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Classification of Polymers:
Polymers may be classified as follows,
according to the mechanical response at elevated temperatures
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Thermoplasts
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Thermosets
a) Thermoplasts:
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Thermoset polymers soften when heated
and harden when cooled. Simultaneous application of heat and
pressure is required to fabricate these materials.
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On the molecular level, when the
temperature is raised, secondary bonding forces are diminished so
that the relative movement of adjacent chains is facilitated when
a stress is applied.
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Most Linear polymers and those having
branched structures with flexible chains are thermoplastics.
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Thermoplastics are very soft and
ductile.
The commercial available thermoplasts
are
b) Thermosets:
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Thermosetting polymers become soft
during their first heating and become permanently hard when
cooled. They do not soften during subsequent heating. Hence, they
cannot be remolded/reshaped by subsequent heating.
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In thermosets, during the initial
heating, covalent cross-links are formed between adjacent
molecular chain. These bonds anchor the chains together to resist
the vibration and rotational chain motions at high temperatures.
Cross linking is usually extensive in that 10 to 15% of the chain
mer units are cross linked. Only heating to excessive temperatures
will cause severance of these crosslink bonds and polymer
degradation.
-
Thermoset polymers are harder,
stronger, more brittle than thermoplastics and have better
dimensional stability.
-
They are more usable in processes
requiring high temperatures
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Most of the cross linked and network
polymers which include
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Vulcanized rubbers
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Epoxies
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Phenolic
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Polyester resins
are thermosetting.
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Thermosets cannot be recycle, do not
melt, are usable at higher temperatures than thermoplastics, and
are more chemically inert
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