Elastic impression materials

ELASTIC
IMPRESSION
MATERIALS
By
Dr. Swapnaneel
Pradhan,
MDS

CONTENTS
• Definition
• Classification
• History
• Hydrocolloids
• Elastomeric impression materials
• Summary
• Conclusion
• References

Impression material
“Any substance or combination of
substances used for making an impression or
negative reproduction.” – GPT 9
Impression
“A negative likeness or
copy in reverse of the surface of
an object; an imprint of the teeth
and adjacent structures for use
in dentistry.” –GPT 9

HISTORY OF IMPRESSION
MATERIALS

Year /
Decade Event
1730s Sealing wax and plaster casts were introduced
1820 Impression tray was made
1844 Plaster impressions were developed
1857 Modeling compound was developed
1883 E.C.C. Stanford, a British pharmacist, discovered algin, the precursor for alginate
1925 Reversible hydrocolloid introduced by Alphons Poller, an Austrian who patented it as “Nogacoll”
1929 Commercial production of alginate by a company named “Kelco” in California
1931 Hydrocolloid “Denticole” was first marketed
1935 A.W. Sears promoted the use of agar as an impression material for fixed partial dentures
1953 Polysulfide impression material was introduced
1960s Polyether impression material was introduced
1970s Condensation silicone was introduced
1980s Addition silicone material was introduced

CLASSIFICATION OF
IMPRESSION
MATERIALS

Classification by S. Mahalaxmi
I. Based on setting mechanism
A. Reversible, e.g., impression compound, dental waxes, and agar hydrocolloids
B. Irreversible, e.g., alginate, zinc oxide eugenol, and elastomers
C. Thermoset, e.g., polyether and silicones
D. Thermoplastic: These materials can be transformed from a hard solid material
into a softened moldable material simply by raising their temperature to an
appropriate level. This process can be reversed by cooling it to room
temperature, e.g., impression compound.
II. Based on flexibility
A. Inelastic/rigid, e.g., plaster of Paris, impression compound, and zinc oxide
eugenol
B. Elastic, e.g., elastomers, alginates, and agar hydrocolloids.

Classification by S. Mahalaxmi
III. Based on amount of pressure
A. Mucostatic impression material, e.g., zinc oxide eugenol impression paste and
impression plaster.
B. Mucocompressive impression material, e.g., impression compound and
impression waxes.
IV. Based on type of tray
A. Perforated metal tray—alginate hydrocolloid.
B. Water-cooled metal tray—agar hydrocolloid.
C. Custom tray—zinc oxide eugenol, impression plaster, elastomeric impression
material.

Classification by Anusavice, Shen and
Rawls

Colloid: “A solid, liquid or gaseous substance made up of
large molecules or masses of smaller molecules that remain
in a suspension in a surrounding continuous medium of
different matter.”
A colloid that contains water as the dispersion phase is
called Hydrocolloid.
By Anusavice KJ: Philips’ Science of Dental materials
11th Edition
Definition:

Particles of size: between 10−4 and 10−7 cm and dispersed
in another medium.
Colloids have two phases:
• Dispersed phase (dispersed particle): It is a substance
which is distributed in the form of colloidal particles
(particles of size: between 10−4 and 10−7 cm) and is
dispersed in a suitable dispersion medium.
• Dispersion phase (dispersion medium): It is a medium
in which colloidal particles are dispersed.

Basic terminologies associated with hydrocolloids:
Sol: Colloidal suspension of very small solid particles in a continuous
liquid medium
Gel: Network of fibrils that form a weak, slightly elastic brush heap
structure
Gelation: Transformation from sol to gel
Liquefaction temperature: The temperature at which the gel converts
to the sol state
Syneresis: Fluid exuded when gel structures reconfigure to achieve
equilibrium through stress relaxation
Imbibition: Absorption of water
Micelles: The dispersed phase agglomerates to form chains or fibrils in the gel
state called micelles

If an hydrocolloid can be easily changed by cooling into gel form and
back into the sol form by heating, the material is known as reversible
hydrocolloid. This process can be repeated several times by temperature
changes.
Gel to sol: On heating, the gel converts to sol form. As the temperature
rises, the kinetic energy of the molecules in the fibrils increases and the
fibrils separate from each other to form sol.
Sol to gel: When the temperature is reduced by cooling, the secondary
intermolecular forces once again come into play and molecules join
together to form fibrils and gel.

• Agar-agar (so called in
Malay) is an edible marine
red alga (seaweed) from
which agar material is
extracted.
• It was first discovered in
1925 by Alphons Poller,
an Austrian. It was later
introduced into dentistry in
1928

Agar is an organic hydrophilic polysaccharide polymer of
two different sugars in alternating sequence 300–400 units
long. The chemical name of this polymer is “agarose.” It is
a sulfuric ester of linear polymer of galactose. It is the first
successful elastic impression material used in dentistry.
The agar hydrocolloid impression material consists of this
agar in the concentration of 12%–17%.

Ingredient Concentration Use
Agar 13% - 17% Main Ingredient
Borate 0.2% - 0.5% Strengthens the set gel
Potassium
sulfate
1% - 2%
Accelerator to counteract the retarding effects of
borax and water on setting of gypsum
Fillers 0.5% - 1%
Commonly added fillers are diatomaceous earth,
silica, clay, rubber, wax, etc. They give strength,
viscosity, and rigidity to the material.
Alkyl benzoate 0.10%
Prevents growth of mold in the material during
storage (Preservative).
Water about 80%
Controls the flow properties of the sol and the
physical properties of the gel.
Ingredients

Agar is supplied in various forms such as:
1. gels in collapsible tubes,
2. as gel sticks (cartridges), or
3. in bulk containers.
Agar is a thixotropic material, which has the property
to flow under application of pressure or force. The
thixotropic property is advantageous when making
impressions for mandibular arch because the material does
not flow until placed over the arch.

