D09. Discuss the influence of impurities and oxygen on addition polymerization and state the purpose and the influence of adding hydroquinone to the monomers.

This topic was treated under D07. Below follows a review.

Inhibition

Inhibitors added to improve storage and increase setting time.

To avoid spontaneous polymerization during storage, manufacturers add different inhibitors to dental resins. Common inhibitors include hydroquinone. The function of hydroquinone is to react with any free radical that is formed. Thus, before the free radical is allowed to initiate a chain growth, the inhibitor "kills" the further growth. It is first when all hydroquinone has been consumed as chain propagation can occur. By increasing the hydroquinone concentration, the working time will be increased.  

Oxygen inhibition

Oxygen inhibition works differently than hydroquinone. If chain propagation has started and oxygen molecules are present, the two oxygen atoms attach to the growing chain. However, the free radical end of the attached oxygen is much less reactive than the regular free radicals present at the carbon atoms of other growing chains. Because of the latter, the polymerization reaction of oxygen inhibited resins proceeds much slower than that of the regular chains. The slower reaction caused by the presence of oxygen inhibition is both good and bad.

It is good when composite increments are added during light curing. The slower reactions means that reactive sites are available when the new layer is added, which results in a good bond between the two layers. If, on the other hand, the first layer would be completely cured, the second layer would not be able to form a significant number of covalent bonds between the two material layers.

It is bad when bonding agents are excessively air thinned. Air-thinning may delay the cure at the bonding resin interface, and when the rest of the material is cured and shrinks, the poorly cured adhesive resin may not be able to resist the shrinkage stresses that develop around the margins. There is also a risk that an oxygen-inhibited layer is left on a surface of a composite and a sealant, and that this layer is dissolved in oral fluids and swallowed.