RUBBER COMPOUND FORMULATION
The two major ingredients in a rubber compound are the rubber itself and the filler, combined in such a way as to achieve different objectives. Depending on the intended use of the tire, the objective may be to optimize performance, to maximize traction in both wet and dry conditions, or to achieve superior rolling resistance. The desired objective can be achieved through the careful selection of one or more types of rubber, along with the type and amount of filler to blend with the rubber.
In general, there are four major rubbers used: natural rubber, styrene-butadiene rubber (SBR), polybutadiene rubber (BR), and butyl rubber (along with halogenated butyl rubber). The first three are primarily used as tread and sidewall compounds, while butyl rubber and halogenated butyl rubber are primarily used for the inner liner or the inside portion that holds the compressed air inside the tire.
The most popular fillers are carbon black and silica, and there are several types of each. The selection depends on the performance requirements, as they are different for the tread, sidewall, and apex. Other ingredients also come into play to aid in the processing of the tire or to function as antioxidants, antiozonants, and anti-aging agents. In addition, the “cure package”—a combination of curatives and accelerators—is used to form the tire and give it its elasticity.
RUBBER COMPOUND MIXING
Once the compound is determined, the next challenge is how to mix it all together. The mixing operation is typically a batch operation, with each batch producing more than 200 kilograms of rubber compound in less than three to five minutes. The mixer is a sophisticated piece of heavy equipment with a mixing chamber that has rotors inside. Its main function is to break down the rubber bale, fillers and chemicals and mix them with other ingredients.
The sequence in which the ingredients are added is critical, as is the mixing temperature, which can rise as high as 160-170 degrees Celsius. If the temperature is too high, the compound can be damaged, so the mixing operation is typically accomplished in two stages. The curative package is normally added in the final stage of mixing, and the final mixing temperature cannot exceed 100-110 degrees Celsius or scorching may occur.
Once the mixing is completed, the batch is dumped out of the mixer and sent through a series of machines to form it into a continuous sheet called a “slap.” The slap is then transferred to other areas for bead wire assembly preparation, inner liner calendering, steel and/or fabric belt/ply cord calendering, tire sidewall extrusion, and tire tread extrusion.