Vulcanization

Vulcanization is the process by which rubber molecules (polymers or macromolecules made of repeating units or monomers called isoprene) are cross-linked with each other by heating the liquid rubber with sulfur. Cross-linking increases the elasticity and the strength of rubber by about ten-fold, but the amount of cross-linking must be controlled to avoid creating a brittle and inelastic substance. The process of vulcanization was discovered accidentally in 1839 by the American inventor Charles Goodyear (1800-1860) when he dropped some rubber containing sulfur onto a hot stove. Goodyear followed up on this discovery and subsequently developed the process of vulcanization. In 1844, Goodyear was issued United States Patent #3644.

Figure 1. The process of vulcanization. Illustration by Hans & Cassidy. Courtesy of Gale Group.

Natural rubber comes from the rubber tree (Hevea brasiliensis) and is a white, milky liquid called latex. Most rubber comes from Malaysia and other nations in East Asia. Latex can also be seen as the white fluid in dandelion stalks. The latex from the tree is actually a suspension of rubber particles in water. Rubber is a polymer (long chain made of repeating units) of isoprene. Natural rubber is relatively reactive, and is especially vulnerable to oxidation.

In the process of vulcanization, the added sulfur allows some C-H bonds to be broken and replaced by C-S bonds. The process of vulcanization cross-links the chains or polyisoprene to each other. The cross-linked molecules create a three-dimensional network of rubber. Each cross-link is a chain of about eight sulfur atoms between two long chains of polyisoprene.

Vulcanized rubber is about 10 times stronger than natural rubber and is also about 10 times more rigid. However, it is still very elastic, which means that is can be stretched reversibly. Polymers that are elastic are sometimes called elastomers. The optimum amount of sulfur to be added to the rubber is about 10% by weight. Adding an excess of sulfur produces a very brittle and inelastic substance called ebonite. Man-made or synthetic rubber can also be vulcanized, and the process is similar.

Figure 1 shows what happens to rubber when the long chains of polyisoprene are cross-linked. In part a, the macromolecules are bent and randomly arranged. In part b, the chains are cross-linked but still randomly arranged. The molecules become aligned when the rubber is stretched. If the individual chains were not crosslinked, each chain could slide freely past each other.