EP1404755A1 - Tyre tread reinforced with silica having a very low specific surface area - Google Patents

Abstract

The invention relates to a tyre tread comprising an elastomer composition reinforced with silica and made from at least (pce = parts by weight per 100 parts of diene elastomer): (i) a diene elastomer; (ii) by way of a reinforcing filler, more than 80 pce of an inorganic filler consisting wholly or partly of a silica (called LS) having the following characteristics: (a) a BET specific surface area of between 50 and 100 m<2>/g; (b) an average particle size dw of between 50 and 350 nm; (iii) a coupling agent that is used to link the reinforcing inorganic filler and the diene elastomer; (iv) a sulphur-based vulcanisation system. Preferably, said LS silica has a deagglomeration speed alpha that is greater than 5.10 -3 mu m-1/min (measured with the deagglomeration test using ultrasound). In comparison to a standard tyre tread for green tyres reinforced with a silica having a higher specific surface area, the inventive tyre tread is provided with an improved performance trade-off in terms of wear resistance, rolling resistance, adhesion and drift thrust.

Description

 TIRE TREAD

REINFORCED WITH A VERY LOW SPECIFIC SILICA

The present invention relates to tire treads, more particularly to treads reinforced with an inorganic filler, in particular silica, and to tires comprising such treads.

The tire treads must in a known manner obey numerous technical requirements, often contradictory, in the first place having a high resistance to wear or abrasion and a high grip while offering the tire a very good level of road behavior ( "handling") on a motor vehicle, requiring in particular a high drift thrust or cornering.

In order to obtain the optimum reinforcement properties conferred by a load in a tire tread and thus a high resistance to wear, it is known that it is generally necessary for this load to be present in the elastomeric matrix under a final form which is both as finely divided as possible and distributed as homogeneously as possible. However, such conditions can only be achieved insofar as this filler has a very good ability, on the one hand to be incorporated into the matrix during mixing with the elastomer and to disaggregate, on the other hand to disperse homogeneously in this matrix.

In known manner, carbon black exhibits such aptitudes, which is generally not the case for inorganic fillers, in particular silicas. Indeed, for reasons of reciprocal affinities, these particles of inorganic charge have an unfortunate tendency, in the elastomeric matrix, to agglomerate between them. These interactions have the harmful consequence of limiting the dispersion of the filler and therefore the reinforcing properties to a level substantially lower than that which would theoretically be possible to achieve if all the bonds (inorganic filler / elastomer) capable of being created. during the mixing operation, were actually obtained; on the other hand, these interactions tend to increase the consistency in the raw state of the rubber compositions and therefore to make their implementation ("processability") more difficult than in the presence of carbon black.

Since fuel economy and the need to protect the environment have become a priority, however, it has become necessary to produce tires with reduced rolling resistance without penalizing their wear resistance.

This has been made possible in particular thanks to the use, in the treads of these tires, of new rubber compositions reinforced with inorganic fillers, in particular of silicas, specific of the highly dispersible type, capable of competing from the point of view of reinforcing with conventional pneumatic grade carbon black, while offering these compositions a lower hysteresis, synonymous with lower rolling resistance for the tires comprising them, as well as improved grip on wet, snowy or icy conditions. . Treads loaded with such highly dispersible silicas, which can be used in tires with low rolling resistance, sometimes qualified as "Green Tires" for the energy saving offered to the user ("Green Tire concept"), have been extensively described. Reference is made in particular to patent applications EP 501 227, EP 692 492, EP 692 493, EP 735 088, EP 767 206, EP 786 493, EP 881 252, WO99 / 02590, WO99 / 02601, O99 / 02602, WO99 / 06480, WO00 / 05300, WO00 / 05301.

All these documents of the prior art teach the need, in order to obtain sufficient wear resistance, to use silicas which not only are sufficiently dispersible but also have a high BET specific surface area, between 100 and 250 m ² / g, in fact typically greater than 150 m ² / g (see in particular EP 501 227 cited above). A silica with a high specific surface area making reference in the field of "Pneus Nerts" is in particular the silica "Zeosil 1165 MP" (BET surface area equal to approximately 160 m ² / g) sold by the company Rhodia (see above-mentioned documents).

The use of these specific silicas with a high specific surface, as reinforcing filler, has certainly reduced the difficulties of processing the rubber compositions containing them, but this processing nevertheless remains delicate, more difficult than for loaded compositions conventionally carbon black.

It is first of all necessary to use a coupling agent, also called bonding agent, which has the function of ensuring the connection or bonding entered the surface of the particles (silanol groups Si-OH) of silica and the elastomer , while facilitating the dispersion of this charge within the elastomeric matrix, the high specific surface recommended for these silicas being precisely intended to increase the number and quality of the connections between the silica and the coupling agent, in order to reach the top expected level of reinforcement.

Such coupling agents are well known to those skilled in the art, essentially polyfunctional organosilanes or polysiloxanes. The best known are polysulphurized alkoxysilanes, in particular bis- (alkoxylsilylalkyl) polysulphides such as bis- (alkoxylsilylpropyl) polysulphides, more particularly bis 3-triethoxysilylpropyl tetrasulphide and disulphide (in short TESPT and TESPD, respectively) generally considered like products providing, for silica-laden treads, the best compromise in terms of safety in roasting, ease of implementation and reinforcing power. As such, they are the coupling agents most used today in "Green Tires".

These polysulphurized alkoxysilanes have the known drawback, however, that they are very expensive and must moreover be used in a relatively large amount, typically of the order of 8 to 12% by weight relative to the weight of silica, which is equivalent to rates generally between 5 and 10 phr (phr = parts by weight per hundred parts of diene elastomer). To reduce these alkoxysilane levels, coupling activators have been proposed in particular (see applications WO00 / 05300 and WO00 / 05301 above). Another factor affecting the processing of rubber compositions in the raw state, compared with the use of carbon black, is linked to the strong physicochemical interactions which may be established between these reinforcing silicas, because of their high surface area. reactive, and other ingredients of the rubber compositions, in particular with the vulcanization system. This drawback is the cause of a loss of yield and a reduction in the vulcanization kinetics. To counteract these parasitic effects, the rates of vulcanizing agents, in particular accelerators, had to be increased, other types of accelerators (secondary accelerators) added, compared to conventional formulations based on carbon black. .

In summary, if the treads loaded with dispersible silicas with a high specific surface area have certainly made it possible to achieve a compromise in properties, in terms of rolling resistance, wear resistance and grip, not accessible with a black of conventional carbon for tires, such a result could only be obtained at the cost of a more difficult implementation and an additional industrial cost linked in particular to the use of relatively large quantities of coupling agents, in particular of polysulfurized alkoxysilanes, and an increase in the levels of vulcanizing agents.

However, the Applicants have discovered during their research that another category of silicas can not only lead to an excellent compromise in performance for the tire treads containing them, even globally superior to that offered by silicas with a high specific surface. , but still make it possible to alleviate, at least in part, the aforementioned drawbacks posed by silicas with a high specific surface used in the treads of "Green Tires".

Consequently, a first subject of the invention relates to a tread incorporating an elastomeric composition reinforced with an inorganic filler, based on at least (pce = parts by weight per hundred parts of diene elastomer):

(i) a diene elastomer; (ii) by way of reinforcing filler, more than 80 phr of an inorganic filler constituted in whole or in part of a silica having the following characteristics (hereinafter "LS" silica):

(a) a BET specific surface area of between 50 and 100 m ² / g;

(b) an average particle size d _w of between 50 and 350 nm; (iii) a coupling agent ensuring the connection between the reinforcing inorganic filler and the diene elastomer; (iv) a sulfur-based vulcanization system.

Such silicas denoted "LS" (for "Low Surface") are known and could have been used, as adhesion promoting fillers or additives, in certain parts of tires, essentially in internal mixtures based on natural rubber intended in particular for calendering plies of crown reinforcement or tire carcass. However, to the knowledge of the Applicants, no document of the state of the art describes or especially does not explain the use of such LS silicas, sometimes qualified as "coarse" because of their very low specific surface and a reinforcing power deemed consequently insufficient, in tire treads. The invention also relates to the use as reinforcing filler, in a tire tread, of a silica LS having the characteristics a and b above.

The invention also relates to the use of a tread according to the invention for the manufacture or retreading of tires, as well as these tires themselves when they comprise a tread according to the invention . The tires of the invention are particularly intended for equipping vehicles capable of traveling at high speed in a sustained manner such as passenger vehicles, 4x4 vehicles (4-wheel drive), SUV ("Sport Utility Vehicles"), two wheels (in particular motorcycles), vans, "Heavy goods vehicles" (notably metro, bus, road transport equipment such as trucks, tractors, trailers).

