elastomers Flashcards
give two types of elastomer
polyether
addition silicones
what is elastic behaviour
material deforms but will recover to original dimensions
how are elastomers formed
polymerisation with cross linking of polymerisation chains
polymerisation may produce by products such as water hydrogen and alcohol - what effects do these have on the elastomer
affect dimensional stability
5 ideal properties of an elastomer
viscosity - flows
low contact angle/ high wettability
elastic recovery - full recovery from deformation
tear strength
what is the shore A hardness test used for
hardness test for impression materials
what does the contact angle refer to
determines how well the material envelopes/wets the hard and soft tissue surface to record fine detail
what happens when there is a large contact angle in elastomer
results in spaces between globules of impression material and some areas not replicated
what is the international standard test for surface reproduction of impression material
ISO 4823:2000
what is the minimum width of groove that elastomer can reproduce according to ISO 4823:2000
50 microns
draw a stress strain curve for viscosity elastic material
gradually reaches the strain then comes back down but not to original level - permanent strain
how can you reduce the permanent deformation of an impression material
reduce load time - remove with a sharp pull
what is the shark fin test used for
to measure flow under pressure
large fin - high flow will flow into undercuts
short fin - low flow
what is tear strength
stress material will withstand before fracturing
what property is equal to stress/strain ration
rigidity
stress needed to cause material to change shape
