Elastomers
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what are the 2 types of elastomer
polyether addition silicones
how are elastomers formed
polymerisation with cross linking polymer chains
what does cross linking cause
generation of elastic properties fluid to solid transition
what do the byproducts of polymerisation affect
dimensional stability and cast compatibility
how do you assess the material properties of an elastomer
surface detail flow/viscosity contact angle elastic recovery stiffness tear strength mixing time working time shore A hardness shark fin test setting shrinkage dimensional stability thermal expansion coefficient biocompatability
what are the ideal properties of an impression material
quality of surface interaction between material and tooth/soft tissues accuracy dealing with removal and undercuts dimensional stability
what should viscosity of elastomers be
able to flow readily
what should surface wetting be
make intimate contact
what is a contact angle
determines how well material envelopes the hard/soft tissue surface
what is viscosity
ability of materials ability to flow and how well it records surface detial
what does a large contact angle result in
spaces between globules of impression material so some of tooth surface not replicated
what type of contact angle do we want
small contact angle
what do hydrophilic silicones do
wet tooth surface
what are the standard notch dimensions for reproduction of surface detail
20um 50um 75um
what is visco elastic recovery
impression material will have some permanent strain once load is removed
what is the ideal behaviour for elasticity
100% elastic recovery and no permanent strain
how do you decrease permanent strain
make load time less and remove with sharp pull
what is the shark fin test
flow under pressure
what does a large fin mean
will flow readily into sulcus/undercuts
what is tear strength
stress material will withstand before fracturing
what should rigidity be for an impression material
low for ease of removal of material from undercut regions