The present invention relates to a process for producing pneumatic tires having high uniformity.
As is well known, cords formed by twisting multi-filaments of rayon, nylon, polyester or the like, or steel wires are used as reinforcing materials for pneumatic tires to be used on a car, and these cords are coated with rubber to provide the carcass plies of the tire.
Tires produced using nylon cords, however, have the problem that when a car with such tires thereon is left standing for a long period of time, a so-called flat spot, i.e., a deformation due to load, is formed in the tires, which leads to an uncomfortable ride at the initial stage of driving the car after sitting. Also, rayon cords have the problem that they have poor strength, and steel cords have the problem that their weight per unit strength is large relative to other types of cords.
On the other hand, polyester cords are free from the foregoing problems and, therefore, they have gained increasingly wider use. However, with extension and improvement of roads and high speed driving, some problems have become apparent in tires comprising polyester cords, such as poor tire uniformity. Thus, it would be very desirable, with respect to carcass plies composed of polyester cord, to provide a process for producing improved dimensional stability and tire uniformity of polyester cord tires without degrading fatigue resistance.
When using tires composed of carcass plies of polyester cords, so-called inflation growth occurs, i.e., the cord is stretched by tension, thus resulting in expansion of the tire. When the inflation growth occurs, the tread rubber and side wall rubber are also stretched. This leads to a reduction in the abrasion resistance and cracking resistance of the tire. In order to prevent inflation growth, it is desirable to reduce the elongation under load of the polyester cord.
Radial tires, in general, are built as follows:
Cord fabrics are coated with rubber and cut in a direction perpendicular to the length of the cords. These cut fabrics are joined with the cords overlapping in a parallel direction to provide a continuous band by overlapping from 1 to 10 end cords of overlapping pieces of fabric. These bands are wrapped once around a tire building drum, cut a few cm longer than the circumference of the drum and joined along the cords of the wrapped band by overlapping from 1 to 5 end cords of the band, so that the band is formed into a cylindrical carcass ply. The carcass ply is contacted with the bead assemblies on the bead portion of the carcass and the edges of the ply are folded around the bead assemblies. Then, the diameter of the carcass ply is increased while reducing the distance between both bead portions and the carcass ply is shaped into a toroidal form. On the surface of the carcass ply are then superposed a belt ply, tread rubber and side wall rubber to form a "green tire" (an unvulcanized tire is called a green tire). The green tire is placed in a mold and vulcanized by heating both the internal and external surfaces of the tire.
During vulcanizing, the internal cavity of the tire is pressurized with steam or hot water pressure, so that the cords are stretched, and then the vulcanized tire is removed from the mold and cooled while applying post inflation pressure in order to prevent shrinkage of the cords. The above-described process of production is similarly applicable to the production of bias tires.
The stiffness of the joined area of the carcass ply of the tire thus composed is higher than other areas of the carcass ply. Furthermore, if the arrangement of cords and cord angles is not uniform, the areas having a high stiffness may be less extended in the shaping of the carcass ply into the toroidal form than other areas. This amplifies the difference in stiffness between the joined areas and the other areas. Such local unevenness in stiffness of the carcass ply brings about an inequality in tension exerted on the cord during post inflation, and therefore an unevenness in the elongation of the cord occurs. Also, since the temperatures of all portions of the mold are not even (because the thickness of the rubber is not even), the temperature distribution in the tire is not always uniform and, during the vulcanizing process thermal shrinkage of the cord occurs unevenly. This unevenness in the thermal shrinkage, in combination with the local unevenness in stiffness of the carcass ply, cause inequality in the elongation of the cord during post inflation. This inequality in the elongation of the cord amplifies the local unevenness in stiffness of the tire.
When a car with the above-described tires having unevenness in stiffness is driven, abnormal vibration occurs, since the degree of deformation changes periodically. This abnormal vibration leads to an uncomfortable ride. If the unevenness increases, the car swings periodically and becomes unstable. The above-described unevenness formed in the tire during the production thereof is due to the high thermal shrinkage of conventional polyester cords. It is, therefore, desired that the polyester cord be made to have a low thermal shrinkage.
The dimensional stability of the cords is represented by the reciprocal of a dimension unstabilizing factor, which is the sum of the elongation (%) at a load of 2.3 g/d and the thermal shrinkage (%) when heated in dry air for 30 minutes. When elongation at a specific load is increased, the thermal shrinkage is decreased. For this reason, low thermal shrinkage and low elongation under specific load are, in general, not compatible with each other and bear an inverse relationship in a conventional polyester cord. For example, the dimension unstabilizing factor of a cord prepared by twisting two 1,500 denier yarns of polyethylene terephthalate (intrinsic viscosity 0.9) at a twist coefficient of 2,190 is within the range of from 9.0 to 9.5. Even though the degree of elongation at load is reduced by changing the heat treating conditions, the thermal shrinkage is increased, and thus the dimension unstabilizing factor remains unchanged.
In order to produce tires having high uniformity, polyester cords in which both the elongation under load and the thermal shrinkage are small, i.e., the dimension unstabilizing factor is small, are required. Previously, in order to reduce the dimension unstabilizing factor, the cord was subject to a heat treatment for a long period of time, or made from polyester having a relatively low viscosity of polymerization, i.e., an intrinsic viscosity of 0.8 or less, or the twist coefficient of the cord was made lower than cord intended for general use. However, the strength and fatigue resistance of the cord prepared in such a manner were lower than cord intended for general use and, therefore the overall practical utility of the tire cord was inferior.