably greater than 95%. The cis content of the polybutadiene rubber is usually provided by the supplier and may be determined by the FTIR method. The method is based on the calculation of the ratio between the intensity of the bands attributable to the 1 ,4-trans and 1 , 2-vinyl isomers and a reference band (internal standard) which falls at 1312 cm ^-1 (L.J. Bellamy, The Infrared Spectra of Complex Molecules, Vol. 1 Third Edition, Chapman and Hall). The 1 ,4-cis content is determined by the difference to 100. The preparation of the sample is carried out on a polybutadiene film, obtained starting from a solution, evaporated on a KBr window.

Advantageously, the butadiene polymer has a Tg of between -110°C and - 90°C.

Advantageously, the elastomeric composition used in the tire tread according to the present invention comprises from 10 to 40 phr of at least one butadiene polymer (BR).

Commercial examples of polybutadiene rubber include ND-PBR (SKDN) from PJSC Nizhnekamskneftekhim (Russia), Buna® CB 22 from Arlanxeo (Holland), Nd BR 40 from KUMHO (South Korea). The elastomeric composition used for making the tire tread according to the present invention comprises from 15 to 100 phr of a mixture of oil and resins which comprises, preferably consists of, at least one low Tg resin with a glass transition temperature lower than -10°C in an amount of from 5 to 40 phr, at least one high Tg resin with a glass transition temperature above 40°C in an amount of from 5 to 20 phr, and at least one oil in an amount of from 5 to 40 phr, where the weight ratio between the total amount of said low and high Tg resins and the amount of said oil is equal to or greater than 1 :1 , preferably between 1 .5:1 and 5:1 .

Advantageously, the elastomeric composition used for making the tire tread according to the present invention comprises from 15 to 55 phr of a mixture of oil and resins which comprises, preferably consists of, at least one low Tg resin with a glass transition temperature lower than -10°C in an amount of from 5 to 20 phr, at least one high Tg resin with a glass transition temperature above 40°C in an amount of from 5 to 15 phr, and at least one oil in an amount of from 5 to 20 phr, where the weight ratio between the total amount of said low and high Tg resins and the amount of said oil is equal to or greater than 1 :1 , preferably between 1.5:1 and 5:1.

Advantageously, the weight ratio between the total amount of said low and high Tg resins and the amount of said oil is between 1.5:1 and 3:1 , more preferably between 1 .5:1 and 2:1 .

Preferably, the low Tg resin has a glass transition temperature of between -40°C and -10°C.

Preferably, the high Tg resin has a glass transition temperature of between 40°C and 90°C.

According to a preferred aspect, the weight ratio between the amount of low Tg resin and the amount of high Tg resin is between 3:1 and 1 :3, preferably between 2:1 and 1 :2.

Advantageously, the weight ratio between the amount of low Tg resin and the amount of high Tg resin is between 2:1 and 1 :1.

The glass transition temperature (Tg) of the resin may advantageously be measured using a differential scanning calorimeter (DSC) according to methods well known to those skilled in the art, such as the ASTM D 6604 method (Glass Transition Temperatures of Hydrocarbon Resins by Differential Scanning Calorimetry). Preferably, the resins have a weight average molecular weight (Mw) of between 200 and 5,000 g/mol, preferably between 400 and 4,000 g/mol.

The weight average molecular weight (Mw) of resins may be measured according to known techniques in the field such as, for example, by SEC (Size- Exclusion Chromatography) according to the ASTM D6579-11 method “Standard Practice for Molecular Weight Averages and Molecular Weight Distribution of Hydrocarbon, Rosin and Terpene Resins by Size-Exclusion Chromatography”.

The resins are preferably non-reactive resins, i.e. a non-cross-linkable polymer, preferably selected from the group comprising hydrocarbon resins, phenolic resins, natural resins and mixtures thereof.

The hydrocarbon resin may be aliphatic, aromatic or combinations thereof, meaning that the base polymer of the resin may consist of aliphatic and/or aromatic monomers.

The hydrocarbon resin may be natural (e.g. vegetable) or synthetic or derived from petroleum.

Preferably, the hydrocarbon resin is selected from homo- or copolymers of butadiene, homo- or copolymers of cyclopentadiene (CPD), dicyclopentadiene (DCPD), homo- or copolymers of terpene, homo- or copolymers of the C5 fraction and mixtures thereof, preferably DCPD/vinyl aromatic copolymers, DCPD/terpene copolymers, DCPD/C5 fraction copolymers, terpene/vinyl aromatic copolymers, C5 fractions/vinyl aromatic copolymers and combinations thereof.

