Mechanical, physical and aesthetic properties of dental

156
MECHANICAL, PHYSICAL AND
AESTHETIC PROPERTIES OF DENTAL
MATERIALS
1
Presented by,
Dr. Chaithra Prabhu B
1st year post graduate student
Department Of Prosthodontics

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CONTENTS
• Introduction
• Definition
• Force
• Stress
• Strain
• Stress strain relation
• Strength properties
• Proportional limit
• Elastic limit
• Yield strength
• Ultimate tensile, compressive,
shear, flexural strengths
• Fatigue
• Impact strength
• Elastic properties
• Young’s modulus
• Dynamic young’s
modulus
• Flexibility
• Resilience
• Other properties
• Toughness
• Fracture toughness
• Brittleness
• Ductility & malleability
• Surface mechanical
properties
• Hardness
• Friction
• Wear

156 3
CONTENTS
• Optical properties
• Hue
• Chroma
• Value
• Color measurement
• Pigmentation
• Metamerism
• Fluorescence
• Opacity, transparency,
translucency
• Rheological properties
• Viscosity
• Creep
• Flow
• Wetting & contact
angle
• Thermal properties
• Temperature
• Thermal
conductivity
• Thermal diffusivity
• Coefficient of
thermal expansion

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INTRODUCTION
pH of
saliva
Salivary flow
Mechanical
loading
4
By applying concepts
basic fundamentals of
material science
Better restorative materials
can be developed
Hostile
environment

156
 Therefore understanding properties of these restorative
materials becomes important for better selection and design
 It is also important to remember that the success of the
restoration depends not only on its properties but also the
biophysical and physiological properties of the supporting
tissue as well.
5

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 Mechanical properties
Defined by the law of mechanics- that is the physical
science dealing with forces that act on bodies and the
resultant motion, deformation or stresses that those bodies
experience -Phillips
6

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 One body interacting with another generates force.
 The result is translation or deformation of the body
depending on whether the body is constrained or rigid/
deformable.
 It is defined by 3 characteristics:-
1) Point of application.
2) Magnitude.
3) Direction of application.
8
S.I. unit of force – newton.
One pound force = 4.4N
Craig's Restorative dental materials 13th ed

156 9
Patient with RPD generate occlsal forces:65 to 235N.
Patient with complete dentures :
forces on molars and bicuspids:100N.
Forces on incisors : 40N.
Craig's restorative dental materials 13th ed

156
 When an external force is applied to a body, an internal
force, equal in magnitude and opposite in direction to the
applied force is set up in the body.
 This internal resistance to the external force is called
“STRESS”
 Denoted by “S” or “σ”
10Craig's restorative dental materials 13th ed
force

156
 Stress cannot be directly measured ( ratio of F/A )
 Commonly stress is expressed in terms of Pascal. Where,
Pascal = 1 N / m².
 DENTAL CONSIDERATION
 Stress----directly proportional to force and inversely to
the area ( e.g premature contacts----small area----heavy
damaging force)

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 Forces can be applied in from any angle or direction and
several forces combine to develop a complex forces.
 Axial ( tensile and compressive )
 Shear
 Bending or flexural
 Torsional

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Tensile and compressive stresses along with shear, are the
three simple examples of stress which form the basis of all
other more complex stress patterns.
FLEXURAL STRESS:
 Also called as bending stress.
 Produced by bending forces over the dental appliance.

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 Each type of stress is capable of producing corresponding
deformation in a body.
 The numerical value of strain is given by the expression
Strain ℇ = change in length / original length
 Strain is dimensionless quantity because a unit length is
divided by unit length.
Expressed in percentage

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Strain is an important consideration in dental restorative
materials.
 Orthodontic wires.
 Implant screws.
 Impression materials.

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 Each type of stress is capable of producing a corresponding
deformation in the body.
 Therefore, stress and strain are not independent and
unrelated properties, but they are closely related and may be
seen as cause and effect.
 The relationship of stress and strain is often used to
characterize the mechanical properties of materials.
18JohnF McCabe applied dental materials 9th ed

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 Stress- strain data are generally obtained using a mechanical
testing machine, which enables strain to be measured as a
function of stress and recorded automatically in the form of
a graph called stress strain curve.

