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Impression materials (requirement and classification)


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Title: Impression materials (requirement and classification)

Impression materials (requirement and
  • Dr. Waseem Bahjat Mushtaha
  • Specialized in prosthodontics

  • The function of an impression material is to
    accurately record the dimensions of the oral
    tissues and their related structures.
  • The impression gives a negative reproduction of
    these tissues.
  • The positive reproduction is called a model or

  • 1) Good dimensional accuracy.
  • 2) Adequate flow properties.
  • 3) Sufficient mechanical strength not to tear or
    permanently deform during removal.
  • 4) Suitable setting time
  • 5) Ease of manipulation and reasonable cost.
  • 6) Acceptability to the patient.
  • 7) Safety non toxic or irritating
  • 8) Compatibility with die and cast materials

  • According to uses
  • I- Single tooth impression
  • 1) Rubber 2) copper band compound
  • II- Partial denture
  • 1) Primary Alginate
  • 2) Secondary a) Alginate b) Rubber c)2 pieces
  • III- complete denture
  • 1) Primary a) compound b) Alginate
  • 2) Secondary a) Rubber b) Zinc oxide Eugenol
    c) plaster of parise

  • According to behavior of material after
  • I- Non Elastic Impression
  • 1) Plaster of Paris 2) compound 3) Zinc oxide
    eugenol 4) waxes
  • 2) Elastic Impression
  • 1) Alginat 2) Agar, Agar 3) Rubber

Hydrocolloid impression materials
  • A colloid must be distinguished from a solution
    and suspension. A solution is a homogenous
    mixture. For example, in an aqueous solution the
    solute exists as small molecules or ions in the
    solvent. In contrast to this, a suspension is
    heterogeneous, consisting of particles of at
    least sufficient size to be seen microscopically,
    dispersed in a medium. Thus a suspension is
    two-phase system.

  • Colloids fall between these two extremes. They
    are heterogeneous, ( 2 phase systems), like
    suspensions, but the particle size of the
    dispersed phase is smaller, usually in the range
    1-200 nm. However, it is not always possible to
    distinguish between a colloid and a solution on
    the one hand and a colloid and suspension on the
  • When the dispersion medium of a colloid is water,
    it is termed a hydrocolloid.

  • Colloid may exist in the sol and gel state. In
    the sol state, the material is a viscous liquid.
    A sol can be converted into a gel- a material of
    gelatin-like consistency, due to agglomeration of
    the molecules of the dispersed phase, to form
    fibrils, or chains of molecules, in a network
    pattern. Theses fibrils enclose the dispersion
    medium, for example water.

  • A sol may be converted into a gel in one of two
  • 1) By a reduction in temperature such processes
    are reversible, since on heating, a sol is formed
    again an example of this is agar. In such a gel
    the fibrils are held together by Van Der Waals
  • 2) Other materials can form a gel by a chemical
    reaction, which is irreversible (for example,

  • The strength or toughness of gel depends on
  • 1) The concentration of fibrils the greater the
    concentration, the stronger the material
  • 2) The concentration of fillers inert powders
    can be added to a gel to render it less flexible.
  • A gel can lose or take up water or other fluids.
    Loss of water can occur by evaporation. Syneresis
    can also occur this happened when the gel
    molecules are drawn closer together, for example
    by continuation of an setting reaction. As a
    result a fluid exudate appears on the surface of
    the gel. Uptake of water is called imbibition.

  • In the use of hydrocolloids for dental
    impression, the material is inserted in the mouth
    in sol state, when it is sufficiency fluid to
    record detail. No gelation should have occurred
    at this stage. It is removed from the tissues
    after the gel is formed, when it exhibits elastic
  • Clearly evaporation form, and imbibition by the
    gel should be avoided, as the former is
    associated with shrinkage and the latter results
    in expansion.

Agar ( Composition )
  • Constituent Approximate percentage
  • Agar 14 colloid
  • Borax 0.2 strengthens
    the gel, but
    retards the setting of
    stone model
  • Potassium sulphate 2 To accelerate the
    setting of
  • Water 83.8 Dispersion

  • a) The material is supplied in sealed containers
    to prevent evaporation of water.
  • b) It is brought to a fluid state by heating the
    tube in boiling water in a processing unit for
    10-45 minutes.
  • c) After boiling, the material can be stored at
    65ºC for up to 8 hours
  • d) It is important that the entire tube of
    material is softened
  • e) The material is extruded onto metal trays with
    mechanical interlocking.
  • f) The filled tray is placed in a tempering bath
    at 45ºC for 2 minutes, before insertion into
    patients mouth.
  • g) With water-cooled trays, the agar material set
  • A higher temperature is required for the
    conversion of gel to sol than for reverse

  • 1) Accuracy
  • The material can be suffiently fluid to record
    fine details if it has been correctly
  • The first material to set is that which is in
    contact with the tray (contrast with alginates)
    since this is cooler than the tissues. Thus the
    material in contact with the tissues stays liquid
    for the longest time, and can flow to compensate
    for any inaccuracy due to dimensional changes, or
    to inadvertent movement of the tray.

