Physical Properties of Dental Materials Presented by Karma Yangchen JR-1

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Dr Deeksha Arya Assistant Professor Department of
Prosthodontics
BDS Ist Year Dental Material Monday 1000 to
1100 AM
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• Introduction
• Physical properties of Dental Materials
• References

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• Physical properties of Dental Materials are based
  on
• The laws of mechanics-Creep flow , abrasion
  and abrasion resistance , viscosity
• Thermodynamics e.g. Thermal conductivity ,
  Thermal diffusivity and Coefficient of thermal
  expansion

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• Electricity Magnetism e.g tarnish and corrosion
• Optics Metamerism ,Hue , chroma value
• Radiation Laser

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• Creepis defined as the time dependent plastic
  strain of a material under a static load or
  constant stress.
• The magnitude of deformation depends on
• the materials
• the temperature
• the load
• the time of application of the load.

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• Creep may cause unacceptable deformation of
  dental restorations .eg.-low-copper dental
  amalgam.
• According to ADA SP. No 1 creep should be below
  3.

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• Flow
• Ability of material to displace itself under
  load.
• Flow is dependent on
• Temperature of the wax.
• Force applied.
• Length of time the force is applied.
• Flow increases as the melting point of the wax is
  approached.

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• Abrasion is defined as the wearing away of a
  substance or structure through a mechanical
  process such as grinding, rubbing or scraping.

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• Hardness has been used as an index of the ability
  of the material to resist abrasion or wear.

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• Viscosity -
• Resistance of a liquid to flow.
• Dental material have different viscosities
  depending on the preparation for their intended
  clinical application.
• measured in units of MPa per second, or
  centipoise (cP).
• most liquids decreases rapidly with increasing
  temperature.

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• Pseudoplastic behavior
• Plastic
• Thixotropic Dental prophylaxis paste,Plaster of
  paris,Resin cements, impression material

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• Properties which are not related to force
  application-
• Mass-related properties
• Density
• Thermal properties
• Thermal conductivity
• Thermal diffusivity
• Coefficient of thermal expansion and contraction
• Heat of fusion latent heat of solidification
• Melting and freezing temperature
• Specific heat

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• Less specific properties
• Water sorption.
• Optical properties
• Color

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• It is the mass per unit volume of the material.
• Units are gm/cm3 .
• Clinical importance
• 1- Retention of the upper denture.
• 2- Weight of complete or partial
• denture.
• 3- During casting.

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• Thermal Conductivity
• Defined as the ability of a material to
  transmit heat or cold.
• A low thermal conductivity is desired in
  restorative materials used on the tooth
• High thermal conductivity is desirable where
  the material covers soft tissue.

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• Thermal diffusivityThermal conductivity
• Density x
  specific heat
• Clinical Importance
• The value of thermal diffusivity of a materials
  controls the time rate of temperature change as
  heat passes through a material.
• Cements which have low thermal diffusivity are
  used for pulpal protection.

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Material Density (g cm-3 ) Specific heat (Cal g-1 K-1) Thermal Conductivity (W m-1 K-1) Thermal diffusivity (cm2 s-1)
Water 1.00 1.00 0.44 0.0014
Dentin 2.14 0.30 0.57 0.0018-0.0026
Glass ionomer 2.13 0.27 0.51-0.72 0.0022
Zinc Phosphate 2.59 0.12 1.05 0.0030
Composite 1.6-2.4 0.20 1.09-1.37 0.0019-0.0073
Enamel 2.97 0.18 0.93 0.0047
Amalgam 11.6 0.005 22.6 0.96
Pure gold 19.3 0.03 297 1.18

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• Metallic fillings in close proximity to the
  dental pulp, causes thermal irritation of the
  pulp through conductors of heat and cold from
  food and drinks when not properly insulated.

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• For effective thermal protection the base should
  have minimal thickness of 0.75 mm.

