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| An alginate impression of the upper arch in a customised tray |
An alginate impression of the lower arch |
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Clinical Use:
The container of powder should be shaken before use to get an even distribution of constituents. Powder and water should be measured to manufactures instructions. Water at room temperature should be used, this gives a reasonable working time of a couple of minutes. Faster or slower setting times can be achieved by using warm or cold water respectively. The material nearer the tissues sets first (cf. agar). Retention is needed to the impression tray and is provided by perforations in the tray and/or adhesives. Once removed from the mouth the impression should be rinsed with cold water to remove any saliva or blood. The impression should then be soaked in hypochlorite for 60 seconds and be covered in a damp gauze/napkin to prevent syneresis (not placed in water which would cause imbibition-expansion). The disinfected impression should then be cast as soon as possible.
Chemistry:
On mixing the powder with water a sol is formed, a chemical reaction takes place and a gel is formed.
The powder contains...
1. Alginate salt (e.g. sodium alginate)
2. Calcium salt (e.g. calcium sulphate)
3. Trisodium phosphate
When water is mixed with the powder the following reaction occurs...
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| SODIUM ALGINATE |
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SODIUM SULPHATE |
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| CALCIUM SULPHATE |
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CALCIUM ALGINATE |
| The above reaction occurs too quickly often during mixing or loading of the impression tray. It can be slowed down by adding trisodium phosphate to the powder. This reacts with the calcium sulphate to produce calcium phosphate, preventing the calcium sulphate reacting with the sodium alginate to form a gel.
This second reaction occurs in preference to the first reaction until the trisodium phosphate is used up, then the alginate will set as a gel.
There is a well-defined working time during which there is no change in viscosity.
Properties:
- Good surface detail
- Reaction is faster at higher temperatures
- Elastic enough to be drawn over the undercuts, but tears over the deep undercuts
- Not dimensionally stable on storing due to evaporation
- Non toxic and non irritant
- Setting time can depend on technique
- Alginate powder is unstable on storage in presence of moisture or in warm temperatures
Advantages:
1. Non toxic and non irritant
2. Good surface detail
3. Ease of use and mix
4. Cheap and good shelf life
5. Setting time can be controlled with temperature of water used
Disadvantages:
1. Poor dimensional stability
2. Incompatibility with some dental stones
3. Setting time very dependent on operator handling
4. Messy to work with
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