Dental Materials (1)
Dr. Hiba Al-Helou
Impression materials are used to make replicas (models or casts) of teeth and other
oral tissues (supporting structures) include gingiva, alveolar bone or residual ridge,
hard and soft palate.
The replicas are used to construct restorations and other appliances. The impression
is a negative reproduction, whereas the replica (model or cast) is a positive
The impression must be an accurate duplication of the hard and soft tissues and be
stable enough to allow disinfection and production of a model.
Impression materials are used for many purposes. Some uses are simply to produce a
physical model of the oral tissues for study, called a study model. Study models are
used in diagnosis and treatment planning. Other uses require very exact replication of
the size and shape of a preparation for the construction of a restoration or appliance.
These replicas are called casts. A replica of a single tooth is called a die.
Classification of Impression Materials
There are several classifications:
according to mode of setting:
- Set by chemical reaction (irreversible): alginate, rubber
- Set by temperature change (reversible): agar, impression Compound
according to their uses in dentistry:
- They are used for complete denture prosthesis: impression plaster, impression
compound, zinc oxide eugenol impression paste..
- They are used for dentulous mouth: alginate, rubber...
A more widely used classification of impression materials involves
consideration of the properties of the set material.
- The properties which are most important are rigidity and elasticity,
- Hence impression materials are classified as being elastic or non-elastic.
- Since they determine whether an impression material can be used to record
- The rigid impression materials cannot be removed from undercuts without
the impression being fractured or distorted
The ideal impression material
Easy to mix and handle.
Suitable working time.
Suitable setting time.
Adequate shelf life.
Ready to disinfect without loss of accuracy.
The requirements of the impression materials can be conveniently discussed under
three main headings:
1- Factors which affect the accuracy of the impression.
2- Factors which affect the dimensional stability of the impression, that is, the
way in whichthe accuracy varies with time after recording the impression.
3- It must be strong enough to resist tearing during removal (“tear resistance”)
There are two basic setting mechanisms: reversible and irreversible. Irreversible
implies that chemical reactions have occurred and that the material cannot revert to a
previous state in the dental office. For example, alginate, zinc oxide– eugenol (ZOE)
impression paste, impression plaster, and elastomeric impression materials, which set
by chemical reactions, are irreversible. On the other hand, Reversible materials,
such as agar and impression compound, soften upon heating and solidify slightly
above body temperature with no chemical change taking place.
Hydrocolloid Impression Materials
The word colloid is derived from the word kola, meaning glue, and oid, meaning like.
Thus a colloid has a glue-like physical character.
The colloid can exist in the form of a viscous liquid, known as a sol or a solid, described
as a gel.
If the particles are suspended in water, then the suspension is called a hydrocolloid, with
the liquid being a hydrosol and the solid a hydrogel.
Two colloidal impression materials made from polysaccharide particles suspended in
water are used in dentistry, based on the mode of gelation, they are classified as:
1. Reversible hydrocolloids, e.g. agar: they are called reversible because their physical
state can be reversed.
2. Irreversible hydrocolloids, e.g. alginate: once these set, it is usually permanent, and
so is known as irreversible.
I. Agar (Reversible Hydrocolloid)
Agar–agar is a sol at elevated temperature and forms a gel on cooling; it requires
a special equipment to store and heat the materials and water-cooled trays for
The special equipment limits the popularity of this excellent material.
Reversible hydrocolloid itself is very inexpensive and results in a very accurate
Making the Agar Impression
It must be prepared for use before taking the impression. The equipment has
a) In the first compartment, the reversible hydrocolloid is boiled to change the
rubbery material (the gel) into a viscous liquid (the sol) in (100°C) for a
minimum of 10 minutes.
b) In the second compartment (storage bath), the material is stored in a (65°C)
water bath until needed. Material can be stored for several days,
c) Several minutes before it is to be used for an impression, the reversible
hydrocolloid is placed in a (45°C) water bath about 3 to 10 min.
For the immediate preparation step, the impression tray is filled with
hydrocolloid sol from the tube taken from the storage bath, and the tray is placed
in the water-filled compartment (at about 45 °C). In any case, the loaded tray
should never be left in this bath for more than 10 minutes as partial gelation can
occur, thereby making the material unusable.
The syringe material is taken directly from the storage compartment and
applied to the prepared teeth. The syringe material is first applied to the base of
the preparation; then the remainder of the prepared tooth is covered. The tray is
immediately brought into position, seated with light pressure.
Gelation is accelerated by circulating cool water (18 °C to 21 °C) through the
tray for 3 to 5 min.
II. Alginate (Irreversible Hydrocolloid) Impression
Alginates are the most widely used impression materials in dentistry, because it is
simpler to use.
They are used for making impressions for:
1. preliminary impressions for complete dentures
2. removable partial dentures with clasps
3. orthodontic models
4. Study models used to plan treatment.
5. For impression the opposing teeth to restoration
6. temporary restorations
They are not accurate enough for fixed partial denture impressions.
Composition of Alginate
The alginates are based on alginic acid, which is derived from a marine plant. Some
of the hydrogen molecules on the carboxyl groups are replaced by sodium, thus
forming a water soluble salt.Manufacturers supply alginate as a powder containing:
Ingredients %wt Functions
Sodium or potassium alginate 12% to 15% Dissolves in water and reacts
with calcium ions
Calcium sulfate dehydrate
8% to 12% Reacts with potassium
alginate and forms insoluble
Inert filler-such as diatomaceous
70% to control the stiffness of the
Sodium phosphate (retarder) 2% Reacts preferentially with
compounds or chlorhexidine
traces to provide self-disinfection
organic glycol traces to coat the powder particles
to minimize dust during
When alginate powder is mixed with water a sol is formed which later sets to a gel
by a chemical reaction.
The final gel, insoluble calcium alginate is produced when soluble sodium alginate
reacts with calcium sulphate (reactor). However, this reaction proceeds too fast.
There is not enough working time.
So the reaction is delayed by addition of a retarder (sodium phosphate) by the
Calcium sulphate prefers to react with the retarder first. Only after the supply of
the retarder is over, the calcium sulphate react with sodium alginate.
This delays the reaction and ensures adequate working time for the dentist.
In other words, two main reactions occur during setting:
2 Na3PO4 + 3CaSO4 Ca3(PO4)2 + 3Na2SO4
Na alginate + CaSO4 Ca alginate + Na2SO4
( powder) ( gel)
First, sodium phosphate reacts with the calcium sulphate to provide adequate
Next, after the sodium phosphate is used up, the remaining calcium sulphate reacts
with sodium alginate to form insoluble calcium alginate which forms a gel with
The sodium phosphate acts as a reta