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BDS2 - DMS > Gypsum > Flashcards

Gypsum Flashcards

1
Q

What is a study model/case a positive replica of?

A

A patient dentition

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2
Q

What is a study cast produced from?

A
  • From the IMPRESSION (negative representation of the patients dentition)
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3
Q

What are the purposes of a study model? (3 points)

A
  • Records the position and shape of teeth
  • Aids visualisation/assessment of dentition
  • Enables manufacture of dental prostheses e.g. partial dentures
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4
Q

How is gypsum manufactured?

A

Calcium sulphate dihydrate is converted into calcium sulphate hemihydrate by using a heating proces and water is released

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5
Q

What is the chemical name of the gypsum used by technicians?

A

Calcium sulphate hemihydrate

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6
Q

What are the 3 different types of gypsum?

A
  • Plaster (beta-hemihydrate)
  • Dental stone (alpha-hemihydrate)
  • Densite (improved stone)

(3 different types depending on how the heating process is carried out)

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7
Q

What is the heat process used to produce plaster?

A
  • Heated in an open vessel
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8
Q

What is the structure of plaster?

A
  • Large porous

- Irregular crystals

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9
Q

What is the heating process used to produce dental stone?

A

Heated in an autoclave

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10
Q

What is the structure of dental stone?

A
  • non-porous, regular crystals, requires less water
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11
Q

What is the heating process sued to produce densite?

A

Heated in presence of Ca & Mg chloride

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12
Q

What is the structure of densite?

A
  • Compact, smoother particles
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13
Q

What does the different crystalline properties of the different forms of gypsum determine?

A

Determines the properties of the material

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14
Q

What is the setting reaction of gypsum?

A
  • Reverse of manufacture

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

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15
Q

How much water should be used when mixing with 100g of plaster?

A

50-60ml

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16
Q

How much water should be used when mixing with 100g of stone?

A

20-35ml

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17
Q

What is the first stage in the setting process of gypsum? (4 points)

A
  • Hemihydrate dissolves
  • Dihydrate forms
  • Dihydrate solubility low - supersaturated solution
  • Impurities present
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18
Q

What is involved in the second stage of the setting process of gypsum? (3 points)

A
  • Dihydrate crystals precipitate on impurities as crystals
  • More hemihydrate dissolves
  • Continues until all hemihydrate is dissolved
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19
Q

What is involved in the third stage of the setting reaction of gypsum and what is this stage called? (5 points)

A
  • Initial set
  • Dihydrate crystals come into contact i.e. push apart
  • Expansion starts
  • Properties of weak solid and will not flow
  • Can be carved
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20
Q

What is involved in the 4th stage of the setting reaction of gypsum and what is this stage called? (3 points)

A
  • Final set
  • Strong and hard enough to be worked
  • Strength continues to develop
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21
Q

What are initial and final setting times of gypsum measured using?

A
  • Measured using Gilmore needles
22
Q

Suring setting of gypsum where is excess water trapped?

A

Trapped in the powder mass

23
Q

On completion of setting of gypsum excess water evaporates. What does this cause?

A
  • Voids are produced (porosity)
24
Q

What is the compressive strength of gypsum?

A

(~20-35MPa) - 1/10 of composite

25
Q

What is the hardness gypsum like?

A

Low

  • Refers to the surface of gypsum
26
Q

Why do you want a good setting time of gypsum?

A

For convenience

27
Q

What would be the ideal expansion for gypsum?

A

0

28
Q

What is the expansion for plaster?

A

0.2-0.3%

29
Q

What is the expansion for stone?

A

0.08-0.1%

30
Q

What is the expansion for densite?

A

0.05-0.07%

31
Q

Which type of gypsum has the best expansion properties?

A
  • Densite
32
Q

How long does it take for the strength of gypsum to develop?

A
  • Develops over a 24hr period
33
Q

Which type of gypsum is the strongest?

A

Improved stone (densite)

34
Q

Why is stone stronger than plaster?

A

It requires less water for a workable mix so is less porous

35
Q

What does increasing the amount of powder in the mix do to the setting time and expansion of the material?

A
  • Decreases setting time

- Increases expansion

36
Q

What does increasing the spatulation of the mix do to the setting time and expansion of the material?

A
  • Decreases setting time

- Increases expansion

37
Q

What does increasing the amount of impurities in the mix do to the setting time and expansion of the material?

A
  • Decreases setting time

- Increases expansion

38
Q

What does increasing temperature of the mix do to the setting time of the material?

A

Can increase or decrease

39
Q

What does increasing the amount of chemical (borax, NaCl) in the mix do to the setting time and expansion of the material?

A
  • Can increase or decrease setting time

- Decreases the expansion

40
Q

What does increased spatulation do to the crystals in gypsum?

A
  • Breaks down the growing crystals
41
Q

What do the fragments of crystals caused by increased spatulation of the material act as?

A
  • Nuclei of crystallisation

more growing crystals = come into contact sooner

42
Q

Increase spatulation of the material causes more growing crystals which will come into contact sooner. How does this affect setting time and expansion?

A
  • Setting time decreased

- Expansion increased

43
Q

What happens to the mixture when you increase the amount of powder in it and what effect does this have on setting time and expansion? (3 points)

A
  • More nuclei of crystallisation per unit volume
  • Crystals come into contact sooner
  • Faster set & greater expansion
  • It is the converse when decreasing the powder
44
Q

Does the rate of diffusion of ions increase or decrease when increasing the temperature of the material?

A

Increases

45
Q

Does the solubility of hemihydrate increase or decrease when increasing the temperature of the material?

A

Decreases

46
Q

What does the addition of potassium sulphate do to the material?

A
  • Produces syngenite
  • Crystallises rapidly - encourages growth of more crystals
  • DECREASES the setting time
47
Q

What does the addition of borax do to the material?

A
  • Forms calcium borate - deposits on dihydrate crystals

- Delays setting process (increases setting time)

48
Q

Dental stone model surface detail depends on the type of impression material used. What are the requirements of this? (2 points)

A
  • Needs to be chemically compatible

- Must ‘wet’ the impression material (i.e. no resistance to flow over surface; avoid bubbl e formation)

49
Q

Ideally gypsum should reproduce the fine details on the impression material BUT gypsum is inherently porous, resulting in a relatively rough surface. How rough is the surface and what is the standard requirement for this?

A
  • About 28-40um

- Standard is 50um wide so gypsum is slightly better than the standard

50
Q

What are the 3 main advantages of gypsum?

A
  • Dimensionally accurate and stable
  • Low expansion (<0.1%) of stone/densite
  • Good colour contrast
51
Q

What are the 5 main 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 - DMS (14 decks)

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