Elastic impression materials like alginate and agar are able to reproduce undercuts accurately due to their elasticity. Alginate is an irreversible hydrocolloid that sets via a chemical reaction between soluble alginate and calcium sulfate. It forms a brush heap gel structure and has adequate strength and accuracy for dental impressions. Agar is a reversible hydrocolloid that transitions between sol and gel states based on temperature. Both require special equipment for manipulation but provide elastic impressions suitable for edentulous and dentulous arches.

1. ELASIC IMPRESSSION
MATERIALS

2. ELASTIC :- Material susceptible to
being stretched compressed or
distorted and then tending to resume
the original shape.

3. ADVANTAGE OF ELASTIC IMPRESSION
MATERIAL OVER RIGID IMPRESSION
MATERIAL
Elastic impression material can be used in
both dentulous and edentulous cases with
undercuts.
The distortion of elastic impression
material when removed from an undercut is
minimal.

4. CLASSIFICATION OF ELASTIC
IMPRESSION MATERIAL
1. REVERSIBLE HYDROCOLLOIDS
# Agar
2. IRREVERSIBLE HYDROCOLLOIDS
# Alginate
3. ELASTOMERIC IMPRESSION MATERIAL
# Polysulphide
# Condensation polymerizing silicon
# Poly ether
# Addition polymerizing silicon

5. COLLOIDS
A solid liquid or gaseous substance made
up of large molecules or masses of smaller
molecules that remain in suspension in a
surrounding continuous medium of different
matter.
TYPES OF COLLOIDS
Aerosols – liquids or solids in air
Lyosol – gas or liquid or solid in liquid.
gas, liquid or solid in solid

6. HYDROCOLLOID
A colloid that contains
water as the disperse
phase

7. GEL
A network of fibrils that
form a weak slightly elastic
brush heap structure of
hydrocolloid

8. SOL GEL TRANSFORMATION
If a hydrocolloid contains an adequate
concentration of disperse phase, a sol under
certain conditions may change to semisolid
material known as gel. In the gel state the
disperse phase agglomerates to form chain of
fibrils also called as micelles. The fibrils may
branch and intermesh to form a brush heap
structure. The disperse medium is held in the
interstices between the fibrils by capillary
attraction or adhesion .

9. For agar secondary bonds hold the
fibrils together, these bonds break at
slightly elevated temperature and become
re-established as the hydrocolloid cools to
room temperature. This process is
reversible. In case of alginate the fibrils
are formed by chemical action and the
transformation is not reversible.

10. GELATION
Setting of reversible hydrocolloid
THERE IS A PHASE CHANGE FROM
SOL GEL

11. LIQUIFACTION TEMPERATURE
Temperature at which gel changes to sol.
(70 -100°c)
GELATION TEMPERATURE
Temperature at which sol changes to
gel.(37-50°c)

12. HYSTERESIS
The temperature lag between the
liquefaction temperature and the gelation
temperature.
IMBIBITION
The process of water sorption i.e. the gel
swells when placed in water.
SYNERESIS
Expression of fluid on to the surface of gel
structure.

13. AGAR
Agar is a organic hydrophilic colloid
(polysaccharide) extracted from a certain
type of sea weed. It is a sulphuric ester
of a linear polymer of galactose.

14. COMPOSITION
 Agar --------------------- 13-17%
 Borates------------------ 0.2-0.5%
 Sulphates----------------- 1-2%
 Wax----------------------- 0.5-1%
 Thixotropic material---- 0.3-0.5%
 Water--------------------- more than 80%

15. AGAR It forms the main constituent but
not by weight. Provides the
dispersed phase of the sol and the
continuous fibril structure to the
gel.
WATER It provides the continuous phase
in the sol and the second
continuous phase in the gel. The
amount controls the flow property
of the sol and the physical
properties of the gel.
BORAX Is added to increase the strength
or density of the gel by
intermolecular attraction.

