Gypsum Flashcards
3 names for gypsum
dental plaster
dental stone
densite
what is a study model/cast
a positive replica of dentition
produced from the impression (i.e. negative representation) of pt dentition
3 purposes of the study model/cast
- records the position, shape of teeth
- aids visualisation/assessment of dentition
- enable manufacture of dental prostheses (e.g. partial dentures, crown etc)
3 ways to determine that gypsum is fit for purpose
accuracy
ease of use
limitations
4 uses of gypsum
cast (plaster/stone)
die (stone / improved stone)
mould material (stone)
investment binder (stone)
manufacture of gypsum
CaSO4.2H2O -> (CaSO4)2 . H2O + H2O
Calcium sulphate dihydrate [heat] -> calcium sulphate hemihydrate
Heat manufacturing (3 ways) determines type
what determines the type of gypsum made
heat manufacturing
crystalline structure determines properties
plaster
beta-hemihydrate
dental stone
alpha-hemihydrate
densite
improved stone
plaster (beta hemihydrate) heating process
heated in open vessel
large porous, irregular crystals
dental stone (alpha-hemihydrate) heating process
heated in an autoclave
non-porous, regular crystals, requires less water
micro crystals
densite (improved stone) heating process
heated in presence of Ca & Mg chloride
compact smoother particles
- can pack more readily, more dense
what determines the properties of gypsum
crystalline structure
differences occur due to heating process used
setting reaction of gypsum
reverse of manufacture
(CaSO4)2 . H2O + 3H2O -> (CaSO4)2 . 2H2O
calcium sulphate hemihydrate + water -> Calcium sulphate dihydrate (study cast)
crystalline structure of plaster (beta-hemihydrate)
large, porous irregular crystals
crystalline structure of dental stone (alpha-hemihydrate)
non-porous, regular crystal, requires less water
micro crystals
crystalline structure of densite (improved stone)
compact smoother particles
- can pack more readily, more dense
why is excess water needed for the ratio of water:powder compared to theoretical ratio?
need excess water for workable mass, affects properties
plaster - 50-60ml water to 100g powder
stone - 20-25ml water to 100g powder
theoretical - 18.6ml water to 100g powder
setting process
hemihydrate dissolves
- finds the impurities
dihydrate forms
dihydrate solubility low - supersaturated solution
dihydrate crystals precipitate on impurities as crystals
more hemihydrate dissolved
- Become smaller
continues until all hemihydrate dissolved
initial set
- dihydrate crystals come into contact – i.e. push apart
- —-small to nil hemihydrate
- expansion starts
- properties of weak solid and will not flow
- ——can be carved and manipulated
final set
- strong and hard enough to be worked
- strength continues to develop
- —–large irregular sized dihydrate crystals
Initial & final setting times measured using Gilmore needles
initial set of gypsum
- dihydrate crystals come into contact – i.e. push apart
- —-small to nil hemihydrate
- expansion starts
- properties of weak solid and will not flow
- ——can be carved and manipulated
final set of gypsum
- strong and hard enough to be worked
- strength continues to develop
- —–large irregular sized dihydrate crystals
reaction with water during gypsum setting
excess water is trapped in the powder mass
reaction with water on completion of setting
excess water evaporated
voids produced (porosity) - issue as relatively weak
compressive strength of gypsum
~20-35MPa
- 1/10 of composite resin
- Same rough bond strength of composite to enamel
hardness of gypsum
low
refers to surface ability to avoid abrasion
typical expansion value of dental plaster
0.2-0.3%
typical expansion value of dental stone
0.08-0.1%
typical expansion value of densite
0.05-0.07%
what is the order of different gypsum forms in strength
densite strongest (35MPa)
stone (30MPa)
plaster (12MPa)
spatulation effect on gypsum qualities
increased spatulation breaks down growing crystals
- fragments act as nuclei of crystallisation
- —-more centres for crystallisation to take place
more growing crystals -come into contact sooner
hence decreased setting time and increased expansion
increased spatulation of gypsum leads to
decreased setting time and increased expansion
increasing powder in powder:water ratio of gypsum
more nuclei of crystallisation per unit volume
crystals come into contact sooner
faster set & greater expansion
decreasing powder in powder:water ratio of gypsum
fewer nuclei of crystallisation per unit volume
crystals take longer to come into contact
slower set and less expansion
why do you want the gypsum model to be slightly large
small expansion %
Allows for model to be a little big
- crowns, bridges and dentures won’t be too tight a fit when placed in mouth
temperature effect on gypsum
confusing
rate of diffusion of ions increased with increased temp.
solubility of hemihydrate decreased with increased temp.
2 chemical additives in gypsum
potassium sulphate (K2SO4)
borax
role of potassium sulphate in gypsum
K2SO4
produces syngenite (K2(CaSO4)2 .H2O )
crystallises form rapidly - encourages growth of more crystals
decreases setting time
role of borax in gypsum
forms calcium borate - deposits on dihydrate crystals
delays setting process (increases setting time)
why would you add chemical additives to gypsum
to be able to control how fast gypsum sets
what must be the relationship between dental stone and impression material
needs to be chemically compatible
must “wet” the impression material
i.e. no resistance to flow over surface; avoid bubble/void formation
surface roughness of gypsum
relatively porous
relatively rough surface – about 28 to 40um
- good enough for most applications
- grainy appearance
3 advantages of gypsum
dimensionally accurate and stable
low expansion (<0.1%) of stone/densite
good colour contrast
5 disadvantages of gypsum
low tensile strength
poor abrasion resistance
very brittle
surface detail less than elastomer impression (~20um groove)
poor “wetting” of some impression materials
- ensure materials used in combination with one another are compatible
