Abstract:
A tire having a tread composed of two non-conductive layers, each layer containing a circumferential insert of conductive mix, the inserts having a wider interface at the interface of the two layers. An apparatus for extruding a layer, provided with a conductive insert, including a main extruder for extruding a non-vulcanized, non-conductive layer, and a micro-extruder having an extrusion head, provided at its end with a nozzle, for extruding a conductive insert in the layer.

Description:
This is a continuation of PCT/EP99/01044, Filed Feb. 17, 1999. 

   BACKGROUND OF THE INVENTION 
   The subject of the present invention is a tire composed of a plurality of mixes comprising as majority filler a non-conductive filler such as silica, or mixes having a low content of black filler, at least one of these mixes being the tread. It relates more particularly to a tire which may exhibit not insignificant internal temperature increases during travel, such as a tire intended, for example, to bear heavy loads. It also relates to a process for making such a tire, and an apparatus for carrying out such a process. 
   Since environmental problems are becoming ever more crucial, and savings in terms of fuel and the fight against the pollution produced by motor vehicles have become a priority, one of the aims of tire manufacturers is to produce a tire having all of the following: very low rolling resistance, excellent adhesion both on dry ground and on wet or snow-covered or icy ground, very good wear resistance, and finally reduced running noise. 
   To achieve this aim, a tire having a tread comprising silica as main reinforcing filler was proposed in European Patent Application No. EP A 501 227. Although this solution makes it possible to obtain the best compromise between all the very contradictory properties mentioned above, it has however proved that, depending on the vehicles, tires using a tread comprising silica as main reinforcing filler have the disadvantage of accumulating static electricity to a greater or lesser extent, this being formed by friction of the tire on the road during travel of the vehicle due to the lack of electrical conductivity of the silica. 
   The static electricity thus built up in a tire, when certain specific conditions are combined, is capable of giving the occupant of a vehicle an unpleasant electric shock when he has cause to touch the body of the vehicle. It is furthermore capable of accelerating ageing of the tire due to the ozone generated by the electrical discharge. It may also be the cause, depending on the type of ground and the vehicle, of poor operation of the onboard radio owing to the interference which it produces. 
   This problem of buildup of static electricity in a tire and of the majority of the disadvantages linked thereto is a very old one, and occurred even in the days when the reinforcing filler used was carbon black. 
   Application No. EP 0 658 452 A1 describes the adaptation of principles which have been known for a long time to a so-called “modern” tire, which adaptation makes it possible to solve the main problems pertaining to the solutions proposed in various older documents and, in particular, the harmful heterogeneity introduced into tire structures. The solution proposed consists in inserting a strip of conductive rubber mix or insert, extending preferably over the entire circumference of the tire and connecting the surface of the tread either to one of the crown plies, or to the carcass reinforcement, or to any other part of the tire which is sufficiently electrically conductive, the necessary electrical conductivity being imparted by the presence of a suitable carbon black. 
   U.S. Pat. No. 5,017,118 describes a co-extrusion apparatus comprising a single extruder which has a plurality of flow channels which open respectively on to an orifice formed by extrusion blades. Thus, as shown in  FIGS. 7 and 8  of the document, the insert  120  parallel to the mixes  114 ,  118  and  122  arrives via a flow channel in the orifice  94  formed by the blades  106  and  98  on the surface  116  of the roller. 
   Although such a solution is perfectly viable for a tire comprising a tread formed of a single non-conductive mix, for example, a tread for a passenger vehicle, the same does not apply in the case of a tire comprising a plurality of layers of rubber mixes above the crown reinforcement and layers of rubber between the crown reinforcement and the carcass reinforcement, as is the case for any tire liable to travel at a high stabilized operating temperature, such as heavy vehicles or high-speed vehicles. 
   In fact, if for any reason one has cause to provide such a tire with a non-conductive layer, or inner part of the tread (part not in contact with the ground), between the crown reinforcement and the outer part (part in contact with the ground) of the tread which has been rendered conductive by the presence of a circumferential insert or striping, said inner part will have to be rendered conductive. Likewise, a layer between the carcass reinforcement and the crown reinforcement, having manifest oversize in the regions of the edges of crown plies, will also have to be rendered conductive if it is not initially so. 
