FIELD OF THE INVENTION
The present invention concerns an extruded profiled element consisting of a cross-linkable rubber composition, a process for obtaining the said profiled element, a tire tread based on the said extruded and cross-linked profiled element, and a tire comprising the said tread.
BACKGROUND OF THE INVENTION
The invention is of use in particular to ensure the proper operation of electronic equipment fitted in a vehicle provided with such tires, particularly a vehicle of the passenger car type. Thus, the invention is useful, for example, in ensuring the reception quality of radio waves by a radio provided within such a vehicle, and/or the reliability of an electronic device provided inside a vehicle or a tire fitted on the vehicle.
As is known, a vehicle's tires become charged and are discharged by virtue of a triboelectric effect during rolling, and in certain meteorological conditions the corresponding charge and discharge sometimes interfere via electromagnetic perturbations with electronic equipment fitted in the vehicle, for example with an on-board radio, and more particularly when the said radio is used in the amplitude modulation mode.
Notably, when moving from a first rolling stretch to a second rolling stretch after it which has physical characteristics different from those of the first stretch, for example different electric conductivity and/or structure and/or relief, there is a sudden discharge from the tread of each tire, of the charge accumulated along the said first stretch.
Such successive rolling stretches can for example consist respectively of an electrically insulating material, such as asphalt, and an electrically conducting material such as that used for the metallic junctures of a bridge, for manhole covers, or for train rails.
It is also known that when changing from rolling on a given first stretch to a given second stretch, the sudden discharges and the electromagnetic perturbations resulting from them are the more marked, in particular the more electrically insulating is the material constituting the tread.
Now, many tires in current use are characterized by a high content of reinforcing filler which is not electrically conductive, such as silica, the advantageous effect of this being to reduce hysteresis losses during rolling and consequently to decrease the rolling resistance of the tires, so that the fuel consumption of the corresponding vehicles also reduced.
A disadvantage of such tires lies in the relatively high resistivity of the tread material, whose effect is sometimes to generate the said electromagnetic perturbations in certain meteorological conditions.
The document of Japanese patent JP-A-10 237223 discloses a tire tread composition with low electrical resistivity, which contains essentially a diene elastomer, silica, and an alkali or alkali-earth metal salt such as lithium perchlorate, this salt being solvated by a diester-based compound such as a diester of adipic acid.
The document of U.S. Pat. No. 6,075,092 discloses a rubber composition with improved hysteresis and service properties, as well as anti-static properties. This composition comprises silica and a polyether consisting of a terpolymer of ethylene oxide, propylene oxide and an unsaturated epoxide such as epichlorohydrin.
The document of European patent EP-A-925 903 discloses an automobile tire designed in particular to enable the electrostatic charge on the vehicle body to be discharged to the ground over which the vehicle is rolling.
For that purpose, the tread of the said tire has all the way round its circumference and emerging on its radially outer surface a radial strip with high electrical conductivity based on a rubber composition reinforced with carbon black, the remainder of the tread being based on another rubber composition whose electrical conductivity is low.
This tread is obtained in the unvulcanized condition by extruding the two rubber compositions in parallel in two separate extruders, and bringing the two compositions that emerge from the said extruders into contact at the downstream end of an extrusion head common to the two extruders, to obtain at the outlet of the said head an extrudate containing the said radial strip within the tread.
Note that the tread obtained only enables the electrostatic charge on the vehicle body to be discharged to the ground, but does not reduce radio interference, for example when rolling over a metal plate.
The document of International patent WO-A-00/27655 in the name of the applicant discloses a tire which minimizes the power of the electrostatic discharges from the tread when moving from a first rolling stretch to a second one with different physical characteristics.
The tire described in that document has a tread comprising on its circumference at least one electrically conducting layer which essentially connects the lateral faces of the said tread to one another, the said layer having higher conductivity than that of the remainder of the tread.
A purpose of the present invention is to propose an extruded profiled element based on a cross-linkable rubber composition, the said profiled element being intended to constitute a tire tread in the cross-linked condition and being delimited in width by two lateral faces which, between them, connect radially inner and outer faces of the said tread, with conducting means provided in the said profiled element to connect the said inner face electrically to the said outer face between the said lateral faces and along the length of the said profiled element, the remainder of the profiled element being based on an electrically insulating material, which again enables the power of the electrostatic discharges from the said tread when rolling on the said successive stretches, and hence the aforesaid electrostatic perturbations, to be minimized.
