properties of dm

1. GOOD MORNING

2. • Physical properties determine how materials
respond to changes in their environments.
• properties are the basis for the selection of
materials to be used in particular dental
procedures and restoration.
• The relationships among the various properties
are classified as:

4. Mechanical
properties

5. Mechanical properties are defined by the laws
of mechanics (physical science that deals with
energy and forces and their effects on bodies).
Mechanical properties of materials under an
applied force.
1. elastic responses (reversible on forces
removal)
2. plastic responses (irreversible)
 Mechanical properties are expressed most
often
1. stress
2. strain.

6. Stress:
• When an external force is applied to a body or
specimen of material under test, an internal
force, equal in magnitude but opposite in
direction is set up in the body.
• Stress =Force /Area

8. • E.g.:
if a wire of 0.000002 m2 is stretched with 200N
force. Then the stress (σ) will be:
σ = force/area = 200/0.000002
= 100x106 N/m2 = 100Mpa
• The SI unit of stress is the ‘Pascal’.

9.  By means of direction of force application
three types of ‘simple’ stresses
1. Tensile
2. Compressive
3. Shear.
‘Complex’ stress conditions are also produced
i.e. flexural stress.

10. Tensile stress:
• It is caused by a load that tends to stretch or
elongate a body.
• It always accompanied by tensile strain.

15. Compressive stress:
• If a body is placed under a load that tends to
compress or shorten it, the internal resistance to
such a load is called a compressive stress.
• It is associated with a compressive strain.

25. Shear stress:
• Resist the sliding or twisting of one portion of a
body over another.
• Shear stress can also be produced by a twisting or
torsional action on a material.
• For e.g., if a force is applied along the surface of a
tooth enamel by a sharp-edged instrument parallel
to the interface between the enamel and
orthodontic bracket, the bracket may debond by
shear stress failure of the resin luting agent.

29. Flexural (bending) stress:
• Example of flexural stress that is produced in a
three unit bridge or a two unit cantilever bridge.
• It is produced by bending forces in dental
appliances in one of two ways:
1. By subjecting a structure such as a fixed partial
denture to three point loading, where by the end
points are fixed and a force is applied between
these end points.

30. 2. By subjecting a cantilevered structure that is
supported at only one end to a load along any
part of the unsupported section.

31. Test methods for stress, used for
dental material are
1. Three point bending test or transverse test.
2. Diametral compressive tensile test

32. 3 point bending test/ transverse test:
• Here numerical value of stress is given by :
Stress= 3FL/2bd2
F- force
L- distance between the
supports
b- width
d- depth

33. Diametral compressive tensile test:
• Commonly used when conventional tensile
testing is difficult in brittle
materials because of
alignment and gripping
problems.
Cannot used in non-brittle material
due to Increased area of contact
between machine platen and material.

34. • The value of stress is given by:
stress = 2P/π x D x t
Where,
P = load
D = diameter
t = thickness

35. Strain:
• The application of an external force to a body or
test specimen results in a change in dimension
of that body.
• If the wire is 0.1 m long, and if it stretches 0.001
m under the load, the strain (Є) will be:
Є = Δl/lo = 0.001m/0.1m = 0.0001m/m
= 0.0001 = 0.01%

36. • When strain becomes large, the dimension of the
test specimens may change in a direction at 90o
to that of the applied force.
• E.g., a cylinder of material can undergo
barrelling in addition to uni-axial compression,
while in tensile stress specimen may become
thinner in cross section as the extension occurs.

37. • Poisson’s ratio is required, In order to monitor
the way in which the stress changes with these
alterations in specimen shape.
• It will give the ratio between strain occurring at
90o to the direction of the applied force and
strain occurring in the direction of force.

38. • The strain may be recoverable, the material may
remain deformed i.e. non-recoverable or strain
may be partially recoverable or that the recovery
is time-dependent.
• The extent of recovery and/or rate of recovery is
a function of the elastic properties of
material.

