Hydrocolloid Impression Materials Overview

Types of Mixtures

• A true solution is a mixture where the dispersed phase is completely dissolved in the solvent, making it invisible to the naked eye.
• A suspension is a mixture where the dispersed phase is insoluble in the dispersion medium, with particles large enough to be seen by the naked eye.
• An emulsion is a mixture where the dispersed phase is liquid, such as oil in water.
• A colloid is a mixture where the particle size of the dispersed phase is smaller (1-200 nm) and not seen by the naked eye.

Hydrocolloids

• Hydrocolloids are colloids where the dispersion medium is water.
• They can exist in sol or gel states.
• Sol is a viscous liquid, while gel is a semi-solid gel.
• Hydrocolloids are inserted in the mouth in sol state, and removed after gelation, which has elastic properties.

Sol-Gel Transformation (Gelation)

• Causes of agglomeration include:
  • Decrease in temperature, resulting in formation of secondary bonds (weak, reversible).
  • Chemical reaction, resulting in formation of primary bonds (strong, irreversible).
• Molecules of the dispersed phase agglomerate to form fibrils, which form network structures that hold the dispersion medium in interstices by capillary action.

Types of Hydrocolloids

• Irreversible gelation occurs through a chemical reaction, as seen in alginate impression materials.
• Reversible gelation occurs through a physical reaction, as seen in agar impression materials.

Alginate Impression Materials

• Definition: A visco-elastic impression material that changes from a sol to a gel state through a chemical reaction and cannot be reversed again to the sol state.
• Uses:
  • Obtaining primary impressions for diagnostic casts, opposing models for crowns or bridges, and construction of night guards and fluoride applications.
  • Not used for crown and bridge, inlays, or onlays due to lack of fine details.
• Presentation forms:
  • Bulk form with suitable measures for dispensing correct quantity of powder and water.
  • Small sealed packets with pre-weighed powder and measured quantity of water.
• Composition:
  • Na alginate or K alginate (main reaction ingredient).
  • Reactor: CaSO4.2H2O (source of Ca2+ ions).
  • Na3PO4 (Sodium triphosphate) (retarder).
  • K Ti fluoride (gypsum accelerator and hardener).
  • Flavoring, coloring, and disinfecting agents (chlorhexidine).
• Setting reaction:
  • Gelation reaction is a chemical reaction (irreversible).
  • On mixing powder with water, a sol is formed.
  • The reactor (Ca sulfate) reacts with Sodium triphosphate (retarder), delaying gel formation and increasing working time allowed for manipulation.
  • Calcium sulfate + Sodium triphosphate → Calcium phosphate + Sodium sulfate.
  • When Sodium triphosphate is consumed, the setting reaction occurs, resulting in the formation of calcium alginate gel.