16. SULPHATES Acts as a plaster hardener i.e.
counteracts the inhibiting effect of
borax and agar on the setting of
gypsum material.
FILLERS Added to control the strength
viscosity and rigidity of the
material. E.g. diatomaceous earth,
silica, wax rubber and similar
inert powder.
COLOR AND
FLAVOUR
To improve the appearance and
taste.

17. GELATION TEMPERATURE:
The gelation temperature of agar is
approximately 37°C if the gelation temperature is
too high it is possible that injury may result to the
oral tissues involved , a sever surface stress may
also develop . if the gelation temperature is too low
it will be difficult or even impossible to chill the
material to a temperature sufficiently low to obtain
a firm gel adjacent to the oral tissues .
According to ADA specification no. 11
gelation temperature must not be less then 37°C or
more then 45°C.

18. GELATION TIME:
Gelation of the reversible hydrocolloids is a
function of both temperature and time.
Available forms:
Syringe material
Tray material
The only difference between the syringe and the
tray material is colour and the greater fluidity of
the syringe material.

19. Manipulation:
Agar hydrocolloid requires special equipment.
Hydrocolloid conditioner
Water cooled rim lock trays.
PREPARATION OF THE MATERIAL :
First step is to reverse the hydrocolloid gel to the sol form.
Usually done at 100C for 10min
3 min should be added to this time whenever the material
is being reused. As it becomes difficult to break down
the agar brush heap structure.
After liquefaction material may be stored in sol condition.
Storage temperature: 65C to 68C.

20. CONDITIONING OF THE MATERIAL:
This refers to the cooling of the material (also called as
‘tempering’ of the material)
Tempering is usually done at 43C for 7min. Although
tempering time and temperature varies according to different
product specifications and also operator choice but in any
case tempering of the material should not exceed 10min
since the gelation may proceed too far.
Uses of tempering:
It increases the viscosity of the material so that the hydrocolloid
does not flow out.
It reduces the temperature of the material so that it is not
uncomfortable for the patient.

21. IMPRESSION:
The syringe material is first filled in the prepared cavity or the
desired area of impression.
The tempered tray material in a tray is then seated in the oral
cavity covering the already placed syringe material with
passive pressure. Excess water from the surface of the tray
material should be removed to facilitate proper union of the
tray and the syringe material.
Gelation is accomplished by circulating cool water at 18C to
21C through the tray for at least 5min. Care should be
taken to prevent the movement of the tray during gelation.
After complete gelation the impression is removed from the
mouth with a single jerk to prevent the tearing of the
impression.

22. Disinfection of the impression:
Disinfection of agar is very important to prevent cross
infection since the material can be re-used.
Disinfectant choice varies with the product according to
the specifications of the manufacturer. Commonly used ones
are iodophor, bleach, or glutaraldehyde

23. Dimensional Stability of the impression:
Gels are invariably subject to changes in dimension by
syneresis and imbibition.
Syneresis is the loss of water by evaporation from the surface
of the gel or by exuding of fluids.
Imbibition is the sorption of water which results in swelling up
of the gel.
Dimensional changes begin as soon as the impression is
removed from the oral cavity.
Storage of the impression in 100% relative humidity is
suggested to prevent dimensional changes.

24. MECHANICAL PROPERTIES:
According to the ADA specification number:- 11 the
compressive strength should not be less the 0.245 MPa
1) Tear strength ----------- 800 to 900 gms/cm2
2) Flexibility ------------ 4 to 15 %
American Dental Association specification number 11.
Compressive strength : not less than 0.245 MPa (35.6psi)
Working time is 7-15 min
Setting time is approximately 5 min
Elasticity and elastic recovery : recovery occurs upto 98.8%

25. Advantages:
Accurate dies can be prepared.
Due to good elasticity reproduction of undercuts is accurate.
It gives good model surface as it is not hydrophobic.
It is palatable and well tolerated by patients.
It can be re-used

26. Disadvantages:
It cannot be electroplated.
Material tears relatively easily.
Only one model can be prepared.
Extensive instrumentation is required to use agar.
A soft surface of the gypsum cast results unless plaster hardener
is used.

