Impression Materials - Alginates

Question 1

What are impression materials

Answer 1

• 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

Question 2

What kind of elastic impression material are alginates

Answer 2

Irreversible hydrocolloids

Question 3

What kind of reaction is used to set alginates

Answer 3

Chemical cross-linking reaction

Question 4

What are alginates extracted from

Answer 4

Seaweed

Question 5

What happens when alginate is mixed with water

Answer 5

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)

Question 6

What are the typical components of an alginate powder

Answer 6

• 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)

Question 7

What part of the sodium alginate structure is where the cross-linking reaction occurs

Answer 7

• COONa

- Bond breaks and forms a new COO-Ca-OOC bond between alginate molecules

Question 8

What happens for the setting reaction of sodium alginate to occur

Answer 8

1. CaSO4 (sparingly soluble) = Ca2+ + SO4^2-
2. Na3PO4 (very soluble) = 3Na+ + PO4^3-
3. Ca2+ + 2PO4^3- = Ca3(PO4)2 (insoluble)
4. After PO4^3- ions are used up, the Ca2+ ions react with the alginate to set the material
5. Sodium alginate + CaSO4 = Calcium alginate + Na2SO4

Question 9

Describe the pH changes during the setting of alginates

Answer 9

Water + Na3PO4 = alkaline pH (~12)

- When setting pH decreases to below pH 3.5 and rises to about 9 on setting

Question 10

Why are pH controllers important in alginates

Answer 10

They decrease the pH to near neutral as an acidic/alkaline set gel will give a poor plaster/stone surface

Question 11

What colour changes occur with chromoclone alginate to indicate that the gel is at the correct pH

Answer 11

• 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

Question 12

What is the mix to set time of chromoclone

Answer 12

90-110 seconds

Question 13

What are the advantages of alginates

Answer 13

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

Question 14

What are the disadvantages of alginates

Answer 14

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

Question 15

What is the disinfection protocol for alginates

Answer 15

• 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

Question 16

Describe the process for the dispensing of alginates

Answer 16

Tub should always be shaken before use and then left for 2 mins

Question 17

Why is there a process to dispense alginates

Answer 17

Silica particles in the dust are a possible health hazard but most alginate powders are dust free now

Question 18

Name a dust free alginate

Answer 18

Triethanolamine alginate

Question 19

What types of alginate are there

Answer 19

• Fast set, regular set and slow set based on the amount of calcium sulfate and sodium phosphate varied
• Sodium alginate
• Potassium alginate
• Mix of both
• Triethanolamine alginate

Question 20

What kind of elastic impression material is Agar and how does it set

Answer 20

Reversible Hydrocolloid - set by a physical process

Question 21

What are the general properties of Agar impression materials

Answer 21

• A solid gel at room temperature
• A viscous liquid at 60C
• On cooling, it reverts to the gel
• Gelling (setting) is a physical process

Question 22

What are the components of Agar impression materials

Answer 22

• Agar - 13-17%
• Borates - 0.2-0.5%
• Potassium Sulphates - 1-2%
• Thixotropic Materials - 0.5-1%
• Water - Balance

Question 23

What is the function of agar in agar impression materials

Answer 23

Colloid

Question 24

What is the function of borates in agar impression materials

Answer 24

Strengthen gel

Question 25

What is the function of agar in Potassium Sulphates impression materials

Answer 25

Accelerates setting of stone

Question 26

What is the function of Thixotropic materials in agar impression materials

Answer 26

Filler

Question 27

What is the function of agar in water impression materials

Answer 27

Dispersion medium

Question 28

Describe the dispensing of agar

Answer 28

- Dispensed in tubes (prevent water loss) - Tubes are put into water bath at 60C - Transferred to a bath at 40C before use - Material is squeezed out onto a special metal tray - seated in the mouth - Cooled by water - circulated via cooling tubes attached on outer surface of tray Note: material sets from tray to mouth

Question 29

What are the advantages of Agar

Answer 29

- Once the equipments' set up, it is easy to use - The material is cheap - Impressions have good surface detail: fine detail recorded, due to setting behaviour

Question 30

What are the disadvantages of agar impression materials

Answer 30

- Must be cast up immediately due to shrinkage from loss of water (syneresis) - Also absorbs water (imbibition) - distortio - Compatibility with model materials - Agar has a poor tear strength (but better than alginates) - Highly viscoelastic material: snap-removal technique needs to be employed - permanent deformation up to 1%