Tire with a tread having improved endurance properties

Abstract

The invention relates to a tire, the tread of which has at least one groove. According to the invention, the tread consists of a first elastomer blend, and at least a portion of the bottom and / or the walls (10) of the at least one groove (7) surrounding the junction zones between the bottom and the two walls of the at least one groove (7) consists of a second elastomer blend over a thickness (E) of at least 1 mm, the first elastomer blend has a tear coefficient of less than 100 N / m, and the second elastomer blend has a tear coefficient that is at least 20% greater than the tear coefficient of the first elastomer blend.

Description

TIRE WITH A TREAD THAT OFFERS IMPROVED ENDURANCE PROPERTIES

[0001] The present invention relates to a tire with a radial carcass reinforcement and more particularly to a tire intended to equip vehicles carrying heavy loads and traveling at sustained speed, such as, for example, trucks, tractors, trailers or road buses.

[0002] In general, in heavy-duty tires, the carcass reinforcement is anchored on both sides in the bead area and is radially surmounted by a crown reinforcement consisting of at least two superimposed layers formed of parallel wires or cables in each layer and crossed from one layer to the next at angles between 10° and 45° with the circumferential direction. These working layers, forming the working reinforcement, may be further covered by at least one protective layer formed of advantageously metallic and extensible reinforcing elements, known as elastic elements.It may also include a layer of low-extensibility wires or cables forming an angle of between 45° and 90° with the circumferential direction. This layer, known as the triangulation layer, is radially positioned between the carcass reinforcement and the first crown layer, known as the working layer, which is formed of parallel wires or cables having angles of no more than 45° in absolute value. The triangulation layer, together with at least the aforementioned working layer, forms a triangulated reinforcement that exhibits minimal deformation under the various stresses it is subjected to. The triangulation layer's essential role is to resist the transverse compression forces exerted on all the reinforcing elements in the crown area of ​​the tire.

[0003] Cables are said to be inextensible when, under a tensile force equal to 10% of the breaking force, said cables exhibit a relative elongation of no more than 0.2%.

[0004] Cables are said to be elastic when, under a tensile force equal to the breaking load, said cables exhibit a relative elongation of at least 3% with a maximum tangent modulus less than 150 GPa.

[0005] Circumferential reinforcement elements are reinforcement elements that make angles with the circumferential direction within the range of +2.5°, -2.5° around 0°.

[0006] The circumferential direction of the tire, or longitudinal direction, is the direction corresponding to the periphery of the tire and defined by the direction of rolling of the tire.

[0007] The transverse or axial direction of the tire is parallel to the axis of rotation of the tire.

[0008] Radial direction is a direction that intersects the axis of rotation of the tire and is perpendicular to it.

[0009] The axis of rotation of the tire is the axis around which it rotates in normal use.

[0010] A radial or meridian plane is a plane that contains the axis of rotation of the tire.

[0011] The circumferential median plane, or equatorial plane, is a plane perpendicular to the axis of rotation of the tire and which divides the tire into two halves.

[0012] Some modern tires, known as "road" tires, are designed for high speeds and increasingly long journeys, due to improvements in road networks and the growth of highways worldwide. The overall conditions under which such a tire is expected to operate undoubtedly allow for an increase in mileage, as tire wear is reduced; however, the tire's durability, and particularly that of the tread, is compromised.

[0013] There are indeed constraints at the level of the top reinforcement and more particularly shear stresses between the top layers, combined with a significant rise in operating temperature at the ends of the axially shortest top layer, which result in the appearance and propagation of cracks in the rubber at said ends.

[0014] In order to improve the endurance of the crown reinforcement of the type of tire studied, solutions relating to the structure and quality of the layers and / or profiles of rubber compounds which are arranged between and / or around the ends of plies and more particularly the ends of the axially shortest plies have already been provided.

[0015] It is known to introduce a layer of rubbery compound between the ends of the working layers to create decoupling between said ends and limit shear stresses. However, such decoupling layers must exhibit very good cohesion. Such layers of rubbery mixtures are described, for example, in patent application WO 2004 / 076204.

