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Properties of dental materials lecture two

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  • 1. U L F A T
  • 2. PROPERTIES OF DENTAL MATERIALS By DR. YAWAR HAYAT KHAN BDS (Pb), M.Sc. (London) Assistant Professor/Head of Department (Dental Materials) ISLAMIC INTERNATIONAL MEDICAL COLLEGE (DENTAL SECTION)
  • 3. PROPERTIES OF DENTAL MATERIALS
    • Many factors must be taken into account when considering which properties are relevant to the successful performance of a material.
    • Several studies evaluate distinct properties of dental materials which can influence and predict their performance.
    • Dental products have been developed very rapidly and, consequently no. of studies designed to evaluate their properties is also increasing.
  • 4. PROPERTIES OF DENTAL MATERIALS
    • No single property can be used to define the quality of a material.
    • Several combined properties determined from standardized laboratory and clinical tests, are employed to give a measure of quality of a material.
  • 5. SCHEMATIC REPRESENTATION OF PROPERTIES USED TO CHARACTERIZE DENTAL MATERIALS MATERIAL PROPERTIES
    • DURING STORAGE
    • (before use)
    • Shelf life
    • DURING MIXING
    • Methods of dispensation
    • Mixing time
    • viscosity
    • DURING SETTING
    • Rate of set
    • Working time
    • Setting time
    • Temperature rise on setting
    • Dimensional changes
    • SET MATERIAL
    • Physical
    • Chemical
    • Biological
    • Mechanical
  • 6. PROPERTIES OF SET MATERIAL
    • PHYSICAL PROPERTIES
    • CHEMICAL PROPERTIES
    • BIOLOGICAL PROPERTIES
    • MECHANICAL PROPERTIES
  • 7. 1. PHYSICAL PROPERTIES
    • OPTICAL PROPERTIES
    • THERMAL PROPERTIES
    • ELECTRICAL PROPERTIES
    • RHEOLOGICAL PROPERTIES
    • MISCELLANEOUS PROPERTIES
  • 8. A. OPTICAL PROPERTIES
    • COLOUR
    • TRANSLUCENCY
    • SURFACE TEXTURE
  • 9. i. COLOUR
      • Colour selection is very important
      • Depends upon no. of physical factors
      • Important parameters used to define colour are hue, chroma and value
      • Hue : Basic colour depending on the wavelength it reflects e.g. blue & green
      • Chroma : Measures the intensity of the colour e.g. low value of chroma indicates a weak colour
      • Value : value is darkness or lightness of the colour
      • Dental applications: Aesthetics , shade guides & fluorescent agents
  • 10. ii. TRANSLUCENCY
      • Transparent : Allows the light to pass through it
      • Translucent : Allows some light to pass through it
      • Opaque : Does not allow light to pass through it
  • 11. iii. SURFACE TEXTURE
      • Whether the material is shiny or matt the surface texture indicates the smoothness of the surface
      • Enamel has a shiny surface hence it is smooth
      • As the surface gets rough, it becomes matt
  • 12. B. THERMAL PROPERTIES
      • Materials placed in the oral environment are constantly subjected to change in temperature
      • Further classified into
          • Temperature
          • Heat of fusion
          • Specific heat
          • Thermal conductivity
          • Thermal diffusivity
          • Coefficient of thermal expansion
  • 13.
      • TEMPERATURE
        • Temperature of a food affects oral temperature
      • HEAT OF FUSION
        • Heat in calories or joules required to convert 1 gm of a material from solid to liquid state at the melting temperature
      • SPECIFIC HEAT
        • Specific heat (C p ) of a substance is the quantity of heat needed to raise the temperature of a unit mass of a substance by 1ºC.
        • C p of metals and alloys should be known (temperature at which it should be melted)
        • When metallic fillings are used, the C p of the materials should be low, this reduces heat conduction
  • 14.
