Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1999-03-03
2001-07-03
Nutter, Nathan M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S181000, C524S491000, C524S474000, C524S481000, C526S348700
Reexamination Certificate
active
06255401
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a rubber composition and, more particularly, to a rubber composition exhibiting an improved high hysteresis loss property without any adverse effects on fracture properties, and to a pneumatic tire which uses the rubber composition and exhibits excellent gripping ability.
DESCRIPTION OF THE RELATED ART
Heretofore, aromatic softeners have frequently been employed to improve the high hysteresis loss property of a rubber composition.
However, this method has a problem in that aromatic softeners must be used in a large amount to obtain the high hysteresis loss property, and as a result, the fracture properties deteriorate.
To overcome the above problem, the use of polyisobutylene type softeners has been studied. However, polyisobutylene exhibits poor compatibility with diene rubbers which are suitably used as the rubber component of a matrix, and bleeding of polyisobutylene occurs at the surface of products obtained after molding. In particular, when a molded product is assembled with other parts to produce a final product, a problem arises in that adhesion between the parts is poor.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a rubber composition having a combination of excellent fracture properties and an excellent high hysteresis loss property, which combination has been difficult to obtain with conventional rubber compositions.
As the result of intensive studies by the present inventor on rubber compositions containing various softeners, it was found that a rubber composition exhibits a combination of excellent fracture properties and an excellent high hysteresis loss property when the rubber composition comprises a polymer obtained by cationic polymerization, and in particular, a polyisobutylene or a copolymer of isobutylene and styrene (hereinafter, such a copolymer may be called as an isobutylene containing copolymer). The present invention has been achieved by utilizing this discovery.
Accordingly, in a first aspect of the present invention, a rubber composition comprising a rubber component and a polymer obtained by cationic polymerization using a Lewis acid catalyst as the initiator, the polymer preferably having a weight-average molecular weight of 2,000 to 50,000, is provided. It is preferable that the amount of the polymer is 5 to 200 parts by weight per 100 parts by weight of the rubber component.
It is preferable that the polymer is substantially a homopolymer of isobutylene or substantially a copolymer of isobutylene and an aromatic vinyl compound. When the polymer is substantially a copolymer of isobutylene and an aromatic vinyl compound, which is preferably styrene, the content of units of the aromatic vinyl compound bound in the copolymer is preferably 20 to 70% by weight of the weight of the copolymer.
It is preferable that the rubber component is a diene rubber. It is more preferable that the rubber component is a homopolymer of a diene compound or a copolymer of a diene compound and an aromatic vinyl compound.
It is preferable that the content of the aromatic vinyl compound bound in the isobutylene containing copolymer is greater than the content of the aromatic vinyl compound bound in the rubber component. It is more preferable that the content of the aromatic vinyl compound bound in the isobutylene containing copolymer is greater, by 15% by weight or more, than the content of the aromatic vinyl compound bound in the rubber component.
In a second aspect of the present invention, a pneumatic tire which is prepared by using any of the above rubber compositions at least in the road-contacting portion of the tread is provided.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The rubber composition of the present invention is obtained by using a polymer obtained by cationic polymerization, such as polyisobutylene or a copolymer of isobutylene and styrene, in combination with the basic component of the rubber composition.
As the rubber component of the rubber composition, diene rubber is preferable, and homopolymers of diene compounds and copolymers of a diene compound and an aromatic vinyl compound are more preferable.
In the preferable embodiment of the present invention, the polymer used in combination with the basic component of the rubber composition can be obtained by cationic polymerization of an isobutylene monomer and an aromatic vinyl hydrocarbon (for example, styrene) in a hydrocarbon solvent by using a Lewis acid catalyst as the initiator. When an isobutylene monomer alone is polymerized, a homopolymer of isobutylene, i.e., polyisobutylene, is obtained. When a mixture of an isobutylene monomer and an aromatic vinyl hydrocarbon is polymerized, a copolymer of isobutylene and the aromatic vinyl hydrocarbon can be obtained.
The above polyisobutylene or isobutylene containing copolymer may further be copolymerized with a conjugated diene monomer such as 1,3-butadiene, isoprene, 1,3-pentadiene, 2,3-dimethylbutadiene, 2-phenyl-1,3-butadiene and 1,3-hexadiene.
In the present invention, examples of the aromatic vinyl compound monomer which is preferably used for preparation of the copolymer of isobutylene and an aromatic vinyl compound include styrene, &agr;-methylstyrene, 1-vinylnaphthalene, 3-vinyltoluene, ethylvinylbenzene, divinylbenzene, 4-cyclohexylstyrene and 2,4,6-trimethylstyrene.
The homopolymer and the copolymer used in the present invention can be obtained by cationic polymerization of the monomers in accordance with, for example, the process described in “Experimental Methods of Polymer Synthesis” (published by Kagaku Dojin Co., Ltd.), page 201, or the process described in Kobunshi Kagaku, Vol. 18, pages 389 to 395.
In the present invention, the molecular weight of the above homopolymer or copolymer is not particularly limited and can be suitably selected in accordance with the object. For example, when the homopolymer or the copolymer is used as an oil which can be used as a softener in the rubber composition, it is preferable that the homopolymer or the copolymer has a relatively low molecular weight such as 2,000 to 50,000. When the molecular weight is less than 2,000, fracture properties tend to deteriorate. When the molecular weight exceeds 50,000, it becomes difficult to obtain the high loss hysteresis property. The molecular weight is more preferably 4,000 to 42,000.
The content of units of the aromatic vinyl compound bound in the copolymer used in the present invention is preferably 20 to 70% by weight, more preferably 30 to 70% by weight and most preferably 40 to 70% by weight of the copolymer. When the content is less than 20%, compatibility with the rubber component deteriorates, and the copolymer tends to bleed. When the content exceeds 70%, the modulus of the rubber composition increases, and the gripping property of the rubber composition (more precisely, the gripping property of a tire formed by using the rubber composition) deteriorates.
From the standpoint of compatibility, it is preferable that the content of the aromatic vinyl compound bound in the copolymer is greater than the content of the aromatic vinyl compound bound in the rubber component of the rubber composition. A difference of 15% by weight or more is most effective.
A solvent generally used in cationic polymerization can be suitably used as the solvent. For example, a hydrocarbon solvent such as an aliphatic hydrocarbon, an aromatic hydrocarbon or a halogenated hydrocarbon can be used. Among these solvents, an aromatic hydrocarbon is preferable, and toluene is more preferable.
Examples of the aliphatic hydrocarbon include pentane and hexane. Examples of the aromatic hydrocarbon include benzene, toluene and xylene. Examples of the halogenated hydrocarbon include chloromethane, chloroethane, methylene chloride, 1,1-dichloroethane, chloroform and 1,2-dichloroethane. These hydrocarbons can be used by itself or in combination of two or more. The aforementioned solvent may be used in combination with small amounts of other solvents. Examples of the other solvent include acetic esters suc
Asinovsky Olga
Bridgestone Corporation
Nutter Nathan M.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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