Abstract:
A tire tread comprising an elongated tread strip of tire tread compound of high resistivity having a transverse width (TR) and, within the transverse width (TR) at least one longitudinally extending conducting strip of low resistivity tire compound with a volume resistivity less than 10 8  ohm-cm, the at least one conducting strip extending from the top to the bottom surface of a tread strip.

Description:
this application is a continuation of application Ser. No. 08/439,981 filed on May 12, 1995, now abandoned. 
    
    
     This invention relates to tire treads and in particular to tire treads which are electrically conducting including those reinforced by silica. 
     BACKGROUND OF THE INVENTION 
     The build-up of static electric charge in moving vehicles can have a potentially adverse effect on electronic circuitry and radio reception, as well as being a safety hazard should spark discharge occur during re-fuelling. Personal discomfort also occurs if charge is earthed through a driver or passenger. 
     Charge is normally conducted to earth through the tires, which requires that the tires themselves have an acceptable level of conductivity--commonly described as anti-static. Such levels of conductivity are normally guaranteed by the use of tire compounds containing significant proportions of carbon black as reinforcing filler. Carbon black reduces the resistivity of the intrinsically non-conductive rubber and so provides a leakage path for the static charge from the vehicle to earth. 
     Anti-static rubber compounds generally require a volume resistivity below 10 8  ohm-cm, which is usually achievable with approx 50 phr or more carbon black in the compound. Such loadings are typical of carbon black-reinforced tires, which are accordingly anti-static. 
     If non-black fillers (e.g. silica) are used, or if the carbon black loading is significantly less than 50 phr, anti-static properties are often not achieved. For example, with silica filler in place of carbon black, the volume resistivity is typically 10 13  ohm-cm. Should such a compound be used in the tread of a tire, for example, to achieve a lower rolling resistance, the vehicle will effectively be insulated from the road and static charge will not be able to leak to earth. 
     It is known that electrical conductivity may be improved by the addition of polar ingredients but the effect is generally of insufficient magnitude. Likewise special grades of superconductive carbon blacks may be added but these need to be present in moderately high loadings, e.g. 10-30 phr, to be significantly effective, and since their other properties are not suited to tire applications, road performance may deteriorate. 
     According to one aspect of the present invention a tire tread comprises an elongated tread strip of tire tread compound of high resistivity having a transverse width (TR) and, within the transverse width (TR) a longitudinally extending conducting strip of low resistivity tire compound with a volume resistivity less than 10 8  ohm-cm, the said conducting strip extending from the top to the bottom surface of the tread strip. 
     By high resistivity is meant tire tread compounds having a volume resistivity greater than 10 89  ohm-cm. Such compounds include, for example, tire tread compounds having a silica filler in place of carbon black of which the volume of resistivity is typically 10 13  ohm-cm. 
     Preferably the conducting strip is positioned at the center of the tire tread strip. 
     Two or more longitudinally extending conducting strips may be provided and when two strips are used they may conveniently be the tread strip wing component which in the finished tire form the tire tread shoulders. This construction is particularly convenient because the tire tread wings are usually extruded of a different material to the main part of the tire tread strip and thus no additional extruded component is necessary. 
     The width of the conducting strip is preferably in the range of 5-25% of the width of the tire tread TR. 
     The compound of the conducting strip more preferably has a volume resistivity of less than 10 6  ohm-cm. 
     Another aspect of the invent ion provides a tire comprising a tire tread strip according to any of the above claimed features. Preferably the width of the conducting strip is greater than the width of the widest longitudinally extending tread groove to ensure that the conducting strip ma y contact the road when the tire is completed and used on a vehicle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further aspects of the present invention will become apparent from the following description, by way of example only, of two embodiments in conjunction with the attached diagrammatic drawings in which: 
     FIG. 1 is a transverse cross-sectional view of an extruded tire tread strip according to the present invention; 
     FIG. 2 shows the tire tread strip of FIG. 1 after tire manufacture; and 
     FIG. 3 shows an alternative tire tread strip. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows the transverse cross-section of a longitudinally extended tire tread strip comprising a main tire tread compound region 1 and a conductive tire sub-tread region 2. This assembly is the standard one where an extruded tread compound strip of the required cross-section is brought into close contact with a flat strip or sheet of sub-tread compound to form a sub-assembly on later tire manufacture. 
     In this cases however the first or main tire tread strip region 1 is in three parts. Firstly a center part 3 having a width of 15% of the tire strip width TR and two main side regions 4. The center region comprises a rubber compound having a volume resistivity of approximately 10 6  ohm-cm being reinforced by carbon black and having the following formulation. 
     
