Elastomers

by polymerisation with cross linking of polymer chains

generates elastic properties causing a fluid to solid transition

may produce by-products e.g. H2O, H2, alcohol which affect dimensional stability and cast compatibility

• silicones - addition cured silicones such as polyvinylsiloxanes • polyethers

• surface detail (reproduction) • flow / viscosity • contact angle / wettability

• elastic recovery (%) • stiffness (flexibility) • tear strength

• mixing time • working time

• shore A hardness - specific hardness test of IM • shark fin test - flow under pressure, relates to ability to deal with undercuts • setting shrinkage - fundamental • dimensional stability - critical for producing accurate positive replica • TEC - important as we cannot eliminate temp gradient between mouth & room temp • biocompatibility - patient safety

• viscosity - must be able to flow readily to record surface detail; range is low, medium, high • surface wetting - must make intimate contact with teeth / mucosa • contact angle- determines how material envelopes hard / soft tissue to record fine detail

• surface reproduction • viscoelasticity / elastic recovery

• flow under pressure i.e. shark fin test • tear strength • rigidity

• setting shrinkage - should be low • thermal expansion coefficient - should be low due to large difference in room temp and mouth temp • storage - can absorb / release moisture which may change its dimensions; this doesn’t affect polyethers or addition silicones

contact angle indicated how readily the IM ‘wets’ the tooth surface large contact angle - spaces between globules of IM so some of tooth surface not replicated small contact angle - no spaces between globules of IM so all of surface is replicated

wets tooth surface more easily wetted by water containing dye materials

standard notch dimensions are 20um, 50um & 75um 50um = normal IM that reproduces at 20um will give you the best surface detail reproduction

load applied at T=0 as when removing impression tray & material stretches instantly to strain required this is maintained until load is removed & IM returns instantly to original dimensions i.e. 100% elastic recovery & no permanent strain

no IM is truly elastic so will exhibit viscoelasticity - load applied and will gradually reach strain required and when load is released its strain level gradually drops but does not return to original dimension - experiences permanent deformation

if load time is less - impression removed with a sharp pull there is less overall permanent strain and so lower deformation

high flow - will flow readily into undercuts & sulcus giving a large fin length low flow - short fin length

stress a material will withstand before fracturing - want IM to have a large tear strength

stress : strain ratio i.e. stress needed to cause material to change shape ideally a low value i.e. flexible for ease of removal of tissue especially from undercut regions

impregum

aquasil ultra virtual flexitime all monophase

polyethers have a quicker setting time & half the working time of addition silicones