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1. REVIEW OF IMPRESSION MATERIALS FOR COMPLETE DENTURES.
DEFINITION
A negative imprint of an oral structure used to produce a positive
replica of the structure to be used as a permanent record or in the
production of a dental restoration or prosthesis.
DESIRABLE PROPERTIES OF IMPRESSION
MATERIALS.
A. COMPATIBILITY WITH PATIENT: 1.Pleasant odor.
2.Pleasant taste.
3.Non-toxic.
4.Non-irritant.
5.Decreased setting time.
6.Esthetic color.
B. EASE OF MANIPULATION: 1. Minimum equipment.
2. Consistency & Satisfactory texture.
3. Adequate flow property.
4. Readily wets oral tissues.
5. Clinically satisfactory setting characteristics & time.
6. Dimensional accuracy: Should have elastic properties with
freedom from permanent deformation after strain.

2. 7. Adequate mechanical strength to resist tearing.
C.
STORAGE:1.Unused
materials
should
have
adequate
shelf
life
for
requirements of storage & distribution.
2.Used/set impression should be dimensionally stable over clinical
& lab procedures for a period long enough to permit production
of cast/die.
D. Impression materials should be economically commensurate with
the results obtained.
E. They should be readily disinfected without loss of accuracy.
CLASSIFICATION OF IMPRESSION MATERIALS:Impression materials can be classified into various types based on
the following characters:
1.
Based on rigidity/elasticity: a. Rigid (non-elastic).
b. Elastic.
2. Based on viscosity: a. Mucostatic.
b. Mucocompressive.
c. Pseudoplastic.
3. Based on setting of material: 1. a. chemical reaction.
b. Physical change of state.

3. 2. a. reversible.
b. Irreversible.
4.Based on interaction with saliva/water: 1. Hydrophobic.
2. Hydrophilic.
5.Based on chemistry:1. Impression Plaster.
2. Impression compound.
3. Metal oxide (zinc oxide eugenol).
4. Reversible hydrocolloid.
5. Irreversible hydrocolloid.
6. Poly sulfides.
7. Condensation silicones.
8. Addition silicones.
9. Polyether.
10. Visible light curing polyether urethane dimethacrylate.
6.Based on use: 1. Primary impression materials.
2. Secondary impression materials.
3. Duplicating materials.
HISORICAL
REVEW
OF
COMPLETE
DENTURE
IMPRESSION
MATERIALS

1756 Bees wax was the first material to be used for the purpose of
impression making.

4. 
1840 Charles De Loude gave the first references to impression
trays.

1842 Montgomery discovered Gutta Percha.

1847 Desirabode gave references to an impression tray.

1848 Gutta percha was introduced as an impression material.
High
working temperature and stiffness made it difficult to
achieve satisfactory results.

1844 Plaster of paris was used for the first time as an impression
material.

1862 Franklin first corrected impression, followed by a
plaster wash.

Until the early 1900s wax or plaster used directly.

1857 Modeling plastics were developed by Charles Stens.

1874 modeling plastics developed by S.S.White.

1900 Green brothers introduced a method for manipulating
modeling plastics.

First to use term posterior dam’in describing posterior
palatal seal.

S.G.Supplee introduced the hot water heater for modeling
plastics.

1915 Rupert Hall perfected the first moderate heat modeling
plastic for making individual impression trays.


1925 Poller introduced Agar for impressions.
Late 1920s first functional waxes were developed. Waxes used
before this time were paraffin and bees wax .

5. 
1930 Ward and Kelly first use ZOE for impressions.

1939Trapozzano introduced an early technique for using
ZOE.


1936 Alginate3 impression material introduced.
1940 Alginate impression material used first time for corrective
wash procedures.

1938 mucostatics PASCAL's law –tissue under a mucostatic
impressions theory developed.

1950 Elastomeric impression materials were introduced.

1955 Pearson reported on polysulfide base materials for use as
an impression for inlays, crowns &FPD’s.
IMPRESSION PLASTER

ADA specification no –25.
Composition:1.Calcined calcium sulfate hemihydrate.
2.Anti-Expansion agents. - Potassium Sulphate was added which had
a tendency to decrease working time.
3.Accelerators –accelerators that were added were
Potassium Sulphate.
Potassium chloride.
4.Retarders.
5.Colloidal materials/Gelatin.
6.Gum – Tragacanth.

6. 7.Pigments.
Uses:
1.Primary impression material.
2.Secondary/corrective impression material.
Water-powder ratio:1.60 ml of water for 100 gm of plaster. Fluidity is required for
recording finer details.
2.
If water-powder ratio is increased, then the following characters were
affected.
a.
It had a thin consistency.
b.
Setting time was increased.
c.
Strength was decreased.
Mixing time:- 60 seconds.
Strength: Impression plaster had less compressive strength and tensile
strength and is considered to be a very brittle material.
Accuracy: Very accurate. It is hydrophilic and has intimate contact with oral
tissues by absorbing surface moisture. Plaster undergoes minimal
dimensional change on setting.
IMPRESSION PLASTER - PRODUCTION
These materials are the result of calcining of calcium
Sulphate Dihydrate or gypsum. Depending on the method of calcination

7. different forms of hemihydrates can be obtained. Commercially gypsum
is ground and subjected to temperatures of 110-120 degrees to drive off
part of the water of crystallization. This corresponds to the first stage of
the equation. As the temperature is further raised, the remaining water
of crystallization is driven off and products are obtained.
The principal constituent of dental plaster or stone is Calcium
Sulphate Hemihydrate. The difference between the two forms is mainly
between the crystal size, surface area & degree of lattice perfection.

Caso4.2H2OCaso4.1/2H2OCaso4
SETTING OF GYPSUM PRODUCTS:

Calcination Of Calcium Sulphate Dihydrate Forms Calcium
Sulphate Hemihydrate

(Caso4).1/2H2O+3H20
--------->
2caso4.2H20+Heat
The product of the reaction is gypsum and the heat evolved in the
exothermic reaction is equivalent to the heat used originally in
calcination.
When hemihydrate is mixed with water, a suspension is formed
that is fluid and workable. Hemihydrate dissolves in it until it forms a
saturated
solution.
This
saturated
hemihydrate
solution
is
supersaturated with dihydrate, so the later precipitates out. As the
dihydrate precipitates, the solution is no longer saturated with
hemihydrate and so it continues to dissolve. The reaction is continuous
and proceeds until no further dihydrate precipitates out of solution.

