Impression Materials and Gypsum

Question 1

Addition reaction silicone
1. Base paste components (1)
2. Catalyst paste components (2)
3. Byproducts?
4. What causes contamination?
5. Reaction type?

Answer 1

1. Base paste: Vinyl-terminated polydimethylsiloxane (vinyl silicone)
2. Catalyst paste: Hydrogen-terminated polydimethylsiloxane and platnium salt catalyst
3. No volatile by-products
4. Sulfur contamination can inhibit the platinum catalyst
5. Addition polymerization (hydrosilylation)

Question 2

Addition Reaction VPS
Base:
Crosslinker:
Catalyst:
Byproduct:

Answer 2

Base: Vinyl polysiloxane

Crosslinker and catalyst: Platnium salt

Reaction produces H2 gas as a secondary byproduct, PT absorbs

Question 3

Polyether

1. Base paste
2. Catalyst paste
3. Reaction type
4. What makes this group of materials the most hydrophilic of elastomeric impression materials?
5. What keeps the reaction going?

Answer 3

1. Base: Polyether polymer (epimine groups)
2. Catalyst: Initiator (aromatic sulfonate ester) and plasticizers
3. Reaction type: Cationic ring-opening polymerization of the epimine rings
4. Ether-dominated polymer backbone
5. As each polymer ring opens, it then itself becomes a cation. It then attacks and opens other rings

Question 4

Rank elastomeric impression materials by tear strength?

Answer 4

Lowest to greatest:
VPS, Polyether, Polysulfide

Question 5

Irreversible hydrocolloid
1. Soluble alginate?
2. Reactor?
3. Filler?
4. Retarder?
5. To combat silicosis risk, alginate manufacturers add what to agglomerate the particles?

Answer 5

1. K/Na alginate
2. Ca sulfate
3. Zinc oxide or diatamaceous earth
4. Sodium phosphate
5. Glycol

Question 6

1. What is calcining?

What gypsum products are made under the following conditions:
2a. If gypsum is heated in open kettle at 110˚C?
2b. If gypsum is dehydrated under pressure and in the presence of water vapor at ~120˚C?

3. How are high-strength low expansion and high expansion stones made? What is their chemical name?

Answer 6

1. Calcining calcium sulfate dihydrate (gypsum) with no to little oxygen. Temps: 110 to 130 degrees C

2a. Plaster
2b. Model plaster (β- calcium sulfate hemihydrate)

3. Calcining in an autoclave for high-strength dental stone α-hemihydrate and with 30% calcium chloride solution for mod α-hemihydrate)

Question 7

Why is dental stone so much denser and stronger than plaster of paris?

How is die stone made?

How are the crystals different?

Answer 7

The α-hemihydrate are more regular and not as porous. So the dihydrate produced is better packed.

Die stone: Calcinating occurs under pressure of 30% calcium chloride solution or in the presence of more than 1% sodium succinate.

Crystals: Hemihydrate crystals are shorter and thicker. Called” modified α-hemihydrate.

Question 8

What wax compromises 40-60% of most dental waxes?
Why is gum dammar added to the above wax?
Carnauba - Three good properties?
What two natural waxes are substituted for Carnauba?

Answer 8

Paraffin - Most dental waxes 40-60%. Not smooth or glossy, so other waxes are added.
Gum dammar - Add to paraffin to improve smoothness, toughness, and luster.
Carnauba wax - Very hard, high melting point, glossy.
Candelilla - Can sub carnauba. Similar but lower melting point and hardness.
Ceresin - White wax, added for hardness, melting range. Can sub with candelilla for carnauba

Question 9

Composition Baseplate wax
Type 1, Type 2, Type 3?

Answer 9

75% paraffin/ceresin

Type 1: Building veneers
Type 2: Normal
Type 3: Tropical climates

Question 10

Why shouldn’t you heat gypsum-bonded investment material beyond 700 degrees celsius?

What two things then occur to the casting?

Answer 10

At 700, you get significant contraction. This is due by decomposition and release of sulfur dioxide.

Results in shrinkage and contamination of the casting (with sulfides of non-noble elements like silver and copper)

Question 11

Phillips Chapter 10, page 202

Why is Silica added to investments?

Answer 11

Gypsum normally shrinks considerably when heating. Silica can lead to expansion due to a change in crystalline form.

Question 12

Do you typically want expansion or contraction of investment material?

What are the two principle components of gypsum-bonded investment?

Answer 12

Expansion, to compensate for the contraction of the alloy

Silica + Calcinated gypsum powder (calcium sulfate hemihydrate, or alpha-hemihydrate)

Question 13

For gypsum bonded investment, as the water powder ratio increases (w/p = .30 to .40 and so on), what happens to expansion of the investment?

Answer 13

As W/P ratio increases, less expansion occurs of the investment.

Question 14

Phosphate-Bonded Investment
- Refractory filler?
- Binder?

Answer 14

Refractor filler: Silica (either cristobalite or quartz)

Binder: Magnesium oxide and phosphate

Question 15

1-3. Three step process for Type 4/5 stones?

4. What is their chemical name?

Answer 15

1. Densite is boiled in a 30% calcium chloride solution
2. The chloride is washed away with hot water
3. The material is ground down to the desired fineness.
4. alpha-calcium sulfate hemihydrate

Question 16

How is Type 3 stone made?

