PHYSICAL PROPERTIES OF DENTAL MATERIALS

1. PHYSICAL PROPERTIES
OF
DENTAL MATERIALS
ASWATI SOMAN
PG STUDENT
DEPT OF PROSTHODONTICS
PCDS
1

2. Introduction
Rheological properties
Thermal properties
Electrical properties
Chemical properties
Optical properties
Conclusion
References
Contents
2

3. INTRODUCTION
 A physical property is any measurable parameter that describes the state of a
physical system
 A thorough knowledge of fundamental principles of physical science serve
to describe transformation of materials subjected to external influences such
as temperature, force, pressure or light
3

4. Physical properties of Dental Materials are based on
The laws of mechanics
• Creep and flow
• Abrasion and abrasion resistance
• Viscosity
Thermodynamics
• Thermal conductivity
• Thermal difussivity
• Co efficient of thermal expansion
Electrical property
• Galvanism
Chemical property
• Tarnish and corrosion
• Color
• Metamerism
• Fluroscence
Optical property
4

5. RHEOLOGY
 Study of deformation and flow characteristics of matter
 The term can be applied to both liquids and solids.
 Flow properties of materials
• Viscosity
• Viscoelasticity
• Creep
• Thixotropic property
5

6. VISCOSITY
 Resistance of a fluid to flow
 Controlled by internal frictional forces within the liquid
 It is the measure of the consistency of a liquid and resistence to flow
 A highly viscous fluid flows slowly
 Unit: megapascal per sec or centipoisec
6

7. VISCOSITY = SHEAR STRESS /SHEAR RATE
Shear stress = F / A
Shear strain = V / d
• As the shear stress force increases , velocity increases
• A curve is obtained for force verses velocity 7

8.  Curve used to characterize the viscous behaviour of different fluids
 4 types
• Newtonion
• Plastic
• Pseudoplastic
• Dilatant
8

9. NEWTONIAN
• ideal fluid
• Shear stress proportional to shear strain
• Greater the force applied, faster the flow
• Exhibit straight line plot on the graph
PSEUDOELASTIC
• Viscosity of some dental material decrease with increasing strain rate untill it
reaches a constant value.
• An increase in shear rate does not lead to a corresponding increase in
shear stress
• The liquid becomes easier to mix at higher shear rates
9

10. DILATANT LIQUID
• Liquid show an increase in viscosity as the shear rate increases
• Faster they are mixed, more viscous and resistant to flow.
PLASTIC
• Materials behave like a rigid body until an initial shear stress is reached
• Exhibit rigid behaviour initially and then attain a constant velocity
ie, flow in a newtonian fashion
10

11. THIXOTROPY
• Thixotropy is a property of gels or other fluid to become less viscous and flow
when shaken, stirred, patted or vibrated
.
• Depend on the previous deformation.
• Also known as shear thinning fluid
• Occurs due to some molecular arrangement during mixing or lack of time for
molecule to return to normal arrangement before mixing again.
• Longer the fluid is mixed at a given shear rate, lower the shear stress and hence
velocity
• When the shear force is decreased to zero, the viscosity increases to the original
value.
11

12. • Eg : Plaster of paris, Fluoride gels, Resin cements, Prophylatic pastes
• Material does not flow out of a mandibular impression tray until placed over tissues.
• Prophylaxis paste does not flow out of the rubber cup unless rotated against the teeth
12

13. CLINICAL APPLICATION
 For dental materials ,Newtonian and pseudoplastic behaviour are commonly
encountered
 Dilatancy is rare
 Waxes are super cooled liquids that can flow plastically under sustained loading.
 Gypsum products: pseudoplastic mix- on vibration or shaking become thinner and
flow better and then transform from slurries into solid structures.
 Pseudoplastic luting cements[znpo4,znoe]- when pressed hard against tooth
become thinner and excess flow out
 Casting alloy liquids: made thinner by adding iridium-to increase flow to avoid
incomplete casting
13

14. VISCOELASTICITY
 The property of materials that exhibit behavior of both viscous liquid and
elastic solid
 Materials having mechanical properties dependent on loading rate
 Polymers like soft relining materials
 Denture base polymers
 Elastomeric impression- Agar-agar, Alginate
 Creep of dental amalgam
14

