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Prosthetics > Gypsum and Refractory Dental Materials Science > Flashcards

Gypsum and Refractory Dental Materials Science Flashcards

1
Q

What are gypsum products in dentistry derived from?

A

Calcium Sulphate Dihydrate (CaSO₄.2H₂O)

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2
Q

Describe the reversible reaction involving gypsum and its products

A

Gypsum (Calcium Sulphate Dihydrate) > Heat > Product (Calcium Sulphate Hemihydrate) + Water

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3
Q

What is the chemical reversible reaction involving gypsum and its products

A

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

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4
Q

Describe the term: Model

A

Represent teeth, alveolar ridges, sulcular anatomy in the mandible or maxillary arch

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5
Q

Describe the term: Die

A

Represents a single tooth preparation such as for a crown or part of a bridge

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6
Q

Describe the term: Investment

A

Material poured around a model or die to enclose it in a mould

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7
Q

Describe the term: Refractory

A

A material used to form a model or die, or as an investment, that will withstand high temperatures

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8
Q

Name 4 gypsum products used commercially

A
  1. Cement
  2. Plasterboard / Plaster
  3. Agriculture
  4. Foodstuffs
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9
Q

Name 5 uses of gypsum products in dentistry

A
  1. Impression materials
  2. Mounting plaster
  3. Model and die materials
  4. Investment materials
  5. Binder in refractory materials
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10
Q

How many types of gypsum products are used in dentistry?

A

5

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11
Q

What is type 1 gypsum?

A

Impression Plaster

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12
Q

What is type 2 gypsum?

A

Model Plaster (Plaster of Paris)

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13
Q

What is a use of type 2 gypsum?

A

Models

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14
Q

What is type 3 gypsum?

A

Dental Strone (Hydrocal)

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15
Q

What is a use of type 3 gypsum?

A

Models

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16
Q

What is type 4 gypsum?

A

Improved Dental Stone (Densite)

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17
Q

What are 2 uses of type 4 gypsum?

A
  1. Models

2. Dies

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18
Q

What is type 5 gypsum?

A

Dental Stone (high strength and high expansion)

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19
Q

What are 2 uses of type 5 gypsum?

A
  1. Used as binder in refractory materials

2. Investment material

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20
Q

Describe the formation of gypsum (hemihydrate)

A
  • Calcined in open oven at 120 degrees to form β-hemihydrate Plaster of Paris
  • Heated in autoclave under steam pressure at 120 degrees forming α-hemihydrate dental stone (hydrocal)
  • Boiled in 30% calcium or magnesium chloride to form high strength improved dental stone (densite)
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21
Q

What is the difference in particles of plaster, stone and improved stone?

A

Particles become increasingly fine and regular resulting in denser product with improved mechanical properties

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22
Q

Describe the setting of gypsum

A
  • Hemihydrate mixed with water forms suspension
  • Hemihydrate dissolves to form a saturated solution
  • Supersaturated with dihydrate which precipitates out from nuclei of crystallization
  • Initial set which is still carvable, followed by final set
  • Small expansion after final set
23
Q

What causes the small expansion of gypsum after the final set?

A

Outward growth of crystals and air porosity

24
Q

Describe hygroscopic expansion

A

Greater expansion of gypsum following the initial set if placed in water

25
Q

Describe the water to powder ratio of plaster, stone and theoretical

A

Plaster - 0.55
Stone - 0.3
Theoretical - 0.186

26
Q

Describe 3 forms of manufacturer control over setting time of gypsum

A
  1. Nucleating agents to promote set
  2. Accelerators
  3. Retarders
27
Q

Name 2 accelerators which may be added to gypsum to control setting time

A
  1. NaCl

2. K₂SO₄

28
Q

Name a retarder added to gypsum to control setting time

A

Borax

29
Q

Describe 3 forms of operator control over setting time of gypsum

A
  1. Temperature
  2. Water : Powder ratio
  3. Mixing time
  4. Nucleating agents
30
Q

Describe anti-expansion solution and gypsum

A
  • Mixed with water when minimal dimension change required
  • K₂SO₄ (4%) reduces expansion but accelerates the set
  • Borax (0.4%) retards the set if K₂SO₄ used as anti-expansion solution
31
Q

Name 2 major requirements of model and die materials

A
  1. Dimensional Accuracy

2. Ideal Mechanical Properties

32
Q

Describe 5 ideal mechanical properties of model and die materials

A
  1. Fluid at the time of pouring
  2. Low contact angle
  3. Strength (fracture resistant)
  4. Hardness (abrasion resistant)
  5. Compatibility with other materials used
33
Q

What is the most common model and die materials?

