Long chain molecules
- There are a wide range of
thermoplastics, some that are rigid and some that are extremely flexible.
- The molecules of thermoplastics
are in lines or long chains with very few entanglements. When heat is
applied the molecules move apart, which increases the distance between
them, causing them to become untangled. This allows them to become soft
when heated so that they can be bent into all sorts of shapes.
- When they are left to cool
the chains of molecules cool, take their former position and the plastic
becomes stiff and hard again. The process of heating, shaping, reheating
and reforming can be repeated many times.
- The molecules of thermosetting
plastics are heavily cross-linked. They form a rigid molecular structure.
- Whereas in thermoplastics
the molecules sit end to end, the molecules in thermoplastics sit end-to-end
- Although they soften when
heated the first time, which allows them to be shaped they become permanently
stiff and solid and cannot be reshaped.
- Thermoplastics remain rigid
and non-flexible even at high temperatures. Polyester resin and urea
formaldehyde are examples of thermosetting plastics.
- Bakelite was a thermosetting
- Each time a thermoplastic
is reheated it will try and return to its original shape, unless it
has been damaged due to overheating or overstretching. This property
is called plastic memory.
- This is why a shape formed
in thermoplastic becomes flat when reheated.
Long chain molecules
and cooling cycle of thermoplastic
cooling cycle of thermosetting plastic
Substances added to plastics
- Certain plastic products
are created in such a way so that they possess specific properties and
qualities depending upon where and how the product will be used.
- Particular products need
have mechanical properties. They may need to be hard, strong and have
a high resistance to impact.
- Other products need to possess
certain environmental properties. They may need to have a resistance
to chemicals such as acids and alkalis.
- Plastics used outside may
need to have a high resistance to ultra violet light and be flame resistant.
For some it may be important that they do not absorb water.
- Before the raw materials
can be converted into finished products they need other substances added
in order to give the required properties.