Elastic Impression Materials

1. Elastic Impression Materials Hydrocolloids

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

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: two phase system. • Dispersed phase. • Dispersion medium  water  Hydrocolloid

4. Sol state: viscous liquid. • Gel state: jelly like • sol •  fibrils in a network pattern. • Brush heap structure. • Solreversible 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 • 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 • Potassium sulfate • to accelerate the setting of gypsum • Fillers • to control the strength, viscosity, and rigidity • Alkyl benzoate • anti-fungal • Pigments and flavors

8. cool to 43 C agar hydrocolloid (hot) agar hydrocolloid (cold) ( sol) heat to 100 C Setting Reaction (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 71-100°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. 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.