What are different types of elastomer?
- polyether
- addition silicone
Describe elastic behaviour.
A material that can undergo elastic strain and elastic recovery
How are elastomers formed?
Polymerisation with cross-linking of polymer chains
What effect does cross-linking have on the elastomer material?
- creates elastic properties
- causes a fluid to solid transition
How do the byproducts produced by polymerisation affect the material?
- water, hydrogen or alcohol
- may affect dimensional stability or cast compatibility
Describe the ideal viscosity of an elastomer.
Must be able to flow readily
Describe the ideal surface wetting of an elastomer.
High - must make intimate contact with teeth and mucosa
Describe the ideal contact angle of an elastomer.
Low contact angle required to envelop tissues and record fine detail
What are hydrophilic silicones?
Incorporate non-ionic surfactant to better wet tooth surface
Describe the ideal accuracy of an elastomer.
- ISO 4832
- ISO norm is 50um
- excellent reproduction of surface detail
Describe the ideal elasticity of an elastomer.
- 100% elastic recovery is ideal
- results in no permanent strain and no distortion of impression
What is visco-elastic recovery?
When there is permanent strain, is influenced by tray removal
What is the shark fin test?
Measures flow under pressure, relevant to removal and undercuts
Describe the ideal flow under pressure of an elastomer.
- long shark fin test is ideal
- ensures material will flow readily into undercuts
Describe the ideal tensile strength of an elastomer.
High tensile stress is required to ensure material doesn’t fracture during removal
Describe the ideal rigidity of an elastomer.
Low rigidity required to allow removal of material from undercuts
Describe the ideal dimensional stability of an elastomer.
- low setting shrinkage
- low thermal expansion/contraction to prevent change in shape when removing from mouth to room temperature
What is an example of an polyether?
Impregum
What is an example of an addition silicone?
- aquasil ultra
- virtual
- flexitime
Describe the difference in setting times between polyethers and addition silicones.
Polyethers set quicker
Describe the difference in working times between polyethers and addition silicones.
Polyethers have shorter working time