8. ADVANTAGES: 1.Good detail reproduction.
2.Inexpensive.
3.Easy to handle.
4.Viscosity can be altered by minor alterations of w/p ratio.
5.Non-toxic.
6.If stored airtight-increased shelf life.
7.Decreased dimensional change on setting.
8.Setting time can be precisely controlled by use of additives.
DISADVANTAGES:-

Requires separating medium for pouring and removing casts.
(Varnish, water glass solution).

Dry sensation in patient’s mouth because it absorbs moisture.
IMPRESSION COMPOUND

ADA specification no –3

Type I: low fusing.

Type II: high fusing, more viscous when soft & more rigid when
hard.
COMPOSITION:Compounds are composed of a mixture of waxes, thermoplastic
resins, filler, and a coloring agent. One of the first substances used, as
an impression material was bees wax. Because such waxes were brittle,

9. substances such as shellac, Stearic acid and gutta percha are added to
improve plasticity and workability.

Rosin:
30 parts. -Thermoplastic material

Copal resin:
30 parts.

Carnauba wax:
10 parts.

Stearic acid:

Talc/soap stone/wax: 15 parts. -Decrease flow; increase strength.

Coloring agents:
5 parts.-increase plasticity.
appropriate amount.
USES:Type-1 -Primary impression, peripheral tracing &tube impression
of single tooth with copper band.
Type-2 -As a tray to support other materials.
MANIPULATION:The compound can be softened with either dry heat or oven or in a
water bath. Care should be taken when compound is softened with dry
heat. The compound should not be over heated since this causes
volatility of its constituents. The compound is broken into small pieces.
This aids in faster and uniform heating and also a uniform mix. If large
amount of compound is heated, it is difficult to heat the compound
uniformly. The compound is heated in a water bath with the help of a
gauze piece. After the mass is removed from the water bath, it is
kneaded. This gives a uniform plasticity to the mass. Prolonged
immersion into water has to be avoided since this causes leaching out of
low molecular weight ingredients.

10. FUSION TEMPERATURE: 1. Approximately 43.5 centigrade.
2. Fusion temperature indicates a definite reduction in plasticity on
cooling. 3.Above these temperatures, fatty acids become liquids and
plasticised
material softens.
THERMAL CONDUCTIVITY:-

The thermal conductivity is low. Hence has to be thoroughly cooled
before removal.

Average linear contraction of compound on cooling from mouth
temperature to room temperature of 25 degrees is 0.3 –0.4 percent.
This magnitude of contraction is unavoidable.
FLOW: After the compound has softened, and during the period it is
impressed against the tissues, the material should flow easily to confirm
to the tissues so that every detail and landmark are recorded accurately.
On the other hand, if the amount of flow at mouth temperature is too
great, distortion can occur.
For type –1:flow at mouth temperature is 6%. When placed against
tissues it
is 65%.
For type –2:flow at mouth temperature is 2%. When placed against
tissues it is 85%.
If the material is older it is uncomfortable for the patient.
ADA Specifications:1. The material should be homogenous.
2. It should have a smooth & glossy appearance on flaming.

11. 3. When trimmed with sharp knife at room temperature margin should
be firm & smooth.
GLASS TRANSITION TEMPERATURE.
The non-crystalline solids do not have a definite melting
temperature but rather they gradually soften as the temperature is
raised & gradually soften as they are cooled. The structural arrangement
of non-crystalline solids does not give a particular melting point to the
compound. They gradually soften as the temperature is raised and
gradually harden as the temperature is lowered. The temperature at
which they form a rigid mass is called the glass transition temperature.
The temperature at which there is as abrupt increase in the
thermal expansion coefficient is an indication of formation of short range
& is called the glass transition temperature. The glass transition
temperature of the compound is 39 degrees or 107 Fahrenheit.
ADVANTAGES:1. Non-toxic.
2. Hardens in reasonably acceptable time.
3. Compatible with gypsum products.
4. No separating medium required.
5. Adequate shelf life.
DISADVANTAGES:1. Although plastic not fluid enough to record fine detail.
2. Distorts over undercuts.
3. Considerable shrinkage on cooling.

12. 4. Dimensional change on storage.
LOW FUSING COMPOUND

They are used to carry the impression material to the depth of the
vestibule.
MODELLING COMPOUND:-

Introduced by Green brothers.

Compound softens easily but remains quite hard at mouth
temperature.

The areas of periphery can be border molded with least possibility
of distortion or breakage.

Very useful for the making of final impressions& when boxing &
pouring the cast.
DIFFERENT TYPES :1.BROWN (highest working temperature)

Stronger at room temperature.

Suitable for extending short borders on the custom tray.

Cake form is used for preliminary impressions.
2.GREEN(lowest working temperature)

It has easy flow & good handling properties.

It is the most popular type.

13. 3.GRAY:-

Average working temperature.

Long period of flow.

Little brittle.
GENERAL CONSIDERATIONS.
1.Modelling compound sticks require relatively high working
temperature.
2.Great care must be taken not to burn the patient.
3.Working time is limited.
4.The fear of harming the patient and delay in seating the tray may lower
the temperature of the material and result in over extended borders.
METALLIC OXIDE PASTES.

ADA specification no.-16

Type-1 (hard)

Type-2 (soft)
USES:1.Impression paste.
2.Cement.
3.Temporary filling.
4.Root canal filling.
5.Bite registration paste.
6.Temporary reline material.

14. 7.Surgical dressing.
AVAILABILITY:1.Two pastes/tubes: zinc oxide (active ingredient) & eugenol+ rosin.
2.Powder (ZnO+rosin) & liquid eugenol.
COMPOSITION:-

Tube-1:
1.ZnO (French processed or u.s.p) :
87%
2.Fixed vegetable oil –olive oil/linseed oil:
13%
3.Plasticiser acts as the vehicle and forms paste. It also helps to
masks irritation effect of eugenol.

