Elastomers

An elastomer is a polymer with viscoelasticity (having both viscosity and elasticity) and very weak inter-
molecular forces, generally having low Young's modulus and high failure strain compared with other
materials. The term, which is derived from elastic polymer, is often used interchangeably with the term
rubber, although the latter 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. Application areas for different
types of rubber are manifold and cover segments as diverse as tires, shoe soles as well as dampening
and insulating elements. The importance rubbers have can be judged from the fact that global revenues
are forecast to rise to US$56 billion in 2020.
Elastomers are usually thermosets (requiring vulcanization) but may also be thermoplastic (see
thermoplastic elastomer). The long polymer chains cross-link during curing, i.e., vulcanizing.
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.

Wikipedia.org
http://en.wikipedia.org/wiki/Elastomer
Tehnic vorbind, elastomerul este un compus macromolecular care, n stare vulcanizat, prezint
hiperelasticitate. Aceast caracteristic este strns legat de tipul de monomer i structura
molecular (NB - dei polietilena i polipropilena sunt materiale plastice, copolimerul etilen-
propilen este un material elastic - cauciuc etilen-propilenic EPM).
O clasificare uzual mparte elastomerii n cteva clase:
cauciuc natural
elastomeri de uz general
elastomeri tehnici
elastomeri speciali
Cauciucul natural este un produs distinct, att ca modalitate de obinere ct i din punct de
vedere al prelucrrii. Este foarte folosit n special n industria de anvelope.
Elastomerii de uz general sunt folosii pe scar larg n industria "de tonaj": anvelope, benzi
transportoare, plci etc. Din aceast categorie fac parte:
IR - izoprenic
SBR - butadien-stirenic
BR - polibutadienic
Elastomerii tehnici sunt folosii n principal pentru articole tehnice: garnituri, chedere, cabluri
etc. Sunt inclui aici:
EPDM - etilen-propilen-dienic
NBR - butadien-acrilonitrilic
(X)IIR - izobutilenic (halogenat)
CR - cloroprenic
C(S)M - polietilen clorurat (clorosulfonat)
EVA - etilen-vinil-acetat
Elastomerii speciali sunt destinai utilizrilor n condiii dificile de funcionare, legate n special
de condiii de temperatur, mediu chimic etc Din aceast categorie fac parte:
HNBR - butadien-acrilonitrilic halogenat
ACM - acrilic
SR - siliconic
SLR - siliconic lichid
FMQ (FVMQ) - siliconic fluorurat
ECO - clorhidrinic
FKM - fluorurat
http://www.elastem.ro/html/rubber/elastomers.html
Proprietati NR SBR IIR EPDM MVQ CR CSM NBR HNBR ECO FPM FMVQ
Duritate
Shore A
25
..
100
40
..
100
40 ..
90
40 ..
90
30 ..
85
40 ..
95
50 ..
95
30 ..
100
30 ..
100
50 ..
90
65 ..
85
65 ..
80
Rezistenta la
tractiune
25
..
25 30 25 8 25 18 25 25 15 15 6
max. N/mm
2
30
Elasticitate 1 2 6 4 3 3 4 3 3 3 4 4
Rezistenta
structurala
1 4 2 4 4 3 3 4 4 3 3 4
Rezistenta la
abraziune
2 2 3 3 4 2 2 2 2 3 3 4
Interval de
temperatura
C
-60
..
+90
-40
..
+90
-35 ..
+150
-35 ..
+150
-90 ..
+200
-35 ..
+100
-50 ..
+130
-40 ..
+100
-40 ..
+150
-40 ..
+140
-30 ..
+200
-60 ..
+200
Stabilitate la
imbatranire
4 3 1 1 1 2 1 3 1 3 1 1
Rezistenta la
apa
3 2 2 1 3 2 2 2 2 3 2 3
substante
alcaline
3 3 3 1 4 3 2 3 3 4 3 2
acizi 3 3 3 2 4 3 2 4 4 4 2 2
uleiuri 6 6 6 6 3 3 3 1 1 1 1 1
solventi
alifatici
6 6 6 6 4 3 4 2 2 1 2 1
solventi
aromatici
6 6 6 6 4 5 5 4 4 3 3 2
solventi
halogenati
6 6 6 6 4 5 5 5 5 5 3 2
combustibili 6 6 6 6 5 5 4 2 3 2 1 2
Proprietati
electrice
pot prezenta atat proprietati de izolare, antistatice sau conductive
1 = excelent
2 = foarte bun
3 = bun
4 = satisfacator
5 = slab
6 = nepotrivit