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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
compressive strength of gypsum
~20-35MPa - 1/10 of composite resin - Same rough bond strength of composite to enamel
26
hardness of gypsum
low refers to surface ability to avoid abrasion
27
typical expansion value of dental plaster
0.2-0.3%
28
typical expansion value of dental stone
0.08-0.1%
29
typical expansion value of densite
0.05-0.07%
30
what is the order of different gypsum forms in strength
densite strongest (35MPa) stone (30MPa) plaster (12MPa)
31
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
32
increased spatulation of gypsum leads to
decreased setting time and increased expansion
33
increasing powder in powder:water ratio of gypsum
more nuclei of crystallisation per unit volume crystals come into contact sooner faster set & greater expansion
34
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
35
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
36
temperature effect on gypsum
confusing rate of diffusion of ions increased with increased temp. solubility of hemihydrate decreased with increased temp.
37
2 chemical additives in gypsum
potassium sulphate (K2SO4) borax
38
role of potassium sulphate in gypsum
K2SO4 produces syngenite (K2(CaSO4)2 .H2O ) crystallises form rapidly - encourages growth of more crystals decreases setting time
39
role of borax in gypsum
forms calcium borate - deposits on dihydrate crystals delays setting process (increases setting time)
40
why would you add chemical additives to gypsum
to be able to control how fast gypsum sets
41
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
42
surface roughness of gypsum
relatively porous relatively rough surface – about 28 to 40um - good enough for most applications - grainy appearance
43
3 advantages of gypsum
dimensionally accurate and stable low expansion (<0.1%) of stone/densite good colour contrast
44
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

BDS2 Dental Materials Sciences (13 decks)

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