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 Strength = degree of stress that is required to cause either
fracture or plastic deformation.
 Strength properties of a material can be described by
1. Proportional limit
2. Elastic limit
3. Yield strength
4. Ultimate Tensile Strength, Compressive strength, Shear
strength, Flexural strength,
20Phillip's Science of dental materials 12th ed

156
Defined as magnitude of elastic stress above which plastic
deformation occurs. Phillip’s
That unit of stresses beyond which deformation is no
longer proportional to the applied load. GPT9
Highest stress at which the stress-strain curve is a straight
line that is, stress is linearly proportional to the strain.
Craig
22

156
Defined as the maximum stress that a material can
withstand without permanent deformation.
For linearly elastic materials (stress directly proportional
to strain) the elastic limit and proportional limit
represent the same stress within the structure and the terms
are often used interchangeably in referring to the stress
involved.
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 Super elastic materials.
 These materials exhibit nonlinear elastic behavior, and their
relationship between stress and strain in the elastic region
does not follow a straight line, but removal of the load
results in a return to zero strain.
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 Also called yield stress or yield point or proof stress
 That property which describes the stress at which material
begins to function in a plastic manner.
 Defined as the stress at which material deforms
plastically and there is defined amount of plastic
strain.
25Craig's Restorative Dental materials 13th ed

156 26
• Select the desired
offset ( e.g 0.2% )
• Draw a parallel line
• Intersecting point is
the yield strength
Craig's Restorative Dental materials 13th ed

156
 Yield strength is slightly higher than proportional limit
(includes slight permanent deformation )
 Elastic limit and yield strength are important properties
because they define transition from elastic to plastic
behavior.
 Important in evaluation of dental materials.
 If the values exceeded by the mastication stress, the
restoration or the appliance may no longer function as
originally designed.

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 In cases of orthodontic wires and clasps of removable
partial dentures to bend the wire or clasps permanently
the forces or stress should be greater than the yield
strength
 In case of clasp the retention is achieved by the small
scale elastic deformation
 Also this elastic deformation explains the function of
elastic bands, o- rings, clasps, implant screws

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 It can be defined as the maximum stress a material can
withstand before failure in compression, tension , shear
or flexure loading.
29Shama Bhat Science of dental materials 2nd ed

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 An alloy that has been stressed to near ultimate strength
will be permanently deformed, so a restoration receiving
that amount of stress during function would be useless.
 The yield strength is often of greater importance than
ultimate tensile strength because it is a estimate of
when a material will start to deform permanently.

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 A safety margin should be incorporated into the design of a
restoration and choice of material to ensure that the Ultimate
strength is not approached in normal function
31Craig's Restorative dental material 13th ed

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 The stress at which a material fractures
 After the maximum tensile force is applied, the specimen begin to
elongate excessively, resulting in necking or a reduction of cross-
sectional area.
32
Fracture strength

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 In engineering stress strain graph, stress is calculated from force
and original cross sectional area and the actual reduction in cross
sectional area is not accounted.
 Accordingly, the stress at the end of the curve is less than at
some intermediate point on the curve.
 Therefore, in materials that exhibit necking, the ultimate
and fracture strengths are different.
 For brittle material like ceramic, they are same points.

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Tensile strength
 For ductile materials, the ultimate tensile strength, can be
measured directly using tensiometer. A dumb bell shaped
cylindrical specimen, is clamped rigidly at the ends and pulled
apart to fracture. UTS is the maximum force per unit area
required to fracture
 Brittle materials may fracture at clamping points due to stress
concentrations.

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Compressive
force ----
vertical axis
Tensile force
Perpendicular
to the vertical
plane
This test is used only
for materials that
exhibit elastic
deformation primarily
and little or no plastic
deformation
Phillip's Science of dental materials 12th ed

156
 Also called transverse strength and modulus of rupture
 For brittle materials such as ceramics, flexural tests are
preferred to the diametral compressive test because they
more closely simulate the stress distributions in dental
prostheses such as cantilevered bridges and multiple unit
bridges as well as the clasp arm of RDP’s

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 Many materials which are used as restoratives or dental
prostheses are subjected to intermittent stresses over a long
period of time – possibly many years.
 Although the stresses encountered may be far too small to
cause fracture of a material when measured in a direct
tensile, compressive or transverse test it is possible that,
over a period of time, failure may occur by a fatigue
process.
37John F McCabe 9th ed

156 38
Fatigue
process
Propagation of the
cracks
Fracture
Micro-crack
formations
because of stress
concentration at
a surface fault or
because of
design of
restoration or
prosthesis
How the
fatigue process
occurs ?
Fatigue strength is the stress level at which a material fails
under repeated loading
John F McCabe 9th ed

156 39
Fatigue property
Fatigue life Fatigue limit
To apply a cyclic
stress at a given
magnitude and
frequency and to
observe the number
of cycles required
for failure.
Select a given number of
stress cycles, say 10 000,
and determine the value
of the cyclic stress which
is required to cause
fracture within this
number of cycles.
John F McCabe 9th ed

156
 Stress applications during mastication may approach
3,00,000 flexures per year, whereas the greater stress
generated by removing and inserting clasp retained RPD
from the mouth amounts to less than 1500 per year.
 Restorations should be designed so the clinical cyclic
stresses are below the fatigue limit.