15.  An elastic solid can be viewed as a spring and fluid as a dashpot
 When a constant load is applied to a spring , an instant strain occurs and remain
constant with time
 When the load is removed , the strain instantaneously decrease to zero
 When a constant load is applied to an ideal viscous element , strain increase linear with
time
 When the load is removed , no further increase or decrease in strain occur
 The elastic element reacts instantaneously to change in load
 Viscous element react after a finite time
MECHANICAL MODELS OF SOLIDS AND FLUIDS
15

16.  Viscoelastic material exhibit both characteristics of solid and fluid
 Viscoelastic behavior explained in terms of combination of simple model a
spring and dashpot
 Two models
MAXWELL MODEL
VOIGOT MODEL
MECHANICAL MODELS OF VISCOELASTICITY
16

17. MAXWELL MODEL
 When a spring and viscous element are connected in series and a fixed load is
applied
 A rapid increase in strain occurs and is followed by a linear increase in strain with
time
 This resultant strain referred as viscoelastic strain
 The rapid increase in strain represents the elastic portion of strain
 Linear increase represents viscous portion strain
 When load is removed , an instantaneously recovery of the elastic strain occurs
 But viscous strain remains.
17

18. 18
KELVIN MODEL 0R VOIGT MODEL
 Spring and viscous element connected in parallel
 When a constant load is applied, a non linear increase in train with time as a
result of the viscous element and reaches a constant value as a result of the
spring
 On removal of the load, the spring act to decrease the strain to zero
 However the strain doesnot instantaneously diminish to zero because of the
action of dashpot
 Eg. Alginate , ploysulfide , silicone

19. 19
CLINICAL IMPORTANCE
 In case of elastic impression materials, they donot immediately lose theri strain
when load is removed
 Therefore on removal from mouth,these materials remain stressed, and thus time
is required for materials to recover before a die can be poured

20. MECHANICAL MODELS OF VISCOELASTICITY
• Combination of a mechanical model of spring and dashpot
20

21. OPTICAL PROPERTIES
• Light is an electromagnetic radiation detected by the human eye
• The eye is sensitive to wavelength from 400 nm (Violet) to 700nm (Dark red)
• Incident light is polychromatic
• For an object to be visible, it must reflect or transmit light incident
• The reflected light intensities and combined intensities of WL present in
incident and reflected light determine the appearance properties.
• Human more sensitive to light in green- yellow region
• Least sensitive to red or blue
21

22. PROPERTIES OF MATERIALS IN RELATION TO LIGHT TRANSMISSION AND
Transparency
 Property of a material, that allows the passage of light in such a manner that little
distortion takes place
 Objects can be clearly seen through them
 e.g. glass, pure acrylic resin.

Translucency
 Property of the material, which allows the passage of some light and scatters or
reflects the rest .
In such manner, the object cannot be clearly seen through them.
 Translucency decreases with increasing the scattering centers.
 e.g. tooth enamel, porcelain, composite and pigmented acrylic resin natural
teeth.
ABSORPTION
22

23. Opacity
 Property of the material that prevents the passage of light.
Opaque material absorbs all of the light.
Objects cannot be seen through them.
Eg. metal-ceramic restoration
•Black color materials absorb all light colors.
•White color materials reflect all light colors.
•Blue color materials absorb all light colors but reflect its color.
23

24.  Sensation induced from light of varying wavelength reaching eye
The perception of the Color of an object is the result of a physiological
response to a physical stimulus.
Albert Munsell described color as a three dimensional phenomenon
 According to one of Grassman’s Laws, eye can distinguish differences in
only three parameters of color
Dimensions are:
 Hue
 Value
 Chroma
COLOR
24

25. HUE
 The dominant wavelength of a color
 It represents the color of the material
 Primary colors - Red, Blue and Green.
 Any other color may be produced by proper combination of these colors.
 This dimension does not tell whether the color is dark or light or strong or weak
 Least important in shade selection
25

26. VALUE
 Value is the most dominant factor of the three color elements
 Relative lightness or darkness of a color
 Also known as grey scale
 Value of 0 = Back
 Value of 10 = White
 Value increases towards the high end and decreases
towards the low end
26