Tube-2:
1. oil of cloves (75%-85% eugenol):
12%
2. Gum/polymerised rosin:
50%.
3.Filler(silica/talc/diatomaceous earth):
4. Kaolin:
20%.
3%.
5. Resinous balsam (Canada balsam):
6. Accelerator solution & color:
10%.
5%.
7. Retarders –Glycerin & petrolatum.
SETTING REACTION: Setting reaction consists of zinc oxide hydrolysis and a
subsequent reaction between zinc hydroxide and eugenol to form a
chelate. Water is needed to initiate the reaction and it is also the byproduct of the reaction. Hence the reaction is called as an autocatalytic
reaction. Hence the reaction occurs more rapidly in humid environment.

15. The setting reaction is accelerated by the presence of zinc acetate
dihydrate, which can supply zinc ions more readily. Acetic acid is a more
active catalyst for the reaction. High atmospheric temperatures also
accelerate the setting reaction.
MIXING: The mixing of the two pastes is generally accomplished on an
oil impervious paper. Two strips of the same length are taken from the
two tubes and a flexible stainless steel spatula is used for mixing. The
pastes are mixed for about a minute till a uniform color is obtained.
SETTING TIME: It should take place within 10 minutes for type 1 paste and within
15 minutes for type 2 pastes.
MIXING TIME: -1 min /30-40 seconds.
Final set is said to have occurred when metal rod of Krebs
penetrometer fails to penetrate more than 0.2mm under 50gm/load.
Control of setting time:1.Setting time decreases with increase in temperature, humidity &
addition of
accelerators.
2.Setting time increases with addition of retarders.
CONSISTENCY & FLOW:
Material should be
1. Homogenous.
2. Flow uniformly.
3. Mucostatic.

16. 4. Flow related to setting time.
DIMENSIONAL STABILITY: The dimensional stability of impression pastes is quite
satisfactory. A negligible shrinkage (<0.1%) may occur during hardening.
Impressions can be preserved indefinitely without change in shape.
REMOVAL OF SET MATERIAL FROM GLASS SLAB/SPATULA.
1.Solvents such as Naphtha and oil of orange can be used.
2. The instrument can be heated and the material can be wiped off.
SURGICAL PASTES:
1.they are less brittle.
2.material is weaker after hardening.
3.it takes a longer time for setting.
4.it should be capable of being formed into a rope for dressing.
5.material contains more eugenol.
ADVANTAGES:1. Adheres well to tray.
2. It is sufficiently fluid to record fine details of tissues.
3. Does not undergo any dimensional change during setting process.
4. Has adequate working time & setting time.
5. Sufficient resistance.
6. It is compatible with gypsum products.
7. No separating medium required for gypsum products.
8. It has a satisfactory shelf life.
9. It is Non-toxic.

17. DISADVANTAGES:1. Burning/tingling sensation of eugenol is a major disadvantage.
2. Persistent taste of eugenol added disadvantage.
3. As it adheres to tissues, lips should be coated with petrolatum jelly.
NON EUGENOL PASTES: One of the chief disadvantages of zinc oxide eugenol pastes is the
possible stinging or burning sensation caused by eugenol when it
contacts soft tissues. Moreover ZoE reaction is never completed with the
result that the eugenol may leach out. Some patients find the taste of
eugenol extremely disagreeable, and in patients who wear surgical pastes
for a long time may develop gastric disturbances.
If zinc oxide reacts with carboxylic acid, the reaction is

ZnO+2RCOOH(RCOO)2Zn+H2O

Most commonly used carboxylic acid is ortho ethoxy benzoic acid.

It is not greatly affected by temperature or humidity.

Bactericides and other medicaments can be incorporated without
interfering with the reaction.

18. ALGINATE(Irreversible Hydrocolloid)

ADA specification no.-18
DEFINITONS OF COLLOID:1. Colloid represents a soluble particle distribution quantitatively
similar to
Molecular liquid in the solvent.
2. A material in which is suspended a constituent in a finely divided
state.
3. A colloid system in which water is the dispersion medium; those
materials described as colloid sols in water.
HYDROCOLLOID is a material consisting of a sol of alginic acid
having a
physical state that is changed by an irreversible chemical
reaction forming insoluble calcium alginate.
COMPOSITION:1. ALGINIC ACID:
15%
2. SOLUBLE SALTS: -Na/k/ammonium salts-
18%.
3. Calcium sulfate dihydrate- (reactor)-
14%.
4. Potassium sulfate/Potassium titanium fluoride/
Silicates/ Borates5. Na/K Oxalates/Carbonates-
10%.
2%.
6. Diatomaceous earth-ZnO/Silicate powder-FILLER- 56%.

19. 7. GLYCOLS/DIHYDRIC ALCOHOL-
small %.
8. WINTER GREEN/PEPPERMINT-
small %.
9. PIGMENTS.
10.REACTION INDICATOR.
The chief ingredient of irreversible hydrocolloid is one of the
soluble alginates such as sodium, potassium, or triethanolamine
alginates.
CHEMISTRY OF HYDROCOLLOID:Colloid state represents the highly dispersed system of fine
particles of one phase in another. The colloidal state of system has
dispersed and dispersion phase. If this has water as the dispersion phase
it is called as a HYDROCOLLOID.
In alginate the dispersion phase is water and the dispersed phase
is alginic acid. The molecular weight of alginic acid is greater than that of
water, hence it does not dissolve in water and forms a gel. If the
concentration of dispersed phase in the hydrocolloid is of the proper
amount, sol changes into gel when temperature is decreased. The
temperature at which this change occurs is called as the GELATION
TEMPERATURE and is in the range of 18-20 degrees. The fibrils of the
gel are formed chemically by primary bonds. Hence these are not affected
by temperature changes. They can be returned to the sol state by
reversal of the reaction and not by heat. Hence these materials
called as irreversible hydrocolloid.
GELATION PROCESS: 
Soluble alginate + CaSo4 Insoluble Ca alginate.
are