Gelati0n or Setting of Agar
The process of converting gel
to sol is known as liquefaction which occurs
at a temperature between 70 and 100 °C.
As the agar sol cools
the dispersed phase
groups to form fibrils
called micelles
The fibrils branch and
intermesh together to
form a brush-heap
structure
The fibrils form weak
covalent bonds with
each other which
break easily at higher
temperatures resulting
in gel turning to sol.

Gelati0n or Setting of Agar
The gelling property of agar-agar is due to the three
equatorial hydrogen atoms on the 3,6-anhydro-L-galactose
residues, which constrain the molecule to form a helix. The
interaction of the helices causes the formation of the gel.

Manipulation of Agar
The liquefaction and gelation
temperatures for agar hydrocolloid are
different. The gel to sol and sol to gel
transformations are dependent on time
and temperature. This is known as
hysteresis. This feature of agar enables its
use for dental impression procedures.

Manipulation of Agar
The major steps involved in manipulation of the agar are as follows:
1. The first step in using agar material is to liquefy and store it as a sol at 100°C.
2. The tray is loaded with this liquefied agar material.
3. Immediately before making the impression, the loaded tray is cooled to lower
the temperature tolerable by the oral tissues by the process called tempering.
4. The final step occurs once the impression is seated against the oral tissues; to
enable the sol to be converted to gel, circulating water around the impression
tray chills the tray and hardens the material.

Equipment for manipulation
A. Agar syringe material
B. Agar tray material in tubes
C. Impression syringes
D. Connecting water hose
E. Water cooled rim lock trays
F. Hydrocolloid conditioner

Hydrocolloid conditioner
Boiling
section or
Liquefaction
section
Ten minutes in boiling water (100 °C). The sol should be homogeneous and free of
lumps. Every time the material is liquefied, three minutes should be added. After
every use the agar brush heap structure gets more difficult to break.
Storage
section
65–68 °C temperature is ideal. It can be stored in the sol condition.
Tempering
section
46 °C for about two minutes with the material loaded in the tray. This reduces the
temperature so that it is tolerated by the sensitive oral tissues. It also makes the
material viscous.

Impression trays for Agar
Rim lock trays with water
circulating devices are used. The rim
lock is a beading on the inside edge
of the tray border which helps to
retain the material (as agar does not
adhere to the tray). It also has an
inlet and outlet for connecting the
water tubes. The tray should allow a
space of 3 mm occlusally and
laterally and extend distally to cover
all teeth.

Working and Setting Time
The working time ranges between 7 minutes and 15
minutes and the setting time is about 5 minutes. Both can be
controlled by regulating the flow of water through the cooling
tubes. Since the cooling tubes are on the periphery, the
material sets from the periphery towards the teeth surfaces.
Removal of Impression
When the agar has gelled, the peripheral seal is broken,
and the impression is removed from the mouth rapidly. The
impression is rinsed thoroughly with water and the excess
water is removed by shaking the impression.

Storage of Agar Impression
Storage of agar impression is to be avoided at all costs. The
cast should be poured immediately. Storage in air results in
dehydration, and storage in water results in swelling of the
impression. Storage in 100% relative humidity results in shrinkage
as a result of continued formation of the agar network
agglomeration. If storage is unavoidable, it should be limited to one
hour in 100% relative humidity.
Separation from Cast
When the gypsum product has set, the agar impression must
be removed promptly since the impression will dehydrate, become
stiff and difficult to remove. Weaker portions of the model may
fracture. In addition, prolonged contact will result in a rougher
surface on the model.

Properties of Agar impression material
1. Hysteresis: Gelation (solidification) occurs at 37 °C approximately, whereas
liquefaction (melting) occurs at a higher temperature, i.e. 60–70 °C higher than
the gelation temperature.
2. dimensional stability: Poor dimensional stability due to imbibition and
syneresis. So cast is poured immediately.
3. Flexibility: applied. A few set materials, however, have a flexibility of 20%. On
an average a flexibility of 11% is desirable.
4. Elasticity and elastic recovery: They are highly elastic, and elastic recovery
occurs to the extent of 98.8%.

Properties of Agar impression material
8. Accuracy and dimensional change: Some contraction takes place during
gelation. If the material is retained well in the tray, the material contracts
towards the tray resulting in larger dies. Agar impressions are highly accurate at
the time of removal from the mouth, but shrink when stored in air or 100%
relative humidity and expand when stored in water. The least dimensional
change occurs when the impressions are stored in 100% humidity (but not for
more than an hour). Thus it is recommended to pour cast in stone immediately.

Cast duplication
With the introduction of alginate, agar slowly lost its appeal as an impression
material. However, it is still popular as a duplicating material primarily because
o When liquefied it flows readily (like a fluid) over the cast to be duplicated. This
makes it an ideal mould material.
o Large quantities can be prepared relatively easily.
o It is economical, because it can be reused.

Cast duplication
In the construction of cast removable partial dentures (RPD) the relieved and
blocked master cast is duplicated in investment material. This is known as a refractory
cast. The master cast to be duplicated is placed in a duplicating flask or mould former
(Fig. C). The agar is broken into small chunks and loaded into the liquefying machine
(Fig. A) where it is liquefied and stored. The liquid agar is poured into a mould former
(Figs. B and C) to create a mould (Fig. D). Later, investment is poured into this to create
a refractory cast (Fig. E) which is used in the fabrication of the cast partial denture
framework.

