Hydrocolloids are impression materials, they have, according to the experimental conditions, or the state of the ground or in the gel state, with the possibility of passing from one state to another.
They are of 2 types:
Are in the form of a gel "AGAR".
Agar, constituting basic reversible hydrocolloids, is extracted from red algae (Rhodophyceae: Sea of Japan in particular).
A - Chemical composition :
They consist essentially of agar (5 to 15% by weight) swollen with water (70 to 84% by weight) in the gel state.
The following formula is given by example:
- Water: 83.5 g
- Agar-Agar: 14, 3 g
- Potassium sulfate: 2g
Compensates the borax retarding effect on the plaster during molding.
- Borax 0.2 g
Increases gel strength and the viscosity of the sol.
- Thymol: Traces.
- Glycerin: Traces.
- Coloring and flavoring agents: ............ Traces.
That make the fingerprint easier to read and more attractive.
B - Indication :
This material was used in full and partial denture and less frequently for unit footprint.
It is also used in the laboratory to make a duplicate in the manufacture of metal frames (Stellite).
C- Properties :
• It is highly elastic, 38.8% yield.
• Their flexibility is 11% and is sufficient to give an accurate impression despite undercuts it entails.
• When subjected to stresses quickly they are more resistant, that is what is recommended for these materials to remove the impression from the mouth suddenly and quickly.
• These materials also have a very high resolution capability.
D- Setting reaction :
The hydrocolloid is solid at room temperature, then it is a gel, to 60-70 ° C it becomes liquid: it is then a soil.
1. Heating :
Heated in water at 100 ° C for 8 to 12 minutes,
Was stored in water at 65 ° C,
Water was tempered at 45 ° C for 2 minutes before taking the impression.
2. Processing the imprint :
After removing the impression from the mouth, it is necessary to wash it to remove saliva because it can interfere with the plaster.
It gets rid of excess water by shaking the footprint and applying a gentle stream of air.
then cast with plaster stone.
3. Dimensional instability :
The agar fingerprints lose their precision during storage.
This is why it is necessary to pour the plaster models without delay.
If the fingerprints agar must be stored for a minimum of moisture on dimensional change for a maximum of one hour, one should avoid:
Imbibition: water absorption phenomenon by the gel which normally causes expansion.
Syneresis: water loss phenomenon which leads to a contraction.
4. Handling characteristics :
Reversible hydrocolloids have a long working time for the manipulation of these materials requires conditioning bathroom.
The gelifraction occurs by circulating cool water in special imprints balcony doors and requires special equipment.
The thermal shock that occurs by the sudden cooling of the hot tray can be painful for the patient if certain teeth wear metal restorations.
5. The plaster :
Reversible hydrocolloids have a retarding effect on the plaster and unit models and study models have poor surface definition.
To compensate for this, the impression can be deceiving in potassium sulfate solution 2%.
But nowadays most of the agar materials contain potassium sulfate acting as an accelerator in the plaster.
• Low mechanical properties, tear easily in areas against masked and remains at the interproximal space.
• As for reversible hydrocolloids, their physicochemical properties requiring immediate casting of the impression because the problem of stability over time to phenomena of inhibition and synérèses are similar.
Conventional hydrocolloids :
We use two hydrocolloids reversible:
- To fill the tray, presented in tube (SuperBody 500)
- For injection in the mouth, presented in prefilled syringe employment (SUPERSYRINGE 255) or as a cartridge or stick.
However, this technique requires the use of a water tank for the liquefaction and a specific tray to cooling by circulating water.