Physical properties of dental materials

1
Physical properties of dental materials
2
The elements of study

• Physical properties include
• 1-density
• 2-thermal properties
• 3-electrical properties
• 4-optical properties

3
Density

• Density is the mass per unite volume of the
  material .its units are gm/cm³ and pound/in³.

4
Examples of density of some dental materials

• a) Acrylic resin 1.2 gm/cm³
• b) Porcelain 2.4 gm/cm³
• c) Gold alloys 17 gm/cm³
• d) Cobalt chromium alloys 8 gm/cm³

5
Importance in dentistry

• 1) Upper denture must be made up of the materials
  that are of low density to help its retention.
• 2) Molten alloys of low density require higher
  pressure to fill the mold cavity during casting
  procedures.

6
(No Transcript)
7
(No Transcript)
8
(No Transcript)
9
Thermal conductivity (K)

• The thermal conductivity of substance is the
  amount of heat in calories, or joules, per second
  passing through a body 1cm thick with a cross
  section of 1 cm² when the temperature difference
  is 1C . The unites are cal/sec/cm²/(C/cm).

10
Examples of (K)

• a) Enamel 0.0022 cal/sec/cm²/(C/cm).
• b) Amalgam 0.055 cal/sec/cm²/(C/cm).
• c) Gold alloys 0.710 cal/sec/cm²/(C/cm).

11
Importance in dentistry

• 1- Metallic filling material e.g. The high
  thermal conductivity of amalgam is a disadvantage
  because if it is near the pulp it may cause
  patient discomfort as a result of temperature
  changes produced by hot or cold foods and
  beverages unless adequate tooth tissue remains or
  non-metallic substances are placed between the
  tooth and filling as a base for insulation. N.B.
  composite and ceramic restorations are non-
  conductive and do not need insulators.

12
Insulation (calcium hydroxide)
13
Amalgam filling
14

• 2) Metallic denture base materials
• Metallic denture base material is an advantage
  because heat transmission causes tissue
  stimulation through vasodilatation and
  vasoconstriction of blood vessels of underlying
  tissues keeping it in a healthy state.

15
Metallic denture base
16
Specific heat (CP)

• How rapidly the temperature increases in a
  material depends upon the specific heat of the
  material, which is defined as the amount of heat
  required to raise the temperature of a unite
  volume by 1C. Thus , its units are Cal.gm-1.C-1

17
Importance in dentistry

• Specific heat of metal or alloys is important
  during the melting process because it indicates
  the amount of heat that must be applied to the
  mass to raise the temperature to the melting
  point.

18
3) Thermal diffusivity (h)

• The transfer of heat to a cold source is
  dependent on both the thermal conductivity and
  the specific heat, with the former regulating the
  rate at which the heat enters and passes through
  the material and the latter determining the rate
  at which the temperature will rise as heat enters
  the material.

19

• This is presented by the thermal diffusivity, h,
  such that
• K
• h ---------------
• Cp P
• P density of material . Cp specific heat
• K thermal conductivity. The thermal diffusivity
  gives a clear indication of the rate of rise of
  temperature at one point due to a heat source at
  another point, and may be considered the most
  important in dental applications. An interesting
  feature is the low diffusivity of water, showing
  it to be an excellent thermal insulator. For this
  reason, Eskimos can be quite warm when sheltering
  in their igloos.

20
(No Transcript)
21
4) Thermal expansion (a)

• When a material is heated, the energy absorbed
  causes the atoms or molecules to vibrate and the
  material expands. the most common way of
  measuring this expansion is by taking a length of
  material, heating it to a certain temperature and
  then measuring the resultant change in length.
  This change in length, per unit length for a 1C
  change in temperature, is called the linear
  coefficient of expansion,a.

22

• L final L original
• a -------------------------------------
  --
• L original x (C final C original)
• Examples
• Enamel 11 x 10-6/C
• Acrylic resin 77 x 10-6/C
• Composite 22 x 10-6/C

23
Importance in dentistry

• In a composite restoration, the seal between the
  composite and tooth may be affected by the
  repeated temperature changes in the mouth which
  cause different amounts of expansion and
  contraction of the composite and the tooth. This
  difference may create gaps between the two
  (marginal percolation) which leads to leakage of
  salivary components into the tooth leading to
  tooth discoloration, recurrent caries, and pain

24
(No Transcript)
25
5) Heat of fusion (L)

• Transitions from one state to another are
  accompanied by absorption or liberation of heat
  and usually by a change in volume. The amount of
  heat in cal or J required to convert 1 gm of a
  material from solid to liquid state.
• Q (total heat
  absorbed)
• L -------------------------
  ------
• m (mass of substance)

26

• As long as the mass is molten, the heat of fusion
  is retained by the liquid. When the liquid is
  frozen, this is liberated (latent heat of
  fusion).
• Latent heat of fusion is the amount of heat in
  cal. Or joule liberated to convert 1 gm of a
  material from liquid to solid state.

27
Electrical properties (electrical conductivity
and resistively)

• The ability of a material to conduct an electric
  current may be stated as either specific
  conductivity or conversely as the specific
  resistitvity, where resistivity is the resistance
  of a material electric current of one cm length
  and a cross sectional area of one cm².

28

• the resistance of a homogenous conductor of
  uniform cross section at a constant temperature
  varies directly with the length and inversely
  with the cross sectional area of the sample
  according to the following equation
• L
• R P --------
• A
• R is the resistance in ohms P is the
  resistivity. L is the length. A is the
  sectional area.

