Elastomer

File:Polymer picture.PNG

A is a schematic drawing of an unstressed polymer, and B is the same polymer under stress. When the stress is removed, B returns to the A configuration. (The dots represent cross-links.)

An elastomer is a polymer with the property of elasticity. In other words, it is a polymer that deforms under stress and returns to its original shape when the stress is removed. The term is a contraction of the words "elastic polymer." There are many types of elastomers, most of which are rubbers. The term elastomer is therefore often used interchangeably with the term rubber. Elastomers are primarily used for seals, adhesives, and molded flexible parts.

Properties

Elastomers are usually thermosets (that require curing), but some are thermoplastic. The principal difference between thermoset elastomers and thermoplastic elastomers is the type of crosslinking bond in their structures. In fact, crosslinking is a critical structural factor which contributes to impart high elastic properties. The crosslink in thermoset polymers is a covalent bond created during the vulcanization process. On the other hand the crosslink in thermoplastic elastomer polymers is a weaker dipole or hydrogen bond or takes place in only in one of the phases of the material.

• The long polymer chains become cross-linked during curing and account for the flexible nature of the material. The molecular form of elastomers has been likened to a "spaghetti and meatball" structure, where the meatballs signify cross-links between the flexible spaghetti strands (polymer chains).

• Each of the monomers that links to form the polymer is usually made of carbon, hydrogen, oxygen, and/or silicon. Elastomers are amorphous polymers existing above their glass transition temperature, so that considerable segmental motion is possible. At ambient temperatures, rubbers are thus relatively soft (E~3MPa) and deformable.

Elastomers are usually thermosets—that is, they require curing by heat, chemical reaction, or irradiation. The long polymer chains become cross-linked during curing.

The molecular structure of elastomers can be imagined as a 'spaghetti and meatball' structure, with the meatballs signifying cross-links.

Some elastomers are thermoplastic.

• While most elastomers are thermosets, thermoplastics are in contrast relatively easy to use in manufacturing, for example, by injection molding. Thermoplastic elastomers show both advantages typical of rubbery materials and plastic materials.

The elasticity is derived from the ability of the long chains to reconfigure themselves to distribute an applied stress. The covalent cross-linkages ensure that the elastomer will return to its original configuration when the stress is removed. As a result of this extreme flexibility, elastomers can reversibly extend from 5 to 700 percent, depending on the specific material. Without the cross-linkages or with short, uneasily reconfigured chains, the applied stress would result in a permanent deformation.

Temperature effects are also present in the demonstrated elasticity of a polymer. Elastomers that have cooled to a glassy or crystalline phase will have less mobile chains, and consequentially less elasticity, than those manipulated at temperatures higher than the glass transition temperature of the polymer.

It is also possible for a polymer to exhibit elasticity that is not due to covalent cross-links, but instead for thermodynamic reasons.

Examples of elastomers

Unsaturated rubbers that can be cured by sulfur vulcanization:

• Natural Rubber (NR)
• Synthetic Polyisoprene (IR)
• Butyl rubber (copolymer of isobutylene and isoprene, IIR)
  • Halogenated butyl rubbers (Chloro Butyl Rubber: CIIR; Bromo Butyl Rubber: BIIR)
• Polybutadiene (BR)
• Styrene-butadiene Rubber (copolymer of polystyrene and polybutadiene, SBR)
• Nitrile Rubber (copolymer of polybutadiene and acrylonitrile, NBR), also called Buna N rubbers
  • Hydrogenated Nitrile Rubbers (HNBR) Therban and Zetpol
• Chloroprene Rubber (CR), polychloroprene, Neoprene, Baypren etc.

(Note that unsaturated rubbers can also be cured by non-sulfur vulcanization if desired).

Saturated Rubbers that cannot be cured by sulfur vulcanization:

• EPM (ethylene propylene rubber, a copolymer of ethylene and propylene) and EPDM rubber (ethylene propylene diene rubber, a terpolymer of ethylene, propylene and a diene-component)
• Epichlorohydrin rubber (ECO)
• Polyacrylic rubber (ACM, ABR)
• Silicone rubber (SI, Q, VMQ)
• Fluorosilicone Rubber (FVMQ)
• Fluoroelastomers (FKM, and FEPM) Viton, Tecnoflon, Fluorel, Aflas and Dai-El
• Perfluoroelastomers (FFKM) Tecnoflon PFR, Kalrez, Chemraz, Perlast
• Polyether Block Amides (PEBA)
• Chlorosulfonated Polyethylene (CSM), (Hypalon)
• Ethylene-vinyl acetate (EVA)

Various other types of elastomers:

• Thermoplastic elastomers (TPE), for example Elastron, etc.
• Thermoplastic Vulcanizates (TPV), for example Santoprene TPV
• Thermoplastic Polyurethane (TPU)
• Thermoplastic Olefins (TPO)
• The proteins resilin and elastin
• Polysulfide Rubber

Mathematical background

Using the laws of thermodynamics, stress definitions, and polymer characteristics, ideal stress behavior may be calculated using the following equation:

$\sigma \ =nkT[\lambda \ _{1}^{2}+\lambda \ _{1}^{-1}]$

where $n$ is the number of chain segments per unit volume, $k$ is Boltzmann's Constant, $T$ is temperature, and $\lambda \ _{1}$ is distortion in the 1 direction.

These findings are accurate for values up to approximately 400 percent strain. At that point, alignment between stretched chains begins to result in crystallization from noncovalent bonding.

Although Young's Modulus does not exist for elastomers because of the nonlinear nature of the stress-strain relationship, a "secant modulus" can be found at a particular strain.

See also

• Ethylene
• Isoprene
• Polymer
• Rubber

References

ISBN links support NWE through referral fees

• Budinski, Kenneth G., and Michael K. Budinski. 2002. Engineering Materials: Properties and Selection, 7th ed. ISBN 0130305332.

• Mark, James E., Burak Erman, and Frederick R. Eirich, eds. 2005. Science and Technology of Rubber. 3rd ed. Amsterdam: Elsevier Academic Press. ISBN 978-0124647862.

• Meyers, Marc A., and Krishan Kumar Chawla. 2007. Mechanical Behavior of Materials. San Diego, CA: M. Meyers. IBSN 978-1427614827.

• Treloar, L. R. G. 2005. The Physics of Rubber Elasticity. 3rd ed. Oxford Classic Texts in the Physical Sciences. Oxford: Clarendon Press. ISBN 978-0198570271.

• Ward, I. M., and J. Sweeney. 2004. An Introduction to the Mechanical Properties of Solid Polymers. Chichester, West Sussex, UK: Wiley. ISBN 978-0471496267.

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