The treads according to the invention can be prepared by a process constituting another object of the present invention. This improved process for preparing a tire vulcanizable with sulfur, with performance compromise (wear resistance / rolling resistance / adhesion / drift thrust), based on diene elastomer, of a reinforcing inorganic filler and of a vulcanization system, comprises the following stages: • incorporating into a diene elastomer, during a first stage known as "non-productive", at least: as reinforcing filler, more than 80 phr of a inorganic filler consisting in whole or in part of a silica having the following characteristics (so-called "LS" silica): - (a) a BET specific surface of between 50 and 100 m ² / g;

- (b) an average particle size d _w of between 50 and 350 nm;

• and a coupling agent ensuring the connection between the silica and the diene elastomer; by thermomechanically kneading the whole, in one or more times, until reaching a maximum temperature of between 110 ° C and 190 ° C; • cool the assembly to a temperature below 100 ° C; then incorporate, during a second so-called "productive" step, sulfur and a primary vulcanization accelerator; • mix everything up to a maximum temperature below 110 ° C; calendering or extruding the elastomeric composition thus obtained in the form of a tire tread.

According to a preferred embodiment of the invention, LS silica constitutes the majority, more preferably still all of the reinforcing inorganic filler. The invention thus makes it possible to significantly reduce the levels of vulcanizing agents, sulfur and vulcanization accelerators.

Another particularly preferred embodiment consists in using, as vulcanization activator, a substantially reduced rate, namely between only 0.5 and 1.5 phr of zinc, provided in particular in the form of zinc oxide or zinc stearate. It has in fact been found that the use of LS silicas has the drawback of significantly increasing the induction times during vulcanization (time required at the start of the vulcanization reaction) and that, unexpectedly, this drawback could be overcome by a reduction in the usual level of zinc.

The invention as well as its advantages will be easily understood in the light of the description and of the exemplary embodiments which follow, as well as of the single figure relating to these examples which represents curves of variation of module (in MPa) in. function of the elongation (in%), recorded for rubber compositions intended for tire treads., conforming or not to the invention.

I. MEASUREMENTS AND TESTS USED

1-1. Characterization of silicas

The silicas described below consist, in known manner, of agglomerates of particles,

; likely to disaggregate into these particles under the effect of an external force, for example under the action of mechanical work or ultrasound. The term "particle" used in the present application must be understood in its usual generic sense of aggregate (also called "secondary particle"), and not in that of elementary particle (also called "primary particle") which may form, the if applicable, part of this aggregate; by "aggregate" means in a known manner the non-breaking set (Le., which cannot be cut, divided, shared) which is produced during the synthesis of the charge, generally formed of aggregated elementary (primary) particles between them. ^"'

These silicas are characterized as indicated below.

A) Specific surface:

The BET specific surface area ("mass area") is determined by gas adsorption using the Brunauer-Emmett-Teller method described in "The Journal of the American Chemical Society" Vol. 60, page 309, February 1938), more precisely according to French standard NF ISO 9277 of December 1996 [multi-point volumetric method (5 points) - gas: nitrogen - degassing: hour at 160 ° C - relative pressure range p / in: 0.05 to 0.17]

The CTAB specific surface is the external surface determined according to French standard NF T 45-007 of November 1987 (method B).

B) Average particle size d w,

The average size (by mass) of the particles, denoted d _w , is measured in a conventional manner after dispersion, by ultrasonic deagglomeration, of the charge to be analyzed in water.

The measurement is carried out using a centrifugal sedimentometer with X-ray detection type "XDC"("X-rays Disk Centrifuge"), sold by the company Brookhaven Instruments, according to the following operating mode. A suspension of 3.2 g of silica sample to be analyzed is produced in 40 ml of water, per action for 8 minutes, at 60% power (60% of the maximum position of the "output control"), a 1500 W ultrasonic probe (3/4 inch Vibracell sonicator sold by the company Bioblock); after sonication, 15 ml of the suspension are introduced into the rotating disc; after sedimentation for 120 minutes, the mass distribution of the particle sizes and the average mass size of the particles d _w are calculated by the "XDC" sedimentometer software (d _w = ∑ (nj di ⁵ ) / ∑ (nj dj ⁴ ) with neither number of objects of the size class or diameter dj).

O disaggregation speed α:

The deagglomeration speed noted α is measured by means of an ultrasonic deagglomeration test, at 100% power of a 600 W (watt) probe, operating here in pulsed mode (i.e.: 1 second ON, 1 second OFF ) to avoid excessive heating of the ultrasound probe during the measurement. This known test, which is in particular the subject of patent application WO99 / 28376 (see also O99 / 28380, WO00 / 73372, WOOO / 73373), makes it possible to continuously measure the change in the average size (in volume) of particle agglomerates during sonication, as indicated below.

The assembly used consists of a laser granulometer (type "Mastersizer S", sold by Malvern Instruments - He-Ne laser source emitting in the red, wavelength 632.8 nm) and its preparer ("Malvern Small Sample Unit MSX1 "), between which has been inserted a continuous flow treatment cell (Bioblock M72410) fitted with an ultrasonic probe (1/2 inch sonicator type Vibracell of 600 W sold by the company Bioblock).

A small amount (150 mg) of silica to be analyzed is introduced into the preparer with 160 ml of water, the circulation speed being fixed at its maximum. At least three consecutive measurements are made to determine according to the known Fraunhofer calculation method (Malvern calculation matrix 3 $$ D) the average initial diameter (in volume) of the agglomerates, denoted d _v [0]. The sonication (pulsed mode: 1 s ON, 1 s OFF) is then established at a power of 100% (ie 100% of the maximum position of the "tip amplitude") and the evolution of the mean diameter is followed for approximately 8 minutes. in volume d _v [t] as a function of time "t" at the rate of a measurement every 10 seconds approximately. After an induction period (approximately 3-4 minutes), it is observed that the inverse of the mean diameter in volume l / d _v [t] varies linearly, or in a substantially linear manner, with time "t" (regime deagglomeration stable). The disagglomeration speed α is calculated by linear regression of the evolution curve of l / d _v [t] as a function of time "t", in the zone of steady disagglomeration regime (in general, between approximately 4 and 8 minutes ). It is expressed in μm'Vmin.

The aforementioned application WO99 / 28376 describes in detail a measuring device which can be used for carrying out this ultrasonic deagglomeration test. This device, it will be recalled, consists of a closed circuit in which a flow of agglomerates of particles in suspension in a liquid can flow. This device essentially comprises a sample preparer, a laser granulometer and a processing cell. A setting at atmospheric pressure, at level of the sample preparer and of the treatment cell itself, allows the continuous elimination of air bubbles which form during sonication (action of the ultrasonic probe).

The sample preparer ("Malvern Small Sample Unit MSX1") is intended to receive the silica sample to be tested (suspended in liquid 3) and to circulate it through the circuit at the preset speed (potentiometer - speed maximum of about 3 1 / min), in the form of a flow of liquid suspension. This preparer simply consists of a receiving tank which contains, and through which circulates, the suspension to be analyzed. It is equipped with an agitation motor, with adjustable speed, in order to avoid sedimentation of the agglomerates of particles in the suspension; a centrifugal mini-pump is intended to ensure the circulation of the suspension in the circuit; the inlet of the preparer is connected to the open air via an opening intended to receive the load sample to be tested and / or the liquid used for the suspension.

The preparer is connected to a laser granulometer ("Mastersizer S") whose function is to continuously measure, at regular time intervals, the volume average size "d _v " of the agglomerates, as the flow passes, thanks to a measurement to which the automatic recording and calculating means of the granulometer are coupled. It will be recalled here briefly that laser granulometers exploit, in a known manner, the principle of diffraction of light by solid objects suspended in a medium whose refractive index is different from that of the solid. According to Fraunhofer's theory, there is a relationship between the size of the object and the angle of diffraction of light (the smaller the object, the larger the angle of diffraction). In practice, it is sufficient to measure the quantity of diffracted light for different diffraction angles to be able to determine the size distribution (in volume) of the sample, d _v corresponding to Ta average size in volume of this distribution (d _v = ∑ ( nj dj ⁴ ) / ∑ (n; dj ³ ) with n. _\ number of objects of the size or diameter class di).

Interposed between the preparer and the laser granulometer is finally a processing cell equipped with an ultrasonic probe, which can operate in continuous or pulsed mode, intended to continuously break up the agglomerates of particles as the flow passes. This flow is thermostatically controlled by means of a cooling circuit arranged, at the level of the cell, in a double envelope surrounding the probe, the temperature being controlled for example by a temperature probe immersed in the liquid at the level of the preparer.

1-2. Characterization of rubber compositions

The rubber compositions are characterized, before and after curing, as indicated below.

A) Mooney plasticity:

An oscillating consistometer is used as described in the French standard NF T 43-005 (1991). The Mooney plasticity measurement is carried out according to the following principle: the composition in the raw state (Le., Before baking) is molded in a cylindrical enclosure heated to 100 ° C. After one minute of preheating, the rotor turns within the test tube at 2 revolutions / minute and the torque useful for maintaining this movement is measured after 4 minutes of rotation. The Mooney plasticity (ML 1 + 4) is expressed in "Mooney unit" (MU, with 1 MU = 0.83 Newton.meter).

B) Toasting time:

The measurements are carried out at 130 ° C, in accordance with French standard NF T 43-005 (1991). The evolution of the consistometric index as a function of time makes it possible to determine the toasting time of the rubber compositions, assessed in accordance with the aforementioned standard by parameter T5 (in the case of a large rotor), expressed in minutes, and defined as being the time necessary to obtain an increase in the consistometric index (expressed in MU) of 5 units above the minimum value measured for this index.

