Vulcanization or vulcanisation (British) is a process of hardening rubber using chemicals and (usually) heat. Originally, vulcanization described the treatment of natural latex rubber with heat and sulfur. While this remains the most common type of vulcanization, the process also applies to synthetic rubber and may involve other chemicals.
History
Many plants produce latex, including the rubber tree (Hevea brasiliensis), bananas, milkweeds, and dandelions. Latex contains the polymer polyisoprene, which is elastic and waterproof. Archeological studies show the Mesoamericans used rubber for balls and waterproofing textiles and containers.
Charles Marie de La Condamine presented samples of American rubber to the Académie Royale des Sciences of France in 1736. In 1770, Joseph Priestley observed the material rubbed pencil marks off paper, leading to the term “rubber.” Rubber found many uses, but it suffered from the effects of temperature of time. Cold caused rubber to harden and break. Heat made it sticky.
In 1839, Charles Goodyear discovered combining rubber with sulfur and heating the mixture hardened the material. Goodyear patented the discovery in the United States in 1844, while Thomas Hancock patented the process in Britain in 1843. Hancock’s friend, William Brockendon, coined the term “vulcanisation” in reference to Vulcan, the Roman god of fire and forging. The American spelling of the word became “vulcanization.”
How Vulcanization Works
Vulcanization hardens or cures rubber by cross-linking its polymer strands. In Goodyear’s process, heat supplies the energy to connect the carbon atoms in polyisoprene molecules by sulfur bridges. The number of sulfur atoms forming the chain between molecules influences the final properties of the vulcanized rubber. For example, short rubber crosslinks yield rubber with high heat resistance. Long chains of crosslinks are more flexible, but have lower heat resistance.
In the original process, the vulcanization agent is sulfur. Other compounds act as vulcanization agents for the synthetic rubbers silicone and chloroprene (neoprene).
The five most common types of vulcanization agents are:
- Sulfur or sulfur compounds
- Metallic oxide (MgO, ZnO, sometimes Pb3O4)
- Peroxides
- Acetoxysilane
- Urethane
Vulcanization of natural rubber requires heat, but it’s not an essential factor in all processes. For example, silicone vulcanization occurs at room temperature. Some processes use radiation instead of heat.
Modern vulcanization often includes a chemical called an accelerator. For example, ethylene thiourea (ETU) is an accelerator using in curing polychloroprene rubber. Carbon black is an accelerator for natural rubber, plus the addition improves properties of the finished product.
Effects of Vulcanization
Vulcanization changes the chemical composition of rubber, so of course it alters the material’s chemical, physical, and mechanical properties. Here are some of the effects:
- Shrinkage: Vulcanization shrinks rubber, as cross-linking draws the polymer chains closer together.
- Irreversible: The effects of vulcanization are permanent and cannot be undone using heat or chemical processes.
- Maintains shape: While vulcanization shrinks rubber, it does not change an object’s shape.
- Increased viscosity
- Increased hardness
- Decreased deformation
Uses of Vulcanized Rubber
Today, most rubber is vulcanized. Vulcanized rubber finds use in vehicle tires, shoe soles, erasers, toys, shoe soles, wet suits, hoses, and belts.
References
- Akiba, M. (1997). “Vulcanization and crosslinking in elastomers”. Progress in Polymer Science. 22 (3): 475–521. doi:10.1016/S0079-6700(96)00015-9
- Engels, Hans-Wilhelm; et al. (2011) “Rubber, 9. Chemicals and Additives”. Ullman’s Encyclopedia of Industrial Chemistry. Wiley-VCH, Weinheim. doi:10.1002/14356007.a23_365.pub3
- Hosler, D.; Burkett, S.L.; Tarkanian, M.J. (1999). “Prehistoric polymers: Rubber processing in ancient Mesoamerica”. Science. 284 (5422): 1988–1991. doi:10.1126/science.284.5422.1988
- Mark, James E.; Erman, Burak (eds.) (2005). Science and Technology of Tubber. ISBN 0-12-464786-3.