Examples of vinyl aromatic monomers include styrene, alpha- methylstyrene, ortho-, meta-, para-methylstyrene, vinyl-toluene, para- terbutylstyrene, methoxy-styrenes, chloro-styrenes, vinyl-mesitylene, divinyl- benzenes, vinyl-naphthalenes, vinyl aromatic monomers derived from the C8- C10 fraction, in particular from C9.

Preferably, the hydrocarbon resin is selected from resins derived from coumarone-indene, styrene-indene, styrene-alkylstyrene, and aliphatic resins.

Specific examples of commercially available hydrocarbon resins are NOVARES resins, from RUTGERS CHEMICAL GmbH (such as Novares TL90 resins (Tg: 45°C) and TT30 (Tg: -15°C), UNILENE A 100 resin (Tg: 55°C) from Braskem, Sylvares SA 85 resin (Tg: 40°C) from Arizona Chemical, Impera R1507 resins (Tg: 65°C), Kristalex F 85 (Tg: 40°C) and Piccotac 1100 (Tg: 45°C) from Eastman, Escorez® 1102 resin (Tg: 52°C) from ExxonMobil and Quintone A 100 resin (Tg: 55°C) from Zeon Chemicals.

The phenolic resin is selected from the alkylphenol-formaldehyde-based resins, rosin-modified alkylphenol resins, alkylphenol-acetylene based resins, modified alkylphenolic resins and terpene-phenol based resins.

Specific examples of commercially available phenolic resins which may be used in the present invention are: OFF APM (Tg: 120°C) from Sinolegend; RESIN SP-1068 octylphenol-formaldehyde resin (Tg: 95°C) from SI GROUP Inc.; DUREZ 32333 terbutylphenol-formaldehyde resin (Tg: 85°C) from Sumitomo Bakelite; KORESIN pt-butylphenol-acetylene resin (Tg: 105°C) from BASF Company; SYLVARES TP 115 terpen-phenolic resin (Tg: 67°C) from Arizona Chemicals.

Natural resins may be terpene or rosin based.

Terpene-based resins are preferably homo or copolymers of alpha-pinene, beta-pinene, limonene, vinyl aromatic monomers (styrene) and/or aromatic monomers (phenol).

Examples of commercial terpene-based natural resins are: Piccolyte F90 (Tg: 45°C) and Piccolyte F105 (Tg: 60°C), from PINOVA; Dercolyte A 115 (Tg: 70°C), Dercolyte TS105 (Tg: 61 °C) and Dercolyte M 115 (Tg: 70°C), from DRT.

The term rosin commonly indicates a mixture of isomeric organic acids (rosinic acids) characterized by a common structure, including three fused C6 rings, double bonds in different numbers and positions and a single carboxylic group, where the main component is abietic acid (C ₂₀ H ₃₀ O ₂ ) and its dihydroabietic (C ₂₀ H ₃₂ O ₂ ) and dehydroabietic (C ₂₀ H ₂₈ O ₂ ) derivatives.

Examples of rosin-based resins are marketed by DRT under the trade name Hydrogral G (Tg: 40°C) and Dertoline P 105 (Tg: 50°C), and by Eastman under the trade name Staybelite, in particular Staybelite Ester 3-E (-19°C).

Advantageously, the elastomeric composition typically also comprises at least one reinforcing filler which may be selected from those commonly used for vulcanised manufactured products, in particular for tires, such as for example: carbon black, silica and silicates, alumina, calcium carbonate, or mixtures thereof. Carbon black, silica and mixtures thereof are particularly preferred. Preferably, said reinforcing filler may be present in the elastomeric composition in an amount of from 10 phr to 120 phr, more preferably from 30 phr to 100 phr.

According to a preferred embodiment, said carbon black reinforcing filler may be selected from those having a surface area of not less than 20 m ² /g (as determined by Statistical Thickness Surface Area - STSA - according to ISO 18852:2005).

According to a preferred embodiment, said silica reinforcing filler may be, for example, precipitated silica.

The silica reinforcing fillers which may advantageously be used in the present invention preferably have a BET surface area of from about 30 m ² /g to 400 m ² /g, more preferably from about 100 m ² /g to about 250 m ² /g, even more preferably from about 120 m ² /g to about 220 m ² /g. The pH of said silica reinforcing filler is generally from about 5.5 to about 7, preferably from about 5.5 to about 6.8.

