Hydrocolloid Impression Materials Overview

Study Notes

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.

Description

Learn about different types of colloids including true solution, suspension, emulsion, and hydrocolloid impression materials in dentistry. This quiz covers the properties and examples of each type of colloid.