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
2000-04-28
2002-10-29
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, C524S495000, C524S496000, C152S209500
Reexamination Certificate
active
06472461
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a rubber composition for tires and a pneumatic tire manufactured by using the rubber composition. More particularly, the present invention relates to a rubber composition which gives a pneumatic tire exhibiting improved braking ability on ice, braking ability on wet roads and driving stability and to a pneumatic tire using the rubber composition.
2. Description of the Related Arts
In recent years, studless tires are used as the tires exhibiting improved performances on ice. For a tread of a studless tire, a rubber composition comprising a polymer, such as polybutadiene, having a glass transition temperature lower than that of polymer used for an ordinary tire is generally used as a main component to enhance friction between the tread and frozen road surfaces. In a tire having a tread portion having a two-layer structure composed of a cap layer and a base layer, a rubber composition which is relatively soft at lower temperatures, for example by comprising a large amount of softeners or by foaming is used for a cap layer which is an outer layer of the tread and contacts road surfaces. However, this method has a drawback in that braking ability on wet surfaces of ordinary roads inevitably deteriorates although performances on ice are improved to some extent.
On the other hand, as a method for increasing the friction coefficient on wet road surfaces, it has heretofore been known that the value of tan &dgr; at 0° C., an indicator of the degree of hysteresis loss which is well correlated to the friction coefficient on wet road surfaces, is increased by using a rubber composition containing a styrene-butadiene copolymer having a relatively high glass transition temperature as the main rubber component. With respect to the performances on ice, however, this method has a drawback in that modulus of the rubber composition increases at a low temperature region which causes the rubber composition failing to follow the change in the shape of the road surface. And further deformation of the tread surface decreases because roughness of frozen road surfaces is generally smaller than that of the road surfaces in the ordinary conditions and, as the result, contribution of energy dissipation (tan &dgr;) between the rubber composition and road surfaces to the braking on ice decreases.
As described above, it has heretofore been difficult that both of the braking ability on ice and the bring ability on wet road surfaces are simultaneously kept at high levels,
SUMMARY OF THE INVENTION
Under the above circumstances, an object of the present invention is to provide a rubber composition which gives a pneumatic tire both a high level of braking ability on ice and a high level of braking ability on wet roads required in the market and further an improved driving stability in the early driving period and a pneumatic tire using the rubber composition. Another object of the present invention is to provide a pneumatic tire which enables remarkable improvement in braking ability on ice even after a base rubber is exposed by abrasion of a tread rubber.
As a result of intensive studies by the present inventors in order to achieve the above objects, the present invention has been completed.
In the first aspect of the present invention, a rubber composition which comprises (1) a rubber component comprising at least one of natural rubber and synthetic diene rubbers and a polybutadiene having a weight-average molecular weight of 5,000 to 30,000 obtained as a weight-average molecular weight of corresponding polystyrene and a content of a cis-1,4 structure of 60 to 98% in an amount of 6% by weight or more, and (2) a short fiber having an average length of 10 mm or shorter is provided.
It is preferable that the rubber component comprises 20% by weight or more of polybutadiene rubber and that the rubber composition comprises a polybutadiene rubber having a weight-average molecular weight of 300,000 or more obtained as a weight-average molecular weight of corresponding polystyrene and a content of the cis-1,4 structure of 80% or more.
It is also preferable that the content of the short fiber is 1 to 20 parts by weight per 100 parts by weight of the rubber component and that the short fiber is polyethylene fiber. It is also preferable that the rubber composition comprises carbon black in combination with silica or aluminum hydroxide in combination as a reinforcing filler and that the rubber composition comprises a blowing agent.
In the second aspect of the present invention, a pneumatic tire comprising the above rubber composition in a tread rubber is provided. It is preferable that, when a tread part of the pneumatic tire has a multi-layered structure, the multi-layered structure comprises a cap layer disposed at the outer side in the radial direction of the tire and a base layer disposed at the inner side in the radial direction of the tire. It is also preferable that a height from a bottom of a groove to an upper end of the base layer is 20% or more of a depth of the groove in a tire tread which has been formed and vulcanized.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The rubber composition of the present invention comprises a rubber component comprising a rubber selected from the group consisting of natural rubber and synthetic diene rubbers, a specific low molecular weight polybutadiene and short fiber.
The low molecular weight polybutadiene used in the rubber composition preferably has a weight-average molecular weight of 5,000 to 80,000 and more preferably 10,000 to 20,000 obtained as the weight-average molecular weight of corresponding polystyrene. It is generally difficult to produce a polybutadiene having a weight-average molecular weight of less than 5,000 industrially. When the weight-average molecular weight exceeds 30,000, the brag ability on wet road surfaces is not sufficiently improved. It is necessary that 6% by weight or more of the low molecular weight polybutadiene be contained in the rubber component. It is preferable that 10 to 40% by weight of the low molecular weight polybutadiene is contained in the rubber component. When the amount of the low molecular weight polybutadiene is less then 6% by weight, the braking ability on wet road surfaces is not sufficiently improved. The above rubber composition comprising the low molecular weight polybutadiene has an improved value of tan &dgr; at 0° C. which is an indicator for the braking ability on wet road surfaces and adverse affects on the braking ability on ice can be suppressed because of the proper characteristic of polybutadiene.
In the rubber composition of the present invention which comprises short fiber as the essential component, the iron coefficient on ice can be increased by the modified form of the surface of the tread and the decrease in the friction coefficient on ice due to an increase in rigidity of tread blocks caused by the presence of the short fiber can be suppressed. The increase in rigidity of tread blocks described above rather advantageously affects the effect of the low molecular weight polybutadiene to enhance the braking ability on wet road surface and thus tan &dgr; at 0° C. further increases. Therefore, in the pneumatic tire in which the rubber composition of the present invention comprising the specific low molecular weight polybutadiene and short fiber is used for the tread, the excellent braking ability on ice and the excellent braking ability on wet road surface can be obtained both at high levels.
When a tread of a pneumatic tire is made from the above rubber composition, in particular, from the rubber composition comprising polyethylene fiber as the short fiber and a blowing agent in combination, the braking ability of the tire on ice is remarkably improved. The mechanism of the improvement can be considered as follows.
When the rubber composition comprising polyethylene short fiber is used for a tread rubber, gases in the rubber are gathered at the polyethylene resin having a decreased viscosity during vulcanization and long closed
Ishikuro Toshiyuki
Nakamura Eiji
Bridgestone Corporation
Cain Edward J.
Sughrue & Mion, PLLC
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