Gypsum Flashcards
when poured what is gypsum? what does this allow?
fluid
captures fine detail
once set what is gypsum like?
hard and easily fractured
what is gypsum compatible with?
all materials
ISO defines gypsum into what types?
1 - impression plaster
2 - dental plaster for models
3 - dental stone for die/model
4 - dental stone, die high strength, low expansion
5 - dental stone, die high strength, high expansion
making model/die use what?
dental plaster
dental stone
densite
what happens to gypsum in the manufacturing process?
gypsum = gypsum product and water
calcium sulphate dihydrate -> calcium sulphate hemihydrate
how to make dental plaster form gypsum?
heat to 120 degrees
how to make dental stone from gypsum?
steam pressure in autoclave to 125 degrees
how to make densite from gypsum?
boil in calcium chloride
dental plaster -> dental stone -> densite as move to each what happens?
porosity decreases and particle size decreases
what does the porosity of materials account for?
the uptake of water
on mixing and air incorporation what happens?
loss of detail
weakness
overcome by vibrating/mechanical mix under vacuum
stages of the rapid setting reaction?
1st - water becomes saturated with hemihydrate 0.8%
2nd - conversion of hemihydrate to dihydrate 0.2% sol.
3rd - dihydrate crystallises out initiated by exposure to nuclei of crystallisation
what is the setting reaction accompanied by?
increased viscosity - initial/final set. gilmore needles.
exothermic up to 30 degrees
expansion 0.15-0.3% because outward thrust of crystals, may enhance in investments, to good effects by hygroscopic expansion
how can manufacturers control the setting time?
increase conc of nucleation agents to speed it up
accelerators - potassium sulphate which increases solubility of hemihydrate
reatrder - borax
what else can be used to control expansion?
accelerators and retardars
how can the operator control the setting time?
temp - accelerates solution process and retards crystallisation
W/P - increase retards setting as decreases conc of crystallisation nuclei
mixing time - increased = increase in set rate because formation of more nuclei of crystallisation as breaks up dihydrate crystals
how are products differentiated?
colours
plaster - white
stone - yellow
die stone - blue/pink
why is stone stronger than plaster?
less porosities
why does strength increase with time?
progressive evaporation of water
process of precipitation of dissolved dihydrate cements crystals of gypsum together
properties of set material?
brittle low flexural strength good dimensional stability good detail reproduction soluble in water
advantages of gypsum?
inexpensive
easy to use
may electroplate surface to strengthen
disadvatges of gypsum?
may interact with borax alginate
hypochlorite may interact if not washed off impression before casting
