Elastomer Types

1. General Purpose
General purpose elastomers are hydrocarbon polymers. They include styrene-butadiene rubber (SBR), butadiene rubber (BR), and polyisoprene rubber - both natural (NR) and synthetic (IR). These "diene" rubbers contain substantial chemical unsaturation in their backbones, causing them to be rather susceptible to attack by oxygen, and especially by ozone. Additionally, they are readily swollen by hydrocarbon fluids. The primary application of these elastomers is in automobile and truck tires.
1.1 Styrene-Butadiene Rubber (SBR)
SBR polymers are widely produced by both emulsion and solution Polymerization with a Tg of approximately -55 °C. Emulsion polymerization is carried out either hot(the polymer formed is highly branched), at about 50°C, or cold (the polymer formed with less branching, and giving stronger vulcanizates), at about 5°C, depending upon the initiating system. SBR made in emulsion usually contains about 23% styrene randomly dispersed with butadiene in the polymer chains. SBR made in solution contains about the same amount of styrene, but both random and block copolymers can be made. Block styrene is thermoplastic and at processing temperatures helps to soften and smooth out the elastomer. SBR was originally produced by the hot emulsion method, and was characterized as more difficult to mill, mix, or calender than natural rubber, and having relatively poor inherent physical properties.
SBR was originally developed as a general purpose elastomer. Its single largest application is in passenger car tires, Substantial quantities are also used in footwear, wire and cable jacketing, belting, hoses, and mechanical goods.

1.2 Polyisoprene (NR, IR)
Natural rubber (NR) Natural rubber is produced from the latex of the Hevea brasiliensis tree. The Tg of NR is about -70 °C and its structure is thought to be completely cis-l,4-polyisoprene, except for the chain ends. Because NR macromolecules are configured identically (stereoregular), they spontaneously pack together as crystallites on standing at low temperature, with a maximum rate at temperatures around -25 °C. NR also crystallizes upon straining. In fact, strain-induced crystallization imparts outstanding green strength and tack, and gives vulcanizates with high resistance to cut growth at severe deformations. NR macromolecules are susceptible to fracture on shearing. High shearing stresses and oxygen promote the rate of molecular chain scission.
Synthetic polyisoprene (IR) IR is produced both anionically and by Ziegler-Natta polymerization. Even though the difference in stereoregularity is small, Ziegler-Natta IR is substantially more crystallizable. However, both types of IR have less green strength and tack than NR. IR compounds have lower modulus and higher breaking elongation than similarly formulated NR compositions. This is due to less strain-induced crystallization with IR, especially at high rates of deformation.
1.3 Polybutadiene (BR)
This elastomer was originally made by emulsion polymerization, It was difficult to process and did not extrude well. Polybutadiene became commercially successful only after it was made by solution polymerization. This provided a polymer with greater than 90% cis-1,4-polybutadiene configuration. This structure hardens at much lower temperatures (with Tg of -100°C) than natural rubber and most other commercial elastomers. This gives better low temperature flexibility and higher resilience at ambient temperatures than most elastomers.
The largest volume use of polybutadiene is in passenger car tires, primarily in blends with SBR or natural rubber to improve hysteresis (resistance to heat buildup), abrasion resistance, and cut growth resistance of tire treads.

2. Specialty Elastomers
In many applications, general purpose elastomers are unsuitable due to their insufficient resistance to swelling, aging, and/or elevated temperatures. Specialty elastomers have been developed to meet these needs.
2.1 Polychloroprene or Neoprene (CR)
Polychloroprene is an emulsion polymer of 2-chlorobutadiene and has a Tg of about -50°C. The electron-withdrawing chlorine atom deactivates the double bond towards attack by oxygen and ozone and imparts polarity to the rubber, making it resistant to swelling by hydrocarbons. Compared to general-purpose elastomers, CR has superior weatherability, heat resistance, flame resistance, and adhesion to polar substrates, such as metals. In addition, CR has lower permeability to air and water vapor.
The microstructure of CR is mostly trans- 1,4. Applications include wire, cable, hose, and some mechanical goods.
2.2 Acrylonitrile-Butadiene Rubber (NBR)
NBR, also termed nitrile rubber, is an emulsion copolymer of acrylonitrile and butadiene. Acrylonitrile content varies from 18 to 50%. Unlike CR, polarity in NBR is introduced by copolymerization with the polar monomer, acrylonitrile, which imparts excellent fuel and oil resistance. With increased acrylonitrile content, there is an increase in Tg, reduction in resilience, lower die swell, decreased gas permeability, increased heat resistance, and increased strength. Because of unsaturation in the butadiene portion, NBR is still rather susceptible to attack by oxygen and ozone. Aging behavior can be improved by blending with small amounts of poly vinyl chloride. Nitrile rubber is widely used for seals and fuel and oil hoses.
2.3 Hydrogenated Nitrile Rubber (HNBR)
Nitrile rubber can be hydrogenated to eliminate most of the unsaturation and hence greatly improve aging and heat resistance. Fuel resistance is maintained. HNBR is used especially in oil field applications, where resistance to hydrocarbons at elevated temperatures is required.
2.4 Butyl Rubber (IIR)
Butyl rubber is a copolymer of isobutylene with a small percentage of isoprene to provide sites for curing. Because IIR is largely saturated, it has excellent aging stability. Another outstanding feature of butyl rubber is its low permeability to gases. Thus, it is widely used in inner tubes and tire innerliners.
2.5 Ethylene-Propylene Rubber (EPR, EPDM)
The first commercial ethylene propylene rubbers were made by the random copolymerization of ethylene and propylene in solution. Since these compounds (EPM) were fully saturated, they were highly resistant to oxidation, ozone, heat, weathering, and polar liquids. They could be cured, however, only by peroxide.
EPDM (ethylene-propylene diene monomer) has a small number of double bonds, external to the backbone, introduced in this way. The ratio of ethylene to propylene in commercial grades varies from 50/50 to 75/25, and a typical Tg is -60 °C. EPRs and EPDMs have excellent resistance to weathering and good heat stability. They can be made partially crystalline to give high green strength, but they possess poor building tack. Applications including seals, gaskets, and hose.
2.6 Silicone Rubber (MQ)
Unlike the previously discussed elastomers, which have carbon-carbon backbones, silicone rubbers contain very flexible siloxane backbones, and have very low glass transition temperatures. The most common silicone elastomer is polydimethyl siloxane with a Tg of -127 °C. Silicone rubbers have both excellent high temperature resistance and low temperature flexibility. Their uses include gaskets, seals, and O-rings.

