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Dental Material Science > Gypsum > Flashcards

Gypsum Flashcards

1
Q

Purpose of study casts (3)

A
  1. Records the position + shape of teeth
  2. Aids assessment of dentition
  3. Enables manufacture of dental prosthesis
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2
Q

List uses of gypsum (4)

A
  1. Cast (plaster/stone)
  2. Die (stone)
  3. Mould material
  4. Investment binder
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3
Q

List different types of gypsum (3)

A
  1. Plaster (B-hemihydrate)
  2. Dental stone (a-hemihydrate)
  3. Densite
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4
Q

Function of the gypsum crystalline structure

A

It determines properties

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5
Q

How is plaster (b-hemihydrate) heated and describe the particles (2)

A
  1. Heated in open vessel

2. Large porous irregular crystals

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6
Q

How is dental stone (a-hemihydrate) heated and describe the particles (2)

A
  1. Heated in an autoclave in the presence of Ca and Mg chloride
  2. Non-porous smooth crystals
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7
Q

How does improved stone particles look?

A

Compact smooth particles

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8
Q

Heating Calcium Sulphate Dihydrate leads to the production of what 3 things?

A
  1. Plaster
  2. Stone
  3. Improved stone
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9
Q

Mixing ratio for plaster

A

WATER: 50-60ml
POWDER: 100g

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10
Q

Mixing ratio for stone

A

WATER: 20-35ml
POWDER: 100g

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11
Q

Theoretical ratio

A

WATER: 18.6ml
POWDER: 100g

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12
Q

How is dihydrate formed?

A

Hemihydrate dissolves

Dihydrate forms

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13
Q

Dihydrate solubility

A

Solubility low - supersaturated solution

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14
Q

Setting process of gypsum (3)

A
  1. Dihydrate crystals precipitate on impurities as crystals
  2. More hemihydrate dissolved
  3. Continues until all hemihydrate dissolved
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15
Q

Describe the initial set process

A
  1. Dihydrate crystals come into contact (push apart)
  2. Expansion starts
  3. As the set is fairly weak solid at this stage, it will not flow
  4. Can be carved as its easy to manipulate
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16
Q

List gypsum advantages (3)

A
  1. Dimensionally accurate and stable
  2. Low expansion (<0.1%) of stone/densite
  3. Good colour contrast
17
Q

List gypsum disadvantages (4)

A
  1. Low tensile strength + very brittle
  2. Poor abrasion resistance
  3. Surface detail less than elastomer impression
  4. Poor wetting of some impression materials
18
Q

What is used to measure initial and final setting times

A

Gilmore needles

19
Q

What happens to excess water during setting?

A
  1. Excess water is trapped in the powder mass
  2. Excess water now has the opportunity to evaporate or else bubbles are formed and will become porous (making the material v weak)
20
Q

What happens on completion of setting?

A
  1. Excess water evaporates

2. Voids are produced (porosity)

21
Q

Typical compressive strength of gypsum

A

Compressive (~20~35MPa)

hardness - low

22
Q

Compare gypsum strength to composite

A

1/10th of composite strength

But roughly the same bonding strength

23
Q

State the typical expansion value of plaster?

A

0.2-0.3%

24
Q

State the typical expansion value of stone?

A

0.08-0.1%

25
Q

State the typical expansion value of densite?

A

0.05-0.07%

26
Q

Why is improved stone stronger?

A

Requires less water for a workable mix (less porous)

27
Q

List the effects of increasing:

  1. Powder
  2. Spatulation
  3. Impurities

On the setting time of gypsum products

A

Decrease setting time

28
Q

List the effects of increasing

  1. Powder
  2. Spatulation
  3. Impurities

On the expansion of gypsum products

A

Increase expansion rate

29
Q

How does spatulation lead to:
Decreased setting time
Increased expansion

A
  1. Increased spatulation breaks down growing crystals
  2. Fragments act as nuclei of crystallisation
  3. More growing crystals come into contact sooner
30
Q

Effect of increasing powder on the powder/water ratio (3)

A
  1. More nuclei of crystallisation per unit volume
  2. Crystals come into contact sooner
  3. Faster set and greater expansion
31
Q

Why should expansion only be a small %? (2)

A
  • Small % otherwise its problematic
  • Allows for model to be a little big so it means crowns, bridges and dentures won’t be too tight a fit when placed in the mouth
32
Q

How does temperature affect the rate of diffusions of ions?

A

Rate of diffusions of ions increases with increased temperature

33
Q

What chemical additives can be added? (2)

A
  1. Potassium sulphate

2. Borax

34
Q

Function of Potassium sulphate (3)

A
  • Produces syngenite
  • Crystallises rapidly encourages growth of more crystals
  • Decreases setting time
35
Q

Function of borax (2)

A
  • Deposits on dihydrate crystals

- Delays setting process (increases setting time)

36
Q

Name a disadvantage of gypsum

A

Porous - so creates a rough surface about 28 to 40um (but good enough for most applications)

Dental Material Science (7 decks)

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