Alginate

Question 1

Two types of trays

Answer 1

1. Stock trays
2. Custom trays

Dentate patient- square
Edentulous patient- oval

Question 2

Desirable properties of impression materials

Answer 2

Biocompatible
Short setting time with long working time
Adequate shelf life (fit for consumption)
Cost effective
Readily disinfected wiithout loss of accuracy
Fluid enough to adapt to oral tissues
Viscous enough to be contained in an impression tray
Should not release by-products during setting

Question 3

Conventional impression materials

Conventional/digital

Answer 3

Elastic (mostly used in undercuts)
Reversible hydrocolloid (agar)
Irreversible hydrocolloid (alginate)
Elastomeric elastic materials
Polysulfide
Silicone Addition/Condesnation
Vinyl Polyether siloxane

Inelastic (used for edentulous patients having no or little undercuts)
Impression waxes
Impression compound
Impression plaster
Metallic pastes
Zinc oxide impression

Question 4

Setting of alginate

chemical reaction

Answer 4

Gelation
Soluble sodium alginate reacts with calcium sulphate to form insoulbe calcium sulphate gel

Calcium ions replace sodium or potassium ions in alginate to form a cross-linked complex polymer chain

Calcium sulphate attacks trisodium phosphate first until all has reacted before reacting with alginate increasing the working time

Question 5

Mode of supply of alginate and the components

Answer 5

1. Bulk powder
2. Two paste system

Components of alginate

1. Sodium/potassium alginate (triethanolamine)
   Thickening agent 15%
2. Calcium phosphate -** reactor**
3. zince oxide- **filler ** 4%
4. Potassium titanium-** Accelerator*/ Gyspsum hardener 3%
5. Diatomaceous earth- filler particles- Filler (strength) 60%
6. Sodium phosphate- **Retarder ** 2%

Question 6

Types of alginate

Answer 6

1. Fast setting (less Trisodium phosphate)
2. Normal setting (more Trisodium phosphate)

Question 7

Advantages and disadvantages of alginate

Answer 7

**Advantages **
Hygienic as not re-used
Easy to use and manipulate
Set impression is flexible
Good surface detail in presence of saliva
Low cost
Comfortable for the patient

**Disadvantages **
Cannot be corrected
Poor tear strength
Cannot be stored for long
Not for high accuracy
Poor dimensional stability due to imbibition, synerisis

Question 8

Factors affecting strength of alginate

Alginate has poor tear strength

Answer 8

W/P ratio
Mixing time
Time of removal of impression