overview of physical properties of dental materials

1. PHYSICAL PROPERTIES OF DENTAL
MATERIALS
Dr. Kriti Trehan
MDS 1ST Year

2. CONTENTS
 Introduction
 Structure of Matter
 Adhesion and bonding
 Concept of stress and strain
 Rheology
 Structural relaxation
 Creep and flow
 Color and optical effects
 Thermal properties
 Electrochemical properties
 Conclusion
 References

3. INTRODUCTION
 Physical properties are based on the laws of mechanics,
optics, thermodynamics, elasticity, magnetism, radiation,
atomic structure, or nuclear phenomena.
 Hue, value and chroma relate to color and perception.
 Thermal conductivity , diffusivity and expansion are
physical properties based on the law of thermodynamics.
 Chemical properties are based on the ways in which
substances interact, combine and change, as governed
by their outer orbital electrons.

4. STRUCTURE OF MATTER
 All matter is composed of indivisible
particles called atoms.
 An atom consists of a nucleus
surrounded by a cloud of negatively
charged electrons.
 The electrons of an atom exist in
different clouds at the various energy
levels.
 An atom becomes a negative ion
when it gains electron(s) or a positive
ion when it loses electron(s).

5.  Two or more atoms can form an electrically neutral entity
called a molecule.
 Attraction between atoms and between molecules result in
materials we can see and touch.
 The transformation between vapor, liquid, and solid is called
the change of state.
 A change from the solid to the liquid state will require
additional energy—kinetic energy— to break loose from the
force of attraction.
 This additional energy is called the latent heat of
fusion. The temperature at which this change occurs is known
as the melting temperature.

6. INTERATOMIC BONDS
 There are different types of forces holding these atoms
and molecules together.
 Primary bonds and secondary bonds.
Primary bonds
 The formation of primary bonds depends on the atomic
structures and their tendency to assume a stable
configuration.
 The strength of these bonds and their ability to reform
after breakage determine the physical properties of a
material.

7. Primary atomic bonds also called chemical bonds, may be
of three different types: (1) ionic (2) covalent and (3) metallic.

8.  The free electrons give the metal its characteristically
high thermal and electrical conductivity.
 These electrons absorb light energy, so that all
metals are opaque to transmitted light.
 The metallic bonds are also responsible for the ability
of metals to deform plastically.
 The free electrons can move through the lattice,
whereas their plastic deformability is associated with
slip along crystal planes.

9. SECONDARY BONDS
 In contrast with primary bonds, secondary bonds do not
share electrons. Instead, charge variations among atomic
groups of the molecule induce dipole forces that attract
adjacent molecules or parts of a large molecule.

10. Hydrogen Bond
 The hydrogen bond is a special case of dipole attraction of
polar compounds.
 Attached to the oxygen atom are two hydrogen atoms.
The protons of the hydrogen atoms pointing away from
the oxygen atom are not shielded efficiently by the
electrons. They become positively charged.
 On the opposite side of the water molecule, the electrons
that fill the outer shell of the oxygen provide a negative
charge.
 Polarity of this nature is important in accounting for the
intermolecular reactions in many organic compounds— for
example, the sorption of water by synthetic dental resins.

12. ARRANGEMENT OF ATOMS
Crystalline
 There are 14 possible lattice types. A space lattice can be
defined as any arrangement of atoms in space in which
every atom is situated similarly to every other atom.
 The type of space lattice is defined by the length of each of
three unit cell edges (called the axes) and the angles
between the edges.
 The simplest and most regular lattice is a cubic.

13. oIt is characterized by axes that are all of equal length and meet
at 90-degree angles.
o Each sphere represents the positions of the atoms. Their
positions are located at the points of intersection of three planes,
each plane (surface of the cube) being perpendicular to the other
two planes. These planes are often referred to as crystal planes.

14.  Most metals used in dentistry
belong to the cubic system.
 For example, iron at room
temperature has an atom at each
corner of the cube and another
atom at the body center of the
cube. This crystal form is called
a bodycentered cubic cell.
 Copper, on the other hand, has
additional atoms at the center of
each face of the unit cell but
none at the center of the cube.
This form is called a face-
centered cubic cell.

15. NONCRYSTALLINE STRUCTURE
 Glass is a typical noncrystalline solid of
SiO2 because its atoms tend to be
arranged in non-repeating units
 This arrangement is also typical of
liquids, such solids are sometimes called
supercooled liquids.
 Because of the complexity of the physical
configuration of polymer chains, the
molecules of resins are not favored to
arrange in orderly repeating patterns.
Therefore, polymeric-based materials
used in dentistry are usually
noncrystalline.