2.7 Polysulfide Rubber (T)
Polysulfide rubbers contain a substantial proportion of sulfur in their structure. For example, the polysulfide rubber made by reacting dichloroethane with sodium tetrasulfide contains about 80% sulfur by weight. This results in high density (1.34 g/cm3) and outstanding resistance to ketones, esters, and most solvents. Major uses of polysulfide rubbers include permanent putties for fuel tank sealants, fuel hose liners, and gaskets.
2.8 Chlorosulfonated Polyethylene (CSM)
When polyethylene is chlorosulfonated, its crystallinity is disrupted and a chemically stable elastomer results. Commercial grades contain 25 to 45% chlorine and 1 to 1.4 % sulfur. These elastomers have excellent weatherability and good flame resistance. Oil resistance increases with increasing chlorine content, while low temperature flexibility and heat aging resistance are improved when the chlorine content is low.
2.9 Chlorinated Polyethylene (CM)
Another modification of polyethylene to produce an elastomer is simple chlorination (25 to 42%, typically about 36%). CMs are less expensive than CSMs and provide vulcanizates with lower compression set. Increased chlorine content improves oil, fuel, and flame resistance, but results in poorer heat resistance. CM has excellent weatherability and heat resistance to 150°C to 175 °C, even when immersed in many types of oil. Hose and wire and cable coverings are typical applications.
2.10 Ethylene-Methyl Acrylate Rubber (AEM)
This elastomer is a terpolymer of ethylene, methyl acrylate, and a small amount of carboxylic monomer as a cure site. Amines and peroxides are used as curatives. AEM has a heat resistance between that of CSM and MQ elastomers. It has poor resistance to strong acids. Weathering and heat aging resistance are good up to 150 °C.
2.11 Acrylic Rubber (ACM)
ACMs are copolymers of a dominant acrylate monomer (ethyl or butyl) and a cure site monomer, such as 2-chloroethyl vinyl ether. Butyl acrylate results in a lower Tg, but poorer oil resistance compared to ethyl acrylate. Copolymerization with acrylonitrile improves oil resistance. Although acrylate rubbers have good heat resistance, they have poor resistance to alkali and acids. Applications include gaskets, O-rings and oil hose.
2.12 Fluorocarbon Rubbers
Fluorocarbon rubbers are made in emulsion and are among the most inert and expensive elastomers. This rubber has a density of 1.85 g/cm3 and has a service temperature exceeding 250 °C. It is little affected by immersion in acids, bases, or aromatic solvents; however, ketones and acetates attack the material. There are many aircraft applications for fluororubbers including O-rings, seals, and gaskets.
2.13 Epichlorohydrin Rubber (CO, ECO)
Two common types are polyepichlorohydrin (CO) and copolymers with ethylene oxide (ECO), which have lower Tg. Epichlorohydrin rubbers are quite resistant to aliphatic and aromatic fluids, and have good building tack. Other notable properties include good ozone resistance, low gas permeability (about one third that of butyl rubber), and heat resistance up to 150°C. Applications include wire and cable jackets, hose and belting, and packings.
2.14 Urethane Rubber
Polyurethane elastomers are produced by reacting a diisocyanate with either a polyether or apolyester. Urethane rubbers can be cured with sulfur or peroxide, and vulcanizates have excellent resistance to weathering, abrasion, and swelling by oil. Some applications are industrial rolls, caster wheels, gaskets, shoe soles, and conveyor belts.