1. Hydrocolloid Impression Materials DA 122 Dental Materials

2. Definition of Hydrocolloid • A hydrocolloid has colloid particles spread throughout water and depending on the quantity of water available can take on different states, e.g., gel ( semi-solid) or sol (liquid). • Dental hydrocolloids exist in two forms: a sol (a more or less viscous liquid) and a gel (an elastic solid). The impression material is introduced into the mouth as a sol and converts into a gel through either a chemical or a thermal process.

3. Definition (continued) • Hydrocolloids can be either irreversible(single-state) or reversible. For example, agar, a reversible hydrocolloid of seaweed extract, can exist in a gel and sol state, and alternate between states with the addition or elimination of heat • Agar: reversible • extracted from cell walls of brown algae • Alginate: irreversible • extracted from certain red algae

4. Reversible Hydrocolloid • Common uses: • Final impressions for fixed prosthodontics • Special Properties: • Made from agar • Thermal reaction causes change from gel to sol • Can go from gel to sol to gel • Accurate • Economical • Requires much equipment

5. Reversible Hydrocolloid Process • Tubes of gel boil for 10 minutes • Boiling unit • Material becomes liquid (sol) and remains in that state in water bath of 150⁰ F until ready to use • Conditioning unit • Tray material is moved to cooler water (110⁰ F) 5 minutes before use • Cooling unit • Material is placed in mouth in trays which are cooled with water flowing through trays during impression

6. Reversible Hydrocolloid • Armamentarium • Conditioning unit • Trays with attachment hoses • Carpules or syringes • Tubes of tray material • Preparation Time: • 10 minutes boiling unit • Hours to days in storage unit • 5 minutes in conditioning unit • Setting Time: • 5 minutes in the mouth

7. Irreversible Hydrocolloid • Common Uses: • Study models (casts) • Counter models • Preliminary impressions for F/F and P/P • Orthodontic appliances • Splints, mouthguards, custom trays • ALGINATE • Ratio: powder/water • 3:3 Maxillary or • 2:2 Mandibular • Special Properties: • Water-based • Economical • Easy to use • Comfortable for patient • Elastic properties • Sets by chemical reaction with water • Made from seaweed and kelp

8. Irreversible Hydrocolloid • Armamentarium • Flexible rubber bowl • Stiff, broad spatula • Powder and scoop • 68⁰F* and measuring vial • Appropriate impression trays • *Note: warmer water sets faster; cooler water sets slower • Clean-up • Let material set; then peel it off spatula and bowl • Wipe bowl with damp paper towel • Be sure bowls and spatulas are completely clean and dry before using

9. Mixing Alginate: Technicallywater in bowl; powder into water • Mix Surface: • Flexible rubber bow • Mix instrument: • Broad, alginate spatula • Mix Time: • Type I (FAST) = 30 – 45 seconds • Type II (regular) = 60 seconds • Work Time: • 2 ¼ minutes • Initial Set: • 2 ½ minutes • Final Set: • 3 ¾ minutes

10. Alginate • Appearance: • Creamy, smooth • Thick • Homogeneous • Rubbery hard when set • Common Brand Names: • Alginate • Jeltrate • Tropicalgin