Elastomers

Question 1

How are elastomers formed? How does this form its properties?

Answer 1

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

Question 2

Give an example of some elastomers

Answer 2

Polysulphides

Silicones

Polyethers

Question 3

How do you assess a material?

Answer 3

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

Question 4

What are the ideal properties of an impression material?

Answer 4

Low viscosity

Low contact angle

High wettability

Low viscoelasticity

Low stiffness

Low thermal expansion

Low polymerisation shrinkage

High tear resistance

Question 5

Why must IMs maintain a high quality of surface interaction between material and tissues? How achieved?

Answer 5

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

Question 6

Why does an IM need accuracy? How achieved? What size of notch can an IM record in a tooth?

Answer 6

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

Question 7

How does an impression material deal with removal and undercuts?

Answer 7

• 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

Question 8

Why does an IM need dimensional stability?

Answer 8

• 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

Question 9

What material has a better setting time and working time out of polyethers and addition silicones?

Answer 9

Polyethers