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
1999-07-14
2001-04-03
Acquah, Samuel A. (Department: 1711)
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...
C525S331700, C525S331800, C525S332100, C525S332500, C525S332600, C525S332800, C525S332900, C525S333100, C525S333200, C525S359100, C525S371000, C524S493000, C524S495000, C524S496000, C524S502000, C524S572000, C524S574000, C524S575500, C524S576000
Reexamination Certificate
active
06211271
ABSTRACT:
DESCRIPTION
1. Background of the Invention
The present invention relates to a process for producing a vulcanizable rubber composition incorporating carbon black-based and silica-based reinforcing fillers dispersed in a cross-linkable unsaturated chain polymer base.
More particularly, the invention relates to a process for producing a vulcanizable rubber composition which has a preferred—though not exclusive—use in the production of treads for vehicle tires.
In the following description and in the subsequent claims, the term: cross-linkable unsaturated chain polymer base, is used to indicate any non cross-linked polymer or polymer mixture, either natural or synthetic, capable of assuming all the chemical-physical and mechanical characteristics typical of elastomers as a result of cross-linking (vulcanization) with sulfur-based systems.
In the same way, in the following description and in the subsequent claims the term: cross-linkable unsaturated chain polymer, is used to indicate any non cross-linked polymer, either natural or synthetic, capable of assuming all the chemical-physical and mechanical characteristics typical of elastomers as a result of cross-linking (vulcanization) with sulfur-based systems.
2. Prior art
In the field of vehicle tire manufacture one of the more difficult objects to be reached has always been that of reducing the rolling resistance of the tire, achieving at the same time a good wear resistance and a satisfactory skid resistance on wet road.
The difficulties for achieving the aforementioned object essentially arise from the fact that the tire rolling resistance on the one hand and the wear resistance and wet skid resistance on the other hand, are affected in an entirely opposite manner by the quantity of reinforcing filler, traditionally mainly constituted by carbon black, used in the rubber composition employed to manufacture the tread of the tire.
In order to reduce the rolling resistance of the tire, in fact, it would be desirable to reduce the quantity of the carbon black-based reinforcing filler used in the rubber composition (for instance, below 70 parts every 100 parts by weight of the polymer base of the rubber composition): in so doing, however, a falling off to unacceptable values of both wet skid resistance and wear resistance of the tire has been observed.
In order to overcome somehow this limitation of the rubber compositions comprising carbon black as main reinforcing filler, the prior art has suggested to partly or fully replace the latter by the so-called “white” fillers, in particular silica, as is described for instance in European Patent application EP 0 501 227.
Even though silica-based reinforcing fillers have allowed to reduce the tire rolling resistance without significantly affecting wear resistance and wet skid resistance, their use is not devoid of drawbacks.
A first severe drawback which has been observed by using reinforcing fillers entirely or mainly constituted by silica is associated to the difficulty of homogeneously and uniformly dispersing silica within the polymer base of the rubber composition. Actually, these reinforcing fillers have per se a poor affinity with the polymer base of the rubber compositions used in the manufacture of tires and require—as such—the use of suitable coupling agents that are able to chemically bind silica to the polymer matrix.
However, the need of using such coupling agents poses a limit to the maximum temperature that may be achieved during the steps of mixing and mechanically working the rubber composition, on pain of an irreversible thermal degradation of the coupling agent.
But the respect of the aforementioned temperature constraint involves a marked reduction in the very mechanical mixing action that is of the essential for an optimum dispersion of silica into the polymer matrix.
The ensuing insufficient and non homogeneous dispersion of silica in the rubber composition causes in turn several drawbacks essentially related to a decrease of the wear resistance and wet skid resistance characteristics, to an extreme variability and non homogeneity of the physical-mechanical characteristics of the rubber composition from zone to zone of the same.
More particularly, a remarkable drawing difficulty and a dimensional variability of the tread have been observed during the manufacture of treads obtained from rubber compositions including silica as main reinforcing filler.
In addition to these drawbacks, additional not negligible shortcomings constituted by a drastic increase in the volume electrical resistivity of the tire tread, which involves an undesired accumulation of electrostatic charges that may in some cases disturb the electronic apparatuses present on board of a vehicle or even cause sudden electrical discharges of high potential, and by an abrasive action of silica on the moving parts and, in general, on the body of the mixing apparatuses used in the production of the rubber composition, which causes an increase in maintenance costs, must also be added.
Lastly, it should be observed that by using reinforcing fillers entirely or mainly constituted by silica, the overall production cost of the finished tire increases to a not negligible extent either because of the much higher cost of silica compared to carbon black, and because of the need of using suitable and expensive coupling agents capable of chemically binding silica to the polymer matrix of the rubber composition.
In order to try to overcome the drawbacks related to the use of silica as main reinforcing filler, it has then been suggested to use both carbon black and silica, dispersing each of said fillers in a corresponding polymer, as disclosed by European Patent application EP-A-0 763 558.
In this way, a substantially heterogeneous rubber composition is obtained in which a first polymer phase wherein carbon black is mainly dispersed and a second polymer phase wherein silica is mainly dispersed may be distinguished.
By operating in accordance with the teaching of the aforesaid patent application, however, the above drawbacks related to difficulties of silica dispersion in its own polymer phase, are still worsened by the high quantities of silica to be used, in particular if low-molecular-weight, low-viscosity polymers are used, such as solution-SBR synthetic rubbers.
The ensuing non homogeneous silica dispersion adversely affects both the wear resistance and wet-skid resistance characteristics, and the physical-mechanical characteristics of the rubber composition from zone to zone of the same.
Furthermore, the necessary mechanical mixing action, essential for an optimum dispersion of silica in its own polymer phase, causes also in this case an undesired marked abrasion action on the body and on the moving parts of the mixing apparatuses, with an increase in maintenance costs.
SUMMARY OF THE INVENTION
The technical problem underlying the present invention is therefore that of providing a process for producing a vulcanizable rubber composition including carbon black-based and silica-based reinforcing fillers which is free from the drawbacks complained with reference to the cited prior art.
According to the invention, the aforesaid technical problem is solved by a process for producing a vulcanizable rubber composition, which is characterized in that it comprises the steps of:
a) intimately mixing at a temperature of from 130° to 180° C.:
i) at least a first cross-linkable unsaturated chain polymer;
ii) a first carbon black-based reinforcing filler, and optionally
iii) one or more non cross-linking ingredients
so as to obtain a first rubber composition having a Mooney viscosity of from 70 to 140 Ms, wherein the first carbon black-based reinforcing filler is dispersed in a substantially homogeneous manner;
b) cooling the first rubber composition obtained from step a) to a temperature not exceeding 100° C.,
c) intimately mixing at a temperature of from 130° to 180° C.:
i) the first high-viscosity rubber composition obtained from step b),
ii) at least a second cross-linkable unsaturated chain polymer,
iii) a second silica-based
Amaddeo Angela
Garro Luciano
Pessina Roberto
Acquah Samuel A.
Finnegan, Henderson, Farabow, Garrett & Dunner. L.L.P.
Pirelli Pneumatici S.p.A.
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