WO2020128208A1

WO2020128208A1 - Optimized tyre for a van

Info

Publication number: WO2020128208A1
Application number: PCT/FR2019/052942
Authority: WO
Inventors: Aurélie JOSEPH; Patrick Bervas; Gérald HUGUENOT; Sébastien GUILHEM
Original assignee: Compagnie Generale Des Etablissements Michelin
Priority date: 2018-12-19
Filing date: 2019-12-05
Publication date: 2020-06-25

Abstract

The invention relates to a tyre for a van and aims to improve the trade-off between rolling resistance and antagonistic performances such as the various types of grip (longitudinal/transversal, on dry/wet ground) and the wear. The tread (2) comprises raised elements (22) having a radial height Hs at least equal to 6.5 mm and at most equal to 9 mm, and having a volume groove rate TEV at least equal to 18% and at most equal to 23%. The at least one elastomeric mixture of the tread (2) has a glass transition temperature Tg at least equal to -18°C and at most equal to -21°C, a Shore A hardness at least equal to 64 and at most equal to 65 and a dynamic loss tgδ at 23°C at least equal to 0.15 and at most equal to 0.27. The tyre also has an intermediate layer of an elastomeric mixture, having a dynamic loss tgδa at 23°C at least equal to 0.05 and at most equal to 0.15, at most equal to the dynamic loss tgδ of the materials of the tread.

Description

Optimized truck tire

The invention relates to a radial tire intended to equip a transport vehicle, usually called a van.

The tire field more particularly concerned is that of tires, the nominal pressure of which is at least equal to 3.2 bar and at most equal to 6 bar, for a load index at least equal to 88 and at most equal to 121 , within the meaning of the European Tire and Rim Technical Organization or “ETRTO” standard. In addition, the diameter at seat D, defining the diameter of the tire mounting rim, is at least equal to 14 inches, and at most equal to 19 inches. The main dimensions of these applications are: 195/70 R15C 104 / 102R, 195/75 R16C 107 / 105R, 215/65 R16C 109 / 107T, 225/70 R15C 112 / 110S, 225/65 R16C 112 / 110R, 215 / 70 R15C 109 / 107S, 205/65 R16C 107 / 105T, 195/65 R16C 104 / 102R, 235/65 R16C 115 / 113R, 205/70 R15C 106 / 104R, 215/60 R16C 103 / 101T, 205/75 R16C 110 / 108R, 195/75 R16C 110 / 108R, 225/75 R16C 121 / 120R, 215/75 R16C 116 / 114R, 215/75 R16C 113/11 IR, 235/65 R16C 121 / 119R. The invention is applicable to them as well as to the other dimensions of this type of application.

In what follows, and by convention, the circumferential directions XX ', axial YY' and radial ZZ 'respectively designate a direction tangent to the running surface of the tire according to the direction of rotation of the tire, a direction parallel to the tire rotation axis and a direction perpendicular to the tire rotation axis. By "radially interior", respectively "radially exterior", is meant "closer to the axis of rotation of the tire", respectively "further from the axis of rotation of the tire". By "axially interior", respectively "axially exterior", is meant "closer to the equatorial plane of the tire", respectively "further from the equatorial plane of the tire", the equatorial plane XZ of the tire being the plane passing through the middle of the tire rolling surface and perpendicular to the axis of rotation of the tire.

In general, a tire comprises a tread, intended to come into contact with the ground via a rolling surface, and connected by by means of two sidewalls with two beads, ensuring the mechanical connection between the tire and the rim on which it is mounted. A radial tire comprises a reinforcing reinforcement, comprising a crown reinforcement, radially internal to the tread, and a carcass reinforcement, generally radially internal to the crown reinforcement.

The tread comprises raised elements separated from each other by recesses. The arrangement of the raised elements and the recesses constitutes the tread pattern. The raised elements extend radially outward from a bottom surface to the running surface. For a tire in new condition, the radial distance between the bottom surface and the running surface is called the radial height of the elements in relief and for the maximum radial height, generally measured at the equatorial plane, tread height Hs. The proportion of hollows in relation to the elements in relief is defined by a rate

TEV volume notch, equal to the ratio between the total volume of the hollows and the sum of the respective total volumes of the hollows and the elements in relief. The total volume of the recesses is the sum of the elementary volumes of the recesses having, typically, an axial width at least equal to 1 mm, that is to say of the recesses which are not simple incisions. The volume measurements are carried out by profile surveys of the tread of the new tire, for example, by laser. The profile of the tread has an axial width equal to the axial width of the tread surface in contact with the ground, when the tire, mounted on its nominal rim, is inflated to its nominal pressure and subjected to its nominal load, said nominal conditions being defined by the ETRTO standard. For the tread of a reference state prior art pick-up tire, the raised elements, in new condition, generally have a tread height Hs of between 9.5 and 11 mm and the rate of notching TEV volume is around 18%.