39. Stress-Strain relationship:
• Stress and strain are closely related and may seen
as an example of cause and effect.
• A plot of the corresponding values of stress and
strain is referred to as a stress-strain curve. Such a
curve may be obtained in compression, tension, or
shear.
• E.g., a stress-strain curve in tension for a dental
gold alloy is shown.

40. • The shape and magnitude of the stress-strain
curve are important in the selection of dental
materials.
• Fig. clearly shows that the curve is a straight line
up to a stress of about 40,000 lbs/in2, after which
it is concave towards the strain axis. The curve
ends at
a stress of
90,000 lbs/in2 and a
strain of 0.2 because
sample ruptured.

41. Proportional limit:
• It indicate the stress at which the material no
longer functions as an elastic solid.
• The strain recovers below the proportional limit,
if the stress is removed, and permanent
deformation of the material occurs above the
proportional limit.
• E.g., connectors which required high proportional
limit. Cobalt-chromium (Co/Cr) is popular for
this application since it can with stand high
stresses without being permanently distorted.

42. Yield strength:
• Used when the proportional limit cannot be
determined with sufficient accuracy.
• E.g., proportional limit for the gold alloy is
40,000lbs/in2 and the yield strength is
47,000lbs/in2. These values indicate that
stresses in excess of 40,000 to 47,000
lbs/in2in the gold alloy will result in
permanent deformation.

43. • The values of proportional limit and yield
strength values are important in the evaluation
of dental materials, because they represent the
stress at which permanent deformation of the
structure begins.
• If they are exceeded by mastication stresses, the
restoration or appliance may no longer function
as originally designed.

44. Ultimate strength / fracture
stress:
• If the material continues to have more and more
weight applied to it, it will break eventually.
• The stress at breakage is called ultimate
strength.

45. Modulus of elasticity (young’s
modulus or elastic modulus)
• The term describes the rigidity of a material,
which is measured by the slope of the elastic
region of the stress-strain diagram.
• The coefficient found by dividing the unit stress,
at any point up to the proportional limit, by its
corresponding unit of elongation or strain.
• Modulus of elasticity = STRESS / STRAIN

46. • As the elastic modulus rises, the material becomes
more rigid.
• A steep slope, giving a high modulus value,
indicates a rigid material, whilst a shallow slope,
giving a low modulus value, indicates flexible
material.
• In another diagram, stress-strain plot for enamel
and dentin that has been subjected to
compression.

47. • It shows that the
 proportional limit,
 ultimate compressive
strength
 elastic modulus
enamel > dentin.

48. • Elastic modulus=stress/strain
• E.g., If a wire is much more difficult to bend than
another of the same shape and size, considerably
higher stress must be induced before a desired
strain or deformation can be produced in the
stiffer wire.
• Such a material would possess a comparatively
high modulus of elasticity.

49. • Units –
 GN/m2 (giganeutons)
or
GPa (gigapascals)

50. Ductility:
• Def.- the ability of a material to withstand
permanent deformation under a tensile load
without rupture.
• Alloy used to form wires must show a high
degree of ductility since they are extended
considerably during the production process.

51. Measurement of Ductility:
There are 3 methods:-
1. The percent elongation after fracture: It is the
simplest and most commonly used method is to
compare the increase in length of a wire or rod
after fracture in tension to its length before
fracture.The ratio of the increase in length after
fracture to the original gauge length, expressed
in percent, is called percent elongation.

52. 2. Reduction in area in the fractured region
ends: the percentage of decrease in cross
sectional area of the fractured end in
comparison to the original area of the wire
or rod is called the reduction in area.
3. Cold bend test: the material is clamped in a
vise and bent around a mandrel of a
specified radius. The number of bends to
fracture is counted, and the greater the
number, the greater the ductility.

53. Malleability:
• Def.- capability of being extended or shaped
with a hammer or with the pressure of rollers.
• Malleability of stainless steel is utilized when
forming a denture base i.e. adapting the sheet of
stainless steel over a preformed cast.