20. The typical sol-gel reaction can be described as a reaction of soluble
alginate with calcium Sulphate and the formation of insoluble calcium
alginate gel. Calcium Sulphate reacts rapidly to produce the insoluble
calcium alginate from the potassium or sodium alginate in an aqueous
solution. The production of calcium alginate is so rapid that it does not
allow sufficient working time. Thus, a third water-soluble salt, such as
trisodium phosphate, is added to the solution to prolong working time.
Calcium sulphate reacts with this in preference to soluble alginate &
hence increases the working time.
Thus, the reaction between the
calcium Sulphate and the soluble alginate is prevented as long as there
is unreacted trisodium phosphate. When the supply of trisodium
phosphate is exhausted, the calcium ions begin to react with the
potassium alginate to produce calcium alginate.
 Insoluble alginate reacts with Ca Sulphate & forms insoluble
calcium alginate gel in the aqueous medium.

2Na3PO4 +3Caso4 -> Ca3 (Po4) 2+3Na2So4
GEL STRUCTURE: 1.Sodium Or Potassium alginate is formed. The cation is attached to the
carboxyl group.
2 .when the insoluble salt formed by the reaction of the sodium alginate
in solution reacts with the calcium salt, calcium ions may replace the
sodium ions in two adjacent molecules to produce cross-linking
between them.
3. As the reaction progresses, cross-linking becomes more extensive and
a complex polymer network is formed. This constitutes the brush heap
structure of the gel.

21. GEL STRENGTH: 1.Stiffness and strength of the gel are directly related to the brush heap
structure.
2.Greater the concentration of the dispersed phase, greater will be the
fibrils formed on gelation.
3.Temperature also affects the gel strength of the material in the case of
reversible hydrocolloid. But temperature does not show any effect on
irreversible hydrocolloid.
4.Lower the temperature, stronger the gel.
5.Gel strength also depends on the presence of modifiers such as fillers
and other chemicals.
6.The gel strength of alginate is 343 Mpa.
7.The type and amount of alginate used also influence the strength.
8.Manipulative factors that affect the strength are
1.too much or too little water affect the strength.
2.insufficient spatulation.
3.overspatulation.
MIXING TIME: 
Type-1 (fast) – 45 seconds.

Type-2(slow)– 4 minutes.
SPATULATION: The measured powder is sifted into premeasured water that has
been placed in a clean rubber bowl. Powder is incorporated into the
water by careful mixing with a metal spatula. Care should be taken to
prevent whipping air into the mix. A vigorous figure- eight motion is best,
with the mix being swiped or stropped against the sides of the rubbermixing bowl with intermittent rotations (180 degrees). Mixing time of

22. 45seconds to 1 minute is generally sufficient. A smooth creamy mix
should be obtained.
Clean equipment is important because many of the problems and
related failures are attributed to dirty contaminated mixing or handling
devices. The bowl should be free of any previous mix of plaster or
alginate.
WORKING TIME: Type-1: 1.5-2 minutes.
Type-2: 3-4 minutes.
45 secs of mixing time + 30-75secs working time  acc.to ADA
sp.No. 18 not <1.5min.
60 secs mixing time + 2-3.5 minutes working
timeaccording to ADA sp.no.18 not < 2 min.
CONTROL OF GELATION TIME: 1. Altering water: powder ratio.
2. Alteration of temperature of water.
STRENGTH: 1. Maximum gel strength is required to prevent fracture and to ensure
recovery of the impression on its removal from the mouth. All
manipulative factors that are under the control of the clinician affect
gel strength.
2. Manipulative factors that are under the control of the clinician are:
a. Proper water: powder ratio.
b. Insufficient spatulation.
c. Over mixing.
VISCOELASTICITY: -

23. Hydrocolloids are strain rate dependent. Thus the tear strength is
increased when the impression is removed with a snap. Usually an
alginate impression does not stick to the oral tissues as strongly as some
of the non-aqueous elastomers, so it is easier to remove alginate
impressions rapidly. It is always best to avoid torquing/tearing the
impression.
ACCURACY: Most alginate impressions are not capable of reproducing the finer
details that are observed in impressions with other Elastomeric
impression materials. Increase in alginic acid results in increased
roughness & does not improve dimensional stability. The roughness of
the impression material is sufficient to distortion at the margins of
prepared teeth. Surfactants can be added to produce a smooth surface,
but the addition of a layer of solutin also obscures the accuracy.
DIMEMSIONAL STABILITY: 1. Dimensional stability is very poor.
2 .If exposed to air at room temperature, shrinkage occurs due to
processes
such as syneresis and evaporation.
3 .If immersed in water, imbibition of water takes place.
4. 2% potassium sulphate or 100% relative humidity are suggested to
solve the problem.
5. Exertion of pressure during gelation process results in the production of
internal stresses. Relaxation of such internal stresses also results in
syneresis & dimensional changes.
6.Thermal changes also contribute to thermal changes because of changes
in room and mouth temperature.

24. GENERAL CONSIDERATIONS:
1.RETENTION OF MATERIAL TO TRAY:
Perforated trays or tray adhesive like molten sticky wax.
2.OPTIMUM BULK/THICKNESS OF MATERIAL:
3-6 mm to reduce the chance of tearing.
3.TRAY EXTENSION / BORDERS:
Refined trays.
4.Less material is loaded posteriorly,patient upright/leaning forward to
prevent posterior flow of material & minimize gagging .
5.Loaded material smoothened with moist finger.
6.REMOVAL: - Break peripheral seal with tissues; then single firm
rapid movement to avoid tearing.
7.Chance for permanent deformation under 10%.
8.Compressive strength – 3500 gm/Sq .cm.
9.Tearing strength – 300-700 gm/cm.
COMPATIBILITY WITH GYPSUM PROCUCTS: The surface of a gypsum cast obtained from the hydrocolloid
impression material may sometimes be too soft for waxing procedures. To
avoid this,
1.Impression is immersed in a solution containing an accelerator for
setting of gypsum products.
2. By incorporating a plaster hardener or accelerator in the material by
the manufacturer.
3. The surface of the impression should not be dried completely.
a. This causes the gel to adhere to the cast on its removal.
b. Many commercially available products give a satisfactory surface
for the stone cast without using any hardeners.

