Elastomers
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What are the two types of elastomers
• polyether - addition silicones
what is meant by elastic behaviour
to what extent does the impression material, after undergoing elastic strain, undergo elastic recovery
How do we decide which impression material to use
• material characteristics - clinical performance (patient acceptance and ease of use)
What material properties do we look out for when assessing materials
• flow/viscosity • surface detail (reproduction) • wettability (contact angle) • elastic recovery (%) • stiffness (flexibility) • tear strength • mixing time (min) • working time (min)
What is the international standards organisation (ISO) standard for impression materials
is that grooves/indentations of either 20µm or 50µm (depending on material viscosity) are replicated
Why would we want an impression material with a good elastic recovery
• easy to remove from mouth - elastically recover from the deformation of removal
why would we want an impression material with a high tear strength
reduces the risk of tearing fine margins
why would we want an impression material with excellent wettability
• spread easily • adapt smoothly to dentin and moist oral tissue • won’t pull away leaving a void
what is a good value for an elastomers linear dimensional change (24hrs)
<0.2% (will shrink by 2% after the removal)
what is a good value for an elastomers recovery from deformation
> 99.5%
what is a good value for an elastomers detail reproduction
20µm
What are the ideal properties for impression materials
• Quality of surface interaction between material and tooth/soft tissue surfaces - viscosity - surface wetting • Accuracy - surface reproduction - visco-elasticity/elastic recovery • Dealing with removal and undercuts - tear/tensile strength - rigidity (flexibility) • Dimensional properties - setting shrinkage - thermal expansion/contraction - storage
Why are there international standards
standards are NOT designed to establish which is the “best performing” material for a given clinical application…(instead) they are designed to EXCLUDE UNSAFE AND POORLY PERFORMING MATERIALS from the market
What properties determine the quality of surface interaction between material and tooth/soft tissue surfaces
viscosity - must be able to flow readily surface wetting - must make intimate contact with teeth/mucosa contact angle
what is viscosity and what does it determine
• a measure of material’s ability to flow • determines a material’s potential for making close contact with hard/soft tissue surfaces • range: low, medium, high
why must surface wetting be good
so all of surface is replicated (impression material must make intimate contact with teeth/mucosa surfaces)
What properties determine accuracy
• surface reproduction - visco-elasticity/elastic recovery
If you have 3 notches; 1 = 20µm, 2= 50µm and 3 = 75µm, and the material flows into 2 and 3, does the material meet the ISO standard for reproduction of surface detail?
yes, meets the 50µm limit
How much elastic recovery do we ideally want
100% elastic recovery no permanent strain
How does the viscoelastic behaviour of impression materials influence tray removal method
the load time should be as little as possible - if the impression is removed with a SHARP PULL, there is less overall permanent strain
do we want a high or low viscoelasticity
low viscoelasticity (small deformation)
What properties determine dealing with removal and undercuts?
• tear/ tensile strength (removal) - rigidity (removal)
what is tear strength
stress material will withstand before fracturing
what is rigidity
stress/ strain • large stress needed to cause material to change shape • ideally impression material is flexible (we want low rigidity to ease its removal from undercut/ interdental regions)