Abstract:
An object is to provide a pneumatic tire which can exhibit a static elimination function in an ensured fashion and be fabricated easily. As a means therefor, there is provided a pneumatic tire in which a conductive layer  12   a  and a non-conductive layer  14   a  are wound alternately in a tire circumferential direction in a partially overlapping fashion at least in part of a tread portion, characterized in that the conductive layer  12   a  is provided wider than the non-conductive layer  14   a.

Description:
TECHNICAL FIELD 
     The present invention relates to a pneumatic tire having a static elimination function. 
     BACKGROUND ART 
     In general, tires are configured in such a manner as to include a plurality of tire rubber members and a plurality of reinforcement members which are mainly made up of cords. In a representative tire, as is shown in  FIG. 7 , respective portions such as an inner liner portion  51 , a tread portion  52 , side wall portions  53 , rim strip portions  54  and the like are formed by rubber members which match properties required for the respective portions and these rubber members are combined with a carcass layer  55  which constitutes a cord-contained reinforcement member, a belt layer  56  and bead portions  57  to thereby make up a tire T 2 . 
     To build rubber members which make up the respective portions, rubber materials were extruded continuously to be built into rubber strips from an extruding machine via dies which match cross sectional shapes of the respective rubber members, and thereafter, the rubber strips so built were cut to constant dimensions to thereby obtain target rubber members. In building a tire, the rubber members were sequentially affixed together on a rotational support element such as a building drum. 
     In addition, in recent years, in order to reduce the rolling resistance of a tire, tread rubbers have been developed which use silica instead of carbon black as a strengthening agent. However, since the tread rubbers have an electric resistance which is higher than that of tread rubbers which is compounded with only carbon black, there has been caused a problem that static electricity conducted from a vehicle body or electricity generated by internal friction when rubber deforms is accumulated. Then, there have been proposed pneumatic tires with a static elimination function which are made up in parallel of a non-conductive rubber which contains silica or the like and a conductive rubber which is compounded with carbon black or the like so that electricity generated in the vehicle body is made to be discharged to the road surface from a tread surface thereof. 
     For example, there has been proposed a pneumatic tire with a static elimination function in which a tread portion is configured by winding sequentially both a non-conductive rubber strip member and a highly conductive rubber strip member along a circumferential direction of a tire in a spiral fashion in such a manner that the non-conductive rubber strip member and the highly conductive rubber strip member are disposed in an alternate fashion (for example, refer to Patent Document No. 1 below). 
     However, in the pneumatic tire, there is caused a fear that the non-conductive rubber strip material covers the highly conductive rubber strip material in the tread portion to thereby exhibit no static elimination function unless winding positions of the non-conductive rubber strip material and the highly conductive rubber strip material are adjusted with good accuracy. Moreover, in the event that the tread portion is formed such as a winding drum by a roller, the non-conductive rubber in the rubber strip material is pressed to expand to cover the conductive rubber during the pressing step, causing the fear that the static elimination function is not exhibited. In addition, at the time of processing a green tire under vulcanization, as with the aforesaid pressing step, the non-conductive rubber flows to cover the conductive rubber, leading to the fear that the static elimination function is not exhibited.
     Patent Document No. 1: JP-A-2004-338621   

     SUMMARY OF THE INVENTION 
     Problem that the Invention is to Solve 
     The invention has been made in view of the problem, and an object thereof is to provide a pneumatic tire which can exhibit the static elimination function in an ensured fashion and moreover which can easily be fabricated. 
     Means for Solving the Problem 
     The invention provides a pneumatic tire in which a conductive layer and a non-conductive layer are wound alternately in a tire circumferential direction in a partially overlapping fashion at least in part of a tread portion, characterized in that the conductive layer is provided wider than the non-conductive layer. 
     In the invention, the non-conductive layer may be a rubber strip material made of a non-conductive rubber, and the conductive layer may be a rubber strip material made of a conductive rubber. Furthermore, the conductive layer and the non-conductive layer are made up, respectively, of a conductive rubber layer made of a conductive rubber and a non-conductive layer made of a non-conductive rubber, and the conductive layer and the non-conductive layer may integrally be laminated into a rubber strip material. 
