Elastic Impression Material / Hydrocolloids (Reversible and Irreversible) / Part 1

Elastic impression materials are those that remain in an elastic or flexible state after they have been removed from the mouth.
Hydrocolloid impression materials used in dentistry are based on colloidal suspensions of polysaccharides in water. A colloidal suspension is characterized by the fact that it behaves neither as a solution, in which the solute is dissolved in the solvent, nor as a true suspension, in which a heterogeneous structure exists with solid particles being suspended in a liquid.
Dental hydrocolloid impression materials exist in two forms: sol or gel form. In the sol form, they are fluid with low viscosity and there is a random arrangement of the polysaccharide chains. In the gel form, the materials are more viscous and may develop elastic properties if the long polysaccharide chains become aligned.
The conversion from sol to gel forms the basis of the setting of the hydrocolloid impression materials. The products are introduced into the patient’s mouth while in the fluid, sol form. When conversion to gel is complete, and elastic properties have been developed, the impression is removed and the model cast
The two common hydrocolloids widely used in dentistry are reversible (agar) and irreversible (alginate) materials.
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