25. c. After the impression is removed from the mouth it is rinsed
under water to remove oral fluids from the surface.
d. Surface of the impression should be shiny but there should not
be any visible moisture.
e. Pouring of the cast should be done from one end to the other of
the impression.
f. The stone should be kept in contact with the impression for a
minimum of 30 minutes & a maximum of 60 minutes.
g. If the cast is allowed to remain in contact with the impression
overnight, a chalky stone surface may be produced.
FAILURES: 1. GRAINY: a. Improper mixing.
b. Prolonged mixing.
c. Under gelation.
d. W: P ratio too low.
2. TEARING: a. Inadequate bulk.
b. Moisture contamination.
c. Premature removal.
d. Severe undercuts.
e. Thin mix.
f. Slow removal.
3.BUBBLES: a. Air incorporation during mixing.

26. b. Undue gelation preventing flow.
4. ROUGH/CHALKY STONE CAST: a. Inadequate cleaning of impression.
b. Excess water left in the impression.
c. Premature removal of cast.
d. Cast left too long.
e. Improper manipulation of stone.
5. DISTORTION: a. Impression 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.
6.DECREASED WORKING TIME/ SETTING TIME: a. Non homogenous mix.
b. Temperature.
c. Contamination- set plaster left in the mixing bowl.
ADVANTAGES: 1. Non-toxic, non-irritant.
2. No special equipment required.
3. Acceptable odor, taste.
4. Sufficiently elastic to be used in undercut area.
5. Sufficiently fluid to record fine detail.
6. Compatible with gypsum products/no-separating medium
required.

27. DISADVANTAGES: 1. Dimensionally unstable.
2. Tears if undercuts are severe.
3. Difficult to sterilize.
4. Poor shelf life if stored.
ELASTOMERIC IMPRESSION MATERIALS
INTRODUCTION TO ELASTOMERS: These materials are classified as synthetic rubbers but mimic natural
rubber. Hence they are called as rubber base materials or rubber
base impression materials or elastomers or Elastomeric
impression materials.
1. Due to the elastic properties, they are called as elastomers.
2. These materials consist of large molecules or polymers that are
joined by small amount of cross-linking.
3. The amount of cross-linking determines the stiffness and the
elastic behavior of the material.
4. The first synthetic rubber materials were developed by the
process of vulcanization.
5. Setting of these materials occurs through a combination of chain
lengthening polymerization or cross-linking or either
condensation or addition reactions.
HANDLING: -

28. The working time of an acceptable material must exceed the time
required for mixing and loading the syringe and the tray.
1. The material distorts soon after removal from the mouth.
2. If the material is not adequately set, the material will not have
sufficient elastic properties to respond to the strain that occurs
when removing it from the mouth.
3. The setting reaction converts them into a visco-elastic solid.
4. The flow behavior of the material is important to obtain an
accurate impression.
5. The ideal impression material accurately records the oral
structures, releases from the mouth undistorted.
6. The impression material should be removed from the mouth
rapidly.
7.while removing the impression, the seal has to be broken and
remove the impression rapidly.
8. Loss of reaction by products and the imposed loads of stone or
plastic used to make cast results in the distortion of the
impression.
DESIRABLE PROPERTIES OF IMPRESSION MATERIALS.
1.should have an infinite shelf life.
2. Be non-toxic.
3. Should have acceptable odor, taste & color.
4. Should have suitable working & setting times.
5.should have strength to resist tearing.
6. Should be compatible with model & die materials.
7. Should be inexpensive.
8. Should be easy to clean up.
9. Should be easy to dispense, proportion& mix.

29. 10. Permit multiple die pours.
11. Facilitate visualization of the finish line.
12. Facilitate the clinical identification of beginning and end of cure.
13. Completely plastic before cure.
14. Sufficient fluidity to record final detail.
15. The ability to wet oral tissues.
16. Dimensional stability.
17. Complete elasticity after cure.
18. Optimal stiffness.

SOLOMON E G R 1973 used silicone material for complete denture
impression – a high viscosity material for border molding and low
viscosity material for secondary impression. He concluded that silicone
impression material was preferable to conventional low fusing impression
compound.
POLYSULFIDES
COMPOSITION: Base paste: 1. Polysulfide polymer: - 80-85%
2. Filler: - Lithopone/Titanium dioxide: 16-18% - for strength.
3. Plasticizer- Di-n-Butyl phthalate- for appropriate viscosity.
4. Sulfur- 0.5% - to enhance reaction.
Reactor paste: 1.Cross-linking agent.
a. Lead oxide.
b. Organic hydro peroxides-T-Butyl hydro peroxides.

30. c. Inorganic hydroxides-Hydrated copper hydroxide.
2.Inert oil or Plasticizer.
3.Filler.
4.Oleic/Stearic acid.
CHEMISTRY
The basic ingredient of polymer paste is polyfunctional mercaptan or
polysulfide polymer. This polymer contains approximately 1 mol% of
branches to provide enough pendant mercaptan groups as chain crosslinking sites. This polymer s usually cross-linked with an oxidizing agent
such as lead dioxide. During the condensation reaction of the lead
dioxide with the polysulfide polymer two reactions take place. They are
1. Chain lengthening polymerization and
2. Cross linking reaction.
Because the pendant group comprises only a small percentage of
available –SH groups, initially, the reaction results in chain lengthening,
which causes viscosity to increase.
The subsequent cross-linking
reactions tie the chains together, forming a three dimensional network.
Curing reaction starts at the beginning of mixing and reaches its
maximum after spatulation is complete, at which stage a resilient
network has started to build. This gives adequate elasticity and strength
to be removed over undercuts readily. The polymerization reaction is
exothermic. Hot, humid conditions accelerate the setting reactions.
MANIPULATION: With the proper length of the two pastes squeezed on to a glass
slab or a mixing pad, the catalyst paste is first collected on a stainless
steel spatula and then distributed over the base paste and the mixture is
spread over the mixing pad. This procedure is continued till the mix is of

