Indian Dental Academy: will be one of the most relevant and exciting training center with best faculty and flexible training programs for dental professionals who wish to advance in their dental practice,Offers certified courses in Dental implants,Orthodontics,Endodontics,Cosmetic Dentistry, Prosthetic Dentistry, Periodontics and General Dentistry.

1. IMPRESSION MATERIALS
First put your patient in a hot baking oven or a searing furnace, then
turn them round and round in a centrifugal machine and pour hot
metal on them. Or better still, heat them above 1000°C in a ceramic
furnace and fine till well done. Don’t look shocked, - not a very
pleasant idea right. Well, that what impression materials same us
from.
Let’s confabulate on this topic under these myriad subheadings:
Introduction
History
Definition and Classification
Ideal requisities
Impression materials and Respective techniques
- Hydrocolloids.
- Elastomeric materials.
- Inelastic or rigid.
Latest Advances
Summary and conclusion
1

2. Bibliography
INTRODUCTION
One of the cornerstones of modern dental practice is the accurate
recording and reproduction of tooth tissue details. Material science
plays a pivotal role in dentistry and impression materials form a vital
core in that foundation. Mimicking the intricate details required in
dental practice in the demanding and challenging oral conditions is an
acid test for any material. Right from the advent of initial crude
impression materials to the modern sophisticated elastomeric
chemistries the quest for the perfect replicating material has spanned
a diverse range of materials, techniques and devices. From dentures to
implants, inlays to crowns, orthodontic to pedodontic to
prosthodontist to restorative and endodontists, there is no branch of
high quality dentistry untouched by the magic of these materials and
no dentist who has not marveled at their ingenuity. Let in delve
deeper into this fascinating and vitally important class of material
science and understand them for their successful and ideal use.
An impression, in general terms is a mark produced on a surface by
pressure. The word impression is divided from latin word
“impression”.
2

3. From impression, it is possible to produce an exact replica of
the dental structures of interest using a cast or die material such as
dental stone or some type of plastic.
There are various categories of impression materials. Each type
processes characteristics which influence the purpose to which it is
best suited, i.e. different types of impression materials have been
developed few different application. These materials have different
physical properties and each has certain advantage or disadvantages.
Thus, an understanding of physical characteristics and
limitation of each material is necessary for its successful use in
clinical dentistry. It would be virtually impossible to perform high
quality restorative dentistry without impression materials.
3

4. History:
The history of dentistry has been influenced a great deal by
development of impression materials. From the cumbersome and
highly unpredictable materials of yore replicating materials now
match highly exacting standards.
In fact the earliest dental impression materials was waxes that
was used in 18th
and 19th
century. Beeswax was apparently the
materials first used in making impression in the mouth.
In 1782 William Rae said that he get the measurement of jaws
in a piece of wax pushed into the gums, afterward making a cast of it
with plaster of paris.
In 1842, Montgemery discovered gutta-percha. In 1848 Colburn
or Blake said that it should thoroughly soaked in boiling water, then
kneaded and moulded in the same way as wax and immediately by
placed in the mouth and firmly pressed to its place.
In 1930 according to Applegate, a series of true physiologic
waxes was developed by cooperative effort of Drs. G.C. Bawles, S.G.
Applegate.
4

5. The first real impetus in the use of the zinc oxide eugenol for
impression materials came from two pioneer dentists, A.W. Ward and
E.B. Kelly, during early 1930’s.
Alginate type materials were experimented with over many
years, however, the first patient was awarded in 1936. This in early
1940’s the first irreversible hydrocolloidal (alginate) impression
material was developed.
In middle 1950’s the elastomeric impression materials were
introduced. Polyethers were introduced to dental profession in
Germany in late 1960’s.
Recently, a new polyether urethane diamethacrylate photo-
initiated elastomeric impression material has entered the market.
Definition and Classification:
Impression material is a dental materials whose function is to
accurately record the dimensions of the oral tissues and their spatial
relationship.
An impression essentially is a negative replica of some
structure. In dentistry, this replica usually is made of teeth or gingival
tissue of maxillary such as mandibular arch.
5

6. Classification:
Impression Materials:
Elastic Non-elastic
Hydrocolloid Rubbers ZOE Impression compound
Agar Alginate
Mercaptam Silicones Polyether
Lead peroxide Clean Condensation addition
Catalyst catalyst
Many criteria may be used to classify impression materials.
They are listed as follows:
1) By their generic chemical same:
For example, one may refer to silicone materials or zinc oxide
eugenol materials or even particular commercial brands of these
materials.
2) According to the manner in which they harden:
A) Set by chemical reaction (Irreversible)
 Plaster of paris
 Zinc oxide eugenol.
 Alginate
 Non-aqueous elastomers.
6

