Elastomeric Materials

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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 was released
• Elastomers are sometimes called rubber or rubbery
  materials
• The term elastomer is often used interchangeably
  with the term rubber
• Elastomers are usually thermosets (requiring
  vulcanization) but may also be thermoplastic (see
  thermoplastic elastomer).

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• All materials have some elastic elongation
• elastic elongation elongation of any material
  when that material is at its yield point
• Ceramic metal- small elastic elongation 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 at room temp,
  however rigid at lower temp (why???)
• They are amorphous polymers existing above their
  glass transition temperature, so that
  considerable segmental motion is possible.
• At ambient temperatures rubbers are thus
  relatively soft (E3MPa) and deformable

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• Most elastomers are crosslink. Atoms between
  crosslink can still move, uncoil and coil.
• The long polymer chains cross-link during curing
  and account for the flexible nature of the
  material.
• Without crosslink, an elastomer may be elongated
  beyond elastic limit, with crosslink, max.
  elongation is set safely within the elastic
  region
• Crosslink density- total number of crosslink in
  the system (less elongation is desired, number of
  crosslink can be increased)

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Natural Rubber

• Rubber tree (Hevea Braziliensis)
• Natural rubber is obtained by drying a latex
  rubber (milk in which the butter fat component is
  suspended 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 tear
  strength, high resilience, resistance to wear, etc

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Polymer repeating groups

• Crude natural rubber was chiefly composed of
  cis-polyisoprena (a polymer chain with carbon
  carbon double bond with repeating unit)
• Cis means that two pendent group (H and CH3) that
  are attached to the two carbons in the carbon
  carbon double bond
• The alternate configuration where the two groups
  are located on the opposite side of the carbon
  carbon double bond is called trans
• The presence of methyl group interfere the
  movement in polyisoprene polymer- restricted
  bending 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 quire different
• Cis is highly elastomeric sensitive to heat
  softening
• Trans materials is called gutta percha, much
  harder than cis isoprena-used for golf balls
• During vulcanization process, sulphur will react
  with carbon carbon double bond

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Synthetic Polyisoprena or Isoprene Rubber (IR)

• Disruption of supplies of natural rubber during
  world war I and II increase needs for
  elastomeric 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 in
  1950s, it was found that 90 pure cis-isoprena
  could be produced by this catalyst system
• However, natural rubber is used mre extensively
  because 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 polybutadiene results
  in porrer strength tear strength than would
  polyisoprena. Resilient is about the same.
  Polybutadiene has poor resistance to solvents

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• Advantages of Polybutadiene low cost,
  improvement in low temp. flexibility,
  compatibility with many other polymeric
  materials, good adhesion to metal
• Butadiene monomer is added to the monomer of the
  other plastic copolymer is created
• Butadiene monomer polystyrene styrene
  butadiene rubber (SBR)
• Bulky Styrene molecules add stiffness and
  intermolecular interference to butadiene while
  butadiene adds flexibility and toughness to
  styrene

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SBR
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BR
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Thermoplastic Elastomer (TPE)

• These materials are not crosslinked, have some
  distinct processing advantages over traditional
  thermoset elastomers and physical properties of
  vulcanised elastomers
• TPEs are able to be molded like thermoplastic
  (injection molding, extrusion, etc)
• Thermoplastic elastomers are more temperature
  sensitive
• Scrap and reject of these materials can be
  recycled-environmetal friendly behavior
• Normal crosslinked polymers cannot be recycled
  because they don't melt. They don't melt because
  the crosslinks tie all the polymer chains
  together, making it impossible for the material
  to flow.

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Silicones, or polysiloxanes

• Silicones, or polysiloxanes, are
  inorganic-organic polymers with the chemical
  formula R2SiOn, where R organic groups such
  as methyl, ethyl, and phenyl.
• These materials consist of an inorganic
  silicon-oxygen backbone (...-Si-O-Si-O-Si-O-...)
  with organic side groups attached to the silicon
  atoms, which are four-coordinate.

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Silicones, or polysiloxanes

• In some cases organic side groups can be used to
  link two or more of these -Si-O- backbones
  together. By varying the -Si-O- chain lengths,
  side groups, and crosslinking, silicones can be
  synthesized with a wide variety of properties and
  compositions.
• They can vary in consistency from liquid to gel
  to rubber to hard plastic. The most common type
  is linear polydimethylsiloxane or PDMS

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Silicones, or polysiloxanes

• Service temperature to about 260C
• Good chemical resistance, low water absorption,
  good electrical properties, available in flame
  retardant grade
• In the plumbing and automotive fields, silicone
  grease is often used as a lubricant. In plumbing,
  the grease is typically applied to O-rings in
  faucets and valves.

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• In the automotive field, silicone grease is
  typically used as a lubricant for brake
  components since it is stable at high
  temperatures, is not water-soluble

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PROCESSING OF ELASTOMER

• Common machine used for rubber compounding-
• Banbury mixer
• 2-roll mill

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APPLICATION OF ELASTOMER

• Bearings - structural joints that are installed
  between a structure and its foundation.
• The bearing is very stiff and strong in the
  vertical direction, but flexible in the
  horizontal direction.

1.0 Introduction
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HOW THE BEARING WORKS
Figure Base-Isolated and Fixed-Base Buildings

• A base isolated structure is supported by a
  series of bearing pads which are placed between
  the building and the building's foundation

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2.0 How The Bearing work?

• As a result of an earthquake, the ground beneath
  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 is actually due to
  inertia.

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2.0 How The Bearing work?

• In addition to displacing toward the right, the
  un-isolated building is also shown to be changing
  its shape-from a rectangle to a parallelogram.
  deforming
• The primary cause of earthquake damage to
  buildings is the deformation which the building
  undergoes as a result of the inertial forces
  acting upon it.

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2.0 How The Bearing work?

• The base-isolated building retains its original,
  rectangular shape.
• It is the elastomeric bearings supporting the
  building that are deformed.
• It implies the inertial forces acting on the
  base-isolated building have been reduced.

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ELASTOMERIC BEARINGS
Fig Basic structure of rubber bearing

• Consist of thin rubber sheets bonded onto thin
  steel plates and combined with an energy
  dissipation mechanism.
• The rubber sheets are vulcanized and bonded to
  the 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
  flexible in horizontal direction.
• Thick mounting steel plates are bonded to the
  bottom and top surfaces allowing the isolator to
  be firmly connected to the foundation below and
  the superstructure above.

3.0 Elastomeric Bearings
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Processing Flow Chart - Seismic Rubber Bearings