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• The change in length per unit length of the
  material for a 1C change in temperature is
  called the linear coefficient of thermal
  expansion(a)
• a L final- L original
• L original x (C final- C original)

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Material a (ppm K-1 ) amaterial / atoothEnamel
Aluminous porcelain 6.6 0.58
Dentin 8.3 0.75
Commercially pure titanium 8.5 0.77
Type II glass ionomer 11.0 0.96
Tooth enamel 11.4 1.00
Gold-palladium alloy 13.5 1.18
Gold(pure) 14.0 1.23
Palladium-silver alloy 14.8 1.30
Amalgam 25.0 2.19
Composite 14-50 1.2-4.4
Denture resin 81.0 7.11
Pit and fissure sealant 85.0 7.46
Inlay wax 400.0 35.1
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• Close matching of the coefficient of thermal
  expansion (a) is important between
• 1-The tooth and the restorative materials to
  prevent marginal leakage.

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• 2-Opening and closing of gap results in breakage
  of marginal seal between the filling and the
  cavity wall, this breakage of seal leads to
• Marginal leakage
• Discoloration
• Recurrent caries
• Hypersensitivity.

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• Heat of fusion is the amount of heat in
  calories or joules required to convert l gm of a
  material from the solid to the liquid state at
  the melting temperature.
• Clinical Importance
• In actual use of pure metal or casting alloy
  must have low specific heat and low heat of
  fusion, so it does not required prolonged heating
  to come to a molten state which may cause
  oxidation of the metal , under conventional
  procedure.

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• Latent heat of solidification is the amount
  of heat in calories or joules liberated when 1 gm
  of a material is converted from liquid to solid
  state.

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• Clinical Importance
• For the fabrication of indirect metallic
  restorations (casting), the melting temperature
  of metals and alloys is important in determining
  the melting machine used for casting
• During casting metal must be heated 100C above
  its melting temperature.

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• The materials that are to be manipulated
  directly in the mouth eg-waxes , impression
  compound etc. should have a softening or melting
  point slightly above the body temperature and
  must harden to a desired degree of rigidity at
  body temperature.
• An ideal solder for user on a particular alloy
  will have a melting temperature 30 C-100 C less
  than that of the alloy.

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• Specific heat is the quantity of heat needed
  to raise the temperature of one gram of the
  substance 1C.
• Metals have low specific heat, while non
  metals have high specific heat .

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• Clinical Importance
• It is very important because it shows how much
  and how long a metal is to be heated to bring it
  to the melting point.
• Because of the low specific heat of dental gold
  alloys, prolonged heating is unnecessary, during
  casting.

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• Water Sorption
• It represents the amount of water adsorbed on
  the surface and absorbed into the body of the
  material.
• Clinical Importance
• 1- Acrylic resin denture base materials have the
  tendency for water sorption.
• 2- Hydrocolloid impression materials will imbibe
  water if immersed in it leading to dimensional
  changes.

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• Tarnish
• Surface discoloration on a metal or even a
  slight loss or alteration of the surface finish
  or lusture.
• Tarnish usually occurs due to-
• Formation of hard and soft deposits on the
  surface of the restorations .e.g.-calculus ,
  mucin and plaque.

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• Corrosion is not a surface discoloration but
  actual deterioration of a metal by reaction with
  the environment .

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• Corrosion may be classified as
• Chemical corrosion is a direct effect on the
  metal surface by oxidation, halogenations or
  sulfurization reaction.
• Acidity and alkalinity may give rise to corrosion
  by direct attack , particularly if there is any
  defect in the constitution of the alloy.

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• Electrolytic corrosion-
• Chemical corrosion leads to electrolytic
  corrosion.
• Electrolytic corrosion may takes place in the
  mouth with saliva acting as an electrolyte. when
  the following condition are present in it
• Different metals and alloys
• Heterogeneous composition of alloys
• Cold work metal
• Difference in oxygen tension

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• Galvanic corrosion
• Stress corrosion
• Concentration cell corrosion or Crevice corrosion

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• Galvanic Corrosion
• Galvanic corrosion occurs when dissimilar metals
  lie in direct physical contact with each other.
• Eg- if a gold restoration comes in contact with
  an amalgam restoration , the amalgam forms the
  anode and starts corroding.
• When two restoration touch causes sharp pain.
• The best precaution is to avoid dissimilar metals
  in contact.

