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
2002-11-22
2004-11-09
Thexton, Matthew A. (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, C524S493000
Reexamination Certificate
active
06815487
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a cross-linkable or cross-linked rubber composition having improved hysteresis properties in the cross-linked state, which is usable to constitute a tire tread or to be used therein. The present invention further relates to a process for the preparation of such a cross-linkable rubber composition, to a tread comprising the cross-linked rubber composition, and to a tire having reduced rolling resistance.
BACKGROUND OF THE INVENTION
Since fuel economies and the need to preserve the environment have become priorities, it has become desirable to produce mixes having good mechanical properties and as low a hysteresis as possible so that they can be processed in the form of rubber compositions useful for the manufacture of various semi-finished products involved in the make-up of tires, including, for example, underlayers, sidewalls or treads. It is also desirable to obtain tires having reduced rolling resistance.
To achieve such objectives, numerous solutions have been proposed, consisting of, in particular, the modification of the structure of diene polymers and copolymers at the end of polymerization by means of functionalizing, coupling or starring agents. The great majority of these solutions have focused on the use of functionalized polymers, which are active with respect to carbon black, with the goal of obtaining a good interaction between the modified polymer and the carbon black.
By way of illustrating the prior art relating to reinforcing fillers formed of carbon black, mention may be made of U.S. Pat. No. 3,135,716, which describes the reaction of living diene polymers at the end of a chain with a polyfunctional organic coupling agent in order to obtain polymers having improved properties. Mention may also be made of U.S. Pat. No. 3,244,664, which discloses the use of tetra-alkoxysilanes as a coupling agent or starring agent for diene polymers.
Silica has been used as a reinforcing filler in cross-linkable rubber compositions, in particular those compositions intended for use in tire treads, for a long time. However, this use has remained very limited because of the unsatisfactory level of certain physical properties of such compositions, in particular, abrasion resistance.
Thus, it has been proposed, in order to overcome these drawbacks, to use functionalized diene polymers, instead of the non-functionalized polymers which were used before, and in particular polymers functionalized by alkoxysilane derivatives, such as tetraethoxysilanes. For example, mention may be made of U.S. Pat. No. 5,066,721, which describes a rubber composition comprising a diene polymer functionalized by an alkoxysilane having at least one non-hydrolyzable alkoxyl radical, which makes it possible to eliminate the polymerization solvent by steam stripping.
One disadvantage of these functionalization reactions lies in the coupling reactions which accompany them, which generally make it necessary to use an excess of alkoxysilane and/or to employ intensive mixing, in order to minimize these coupling reactions.
Another drawback of these reactions lies in the later implementation of the steam stripping operation, which is necessary to eliminate the polymerization solvent. In fact, generally, experience shows that the functionalized polymers obtained undergo changes in macrostructure during this stripping operation, which result in serious degradation of their properties, unless one is limited to using as the functionalizing agent an alkoxysilane belonging to a restricted family, such as that described in the aforementioned U.S. Pat. No. 5,066,721.
Consequently, it may be seen from the above that the use of diene polymers comprising an alkoxysilane function to obtain rubber compositions comprising silica as reinforcing filler is not satisfactory, despite the improved physical properties of these compositions.
Thus, research has been conducted on other functionalization reactions, always with a view to obtaining such rubber compositions.
By way of example, mention may be made of French Patent No. FR 2.740.778, assigned to the Assignee of the present invention, which discloses the incorporation, into rubber compositions that comprise, as the reinforcing filler, silica in a majority proportion (for example, a blend of silica and carbon black), of diene polymers bearing at the chain end a silanol function or a polysiloxane block having a silanol end. For example, a functionalizing agent consisting of a cyclic polysiloxane, such as hexamethylcyclotrisiloxane, may be used. The functionalized polymers obtained can be separated from the reaction medium, resulting in their formation by steam extraction of the solvent, without their macrostructure and consequently their physical properties changing.
Mention may also be made of European Patent No. EP 0 877 047, which discloses the incorporation of such polymers having a silanol function in rubber compositions comprising as the reinforcing filler carbon black having silica fixed to its surface.
It has been possible to establish that these polymers impart rubber properties, in particular, hysteresis and reinforcement properties, in the cross-linked state, which are improved compared with those of control compositions based on non-functionalized diene polymers, and which are at least analogous to those of compositions based on diene polymers comprising an alkoxysilane function.
Mention may also be made of European Patent No. EP 0 692 493, which establishes that diene polymers bearing at the chain end alkoxysilane groups and an epoxy group result in improved reinforcement properties and in reduced hysteresis losses at small and large deformations.
One disadvantage of these polymers, which comprise a functional group that is active for coupling to silica or to carbon black surface-modified by silica, is that the improvement in the hysteresis and reinforcement properties which they impart to the rubber compositions incorporating them is generally accompanied by a processing ability of the non-cross-linked mixes which is compromised when compared to the processing ability of non-functionalized “control” polymers.
Among the other functionalization reactions studied, mention may be made, for example, of the functionalization of the diene polymers along the chain by COOH functions.
The functionalization along the chain can be effected by direct metallation, in the presence of N,N,N′,N′-tetramethylethylenediamine (TMED), by means of butyllithium or metallic sodium (as described in U.S. Pat. Nos. 3,978,161 and 3,976,628, respectively), followed by a carbonation reaction by means of carbonic gas. However, such a process has the disadvantage of generally resulting in cuts in the chain of the modified polymer.
Two specific reagents, of the respective formulae HSCH
2
CO
2
CH
3
and N
2
CHCO
2
CH
2
CH
3
, have also been used to graft COOH functions along the chain of a diene polymer. For the description of the reaction mechanisms relating to the use of these two reagents, reference may be made respectively to the following documents: Sanui et al.,
J. Poly. Sci., Polym. Chem. Ed
. 12:1965 (1974) and Tanaka et al.,
J. Poly. Sci., Polym. Chem. Ed
. 17:2975 (1979). However, one major disadvantage of using one or the other of these two reagents is that it results in significant changes in macrostructure for the modified polymer.
This functionalization along the chain may also be implemented by means of carbon monoxide, either by hydroformylation followed by oxidation of the aldehyde formed (as described in U.S. Pat. No. 4,912,145), or by direct hydrocarboxylation of the polymer (as described in Ajjou et al.,
Macromolecules
29:1784 (1996)). The catalysts used for these reactions are based on rhodium or palladium. One disadvantage of this functionalization by carbon monoxide lies, on one hand, in the drastic nature of the operating conditions and, on the other hand, in the frequent formation of a gel in the reaction medium.
Functionalization by means of maleic anhydride is more widespread. This type of functionaliza
Barbotin Fanny
Favrot Jean-Michel
Laubry Philippe
Robert Pierre
Burns Doane Swecker & Mathis L.L.P.
Michelin & Recherche et Technique S.A.
Thexton Matthew A.
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