1. Field of the Invention
The present invention relates to a steel cord for reinforcing rubber articles such as pneumatic tires, industrial belts and the like. More particularly it relates to rubber articles and their steel cord having enhanced rubber penetration property, enhanced resistance to cut separation indicating tire durability, and good resistance to BES (belt edge separation). The invention also particularly relates to pneumatic radial tires using such steel cord.
2. Description of the Prior Art
Products reinforced with steel cords are liable to suffer from corrosion of the steel filament caused by water entering the products and thereby the durability and life of the products are lowered.
For example, when steel cords used in a belt of a tire have a void and the tire tread are subjected to damage reaching the belt, water entering the belt spreads along the longitudinal direction of the cord through the voids in the steel cords. As a result, rust formed due to water also diffuses and the adhesion between rubber and steel cord is lowered at that portion. Finally, separation phenomena occurs.
In order to prevent such corrosion propagation, there is proposed a cord structure in which rubber can sufficiently penetrate into the inside of the cord through gaps between adjacent metal filaments by vulcanization under pressure.
Japanese Patent Application Laid-open Nos. 8208/1985 and 1790/1984 disclose one of the above-mentioned cord structures, a so-called "1+5 structure" cord composed of one core filament and five sheath filaments, having gaps between sheath filaments and rubber and can easily penetrate into the gaps. Further this cord can be produced by one-step twisting and thereby the productivity is high.
Indeed the average sheath gaps are sufficient in such a cord structure, but deviation occurs in the arrangement of sheath filaments and there are formed attaching portions of the filaments resulting in forming of portions where rubber does not penetrate due to a fluctuation in the manufacturing procedure.
The objects of the present invention are to provide a steel cord having remarkably enhanced rubber penetration property by changing the structure of a core filament and good resistance to BES, and to provide a pneumatic radial tire using the same.