Properties of Impression Plaster

CHEMISTRY

Impression plaster is based on calcined calcium sulphate hemihydrate, which reacts with water to form a hard mass of calcium sulphate dihydrate.
This setting reaction is associated with an expansion of 0.3-0.6%. When this is within the confines of an impression tray it will lead to a significant reduction in accuracy.
Mixing the plaster with anti-expansion solution (containing 4% potassium sulphate and 0.4% borax) will reduce this. The potassium sulphate reduces expansion to 0.05%, but this also accelerates the setting reaction, and borax is added as a retarder, which gives more time to take the impression.

PROPERTIES

• Excellent at recording fine detail (because very fluid when inserted in mouth)
• Dimensionally stable if anti expansion solution used
• Fractures if undercuts present
• Mucostatic
• Needs to be treated with a separating medium (e.g. varnish or soap solution) before being cast in stone or plaster
• Exothermic setting reaction
• On storage dimensionally stable but a small amount of shrinkage may occur
• Non toxic but may be unpleasant due to dryness and heat evolved during setting

ADVANTAGES

1. Good surface detail
2. Excellent dimensional stability
3. Rate of the setting reaction can be controlled by the clinician

DISADVANTAGES

1. Cannot be used for mucocdisplacive impressions
2. Cannot be added to
3. Properties affected by operator handling technique
4. Taste and roughness may cause the patient to vomit