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Elastomeric Materials
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Elastomeric Materials
Common characteristics; Large elastic elongation (i.e.200%) Can be stretched and then immediately return
to their original length when the load wasreleased
Elastomers are sometimes called rubber orrubbery materials
The term elastomer is often usedinterchangeably with the term rubber
Elastomers are usually thermosets(requiring vulcanization) but may also bethermoplastic (see thermoplasticelastomer).
http://en.wikipedia.org/wiki/Rubberhttp://en.wikipedia.org/wiki/Rubberhttp://en.wikipedia.org/wiki/Thermosethttp://en.wikipedia.org/wiki/Thermoplastichttp://en.wikipedia.org/wiki/Thermoplastic_elastomerhttp://en.wikipedia.org/wiki/Thermoplastic_elastomerhttp://en.wikipedia.org/wiki/Thermoplastichttp://en.wikipedia.org/wiki/Thermoplastic_elastomerhttp://en.wikipedia.org/wiki/Thermoplastic_elastomerhttp://en.wikipedia.org/wiki/Thermoplastic_elastomerhttp://en.wikipedia.org/wiki/Thermoplastic_elastomerhttp://en.wikipedia.org/wiki/Thermoplastichttp://en.wikipedia.org/wiki/Thermosethttp://en.wikipedia.org/wiki/Rubber7/29/2019 Week 7 Elastomer
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All materials have some elasticelongation
elastic elongation = elongation ofany material when that material is atits yield point
Ceramic & metal- small elasticelongation; 2%
PE, elastic elongation; 50%
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Stress-strain diagram
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Idealized stress-strain curves for
metals, conventional plastics and
elastomer
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Diagram showing the random,
natural state of elastomer when
under no stress and when stressed
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A material may be elastomeric atroom temp, however rigid at lower
temp (why???) They are amorphous polymers
existing above their glass transition
temperature, so that considerablesegmental motion is possible.
At ambient temperatures rubbers are
thus relatively soft (E~3MPa) anddeformable
http://en.wikipedia.org/w/index.php?title=Amorphous_polymer&action=edithttp://en.wikipedia.org/wiki/Glass_transition_temperaturehttp://en.wikipedia.org/wiki/Glass_transition_temperaturehttp://en.wikipedia.org/wiki/Young%27s_modulushttp://en.wikipedia.org/wiki/Young%27s_modulushttp://en.wikipedia.org/wiki/Glass_transition_temperaturehttp://en.wikipedia.org/wiki/Glass_transition_temperaturehttp://en.wikipedia.org/w/index.php?title=Amorphous_polymer&action=edit7/29/2019 Week 7 Elastomer
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Most elastomers are crosslink. Atomsbetween crosslink can still move, uncoiland coil.
The long polymer chains cross-link duringcuring and account for the flexible natureof the material.
Without crosslink, an elastomer may beelongated beyond elastic limit, withcrosslink, max. elongation is set safelywithin the elastic region
Crosslink density- total number ofcrosslink in the system (less elongation isdesired, number of crosslink can beincreased)
http://en.wikipedia.org/wiki/Cross-linkhttp://en.wikipedia.org/wiki/Cross-linkhttp://en.wikipedia.org/wiki/Cross-linkhttp://en.wikipedia.org/wiki/Cross-link7/29/2019 Week 7 Elastomer
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Natural Rubber Rubber tree (Hevea Braziliensis)
Natural rubber is obtained by drying a latexrubber (milk in which the butter fat component issuspended in water salution)
High temperature stability cooking the crude
natural rubber with sulphur (vulcanization) Vulcanization creates crosslinking between rubber
molecules
Natural rubber is highly elastomeric (elongation
1000% for vulcanized natural rubber) Compared to other elastomeric materials, natural
rubber shows higher tensile strength, high tearstrength, high resilience, resistance to wear, etc
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Polymer repeating groups
Crude natural rubber was chiefly composed ofcis-polyisoprena (a polymer chain with carboncarbon double bond with repeating unit)
Cis means that two pendent group (H and CH3)that are attached to the two carbons in thecarbon carbon double bond
The alternate configuration where the two groupsare located on the opposite side of the carboncarbon double bond is called trans
The presence of methyl group interfere themovement in polyisoprene polymer- restrictedbending and twisting motion (increased stiffness,higher strength, and higher temperature stability
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Polyisoprene structure
Cis-poliisoprena
(Hevea rubber)
Trans-poliisoprena
(Gutta percha)
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Properties of cis and trans are quiredifferent
Cis is highly elastomeric & sensitive to
heat softening Trans materials is called gutta percha,
much harder than cis isoprena-used forgolf balls
During vulcanization process, sulphur willreact with carbon carbon double bond
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Synthetic Polyisoprena or Isoprene Rubber
(IR)
Disruption of supplies of natural rubber duringworld war I and II & increase needs forelastomeric materials- needs for synthetic rubber
Synthetic polyisoprena made in early 1900s, used
for tires for lightweight vehicles Combination of cis and trans molecular forms-
mixture of properties
Ziegler-Natta catalyst system was developed in1950s, it was found that 90% pure cis-isoprenacould be produced by this catalyst system
However, natural rubber is used mre extensivelybecause of its low cost
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Butadiene Rubber (BR) Synthetic rubber Repeating units of both have a backbone of
four carbon atoms including carbon carbon
double bond Polybutadiene has just two hidrogen attached
to the carbon carbon double bond
Absence of methyl group in polybutadieneresults in porrer strength & tear strength thanwould polyisoprena. Resilient is about thesame. Polybutadiene has poor resistance to
solvents
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Advantages of Polybutadiene; low cost,improvement in low temp. flexibility,compatibility with many other polymericmaterials, good adhesion to metal
Butadiene monomer is added to the monomer ofthe other plastic copolymer is created
Butadiene monomer + polystyrene = styrenebutadiene rubber (SBR)
Bulky Styrene molecules add stiffness andintermolecular interference to butadiene whilebutadiene adds flexibility and toughness tostyrene
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SBR
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BR
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Thermoplastic Elastomer (TPE)
These materials are not crosslinked, have somedistinct processing advantages over traditionalthermoset elastomers and physical properties ofvulcanised elastomers
TPEs are able to be molded like thermoplastic(injection molding, extrusion, etc)
Thermoplastic elastomers are more temperaturesensitive
Scrap and reject of these materials can berecycled-environmetal friendly behavior
Normal crosslinked polymers cannot be recycledbecause they don't melt. They don't melt becausethe crosslinks tie all the polymer chains together,making it impossible for the material to flow.
