Patent ID: 12240272

DETAILED DESCRIPTION

The tyre thus described comprises an outer strip comprising two rubber compounds, one disposed in the centre and radially on the outside in the new state of the tyre and the other, which is less rigid, disposed:around the equatorial plane, radially on the inside of the tread occupying a particular percentage of the protective stripon the laterally outer portions of the outer strip and of the tread near the ends of the crown layers, called shoulders, a greater or lesser percentage depending on the targeted performance aspects of the protective strip and of the tread.

This configuration is not necessarily symmetrical on either side of the equatorial plane and may be effective only on one of the two shoulders.

In order to improve the rolling resistance, it is customary to increase the number of rubber materials by using materials with the lowest possible hysteresis on the portions of the tyre that allow this. Thus, the protective strip is usually a compound of very low hysteresis that is able to absorb the energy of an impact due to running over an obstacle, while at the same time being protected from the attack of contact of this obstacle by the material of the tread that has the characteristics suitable for this function.

It is also possible to improve the rolling resistance by reducing the stiffness of materials under certain conditions. At their axial ends, the crown layers have a double curvature, one being circumferential, equivalent to the central part of the tread, and the other being axial due to the curvature of the carcass layer and therefore of the crown layers on this portion. The necessary flattening energy is therefore all the higher. In order to improve the performance in terms of rolling resistance, for a tyre with a given rubber compound for the central part of the tread, it is therefore possible to lower the stiffness of the compounds at the shoulders on the outermost parts of the outer strip within or beyond the limit of the tread surface.

For a significant improvement in rolling resistance, it is necessary for this percentage of the second rubber compound to be at least equal to 60% of the volume of the axially outer parts of the outer strip. This second rubber compound has to have stiffness and grip properties such that it can be in contact with the ground on which the tyre is running either in the new state or in the worn state and therefore such that it has hysteretic properties that are compatible with this function and therefore less favourable a priori than the protective compounds usually used to lower rolling resistance.

The solution with three compounds, one of which is the protective compound, the properties of which are very different from the two other rubber compounds, unacceptably degrades the capacity to recycle these compounds for uses related to tyres in which the compositions are very precise. Furthermore, the use of such compounds for the protective strip degrades the grip of the tyres, in particular at the end of their life. Specifically, in a surprising manner, the grip is not only dependent on the material in contact with the ground but also on the materials of the protective strip.

The invention therefore consists in using a single rubber compound for the protective strip and for the largest portion of the axially outer parts of the outer strip. This said compound has to be of lower stiffness than the rubber compound that is radially outermost at the centre of the tread. This rubber compound has to be suitable for contact with the ground on which the tyre is running either in the new state or in the worn state. The minimum thickness of a protective strip is at least equal to 0.3 mm, preferably at least equal to 0.6 mm, at least equal to 1 mm; the substantially continuous part constituted by said compound between the two axially outer parts of the outer strip has to have the same properties.

For optimum performance in terms of behaviour and uniformity of wear, and to allow recycling of scrap, the first rubber compound has a dynamic shear modulus G*, at 10% deformation at 10 Hz at 23° C. measured in accordance with standard ASTM D 5992-96, at least equal to 1.35 times the dynamic shear modulus G*, at 10% deformation at 10 Hz at 23° C. measured in accordance with standard ASTM D 5992-96, of the second rubber compound and at most equal to 3 times the dynamic shear modulus G*, at 10% deformation at 10 Hz at 23° C. measured in accordance with standard ASTM D 5992-96, of the second rubber compound,

For optimum performance in terms of grip on snow-covered ground, the first rubber compound has a dynamic loss tan D-10_1, measured in accordance with standard ASTM D 5992-96, at a temperature of −10° C. and under a stress of 0.7 MPa at 10 Hz, at least equal to 0.5, preferably at least equal to 0.7.

For optimum performance in terms of rolling resistance, the second rubber compound has a dynamic loss tan D23_2, measured in accordance with the same standard ASTM D 5992-96, at a temperature of 23° C. and under a deformation of 10% at 10 Hz, at most equal to 0.3.

