Beyond the visible tread and sidewall compounds, there are more than a dozen specially formulated compounds that are used in the interior of the tire. Each compound is engineered and has a substantial part in contributing to the tire performance.
· Polymers are the backbone of rubber compounds. They consist of natural or synthetic rubber.
· Fillers reinforce rubber compounds. The most common filler is carbon black although other materials, such as silica, are used to give the compound unique properties.
· Softeners: Petroleum oils, pine tar, resins and waxes are all softeners that are used in compounds principally as processing aids and to improve tack or stickiness of non-vulcanized compounds.
· Antidegradents: Waxes, antioxidants, and antiozonants are added to rubber compounds to help protect tires against deterioration by ozone, oxygen and heat .
· Curatives: During vulcanization or curing, the polymer chains become linked, transforming the viscous compounds into strong, elastic materials. Sulfur along with accelerators and activators help achieve the desired properties.
Material design property balance
Considering the many polymers, carbon blacks, silicas, oils, waxes and curatives, plus specialty materials such as colorants, adhesion promoters, and hardeners, the variety of compounds available seems endless. A typical car tire uses about 60 raw materials. However, the tire compounder quickly learns that adjusting one of the properties often affects other performance areas. The best tread compound for dry traction and handling might be lacking in wet/snow traction, chip/tear resistance, or fuel economy. Thus, compounds must be “engineered” or “balanced” to meet performance criteria for both the original equipment (OE) vehicle manufacturer and the aftermarket customer. . Adding to the complexity, the chosen compound must be cost-competitive and could be processed in manufacturing plants.
Reinforcement materials Purpose
Fillers are solid powders mixed into rubber to provide bulk or to modify properties, such as hardness or specific gravity. Tire compounds use reinforcing fillers which profoundly modify an elastomer’s properties. Carbon black and hydrated silicas with very small particle size and high surface areas are almost exclusively used. Reinforcing fillers interact with themselves as well as with the rubber. Control of filler—filler interaction can lead to physical property improvements.
A tire’s reinforcing materials — tire cord and bead wire — are the predominant load carrying members of the cord-rubber composite. They provide strength and stability to the sidewall and tread as well as contain the air pressure.
Type and common usage
Nylon type 6 and 6,6 tire cords are synthetic long chain polymers produced by continuous polymerization/spinning or melt spinning. The most common usage in radial passenger tires is as cap, or overlay ply, or belt edge cap strip material, with some limited applications as body plies.
Advantages: Good heat resistance and strength; less sensitive to moisture.
Disadvantages: Heat set occurs during cooling (flatspotting); long term service growth.
Polyester tire cords are also synthetic, long chain polymers produced by continuous polymerization/spinning or melt spinning. The most common usage is in radial body plies with some limited applications as belt plies.
Advantages: High strength with low shrinkage and low service growth; low heat set; low cost.
Disadvantages: Not as heat resistant as nylon or rayon.
Rayon is a body ply cord or belt reinforcement made from cellulose produced by wet spinning. It is often used in Europe and in some run-flat tires as body ply material.
Advantages: Stable dimensions; heat resistant; good handling characteristics.
Disadvantages: Expensive; more sensitive to moisture; environmental manufacturing issues.
Aramid is a synthetic, high tenacity organic fiber produced by solvent spinning. It is 2 to 3 times stronger than polyester and nylon. It can be used for belt or stabilizer ply material as a light weight alternative to steel cord.
Advantages: Very high strength and stiffness; heat resistant.
Disadvantages: Cost; processing constraints (difficult to cut).
Steel cord is carbon steel wire coated with brass that has been drawn, plated, twisted and wound into multiple-filament bundles. It is the principal belt ply material used in radial passenger tires.
Advantages: High belt strength and belt stiffness improves wear and handling.
Disadvantages: Requires special processing ; more sensitive to moisture.
Bead wire is carbon steel wire coated with bronze that has been produced by drawing and plating. Filaments are wound into two hoops, one on each side of the tire, in various configurations that serve to anchor the inflated tire to the rim
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