18 ChemMatters | OCTOBER/NOVEMBER 2017
R
S
GR
AP
HX
,
IN
C.
;
S
HU
TTE
RS
TO
CK
A polymer is a chemical chain of
repeating monomers, which can link
hundreds, thousands, or even millions
of times.
If you have ever used super glue, you
have seen polymerization at work. The main
component of super glue is a liquid monomer
called methyl-2-cyanoacrylate. The presence
of water (the moisture in the air is sufficient)
starts the reaction, and the monomers join to
form a solid polymer that is a strong adhesive.
Dental fillings are more com-
plicated than super glue and
involve a different mono-
mer, but the chemistry
behind their reactions
works with the same
polymerization principles.
In both, the monomers
and the polymer have differ-
ent properties—before polym-
erization, the resin behaves like a paste, but
once polymerized it becomes solid and hard.
Primers and blue light
The polymerization of the dental resin has
to start with a molecule that triggers the reaction. This compound, called a primer or an
initiator, works by creating free radicals—
molecules with unpaired electrons—which promote the bonding of monomers. Once started,
polymerization continuously happens, so only
a few molecules of primer are needed to start
the process.
When getting a white filling, you might have
noticed a curious thing happening during the
procedure. Immediately after laying the resin
in the tooth, the dentist points a bright blue
light close to it and asks you to remain still
for a moment. As a patient, you may find it
uncomfortable, but this is how polymerization
is controlled. In this technique, the primer
reacts only in the presence of blue light.
The advantage of such a process is that
dentists can use pre-made resins that have
the monomer and the primer already mixed.
They use blue light to induce polymerization only when the resin is perfectly tucked
inside the cavity. Since the light can travel
only a few millimeters into the filling, the
dentist pours a thin layer of resin first,
applies the light to trigger polymerization,
and then applies another layer of resin and
repeats the procedure until the cavity is
filled.
A COMPOSITE is a material made from
two or more constituent materials
with different physical properties. The
constituent materials join to produce a
composite, which has different properties than the constituent materials. The
composite is stronger and more rigid
than the individual materials. Composites have two parts: 1) the matrix, and
2) reinforcement or filler. In the case of
dental resin, the polymer is the matrix
and silica is the reinforcement material—it is the substance that provides
strength to the composite material.
Strength in diversity
Because the polymer alone
would not be sturdy enough to
withstand chewing, the dental
resins include other materials to
provide strength. Dental resin is
considered a composite material, which means two or more
substances form a structure that
is stronger than the individual
substances. Composite materials
are used in many common substances—concrete and plywood
are composites.
To reinforce dental resins,
particles of silica-based glass are
usually added. The polymer in
dental resin is hydrophobic—it
does not mix with water. Silica-based glass,
however, is hydrophilic (it attracts water) due
to a surface layer of polar hydroxyl groups
(–OH) bound to the silica. Because of these
opposite properties toward water, the polymer and the glass naturally repel each other;
therefore, a third compound is needed to get
the reaction going. This is why the composite
also includes coupling agents, which are compounds that bond the polymer to the silica
(Fig. 2).
Silanes are usually used as coupling agents.
These silane molecules have a hydroxyl group
on one end, and it condenses with a hydroxyl
group on the surface of the glass to form an
Si–O–Si link. The silanes also have a group
that can bond covalently with carbon atoms in
the polymer. Silanes work as a chemical bridge
and hold the polymer and the silica-based
glass together. Resins also include various
pigments, or dyes, so the dentist can choose a
shade to match their patient’s tooth color.
Composite-resin fillings are sophisticated
products that are constantly improving, but
prevention remains the best course of action
against tooth decay. Limiting the consumption
of sugar and carbohydrates, brushing and
flossing every day, and seeing a dental hygienist regularly are still some of the most effective
ways to preserve healthy teeth, and possibly
avoid the need for dental fillings altogether.
SELECTED REFERENCES
Warmflash, D. Tooth Decay: A Delicate Balance.
ChemMatters, Oct/Nov 2015, pp 8–10.
First Data on New Dental Fillings that Will Repair
Tooth Decay. ScienceDaily, University of Queen
Mary London, Sept 26, 2016: https://www.sci-
encedaily.com/releases/2016/09/
160926100014.htm [accessed Aug 2017].
The Tooth Decay Process: How to Reverse It and
Avoid a Cavity. National Institute of Dental and
Craniofacial Research, May 2013: https://www.
nidcr.nih.gov/OralHealth/OralHealthInformation/
ChildrensOralHealth/ ToothDecayProcess.htm
[accessed Aug 2017].
Dental Health and Tooth Fillings. WebMD: http://
www.webmd.com/oral-health/guide/dental-health-
fillings#1 [accessed Aug 2017].
Sergio Pistoi is a science writer who lives
in Arezzo, Italy. This is his first article in
ChemMatters.
www.acs.org/chemmatters
A hand-held wand
that emits blue light
is used to harden the
resin within a dental
patient’s mouth.
Figure 2. These are the structures of silica-based glass, the silane
coupling agent, and the final composite resin. Y represents a
group on the silane that can covalently bond to the polymer and
provide the rigidity to a composite-resin filling.
OO
OO
O Si
OH OH
Si
HO OH
Y
Si
OH
O Si
polymer polymer
Si
Si Si O O O
silica-based glass
hydrophobic,
joins with
polymer
hydrophillic,
joins with silica
composite resin
A thoroughly
cross-linked polymer can
be so interconnected that
it is one molecule. A solid
resin bowling ball
is really one big
molecule!