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Plastics
Plastics have become all
present in our everyday lives. From buckets to frames of computers,
telephones, electrical switches and insulation, etc. are all made of
plastics. Special plastics such as Bakelite are used as transformer cores on
which a transformer is wound. Plastics are all synthetic polymers.
| There are two types of
plastics : |
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Thermoplastics
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Thermosetting
plastics
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Thermoplastics
A plastic material, which
can be repeatedly melted-molded again and again by simple heating, is called
a thermoplastic. Another name of thermoplastic material is thermoplastic
polymer. Thermoplastics become soft and moldable on heating. They can then
be recast in different shapes and cooled. On cooling the thermoplastic
material hardens.
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Polyethylene with a
series of chains of (----CH2---CH2----)n
and is obtained by polymerization of ethene. This we have seen earlier. The
reaction is shown below.
Polymerization of other ethene monomers gives rise to other plastics. For
example by replacing one hydrogen atom with a chlorine atom we get vinyl
chloride (1-chloro ethene) -
CH2 
CH 
Cl.
Polymerization of this molecule leads to a plastic called polyvinyl chloride
or PVC as it is popularly known. The PVC polymer is as shown below.
If hydrogen from an ethene molecule is replaced
by a phenyl group (C6H5),
the monomer is called styrene CH2
CH C6H5
(1 phenyl ethene). Polymerization of this molecule gives rise to a plastic
called polystyrene. The structure of polystyrene is shown below.
Thermosetting
plastics
A
plastic material, which cannot be repeatedly melted-molded again, is called
a thermosetting plastic. Another name of thermosetting plastic material is
thermosetting polymer. Thermosetting plastics are long chain molecules,
which have crosslinks to other long chains. In case of thermoplastics, the
long chain molecules are held to other long chain molecules by van der waals
forces. When heated, these chains
slip off from each other and the material melts. When cooled, the long chain
molecules return back to be held together again. On the other hand, in case
of thermosetting plastics, the cross-links prevent displacement of the
polymer chains on heating. Thus thermosetting plastics do not become soft on
heating and they do not change their shape on heating. Bakelite is one
example of a thermosetting plastic. The diagram below shows the difference
between a thermoplastic such as a polythene and a thermosetting plastic such
as bakelite.
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