7. B) Set by temperature change (reversible)
These materials are again subclassified as:
1) Thermoplastic materials.
- Impression compound.
- Wax.
2) Non-thermoplastic material and agar
3) According to the ability of set material to be withdrawn over
undercuts.
A) Elastic impression materials:
- Alginate.
- Agar.
- Non-acqueous elastomers.
B) Non-elastic or rigid impression materials:
- Impression compound.
- Impression plaster.
- Zinc oxide eugenol.
- Wax.
4) According to the use of material in dentistry:
A) Materials used for obtaining impression of dentulous
mouth.
- Alginate.
- Agar.
- Non-acqueous elastomers.
7

8. These materials are used in crown and bridge, partial denture
and inoperative dentistry.
B) Materials used for obtaining impression of edentulous
mouth:
- Impression compound.
- Impression plaster.
- Zinc-oxide eugenol.
- Wax.
5) According to the viscosity or the tissue displacement:
Materials which are initially vary fluid are often
classified as mucostatic impression materials, because they are
less likely to compress soft tissues, while materials which are
initially more viscous are classified as mucocompressive.
A) Mucostatic materials:
- Impression plaster.
- Agar.
- Alginate.
B) Muco-compressive materials:
- Impression compound.
8

9. Ideal Requirements of Dental impression materials:
To produce an accurate impression, the materials used to
produce replicas of intraoral and some extraoral tissues should fulfill
the following criteria.
1. Pleasant taste, odor and esthetic color.
2. Not contain any toxic or initiating ingredient.
3. Have adequate shelf life for storage and
distribution.
4. Be economical.
5. Be easy to use with minimum equipment.
6. Have adequate setting characteristics that meet
clinical requirements.
7. Possess satisfactory consistency and texture.
8. Adequate strength so that it will not break or tear
while removing from the mouth.
9. Possess elastic properties with freedom from
prominent deformation after strain.
9

10. 10. Exhibit dimensional stability over temperature
and humidity ranges.
11. Readily wets oral tissues.
12. Compatibility with cast and die materials
13. Accuracy and faithful reproduction of details.
14. Ability to be electroplated.
15. Readily disinfected.
16. No release of gas during setting of impression or
cast and die materials.
- They should be fluid enough to adapt to the
oral tissue and viscous enough to remain content in the tray that
delivers impression to the mouth.
- While in the mouth they should transform
(set) into a rubbery solid in a reasonable amount of time
(should be less than 7 minutes).
- The set impression should not distort or tear
when removed from the mouth. Material should dimensionally
stable so the cast can be poured.
10

11. - In this seminar I will be discussing about
the following impression material:
Hydrocolloids
Reversible Irreversible
1) Hydrocolloids – It is a suspension of time part less the 1µm).
Colloids are often classified as a faster state of matter, the
colloidal state, because of their difference in structure, constitution
and reaction. If the particles are large and can be seen by the naked
eye as through a microscope, the system is turned a suspension or
emulsion. There suspended particles do not readily diffuse and tend to
fall out of the suspending medium unless some type of bonding is
employed to maintain the suspension or emulsion.
The molecules of the colloid remain dispersed nature of fact
that they carry small electrical changes and repel one another within
the dispersion medium.
Types of colloids:
With the exception of the gaseais state (two gases), colloidal
sol may be composed of combination of any other states of matter.
For example:
11

12. - Liquid / solid in air (aerosol).
- Liquid / solid in liquid (Lysol).
- Gas /liquid / solid in solid.
All solids are termed as sols, and not just more in which a
liquid is the dispersion medium. Since hydrocolloid impression
materials are solids (polysaccharides) suspended in liquid (water)
they are lyophilic (liquid leaving) solutions. In general, organic
colloids are lyophilic, whereas the metallic dispersion tend to
lyophobic (liquid heating).
Dental hydrocolloid impression materials exist in two form: sol
or gel form. In the sol form they are fluid with low viscosity and there
is random arrangement of polysaccharide chains. In gel form the
materials are more viscous and may develop elastic properties if the
long polysaccharide chain become aligned.
Gels:
If the concentration of the dispersed phase in the hydrocolloid
is proper amount, the sol may be changed to a semisolid material
known as a gel or jelly when the temperature is decreased. The
temperature at which this change occurs is known as gelation
12

13. temperature is about 37°C as slightly higher few agar. The process is
reversible. Thus agar is known as reversible hydrocolloids.
Reversible hydrocolloids:
These impression materials are compounded from reversible
agar gels, when heated, they liquefy argo into the solution state and
on cooling they return to gel state. Since this process can be repeated,
a gel of this type is described as reversible.
Agar impression are dimensionally unstable on standing and so
models should be made so soon as possible after the impression is
taken. If the agar type of impression material is used carefully with an
understanding of its physical properties it is an excellent elastic
impression material of high accuracy in registering fine detail.
Chemical Ingredients:
The main active constituent of a reversible hydrocolloid
impression product is agar-known commercially as agar-agar, which
is a sulfuric ester of a galactan complex. This material forms a colloid
with water which will liquefy between 71°C and 100°C and set a gel
again between 30°C and 50°C.
13