Examples of silica reinforcing fillers which may be used in the present invention and are commercially available are the products known under the names of Hi-Sil® 190, Hi-Sil® 210, Hi-Sil® 233, Hi-Sil® 243, available from PPG Industries (Pittsburgh, Pa.); or the products known by the names of Ultrasil® VN2, Ultrasil® VN3, Ultrasil ® 7000 from Evonik; or the products known by the names of Zeosil® 1165MP and 1115MP from Solvay.

Advantageously, the elastomeric composition comprises at least one silane coupling agent capable of interacting with the reinforcing filler and binding it to the elastomeric polymer during vulcanisation.

The coupling agents which are preferably used are silane-based ones which may be identified, for example, by the following structural formula (VI):

(R ₂ ) ₃ Si-C _t H _2t -X (VI) wherein the R ₂ groups, which may be equal or different from each other, are selected from: alkyl, alkoxy or aryloxy groups or halogen atoms, with the proviso that at least one of the R ₂ groups is an alkoxy or an aryloxy group; t is an integer of between 1 and 6 inclusive; X is a group selected from nitrose, mercapto, amino, epoxide, vinyl, imide, chlorine, -(S) _u C _t H _2t -Si- (R ₂ ) ₃ or -S- COR ₂ , wherein u and t are integers of between 1 and 6, ends included and the R ₂ groups are as defined above. Particularly preferred coupling agents are bis(3- triethoxysilylpropyl)tetrasulphide and bis(3-triethoxysilylpropyl)disulphide. Said coupling agents may be used as such or in a suitable mixture with an inert filler (such as carbon black) so as to facilitate their incorporation into the elastomeric composition.

Preferably, the coupling agent is added to the elastomeric composition in an amount ranging from 1 to 20% by weight, more preferably from 5 to 15% by weight, and even more preferably from 6 to 10% by weight with respect to the weight of silica.

The above elastomeric composition may be vulcanised according to known techniques, in particular with sulphur-based vulcanising systems commonly used for elastomeric polymers. For this purpose, after one or more thermo- mechanical processing steps, a sulphur-based curing agent is incorporated in the composition, together with vulcanisation accelerants. In the final processing step, the temperature is generally kept below 120°C and preferably below 100°C, so as to prevent any undesired pre-vulcanisation phenomenon.

The vulcanisation agent used in the most advantageous manner is sulphur or sulphur-containing molecules (sulphur donors), with vulcanisation activating agents, accelerants and retardants which are known to those skilled in the art.

Said vulcanisation agent is used in the elastomeric composition in an amount of from 0.1 phr to 12 phr, preferably from 0.5 phr to 10 phr, more preferably from 1 phr to 5 phr.

Activators which are particularly effective are the zinc compounds, and in particular ZnO, zinc salts of saturated or unsaturated fatty acids, such as for example zinc stearate, which are preferably formed in situ in the elastomeric composition starting from ZnO and fatty acids. Useful activators may also be oxides or inorganic salts of Fe, Cu, Sn, Mo and Ni as described in patent application EP 1231079. Stearic acid is typically used as an activator with zinc oxide.

Said vulcanisation activating agents are preferably employed in the elastomeric composition in an amount of from about 0.5 phr to about 10 phr, more preferably from 1 phr to 5 phr.

The accelerants which are commonly used may be selected from: dithiocarbamates, guanidine, thiourea, thiazoles, sulphenamides, thiurams, amines, xanthates or mixtures thereof. Said vulcanisation accelerants are preferably used in the elastomeric composition in an amount of from about 0.5 phr to about 10 phr, more preferably from 1 phr to 5 phr.

Vulcanisation retardants which are commonly used may be selected, for example, from: urea, N-cyclohexyl-2-benzothiazolyl sulphenamide, N- cyclohexyl-phthalimide, N-cyclohexylthiophthalimide, N-nitrosodiphenylamine or mixtures thereof.

Said vulcanisation retardants are optionally used in the elastomeric composition in an amount lower than 1 phr, more preferably lower than 0.5 phr and, even more preferably, from about 0.1 phr to about 0.3 phr.

The elastomeric composition may comprise other commonly used additives based on the specific application for which the composition will be used. For example, the following may be added to the elastomeric composition: antioxidant, anti-ageing agents, plasticisers, adhesives, antiozonants (in particular of the p-phenylenediamine type), waxes, modified resins, fibres (for example Kevlar® paste), or mixtures thereof.