Among the hollows of the tread having a total volume Vc, there are so-called main hollows, each having an average width at least equal to 6 mm and having a total volume Vcp. These main recesses have, in particular, the function of storing water in the contact area between the tread and the ground and of discharging it out of the contact area in order to ensure the performance of hydroplaning. On these ranges, despite the tire pressure being higher than that of passenger vehicles and therefore reducing the risk, the speed index is high and therefore the risk of aquaplaning must be taken into account in design. The volume notching rate is specific, overall lower than that of passenger vehicles and overall higher than that of heavy goods vehicles.

These main hollows are qualified as substantially circumferential, or longitudinal hollows, when their mean line forms with the circumferential direction XX ’of the tire an angle at most equal to 45 °. They are qualified as substantially axial, or transverse hollows, when their mean line forms with the direction.

circumferential XX ’of the tire an angle at least equal to 45 °.

The tread comprises at least one polymeric material of the elastomeric blend type, that is to say a polymeric material obtained by mixing at least one elastomer, at least one reinforcing filler and a system. of crosslinking. Most often, the tread is formed by a unique elastomeric mixture.

A usual physical characteristic of an elastomeric mixture is its glass transition temperature Tg, temperature at which the elastomeric mixture passes from a deformable rubbery state to a rigid glassy state. The glass transition temperature Tg of an elastomeric mixture is generally determined when measuring the dynamic properties of the elastomeric mixture, on a viscoanalyzer for example of the Metravib VA4000 type, according to standard ASTM D 5992-96. The dynamic properties are measured on a sample of vulcanized elastomeric mixture, that is to say baked up to a conversion rate of at least 90%, the sample having the shape of a cylindrical test piece having a thickness equal to 2 mm and a section equal to 78.5 mm ² . The response of the sample of elastomeric mixture to a sinusoidal stress in alternating single shear is recorded, having a peak-peak amplitude equal to 0.7 MPa and a frequency equal to 10 Hz. A temperature scan is carried out at rising speed at a constant temperature of + 1.5 ° C / min. The results exploited are generally the complex dynamic shear modulus G *, comprising an elastic part G 'and a viscous part G ”, and the dynamic loss tgô, equal to the ratio G” / G'. The glass transition temperature Tg is the temperature at which the dynamic loss tgô reaches a maximum during the temperature sweep.

The mechanical behavior of an elastomeric mixture can be characterized, statically, by its Shore A hardness, measured in accordance with DIN 53505 or ASTM 2240 standards, and, in dynamics, by its complex dynamic shear modulus G *, such as previously defined, at a given temperature, typically at 23 ° C.

The heat dissipation, or hysteresis, of an elastomeric mixture can be characterized, statically, by its loss at 60 ° C, which is a loss of energy at 60 ° C, measured by rebound at imposed energy measured at sixth shock and whose value, expressed in%, is the ratio between the difference between the energy supplied and the energy restored and the energy supplied. It can also be characterized, in dynamics, by its dynamic loss tgô, as previously defined, at a given temperature, typically at 23 ° C.

The material of the tread of a light truck tire of the reference state of the art is an elastomeric mixture having a glass transition temperature Tg equal to -23 ° C, a Shore A hardness of 68 and plus and a dynamic loss tgô at 23 ° C of approximately 0.30.

The crown reinforcement, radially internal to the tread and most often, radially external to the carcass reinforcement, comprises, radially from the outside towards the interior, a hooping reinforcement comprising at least one hooping layer and a working reinforcement comprising at least two working layers.

The crown reinforcement has the function of resuming both the mechanical inflation stresses generated by the inflation pressure of the tire and transmitted by the carcass reinforcement and the mechanical rolling stresses generated by the rolling tire on a ground and transmitted by the tread. The specific purpose of the working reinforcement is to impart rigidity to the tire in the circumferential, axial and radial directions and, in particular, in road holding. The hoop reinforcement has the specific function of providing additional circumferential rigidity relative to the working reinforcement, in order to limit the radial deformations of the tire. The hooping reinforcement, for a tire for a van, most often comprises a single hooping layer. A hooping layer comprises reinforcements generally made of textile, for example of aliphatic polyamide such as nylon, coated in an elastomeric mixture and parallel to each other. The reinforcements form, with the circumferential direction XX ′ of the tire, an angle AF, measured in the equatorial plane XZ of the tire, at most equal to 5 °, in absolute value.