   A first solution for obtaining a conductive tread consists in coextruding the inner and outer parts of the tread, and providing the assembly thus formed with a conductive circumferential insert. Said solution is not satisfactory for several reasons; two of these may be mentioned: in the type of tire in question, the total thickness of the tread is too great; furthermore, it may be advantageous for the conductive inserts, of the inner and outer layers of the tread respectively, not to be made with the same quality of rubber mix. 
   Another solution, as described in French Application No. FR 97/02276 by the Applicant, consists in providing the electrical connection between two layers which are or have been rendered conductive, separated by a non-conductive layer, by at least one strip of rubber mix of low thickness, width and length, laid between the two faces of the weld of the non-conductive layer and in contact with the means which render conductive the two layers connected by the connection. Although industrially satisfactory, said method requires additional laying of product and involves additional production costs. 
   A third solution consists in providing each non-conductive part with a circumferential insert having a circular rectilinear trace or circular striping after extrusion of said part by conventional extrusion means, and then joining the two products together before laying them on the crown reinforcement. To the extent that the striping is generally very fine in order not to upset the physical properties of the compositions constituting the two parts of the tread, the thickness of these inserts, on the tire viewed in cross-section, being of the order of 0.01 to 2.0 mm, such a solution requires the trace of the striping of the outer part of the tread on the contact wall between the two parts to be perfectly aligned with or centered on the trace on said wall of the striping of the inner part of the same tread. 
   Furthermore, the mechanical properties of the rubber compositions before vulcanizations are very poor, and the uncured rubber mix may be in the form of an extremely soft paste, or conversely an extremely hard agglomerate. Whatever the manner of working such mixes, it is difficult perfectly to control the geometry of the semi-finished product, the assembly of the two non-vulcanized parts of the tread: the concordance and alignment between the respective traces of the two sets of striping on the contact wall between the two parts cause great development difficulties, the solution of two sets of circular striping being unable to be considered as structurally optimized and industrially viable, both from the point of view of cost and of the performance obtained. 
   Thus although one of the aims of the invention is, in a tire having a plurality of non-electrically conductive mixes, to dissipate the electrostatic charges induced by the travel of the tire, without significantly affecting the level of the properties of the tire, the other aim is to be able to obtain a tire as simply as possible and at lower cost, be these material costs and/or manufacturing costs. 
   SUMMARY OF THE INVENTION 
   In a first aspect, the invention proposes a tire comprising at least two layers of non-electrically conductive rubber mixes, said two layers having a common contact wall, characterized in that each layer contains a circumferential insert of conductive mix, having a circular rectilinear trace, the insert of at least one first layer having, when viewed in cross-section, a surface S having two bases, a first base, on the contact wall between the two layers, having a width equal to at least 10 times the width of the first base on said contact surface of the surface S′ of the insert of the second layer, said first bases being on the common contact wall facing each other, and the central axes of the two bases which face each other being parallel to and distant from the equatorial plane of the tire by amounts, the difference between which is at most equal to half the width of the first base of the first layer, such that there is circumferentially between the two traces on the common contact wall a plurality of contact points providing the electrical connection between two conductive elements. 
   In the very great majority of cases, the two non-conductive layers will be the inner and outer parts of the tread, and the electrical connection must be ensured between the ground and the crown reinforcement, which is composed of calendered metal cables in a rubber mix which has been rendered conductive by the presence of carbon black in its composition. The non-conductive layers may also be, firstly, the axially outer part of the profiled element of triangular transverse shape separating the carcass reinforcement and an edge of the crown reinforcement, and secondly the inner and outer parts of the tread, the connection needing to be ensured between the three layers. 
   Preferably, and in the case of a tread, one of the parts of said tread, and more particularly the outer part of the tread, will be provided with an insert having a transverse surface having two bases of different widths, the first base having a width very much greater than the width of the second base which is located on the surface of the tread, whereas the other part, preferably the inner part of the tread, will be provided with striping, the transverse surface of which is rectangular, the two bases of said surface being equal. 
