Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1998-09-24
2001-11-06
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S492000, C264S211180, C264S211210, C264S211220, C264S211240
Reexamination Certificate
active
06313212
ABSTRACT:
The present invention relates to a continuous process for producing a rubber compound which can be hot-crosslinked with sulphur, comprising a polymer base containing a crosslinkable unsaturated chain, supplemented with at least one silica filler and a silica-binding agent containing at least one sulphur atom; more specifically, the invention relates to a process for producing semi-finished products formed from the said compound, in particular tread bands, as well as to tires which can be obtained from the said semi-finished products.
In the following description and in the subsequent claims, the expression polymer base containing a crosslinkable unsaturated chain is intended to refer to any natural or synthetic, non-crosslinked polymer which can acquire all the physico-chemical and mechanical properties typical of elastomers after crosslinking (vulcanization) with sulphur-based systems.
The semi-finished products for tires to which the invention relates are formed of compounds of varying chemical composition, which are appropriately selected in order to obtain, after vulcanization, tires with particular properties and performance levels.
The said semi-finished products are, in particular, tire side walls and tread bands.
In certain applications, semi-finished products for tires with rubber reinforced by so-called “white” fillers are required; purely as a guide, it is pointed out here that these are inorganic-type reinforcing fillers such as chalk, talc, kaolin, bentonite, titanium dioxide, silicates of various types and silica, which are referred to hereinbelow as silica fillers for simplicity. In particular, it is known to use tread rubbers which comprise reinforcing silica fillers in order to reduce the tire's resistance to rolling.
Since silica has little affinity for the polymer base, it is necessary to add a silica-binding agent, advantageously consisting of a silane, which is capable of chemically attaching the silica to the polymer matrix during vulcanization of the compound: the abovementioned chemical attachment is optimally achieved when the silica and the silane are incorporated into the compound together.
However, the need to incorporate the silica and the silane together into the compound places a constraint on the maximum temperature which can be reached during the mechanical processing of the compound by mixing: the temperature needs to be carefully maintained below 165° C., otherwise irreversible thermal degradation of the binding agent takes place.
Unfortunately, in complying with this temperature constraint, there is a substantial reduction precisely in this mechanical mixing action, which is essential for optimum dispersion of the silica in the polymer matrix.
The resulting insufficient dispersion of the silica in the compound in turn gives rise to a whole range of drawbacks essentially related to large variability and non-uniformity of the physico-mechanical properties of the mixture between one region and another.
Moreover, incorporation of the vulcanizing system into the compound also requires that a temperature of 110° C. is not exceeded during processing, and since each mechanical processing operation involves a gradual increase in the temperature of the compound, the abovementioned compounds are produced by the so-called “batchwise” system, that is to say in non-continuous amounts, of about 200-300 kilograms for each load, so as to be able to stop the processing cycle in order to carry out alternating cooling cycles during manufacture of the compound.
A general process for the mechanical processing of a rubber compound with silica and silane is the subject of the publication “Silica based tread compounds: Background and performances”, page 14, table IV, published by Degussa on the occasion of the Tyretech '93 conference which was held in Basilea on Oct. 28-29 1993.
According to that process, the silica and silane are added to the rubber compound at the same time, while keeping the processing temperature below 160-165° C. in order to avoid premature crosslinking of the silane from taking place by exceeding this temperature.
From U.S. Pat. No. 5,227,425, a process is also known for the production of tread bands which are obtained by mixing a polymer base formed of a diene conjugated with a vinylaromatic compound having a vinyl group content of between 5% and 50%, this polymer base having a high silica content and containing a silane.
The base polymer and the silica undergo mechanical processing in a mixer or extruder until a minimum temperature of 130° C. but one not exceeding 180° C., and preferably between 145° C. and 180° C. is reached. According to another example, the polymer and the silica undergo mechanical processing in two distinct phases separated from each other by an intermediate cooling phase. In the first phase, the base polymer, the silica and crosslinking agent undergo mechanical processing until a temperature above 145° C., preferably between 145 and 170° C., is reached.
The blend obtained is cooled to a temperature below 100° C., preferably about 60° C., and, in a second phase, it then undergoes mechanical processing in a Banbury mixer (internal mixer) together with other ingredients, but not the vulcanization system, until a temperature of between 145 and 170° C. is again reached.
Next, the blend thus obtained is again cooled to a temperature below 100° C., preferably about 60° C., after which the vulcanization system is added to the blend by means of a final mechanical processing operation in a two-cylinder mixer (external mixer), while keeping the temperature at a value below 100° C.
Italian patent application No. 95IT-MI000359A, from the same Applicant, which is aimed at improving the dispersion of the silica in the base polymer, describes a process in which, firstly, the base polymer is mixed with the silica in a closed rotary mixer (Banbury) until a temperature of between 165 and 180° C. is reached, after which the blend is cooled to room temperature.
In a second phase, the silane is added and the compound and the silane-binding agent again undergo intimate mixing in a Banbury mixer until a temperature of 135° C. is reached, after which the compound is again cooled to room temperature.
In a final phase, the ingredients of the vulcanization system are added to the compound and further mixing is carried out in a Banbury mixer without exceeding a temperature of 100° C.
A number of continuous processes in which silicone rubbers, reinforcing fillers, including silica, and further ingredients, including silanes, are mixed together, are likewise known from patents EP 0,258,159 B1 and U.S. Pat. No. 5,409,978.
In particular, the process of patent EP 0,258,159 B1 uses a twin-screw mixer followed in series by a single-screw extruder whose outlet is fitted with a filter placed before a die.
The twin-screw mixer comprises two screws arranged side by side which rotate in the same direction.
The charge introduced into the twin-screw mixer is composed of the base polymer, available from silicon product manufacturers, a powdered charge such as silica and silane compounds.
The compound leaves the twin-screw mixer at a temperature of between 150 and 250° C. and is introduced directly into the single-screw extruder, passing, at the end, through the filter and the die, from which it exits at a temperature of between 120 and 220° C.
In a further example, it is mentioned that the compound leaves the twin-screw mixer at 231° C. and at a pressure of 2 bar; the compound is then conveyed to the filtering zone at a pressure of 70 bar and leaves the die at 170° C.
The description gives no indication as to the addition of vulcanization agents.
The process of U.S. Pat. No. 5,409,978 uses two twin-screw mixers for obtaining a silicone rubber compound; the first mixer comprises a pair of screws rotating in the same direction, while in the second the screws rotate in opposite directions. The base polymer, the silica and the silane ingredients are mixed in the first mixer, until a temperature of between 200 and 300° C. is reached.
The extrudate is convey
Caretta Renato
Pessina Roberto
Proni Antonio
Cain Edward J.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Pirelli Coordinamento Pneumatici S.p.A.
Wyrozebski Katarzyna
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