Tire sidewall for a heavy duty civil engineering vehicle

Abstract

A radial tire (10) for a heavy vehicle of construction plant type, and more particularly, the sidewalls thereof (20), arranged to minimize the temperature of the tire while guaranteeing its electrical conductivity. The tread (30) comprises two tread wings (31) and a central portion (32). The bead layer (71), the elastomeric coating compound of the carcass layer (50), the second sidewall layer (22) and the tread wing (31) constitute a preferential conductive pathway of the electric charges between the rim and the ground when the tire is mounted on its rim and flattened on the ground.

Claims

1. A tire for heavy vehicle of construction plant type comprising: a tread comprising two axial end portions or tread wings axially separated by a central portion; two sidewalls connecting the tread wings to two beads, adapted to come into contact with a mounting rim by a bead layer comprised of electrically-conductive elastomeric compound; each said sidewall being axially on the outside of a carcass reinforcement comprising at least one carcass layer having metallic reinforcers that are coated in an electrically-conductive elastomeric coating compound; each said sidewall having a laminate comprising at least two sidewall layers that are at least partly axially superposed and having a total thickness E; the axially outermost first sidewall layer from said sidewall layers having a thickness E.sub.1 and having a first elastomeric compound M.sub.1; the first elastomeric compound M.sub.1 having a viscous shear modulus G″.sub.1 and a thermal conductivity λ.sub.1; the axially innermost second sidewall layer from said sidewall layers having a thickness E.sub.2 and having a second elastomeric compound M.sub.2; the second elastomeric compound M.sub.2 having a viscous shear modulus G″.sub.2, a thermal conductivity λ.sub.2 and an electrical resistivity ρ.sub.2; each said tread wing having a third elastomeric compound M.sub.3 having an elastic dynamic shear modulus G′.sub.3 and an electrical resistivity ρ.sub.3; wherein the thickness E.sub.1 of the first sidewall layer is at least equal to 0.9 times the total thickness E of the laminate, wherein the thickness E.sub.2 of the second sidewall layer is at least equal to the minimum value between 3 mm and 0.1 times the total thickness E of the laminate, wherein the first elastomeric compound M.sub.1 of the first sidewall layer has a viscous shear modulus G″.sub.1 at most equal to 0.165 MPa and a thermal conductivity λ.sub.1 at least equal to 0.190 W/m.Math.K, wherein the second elastomeric compound M.sub.2 of the second sidewall layer has an electrical resistivity ρ.sub.2 of less than or equal to 10.sup.6Ω.Math.cm and a thermal conductivity λ.sub.2 greater than that of the first elastomeric compound M.sub.1 of the first sidewall layer, and wherein the electrical resistivities ρ.sub.2 and ρ.sub.3, respectively, of the second elastomeric compound M.sub.2 of the second sidewall layer and of the third elastomeric compound M.sub.3 of the tread wing are at most equal to 10.sup.6Ω.Math.cm, so that the bead layer, the elastomeric coating compound of the carcass layer, the second sidewall layer and the tread wing constitute a preferential conductive pathway of the electric charges between the rim and the ground when the tire is mounted on its rim and flattened on the ground.

2. The tire according to claim 1, the second sidewall layer being in contact via a radially outer top end with a said tread wing over a length L.sub.h, wherein the length L.sub.h is at least equal to 10 mm.

3. The tire according to claim 1, the second sidewall layer being in contact via a radially inner bottom end with the electrically-conductive elastomeric coating compound of the carcass layer over a length L.sub.b, wherein the length L.sub.b is at least equal to 10 mm.

4. The tire according to claim 1, wherein the thermal conductivity λ.sub.2 of the elastomeric compound of the second sidewall layer is greater than or equal to 0.240 W/m.K.

5. The tire according to claim 1, wherein the third elastomeric compound M.sub.3 of at least one said tread wing is an electrically-conductive rubber composition based at least on polyisoprene, on a crosslinking system, and on at least one reinforcing filler comprising carbon black, having a BET surface area at least equal to 110 m.sup.2/g, and a content at least equal to 30 phr and at most equal to 80 phr.

6. The tire according to claim 1, wherein the second elastomeric compound M.sub.2 of the axially innermost second sidewall layer is an electrically-conductive rubber composition based at least on a mixture of polyisoprene and polybutadiene, on a crosslinking system, and on a reinforcing filler comprising carbon black, having a BET surface area at least equal to 80 m.sup.2/g, and a content at least equal to 40 phr and at most equal to 60 phr.

7. The tire according to claim 1, wherein the first elastomeric compound M.sub.1 of the axially outermost second sidewall layer has a rubber composition based on at least one blend of polyisoprene, natural rubber or synthetic polyisoprene, and polybutadiene, on a crosslinking system, and on a reinforcing filler, at an overall content at most equal to 45 phr, and comprising carbon black, at a content at most equal to 5 phr, and, predominantly, silica, at a content at least equal to 20 phr and at most equal to 40 phr.