   The transverse surface of the insert comprising two bases of different widths may be of several forms. Advantageously, and so as to have the least possible amount of conductive material, in particular in the tread layers of the tire, the transverse surface of the insert of the outer tread layer will be in the shape of an upturned T, with a rectangular part of low width, and a triangular or rectangular part with a base of great width, based on the common wall of the two layers. 
   A layer, be it part of the tread or an intermediate profile between the carcass reinforcement and the crown reinforcement, may be obtained, for example, in the non-vulcanized state by means of extrusion in an extruder which is referred to, in known manner, as a roller die extruder, the profile or section of the extruded product being defined firstly by the surface of the roller on which the rubber mix is extruded and secondly by the fixed wall of an extrusion blade which cooperates with said surface to define an extrusion orifice. The process according to the invention, intended for obtaining an assembly of two conductive layers, for example the inner and outer parts of a tire tread, consists in:
         extruding each layer separately on an extruder comprising an extrusion head provided with a flow channel which opens on to an extrusion orifice comprising a fixed wall which cooperates with a rotary roller,   inserting by coextrusion into each layer, for example, each part of the tread, striping of conductive mix in the non-vulcanized state which is circumferential and has a transverse surface having two bases, by means of the nozzle of a micro-extruder, said nozzle being fixed and applied against a wall of said extrusion apparatus, for example the wall of the roller of the main extrusion apparatus, and   joining the two non-vulcanized layers by the conventional means.       

   The coextrusion can be advantageously effected at three different points: either in the flow channel between the upper arch and the lower arch defining said channel, or between the upper arch and the wall of the rotary roller, or between the upper wall of the extrusion orifice and the wall of the roller. 
   The apparatus according to the invention for implementing the process for obtaining a rubber profiled element with insert or striping of cross-section having two bases, are of two types. 
   The apparatus of the first type comprises a main roller die extruder for extruding the rubber layer or profiled element, for example, the upper part of a non-vulcanized tread, and a micro-extruder, mounted fixed on a support and comprising mainly an extrusion head, provided at its end with a nozzle for extruding, according to the desired profile and trace, the circumferential insert in the profiled element of non-vulcanized mix emerging from the main extruder, the extrusion head and the nozzle being placed in the flow channel or accumulation chamber for the rubber to be extruded either between the upper arch and the lower arch of the main extruder, or between the upper arch and the rotary roller, the nozzle having a head, the end of which is applied against the lower arch or against the roller of the main extruder. The invention also relates to the extrusion nozzle or orifice of the micro-extruder used. Said nozzle is characterized in that it comprises an annular part for bearing on and attachment to the die of the micro-extruder with the attachment elements necessary for fixing, a hollow cylindrical feed chamber, either straight or bent, having on its cylindrical surface an extrusion orifice, through which is extruded the conductive rubber material, and which occupies substantially all the height of said nozzle, and the cross-section of which has a shape adapted to the desired section of the conductive insert: a section having two bases of different widths if an insert having two different bases is being extruded, or a section having two bases of equal widths if an insert of rectangular section is being extruded. 
   The apparatus of the second type likewise comprises a main extruder, having a roller die or flat die, for extruding the rubber layer or profiled element, for example, the inner part of a non-vulcanized tread, and a micro-extruder comprising mainly an extrusion head, provided at its end with a nozzle intended to extrude, according to the desired profile and trace, the circumferential insert in the profiled element of non-vulcanized rubber mix emerging from the main extruder. The invention relates more particularly to the fastening of the micro-extruder and the nozzle or extrusion orifice of the micro-extruder used. The micro-extruder, or at least part of the extrusion head thereof, is inserted and fixed in the upper part of the extrusion blade, said extrusion head being bent at its end to permit application of the nozzle against the roller or against the lower part of the extrusion blade, depending on the type of main extruder used. 
