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BDS3 Dental Material Science > Investment material > Flashcards

Investment material Flashcards

ILO 1.6c Be familiar with the chemical and physical properties as well as the clinical uses of a range of dental materials

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1
Q

What are the clinical uses of investment materials in dentistry (ILO dental uses)

A

To produce metal/alloy inlays, onlays, crowns and bridges

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2
Q

What technique for using investment materials involves….

A

Casting molten alloy under pressure by centrifugal force

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3
Q

What are the stages when using investment materials?

A

1) Wax pattern of the required prosthesis e.g crown, inlay (ie a positive replica)

2) Investment material poured around wax pattern and allowed to set (mould, a negative replica)

3) The wax removed – by burning or with boiling water.

4) Molten alloy:
- Poured into mould cavity
– via the sprue – hollow tubes that allow the alloy to flow in.

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4
Q

What types of investment material are there?

A
  • Dental stone/plaster - acrylic dentures
  • Gypsum bonded materials - gold casting alloys
  • Phosphate bonded materials - base metals/cast ceramics
  • Silica bonded materials - base metal alloys
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5
Q

What are the ideal properties/requirements of an investment material? (ILO physical properties)

A

1) Expands - compensate for cooling shrinkage of alloy
2) Porous - allow escape of trapped gases
3) Strong
- Room Temp - easy of handling (“green strength)
-Casting temp - withstand casting forces

Further ideal properties/requirements
- Smooth finish- for easy finishing
- Chemically stable - porosity, surface detail
- Easy removal from cast
- Handling - not complicated
- Relatively inexpensive - destroyed after use

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6
Q

What are the typical contractions/shrinkage (by volume) for alloy melting point to room temperature of gold alloy, Ni/Cr alloys and Co/Cr alloys

A

Gold alloys: 1.4%.
NiCr alloys: 2%
CoCr alloys: 2.3%

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7
Q

What are the components of investment material?

A

1) Binder - Gypsum, phosphate, silica-bonded
2) Refectory - Silica (quartz or cristobalite)

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8
Q

What is the purpose of in the binder component in investment materials?

A

To form a coherent solid mass/provides substance

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9
Q

What is the purpose of the refractory component in investment materials?

A

To withstand high temperatures
to give expansion

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10
Q

In the refractory component of investment materials, what form does quartz exist in at different temperatures?

A

Below 573oC - alpha-quartz (squashed crystalline lattice structure)
Above 573oC - beta-quartz (explodes to it’s maximum volume)

This is called inversion expansion

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11
Q

What is the composition of Gypsum-bonded Investment? What do some of the element do?

A

Power (mixed with water)
- Silica (60-65%) - withstand high temperatures (refractory component)
- Calcium Sulphate hemihydrate (30-35%) - produces calcium sulphate di-hydrate when mixed with water
- reducing agent for oxides
-chemicals to inhibit heating shrinkage and control setting time (Boric acid, Nacl)

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12
Q

What is the setting reaction for gypsum-bonded investment?

A

(CaSO4)2.H2O +3H20 (hemihydrate) –> 2CaSO4.2H20 (dehydrate)

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13
Q

What dimensional changes does gypsum-bonded investment material undergo

A

Silica - Thermal and inversion
Gypsum - setting expansion:
1) Hygroscopic expansion
2) Contraction above 320oC

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14
Q

In gypsum, what is hygroscopic expansion?

A

Expansion:
- 5X change in volume of gypsum-bonded investment materials.
- Expansion due to capillary forces pulling water molecules into gaps between crystals (of calcium sulphate hemi-hydrate) – forcing them apart.

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15
Q

What factors increase hydroscopic expansion?

A

Lower powder/water ratio
increased silica content
higher water temperature
longer immersion time in water

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16
Q

What temperature does gypsum contract?

A

above 320oC

17
Q

Why does gypsum contract above 320oC?

A

a) water loss
b) presence of sodium chloride and boric acid

18
Q

What are the properties of Gypsum bonded investment material?

A
  • Expansion:
    • expands by 1.4%
    • good enough expansion for gold alloy
  • Smooth surface
    • fine particles
  • manipulation
    • easy
    • setting time controlled
  • Porous
    • good
  • Strength
  • adequate if correct powder/liquid ratio and correct manipulation
19
Q

At what temperature does gypsum-bonding investment have an unwanted reaction and why?

A

above 700oC
Due to wax residue or graphite in the Investment Material leading to production of:
carbon dioxide
sulphur dioxide

CaS04 + 4C –> CaS + 4CO
then
3CaS04 + CaS –> 4CaO + 4SO2

20
Q

What technique it used when unwanted reactions occur in gypsum-bonded investment? And what is it’s purpose?

A

Heat soaking

Important CO and SO2 produced above 700oC can escape. Heat soaking - when the Investment Material is held at a high temperature fore some time allowing the gases to gradually escape

21
Q

At what temperature is a gypsum bonded investment suitable for cast alloy? And why?

A

Below 1200oC - satisfies requirement
Above 1200oC - problems due to production of sulphur trioxide which will produce voids in the cast alloy and contributes to corrosion

CaS04+SiO2 –> CaSiO3 + SO3

22
Q

Gypsum-bonded investment are limited to alloys which which melting point

A

Below 1200oc

23
Q

What is the composition of phosphate bonded investment

A

Powder
- silica
- Magnesium oxide
- ammonium phosphate

Liquid
- Water or colloidal silica

24
Q

What are the properties of colloidal silica?

A

Increases strength
Gives hygroscopic expansion (2%) - compensated for alloy shrinkage

25
Q

What is the setting reaction for phosphate bonded investment?

A

NH4.H2Po4 + MaO + 5H20 ->Mg NH4PO4.6H2O
Ammonium phosphate + magnesium oxide -> magnesium ammonium phosphate

26
Q

What happens when you heat phosphate bonded invest to around 1000oC?

A

At 300oC water and ammonia liberated
at higher temperature complex reactions with silico-phosphates formed - increased strength

27
Q

What are the properties of phosphate bonded investment?

A

High strength
porous
chemically stable
high “green” strength” - don’t need metal casting ring for support
easy to use

28
Q

In investment material, what types of expansion are there? And what is their purpose?

A

Thermal,
Hygroscopic,
inversion

Counter alloy shrinkage on cooling

29
Q

Why is strength important for investment material?

A

To withstand pressures during casting

30
Q

Why is stability important for investment material?

A

Doesn’t degrade

31
Q

What dimensional changes occur for silica-based investment materials?

A
  • contraction at early stages of heating - water and alcohol loss from gel
  • substantial thermal and inversion expansion - lots of silica present
32
Q

What are the properties of silica-based investment materials?

A
  • strength
  • not porous - needs vents (alloy will end up porous and weak otherwise)
    -complicated manipulation

BDS3 Dental Material Science (1 decks)

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