54. • Gold is the most ductile and malleable pure
metal, and silver is second.
• Of all the metals of interest to the dentist,
platinum ranks third in ductility and copper
ranks third in malleability.

55. Resilience:
• Def.- Capability to withstanding shock without
permanent deformation.
• As the interatomic spacing increases, the internal
energy increases. As long as the stress is not greater
than
the proportional limit,
this energy is known as
‘resilience’.
• The term resilience is
associated with
‘springiness’.

56. • Material with the larger elastic area has
the higher resilience.
• E.g., material used to apply a cushioned
lining to a hard denture base are able to
absorb considerable amount of energy
without being permanently distorted.

57. Toughness:
• Def.- ability of a material to withstand stresses
and strains without breaking.
• The total area under
stress-strain
graph gives an
indication of
toughness.

58. • It depends on strength and ductility.
• The higher the strength and the higher the
ductility, the greatest the toughness.
• Its measurement depends on factors such as the
speed with which stress is increased and the
presence of small imperfections on the specimen
surface from which crack can propagate.

59. Fatigue strength:
• Most prosthetic fractures develop progressively
over many stress cycles after initiation of a crack
from a critical flaw and subsequently by
propagation of the crack until a sudden,
unexpected fracture occurs.
• Stress values well below the ultimate tensile
strength can produce premature fracture of a
dental prosthesis because microscopic flaws grow
slowly over many cycles of stress. This
phenomenon is called fatigue failure.

60. • For glasses and certain glass containing ceramics,
the induced tensile stress and the presence of an
aqueous environment cause an extension of the
microscopic flaws by chemical attack and further
reduce the number of cycles to cause dynamic
fatigue failure.
• Fatigue behavior is determined by subjecting a
material to a cyclic stress of a maximum known
value and determining the number of cycles that
are required to produce failure.

61. • For brittle materials with rough surfaces, the
endurance limit (the maximum stress that can be
maintained without failure over an infinite
number of cycles) is lower than it would be if the
surfaces were more highly polished.
• Some materials or prosthetic appliance exhibit
static fatigue, a phenomenon attributed to the
interaction of a constant tensile stress with
structural flaws over time.

62. • For a given flaw size, less stress is required to
produce failure if the stress is dynamically cycled
between high and low values.
• Furthermore, aqueous solutions are known to
corrosively degrade dental ceramics by
lengthening surface flaws over time in the
presence of tensile stress.
• Ceramic orthodontic brackets activated wires
with in the brackets represent a clinical system
that can exhibit static fatigue failure.

63. • The delayed fracture of molar ceramic crowns
that are subjected to periodic cyclic forces may be
caused by dynamic fatigue failure.
• In either case (static or dynamic), the failure
begins as a flaw that slowly propagates until
catastrophic fracture occurs.

64. Impact strength:
• It is defined as the energy required to fracture a
material under an impact force.
• The term impact is used to describe the reaction
of a stationary object to a collision with a moving
object.

65. • It can be measured by:
1. Charpy type impact tester :-
- Usually used.
- A pendulum is released that swings down to
fracture the center of a specimen that is
supported at both the ends.

66. - The energy lost by the pendulum during the
fracture is can be determined by a comparison
of the length of its swing after the impact with
that of its free swing when no impact occurs.
2. Izod impact tester :-
- Specimen is clamped vertically at one end.
- The blow is delivered at a certain distance above
the clamped end.

67. • A moving object possesses a known amount of
kinetic energy. If the struck object is not
permanently deformed, it stores the energy of
the collision in an elastic manner.

68. Mechanical properties of tooth
structure:

69. Brittleness:
• It is the relative inability of a material to sustain
plastic deformation before fracture of a material
occurs.
• For e.g., amalgams, ceramics and composites
are brittle at oral temperature.
• A brittle material fractures at or near its
proportional limit.

