Gypsum Flashcards
What is a study model
a POSITIVE REPLICA of dentition, produced from the impression (negative representation) of a patient’s dentition
what are the purposes of study models
- records the position, shape of teeth
- aids visualisation/ assessment of dentition
- enables manufacture of dental prostheses
what material is often used to create study models
gypsum
what are the uses of gypsum
- cast (plaster/stone)
- die (stone/ improved stone)
- mould material (stone)
- investment binder (stone)
what determines the type of gypsum you end up with
manufacturing conditions
What is the manufacturing process of gypsum
Calcium sulphate dihydrate, heats it, produces calcium sulphate hemihydrate and water
What are the different types of gypsum
- plaster (beta-hemihydrate)
- dental stone (alpha-hemihydrate)
- densite (improved stone)
what determines the properties of the gypsum
crystalline structure
How do you manufacture gypsum to get plaster (beta-haemihydrate)
- heated in open vessel
How do you manufacture gypsum to get dental stone (alpha hemihydrate)
- heat in autoclave
How do you manufacture gypsum to get densite (improved stone)
- heat in the presence of Ca and Mg chloride
What is the crystalline structure like
large porous, irregular crystals
What is the crystalline structure of dental stone like
non-porous, regular crystals, requires less water
What is the crystalline structure of densite like
compact smoother particles
what is the basic reaction gypsum undergoes during the manufacturing process
dihydrate –> hemihydrate
what is the basic setting reaction gypsum undergoes
hemihydrate –> dihydrate
Powder + water = gypsum (study cast)
(reverse of manufacture)
What is the mixing ratio for plaster
water = 50-60ml powder = 100g
What is the mixing ratio for stone
water = 20-35ml powder = 100g
what is the theoretical ratio for the mixing ratio
water = 18.6ml powder = 100g
are the mixing ratios of powder and water the same for all types of gypsum
no
what is excess water needed for
a workable mass, affects properties
what happens during the 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
Initial set
- dihydrate crystals come into contact i.e. push apart
- expansion starts
- properties of weak solid and will not flow
- can be carved
Final set
- strong and hard enough to be worked
- strength continues to develop
During setting:
excess water is trapped in the powder mass
On completion of setting:
- excess water evaporates
- voids produced (porosity)
How are initial and final setting times measured
using gilmore needles
How is the strength of gypsum
- compressive (~20-35MPa)
- low hardness
How is the setting tie of gypsum
convenience
What are the typical expansion values for plaster, stone and densite?
plaster = 0.2-0.3% stone = 0.08%-0.1% densite = 0.05-0.07%
What is the strongest out of plaster, stone and improved stone
improved stone (densite)
when does strength develop
over 24 hr period
why is stone stronger
it requires less water for workable mix (less porous)
What does increase powder do to the setting time and expansion
decreases setting time
increases expansion
What does increase spatulation do to the setting time and expansion
decreases setting time
increases expansion
What does increase impurities do to the setting time and expansion
decreases setting time
increases expansion
What does increase temp do to the setting time and expansion
depends on the brand of gypsum for setting time,
no effect on expansion
What does increase chemicals do to the setting time and expansion
chemicals can increase or decrease the setting time, decreases expansion
What does spatulation do
- breaks down growing crystals
- fragments act as nuclei of crystallisation
- more growing crystals - come into contact sooner
HENCE:
- decreases setting time
- increases expansion
What does increasing the powder in the powder/water ratio do
- more nuclei of crystallisation per unit volume
- crystals come into contact sooner
- faster set and greater expansion
why is it important we don’t get too much expansion on setting
- so crowns, bridges and dentures won’t be too tight a fit when placed in mouth
What are the factors which influence the effect of temperature on setting
- rate of diffusion of ions increase with increasing temp
- solubility of hemihydrate decreases with increasing temp
CONFLICTING mechanisms
What chemical additives are there
- potassium sulphate
- borax
what is the effect of potassium sulphate
- produces syngenite
- crystallises rapidly (encourages growth of more crystals)
- decreases setting time
what is the effect of borax
- forms calcium borate (deposits on dihydrate crystals)
- delays setting process (increases setting time)
Whats important to know about compatibility with impression material
- dental stone model surface detail depends on type of impression material
- needs to be chemically compatible
- must “wet” the impression material i.e. no resistance to flow over surface; avoid bubble formation
What is gypsums role in the reproduction of surface detail
- ideally gypsum reproduces the fine detail on the impression material
- but, gypsum is inherently porous, resulting in a relatively rough surface (about 28-40µm)
what is the impression material standard it has to meet
material must reproduce a 50µm wide groove
Advantages of gypsum
- dimensionally accurate and stable
- low expansion (<0.1%) of stone/ densite)
- good colour contrast
Disadvantages of gypsum
- low tensile strength
- poor abrasion resistance
- very brittle
- surface detail less than elastomer
- poor “wetting” of some impression materials
