Procedure for producing a multi-compound tread for road vehicle tires

[0001] The present invention relates to a process for the manufacture of a multi-compound tread for pneumatic tires for road vehicles.

[0002] In the field of pneumatic tires for road vehicles, it is known that low rolling resistance and good wet grip performance are characteristics that are difficult to reconcile within the same tread. In fact, while in order to confer good rolling resistance to a pneumatic tire treads that are manufactured with compounds with a low hysteresis loss are used, in order to confer good wet grip performance to a pneumatic tire compounds having a relatively higher hysteresis loss become necessary.

[0003] Consequently, an optimal tread compound for simultaneously enhancing all dynamic responses should exhibit opposing hysteresis characteristics, at least over certain required performance limits.

[0004] It is known that the properties of the aforementioned rubber compound influence not only the balance between wet grip performance/rolling resistance, but also other performance of the pneumatic tire, such as the balance between winter/wet grip performance.

[0005] As is known, the tread is composed of a plurality of blocks, which are elements that are delimited by a plurality of grooves that are formed within the tread. These grooves may have differing dimensions depending upon the function that they must fulfill. For example, when the grooves are small in size they are called blades and are intended to provide greater grip on snow and ice.

[0006] In other words, a tread block is a rubber filled portion bounded by a plurality of grooves, independently of whatever the dimensions of the latter may be.

[0007] The inventors of the present invention have found that, if the blocks of the tread are composed of different portions manufactured with respective compounds having differing hysteresis losses, it is possible to obtain a synergistic effect that is capable of imparting significant improvements in terms of wet grip performance, without thereby adversely affect the rolling resistance.

[0008] The inventors of the present invention have implemented a process and an associated plant in order to be able to manufacture, in an effective and productive manner, a tread comprising blocks composed of different portions having differing hysteresis losses.

[0009] The object of the present invention is a process for the manufacture of a tread band, wherein, the blocks thereof comprise different portions having differing hysteresis losses; said process being characterized in that it comprises a step for the extrusion of a preliminary tread band consisting of a first rubber compound and an injection step, wherein the preliminary tread band, once extruded, is subjected to the action of at least one injector arranged such as to penetrate into the layer of the preliminary tread band and to discharge a second rubber compound, which is distributed within the layer consisting of said first compound, maintaining exposed a surface that is suitable for defining a portion of the ground-contacting surface of the resulting tread band; said first and said second compounds having different dynamic properties in terms of: dynamic modulus at 30° C., tan δ at 0° C., tan δ at 30° C. and tan δ at 60° C., said first compound has a dynamic modulus at 30° C. of between 11.0 and 17.0 MPa, a tan δ at 0° C. of between 0.85 and 1.1, a tan δ at 30° C. of between 0.45 and 0.65 and a tan δ at 60° C. of between 0.19 and 0.30; said second compound having a dynamic modulus at 30° C. of between 5.5 and 15 MPa and at least 2 MPa less than that of said first compound, a tan δ at 0° C. of between 0.40 and 0.99 and at least 0.02 less than that of said first compound, a tan δ at 30° C. of between 0.21 and 0.63 and at least 0.02 less than that of said first compound, a tan δ at 60° C. of between 0.10 and 0.28 and at least 0.02 less than that of said first compound.

[0010] Preferably, said extrusion step is continuous and said injection step is discontinuous.

[0011] Preferably, said process comprises an interconnection step, wherein said preliminary tread band, which is produced continuously, is stopped at intervals in order to then be recovered and subjected to said injection step.

[0012] Preferably, said first compound having a dynamic modulus at 30° C. of between 11.0 and 17.0 MPa, a tan δ at 0° C. of between 0.85 and 1.1, a tan δ at 30° C. of between 0.45 and 0.65 and a tan δ at 60° C. of between 0.19 and 0.30, said second compound having a dynamic modulus at 30° C. lower than that of said first compound with a value of between 7.5 and 10.5 MPa, a tan δ at 0° C. lower than that of said first compound with a value of between 0.70 and 0.84, a tan δ at 30° C. lower than that of said first compound with a value of between 0.21 and 0.42 a tan δ at 60° C. lower than that of said first compound with a value of between 0.10 and 0.17.

