Definition: Synthetic elastomeric impression materials are polymer-based elastic materials used above their glass transition temperature (Tg) so they are workable as a paste/flowable material at room temperature, then “set” by cross-linking the polymer chains into a 3D network (rubber-like solid). They are designed to provide high accuracy, large recoverable deformation (removal from undercuts), and better long-term dimensional stability than hydrocolloids.
Summary
Hydrocolloids and rigid materials have limitations for dentate accuracy (alginate weaker/less detail; agar unstable and equipment-heavy; rigid materials cannot be removed from undercuts).
Elastomers solve this by using long-chain polymers whose viscosity is controlled by molecular weight and fillers, then converting the paste into a solid via cross-linking.
The main groups are:
Polysulphides
Polyethers
Silicones (addition- and condensation-cured).
Both addition- and condensation-cured silicones are made of the same polymer backbone of polydimethylsiloxane but with different end groups.
Addition-cured silicones have a vinyl end group
Condensation-cured silicones have a hydroxyl end group.
Clinically, performance differences come from by-products (or not), moisture sensitivity/wetting, stiffness/tear strength, and dimensional stability over time, plus availability of different viscosities and delivery systems.
Useful rankings:
Permanent deformation (most → least): polysulphides > polyethers > condensation-cured silicones > addition-cured silicones
Stiffness (most → least): polyethers > addition-cured silicones > condensation-cured silicones > polysulphides
Material | Core chemistry | By-product | Viscosity | Mechanical behaviour | Dimensional stability | Moisture |
|---|---|---|---|---|---|---|
Polysulphide (PS) | Mercaptan (–SH) groups oxidised → chain lengthening + cross-linking → rubber | Water | Light/medium/heavy; no putty → typically special tray | Most flexible; highest tear strength; more permanent deformation(more viscous flow) | Can contract on storage (esp. low humidity) due to water by-product → pour soon | Hydrophobic; wets less well than polyether |
Polyether (PE) | Cures via imine end groups → cross-linkingnetwork | No by-product | Typically single viscosity; thinner available for wash; usually special tray | Stiffest once set (harder removal from undercuts); intermediate tear | Very stable, but absorbs water in high humidity → expansion(danger: model too small) | Best wetting of elastomers cited (lowest contact angle); keep dry, never store with alginate |
Condensation-cured silicone (CCS) | Hydroxyl-terminated PDMS cross-linked via tetraethyl silicate (TES) | Alcohol (R-OH) | Wide range incl. putty, heavy, medium, light, extra-fine | Low permanent deformation (good elastic recovery); stiffness below PE | Contracts over time(alcohol loss) → pour soon (after elastic recovery ~30–60 min) | Hydrophobic; saliva contamination can lose detail |
Addition-cured silicone (ACS / PVS) | Vinyl-terminated PDMS cross-linking via platinum catalyst | No by-product | Wide range incl. putty, heavy, medium, light; monophase versions for stock trays | Least permanent deformation; can be very stiff in putty form (soft putties exist) | Extremely stable; minimal dimensional change → good for duplicate dies | Most hydrophobic (highest contact angle) unless “hydrophilic” PVS (surfactant added) |