A comprehensive slideshow covering all the basics relating to dental materials and their physical properties. Based on standard text books - Phillips Science of Dental Materials (11th Edition).
2. Index
What are physical properties?
Abrasion and Abrasion resistance.
Viscosity
Structural and Stress Relaxation
Creep and Flow
Color and Color Perception
Thermo-physical Properties
Tarnish and Corrosion
3. What are Physical Properties?
Physical properties are based on the laws of
mechanics, acoustics, optics,
thermodynamics, elasticity, magnetism,
radiation, atomic structure, or nuclear
phenomena.
Examples: color, creep, viscosity etc
4. Abrasion and Abrasion
Resistance
Abrasion is the process of scraping or wearing.
(Phillips Science of Dental Materials, 11th edition)
Hardness: Property of being difficult to indent, cut
or scratch. (Phillips Science of Dental Materials, 11th edition)
Simply resistance to plastic deformation or
permanent deformation of material.
Used within a given classification as an index of
the ability of a material to resist abrasion or wear.
5. Abrasion and Abrasion Resistance
contd…
The surface hardness tests commonly used in
dentistry are :
•Brinell hardness test
•Knoop hardness test
•Vickers hardness test
•Rockwell hardness test
•Barcol hardness test
•Shore hardness test
11. Viscosity
Resistance of a liquid to flow
Viscosity is controlled by internal frictional forces and
is measured in poise (Mpa/sec) or centipoise.
Study of flow characteristics of materials and
deformation is rheology.
The term rheology was coined by Eugene C.
Bingham.
12. Viscosity contd…
To explain viscous nature of some materials,
shear stress v/s shear strain rate curve can be
13. Structural and Stress Relaxation
When force applied on a body, internal resistance
to load is stress.
The value of stress which causes an object to
give way or break is called strength of material.
14. Strain can be defined as change in length per
unit length of the body when subjected to stress.
Has no definite unit.
Strain may be elastic /reverts back to original
form or plastic/ permanent deformation.
Structural and Stress Relaxation
Contd…
16. Creep and Flow
Creep is defined as
the time-dependent
plastic strain of a
material under a static
load or constant
stress.
The term creep
implies a relatively
small deformation
produced by a
relatively
large stress over a
17. Creep and Flow contd…
Flow implies a greater
deformation produced
more rapidly with a smaller
applied stress.
The term flow is generally
been used in dentistry to
describe the rheology of
amorphous materials such
as waxes.
18. Color and Color Perception
Color is a sensation induced from light of varying
wavelengths reaching eye.
Cone cells of retina are responsible for spectral
sensitivity
19. Color Perception
The phenomenon of vision can be explained by
considering the response of the human eye to light
reflected from an object.
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.
20. The Three Dimensions of Color
Color perception is
described by three
objective variables:
Hue
Value
Chroma
These three parameters
constitute the three
dimensions of “color
space”.
21. HUE
Hue is defined as the
particular variety of a
color, shade, or tint.
E.g. RED, GREEN
YELLOW
23. Value
Value is defined as the
relative lightness or
darkness of a color or the
brightness of an object
• Value of 0 = black
• Value of 10 = white
24.
25. CIELAB Color System
The Commission Internationale de l’Eclairage
(CIE), an international color research group
published the CIELAB
color system in 1976.
26. Color Terminology
Opacity - The measure of
impenetrability of visible
light.
Translucency-Diffused
transmission of visible light
Transparency- capable of
transmitting light
28. Thermophysical Properties
Thermophysical properties can be simply
defined as material properties that vary with
temperature without altering the material's
chemical identity.
However, it has become customary to limit the
scope of the term to properties having a bearing
on the transfer and storage of heat
These are:
Thermal Conductivity
Thermal Diffusivity
Coefficient of thermal expamsion
30. Thermal Diffusivity
It is a measure of the rate at which a body with a
no uniform temperature reaches a state of
thermal equilibrium
The thermal conductivity of zinc oxide-eugenol is
slightly less than that of dentin, its thermal
diffusivity is more than twice that of dentin.
