Elastomers Flashcards
What type of forces are the macromolecules in elastomers held by?
VDW
What is elastic deformation per unit of applied stress
When Individual polymer chains can straighten or slip through planes when external stress is applied
What is ISO 4823
Dictates consistency 0= very high (putty 1= high consistency (heavy body) 2= medium consistency 3= low consistency (light body)
Name 4 properties of the light body
- Least dimensionally stable
- Least viscous
- most setting contraction (so shrinks on cooling)
- Most thermal contraction
Ingredients in condensation silicone and addition
Condensation= Hydroxy terminated polydimethyl siloxane
Addition= vinyl terminated polydimethyl siloxane
The other ingredients in conesnation silicone
Silica= Body
Alkyl Silicate= Cross linker liquid
Tin= Catalyst
Cross linking between alkyl silicate and polydimethyl siloxane
What is lost in the condensation reaction
Loss of 2 ethyl molecules
in ADDition polymers what does the polydimethyl siloxane react with?
Silicone with a hydride group
Why is hydrogen needed in addition polymer
For cross-linking
Why shouldnt rubber gloves be used with addition silicones
As they can poison the Pt catalyst
6 positive properties of addition silicone
- Accurate detailed impression
- Dimensionally stable
- Available in different viscosity
- good tear strength
- Sets quickly
- Lack of unpleasant odour/taste
4 disadvantages od addition silicone
- Pryoplastic as flow properties can differ under different stress
- Too accurate sometimes which means cast may not be oversized enough
- Hydrogen released can cause surface pitting
- Hard to remove the impression
3 negatives of condesnation silicone
- Loss of alcohol can cause shrinkage of the impression, so needs to be cast quickly
- Hydrophobic material so we need a dry mouth and the fact it is hydrophobic causes blow holes
- Alkyl silicate can cause irritation
4 positives of condenasation silicone
-Good elastic recovery (so wont really fracture)
-Good tear resistance
-Sets quickly
Accurate on severe undercuts
2 reasons why a addition silcone better than condensation
- Addition= better accuracy with stock tray
- Addition- =Better dimensional stability (doesnt need be cast as quickly)
