Introduction Rubberlike elastic impression materials Currently
referred to as elastomers or elastomeric impression materials.
Elastomeric material consists of large molecules or polymers that
are joined by a small amount of cross- linking Cross linking form a
three-dimensional network The amount of cross-linking determines
the stiffness and elastic behaviour of the material
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Requirements Accuracy- must reproduce the entire surface upon
which the prosthesis to be made will fit Elasticity- low elastic
modulus and high elastic range Dimensional stability- the way in
which accuracy varies with time after recording the impression Ease
of handling Good tear strength Non-toxic and non-irritating
Compatible with die and model materials
Classification The ADA considers three types of elastomeric
material and each type can be further divided into four viscosity
classes: light body medium body heavy body putty
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Polysulphides Supplied in the form of two pastes- a base and a
catalyst Examples: Permlastic (SDS/Kerr), Coe-Flex, Omniflex (GC
America) The base paste consists of: Polysulphide polymer Suitable
filler (titanium dioxide or lithopone) Plasticizer ( eg. dibutyl
phthalate) Sulphur small amount that enhances reaction (
catalyst)
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Polysulphides The catalyst paste: Lead dioxide Plasticizer-
chlorinated parafin Approx same amount of filler as in base paste
Oleic or stearic acid- retarders- control the rate of setting
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Polysulphides Must be used with a special tray Reasonable tear
strength and good elastic properties Unpleasant odor and taste Work
well in the presence of moisture- moderately hydrophilic Used
commonly for crown and bridge impressions Casts should be poured
within 24hrs of impression but not immediately as to allow for full
elastic recoil Reported incidences of allergic reaction to patients
sensitive to latex Impressions made from polysulfide materials
should be poured within 30 minutes following removal from the
mouth, because in the first hour they undergo 50% of their 24-hour
shrinkage.
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Polysulphides AdvantagesDisadvantages Good tear strengthPoor
dimension stability Good elasticityOffensive odor Low costStains
clothing Long setting time
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Polysulphides
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Silicones Condensation silicone impression material Supplied in
a base paste and low viscosity liquid Examples: Speedex
(Coltene/Whaledent), Accoe (GC America) McCabe and Walls, 2008
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Silicones Condensation silicone impression material Setting
characteristics more favourable than polysulphide Setting time
shorter and elasticity developed earlier Very hydrophobic- repelled
by water or saliva therefore necessary to dry areas of the mouth
for an acurate impression Adequate tear resistance for most
purposes Models must be poured as soon as possible after impression
recordings The increased use of addition silicones has lead to a
gradual decline in the use of addition silicone
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Silicones Condensation silicone impression material
AdvantagesDisadvantages Moderate tear strengthPoor dimensional
stability Good working timePoor wetability Good setting time
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Silicones Addition silicone impression material Also supplied
as two paste The hydroxyl group from the condensation silicone is
replaced by vinyl group Examples: Extrude, Take 1 (SDS/Kerr),
President (Coltene/Whaledent), Express, Imprint II (3M ESPE),
Examix, Exaflex (GC America), Reprosil, Aquasil, Hydrosil
(Dentsply/Caulk)
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Silicones Addition silicone impression material Properties are
similar to condensation silicones They have near ideal elasticity
with adequate tear resistance and setting characteristics The use
of the light body and putty enables accurate impressions to be
recorded Dimensional stability is greater than condensation
silicones Also inherently hydrophobic therefore a dry field is
required during impression recording Although surface active agents
have been added to newer materials to make it less hydrophobic
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Silicones Addition silicone impression material Handling
characteristics similar to condensation silicone Routinely used for
crown and bridge impressions Preferred to condensation silicones
due to its dimensional stability.
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Silicones Addition silicone impression material
AdvantagesDisadvantages Excellent dimensional stabilityHydrogen gas
release Good tear strength Inhibition of setting by sulphur
containing materials Good working and setting timeExpensive
Excellent wettabilityStiff Automix system
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Silicones Addition silicone impression material
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Polyethers Also supplied as two pastes and automix Examples:
Impregum F, Permadyne (3M ESPE), Polyjel (Dentsply/Caulk) McCabe
and Walls, 2008
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Polyethers Tear resistance and elasticity approaching that of
silicones Relatively rigid when set It is dimensionally stable
under relatively low humidity conditions The set material is
hydrophilic and absorbs moisture under high humidity conditions Can
record accurate impression even when perfect moisture control is
not achieved
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Polyethers The major disadvantage is its rigidity during its
set state- but this is also an advantage, e.g. when taking
impressions for implants AdvantagesDisadvantages Excellent
wettabillityPoor tear strength Good dimensional stability Short
working and setting time Stiff Difficult to disinfect Expensive
Patient sensitivity
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Polyethers
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Comparison of the properties of elastomeric impression material
McCabe and Walls, 2008
