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The Structure of Polymers. What are Cross Links?

Polymers may be classified as thermosoftening or thermosetting.

What is a Thermosetting Polymer?

Thermosetting polymers have their chains cross linked by
covalent bonds. The starting materials are placed into
a mould to form the desired shape. The polymer is then
heated (or initiated with uv light) and chemical reactions
occur to form the cross links between the chains.
The resulting three dimensional solid structure cannot then be
changed. Further heating will not cause the polymer to soften,
melt or change shape (unlike thermosoftening polymers).

Examples of thermosetting polymers are

1.  Melamine resin - used in furniture.

2.  Bakelite - used for saucepan handles
and electric light fittings.

3.  Epoxy resins - used in many glues.

The picture below shows a typical structure
for a thermosetting polymer. The red lines
represent the cross links between the chains.
Thermosetting Polymer

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