Impression disinfection
Since the impression has to be sent to the laboratory, the need to disinfect it is very
important. Most manufacturers recommend a specific disinfectant. The agent may be
iodophor, bleach or glutaraldehyde. Apparently little distortion occurs if the
recommended immersion time is followed and if impression is poured promptly.

Advantages
• Accurate dies can be prepared, if the material is properly handled.
• Good elastic properties help reproduce most undercut areas.
• It has good recovery from distortion.
• Hydrophilic, moist mouth not a problem. It also gives a good model surface.
• It is palatable and well tolerated by the patient.
• It is economical when compared to synthetic elastic materials.
• It can be reused when used as a duplicating material (reuse is not recommended when
used as impression material).
• Low cost because it can be reused.

Disadvantages
• Does not flow well when compared to newly available materials.
• It cannot be electroplated.
• During insertion or gelation, the patient may experience thermal discomfort.
• Tears relatively easily. Greater gingival retraction is required for providing adequate
thickness of the material.
• Only one model can be poured.
• Has to be poured immediately. Cannot be stored for too long.
• Requires special and expensive equipment.
• A soft surface of the gypsum cast results unless a plaster hardener is used.
• Although it can be reused, it is impossible to sterilize this material. Also, with repeated
use there may be contamination of the materials and a deterioration in its properties.

Irreversible Hydrocolloids
If a hydrocolloid changes from colloidal solution (sol) to an elastic
gel by a chemical reaction, the resultant gel cannot be converted back
to its original sol state. Such an hydrocolloid material is described as
irreversible hydrocolloid.
The molecules in the irreversible hydrocolloid are joined together
by primary valency bonds. These bonds are very strong and cannot be
affected by temperature changes except at which decomposition takes
place.
For example, alginate impression material: Sodium alginate sol in
water reacts with calcium sulphate, as
Sodium alginate + calcium sulphate → calcium alginate (gel) + sodium sulphate

Alginate is based on alginic acid which is prepared from a
brown seaweed, algae, a marine plant. Chemically, it is a linear
polymer of anhydro β-d mannuronic acid of high molecular
weight. Alginic acid is insoluble in water but the salts obtained
with sodium and ammonium are soluble. Sodium potassium or
triethanol amine alginates are used in dental impression
materials

Dispensing
The material is supplied as powder in
sealed bulk containers or in weighed small
packets or sachets of plastic or metal foils
for longer storage time.
As per ADA No. 18, there are two types:
• Type I-fast set, with setting time, 1–2 min
• Type II-normal set with setting time 2–4
min.
Type I Type II

Composition of Alginate Impression Powder
Ingredients % Functions
1 Soluble salts of alginates
of (Na, K, ammonium or
triethanol amine alginate)
15% Main reactive ingredient, Forms sol with water,
Reacts with calcium to form a gel of calcium
alginate
2 Calcium sulphate
dihydrate insoluble
16% Reactor-releases calcium ions to react with
soluble alginate to form calcium alginate gel
(accelerator).
3 Trisodium phosphate 2% Retarder-to react preferentially with calcium ions,
delay gelation and increase working time.
4 Diatomaceous earth 60% Filler-to increase the strength and stiffness of the
gel structure (that is not tacky) and controls the
viscosity of the mix.

5 Zinc oxide 4% Filler-has some influence on physical properties
and setting time of the gel.
6 Potassium titanium
fluoride
3% Gypsum hardener-to counter act inhibiting effect
of alginate on setting of dye materials and
improves surface of the stone model.
7 Flavouring agent Trace To provide pleasant taste to make it more (winter
green or peppermint) acceptable to the patient.
8 Colour pigments Trace To provide characteristic colour, sometimes
changing.
Composition of Alginate Impression Powder

Gelation Reaction
On mixing the powder with water, a sol is formed and the alginate,
calcium salt and tri sodium phosphate begin to dissolve. Calcium sulphate
rapidly reacts with soluble alginate to produce an insoluble calcium alginate
gel. The production of calcium alginate gel is so rapid that it does not allow
sufficient working time.
Trisodium phosphate reacts with calcium sulphate in preference to
the soluble alginate to give a precipitate of calcium phosphate. This reaction
delays the supply of calcium ions required for the gelation reaction and
thereby increases the working time.
2 Na3PO4 + 3CaSO4 → Ca3 (PO4)2 + 3 Na2SO4

Gelation Reaction
When all the sodium phosphate has reacted,
the calcium ions begin to react with soluble alginate
quickly to produce calcium alginate as a gel. As the
reaction proceeds, the degree of cross linking
increases and gel develops elastic properties.
The set material is an intermeshed brush heap structure of fibrils of
calcium alginate enclosing unreacted sodium alginate, excess water, filler
particles and reaction by products. Calcium ions replace two sodium ions of
NanAlg molecules. This cross-linking causes gelation.

Biological Properties
• They are nontoxic and nonirritant to the oral tissues (alginate-has
diatomaceous earth as filler has finely divided silica particles.
Some of these particles are present in the alginate dust which
rises from the tin or container, after while tumbling, fluffing or
shaking. These silica particles are found to be a source of health
hazard, silicosis if inhaled.
Remedy: The container is allowed to settle for a while after
tumbling, then the container is held away from the face while
opening to avoid breathing the dust or use dust free alginates.
• They have pleasant taste and odour.