16. INTERATOMIC BOND DISTANCE AND THERMAL ENERGY
 Between any two atoms, there are forces of
attraction drawing them together and forces of
repulsion pushing them apart.
 Both forces increase as the distance between the
atoms decreases. The force of repulsion increases
much more than the force of attraction as the atoms
get closer.
 Bond Distance- The position at which both forces
are equal in magnitude (but opposite in direction) is
considered the equilibrium position of the atoms

17.  The interatomic distance at equilibrium represents the
distance between the centers of the two adjacent atoms.

18.  Bonding Energy-.
 Amount of energy that has to be supplied to separate the
two atoms .
 Energy is defined as the product of force and distance.
Integration of the interatomic force over the interatomic
distance yields the interatomic energy.
 Generally covalent bonds are the strongest, followed by the
iconic bonds, then metallic bonds.

19. Thermal Energy
 The atoms in a crystal at temperatures above absolute
zero are in a constant state of vibration, and the average
amplitude is dependent on the temperature.
 As the temperature increases, the amplitude of the atomic
(or molecular) vibration increases.
 It follows also that the mean interatomic distance increases
as well as the internal energy. The overall effect is the
phenomenon known as thermal expansion.

20. ADHESION AND BONDING
 When the molecules of one substrate adhere or are attracted
to molecules of the other substrate, the force of attraction is
called adhesion when unlike molecules are attracted and
cohesion when the molecules involved are of the same kind.

21. SURFACE AND SURFACE
ENERGY
 Solids or liquids are made up of a
finite number of atoms or
molecules bonded by primary
and/or secondary bonds.
 This means that their surface is
populated by atoms or molecules
that are ready to attract other
atoms or molecules approaching
the surface.

22.  This energy quantifies the work needed to disrupt
intermolecular bonds resulting a new surface. Thus, it is
called the surface energy.
 The functional chemical groups available or the type of
crystal plane of a space lattice present at the surface may
affect the surface energy and thus the adhesion.
 The energy on the surface per unit area is referred to as the
surface energy (in mJ/m2 ) or surface tension (in mN/m).
 Any acquired surface impurity- such as an adsorbed gas, an
oxide, or human secretions-can cause a reduction in the
surface energy and adhesive qualities of a given
solid as these impurities constitute the new surface.

23. WETTING
 To produce adhesion on any targeted surface, the liquid
must flow easily over the entire surface and adhere to the
solid. This characteristic is known as wetting.
 The ability to wet the substrate is the dominating
contributor to the adhesive bond when the adhesive sets
from liquid to solid.
 The ability of an adhesive to wet the surface of the
adherend is influenced by a number of factors.
 The cleanliness of the surface
 Surface energy.
 Impurity-free metal surfaces.

24.  The tangent line drawn relative to the curvature of the
liquid profile and the solid surface constitute an angle; this
is called the contact angle
 A small contact angle indicates that the adhesive forces at
the interface are stronger than the cohesive forces
holding the molecules of the adhesive together.
 Complete wetting occurs at a contact angle of 0° and no
wetting occurs at an angle of 180°

25.  Dental professionals encounter wetting issues on a daily
basis.
 When gypsum products are mixed with water to pour dental
models in various types of impressions, wetting must occur
between gypsum and the impression to ensure good
surface quality of the gypsum model.
 To improve the wettability of the set elastomeric impression
material by a gypsum-water mixture, the operator usually
sprays a surfactant (also called debubblizer).
 The wetting agent migrates to the solid surface and
accommodates surface wetting by the aqueous gypsum
forming mixture.

26. CONCEPT OF STRESS AND STRAIN
 STRESS- Force induced by or resisting an external force.
 Stress= Force per unit area
 Stress is equal and opposite in direction to the load or
external force.
 TYPES OF STRESS
 Tensile
 Compressive
 Shear

27.  STRAIN - Can be defined as change in length per unit
length of the body when subjected to stress.
 Strain can either be elastic or plastic.
 Elastic strain is strain that totally disappears once the
external load that caused it is removed.
 Plastic strain is strain that permanently remains once the
external load that caused it is removed.
 It occurs when the force applied to the atoms moves them
so far from their equilibrium position that they do not return
to it once the force is removed.

