Sulfur Vulcanization of Rubbers (polydienes)

Vulcanization is a chemical process that converts natural rubber and other polydiene elastomers into cross-linked polymers. The most common vulcanization agent is sulfur. It forms bridges between individual polymer molecules when heated with rubber. Often a catalyst and initiator is added to accelerate the vulcanization process. The cross-linked elastomers have much improved mechanical properties. In fact, unvulcanized rubber has poor mechanical properties and is not very durable.

The cross-linking process is rather complicated and involves a sequence of reactions. A free-radical mechanisms was originally assumed which has been described in great detail by Paul J. Flory (1953)1 and others. However, all evidence points to an ionic mechanism because neither radicals could be detected nor do free-radical inhibitors and retarders effect sufur vulcanization whereas organic acids, bases as well as solvents of high dielectric constant accelerate sulfur cure.2 A possible reaction mechanism for polyisoprene (natural rubber) is shown below.3 The process starts with the formation of a persulfonium ion (I) by reaction of an isoprene unit with a polarized sulfur molecule or with a sulfur cation. The persulfonium ion reacts than with another isoprene unit by allylic hydrogen abstraction to produce a polymeric allylic carbocation (II). In a third step a sulfur molecule (cyclic S8) combines with the allylic cation to produce another sulfonium ion (IV) which releases (monomeric) sulfur. The polymeric sulfonium ion then undergoes crosslinking by anionic addition to a polymeric double bond to create a new carbocation (V) which then either reacts with another sulfur molecule or it abstracts a hydrogen from another isoprene unit to create a new polymeric cation (II).

 

Rubber Vulcanization

 

The polysulfide crosslinks formed by these reactions may contain four to six sulfur atoms at low temperatures whereas at higher reaction temperatures shorter sulfur bridges are formed.

Vulcanization of rubber by sulfur alone is extremely slow and can take several hours at elevated temperatures (140°C or more). This is problematic because long exposure to temperature and oxygen leads to oxidative degradation which, in turn, results in poor mechanical properties. It is also not very economical. To minimize rubber degradation and to speed-up the vulcanization process, accelerators are usually employed. An accelerator is defined as a compound that increases the speed of vulcanization and that enables vulcanization to proceed at lower temperature and with greater efficiency. Accelerator also decreases the amount of sulfur needed to cross-link the polydiene thus improving the aging properties of the vulcanized rubber.

References and Notes
  1. Paul J. Flory, Principles of Polymer Chemistry, 1st Edition 1953 Cornell University
  2. A. Oae, Organic Chemistry of Sulfur, Plenum Press, New York 1977
  3. The actual reactions and structure of the sulfur intermediates are uncertain and most likely depend on the temperature and composition.
  4. A.M. Joseph, B. George, K.N. Madhusoosaban, and R. Alex, Rubber Science, 28(1), 82-121 (2015)
  • Summary

    The Invention of Vulcanization

    The process of vulcanization was discovered by Goodyear in 1839. He found, by accident, that sulfur and white lead vulcanizes natural rubber. His discovery was a major technological breakthrough in the rubber-good industry and made the production of durable and waterproof rubber goods possible.

  • The cross-linking reaction between sulfur and rubber occurs mainly at the carbon-carbon double bonds (C=C) in large excess of sulfur. Often accelerators are added to speed up the vulcanization and to reduce the amount of free sulfur which has a detrimental effect on the properties of the cured rubber.

  • Sulfur cure is the most common and economical curative but it is limited to elastomers that contain unsaturations such as polybutadiene and polyisoprene.

  • Some elastomers like EPDM and butyl rubber do not contain double bonds. These polymers are often chemically modified by copolymerization with small amounts of dienes to allow for vulcanization.

  • Sulfur vulcanization proceeds via ionic vinyl polymerization. The process involves the reaction of sulfur with carbon-carbon double bonds to form persulfonium ion intermediates which then undergo a sequence of ionic reactions that lead to cross-linked rubber.

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