Elastomers Flashcards Preview

BDS3 Dental Materials Science > Elastomers > Flashcards

Flashcards in Elastomers Deck (25)
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1
Q

how are elastomers formed

A

by polymerisation with cross linking of polymer chains

2
Q

what does cross linking do

A

generates elastic properties causing a fluid to solid transition

3
Q

main issue with polymerisation

A

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

4
Q

main elastomers we use

A
  1. silicones - addition cured silicones such as polyvinylsiloxanes
  2. polyethers
5
Q

factors affecting the accuracy by which surface features are recorded

A
  1. surface detail (reproduction)
  2. flow / viscosity
  3. contact angle / wettability
6
Q

factors affecting the accuracy of dimensions and shape of final impression

A
  1. elastic recovery (%)
  2. stiffness (flexibility)
  3. tear strength
7
Q

practical considerations of elastomer material

A
  1. mixing time
  2. working time
8
Q

6 things to assess in elastomers

A
  1. shore A hardness - specific hardness test of IM
  2. shark fin test - flow under pressure, relates to ability to deal with undercuts
  3. setting shrinkage - fundamental
  4. dimensional stability - critical for producing accurate positive replica
  5. TEC - important as we cannot eliminate temp gradient between mouth & room temp
  6. biocompatibility - patient safety
9
Q

IM ideal properties for quality surface interaction between material & tooth / soft tissues

A
  1. viscosity - must be able to flow readily to record surface detail; range is low, medium, high
  2. surface wetting - must make intimate contact with teeth / mucosa
  3. contact angle- determines how material envelopes hard / soft tissue to record fine detail
10
Q

IM ideal properties for accuracy

A
  1. surface reproduction
  2. viscoelasticity / elastic recovery
11
Q

IM ideal properties for dealing with undercuts

A
  1. flow under pressure i.e. shark fin test
  2. tear strength
  3. rigidity
12
Q

IM ideal properties for dimensional stability

A
  1. setting shrinkage - should be low
  2. thermal expansion coefficient - should be low due to large difference in room temp and mouth temp
  3. storage - can absorb / release moisture which may change its dimensions; this doesn’t affect polyethers or addition silicones
13
Q

wettability & contact angle

A

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

14
Q

why do hydrophilic silicones incorporate non-ionic surfactant

A

wets tooth surface
more easily wetted by water containing dye materials

15
Q

surface reproduction ISO4823

A

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

16
Q

describe ideal elastic behaviour

A

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

17
Q

describe viscoelastic behaviour

A

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

18
Q

how does tray removal influence viscoelastic behaviour

A

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

19
Q

describe the shark fin test (flow under pressure)

A

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

20
Q

what is tear strength

A

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

21
Q

what is rigidity and how does it affect IM

A

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

22
Q

eg of a polyether IM

A

impregum

23
Q

eg of an addition silicone IM

A

aquasil ultra
virtual
flexitime
all monophase

24
Q

compare setting time & working times of polyethers and addition silicones

A

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

25
Q

key properties for IMs

A

viscosity = low
contact angle = low
wettability = high
viscoelasticity = low
stiffness = low
TEC = low
polymerisation shrinkage = low
tear strength = high