27. IRREVERSIBLE HYDROCOLLOID-
ALGINATE

28. INTRODUCTION
 At the end of the last century, a chemist from
Scotland noticed that certain brown sea weed
(algae) yielded a peculiar mucous extraction.
He named it "algin" . This nature substance
was later identified as anhydrous -B-d-
mannuronic acid (alginic acid)
 In England 40 years later another chemist
S.Willing Wilding received a basic patent for
the use of algin as a dental impression
material.

29.  TYPES
 1. Type I - fast setting
 2. Type II - Normal setting

30. MODE OF SUPPLY
It is supplied as a powder that is packed
In bulk or in tins or in sachets.
In pre-weighed individual containers
A plastic scoop is supplied for dispensing the
bulk powder and a plastic cylinder is supplied for
measuring the water required for the bulk or the
pre-weighed alginate powder.

31.  COMPOSITION:
 The chief active ingredient is one of the
soluble alginates such as sodium potassium
or triethonalamine alginate. When mixed
with water they form a viscous sol.
 Calcium sulfate dihydrate is used as a
reactor. It reacts with the soluble alginate
to form a insoluble calcium alginate gel.

32.  Diatomaceous earth is added as filler. It
increases the strength and stiffness of the
alginate gel, Produces a smooth texture and
ensures a firm gel surface that is not tacky. It also
aids in forming the sol by dispersing the alginate
powder particles.
 Zinc oxide also acts as a filler and influences
some of the physical properties and setting time
of the gel.
 A fluoride such as potassium titanium fluoride , is
added as an accelerator for the setting of the
stone to ensure that a heart, dense, stone cast
surface is produced against the impression.

33. A typical manufacturer's formula for the
alginate powder is as follows
COMPONENT FUNCTION WEIGHT %
Potassium alginate
Calcium sulfate
Diatomaceous earth
Zinc oxide
Potassium titanium
fluoride
Sodium phosphate
Coloring and
Flavoring agents
Soluble alginate
Reactor
Filler
Filler
Accelerator
Retarder
15%
16%
60%
4%
3%
2%
Traces

34. MODIFIED ALGINATES
 Traditionally alginate is used as a two
component system, a powder and water.
However other forms are also available.
 The alginate is available in the form of a sol
containing the water but no source calcium ions.
A reactor of plaster of Paris can then be added to
the sol.
 As a two paste system. One contains the alginate
sol, while the second contains the calcium
reactor.They may be supplied in both tray and
syringe viscosity.

35. SHELF LIFE
 Storage temperature and moisture contamination
are the two major factors that effect the shelf life of
alginate impression materials.
 2 The material deteriorates at elevated
temperatures therefore it should be stored in cool,
dry environment.
 2 The lid of the bulk package can, should be firmly
replaced as soon as possible after each use, so as to
minimize moisture contamination

36. GELATION PROCESS
 The typical sol - gel reaction can be described simply as a
reaction of soluble alginate with calcium sulfate to form an
insoluble calcium alginate gel. The production of calcium
alginate is so rapid that it does not allow sufficient working
time, thus a third water soluble salt, such as tri sodium
phosphate, is added to prolong the working time. The
strategy is that the calcium sulfate will react with the
trisodium phosphate in preference to the sodium alginate and
delay the formation of the insoluble calcium alginate.The
reaction occurs as a follows
 2 Na3 Po4 + 3 Ca S04 - Ca, (P04) z+ 3 Na2 S04
 When the tri sodium phosphate is exhausted , the calcium
ions begins to react with the potassium alginate to produce
calcium alginate as follows
 K2„Alg + n CaSo4 -> n Kz 504 + Can Alg

37.  GEL STRUCTURE
 The gel structure can be envisioned as a brush
heap of calcium alginate fibril network
enclosing unreacted sodium alginate sol,
excess water , filler particles and reactions
byproducts, such as sodium sulfate and
calcium phosphate.

