Hydrocolloid

impression materials

Colloids: Classified as a fourth state of matter and formed from two phases: the dispersed phase and the dispersion medium.

The dispersed phase consists of molecules held together either by primary or secondary forces. The sizes of the colloid particles range from 1 to 200 nm.

Sol-gel transformation

In the gel state, the dispersed phase agglomerates to form chains or fibrils.

If dispersion medium is water, it is called hydrocolloids.

According to the mode of transforming the sol form into a gel one, hydrocolloids are classified into

Reversible (Agar imp.): changed from sol to gel by cooling and returned back to sol by heating (physical change).

Irreversible (Alginate imp.): changed from sol to gel by chemical reaction.

Alginate imp. material

It is elastic irreversible hydrocolloid.

Used as a primary imp. to construct a custom tray or to make study models.

This material is a linear polymer that extracted from certain brown seaweed which is called alginic acid.

Composition of powder component of an alginate imp.:

Potassium alginate To dissolve in water and react with

calcium ions.

Calcium sulfate To react with potassium alginate to form

an insoluble calcium alginate gel.

Zinc oxide Filler, affect physical prop. and setting

time.

Potassium titanium fluoride Accelerator for the setting of the stone.

Diatomaceous earth To control the consistency of the mixed

alginate and the flexibility of the set imp.

Sodium phosphate To react with calcium ions to provide

working time before gelation.

Glycerin To make the powder dustless.

Flavoring agent To produce a pleasant taste.

Disinfectants(chlorhexidine) To produce a disinfectant effect.

Gelation process

The typical sol-gel reaction is the reaction of soluble alginate with calcium sulfate and formation of an insoluble calcium alginate gel.

The production of the calcium alginate is so rapid→ insufficient

working time.

A retarder is added to the solution to prolong the working time.

2Na3PO4 + 3CaSO4 → Ca3(PO4)2 + 3Na2SO4

When trisodium phosphate is exhausted , calcium ion begin to react with potassium alginate .

K2nAlg + n CaSO4 →H2O

→ n K2SO4 + n CaAlg

Controlling setting time

Powder particle size: smaller particles are easily dissolved and ionized so accelerate the

reaction.

W/P ratio. This can affect the properties of the gel, the tear strength, and the elasticity.

Mixing time: ↑it (within limits)→ ↓the

setting time.

The temp of the water: using warm water → ↓ setting time.

Retarder: this controlled by the manufacture.

Fast setting alginate → 1.5-3 min.

Normal setting alginate → 3-4.5 min.

Manipulation

Water is inserted into a clean rubber bowel, then the powder is incorporated into the water by mixing with a metal or plastic spatula.

Mixing time is 45 second to 1 min. depending on whether fast set or regular set.

The result should be creamy, smooth mix.

Contaminants, such as small amounts of gypsum left in the bowel from a previous mix → accelerate the set.

Making the imp.

Perforated, rim lock tray is selected. A thin layer of alginate adhesive is applied and allowed to dry before loading the alginate.

The thickness of the alginate imp. between the tray and the tissues should be at least

3 mm.

The imp. should not be removed from the mouth for at least 3 min. after gelation → improve the compressive

strength, and elasticity.

The tear strength is increased when the imp. is removed with a snap as alginate materials are strain rate dependant.

Properties

Strength

Maximum gel strength is required to prevent tearing

and to ensure elastic recovery of

the imp. upon its removal from the mouth. All manipulative factors affect gel strength.

If too much or too little water is used in mixing → weak and less

elastic gel.

Insufficient spatulation →

chemical reactions are not uniform throughout the mass.

Overmixing → breaks up the calcium alginate gel network →

reduce the strength.

Accuracy

Most alginate imp. are not capable of reproducing the finer details that are observed in imp. with other elastomeric imp. materials.

Biocompatibility

No chemical or allergic reactions are observed. Inhaling fine airborne particles → silicosis and pulmonary

hypersensitivity. Dustless alginate is preferred.

Dimensional stability

Once the imp. is removed from the mouth and exposed to the air at room temp. → shrinkage associated with syneresis and evaporation.

If the imp. is immersed in water→ swelling due to imbibation.

Alginate should be poured up in the model material as soon as possible, If not , the imp. should be stored in 100% relative humidity in a plastic bag or wrapped in a damp paper towel.

Disinfection

The imp. must be rinsed well in cold water to remove saliva and any blood. Disinfection procedure should be relatively rapid to prevent dimensional changes.

The disinfectant (household bleach, iodophors, or synthetic phenols) can be sprayed on the exposed surface. The imp. is then wrapped in a disinfectant-soaked paper towel and placed in a sealed plastic bag for 10 min.

Immersion of the imp. should not exceed 10 min.

Compatibility with gypsum Water content of the hydrocolloid imp. and

sodium sulfate inhibit the setting of the gypsum at the surface. This can be overcome by:

immersing the imp. in a solution containing gypsum accelerator, such as 2% potassium sulfate solution.

using product that contain a gypsum hardener as potassium titanium fluoride.

A rough stone surface will result if excess rinsing water has collected on the surface of the imp. at the time the stone mixture is poured.

Shelf life

Storage temp. and moisture contamination affect shelf life. High temp.(even at 54 ˚C)→

deterioration because of depolymerization of alginate. The material should be stored in a cool, dry environment.

Advantages of elastomeric impression materials over the hydrocolloid

Elastomeric impression materials have higher tear strength than hydrocolloid.

Elastomers have no synersis or imbibition like hydrocolloids. They are more dimensionally stable on storage.

Most of elastomers can be electroplated.

Modified alginates:

Alginate can be present in the form of a sol, containing the water. A reactor of Plaster of Paris can then be added to the sol.

Alginate is available in the form of two pastes, one containing the alginate sol and the second containing the calcium reactor.

Color-changing alginate → visual

indication of working time and setting time the mechanism of the color change is a pH-related change of the dye. It changed from light pink to white.

What is the meaning of hydrocolloid material

Write short notes on:

Types of gelation reactions

Composition of alginate imp. material

Factors affecting setting time of alginate imp. Material

Dimensional changes and disinfection of alginate imp. material