C) Rheometry:

The measurements are carried out at 150 ° C. with an oscillating chamber rheometer, according to standard DIN 53529 - part 3 (June 1983). The evolution of the rheometric torque as a function of time describes the evolution of the stiffening of the composition as a result of the vulcanization reaction. The measurements are processed according to DIN 53529 - part 2 (March 1983): tj is the induction time, that is to say the time necessary for the start of the vulcanization reaction; t _α (for example t ₉₀ or t ₉₉ ) is the time necessary to reach a conversion of α%, i.e. α% (for example 90% or 99%, respectively) of the difference between the couples minimum and maximum. The conversion speed constant denoted K (expressed in min ^'1 ), of order 1, calculated between 30% and 80% of conversion, is also measured, which makes it possible to assess the vulcanization kinetics.

D) Tensile tests:

These tests make it possible to determine the elasticity stresses and the breaking properties. Unless otherwise indicated, they are carried out in accordance with French standard NF T 46-002 of September 1988. The second secant modules (or apparent stresses, or apparent stresses, in MPa) are measured at 10% (ie after an accommodation cycle) at 10% elongation (denoted MA10), 100% elongation (denoted MA100) and 300% elongation (denoted MA300). The stresses at break (in MPa) and the elongations at break (in%) are also measured. All these traction measurements are carried out under normal temperature (23 ± 2 ° C) and hygrometry (50 + 5% relative humidity) conditions, according to French standard NF T 40-101 (December 1979).

A processing of the traction records also makes it possible to plot the module curve as a function of the elongation (see attached figure), the module used here being the true secant module measured in first elongation, calculated by reducing to the real section of the 'test tube and not in the initial section as previously for the nominal modules.

E) Dynamic properties: The dynamic properties ΔG * and tan _max (δ) are measured on a viscoanalyzer (Metravib VA4000), according to standard ASTM D 5992-96. The response of a sample of vulcanized composition (4 mm thick cylindrical test piece and 400 mm ² section) is recorded, subjected to a sinusoidal stress in alternating single shear, at the frequency of 10 Hz, under normal conditions of temperature (23 ° C) according to ASTM D 1349-99, or as the case may be at a different temperature. A deformation amplitude sweep is carried out from 0.1 to 50%) (outward cycle), then from 50%) to 1% (return cycle). The exploited results are the complex dynamic shear modulus (G *) and the loss factor tan (δ). For the return cycle, we indicate the maximum value of tan (δ) observed, noted tan (δ) _max , as well as the difference in complex modulus (ΔG *) between the values at 0.15 and 50% of deformation (Payne effect) .

F) Shore A hardness:

The Shore A hardness of the compositions after curing is assessed in accordance with standard ASTM D 2240-86.

1-3. Characterization of tires or treads

A) Rolling resistance:

Rolling resistance is measured on a steering wheel, according to ISO 87-67, (1992). A value greater than that of the control, arbitrarily set to 100, indicates an improved result, that is to say a lower rolling resistance.

B) Resistance to wear:

The tires are subjected to actual rolling on the road, on a specific motor vehicle, until the wear due to rolling reaches the wear indicators arranged in the grooves of the tread. A value greater than that of the witness, arbitrarily fixed at 100, indicates an improved result, that is to say a higher mileage traveled.

C) Braking on dry ground:

The tires are mounted on a motor vehicle equipped with an ABS braking system and the distance required to go from 100 km / h to 0 km / h is measured during brutal braking on dry ground (bituminous concrete). A value greater than that of the control, arbitrarily fixed at 100, indicates an improved result, that is to say a shorter braking distance.

D) Braking on wet ground:

The tires are mounted ^'on a motor vehicle equipped with a braking system

ABS and we measure the distance required to go from 50 km h to 10 km / h during brutal braking on watered ground (bituminous concrete). A value higher than that of the witness, arbitrarily set to 100, indicates an improved result i.e. a shorter braking distance.

E) Braking on ice:

The tires are mounted on a motor vehicle equipped with an ABS braking system on all four wheels, and the distance required to go from 20 km / h to 5 km h is measured when braking on ice. A value greater than that of the control, arbitrarily set at 100, indicates an improved result, that is to say a shorter braking distance.

F) Grip on wet ground:

To assess the grip performance on wet ground, the behavior of the tires mounted on a given motor vehicle is analyzed, under speed limit conditions, on a circuit that is highly virulent and watered so as to keep the ground moist.

On the one hand, we measure the minimum time necessary to cover the entire circuit; a value greater than that of the control, arbitrarily fixed at 100, indicates an improved result, that is to say a shorter travel time.

The professional driver of the vehicle also gives a global, subjective rating of the road behavior of the vehicle - and therefore of the tires - on this watered virgin circuit; a score higher than that of the witness, arbitrarily set at 100, indicates improved overall behavior.

G) Drift thrust:

Each tire tested is mounted on a wheel of suitable size and inflated to 2.2 bars. It is made to run at a constant speed of 80 km / h on an appropriate automatic machine ("ground-plane" type machine sold by the company MTS). The load denoted "Z" is varied, under a drift angle of 1 degree, and the stiffness or drift thrust denoted "D" (corrected for zero drift thrust) is measured in a known manner, by recording at using sensors the transverse force on the wheel as a function of this load Z. The edge thrust indicated in the tables is the slope at the origin of the curve D (Z); a value greater than that of the control, arbitrarily set at 100, indicates an improved result, that is to say a stronger drift thrust. An increase in drift thrust is favorable to road behavior on dry ground.

IT CONDITIONS FOR CARRYING OUT THE INVENTION

The tire treads according to the invention are therefore formed, in whole or in part, of a rubber composition based on at least: (i) a diene elastomer; (ii) by way of reinforcing filler, more than 80 phr of an inorganic filler constituted in whole or in part of a silica having the following characteristics:

(a) a BET specific surface area of between 50 and 100 m ² / g;

(b) _. an average particle size d _w of between 50 and 350 nm; (iii) a coupling agent (inorganic filler / diene elastomer) ensuring the connection between the inorganic filler and the diene elastomer; (iv) a sulfur-based vulcanization system.

Of course, by the expression composition "based on" is meant a composition comprising the mixture and / or the in situ reaction product of the various constituents used, some of these base constituents being capable of, or intended to react between them, at least in part, during the different manufacturing phases of the treads and tires, in particular during their vulcanization.

II- 1. Diene elastomer

By "diene" elastomer (or rubber) is understood in known manner an elastomer derived at least in part (ie, a homopolymer or a copolymer) from diene monomers, that is to say from monomers carrying two carbon double bonds -carbon, conjugated or not. By "essentially unsaturated" diene elastomer is meant here a diene elastomer derived at least in part from conjugated diene monomers, having a proportion of units or units of diene origin (conjugated dienes) which is greater than 15% (% by moles) . In the category of “essentially unsaturated” diene elastomers, the expression “highly unsaturated” diene elastomer is understood in particular to mean a diene elastomer having a content of units of diene origin (conjugated dienes) which is greater than 50%.

These general definitions being given, a person skilled in the art of the tire will understand that the present invention is firstly implemented with highly unsaturated diene elastomers, in particular with:

(a) - any homopolymer obtained by polymerization of a conjugated diene monomer having from 4 to 12 carbon atoms;

(b) - any copolymer obtained by copolymerization of one or more dienes conjugated together or with one or more vinyl aromatic compounds having from 8 to 20 carbon atoms.

By way of conjugated dienes, 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-di-alkyl (Cι-C ₅ ) -1, 3-butadienes, such as for example, are suitable. 2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene, 2-methyl-3-isopropyl-1 , 3-butadiene, an aryl-1,3-butadiene, 1,3-pentadiene, 2,4-hexadiene. Suitable vinyl-aromatic compounds are, for example, styrene, ortho-, meta-, para-methylstyrene, the commercial "vinyl-toluene" mixture, para-tertiobutylstyrene, methoxystyrenes, chlorostyrenes, vinyl mesitylene, divinylbenzene. , vinylnaphthalene.

The copolymers can contain between 99% and 20% by weight of diene units and between 1% and 80% or by weight of vinyl-aromatic units. Elastomers can have any microstructure which is a function of the polymerization conditions used, in particular the presence or absence of a modifying and / or randomizing agent and the quantities of modifying and / or randomizing agent used. The elastomers can for example be block, statistical, sequence, microsequenced, and be prepared in dispersion or in solution; they can be coupled and / or stars or functionalized with a coupling and / or star-forming or functionalizing agent.

The diene elastomer of the tread according to the invention is preferably made, in whole or in part, in an amount of at least 40 phr, more preferably still at least 50 phr, by a highly unsaturated elastomer of the type butadiene, that is to say chosen from the group of highly unsaturated diene elastomers constituted by polybutadienes (BR), butadiene copolymers and mixtures of these elastomers. These butadiene copolymers are in particular butadiene-styrene copolymers (SBR), isoprene-butadiene copolymers (BIR), isoprene-butadiene-styrene copolymers (SBIR).