Rheological properties
• Gelation time
It is the time from the beginning of mixing until the gelation occurs. It is measured as
the time from the beginning of mixing until the material is no longer tacky or sticky
when it is touched with clean, dry fingers. Based on the gelation time, there are two
types of alginates.
Type I–2 minutes)
Type II–normal set (2–4 minutes)
Control of gelation time
• By altering the w/p ratio or mixing time (not recommended as these methods affect
other properties).
• By adding retarder trisodium phosphate to the material, as controlled by the
manufacturer.
• By decreasing the temperature of mixing water to 18°C or 20°C, setting time can be
increased and this method is used in the clinics.

Elastic recovery and permanent
deformation
These materials are classified as elastic but they are not perfectly
elastic. They undergo a small amount of deformation known as permanent set
due to their visco elastic behavior. It is measured as the percentage of
deformation that occurs in a cylindrical sample after it is compressed by 10%
strain for 30 seconds. According to ADA No. 18, it should be less than 3%.
Elastic recovery (ER) >100–3 or >97%
It is a time dependent property and is a function of
• Percentage compression
• Time under compression
• Time after removal of compressive load
• Severity of undercuts

Gel Strength
According to ADA specification No. 18, it should be more
than 0.343 MPa (about 0.5–0.8 MPa).
Factors affecting strength:
• Decrease in w/p ratio within limits, increases strength
• Both under or over spatulation decrease strength
• Higher rate of loading increase strength

Tear strength
Varies from 300–700 gm/cm2.
Hence thickness should be between 3–5 mm.
Flexibility
According to ADA specificatoin No. 18, it should be between 5–20%. It is
measured as the amount of strain produced when a sample is stressed
between 100–1000 gm/cm2. Most alginates have values of 12–14%.

Dimensional stability
These are dimensionally unstable due to syneresis and imbibition, so cast
should be poured immediately after recording the impression.
Minor properties
• Compatible with gypsum dye materials
• Electroplating cannot be done
• Trays used should be perforated for mechanical retention
• Shelf-life is quite short: They deteriorate rapidly at higher temperatures.
Therefore, it is better not to stock the material more than one year and it
should be stored in cool dry environment.

Instruments
Plastic mixing bowl, alginate mixing spatula with curved end and perforated
tray of suitable size.
Proportioning
The container of the powder should be shaken before use, to get uniform distribution
of components. Water powder ratio is taken 3:1 by volume as per manufacturer’s instruction
using the measures supplied for examples,
• For maxillary impressions-2 scoops of powder (15 g) + 2 measure of water (48 ml)
• For mandibular impressions-1 scoop of powder (7.5 g) + 1 measure of water (24 ml).

Mixing
Mechanical mixing instruments are supplied by manufacturers which have controls of
speed and time of mixing. This gives reproducible mix.
Measured powder is
shifted into a clean
rubber bowl
measured quantity of
water is added
mixed with a vigorous
figure 8 motion or
stropped between the
blade of the spatula and
sides of the mixing bowl
intermittent rotation of
the bowl in the opposite
(anticlockwise) direction
Mix is collected and
again stropped
repeatedly to form
homogeneous
conistancy
Hand mixing
Mixing time is about 45 seconds.

Under spatulation
• Inadequate wetting and lack of homogeneity
• Mix will be grainy and poor recording of details and mechanical properties.
Over spatulation
• Reduction in working time
• Reduction in strength due to destruction of gel fibrils as they form and intermesh.
The final mix should be smooth and creamy that does not drip off the
spatula when it is raised from the bowl.

Loading Tray
The mixed alginate is transferred to a perforated tray by using mixing spatula and
is generally added to the posterior portion of the tray and pushed towards anterior portion.
Impression recording
The loaded tray is carried to the patients mouth to record the impression. The
posterior portion of the tray is usually seated first then the anterior portion. The tray
should be held gently until the alginate sets.
Removal of Impression
After the seal between impression and peripheral tissue is broken, the tray
and the impression should be removed with a single sudden jerk to
minimize the permanent deformation.

Disinfection of impression
• It should be washed under running tap water and excess water should be
shaken off.
• The impression can be disinfected by immersing it in 1% sodium
hypochlorite or 2% glutaraldehyde solution or iodophor for 10 minutes.
• The disinfectants can also be sprayed on the impression.
Construction of cast
Cast should be poured immediately as they are not dimensionally
stable, due to syneresis and imbibition.

• The instruments must be absolutely clean. Small amounts of
gypsum impurities left in the bowl, will accelerate the reaction.
• After tumbling, the powder container, the dust coming out
should not be inhaled which may lead to health hazard.
• Correct water/powder ratio as specified by the manufacturer is
to be followed. Variation in W/P ratio effects setting time,
permanent deformation, flexibility and strength.
• Air should not be incorporated during mixing.
• Both under and over spatulation should be avoided.
• The temperature of water used for mixing should be between
18–23°C.

• The thickness of the impression should be 3–5 mm to avoid
tearing of the material.
• Tray should not be disturbed during gelation.
• Impression should be held in the mouth for at least 2–3 minutes
after the material has gelled, because strength and elasticity of
the gel increases with time thus permitting superior
reproduction of undercuts.
• Dislodge the impression with a single sudden jerk.
• Cast should be poured immediately. For shorter periods it can be
stored in 100% humidity or its surface can be covered with damp
napkin or wet cotton.

• Reproduces excellent surface details
• High elastic recovery
• Records undercuts fairly accurately
• Comfortable to the patient
• Hygienic since fresh material is used each time
• Not very expensive
• Easy manipulation procedure.

• Dimensionally unstable
• Low tear strength
• Cannot be electroplated
• No proper storage medium
• Cannot be added in increments, if faulty.

• It is used to record the impressions of dentulous arches in
preparation of crowns and bridges, partial dentures to limited
extent.
• To record preliminary impression in preparation of complete
dentures.
• To record the impression in orthodontia to prepare study
models
• To record the impression to construct athletic mouth
protectors
• For duplicating cast and models.

