Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2002-02-11
2003-02-18
Teskin, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S087000, C526S173000, C526S180000, C525S105000, C525S237000, C525S332900, C524S575000
Reexamination Certificate
active
06521733
ABSTRACT:
TECHNICAL FIELD
This invention relates to a tapered aromatic vinyl-conjugated diene copolymer rubber, a process for producing the copolymer rubber, and a rubber composition comprising the copolymer rubber. More particularly, it relates to a tapered aromatic vinyl-conjugated diene copolymer rubber which, when the tapered copolymer rubber is mixed with natural rubber or a synthetic rubber, gives a rubber material for tire exhibiting a reduced heat-build up and having excellent wet skid resistance, tensile strength and abrasion resistance; a process for producing the tapered copolymer rubber; and a rubber composition comprising the material for rubber.
BACKGROUND ART
A serious view has been taken for saving resources and cleaning up the environment, and thus, the reduction of fuel consumption of automobiles is severely required. 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.
For enhancing safety of automobiles, tires having a reduced braking distance are required. To reduce the braking distance on a wet road, namely, to improve a wet skid braking property, a crosslinked rubber exhibiting an enhanced heat build-up is generally used.
To meet the contradictory requirements of reduction of rolling resistance and improvement of wet skid resistance, modified rubbers have been proposed, which include, for example, a modified rubber prepared by allowing a diene rubber molecule to react with a benzophenone compound (for example, Japanese Unexamined Patent Publication [hereinafter abbreviated to “JP-A”] No. S58-189203); and a modified rubber composition comprising a diene rubber having introduced therein a substituted amino group or an alkoxysilyl group, and silica as a reinforcing agent (for example, JP-A S64-22940, JP-A H9-151275, JP-A H9-151276, JP-A H9-208621, JP-A H9-227628 and JP-A H9-235324). However, these modified rubbers do not exhibit a balanced property between the reduced heat build up and the enhanced heat build up, namely, the contradictory requirements for reduced fuel consumption and wet skid braking property are not satisfied.
Further, to provide a rubber having good and balanced reduced fuel consumption and wet skid braking property, a styrene-butadiene copolymer rubber has been proposed wherein the content of styrene units varies continuously depending upon the molecular weight of a copolymer molecule (for example, JP-A S57-53511). It is to be noted, however, that the styrene-butadiene copolymer rubber is fundamentally a polyblend comprised of a plurality of copolymer rubbers. It is described in the patent publication that the styrene-butadiene copolymer rubber is made by a polymerization process comprising sequential polymerization steps wherein a fresh monomer mixture comprised of a polymerization initiator and monomers is added in each polymerization step. A polymerization mixture at each step in this polymerization process is a mixture comprising polymer molecules growing continually from the previous step and fresh polymer molecules growing afresh. Thus the resulting copolymer rubber is a polyblend comprised of a plurality of rubbers. Therefore copolymer rubbers having a constant quality are difficult to make, and alteration of quality of copolymer rubbers also is difficult. When a rubber composition comprising this styrene-butadiene copolymer and natural rubber is used as a material for tire, the resulting tire has good abrasion resistance and wet skid braking property, but does not satisfy a severe requirement for reduced fuel consumption.
A tapered styrene-butadiene copolymer rubber wherein the content of vinyl bond units in the butadiene units has been proposed as a rubber material exhibiting good compatibility with natural rubber and giving a mixture of the copolymer rubber with natural rubber which is used as rubber vibration isolator (JP-A H1-167346). However, the rubber composition comprising this copolymer rubber and natural rubber is characterized in that the temperature dispersion profile of the dissipation factor has a one-peak and the peak of the dissipation factor is positioned at the low temperature side relative to that of the copolymer rubber, and thus, the dissipation factor around 0° C. of the rubber composition is low. A tire made from the rubber composition has very poor wet skid resistance and is of no practical use.
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide a tapered aromatic vinyl-conjugated diene copolymer rubber used as a material giving tires having reduced heat-build up, enhanced wet skid resistance, high tensile strength and good abrasion resistance.
Another object of the present invention is to provide a process for producing the above-mentioned tapered aromatic vinyl-conjugated diene copolymer rubber.
Still another object of the present invention is to provide a rubber composition comprising the above-mentioned tapered aromatic vinyl-conjugated diene copolymer rubber, which is useful as a material for tire.
The present inventor made an extensive research to obviate the problems of prior art and found that a tapered aromatic vinyl-conjugated diene copolymer rubber wherein the content of aromatic vinyl monomer units in a polymer molecule varies in a specific fashion has characteristics such that, when the copolymer rubber is mixed with natural rubber or a synthetic rubber, a part of polymer molecule of the copolymer rubber exhibits good compatibility but the remaining part does not exhibit compatibility with natural rubber or a synthetic rubber, and thus, the tapered copolymer rubber gives a rubber composition having reduced heat-build up, enhanced wet skid resistance, high tensile strength and good abrasion resistance. Based on this finding, the present invention has been completed.
Thus, in accordance with the present invention, there is provided a tapered aromatic vinyl-conjugated diene copolymer rubber characterized by a tapered copolymer comprising aromatic vinyl monomer units and conjugated diene monomer units, and having a ratio W
0.6Mp
/W
Mp
in the range of from 0.65 to 0.9 or from 1.1 to 1.3 wherein Mp is molecular weight of a copolymer molecule corresponding to the peak of a molecular weight distribution curve obtained by gel permeation chromatography, W
Mp
is a weight fraction of aromatic vinyl monomer units in the copolymer molecule having a molecule weight Mp, and W
0.6Mp
is a weight fraction of aromatic vinyl monomer units in a copolymer molecule having a molecular weight of Mp×0.6.
In accordance with the present invention, there is further provided a process for producing a tapered aromatic vinyl-conjugated diene copolymer rubber wherein an aromatic vinyl monomer and a conjugated diene monomer are copolymerized while the composition of a monomer mixture comprising the aromatic vinyl monomer and the conjugated diene monomer is varied with a progress of polymerization, characterized in that the copolymerization is carried out by using an organic active metal as an initiator in the presence of a polar compound in a hydrocarbon medium, while, with a progress of polymerization, (i) the ratio of the aromatic vinyl monomer within the monomer mixture is increased so that a tapered copolymer having a ratio W
0.6Mp
/W
Mp
in the range of from 0.65 to 0.9 is produced, or, (ii) the ratio of the aromatic vinyl monomer within the monomer mixture is decreased so that a tapered copolymer having a ratio W
0.6Mp
/W
Mp
in the range of from 1.1 to 1.3 is produced, wherein Mp is molecular weight of a copolymer molecule corresponding to the peak of a molecular weight distribution curve obtained by gel permeation chromatography, W
Mp
is a weight fraction of aromatic vinyl monomer units in the copolymer molecule having a molecular weight Mp, and W
0.6Mp
is a weight fraction of aromatic vinyl monomer units in a copolymer molecule having a molecular weight of Mp×0.6.
In acc
Armstrong Westerman & Hattori, LLP
Teskin Fred
Zeon Corporation
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