2. Rubbery polymers, which are either
chemically or physically cross linked. They
can be easily stretched and rapidly recover
their original dimensions when the applied
stress is released
Chemically, there are four kinds of
elastomers used as impression materials are:
polysulfide, condensation polymerizing
silicon, addition polymerizing silicon &
polyether.
4. Composition(Base paste)
Ingredients Weight (%) Function
Polysulfide
ploymer
80-85 Main component
Lithopone/titaniu
m dioxide/zinc
sulphate/copper
carbonate/silica
16-18 Filler (to provide
required
strength)
dibutyl phthalate 2-4 Plasticizer
5. Catalyst paste
Ingredient Weight (%) Function
lead dioxide 60-68 Oxidizing agent
(to initiate the
polymerization)
dibutyl phthalate 30-35 Plasticizer
Sulphur 3 Accelerator
oleic acid/stearic
acid
2 Retarder
6. Setting reaction
During setting reaction terminal hydrogen
atoms of the polisulfide polymer are
oxidized by the lead dioxide with a resultant
increase in the degree of polymerization
7. The reaction is slightly exothermic, with a
typical increase in temperature of 3-4C.
mixes set to rubber consistency in about 10-
20 min but polymerisation continues & the
properties change for a number of hours
after the material sets
8. hot & humid conditions accelerate the
setting of polysulfide impression material.
The reaction yields water as a by product of
reaction. Loss of this molecule from the set
material has a significant effect on the
dimensional stability of the impression.
9. do not adhere to impression tray-
adhesives:rubber solution in acetone
available in three consistencies
– Low (syringe or wash)
– Medium (regular)
– High (tray)
10. Condensation silicone
Base paste: Polydimethyl siloxane:main
constituent & has reactive terminal
hydroxyl groups.
– Fillers: calcium carbonate or silica in
concentration of 35% for low consistencies to
75% for putty like consistencies.
Accelerator: liquid that consists of
stannous octate suspension & alkyl
silicate/paste by adding a thickening agent.
11. Setting reaction
polydimethyl siloxane & accelerator are
mixed leading to production of a three
dimensional network with the liberation of
ethyl alcohol (byproduct) & an exothermic
temperature rise of about 1C.
release of ethyl alcohol causes a shrinkage
that is greater in the low consistency than in
the putty like consistency.
12. Available consistencies
– Low
– Putty
Catalyst of Condensation silicones can be
supplied as a putty or liquid
do not adhere to impression tray-adhesive
solutions
13.
14. Addition silicone
Base paste: polymethyl hydrogen siloxane& other
siloxane prepolymers. If catalyst paste contains
platinum salt activator, then base paste must
contain hybrid silicone. Fillers are also present.
Accelerator paste/ catalyst paste: divinyl
polydimethyl siloxane & other siloxane
prepolymers. Platinum salt activator may be
added. Fillers are also present.
15. Setting reaction
no reaction byproducts develop as long as
the direct proportions of vinyl silicone &
hydride silicone are maintained & there are
no impurities.
However, a secondary reaction between
moisture & residual hydrides of the base
polymer can lead to the development of
hydrogen gas.
19. Setting reaction
During setting reaction cross linking
reaction occurs with the aziridino groups of
the base and the end product is a rubber-like
polymer
26. Setting properties
Viscosity: is the resistance of a fluid to flow
Polysulfides have the lowest viscosity &
ranks as one of the least stiff of elastomeric
impression materials
The most rapid increase in viscosity with
time occurred with silicones & polyether
materials
28. Working & setting times
working time is measured at room
temperature where as, setting time is
measured at mouth temperature
An increase in the temperature accelerates
the curing rate of all elastomers & thus
decreases both setting & working time
29. Working & setting times
Cooling is a practical method of increasing
the working time
Working &setting time decreases with
increase in viscosity.
Altering the base/catalyst ratio will change
the curing rate.Mechanical properties can be
adversely affected when marked change in
base/catalyst ratio occur.
30. – Polysulfides have the longest working &
setting times followed by silicones &
polyethers.
– Polyethers show a clearly defined
working time with a sharp transition into
the setting phase. This behaviour is often
called snap test.
31. Viscosity Max. mixing
time (min)
Max.
working
time (min)
Low 1 2
Medium 1 2
High 1 2
Very high 1 2
Requirements by ANSI/ADA specification no. 19
32. Dimensional change
– Polymerization shrinkage
– Loss of byproduct (water or ethyl alcohol)
– Thermal contraction from oral to room
temperature
– Imbibitions when exposed to water, disinfectant
or a high humidity environment over a period
of time
– Incomplete recovery of deformation because of
viscoelastic behaviour
33. In ANSI/ADA specification number 19 for
elastomeric impression materials, a disk of
impression material is placed on a talc
covered glass plate. At the end of 24 hours,
the contraction should not exceed o.5-1.0%
The change is greater in magnitude for the
polysulfide & condensation silicone.
34. One property that has a negative effect on
the polyether impression is the absorption
of water or fluids & the simultaneous
leaching of water soluble plasticizer. Thus
the stored impression must be kept in a dry,
cool environment to maintain its accuracy.
