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BDS2: DM > Gypsum Products > Flashcards

Gypsum Products Flashcards

1
Q

What is Gypsum?

A

Calcium Sulphate Dihydrate

(CaSO4)2H2O

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2
Q

What can Gypsum products be used for? 5

A
  • Impression Materials
  • Models (Positive impression)
  • Dies (Positive replicas of individual teeth)
  • Moulds (Dentures)
  • Refractory Investments
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3
Q

What do manufacturers convert Gypsum into?

A

Hemihydrate version (CaSO4)H2O

  • When mixed with water on setting converts back to Dihydrate
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4
Q

Depending on the type of conversion treatment what can the hemihydrate lead to?

A
  • Plaster of Paris
  • Stone
  • Improved Stone
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5
Q

What does treating Dihydrate with 120˚C heat from open kettles form?

A

Plaster of Paris

- Beta Hemihydrate

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6
Q

What does treating Dihydrate with 120-130˚C heat with steam and pressure form?

A

Autoclaved Artificial Stone / Hydrocal Calcined

- Alpha Hemihydrate

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7
Q

What does treating Dihydrate with a 30% solution of CaCl2 or MgCl2 form?

A 
Improved Stone (Densite)
- Alpha Hemihydrate
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8
Q

Comment on the strength of Beta Hemihydrate when mixed with water?

A
  • Forms a weak product

- Due to large irregular and porous particles which do not pack closely, leaving large gaps.

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9
Q

Comment on the strength of Alpha Hemihydrate when mixed with water?

A
  • Forms a stronger product than beta

- Due to its small regular and non porous particles, which pack closely together

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10
Q

Where is Plaster used?

A
  • Mounting models into articulators
  • Flasking procedures for dentures
  • Base for models

Used where strength is not a critical requirement

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11
Q

Advantages of Plaster 3

A
  • Soft
  • Cheap
  • Easy to use and shape
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12
Q

Where is Stone used?

A
  • Replica of individual teeth
  • Construction of crowns, bridges and dentures
  • Study casts
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13
Q

When maximum strength is essential for dentate casts what is used?

A

Dental Stone by itself

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14
Q

Composition of Gypsum

A
  • Hemihydrate a or b ( 75-85%)
  • Unchanged Gypsum (5-8%)
  • Insoluble Anhydrites (5-8%)
  • Impurities (≈4%)
  • Accelerators/ retarders (≈4%)
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15
Q

What are accelerators/retarders?

A

Chemicals that increase/decrease the solubility of gypsum in water, thus affecting it’s setting time.

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16
Q

Name an Accelerator for Gypsum

What does this produce?

A

K2SO4

Syngenite [K2(CaSO4)2.H2O]

17
Q

How does Syngenite work?

A
  • Crystallises very rapidly
  • Reduces overall expansion
  • Accelerates setting reaction
18
Q

3 Examples of Accelerators for Gypsum

A
  • K2SO4
  • CaSO4.2H2O
  • NaCl (<20%)
19
Q

3 Examples for retarders for Gypsum

A
  • NaCL(>20%)
  • Borax
  • Potassium Citrate
20
Q

Which is more soluble: hemihydrate or Dihydrate?

A

Hemihydrate

21
Q

What happens during the setting reaction for Hemihydrate?

A
  • Hemihydrate slowly forms Dihydrate
  • Some Hemihydrate dissolves in water and forms dihydrate
  • The dihydrates lower solubility results in a supersaturated, UNSTABLE solution
  • Thus forming STABLE crystals
  • As crystals form MORE hemihydrate dissolves until set
22
Q

What does unchanged Gypsum do in the mechanism?

A
  • Acts as a crystallisation nuclei for growth of Dihydrate Crystals
23
Q

How much water is needed to mix 100g Gypsum products?

A

50ml for PLASTER
20ml for STONE
20ml for IMPROVED STONE

24
Q

What will happen if you add MORE WATER to the mixing process of gypsum products?

A
  • Slower Setting time
  • Runny mix
  • Weaker model
25
Q

What will happen if you add MORE POWDER to the mixing process of gypsum products?

A
  • Difficult to mix

- Porosity within set material

26
Q

What will have if you increase SPATULATION TIME to the mixing process of gypsum products?

A
  • Reduced setting time

- Increased setting expansion

27
Q

What is the optimum temperature for the mixing process of gypsum products?

A

0-50˚C with little change

28
Q

What will have if you increase TEMPERATURE in the mixing process of gypsum products?

A

> 50˚C Gradual Retardation takes place

≥100˚C No reaction

29
Q

What can you measure setting time of Gypsum Products with?

A

Vicat Needles

Gillmore Needles (1/4lb and 1lb in weight)

30
Q

Why is setting expansion important in Gypsum Products?

A
  • Models are slightly bigger than the oral anatomy so crowns and dentures aren’t to tight
31
Q

Comment on Dry strength compared to Wet strength of Gypsum Products

A

Dry strength is roughly twice as strong as wet strength

32
Q

Tensile Strength of Plaster

A

Very Low 2MPa

33
Q

Tensile Strength of Stone

A

Double Strength of Plaster

34
Q

Place in order of surface hardness
Plaster
Improved Stone
Stone

A

Plaster - Weakest
Stone
Improved Stone

35
Q

Setting time for Plaster as an impression material

A

3-5 Mins

36
Q

Setting time for Stone

A

7-15 Mins

37
Q

Setting time for Improved Stone

A

5-12 Mins

38
Q

Setting time for Plaster as a laboratory material

A

5-10 Mins

BDS2: DM (6 decks)

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