   Said nozzle is characterized in that it comprises an annular part for bearing on and attachment to the die of the micro-extruder with the attachment elements necessary for fixing, a hollow cylindrical feed chamber, either straight or bent, having on its cylindrical surface an extrusion orifice, through which is extruded the conductive rubber material, and which occupies substantially all the height of said nozzle, and the cross-section of which has a shape adapted to the desired section of the conductive insert: In this apparatus of the second type, the extrusion nozzle is advantageously provided with a die which is mobile in translation along an axis substantially perpendicular to the axis of the roller, and making it possible, under the action of the pressure of the conductive mix which passes through it, to manufacture products of different thicknesses, while guaranteeing, during their manufacture, a minimum distance between the end of the nozzle and the roller of the main extruder. 

   
     DESCRIPTION OF THE DRAWINGS 
     The characteristics of the present invention will be better understood with reference to the drawings appended to the following description, in which drawings: 
       FIG. 1A  shows, in perspective view, the crown part of a tire having a non-conductive tread, and  FIG. 1B  is a plan view of part of the tread of said tire; 
       FIG. 2  shows the main components of an apparatus used according to the invention; 
       FIG. 3  is an enlarged view of the apparatus of  FIG. 2 ; 
       FIGS. 4 and 5  show the components of other extrusion apparatus capable of being used for forming a conductive insert; 
       FIG. 6  is an enlarged view of an extrusion nozzle which can be used on the apparatus of  FIG. 5 . 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   The tire of dimension 315/80 R 22.5, which is designed to have a low rolling resistance, comprises a carcass reinforcement  1  composed of a metal ply formed of inextensible metal cables coated in a rubber calendering mix, which is rendered conductive of electrostatic charges by means of a carbon black commonly used as a reinforcing filler in mixes. Said carcass reinforcement  1  is, as is known, anchored to at least one bead wire within each bead to form an upturn (not shown). Within said carcass reinforcement  1  there are the conventional reinforcement layers, and the so-called inner layers formed of mixes which are generally impermeable to the known inflation gases. The end(s) of this (these) inner layer(s) is (are) generally covered by the axially inner part of the protective layer of the bead (not shown), a wear layer, the axially outer part of which bears on the mounting rim, said layer generally having a very high content of carbon black filler, and therefore being highly conductive. 
   The carcass reinforcement  1  is surmounted in its crown by a crown reinforcement composed, in the example described, of two so-called triangulation half-plies  20  formed of inextensible metal cables which are oriented at an angle which may be between 30° and 90° relative to the circumferential direction; then, radially above, of two so-called working plies  21  and  22  composed of inextensible metal cables crossed from one ply to the next, forming angles which may or may not be identical, and of absolute value of between 10° and 30°, with the circumferential direction; then generally, lastly of at least one so-called protective ply  23  formed of elastic cables which form with the circumferential direction an angle equal in direction and value to the angle of the radially outermost working ply. All the cables of this crown reinforcement  2  are coated in one or more rubber calendering mix(es) which are conductive of electrostatic charges due to a carbon black commonly used as reinforcing filler in the mixes. 
   The crown reinforcement  2 , which does not have the same transverse profile as that of the subjacent carcass reinforcement  1 , is separated from said carcass reinforcement  1  by a rubber assembly of low thickness on the central axial part  61 , where the two reinforcements are substantially parallel and, on either side of said central part  61 , of increasing thickness towards the outside of the tire in order to form wedges or triangular profiled members  62 . For reasons of heating, said rubber wedges  62  and the axial part  61  are not made with conductive rubber mixes, because they have a very low carbon black filler content. Radially above the crown reinforcement  2 , there is arranged the tread  3  composed of the radially inner layer  31 , referred to as “underlayer”, formed of a mix filled with a majority of silica, which underlayer  31  itself is surmounted radially by the radially outer layer  32 , or rolling layer, having a very high silica filler content. The tread  3  is connected to the beads by the sidewall mixes  4 , which also have a very high silica filler content. 