The working frame, for a van tire, usually comprises two working layers, radially superimposed, comprising metal reinforcements coated in an elastomeric mixture, mutually parallel in each layer and crossed from one layer to the next. , training, with management

circumferential XX ’of the tire, angles (ATI, AT2), measured in the equatorial plane XZ of the tire, the absolute value of which is generally 22 °.

The metal reinforcements of the working layers are cables formed by an assembly of metal wires. By way of example, the metal reinforcements used for the prior art pick-up tire, taken as a reference, is a cable 4.32 constituted by a twisted assembly of four metal wires each having a diameter equal to 0.32 mm.

Radially inside the crown reinforcement, the carcass reinforcement comprises at least one carcass layer comprising reinforcements, most often made of textile material, coated in an elastomeric material and parallel to each other. The reinforcements of a carcass layer form, with the circumferential direction XX ’of the tire, an angle AC at least equal to 85 ° and at most equal 95 °. Textile materials commonly used for reinforcements of a carcass layer, for a van tire of the prior art, are a polyester such as a polyethylene terephthalate (PET) or an aromatic polyamide, such as aramid .

A carcass layer can be turned over or not turned over. A carcass layer is said to be inverted, when it comprises a main part, connecting the two beads of the tire to each other, and is wound, in each bead, from the inside to the outside of the tire around a circumferential reinforcement or rod, to form a reversal having a free end. A carcass layer is not returned, when it consists only of a main part, connecting the two beads together, without wrapping around a rod.

The main performances targeted for a van tire are, in a non-exhaustive manner, the longitudinal and transverse adhesions on wet ground and on dry ground, the behavior, in particular at low transverse acceleration, the wear and the rolling resistance. .

Currently, car manufacturers are expressing strong demand for a reduction in rolling resistance, in order to reduce fuel consumption. However, it is known to those skilled in the art that a drop in rolling resistance is generally achieved at the expense of other key performances such as the various types of grip (longitudinal / transverse, on dry / wet ground) or behavior and wear.

The inventors have given themselves the objective of improving, compared to a van tire of the prior art as previously described, the compromise between rolling resistance and antagonistic performance such as the various types of adhesion (longitudinal / transverse, on dry / wet ground) and wear.

This object was achieved, according to the invention, by a tire for a transport vehicle whose nominal pressure is at least equal to 3.2 bar and at most equal to 6 bar, for a load index at least equal to 88 and at most equal to 121 comprising:

-a tread, intended to come into contact with a ground by means of a tread surface and comprising elements in relief extending radially outward from a bottom surface to the running surface over a maximum radial height Hs, the tread comprising at least one wear indicator,

the elements in relief having a total volume Vp and being separated by hollows having a total volume Vc _. a part of the hollows being main hollows, each having an average width at least equal to 6 mm and all of the main hollows having a total volume Vcp,

-the tread having a TEV volume notching rate, defined as the ratio between the total volume Vc of the hollows and the sum of the total volume Vc of the hollows and the total volume Vp of the elements in relief, at least equal to 18% and at most equal to 23%, the tread further comprising at least one elastomeric mixture having a glass transition temperature Tg, a Shore A hardness and a dynamic loss tg6 at 23 ° C,

an intermediate layer radially inside the tread further comprising an elastomeric mixture having a dynamic loss tgôa at 23 ° C,

a hooping reinforcement, radially internal to the intermediate layer, comprising at least one hooping layer comprising textile reinforcements coated in an elastomeric mixture, parallel to each other and forming, with a direction

circumferential XX ’of the tire, an angle AF, measured in the equatorial plane XZ of the tire, at most equal to 5 ° in absolute value,

-a working reinforcement, radially inside the hooping reinforcement, comprising at least two working layers, radially superimposed, comprising metallic reinforcements coated in an elastomeric mixture, mutually parallel in each layer and crossed from one layer to the following, training, with management

circumferential XX ’of the tire, an angle (ATI, AT2), measured in the equatorial plane XZ of the tire, the absolute value of which is at least equal to 23 ° and at most equal to 30 °,

a carcass reinforcement comprising at least one carcass layer and at most two carcass layers, each carcass layer comprising textile reinforcements coated in an elastomeric material, mutually parallel and forming, with the circumferential direction XX ′ of the tire, a angle AC at least equal to 85 ° and at most equal 95 °,

- elastomeric sidewall reinforcement mixtures (XX) axially external to the most radially external carcass layer, of an axial thickness Epf, measured at the most axially external point of the tire,

the at least one elastomeric mixture of the tread having a glass transition temperature Tg at least equal to -21 ° C and at most equal to -18 ° C, a Shore A hardness at least equal to 64 and at most equal at 65 and a dynamic loss tgo at 23 ° C at least equal to 0.15 and at most equal to 0.27,

- the elastomeric mixture of the intermediate layer having a dynamic loss tgôa at 23 ° C at most equal to the dynamic loss tgô of the elastomeric mixture (s) of the tread, the sculpture height Hs being at least equal to 6.5 mm and at most equal to 9 mm, the axial thickness Epf being at least equal to 3 mm and at most equal to 7 mm.