To that end, an extruded profiled element according to the invention is such that the said conducting means, viewed in a cross-section of the said profiled element, have a layered structure comprising electrically conducting layers which are essentially concentric and are curved in the direction of at least one of the said inner and outer faces, with at least one of the said layers emerging at the surface of the said outer face.
When used in a set of tires fitted to a vehicle with an on-board radio receiver, this layered tread structure results in particular in a significant reduction of the radio interferences that can be perceived in the amplitude modulation mode when rolling on electrically conductive road elements in certain meteorological conditions, and this even when the said tires are substantially worn.
According to one example embodiment of the invention, the said electrically conducting layers, viewed in a cross-section of the said profiled element, describe a plurality of turns essentially around the longitudinal axis of symmetry of the said profiled element.
In an embodiment of the invention, the said electrically conducting layers are globally wound in a spiral around the said longitudinal axis.
According to another characteristic of this embodiment, along the length of the said profiled element the electrically conducting layers have a filament shape comprising a plurality of helicoidal filaments centered on the said axis of symmetry.
More precisely, along the said profiled element this filamentation comprises a succession of identical sections Ti (i=1 to n) each consisting of a plurality of conical filaments Fj (j=1 to m) inscribed on cone sections essentially parallel to one another and centered on the said axis of symmetry.
In a known way, conical filament means a helicoidal filament inscribed on a cone section, i.e. rolled in a conical spiral on the outer surface of a truncated cone.
Moreover, each conical filament Fj of each of the said sections Ti is inscribed on a cone section that converges towards the inside of the cone section on which the same conical filament Fj of an immediately consecutive section Ti+1 is inscribed.
Advantageously, in a cross-section of the said profiled element the said layers can have essentially the shape of an arc of a flattened ellipse whose major axis corresponds to the transverse direction of the said profiled element.
Preferably, the said electrically conducting layers, viewed in cross-section in the said profiled element, describe a number of turns between 30 and 70 and, more preferably still, between 40 and 60. Each of the conducting layers then has a thickness essentially between 0.05 and 0.15 mm.
According to an example embodiment of the invention, at least one of the said electrically conducting layers emerges at the surface of one or of each of the lateral faces of the said profiled element.
Note that these on the surface of the lateral faces, as also the aforesaid points of emergence on the surface of the outer face of the profiled element, can advantageously allow colour contrasts or shades to be formed on the tire tread consisting of the profiled element, for example when a colored rubber composition is used for the said insulating material.
According to a preferred embodiment of the invention, the said conducting means also comprise a conducting film at the position of one or of each lateral face of the said profiled element.
This or these conducting film(s) further minimize(s) the power of the aforesaid electrostatic discharges and so too, therefore, the resultant radio interference.
According to a first example embodiment of the invention, the said electrically conducting layers consist of a rubber composition based on at least one diene elastomer, containing carbon black as the reinforcing filler and having an electrical resistivity lower than 108-Ω.cm, for example between 104 and 105 Ω.cm, the resistivity of the said insulating material necessarily being above 108 or 109 Ω.cm. The proportion of carbon black in this composition is determined as a function of the resistivity desired.
Suitable carbon blacks are any of those conventionally used in tires and especially in tire treads, in particular carbon blacks of the HAF, ISAF or SAF type. As non-limiting examples of such carbon blacks, N115, N134, N234, N339, N347 and N375 can be mentioned.
According to a second example embodiment of the invention, the said electrically conducting layers consist of a rubber composition based on at least one diene elastomer, comprising an inorganic reinforcing filler and a conducting ionic solution comprising:
a polyether which is a copolymer of oxyethylene and oxypropylene with a majority of oxyethylene units,
an ionic salt of a monovalent or divalent metal, such as lithium perchlorate or zinc dichloride, and
a polar solvent such as polypropylene glycol carbonate.
Note that in this case the said conducting layers comprise an electrolyte solution that imparts to them ionic conduction properties (by virtue of the migration of ionic charges), in contrast to rubber compositions with a carbon black filler, which are characterized by electronic conduction (by electron drift). The electrical resistivity of such conducting layers can range from 106 to 109 Ω.cm.