LAMINATE TECHNIQUE
(AGAR–ALGINATE
COMBINATION TECHNIQUE)
After injecting the syringe agar
on to the area to be recorded, an
impression tray containing a mix of
chilled alginate that will bond with the
agar is positioned over it. The alginate
gels by a chemical reaction, whereas
the agar gels through contact with the
cool alginate, rather than the water
circulating through the tray.
Advantages
1. The syringe agar gives better details
than alginate.
2. Less air bubbles.
3. Water cooled trays are not required and
therefore more convenient.
4. It sets faster than the regular agar
technique.

WET FIELD TECHNIQUE
• In this technique the areas to be recorded are actually flooded
with warm water.
• Then the syringe material is introduced quickly, liberally, and in
bulk to cover the occlusal and/or incisal areas only.
• While the syringe material is still liquid, the tray material is
seated. The hydraulic pressure of the viscous tray materials
forces the fluid syringe hydrocolloid down into the areas to be
recorded.
• This motion displaces the syringe materials as well as blood
and debris throughout the sulcus.

Elastomeric Impression
materials

ELASTOMER :
A POLYMER THAT HAS A GLASS TRANSITION TEMPERATURE THAT IS BELOW
ITS SERVICE TEMPERATURE (USUALLY ROOM TEMPERATURE); THESE
MATERIALS ARE CHARACTERIZED BY LOW STIFFNESS AND EXTREMELY LARGE
ELASTIC STRAINS.
ELASTOMERIC IMPRESSION MATERIAL:
A GROUP OF FLEXIBLE CHEMICAL POLYMERS THAT ARE EITHER
CHEMICALLY OR PHYSICALLY CROSS-LINKED; GENERALLY, THEY CAN BE EASILY
STRETCHED AND RAPIDLY RECOVER THEIR ORIGINAL DIMENSIONS WHEN
APPLIED STRESSES ARE RELEASED.
Defination:
According to GPT 9

 After World War II (1950s), a group of synthetic rubbery materials
called elastomers Polysulphides and Condensation Silicones) ,
which are capable of making impressions of both soft and hard
tissues are developed.
 1960s : Polyether impression material developed in Germany
 1970s : Addition silicone was introduced as a dental impression
material
 1988 : Latest addition and light cure elastomers
 1990-2000 : New auto devices and delivery systems

 Sufficiently fluid to adapt to the oral tissues
 Viscous enough to be contained in a tray
 Able to transform (set) into a rubbery or rigid solid in the mouth in a reasonable
time (less than 7 min),
 Resistant to distortion or tearing when removed from the mouth,
 Dimensionally stable long enough to allow one or more casts to be poured
 Biocompatible
 Cost-effective in terms of time as well as the expense of the associated
processing equipment.

Impression material for all applications including:
Fixed partial dentures
Dentulous and edentulous impressions
Border moulding of special trays(polyether)
Bite registration
As duplicating material for refractory casts

Polysulphide
Addition
Silicon
rubbers
Condensation
Silicon
rubbers
Polyether

 Consistency is measured by pressing 0.5 ml of
mixed material between two flat plates by
applying a force of 1.5N.
 Consistency is defined by average diameter
of the resulting disc of the material.
 Diameter ∝ viscosity

Consistency of test disc
diameter(mm)
Type Description Min Max
0 Very high
consistency
(putty like)
35
1 High
consistency
(heavy
bodied)
35
2 Medium
consistency
(medium
bodied)
31 41
3 Low
consistency
(light
bodied)
36

Type Max permanent
deformation
Max flow in
compression
Max
dimensional
change
in 24 hrs
I 2.5 0.5 -0.5
II 2.5 0.5 -1
III 5.5 2 -0.5

 Workability
 Dimensional stability
 Accuracy
 Rheological properties
 Elasticity
 Tear strength
 Biocompatibility
 Shelf life

Impression
material
Mean working time (min) Mean setting time (min)
23℃ 37℃ 23℃ 37℃
Polysulfide 6 4.3 16 12.5
Condensation
silicon
3.3 2.5 11 8.9
Addition
silicon
3.1 1.8 8.9 5.9
Polyether 3.3 2.3 9 8.3

Material %decrease in
working time
when temp
increased
(mean)
%decrease in
setting time
when temp
increased
(mean)
Polysulfide 30 23
condensation
silicon
16 15.5
Addition silicon 38 31
Polyether 31 8.5

WORKING & SETTING
TIME
Curing of polyether is less sensitive to temperature
 modification of base/accelarator paste
 thinner - WT, slight ST
temperature -
Viscosity
Humidity

AN IMPRESSION MATERIAL SUSTAINS SOME DEFORMATION AS IT IS REMOVED
FROM THE MOUTH BUT IT MUST REBOUND TO ITS PRE- REMOVAL
DIMENSIONS.
 An impression with a sufficiently high elastic limit should
not sustain permanent deformation.
 The elastic properties of these elastomeric impression
materials improve with an increase in curing time in
the mouth
 An extra time of 1 or 2 min before removal may be
beneficial.