   The rolling layer  32  is rendered conductive by means of the rubber insert  11 , which is in the form of a circumferential ring over the entire height of the rolling layer  32  to connect the surface of the tread which comes into contact with the ground with the radially upper face  310  of the underlayer  31 . This insert  11  of very low axial width e on the surface of the tread, equal to 0.5 mm, in the present case, is a single one and is centered theoretically on the equatorial plane XX′ of the tire, and its trace on the contact surface  310  between the two parts  31  and  32  of the tread is rectilinear and circular. The insert  11  could be off-center, in particular in the case of the presence of a central groove on the tread; there could be two inserts  11 , placed, for example, symmetrically to the equatorial plane, or more, but in any case placed axially such that the contact with the ground can be established whatever the amount of wear of the tread. This insert, when viewed in cross-section, has a surface having two bases  110  and  111 , the base  110  being the base of low width e on the tread surface. As for the other base  111 , on the contact surface between the two parts  31  and  32  of the tread, it is comparatively very wide since its width E is equal, in the example described, to 20 mm, that is to say, 40 times the width e, and the cross-section of the insert  11  may be considered as the radial superposition of a rectangular part of low width e and great thickness on a rectangular part of great width E and low thickness  FIG. 1B . The electrical connection between the running layer  32 , or more accurately between the insert(s)  11  and the radially outer ply  23  of the crown reinforcement  2 , which is formed of metal cables coated in a rubber mix filled with a conventional, conductive, carbon black, is effected by means of a circumferential insert  12  over the entire height of the part  31  of the tread, and the trace of which, of width e′, on the contact wall  310  between the two parts  31  and  32  of the tread  3 , is also circular, the width e′, in the case described, being equal to the width e. The rubber composition constituting the conductive connection  12  for the electrostatic charges, like the composition of the insert  11 , is based on a natural rubber and/or synthetic rubbers, which are conventionally used in building tires and particularly treads, and having as reinforcing filler a conductive carbon black preferably conventionally used in the manufacture of tires. As shown in  FIGS. 1A and 1B , said insert  12 , like the insert  11 , has a trace or line, on the contact surface between the two layers  31  and  32 , of circular shape, the transverse surface of which is in the form of a rectangular surface of width e′ and of a depth equal to the thickness of the layer  31 . The inserts  11  and  12  have, on the aforementioned contact wall, circumferential surfaces such that the surface of width e′ is included in the surface of width E, which is substantially larger. The arrangement thus adopted makes it possible, after positioning the two parts of the tread, be it on a tire building drum or on any other means of joining said two parts, to have between the two inserts  11  and  12  which are in contact, whatever the normally accepted errors in positioning of the two parts of the tread relative to each other, or errors of centering during manufacture (offset of the center axes of the two inserts  11  and  12  which is visible in  FIGS. 1A and 1B ). 
   The electrical connection between the crown reinforcement  2  and the carcass reinforcement  1  is effected, in the case described, by means of an insert  13  of rectangular cross-section having rectilinear and circular traces of a width e′ equal to e and to 0.5 mm on the surface common with the carcass reinforcement and the surface common with the crown reinforcement, said insert  13  possibly being circumferentially continuous or discontinuous. 
   The insert(s)  11 ,  12  which conduct(s) electrostatic charges, owing to its (their) rubber composition having a high content of carbon black, may be produced by any suitable means, both at the time of building of the tire in the non-vulcanized state and after vulcanization. The conductive insert or inserts  11  may be produced, for example, by extrusion during the building of the radially upper layer  32  of the tread  3  in the non-vulcanized state. The same applies to the inserts  12  and  13  for the radially inner layer  31 , which is obtained separately by extrusion, or for the layer  6 . 