As preferred butadiene elastomers, in particular, the BRs having a content of -1.2 units between 4% and 80% or those having a cis-1,4 content greater than 80%), the SBRs having a content in styrene between 5%> and 50% by weight and more particularly between 20% and 40%>, a content of -1,2 bonds of the butadiene part between 4% and 65%, a content of trans-1 bonds , 4 between 20%> and 80%>, the BIRs having an isoprene content of between 5%> and 90%> by weight and a glass transition temperature ("Tg" measured according to standard ASTM D3418-82) of - 40 ° C to -80 ° C. In the case of SBIR copolymers, especially those having a styrene content of between 5% and 50% by weight and more particularly of between 10%) and 40%, an isoprene content of between 15%) and 60% by weight are suitable; more particularly between 20% and 50%, a content of butadiene between 5% and 50%) by weight and more particularly between 10% and 40%>, a content of units -1,2 of the butadiene part of between 4 % and 85% o, a content in trans units -1.4 of the butadiene part between 6% and 80%, a content in units -1.2 plus -3.4 of the isoprenic part between 5% and 70 % and a content in trans units -1.4 of the isoprenic part between 10% and 50%), and more generally any SBIR having a Tg ime between -20 ° C and -70 ° C.

The butadiene elastomer is particularly chosen from BR, SBR and mixtures of these elastomers.

Preferably, in the case of a tire tread for a passenger vehicle, the butadiene elastomer is mainly an SBR elastomer, whether it is an SBR prepared in emulsion ("ESBR") or a SBR prepared in solution ("SSBR"), or a mixture of SBR and another diene elastomer, in particular butadiene, for example a blend of SBR and BR, of SBR and NR (natural rubber), of SBR and IR (synthetic polyisoprene).

Use is made in particular of an SBR having a styrene content of between 10% and 30%> by weight, a content of vinyl bonds of the butadiene part of between 15%) and 65% o, a content of trans-1,4 bonds included between 15%) and 75%) and a Tg of between -20 ° C and -55 ° C. Such an SBR copolymer, preferably an SSBR, is optionally used in admixture with BR preferably having more than 90% of cis-1,4 bonds. The compositions of the treads of the invention may contain a single diene elastomer or a mixture of several diene elastomers, the diene elastomer or elastomers being able to be used in combination with any type of synthetic elastomer other than diene, or even with other polymers. as elastomers, for example thermoplastic polymers.

H-2. Reinforcing inorganic filler ("LS" silica)

The essential characteristic of the tread according to the invention is that it is reinforced by more than 80 phr of a reinforcing inorganic filler comprising, preferably in the majority, a specific silica with very low specific surface having the following characteristics:

- (a) a BET specific surface area of between 50 and 100 m ² / g;

- (b) an average particle size d _w of between 50 and 350 nm.

Silicas with a small BET surface capable of meeting this definition are known and have been described in particular in applications EP 157 703, EP 396 450 or EP 722 977. As already indicated, their known application in tires has hitherto been limited to parts of the tire other than its tread, in particular in internal mixtures used, for example, for calendering plies of crown or carcass reinforcement. ^•

It is recalled that by "reinforcing inorganic filler" must be understood in known manner an inorganic or inorganic filler, whatever its color and its origin (natural or synthetic), also called "white" filler or sometimes "clear" filler by opposition to carbon black, capable of reinforcing on its own, without other means than an intermediate coupling agent, a rubber composition intended for the manufacture of tires, in other words capable of replacing, in its reinforcing function, a conventional charge of pneumatic grade carbon black.

This specific silica "LS" (for "Low Surface") firstly has an unusual BET surface for a tread application, between 50 and 100 m ² / g. For a BET surface area of less than 50 m ² / g, the rubber compositions certainly exhibit easier processing as well as reduced hysteresis, but there is a decline in the properties at break and resistance to wear, in tires , which decreases prohibitively. For a BET surface area greater than 100 m ² / g, in particular between 150 and 250 m ² / g, there is the disadvantage of the usual silicas with a high specific surface for "Green Tires" treads, namely on the one hand, reduced dispersibility in the rubber matrix and difficulties in processing in the raw state, due to parasitic interactions with certain other ingredients of the rubber compositions (in particular vulcanization system), on the other hand the need for '' use a higher level of coupling agent.

The silica LS must on the other hand have a size d _w of between 50 and 350 nm. For excessively large sizes d _w , greater than 350 nm, the particles behave like defects which locate the constraints and are harmful to wear; sizes d _{w that are} too small, less than 50 nm, on the other hand penalize implementation in the raw state and the dispersion of the charge during this implementation.

Finally, for applications where the highest level of reinforcement is targeted, the LS silica used will also preferably have a high intrinsic dispersibility, illustrated by ¹ a deagglomeration speed α (measured in the ultrasonic deagglomeration test described in previous chapter I) greater than 5.10 ^-3 μπrVmin, more preferably at least equal to 1.10 ^-2 μπrVmin. For such a disaggregation speed, it was found that the silica LS exhibited a very high dispersibility, that is to say that few micron agglomerates are observed by reflection under optical microscopy on a section of rubber composition prepared according to the rules. art.

For the various reasons indicated above, the silica LS selected preferably checks at least one, more preferably all, of the following characteristics:

a BET surface in a range of 60 to 90 m ² / g; a particle size d _w of between 100 and 300 nm; a disagglomeration speed α greater than 5.10 " ³ μm'Vmin.

Even more preferably, this LS silica verifies all of the following characteristics:

a BET surface in a range of 60 to 90 m ² / g; - A particle size d _w in a range of 150 to 250 nm; a disagglomeration speed α at least equal to 1.10 " ² μnrVmin.

The physical state under which LS silica may appear is indifferent, whether in the form of powder, microbeads, granules, pellets, beads or any other densified form; it can be a precipitated silica such as a pyrolysis silica. Its BET / CTAB surface ratio is preferably within a range from 1.0 to 1.5, more preferably from 1.0 to 1.2.

The silica LS above can advantageously constitute all of the reinforcing inorganic filler.

However, this LS silica may optionally be associated with another reinforcing inorganic filler, for example a conventional reinforcing silica with a higher specific surface. In such a case, the silica LS preferably constitutes at least 50% by weight of the total reinforcing inorganic filler, more preferably still more than 80%) by weight of this total reinforcing inorganic filler.

Preferably, the rate of reinforcing inorganic filler, that is to say of LS silica when the latter constitutes the totality of the reinforcing inorganic filler, is greater than 90 phr, more preferably comprised within a range of 100 to 150 phr. , the optimum of course being different depending on the type of tire concerned. Silica LS can also be combined with a conventional carbon black of pneumatic grade, chosen in particular from blacks of the HAF, ISAF, SAF type conventionally used in tire treads (for example, blacks NI 15, N134, N234, N330, N339, N347, N375). This carbon black is then preferably used in small proportion, at a rate preferably between 2 and 20 phr, more preferably in a range of 5 to 15 phr. In the indicated intervals, the coloring properties (black pigmentation agent) and anti-UV properties of the carbon blacks are benefited, without, moreover, penalizing the typical performances provided by LS silica.

Finally, a person skilled in the art will understand that, as an equivalent charge of a reinforcing inorganic filler, a reinforcing filler of the organic type could be used, in particular a carbon black for tires, covered at least in part with an inorganic layer. , in particular of silica, requiring as for it the use of a coupling agent to ensure the connection with the elastomer.

II-3. Coupling agent

It is recalled here that by "coupling agent" (inorganic filler / elastomer) is meant, in known manner, an agent capable of establishing a sufficient bond, of chemical and / or physical nature, between the inorganic filler and the diene elastomer ; such a coupling agent, at least bifunctional, has for example as simplified general formula "Y-T-X", in which:

Y represents a functional group (“Y” function) which is capable of physically and / or chemically binding to the inorganic charge, such a bond being able to be established, for example, between a silicon atom of the coupling agent and surface hydroxyl groups (OH) of the inorganic filler (for example surface silanols when it is silica); - X represents a functional group ("X" function) capable of physically and / or chemically bonding to the diene elastomer, for example by means of a sulfur atom;

T represents a divalent organic group making it possible to connect Y and X.

It will be recalled that the coupling agents should not be confused with simple agents for recovering the inorganic charge which, in known manner, may include the "Y" function active with respect to the inorganic charge but are devoid of the "X" function active with respect to the diene elastomer.

