DM- Ch 3 Physical and Mechanical Properties of Dental Materials

Physical properties

• Based on the laws of physics

• Describe mass, energy, force, light, heat, electricity, and other properties

Mechanical Properties

Describe a material's ability to resist forces

Dependent on:

• Amount of the material

• Size of the object

• Shape of the object

Chemical Properties:

Describe the:

• Setting reaction of materials

• Decay or degradation of materials

Biologic Properties:

• Describe the effects the material has on living tissue

Density (PP)

• Material mass per given volume

• Measures how tightly matter is packed together in a certain amount of space

• "how much stuff is in a given amount of space"

Boiling and Melting Points (PP)

• Breaking of atomic bonds between atoms by thermal energy

• Temperature range vs. specific temperature

Vapor pressure (PP)

• Measure of a liquid's tendency to evaporate and become a gas

• As temperature increases, vapor pressure increases

Thermal conductivity (PP)

• Rate of heat flow through a

material

• Measured as heat flow over time

Depends on:

• Distance heat travels

• Area in which heat travels

• Temperature difference between source and destination

Heat Capacity (Thermal capacity) (PP)

Amount of heat that must be applied to an object to produce a change in its temperature

Specific heat capacity

• The number of heat units needed to raise the temperature of something by

one degree

Heat of Fusion and Vaporization (PP)

Heat of Fusion

• Amount of energy required to melt a material

Heat of Vaporization

• Amount of energy required to boil a material

Coefficient of Thermal Expansion (PP)

A measure of how the volume or length of an object changes in relation to the change in temperature

• Materials shrink when cooled, expand when heated

Close matching of the coefficient of thermal expansion is important

between the tooth and restorative materials to prevent marginal leakage

Percolation

Opening and closing of the gap between tooth structure and a restoration due to differences in coefficients of thermal expansion

Percolation results in

• Microleakage

• Tooth sensitivity

• Recurrent decay

• Discoloration

Interface

Gap between tooth structure and a restoration

Electrical Conductivity (PP)

How well a material allows electricity to flow through it

Good electrical conductors

Metals

Poor electrical conductors (insulators)

• Polymers

• Ceramics

Viscosity (PP)

Ability of a material to flow

High viscosity

poor wetting

• Low flow

Low viscosity

good wetting

• High flow

Wetting (PP)

• The ability of a liquid to wet or intimately contact a solid surface.

• Water beading on a waxed car is an example of poor wetting

Hardness (PP)

Measure of resistance of a solid towards deformation

A microhardness test that measures a material's resistance to indentation

Knoop hardness

• Enamel = 350

• Dentin = 70

• Porcelain = 400-500

• Acrylic denture teeth = 20

• Diamond = 7000

Durometer Measurements

Standardized way of measuring the hardness of materials like rubber or plastic

Durometer

A tool used to apply a consistent force to a material and measure the resulting indentation

Durometer determines the hardness of

• Impression materials

• Other elastic polymers

Abrasion Resistance (PP)

An ability to withstand the wear and tear of friction caused by mechanical force

- Goldilocks Principle—not too hard, not too soft,

just right

Restorative material too hard

wears away opposing teeth

• Porcelain vs. enamel

Restorative material is too soft

restoration wears away

Solubility (PP)

Amount of material that dissolves in liquid

Materials placed in the oral cavity

are exposed to various aqueous fluids

Excessive solubility in the mouth leads to

• Loss of material

• Recurrent decay

Water Sorption (PP)

Weight gained when a dental material is immersed in water

Material absorbs water and swells:

• Ex: cookie dunked in milk

Color (PP)

Describes how a substance interacts with light by absorbing and reflecting specific wavelengths of the visible spectrum

- Standardization by using shade guides when working with dental materials

Interaction of Materials with X-Rays Based on Atomic Number (PP)

Radiolucent or radiopaque

Radiolucent

• Some ceramic materials

• Denture acrylic resin

• Dark/black on radiographs

Radiopaque

• Metal restorations

• Materials with radiopaque fill

• Light/white on radiographs

Force (MP)

Weight or load applied to an object

• Push and pull on an object resulting from the objects interaction with another

object

Stress (MP)

Force divided by the area in which it is applied

• An internal resistance to deformation when under force

Elasticity (MP)

The ability of an object or material to resume its normal

shape after being stretched of compressed

Strain (MP)

The deformation of a material from stress

- The change in length divided by the original length

Strain measured as

• Fraction

• Percentage

Stress

Force per unit area

• Force that develops in a loaded object

• Proportional to load

• Related to size of the object

Stress formula

load/area

Stress is measured in

• Pounds per square inch (psi)

• Pascals (metric

Relationship of Stress and Strain (MP)

Distortion or deformation that occurs when an object cannot resist a stress

Proportional relationship

Strain is directly proportional to the stress applied

- Always occur together

Modulus of elasticity

• Stress/strain

• Characteristic of a material and its atomic bonds

• High modulus of elasticity = stiff material

Stress-strain Plot/Graph (MP)

A graphical depiction of a materials behavior when subjected to increasing loads

Stress-strain Plot/Graph shows

• Elastic deformation

• Plastic/permanent deformation

• Elastic limit, proportional limit, or yield point

• Ultimate strength

Types of stress (MP)

compression, tension, shear, torsion, bending

Compression

pushing stress

Tension

pulling stress

Shear (slip)

sliding stress

Torsion

twisting stress

Bending

compression + tension stress

Poisson Ratio (MP)

A mechanical ratio of strain in the direction of stress to the strain in the direction perpendicular to the stress

• A mathematical equation is used

Ex of poisson ratio

If you stretch a rubber band it gets longer and longitudinal strain is caused.

As the rubber band gets longer, it gets thinner laterally causing lateral strain

Resilience (MP)

Material has the ability to absorb energy without becoming deformed

Toughness (MP)

Energy absorbed up to the failure point on the stress-strain plot

Fracture Toughness (MP)

Measure of the energy required to fracture a material when a crack is present

Low-fracture toughness

• Glass

• Dental porcelain

High-fracture toughness

• Metals

Fatigue (MP)

Material/object failure due to being stressed repetitively for a long time, stress over time

Time-Dependent Properties (MP)

creep and stress relaxation

Creep

Small change in shape when an object is under continuous compression

Stress relaxation

Slow decrease in force over time