A method for reducing the working temperature of a tire tread for vehicles and some tire treads capable of performing said method

Abstract: A method for reducing the working temperature of a tire tread (10) for vehicles utilizing a tire tread (10) consisting of a first portion (A), comprising 100 parts by weight of an elastomeric material, 40-120 parts by weight of a filler, comprising from 50 to 100% by weight of carbon black and from 0 to 50% of silica, and 3-40 parts by weight of at least a conventional additive, and a second portion (B), comprising 100 parts by weight of an elastomeric material, 40-120 parts by weight of a filler, comprising from 30 to 100% by weight of silica and from 0 to 70% of carbon black, and 3-40 parts by weight of at least a conventional additive provided, however, that the silica content in portion (B) is at least 20% higher than in portion (A). The tires where the area of said portion A exceeds 37,1% of the total area of the tire tread are novel and are a further object of the invention.

Description: The present invention relates to a method for making a vehicle tire having a tread showing a reduced working temperature in operation over routes and some tire made according to said method. More in particular, the present invention relates to a method for reducing the working temperature of a tire tread for vehicles utilizing a tire tread consisting of a first and a second portion made of an elastomeric mixture comprising a filler, wherein the filler present in said first portion is mainly composed of carbon black while a substantial part of the filler present in said second portion is composed of silica. Modern tires are required to show not only a good performance when running on any type of surface, dry, wet, snowy, but also a good qualitative level referred to other functional features such as, for example, comfort, smoothness and mileage, and these features have to be maintained even when the tire runs under critical or extreme conditions. For example, a too high speed compared to the type of road, especially when combined with a high room temperature. The performance of a tire depends on circumferential and/or transversal grooves and notches in the tread that form seams and/or blocks, variously arranged so as to form a specific tread pattern designed in view of the type of use expected for the tire. Typically, a tread pattern is said to be symmetrical when its outlook is the same irrespective from the rolling direction of the tire. By converse, it is said to be asymmetrical when its outlook changes depending on the rolling direction. Furthermore, a tread pattern is said to be directional when it identifies a particular, preferred direction of rotation of the tire, that is when it is specular with respect to the equatorial plane of the same. Furthermore the performance of a tire is affected to a considerable extent by the working temperature. One of the most difficult problems is to reconcile a good resistance to wear with an adequate road-holding both at normal (i.e. 30°C - 70°C, generally 30-60°C, typically 30-50°C) and high (i.e. more than 70°C, and even more than 100°C in case of a very fast car) working temperatures, the latter being caused by the high thermomechanical stresses produced by driving in extreme conditions. In fact, in order to achieve a good resistance to wear and low resistance to rolling it is necessary to use elastomeric mixtures showing a low level of working temperature. By converse, in order to achieve a good road-holding, it is necessary to use elastomeric mixtures having a high level of working temperature thereby dissipating a high amount of energy and ensuring high adherence of the tread to the ground. Hence, the theoretical models for an optimal tread require conflicting working temperature levels. So far, the attempts made in the art to improve the performance of a tire in a temperature range other than that of normal usage (i.e. of from about 10 to about 40°C in winter season and of from about 30 to about 60°C in summer season) resulted either in a substantial and undesired loss of road-holding and/or in a substantial worsening of the resistance to abrasion of the tread. Now, it has been unexpectedly found a valuable solution to the above mentioned problem. Initially the applicant faced the problem of producing tires having some colored portions, and more particularly tires whose tread includes at least one colored insert. In fact, a two-colored tire, in addition to exhibiting a pleasant and unusual aesthetic appearance, allowing for example the customization of a vehicle with preferred colors, would also have some practical advantages. For example, a first advantage would be to make the correct assembly of a tire easier on the vehicle in case of symmetrical tires. A second advantage, would be to allow an immediate identification of each type of many tires stored in a warehouse. The applicant departed from the consideration that the production of colored elastomeric mixtures precludes the addition, even in a minimal proportion, of carbon black as a filler for the elastomeric mixture since the coloring power of carbon black is so high that