Elastomeric impression materials (elastomers) Flashcards
What is an elastomer?
A polymer with elastic (rubber-like) properties
Elastomers can be natural or synthetic - which are only used in dentistry?
Synthetic
In what form do elastomeric impression materials come? Explain
Liquid polymers (pre-polymers (short chains thin grow via polymerisation or cross linking)
Elastomers often have repeating units of Si-O or C-O
Why is this better than C=C?
More flexible
Why is filler added to liquid polymers?
Make a paste
What property of the liquid polymer makes the elastomer?
Crosslinking
Why were elastomers developed in terms of addressing problems with alienates?
Alginates not accurate enough for crown and bridge work
Large dimensional changes
poor tear resistance - problems in thin sections
Name 2 useful elastomers?
Silicones
Polyether
By what reaction are condensation silicone elastomers formed?
Condensation reaction
Describe the condensation reaction to form silicones?
a) reactors
b) catalyst
c) setting reaction
d) initial and final characteristic
e) by-product
a) silicone prepolymers and silica to from paste
b) tin
c) chain extension and cross linking
d) fluid to elastic
e) alcohol
Significance of alcohol by product in silicone setting reaction?
Significant dimensional changes over time - pour cast as soon as possible
Why are addition silicones used more than condensation silicones?
Less dimensional changes
What are the 3 paste systems that addition silicones come in?
Hand mixed
Cartridge - expensive, more waste
Mechanical mixers - even more expensive and more waste
Advantages of addition silicones?
Addition reaction = no-by product = no dimensional changes
Why wait a certain amount of time before pouring the impression of addition silicones?
hydrogen may be evolved - form pits
How is the viscosity of addition silicones controlled?
Amount of filler
Makes ultra-light, light, regular, heavy and putty
Why is it necessary to have different viscosities of silicones?
Use a mix of different viscosities to get high level of accuracy e.g. place putty into impression tray and syringe low viscosity onto teeth
Why not use light silicone at all times?
Light body = low viscosity = finest detail but highest shrinkage (less filler)
Heavy vs light body silicone?
Heavy = less shrinkage on setting, less thermal contraction, more stable, easier to handle, no special tray (acts like tray for light body)
Physical properties of addition silicones?
Good dimensional stability
elastic when set
tear resistance
Problems with addition silicones?
Hydrophobic - don’t interact well with moist tissues = impression faults
Need a dry field (surface treatment)
Polymerisation of polyether?
Addition polymerisation
No by product
Chain extension and cross linking to make it elastic
Polyether properties
a) setting characteristics (WT and ST)
b) properties when set
c) properties before setting
d) dimensional stability
a) enough WT, short ST
b) rigid - difficult to remove from mouth - can remove mobile teeth
c) fluid and pseudoplastic (good detail), hydrophilic
d) good when dry but can absorb water and swell - model made quickly so not a problem
What can elastomers be used for?
undercuts e.g. crowns, bridges
Addition silicones vs polyethers uses?
Addition = very popular and material of choice but hydrophobicity is a problem - if you are careful e.g. dry field - this is not a problem
Polyethers - can be used for implants as hydrophilic