14. Composition and function:
1) Agar (12.5%) – To prevent the dispersed phase of the solution
and the continuous fibril structure of the gel.
2) Potassium sulfate (1.7%) – To counteract the inhibiting effect
of beewax and agar on the setting of gypsum model materials.
3) Borax (0.02%) – To produce intermolecular attraction in order
to improve the strength of the gel.
4) Alkyl Benzoate (0.1%) – To prevent the growth of the mold in
the impression material during storage.
5) Water (85.5%) – To provide the continuous phase in the
solution and second continuous phase is the gel, the amount
centrals the flaws properties of the solution and the physical
properties of the gel.
6) Colour and flavour (trace) : To improve the appearance and
taste.
The agar content is reduced in the syringe type of materials so
that it is much more fluid at the time of injection than the tray
material at the time of insertion.
14

15. Properties:
1) Viscosity of the sol:
In sol form agar is sufficiently fluid to allow detailed
reproduction of hard and soft tissues. Its low viscosity classifies it as
a mucostatic materials as it does not compress or displace soft tissues.
Agar is a visco-elastic material.
2) Strength:
The strength values of importance for agar impression are tear
strength and the compressive strength. Since the agar impression are
viscoelastic the strength preparation are time dependent and higher
compression and tear strong occur at higher rates of loading.
The compressive strength of a typical agar impression material
is 8000gm/cm2
(0.245Mpa or 35.6Psi). The tear strength is about
700gm/cm though there is no ANSI/ADA specification requirement.
3) Gelation temperature:
The temperature at which the hydrocolloid impression material
sets to a gel is important. The gel must be heated to a higher
temperature. Known as liquefaction temperature (70°C to 100°C) to
return to its sol condition.
15

16. It transforms into a gel between 37° and 50°C. The exact
gelation temperature depends on several factors, including the
molecular weights, the purity of agar and the ratio of agar to other
ingredients.
4) Dimensional stability:
Storage
condition
Dimensional
change
Causes
1) Air Shrinkage Evaporation of water foam
gel
2) Water Expansion Inhibition and absorption
of water
3) 100% relative
humidity
Shrinkage Syneresis
4) Inorganic salt
solution
Expansion /
Shrinkage
Depends on relationship of
electrolyte in gel and
solution.
When stored in air, agar gels losses water and contract, when
they started in water it results in absorption and swetting. So agar
impression are best stored in 100% relative humidity not more than 1
hour.
5) Flexibility:
The ADA specification requirement for flexibility allows a
range of 4% to 15% when a stresses of 14.2 psi (1000gm/cm2
) and
16

17. most agar impression materials meet this requirement. A few hard
setting materials have a flexibility of 1% to 2% agar gel has very poor
mechanical properties and tears at very low levels of stress. Inter
proximal and subgingival areas are very difficult to record with this
type of impression materials.
Advantages:
17. Hydrophilic impression materials since it
respondless critically to moisture, fluids in the sulcus are less
tolerated.
18. Long working time.
19. It does not require any custom tray.
20. Because there is no mixing of separated
components, the potential for errors of measurement are
eliminated.
21. It is clean and pleasant.
22. It is compatible with die stone, enabling bubble
fice casts.
23. It is a material of high accuracy and registers
fine detail.
17

18. 24. Casts are easily removed.
25. Reliability.
18

19. Disadvantages:
1. Low tear resistance.
2. Low dimensional stability and the special
handling to prevent dimensional change.
3. Potential hazard of a bown to the patient.
4. Initial purchase of the conditioning unit.
5. Easily distorted as a result of movement
during gelation.
6. Rapid cooling can cause concentration of
stresses near the tray during gelation.
Clinical Presentation:
The agar impression material is supplied as a gel in a metal,
plastic or other types of collapsible disposable or as a number of
cylinders in a glass jar. The first form is used with a water cooled
impression tray and the second for injector with a syringe. The
syringe material may be used in combination with a tray material or a
copper-band technique as with impression compound.
19

20. Types of failure:
Type Cause
1. Grainy material. A. Inadequate boiling.
B. Storage temperature
too low.
C. Storage time too low.
2. Seperation of tray and
syringe material
A. Water-
soaked layer of tray material
not removed.
A. Gelation of either
syringe or tray material.
3. Tearing A. Inadequate bulk.
B. Premature removal
from mouth.
C. Syringe material
partially gelated when tray
seated.
4. External bubbles A. Gelation of syringe
material preventing flaw.
5. Irregularly shaped voids A. Material too cool or
grainy.
Laminate technique (Agar alginate combination impression):
In a recent modified procedure, the tray hydrocolloid is
replaced with a mix of chilled alginate that will bond with the syringe
agar. The alginate gels by a chemical reaction, where the agar gels by
20

