DML7: Gypsum products - plaster and stone Flashcards
What is gypsum
Calcium sulphate dihydrate = (CaSO4)2H20 found naturally
How can gypsum based products be used
- Impression materials to give negative replicas
- Models = study casts; poured from an impression to give a positive replica
- Dies = positive replica of an individual tooth
- Moulds; for denture construction
- 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
- Open kettle heating at 120 = B-hemihydrate (plaster of Paris)
- Autoclave under steam and pressure = a-hemihydrate (stone or hydrocal calcined)
- Boiling in 30% CaCl2/MgCl2 = a-hemihydrate (improved stone)
- Heat to 130-200 giving hexagonal CaSO4 = soluble anhydrite
- 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
