DM4 - Gypsum Flashcards
what is type 1 ISO standard dental gypsum products?
impression plaster
what is type 2 ISO standard dental gypsum products?
dental plaster for models
what is type 3 ISO standard dental gypsum products?
dental stone for die/models
what is type 4 ISO standard dental gypsum products?
- dental stone
- die high strength
- low expansion
what is type 5 ISO standard dental gypsum products?
- dental stone
- die high strength
- high expansion
What is the name for how easily a substance spreads?
good wetting
What makes a good wetting?
small contact angle
what products are used models/dies?
dental plaster
dental ston e
densite
how does gypsum turn to plaster?
120 degrees
how does gypsum turn to stone?
By steam pressure in an autoclave to 125 degrees
how does gypsum turn to densite?
boil in CaCl2
What accounts for the uptake of water in gypsum?
porosity §
On mixing what does air incorporation lead to? (3)
- loss os detail
- weakness
- may overcome by vibrating/mechanical mix under vacuum
what is porosity?
measure of the void spaces in a material
what is the first stage of setting?
water becomes saturated with hemihydrate
what is the 2nd stage of setting?
conversion of hemihydrate to dihydrate
what is the 3rd stage of setting?
dihydrate crystallises out initiated by exposure to nuclei of crystallisation
what is setting accompanied wit?
- increased viscosity
- exothermic
- expansion
What kind of set does a Gilmore needle give you?
The intermediate set but NOT the final set
How would the manufacturer control setting time? (3)
- increase concentration of nucleating agents to speed up
- use K2SO4 as an accelerator which increases solubility of hemihydrate
- can use borax as a retarder
How would the operator control setting time? (3)
- Increase the water to powder ratio. This decreases setting as decreases concentration of crystallisation nuclei
- increase mixing time. This increases set rate due to formation of more nuclei of crystallisation as breaks up dihydrate crystals
- increase temperature. accelerates solution process and retards crystallisation
what colour is plaster?
white
what colour is stone?
yellow
what colour is die stone?
blue/pink
What are the set material properties?
- brittle
- low flexural strength
- good dimensional stability
- good detail reproduction
- soluble in water
why is stone stronger than plaster?
less porosity
What are the advantages of setting materials? (2)
- inexpensive
- easy to use
What are the disadvantages of setting materials? (2)
- may interact with borax in alginate
- hypochlorite if not rinsed off impression before casting
