L10 Dental Gypsum Products

Question 1

What is a model?

Answer 1

Replica of several teeth and their associated soft tissues

Question 2

What is a die?

Answer 2

Replica of a single tooth

Question 3

What are the uses of dental gypsum (dental stone/plaster)?

Answer 3

Models - full or partial dentures
Dies - crowns and bridges
Impression material - increase K2SO4 to hamper growth of crystals to reduce amount of expansion
Moulds - denture construction
Refractory investments - cast alloys

Question 4

What are the key material requirements for dental gypsum?

Answer 4

Dimensional accuracy - fluidity/viscosity to record fine detail
Dimensional stability - during or after setting
Mechanical properties - resist accidental fracture, abrasion resistance
Compatibility - with impression, good colour contrast to wax

Question 5

What is gypsum?

Answer 5

Naturally occurring white powder mineral. Calcium sulphate dihydrate.

Question 6

What are dental gypsum compared to gypsum?

Answer 6

Adaptation by dehydration to become calcium sulphate hemihydrate

Question 7

Briefly describe gypsum production

Answer 7

Driving off part of the water of crystallisation
Open kettle to form large porous particles, like dental plaster
Pressure vessels for small dense particles, like dental stones
Adding defloculants for small very dense particles, like diestone

Question 8

What is dental plaster?

Answer 8

Plaster of paris

Question 9

What is calcination

Answer 9

To dessicate without melting

Question 10

What are the properties of dental plaster?

Answer 10

Setting expansion - 0.2-0.3%
Compressive strength - 8.8MPa
Setting time - 12+-4mins (initial hardening)

Question 11

What is the particle morphology of dental plaster?

Answer 11

Irregular large particles

Question 12

Dental stone is chemically identical to dental plaster, how is it physically dissimilar?

Answer 12

More regular, less porous hemihydrate formed. Lower gauging volume - the amount of water needed to fill up all the gaps between the particles is lowered. Powder to liquid ratio can decrease, hence increasing mechanical properties.

Question 13

What are the properties of dental stone?

Answer 13

Setting expansion - 0.08-0.1% (decreased)
Compressive strength - 20MPa (increased)
Setting time - 12+-4 mins
(Optimised process)

Question 14

Particle morphology of dental stone

Answer 14

Image on pg54

Question 15

How is diestone different?

Answer 15

Improved dental stone with improved mechanical properties.
Cl- ions act as a deflocculant to prevent agglomeration of particles and high density fine particulate is formed.

Question 16

What are the properties of diestone?

Answer 16

Setting expansion - 0.05-0.07%
Compressive strength - 34MPa
Setting time - 12+-4mins

Question 17

Basic setting reaction of dental gypsum

Answer 17

Hemihydrate and water to form dihydrate
Plaster slurry to solid and resultant dimensional change

Question 18

Why is the gypsum reaction rapid?

Answer 18

Because it is very soluble in water

Question 19

Describe the setting process for plaster

Answer 19

Hemihydrate dissolves very quickly
Dihydrate precipitation is delayed - quite unstable phase
Solubility level of dihydrate exceeded - nucleation and crystallisation
Crystals precipitate from solution - more hemihydrate dissolves until most is converted to dihydrate

Question 20

Dimensional stability, how does the plaster expand on setting?

Answer 20

Spherulites grow from nuclei of crystallisation, which impinge on one another. Force is exerted as spherulites grow forming spaces between the crystals resulting in porosity and expansion

Question 21

Why could expansion of plaster be advantageous if controlled?

Answer 21

Investment materials - makes up shrinkage of metal castings on cooling
Crowns, bridges and dentures - don’t fit too closely

Question 22

What factors can control the setting reaction?

Answer 22

Water/powder ratio
Spatulation
Accelerators
Retarders
Temperature

Question 23

How does the water/plaster ratio affect setting characteristics?

Answer 23

Dental plaster 50ml water per 100gm powder
Stone 20-35ml water per 100gm powder

Question 24

How does the spatulation affect setting characteristics?

Answer 24

The way you mix the materials affects them. The more you mix, the faster the reaction becomes due to increasing sites of nucleation. Increasing mixing time increases setting time.

Question 25

How do the accelators affect setting characteristics?

Answer 25

Increase rate of reaction:
Potassium sulphate encourages growth of more crystals and controls expansion
Gypsum - adding more sites of nucleation, more dihydrate
Sodium chloride (small amounts) - analogous to using gypsum, increases density and nucleation sites

Question 26

How do retarders affect setting characteristics?

Answer 26

Slow rate of reaction:
Borax - forms calcium tetraborate on the crystals as they form, stopping them from growing as much and slowing the groups
Potassium citrate
Sodium chloride (larger amounts) - analogous to others

Question 27

How does temperature affect setting characteristics?

Answer 27

Limited effect
Process of dissolving the hemihydrate will increase with increased temperature, but it will retard crystallisation too, having a counter effect

Question 28

How does crystallisation affect setting time?

Answer 28

Increasing points of crystallisation by increasing powder liquid ratio, setting time increases

Question 29

How does mixing time affect setting time?

Answer 29

Increasing mixing time breaks up dihydrate crystals and creates more nucleation sites, decreasing setting time

Question 30

How does the water/powder ratio affect compressive strength?

Answer 30

Increasing amount of powder, compressive strength increases as water powder ratio goes up. More powder means more sites of nucleation so more crystals and higher density and reduced porosity to increase compressive strength.
Increasing amount of water does the opposite