     According to the invention, in the conductive layer and the non-conductive layer which make up the tread portion, since the conductive layer is provided wider than the non-conductive layer, the conductive layer is provided on a surface of the tread portion, whereby the static elimination function is allowed to be exhibited in an ensured fashion. In addition, when the conductive layer and the non-conductive layer are folded back to form the tread portion, the conductive layer can be made to intersect itself at an interface so that the conductive layer is brought into contact with itself, whereby the static elimination function can be exhibited in an ensured fashion. 
     Advantage of the Invention 
     According to the invention, since the adjacent conductive layers can be brought into contact with each other in the ensured fashion, the pneumatic tire with the static elimination function can easily be fabricated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  A sectional view showing a pneumatic tire according to an embodiment of the invention. 
         FIG. 2A  A sectional view of a rubber strip material employed in a fabrication method according to the embodiment. 
         FIG. 2B  A sectional view of a rubber strip material employed in a fabrication method according to the embodiment. 
         FIG. 3  A drawing explaining a method for building a tread portion by winding a rubber strip material which is extruded by an extruding machine. 
         FIG. 4  A plan view explaining a winding method of the rubber strip material. 
         FIG. 5  A sectional view showing a method for building a tread portion by employing the rubber strip materials. 
         FIG. 6  A sectional view showing a method of winding the rubber strip materials. 
         FIG. 7  A sectional view showing an example of a conventional pneumatic tire. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, a first embodiment of the invention will be described based on the drawings. 
       FIG. 1  is a sectional view showing an example of a pneumatic tire T 1  according to the embodiment, and FIG.  2 A, 2 B shows sectional views of rubber strip members  10  which make up a tread portion  2  of the pneumatic tire T 1 . 
     A pneumatic tire (hereinafter, referred to as a tire) T 1  according to the embodiment includes, for example, as is shown in  FIG. 1 , a pair of bead portions  7 , rim strip portions  4  and side wall portions  3  which extend radially outwards of the tire from the bead portions  7 , respectively, and a tread portion  2  which is provided between the side wall portions  3 , and a carcass layer  5  which is made up of carcass plies establishes a reinforcement between the bead portions  7 . An inner liner portion  1  and a belt layer  6  in which two internal and external belt plies are laminated together are provided, respectively, on an inner circumference and an outer circumference of the carcass layer  5  so as to hold an air pressure. 
     The tread portion  2  is made up of shoulder portions  2   a ,  2   a  which are provided on both sides in a tire width direction and a tread central portion  2   b  which is provided between both the shoulder portions  2   a ,  2   a , and the shoulder portions  2   a  and the tread central portion  2   b  are built by a rubber strip member  10  being wound in an overlapping fashion. 
     This rubber strip member  10  is formed into a ribbon shape having a flat cross sectional shape such as a substantially crescent shape, a flat substantially triangular shape or a flat substantially trapezoidal shape in which mainly a widthwise central portion is made thickest and the thickness is gradually reduced from this central portion towards both sides. 
     To describe in greater detail, in a rubber strip member  10   a  which makes up the shoulder portions  2   a ,  2   a , as is shown in  FIG. 2A , a conductive layer  12   a  made of a conductive rubber material and a non-conductive layer  14   a  made of a non-conductive rubber material are laminated integrally, and the conductive layer  12   a  and the non-conductive layer  14   a  are provided in such a manner that a width dimension W 1  of the conductive layer  12   a  becomes wider than a width dimension W 2  of the non-conductive layer  14   a . On the other hand, a rubber strip member  10   b  which makes up the tread central portion  2   b  is made up of a non-conductive rubber layer  14   b  only which is made up of a non-conductive rubber material shown in  FIG. 2B , and does not have a conductive rubber layer made of a conductive rubber material. 
     An example of a rubber strip material  10  which is used in this embodiment will be described by reference to  FIG. 2A . A cross section of the rubber strip material  10  has a substantially crescent shape having a thickness dimension T 1  of 0.5 to 30 mm, and the width dimension W 1  of the conductive layer  12   a  of the rubber strip material  10   a  becomes 5 to 50 mm and a thickness dimension T 1  thereof becomes 0.05 to 0.2 mm, while the width dimension W 2  of the non-conductive layer  14   a  becomes 4 to 45 mm. In addition, although the dimensions and shape of the rubber strip is not specifically limited to those described above, the cross sectional shape is preferably small from the viewpoint of dimensional accuracy of the tread portion and weight balance and uniformity of the tire. 
