Elastomers Flashcards
What are the two types of elastomers?
- Polyether
- Addition silicones
What is meant by elastic behaviour for impression materials?
- When removing IM from contact with tooth after it has set
- IM has to stretch and flares out at sides to overcome bulbous aspect of tooth
- Ideally it recovers to original dimension, replicating shape of tooth
- Assumes perfectly elastic
How are elastomers formed?
- Formed by polymerisation with cross-linking of polymer chains
- Polymerisation may produce by-products like water or hydrogen which affect dimensional stability and cast compatibility
What does the cross-linking generate?
- Elastic properties
- Causes fluid to solid transition
What 3 mechanical properties of elastomers affect the accuracy by which the surface features are recorded?
- Surface details (reproduction)
- Flow/viscosity
- Contact angle/ wettability
What 3 mechanical properties affect accuracy of the dimensions and shape of final impression?
- Elastic recovery %
- Stiffness (flexibility)
- Tear strength
What is Shore A hardness?
- Specific hardness test for IM
What is the shark fin test?
- Relates to ability of IM to deal with undercuts
What other mechanical properties are important for Elastomers?
- Shore A hardness
- Shark fin test (flow under P)
- Setting shrinkage
- Dimensional stability
- Thermal expansion coefficient
- Biocompatibility
What is VIRTUAL?
- An addition polyvinylsiloxane
- Made by IVPCLAR-VIVADENT
What are the important properties of VIRTUAL?
- Has good hydrophillic properties during impression taking ( good for fine detail of oral hard and soft tissues)
- Good compatibility with moist dentin and gingival tissue
- Exhibit very low contact angles with excellent wettability
- Excellent moisture displacement
- Colour contrast gives margins that are easy to read
- Precise impressions
- High tear strength
- Hardness adjusted to clinical needs
- Good elastic recovery
What is Viscosity?
- Measure of materials ability to flow
- Determines a materials potential for making close contact with hard/soft tissue surface so how well it records fine detail
- Range can be low, medium or high
What does the contact angle indicate?
- How readily ‘wet’ the tooth surface is and how closely the IM envelops the tooth surface
- Low contact angle means large % of its vol make contact with target surface = no space between globules of IM = ideal
- Large contact angle = spaces between globules of IM = some surface not replicated
What are hydrophilic silicones?
- Non ionic surfactant
- Incorporated into addition silicones as they have difficulty making good contact with moist tooth surface
- They wet tooth surface
What is the ISO 4823?
- International standard measure for surface reproduction of IM
- Measures how accurately surface is represented
- Test involves placing IM along surface which has grooves of 20, 50 and 75um
- Uniform pressure applied across width of IM
- An IM that record the 20um grooves gives most accurate surface detail
Are Impression material truly elastic?
- No they are all Visco-elastic to some extent
- When load applied IM will gradually reach strain required
- When load released strain gradually drops
- Does not return to original dimension and experiences permanent deformation (aka permanent strain)
What is definition of Viscoelastic behaviour?
- Occurs when, after being stretched or compressed, a material fails to return to its original dimensions/shape and there is permanent deformation
How is viscoelastic behaviour influenced by tray removal method?
- If load time is less and impression tray removed with sharp pull then less overall permanent strain (lower deformation)
When does the IM elasticity begin to develop?
- IM ability to flow diminishes
- Only when setting reaction progressed to certain extent does materials elasticity begin to develop fully
- Even when IM firm to touch it is still developing elasticity
- Wait a few mins before removing it
What is the shark fin test?
- Used to record an undercut
- Undercut can only be recorded when IM reaches the narrow zone (between the gingivae and tooth surface)
- IM inserted into cylindrical chamber that has slot of specified depth and forced down (pressure applied)
What does large fin length and short fin length mean on shark fin test?
- Large fin length means high flow and will flow readily into sulcus and undercuts
- Short fin length has low flow
What is the tear strength?
- Stress material will withstand before fracturing
What is rigidity?
- stress/strain ratio
- Stress needed to cause material to change shape
What is an example of Polyether elastomer?
- Impregum Penta soft (medium)
What are some examples of addition silicone elastomer?
Aquasil Ultra (monophase)
VIRTUAL (monophase)
Flexitime (monophase)
How to use the best decision-making approach when choosing IM?
- Know KEY material properties
- be prepared for new terminology, & sales pitch. - Review product specification data
– cf best rival products? - Know typical values expected for specific properties
- Identify properties NOT mentioned
- REJECT claims NOT supported with scientific/clinical data
What are the key properties summary?
Viscosity - Flow across surface - Low
Contact angle (wettability) - Engagement with tooth surface/surface quality - Low (high)
Viscoelasticity - Deformation on removal - Low
Stiffness - Ease of removal from undercut - Low
Thermal expansion - contraction - mouth room temp - Low
Polymerisation shrinkage - Contraction during setting - Low
Tear resistance - ability to withstand large stresses during removal - High but not too high
