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Materials > Chapter 2: Physical Properties > Flashcards

Chapter 2: Physical Properties Flashcards

1
Q

list the physical properties

A

(not completed ans)
- Mass related Properties
- Thermal Properties
- Rheological Properties
- Optical Properties
- Electrical properties
- Other properties

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2
Q

list all the thermal properties

A
  1. Thermal conductivity.
  2. Specific heat.
  3. Thermal diffusivity.
  4. Coefficient of thermal expansion + contraction.
  5. Melting and Freezing point.
  6. Heat of Fusion.
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3
Q

Mass Related Properties: Density
(def and unit)

A

➢ It is the mass per unit volume
➢ Unit: gm/cc OR gm/cm3

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4
Q

what is the importance of density in dentistry??

A
  1. Retention of upper denture:
    Denture w/ lighter weight will help in retention of denture.
    i. Nonmetallic denture base is lighter than metallic one.
    ii. Base metal alloys are lighter than the gold alloys.
  2. During Casting:
    Lighter alloys require more casting force to allow rapid filling of the mold
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5
Q

Thermal Properties:
define thermal conductivity:

A
  • it is the quantity of heat in calories per second
  • passing through a body of 1 cm thick w/ a cross section of 1 cm2
  • when temperature difference is 1*C.
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6
Q

what is the unit of thermal conductivity?

A

Cal/Sec/cm2 ( degrees C/cm).

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7
Q

what is the importance of thermal conductivity in dentistry?

A
  1. Metallic denture base is preferred than non-metallic denture base, as they provide physiological stimulation (VC and VD) to oral tissue to maintain them in good health
    (thermal conductivity is an advantage).

2.Metallic restoration in deep cavities should be preceded by a protective base to protect the dental pulp form thermal shock
(thermal conductivity is a disadvantage).

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8
Q

define specific heat….

A

It is the quantity of heat needed to raise the temperature of a 1 gm of the material 1degrees C.

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9
Q

what is the importance of specific heat in dentistry?

A

Prolonged heating of gold during casting is unnecessary because gold has low specific heat

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10
Q

Thermal Diffusivity Definition??

A

it is the rate at which a body w/ non-uniform temp. approaches thermal equilibrium.

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11
Q

Thermal Diffusivity unit and equation:

A

➢ Unit: mm2/Sec.

➢ Equation:
Thermal conductivity / Specific heat X Density

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12
Q

what is the importance of thermal diffusivity in dentistry??

A
  1. Low specific heat combined w/ high thermal conductivity of dental amalgam ➔create thermal shock
  2. The thickness of remaining dentine is very important to prevent thermal pulp
    shock ➔preserve the sound tooth structure.
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13
Q

Coefficient of Thermal Expansion and Contraction Definition:

A
  • it is the change in length per unit length
  • of the material for 1 degrees C change in temp.

It is cld linear thermal coefficient of expansion and contraction (α).

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14
Q

equation of linear thermal coefficient of expansion and contraction (α).

A

α = Lf - Lo / Lo (C^o f – C^o o)

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15
Q

what is the importance of Coefficient of Thermal Expansion and Contraction in dentistry?

A
  1. The high difference in α between tooth + restoration leads to marginal leakage at tooth-restoration interface, This leads to:
    -Hypersensitivity
    -Recurrent carries
    -Marginal discoloration
    * then Marginal percolation (pumping in and out of food and saliva between tooth and restoration due to high α difference).
  2. In porcelain fused to metal crown, slight difference in α b2ian metal and porcelain is important to allow strong compressive bond.
  3. The high difference in α between artificial teeth and acrylic resin denture base should be avoided to prevent crazing of the denture base.
  4. The high α of wax used for wax pattern construction may lead to its
    distortion after cooling of the molten wax.
  5. Thermal expansion of investment material is important to compensate the thermal shrinkage of molten metal.
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16
Q

Melting and Freezing Temperature Definition:

A

It is the temperature at which a material melts or freezes.

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17
Q

Importance of Melting and Freezing Temperature in Dentistry:

A
  1. Determine the melting machine for melting dental casting alloys.
  2. Determine the type of investment material.
  3. Avoid over heating of materials to avoid evaporation of specific ingredients from it.
  4. Waxes used in molten state inside the oral cavity should have low softening point to avoid burning of living tissues.
  5. The solder should melt at temperature lower than soldered metals by 50-100 degrees C to avoid distortion of soldered structures.
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18
Q

Heat of Fusion Definition:

A
  • it is the amount of heat in calories
  • required to convert 1 gm of a material
  • from solid state to the liquid state
  • at melting temperature.
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19
Q

When the liquid is cooled, it will liberate the same amount of heat and cld Latent heat of fusion

okay define it ba’aa:

A
  • It is the amount of heat liberated
  • during converting 1 gm of the material
  • from liquid to solid state at freezing temperature
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20
Q

what is the importance of heat of fusion in dentistry?

A
  • Heating the molten alloys 100 degrees C
  • above its meting temperature
  • to give time of molten alloy to completely fill mold
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21
Q

Rheological Properties def:

A

It is the study of the flow of the material.

