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Elastic Impression Materials. Hydrocolloids. Prof Amal Ezzeldin Abdel Aziz Professor of Dental Biomaterials. Agar (reversible). Aqueous Hydrocolloids. Alginate ( irreversible ). Elastic. Polysulfide. Condensation. Non-aqueous Elastomers. Silicones. Addition. Polyether.

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Elastic Impression Materials


Prof AmalEzzeldin Abdel Aziz

Professor of Dental Biomaterials


Agar (reversible)

Aqueous Hydrocolloids

Alginate (irreversible)




Non-aqueous Elastomers




O’Brien Dental Materials & their Selection 1997


Alginate Impression material

ِIrreversible Hydrocolloids


Elastic Impression Materials


  • 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

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.


Agglomeration of molecules


reduction in temperature


Irreversible Hydrocolloids

Alginate Impression Material


Sol  gel by chemical reaction

  • Advantages:
  • Easy to manipulate.
  • Does not require special equipment.
  • Relatively inexpensive.
  • Comfortable for the patient.


  • Potassium alginate.(soluble sol)
  • Calcium sulphatedihydrate.(reactor)
  • Potassium sulphate.
  • Sodium phosphate: retarder.
  • Diatomaceous earth: filler.
  • Glycols: dustless.
  • Pigments.
  • Flavours.
  • Quaternary ammonium salts: Disinfectants
  • 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.Potassium salt form of alginic acid (Potassium alginate) react with calcium sulfate dihydrate and water to form a calcium alginate gel.



  • 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

g el structure
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.


  • Proportioning.
  • Mixing: vigorous mix – against sides of the rubber bowl.
  • Mixing time: 45 seconds (fast set).

One minute (regular set).


Gelation time:

  • Timefrom 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
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.

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 suchDimensional 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.
compatibility with gypsum
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 beforeone hour.
d isinfection
  • 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.



    • 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