Post on 02-Feb-2023
Physical properties and chemical properties of materials
Shaista Afroz
(MDS, PhD)
Associate professor
Prosthodontics
Dr Z A Dental College
Aligarh Muslim University
Physical properties are based on the laws of
• Mechanics
• Acoustics
• Optics
• Thermodynamics
• Electricity
• Rheology
• Magnetism
• Radiation
• Atomic Structure
THERMAL PROPERTIES
- Thermal conductivity
- Coefficient of thermal expansion and contraction
- Heat of fusion & latent heat of fusion
- Melting and freezing temperature
- Specific heat
THERMAL CONDUCTIVITY
It is the characteristic that determines the rate at
which heat flows through a material.
It is a function of the
• composition which determines the heat capacity
• magnitude of the temperature change
• thickness of the object.
Clinical importance in Dentistry:
1- Metallic filling materials.
2- Metallic denture base materials.
THERMAL DIFFUSIVITY
It is a measure of the rate at which a body with a non uniform
temperature reaches a state of thermal equilibrium.
Clinical importance in Dentistry:
Thermal insulation → Thickness of liner or base
→ 1 / thermal diffusivity
COEFFICIENT OF THERMAL EXPANSION
The change in length per unit length of the material
for a 1°C change in temperature is called the
linear coefficient of thermal expansion(α)
= L final- L original
L original x (°C final- °C original)
Clinical importance in Dentistry:
Close matching of the coefficient of thermal expansion (α) is important
between:
1. The tooth and the restorative materials to prevent marginal leakage.
Opening and closing of gap results in breakage of seal
(marginal percolation):
1. Marginal leakage
2. Discoloration
3. Recurrent caries
4. Hypersensitivity.
2. Porcelain and metal in ceramo-metallic
restorations (crowns and bridges) to provide
metal ceramic bonding.
3. Artificial tooth and denture base to avoid crazing.
HEAT OF FUSION (L)
The amount of heat in calories or joules required to
convert 1 gm of a material from the solid to the
liquid state at the melting temperature.
As long as the mass is molten, the heat of fusion is
retained by the liquid.
LATENT HEAT OF FUSION
It is the amount of heat in calories or joules liberated
when 1 gm of a material is converted from liquid to
solid state.
Importance in dentistry:
During casting, the metal must be heated 100 °C more
than its melting temperature for proper melting
SPECIFIC HEAT
It is the quantity of heat needed to raise the temperature of
one gram of the substance 1°C
Metals have low specific heat, while non metals have high
specific heat
Importance in dentistry:
- Because of the low specific heat of dental gold alloys,
prolonged heating is unnecessary, during casting
The science of study of flow and deformation of matter.
Importance in dentistry:
I. Many dental materials are mixed as fluid pastes which
subsequently solidify.
II. The mixed pastes are adapted to the required shape
III. The setting of such materials initially involves a change
in viscosity with time and then the development of an
elastic modulus on solidification
RHEOLOGICAL PROPERTIES
VISCOSITY
Viscosity of a fluid is its resistance to flow.
Viscosity of fluids depends on many factors i.e. the
nature of the substance, temperature, pressure,
etc.
• Newtonian fluid : Ideal fluid demonstrates a
shear stress that is proportional to the strain rate
and thus plot a straight line
Non – Newtonian fluids.
1. Plastic
Some classes of materials behave like a rigid body
until minimum value of shear stress is reached
known as PLASTIC.
E.g. Ketchup – a sharp blow to the bottle is usually
required to produce an initial flow.
2. Psuedoplastic behavior-- Viscosity decreases
with increasing shear until it reaches a nearly
constant value ( Shear thinning ) e.g. polymer,
natural resins , impression materials.
3. Dilatants – Materials whose viscosity increases
with increasing shear rate until constant value
(shear hardening) i.e. becomes more rigid as
rate of deformation increases.
Thixotropic behavior
When some fluids are sheared at a steady rate
viscosity decreases with time. On standing , the
fluid regains its original viscosity, this is called
THIXOTROPY, and is caused by structural
breakdown and reformation.
e.g. fluoride gels, prophylaxis paste, resin
cements, plaster of paris, impression materials
CREEP
Time dependent plastic strain of a material under a
static load or constant stress.
