elastomeric impression materials Flashcards
chemistry
polymerisation - cross-linking of polymer chains
- generates elastic properties
- fluid - solid transition
may produce by-products
- H2O, H2, alcohol
- affect dimensional stability and cast compatibility
elastomers
polysulphides silicones - addition curing - condensation curing (conventional) polyethers
conventional silicone e.g. Verone chemistry - base paste
silicone prepolymer with terminal hydroxyl groups
filler
conventional silicone e.g. Verone chemistry - catalyst paste (or liquid)
cross-linking agents (alkoxyorthosilicate or organohydrogen siloxane)
activator - organo-tin compound
conventional silicone e.g. Verone chemistry - different setting reactions
different formulations have different setting reaction depending on cross-linking agents
- alkoxyl-orthosilicate
- organohydrogen siloxane
conventional silicone e.g. Verone chemistry - setting reactions
silicone polymers + organohydrogensiloxane - cross-linked polymer + H2
silicone polymers + alkoxyorthosilicate - cross-linked polymer + alcohol
addition cured silicone e.g. PVS e.g. extrude chemistry
base paste and catalyst paste
no byproducts
hydrophobic
addition cured silicone e.g. PVS e.g. extrude chemistry - base paste
polydimethylsiloxane - some methyl groups replaced by hydrogen
filler - variations change viscosity
addition cured silicone e.g. PVS e.g. extrude chemistry - catalyst paste
polydimethylsiloxane - some methyl groups replaced by vinyl
filler - variations change viscosity
platinum catalyst e.g. chloroplatinic acid
hydrophillic silicones
incorporate non-ionic surfactant
- wets tooth surface
- more easily wetted by water-containing die materials
polyethers e.g. Impregum chemistry
base paste
catalyst paste
polyether + sulphonate ester = cross-linked material
polyethers e.g. Impregum chemistry - base paste
imine terminated pre-polymer - cross-linking
inert filler - viscosity, strength
polyethers e.g. Impregum chemistry - catalyst paste
ester derivative of aromatic sulphonic acid - initiates polymerisation
inert oils - form paste
inert fillers - form paste
polyethers setting reactions
activation
initiation
propagation
material properties
flow/viscosity surface detail (reproduction) contact angle/wettability elastic recovery (%) stiffness (flexibility) tear strength mixing time (min) working time (min) Shore A hardness thermal contraction Shark fin test (flow under pressure) setting shrinkage dimensional stability thermal expansion coefficient biocompatibility
ideal properties
quality of surface interaction between material and tooth/soft tissue surfaces
accuracy
dealing with removal and undercuts
dimensional stability
ideal properties - quality of surface interaction between material and tooth/soft tissue surfaces
viscosity
surface wetting
contact angle
ideal properties - accuracy
surface reproduction (ISO) viscoelasticity/elastic recovery
ideal properties - dealing with removal and undercuts
rigidity
flow under pressure “shark fin test” - flows readily under pressure - large shark fin length
tear/tensile strength
- removing from undercuts
- can have a good shark fin length but if tears easily then not good when you come to remove it
- not too high - if in deep undercut might not be able to remove it
ideal properties - dimensional stability
setting shrinkage
thermal expansion/contraction
- contraction when you remove it from warmer oral cavity to cooler room temp
viscosity
measure of a materials ability to flow
determines a material’s potential for making close contact with surfaces
how well it records surface detail
contact angle
determines how well material envelop the hard/soft tissue surface (to record fine detail)
surface wetting
must make intimate contact with teeth/mucosa
is material hydrophillic?
small contact angle
no spaces between globules of impression material so all of surface is replicated
large contact angle
spaces between globules of impression material, so some of tooth surface not replicated
reproduction of surface detail - ISO
standard notch dimensions
A - 20um
B - 50um ISO norm
C - 75um
elasticity ideal behaviour
no permanent deformation - once load (strain) removed material back to original dimensions
flow under pressure - shark fin test
force - impression material - chamber with slot
high flow = large fin length (will flow rapidly into sulcus, undercuts)
low flow = short fin length
tear/tensile strength
stress material will withstand before fracturing
rigidity
stress/strain ratio i.e. stress needed to cause material to change shape
ideally low value i.e. flexible - for ease of removal of material, esp from undercut regions
viscoelastic behaviour
occurs when after being stretched/compressed a material fails to return to its original dimensions/shape
i.e. there is a permanent deformation
minimising permanent deformation with viscoelastic behaviour
remove tray quickly with a sharp pull
- if load time is less there is less overall permanent strain (lower deformation)
leave before pouring - recovery takes time
elasticity only develops after material is firm - don’t remove tray too soon
setting shrinkage
low
thermal expansion/contraction
a large difference between mouth and room temp may cause a change in shape
ppm/degrees should be low
storage
some materials absorb/release moisture causing a change in its dimensions
extrude
PVS addition cured
impregum
polyether
aquasil
addition silicone
virtual
addition silicone
flexitime
addition silicone
best elastic recovery
virtual
best tear strength
virtual
viscosity depends on
molecular weight of polymer
additives e.g. fillers
cross-linking
binding chains to form a 3D network
