gypsum Flashcards
what is a study model?
positive replica of dentition
produced from the impression
what is the purpose of a study model?
records position, shape and dimensions of teeth
aids visualisation/assessment of dentition
enables manufacture of dental prostheses
-partial denture framework, crowns
what are the uses of gypsum?
cast (plaster/stone)
die (stone/improved stone)
mould material (stone)
investment binder (stone)
describe the manufacture of gypsum
calcium sulphate dihydrate heated to become calcium sulphate hemihydrate and release water
what are the 3 types of gypsum?
plaster
dental stone
densite (improved stone)
what determines gypsum properties?
crystalline structure
describe plaster
heated in open vessel
large porous, irregular crystals
describe stone
heated in autoclave
non-porous, regular crystals, requires less water
describe densite
heated in presence of Ca and Mg chloride
compact smoother particles
how does gypsum become the different types?
processed in different ways influences structure
describe the setting reaction (reverse of manufacture)
hemihydrate powder + water = gypsum
what is the water to powder ratio for plaster?
50-60ml to 100g
what is the water to powder ratio for stone?
20-35 ml to 100g
what happens during the setting process?
hemihydrate dissolves
dihydrate forms
dihydrate solubility low-supersaturated solution
dihydrate crystals precipitate on impurities as crystals
more hemihydrate dissolved
continues until all hemihydrate dissolved
describe the initial set
dihydrate crystals come together -grow until contact each other expansion starts properties of weak solid will not flow can be carved can be carved
describe the final set
strong and hard enough to be worked
strength continues to develop
crystals large dif size & shape
what happens to water during setting?
excess water trapped in the powder mass
what happens to water on completion of setting?
excess water evaporates
voids produced-porosity
describe gypsums reproduction of surface detail
inherently porous resulting in relatively rough surface
-28-40um
good enough for most applications
describe gypsums strength
compressive strength -28-38mPa develops over 24h -75MPa stone stronger as requires less water for workable mix improved stone strongest
describe its surface hardness
low
-surface easy to abrade
-not ideal
must be careful when handling
describe setting time
must be convenient for user
user must be aware of factors that affect this and use accordingly
what factors decrease setting time?
more powder, spatulation, impurities
what factors can increase or decrease setting time?
temperature
chemicals
-borax, K2SO4
what factors increase expansion?
more powder, spatulation, impurities
what factor decreases expansion?
chemicals
why does spatulation decrease setting time and increase expansion?
breaks down growing crystals
fragments act as nuclei of crystallisation
more growing crystals come in contact sooner
why does increasing powder cause faster set and increase expansion?
more nuclei of crystallisation per unit volume
crystals come into contact sooner
describe expansion on setting
should be small % otherwise problematic plaster 0.2-0.2 stone 0.08 -0.1 densite 0.05-0.07 allows model to be a little big so crowns etc. not too tight in mouth
what happens when temp increased?
rate of diffusions of ions increase with temp
solubility of hemihydrate decreased with increased temp
why does potassium sulphate decrease setting time?
produces syngenite
crystallises rapidly-encourages growth of more crystals
why does borax increase setting time?
forms calcium borate- deposits dihydrate crystals
describe compatibility with impression material
dental stone model surface detail depends on type of impression material
- needs to be chemically compatible
- must be wet
- avoid bubble formation
describe bittleness
flexural strength 15-20MPa
much more brittle than composite resin 150MPa
