Definition: Condensation-cured silicones are synthetic elastomeric impression materials based on hydroxyl-terminated polydimethyl siloxane (PDMS) polymers. They set by cross-linking using tetraethyl silicate (TES) as a cross-linking agent, forming an elastic rubber. A key feature is that the setting reaction produces an alcohol (R–OH) by-product, which can be lost over time and therefore compromises storage dimensional stability.
Clinical Case
A 48-year-old patient needs a crown impression for a lower first molar. You choose a silicone putty/wash technique in a stock tray for convenience, using condensation-cured silicone because it is available in multiple viscosities. You ensure meticulous mixing and prompt pouring of the model after elastic recovery, recognising that this material can contract with time due to alcohol by-product loss.
Uses
Crowns, bridges and inlays/onlays where elastomer accuracy is required
Putty/wash or twin-mix techniques where a stock tray is desirable
Situations where a range of viscosities (including extra-fine wash) is helpful
Best practice caveat: due to time-dependent contraction, these impressions are generally preferred when the model can be poured soon after taking the impression.
How to use condensation-cured silicones in clinical practice
Common techniques
Putty/wash (single- or two-step): putty provides bulk and tray control; wash captures surface detail.
Twin-mix (double mix): wash syringed around critical areas while heavy/putty is loaded in the tray; both set together.
Handling essentials
Moisture control: silicones are hydrophobic—saliva contamination can prevent wetting and cause detail loss.
Mixing quality matters: avoid air incorporation and ensure a homogeneous mix; improper mixing can leave tacky/unset areas.
Removal: elastomers are viscoelastic → remove with a sharp tug/snap to minimise permanent deformation.
Tips:
Pour early: because condensation silicones can contract over time, prepare models as soon as possible after allowing for elastic recovery—commonly between 30 minutes and 1 hour after taking the impression (per the text).
Keep wash material to the minimum needed (general elastomer principle: lower filler content → more contraction).
Watch for erratic setting: non-uniform mixing and TES issues can cause incomplete set.
Avoid saliva contamination → hydrophobic materials are prone to blow holes and loss of detail if they cannot wet the surface.
Constituents
Two-paste system:
Base paste: silicone fluid (hydroxyl-terminated PDMS) + filler
Activator paste: tetraethyl silicate (TES) (cross-linker) + components as supplied
Viscosities available: very wide range—putty, heavy-, medium-, light-body, and extra-fine low-viscosity materials. This supports many tray/technique combinations, including stock-tray putty/wash approaches.
Important handling note (TES control): The amount/condition of TES is critical.
Too little TES → incomplete cure → poorer mechanical properties (e.g., higher permanent set).
Too much TES can also lead to incomplete cure, leaving unreacted ethyl groups.
TES is susceptible to hydrolysis if contaminated with moisture, which can make it inactive and lead to setting problems.