Polymerization
1. Linear Polymer
Crosslinked Polymer
Branched Polymer
Monomers
nc
2.
3.
ChemMatters | OCTOBER/NOVEMBER 2017 17
IF YOU HAVE HEALTHY TEETH NOW, KEEP TAKING CARE OF THEM. But don’t get your hopes up too high for the future: During a lifetime, 92% of people eventually experience tooth decay, according to the National
Institute of Dental and Craniofacial Research. You may already be familiar with the problem, as
approximately half of high school students in the United States have tooth decay.
Dental fillings are the unsung heroes in the fight against
tooth decay. As the name suggests, the role of these substances is to seal any parts of a
tooth that are damaged or lost as a result of tooth decay, restoring the tooth’s function and preventing further decay.
What materials are used in dental fillings, and what do dentists do to make them stay inside
teeth for a long time?
Different types of fillings
Sealing a tooth isn’t easy. An ideal dental
filler needs to be soft and malleable when
applied, and must harden once in place. It must
be resistant to chewing (a human bite can exert
up to the equivalent of 275 pounds of pressure!), chemically stable, and biocompatible.
Gold was used as dental filler in the past.
As a metal, it can be melted and cast inside
a cavity. Gold is extremely durable but is
expensive and not everybody likes it. Unless
you are among the rich and famous, your
dentist probably won’t offer you the gold filling option.
Amalgam fillings, also known as silver
fillings, have been used since the late 19th
century. They are alloys of mercury and other
metals, such as silver, tin, or copper. Amalgam
fillings are durable and cost-effective, and are
still used today. As a drawback, the dark metal
color of amalgam is unattractive in a tooth.
Also, because the amalgam does not stick to
the tooth, dentists sometimes need to remove
healthy portions to anchor the amalgam. Con- R S G R
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A Reaction in Your Mouth
cerns have been raised about mercury (a toxic
metal) leaking from fillings. The American
Dental Association, however, says the amount
of mercury released in the mouth under the
pressure of chewing
and grinding is small
and no cause for alarm.
Composite resins,
also known as white
fillings, became
widespread during
the 1990s and are the fillings dentists use
today. White fillings look better than amalgam
because they can match the patient’s tooth
color. Resins are soft and can be easily applied
to fill the hole. Inside a tooth, resins quickly
solidify to become as hard as rock, and they
last a long time thanks to a chemical reaction
known as polymerization.
Glue in your teeth?
Generally speaking, polymerization (Fig. 1)
occurs when many individual molecules,
called monomers, react to form covalent
bonds and produce a bigger molecule called
a polymer.
Monomer: from the
Greek word mono, mean-
ing one, and méros, mean-
ing part (one part).
Polymer: from the Greek
word poly, meaning many
(many parts).
Figure 1. Individual monomers chemically combine to form one large
polymer. The monomers can join: 1) to form one long chain in a linear
fashion; 2) to make smaller chains that link to make a branched polymer;
or 3) to make chains that form a complicated cross-linked structure.
By Sergio Pistoi
