Gypsum Flashcards
What is a study cast
a positive replica of the dentition
What are study casts produced from
impressions
What is the purpose of a study cast
records the position, shape of teeth
aids visualisation/assessment of dentition
enables manufacture of dental prostheses (e.g partial dentures, crowns etc)
What is the use of gypsum
cast (plaster/stone)
die (stone/improved stone)
mould material (stone)
investment binder (stone)
How is gypsum made
calcium sulphate dihydrate is heated until calcium sulphate hemihydrate is produced and it is surrounded by water
What determines the type of gypsum
manufacturing conditions
What are the types of gypsum
plaster
dental stone
densite
What determines the properties of the different types of gypsum
crystalline structure
What is beta-hemihydrate
plaster
What is alpha-hemihydrate
dental stone
What is improved stone
densite
What is plaster heated in
an open vessel
What is the crystallite structure of plaster
large, porous, irregular crystals
What is the crystallite structure of dental stone
non-porous, regular crystals, requires less water
How is dental stone heated
in an autoclave
How is densite heated
in presence of calcium and magnesium chloride
What is the structure of densite
compact smoother particles
What is the setting reaction
reverse of the manufacturer
hemihydrate + 3H2O –> dihydrate.2H2O
powder + water –> gypsum (study cast)
Why is excess water used in the mixing ratios
it is needed for a workable mass
affects properties
What is the mixing ratio for plaster
50-60ml
100g
What is the mixing ratio for stone
20-35ml
100g
What is the theoretical ratio
18.6ml
100g
Describe the setting process
hemihydrate dissolves
dihydrate forms but its solubility is low resulting in a super saturated solution
the dihydrate crystals precipitate on impurities as crystals
more hemihydrate dissolves and this continues until all hemihydrate is dissolved
Describe the initial set of gypsum in the setting process
dihydrate crystals come into contact and start to push each other apart
expansion starts
properties of weak solid and will not flow
can be carved
Describe the final set of gypsum
strong and hard enough to be worked
strength continues to develop
initial and final setting times measured using gilmore needles
What happens to water during setting
excess water is trapped in the powder mass
What happens to water on completion of setting
excess evaporates
voids produced - not good
What is the compressive strength of gypsum
20-35 MPa
What gypsum has the strongest compressive strength
densite with around 38 MPa
Why is stone stronger than plaster
stone stronger as it requires less water for workable mix making it less porous
Which has the lowest expansion
densite
Which has the greatest expansion
plaster
How does increased powder effect setting time and expansion
decreased setting time
increased expansion
How does increased spatulation effect setting time and expansion
decreased setting time and increased expansion
How does increased impurities effect the setting time and expansion
decreased setting time
increased expansion
How does an increase in temperature effect setting time
not very predictable
different temperatures have different effects
What do chemicals such as borax and NCl effect
increase/decrease setting time
decrease expansion
What does increased spatulation do
increased spatulation breaks down growing crystals
fragments act as nuclei of crystallization
more growing crystals - come into contact sooner
What does increasing powder do
more nuclei of crystallization per unit volume
crystals come into contact sooner
faster set and greater expansion
What is the effect of expansion
can result in a bigger model
can allow for slightly bigger so crowns bridges etc won’t be too tight
What is the effect of an increase temperature on the rate of diffusion of ions
increase in rate of diffusion
What is the effect of an increase in temperature on solubility of hemihydrate
decrease in solubility
What are the chemical additives
potassium sulphate
borax
What does potassium sulphate do
produces syngenite (K2(CaSO4)2.H2) crystallites rapidly and encourages growth of more crystals so decreases setting time
What does borax do
forms calcium borate which deposits on dihydrate crystals and this delays the setting process which increases the setting time
Why is compatibility of impression material important
dental stone model surface detail depends on type of impression material
needs to be chemically compatible
must ‘wet’ the impression material with no resistance to flow
What should gypsum ideally reproduce
fine detail on the impression material
but gypsum is inherently porous resulting in a relatively rough surface
What are the advantages of gypsum
dimensionally accurate and stable
low expansion of stone/densite
good color contrast
What are the disadvantages of gypsum
low tensile strength
poor abrasion resistance
very brittle
surface detail less than elastomer impression
poor ‘wetting’ of some impression materials
