Elastomers Flashcards
how are elastomers formed
by polymerisation with cross linking of polymer chains
what does cross linking do
generates elastic properties causing a fluid to solid transition
main issue with polymerisation
may produce by-products e.g. H2O, H2, alcohol which affect dimensional stability and cast compatibility
main elastomers we use
- silicones - addition cured silicones such as polyvinylsiloxanes
- polyethers
factors affecting the accuracy by which surface features are recorded
- surface detail (reproduction)
- flow / viscosity
- contact angle / wettability
factors affecting the accuracy of dimensions and shape of final impression
- elastic recovery (%)
- stiffness (flexibility)
- tear strength
practical considerations of elastomer material
- mixing time
- working time
6 things to assess in elastomers
- 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
IM ideal properties for quality surface interaction between material & tooth / soft tissues
- 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
IM ideal properties for accuracy
- surface reproduction
- viscoelasticity / elastic recovery
IM ideal properties for dealing with undercuts
- flow under pressure i.e. shark fin test
- tear strength
- rigidity
IM ideal properties for dimensional stability
- 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
wettability & contact angle
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
why do hydrophilic silicones incorporate non-ionic surfactant
wets tooth surface
more easily wetted by water containing dye materials
surface reproduction ISO4823
standard notch dimensions are 20um, 50um & 75um
50um = normal
IM that reproduces at 20um will give you the best surface detail reproduction
describe ideal elastic behaviour
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
describe viscoelastic behaviour
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
how does tray removal influence viscoelastic behaviour
if load time is less - impression removed with a sharp pull there is less overall permanent strain and so lower deformation
describe the shark fin test (flow under pressure)
high flow - will flow readily into undercuts & sulcus giving a large fin length
low flow - short fin length
what is tear strength
stress a material will withstand before fracturing - want IM to have a large tear strength
what is rigidity and how does it affect IM
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
eg of a polyether IM
impregum
eg of an addition silicone IM
aquasil ultra
virtual
flexitime
all monophase
compare setting time & working times of polyethers and addition silicones
polyethers have a quicker setting time & half the working time of addition silicones
key properties for IMs
viscosity = low
contact angle = low
wettability = high
viscoelasticity = low
stiffness = low
TEC = low
polymerisation shrinkage = low
tear strength = high