An elastomeric tread mixture with low hysteresis contributes to low rolling resistance, this effect being reinforced by the limited radial height of the raised tread elements. Furthermore, in this type of application, it is necessary to achieve the desired performance to have an intermediate layer under the tread, the dynamic loss of which is also low and at most equal to that of the elastomeric mixture or mixtures included in the band. and intended to be in contact with the road surface during normal use of the tire on a vehicle.

Furthermore, an elastomeric tread mixture with reduced hardness but over a narrow range of values, therefore rather softer while retaining rigidity, makes it possible to increase the grip on wet ground but degrades the behavior . However, the negative impact of the soft elastomeric mixture on the behavior is at least partially offset by a low radial height of the raised tread elements.

In this type of vehicle frequently used for city trips, the tires are subjected to contacts on the sidewalls along the sidewalks. This requires very particular care in the design of the sidewalls of the tire, and in particular in the axial thickness Epf of elastomeric mixtures of sidewall reinforcements which constitute the elastomeric mixtures radially external to the most axially external carcass layer, this while optimizing this thickness so as not to deteriorate the rolling resistance performance.

Concerning the glass transition temperature Tg, a narrow range of values makes it possible to adjust the compromise between rolling resistance and grip on wet ground: a rather low glass transition temperature Tg is favorable to rolling resistance , whereas a rather high glass transition temperature Tg is favorable for adhesion.

Thus the elastomeric mixture or mixtures of the tread have a glass transition temperature Tg at least equal to -18 ° C and at most equal to -21 ° C. This range of glass transition temperature values Tg makes it possible to achieve a satisfactory compromise between rolling resistance and grip on wet surfaces, with the particular operating point of light truck tires. The glass transition temperature Tg is high, favorable to grip on wet ground, but sufficiently low, which is favorable to the rolling resistance, to the pressures and to the loads of use of these tires.

Finally, a total volume Vcp of all of the main hollows adapted to the pressure of these tires contributes to good grip on wet ground, in particular with a high water level, thanks to its storage effect and d evacuation of water.

A preferred embodiment for obtaining the characteristics of the tread material whose glass transition temperature Tg is at most equal to - 18 ° c and at least equal to -21 ° C is that the elastomeric mixture is a modified diene elastomer comprising at least one functional group comprising a silicon atom, the latter being located within the main chain, including the chain ends. The expression “atom located within the main chain of Telastomer, including the chain ends”, is understood here as not being an atom hanging (or sideways) along the main chain of Telastomer but being an atom integrated into the main chain.

From the point of view of its chemical composition, the modified diene elastomeric mixture comprises 100 phr (parts per cent of elastomer) of an elastomer matrix comprising a modified diene elastomer. By definition, a diene elastomer is a homopolymer or a copolymer, derived at least in part from diene monomers, that is to say from monomers carrying two carbon-carbon double bonds, conjugated or not. Preferably, the elastomeric mixture comprises diene telastomer modified at a rate at least equal to 20 phr.

The chemical composition of the elastomeric mixture of the tread according to the invention contributes to an improvement in rolling resistance and / or wear and / or grip on wet ground or a shift in the compromise between these performances. . According to certain variants of the chemical composition of the elastomeric mixture of the tread, the functional group comprising a silicon atom is located at one end of the main chain of the elastomer.

According to implementations of these variants, the functional group comprises a silanol function. According to other implementations, the silicon atom of the functional group is substituted by at least one alkoxy function, optionally totally or partially hydrolyzed to hydroxyl.

According to other variants, the functional group is located in the main elastomer chain, it will then be said that the diene elastomer is coupled or even functionalized in the middle of the chain. The functional group silicon atom then links the two branches of the main chain of the diene elastomer. According to these variants, the silicon atom of the functional group is substituted by at least one alkoxy function, optionally totally or partially hydrolyzed to hydroxyl.

According to variants of the invention according to which the functional group comprises a silanol function at the chain end, the functional group can be a silanol function or else a polysiloxane group having a silanol end. Corresponding modified diene elastomers are described in particular in documents EP 0 778 311 A1, WO 2011/042507 Al.