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 This property may be 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
 Devices used to measure impact strength
1. Charpy- test impact tester
2. Izod impact tester

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 Denture base materials should have high impact strengths to
protect from fractures by dynamic masticating forces
accidental falls or trauma .The PMMA denture base resin has
low impact strengths of about 0.25 Joules, which is its main
drawback. When it is modified by butadiene rubber, the
impact strength becomes more than double, about 0.6 Joules.

156 43
Conclusion
Among the different denture base materials used in the study Glass
fibre reinforced proved to have better transverse and impact strength

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 Mechanical properties and parameters that are measure of
the elastic strain or plastic strain behavior of dental materials
 These include
1. Elastic modulus
2. Dynamic young’s modulus
3. Flexibility
4. Resiliency
5. Poisson’s ratio
44Phillip's Science od dental materials 12th ed

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 Word modulus means ratio.
 Also called modulus of elasticity / young’s modulus
 Represents stiffness of a material.
 Calculated as ratio of elastic stress to elastic
  ℇ  =E ℇ E= ℇ
 Expressed in MPa or GPa
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 Intermolecular or interatomic forces of a material
responsible for its property of elasticity
 Stronger the basic attraction forces-------greater the
modulus value---------stiffer material
 Independent of any heat or mechanical treatments that the
metal or alloy has received
 Rather Dependent on composition.
Material
insensitivity

156 47Craig's restorative dental materials 13th ed

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 Defined as ratio of stress to strain for small cyclical
deformations at a given frequency and at a particular
point on the stress strain curve.
 Dynamic Young’s modulus of elasticity is more realistic
than the static values..

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It is the maximum flexural strain that occurs when a material
is stressed to proportional limit. Phillips
50

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 Restorative materials should withstand high stresses and
show minimum distortion or should have minimum
flexibility.
 Impression materials should have large flexibility or elastic
deformation to withdraw through severe undercuts without
permanent deformation.
 Maxillofacial materials and soft denture liners should have
high flexibility.
51Shama Bhat 2nd ed

156
It is amount of energy absorbed within a unit volume of a
structure when it is stressed to proportional limit. Phillip’s
It indicates the amount of energy necessary to deform the
material to its proportional limit Craig
52

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 Resilience has particular importance in the evaluation of
orthodontic wires . It determines the magnitude of the force
that can be applied to the tooth and how far the tooth can
move before the spring is no longer effective.
53

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 A high value of resilience is one parameter often used to
characterize elastomers. Such materials which may, for
example, be used to apply a cushioned lining to a hard
denture base are able to absorb considerable amounts of
energy without being permanently distorted. The energy is
stored and released when the material springs back to its
original shape after removal of the applied stress.

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 When a force is applied along one axis to produce elongation,
compressive strain is produced at right angles,
proportionately.
 Within elastic range the ratio of lateral to the axial strain is
called Poisson's ratio.
56

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 In tensile loading, poison’s ratio indicates that reduction
in cross section is proportional to the elongation during
elastic deformation and continues till material fractures.
 under compressive load, there is increase in cross
section.
 Dental materials have Poisson's ratio value in range of
0.3 to 0.5.

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TOUGHNESS
Defined as ability of a material to absorb elastic energy & to
deform plastically before fracturing

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 Fracture toughness is the resistance of a brittle material to
catastrophic propagation of flaws under an applied
stress.
 Fracture toughness is an indication of the amount of stress
required to propagate a preexisting flaw.

156 60
 Flaws or cracks may arise naturally in a material or nucleate
after time in service.
 In either case, any defect generally weakens a material, and as
a result, sudden fractures can arise at stresses below the yield
stress.
 Sudden catastrophic fractures typically occur in brittle
materials that do not have ability to plastically deform and
redistribute stresses.

156 61
Dispersion toughening of porcelain: Addition of hard (tough)
materials like zirconia (ZrO2), alumina (Al2O3), leucite, lithia
disilicate crystals, etc. to porcelain, can resist crack propagation
and increase fracture toughness, up to about 3.3. MPa·m½ (refer
ceramics).
Shama Bhat 2nd ed
Application

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 Brittleness is the relative inability of a material to sustain
plastic deformation before fracture of a material occurs
 Example - amalgam, ceramics, composites
62Phillip's Sciene of dental materials 12th ed