Coupling agents (silica / diene elastomer), of variable effectiveness, have been described in a very large number of documents and are well known to those skilled in the art. Any known coupling agent can be used capable of effectively ensuring, in diene rubber compositions which can be used for the manufacture of tire treads, the connection between a reinforcing inorganic filler such as silica and a diene elastomer, in particular organosilanes or polyfunctional polyorganosiloxanes carrying the functions X and Y above. Use is made in particular of polysulphurized silanes, called "symmetrical" or "asymmetrical" according to their particular structure, as described for example in patents or patent applications FR 2 149 339, FR 2 206 330, US 3 842 111, US 3 873 489, US 3,978 103, US 3,997,581, US 4,002,594, US 4,072,701, US 4,129,585, US 5,580,919, US 5,583,245, US 5,650,457, US 5,663,358, US 5,663 395, US 5 663 396, US 5 674 932, US 5 675 014, US 5 684 171, US 5 684 '172, US 5 696 197, US 5 708 053, US 5 892 085, EP 1 043 357., _β

Particularly suitable for implementing the invention, without the following definition being limiting, symmetrical polysulphurized silanes corresponding to the following general formula (I):

(I) Z - A - S _n - A - Z, in which:

- n is an integer from 2 to 8 (preferably from 2 to 5);

- A is a di valent hydrocarbon radical (preferably C 1 -C ₈ alkylene groups or C ₆ -C ₁ arylene groups, more particularly C ₁ -C 10, especially C ₁ -C ₄ alkylene groups, in particular propylene);

- Z meets one of the formulas below:

R1 R1 R2

- If— R1; - If— R2; - If— R2,

R2 R2 R2

in which:

- the radicals R ¹ , substituted or unsubstituted, identical or different from each other, represent a Cι-Cι ₈ alkyl, C ₅ -C ₁₈ cycloalkyl or C ₆ -C ₁₈ aryl group (preferably Cι alkyl groups -C ₆ , cyclohexyl or phenyl, in particular Cι-C ₄ alkyl groups, more particularly methyl and / or ethyl).

- the radicals R ^2, substituted or unsubstituted, identical or different, represent a hydroxyl, alkoxyl or Ci-C ₁₈ cycloalkoxy, C ₅ -C ₁₈ (preferably a group selected from hydroxyl, alkoxyl Cι-C ₈ and C ₅ -C ₈ cycloalkoxyls, more preferably still a group chosen from hydroxyl and Cι-C alkoxyls, in particular methoxyl and ethoxyl).

In the case of a mixture of polysulphurized silanes corresponding to formula (I) above, in particular the usual mixtures commercially available, it will be understood that the average value of "n" is a fractional number, preferably included in a range of 2 to

5.

By way of examples of polysulphide silanes, mention will be made more particularly of the polysulphides (in particular disulphides, trisulphides or tetrasulphides) of bis- (alkoxyl (Cι-C ₄ ) -alkyl (C ₁ -C ₄ ) silyl- alkyl (Cι-C ₄ )), such as for example the polysulphides of bis (3-trimethoxysilylpropyl) or bis (3-triethoxysilylpropyl). Among these compounds, use is made in particular of bis (3-tri-ethoxysilylpropyl) tetrasulfide, abbreviated as TESPT, of formula [(C ₂ H ₅ O) ₃ Si (CH ₂ ) ₃ S ₂ ] ₂ or bis disulfide - (triethoxysilylpropyle), abbreviated TESPD, of formula [(C ₂ H ₅ θ) Si (CH) ₃ S] ₂ .

The TESPD is marketed for example by the company Degussa under the name Si75 (in the form of a mixture of disulfide - at 75% by weight - and polysulfides), or also by the company Witco under the name Silquest Al 589. TESPT is marketed for example by the company Degussa under the name Si69 (or X50S when it is supported at 50%) by weight on carbon black), or also by the company Osi Specialties under the name Silquest A1289 (in both cases , commercial mixture of polysulphides with an average value for n which is close to 4).

Those skilled in the art will be able to adjust the content of coupling agent in the compositions of the invention, depending on the intended application, the nature of the elastomer used and the amount of LS silica, supplemented if necessary with any other inorganic filler used as additional reinforcing filler.

It has been found that the use of LS silica, in the treads according to the invention, makes it possible to significantly reduce the level of coupling agent, in particular of polysulphurized silane, compared to the usual levels. practiced in the presence of a conventional silica with a higher specific surface. Thus, in these treads of the invention, the level of coupling agent, in particular of polysulphurized silane, is preferably between 2 and 5 phr, more preferably comprised within a range of 3 to 4.5 phr. Reduced to the weight of reinforcing inorganic filler, in particular of LS silica when the latter constitutes the totality of the reinforcing inorganic filler, this rate of coupling agent is advantageously less than 8%>, more preferably less than 6% by weight relative the weight of reinforcing inorganic filler.

The coupling agent used could be grafted beforehand (via the "X" function) on the diene elastomer of the composition of the invention, the elastomer thus functionalized or

"precoupled" then comprising the free "Y" function for the reinforcing inorganic filler.

The coupling agent could also be grafted beforehand (via the "Y" function) onto the reinforcing inorganic filler, the filler thus "precoupled" can then be linked to the diene elastomer via the free "X" functions. However, it is preferred to use the coupling agent in the free state (Le., Not grafted) or grafted on the reinforcing inorganic filler, in particular for reasons of better implementation of the compositions in the raw state.

Finally, the coupling agent can optionally be combined with an appropriate "coupling activator", that is to say a body (single compound or combination of compounds) which, mixed with this coupling agent, increases the efficiency of the latter (see for example applications WO00 / 5300 and WO00 / 5301 above).

H-4. Vulcanization system The basic vulcanization system consists of sulfur and a primary vulcanization accelerator. To this basic vulcanization system are added, incorporated during the first non-productive phase and / or during the productive phase, various known secondary accelerators or activators of vulcanization.

The primary vulcanization accelerator is preferably an accelerator of the sulfenamide type. The use of LS silica makes it possible to significantly reduce the overall level of sulfur and of sulfenamide accelerator to a preferential value of between 1.25 and 2.75 phr, more preferably within a range of 1.5 to 2.5 phr, sulfur and sulfenamide accelerator being also each used, even more preferably, at a rate of between 0.5 and 1.5 phr.

As secondary vulcanization accelerator, a guanidine derivative is preferably used, in particular there diphenylguanidine (DPG), incorporated during the first non-productive phase (preferred mode of the invention) and / or during the productive phase. This guanidine derivative also plays the advantageous role of agent for covering silica LS. The use of LS silica again makes it possible to advantageously reduce the overall level of sulfur, sulfenamide and guanidine derivative to a preferential value comprised in a range from 1.75 to 4.25, more preferably in a range from 2 to 4 phr.

When curing treads or tires in accordance with the invention, it has been observed that the use of an LS silica, in particular when the latter constitutes all of the reinforcing inorganic filler, has the disadvantage of Significantly increase the induction times (time required at the start of the vulcanization reaction), so as to be considered prohibitive with regard to industrial cooking conditions.

The above drawback has been unexpectedly overcome by a substantial reduction in the level of zinc in the rubber composition.

Thus, according to a particularly preferred embodiment of the invention, a very small amount of zinc is used as vulcanization activator, between 0.50 and 1.5 phr, more preferably within a range from 0.7 to 1.3 pc.

This specific quantity of zinc can be added to the rubber composition in any manner known to those skilled in the art, preferably in the form of zinc oxide, in this case therefore used at an equivalent preferential rate of between 0.6 and 1.9 pce, more preferably within a range of 0.9 to 1.6 pce.

With this zinc oxide is preferably associated a fatty acid, more preferably stearic acid, present at a preferential rate of 0.5 to 3 phr, more preferably of 1 to 3 phr.

All or part of the zinc used can also be incorporated into the treads and their compositions in the form of a zinc salt of fatty acid, in particular in the form of zinc stearate, or of another active zinc donor compound. with regard to vulcanization. Induction times and therefore reduced cooking times are particularly advantageous for. treads intended for retreading, whether it is "cold" retreading (use of a prebaked tread) or conventional "hot" retreading (using a tread in the state In the latter case, a reduced baking time, in addition to reducing production costs, limits the overcooking (or postcooking) imposed on the rest of the casing ("carcass") of the used tire (already vulcanized).

II-5. Miscellaneous additives

Of course, the elastomeric compositions of the treads according to the invention. also comprise all or part of the usual additives used in rubber compositions intended for the manufacture of tire treads, such as for example extension oils, plasticizers, protective agents such as anti-ozone waxes, chemical ozonants, antioxidants, anti-fatigue agents, coupling activators, reinforcing resins, methylene acceptors and / or donors. Silica LS can also be associated, if necessary, with a conventional white filler that is not very reinforcing or not, for example particles of clay, bentonite, talc, chalk, kaolin, which can be used, for example, in colored tire treads.

The elastomeric compositions can also contain, in addition to the agents. coupling agents described above, recovery agents for inorganic fillers, comprising, for example, the only Y function, or more generally, agents for aid in implementation which are capable in known manner, thanks to an improvement in the dispersion of the inorganic filler reinforcing in the rubber matrix and lowering the viscosity of the compositions, improving their ability to be used in the raw state; these agents, used at a preferential rate of between 0.5 and 3 phr, are for example alkylalkoxysilanes, in particular alkyltriethoxysilanes, such as 1-octyl-triethoxysilane sold by the company Degussa-Hϋls under the name Dynasylan Octeo or 1- hexa-decyl-triethoxysilane sold by the company Degussa-Hϋls under the name Si216, polyols, polyethers (for example polyethylene glycols), primary, secondary or tertiary amines (for example trialcanol-amines), hydroxylated or hydrolyzable polyorganosiloxanes , for example α, ω-dihydroxy-polyorganosiloxanes (in particular α, ω-dihydroxy-polydimethylsiloxanes).

