Synthetic rubber
Synthetic rubber, also known as elastomers, is a type of polymer that exhibits elastic properties, allowing it to return to its original shape after being stretched. Unlike natural rubber, which is derived from latex, synthetic rubber is primarily produced from petroleum feedstocks and various chemical processes. Its production involves the polymerization of monomers like butadiene and styrene, which creates commonly used forms such as styrene-butadiene rubber (SBR), accounting for a significant portion of tire rubber.
One of the earliest synthetic rubbers, neoprene, offers enhanced resistance to solvents, making it suitable for applications in protective clothing and electrical insulation. Other variants, such as acrylonitrile-butadiene rubber (NBR), are known for their exceptional solvent resistance and are used in products like oil hoses and gaskets. Advances in synthetic rubber technology have led to improved formulations that enhance properties like abrasion resistance and durability, particularly in tire manufacturing.
While natural rubber remains competitive in terms of price, synthetic rubbers have become essential in various industries, including automotive and manufacturing, due to their unique properties and versatility.
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Synthetic rubber
Synthetic rubbers of more than two dozen types have been manufactured since the late 1920s. Worldwide production of synthetic rubbers totals approximately 14 million metric tons annually, more than double that of natural rubber.
Definition
Rubbers, more properly called “elastomers,” are composed of extremely long-chain molecules (in natural rubber the molecules contain about twenty thousand repeating five-carbon units) that are bonded to each other so that they cannot flow. The molecules assume a coiled shape until they are stretched; then they straighten out. The tendency to reassume the coiled form accounts for the elasticity of these materials—that is, their resumption of their original shape when stress is removed. Natural rubber is made up of units of isoprene. The residual double bonds make it possible to “vulcanize” the natural elastomer—to heat it with 1 to 3 percent sulfur to form -S-S- “cross-links” that hold adjacent molecules together so that they cannot slip and flow away. The double bonds also make the rubber vulnerable to deterioration by reaction with atmospheric oxygen and ozone. Resources used to create synthetic rubber include feedstocks, alcohol from grain, carbon black from petroleum or natural gas, finely divided silica, sulfur, and various organic and inorganic chemicals as curing agents and accelerators.
Overview
One of the earliest successful synthetic elastomers was neoprene (ASTM code CR), made of chloroprene, which resembles isoprene in molecular shape. Neoprene proved to be resistant to solvents such as gasoline and oils, unlike natural rubber, but it was expensive. It found applications in specialty tubing, electrical insulation, gaskets and seals, and protective clothing.
Both the Germans and the Russians used 1,3-butadiene (CH2=CHCH=CH2) in the 1930s for synthetic rubber, but the product was inferior until about 25 percent styrene (C6H5CH=CH2) was included in the reaction mixture. This produced styrene-butadiene rubber (SBR), which is the most common type of synthetic elastomer in use today. In slightly varying formulations, and always with about one-third carbon black (sometimes powdered silica) as a filler and strengthener, SBR accounts for most of the tire rubber currently in use—which means about 75 percent of all rubber produced.
A reaction of butadiene with acrylonitrile (CH2=CH-CN) rather than with styrene produces acrylonitrile-butadiene rubber (NBR), which has extreme solvent resistance and is used in oil hoses, oil well parts, fuel tank liners, gaskets, shoe soles, printing rolls, and even as a binder in rocket propellants. A hydrogenated form of NBR, with the residual double bonds eliminated by reaction with hydrogen, is highly resistant to air and forms films that prevent passage of gases.
The poor quality of butadiene rubber (BR) was overcome in the 1960s by the discovery of special catalysts for the rubber-producing reaction that made the geometry uniform about the double bond. This produced BR with high resistance to and cracking and with low heat buildup with flexing, qualities that have been useful in tire treads, particularly in the giant tires used on construction equipment.
Many specialty elastomers, such as ethylene-propylene copolymer (EPM), silicone rubber (MQ), fluorocarbon elastomers (FPM), epichlorohydrin elastomers (CO or ECO), and polyurethanes (PU), are produced for their special physical or chemical (resistant) properties. Natural rubber still generally holds the market edge in price, but synthetic elastomers have taken over large parts of the automotive and manufacturing markets.
"How Is Synthetic Rubber Made?" Polymer Trade Manufacturing, Nov. 2024, epdm.co.uk/2024/11/05/how-is-synthetic-rubber-made/. Accessed 6 Jan. 2025.
"Synthetic Rubber Project." Library of Congress, 10 Oct. 2024, guides.loc.gov/technical-reports/rubber. Accessed 6 Jan. 2025.
"What Is Synthetic Rubber?" ACE Laboratories, 21 Oct. 2022, www.ace-laboratories.com/what-is-synthetic-rubber/. Accessed 6 Jan. 2025.