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Vulcanization Of Natural Rubber

Vulcanization or vulcanisation is a chemical process for converting natural rubber or related polymers into more durable materials via the addition of sulfur[1] or other equivalent curatives or accelerators. These additives modify the polymer by forming cross-links (bridges) between individual polymer chains.[2][3] Vulcanized materials are less sticky and have superior mechanical properties. The term vulcanized fibre refers to cellulose that has been treated in a zinc chloride solution to cross-link the cellulose fibers.
Although the curing of rubber has been carried out since prehistoric times, the modern process of vulcanization, named after Vulcan, the Roman god of fire, was not developed until the 19th century. Today, a vast array of products are made with vulcanized rubber including tires, shoe soles, hoses, and conveyor belts. Hard vulcanized rubber is sometimes sold under the brand names ebonite or vulcanite, and is used in making articles such as clarinet and saxophone mouth pieces, bowling balls and hockey pucks.
In contrast with thermoplastic processes (the melt-freeze process that characterize the behavior of most modern polymers) vulcanization, in common with the curing of other thermosetting polymers, is generally irreversible. The cross-linking is usually achieved by the addition of 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 car 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 less 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.
A variety of methods exist for vulcanization. The economically most important method (vulcanization of tires) uses high pressure and temperature after the curative has been added to the rubber. A typical vulcanization temperature for a passenger tire is 10 minutes at 170 °C. This process employs the technique known as compression molding, where 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:
Metallic oxides
Sulfur systems
Urethane crosslinkers