The vulcanisable elastomeric compound resulting from the elastomeric composition and the addition of the above additives may be prepared by mixing together the basic elastomeric components together with the other optionally present additives, according to the techniques known in the art. The mixing steps may be carried out, for example, using an open mixer of the cylinder type or an internal mixer of the type with tangential rotors (Banbury) or with interpenetrating rotors (Intermix), or in continuous mixers of the Ko-Kneader type (Buss) or of the co-rotating or counter-rotating twin-screw type.

DRAWINGS

Figure 1 schematically shows a semi-sectional view of a tire for vehicle wheels according to the present invention.

Figure 2 shows a diagram of the Tan5 values in the temperature range from -70°C to 30°C of the elastomeric compounds A and B of Example 1 .

Figure 3 shows a diagram of the values of E’ in the temperature range from -70°C to 30°C of the elastomeric compounds A and B of Example 1 . DETAILED DESCRIPTION OF THE INVENTION

The present invention will be illustrated in further detail by means of an illustrative embodiment with reference to the accompanying Figure 1 , where “a” indicates an axial direction and “r” indicates a radial direction. For simplicity, Figure 1 shows only a part of the tire, the remaining part not shown being identical and disposed symmetrically with respect to the radial direction “r”.

The reference numeral 100 indicates in Figure 1 a tire for vehicle wheels, which generally comprises a carcass structure 101 having respectively opposite end flaps engaged with respective annular anchoring structures 102, called bead cores, possibly associated with a bead filler 104. The tire area comprising the bead core 102 and the filler 104 forms a bead structure 103 intended for anchoring the tire onto a corresponding mounting rim, not shown. Each bead structure 103 is associated to the carcass structure by folding back of the opposite lateral edges of the at least one carcass layer 101 around the bead core 102 so as to form the so-called carcass flaps 101 a as shown in Figure 1 .

The carcass structure 101 is possibly associated with a belt structure 106 comprising one or more belt layers 106a, 106b placed in radial superposition with respect to one another and with respect to the carcass structure 101 , having typically metal reinforcing cords. Such reinforcing cords may have crossed orientation with respect to a circumferential extension direction of the tire 100. By “circumferential” direction we mean a direction generally facing according to the direction of rotation of the tire, or in any case slightly inclined with respect to the direction of rotation of the tire.

The belt structure 106 further comprises at least one radially external reinforcing layer 106c with respect to the belt layers 106a, 106b. The radially external reinforcing layer 106c comprises textile or metal cords, disposed according to a substantially zero angle with respect to the circumferential extension direction of the tire and immersed in the elastomeric material. Preferably, the cords are disposed substantially parallel and side by side to form a plurality of turns. Such turns are substantially oriented according to the circumferential direction (typically with an angle of between 0° and 5°), such direction being usually called “zero degrees” with reference to the laying thereof with respect to the equatorial plane X-X of the tire. By “equatorial plane” of the tire it is meant a plane perpendicular to the axis of rotation of the tire and which divides the tire into two symmetrically equal parts. In a radially external position with respect to the carcass structure 101 and/or if present (as in the illustrated case) to the belt structure 106 a tread band 109 in vulcanised elastomeric compound obtained by vulcanisation of the vulcanisable elastomeric compound according to the present invention is applied.

In a radially external position, the tread band 109 has a rolling portion 109a intended to come into contact with the ground. Circumferential grooves, which are connected by transverse notches (not shown in Figure 1 ) so as to define a plurality of blocks of various shapes and sizes distributed in the rolling portion 109a, are generally made in this portion 109a, which for simplicity is represented smooth in Figure 1 .

To optimise the performance of the tread, the tread band may be made in a two-layer structure.

Such two-layer structure comprises the rolling layer or portion 109a (called cap) and a substrate 111 (called base) forming the so-called cap-and-base structure. It is thus possible to use an elastomeric material capable of providing a low rolling resistance for the cap 109a and at the same time high resistance to wear and to the formation of cracks while the elastomeric material of the substrate 111 may be particularly aimed at a low hysteresis to cooperate in reducing rolling resistance. One or both layers of the cap-and-base structure may be made with a vulcanised elastomeric compound obtained by vulcanising the vulcanisable elastomeric compound according to the present invention. The under-layer 111 of vulcanised elastomeric compound may be disposed between the belt structure 106 and the rolling portion 109a.