156 63
 Materials that are every brittle have the tensile strength lower
than the corresponding compressive strength because of their
inability to plastically deform.
 Brittle materials need not necessarily be weak.
 Co-Cr-Ni partial denture % elongation-1.5% UTS- 870MPa.
 If a glass is drawn into fiber with very smooth surfaces &
insignificant internal flaws, its tensile strength may be as high as
2800MPa but it will have no ductility.
 Thus dental material with low or 0% elongation including
amalgam, composites, ceramics & nonresin luting cements will
have little or no burnishability because they have no plastic
deformation potential.
Phillip's Sciene of dental materials 12th ed

156 64Phillip's Sciene of dental materials 12th ed

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 Two significant properties of metals & alloys.
 DUCTILITY of a material represents its ability to be drawn &
shaped into wires under tension
 When a tensile load is applied,
wire is formed by permanent deformation
 MALLEABILITY of a
material represents its ability
to be hammered or rolled into sheets
without fracturing

156 66
• Gold is the most ductile and malleable pure metal
• Silver is second
• Of the metals of interest to the dentist,
• Platinum ranks third in ductility
• Copper ranks third in malleability

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1. Percentage elongation after fracture
2. Reduction in area of tensile test specimen
3. The maximum number of bends performed in a cold bend
test

156 68
For dental
materials the
standard
gauge length
is 51 mm

156 69
The % decrease in cross-sectional area of the fractured end
in comparison with the original area of the specimen is
referred to as relative reduction in the area.

156
 The material is clamped and bent around a mandrel of
specified radius, the number of bends to fracture is counted,
with the greater the number, the greater is the ductility of
the material.

156 71Phillip's Sciene of dental materials 12th ed

156
 Mechanical properties that are more a function of the
surface condition of a material
72
Hardness
Friction
Wear
Craig's Restorative Dental Material 12th ed

156
 It’s the measure of resistance to plastic deformation which is
typically produced by an indentation force
 Hardness test used in dentistry are
 BRINELL
 KNOOP
 VICKERS
 ROCKWELL
 BARCOL
 SHORE A
73Craig's Restorative Dental Material 12th ed
Micro-surface
hardness tests

156
 Oldest test.
 Used for metals and alloys in dentistry.
 It is based on resistance to penetration by a steel or tungsten ball.
 Disadvantages
 This method cannot be used for:
 Brittle materials-like ceramics, gypsum products
 Elastically recovering materials, as ‘d’ decreases on removal of
indentor

156
 Steel or tungsten ball of
diameter 1.6mm is used
 Load of 123N is applied
 The indenter is kept in contact
of test specimen for 30 s
 BHN = load / area of the
indentation
 Small area = increased value

156
 Developed to fulfill the needs of micro indentation test
methods
 Used for thin plastic or metal sheets or brittle materials
 Advantage
 Materials with a great range of hardness can be tested by
varying the test load.
 Disadvantage
 It needs very high polished and flat surface.

156
 Rhombic shaped diamond indenter is used
 Applied load should not exceed 35N or 3.5KgF
( 0.1KgF – 1KgF )
 Indentation area varies with
 Load applied
 Nature of test specimen
 KHN = load / measurement of the largest diagonal

156 78Craig's Restorative Dental Material 12th ed

156
 Method is similar in principle to the Knoop and Brinell
 Except that it uses square shaped diamond indenter of 136
degrees
 Square shape indentation is produced
 The diagonals are measured and average value taken
 Used for
 Brittle materials
 Dental casting alloys
 Tooth structures

156 80
Material VHN
ENAMEL 300
DENTIN 60
TITANIUM ALLOY 125- 350
NOBLE METAL
ALLOY
175- 400
NICKEL
CHROMIUM ALLOY
210- 380
COBALT
CHROMIUM ALLOY
300- 465
PORCELAIN 400- 700
ALUMINA 1800
Shama Bhat 2nd ed

156
 Developed as a rapid method for hardness determination.
 A ball or metal cone indenter is used & the depth of the
indentation is measured with a sensitive dial micrometer.
 The indenter balls or cones are of
different diameters and load applications
( 60-150 KgF or 588- 1470N )

156
 This superficial rockwell method has been used to test
plastics used in dentistry.
 The test is made by first applying a preload of 29.4N OR
3KgF and then a major load of 294N OR 30KgF is applied
to the specimen for 10mins before reading is taken

156
 Advantages
 Hardness is read directly
 It is good for testing viscoelastic materials.
 Disadvantages
 Preload is needed
 Greater time required
 The indentation may disappear immediately on removal
of load.

156
 Used to study depth of cure of resin composite.
 The barcol indenter is a spring loaded needle with a diameter of
1mm that is pressed against the surface to be tested.
 The reading on scale decreases as the indenter penetrates the
surface.

156
 Depth of cure of resin composite is tested by preparing specimens
varying in thickness from 0.5 to 6mm.
 Then top of the specimen is activated by light curing unit.
 The barcol hardness of the top surface is compared with that of
bottom surface.
 The depth of cure is defined as the maximum thickness at which the
barcol reading of the bottom surface does not change by more than
10% of the reading of the top surface.