Preferably, although there are rubber compounds that are able to be in contact with the ground that are less stiff, the second rubber compound has a dynamic shear modulus G*, at 10% deformation at 10 Hz at 23° C. measured in accordance with standard ASTM D 5992-96, at least equal to 0.8 MPa and at most equal to 4 MPa, preferably at least equal to 1 MPa and at most equal to 2.5 MPa, for good performance in terms of wear and behaviour.

For uses with high speed ratings and therefore generally drift rates that are also higher, it is preferred that the second rubber compound has a secant extension modulus MA300 at 300% deformation, measured at 23° C. in accordance with standard ASTM D 412-16, at least equal to 1.7 in order to ensure a correct wear pattern on the axially outer part of the outer strip for this use.

The properties of the rubber compounds are measured in accordance with standard ASTM D 5992-96. For example, the dynamic shear moduli G* and tan δ are measured at 23° C. at 10% deformation at 10 Hz, in accordance with standard ASTM D 5992-96. The response of a sample of cross-linked composition (preferably cylindrical test specimen with a thickness of 4 mm and a cross section of 400 mm2), subjected to a simple alternating sinusoidal shear stress, at a frequency of 10 Hz, at 23° C. in accordance with standard ASTM D 5992-96, is recorded. A peak-to-peak strain amplitude sweep is carried out from 0.1 to 100% (outward cycle) and then from 100 to 0.1% (return cycle). The result that is made use of is the loss factor (tan(δ)). For the return cycle, the maximum value of tan(δ) observed (tan(δ)max at 23° C.) and the modulus G* are indicated.

Surprisingly, by coupling the axially outer parts of the outer strip and protective strip through the use of a single material, the rolling resistance is improved compared with a solution with two rubber compounds, one for the tread and the other for the protective strip, and retained or slightly improved compared with a solution with three rubber compounds including a specific one for the protective strip. Furthermore, the invention brings about an obvious improvement in recycling capacity.

Since the tyre is toric, the volume ratios between the two rubber compounds of the outer strip are evaluated from the surface areas of these materials, which are measured on one or more meridian sections, taking into account the volumetric void ratio of the tread pattern. This practice is well known to those skilled in the art.

When the first rubber compound is axially on the inside of the second rubber compound, it is necessary to ensure that the interface between the two first and second compounds of the outer strip of the tyre is maintained. To this end, the interface between the first and the second rubber compounds has to form an angle with the normal to the tread surface at the point of intersection of the interface with the tread surface that is at most equal to 60° in terms of absolute value, preferably at most equal to 20° in terms of absolute value. For angles outside these ranges, the shear forces under transverse cornering forces subject the interface of the two products to deformations risking progressive microcracking of the interface at the zone of contact with the ground on which the tyre is running. The parts axially on the outside of these microcracks are gradually torn away, generating an irregular wear zone.

For application to tyres intended for sports cars for which the behaviour is a favoured performance aspect, it is preferred that the layer of the second rubber compound, which is continuous from one outer part of the outer strip to the other, is radially on the inside, in the central part of the tread, of the bottom surfaces of the grooves or of the circumferential furrows of the tread. In this case, the central part of the outer strip is predominantly made up of the first rubber compound, which is the most rigid, and this ensures a high stiffness of the tread pattern.

In order to improve wear and grip at the end of life, the invention may comprise grooves or circumferential furrows that are configured such that the void ratio of the tread surface increases with wear of the tyre, such as droplet-shaped knife cuts, voids that are hidden in the new state, angles of the lateral faces of the cuts that are configured such that the surface void ratio of said cut increases with wear.

In order to achieve the targeted mechanical properties of the first and second rubber compounds, in particular for their properties in terms of rolling resistance and grip, it is preferred that the first and second rubber compounds have reinforcing fillers made up of at least 80% silica, expressed as a percentage of the total mass of the reinforcing fillers.

For ease of fitting on the vehicle, and in particular for symmetrical, non-directional tyres, it is preferred that the volumes of first and second rubber compounds of the outer strip are substantially symmetrical with respect to the equatorial plane.

