Impression Materials Flashcards
What can IMs be split into?
Elastic and Rigid (non-elastic)
Give examples of rigid IMs
Impression plaster
Impression compound
Zinc oxide eugenol pastes
What are the different types of elastic IMs
Synthetic elastomers and hydrocolloids
Give examples of synthetic elastomers
Polysulphide
Polyethers
Silicones - addition cured or condensation cured
Give examples of hydrocolloids
Reversible (agar)
Irreversible (alginate)
Describe the ideal behaviour of an IM
At time=0, no strain
Load applied uniformly
Load removed
Complete elastic recovery as strain goes back to 0
Describe the actual behaviour of an IM
Load is applied over a longer period of time Material gradually recovers Does not completely recover Permanent deformation occurs Material is viscoelastic
How can permanent deformation be reduced?
By removing the impression tray with a sharp pull
What are the ideal properties of an IM?
Flow - low viscosity No dimension change upon setting Low thermal expansion High tear strength Complete elastic recovery No dimensional change during storage Non toxic, non irritant Acceptable smell and taste
What are the properties of alginate?
Low viscosity - flow No dimension change upon setting Complete elastic recovery upon removal - not 100% Dimensional change during storage Non toxic, non irritant Acceptable smell and taste
How does alginate undergo dimensional change upon storage?
Syneresis - the release of water
Imbibition - the uptake of water
What is viscoelastic behaviour?
When IM is stretched/compressed on removal there is strain
It fails to return to its original dimensions and permanent deformation occurs
What are the advantages of elastomers over alginate?
High tear strength - less likely to distort on removal
High elastic recovery - more accurate
