Elastomers

• Polyether • Addition silicones

• 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

• 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

• Elastic properties • Causes fluid to solid transition

• Surface details (reproduction) • Flow/viscosity • Contact angle/ wettability

• Elastic recovery % • Stiffness (flexibility) • Tear strength

• Specific hardness test for IM

• Relates to ability of IM to deal with undercuts

• Shore A hardness • Shark fin test (flow under P) • Setting shrinkage • Dimensional stability • Thermal expansion coefficient • Biocompatibility

• An addition polyvinylsiloxane • Made by IVPCLAR-VIVADENT

• 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

• 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

• 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

• Non ionic surfactant • Incorporated into addition silicones as they have difficulty making good contact with moist tooth surface • They wet tooth surface

• 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

• 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)

• Occurs when, after being stretched or compressed, a material fails to return to its original dimensions/shape and there is permanent deformation

• If load time is less and impression tray removed with sharp pull then less overall permanent strain (lower deformation)

• 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

• 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)

• Large fin length means high flow and will flow readily into sulcus and undercuts • Short fin length has low flow

• Stress material will withstand before fracturing

• stress/strain ratio • Stress needed to cause material to change shape

• Impregum Penta soft (medium)