1. 1
DENTAL MATERIALS
ASSIGNMENT
HIBA HAMID OKARVI
BDS 1ST
YEAR
ROLL # 32
COMPARISON OF SYNTHETIC ELASTOMERS
Comparison
topic
Polysulphides Condensation silicones Addition silicones Polyethers
Composition Two paste system dispensed
from tubes – one paste normally
base paste and other catalyst
paste.
Base paste contains:
- polysulphide prepolymer with
terminal and pendant thiol
groups
- plasticizer
- inert filler
Catalyst paste contains:
- lead oxide
- sulphur
- inert oil normally paraffinic oil
Two paste system or paste and
liquid.
Paste contains:
- hydroxyl terminated
polydimethylsiloxane
- inert filler such as silica
Liquid contains:
- alkyl silicate such as
tetraethylsilicate
- tin compound such as dibutyl tin
dilaurate
Supplied as two pastes.
- each paste contains a liquid
silicone prepolymer and filler.
- one of the paste contains a
catalyst
- one paste contains a
polydimethylsiloxane prepolymer
in which some methyl groups are
replaced by hydrogen
- other paste contains a
prepolymer in which methyl
groups are replaced by vinyl
groups
- catalyst in the paste is normally
platinum containing compound
such as chloroplatinic acid
Supplied as two pastes
Base paste (large tube) contains:
- imine-terminated prepolymer
- inert filler – silica
Catalyst paste (small tube)
contains:
- ester derivative of aromatic
sulphonic acid
- inert filler – silica
- plasticizer – phthalate
Viscosity Available in three viscosities;
light-bodied, regular-bodied and
heavy-bodied; no putty
Available in all four viscosities:
light-bodied, regular-bodied,
heavy-bodied and putty material
Available in all four viscosities:
light-bodied, regular-bodied,
heavy-bodied and putty material
Available in single viscosity
(regular-bodied).
Setting reaction On mixing two pastes, terminal
and pendant thiol groups of
prepolymer undergo reaction
with lead oxide. Reaction results
in chain extension and cross
linking. This reaction is of
condensation polymerization
type as a water molecule is
produced as a byproduct of each
reaction stage
On mixing, reaction begins
immediately in which terminal
hydroxyl groups of prepolymer
chains react with the cross linking
agent under influence of catalyst.
Due to the each molecule of cross
linking agent reacting with four
prepolymer chain, extensive
cross-linking occurs. Each
reaction stage produces one
On mixing two pastes, platinum-
catalysed addition reaction
occurs, causing cross-linking
between two types of siloxane
prepolymer. Reaction does not
produce any byproducts although
it has been reported that these
materials occasionally evolve
hydrogen
When two pastes mixed
together; cationic, ring opening
addition reaction occurs. Ionized
form of sulphonic acid ester
provides initial source of cations
and each stage of reaction
involves opening of an epimine
ring and production of a fresh
cation. Reaction is of addition
type with no by-product being

2. 2
molecule of ethyl alcohol as a
byproduct
formed.
Tear resistance Very good tear resistance.
Typically can withstand about
700% tensile strain before
tearing
Adequate tear resistance.
Undergo only about 300%
extension before fracturing.
Adequate tear resistance Adequate tear resistance
Elasticity Viscoelastic materials. Recover
only slowly and not completely
after being compressed or
stretched.
Elastic properties which most
closely approach the ideal of
complete and instantaneous
recovery following stretching or
compression
Near ideal elasticity Elastic properties near to those
of silicones. Adequate elastic
properties
Accuracy Heavy-bodied materials are
potentially more accurate, since
they exhibit lest setting and
thermal contraction. Their high
viscosity means that they are
unable to record the fine details
as the more fluid, light-bodied
materials.
Very hydrophobic materials and
are readily repelled by water or
saliva. As a result, it is necessary
to dry areas of the mouth for
which an accurate impression is
required. If a dry field is not
secured “blow holes” are likely to
occur in the impression because
the material will not be able to
drive away the residual moisture.
To obtain maximum accuracy,
model should be cast as soon as
possible after recording the
impression
The combined use of putty and
light-bodied materials enables
accurate impressions to be
recorded. Inherently hydrophobic
in nature, which can cause
imperfections in impressions if
the area to be recorded is not
thoroughly dried. Repelled by
moisture which can result in the
formation of blow holes.
Accuracy of polyether
impressions compares
favourably with other light-
bodied elastomers. The lack of
heavy-bodied and putty pastes,
however, precluded the use of
techniques using combined
viscous/fluid pastes which are
commonly used with other
elastomers to optimize accuracy
Dimensional
stability
Dimensional changes occur after
apparent setting of polysulphides
due to two major factors:
- continued reaction occurs for
some time after the apparent
setting time, causing further
shrinkage of the impression
- water produced as the
byproduct of the setting reaction
may be lost by evaporation from
the surface. In this case
dimensional change associated
Dimensional changes after setting
may be continued due to loss of
alcohol produced as byproduct of
setting reaction. This effect
produces measurable weight loss
accompanied by shrinkage of
impression material.
Dimensional changes of regular-
bodied condensation silicones
slightly greater than those of
regular-bodied polysulphides.
Production of little or no
byproduct in the cross-linking
reaction of the addition curing
material results in a very stable
impression.
Under conditions of low
humidity, have very good
dimensional stability. Material
contains no volatile constituents
and sets by an addition reaction
which produces no volatile
byproducts. Set material is
relatively hydrophilic and
absorbs water under conditions
of high humidity. This causes
impression to swell and distort.

3. 3
with change in weight of the
material
Setting time Take longer to set Less setting time as compared to
polysulphides
Less setting time as compared to
polysulphides
Distinctive transition from
plastic to elastic behavior which
can be helpful in estimating the
earliest safe time for removal
from the mouth
Decontamination Soaking in hypochlorite or
glutaraldehyde to decontaminate
has little effect and does not
harm the impression due to the
hydrophobicity of the material.
Soaking in hypochlorite or
glutaraldehyde to decontaminate
has little effect and does not harm
the impression due to the
hydrophobicity of the material.
Soaking in hypochlorite or
glutaraldehyde to decontaminate
has little effect and does not harm
the impression due to the
hydrophobicity of the material.
As these materials are
hydrophilic, great care must be
taken when soaking impressions
in aqueous solutions – lest the
material absorbs water and
swells.
Clinical use Materials most commonly used
in two viscosities, high and low,
and must be used in a special
tray. Commonly used for crown
and bridge impressions.
Limited reports of allergic
responses to these materials,
from people with and allergy to
latex products
Available in range of viscosities
from light-bodied materials used
to record accurate surface detail
on prepared tooth surfaces,
through medium viscosities,
commonly used a monophase
material during either crown and
bridgework or denture
manufacture, and heavy-bodied
materials used to support light
body in stock trays for crown and
bridge impressions, to putties
which are now available in both
soft and hard format
Preferred due to their greater
dimensional stability.
Available in range of viscosities
from light-bodied materials used
to record accurate surface detail
on prepared tooth surfaces,
through medium viscosities,
commonly used a monophase
material during either crown and
bridgework or denture
manufacture, and heavy-bodied
materials used to support light
body in stock trays for crown and
bridge impressions, to putties
which are now available in both
soft and hard format
Commonly used as a monophase
where a single viscosity of
material is used for both the bulk
of the impression in the tray and
to be syringed around the
prepared teeth in the mouth.
Greatest disadvantage from a
clinical standpoint is the rigidity
of these materials when set. This
can make removal of the
impressions very difficult in
dentate patients when there are
marked undercuts present around
the teeth.