[0016] The tires produced in this way effectively improve performance, particularly in terms of endurance.

[0017] Furthermore, it is known for producing tires with very wide treads or for increasing the load-bearing capacity of tires of a given size by incorporating a layer of circumferential reinforcing elements. Patent application WO 99 / 24269, for example, describes the presence of such a layer of circumferential reinforcing elements.

[0018] The layer of circumferential reinforcing elements is usually made up of at least one wire rope wound to form a spiral whose angle of laying with respect to the circumferential direction is less than 2.5°.

[0019] In combination with this internal tire structure, it is known to equip the tread—that is, the part of the tire that comes into contact with the ground while driving and wears down—with a tread pattern formed of relief elements delimited by cuts that are circumferential, transverse, or oblique. The purpose of such a tread pattern is to give the tread good performance on dry pavement and on wet pavement, particularly in rainy weather.

[0020] For the purposes of this invention, a cut-out generically refers to either a groove or an incision and corresponds to the space delimited by opposing walls of material separated by a non-zero distance (called the "cut-out width"). What differentiates an incision from a groove is precisely this distance; in the case of an incision, this distance is suitable to allow at least partial contact between the opposing walls delimiting said incision, at least during contact with the road surface. In the case of a groove, the walls of this groove cannot come into contact with each other under normal driving conditions.

[0021] For the purposes of this invention, a circumferential or longitudinal cutout is a cutout in which the mean plane of at least a portion of the walls of said cutout forms an angle of less than 10° with a longitudinal plane. This angle formed with a longitudinal plane may be oriented in either direction with respect to said plane. longitudinal. A cut with a longitudinal orientation can also be a cut whose walls undulate or zigzag around a mean plane such as has just been described.

[0022] For the purposes of this invention, a transversely oriented cutout is a cutout in which the mean plane of at least a portion of the walls of said cutout forms an angle of less than 35° with a radial plane. This angle formed with a radial plane may be oriented in either direction with respect to said radial plane. A transversely oriented cutout may also be a cutout that runs continuously on either side of a mean plane as described above; it may also be a cutout whose walls undulate or zigzag around a mean plane as described above.

[0023] For the purposes of this invention, an obliquely oriented cutout is a cutout in which the mean plane of at least a portion of the walls of said cutout forms an angle with a radial plane of between 35° and 80°. This angle formed with a radial plane may be oriented in either direction with respect to said radial plane. An obliquely oriented cutout may also be a cutout that runs continuously on either side of a mean plane as described above; it may also be a cutout whose walls undulate or zigzag around a mean plane as described above.

[0024] During tire tests carried out under particularly demanding stresses on the tread, especially in terms of violent impacts for example on sidewalks, damage to the tread appeared, resulting in particular in the tearing off of blocks of rubber.

[0025] One aim of the invention is to provide tires whose tread endurance properties are improved regardless of use, the wear and overall endurance properties of the tire being maintained for normal uses.

[0026] This objective is achieved according to the invention by a radially reinforced tire comprising a crown reinforcement, itself radially capped by a tread, said tread being joined to two beads by means of two sidewalls, said tread having a tread surface intended to come into contact with a road surface and form a contact patch, said tread having a plurality of cutouts consisting of at least one groove made up of two walls opening onto the tread surface and a surface connecting the two walls forming the bottom of said at least one groove, said bottom of said at least one groove being connected to said two walls by junction zones, the tread being made up of at least a first elastomeric compound forming at least part of the tread surface when the tire is new, at least part of the base and / or the walls of said at least one groove surrounding the junction zones between said base and said two walls of said at least one groove being made up of a second elastomeric compound with a thickness of at least 1 mm, the first elastomeric compound having a tear coefficient of less than 100 N / m and the second elastomeric compound having a tear coefficient (expressed in N / m) greater than at least 25% than the tear coefficient of the first elastomeric compound, the tear coefficient being the product of the breaking force per unit thickness (expressed in N / mm of thickness) by the elongation at break (expressed in %), measured at 100 °C.

