Elastomers

• An elastomer is a polymer that displays rubber-like properties. It can be a natural polymer (i.e. rubber) or synthetic. Only synthetic polymers are used as elastomeric impression materials.
• Elastomeric impression materials:

  • Liquid polymers
  • Often come as "pre-polymers"
  • Short polymer chains that are not terminated
  • Polymerisation occurs by the pre-polymer chains growing (chain extension) or crosslinking
  • Flexible repeating units
  • Si-O-Si-O-... etc
  • C-O-C-O-... etc
  • Add filler to make paste - filler concentration is related to viscosity
  • Cross-linking converts to elastomer
• Elastomers were developed to address problems with alginates:

  • Accuracy - not sufficient for crown and bridge work
  • Dimensional changes - problems due to syneresis
  • Poor tear resistance - problems in areas with thin sections
• Silicones - condensation reaction:

  • Available as 2 components system: paste/paste or paste/liquid
  • Condensation reaction:
  • Silicone pre-polymer chains used
  • Silica added to make a paste
  • Catalysed by a tin compound
  • Setting reaction: chain extension and cross-linking
  • Initially fluid -> became elastic (rubbery)
  • Alcohol formed as by-product
• Silicones - dimensional stability:

  • Better than hydrocolloids and polysulphides
  • Loss of alcohol (reaction by-product)
  • Significant dimensional change over time
  • Pour cast as soon as possible
• Addition silicones - polyvinylsiloxanes:

  • Developed to improve on condensation silicones
  • Supplied as 2 pastes
  • Hand-mixed (colour contrast between pastes)
  • Cartridge mix - more expensive equipment, more waste
  • Mechanical mixers - even more expensive, more waste
  • Paste 1:
  • Liquid silicone polymer with Si-H groups
  • Filler
  • Paste 2:
  • Liquid silicone polymer with Si-CH=CH₂ groups
  • Filler
  • Chloroplatinic acid (platinum catalyst)
  • Addition reaction
  • No by-product
  • BUT hydrogen may be evolved - wait to pour model
• Polyvinylsiloxanes - viscosities:

  • Amount of filler related to viscosity
  • Available in many viscosities
  • Ultra-light, light, regular, heavy, putty
  • For putty only hand-mixing is possible
  • Use of different viscosities:
• Polyvinylsiloxanes - viscosities:

  • Amount of filler related to viscosity
  • Available in many viscosities
  • Ultra-light, light, regular, heavy, putty
  • For putty only hand-mixing is possible
  • Use of different viscosities:
  • General work - use single viscosity - regular
  • Where high accuracy is needed
  • Use 2 viscosities - putty + light
  • Place putty into impression tray
  • Syringe low viscosity material onto teeth
• Why we don't just use light-bodied silicone all the time:

  • Light body - lowest viscosity so produces finest detail

  BUT

  • Light body shrinks the most during polymerisation
  • Remember from composites: more filler = lower shrinkage

  So...

  • Heavy body or putty:
  • Less shrinkage on setting
  • Less thermal contraction
  • More stable
  • Easier to handle in bulk
  • No special tray (acts like a tray for light body)
• Polyvinylsiloxanes - properties:

  • Physical properties
  • Good dimensional stability
  • Elastic (when set!)
  • Resist tearing
  • Accuracy and fine detail
  • Small setting contraction - combination of viscosity
  • Hydrophobic
  • Doesn't wet moist tissue - can lead to impression faults
  • Needs a dry field - surface treatment sprays available
  • "Hydrophilic" materials available - beware not much improvement
  • Care in pouring cast
  • Catalyst problems - can prevent polymerisation
  • Latex gloves "pollute" catalyst
  • Freshly placed methacrylates "pollute" catalyst
• Polyether:

  • Available as 2-paste systems
  • Hand mix
  • Cartridge mix
  • Pentamix
  • Viscosity
  • Monophase - regular
  • Heavy body/light body
  • Tray - stock tray
• Polyether - composition:

  • Paste 1
  • Imine terminated polyether (liquid polymer)
  • Plasticiser
  • Filler
  • Paste 2
  • Aromatic sulphonate
  • Plasticiser
  • Filler
  • Ring-opening cationic polymerisation
  • Addition polymerisation of the liquid pre-polymer
  • No by-product
  • Chain extension and crosslinking occurs
  • Sets to be rubber-like
• Polyether - properties:

  • Good setting characteristics - enough working time, short setting time, clear transition
  • Quite rigid when set
  • Difficult to remove from mouth and cast
  • Can remove mobile standing teeth!
  • Before setting polyethers are:
  • Fluid and pseudoplastic - good detail reproduction
  • Hydrophilic - do not require dry field
  • Elastic when set
  • Similar to silicones
  • Suitable for crown and bridge work
  • Often material of choice for implant work
  • Dimensional stability
  • Good when dry but can absorb water - swell
  • Difficult to decontaminate
• Uses of elastomers:

  • Elastic materials
  • Suitable for use with undercuts
  • Eg crowns and bridges
  • BUT both can still tear in thin sections - materials pigmented to produce good colour contrast with oral tissues
  • Addition silicones
  • Very popular, often considered the "material of choice"
  • BUT hydrophobicity is a problem - impression faults common
  • Polyether
  • Hydrophilic - no dry field required
  • May be the only choice for implants
  • BUT very rigid when set - difficult to remove and may remove mobile teeth