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    Dental Materials (1)

    Impression Materials.1

    Dr. Hiba Al-Helou

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    Impression Materials

    General Comments 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.

    (die) (casts)

    (study model)

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    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 strength.

    Adequate shelf life. Economical 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”)

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    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 Colloids 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


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    Making the Agar Impression

    It must be prepared for use before taking the impression. The equipment has

    three compartments:

    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.

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    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

    calcium alginate

    Inert filler-such as diatomaceous


    70% to control the stiffness of the

    set gel

    Sodium phosphate (retarder) 2% Reacts preferentially with

    calcium sulphate

    Quarternary ammonium

    compounds or chlorhexidine

    traces to provide self-disinfection

    organic glycol traces to coat the powder particles

    to minimize dust during


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    Setting reaction

    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

    working time.

    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 retarder, and the amount included can be varied to

    produce regular(or normal) and fast setting versions of this impression material.

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    Elasticity and elastic recovery

    Alginate hydrocolloids are highly elastic (but less when compared to agar) and

    about 97.3% elastic recovery occurs. Thus permanent deformation is more for

    alginate. Permanent deformation is less if the set impression is removed from

    the mouth quickly.

    The permanent deformation can be minimized:

    - By ensuring that there are no deep undercuts, as the deeper the undercut the

    greater the amount of compression.

    - Using a snap removal will ensure that the time for which the material is

    under compression is as short as possible, Because the longer the material is

    under compression.


    Alginate gels have poor mechanical properties and are liable to tear when removed

    from deep undercuts, particularly in interproximal and subgingival areas.

    Factors affecting gel strength;

    - Water/powder ratio, too much or too little water reduces gel strength.

    - Mixing time, over and under mixing both reduce strength.

    - Time of removal of impression, strength increases if the time of removal is

    delayed for few minutes after setting

    Dimensional stability

    Set alginates have poor dimensional stability due to evaporation, syneresis and

    imbibition. Therefore, the cast should be poured immediately. If storage is

    unavoidable, keeping in a humid atmosphere of 100% relative humidity results in

    the least dimensional change.

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    Alginate does not adhere well to the tray. Good adhesion is important for the

    accuracy of the impression. Retention to the tray is achieved by mechanical

    locking features in the tray or by applying an adhesive.

    Advantages of alginate

    1. It is easy to mix and manipulate. 2. Minimum requirement of equipment. 3. Flexibility of the set impression. 4. Low cost 5. Hydrophilic

    - displace moisture, blood, fluids

    6. Stock trays


    1. Tears easily 2. Dimensionally unstable

    - Immediate pour

    - Single cast

    3. Lower detail reproduction 4. unacceptable for fixed pros 5. High permanent deformation 6. Difficult to disinfect


    - Fluff the powder by inverting the can several times. This ensures uniform

    distribution of the filler before mixing. The top of the can should be taken

    off carefully to prevent the very fine silica particles from being inhaled. -

    For mixing, we need:

    - A clean flexible plastic bowl and

    - A clean wide bladed, reasonably stiff metal spatula.

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    Note: it is better to use separate bowls for plaster and alginate as plaster

    contamination can accelerate setting.

    - The proper W/P ratio as specified by the manufacturer should be used (usually

    one measure water with one scoop of powder). The water measure and scoop are

    supplied by the manufacturer.

    - Add powder to the bowl first

    - Then add water to powder.

    - The lid of the metal can is replaced immediately.

    - The mixing is stated with a stirring motion to wet the powder with water.

    - Once the powder has been moistened, rapid spatulation by swiping or

    stropping against the side of the bowl is done.

    - A vigorous figure- eight motion can also be used.

    This helps:

    - remove most of the air bubbles.

    - Wipe dissolved align from the surface of the yet undissolved align

    thereby promoting complete dissolution.

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    Mixing time

    - For fast set alginate is about 45 seconds

    - For normal set alginate is about 60 seconds.

    Over mixing results in

    - Reductions in final strength as the gel fibrils are destroyed.

    - Reduction in working time.

    Under mixing results in

    - Inadequate wetting, lack of homogeneity and reduced strength.

    - The mix being grainy and poor recording of detail.

    Working time

    - Fast set alginate is about 1 1/4


    - Normal set alginate is about 2 minutes.

    Control of gelation time

    Ideal gelation time is 3-4 minutes (at 20o C room temperature).

    Gelation time is best controlled by adding retarders (which is in

    manufacturer’s hands).

    The dentist can best control the setting time by altering the temperature of

    the water for mixing alginate material.

    - Colder the water--- longer is the gelation time.

    - Warmer the water--- shorter is the gelation time.

    Even the mixing bowl and spatula can be cooled.

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    Tray selection

    Since alginate has poor adhesion, tray selection is very important. Alginate can be

    retained by:

    Mechanical locking features in the tray - Perforations in the tray

    Applying a suitable adhesive ( available as liquid or sprays) A combination of the above.

    The tray should cover the entire impression areas and provide a space of at least 3

    mm on all sides.

    Time of removal

    The alginate impression should be left in the mouth for at least 2-3 minutes after

    initial gelation. The strength and elasticity of the alginate gel continues to increase

    for several minutes after initial gelation.

    Storage of alginate impression

    Alginate impressions must be poured as soon as possible. If it becomes necessary

    to store the impression, the following methods may be used:

    Wrap the impression lightly in a wet paper towel and cover with a rubber bowl.

    Or keep the impression in a plastic bag. Note: even under these conditions storage should not be done for more than one

    hour. Care should be taken not to use a “soaking wet” paper towel as it can cause

    inbibition of water.

  • Impression disinfection

    Disinfection of impression is a concern because of viral diseases such as

    hepatitis B, AIDS and herpes simplex. The viruses can contaminate the

    gypsum models and present a risk to dental laboratory and operating


    The irreversible hydrocolloids may be disinfected by 10 minutes immersion

    in, or spraying of, some antimicrobial agent (e.g. sodium hypochlorite,

    glutaraldehyde) without significant dimensional changes.