Properties of Zinc-Oxide Eugenol
CHEMISTRY
Constituents of a typical zinc oxide eugenol paste are:
BASE PASTE
- Zinc oxide
- Inert oils (plasticiser)
- Hydrogenated resins (increases setting time and improves cohesion)
REACTOR PASTE
- Eugenol
- Zinc acetate (accelerator)
- Fillers (talc or kaolin)
Some pastes contain a substitute for eugenol e.g. a carboxylic acid.
The 2 pastes come in contrasting colours and are dispensed in a 1:1 ratio. They are mixed to give a paste of even colour. The setting reaction is shown below:
| ZINC OXIDE (excess) |
|
ZINC OXIDE (UNREACTED) |
| + |
® |
+ |
| EUGENOL |
|
EUGENOL |
The set material contains both some unreacted zinc oxide and eugenol.
Any movement of the tray as the paste is hardening will lead to a deformed, inaccurate impression.
Setting time depends on:
1. Accelerator additives (e.g. zinc acetate, acetic acid)
2. Exposure to moisture on mixing or the addition of water will accelerate the reaction
3. Increasing temperature causes a faster setting reaction
Setting time is normally 4-5 minutes.
PROPERTIES
- Non toxic
- Adherence to tissues
- Mucostatic or mucocodisplacive (depending on brand used)
- Good surface detail in thin section
- Good dimensional stability (little or no dimensional change on setting, 0.1% dimensional change during setting)
- Can be added to with fresh zinc oxide eugenol
- Stable on storage and good shelf life
ADVANTAGES
1. Dimensional stability
2. Good surface detail
3. Can be added to
4. Mucostatic or mucocodisplacive
DISADVANTAGES
1. Cannot be used in very deep undercuts
2. Only sets quickly in thin section
3. Eugenol allergy in some patients
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