Asynthetic rubber is an artificialelastomer. They arepolymers synthesized frompetroleum byproducts. About 32 million tonnes (35 million short tons; 31 million long tons) of rubber is produced annually in the United States, and of that amount two thirds are synthetic. Synthetic rubber, just likenatural rubber, has many uses in theautomotive industry fortires, door and window profiles,seals such asO-rings andgaskets,hoses,belts,matting, andflooring. They offer a different range of physical and chemical properties which can improve the reliability of a given product or application. Synthetic rubbers are superior to natural rubbers in two major respects: thermal stability, and resistance to oils and related compounds.[1] They are more resistant to oxidizing agents, such asoxygen andozone which can reduce the life of products like tires.
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The expanded use of bicycles, and particularly their pneumatictires, starting in the 1890s, created increased demand for rubber. In 1909, a team headed byFritz Hofmann, working at theBayer laboratory inElberfeld,Germany, succeeded in polymerizingisoprene, making the first synthetic rubber.[2][3]
Studies published in 1930 written independently by the RussianSergey Lebedev, the AmericanWallace Carothers and the German scientistHermann Staudinger led in 1931 to one of the first successful synthetic rubbers, known asneoprene, which was developed atDuPont under the direction ofE. K. Bolton. Neoprene is highly resistant to heat and chemicals such asoil andgasoline, and is used in fuel hoses and as an insulating material in machinery. The companyThiokol applied their name to a competing type of rubber based onethylene dichloride.[4]
In 1935, German chemists synthesized the first of a series of synthetic rubbers known asBuna rubbers. These werecopolymers, meaning thepolymers were made up from twomonomers in alternating sequence. Other brands includedKoroseal, whichWaldo Semon developed in 1935, andSovprene, which Soviet researchers created in 1940.[5]
Production of synthetic rubber in the United States expanded greatly duringWorld War II since theAxis powers controlled nearly all the world's limited supplies of natural rubber by mid-1942, following the Japanese conquest of most of Asia, particularly in the Southeast Asian colonies ofBritish Malaya (Malaysia) and theDutch East Indies (Indonesia) from where much of the global supply of natural rubber was sourced.[6] However, production of synthetic rubber was not sufficient to eliminate the threat that the U.S. would run out of rubber.[7]
Operation Pointblank bombing targets ofNazi Germany included theSchkopau (50,000 tons/yr) plant and the Hüls synthetic rubber plant nearRecklinghausen (30,000, 17%),[8] and the Kölnische Gummifäden Fabrik tire and tube plant atDeutz on the east bank of the Rhine.[9] TheFerrara, Italy, synthetic rubber factory (near a river bridge) was bombed August 23, 1944.[10] Three other synthetic rubber facilities were atLudwigshafen/Oppau (15,000),Hanover/Limmer (reclamation, 20,000), andLeverkusen (5,000). A synthetic rubber plant atOświęcim, in Nazi-occupied Poland, was under construction on March 5, 1944[11] operated byIG Farben and supplied with slave labor, by the SS, from the associated campAuschwitz III (Monowitz).[12][13]
The most prevalent synthetic rubber isstyrene-butadiene rubbers (SBR) derived from thecopolymerization ofstyrene and1,3-butadiene. Other synthetic rubbers include:
Many variations of these can be prepared with mixtures of monomers and with various catalysts that allow for control ofstereochemistry.[14]
Polyisobutylene or butyl rubber is commonly used in tire inner tubes or linings because of its resistance to diffusion of air through the lining. It is a much less resilient material than cis-polybutadiene which is frequently used in tire sidewalls to minimize energy losses and heat build-up. It is so resilient that it is used insuper balls. An elastomer widely used for external sheet such as roof coverings isHypalon or chlorosulphonatedpolyethylene. Synthetic rubbers likeEPR can also be used for electrical insulation.
Silicone rubber is also a synthetic elastomer composed ofsilicone polymers. Silicone rubbers are widely used in industry, and there are multiple formulations. Silicone rubbers are often one- or two-part polymers, and may contain fillers to improve properties or reduce cost. Silicone rubber is generally non-reactive, stable, and resistant to extreme environments and temperatures.
Synthetic rubber is produced by polymerizing petroleum-based monomers. The manufacturing process has control over the molecular weight and properties of the synthetic rubber molecule (unlike in natural rubber). The synthesis mainly occurs throughstep-growth andchain-growth polymerization.[15]
Instep-growth polymerization, monomers or oligomers combine to form polymers through reactions such ascondensation (releasing small byproducts) orpolyaddition (without byproducts).
Inchain-growth polymerization, polymer chains grow by adding monomers to reactive sites, initiated by radicals, ions, or coordination catalysts. This method includesinitiation,propagation, andtermination steps.
Natural rubber, coming fromlatex ofHevea brasiliensis, is mainly poly-cis-isoprene.
Synthetic rubber, like most otherpolymers, is made from various petroleum-basedmonomers.
Some synthetic rubbers are less sensitive toozone cracking than natural rubber. Natural rubber is sensitive because of the double bonds in its chain structure, but some synthetic rubbers do not possess these bonds and so are more resistant to ozone cracking. Examples includeViton rubber,ethylene propylene diene monomer (EPDM), andbutyl rubber.
A new class of synthetic rubber is thethermoplastic elastomers which can be moulded easily unlike conventional natural rubbervulcanized rubber. Their structure is stabilized bycross-linking bycrystallites in the case ofpolyurethanes or by amorphous domains in the case of SBSblock copolymers.
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