Tyre treads and tyres incorporating them

Abstract: A tyre tread comprising an elongated strip of tyre tread compound of high resistivity having a transverse width (TR) characterised by within the transverse width (TR) a longitudinally extending conducting strip (3,6,7) of low resistivity tyre compound with a volume resistivity less than 10 8 ohm cm said conducting strip extending from the top to the bottom surface of the tread strip.

Description: This invention relates to tyres having treads and in particular treads which are electrically conducting including those reinforced by Silica. 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 tyres, which requires that the tyres themselves have an acceptable level of conductivity - commonly described as anti-static. Such levels of conductivity are normally guaranteed by the use of tyre compounds containing significant proportions of carbon black as reinforcing filler. Carbon black reduces the resistivity of the intrinsically nonconductive 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 50phr or more carbon black in the compound. Such loadings are typical of carbon black-reinforced tyres, which are accordingly anti-static. If non-black fillers (e.g. silica) are used, or if the carbon black loading is significantly less than 50phr, 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 tyre, 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. 10-30phr, to be significantly effective, and since their other properties are not suited to tyre applications, road performance may deteriorate. A tyre according to the preamble of Claim 1 is known from GB-A-544 757. According to one aspect of the present invention a tyre comprises an elongated main tread strip of high resistivity tread compound having a transverse width TR characterised by a sub-tread disposed in contact with the radially inner surface of the main tread strip and two longitudinally extending conducting strip of low resistivity tyre compound having a volume resistivity of less than 10 8 ohm-cm disposed one at each axial edge of the transverse width TR of the main tread strip, the conducting strips extending from the radially outer surface of the main tread strip to its radially inner surface to contact the sub-tread, the sub-tread extending axially over the whole transverse width TR of the main tread strip. By high resistivity is meant tyre tread compounds having a volume resistivity greater than 10 8 ohm cm. Such compounds include for example tyre tread compounds having a silica filler in place of carbon black of which the volume of resistivity is typically 10 13 ohm-cm. The conducting strips may conveniently be the tread strip wing component which in the finished tyre form the tyre tread shoulders. This construction is particularly convenient because the tyre tread wings are usually extruded of a different material to the main part of the tyre tread strip and thus no additional extruded component is necessary. The width of each of the conducting strip is preferably in the range of 5-25% of the width of the tyre tread TR. The compound of the conducting strip more preferably has a volume resistivity of less than 10 6 ohm cm. Further aspects of the present invention will become apparent from the following description, by way of example only, of one embodiment in conjunction with the attached diagrammatic drawings in which: Figure 1 shows a tyre tread strip according to the present invention. In the embodiment shown in Figure 1 high resistivity silica compound forms the center region of the tyre tread strip. A sub-tread 2 is provided and the wing portions 6 and 7 of the tyre tread strip comprise low resistivity materials. The longitudinally extending conducting wing portions 6 and 7 comprise 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%, oil 27.3% 96 BR, 96% 1,4-cls 30 N234 Carbon Black 80 Silica Ultrasil VN3 - TESPT/N330 1:1 (X50-S) - Zinc Oxide 3 Stearic acid 2 Aromatic oil 10 CBS 1.5 DPG - Sulfur 1.5 The center region is of a rubber compound reinforced by silica having a volume of resistivity approximately 10 13 ohm-cm. The formulation of the side region tread compound is as follows. SBR 1712, styrene 23.5%, oil 27.3% 96 BR, 96% 1,4-cls 30 N234 Carbon Black - Silica Ultrasil VN3 80 TESPT/N330 1:1 (X50-S) 12.8 Zinc Oxide 3 Stearic acid 2 Aromatic oil 10 CBS 1.5 DPG 2 Sulfur 1.5 The tyre tread strip is fitted to tyre a carcass on top of a tread reinforcing breaker in the normal way and the tyre is vulcanised in a tyre curing press where the tyre tread pattern is formed. 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 tyre the properties of the silica reinforced tyre 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 tyre 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 tyre tread strip can be manufactured using conventional extrusion techniques bringing the various components together to form a handlable tyre tread strip. The tyre tread strip may be spooled and used in continuous lengths or cut to the required length for each tyre to be manufactured.

Claims: A tyre comprising an elongated main tread strip (1) of high resistivity tread compound having a transverse width TR characterised by a sub-tread (2) disposed in contact with the radially inner surface of the main tread strip (1) and two longitudinally extending conducting strips (6,7) of low resistivity tyre compound having a volume resistivity of less than 10 8 ohm-cm disposed one at each axial edge of the transverse width of the main tread strip (1), the conducting strips extending from the radially outer surface of the main tread strip (1) to its radially inner surface to contact the sub-tread (2), the sub-tread (2) extending axially over the whole transverse width TR of the main tread strip (1). A tyre according to claim 1, characterised in that the conducting strips (6,7) are the tread wings which form the tyre tread shoulders after tyre manufacture. A tyre according to either of claims 1 or 2, characterised in that the width of each conducting strip (6,7) is in the range of 5 to 25% of the width of the tyre tread. A tyre according to any of claims 1 to 3, characterised in that each of the conducting strips (6,7) has a longitudinally extending tread groove formed therein. A tyre according to any of claims 1 to 4, characterised in that each of the conducting strips (6,7) has a width greater than the width of the widest longitudinally extending tread groove. A tyre according to any of claims 1 to 5, characterised in that the volume resistivity of the compound of the conducting strips (6,7) is less than 10 6 ohm cm. A tyre according to any of claims 1 to 6, characterised in that the high resistivity compound comprises silica as the major reinforcement or filler material.

Inventors: POWELL BRIAN DAVID WILLIAM, POWELL, BRIAN DAVID WILLIAM
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES, LTD.
Application year: 1995
Main classification: B60C19
IPC codes: B60C19