week 7 elastomer

7/29/2019 Week 7 Elastomer

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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/Rubber


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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=edit


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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-link


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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/Polymer


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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/Polydimethylsiloxane


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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%29


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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/Brake


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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.

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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

week 7 elastomer

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