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BDS2 - dental material science > Gypsum > Flashcards

Gypsum Flashcards

1
Q

what is a study model / cast

A

A positive replica of dentition produced from the impression (which is a negative representation of a patient’s dentition)

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2
Q

what are the purposes of a study model / cast

A

○ Records the position, shape of teeth
○ Aids visualisation / assessment of dentition
○ Enables manufacture of dental prostheses § Eg partial dentures, crowns etc

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3
Q

name the uses of gypsum

A

• Cast
○ Plaster / stone

• Die
○ Stone / improved stone

• Mould material
○ Stone

• Investment binder
Stone

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4
Q

what does the manufacturing determine with gypsum

A

what type of gypsum is produced

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5
Q

what is the reaction that occurs with gypsum manufacture

A

CaSO4.2H2O. —[heat]—> (CaSO4)2.H2O + H2O

Calcium sulphate Dihydrate –[heat]—> Calcium sulphate hemihydrate

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6
Q

what are the types of gypsum

A

• Plaster
○ Beta-hemihydrate

• Dental stone
○ Alpha-hemihydrate

• Densite
Improved stone

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7
Q

what does crystalline structure determine

A

properties

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8
Q

how is plaster (beta-hemihydrate) made and what is it like

A
  • Heated in open vessel

* Large porous, irregular crystals

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9
Q

how is dental stone (alpha hemihydrate) made and what is it like

A
  • Heated in an autoclave

* Non-porous, regular crystals, requires less water

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10
Q

how is densite (improved stone) made and what is it like

A
  • Heated in presence of Ca and Mg chloride

* Compact smoother particles - Smoother then it can be packed in more readily and made more dense

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11
Q

what is the setting reaction

A

reverse of manufacture

(caSO4)2.H2O + 3H2O —> 2CaSO4.2H2O

hemihydrate —> dihydrate

powder + water = gypsum

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12
Q

what are the mixing ratios

A

Water Powder
Plaster 50-60ml 100g
Stone 20-35ml 100g
Theoretic18.6ml 100g

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13
Q

why does the theoretical mixing ratios differ from the actual mixing ratios

A

Practise and experience tells you that these ratios are what are needed to mix the material with water rather than the theoretic ratio
Excess water
- Needed for a workable mass
- Affects properties

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14
Q

what is the setting process for gypsum

A

1)
- hemihydrate dissolves
- dihydrate forms
- dihydrate solubility low - supersaturated solution
- impurities present

2)

  • dihydrate crystals precipitate on impurities as crystals
  • more hemihydrate dissolved
  • continues until all hemihydrate dissolved (until you are just left with dihydrate crystals)

3)
initial set
- dihydrate crystals come into contact (ie push apart)
- expansion starts
- properties of weak solid and will not flow
- can be carved / manipulated / shaped

4)
final set
- strong and hard enough to be worked (large dihydrate crystals of different sizes all packed together)
- strength continues to develop

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15
Q

what are initial and final setting times measured using

A

gilmore needles

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16
Q

where is excess water found during setting

A

trapped in the powder mass

17
Q

what happens to the excess water on completion of setting

A

excess water evaporates
voids are produced (porosity)
this brings a problem as any porous material is relatively weak

18
Q

what is the compressive strength of gypsum

A

Compressive (~20-35MPa)

This is around a tenth of the compressive strength of composite resin

19
Q

what is the hardness like of gypsum

A

• Hardness - low
○ Remember hardness is not strength as it refers to just the surface
○ With gypsum you don’t want the surface to scrape parts off or be worn off as it is passed between clinic and the lab etc
Want the surface to remain in tact but as it has low hardness then you need to handle it with care to protect it from chipping

20
Q

what are the typical expansion values of gypsum

A

Plaster = 0.2-0.3%
Stone = 0.08-0.1%
Densite (improved stone) = 0.05-0.07%

expansion should ideally be 0%

21
Q

explain the strengths of gypsum

A
  • Develops over 24 hour period
  • Stone stronger as it requires less water for workable mix (less porous)
  • Improved stone (deniste) strongest (~35MPa) but Plaster is weakest (~12MPa)
22
Q

what happens to the setting time and expansion of gypsum if you increase powder, spatulation and impurities

A

setting time decrease

expansion increases

23
Q

what effect down chemicals have on the setting time and expansion

A

can either increase or decrease the setting time but will always decreases the expansion

24
Q

how does spatulation affect the gypsum setting

A

• Increase spatulation breaks down growing crystals
• Fragments act as nuclei of crystallisation
○ Centres around which the crystallisation takes place
○ More crystals come into contact more regularly
• More growing crystals - come into contact sooner

• Hence
○ Decreased setting time
○ Increase expansion

Spatulation = how you mix

25
Q

how does the powder / water ratio affect gypsum setting

A

• Increasing powder
○ More nuclei of crystallisation per unit volume
○ Crystals come into contact sooner
○ Faster set and greater expansion

• Decreasing powder
○ Converse

26
Q

why should expansion on setting be a small percentage

A

• Allows for model to be a little big

So crowns, bridge and dentures wont be too tight a fight when placed in the mouth

27
Q

how does temperature affect setting times for gypsum

A
  • Conflicting mechanisms
  • Rate of diffusion of ions increases with increase in temperature
  • Solubility of hemihydrate decreases with an increased temperature
  • Large and unpredictable behaviour

Be careful with temperature

28
Q

how does potassium sulphate K2SO4 affect setting time

A

Produces syngenite (K2 (CaSO4)2 . H2O)
Crystallises rapidly - encourages growth of more crystals
Decreases setting time

29
Q

how does borax affect setting time

A

Forms calcium borate - deposits on dihydrate crystals
Delays setting process (increases setting time)

Very rarely have pure
materials - collaboration of different materials

30
Q

is gypsum compatible with impression material

A

Dental stone mode surface detail depends on type of impression material
○ Needs to be chemically compatible
○ Must “wet” the impression material § Ie no resistance to flow over surface, avoid bubble formation § Don’t want deformation

31
Q

what is the problem with gypsum with regards to surface detail

A

• Ideally gypsum reproduces the fine detail on the impression material
• But gypsum is inherently porous, resulting in a relatively rough surface - about 28 to 40um (good enough for most applications)
○ Grainy sort of appearance
• cf impression material standard
○ Material must reproduce a 50um wide groove
Slightly better than that standard / good enough

32
Q

what are the advantages of gypsum

A

○ Dimensionally accurate and stable § Worst will be 0.3% bigger than it should be
○ Low expansion (<0,1%) of stone / densite
○ Good colour contrast

33
Q

what are the 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
§ Materials need to be compatible, Don’t assume - ask / find out

BDS2 - dental material science (13 decks)

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