Gypsum Products Flashcards
What is the chemical composition of gypsum?
- Calcium sulphate dihydrate
- (CaSO4)2H2O
What can gypsum based materials be used for? (5)
- Impression materials
- Model (poured from an impressionl)
- Dies
- Moulds (construction of dentures)
- Refractory investments
Where is gypsum obtained from?
White Cliffs of Dover
What is gypsum converted to by the manufacturers?
- Calcium sulphate hemihydrate
- (CaSO4) H2O
Depending on the treatment process what can conversion too hemihydrate result in? (3)
- Plaster
- Stone
- Improved stone
Heating gypsum in an open kettle at 120° results in production of?
Plaster of Paris
b-hemihydrate
(CaSO4)2H2O
Heating gypsum in an autoclave under steam and pressure at 120/130° results in production of? (2)
- Autoclaved Artificial Stone
- Hydrocal Calcined
α-hemihydtare
(CaSO4)2H2O
Heating gypsum in 30% boiling solution of magnesium or calcium chloride results in production of?
Improved stone (densite)
α-hemihydrate
(CaSO4)2H2O
Comment on the strength of product produced when a b-hemihydrate is mixed with water
Why is this?
- Forms a weak product
- Due to the large irregular & porous particles, which do not pack closely, thus resulting in large pores within the set material
Comment on the strength of product produced when a α-hemihydrate is mixed with water
Why is this?
- Forms a strong product
- Due to it’s small, regular & non-porous particles, which pack closely together within the set material
What are the advantages of plaster? (3)
- Cheap
- Soft
- Easy to use and shape
When is plaster generally used? (3)
- Mounting models into articulators
- Flasking procedures
- Basing models
What is stone generally used for?
Models of the mouth
- Replica of individual teeth
- Construction of crowns bridges and dentures
What is used in dentate casts when maximum strength is essential?
Dental stone
What is the general composition of gypsum products? (5)
Give rough percentages of these
- Hemihydrate (75-80%)
- Dihydrate (5-8%)
- Insoluble anhydrites (5-8%)
- Impurities (4%)
- Accelerators/retarders (4%)
How do accelerators and retarders affect setting time?
These chemicals increase or decrease the solubility of gypsum in water, thus affecting it’s setting time
Name an accelerator for gypsum
What product is produced because of this?
- 2% K2SO4 solution
- Syngenite [K2(CaSO4)2H2O]
How does syngenite speed up a reaction? (3)
- Rapid crystallisation
- Reduces overall expansion
- Accelerates setting reaction
Name 3 retarders for gypsum
- > 20% NaCl
- Borax
- Potassium citrate
Comment on the solubility of hemihydrate over dihydrate
Hemihydrate is more soluble
What happens during the setting reaction of hemihydrate
- Water is added
- Some hemihydrate dissolves in water and forms dihydrate
- The dihydrate’s lower water solubility results in a supersaturated solution being formed
- This is unstable so the dihydrate precipitates to form stable crystals
What is the role of unchanged gypsum dihydrate in the setting mechanism?
Acts as crystallisation nuclei for growth of dihydrate crystals
How much water is required to set 100g of
Plaster?
Stone?
Improved stone?
- 50ml
- 20ml
- 20ml
What will occur if you add too much water in the setting process of gypsum products?
Setting time?
Consistency of mix?
Model strength?
- Slower setting time
- The mix will be runny
- Model will be weaker
What will occur if you add too much powder in the setting process of gypsum products?
- Difficult to mix
- Result in porosity within the set material
Increasing spatulation time and or rate in gypsum products will result in? (2)
- Reduction in setting time
- Increases expansion
What is the result of increasing temperature of the water in gypsum product setting?
What is the ideal temperature water?
- Slow the reaction down
- Will stop completely at 100°
- 0°-50°
How is the setting time of different gypsum products measured? (2)
- Vicant needles
- Gillmore needles
¼lbs needle for initial
(1lbs needle for final)
Why is setting expansion in gypsum products important?
Allowing models and dies to be slightly larger than oral anatomy so crowns and dentures aren’t too tight
Comment on the dry strength of gypsum products compared to wet strength
Dry strength is twice that of wet strength
What is the tensile strength of plaster
Very low 2Mpa
What is the tensile strength of stone
4 Mpa
Comment on the surface hardness of
- Plaster
- Stone
- Improved stone
- Low
- Improved
- Best out of 3
What is setting time for plaster as an impression material
3-5 minutes
What is the setting time for plaster in the laboratory
5-10 minutes
What is the setting time for stone
7-15 minutes
What is the setting time for improved stone
5-12 minutes
