ELASTOMERIC IMPRESSION MATERIAL

ALGINATE

IT IS A NEGATIVE LIKENESS OR COPY IN REVERSE OF THE SUBJECT OR OBJECT.

EXAMPLE: AN IMPRINT OF THE TEETH AND ADJASCENT STRUCTURES FOR USE IN

DENTISTRY

WHAT IS AN IMPRESSION

ALGINATE

Alginate is classified as irreversible hydrocolloid. Hydrocolloid because it consists of particles of a

gelatinous (colloidal) state in water (hydro) and irreversible because once it has jelled it cannot be returned to a liquid solution.

The Word Alginate comes from the term “ALGIN”. It is a peculiar mucous extract yielded by certain

brown seaweed ( algae ) . The term „ALGIN‟ was coined by chemist from

Scotland at the end of 19th century.

TYPES

Type I – Fast setting. Type II – Normal setting.

MODES OF SUPPLY

It is supplied as a powder that is packed: a) In bulk or in tins or in sachets (or)b) In pre-weighed individual containers. A plastic scoop is supplied for dispensing the bulk

powder, & a plastic cylinder is supplied for measuring the water .

APPLICATIONS

1) Used for impression making : a) when there are undercuts b) In mouth with excessive flow of saliva c ) For partial dentures with clasps.

2) For making preliminary impressions for complete denture.

3) For orthodontic and study models.

4) For duplicating models.

COMPOSITION

Potassium or sodium alginate(15%)

Dissolves in water and reacts with ca+ ions Calcium sulfate(16%)

Reacts with the potassium alginate to form the gel.Sodium phosphate(2%)

Reacts preferentially with calcium sulphate/ retarder

Diatomaceous earth( 60 %)

filler that adds bulk to the material. Zinc oxide (4%)

Adds bulk to the material / fillerPotassium titanium fluoride(3%)

gypsum hardenerColoring and flavoring agent(traces)

Peppermint, orange, etc

PHYSICAL PHASES OF ALGINATE

The first phase is a sol (as in solution). In the sol phase, the material is in a liquid or semi liquid form.

The second phase is a gel. In the gel phase, the material is semisolid, similar to a gelatin dessert.

SETTING REACTION

Reaction occurs by a chemical reactionSoluble alginate reacts with calcium sulphate to

produce insoluble calcium alginate as a gel The production of calcium alginate is delayed by the

addition of a third soluble salt (tri sodium phosphate) to the solution, with which the calcium sulphate will react in preference to the soluble alginate to form an insoluble calcium salt.

In a nutshell , when powder is mixed with water to obtain a paste ,two main reaction occur during setting :

a. 2Na3P04 + 3CaSo4---> Ca3(Po4)2+3 Na2SO4

b. sodium alginate(Powder ) + CaSO4 + H2O --> Ca alginate + Na2SO4(Gel)

First , sodium phosphate reacts with the calcium sulphate to provide adequate working time.

Second ,after the sodium phosphate has reacted, the remaining calcium sulphate reacts with sodium alginate to form an insoluble calcium alginate which forms a gel with water.

WATER-TO-POWDER RATIO

An adult mandibular impression generally requires two scoops of powder and two measures of water.

An adult maxillary impression generally requires three scoops of powder and three measures of water.

MANIPULATION

Fluff or shake the powder to distribute the powder particles evenly

Use the specific measuring devices (water & powder) provided by the manufacturer for mixing

Follow the manufacturer's direction regarding the ratio of water to powder.

Use exact measurements

After the water and powder have been measured, place the water in a clean dry bowl. Shift the powder into the water. Adding the powder to the water ensures the powder particles are wet evenly. If mixed in reverse (the water is added to the powder) the chemical reaction will start early with some particles setting faster than others.

Mix the alginate for the specific amount of time and using a stiff spatula “swipe” the alginate mass against the sides of the bowl to avoid entrapment of air in the mix.

ALGINATE SETTINGS

Working time

The time allowed for mixing the alginate, loading the tray, and positioning the tray in the patient's mouth.