35. Mechanical properties
Elastic recovery
– The relative amount of permanent deformation
following strain in compression increases in the
following order: addition silicone, condensation
silicone, polyether & polysulfide.
– Addition silicones are the most ideally elastic.
Distortion on removal from undercuts is virtually
nonexistent because these materials exhibit the lowest
permanent distortion after strain in compression
36. Despite the possibility of a large
dimensional change occurring when a
polysulfide impression is removed from the
mouth, there is no advantage to “bench
cure” the material.
37. Minimum elastic recovery required for
elastomeric material is 96.5% & minimum
strain in compression required is 0.8% &
maximum is 20%.
38. Strain in compression
The stiffness of elastomers increase in order
of: polysulfide, condensation silicone,
addition silicone & polyether.
39. Flow
It is measured on a cylindrical specimen
1 hour old & the %age flow is determined
15 minutes after a load of 1N is applied.
Silicones & polyether have the lowest
values of flow & polysulfides have the
highest.
40. Hardness
The shore A hardness increases from low to
high consistency.
The low, medium & high viscosity addition
silicones & polysulfides don’t change
hardness significantly with time, whereas
hardness of condensation silicone, addition
silicone putties & polyethers does increase
with time.
41. Low flexibility & high hardness can be
compensated for clinically by producing
more space for the impression material
between the tray & the teeth.
This can be accomplished with additional
block-out for custom trays or by selecting a
larger tray when using disposable trays.
42. Tear strength
It indicates the ability of a material to
withstand tearing in thin interproximal areas
& margins of periodontally involved teeth
The ranking of tear strength from the lowest
to highest of all impression materials
generally is as follows: hydrocolloids (agar
& alginate), silicones, polyether & then
polysulfide
43. In addition to chemical composition of the
material, tear strength is also influenced by
the consistency & manner of removal of the
material
45. The flatness or parallelism of the curves
with respect to the time axis indicates low
permanent deformation during the removal
of an impression material; polyether &
addition silicones have the best elastic
recovery followed by condensation
silicones & then polysulfides
46. Detail reproduction
Expect for the very high viscosity products,
all should reproduce a V-shaped groove & a
0.02 mm wide line in elastomeric.
The rubber impression materials are capable
of reproducing detail more acuurately than
can be transferred to the stone or die cast
50. All the elastomeric impression materials
possess advancing & receeding contact
angle greater than 45 degrees.
There are however, differences in wetting
among & with in types of elastomers.
Traditional addition silicone is not as
wettable as polyether.
52. diffusion controlled transfer of surfactant
molecules from the polyvinyl siloxane into
the aqueous phase, thereby altering the
surface tension of surrounding liquid.
By observing water droplets on impression
surfaces, it has been shown that
hydrophilized addition silicone & polyether
are wetted the best, & condensation
silicones & traditional addition silicones the
least
53. Ability of polyether & addition
silicone to reproduce detail under
wet & dry condition
55. BIOCOMPATIBILITY
Cell cytotoxicity test for different materials
reveals that polysulfides result in the lowest
cell death count & the set polyethers
produce highest cell cytotxicity scores.
problem occurs when a segment of
impression material is lodged in patient’s
gingival sulcus. It may cause gingival
inflammation & can be misdiagnosed.
56. SHELF LIFE
Properly compounded impression material
doesn’t deteriorate appreciably in the tube
or container when it is stored in a dry, cool
environment.
The tubes should always be kept tightly
closed when they are not in use.
57. COMMON FAILURES
Rough/uneven surface of impression
– Incomplete polymerization caused by premature
removal from mouth
– Improper ratio/mixing of components
– Presence of oil/other organic material on teeth
– Contamination with latex gloves for addition
silicones
– Too rapid polymerization from high humidity or
temperature
– Excessive high accelerator/base ratio with
condensation silicone
58. Bubbles
– Too rapid polymerization, preventing flow
– Air incorporated during mixing
–Irregularly shaped voids
• Moisture or debris on surface of teeth
59. Rough/chalky stone casts
Inadequate cleaning of impression
Excess water left on the surface of
impression
Excess wetting agent on the impression
Premature removal of cast/improper
manipulation of stone
Failure to delay pour of addition silicone at
least 20 min
60. Distortion
Resin tray not aged sufficiently; still
polymerization going on
Lack of adhesion of rubber to impression tray
caused by not applying enough coats of
adhesive/filling the tray with material too soon
after adhesive application or using wrong adhesive
Lack of mechanical retention for those materials
where adhesive is ineffective
Development of elastic properties in material
before the tray is seated
61. Excessive bulk of material
Insufficient relief for the reline material (if
such technique is used)
Continue pressure against impression
material that has developed elastic
properties
Movement of tray during polymerization
Premature removal from the mouth
Improper removal from the mouth
Delayed pouring of the polysulfide or
condensation silicone impression
69. Disadvantages
Stiff / high modulus of elasticity
Bitter taste
Needs to block undercuts
Absorbs water
Leaches components
High cost
70. “ No impression material fulfills
all the requirements”
the selection of the material best
suited for a particular clinical situation
and technique rests with the dentist.