it conceal any other colored pigment. Only the so-called white charges can, in fact, be colored with suitable pigments. Typical white charges for use in tires, in particular in the tread, are made of silicon compounds, preferably silica. However, silica is known to provide an elastomeric mixture for a tire having a low resistance to rolling and a good road-holding on snowy and wet roads while reduces both road-holding on dry roads in highly severe conditions and resistance to wear. Hence, the use of silica as a filler for the elastomeric mixture in tire tread implies careful evaluations. By examining the road test results from a number of tire prototypes having the tread made of two different circumferential and axially side-by-side portions, the first made of an elastomeric mixture charged mainly with carbon black (black elastomeric mixture) the second made of an elastomeric mixture charged only with a white charge (white elastomeric mixture), the inventors have unexpectedly perceived that the performances on a dry road of this tire are superior compared to both a first comparison tire wherein the very same tread is made from the very same elastomeric mixture charged with carbon black only and a second comparison tire wherein the very same tread is made from the very same elastomeric mixture charged with silica only. In order to understand the reason for this unexpected and inexplicable effect, the applicant has also performed thermographies on the tread of the tires under test and has unexpectedly found that the two-colored tires always ran at a colder temperature than the comparison black tires. Even more unexpectedly, also the black portion per se of said prototypes leant to work at a lower temperature compared to the corresponding tread portion in the comparison black tires. Without intending to be bound in any way by this theory, it has been assumed that the portion of white elastomeric mixture develops a synergic action that keeps cooler than usual the adjacent portion of black elastomeric mixture as well, thereby reducing the working temperature of all the tire in its entirety. US-A-5 225 011 discloses a tire having superior cut resistance and less belt separation comprising a tread with steel cords belts in which a center rubber for middle part of the tread and a side rubber for both sides thereof are respectively formed of different rubber composition, and in which at least an inner portion of the center rubber is composed of a rubber composition of superior cut resistance wherein either a natural rubber of a SBR blended natural rubber is compounded with carbon black of large iodine absorption number, silica and silane coupling agent expressed by a formula of (OR) 3 Si CnH 2 nSmCn H 2 nSi(OR) 3 or (OR) 3 Si CnH 2 nX, while the side rubber is composed of a rubber composition having small heat build up. This document does not disclose a tire having only two circumferential tread bands wherein one tread band has only silica as a filler and is placed on the sidewall of the tire facing the vehicle. Thus, said tire does not show a reduced working temperature in operation over routes and concurrently improved performances on the road under extreme conditions. EP-A-0 658 452 discloses a tire wherein the tread band has a low content of carbon black and a high content of silicon (or other non conductive fillers) thereby being antistatic in nature. In order to increase the electric conductivity thereof, said tire further comprises at least one conductive insert made of an elastomeric material incorporated in the tread band and extending through the whole tread band thickness, being then exposed externally of the rolling surface, wherein the elastomeric material forming said conductive insert exhibits an electric resistivity lower than that of the elastomeric material forming the tread band. When said inserts are two or more their overall width measured parallel of the tire axis is of from 4 to 50 mm. Since the total width of a tread band ranges from about 135 mm to about 285 mm, the maximum width of the conductive inserts is of 37% of the width of the tread band. Similarly, EP-A-0 681 931 discloses a tire tread which is electrically conducting. More particularly, this document teaches to make an electrically conducting tire tread comprising an elongated 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, said conducting strip extending from the top to the bottom surface of the tread strip. In a specific embodiment, the high resistivity compound comprises silica as reinforcement or filler material while the low resistivity compound comprises carbon black as reinforcing filler. The width of the conducting strip is preferably in the range of 5-25% of the width of the tire tread (TR). The working temperature of said tire tread is never mentioned. In a first aspect, this invention refers to a method for making a vehicle tire, as defined in claim 1. Typically, the amount of filler in said first portion (A) is of from 60 to 80 parts by weight. Preferably, the percentage of carbon black in said filler of said first portion (A) is of from 80 to 100% by weight. Typically, the amount of filler in said second