RELATIVE AMOUNT OF PERMANENT DEFORMATION IN COMPRESSION
FOLLOWING STRAIN INDUCED DURING REMOVAL INCREASES IN THE
FOLLOWING ORDER
Addition
silicon
Condensation
silicon
Polyether Polysulphide

Polysulphide
Condensation
silicone
Addition
silicone
Polyether

 Ideally-should flow freely and wet the tissue as it is being
injected to achieve adaptation - then resist flow away from
the intended surface areas.
 This will facilitate spreading of heavy-body material on the
impression tray and retain it in the tray. This phenomenon
is called shear thinning
 Polyether – rigid – problem preparation on periodontally
weak tooth - # of dies & tearing of impression material at the
sulcus

 Polymerization shrinkage
 Loss of byproduct
 Thermal contraction from oral temperature to
room temperature
 Imbibitions
 Incomplete recovery of deformation(visco elastic
nature)
 Pour within 30 mins – polysulfide & condensation
silicon

THE AMOUNT OF FORCE NEEDED TO TEAR A SPECIFIED TEST
SPECIMEN DIVIDED BY THE THICKNESS OF THE SPECIMEN IS CALLED
THE TEAR STRENGTH.
 The ranking of tear strength from the lowest to highest of
the impression materials is as follows
silicones polyether polysulfide

 Probability of allergic reactions is low
 Polysulfide hasthe lowest cell deathcount
 Polyether has the highest cell death count,
toxicity and contact dermatitis amongthe class.
 The most likely problem islodgment of
impression material in gingival sulcusresulting
in severeinflammation,

 Subgingival regions are very thin – material
cantear
 Residualsegment of impression material
difficult to detect
➡️radio opacity of polysulfide can help
 Severegingival inflammation.
 Examinethe gingival sulcusimmediately
after impression removal and alsothe
impression for any evidence of tearing

polysulfide 2 yrs
Condensation silicon stannous octoate oxidizes
Orthoethyl silicate is not stable
in presence of tin ester
Addition silicon 1-2 yrs
Poly ether > 2yrs
Storage:
Cool, dry environment Tubes always tightly sealed
Container closed

Property Polysulphides Condensation
silicones
Addition
silicones
Polyethers
Viscosity 3
viscosities (no
putty)
4
viscosities
including
putty
4 viscosities
including putty
single
viscosity(regul
ar) + diluent +
putty
Tear
resistance
Adequate Adequate Adequate Adequate
Elasticity Visco elastic
material
Very good Very good Adequate
Accuracy Good with
special trays
Acceptable
with stock
trays
Good with
stock trays
Good with
special trays
Dimensional
stability
Adequate Poured as
quickly as
possible
Very good Very good in
low humidity

 First synthetic elastomeric impression
material
 Also known as MERCAPTAN or THIOKOL
 Mode of supply
 Collapsible tubes
 One labeled Base paste and
 Other labeled Accelerator paste
 Consistencies
 Light body(syringe or wash)
 Medium body (regular)
 Heavy body

Base paste •Polysulphide
prepolymer with terminal
and pendanthiol (-SH
groups)-80-85%
•Plasticizer – di-n-butyl
phthalate
•Inert filler- possible
chalk or titanium dioxide-
16-18%
Polymerized and cross
linked to form rubber
To control viscosity
To give ‘body’ control
viscosity and modify
physical properties
Catalyst paste •PbO2 / other alternative
oxidizing agent-60-68%
•Sulphur-0.5%
•Inert oil- paraffin type/
di-n-butyl phthalate
To react with thiol
groups- setting
Setting reaction
To form a paste with
PbO2 and sulphur

Polysulfide
prepolymer
Lead
dioxide
polysulphide water

 working time – 4-7 mins
 Setting time – 7-10 mins
Colder climate- increases setting time
A drop of water accelerates the reaction.
 Lowest viscosity
 excellent reproduction of details
 Dimensional stability -
Percent contraction (at 24hrs) – 0.40%-0.45%
Shrinkage is due to loss of polymerization byproduct
such as water

 Deformation on removal
caused by rocking the impression while
removal; it should be removed with a single
swift pull
 High tear strength – 2500-7000 gm/cm2
 Biocompatibility – lowest cell death count
 Moderately hydrophilic
 Unpleasant odor and taste
 Can be electroplated with copper sulphate

- Long working time
- Good tear strength
- Radiopaque
- High flexibility
- Lower cost
- Good reproduction of surface
details
- Requires custom tray
- Obnoxious odor
- Tendency to run down patient’s
throat
- Stains clothing (pbO2) & messy to
work with
- Must be poured within 1 hour
- Hydrophobic so impression area
has to be dry
- Long setting time

• FIRST TYPE OF SILICONE IMPRESSION MATERIAL
 Also known as conventional silicone’
 The setting occurs in room temperature so called as RTV
silicones (room temperature vulcanization)
Mode of supply
 Collapsible tubes
 Base paste
 Accelerator paste / liquid
 Putty is supplied in jars
 low, medium, high, and very high (putty) consistencies

Composition Function
Paste •α- ω hydroxyl-
terminated
polydimethyl siloxane
(liquid silicon
prepolymer)
•Inert filler - silica
Undergoes cross
linking to form rubber
Gives ‘body’,
controls viscosity &
modifies physical
properties
Liquid •Alkyl silica – tetra
ethyl silicate
•Tin compound –
dibutyl tin dilaurate/ tin
octate
Cross linking agent
Reaction catalyst

Dimethyl
siloxane
Tetra ethyl
orthosilicate
Silicon
rubber
Ethyl
alcohol
Stannous octate

 Working time- 2.5 – 4 mins
 Setting time – 6-8 mins
 Tear strength – 2300-2600 N/m
 % contraction at 24 hrs - 0.38-0.60% caused by
Polymerization and evaporation of the alcohol
 Hydrophobic
 Can be electroplated with silver and copper
 Stiffer and harder than polysulfide

• High polymerization
shrinkage
• Volatile alcohol byproduct
• Low tear strength
• Hydrophobic Pour
immediately
• Clean and pleasant
• Good working time
• Easily seen margins