[0013] The dynamic properties were measured in accordance with the ISO 4664 standard. As is known to a person skilled in the art, the tan δ values at 60° C. are strictly correlated to the property of rolling resistance: the lower the tan δ value at 60° C., the better the rolling resistance.

[0014] A further object of the present invention is a plant for the manufacture of a tread band wherein the blocks thereof comprise different portions having differing hysteresis losses; said plant being characterized in that it comprises an extrusion station wherein a preliminary tread band is formed, an injection station wherein, within said preliminary tread band portions of a second rubber compound are injected, and movement means that are suitable for moving said preliminary tread band from said extrusion station to said injection station; said extrusion station comprising at least one extruder and at least supply means that are suitable for supplying said extruder with a first rubber compound; said injection station comprising a dispenser (10) of said second rubber compound and a plurality of injectors (11) connected to said dispenser (10) for receiving a determined quantity of said second rubber compound to be injected within said preliminary tread band.

[0015] Preferably, said plant comprises an interconnection station, wherein said preliminary tread band is stopped at intervals in order to then be recovered and transported to said injection station.

[0018] The plant 1 comprises an extrusion station 2, an interconnection station 3, an injection station 4 and movement means 5 responsible for the advancement of a preliminary tread band from the interconnection station 3 to the injection station 4.

[0019] The extrusion station 2 comprises a hopper 6, whereinto a first rubber compound is loaded, and an extruder 7 that is capable of producing, with the first rubber compound, a preliminary tread band 8.

[0020] The interconnection station 3 comprises two rollers 9 arranged such as to rotate idling and wherebetween a loop is implemented that is variable according to the action of the movement means 5.

[0021] The injection station 4 comprises a dispenser 10, connected to a supply line 10a of a second rubber compound and a plurality of injectors 11, wherein each thereof extends from said dispenser 10, from which it receives a determined quantity of the second rubber compound. Each of the injectors 11 has a free dispensing end that is capable of penetrating into the layer of the preliminary tread band and releasing within the layer itself the quantity of the second rubber compound.

[0022] The movement means 5 comprise a plurality of rollers 12 that are moved by actuator means (known and not described in detail). In essence, the rollers 12 function as a conveyor belt.

[0023] The first and second compounds have differing dynamic properties in terms of: dynamic modulus at 30° C., tan δ at 0° C., tan δ at 30° C. and tan δ at 60° C.

[0024] In particular, the first compound has a dynamic modulus at 30° C. of between 11.0 and 17.0 MPa, a tan δ at 0° C. of between 0.85 and 1.1, a tan δ at 30° C. of between 0.45 and 0.65 and a tan δ at 60° C. of between 0.19 and 0.30; whilst the second compound has a dynamic modulus at 30° C. lower than that of the first compound with a value of between 7.5 and 10.5 MPa, a tan δ at 0° C. lower than that of the first compound with a value of between 0.70 and 0.84, a tan δ at 30° C. lower than that of the first compound with a value of between 0.21 and 0.42 a tan δ at 60° C. lower than that of the first compound with a value of between 0.10 and 0.17.

[0025] As may appear obvious from the description above, the process and plant of the present invention ensure the effective preparation and high productivity of tread bands, wherein the blocks thereof are composed of parts having differing hysteresis losses.

[0026] In this way it will be possible to produce a tread band wherein the technical characteristics thereof are such as to ensure significant improvement effects in terms of wet grip performance, maintaining, at the same time, the advantages obtained in terms of rolling resistance. In particular, the tread band manufactured with the process and plant of the present invention relates to a kind of “multi-pin tread” for winter/all seasons applications. In fact, those elements that are manufactured using the second rubber compound (a stiffer compound) are incorporated into the preliminary tread band, manufactured using the first rubber compound (a softer compound), and act as studs when on roads with ice and snow.