  • The set material can be withdrawn over
    undercuts. The adhesion of agar to metal is poor,
    so perforated trays are used.
  • Models should be cast up immediately from agar
    impressions, to avoid the possibility of the
    evaporation or imbibition. If this is not
    possible, it has been claimed that stability is
    achieved by storage in 2 K2SO4 solution or

  • The compatibility with cast materials depends
    on the chemicals in the impression material.
    Without an accelerator for the setting of stone
    (e.g. K2SO4) a soft surface may be obtained.
  • 2) These materials are non-toxic and
  • 3) Their setting time is rather slow, unless
    efficient cooling is achieved.
  • 4) Tear resistance is poor.
  • 5) Their shelf-life is adequate. The material can
    be reused and can be sterilized. Loss of water,
    with an increase in viscosity of the sol, may
    occur. Water can be added if required.
  • 6) Can be sterilized by immersion for 10 minutes
    in dilute aqueous solutions of sodium
    hypochlorite or glutaraldehyde.

  • These materials may be used to some extend for
    prosthetic impressions, and in crown and bridge
    work. Agar can be employed in the laboratory, for
    model duplication, since they can be re-used many
    times, due to the reversible nature of the

Alginates (composition and setting)
  • Constituent
  • Sodium potassium
    12 react with ca2 to give calcium

  • Slowly soluble calcium salt caso4 12
    releases ca2 to react with

  • Trisodium phosphaste
    2 reacts with ca2 to give

    ca3(po4)2 to delayed gel

  • Filler (diatomaceous earth)
    70 increases cohesion of mix and

    strengths gel
  • Silicon fluorides small
    quantity improves surface of stone model
  • Flavouring agents small
    quantity makes materials more acceptable

  • Chemical indicator small
    quantity changes color with PH change, to

    different stages in

    manipulation, e.g.
    violet color

    during spatulation, pink when

    ready to load the tray, white when

    ready for insertion
    into the mouth.

  • On mixing the powder with water a sol is formed,
    and the alginate, the calcium salt and the
    phosphate begin to dissolve. The following
    reaction occurs to form an elastic gel of calcium
  • NaAlg CaSO4 Na2So4 CaAlg..1
  • Only the outer layer of each particle of sodium
    alginate dissolves and reacts.
  • However, the mixing and tray loading procedures.
    This is obviously undesirable, since the material
    should deform plastically, not elastically, on
    insertion into the mouth. Gel formation is
    delayed by trisodium phosphate, which reacts with
    calcium sulphate to give a precipitate of calcium
    phosphate, as following
  • 2Na3Po4 3CaSo4 Ca3(Po4)2
  • This latter reaction does not contribute any
    elastic properties to the material.
  • Reaction 2 occurs in preference to 1, no
    substantial quantity of calcium alginate gel is
    formed until the trisodium phosphate is used up.
    The manufacture can therefore control the setting
    time of this product by adjusting the quantity of
    this constituent.

  • The following points should be observed in order
    to obtain the best results
  • 1) The container of powder should be shaken
    before use to get an even distribution of
  • 2) The powder and water should be measured, as
    directed by the manufacture. On brand of powder
    has been supplied in water soluble sachets, which
    help to ensure a uniform consistency of mix.

  • 3) Room temperature water is usually used, slower
    or faster setting times can be achieved, if
    required, by using cooler or warmer water
  • 4) Retention to the tray is achieved by one or
    both of two means
  • a) Perforated tray
  • b) An adhesive such as molten sticky wax or
    methyl cellulose.

  • 5) There should be vigorous mixing by spreading
    the material against the side of the bowel (one
  • 6) An alginate impression should be displaced
    sharply from the tissue- this sudden displacement
    ensure the best elastic behavior. The impression
    is removed about 2 minutes after set.
  • 7) On removal from the mouth, the impression
    should be
  • a) wash with cold water to remove saliva.
  • b) Converted with a damp napkin to prevent
  • c) Cast up as soon as possible, preferably not
    more than 15 minutes after taking the impression.

