Alginate is classified as irreversible hydrocolloid. It is hydrocolloid because it consists of particles of a gelatinous (colloidal) state in water (hydro); and irreversible because once it has jelled it cannot be returned to a liquid solution.
It is supplied in powder form and mixed with water. When set, the material is a flexible gel resembling rubber.The impression is made directly in the patient’s mouth producing a negative replica, then poured in dental stone, producing a positive cast.
These materials are elastic enough to be withdrawn from the undercuts without permanent deformation or distortion. It is extremely accurate in tissue details when handled properly.
Other elastic impression materials:
- Agar hydrocolloid
- Condensation silicone
- Addition silicone
The basic components are a soluble alginate (either potassium alginate or sodium alginate) and a reactor (calcium sulfate), which causes the alginate to gel.
Mixing the alginate:
- Use the specific measuring devices (water & powder) provided by the manufacturer for mixing.
- Add the powder into the water. This ensures the powder particles are wet evenly.
- If mixed in reverse (the water is added to the powder) the chemical reaction will start early with some particles setting faster than others.
Setting Time of Alginate:
Temperature is a major factor in the setting time. The colder the temperature of the water the longer it takes to set.
Alginate has a tendency, after it sets to lose (syneresis) or absorb (imbibition) water, depending on the atomosheric conditions surrounding it.
If conditions are dry it loses water & shrinks (syneresis) ; if immersed in water, it imbibes moisture or swells. (imbibition)
- It makes an accurate impression if handled properly
- Elastic to be withdrawn from undercuts without distortion
- Easy to work with, not time consuming
- Subject to dimensional changes if not stored properly before pouring (Syneresis or Imbibition)
- Cannot be added to (add another layer)
The disinfection of alginate impressions must be carried out with regard for the dimensional instability of the impression material.
Excessive immersion in aqueous solutions may cause swelling of the material, and should be avoided.
Immersing alginates in glutaraldehyde solutions for a relatively short term (approximately 10 minutes) can be effective without causing undue dimensional change.
Manipulation of Alginate Impression
- Should not be exposed to air (dehydration).
- Should not be immersed in water (imbibition).
- Should be poured immediately OR
- Should be stored in a humid atmosphere by wrapping in a damp paper towel or in 100% relative humidity (humidor).
- No separating medium is needed for pouring.
Forming the Cast (without boxing)
- Remove the alginate impression from the damp paper towel and shake out any moisture.
- Add gypsum product (powder) into water “according to manufacturer’s instructions” and spatulate. Place the bowl on the vibrator to escape the entrapped air.
- Hold the impression tray against the vibrator and add a small amount of mixed gypsum. Continue to add small increments until impressions of the teeth have been filled.
- Continue to add gypsum in larger portions until the impression is completely filled.
- Let the gypsum reach its initial set.
- The base of the cast can then be formed with a new mix of gypsum. The base should be at least 15 mm thick.
Forming the Cast (with boxing)
- Apply beading wax to the periphery of the impression. It should be placed 2-3 mm from the borders of the impression and should be 4 mm wide.
- Box the impression with boxing wax sheet.
- Hold the boxed impression on a vibrator and add mixed gypsum product in small increments until completely filled.
- After the gypsum has completely set & the exo-therm completed, peal off the boxing wax and remove the beading wax.
- Remove the impression gently off the cast with a plaster knife. For the border molded impression, the cast should be dipped in warm water for easy removal.
The sides of the cast are trimmed to be parallel, any stone nodules are carefully removed. The base can be trimmed for either orthodontic specifications (for a record cast) or to remove excess stone only ( for a master or a working cast ).
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