Download Physical properties

Physical properties
BY
Dr. Reham Mohammed Abdallah
Items to be covered
I. Definition
II. Classification of physical properties
1. Optical properties
• Color
• Color parameters
• Factors affecting color matching
• Color matching in dentistry
2. Thermal properties
• Glass transition temperature
• Melting temperature
• Heat of fusion
• Thermal conductivity
• Thermal diffusivity
• Coefficient of thermal expansion
3. Electrical properties
• Electrical conductivity and resistivity
• Electromotive series
• Corrosion
• Physical property is a property that can
change without involving a change in the
chemical composition.
• Unlike mechanical, physical properties are
not concerned with the application of force
to a body.
A. OPTICAL PROPERTIES
Color
• One of the most desirable characteristics of
dental restoration is a natural tooth-like color.
• For an object to be visible, it must emit, reflect or
transmit light.
• Light is an electromagnetic radiant energy of
certain wavelength.
• Human eye is sensitive to wavelengths ranging
between 400 nm(violet) to700 nm (red).
Color parameters
• The eye can distinguish between 3 color
parameters:
a. Hue
• It is the dominant wavelength of light (ex: red,
blue or green).
b. Value
• It is the lightness or darkness of color (lighter
shade has higher value).
c. Chroma
• It is the degree of saturation (intensity) of a
particular hue.
Factors affecting color matching
1. Surface finish
• Rough surface appears lighter than smooth
surface of the same material.
2. Pigmentation
• Esthetic effects are sometimes produced by
incorporating colored pigments in dental
materials such as composite resin and acrylic
denture.
3. Metamerism
• A phenomena in which objects appear colormatched under one type of light, and different
under another light source.
Clinical Significance
• Color matching should be done under 2 light
sources, one of which is sun light (in the dental
clinic, color matching could be done using either
white light or sun light).
4. Fluorescence
• It is the ability of a material to absorb color of certain
wave length and reemit light of a different wave length.
Example
• Natural teeth can absorb U.V. and near U.V. radiation
(invisible light) and emit visible light of different
wavelength, so teeth appear as a light source.
(Photoflash lamps, vapor lamps, and sunlight contain
near U.V radiation).
Clinical Significance
• Dental porcelains are formulated with fluorescing
agents (as Cerium oxide) to produce the natural
appearance of tooth structure, while acrylic resin
doesn’t have the same property.
5. Opacity, translucency and transparency
Opacity: The property of a material that prevents
passage of light (by absorption, reflection or
both).
Translucency: Is the property of a material
that permits passage of light but with
dispersion, so objects cannot be seen clearly
through it, e.g. Tooth enamel, dental ceramics.
Transparency: Is the property of a material
that allows passage of light with only little
distortion that objects can be seen clearly
through it. e.g. Glass
Color matching in dentistry
• The most common method used for color
matching in dentistry is the “shade guide”(see
the following figure).
Figure 1: Color matching
by shade guide selection.
B. THERMAL PROPERTIES
1. Glass Transition temperature Tg
• This
property
is
characteristic
to
non-
crystalline structures (as polymers & waxes)
that do not have definite melting point but
rather gradually soften as their temperature is
raised.
• So, glass transition temperature (Tg) can be
defined as:
The temperature at which there is an abrupt
increase in the coefficient of thermal
expansion.
2. Melting temperature
• It is the temperature at which the material
transforms from solid to liquid (for crystalline
materials).
3. Heat of fusion
• It is the amount of heat in calories or joules that is
required to convert 1 gram of a material from
solid to liquid state at the melting temperature.
4. Thermal conductivity
• It is the quantity of heat in calories or joules, per
second passing through a body 1 cm thick with a
cross section of 1 cm2 when the temperature
difference is 1° C.
Clinical Significance
1. In case of metallic restorations with high thermal
conductivity as amalgam, extremes in temperature
would produce pain & injury to the tooth pulp. Thus, an
insulating base is needed under metallic restorations to
prevent such thermal shocks.
2. Low thermal conductivity of resin denture base
materials tends to prevent heat exchange between the
oral cavity and soft tissues.
This exchange is important to keep vitality of
soft tissues.
• Furthermore, these materials don’t allow the
patient to have the normal sense of heat and
cold while swallowing food and drinks. So
metallic denture base would be better is this
instance.
5. Thermal diffusivity
• It is the rate at which a body of non-uniform
temperature approaches thermal equilibrium.
Clinical Significance
• Gold or amalgam restorations may cause high
thermal shock due to their high thermal
diffusivity
(as
they
have
high
conductivity and low specific heat).
thermal
6. Coefficient of thermal expansion (CTE)
It is the change in length per unit length of a
material for 1° C change in temperature.
Clinical Significance
1. Ideally, filling materials should have similar CTE
to that of the tooth structure to avoid breaking the
marginal seal and subsequent leakage between the
filling & the tooth during thermal changes.
2. During denture construction, matching in CTE
between denture base and artificial teeth is
essential to avoid their separation during
thermal changes.
C. ELECTRICAL PROPERTIES
Electrical conductivity and resistivity
Electrical conductivity
• It is the ability of the material to conduct an
electric current.
Electrical resistivity
• Is the resistance of material to conduct electric
current.
Clinical Significance
• Cements to be used as insulating bases under
metallic restorations, should have high electrical
resistivity to protect the underlying tooth tissues
from galvanism.
2. Electromotive series
• It is a listing of electrode potentials of metals
according to their decreasing tendency to oxidize
in solution.
Metal as Zn, tends to oxidize & corrode relatively easily,
while those such as Pt& Au resist oxidation.
3.Corrosion
• It is the deterioration of a metal caused by reaction with
its environment.
Galvanic corrosion
• It is an electrochemical process in which one metal
corrodes preferentially to another when both metals are
in electrical contact and immersed in an electrolyte.
Significance
• Avoid using different metallic restorations
(e.g. gold and amalgam) in opposing or
adjacent teeth as they can form an electric cell
in conjunction with saliva, leading to galvanic
corrosion of the more anodic metal.