Introduction
Impression materials are plastic when placed, but they differ in their state when the impression is finished. Elastic materials are needed wherever there are undercuts, such as natural teeth with a cervix narrower than the equator, or overgrown alveolar ridges. They deform on removal and then recover their shape.
Hydrocolloids
These are colloidal suspensions of polysaccharides in water, placed as a sol and turned into an elastic gel.
- Reversible: agar-agar, which gels on cooling.
- Irreversible: alginate, which gels chemically.
They are very accurate, thanks to low viscosity and good wetting of moist surfaces, but they gain and lose water easily.
Elastomers
Elastomers are synthetic polymers that become elastic through partial cross-linking. Polymerization gives little shrinkage, while polycondensation gives more because a by-product leaves. Replacing part of the polymer with inert fillers limits shrinkage and sets the viscosity: heavy-bodied, medium-bodied or light-bodied.
Requirements for elastic materials
They must meet the general requirements for impression materials, and in addition:
- Minimal residual deformation: no more than 3%, so that the undercut is not enlarged on the model.
- Strength: they must not tear in thin areas.
- Compensable volume change: shrinkage within the 0.1 to 0.3% expansion of gypsum.
- Tray retention: elastomers do not stick to metal or plastic, so use perforations, tape or adhesive.
- Stability in storage and disinfection: elastomers manage this well, but hydrocolloids do not.