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Silicones, orpolysiloxanes Silicones, or polysiloxanes, are inorganic-
organic polymers with the chemical formula[R2SiO]n, where R = organic groups such asmethyl, ethyl, and phenyl.
These materials consist of an inorganicsilicon-oxygen backbone (...-Si-O-Si-O-Si-O-...) with organic side groups attached to thesilicon atoms, which are four-coordinate.
http://en.wikipedia.org/wiki/Polymerhttp://en.wikipedia.org/wiki/Polymerhttp://en.wikipedia.org/wiki/Polymerhttp://en.wikipedia.org/wiki/Polymerhttp://en.wikipedia.org/wiki/Functional_grouphttp://en.wikipedia.org/wiki/Methylhttp://en.wikipedia.org/wiki/Ethylhttp://en.wikipedia.org/wiki/Functional_grouphttp://en.wikipedia.org/wiki/Phenylhttp://en.wikipedia.org/wiki/Methylhttp://en.wikipedia.org/wiki/Ethylhttp://en.wikipedia.org/wiki/Phenylhttp://en.wikipedia.org/wiki/Siliconhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Siliconhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Siliconhttp://en.wikipedia.org/wiki/Phenylhttp://en.wikipedia.org/wiki/Ethylhttp://en.wikipedia.org/wiki/Methylhttp://en.wikipedia.org/wiki/Functional_grouphttp://en.wikipedia.org/wiki/Polymer7/29/2019 Week 7 Elastomer
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In some cases organic side groupscan be used to link two or more ofthese -Si-O- backbones together. By
varying the -Si-O- chain lengths, sidegroups, and crosslinking, siliconescan be synthesized with a widevariety of properties and
compositions. They can vary in consistency from
liquid to gel to rubber to hard plastic.
The most common type is linear
Silicones, orpolysiloxanes
http://en.wikipedia.org/wiki/Polydimethylsiloxane7/29/2019 Week 7 Elastomer
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Service temperature to about 260C
Good chemical resistance, low waterabsorption, good electricalproperties, & available in flameretardant grade
In the plumbing and automotivefields, silicone grease is often usedas a lubricant. In plumbing, thegrease is typically applied to O-rings
in faucets and valves.
Silicones, orpolysiloxanes
http://en.wikipedia.org/wiki/Grease_%28lubricant%29http://en.wikipedia.org/wiki/Grease_%28lubricant%297/29/2019 Week 7 Elastomer
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In the automotive field, siliconegrease is typically used as a lubricantfor brake components since it is
stable at high temperatures, is notwater-soluble
http://en.wikipedia.org/wiki/Brakehttp://en.wikipedia.org/wiki/Brake7/29/2019 Week 7 Elastomer
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PROCESSING OF ELASTOMER
Common machine used for
rubber compounding:-1. Banbury mixer
2. 2-roll mill
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Bearings - structural joints
that are installed between astructure and its foundation.
The bearing is very stiff andstrong in the vertical direction,
but flexible in the horizontaldirection.
1.0
Introduction
APPLICATION OF ELASTOMER
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Figure: Base-Isolated and Fixed-Base Buildings
A base isolated structure is supported by aseries of bearing pads which are placed
between the building and the building's
foundation
HOW THE BEARING WORKS
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As a result of an earthquake, the groundbeneath each building begins to move.Each building responds with movement
which tends toward the right.The building's displacement in the
direction opposite the ground motion isactually due to inertia.
2.0
How
The
Bearing
work?
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In addition to displacing toward the right, theun-isolated building is also shown to bechanging its shape-from a rectangle to a
parallelogram. deforming
The primary cause of earthquake damage tobuildings is the deformation which the building
undergoes as a result of the inertial forcesacting upon it.
2.0
How
The
Bearing
work?
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The base-isolated building retains itsoriginal, rectangular shape.
It is the elastomeric bearings supporting
the building that are deformed.
It implies the inertial forces acting onthe base-isolated building have been
reduced.
2.0
How
The
Bearing
work?
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Consist of thin rubber sheets bonded onto thinsteel plates and combined with an energydissipation mechanism.
The rubber sheets are vulcanized and bonded tothe thin steel plates under pressure and heat.
it is designed in such a way that bearing is very
stiff and strong in vertical direction, but flexiblein horizontal direction.
Thick mounting steel plates are bonded to thebottom and top surfaces allowing the isolator tobe firmly connected to the foundation below andthe superstructure above.
3.0
ElastomericBearings
Fig: Basic structure ofrubber bearing
ELASTOMERIC BEARINGS
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ProcessingFlow Chart -SeismicRubberBearings