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BDS2 Dental Materials Sciences > Gypsum > Flashcards

Gypsum Flashcards

1
Q

3 names for gypsum

A

dental plaster

dental stone

densite

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2
Q

what is a study model/cast

A

a positive replica of dentition

produced from the impression (i.e. negative representation) of pt dentition

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3
Q

3 purposes of the study model/cast

A
  • records the position, shape of teeth
  • aids visualisation/assessment of dentition
  • enable manufacture of dental prostheses (e.g. partial dentures, crown etc)
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4
Q

3 ways to determine that gypsum is fit for purpose

A

accuracy

ease of use

limitations

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5
Q

4 uses of gypsum

A

cast (plaster/stone)

die (stone / improved stone)

mould material (stone)

investment binder (stone)

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6
Q

manufacture of gypsum

A

CaSO4.2H2O -> (CaSO4)2 . H2O + H2O

Calcium sulphate dihydrate [heat] -> calcium sulphate hemihydrate

Heat manufacturing (3 ways) determines type

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7
Q

what determines the type of gypsum made

A

heat manufacturing

crystalline structure determines properties

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8
Q

plaster

A

beta-hemihydrate

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9
Q

dental stone

A

alpha-hemihydrate

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10
Q

densite

A

improved stone

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11
Q

plaster (beta hemihydrate) heating process

A

heated in open vessel

large porous, irregular crystals

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12
Q

dental stone (alpha-hemihydrate) heating process

A

heated in an autoclave

non-porous, regular crystals, requires less water

micro crystals

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13
Q

densite (improved stone) heating process

A

heated in presence of Ca & Mg chloride

compact smoother particles
- can pack more readily, more dense

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14
Q

what determines the properties of gypsum

A

crystalline structure

differences occur due to heating process used

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15
Q

setting reaction of gypsum

A

reverse of manufacture

(CaSO4)2 . H2O + 3H2O -> (CaSO4)2 . 2H2O

calcium sulphate hemihydrate + water -> Calcium sulphate dihydrate (study cast)

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16
Q

crystalline structure of plaster (beta-hemihydrate)

A

large, porous irregular crystals

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17
Q

crystalline structure of dental stone (alpha-hemihydrate)

A

non-porous, regular crystal, requires less water

micro crystals

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18
Q

crystalline structure of densite (improved stone)

A

compact smoother particles

- can pack more readily, more dense

19
Q

why is excess water needed for the ratio of water:powder compared to theoretical ratio?

A

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

20
Q

setting process

A

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

21
Q

initial set of gypsum

A
  • 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
22
Q

final set of gypsum

A
  • strong and hard enough to be worked
  • strength continues to develop
  • —–large irregular sized dihydrate crystals
23
Q

reaction with water during gypsum setting

A

excess water is trapped in the powder mass

24
Q

reaction with water on completion of setting

A

excess water evaporated

voids produced (porosity)
- issue as relatively weak
25
Q

compressive strength of gypsum

A

~20-35MPa

  • 1/10 of composite resin
  • Same rough bond strength of composite to enamel
26
Q

hardness of gypsum

A

low

refers to surface ability to avoid abrasion

27
Q

typical expansion value of dental plaster

A

0.2-0.3%

28
Q

typical expansion value of dental stone

A

0.08-0.1%

29
Q

typical expansion value of densite

A

0.05-0.07%

30
Q

what is the order of different gypsum forms in strength

A

densite strongest (35MPa)
stone (30MPa)
plaster (12MPa)

31
Q

spatulation effect on gypsum qualities

A

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

32
Q

increased spatulation of gypsum leads to

A

decreased setting time and increased expansion

33
Q

increasing powder in powder:water ratio of gypsum

A

more nuclei of crystallisation per unit volume

crystals come into contact sooner

faster set & greater expansion

34
Q

decreasing powder in powder:water ratio of gypsum

A

fewer nuclei of crystallisation per unit volume

crystals take longer to come into contact

slower set and less expansion

35
Q

why do you want the gypsum model to be slightly large

A

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

36
Q

temperature effect on gypsum

A

confusing

rate of diffusion of ions increased with increased temp.
solubility of hemihydrate decreased with increased temp.

37
Q

2 chemical additives in gypsum

A

potassium sulphate (K2SO4)

borax

38
Q

role of potassium sulphate in gypsum

A

K2SO4

produces syngenite (K2(CaSO4)2 .H2O )

crystallises form rapidly - encourages growth of more crystals

decreases setting time

39
Q

role of borax in gypsum

A

forms calcium borate - deposits on dihydrate crystals

delays setting process (increases setting time)

40
Q

why would you add chemical additives to gypsum

A

to be able to control how fast gypsum sets

41
Q

what must be the relationship between dental stone and impression material

A

needs to be chemically compatible

must “wet” the impression material
i.e. no resistance to flow over surface; avoid bubble/void formation

42
Q

surface roughness of gypsum

A

relatively porous

relatively rough surface – about 28 to 40um

  • good enough for most applications
  • grainy appearance
43
Q

3 advantages of gypsum

A

dimensionally accurate and stable

low expansion (<0.1%) of stone/densite

good colour contrast

44
Q

5 disadvantages of gypsum

A

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

BDS2 Dental Materials Sciences (13 decks)

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