38.  CONTROL OF GELATIONTIME
The gelation time measured from the beginning
of mixing until gelation, must allow sufficient
time for the dentist to mix the material, load
the tray and place it in the patient's mouth.
Once gelation starts the impression material
must not be disturbed because the growing
fibrils will fracture and the impression would be
significantly weakened.

39.  Probably the optimal gelation time is between 3-4 minutes
at room temperature of 20° C. Gelation time is best
regulated by the amount of retarders added during
manufacturing.
 The dentist can best control the gelation time by altering
the temperature of water used for mixing the alginate
material.The higher the temperature the shorter is the
gelation time and vice versa.
 -The mixing bowl and spatula can be cold to prolong the
gelation time.
 Altering the gelation time by altering theW/ P ratio and
mixing time can have marked effects on the properties of
the gel, impairing the tear strength and elasticity.
 The practical method of determining gelation time is to
observe the time from the start of mixing until the material
is no longer tacky or sticky when touched with clean dry,
gloved finger tip.

40.  MANIPULATION
 Preparing the mix:
 The measured powder is sifted into pre measured water
that as been placed in a clean rubber bowl .The powder
incorporated in to the water by careful mixing with a
curved ,clean, metal spatula. Care should be taken to
avoid whipping air into the mix.A vigorous figure eight
motion is best, with the mix being swiped or stropped
against the sides of the rubber bowl with
 intermittent rotations ( 180 degrees) of the spatula to
press out air bubbles. It is important to get all the powder
dissolved, if residual powder remains, a good gel cannot
form and the properties are compromised.

41.  A mixing time of 45 seconds to 1minute is
sufficient, depending on the brand and type
of alginate. Strength of the gel is reduced if
the mixing is not complete the result should
be a smooth, creamy mixture that does not
drip off the spatula when it is raised from the
bowl A variety of mechanical devices are also
available for spatulating the alginate
materials.Their principal benefit is
convenience, speed and elimination of the
human variable.

42.  Making the impression:
 The mixture is placed in a suitable tray, which is
carried into place in the mouth. It is imperative
that the impression adhere to the tray, so that
the impression can be with drawn from around
the teeth.Therefore a perforated tray is generally
used.
 If a plastic or a metal rim lock tray is used, a thin
layer of tray adhesive should be applied and
allowed to dry completely before loading the
tray.

43.  Thin layers of alginate are weak; therefore the
tray must fit the patient's arch so that there is
sufficient bulk of the material (at least 3 mm)
between the tray and the tissues.
 The alginate impression should not be
removed from the mouth for at least 2-3
minutes after the gelation has occurred i.e.,
the time by which the material loses its
tackiness. If the impression is left for too long
significant distortion results.

44.  Before seating the impression, the material
should have developed sufficient body so that it
does not flow out of the tray and choke the
patient
 Thin layers of alginate are weak; therefore the
tray must fit the patient's arch so that there is
sufficient bulk of the material (at least 3 mm)
between the tray and the tissues.
 The alginate impression should not be removed
from the mouth for at least 2-3 minutes after the
gelation has occurred i.e., the time by which the
material loses its tackiness. If the impression is
left for too long significant distortion results.

45. STRENGTH
Maximum gel strength is required to prevent fracture and
to ensure elastic recovery of the impression on its removal
form the mouth. Al I manipulative factors that are under
the control of the clinician affect the gel strength. For
e.g., If too much too little water is used in mixing the final
gel will weakened, making it less elastic. The proper
water- powder ratio should be employed as specified ;by
the manufacturer. Insufficient spatulation results in failure
of the ingredients to dissolve sufficiently so that the
chemical reaction can proceed uniformly throughout the
mass. Over mixing breaks up the gel network as it is
forming and reduces its strength. The directions supplied
with the product should be followed in all respects.