27. Most relevant parameter in shade matching.
 Teeth and other objects can be separated into lighter and darker shades
 depends on
Incident light
Surface finish of object and the background
 Value can be measured independently of hue.
 Shade guide is arranged on the basis of the Value parameter.
27

28. CHROMA
 It represents the strength of the color or degree of saturation of the color
 Higher chroma more intense the color
 Low chroma appear dull
28

29. MUNSELL COLOR SYSTEM
 A popular system for visual determination of color
 A color system that specifies colors based on three color dimension - Hue , Value , chroma
 Proposed by Albert . H. Munsell
 Every color is alpha numerically labeled with letter for hue , a no. for value, a no. for chroma
 If two munsell color samples are equal on one variable, they will appear the same in that
attribute
29

30. 30

31. VALUE
 The quality by which we distinguish a light color from a dark one
 It is the vertical axis is to our circle of hue
 Value ranges from zero to ten
 Black at the lower pole and white at the top
 This representing total absence of light in lower end and
pure light in the upper end
31

32. CHROMA
 Measured radially outwards from the neutral vertical axis
 Number of steps away from grey
 Colors in centre are dull or grey
 Range 0-10
 Represented by horizontal bar
 Increases along radius
32

33. HUE
 The quality by which we distinguish one color from another as, red from yellow , a
green , a blue from purple
 measured by degrees around horizontal circles
 Measured on a scale from 2.5 to 10 in increments of 2.5 for each of the 10 color
families
 Divided into 10 gradations
Red
Yellow red
Yellow
Green- yellow
Green
Blue- green
Blue
Purple -blue
Purple
Red- purple
33

34. 34

35.  Value is determined first by the selection of a tab that most nearly correspond
with white or darkness of the color.
 Ranges from 0 – 10
 Chroma is determined next with tabs that are close to measured value , but are
of increasing saturation
 Ranges from achromatic or grey to highly saturated color
 Hue is determined by matching with color tabs of value and chroma already
determined
35
SHADE SELECTION

36. SHADE GUIDE
 Shade guides are used in determining the color of natural teeth
 So that the artificial teeth will posses the similar color
 Shade selection involves “ direct visual comparison of different color samples present in
shade guides with the natural teeth and determination of which one suits the teeth
VITAPAN Classical shade guide
 Introduced in 1956
 Widely used in dentistry for color matching in dentistry
 It has 16 shade samples
36

37. VITA SYSTEM 3D MASTER
 It has 26 shades
 Divided into group 1 to 5
 Tabs are marked using a number letter number combination
 First no. 1 -5 represents “Value”
 Letter L ,M , R represents “Hue”
 The second no represents “Chroma”
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38. 38

39. 39

40. 40

41. NATURAL TEETH- DIMENSIONS OF COLOR
 HUE: yellow red to yellow portion of the spectrum
566-586 nm
 CHROMA: low [ .35-.4]
 VALUE: high [36-45]
41

42. Tips for selecting shade
 Ensure the tooth condition is appropriate for matching
 Tooth shade should be determined under daylight or under standardized daylight lamps
 Not under operating lamps
 Eyes usually tire after 5 -7 sec , it is recommended to select quickly
 Avoid bright colors in shade taking environment. ie , lipstick, tinted eyeglasses, no bright
colored clothes.
 Selection distance - selection made at 1 – 2 feet is generally more useful than one made in
close proximity to the teeth
 Evaluate prospective shade guide specimen one at a time by holding them net to the tooth
being matched
42

43. FACTORS AFFECTING COLOR APPEARANCE AND SELECTION
 Shade guide is used for color matching. So, it is important to match colors under
appropriate conditions.
Source
 Different sources have different color content.
 i.e. Incandescent light has a color content different from that of fluorescent light.
43

44. Surrounding
 Colors of wall, lips or clothes of the patient modify the type of light reaching
the object.
Object
 Translucency:
It controls lightness or darkness of color.
High translucency gives a lighter color appearance. i.e more vital tooth
appearance
 Surface texture
Determines the relative amount of light reflected from the surface
Smooth surface appears brighter than rough surface.
 Presence of scattering centers as inclusions or voids
This increase opacity and lower the value (more dark)
44