Also known aspolyvinyl siloxaneor vinyl polysiloxane
Mode of Supply:
 Collapsible tubes
 Base paste and
 Accelerator paste
 Putty supplied in Jars
Consistencies:
 Light body (syringe or
wash)
 Medium body (regular)
 Heavy body
 Putty

Composition Function
Base paste •Poly(methyl hydrogen Undergoes cross
siloxane) linking
• Other siloxane
prepolymers
• Fillers- colloidal silica controls viscosity
35-75%
Accelerator •Di vinyl polysiloxane
prepolymers
•Platinum salt
(chloroplatinic acid)
• Palladium
• Retarders
• Fillers
Cross linking agents
catalyst
Hydrogen absorber

Polymethy
lhydrosilo
xane
Divinylpoly
siloxane
Chloropl
atinic
acid
Silicon
rubber

 Working time – 2-4 mins
 Setting time – 4-6.5 mins
 Tear strength – 1500-4300 N/m
 Percent contraction- 0.14-0.17%
 Pseudo plastic
 Exhibits lowest permanent distortion

 Sulphur contamination- inhibits setting
 Vinyl gloves also – sulphur containing stabilizer
used in the manufacturing process
 Even touching the tooth with the gloves before
seating impression – inhibits setting
 Inhibition of polymerization reaction  distortion
 Contact of internal surface of impression with
gloved hands :-
 Failure of the material adjacent to the tray to
polymerize
 Separation of the tray from the impression
material

 Aluminum sulfate and ferric sulfate  gingival retraction
cord  retardation
 Residues from acrylics, methacrylates and petroleum
jelly lubricants may interfere with setting reaction of
material

 Danuta Nowakowska , et al conducted a study on
Polymerization time compatibility index of polyvinyl
siloxane impression materials with conventional
and experimental gingival margin displacement
agents
 They concluded that all of the evaluated displacement
agents at laboratory and intraoral temperatures
induced changes in the polymerization time of PVS.
Therefore, chemical displacement agents should not
come into direct contact with PVS impression
materials.
(J Prosthet Dent 2014;112:168-175)

- Highly accurate
- High dimensional stability
- Pleasant to use
- Short setting time
- Auto mix available
- If hydrophilic, good
compatibility with gypsum
- Hydrophobic
- Expensive
- Hydrogen gas evaluation
in some materials
- Hydrophilic formulations
imbibe moisture
- Sulfur contamination by
latex glove

First elastomer to be developed primarily to function as
an impression material
Mode of supply
 Collapsible tubes
 Base paste
 Accelerator paste
 Third tube containing thinner may be supplied
Consistencies
 Light bodied(syringe or wash)
 Medium bodied (regular)
 Heavy bodied

Base paste
(large tube)
•Imine-terminated
prepolymer
•Inert filler- silica
•Plasticizer-
phthalate
Cross linked to
form rubber
To give body’
control viscosity
and physical
properties
To aid mixing
Catalyst paste
(small tube)
•Ester derivative
of aromatic
sulphonic acid
•Inert filler – silica
•Plasticizer-
phthalate
Initiate cross
linking
To form paste

polyether
Sulfonic
ester
Cross linked rubber

 Working time – 3 mins
 Setting time – 6 mins
 Tear strength – 1800- 4800 N/m
 Percent contraction – 0.19 – 0.24%
 least amount of distortion
 Pseudo plastic
 Biocompatibility – contact dermatitis

- Dimensional stability
- Accuracy
- Shorter setting time
- Automix available
- Set material very stiff
Imbibition
- Short working time.
- Allergic hypersensitivity
in some cases.

o In early 1988, a visible light cured impression was
introduced(Genesis L.D. caulk).
o Two viscosities - Light and heavy bodied
Composition :-
Polyether urethane dimethacrylate
Photoinitiators (camphoroquinone)
Photoaccelerators (Diethyl amino
ethyl methacrylate)
Silicone dioxide (Filler)

Properties :-
 Long working time and short setting time
 Blue light is used for curing with transparent
impression trays
 Tear strength-6000-7500 gm/cm2 (Highest
among elastomers)
 Dimensional stability, flow, detail reproduction,
permanent deformation, wettability, compatibility
with
 cast and die materials and electroforming is
similar to addition silicone

Manipulation :-
 Light body is syringed into the sulcus and over the
preparation
 Heavy body is loaded onto a clear tray and seated over
the light body
 Both are simultaneously cured with a visible light curing
unit having an 8mm or larger diameter probe
 Curing time is approximately 3 mins

- Controlled working time
- Excellent properties
- Ease of cold disinfection
without loss of quality.
- The impression material is
also compatible with gypsum
and silver or copper
metallizing baths
- Need special transparent
trays
- Difficult to cure in remote
area

• Surfactants are added to reduce the contact angle; dilute
solution of soap
• Most commonly used – non-ionic surfactants
Oligoether or polyether substructure
Hydrophilic part
silicon compatible hydrophobic part

Diffusion – controlled transfer of surfactant molecules
from PVS to aqueous phase
Reduction in surface tension
Greater wettability

 used for making intraoral or extraoral occlusal bite
registrations for fixed or removable restoration
and implants.
 Fast intraoral set time of 20 secs – 1 min
 Doesnot slump or drip
 Supplied as cartridges to be used
With a caulking gun
 Commercial name;
Exabyte – Gc
Jet bite – Coltene whaledent

 This latest technique consists of a double barrel caulking
gun with mixing tip. The tip contains spirals on the inside.
Forcing of the base & accelerator results in its mixing.
 e.g. Volume mixer (Kerr), Pentamix(3M ESPE)
Advantages :-
More uniform mix
Less air bubbles
Reduced working time

HYBRID POLYETHER / POLYSILOXANE MATERIAL THAT HAS
BOTH HYDROPHILICITY AND DIMENSIONAL ACCURACY,
BEFORE, DURING AND AFTER SET.

