IRREVERSIBLE HYDROCOLLOIDALGINATE

DR AMIT AGRAWAL. 1ST PG PROSTHO DEPT.

JAIPUR DENTAL COLLEGE

IMPRESSION MATERIAL

DEFINATION A negative likeness or copy in reverse of

the surface or object.

An imprint of the teeth and adjacent structures for use in dentistry

Non-elastic

Elastic

Aqueous Hydrocolloids

Non-aqueous Elastomers

Polysulfide

Silicones

Polyether

Condensation

Addition

Agar (reversible)

Alginate (irreversible)

Plaster

Compound

ZnO - Eugenol

Waxes

O’Brien, Dental Materials & their Selection 1997

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 @ 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 sackets (or)b) In preweighed 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 alginate(15%) Comes from seaweed; is used as a

thickening agent. Calcium sulfate(16%)

Reacts with the potassium alginate to form the gel.

Trisodium phosphate (2%) Added to slow down the reaction time for

mixing.

Diatomaceous earth( 60 %) A filler that adds bulk to the material.

Zinc oxide (4%) Adds bulk to the material.

Potassium titanium fluoride(3%) Added so as not to interfere with the

setting and surface strength.

PHYSICAL PHASES OF ALGINATE

The first phase is a sol (as in solution). In the sol phase, the material is in a liquid or semiliquid 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 reaction. Soluble 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 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 :

1) 2Na3P04 + 3CaSo4---> Ca3(Po4)2+3 Na2SO4

2) sodium alginate + CaSO4 + H2O -------- > Ca alginate +

Na2SO4

(Powder ) (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.

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 loacking 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 movemnets.

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.

ALGINATE SETTINGS

Normal set alginate Working time of 2 minutes and a setting time of

up to 41/2 minutes after mixing. Fast set alginate

Working time of 11/4 minutes and a setting time of 1 to 2 minutes.

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.

TAKING AN ALGINATE IMPRESSION

Explain the procedure to the patient: The material will feel cold, there is no

unpleasant taste, and the material will set quickly.

Breathe deeply through your nose to help you relax and be more comfortable.

Use hand signals to communicate any discomfort.

AN ACCEPTABLE ALGINATE IMPRESSION

There is a complete "peripheral roll," which includes all of the vestibular areas.

The tray is not "overseated," which would result in exposure of areas of the impression tray.

The impression is free from tears or voids. There is sharp anatomic detail of all teeth and soft

tissues. The retromolar area, lingual frenum, tongue space,

and mylohyoid ridge are reproduced in the mandibular impression.

The hard palate and tuberosities are recorded in the maxillary impression.

CAUSES FOR DISTORTION AND DIMENSIONAL CHANGE OF ALGINATE

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.

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.

RECENT ADVANCES IN 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 chlorohexamine are

added to alginate powder.

Hard and soft set alginates- by adjusting the amount of fillers

PROPERTIES

1)FLEXIBILITY It is about 14% at a stress of 1000g/cm2.

Lower w/p ratio results in lower flexibility. 2)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.

3) STRENGTH The compressive strengths of alginate

gels range from 5000 to 8000 g/cm2.The tear strengths vary from 350 to 700

g/cm2.Both of these properties are time

dependent with higher values being obtained if the time of removal is delayed.

4) 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.

5)SHELF LIFEAlginate 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 .

ADVANTAGES OF ALGINATE

It makes an accurate impression It allows for undercuts The process is not time consuming It is easy to work with, has good

viscosity & is low in adhesive qualities It is comfortable to the patient Inexpensive to the dentist Requires little armamentarium

DISADVANTAGES

1) Can’t be corrected.2) Cannot be used alone for compressing

the tissues.3) It is not accurate enough for crown

and bridge impressions.

THANK YOU…