3Elastic Impression Materials HydrocolloidsSolution: 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
4Agglomeration of molecules Sol state: viscous liquid.Gel state: jelly likesol fibrils in a network pattern.Brush heap structure.Sol reversible hydrocolloidOr by: chemical reaction:irreversible hydrocolloid.gelAgglomeration of moleculesGelreduction in temperature
5Reversible hydrocolloids: Agar-Agar Liquefaction temperature: 71°C-100°C.Gelation temperature 37°C-43°C.Hysteresis: temperature lag or different liquefaction and gelation temperatureSolgel: physical change.
6Agar hydrocolloid Impression Materials UsesComposition: Agar 12.5% by wt.BoraxPotassium sulphateAlkyl benzoateWater 85.5% by wt.Syringes: low viscosity – high flow.Tubes: high viscosity – heavy body: low flow.
7Agar hydrocolloid Potassium sulfate Agar (8-15%) Fillers to accelerate the setting of gypsumFillersto control the strength, viscosity, and rigidityAlkyl benzoateanti-fungalPigments and flavorsAgar (8-15%)Polysaccharide from seaweedProvides matrix phase which forms fibrils in the “gel” stage.Water (>80%)Reaction medium, controls flow propertiesBoraxto strengthen the gelretarder for the setting of gypsum
8Setting Reaction (gel) ( sol) agar hydrocolloid (hot) agar hydrocolloid (cold)( sol)cool to 43 Cheat to 100 C(gel)
9Clinical applicationThe 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).
10Preparing 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 chamber2nd 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.
12Gelation: Use of water cooling system tray with means of retention (perforations). Avoid iced H2O: concentration of stresses.
13Avoid exerting pressure on the tray or moving it. Removal impression: one single stroke or sudden pull. reduce permanent deformation and tearing of impression
14Loss of water by Evaporation Dimensional changes:Loss of water by EvaporationSynersis: Exuding of fluid at surface of the gel. fibril cross linking continuescontracts with time exudes waterImbibition: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
16PropertiesFlows well and adapts readily to hard and soft tissue contoursAccurate 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 immediatelyDistortion is more likely to occur if impression not poured within an hour.
17Tear 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.