Dental Materials - Properties of materials

1. Physical properties
of
dental materials
By Dr Ambalika Raj
First Year PGT,
Department of conservative dentistry and endodontics

2. CONTENTS -
• Introduction
• Properties of importance in manufacturing or finishing process
• Optical properties and parameters
• Thermal properties and parameters
• Electrochemical properties
• Conclusion

3. INTRODUCTION
• A physical property is any measurable parameter that describes the
state of a physical system.
• Physical properties are based on the laws of mechanics , acoustics,
optics, thermodynamics, electricity , magnetism , radiation , atomic
structure or nuclear phenomenon .
• Dental materials may fall into any of
the following classes: metals, ceramics, polymers, or composites.

4. • In order to ensure the successful performance of the dental
material , it is imperative to know about their properties.For
example -
• The physical properties of color and thermal expansion are of
particular importance to the performance of dental ceramics .
• Flow and viscosity (the resistance of a fluid to flow) are essential
properties of impression materials .
• Creep (slow deformation under a static load) is relevant to the
clinical performance of amalgam .
• Tarnish and corrosion are electrochemical properties that strongly
affect the performance of metals and their alloys.

5. • Physical properties can be divided into
the following categories –
1. Properties important in manufacturing
and finishing process
2. Optical properties
3. Thermal properties

6. 1.
PROPERTIES OF IMPORTANCE IN MAN
UFACTURING OR FINISHING PROCESSES
Castability Brittleness Creep resistance Hardness
Melting temperature
or melting
temperature range
Flowability under hot-
isostatic-pressing
(HIP) temperature and
pressure conditions
Machinability Polishability

7. 1 . Rheology —
It deals with deformation and flow of matter , whether solid or liquid .
It includes viscosity.
Viscosity is the resistance of a fluid to flow.
High viscosity – material will flow slowly and vice versa.
Most dental materials are initially in a fluid state so that they can be placed
and shaped as required; then they undergo transformation to a solid state, in which
they are durable and perform their function.

8. • Viscosity (h) can be calculated by shear stress divided by strain rate.
• h = τ/ε
• Stress is the force per unit area that develops within a structure when an external
force is applied. This stress causes a deformation, or strain, to develop.
• Strain is calculated as a change in length divided by the initial reference length. If
the two surfaces have an area (A) in contact with the liquid, a shear stress (τ) can be
defined as τ = F/A.
• Strain = Change in length / Original length
• Shear stress is the external force acting on an object or surface parallel to the
slope or plane in which it lies; the stress tending to produce shear.
• The shear strain rate, or rate of change of deformation, is ε = V/d, where d is the
shear distance of the upper surface relative to the fixed lower surface and V is the
velocity of the moving surface . It is the speed or velocity at which deformation of
an object from its original form occurs.

9. 2.
OPTICAL PROPERTIES AND
PARAMETERS
Absorptivity
Color
Fluorescence
Luminescence
Opacity
Photosensitivity
Reflectivity
Refractive index
Translucency
Transmittance

10. COLOR
• An important aspect of dentistry is esthetics – color and
appearance of the dentition.
• The dental materials used should match the natural hard and
soft tissues in appearance.
• Colour is a somewhat subjective phenomenon which may
be judged differently by different observers.
• The eye is sensitive to wavelengths from approximately 400
nm (violet) to 700 nm (dark red) .
• Using the CIE (Commission International de l’Eclairage)
method of colour measurement colour is defined by three
parameters, L, a and b .
Spectrum of visible light ranging in wavelength from 400
nm ( violet ) to 700 nm ( red )most visually perceptible
region of the equal energy spectrum under daylight
conditions is between wavelengths of 540 and 570 nm,
with a maximum value of visual perceptibility at 555 nm
INFRARED
ULTRAVIOLET

11. • The dominant wavelength or hue is represented by the
relative values of a and b and their signs. It is defined as
the dominant color of an object; for example, red, green,
or blue.
• The brightness ( value ) is represented by the value of L,
which indicates the position on the vertical column .
relative lightness or darkness of a color.
• Value is also expressed by the “lightness” factor with
varying levels of gray between the extremes of white and
black.
• Gray scale.
• The colour intensity or chroma is represented by
the distance from the centre of the chart as indicated by
the magnitude of the values of a or b. Defined as 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.The higher the
chroma, the more intense the color.
Three dimensions of color space –
Hue , Value and Chroma

12. •The three dimensions of color space-
•Value increases from black at the bottom center
to white at the top center. Chroma increases from
the center radially outward, and changes in hue
occur in a circumferential direction.
•A, 3-D Munsell Color Space.
•B, Partial color space revealing hue, value, and
chroma regions. (Courtesy of Minolta
Corporation, Instrument Systems Division,
Ramsey, NJ.)