31. Thermal Diffusivity contd…
Material Thermal
Diffusivity (in
10-4 cm2 /sec)
Pure Gold 11,800
Amalgam 960
Composite 19 to 73
Water 14
Glass Ionomer 22
Dentin 18 to 26
Enamel 47
Zinc Phosphate 30
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.
32. Coefficient of Thermal Expansion
The coefficient of thermal expansion describes
how the size of an object changes with a change in
temperature.
It measures of the fractional change in size per
degree change in temperature at a constant
pressure.
33. Tarnish and Corrosion
TARNISH:
• Tarnish is a surface
discoloration of the metal or
even a slight loss of the
lustre.
• This surface discoloration
is due to either, the
formation of hard and soft
deposits on the surface, or
the formation of thin films of
oxides, sulphides or
chlorides.
• Tarnish is often the first
step of corrosion
34. Tarnish and Corrosion contd…
CORROSION:
Corrosion is not only a surface discoloration but
is a disintegration of a metal by reaction with its
environment.
37. Dry Corrosion
In which there is a direct
combination of metallic
and non-metallic
elements
Electrolytes are absent
e.g. oxidation,
halogenations, or
sulfarization reaction.
39. Dissimilar Metals
Galvanism is results of
difference in potential
between dissimilar
restoration in opposing or
adjacent tooth.
40. Heterogeneous Surface Composition
• A second type of electrolytic
corrosion is that due to
heterogeneous compositions
of the metal surface. Example
of this type may be the
intectic and peritectic alloys.
• When such an alloy is
immersed is an electrolyte,
the metallic grains with the
less electrode potential are
attacked and corrosion
results.
• A common situation for this
type of corrosion would be an
amalgam restorations with
polished and unpolished
area.
41. Stress Corrosion
Degradation by the combined effect of
mechanical stress and corrosive environment,
usually in form of cracking.
42. Concentration Cell Corrosion
Example: A metallic
restoration which is partly
covered by food debris will
differ from that of saliva, and
this can contribute to the
corrosion of the restoration.
43. Protection Against Corrosion
Avoid using dissimilar metals.
The surface of any dental restoration should be
smooth and lustrous. A polished, smooth surface
provides easier cleaning and prevents
accumulation of debris.
44. Clinical Significance of Galvanic
Currents
As long as metallic dental restoration materials
are employed there seems to be little possibility
that the galvanic currents can be eliminated.
Galvanic Shock A pain sensation caused by
electric current
generated by a contact between two dissimilar
metal forming a galvanic cell in oral environment.
It can be a real source of discomfort to an
occasional patient. Such post operative pain
usually occurs immediately after insertion of a
new restoration and generally is gradually
subsides and disappears in a few days.
45. 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.
When tooth shade is selected using conventional means,
knowledge & skill of practitioner comes into play.
Technique based system provide dentist with distinct
advantage in creating highly aesthetics, natural looking
restoration.
46. References:
CONTENT:
Phillips science of dental material 10th & 11th edition
Restorative dental materials – craig
13th edition
Dental materials and their selection- 3rd edition by
william j. O'brien
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Notas do Editor
Oldest methods used to test metals and alloys
Small steel or tungsten carbide ball, typically 1.6 mm in diameter, subjected to a weight of 123 N and it remain in contact to specimen for 30 sec
Smaller the area of indentation, harder the material and larger the BHN value
The method is similar to BHN except that a 136 degree diamond pyramid shaped indenter is forced into the material with a definite load application.
The indenter produces a square indentation, the diagonals of which are measured. It is suitable for brittle materials so it is used for the measurement of hardness of tooth structures and also of dental casting gold alloys.
A significant advantage of this test is that it can be used for testing very small specimens because the indenter tip is small.
Also, because the load applied to the specimen can be varied, the test can be used on materials that are soft as well as on those that are hard.
Knoop Indenter: Highly polished, Rhombic-based, pyramidal diamond that produces a diamond shaped indentation.
