The dimensional changes are important in dentistry as any changes in dimension of impression after it has been removed from the mouth is a source of error.
IRREVERSIBLE HYDROCOLLOID
ALGINATE (IRREVERSIBLE HYDROCOLLOIDS)
It is extracted from certain brown seaweed. The substance is called anhydro-beta-d mannuronic acid or alginic acid.
TYPES:-
1) Type I – Fast setting (1-2 min)
2) Type II- Normal setting ( 2-4.5 min)
COMPOSITION
| INGREDIENT | WEIGHT(%) | FUNCTION |
| Potassium alginate | 15% | Dissolves in water and reacts with calcium ions |
| Zinc Oxide | 4% | Acts as filler |
| Potassium titanium fluoride | 3% | Accelerator |
| Diatomaceous earth | 60% | Filler |
| Sodium phosphate | 2% | Retarder |
| Coloring and Flowering agents | TRACES |
CHEMISTRY AND SETTING REACTION
The chief ingredient of irreversible hydrocolloid is one of the soluble alginates. Reaction occurs by a chemical reaction. Soluble alginates reacts with Calcium sulfate to produce insoluble calcium alginate as a gel.
The production of calcium alginate is delayed by the addition of a third soluble salt to the solution with which calcium sulfate will react in preference to the soluble alginate to form an insoluble calcium salt.
Thus, the reaction between the calcium sulfate and the soluble alginate is prevented as long as any of the added salt is left. The added salt is called retarder.
The reactions that occur are
2Na3 PO4 + 3 CaS04 à Ca3 (Po4)2 + 3 Na2 So4 When the supply of retarder ( trisodium phosphate) is exhausted, Calcium ions begins to react with potassium alginate to produce calcium alginate.
Sod. Alg + n CaSo4 + H2Oà nNa2So4 + Cal Alg
GEL STRUCTURE
The gel structure consists of a brush heap of calcium alginate fibril network enclosing unreacted sodium alginate sol, excess water, filler particles and reaction by products.
It is a cross linked structure where calcium is responsible for cross linking.
CONTROL OF SETTING TIME:
Setting time can be altered by altering the amount of water.
As the temperature increases, the setting time decreases.
For 10 rise of temperature 1 min reduction in setting time occurs.
Bowl and spatula also can be cooled to increase setting time.
MANIPULATION:
Gypsum can cause acceleration of setting of alginate
Alginate contamination can decrease strength of casts
For manipulations a clean bowl and metal spatula.
The First step in the manipulation is to prepare a proper mixture of water and powder. Water is poured in to bowl and specified weight of powder is added and powder is carefully incorporated into water with spatula.
A vigorous figure 8 motion is best with the mix being stopped against the sides of bowl with intermittent rotations of the spatula to press out air bubbles. The final mix should be smooth creamy mixture that does not drip off the spatula when it is raised. Mechanical devices are also available for mixing. Their advantages are speed convenience and eliminating human errors.