28. RHEOLOGY
 The study of the deformation and flow characteristics of
matter.
 Viscosity is the resistance of a fluid to flow which is
controlled by internal frictional forces within the liquid.
 Most dental materials are initially in a fluid state so that they
can be placed and shaped as required.
 Cements and impression materials undergo fluid-to-solid
transformation in the mouth. Gypsum products are
transformed extraorally.

29.  Curves depicting shear stress
versus shear strain rate are
used to characterize the
viscous behavior of fluids.
 An “ideal” fluid produces a
shear stress proportional to
the strain rate.
 That is, the greater the force
applied, the faster the fluid
flows and the plot is a straight
line. This is known as
Newtonian viscosity

30.  The viscosity of many dental materials decreases
with increasing strain rate until it reaches a nearly
constant value. This is pseudoplastic viscosity.
 Dilatants show opposite behavior of pseudoplastic
fluids and become more rigid as the rate of
deformation (shear strain rate) increases.
 The fluids which exhibit rigid behavior initially and
then attain constant viscosity, are referred to as
plastic.
 Fluids that become less viscous and more flowable
upon repeated applications of pressure and are
termed thixotropic.

31. CREEP AND FLOW
 As flow of liquids is measured by viscosity for solids it
can be done by creep.
 Time dependent plastic deformation of a material under
static load or constant stress over time is called creep.
 Metal creep occurs at temperatures near melting point.
As a result, dental amalgams can undergo creep at
restored tooth site under periodic sustained stress.
 Flow is used to define creep of amorphous materials
such as waxes. The flow of wax is a measure of its
potential to deform under a small static load.

32. STRUCTURAL RELAXATION
 After a substance has been permanently deformed (plastic
deformation), there are trapped internal stresses. The
displaced atoms are not in equilibrium positions and are
therefore unstable.
 Through a solid-state diffusion process driven by thermal
energy, the atoms can slowly return to their equilibrium
positions.
 This results in change in shape or contour of solid and is
called stress relaxation which can lead to distortions in the
impression and subsequent lack of fit of the material.
 Such is a problem with elastomeric impression materials.

33. COLOR AND OPTICAL EFFECTS
 An important goal of dentistry is to restore or improve
esthetics- colour and apperance of natural dentition.
 Esthetic dentistry imposes severe demands on artistic abilities
of dentist and technician, knowledge of underlying scientific
principles of color and other optical effects are essential.
 For an object to be visible, it must reflect or transmit light
incident on it from an external source.
 Light from an object that is incident on the eye is focused in the
retina and is converted into nerve impulses, which are
transmitted to the brain.
 Cone-shaped cells in the retina are responsible for color vision.

34. THREE DIMENSIONS OF COLOR
Hue:
 The dominant color of an
object, for example red,
green, or blue.
 This refers to the
dominant wavelengths
present in the spectral
distribution.

35. Value
 Value is also known as
the gray scale. It is the
vertical, or Z-axis. Value
increases toward the high
end (lighter) and
decreases toward the low
end (darker).
 For a light-diffusing and
light-reflecting object such
as a tooth or dental crown,
value identifies the
lightness or darkness of a
color, which can be
measured independently
of the hue.

36. Chroma
 Chroma is the degree of
saturation of a particular
hue. For example, red can
vary from “scarlet” to light
pink, where scarlet has a
high saturation and pink
has a low saturation.
 In other words, the higher
the chroma, the more
intense the color.

37. Measurement of color
 One of the most commonly used method to define and
measure colour quantitatively is the “munsell system”.
 It is a coordinate system which can be viewed as a
cylinder.
 The hues are arranged sequentially around the
perimeter of cylinder while chroma increases along a
radius from the axis
 The values coordinate varies along length of cylinder
from back at the bottom to neutral grey at the center to
white at top.

39. Metamerism
 Phenomenon in which the color of an object under one
type of light appears to change when illuminated by
different light source.
 Because the spectral distribution of the light reflected from
or transmitted through an object is dependent on the
spectral content of the incident light, the appearance of an
object is dependent on the nature of the light in which the
object is viewed.
 Clinical significance: If possible, color matching should
be done under two or more different light sources, one of
which should be daylight, and the laboratory shade
matching procedures should be performed under the same
lighting conditions.

