Dimesnional accuracy
Fluid when poured
Good wetting (how well material spreads)
Once set is hard and fracture resistant
Compatible with all materials it contacts
List the requirements of model/die materials
Low contact angle
Good wetting results in what kind of contact angle?
High contact angle
Bad wetting results in what kind of contact angle?
As the type increases in number the materials toughness also increases
(eg type 1 = impression plaster
Type 5 = dental stone )
Describe the ISO classification of dental gypsum materials
Gypsum -> Gypsum product + water
What is the word equation for the manufacture of gypsum
A hydrated salt from calcium sulphate
What is gypsum?
Dental plaster
Dental stone
Densite
Name 3 products that originate from gypsum
Dental plaster
Dental stone
Densite
List gypsum products in order of weakness to toughness
Decrease
As the toughness of a gypsum product increases what happens to the porosity (amount of air) and particle size?
Porosity
What factor accounts for the uptake of water?
Loss of detail
Weakness
(However may be overcome by vibrating/mechanical mix under vacuum)
Mixing air incorporation leads to what?
1st stage - water saturated with hemihydrate
2nd stage - dihydrate
3rd stage - crystallises out initiated by exposure to nuclei of crystallisation (impurities present in plaster)
Describe the 3 stages of the setting reaction of gypsum
Increased viscosity
Exothermic
Expansion
What is setting also accompanied by in gypsum?
Manufacturer
- increased concentration of nucleating agents
- accelerators increase solubility of hemihydrate
Accelerators and retarders can be used to control expansion
Operator
-Temperature has no effect on expansion
-water/powder
What manufacturing factors effect the setting time?
Temperature - little effect as accelerates solution process and retards crystallisation. And no effect in expansion
Water/powder - increase retard setting as decreases concentration of recrystallisation nuclei
Mixing time - increase time then increase set
For the operator what factors effect the setting time?
True
True or false
Temperature has no effect on expansion
White
What colour of dye is added to plaster?
Yellow
What colour of dye is added to stone?
Blue/pink
What colour of dye is added to die stone?
Stone stronger than plaster as less porous
Strength increases in time as progressive evaporation of water
Brittle
Low flexural strength
Good dimensional stability
Good detail reproduction
Soluble in water
List set material properties
Inexpensive
Easy to use
Advantages of gypsum
May interact with borax in alginate
Hypochlorite may interact if not rinsed off impression before casting
Disadvantages of gypsum