Ratio between the long and short diagonals of about 7:1. the Pyramid shape has an angle, Longitudinally of 172 and transverse of 130
•Used for thin plastic or metal sheets •Advantage of this method is that materials with a great range of hardness can be tested by varying the test load.•Disadvantage is that it needs very high polished and flat surface.
This was developed as a rapid method for hardness determination. A ball or metal cone indenter is normally used and 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 to 150 kg)
The advantages of Rockwell system is that hardness is read directly and it is good for testing viscoelastic materials. The disadvantages are that a preload is needed, greater time required and the indentation may disappear immediately on removal of the load.
This method is used to study the depth of cure of resin composites.The Barcol indenter is a spring loaded needle with a diameter of 1 mm that is pressed against the surface to be tested. If no penetration of the needle into the surface occurs the scale reads 100. The reading on the scale decreases as the indenter penetrates the surface.
Depth of cure of a resin composite is tested by preparing specimens varying in thickness from 0.5 to 6.0 mm. Then the top surface of a specimen is activated by a light curing unit. The Barcol hardness of the top surface is compared with that of the bottom surface. 10% decrease in Barcol hardness of a resin composite results in a 20% decrease in the flexural strength.
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.
Materials like cements and impression materials undergo a liquid-to-solidtransformation in the mouth.
Gypsum products used in the fabrication of models and dies are transformed from slurries into solid structuresAmorphous materials such as waxes and resins appear solid but actually are supercooled liquids that can flow plastically under sustained loading ordeform elastically under small stresses.
NEWTONIAN FLUID:- an ideal fluid - Shear stress proportional to strain rate -Straight line on 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. e.g. Ketchup, blood, nail-polish
DILATENT FLUIDViscosity increase with increasing stress. The material become more rigid under stress(disadvantage) e.g.-Acrylic denture base material, sand in waterPLASTIC FLUIDMaterial behaves rigid until a minimum of stress is applied ,then it starts behaving like Newtonian fluid. e.g.- clay suspension, composite material
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.
We can see here that the paper clip, made of metal has been permanently deformed, now there are trapped internal stresses. The atoms in the crystal structure are displaced and the system is not in equilibrium.
In amorphous structures, some molecules are too close together and others too far apart when the substance is permanently deformed.
The displaced atoms are in unstable positions. They can slowly return to their equilibrium positions by a solid-state diffusion which is dependent on thermal energy. The result is a change in the shape or contour of the solid. The material warps or distorts. Such stress relaxation can be a problem with elastomeric impression materials and can lead to distortions in the impression.
The rate of relaxation increases with an increase in temperature. In this example, the clip tends to straighten out the moment it is dropped in hot water, but at room temperature, there is negligible change.
There are many non-crystalline dental materials (such as waxes, resins) that, when manipulated and cooled, can undergo relaxation with consequent distortion at an elevated temperature. This may result in improper fit of prosthesis.
Metal creep usually occurs as the temperature increases to within a few hundred degrees of the melting range. Metals used in dentistry for cast restorations or substrates for porcelain veneers have melting points (for pure metals) or melting ranges (for alloys) that are much higher than mouth temperatures, and they are not susceptible to creep deformation intraorally.
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.
The flow of wax is a measure of its potential to deform under a small static load even that associated with its own mass.
An important goal of dentistry is to restore or improve esthetics—the color and appearance of natural dentition. Esthetic considerations in restorative and prosthetic dentistry have received increasingly greater emphasis in recent times.
For an object to be visible, it must reflect or transmit light incident on it from an external source. The incident light is usually polychromatic; that is, a mixture of the various wavelengths, commonly known as “white” light. Incident light is selectively absorbed or scattered (or both) at certain wavelengths.
Cone-shaped cells in the retina are responsible for color vision. These cells have a threshold intensity required for color vision and respond to wavelengths for both normal color vision and color-deficient vision. Someone with normal vision has maximum sensitivity in the green-yellow region at about 550 nm and is least sensitive in the red and blue-violet regions of the spectrum.