[0027] The thickness of the second elastomeric mixture is measured locally in a direction normal to the surface of the wall of said at least one circumferential groove.

[0028] The tensile strength per unit thickness and the elongation at break, in terms of tearability, are measured on a specimen stretched at 500 mm / min to induce breakage on a tensile testing machine equipped with a system for measuring and acquiring the force and displacement of the moving crosshead. The tensile specimen consists of a parallelepiped-shaped rubber plate, 2.5 mm thick, 145 mm long, and 10 mm wide. Before starting the test, three very fine cuts perpendicular to the length of the specimen are made with a razor blade to a depth of 3 mm on one edge of the specimen: one in the middle and the other two on either side of the first, 6 mm apart. The force (expressed in N per mm of thickness of the specimen) to be exerted to obtain the break is determined and the elongation at break (expressed in %) is measured.The test was conducted in air at a temperature of 100°C. High values ​​indicate good cohesion of the rubber composition, although crack initiation is present.

[0029] According to a first embodiment of the invention, said at least one groove is a groove with longitudinal orientation.

[0030] According to a second embodiment of the invention, said at least one groove is a transversely oriented groove.

[0031] According to a third embodiment of the invention, said at least one groove is an obliquely oriented groove.

[0032] The tires thus made according to the invention effectively allow driving in particularly demanding conditions without any tearing appearing on the tread compared to tires of more conventional design.

[0033] The inventors believe they have demonstrated that during driving under particularly demanding conditions for the tread with conventionally designed tires, the tears that occur on the tread appear to originate at the junction zones between the base and the sides of a groove. The inventors believe they can explain this phenomenon by the fact that these junction zones are prone to longitudinal breaks during violent impacts. During driving, as the tire is subjected to impacts on the tread, these breaks lead to the tearing of a portion of the tread.

[0034] The presence of a mixture conforming to the invention at least locally in the area sensitive to the appearance of cracks makes it possible to limit or at least delay these appearances of cracks and therefore reduces the risks of tearing off part of the tread regardless of the driving conditions.

[0035] According to a preferred embodiment of the invention, the entire bottom and walls of said at least one groove are made up of the second elastomeric mixture to a thickness of at least 1 mm such that the second mixture forms the complete surface of said at least one groove.

[0036] This preferred embodiment of the invention simplifies the production of the tire according to the invention, the second mixture being positioned before molding according to the knowledge of the person skilled in the art to obtain the tire according to the invention after curing and molding of the tread forming the design.

[0037] According to a preferred embodiment of the invention, the thickness of the second elastomeric compound is less than 5 mm, without taking into account the volume of the protrusion that may be formed by this second compound. The volume of the second compound thus remains limited compared to the volume of the first compound, which therefore contributes predominantly to the desired properties of the tread, such as grip and wear.

[0038] According to a preferred embodiment of the invention, the second elastomeric mixture has a tear coefficient that is at least 50% higher and preferably at least 100% higher than the tear coefficient of the first elastomeric mixture.

[0039] According to an advantageous embodiment of the invention, the first elastomeric mixture has a maximum value of tan(ô), denoted tan(ô)max, less than 0.25.

[0040] The loss factor tan(φ) is a dynamic property of the rubber compound layer. It is measured on a viscoelastic analyzer (Metravib VA4000), according to ASTM D 5992-96. The response of a vulcanized composition sample (cylindrical specimen 2 mm thick and 78 mm long) is recorded. 2 of section), subjected to a sinusoidal alternating simple shear load at a frequency of 10 Hz and a temperature of 60°C. A strain amplitude sweep is performed from 0.1 to 50% (forward cycle), then from 50% to 1% (reverse cycle). For the reverse cycle, the maximum observed value of tan(φ) is indicated, denoted tan(φ). ma x.