The invention requires that the outer strip is made up, to at least 90% by volume, of the two rubber compounds, the remaining 10% being provided for the possible use of other rubber compounds so as to fulfil other functions such as for example the electrical conductivity of the tyre or ensure the bonding of the outer strip to the crown reinforcement.

The optimal solution is that the tread is made up of two rubber compounds and one of these rubber compounds is an electrical conductor and is configured such that the electrical resistance of the tyre, measured in accordance with standard ISO 16392:2017, is at most equal to 1010ohms, preferably at most equal to 108ohms.

In the case in which the first and second rubber compounds of the outer strip are not sufficiently conductive to comply with standard ISO 16392:2017, an advantageous solution is that the outer strip is made up of three rubber compounds, the third rubber compound being disposed between the radially outermost layer of reinforcing elements of the crown reinforcement and the tread surface and configured such that the electrical resistance of the tyre, measured in accordance with standard ISO 16392:2017, is at most equal to 1010ohms, preferably at most equal to 108ohms. This rubber compound is a compound designed to be incorporated into the tread while at the same time containing a portion of conductive filler, usually carbon black. Its properties mean that it is able to be recycled with the first or second rubber compound of the tread, all the more so if its volume is low. In this case, there is a very localized discontinuity contingent on the electrical conductivity function in the layer of the second rubber compound, this being the only exception to the continuity of this said layer indicated by the term “substantially” continuous.

In order to obtain optimum performance in terms of rolling resistance, it is common for the rubber compounds that are able to be in contact with the ground on which the tyre is running in the new state, or in the worn state, to have a high content of silica, and this can lead to inadequate performance in terms of bonding on the crown layers during the manufacture of the tyre, before it is cured. It is customary in this case to use an intermediate rubber called a bonding rubber to allow the adhesion of the compounds of the protective strip and the tread to the crown layers. This bonding layer never has a radial thickness greater than 0.6 mm.

In the case in which there is a problem of tack in manufacture between the second rubber compound and the radially outermost crown layer, and the first and second compounds are electrically conductive to the point of complying with standard ISO 16392:2017, the preferred solution is that the outer strip is made up of three rubber compounds and the third rubber compound is disposed between the radially outermost layer of reinforcing elements of the crown reinforcement and the second rubber compound, the maximum thickness of this third rubber compound being at most equal to 0.6 mm, preferably at most equal to 0.4 mm.

In the case in which there is a problem of tack in manufacture between the second rubber compound and the radially outermost crown layer, and the first and second compounds are not sufficiently electrically conductive to comply with standard ISO 16392:2017, an advantageous solution is that the outer strip is made up of four rubber compounds, wherein the third rubber compound is disposed between the radially outermost layer of reinforcing elements of the crown reinforcement and the second rubber compound, the maximum thickness of this third rubber compound is at most equal to 0.4 mm, and wherein the fourth rubber compound is disposed between the radially outermost layer of reinforcing elements of the crown reinforcement and the tread surface and designed such that the electrical resistance of the tyre, measured in accordance with standard ISO 16392:2017, is at most equal to 1010ohms, preferably at most equal to 108ohms.

In order to improve the recyclability of the compounds, the total volume of the second rubber compound represents at least 25% and at most 50% of the volume of the outer strip.

Manufacturing the tyre generates extruded materials of rubber compounds that will not be used for the manufacture of tyres, called scrap. For example, the adjustment of a machine during a change of size generates such scrap and in particular scrap made up of rubber compounds of the outer strip, in particular of the tread and of the protective strip. In the case of configurations with a plurality of rubber compounds in the outer strip, the scrap is called mixed scrap. In order to reduce the scrap that is not put to profitable use in the tyre, receiving volumes in a rubber compound called the “host” compound are defined in the compounds of the outer strip. Those skilled in the art are aware of the complexity of mixing the host compound and the mixed scrap, which requires machine occupation time for the homogenization of the host compound with the scrap. This implies that a single rubber compound is a “host” compound, unless there is a loss in productivity or in recycling rate.