II-5. Preparation of compositions and treads

The elastomeric compositions are produced in suitable mixers, using two successive preparation phases well known to those skilled in the art: a first working phase or thermo-mechanical kneading (sometimes called a "non-productive" phase) at high temperature. , up to a maximum temperature (noted T _max ) of between 110 ° C and 190 ° C, preferably between 130 ° C and 180 ° C, followed by a second phase of mechanical work (sometimes qualified as a "productive" phase ) at a lower temperature, typically less than 110 ° C., for example between 40 ° C. and 100 ° C., the finishing phase during which the basic vulcanization system is incorporated; such phases have been described for example in applications EP 501 227, EP 735 088, WO00 / 05300 or WO00 / 05301 mentioned above. The process for manufacturing the compositions according to the invention is characterized in that at least the LS silica (associated or not with another reinforcing inorganic filler or with a carbon black) and the coupling agent are incorporated by kneading with diene elastomer during the first so-called non-productive phase, that is to say that one introduces into the mixer and that one thermomechanically kneads, in one or more times, at least these different basic constituents until a maximum temperature between 110 ° C and 190 ° C is reached, preferably between 130 ° C and 180 ° C.

All or part of the vulcanization activator, consisting of the very small amount of zinc of between 0.50 and 1.5 phr, can be introduced during the non-productive phase or else during the productive phase.

For example, the first (non-productive) phase is carried out in a single thermomechanical step during which all the necessary basic components are introduced into a suitable mixer such as a conventional internal mixer. (diene elastomer, reinforcing inorganic filler and coupling agent), then in a second step, for example after one to two minutes of mixing, any additional covering or implementing agents and other various additives, including in particular zinc and DPG, with the exception of the basic vulcanization system consisting of sulfur and primary accelerator, in particular sulfenamide; the apparent density of LS silica being generally low, it may be advantageous to split its introduction into two or more parts.

A second step (or even several) of thermomechanical work can be added to this internal mixer, after the mixture has fallen and intermediate cooling (cooling temperature preferably less than 100 ° C.), with the aim of subjecting the compositions to a complementary thermomechanical treatment , in particular to further improve the dispersion in the elastomeric matrix of the reinforcing inorganic filler and its coupling agent. The total duration of the kneading, in this non-productive phase, is preferably between 2 and 10 minutes.

After cooling the mixture thus obtained, sulfur and primary accelerator are then incorporated at low temperature, generally in an external mixer such as a cylinder mixer; the whole is then mixed (productive phase) for a few minutes, for example between 5 and 15 minutes.

The final composition thus obtained is then calendered, for example in the form of plates (thickness of 2 to 3 mm) or of thin sheets of rubber for the measurement of its physical or mechanical properties, in particular for a characterization in the laboratory, or also extruded to form rubber profiles used directly, after cutting or assembling to the desired dimensions, as tire treads.

In summary, the method for preparing a tire tread according to the invention comprises the following steps: • incorporating into a diene elastomer, during a first step called "non-productive": . by way of reinforcing filler, more than 80 phr of an inorganic filler constituted in whole or in part of silica having the following characteristics:

- (a) a BET specific surface area of between 50 and 100 m ² / g; - (b) an average particle size d _w of between 50 and 350 nm;

. and a coupling agent ensuring the connection between the silica and the diene elastomer; by thermomechanically kneading the whole, in one or more times, until reaching a maximum temperature of between 110 ° C and 190 ° C; • cool the assembly to a temperature below 100 ° C; then incorporate, during a second so-called "productive" step, sulfur and a primary vulcanization accelerator; knead everything up to a maximum temperature below 110 ° C; • calendering or extruding the elastomeric composition thus obtained in the form of a tire tread.

The vulcanization or curing of the tread or of the tire is carried out in a known manner at a temperature preferably between 130 ° C. and 200 ° C., preferably under pressure, for a sufficient time which can vary for example between 5 and 90 min depending in particular on the baking temperature, the vulcanization system adopted, the vulcanization kinetics of the composition considered and the size of the tire.

The rubber compositions described above based on LS silica generally constitute the entire tread according to the invention. However, the invention also applies to cases where these rubber compositions form only part of a tread of the composite type, consisting for example of transversely different adjacent bands, or else of two radially superposed layers of different constitutions, the part loaded with silica LS which can constitute, for example, the radially external layer of the tread intended to come into contact with the ground from the start of rolling of the new tire, or on the contrary its radially internal layer intended to come into contact with the ground later.

It goes without saying that the present invention relates to the treads and tires previously described both in the so-called "raw" state (Le., Before baking) and in the so-called "cooked" or vulcanized state (Le., After vulcanization).

III. EXAMPLES OF EMBODIMENT OF THE INVENTION

III- 1. Charges used

The characteristics of the fillers used in the following examples are summarized in Table 1. The filler denoted A is a conventional reinforcing silica, with high specific surface (BET of about 160 m ² / g), reference inorganic filler for reinforcement "Green Tires" treads ("Zeosil 1165MP" silica from Rhodia). Filler B is a silica with very low specific surface (BET close to 90 m ² / g) intended usually with internal mixtures for calendering of tire crown reinforcement plies (BET / CTAB ratio equal to 1.1).

The BET surface and size d _{w characteristics} therefore clearly differentiate the two charges, charge B having a surface area twice as low per unit mass and an average particle size d _w more than twice as large. The two silicas are also characterized both by a high intrinsic dispersibility, illustrated by a speed of

9 1 9 1 very high disaggregation α (approximately 1.25.10 ^" μm ^" / min for A and 1.10 ^" μm ^" / min for B).

We note that LS silica verifies all of the following preferential characteristics:

a BET surface in a range of 60 to 90 m ² / g; a particle size d _w within a range of 150 to 250 nm; - a deagglomeration speed α at least equal to 1.10 " ² μm'Vmin.

III-2. Preparation of the compositions

The following tests are carried out as follows: a diene elastomer (or mixture of elastomers) is introduced into an internal mixer, filled to 70%> and whose initial tank temperature is approximately 60 ° C. diene, if applicable), the reinforcing filler, the coupling agent, then, after one to two minutes of mixing, the various other ingredients with the exception of sulfur and the primary accelerator sulfenamide. Thermomechanical work (non-productive phase) is then carried out in one or two stages (total mixing time equal to approximately 7 min), until a maximum "fall" temperature of approximately 160-165 ° C. is reached.

The mixture thus obtained is recovered, cooled, then sulfur and sulfenamide accelerator are added to an external mixer (homo-finisher) at 30 ° C, mixing everything (productive phase) for 3 to 4 minutes.

The compositions are then either calendered in the form of plates (thickness of 2 to 3 mm) for the measurement of their physical or mechanical properties, or directly extruded in the form of tire treads.

In the following tests, LS silica advantageously constitutes all of the reinforcing inorganic filler, associated with a low level of carbon black (less than 10 phr).

III -3. testing

The purpose of this test is to demonstrate the improved performance of an elastomeric composition based on LS silica, compared to a control composition using a conventional silica (high specific surface) for a "Green Tire" tread.

For this, three butadiene rubber compositions (SBR / BR cutting) are compared intended for the manufacture of treads for passenger car tires: composition noted Cl (control): silica A (60 phr) with TESPT coupling agent; composition C-2 (invention): silica B (85 phr) with TESPT coupling agent; composition C-3 (invention): silica B (85 phr) with TESPD coupling agent.

The butadiene elastomer consists of an SSBR comprising 25%) of styrene, 58%> of polybutadiene units 1-2 and 23% of polybutadiene units 1-4 trans, with which is associated a BR having more than 90% of cis bonds -1.4. The rate of total reinforcing filler, constituted by silica LS and a small amount of carbon black (less than 10 phr) is greater than 90 phr in the compositions according to the invention.

Tables 2 and 3 show in succession the formulation of the different compositions _(Table. 2 - rate of the different products expressed in phr), and their properties before and after curing at 150 ° C for 40 minutes (Table 3). The appended figure reproduces the module curves (in MPa) as a function of the elongation (in%>); these curves are denoted C1 to C3 and correspond respectively to compositions C1 to C-3.

The silica content is therefore more than 40% higher> (85 phr instead of 60 phr) in the compositions according to the invention (C-2 and C-3) which, compared to the control composition (Cl), have moreover the following advantageous characteristics, in particular from the point of view of costs:

a much reduced weight of coupling agent, relative to the weight of silica (less than 5% compared to 8%> for the control composition); - a reduced overall sulfur and accelerator sulfenamide level (2.6 phr instead of 3.1 phr), as does the overall sulfur content, sulfenamide accelerator and guanidine derivative (3.6 phr instead of 4.6 phr) .

The level of ZnO was reduced by 40%> (1.5 phr instead of 2.5 phr in the control composition) in compositions C-2 and C-3 in order to reduce their induction time (tj) to a value comparable to that of the control, a rate of 2.5 phr leading, for example, for composition C-2 to an induction time increased by almost 50%, an increase considered prohibitive with regard to industrial cooking conditions .