8. The tire according to claim 1, the central tread portion being formed by a fourth elastomeric compound M.sub.4, wherein the fourth elastomeric compound M.sub.4 of the central tread portion is a rubber composition based on at least one diene elastomer, on a crosslinking system, and on a reinforcing filler comprising carbon black, having a BET surface area at most equal to 115 m.sup.2/g, and a content at most equal to 40 phr, and silica, at a content at most equal to 20 phr.

9. The tire according to claim 1, the central tread portion being formed by a fourth elastomeric compound M.sub.4, wherein the fourth elastomeric compound M.sub.4 of the central tread portion is a rubber composition based on at least one diene elastomer, on a crosslinking system, and on a reinforcing filler, at an overall content at most equal to 40 phr, and comprising carbon black, and silica.

10. The tire according to claim 1, the central tread portion being formed by an elastomeric compound M.sub.4, wherein the fourth elastomeric compound M.sub.4 of the central tread portion is an electrically-conductive rubber composition based on at least one diene elastomer, on a crosslinking system, and on a reinforcing filler comprising carbon black, having a BET surface area at least equal to 120 m.sup.2/g, and a content at least equal to 35 phr and at most equal to 80 phr, and silica, at a content at most equal to 20 phr.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The architecture of the tire according to the invention will be better understood with reference to FIG. 1, not to scale, which represents a meridian half section of a tire.

(2) FIG. 1 schematically represents a tire 10 intended to be used on Dumper type vehicles. FIGS. 2, 3, and 4 represent the various possible configurations of the tread wings relative to the central portion. FIG. 5 shows the curves of thermal conductivities as function of the amount of reinforcing fillers in phr.

DETAILED DESCRIPTION OF THE DRAWINGS

(3) In FIG. 1, the tire 10 comprises a radial carcass reinforcement 50, anchored in two beads 70 and turned up, in each bead, around a bead wire 60. Each bead 70 comprises a bead layer 71 intended to come into contact with a rim flange. The carcass reinforcement 50 is generally formed of a single carcass layer, consisting of metal cords coated in an elastomeric coating compound. Positioned radially on the outside of the carcass reinforcement 50 is a crown reinforcement (not referenced), itself radially on the inside of a tread 30. The tread 30 comprises, at each axial end, an axial end portion or tread wing 31, axially on the outside of a central tread portion 32. Each tread axial end portion 31 is connected to a bead 70 via a sidewall 20.

(4) Each sidewall 20 includes a laminate comprising two sidewall layers that are at least partly axially superposed and having a total thickness E. The axially outer first sidewall layer 21 has a thickness E.sub.1 and the axially inner second sidewall layer 22 has a thickness E.sub.2.

(5) The thicknesses E.sub.1 and E.sub.2 respectively of the first and second sidewall layers 21 and 22, constituting the sidewall 20, are measured along the direction normal to the carcass reinforcement 50, defined by the axis 80, in the middle of the height of the sidewall. The sidewall height of a tire for a construction plant vehicle is standardized and defined, for example, in the ETRTO (European Tires and Rim Organisation) manual. The measurement points correspond to the positions determined by the intersections of the axis 80 with the faces of said sidewall layers.

(6) The thickness E.sub.1 of the first sidewall layer 21 is at most less than 0.9 times the total thickness E of the laminate, and the thickness E.sub.2 of the second sidewall layer 22 is at least equal to the minimum value between 3 mm and 0.1 times the total thickness E of the laminate.

(7) The radially outer top end 221 of the axially inner second sidewall layer 22 is advantageously in contact with the tread wing over a length L.sub.h at least equal to 10 mm. In the same way, its radially inner bottom end 222 is also advantageously in contact with the elastomeric coating compound of the carcass layer 50 over a length L.sub.b at least equal to 10 mm.

(8) Similarly, the radially outer top end of the axially outer first sidewall layer 21 is in contact with the axially inner second sidewall layer 22. Its radially inner bottom end is in contact with the bead layer 71. Here too, the contact lengths are at least equal to 10 mm.

(9) The radially outer top end of the tread wing 31 is in contact with the central tread portion 32 over its entire thickness. Its radially inner bottom end is in contact with the axially inner second sidewall layer 22 over a length at least equal to 10 mm.

(10) The objective is to ensure a permanent contact between the electrically-conductive elastomeric compounds, in twos, in order to guarantee the continuity of the pathway for discharging the electrostatic charges, taking into account the manufacturing tolerances.

