THE VULCANIZATION OF RUBBER

The picture show a bowling ball that have undergo vulcanization process.Charles Goodyear (1800–1860) invented vulcanization of rubber when he was experimenting, and heated a mixture of rubber and sulfur.Vulcanization or vulcanisation is a chemical process for converting rubber or related polymers into more durable materials via the addition of sulfur or other equivalent "curatives." These additives modify the polymer by forming crosslinks (bridges) between individual polymer chains.^[1] Vulcanized material is less sticky and has superior mechanical properties. A vast array of products are made with vulcanized rubber including tires, shoe soles, hoses, and hockey pucks. The process is named after Vulcan, Roman god of fire. Hard vulcanized rubber is sometimes sold under the brand names ebonite or vulcanite, and is used to make hard articles such as bowling balls and saxophone mouth pieces.

Process

Vulcanization is generally irreversible, similar to other thermosets and in contrast to thermoplastic processes (the melt-freeze process) that characterize the behavior of most modern polymers. The cross-linking is usually done with sulfur, but other technologies are known, including peroxide-based systems.
The main polymers subjected to vulcanization are polyisoprene (natural rubber) and styrene-butadiene rubber (SBR), which are used for most passenger tires. The "cure package" is adjusted specifically for the substrate and the application. The reactive sites—"cure sites"—are allylic hydrogen atoms. These C-H bonds are adjacent to carbon-carbon double bonds. During vulcanization, some of these C-H bonds are replaced by chains of sulfur atoms that link with a cure site of another polymer chain. These bridges contain between one and eight atoms. The number of sulfur atoms in the crosslink strongly influences the physical properties of the final rubber article. Short crosslinks give the rubber better heat resistance. Crosslinks with higher number of sulfur atoms give the rubber good dynamic properties but with lesser heat resistance. Dynamic properties are important for flexing movements of the rubber article, e.g., the movement of a side-wall of a running tire. Without good flexing properties these movements rapidly form cracks and, ultimately, make the rubber article fail.

 Vulcanization methods

A variety of methods exist for vulcanization. The economically most important method (vulcanization of tires) uses high pressure and temperature. A typical vulcanization temperature for a passenger tire is 10 minutes at 170 °C. This type of vulcanization is called compression molding. The rubber article is intended to adopt the shape of the mold. Other methods, for instance to make door profiles for cars, use hot air vulcanization or microwave heated vulcanization (both continuous processes).
Five types of curing systems are in common use. They are:

1. Sulfur systems
2. Peroxides
3. Urethane crosslinkers
4. Metallic oxides
5. Duece

   that how the vulcanization process conducted in industry scale.