156
 Shore A durometer is used in rubber industry to determine
the hardness of elastomers, where the hardness is measured
in terms of material elasticity.
 The instrument consist of blunt point indenter 0.8mm in
diameter that tapers to a cylinder of 1.6mm.

156
 The usual method is to press down firmly and quickly on
the indenter and record the maximum reading.
 If indenter completely penetrates the specimen, a
reading of 0 is obtained.
 If no penetration occurs, a reading of 100 units results.
 Accurate reading is difficult to obtain.
 It is used to evaluate soft denture liners, mouth protector
and maxillofacial elastomers.

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156
 Friction is the resistance between
contacting bodies when one moves
relative to another.
 Frictional force is proportional to the normal force between
the surfaces and coefficient of friction.
 Coefficient of friction varies between 0 and 1 and is a
function of two materials in contact, their composition,
surface finish and lubrication.
 Materials in contact---------- greater cof.
 If lubricating medium exist---------reduced cof.

156
 It is defined as loss of material resulting from removal
and relocation of materials through the contact of two or
more materials.
 It can produce
 Inflammatory response
 Shape changes.
 Presence of lubricating film, such as saliva, separates
surfaces during relative motion and reduces frictional
forces and wear.

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Adhesive wear
Corrosive wear
Surface fatigue wear
Abrasive wear

156 92
ADHESIVE WEAR:-
• Characterized by formation and
disruption of micro-junction.
• Micro-regions are pulled from one
object and transferred to other.
CORROSIVE WEAR:-
• Its is secondary to physical
removal of a protective layer
and is related to chemical
activity of the wear surface.
• Sliding action of the surfaces
accelerate corrosion.
FATIGUE WEAR:-
• Free particles with small areas of contact
contribute to high localized stresses and
produce surface or subsurface cracks.
• Particles break off under cyclic loading and
sliding.
ABRASIVE WEAR:-
• Involves a harder material cutting or plowing
into a softer material.
• Two types
 Two body wear
 Three body wear
Craig's Restorative Dental Material 12th ed

156
MECHANICAL, PHYSICAL AND
AESTHETIC PROPERTIES OF DENTAL
MATERIALS
95
Guided by,
Dr. Jnanadev K.R.
Mrs. Savitha P Rao
Presented by,
Chaithra Prabhu B
1st year post graduate student

156 96
CONTENTS
• Optical properties
• Hue
• Chroma
• Value
• Color measurement
• Pigmentation
• Metamerism
• Fluorescence
• Opacity, transparency,
translucency
• Physical properties
• Viscosity
• Creep
• Flow
• Wetting & contact
angle
• Thermal properties
• Temperature
• Thermal
conductivity
• Thermal diffusivity
• Coefficient of
thermal expansion

156
 Physical properties are based on the laws of mechanics,
acoustics, optics, thermodynamics, elasticity, magnetism,
radiation, atomic structure, or nuclear
97Craig's restorative dental materiala 13th ed

156
 Viscosity is the resistance of a fluid to flow.
 The study of the deformation and flow characteristics of
matter, whether liquid or solid is known as ‘RHEOLOGY’
 It is measured in MPa/second or centipoise.
98Phillip's 12th ed and Craig's 13th ed

156 99
 Materials like cements and impression materials undergo a
liquid-to-solid transformation in the mouth.
 Gypsum products used in the fabrication of models and dies
are transformed from slurries into solid structures.
 The success or failure of a given material may be as
dependent on its properties in the liquid state as it is on its
properties as a solid.
Phillip's science of dental materials 12th ed

156
NEWTONIAN FLUID:
 An ideal fluid
 Shear stress proportional to strain rate -Straight line on curve
 Resembles elastic portion of stress strain curve.
 Viscosity(η)= shear stress(τ)/strain rate (ε)
 Constant velocity.
PSEUDOPLASTIC FLUID:
 viscosity decreases with increasing
strain rate, until it reaches a nearly
constant value.
10
0
Phillip's 12th ed and Craig's 13th ed

156
DILATENT FLUID
 Viscosity increase with increasing stress.
 The material become more rigid under stress(disadvantage)
 e.g.-Acrylic denture base material.
PLASTIC FLUID
 Material behaves rigid until a
minimum of stress is applied ,
then it starts behaving like Newtonian fluid.
 e.g.- ketchup, sharp blow is required to produce initial
flow.
10
1

156
 The viscosity of most liquid decreases with increasing
temperature and becomes more fluid under repeated
application of pressure is referred to as thixotropic.
 Dental prophylaxis paste
 Plaster of Paris
 Resin cements
 Impression materials
10
2

156
 Creep- it is defined as time dependent plastic strain of a
material under a static load or constant stress.
Phillip’s
If a metal is held at a temperature near its melting point & is
subjected to a constant applied stress, the resulting strain will
increase over time.
10
3

156
10
4

156
10
5
The term flow, rather than creep, has generally been used in
dentistry to describe the rheology of amorphous materials such
as waxes.
The flow of wax is a measure of its potential to deform
under a small static load even that associated with its own
mass.