13. • Chroma is not considered separately in dentistry. It is always
associated with the hue and value of dental tissues, restorations, and
prostheses.
• The hue and chroma are inherent properties of materials whereas the
brightness may be affected by factors such as surface finish.
• In many materials (e.g. resin-based products) the initial hue and
chroma are controlled by the manufacturer through the incorporation
of pigments and the use of fillers having varying translucency/opacity
.
• To distinguish these two processes the terms intrinsic and extrinsic
staining are often used. The latter is normally related to the surface
roughness of the material .

14. • Reflection of light from the surface of material and tooth , absorption,
refraction, or transmission (i.e, by passing through unchanged) determine
the opacity, translucency, or transparency of the object.
• Opaque material – absorbs all light falling on it , reflects none.
• Translucent material – absorbs some light and reflects most . E.g. Enamel
• Transparent material - absorb no lights , transmits 100 % .

15. COLOR MATCHING
• In dental practice, color matching is most
often performed with the use of a shade guide
• The individual shades are grouped according
to hue -
1. A – RED BROWN
2. B – RED YELLOW
3. C - GRAY
4. D – RED GRAY , followed by value ( 1 – 4 ;
lightest to darkest )
• This arrangement follows the “classical” order
originated by Vita for porcelain .
• Recent trend is to arrange the shades in
decreasing order of value (lightest to darkest:
B1, A1, B2, D2, A2, C1, C2, D4, A3, D3, B3,
A3.5, B4, C3, A4, C4) .
Tab arrangements of the Vitapan classical shade guide.
A, Manufacturer’s arrangement No. 1: group division according to hue.
B, Manufacturer’s arrangement No. 1: “value scale,” no group division.
C, Alternative arrangement: according to color difference in relation to
the Lightest tab, group division
A
B
C

16. 1. Send drawings , descriptions and
photographs
2. Visual inspection
3. Subjective preference of the patient
4. Effect of the observer
• Bezold-Brucke effect – At low light , rods are
more active than cones , so color perception lost.
As brightness increase , color appears to change
.
A, Two central incisor metal-ceramic crowns
with porceain margins. The value (L*) of these
crowns is higher than that of the
adjacent lateral incisor teeth.
B, Closeup view of the metal ceramic crowns
on the left.
Factors to consider while shade selection

17. • According to a recent study on the repeatability of the human eye compared
to an intraoral scanner in dental shade matching , it was concluded that the
TRIOS intraoral scanner ensured better repeatability than the visual method
in dental shade matching .
• Also ambient lighting had a direct effect on the repeatability of the shade
selection for the visual method, whereas the observer's sex and clinical
experience did not .
• For the visual method, the repeatability in dental shade matching depended
on the dimension studied, with the best results in value, followed by hue and
chroma; however, such dependence was not detected for the intraoral
scanner. ("Repeatability of the human eye compared to an intraoral scanner in dental shade matching " ; Juan Reyes Pamela Acosta Dalina
VenturaAlumnus, Stomatology School, Pontificia Universidad Catolica Madre y Maestra, Santiago de los Caballeros, Santiago, Dominican Republic )

18. 5. The effect of light source -
• the appearance of an object is dependent on the nature of the light in which the
object is viewed. ( spectral content )
• Common sources of light in the dental operatory - Daylight, incandescent, and
fluorescent lamps
• Metamerism
• Color matching should be done under two or more different light sources,
one of which should be daylight .

19. • 6 . Fluorescence - It is the phenomenon in which a TOOTH absorbs
energy ( light ) of shorter wavelength and converts them into light of
longer wavelength. The emitted light, a blue-white color, is primarily
in the 400- to 450-nm range .
• Affects the brightness and vital appearance of a human tooth .
• 7. Radiopacity - the interaction of dental biomaterials with x-radiation
may be classified as an optical property .
• Sufficient radiographic contrast is required in an x-ray image in
order to assess restorations for marginal defects or breakdown,
help differentiate composite restorations from dental caries, and detect
microleakage.

20. • Polymers and resins are inherently radiolucent, whereas metals with atomic
numbers above about 19 (potassium) are inherently radiopaque. To impart
radiopacity, restorative resins often utilize strontium- or barium-containing glass
reinforcing particles ; denture polymers may (but rarely contain barium-sulfate
or other heavy-metal compound additives to render them radiopaque .
• Radiopacity = too low, the resin will not be visible on an x-ray image; when =
too high, it may block out and obscure details of adjacent anatomy.

21. THERMAL PROPERTIES

22. 3. THERMAL PROPERTIES AND PARAMETERS
Coefficient of thermal expansion or contraction
Eutectic temperature
Fusion temperature
Glass transition temperature
Heat of vaporization
Heat of fusion
Liquidus temperature

23. Melting point
Softening point
Solidus temperature
Specific heat
Thermal conductivity
Thermal diffusivity
Vapor pressure
Viscosity

24. • The transmission of a certain amount of thermal energy is desirable to convey
the sensations of heat and cold associated with food and beverages .
• However large amounts of transmitted heat to the pulp can result in thermal
shock and injury.
• These depend on the thermal properties of conductivity and diffusivity.
1. 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 l K (1 °C) and is measured under steady-state conditions in which
the temperature gradient does not change.

