Elastomer

An elastomer is a polymer with the property of elasticity. The term, which is derived from elastic polymer, is often used interchangeably with the term rubber, and is preferred when referring to vulcanisates. Each of the monomers which link 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. Their primary uses are for seals, adhesives and molded flexible parts.

Background

File:Polymer picture.PNG

A is a schematic drawing of an unstressed polymer. The dots represent cross-links. B is the same polymer under stress. When the stress is removed, it will return to the A configuration.

Elastomers are usually thermosets (requiring vulcanization) but may also be thermoplastic (see thermoplastic elastomer). The long polymer chains cross-link during curing. The molecular structure of elastomers can be imagined as a 'spaghetti and meatball' structure, with the meatballs signifying cross-links. 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 5-700%, 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.

Mathematic justifications

Using the laws of thermodynamics, stress definitions and polymer characteristics (complete derivation in [1], pages103-105), we find ideal stress behavior:

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

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 of up to approximately 400% strain. At this point, alignment between stretched chains begins to result in crystallization from noncovalent bonding.

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

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

References

ISBN links support NWE through referral fees

• Treloar L.R.G., The Physics of Rubber Elasticity, Oxford University Press, 1975. ISBN 0-19-85027-9.
• Meyers and Chawla. Mechanical Behaviors of Materials, Prentice Hall, Inc. (Pearson Education) 1999.
• Budinski, Kenneth G., Budinski, Michael K., Engineering Materials: Properties and Selection, 7th Ed, 2002. ISBN 0-13-030533-2.

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