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• The perception of the Color of an object is
  the result of a physiological response to a
  physical stimulus (light).
• Light is an electromagnetic radiation that
  can be detected by the human eye. It can be seen
  that the visible electromagnetic radiation is in
  the range from 400-700 nanometers.

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• Transparency
• is a property of a material, that allows the
  passage of light in such a manner that little
  distortion takes place so that objects can be
  clearly seen through them
• e.g. glass, pure acrylic resin.

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• Translucency
• Property of the material, which allows the
  passage of some light and scatters or reflects
  the rest . In such manner, the object cannot be
  clearly seen through them
• Translucency decreases with increasing the
  scattering centers.
• e.g. tooth enamel, porcelain, composite and
  pigmented acrylic resin natural teeth.
• 
  
  
  

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• Opacity
• is a property of the material that prevents
  the passage of light. Opaque material absorbs all
  of the light. Objects cannot be seen through
  them.
• Eg. metal-ceramic restoration
• Black color materials absorb all light colors.
• White color materials reflect all light colors.
• Blue color materials absorb all light colors
  but reflect its
  color.

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• Ideal restorative material should match the color
  of the tooth it restores.
• In maxillofacial prosthetics the color of the
  gums ,external skin and eyes have to be
  duplicated.
• Clinically in the operatory or dental lab ,
  color selection is usually done by the use to
  shade guides.

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• COLOR
• Color parameters
• A- Hue
• It is the dominant wave length. It
  represents the color of the material, i.e yellow,
  green, red and blue.
• B- Chroma
• It represents the strength of the color or
  degree of saturation of the color (color
  intensity).

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• C- Value
• It represents the lightness or darkness of
  color (the amount of grayness).
• A tooth of low value appears gray and
  nonvital DEAD,
• Therefore, it is the most important
  parameter. Because it is intimately related to
  the aspect of vitality in human teeth.

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• Shade guide is used for color matching. So, it
  is important to match colors under appropriate
  conditions.
• 1- Source
• Different sources have different color
  content. i.e. Incandescent light has a color
  content different from that of fluorescent light.
  

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• 2- Surrounding
• Colors of wall, lips or clothes of the patient
  modify the type of light reaching the object.
• 3- Object
• A- Translucency
• It controls lightness or darkness of color.
• High translucency gives a lighter color
  appearance (higher value) i.e more vital tooth
  appearance

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• B- Surface texture (surface finish)
• This determines the relative amount of light
  reflected from the surface, smooth surface
  appears brighter than rough surface.
• C- Presence of scattering centers as inclusions
  or voids
• This increase opacity and lower the value
  (more dark)

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• D- Fluorescence
• It makes the teeth bright and vital, as it
  increases the brightness.
• Light used in discotheques or even under some
  fluorescent light under such conditions
  artificial teeth or restorations without any
  fluorescence completely black out.

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• E- Thickness
• The thickness of a restoration can affect its
  appearance. Increase in thickness, increase
  opacity, and lower the value.
• F- Metamerism
• It is the change of color matching of two
  objects under different light sources.

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• 4- Observer
• A- Color response
• Eye responds differently among individuals.
• B- Color Vision
• Some individuals may have color blindness
  and inability to distinguish certain colors.
• C- Color Fatigue
• Constant stimulus of one color decreases the
  response to that color.

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• Light Amplification by Stimulate Emission of
  Radiation.
• The principle of laser production is simply
  that an element or compound (medium) can be
  excited by high energy to produce a special type
  of light called laser

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• Characteristics of laser beam
• a- Monochromatic
• all photons have the same wave length.
• b- Coherent
• all waves are bin phase (have the same speed ).
• c- Collimated
• all waves are parallel ( minimum divergence )

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• Surgery for removal of soft tissues.
• Removal of initial carious lesions.
• Curing of composite resin.

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• Philips Science Of Dental Material by Kenneth
  J. Anusavice DMD PhD (Jun 17, 2003)
• Notes on Dental Materials (Dental Series) by E.
  C. Combe (Nov 1992)

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