(11) FIG. 2 represents a tread that is symmetrical relative to the equatorial plane comprising two axial end portions or tread wings that are actually separated by a central portion. The inner end of the tread wing, in the axial direction, is located at a given distance L.sub.1 relative to the equatorial plane. The other outer end of the tread wing, still in the axial direction, is positioned at a distance of L.sub.2 of the same equatorial plane. The reference 100 from FIG. 2 represents the exterior side of the vehicle when the tire is mounted on this vehicle and the reference 110 represents the interior side of the vehicle.

(12) FIGS. 3 and 4 represent a tread that is not symmetrical relative to the equatorial plane. In FIG. 3, the tread wing is positioned only on the vehicle exterior side (reference 100), and in FIG. 4, it is positioned only on the vehicle interior side (reference 110).

(13) The invention has more particularly been studied on a tire for a Dumper type vehicle, of dimensions 59/80 R63, comprising a sidewall having two sidewall layers, and a tread comprising two tread wings that are axially separated by a central portion.

(14) The results calculated on the tire produced according to embodiments of the invention are compared to those obtained for a reference tire of the same dimensions, comprising a sidewall having a single sidewall layer, and a tread made of a single portion. The elastomeric compounds associated with the sidewall and with the tread of the reference tire are of standard composition for a person skilled in the art.

(15) The inventors have established the connection between the chemical composition of the elastomeric compounds and the physical parameters such as the electrical resistivity, the thermal conductivity, and the viscoelastic loss. By way of example, represented on the graph from the appended FIG. 5, for the two elastomeric compounds of the sidewall, are the curves of thermal conductivities as a function of the amount of reinforcing fillers in phr. These curves show that the elastomeric compound of the axially outer first sidewall layer filled with silica is optimized for the hysteresis, but with a thermal conductivity that is relatively lower than the elastomeric compound of the axially inner second sidewall layer filled with carbon black, for which the electrical conductivity property is favoured.

(16) According to the curve from FIG. 5, for a given content of filler, for example carbon black, it is possible to predict the value of the thermal conductivity of the elastomeric compound. The thermal conductivities are measured at an ambient temperature of from 23° C. to 25° C. The dependency of the thermal conductivity relative to the temperature is not taken into account here.

(17) The inventors determined the composition of the elastomeric compounds, constituting the sidewall layers, by finding a compromise between the following physical parameters: the dynamic viscoelastic loss or the viscous shear modulus which are directly connected to the viscoelastic heat sources; the thermal conductivity which controls the thermal conduction of the heat in the compounds; the electrical conductivity which must be at a level sufficient for discharging electrostatic charges.

(18) In the example studied, the compositions of the elastomeric compounds, resulting from this compromise, are summarized in Table 1 below:

(19) TABLE-US-00001 TABLE 1 Elastomeric Elastomeric compound M.sub.1 compound M.sub.2 Elastomeric of the axially of the axially Elastomeric compound M.sub.4 outer first inner second compound M.sub.3 of the central Composition sidewall layer sidewall layer of the tread wing tread portion Elastomer NR 50 50 100  100 *   (Natural Rubber) Elastomer BR 50 50 NC NC (Butadiene Rubber) Carbon black N330 NC 55 NC NC Carbon black N234 3 NC 35  35 *   Silica (2) 29 NC 10 10   Plasticizer (3) 10 18 NC NC Wax 1 1 NC NC Antioxidant 3 3 3 3   ZnO 2.5 2.5 2.7 2.7 Stearic acid 1 1 2.5 2.5 Sulfur 1 0.9 1.25  1.25 Accelerator 0.8 0.6 1.4 1.4 * elastomeric compound M4 obtained via a liquid route (2) “Zeosil 1165MP” silica sold by Rhodia (3) “Vivatec 500” TDAE oil from Klaus Dahleke

(20) Table 2 brings together the physical parameters of the elastomeric compounds, measured on test specimens and resulting from choices of chemical composition:

(21) TABLE-US-00002 TABLE 2 Elastomeric Elastomeric compound M.sub.1 compound M.sub.2 Elastomeric of the axially of the axially Elastomeric compound M.sub.4 outer first inner second compound M.sub.3 of the central Composition sidewall layer sidewall layer of the tread wing tread portion Thermal 0.208 0.265 0.240 0.240 conductivity at 25° C. (W/m.K) Electrical 11.6 4.4 5.7 10.4 resistivity in Log (Ω.cm) Viscous shear 0.125 0.300 NC NC modulus G″max at 60° C. and 10 Hz (in MPa) Elastic shear NC NC 1.33 1.16 modulus G*max (50%, 100° C. and 10 Hz) Dynamic loss NC NC 0.10 0.06 tgδ.sub.max (50%, 100° C. and 10 Hz)

(22) In a construction plant tire, the amount of elastomeric compound of the tread represents around 35% to 40% of the total mass of elastomeric compounds of the tire. The tread is thus one of the main sources of hysteresis, and it therefore contributes greatly to the increase in temperature of the tire. Consequently, the elastomeric compound M.sub.4 of the central tread portion is designed to have a low hysteresis with a dynamic viscoelastic loss of the order of 0.06, measured at a temperature of 100° C., and at a frequency of 10 Hz.