Type 3 stones are also called:
2. (Hint, begins with a greek letter)
3. (Hint, it begins with “H”)

Answer 16

1. Gypsum is dehydrated under pressure in the presence of water vapor at ~125˚ C.
2. ∂-calcium sulfate hemihydrate
3. Hydrocal

Question 17

How is dental plaster (Type 1 stones) made?

What’s the hemihydrate produced called?

Answer 17

Gypsum is heated in an open kettle at 110˚ C

ß-calcium sulfate hemihydrate

Question 18

How much µm detail are the five types of stones required to reproduce per ANSI/ADA spec No. 25

Answer 18

1-3: 75 plus or minus 8

4-5: 50 plus or minus 8

Question 19

Elastomeric impression materials are ADA specification no?

• Impression and die stone materials are compatible if it can reproduce what?

Answer 19

ADA spec no 19

20 µm

Question 20

If typical, high strength dental stone is set in water, it has additional expansion.

How much more?
What is this called?
Why?

Answer 20

1. From .08% to .10%
2. Hygroscopic expansion
3. The water allows crystals to expand further and wider

Question 21

How do you lower value?

How do you raise chroma?

How do you lower chroma?

Answer 21

Lower value with complimentary color

Raise chroma with dominant hue

Lower chroma with complimentary color

Question 22

Plaster/dental stone
- Formula for making gypsum?

• How is plaster of Paris made?
• How is Type III dental stone made?
• How is Type IV dental stone made?

Answer 22

1.Calcium sulfate dihydrate + heat -> calcium sulfate hemihydrate
2. Plaster - β - open kettle
3. Type 3 - α - autoclave under steam
4. Type 4 - modified α - autoclave with 30% CaCl2

Question 23

Compressive strength

• Plaster
• Stone
• Improved stone

Answer 23

1. Plaster 12.5 MPa
2. Stone 34 MPa
3. Improved stone 45 MaP

Question 24

Dimensional changes

Rank alginate, addition reaction silicone, polyether, and polysulfide in terms of least to most

Answer 24

ARS (0.05%) < PE (0.1%) < PS < Alginate

Question 25

Tear Strength: Rank alginate, addition reaction silicone, polyether, and polysulfide from lowest to highest

Answer 25

Alginate < polyether < PVS < Polysulfide

Question 26

Water power ratio, expansion, and compressive strength for Type 1, Type 3 and Type 4 stone?

Answer 26

Type 1 (Impression plaster .50 W/P, .15%, 4 MPa) Mounting Stone (type 3) 26 ml water to 100 g powder (.28 W/P) .08% expansion 59 MPa Microstone 28 ml to 100g (.28 W/P) .12% expansion 59 MPa Silky Rock 16 ml water to 70 g stone (0.23 W/P ratio) .09% expansion Compressive strength: 90 MPa

Question 27

Gypsum products chemical reaction

Answer 27

[CaSO4 * (1/2)H2O] + [(3/2)H2O] ----------> CaSO4 * (2)H2O + HEAT Plaster, stone, die stone + water ---> Gypsum + Heat Calcium sulfate dehydrate = CaSO4 * (2) H2O (Gypsum)

Question 28

Gypsum product setting mechanism (3 steps)

Answer 28

1. Dissolution of hemihydrate crystals into water 2. Nucleation and growth of new crystals 3. Interlocking of crystals during hardening

Question 29

What's added to gypsum as an accelerator, retarder, and as a filler?

Answer 29

1. Accelerator: Potassium sulfate or slurry water (calcium sulfate dehydrate) 2. Retarder: Borax 3. Fillers: Silica, resin

Question 30

Scientific names for dental stone and gypsum?

Answer 30

Dental stone: alpha calcium sulfate hemihydrate gypsum: calcium sulfate dihydrate

Question 31

If the calcium sulfate dihydrate reaction occurred underwater, what would occur to setting expansion? What is this process called?

Answer 31

Setting expansion would more than double. (Crystals form freely and not constrained by air surface tension.) Hygroscopic setting expansion

Question 32

What kind of 3d printer printed your casts? What other types are there?

Answer 32

Resin-based stereolithography (SLA) 3d Printer Fused deposition modeling (extruded filament) Digital light processing (uses a projector to cure a layer at once) Material Jetting (jets droplets of photopolymer resin and cures them

Question 33

Why is it that Type 4 gypsum needs less water than Type 3, and so on?

Answer 33

Alpha-hemihydrate stone has a more regular, uniform, and dense crystal structure compared to beta. That means less water required for wetting and lubrication.

Question 34

GPT Definition of PVS

Answer 34

An addition reaction silicone elastomeric impression material of silicone polymers having terminal vinyl groups that cross-link with silanes on activation by a platinum or palladium salt catalyst.

Question 35

If you increase water in gypsum reaction, what happens to setting time, setting expansion, and compressive strength?

Answer 35

Longer setting time, less expansion, less compressive strength

Question 36

If you spatulate faster, what happens to setting time, setting expansion, and compressive strength?

Answer 36

Shorter setting time, more expansion, no effect on compressive strength

Question 37

If you use hot water, what happens to setting time, setting expansion, and compressive strength?

Answer 37

Decrease setting time, increase setting expansion, no effect on compressive strength