Gypsum Flashcards
what is gypsum used for?
to provide a positive replica of a patients dentition from the impression
why are positive replicas of dentition required?
- records the position, shape & dimensions of teeth
- aids visualisation/assessment of dentition
- enables manufacture of dental prostheses
what is a cast?
a replica of the patients full dentition (made from plaster/stone)
what is a die?
a replica of an individual tooth (made from stone/improved stone)
what are the different types of gypsum?
- plaster (ß-hemihydrate)
- dental stone (α-hemihydrate)
- densite (improved stone)
what do the different gypsum types depend on?
dependant on the heating process
what does the crystalline structure of gypsum determine?
the properties of the material
what type of gypsum is heated in an open vessel?
plaster (beta-hemihydrate)
what is the structure of plaster?
- large porous
- irregular crystals
what type of gypsum is heated in an autoclave?
dental stone (alpha-hemihydrate)
what is the structure of dental stone?
- non-porous
- regular crystals
- requires less water
what is an autoclave?
closed vessel
how is densite (improved stone) heated?
in the presence of calcium chloride & magnesium chloride
what is the structure of densite?
- compact smoother particles
what is the simplified setting reaction of gypsum?
powder + water = gypsum (study cast)
what is the mixing ratio for plaster?
50-60ml water : 100g powder
what is the mixing ratio for stone?
20-35ml water : 100g powder
what occurs during the overall setting process of gypsum?
- hemihydrate dissolves in water
- dihydrate forms & are pulled towards impurities
- the dihydrate precipitate on impurities as crystals
- more hemihydrate is dissolved
- continues until all hemihydrate is dissolved
what do the impurities act as in the setting process of gypsum?
they act as nucleating agents (centres of crystal growth)
explain the initial set of the gypsum:
- dihydrate crystals continue to grow and come into contact with each other
- expansion of material starts
what are the properties of gypsum like during the initial set?
- weak solid
- will not flow
- can be carved
explain the properties of final set gypsum:
- strong and hard enough to be worked
- strength continues to develop
what are initial & final setting times of gypsum measured with?
Gilmore needles
water is present during the setting stage of gypsum, how does this differ on completion of setting?
- excess water evaporates
- voids produced (porosity)
does gypsum reproduce fine detail on the impression material?
- gypsum is porous, resulting in a rough surface (28-49um)
- however this is good enough
what is the compressive strength of set gypsum after 1 hour?
28 MPa - 38 MPa
what is the compressive strength of gypsum after 24 hours?
approx. 75 MPa
what is the strongest type of gypsum?
improved stone (densite)
what is the weakest type of stone?
plaster
how can the surface hardness of gypsum be described?
- surface hardness is low (easy to abrade)
what does an increase in powder (during the mix) result in?
- reduced setting time
- increased expansion
what methods can be used to decrease setting time of gypsum?
- increase powder
- increase spatulation
- increase impurities
what does spatulation do to gypsum?
breaks down growing crystals
- decreased setting time
- increased expansion
how does increasing the temperature affect the solubility of hemihydrate?
lower solubility
what are the purpose of chemical additives in gypsum?
used as antiexpansion agents
what chemical additives are used in gypsum?
- potassium sulphate
- borax
what is the function of potassium sulphate in gypsum?
- produces syngenite
- crystallises rapidly & encourages growth of more crystals
- DECREASES SETTING TIME
what is the function of borax in gypsum?
- forms calcium borate
- deposits on dihydrate crystals (restricting their growth)
- delays setting process (INCREASES SETTING TIME)
why must you ‘wet’ the impression surface before casting?
- so there is no resistance to flow over surface
- avoids bubble formation
what is a weakness of gypsum?
- gypsum is brittle
- has a flexural strength of 15 to 20 MPa
what is meant by a brittle material?
a brittle material will readily fracture when subjected to a small amount of strain
compare the flexural strength of gypsum to that of composite resin? what does this indicate?
- gypsum = 15 to 20MPa
- composite resin = 150MPa
therefore gypsum is much more brittle than composite resin
what are the advantages of gypsum?
- dimensionally accurate and stable
- low expansion (<0.1%) of stone/densite
- good colour contrast
what are the disadvantages of gypsum?
- VERY BRITTLE
- low tensile strength
- poor abrasion resistance
- surface detail less than elastomer impression
- poor ‘wetting’ of some impression materials
