Phillip's Dental Materials - Impression Materials Flashcards
Polysulfide - Base paste composition
- What is the multi-functional polymer?
- What is the suitable filler to add strength?
- What is the plasticizer for viscosity?
- What is the accelerator (cross linking agent)?
- Polysulfide polymer (mercaptan, “-SH”)
- Titanium dioxide
- Dibutyl phthalate
- Sulfur (approx 0.5%)
Polysulfide - Catalyst paste
- What four components?
- Lead dioxide or copper dioxide
- Filler (like titanium dioxide)
- Plasticizer (like dibutyl phthalate)
- Retarder (like oleic acid) to control the setting
Polysulfide - What gives it its characteristic brown color? (Permlastic)
- What is the teal variation? (Coe-Flex)
Lead dioxide (the reactor/catalyst//initator/accelerator)
Teal variation: Copper dioxide
Polysulfide - Addition or condensation reaction?
Condensation: Produces water as by product
This reaction leads to loss of dimensional stability
Condensation silicone - addition or condensation reaction?
Condensation: Produces ethyl alcohol (does not produce water)
This reaction leads to loss of dimensional stability
Condensation silicone - Base paste
- Catalyst paste
dimethylsiloxane with reactive -OH groups
Occurs in the presence of tin octoate and alkyl silicate as a catalyst
Results in silicone rubber + EtOH
Addition silicone (PVS)
- Reaction between what components
- Catalyst/crosslinking agent?
Vinyl terminal siloxane crosslinks with silane terminal siloxane
Catalyst is also referenced as Chloroplatinic acid
Addition silicone (PVS)
- Reaction by-products?
- How does this occur?
Technically, hydrogen gas (does not affect dimensional stability)
This results from the residual polymethylhydrosiloxane reacting with itself or moisture
Addition silicone (PVS) - What can you add to minimize the release of reaction by-products?
A noble metal such as palladium as a scavenger for the released hydrogen gas
Disadvantage of condensation and addition silicones?
What can you add to improve upon this?
Hydrophobic nature
A nonionic surfactant to render hydrophilicity
Contamination from what office item can inhibit the setting of addition silicone?
Sulfur contamination from latex or vinyl gloves
Polyether
- Polymer?
- Filler?
- Plasticizer?
- Catalyst/cross-linking agent?
Base: Ethylene Imine rings
Filler - silica
Plasticizer - phthalate
Catalyst - Aromatic sulfonate
Polyether - Why the most hydrophilic material?
Ether-dominated polymer backbones
Irreversible hydrocolloid - Components
Sodium alginate (soluble alginate) Calcium sulfate (reactor) Zinc oxide (filler particle) Potassium titanium fluoride (gypsum hardener) sodium phosphate (retarder) Diatomaceous earth (filler) Polypropylene glycol (agglomerate)
What is the risk of breathing in freshly fluffed alginate powder?
What do manufacturers add to limit this?
Can breathe in the filler (diatomaceous earth), which can lead to silicosis
Polyproplyene glycol to densify the powder
Why does alginate have a “gypsum hardener” and what is it?
Alginate gelation process produces sodium sulfate which is a gypsum-setting accelerator at low concentration but also a retarder at high concentrations
Gypsum hardener: Potassium titanium fluoride
What is the most widely accepted mechanism for gypsum setting?
Dissolution-precipitation mechanism
Dissolution of hemihydrate particles in water followed by instant recrystallization to the dihydrate
How can you quicken the setting of “α” calcium sulfate hemihydrate?
Adding fine gypsum particles (increasing nuclei)
Breaking up gypsum crystals as they form (increasing nuclei)
Decreasing water (increasing the number of nuclei per unit volume)
Does increasing water temperature lead to faster setting and formation of “α” calcium dihydrate?
No.
Between 0 to 50° C, no change in setting time
As temp of plaster-water mixture exceeds 50° C, a retardation occurs. As it approaches 100°, no reaction takes place
Mechanism of “α” calcium sulfide hemihydrate setting expansion
Crystals growing from the nuclei can intermesh with and obstruct the growth of adjacent crystals. When this repeats itself with thousands of crystals during growth, an outward stress or thrust develops, producing an expansion of the entire mass.
Why is the term hygroscopic expansion not accurate for gypsum setting?
Because hygroscopic means absorption/adsorption of water molecules.
In normal expansion, as the crystals of hemihydrate surrounded by water begin to form on the nuclei the water around the particles decreases. This loss of water contracts the crystals together which counteract the expansion. As the dihydrates grow, they will eventually contact each other and the expansion stops.
In hygroscopic expansion, the water around the forming crystals does not decrease. This means the crystals do not contract inwards.
Byproducts of:
- Polyether
- Polysulfide
- PVS addition
- PVS condensation
- Alginate
PE: None PS: Water PVS addition: Hydrogen PVS condensation: Ethyl alcohol Alginate: None
Accuracy rankings
Alg < PS < Pe < PVS
Dimensional stability rankings
Alg < PS < PE/PVS
Tear resistance
Alg/PE < PVS < PS
Polyether polymerization reaction is chain or stepwise?
Condensation or addition?
PE: Stepwise
Addition
Addition polyvinyl siloxane reaction is chain or stepwise?
Condensation or addition?
PVS: Chain reaction polymerization
Condensation - H2
Polysulfide reaction is chain or stepwise reaction?
Condensation or addition?
PS: Stepwise
Condensation - H2O
Chemical formulas for:
- Gypsum
- Plaster
- Dental stone
- Calcium sulfate dihydrate
- β form of calcium sulfate hemihydrate
- α form of calcium sulfate hemihydrate
