Dental Materials · MU Sofia

49. Silicone impression materials

Explains what silicone impression materials are, their polydimethylsiloxane base, and the two setting families: condensation-setting C-silicones and addition-setting A-silicones. Covers consistencies and filler content with a table, two-phase impressions, shrinkage, working and setting times, and handling. Also describes wetting of the field, a side-by-side comparison table of the two families, latex and drug inhibition, and uses for bite registration and model duplication.

Inside: 7 sections · 2 labelled pictures · 2 tables · 12 flashcards · 4 min

Topic 49 of 52 in the MU Sofia syllabus for Dental Materials (Dental Medicine, Year 1). The free AceMed note covers it in 7 sections, about 464 words (4 min of reading), with 12 flashcards.

In this note

  1. Introduction
  2. Viscosity and fillers
  3. C-silicones (condensation)
  4. A-silicones (addition)
  5. Wetting
  6. C-silicones and A-silicones compared
  7. Other uses of A-silicones

Introduction

Silicone impression materials, or polysiloxanes, are elastomers with a backbone of alternating silicon and oxygen atoms carrying organic groups. The base polymer is usually polydimethylsiloxane (PDMS). As the chains link up, the paste turns into a rubber that records fine detail. How they link up divides silicones into two families: C-silicones, which set by condensation, and A-silicones, which set by addition.

Viscosity and fillers

Both families come in several consistencies, and the amount of filler decides which. More filler means less shrinkage and a harder, stronger material, but less accuracy.

A two-phase impression combines the two extremes. A bulk of putty gives strength and limits shrinkage, and a thin layer of light-bodied material gives accuracy. The two bond chemically.

C-silicones (condensation)

  • Base: hydroxyl-terminated PDMS with silica or copper carbonate fillers of 2 to 8 µm.
  • Catalyst: tetraethyl orthosilicate, which is the cross-linker, supplied as drops or a paste.
  • Setting: polycondensation, which releases ethanol as a by-product.
  • Dimensional change: shrinkage is about 0.6% for light-bodied, 0.4% for medium-bodied and 0.2% for heavy-bodied material, and about 0.3% for a two-phase impression.
  • Times: working time 1.5 to 3 minutes, setting time 3 to 7 minutes.
  • Elastic recovery: about 99% for putty and 98% for light-bodied material.
  • Handling: the catalyst dose follows the manufacturer's instructions. Pastes are mixed until the color is even. Pour the model within 24 hours, because shrinkage slowly increases.

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