45. Thickness
 The thickness of a restoration can affect its appearance. Increase in thickness, increase
opacity, and lower the value.
Observer
 Color response
Eye responds differently among individuals.
 Color Vision
Some individuals may have color blindness and inability to distinguish certain colors.
 Color Fatigue
Constant stimulus of one color decreases the response to that color.
45

46. Metamerism
 It is the change of color matching of two objects under different light sources.
 objects under one type of light appears to change when illuminated by different light source
Fluorescence
 Absorption of light by a material and the spontaneous emission of light in a longer wavelength
 Thus tooth actually becomes a light source
 It makes the teeth bright and vital, as it increases the brightness.
 In natural source, it primarily occurs in the dentine because of the higher amount of organic
material
 Under some fluorescent light conditions artificial teeth or restorations without any fluorescence
completely black out.
 Anterior restorative materials have fluorescing agent[ rare earth elements]
Dental porcelain-cerium oxide
46

47. THERMAL PROPERTIES
 Materials placed in oral environment are constantly subjected to change in temperature
 The response a material to a source of heat depends on the ease with which heat is
transferred through the material
 when restorative materials are placed in deep cavities, heat transmitted to the vital pulp
must be limited to prevent thermal shock
 Denture base in contact with mucosal surface – transmission of certain amount of heat
is desirable for sensation of heat and cold
 So it is necessary to understand the thermal properties of dental materials
47

48. THERMAL CONDUCTIVITY
 Physical property that governs the heat transfer through a material by conductive
flow
 Involves transfer of thermal energy from one part of material to another across a
temperature gradient
 It is defined as the quality of heat in calories per sec passing through a material 1cm
thick with cross section of 1cm2 having a temperature difference of 1k
 SI unit – watts per meter per kelvin
48

49.  According to 2nd law of thermodynamics, heat flows from higher temperature point to lower
temperature point
 Materials with high thermal conductivity - Thermal conductors
 Materials with low thermal conductivity - Thermal insulators
 Higher thermal conductivity , greater ability to conduct heat
 Lower thermal conductivity , less ability to conduct heat
Thermal conductivity increases in the order
Polymer < Ceramic < Metals
49

50. MATERIALS THERMAL CONDUCTIVITY
Water 0.44
Dentine 0.57
GIC 0.51 – 0.72
Zinc phosphate 1.05
Composite 1.09 – 1.37
Enamel 0.93
Amalgam 22.6
Pure gold 297
50

51. THERMAL DIFFUSIVITY
 Measure of the speed with which a temperature change will spread through an object
when one surface is heated.
 Calculated from thermal conductivity divided by product of density and heat
h =
k
Cp X p
• h = thermal diffusivity
• k = thermal conductivity
• Cp = heat capacity at constant pressure
• p = temperature dependent density in gram per cm3
51

52. CLINICAL APPLICATION
 In oral environment , rapid change in temperature occurs during ingestion of hot and cold
foods and liquids
 Such conditions thermal diffusivity play an important role
 Enamel and dentine are effective thermal insulators
 The thermal diffusivity and conductivity is comparable with cementing materials like GIC,
composite, zinc phosphate
 When the remaining dentine between the cavity and pulp is too thin , an insulating base
material should be placed below the metallic restoration
 Thermal conductivity of Zinc Oxide Eugenol is slightly less than dentin but its diffusivity
is more than twice that of dentin , therefore greater thickness is required on placement. 52

53. High density and high specific heat
Low thermal diffusivity
Material change temperature slowly
Low heat capacity & high conductivity
Temperature changes rapidly through the material
High thermal diffusivity
o When the product of heat capacity and density high , thermal diffusivity may be low
53

54. MATERIALS THERMAL DIFFUSIVITY
Water 0.0014
Dentine 0.0018 -0.0026
GIC 0.0022
Zinc phosphate 0.003
Composite 0.0019 – 0.0073
Enamel 0.0047
Amalgam 0.96
Pure gold 1.18
54

