Laurence Bohan
Content Writer
Laurence Bohan
Content Writer

Laurence is a copywriter and trained journalist. His passion is creating insightful and educational content for dental professionals of every level. Outside of work, he enjoys watching sport, quizzing and spending time with his family.

Silicone is one of the most widely used impression materials for fixed and removable dental prostheses, such as crowns and bridges. There are two main types of silicone impression. These are addition silicone, often shortened to a-silicone, and condensation silicone, often shorted to c-silicone. The two materials share many similarities but also some key differences that are essential to understand in order to best decide which one to use.

How are addition silicone and condensation silicone different?

The key difference between addition silicone and condensation silicone is in how they are formed. This is what gives the products their own unique advantages and disadvantages. As the names suggest, condensation silicone forms from a condensation chemical reaction while addition silicone forms from an addition chemical reaction. Though both products are created when a base paste mixes with an accelerator paste, the chemical reactions take place at different speeds and temperatures because of their different compositions.

The setting of condensation silicone normally occurs at room temperature and is hence sometimes referred to as RTC (room temperature vulcanisation) silicone.

Advantages and Weaknesses

  • Most dimensionally stable impression material
  • Releases no by-products when set, so virtually no shrinkage
  • Best fine detail
  • Excellent stability allows multiple models to be made up at once
  • Most time to pour (as long as 60 minutes after setting)
  • Tasteless and no odour
  • Best elastic recovery of all elastomers
  • Adequate tear strength
  • Hydrophobic – not susceptible to imbibition (swelling) or syneresis (shrinking)
  • Short setting time (5-9 minutes) and adequate working time (2:15 minutes)
  • Thixotropic (stays where you need it and flowable when you want it)
  • Works better in high temperatures compared to c-silicone
  • Higher priced versus alginate and condensation silicone
  • Hydrophobic – excellent moisture control is required
  • Inhibited by the sulphur in latex gloves and rubber dam, so non-latex gloves and dams should be used if necessary
  • Susceptible to chemicals like nitrogen and phosphorus
  • More technique sensitive versus c-silicone
  • More accurate than alginate
  • High elasticity
  • Adequate tear strength – better than addition silicone
  • Adequate working time (2:30 seconds) and setting time (8-9 minutes)
  • Hydrophobic – not susceptible to imbibition (swelling) or syneresis (shrinking)
  • Tasteless and no odour
  • Less messy than alginate
  • Less expensive than a-silicone
  • Biodegradable
  • Releases an alcohol by-product that can cause shrinkage
  • Not as accurate as addition silicone
  • Hydrophobic – excellent moisture control is required
  • Higher priced than alginate