These comments on the formulation having been made, the study of the results in Table 3 demonstrates, for the compositions based on LS silica, compared with the control composition:

a viscosity in the raw state, admittedly increased, but satisfactory (at most 95 MU), in any case lower than that which would be observed for the control composition with isotals (85 phr) of conventional silica with high specific surface (in the latter case , more than 100

MU for the control solution); rheometric properties at least equivalent, if not improved for composition C-2 based on TESPT coupling agent, as illustrated by the values T5, ti, t ₉₀ , t ₉₀ - tj and finally K; - equivalent hysteresis properties (ΔG * and tan (δ) _max ); finally and above all, even more unexpectedly for a person skilled in the art, reinforcement properties after curing which are at least equal to or even superior to those of the control solution: equivalent Shore hardness but modules under strong deformation (MA100, MA300) and significantly higher MA300 / MA100 ratio, as many clear indicators of a high quality of the reinforcement provided by LS silica.

The appended figure confirms these results well with, for elongations of 100%) and more, a higher modulus in the case of compositions C-2 and C-3 (curves C2 and C3 above the curve Cl), proof of '' a strong interaction between the diene elastomer and silica LS.

Such results must now be confronted with real tests of rolling of the treads, as exposed in test 2 which follows.

B) Test 2

The above compositions Cl and C-2 are used in this test as treads for passenger car tires with a radial carcass, of dimension 175/70 RI 4 (speed index T), conventionally manufactured and in all points identical except for the rubber composition constituting the tread: composition Cl for the "Green Tires" witnesses (denoted Pl), composition C-2 for the tires of the invention (denoted P-2).

The tires were first tested on a machine to determine their rolling resistance and their drift thrust, then mounted on the vehicle for further testing.

All the driving results are summarized in Table 4.

First of all, it can be seen that the rolling resistance of the two types of tire is equivalent, synonymous with low fuel consumption for a vehicle fitted with tires in accordance with the invention.

The tires are then subjected to road rolling, on a passenger vehicle of the Citroën Xsara brand, to determine the resistance to wear. In known manner, the wear resistance of the tread, during the rolling of the tire, is directly correlated to the level of reinforcement provided by the reinforcing filler and its associated coupling agent. In other words, the measurement of the resistance to wear is an excellent indicator, if not the best since evaluated on the final manufactured product, of the overall performance of the inorganic filler used. It can then be seen that the tire according to the invention shows a performance identical to that of the control tire.

Thus, despite the use of silica with a very low specific surface, a reduced rate of coupling agent and vulcanizing agents, it is possible to maintain an excellent compromise (rolling resistance / resistance to wear), at least as good as that available on conventional "Green Tires" reinforced with silica with a higher specific surface, which is already a remarkable and unexpected result for those skilled in the art. It can also be seen that the drift thrust, which is indicative for those skilled in the art of road behavior on dry ground, is significantly increased, with a recorded gain of 4%>.

The tires are finally mounted on another passenger vehicle to be subjected to the braking and grip tests described in paragraph 1-3, under the following specific conditions:

braking (on dry or wet ground): VW vehicle "Polo GT" model (nominal pressures at the front and rear), the tires to be tested being fitted at the front of the vehicle; - driving on a virage wet circuit: VW "Golf" model vehicle (nominal pressures at the front and rear), the tires to be tested being fitted at the front and rear of the vehicle.

The P-2 tires firstly show a significant gain of 4%> in braking on wet surfaces. The running test on a wet and bumpy track confirms that the use of LS silica results in a significant improvement in grip, illustrated both by a reduction in the minimum time required to cover the track in speed conditions limit (journey time reduced by one second per lap) only by the change in the behavior score awarded by the pilot (8% increase>), these two variations being very significant for such a test.

In summary, the tire of the invention shows a compromise (rolling resistance / resistance to wear / behavior / adhesion) which is generally improved compared to the reference constituted by the treads of the "Green Tires" controls Pl, with in particular an improvement both in road behavior and grip on wet, snowy or icy conditions.

C) Trial 3

This test confirms the advantage of lime silica LS in a tread whose diene matrix is different from the previous one, consisting of a blend of butadiene elastomer (BR) and natural rubber. The tread is intended for passenger tires of the "winter" type with high grip on snow and ice.

The two compositions tested are essentially differentiated by the following characteristics:

composition C-4 (control): silica A (65 phr); composition C-5 (according to the invention): silica B (85 phr).

Tables 5 and 6 successively give the formulation of the different compositions (Table 5 - rate of the various products expressed in phr), their properties before and after cooking at 150 ° C. for 40 minutes (Table 6).

The level of silica is therefore significantly higher in the tread according to the invention, the rate of total reinforcing charge (LS silica plus carbon black) is greater than 90 pce. The coupling agent is TESPT in both cases, its weight ratio relative to the weight of silica being however significantly reduced in the tread according to the invention (less than 5% compared to 8.7% for the control solution ).

The study of the results in Table 6 demonstrates that the composition based on LS silica, compared with the control composition, is characterized in particular by:

an equivalent raw viscosity; a roasting time T5 certainly increased, but which can be corrected by a reduction in the level of zinc (T5 <20 min for 1.5 pce of ZnO); after baking, reinforcing properties which appear to be improved compared to those of the control solution (MAI 00, MA300 and higher MA300 / MA100 ratio), a clear indicator of a high level of reinforcement and therefore of wear resistance for the tread of the invention; - finally, hysteresis properties which are significantly improved (markedly lower values for ΔG * and tan (δ) _max ), suggesting reduced rolling resistance.

The compositions C-4 and C-5 were then tested as winter tire treads, dimension 175/70 RI 4, conventionally manufactured and in all respects identical except for the composition used: C-4 for the control tires (noted P-4), C-5 for the tires of the invention (denoted P-5). The tires were tested on a machine and on a vehicle, in accordance with the indications of the previous test 2 and of paragraph 1-3, to determine on the one hand their rolling resistance on the one hand, and on the other hand their grip. (brake test) both on wet ground and on ice.

The running results in Table 7 show the superiority of the tires according to the invention, the rolling resistance, braking on wet ground and braking on ice performance being all improved, a particularly advantageous result for tires of the winter type.

In summary, the specific silicas meeting the characteristics described above, with a very low specific surface and preferably with high dispersibility, when they are used in treads at a rate of reinforcing inorganic filler as high as that recommended, are unexpectedly revealed to those skilled in the art, capable of offering these treads an overall compromise in improved rolling performance, while making it possible to reduce their additional cost thanks to a possible reduction in the rates of coupling agent like those of vulcanizers.

Compared to silicas with a high specific surface, the previously described LS silicas, thanks to their markedly reduced specific surface, have many advantages:

less parasitic interactions, on the one hand between the silica particles themselves (reduced risk of re-agglomeration in the rubber matrix), on the other hand between the silica particles and the other rubber additives; . overall improvement in dispersibility in the diene matrix, during mixing operations, and therefore in the use of the compositions in the raw state; possible reduction in the quantities of coupling agent and vulcanizing agents, in particular of sulfur and of sulfenamide accelerator, which leads to a reduction in the costs of the formulation.

If, on the other hand, these LS silicas certainly have the drawback of lengthening the induction times when the treads are baking, it has been found that this problem could be resolved by using a very low amount of zinc in the compositions, thereby contributing to the reduction of industrial costs.

Table 1

(A) high specific BET surface silica (CTAB: 157.5 m ² / g);

(B) silica with very low BET specific surface (CTAB: 81 m ² / g).

Table 2

(1) SSBR with 59.5% of polybutadiene 1-2 units; 26.5% styrene; Tg = -29 ° C; 75 pce dry SBR extended with 18% by weight of aromatic oil (a total SSBR + oil equal to 88.5 pce);

(2) BR with 4.3% of 1-2; 2.7% trans; 93% cis 1-4 (Tg = -106 ° C);

(3) carbon black N234;

(4) aromatic oil in free form ("Enerflex 65" from the company BP);

(5) TESPT ("Si69" from Degussa);

(6) TESPD ("Si75" from Degussa);

(7) diphenylguanidine ("Vulcacit D" from the company Bayer);

(8) zinc supplied in the form of ZnO (rubber grade);

(9) mixture of macro- and micro-crystalline anti-ozone waxes;

(10) N-1,3-dimethylbutyl-N-phenyl-para-phenylenediamine ("Santoflex 6-PPD" from the company Flexsys);

(11) N-cyclohexyl-2-benzothiazyl-sulfenamide (Flexsys company: "Santocure" CBS). Table 3

Table 4

(a value greater than 100 indicates improved performance compared to the control - base 100) Table 5

(1) natural rubber; (3) carbon black N330; (6) "Flexon 815" from Shell; (other indices: same as table 1)

Table 6

Table 7

(a value greater than 100 indicates improved performance compared to the control - base 100)

Claims

1. Tire tread incorporating an elastomeric composition reinforced with an inorganic filler, based on at least (phr = parts by weight per hundred parts of diene elastomer):

(i) a diene elastomer;

(ii) by way of reinforcing filler, more than 80 phr of an inorganic filler constituted in whole or in part of a silica called "LS" having the following characteristics: (a). a BET specific surface of between 50 and 100 m / g;

(b) an average particle size d _w of between 50 and 350 nm;

(iii) an agent of. coupling ensuring the connection between the reinforcing inorganic filler and the diene elastomer; (iv) a sulfur-based vulcanization system.

