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Dental Materials > Gypsum > Flashcards

Gypsum Flashcards

1
Q

What are study casts and what are their uses

A

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

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2
Q

What are the uses of gypsum

A

Cast - plaster/stone
Die - stone/improved stone
Mould material - stone
Investment binder - stone

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3
Q

How does gypsum occur naturally

A

As calcium sulphate dihydrate

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4
Q

What happens chemically during the manufacturing stage of gypsum powder

A

The dihydrate form of calcium sulphate is heated to produce calcium sulphate hemihydrate and the release of water

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5
Q

What are the different types of gypsum that can be produced on heating

A

Plaster - B-hemihydrate
Dental stone - a-hemihydrate
Densite - improved stone

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6
Q

What determines the properties of gypsum

A

The crystalline structure

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7
Q

How is plaster (B-hemihydrate) produced

A

Gypsum is heated in an open vessel (with air readily available)
This creates a powder with a large porous and irregular structure

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8
Q

How is dental stone (a-hemihydrate) produced

A

Heated in an autoclave (a closed vessel)

This produces non-porous and regular shaped crystals

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9
Q

How is densite (improved stone) produced

A

Heating process is performed with CaCl and MgCl present

This produces a more compact, less porous material with smoother particles

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10
Q

Describe the setting reaction of gypsum

A

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

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11
Q

What are the water/powder ratios of plaster and stone, and what is the theoretical ratio

A

Plaster - 50-60 ml to 100g
Stone - 20-35 ml to 200g
Theoretical ratio - 18.6 ml to 100g

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12
Q

Why is excess water used in the mixing process

A

To allow the powder and water to be mixed easily and achieve the right consistency

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13
Q

What happens in the setting process of gypsum

A

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

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14
Q

What role do impurities play in the setting process of gypsum

A

They are nucleating agents and the centres of crystal growth

Dihydrate is pulled towards them

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15
Q

Describe the initial set of gypsum

A

Dihydrate crystals come into contact and push apart causing expansion
Properties of a weak solid and will not flow

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16
Q

Describe the final set of gypsum

A

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

17
Q

What happens to the excess water after setting of gypsum

A

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

18
Q

What sizes of grooves can be replicated by gypsum

A

Grooves between 28 and 40 um

19
Q

Rank the different types of gypsum from lowest to highest on compressive strength

A

Plaster
Stone
Densite

20
Q

What is the average compressive strength of gypsum

A

After 1 hour is between 28 and 38 MPa

Develops over 24 hours to around 75MPa

21
Q

Why do different types of gypsum have a higher compressive strength than others

A

Some types require less water (stone) for a workable mix and is less porous

22
Q

Describe the surface hardness of gypsum

A

Is low so the surface is easily abrasive which isn’t ideal

23
Q

What is the ideal expansion on setting of gypsum

A

Should be low to minimise dimensional inaccuracy

24
Q

What can be done to reduce the setting time of gypsum

A

Increasing powder quantity
Increase spatulation
Adding impurities
Raising the temperature above 40*C

25
Q

Describe how spatulation affects mixing time

A

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

26
Q

What happens when increasing powder quantity is used in the powder/water ratio

A

More nuclei of crystallisation are created

Crystals contact one another sooner leading to a faster setting reaction and greater expansion

27
Q

What is typical for expansion on setting for different types of gypsum

A

Plaster - 0.3-0.3%
Stone - 0.08-0.1%
Densite - 0.05-0.07%

28
Q

What will expansion on setting cause

A

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

29
Q

What mechanisms cause temperature to have an effect on setting

A

Rate of diffusion of ions increases with increasing temperature
Solubility of hemihydrate decreases with increasing temperature

30
Q

What effects does changing temperature have on setting

A

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

31
Q

What effect does adding potassium sulphate have on gypsum

A

Produces syngenite which encourages growth of more crystals

This decreases setting time

32
Q

What chemical additives are commonly used in gypsum

A

Potassium sulphate

Borax

33
Q

What effect does adding borax have on gypsum

A

Forms calcium borate which deposits on dihydrate crystals

This delays the setting process increasing setting time

34
Q

Describe gypsums compatibility with impression material

A

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

35
Q

Describe the storage of gypsum

A

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

36
Q

How brittle is gypsum

A

Flexural strength is between 15 and 20 MPa which is very low so it is very brittle

37
Q

What are the advantages of gypsum

A 
Dimensionally accurate
Low expansion
Can replicate most grooves well
Has sufficient compressive strength
Maintains dimensions within storage
38
Q

What are the disadvantages of gypsum

A

Doesn’t withstand large tensile stresses
Surface can be abraded quite easily
Brittle so may fracture easily
Cannot reproduce very fine surface features

Dental Materials (19 decks)

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