31. uniform color with no streaks of the base or the catalyst paste appearing
in the mixture. If mix is not homogenous, curing will not be uniform.
MIXING TIME: 45 seconds – 4 minutes.
SETTING TIME: 8 minutes.
WORKING TIME: 5-7 minutes.
WORKING AND SETTING TIMES: 1. Measured by oscillating rheometer.
2. Increase in temperature decreases both working & setting times.
3. Cooling the material is a practical method of increasing the working
time & when the material is carried to the mouth, setting time is
decreased by higher oral temperature.
4. Adding a drop of water accelerates curing time.
ELASTICITY: Elastic properties of these materials improve with curing time.
Longer the impression can remain in the mouth before removal, greater
the accuracy. The impression material must undergo some distortion as
it is removed from the mouth, but the elastic properties of the impression
material help minimize this distortion. Distortion can occur if the tray is
torqued. Recovery of elastic deformation after strain rate is less rapid for
polysulphides than for other kinds of materials. Polysulphides exhibit the
most permanent deformation following strain in compression compared
with the other materials. Polysulphides also sustain more distortion
when the strain rate is slow.

32. RHEOLOGY
Polysulfide is one of the least stiff of the Elastomeric impression
materials. This flexibility allows the set material to release from undercut
areas with minimum stress. Despite the lack of stiffness the unset
material has high level of viscosity. This thick consistency of the uncured
material helps displace any unwanted fluid present while seating the
impression. Due to the high level of viscosity the material does not flow
out of the tray when it is placed in the mouth.
TEAR SRENGTH: Polysulphides have highest tear resistance – About 4000 gm/cm (8
times that of hydrocolloid).
DIMENSIONAL STABILITY
The stone cast must be poured immediately since the impression is
most accurate immediately after removing it from the mouth. Sources of
dimensional change are
 During setting, most polymers contract slightly due to cross-linking.
 After setting, the by-product of condensation reaction (water) is lost
which causes shrinkage.
 After setting, there is incomplete recovery of deformation because of
the visco-elastic properties.
 Dimensional changes are greater for polysulfide than for Polyether &
for addition silicone.
 If maximum accuracy is to be maintained, the stone die or cast
should be constructed within 30 minutes. Although the material is
fluid repellant, it can absorb water, disinfectant etc. when exposed for
a long time.

33. BIOCOMPATIBILITY: 1. Probability of allergic or toxic reactions.
2. Cytotoxic.
HANDLING OF TRAY: One way to minimize the effects of polymerization shrinkage, loss
of by products, and deformation associated with distortion is to minimize
the amount of material that is used to make the impression. The most
accurate polysulphides impressions are made by using a custom acrylic
tray, because uniform thickness of material can be obtained.
A stone cast is constructed from an impression of the tissues and a
custom tray is fabricated. Important parts of the cast, such as prepared
teeth are covered with one or two layers of base plate wax & tin foil to act
as spacer for the impression material. Chemical curing or light curing
resin is used to prepare the tray. Adhesion can be obtained by the
application of minimal, uniform thickness of adhesive to prepared tray,
before the insertion of the impression material. The adhesive then forms
a tenacious bond between the rubber material and the tray.
HANDLING TECHNIQUES.
The impression material is currently available in two consistencies:

Available in 2 consistencies:
1. Tray material.
2. Syringe material.
Syringe material may have longer working & setting time. Syringe
material contains lesser filler particles, hence has greater polymerization
shrinkage and more thermal contraction. Hence it is not advisable to use
syringe material alone.

34. 
THE TECHNIQUE OF USING BOTH TRAY AND SYRINGE
MATERIAL IS CALLED “MULTIPLE MIX”
1. Mix the tray material first & fill the tray with a uniform thickness of
the material & set it aside.
2. Second person begins mixing &filling the syringe.
3.
The material is injected from the filled syringe within around the
prepared teeth.
4. The filled tray is then placed over the syringe material so that both
materials cure together.
DISINFECTION OF IMPRESSIONS: Polysulphides can be disinfected by most of the various anti
microbial solutions without adverse dimensional changes, provided the
disinfection’s time is short. Prolonged immersion may produce minimal
distortion. One recommended procedure is a 10-minute immersion in a
10% solution of sodium hypochlorite.
SHELF LIFE: 1. The material does not deteriorate appreciably when stored
under normal environment.
2. The tubes should be kept tightly closed when not in use.
3. Storage in a cool environment is advisable.
ADVANTAGES: 1. No special equipment required.
2. Superior strength in deep sulcus.
3. Finish line can be easily read.
4. Cast pouring can be delayed up to one hour.
5. Can be poured more than once.

35. 6. Adequate shelf life.
DISADVANTAGES: 1. Custom trays required.
2. Hydrophobic.
3. Sensitive to heat & humidity.
4. Severe undercuts must be blocked.
5. Objectionable odor.
6. Long setting time.
7. Moderately high shrinkage.
8. Fairly high permanent deformation.
RECENT DEVELOPMENTS.

POLYETHER PUTTY MATERIAL + POLYSULPHIDES ARE USED AS
WASH MATERIAL.
CONDENSATION SILICONES.

COMPOSITION:
The condensation silicone impression materials are supplied as
a base paste and a low viscosity liquid or catalyst paste. Because the
silicone polymer is a liquid, colloidal silica is added as a filler to form a
paste. The particles should be within the optimum range of 5-10
microns.
BASE PASTE:
1.Poly dimethyl siloxane/Liquid silicone polymer.

36. 2.FILLER-Colloidal silica/Micronised metal oxide.
REACTER PASTE:
1.Tri/Tetra functional alkyl silicates.
2.Tin compound – Stannous Octoate.

Properties of impression material are influenced by properties of
filler, according to Law Of Mixtures.

