Additionally, in response to increasing social demands for energy and resource saving, there is an increasing need for a rubber material that is excellent in weather resistance and crack growth resistance to meet the requirements for improving durability of tires in order to reduce fuel consumption of automobiles. In addition, in view of the recent surge in price of butadiene, it is expected that the price of raw materials will even more dramatically rise in the future. As such, there is a need to make use of inexpensive olefin resource even in tire materials. To address this issue, some techniques have been proposed to improve weather resistance by blending an ethylene-propylene-diene rubber (EPDM) containing olefin as a principal component. These techniques, however, suffer from drawbacks, such as lower fracture resistance and lower crack growth resistance (see, for example, JP 2000-063639 A (PTL 1)).
In addition, applying a copolymer of a conjugated diene and a non-conjugated olefin to the compounded rubber results in fewer double bonds in the conjugated diene unit (conjugated diene compound-derived unit) in the copolymer as compared with conjugated polymers, and therefore, improved ozone resistance. In addition, the characteristics other than ozone resistance required when a rubber composition is applied to various applications (such as tires, conveyor belts or anti-vibration rubber) include good fracture resistance and good crack growth resistance. It is well known that coordination anionic polymerization using catalyst systems represented by a Ziegler-Natta catalyst allows for homopolymerization of olefins and dienes. However, it was difficult to provide efficient copolymerization of olefins and dienes using such polymerization systems.
To address this issue, for example, JP 2000-154210 A (PTL 2) discloses such a catalyst for polymerizing a conjugated diene that contains a transition metal compound of group IV of the periodic table having a cyclopentadiene ring structure, and also discloses α-olefins such as ethylene as monomers which can be copolymerized with the conjugated diene. However, PTL 2 does not provide a specific description of copolymerization of a conjugated diene compound and a non-conjugated olefin. Obviously, there is no description or suggestion that a rubber which is excellent in weather resistance, fracture resistance and crack growth resistance can be obtained by mixing: a conjugated diene compound/non-conjugated olefin copolymer containing a conjugated diene compound-derived unit by at least 40 mol %; a conjugated diene-based polymer; and a non-conjugated diene compound/non-conjugated olefin copolymer containing an ethylene-propylene-diene rubber.
For example, JP 2006-249442 A (PTL 3) discloses a catalyst for polymerizing olefins that consists of a transition metal compound such as a titanium compound and a co-catalyst, and also discloses a copolymer of an α-olefin and a conjugated diene compound. However, there is no description or suggestion in PTL 3 that a rubber which is excellent in weather resistance, fracture resistance and crack growth resistance can be obtained by mixing: a conjugated diene compound/non-conjugated olefin copolymer containing a conjugated diene compound-derived unit by at least 40 mol %; a conjugated diene-based polymer; and a non-conjugated diene compound/non-conjugated olefin copolymer containing an ethylene-propylene-diene rubber.
In addition, JP 2006-503141 A (PTL 4) discloses a copolymer of ethylene and butadiene that is obtained by synthesizing ethylene and butadiene as a starting material by means of a special organic metal complex as a catalytic component. However, there is no description or suggestion in PTL 4 that a rubber which is excellent in weather resistance, fracture resistance and crack growth resistance can be obtained by mixing: a conjugated diene compound/non-conjugated olefin copolymer containing a conjugated diene compound-derived unit by at least 40 mol %; a conjugated diene-based polymer; and a non-conjugated diene compound/non-conjugated olefin copolymer containing an ethylene-propylene-diene rubber.
In addition, JP 2000-086857 A (PTL 5) discloses a butadiene polymer having cis content of 92%, vinyl content of 6% and ethylene content of 3% or 9%. However, there is no description or suggestion in PTL 5 that a rubber which is excellent in weather resistance, fracture resistance and crack growth resistance can be obtained by mixing: a conjugated diene compound/non-conjugated olefin copolymer containing a conjugated diene compound-derived unit by at least 40 mol %; a conjugated diene-based polymer; and a non-conjugated diene compound/non-conjugated olefin copolymer containing an ethylene-propylene-diene rubber.
In addition, JP 2000-154279 (PTL 6) discloses a rubber composition including: a butadiene-ethylene block copolymer having a cis content of 92% and ethylene segments in an amount of 4.8 mass % of the total; polybutadiene having a cis content of 95.2% and a vinyl content of 2.5%; and carbon black N220. However, there is no description or suggestion in PTL 6 that a rubber which is excellent in weather resistance, fracture resistance and crack growth resistance can be obtained by mixing: a conjugated diene compound/non-conjugated olefin copolymer containing a conjugated diene compound-derived unit by at least 40 mol %; a conjugated diene-based polymer; and a non-conjugated diene compound/non-conjugated olefin copolymer containing an ethylene-propylene-diene rubber.
In addition, JP 11-228743 (PTL 7) discloses an olefin-rich, olefin-diene copolymer. However, there is no description or suggestion in PTL 7 that a rubber which is excellent in weather resistance, fracture resistance and crack growth resistance can be obtained by mixing: a conjugated diene compound/non-conjugated olefin copolymer containing a conjugated diene compound-derived unit by at least 40 mol %; a conjugated diene-based polymer; and a non-conjugated diene compound/non-conjugated olefin copolymer containing an ethylene-propylene-diene rubber.