Impression materials (requirement and classification)

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Impression materials (requirement and
classification)

• Dr. Waseem Bahjat Mushtaha
• Specialized in prosthodontics

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Definition

• 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
  cast.

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Requirements

• 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

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Classification

• 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
  impression
• III- complete denture
• 1) Primary a) compound b) Alginate
• 2) Secondary a) Rubber b) Zinc oxide Eugenol
  c) plaster of parise

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• According to behavior of material after
  setting
• 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

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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.

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• 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
  other.
• When the dispersion medium of a colloid is water,
  it is termed a hydrocolloid.

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• 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.

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• A sol may be converted into a gel in one of two
  ways
• 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
  forces.
• 2) Other materials can form a gel by a chemical
  reaction, which is irreversible (for example,
  alginates)

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• 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.

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• 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
  properties.
• Clearly evaporation form, and imbibition by the
  gel should be avoided, as the former is
  associated with shrinkage and the latter results
  in expansion.

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Agar ( Composition )

• Constituent Approximate percentage
  Function
• Agar 14 colloid
• Borax 0.2 strengthens
  the gel, but
  retards the setting of
  dental
  stone model
  materials
• Potassium sulphate 2 To accelerate the
  setting of
  stone
• Water 83.8 Dispersion
  medium

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Manipulation

• 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
  quickly.
• A higher temperature is required for the
  conversion of gel to sol than for reverse
  reaction.

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Properties

• 1) Accuracy
• The material can be suffiently fluid to record
  fine details if it has been correctly
  manipulated.
• 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.

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• 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
  humidor.

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• 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
  non-irritant.
• 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.

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Applications

• 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
  reaction.

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Alginates(composition and setting)

• Constituent
  approx.
  function
• Sodium potassium
  12 react with ca2 to give calcium
  
  
  gel
• Slowly soluble calcium salt caso4 12
  releases ca2 to react with
  
  
  alginate
• Trisodium phosphaste
  2 reacts with ca2 to give
  
  
  ca3(po4)2 to delayed gel
  
  formation
  
• 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
  
  to
  patient
• Chemical indicator small
  quantity changes color with PH change, to
  
  indicate
  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.

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• 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
  alginate
• 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
  3Na2So42
• 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.

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Manipulation

• 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
  constituents.
• 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.

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• 3) Room temperature water is usually used, slower
  or faster setting times can be achieved, if
  required, by using cooler or warmer water
  respectively.
• 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.

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• 5) There should be vigorous mixing by spreading
  the material against the side of the bowel (one
  minute)
• 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
  syneresis.
• c) Cast up as soon as possible, preferably not
  more than 15 minutes after taking the impression.

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Properties

• 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.

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

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• 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.

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Application

• 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.

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Elastomeric Impression Materials

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

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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.

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Chemistry of the polysulphides

• Alternative names rubber-base, mercaptan,
  Thiokol.
• Composition
• The materials supplied as two past
• 1) The base past
• 2) The reactor past

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

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

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Setting

• The SH groups can be oxidised by PbO2, giving
  S-S linkages, as follows
• R-SHPbO2HS-R R-S-S-RPbOH2O
• 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
  polymer.

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• 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.

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• 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.

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Silicon rubber impression materials

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

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Composition

• 1) Base
• - Past contains low molecular weight silicon
  liquid\ dimethyl siloxane which has terminal
  reactive OH gp.
• - Reinforcing agents silica
• 1) Proper consistency to the mix.
• 2) Stiffness to the set rubber.

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• 2) Accelerator
• Usually supplied as a liquid but sometimes as a
  past by the use of thickening agent tin octate
  and an alkyl silicate such as artho -ethyl
  silicate.
• 3) Silicone pastes supplied
• Consistency controlled by a) concentration of
  reinforcing agents.
• b) Molecular weight of dimethyle siloxan
• light.. 35 reinforcing agent
• regular.
• heavy-bodiesmol.w
• puttymol.w..75

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Setting reaction

• 1) Dimethyl siloxan ortho-ethyl silicate tin
  octacle silicone rubber ethylac.
• 2) The multifunctional ethyl silicate produces a
  cross-linked structure that partly accounts for
  the low value of permanent deformation and flow
  of silicon rubber.
• 3) Ethylac (by product) evaporates gradually
  reasonable for shrinkage during 24 hrs after
  setting.
• 4) Setting reaction is more sensitive than
  polysulphid to moisture and heat-increase either
  of them the setting and working time( normally
  the silicon setting and working times are shorter
  than polysulphides)

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Addition type

• 1) Silane containing siloxane vinyl-terminal
  siloxane chloroplatinic acid silicone
  rubber (by volatile byproducts)
  minimal dimensional changes during polymerize
  action
• 2) In Temp the rate of reaction and
  shorten the setting time.
• 3) If hydroxyl gp. Present in the addition
  silicon
• Side reaction occurs with the result of H2
  gradually released from the set. Impression
  bubbles in the gypsum models prepared after less
  than 1hr often impression taken. Overcome by
• 1) Control the presence of hydroxyl gp.
• 2) Put H2 absorber such as palladium

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Polyether rubber impression materials

• It offers mechanical properties than
  polysulphides and dimensional changes the
  condensation silicone. However it possesses
  limitations such short working time and high
  stiffness.

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Composition and setting reaction

• 1) Base
• 1) Lower molecular weight polyether contain
  ethyleneneamin terminal gp.
• 2) Plasticizers
• 3) Filler
• 2) Catalyst
• Aromatic sulphonate acid ester these terminal gps
  reacted together by the action of catalyst to
  form cross-linked mol.w.rubber.
• 2) Plasticizer
• 3)Filler
• Polyether sulphonic ester
  cross linked rubber

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Visible light-cure impression material

• 1) Polyether urethanedimethacrylate resin which
  visible light-cure photo initiator and
  photoaccelerater added.
• 2) The silicon dioxide filler has a refractive
  index close to that of the resin in order to
  provide the translucency for maximum depth of
  cure.