Elastic Impression Materials Hydrocolloids Prof Amal Ezzeldin Abdel Aziz Professor of Dental Biomaterials

Aqueous Hydrocolloids Elastic Aqueous Hydrocolloids Non-aqueous Elastomers Polysulfide Silicones Polyether Condensation Addition Agar (reversible) Alginate (irreversible) O’Brien Dental Materials & their Selection 1997

Alginate Impression material ِIrreversible Hydrocolloids

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

Irreversible Hydrocolloids Alginate Impression Material Uses Sol  gel by chemical reaction Advantages: Easy to manipulate. Does not require special equipment. Relatively inexpensive. Comfortable for the patient.

Composition: Potassium alginate.(soluble sol) Calcium sulphate dihydrate.(reactor) Potassium sulphate. Sodium phosphate: retarder. Diatomaceous earth: filler. Glycols: dustless. Pigments. Flavours. Quaternary ammonium salts: Disinfectants

Composition Soluble alginate Potassium alginate (15%) Soluble alginate Calcium sulfate dihydrate (16%) Reactor Diatomaceous earth (60%) Fillers Strength, Stiffness, Texture Potassium titanium fluoride (3%) Accelerator for the setting of the stone Sodium phosphate (2%) Retartder - reacts with calcium ions to slow the working time Glycols Decrease dust formation Quaternary ammonium salts Disinfectants Pigments, taste

Substance 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.

Reaction: CaSO4.2H2O + Na3PO4 CaSO4.2H2O + kalginate K2SO4 + calcium alginate (insoluble gel) Types: amount of Na3PO4 Fast set: less amount of retarder. Regular set. + H2O Ca3PO4 + Na2SO4

Gel Structure The final structure cross-linked calcium alginate gel. Cross-linking of Sodium alginate

Concentration 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: 45 seconds (fast set). One minute (regular set).

Manipulation of Alginate impression

Avoid 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 .

Gelation Time Once gelation starts, the impression material must not be disturbed because the growing fibrils will fracture and the impression would be significantly weakened. Fast setting = 1.5 -3 mins. Normal setting = 3 – 4.5 mins Best regulated by the amount of retarder added during manufacturing.

Fast 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 = 1.5 -3 mins. Normal setting = 3 – 4.5 mins

Removal 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.

Avoid torquing or twisting when remove the impression Pouring impression: washing under running tap H2O, to remove blood, saliva. No: separating medium.

How 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.

Sealed plastic bag

Compatibility 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.

2-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.

Disinfection Certain disinfectants my result in gypsum casts that have a lower surface hardness or diminished surface detail Current protocol Impression --> thoroughly rinsed --> spray disinfectant --> wrap in a disinfectant-soaked paper towel --> place in a sealed plastic bag for 10 minutes Iodophor, Sodium hypochlorite, Glutaraldehyde

Agar-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.

Advantages: 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