The document discusses dental impression materials. It defines a dental impression as a negative record of the tissues in the mouth. Impressions are used to reproduce the teeth and surrounding areas to create positive models or casts. There are two main types of impression materials - rigid materials that cannot engage undercuts, and elastic materials that can engage undercuts. Common elastic materials discussed include alginate, polysulfide rubber, silicone, and polyether. The document outlines the characteristics and properties required of good impression materials.

1. IMPRESSION MATERIAL
Dr.Himansgu Tiwari
Assistant Professor
Dept. Of Prosthodontics
and Crown & Bridge

2. Dental Impression: It is a negative record of tissue of the mouth.
It is used to reproduce the form of the teeth and surrounding
tissues.
*A positive reproduction is obtained by pouring dental stone or
other suitable material into the impression and allowing it to
harden.

3. **The positive reproduction of a single tooth
is described as die, and when several teeth or
a whole arch is reproduced, it is called cast or
model.

4. ***The impression material is carried to the mouth
in a tray, which either stock tray or special tray.

5. Requirements (desirable properties) of impression materials:
1- Accurate reproduction of surface details.
2- A pleasant odor, taste, and esthetic color.
3- Absence of toxic or irritant constituents.
4- Adequate shelf life for requirements of storage and distribution.
5- Reasonable cost.
6- Easy to use with the minimum of equipment.
7- Setting characteristics that meet clinical requirements.
8- Satisfactory consistency and texture.
9- Readily wets oral tissues.
10- Elastic properties with freedom from permanent deformation after strain.
11- Adequate strength so it will not break or tear on removal from the mouth.
12- Dimensional stability over temperature and humidity ranges normally found in clinical
and laboratory procedures for a period long enough to permit the production of a cast or die.
13- Compatibility with cast and die materials.
14- Readily disinfected without loss of accuracy.
15- No release of gas during the setting of the impression or cast and die materials.

6. CLASSIFICATIONS OF IMPRESSION MATERIALS
According to the properties of set material (elasticity)
1- RIGID IMPRESSION MATERIALS
They cannot engage undercuts, so their use is restricted to edentulous patient without undercut.
a- Impression plaster.
b- Impression compound.
c- Zinc oxide eugenol.
d- Impression wax.
2- ELASTIC IMPRESSION MATERIALS
They can engage undercuts, and they may be used in edentulous, partially dentate, and fully dentate patients.
• Hydrocolloid:
a- Reversible hydrocolloid (agar-agar).
b- Irreversible hydrocolloid (alginate).
• Elastomers
a- Polysulfide.
b- Silicone:
- Condensation polymerizing silicone.
- Addition polymerizing silicone.
c- Polyether.

7. Impression Plaster
• The chemical formula is (Ca SO 4 .1/2 H 2 O),
• this impression material is known to be very accurate with
minimum setting expansion (<0.06%).
• The water/powder ratio is higher than that for dental plaster.
• This impression material contains additives that control and
regulate both setting time and expansion (K 2 SO 4 ).

8. Impression Compound
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.
They supplied as sheet, stick, and cake.

9. Types:-
1- Type I Impression Compound (Lower Fusing)
*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.

10. 2- Type II Tray Compound (Higher Fusing)
It is used to prepare a tray for making an impression. It is
generally stiffer and has
less flow than regular impression compound.

11. Zinc Oxide Eugenol Impression Material
Classification
1- Type I (Hard).
2- Type II (Soft).
Available as
1- base paste (white in color).
2- Accelerator or reactor or catalyst paste (red in
color).

12. Factors Controlling the Setting Time
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 dropping 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 dropping of oil or wax, the setting time can be increased.

13. Reversible hydrocolloid (agar-agar).
• This impression material is liquid state at elevated temperature
that changes into viscoelastic state upon cooling.
• The material preparation requires special equipment as well as
special water-cooled trays for impression making.
• Although it is an excellent impression material and yields
accurate impressions, presently it has been largely replaced by
alginate hydrocolloid and rubber impression materials.

14. Reversible hydrocolloid (Alginate impression material)
• Alginate was developed as a substitute for agar.
• Currently, alginate is more popular than agar for primary impression.
• Its good elastic property, its long term stability is quite poor.
• Hand mixing is required and the powder/water ratio can affect the setting
reaction, strength and consistency.

15. Polysulfide Impression Material
• Polysulfide rubber impression material has a polysulfide
polymer base that reacts with sulfur with help of a catalyst.
• This impression material has good elastic property
• and excellent tear strength.
• However, this impression material has unpleasant smell.
• It is available in different consistencies.

16. Silicone Impression Material
a- Condensation - cured silicone:
They consist of a base material and a catalyst.
The base material is a dimethyl siloxane with 35- 70% silica filler,
with the help of alkyl silicate catalyst
the polymerization is carried out by liberation alcohol. The
material has rapid setting reaction but high setting
shrinkage.

17. b- Addition –cured silicone:
They are known as polysiloxanes, this impression material is also
provided as a base and a catalyst tubes with no byproduct resulted
upon reaction. This material has long term stability as well as
being hydrophobic with excellent elastic properties.

18. Polyether Impression Material
The base is a polyether with terminal imine rings that
undergo cross-linkage by 2,5-dichlorobezene sulfonate. It
has a rapid setting time, with moderate dimensional
change on setting, good elastic recovery. It can induce
allergic reaction.