21. means of contact with the cool alginate rather than the water
circulating through the tray.
The equipment needed for taking an agar impression can be
minimized by use of agar alginate syringe tray combination
impression. In this procedure a syringe type of agar in a cartridge is
heated in boiling water for 6 minutes and stored in 65°C water bath
10 minutes before use. The tray alginate of the regular set type is
mixed with 10% more water than normally recommended and it is
placed in a tray. The agar is injected around the preparation, and the
mixed alginate is promptly seated on the top of agar. The alginate sets
in about 3 minutes and agar sets with this time as a result of being
cooled by alginate. During the setting of alginate and gelling of the
agar a bond forms between them. The impression may be removed in
about 4 minutes.
The accuracy of the agar-alginate impressions was determined
with a laboratory model. Impressions were taken and paired in high
strength stone. The accuracy of 1) The interpreparation distance, 2)
buccolingual diameter and 3) the preparation height of the models
were measured and compared to values obtained with polysulfide
condensation silicone, polyether and addition silicone impression
21

22. materials. Except for the interpreparation distance the agar-alginate
system had same order of accuracy as rubber impression materials.
The advantage of agar-alginate combination impression
compared to agar system alone is the simplification of the heating
equipment, the elimination of water cooled impression trays and the
overall simplification of the procedure. In addition, the agar is more
compatible with gypsum model materials than alginate, the accuracy
is acceptable and the cost of the material is low.
Wet field technique:
Another recent technique has become popular for making
impression in a wet field. It differs in than the tooth surface and tissue
are purposely left wet. The areas 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
hydrolic pressure of the viscous tray material forces the fluid syringe
hydrocolloide down into the areas to be restored. This motion
displaces the syringe material, blood and debris with the stronger tray
material throughout the sulcus.
Irreversible hydrocolloide:
22

23. At the end of the last century, a chemist from Scotland noticed
that certain brown seated (algal). He named it algin. This natural
substance was later identified as a linear polymer with numerous
carboxyl acid groups.
When agar impression material become scarce because of
World war II (Japan was a pumice scarce of agar), research was done
to find a suitable substitute. The result was of cause the present
irreversible hydrocolloid, or alginate.
Alginate is an elastic mucostatic impression material. It is more
widely cured than any other elastic impression material. The principle
factors responsible for the success of this type of impression material
are:
1. Early to manipulate.
2. It is comfortable to the patient.
3. It is inexpensive.
4. Does not require elaborate equipment.
Uses:
1. Used widely in complete and partial denture prosthesis and
orthodontics.
23

24. 2. To prepare study models of either the entire dental arch or a
segment of it.
3. For master impression in rigid or in divided trays.
4. Whenever there is undercut and not suitable for rigid materials.
5. Useful to a limited extent in inlay, crown and bridge
procedures.
6. In mouth where there is excessive flow of saliva.
7. As a duplicating mateial.
8. To prepare gypsum models of patients for the preparation of
alternate in mouth protectors.
Composition:
Ingredients Function
1. Potassium alginate 18% To dissolve in water and react
with calcium ions.
2. Calcium sulfate dehydrate 14% To react with potassium alginate
to form an insoluble calcium
alginate gel.
3. Potassium sulfate, potassium
zinc fluoride silicate or borates
10%`
To counteract the inhibiting effect
of the hydrocolloids on the setting
of gypsum, giving a high quality
surface to the die.
24

25. 4. Sodium phosphate 2% To react preferentially with
calcium ions to provide working
time before gelation.
5. Diatomaceous earth or silicate
powder 56%
To control the consistency of the
mixed alginate and the flexibility
of set impression.
6. Glycols-small amount To make powder distlys.
7. Winter gum, peppermint trace To produce a pleasant taste.
8. Pigments – Trace To provide colour.
Change in the water / powder ration will alter the consistency
and setting times of the mixed material. Mixing time for regular set is
1 minute. Fast set alginate should be mixed with water for 45 seconds.
According to ADA specification No 18 for alginate impression
material requires that it no less than 1.25 minutes.
Flexibility:
The ANSI/ADA specification permits a range of 10% to 20% at
a stress of 1000gm/cm2
. The compressive strength ranges from 5000-
8000gm/cm2
.
ADA specification requires a compressive strength of atleast
3500gm/cm2
. The tear strength vary from 350 to 600gm/cm.
Types of failure:
25

26. Type Cause
1. Grainy material a. Improper mixing.
b. Prolonged mixing.
c. Undue gelation.
d. Water / powder ratio
too low.
2. Tearing a. In
adequate bulk.
b. M
oisture contamination.
c. Pr
emature removal from mouth.
d. Pr
olonged mixing.
c. Bubbles a. Undue gelation, preventing
flow.
4.Irregularly shaped voids a. Air incorporated during
mixing.
b. Moisture or debris on tissue.
5. Rough or chalky stone cast. a. Inadequate cleaning of
impression.
b. Excess water left in
impression.
c. Premature removal of cast.
d. Learning cast in the
impression too long.
6. Distortion a. Impression not paired
immediately.
b. Movement of tray during
gelation.
c. Premature removal from
26

