Impression Materials - Alginates Flashcards
What are impression materials
- Used to produce a detailed replica of the teeth and oral tissues
- A model is made from this impression
- The model is then used in the construction of full and partial dentures, crowns, bridges and inlays, or as study casts depending on the impression material used
What kind of elastic impression material are alginates
Irreversible hydrocolloids
What kind of reaction is used to set alginates
Chemical cross-linking reaction
What are alginates extracted from
Seaweed
What happens when alginate is mixed with water
When alginate is mixed with water, it forms a colloidal suspension in it, and then it sets by a chemical cross-linking reaction (irreversible hydrocolloid)
What are the typical components of an alginate powder
- Sodium/potassium alginate - 12%
- Diatomaceous earth (filler) - 70%
- CaSO4 (cross-linking agent) - 12%
- Na3PO4/Na2CO3 (retarding agent)
- Sodium silicofluoride/fluorotintinate (pH controller) - 4%
- Magnesium Oxide (pH controller)
What part of the sodium alginate structure is where the cross-linking reaction occurs
- COONa
- Bond breaks and forms a new COO-Ca-OOC bond between alginate molecules
What happens for the setting reaction of sodium alginate to occur
- CaSO4 (sparingly soluble) = Ca2+ + SO4^2-
- Na3PO4 (very soluble) = 3Na+ + PO4^3-
- Ca2+ + 2PO4^3- = Ca3(PO4)2 (insoluble)
- After PO4^3- ions are used up, the Ca2+ ions react with the alginate to set the material
- Sodium alginate + CaSO4 = Calcium alginate + Na2SO4
Describe the pH changes during the setting of alginates
Water + Na3PO4 = alkaline pH (~12)
- When setting pH decreases to below pH 3.5 and rises to about 9 on setting
Why are pH controllers important in alginates
They decrease the pH to near neutral as an acidic/alkaline set gel will give a poor plaster/stone surface
What colour changes occur with chromoclone alginate to indicate that the gel is at the correct pH
- When water is added the alginate changes to purple
- At this stage mix thoroughly and completely until the material changes to pink
- Pink - the material is loaded into the tray
- Light peach - insert the tray in the mouth
- This prevents unnecessary time in the oral cavity
What is the mix to set time of chromoclone
90-110 seconds
What are the advantages of alginates
V good setting behaviour:
- Na3PO4 suppresses setting at first so material is a viscous paste while impression tray is seated in the mouth
- When the setting reaction begins, it is completed very quickly so it minimises the time of impression taking
- The material is cheap and reliable
What are the disadvantages of alginates
Alginates in air loses water, which results in:
- Continual shrinkage in air
- Poor dimensional stability in air
- Impression must be covered with a damp napkin/gauze and placed in a plastic bag, sealed (only a few hours)
Alginate immersed water/disinfecting solution
- Initially swells, then shrinks
- Poor dimensional stability in solution; immersion in disinfecting solution - unsolved problem
- MUST follow disinfection protocol
- Alginates have a poor tear strength
Highly viscoelastic material:
- Snap-removal technique need to be employed
- Permanent deformation up to 1.5%
- Latter can be mined if undercuts are not deep
Doesn’t adhere to trays well - retention to tray is achieved by mechanical locking features ie. perforated trays or by applying an adhesive
What is the disinfection protocol for alginates
- Remove set alginate/tray from mouth
- Rinse in tap water to remove blood/saliva
- Immerse in disinfecting solution (e.g. sodium hypochlorite) for only 10mins
- Remove from disinfecting solution and rinse in tap water
- Wrap a damp napkin/gauze and seal in a polythene bag
