It presents as powder mixed with water in water/powder ratio
(W/P= 0.60), 100 g powder/60 ml water.
1- Calcium sulfate β-hemihydrate.
2- Potassium sulfate: to reduce expansion, and to accelerate the setting
reaction.
3- Borax: to reduce the rate of setting.
4- Starch: to help disintegration of impression on separation from the
plaster or stone cast.
After cast hardening, the impression and cast are put in hot water. The
starch swells and the impression disintegrates, making it easy to separate
the cast from the impression.
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Setting time (5 minutes).
The mixed material has a very low viscosity, so it is mucostatic.
It is hydrophilic.
It adapts to the soft tissue and recording their surface detail with great
accuracy.
The dimensional stability is very good (a dimensional change during
setting is 0.06 %).
A separating medium must be used between the impression plaster
and the pouring plaster or stone.
The material is rigid once set, and thus unable to record undercuts.
Patient complains very dry sensation after having impression
recorded because of water absorbing nature of this material.
The material is best used in a special tray, made from acrylic (1.5 mm
spacer).
1- Final impression for completely edentulous arch.
2- Occlusal bite registration.
Impression compound is described as a rigid, reversible impression
material which sets by physical changes. On applying heat, it softens and
on cooling it hardens.
Figure (4-1): (A) This shows examples of dental compound in the form
of either cake or sheet or in the form of sticks. The slabs are used to
make impressions of edentulous areas in the mouth whilst the sticks are
used as tray extension materials or for extending special trays. (B) This
shows a typical edentulous impression recorded in impression
compound. Note the lack of any fine detail in this impression due to the
very high viscosity of the material.
They supplied as sheet, stick, and cake.
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Thermoplastic resins.
Wax.
Plasticizer: stearic acid: addition of plasticizer to overcome brittleness.
Filler: talc, calcium carbonate added to:
a- Overcome tackiness.
b- Control degree of flow.
c- Minimize shrinkage due to thermal contraction.
d- Improve rigidity of impression material.
 Sheet form material: it is softened using water bath, a temperature in
range (55-60 °C), knead the material after it has been heated in water
to ensure its being at a uniform temperature. Storage in hot water
should not be long that important constituents such as stearic acid may
be leached out. Overheating make the compound sticky and difficult
to handle.
 Stick form material: it is softened over a flame. The compound should
not be allowed to boil; otherwise, the plasticizers are volatilized.
It is used to prepare a tray for making an impression. It is generally
stiffer and has less flow than regular impression compound.
1- It is mucocompressive.
2- Because of high viscosity and low flow; therefore, the reproduction of
surface detail is not very good.
3- It is not used to record the undercut, because it is rigid once cooled.
4- Poor dimensional stability. It has high value of coefficient of thermal
expansion and undergoes considerable shrinkage on removal from the
mouth. Also because pressure is applied during formation of an impression
(mucocompressive), residual stress exists in cool impression, the gradual
relief of internal stresses may cause distortion of impression (the cast
should be poured as soon as possible or at least within the hour).
5- Impression compound has low thermal conductivity, therefore, time must
be allowed during heating or cooling to allow impression compound to
come to uniform softening.
6- This material can be reused a number of times for the same patient only, in
case of errors.
7- The material has sufficient body to support itself to an extent especially in
the peripheral portions.
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Difficult to record details because of its high viscosity.
Compress soft tissues while making impression.
Distortion due to its poor dimensional stability.
Difficult to remove it if there are severe undercuts.
There is always the possibility of overextension especially in the peripheral
portions.
1- Type I sheet form: It is used for recording primary impression of
edentulous ridges using stock tray.
2- Type I stick form: It is used for border molding of an acrylic special tray
during fitting of the tray.
3- Type II tray compound: It is used to make a special tray (now largely
replaced by acrylic tray).
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Cementing and insulating medium.
Temporary filling.
Root canal filling material.