70. • Dental materials with low or zero percent
elongation, including amalgams, composites,
ceramics and non-resin luting agents, will have
little or no burnishability, because they have no
plastic deformation potential

71. Hardness:
• Def.- Resistance offered by the surface of a
material to scratching, abrasion, indentation or
penetrations.
• The value of hardness, often referred to as the
hardness number, depends on the method used
for its evaluation.
• High values of hardness number indicate a hard
material.

72. Methods of measuring surface
hardness:
1. Moh’s scratch test
2. Indentation methods :
Brinell’s,
Rockwell’s
Vicker’s and
Knoop’s
3. Penetration methods
Shore A
Barcol test

73. 1. Moh’s scratch tests:
He compared the hardness of various mineral
ores by scratching one ore by another ore. This
method is inaccurate and could not be used for
many materials.
2. Indentation methods:
Brinnel’s hardness test (BHN)
• A hardened steel or tungsten carbide ball of
diameter D mms is forced
into the surface of a
material under a specified
load.

74. • This indentation process leaves a round dent in
the material, and hardness is determined by
measuring the diameter of the dent.
• It is a simple method.
• This method cannot be used for Brittle materials
like ceramics, gypsum products, elastomers,
hydrocolloids. Getting accurate real values of
hardness of surfaces is difficult as the depth or
area of indentation is quite large.

75. Rockwell’s hardness test
• This test is used primarily for determining the
hardness of steels in United States.
• A hardened steel ball of 12.7 mm diameter or a
conical diamond point is held on the surface.
• Then the major load of 30kgms is applied for 10
min and the depth is measured with a micrometer
dial gauge.

76. • It is also quite simple and RHN is directly
obtained from different scales.
• It can be used for hard, ductile and brittle
materials.
• On the same time, it cannot be used for very hard
base metal alloys, large elastically recovering
materials.
• It does not give accurate real hardness of surface,
as depth of indentation is quite large.

77. Vicker’s hardness test:
• It is devised in 1920’s by engineers at vickers Ltd.
in the United Kingdom.
• The indenter used in this test is a square, pyramid-
shaped diamond of a large angles 136o.
• The hardness is determined by measuring the
diagonals of the square and taking the average of
the two dimensions.

78. • This test is also called the diamond pyramid
hardness test.
• This test is used to measure the hardness of dental
alloys, and expressed in vickers hardness numbers
• To compare the VHN and BHN, use the following
relationship:
VHN= 1.05 X BHN

80. Knoop hardness
• The indenter used in this test is also made of a
diamond, but its outline is somewhat different
from the Vickers indenter, that is one diagonal is
much longer than the other.
• Only the longer diagonal is measured to
determine the Knoop hardness number.
• It is used to measure the hardness of both
exceedingly hard and soft materials

81. e.g.,
• VHN, KHN tests are microhardness
• Brinell and Rockwell macrohardness tests

82. Shore A
• This test is used to measure the hardness of
rubbers and soft plastics.
• The shore A scale is between 0 to 100 units, with
complete penetration of the of the material by
the indenter yielding a value of 0, and no
penetration yielding a value of 100.

83. Viscoelastic behavior:
• When a material undergoes full elastic recovery
immediately after removal of an applied load it is
elastic, while if the recovery takes place slowly, or
if a degree of permanent deformation remains, the
material is said to be viscoelastic.

84. • In the fig. a model explain this
behavior. It is called spring
and dashpot (or voigt) model.
• As the model is stretched, the
spring component stretches
and the piston of the dashpot also moves through
the viscous liquid.

85. • The strain under stress and after release of the
stress is time dependent.
• The strain gradually builds up until the release
of stress, and then it gradually goes back to zero
as the spring element returns to its original
length but is dampened by the dashpot element.

86. • Elastic impression materials are viscoelastic.
Initially they are strained upon removal from the
mouth and require a short period of time to
recover before models or dies are poured.
• Creep and stress relaxation are the two
phenomena which can be explained using the
viscoelastic models.

