Elastic Impression Materials Hydrocolloids

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

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

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

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.

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.

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

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

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).

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.

Manipulation Heating and Conditioning Bath

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

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

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

Syneresis and Imbibition H2O H2O

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.

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.