Impression Materials and Gypsum Flashcards
Polysulfide
- Primary component of polymer
- Filler? And what does it do?
- Plasticizer? What does it do?
- Catalyst/reactor?
- Accelerator?
- Retarder?
- What gives polysulfide its characteristic brown color?
- Multi-functional mercaptan (-SH) aka Polysulfide polymer
- Filler (titanium dioxide) - strength
- Plasticizer (dibutyl-phthalate) - viscosity
- Catalyst - lead dioxide
- Accelerator - Sulfur
- Retarder - oleic acid
- Lead Dioxide
Polysulfide
- Base paste
- Catalyst paste
- Reaction
- Type of reaction
- Byproduct
- Polysulfide polymers with terminal thiol (-SH) groups
- Catalyst: Lead dioxide and a sulfur compound (mercaptan) as a cross-linking agent
- Chemical reaction: Hydrogen sulfide (SH) groups interact with oxygen released from lead dioxide and water
- Condensation polymerization (like condensation silicone)
- Water (cause shrinkage or dimensional instability)
Condensation Silicone
- Base paste
- Catalyst paste and cross-linking agent
- Reaction type
- By-product
- Polydimethylsiloxane (PDMS) with terminal hydroxyl groups
- Tin-based catalyst (e.g. stannous octoate) and cross-linking agent (alkyl silicate)
- Reaction type: Condensation polymerization
- Ethanol molecules (causes shrinkage)
Dimethylsiloxane with reactive -oH groups, silica added for consistency and stiffness. The material cross-links between terminal groups of the silicone polymers and the alkyl silicate to form a three-dimensional network
Addition reaction silicone
1. Base paste components (1)
2. Catalyst paste components (2)
3. Byproducts?
4. What causes contamination?
5. Reaction type?
- Base paste: Vinyl-terminated polydimethylsiloxane (vinyl silicone)
- Catalyst paste: Hydrogen-terminated polydimethylsiloxane and platnium salt catalyst
- No volatile by-products
- Sulfur contamination can inhibit the platinum catalyst
- Addition polymerization (hydrosilylation)
Addition Reaction VPS
Base:
Crosslinker:
Catalyst:
Byproduct:
Base: Vinyl polysiloxane
Crosslinker and catalyst: Platnium salt
Reaction produces H2 gas as a secondary byproduct, PT absorbs
Polyether
- Base paste
- Catalyst paste
- Reaction type
- What makes this group of materials the most hydrophilic of elastomeric impression materials?
- What keeps the reaction going?
- Base: Polyether polymer (epimine groups)
- Catalyst: Initiator (aromatic sulfonate ester) and plasticizers
- Reaction type: Cationic ring-opening polymerization of the epimine rings
- Ether-dominated polymer backbone
- As each polymer ring opens, it then itself becomes a cation. It then attacks and opens other rings
Rank elastomeric impression materials by tear strength?
Lowest to greatest:
VPS, Polyether, Polysulfide
Irreversible hydrocolloid
1. Soluble alginate?
2. Reactor?
3. Filler?
4. Retarder?
5. To combat silicosis risk, alginate manufacturers add what to agglomerate the particles?
- K/Na alginate
- Ca sulfate
- Zinc oxide or diatamaceous earth
- Sodium phosphate
- Glycol
- What is calcining?
What gypsum products are made under the following conditions?
2a. If gypsum is heated in open kettle at temp of 110˚C?
2b. If gypsum is dehydrated under pressure and in the presence of water vapor at ~120˚C?
2c. How are high-strength low expansion and high expansion stones made? What is their chemical name?
- 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)
2c. Calcining in an autoclave for high-strength dental stone α-hemihydrate and with 30% calcium chloride solution for mod α-hemihydrate)
Why is dental stone so much denser and stronger than plaster of paris?
How is die stone made?
How are the crystals different?
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.
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?
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
Composition Baseplate wax
Type 1, Type 2, Type 3?