87. Creep:
• Def.- it is the slow change in dimensions of an
object due to prolonged exposure to high
temperature or stress.
• It is distinguished from the flow by the extent of
deformation and the rate at which it occurs.
Whereas flow implies a greater deformation
produced more rapidly with a smaller applied
stress.
• In metal, creep usually occurs at very high
temperature, near to metal melting point.

88. • Metal used for cast restorations or substrates for
porcelain veneers are not susceptible to creep
deformation due to melting points.
• The most important exception is dental
amalgam, which has components with melting
points only slightly above the room temperature.
• Because of its low melting range, dental
amalgam can slowly creep from a restored tooth
site under periodic sustained stress such as,
would be imposed by patients who clench their
teeth.

89. • Flow is applied to amorphous materials.
• E.g.,
Silly putty- it fractures at a rapid stretching rate,
however if it is placed as a sphere on the table, it
will flatten out under its own weight over time.
waxes- the flow is a measure of its potential to
deform under a small static load, even that
associated with its own mass.

90. Color and color perception:
• Light is an
electromagnetic
radiation that can be
detected by the human
eye.
• The eye is sensitive to
wavelength from
approx. 400nm (violet)
to 700nm (dark red).

91. • The reflected light intensity and the combined
intensity of the wavelengths present in incident
light and reflected light determines the
appearance properties.
• For an object to be visible, it must reflect or
transmit light incident on it from an external
source.

92. • The incident light is usually polychromatic.
• Incident light is selectively absorbed or scattered
at certain wavelength.
• The spectral distribution of the transmitted or
reflected light resembles that of incident light,
although certain wavelengths are reduced in
magnitude.

93. Phenomenon of vision:
• Light from an object that is incident on the eye is
focused in the retina and is converted into nerve
impulses that are transmitted to the brain.
• Cone-shaped cells in the retina are responsible
for color vision.

94. • These, cone-shaped cells have a threshold
intensity required for color vision.
• These cells also exhibit a response curve related
to the wavelength of the incident light.

95. This is a curves for
individual with
normal color vision
and for individual
With color deficient
vision.
It indicates that the eye is most sensitive to
light in the green- yellow region (at 550nm)
and least sensitive at the red or blue regions of
color spectrum.

96. • Because a neural response is involved in color
vision, constant stimulation by single color may
result in color fatigue and a decrease in the eye’s
response.
• In a scientific sense, human eye also denoted as
an exceptionally sensitive differential
calorimeter.
• Although the calorimeter is more precise than
the human eye in measuring slight differences in
colored object.

97. Three dimensions of color:
• Verbal descriptions of color are not precise
enough to describe the appearance of teeth.
• Three variables must be measured to accurately
describe our perception of light reflected from a
tooth or restoration surface: hue, value and
chroma.

98. Munsell color system:
• The color
considered is
compared with a
large set of color
tabs.
Munsell scale

99. Value:
• It is the lightness or darkness of a color.
• E.g. yellow of a lemon is lighter than the red of a
sweet cherry.
• This lightness can be measured independent of
the color hue.

100. • Value is determined first by the selection of a
color tab that most nearly corresponds with the
lightness or darkness of the color.
• Value ranges from white (10/) to black (0/).

101. Chroma:
• It is a measurement of color intensity, that is the
amount of hue saturation in a color.
• E.g. a beaker of water containing one drop of
colorant; it is lower in chroma than a beaker of
water containing ten drops of the same colorant.

102. • Chroma is determined next with tabs that are
close to the measured value but are of increasing
saturation of color.
• Chroma ranges from gray (/0) to a highly
saturated color (/18).

