alginate, irreversible impression material. elastomeric in nature

1. ELASTOMERIC IMPRESSION MATERIAL
ALGINATE

2. I T I S A N E G A T I V E L I K E N E S S O R C O P Y I N
R E V E R S E O F T H E S U B J E C T O R O B J E C T .
E X A M P L E : A N I M P R I N T O F T H E T E E T H A N D
A D J A S C E N T S T R U C T U R E S F O R U S E I N
D E N T I S T R Y
WHAT IS AN IMPRESSION

4. 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.

5.  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.

6. TYPES
 Type I – Fast setting.
 Type II – Normal setting.

7. 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 .

9. 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.

10. 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

11.  Diatomaceous earth( 60 %)
filler that adds bulk to the material.
 Zinc oxide (4%)
Adds bulk to the material / filler
 Potassium titanium fluoride(3%)
gypsum hardener
 Coloring and flavoring agent(traces)
Peppermint, orange, etc

12. 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.

13. 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 (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.

14. 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)

15.  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.

16. 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.

17. 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

18.  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.

19.  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.

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

22. 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.

23. 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.

24. 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.

25. 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.

27. 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 fillers
 Chromatic Alginates
Alginates which change color on setting

28. 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.

29.  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.

30.  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.

31.  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 .

32. 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.

33. ADVANTAGES OF ALGINATE
 It is easy to mix and manipulate
 Minimum requirement of equipments
 Flexibility of the set impression
 Accuracy if properly handled
 Low cost
 Comfortable to the patient
 Gives a good surface detail even in the presence of
saliva

34. DISADVANTAGES
 Can’t be corrected.
 Cannot be electroplated so metal dies are not
possible
 Poor dimensional stability
 Poor tear strength
 It is not recommended where higher degree of
accuracy is required, example: for RPD, FPD,
Crowns etc.

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