Vulcanization

vulcanization

[‚vəl·kə·nə′zā·shən]

(chemical engineering)

A chemical reaction of sulfur (or other vulcanizing agent) with rubber or plastic to cause cross-linking of the polymer chains; it increases strength and resiliency of the polymer. Also known as cure.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

vulcanization

An irreversible process during which a rubber compound, through a change in its chemical structures, becomes less plastic, more resistant to swelling by organic liquids, and more elastic (or the elastic properties are extended over a greater range of temperature).

McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.

The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Vulcanization

 

a technological process in rubber production in which raw rubber is made into cured rubber. Vulcanization increases the durability, hardness, elasticity, and heat and cold resistance of raw rubber and lowers its degree of swelling and solubility in organic solvents.

The essence of vulcanization is the joining of the linear macromolecules of raw rubber into a single, “sewn” system—the so-called vulcanization network. As a result of vulcanization, cross-links are formed between the macromolecules; the number and structure of the cross-links depend on the method of vulcanization. During vulcanization certain properties of the vulcanized mixture change with time, but they pass through a maximum or minimum rather than change constantly. The degree of vulcanization at which the rubber achieves the best combination of various physical and mechanical properties is called the optimal vulcanization.

Mixtures of raw rubber with various substances that ensure the necessary useful qualities of the cured rubber (fillers such as carbon black, chalk, and kaolin; softeners; and preservatives) are usually vulcanized.

In most instances, raw rubber for general use (natural rubber, butadiene, or butadiene-styrene) is vulcanized by heating it with elemental sulfur to 140°-160° C (sulfur vulcanization). The intermolecular cross-links that form are made up of one or several sulfur atoms. If 0.5 to 5 percent sulfur is added to the raw rubber, a soft vulcanizate (for automotive inner tubes and tire casings, balls, tubes, and so on) is formed. The addition of 30 to 50 percent sulfur leads to the formation of a hard, inelastic substance, ebonite. Sulfur vulcanization can be accelerated by the addition of small quantities of organic compounds—so-called vulcanization accelerators such as kaptaks or thiuram. These substances are fully active only in the presence of metal oxides (most often zinc oxide), which are activators. In industry sulfur vulcanization is accomplished by heating the articles being vulcanized in molds under high pressure or in the form of unformed articles (in “free” shape) in boilers, autoclaves, individual vulcanizers, or apparatus for continuous vulcanization. In these devices heating is done by steam, air, superheated water, electricity, or high-frequency current. The forms are usually put between the heated plates of a hydraulic press. Vulcanization with sulfur was discovered by C. Goodyear (USA, 1839) and T. Hancock (Great Britain, 1843). To vulcanize raw rubber for special uses, organic peroxides (such as benzoyl peroxide), synthetic resins (for example, phenol-formaldehyde), and nitro and diazo compounds are used. The conditions of vulcanization are the same as for sulfur vulcanization.

Vulcanization is also possible under the action of ionizing radiation (gamma radiation from radioactive cobalt) and streams of fast electrons (radiation vulcanization). Methods of sulfurless and radiation vulcanization allow the production of rubbers with high thermal and chemical resistance.

REFERENCES

Koshelev, F. F., A. E. Kornev, and N. S. Klimov. Obshchaia tekhnologiia reziny. Moscow, 1968.
Dogadkin, B. A. Khimiia elastomerov. Moscow, 1972.
Hofmann, W. Vulkanizatsiia i vulkanizuiushchie agenty. Moscow, 1968. (Translated from German.)

A. N. TARASOVA

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.