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
2000-09-29
2002-01-29
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...
C524S493000, C526S222000, C526S224000
Reexamination Certificate
active
06342559
ABSTRACT:
This application is the national phase under 35 U.S.C. § 371 of PCT International Application No. PCT/JP99/01625 which has an International filing date of Mar. 30, 1999, which designated the United States of America.
TECHNICAL FIELD
This invention relates to a conjugated diene rubber exhibiting good dispersibility when silica is incorporated therein. More particularly, it relates to a conjugated diene rubber suitable for a silica-incorporated rubber composition giving tires having a reduced rolling resistance.
BACKGROUND ART
In recent years, a serious view has been taken for saving resources and cleaning up the environment, and thus, the reduction of fuel consumption of automobiles is required severely. For the reduction of fuel consumption, reduction of rolling resistance of tires is desired. The reduction of rolling resistance of tires is attained usually by using a rubber material capable of providing a rubber vulcanizate exhibiting a reduced heat build-up.
Incorporation of silica as a reinforcer, instead of carbon black, in a diene rubber for tires has heretofore been proposed to give tires exhibiting a reduced heat build-up. Nevertheless, the silica-incorporated diene rubber composition has a problem such that the tensile strength and abrasion resistance are poor as compared with a carbon black-incorporated rubber composition. This would be due to the fact that silica has poorer affinity than carbon black to a diene rubber, and thus, silica does not exhibit a high reinforcing effect.
To enhance the affinity of silica to a diene rubber, a proposal of incorporating a silane-coupling agent has been made, for example, in Japanese Unexamined Patent Publication (hereinafter abbreviated to “JP-A”) No. H3-252431 and JP-A H3-252433. In this proposal, an expensive silane-coupling agent must be used in a salient amount for enhancing the affinity of silica to the desired extent.
As another proposal, a conjugated diene rubber having introduced therein a substituent exhibiting an enhanced affinity to silica has been proposed. For example, among conjugated diene rubbers prepared by an emulsion polymerization procedure, a conjugated diene rubber having a tertiary amino group is described in JP-A H1-101344. Among conjugated diene rubbers prepared by an anion polymerization procedure, a conjugated diene rubber having an alkylsilyl group is described in JP-A H1-188501, a conjugated diene rubber having a halogenated silyl group is described in JP-A H5-230286, and a conjugated diene rubber having a substituted-amino group is described in JP-A S64-22940.
Most of the conjugated diene rubbers having the above-recited substituents have problems such that, when these rubbers are mixed with silica, they strongly agglomerate together with silica and do not allow silica to uniformly disperse, and consequently, processability is reduced, and characteristics such as low heat build-up, tensile strength and abrasion resistance are not improved to the desired extent.
To enhance dispersibility of silica in conjugated diene rubber, and improve characteristics such as low heat build-up, tensile strength and abrasion resistance, a proposal of using a conjugated diene rubber having introduced therein a polar group by copolymerization with a monomer having a polar group was made. However, due to the presence of a polar group in the main structure, this conjugated diene rubber has poor stability with time, tends to be easily gelled and the Mooney viscosity is apt to increase at a high temperature. Therefore, the conjugated diene rubber still has problems such that, when silica and other fillers are kneaded together, uniform kneading becomes difficult, and, because of high viscosity, processability is low and the handling is difficult.
In general, in radical polymerization of a conjugated diene monomer, branched chain formation or galling occurs with an increase of polymerization conversion. Consequently, when the resulting rubber is crosslinked with a crosslinking agent, a high crosslinking efficiency cannot be obtained and thus crosslinking properties such as tensile stress and impact resilience are deteriorated. To avoid these disadvantages, a molecular weight modifier is usually added to suppress the galling at the polymerization step. As the molecular weight modifier, t-dodecyl mercaptan is conventionally used in radical polymerization for a conjugated diene rubber, which is A mixture of isomers of alkylthiol compounds having 9 to 16 carbon atoms. The conjugated diene rubber obtained by using this molecular weight modifier exhibits poor high-rate crosslinkability when crosslinking is effected at a high temperature with a short crosslinking time, and thus, crosslinking of a desired degree is difficult to obtain.
A polymerization process for the production of a conjugated diene-aromatic vinyl compound copolymer rubber by using as a molecular weight modifier an alkylthiol compound having 12 to 16 carbon atoms, which include at least three tertiary carbon atoms, and having a sulfur atom which is directly bound to at least one of the tertiary carbon atoms, for example, 2,2′,4,6,6′-pentamethylheptana-4-thiol is described in JP-A H8-104717. This polymerization process results in a rubber exhibiting a high-rate crosslinkability. However, It is not known to use the alkylthiol molecular weight modifier in polymerization for the production of a conjugated diene rubber having a polar group. It could not be foreseen what type of rubber vulcanizate would be obtained when a composition comprising the conjugated diene rubber having a polar group and silica is crosslinked.
SUMMARY OF THE INVENTION
A primary object of the present invention is to provide a conjugated diene rubber composition characterized as exhibiting uniform dispersibility of silica therein, good stability with time, stable Mooney viscosity at high temperature, good processability, and low heat build-up.
The present inventors made extensive researches to solve the above-mentioned problems of the prior art, and have found that the object of the invention can be achieved by copolymerizing a conjugated diene monomer, an aromatic vinyl monomer and a monomer having a polar group in the presence of a specific molecular weight modifier. Based on this finding, the present invention has been completed,
In one aspect of the invention, there is provided a diene rubber which is a copolymer comprising (1) 30 to 85% by weight of conjugated diene monomer units, (2) 15 to 50% by weight of aromatic vinyl monomer units and (3) 0.05 to 20% by weight of units of a vinyl monomer having at least one kind of polar group selected from the group consisting of N,N-disubstituted-aminoalkyl acrylates, N,N-disubstituted-aminoalkyl methacrylates, N,N-disubstituted-aminoalkyl acrylamides, N,N-disubstituted-aminoalkyl methacrylamides, hydroxyl-substituted-alkyl acrylates and hydroxyl-substituted-alkyl methacrylates; said diene rubber having in the molecule an alkylthio group having 12 to 16 carbon atoms, which include at least three tertiary carbon atoms, and having a sulfur atom which is directly bound to at least one of the tertiary carbon atoms.
In another aspect of the invention, there is provided a process for producing a diene rubber which comprises copolymerizing (1) a conjugated diene monomer, (2) an aromatic vinyl monomer and (3) a vinyl monomer having at least one kind of polar group selected from the group consisting of N,N-disubstituted-aminoalkyl acrylates, N,N-disubstituted-aminoalkyl methacrylates, N,N-disubstituted-aminoalkyl acrylamides, N,N-disubstituted-aminoalkyl methacrylamides, hydroxyl-substituted-alkyl acrylates and hydroxyl-substituted-alkyl methacrylates, in the presence of (a) an alkylthiol compound having 12 to 16 carbon atoms, which include at least three tertiary carbon atoms, and having a sulfur atom which is directly bound to at least one of the tertiary carbon atoms, and (b) a free-radical initiator.
In a further aspect of the invention, there is provided a rubber composition comprising a rubber ingredient comprising the above-mentioned diene r
Cain Edward J.
Zeon Corporation
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