The diene elastomer used can be any homopolymer or copolymer obtained by polymerisation of a conjugated diene monomer having 4 to 12 carbon atoms, or several such diene monomers conjugated together, or else one or more such diene monomers conjugated with one or more vinylaromatic compounds each having 8 to 20 carbon atoms.
Suitable conjugated diene monomers are in particular butadiene-1,3, 2-methyl-1,3-butadiene, the 2,3-di(alkyl with C1 to C5)-1,3-butadienes such as 2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene, 2-methyl-3-isopropyl-1,3-butadiene, an aryl-1,3-butadiene, 1,3-pentadiene, or 2,4-hexadiene.
Suitable vinylaromatic compounds are for example styrene, ortho-, meta- and para-methylstyrene, the commercial mixture “vinyltoluene”, para-tertiobutylstyrene, or divinylbenzene. Note that styrene is used for preference.
Preferably, butadiene-1,3 and/or isoprene is/are used as the conjugated diene(s) and styrene as the vinylaromatic monomer, to obtain butadiene/isoprene, styrene/butadiene, styrene/isoprene, or butadiene/styrene/isoprene copolymers.
The polyether that can be used according to the invention has a high inherent viscosity which, measured at 25° C. in toluene, is greater than 4 dl/g and preferably between 4 and 8_dl/g.
More preferably still, this polyether contains oxyethylene units in a mole fraction between 85 and 95%, and oxypropylene units in a mole fraction between 5 and 15%.
The polyether can be used in the said rubber composition for the conducting layers, in an amount ranging between 20 and 50 phr (phr: parts by weight per 100 parts of the elastomer(s)).
The ionic salt of a monovalent metal used can for example be a salt of lithium, sodium, potassium, caesium or silver.
The ionic salt of a divalent metal used can for example be a salt of magnesium, calcium, copper or zinc.
For example, the following anions can be used with the cations of the aforesaid metals:
Cl−, Br−, I−, NO3 −, SCN−, CO4 −, CF3SO3 −.
The ionic salt can be used in the said rubber composition in an amount between 5 and 30 phr depending on the salt used, and preferably between 10 and 20 phr.
As is known, “inorganic filler” means an inorganic or mineral filler, whatever its colour and origin (natural or synthesised), also known as a white filler or sometimes a light filler as opposed to carbon black.
Again in a known way, “inorganic reinforcing filler” means an inorganic filler capable, on its own and without any other means than an intermediate coupling agent, of reinforcing a rubber composition intended for the production of tyres, in other words capable of replacing in its reinforcing function a conventional filler of tyre-grade carbon black.
For the inorganic reinforcing filler, for example any reinforcing silica among the ones familiar to those with knowledge of the field can be used, in particular any precipitated silica with a BET area and a CTAB specific surface area both below 450 m2/g, although the highly dispersible precipitated silicas are preferred (the BET specific surface area being determined in a known way in accordance with the Brunauer-Emmett-Teller method described in “Journal of the American Chemical Society”, Vol. 60, page 309, February 1938 and corresponding to the standard AFNOR—NFT-45007 (November 1987), and the CTAB specific surface area being the external surface area determined in accordance with the same standard AFNOR—NFT-45007 of November 1981).
More preferably still, the silica used has BET or CTAB specific surface areas both ranging between 80 m2/g and 260 m2/g.
“Highly dispersible silica” means a silica with a very pronounced ability to disagglomerate and become dispersed in an elastomer matrix, as observed in a known way by electron or optical microscopy of thin sections.
As non-limiting examples of such highly dispersible silicas that are preferred, the following can be mentioned: Perkasil KS 430 silica available from AKZO, BV 3380 silica available from Degussa, Zeosil 1165 MP and 1115 MP silicas from Rhodia, Hi-Sil 2000 from PPG, Zeopol 8741 or 8745 silicas from Huber, or treated precipitated silicas such as the silicas “doped” with aluminium described in patent application EP-A-0 735 088.
Of course, inorganic reinforcing filler can also mean mixtures of inorganic reinforcing fillers, in particular of highly dispersible silicas such as those described above.
In a non-limiting way, it is also possible to use aluminas (of formula Al2O3) such as the highly dispersible aluminas described in European patent document EP-A-810 258, or aluminium hydroxydes such as those described in International patent document WO-A-99/28376.