Moreover, respective sidewalls 108 of vulcanised elastomeric compound are further applied in an axially external position to said carcass structure 101 , each extending from one of the lateral edges of the tread band 109 up to the respective bead structure 103.

A strip consisting of elastomeric compound 110, commonly known as “mini- sidewall”, of vulcanised elastomeric compound may optionally be provided in the connecting zone between sidewalls 108 and the tread band 109, this mini- sidewall generally being obtained by co-extrusion with the tread band 109 and allowing an improvement of the mechanical interaction between the tread band 109 and the sidewalls 108. Preferably, the end portion of sidewall 108 directly covers the lateral edge of the tread band 109. In some specific embodiments, such as the one illustrated and described herein, the stiffness of the bead 103 may be improved by providing a reinforcing layer 120 generally known as a “flipper” in the tire bead.

The flipper 120 is wrapped around the respective bead core 102 and the bead filler 104 so as to at least partially surround them. The flipper 120 is disposed between the carcass layer 101 and the bead structure 103. Usually, the flipper 120 is in contact with the carcass layer 101 and said bead structure 103. The flipper 120 typically comprises a plurality of metal or textile cords incorporated in a vulcanised elastomeric compound.

In some specific embodiments, such as the one illustrated and described herein, the bead structure 103 may further comprise a further protective layer 121 which is generally known by the term of “chafer”, or protective strip, and which has the function to increase the rigidity and integrity of the bead structure 103.

The chafer 121 usually comprises a plurality of cords incorporated in a vulcanised elastomeric compound; such cords are generally made of textile material (for example aramid or rayon), or of metallic material (for example steel cords).

Optionally, an anti-abrasive strip 105 is disposed so as to wrap the bead structure 103 along the axially internal and external and radially internal areas of the bead structure 103, thus interposing itself between the latter and the wheel rim when the tire 100 is mounted on the rim.

Moreover, a radially internal surface of tire 100 is preferably internally lined by a layer of substantially airtight elastomeric material, or so-called liner 112.

According to an embodiment not shown, the tire may be a tire for motorcycle wheels. The profile of the straight section of the tire for motorcycle (not shown) has a high transversal curvature since it must guarantee a sufficient footprint area in all the inclination conditions of the motorcycle. The transverse curvature is defined by the value of the ratio between the distance f of the ridge of the tread from the line passing through the laterally opposite ends of the tread itself, measured on the equatorial plane of the tire, and the width C defined by the distance between the laterally opposite ends of the tread itself. A tire with high transverse curvature indicates a tire whose transverse curvature ratio (f/C) is at least 0.20. The building of the tire 100 as described above is carried out by assembling respective semi-finished products onto a forming drum, not shown, by at least one assembly device.

At least a part of the components intended to form the carcass structure 101 of the tire 100 is built and/or assembled on the forming drum. More particularly, the forming drum is intended to first receive the possible liner 112, and then the carcass ply 101. Thereafter, devices non shown coaxially engage one of the annular anchoring structures 102 around each of the end flaps, position an external sleeve comprising the belt structure 106 and the tread band 109 in a coaxially centred position around the cylindrical carcass sleeve and shape the carcass sleeve according to a toroidal configuration through a radial expansion of the carcass ply 101 , so as to cause the application thereof against a radially internal surface of the external sleeve.

After building of the green tire 100, a moulding and vulcanisation treatment is generally carried out in order to determine the structural stabilisation of the tire 100 through vulcanisation of the elastomeric compounds, as well as to impart a desired tread pattern on the tread band 109 and to impart any distinguishing graphic signs at the sidewalls 108.

The present invention will be further illustrated below by means of a number of preparatory examples, which are provided for indicative purposes only and without any limitation of the present invention.

EXAMPLES

Methods of analysis

Scorching time (Scorch): it is the time required, expressed in minutes, to have a +5 points increase in Mooney viscosity, measured according to ISO 289-2 (1994), at 127°C.

Viscosity: the measurement was carried out at 100°C on the final elastomeric composition before vulcanisation according to the ISO 289-1 (1994) procedure.

MDR rheometric analysis: the analysis was carried out according to the ISO 6502 method, with an Alpha Technologies model MDR2000 rheometer, at 170°C and for 30 minutes.

