Properties of Elastomeric Impression Materials

Elastomeric impression materials are used to create accurate molds of oral structures. Their properties ensure precision, stability, and ease of use in clinical practice. Here’s an overview of their key characteristics:

1. Working and Setting Times

• Working time is the period from mixing to the onset of setting, during which the material must be injected and the tray seated.
• Setting time is the time needed for the material to cure enough to be removed without distortion.
• Heat speeds up polymerization, while increasing filler content shortens both times.
• Changing the base/catalyst ratio affects setting time, though it may reduce efficiency and predictability.

2. Detail Reproduction

• Elastomers can record fine surface details very accurately.
• However, detail transfer may be limited by the die material (e.g., dental stone), not the impression itself.
• Hydrophobicity of some materials may reduce accuracy in the moist oral environment.

3. Flow and Rheology

• These materials exhibit shear-thinning behavior: they flow under pressure but become more viscous at rest.
• Thixotropy is when materials flow only under impact or vibration and return to their original state after a delay.
• This helps them stay in place once applied, although the benefit in practice is still debated.

4. Elasticity and Viscoelasticity

• Elastomeric materials must stretch during removal and return to their original shape.
• Materials vary in elastic recovery:
  Best to worst: Addition silicone > Condensation silicone > Polyether > Polysulfide.
• Removal should be quick (“snap removal”) to reduce permanent deformation.
• Viscoelastic behavior explains how deformation occurs and partially recovers over time.

5. Tear Strength

• Tear strength is critical for subgingival and interproximal areas.
• Ranking from lowest to highest: Silicones < Polyether < Polysulfide.
• Polysulfides resist tearing best, but are prone to distortion instead.
• Fast removal improves tear resistance; however, strain during removal can still cause issues.

6. Dimensional Stability

Dimensional accuracy: how stable the impression is right after curing.

• Dimensional stability: how it maintains shape over time.
• Changes may result from polymerization shrinkage, temperature change, or water absorption.
• Best long-term stability: Addition silicone and polyether.
  They can be poured later without compromising accuracy.
• Impressions should be stored dry and cool, especially polyethers.

7. Disinfection

• Most elastomeric materials can be disinfected with standard EPA-registered solutions.
• Avoid prolonged immersion—especially with polyethers and hydrophilic silicones—to prevent dimensional changes.
• Follow the disinfectant manufacturer’s recommended immersion time.

8. Wettability and Hydrophilization

• Traditional silicones are hydrophobic, leading to voids in casts.
• Hydrophilized PVS includes surfactants to improve wettability.
• Surfactants migrate to the surface during setting, improving gypsum flow and reducing voids.
• However, clinical benefits remain debated due to varying results in studies.

9. Biocompatibility

• Polysulfides show the lowest cytotoxicity; polyethers show the highest in vitro.
• Residual fragments can cause inflammation or mimic other clinical problems.
• Polyether materials may cause contact dermatitis in rare cases.
• Radiopacity of polysulfide is helpful for detecting fragments in tissues.

10. Shelf Life

• These materials remain stable until the expiration date if stored properly (cool and dry).
• Presence of a clear liquid in the mix indicates degradation—typically due to improper storage.