Setting time

The time required for the chemical action to be completed.

Normal set alginate Working time of 2 minutes and a setting time of up to 41/2 minutes

after mixing. Fast set alginateWorking time of 11/4 minutes and a setting time of 1 to 2 minutes.

SETTING TIME OF ALGINATE

Since alginate reacts or gels chemically, temperature is a major factor in the setting time. The colder the temperature of the water the longer it takes to set; conversely, the higher the water temperature the faster it sets.

Ideal water temperature is 68 degrees or room temperature.

LOADING OF TRAY

A perforated tray is used so that the material is forced out slightly through the holes in the tray during loading, thereby locking itself mechanically into the tray

The surface of the alginate in the tray may be smoothened out by moistening the finger with water and running it over the surface of the alginate.

SEATING THE TRAY

Before making the upper impression, saliva should be wiped off from the palatal region 

In order to prevent the material from flowing along the throat and causing nausea to the patient, the posterior portion may be seated first and then the anterior portion seated properly

Since the material sets from tissues towards periphery, any movement during gelation may result in distortion.

So once the tray is seated ,it must be held in place firmly without any movements.

REMOVAL OF THE IMPRESSION

An alginate impression when set develops a very effective peripheral seal

This seal should be freed by running the finger around the periphery.

The impression must be removed suddenly ,with a jerk.

After removal from the mouth, washed with water to remove saliva. cast should be poured as soon as possible.

MODIFIED ALGINATES

Dust free alginates

De-dusting agent (glycerin or glycol) Siliconized alginates

silicon polymers are added to strengthen the material Alginates containing disinfectants

eg :quaternary ammonium salts or chlorhexamine are added to alginate powder.

Hard and soft set alginates

by adjusting the amount of fillersChromatic Alginates

Alginates which change color on setting

PROPERTIES

FLEXIBILITY

It is about 14% at a stress of 1000g/cm2. Lower w/p ratio results in lower flexibility.

ELASTICITY AND ELASTIC RECOVERY

Alginate are highly elastic but less when compared to the agar. 97.3% elastic recovery occurs.

Permanent deformation is less if the impression is removed from the mouth quickly.

STRENGTH

The compressive strengths of alginate gels range from 0.5 TO 0.9 MPa

The tear strengths vary from 0.4 to 0.7 MPa

Both of these properties are time dependent with higher values being obtained if the time of removal is delayed.

DIMENSIONAL STABILITY :

Alginate impression loses water by evaporation and shrinks on standing in air. If it is placed in water it absorbs water and swells. Therefore ,cast should be poured immediately after making the impression.

BIOLOGICAL PROPERTIES

No known chemical or allergic reaction have been identified. However , avoid breathing in Silica particles after fluffing alginate powder.

ADHESION

Alginate does not adhere well to the tray, retention is obtained by mechanical locking features in the tray or by applying an adhesive.

SHELF LIFE

Alginate impression material have a short shelf life. They deteriorate rapidly when stored at elevated temperatures in a humid environment. Therefore ,it is better not to stock more than 1 year supply .

CAUSES FOR DISTORTION AND DIMENSIONAL CHANGE OF ALGINATE

IMBIBITION

If an alginate impression is stored in water or in a very wet paper towel, the alginate will absorb additional water and expand. This condition is called imbibition.

SYNERESIS

If an alginate impression remains in the open air, moisture will evaporate from the material, causing it to shrink and distort. This condition is called syneresis.

ADVANTAGES OF ALGINATE

It is easy to mix and manipulateMinimum requirement of equipmentsFlexibility of the set impressionAccuracy if properly handledLow costComfortable to the patientGives a good surface detail even in the presence of

saliva

DISADVANTAGES

Can’t be corrected. Cannot be electroplated so metal dies are not

possiblePoor dimensional stabilityPoor tear strengthIt is not recommended where higher degree of

accuracy is required, example: for RPD, FPD, Crowns etc.

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