103. Hue:
• It is determined last by matching with color tabs
of the value and chroma.
• Hue is measured on a scale from 2.5 to 10 in
increments of 2.5 for each of the 10 color
families (red, yellow-red, yellow, green-yellow,
green, blue-green, blue, purple-blue, purple, red-
purple)

104. • E.g. the color of the attached gingiva of a healthy
patient has been measured as
5R 6/4, where,
hue – 5R
value – 6
Chroma – 4

105. Color measurement:
Spectrophotometers:
• It measure the amount of
light reflected at each
wavelength.
• It has been used to
evaluate the color
parameters for restorative
resins, denture teeth,
porcelains, etc.

106. Colorimeters:
• It measure the amount of light reflected at
selected color.
• But it can be extremely inaccurate when used on
rough or curved surfaces.

107. • Major problem in using such instruments to
measure the color parameters of teeth is the
‘edge effect’.
• Enamel is a translucent material that scatters
the incident light from the instrument.
• As a result, the light reflected back into the
instrument for analysis is not reliable and
presents major errors in shade selection.

108. • To over come that photcolorimetry method is
used.
• A new approach tooth shade selection is the use
of photography in combination with a
spectrophotometer.
It involves following steps:-
• 3 or 4 closest matching shade guide teeth is held
next to the patient’s teeth and photographs are
taken.

109. • These photographs
are sent to the
laboratory along with
the impression.
• The technician
measures the color of
the patient’s teeth
and shade guide teeth
in the photograph
and closest match is
selected with the help
of computer program.

110. Metamerism:
• The change in
color matching of
two objects under
different light
sources is called
metamerism.

111. • E.g. when shade guide tooth matches the tooth
under fluorescent light but not under
incandescent light.

112. Stress relaxation:
• After a substance has been permanently deformed
(plastic deformation), there are trapped internal
stresses.
• For eg. In a crystalline substance, the atoms in the
space lattice are displaced and the system is not in
equilibrium. Similarly, in amorphous structures,
like waxes, some molecules are too close together
and

113. • others too far apart after the substance has been
permanently deformed.
• Such a situation is unstable.
• Through thermal energy, they slowly move back
to their equilibrium position.
• The result will be rearrangement in atomic or
molecular position.

114. • As a result material will warps or distorts.
• Such a relief of stress is known as ‘relaxation’.
• The rate of relaxation increases with an increase in
temperature.
• For eg. If a wire is bent, it may tend to straighten
out if it is heated to a high temperature.
• Many non-crystalline dental materials, such as
waxes, resins etc. that when manipulated and
cooled, can then undergo relaxation at any elevated
temperature.

115. • Any dimensional changes by relaxation may
result in an inaccurate fit of the dental appliance.

116. Viscosity:
• The study of matter flow characteristics is the
basis for the science of Rheology.
• A liquid at rest cannot support a shear stress,
most liquids, when placed in motion, resist
imposed forces that cause them to move.
• This resistance to motion is called viscosity

117. • Viscosity is controlled by the internal frictional
forces with in the liquid.
• Dental materials have different viscosities when
prepared for their intended clinical application
for e.g. when we compare the flow properties of
GIC, which is more viscous than Zinc phosphate
cement when mixed as luting agent.

118. • This concept can be explained
quantitatively as:
Two metal plates and
Liquid between. Lower
plate is fixed and upper
plate is moving.

119. • Upper plate is moving with certain velocity
(V). A force (F) is required for the
movement, which produced by the friction
(viscosity) of the liquid. Stress develop
within a structure when an external force is
applied, Which causes strain to develop.
If the plate have area (A),
Shear stress = F/A and
Shear strain rate or rate of change of
deformation
= V/d,
where,
V = velocity of liquid
d = distance between 2 plates.

120. • So,
viscosity = shear stress / shear strain
• Further characterization of the rheological
properties of materials is obtained by reference
to the equation:
shear stress = K (strain rate)n

121. • Where K and n is constant and n is referred to as
the flow index.
Newtonian - In case where n=1, the shear
stress that is proportional to the strain rate, and
viscosity of the material is constant.