     Here, the conductive rubber material which makes up the conductive rubber layer denotes a conductive rubber compound having a specific volume resistance of less than 10 8 Ω·cm, and for example, a rubber compound can be raised as an example which contains much carbon black as a strengthening agent. The conductive rubber material can be obtained by compounding, other than carbon black, a predetermined amount of a known conductivity imparting material such as a carbon-based material including carbon fiber, graphite or the like and a metal-based material including metallic powder, metallic oxide, metallic flakes, metallic fiber or the like. In addition, the non-conductive rubber material which makes up the non-conductive rubber layer denotes a non-conductive or insulating rubber compound having a specific volume resistance of 10 8 Ω·cm or more, and for example, a rubber compound can be raised as an example which is compounded with, in place of carbon black, silica in a high proportion as the strengthening agent. The non-conductive rubber of this embodiment is such as to contain 40 to 100% of silica by weight ratio. 
     Next, a building method of the tread portion  2  of the tire T will be described. 
       FIG. 3  is a drawing explaining a method for building the tread portion  2  by winding a rubber strip member  10  which is extruded by an extruding machine  100 ,  FIG. 4  is an exemplary drawing explaining a method for winding the rubber strip member  10  on to a rotational support element  120 , and  FIG. 5  is a sectional view showing a method for building the tread portion  2  by the use of the rubber strip member  10 . 
     In a fabricating process of the tire T, the tread portion  2  of the tire T is formed by winding the rubber strip member  10  which is extruded from the extruding machine  100  as shown in  FIG. 3  on to the rotational support element  120  such as a building drum or a green tire (not shown) which is supported at bead portions. 
     To describe in detail, the extruding machine  100  which extrudes the rubber strip member  10  includes, as is shown in  FIG. 3 , a pair of main body cases  102 ,  103  which are each formed into a cylindrical shape and in which rubber feeding screw shafts  107 ,  109  are provided respectively in interiors thereof, a pair of head portions  104 ,  105  which have gear pumps which are provided consecutively to distal ends of the main body cases  102 ,  103 , respectively, a rubber coalescence portion  106  provided in common at distal ends of the head portions  104 ,  105  and an extruding die  108  which is added to a distal end of the rubber coalescence portion  106 , so that a non-conductive rubber material Q 2  is made to be supplied into an interior of the main body case  102  from a hopper  110  and a conductive rubber material Q 1  is made to be supplied into an interior of the main body case  103  from a hopper  111 . Both the rubber materials Q 1 , Q 2  which are so supplied to the main body cases  102 ,  103  are fed forwards by virtue of rotation of the screw shafts  107 ,  109 , respectively, and are then fed to the rubber coalescence portion  106  by the gear pumps of the head portions  104 ,  105  in such a manner as to realize required flow rates. 
     At the rubber coalescence portion  106 , the rubber material Q 1  and the rubber material Q 2  are formed into shapes which correspond, respectively, to the conductive layer  12   a  and the non-conductive layer  14   a  and are then coalesced together, whereby a ribbon-shaped rubber strip member  10   a  in which a conductive rubber layer  12   a  and a non-conductive layer  14   a  are laminated integrally as is shown in  FIG. 2   a  is continuously extruded via the extruding die  108  whose discharge port  108   a  is formed into a shape matching the cross sectional shape of the rubber strip material  10 . 
     In the extruding machine  100  configured as described above, by stopping the operations of the screw shaft  109  in the main body case  103  into which the conductive rubber material Q 1  is supplied and the gear pump  105  from a working state in which the rubber strip member  10   a  is extruded and controlling the screw shaft  107  in the other main body case  102  into which the non-conductive rubber material Q 2  is supplied and the gear pump  104  in such a manner as to realize a predetermined flow rate of the rubber material which is fed to the rubber coalescence portion  106 , a rubber strip member  10   b  which is made up of only a non-conductive rubber layer  14   b  is continuously extruded from the extruding machine  100  without stopping the operation of the extruding machine  100 . On the other hand, by causing the screw shaft  109  in the main body case  103  and the gear pump  105  of the extruding machine  100  which have been stopped from rotating from the working state in which the rubber strip material  10   b  is extruded to rotate at a predetermined speed and controlling the screw shaft  107  in the other main body case  102  into which the non-conductive rubber material Q 2  is supplied and the gear pump  104  to reduce their rotational speeds in such a manner as to realize the predetermined flow rate of the rubber material which is fed to the rubber coalescence portion  106 , the rubber strip member  10   a  whose cross section is made up of the conductive layer  12   a  and the non-conductive layer  14   a  which are laminated integrally is continuously extruded without stopping the operation of the extruding machine  100 . 
