Elastomers
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How are elastomers formed? How does this form its properties?
Polymerisation and cross linking of polymer chains Cross linking - generates elastic properties Polymerisation can produce byproducts such as water or alcohol which affects dimensional stability
Give an example of some elastomers
Polysulphides Silicones Polyethers
How do you assess a material?
Surface detail Flow/viscosity Contact angle Elastic recovery Stiffness Tear strength Mixing time and working time Setting shrinkage Dimensional stability Thermal expansion Biocompatibility Shark fin test (flow under pressure) Shore A hardness
What are the ideal properties of an impression material?
Low viscosity Low contact angle High wettability Low viscoelasticity Low stiffness Low thermal expansion Low polymerisation shrinkage High tear resistance
Why must IMs maintain a high quality of surface interaction between material and tissues? How achieved?
To get high accuracy • low contact angle and high wettability is needed so the IM makes contact with as much tooth/mucosa as possible • viscous so it flows readily
Why does an IM need accuracy? How achieved? What size of notch can an IM record in a tooth?
50um Needs accuracy so the impression is as close as an exact replica as possible • achieved through viscosity • visco-elastic recovery into its normal position after removal from undercuts / patients mouth
How does an impression material deal with removal and undercuts?
• High shark fin test so the material will readily flow into undercuts and sulci • high tear strength so material doesn’t tear when removing from undercuts • have low rigidity so it can be easily removed
Why does an IM need dimensional stability?
• so little to no setting shrinkage affecting accuracy of impression • thermal expansion / contraction should be close to mouth and room temperature so that accuracy is not affected during setting • so that dimensions do not change in storage
What material has a better setting time and working time out of polyethers and addition silicones?
Polyethers