Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2000-10-30
2002-03-26
Michl, Paul R. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
Reexamination Certificate
active
06362253
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to new rubber compositions intended for the manufacture of tire casings based on precipitated silicas containing a reinforcing additive based on a functionalized polyorganosiloxane and an organosilane compound.
Since the economies of fuel and the need to protect the environment have become a priority, it is desirable to produce polymers which have good mechanical properties and a hysteresis which is as low as possible in order to enable them to be used in the form of rubbery compositions which can be employed for the manufacture of various semifinished products forming part of the constitution of tire casings, such as, for example, underlinings, calendering or sidewall rubbers or treads and to obtain tires with improved properties, which have in particular a reduced rolling resistance.
Many solutions have been proposed to meet such an objective, consisting especially in modifying, among others, the nature of the diene polymers and copolymers at the end of polymerization by means of coupling or starring or functionalizing agents. All these solutions have concentrated essentially on the use of the modified polymers with carbon black as reinforcing filler with the aim of obtaining a good interaction between the modified polymer and the carbon black. It is known, in general, that in order to obtain optimum reinforcing properties which are imparted by a filler it is appropriate that the latter should be present in the elastomer matrix in a final form which is both as finely divided as possible and distributed as homogeneously as possible. However, such conditions can be achieved only insofar as, on the one hand, the filler has a very good capacity for being incorporated into the matrix during the mixing with the elastomer and for being deaggregated or deagglomerated and for being dispersed homogeneously in the elastomer. The use of white reinforcing fillers, and especially of silica, has been found inappropriate because of the low level of some properties of such compositions and consequently of some properties of the tires utilizing these compositions.
In addition, for reasons of mutual affinities, silica particles have an unfortunate tendency, in the elastomer matrix, to agglomerate together. These silica/silica interactions have the detrimental consequence of limiting the reinforcing properties to a level which is appreciably lower than that which it would be theoretically possible to attain if all the silica/elastomer interactions capable of being created during the mixing operation were actually obtained.
What is more, the use of silica gives rise to difficulties in processing which are due to the silica/silica interactions which tend, in the raw state (before curing), to increase the consistency of the rubbery compositions and, in any event, to make the processing more difficult than the processing of carbon black.
Finally, the interactions between the silica and the crosslinking system, when the latter is sulfur-based, and the accelerators usually employed in the case of sulfur reduce the rate and efficiency of crosslinking.
In the case of silica-reinforced compositions interest has been revived with the publication of European Patent Application EP-A-0 501 227, which discloses a sulfur-vulcanizable rubber composition obtained by thermomechanical working of a copolymer of conjugated diene and of an aromatic vinyl compound, prepared by polymerization in solution with 30 to 150 parts by weight, per 100 parts by weight of elastomer, of a particular precipitated silica. The use of such a silica has undoubtedly reduced the difficulties in processing the mixtures containing it, predominantly or otherwise, as reinforcing filler, but the processing of such rubbery compositions nevertheless remains more difficult than the processing of carbon black.
It is known to a person skilled in the art that a coupling or bonding agent which reacts with silica must be employed to create good interactions between the surface of the silica and the elastomer while promoting the dispersion of the silica, and the compositions described in European Patent Application EP-A-0 501 227 are also subject to this need.
One objective of a person skilled in the art consists in improving the processing of the diene rubber compositions including silica as reinforcing filler which are intended for the manufacture of tire casings and, on the other hand, to reduce the quantity of coupling and/or reinforcing agent needed, without degrading the properties of such compositions.
Thus, in U.S. Pat. No. 3,350,345 it has been proposed to employ, in rubber compositions including silica, a hydrolyzable silane and in particular a mercaptosilane as elastomer/silica coupling agent. In Patent Application FR-A-2,094,859 it was subsequently proposed to employ rubber compositions including silica and a mercaptosilane as coupling agent for the manufacture of tire treads, because of the improved properties exhibited by such compositions. It was rapidly demonstrated and known to a person skilled in the art that mercaptosilanes and in particular &ggr;-mercaptopropyltrimethoxysilane and &ggr;-mercaptopropyltriethoxysilane were capable of providing the best silica/elastomer coupling properties, but that the industrial use of these coupling agents was not possible because of the high reactivity of the SH functional groups resulting very rapidly, during the preparation of a composition in an internal mixer, in premature vulcanizations, also called “scorching”, with very high Mooney plasticities and, all things considered, in compositions which were virtually impossible to work and to process on industrial scale.
To illustrate this impossibility of employing such coupling agents and the compositions containing them on industrial scale, Patent Application FR-A-2,206,330 and U.S. Pat. No. 4,002,594 may be mentioned.
To overcome this disadvantage, in Patent Application FR-A-2,206,330 it has been proposed to employ as coupling agent organosilane polysulfides, including bis-3-triethoxysilylpropyl tetrasulfide, which are found to give the best compromise, in the case of silica-filled vulcanizates, in terms of scorch safety, ease of processing and reinforcing power. However, this coupling agent is very costly and must be employed in a relatively large quantity, of the order of 2 to 3 times greater than the quantity of &ggr;-mercaptopropyltrimethoxysilane needed to obtain equivalent coupling property levels.
Consequently, it therefore appears desirable from an economical viewpoint to have the ability to develop on industrial scale silica-reinforced rubber compositions including low contents of reinforcing additives which are as effective as mercaptosilanes, but while avoiding premature is scorching and problems in processing which are related to an excessive viscosity of the compositions.
An attempt in this direction was described in U.S. Pat. No. 4,474,908 which discloses the use of the mixture of a mercaptosilane and an alkoxysilane as reinforcing additive for a rubber composition.
However, this route is not a satisfactory remedy to the problem of scorching and of processing and, in addition, it is costly.
Another attempt has been described in Japanese Patent Application JP-A-06,248,116, which discloses rubber compositions intended for the manufacture of tire casings including, as reinforcing filler, a blend of carbon black and of silica surface treated with unfunctionalized silicone oils (generally and usually called PDMS by a person skilled in the art), as well as a silane as coupling agent. This route does not enable the problem faced by a person skilled in the art to be solved, whether the filler consists of a black/silica dilution or silica alone. In fact, the solution described in this application requires the pretreatment of the silica with the silicone oil at a high temperature (approximately 250° C.) and for an extended period (approximately 1 hour) before it is incorporated into the elastomer and into the coupling agent.
The present invention remedies the problem presented by t
Baker & Botts L.L.P.
Compagnie Generale des Establissements Michelin--Michelin & Cie
Michl Paul R.
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