The applied oscillation frequency was 1 ,66Hz with an oscillation width of ± 0.5°. The time required to achieve an increase of two rheometric units (TS2) and to respectively reach 30% (T30), 60% (T60) and 90% (T90) of the maximum torque MH was measured. The maximum torque value MH and the minimum torque value ML were also measured.

IRHD hardness: IRHD hardness (23°C) was measured on vulcanised compounds according to ISO 48:2007.

Glass transition temperature (Tq): The glass transition temperature Tg of the elastomeric polymers and of the vulcanised compounds, determined on the peak value of the Tan Delta, was measured by dynamo-mechanical analysis (DMA). In detail, the samples were analysed with an EPLEXOR® 150 (GABO) device by carrying out a temperature scan from -80°C to + 30°C with temperature increases of 2°C/min, applying a dynamic tensile deformation of the 0.1 % at a frequency of 1 Hz. The specimens had the following dimensions: thickness 1 mm, width 10 mm, length 46mm, reference length 29mm (represents the free length that participates in the deformation while the two clamps block the ends of the specimen).

Stress deformation properties: the static mechanical properties were measured according to ISO 37:2005, on O-rings. Strength was evaluated at different elongations (100% and 300%, respectively, CA1 and CA3). CR (tensile strength) and AR (elongation at break) were also measured.

Dynamic Mechanical Analysis (MTS): the dynamic mechanical properties were measured using an Instron dynamic device in compression and tension operation by the following method. A specimen of vulcanised elastomeric compositions of cylindrical shape (height = 25 mm; diameter = 14 mm), preload in compression up to 25% of longitudinal deformation with respect to the initial length and maintained at the predetermined temperature (-10°C, 23°C, 70°C or 100°C) during the test was subjected to a dynamic sinusoidal tension of amplitude ± 3.5% with respect to the length of the preload, at a frequency of 10Hz for the test at 10°C, and 100Hz for the tests at 23°C, 70°C, and 100°C.

The dynamic mechanical properties are expressed in terms of dynamic elastic modulus (E') and Tan delta (loss factor). The Tan delta value was calculated as the ratio between the viscous dynamic module (E”) and the dynamic elastic modulus (E').

Abrasion: abrasion resistance was evaluated according to DIN 53516, where the sample is forced against a rotating drum and the weight loss (mg) is measured. The lower the value, the greater the abrasion resistance of the sample.

Example 1

Preparation of elastomeric compounds for all-season tire treads

The composition of the elastomeric compounds A-D for high-performance tire treads is shown in the following Table 1. All values are expressed in phr.

TABLE 1

NR: natural rubber (Standard Thai Rubber STR 20 - Thaiteck Rubber);

BR: butadiene rubber (ND-PBR (SKDN) - PJSC Nizhnekamskneftekhim, Russia);

SBR: styrene-butadiene rubber comprising 25% by weight of styrene and 28% by weight of vinyl with respect to the butadiene content, produced by solution polymerisation, extended with 25 parts of TDAE oil per 100 parts of dry polymer (HPR621 - JSR Corporation; Tg: -47°C);

Carbon black: N330 from Cabot Corporation;

Silica: ZEOSIL® 1165 MP, standard grade with surface area of approx. 175 m ² /d from Solvay;

Coupling agent: bis[3-(triethoxysilyl)propyl]tetrasulphide JH-S69 from ChemSpec Ltd.;

Oil: MES (Mild Extract Solvated) CLEMATIS MS from ENI;

Resin 1: Low Tg resin - Staybelite Ester 3-E - triethylene glycol (TEG) ester of hydrogenated rosin resin (Tg: -19°C);

Resin 2: High Tg resin - Impera R1507 - aliphatic resin (Tg: 65°C);

ZnO: Standard Zn oxide from A-Esse;

Antidegradant: N-(1 ,3-dimethylbutyl)-N’-phenyl-p-phenylene-diamine SANTOFLEX 6PPD from EASTMAN;

Accelerant: N-cyclohexylbenzothiazole-2-sulphenamide RUBENAMID C EG/C from GENERAL QUIMICA.

Evaluation of the elastomeric compounds A-B performance

Starting from the elastomeric compositions shown in Table 1 , the corresponding elastomeric compounds were prepared according to the following process. The mixing of the components was carried out in two steps using an internal mixer (Banbury, Intermix or Brabender)

In the first mixing step (1 ), all the ingredients were introduced with the exception of vulcanisers and accelerants. The mixing was continued for a maximum time of 5 minutes, reaching a temperature of approximately 145°C. Subsequently, in the second mixing step (2), again carried out using an internal mixer, the vulcanisers and accelerants were added, and the mixing was continued for about 4 minutes while maintaining the temperature below 100°C. The compounds were then unloaded. After cooling and at least 12 hours from preparation, some samples of the compounds were vulcanised in a press at 170°C for 10 min to give the specimens useful for mechanical characterisations.

