Biomaterials - Gypsum

Question 0

Define die

Answer 0

a model of a single tooth, prepared from an impression

Question 1

Define cast

Answer 1

a replica of the teeth and/or associated bony structures of one arch (prepared from an impression)

Question 2

Requirements of cast and die materials

Answer 2

“My Dad Does Effortless Casts and Crowns”

• mechanical (bulk properties - strength; surface properties - hardness)
• detail reproduction
• dimensional accuracy and reliability
• economical
• compatibility with impression materials
• color contrast

Question 3

What form of calcium sulfate is gypsum?

Answer 3

the dihydrate form

Question 4

What is the powder supplied in the dental lab/office (and what we use) exactly?

Answer 4

It is not gypsum, but a gypsum product (a partially dehydrated form) - it is made of calcium hemihydrate.
(it is obtained by heating gypsum)

Question 5

What are the 2 forms of hemihydrate and how are they obtained?

Answer 5

1. Calcined hemihydrate (also known as beta-hemihydrate, or dental plaster) - dry process; obtained through calcination of gypsum
2. Autoclaved hemihydrate (also known as alpha-hemihydrate, or dental stone) - wet process; obtained through autoclaving of gypsum

Question 6

How are dihydrate and hemihydrates “interchanged” from one form to the other?

Answer 6

When you heat a dihydrate, it becomes a hemihydrate (plaster/stone powder we have in lab)
Allow hemihydrate to set/crystallize (i.e., mix with water) and it becomes a dihydrate (set cast/die)

Question 7

Name the classes of gypsum materials

Answer 7

Type 1 - impression plaster
Type 2 - model plaster 
Type 3 - dental stone
Type 4 - high strength dental stone
Type 5 - high strength, high expansion dental stone

Question 8

In the setting reaction of calcium sulfate hemihydrate with water, what are the 2 functions that water has?

Answer 8

1. hydration

2. to give a smooth workable mix (make sure you follow directions for correct amount)

Question 9

What are the stages of the setting reaction (for both alpha and beta)?

Answer 9

1. dissolution of some of the hemihydrate (dihydrate formed in solution, diffusion of Ca and SO4 ions)
2. crystal growth of dihydrate from crystal nuclei (grow outward to form a sphere)
3. more hemihydrate dissolves
4. interlocking crystals give rigidity and strength (this is the mechanical setting mechanism)
5. porosity results because of excess water
6. set material is almost entirely calcium sulfate dihydrate (no more water left)

Question 10

What does volumetric expansion describe?

Answer 10

the expansion, or “thrust” that occurs when the dihydrate crystals grow from the nuclei (they look star-shaped)

Question 11

Differences between calcined (beta) and autoclaved (alpha) hemihydrate

Answer 11

1. manufacture (beta: dry calcination vs alpha: wet autoclave)
2. particle size and shape (beta: more rounded vs alpha: more prismatic)
3. water/powder ratio (beta: dental plaster mixed with more water than alpha: dental stone)
4. set material - density (plaster is more porous than stone)
5. set material - mechanical properties (plaster is more brittle and weaker than stone)
6. applications (beta/plaster: used for mounting casts, study casts - strength/hardness not as important) (alpha/stone: used for working casts and dies - strength/hardness very important)

Question 12

Compressive strength of plaster vs stone

Answer 12

when dry:
plaster = about 20 MPa
stone = about 70 MPa

Question 13

What are the additives in gypsum products made to do? What is the most important type?

Answer 13

1. effect on the setting time (most important)
2. effect on setting expansion
3. effect on mechanical properties

Question 14

What are the specific types of additives that affect setting time of gypsum products and examples?

Answer 14

1. Accelerators (gypsum particles)

2. Retarders (borax)

Question 15

What effects on setting expansion can additives have on gypsum products and examples?

Answer 15

1. increase - calcium acetate

2. decrease - potassium sulfate

Question 16

What is an additives usual effect on mechanical properties of gypsum products?

Answer 16

usually weakening (one reason to control the amount of additives)

Question 17

Is the setting time of gypsum products affected by temperature?

Answer 17

Not greatly - but depends on type of material, water/powder ratio, and mixing time