      • THERMAL CONDUCTIVITY
        • The factor which determines the ease with which the heat is transferred to a material is its thermal conductivity (K)
        • It is defined as rate of heat flow per unit temperature gradient. Units are cal/ sec/ cm 2
        • Different materials have different values of K
        • Materials with high K -> good conductors of heat and cold
        • Materials with low K -> bad conductors of heat and cold
        • Metals are good conductors as compared to plastics and ceramics
        • Composites and zinc phosphates have same values of K as that of enamel and dentine
        • Dental considerations
          • A metallic restoration conducts heat -> transferred to pulp -> pain and discomfort -> can be avoided by placing insulating bases (cements) as they are poor conductors
          • Denture bases can be made up of metals or resins. Metallic is better as it conducts heat and maintains tissue health.
  • 15.
      • THERMAL DIFFUSIVITY
        • It is a measure of transient heat flow
        • Thermal diffusivity = Thermal Conductivity
        • Density x specific heat
        • Δ = K
            • ρ x C P
        • Units are mm 2 /S
        • It is related to the thickness of material, e.g., liners should have adequate thickness when used below restorations
        • In case of metallic restorations, high thermal diffusivity than the tooth structure results in thermal sensitivity, e.g., amalgam fillings, gold inlays and crowns
  • 16.
      • COEFFICIENT OF THERMAL EXPANSION
        • When a materials is heated the extra energy it absorbs causes the atoms and molecules to vibrate with an increased amplitude. As a result the material expands.
        • Most common way of measuring this expansion is by taking the length of the material, heating it to a certain temperature and then measure resultant change in length
        • Most often linear thermal expansion is expressed as coefficient of thermal expansion ( ά ).
        • Defined as change in length per unit length of the material for 1 ºC change in temperature. Units are ppm/ ºC.
        • For restorative materials it should be similar or close to the tooth structure, otherwise thermal mismatch can give rise to marginal gap formation, breakdown of adhesive bonds and finally failure of restoration.
  • 17. C. ELECTRICAL PROPERTIES
    • Resistivity & conductivity
    • Dielectric Constant
    • Galvanism
    • Electrochemical Corrosion
  • 18. i. Resistivity & Conductivity
    • The ability to conduct electric current may be stated either as “specific conductance” or “conductivity” and conversely as the “specific Resistance” or “resistivity”.
    • The sensitivity of the tooth structure depends upon the electrical resistance of the tooth structure. Carious teeth shows less resistance than normal.
    • Zinc Oxide Eugenol has the highest electrical resistivity.
    • Conductivity is important in case of restorative materials with GIC being the most conductive.
  • 19. ii. Dielectric Constant
    • A material that provides electrical insulation is known as dielectric constant.
    • The value of dielectric of a dental cement decreases as it hardens.
    • Electrical insulation by cement bases is especially important in case of galvanism.
  • 20. iii. Galvanism
    • The presence of metallic restoration in mouth may cause a phenomenon called as Galvanic action or galvanism.
    • It is the generation of electrical current that the patient can feel
    • It results from difference in potential between dissimilar materials in opposing or adjacent teeth
    • Examples:
      • Metals placed in an electrolyte (liquid containing ions) have various tendencies to go into solution. Aluminium the alloys of which are used as temporary crowns has great tendency to go into solution and has electrode potential of +1.33 volts.
      • Gold on the other hand has little tendency to go into the solution as indicated by the electrode potential of -1.36 volts.
  • 21. iii. Galvanism
    • In this situation, two opposing teeth where oral fluids acts as electrolytes. The system is similar to that of an electrical cell. When the two restorations touch the current flows because the potential difference is 2.69 volts and the patient feels pain, discomfort and metallic taste.
    • The same effect is experienced if an aluminium foil from a baked potato becomes wedged between two teeth and contacts a gold restoration. Temporary plastic crowns are used to prevent this problem since they are poor electrical conductors.
  • 22. iv. Electro Chemical Corrosion
    • Corrosion is dissolution of metals in the mouth.
    • As a result of galvanic action material goes into solution and roughness & pitting occurs.
