Elastomers Flashcards
What are the ideal properties of elastomers?
Low viscosity
Low contact angle
High wettability
Low viscoelasticity
Low stiffness
Low thermal expansion
Low polymerisation shrinkage
High tear resistance (not too high)
What are the different types of elastomers?
Polyether
Addition silicones
How are elastomers formed?
Polymerisation with cross linking of polymer chains
Cross linking generates elastic properties and causes fluid to solid transition
May produce byproducts which affect dimensional stability
What are the different types of elastomers?
Polysulphides
Silicones - addition curing and condensation curing
Polyethers
How are elastomers packaged?
2 cartridges, 1 base and 1 catalyst
Describe the ideal viscosity of elastomers
Determines the potential for making close contact with hard/soft tissue surfaces - how well it records surface detail
Ideally low
Describe what a high contact angle means
Results in spaces between globules of impression material, so some of tooth surface isn’t replicated
Describe what a low contact angle means
No spaces between globules of impression material so all of surface is replicated
What is used to judge reproduction of surface detail?
ISO 4823
Describe ISO 4823
Place an IM on surface with different sized grooves
Apply uniform pressure along IM
See which grooves are filled by the IM
How is elastic recovery dependent on clinical technique?
If load time is less and impression is removed with a sharp pull, there is less overall permanent strain
Describe viscoelastic behaviour
After material is stretched or compressed, a material fails to return to its original dimensions and there is permanent deformation
Why should you wait even when an IM is firm to touch to remove the impression?
The IM is still developing elasticity
What is the Shark Fin test?
An objective measure of flow under pressure
Describe the Shark Fin test
Cylindrical chamber with a slot of certified depth
IM inserted in upper part of cylindrical chamber - and has to have a depth greater than that of the slot
Then it is forced downwards as pressure is applied
A large fin length means high flow - will flow readily into sulcus, undercuts