     In this way, the extruding machine  100  can extrude the two types of rubber strip materials  10   a ,  10   b  while switching therebetween at an arbitrary timing without stopping the operation of the extruding machine  100  by controlling the flow rates of the conductive rubber material Q 1  and the non-conductive rubber material Q 2  which are fed to the rubber coalescence portion  106 . 
     The rubber strip materials  10   a ,  10   b  which are extruded in the way described above are then wound on to the rotational support element  120  which is disposed in such a manner as to confront the extruding machine  100  via rolls  114  which introduce the rubber strip materials  10   a ,  10   b  which are each extruded into a ribbon shape having a predetermined cross sectional shape from the extruding machine  100  to the rotational support element  120  while shaping properly the cross sectional shapes of the rubber strip materials  10   a ,  10   b . The rotational support element  120  can rotate about a shaft  120   a , and the rubber strip members  10   a ,  10   b  are wound along a tire circumferential direction while rotating the rotational support element  120  in a direction indicated by an arrow K in  FIG. 3 . The rubber strip materials  10   a ,  10   b  which are so wound on to the rotational support element  120  are pressed against a winding surface  120   b  of the rotational support element  120  by a roller  116 . 
       FIG. 4  is a view resulting when the rotational support element  120  of the building drum is viewed thereabove, and an arrow A denotes the tire circumferential direction, and an arrow B denotes a tire width direction (axial direction). When winding spirally the rubber strip materials  10   a ,  10   b  along the tire circumferential direction, not only by rotating the rotational support element  120  but also by shifting either the extruding machine  100  or the rotational support element  120  along the tire width direction so as to relatively shift the extruding machine  100  along the tire width direction B, the adjacent rubber strip materials  10   a ,  10   b  are wound in a partially overlapping fashion. As this occurs, as is shown in  FIG. 6 , by controlling the relative shifting speed in the tire width direction B, an overlapping amount S between the adjacent rubber strip materials is adjusted, so as to control an inclination angle β of the rubber strip materials  10   a ,  10   b  relative to the winding surface  120   b  of the rotational support element  120  in such a manner as to become a predetermined value. The operations of the extruding machine  100  and the rotational support element  120  are controlled by a control unit  130 . 
     In building the tread portion  2  on a belt portion  6  formed on the winding surface  120   b  of the rotational support element  120  by the use of the extruding machine  100  constituted as described above, firstly, by controlling the screw shaft  107  in the other main body case  102  into which the non-conductive rubber material Q 2  is supplied and the gear pump  104  in such a manner as that the rubber material is fed to the rubber coalescence portion  106  at the predetermined flow rate, as is shown in  FIG. 5 , the rubber strip material  10   b  is extruded by the extruding machine  100 , and the rubber strip material  10   b  so extruded is wound on to the rotational support element  120  while shifting the rubber strip material  10   b  from a winding starting position P 1  at a central portion of the rotational support element  120 , which corresponds to a central portion of the tread portion  2 , towards one end portion (for example, a right end portion) in the tire width direction. 