[0041] In the case where the thickness of the material is between 1 and 2 mm, the loss factor tan(ô) is measured according to the same method and under the same conditions, as described previously, on a sample of vulcanized composition which is in the form of a cylindrical specimen 1 mm thick and 78 mm2 in cross-section.

[0042] Rolling resistance is the resistance that arises when a tire is rolling. It is represented by the hysteretic losses related to the tire's deformation during one revolution. The frequency values ​​associated with the tire's revolution correspond to tan(φ) values ​​measured between 30 and 100°C. The tan(φ) value at 60°C thus corresponds to an indicator of the tire's rolling resistance.

[0043] Furthermore, the tests carried out have shown that the rolling resistance properties are maintained or even improved by the choice of the second elastomeric mixture, which can present better rolling resistance performance than the first elastomeric mixture.

[0044] Advantageously according to the invention, the first elastomeric mixture is a rubber composition based on at least one elastomeric matrix comprising a styrene-butadiene based copolymer and at least one reinforcing filler.

[0045] By "elastomeric matrix" or "elastomeric matrix", we mean the entire set of elastomer(s) present in the rubber composition.

[0046] By "diene" elastomer (or indistinctly rubber), whether natural or synthetic, must be understood an elastomer consisting at least in part (i.e., a homopolymer or a copolymer) of diene monomer(s) (i.e., bearing two carbon-carbon double bonds, conjugated or not).

[0047] By styrene-butadiene copolymer, we mean here a copolymer of at least one styrene monomer and at least one butadiene monomer (and of course also any mixture of such copolymers); in other words, said styrene-butadiene copolymer by definition comprises at least styrene units (from the styrene monomer) and butadiene units (from the butadiene monomer).

[0048] Advantageously according to the invention, the second elastomeric blend is a rubber composition based on at least one elastomeric matrix comprising predominantly natural rubber for at least 80% and at least one reinforcing filler.

[0049] According to a preferred embodiment of the invention, the second elastomeric blend is a rubber composition based on at least one elastomeric matrix comprising 100% natural rubber.

[0050] The rubber compositions according to the invention of the first mixture or the second mixture may include one or more reinforcing fillers.

[0051] Any type of so-called reinforcing filler can be used, known for its ability to strengthen a rubber composition usable in particular for the manufacture of tires, for example an organic filler such as carbon black, an inorganic filler such as silica or a mixture of these two types of fillers.

[0052] All carbon blacks are suitable, including those conventionally used in tires or their treads. Among these, particularly the reinforcing carbon blacks of the 100, 200, and 300 series, or the blacks of the 500, 600, or 700 series (ASTM D-1765-2017 grades), such as NI 15, N134, N234, N326, N330, N339, N347, N375, N550, N683, and N772.

[0053] The term "reinforcing inorganic filler" here refers to any inorganic or mineral filler, regardless of its color or origin (natural or artificial). (synthesis), also called "white" filler, "light" filler, or even "non-black" filler ("non-black filler") as opposed to carbon black; this inorganic filler is capable of reinforcing, on its own and without any means other than an intermediate coupling agent, a rubber composition intended for the manufacture of pneumatic tires; in other words, it is capable of replacing, in its reinforcing function, a conventional carbon black of pneumatic grade. Such a filler is generally characterized, in a known way, by the presence of hydroxyl groups (-OH) on its surface, requiring, in order to be used as a reinforcing filler, the use of a coupling agent or system designed to ensure a stable chemical bond between the filler and the elastomeric matrix.

[0054] Suitable inorganic reinforcing fillers include siliceous mineral fillers, preferably silica (SiCh). The silica used may be any reinforcing silica known to those skilled in the art, particularly any precipitated or fumed silica with a BET surface area and a CTAB specific surface area both below 450 m² 2 / g, preferably from 30 to 400 m 2 / g, especially between 60 and 300 m 2 / g.

