Gypsum Flashcards
What are study casts and what are their uses
Positive replica of the dentition
Records the position, shape and dimensions of all the teeth present and allows visualisation of the patients dentition
Enables manufacture of dental prostheses
What are the uses of gypsum
Cast - plaster/stone
Die - stone/improved stone
Mould material - stone
Investment binder - stone
How does gypsum occur naturally
As calcium sulphate dihydrate
What happens chemically during the manufacturing stage of gypsum powder
The dihydrate form of calcium sulphate is heated to produce calcium sulphate hemihydrate and the release of water
What are the different types of gypsum that can be produced on heating
Plaster - B-hemihydrate
Dental stone - a-hemihydrate
Densite - improved stone
What determines the properties of gypsum
The crystalline structure
How is plaster (B-hemihydrate) produced
Gypsum is heated in an open vessel (with air readily available)
This creates a powder with a large porous and irregular structure
How is dental stone (a-hemihydrate) produced
Heated in an autoclave (a closed vessel)
This produces non-porous and regular shaped crystals
How is densite (improved stone) produced
Heating process is performed with CaCl and MgCl present
This produces a more compact, less porous material with smoother particles
Describe the setting reaction of gypsum
Gypsum powder in the lake is the hemihydrate form which is mixed with water to produce the gypsum material used to create study casts or die
Reverse of manufacture
What are the water/powder ratios of plaster and stone, and what is the theoretical ratio
Plaster - 50-60 ml to 100g
Stone - 20-35 ml to 200g
Theoretical ratio - 18.6 ml to 100g
Why is excess water used in the mixing process
To allow the powder and water to be mixed easily and achieve the right consistency
What happens in the setting process of gypsum
Hemihydrate dissolves and dihydrate forms
Dihydrate precipitate on impurities as crystals
More hemihydrate is dissolved
The crystal growth process continues until all the hemihydrate is consumed
What role do impurities play in the setting process of gypsum
They are nucleating agents and the centres of crystal growth
Dihydrate is pulled towards them
Describe the initial set of gypsum
Dihydrate crystals come into contact and push apart causing expansion
Properties of a weak solid and will not flow
Describe the final set of gypsum
Crystals that are quite large, of different sizes and shape CaSO4s coming into direct contact with one another
Now strong and hard but strength will continue to develop over the next few hours
What happens to the excess water after setting of gypsum
During the setting the water is trapped in the powder
When setting is over the water evaporates leaving voids
Evaporation may cause any dissolved dihydrate crystals to cement together
What sizes of grooves can be replicated by gypsum
Grooves between 28 and 40 um
Rank the different types of gypsum from lowest to highest on compressive strength
Plaster
Stone
Densite
What is the average compressive strength of gypsum
After 1 hour is between 28 and 38 MPa
Develops over 24 hours to around 75MPa
Why do different types of gypsum have a higher compressive strength than others
Some types require less water (stone) for a workable mix and is less porous
Describe the surface hardness of gypsum
Is low so the surface is easily abrasive which isn’t ideal
What is the ideal expansion on setting of gypsum
Should be low to minimise dimensional inaccuracy
What can be done to reduce the setting time of gypsum
Increasing powder quantity
Increase spatulation
Adding impurities
Raising the temperature above 40*C
Describe how spatulation affects mixing time
Increased spatulation breaks down growing crystals meaning more nuclei of crystallisation are formed and subsequently more crystals are created
These crystals come into contact with one another at an earlier stage causing greater expansion and a decreased setting time
What happens when increasing powder quantity is used in the powder/water ratio
More nuclei of crystallisation are created
Crystals contact one another sooner leading to a faster setting reaction and greater expansion
What is typical for expansion on setting for different types of gypsum
Plaster - 0.3-0.3%
Stone - 0.08-0.1%
Densite - 0.05-0.07%
What will expansion on setting cause
Study casts will be slightly too large so crowns, bridges and dentures produced from the casts will be slightly too large and so not too tight a fit when located in the mouth
What mechanisms cause temperature to have an effect on setting
Rate of diffusion of ions increases with increasing temperature
Solubility of hemihydrate decreases with increasing temperature
What effects does changing temperature have on setting
Changing water temperature from 10-40C will reduce setting time
Raising the temperature from 40-80C will increase the setting time
Raising the temperature above 80*C will rapidly increase setting time
What effect does adding potassium sulphate have on gypsum
Produces syngenite which encourages growth of more crystals
This decreases setting time
What chemical additives are commonly used in gypsum
Potassium sulphate
Borax
What effect does adding borax have on gypsum
Forms calcium borate which deposits on dihydrate crystals
This delays the setting process increasing setting time
Describe gypsums compatibility with impression material
Must be compatible with the selected impression material so that it wets the surface, making close contact and avoiding the formation of voids and bubbles
Describe the storage of gypsum
Once study casts are made they can be stored for a considerable time
Long term dimensional stability is good so it can be used as a positive replica for a long time
How brittle is gypsum
Flexural strength is between 15 and 20 MPa which is very low so it is very brittle
What are the advantages of gypsum
Dimensionally accurate Low expansion Can replicate most grooves well Has sufficient compressive strength Maintains dimensions within storage
What are the disadvantages of gypsum
Doesn’t withstand large tensile stresses
Surface can be abraded quite easily
Brittle so may fracture easily
Cannot reproduce very fine surface features
