Elastomer 842224 214598596 2008-05-24T09:32:16Z Numbo3-bot 7118326 robot Modifying: [[eo:Elastomero]] 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 [[vulcanization|vulcanisates]]. Each of the [[monomer]]s which link to form the polymer is usually made of carbon, hydrogen, oxygen and/or silicon. Elastomers are [[amorphous polymer]]s existing above their [[glass transition temperature]], so that considerable segmental motion is possible. At [[ambient temperature]]s rubbers are thus relatively soft ([[Young's modulus|E]]~3MPa) and deformable. Their primary uses are for [[Seal (mechanical)|seal]]s, [[adhesive]]s and molded flexible parts. ==Background== [[Image:Polymer picture.PNG|frame|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 [[thermoset]]s (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 theory of polymer elasticity|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: <math> \sigma\ = n k T [ \lambda\ _ 1 ^ 2 + \lambda\ _ 1 ^ {-1} ] </math> where <math>n</math> is the number of chain segments per unit volume, <math>k</math> is [[Boltzmann's Constant]], <math>T</math> is temperature, and <math> \lambda\ _ 1 </math> 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) **[[Halogenation|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 [[Plastic#Synthetic rubber|Buna N rubbers]] **[[Hydrogenation|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 elastomer]]s (TPE), for example Elastron, etc. *Thermoplastic Vulcanizates (TPV), for example Santoprene TPV *Thermoplastic [[Polyurethane]] (TPU) *Thermoplastic Olefins (TPO) *The [[protein]]s [[resilin]] and [[elastin]] *[[Polysulfide]] Rubber ==References== *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. [[Category:Elastomers| ]] [[Category:Materials science]] [[Category:Polymer physics]] [[ar:وحدة مرنة]] [[de:Elastomer]] [[es:Elastómero]] [[eo:Elastomero]] [[fa:الاستومر]] [[fr:Élastomère]] [[it:Elastomero]] [[nl:Elastomeer]] [[pl:Elastomery]] [[pt:Elastômero]] [[ru:Эластомер]] [[sl:Elastomer]] [[fi:Elastomeeri]] [[sv:Elastomer]] [[uk:Еластомери]]