         ______________________________________SBR 1712, styrene 23.5%,              96oil 27.3%BR, 96% 1,4-cls    30N234 Carbon Black  80Silica Ultrasil VN3              --*TESPT/N330 1:1 (X50-S)              --Zinc Oxide          3Stearic acid        2Aromatic oil       10CBS                  1.5DPG                --Sulfur               1.5______________________________________ *bis  3triethoxysilylpropyl! tetrasulfane/N330 1:1 blend marketed by Degussa as X50S. 
    
     The side regions 4 are of a rubber compound reinforced by silica having a volume of resistivity approximately 10 13  -cm. The formulation of the side region tread compound is as follows. 
     
         ______________________________________SBR 1712, styrene 23.5%,              96oil 27.3%BR, 96% 1,4-cls    30N234 Carbon BlackSilica Ultrasil VN3              80*TESPT/N330 1:1 (X50-S)                12.8Zinc Oxide          3Stearic acid        2Aromatic oil       10CBS                  1.5DPG                 2Sulfur               1.5______________________________________ *bis  3triethoxysilylpropyl! tetrasulfane/N330 1:1 blend marketed by Degussa as X50S. 
    
     The tire tread strip is fitted to tire a carcass on top of a tread reinforcing breaker in the normal way and the tire is vulcanised in a tire curing press where the tire tread pattern is formed. FIG. 2 shows a short section of the tire tread strip after such vulcanisation in a tire mould and for simplicity shows the three longitudinally extending zig-zag grooves as the tire tread pattern. As can be seen the central region 3 having low resistivity has a large center groove 5 formed therein but the width of the center region 3 is such that there is still low resistivity compound at the tire road contacting surface and the low resistivity material extends to the sub-tread so providing a path for charge to dissipate from the tire to the road. The two side main grooves 5A do not have this low resistivity material but are formed in the silica, comparatively high resistivity, material. 
     In the embodiment shown in FIG. 3 the high resistivity silica compound forms the main region 1 of the tire tread strip. Once again a sub-tread 2 is provided out in this case the wing portions 6 and 7 of the tire tread strip comprise the low resistivity materials. Similar compounds to the previous one may be used for this embodiment and it will be appreciated that in this case it is through the shoulder regions of the tire that the electrical path is provided. 
     In a further modification to the constructions embodied in FIGS. 1, 2 and 3, the sub-tread component (region 2) may be omitted. 
     The formulation for the conducting or low volume resistivity components is given purely by way of example only and any of the normal low resistivity formulations known in the art may be used. Likewise the formulation of the non-conducting or high volume resistivity component is given purely by way of example only and any of the high resistivity formulations known in the art may be used. 
     In the resultant tire, the properties of the silica reinforced tire exist in full measure giving wear, wet grip, ice grip etc according to the silica compound formulation chosen. The disadvantage of silica compounds, however, in having too high a volume resistivity whereby static charge may be maintained on the vehicle so causing the problems mentioned above is overcome by the conducting paths of the low volume resistivity material provided in the tire tread strip. 
     A most important advantage of the construction of the present invention however is that the strips of low resistivity material comprise extrudable sections and the tire tread strip can be manufactured using conventional extrusion techniques bringing the various components together to form a handlable tire tread strip. The tire tread strip may be spooled and used in continuous lengths or cut to the required length for each tire to be manufactured.