The slow change in the dimensions of an object due
to prolonged (i.e. time dependent) exposure to
stress and high temperature.
FLOW.
Rheology of amorphous material
e.g. waxes has potential to deform under small static
load (own mass)
Metals used in dentistry , for cast restorations or substances for
porcelain veneers have melting points that are much higher than
mouth temperature, and thus are not susceptible to creep
deformation.
An exception is dental amalgam having components with melting
point slightly above room temperature, can slowly creep from
restored tooth
Importance in dentistry:
Galvanism is defined as accelerated corrosion of a metal due
to electrical contact with a more noble metal in a corrosive
electrolyte.
Also called as Wet corrosion.
The resulting current flow can produce nerve stimulation,
unpleasant taste and other physiological reaction.
ELECTROCHEMICAL PROPERTIES
The electrochemical properties of dental materials causes
important phenomenons known as TARNISH and CORROSION.
TARNISH is defined as a process by which a metal surface is
dulled in brightness or discolored through the formation of
chemical film , such as sulfide and an oxide.
CORROSION is defined as the action , process, or effect of
corroding ie, the loss of elemental constituents to the adjacent
environment.
CHEMICAL CORROSION
Direct combination of metal and non metal
components to yield chemical compound.
This is also called as Dry corrosion.
e.g. oxidation of silver-copper alloy particles med
with mercury to prepare dental amalgam
Importance in dentistry:
Stress corrosion
Due to fatigue or cyclic loading in oral cavity.
Concentration cell corrosion
e.g. pitting corrosion or crevice corrosion
The oxygen concentration is lower around amalgam margins
within the crevicular area as compared to outside, so acidity is
greater and plaque accumulation further increases the acidity
which causes corrosion .
Corrosion can be considered somewhat beneficial in case
of amalgam because corrosion products seal the bacterial
leakage from saliva thus improving the longevity of
restoration by decreasing the recurrent caries.
Metallic ions may also serve as bacteriostatic and
bactericidal agents.
However, this type of deterioration can also be harmful
when it occurs in thin sections at solder joints of
appliances .
According to GPT-8, color is defined as
• A phenomenon of light or visual perception that enables one to
differentiate otherwise identical objects.
• The quality of an object or substance with respect to light
reflected or transmitted by it.
• A visual response to light consisting of the three dimensions of
hue, value, and saturation.
• The perception of the Color of an object is the result of a physiological
response to a physical stimulus (light).
• Light is an electromagnetic radiation that can be detected by the human
eye. It can be seen that the visible electromagnetic radiation is in the range
from 400-700 nanometers.
Properties of materials in relation to light transmission and absorption-
Transparency - that allows the passage of light in such a manner that little
distortion takes place.
e.g. glass, pure acrylic resin.
Translucency- which allows the passage of some light and scatters or reflects
the rest . In such manner, the object cannot be clearly seen through them .
e.g. tooth enamel, porcelain, composite and pigmented acrylic resin.
Opacity- that prevents the passage of light. Opaque material absorbs all of
the light. Objects cannot be seen through them.
1- Reflection:
Specular Reflection-
Smooth surface: angle of incidence = angle of reflection
The restoration should have a highly smooth and polished surface to simulate
the tooth structure.
Diffuse Reflection-
Rough surface- the surface appears to have little gloss.
2- Refraction:
It is the change of the direction of a beam of light on entering second
medium.
Refraction results from the difference in refractive indices of the two media.
For perfect matching the refractive index of the restoration should be equal
to the refractive index of the tooth.
Example: Control of refractive index of the filler and matrix phases in
composite resins and porcelain.
3- Scattering
If light rays passing through a medium are obstructed by any different
inclusions it will be redirected in another direction and is attenuated.
↑ OPACITY ↓ TRANSLUCENCY.
Importance in dentistry:
Opacifiers & pigments added to composite resins act as scattering centers
that give rise to opaque shades of the material.
4. Transmission:
Light passing through an optical medium without attenuation
→ completely transmitted.
Total transmission occurs in perfectly transparent materials.
If part of the light is transmitted and part is reflected
( i.e. diffuse transmission), the material appears translucent.
THE THREE DIMENSIONS OF COLOR
Color parameters:
A- Hue: It is the dominant wave length. It represents the color of the material,
i.e yellow, green, red and blue.