    • May also be observed when a gold alloy is contaminated with iron during handling in the dental laboratory
  • 23. D. RHEOLOGICAL PROPERTIES
    • They are important as they have a major influence on the handling characteristics of the material
        • Viscosity
        • Viscoelasticity
        • Time - dependent properties
          • Creep and flow
          • Stress relaxation
  • 24. i. Viscosity
    • The viscosity of a fluid is its resistance to flow and is equal to the shear stress divided by the strain rate.
    • η = shear stress/ strain
    • Units are Pascal seconds (Pas)
    • The viscosity of a fluid depends upon number of factors
      • Nature of substance
      • Temperature
      • Pressure
      • Shear rate
    • Two types of flow behaviour: Newtonian & Non-Newtonian
  • 25. i. Viscosity
    • Newtonian Flow Behaviour
      • Fluids which depends upon shear stress
      • Only 10% of liquids are Newtonian
    • Non - Newtonian Flow Behaviour
      • When it does not follow the Newton’s law
      • It has the following types
        • Bingham flow e.g. composite filling materials
        • Plastic Flow
        • Pseudoplastic or dilatent e.g.
          • Polymers
          • Natural resins
          • Blood
          • Impression materials
          • Fluorides gels
        • Thixotropy when some fluids are sheared at a steady rate viscosity decreases with time, on standing the fluid regain its original viscosity. It occurs due to structural breakdown and reformation. It is exhibited by many dental materials extremely useful in impression materials
  • 26. i. Viscosity
    • Dental applications
      • Materials should have good viscosity and flow to be easily manipulated
      • Material has poor rheological properties when there is no well defined working time
      • Material has ideal rheological properties when it has well defined working time followed by sudden and complete setting
    • Measurement of Viscosity
      • Capillary viscometers
      • Falling body viscometers
      • Rotational viscometers
      • Parallel plate plastometers
  • 27. ii. Viscoelasticity
    • Materials showing combination of viscous & elastic properties are called as viscoelastic
    • It can be explained by models
      • Spring
      • Dashpot
      • Maxwell (combination)
    • Dental applications
      • Polymers like soft relining materials & denture base polymers exhibit viscoelastic behaviour
      • Elastomeric impression materials agar-agar; alginate
      • Creep of dental amalgam
  • 28. iii. Time Dependent Properties
    • Creep and Flow
      • Creep is a time dependent plastic deformation. It is of two types- static and dynamic, e.g., dental amalgam
      • Flow similar term to creep but deformation which occurs at a smaller stress, e.g., amorphous materials like waxes and resins
    • Stress Relaxation
      • It is a slow change in shape with a decrease in force overtime
      • Examples:
        • When orthodontic arch wire is bent, it can alter tooth position due to stress relaxation as it regain its original contour
        • Orthodontic elastics have to be frequently changed because loss of force occurs due to stress relaxation
        • Waxes like inlay wax or impression wax when left for a long time undergo distortion due to stress relaxation
  • 29. E. MISCELLANEOUS PROPERTIES
    • Dimensional changes
    • Density
    • i. Dimensional changes :
      • Dimensional change is the percentage of shrinkage or expansion of a material
      • Maintaining dimensional changes during dental procedures such as taking impressions and pouring models is highly important in accuracy of dental restorations
      • Dimensional changes may occur during setting as a result of chemical reaction, as with rubber impression materials or composites and from cooling of wax patterns, gold restorations during fabrication
      • Ideally there should be no dimensional changes on setting
  • 30. E. MISCELLANEOUS PROPERTIES
    • ii. Density:
      • Fundamental property which affects design aspect of dental appliances
      • it is necessary to consider density for choosing an alloy with which to construct components of an upper denture
      • A bulky design in a heavy alloy would result in large displacing forces making retention difficult
      • In order to reduce such destabilizing forces one could choose to use a lower density alloy and to keep the alloy bulk to a minimum.
  • 31. THANK YOU To be continued ….

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