portion (B) is of from 60 to 80 parts by weight. Since the percentage of silica in said filler of said second portion (B) is of 100% by weight, the elastomeric mixture of portion (B) preferably comprises also at least one coloring pigment selected from the well-known pigments usually added to the elastomeric mixtures. Typically, said elastomeric material is selected from natural rubber, 1,4- cis polybutadiene, polychloroprene, 1,4- cis polyisoprene, optionally halogenated isoprene/isobutene copolymers, butadiene-acrylonitrile rubber, styrene-butadiene rubber, styrene-butadiene-isoprene terpolymers prepared either by solution or emulsion polymerization, ethylene-propylene-diene terpolymers, and mixtures thereof. Preferably, said conventional additives are selected from vulcanizers, vulcanization accelerating agents, ZnO, stearic acid, antioxidants, plasticizers, antiaging agents, antifatigue agents, gypsum, talc, kaolin, bentonite, TiO 2 , silicates, colored pigments, binding agents capable of chemically reacting with silica in order to bind the latter to the elastomeric mixture during vulcanization, and mixtures thereof. A typical example of binding agent is the silanic compound Si69 by Degussa, Germany. For the purpose of this invention, the expression "total area of the tire tread" is intended to mean the circumferential portion of the tire whose width (L in Figg. 1,3 and 5, circle arch C-C in Fig. 2) is delimited by the two lines running where the tread intersects the two sidewalls of the tire. In a second aspect, this invention refers to a tire for vehicles as defined in claim 7. Preferably, the area of said portion A is of from 40% to 70%, and even more preferably of from 50% to 60% of the total area of the tire tread. The invention will now become more evident from the following description and the enclosed drawings, having the aim of describing and showing some preferred embodiments without, however, limiting it in any way. Fig. 1 is an axial cross-section of a tire of this invention for a four-wheeled vehicle; Fig. 2 is a front view of a tire of this invention of the asymmetrical type; Fig. 3 shows thermographic images of a tire tread of this invention compared to that of a conventional tire tread having the very same tread pattern, of the asymmetrical type, after having worked under the same conditions. Fig. 1 shows the structure of a tire of this invention for a four-wheeled vehicle assembled on a rim. Such structure is conventional in nature except for the composition of the tread as described hereinbelow. The tire 1 comprises a carcass 2 having a resistant structure formed by one ply 3 of a rubber-coated fabric comprising reinforcing cords, made of textile or metal thread, sunk in the elastomeric material of the fabric, having its ends 3a each turned down round an anchoring bead wire 4, the latter being provided on its radially external surface with a rubber element 5. Preferably the turned down flaps 3a of the plies of the carcass go back up radially towards the outside along at least part of the sidewall of the above mentioned rubber element. As is known, the area of the tire comprising the bead wire 4 and the element 5, that is the radially internal part of the sidewall of the tire, forms the bead, aimed to anchor the tire to a corresponding assembly rim 20. On the carcass, preferably of the radial type, that is with the reinforcing cords lying in planes containing the axis of rotation of the tire, there is arranged in a known manner a tread 10, provided with a pattern in relief for the rolling contact of the tire on the ground. The width L of such tread is identified by the distance between the points of intersection of the curvatures of the tread and of the sidewalls. Such tread pattern comprises a plurality of seams and/or blocks 11, 12 separated one from the other by corresponding notches and grooves 13, 14, directed both circumferentially and transversally, said seams and/or blocks may be provided with various slits and cuts. The combination of said structural elements may generate many different configurations well-known to the persons skilled in the art. The tire 1 can also comprise a belt-like structure 6 arranged in crown to the carcass 2, interposed between carcass and tread, and extended from one sidewall to the other of the tire, that is substantially as wide as the tread. Said belt-like structure 6 comprises two radially superimposed layers 7, 8 of reinforcing cords, preferably metallic, wherein said reinforcing cords are parallel to one another in each layer and cross those of the adjacent layer with respect to the equatorial plane of the tire. Preferably, said belt-like structure 6 further comprises an additional external layer 9 of reinforcing cords, preferably made of textile threads, and even more preferably of heat-shrinking material (nylon), oriented at 0°, that is in the circumferential direction. The tread of tire 1 comprises axially adjacent, circumferential portions: a first portion A and a second portion B, respectively. A suitable elastomeric mixture for portion A has the following composition. The amounts are given in parts by weight. Components Amounts - elastomeric