Body plies: more than meets the eye
Not much is said about the use of textile fiber cords in tires, yet the technology involved is remarkable
In the first few motoring decades, technology made great strides in a number of sectors. From the design and size of the main components, to materials, processes and treatments, as well as ignition systems, lubricants and fuels. On the other hand, for tires, though production volumes soon reached considerable figures and manufacturers quickly grew in size, technological developments were rather slow in coming, particularly with regards to textile fibers, a fundamental component of a tire’s main structure, the carcass or casing.
Until the latter part of the thirties, the scene was totally dominated by cotton, just as at the beginning of the century. Then, steel and rayon quietly made an appearance, with the latter being a cellulose derivative with excellent characteristics, produced in different types, which soon completely replaced cotton and still finds many useful applications in our industry today.
In the late forties, while nylon started becoming commercially available and widely distributed, radial tires made their appearance on the market and steel began to be used for their belts. Its remarkable rigidity and its high mechanical strength in fact guaranteed maximum stability to the structure found immediately under the tread area. Just over a decade later, in the early sixties, a new synthetic polymeric fiber made its appearance. Soon to be widely used in the production of tires, polyester found also a large number of applications in other industrial sectors as well.
The first use of an aramid synthetic fiber (Kevlar being by far the best known example of this) dates back to shortly before the middle of the following decade. Technical developments did not stop there though, things quickly evolved leading to the creation of new generations of rayon and polyester. Today, a metal (steel) and four distinct groups of textile fibers are largely used in the tire industry.
As regards these latter ones, filaments are twisted in order to form the yarns, which in turn are twisted in the opposite sense to form the cords, soon to be joined to an elastomeric matrix to form the cap plies. Nowadays, a combination of polyester casings and steel belts are a rather popular solution on passenger cars.
Due to its low density, great weight/size ratio and good mechanical characteristics, nylon makes the production of lighter tires possible, hence the large use of it by manufacturers. On the other hand, rayon is experiencing a revival on high performance tires. Although radial belts are in most cases made of steel, aramid and para-aramid synthetic fibers are sometimes used too.
Due to its good mechanical resistance, remarkable stiffness and low cost, polyester has reached huge popularity among manufacturers. However, despite all these positive characteristics, it is not suitable for all applications since the above mentioned qualities undergo a drastic deterioration operating at high temperatures (as in the case of heavy duty or industrial applications). On the contrary, Rayon, whose mechanical properties have greatly increased over the years thanks to research and development, can work at fairly high temperatures, albeit at a much higher cost.
Nylon 66, the most widely used polyamide fiber, has a relatively low stiffness but an excellent resistance to impact and good tensile strength, perfect for the production of sturdy yet light casings; these are extensively used in tires for industrial and aeronautical vehicles. Aramid fibers, on the other hand, are characterized by a high breaking point, a considerable stiffness and a good resistance to high temperatures. However, the high costs involved strongly limit their use to racing tires and aircrafts.
The cords should have characteristics such as high tensile strength, stiffness and resistance to wear in order to ensure great dimensional stability, a long life span (which depends largely on resistance to mechanical stress and a reduced generation of heat during use) and adequate traction and grip.
The production process starts with a spinning phase, performed with different techniques: melt spinning (from molten material) for nylon and polyester, and wet- solution spinning (with solvents) for rayon and aramid fibers, which have a higher melting point. The threads are twisted together in yarns and then cabled into cords (6 to 12 turns per inch indicatively). The ply is then created by weaving the cords parallel to each other and spacing them evenly. At this point, the roll of fabric obtained is dipped into an adhesive that must adequately penetrate the cords and properly "wet" the fibers that constitute them, followed by the vulcanizing or curing process.
Steel, the main material along with aramid fibers for radial belts, is reduced into wires with a diameter in the order of 0.15 to 0.40 mm, which are then joined and twisted to form first the strands and then the cords. Generally high carbon steels (0.65 to 0.95%) are used. To facilitate the bonding between steel and rubber and to ensure a good protection against corrosion, the wires are first electroplated with copper and then zinc; since bonding steel and rubber is a rather critical step, the next procedure provides for a metal interdiffusion that forms a less than one micron protective layer of brass.