  • 1) Alginate are sufficiently fluid to record fine
    detail in the mouth.
  • 2) During setting of the material it is important
    that the impression should not be removed. The
    reaction is faster at higher temperatures and so
    the material in contact with the tissues set
    first any pressure on the gel due to movement of
    the tray will set up stresses within the
    material, which will distort the alginate after
    its removal from the mouth.

  • 3) The material is sufficiently elastic to be
    withdrawn over undercuts, tearing of the
    impression material may occur with sever
  • 4) Alginate are not dimensionally stable on
    storage, because of evaporation.
  • 5) Compatibility with plaster and stone can be
  • 6)The materials are non toxic and non irritant,
    their taste and odor are usually acceptable

  • 7) The setting time depends on the composition
    and temperature of mixing
  • 8) The alginate powder is not stable on storage
    in the presence of moisture.
  • 9) Difficult to sterilize, spray disinfection
    diminish the sharpness of surface detail, while
    immersion in solutions adversely affects
    dimensional accuracy.

  • These materials are not generally used for
    impression for inlay, crown and bridge work, but
    are applied with great success for prosthetic and
    orthodontic purposes. Alginates are dimensionally
    less stable than the elastomer.

Elastomeric Impression Materials
  • Chemically there are 4 kinds
  • 1) polysulphid
  • 2) Condensation polymerizing silicone
  • 3) Additional polymerizing silicone
  • 4) Polyether

1- Polysulphide rubber impression materials
  • 1) The first rubber impression material.
  • 2) Didnt have major changes during storage that
    agar and alginate have.
  • 3)Impression was much stronger and more resistant
    to tearing than agar and alginate.
  • 4) Rubber could be electroplated therefore metal
    die as well as gypsum models could be prepaired.

Chemistry of the polysulphides
  • Alternative names rubber-base, mercaptan,
  • Composition
  • The materials supplied as two past
  • 1) The base past
  • 2) The reactor past

  • 1) past base contains
  • 80 low molecular weight organic polymer
    containing reactive mercaptan gp (-SH)
  • 20 reinforcing agents (titanium oxides- zinc
    sulphate, copper carbonte, or silica.
  • 2) The reactor past ( acceleator or catalyst
  • Lead dioxide (PbO2) this causes(polymerization
    and cross-linking), dioxide using it result in
    paste being dark brown to dark gray.

  • Sulphur
  • An inert oil (dibutyle or dicotyle phthalate)
  • To make past.
  • Other catalyst system use copper hydroxide,
    organic peroxides (not used since they are
  • N.B. the viscosity of the material is controlled
    by the molecular weight of the mercaptan and by
    the selection of the reinforcing agents.

  • The SH groups can be oxidised by PbO2, giving
    S-S linkages, as follows
  • This linear polymer contains approximately 1mol
    of branches to provide enough pendant mercaptan
    groups as chain cross-linking sites. This polymer
    is usually cross-linked with an oxidizing agent
    such as lead dioxide. It is the lead dioxide that
    gives polysulfide its characteristic brown color.
    During the condensation reaction of the lead
    dioxide with the SH groups of the polysulphide

  • Two phenomena occur
  • 1) Chain- lengthening polymerization from the
    reaction with the terminal-SH groups
  • 2) Cross-linking from the reaction with the
    pendant-SH groups.
  • Because the pendant groups compose only a small
    percentage of the available SH groups,
    initially, the polymerization reactions result in
    chain lengthening, which causes the viscosity to
    increase. The subsequent cross-linking reactions
    tie the chains together, forming a
    three-dimensional network that confers elastic
    properties to the material.

  • The initial increase in viscosity affects the
    working time of the material and is a change the
    dentist becomes accustomed to when handling this
  • The curing reaction starts at the beginning of
    mixing and reaches its maximum rate soon after
    the spatulation is complete, at which stage a
    resilient network has started to build. During
    the final set, a material of adequate elasticity
    and strength is formed that can be removed over
    undercuts quite readily.

  • The polymerization reaction of polysulfide
    polymer is exothermic, the amount of heat
    generated depends on the amount of total material
    and the concentration of initiators. Moisture and
    temperature exert a significant effect setting of
    polysulfide impression material. The condensation
    reaction by-product is water. Loss of even this
    small molecule from the set material has a
    significant effect on the dimensional stability
    of the impression.

Silicon rubber impression materials
  • Were developed to over come the disadvantages of
    polysulphides which are
  • 1) Objectionable odour.
  • 2) Staining clothes by the lead dioxides.
  • 3) Long setting times.
  • 4) Fairly high permanent deformation.