2. A tread according to claim 1, the diene elastomer comprising at least 40 phr, preferably at least 50 phr of a butadiene elastomer.

3. A tread according to claim 2, the butadiene elastomer being chosen from polybutadienes (BR), styrene-butadiene copolymers (SBR) and mixtures of these elastomers.

4. Tread according to any one of claims 1 to 3, the rate of reinforcing inorganic filler being greater than 90 phr.

5. A tread according to claim 4, the rate of reinforcing inorganic filler being between 90 phr and 150 phr.

6. A tread according to any one of claims 1 to 5, the silica LS representing at least 50% by weight of the reinforcing inorganic filler.

7. A tread according to claim 6, the silica LS representing more than 80% by weight of the reinforcing inorganic filler.

8. A tread according to claim 7, the silica LS representing all of the reinforcing inorganic filler.

9. A tread according to any one of claims 1 to 8, the coupling agent being chosen from polyfunctional silanes and polysiloxanes.

10. The tread according to claim 9, the coupling agent being a polysulphurized silane.

11. A tread according to claim 10, the quantity of polysulphurized silane being between 2 and 5 phr.

12. A tread according to claim 11, the amount of polysulphurized silane being in a range from 3 to 4.5 phr.

13. A tread according to any one of claims 1 to 12, comprising carbon black.

14. A tread according to claim 13, the carbon black being present at a rate of between 2 and 20 phr.

15. A tread according to claim 14, the carbon black being present at a rate comprised within a range of 5 to 15 phr.

16. A tread according to any one of claims 1 to 15, the silica LS having at least one of the following characteristics:

a BET surface in a range of 60 to 90 m ² / g; a particle size d _w of between 100 and 300 nm; a disagglomeration speed α greater than 5.10 " ³ μnrVmin.

17. A tread according to claim 16, the silica LS having at least one of the following characteristics:

a particle size d _w within a range of 150 to 250 nm; a disagglomeration speed α at least equal to 1.10 " ² μπrVmin.

18. A tread according to claim 17, the silica LS having all of the following characteristics:

a BET surface in a range of 60 to 90 m ² / g; - A particle size d _w in a range of 150 to 250 nm; a disagglomeration speed α at least equal to 1.10 " ² μm" Vmin.

19. A tread according to any one of claims 1 to 18, the butadiene elastomer constituting the whole of the diene elastomer.

20. A tread according to any one of claims 2 to 19, the butadiene elastomer comprising, preferably in the majority, an SBR elastomer.

21. The tread according to claim 20, the SBR being an SBR having a styrene content of between 20% and 30%> by weight, a vinyl bond content of the butadiene part of between 15%> and 65%, a content of trans-1,4 bonds between 20% and 75% o and a glass transition temperature between -20 ° C and -55 ° C.

22. A tread according to claims 20 or 21, the SBR being an SBR prepared in solution (SSBR).

23. A tread according to any one of claims 20 to 22, the SBR being used in admixture with a BR elastomer.

24. The tread according to claim 23, the elastomer BR having more than 90%) of cis-1,4 bonds.

25. A tread according to any one of claims 1 to 24, the vulcanization system comprising a primary sulfenamide accelerator, the overall rate of sulfur and sulfenamide accelerator being within a range of 1.25 to 2.75 phr .

26. A tread according to claim 25, the vulcanization system further comprising a guanidine derivative, the overall level of sulfur, sulfenamide and guanidine derivative being within a range of 1.75 to 4.25 phr.

27. A tread according to claim 26, the guanidine derivative being DPG (diphenylguanidine).

28. A tread according to any one of claims 1 to 27, the vulcanization system comprising, as vulcanization activator, between 0.5 and 1.5 phr of zinc.

29. A tread according to claim 28, the zinc being provided in the form of zinc oxide, preferably in the presence of a fatty acid.

30. A tread according to claim 29, the fatty acid being stearic acid, present at a preferential rate of 0.5 to 3 phr.

31. A tread according to any one of claims 28 to 30, the amount of zinc being in a range from 0.7 to 1.3 phr.

32. Tread according to any one of claims 1 to 31, characterized in that it is in the vulcanized state.

33. Process for preparing a tread of vulcanizable sulfur tire, with performance compromise (wear resistance / rolling resistance / adhesion / drift thrust) improved, based on diene elastomer, of an inorganic filler reinforcing and of a vulcanization system, comprising the following stages (pce = parts by weight percent of diene elastomer):

• incorporate into a diene elastomer, during a first step called "non-productive", at least:. by way of reinforcing filler, more than 80 phr of an inorganic filler constituted in whole or in part of a silica (hereinafter "LS" silica) having the following characteristics:

- (a) a BET specific surface area of between 50 and 100 m ² / g;

- (b) an average particle size d _w of between 50 and 350 nm; . and a coupling agent ensuring the connection between the silica and the diene elastomer, • by thermomechanically mixing everything, in one or more times, until reaching a maximum temperature between 110 ° C and 190 ° C; cool the assembly to a temperature below 100 ° C;

• then incorporate, during a second so-called "productive" step, sulfur and a primary vulcanization accelerator;

• mix everything up to a maximum temperature below 110 ° C;

• calendering or extruding the elastomeric composition thus obtained in the form of a tire tread.

34. The method of claim 33, the diene elastomer comprising at least 40 phr, preferably at least 50 phr of a butadiene elastomer.

35. The method of claim 34, the diene elastomer being chosen from polybutadienes (BR), styrene-butadiene copolymers (SBR) and mixtures of these elastomers.

36. Method according to any one of claims 33 to 35, the rate of reinforcing inorganic filler being greater than 90 phr.

37. The method of claim 36, the rate of reinforcing inorganic filler being between 90 phr and 150 phr.

38. Method according to any one of claims 33 to 37, the silica LS representing at least 50% by weight of the reinforcing inorganic filler.

39. The method of claim 38, the silica LS representing more than 80% by weight of the reinforcing inorganic filler.

40. The method of claim 39, the silica LS representing all of the reinforcing inorganic filler.

41. Process according to any one of claims 33 to 40, the primary accelerator being a sulfenamide accelerator and the overall rate of sulfur and sulfenamide accelerator being within a range of 1.25 to 2.75 phr.

42. The method of claim 41, a guanidine derivative being further incorporated, the _. overall level of sulfur, sulfenamide and guanidine derivative being within a range of 1.75 to 4.25 phr.

43. The method of claim 42, the guanidine derivative being incorporated during the non-productive phase.

44. The method of claims 42 or 43, the guanidine derivative being DPG (diphenylguanidine).

45. Method according to any one of claims 33 to 44, the vulcanization system comprising, as vulcanization activator, between 0.5 and 1.5 phr of zinc.

46. The method of claim 45, the zinc being provided in the form of zinc oxide, preferably in the presence of a fatty acid.

47. The method of claim 46, the fatty acid being stearic acid, present at a preferential rate of 0.5 to 3 phr.

48. Method according to any one of claims 33 to 47, the coupling agent being chosen from polyfunctional silanes and polysiloxanes.

49. The method of claim 48, the coupling agent being a polysulphurized silane.

50. The method of claim 49, the amount of polysulfurized silane being between 2 and 5 phr.

51. Method according to any one of claims 33 to 50, carbon black being additionally incorporated during the non-productive phase, at a preferential rate comprised within a range of 5 to 15 phr.

52. Method according to any one of claims 33 to 51, the silica LS having at least one of the following characteristics:

a BET surface in a range of 60 to 90 m ² / g; a particle size d _w of between 100 and 300 nm; - a deagglomeration speed α greater than 5.10 " ³ μπrVmin.

53. The method of claim 52, the LS silica having at least one of the following characteristics:

- A particle size d _w in a range of 150 to 250 nm; a disagglomeration speed α at least equal to 1.10 " ² μπrVmin.

54. The method of claim 53, the LS silica having all of the following characteristics:

a BET surface in a range from .60 to 90 m ² / g; a particle size d _w within a range of 150 to 250 nm; a disagglomeration speed α at least equal to 1.10 " ² μm" Vmin.

55. Use, as reinforcing inorganic filler, in a tire tread based on diene elastomer, of a silica having the following characteristics:

(a) a BET specific surface area of between 50 and 100 m ² / g;

(b) an average particle size (by mass) of between 50 and 350 nm;

56. Use according to claim 55, the silica having at least one of the following characteristics:

a BET surface in a range of 60 to 90 m ² / g; a particle size d _w of between 100 and 300 nm; a disagglomeration speed α greater than 5.10 " ³ μm" Vmin.

57. Use according to claim 56, the silica having at least one of the following characteristics:

a particle size d _w within a range of 150 to 250 nm; a disagglomeration speed α at least equal to 1.10 " ² μm" Vmin.

58. Use according to claim 57, the silica having all of the following characteristics:

a BET surface in a range of 60 to 90 m / g; a particle size d _w within a range of 150 to 250 nm; 'a deagglomeration speed α at least equal to 1.10' ² μm "Vmin.

59. Use of a tread according to any one of claims 1 to 32 for the manufacture or retreading of tires.

60. A tire comprising a tread according to any one of claims 1 to 32.