Gypsum Flashcards
what produces a positive replica of the dentition?
study model/cast
what is the purpose of a study model?
to record position, shape and dimensions of teeth, aid visualisation/assessment of dentition, enables manufacture of dental prostheses
what are the uses of gypsum?
cast, die, mould material, investment binder
what are the 3 types of gypsum?
plaster, dental stone, densite (improved stone)
what determines the properties of gypsum?
the crystalline structure
where is plaster heated?
in an open vessel
what does plaster look like?
large porous, irregular crystals
where is dental stone heated?
in an autoclave
what does dental stone look like?
non-porous, regular crystals, requires less water
where is densite heated?
in presence of Ca and Mg chloride
what does densite look like?
compact smoother particles
how are the different types of gypsum produced?
dihydrate is heated in different environments to produce the different types of gypsum which is present in hemihydrate form
what happens in the setting reaction of gypsum?
the reverse of the manufacture reaction - hemihydrate turns to dihydrate
with 100g of powder how much water is needed to make plaster?
50-60ml
with 100g of powder how much water is needed to make stone?
20-35ml
what is the theoretical ratio for mixing powder and water?
18.6ml water and 100g powder
what happens in the setting process of gypsum?
hemihydrate dissolved, dihydrate forms, dihydrate solubility is low-supersturated
when the setting process occurs what do dihydrate crystals do?
precipitate on impurities as crystals
what occurs in the initial set of gypsum?
dihydrate crystals come into contact, expansion starts, properties of weak solid and will not flow, can be carved
what is the state of the gypsum when it has finally set?
strong and hard enough to be worked and strength continues to develop
what happens to water during setting?
excess water is trapped in the powder mass
what happens to water on completion of setting?
excess water evaporates, voids produced so porosity occurs
what are the properties needed to consider with gypsum?
reproduction of surface detail, strength, surface hardness, setting time, expansion
what results from gypsum being inherently porous?
a rough surface f about 28-40um
what is the impression material standard?
material must reproduce a 50um wide groove
what type of gypsum is the strongest?
improved stone
what is the final compressive strength of gypsum?
75MPa
what is the surface hardness?
low so easily abraded which is not ideal
what should expansion on setting be?
low to minimise dimensional inaccuracy
what does increased spatulation do?
breaks down growing crystals to nuclei of crystallisation allowing more growing crystals to come into contact sooner and decrease setting time with increased expansion
what does increasing the powder do?
introduce more nuclei of crystallisation so the crystals come into contact sooner and have a faster set and greater expansion
which type of gypsum has the smallest expansion on setting percentage?
densite
what does expansion on setting allow?
for models to be a little big so crowns and bridges and dentures wont be too tight
what happens to the rate of diffusion of ions with increasing temperature?
increases
what happens to the solubility of hemihydrate with increasing temperature
decreases
what are the chemical additives added to gypsum?
potassium sulphate and borax
what does potassium sulphate do?
produces syngenite, crystallises rapidly - encourages growth of more crystals, decreases setting time
what does borax do?
forms calcium borate and delays setting process
what does dental stone model surface detail depend on?
type of impression material
what is a weakness of gypsum?
it is brittle
what does brittle mean?
it will readily fracture when subjected to a small amount of strain
how is brittleness measured?
flexural strength
what is the flexural strength of gypsum?
15 to 20MPa
what are the advantages of gypsum?
dimensionally accurate and stable, low expansion of stone/densite, good colour 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
