Definition: Addition-cured silicones are synthetic elastomeric impression materials based on polydimethyl siloxane polymers with vinyl terminal groups (polyvinyl siloxane, PVS). They are used as a paste at room temperature and set by cross-linking into a rubbery solid via a platinum-catalysed addition reaction with no volatile by-product, giving excellent dimensional stability.
Clinical Case
A 52-year-old patient requires a crown for a heavily restored upper first molar. You need high detail at the finish line and excellent dimensional stability because the impression may be poured later and you may require a duplicate die. You choose an addition-cured silicone in a putty/wash technique, syringing a light-body around the preparation and seating a tray loaded with heavy-body/putty to capture fine detail with stable dimensions.
Uses
Crowns, bridges, inlays/onlays (fixed prosthodontics requiring high accuracy)
Implant impressions (where high dimensional stability is critical)
Situations where the impression may need delayed pouring
Cases requiring duplicate stone dies (very stable after set)
How to use addition-cured silicones in clinical practice
Common techniques:
Twin-mix (double mix) technique: light-body syringe around critical surfaces while heavy-body is loaded in tray; both set together.
Putty/wash technique (single- or two-step): putty provides bulk and tray control; wash captures detail.
Monophase technique (single-step): some PVS materials are formulated to resist slumping at rest in the tray but flow under pressure, enabling stock-tray use.
Handling essentials:
Moisture control: silicones are hydrophobic; saliva contamination can prevent wetting and cause loss of detail.
Mixing: avoid air incorporation; non-homogeneous mixing can leave tacky unset areas.
Removal: all elastomers are viscoelastic → remove with a sharp tug/snap for near-elastic recovery and minimal distortion.
Tips:
Keep light-body (wash) to the minimum necessary: lower filler content means more shrinkage than heavily filled materials (general elastomer principle from the text).
Be cautious with very stiff putties: removal can be difficult if material flows into interdental spaces; “soft putties” reduce this issue.
Latex glove inhibition: addition-cured silicone putties can be inhibited by latex; switch to nitrile if you see incomplete set.
Aim for excellent field control: hydrophobicity means poor wetting of saliva-contaminated surfaces can produce blow holes and detail loss.
Use delivery systems (e.g., gun systems) where available to reduce mixing errors and air bubbles.
Constituents
Two-paste system:
Base paste: polyvinyl siloxane (vinyl-terminated PDMS), silanol, filler
Catalyst paste: polyvinyl siloxane, platinum catalyst, filler
Viscosities available: putty, heavy-, medium-, light-body (and monophase variants). Viscosity is governed by polymer chain length (molecular weight) and especially filler loading.
Chemistry
Why elastomers can be fluid then set?
Elastomer impression materials are used as polymers above Tg, so they can be workable as a paste. Setting occurs when polymer chains are bound into a 3D network by cross-linking, producing an elastic rubber.