Elastomeric Materials
Elastomeric Materials
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Common characteristics;
• Large elastic elongation (i.e.200%)
• Can be stretched and then immediately return
to their original length when the load was
released
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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%
Stress-strain diagram
Idealized stress-strain curves for
metals, conventional plastics and
elastomer
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 (E~3MPa) 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)
Natural Rubber
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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
Polymer repeating groups
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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
Polyisoprene structure
Cis-poliisoprena
Trans-poliisoprena
(Hevea rubber)
(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
Synthetic Polyisoprena or Isoprene Rubber
(IR)
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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 formsmixture 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
Butadiene Rubber (BR)
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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
SBR
BR
Thermoplastic Elastomer (TPE)
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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.
Silicones, or polysiloxanes
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Silicones, or polysiloxanes, are inorganicorganic polymers with the chemical formula
[R2SiO]n, 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.
Silicones, or polysiloxanes
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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
Silicones, or polysiloxanes
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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
PROCESSING OF ELASTOMER
Common machine used for
rubber compounding:1.
2.
Banbury mixer
2-roll mill
1.0 Introduction
APPLICATION OF ELASTOMER
Bearings - structural joints
that are installed between a
structure and its foundation.
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The bearing is very stiff and
strong in the vertical direction,
but flexible in the horizontal
direction.
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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
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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3.0 Elastomeric Bearings
ELASTOMERIC BEARINGS
Fig: Basic structure of
rubber bearing
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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.
Processing
Flow Chart Seismic
Rubber
Bearings