The process according to the invention for obtaining the aforesaid extruded profiled element consists in the following:
on the one hand, the said electrically insulating tread material is introduced into an inlet of a main extruder which opens coaxially into an extrusion head, and on the other hand, the said electrically conducting material intended to constitute the said conducting means with a layered structure is introduced into an inlet of at least one satellite extruder which opens radially upstream from the said extrusion head inside the said main extruder,
the insulating material and the conducting material are propelled through the extruders,
inside the said main extruder and upstream from the said extrusion head, a uniformly distributed mixture of the said insulating material and the said conducting material is produced, the mass fraction of the said insulating material being equal to or greater than 80% and that of the said conducting material being less than or equal to 20% in the said mixture, and
the said mixture is passed into a channel of the said extrusion head to obtain, at the outlet of an extrusion orifice of the said extrusion head, the extruded and cross-linkable profiled element for the tread.
Note that the diameter of the orifice via which the satellite extruder communicates with the main extruder, and the respective feed flow rates of insulating material and conducting material into these extruders, determine the degree of homogenisation of the mixture obtained and the geometry of the layered structure formed in the extruded profiled element.
According to another characteristic of the invention, the temperature inside each extruder is between 70 and 90° C.
According to a further characteristic of the invention, the absolute pressure inside the said extrusion head is between 20 and 30 bars.
Note that this process according to the invention can be implemented by positioning the satellite extruder at a variable distance upstream from the extrusion head, as a function of the dimensional characteristics desired for the layered structure of conducting means to be formed in the extruded profiled element (for example, the number of turns of the spiral obtained and/or their thickness).
To do this, a plurality of orifices can be provided along the main extruder for its communication with the satellite extruder, the orifices not used for this being of course blocked.
It should also be noted that this process according to the invention is not limited to the production, inside the said main extruder and upstream from the said extrusion head, of a uniformly distributed mixture consisting of the insulating material and just one conducting material. For example, it would be possible to mix the insulating material with several conducting materials by means of a plurality of satellite extruders respectively intended to extrude these conducting materials and each opening radially within the main extruder, in all cases upstream from the extrusion head.
Note also that the extruded profiled element obtained according to the invention can be split (i.e. the profiled element can be divided in the direction of its length), for example if it were desired to obtain particular coloured designs at the positions of the lateral faces of the tread.
A cross-linkable or cross-linked tyre tread according to the invention is one that consists of the extruded profiled element defined above, and a tyre according to an embodiment of the invention is one that comprises the said tread in the cross-linked condition.
A second aspect of the invention is to propose a cross-linkable or cross-linked tyre tread delimited at the sides by two lateral faces that connect together the radially inside and outside faces, the said tread being based on an electrically insulating material and comprising at its circumference at least one axial conducting layer which essentially connects the said lateral faces together and which has a resistivity lower than that of the said insulating material, this being provided radially on both sides of the said layer in the said tread, which also allows the power of the electrostatic discharges from the tread when rolling on the aforesaid successive stretches, and hence the resultant electrostatic disturbances, to be minimized.
The tread according to this second aspect of the invention is such that the said axial conducting layer consists of a rubber composition such as that described above with reference to the first example embodiment of electrically conducting layers that form the layered structure according to the invention. This composition is thus based on a diene elastomer and comprises an inorganic reinforcing filler and an ionic solution comprising a polyether (copolymer of oxyethylene and oxypropylene with a majority of oxyethylene units), an ionic salt of a monovalent or divalent metal, such as lithium perchlorate or zinc dichloride, and a polar solvent, such as polypropylene glycol, in the quantities indicated previously.
Reference should be made to what was said earlier (including the quantities in phr) for a detailed description of the diene elastomer(s), the inorganic reinforcing filler and the conducting ionic solution that can be used to obtain the said axial conducting layer.
In particular, when used for a set of tyres fitted to a vehicle with an on-board radio receiver, this tread significantly reduces the radio interference that can be perceived in amplitude modulation mode in certain meteorological conditions when rolling over electrically conducting road elements.
Advantageously, the tread according to this second aspect of the invention also comprises a conducting film at the position of one or both lateral faces, which connects the said inner and outer faces electrically to one another.
The conducting film(s) enable(s) the power of the aforesaid electrostatic discharges and consequently the resulting radio interferences to be minimised still further.