The portions of the radially outer compounds that have a high level of contact with the road cannot be used for incorporating scrap so as not to modify or impair the performance of the tyre. Thus, for configurations with two materials, one for the protective strip and one for the tread, only the protective strip can be a host compound, with a volume of host compound at most equal to 20% of the volume of the outer strip. For configurations with three rubber compounds, one protective strip and two different compounds in the tread, only the protective strip can be a host compound, with a volume of host compound at most equal to 20% of the volume of the outer strip. For the tyre according to the invention, the second rubber compound constituting the volume of the protective strip and a part of the tread and having a low level of contact, only in cornering under high lateral force, constitutes a volume at least equal to 25% of the volume of the outer strip, preferably at least equal to 40%. This greater volume naturally allows a greater volume of mixed scrap to be incorporated.

The usable volume of mixed scrap per tyre is defined by the receiving volume and the maximum admissible content of mixed scrap in this rubber compound, because although it is possible to incorporate mixed scrap into the host rubber compound, it is not possible to integrate too high a content thereof at the risk of seeing its properties modified beyond what is desirable. It is thus possible to calculate the recycling power Pr, which is equal to the admissible percentage of mixed scrap in the total volume of the outer strip. Pr is equal to the volume of the host rubber compound multiplied by the maximum percentage of mixed scrap in the host compound.

By taking a maximum percentage of mixed scrap in the host compound of 10% by volume, the invention makes it possible to recycle at least 20% more mixed scrap and up to 100% more compared with the prior art.

The features and the other advantages of the invention will be understood better with the aid ofFIGS.1to3, said figures not being drawn to scale but in a simplified manner so as to make it easier to understand the invention.FIGS.1to3are parts of a tyre, in particular the crown reinforcement and outer strip and sidewall thereof.

FIG.1schematically depicts the meridian half-section through the crown of the tyre according to the invention. It illustrates in particular the crown1comprising an outer strip2comprising a tread21that is intended to come into contact with the ground via a tread surface SR of width LA. The crown1also comprises a crown reinforcement3, comprising three layers of reinforcing elements31,32,33radially on the outside of the carcass layer30. The outer strip2is radially on the outside of the radially outermost layer of reinforcing elements33of the crown reinforcement3and axially on the inside of the part of the sidewalls4that is radially on the outside of the crown reinforcement3.

The outer strip is made up in this example of three rubber compounds M1, M2and M3. The first rubber compound M1is radially on the outside of the second rubber compound M2and they represent at least 90% of the volume of the outer strip2—in this case 97%, the remaining 3% consisting of M3. The outer strip has a central part with a width equal to 80% of the tread surface SR and two axially outer parts. The axially outer parts of the outer strip are made up of at least 60% by volume of the second compound M2. The second compound M2is substantially continuous from one outer part of the outer strip to the other, i.e. it is continuous except if a compound M3binds the crown layers to the tread surface in order to ensure the compliance of the tyre with the standard on the electrical conductivity of tyres.

InFIG.1, the axial borders of the tread surface are determined in a meridian plane, which axial borders make it possible to measure the tread width in this meridian plane. In some cases, the width of the tread surface is trivially determined by those skilled in the art, since the axially outermost rib of the tread on either side of the equatorial plane has a clear discontinuity allowing simple measurement. InFIG.1, which shows the case of a number of tyres for passenger vehicles in which the tread surface SR is continuous with the outer surface of the sidewall, the tangent24to the tread surface SR at any point on said tread surface in the region of transition towards the sidewall is plotted on a meridian section of the tyre, which is in the fitted position and inflated to nominal pressure. On each side of the equatorial plane, the axial border passes through the point for which the angle between said tangent24and an axial direction ZZ′ is equal to 60°. When, in a meridian plane, there are several points on one and the same side of the equatorial plane for which the angle between said tangent and an axial direction ZZ″ is equal to 60°, it is the radially outermost point that is adopted. The width of the tread, at the meridian plane, is the axial distance between the two points of the two axial borders of the tread surface. The width of the tread of the tyre is the maximum value of the widths of the tread over all the meridians.

