1. Elastic Impression Materials
Hydrocolloids

2. Elastic
Aqueous Hydrocolloids
Non-aqueous Elastomers
Polysulfide
Silicones
Polyether
Condensation
Addition
Agar (reversible)
Alginate (irreversible)

3. Elastic Impression Materials
Hydrocolloids
Solution: homogenous mixture:
one phase system.
Suspension: hetrogenous mixture:
two phase system.
Colloids: hetrogenous, particle size of the particles are smaller than in suspension:
Dispersed phase.
Dispersion medium  water  Hydrocolloid

4. Agglomeration of molecules
Sol state: viscous liquid.
Gel state: jelly like
sol
 fibrils in a network pattern.
Brush heap structure.
Sol reversible hydrocolloid
Or by: chemical reaction:
irreversible hydrocolloid.
gel
Agglomeration of molecules
Gel
reduction in temperature

5. Reversible hydrocolloids: Agar-Agar
Liquefaction temperature: 71°C-100°C.
Gelation temperature 37°C-43°C.
Hysteresis: temperature lag or different liquefaction and gelation temperature
Sol
gel: physical change.

6. Agar hydrocolloid Impression Materials
Uses
Composition: Agar 12.5% by wt.
Borax
Potassium sulphate
Alkyl benzoate
Water 85.5% by wt.
Syringes: low viscosity – high flow.
Tubes: high viscosity – heavy body: low flow.

7. Agar hydrocolloid Potassium sulfate Agar (8-15%) Fillers
to accelerate the setting of gypsum
Fillers
to control the strength, viscosity, and rigidity
Alkyl benzoate
anti-fungal
Pigments and flavors
Agar (8-15%)
Polysaccharide from seaweed
Provides matrix phase which forms fibrils in the “gel” stage.
Water (>80%)
Reaction medium, controls flow properties
Borax
to strengthen the gel
retarder for the setting of gypsum

8. Setting Reaction (gel) ( sol)
agar hydrocolloid (hot) agar hydrocolloid (cold)
( sol)
cool to 43 C
heat to 100 C
(gel)

9. Clinical application
The use of agar requires special equipment consisting of hydrocolloid conditioner (has 3 water bath chambers) and water -cooled tray connected to a rubber hose that delivers water.
Agar is supplied in two viscosities, thick and thin (depending on the amount of agar).

10. Preparing the material for impression:
Gel heated to °C, becomes liquid (sol)
Tray and agar syringe are liquefied in 8-12 minutes in boiling water in 1st chamber
2nd chamber is used to store the agar at 60-66°C.
3rd camber kept at 45-47°C to temper agar before use to prevent burns.
Tray is loaded and water hose attached and tray is seated on teeth.

11. Manipulation
Heating and Conditioning Bath

12. Gelation: Use of water cooling system tray with means of retention (perforations).
Avoid iced H2O: concentration of stresses.

13. Avoid exerting pressure on the tray or moving it.
Removal impression: one single stroke or sudden pull.
 reduce permanent deformation and tearing of impression

14. Loss of water by Evaporation
Dimensional changes:
Loss of water by Evaporation
Synersis: Exuding of fluid at surface of the gel. fibril cross linking continues
contracts with time exudes water
Imbibition:
If the gel takes water  expansion or swelling.
Stresses:
Pressure on tray during gelation.
Use of iced H2O during initial stages of gelation  rapid cooling  internal stresses.
Shrinkage

15. Syneresis and Imbibition
H2O
H2O

16. Properties
Flows well and adapts readily to hard and soft tissue contours
Accurate reproduction (hydrophilic nature), however, the impression need to be poured immediately.
Need to be stored in 100% humidity for short period of time if not poured immediately
Distortion is more likely to occur if impression not poured within an hour.

17. Tear strength is not high.
Material suffers from loss or gain of water:
Syneresis: a process by which the gel contracts and some of the liquid is squeezed out, forming an exudate on the surface.
Evaporation: loss of water which causes the material to shrink, and impression is distorted.
Imbibition: uptake of water, this will swell and distort the impression.