75% paraffin/ceresin
Type 1: Building veneers
Type 2: Normal
Type 3: Tropical climates
Why shouldn’t you heat gypsum-bonded investment material beyond 700 degrees celsius?
What two things then occur to the casting?
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)
Phillips Chapter 10, page 202
Why is Silica added to investments?
Gypsum normally shrinks considerably when heating. Silica can lead to expansion due to a change in crystalline form.
Do you typically want expansion or contraction of investment material?
What are the two principle components of gypsum-bonded investment?
Expansion, to compensate for the contraction of the alloy
Silica + Calcinated gypsum powder (calcium sulfate hemihydrate, or alpha-hemihydrate)
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?
As W/P ratio increases, less expansion occurs of the investment.
Phosphate-Bonded Investment
- Refractory filler?
- Binder?
Refractor filler: Silica (either cristobalite or quartz)
Binder: Magnesium oxide and phosphate
1-3. Three step process for Type 4/5 stones?
- What is their chemical name?
- Densite is boiled in a 30% calcium chloride solution
- The chloride is washed away with hot water
- The material is ground down to the desired fineness.
- alpha-calcium sulfate hemihydrate
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”)
- Gypsum is dehydrated under pressure in the presence of water vapor at ~125˚ C.
- ∂-calcium sulfate hemihydrate
- Hydrocal
How is dental plaster (Type 1 stones) made?
What’s the hemihydrate produced called?
Gypsum is heated in an open kettle at 110˚ C
ß-calcium sulfate hemihydrate
How much µm detail are the five types of stones required to reproduce per ANSI/ADA spec No. 25
1-3: 75 plus or minus 8
4-5: 50 plus or minus 8
Elastomeric impression materials are ADA specification no?
- Impression and die stone materials are compatible if it can reproduce what?
ADA spec no 19
20 µm
If typical, high strength dental stone is set in water, it has additional expansion.
How much more?
What is this called?
Why?
- From .08% to .10%
- Hygroscopic expansion
- The water allows crystals to expand further and wider
How do you lower value?
How do you raise chroma?
How do you lower chroma?
Lower value with complimentary color
Raise chroma with dominant hue
Lower chroma with complimentary color
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?
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
Compressive strength
- Plaster
- Stone
- Improved stone
- Plaster 12.5 MPa
- Stone 34 MPa
- Improved stone 45 MaP
Dimensional changes
Rank alginate, addition reaction silicone, polyether, and polysulfide in terms of least to most
ARS (0.05%) < PE (0.1%) < PS < Alginate
Tear Strength: Rank alginate, addition reaction silicone, polyether, and polysulfide from lowest to highest
Alginate < polyether < PVS < Polysulfide
Water power ratio, expansion, and compressive strength for Type 1, Type 3 and Type 4 stone?
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
Gypsum products chemical reaction
[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)
Gypsum product setting mechanism (3 steps)
- Dissolution of hemihydrate crystals into water
- Nucleation and growth of new crystals
- Interlocking of crystals during hardening
What’s added to gypsum as an accelerator, retarder, and as a filler?
- Accelerator: Potassium sulfate or slurry water (calcium sulfate dehydrate)
- Retarder: Borax
- Fillers: Silica, resin
Scientific names for dental stone and gypsum?
Dental stone: alpha calcium sulfate hemihydrate
gypsum: calcium sulfate dihydrate
If the calcium sulfate dihydrate reaction occurred underwater, what would occur to setting expansion? What is this process called?
Setting expansion would more than double. (Crystals form freely and not constrained by air surface tension.)
Hygroscopic setting expansion
Composition of gypsum-bonded investment (two)
- α-hemihydrate of gypsum (binder to cast gold alloys)
- Quartz (refractory component, regulate thermal expansion)
Which impression material does not produce byproducts?
Polyether
During polymerization shrinkage, which direction does the distortion occur?
Towards the impression material tray
