1. Properties of Materials
DHYG 113
Restorative Dentistry I

2. Objectives
Describe the physical, chemical, biologic, and mechanical properties of commonly used dental materials
Explain the application of properties in the use of materials to insulate pulp
Define coefficient of thermal expansion
Define viscosity and wetting and apply to the science of dental materials
Explain how hardness is measured and relate to biocompatibility of various materials

3. Objectives
Define modulus of elasticity and its relationship to stress and strain
Explain how plastic deformation, elastic limit, proportional limit, and yield strength affect materials
Describe the different types of stress in relation to materials
Explain the differences between resilience and toughness

4. Physical Properties Based on Laws of Physics Mass Energy Force Light
Heat
Electricity
Other physical phenomena

5. Density Mass of a material in a given volume
Common value is grams/cubic centimeter
Depends on the type of atoms present
High density metals feel heavy
High atomic numbers
Atoms closely packed

6. Density In the example to the right, both cans have
Comparison of:
 Classic Coke
 Diet Coke
 355 mL
 Water = 355 g
 Sugar = 39 g
 Sugar = 0 g
 Nutra Sweet = 0 g
Nutra Sweet = 0.1g
 Tot. Wgt. = 394 g
Tot. Wgt. = g
In the example to the
right, both cans have
the same volume, but
the classic coke is
more dense because the
sugar weighs more than nutrasweet.
The density of classic coke is 1.11 g/ml,
and the density of diet coke is 1.00 g/ml

7. Properties of Materials
Physical Properties
Based on laws of physics
Mechanical Properties
Material’s ability to resist forces
Chemical Properties
Setting reactions, setting & degradation
Biologic Properties
Effects on living tissues

8. Thermal & Electrical Properties
Materials that conduct electricity need to have insulation from the pulp
Electrical current generation
Usually by means of different metals in contact with each other (Galvanism)
Saliva facilitates flow of electrons between metals, producing an electrical current like a battery
Pain reaction to electrical current in tooth with deep filling (little insulating dentin)
Normally, remaining dentin in a cavity preparation insulates the pulp.
When little dentin remains (within 1 mm of pulp), cement bases can be used to insulate pulp.
Composite and ceramic restorations are nonconductive and do not need insulators.

9. Pulp Insulation
Normally, remaining dentin in a cavity preparation insulates the pulp.
When little dentin remains (within 1 mm of pulp), cement bases can be used to insulate pulp.
Composite and ceramic restorations are nonconductive and do not need insulators.

10. Boiling and Melting Points
Help identify chemicals
Mixtures have boiling range rather than a specific boiling point
Atomic bonds broken by thermal energy
Some materials don’t melt or boil…
Decompose (burn) – wood, cookie dough

11. Vapor Pressure Measure of tendency to evaporate
Higher temperature increases vapor pressure
Molecules escape from liquid to form gas
Useful as solvents
Solvent evaporates, leaving a film of desired material (Copal varnish, etc.)

12. Thermal Conductivity The rate that heat flows through a material
Metals have low heat capacity
Readily warms up and transmits heat
Example: Temperature change of hot food (55°C) and pulp (37°C) provides strong stimulus
Insulating base of .75 – 1mm minimize effects of rapid temperature change

13. Thermal Conductivity Measurement depends on: Distance the heat travels
Difference in temperature between source and destination (water pipe)
Measured in heat flow over time
Calories/second·meter·degree
Insulating material needed to protect pulp with deep metal restoration

14. Thermal Conductivity Measure of heat transfer Rate of heat flow
Thermal Conductivity of Dental Materials
Material
Thermal Conductivity
(cal/sec/cm2 [ºC/cm])
Human enamel
0.0022
Amalgam
0.055
Gold alloy
0.710

15. Heat Capacity
Amount of energy it takes to raise the temperature of that object 1°
Specific heat capacity is the amount of energy it takes to raise the temperature of 1 unit of mass of that material 1°

16. Heat of Fusion & Vaporization
Amount of energy needed to melt a material = heat of fusion
Need 80 times more energy to melt ice than to raise the temperature of water 1°
Amount of energy needed to boil a material
= heat of vaporization
Need 540 times the energy to boil the same quantity of water

17. Coefficient of Thermal Expansion
Measurement of change in volume in relation to a change in temperature
Cooling results in shrinkage/contraction
Compare dental material to tooth
Restoration will shrink with cold and expand with heat
Opens gaps between restoration and tooth = microleakage (may cause recurrent decay)
Opening and closing gap = percolation
Dental amalgam – percolation decreases over
time due to corrosion products from the
amalgam filling the space

18. K ([mcal · cm]/[cm2 · sec · °C])
Thermal Properties
Material
Thermal Expansion
α(x10-6/°C)
Thermal Conductivity
K ([mcal · cm]/[cm2 · sec · °C])
Tooth
8-11
1-2
Porcelain
6-15
2-3
Dental Cement
10-12
1-3
Gold
14-16
710
Amalgam
22-28 55
Composite
20-50
Wax 1

19. Electrical Conductivity
Metals are good conductors
Polymers and ceramics are poor conductors – insulators
Affects corrosion of metals
Electric pulp testing – need to know what material is in or on the tooth

20. Viscosity Ability to flow Measured in grams/meter·second, or poise (P)
Temperature-dependent property
Thick = flow poorly (cold syrup)
Thin = flow easily (warm syrup)
Water at 20ºC = 0.01 P (1 cP)
Impression materials between 100,000 and 1,000,000 cP