     Following this, when the rubber strip material  10   b  reaches a position P 2  which corresponds to the shoulder portion  2   a , from the midst of the winding operation of the rubber strip material  10   b , the screw shaft  109  in the man body case  103  into which the conductive rubber material Q 1  is supplied and the gear pump  105  of the extruding machine  100  are caused to rotate at a predetermined speed, while the screw shaft  107  in the other main body case  102  into which the non-conductive rubber material Q 2  is supplied and the gear pump  105  are controlled to reduce their rotational speeds in such a manner that the predetermined flow rate of the rubber material that is fed to the rubber coalescence portion  106  is realized. By controlling the extruding machine  100  in the way described above, the rubber strip material  10   a  is extruded from the extruding machine  100  without any interruption so as to be supplied to the rotational support element  120 , and when the rubber strip material  10   a  reaches a right end, the rubber strip material  10   a  is folded to a tire outer circumferential side, so as to continue to be wound while being shifted from the right end towards the other end portion (a left end portion) in the tire width direction, whereby the shoulder portion  2   a  of the tread portion  2  is built. In the shoulder portion  2   a  so formed, since the spiral direction becomes opposite before and after where the rubber strip portion  10   a  is folded back, there is produced a portion where the rubber strip material  10   a  intersects itself. At this intersecting portion, by the conductive layer  12   a  of the rubber strip member  10   a  being brought into contact with itself as a result of the rubber strip member  10   a  being so folded back, a conductive path  2   c  is formed for releasing static electricity from a belt layer  6  lying underneath the tread portion  2  to the surface of the tread. 
     Following this, when the rubber strip material  10   a  reaches a position P 3  which corresponds to the tread central portion  2   b , from the midst of the winding operation of the rubber strip material  10   a , the screw shaft  109  in the main body case  103  into which the conductive rubber material Q 1  is supplied and the gear pump  104  of the extruding machine  100  are stopped from operating, while the screw shaft  107  in the other main body case  102  into which the non-conductive rubber material Q 2  is supplied and the gear pump  104  are controlled in such a manner that the predetermined flow rate of the rubber material that is fed to the rubber coalescence portion  106  is realized. By controlling the extruding machine  100  in the way described above, a rubber strip member  10   b  is extruded from the extruding machine  100  without any interruption so as to be supplied to the rotational support element  120 , and the rubber strip member  10   b  is wound on to the rotational support element  120  while being shifted leftwards towards the left end portion, so that the tread central portion  2   b  of the tread portion  2  is built. 
     Following this, when the rubber strip material  10   b  reaches a position P 4  which corresponds to the shoulder portion  2   a , from the midst of the winding operation of the rubber strip material  10   b , by controlling the extruding machine  100  in such a manner that the screw shafts  107 ,  109  and the gear pumps  104 ,  105  are caused to rotate at the predetermined speeds in the way described above so as to realize predetermined flow rates of the rubber materials Q 1 ,Q 2  which are fed to the rubber coalescence portion  106 , a rubber strip material  10   a  is extruded from the extruding machine  100  so as to be supplied on to the rotational support element  120  and is wound on thereto while being shifted from the right to the left. When the rubber strip material  10   a  reaches the left end, the rubber strip material  10   a  is folded towards the tire outer circumferential side, so as to continue to be wound while being shifted from the left to the right, whereby the shoulder portion  2   a  of the tread portion  2  is built. As with what has been described above, in the shoulder portion  2   a  so built, in the portion where the rubber strip material  10   a  intersects itself, by the conductive layer  12   a  of the rubber strip material  10   a  being brought into contact with itself as a result of the rubber strip material  10   a  being so folded back, a conductive path  2   c  is formed for releasing static electricity from a belt layer  6  lying underneath the tread portion  2  to the surface of the tread. 
     Following this, when the rubber strip material  10   a  reaches a position P 5  which corresponds to the tread central portion  2   b , from the midst of the winding operation of the rubber strip material  10   a , by controlling the extruding machine  100  in the way described above, a rubber strip material  10   b  is extruded from the extruding machine  100  so as to be supplied on to the rotational support element  120 , and the rubber strip material  10   b  is wound while being shifted to the right, whereby the tread central portion  2   b  of the tread portion  2  is built. Thus, the tread portion  2  can be formed which includes the shoulder portions  2   a  which are made up of the conductive rubber material and the non-conductive rubber material and which are imparted the static elimination function and the tread central portion  2   b  which is made up of the non-conductive rubber material. 
     As has been described above, in the rubber strip material  10   a  which makes up the shoulder portions  2   a ,  2   a , since the conductive layer  12   a  is provided wider than the non-conductive layer  14   a , when the rubber strip material  10   a  is extruded from the extruding machine  100 , the conductive rubber material Q 1  can be made to flow through in the vicinity of both end portions of the discharge port  108   a  where controlling of the rubber material which flows therethrough is difficult, whereby the conductive rubber can be provided at both the end portions of the rubber strip material  10   a  at all times. 
