Elastomers Flashcards
1
Q
What are the two types of elastomers?
A
- Polyether
- Addition silicones
2
Q
What is meant by elastic behaviour for impression materials?
A
- 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
3
Q
How are elastomers formed?
A
- 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
4
Q
What does the cross-linking generate?
A
- Elastic properties
- Causes fluid to solid transition
5
Q
What 3 mechanical properties of elastomers affect the accuracy by which the surface features are recorded?
A
- Surface details (reproduction)
- Flow/viscosity
- Contact angle/ wettability
6
Q
What 3 mechanical properties affect accuracy of the dimensions and shape of final impression?
A
- Elastic recovery %
- Stiffness (flexibility)
- Tear strength
7
Q
What is Shore A hardness?
A
- Specific hardness test for IM
8
Q
What is the shark fin test?
A
- Relates to ability of IM to deal with undercuts
9
Q
What other mechanical properties are important for Elastomers?
A
- Shore A hardness
- Shark fin test (flow under P)
- Setting shrinkage
- Dimensional stability
- Thermal expansion coefficient
- Biocompatibility
10
Q
What is VIRTUAL?
A
- An addition polyvinylsiloxane
- Made by IVPCLAR-VIVADENT
11
Q
What are the important properties of VIRTUAL?
A
- 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
12
Q
What is Viscosity?
A
- 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
13
Q
What does the contact angle indicate?
A
- 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
14
Q
What are hydrophilic silicones?
A
- Non ionic surfactant
- Incorporated into addition silicones as they have difficulty making good contact with moist tooth surface
- They wet tooth surface
15
Q
What is the ISO 4823?
A
- 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