122. Pseudoplastic –
• when the flow index value is less than unity, an
increase in strain rate produces a less than
proportionate increase in shear stress. Thus the
viscosity decrease with increasing strain rate.
• For e.g., elastomeric impression materials when
loaded into a tray, in mouth shows a higher
viscosity, whereas the same material when
extruded under pressure through a syringe tip
shows more fluidity.

123. Dilatant –
• when the flow index value is greater than unity an
increase in strain rate produces a more
proportionate increase in shear stress, thus
viscosity increases.
• These liquids shows higher viscosity as shear rate
increases for e.g., fluid denture base resins.

124. Plastic –
• These materials behave like a rigid body until
some minimum value of shear stress is reached.
For e.g., ketchup – a sharp blow to the bottle is
usually required to produce an initial flow.

126. Thermal
properties

127. • Wide temperature fluctuations occur in the oral
cavity due to ingestion of hot or cold food and
drink.
• Dental pulp is very sensitive to temperature
change and in the healthy tooth is surrounded by
dentin and enamel, which are relatively good
thermal insulators.

128. • Materials used to restore teeth should not only
offer a similar degree of insulation but also
should not under go a large temperature rise
when setting in situ.
• Other consequence of thermal change is
dimensional change.
• Materials generally expand when heated and
contract when cooled.

129. Thermal conductivity:
• Def.- it is a measure of the speed at which heat
travels through a given thickness of material,
when one side of the material is maintained at a
constant temperature that is 1o C higher than the
other side.
• Units – cal/cm.sec.oC
• It is also define as rate of heat flow per unit
temperature gradient.

130. • Thus, good conductors have high values of
conductivity.
• More heat is conducted through metals and alloys
than through polymer such as acrylic resin.
• High value of conductivity for dental amalgam
indicates that this material could not provide
satisfactory insulation of the pulp.

131. • Therefore, cavity base of a cement is used such
as zinc phosphate which has a lower thermal
conductivity value.

132. Thermal diffusivity:
• The thermal diffusivity of a material controls the
time rate of temperature change as heat passes
through a material.
• It is a measure of the rate at which a body with a
non-uniform temperature reaches a state of
thermal equilibrium.

133. • Thermal diffusivity (D)
D = k/cpρ
Where,
K = thermal conductivity
Cp = temperature dependent specific heat
capacity= specific heat (heat required
to raise any substance through 10 of
temperature)
ρ = density
Units = m2/sec or
cm2/sec

134. • Measurement of thermal diffusivity are often
made by embedding a thermocouple in a
specimen of material and plugging the specimen
into a hot or cold liquid.
• If the temperature recorded by the thermocouple
rapidly reaches that of the liquid, this indicates
high value.

135. • A slow response, indicates a lower value.
• E.g., In the upper denture, base covers most of the
hard palate. Its low thermal conductivity tends to
prevent heat exchange between the supporting soft
tissues and the oral cavity itself.
• Thus, the patient partially losses the sensation of
hot and cold while eating and drinking. The use of
a metal denture base

136. may be more comfortable and pleasant from this
standpoint.

137. Thermal expansion:
• It is defined as the change in length per unit of
the original length of a material when its
temperature is raised 1oC.
• The unit is expressed as μm/cmoC.
• Inlay waxes have high expansion coefficient
because it is highly susceptible to temperature
changes.

138. • e.g., an accurate wax pattern that fits a prepared
tooth contracts significantly when it is removed
from the tooth or from a die in a hot area and
then stored in a cooler area. This dimensional
change is transferred to a cast restoration which
is made from that wax pattern.

139. • Although these thermal stresses cannot be
eliminated completely, they can be reduced
appreciably by selection of materials whose
expansion or contraction coefficients are
matched closely within 4%.

140. References:-
• Anusavice K.J. Phillip’s science of dental
materials - tenth edition
• Craig R.G. Dental materials – sixth edition.
• McCane J.F. Applied dental materials – eighth
edition.
• Glossary of prosthodontics- 8th edition