Different colors are available: Pastel pink, blue, Green, Purple.
CHEMISTRY: 1.The polymer consists of a hydroxy terminated polydimethyl siloxane.
Condensation polymerization reaction of this material involves a
reaction
with
trifunctional
and
tetra
functional
alkyl
silicates,
commonly tetraethyl ortho silicate, in the presence of stannous octoate.
These reactions can take place at room temperature & hence these
materials
are
called
as
“ROOM
TEMPERATURE
VULCANISATION” silicones.
Formation of the elastomer occurs through a cross-linking
between terminal groups of the silicone polymer & the alkyl silicate to
form a three dimensional network. Ethyl alcohol is the reaction by
product. Its evaporation accounts for the shrinkage seen in the set
polymer.
MANIPULATION: 1. Supplied as base paste & liquid catalyst.
2. A length of the base paste is dispensed onto a graduated mixing
pad.
3. One drop of liquid catalyst is added for each unit length of base.
4. Both pastes are mixed till a uniform color is obtained.

37. 5. Putty material – very thick paste and a liquid accelerator.
Manufacturer’s directions are followed to mix the material.
6. Two-paste putty system-best mixing technique is to knead the
material with the fingers.
7. Wearing gloves adds another complication; some latex gloves
contain sulfur component & this inhibits setting.
ELASTICITY: Condensation silicones impression materials are more ideally
elastic than polysulphides. They exhibit minimal permanent deformation
and recover more rapidly when strained.
RHEOLOGY: The material is more likely to respond as elastic if it is strained
rapidly. Hence, impressions must be removed quickly so that the
deformation is elastic and recoverable.
TEAR STRENGTH: Tear resistance is low. They must be handled carefully to avoid
ruining a margin of a crown preparation. Applying a force rapidly
ensures the highest tear resistance. 3000 gm/cm.
WORKING TIME: -up to 5 minutes
SETTING TIME: -10-12 minutes.
Chilling the material or mixing on cool slab slows reaction rate.

38. DIMENSIONAL STABILITY:
1. Material exhibits excessive polymerization shrinkage. Hence a puttywash technique is used.
2. The amount of linear contraction is 2-4 times greater than others.
3. This is caused by release of ethyl alcohol as an end product.
4. Polymerization reaction continues after material is clinically set.
5.Accurate
model
is
obtained
by
pouring
up
the
impression
immediately-within 30minutes.
HANDLING TECHNIQUE: Because the putty wash impression technique is used with this
material, custom tray fabrication is not necessary. Disposable stock
trays can be used to support the putty material.
1. Thick putty material placed in the tray & preliminary impression is
made (Intra oral custom tray).
2. Space for light body wash material is provided by scraping the tray
putty or polyethylene sheet used as a spacer.
3. Thin consistency wash impression material is placed over the putty
impression.
This is called as TWO STAGE PUTTY WASH TECHNIQUE OR
RELINE TECHNIQUE.
ADVANTAGES: 1. No special equipment.
2. Finish lines easily read.
3. Pleasant odor & appearance.

39. 4. Adequate shelf life.
DISADVANTAGES: 1. Requires special care in pouring.
2. Should be poured immediately after removal.
3. Easily distorted.
POLYETHER
INTRODUCED IN GERMANY DURING LATE 1960’S
COMPOSITION: 1. Low molecular weight Imine terminated prepolymers.
2.Inert filler.
3.plasticiser-Glycol ether phthalate.
4.Ester derivative of aromatic sulphonic acid.
Polyether is supplied as two pastes. Base paste contains
Polyether polymer, colloidal silica as filler, and a plasticizer such as
glycolether or phthalate. The accelerator paste contains the alkyl
aromatic suldonate in addition to the aforementioned filler and
plasticizer.
WORKING TIME: 5-7 minutes.
SETTING TIME: 5-6 minutes.
The curing time of Polyether is less sensitive to temperature
changes.
MIXING TIME: 30-45 seconds.
Working time can be altered by Base: accelerator ratio.

40. ELASTICITY:
Polyethers have been considered to be the stiffest of all the
materials. The original material was extremely difficult to remove from
undercut areas because of the high modulus of elasticity.
TEAR STRENGTH: Tear resistance is better than that of condensation silicones. But
Polyether is more prone to tearing than polysulphides. Because of this,
the margin should be carefully inspected immediately after removing the
impression.
DIMENSIONAL STABILITY: The dimensional changes of Polyether are relatively small. They
have no reaction by product. Although the residual polymerization
continues beyond the clinical time, it is much shorter than that of
polysulphides. The stiffness of the material means that the force needed
to remove the impression is greater for Polyether but the recovery is
nearly complete.
BIOCOMPATIBILITY: 1.Hypersensitivity.
2.Contact dermatitis.
3.High cell toxicity.
SHELF LIFE: Good shelf life: - Storage at room temperature.
Storing in a cool dry environment prolongs shelf life.
ADVANTAGES: -

41. 1. No special equipment required.
2. Finish line easily read.
3. Superior Dimensional stability.
4. Fast setting.
5. Cast can be poured 1-7 days later.
6. Pleasant odor & appearance.
7. Good shelf life.
DISADVANTAGES: 1. Custom tray required.
2. Very stiff.
3. Short working time.
4. Least tear strength.
5. More expensive.
6. Aromatic sulphonic acid ester catalyst – Skin irritant.
ADDITION SILICONES
COMPOSITION: -


BASE: - polyvinyl siloxane or vinyl poly siloxane.
ACCELERATOR: 1.divinyl siloxane
2.Platinum salt-catalyst (chloroplatinic acid)
3.Palladium- hydrogen absorber.
4.Retarders.
5. Fillers.