material 100.0 - carbon black 68.0 - ZnO 2.0 - stearic acid 1.0 - antioxidants 2.5 - antifatigue agents 1.0 - plasticizers 15.0 - sulphur 1.5 - vulcanization accelerating agents 1.8 A suitable elastomeric mixture for portion B has the following composition. The amounts are given in parts by weight. Components Amounts - elastomeric material 100.0 - silica 70.0 - ZnO 2.0 - stearic acid 1.0 - antioxidants 2.5 - antifatigue agents 1.0 - plasticizers 15.0 - sulphur 1.2 - vulcanization accelerating agents 2.5 Since the second portion B is more sensitive to wear and more prone to cause loss of road-holding on dry road, in those tires which are not made to work in extreme conditions said second portion B is preferably arranged on the internal tread side facing with the vehicle (hereinafter identified as B/A combination), thereby minimizing the stress of portion B on a bend. Fig. 2 is a front view of a tire provided with a tread of this invention stamped according to a pattern of the asymmetrical type. The tire has a first side-wall I opposite to a second side-wall E. This tire is assembled on the vehicle with the side-wall I facing with the vehicle and the side-wall E towards the outside. In these kinds of pattern, portion B (shaded) will preferably be arranged on a lateral position of the pattern and, even more preferably, by the side-wall I (B/A combination). The working temperature of the tire of Fig. 2 has been compared by thermography with the working temperature of an identical tire (size: 175/70 R 13) except that the entire tread thereof was made of composition A only . Such comparison tire is available from Pirelli under the trademark "P200 CHRONO ENERGY ™". The elastomeric mixtures of the portions A and B had the following compositions where the amounts of the components are given in parts by weight: Amounts Components Portion A Portion B - SBR 1712 80 - SBR 1500 20 - OE-SSBR 80 - BR 33 - Carbon black N 220 45 - silica VN3 by Degussa 20 70 - binding agent 1.6 5.6 - ZnO 2.5 2.5 - stearic acid 2 2 - antioxidants 1.5 1.5 - antifatigue agents 1 1 - plasticizers 10 5 - sulphur 1 1.4 - vulcanization accelerating agents 2.3 3.8 The tires under evaluation were driven in rotation by friction against a drum rotating at a pre-established speed. Such drum had diameter of 1,700 mm and was driven at a speed of 120 km/h. The tires were set on a proper rim of the type J 5.5, inflated to a pressure of 2.2 bar and loaded with 450 kg which corresponds to the nominal working load, according to "E.T.R.T.O. - European Tire and Rim Technical Organization" specifications. The inner temperature of the tire of this invention stabilized at 53°C after 60 minutes while in the comparison tire stabilized at 58°C after 60 minutes. When the temperature was stabilized, the treads were photographed under infrared light while the tire was kept still in rotation. It resulted an image of the tire tread suitably reproducible in color or in black and white wherein the different colors or the different shades of grey identify, point by point, the different temperatures of the tread both at the surface and at the notches bottom. In the chromatic scale selected by the applicant the color ranges from blue to red, passing through light blue, green and yellow, as the temperature rises from 25°C to 65°C. Fig. 3 shows the thermographic images in color of the tread of Fig. 2 compared to the above mentioned comparison tire. The image on the right relates to the tire tread of this invention while that on the left relates to the comparison tire. Fig. 3 shows that the light blue portion, colder than the green one, spans an appreciably larger area in the tire of the invention than in the comparison tire. In fact, in the tire of this invention such light blue portion spans also the left-hand shoulder made of elastomeric mixture A. Additionally, the tire of Fig. 2, according to this invention, has been compared with the above mentioned comparison tire in several driving tests on the road. The results are shown in the following table. Performance Tire 1 2 Straight run Directional stability 6.5 6.5 Steering rigidity 6.5 6.5 Fast run Empty on the center 6.0 6.5 Steering response speed 6.5 7.0 Steering response progressivity 5.5 6.0 Directional stability on a bend 6.5 6.5 Realignment 5.5 6.5 Extreme conditions Yield 7.0 7.0 Release on bends 6.5 7.0 Controllability 7.0 7.0 1 is the comparison tire (P 200 CHRONO ENERGY™). 2 is the tire of the invention. The tires under evaluation were assembled on a VW Polo 1.4 car. The performance tests were carried out on a mixed run including a part on a normal road and a part on a track, having a dry surface, at a room temperature of 14°C. The marks, on a scale of from 1 to 10, reflect the opinion of the test driver. As higher is the mark, the better is the result. The table shows that the performance of the tire of the invention is substantially the same as that of the comparison tire in a normal straight run. That is, when the elastomeric mixture of the tread does not undergo a substantial stress. By converse, the performance of the tire of the invention is unexpectedly substantially better as the severity of run increases.