156
 Creep compliance J𝑡
Defined as strain divided by stress at a given time
Once the creep curve is obtained, the corresponding creep
compliance curve can also be obtained
Equation for creep compliance
10
7
Craig's restorstive dental materials 13th ed

156
10
8
Craig's restorstive dental materials 13th ed

156
10
9
Application
Polysulfide is most flexible and
polyether is least flexible.
Parallelism of curve indicates low
permanent deformation and
excellent recovery during removal
of an impression.
Additional silicones and polyether
have the best elastic recovery.
Additional silicone have lowest
viscoelastic properties and requires
less time to recover viscoelastic
deformation followed by polyether.

156 110
Concluded that the newer material PVES tested was
found to be more flexible with high-tensile
energy.This material canbe preferred in cases
with undercut areas, favoring the removal of
impressions without tear and distortion.
Pragya Pandey, Sneha Mantri, Abhilasha
Bhasin Suryakant C. Deogade
Contemporary Clinical Dentistry |
Published by Wolters Kluwer - Medknow

156
 Surface tension : a property of liquids in which the exposed
surface tends to contract to the smallest possible area, as in the
spherical formation of drops; a phenomenon attributed to the
attractive forces, or cohesion, between the molecules of the
liquid GPT9
 Measured in dynes per centimeter
111

156 112
Liquid Surface tension
Water at 20℃ 72.8 dynes/cm
Water at 0℃ 76 dynes/cm
Water 100℃ 59dynes/cm
benzene 29 dynes/cm
Alcohol 22 dynes/cm
Mercury 465 dynes/cm
Factors affecting surface tension
1. Temperature (inversely proportional)
2. Presence of impurity / surface active agents eg sodium
lauryl sulfate

156 113
 The wetting power of a liquid is represented by its tendency to
spread on the surface of the solid.
 In restorative dental laboratory procedures, wax patterns are
formed that are to be wetted by water or water suspensions of a
casting investment.
 Wax is not well wetted by water, so a wetting agent such as
aerosol is first painted on the wax in small quantity to aid in
spreading of casting investment.
 Without adequate wetting, the investment could not flow over
the surface of the wax and replicate fine details
 Much regarding wettability can be learned by studying contact
angle between the liquid and the solid surface.

156
 It is defined as the angle formed by tangent drawn to the
drop of liquid and the solid surface.
GPT9
 It results from a balance of
surface and interfacial energies.
 Greater the tendency to wet the surface,
the lower the contact angle,
until complete wetting occurs
at an angle equal to zero.
114

156
 Contact angle of water and saliva on complete dentures relate
to the retention of dentures.
 If the saliva is allowed to stand on the acrylic dentures for
a longer time, contact angle is reduced and wettability is
improved.
115

156
 Contact angle provides information regarding the wettablility
and ease of pouring of dental stone mix
 Some elastomeric impressions
are hydrophobic and air gets
collected when stone is poured
 Surfactants can be added to the
Surface to artificially reduced the
Contact angle
116

156
 The surface tension of metals are relatively high when
compared with other liquids because of increased cohesive
force between the liquid metal atoms in liquid air interface
than in water.
 Contact angle is important because it defines the ease of
casting and reproduction of finer details
 Same applies on spreading of molten flux on hot metal
during soldering , if the contact angle of the solder is
greater, it will not penetrate into the fine details of the
structures to be joined.
117

156
1. Contact angle is less
2. Surface energy of solid is more
3. Surface tension of liquid is less
4. Surface is clean without oxide layer or contamination
118Shama Bhat 2nd ed

156
 When a denture base is in contact with the mucosal surface,
the transmission of certain amount of thermal energy is
desirable to convey the sensations of heat and cold
associated with food and beverages
 These are governed by thermal conductivity and thermal
diffusivity.
 Another category of thermal behavior is contraction when
cooled and expansion when heated
 This is governed by the COTE.

