Elastic impression materials

1. Impression materials
A brief introductionA brief introduction
Dr saransh malotDr saransh malot

2. Impression materials
Elastic
Chemical reactions
Irreversible
Alginate
Elastomers
Polysulphides
Polyethers
Condensation silicon
Addition silicon
Temperature change
Reversible
Agar hydrocolloid
Chemical reactions
Irreversible
Plaster of Paris
ZnO Eugenol
Temperature change
Reversible
Impression compound
Waxes
Rigid / Inelastic

3. ELASTIC
Material susceptible to being stretched compressed or
distorted and then tending to resume the original shape.
Elastic impression materials are capable of accurately
reproducing both the hard and the soft structures of the
mouth including the undercut areas and interproximal
spaces

4. 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.

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

6. 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
Lysosols ------gas or liquid or solid in liquid.
Foams---------gases in solid
Solid emulsion – liquids in solid
Solid suspension -- solids in solid

7. HYDROCOLLOID
A colloid that contains
water as the dispersion
phase

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

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

10. For agar secondary bonds hold the
fibrils together. These bonds break at
slightly elevated temperatures 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.

11. GELATION
Setting of reversible hydrocolloid
THERE IS A PHASE CHANGE FROM

SOL GEL

12. LIQUIFACTION TEMPERATURE
Temperature at which gel changes to sol.
(70 -100°c)

GELATION TEMPERATURE
Temperature at which sol changes to gel.(37-50°c)
HYSTERESIS
The temperature lag between the liquefaction
temperature and the gelation temperature.

13. 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.

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

15. 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%

16. 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.

17. 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.

18. 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.

19. 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.

20. Manipulation:
Agar hydrocolloid requires special equipment.
Hydrocolloid conditioning unit
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.

21. 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.

22. 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.

23. 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

24. 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.

25. 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
3) Working time is 7-15 min
4) Setting time is approximately 5 min
8)Elasticity and elastic recovery : recovery occurs upto
98.8%

26. 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

27. 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.