According to any variant of the invention according to which the silicon atom of the functional group is substituted by at least one alkoxy function, optionally totally or partially hydrolyzed to hydroxyl, the silicon atom can also be substituted , directly or via a divalent hydrocarbon radical, by at least one other function comprising at least one heteroatom chosen from N, S, O, P. Preferably, the silicon atom is substituted by at least one other function via a divalent hydrocarbon radical, more preferably a linear Cl-Cl 8 aliphatic radical. Mention may be made, by way of example, among these other functions, of primary, secondary or tertiary amines, cyclic or not, isocyanates, imines, cyano, thiols, carboxylates, epoxides, primary, secondary or tertiary phosphines. The other function is preferably a tertiary amine, more preferably a diethylamino- or dimethylamino- group. The alkoxy function is preferably methoxy, ethoxy, butoxy or propoxy. Of modified diene elastomers corresponding to these variants are described in particular in documents WO 2009/133068 A1, WO 2015/018743 A1.

The modification of the diene elastomer by at least one functional group comprising a silicon atom does not exclude another modification of the elastomer, for example at the chain end by an amine function introduced during the initiation of the polymerization, such as described in WO 2015/018774 Al, WO 2015/018772 Al.

Preferably, the diene elastomer modified according to the invention is a 1,3-butadiene polymer, more preferably a copolymer of 1,3-butadiene and styrene (SBR)

Modified diene elastomer according to the invention can be, according to different variants, used alone in the elastomeric mixture or in blending with at least one other diene elastomer conventionally used in tires, whether it is star-shaped, coupled, functionalized by example with Pétain or silicon, or not.

Also from the point of view of its chemical composition, the elastomeric mixture of the tread according to the invention comprises a plasticizing resin of the thermoplastic resin type at a rate at least equal to 20 phr.

Preferably, the carcass reinforcement can be made up of an inverted and engaged carcass layer. In this case, the reversal of the carcass layer starts again from each bead, crosses the sidewalls of the tire towards the top. If the free end of the carcass layer is axially internal to the axial end of one of the layers of the crown reinforcement, it is said to be engaged. This solution is particularly advantageous in the use of tires for vans to resist internal pressure and optimize the mass of the tire.

Another preferred embodiment is that the carcass reinforcement is made up of two carcass layers to improve the impact resistance against sidewalks while facilitating manufacture compared to other solutions.

The maximum radial height of the raised elements of the tread, or sculpture height Hs, is at least equal to 6.5 mm and at most equal to 9 mm. This range of radial height values makes it possible to obtain a satisfactory compromise between the performances of rolling resistance, behavior and grip on the ground. wet with the glass transition values Tg chosen for the tread material (s). Preferably, the maximum radial height of the elements in relief of the tread, or sculpture height Hs, is at least equal to 7 mm and at most equal to 8.5 mm.

The TEV volume notching rate of the tread is at least 18% and at most 23%. This range of scoring rate values ensures good adhesion performance on wet ground, thanks to a tread scoring rate high enough to compensate for a limited tread height. Preferably, the TEV volume notching rate of the tread is at least equal to 19% and at most equal to 22%.

Preferably, to obtain a good balance between hydroplaning performance and behavior, the main recesses are oriented in the circumferential direction, preferably, the tread comprises at least 3 at most 4 circumferential main recesses.

An advantageous solution is that the volume of main recesses is relatively small and at least equal to 50% and at most equal to 79% in order to stiffen the sculpture to preserve the performance in wear. This relatively low value is made possible by the specific pressure ranges of this range of tires.

A preferred solution is that the parts of the tread axially external to the main recesses oriented in the circumferential direction the most radially external comprise recesses opening on the outside of the

tire with a width at least equal to 2.5mm and at most equal to 4.5mm. This type of hollow in the shoulders allows efficient evacuation of water out of the contact area and improves grip.

Advantageously, the tread comprises recesses, forming an angle with the longitudinal direction at least equal to 40 ° with the longitudinal axis XX ', having a variable width and an average value at most equal to 2 mm . The average width at most equal to 2 mm allows these recesses to close in the passage of contact with the running surface and to maintain, despite these recesses, a rigidity of the tread favorable to rolling resistance. The variable nature of this thickness makes it possible to increase this effect.

Advantageously on either side of the equatorial plane, the most axially exterior point of the hooping layer is axially exterior to the most axially exterior point of the most radially exterior working layer. This architecture is favorable to the rigidification of the structure and therefore to the behavior.