156
 It is defined as quantity of heat in calories per second passing
through a material 1cm thick with a cross section of 1cm sq.
having a temperature difference of 1K and is measure under
steady state conditions in which the temperature gradient
does not change Phillip’s
 S.I Unit is watts per meter per kelvin
 It is the physical property that governs heat transfer through a
material by conductive flow
12
0

156
12
1
• According to the second law of thermodynamics, heat flows
from points of higher temperature to points of lower
temperature.
• Materials that have a high thermal conductivity are called
conductors.
• Materials of low thermal conductivity are called insulators.
• Higher the thermal conductivity, greater is the ability of the
substance to transmit thermal energy, and vice versa.
• Polymers< ceramics< metals

156
12
2
Amalgam restoration
close proximity to pulp
can cause discomfort
Suitable Liners / base
PMMA poor
conductors of
heat
Shama Bhat 2nd ed

156
 It is a measure of the speed with which a temperature
change will spread through an object when one surface is
heated Phillip’s
 It is calculated by
12
3

156
 The materials with high density and specific heat will have
low thermal diffusivity. Such materials will changes its
temperature very slowly.
 When the product of heat capacity and density is high, the
thermal diffusivity may be low, even though thermal
conductivity is relatively high
 Therefore thermal conductivity and diffusivity are important
parameters in predicting the transfer of thermal energy
12
4

156
 Defined as the change in length per unit of the original length
of a material when its temperature is raised 1℃ / 1 K
Phillip’s
 When materials undergo a temperature increase, the vibrational
motion of atoms and mean interatomic distance increase . This
results an increase in volume—an expansion
12
5

156
 A tooth restoration may expand or contract more than the
tooth during a change in temperature thus there may be
marginal micro-leakage adjacent to the restoration or the
restoration may de-bond from the tooth.
12
6

156
 High coefficient of thermal expansion of inlay wax is also
important, as it is highly susceptible to temperature changes.
 Eg:- an accurate wax pattern that fits a prepared tooth
contracts when it is removed from the tooth or die in a
warmer area and then stored in cooler area
12
7

156
Aesthetic consideration in restorative & prosthetic dentistry
have received greater emphasis over the past several decades.
Since esthetic dentistry imposes several demands on the
artistic abilities of the dentist and technician, knowledge of
the underlying scientific principles of color is essential
12
8

156
 Verbal description of color are not precise enough to
describe appearance of teeth
example puce by definition “ a brilliant purplish-red color”
 These definitions are too variable, complex & imprecise to
describe a desired color of a dental crown to a laboratory
technician
12
9

156
 Color perception is described by three objective variables
1. Hue
2. Value
3. Chroma
These three parameters constitutes the 3 Dimensions of color
space

156
 This refers to the dominant
wavelengths present in the spectral
distribution
 Light having shorter wavelength
(400nm) is violet in color, and light
having long wavelength( 700nm) is red .
 Between these two wavelengths are the
dominant wavelength present in the
spectral distribution, which describe the
solid color of an object.
 Eg:- red, green, or blue.

156
 It is the luminous reflectance of a color of a surface
 It represents the lightness or darkness of color (the amount
of grayness). A black standard is assigned a value of 0,
whereas a white standard is assigned 10.
 It is independent of HUE

156
 It represents the strength of the color or degree of
saturation of the color (color intensity).
 it varies radially.
 Colors in the center are dull.
 The higher the chroma, more intense the color.

156
 The color of the dental restorative materials are most
commonly measured in reflected light using
1. visual method ------- Munsell’s color system
2. Color measuring instrument
134

156
 Munsell color system is a popular system for the visual
determination of color
135

156
13
6
Value is selected first .
Ranges from 0/ (black)
to 10/ (white)
Chroma----- varies from
/ 0 gray or to a highly
saturated color
/18
Hue is determined lastly
It is measured on a scale
of 2.5 to 10 with an
increment of 2.5 for each
hue
Hues considered here are
R Y G B P principle hues
with the intermediate hue
Craigs restorative dental materials 13th ed

156
13
7
The Munsell color
system, showing 5PB5/6
a circle of hues at value
5 chroma 6; and the
chroma of purple-blue
(5PB) at value 5.
EXAMPLE

156
13
8
Commission Internationale del’Eclairage
L*a* b*
color
coordinate
system
Color measuring instrument
system

156
 An extremely rough surface appears lighter than a smooth
surface of the same material.
 Eg:- unpolished or worn glass ionomer and resin composite
restorations.
 The thickness of a restoration can affect its appearance.
 Eg:- as thickness of composite restoration placed against a
white background increases, the lightness decreases.
 This is observed as an increase in opacity as the thickness
increases.

156
 Esthetic effect are sometimes produced in restoration by
incorporating color pigments in non-metallic materials like
resin composites, denture acrylics, silicone maxillofacial
materials and dental ceramics.
 Inorganic pigments are often preferred than organic dyes
because they are more permanent and durable in their color
qualities.

156
 To match tooth tissue, various shades of yellow and grey are
blended into the white base material, and occasionally some
blue or green pigments are added.