     Because of this, even in the event that the rubber strip material  10   a  is pressed against by the roller  116  when the rubber strip material  10   a  is wound on to the winding surface  120   b  of the rotational support element  120 , the conductive rubber which lies at both the end portions is pressed to be expanded, and hence, there is caused no such situation that the non-conductive rubber covers the conductive rubber, and since there is no possibility that the non-conductive rubber flows to cover the conductive rubber during vulcanization of a green time, the static elimination function can be made to be exhibited in an ensured fashion. 
     In addition, in the tire T 1  of this embodiment, while the shoulder portions  2   a ,  2   a  of the tread portion  2  are made up of the rubber strip material  10   a  which is made up of the conductive layer  12   a  and the non-conductive layer  14   a , the invention is not limited thereto, and hence, the whole area of the tread portion  2  may be made of the rubber strip material  10   a  which is made up of the conductive layer  12   a  and the non-conductive layer  14   a.    
     Additionally, in the tire T 1  of this embodiment, while the tread portion  2  is formed by the use of the rubber strip material  10   a  into which the conductive layer  12   a  made of the conductive rubber material and the non-conductive layer  14   a  made of the non-conductive rubber material are integrated, a conductive rubber strip material made of a conductive rubber material and a non-conductive rubber strip material made of a non-conductive rubber material may be wound on to the winding surface  120   b  while being laminated on the rotational support element  120 . 
     Furthermore, a conductive layer may be formed on a surface of the non-conductive layer made of the non-conductive rubber strip material by applying a conductive liquid material. Namely, a conductive layer may be formed by applying a conductive liquid material to the surface of the non-conductive layer which is wound on to the winding surface  120   b  of the rotational support element  120  with a brush or a roller in such a manner that the conductive layer so applied becomes wider than the non-conductive layer, or by winding a non-conductive rubber strip material on the whole surface of which a conductive layer is formed by applying a conductive liquid material thereto on the building drum. 
     Here, there is no specific limitation on the conductive liquid material used, and any conductive liquid material can be used provided that the liquid material has superior adhesion to rubber and its conductivity is not damaged even when subjected to a vulcanization process. As such conductive liquid materials, for example, a rubber glue or cement can be raised which is mixed with a rubber compound which is compounded with carbon black in a high proportion. 
     With the conductive liquid material described above, even in the event that the liquid material is applied to the surface of a non-conductive rubber strip material while the viscosity thereof being adjusted as required so as to form a conductive layer thereon, a conductive layer can easily be formed wider than the non-conductive rubber strip material. 
     In addition, when a conductive layer is formed on the surface of a non-conductive rubber strip material, by applying a conductive liquid material while appropriately adjusting the viscosity thereof, at least part of the conductive liquid material so applied stays on the surface of the non-conductive rubber strip material, whereby a conductive layer can easily be formed wider than the non-conductive rubber strip material. 
     In this way, even when the conductive rubber strip material and the non-conductive rubber strip material are laminated one on the other, by providing the conductive rubber strip material wider than the non-conductive rubber strip material, the conductive rubber material can be made to be exposed, in an ensured fashion, to the surface of the rubber strip materials which are laminated without adjusting the positions of both the rubber strip materials with good accuracy, whereby the static elimination function can be exhibited in an ensured fashion. 
     DESCRIPTION OF REFERENCE NUMERALS AND CHARACTERS 
     
         
           1  . . . inner liner rubber portion 
           2  . . . tread rubber portion 
           2   a  . . . shoulder portion 
           2   b  . . . tread central portion 
           2   c  . . . conductive path 
           3  . . . side wall rubber portion 
           4  . . . rim strip rubber portion 
           5  . . . carcass layer 
           6  . . . belt layer 
           7  . . . bead portion 
           8  . . . base rubber portion 
           10 ,  10   a ,  10   b  . . . rubber strip material 
           12   a  . . . conductive layer 
           14   a  . . . non-conductive layer 
           14   b  . . . non-conductive rubber layer 
           100  . . . extruding machine