[0055] Of course, the term "inorganic reinforcing filler" also refers to mixtures of different reinforcing inorganic fillers, particularly highly dispersible silicas as described above, or a mixture of siliceous and non-siliceous inorganic fillers. Examples of non-siliceous inorganic fillers include aluminous mineral fillers, particularly alumina (Al₂O₃) or aluminum (oxide)hydroxides, or reinforcing titanium oxides, for example, those described in US 6,610,261 and US 6,747,087. When present, non-siliceous inorganic fillers constitute a minor component of the reinforcing filler.

[0056] The physical state in which the inorganic reinforcing charge is presented is irrelevant, whether it is in the form of powder, microbeads, granules, or balls.

[0057] Those skilled in the art will understand that, as an equivalent to the reinforcing inorganic filler described in this paragraph, a reinforcing filler of another nature, particularly an organic one such as carbon black, could be used, provided that this reinforcing filler is covered with an inorganic layer such as silica, or has functional sites on its surface, particularly hydroxyl sites, requiring the use of a coupling agent to establish the bond between the filler and the elastomer. As an example, we can cite for instance carbon blacks for tires such as described for example in patent documents WO 96 / 37547, WO 99 / 28380.

[0058] According to one embodiment of the invention, the reinforcing filler of the first mixture and / or the second mixture is predominantly carbon black, that is, it comprises more than 50% (>50%) by weight of carbon black relative to the total weight of reinforcing filler. Optionally, according to this embodiment, the reinforcing filler may also include silica or another inorganic reinforcing filler.

[0059] According to another variant of the invention, the reinforcing filler of the first mixture and / or the second mixture consists of carbon black.

[0060] According to another embodiment of the invention, the reinforcing filler of the first and / or second mixture is predominantly an inorganic reinforcing filler (preferably silica), that is, it comprises more than 50% (>50%) by weight of an inorganic reinforcing filler such as silica relative to the total weight of the reinforcing filler. Optionally, according to this embodiment, the reinforcing filler also includes carbon black. According to this option, the carbon black is used at a rate less than or equal to 20%, more preferably less than or equal to 10% (for example, the carbon black content may be in the range of 0.5 to 20%, in particular from 1 to 10%). Within the indicated ranges, the coloring (black pigmenting agent) and UV-resistant properties of carbon black are benefited without compromising the typical performance provided by the inorganic reinforcing filler.

[0061] The elastomeric compositions of the tread of the tire according to the invention may optionally also include all or part of the usual additives known to those skilled in the art and commonly used in treads, such as processing aids, fillers (reinforcing or non-reinforcing, other than those mentioned above), pigments, protective agents such as anti-ozone waxes, chemical anti-ozonants (6PPD, TMQ), antioxidants, anti-fatigue agents and reinforcing resins (such as those described for example in application WO 02 / 10269).

[0062] According to one embodiment of the invention, the top reinforcement of the tire is formed of at least two working top layers of reinforcing elements.

[0063] According to a preferred embodiment of the invention, the reinforcement elements of the top working layers are inextensible metal cables.

[0064] The metallic components are preferably steel cables.

[0065] Advantageously according to the invention, the reinforcing elements of the top working layers of the top reinforcement are crossed from one layer to another by making angles with the circumferential direction between 10° and 45°.

[0066] Advantageously according to the invention, the working top reinforcement comprises a layer of circumferential reinforcing elements, preferably arranged radially between two working top layers.

[0067] According to an advantageous embodiment of the invention, the reinforcing elements of at least one layer of circumferential reinforcing elements are metallic reinforcing elements having a secant modulus at 0.7% elongation between 10 and 120 GPa and a maximum tangent modulus less than 150 GPa.

[0068] A preferred embodiment of the invention further provides that the top reinforcement is completed radially on the outside by at least one additional layer, called a protective layer, of so-called elastic reinforcing elements, oriented with respect to the circumferential direction with an angle between 10° and 45° and in the same direction as the angle formed by the inextensible elements of the working layer which is radially adjacent to it.

[0069] According to any one of the embodiments of the invention mentioned above, the top reinforcement can be further completed, radially inside between the carcass reinforcement and the radially inner working layer closest to said carcass reinforcement, by a triangulation layer of inextensible metallic steel reinforcing elements making, with the circumferential direction, an angle greater than 45° and in the same direction as that of the angle formed by the reinforcing elements of the layer radially closest to the carcass reinforcement.