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22
Q

Fluidity def?

A

it is the tendency of the fluids to flow

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23
Q

Viscosity def?

A

it is the resistance of fluids to flow

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24
Q

Viscoelasticity def?

A

it the deformation of solids

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25
Q

Rheological properties of matter can be classified to:

A
  1. Newtonian
  2. Dilatant
  3. Pseudo-plastic
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26
Q

Newtonian def

A

The viscosity is constant with application of Shear
stresses.

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27
Q

Dilatant def:

A

The Viscosity increased with increasing shear
stresses.

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28
Q

Pseudo-plastic def:

A

The viscosity decreased with increasing shear stresses.

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29
Q

what is the importance of rheological properties in dentistry?

A

Many impression materials and cements are subjected to shear stresses during their extrusion just before clinical use to increase their flow.

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30
Q

GR: Plastic (acrylic) upper dentures are preferred than metallic dentures.

A

Due to the lower density of the plastic helps in the retention of the denture

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31
Q

GR: Metallic dentures are preferred than plastic dentures.

A

BC metals conduct heat to underlying tissues providing physiologic stimulation (VC and VD) so keep them in a good health.

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32
Q

GR: Casting of gold alloys is easier than casting of base metal alloys.

A

BC the higher density of gold alloys allows rapid filling of the mold.

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33
Q

GR: Large metallic restorations should be preceded by an insulating base.

A

To prevent heat conductivity by metallic restoration to the dental pulp so protect it from thermal shock

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34
Q

GR: Prolonged heat of gold alloys during casting is unnecessary.

A

BC gold has low specific heat so it requires little amount of heat required to raise its temperature

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35
Q

GR: Dental amalgam can cause thermal shock to the dental pulp

A

BC it has high thermal diffusivity (low specific heat and high thermal conductivity)

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36
Q

GR: Preservation of sound tooth structure is very important during cavity preparation.

A

BC dentine is the best thermal insulator.

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37
Q

GR: The coefficient of thermal expansion and contraction of the tooth and restoration should be matched.

A

To prevent marginal leakage b2ian tooth and restoration during changes in the temperature.

38
Q

GR: n porcelain fused to metal crowns, the coefficient of thermal expansion and contraction of metal should be slightly lower than the porcelain.

A

To provide strong compressive bond between metal and porcelain.

39
Q

GR: In removable dentures, the artificial teeth and denture base should have the same coefficient of thermal expansion and contraction.

A

To avoid crazing of denture base due to temperature changes.

40
Q

GR: Overheating of the materials should be avoided.

A

To avoid evaporation of its ingredients to preserve its properties.

41
Q

GR: Waxes used inside in molten stages the oral cavity should have lower melting temperature.

A

To avoid burning of vital tissues

42
Q

GR: The solder should have lower melting temperature than soldered structures.

A

To avoid distortion of the soldered structures

43
Q

GR: During solidification of metals, the temperature of the molten metal remains constant while it transforms from liquid to solid.

A

Due to liberation of latent heat of fusion

44
Q

GR: During casting, the molten alloys should be heated 100 °C over its melting temperature

A

To give time of the molten alloy to completely fill the mold while it is still molten.

45
Q

GR: Many dental impression materials and cements should be subjected to shear stresses during their mixing.

A

To increase their fluidity as they are pseudo-plastic materials ( INCREASE shear stress INCREASES fluidity)

46
Q

list the optical properties:

A

(not sure about this one)
1. Light
2. Color

47
Q

what is the def of light?

A
  • it is an electromagnetic radiation
  • that can be detected by human eye.
48
Q

When incident light travelling form one medium to another, one of the following results:

A
  1. Reflection:
  2. Refraction:
  3. Scattering:
  4. Transmission:
49
Q

Reflection:

A

➢ If light falls on smooth surface:
- it reflects regularly
- angle of incidence = angle of reflection this is cld specular reflection
- hence the surface appears shiny.

➢ If light falls on rough surface:
- it reflects in all directions
- is cld diffuse reflection
- hence the surface appears dull

50
Q

What is the Importance of reflection in Dentistry?

A

The surface of aesthetic restoration should be smooth
- to produce specular reflection
- so producing shiny surface

51
Q

def of refraction??

A

It is the change of light direction on entering
second medium.

52
Q

what is the reason behind refraction??

A

It results from difference in refractive indices of 2
media.

53
Q

what is the importance of refraction in dentistry??

A

The refractive indices of aesthetic restorative material (composite resin, porcelain) should be matched with tooth structure.

54
Q

why would a material will appear opaquer??

A
  • If a light beam (passing through a medium)
  • found a scattering center (opacifiers or air bubbles)
  • it will emerge in all directions and it will weaken.
55
Q

what is the importance of scattering in dentistry?

A

1.Opacifiers are added to esthetic restorative materials to obtain different shades. (advantage)

  1. Incorporation of air bubbles during mixing of
    restoration will give more opaque restoration. (disadvantage)
56
Q

What are the Properties of materials in Relation to Light Transmission:

A
  1. Transparency:
  2. Translucency:
  3. Opacity:
57
Q

what happens in Transparency??