(23) In one preferred embodiment of the invention, the elastomeric compound M.sub.4 of the central tread portion has a composition which comprises at least one diene elastomer and a reinforcing filler consisting of carbon black and silica, so that the carbon black has a content at most equal to 40 phr and a BET surface area at most equal to 115 m.sup.2/g and the silica has a content at most equal to 20 phr. The elastomer and carbon black mixture is obtained beforehand preferentially via a liquid route. In this embodiment, the central tread portion is electrically insulating. The discharging of the electrostatic charges is then carried out along the conduction pathway defined by the invention which passes through the tread wings in contact with the ground and which are always electrically conductive.

(24) For the elastomeric compound M.sub.3 of the running tread wings, the overall filler content being 45 phr, with 35 phr of carbon black and 10 phr of silica, guarantees an electrical resistivity of less than or equal to 10.sup.6′Ω.Math.cm, and a suitable thermal conductivity. In the example dealt with here, the thermal conductivity of the tread wing is equal to 0.240 W/m.Math.K. The same elastomeric compound M3 is used for the two tread wings positioned at the two ends of the tread, but the invention still remains valid if different materials are used. The required condition is to have at least, at one of the two axial ends of the tread, an elastomeric compound with an electrical resistivity of less than or equal to 10.sup.6′Ω.Math.cm.

(25) In a tire for a construction plant vehicle, the mass of the elastomeric compounds of the sidewalls is of the order of 15% of the total mass of compounds of the tire. The option selected by the inventors is to have a laminate of two sidewall layers to ensure both a low hysteresis and an electrical conductivity of less than or equal to 10.sup.6′Ω.Math.m. Combined with the thickest and axially outer first sidewall layer is an elastomeric compound of low hysteresis with a viscous shear modulus of 0.125 MPa. An electrically-conductive elastomeric compound, with an electrical resistivity of the order of 10.sup.44′Ω.Math.cm, corresponds to the axially inner second sidewall layer.

(26) The results on tires were obtained by finite element calculations is in order to determine the viscoelastic heat sources, the temperature and the electrical resistivity.

(27) Finite element calculations were carried out on the tires of the invention and reference tires respectively. The results of calculations, for the reference tire, comprising a single sidewall layer (compound M.sub.2), and a tread (compound M.sub.3) made of a single portion, are represented below in Table 3:

(28) TABLE-US-00003 TABLE 3 Results Single sidewall layer Tread made of one portion Electrical 4.4 5.7 resistivity Log (Ω.cm) Viscoelastic 4520 5100 sources (W) Maximum 99.8 90 temperature° C.

(29) The reference tire is electrically conductive with an average operating temperature of the order of 90.4° C.

(30) For the tire of the invention, the results of the finite element calculations are summarized in Table 4:

(31) TABLE-US-00004 TABLE 4 Axially outer Axially inner Central first sidewall second tread Results layer sidewall layer Tread wing portion Electrical 11.6 4.4 5.7 10.4 resistivity Log (Ω.cm) Viscoelastic 2080 580 696 3628 sources (W) Maximum 91.8 93.5 65.5 86.6 temperature° C.

(32) The finite element calculations confirm the electrically insulating nature of the axially outer first sidewall layer and of the central tread portion. The tread wing in contact with the ground and the axially inner second sidewall layer are, on the other hand, electrically conductive. The evaluation of the electric potential confirms the conduction pathway with levels of electrical resistivity ranging from 10.sup.4′Ω.Math.cm to 10.sup.6′Ω.Math.m for the elastomeric compounds constituting the pathway for discharging the electrostatic charges.

(33) For the tire of the invention, relative to the reference tire, the viscoelastic loss sources were halved in the sidewall of the tire, and in the tread the reduction is also significant.

(34) As a consequence of the drop in the viscoelastic loss sources, the calculation of the temperature field of the tire of the invention gives an average level of 92° C., which corresponds to a difference of 8% relative to the reference tire. This difference is sufficient for a significant improvement in the endurance of the tire of the invention by prolonging its service life by around 30%.

(35) The invention has been presented for a tire for a construction plant vehicle, but it can in fact be extrapolated to other types of tire.

(36) The scope of protection of the invention is not limited to the examples given hereinabove. The invention is embodied in each novel characteristic and each combination of characteristics, which includes every combination of any features which are stated in the claims, even if this feature or combination of features is not explicitly stated in the examples.