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BDS2 CLINICAL Dental Materials > Gypsum > Flashcards

Gypsum Flashcards

1
Q

What is a study cast

A

a positive replica of the dentition

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2
Q

What are study casts produced from

A

impressions

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3
Q

What is the purpose of a study cast

A

records the position, shape of teeth
aids visualisation/assessment of dentition
enables manufacture of dental prostheses (e.g partial dentures, crowns etc)

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4
Q

What is the use of gypsum

A

cast (plaster/stone)
die (stone/improved stone)
mould material (stone)
investment binder (stone)

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5
Q

How is gypsum made

A

calcium sulphate dihydrate is heated until calcium sulphate hemihydrate is produced and it is surrounded by water

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6
Q

What determines the type of gypsum

A

manufacturing conditions

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7
Q

What are the types of gypsum

A

plaster
dental stone
densite

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8
Q

What determines the properties of the different types of gypsum

A

crystalline structure

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9
Q

What is beta-hemihydrate

A

plaster

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10
Q

What is alpha-hemihydrate

A

dental stone

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11
Q

What is improved stone

A

densite

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12
Q

What is plaster heated in

A

an open vessel

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13
Q

What is the crystallite structure of plaster

A

large, porous, irregular crystals

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14
Q

What is the crystallite structure of dental stone

A

non-porous, regular crystals, requires less water

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15
Q

How is dental stone heated

A

in an autoclave

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16
Q

How is densite heated

A

in presence of calcium and magnesium chloride

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17
Q

What is the structure of densite

A

compact smoother particles

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18
Q

What is the setting reaction

A

reverse of the manufacturer
hemihydrate + 3H2O –> dihydrate.2H2O
powder + water –> gypsum (study cast)

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19
Q

Why is excess water used in the mixing ratios

A

it is needed for a workable mass

affects properties

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20
Q

What is the mixing ratio for plaster

A

50-60ml

100g

21
Q

What is the mixing ratio for stone

A

20-35ml

100g

22
Q

What is the theoretical ratio

A

18.6ml

100g

23
Q

Describe the setting process

A

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

24
Q

Describe the initial set of gypsum in the setting process

A

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

25
Q

Describe the final set of gypsum

A

strong and hard enough to be worked
strength continues to develop
initial and final setting times measured using gilmore needles

26
Q

What happens to water during setting

A

excess water is trapped in the powder mass

27
Q

What happens to water on completion of setting

A

excess evaporates

voids produced - not good

28
Q

What is the compressive strength of gypsum

A

20-35 MPa

29
Q

What gypsum has the strongest compressive strength

A

densite with around 38 MPa

30
Q

Why is stone stronger than plaster

A

stone stronger as it requires less water for workable mix making it less porous

31
Q

Which has the lowest expansion

A

densite

32
Q

Which has the greatest expansion

A

plaster

33
Q

How does increased powder effect setting time and expansion

A

decreased setting time

increased expansion

34
Q

How does increased spatulation effect setting time and expansion

A

decreased setting time and increased expansion

35
Q

How does increased impurities effect the setting time and expansion

A

decreased setting time

increased expansion

36
Q

How does an increase in temperature effect setting time

A

not very predictable

different temperatures have different effects

37
Q

What do chemicals such as borax and NCl effect

A

increase/decrease setting time

decrease expansion

38
Q

What does increased spatulation do

A

increased spatulation breaks down growing crystals
fragments act as nuclei of crystallization
more growing crystals - come into contact sooner

39
Q

What does increasing powder do

A

more nuclei of crystallization per unit volume
crystals come into contact sooner
faster set and greater expansion

40
Q

What is the effect of expansion

A

can result in a bigger model

can allow for slightly bigger so crowns bridges etc won’t be too tight

41
Q

What is the effect of an increase temperature on the rate of diffusion of ions

A

increase in rate of diffusion

42
Q

What is the effect of an increase in temperature on solubility of hemihydrate

A

decrease in solubility

43
Q

What are the chemical additives

A

potassium sulphate

borax

44
Q

What does potassium sulphate do

A 
produces syngenite (K2(CaSO4)2.H2)
crystallites rapidly and encourages growth of more crystals so decreases setting time
45
Q

What does borax do

A

forms calcium borate which deposits on dihydrate crystals and this delays the setting process which increases the setting time

46
Q

Why is compatibility of impression material important

A

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

47
Q

What should gypsum ideally reproduce

A

fine detail on the impression material

but gypsum is inherently porous resulting in a relatively rough surface

48
Q

What are the advantages of gypsum

A

dimensionally accurate and stable
low expansion of stone/densite
good color contrast

49
Q

What are the disadvantages of gypsum

A

low tensile strength
poor abrasion resistance
very brittle
surface detail less than elastomer impression
poor ‘wetting’ of some impression materials

BDS2 CLINICAL Dental Materials (15 decks)

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