Fixed - Impression Materials Flashcards
What is accuracy?
Accuracy: Ability to replicate the intraoral surface details.
What is dimensional stability?
Dimensional Stability: Ability to retain its absolute dimensional size over time.
What is tear resistance?
Tear Resistance: Ability to resist tearing in thin sections (such as through feather-edged material within the gingival sulcus.)
What are the non-water based rigid impression materials and which one is the only physical reversible material?
- impression plaster
- impression compound
- zinc oxide eugenol
*impression compound = only physical set, reversible rxn rigid material
What is the most common impression material and what is its shortcomings?
Alginate (irreversible hydrocolloid that sets up due to irreversible chemical rxn.)
- shortcoming = *not accurate enough for fixed and removable restorations.
What are the 2 types of water-based rigid impression materials? And what type of reactions and sets are they?
- alginate - irreversible, chemical
2. agar-agar - reversible, physical
What does sodium phosphate do in alginate?
React preferentially with calcium ions to provide working time before gelation
What does diatomaceous earth or silicate powder do in alginate?
Control the consistency of the mixed alginate and the flexibility of the set impression
What does organic glycols do in alginate?
Makes the powder dustless
Changes in water/powder ratio will alter what?
consistency & setting times of the mixed material
What do you add to reduce setting time or slow the reaction down?
trisodium phosphate
What are the 2 main rxns of the alginate process?
1st: 2Na3PO4 + 3CaSO4 Ca(PO4)2 + 3Na2SO4
2nd: K2Alginate (powder) + CaSO4 Powder Water K2SO4 + CaAlginate (gel)
what are the general properties of alginates?
- Highly hydrophilic.
- Expand slightly during setting rxn.
- Sensitive to ambient humidity
- Low tear strength.
- Short shelf life (<1year).
- Accuracy & reprod. of detail is limited.
- Dimensionally unstable–remember water can come in and out depending on the storage environment.
What are the 4 types of elastomeric impression material?
- polysulfide (rubber base, thiokol)
- silicone (conventional, condensation)
- polyether
- polyvinyl siloxane (addition silicone, vinyl polysiloxane)
*all flexible properties, irreversible rxn, chemical set
ISO 4823 requirements?
- consistency
- detail reprod.
- linear dimensional change
- compatibility w/ gypsum
- strain in compression
- elastic recovery
- report working time & setting time
What are the 4 types of ISO consistencies?
Type 0: putty consistency
Type 1: heavy-bodied consistency
Type 2: medium body consistency
Type 3: light-bodied consistency
What does the continuous phase involved?
Flexible matrix
• Multifunctional pre-polymer or polymer.
• Crosslinking agent.
• Curing agent (catalyst or initiator).
• Modifiers (accelerators, retarders, plasticizers, flavoring agents, colorants).
What does the dispersed phase involve?
filler or extender
Would a filled polymer have high or low shrinkage?
low shrinkage.
- unfilled polymer has high shrinkage
What are 2-step techniques you can use to manage shrinkage?
- light-heavy body
2. wash-putty
During setting shrinkage, distortion of impression is toward the ____.
tray
Dies tend to be slightly ____, so the casting is also slightly ____.
oversized, oversized
How would you do the light-heavy body technique?
cover prep w/ light-bodied IM from syringe; load tray w/ heavy bodied-IM
*this is what we do in lab
The ____ the removal of the tray the less distorion. (slower/faster)
faster; use very fast snap!
What are the mixing options for elastomeric impression materials?
Mixing Options:
• 2 Pastes on a mixing pad.
• 2 Pastes in mixing gun (Kenics mixing tip).
• 2 Pastes in mixing machine (Kenics mixing tip).
A _____ material is one in which viscosity increases with the rate of shear strain.
dilatant (also termed shear thickening)
- mix pad
_____ is an effect where a fluid’s viscosity—the measure of a fluid’s resistance to flow—decreases with an increasing rate of shear stress.
Shear thinning (pseudoplastic)
- auto-mix gun or machine
what are the general properties of polysulfide IM?
aka rubber base, mercaptan rubber; works best in custom tray; not very good w/ pts (smelly, bad taste, stains clothes)
Examples of shear thinning IM’s?
polyether, polyvinylsiloxane
Examples of shear thickening IM’s?
silicone rubber, polysulfide rubber
what are the general properties of silicone rubber IM?
- Limited shelf-life: Unstable in tubes.
- Requires mechanical retention or special tray adhesives.
- No syneresis or imbibition, but continued polymerization shrinkage.
- Better dimensional stability than reversible hydrocolloid but more expensive.
- Pouring of models:
- More flexible so more chance for distortion during removal.
- Wait 20-30 minutes before pour for stress relaxation to occur.
For polyether rubber, what is involved in the continuous phase & dispersed phase?
CONTINUOUS PHASE:
• Polymer: Amine-terminated polyether.
• Crosslinking agent in the catalyst: Aromatic sulfonate.
• Modifiers: Colorants, glycol plasticizers, flavorants.
DISPERSED PHASE:
• Fillers: Silica.
Why is silica put in as a filler during the dispersed phase?
to improve stability of the iM
What is the setting rxn for polyether rubber?
SETTING REACTION: Stepwise (relatively slow); exothermic; affected by temperature
what are the general properties of silicone rubber IM?
- Excellent impression accuracy and dimensional stability.
- Stiff and therefore difficult to remove without rocking.
- Break seal and rock slightly to prevent tearing: Low tear resistance.
- Negatively affected by H2O, saliva, and blood.
- Since hydrophobic, moisture increases marginal discrepancy.
- Increased water absorption occurs if use thinning agents.
- Can be dispensed from automated extruder and mixer (ESPE PentaMix).
For polyvinyl siloxane, what is involved in the continuous phase & dispersed phase?
CONTINUOUS PHASE:
• Polymer: Double-bond-functional silicone polymer.
• Crosslinking agent and catalyst: Chloroplatinic acid.
• Modifiers: Colorants, flavorants, plasticizers.
DISPERSED PHASE: Fillers: Silica.
What is the setting rxn for polyvinyl siloxane?
SETTING REACTION:
Chain (very fast), exothermic; affected by temperature; hydrogen gas released by decomposition of crosslinking agent.
What is good about polyvinyl siloxane’s setting process?
The reaction is essentially complete at the time of removal = allows for no distortion (or
very limited)
what are the general properties of polyvinyl siloxane IM?
- BEST impression material for dimensional stability:
- Pouring should be delayed for H2 out-gassing.
- Pouring can be delayed up to7-to-10days (or indefinitely).
- Stiffness makes removal difficult.
- Most material dispensed using auto-mixing gun and mixingtips.
Accuracy of IM in order of good to poor for wetting tissues?
Wetting of tissues: RHC, PVS => PE > PS, Sil > ALG
Accuracy of IM in order of good to poor for wetting stone?
RHC, ALG => PE > PS > Sil, PVS
Dimensional stability of IM in order of good to poor? (involves polymerization shrinkage, loss of components, thermal shrinkage, stress relaxation)
PE, PVS >PS, Sil >RHC >ALG
Tear resistance of IM in order of good to poor for elastic deformation & strain rate sensitivity?
PS>Sil,PVS >PE,RHC,ALG
Remember: PVS’s qualities as far as accuracy, dimensional stability, tear resistance.
PVS = 2nd from top in accuracy PVS = 2nd from top in dimensional stability PVS = was 2nd/3rd spot from top of list in tear resistance.
*PVS = intermediate in all 3 categories; best trait = tear resistance