 Specialised addition silicone
 Used for checking errors in the internal surface of crowns
and fpd
 Available as two paste system
 Areas of premature contacts are revealed as bare areas,
which are marked and removed
 Commercial name
Fit Checker - GC

1. Preparing a tray
2. Managing tissue
3. Preparing the material
4. Making an impression
5. Removing the impression
6. Preparing stone casts and dies

1. Paint on adhesives
Eg Coltene, Kerr Universal VPS.
2. Spray adhesives
Eg Sili spray

 Polysulfide
 Butyl rubber
 Styrene / acrylonitrile
Dissolved in volatile solvent such as
chloroform or ketone
 Silicones
 Polydimethyl siloxane / similar reactant like silicon & ethyl
silicate
 Hydrated silica forms of ethyl silicate – bonds with the
tray
 Chemical bond between tray material and Polydimethyl
siloxane.

 A. Peregrina et al, conducted a study on the effect of
different adhesives on vinyl polysiloxane bond strength
to two tray materials .
 Conclusion : The use of GC paint-on universal adhesive
provided significantly higher adhesive values than those
obtained with the adhesives supplied by the
manufacturers of the impression materials tested, with
the exception of the Kerr impression and adhesive
material combination where no significant differences
were found
(J Prosthet Dent 2005;94:209-13.)

 Gingival retraction cord
 double-cord technique is used when the margin is very
close to the gingival attachment.
 Retraction cords - impregnated with a hemostatic agent
(epinephrine)
 An electrosurgical unit
 Or a soft tissue laser

 Hand mixing
 Static mixing
 Dynamic mechanical mixing

 Multiple mix technique
 Mono phase technique
 Putty wash technique

Only onemix ismade-Partof it isplacedin thetray
Another portion isplacedin syringefor injection
Medium viscosityof addition andpolyether canbe used.

 Giuseppe Varvara et al, conducted an invitro study on
Evaluation of defects in surface detail for monophase,
2- phase, and 3-phase impression techniques
 They concluded that the 3-phase, 2-step impression
injection technique provides improved defect-free
reproduction of detail, showing fewer defects than
other impression techniques.
(J Prosthet Dent 2015;113:108-113)

ALL ELASTOMERIC IMPRESSION MATERIALS ARE VISCOELASTIC,
AND IT IS NECESSARY TO USE A QUICK SNAP TO MINIMIZE
PLASTIC DEFORMATION

 Debubblizers, a dilute solution of soap - wettability of
the silicone impression material for the stone slurry
 Excellent dimensional stability of addition silicone and
polyether impression - construct two or three casts or
dies

• Immersion
• Gluteraldehyde , chlorine compounds,
iodophors, phenolics
• Disinfectants requiring more than 30
mins are not recommended
POLYSULPHIDES
&
SILICONES
• Immersion with caution
• Chlorine compounds , iodophors
• Short term exposure – avoid distortion
Polyether

YOUNG S. KANG , ET AL CONDUCTED A STUDY ON EFFECTS OF CHLORINE-
BASED AND QUATERNARY AMMONIUM-BASED DISINFECTANTS ON THE
WETTABILITY OF A POLYVINYL SILOXANE IMPRESSION MATERIAL
THEY CONCLUDED THAT:
1.A QAB disinfectant product is more effective at
removing surfactant than a CLB disinfectant product.
Therefore, a CLB disinfectant provides more time and
control.
2.A wetting agent can reverse the hydrophobicity of a
disinfected PVS impression material if the duration of
cold disinfection is less than 6 hours.
(J Prosthet Dent 2017;117:266-270)

1. ROUGH / UNEVEN SURFACE
 Premature removal
 Improper mixing ratio
 Too rapid polymerisation
 Excessive high accelerator / base
ratio – condensation silicon

2. BUBBLES
 Too rapid polymerisation preventing
flow
 Air incorporation

3.IRREGULARLY SHAPED VOIDS
 Moisture / debris

4.ROUGH / CHALKY STONE CAST
 Inadequate cleaning
 Excess water left on surface
 Excess wetting agent
 Premature removal
 Improper manipulation
 Failure to delay pour

5.DISTORTION
 Resin tray not aged sufficiently
 Lack of adhesion
 Lack of mechanical retention
 Premature development of elastic properties
 Excessive bulk
 Insufficient relief
 Continued pressure
 Movement of tray
 Premature/improper removal from mouth
 Delayed pouring

6. FAULTY ELECTROPLATING
 Dimensional change in the elastomer – continued
polymerisation during electroplating
 Electrodeposited metal tend to contract during deposition
 Flat surface tend to become curved and sharp angles
rounded
 Improper adherence of the electroformed metal to
impression material – greater distortion

1. Philips science of dental materials 12thedition
2. Craig’srestorative dental materials 13th edition
3. Mc Cabe and walls’ applied dental materials 9th edition
4. William J O'Brien Dental materials selection 3rd edition
5. Effects of chlorine-based and quaternary
ammonium- based disinfectants on the wettability of
a polyvinyl siloxane impression material (J Prosthet
Dent 2017;117:266-270)
6. Evaluation of defects in surface detail for monophase,
2- phase, and 3-phase impression techniques (J
Prosthet Dent 2015;113:108-113)
7. The dimensional stability of a vinyl polyether silicone
impression material over a prolonged storage period
(J Prosthet Dent 2013;109:172-178)

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