Elastomeric Impression Materials
On this page
Direct answer
Elastomeric impression materials — polysulphide, condensation silicone, addition silicone (polyvinyl siloxane) and polyether — are cross-linking polymers that cure into elastic rubbers capable of reproducing a preparation margin precisely enough to cast a crown. Addition silicone is the modern benchmark: its platinum-catalysed hydrosilylation produces no by-product, so it shows the least dimensional change and permanent deformation, tolerates repeated pours, and disinfects without complaint. Polyether answers its one weakness with genuine hydrophilicity but is stiff and moisture-sensitive in storage; polysulphide still wins tear strength; condensation silicone, releasing alcohol as it cures, is now largely history. Selection among them is really a decision about the field, the sulcus depth and how many casts the laboratory will need.
What you must remember
- Curing divides the family: condensation cures (polysulphide, condensation silicone) generate by-products — water and lead sulphide, or ethanol — whose loss shrinks the material; addition cures (PVS, and polyether's own anhydride-opening mechanism) create no small molecule, hence superior stability.
- Polysulphide (rubber base): lead dioxide catalyst, notoriously slow, smelly, staining; the highest tear strength and longest working time make it the classic for severe undercuts and deep flanges.
- Condensation silicone: orthoethyl silicate and tin catalyst, alcohol by-product, 0.4–0.6% shrinkage; quick and clean but dimensionally the weakest — pour immediately if used at all.
- Addition silicone (PVS): chloroplatinic acid catalyst; lowest permanent deformation (well under 0.05%), dimensional change under 0.1%, multiple accurate pours — the standard for fixed prosthodontics; hydrogen gas evolved on setting can pit gypsum casts, so wait about half an hour before pouring.
- PVS contamination quirk: sulphur from latex gloves and aluminium chloride from retraction cords inhibit its polymerisation — wash the field, use non-latex gloves, and never knead putty with bare latex hands.
- Polyether: sulphonic acid-terminated, genuinely hydrophilic, stiff (hardest to remove and to pour), short working time; it absorbs water in storage, so never soak it; documented allergy among dental staff.
- Consistency ladder: putty, heavy, medium (monophase), light — used as two-step (putty plus light wash, with or without a spacer) or single-step putty-wash; automix cartridges beat hand-mixing for homogeneity and bubble control.
- Disinfection: all four tolerate ten minutes in glutaraldehyde or hypochlorite; polyether should be sprayed, not immersed.
- Property rankings worth memorising: permanent deformation lowest in PVS and polyether; tear strength highest in polysulphide; wettability best in polyether; detail reproduction excellent across the family.
Selecting an elastomer for a wet field
A crown preparation with a 3 mm subgingival margin and a bleeding sulcus is a materials examination before it is a clinical one. Addition silicone in a putty-wash single-mix is the default — but the field must be dry, and the assistant must not wear latex gloves while handling the wash, because sulphur inhibition at the margin produces the perfect-looking impression whose die is mysteriously short. Cords soaked in aluminium chloride carry the same poison, so the cord is removed, the sulcus rinsed and dried, and the light-bodied material syringed from the margin outward while the putty tray follows immediately; the two viscosities cure chemically together. Half an hour later the laboratory pours stone, avoiding hydrogen bubble artefacts, and may pour twice more over the week without drift — the property that made PVS the fixed-prosthodontic standard.
Change the patient and the choice changes. An edentulous ridge with severe undercutting and a tender mucosa argues for polysulphide's tear strength despite the smell and the twelve-minute wait; a bruxing patient who clenches through removal argues against polyether, whose stiffness can fracture on removal over deep undercuts. The reasoning chain — field wetness, undercut severity, number of pours, removal forces — is precisely the viva the examiner is preparing.
MCQ traps in elastomers
The single-best-answer bank loves four contrasts. Which material releases hydrogen on setting and why does it matter? Addition silicone, and gypsum pitting — hence the half-hour pouring delay. Which is inhibited by latex gloves and astringent cords? Addition silicone again. Which is hydrophilic? Polyether. Which has the highest tear strength? Polysulphide. Reversing any pair is the trap. Expect also the matching question pairing by-product with material (ethanol — condensation silicone; water — polysulphide; none — PVS), and the clinical stem describing a pitted die asking for the cause. The putty-wash question — one-step versus two-step with a spacer film — probes why the spacer exists: to leave room for the wash and prevent the putty's own setting distortion from locking out the light body.
Frequently asked questions
Why does addition silicone give the most dimensionally stable impression?
Its platinum-catalysed addition cure forms no volatile by-product, so there is nothing to evaporate or leach out and the cured rubber barely changes dimension over days.
Why must addition silicone not be handled with latex gloves?
Sulphur compounds in latex inhibit the platinum catalyst, leaving sticky, unset spots at critical margins; non-latex gloves are used instead.
Which elastomer suits a wet field best, and why?
Polyether, whose hydrophilic sulphonic groups wet a moist surface and displace sulcular fluid — though the field is still dried for best margin detail.
Why is a two-step putty-wash technique used with a spacer?
The spacer creates room for a uniform layer of light-bodied wash so the stiff putty records gross contour while the wash captures the margin without being displaced.