According to a preferred embodiment of the hooping frame, the textile reinforcements of the at least one hooping layer of the hooping frame comprise an aromatic polyamide, such as aramid. Aramid has a high extension module and therefore low deformability, which is favorable for a hooping function of the tire.

According to a first variant of the preferred embodiment of the hooping frame, the textile reinforcements of the at least one hooping layer of the hooping frame comprise a combination of an aromatic polyamide, such as aramid , and an aliphatic polyamide, such as nylon. Such reinforcements are also called hybrid reinforcements and have the advantage of having a mechanical behavior in extension known as “bi-module”, characterized by a low extension module, that of nylon, and therefore great deformability, for small elongations, and a high extension module and therefore lower deformability, for large elongations.

According to a variant of the preferred embodiment of the hooping frame, the textile reinforcements of the at least one hooping layer of the hooping frame comprise a combination of an aromatic polyamide, such as aramid, and a polyester, such as polyethylene terephthalate (PET). Such reinforcements are also hybrid reinforcements which have the same advantages as those described above.

Alternatively, the most radially outer point of the layer

intermediate is radially outside at the most radially inside point of the bottom surface of the tread, and preferably radially inside at the most radially outside point of the wear indicator. This variant is very favorable to rolling resistance because it reduces the average hysteresis of the block top while ensuring that only a material designed for this purpose is in contact with the road surface.

Advantageously, the intermediate layer is profiled at its axial ends and of an average radial thickness at least equal to 3 mm and at most equal to 5 mm. This progressive thickness of the intermediate layer allows axial management of the stiffnesses of the tread. Near the equator plane, under the elements in relief, the intermediate layer is of maximum thickness and this thickness can be variable to decrease under the main hollows and increase or not under elements in relief of the tread axially offset from the plane Ecuador. It is preferred that the thickness decreases progressively and continuously at the ends of the intermediate layer in order to compensate for the loss of rigidity of the working layers due to the edge effects of their own axial ends. For a rolling resistance balanced with the behavior, a thickness of between 3 and 5 mm is preferred.

Preferably the intermediate layer consists of an elastomeric mixture of a dynamic loss tgôa at 23 ° C at least equal to 0.05 and at most equal to 0.15.

In these ranges to facilitate the use of tires, it is preferred that no preferred mounting direction is indicated.

It is the balance between the glass transition temperature Tg, the rigidity of the elastomeric mixture of the tread, the dynamic losses of the elastomeric mixtures of the tread and of the intermediate layer, associated with a sculpture. and to an optimized architecture which makes it possible to obtain improved performances of rolling resistance, grip and behavior. The other characteristics mentioned make it possible to further improve the desired performance compromise.

All of these design parameters should be adapted to the usual pressures and loads of this type of vehicle, in particular to the temperatures obtained at the contact surface.

In conclusion, the combination of the essential characteristics of the invention makes it possible to provide an advantage in rolling resistance without degrading the grip and wear. . The invention is illustrated by FIG. 1, a figure in perspective section of a tire according to the invention, not shown to scale and described below.

[0062] FIG. 1 represents a meridian section in perspective, in a meridian plane YZ, of a tire for a van vehicle according to the invention. The tire comprises, most radially on the outside, a tread 2, intended to come into contact with a ground via a tread surface 21 and comprising elements in relief 22 extending radially towards the exterior from a bottom surface 23 to the rolling surface 21 over a tread height Hs of the tire, at least equal to 6.5 mm and at most equal to 9 mm. The elements in relief 22 have a total volume Vp and are separated by recesses 24 having a total volume Vc. Part of the recesses 24 are main recesses, each having an average width at least equal to 6 mm and all of the main recesses having a total volume Vcp. The tread 2 has a rate of volume notching TEV, defined as the ratio between the total volume Vc of the recesses 24 and the sum of the total volume Vc of the recesses 24 and the total volume Vp of the elements in relief 22, at least equal at 18% and at most equal to 23%. The tread 2 is constituted by an elastomeric mixture having a glass transition temperature Tg at least equal to -21 ° C and at most equal to -18 ° C, a Shore A hardness at least equal to 64 and at most equal to 65 and a dynamic loss tgô at 23 ° C at least equal to 0.15 and at most equal to 0.27. The tire also comprises an intermediate layer 6 comprising an elastomeric mixture and positioned radially inside the tread 2 and whose dynamic loss tgôa at 23 ° C at least equal to 0.05 and at most equal to 0.15 and at more equal to the dynamic loss tgô at 23 ° C of the tread material (s). The tire also comprises a hooping reinforcement 3, radially inside the intermediate layer 6 and comprising a hooping layer 31, a working reinforcement 4, radially inside the hooping reinforcement 3 and comprising two working layers (41, 42 ) radially superimposed, and, finally, a carcass reinforcement 5 comprising a carcass layer 51.