21. Wetting
Low viscosity and ability to wet a surface are important in dental materials
Measured by determining the contact angle of a liquid or solid
Low contact angle = good wetting
Example: drop of water on ice cube
High contact angle = poor wetting
Example: drop of water on plastic

22. Wetting Partial wetting-contact angle around 90 degrees
Non wetting-close to 180 degree contact angle

23. Hardness
Measured by pressing a hard shaped tip into the surface of a material
Brinell, Rockwell, Vickers, Knoop
Calculated based on:
Size of indentation
Load on the tip
Shape of the tip
Knoop (KHN): enamel = 350, dentin = 70, porcelain = , acrylic denture teeth = 20

24. Abrasion Resistance Goldilock’s Principle (Just Right!)
Wear resistance of dental materials to food and opposing teeth
Hard enough to wear well, but not wear away opposing teeth

25. Solubility
Calculated by amount of material that dissolves in a given amount of liquid in a given time
Test by immersing in water
Sample weighed before and after
Weight difference is solubility
Dental material should be nearly zero

26. Water Sorption Ability to absorb water Measured much like solubility
Weight gained is
the water sorption

27. Color Complex phenomenon Psychologic response to a physical stimulus
Perception of color may differ between people
Color depends on light (hard to match restorative material to adjacent teeth)
Measured by matching against color tabs
Spectrophotometer (not useful in clinical dentistry)
Fluorescence is important
Color of teeth is in the yellow range

28. Interaction with X-Rays
Some materials are radiolucent
Not seen in XR
Radiopaque – metals
Some materials match radiopacity of enamel to allow diagnosis of recurrent caries – makes them hard to detect on XR, though!

29. Mechanical Properties
Subgroup of physical properties
Describe a material’s ability to resist forces
Elasticity, stress, strain

30. Biting Forces Force: Any push or pull upon matter
Stress: The reaction within the material to an externally applied force
Strain: The change produced within the material as the result of stress

31. Types of Forces Compression – pushing or crushing stress
Average biting force in posterior is ~170 lbs. or about 28,000 psi on a single cusp of a molar

32. ↑ ↓ Forces Tension – pulling stress (tug of war)
Shear – parts of an object slide by each other
Torsion – twisting force
Bending – combination of several types of stresses
One side stretched, other side compressed ↑ ↓

33. Forces in single dimensions
Compression
Tension
Shear
Torsion
Flexure
Diametrical tension/compression
This video also available separately on ANGEL

34. Stress and strain When force is applied to an object, it deforms
Stress-load per unit of cross-sectional area (eg. pounds per square inch); the resistance a material makes to an applied load
Strain-deformation per
unit length; the change in
shape (deformation) a
material makes in
response to stress

35. Stress and strain
If a pile of books is placed on a shelf, the weight of the books exerts a downward force on the shelf. The shelf does not fall down, the shelf resists the weight of the books. This resisting force is stress. If the shelf were to change shape (eg. sag in the middle) as a result of the weight of the books, the amount of change would be the strain.

36. Strain & Stress
Strain : Change in length divided by the original length
Fractions (0.02) or percent (2%)
Stress : Force that develops in loaded object (load)
Stress = load/area
Pounds/square inch (psi)
Stress and strain are proportional

37. Elasticity
When force is removed, the object returns to its original shape
Atomic bonds = microscopic springs
Bending = stretching + compression of atomic bonds
Compression or elongation of a loaded object – measured in terms of change in length

38. Young’s modulus Modulus of elasticity
Measure of the material’s rigidity or stiffness
Resistance of the material to strain or deformation
High modulus = stiff material (enamel)
Low modulus = more flexible (rubber band)
Units are psi, but larger (psi x 106 or gigapascals)

39. Strain
Elastic strain: deformation/strain that is reversible (eg. stretching an elastic band a little and it bounces back to its original shape and size)
Plastic strain: some permanent deformation caused (eg. stretching an elastic band really wide to the point that when it relaxes, it remains a little stretched out)
The tipping point between elastic and plastic strain is the elastic limit

40. Plastic Deformation Stress no longer proportional to strain
Spring doesn’t return to original length
Elastic limit, proportional limit, yield point
Ultimate tensile strength – point where material breaks (failure occurs)
Highest stress on the graph
Bad for bridges – road or dental ones
Ultimate strength = highest stress measured
Compressive test shows compressive (tensile) strength

41. Mechanical Properties of Dental Materials
Resilience – ability to absorb energy and
not be deformed (mouthguard)
Toughness – energy absorbed up to the
failure point on stress/strain diagram (helmet)
Fracture toughness – energy it takes to
fracture a material when a crack is present
Glass = low Metals = high

42. Fatigue Fatigue – testing replicates real world applications
Materials used multiple times – things fail eventually
Testing predicts amount of stress the material can endure without breaking

43. Time-dependent Properties
Creep – very slow flow
Small change in shape when an object is under continuous compression (amalgam)
Takes place over a long period of time
Temperature dependent
Stress relaxation – similar to creep
Slow decrease in force over time (ortho elastics)

44. Stress Concentration Stress focuses around defect
Glass cutter scratches surface (defect)
Bending stress applied; fracture occurs
Control the defects – it’s important in dentistry to handle materials properly
Remove surface defects that concentrate stress
Polish restorations, proper design, glaze porcelain

45. Chemical Properties Decay or degradation Setting Reactions
Gypsum products set by precipitation
Composites polymerize

46. Biologic Properties
Effects of a material on living tissue