B- Chroma: It represents the strength of the color or degree of saturation of the
color (color intensity).
C- Value:
It represents the lightness or darkness of color.
A black standard is assigned a value of 0, whereas a white standard is
assigned 10.
A tooth of low value appears gray and non–vital = DEAD,
Therefore, it is the most important parameter. Because it is intimately
related to the aspect of vitality in human teeth.
Factors affecting color appearance and selection:
According to GPT-8 :
Shade is a term used to describe a particular hue, or variation of a
primary hue such as a greenish shade of yellow.
Tooth color selection is the determination of the color and other attributes
of appearance of an artificial tooth or set of teeth for a given individual.
Shade guide is used for color matching. So, it is important to match colors
under appropriate conditions.
1- Source-
Different sources have different color content.
i.e Incandescent light OR fluorescent light.
Metamerism: It is the change of color matching of two objects under different
light sources.
Metameric pairs: Two objects that are matched in color under one light
source but are not matched under other light sources form metameric pair.
Isomeric pair: They are color matched under all light sources.
Color matching should be done under two or more different light sources.
2- Surroundings:
Colors of wall, lips or clothes of the patient modify the type of
light reaching the object.
3- Object:
• Translucency: High translucency gives a lighter color appearance (higher value)
i.e more vital tooth appearance.
• Surface texture (surface finish): smooth surface appears brighter than rough.
• Presence of scattering centers as inclusions or voids: This increase opacity and
lower the value (more dark).
• Fluorescence: It makes the teeth bright and vital.
• Thickness: The thickness of a restoration can affect its appearance. Increase in
thickness, increase opacity, and lower the value.
• Metamerism
4- Observer:
Eye and color perception
Under low lighting conditions, only the rods are used (scotopic vision).
These receptors allow interpretation of the brightness (but not the color)
of objects to be made. They are most sensitive to blue-green objects.
Color vision is dependent on the cones, which are active under higher
lighting conditions (photopic vision).
The change from photopic to scotopic vision is called Dark Adaptation.
A- Color response: Eye responds differently among individuals.
B- Color Vision: Some individuals may have color blindness and
inability to distinguish certain colors.
C- Color Fatigue: Constant stimulus of one color decreases the
response to that color.
It is measure of the affinity of the liquid for a solid as indicated by
spreading of a drop.
e.g. Wetting of denture base by saliva, wetting of enamel by pit and
fissure sealants .
Wettability of solid by a liquid can be observed by the shape of a
drop of liquid on the solid surface. Shape of the drop can be
identified by contact angle.
.
The degree of wetting depends on the relative surface energies of
the solids and liquids and on their intermolecular attraction.
High energy solids and low energy liquids encourage good wetting
thus liquids generally wet higher energy solids well. e.g. water on
metals and oxides.
On the other hand, liquids bead up on the lower energy solids-
e.g. wax, Teflon, and polymers.
The contact angle is the angle formed by the adhesive with the adherent at their interface.
Hydrophobic surface Hydrophilic surface
The contact angle of water and saliva on denture : it relates to the
retention of the denture.
Contact angle for water on wax is about 110 0 and for water on acrylic is
around 75 0
The spreading of molten solder on the surface of the parts to be
assembled : If the wetting is not adequate, soldering may fail, and if
contact angle of solder is too great, it will not penetrate into the fine
details of metals to be soldered.
ADHESION is defined as the property of remaining in close proximity, as
that resulting from the physical attraction of molecules to a substance or
molecular attraction existing between the surfaces in contact.
BONDING is defined as the procedure of using an adhesive, cementing
material of fusible ingredient to combine, unite, or strengthen .
Attachment of plaque or calculus to the tooth surface can be partially
explained by adhesion attachment.
➢ Leakage adjacent to dental restorative material is severely affected by poor
adhesion and bonding.
➢ Retention of artificial dentures depends on adhesion between the denture and
saliva, and between saliva and soft tissue.
➢ Strong adhesion increases the likelihood that a given restoration or appliance
will be retained on the teeth.
➢ Strong chemical adhesion eliminates the need for excessive removal of tooth
structure to ensure retention, through mechanical undercuts thus chemical
adhesion conserves tooth structures.