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Polymers - Dentistry - White Fillings - Composite - UV - Amalgam.

Polymers are used in dentistry to make white fillings for teeth.

Teeth are mainly made from hydroxyapatite (or hydroxylapatite),
which is a type of calcium phosphate with the formula Ca10(PO4)6(OH)2.
The outside layer of teeth is called enamel which contains over 90%
 hydroxyapatite. Enamel is the hardest substance in the human body.

Bacteria which live in the mouth produce acid in the presence of starch
and sugar. The acid can cause small holes to form in the tooth's enamel.
Eventually these holes become big enough to reach the nerve inside
the tooth and it becomes painful. A dentist can fill these holes
before they reach the nerve to prevent further decay.

For many years teeth have been filled with a mixture of metals called
an amalgam. The word amalgam means an alloy of mercury.
The amalgam used for fillings contains mercury, silver and other metals.
The amalgam is strong enough to withstand the pressures put on the
back teeth (called molars) during crunching and chewing.
It has the disadvantage that it looks like dark silver and is a good
conductor of heat which can make eating hot or cold things unpleasant.

More recently dentists have used a white composite for filling teeth.
A composite is a substance made by combining two or more materials.
White fillings are a composite of a polymer filled with glass.
The filling is put on the tooth in the form of a paste and then a beam
 of ultraviolet (uv) light is shone on it. The uv light initiates (starts)
chemical reactions in the paste which form cross links between the
polymer chains. The polymer containing the glass filler becomes a solid
three dimensional cross linked network which fills the hole in the tooth.

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