[0070] Other advantageous details and features of the invention will become apparent from the description of examples of embodiments of the invention with reference to Figures 1 and 2, which represent: Figure 1, a meridian view of a diagram of a tire according to the invention, Figure 2, a radial cross-sectional view of a groove of a tire according to the invention.

[0071] The figures are not shown to scale to simplify understanding.

[0072] In Figure 1, the tire 1, of size 315 / 70 R 22.5, comprises a radial carcass reinforcement 2 anchored in two beads 3 by turning around beads 4. The carcass reinforcement is formed of a single layer of metal cables. This carcass reinforcement 2 is reinforced by a top reinforcement 5, formed radially from the inside to the outside: of a first working layer formed of inextensible metal cables 9.35 not reinforced, continuous over the entire width of the layer, oriented at an angle equal to 22°, of a layer of circumferential reinforcing elements formed of steel metal cables 21x23, of a second working layer formed of inextensible metal cables 9.35 not reinforced, continuous over the entire width of the layer, oriented at an angle equal to 18° and crossed with the metal cables of the first working layer, of a protective layer formed of elastic metal cables 6.35.

[0073] The entirety of these layers forming the vertex 5 reinforcement is not shown in the figures.

[0074] The top reinforcement is itself capped with a tread having a surface 6 intended to come into contact with the ground. The surface 6 of the tread is formed of four circumferential grooves 7 forming circumferential ribs 8 constituting the tread.

[0075] Figure 2 schematically represents a circumferential groove 7 in section along a radial plane.

[0076] The tread is mainly made up of a first compound. Figure 2 also shows a thickness E of a second compound forming the wall 10 of the circumferential groove 7.

[0077] The average thickness E of the second mixture is equal to 2 mm. This thickness E is measured along the direction normal to the inner surface of the circumferential groove 7.

[0078] Tests are carried out with tires according to the invention.

[0079] The same tests are carried out with reference tires. The reference tires differ from the tires according to the invention by the absence of the second compound, i.e. the tread is entirely made up of the first compound.

[0080] The different compounds used for tire treads are listed below: (1) Natural rubber (2) Neodymium polybutadiene 98% of 1,4-cis motif; Tg=-108°C (3) Anionic styrene-butadiene copolymer containing 15% by weight of styrene motif and 24% vinyl of the butadiene part (Tg -65°C) (4) N 134 grade carbon black according to ASTM D-1765-2017 (5) N115 grade carbon black according to ASTM D-1765-2017 (6) SER 6266 from SER (7) N-(l,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine “Santaflex 6PPD” from the company Flexys (8) N-cyclohexyl-2-benzothiazol-sulfenamide “Santicure CBS” from Flexsys (9) N-cyclohexylthio-phthalimide (CTP / PVI) marketed by Shandong Derek New Materials Co. (10) Stearic acid “Pristerene 4931” from the company Uniqema (11) Industrial grade zinc oxide from Umicore

[0081] The values ​​of the constituents are expressed in pce (parts by weight per hundred parts of elastomers).

[0082] The measured properties are expressed in the following table for each of the mixtures:

[0083] Initial endurance tests were carried out on a test machine, imposing on each tire a straight line run at a speed equal to the maximum speed index prescribed for said tire (speed index) under an initial load of 4000 Kg, progressively increased to reduce the duration of the test.

[0084] Further endurance tests were carried out on a test machine that cyclically applied transverse force and dynamic overload to the tires. The tests were performed on the tires according to the invention under conditions identical to those applied to the reference tires.

[0085] The tests thus carried out showed that the distances covered during each of these tests are at least as great as, or even greater than, the tires according to the invention than the reference tires.

[0086] A final test aimed at reproducing curb impacts that tires may experience was carried out.

[0087] This test consists of a preliminary driving stage of one and a half hours at 120 km / h, with the tires subjected to a load of 4000 kg.