40. Fluorescence
 It is the absorption of light by a material and the
spontaneous emission of light in a longer
wavelength.
 Fluorescence makes a definite contribution to the
brightness and vital appearance of a human tooth.
 UV light is absorbed and fluoresced back as light
primarily in the blue end of the spectrum.
 Ceramic crowns or composite restoration that lack
a fluorescing agent appear as missing with when
viewed under a black light.

41.  The color of an object is also modified by the
translucency or opacity of the object.
 Opacity is a property of materials that prevents the
passage of light.
 Translucency is a property of substances that permits
the passage of light but disperses the light, so objects
cannot be seen through the material.
 Opalescent materials should be used to mimic a
natural tooth and they appear brown/yellow under
transmitted light, whereas shades of blue are
perceptible under reflected light.

42. THERMAL PROPERTIES
 Thermal conductivity (κ) is the physical property that governs
heat transfer through a material by conductive flow.
 It is defined as the quantity of heat in calories per second
passing through a material l cm thick with a cross section of 1
cm2 having a temperature difference of 1 °C and is measured
under steady-state conditions.
 The International System (SI) unit or measure for thermal
conductivity is watts per meter per kelvin (W × m−1 × K−1 ).
 In general, thermal conductivities increase in the
following order: polymers < ceramics < metals.

43.  Materials that have a high thermal conductivity are called
conductors, whereas materials of low thermal conductivity are
called insulators.
 The higher its thermal conductivity, the greater the ability of a
substance to transmit thermal energy.
 Thermal diffusivity is a measure of the speed with which a
temperature change will spread through an object when one
surface is heated.
 It is calculated from the thermal conductivity divided by the
product of density and heat capacity:
h = κ cp ρ

44. • In the oral environment, temperatures are not constant
during the ingestion of foods and liquids. Under such
conditions, thermal diffusivity is important.

45.  Thus for a patient drinking ice water, the low specific heat of
amalgam and its high thermal conductivity suggest that the
higher thermal diffusivity favors a thermal shock situation
more than that is likely to occur when only natural tooth
structure is exposed to the cold liquid.
 The low thermal conductivity of enamel and dentin aids in
reducing thermal shock and pulpal pain when hot or cold
foods are taken into the mouth.

46.  When materials undergo a
temperature increase, the
vibrational motion of atoms
and mean interatomic (bond)
distances increase. This
results is an increase in
volume—an expansion.
 The increase is the coefficient
of thermal expansion, α,
which is defined as the
change in length per unit of
the original length of a
material when its temperature
is raised 1 °C (1 K).
COEFFICIENT OF THERMAL EXPANSION

47.  Clinical Significance
 Close matching of the coefficient of thermal expansion (α) is
important between the tooth and the restorative materials to
prevent marginal leakage.
 Opening and closing of gap results in breakage of marginal
seal between the filling and the cavity wall, this breakage of
seal leads to marginal leakage, discoloration &
hypersensitivity.

48. ELECTROCHEMICAL PROPERTIES
 Tarnish is a surface discoloration on a metal or a slight
loss or alteration of the surface finish or luster.
 In oral environment tarnish occurs due to:
 Formation of hard substance- calculus and soft
substance-plaque.
 Formation of thin films such as oxides, sulfides or
chlorides.
 The latter phenomenon may be only a simple surface
deposit, and such a film may even be protective. However,
it is often an early indication and precursor
of corrosion.

49.  Corrosion is a process whereby deterioration of a metal
is caused by reaction with its environment.
 Corrosive disintegration can take place through the
action of moisture, atmosphere, acid or alkaline solutions,
and certain chemicals.
 Corrosion occurs because most commonly used metals
and alloys are not in their thermodynamically most stable
state.
 Thus, pure metals spontaneously convert to a highly
reacted, oxidized state by reacting with oxygen, sulfur, or
chlorine in order to revert to their lowest energy

50. 1. Non aqueous (dry) or chemical corrosion:
o In which there is a direct combination of metallic and
non-metallic elements .
o Occurs in the absence of water or another fluid
electrolyte e.g. oxidation, halogenations, or sulfarization
reaction.
o This type of corrosion is less susceptible to occur in the
mouth.
o Example is the discoloration of silver by sulfur, where
silver sulfide forms by chemical corrosion.

51. 2. Aqueous (wet) or electrolytic corrosion:
 When a metal is in contact with a fluid electrolyte, the chemical
potential causes enough ions to dissolve to form a saturated
solution and produce an equal number of free electrons.
 The loss of electrons by a metal is known as oxidation and is
the initial electrochemical event in the corrosion process.