Surgical pack in periodontal surgical procedures.
Bite registration paste.
Temporary relining material for dentures.
Impression material for edentulous area.
1- Type I (Hard).
2- Type II (Soft).
1- Base paste (white in color).
2- Accelerator or reactor or catalyst paste (red in color).
Figure (4-2): This shows a
typical example of impression
paste materials. They consist
of two pastes which are
extruded out onto the mixing
slab and mixed together by
hand using a spatula. The
main active ingredient of one
paste is zinc oxide whilst the
main active ingredient of the
other paste is eugenol.
.
Zinc oxide (reactive component)
(87%).
Fixed vegetable or mineral oil
(act as plasticizer, and aids in
masking the action of eugenol as
an irritant) (13%)
Oil of cloves or eugenol (reactive
component) (12%).
Gum (speed the reaction) (50%).
Filler (20%).
Lanolin (3%).
Resinous Balsam (improve flow
and mixing properties) (10%).
CaCl2 (accelerator solution) and
coloring agent (5%).
The setting reaction is a typical acid-base reaction to form a chelate. This
reaction called chelation and the product is called zinc eugenolate.
1- ZnO + H2O
2- Zn(OH)2 + 2HE
Zn(OH)2
ZnE2 + 2H2O
The set material consists of a matrix of amorphous zinc eugenolate
surrounding and holds the unreacted zinc oxide particles.
Type I (Hard)
Type (Soft)
Initial setting time
3-6 minutes
3-6 minutes
Final setting time
10 minutes
15 minutes
a- Particles size of zinc oxide powder: if the particle size is small, the
setting time is less.
b- By varying the lengths of the two pastes.
c- By adding a drop of water, the setting time can be decreased.
d- Longer the mixing time, shorter is the setting time.
e- High atmospheric temperature and humidity decrease the setting time.
f- Cooling the mixing slab, spatula increase the setting time.
g- By adding a drop of oil or wax, the setting time can be increased.
2- It registers surface details accurately due to its good flow.
3- The material has mucostatic properties.
4- The material is rigid once set and cannot be used for making
impression of teeth and undercut areas.
5- It requires a special tray for impression making; it has adequate
adhesion to acrylic tray.
6- It is dimensionally stable, a negligible shrinkage (less than 0.1%) may
occur during hardening.
7- No separating medium is required before the cast is poured because it
does not stick to the cast material.
8- The paste tends to adhere to the skin, so the skin around the lips
should be protected with Vaseline to make the cleaning process much
easier.
9- Eugenol can cause burning sensation and tissue irritation. Non
eugenol paste were developed, here the zinc oxide is reacted with a
carboxylic acid.
10- It can be checked in the mouth repeatedly, and minor defects can be
corrected locally without discarding a good impression.
The mixing is done on oil impervious or glass slab. Equal length of base
paste and catalyst paste squeezed on to mixing slab and mixed until a
uniform color is observed. The mixing time is 1 minute.
1- Final impression of edentulous ridge.
2- Occlusal bite registration.
Figure (4-3): This shows the
two pastes of zinc oxide and
eugenol being mixed
together. Here we see the
advantage of using pastes of
different colors since it is
possible to tell when proper
mixing has been achieved. In
this case there are still
obvious streaks of the two
individual pastes showing
that mixing is incomplete
Impression waxes are rarely used to record complete impression but are
used to correct small imperfection in other impression. Waxes are
generally used in combination with other impression materials
These materials consist of a mixture of low melting paraffin wax and
beeswax in ratio about 3:1. It may also contain metal particles. The flow
at 37°C is 100 %. These waxes are subjected to distortion during removal
from the mouth. They should be poured immediately.
Waxes have larger coefficient of thermal expansion of any material used
in restorative dentistry.
1- To make functional impression of free end saddles (class I and class II
removable partial dentures).
2- To record posterior palatal seal in dentures.
3- Functional impression for obturators.