ChemMatters - October 2017

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.