Claims: A method for making a vehicle tire having a tread (10) showing a reduced working temperature in operation over routes, comprising: making a first circumferential portion (A) of said tread (10) comprising: 100 parts by weight of an elastomeric material, 40-120 parts by weight of a filler comprising from 50 to 100% by weight of carbon black and from 0 to 50% of silica, and 3-40 parts by weight of at least a conventional additive; characterized in that it further comprises making a second circumferential portion (B) of said tread (10) comprising: 100 parts by weight of an elastomeric material, 40-120 parts by weight of a filler comprising 100% by weight of silica, and 3-40 parts by weight of at least a conventional additive, arranging said first and second circumferential portions axially side-by-side, positioning said first circumferential portion to encompass a first outside edge of said tread and said second circumferential portion to encompass a second outside edge of said tread, and arranging said second circumferential portion on a sidewall facing the vehicle. A method according to claim 1, wherein the amount of filler in said first portion (A) is of from 60 to 80 parts by weight. A method according to claim 1 or 2, wherein the percentage of carbon black in said filler of said first portion (A) is of from 80 to 100% by weight. A method according to any of claims 1 to 3, wherein the amount of filler in said second portion (B) is of from 60 to 80 parts by weight. A method according to any of claims 1 to 4, wherein said elastomeric material is selected from natural rubber, 1,4- cis polybutadiene, polychloroprene, 1,4- cis polyisoprene, optionally halogenated isoprene/isobutene copolymers, butadiene-acrylonitrile rubber, styrene-butadiene rubber, styrene-butadiene-isoprene terpolymers prepared either by solution or emulsion polymerization, ethylene-propylene-diene terpolymers, and mixtures thereof. A method according to any of claims 1 to 5, wherein said conventional additives are selected from vulcanizers, vulcanization accelerating agents, ZnO, stearic acid, antioxidants, plasticizers, antiaging agents, antifatigue agents, binding agents, gypsum, talc, kaolin, bentonite, TiO 2 , silicates, colored pigments, and mixtures thereof: A tire for vehicles having a tread (10) consisting of: a first circumferential portion (A) comprising: 100 parts by weight of an elastomeric material, 40-120 parts by weight of a filler comprising from 50 to 100% by weight of carbon black and from 0 to 50% of silica, and 3-40 parts by weight of at least a conventional additive; characterized in that a second circumferential portion (B) of said tread (10) comprises: 100 parts by weight of an elastomeric material, 40-120 parts by weight of a filler comprising 100% by weight of silica, and 3-40 parts by weight of at least a conventional additive, the area of said portion A is of from 37,1 % to 80% of the total area of the tire tread, said first and second circumferential portions are axially side-by-side, said first circumferential portion encompasses a first outside edge of said tread and said second circumferential portion encompasses a second outside edge of said tread, and said second circumferential portion is mounted on a sidewall facing the vehicle. A tire according to claim 7, characterized in that the area of said portion A is of from 40% to 70% of the total area of the tire tread. A tire according to claim 7, characterized in that the area of said portion A is of from 50% to 60% of the total area of the tire tread. A tire according to any claim from 7 to 9, characterized in that the amount of filler in said first portion (A) is of from 60 to 80 parts by weight. A tire according to any claim from 7 to 10, characterized in that the percentage of carbon black in said filler of said first portion (A) is of from 80 to 100% by weight. A tire according to any claim from 7 to 11, characterized in that the amount of filler in said second portion (B) is of from 60 to 80 parts by weight. A tire according to claim 7, characterized in that the elastomeric mixture of said second portion (B) further comprises at least one coloring pigment. A tire according to any claim from 7 to 13, characterized in that said elastomeric material is selected from natural rubber, 1,4- cis polybutadiene, polychloroprene, 1,4- cis polyisoprene, optionally halogenated isoprene/isobutene copolymers, butadiene-acrylonitrile rubber, styrene-butadiene rubber, styrene-butadiene-isoprene terpolymers prepared either by solution or emulsion polymerization, ethylene-propylene-diene terpolymers, and mixtures thereof. A tire according to any claim from 7 to 13, characterized in that said conventional additives are selected from vulcanizers, vulcanization accelerating agents, ZnO, stearic acid, antioxidants, plasticizers, antiaging agents, antifatigue agents, binding agents, gypsum, talc, kaolin, bentonite, TiO 2 , silicates, colored pigments, and mixtures thereof.

Inventors: CARETTA RENATO, NAHMIAS NANNI MARCO, CARETTA, RENATO, NAHMIAS NANNI, MARCO
Applicants: PIRELLI, PIRELLI PNEUMATICI SOCIETA' PER AZIONI
Application year: 1997
Main classification: B60C11
IPC codes: B60C11, B29D30, B29K503, B60C1, B29K21, B29K507