27. mouth.
d. Improper removal from
mouth.
e. Tray held in mouth too
long.
Development in alginate:
1) Dustless alginate:
Many materials have been formulated which give off
little or no dust particles, so avoiding dust inhalation. This can
be achieved by coating the material with a glycol / glycerine.
2) Chromatic alginates:
During the setting reaction, the pH value of the fluid
mass changes. Because of this, some manufacturers include
acid/base indications in their formulation so that a colour
change of the setting mass indicates that a certain point has
been reached, usually the point at which the tray should be
loaded or inserted into the mouth.
27

28. 3) Modified alginate:
The traditional alginate is supplied as powder and water.
There is no reaction until the water is added to the powder to
initiate the reaction.
There is yet another modification of this concept. The
two component system may be in the form of two pastes. One
certain alginate sol, while the second contain the calcium react
or impression materials of this type are said to contain (9/50)
silicone and humeetants to stabilize. It is said to be better than
water / powder alginate.
4) Silicone alginates:
Alginate modified by the incorporation of silicone
polymers have been developed. There are supplied as two parts
which are mixed together. The materials are considered as
hybrids of alginates and silicone elastomers but their properties
are closely related to more of alginates.
28

29. Next we come to the most important categories of impression
material:
Elastomers or rubber dam impression materials:
Non-aqueous elastomeric dental impression materials as per ADA Sp.
No. 19 are liquid polymers that cross link or polymerize with various
reagents to become solid elastic rubber at room temperature. They are
essential in today’s high teeth dental age of metal free ceramics and
high precision castings.
Rubber impression materials:
Three major types of rubber impression materials are used to
record dental impressions. They are polysulfides, silicones (poly
siloxanes) and polyethers. The silicone type is subdivided into two
classes, condensation and addition or vinyl polysiloxane.
Polysulfide:
These materials are supplied in three consistencies:
1. Low (syringe or wash).
2. Medium (regular).
3. High (tray).
29

30. These are supplied as two pastes in collapsible when, one
labeled base and the other labeled accelerator or catalyst.
Base:
- Polysulphide polymer – 80-85% weight.
- Titanium dioxide, zinc sulfate, copper
carbonate or silica – 16-18% weight.
Accelerator:
- Lead dioxide – 60-68%.
- Dibutyl or dioctyl – 30-35%.
- Sulfur – 3%
Other substances such as Magnesium stearate and deodorants
– 2%
Properties:
1. The elastic properties of these rubber impression materials
improve with curing time (i.e. the longer the impression can
remain in the mouth before removal, the greater the
accuracy).
30

31. 2. Polysulfide ranks as one of the best stiff of elastomeric
impression materials.
3. They have the highest resistance to tearing (thin sections of
polysulfide impression material are less likely to tear than
similar thickness of polyether or silicone impression
material).
4. The stone should be poured immediately because, the
impression in the most accurate immediately after removing
it from mouth.
5. The ADA specification for testing biocompatibility includes
dental impression materials, despite the fact that the
probability of allergic or toxic reactions from impression
materials or their components is small.
6. Perhaps the most likely elastomer induced biocompatibility
problem occurs when a piece of the impression material is
left in the gingival sulcus. The irritation can range from
minor to severe. The radioopacity of the load containing
polysulfide materials is an advantage in these situations, as
in the materials resistance to tearing.
31

32. 7. One way to minimize the effects of polymerization
shrinkage, less reaction by-product and deformation
associated with distortion is to minimize the amount of
material that is used to make the impression. The most
accurate polysulfide impression are made by using a custom
acrylic tray.
8. The polysulphide polymer has a molecular weight of 2000
to 7000 with terminal and pendant mercaplan groups. The
terminal and pendant groups of adjacent molecules are
oxidized by the accelerator to produce chain extension and
crosslinking. The reaction results in a rapid increase in
molecular weight, and the mixed part is converted to a
rubber. The weight percent of the filler in the base increases
from low medium to high consistencies. The particle size of
the filler is about 0.3µm. Although the most common active
ingredient in the accelerator is lead dioxide, some
magnesium oxide may also be present.
32

33. Other oxidizing agents such as hydrated copper oxide or
organic peroxides such as amine hydroperoxide have been used
as a substitute for lead dioxide.
Properties:
1. Condensation silicone impression materials are more ideally
elastic than polysulfides. They exhibit minimal permanent
deformation and recover rapidly when strained. Like
polysulfides, there materials are not very stiff, which means it
is not difficult to remove them from undercuts without
distortion.
2. The viscoelastic characteristics of these materials suggest that
they can respond elastically as an viscous liquids that easily
sustain permanent deformation.
3. Tear resistance is low for condensation silicone impression
materials. Although they do not tear as easily as alginates or
agar hydrocolloids, they must be handled carefully to avoid
relining a margin of a crown preparation when it is run.
4. The excessive polymerization shrinkage of the condensation
silicones requires a modification of the impression making
33

