Gypsum Products Flashcards
What is gypsum?
Calcium sulphate
Unset material: calcium sulphate hemihydrate (CaSO4 H2O)
Set material: calcium sulphate dihydrate (tCaSO4 2H2O)
Bidirectional reaction between hemihydrate and dihydrate. Just add water or put in oven to remove water
List out the types of Gypsum Products according ADA classification:
I: impression plaster (weaker)
II: Model Plaster
III: Dental stone (green stone)
IV: Dental stone, high strength (silky rock)
V: Dental stone, high strength & expansion (compensate for cast alloy shrinkage)
W decreasing W/P ratio from I to V
List the factors that affect setting time of Gypsum Products:
- Particle size
- Impurities
- Water/Powder ratio
- Mixing time
- Accelerators & retarders
How does particle size affect setting time?
Smaller particle size = increased surface area => decreased setting time
How does mixing time affect setting time of gypsum products?
Longer mixing time = higher no. of nuclei => shorter setting time
The ______ impurities, the shorter the setting time.
More impurities => shorter setting time
How does water/powder ratio affect setting time?
The less water, the shorter the setting time
Porosity: is 1. Dental Plaster & 2. Dental Stone porous?
- Dental Plaster
- particles are porous
- more water required as water is absorbed by the porous particels - Dental Stone
- particles are non-porous
- less water is required
What is the initial and final setting time for Dental Plaster?
Initial Setting time: approx 7-10min
- stage where u can no longer work w the gypsum anymore. If u try to change the shape of the gypsum, ull break it
Final Setting time: approx 20-30+ mins
- when crystallisation is over n u can remove it
Initial: can separate from impression
Final: can start carving
Name some retarders of dental plaster/stone:
Borax, acetates, colloids (e.g. blood)
- decreases rate of crystallisation by poisoning nuclei
