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
1997-03-27
2002-10-29
Sellers, Robert E. L. (Department: 1712)
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...
C525S099000, C525S313000, C525S332900
Reexamination Certificate
active
06472464
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a styrene-butadiene copolymer and a rubber composition comprising the copolymer. More particularly, the present invention relates to a styrene-butadiene copolymer which has low hysteresis loss characteristic and excellent wet skid resistance, and which is very excellent in fracture characteristics and abrasion resistance, and is advantageously used for tire tread rubbers, and a rubber composition comprising the copolymer.
2. Description of Related Art
For automobiles, low fuel consumption and safety are both required. Therefore, improvement in low hysteresis loss characteristic (low rolling resistance), wet skid resistance, fracture characteristics, and abrasion resistance are strongly required for styrene-butadiene copolymers used for the tread part of tires and for rubber compositions comprising the copolymers.
Various attempts have been made to satisfy the requirements for tire tread rubbers. As one of such attempts, styrene-butadiene copolymers comprising blocks of styrene sequences have been examined. Among these styrene-butadiene copolymers, styrene-butadiene copolymers comprising blocks of styrene sequences of 4 to 20 styrene units (hereinafter referred to as short styrene blocks) have been noted in view of improvement in the abrasion resistance.
In Japanese Patent Application Laid-Open No. Showa 57(1982)-179212, a styrene-butadiene copolymer which comprises 5% by weight or less of long styrene blocks and 50% by weight or more of the isolated single styrene unit based on the total styrene content in the copolymer and 10 to 30% by weight of the total styrene based on the copolymer and shows a high abrasion resistance is described. However, this copolymer does not show sufficient fracture characteristics, abrasion resistance, and low hysteresis loss characteristic.
In Japanese Patent Application Laid-Open No. Showa 60(1985)-23409, a styrene-butadiene copolymer is obtained by a multi-step polymerization process in which the monomers are divided into portions and added successively. The obtained copolymer which comprised less than 50% by weight of styrene sequences having 1 to 3 styrene units, 30% by weight or more of styrene sequences having 4 to 20 styrene units, and 20% by weight or less of styrene sequences having more than 20 styrene units all based on the total styrene content in the copolymer, and 10 to 45% by weight of the total styrene based on the copolymer, and which had tin-carbon bond exhibited a high strength at break, a high abrasion resistance, and a low hysteresis loss.
However, controlling the content of the short styrene block is difficult in this process, and long styrene blocks which are inferior in the low hysteresis loss characteristic tend to be formed. The content of short styrene blocks is measured by the ozonolysis-gel permeation chromatography (GPC) method in the above patent application. This method provides a different value from the method in the present invention which uses the nuclear magnetic resonance spectroscopy (NMR) and GPC in combination. For example, a styrene-butadiene copolymer obtained in an example of the above patent application had a content of short styrene blocks of 48% by weight. However, when the measurement is made on this copolymer by the method using NMR and GPC in combination, the copolymer contains 78% by weight of short styrene blocks. (This is described below with reference to data.) Therefore, a greater amount of long styrene blocks are formed in accordance with the process described in the above patent application to cause inferior low hysteresis loss characteristic.
In Japanese Patent Application Laid-Open Heisei 1(1989)-230647, a styrene-butadiene copolymer was obtained by using an organolithium initiator, a potassium compound, and a tin compound. The obtained copolymer comprises 40% by weight or less of the isolated single styrene unit, 60% by weight or more of styrene sequences having 2 to 7 styrene units, and 10% by weight or less of styrene sequences having 8 or more styrene units all based on the total styrene content, and 5 to 50% by weight of the total styrene based on the copolymer, and showed a high strength at break, a high abrasion resistance, and a high resilience. However, this copolymer was insufficient in the wet skid resistance and the low hysteresis loss characteristic.
It is generally known that, for incorporating short styrene blocks into a styrene-butadiene copolymer, styrene and butadiene are copolymerized by using an organolithium initiator and a potassium compound. However, controlling the amount of short styrene blocks is difficult in this process. In order to satisfy the requirement for the low hysteresis loss characteristic, it is necessary that the distribution of styrene in the main chain be made uniform. This requirement can be sa tisfied by using a suitable amount of a potassium compound. However, the amount of short styrene blocks is increased, for example, to 70% or more under such a condition, and the requirement for the low hysteresis loss characteristic cannot be satisfied.
On the other hand, as the process for producing a styrene-butadiene copolymer containing short styrene blocks, the process described in the U.S. Pat. No. 5,393,721 has been known. This patent discloses that styrene and butadiene are copolymerized by using a soluble initiator comprising a mixture of an organolithium compound having a cyclic amine group, an organopotassium compound, and a chelating agent such as an ether, and the resultant copolymer is treated for coupling with a tin compound to obtain a styrene-butadiene copolymer excellent in the fracture characteristics, the abrasion resistance, and the low hysteresis loss characteristic. However, when a high temperature is used for increasing the productivity in this process, the lithium at the end tends to be deactivated, and efficiency of the coupling is decreased. Thus, the low hysteresis loss characteristic of this copolymer becomes insufficient. This copolymer has another drawback in that the copolymer is inferior in the processability because amine is bonded at the end of the copolymer molecule. Thus, this copolymer sometimes is not advantageous in practical application.
SUMMARY OF THE INVENTION
The present invention has been made under the above circumstances and has objects of providing a styrene-butadiene copolymer which is excellent in the processability, has the low hysteresis loss characteristic and the wet skid resistance held at high levels, shows remarkably improved fracture characteristics and abrasion resistance, and used for tire treads, and also providing a rubber composition comprising the copolymer.
The present inventors paid attention to the molecular structure of the styrene-butadiene copolymer, particularly to the sequences of styrene units, in order to satisfy the above requirements, and conducted extensive studies. As the result, it was discovered that the above requirements can simultaneously be satisfied by the following copolymer and composition, and the present invention was completed on the basis of the discovery.
Accordingly, the present invention provides a styrene-butadiene copolymer wherein:
a total styrene content is in the range of 33 to 42% by weight of the styrene-butadiene copolymer;
a content of styrene sequences having 4 to 20 styrene units is in the range of 40 to 65% by weight of the total styrene content, as measured by nuclear magnetic resonance spectroscopy and gel permeation chromatography in combination;
a content of styrene sequences having more than 20 styrene units is in the range of 5% by weight or less of the total styrene content, as measured by gel permeation chromatography;
a content of vinyl unit is in the range of 30% by mol or less of the butadiene part of the styrene-butadiene copolymer;
a sum of the total styrene value and ½ of the content of vinyl units is in the range of 46 to 52%; and
an inner or end structure ha s a bond selected from the group consisting of tin-carbon bond, carbon-carbon bond, carb
Kondo Hajime
Morita Koichi
Ozawa Yoichi
Bridgestone Corporation
Sellers Robert E. L.
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