34. technique to produce accurate impressions. A putty-wash
technique is used for condensation silicones.
5. Silicone is one of the most biologically inert materials.
6. The alkyl silicates are slightly unstable, particularly if they are
mixed with a tin compound to form a single catalyst liquid.
Thus a limited shelf life may result because of oxidation of the
tin component within the catalyst.
7. The condensation silicones are compatible with all gypsum
products.
II) Silicone Rubber Impression materials:
Developed to overcome disadvantages of polysulfide material, these
are based on silicone technology and are of 2 types:
- Condensation silicones.
- Addition silicones.
Available in various color and viscosities.
Silicones (Polysiloxane):
Condensation silicones are usually supplied in low and putty
like consistency. These are supplied as base and an accelerator. The
34

35. base contains a moderately low molecular weight silicone called a
dimethyl siloxane which have reactive terminal hydroxyl groups.
Fillers may be copper carbonate or silica having particle size from 2
to 8µm in concentrations from 35% to 75% for low to putty
consistencies. The accelerator may be a liquid that consists of
stannous octoate suspension and alkyl silicate or it may be supplied as
a paste. The reaction proceeds a three dimensional network rubber
with the liberation of ethyl alcohol and an exothermic temperature
vice of about 1°C. Besides viscosities common to the polysulfides the
condensation silicones are also supplied in an extremely high
viscosity or putty material. These materials are used in the putty wash
techniques.
Polyethers:
Polyether impression materials are supplied as a medium
consistency type in a base and an accelerator tube. The base is a
moderately low molecular weight polyether with ethylene imine rings.
Properties:
1. The polyethers have always been considered the stiffest of the
impression materials, excluding the high viscosity putties.
35

36. Some of the new formulations of regular or medium-bodied
material are actually less stiff.
2. The pseudoplastic characteristics allows the original single-
viscosity materials to be used as both syringe and tray
materials.
3. Tear resistance is better than that of the condensation silicone
impression materials. However, polyether is more prone to
tearing than polysulfide.
4. The dimensional change of the polyether impression material is
small. Like the addition silicones, polyether have no by-
product.
5. The most likely elastomeric induced problem for the patient
arises from pieces of the impression materials being left in the
sulcus. The irritation can range from mild to severe.
6. Storing in a cool, dry environment prolongs the shelf life.
The terminal groups. The catalyst paste contains 2.5-dichloro
benzine sulfonate as a cross-linking agent, along with a thickening
agent. A separate tube contains a thicker that includes actylpindate
and about 5% methyl cellulose as a thickening agent. The rubber is
36

37. formed by cationic polymerization and opening of imine rings. The
setting reaction is slightly more exothermic than that of the other
rubber impression materials with a temperature rice of about 4°C.
Addition silicone (poly vinylsiloxanes):
The addition type is available in low, medium, heavy and very
heavy putty consistencies and is also polysiloxane. The base contains
a moderately low molecular weight polymer with siloxane (-Si-H)
groups and filler. The accelerator (or catalyst) contains a moderately
low molecular weight polymer with vinyl terminal groups, pulls filler
and chloroplatine acid catalyst. Several products contain finely
divided palladium or platinum, which absorbs hydrogen.
A retarder, a liquid low molecular weight polymer of the same
type as the base polymer is available to extend the working and
setting time. Silicone rubber impression are hydrophobic when mixes
of gypsum products are poured into them.
Properties:
1. As one of the most pseudoplastic impression materials, the
effect of increased strain rate on the unset material is quite
pronounced for vinyl polysiloxane.
37

38. 2. The resistance to tearing is adequate, similar to that of
condensation silicone. If not handled correctly, these materials
will fear rather than stretch like polysulfides.
3. Vinyl polysiloxane impression materials are most
dimensionally stable of all existing materials. No volatile by-
product is released to cause the material to shrink.
The base and catalyst putty of addition silicones are mixed by
hand. If they are mixed by the operator while later gloves are being
warm, the setting time is lengthened or the material will not set.
Sulphur compounds used in the vulcanization of latex rubber gloves
can migrate to the surface on storage during mixing of the two putties,
there compounds are incorporated into the mix and poison the
platinum containing catalyst resulting in retarded or no
polymerization.
In general polysulfides have the longest working time, followed
by silicones and polyethers.
38

39. Comparison of certain characteristics of elastomeric dental
impression:
Polysulfide
Condensation
silicone
Polyether
Addition
silicone
Mixing Fair to easy Fair to easy Easy Easy
Flow Variable Good Good Good
Stock Fair to good Fair Good
Fair to
Good
Elastic
recovery
Fair Very good Very good Excellent
Advanced Inpleaus Acceptable Acceptable Acceptable
Clean up Difficulties Easy Easy Easy
Types of failures:
Type Cause
1. Rough or uneven
surface on impression
a. Inc
omplete polymerization caused by
premature removal from mouth.
b. To
o rapid polymerization from high
humidity or temperature
2 Bubbles a. Too rapid polymerization preventing
flow.
b. Air incorporated during mixing.
39