52.  The anode is the surface or site on a surface where
positive ions (M+ ) are formed (i.e., the metal surface that
is undergoing an oxidation reaction and corroding) with
the production of free electrons.
 The cathode is the surface or sites on a surface where
metal ions are deposited from a saturated solution and
consume free electrons produced at the anode.
 The electrolyte supplies the ions needed at the cathode
and carries away the corrosion products at the anode.
 The external circuit serves as a conduction path to carry
electrons (the electrical current) from the anode to the
cathode

53.  In order for electrochemical
corrosion to be an ongoing
process, the production of
electrons must be exactly
balanced by the
consumption of electrons.
 Different metals have
different tendencies for
oxidation because of their
differences in electronic
structure; this tendency to
oxidize (ionize) is measured
by the electrode potential
expressed in volts or
millivolts.

54.  Many types of electrochemical corrosion are possible in
the oral environment because saliva, with the salts it
contains, is a weak electrolyte.
 The electrochemical properties of saliva depend on the
concentrations of its components, pH, surface tension,
and buffering capacity.
 Each of these factors may influence the strength of any
electrolyte. Thus, the magnitude of the resulting corrosion
process will be controlled by these variables.

55. 1. Galvanic cell corrosion
o When combinations of two dissimilar
metals are in direct physical contact,
it may produce galvanic corrosion
through the flow of galvanic currents
which may be in either continuous or
intermittent contact.
o When the two restorations are
brought into contact, there is a
sudden short-circuit through the two
alloys. This can result in a sharp
pain, called galvanic shock.
Types of electrochemical corrosion:

56. 2. Stress corrosion
 Since the imposition of stress increases the
internal energy of an alloy when permanent
deformation occurs, the tendency to undergo
corrosion will be increased.
 For most metallic dental appliances, the
deleterious effects of stress and corrosion, called
stress corrosion, are most likely to occur during
fatigue or cyclic loading in the oral environment.
 Small surface irregularities, such as notches or
pits, act as sites of stress concentration so that
ordinary fatigue failure (in the absence of
corrosion) occurs at nominal stresses.

57.  Any cold working of an alloy by bending, burnishing, or
malleting causes localized permanent deformation in
some parts of the appliance.
 Electrochemical cells consisting of the more deformed
metal regions (anodic), saliva, and undeformed or less
deformed metal regions (cathodic) are created, and the
deformed regions will experience corrosion attack.
 This is one reason why excessive burnishing of the
margins of metallic restorations is contraindicated.

58. 3. Concentration cell corrosion
 Type of electrochemical corrosion which occurs
whenever there are variations in the electrolytes or in
the composition of the given electrolyte within the
system.
 A similar type of attack may occur from differences in
the oxygen concentration between parts of the same
restoration, with the greatest attack at the areas
containing the least oxygen (the anode).
 Irregularities—such as pits, scratches, and cracks—in
restoration surfaces are important examples of this
phenomenon.

59.  The region at the bottom of such a
defect is oxygen-deprived and
becomes the anode.
 The alloy surface around the rim of
a scratch or pit becomes the
cathode.
 Consequently metal atoms at the
base of the pit ionize and go into
solution, causing the pit to deepen.
 Thus, to protect against such
pitting corrosion, all metallic dental
restorative materials should be
polished.

60. PROTECTION AGAINST CORROSION
 A highly effective protection utilizes certain metals that
develop thin, adherent highly protective film by reaction with
environment such metal is said to be passive.
 Example: A thin surface oxide formed on chromium, stainless
Steel which contains sufficient amount of chromium is added
to iron and its alloy to passivate the alloy.
 Titanium and its alloy are widely used because of its
favorable combination of physical chemical and biological
properties as well as their resistance to corrosion,
 Iron, steel, and certain other metals that are subject to
corrosion can be electroplated with nickel followed by
chromium for corrosion protection and esthetic reasons.

61. CONCLUSION
 A proper knowledge of physical properties of dental
materials helps us in making correct choice for
various clinical restorations. This in turn increases
the durability and life span of the restoration.
 This will also enable us to select a material that will
have properties close to that of natural tooth
surface.
 Technique based system provide dentist with
distinct advantage in creating highly esthetics ,
natural looking restoration

62. REFERENCES
 Phillips science of dental material -12th edition .Pg
20-48.
 Craig. Restorative dental materials –13th
edition.Pg 35-59