     FIG. 2  shows a first extruder  10  which, using the extrusion screw  101 , delivers the non-vulcanized rubber mix intended for the manufacture of the upper part  32  of the tread, in an extrusion head  102 , defined by upper  104  and lower  103  arches, and comprising a flow channel  105  which firstly carries the mix to the rotary roller  15  which is associated with the extruder  10 , and secondly carries said mix towards an extrusion orifice  107  defined by the cylindrical surface  150  of the roller  15  and the fixed wall of a blade  106 , the extrusion orifice making it possible to impart the desired profile to the extruded mix. With said first extruder and roller there is associated a micro-extruder  40 , mounted fixedly on a support  41 . This extruder, equipped with a screw  401  and an extrusion head  402 , comprises at its end a nozzle  403  fixed to the die of the micro-extruder  40 , and intended, depending on the desired profiles and traces, to extrude the circumferential insert  11  in the profiled member of hot, non-vulcanized mix emerging from the orifice  107  of the extruder  10 . The support  41  is such that the extrusion head  402  and the nozzle  403  of said micro-extruder are placed in the flow channel  105  of the main extruder  10 , upstream of the extrusion orifice  107 . The conductive rubber mix is thus extruded in the flow of non-conductive mix. 
   As shown in  FIG. 3 , which shows an enlargement of the extrusion nozzle  403  of the micro-extruder  40  of  FIG. 2 , said nozzle  403  of the extrusion head  402  placed in the flow channel  105  of the main extruder  10  comprises an annular part  404  which makes it possible for the nozzle to bear on and be attached to the die of the extrusion head  402  of the micro-extruder  40 , owing to the presence of a plurality of attachment holes  405 . A hollow cylindrical feed chamber  406  has on its cylindrical surface an extrusion orifice  407  through which the conductive rubber material is extruded and which occupies virtually the entire height of said nozzle, and the cross-section of which is a section having two bases of different widths. The extrusion nozzle  403  is placed in the flow channel  105  such that the flow F of the rubber mix to be extruded by the main extruder  10  initially hits the side of the hollow body opposite the place where the extrusion orifice is located. Since the end  408  of the nozzle  403  is applied against the surface  150  of an extrusion roller  15  of the main extruder  10 , the conductive mix to be extruded adopts precisely the shape imparted by the extrusion orifice. 
   As for the nozzle  403  shown in  FIG. 4 , it differs primarily from the nozzle of  FIG. 3  in that the feed chamber for conductive mix is of bent shape such that the end  408  of the nozzle  403  is applied, not against the roller  15  of the extruder  10 , but against the wall of the lower arch  103  of the extrusion head  102  of said main extruder  10 . 
     FIG. 5  shows merely the upper arch  104 , the flow channel  105  bringing in the mix towards an extrusion orifice  107 , defined by the cylindrical surface  150  of the roller  15  and the fixed wall of a blade  106 , and thus on to said rotary roller  15 , which is associated with the extruder  10 , said extrusion orifice making it possible to impart the desired profile to the extruded mix. With said first extruder and roller there is associated a micro-extruder  40 , mounted fixedly on the extrusion blade  106 . This extruder, equipped with a screw  401  and an extrusion head  402 , comprises at its end a nozzle  403  intended, depending on the desired profile and trace, to extrude the circumferential insert  12  in the profiled member of hot, non-vulcanized mix emerging from the orifice  107  of the extruder  10 . The extrusion head  402  is bent at its end, so that the extrusion nozzle  403  fastened to the die of the extrusion head and placed in the extrusion orifice  107  of the main extruder can be placed against the surface  150  of the roller  15 . The conductive rubber mix is thus extruded in the flow of non-conductive mix. As can easily be understood, the principle of fixing the micro-extruder to the extrusion blade of the main extruder also advantageously applies to a flat-die extruder, said extruder differing from the preceding one by the fact that there is no longer a roller, and that the extrusion orifice  107  is defined by the lower and upper walls of the extrusion blade (not shown). 
     FIG. 6  shows an enlarged view of an extrusion nozzle  403  having a mobile die used with the micro-extruder of  FIG. 5 , said die being in a position of contact with the roller of the main extruder. The extrusion blade  106  receives the nozzle  403  with mobile die, slotted along an appropriate section over part of its height, in a bore perpendicular to the extrusion head  402 . The contact between the base  408  of the nozzle with mobile die  403  and the wall  150  of the roller  15  is maintained by the pressure of the conductive mix acting on the section  409 , which may possibly permit extrusion of the conductive mixture discontinuously.