vulcanization

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vulcanization

(vŭl'kənəzā`shən), treatment of rubberrubber,
any solid substance that upon vulcanization becomes elastic; the term includes natural rubber (caoutchouc) and synthetic rubber. The term elastomer is sometimes used to designate synthetic rubber only and is sometimes extended to include caoutchouc as well.
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 to give it certain qualities, e.g., strength, elasticity, and resistance to solvents, and to render it impervious to moderate heat and cold. Chemically, the process involves the formation of cross-linkages between the polymer chains of the rubber's molecules. Vulcanization is accomplished usually by a process invented by Charles GoodyearGoodyear, Charles,
1800–1860, American inventor, b. New Haven, Conn., originator of vulcanized rubber. He failed in his earlier business ventures and was in jail for debt when he began his experiments with rubber, searching for a way to prevent it from sticking and melting
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 in 1839, involving combination with sulfur and heating. A method of cold vulcanization (treating rubber with a bath or vapors of a sulfur compound) was developed by Alexander Parkes in 1846. Rubber for almost all ordinary purposes is vulcanized; exceptions are rubber cement, crepe-rubber soles, and adhesive tape. Hard rubber is vulcanized rubber in which 30% to 50% of sulfur has been mixed before heating; soft rubber contains usually less than 5% of sulfur. After the sulfur and rubber (and usually an organic accelerator, e.g., an aniline compound, to shorten the time or lower the heat necessary for vulcanization) are mixed, the compound is usually placed in molds and subjected to heat and pressure. The heat may be applied directly by steam, by steam-heated molds, by hot air, or by hot water. Vulcanization can also be accomplished with certain peroxides, gamma radiation, and several other organic compounds. The finished product is not sticky like raw rubber, does not harden with cold or soften much except with great heat, is elastic, springing back into shape when deformed instead of remaining deformed as unvulcanized rubber does, is highly resistant to abrasion and to gasoline and most chemicals, and is a good insulator against electricity and heat. Many synthetic rubbers undergo processes of vulcanization, some of which are similar to that applied to natural rubber. The invention of vulcanization made possible the wide use of rubber and aided the development of such industries as the automobile industry.

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

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.

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).
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