[0088] The vehicle then follows a trajectory which leads it to pass the tires over a curb which has a height of between 15 cm and 20 cm.

[0089] The tires are made to follow three different trajectories in succession in order to have overlaps of a few centimeters, more or less pronounced, of the tire tread on the sidewalk at a speed of 10 km / h. These three successive trajectories are reproduced four times in order to complete twelve passes on the sidewalk.

[0090] The entire test is repeated so that each tire undergoes it twice.

[0091] At the end of these tests, the tires are inspected using shearography and dissected to analyze any damage. This visual analysis allows for comparison of any cracks and their propagation. The tires are then scored and compared. A score above 100 corresponds to a less damaged tire. A score of 100 is assigned to the most damaged reference tire.

[0092] At the end of the test, the tires according to the invention show less extensive damage than the reference tires.

[0093] In addition, rolling resistance measurements were carried out.

[0094] The test results are presented in the following table. Rolling resistance measurements are expressed in kg / t, with a value of 100 assigned to the reference tire. Values ​​above 100 indicate better rolling resistance performance.

[0095] These tests show that the tires according to the invention make it possible to maintain, or even slightly improve, performance in terms of rolling resistance compared to the reference tire.

Claims

CLAIMS 1 - A tire (1) with a radial carcass reinforcement (2), comprising a crown reinforcement (5), itself radially capped with a tread, said tread being joined to two beads (3) by means of two sidewalls, said tread comprising a tread surface (6) intended to come into contact with a roadway and form a contact surface, said tread having a plurality of cutouts consisting of at least one groove (7) consisting of two walls opening onto the tread surface (6) and a surface connecting the two walls forming the bottom of said at least one groove, said bottom of said at least one groove being connected to said two walls by junction zones, characterized in that the tread consists of at least a first elastomeric mixture forming at least part of the tread surface when the tire is new,in that at least a portion of the bottom and / or walls of said at least one groove (7) surrounding the junction zones between said bottom and said two walls of said at least one groove (7) is made of a second elastomeric mixture over a thickness E of at least 1 mm, in that the first elastomeric mixture has a tear coefficient of less than 100 N / m and in that the second elastomeric mixture has a tear coefficient (expressed in N / m) at least 25% greater than the tear coefficient of the first elastomeric mixture, the tear coefficient being the product of the breaking force per unit of thickness (expressed in N / mm of thickness) by the elongation at break (expressed in %), measured at 100°C., 2 - Tire (1) according to claim 1, characterized in that said at least one groove (7) is a longitudinally oriented groove. 3 - Tire (1) according to claim 1, characterized in that said at least one groove is a transversely oriented groove. 4 - Tire (1) according to claim 1, characterized in that said at least one groove is an obliquely oriented groove. 5 - Tire (1) according to one of claims 1 to 4, characterized in that the entire bottom and walls of said at least one groove (7) are made of the second elastomeric mixture over a thickness of at least 1 mm. 6 - Tire (1) according to one of claims 1 to 5, characterized in that the thickness E of the second elastomeric mixture is less than 5 mm. 7 - Tire (1) according to one of the preceding claims, characterized in that the second elastomeric mixture has a tear coefficient at least 50% higher than the tear coefficient of the first elastomeric mixture. 8 - Tire (1) according to one of the preceding claims, characterized in that the first elastomeric mixture has a maximum value of tan(ô), noted tan(ô) ma x, less than 0.25, the measurement of tan(ô) being carried out at 60°C according to standard ASTM D 5992-96. 9 - Tire (1) according to one of the preceding claims, characterized in that the first elastomeric mixture is a rubber composition based on at least one elastomeric matrix comprising a copolymer based on styrene and butadiene and at least one reinforcing filler. 10 - Tire (1) according to one of the preceding claims, characterized in that the second elastomeric mixture is a rubber composition based on at least one elastomeric matrix comprising, as a majority, natural rubber for at least 80% and at least one reinforcing filler.

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