3Alginate Impression material ِIrreversible Hydrocolloids
4Elastic 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
5Agglomeration of molecules Sol state: viscous liquid.Gel state: jelly likesol fibrils in a network pattern.Brush heap structureSOLreversible hydrocolloidOr by: chemical reaction:irreversible hydrocolloid.gelAgglomeration of moleculesGelreduction in temperature
6Irreversible Hydrocolloids Alginate Impression Material UsesSol gel by chemical reactionAdvantages:Easy to manipulate.Does not require special equipment.Relatively inexpensive.Comfortable for the patient.
8Composition Soluble alginate Potassium alginate (15%)Soluble alginateCalcium sulfate dihydrate (16%)ReactorDiatomaceous earth (60%)FillersStrength, Stiffness, TexturePotassium titanium fluoride (3%)Accelerator for the setting of the stoneSodium phosphate (2%)Retartder - reacts with calcium ions to slow the working timeGlycolsDecrease dust formationQuaternary ammonium saltsDisinfectantsPigments, taste
9Substance is alginic acid, which was extracted from seaweed Substance is alginic acid, which was extracted from seaweed. Potassium salt form of alginic acid (Potassium alginate) react with calcium sulfate dihydrate and water to form a calcium alginate gel.
10Reaction: CaSO4.2H2O + Na3PO4 CaSO4.2H2O + kalginate K2SO4 + calcium alginate (insoluble gel)Types: amount of Na3PO4Fast set: less amount of retarder.Regular set.+ H2OCa3PO4 + Na2SO4
11Gel Structure The final structure cross-linked calcium alginate gel. Cross-linking of Sodium alginate
12Concentration of filler: Controls flexibility of set impression.Soft set: less amount of filler.Hard set.Manipulation:Proportioning.Mixing: vigorous mix – against sides of the rubber bowl.Mixing time: seconds (fast set).One minute (regular set).
14Avoid movement of the tray during setting. Gelation time:Time from the start of mix till material is no longer tacky when touched with finger.Avoid movement of the tray during setting.Remove the tray after additional/2-3 minutes as elasticity is improved with time and permanent deformation is .
15Gelation TimeOnce gelation starts, the impression material must not be disturbed because the growing fibrils will fracture and the impression would be significantly weakened.Fast setting = mins.Normal setting = 3 – 4.5 minsBest regulated by the amount of retarder added during manufacturing.
16Fast setting = 1.5 -3 mins. Normal setting = 3 – 4.5 mins Control of the S.T:Do not reduce powder/H2O ratio as this affects consistency, accuracy and strength.To prolong S.T: reduce temperature of H2O *not below 18°C.temperature of water should be between 18-24°C.Better to choose the suitable type of alginate.Fast setting = mins.Normal setting = 3 – 4.5 mins
17Removal of the impression: one sudden pull-single stroke. As the material is Visco Elastic:Mechanical properties are time dependant and depends upon rate of load application.Rapid rate of removal: Tear Strength 4 times by sudden pull removal.
18Avoid torquing or twisting when remove the impression Pouring impression: washing under running tap H2O, to remove blood, saliva.No: separating medium.
19How to avoid such Dimensional changes How to avoid such Dimensional changes.(synersis –imbibition –evaporation)Immediate pouring of the impression.Storing in a humidor: 100% relative humidity or wrapping in paper towel saturated with water.This does not prevent synersis.Done only for one hour maximum.
21Compatibility with Gypsum Surface Hardness Of The Cast (Gypsum casts poured against hydrocolloids sometimes have rough or chalky surface which causes inaccuracy or lack of sharp details.Causes:1-Impression not washed from blood or saliva after removal from the mouth as blood and saliva retard the setting of gypsum.
222-Excess H2O left in impression surface before pouring dilute surface of the model. 3-Impression stored in humidor for ½ or 1 hour, not washed to remove exudate.4-Removal of the model after a long time (several hours)…contact of the gel containing H2O with gypsum which is slightly soluble. CaSO4.2H2O chalky surface.5-Premature removal of the model: not before one hour.
23DisinfectionCertain disinfectants my result in gypsum casts that have a lower surface hardness or diminished surface detailCurrent protocolImpression --> thoroughly rinsed --> spray disinfectant --> wrap in a disinfectant-soaked paper towel --> place in a sealed plastic bag for 10 minutesIodophor, Sodium hypochlorite, Glutaraldehyde
24Agar-Alginate Combination Impression Syringed agar: boiled for 8 minutes cooled to 65°C for 10 minutes, injected around preparation.Alginate: mixed in tray over agar.Setting time: 4 minutes.
25Advantages:Simple heating equipment.No water cooling system trays.Fine details of preparation.Agar: more compatible with gypsum model materials than alginate, & has higher tear strength than algiante