46. Viscoelasticity:
Hydrocolloids are strain -rate dependent. Thus, the tear
strength is increased when the impression is removed
with a snap. The speed of removal must be a compromise
between a rapid movements and the comfort of the
patient. Usually, an alginate impression does not adhere
to the oral tissues as strongly as some of the nonaqueous
elastomers, so it is easier to remove the alginate
impression rapidly. However ;it is always best avoid
torquing or twisting the impression in an effort to remove
it quickly.

47. Dimensional stability:
Gels are invariably subject to changes in dimension by
syneresis , evaporation and inhibition. Once the
impression is removed from the mouth and exposed to the
air at room temperature , shrinkage associated with
synerisis and evaporation is bond to occur. Conversely if
impression is immersed in water, swelling will occur.
Therefore the impression should be exposed to air for as
short a time as possible and a cast should be constructed
immediately. If pouring the impression must be delayed ,
it should be rinsed in tap water and wrapped in paper
towel. Saturated with water and placed in a closed
container to create 100% humid environment.

48. Disinfection:
The need to disinfect impressions is well
established. Because the Hydrocolloid impression must
be poured within a short time after removal from the
mouth, the disinfection procedure should be relatively
rapid to prevent dimensional change.
The irreversible hydrocolloids may be disinfected
by 10- minutes immersion in, or spraying with, an
antimicrobial agent such as sodium hypochlorite and
glutaraldehyde without significant dimensional changes.
However, certain disinfectants may result in gypsum casts
that have a lower surface hardness or diminished surface
detail.

49. Compatibility with gypsum:
Gypsum casts are not compatible with hydrocolloid
impressions. They may be too soft for waxing procedures.
This disadvantage can be over come in two ways
1) By immersing the impression in a solution containing
an accelerator for the setting of the gypsum cast.
2) By incorporating a plaster hardener or accelerator in
material by the manufacturer.

50. Alginate impression material is available in the market by
some of the following trade names
TRADE NAMES:
1) Jeltrate
2) Zelgan
3) Algitex
4) Vericol aroma etc.

51. ADVANTAGES
1) It is easy to mix and manipulate
2) Minimum requirement of equipment
3) Flexibility of the set impression
4) Accuracy if properly handled.
5) Low cast
6) Comfortable to the patient.
7) It is hygienic, as fresh material must be used for
each impression
8) It is gives a good surface detail even in very wet
mouths.

52. DISADVANTAGES
1. It is not possible to prepare metal dies which have
a higher resistance to abrasion than does gypsum.
2. It is not accurate enough for crown and bridge
impression
3. It cannot be used alone for compressing the
tissues.
4. It cannot be corrected
5. Distortion may occur without it being obvious if
the material is not held stationary in relation to
the tissues throughout its setting period.

53. TYPE CAUSE
1. Grainy material a. Improper mixing
b. Prolonged mixing
c. Undue gelation
d. Water ; powder ratio too low
2. Tearing a. Inadequate bulk
b. Moisture contamination
c. Premature removal from
mouth
d. Prolonged mixing
3. Bubbles a. Undue gelation, preventing
flow
b. Air incorporated during
mixing.
4. Irregularly shaped voids Moisture or debits on tissues.
TYPES OF FAILURES IN ALGINATE

54. 5. Rough or chalky stone cast a. Adequate cleaning of
impressions.
b. Excess water left in
impressions.
c. Premature removal of cast
d. Leaving cast in impression too
long
e. Improper manipulation of
stone.
6. Distortion a. Impressions not poured
immediately.
b. Movement of tray during
gelation.
c. Premature removal from
mouth.
d. Improper removal from mouth.
e. Tray held in mouth for too
long.

55. REFERENCES
 Kenneth J,. Anusavice- PhilipsScience of
Dental materials- 10th edition
 Robert G Craig- Restorative dental
Materials- 11th Edition
 William J. o brien- Dental matereials and
their selection- 3rd edition
 Notes on Dental Materials, E. C Combe- 6th
edition