[Fig 1]

The hooping layer 31 comprises textile reinforcements 311 coated in an elastomeric mixture, mutually parallel and forming, with a direction circumferential XX 'of the tire, an angle AF, measured in the equatorial plane XZ of the tire, at most equal to 5 ° in absolute value. The two working layers (41, 42) each include metal reinforcements (411, 421) coated in an elastomeric mixture, mutually parallel in each layer and crossed from one layer to the next, forming, with the circumferential direction XX 'of the tire, an angle (ATI, AT2), measured in the equatorial plane XZ of the tire, the absolute value of which is at least equal to 23 ° and at most equal to 30 °. The carcass layer 51 comprises textile reinforcements 511 coated in an elastomeric material, mutually parallel and forming, with the circumferential direction XX ′ of the tire, an angle AC at least equal to 85 ° and at most equal 95 °.

The invention has been more particularly studied for a van tire of size 235 / 65R16, with a nominal pressure of 4.75b and a load index 115. A reference tire Ref has been compared to a tire Inv according to the invention.

The tread of the reference tire Ref comprises elements in relief extending radially over a tread height Hs equal to 9.6 mm. The TEV volume notching rate of the tread is 19%. The elastomeric mixture of the tread has a glass transition temperature Tg equal to -23 ° C, a Shore A hardness equal to 68 and a dynamic loss tgô at 23 ° C equal to 0.29. The reference tire Ref does not include an intermediate layer radially inside the tread. The hooping reinforcement comprises a hooping layer, the textile reinforcements of which are made of nylon 140/2 (assembly of 2 additional 140 tex each, 1 tex being the mass in g of 1000 m of thread) and distributed in the layer hooping with a density of 98 fi / dm. The working frame comprises two radially superimposed working layers. The metallic reinforcements of formula 4.32 (twisted assembly of four metallic wires each having a diameter equal to 0.32 mm) of the radially innermost working layer and of the radially outermost working layer are distributed at a pitch equal to 1.4 mm and form, with the circumferential direction XX ′ of the tire, an angle (ATI, AT2), measured in the equatorial plane XZ of the tire, respectively equal to + 22 ° and -22 °. The average radial thickness EpT of each working layer is equal to 1.23 mm. The frame of carcass is made up of two carcass layers, the textile reinforcements of which are made of polyethylene terephthalate (PET) with a 144/2 title (assembly of two additional 144 tex each), with a twist of 420 rpm and distributed in the carcass layer with a density of 104 threads / dm. The thickness of the elastomeric sidewall reinforcement mixtures axially external to the most radially external carcass layer with an axial thickness Epf, measured at the most axially external point of the tire, is equal to 3.7 mm.

The tread of the tire according to the invention Inv includes elements in relief extending radially over a tread height Hs equal to 8.65 mm. The TEV volume entablature rate of the tread is 21%. The elastomeric mixture of the tread has a glass transition temperature Tg equal to -20 ° C, a Shore A hardness equal to 64 and a dynamic loss tgô at 23 ° C equal to 0.27. The tire according to the invention also comprises an intermediate layer, radially inside the tread and radially outside the hooping reinforcement, constituted by an elastomeric mixture having a dynamic loss tgô at 23 ° C equal to 0.14. The hoop reinforcement and the working and carcass layers are not modified between the reference tire Ref and the

tire according to the invention Inv, except for the angle of the working layers, the value of which (ATI, AT2), measured in the equatorial plane XZ of the tire, equal to + 24 ° and -24 ° respectively.

The tread of the reference tire Ref does not comprise a modified diene elastomer comprising at least one functional group, it is a conventional elastomeric mixture for this type of application. The tread of the tire according to the invention Inv comprises a modified diene elastomer comprising at least one functional group comprising a silicon atom, the latter being located at the end of the chain.

The Inv and Ref tires were subjected to various tests and comparative measurements. The results, presented below, are expressed respectively for Inv with respect to Ref (reference or base 100):

- Rolling resistance: -1.3 kg / t

- Longitudinal grip (braking distance) on dry ground (base 100): 100 - Longitudinal grip (braking distance) on wet ground (base 100): 105

- Wear performance (average use) (base 100): 110

Compared with the tire of the state of the art Ref taken as a reference, the tire Inv according to the invention has a reduction in rolling resistance of 1.3 kg / t, with performance in longitudinal grip at least equivalent, improved on wet ground and a gain in wear of 10%.