40. 3. Irregularly shaped
voids
a. Inadequate cleaning of impression.
b. Excess water left on the surface of the
impression.
c. Premature removal of cast.
d. Improper manipulation of stone.
4. Distortion a. Lack of adhesion of rubber to the tray
caused by not enough coats of
adhesive, filling tray with material too
soon after applying adhesive, or using
wrong adhesive.
b. Lack of mechanical retention for more
materials where adhesive is
ineffective.
c. Excessive bulk of material.
d. Movement of tray during
polymerization.
6. Improper removal
from mouth
a. Premature removal from mouth
Advantages of the elastomeric impression material:
1. They are vary elastic in nature.
2. They have good dimensional stability.
40

41. 3. The impression material do not have to be paired within
half hour unlike hydrocollides.
4. More than one cast can be made successfully within or
half hour.
5. Exceptionally smooth dies can be made.
6. Gum retraction or gingival retraction is not always
necessary.
7. Accuracy is comparable to that of the hydrocollides.
Visible light cured impression material:
In early 1988, a visible light cured impression material was
introduced (Genesis, L.D. Caulk). This material is available in two
viscosities. The light body material is packed in disposable syringe
and the heavy body material is packed in tubes. This material has
excellent elasticity and very low dimensional shrinkage upon storage.
It may be poured immediately or upto 2 weeks later. The
material is rigid and it is recommended that severe undercuts should
be blocked out to case removal of the impression.
41

42. Manipulation:
No mixing as syringe loading is necessary. The light body
material is syringed into the sulcus, around and over the preparation
and portion of the adjacent teeth. A clear tray is loaded to the full
time with heavy body material. After the tray is seated in the mouth,
both viscosities are cured simultaneously using a visible light curing
unit having an 8mm or larger diameter probe. The curing time is
approximately 3 minutes.
Advantages:
1. Control over the working time.
2. Curing time is relatively short (3 minutes).
3. Excellent clinical, physical and mechanical properties.
Disadvantages:
1. Need for special tray which should be transparent to the
visible light received to case the material.
2. If delay occurs before placement, the material should be
stored in a dark place away from light.
3. Difficult to light cure the remote area.
42

43. III) Polyethers:
Introduced in Germany in late 1960’s it has good mechanical
properties and dimensional stability but is expensive.
Again available in different viscosities and a base and
accelerator.
Composition:
Base:
Polyether polymer.
Colloidal silica – filler.
Glycolether or phthalate – plasticizer.
Accelerator:
Aromatic sulfonate ester – cross linking agents.
Colloidal silica-filler.
Phthalate or glycolether – plasticizer
Chemistry and setting reaction:
It is cured by the reaction between azinidine rings which are at the
end of branched polyether molecule. The main chain is a copolymer
of ethylene oxide and tetrahydrofuran. Cross linking is via the
43

44. aromatic sulfonate ester via the imina end gps reaction is exothermic
(4-5°C↑).
Properties:
1. Pleasant odor and taste.
2. Mixing time of 30 seconds, setting time of 8.3 minutes.
3. Curing shrinkage is low (0.24%) permanent deformation is
also low (1-2%) can absorb water and change dimension.
4. Very stiff (flexibility of 3%) requires extraspacing of upto
4mm.
5. Tear strength is good (3000gm/cn2).
6. It is hydrophilic, so moisture control is not a critical. Has
best compatibility with stone.
7. Can be electroplated with silver and copper.
8. Excellent shelf tip above 2 years.
The material should not be used with patients with a known always or
sensitivity to urethanes, acrylic or methacrylates.
Impression techniques:
There are two techniques:
44

45. - Multiple mix technique.
- Reline technique.
a) Multiple mix technique:
Two consistencies of material are provided one for use with the
tray and other for use with the syringe type has longer working and
setting time, and a greater polymerization shrinkage and thermal
contraction.
In the multiple mix technique, both the syringe and tray
material are used for same impression. The tray material is usually
mixed first and filled into the tray to a uniform thickness and set
aside. The syringe material is mixed on a separate mixing pad, loaded
into a syringe and injected into the prepared teeth. The filled tray is
then carried to place.
b) Reline technique (putty-wash technique):
Here a preliminary impression is taken with a putty silicone is a stock
impression tray. This forms a custom made tray in which by cutting
away some of the tray silicone or by using thin resin, rubber or wax
sheet as spaces between the teeth and the silicone. This area is then
filled with a thinner consistency silicone and the tray is repeated into
45