42. Both the base paste and the catalyst paste contain a form of vinyl
silicone. The base paste contains polymethyl hydrogen siloxane as well
as other siloxane prepolymers. The catalyst paste contains divinyl
polydimethyl siloxane other siloxane polymers. If the catalyst contains
the platinum salt activator, then the paste labeled base must contain the
hybrid silicone.
CHEMISTRY: Addition silicones are hydrophilic in contrast to all other silicone
impression materials. In contrast to the condensation silicones, the
addition polymer is terminated with vinyl groups and is cross-linked with
hybrid groups activated by platinum salt- catalyst. Actually, there will
not be any by-products as long as correct proportions of vinyl silicones
and hybrid silicones are present. If the proportions are out of balance
then hydrogen gas is produced as the by-product. The hydrogen gas that
evolves from the set material can result in pinpoint voids in the stone
cast.
MANIPULATION: The light body and medium body Vinyl poly siloxane are supplied
as two pastes and the Putty is supplied as two jars of high viscosity base
and catalyst materials. These materials are suitable for an automatic
dispensing & mixing device. With the mechanical mixing device there is
greater uniformity in proportioning & mixing, less air is incorporated into
mix and mixing time is reduced. The mixed impression material is
ejected directly into adhesive coated tray or onto the prepared tooth. The
basic automatic mixers sold by manufacturers are interchangeable. The
tips vary in diameter, length & perhaps more importantly in number of
spiral units within the tips. More units provide more thorough mixing.

43. Thus an impression material that is adequately mixed in a spiral unit
may be inadequately mixed with another spiral tip.
WORKING AND SETTING TIMES.
More sensitive to temperature than polysulphides.Adding suitable
retarders & cooling the mixing slab can alter both. Working and setting
times can be extended by the addition of a retarder and by cooling the
mixing slab.
ELASTICITY: The vinyl polysiloxane impression materials are the most ideally
elastic of all the currently available materials. Distortion on removal from
undercuts is virtually non-existent. Material is extremely difficult to
remove from undercut areas because of high molecular elasticity.
stiffness is proportional to consistency of the material.
RHEOLOGY: As one of the most pseudoplastic impression materials, the effect
of increased strain rate on the unset material is quite pronounced for
vinyl polysiloxane. This large discrepancy between flow properties of the
material under strong force such as syringing, and light force such as
during seating the tray can be used for advantage as a one step material.
Medium body material is used to capture the fine detail and is available
as a one-stage material. The basic difference between the material used
for injection and tray is the viscosity.
TEAR ENERGY: The resistance to tearing is adequate. These materials if not
handled properly will tear rather than stretch like polysulphides.
Polyether is more prone to tearing than polysulphides.

44. DIMENSIONAL STABILITY: The
vinyl
polysiloxane
impression
materials
are
the
most
dimensionally stable of all the current materials. No volatile by product is
released to cause shrinkage of the material. The clinically set material is
close to being completely cured, so that little residual polymerization
occurs later to contribute to polymerization shrinkage. Impression does
not have to be poured immediately.
BIOCOMPATIBILITY: The danger of leaving a piece of the material during removal of the
impression can be avoided by proper handling. A foreign body of
impression material can cause severe gingival inflammation.
HANDLING OF THE TRAY: Stock tray is used for reline material. The primary putty
impression actually serves as a custom tray for wash or reline material.
HANDLING TECHNIQUE: Putty wash technique has been popular for vinyl siloxane
impression material. Putty wash technique is more convenient and the
bulk of the material is formed by the highly filled putty material that has
relatively low polymerization shrinkage and a low thermal contraction
coefficient. Supplied as single phase or monophase.
DISINFECTION: These impression materials are easily disinfected by immersing it in
either of the following solutions.
1.10% Hypochlorite.
2. 2% glutaraldehyde.

45. SHELF LIFE: Material has a shelf life of about 2 years. The containers must be
tightly closed. Viscosity not affected by temperature. shelf life can be
prolonged by storing it in a cool, dry environment.
ADVANTAGES: 1. Superior dimensional stability.
2. Can be poured more than once.
3. Pleasant odor and appearance.
4. Good shelf life.
5. Shorter setting time.
6. Adequate tear strength.
7. Extremely high accuracy.
8. Less distortion on removal.
DISADVANTAGES: 1. Custom tray required for double mix technique.
2. Hydrogen gas may evolve from some materials.
3. Most difficult to pour, requires special care.
4. Expensive.
RECENT ADVANCES

Visible light cured polymer urethane dimethacrylate.

The advantages of this material are that its working time is
controlled by the operator.

ion: 1.polyether urethane dimethacrylate.
2. Photo initiators.
Composit

46. 3. Photo accelerators.
4.silicone dioxide fillers.
SETTING PROPERTIES OF ELASTOMERS
Material
Condensation
silicones
Addition
silicones
Poly ether
Viscosity
W.T
S.T
Dimensional
(in Cp)
Polysulfides
Consistency
(min)
(min)
change in
Low
Medium
High
Low
60,000
110,000
450,000
70,000
4-7
3-6
3-6
2.5-4
7-10
6-8
6-8
6-8
24 hours
0.40
0.45
0.40
0.60
High
70,000
2-2.5
3-6
0.38
Low
Medium
High
Very high
Low
Medium
Medium+
150,000
150,000
150,000
150,000
130,000
130,000
130,000
2-4
2-4
2.5-4
1-4
2.5
2-3
3-4
4-6.5
4-6.5
4-6.5
3-5
4.5
3.4-5
4.5-5
0.17
0.15
0.14
0.15
0.23
0.24
0.23
thin
High
130,000
2.5
4-5
0.19

47. MECHANICAL PROPERTIES OF ELASTOMERS
Material
Consistency
Permanent
Flow
Hardness
deformation
Polysulfides
Low
Medium
Condensation
High
Low
3-4
3-5
strength
0.5-2
0.5-1
20
(gm/cm)
2500-
30
7000
3000-
3-6
1-2
0.5-1 35
0.05-0.115-30
7000
---2300-
2-3
0.02-
50-65
2600
----
0.05-0.4
0.05
0.01-
35
1500-
0.05-0.3
0.03
0.01-
50
3000
2200-
0.2-0.5
0.03
0.01-
75
3500
2500-
1.5
1-2
0.1
0.03
0.02
35-40
35-60
4300
1800
2800-
30-50
40-50
4800
2500
3000
silicones
Very high
Addition
Low
silicones
Medium
Very high
Polyether
Low
Medium
Medium+ thin
High
Tear
2
2
0.04
0.02