Rubber is a plastic polymer that arises from a milky emulsion, known as latex, in plant salvia, although more than half of the rubber used today is synthetic. The vulcanization of rubber is an irreversible process by which raw rubber is heated together with sulfur in order to make it harder and more resistant to cold, as Goodyear verified in 1839.
During vulcanization, closely parallel linear polymers form cross-linking bridges with each other. The end result is that the elastic rubber molecules are linked together to a greater or lesser extent, which forms a more stable, hard rubber, more resistant to chemical attack and without losing its natural elasticity. It also transforms the sticky surface of the material into a smooth surface that will not stick to metal or plastic substrates.
Normally, chemical crosslinking is done with sulfur, but there are other technologies such as systems based on organic peroxides, polychloroprene or silicone vulcanization. They are often used in combination with accelerating and retarding agents.
The most widespread method uses high pressure and temperature: around 10 minutes at 170°C. This type of vulcanization uses the so-called compression molding; the rubber article is forced into the shape of the mold.
VULCANIZATION
Vulcanization is an irreversible process during which, through chemical modification, the polymer becomes less plastic, acquires or improves its elastic properties and assumes greater resistance to swelling if it is kept in contact with organic compounds.
Vulcanization was discovered in 1839 by Charles Goodyear. Goodyear discovered a vulcanization process for rubber using sulfur and basic lead carbonate. He found that when a mixture of natural rubber, sulfur, and lead carbonate was heated, the rubber transformed from a thermoplastic material into an elastomer. He then discovered the ability of rubber latex to bind sulfur at high temperature and transform into a product with mechanical and physical properties superior to those of raw rubber (it also totally lost its stickiness), and considerably improve its elastic capabilities.
Most of the rubber objects, once the manufacture is finished, are subjected to vulcanization under conditions of high temperature and pressure. In some cases, the objects are vulcanized in molds pressed by hydraulic presses, in other cases they are subjected to internal or external steam pressure during heating.
Vulcanization is a temperature and time dependent process: as the temperature increases, a consequent increase in the vulcanization rate is required. However, using a higher temperature to reduce vulcanization times is an approach that has limits: in fact, vulcanized products can be damaged by excessive temperatures, the so-called reversion phenomenon can occur, which consists of a chemical decomposition of the vulcanized product with the consequent softening and loss of elasticity thereof.
Regardless of the vulcanization process, shape and thickness of the product, vulcanization temperatures can vary from room temperature to approximately 300°C, while times vary from a few seconds to several hours.
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