46. the mouth for reproduction of sharp angles accurately, a light bodied
silicone is injected.
The latest technique is mixing is the use of automatic spanars and
mixes. These consist of a double barrel caulking gun with mixing tip.
The tip contains spirals on the inside. Foving of the base and
accelerator through these spirals results in mixing. Advantages
include improved properties, more uniform mix, lesser air bubbles
and reduced working time.
Finally, the impression is removed after chocking is set by providing
with a blunt instrument. It become firm and returns to its original
contour. Removal is done quickly and is one motion for best result
disinfection is done by 10 minutes in 2% glutaraldehyde or 3 minutes
in chlorine dioxide solution, phobe iodophor can also be used.
Recent Advances in Elastomers:
Visible light cured polyether urethane dimethacrylate:
In early 1988, a visible light cured impression material was
introduced (Genesis, L.D. Caulk).
Available in 2 viscosities – Light and heavy bodied.
Composition:
46

47. 1. Polyether urethane dimethacrylate.
2. Photoinitiates.
3. Photo-accelerators.
4. Silicone dioxide filler.
Properties:
They have long working but short setting times. Blue light is used for
curing along with transparent impression trays. Highest resistance to
tearing among the elastomers (tear strength of 6000-7500gm/cm2).
Dimensional stability, flow, detail reproduction, permanent
deformation, wettability, compatibility with cast materials and
electroforming is similar to addition silicones. The material is rigid
and severe undercuts should be blocked to ease impression removal.
Manipulation: Light body is syringed into the sulcus and over the
preparation while heavy body is loaded onto the 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 minutes.
Advantages include – Controlled working time.
Excellent properties.
47

48. Disadvantages include;
1. Need for special transparent trays.
2. Difficult to cure in remote areas.
It is contraindicated in patients with a known allergy or sensitivity to
wethers, acrylics or methacrylates.
Lastly, we come to the inelastic impression materials – due to this
limited use in operative dentistry, we shall have a brief overview of
those:
1. Impression plaster – Type I gypsum i.e. calcium sulfate with
modifier was used earlier but is really used now. It is brittle
and rigid. It may be used as a final or wash impression in
complete denture prosthesis.
2. Impression compound or modeling plastic – A thermoplastic
material, it is used primarily for edentulous complete denture
primary impression and for single tooth tube impression with a
copper band, greenstick compound, a type of impression
compound is used for border moulding.
It is composed of thermoplastic resins, copal resins, carnauba wax,
steam acid, talu, coloring agents and fillers along with plasticizers.
48

49. The compound softens at 39°C(glass transition) and is manipulate at
43.5°C (fusion temperature). It can be softened over a flame or in
warm water. It is then baded on to tray and binding seated till rigid.
Dimensional stability is less with distortions occurring and surface
details reproduction is comparatively less. Casts should be poured
immediately.
Advantages include repeated cure and reparability. Disadvantages
include distortions and difficult manipulation as well as rigidity.
3) Zinc oxide eugenol pastes: Available as 2 pastes, composition is as
follows:
Base paste Accelerator
Zinc oxide – 87%
Fixed vegetable or mixed oil –
13%
Oil of cloves or eugenol – 12%
Gum or polymerized resin – 50%
Filter (silica type) – 20%
Lasolin – 3%
Resinous bulsam – 10%
Accelerator solution (CaCl2) –
5%
Coloring agents.
Setting reaction is an acid base reaction forming zinc eugenolate.
Final setting range from 10-15 minutes.
49

50. Manipulate – equal length are dispersed, mixed quickly (1 minute
mixing time) and loaded onto the tray, seated till set and removed.
Advantages include good accuracy, dimensional stability and
compatability with casts. Disadvantages include requirement of
special tray, burning reaction of eugenol and inability register
undercuts.
Non eugenol pastes have been developed to overcome eugenol
initiation by adding carboxylic acids like orthoetching benzoic acid.
They can also be used as bite registration pastes.
4) Lastly, a material not brief an impression material but used as such
inlay wax used in direct or indirect techniques to record single tooth
impressions. It has type I and II and is composed of paraffin wax,
gum damer, canaculi were and coloring agents. Candidia wax, natural
resins and other waxed are also added.
The wax softens at about 40-45°C and flows at 56°C or higher and
vaporizes at 500°C. it is heat flamed, softened and manipulated as
desired, and invested immediately to avoid distortion.
CONCLUSION:
50

51. The famous saying goes “The first impression is the best impression”.
That should also be the endeavor of every dental surgeon. Realizing
that a restoration or prosthesis can only be as good as the preparation
and the impression will encourage dentists to master the art and
science of impression making and recording. This can only be
fulfilled by having an indepth information of material science and
unraveling their intricacies as well as being update on the driving
technologies and techniques governing those materials. Only this
holistic knowledge will enable clinicians deliver ideal dental care and
“impress” the patient.
51

52. Bibliography:
1. Philipps Science of Dental Materials.
2. Restorative Dental Materials – Craig.
3. Materials in Dentistry – Jack L. Ferracane.
4. Basic dental material – John J. Manappallil.
5. Notes on dental materials – C. Combe.
6. Dental materials. – Richard Van Noort.
52