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BDS2; Dental materials > DML7: Gypsum products - plaster and stone > Flashcards

DML7: Gypsum products - plaster and stone Flashcards

1
Q

What is gypsum

A

Calcium sulphate dihydrate = (CaSO4)2H20 found naturally

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2
Q

How can gypsum based products be used

A
  1. Impression materials to give negative replicas
  2. Models = study casts; poured from an impression to give a positive replica
  3. Dies = positive replica of an individual tooth
  4. Moulds; for denture construction
  5. Refractory investments
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3
Q

What is calcium sulphate dihydrate converted to for the use in gypsum products

A

It is converted into hemihydrate which results in a

  • plaster = soft white powder
  • stone = harder yellow powder
  • improved stone
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4
Q

What happens when plaster, stone or improved stone are mixed with water

A

The hemihydrate is converted back into calcium sulphate dihydrate on setting

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5
Q

How can gypsum be treated to produce hemihydrate

A
  1. Open kettle heating at 120 = B-hemihydrate (plaster of Paris)
  2. Autoclave under steam and pressure = a-hemihydrate (stone or hydrocal calcined)
  3. Boiling in 30% CaCl2/MgCl2 = a-hemihydrate (improved stone)
  4. Heat to 130-200 giving hexagonal CaSO4 = soluble anhydrite
  5. Heat >200 giving orthorhombic CaSO4 = insoluble anhydrite
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6
Q

Why is heating with CaCl2/MgCl2 better

A

Because it separates the crystals giving a smoother powder (but this will need separation afterwards)

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7
Q

Why do plaster, stone and improve stone differ

A

They are chemically identical hemihydrates but have different physical properties which is dependant on the preparation process (heating)

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8
Q

Outline properties of plaster

A

Large irregular porous particles

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9
Q

Outline properties of stone/improved stone

A

Small regular non-porous particles

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10
Q

Outline properties of improved stone

A

Expensive but more scratch resistant compared to stone

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11
Q

What is the difference between a-hemihydrate and B-hemihydrate

A

Differ in particle/crystal size and surface area

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12
Q

Outline the properties of a-hemihydrate on mixing with water and subsequent setting

A

Forms a stronger product due to smaller, regular and non-porous particles which can pack closely together in the set material

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13
Q

Outline the properties of B-hemihydrate on mixing with water and subsequent setting

A

Forms a weaker product due to irregular, larger and porous particles which cannot pack closely together in the set material causing large pores

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14
Q

What are the uses of plaster

A

It is soft, cheap and easy to shape - it is used for mounting models into articulators because strength isn’t critical for this

  • flasking procedures for complete and partial denture processing
  • rudimentary functions such as basing models
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15
Q

What are the uses of stone/ improved stone

A

This is used when strength is important

  • dies
  • models of mouth; study casts, dentate casts
  • construction of crowns, bridges, dentures
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16
Q

What is an accelerator/retarder

A

Chemical which increases/decreases the solubility of gypsum in water and so affects its setting time

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17
Q

Give an example of an accelerator and how it works

A

K2SO4 = potassium sulphate 2% solution

  • reacts with water/hemihydrate
  • to give syngenite [K2(CaSO4)2.H20]
  • this crystalises rapidly and encourages crystal growth
  • reduces overall expansion
  • accelerates setting reaction from 10 to 4 mins
18
Q

What is the role of CaSO4.2H20 in setting

A

It provides additional nucleation sites for crystal growth and reduces the working and setting times

19
Q

What is the role of NaCl (<20%) in setting

A

Provides additional sites for crystallisation and increases the reaction rate while reducing the observed expansion

20
Q

Give examples of retarders

A

NaCl (>20%) = slows down setting rate by depositing on crystals and preventing growth

Borax = counteracts the increased setting rate

Potassium citrate (gum arabic, acetates)

21
Q

Describe how the addition of water to powder causes the setting reaction

A

On addition of water to the powder, a wet slurry is formed which subsequently hardens

  • hemihydrate slowly forms the dihydrate
  • some CaSO4 hemihydrate dissolves in water and reacts to form dihydrate
  • dihydrate is lower in solubility so eventually a supersaturated solution will form which is unstable
  • the unstable supersaturated solution will form stable crystals of CaSO42H2o precipitate
  • setting begins when growing monoclinic crystals interlock and this causes the expansion of the gypsum product
22
Q

In practice why is more water used to make gypsum products than expected

A

Because more water is needed for a smooth workable mix

23
Q

What does increasing the liquid in the powder/liquid ratio cause in gypsum products

A

This results in slower setting time because it will take longer for saturation, the mix will be runny and the model weaker

24
Q

What does increasing the powder in the powder/liquid ratio cause in gypsum products

A

This makes the product difficult to mix and so it will have porosity

25
Q

What does increasing the spatulation time for gypsum products result in

A

This reduces the setting time because it will break up the formed crystals which can then act as new sites for crystal growth; this also increases the setting expansion

26
Q

How does temperature affect the setting of gypsum products

A

Little change when increasing from 0 - 50 degrees C
Over 50 degrees = gradual retardation
At 100 degrees = no reaction

27
Q

How is setting time measured

A
  1. Vicat needles

2. Gillmore needles

28
Q

What is the initial set of gypsum products

A

Beyond the working time where the material can no longer be poured; semi-hard but can still be carved (not moulded)

29
Q

What is the final set of gypsum products

A

The final hardness of the material where it can be removed from the impression without distortion/fracture

The surface loses its gloss/shine

30
Q

What is the working and setting time for model plaster and stone

A

Model plaster; 2-3 mins, 5-10 mins

Stone; 5-10 mins, 20 mins

31
Q

What causes an increased setting expansion

A

Increased powder/liquid ratio

Increased spatulation time

32
Q

Why does setting expansion occur

A

Because crystals impinge on one another as they grow and push each other apart causing expansion which leads to porosity

33
Q

What is hygroscopic expansion

A

When crystals grow freely in water and expand - by immersing material in water while setting increases the setting expansion and this is used to expand some gypsum bonded refractory investments (for casting alloys or materials with high expansion and shrinkage on solidification)

34
Q

Outline the properties of gypsum products

A
  • Once set there is little dimensional change
  • Excellent storage
  • XS water = inferior strength
  • XS powder = porosity and incomplete reaction
  • Dry strength is double wet strength
35
Q

How does tensile strength of plaster compare to stone

A

This is the strength required to pull something before it breaks

  1. Plaster is very low, teeth and margins can break
  2. Stone is double strength of plaster, crowns and bridges, dies
36
Q

How does the surface hardness and scratch/abrasion resistance of plaster compare to stone and improved stone

A

Plaster = low surface hardness, susceptible to scratches
Stone = better surface hardness, better scratch resistance
Improved stone = even better than stone

37
Q

Type I gypsum products

A

Dental plaster; impression plaster

38
Q

Type II gypsum products

A

Dental plaster; models

39
Q

Type III gypsum products

A

Dental stone; models

40
Q

Type IV gypsum products

A

Dental stone; dies, high strength, low expansion

41
Q

Type V gypsum products

A

Dental stone; dies, high strength, high expansion

BDS2; Dental materials (11 decks)

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