55. COEFFICIENT OF THERMAL EXPANSION
 The linear coefficient of thermal expansion of materials can be measured by
determination of difference in length of a specimen at two different temperature.
 The change in length(L final – L original) per unit length material for a 1°C
change in temperature is called linear coefficient of expansion
 To make comparisons at different temperature,easier,it is expressed as a
coefficient,
α=L (final)-L(original)/L(original)*(t2-t1)
t - temperature
l - length
55

56. MATERIAL COEFFICIENT(*10¯6)
Human teeth 10-15
Dental amalgam 22-28
Composite 25-68
Gold alloys 12-15
Unfilled plastics and sealants 70-100
Porcelain 8
Inlay wax 300-1000
56

57.  Surface discoloration on a metal or even a slight loss or alteration of the surface finish or
lusture.
 In the oral environment, tarnish often occurs from the formation of hard and soft
deposits on the surface of the restoration.
 Calculus is the principal hard deposit, and its color varies from light yellow to brown.
 Also arise from the formation of thin films of sulfides , oxides or chlorides
TARNISH
57

58.  Corrosion is not a surface discoloration but actual deterioration of a metal by reaction with the
environment .
 Cause severe disintegration of metals
 Disintegration of a metal by the action of corrosion may occur through the action of moisture,
atmosphere, acid or alkaline solutions and certain chemicals.
 Corrosion often preceded by tarnish
 Tarnish film accumulates components that chemically attack the metallic surface.
CORROSION
58

59. CLASSIFICATION
59

60.  In this type of corrosion there is direct combination of metallic and non metallic
elements to yield a chemical compound through the processes
 Oxidation
 Halogenation
 Sulferization
 Also referred as dry corrosion
 Eg. discoloration of silver by sulfide forms by chemical corrosion
 Oxidation of silver-copper alloy particles
CHEMICAL CORROSION
60

61.  Chemical corrosion is seldom isolated and is almost invariably accompanied by
electrochemical corrosion.
 Also referred as “wet corrosion” (requires the presence of water or some other
fluid electrolyte).
 In order to continue the process it requires the pathway for the transport of
electrons (electric current).
 More important for dental restorations.
ELECTROCHEMICAL CORROSION
61

62.  Electrolytic corrosion may takes place in the mouth with saliva acting as an
electrolyte , when the following condition are present in it:
Different metals and alloys
Heterogeneous composition of alloys
Cold work metal
Difference in oxygen tension
 E.g. anode can be dental amalgam, cathode may be gold alloy restoration and
saliva as electrolyte.
62

63.  Galvanic corrosion occurs when dissimilar metals lie in direct physical contact with
each other.
 Eg- if a gold restoration comes in contact with an amalgam restoration , the
amalgam forms the anode and starts corroding.
 When two restoration touch causes sharp pain.
 The best precaution is to avoid dissimilar metals in contact
GALVANIC CORROSION
63

64.  Since the imposition of stress increases the internal energy of an alloy
 Either through the elastic displacement of atoms or the creation of microstrain fields
associated with dislocations, the tendency to undergo corrosion will be increased.
 For most metallic appliances, the deleterious effects of stress and corrosion, called
stress corrosion, are most likely to occur during fatigue or cyclic loading.
 Electrochemical cells consisting of the more deformed metal regions(anodic),saliva
and underformed 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.
STRESS CORROSION
64

65.  Occurs whenever there is variation in electrolytes or in composition of given electrolyte in
a system.
 e.g. Electrolytes produced by food debris.
 Difference in oxygen concentration.
CONCENTRTION CELL CORROSION
65

66. 66
 The delicate tooth pulp should be protected from electric, thermal and chemical
insults and trauma.
 Knowledge of properties and behavior of materials is essential for correct
selection of material, manipulation techniques and precautions which serves in
designing and constructing effective dental prosthesis
CONCLUSION

67. 67
 Phillips ‘Science of Dental materials.11th Ed
by Anusavice
 Restorative Dental Materials .11th Ed
by Robert G.Craig and John M Powers
 Dental Materials-Properties and Manipulation
by Robert G.Craig, John M Powers and John C.Wataha
 Dental Materials and their selection-4th Ed-William J Obrien
 Clinical Aspects of dental materials;Marcia Gladwin and Michael Bagby
REFERRENCE