A

Calcium Sulphate Dihydrate (Gypsum)

34
Q

Name 4 alternative model and die materials to gypsum

A
  1. Metallic Dies - Painted on dental stone or amalgam
  2. Resins - Polymeric materials e.g. Rhino Rock
  3. Phosphate-bonded Materials
  4. Virtual Models - 3D printing using a thermoplastic polymer
35
Q

What are refractory investment materials used for?

A

Production of metal coverings

36
Q

Describe the ideal properties for refractory investment materials

A
  • Fine particle size (smooth surface on casting)
  • Suitable setting time
  • Retain integrity at casting temperature
  • Expand to compensate for alloy shrinkage
  • Permeable to allow air to escape as allow enters
  • Strong to withstand force of molten metal entering
37
Q

Describe the use of dental plaster or stone

A

Mould for acrylic dentures

38
Q

Describe the use of gypsum-bonded refractory materials

A

Mould for gold casting alloys

39
Q

Describe 3 uses of phosphate-bonded refractory materials

A
  1. Mould for base metal and precious metal casting allows
  2. Mould for cast ceramics
  3. Die for ceramic build-up
40
Q

Describe the use of silica-bonded refractory materials

A

Mould for base metal casting alloys

41
Q

Describe the preparation of mould for acrylic dentures

A
  • Wax trial dentures placed on stone model
  • Plaster of Paris mixed and covers the trial dentures
  • Flasks placed together and investment materials allowed to harden
  • Plaster of Paris separated and wax is boiled away leaving mould shape
  • Mix of acrylic placed into the mould so takes shape of original wax
42
Q

Describe the 7 steps of lost wax casting techniques

A
  1. Model or die taken from patient
  2. Wax pattern
  3. Sprue the pattern
  4. Assemble in casting flask (ring)
  5. Investment of the pattern
  6. Removal of the wax
  7. Fill the mould with alloy
43
Q

What are the alloys and casting temperatures for gold, high palladium, nickel-chromium, cobalt-chromium?

A

Gold - 1035⁰C
High Palladium - 1380⁰C
Nickel-Chromium - 1400⁰C
Cobalt-Chromium - 1450⁰C

44
Q

Name 4 ways expansion is compensated for in investment materials

A
  1. Setting expansion
  2. Thermal expansion
  3. Silica transformation expansion
  4. Hygroscopic expansion
45
Q

What are 3 allotropic forms of silica?

A
  1. Quartz
  2. Cristobalite
  3. Tridymite
46
Q

What is the difference between the allotropic forms of silica?

A

Chemically identical but differ in crystalline form

47
Q

Name 2 uses for silica as cristobalite or quartz

A
  1. Act as refractory

2. Provide mould expansion

48
Q

Name 3 uses of autoclaved calcium sulphate hemihydrate

A
  1. Binder
  2. Gives strength
  3. Gives mould expansion
49
Q

Name 5 dimensional changes of moulds on casting

A
  1. Setting expansion
  2. Thermal expansion
  3. Hygroscopic expansion
  4. Shrinkage of gypsum on high heating
  5. Casting shrinkage of alloy
50
Q

Name 2 limitations of bonded gypsum

A
  1. Not suitable for higher fusing dental alloys

2. Above 1200⁰C reaction between silica and calcium sulphate releases sulphur trioxide

51
Q

What are 3 issues with sulphur trioxide when released during a gypsum and silica reaction?

A
  1. Weakens investment
  2. Causes porosity in casting
  3. Contributes to corrosion
52
Q

What is the most commonly used phosphate bonded investment for refractory?

A

Cristobalite or quartz

53
Q

What is the most commonly used phosphate bonded investment for binders?

A

Magnesium oxide and ammonium phosphate rxn. product

54
Q

Describe the setting reaction of magnesium oxide

A

MgO + NH₄.H₂PO₄ + 5H₂O > Mg.NH₄PO₄.6H₂O

Prosthetics (16 decks)

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