FIG.2gives a variant of the invention with respect toFIG.1, in which the rubber compound M2opens onto the tread surface and in which the interface with the rubber compound M1is radially on the outside of the radially innermost points of the bottom surfaces of the circumferential furrows and the grooves25. In versions in which rolling resistance would be the most desirable performance aspect, this interface could be radially on the outside of the wear indicator26. Specifically, M2is configured to be in contact with the ground on which the tyre is running.

FIG.3shows a detail of the outer strip2and the interface27between the first rubber compound M1and the second rubber compound M2in the axially outer part of the outer strip. The first rubber compound M1is axially on the inside of the second rubber compound M2, and therefore the second rubber compound M2opens onto the external surface of the outer strip2at the tread surface SR or onto the external surface of the outer strip outside the tread surface. The interface between the first and the second rubber compounds forms an angle with the normal28to the tread surface at the point of intersection of the interface with the outer surface of the outer strip that is at most equal to 60° in terms of absolute value.

A meridian section through the tyre is obtained by cutting the tyre on two meridian planes.

The invention has been carried out on a tyre of size 225/45 ZR17 intended to be fitted to a passenger vehicle satisfying the conditions for being marked with the symbol 3PMSF authorizing its use on snow-covered roads on European territory. The depths D of the grooves of the tread pattern are between 4 and 8 mm.

The inventors have a plurality of rubber compounds that originate from the same manufacturing technology and from the same raw materials and that are able to constitute the central part of the outer strip. The materials in question vary in terms of properties between a rather rigid, adherent and hysteretic rubber compound MT1and a material MT2that is less rigid, less adherent and less hysteretic than MT1.

The rubber compound MT1has an elastic shear modulus G*, at 10% deformation at 10 Hz at 23° C. measured in accordance with standard ASTM D 5992-96, equal to 2.65 MPa, a dynamic loss tan D-10_1, measured in accordance with standard ASTM D 5992-96, at a temperature of −10° C. and under a stress of 0.7 MPa at 10 Hz, equal to 0.73 and a dynamic loss tan D23_2, measured in accordance with the same standard ASTM D 5992-96, at a temperature of 23° C. and under a deformation of 10% at 10 Hz, equal to 0.55.

The rubber compound MT2has an elastic shear modulus G*, at 10% deformation at 10 Hz at 23° C. measured in accordance with standard ASTM D 5992-96, equal to 1.45 MPa, a dynamic loss tan D-10_2, measured in accordance with standard ASTM D 5992-96, at a temperature of −10° C. and under a stress of 0.7 MPa at 10 Hz, equal to 0.31 and a dynamic loss tan D23_2 measured in accordance with the same standard ASTM D 5992-96, at a temperature of 23° C. and under a deformation of 10% at 10 Hz, equal to 0.19.

For tyres according to the prior art, the compound that is able to be in contact with the ground is combined with a rubber compound for the protective strip of an almost constant thickness equal to 2.1 mm on average, representing 20% of the volume of the outer strip, and of which the dynamic loss tan D23_2 measured in accordance with the same standard ASTM D 5992-96, at a temperature of 23° C. and under a deformation of 10% at 10 Hz, is equal to 0.12, with the objective of best optimizing the rolling resistance.

By varying the properties of the material that is able to be in contact with the ground between the properties of the rubber compounds MT1and MT2, a person skilled in the art can design tyres according to a certain compromise between rolling resistance and grip.

The inventors have also produced a tyre according to the invention in which the radially outer compounds are made up of the first rubber compound MT1and the second rubber compound MT2. The axially outer parts of the outer strip of the tyre comprise at least 85% by volume of the second rubber compound MT2. The rubber compound MT2is continuous from one axially outer part to the other of the outer strip2over an almost constant thickness of 2.1 mm. In total, the volume of the first rubber compound MT1represents 57% of the total volume of the outer strip.

Compared to tyres according to the prior art, the invention either, for identical grip, in particular on snow-covered ground, improves the compromise in terms of rolling resistance by approximately 5%, or for the same rolling resistance and the same grip on snow-covered ground, improves wet grip by 3%, the performance being measured on circuits.

Furthermore, the recycling capacity of the MT1and MT2compounds is increased by 100% compared with tyres outside the invention.