Claims

1. Tire for transport vehicle whose nominal pressure is at least equal to 3.2 bar and at most equal to 6 bar, for a load index at least equal to 88 and at most equal to 121 comprising:

-a tread (2), intended to come into contact with a ground via a tread surface (21) and comprising elements in relief (22) extending radially outward from '' a bottom surface (23) to the running surface (21) over a maximum radial height Hs, the raised elements (22) having a total volume Vp and being separated by recesses (24) having a volume total Vc _. a part of the hollows (24) being main hollows, each having an average width (w) at least equal to 6 mm and all of the main hollows having a total volume Vcp,

the tread (2) having a TEV volume notching rate, defined as the ratio between the total volume Vc of the recesses (24) and the sum of the total volume Vc of the recesses (24) and the total volume Vp of the elements in relief (22), at least equal to 18% and at most equal to 23%, the tread comprising at least one wear indicator,

the tread (2) further comprising at least one mixture

elastomeric having a glass transition temperature Tg, a Shore A hardness and a dynamic loss tgô at 23 ° C,

an intermediate layer (6) radially inside the tread (2) further comprising at least one elastomeric mixture having a dynamic loss tgôa at 23 ° C,

a hooping reinforcement (3), radially inside the intermediate layer (6), comprising at least one hooping layer (31) comprising textile reinforcements (311) coated in an elastomeric mixture, parallel to each other and forming, with a circumferential direction XX 'of the tire, an angle AF, measured in the equatorial plane XZ of the tire, at most equal to 5 ° in absolute value, -a working frame (4), radially inside the hooping frame (3) , comprising at least two working layers (41, 42), radially superposed, comprising metallic reinforcements (411, 421) coated in an elastomeric mixture, mutually parallel in each layer and crossed from one layer to the next, forming, with the circumferential direction XX 'of the tire, an angle (ATI, AT2) , measured in the equatorial plane XZ of the tire, the absolute value of which is at least equal to 23 ° and at most equal to 30 °,

a carcass reinforcement (5) comprising at least one carcass layer (51) and at most two carcass layers, each carcass layer comprising textile reinforcements (511) coated in an elastomeric material, mutually parallel and forming, with the circumferential direction XX ′ of the tire, an angle AC at least equal to 85 ° and at most equal 95 °,

- characterized in that the at least one elastomeric mixture of the tread (2) has a glass transition temperature Tg at least equal to -21 ° C and at most equal to -18 ° C, a Shore A hardness at least equal to 64 and at most equal to 65 and a dynamic loss tgô at 23 ° C at least equal to 0.15 and at most equal to 0.27,

- in that the elastomeric mixture of the intermediate layer (6) has a dynamic loss tgôa at 23 ° C at most equal to the dynamic loss tgô of the elastomeric mixture (s) of the tread,

- and in that the sculpture height Hs is at least equal to 6.5 mm and at most equal to 9 mm, the axial thickness Epf is at least equal to 3mm and at most equal to 7mm.

2. A tire according to claim 1, in which the carcass reinforcement (5) consists of two carcass layers (51).

3. A tire according to one of claims 1 or 2, in which on either side of the equatorial plane, the most axially outer point of the hooping layer (31) is axially outer at the most axially outer point of the most radially outer working layer (42).

4. Tire according to any one of claims 1 to 3, in which the most radially outer point of the intermediate layer (6) is radially outside at the most radially inside point of the bottom surface (23) of the nearest tread (2), and preferably radially inside at the most radially outside point of the wear indicator.

5. Tire according to one of claims 1 to 4, in which the layer

intermediate (6) is profiled at its axial ends and of an average radial thickness at least equal to 3 mm and at most equal to 5 mm and consists of an elastomeric mixture of a dynamic loss tgôa at at least 23 ° C equal to 0.05 and at most equal to 0.15.

6. Tire according to any one of claims 1 to 5, in which the tread comprises a modified diene elastomer comprising at least one functional group comprising a silicon atom, the latter being located within the main chain of the elastomer including chain ends.

7. A tire according to any one of claims 1 to 6, in which

The modified diene elastomer of the elastomeric mixture of the tread (2) comprises a functional group comprising a silicon atom at one end of the main chain of the elastomer.

8. A tire according to any one of claims from 1 to 6, the modified diene elastomer of the elastomeric mixture of the tread (2) comprises a functional group comprising a silicon atom in the middle of the chain.

9. A tire according to any one of claims 1 to 8 in which the tread comprises recesses forming an angle with the longitudinal direction at least equal to 40 °, having a variable width from an average value at most equal to 2 mm.

10. A tire according to any one of claims 1 to 9, in which the maximum radial height Hs, is at least equal to 7 mm and at most equal to 8.5 mm.