A

➢ Most of light passed through the material
➢ Object can be seen clearly through it.
➢ e.g: glass & acrylic resin.

58
Q

what is an eg of materials with transparency?

A

glass & acrylic resin.

59
Q

what happens in Translucency??

A

➢ Some of light passed and the rest scattered or reflected
➢ Object can’t be seen clearly.

60
Q

what is an eg of materials with translucency??

A
  • tooth enamel
  • porcelain
  • composite resin
  • pigmented acrylic resin
61
Q

what happens in opacity??

A

➢ All of light is absorbed, the material prevents light passage through it.
➢ The objects can’t be seen through it.

62
Q

what is an eg of materials w/ opacity???

A
  • Metals
  • alloys
  • gypsum products.
63
Q

what is the meaning of Fluorescence?

A

the human teeth absorb light
and
emit it with longer wavelength.

64
Q

what is the importance of fluorescence in dentistry??

A
  1. Fluorescence gives the tooth brightness + vital appearance.
  2. Tooth structure emits fluorescent light when exited by ultraviolet radiation.
    So, aesthetics restorative materials should simulate natural tooth structure.
  3. Fluorescence helps in diagnosis of initial caries.
65
Q

what happens in Opalescence:

A

It is a light scattering effects that is occurs @ tooth surface especially @ the incisal edges.

66
Q

what is the importance of opalescence in dentistry??

A

This effect creates bluish-white color
when the tooth is seen at different angles.

67
Q

def of color??

A

It is a physiological response to a physical stimulus.

68
Q

Munsell color system described the color by three parameters:

A
  • hue
  • chroma
  • value
69
Q

what about hue??

A

➢ It is the dominant wave length.
➢ It represents the color of material
(red, yellow, blue, …)

70
Q

what about chroma??

A

➢ It represents the strength of color or degree of saturation or measurement
of color intensity.
➢ EG: Glass of water contains 10 ink drops have more chroma than a glass contains
just 1 drop.

71
Q

what about value??

A

➢ It represents the lightness or darkness of color.
➢ The lightest materials have a value = 10, while the darkest materials have value = 0
➢ It is most important color parameter as it represents vitality of tooth
(non-vital tooth has low value and appears gray).

72
Q

What r the Factors Affecting Color Appearance and Selection??

A
  1. Light Source:
  2. Surroundings:
  3. Object:
  4. Observer:
73
Q

what about light source??

A

➢ Different light sources have different color content

74
Q

define metamerism??

A

The change of color matching of two objects under diff light sources.

75
Q

what is the clinical importance of the list source in color apperance and selection??

A

Selection of tooth color should be done under day light, as it contains almost all visible wavelengths.

76
Q

what is the effect of object translucency??

A

➢ It controls the lightness or darkness of color.
➢ High translucency gives a lighter color appearance (higher value)
i.e. more vital tooth appearance

77
Q

what happens to the object when scattering centers r present?

A

Increase opacity and lower the value (darker).

78
Q

what is the effect of thickness of the object on its appearance??

A

increase opacity, and lower the value.

79
Q

list the factors of the observer??

A

a. Color Response:
b. Color Blindness:
c. Color Fatigue:

80
Q

TRUE OR FALSE??
Constant stimulation of one color decreases the response of the eye to that color.

A

TRUE

81
Q

Def of water sorption:

A

it is the amount of water:
- adsorbed on the surface
- absorbed into the body
of the material.

82
Q

what Is the clinical importance of water sorption:

A
  1. Water sorption of acrylic resin denture base will compensate cooling shrinkage.
  2. Hydrocolloids impression materials should not be immersed into water to avoid dimensional changes.
83
Q

who is zat shelf life??

A

It is a term applied to the general deterioration and change in quality of materials during shipment and storage.

84
Q

GR: The surface of aesthetic restorations should be smooth and polished.

A

To produce specular reflection so gives shiny and bright appearance.

85
Q

GR: The refractive index of the aesthetic restorative material should be matched with the tooth structure.

A

To simulate the natural tooth appearance:
- by avoiding different refraction patterns of tooth and restoration.

86
Q

GR: Opacifiers are added to aesthetic restorative materials

A

To obtain different shades of restoration.

87
Q

GR: Incorporation of air voids inside esthetic restorations should be avoided

A

BC voids will give more opaque appearance to the restoration.

88
Q

GR: Fluorescence agent should be added to aesthetic restorations.

A

To simulate the fluorescence nature of the natural tooth.

89
Q

GR: Value is the most important color parameter in dentistry

A

Because it represents vitality of the tooth

90
Q

GR: Shade selection of the tooth should be done under day light.

A

To avoid metamerism as day light contains almost all visible wavelengths

91
Q

GR: Time of shade selection should not exceed 5 seconds.

A

To avoid eye fatigue which decrease the response of the eye to color.

92
Q
A

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