Plastic and nonmetallic article shaping or treating: processes – Forming continuous or indefinite length work – Layered – stratified traversely of length – or multiphase...
Reexamination Certificate
1998-12-28
2001-09-25
Maki, Steven D. (Department: 1733)
Plastic and nonmetallic article shaping or treating: processes
Forming continuous or indefinite length work
Layered, stratified traversely of length, or multiphase...
C152S152100, C152S209500, C152SDIG002, C156S110100, C264S173170, C264S174110, C425S131100, C425S133500, C425S462000
Reexamination Certificate
active
06294119
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a pneumatic tire which includes a tread rubber capable of discharging electrostatic charges of the vehicle body to the road surface. It also relates to method and apparatus for producing unvulcanized tread rubbers for such tires.
2. Description of the Related Art
As known widely, electrostatic charges generated in the vehicle body of passenger cars, trucks or busses cause not only electroshock, but also occasional failure of the vehicle functions due to spark discharge. Such problems do not arise when the tires are comprised of a rubber which contains a large amount of carbon black because the tire has a sufficiently is high conductivity, and the electrostatic charges generated in the vehicle body can be discharged to the road surface through the tire.
There is an increasing demand for tires capable of minimizing fuel consumption of vehicles not only in view of economics, but also from environmental considerations. In order to cope with such demand, it would be desirable to employ low rolling resistance tires in which a large part of carbon black in the tire rubber is replaced with silica. However, by decreasing the amount of carbon black and increasing the amount of silica, it is inevitable that the electric resistance value of the tread rubber increases, making it difficult to discharge electrostatic charges of the vehicle body to the road surface.
In order to discharge electrostatic charges of the vehicle body to the road surface, a discharge means typically in the form of a metal strip may be directly connected to the conductive material of the vehicle body. However, such metal strip rubs or collides with the road surface and has to be exchanged whenever it is worn or broken; otherwise, the above-mentioned problems arising from the electrostatic charges of the vehicle body would be inevitable.
In order to overcome the above-mentioned disadvantages, there has been proposed, for example in JP-08-34204 A, a tire having an extruded tread rubber, in which a tread rubber comprises a conventional high conductive rubber containing carbon black, which is arranged on the center region, and a low conductive rubber containing silica, which is arranged at the remaining regions. According to this proposal, the high conductive rubber region in the tread rubber is connected to an ordinary base rubber containing carbon black, to ensure that the tire exhibits the desired electrostatic discharge function.
However, rubber-reinforcing effect achieved by silica is lower than that by carbon black, and there is a tendency that the tread rubber undergoes non-uniform wear as running of the tire proceeds. More specifically, the silica-blended low conductive rubbers on both sides of the center region exhibit a large amount of wear as compared to the center region comprised of high conductive rubber which contains carbon black. As a result, there tends to be caused a level difference, between the high conductive rubber portion and the low conductive rubber portions on both sides thereof. Such a level difference becomes significant as running proceeds and spreads in the width direction of the tread, causing a marked irregular wear and thereby reducing the service life of the tire.
Furthermore, it has been confirmed that the tread rubber tends to be twisted due to the slip angle which is applied to the tread rubber during cornering of the vehicle, and the high conductive rubber in the center region of the tread projects outwards and apply a large tearing force to the junction between the high conductive rubber and the silica-blended rubber adjacent to the high conductive rubber. Such tearing force tends to cause cracks in the silica-blended rubber which does not provide a sufficient rubber-reinforcing effect. Furthermore, the high conductive rubber occupies a large volume in the tread rubber which results in a degraded rolling resistance property. This shows that the pneumatic tires of this kind requires further improvement.
SUMMARY OF THE INVENTION
It is therefore one object of the present invention to provide an improved pneumatic tire having a low rolling resistance and capable of effectively discharging electrostatic charges of the vehicle body without problems of irregular wear or cracks of the tread rubber.
Another object of the present invention is to provide method and apparatus for producing unvulcanized tread rubbers in economically advantageous manner. The method and apparatus according to the present invention can be applied to the production of both conventional pneumatic tires and pneumatic tires having low rolling resistance property and high conductivity, and requires only slight remodeling of a conventional apparatus for producing unvulcanized tread rubbers in the form of a dual-type extruder in which two extruder units are connected to a common extrusion die head, or a triple-type extruder in which three extruder units are connected to a common extrusion die head.
According to one aspect of the present invention, there is provided a method for producing an unvulcanized tread rubber for pneumatic tires, wherein at least one kind of low conductive unvulcanized rubber and at least one kind of high conductive unvulcanized rubber are supplied to respective extend units on upstream side of a common extrusion head, and thereby caused to flow along respective passages in the extrusion head and then merged with each other at a back side of an extrusion orifice in the extrusion head, before they are continuously extrude from the extrusion orifice as an integrated composite body corresponding to the unvulcanized tread rubber.
In the method according to the present invention, the low conductive unvulcanized rubber is passed through a first passage extending from first extruder unit to the back side of the extrusion orifice, so as to form at least a cap rubber portion of a tread rubber in a product tire; the high conductive unvulcanized rubber is passed through a second passage extending from a second extruder unit to the back side of the extrusion orifice, so as to form at least part of the remaining portion of the tread rubber, and the high conductive unvulcanized rubber while passing through the second passage is partly introduced into a narrow third passage branched from the second passage, and passed through a slit-like opening having a cross-sectional shape which is substantially straight in a height direction of the extrusion head and situated in the vicinity of the back side of the extrusion orifice, so as to form a narrow strip of the high conductive rubber in said composite body of unvulcanized rubbers, said narrow strip extending in the low conductive rubber portion of the tread substantially over the entire thickness and circumference thereof.
According to another aspect of the present invention, there is provided an apparatus for producing an unvulcanized tread rubber for pneumatic tires, wherein at least one kind of low conductive unvulcanized rubber and at least one kind of high conductive unvulcanized rubber are supplied to respective extruder units on upstream side of a common extrusion head, and thereby caused to flow along respective passages in the extrusion head and then merged with each other at a back side of an extrusion orifice in the extrusion head, before they are continuously extruded from the extrusion orifice as an integrated composite body corresponding to the unvulcanized tread rubber.
The apparatus according to the present invention comprises: a first passage extending from a first extrusion unit to the back side of the extrusion orifice, for allowing the low conductive unvulcanized rubber to be passed therethrough so as form at least a cap rubber portion of a tread rubber in a product tire; a second passage extending from a second extrusion unit to the back side of the extrusion orifice, for allowing the high conductive unvulcanized rubber to be passed therethrough so as to form at least part of the remaining portion of the tread rubber, and a narrow third passage branched from
Bridgestone Corporation
Maki Steven D.
Sughrue Mion Zinn Macpeak & Seas, PLLC
LandOfFree
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