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.
A. N. TARASOVA