29
Importance in dentistry

• Zinc oxide eugenol cements have the highest
  resistivity followed by zinc polyacrylate and
  zinc phosphate cements. The glass ionomer cements
  are the most conductive.

30
Zinc oxide eugenol cements
31
Polyacrylate cement
32
zinc phosphate cement
33
glass ionomer cements
34
(No Transcript)
35
4) Optical propertied

• The restoration of teeth has moved more and more
  from the purely functional towards the aesthetic.
  Consequently, the optical properties of the
  materials that are selected and used by the
  dentist have become of great importance.
• The perception of color is a physiological
  response to a physical stimulus (light). For
  example, the choice of color differs from person
  to person, and presents a real problem for those
  who suffer from color blindness.

36

• Light is electromagnetic radiation that can be
  detected by the human eye. It can be seen that
  the visible electromagnetic radiation is the
  range from 400 700 nanometers.

37
Color parameters

• a) Hue this represents the color of the
  material . E.g. blue, green, yellow , orange,
• b) Chroma it is strength of color, or it is a
  measurement of color intensity. A beaker of water
  containing one drop of colorant is lower in
  chroma than a beaker of water containing ten
  drops of the same colorant.
• c) Value it represents the lightness or
  darkness of a color. A black standard is assigned
  a value of 0, where as a white standard is
  assigned 100. a tooth of low value appears gray
  and non-vital therefore it is the most important
  parameter.

38

• Primary colors
• The primary color are red, green, and blue, from
  which all other colors can be produced. This fact
  is used in TV sets to create a full color picture
  from only three colored sets of dots. Combining
  the three primary colors results in white.
• Secondary colors each secondary color- cyan,
  magenta, and yellow results from the
  combination of two primary colors, e.g green and
  red gives yellow.

39
(No Transcript)
40

• Complementary colors two colors are
  complementary to each other when their
  combination results in white e.g. yellow is the
  complementary color of blue.
• Green
• Cyan Yellow
• 
  White
• Blue Red
• Magenta

41
Transparency, translucency, and opacity

• Transparency is the property of materials that
  allow the passage of light in such a manner that
  objects can be clearly seen through them. e,.g.
  glass and clear acrylic resin.
• Translucency is the property of materials that
  allow the passage of some light but disperses
  ( scatters or reflects) the rest so that objects
  cannot be clearly seen through them e.g. dental
  porcelain, composite resin and pigmented acrylic
  resin .

42
(No Transcript)
43
(No Transcript)
44
(No Transcript)
45

• Opacity is the property pf materials that
  prevents the passage of light. An opaque material
  either absorbs or reflects all of the light. When
  all light is absorbed the material appears black.
• Examples of opaque materials are metals.

46
(No Transcript)
47

• Metamerism the color of objects under one type
  of light may appear different under another light
  source. This phenomenon is called metamerism.
• Fluorescence natural tooth structure absorbs
  light at wavelengths too short i.e. not visible
  to the human eye between 300 and 400 nm
  (near-ultraviolet radiation). Natural sun light
  and ultraviolet lights used in decorative
  lighting are sources containing amounts of near
  ultraviolet radiation.

48
(No Transcript)
49

• The energy that the tooth absorbs is converted
  into light with longer wavelengths, in this case
  the tooth actually becomes a light source. This
  phenomenon is called fluorescence. Fluorescence
  contributes to the brightness and vital
  appearance of a human tooth. A person with
  porcelain crowns or composite restorations that
  lack a fluorescing agent appears to be missing
  teeth when viewed under a black light .
• Therefore, some anterior restorative materials
  and dental porcelains are recently formulated
  with fluorescing agent to reproduce the natural
  appearance of tooth structure.

50
(No Transcript)
51
Propagation of light

• 1) Reflection light is reflected in all
  directions, this is called diffuse reflection.
  Rough surfaces undergo diffuse reflection when
  the roughness have dimensional larger than the
  wavelength of the reflected wave. Smooth surfaces
  reflect light in one direction only, this is
  called specular reflection . Highly polished
  surfaces (mirrors) reflect all light in one
  direction where the angle of incidence equals the
  angle of reflection.

52
(No Transcript)
53
(No Transcript)
54

• 2) Refraction
• As light becomes incident on a surface separating
  two different media, a part is reflected and a
  part is refracted( i.e. there is a change in the
  direction of light as it enters the other
  medium). The ratio of the angle of incidence and
  sine angle of refraction is a constant called
  refractive index.

55
(No Transcript)
56

• 3) Scattering this occurs when light passing
  through an optical medium redirects some of the
  radiation into direction other than that of the
  beam. Thus, the original beam is weakened by
  scattering in a direction away from the
  observers eye. As scattering of light increases,
  the body appears more dull and opaque.
• 4) transmission light passing through an optical
  medium without attenuation is said to be
  completely transmitted. Total transmission is
  undergone by perfectly transparent material

57
(No Transcript)
58
Clinical considerations

• 1) Rough composite fillings scatter more light
  thus appear opaque where as smooth highly
  polished filling do not.
• 2) Opacifires added to composite resins act as
  scattering centers that give rise to opaquer
  shades of the material.
• 3) Incorporated air bubbles in a restoration as
  well, act as scattering centers.

59
(No Transcript)