This document discusses thermoplastic elastomers (TPEs). TPEs have both thermoplastic and elastomeric properties. They can be melt-processed like thermoplastics but are flexible and elastic like vulcanized rubbers. The most common TPE is a styrene-butadiene block copolymer, which has rigid polystyrene end blocks and soft polybutadiene mid blocks. This structure allows it to behave like a rubber at low temperatures but melt and flow like a thermoplastic at higher temperatures. Common applications of TPEs include automotive parts, medical devices, shoes, and cables due to advantages like recyclability and simpler processing compared to thermoset rubbers
2. THERMOPLASTIC ELASTOMERS (TPE)
INTRODUCTION
Elastomers:
An
Elastomer is a polymeric material that has
elongation greater than 100% and a significant amount
of resilience.
For elastomers to experience relatively large elastic
deformations, the onset of plastic deformation must be
delayed.
Restricting the motions of chains past one another by
cross-linking accomplishes this objective.
Cross-linking in many elastomers is carried out in a
process called vulcanization (heating with Sulphur
about 140oC).
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4. THERMOPLASTIC ELASTOMERS (TPE)
THERMOPLASTIC ELASTOMERS:
This
is a special group of polymers that do not rely
on cross-linking to produce a large amount of
elastic deformation.
TPEs
have
Rubber
and
thermoplastic
characteristics.
They are flexible like vulcanized rubbers and are
melt processible like a thermoplastic.
Many of the TPEs have block copolymer structure.
Block copolymers contain rigid thermoplastic and
soft thermoset polymer segments within the
polymer chain
Example: copolymer of polystyrene and butadiene
rubber.
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7. THERMOPLASTIC ELASTOMERS (TPE)
The
structure of a styrene-butadiene block
copolymer:
The styrene repeat units are located only at the
ends of the chains. Approximately 25% of the
chain is composed of styrene.
The styrene ends of several chains form sphericalshaped domains. The styrene has a high glasstransition temperature; consequently, the domains
are strong and rigid and tightly hold the chains
together.
The Butadiene repeat units are located between
the styrene domains. The Butadiene portions have
a glass-transition temperature below room
temperature and therefore behave in a soft,
rubbery manner.
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8. THERMOPLASTIC ELASTOMERS (TPE)
The
structure of a styrene-butadiene block
copolymer:
Elastic deformation occurs by recoverable
movement of the chains; sliding of the chains
at normal temperatures is prevented by the
styrene domains (Rigid).
When the thermoplastic elastomer is heated,
the styrene heats above the glass transition
temperature, the domains are destroyed, the
polymer deforms in a viscous manner and it
behaves as thermoplastic, can be fabricated
very easily.
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9. THERMOPLASTIC ELASTOMERS (TPE)
The
structure of a styrene-butadiene block
copolymer:
When the polymer cools, the domains reform
and the polymer reverts to its elastomeric
characteristics.
The thermoplastic elastomers consequently
behave as ordinary thermoplastics at elevated
temperatures and as elastomers at low
temperatures.
This behaviour also permits thermoplastic
elastomers to be more easily recycled than
conventional elastomers.
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10. THERMOPLASTIC ELASTOMERS (TPE)
Advantages
of TPEs compared with conventional
thermoset elastomers:
No chemical cross-linking
Simpler processing and faster cycle times
Can be processed by Injection molding, Blow molding,
thermoforming and extrusion.
Undercuts and complex shapes are possible with
simple tooling.
Scrap and waste can be recycled. Normal cross-linked
polymers cannot be recycled because they don't melt
due to the cross-links tie all the polymer chains
together, making it impossible for the material to flow.
Limitations:
They may have lower temperature resistance than
most thermoset rubbers.
Creep resistance inferior to thermoset rubbers.
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19. THERMOPLASTIC ELASTOMERS (TPE)
References
:
Donald
R. Askeland, Pradeep P. Fulay, Wendelin J. Wright,
The Science and Engineering of Materials, Sixth Edition.
Kenneth
G Budinski and Michael K Budinski, Engineering
Materials - Properties and selection, Eighth edition,
Prentice-Hall Inc.
William
D.Callister, Fundamentals of Materials Science
and Engineering, Fifth edition.
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