156
 It is defined as phenomenon in which the color of an object
under one type of light source appears to change when
illuminated by a different light source.
 The quality and intensity of light are factors that must be
controlled when matching color in dental restorations.
 Because the light spectrum of incandescent lamps,
fluorescent lamp and the sun differ from each other.

156
 Recommended that shade matching to be done under two or
more different light sources, one of which should be day light
 Shade matching should be in conditions where patients most
of the activity takes place.
14
4

156
FLUROSCENCE
 It is the emission of luminous energy by a material when a
beam of light is shone on it.
 Sound human teeth emit fluorescent light when excited by
ultraviolet or blue light.

156
14
6
Fluorescence makes a definite contribution to brightness and vital
appearance of a human tooth
Eg : ceramic crowns or composite restoration that lack a
fluorescing agent appear as missing teeth when viewed under a
black light

156
OPACITY, TRANSLUCENCY,
TRANSPARENCY AND
OPALESCENCE
147

156
OPACITY
 It is the property of a material that prevents
the passage of light.
 When all the colors of the spectrum from a
white light source such as sunlight are reflected
from an object with the same intensity as
received, object appears white.
 When all the colors are absorbed equally,
object appears black.
 An opaque material absorb some of the light
and reflect the remainder.

156
 It is property of a material that permits the passage of light
but disperses the light, so the object cannot be seen through
the materials.
 Eg:- Ceramics
 Resin composites
 Acrylics.

156
 Transparent materials allow the passage of light
 Objects may be clearly seen through them
 Eg Glass
 If a piece of glass absorbed all wavelengths except red then it
would appear red by the transmitted light
 If a light be with no red wavelength is shone on the piece of
glass, then it would appear opaque as remaining wavelengths
would be absorbed

156
 Materials such as dental
enamel, are able to
scatter shorter
wavelength of light
 Under transmitted light,
the appear brown
yellow.
 Under reflected light,
appear blue.

156
15
2
Optical properties of zirconia ceramics for esthetic dental
restorations: A systematic review.
J Prosthet Dent. 2018 Jan;119(1):36-46
PURPOSE:-
The purpose of the present systematic review was to assess
information on the mechanical, chemical, and optical
requirements of monolithic zirconia dental restorations.
CONCLUSION:-
Although zirconia-toughened lithium silicate offers the best
esthetic outcomes, transformation-toughened zirconia offers
the best mechanical properties and long-term stability; cubic
stabilized zirconia offers a potential compromise. The
properties of these materials can be altered to some extent
through the appropriate application of intrinsic (such as,
annealing) and extrinsic (such as, shade-matching) parameters

156
15
3
CONCLUSION : The study concluded that high translucency lithium
disilicate is the most translucent material amongst the materials studied.
High translucent zirconia is significantly more translucent than
conventional zirconia. However, the increase in transmittance achieved
with high translucency zirconia is significantly less compared to even
conventional lithium disilicate.
AIM : Evaluate the light transmittance of this translucent variety
of 3Y-TZPs at different wavelengths and compare it to lithium
disilicate

156
 Physical, mechanical and optical properties of each
dental material form the key stone, for a clinician
,regarding the selectivity of a particular product. This in turn
increases the durability and life span of the restoration.
 When tooth shade is selected using conventional means,
knowledge & skill of practitioner comes into play.
 It form the back bone of the handling, manipulation, and
storage characteristics of a dental material and a thorough
knowledge in this aspect is a must for a successful
clinician.
15
4

156
Craig’s Restorative Dental Materials 13th edition
 Philips Science Of Dental Materials 12th edition
Shama Bhat Science Of Dental Materials 2nd edition
John F McCabe Applied dental materials 9th edition
155

156
 A Comparative Evaluation of Flexure and Impact Strength of Three
Different Denture Base Materials: An In Vitro Study
Vineet Kumar1,Tanvi Bihani, Karandeep Singh Arora,Veer Kapilash,
Kapil Sharma
 The influence of elastic modulus mismatch between tooth and post and
core restorations on root fracture
M. Ona, N. Wakabayashi,T. Yamazaki, A. Takaichi & Y. Igarashi
 Optical properties of zirconia ceramics for esthetic dental restorations:
A systematic review.
J Prosthet Dent. 2018 Jan;119(1):36-46
 Mechanical Properties of a New Vinyl Polyether Silicone in Comparison
to Vinyl Polysiloxane and Polyether Elastomeric Impression Materials
Pragya Pandey, Sneha Mantri, Abhilasha Bhasin, Suryakant C. Deogade
156

156
15
7
Craig's Restorative dental materials 13th ed

Mechanical, physical and aesthetic properties of dental

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