25. • In general, thermal conductivities increase in the following order: polymers <
ceramics < metals .
• Materials with high thermal conductivity are called conductors, whereas
materials of low thermal conductivity are insulators.
• The higher its thermal conductivity, the greater the ability of a substance to
transmit thermal energy and vice versa.

26. 2. 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 x ρ
• where h = thermal diffusivity, κ is thermal conductivity, cp = heat capacity at
constant pressure, and ρ = temperature dependent density in grams per cm3 .
• CHANGE IN TEMPERATURE GRADIENT INSIDE THE MOUTH -
THERMAL DIFFUSIVITY – determines the amount of heat transferred from
the material to the tooth.
• NO CHANGE IN TEMPERATURE GRADIENT -
THERMAL CONDUCTIVITY

27. 3. Specific heat - It is the quantity of heat needed to raise the temperature of a
unit mass by l °C.
• If a material has high specific heat and high density , it will take more time
to change its temperature and it will have low thermal diffusivity .
• If a material has high thermal conductivity and high diffusivity , temperature
change will occur rapidly an transmitted rapidly too. This can cause
thermal damage to pulp as it can withstand moderate slowly changing / rise
in temperature.
• Glass ionomer cement - thermal conductivity and diffusivity almost similar to
that of dentin .

29. • Ideally dental restorative materials should have low diffusivity .​
• Metallic restorations like Gold and Amalgam also have high thermal
conductivity and diffusivity .Therefore it is important to use a liner or
protective base before placing these materials.​
• Thermal diffusivity of a dental restorative material is more important than its
thermal conductivity.
• 4. COEFFICIENT OF THERMAL EXPANSION - defined as the change
in length per unit of the original length of a material when its temperature is
raised 1 °C (1 K) .​
• The result is expansion in volume of the material.​
• This has an important implication in cast restorations , amalgam and composite
restorations , metal-ceramic crowns and bridges and while preparing wax
patterns.​

31. • A tooth restoration may expand or contract more than the tooth during a
change in temperature; thus there may be marginal microleakage adjacent to
the restoration, or the restoration may debond from the tooth.
• Aluminous porcelain and Commercially pure titanium have coefficient of
thermal expansion values similar to that of dentin while Type II glass ionomer
• Gold-palladium alloy , Gold (pure) , Palladium-silver alloy, Amalgam and
Composite have COEF values higher than dentin.

32. ELECTRO CHEMICAL PROPERTIES

33. • One of the main factors which determines the durability of a material used in
the mouth is its chemical stability. Materials should not dissolve, erode or
corrode, nor should they leach important or toxic constituents into the oral
fluids.
• Tarnish—Process by which a metal surface is dulled or discolored when a
reaction with a sulfide, oxide, chloride, or other chemical causes surface
discoloration through formation of a thin oxidized film.
• Early indication and precursor of corrosion.

34. • The most important electrochemical effects are toxic and allergic responses to
metal ions released by corrosion, which may affect both nearby tissues and
distant organs .
• BENEFICIAL IN AMALGAM RESTORATION within a limit because it
reduces microleakage by sealing the gap at the margins.
• Corrosion—Chemical or electrochemical process in which a solid, usually
a metal, is attacked by an environmental agent, resulting in partial or
complete dissolution .​
• Corrosion and its influence on durability and appearance are the major ways
in which electrochemistry affects oral well-being.​

35. • BASIS OF CORROSION
Gold - Exists in free form
Ceramics - are already fully oxidized
EXCEPTIONS :-

36. Corrosion
Electrochemical Chemical

37. • Chemical corrosion is the direct combination of metallic and
nonmetallic elements to yield a chemical compound through
oxidation reactions.
• A good example is the discoloration of silver by sulfur, where silver
sulfide forms by chemical corrosion. It can also be a corrosion
product of dental gold alloys that contain silver.
• Dry corrosion

38. • Electrochemical corrosion, also known as
galvanic corrosion, requires the presence
of water or some other fluid electrolyte and
a pathway for the transport of electrons
(i.e., an electrical current).
• It is also referred to as wet corrosion,
since it requires a fluid electrolyte .

39. Electromotive /
galvanic series
Electrode potential
+ ve EP = less
susceptible to
corrosion .
More –ve EP = lose
electrons, corrode
easily
Cathode – Oxidation Anode – Reduction Galvanic corrosion
Single metallic
restoration - GC
Heterogenous surface
composition – GC
Dissimilar metals - GC
Concentration cell
corrosion
Crevice corrosion
Pitting corrosion

41. An electrolytic cell involving different phases within one
alloy. Saliva acts as the electrolyte. Phase A is more
electronegative than phase B. Arrows represent
flow of ELECTRONS

42. Conclusion
• The dental materials used in different clinical conditions depend
on their physical properties which also affect their handling and
storage properties.
• Therefore knowledge of their properties is a must to ensure their
long term survival inside the oral cavity.

43. THANK YOU....