The features of each elastomeric compound A-B were evaluated as previously described in the section “methods of analysis” and the results are summarised in the following Table 2.

TABLE 2

The stiffness (E’) and hysteresis (Tanδ) values in the temperature range between -70°C and 30°C are shown in Figure 2.

The results obtained showed that: • compounds A and B show vulcanisation features in line with the applied conditions;

• compound B shows dynamic mechanical properties in line with or better than the reference compound A, in particular it shows a comparable trend as regards the stiffness (E’) at the various temperatures, while it shows a significant improvement in hysteresis

(Tanδ) in the range of interest (from -10°C to 30°C) predictive of a substantial improvement in performance on wet asphalt;

• compound B shows slightly better static mechanical properties than the reference compound A, in particular in the elongation at break (AR) and tensile strength (CR) values, thus improving the reinforcement of the compound;

• compound B shows better abrasion resistance, allowing thinner treads to be made, thus balancing the increase in rolling resistance on dry resulting from the higher Tan5 value at 70°C.

Example 2

Driving tests

Car tires with tread band prepared by vulcanisation of the reference elastomeric composition A and according to the invention B were produced and subjected to driving tests.

All tires had size 235/60R18 107V, with 8.0Jx18 rim and 2.0 bar inflation pressure for the front and rear tires.

The tests were carried out by equipping an Audi Q5 50 TDI car for the tests on dry and wet roads, while a BMW X5 (F15 2WD) was used for the snow tests.

Braking tests were carried out on wet road surfaces (wet braking), driving behaviour tests in normal conditions on dry (soft handling) and wet roads (wet handling), driving behaviour tests in extreme conditions (hard handling) on dry road surfaces and traction and braking tests on snow-covered roads (snow traction and snow braking).

The braking test took place with tires fitted to a vehicle equipped with an anti-lock braking system (A.B.S.).

The wet braking test was carried out on a straight stretch of asphalt, measuring the space needed to go from an initial speed of 80 km/h to 10 km/h.

The braking test on snow-covered road was carried out subjecting the vehicle to deceleration from 35 to 10 km/h using both the wheel anti-blocking system (A.B.S.) and the drive with blocked wheels.

The driving behaviour test, on dry or wet surfaces, was carried out on predefined routes, typically circuits closed to traffic. By simulating some typical manoeuvres (such as changing lanes, overtaking, slalom between skittles, entering and exiting comers) performed at a constant speed, and in acceleration and deceleration, the test driver evaluated the tire performance giving a numerical evaluation of the behaviour of the latter during the above manoeuvres. In particular, the driving behaviour in normal conditions expresses the car's response to steering in straight-line driving conditions at low/medium speed, while the driving behaviour in extreme conditions expresses the set of features of the car/tire set either in conditions of emergency manoeuvres or with very sporty or exasperated driving, which imply reaching the grip limit.

The traction test on a snow-covered road was carried out by subjecting the vehicle to acceleration from 10 to 35 km/h and measuring the traction force exerted by the tire on the snow-covered road surface with accelerometers.

The results of the driving tests, where the values of the assessments were re-measured by setting the values relating to the reference tire A to 100, are shown in the following Table 3:

TABLE 3

The results shown in Table 2 show that the driving performance of the tire according to the invention, the tread of which consisted of compound B, was significantly improved, both as regards handling in normal and extreme conditions in wet and dry conditions and in wet braking tests, without significantly penalising the traction and braking performance on snow-covered roads.

In conclusion, the examples described herein show that compounds for tire treads comprising a specific combination of high Tg resins and low Tg resins, the latter in partial replacement of the oil conventionally used in compounds for all-season tires, result in a particular hysteresis trend as the temperature varies, i.e. an increase in values starting from the temperature of -30°C and gradually more pronounced for higher temperatures, included in the temperature range relevant for wet performance of the tire. This particular hysteretic behaviour of the compounds according to the invention has the advantage of guaranteeing the mobility of the rubber in the range of very low temperatures for snow, and extending the working range of the tire in the area of performance in the wet, particularly relevant for four- season tires, while providing good wear resistance.