DML7: Gypsum products - plaster and stone Flashcards Preview

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Flashcards in DML7: Gypsum products - plaster and stone Deck (41)
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What is gypsum

Calcium sulphate dihydrate = (CaSO4)2H20 found naturally


How can gypsum based products be used

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


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

It is converted into hemihydrate which results in a
- plaster = soft white powder
- stone = harder yellow powder
- improved stone


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

The hemihydrate is converted back into calcium sulphate dihydrate on setting


How can gypsum be treated to produce hemihydrate

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


Why is heating with CaCl2/MgCl2 better

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


Why do plaster, stone and improve stone differ

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


Outline properties of plaster

Large irregular porous particles


Outline properties of stone/improved stone

Small regular non-porous particles


Outline properties of improved stone

Expensive but more scratch resistant compared to stone


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

Differ in particle/crystal size and surface area


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

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


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

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


What are the uses of plaster

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


What are the uses of stone/ improved stone

This is used when strength is important
- dies
- models of mouth; study casts, dentate casts
- construction of crowns, bridges, dentures


What is an accelerator/retarder

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


Give an example of an accelerator and how it works

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


What is the role of CaSO4.2H20 in setting

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


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

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


Give examples of retarders

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

Borax = counteracts the increased setting rate

Potassium citrate (gum arabic, acetates)


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

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


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

Because more water is needed for a smooth workable mix


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

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


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

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


What does increasing the spatulation time for gypsum products result in

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


How does temperature affect the setting of gypsum products

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


How is setting time measured

1. Vicat needles
2. Gillmore needles


What is the initial set of gypsum products

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


What is the final set of gypsum products

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

The surface loses its gloss/shine


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

Model plaster; 2-3 mins, 5-10 mins
Stone; 5-10 mins, 20 mins