The signals from the retina are processed by the brain to produce the psychophysiological perception of color. Because a neural response is involved in color vision, constant stimulation by a single color may result in color fatigue and a decrease in the eye’s response to that color. Defects in certain portions of the color-sensing receptors result in the different types of color blindness; thus humans vary greatly in their ability to distinguish colors.
Verbal descriptions of color are not precise enough to describe the appearance of teeth.
These definitions are too variable, complex, and imprecise to describe a desired color of a dental crown to a laboratory technician.
Professor Albert H. Munsell•Basic principles first published in 1905•Hues are divide into 10 gradations:yellow, yellow-red, red, red-purple,purple, purple-blue, blue, blue-green, green
•3-dimensional color system• L* refers to brightness (0 to 100)• a* represents red (+a*) vs. green (-a*)• b* indicates yellow (+b*) vs. blue (-b*).•When a* and b* are zero, the L value represents the continuum of black to white
Metamerism -Phenomenon in which the color of an object under one type of light appears to change when illuminated by different light source.
Different light sources interact differently with different materials.
Fluorescence is the absorption of light by a material and the spontaneous emission of light in a longer wavelength.•In a natural tooth, it primarily occurs in the dentin because of the higher amount of organic material.•UV light is absorbed and fluoresced back as light primarily in the blue end of the spectrum.
• Heat transfer through solid substances is most commonly occur by means of conduction.• The conduction of heat through metals occurs through the interactions of crystal lattice vibrations and by the motion of electrons and their interaction with atoms.
In the oral environment, temperatures are not constant during the ingestion of foods and liquids. Under such conditions, thermal diffusivity is important.
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.
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.
• Stain or discoloration arises from pigment – producing bacteria, drugs containing chemicals such as iron or mercury & absorbed food debris.
• Although deposits are the main cause of the tarnish in the oral environment, surface discoloration may also arise on a metal from the formation of thin films, such as oxides, sulfides, or chlorides.
• Corrosion may occur due to moisture, atmosphere, acid or alkaline solutions, & certain chemicals.• Common e.g. rusting of iron, a complex chemical reaction in which iron combines with oxygen in air & water to form hydrated oxide of iron.
Eggs & other food contain significant amount of sulfur, various sulfides (hydrogen or ammonium sulfide) corrode silver, copper, mercury & similar metals present in dental alloys.
Water, oxygen & chlorine ions present in saliva.
Various acidic solutions such as phosphoric , acetic, & lactic acids at proper concentration & pH promote corrosion.
This type of corrosion is less susceptible to occur in the mouth.
Example, oxidation of metal surface during soldering and heat treatmentprocedures.
Galvanic corrosion (also called bimetallic corrosion) is an electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another, in the presence of an electrolyte.
In a solid solution, any cored structure is less resistant to corrosion than the homogenized structure because of difference in electrode potential.
Even an homogenized solid solution in some what susceptible to corrosion because of the difference in structure between the grains and their boundaries.
The grains boundaries may act as anodes and the interior of the grains as the cathode. This results in the corrosion of the material in the anodic region at the grain boundaries.
Example, a metal which has been stressed by burnishing produces the localized stress in some part of structure If stressed and under stressed metals are in contact in an electrolyte the stressed area will become anode and will corrode.
A homogeneous metal or alloy can undergo electrolytic corrosion wherethere is a difference in electrolyte concentration across the specimen.
• Certain metals develop a thin ,adherent , highly protective film by reaction with the environment, such a metal is said to be passive.
• A thin surface oxide forms on chromium , a good e.g. of a passivating metal, & stainless steel contain sufficient amounts of chromium to passivate the alloy.
The cement base itself although it is a good thermal insulator, has little effect in minimizing the current that is carried into the tooth and through the pulp.
It is likely that the physiologic condition of the tooth is the primary factor responsible for the pain resulting from this current flow.
Once the tooth has recovered from the injury of preparing the cavity and has returned to a more normal physiological condition the current flow then produces no response